Internet Engineering Task Force                             Eddie Kohler   
INTERNET-DRAFT                                                      UCLA   
draft-ietf-dccp-spec-08.txt                                 Mark Handley   
draft-ietf-dccp-spec-07.txt                                 Mark Handley
Expires: 25 April 2005                                               UCL   
Expires: April 2005                                                  UCL
                                                             Sally Floyd   
                                                                    ICIR   
                                                         25 October 2004   
                                                                           
                                                                           
              Datagram Congestion Control Protocol (DCCP)                  
                                                                           
                                                                           
Status of this Memo                                                        
                                                                           
    This document is an Internet-Draft and is subject to all provisions    
    This document is an Internet-Draft.                                 
    of section 3 of RFC 3667.  By submitting this Internet-Draft, each     
    By submitting this Internet-Draft, we certify that any applicable   
    author represents that any applicable patent or other IPR claims of    
    patent or other IPR claims of which we are aware have been          
    which he or she is aware have been or will be disclosed, and any of    
    disclosed, or will be disclosed, and any of which we become aware   
    which he or she become aware will be disclosed, in accordance with     
    will be disclosed, in accordance with RFC 3668 (BCP 79).            
    RFC 3668.                                                              
    By submitting this Internet-Draft, we accept the provisions of      
                                                                           
    Section 3 of RFC 3667 (BCP 78).                                     
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    Drafts.                                                                
                                                                           
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    http://www.ietf.org/1id-abstracts.html                              
                                                                           
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    http://www.ietf.org/shadow.html                                     
                                                                           
    This Internet-Draft will expire on 25 April 2005.                      
                                                                           
Copyright Notice                                                           
                                                                           
    Copyright (C) The Internet Society (2004). All Rights Reserved.        
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
Kohler/Handley/Floyd                                            [Page 1]   
                                                                          
INTERNET-DRAFT           Expires: 25 April 2005             October 2004   
                                                                           
                                                                           
Abstract                                                                   
                                                                           
    The Datagram Congestion Control Protocol (DCCP) is a transport         
    protocol that provides bidirectional unicast connections of            
    protocol that implements bidirectional, unicast connections of      
    congestion-controlled unreliable datagrams.  DCCP is suitable for      
    congestion-controlled, unreliable datagrams.  It should be suitable 
    applications that transfer fairly large amounts of data, but can       
    for use by applications such as streaming media, Internet telephony,
    benefit from control over the tradeoff between timeliness and          
    and on-line games.                                                  
    reliability.                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
Kohler/Handley/Floyd                                            [Page 2]   
                                                                          
INTERNET-DRAFT           Expires: 25 April 2005             October 2004   
                                                                           
                                                                           
    TO BE DELETED BY THE RFC EDITOR UPON PUBLICATION:                      
                                                                           
    Changes since draft-ietf-dccp-spec-07.txt:                             
                                                                           
    * Many changes, not listed here, for WGLC.                             
                                                                           
    * The more stringent Sequence Number checks on DCCP-Sync and DCCP-     
    SyncAck packets become SHOULD, not MAY.                                
                                                                           
    Changes since draft-ietf-dccp-spec-06.txt:                             
                                                                           
    * Change extended sequence numbers.  Now 48-bit sequence numbers are   
    MANDATORY, and all packet types except Data, Ack, and DataAck always   
    use 48-bit sequence numbers.  This change improves DCCP's robustness   
    against blind attacks.                                                 
                                                                           
    * Removed empty Change options.                                        
                                                                           
    * Allow preference list changes during feature negotiations (this      
    seems easier to implement than the alternative).  This required a      
    new feature negotiation state, UNSTABLE.                               
                                                                           
    * Add Minimum Checksum Coverage feature.                               
                                                                           
    * Add Reset Congestion State option.                                   
                                                                           
    * Simplify the implementation of CCID-specific option processing: no   
    need to check whether the CCID feature is being negotiated.            
                                                                           
    * Many more minor changes.                                             
                                                                           
    Changes since draft-ietf-dccp-spec-05.txt:                             
                                                                           
    * Organization overhaul.                                               
                                                                           
    * Add pseudocode for event processing.                                 
                                                                           
    * Remove # NDP; replace with Ack Count.                                
                                                                           
    * Remove Identification, Challenge, ID Regime, and Connection Nonce.   
                                                                           
    * Data Checksum (formerly Payload Checksum) uses a 32-bit CRC.         
                                                                           
    * Switch location of non-negotiable features to clarify                
    presentation; now the feature location controls its value.             
                                                                           
    * Rename "value type" to "reconciliation rule".                        
                                                                           
                                                                           
                                                                           
                                                                           
Kohler/Handley/Floyd                                            [Page 3]   
                                                                          
INTERNET-DRAFT           Expires: 25 April 2005             October 2004   
                                                                           
                                                                           
    * Rename "Reset Reason" to "Reset Code".                               
                                                                           
    * Mobility ID becomes 128 bits long.                                   
                                                                           
    * Add probabilities to Mobility ID discussion.                         
                                                                           
    * Add SyncAck.                                                         
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
Kohler/Handley/Floyd                                            [Page 4]   
                                                                          
INTERNET-DRAFT           Expires: 25 April 2005             October 2004   
                                                                           
                                                                           
                             Table of Contents                             
                                                                           
    1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . .  10   
    1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   7
    2. Design Rationale. . . . . . . . . . . . . . . . . . . . . . .  11   
    2. Design Rationale. . . . . . . . . . . . . . . . . . . . . . .   8
    3. Conventions and Terminology . . . . . . . . . . . . . . . . .  12   
    3. Conventions and Terminology . . . . . . . . . . . . . . . . .   9
       3.1. Numbers and Fields . . . . . . . . . . . . . . . . . . .  12   
       3.1. Numbers and Fields . . . . . . . . . . . . . . . . . . .   9
       3.2. Parts of a Connection. . . . . . . . . . . . . . . . . .  13   
       3.2. Parts of a Connection. . . . . . . . . . . . . . . . . .   9
       3.3. Features . . . . . . . . . . . . . . . . . . . . . . . .  13   
       3.3. Features . . . . . . . . . . . . . . . . . . . . . . . .  10
       3.4. Round-Trip Times . . . . . . . . . . . . . . . . . . . .  14   
       3.4. Round-Trip Times . . . . . . . . . . . . . . . . . . . .  10
       3.5. Security Limitation. . . . . . . . . . . . . . . . . . .  14   
       3.5. Security Limitation. . . . . . . . . . . . . . . . . . .  11
       3.6. Robustness Principle . . . . . . . . . . . . . . . . . .  14   
       3.6. Robustness Principle . . . . . . . . . . . . . . . . . .  11
    4. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . .  14   
    4. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . .  11
       4.1. Packet Types . . . . . . . . . . . . . . . . . . . . . .  15   
       4.1. Packet Types . . . . . . . . . . . . . . . . . . . . . .  11
       4.2. Sequence Numbers . . . . . . . . . . . . . . . . . . . .  16   
       4.2. Sequence Numbers . . . . . . . . . . . . . . . . . . . .  13
       4.3. States . . . . . . . . . . . . . . . . . . . . . . . . .  17   
       4.3. States . . . . . . . . . . . . . . . . . . . . . . . . .  13
       4.4. Congestion Control . . . . . . . . . . . . . . . . . . .  19   
       4.4. Congestion Control . . . . . . . . . . . . . . . . . . .  15
       4.5. Features . . . . . . . . . . . . . . . . . . . . . . . .  19   
       4.5. Features . . . . . . . . . . . . . . . . . . . . . . . .  16
       4.6. Differences From TCP . . . . . . . . . . . . . . . . . .  20   
       4.6. Differences From TCP . . . . . . . . . . . . . . . . . .  17
       4.7. Example Connection . . . . . . . . . . . . . . . . . . .  21   
       4.7. Example Connection . . . . . . . . . . . . . . . . . . .  18
    5. Packet Formats. . . . . . . . . . . . . . . . . . . . . . . .  23   
    5. Header Formats. . . . . . . . . . . . . . . . . . . . . . . .  19
       5.1. Generic Header . . . . . . . . . . . . . . . . . . . . .  23   
       5.1. Generic Header . . . . . . . . . . . . . . . . . . . . .  20
       5.2. DCCP-Request Packets . . . . . . . . . . . . . . . . . .  27   
       5.2. DCCP-Request Header. . . . . . . . . . . . . . . . . . .  23
       5.3. DCCP-Response Packets. . . . . . . . . . . . . . . . . .  28   
       5.3. DCCP-Response Header . . . . . . . . . . . . . . . . . .  23
       5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Packets. . . . . .  28   
       5.4. DCCP-Data, DCCP-Ack, and DCCP-DataAck Headers. . . . . .  24
       5.5. DCCP-CloseReq and DCCP-Close Packets . . . . . . . . . .  30   
       5.5. DCCP-CloseReq and DCCP-Close Headers . . . . . . . . . .  26
       5.6. DCCP-Reset Packets . . . . . . . . . . . . . . . . . . .  30   
       5.6. DCCP-Reset Header. . . . . . . . . . . . . . . . . . . .  26
       5.7. DCCP-Sync and DCCP-SyncAck Packets . . . . . . . . . . .  33   
       5.7. DCCP-Sync and DCCP-SyncAck Headers . . . . . . . . . . .  29
       5.8. Options. . . . . . . . . . . . . . . . . . . . . . . . .  34   
       5.8. Options. . . . . . . . . . . . . . . . . . . . . . . . .  30
          5.8.1. Padding Option. . . . . . . . . . . . . . . . . . .  36   
          5.8.1. Padding Option. . . . . . . . . . . . . . . . . . .  31
          5.8.2. Mandatory Option. . . . . . . . . . . . . . . . . .  36   
          5.8.2. Mandatory Option. . . . . . . . . . . . . . . . . .  31
    6. Feature Negotiation . . . . . . . . . . . . . . . . . . . . .  37   
    6. Feature Negotiation . . . . . . . . . . . . . . . . . . . . .  32
       6.1. Change Options . . . . . . . . . . . . . . . . . . . . .  37   
       6.1. Change Options . . . . . . . . . . . . . . . . . . . . .  33
       6.2. Confirm Options. . . . . . . . . . . . . . . . . . . . .  38   
       6.2. Confirm Options. . . . . . . . . . . . . . . . . . . . .  33
       6.3. Reconciliation Rules . . . . . . . . . . . . . . . . . .  38   
       6.3. Reconciliation Rules . . . . . . . . . . . . . . . . . .  34
          6.3.1. Server-Priority . . . . . . . . . . . . . . . . . .  38   
          6.3.1. Server-Priority . . . . . . . . . . . . . . . . . .  34
          6.3.2. Non-Negotiable. . . . . . . . . . . . . . . . . . .  39   
          6.3.2. Non-Negotiable. . . . . . . . . . . . . . . . . . .  34
       6.4. Feature Numbers. . . . . . . . . . . . . . . . . . . . .  39   
       6.4. Feature Numbers. . . . . . . . . . . . . . . . . . . . .  35
       6.5. Examples . . . . . . . . . . . . . . . . . . . . . . . .  40   
       6.5. Examples . . . . . . . . . . . . . . . . . . . . . . . .  35
       6.6. Option Exchange. . . . . . . . . . . . . . . . . . . . .  41   
       6.6. Option Exchange. . . . . . . . . . . . . . . . . . . . .  37
          6.6.1. Normal Exchange . . . . . . . . . . . . . . . . . .  42   
          6.6.1. Normal Exchange . . . . . . . . . . . . . . . . . .  37
          6.6.2. Processing Received Options . . . . . . . . . . . .  42   
          6.6.2. Processing Received Options . . . . . . . . . . . .  38
          6.6.3. Loss and Retransmission . . . . . . . . . . . . . .  44   
          6.6.3. Loss and Retransmission . . . . . . . . . . . . . .  40
          6.6.4. Reordering. . . . . . . . . . . . . . . . . . . . .  45   
          6.6.4. Reordering. . . . . . . . . . . . . . . . . . . . .  41
          6.6.5. Preference Changes. . . . . . . . . . . . . . . . .  46   
          6.6.5. Preference Changes. . . . . . . . . . . . . . . . .  42
          6.6.6. Simultaneous Negotiation. . . . . . . . . . . . . .  46   
          6.6.6. Simultaneous Negotiation. . . . . . . . . . . . . .  42
          6.6.7. Unknown Features. . . . . . . . . . . . . . . . . .  46   
          6.6.7. Unknown Features. . . . . . . . . . . . . . . . . .  42
          6.6.8. Invalid Options . . . . . . . . . . . . . . . . . .  47   
          6.6.8. Invalid Options . . . . . . . . . . . . . . . . . .  43
          6.6.9. Mandatory Feature Negotiation . . . . . . . . . . .  48   
          6.6.9. Mandatory Feature Negotiation . . . . . . . . . . .  43
                                                                           
          6.6.10. Out-of-Band Agreement. . . . . . . . . . . . . . .  44
                                                                           
    7. Sequence Numbers. . . . . . . . . . . . . . . . . . . . . . .  44
                                                                           
       7.1. Variables. . . . . . . . . . . . . . . . . . . . . . . .  44
Kohler/Handley/Floyd                                            [Page 5]   
       7.2. Initial Sequence Numbers . . . . . . . . . . . . . . . .  45
                                                                          
INTERNET-DRAFT           Expires: 25 April 2005             October 2004   
       7.3. Quiet Time . . . . . . . . . . . . . . . . . . . . . . .  46
                                                                           
       7.4. Acknowledgement Numbers. . . . . . . . . . . . . . . . .  46
                                                                           
       7.5. Validity and Synchronization . . . . . . . . . . . . . .  47
    7. Sequence Numbers. . . . . . . . . . . . . . . . . . . . . . .  48   
          7.5.1. Sequence-Validity Rules . . . . . . . . . . . . . .  47
       7.1. Variables. . . . . . . . . . . . . . . . . . . . . . . .  49   
          7.5.2. Handling Sequence-Invalid Packets . . . . . . . . .  49
       7.2. Initial Sequence Numbers . . . . . . . . . . . . . . . .  49   
          7.5.3. Sequence and Acknowledgement Number                    
       7.3. Quiet Time . . . . . . . . . . . . . . . . . . . . . . .  50   
          Windows. . . . . . . . . . . . . . . . . . . . . . . . . .  50
       7.4. Acknowledgement Numbers. . . . . . . . . . . . . . . . .  51   
          7.5.4. Sequence Window Feature . . . . . . . . . . . . . .  51
       7.5. Validity and Synchronization . . . . . . . . . . . . . .  51   
          7.5.5. Sequence Number Attacks . . . . . . . . . . . . . .  52
          7.5.1. Sequence and Acknowledgement Number                       
          7.5.6. Examples. . . . . . . . . . . . . . . . . . . . . .  53
          Windows. . . . . . . . . . . . . . . . . . . . . . . . . .  52   
       7.6. Short Sequence Numbers . . . . . . . . . . . . . . . . .  54
          7.5.2. Sequence Window Feature . . . . . . . . . . . . . .  53   
          7.6.1. Allow Short Sequence Numbers Feature. . . . . . . .  54
          7.5.3. Sequence-Validity Rules . . . . . . . . . . . . . .  53   
          7.6.2. When to Avoid Short Sequence Numbers. . . . . . . .  55
          7.5.4. Handling Sequence-Invalid Packets . . . . . . . . .  55   
       7.7. NDP Count and Detecting Application Loss . . . . . . . .  55
          7.5.5. Sequence Number Attacks . . . . . . . . . . . . . .  56   
          7.7.1. Usage Notes . . . . . . . . . . . . . . . . . . . .  56
          7.5.6. Examples. . . . . . . . . . . . . . . . . . . . . .  57   
          7.7.2. Send NDP Count Feature. . . . . . . . . . . . . . .  57
       7.6. Short Sequence Numbers . . . . . . . . . . . . . . . . .  58   
    8. Event Processing. . . . . . . . . . . . . . . . . . . . . . .  57
          7.6.1. Allow Short Sequence Numbers Feature. . . . . . . .  59   
       8.1. Connection Establishment . . . . . . . . . . . . . . . .  57
          7.6.2. When to Avoid Short Sequence Numbers. . . . . . . .  59   
          8.1.1. Client Request. . . . . . . . . . . . . . . . . . .  57
       7.7. NDP Count and Detecting Application Loss . . . . . . . .  60   
          8.1.2. Service Codes . . . . . . . . . . . . . . . . . . .  58
          7.7.1. Usage Notes . . . . . . . . . . . . . . . . . . . .  61   
          8.1.3. Server Response . . . . . . . . . . . . . . . . . .  59
          7.7.2. Send NDP Count Feature. . . . . . . . . . . . . . .  61   
          8.1.4. Init Cookie Option. . . . . . . . . . . . . . . . .  60
    8. Event Processing. . . . . . . . . . . . . . . . . . . . . . .  61   
          8.1.5. Handshake Completion. . . . . . . . . . . . . . . .  61
       8.1. Connection Establishment . . . . . . . . . . . . . . . .  62   
       8.2. Data Transfer. . . . . . . . . . . . . . . . . . . . . .  62
          8.1.1. Client Request. . . . . . . . . . . . . . . . . . .  62   
       8.3. Termination. . . . . . . . . . . . . . . . . . . . . . .  62
          8.1.2. Service Codes . . . . . . . . . . . . . . . . . . .  63   
          8.3.1. Abnormal Termination. . . . . . . . . . . . . . . .  64
          8.1.3. Server Response . . . . . . . . . . . . . . . . . .  64   
       8.4. DCCP State Diagram . . . . . . . . . . . . . . . . . . .  64
          8.1.4. Init Cookie Option. . . . . . . . . . . . . . . . .  65   
       8.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . .  65
          8.1.5. Handshake Completion. . . . . . . . . . . . . . . .  66   
    9. Checksums . . . . . . . . . . . . . . . . . . . . . . . . . .  69
       8.2. Data Transfer. . . . . . . . . . . . . . . . . . . . . .  66   
       9.1. Header Checksum Field. . . . . . . . . . . . . . . . . .  69
       8.3. Termination. . . . . . . . . . . . . . . . . . . . . . .  67   
       9.2. Header Checksum Coverage Field . . . . . . . . . . . . .  70
          8.3.1. Abnormal Termination. . . . . . . . . . . . . . . .  69   
          9.2.1. Minimum Checksum Coverage Feature . . . . . . . . .  71
       8.4. DCCP State Diagram . . . . . . . . . . . . . . . . . . .  69   
       9.3. Data Checksum Option . . . . . . . . . . . . . . . . . .  71
       8.5. Pseudocode . . . . . . . . . . . . . . . . . . . . . . .  70   
          9.3.1. Check Data Checksum Feature . . . . . . . . . . . .  72
    9. Checksums . . . . . . . . . . . . . . . . . . . . . . . . . .  74   
          9.3.2. Usage Notes . . . . . . . . . . . . . . . . . . . .  73
       9.1. Header Checksum Field. . . . . . . . . . . . . . . . . .  75   
    10. Congestion Control IDs . . . . . . . . . . . . . . . . . . .  73
       9.2. Header Checksum Coverage Field . . . . . . . . . . . . .  76   
       10.1. Unspecified Sender-Based Congestion Control . . . . . .  74
          9.2.1. Minimum Checksum Coverage Feature . . . . . . . . .  77   
       10.2. TCP-like Congestion Control . . . . . . . . . . . . . .  75
       9.3. Data Checksum Option . . . . . . . . . . . . . . . . . .  77   
       10.3. TFRC Congestion Control . . . . . . . . . . . . . . . .  76
          9.3.1. Check Data Checksum Feature . . . . . . . . . . . .  78   
       10.4. CCID-Specific Options, Features, and Reset                 
          9.3.2. Usage Notes . . . . . . . . . . . . . . . . . . . .  78   
       Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . .  76
    10. Congestion Control . . . . . . . . . . . . . . . . . . . . .  79   
    11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  78
       10.1. TCP-like Congestion Control . . . . . . . . . . . . . .  80   
       11.1. Acks of Acks and Unidirectional Connections . . . . . .  78
       10.2. TFRC Congestion Control . . . . . . . . . . . . . . . .  80   
       11.2. Ack Piggybacking. . . . . . . . . . . . . . . . . . . .  80
       10.3. CCID-Specific Options, Features, and Reset                    
       11.3. Ack Ratio Feature . . . . . . . . . . . . . . . . . . .  80
       Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . .  80   
       11.4. Ack Vector Options. . . . . . . . . . . . . . . . . . .  82
       10.4. CCID Profile Requirements . . . . . . . . . . . . . . .  83   
          11.4.1. Ack Vector Consistency . . . . . . . . . . . . . .  84
       10.5. Congestion State. . . . . . . . . . . . . . . . . . . .  83   
          11.4.2. Ack Vector Coverage. . . . . . . . . . . . . . . .  85
    11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  84   
       11.5. Send Ack Vector Feature . . . . . . . . . . . . . . . .  86
       11.1. Acks of Acks and Unidirectional Connections . . . . . .  84   
       11.6. Slow Receiver Option. . . . . . . . . . . . . . . . . .  86
       11.2. Ack Piggybacking. . . . . . . . . . . . . . . . . . . .  86   
       11.7. Reset Congestion State Option . . . . . . . . . . . . .  87
                                                                           
       11.8. Data Dropped Option . . . . . . . . . . . . . . . . . .  87
                                                                           
          11.8.1. Data Dropped and Normal Congestion                    
                                                                           
          Response . . . . . . . . . . . . . . . . . . . . . . . . .  90
Kohler/Handley/Floyd                                            [Page 6]   
          11.8.2. Particular Drop Codes. . . . . . . . . . . . . . .  90
                                                                          
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    12. Explicit Congestion Notification . . . . . . . . . . . . . .  91
                                                                           
       12.1. ECN Capable Feature . . . . . . . . . . . . . . . . . .  92
                                                                           
       12.2. ECN Nonces. . . . . . . . . . . . . . . . . . . . . . .  92
       11.3. Ack Ratio Feature . . . . . . . . . . . . . . . . . . .  86   
       12.3. Other Aggression Penalties. . . . . . . . . . . . . . .  93
       11.4. Ack Vector Options. . . . . . . . . . . . . . . . . . .  88   
    13. Timing Options . . . . . . . . . . . . . . . . . . . . . . .  94
          11.4.1. Ack Vector Consistency . . . . . . . . . . . . . .  90   
       13.1. Timestamp Option. . . . . . . . . . . . . . . . . . . .  94
          11.4.2. Ack Vector Coverage. . . . . . . . . . . . . . . .  92   
       13.2. Elapsed Time Option . . . . . . . . . . . . . . . . . .  94
       11.5. Send Ack Vector Feature . . . . . . . . . . . . . . . .  92   
       13.3. Timestamp Echo Option . . . . . . . . . . . . . . . . .  95
       11.6. Slow Receiver Option. . . . . . . . . . . . . . . . . .  93   
    14. Maximum Packet Size. . . . . . . . . . . . . . . . . . . . .  96
       11.7. Data Dropped Option . . . . . . . . . . . . . . . . . .  93   
    15. Forward Compatibility. . . . . . . . . . . . . . . . . . . .  99
          11.7.1. Data Dropped and Normal Congestion                       
    16. Middlebox Considerations . . . . . . . . . . . . . . . . . .  99
          Response . . . . . . . . . . . . . . . . . . . . . . . . .  96   
    17. Relations to Other Specifications. . . . . . . . . . . . . . 101
          11.7.2. Particular Drop Codes. . . . . . . . . . . . . . .  97   
       17.1. DCCP and RTP. . . . . . . . . . . . . . . . . . . . . . 101
    12. Explicit Congestion Notification . . . . . . . . . . . . . .  98   
       17.2. Multiplexing Issues . . . . . . . . . . . . . . . . . . 102
       12.1. ECN Incapable Feature . . . . . . . . . . . . . . . . .  98   
    18. Security Considerations. . . . . . . . . . . . . . . . . . . 102
       12.2. ECN Nonces. . . . . . . . . . . . . . . . . . . . . . .  99   
       12.3. Other Aggression Penalties. . . . . . . . . . . . . . . 100   
    13. Timing Options . . . . . . . . . . . . . . . . . . . . . . . 100   
       13.1. Timestamp Option. . . . . . . . . . . . . . . . . . . . 101   
       13.2. Elapsed Time Option . . . . . . . . . . . . . . . . . . 101   
       13.3. Timestamp Echo Option . . . . . . . . . . . . . . . . . 102   
    14. Maximum Packet Size. . . . . . . . . . . . . . . . . . . . . 103   
       14.1. Measuring PMTU. . . . . . . . . . . . . . . . . . . . . 104   
       14.2. Sender Behavior . . . . . . . . . . . . . . . . . . . . 105   
    15. Forward Compatibility. . . . . . . . . . . . . . . . . . . . 106   
    16. Middlebox Considerations . . . . . . . . . . . . . . . . . . 107   
    17. Relations to Other Specifications. . . . . . . . . . . . . . 108   
       17.1. RTP . . . . . . . . . . . . . . . . . . . . . . . . . . 108   
       17.2. Congestion Manager and Multiplexing . . . . . . . . . . 109   
    18. Security Considerations. . . . . . . . . . . . . . . . . . . 110   
       18.1. Security Considerations for Partial                           
       Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 110   
       Checksums . . . . . . . . . . . . . . . . . . . . . . . . . . 103
    19. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 111   
    19. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 104
       19.1. Packet Types. . . . . . . . . . . . . . . . . . . . . . 111   
    20. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
       19.2. Reset Codes . . . . . . . . . . . . . . . . . . . . . . 111   
    A. Appendix: Ack Vector Implementation Notes . . . . . . . . . . 105
       19.3. Option Types. . . . . . . . . . . . . . . . . . . . . . 112   
       A.1. Packet Arrival . . . . . . . . . . . . . . . . . . . . . 107
       19.4. Feature Numbers . . . . . . . . . . . . . . . . . . . . 112   
          A.1.1. New Packets . . . . . . . . . . . . . . . . . . . . 107
       19.5. Congestion Control Identifiers. . . . . . . . . . . . . 112   
          A.1.2. Old Packets . . . . . . . . . . . . . . . . . . . . 108
       19.6. Ack Vector States . . . . . . . . . . . . . . . . . . . 113   
       A.2. Sending Acknowledgements . . . . . . . . . . . . . . . . 109
       19.7. Drop Codes. . . . . . . . . . . . . . . . . . . . . . . 113   
       A.3. Clearing State . . . . . . . . . . . . . . . . . . . . . 110
       19.8. Service Codes . . . . . . . . . . . . . . . . . . . . . 113   
       A.4. Processing Acknowledgements. . . . . . . . . . . . . . . 111
    20. Thanks . . . . . . . . . . . . . . . . . . . . . . . . . . . 113   
    B. Appendix: Design Motivation . . . . . . . . . . . . . . . . . 112
    A. Appendix: Ack Vector Implementation Notes . . . . . . . . . . 114   
       B.1. CsCov and Partial Checksumming . . . . . . . . . . . . . 112
       A.1. Packet Arrival . . . . . . . . . . . . . . . . . . . . . 116   
    Normative References . . . . . . . . . . . . . . . . . . . . . . 113
          A.1.1. New Packets . . . . . . . . . . . . . . . . . . . . 116   
    Informative References . . . . . . . . . . . . . . . . . . . . . 114
          A.1.2. Old Packets . . . . . . . . . . . . . . . . . . . . 117   
    Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 116
       A.2. Sending Acknowledgements . . . . . . . . . . . . . . . . 118   
    Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 116
       A.3. Clearing State . . . . . . . . . . . . . . . . . . . . . 119   
    Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 116
       A.4. Processing Acknowledgements. . . . . . . . . . . . . . . 120   
    B. Appendix: Design Motivation . . . . . . . . . . . . . . . . . 121   
       B.1. CsCov and Partial Checksumming . . . . . . . . . . . . . 121   
                                                                           
                                                                           
                                                                           
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    Normative References . . . . . . . . . . . . . . . . . . . . . . 122   
    Informative References . . . . . . . . . . . . . . . . . . . . . 123   
    Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 125   
    Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 125   
    Intellectual Property. . . . . . . . . . . . . . . . . . . . . . 125   
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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                               List of Tables                              
                                                                           
    Table 1: DCCP Packet Types . . . . . . . . . . . . . . . . . . .  25   
    Table 2: DCCP Reset Codes. . . . . . . . . . . . . . . . . . . .  33   
    Table 3: DCCP Options. . . . . . . . . . . . . . . . . . . . . .  35   
    Table 4: DCCP Feature Numbers. . . . . . . . . . . . . . . . . .  39   
    Table 5: DCCP Congestion Control Identifiers . . . . . . . . . .  79   
    Table 6: DCCP Ack Vector States. . . . . . . . . . . . . . . . .  88   
    Table 7: DCCP Drop Codes . . . . . . . . . . . . . . . . . . . .  95   
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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1.  Introduction                                                           
                                                                           
    The Datagram Congestion Control Protocol (DCCP) is a transport         
    protocol that implements bidirectional, unicast connections of         
    congestion-controlled, unreliable datagrams.  Specifically, DCCP       
    provides:                                                              
                                                                           
    o  Unreliable flows of datagrams, with acknowledgements.               
                                                                           
    o  Reliable handshakes for connection setup and teardown.              
                                                                           
    o  Reliable negotiation of options, including negotiation of a         
       suitable congestion control mechanism.                              
                                                                           
    o  Mechanisms allowing servers to avoid holding state for              
       unacknowledged connection attempts and already-finished             
       connections.                                                        
                                                                           
    o  Congestion control incorporating Explicit Congestion Notification   
       (ECN) and the ECN Nonce, as per [RFC 3168] and [RFC 3540].          
                                                                           
    o  Acknowledgement mechanisms communicating packet loss and ECN        
       information.  Acks are transmitted as reliably as the relevant      
       congestion control mechanism requires, possibly completely          
       reliably.                                                           
                                                                           
    o  Optional mechanisms that tell the sending application, with high    
       reliability, which data packets reached the receiver, and whether   
       those packets were ECN marked, corrupted, or dropped in the         
       receive buffer.                                                     
                                                                           
    o  Path Maximum Transmission Unit (PMTU) discovery, as per [RFC        
    o  Path Maximum Transfer Unit (PMTU) discovery, as per [RFC 1191].  
       1191].                                                              
    DCCP is intended for applications, such as streaming media and      
                                                                           
    Internet telephony, where the costs of long delays can exceed the   
    o  A choice of modular congestion control mechanisms.  Two             
    costs of loss and out-of-order delivery.  TCP is not well-suited for
       mechanisms are currently specified, TCP-like Congestion Control     
    these applications, since its reliable in-order delivery, combined  
       [CCID 2 PROFILE] and TFRC (TCP-Friendly Rate Control) Congestion    
    with congestion control, can cause arbitrarily long delays.  UDP    
       Control [CCID 3 PROFILE], but DCCP is easily extensible to          
    avoids long delays, but UDP applications must implement congestion  
       further forms of unicast congestion control.                        
    control on their own.  DCCP provides built-in congestion control,   
                                                                           
    including ECN support, for unreliable datagram flows.  DCCP avoids  
    DCCP is intended for applications that can benefit from control over   
    the arbitrary delays associated with TCP.  It also implements       
    the tradeoffs between delay and reliable in-order delivery.  Such      
    reliable connection setup, teardown, and feature negotiation, and   
    applications include streaming media and Internet telephony.  TCP is   
    provides a choice of congestion control mechanisms.                 
    not well-suited for these applications, since reliable in-order        
    delivery and congestion control can cause arbitrarily long delays.     
    UDP avoids long delays, but UDP applications that implement            
    congestion control must do so on their own.  DCCP provides built-in    
    congestion control, including ECN support, for unreliable datagram     
                                                                           
                                                                           
                                                                           
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    flows, avoiding the arbitrary delays associated with TCP.  It also     
    implements reliable connection setup, teardown, and feature            
    negotiation.                                                           
                                                                           
2.  Design Rationale                                                       
                                                                           
    One DCCP design goal was to give most streaming UDP applications       
    Most streaming UDP applications should have little reason not to    
    little reason not to switch to DCCP, once it is deployed.  To          
    switch to DCCP, once it is deployed.  To facilitate this, DCCP was  
    facilitate this, DCCP was designed to have as little overhead as       
    designed to have as little overhead as possible, both in terms of   
    possible, both in terms of the packet header size and in terms of      
    the packet header size and in terms of the state and CPU overhead   
    the state and CPU overhead required at end hosts.  Only the minimal    
    required at end hosts.  Only the minimal necessary functionality was
    necessary functionality was included in DCCP, leaving other            
    included in DCCP, leaving other functionality, such as forward error
    functionality, such as forward error correction (FEC), semi-           
    correction (FEC), semi-reliability, and multiple streams, to be     
    reliability, and multiple streams, to be layered on top of DCCP as     
    layered on top of DCCP as desired.  This desire for minimal overhead
    desired.                                                               
    is also one of the reasons to avoid proposing an unreliable variant 
                                                                           
    of the Stream Control Transmission Protocol (SCTP, [RFC 2960]).     
    Different forms of conformant congestion control are appropriate for   
    different applications.  For example, on-line games might want to      
    make quick use of any available bandwidth, while streaming media       
    might trade off this responsiveness for a steadier, less bursty        
    rate.  (Sudden rate changes can cause unacceptable UI glitches, such   
    as audible pauses or clicks in the playout stream.)  DCCP thus         
    allows applications to choose from a set of congestion control         
    allows applications to choose between several forms of congestion   
    mechanisms.  One alternative, TCP-like Congestion Control, halves      
    control.  One choice, TCP-like Congestion Control, halves the       
    the congestion window in response to a packet drop or mark, as in      
    congestion window in response to a packet drop or mark, as in TCP.  
    TCP.  Applications using this congestion control mechanism will        
    Applications using this congestion control mechanism will respond   
    respond quickly to changes in available bandwidth, but must tolerate   
    quickly to changes in available bandwidth, but must tolerate the    
    the abrupt changes in congestion window typical of TCP.  A second      
    abrupt changes in congestion window typical of TCP.  A second       
    alternative, TCP-Friendly Rate Control (TFRC, [RFC 3448]), a form of   
    equation-based congestion control, minimizes abrupt changes in the     
    sending rate while maintaining longer-term fairness with TCP.  Other   
    sending rate while maintaining longer-term fairness with TCP.       
    alternatives can be added as future congestion control mechanisms      
    are standardized.                                                      
                                                                           
    DCCP also lets unreliable traffic safely use ECN.  A UDP kernel API    
    might not allow applications to set UDP packets as ECN-capable,        
    since the API could not guarantee the application would properly       
    detect or respond to congestion.  DCCP kernel APIs will have no such   
    issues, since DCCP implements congestion control itself.               
                                                                           
    We chose not to require the use of the Congestion Manager [RFC         
    3124], which allows multiple concurrent streams between the same       
    sender and receiver to share congestion control.  The current          
    Congestion Manager can only be used by applications that have their    
    own end-to-end feedback about packet losses, but this is not the       
    case for many of the applications currently using UDP.  In addition,   
    the current Congestion Manager does not easily support multiple        
    congestion control mechanisms, or lend itself to the use of forms of   
                                                                           
                                                                           
                                                                           
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    TFRC where the state about past packet drops or marks is maintained    
    at the receiver rather than at the sender.  DCCP should be able to     
    make use of CM where desired by the application, but we do not see     
    any benefit in making the deployment of DCCP contingent on the         
    deployment of CM itself.                                               
                                                                           
    We intend for DCCP's protocol mechanisms, which are described in       
    this document, to suit any application desiring unicast congestion-    
    controlled streams of unreliable datagrams.  The congestion control    
    mechanisms currently approved for use with DCCP, which are described   
    in separate Congestion Control ID Profiles [CCID 2 PROFILE] [CCID 3    
    PROFILE], may, however, cause problems for some applications,          
    including high-bandwidth interactive video.  These applications        
    should be able to use DCCP once suitable Congestion Control ID         
    Profiles are standardized.                                             
                                                                           
3.  Conventions and Terminology                                            
                                                                           
    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",    
    "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in       
    this document are to be interpreted as described in [RFC 2119].        
                                                                           
3.1.  Numbers and Fields                                                   
                                                                           
    All multi-byte numerical quantities in DCCP, such as port numbers,     
    Sequence Numbers, and arguments to options, are transmitted in         
    network byte order (most significant byte first).                      
                                                                           
    We occasionally refer to the "left" and "right" sides of a bit         
    field.  "Left" means towards the most significant bit, and "right"     
    means towards the least significant bit.                               
                                                                           
    Random numbers in DCCP are used for their security properties, and     
    SHOULD be chosen according to the guidelines in [RFC 1750].            
    MUST be chosen according to the guidelines in [RFC 1750].           
                                                                           
    All operations on DCCP sequence numbers, and comparisons such as       
    "greater" and "greatest", use circular arithmetic modulo 2**48.        
    This form of arithmetic preserves the relationships between sequence   
    numbers as they roll over from 2**48 - 1 to 0.  We note that the       
    numbers as they roll over from 2**48 - 1 to 0.                      
    two's-complement trick for implementing circular comparison --         
    namely, A < B in the circular comparison sense if and only if          
    (A - B) < 0 in the conventional arithmetic sense -- applies directly   
    to DCCP sequence numbers, as long as they are stored in the most       
    significant 48 bits of 64-bit integers.                                
                                                                           
    Reserved bitfields in DCCP packet headers MUST be set to zero by       
    senders, and MUST be ignored by receivers, unless otherwise            
    specified.  This is to allow for future protocol extensions.  In       
                                                                           
                                                                           
                                                                           
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    particular, DCCP processors MUST NOT reset a DCCP connection simply    
    because a Reserved field has non-zero value [RFC 3360].                
                                                                           
3.2.  Parts of a Connection                                                
                                                                           
    Each DCCP connection runs between two hosts, which we often name       
    Each DCCP connection runs between two endpoints, which we often name
    DCCP A and DCCP B.  Each connection is actively initiated by one of    
    DCCP A and DCCP B.                                                  
    the hosts, which we call the client; the other, initially passive      
    DCCP connections are actively initiated by one endpoint.  The active
    host is called the server.  The term "DCCP endpoint" is used to        
    endpoint is called the client, and the passive endpoint is called   
    refer to either of the two hosts explicitly named by the connection    
    the server.                                                         
    (the client and the server).  The term "DCCP processor" refers more    
    DCCP connections are bidirectional; data may pass from either       
    generally to any host that might need to process a DCCP header; this   
    includes the endpoints and any middleboxes on the path, such as        
    firewalls and network address translators.                             
                                                                           
    DCCP connections are bidirectional: data may pass from either          
    endpoint to the other.  This means that data and acknowledgements      
    may be flowing in both directions simultaneously.  Logically,          
    however, a DCCP connection consists of two separate unidirectional     
    connections, called half-connections.  Each half-connection consists   
    of the application data sent by one endpoint and the corresponding     
    acknowledgements sent by the other endpoint.  We can illustrate this   
    as follows:                                                            
                                                                           
     +--------+  A-to-B half-connection:         +--------+                
     |        |    -->  application data  -->    |        |                
     |        |    <--  acknowledgements  <--    |        |                
     | DCCP A |                                  | DCCP B |                
     |        |  B-to-A half-connection:         |        |                
     |        |    <--  application data  <--    |        |                
     +--------+    -->  acknowledgements  -->    +--------+                
                                                                           
    Although they are logically distinct, in practice the half-            
    connections overlap; a DCCP-DataAck packet, for example, contains      
    application data relevant to one half-connection and acknowledgement   
    information relevant to the other.                                     
                                                                           
    In the context of a single half-connection, the terms "HC-Sender"      
    and "HC-Receiver" denote the endpoints sending application data and    
    acknowledgements, respectively.  For example, DCCP A is the HC-        
    Sender and DCCP B is the HC-Receiver in the A-to-B half-connection.    
                                                                           
3.3.  Features                                                             
                                                                           
    A DCCP feature is a connection attribute on whose value the two        
    endpoints agree.  Many properties of a DCCP connection are             
    controlled by features, including the congestion control mechanisms    
    in use on the two half-connections.  The endpoints achieve agreement   
    in use on the two half-connections.  The endpoints can achieve      
                                                                           
    agreement through the exchange of feature negotiation options in    
                                                                           
    DCCP headers, or through out-of-band communication.                 
                                                                           
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    through the exchange of feature negotiation options in DCCP headers.   
                                                                           
    DCCP features are identified by a feature number and an endpoint.      
    The notation "F/X" represents the feature with feature number F        
    located at DCCP endpoint X.  Each valid feature number thus            
    corresponds to two features, which are negotiated separately and       
    need not have the same value.  The two endpoints know, and agree on,   
    the value of every valid feature.  DCCP A is the "feature location"    
    for all features F/A, and the "feature remote" for all features F/B.   
                                                                           
3.4.  Round-Trip Times                                                     
                                                                           
    DCCP round-trip time measurements are performed by congestion          
    We sometimes refer to round-trip times -- for setting timers, for   
    control mechanisms; different mechanisms may measure round-trip time   
    example.  If no useful round-trip time estimate is available, a DCCP
    in different ways, or not measure it at all.  However, the main DCCP   
    implementation SHOULD use 0.1 seconds instead.                      
    protocol does use round-trip times occasionally, such as in the        
    initial values for certain timers.  Each DCCP implementation thus      
    defines a default round-trip time for use when no estimate is          
    available; this parameter should default to not less than              
    0.2 seconds, a reasonable median round-trip time for Internet TCP      
    connections.  Protocol behavior specified in terms of "round-trip      
    time" values actually refers to "a current round-trip time estimate    
    taken by some CCID, or, if no estimate is available, the default       
    round-trip time parameter".                                            
                                                                           
    The maximum segment lifetime, or MSL, is the maximum length of time    
    a packet can survive in the network.  The DCCP MSL should equal that   
    a packet can survive in the network.  The default MSL is two minutes
    of TCP, which is normally two minutes.                                 
    for this specification.                                             
                                                                           
3.5.  Security Limitation                                                  
                                                                           
    DCCP provides no protection against attackers who can snoop on a       
    DCCP is not robust against attackers who can snoop on a connection  
    connection in progress, or who can guess valid sequence numbers in     
    in progress, or who can guess valid sequence numbers in other ways. 
    other ways.  Applications desiring stronger security should use        
    Applications desiring stronger security should use IPsec or         
    IPsec [RFC 2401]; depending on the level of security required,         
    application-level cryptography.                                     
    application-level cryptography may also suffice.  These issues are     
    discussed further in Sections 18 and 7.5.5.                            
                                                                           
3.6.  Robustness Principle                                                 
                                                                           
    DCCP implementations will follow TCP's "general principle of           
    robustness": "be conservative in what you do, be liberal in what you   
    accept from others" [RFC 793].                                         
                                                                           
4.  Overview                                                               
                                                                           
    DCCP's high-level connection dynamics echo those of TCP.               
    Connections progress through three phases: initiation, including a     
                                                                           
                                                                           
                                                                           
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    three-way handshake; data transfer; and termination.  Data can flow    
    both ways over the connection.  An acknowledgement framework lets      
    senders discover how much data has been lost, and thus avoid           
    unfairly congesting the network.  Of course, DCCP provides             
    unreliable datagram semantics, not TCP's reliable bytestream           
    semantics.  The application must package its data into explicit        
    frames, and must retransmit its own data as necessary.  It may be      
    useful to think of DCCP as TCP minus bytestream semantics and          
    reliability, or as UDP plus congestion control, handshakes, and        
    acknowledgements.                                                      
                                                                           
4.1.  Packet Types                                                         
                                                                           
    Ten packet types implement DCCP's protocol functions.  For example,    
    every new connection attempt begins with a DCCP-Request packet sent    
    by the client.  A DCCP-Request packet thus resembles a TCP SYN; but    
    DCCP-Request is a packet type, not a flag, so there's no way to send   
    an unexpected combination such as TCP's SYN+FIN+ACK+RST.               
                                                                           
    Eight packet types occur during the progress of a typical              
    connection, shown here.  Note the three-way handshakes during          
    initiation and termination.                                            
                                                                           
       Client                                      Server                  
       ------                                      ------                  
                        (1) Initiation                                     
       DCCP-Request -->                                                    
                                        <-- DCCP-Response                  
       DCCP-Ack -->                                                        
                        (2) Data transfer                                  
       DCCP-Data, DCCP-Ack, DCCP-DataAck -->                               
                    <-- DCCP-Data, DCCP-Ack, DCCP-DataAck                  
                        (3) Termination                                    
                                        <-- DCCP-CloseReq                  
       DCCP-Close -->                                                      
                                           <-- DCCP-Reset                  
                                                                           
    The two remaining packet types are used to resynchronize after         
    bursts of loss.                                                        
                                                                           
    Every DCCP packet starts with a 12-byte generic header.  Particular    
    packet types include additional fixed-size header data; for example,   
    DCCP-Acks include an Acknowledgement Number.  DCCP options and any     
    application data follow the fixed-size header.                         
                                                                           
    The packet types are as follows:                                       
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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    DCCP-Request                                                           
        Sent by the client to initiate a connection (the first part of     
        the three-way initiation handshake).                               
                                                                           
    DCCP-Response                                                          
        Sent by the server in response to a DCCP-Request (the second       
        part of the three-way initiation handshake).                       
                                                                           
    DCCP-Data                                                              
        Used to transmit application data.                                 
                                                                           
    DCCP-Ack                                                               
        Used to transmit pure acknowledgements.                            
                                                                           
    DCCP-DataAck                                                           
        Used to transmit application data with piggybacked                 
        acknowledgements.                                                  
                                                                           
    DCCP-CloseReq                                                          
        Sent by the server to request that the client close the            
        connection.                                                        
                                                                           
    DCCP-Close                                                             
        Used by the client or the server to close the connection;          
        Used to close the connection; elicits a DCCP-Reset in response. 
        elicits a DCCP-Reset in response.                                  
                                                                           
    DCCP-Reset                                                             
        Used to terminate the connection, either normally or abnormally.   
                                                                           
    DCCP-Sync, DCCP-SyncAck                                                
        Used to resynchronize sequence numbers after large bursts of       
        loss.                                                              
                                                                           
4.2.  Sequence Numbers                                                     
                                                                           
    Each DCCP packet carries a sequence number, so that losses can be      
    detected and reported.  Unlike TCP sequence numbers, which are byte-   
    detected and reported.  Unlike TCP's byte-based sequence numbers,   
    based, DCCP sequence numbers increment by one per packet.  For         
    DCCP sequence numbers are packet-based: each packet sent increments 
    example:                                                               
    the sequence number by one.  For example:                           
                                                                           
       DCCP A                                      DCCP B                  
       ------                                      ------                  
       DCCP-Data(seqno 1) -->                                              
       DCCP-Data(seqno 2) -->                                              
                          <-- DCCP-Ack(seqno 10, ackno 2)                  
       DCCP-DataAck(seqno 3, ackno 10) -->                                 
                                  <-- DCCP-Data(seqno 11)                  
                                                                           
                                                                           
                                                                           
                                                                           
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    Every DCCP packet increments the sequence number, whether or not it    
    Even DCCP-Ack pure acknowledgements increment the sequence number.  
    contains application data.  DCCP-Ack pure acknowledgements increment   
    In the example, DCCP B's second packet uses sequence number 11,     
    the sequence number, for instance: DCCP B's second packet above uses   
    since sequence number 10 was used for an acknowledgement.  This lets
    sequence number 11, since sequence number 10 was used for an           
    endpoints detect lost acknowledgements.  It also means that         
    acknowledgement.  This lets endpoints detect all packet loss,          
    endpoints can get out of sync after long bursts of loss; the DCCP-  
    including acknowledgement loss.  It also means that endpoints can      
    Sync and DCCP-SyncAck packet types are used to recover (Section     
    get out of sync after long bursts of loss; the DCCP-Sync and DCCP-     
    7.5).                                                               
    SyncAck packet types are used to recover (Section 7.5).                
                                                                           
    Since DCCP provides unreliable semantics, there are no                 
    retransmissions, and it doesn't make sense to have a TCP-style         
    cumulative acknowledgement field.  DCCP's Acknowledgement Number       
    field equals the greatest sequence number received, rather than the    
    smallest sequence number not received.  Separate options indicate      
    any intermediate sequence numbers that weren't received.               
                                                                           
4.3.  States                                                               
                                                                           
    DCCP endpoints progress through different states during the course     
    of a connection, corresponding roughly to the three phases of          
    initiation, data transfer, and termination.  The figure below shows    
    the typical progress through these states for a client and server.     
                                                                           
       Client                                             Server           
       ------                                             ------           
                         (0) No connection                                 
       CLOSED                                             LISTEN           
                                                                           
                         (1) Initiation                                    
       REQUEST      DCCP-Request -->                                       
                                    <-- DCCP-Response     RESPOND          
       PARTOPEN     DCCP-Ack or DCCP-DataAck -->                           
                                                                           
                         (2) Data transfer                                 
       OPEN          <-- DCCP-Data, Ack, DataAck -->      OPEN             
                                                                           
                         (3) Termination                                   
                                    <-- DCCP-CloseReq     CLOSEREQ         
       CLOSING      DCCP-Close -->                                         
                                       <-- DCCP-Reset     CLOSED           
       TIMEWAIT                                                            
       CLOSED                                                              
                                                                           
    The nine possible states are as follows.  They are listed in           
    The nine possible states are as follows.  Section 8 describes them  
    increasing order, so that "state >= CLOSEREQ" means the same as        
    in more detail.                                                     
    "state = CLOSEREQ or state = CLOSING or state = TIMEWAIT".  Section    
    8 describes the states in more detail.                                 
                                                                           
                                                                           
                                                                           
                                                                           
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    CLOSED                                                                 
        Represents nonexistent connections.                                
                                                                           
    LISTEN                                                                 
        Represents server sockets in the passive listening state.          
        LISTEN and CLOSED are not associated with any particular DCCP      
        connection.                                                        
                                                                           
    REQUEST                                                                
        A client socket enters this state, from CLOSED, after sending a    
        DCCP-Request packet to try to initiate a connection.               
                                                                           
    RESPOND                                                                
        A server socket enters this state, from LISTEN, after receiving    
        a DCCP-Request from a client.                                      
                                                                           
    PARTOPEN                                                               
        A client socket enters this state, from REQUEST, after receiving   
        a DCCP-Response from the server.  This state represents the        
        third phase of the three-way handshake.  The client may send       
        application data in this state, but it MUST include an             
        Acknowledgement Number on all of its packets.                      
                                                                           
    OPEN                                                                   
        The central, data transfer portion of a DCCP connection.  Client   
        and server sockets enter this state from PARTOPEN and RESPOND,     
        respectively.  Sometimes we speak of SERVER-OPEN and CLIENT-OPEN   
        states, corresponding to the server's OPEN state and the           
        client's OPEN state.                                               
                                                                           
    CLOSEREQ                                                               
        A server socket enters this state, from SERVER-OPEN, to signal     
        that the connection is over, but the client must hold TIMEWAIT     
        state.                                                             
                                                                           
    CLOSING                                                                
        Server and client sockets can both enter this state to close the   
        connection.                                                        
                                                                           
    TIMEWAIT                                                               
        A server or client socket remains in this state for 2MSL (4        
        A socket remains in this state for 2MSL (4 minutes) after the   
        minutes) after the connection has been torn down, to prevent       
        connection has been torn down, to prevent mistakes due to the   
        mistakes due to the delivery of old packets.  Only one of the      
        delivery of old packets.                                        
        endpoints need enter TIMEWAIT state (the other can enter CLOSED    
        state immediately), and a server can request its client to hold    
        TIMEWAIT state using the DCCP-CloseReq packet type.                
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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4.4.  Congestion Control                                                   
                                                                           
    DCCP connections are congestion controlled, but unlike in TCP, DCCP    
    applications have a choice of congestion control mechanism.  In        
    fact, the two half-connections can be governed by different            
    mechanisms.  Mechanisms are denoted by one-byte congestion control     
    identifiers, or CCIDs.  The endpoints negotiate their CCIDs during     
    connection initiation.  Each CCID describes how the HC-Sender limits   
    data packet rates, how the HC-Receiver sends congestion feedback via   
    acknowledgements, and so forth.  CCIDs 2 and 3 are currently           
    defined; CCIDs 0, 1, and 4-255 are reserved.  Other CCIDs may be       
    defined; CCID 0 is reserved, and CCID 1 is used for special         
    defined in the future.                                                 
    purposes.                                                           
                                                                           
    CCID 2 provides TCP-like Congestion Control, which is similar to       
    that of TCP.  The sender maintains a congestion window and sends       
    packets until that window is full.  Packets are acknowledged by the    
    receiver.  Dropped packets and ECN [RFC 3168] indicate congestion;     
    the response to congestion is to halve the congestion window.          
    Acknowledgements in CCID 2 contain the sequence numbers of all         
    received packets within some window, similar to a selective            
    acknowledgement (SACK) [RFC 2018].                                     
    acknowledgement (SACK) [RFC 3517].                                  
                                                                           
    CCID 3 provides TFRC Congestion Control, an equation-based form of     
    congestion control intended to respond to congestion more smoothly     
    than CCID 2.  The sender maintains a transmit rate, which it updates   
    using the receiver's estimate of the packet loss and mark rate.        
    CCID 3 behaves somewhat differently from TCP in the short term, it     
    is designed to operate fairly with TCP over the long term.             
                                                                           
    Section 10 describes DCCP's CCIDs in more detail.  The behaviors of    
    CCIDs 2 and 3 are fully defined in separate profile documents [CCID    
    2 PROFILE] [CCID 3 PROFILE].                                           
                                                                           
4.5.  Features                                                             
                                                                           
    DCCP endpoints use Change and Confirm options to negotiate and agree   
    DCCP endpoints generally use Change and Confirm options to negotiate
    on feature values.  Feature negotiation will almost always happen on   
    and agree on feature values, although agreement may also be achieved
    the connection initiation handshake, but it can begin at any time.     
    using an out-of-band signalling channel.  Feature negotiation will  
                                                                           
    almost always happen on the connection initiation handshake, but it 
    There are four feature negotiation options in all: Change L,           
    can begin at any time.                                              
    Confirm L, Change R, and Confirm R.  The "L" options are sent by the   
    feature location, and the "R" options are sent by the feature          
    remote.  A Change R option says to the feature location, "change       
    this feature value as follows".  The feature location responds with    
    Confirm L, meaning "I've changed it".  Some features allow Change R    
    options to contain multiple values, sorted in preference order.  For   
    example:                                                               
                                                                           
                                                                           
                                                                           
                                                                           
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       Client                                        Server                
       ------                                        ------                
       Change R(CCID, 2) -->                                               
                                     <-- Confirm L(CCID, 2)                
                  * agreement that CCID/Server = 2 *                       
                                                                           
       Change R(CCID, 3 4) -->                                             
                                <-- Confirm L(CCID, 4, 4 2)                
                  * agreement that CCID/Server = 4 *                       
                                                                           
    Both exchanges negotiate the CCID/Server feature's value, which is     
    In the second exchange, the client requests that the server use     
    the CCID in use on the server-to-client half-connection.  In the       
    either CCID 3 or CCID 4, with 3 preferred.  The server chooses 4 and
    second exchange, the client requests that the server use either        
    CCID 3 or CCID 4, with 3 preferred; the server chooses 4 and           
    supplies its preference list, "4 2".                                   
                                                                           
    The Change L and Confirm R options are used for feature negotiations   
    initiated by the feature location.  In the following example, the      
    server requests that CCID/Server be set to 3 or 2, with 3 preferred,   
    and the client agrees.                                                 
                                                                           
       Client                                       Server                 
       ------                                       ------                 
                                   <-- Change L(CCID, 3 2)                 
       Confirm R(CCID, 3, 3 2)  -->                                        
                  * agreement that CCID/Server = 3 *                       
                                                                           
                                                                           
    Section 6 describes the feature negotiation options further,           
    including the retransmission strategies that make negotiation          
    reliable.                                                              
                                                                           
4.6.  Differences From TCP                                                 
                                                                           
    Differences between DCCP and TCP apart from those discussed so far     
    include:                                                               
                                                                           
    o  Copious space for options (up to 1008 bytes or the PMTU).           
    o  Copious space for options (up to 1008 bytes).                    
                                                                           
    o  Different acknowledgement formats. The CCID for a connection     
    o  Different acknowledgement formats.  The CCID for a connection       
       determines how much acknowledgement information needs to be         
       transmitted. For example, in CCID 2 (TCP-like), this is about one   
       transmitted. In CCID 2 (TCP-like), this is about one ack per 2   
       ack per 2 packets, and each ack must declare exactly which          
       packets, and each ack must declare exactly which packets were    
       packets were received; in CCID 3 (TFRC), it's about one ack per     
       received; in CCID 3 (TFRC), it's about one ack per RTT, and acks 
       round-trip time, and acks must declare at minimum just the          
       must declare at minimum just the lengths of recent loss          
       lengths of recent loss intervals.                                   
       intervals.                                                       
                                                                           
    o  Denial-of-service (DoS) protection. Several mechanisms help limit
                                                                           
       the amount of state possibly-misbehaving clients can force DCCP  
                                                                           
       servers to maintain.  An Init Cookie option, analogous to TCP's  
                                                                           
       SYN Cookies [SYNCOOKIES], avoids SYN-flood-like attacks.  Only   
                                                                           
       one connection endpoint need hold TIMEWAIT state; the DCCP-      
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    o  Denial-of-service (DoS) protection.  Several mechanisms help        
       limit the amount of state possibly-misbehaving clients can force    
       DCCP servers to maintain.  An Init Cookie option, analogous to      
       TCP's SYN Cookies [SYNCOOKIES], avoids SYN-flood-like attacks.      
       Only one connection endpoint need hold TIMEWAIT state; the DCCP-    
       CloseReq packet, which may only be sent by the server, passes       
       that state to the client.  Various rate limits let servers avoid    
       attacks that might force extensive computation or packet            
       generation.                                                         
                                                                           
    o  Distinguishing different kinds of loss.  A Data Dropped option      
       (Section 11.7) lets an endpoint declare that a packet was dropped   
       (Section 11.8) lets an endpoint declare that a packet was dropped
       because of corruption, because of receive buffer overflow, and so   
       on.  This facilitates research into more appropriate rate-control   
       responses for these non-network-congestion losses (although         
       currently such losses will cause a congestion response).            
                                                                           
    o  Acknowledgeability.  In TCP, a packet may be acknowledged only      
    o  Acknowledgement readiness. In TCP, a packet is acknowledged only 
       once the data is reliably queued for application delivery.  This    
       when the data is queued for delivery to the application.  This   
       does not make sense in DCCP, where an application might, for        
       does not make sense in DCCP, where an application might request a
       example, request a drop-from-front receive buffer.  A DCCP packet   
       drop-from-front receive buffer, for example.  DCCP acknowledges a
       may be acknowledged as soon as its header has been successfully     
       packet when its options have been processed.  The Data Dropped   
       processed.  Concretely, a packet becomes acknowledgeable at         
       option may later report that the packet's payload was discarded. 
       Step 8 of Section 8.5's packet processing pseudocode.               
       Acknowledgeability does not guarantee data delivery, however: the   
       Data Dropped option may later report that the packet's              
       application data was discarded.                                     
                                                                           
    o  No receive window.  DCCP is a congestion control protocol, not a    
       flow control protocol.                                              
                                                                           
    o  No simultaneous open.  Every connection has one client and one      
       server.                                                             
                                                                           
    o  No half-closed states.  DCCP has no states corresponding to TCP's   
       FINWAIT and CLOSEWAIT, where one half-connection is explicitly      
       closed while the other is still active.  The Data Dropped           
       closed while the other is still active.                          
       option's Drop Code 1, Application Not Listening (Section 11.7),     
       can achieve a similar effect, however.                              
                                                                           
4.7.  Example Connection                                                   
                                                                           
    The progress of a typical DCCP connection is as follows.  (This        
    description is informative, not normative.)                            
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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           Client                                  Server                  
           ------                                  ------                  
       0.  [CLOSED]                              [LISTEN]                  
       1.  DCCP-Request -->                                                
       2.                               <-- DCCP-Response                  
       3.  DCCP-Ack -->                                                    
|| TEXT DELETED ||                                                         
                                             <-- DCCP-Ack               
       4.  DCCP-Data, DCCP-Ack, DCCP-DataAck -->                           
                    <-- DCCP-Data, DCCP-Ack, DCCP-DataAck                  
       5.                               <-- DCCP-CloseReq                  
       6.  DCCP-Close -->                                                  
       7.                                  <-- DCCP-Reset                  
       8.  [TIMEWAIT]                                                      
                                                                           
                                                                           
    1.  The client sends the server a DCCP-Request packet specifying the   
        client and server ports, the service being requested, and any      
        features being negotiated, including the CCID that the client      
        would like the server to use.  The client may optionally           
        piggyback an application request on the DCCP-Request packet,       
        which the server may ignore.                                       
                                                                           
    2.  The server sends the client a DCCP-Response packet indicating      
        that it is willing to communicate with the client.  This           
        response indicates any features and options that the server        
        agrees to, begins other feature negotiations as desired, and       
        optionally includes an Init Cookie that wraps up all this          
        information and which must be returned by the client for the       
        connection to complete.                                            
                                                                           
    3.  The client sends the server a DCCP-Ack packet that acknowledges    
        the DCCP-Response packet.  This acknowledges the server's          
        initial sequence number and returns the Init Cookie if there was   
        one in the DCCP-Response.  It may also continue feature            
        negotiation.  The client may piggyback an application-level        
        request on its final ack, producing a DCCP-DataAck packet.         
                                                                           
    4.  The server and client then exchange DCCP-Data packets, DCCP-Ack    
        packets acknowledging that data, and, optionally, DCCP-DataAck     
        packets containing data with piggybacked acknowledgements.  If     
        the client has no data to send, then the server will send DCCP-    
        Data and DCCP-DataAck packets, while the client will send DCCP-    
        Acks exclusively.  (However, the client may not send DCCP-Data     
        Acks exclusively.                                               
        packets before receiving at least one non-DCCP-Response packet     
        from the server.)                                                  
                                                                           
    5.  The server sends a DCCP-CloseReq packet requesting a close.        
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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    6.  The client sends a DCCP-Close packet acknowledging the close.      
                                                                           
    7.  The server sends a DCCP-Reset packet with Reset Code 1,            
        "Closed", and clears its connection state.  DCCP-Resets are part   
        of normal connection termination; see Section 5.6.                 
                                                                           
    8.  The client receives the DCCP-Reset packet and holds state for      
    8.  The client receives the DCCP-Reset packet and holds state for a 
        two maximum segment lifetimes, or 2MSL, to allow any remaining     
        reasonable interval of time to allow any remaining packets to   
        packets to clear the network.                                      
        clear the network.                                              
                                                                           
    An alternative connection closedown sequence is initiated by the       
    client:                                                                
                                                                           
    5b. The client sends a DCCP-Close packet closing the connection.       
                                                                           
    6b. The server sends a DCCP-Reset packet with Reset Code 1,            
        "Closed", and clears its connection state.                         
                                                                           
    7b. The client receives the DCCP-Reset packet and holds state for      
    7b. The client receives the DCCP-Reset packet and holds state for a 
        2MSL to allow any remaining packets to clear the network.          
        reasonable interval of time to allow any remaining packets to   
                                                                           
        clear the network.                                              
5.  Packet Formats                                                         
5.  Header Formats                                                      
                                                                           
    The DCCP header can be from 12 to 1020 bytes long.  The initial 12     
    bytes of the header have the same semantics for all currently-         
    bytes of the header have the same semantics for all packet types.   
    defined packet types.  Following this comes any additional fixed-      
    Following this comes any additional fixed-length fields required by 
    length fields required by the packet type, and then a variable-        
    the packet type, and then a variable-length list of options.  Some  
    length list of options.  The application data area follows the         
    packet types allow application data to follow the header.           
    header.  In some packet types, this area contains data for the         
    application; in other packet types, its contents are ignored.          
                                                                           
     +---------------------------------------+  -.                         
     |            Generic Header             |   |                         
     |             Generic Header            |   |                      
     +---------------------------------------+   |                         
     | Additional Fields (depending on type) |   +- DCCP Header            
     +---------------------------------------+   |                         
     |          Options (optional)           |   |                         
     |           Options (optional)          |   |                      
     +=======================================+  -'                         
     |         Application Data Area         |                             
     |      Application Data (optional)      |                          
     +---------------------------------------+                             
                                                                           
                                                                           
5.1.  Generic Header                                                       
                                                                           
    The DCCP generic header takes different forms depending on the value   
    of X, the Extended Sequence Numbers bit.  If X is one, the Sequence    
    Number field is 48 bits long and the generic header takes 16 bytes,    
    as follows.                                                            
                                                                           
                                                                           
                                                                           
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      0                   1                   2                   3        
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |          Source Port          |           Dest Port           |     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |  Data Offset  | CCVal | CsCov |           Checksum            |     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |     |       |X|               |                               .     
     |     |X|       |                                               .  
     | Res | Type  |=|   Reserved    |  Sequence Number (high bits)  .     
     | Res |=| Type  |          Sequence Number (high bits)          .  
     |     |       |1|               |                               .     
     |     |1|       |                                               .  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     .                  Sequence Number (low bits)                   |     
     .          Sequence Number (low bits)           |   Reserved    |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
                                                                           
    If X is zero, only the low 24 bits of the Sequence Number are          
    transmitted, and the generic header is 12 bytes long.                  
                                                                           
      0                   1                   2                   3        
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |          Source Port          |           Dest Port           |     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |  Data Offset  | CCVal | CsCov |           Checksum            |     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |     |       |X|                                               |     
     |     |X|       |                                               |  
     | Res | Type  |=|          Sequence Number (low bits)           |     
     | Res |=| Type  |          Sequence Number (low bits)           |  
     |     |       |0|                                               |     
     |     |0|       |                                               |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
                                                                           
                                                                           
    The generic header fields are defined as follows.                      
                                                                           
    Source and Destination Ports: 16 bits each                             
        These fields identify the connection, similar to the               
        corresponding fields in TCP and UDP.  The Source Port represents   
        the relevant port on the endpoint that sent this packet, the       
        Destination Port the relevant port on the other endpoint.  When    
        Destination Port the relevant port on the other endpoint.       
        initiating a connection, the client SHOULD choose its Source       
        Source Ports SHOULD be chosen randomly, to reduce the likelihood
        Port randomly to reduce the likelihood of attack.                  
        of attack.                                                      
                                                                           
        DCCP APIs should treat port numbers similarly to TCP and UDP       
        port numbers.  For example, machines that distinguish between      
        "privileged" and "unprivileged" ports for TCP and UDP should do    
        the same for DCCP.                                                 
                                                                           
    Data Offset: 8 bits                                                    
        The offset from the start of the packet's DCCP header to the       
        The offset from the start of the DCCP header to the beginning of
        start of its application data area, in 32-bit words.  The          
        the packet's application data, in 32-bit words.                 
                                                                           
                                                                           
                                                                           
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        receiver MUST ignore packets whose Data Offset is smaller than     
        the minimum-sized header for the given Type, or larger than the    
        DCCP packet itself.                                                
                                                                           
    CCVal: 4 bits                                                          
        Used by the HC-Sender CCID.  For example, the A-to-B CCID's        
        sender, which is active at DCCP A, MAY send 4 bits of              
        information per packet to its receiver by encoding that            
        information in CCVal.  The sender MUST set CCVal to zero unless    
        its HC-Sender CCID specifies otherwise, and the receiver MUST      
        ignore the CCVal field unless its HC-Receiver CCID specifies       
        otherwise.                                                         
                                                                           
    Checksum Coverage (CsCov): 4 bits                                      
        Checksum Coverage determines the parts of the packet that are      
        covered by the Checksum field.  This always includes the DCCP      
        header and options, but some or all of the application data may    
        be excluded.  This can improve performance on noisy links for      
        applications that can tolerate corruption.  See Section 9.         
                                                                           
    Checksum: 16 bits                                                      
        The Internet checksum of the packet's DCCP header (including       
        options), a network-layer pseudoheader, and, depending on          
        Checksum Coverage, all, some, or none of the application data.     
        Checksum Coverage, some or all of the application data.  See    
        See Section 9.                                                     
        Section 9.                                                      
                                                                           
    Reserved (Res): 3 bits                                                 
        Senders MUST set this field to all zeroes on generated packets,    
        and receivers MUST ignore its value.                               
                                                                           
    Type: 4 bits                                                           
        The Type field specifies the type of the packet.  The following    
        values are defined:                                                
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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                          Type   Meaning                                   
        Type   Meaning                                                  
                          ----   -------                                   
        ----   -------                                                  
                            0    DCCP-Request                              
          0    DCCP-Request                                             
                            1    DCCP-Response                             
          1    DCCP-Response                                            
                            2    DCCP-Data                                 
          2    DCCP-Data                                                
                            3    DCCP-Ack                                  
          3    DCCP-Ack                                                 
                            4    DCCP-DataAck                              
          4    DCCP-DataAck                                             
                            5    DCCP-CloseReq                             
          5    DCCP-CloseReq                                            
                            6    DCCP-Close                                
          6    DCCP-Close                                               
                            7    DCCP-Reset                                
          7    DCCP-Reset                                               
                            8    DCCP-Sync                                 
          8    DCCP-Sync                                                
                            9    DCCP-SyncAck                              
          9    DCCP-SyncAck                                             
                          10-15  Reserved                                  
        10-15  Reserved                                                 
                                                                           
                      Table 1: DCCP Packet Types                           
                                                                           
        Receivers MUST ignore any packets with reserved type.  That is,    
        packets with reserved type MUST NOT be processed and they MUST     
        NOT be acknowledged as received.                                   
|| TEXT DELETED ||                                                         
    Reserved (Res): 3 bits                                              
                                                                           
        Senders MUST set this field to all zeroes on generated packets, 
    Extended Sequence Numbers (X): 1 bit                                   
        and receivers MUST ignore its value.                            
        Set to one to indicate the use of an extended generic header       
        with 48-bit Sequence and Acknowledgement Numbers.  DCCP-Data,      
        DCCP-DataAck, and DCCP-Ack packets MAY set X to zero or one.       
        All DCCP-Request, DCCP-Response, DCCP-CloseReq, DCCP-Close,        
        DCCP-Reset, DCCP-Sync, and DCCP-SyncAck packets MUST set X to      
        one; endpoints MUST ignore any such packets with X set to zero.    
        High-rate connections SHOULD set X to one on all packets to gain   
        increased protection against wrapped sequence numbers and          
        attacks.  See Section 7.6.                                         
                                                                           
    Sequence Number: 48 or 24 bits                                         
    Sequence Number: 24 or 48 bits                                      
        Identifies the packet uniquely in the sequence of all packets      
        the source sent on this connection.  Sequence Number increases     
        by one with every packet sent, including packets such as DCCP-     
        Ack that carry no application data.  See Section 7.                
                                                                           
    All currently defined packet types except DCCP-Request and DCCP-Data   
    carry an Acknowledgement Number Subheader in the four or eight bytes   
    carry an Acknowledgement Number in the four or eight bytes          
    immediately following the generic header.  When X=1, its format is:    
                                                                           
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |           Reserved            |    Acknowledgement Number     .     
     |   Reserved    |       Acknowledgement Number (high bits)      .  
     |                               |          (high bits)          .     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     .               Acknowledgement Number (low bits)               |     
     .       Acknowledgement Number (low bits)       |   Reserved    |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
                                                                           
                                                                           
                                                                           
                                                                           
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    When X=0, only the low 24 bits of the Acknowledgement Number are       
    transmitted, giving the Acknowledgement Number Subheader this          
    transmitted.                                                        
    format:                                                                
                                                                           
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |   Reserved    |       Acknowledgement Number (low bits)       |     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
                                                                           
                                                                           
    Reserved: 16 or 8 bits                                                 
    Acknowledgement Number: 24 or 48 bits                               
        Senders MUST set this field to all zeroes on generated packets,    
        and receivers MUST ignore its value.                               
                                                                           
    Acknowledgement Number: 48 or 24 bits                                  
        Generally contains GSR, the Greatest Sequence Number Received on   
        any acknowledgeable packet so far.  A packet is acknowledgeable    
        if and only if its header was successfully processed by the        
        if and only if its header options were processed by the         
        receiver; Section 7.4 describes this further.  Options such as     
        receiver.  Section 7.4 describes this further.  Options such as 
        Ack Vector (Section 11.4) combine with the Acknowledgement         
        Number to provide precise information about which packets have     
        arrived.                                                           
                                                                           
        Acknowledgement Numbers on DCCP-Sync and DCCP-SyncAck packets      
        need not equal GSR.  See Section 5.7.                              
        need not equal GSR; see Section 5.7.                            
                                                                           
    Reserved: 8 bits                                                    
5.2.  DCCP-Request Packets                                                 
        Senders MUST set this field to all zeroes on generated packets, 
                                                                           
        and receivers MUST ignore its value.                            
    A client initiates a DCCP connection by sending a DCCP-Request         
5.2.  DCCP-Request Header                                               
    packet.  These packets MAY contain application data, and MUST use      
    packet.  These packets MAY contain application data.                
    48-bit sequence numbers (X=1).                                         
                                                                           
      0                   1                   2                   3        
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /            Generic DCCP Header with X=1 (16 bytes)            /     
     /                   with Type=0 (DCCP-Request)                  /     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |                         Service Code                          |     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /                      Options and Padding                      /     
     |                     Options                   /    Padding    |  
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+     
     /                       Application Data                        /     
     |                        Application Data                       |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |                              ...                              |  
                                                                           
                                                                           
    Service Code: 32 bits                                                  
        Describes the application-level service to which the client        
        Describes the service to which the client application wants to  
        application wants to connect.  Service Codes are intended to       
        connect.  Examples might include RTSP and DOOM.  Service Codes  
                                                                           
        are intended to make application protocols independent of well- 
                                                                           
        known ports, and help middleboxes identify the protocol used on 
                                                                           
        a given connection.  See Section 8.1.2.                         
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5.3.  DCCP-Response Header                                              
                                                                          
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        provide information about which application protocol a             
        connection intends to use, and thus aiding middleboxes and         
        reducing reliance on globally well-known ports.  See Section       
        8.1.2.                                                             
                                                                           
5.3.  DCCP-Response Packets                                                
                                                                           
    The server responds to valid DCCP-Request packets with DCCP-Response   
    packets.  This is the second phase of the three-way handshake.         
    DCCP-Response packets MAY contain application data, and MUST use       
    DCCP-Response packets MAY contain application data.                 
    48-bit sequence numbers (X=1).                                         
                                                                           
      0                   1                   2                   3        
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /            Generic DCCP Header with X=1 (16 bytes)            /     
     /                  with Type=1 (DCCP-Response)                  /     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /          Acknowledgement Number Subheader (8 bytes)           /     
     |   Reserved    |       Acknowledgement Number (high bits)      .  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  
     |                         Service Code                          |     
     .       Acknowledgement Number (low bits)       |   Reserved    |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /                      Options and Padding                      /     
     |                     Options                   /    Padding    |  
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+     
     /                       Application Data                        /     
     |                        Application Data                       |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |                              ...                              |  
                                                                           
                                                                           
    Acknowledgement Number: 48 bits                                        
        Contains GSR.  Since DCCP-Responses are only sent during           
        connection initiation, this will always equal the Sequence         
        Number on a received DCCP-Request.                                 
                                                                           
    Service Code: 32 bits                                                  
        MUST equal the Service Code on the corresponding DCCP-Request.     
        Echoes the Service Code on a received DCCP-Request.             
                                                                           
5.4.  DCCP-Data, DCCP-Ack, and DCCP-DataAck Headers                     
5.4.  DCCP-Data, DCCP-Ack, and DCCP-DataAck Packets                        
                                                                           
    The central data transfer portion of every DCCP connection uses        
    DCCP-Data, DCCP-Ack, and DCCP-DataAck packets.  These packets MAY      
    DCCP-Data, DCCP-Ack, and DCCP-DataAck packets.  DCCP-Data packets   
    use 24-bit sequence numbers, depending on the value of the Allow       
    carry application data.                                             
    Short Sequence Numbers feature (Section 7.6.1).  DCCP-Data packets     
    carry application data without acknowledgements.                       
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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      0                   1                   2                   3        
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /              Generic DCCP Header (16 or 12 bytes)             /     
     /              Generic DCCP Header (12 or 16 bytes)             /  
     /                    with Type=2 (DCCP-Data)                    /     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /                      Options and Padding                      /     
     |                     Options                   /    Padding    |  
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+     
     /                       Application Data                        /     
     |                        Application Data                       |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |                              ...                              |  
                                                                           
    DCCP-Ack packets dispense with the data, but contain an                
    Acknowledgement Number.  They are used for pure acknowledgements.      
                                                                           
      0                   1                   2                   3        
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /              Generic DCCP Header (16 or 12 bytes)             /     
     /              Generic DCCP Header (12 or 16 bytes)             /  
     /                    with Type=3 (DCCP-Ack)                     /     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /        Acknowledgement Number Subheader (8 or 4 bytes)        /     
     |   Reserved    |            Acknowledgement Number             |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
    (+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+) 
     /                      Options and Padding                      /     
    (.       Acknowledgement Number (low bits)       |   Reserved    |) 
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  
     /                Application Data Area (Ignored)                /     
     |                     Options                   /    Padding    |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  
                                                                           
    (The parenthesized fields appear only when the header's Extended    
    DCCP-DataAck packets carry both application data and an                
    Sequence Numbers field is 1.)  DCCP-DataAck packets carry both      
    Acknowledgement Number: acknowledgement information is piggybacked     
    application data and an Acknowledgement Number: acknowledgement     
    on a data packet.                                                      
    information is piggybacked on a data packet.                        
                                                                           
      0                   1                   2                   3        
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /              Generic DCCP Header (16 or 12 bytes)             /     
     /              Generic DCCP Header (12 or 16 bytes)             /  
     /                  with Type=4 (DCCP-DataAck)                   /     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /        Acknowledgement Number Subheader (8 or 4 bytes)        /     
     |   Reserved    |            Acknowledgement Number             |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
    (+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+) 
    (.       Acknowledgement Number (low bits)       |   Reserved    |) 
     /                      Options and Padding                      /     
     |                     Options                   /    Padding    |  
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+     
     /                       Application Data                        /     
     |                        Application Data                       |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |                              ...                              |  
                                                                           
    A DCCP-Data or DCCP-DataAck packet may have a zero-length              
    application data area, which indicates that the application sent a     
    zero-length datagram.  This differs from DCCP-Request and DCCP-        
    Response packets, where an empty application data area indicates the   
                                                                           
                                                                           
                                                                           
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    absence of application data (not the presence of zero-length           
    absence of application data (as opposed to the presence of zero-    
    application data).  The API SHOULD report any received zero-length     
    length application data).                                           
    datagrams to the receiving application.                                
    Receivers MUST ignore the application data area in DCCP-Ack packets.
                                                                           
    DCCP-Ack senders will generally leave this area empty.              
    A DCCP-Ack packet MAY have a non-zero-length application data area,    
    which essentially pads the DCCP-Ack to a desired length.  Receivers    
    MUST ignore the content of the application data area in DCCP-Ack       
    packets.                                                               
                                                                           
    DCCP-Ack and DCCP-DataAck packets often include additional             
    acknowledgement options, such as Ack Vector, as required by the        
    congestion control mechanism in use.                                   
                                                                           
5.5.  DCCP-CloseReq and DCCP-Close Packets                                 
5.5.  DCCP-CloseReq and DCCP-Close Headers                              
                                                                           
    DCCP-CloseReq and DCCP-Close packets begin the handshake that          
    normally terminates a connection.  Either client or server may send    
    a DCCP-Close packet, which will elicit a DCCP-Reset packet.  Only      
    the server can send a DCCP-CloseReq packet, which indicates that the   
    server wants to close the connection, but does not want to hold its    
    TIMEWAIT state.  Both packet types MUST use 48-bit sequence numbers    
    TIMEWAIT state.                                                     
    (X=1).                                                                 
                                                                           
      0                   1                   2                   3        
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /            Generic DCCP Header with X=1 (16 bytes)            /     
     /         with Type=5 (DCCP-CloseReq) or 6 (DCCP-Close)         /     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /          Acknowledgement Number Subheader (8 bytes)           /     
     |   Reserved    |       Acknowledgement Number (high bits)      .  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /                      Options and Padding                      /     
     .       Acknowledgement Number (low bits)       |   Reserved    |  
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  
     /                Application Data Area (Ignored)                /     
     |                     Options                   /    Padding    |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
                                                                           
    As with DCCP-Ack packets, DCCP-CloseReq and DCCP-Close packets MAY     
    Receivers MUST ignore the application data area in DCCP-CloseReq and
    have non-zero-length application data areas, whose contents            
    DCCP-Close packets.                                                 
    receivers MUST ignore.                                                 
5.6.  DCCP-Reset Header                                                 
                                                                           
5.6.  DCCP-Reset Packets                                                   
                                                                           
    DCCP-Reset packets unconditionally shut down a connection.             
    Connections normally terminate with a DCCP-Reset, but resets may be    
    sent for other reasons, including bad port numbers, bad option         
    behavior, incorrect ECN Nonce Echoes, and so forth.  DCCP-Resets       
    behavior, incorrect ECN Nonce Echoes, and so forth.                 
    MUST use 48-bit sequence numbers (X=1).                                
                                                                           
                                                                           
                                                                           
                                                                           
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      0                   1                   2                   3        
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /            Generic DCCP Header with X=1 (16 bytes)            /     
     /                   with Type=7 (DCCP-Reset)                    /     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /          Acknowledgement Number Subheader (8 bytes)           /     
     |   Reserved    |       Acknowledgement Number (high bits)      .  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  
     |  Reset Code   |    Data 1     |    Data 2     |    Data 3     |     
     .       Acknowledgement Number (low bits)       |   Reserved    |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /                      Options and Padding                      /     
     |                     Options                   /    Padding    |  
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+     
     /              Application Data Area (Error Text)               /     
     |                          Error Text                           |  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     |                              ...                              |  
                                                                           
                                                                           
    Reset Code: 8 bits                                                     
        Represents the reason that the sender reset the DCCP connection.   
                                                                           
    Data 1, Data 2, and Data 3: 8 bits each                                
        The Data fields provide additional information about why the       
        sender reset the DCCP connection.  The meanings of these fields    
        depend on the value of Reset Code.                                 
        depend on the value of Reason.                                  
                                                                           
    Error Text (application data area)                                  
    Application Data Area: Error Text                                      
        If present, Error Text is a human-readable text string,         
        If present, Error Text is a human-readable text string encoded     
        preferably in English and encoded in Unicode UTF-8, that        
        in Unicode UTF-8, and preferably in English, that describes the    
        describes the error in more detail.  For example, a DCCP-Reset  
        error in more detail.  For example, a DCCP-Reset with Reset Code   
        with Reset Code 11, "Aggression Penalty", might contain Error   
        11, "Aggression Penalty", might contain Error Text such as         
        Text such as "Aggression Penalty: Received 3 bad ECN Nonce      
        "Aggression Penalty: Received 3 bad ECN Nonce Echoes, assuming     
        Echoes, assuming misbehavior".                                  
        misbehavior".                                                      
                                                                           
    The following Reset Codes are currently defined.  Unless otherwise     
    specified, the Data 1, 2, and 3 fields MUST be set to 0 by the         
    sender of the DCCP-Reset and ignored by its receiver.  Section         
    references describe concrete situations that will cause each Reset     
    Code to be generated; they are not meant to be exhaustive.             
                                                                           
    0, "Unspecified"                                                       
        Indicates the absence of a meaningful Reset Code.  Use of Reset    
        Code 0 is NOT RECOMMENDED: the sender should choose a Reset Code   
        that more clearly defines why the connection is being reset.       
                                                                           
    1, "Closed"                                                            
        Normal connection close.  See Section 8.3.                         
                                                                           
    2, "Aborted"                                                           
        The sending endpoint gave up on the connection because of lack     
                                                                           
                                                                           
                                                                           
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        of progress.  See Sections 8.1.1 and 8.1.5.                        
                                                                           
    3, "No Connection"                                                     
        No connection exists.  See Section 8.3.1.                          
                                                                           
    4, "Packet Error"                                                      
        A valid packet arrived with unexpected type.  For example, a       
        An unexpected packet type arrived; for example, a DCCP-Data     
        DCCP-Data packet with valid header checksum and sequence numbers   
        packet arrived at a connection in the REQUEST state.  See       
        arrived at a connection in the REQUEST state.  See Section         
        Section 8.3.1. The Data 1 field equals the offending packet     
        8.3.1.  The Data 1 field equals the offending packet type as an    
        type.                                                           
        eight-bit number; thus, an offending packet with Type 2 will       
        result in a Data 1 value of 2.                                     
                                                                           
    5, "Option Error"                                                      
        An option was erroneous, and the error was serious enough to       
        warrant resetting the connection.  See Sections 6.6.7, 6.6.8,      
        and 11.4.  The Data 1 field equals the offending option type;      
        Data 2 and Data 3 equal the first two bytes of option data (or     
        zero if the option had less than two bytes of data).               
                                                                           
    6, "Mandatory Error"                                                   
        The sending endpoint could not process an option O that was        
        The sending endpoint could not process an option marked         
        immediately preceded by Mandatory.  The Data fields report the     
        Mandatory.  The Data fields report the option type and data of  
        option type and data of option O, using the format of Reset Code   
        the unprocessed option (not the Mandatory option), using the    
        5, "Option Error".  See Section 5.8.2.                             
        format of Reset Code 5, "Option Error".  See Section 5.8.2.     
                                                                           
    7, "Connection Refused"                                                
        The Destination Port didn't correspond to a port open for          
        listening.  Sent only in response to DCCP-Requests.  See Section   
        8.1.3.                                                             
                                                                           
    8, "Bad Service Code"                                                  
        The Service Code didn't equal the service code attached to the     
        Destination Port.  Sent only in response to DCCP-Requests.  See    
        Section 8.1.3.                                                     
                                                                           
    9, "Too Busy"                                                          
        The server is too busy to accept new connections.  Sent only in    
        response to DCCP-Requests.  See Section 8.1.3.                     
                                                                           
    10, "Bad Init Cookie"                                                  
        The Init Cookie echoed by the client was incorrect or missing.     
        See Section 8.1.4.                                                 
                                                                           
    11, "Aggression Penalty"                                               
        This endpoint has detected congestion control-related              
        misbehavior on the part of the other endpoint.  See Sections       
        12.2 and 13.2.                                                     
                                                                           
                                                                           
                                                                           
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    12-127, Reserved                                                       
        Receivers should treat these codes like Reset Code 0,              
        "Unspecified".                                                     
                                                                           
    128-255, CCID-specific codes                                           
        Semantics depend on the connection's CCIDs.  See Section 10.3.     
        Semantics depend on the connection's CCIDs.  See Section 10.4.  
        Receivers should treat unknown CCID-specific Reset Codes like      
        Reset Code 0, "Unspecified".                                       
                                                                           
    The following table summarizes this information.                       
                                                                           
          Reset                                                            
     Reset                                                              
          Code   Name                    Data 1     Data 2 & 3             
     Code   Name                    Data 1     Data 2 & 3               
          -----  ----                    ------     ----------             
     -----  ----                    ------     ----------               
            0    Unspecified               0            0                  
       0    Unspecified               0            0                    
            1    Closed                    0            0                  
       1    Closed                    0            0                    
            2    Aborted                   0            0                  
       2    Aborted                   0            0                    
            3    No Connection             0            0                  
       3    No Connection             0            0                    
            4    Packet Error           pkt type        0                  
       4    Packet Error           pkt type        0                    
            5    Option Error           option #   option data             
       5    Option Error           option #   option data               
            6    Mandatory Error        option #   option data             
       6    Mandatory Error        option #   option data               
            7    Connection Refused        0            0                  
       7    Connection Refused        0            0                    
            8    Bad Service Code          0            0                  
       8    Bad Service Code          0            0                    
            9    Too Busy                  0            0                  
       9    Too Busy                  0            0                    
           10    Bad Init Cookie           0            0                  
      10    Bad Init Cookie           0            0                    
           11    Aggression Penalty        0            0                  
      11    Aggression Penalty        0            0                    
          12-127 Reserved                                                  
     12-127 Reserved                                                    
         128-255 CCID-specific codes                                       
    128-255 CCID-specific codes                                         
                                                                           
5.7.  DCCP-Sync and DCCP-SyncAck Headers                                
                        Table 2: DCCP Reset Codes                          
                                                                           
    Options on DCCP-Reset packets are processed before the connection is   
    shut down.  This means that certain combinations of options,           
    particularly involving Mandatory, may cause an endpoint to respond     
    to a valid DCCP-Reset with another DCCP-Reset.  This cannot lead to    
    a reset storm; since the first endpoint has already reset the          
    connection, the second DCCP-Reset will be ignored.                     
                                                                           
5.7.  DCCP-Sync and DCCP-SyncAck Packets                                   
                                                                           
    DCCP-Sync packets help DCCP endpoints recover synchronization after    
    bursts of loss, or recover from half-open connections.  Each valid     
    received DCCP-Sync immediately elicits a DCCP-SyncAck.  Both packet    
    received DCCP-Sync immediately elicits a DCCP-SyncAck.              
    types MUST use 48-bit sequence numbers (X=1).                          
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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      0                   1                   2                   3        
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /            Generic DCCP Header with X=1 (16 bytes)            /     
     /          with Type=8 (DCCP-Sync) or 9 (DCCP-SyncAck)          /     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     /          Acknowledgement Number Subheader (8 bytes)           /     
     |   Reserved    |       Acknowledgement Number (high bits)      .  
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  
     .       Acknowledgement Number (low bits)       |   Reserved    |  
     /                      Options and Padding                      /     
     |                     Options                   /    Padding    |  
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+     
     /                Application Data Area (Ignored)                /     
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     
                                                                           
    The Acknowledgement Number field has special semantics for DCCP-Sync   
    and DCCP-SyncAck packets.  First, the packet corresponding to a        
    DCCP-Sync's Acknowledgement Number need not have been                  
    acknowledgeable.  Thus, receivers MUST NOT assume that a packet was    
    processed simply because it appears in the Acknowledgement Number      
    field of a DCCP-Sync packet.  This differs from all other packet       
    types, where the Acknowledgement Number by definition corresponds to   
    an acknowledgeable packet.  Second, the Acknowledgement Number on      
    any DCCP-SyncAck packet MUST correspond to the Sequence Number on an   
    acknowledgeable DCCP-Sync packet.  In the presence of reordering,      
    this might not equal GSR.                                              
                                                                           
    As with DCCP-Ack packets, DCCP-Sync and DCCP-SyncAck packets MAY       
    Receivers MUST ignore the application data area in DCCP-Sync and    
    have non-zero-length application data areas, whose contents            
    DCCP-SyncAck packets.  Endpoints may find it useful to pad DCCP-Sync
    receivers MUST ignore.  Padded DCCP-Sync packets may be useful when    
    packets with "application data" when performing PMTU discovery; see 
    performing Path MTU discovery; see Section 14.                         
    Section 14.                                                         
                                                                           
5.8.  Options                                                              
                                                                           
    Any DCCP packet may contain options, which occupy space at the end     
    of the DCCP header.  Each option is a multiple of 8 bits in length.    
    Individual options are not padded to multiples of 32 bits, and any     
    The combination of all options MUST add up to a multiple of 32 bits.
    option may begin on any byte boundary.  However, the combination of    
    Individual options are not padded to multiples of 32 bits, however; 
    all options MUST add up to a multiple of 32 bits; Padding options      
    any option may begin on any byte boundary.  Any options present are 
    MUST be added as necessary to fill out option space to a word          
    included in the header checksum.                                    
    boundary.  Any options present are included in the header checksum.    
                                                                           
    The first byte of an option is the option type.  Options with types    
    0 through 31 are single-byte options.  Other options are followed by   
    a byte indicating the option's length.  This length value includes     
    the two bytes of option-type and option-length as well as any          
    option-data bytes, and must therefore be greater than or equal to      
    two.                                                                   
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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    Options are processed sequentially, starting at the first option in    
    the packet header.  Options with unknown types, and options with       
    the packet header.                                                  
    invalid lengths (length byte less than two or more than the            
    remaining space in the options portion of the header), MUST be         
    ignored.                                                               
                                                                           
    The following options are currently defined:                           
                                                                           
               Option                           DCCP-  Section             
             Option                            Section                  
       Type    Length     Meaning               Data?  Reference           
     Type    Length     Meaning               Reference                 
       ----    ------     -------               -----  ---------           
     ----    ------     -------               ---------                 
         0        1       Padding                 Y      5.8.1             
       0        1       Padding                 5.8.1                   
         1        1       Mandatory               N      5.8.2             
       1        1       Mandatory               5.8.2                   
         2        1       Slow Receiver           Y      11.6              
       2        1       Slow Receiver           11.6                    
       3-31       1       Reserved                                         
       3        1       Reset Congestion State  11.7                    
        32     variable   Change L                N      6.1               
     4-31       1       Reserved                                        
        33     variable   Confirm L               N      6.2               
      32     variable   Change L                6.1                     
        34     variable   Change R                N      6.1               
      33     variable   Confirm L               6.2                     
        35     variable   Confirm R               N      6.2               
      34     variable   Change R                6.1                     
        36     variable   Init Cookie             N      8.1.4             
      35     variable   Confirm R               6.2                     
        37       3-5      NDP Count               Y      7.7               
      36     variable   Init Cookie             8.1.4                   
        38     variable   Ack Vector [Nonce 0]    N      11.4              
      37       4-5      NDP Count               7.7                     
        39     variable   Ack Vector [Nonce 1]    N      11.4              
      38     variable   Ack Vector [Nonce 0]    11.4                    
        40     variable   Data Dropped            N      11.7              
      39     variable   Ack Vector [Nonce 1]    11.4                    
        41        6       Timestamp               Y      13.1              
      40     variable   Data Dropped            11.8                    
        42      6/8/10    Timestamp Echo          Y      13.3              
      41        6       Timestamp               13.1                    
        43       4/6      Elapsed Time            N      13.2              
      42       6-10     Timestamp Echo          13.3                    
        44        6       Data Checksum           Y      9.3               
      43       4-6      Elapsed Time            13.2                    
       45-127  variable   Reserved                                         
      44        4       Data Checksum           9.3                     
      128-255  variable   CCID-specific options   -      10.3              
     45-127  variable   Reserved                                        
                                                                           
    128-255  variable   CCID-specific options   10.4                    
                        Table 3: DCCP Options                              
                                                                           
    Not all options are suitable for all packet types.  For example,       
    since the Ack Vector option is interpreted relative to the             
    Acknowledgement Number, it isn't suitable on DCCP-Request and DCCP-    
    Data packets, which have no Acknowledgement Number.  If an option      
    occurs on an unexpected packet type, it MUST generally be ignored;     
    any such restrictions are mentioned in each option's description.      
    The table summarizes the most common restriction: when the DCCP-       
    Data? column value is N, the corresponding option MUST be ignored      
    when received on a DCCP-Data packet.                                   
                                                                           
    This section describes two generic options, Padding and Mandatory.     
    Other options are described later.                                     
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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5.8.1.  Padding Option                                                     
                                                                           
    +--------+                                                             
    |00000000|                                                             
    +--------+                                                             
      Type=0                                                               
                                                                           
    Padding is a single-byte "no-operation" option used to pad between     
    Padding is a single byte option used to pad between or after        
    or after options.  If the length of a packet's other options is not    
    options.  It either ensures the application data begins on a 32-bit 
    a multiple of 4, then Padding options are REQUIRED to pad out the      
    boundary (as required), or ensures alignment of following options   
    options area to the length implied by Data Offset.  Padding may also   
    (not mandatory).                                                    
    be used between options -- for example, to align the beginning of a    
    subsequent option on a word boundary.  There is no guarantee that      
    senders will use this option, so receivers must be prepared to         
    process options even if they do not begin on a word boundary.          
                                                                           
5.8.2.  Mandatory Option                                                   
                                                                           
    +--------+                                                             
    |00000001|                                                             
    +--------+                                                             
      Type=1                                                               
                                                                           
    Mandatory is a single byte option that marks the immediately           
    following option as mandatory.  Say that the immediately following     
    option is O.  Then the Mandatory option has no effect if the           
    option is OP.  Then the Mandatory option has no effect if the       
    receiving DCCP endpoint understands and processes O.  If the           
    receiving DCCP endpoint understands and processes OP.  If the       
    endpoint does not understand or process O, however, then it MUST       
    endpoint does not understand or process OP, however, then it MUST   
    reset the connection using Reset Code 6, "Mandatory Failure".  For     
    instance, the endpoint would reset the connection if it did not        
    understand O's type; if it understood O's type, but not O's data; if   
    understand OP's type; if it understood OP's type, but not OP's data;
    O's data was invalid for O's type; if O was a feature negotiation      
    if OP's data was invalid for OP's type; if OP was a feature         
    option, and the endpoint did not understand the enclosed feature       
    negotiation option, and the endpoint did not understand the enclosed
    number; if the endpoint understood O, but chose not to perform the     
    feature number; if the endpoint understood OP, but chose not to     
    action O implies; and so forth.                                        
    perform the action OP implies; and so forth.                        
                                                                           
    Mandatory options MUST NOT be sent on DCCP-Data packets, and any       
    Mandatory options received on DCCP-Data packets MUST be ignored.       
                                                                           
    The connection is in error and should be reset with Reset Code 5,      
    "Option Error" if option O is absent (Mandatory was the last byte of   
    "Option Error" if option OP is absent (Mandatory was the last byte  
    the option list), or if option O equals Mandatory.  However, the       
    of the option list), or if option OP equals Mandatory.  However, the
    combination "Mandatory Padding" is valid, and MUST behave like two     
    bytes of Padding.                                                      
                                                                           
    Section 6.6.9 describes the behavior of Mandatory feature              
    negotiation options in more detail.                                    
                                                                           
                                                                           
                                                                           
                                                                           
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6.  Feature Negotiation                                                    
                                                                           
    Four DCCP options, Change L, Confirm L, Change R, and Confirm R, are   
    Four DCCP options, Change L, Confirm L, Change R, and Confirm R,    
    used to negotiate feature values.  Change options initiate a           
    implement in-band feature negotiation.  Change options initiate a   
    negotiation; Confirm options complete that negotiation.  The "L"       
    options are sent by the feature location, and the "R" options are      
    sent by the feature remote.  Change options are retransmitted to       
    ensure reliability.                                                    
                                                                           
    All these options have the same format.  The first byte of option      
    data is the feature number, and the second and subsequent data bytes   
    hold one or more feature values.  The exact format of the feature      
    hold one or more feature values.  The feature values are generally  
    value area depends on the feature type; see Section 6.3.               
    arranged in a linear preference list, where the first value is most 
                                                                           
    preferred.                                                          
    +--------+--------+--------+--------+--------                          
    |  Type  | Length |Feature#| Value(s) ...                              
    +--------+--------+--------+--------+--------                          
                                                                           
    Together, the feature number and the option type ("L" or "R")          
    uniquely identify the feature to which an option applies.  The exact   
    format of the Value(s) area depends on the feature number.             
                                                                           
    Feature negotiation options MUST NOT be sent on DCCP-Data packets,     
    and any feature negotiation options received on DCCP-Data packets      
    MUST be ignored.                                                       
                                                                           
6.1.  Change Options                                                       
                                                                           
    Change L and Change R options initiate feature negotiation.  The       
    Change L and Change R options initiate feature negotiation.  Which  
    option to use depends on the relevant feature's location: To start a   
    option to use depends on where the negotiated feature is located.   
    negotiation for feature F/A, DCCP A will send a Change L option; to    
    To start a negotiation for feature F/A, DCCP A must send a Change L 
    start a negotiation for F/B, it will send a Change R option.  Change   
    option; to start a negotiation for F/B, it must send a Change R     
    options are retransmitted until some response is received.  They       
    option.  Change options are retransmitted until some response is    
    contain at least one Value, and thus have length at least 4.           
    received.  Change options contain at least one Value, and thus have 
                                                                           
    length at least 4.                                                  
               +--------+--------+--------+--------+--------               
    Change L:  |00100000| Length |Feature#| Value(s) ...                   
               +--------+--------+--------+--------+--------               
                Type=32                                                    
                                                                           
               +--------+--------+--------+--------+--------               
    Change R:  |00100010| Length |Feature#| Value(s) ...                   
               +--------+--------+--------+--------+--------               
                Type=34                                                    
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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6.2.  Confirm Options                                                      
                                                                           
    Confirm L and Confirm R options complete feature negotiation, and      
    are sent in response to Change R and Change L options, respectively.   
    Confirm options MUST NOT be generated except in response to Change     
    options.  Confirm options need not be retransmitted, since Change      
    options.  Any packet including a Confirm option MUST carry an       
    options are retransmitted as necessary.  The first byte of the         
    Acknowledgement Number; thus, Confirm options are not allowed on    
    Confirm option contains the feature number from the corresponding      
    DCCP-Request and DCCP-Data packets.  Confirm options need not be    
    Change.  Following this is the selected Value, and then possibly the   
    retransmitted, since Change options are retransmitted as necessary. 
    sender's preference list.                                              
    Normal Confirm options contain the selected Value, possibly followed
                                                                           
    by the sender's preference list.                                    
               +--------+--------+--------+--------+--------               
    Confirm L: |00100001| Length |Feature#| Value(s) ...                   
               +--------+--------+--------+--------+--------               
                Type=33                                                    
                                                                           
               +--------+--------+--------+--------+--------               
    Confirm R: |00100011| Length |Feature#| Value(s) ...                   
               +--------+--------+--------+--------+--------               
                Type=35                                                    
                                                                           
    If an endpoint receives an invalid Change option -- with an unknown    
    feature number, or an invalid value -- it will respond with an empty   
    Confirm option containing the problematic feature number, but no       
    Confirm option containing no value.  Such options have length 3.    
    value.  Such options have length 3.                                    
                                                                           
6.3.  Reconciliation Rules                                                 
                                                                           
    Reconciliation rules determine how the two sets of preferences for a   
    given feature are resolved into a unique result.  The reconciliation   
    rule depends only on the feature number.  Each reconciliation rule     
    must have the property that the result is uniquely determined given    
    the contents of Change options sent by the two endpoints.              
                                                                           
    All current DCCP features use one of two reconciliation rules,         
    server-priority ("SP") and non-negotiable ("NN").                      
                                                                           
6.3.1.  Server-Priority                                                    
                                                                           
    The feature value is a fixed-length byte string (length determined     
    by the feature number).  Each Change option contains a list of         
    by the feature number).  Each Change option contains a preference   
    values ordered by preference, with the most preferred value coming     
    list of values, with the most preferred value coming first.  Each   
    first.  Each Confirm option contains the confirmed value, followed     
    Confirm option contains the confirmed value, followed by the        
    by the confirmer's preference list.  Thus, the feature's current       
    confirmer's preference list.  Thus, the feature's current value will
    value will generally appear twice in Confirm options' data, once as    
    generally appear twice in Confirm options' data, once as the current
    the current value and once in the confirmer's preference list.         
    value and once in the confirmer's preference list.                  
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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    To reconcile the preference lists, select the first entry in the       
    server's list that also occurs in the client's list.  If there is no   
    shared entry, the feature's value MUST NOT change, and the Confirm     
    option will confirm the feature's previous value (unless the Change    
    option was Mandatory; see Section 6.6.9).                              
|| TEXT DELETED ||                                                         
    A single feature negotiation may, because of loss or delay, contain 
                                                                           
    retransmitted Change options and multiple Confirm options.  Each of 
6.3.2.  Non-Negotiable                                                     
    the retransmitted Change options MUST contain the same payload; see 
                                                                           
    Section 6.6.3.  For server-priority features, this means that an    
    The feature value is a byte string.  Each option contains exactly      
    endpoint sending Change options MUST NOT change its preference list 
    one feature value.  The feature location signals a new value by        
    during a negotiation.  However, the other endpoint MAY change its   
    sending a Change L option.  The feature remote MUST accept any valid   
    preference list at will, assuming it hasn't recently sent a Change  
    value, responding with a Confirm R option containing the new value,    
    option for the same feature.  Reordering protection (Section 6.6.4) 
    and it MUST send empty Confirm R options in response to invalid        
    ensures that agreement is reached.                                  
    values (unless the Change L option was Mandatory; see Section          
    6.6.9).  Change R and Confirm L options MUST NOT be sent for non-      
    negotiable features; see Section 6.6.8.  Non-negotiable features use   
    negotiable features.  Non-negotiable features use the feature       
    the feature negotiation mechanism to achieve reliability.              
    negotiation mechanism to achieve reliability.                       
                                                                           
6.4.  Feature Numbers                                                      
                                                                           
    This document defines the following feature numbers.                   
                                                                           
                                           Rec'n Initial        Section    
    Number   Meaning                       Rule   Value  Req'd Reference   
    ------   -------                       -----  -----  ----- ---------   
       0     Reserved                                                      
       1     Congestion Control ID (CCID)   SP      2      Y     10        
       2     Allow Short Seqnos             SP      1      Y     7.6.1     
       3     Sequence Window                NN     100     Y     7.5.2     
       3     Sequence Window                NN     100     Y     7.5.4  
       4     ECN Incapable                  SP      0      N     12.1      
       4     ECN Capable                    SP      1      Y     12.1   
       5     Ack Ratio                      NN      2      N     11.3      
       6     Send Ack Vector                SP      0      N     11.5      
       7     Send NDP Count                 SP      0      N     7.7.2     
       8     Minimum Checksum Coverage      SP      0      N     9.2.1     
       9     Check Data Checksum            SP      0      N     9.3.1     
     10-127  Reserved                                                      
    128-255  CCID-specific features                              10.3      
    128-255  CCID-specific features                              10.4   
                                                                           
                       Table 4: DCCP Feature Numbers                       
                                                                           
                                                                           
    Rec'n Rule     The reconciliation rule used for the feature.  SP is    
                   server-priority and NN is non-negotiable.               
                                                                           
    Initial Value  The initial value for the feature.  Every feature has   
                   a known initial value.                                  
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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    Req'd          This column is "Y" if and only if every DCCP            
    Req'd          This column is "Y" iff every DCCP implementation MUST
                   implementation MUST understand the feature.  If it is   
                   understand the feature.  If it is "N", then the      
                   "N", then the feature behaves like an extension (see    
                   feature behaves like an extension (see Section 15),  
                   Section 15), and it is safe to respond to Change        
                   and it is safe to respond to Change options for the  
                   options for the feature with empty Confirm options.     
                   feature with empty Confirm options.  Of course, a    
                   Of course, a CCID might require the feature; a DCCP     
                   CCID might require the feature; a DCCP that          
                   that implements CCID 2 MUST support Ack Ratio and       
                   implements CCID 2 MUST support Ack Ratio and Send Ack
                   Send Ack Vector, for example.                           
                   Vector, for example.                                 
                                                                           
6.5.  Examples                                                             
    Here are three example feature negotiations for features located at    
    the server, the first two for the Congestion Control ID feature, the   
    last for the Ack Ratio.                                                
                                                                           
                Client                     Server                          
                ------                     ------                          
     1. Change R(CCID, 2 3 1)  -->                                         
        ("2 3 1" is client's preference list)                              
     2.                        <--  Confirm L(CCID, 3, 3 2 1)              
                              (3 is the negotiated value;                  
                              "3 2 1" is server's pref list)               
                 * agreement that CCID/Server = 3 *                        
                                                                           
                                                                           
     1.                   XXX  <--  Change L(CCID, 3 2 1)                  
     2.                             Retransmission:                        
                               <--  Change L(CCID, 3 2 1)                  
     3. Confirm R(CCID, 3, 2 3 1)  -->                                     
                 * agreement that CCID/Server = 3 *                        
                                                                           
                                                                           
     1.                        <--  Change L(Ack Ratio, 3)                 
     2. Confirm R(Ack Ratio, 3)  -->                                       
              * agreement that Ack Ratio/Server = 3 *                      
                                                                           
    This example shows a simultaneous negotiation.                         
                                                                           
                Client                     Server                          
                ------                     ------                          
    1a. Change R(CCID, 2 3 1)  -->                                         
     b.                        <--  Change L(CCID, 3 2 1)                  
    2a.                        <--  Confirm L(CCID, 3, 3 2 1)              
     b. Confirm R(CCID, 3, 2 3 1)  -->                                     
                 * agreement that CCID/Server = 3 *                        
                                                                           
    Here are the byte encodings of several Change and Confirm options.     
    Each option is sent by DCCP A.                                         
                                                                           
                                                                           
                                                                           
                                                                           
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    Change L(CCID, 2 3) = 32,5,1,2,3                                       
        DCCP B should change CCID/A's value (feature number 1, a server-   
        priority feature); DCCP A's preferred values are 2 and 3, in       
        that preference order.                                             
                                                                           
    Change L(Sequence Window, 1024) = 32,6,3,0,4,0                         
        DCCP B should change Sequence Window/A's value (feature number     
        3, a non-negotiable feature) to the 3-byte string 0,4,0 (the       
        value 1024).                                                       
                                                                           
    Confirm L(CCID, 2, 2 3) = 33,6,1,2,2,3                                 
        DCCP A has changed CCID/A's value to 2; its preferred values are   
        2 and 3, in that preference order.                                 
                                                                           
    Empty Confirm L(126) = 33,3,126                                        
        DCCP A doesn't implement feature number 126, or DCCP B's           
        proposed value for feature 126/A was invalid.                      
                                                                           
    Change R(CCID, 3 2) = 34,5,1,3,2                                       
        DCCP B should change CCID/B's value; DCCP A's preferred values     
        are 3 and 2, in that preference order.                             
                                                                           
    Confirm R(CCID, 2, 3 2) = 35,6,1,2,3,2                                 
        DCCP A has changed CCID/B's value to 2; its preferred values       
        were 3 and 2, in that preference order.                            
                                                                           
    Confirm R(Sequence Window, 1024) = 35,6,3,0,4,0                        
        DCCP A has changed Sequence Window/B's value to the 3-byte         
        string 0,4,0 (the value 1024).                                     
                                                                           
    Empty Confirm R(126) = 35,3,126                                        
        DCCP A doesn't implement feature number 126, or DCCP B's           
        proposed value for feature 126/B was invalid.                      
                                                                           
6.6.  Option Exchange                                                      
                                                                           
    A few basic rules govern feature negotiation option exchange.          
                                                                           
    1.  Every non-reordered Change option gets a Confirm option in         
        response.                                                          
                                                                           
    2.  Change options are retransmitted until a response for the latest   
        Change is received.                                                
                                                                           
    3.  Feature negotiation options are processed in strictly increasing   
        order by Sequence Number.                                          
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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    The rest of this section describes the consequences of these rules     
    in more detail.                                                        
                                                                           
6.6.1.  Normal Exchange                                                    
                                                                           
    Change options are generated when a DCCP endpoint wants to change      
    the value of some feature.  Generally, this will happen at the         
    beginning of a connection, although it may happen at any time.  We     
    say the endpoint "generates" or "sends" a Change L or Change R         
    option, but of course the option must be attached to a packet.  The    
    endpoint may attach the option to a packet it would have generated     
    anyway (such as a DCCP-Request), or it may create a "feature           
    anyway (such as a DCCP-Request).  Alternatively, it may create a    
    negotiation packet", often a DCCP-Ack or DCCP-Sync, just to carry      
    "feature negotiation packet", often a DCCP-Ack or DCCP-Sync, just to
    the option.  Feature negotiation packets are controlled by the         
    carry the option.  Feature negotiation packets MUST be rate-limited 
    relevant congestion control mechanism.  For example, DCCP A may send   
    by the relevant congestion control mechanisms.  In addition, an     
    a DCCP-Ack or DCCP-Sync for feature negotiation only if the B-to-A     
    endpoint SHOULD generate at most one feature negotiation packet per 
    CCID would allow sending a DCCP-Ack.  In addition, an endpoint         
    round-trip time (0.1 seconds, if no RTT is available).              
    SHOULD generate at most one feature negotiation packet per round-      
    trip time.                                                             
                                                                           
    On receiving a Change L or Change R option, a DCCP endpoint examines   
    the included preference list, reconciles that with its own             
    preference list, calculates the new value, and sends back a            
    Confirm R or Confirm L option, respectively, informing its peer of     
    the new value or that the feature was not understood.  Every non-      
    the new value.  Every non-reordered Change option MUST result in a  
    reordered Change option MUST result in a corresponding Confirm         
    corresponding Confirm option, and any packet including a Confirm    
    option, and any packet including a Confirm option MUST carry an        
    option MUST carry an Acknowledgement Number.  Generated Confirm     
    Acknowledgement Number.  Generated Confirm options may be attached     
    options may be attached to packets that would have been sent anyway 
    to packets that would have been sent anyway (such as DCCP-Response     
    (such as DCCP-Response or DCCP-SyncAck), or to new feature          
    or DCCP-SyncAck), or to new feature negotiation packets, as            
    negotiation packets, as described above.                            
    described above.                                                       
                                                                           
    The Change-sending endpoint MUST wait to receive a corresponding       
    Confirm option before changing its stored feature value.  The          
    Confirm-sending endpoint changes its stored feature value as soon as   
    it sends the Confirm.                                                  
                                                                           
    A packet MAY contain more than one feature negotiation option, as      
    Endpoints MUST NOT send packets that contain more than one feature  
    long as no two options refer to the same feature.  Note, however,      
    negotiation option referring to the same feature.  Note, however,   
    that a packet is allowed to contain one L option and one R option      
    with the same feature number, since the two options actually refer     
    with the same feature number F, since the two options actually refer
    to different features (F/A and F/B).                                   
                                                                           
6.6.2.  Processing Received Options                                        
                                                                           
    DCCP endpoints exist in one of three states relative to each           
    feature.  STABLE is the normal state, where the endpoint knows the     
    feature's value and thinks the other endpoint agrees.  An endpoint     
                                                                           
                                                                           
                                                                           
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    enters the CHANGING state when it first sends a Change for the         
    feature, and returns to STABLE once it receives a corresponding        
    Confirm.  The final state, UNSTABLE, indicates that an endpoint in     
    CHANGING state changed its preference list, but has not yet            
    transmitted a Change option with the new preference list.              
                                                                           
    Feature state transitions at a feature location are implemented        
    Feature-related state transitions at the feature location are       
    according to this diagram.  The diagram ignores sequence number and    
    implemented as shown in the diagram below.  For feature-related     
    option validity issues; these are handled explicitly in the            
    state transitions at the feature remote, switch the "L"s and "R"s.  
    pseudocode that follows.                                               
    The diagram ignores sequence number and option validity issues;     
                                                                           
    these are handled explicitly in the pseudocode that follows the     
                                                          timeout/         
    diagram.                                                            
 rcv Confirm R      app/protocol evt : snd Change L       rcv non-ack      
 : ignore      +---------------------------------------+  : snd Change L   
      +----+   |                                       |  +----+           
      |    v   |                   rcv Change R        v  |    v           
   +------------+  rcv Confirm R   : calc new value, +------------+        
   |            |  : accept value    snd Confirm L   |            |        
   |   STABLE   |<-----------------------------------|  CHANGING  |        
   |            |        rcv empty Confirm R         |            |        
   +------------+        : revert to old value       +------------+        
       |    ^                                            |    ^            
       +----+                                  pref list |    | snd        
 rcv Change R                                  changes   |    | Change L   
 : calc new value, snd Confirm L                         v    |            
                                                     +------------+        
                                                 +---|            |        
                            rcv Confirm/Change R |   |  UNSTABLE  |        
                            : ignore             +-->|            |        
                                                     +------------+        
                                                                           
    Feature locations SHOULD use the following pseudocode, which           
    Endpoints SHOULD use the following pseudocode, which corresponds to 
    corresponds to the state diagram, to react to each feature             
    the state diagram, to react to each feature negotiation option on   
    negotiation option on each valid packet received.  The pseudocode      
    each valid packet received.  The pseudocode refers to "P.seqno" and 
    refers to "P.seqno" and "P.ackno", which are properties of the         
    "P.ackno", which are properties of the packet; "O.type", and        
    packet; "O.type", and "O.len", which are properties of the option;     
    "O.len", which are properties of the option; "FGSR" and "FGSS",     
    "FGSR" and "FGSS", which are properties of the connection, and         
    which are properties of the connection, and handle reordering as    
    handle reordering as described in Section 6.6.4; "F.state", which is   
    described in Section 6.6.4; "F.state", which is the feature's state 
    the feature's state (STABLE, CHANGING, or UNSTABLE); and "F.value",    
    (STABLE, CHANGING, or UNSTABLE); and "F.value", which is the        
    which is the feature's value.                                          
    feature's value.                                                    
                                                                           
    First, check for unknown features (Section 6.6.7);                     
       If F is unknown,                                                    
       If F is unknown:                                                 
          If the option was Mandatory,   /* Section 6.6.9 */               
          If option was Mandatory:   /* Section 6.6.9 */                
             Reset connection and return                                   
          Otherwise, if O.type == Change R,                                
          Otherwise, if O.type == Change R:                             
             Send Empty Confirm L on a future packet                       
          Return                                                           
                                                                           
                                                                           
                                                                           
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    Second, check for reordering (Section 6.6.4);                          
       If F.state == UNSTABLE or P.seqno <= FGSR                           
               or (O.type == Confirm R and P.ackno < FGSS),                
               or (O.type == Confirm R and P.ackno < FGSS)              
          Ignore option and return                                         
                                                                           
    Third, process Change R options;                                       
       If O.type == Change R,                                              
       If O.type == Change R:                                           
          If the option's value is valid,   /* Section 6.6.8 */            
          If option's value is valid:   /* Section 6.6.8 */             
             Calculate new value                                           
             Send Confirm L on a future packet                             
             Set F.state := STABLE                                         
          Otherwise, if the option was Mandatory,                          
          Otherwise, if option was Mandatory:                           
             Reset connection and return                                   
          Otherwise,                                                       
          Otherwise:                                                    
             Send Empty Confirm L on a future packet                       
             /* Remain in existing state.  If that's CHANGING, this        
                endpoint will retransmit its Change L option later. */     
                                                                           
    Fourth, process Confirm R options (but only in CHANGING state).        
       If F.state == CHANGING and O.type == Confirm R,                     
       If F.state == CHANGING and O.type == Confirm R:                  
          If O.len > 3,   /* nonempty */                                   
          If O.len > 3:   /* nonempty */                                
             If the option's value is valid,                               
             If option's value is valid:                                
                Set F.value := new value                                   
             Otherwise,                                                    
             Otherwise:                                                 
                Reset connection and return                                
          Set F.state := STABLE                                            
                                                                           
    Of course, every DCCP endpoint is both a feature location and a        
    feature remote.  A similar diagram and pseudocode applies to feature   
    remotes; simply switch the "L"s and "R"s, so that the relevant         
    options are Change R and Confirm L.                                    
                                                                           
6.6.3.  Loss and Retransmission                                            
                                                                           
    Packets containing Change and Confirm options might be lost or         
    delayed by the network.  Therefore, Change options are repeatedly      
    delayed by the network.  Therefore, Change options are retransmitted
    transmitted to achieve reliability.  We refer to this as               
    to achieve reliability.                                             
    "retransmission", although of course there are no packet-level         
    retransmissions in DCCP: a Change option that is sent again will be    
    sent on a new packet with a new sequence number.                       
                                                                           
    A CHANGING endpoint transmits another Change option once it realizes   
    that it has not heard back from the other endpoint.  The new Change    
    option need not contain the same payload as the original; reordering   
    protection will ensure that agreement is reached based on the most     
    recently transmitted option.                                           
    recently transmitted option.  The endpoint may piggyback its Change 
                                                                           
    options on packets it would have sent anyway.  If it generates new  
                                                                           
    packets for feature negotiation, it MUST use an exponential-backoff 
                                                                           
    timer.  The timer is initially set to approximately one or two      
                                                                           
    round-trip times (or 0.1-0.2 seconds, if no RTT is available), and  
                                                                           
    pinned at roughly 32 RTTs.                                          
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    A CHANGING endpoint MUST continue retransmitting Change options        
    until it gets some response or the connection terminates.              
                                                                           
    Endpoints SHOULD use an exponential-backoff timer to decide when to    
    Endpoints SHOULD NOT send Change options for a given feature more   
    retransmit Change options.  (Endpoints that generate packets           
    frequently than once per RTT.  Otherwise, the reordering protection 
    specifically for feature negotiation MUST use such a timer.)  The      
    algorithms described in the next subsection may delay agreement,    
    timer interval is initially set to not less than one round-trip        
    since no received Confirm option would acknowledge the most recently
    time, and should back off to not less than 64 seconds.  The backoff    
    transmitted Change.                                                 
    protects against delayed agreement due to the reordering protection    
    algorithms described in the next section.  Again, endpoints may        
    piggyback Change options on packets they would have sent anyway, or    
    create new packets to carry the options; any such new packets are      
    controlled by the relevant congestion-control mechanism.               
                                                                           
    Confirm options are never retransmitted, but the Confirm-sending       
    endpoint MUST generate a Confirm option after every non-reordered      
    Change.                                                                
                                                                           
6.6.4.  Reordering                                                         
                                                                           
    Reordering might cause packets containing Change and Confirm options   
    to arrive in an unexpected order.  Endpoints MUST ignore feature       
    negotiation options that do not arrive in strictly-increasing order    
    by Sequence Number.  The rest of this section presents two             
    algorithms that fulfill this requirement.                              
                                                                           
    The first algorithm introduces two sequence number variables that      
    each endpoint maintains for the connection.                            
                                                                           
    FGSR      Feature Greatest Sequence Number Received: The greatest      
              sequence number received, considering only valid packets     
              that contained one or more feature negotiation options       
              (Change and/or Confirm).  This value is initialized to       
              ISR - 1.                                                     
                                                                           
    FGSS      Feature Greatest Sequence Number Sent: The greatest          
              sequence number sent, considering only packets that          
              contained one or more non-retransmitted Change options.      
              (Retransmitted Change options MUST have exactly the same     
              contents as previously transmitted options, so limited       
              reordering can safely be tolerated.)  This value is          
              initialized to ISS.                                          
                                                                           
    Each endpoint checks two conditions on sequence numbers to decide      
    whether to process received feature negotiation options.               
                                                                           
    1.  If a packet's Sequence Number is less than or equal to FGSR,       
        then its Change options MUST be ignored.                           
                                                                           
                                                                           
                                                                           
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    2.  If a packet's Sequence Number is less than or equal to FGSR, OR    
        it has no Acknowledgement Number, OR its Acknowledgement Number    
        is less than FGSS, then its Confirm options MUST be ignored.       
                                                                           
    Alternatively, an endpoint MAY maintain separate FGSR and FGSS         
    values for every feature.  FGSR(F/X) would equal the greatest          
    sequence number received, considering only packets that contained      
    Change or Confirm options applying to feature F/X; FGSS(F/X) would     
    be defined similarly.  This algorithm requires more state, but is      
    slightly more forgiving to multiple overlapped feature negotiations.   
    Either algorithm MAY be used; the first algorithm, with connection-    
    wide FGSR and FGSS variables, is RECOMMENDED.                          
                                                                           
    One consequence of these rules is that a CHANGING endpoint will        
    ignore any Confirm option that does not acknowledge the latest         
    Change option sent.  This ensures that agreement, once achieved,       
    used the most recent available information about the endpoints'        
    preferences.                                                           
                                                                           
6.6.5.  Preference Changes                                                 
                                                                           
    Endpoints are allowed to change their preference lists at any time.    
    However, an endpoint that changes its preference list while in the     
    CHANGING state MUST transition to the UNSTABLE state.  It will         
    transition back to CHANGING once it has transmitted a Change option    
    with the new preference list.  This ensures that agreement is based    
    on active preference lists.  Without the UNSTABLE state,               
    simultaneous negotiation -- where the endpoints began independent      
    negotiations for the same feature at the same time -- might lead to    
    the negotiation terminating with the endpoints thinking the feature    
    had different values.                                                  
                                                                           
6.6.6.  Simultaneous Negotiation                                           
                                                                           
    The two endpoints might simultaneously open negotiation for the same   
    feature, after which an endpoint in the CHANGING state will receive    
    a Change option for the same feature.  Such received Change options    
    can act as responses to the original Change options.  The CHANGING     
    endpoint MUST examine the received Change's preference list,           
    reconcile that with its own preference list (as expressed in its       
    generated Change options), and generate the corresponding Confirm      
    option.  It can then transition to the STABLE state.                   
                                                                           
6.6.7.  Unknown Features                                                   
                                                                           
    Endpoints may receive Change options referring to feature numbers      
    they do not understand -- for instance, when an extended DCCP          
    converses with a non-extended DCCP.  Endpoints MUST respond to         
                                                                           
                                                                           
                                                                           
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    unknown Change options with Empty Confirm options (that is, Confirm    
    options containing no data), which inform the CHANGING endpoint that   
    the feature was not understood.  However, if the Change option was     
    Mandatory, the connection MUST be reset; see Section 6.6.9.            
    preceded by a Mandatory option, the connection MUST be reset; see   
                                                                           
    Section 6.6.9.                                                      
    On receiving an empty Confirm option for some feature, the CHANGING    
    endpoint MUST transition back to the STABLE state, leaving the         
    feature's value unchanged.  Section 15 suggests that the default       
    value for any extension feature should correspond to "extension not    
    available".                                                            
                                                                           
    Some features are required to be understood by all DCCPs (see          
    Section 6.4).  The CHANGING endpoint SHOULD reset the connection       
    (with Reset Code 5, "Option Error") if it receives an empty Confirm    
    option for such a feature.                                             
                                                                           
    Since Confirm options are generated only in response to Change         
    options, an endpoint should never receive a Confirm option referring   
    to a feature number it does not understand.  Nevertheless, endpoints   
    to a feature number it does not understand.  Endpoints MUST ignore  
    MUST ignore any such options they receive.                             
    such options.                                                       
                                                                           
6.6.8.  Invalid Options                                                    
                                                                           
    A DCCP endpoint might receive a Change or Confirm option that lists    
    one or more values that it does not understand.  Some, but not all,    
    such options are invalid, depending on the relevant reconciliation     
    rule (Section 6.3).  For instance:                                     
                                                                           
    o  All features have length limitiations, and options with invalid     
       lengths are invalid.  For example, the Ack Ratio feature takes      
       16-bit values, so valid "Confirm R(Ack Ratio)" options have         
       option length 5.                                                    
                                                                           
    o  Some non-negotiable features have value limitations.  The Ack       
       Ratio feature takes two-byte, non-zero integer values, so a         
       "Change L(Ack Ratio, 0)" option is never valid.  Note that          
       server-priority features do not have value limitations, since       
       unknown values are handled as a matter of course.                   
                                                                           
    o  Any Confirm option that selects the wrong value, based on the two   
       preference lists and the relevant reconciliation rule, is           
       invalid.                                                            
                                                                           
    o  However, unexpected Confirm options -- that refer to unknown        
       feature numbers, or that don't appear to be part of a current       
       negotiation -- are considered valid, although they are ignored by   
       the receiver.                                                       
                                                                           
                                                                           
                                                                           
                                                                           
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    An endpoint receiving an invalid Change option MUST respond with the   
    corresponding empty Confirm option.  An endpoint receiving an          
    invalid Confirm option MUST reset the connection, with Reset Code 5,   
    "Option Error".                                                        
                                                                           
6.6.9.  Mandatory Feature Negotiation                                      
                                                                           
    Change options may be preceded by Mandatory options (Section 5.8.2).   
    Mandatory Change options are processed like normal Change options,     
    except that the following failure cases will cause the receiver to     
    reset the connection with Reset Code 6, "Mandatory Failure", rather    
    than send a Confirm option.  The connection MUST be reset if:          
                                                                           
    o  The Change option's feature number was not understood;              
                                                                           
    o  The Change option's value was invalid, and the receiver would       
       normally have sent an empty Confirm option in response; or          
                                                                           
    o  For server-priority features, there was no shared entry in the      
       two endpoints' preference lists.                                    
                                                                           
    There's no reason to mark Confirm options as Mandatory in this         
    version of DCCP, since Confirm options are sent only in response to    
    Change options and therefore can't mention potentially-invalid         
    values or unexpected feature numbers.                                  
|| TEXT DELETED ||                                                         
6.6.10.  Out-of-Band Agreement                                          
                                                                           
    An endpoint MUST NOT unilaterally change the value of any DCCP      
7.  Sequence Numbers                                                       
    feature.  However, endpoints MAY cooperatively change DCCP feature  
                                                                           
    values without using in-band feature negotiation options.  For      
    DCCP uses sequence numbers to arrange packets into sequence, detect    
    example, features MAY be changed via negotation over a separate     
    losses and network duplicates, and protect against attackers, half-    
    signaling channel, for example.                                     
    open connections, and the delivery of very old packets.  Every         
    packet carries a Sequence Number; most packet types carry an           
    Acknowledgement Number as well.                                        
                                                                           
    DCCP sequence numbers are packet-based.  That is, the packets          
    generated by each endpoint have Sequence Numbers that increase by      
    one, modulo 2^48, for every packet.  Even DCCP-Ack and DCCP-Sync       
    packets, and other packets that don't carry user data, increment the   
    Sequence Number.  Since DCCP is an unreliable protocol, there are no   
    true retransmissions; but effective retransmissions, such as           
    retransmissions of DCCP-Request packets, also increment the Sequence   
    Number.  This lets DCCP implementations detect network duplication,    
    retransmissions, and acknowledgement loss, and is a significant        
    departure from TCP practice.                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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7.1.  Variables                                                            
                                                                           
    DCCP endpoints maintain a set of sequence number variables for each    
    connection.                                                            
                                                                           
    ISS     The Initial Sequence Number Sent by this endpoint.  This       
            equals the Sequence Number of the first DCCP-Request or        
            DCCP-Response sent.                                            
                                                                           
    ISR     The Initial Sequence Number Received from the other            
            endpoint.  This equals the Sequence Number of the first        
            DCCP-Request or DCCP-Response received.                        
                                                                           
    GSS     The Greatest Sequence Number Sent by this endpoint.  Here,     
            and elsewhere, "greatest" is measured in circular sequence     
            space.                                                         
                                                                           
    GSR     The Greatest Sequence Number Received from the other           
            endpoint on an acknowledgeable packet.  (Section 7.4 defines   
            this term.)                                                    
            "acknowledgeable" packets.)                                 
                                                                           
    GAR     The Greatest Acknowledgement Number Received from the other    
            endpoint on an acknowledgeable packet that was not a DCCP-     
            Sync.                                                          
                                                                           
    Some other variables are derived from these primitives.                
                                                                           
    SWL and SWH                                                            
            (Sequence Number Window Low and High)  The extremes of the     
            validity window for received packets' Sequence Numbers.        
                                                                           
    AWL and AWH                                                            
            (Acknowledgement Number Window Low and High)  The extremes     
            of the validity window for received packets' Acknowledgement   
            Numbers.                                                       
                                                                           
7.2.  Initial Sequence Numbers                                             
                                                                           
    The endpoints' initial sequence numbers are set by the first DCCP-     
    Request and DCCP-Response packets sent.  Initial sequence numbers      
    MUST be chosen to avoid two problems:                                  
                                                                           
    o  Delivery of old packets, where packets lingering in the network     
       from an old connection are delivered to a new connection with the   
       same addresses and port numbers.                                    
                                                                           
    o  Sequence number attacks, where an attacker can guess the sequence   
       numbers that a future connection would use [M85].                   
                                                                           
                                                                           
                                                                           
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    These problems are the same as problems faced by TCP, and DCCP         
    implementations SHOULD use TCP's strategies to avoid them [RFC 793]    
    [RFC 1948].  The rest of this section explains these strategies in     
    more detail.                                                           
                                                                           
    To address the first problem, an implementation MUST ensure that the   
    initial sequence number for a given <source address, source port,      
    destination address, destination port> 4-tuple doesn't overlap with    
    recent sequence numbers on previous connections with the same          
    4-tuple.  ("Recent" means sent within 2 maximum segment lifetimes,     
    or 4 minutes.)  The implementation MUST additionally ensure that the   
    lower 24 bits of the initial sequence number don't overlap with the    
    lower 24 bits of recent sequence numbers (unless the implementation    
    plans to avoid short sequence numbers; see Section 7.6).  An           
    implementation that has state for a recent connection with the same    
    4-tuple can pick a good initial sequence number explicitly.            
    Otherwise, it could tie initial sequence number selection to some      
    clock, such as the 4-microsecond clock used by TCP [RFC 793].  Two     
    separate clocks may be required, one for the upper 24 bits and one     
    for the lower 24 bits.                                                 
                                                                           
    To address the second problem, an implementation MUST provide each     
    4-tuple with an independent initial sequence number space.  Then       
    opening a connection doesn't provide any information about initial     
    sequence numbers on other connections to the same host.  RFC 1948      
    achieves this by adding a cryptographic hash of the 4-tuple and a      
    secret to each initial sequence number.  For the secret, RFC 1948      
    recommends a combination of some truly-random data [RFC 1750], an      
    administratively-installed passphrase, the endpoint's IP address,      
    and the endpoint's boot time, but truly-random data is sufficient.     
    Care should be taken when changing the secret; such a change alters    
    all initial sequence number spaces, which might make an initial        
    sequence number for some 4-tuple equal a recently sent sequence        
    number for the same 4-tuple.  To avoid this problem, the endpoint      
    might remember dead connection state for each 4-tuple or stay quiet    
    for 2 maximum segment lifetimes around such a change.                  
                                                                           
7.3.  Quiet Time                                                           
                                                                           
    DCCP endpoints, like TCP endpoints, must take care before initiating   
    connections when they boot.  In particular, they MUST NOT send         
    packets whose sequence numbers are close to the sequence numbers of    
    packets lingering in the network from before the boot.  The simplest   
    way to enforce this rule is for DCCP endpoints to avoid sending any    
    packets until one maximum segment lifetime (2 minutes) after boot.     
    Other enforcement mechanisms include remembering recent sequence       
    numbers across boots, and reserving the upper 8 or so bits of          
    initial sequence numbers for a persistent counter that decrements by   
                                                                           
                                                                           
                                                                           
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    two each boot.  (The latter mechanism would require disallowing        
    packets with short sequence numbers; see Section 7.6.1.)               
                                                                           
7.4.  Acknowledgement Numbers                                              
                                                                           
    Cumulative acknowledgements are meaningless in an unreliable           
    protocol.  Therefore, DCCP's Acknowledgement Number field has a        
    different meaning than TCP's.                                          
                                                                           
    A received packet is classified as acknowledgeable if and only if      
    A packet is classified as "acknowledgeable" if and only if its      
    its header was succesfully processed by the receiving DCCP.  In        
    options were processed by the receiving DCCP.  This means, for      
    terms of the pseudocode in Section 8.5, a received packet becomes      
    example, that all acknowledgeable packets have valid header         
    acknowledgeable when the receiving endpoint reaches Step 8.  This      
    checksums and sequence numbers.  The Acknowledgement Number MUST    
    means, for example, that all acknowledgeable packets have valid        
    equal GSR, the Greatest Sequence Number Received on an              
    header checksums and sequence numbers.  The Acknowledgement Number     
    MUST equal GSR, the Greatest Sequence Number Received on an            
    acknowledgeable packet, for all packet types except DCCP-Sync and      
    DCCP-SyncAck.                                                          
                                                                           
    "Acknowledgeable" does not refer to data processing.  Even             
    acknowledgeable packets may have their application data dropped, due   
    to receive buffer overflow or corruption, for instance.  Data          
    Dropped options report these data losses when necessary, letting       
    congestion control mechanisms distinguish between network losses and   
    endpoint losses.  This issue is discussed further in Sections 11.4     
    and 11.7.                                                              
    and 11.8.                                                           
                                                                           
    DCCP-Sync and DCCP-SyncAck packets' Acknowledgement Numbers differ     
    as follows: The Acknowledgement Number on a DCCP-Sync packet           
    corresponds to a received packet, but not necessarily an               
    acknowledgeable packet; in particular, it might correspond to an       
    out-of-sync packet whose options were not processed.  The              
    Acknowledgement Number on a DCCP-SyncAck packet always corresponds     
    to an acknowledgeable DCCP-Sync packet; it might be less than GSR in   
    the presence of reordering.                                            
                                                                           
7.5.  Validity and Synchronization                                         
                                                                           
    Any DCCP endpoint might receive packets that are not actually part     
    of the current connection.  For instance, the network might deliver    
    an old packet, an attacker might attempt to hijack a connection, or    
    the other endpoint might crash, causing a half-open connection.        
                                                                           
    DCCP, like TCP, uses sequence number checks to detect these cases.     
    Packets whose Sequence and/or Acknowledgement Numbers are out of       
    range are called sequence-invalid, and are not processed normally.     
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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    Unlike TCP, DCCP requires a synchronization mechanism to recover       
    from large bursts of loss.  One endpoint might send so many packets    
    during a burst of loss that when one of its packets finally got        
    through, the other endpoint would label its Sequence Number as         
    invalid.  A handshake of DCCP-Sync and DCCP-SyncAck packets recovers   
    from this case.                                                        
                                                                           
7.5.1.  Sequence and Acknowledgement Number Windows                        
7.5.1.  Sequence-Validity Rules                                         
                                                                           
    Each DCCP endpoint defines sequence validity windows that are          
    subsets of the Sequence and Acknowledgement Number spaces.  These      
    windows correspond to packets the endpoint expects to receive in the   
    next few round-trip times.  The Sequence and Acknowledgement Number    
    windows always contain GSR and GSS, respectively.  The window widths   
    are controlled by Sequence Window features for the two half-           
    connections.                                                           
                                                                           
    The Sequence Number validity window for packets from DCCP B is [SWL,   
    SWH].  This window always contains GSR, the Greatest Sequence Number   
    Received on a sequence-valid packet from DCCP B.  It is W packets      
    wide, where W is the value of the Sequence Window/B feature.  One-     
    fourth of the sequence window, rounded down, is less than or equal     
    to GSR, and three-fourths is greater than GSR.  (This asymmetric       
    placement assumes that bursts of loss are more common in the network   
    than significant reordering.)                                          
                                                                           
      invalid  |       valid Sequence Numbers        |  invalid            
    <---------*|*===========*=======================*|*--------->          
          GSR -|GSR + 1 -   GSR                 GSR +|GSR + 1 +            
     floor(W/4)|floor(W/4)                 ceil(3W/4)|ceil(3W/4)           
                = SWL                           = SWH                      
                                                                           
    The Acknowledgement Number validity window for packets from DCCP B     
    is [AWL, AWH].  The high end of the window, AWH, equals GSS, the       
    Greatest Sequence Number Sent by DCCP A; the window is W' packets      
    wide, where W' is the value of the Sequence Window/A feature.          
                                                                           
      invalid  |    valid Acknowledgement Numbers    |  invalid            
    <---------*|*===================================*|*--------->          
       GSS - W'|GSS + 1 - W'                      GSS|GSS + 1              
                = AWL                           = AWH                      
                                                                           
    SWL and AWL are initially adjusted so that they are not less than      
    the initial Sequence Numbers received and sent, respectively:          
                 SWL := max(GSR + 1 - floor(W/4), ISR),                    
                 AWL := max(GSS - W' + 1, ISS).                            
    These adjustments MUST be applied only at the beginning of the         
    connection.  (Long-lived connections may wrap sequence numbers so      
                                                                           
                                                                           
                                                                           
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    that they appear to be less than ISR or ISS; the adjustments MUST      
    NOT be applied in that case.)                                          
                                                                           
7.5.2.  Sequence Window Feature                                            
                                                                           
    The Sequence Window/A feature determines the width of the Sequence     
    Number validity window used by DCCP B, and the width of the            
    Acknowledgement Number validity window used by DCCP A.  DCCP A sends   
    a "Change L(Sequence Window, W)" option to notify DCCP B that the      
    Sequence Window/A value is W.                                          
                                                                           
    Sequence Window has feature number 3, and is non-negotiable.  It       
    takes 48-bit (6-byte) integer values, like DCCP sequence numbers,      
    but 1- to 5-byte values are also allowed in options -- they are        
    padded on the left with zero bytes as necessary to total 48 bits.      
    Change and Confirm options for Sequence Window are therefore between   
    4 and 9 bytes long.  New connections start with Sequence Window 100    
    for both endpoints.  The maximum valid Sequence Window value is        
    Wmax = 2^46 - 1 = 70368744177663; circular sequence number             
    comparisons would stop working absent this constraint.  Change         
    options suggesting larger Sequence Window values are invalid and       
    MUST be handled accordingly.                                           
                                                                           
    A proper Sequence Window/A value should reflect how many packets       
    DCCP A expects to be in flight.  Only DCCP A can anticipate this       
    number.  Too-small values increase the risk of the endpoints getting   
    out sync after bursts of loss; too-large values increase the risk of   
    connection hijacking.  (The next section quantifies this risk.)  One   
    good guideline is for each endpoint to set Sequence Window to about    
    five times the maximum number of packets it expects to send in a       
    round-trip time.  This value may not be available at connection        
    initiation, when the round-trip time is unknown, but the endpoint      
    can always send updates as the connection progresses.                  
                                                                           
7.5.3.  Sequence-Validity Rules                                            
                                                                           
    Sequence-validity depends on the received packet's type.  This table   
    shows the sequence and acknowledgement number checks applied to each   
    packet; a packet is sequence-valid if it passes both tests, and        
    sequence-invalid if it does not.  Many of the checks refer to the      
    sequence and acknowledgement number validity windows [SWL, SWH] and    
    sequence and acknowledgement number windows [SWL, SWH] and [AWL,    
    [AWL, AWH], which are defined in Section 7.5.1.                        
    AWH], which are defined in Section 7.5.3.                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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                                              Acknowledgement Number       
    Packet Type      Sequence Number Check    Check                        
    -----------      ---------------------    ----------------------       
    DCCP-Request     SWL <= seqno <= SWH (*)  N/A                          
    DCCP-Response    SWL <= seqno <= SWH (*)  AWL <= ackno <= AWH          
    DCCP-Data        SWL <= seqno <= SWH      N/A                          
    DCCP-Ack         SWL <= seqno <= SWH      AWL <= ackno <= AWH          
    DCCP-DataAck     SWL <= seqno <= SWH      AWL <= ackno <= AWH          
    DCCP-CloseReq    GSR <  seqno <= SWH      GAR <= ackno <= AWH          
    DCCP-Close       GSR <  seqno <= SWH      GAR <= ackno <= AWH          
    DCCP-Reset       GSR <  seqno <= SWH      GAR <= ackno <= AWH          
    DCCP-Sync        SWL <= seqno             AWL <= ackno <= AWH          
    DCCP-Sync        seqno >= SWL             AWL <= ackno <= AWH       
    DCCP-SyncAck     SWL <= seqno             AWL <= ackno <= AWH          
    DCCP-SyncAck     seqno >= SWL             AWL <= ackno <= AWH       
                                                                           
    (*) Check not applied if connection is in LISTEN or REQUEST state.     
                                                                           
    In general, packets are sequence-valid if their Sequence and           
    Acknowledgement Numbers lie within the corresponding valid windows,    
    [SWL, SWH] and [AWL, AWH].  The exceptions to this rule are as         
    follows:                                                               
                                                                           
    o  Since DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets end a       
       connection, they cannot have Sequence Numbers less than or equal    
       to GSR, or Acknowledgement Numbers less than GAR.                   
                                                                           
    o  DCCP-Sync and DCCP-SyncAck Sequence Numbers are not strongly        
       checked.  These packet types exist specifically to get the          
       endpoints back into sync; checking their Sequence Numbers would     
       endpoints back into sync after bursts of loss; checking their    
       eliminate their usefulness.                                         
       Sequence Numbers would eliminate their usefulness.               
                                                                           
    The lenient checks on DCCP-Sync and DCCP-SyncAck packets allow      
    Although the lenient checks on DCCP-Sync and DCCP-SyncAck packets      
    continued operation after unusual events, such as endpoint crashes  
    allow continued operation after unusual events, such as endpoint       
    and large bursts of loss.  There's no need for leniency when the    
    crashes and large bursts of loss, there's no need for leniency when    
    endpoints are actively sending packets to one another.  Therefore,  
    the endpoints are actively sending packets to one another.             
    DCCP implementations SHOULD use the following, more stringent checks
    Therefore, DCCP implementations SHOULD use the following, more         
    for active connections.  A connection is considered active if it has
    stringent checks for active connections.  A connection is considered   
    received valid packets from the other endpoint within the last      
    active if it has received valid packets from the other endpoint        
    several round-trip times, or 0.5 seconds, if the RTT is not known.  
    within the last five round-trip times.                                 
                                                                           
                                              Acknowledgement Number       
    Packet Type      Sequence Number Check    Check                        
    -----------      ---------------------    ----------------------       
    DCCP-Sync        SWL <= seqno <= SWH      AWL <= ackno <= AWH          
    DCCP-SyncAck     SWL <= seqno <= SWH      AWL <= ackno <= AWH          
                                                                           
    Finally, an endpoint MAY apply the following more stringent checks     
    to DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets, further          
    lowering the probability of successful blind attacks using those       
                                                                           
                                                                           
                                                                           
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    packet types.  Since these checks can cause extra synchronization      
    overhead and delay connection closing when packets are lost, they      
    should be considered experimental.                                     
                                                                           
                                              Acknowledgement Number       
    Packet Type      Sequence Number Check    Check                        
    -----------      ---------------------    ----------------------       
    DCCP-CloseReq    seqno == GSR + 1         GAR <= ackno <= AWH          
    DCCP-Close       seqno == GSR + 1         GAR <= ackno <= AWH          
    DCCP-Reset       seqno == GSR + 1         GAR <= ackno <= AWH          
                                                                           
    Note that sequence-validity is only one of the validity checks         
    applied to received packets.                                           
                                                                           
7.5.4.  Handling Sequence-Invalid Packets                                  
7.5.2.  Handling Sequence-Invalid Packets                               
                                                                           
    Sequence-invalid DCCP-Sync and DCCP-SyncAck packets MUST be ignored.
    Endpoints MUST ignore sequence-invalid DCCP-Sync and DCCP-SyncAck      
    On receiving any other sequence-invalid packet, an endpoint (say,   
    packets, and MUST respond to other sequence-invalid packets with       
    DCCP A) MUST reply with a DCCP-Sync packet.  This packet MUST       
    (possibly rate-limited) DCCP-Sync packets.  Each DCCP-Sync packet      
    acknowledge the sequence-invalid packet's Sequence Number, not GSR. 
    MUST acknowledge the corresponding sequence-invalid packet's           
    The DCCP-Sync MUST use a new Sequence Number, and thus will increase
    Sequence Number, not GSR.  The DCCP-Sync MUST use a new Sequence       
    GSS; GSR will not change, however, since the received packet was    
    Number, and thus will increase GSS; GSR will not change, however,      
    sequence-invalid.  DCCP A MUST NOT otherwise process sequence-      
    since the received packet was sequence-invalid.                        
    invalid packets.  For instance, it MUST NOT process their options.  
                                                                           
    On receiving a sequence-valid DCCP-Sync, the peer endpoint (DCCP B) 
    On receiving a sequence-valid DCCP-Sync packet, the peer endpoint      
    MUST either respond with a DCCP-Reset packet, or update its GSR     
    (say, DCCP B) MUST update its GSR variable and reply with a DCCP-      
    variable and reply with a DCCP-SyncAck packet.  The DCCP-SyncAck    
    SyncAck packet.  The DCCP-SyncAck packet's Acknowledgement Number      
    packet's Acknowledgement Number will equal the DCCP-Sync's Sequence 
    will equal the DCCP-Sync's Sequence Number, not necessarily GSR.       
    Number, not necessarily GSR.  Upon receiving this DCCP-SyncAck,     
    Upon receiving this DCCP-SyncAck, which will be sequence-valid since   
    which will be sequence-valid since it acknowledges the DCCP-Sync,   
    it acknowledges the DCCP-Sync, DCCP A will update its GSR variable,    
    DCCP A will update its GSR variable, and the endpoints will be back 
    and the endpoints will be back in sync.  As an exception, if the       
    in sync.                                                            
    peer endpoint is in the REQUEST state, it MUST respond with a DCCP-    
    A DCCP endpoint MAY temporarily preserve sequence-invalid packets in
    Reset instead of a DCCP-SyncAck.  This serves to clean up DCCP A's     
    case they become valid later.  This can reduce the impact of bursts 
    half-open connection.                                                  
    of loss by delivering more packets to the application.  In          
                                                                           
    particular, an endpoint MAY preserve sequence-invalid packets for up
    To protect against denial-of-service attacks, DCCP implementations     
    to 2 round-trip times (or 0.2 seconds, if the RTT is unknown); if,  
    SHOULD impose a rate limit on DCCP-Syncs sent in response to           
    within that time, the relevant sequence windows change so that the  
    sequence-invalid packets, such as not more than eight DCCP-Syncs per   
    packets becomes sequence-valid, the endpoint MAY process the packets
    second.                                                                
    again.                                                              
                                                                           
    To protect itself against denial-of-service attacks (where an       
    DCCP endpoints MUST NOT process sequence-invalid packets except,       
    attacker sends many sequence-invalid packets, trying to force the   
    perhaps, by generating a DCCP-Sync.  For instance, options MUST NOT    
    receiver to send many DCCP-Syncs), a DCCP implementation SHOULD     
    but processed.  An endpoint MAY temporarily preserve sequence-         
    rate-limit the DCCP-Syncs sent in response to sequence-invalid      
    invalid packets in case they become valid later, however; this can     
    packets.                                                            
    reduce the impact of bursts of loss by delivering more packets to      
    the application.  In particular, an endpoint MAY preserve sequence-    
    invalid packets for up to 2 round-trip times.  If, within that time,   
    the relevant sequence windows change so that the packets become        
                                                                           
                                                                           
                                                                           
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    sequence-valid, the endpoint MAY process them again.                   
                                                                           
    Note that sequence-invalid DCCP-Reset packets cause DCCP-Syncs to be   
    generated.  This is because endpoints in an unsynchronized state       
    (CLOSED, REQUEST, and LISTEN) might not have enough information to     
    generate a proper DCCP-Reset on the first try.  For example, if a      
    peer endpoint is in CLOSED state and receives a DCCP-Data packet, it   
    cannot guess the right Sequence Number to use on the DCCP-Reset it     
    generates (since the DCCP-Data packet has no Acknowledgement           
    Number).  The DCCP-Sync generated in response to this bad reset        
    serves as a challenge, and contains enough information for the peer    
    to generate a proper DCCP-Reset.  However, the new DCCP-Reset may      
    carry a different Reset Code than the original DCCP-Reset; probably    
    the new Reset Code will be 3, "No Connection".  The endpoint SHOULD    
    use information from the original DCCP-Reset when possible.            
|| TEXT DELETED ||                                                         
7.5.3.  Sequence and Acknowledgement Number Windows                     
                                                                           
    Each DCCP endpoint defines sequence validity windows that are       
7.5.5.  Sequence Number Attacks                                            
    subsets of the Sequence and Acknowledgement Number spaces.  These   
                                                                           
    windows correspond to packets the endpoint expects to receive in the
    Sequence and Acknowledgement Numbers form DCCP's main line of          
    next few round-trip times.  The Sequence and Acknowledgement Number 
    defense against attackers.  An attacker that cannot guess sequence     
    windows always contain GSR and GSS, respectively.  The window widths
    numbers cannot easily manipulate or hijack a DCCP connection, and      
    are controlled by Sequence Window features for the two half-        
    requirements like careful initial sequence number choice eliminate     
    connections.                                                        
    the most serious attacks.                                              
    The Sequence Number validity window for packets from DCCP B is [SWL,
                                                                           
    SWH].  This window always contains GSR, the Greatest Sequence Number
    An attacker might still send many packets with randomly chosen         
    Received on a sequence-valid packet from DCCP B.  It is W packets   
    Sequence and Acknowledgement Numbers, however.  If one of those        
    wide, where W is the value of the Sequence Window/B feature.  One-  
    probes ends up sequence-valid, it may shut down the connection or      
    fourth of the sequence window, rounded down, is less than or equal  
    otherwise cause problems.  The easiest such attacks to execute are:    
    to GSR, and three-fourths is greater than GSR.  (This asymmetric    
                                                                           
    placement assumes that bursts of loss are more common in the network
    o  Send DCCP-Data packets with random Sequence Numbers.  If one of     
    than significant reordering.)                                       
       these packets hits the valid sequence number window, the attack     
      invalid  |       valid Sequence Numbers        |  invalid         
       packet's application data may be inserted into the data stream.     
    <---------*|*===========*=======================*|*--------->       
                                                                           
          GSR -|GSR + 1 -   GSR                 GSR +|GSR + 1 +         
    o  Send DCCP-Sync packets with random Sequence and Acknowledgement     
     floor(W/4)|floor(W/4)                 ceil(3W/4)|ceil(3W/4)        
       Numbers.  If one of these packets hits the valid acknowledgement    
                = SWL                           = SWH                   
       number window, the receiver will shift its sequence number window   
    The Acknowledgement Number validity window for packets from DCCP B  
       accordingly, getting out of sync with the correct endpoint --       
    is [AWL, AWH].  The high end of the window, AWH, equals GSS, the    
       perhaps permanently.                                                
    Greatest Sequence Number Sent by DCCP A; the window is W' packets   
                                                                           
    wide, where W' is the value of the Sequence Window/A feature.       
    The attacker has to guess both Source and Destination Ports for any    
      invalid  |    valid Acknowledgement Numbers    |  invalid         
    of these attacks to succeed.  Additionally, the connection would       
    <---------*|*===================================*|*--------->       
    have to be inactive for the DCCP-Sync attack to succeed, assuming      
       GSS - W'|GSS + 1 - W'                      GSS|GSS + 1           
    the victim implemented the more stringent checks for active            
                = AWL                           = AWH                   
    SWL and AWL are initially adjusted so that they are not less than   
    the initial Sequence Numbers received and sent, respectively:       
                 SWL := max(GSR + 1 - floor(W/4), ISR),                 
                 AWL := max(GSS - W' + 1, ISS).                         
    These adjustments MUST be applied only at the beginning of the      
    connection.  (Long-lived connections may wrap sequence numbers so   
    that they appear to be less than ISR or ISS; the adjustments MUST   
    NOT be applied in that case.)                                       
7.5.4.  Sequence Window Feature                                         
    The Sequence Window/A feature determines the width of the Sequence  
    Number validity window used by DCCP B, and the width of the         
    Acknowledgement Number validity window used by DCCP A.  DCCP A sends
    a "Change L(Sequence Window, W)" option to notify DCCP B that the   
    Sequence Window/A value is W.                                       
    Sequence Window has feature number 3, and is non-negotiable.  It    
    takes 3- or 6-byte integer values, like DCCP sequence numbers.      
    Change and Confirm options for Sequence Window are therefore either 
    6 or 9 bytes long.  New connections start with Sequence Window 100  
    for both endpoints.                                                 
    A proper Sequence Window/A value should reflect how many packets    
    DCCP A expects to be in flight.  Only DCCP A can anticipate this    
    number.  Too-small values increase the risk of the endpoints getting
    out sync after bursts of loss; too-large values increase the risk of
    connection hijacking.  (The next section quantifies this risk.)  One
    good guideline is for each endpoint to set Sequence Window to about 
    five times the maximum number of packets it expects to send in a    
    round-trip time.  This value may not be available at connection     
    initiation, when the round-trip time is unknown, but the endpoint   
    can always send updates as the connection progresses.               
    connections recommended in Section 7.5.3.                              
    connections recommended in Section 7.5.1.                           
                                                                           
    To quantify the probability of success, let N be the number of         
    attack packets the attacker is willing to send, W be the relevant      
    sequence window width, and L be the length of sequence numbers (24     
                                                                           
                                                                           
                                                                           
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    or 48).  The attacker's best strategy is to space the attack packets   
    evenly over sequence space.  Then the probability of hitting one       
    sequence number window is P = WN/2^L.                                  
                                                                           
    The success probability for a DCCP-Data attack using short sequence    
    For N = 1000, W = 100, and L = 24, P is about 0.006.  This is the   
    numbers thus equals P = WN/2^24.  For W = 100, then, the attacker      
    probability of a successful DCCP-Data attack using short sequence   
    must send more than 83,000 packets to achieve a 50% chance of          
    numbers.  (For reference, the easiest TCP attack -- sending a SYN   
    success.  For reference, the easiest TCP attack -- sending a SYN       
    with a random sequence number, which will cause a connection reset     
    if it falls within the window -- has W = 8760 (a common default) and   
    if it falls within the window -- will succeed with probability 0.002
    L = 32, and requires more than 245,000 packets to achieve a 50%        
    for N = 1000, W = 8760 [a common default], and L = 32.)  A          
    chance of success.                                                     
    connection can reduce this probability by requiring long sequence   
                                                                           
    numbers; see Section 7.6.1.                                         
    A fast connection's W will generally be high, increasing the attack    
    success probability for fixed N.  If this probability gets             
    uncomfortably high with L = 24, the endpoint SHOULD prevent the use    
    of short sequence numbers by manipulating the Allow Short Sequence     
    Numbers feature (see Section 7.6.1).  The probability limit depends    
    on the application, however.  Some applications, such as those         
    already designed to handle corruption, are quite resilient to data     
    injection attacks.                                                     
                                                                           
    The DCCP-Sync attack has L = 48, since DCCP-Sync packets use long      
    sequence numbers exclusively; in addition, the success probability     
    sequence numbers exclusively, and attacks correspondingly have a    
    is halved, since only half the Sequence Number space is valid.         
    smaller probability of success.  For N = 10,000, W = 2000, and L =  
    Attacks have a correspondingly smaller probability of success.  For    
    48, a DCCP-Sync attack will succeed with probability 7*10^-8.       
    a large W of 2000 packets, then, the attacker must send more than      
    Attacks involving DCCP-CloseReq, DCCP-Close, and DCCP-Reset packets 
    10^11 packets to achieve a 50% chance of success.                      
    are more difficult still, since 48-bit Sequence and Acknowledgement 
                                                                           
    Numbers must both be guessed.                                       
    Attacks involving DCCP-Ack, DCCP-DataAck, DCCP-CloseReq, DCCP-Close,   
    and DCCP-Reset packets are more difficult, since Sequence and          
    Acknowledgement Numbers must both be guessed.  The probability of      
    attack success for these packet types equals P = WXN/2^(2L), where W   
    is the Sequence Number window, X is the Acknowledgement Number         
    window, and N and L are as before.                                     
                                                                           
    Since DCCP-Data attacks with short sequence numbers are by far the     
    easiest for attackers to execute, DCCP has been engineered to          
    prevent such data injection attacks from escalating to reset attacks   
    or other, more serious attacks.  In particular, any options whose      
    processing might cause the connection to be reset are ignored when     
    they appear on DCCP-Data packets.                                      
                                                                           
7.5.6.  Examples                                                           
                                                                           
    In the following example, DCCP A and DCCP B recover from a large       
    burst of loss that runs DCCP A's sequence numbers out of DCCP B's      
    appropriate sequence number window.                                    
                                                                           
                                                                           
                                                                           
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    DCCP A                                           DCCP B                
    (GSS=1,GSR=10)                                   (GSS=10,GSR=1)        
                -->   DCCP-Data(seq 2)     XXX                             
                          ...                                              
                -->   DCCP-Data(seq 100)   XXX                             
                -->   DCCP-Data(seq 101)           -->  ???                
                                                     seqno out of range;   
                                                     send Sync             
       OK       <--   DCCP-Sync(seq 11, ack 101)   <--                     
                                                     (GSS=11,GSR=1)        
                -->   DCCP-SyncAck(seq 102, ack 11)   -->   OK             
    (GSS=102,GSR=11)                                 (GSS=11,GSR=102)      
                                                                           
    In the next example, a DCCP connection recovers from a simple blind    
    attack.                                                                
                                                                           
    DCCP A                                           DCCP B                
    (GSS=1,GSR=10)                                   (GSS=10,GSR=1)        
                 *ATTACKER*  -->  DCCP-Data(seq 10^6)  -->  ???            
                                                     seqno out of range;   
                                                     send Sync             
       ???      <--   DCCP-Sync(seq 11, ack 10^6)  <--                     
    ackno out of range; ignore                                             
    (GSS=1,GSR=10)                                   (GSS=11,GSR=1)        
                                                                           
    The final example demonstrates recovery from a half-open connection.   
                                                                           
    DCCP A                                           DCCP B                
    (GSS=1,GSR=10)                                   (GSS=10,GSR=1)        
    (Crash)                                                                
    CLOSED                                               OPEN              
    REQUEST     -->   DCCP-Request(seq 400)        -->   ???               
    !!          <--   DCCP-Sync(seq 11, ack 400)   <--   OPEN              
    REQUEST     -->   DCCP-Reset(seq 401, ack 11)  -->   (Abort)           
    REQUEST                                              CLOSED            
    REQUEST     -->   DCCP-Request(seq 402)        -->   ...               
                                                                           
                                                                           
7.6.  Short Sequence Numbers                                               
                                                                           
    DCCP sequence numbers are 48 bits long.  This large sequence space     
    protects DCCP connections against some blind attacks, such as the      
    injection of DCCP-Resets into the connection.  However, DCCP-Data,     
    DCCP-Ack, and DCCP-DataAck packets, which make up the body of any      
    DCCP connection, may reduce header space by transmitting only the      
    lower 24 bits of the relevant Sequence and Acknowledgement Numbers.    
    The receiving endpoint will extend these numbers to 48 bits using      
    the following pseudocode:                                              
                                                                           
                                                                           
                                                                           
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    procedure Extend_Sequence_Number(S, REF)                               
       /* S is a 24-bit sequence number from the packet header.            
          REF is the relevant 48-bit reference sequence number:            
          GSS if S is an Acknowledgement Number, and GSR if S is a         
          Sequence Number. */                                              
       Set REF_low := low 24 bits of REF                                   
       set REF_low := low 24 bits of REF                                
       Set REF_hi := high 24 bits of REF                                   
       set REF_hi := high 24 bits of REF                                
       If REF_low (<) S           /* circular comparison mod 2^24 */       
       if REF_low (<) S           /* CIRCULAR comparison mod 2^24 */    
             && S |<| REF_low,    /* conventional, non-circular            
             && S |<| REF_low:    /* NON-CIRCULAR comparison */         
                                     comparison */                         
          return ((REF_hi + 1) << 24) | S                               
          Return (((REF_hi + 1) mod 2^24) << 24) | S                       
       otherwise:                                                       
       Otherwise,                                                          
          return (REF_hi << 24) | S                                     
          Return (REF_hi << 24) | S                                        
                                                                           
    The two different kinds of comparison in the if statement detect       
    when the low-order bits of the sequence space have wrapped.  (The      
    when the low-order bits of the sequence space have wrapped.  When   
    circular comparison "REF_low (<) S" returns true if and only if        
    this happens, the high-order bits are incremented.                  
    (S - REF_low), calculated using two's-complement arithmetic and then   
    represented as an unsigned number, is less than or equal to 2^23       
    (mod 2^24).)  When this happens, the high-order bits are               
    incremented.                                                           
                                                                           
7.6.1.  Allow Short Sequence Numbers Feature                               
                                                                           
    Endpoints can require that all packets use long sequence numbers by    
    setting the Allow Short Sequence Numbers feature to false.  This can   
    reduce the risk that data will be inappropriately injected into the    
    connection.  DCCP A sends a "Change R(Allow Short Seqnos, 0)" option   
    to ask DCCP B to send only long sequence numbers.                      
                                                                           
    Allow Short Sequence Numbers has feature number 2, and is server-      
    priority.  It takes one-byte Boolean values.  DCCP B MUST NOT send     
    packets with short sequence numbers when Allow Short Seqnos/B is       
    zero.  Values of two or more are reserved.  New connections start      
    with Allow Short Sequence Numbers 1 for both endpoints.                
                                                                           
7.6.2.  When to Avoid Short Sequence Numbers                               
                                                                           
    Short sequence numbers reduce the rate DCCP connections can safely     
    Short sequence numbers increase the risks of certain kinds of       
    achieve, and increase the risks of certain kinds of attacks,           
    attacks, including blind data injection, and reduce the rate DCCP   
    including blind data injection.  Very-high-rate DCCP connections,      
    connections can safely achieve.  Very-high-rate DCCP connections,   
    and connections with large sequence windows (Section 7.5.2), SHOULD    
    and connections with large sequence windows (Section 7.5.4), SHOULD 
    NOT use short sequence numbers on their data packets.  The attack      
    NOT use short sequence numbers on their data packets.               
    risk issues have been discussed in Section 7.5.5; we discuss the       
    The rate limitation imposed by short sequence numbers is easy to    
    rate limitation issue here.                                            
    calculate.  The sequence-validity mechanism assumes that the network
                                                                           
    does not deliver extremely old data.  In particular, it assumes that
    The sequence-validity mechanism assumes that the network does not      
    the network must have dropped any packet by the time the connection 
    deliver extremely old data.  In particular, it assumes that the        
    wraps around and uses its sequence number again.  We can easily     
                                                                           
    calculate the maximum connection rate that can be safely achieved   
                                                                           
    given this constraint.  Let MSL equal the maximum segment lifetime, 
                                                                           
    P equal the average DCCP packet size in bits, and L equal the length
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    of sequence numbers (24 or 48 bits).  Then the maximum safe rate, in
                                                                          
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    bits per second, is R = P*(2^L)/2MSL.                               
                                                                           
                                                                           
    network must have dropped any packet by the time the connection        
    wraps around and uses its sequence number again.  This constraint      
    limits the maximum connection rate that can be safely achieved.  Let   
    MSL equal the maximum segment lifetime, P equal the average DCCP       
    packet size in bits, and L equal the length of sequence numbers (24    
    or 48 bits).  Then the maximum safe rate, in bits per second, is R =   
    P*(2^L)/2MSL.                                                          
                                                                           
    For the default MSL of 2 minutes, 1500-byte DCCP packets, and short    
    sequence numbers, the safe rate is therefore approximately 800 Mb/s.   
    Although 2 minutes is a very large MSL for any networks that could     
    Of course, 2 minutes is a very large MSL for any networks that could
    sustain that rate with such small packets, long sequence numbers       
    sustain that rate with such small packets.  Nevertheless, long      
    allow much higher rates under the same constraints: up to              
    sequence numbers allow much higher rates, up to 14 petabits a second
    14 petabits a second for 1500-byte packets and the default MSL.        
    for 1500-byte packets and the default MSL.                          
                                                                           
    The probability of data injection attack success P = WN/2^L,        
7.7.  NDP Count and Detecting Application Loss                             
    discussed in Section 7.5.5, may also be relevant when deciding      
                                                                           
    whether to use short sequence numbers.  A fast connection will      
    DCCP's sequence numbers increment by one on every packet, including    
    generally have a relatively high W (sequence window size),          
    non-data packets (packets that don't carry application data).  This    
    increasing the attack success probability for fixed N (number of    
    makes DCCP sequence numbers suitable for detecting any network loss,   
    attack packets); if the probability gets uncomfortably high with L =
    but not for detecting the loss of application data.  The NDP Count     
    24, the connection should avoid short sequence numbers entirely.    
    option reports the length of each burst of non-data packets.  This     
    lets the receiving DCCP reliably determine when a burst of loss        
    lets the receiving DCCP reliably determine when bursts of loss      
    included application data.                                             
                                                                           
    +--------+--------+-------- ... --------+                              
    |00100101| Length |      NDP Count      |                              
    +--------+--------+-------- ... --------+                              
     Type=37  Len=3-5       (1-3 bytes)                                    
                                                                           
    If a DCCP endpoint's Send NDP Count feature is one (see below), then   
    that endpoint MUST send an NDP Count option on every packet whose      
    immediate predecessor was a non-data packet.  Non-data packets         
    consist of DCCP packet types DCCP-Ack, DCCP-Close, DCCP-CloseReq,      
    DCCP-Reset, DCCP-Sync, and DCCP-SyncAck.  The other packet types,      
    namely DCCP-Request, DCCP-Response, DCCP-Data, and DCCP-DataAck, are   
    considered data packets, although not all DCCP-Request and DCCP-       
    Response packets will actually carry application data.                 
                                                                           
    The value stored in NDP Count equals the number of consecutive non-    
    data packets in the run immediately previous to the current packet.    
    Packets with no NDP Count option are considered to have NDP Count      
    zero.                                                                  
                                                                           
    The NDP Count option can carry one to three bytes of data.  The        
    smallest option format that can hold the NDP Count SHOULD be used.     
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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    With NDP Count, the receiver can reliably tell only whether a burst    
    of loss contained at least one data packet.  For example, the          
    receiver cannot always tell whether a burst of loss contained a non-   
    data packet.                                                           
                                                                           
7.7.1.  Usage Notes                                                        
                                                                           
    Say that K consecutive sequence numbers are missing in some burst of   
    loss, and the Send NDP Count feature is on.  Then some application     
    data was lost within those sequence numbers unless the packet          
    following the hole contains an NDP Count option whose value is         
    greater than or equal to K.                                            
                                                                           
    For example, say that an endpoint sent the following sequence of       
    non-data packets (Nx) and data packets (Dx).                           
                                                                           
    N0  N1  D2  N3  D4  D5  N6  D7  D8  D9  D10 N11 N12 D13                
                                                                           
    Those packets would have NDP Counts as follows.                        
                                                                           
    N0  N1  D2  N3  D4  D5  N6  D7  D8  D9  D10 N11 N12 D13                
    -   1   2   -   1   -   -   1   -   -   -   -   1   2                  
                                                                           
    NDP Count is not useful for applications that include their own        
    sequence numbers with their packet headers.                            
                                                                           
7.7.2.  Send NDP Count Feature                                             
                                                                           
    The Send NDP Count feature lets DCCP endpoints negotiate whether       
    they should send NDP Count options on their packets.  DCCP A sends a   
    "Change R(Send NDP Count, 1)" option to ask DCCP B to send NDP Count   
    options.                                                               
                                                                           
    Send NDP Count has feature number 7, and is server-priority.  It       
    takes one-byte Boolean values.  DCCP B MUST send NDP Count options     
    as described above when Send NDP Count/B is one, although it MAY       
    send NDP Count options even when Send NDP Count/B is zero.  Values     
    of two or more are reserved.  New connections start with Send NDP      
    Count 0 for both endpoints.                                            
                                                                           
8.  Event Processing                                                       
                                                                           
    This section describes how DCCP connections move between states, and   
    which packets are sent when.  Note that feature negotiation takes      
    place in parallel with the connection-wide state transitions           
    described here.                                                        
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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8.1.  Connection Establishment                                             
                                                                           
    DCCP connections' initiation phase consists of a three-way             
    handshake: an initial DCCP-Request packet sent by the client, a        
    DCCP-Response sent by the server in reply, and finally an              
    acknowledgement from the client, usually via a DCCP-Ack or DCCP-       
    DataAck packet.  The client moves from the REQUEST state to            
    PARTOPEN, and finally to OPEN; the server moves from LISTEN to         
    RESPOND, and finally to OPEN.                                          
                                                                           
      Client State                             Server State                
         CLOSED                                   LISTEN                   
    1.   REQUEST   -->       Request        -->                            
    2.             <--       Response       <--   RESPOND                  
    3.   PARTOPEN  -->     Ack, DataAck     -->                            
    4.             <--  Data, Ack, DataAck  <--   OPEN                     
    5.   OPEN      <->  Data, Ack, DataAck  <->   OPEN                     
                                                                           
                                                                           
8.1.1.  Client Request                                                     
                                                                           
    When a client decides to initiate a connection, it enters the          
    REQUEST state, chooses an initial sequence number (Section 7.2), and   
    sends a DCCP-Request packet using that sequence number to the          
    intended server.                                                       
                                                                           
    DCCP-Request packets will commonly carry feature negotiation options   
    that open negotiations for various connection parameters, such as      
    preferred congestion control IDs for each half-connection.  They may   
    also carry application data, but the client should be aware that the   
    server may not accept such data.                                       
                                                                           
    A client in the REQUEST state SHOULD send use an exponential-backoff   
    A client in the REQUEST state SHOULD send new DCCP-Request packets  
    timer to send new DCCP-Request packets if no response is received.     
    after some timeout if no response is received.  The retransmission  
    The first retransmission should occur after approximately one          
    strategy SHOULD be similar to that for retransmitting TCP SYNs; for 
    second, backing off to not less than one packet every 64 seconds; or   
    instance, a first timeout on the order of a second, with an         
    the endpoint can use whatever retransmission strategy is followed      
    exponential backoff timer.  Each new DCCP-Request MUST increment the
    for retransmitting TCP SYNs.  Each new DCCP-Request MUST increment     
    Sequence Number by one, and MUST contain the same Service Code and  
    the Sequence Number by one, and MUST contain the same Service Code     
    application data as the original DCCP-Request.                      
    and application data as the original DCCP-Request.                     
    A client MAY give up after some number of DCCP-Requests.  If so, it 
                                                                           
    SHOULD send a DCCP-Reset packet to the server with Reset Code 2,    
    A client MAY give up on its DCCP-Requests after some time              
    "Aborted", to clean up state in case one or more of the Requests    
    (3 minutes, for example).  When it does, it SHOULD send a DCCP-Reset   
    actually arrived.  A client in REQUEST state has never received an  
    packet to the server with Reset Code 2, "Aborted", to clean up state   
    initial sequence number from its peer, so the DCCP-Reset's          
    in case one or more of the Requests actually arrived.  A client in     
    Acknowledgement Number should be set to zero.                       
    REQUEST state has never received an initial sequence number from its   
    peer, so the DCCP-Reset's Acknowledgement Number MUST be set to        
    zero.                                                                  
                                                                           
                                                                           
                                                                           
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    The client leaves the REQUEST state for PARTOPEN when it receives a    
    DCCP-Response from the server.                                         
                                                                           
8.1.2.  Service Codes                                                      
                                                                           
    Each DCCP-Request contains a 32-bit Service Code, which identifies     
    the application-level service to which the client application is       
    the service to which the client application is trying to connect.   
    trying to connect.  Service Codes should correspond to application     
    Service Codes should correspond to application services and         
    services and protocols.  For example, there might be a Service Code    
    protocols.  For example, there might be a Service Code for HTTP     
    for SIP control connections and one for RTP audio connections.         
    connections, one for FTP control connections, and one for FTP data  
    Middleboxes, such as firewalls, can use the Service Code to identify   
    connections.  Middleboxes, such as firewalls, can use the Service   
    the application running on a nonstandard port (assuming the DCCP       
    Code to identify the application running on a nonstandard port      
    header has not been encrypted).                                        
    (assuming the DCCP header has not been encrypted).                  
                                                                           
    Endpoints MUST associate a Service Code with every DCCP socket, both   
    actively and passively opened.  The application will generally         
    supply this Service Code.  Each active socket MUST have exactly one    
    Service Code.  Passive sockets MAY, at the implementation's            
    Service Code, while passive sockets MAY have more than one; this    
    discretion, be associated with more than one Service Code; this        
    might let multiple applications listen on the same port,            
    might let multiple applications, or multiple versions of the same      
    differentiated by Service Code.  If the DCCP-Request's Service Code 
    application, listen on the same port, differentiated by Service        
    doesn't match any of the server's Service Codes for the given port, 
    Code.  If the DCCP-Request's Service Code doesn't match any of the     
    the server MUST reject the request by sending a DCCP-Reset packet   
    server's Service Codes for the given port, the server MUST reject      
    with Reset Code 8, "Bad Service Code".  A middlebox MAY also send   
    the request by sending a DCCP-Reset packet with Reset Code 8, "Bad     
    such a DCCP-Reset in response to packets whose Service Code is      
    Service Code".  A middlebox MAY also send such a DCCP-Reset in         
    considered unsuitable.                                              
    response to packets whose Service Code is considered unsuitable.       
    Service Codes are allocated by IANA.  Following the policies        
                                                                           
    outlined in [RFC 2434], most Service Codes are allocated First Come 
    Service Codes are not intended to be DCCP-specific, and are            
    First Served, subject to the following guidelines.                  
    allocated by IANA.  Following the policies outlined in [RFC 2434],     
    most Service Codes are allocated First Come First Served, subject to   
    the following guidelines.                                              
                                                                           
    o  Service Codes are allocated one at a time, or in small blocks.  A   
       short English description of the intended service is REQUIRED to    
       short English description of the intended service is required to 
       obtain a Service Code assignment, but no specification,             
       standards-track or otherwise, is necessary.  IANA maintains an      
       association of Service Codes to the corresponding phrases.          
                                                                           
    o  Users request specific Service Code values.  We suggest that        
       users request Service Codes that can be interpreted as meaningful   
       four-byte ASCII strings.  Thus, the "Frobodyne Plotz Protocol"      
       might correspond to "fdpz", or the number 1717858426.  The          
       canonical interpretation of a Service Code field is numeric.        
                                                                           
    o  Service Codes whose bytes each have values in the set {32, 45-57,   
       65-90} use a Specification Required allocation policy.  That is,    
       these Service Codes are used for international standard or          
       standards-track specifications, IETF or otherwise.  (This set       
                                                                           
                                                                           
                                                                           
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       consists of the ASCII digits, uppercase letters, and characters     
       space, '-', '.', and '/'.)                                          
                                                                           
    o  Service Codes whose high-order byte equals 63 (ASCII '?') are       
       reserved for Private Use.                                           
                                                                           
    o  Service Code 0 represents the absence of a meaningful Service       
       Code, and MUST never be allocated.                                  
       Code, and should never be allocated.                             
                                                                           
    This design for Service Code allocation is based on the allocation     
    of 4-byte identifiers for Macintosh resources, PNG chunks, and         
    TrueType and OpenType tables.                                          
                                                                           
8.1.3.  Server Response                                                    
                                                                           
    In the second phase of the three-way handshake, the server moves       
    from the LISTEN state to RESPOND, and sends a DCCP-Response message    
    to the client.  In this phase, a server will often specify the         
    features it would like to use, either from among those the client      
    requested, or in addition to those.  Among these options is the        
    congestion control mechanism the server expects to use.                
                                                                           
    The server MAY respond to a DCCP-Request packet with a DCCP-Reset      
    The receiver MAY respond to a DCCP-Request packet with a DCCP-Reset 
    packet to refuse the connection.  Relevant Reset Codes for refusing    
    a connection include 7, "Connection Refused", when the DCCP-           
    Request's Destination Port did not correspond to a DCCP port open      
    for listening; 8, "Bad Service Code", when the DCCP-Request's          
    Service Code did not correspond to the service code registered with    
    the Destination Port; and 9, "Too Busy", when the server is            
    currently too busy to respond to requests.  The server SHOULD limit    
    the rate at which it generates these resets, for example to not more   
    the rate at which it generates these resets.                        
    than 1024 per second.                                                  
    The receiver SHOULD NOT retransmit DCCP-Response packets; the sender
                                                                           
    The server SHOULD NOT retransmit DCCP-Response packets; the client     
    will retransmit the DCCP-Request if necessary.  (Note that the         
    "retransmitted" DCCP-Request will have, at least, a different          
    sequence number from the "original" DCCP-Request.  The server can      
    sequence number from the "original" DCCP-Request; the receiver can  
    thus distinguish true retransmissions from network duplicates.)  The   
    server will detect that the retransmitted DCCP-Request applies to an   
    responder will detect that the retransmitted DCCP-Request applies to
    existing connection because of its Source and Destination Ports.       
    an existing connection because of its Source and Destination Ports. 
    Every valid DCCP-Request received while the server is in the RESPOND   
    state MUST elicit a new DCCP-Response.  Each new DCCP-Response MUST    
    increment the server's Sequence Number by one, and MUST include the    
    increment the responder's Sequence Number by one, and MUST include  
    same application data, if any, as the original DCCP-Response.          
    the same application data, if any, as the original DCCP-Response.   
                                                                           
    The responder MUST accept at most one piece of DCCP-Request data per
    The server MUST NOT accept more than one piece of DCCP-Request         
    connection.  In particular, the DCCP-Response sent in reply to a    
    application data per connection.  In particular, the DCCP-Response     
    retransmitted DCCP-Request with data SHOULD contain a Data Dropped  
    sent in reply to a retransmitted DCCP-Request with application data    
    option, in which the retransmitted DCCP-Request is reported as "data
                                                                           
    dropped due to protocol constraints" (Drop Code 0).  The original   
                                                                           
    DCCP-Request SHOULD also be reported in the Data Dropped option,    
                                                                           
    either in a Normal Block (if the responder accepted the data, or    
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    there was no data), or in a Drop Code 0 Drop Block (if the responder
                                                                          
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    refused the data the first time as well).                           
                                                                           
                                                                           
    SHOULD contain a Data Dropped option, in which the retransmitted       
    DCCP-Request data is reported with Drop Code 0, Protocol               
    Constraints.  The original DCCP-Request SHOULD also be reported in     
    the Data Dropped option, either in a Normal Block (if the server       
    accepted the data, or there was no data), or in a Drop Code 0 Drop     
    Block (if the server refused the data the first time as well).         
                                                                           
    The Data Dropped and Init Cookie options are particularly useful for   
    DCCP-Response packets (Sections 11.7 and 8.1.4).                       
    DCCP-Response packets (Sections 11.8 and 8.1.4).                    
                                                                           
    The server leaves the RESPOND state for OPEN when it receives a        
    valid DCCP-Ack from the client, completing the three-way handshake.    
    It MAY also leave the RESPOND state for CLOSED after a timeout of      
    not less than 4MSL (8 minutes); when doing so, it SHOULD send a        
    DCCP-Reset with Reset Code 2, "Aborted", to clean up state at the      
    client.                                                                
                                                                           
8.1.4.  Init Cookie Option                                                 
                                                                           
    +--------+--------+--------+--------+--------+--------                 
    |00100100| Length |         Init Cookie Value   ...                    
    +--------+--------+--------+--------+--------+--------                 
     Type=36                                                               
                                                                           
                                                                           
    The Init Cookie option lets a DCCP server avoid having to hold any     
    state until the three-way connection setup handshake has completed,    
    state until the three-way connection setup handshake has completed. 
    in a similar fashion as TCP SYN cookies [SYNCOOKIES].  The server      
    The server wraps up the service code, server port, and any options  
    wraps up the Service Code, server port, and any options it cares       
    it cares about from both the DCCP-Request and DCCP-Response in an   
    about from both the DCCP-Request and DCCP-Response in an opaque        
    opaque cookie.  Typically the cookie will be encrypted using a      
    cookie.  Typically the cookie will be encrypted using a secret known   
    secret known only to the server and include a cryptographic checksum
    only to the server and include a cryptographic checksum or magic       
    or magic value so that correct decryption can be verified.  When the
    value so that correct decryption can be verified.  When the server     
    server receives the cookie back in the response, it can decrypt the 
    receives the cookie back in the response, it can decrypt the cookie    
    cookie and instantiate all the state it avoided keeping.  In the    
    and instantiate all the state it avoided keeping.  In the meantime,    
    meantime, it need not move from the LISTEN state.                   
    it need not move from the LISTEN state.                                
    This option is permitted in DCCP-Response, DCCP-Data, DCCP-Ack,     
                                                                           
    DCCP-DataAck, DCCP-Sync, and DCCP-SyncAck packets.  The server MAY  
    The Init Cookie option MUST NOT be sent on DCCP-Request or DCCP-Data   
    include an Init Cookie option in its DCCP-Response.  If so, then the
    packets, and any such options received on DCCP-Request or DCCP-Data    
    client MUST echo the same Init Cookie option in each succeeding DCCP
    packets MUST be ignored.  The server MAY include an Init Cookie        
    packet until one of those packets is acknowledged, meaning the      
    option in its DCCP-Response.  If so, then the client MUST echo the     
    three-way handshake has completed, or the connection is reset.  The 
    same Init Cookie option in each succeeding DCCP packet until one of    
    server SHOULD design its Init Cookie format so that Init Cookies can
    those packets is acknowledged, meaning the three-way handshake has     
    be checked for tampering; it SHOULD respond to a tampered Init      
    completed, or the connection is reset.  (As a result, the client       
    Cookie option by resetting the connection with Reset Code 10, "Bad  
    MUST NOT use DCCP-Data packets until the three-way handshake           
    Init Cookie".                                                       
    completes or the connection is reset.)  The server SHOULD design its   
    Init Cookie format so that Init Cookies can be checked for             
    tampering; it SHOULD respond to a tampered Init Cookie option by       
                                                                           
                                                                           
                                                                           
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    resetting the connection with Reset Code 10, "Bad Init Cookie".        
                                                                           
    The precise implementation of the Init Cookie does not need to be      
    specified here; since Init Cookies are opaque to the client, there     
    are no interoperability concerns.                                      
                                                                           
    Init Cookies are limited to at most 253 bytes in length.               
                                                                           
8.1.5.  Handshake Completion                                               
                                                                           
    When the client receives a DCCP-Response from the server, it moves     
    from the REQUEST state to PARTOPEN and completes the three-way         
    from the REQUEST state to PARTOPEN, and completes three-way         
    handshake by sending a DCCP-Ack packet to the server.  The client      
    remains in PARTOPEN until it can be sure that the server has           
    remains in the PARTOPEN state until it can be sure that the server  
    received some packet the client sent from PARTOPEN (either the         
    has received this DCCP-Ack, or another packet sent later.  Clients  
    initial DCCP-Ack or a later packet).  Clients in the PARTOPEN state    
    in the PARTOPEN state that want to send data MUST do so using DCCP- 
    that want to send data MUST do so using DCCP-DataAck packets, not      
    DataAck packets, not DCCP-Data packets.  This is because DCCP-Data  
    DCCP-Data packets.  This is because DCCP-Data packets lack             
    packets lack Acknowledgement Numbers, so the server can't tell from 
    Acknowledgement Numbers, so the server can't tell from a DCCP-Data     
    a DCCP-Data packet whether the client saw its DCCP-Response.        
    packet whether the client saw its DCCP-Response.  Furthermore, if      
    Furthermore, if the DCCP-Response included an Init Cookie, that Init
    the DCCP-Response included an Init Cookie, that Init Cookie MUST be    
    Cookie MUST be included on every packet sent in PARTOPEN.           
    included on every packet sent in PARTOPEN.                             
                                                                           
    The single DCCP-Ack sent when entering the PARTOPEN state might, of    
    course, be dropped by the network.  The client SHOULD ensure that      
    some packet gets through eventually.  The preferred mechanism would    
    be a roughly 200-millisecond timer, set every time a packet is         
    be a delayed-ack-like 200-millisecond timer, set every time a packet
    transmitted in PARTOPEN.  If this timer goes off and the client is     
    is transmitted in PARTOPEN.  If this timer goes off and the client  
    still in PARTOPEN, the client generates another DCCP-Ack and backs     
    is still in PARTOPEN, the client generates another DCCP-Ack and     
    off the timer.  If the client remains in PARTOPEN for more than 4MSL   
    backs off the timer.  If the client remains in PARTOPEN for more    
    (8 minutes), it SHOULD reset the connection with Reset Code 2,         
    than 4MSL (8 minutes), it SHOULD reset the connection with Reset    
    "Aborted".                                                             
    Code 2, "Aborted".                                                  
                                                                           
    The client leaves the PARTOPEN state for OPEN when it receives a       
    valid packet other than DCCP-Response, DCCP-Reset, or DCCP-Sync from   
    packet other than DCCP-Response or DCCP-Reset from the server.      
    the server.                                                            
                                                                           
8.2.  Data Transfer                                                        
                                                                           
    In the central data transfer phase of the connection, both server      
    and client are in the OPEN state.                                      
                                                                           
    DCCP A sends DCCP-Data and DCCP-DataAck packets to DCCP B due to       
    application events on host A.  These packets are congestion-           
    controlled by the CCID for the A-to-B half-connection.  In contrast,   
    DCCP-Ack packets sent by DCCP A are controlled by the CCID for the     
    B-to-A half-connection.  Generally, DCCP A will piggyback              
    acknowledgement information on DCCP-Data packets when acceptable,      
                                                                           
                                                                           
                                                                           
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    creating DCCP-DataAck packets.  DCCP-Ack packets are used when there   
    is no data to send from DCCP A to DCCP B, or when the congestion       
    state of the A-to-B CCID will not allow data to be sent.               
                                                                           
    DCCP-Sync and DCCP-SyncAck packets may also occur in the data          
    transfer phase.  Some cases causing DCCP-Sync generation are           
    discussed in Section 7.5.  One important distinction between DCCP-     
    Sync packets and other packet types is that DCCP-Sync elicits an       
    immediate acknowledgement.  On receiving a valid DCCP-Sync packet, a   
    DCCP endpoint MUST immediately generate and send a DCCP-SyncAck        
    response (subject to any implementation rate limits); the              
    response; and the Acknowledgement Number on that DCCP-SyncAck MUST  
    Acknowledgement Number on that DCCP-SyncAck MUST equal the Sequence    
    equal the Sequence Number of the DCCP-Sync.                         
    Number of the DCCP-Sync.                                               
                                                                           
    A particular DCCP implementation might decide to initiate feature      
    negotiation only once the OPEN state was reached, in which case it     
    might not allow data transfer until some time later.  Data received    
    during that time SHOULD be rejected and reported using a Data          
    Dropped Drop Block with Drop Code 0, Protocol Constraints (see         
    Dropped Drop Block with Drop Code 0.                                
    Section 11.7).                                                         
                                                                           
8.3.  Termination                                                          
                                                                           
    DCCP connection termination uses a handshake consisting of an          
    optional DCCP-CloseReq packet, a DCCP-Close packet, and a DCCP-Reset   
    packet.  The server moves from the OPEN state, possibly through the    
    CLOSEREQ state, to CLOSED; the client moves from OPEN through          
    CLOSING to TIMEWAIT, and after 2MSL wait time (4 minutes), to          
    CLOSED.                                                                
                                                                           
    The sequence DCCP-CloseReq, DCCP-Close, DCCP-Reset is used when the    
    server decides to close the connection, but doesn't want to hold       
    TIMEWAIT state:                                                        
                                                                           
      Client State                             Server State                
         OPEN                                     OPEN                     
    1.             <--       CloseReq       <--   CLOSEREQ                 
    2.   CLOSING   -->        Close         -->                            
    3.             <--        Reset         <--   CLOSED (LISTEN)          
    4.   TIMEWAIT                                                          
    5.   CLOSED                                                            
                                                                           
    A shorter sequence occurs when the client decides to close the         
    connection.                                                            
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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      Client State                             Server State                
         OPEN                                     OPEN                     
    1.   CLOSING   -->        Close         -->                            
    2.             <--        Reset         <--   CLOSED (LISTEN)          
    3.   TIMEWAIT                                                          
    4.   CLOSED                                                            
                                                                           
    Finally, the server can decide to hold TIMEWAIT state:                 
                                                                           
      Client State                             Server State                
         OPEN                                     OPEN                     
    1.             <--        Close         <--   CLOSING                  
    2.   CLOSED    -->        Reset         -->                            
    3.                                            TIMEWAIT                 
    4.                                            CLOSED (LISTEN)          
                                                                           
                                                                           
    In all cases, the receiver of the DCCP-Reset packet holds TIMEWAIT     
    state for the connection.  As in TCP, TIMEWAIT state, where an         
    endpoint quietly preserves a socket for 2MSL (4 minutes) after its     
    connection has closed, ensures that no connection duplicating the      
    current connection's source and destination addresses and ports can    
    start up while old packets might remain in the network.                
                                                                           
    The termination handshake proceeds as follows.  The receiver of a      
    valid DCCP-CloseReq packet MUST respond with a DCCP-Close packet.      
    valid DCCP-CloseReq packet MUST respond with a DCCP-Close packet;   
    The receiver of a valid DCCP-Close packet MUST respond with a DCCP-    
    that receiving endpoint will expect to hold TIMEWAIT state after    
    Reset packet, with Reset Code 1, "Closed".  The receiver of a valid    
    later receiving a DCCP-Reset.  The receiver of a valid DCCP-Close   
    DCCP-Reset packet -- which is also the sender of the DCCP-Close        
    packet MUST respond with a DCCP-Reset packet, with Reset Code 1,    
    packet and the receiver of any DCCP-CloseReq packet -- will hold       
    "Closed"; the endpoint that originally sent the DCCP-Close will hold
    TIMEWAIT state for the connection.                                     
    TIMEWAIT state.  The endpoint that receives a valid DCCP-Reset      
                                                                           
    packet will hold TIMEWAIT state for the connection.                 
    A DCCP-Reset packet completes every DCCP connection, whether the       
    termination is clean (due to application close; Reset Code 1,          
    "Closed") or unclean.  Unlike TCP, which has two distinct              
    termination mechanisms (FIN and RST), DCCP ends all connections in a   
    uniform manner.  This is justified because some aspects of             
    uniform manner.  This is justified because some responses to        
    connection termination are the same independent of whether             
    connection termination are the same no matter whether termination   
    termination was clean.  For instance, the endpoint that receives a     
    was clean.  For instance, the endpoint that receives a valid DCCP-  
    valid DCCP-Reset SHOULD hold TIMEWAIT state for the connection.        
    Reset SHOULD hold TIMEWAIT state for the connection.  Processors    
    Processors that must distinguish between clean and unclean             
    that must distinguish between clean and unclean termination can     
    termination can examine the Reset Code.  DCCP-Reset packets MUST NOT   
    examine the Reset Code.  DCCP-Reset packets MUST NOT be generated in
    be generated in response to received DCCP-Reset packets.  DCCP         
    response to received DCCP-Reset packets.  DCCP implementations      
    implementations generally transition to the CLOSED state after         
    generally transition to the CLOSED state after sending a DCCP-Reset 
    sending a DCCP-Reset packet.                                           
    packet.                                                             
                                                                           
    Endpoints in the CLOSEREQ and CLOSING states MUST retransmit DCCP-     
    CloseReq and DCCP-Close packets, respectively, until leaving those     
                                                                           
                                                                           
                                                                           
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    states.  The retransmission timer should initially be set to go off    
    in two round-trip times, and should back off to not less than once     
    in two RTTs, or 0.2 seconds if the RTT is not known, and should back
    every 64 seconds if no relevant response is received.                  
    off to not less than once every 64 seconds if no relevant response  
                                                                           
    is received.                                                        
    Only the server can send a DCCP-CloseReq packet or enter the           
    CLOSEREQ state.  A server receiving a sequence-valid DCCP-CloseReq     
    CLOSEREQ state.                                                     
    packet MUST respond with a DCCP-Sync packet, and otherwise ignore      
    the DCCP-CloseReq.                                                     
                                                                           
    DCCP-Data, DCCP-DataAck, and DCCP-Ack packets received in CLOSEREQ     
    or CLOSE states MAY be either processed or ignored.                    
                                                                           
8.3.1.  Abnormal Termination                                               
                                                                           
    DCCP endpoints generate DCCP-Reset packets to terminate connections    
    abnormally; a DCCP-Reset packet may be generated from any state.       
    Resets sent in the CLOSED, LISTEN, and TIMEWAIT states use Reset       
    Code 3, "No Connection", unless otherwise specified.  Resets sent in   
    the REQUEST or RESPOND states use Reset Code 4, "Packet Error",        
    unless otherwise specified.                                            
                                                                           
    DCCP endpoints in CLOSED or LISTEN state may need to generate a        
    DCCP-Reset packet in response to a packet received from a peer.        
    Since these states have no associated sequence number variables, the   
    Sequence and Acknowledgement Numbers on the DCCP-Reset packet R are    
    taken from the received packet P, as follows.                          
                                                                           
    1.  If P.ackno exists, then set R.seqno := P.ackno + 1.  Otherwise,    
        set R.seqno := 0.                                                  
                                                                           
    2.  Set R.ackno := P.seqno.                                            
                                                                           
    3.  If the packet used short sequence numbers (P.X == 0), then set     
        the upper 24 bits of R.seqno and R.ackno to 0.                     
                                                                           
8.4.  DCCP State Diagram                                                   
                                                                           
    The most common state transitions discussed above can be summarized    
    in the following state diagram.  The diagram is illustrative; the      
    text in Section 8.5 and elsewhere should be considered definitive.     
    For example, there are arcs (not shown) from every state except        
    CLOSED to TIMEWAIT, contingent on the receipt of a valid DCCP-Reset.   
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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    +---------------------------+    +---------------------------+         
    |                           v    v                           |         
    |                        +----------+                        |         
    |          +-------------+  CLOSED  +------------+           |         
    |          | passive     +----------+  active    |           |         
    |          |  open                      open     |           |         
    |          |                         snd Request |           |         
    |          v                                     v           |         
    |     +----------+                          +----------+     |         
    |     |  LISTEN  |                          | REQUEST  |     |         
    |     +----+-----+                          +----+-----+     |         
    |          | rcv Request            rcv Response |           |         
    |          | snd Response             snd Ack    |           |         
    |          v                                     v           |         
    |     +----------+                          +----------+     |         
    |     | RESPOND  |                          | PARTOPEN |     |         
    |     +----+-----+                          +----+-----+     |         
    |          | rcv Ack/DataAck         rcv packet  |           |         
    |          |                                     |           |         
    |          |             +----------+            |           |         
    |          +------------>|   OPEN   |<-----------+           |         
    |                        +--+-+--+--+                        |         
    |       server active close | |  |   active close            |         
    |           snd CloseReq    | |  | or rcv CloseReq           |         
    |                           | |  |    snd Close              |         
    |                           | |  |                           |         
    |     +----------+          | |  |          +----------+     |         
    |     | CLOSEREQ |<---------+ |  +--------->| CLOSING  |     |         
    |     +----+-----+            |             +----+-----+     |         
    |          | rcv Close        |        rcv Reset |           |         
    |          | snd Reset        |                  |           |         
    |<---------+                  |                  v           |         
    |                             |             +----+-----+     |         
    |                   rcv Close |             | TIMEWAIT |     |         
    |                   snd Reset |             +----+-----+     |         
    +-----------------------------+                  |           |         
                                                     +-----------+         
                                                  2MSL timer expires       
                                                                           
                                                                           
8.5.  Pseudocode                                                           
                                                                           
    This section presents an algorithm describing the processing steps a   
    DCCP endpoint must go through when it receives a packet.  A DCCP       
    implementation need not implement the algorithm as it is described     
    here, but any implementation MUST generate observable effects          
    exactly as indicated by this pseudocode, except where allowed          
    (meaning packets) exactly as indicated by this pseudocode, except   
    otherwise by another part of this document.                            
    where allowed otherwise by another part of this document.           
                                                                           
                                                                           
                                                                           
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    The received packet is written as P, the socket as S.                  
    Packet variables P.seqno and P.ackno are 48-bit sequence numbers.      
    Socket variables:                                                      
    S.SWL - sequence number window low                                     
    S.SWH - sequence number window high                                    
    S.AWL - acknowledgement number window low                              
    S.AWH - acknowledgement number window high                             
    S.ISS - initial sequence number sent                                   
    S.ISR - initial sequence number received                               
    S.OSR - first OPEN sequence number received                            
    S.GSS - greatest sequence number sent                                  
    S.GSR - greatest valid sequence number received                        
    S.GAR - greatest valid acknowledgement number received on a            
            non-Sync; initialized to S.ISS                                 
    "Send packet" actions always use, and increment, S.GSS.                
                                                                           
    Step 1: Check header basics                                            
    First, check the header basics;                                     
       /* This step checks for malformed packets.  Packets that fail       
       If the header checksum is incorrect, drop packet and return      
          these checks are ignored -- they do not receive Resets in        
          response */                                                      
       If the packet is shorter than 12 bytes, drop packet and return      
       If the packet type is not understood, drop packet and return        
       If P.Data Offset is too small for packet type, or too large for     
             packet, drop packet and return                                
|| TEXT DELETED ||                                                         
       If P.CsCov is too large for the packet size, drop packet and     
       If P.type is not Data, Ack, or DataAck and P.X == 0 (the packet     
             return                                                     
             has short sequence numbers), drop packet and return           
       If the header checksum is incorrect, drop packet and return         
    Second, check ports and process TIMEWAIT state;                     
       If P.CsCov is too large for the packet size, drop packet and        
       Look up flow ID; get socket.                                     
             return                                                        
                                                                           
    Step 2: Check ports and process TIMEWAIT state                         
       Look up flow ID in table and get corresponding socket               
       If no socket, or S.state == TIMEWAIT,                               
          Generate Reset(No Connection) unless P.type == Reset             
          Drop packet and return                                           
                                                                           
    Step 3: Process LISTEN state                                           
    Third, process LISTEN state;                                        
       If S.state == LISTEN,                                               
          If P.type == Request or P contains a valid Init Cookie option,   
          If P.type == Request or P contains a valid Init Cookie,       
             /* Must scan the packet's options to check for an Init        
                Cookie.  Only the Init Cookie is processed here,           
                however; other options are processed in Step 8.  This      
                scan need only be performed if the endpoint uses Init      
                Cookies */                                                 
             /* Generate a new socket and switch to that socket */         
             Set S := new socket for this port pair                        
             S.state = RESPOND                                             
             Choose S.ISS (initial seqno) or set from Init Cookie          
                                                                           
                                                                           
                                                                           
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             Set S.ISR, S.GSR, S.SWL, S.SWH from packet or Init Cookie     
             Continue with S.state == RESPOND                              
             Continue (with S.state == RESPOND)                         
             /* A Response packet will be generated in Step 11 */          
          Otherwise,                                                       
             Generate Reset(No Connection) unless P.type == Reset          
             Drop packet and return                                        
                                                                           
    Step 4: Prepare sequence numbers in REQUEST                            
    Fourth, process Reset;                                              
       If S.state == REQUEST,                                              
       If P.type == Reset,                                              
          If S.GSR < P.seqno <= S.SWH                                   
                and S.GAR <= P.ackno <= S.AWH,                          
             Tear down connection                                       
             S.state := TIMEWAIT                                        
             Set TIMEWAIT timer                                         
             Drop packet and return                                     
          Otherwise,                                                    
             Send Sync packet acknowledging P.seqno                     
             Drop packet and return                                     
    Fifth, process REQUEST state;                                       
          If (P.type == Response or P.type == Reset)                       
          If P.type == Response and S.AWL <= P.ackno <= S.AWH,          
                and S.AWL <= P.ackno <= S.AWH,                             
             /* Set sequence number variables corresponding to the         
                other endpoint, so P will pass the tests in Step 6 */      
             Set S.GSR, S.ISR, S.SWL, S.SWH                                
             /* Response processing continues in Step 10; Reset            
                processing continues in Step 9 */                          
          Otherwise,                                                       
             /* Only Response and Reset are valid in REQUEST state */      
             Generate Reset(Packet Error)                                  
             Drop packet and return                                        
                                                                           
    Step 5: Prepare sequence numbers for Sync                              
    Sixth, process Sync sequence numbers;                               
       If P.type == Sync or P.type == SyncAck,                             
          If S.AWL <= P.ackno <= S.AWH and P.seqno >= S.SWL,               
             /* P is valid, so update sequence number variables            
                accordingly.  After this update, P will pass the tests     
                in Step 6.  A SyncAck is generated if necessary in         
                Step 15 */                                                 
             Update S.GSR, S.SWL, S.SWH                                    
          Otherwise,                                                       
             Drop packet and return                                        
                                                                           
    Step 6: Check sequence numbers                                         
    Seventh, check sequence numbers;                                    
       Let LSWL = S.SWL and LAWL = S.AWL                                   
       If P.type == CloseReq or P.type == Close or P.type == Reset,        
       If P.type == CloseReq or P.type == Close,                        
          LSWL := S.GSR + 1, LAWL := S.GAR                                 
       If LSWL <= P.seqno <= S.SWH                                         
             and (P.ackno does not exist or LAWL <= P.ackno <= S.AWH),     
          Update S.GSR, S.SWL, S.SWH                                       
          If P.type != Sync,                                               
             Update S.GAR                                                  
       Otherwise,                                                          
          Send Sync packet acknowledging P.seqno                           
          Drop packet and return                                           
                                                                           
    Step 7: Check for unexpected packet types                              
    Eighth, check packet type;                                          
       If (S.is_server and P.type == CloseReq)                             
            or (S.is_server and P.type == Response)                        
                                                                           
                                                                           
                                                                           
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            or (S.is_client and P.type == Request)                         
            or (S.state >= OPEN and P.type == Request                      
                and P.seqno >= S.OSR)                                      
            or (S.state >= OPEN and P.type == Response                     
                and P.seqno >= S.OSR)                                      
            or (S.state == RESPOND and P.type == Data),                    
          Send Sync packet acknowledging P.seqno                           
          Drop packet and return                                           
                                                                           
    Step 8: Process options and mark acknowledgeable                       
    Ninth, process options;                                             
       /* Option processing is not specifically described here.            
       /* May involve resetting connection, etc. */                     
          Certain options, such as Mandatory, may cause the connection     
       Mark packet as "received" for acknowledgement purposes           
          to be reset, in which case Steps 9 and on are not executed */    
    Tenth, process RESPOND state;                                       
       Mark packet as acknowledgeable (in Ack Vector terms, Received       
            or Received ECN Marked)                                        
                                                                           
    Step 9: Process Reset                                                  
       If P.type == Reset,                                                 
          Tear down connection                                             
          S.state := TIMEWAIT                                              
          Set TIMEWAIT timer                                               
          Drop packet and return                                           
                                                                           
    Step 10: Process REQUEST state (second part)                           
       If S.state == REQUEST,                                              
          /* If we get here, P is a valid Response from the server (see    
             Step 4), and we should move to PARTOPEN state.  PARTOPEN      
             means send an Ack, don't send Data packets, retransmit        
             Acks periodically, and always include any Init Cookie from    
             the Response */                                               
          S.state := PARTOPEN                                              
          Set PARTOPEN timer                                               
          Continue with S.state == PARTOPEN                                
          /* Step 12 will send the Ack completing the three-way            
             handshake */                                                  
                                                                           
    Step 11: Process RESPOND state                                         
       If S.state == RESPOND,                                              
          If P.type == Request,                                            
             Send Response, possibly containing Init Cookie                
             If Init Cookie was sent,                                      
                Destroy S and return                                       
                /* Step 3 will create another socket when the client       
                /* Step Three will create another socket when the client
                   completes the three-way handshake */                    
                   responds. */                                         
          Otherwise,                                                       
             S.OSR := P.seqno                                              
             S.state := OPEN                                               
                                                                           
                                                                           
                                                                           
                                                                           
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    Step 12: Process PARTOPEN state                                        
    Eleventh, process REQUEST state;                                    
       If S.state == PARTOPEN,                                             
       If S.state == REQUEST,                                           
          If P.type == Response,                                           
          S.state := PARTOPEN                                           
             Send Ack                                                      
          /* PARTOPEN means don't send Data packets, retransmit         
             Acks periodically, and include any Init Cookie on          
             every packet sent */                                       
          Set PARTOPEN timer                                            
    Twelfth, process PARTOPEN state;                                    
          Otherwise, if P.type != Sync,                                    
          Otherwise,                                                    
             S.OSR := P.seqno                                              
             S.state := OPEN                                               
                                                                           
    Step 13: Process CloseReq                                              
    Thirteenth, process CloseReq;                                       
       If P.type == CloseReq and S.state < CLOSEREQ,                       
          Generate Close                                                   
          S.state := CLOSING                                               
          Set CLOSING timer                                                
                                                                           
    Step 14: Process Close                                                 
    Fourteenth, process Close;                                          
       If P.type == Close,                                                 
          Generate Reset(Closed)                                           
          Tear down connection                                             
          Drop packet and return                                           
                                                                           
    Step 15: Process Sync                                                  
    Fifteenth, process Sync;                                            
       If P.type == Sync,                                                  
          Generate SyncAck                                                 
                                                                           
    Step 16: Process data                                                  
    Sixteenth, process data.                                            
       /* At this point any application data on P can be passed to the     
       Do not deliver data from more than one Request or Response       
          application, except that the application MUST NOT receive        
          data from more than one Request or Response */                   
                                                                           
9.  Checksums                                                              
                                                                           
    DCCP uses a header checksum to protect its header against              
    corruption.  Generally, this checksum also covers any application      
    data.  DCCP applications can, however, request that the header         
    checksum cover only part of the application data, or perhaps no        
    application data at all.  Link layers may then reduce their            
    protection on unprotected parts of DCCP packets.  For some noisy       
    links, and applications that can tolerate corruption, this can         
    greatly improve delivery rates and perceived performance.              
                                                                           
    Checksum coverage may eventually impact congestion control             
    If checksum coverage is complete, packets with corrupt application  
    mechanisms as well.  A packet with corrupt application data and        
    data must be treated as network losses, thus incurring a loss       
    complete checksum coverage is treated as lost.  This incurs a heavy-   
    response from the sender's congestion control mechanism.  Such a    
    duty loss response from the sender's congestion control mechanism,     
    heavy-duty response may unfairly penalize connections on links with 
    which can unfairly penalize connections on links with high             
    high background corruption.  It is to the application's benefit to  
    background corruption.  The combination of reduced checksum coverage   
    report corruption losses differently from network losses.           
    and Data Checksum options may let endpoints report packets as          
    Therefore, even applications that demand correct data can make use  
    corrupt rather than dropped, using Data Dropped options and Drop       
    of reduced checksum coverage, by including a Data Checksum option.  
                                                                           
    Data Checksum holds a strong checksum of the application data.  The 
                                                                           
    combination of reduced checksum coverage and Data Checksum can drop 
                                                                           
    corrupt application data, but report such drops as corruption, not  
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    Code 3 (see Section 11.7).  This may eventually benefit                
    applications.  However, further research is required to determine an   
    appropriate response to corruption, which can sometimes correlate      
    with congestion.  Corrupt packets currently incur a loss response.     
                                                                           
    The Data Checksum option, which contains a strong CRC, lets            
    endpoints detect application data corruption.  An API can then be      
    used to avoid delivering corrupt data to the application, even if      
    links deliver corrupt data to the endpoint due to reduced checksum     
    coverage.  However, the use of reduced checksum coverage for           
    applications that demand correct data is currently considered          
    experimental.  This is because the combined loss-plus-corruption       
    rate for packets with reduced checksum coverage may be significantly   
    higher than that for packets with full checksum coverage, although     
    the loss rate will generally be lower.  Actual behavior will depend    
    on link design; further research and experience is required.           
                                                                           
    Reduced checksum coverage introduces some security considerations;     
    see Section 18.1.  See Appendix B.1 for further motivation and         
    discussion.  DCCP's implementation of reduced checksum coverage was    
    inspired by UDP-Lite [RFC 3828].                                       
                                                                           
9.1.  Header Checksum Field                                                
                                                                           
    DCCP uses the TCP/IP checksum algorithm.  The Checksum field in the    
    DCCP generic header (see Section 5.1) equals the 16 bit one's          
    complement of the one's complement sum of all 16 bit words in the      
    DCCP header, DCCP options, a pseudoheader taken from the network-      
    layer header, and, depending on the value of the Checksum Coverage     
    field, some or all of the application data.  When calculating the      
    checksum, the Checksum field itself is treated as 0.  If a packet      
    contains an odd number of header and payload bytes to be               
    contains an odd number of header and text bytes to be checksummed, 8
    checksummed, 8 zero bits are added on the right to form a 16 bit       
    zero bits are added on the right to form a 16 bit word for checksum 
    word for checksum purposes.  The pad byte is not transmitted as part   
    purposes.  The pad byte is not transmitted as part of the packet.   
    of the packet.                                                         
                                                                           
    The pseudoheader is calculated as for TCP.  For IPv4, it is 96 bits    
    long, and consists of the IPv4 source and destination addresses, the   
    IP protocol number for DCCP (padded on the left with 8 zero bits),     
    and the DCCP length as a 16-bit quantity (the length of the DCCP       
    header with options, plus the length of any data); see Section 3.1     
    of [RFC 793].  For IPv6, it is 320 bits long, and consists of the      
    IPv6 source and destination addresses, the DCCP length as a 32-bit     
    quantity, and the IP protocol number for DCCP (padded on the left      
    with 24 zero bits); see Section 8.1 of [RFC 2460].                     
                                                                           
    Packets with invalid header checksums MUST be ignored.  In             
    particular, their options MUST NOT be processed.                       
                                                                           
                                                                           
                                                                           
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9.2.  Header Checksum Coverage Field                                       
                                                                           
    The Checksum Coverage field in the DCCP generic header (see Section    
    5.1) specifies what parts of the packet are covered by the Checksum    
    field, as follows:                                                     
                                                                           
    CsCov = 0      The Checksum field covers the DCCP header, DCCP         
                   options, network-layer pseudoheader, and all            
                   application data in the packet, possibly padded on      
                   the right with zeros to an even number of bytes.        
                                                                           
    CsCov = 1-15   The Checksum field covers the DCCP header, DCCP         
                   options, network-layer pseudoheader, and the initial    
                   (CsCov-1)*4 bytes of the packet's application data.     
                                                                           
    Thus, if CsCov is 1, none of the application data is protected by      
    the header checksum.  The value (CsCov-1)*4 MUST be less than or       
    equal to the length of the application data.  Packets with invalid     
    CsCov values MUST be ignored; in particular, their options MUST NOT    
    be processed.  The meanings of values other than 0 and 1 should be     
    considered experimental.                                               
                                                                           
    Values other than 0 specify that corruption is acceptable in some or   
    all of the DCCP packet's application data.  In fact, DCCP cannot       
    even detect corruption in areas not covered by the header checksum,    
    unless the Data Checksum option is used.  Applications should not      
    make any assumptions about the correctness of received data not        
    covered by the checksum, and should if necessary introduce their own   
    validity checks.                                                       
                                                                           
    A DCCP application interface should let sending applications suggest   
    a value for CsCov for sent packets, defaulting to 0 (full coverage).   
    The Minimum Checksum Coverage feature, described below, lets an        
    endpoint refuse delivery of application data on packets with partial   
    checksum coverage; by default, only fully-covered application data     
    is accepted.  Lower layers that support partial error detection MAY    
    use the Checksum Coverage field as a hint of where errors do not       
    need to be detected.  Lower layers MUST use a strong error detection   
    mechanism to detect at least errors that occur in the sensitive part   
    of the packet, and discard damaged packets.  The sensitive part        
    consists of the bytes between the first byte of the IP header and      
    the last byte identified by Checksum Coverage.                         
                                                                           
    For more details on application and lower-layer interface issues       
    relating to partial checksumming, see [RFC 3828].                      
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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9.2.1.  Minimum Checksum Coverage Feature                                  
                                                                           
    The Minimum Checksum Coverage feature lets a DCCP endpoint determine   
    whether its peer is willing to accept packets with reduced Checksum    
    Coverage.  For example, DCCP A sends a "Change R(Minimum Checksum      
    Coverage.  DCCP A sends a "Change R(Minimum Checksum Coverage, 1)"  
    Coverage, 1)" option to DCCP B to check whether B is willing to        
    option to DCCP B to check whether B is willing to accept packets    
    accept packets with Checksum Coverage set to 1.                        
    with Checksum Coverage set to 1.                                    
                                                                           
    Minimum Checksum Coverage has feature number 8, and is server-         
    priority.  It takes one-byte integer values between 0 and 15; values   
    of 16 or more are reserved.  Minimum Checksum Coverage/B reflects      
    values of Checksum Coverage that DCCP B finds unacceptable.  Say       
    that the value of Minimum Checksum Coverage/B is MinCsCov.  Then:      
                                                                           
    o  If MinCsCov = 0, then DCCP B only finds packets with CsCov = 0      
       acceptable.                                                         
                                                                           
    o  If MinCsCov > 0, then DCCP B additionally finds packets with        
       CsCov >= MinCsCov acceptable.                                       
                                                                           
    DCCP B MAY refuse to process application data from packets with        
    unacceptable Checksum Coverage.  Such packets SHOULD be reported       
    using Data Dropped options (Section 11.7) with Drop Code 0, Protocol   
    using Data Dropped options (Section 11.8) with Drop Code 0,         
    Constraints.  New connections start with Minimum Checksum Coverage 0   
    "Protocol Constraints".  New connections start with Minimum Checksum
    for both endpoints.                                                    
    Coverage 0 for both endpoints.                                      
                                                                           
9.3.  Data Checksum Option                                                 
                                                                           
    The Data Checksum option holds a 32-bit CRC-32c cyclic redundancy-     
    check code of a DCCP packet's application data.                        
                                                                           
    +--------+--------+--------+--------+--------+--------+                
    |00101100|00000110|              CRC-32c              |                
    +--------+--------+--------+--------+--------+--------+                
     Type=44  Length=6                                                     
                                                                           
    The sending DCCP computes the CRC of the bytes comprising the          
    Data Checksum is intended for packets containing application data,  
    application data area and stores it in the option data.  The CRC-32c   
    such as DCCP-Request, DCCP-Response, DCCP-Data, and DCCP-DataAck,   
    algorithm used for Data Checksum is the same as that used for SCTP     
    but it may be included on any packet.  The sending DCCP computes the
    [RFC 3309]; note that the CRC-32c of zero bytes of data equals zero.   
    CRC of the bytes comprising the application data area and stores it 
    The DCCP header checksum will cover the Data Checksum option, so the   
    in the option data.  The CRC-32c algorithm used for Data Checksum is
    data checksum must be computed before the header checksum.             
    the same as that used for SCTP [RFC 3309]; note that the CRC-32c of 
                                                                           
    zero bytes of data equals zero.  The DCCP header checksum will cover
    A DCCP endpoint receiving a packet with a Data Checksum option         
    the Data Checksum option, so the data checksum must be computed     
    SHOULD compute the received application data's CRC-32c, using the      
    before the header checksum.                                         
    same algorithm as the sender, and compare the result with the Data     
    Checksum value.  (The endpoint can indicate its willingness to check   
    Checksum value.  (The endpoint can indicate whether it will is      
    Data Checksums using the Check Data Checksum feature, described        
    willing to check Data Checksums using the Check Data Checksum       
                                                                           
    feature, described below.)  If the CRCs differ, the endpoint reacts 
                                                                           
    in one of two ways.                                                 
                                                                           
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    below.)  If the CRCs differ, the endpoint reacts in one of two ways.   
                                                                           
    o  The receiving application may have requested delivery of known-     
       corrupt data via some optional API.  In this case, the packet's     
       data MUST be delivered to the application, with a note that it is   
       known to be corrupt.  Furthermore, the receiving endpoint MUST      
       report the packet as delivered corrupt using a Data Dropped         
       option (Drop Code 7, Delivered Corrupt).                            
       option (Drop Code 7).                                            
                                                                           
    o  Otherwise, the receiving endpoint MUST drop the application data,   
       and report that data as dropped due to corruption using a Data      
       and report the packet as dropped due to corruption using a Data  
       Dropped option (Drop Code 3, Corrupt).                              
       Dropped option (Drop Code 3).                                    
                                                                           
    In either case, the packet will be reported as Received or Received 
    In either case, the packet is considered acknowledgeable (since its    
    ECN Marked by Ack Vector or similar options.                        
    header was processed), and will therefore be acknowledged using the    
    equivalent of Ack Vector's Received or Received ECN Marked states.     
                                                                           
    Although Data Checksum is intended for packets containing              
    application data, it may be included on other packets, such as DCCP-   
    Ack, DCCP-Sync, and DCCP-SyncAck.  The receiver SHOULD calculate the   
    application data area's CRC-32c on such packets, just as it does for   
    DCCP-Data and similar packets; and if the CRCs differ, the packets     
    similarly MUST be reported using Data Dropped options (Drop Code 3),   
    although their application data areas would not be delivered to the    
    application in any case.                                               
                                                                           
9.3.1.  Check Data Checksum Feature                                        
                                                                           
    The Check Data Checksum feature lets a DCCP endpoint determine         
    whether its peer will definitely check Data Checksum options.          
    whether its peer can check Data Checksum options.  DCCP A sends a   
    DCCP A sends a Mandatory "Change R(Check Data Checksum, 1)" option     
    Mandatory "Change R(Check Data Checksum, 1)" option to DCCP B to    
    to DCCP B to require it to check Data Checksum options (the            
    require B to check Data Checksum options (the connection will be    
    connection will be reset if it cannot).                                
    reset if DCCP B cannot).                                            
                                                                           
    Check Data Checksum has feature number 9, and is server-priority.      
    It takes one-byte Boolean values.  DCCP B MUST check any received      
    Data Checksum options when Check Data Checksum/B is one, although it   
    MAY check them even when Check Data Checksum/B is zero.  Values of     
    two or more are reserved.  New connections start with Check Data       
    Checksum 0 for both endpoints.                                         
                                                                           
9.3.2.  Usage Notes                                                        
                                                                           
    Internet links must normally apply strong integrity checks to the      
    packets they transmit [RFC 3828] [RFC 3819].  This is the default      
    case when the DCCP header's Checksum Coverage value equals zero        
    (full coverage).  However, the DCCP Checksum Coverage value might      
    not be zero.  By setting partial Checksum Coverage, the application    
                                                                           
                                                                           
                                                                           
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    indicates that it can tolerate corruption in the unprotected part of   
    the application data.  Recognizing this, link layers may reduce        
    error detection and/or correction strength when transmitting this      
    unprotected part.  This, in turn, can significantly increase the       
    likelihood of the endpoint receiving corrupt data; Data Checksum       
    lets the receiver detect that corruption with very high probability.   
                                                                           
10.  Congestion Control                                                    
10.  Congestion Control IDs                                             
                                                                           
    Each congestion control mechanism supported by DCCP is assigned a      
    congestion control identifier, or CCID: a number from 0 to 255.        
    During connection setup, and optionally thereafter, the endpoints      
    negotiate their congestion control mechanisms by negotiating the       
    values for their Congestion Control ID features.  Congestion Control   
    ID has feature number 1.  The CCID/A value equals the CCID in use      
    for the A-to-B half-connection.  DCCP B sends a "Change R(CCID, K)"    
    option to ask DCCP A to use CCID K for its data packets.               
                                                                           
    CCID is a server-priority feature, so CCID negotiation options can     
    list multiple acceptable CCIDs, sorted in descending order of          
    priority.  For example, the option "Change R(CCID, 2 3 4)" asks the    
    priority.  For example, the option "Change R(CCID, 1 2 3)" asks the 
    receiver to use CCID 2 for its packets, although CCIDs 3 and 4 are     
    receiver to use CCID 1 for its packets, although CCIDs 2 and 3 are  
    also acceptable.  (This corresponds to the bytes "35, 6, 1, 2, 3,      
    also acceptable.  (This corresponds to the bytes "35, 6, 1, 1, 2,   
    4": Change R option (35), option length (6), feature ID (1), CCIDs     
    3": Change R option (35), option length (6), feature ID (1), CCIDs  
    (2, 3, 4).)  Similarly, "Confirm L(CCID, 1, 2 3 4)" tells the          
    (1, 2, 3).)  Similarly, "Confirm L(CCID, 1, 1 2 3)" tells the       
    receiver that the sender is using CCID 2 for its packets, but that     
    receiver that the sender is using CCID 1 for its packets, but that  
    CCIDs 3 and 4 might also be acceptable.                                
    CCIDs 2 or 3 might also be acceptable.                              
                                                                           
    Currently allocated CCIDs are as follows.                              
                                                                           
              CCID   Meaning                      Reference                
         CCID   Meaning                                                 
              ----   -------                      ---------                
         ----   -------                                                 
               0-1   Reserved                                              
           0    Reserved                                                
                2    TCP-like Congestion Control  [RFC TBA]                
           1    Unspecified Sender-Based Congestion Control             
                3    TFRC Congestion Control      [RFC TBA]                
           2    TCP-like Congestion Control                             
              4-255  Reserved                                              
           3    TFRC Congestion Control                                 
                                                                           
         4-255  Reserved                                                
              Table 5: DCCP Congestion Control Identifiers                 
                                                                           
    New connections start with CCID 2 for both endpoints.  If this is      
    unacceptable for a DCCP endpoint, that endpoint MUST send Mandatory    
    Change(CCID) options on its first packets.                             
                                                                           
    All CCIDs standardized for use with DCCP will correspond to            
    congestion control mechanisms previously standardized by the IETF.     
    We expect that for quite some time, all such mechanisms will be TCP-   
    friendly, but TCP-friendliness is not an explicit DCCP requirement.    
                                                                           
                                                                           
                                                                           
                                                                           
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    A DCCP implementation intended for general use, such as an             
    implementation in a general-purpose operating system kernel, SHOULD    
    implement at least CCID 2. The intent is to make CCID 2 broadly        
    implement at least CCIDs 1 and 2.  The intent is to make these CCIDs
    available for interoperability, although particular applications       
    broadly available for interoperability, although particular         
    might disallow its use.                                                
    applications might disallow their use.                              
                                                                           
10.1.  Unspecified Sender-Based Congestion Control                      
10.1.  TCP-like Congestion Control                                         
    CCID 1 denotes an unspecified sender-based congestion control       
                                                                           
    mechanism.  This provides a limited, controlled form of             
    CCID 2, TCP-like Congestion Control, denotes Additive Increase,        
    interoperability for new IETF-approved CCIDs: with CCID 1, an HC-   
    Multiplicative Decrease (AIMD) congestion control with behavior        
    Sender can use a new sender-based congestion control mechanism whose
    modelled directly on TCP, including congestion window, slow start,     
    details the HC-Receiver does not understand.                        
    Some congestion control mechanisms require only generic behavior    
    from the receiver.  For example, CCID 2, TCP-like Congestion        
    Control, requires that the receiver (1) send Ack Vectors and (2)    
    respond to Ack Ratio.  Both of these requirements use generic       
    mechanisms described in this document.  Thus, a CCID 2 HC-Receiver  
    doesn't really need to understand the details of CCID 2.            
    CCID 1 uses this insight to support forward compatibility for       
    sender-based congestion control mechanisms.  An HC-Sender proposes  
    CCID 1 as a proxy for a sender-based mechanism whose details the HC-
    Receiver doesn't need to understand.  The HC-Receiver can then agree
    to CCID 1, and provide generic acknowledgement feedback as requested
    by other features (such as Send Ack Vector).  Individual CCID       
    profile documents say whether or not they can masquerade as CCID 1. 
    For example, say that CCID 98, a new sender-based congestion control
    mechanism using Ack Vector for acknowledgements, has entered the    
    IETF standards process, and the IETF has approved the use of CCID 1 
    as a proxy for CCID 98.  Now, say DCCP A would like to use CCID 98  
    for its data packets.  It should therefore send a "Change L(CCID, 98
    1)" option to open a CCID negotiation.  98 comes first, since that  
    is the preferred CCID; 1 comes next, as a potential proxy for 98.   
    If DCCP B understands CCID 98, it will respond with "Confirm R(CCID,
    98, ...)" and all is well.  But if it does not understand CCID 98,  
    it may respond with "Confirm R(CCID, 1, ...)", still allowing DCCP A
    to use CCID 98.  DCCP A will separately negotiate Send Ack Vector,  
    and thus DCCP B will provide the feedback DCCP A requires, namely   
    Ack Vector, without needing to understand the operation of CCID 98. 
    Implementors MUST NOT use CCID 1 in production environments as a    
    proxy for congestion control mechanisms that have not entered the   
    IETF standards process.  We intend that any production use of CCID 1
    would have to be explicitly approved first by the IETF.  Middleboxes
    MAY choose to treat the use of CCID 1 as experimental or            
    unacceptable.                                                       
    Since CCID 1 should be used only as a proxy for other, defined      
    CCIDs, an HC-Sender MUST NOT report a preference list consisting    
    only of CCID 1, and the option "Change L(CCID, 1)" is illegal.      
    Receiving such an option SHOULD result in connection reset with     
    Reset Code 5, "Option Error".  An HC-Receiver MAY suggest CCID 1    
    exclusively: the option "Change R(CCID, 1)" is not illegal.         
    If CCID 1 is the result of a CCID feature negotiation, the HC-Sender
    determines which CCID to actually use by picking the earliest CCID  
    in its preference list that can masquerade as CCID 1.  The HC-Sender
    MUST pick a CCID that appeared explicitly in its preference list.   
    Many DCCP APIs will allow applications to suggest preferred CCIDs   
    for sending and receiving data.  Such APIs might let applications   
    allow or prevent the use of CCID 1 for receiving, but they should   
    not let applications suggest the use of CCID 1 for sending.  The    
    code implementing a particular CCID should add CCID 1 to the HC-    
    Sender's CCID preference list when appropriate, unless the          
    application disagrees.  The default for both sender and receiver    
    should be to allow CCID 1 when possible.                            
    CCID 1 places no restrictions on how often the HC-Receiver may send 
    DCCP-Ack packets.  A careful implementation SHOULD implement a      
    liberal rate limit on DCCP-Acks to prevent ack storms.              
10.2.  TCP-like Congestion Control                                      
    timeouts, and so forth [RFC 2581].  CCID 2 achieves maximum            
    timeouts, and so forth.  CCID 2 achieves maximum bandwidth over the 
    bandwidth over the long term, consistent with the use of end-to-end    
    long term, consistent with the use of end-to-end congestion control,
    congestion control, but halves its congestion window in response to    
    but halves its congestion window in response to each congestion     
    each congestion event.  This leads to the abrupt rate changes          
    event.  This leads to the abrupt rate changes typical of TCP.       
    typical of TCP.  Applications should use CCID 2 if they prefer         
    Applications should use CCID 2 if they prefer maximum bandwidth     
    maximum bandwidth utilization to steadiness of rate.  This is often    
    utilization to steadiness of rate.  This is often the case for      
    the case for applications that are not playing their data directly     
    applications that are not playing their data directly to the user.  
    to the user.  For example, a hypothetical application that             
    For example, a hypothetical application that transferred files over 
    transferred files over DCCP, using application-level retransmissions   
    DCCP, using application-level retransmissions for lost packets,     
    for lost packets, would prefer CCID 2 to CCID 3.  On-line games may    
    would prefer CCID 2 to CCID 3.  On-line games may also prefer CCID  
    also prefer CCID 2.                                                    
    2.                                                                  
                                                                           
    CCID 2 is further described in [CCID 2 PROFILE].                       
                                                                           
10.2.  TFRC Congestion Control                                             
10.3.  TFRC Congestion Control                                          
                                                                           
    CCID 3 denotes TCP-Friendly Rate Control (TFRC), an equation-based     
    rate-controlled congestion control mechanism.  TFRC is designed to     
    be reasonably fair when competing for bandwidth with TCP-like flows,   
    where a flow is "reasonably fair" if its sending rate is generally     
    within a factor of two of the sending rate of a TCP flow under the     
    same conditions.  However, TFRC has a much lower variation of          
    throughput over time compared with TCP, which makes CCID 3 more        
    suitable than CCID 2 for applications such as telephony or streaming   
    media where a relatively smooth sending rate is of importance.         
                                                                           
    CCID 3 is further described in [CCID 3 PROFILE].  The TFRC             
    congestion control algorithms were initially described in [RFC         
    3448].                                                                 
                                                                           
10.3.  CCID-Specific Options, Features, and Reset Codes                    
10.4.  CCID-Specific Options, Features, and Reset Codes                 
                                                                           
    Half of the option types, feature numbers, and Reset Codes are         
    reserved for CCID-specific use.  CCIDs may often need new options,     
    for communicating acknowledgement or rate information, for example;    
    reserved option spaces let CCIDs create options at will without        
    polluting the global option space.  Option 128 might have different    
                                                                           
                                                                           
                                                                           
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    meanings on a half-connection using CCID 4 and a half-connection       
    using CCID 8.  CCID-specific options and features will never           
    conflict with global options and features introduced by later          
    versions of this specification.                                        
                                                                           
    Any packet may contain information meant for either half-connection,   
    so CCID-specific option types, feature numbers, and Reset Codes        
    explicitly signal the half-connection to which they apply.             
                                                                           
    o  Option numbers 128 through 191 are for options sent from the HC-    
       Sender to the HC-Receiver; option numbers 192 through 255 are for   
       options sent from the HC-Receiver to the HC-Sender.                 
                                                                           
    o  Reset Codes 128 through 191 indicate that the HC-Sender reset the   
       connection (most likely because of some problem with                
       acknowledgements sent by the HC-Receiver); Reset Codes 192          
       through 255 indicate that the HC-Receiver reset the connection      
       (most likely because of some problem with data packets sent by      
       the HC-Sender).                                                     
                                                                           
    o  Finally, feature numbers 128 through 191 are used for features      
       located at the HC-Sender; feature numbers 192 through 255 are for   
       features located at the HC-Receiver.  Since Change L and            
       Confirm L options for a feature are sent by the feature location,   
       we know that any Change L(128) option was sent by the HC-Sender,    
       while any Change L(192) option was sent by the HC-Receiver.         
       Similarly, Change R(128) options are sent by the HC-Receiver,       
       while Change R(192) options are sent by the HC-Sender.              
                                                                           
    For example, consider a DCCP connection where the A-to-B half-         
    connection uses CCID 4 and the B-to-A half-connection uses CCID 5.     
    Here is how a sampling of CCID-specific options are assigned to        
    half-connections.                                                      
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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                                    Relevant    Relevant                   
         Packet  Option             Half-conn.  CCID                       
         ------  ------             ----------  ----                       
         A > B   128                  A-to-B     4                         
         A > B   192                  B-to-A     5                         
         A > B   Change L(128, ...)   A-to-B     4                         
         A > B   Change R(192, ...)   A-to-B     4                         
         A > B   Confirm L(128, ...)  A-to-B     4                         
         A > B   Confirm R(192, ...)  A-to-B     4                         
         A > B   Change R(128, ...)   B-to-A     5                         
         A > B   Change L(192, ...)   B-to-A     5                         
         A > B   Confirm R(128, ...)  B-to-A     5                         
         A > B   Confirm L(192, ...)  B-to-A     5                         
                                                                           
         B > A   128                  B-to-A     5                         
         B > A   192                  A-to-B     4                         
         B > A   Change L(128, ...)   B-to-A     5                         
         B > A   Change R(192, ...)   B-to-A     5                         
         B > A   Confirm L(128, ...)  B-to-A     5                         
         B > A   Confirm R(192, ...)  B-to-A     5                         
         B > A   Change R(128, ...)   A-to-B     4                         
         B > A   Change L(192, ...)   A-to-B     4                         
         B > A   Confirm R(128, ...)  A-to-B     4                         
         B > A   Confirm L(192, ...)  A-to-B     4                         
                                                                           
    Using CCID-specific options and feature options during a negotiation   
    for that CCID feature is NOT RECOMMENDED, since it is difficult to     
    predict the CCID that will be in force when the option is processed.   
    For example, if a DCCP-Request contains the option sequence            
    "Change L(CCID, 3), 128", the CCID-specific option "128" may be        
    processed either by CCID 3 (if the server supports CCID 3) or by the   
    default CCID 2 (if it does not).  However, it is safe to include       
    CCID-specific options following certain Mandatory Change(CCID)         
    options.  For example, if a DCCP-Request contains the option           
    sequence "Mandatory, Change L(CCID, 3), 128", then either the "128"    
    option will be processed by CCID 3 or the connection will be reset.    
                                                                           
    Servers that do not implement the default CCID 2 might nevertheless    
    receive CCID 2-specific options on a DCCP-Request packet.  (Such a     
    receive CCID 2-specific options on a DCCP-Request packet.  (Since   
    server MUST send Mandatory Change(CCID) options on its DCCP-           
    the server MUST send Mandatory Change(CCID) options on its DCCP-    
    Response, so CCID-specific options on any other packet won't refer     
    Response, these options can't appear on any other packet.)  The     
    to CCID 2.)  The server MUST treat such options as non-understood.     
    server MUST treat such options as non-understood.  Thus, it will    
    Thus, it will reset the connection on encountering a Mandatory CCID-   
    reset the connection on encountering a Mandatory CCID-specific      
    specific option, send an empty Confirm for a non-Mandatory Change      
    option, send an empty Confirm for a non-Mandatory Change option for 
    option for a CCID-specific feature, and ignore other options.          
    a CCID-specific feature, and ignore other options.                  
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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10.4.  CCID Profile Requirements                                           
                                                                           
    Each CCID Profile document MUST address at least the following         
    requirements:                                                          
                                                                           
    o  The profile MUST include the name and number of the CCID being      
       described.                                                          
                                                                           
    o  The profile MUST describe the conditions in which it is likely to   
       be useful.  Often the best way to do this is by comparison to       
       existing CCIDs.                                                     
                                                                           
    o  The profile MUST list and describe any CCID-specific options,       
       features, and Reset Codes, and SHOULD list those general options    
       and features described in this document that are especially         
       relevant to the CCID.                                               
                                                                           
    o  Any newly defined acknowledgement mechanism MUST include a way to   
       transmit ECN Nonce Echoes back to the sender.                       
                                                                           
    o  The profile MUST describe the format of data packets, including     
       any options that should be included and the setting of the CCval    
       header field.                                                       
                                                                           
    o  The profile MUST describe the format of acknowledgement packets,    
       including any options that should be included.                      
                                                                           
    o  The profile MUST define how data packets are congestion             
       controlled.  This includes responses to congestion events, idle     
       and application-limited periods, and responses to the DCCP Data     
       Dropped and Slow Receiver options.  CCIDs that implement per-       
       packet congestion control SHOULD discuss how packet size is         
       factored in to congestion control decisions.                        
                                                                           
    o  The profile MUST specify when acknowledgement packets are           
       generated, and how they are congestion controlled.                  
                                                                           
    o  The profile MUST define when a sender using the CCID is             
       considered quiescent.                                               
                                                                           
    o  The profile MUST say whether its CCID's acknowledgements ever       
       need to be acknowledged, and if so, how often.                      
                                                                           
10.5.  Congestion State                                                    
                                                                           
    Most congestion control algorithms depend on past history to           
    determine the current allowed sending rate.  In CCID 2, this           
    congestion state includes a congestion window and a measurement of     
                                                                           
                                                                           
                                                                           
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    the number of packets outstanding in the network; in CCID 3, it        
    includes the lengths of recent loss intervals; and both CCIDs use an   
    estimate of the round-trip time.  Congestion state depends on the      
    network path, and is invalidated by path changes.  Therefore, DCCP     
    senders and receivers SHOULD reset their congestion state --           
    essentially restarting congestion control from "slow start" or         
    equivalent -- on significant changes in end-to-end path.  For          
    example, an endpoint that sends or receives a Mobile IPv6 Binding      
    Update message [RFC 3775] SHOULD reset its congestion state for any    
    corresponding DCCP connections.                                        
                                                                           
11.  Acknowledgements                                                      
                                                                           
    Congestion control requires receivers to transmit information about    
    packet losses and ECN marks to senders.  DCCP receivers MUST report    
    all congestion they see, as defined by the relevant CCID profile.      
    Each CCID says when acknowledgements should be sent, what options      
    they must use, and so on.  DCCP acknowledgements are congestion        
    they must use, how they should be congestion controlled, and so on. 
    controlled, although it is not required that the acknowledgement       
    stream be more than very roughly TCP-friendly; each CCID defines how   
    acknowledgements are congestion controlled.                            
                                                                           
    Most acknowledgements use DCCP options.  For example, on a half-       
    connection with CCID 2 (TCP-like), the receiver reports                
    acknowledgement information using the Ack Vector option.  This         
    section describes common acknowledgement options and shows how acks    
    using those options will commonly work.  Full descriptions of the      
    ack mechanisms used for each CCID are laid out in the CCID profile     
    specifications.                                                        
                                                                           
    Acknowledgement options, such as Ack Vector, generally depend on the   
    DCCP Acknowledgement Number, and are thus only allowed on packet       
    types that carry that number (all packets except DCCP-Request and      
    DCCP-Data).  Detailed acknowledgement options are not necessarily      
    required on every packet that carries an Acknowledgement Number,       
    however.                                                               
                                                                           
11.1.  Acks of Acks and Unidirectional Connections                         
                                                                           
    DCCP was designed to work well for both bidirectional and              
    unidirectional flows of data, and for connections that transition      
    between these states.  However, acknowledgements required for a        
    unidirectional connection are very different from those required for   
    a bidirectional connection.  In particular, unidirectional             
    connections need to worry about acks of acks.                          
                                                                           
    The ack-of-acks problem arises because some acknowledgement            
    mechanisms are reliable.  For example, an HC-Receiver using CCID 2,    
                                                                           
                                                                           
                                                                           
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    TCP-like Congestion Control, sends Ack Vectors containing completely   
    reliable acknowledgement information.  The HC-Sender should            
    occasionally inform the HC-Receiver that it has received an ack.  If   
    it did not, the HC-Receiver might resend complete Ack Vector           
    information, going back to the start of the connection, with every     
    DCCP-Ack packet!  However, note that acks-of-acks need not be          
    reliable themselves: when an ack-of-acks is lost, the HC-Receiver      
    will simply maintain, and periodically retransmit, old                 
    acknowledgement-related state for a little longer.  Therefore, there   
    is no need for acks-of-acks-of-acks.                                   
                                                                           
    When communication is bidirectional, any required acks-of-acks are     
    automatically contained in normal acknowledgements for data packets.   
    On a unidirectional connection, however, the receiver DCCP sends no    
    data, so the sender would not normally send acknowledgements.          
    Therefore, the CCID in force on that half-connection must explicitly   
    say whether, when, and how the HC-Sender should generate acks-of-      
    acks.                                                                  
                                                                           
    For example, consider a bidirectional connection where both half-      
    connections use the same CCID (either 2 or 3), and where DCCP B goes   
    "quiescent".  This means that the connection becomes unidirectional:   
    DCCP B stops sending data, and sends only sends DCCP-Ack packets to    
    DCCP A.  For example, in CCID 2, TCP-like Congestion Control, DCCP B   
    DCCP A.  For CCID 2, TCP-like Congestion Control, DCCP B uses Ack   
    uses Ack Vector to reliably communicate which packets it has           
    Vector to reliably communicate which packets it has received.  As   
    received.  As described above, DCCP A must occasionally acknowledge    
    described above, DCCP A must occasionally acknowledge a pure        
    a pure acknowledgement from DCCP B, so that B can free old Ack         
    acknowledgement from DCCP B, so that B can free old Ack Vector      
    Vector state.  For instance, A might send a DCCP-DataAck packet        
    state.  For instance, A might send a DCCP-DataAck packet every now  
    every now and then, instead of DCCP-Data.  In contrast, in CCID 3,     
    and then, instead of DCCP-Data.  In contrast, for CCID 3, TFRC      
    TFRC Congestion Control, DCCP B's acknowledgements generally need      
    Congestion Control, DCCP B's acknowledgements generally need not be 
    not be reliable, since they contain cumulative loss rates; TFRC        
    reliable, since they contain cumulative loss rates; TFRC works even 
    works even if every DCCP-Ack is lost.  Therefore, DCCP A need never    
    if every DCCP-Ack is lost.  Therefore, DCCP A need never acknowledge
    acknowledge an acknowledgement.                                        
    an acknowledgement.                                                 
                                                                           
    When communication is unidirectional, a single CCID -- in the          
    example, the A-to-B CCID -- controls both DCCPs' acknowledgements,     
    in terms of their content, their frequency, and so forth.  For         
    bidirectional connections, the A-to-B CCID governs DCCP B's            
    acknowledgements (including its acks of DCCP A's acks), while the B-   
    to-A CCID governs DCCP A's acknowledgements.                           
                                                                           
    DCCP A switches its ack pattern from bidirectional to unidirectional   
    when it notices that DCCP B has gone quiescent.  It switches from      
    unidirectional to bidirectional when it must acknowledge even a        
    single DCCP-Data or DCCP-DataAck packet from DCCP B.                   
                                                                           
    Each CCID defines how to detect quiescence on that CCID, and how       
    that CCID handles acks-of-acks on unidirectional connections.  The     
                                                                           
                                                                           
                                                                           
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    B-to-A CCID defines when DCCP B has gone quiescent.  Usually, this     
    happens when a period has passed without B sending any data packets;   
    in CCID 2, for example, this period is the maximum of 0.2 seconds      
    for CCID 2, this period is the maximum of 0.2 seconds and two round-
    and two round-trip times.  The A-to-B CCID defines how DCCP A          
    trip times.  The A-to-B CCID defines how DCCP A handles acks-of-acks
    handles acks-of-acks once DCCP B has gone quiescent.                   
    once DCCP B has gone quiescent.                                     
                                                                           
11.2.  Ack Piggybacking                                                    
                                                                           
    Acknowledgements of A-to-B data MAY be piggybacked on data sent by     
    DCCP B, as long as that does not delay the acknowledgement longer      
    than the A-to-B CCID would find acceptable.  However, data             
    acknowledgements often require more than 4 bytes to express.  A        
    large set of acknowledgements prepended to a large data packet might   
    exceed the allowed maximum packet size.  In this case, DCCP B SHOULD   
    send separate DCCP-Data and DCCP-Ack packets, or wait, but not too     
    long, for a smaller datagram.                                          
                                                                           
    Piggybacking is particularly common at DCCP A when the B-to-A half-    
    connection is quiescent -- that is, when DCCP A is just                
    acknowledging DCCP B's acknowledgements.  There are three reasons to   
    acknowledge DCCP B's acknowledgements: to allow DCCP B to free up      
    information about previously acknowledged data packets from A; to      
    shrink the size of future acknowledgements; and to manipulate the      
    rate at which future acknowledgements are sent.  Since these are       
    secondary concerns, DCCP A can generally afford to wait indefinitely   
    for a data packet to piggyback its acknowledgement onto; if DCCP B     
    wants to elicit an acknowledgement, it can send a DCCP-Sync.           
                                                                           
    Any restrictions on ack piggybacking are described in the relevant     
    CCID's profile.                                                        
                                                                           
11.3.  Ack Ratio Feature                                                   
                                                                           
    The Ack Ratio feature lets HC-Senders influence the rate at which      
    HC-Receivers generate DCCP-Ack packets, thus controlling reverse-      
    path congestion.  This differs from TCP, which presently has no        
    congestion control for pure acknowledgement traffic.  Ack Ratio        
    reverse-path congestion control does not try to be TCP-friendly.  It   
    just tries to avoid congestion collapse, and to be somewhat better     
    than TCP in the presence of a high packet loss or mark rate on the     
    reverse path.                                                          
                                                                           
    Ack Ratio applies to CCIDs whose HC-Receivers clock acknowledgements   
    off the receipt of data packets.  The value of Ack Ratio/A equals      
    the rough ratio of data packets sent by DCCP A to DCCP-Ack packets     
    sent by DCCP B.  Higher Ack Ratios correspond to lower DCCP-Ack        
    rates; the sender raises Ack Ratio when the reverse path is            
    congested and lowers Ack Ratio when it is not.  Each CCID profile      
    congested and lowers Ack Ratio when it is not.  CCID 2, TCP-like    
                                                                           
    Congestion Control, use Ack Ratio for acknowledgement congestion    
                                                                           
    control.  Other CCIDs can ignore Ack Ratio if they perform          
                                                                           
    congestion control on acknowledgements in some other way.           
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    defines how it controls congestion on the acknowledgement path, and,   
    particularly, whether Ack Ratio is used.  CCID 2, for example, uses    
    Ack Ratio for acknowledgement congestion control, but CCID 3 does      
    not.  However, each Ack Ratio feature has a value whether or not       
    that value is used by the relevant CCID.                               
                                                                           
    Ack Ratio has feature number 5, and is non-negotiable.  It takes       
    two-byte integer values.  An Ack Ratio/A value of four means that      
    two-byte integer values.  If Ack Ratio/A is four, then DCCP B will  
    DCCP B will send at least one acknowledgement packet for every four    
    send at least one acknowledgement packet for every four data packets
    data packets sent by DCCP A.  DCCP A sends a "Change L(Ack Ratio)"     
    sent by DCCP A.  DCCP A sends a "Change L(Ack Ratio)" option to     
    option to notify DCCP B of its ack ratio.  An Ack Ratio value of       
    notify DCCP B of its ack ratio.  An Ack Ratio value of zero         
    zero indicates that the relevant half-connection does not use an Ack   
    indicates that the relevant half-connection does not use an Ack     
    Ratio to control its acknowledgement rate.  New connections start      
    with Ack Ratio 2 for both endpoints; this Ack Ratio results in         
    acknowledgement behavior analogous to TCP's delayed acks.              
                                                                           
    Ack Ratio should be treated as a guideline rather than a strict        
    requirement.  We intend Ack Ratio-controlled acknowledgement           
    behavior to resemble TCP's acknowledgement behavior when there is no   
    reverse-path congestion, and to be somewhat more conservative when     
    there is reverse-path congestion.  Following this intent is more       
    important than implementing Ack Ratio precisely.  In particular:       
                                                                           
    o  Receivers MAY piggyback acknowledgement information on data         
       packets, creating DCCP-DataAck packets.  The Ack Ratio does not     
       apply to piggybacked acknowledgements.  However, if the data        
       packets are too big to carry acknowledgement information, or the    
       data sending rate is lower than Ack Ratio would suggest, then       
       DCCP B SHOULD send enough pure DCCP-Ack packets to maintain the     
       rate of one acknowledgement per Ack Ratio received data packets.    
                                                                           
    o  Receivers MAY rate-pace their acknowledgements, rather than         
       sending acknowledgements immediately upon the receipt of data       
       packets.  Receivers that rate-pace acknowledgements SHOULD pick a   
       rate that approximates the effect of Ack Ratio, and SHOULD          
       include Elapsed Time options (Section 13.2) to help the sender      
       calculate round-trip times.                                         
                                                                           
    o  Receivers SHOULD implement delayed acknowledgement timers like      
       TCP's, whereby any packet's acknowledgement is delayed by at most   
       TCP's, whereby each packet is acknowledged within at most T      
       T seconds.  This delay lets the receiver collect additional         
       seconds of its receipt.  The default value of T should be        
       packets to acknowledge, and thus reduce the per-packet overhead     
       0.2 seconds, as is common in TCP implementations.  This may lead 
       of acknowledgements; but if T seconds have passed by and the ack    
       to sending more acknowledgement packets than Ack Ratio would     
       is still around, it is sent out right away.  The default value of   
       suggest.                                                         
       T should be 0.2 seconds, as is common in TCP implementations.       
       This may lead to sending more acknowledgement packets than Ack      
       Ratio would suggest.                                                
                                                                           
                                                                           
                                                                           
                                                                           
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    o  Receivers SHOULD send acknowledgements immediately on receiving     
       packets marked ECN Congestion Experienced, or packets whose out-    
       marked packets, or packets whose out-of-order sequence numbers   
       of-order sequence numbers potentially indicate loss.  However,      
       potentially indicate loss.  However, there is no need to send    
       there is no need to send such immediate acknowledgements for        
       such immediate acknowledgements for marked packets more than once
       marked packets more than once per round-trip time.                  
       per round-trip time.                                             
                                                                           
    o  Receivers MAY ignore Ack Ratio if they perform their own            
       congestion control on acknowledgements.  For example, a receiver    
       that knows the loss and mark rate for its DCCP-Ack packets might    
       maintain a TCP-friendly acknowledgement rate on its own.  Such a    
       receiver MUST either ensure that it always obtains sufficient       
       receiver MUST ensure that it always obtains sufficient           
       acknowledgement loss and mark information, or fall back to Ack      
       acknowledgement loss and mark information or fall back to Ack    
       Ratio when sufficient information is not available, as might        
       happen during periods when the receiver is quiescent.               
                                                                           
11.4.  Ack Vector Options                                                  
                                                                           
    The Ack Vector gives a run-length encoded history of data packets      
    received at the client.  Each byte of the vector gives the state of    
    that data packet in the loss history, and the number of preceding      
    packets with the same state.  The option's data looks like this:       
                                                                           
    +--------+--------+--------+--------+--------+--------                 
    |0010011?| Length |SSLLLLLL|SSLLLLLL|SSLLLLLL|  ...                    
    +--------+--------+--------+--------+--------+--------                 
    Type=38/39         \___________ Vector ___________...                  
                                                                           
    The two Ack Vector options (option types 38 and 39) differ only in     
    the values they imply for ECN Nonce Echo.  Section 12.2 describes      
    this further.                                                          
                                                                           
    The vector itself consists of a series of bytes, each of whose         
    encoding is:                                                           
                                                                           
     0 1 2 3 4 5 6 7                                                       
    +-+-+-+-+-+-+-+-+                                                      
    |Sta| Run Length|                                                      
    +-+-+-+-+-+-+-+-+                                                      
                                                                           
    Sta[te] occupies the most significant two bits of each byte, and can   
    have one of four values, as follows.                                   
    have one of four values:                                            
                                                                           
        0   Packet received (and not ECN Congestion Experienced).       
                                                                           
        1   Packet received with ECN Congestion Experienced ("ECN       
                                                                           
            marked" for short).                                         
                                                                           
        2   Reserved.                                                   
                                                                           
        3   Packet not yet received.                                    
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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                       State  Meaning                                      
                       -----  -------                                      
                         0    Received                                     
                         1    Received ECN Marked                          
                         2    Reserved                                     
                         3    Not Yet Received                             
                                                                           
                     Table 6: DCCP Ack Vector States                       
                                                                           
    The term "ECN marked" refers to packets with ECN code point 11, CE     
    (Congestion Experienced); packets received with this ECN code point    
    MUST be reported using State 1, Received ECN Marked.  Packets          
    received with other ECN code points 00, 01, or 10 (Non-ECT, ECT(0),    
    or ECT(1), respectively) MUST be reported using State 0, Received.     
                                                                           
    Run Length, the least significant six bits of each byte, specifies     
    how many consecutive packets have the given State.  Run Length zero    
    says the corresponding State applies to one packet only; Run Length    
    63 says it applies to 64 consecutive packets.  Run lengths of 65 or    
    more must be encoded in multiple bytes.                                
                                                                           
    The first byte in the first Ack Vector option refers to the packet     
    indicated in the Acknowledgement Number; subsequent bytes refer to     
    older packets.  (Ack Vector MUST NOT be sent on DCCP-Data and DCCP-    
    Request packets, which lack an Acknowledgement Number.)  An Ack        
    Request packets, which lack an Acknowledgement Number.)  If an Ack  
    Vector containing the decimal values 0,192,3,64,5 and the              
    Vector contains the decimal values 0,192,3,64,5 and the             
    Acknowledgement Number is decimal 100 indicates that:                  
    Acknowledgement Number is decimal 100, then:                        
                                                                           
        Packet 100 was received (Acknowledgement Number 100, State 0,      
        Run Length 0).                                                     
                                                                           
        Packet 99 was lost (State 3, Run Length 0).                        
                                                                           
        Packets 98, 97, 96 and 95 were received (State 0, Run Length 3).   
                                                                           
        Packet 94 was ECN marked (State 1, Run Length 0).                  
                                                                           
        Packets 93, 92, 91, 90, 89, and 88 were received (State 0, Run     
        Length 5).                                                         
                                                                           
    A single Ack Vector option can acknowledge up to 16192 data packets.   
    Should more packets need to be acknowledged than can fit in 253        
    bytes of Ack Vector, then multiple Ack Vector options can be sent;     
    the second Ack Vector begins where the first left off, and so forth.   
                                                                           
    Ack Vector states are subject to two general constraints.  (These      
    principles SHOULD also be followed for other acknowledgement           
    mechanisms; referring to Ack Vector states simplifies their            
                                                                           
                                                                           
                                                                           
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    explanation.)                                                          
                                                                           
    1.  Packets reported as State 0 or State 1 MUST be acknowledgeable:    
    1.  Packets reported as State 0 or State 1 MUST have been processed 
        their options have been processed by the receiving DCCP stack.     
        by the receiving DCCP stack.  In particular, their options must 
        Any data on the packet need not have been delivered to the         
        have been processed.  Any data on the packet need not have been 
        receiving application; in fact, the data may have been dropped.    
        delivered to the receiving application; in fact, the data may   
                                                                           
        have been dropped.                                              
    2.  Packets reported as State 3 MUST NOT be acknowledgeable.           
    2.  Packets reported as State 3 MUST NOT have been received by DCCP.
        Feature negotiations and options on such packets MUST NOT have     
        been processed, and the Acknowledgement Number MUST NOT            
        correspond to such a packet.                                       
                                                                           
    Packets dropped in the application's receive buffer MUST be reported   
    Packets dropped in the application's receive buffer SHOULD be       
    as Received or Received ECN Marked (States 0 and 1), depending on      
    reported as Received or Received ECN Marked (States 0 and 1),       
    their ECN state; such packets' ECN Nonces MUST be included in the      
    depending on their ECN state; such packets' ECN Nonces MUST be      
    Nonce Echo.  The Data Dropped option informs the sender that some      
    included in the Nonce Echo.  The Data Dropped option informs the    
    packets reported as received actually had their application data       
    sender that some packets reported as received actually had their    
    dropped.                                                               
    application data dropped.                                           
                                                                           
    One or more Ack Vector options that, together, report the status of    
    more packets than have actually been sent SHOULD be considered         
    invalid.  The receiving DCCP SHOULD either ignore the options or       
    reset the connection with Reset Code 5, "Option Error".  Packets       
    that haven't been included in any Ack Vector option SHOULD be          
    treated as "not yet received" (State 3) by the sender.                 
                                                                           
    Appendix A provides a non-normative description of the details of      
    DCCP acknowledgement handling, in the context of an abstract Ack       
    Vector implementation.                                                 
                                                                           
11.4.1.  Ack Vector Consistency                                            
                                                                           
    A DCCP sender will commonly receive multiple acknowledgements for      
    some of its data packets.  For instance, an HC-Sender might receive    
    two DCCP-Acks with Ack Vectors, both of which contained information    
    about sequence number 24.  (Information about a sequence number is     
    generally repeated in every ack until the HC-Sender acknowledges an    
    ack.  In this case, perhaps the HC-Receiver is sending acks faster     
    than the HC-Sender is acknowledging them.)  In a perfect world, the    
    two Ack Vectors would always be consistent.  However, there are many   
    reasons why they might not be.  For example:                           
                                                                           
    o  The HC-Receiver received packet 24 between sending its acks, so     
       the first ack said 24 was not received (State 3) and the second     
       said it was received or ECN marked (State 0 or 1).                  
                                                                           
    o  The HC-Receiver received packet 24 between sending its acks, and    
       the network reordered the acks.  In this case, the packet will      
                                                                           
                                                                           
                                                                           
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       appear to transition from State 0 or 1 to State 3.                  
                                                                           
    o  The network duplicated packet 24, and one of the duplicates was     
       ECN marked.  This might show up as a transition between States 0    
       and 1.                                                              
                                                                           
    To cope with these situations, HC-Sender DCCP implementations SHOULD   
    combine multiple received Ack Vector states according to this table:   
                                                                           
                                Received State                             
                                  0   1   3                                
                                +---+---+---+                              
                              0 | 0 |0/1| 0 |                              
                        Old     +---+---+---+                              
                              1 | 1 | 1 | 1 |                              
                       State    +---+---+---+                              
                              3 | 0 | 1 | 3 |                              
                                +---+---+---+                              
                                                                           
    To read the table, choose the row corresponding to the packet's old    
    state and the column corresponding to the packet's state in the        
    newly received Ack Vector, then read the packet's new state off the    
    table.  For an old state of 0 (received non-marked) and received       
    state of 1 (received ECN marked), the packet's new state may be set    
    to either 0 or 1.  The HC-Sender implementation will be indifferent    
    to ack reordering if it chooses new state 1 for that cell.             
                                                                           
    The HC-Receiver should collect information about received packets,     
    which it will eventually report to the HC-Sender on one or more        
    acknowledgements, according to the following table:                    
                                                                           
                               Received Packet                             
                                  0   1   3                                
                                +---+---+---+                              
                              0 | 0 |0/1| 0 |                              
                      Stored    +---+---+---+                              
                              1 |0/1| 1 | 1 |                              
                       State    +---+---+---+                              
                              3 | 0 | 1 | 3 |                              
                                +---+---+---+                              
                                                                           
    This table equals the sender's table, except that when the stored      
    state is 1 and the received state is 0, the receiver is allowed to     
    switch its stored state to 0.                                          
                                                                           
    A HC-Sender MAY choose to throw away old information gleaned from      
    the HC-Receiver's Ack Vectors, in which case it MUST ignore newly      
    received acknowledgements from the HC-Receiver for those old           
                                                                           
                                                                           
                                                                           
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    packets.  It is often kinder to save recent Ack Vector information     
    for a while, so that the HC-Sender can undo its reaction to presumed   
    congestion when a "lost" packet unexpectedly shows up (the             
    transition from State 3 to State 0).                                   
                                                                           
11.4.2.  Ack Vector Coverage                                               
                                                                           
    We can divide the packets that have been sent from an HC-Sender to     
    an HC-Receiver into four roughly contiguous groups.  From oldest to    
    youngest, these are:                                                   
                                                                           
    1.  Packets already acknowledged by the HC-Receiver, where the HC-     
        Receiver knows that the HC-Sender has definitely received the      
        acknowledgements.                                                  
                                                                           
    2.  Packets already acknowledged by the HC-Receiver, where the HC-     
        Receiver cannot be sure that the HC-Sender has received the        
        acknowledgements.                                                  
                                                                           
    3.  Packets not yet acknowledged by the HC-Receiver.                   
                                                                           
    4.  Packets not yet received by the HC-Receiver.                       
                                                                           
    The union of groups 2 and 3 is called the Acknowledgement Window.      
    Generally, every Ack Vector generated by the HC-Receiver will cover    
    the whole Acknowledgement Window: Ack Vector acknowledgements are      
    cumulative.  (This simplifies Ack Vector maintenance at the HC-        
    Receiver; see Appendix A, below.)  As packets are received, this       
    window both grows on the right and shrinks on the left.  It grows      
    because there are more packets, and shrinks because the data           
    packets' Acknowledgement Numbers will acknowledge previous             
    acknowledgements, moving packets from group 2 into group 1.            
                                                                           
11.5.  Send Ack Vector Feature                                             
                                                                           
    The Send Ack Vector feature lets DCCPs negotiate whether they should   
    use Ack Vector options to report congestion.  Ack Vector provides      
    detailed loss information, and lets senders report back to their       
    applications whether particular packets were dropped.  Send Ack        
    Vector is mandatory for some CCIDs, and optional for others.           
                                                                           
    Send Ack Vector has feature number 6, and is server-priority.  It      
    takes one-byte Boolean values.  DCCP A MUST send Ack Vector options    
    on its acknowledgements when Send Ack Vector/A has value one,          
    although it MAY send Ack Vector options even when Send Ack Vector/A    
    is zero.  Values of two or more are reserved.  New connections start   
    with Send Ack Vector 0 for both endpoints.  DCCP B sends a             
    "Change R(Send Ack Vector, 1)" option to DCCP A to ask A to send Ack   
                                                                           
                                                                           
                                                                           
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    Vector options as part of its acknowledgement traffic.                 
                                                                           
11.6.  Slow Receiver Option                                                
                                                                           
    An HC-Receiver sends the Slow Receiver option to its sender to         
    indicate that it is having trouble keeping up with the sender's        
    data.  The HC-Sender SHOULD NOT increase its sending rate for          
    approximately one round-trip time after seeing a packet with a Slow    
    Receiver option.  After one round-trip time, the effect of Slow        
    Receiver option.  However, the Slow Receiver option does not        
    Receiver disappears and the HC-Sender may again increase its rate,     
    indicate congestion, and the HC-Sender need not reduce its sending  
    so the HC-Receiver SHOULD continue to send Slow Receiver options if    
    rate.  (If necessary, the receiver can force the sender to slow down
    it needs to prevent the HC-Sender from going faster in the long        
    by dropping packets, with or without Data Dropped, or reporting     
    term.  The Slow Receiver option does not indicate congestion, and      
    false ECN marks.)  APIs should let receiver applications set Slow   
    the HC-Sender need not reduce its sending rate.  (If necessary, the    
    Receiver, and sending applications determine whether or not their   
    receiver can force the sender to slow down by dropping packets, with   
    receivers are Slow.                                                 
    or without Data Dropped, or reporting false ECN marks.)  APIs should   
    let receiver applications set Slow Receiver, and sending               
    applications determine whether or not their receivers are Slow.        
                                                                           
    Slow Receiver is a one-byte option.                                    
                                                                           
    +--------+                                                             
    |00000010|                                                             
    +--------+                                                             
     Type=2                                                                
                                                                           
    Slow Receiver does not specify why the receiver is having trouble      
    keeping up with the sender.  Possible reasons include lack of buffer   
    space, CPU overload, and application quotas.  A sending application    
    might react to Slow Receiver by reducing its sending rate, for         
    might react to Slow Receiver by reducing its sending rate or by     
    example.                                                               
    switching to a lossier compression algorithm.                       
                                                                           
    The sending application should not react to Slow Receiver by sending   
    more data, however.  The optimal response to a CPU-bound receiver      
    might be to increase the sending rate, by switching to a less-         
    compressed sending format, since a highly-compressed data format       
    might overwhelm a slow CPU more seriously than the higher memory       
    requirements of a less-compressed data format.  This kind of format    
    requirements of a less-compressed data format.  The Slow Receiver   
    change should be requested at the application level, not via the       
    option is not appropriate for this case; a CPU-bound receiver should
    Slow Receiver option.                                                  
    not ask for Slow Receiver options to be sent.                       
                                                                           
    Slow Receiver implements a portion of TCP's receive window             
    functionality.                                                         
                                                                           
11.7.  Data Dropped Option                                                 
11.7.  Reset Congestion State Option                                    
                                                                           
    An HC-Receiver sends the Reset Congestion State option to its sender
    The Data Dropped option indicates that the application data on one     
    to force the sender to reset its congestion state -- that is, to    
    or more received packets did not actually reach the application.       
    "slow start", as if the connection were beginning again.  Reset     
                                                                           
    Congestion State is a one-byte option.                              
                                                                           
    +--------+                                                          
                                                                           
    |00000011|                                                          
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     Type=3                                                             
                                                                           
    The Reset Congestion State option is reserved for the very few cases
                                                                           
    when an endpoint knows that the congestion properties of a path have
    Data Dropped additionally reports why the data was dropped: perhaps    
    changed.  Currently, this reduces to mobility: a DCCP endpoint on a 
    the data was corrupt, or perhaps the receiver cannot keep up with      
    mobile host MUST send Reset Congestion State to its peer after the  
    the sender's current rate and the data was dropped in some receive     
    mobile host changes address or path.  DCCP endpoints MUST NOT use   
    buffer.  Using Data Dropped, DCCP endpoints can discriminate between   
    Reset Congestion State for other purposes.                          
    different kinds of loss; this differs from TCP, in which all loss is   
11.8.  Data Dropped Option                                              
    reported the same way.                                                 
                                                                           
    Unless explicitly specified otherwise, DCCP congestion control         
    mechanisms MUST react as if each Data Dropped packet was marked as     
    ECN Congestion Experienced by the network.  We intend for Data         
    Dropped to enable research into richer congestion responses to         
    corrupt and other endpoint-dropped packets, but DCCP CCIDs MUST        
    react conservatively to Data Dropped until this behavior is            
    react conservatively to Data Dropped until this research is done.   
    standardized.  Section 11.7.2, below, describes congestion responses   
    Section 11.8.2, below, describes congestion responses for all       
    for all current Drop Codes.                                            
    current Drop Codes.                                                 
                                                                           
    If a received packet's application data is dropped for one of the      
    reasons listed below, this SHOULD be reported using a Data Dropped     
    option.  Alternatively, the receiver MAY choose to report as           
    "received" only those packets whose data were not dropped, subject     
    to the constraint that packets not reported as received MUST NOT       
    have had their options processed.                                      
                                                                           
    The option's data looks like this:                                     
                                                                           
    +--------+--------+--------+--------+--------+--------                 
    |00101000| Length | Block  | Block  | Block  |  ...                    
    +--------+--------+--------+--------+--------+--------                 
     Type=40          \___________ Vector ___________ ...                  
                                                                           
    The Vector consists of a series of bytes, called Blocks, each of       
    whose encoding corresponds to one of two choices:                      
                                                                           
     0 1 2 3 4 5 6 7                  0 1 2 3 4 5 6 7                      
    +-+-+-+-+-+-+-+-+                +-+-+-+-+-+-+-+-+                     
    |0| Run Length  |       or       |1|DrpCd|Run Len|                     
    +-+-+-+-+-+-+-+-+                +-+-+-+-+-+-+-+-+                     
      Normal Block                      Drop Block                         
                                                                           
    The first byte in the first Data Dropped option refers to the packet   
    indicated in the Acknowledgement Number; subsequent bytes refer to     
    older packets.  (Data Dropped MUST NOT be sent on DCCP-Data or DCCP-   
    Request packets, which lack an Acknowledgement Number, and any Data    
    Request packets, which lack an Acknowledgement Number.)  Normal     
    Dropped options received on these packet types MUST be ignored.)       
    Blocks, which have high bit 0, indicate that any received packets in
    Normal Blocks, which have high bit 0, indicate that any received       
    the Run Length had their data delivered to the application.  Drop   
    packets in the Run Length had their data delivered to the              
    Blocks, which have high bit 1, indicate that received packets in the
    application.  Drop Blocks, which have high bit 1, indicate that        
    Run Len[gth] were not delivered as usual.  The 3-bit Drop Code      
    received packets in the Run Len[gth] were not delivered as usual.      
    [DrpCd] field says what happened; generally, no data from that      
                                                                           
    packet reached the application.  Packets reported as "not yet       
                                                                           
    received" MUST be included in Normal Blocks; packets not covered by 
                                                                           
    any Data Dropped option are treated as if they were in a Normal     
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        0   Packet data dropped due to protocol constraints.  For       
                                                                           
            example, the data was included on a DCCP-Request packet, but
                                                                           
            the receiving application does not allow such piggybacking; 
    The 3-bit Drop Code [DrpCd] field says what happened; generally, no    
            or the data was included on a packet with inappropriately   
    data from that packet reached the application.  Packets reported as    
            low Checksum Coverage.                                      
    "not yet received" MUST be included in Normal Blocks; packets not      
        1   Packet data dropped because the application is no longer    
    covered by any Data Dropped option are treated as if they were in a    
            listening.  See Section 11.8.2.                             
    Normal Block.  Defined Drop Codes for Drop Blocks are as follows.      
        2   Packet data dropped in a receive buffer.  See Section       
                                                                           
            11.8.2.                                                     
                   Drop Code  Meaning                                      
        3   Packet data dropped due to corruption.  See Section 9.3.    
                   ---------  -------                                      
        4-6 Reserved.                                                   
                       0      Protocol Constraints                         
        7   Packet data corrupted, but delivered to the application     
                       1      Application Not Listening                    
            anyway.  See Section 9.3.                                   
                       2      Receive Buffer                               
    For example, if a Data Dropped option contains the decimal values   
                       3      Corrupt                                      
    0,160,3,162, the Acknowledgement Number is 100, and an Ack Vector   
                      4-6     Reserved                                     
    reported all packets as received, then:                             
                       7      Delivered Corrupt                            
                                                                           
                        Table 7: DCCP Drop Codes                           
                                                                           
    To go into more detail:                                                
                                                                           
        0   The packet data was dropped due to protocol constraints.       
            For example, the data was included on a DCCP-Request packet,   
            but the receiving application does not allow such              
            piggybacking; or the data was included on a packet with        
            inappropriately low Checksum Coverage.                         
                                                                           
        1   The packet data was dropped because the application is no      
            longer listening.  See Section 11.7.2.                         
                                                                           
        2   The packet data was dropped in a receive buffer, probably      
            because of receive buffer overflow.  See Section 11.7.2.       
                                                                           
        3   The packet data was dropped due to corruption.  See Section    
            9.3.                                                           
                                                                           
        7   The packet data was corrupted, but delivered to the            
            application anyway.  See Section 9.3.                          
                                                                           
    For example, assume a packet arrives with Acknowledgement Number       
    100, an Ack Vector reporting all packets as received, and a Data       
    Dropped option containing the decimal values 0,160,3,162.  Then:       
                                                                           
        Packet 100 was received (Acknowledgement Number 100, Normal        
        Block, Run Length 0).                                              
                                                                           
        Packet 99 was dropped in a receive buffer (Drop Block, Drop Code   
        2, Run Length 0).                                                  
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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        Packets 98, 97, 96, and 95 were received (Normal Block, Run        
        Length 3).                                                         
                                                                           
        Packets 95, 94, and 93 were dropped in the receive buffer (Drop    
        Block, Drop Code 2, Run Length 2).                                 
                                                                           
    Run lengths of more than 128 (for Normal Blocks) or 16 (for Drop       
    Blocks) must be encoded in multiple Blocks.  A single Data Dropped     
    option can acknowledge up to 32384 Normal Block data packets,          
    although the receiver SHOULD NOT send a Data Dropped option when all   
    relevant packets fit into Normal Blocks.  Should more packets need     
    to be acknowledged than can fit in 253 bytes of Data Dropped, then     
    multiple Data Dropped options can be sent.  The second option will     
    begin where the first left off, and so forth.                          
                                                                           
    One or more Data Dropped options that, together, report the status     
    of more packets than have been sent, or that change the status of a    
    packet, or that disagree with Ack Vector or equivalent options (by     
    reporting a "not yet received" packet as "dropped in the receive       
    buffer", for example), SHOULD be considered invalid.  The receiving    
    DCCP SHOULD either such options, or respond by resetting the           
    DCCP SHOULD respond to invalid Data Dropped options by ignoring     
    connection with Reset Code 5, "Option Error".                          
    them, or by resetting the connection with Reset Code 5, "Option     
                                                                           
    Error".                                                             
    A DCCP application interface should let receiving applications         
    specify the Drop Codes corresponding to received packets.  For         
    example, this would let applications calculate their own checksums,    
    but still report "dropped due to corruption" packets via the Data      
    Dropped option.  The interface SHOULD NOT let applications reduce      
    Dropped option.  The interface should not let applications reduce   
    the "seriousness" of a packet's Drop Code; for example, the            
    application should not be able to upgrade a packet from delivered      
    corrupt (Drop Code 7) to delivered normally (no Drop Code).            
                                                                           
    Data Dropped information is transmitted reliably.  That is,            
11.8.1.  Data Dropped and Normal Congestion Response                    
    endpoints SHOULD continue to transmit Data Dropped options until       
    receiving an acknowledgement indicating that the relevant options      
    have been processed.  In Ack Vector terms, each acknowledgement        
    should contain Data Dropped options that cover the whole               
    Acknowledgement Window (Section 11.4.2), although when every packet    
    in that window would be placed in a Normal Block no actual option is   
    required.                                                              
                                                                           
11.7.1.  Data Dropped and Normal Congestion Response                       
                                                                           
    When deciding on a response to a particular acknowledgement or set     
    of acknowledgements containing Data Dropped options, a congestion      
    of acknowledgements containing Data Dropped packets, a congestion   
    control mechanism MUST consider dropped packets and ECN Congestion     
    control mechanism MUST consider dropped packets and ECN marks       
    Experienced marks (including marked packets that are included in       
    (including ECN-marked packets that are included in Data Dropped), as
    Data Dropped), as well as the packets singled out in Data Dropped.     
    well as the Data Dropped packets.  For window-based mechanisms, the 
                                                                           
    valid response space is defined as follows.                         
                                                                           
                                                                           
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    For window-based mechanisms, the valid response space is defined as    
    follows.                                                               
                                                                           
    Assume an old window of W.  Independently calculate a new window       
    W_new1 that assumes no packets were Data Dropped (so W_new1 contains   
    only the normal congestion response), and a new window W_new2 that     
    assumes no packets were lost or marked (so W_new2 contains only the    
    Data Dropped response).  We are assuming that Data Dropped             
    recommended a reduction in congestion window, so W_new2 < W.           
                                                                           
    Then the actual new window W_new MUST NOT be larger than the minimum   
    of W_new1 and W_new2; and the sender MAY combine the two responses,    
    by setting                                                             
          W_new = W + min(W_new1 - W, 0) + min(W_new2 - W, 0).             
    W_new = W + min(W_new1 - W, 0) + min(W_new2 - W, 0).                
                                                                           
    Non-window-based congestion control mechanisms MUST behave          
    The details of how this is accomplished are specified in CCID          
    analogously.                                                        
    profile documents.  Non-window-based congestion control mechanisms     
11.8.2.  Particular Drop Codes                                          
    MUST behave analogously; again, CCID profiles define how.              
    Drop Code 0 ("protocol constraints") does not indicate any kind of  
                                                                           
    congestion, so the sender's CCID SHOULD react to non-marked packets 
11.7.2.  Particular Drop Codes                                             
    with Drop Code 0 as if they were received.  However, the sending    
                                                                           
    endpoint SHOULD NOT send data until it believes the protocol        
    Drop Code 0, Protocol Constraints, does not indicate any kind of       
    constraint isn't relevant any longer.                               
    congestion, so the sender's CCID SHOULD react to packets with Drop     
    Drop Code 1 ("application no longer listening") means the           
    Code 0 as if they were received (with or without ECN Congestion        
    application running at the endpoint that sent the option is no      
    Experienced marks, as appropriate).  However, the sending endpoint     
    longer listening for data.  For example, a server might close its   
    SHOULD NOT send data until it believes the protocol constraint no      
    receiving half-connection to new data after receiving a complete    
    longer applies.                                                        
    request from the client.  This would limit the amount of state      
                                                                           
    available at the server for incoming data, and thus reduce the      
    Drop Code 1, Application Not Listening, means the application          
    potential damage from certain denial-of-service attacks.  A Data    
    running at the endpoint that sent the option is no longer listening    
    Dropped option containing Drop Code 1 SHOULD be sent whenever       
    for data.  For example, a server might close its receiving half-       
    received data is ignored due to a non-listening application.  Once  
    connection to new data after receiving a complete request from the     
    an endpoint reports Drop Code 1 for a packet, it SHOULD report Drop 
    client.  This would limit the amount of state available at the         
    Code 1 for every succeeding data packet on that half-connection;    
    server for incoming data, and thus reduce the potential damage from    
    once an endpoint receives a Drop State 1 report, it SHOULD expect   
    certain denial-of-service attacks.  A Data Dropped option containing   
    that no more data will ever be delivered to the other endpoint's    
    Drop Code 1 SHOULD be sent whenever received data is ignored due to    
    application, so it SHOULD NOT send more data.                       
    a non-listening application.  Once an endpoint reports Drop Code 1     
    Drop Code 2 ("receive buffer drop") indicates congestion inside the 
    for a packet, it SHOULD report Drop Code 1 for every succeeding data   
    packet on that half-connection; once an endpoint receives a Drop       
    State 1 report, it SHOULD expect that no more data will ever be        
    delivered to the other endpoint's application, so it SHOULD NOT send   
    more data.                                                             
                                                                           
    Drop Code 2, Receive Buffer, indicates congestion inside the           
    receiving host.  For instance, if a drop-from-tail kernel socket       
    buffer is too full to accept a packet's application data, that         
    packet should be reported as Drop Code 2.  For a drop-from-head or     
    more complex socket buffer, the dropped packet should be reported as   
                                                                           
                                                                           
                                                                           
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    Drop Code 2.  DCCP implementations may also provide an API by which    
    applications can mark received packets as Drop Code 2, indicating      
    applications can mark received packets as Drop Code 2, incidicating 
    that the application ran out of space in its user-level receive        
    buffer.  (However, it is not generally useful to report packets as     
    dropped due to Drop Code 2 after more than a couple round-trip times   
    have passed.  The HC-Sender may have forgotten its acknowledgement     
    state for the packet by that time, so the Data Dropped report will     
    have no effect.)  Every packet newly acknowledged as Drop Code 2       
    SHOULD reduce the sender's instantaneous rate by one packet per        
    round-trip time.  Each CCID profile defines the CCID-specific          
    round trip time, using whatever mechanism is appropriate for the    
    mechanism by which this is accomplished.                               
    relevant CCID.  Further details may be available in CCID documents. 
                                                                           
    The other Drop Codes, namely Drop Code 3 ("corrupt"), Drop Code 7   
    Currently, the other Drop Codes, namely Drop Code 3, Corrupt, Drop     
    ("delivered corrupt"), and reserved Drop Codes 4-6, MUST currently  
    Code 7 Delivered Corrupt, and reserved Drop Codes 4-6, MUST cause      
    be treated like ECN Congestion Experienced marks.                   
    the relevant CCID to behave as if the relevant packets were ECN        
    marked (ECN Congestion Experienced).                                   
                                                                           
12.  Explicit Congestion Notification                                      
                                                                           
    The DCCP protocol is fully ECN-aware [RFC 3168].  Each CCID            
    specifies how its endpoints respond to ECN marks.  Furthermore,        
    DCCP, unlike TCP, allows senders to control the rate at which          
    acknowledgements are generated (with options like Ack Ratio); since    
    acknowledgements are generated (with options like Ack Ratio); this  
    acknowledgements are congestion-controlled, they also qualify as       
    means that acknowledgements are generally congestion-controlled, and
    ECN-Capable Transport.                                                 
    may have ECN-Capable Transport set.                                 
                                                                           
    A CCID profile describes how that CCID interacts with ECN, both for    
    data traffic and pure-acknowledgement traffic.  A sender SHOULD set    
    ECN-Capable Transport on its packets' IP headers, unless the           
    ECN-Capable Transport on its packets whenever the receiver has its  
    receiver's ECN Incapable feature is on or the relevant CCID            
    ECN Capable feature turned on and the relevant CCID allows it,      
    disallows it.                                                          
    unless the sending application indicates that ECN should not be     
                                                                           
    used.                                                               
    The rest of this section describes the ECN Incapable feature and the   
    The rest of this section describes the ECN Capable feature and the  
    interaction of the ECN Nonce with acknowledgement options such as      
    Ack Vector.                                                            
                                                                           
12.1.  ECN Incapable Feature                                               
12.1.  ECN Capable Feature                                              
                                                                           
    The ECN Capable feature lets a DCCP inform its peer that it cannot  
    DCCP endpoints are ECN-aware by default, but the ECN Incapable         
    read ECN bits from received IP headers, so the peer must not set    
    feature lets an endpoint reject the use of Explicit Congestion         
    ECN-Capable Transport on its packets.                               
    Notification.  The use of this feature is NOT RECOMMENDED.  ECN        
    ECN Capable has feature number 4, and is server-priority.  It takes 
    incapability both avoids ECN's possible benefits and prevents          
    one-byte Boolean values.  DCCP A MUST be able to read ECN bits from 
    senders from using the ECN Nonce to check for receiver misbehavior.    
    received frames' IP headers when ECN Capable/A is one.  (This is    
    A DCCP stack MAY therefore leave the ECN Incapable feature             
    independent of whether it can set ECN bits on sent frames.)  DCCP A 
    unimplemented, acting as if all connections were ECN capable.  It is   
    thus sends a "Change L(ECN Capable, 0)" option to DCCP B to inform  
    worth noting that the inappropriate firewall interactions that         
    it that A cannot read ECN bits.  New connections start with ECN     
    dogged TCP's implementation of ECN [RFC 3360] involve TCP header       
    Capable 1 (that is, ECN capable) for both endpoints.  Values of two 
    bits, not the IP header's ECN bits; we know of no middlebox that       
    or more are reserved.                                               
                                                                           
                                                                           
                                                                           
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    would block ECN-capable DCCP packets, but allow ECN-incapable DCCP     
    packets.                                                               
                                                                           
    ECN Incapable has feature number 4, and is server-priority.  It        
    takes one-byte Boolean values.  DCCP A MUST be able to read ECN bits   
    from received frames' IP headers when ECN Incapable/A is zero.         
    (This is independent of whether it can set ECN bits on sent frames.)   
    DCCP A thus sends a "Change L(ECN Inapable, 1)" option to DCCP B to    
    inform it that A cannot read ECN bits.  If the ECN Incapable/A         
    feature is one, then all of DCCP B's packets MUST be sent as ECN       
    incapable.  New connections start with ECN Incapable 0 (that is, ECN   
    capable) for both endpoints.  Values of two or more are reserved.      
                                                                           
    If a DCCP is not ECN capable, it MUST send Mandatory "Change L(ECN     
    Incapable, 1)" options to the other endpoint until acknowledged (by    
    Capable, 0)" options to the other endpoint until acknowledged (by   
    "Confirm R(ECN Incapable, 1)") or the connection closes.               
    "Confirm R(ECN Capable, 0)") or the connection closes.  Furthermore,
    Furthermore, it MUST NOT accept any data until the other endpoint      
    it MUST NOT accept any data until the other endpoint sends          
    sends "Confirm R(ECN Incapable, 1)".  It SHOULD send Data Dropped      
    "Confirm R(ECN Capable, 0)".  It SHOULD send Data Dropped options on
    options on its acknowledgements, with Drop Code 0 ("protocol           
    its acknowledgements, with Drop Code 0 ("protocol constraints"), if 
    constraints"), if the other endpoint does send data inappropriately.   
    the other endpoint does send data inappropriately.                  
                                                                           
12.2.  ECN Nonces                                                          
                                                                           
    Congestion avoidance will not occur, and the receiver will sometimes   
    get its data faster, if the sender isn't told about congestion         
    events.  Thus, the receiver has some incentive to falsify              
    acknowledgement information, reporting that marked or dropped          
    packets were actually received unmarked.  This problem is more         
    serious with DCCP than with TCP, since TCP provides reliable           
    transport: it is more difficult with TCP to lie about lost packets     
    without breaking the application.                                      
                                                                           
    ECN Nonces are a general mechanism to prevent ECN cheating (or loss    
    cheating).  Two values for the two-bit ECN header field indicate       
    ECN-Capable Transport, 01 and 10.  The second code point, 10, is the   
    ECN Nonce.  In general, a protocol sender chooses between these code   
    points randomly on its output packets, remembering the sequence it     
    chose.  The protocol receiver reports, on every acknowledgement, the   
    number of ECN Nonces it has received thus far.  This is called the     
    ECN Nonce Echo.  Since ECN marking and packet dropping both destroy    
    the ECN Nonce, a receiver that lies about an ECN mark or packet drop   
    has a 50% chance of guessing right and avoiding discipline.  The       
    sender may react punitively to an ECN Nonce mismatch, possibly up to   
    dropping the connection.  The ECN Nonce Echo field need not be an      
    integer; one bit is enough to catch 50% of infractions, and the        
    integer; one bit is enough to catch 50% of infractions.             
    probability of success drops exponentially as more bits are sent       
    [RFC 3540].                                                            
                                                                           
                                                                           
                                                                           
                                                                           
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    In DCCP, the ECN Nonce Echo field is encoded in acknowledgement        
    options.  For example, the Ack Vector option comes in two forms, Ack   
    Vector [Nonce 0] (option 38) and Ack Vector [Nonce 1] (option 39),     
    corresponding to the two values for a one-bit ECN Nonce Echo.  The     
    Nonce Echo for a given Ack Vector equals the one-bit sum (exclusive-   
    or, or parity) of ECN nonces for packets reported by that Ack Vector   
    as received and not ECN marked.  Thus, only packets marked as State    
    0 matter for this calculation (that is, valid received packets that    
    were not ECN marked).  Every Ack Vector option is detailed enough      
    for the sender to determine what the Nonce Echo should have been.      
    It can check this calculation against the actual Nonce Echo, and       
    complain if there is a mismatch.  (The Ack Vector could conceivably    
    report every packet's ECN Nonce state, but this would severely limit   
    Ack Vector's compressibility without providing much extra              
    protection.)                                                           
                                                                           
    Each DCCP sender SHOULD set ECN Nonces on its packets, and remember    
    Given an A-to-B half-connection, DCCP A SHOULD set ECN Nonces on its
    which packets had nonces.  When a sender detects an ECN Nonce Echo     
    packets, and remember which packets had nonces, whenever DCCP B     
    mismatch, it SHOULD behave as if the receiver had reported one or      
    reports that it is ECN Capable.  An ECN-capable endpoint MUST       
    more packets as ECN-marked (instead of unmarked).  It MAY take more    
    calculate and use the correct value for ECN Nonce Echo when sending 
    punitive action, such as resetting the connection with Reset Code      
    acknowledgement options.  An ECN-incapable endpoint, however, SHOULD
    11, "Aggression Penalty".  Each DCCP receiver MUST calculate and use   
    treat the ECN Nonce Echo as always zero.  When a sender detects an  
    the correct value for ECN Nonce Echo when sending acknowledgement      
    ECN Nonce Echo mismatch, it SHOULD behave as if the receiver had    
    options.                                                               
    reported one or more packets as ECN-marked (instead of unmarked).   
                                                                           
    It MAY take more punitive action, such as resetting the connection  
    ECN incapability, as indicated by the ECN Incapable feature, is        
    with Reset Code 11, "Aggression Penalty".                           
    handled as follows: An endpoint sending packets to an ECN-incapable    
    An ECN-incapable DCCP SHOULD ignore received ECN nonces and generate
    receiver MUST send its packets as ECN incapable, and an ECN-           
    ECN nonces of zero.  For instance, out of the two Ack Vector        
    incapable receiver MUST use the value zero for all ECN Nonce Echoes.   
    options, an ECN-incapable DCCP SHOULD generate Ack Vector [Nonce 0] 
                                                                           
    (option 38) exclusively.  (Again, the ECN Capable feature MUST be   
12.3.  Other Aggression Penalties                                          
    set to zero in this case.)                                          
                                                                           
    The ECN Nonce provides one way for a DCCP sender to discover that a    
    receiver is misbehaving.  There may be other mechanisms, and a         
    receiver or middlebox may also discover that a sender is misbehaving   
    -- sending more data than it should.  In any of these cases, the       
    entity that discovers the misbehavior MAY react by resetting the       
    connection with Reset Code 11, "Aggression Penalty".  A receiver       
    that detects marginal (meaning possibly spurious) sender misbehavior   
    MAY instead react with a Slow Receiver option, or by reporting some    
    packets as ECN marked that were not, in fact, marked.  A large of      
    packets as ECN marked that were not, in fact, marked.               
    range of alternate strategies are available, including priority        
    queueing, rate limiting, and so forth.                                 
                                                                           
13.  Timing Options                                                        
                                                                           
    The Timestamp, Timestamp Echo, and Elapsed Time options help DCCP      
    endpoints explicitly measure round-trip times.                         
                                                                           
                                                                           
                                                                           
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13.1.  Timestamp Option                                                    
                                                                           
    This option is permitted in any DCCP packet.  The length of the        
    option is 6 bytes.                                                     
                                                                           
    +--------+--------+--------+--------+--------+--------+                
    |00101001|00000110|          Timestamp Value          |                
    +--------+--------+--------+--------+--------+--------+                
     Type=41  Length=6                                                     
                                                                           
    The four bytes of option data carry the timestamp of this packet.      
    The four bytes of option data carry the timestamp of this packet in 
    The timestamp is a 32-bit integer that increases monotonically with    
    some undetermined form.  A DCCP receiving a Timestamp option SHOULD 
    time, at a rate of 1 unit per 10 microseconds.  At this rate,          
    respond with a Timestamp Echo option on the next packet it sends.   
    Timestamp Value will wrap approximately every 11.9 hours.  Endpoints   
    need not measure time at this fine granularity; for example, an        
    endpoint that preferred to measure time at millisecond granularity     
    might send Timestamp Values that were all multiples of 100.  The       
    precise time corresponding to Timestamp Value zero is not specified:   
    Timestamp Values are only meaningful relative to other Timestamp       
    Values sent on the same connection.  A DCCP receiving a Timestamp      
    option SHOULD respond with a Timestamp Echo option on the next         
    packet it sends.                                                       
                                                                           
13.2.  Elapsed Time Option                                                 
                                                                           
    This option is permitted in any DCCP packet that contains an           
    Acknowledgement Number (such options received on other packet types    
    Acknowledgement Number.  It indicates how much time, in tenths of   
    MUST be ignored).  It indicates how much time has elapsed, in          
    milliseconds, has elapsed since the packet being acknowledged -- the
    hundredths of milliseconds (or, equivalently, multiples of             
    packet with the given Acknowledgement Number -- was received.  The  
    10 microseconds), since the packet being acknowledged -- the packet    
    option may take 4 or 6 bytes, depending on the size of the Elapsed  
    with the given Acknowledgement Number -- was received.  The option     
    Time value.  Elapsed Time helps correct round-trip time estimates   
    may take 4 or 6 bytes, depending on the size of the Elapsed Time       
    when the gap between receiving a packet and acknowledging that      
    value.  Elapsed Time helps correct round-trip time estimates when      
    packet may be long -- in CCID 3, for example, where acknowledgements
    the gap between receiving a packet and acknowledging that packet may   
    are sent infrequently.                                              
    be long -- in CCID 3, for example, where acknowledgements are sent     
    infrequently.                                                          
                                                                           
    +--------+--------+--------+--------+                                  
    |00101011|00000100|   Elapsed Time  |                                  
    +--------+--------+--------+--------+                                  
     Type=43    Len=4                                                      
                                                                           
    +--------+--------+--------+--------+--------+--------+                
    |00101011|00000110|            Elapsed Time           |                
    +--------+--------+--------+--------+--------+--------+                
     Type=43    Len=6                                                      
                                                                           
    The option data, Elapsed Time, represents an estimated upper bound     
                                                                           
                                                                           
                                                                           
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    on the amount of time elapsed since the packet being acknowledged      
    was received, with units of tenths of milliseconds.  If Elapsed Time   
    is less than a half-second, the first, smaller form of the option      
    is less than a second, the first, smaller form of the option SHOULD 
    SHOULD be used.  Elapsed Times of more than 0.65535 seconds MUST be    
    be used.  Elapsed Times of more than 6.5535 seconds MUST be sent    
    sent using the second form of the option.  The special Elapsed Time    
    using the second form of the option.  DCCP endpoints MUST NOT report
    value 4294967295, which corresponds to approximately 11.9 hours, is    
    Elapsed Times that are significantly larger than the true elapsed   
    used to represent any Elapsed Time greater than 42949.67294 seconds.   
    times.  A connection MAY be reset with Reset Code 11, "Aggression   
    DCCP endpoints MUST NOT report Elapsed Times that are significantly    
    Penalty", if one endpoint determines that the other is reporting a  
    larger than the true elapsed times.  A connection MAY be reset with    
    much-too-large Elapsed Time.                                        
    Reset Code 11, "Aggression Penalty", if one endpoint determines that   
    Elapsed Time is measured in tenths of milliseconds as a compromise  
    the other is reporting a much-too-large Elapsed Time.                  
    between two conflicting goals.  First, it provides enough           
                                                                           
    Elapsed Time is measured in hundredths of milliseconds as a            
    compromise between two conflicting goals.  First, it provides enough   
    granularity to reduce rounding error when measuring elapsed time       
    over fast LANs; second, it allows many reasonable elapsed times to     
    over fast LANs; second, it allows most reasonable elapsed times to  
    fit into two bytes of data.                                            
                                                                           
13.3.  Timestamp Echo Option                                               
                                                                           
    This option is permitted in any DCCP packet, as long as at least one   
    packet carrying the Timestamp option has been received.  Generally,    
    a DCCP endpoint should send one Timestamp Echo option for each         
    Timestamp option it receives; and it should send that option as soon   
    as is convenient.  The length of the option is between 6 and 10        
    bytes, depending on whether Elapsed Time is included and how large     
    it is.                                                                 
                                                                           
    +--------+--------+--------+--------+--------+--------+                
    |00101010|00000110|           Timestamp Echo          |                
    +--------+--------+--------+--------+--------+--------+                
     Type=42    Len=6                                                      
                                                                           
    +--------+--------+------- ... -------+--------+--------+              
    |00101010|00001000|  Timestamp Echo   |   Elapsed Time  |              
    +--------+--------+------- ... -------+--------+--------+              
     Type=42    Len=8       (4 bytes)                                      
                                                                           
    +--------+--------+------- ... -------+------- ... -------+            
    |00101010|00001010|  Timestamp Echo   |    Elapsed Time   |            
    +--------+--------+------- ... -------+------- ... -------+            
     Type=42   Len=10       (4 bytes)           (4 bytes)                  
                                                                           
    The first four bytes of option data, Timestamp Echo, carry a           
    Timestamp Value taken from a preceding received Timestamp option.      
    Usually, this will be the last packet that was received -- the         
    packet indicated by the Acknowledgement Number, if any -- but it       
    might be a preceding packet.  Each Timestamp received will generally   
    might be a preceding packet.                                        
                                                                           
                                                                           
                                                                           
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    result in exactly one Timestamp Echo transmitted.  If an endpoint      
    has received multiple Timestamp options since the last time it sent    
    a packet, then it MAY ignore all Timestamp options but the one         
    included on the packet with the greatest sequence number;              
    alternatively, it MAY include multiple Timestamp Echo options in its   
    response, each corresponding to a different Timestamp option.          
                                                                           
    The Elapsed Time value, similar to that in the Elapsed Time option,    
    indicates the amount of time elapsed since receiving the packet        
    whose timestamp is being echoed.  This time MUST be in hundredths of   
    whose timestamp is being echoed.  This time MUST be in tenths of    
    milliseconds.  Elapsed Time is meant to help the Timestamp sender      
    separate the network round-trip time from the Timestamp receiver's     
    processing time.  This may be particularly important for CCIDs where   
    acknowledgements are sent infrequently, so that there might be         
    considerable delay between receiving a Timestamp option and sending    
    the corresponding Timestamp Echo.  A missing Elapsed Time field is     
    equivalent to an Elapsed Time of zero.  The smallest version of the    
    option SHOULD be used that can hold the relevant Elapsed Time value.   
                                                                           
14.  Maximum Packet Size                                                   
                                                                           
    A DCCP implementation MUST maintain the maximum packet size (MPS)      
    allowed for each active DCCP session.  The MPS is influenced by the    
    maximum packet size allowed by the current congestion control          
    mechanism (CCMPS), the maximum packet size supported by the path's     
    links (PMTU, the Path Maximum Transmission Unit) [RFC 1191], and the   
    links (PMTU, the Path Maximum Transfer Unit) [RFC 1191], and the    
    lengths of the IP and DCCP headers.                                    
                                                                           
    A DCCP application interface SHOULD let the application discover       
    A DCCP application interface should let the application discover    
    DCCP's current MPS.  Generally, the DCCP implementation will refuse    
    DCCP's current MPS.  DCCP applications should use the API to        
    to send any packet bigger than the MPS, returning an appropriate       
    discover the MPS.  Generally, the DCCP implementation will refuse to
    error to the application.  A DCCP interface MAY allow applications     
    send any packet bigger than the MPS, returning an appropriate error 
    to request fragmentation for packets larger than PMTU, but not         
    to the application.                                                 
    larger than CCMPS (packets larger than CCMPS MUST be rejected in any   
    A DCCP interface may allow applications to request that packets     
    case).  Fragmentation SHOULD NOT be the default, since it decreases    
    larger than PMTU be fragmented on IPv4 networks.  This only matters 
    robustness: an entire packet is discarded if even one of its           
    when CCMPS > PMTU; packets larger than CCMPS MUST be rejected       
    fragments is lost.  Applications can usually get better error          
    regardless.  Fragmentation should not be the default.  The rest of  
    tolerance by producing packets smaller than the PMTU.                  
    this section assumes the application has not requested              
                                                                           
    fragmentation.                                                      
    The MPS reported to the application SHOULD be influenced by the size   
    expected to be required for DCCP headers and options.  If the          
    application provides data that, when combined with the options the     
    DCCP implementation would like to include, would exceed the MPS, the   
    implementation should either send the options on a separate packet     
    (such as a DCCP-Ack) or lower the MPS, drop the data, and return an    
    appropriate error to the application.                                  
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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14.1.  Measuring PMTU                                                      
    The PMTU SHOULD be initialized from the interface MTU that will be  
                                                                           
    used to send packets.  The MPS will be initialized with the minimum 
    Each DCCP endpoint MUST keep track of the current PMTU for each        
    of the PMTU and the CCMPS, if any.                                  
    connection, except that this is not required for IPv4 connections      
    To perform classical PMTU discovery, the DCCP sender sets the IP    
    whose applications have requested fragmentation.  The PMTU SHOULD be   
    Don't Fragment (DF) bit.  However, it is undesirable for MTU        
    initialized from the interface MTU that will be used to send           
    discovery to occur on the initial connection setup handshake, as the
    packets.  The MPS will be initialized with the minimum of the PMTU     
    connection setup process may not be representative of packet sizes  
    and the CCMPS, if any.                                                 
    used during the connection, and performing MTU discovery on the     
                                                                           
    initial handshake might unnecessarily delay connection              
    Classical PMTU discovery uses unfragmentable packets.  In IPv4,        
    establishment.  Thus, DF SHOULD NOT be set on DCCP-Request and DCCP-
    these packets have the IP Don't Fragment (DF) bit set; in IPv6, all    
    Response packets. In addition DF SHOULD NOT be set on DCCP-Reset    
    packets are unfragmentable.  As specified in [RFC 1191], when a        
    packets, although typically these would be small enough to not be a 
    router receives a packet with DF set that is larger than the next      
    problem.  On all other DCCP packets, DF SHOULD be set.              
    link's MTU, it sends an ICMP Destination Unreachable message back to   
    As specified in [RFC 1191], when a router receives a packet with DF 
    the source whose Code indicates that an unfragmentable packet was      
    set that is larger than the next link's MTU, it sends an ICMP       
    too large to forward (a "Datagram Too Big" message).  When a DCCP      
    Destination Unreachable message to the source of the datagram with  
    implementation receives a Datagram Too Big message, it decreases its   
    the Code indicating "fragmentation needed and DF set" (also known as
    PMTU to the Next-Hop MTU value given in the ICMP message.  If the      
    a "Datagram Too Big" message).  When a DCCP implementation receives 
    MTU given in the message is zero, the sender chooses a value for       
    a Datagram Too Big message, it decreases its PMTU to the Next-Hop   
    PMTU using the algorithm described in Section 7 of [RFC 1191].  If     
    MTU value given in the ICMP message.  If the MTU given in the       
    the MTU given in the message is greater than the current PMTU, the     
    message is zero, the sender chooses a value for PMTU using the      
    Datagram Too Big message is ignored, as described in [RFC 1191].       
    algorithm described in Section 7 of [RFC 1191].  If the MTU given in
    (We are aware that this may cause problems for DCCP endpoints behind   
    the message is greater than the current PMTU, the Datagram Too Big  
    certain firewalls.)                                                    
    message is ignored, as described in [RFC 1191].  (We are aware that 
                                                                           
    this may cause problems for DCCP endpoints behind certain           
    A DCCP implementation may allow the application to occasionally        
    firewalls.)                                                         
    request that PMTU discovery be performed again.  This will reset the   
    If the DCCP implementation has decreased the PMTU, and the sending  
    PMTU to the outgoing interface's MTU.  Such requests SHOULD be rate    
    application attempts to send a packet larger than the new MPS, the  
    limited, to one per two seconds, for example.                          
    API must refuse to send the packet and return an appropriate error  
                                                                           
    to the application.  The application should then use the API to     
    A DCCP sender MAY treat the reception of an ICMP Datagram Too Big      
    query the new value of MPS.  The kernel might have some packets     
    message as an indication that the packet being reported was not lost   
    buffered for transmission that are smaller than the old MPS, but    
    larger than the new MPS.  It MAY send these packets with the DF bit 
    cleared, or it MAY discard these packets; it MUST NOT transmit these
    datagrams with the DF bit set.                                      
    due to congestion, and so for the purposes of congestion control it    
    due congestion, and so for the purposes of congestion control it MAY
    MAY ignore the DCCP receiver's indication that this packet did not     
    ignore the DCCP receiver's indication that this packet did not      
    arrive.  However, if this is done, then the DCCP sender MUST check     
    the ECN bits of the IP header echoed in the ICMP message, and only     
    perform this optimization if these ECN bits indicate that the packet   
    did not experience congestion prior to reaching the router whose       
    link MTU it exceeded.                                                  
                                                                           
    A DCCP implementation SHOULD ensure, as far as possible, that ICMP     
    Datagram Too Big messages were actually generated by routers, so       
    that attackers cannot drive the PMTU down to a falsely small value.    
    The simplest way to do this is to verify that the Sequence Number on   
    the ICMP error's encapsulated header corresponds to a Sequence         
    Number that the implementation recently sent.  (Routers are not        
    required to return more than 64 bits of the DCCP header [RFC 792],     
    but most modern routers will return far more, including the Sequence   
                                                                           
                                                                           
                                                                           
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    Number.)  ICMP Datagram Too Big messages with incorrect or missing     
    Sequence Numbers may be ignored, or the DCCP implementation may        
    lower the PMTU only temporarily in response.  If more than three odd   
    Datagram Too Big messages are received and the other DCCP endpoint     
    reports more than three lost packets, however, the DCCP                
    reports commensurate loss, however, the DCCP implementation SHOULD  
    implementation SHOULD assume the presence of a confused router, and    
    assume the presence of a confused router, and either obey the ICMP  
    either obey the ICMP messages' PMTU or (on IPv4 networks) switch to    
    messages' PMTU or (on IPv4 networks) switch to allowing             
    allowing fragmentation.                                                
    fragmentation.                                                      
                                                                           
    DCCP also allows upward probing of the PMTU [PMTUD], where the DCCP    
    endpoint begins by sending small packets with DF set, then gradually   
    increases the packet size until a packet is lost.  This mechanism      
    does not require any ICMP error processing.  DCCP-Sync packets are     
    the best choice for upward probing, since DCCP-Sync probes do not      
    risk application data loss.  The DCCP implementation inserts           
    arbitrary data into the DCCP-Sync application area, padding the        
    packet to the right length; and since every valid DCCP-Sync            
    generates an immediate DCCP-SyncAck in response, the endpoint will     
    have a pretty good idea of when a probe is lost.                       
                                                                           
14.2.  Sender Behavior                                                     
                                                                           
    A DCCP sender SHOULD send every packet as unfragmentable, as           
    described above, with the following exceptions.                        
                                                                           
    o  On IPv4 connections whose applications have requested               
       fragmentation, the sender SHOULD send packets with the DF bit not   
       set.                                                                
                                                                           
    o  On IPv6 connections whose applications have requested               
       fragmentation, the sender SHOULD use fragmentation extension        
       headers to fragment packets larger than PMTU into suitably-sized    
       chunks.  (Those chunks are, of course, unfragmentable.)             
                                                                           
    o  It is undesirable for PMTU discovery to occur on the initial        
       connection setup handshake, as the connection setup process may     
       not be representative of packet sizes used during the connection,   
       and performing MTU discovery on the initial handshake might         
       unnecessarily delay connection establishment.  Thus, DCCP-Request   
       and DCCP-Response packets SHOULD be sent as fragmentable.  In       
       addition, DCCP-Reset packets SHOULD be sent as fragmentable,        
       although typically these would be small enough to not be a          
       problem.  For IPv4 connections, these packets SHOULD be sent with   
       the DF bit not set; for IPv6 connections, they SHOULD be            
       preemptively fragmented to a size not larger than the relevant      
       interface MTU.                                                      
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
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    However, applications are not advised                                  
                                                                           
    If the DCCP implementation has decreased the PMTU, the sending         
    application has not requested fragmentation, and the sending           
    application attempts to send a packet larger than the new MPS, the     
    API MUST refuse to send the packet and return an appropriate error     
    to the application.  The application should then use the API to        
    query the new value of MPS.  The kernel might have some packets        
    buffered for transmission that are smaller than the old MPS, but       
    larger than the new MPS.  It MAY send these packets as fragmentable,   
    or it MAY discard these packets; it MUST NOT send them as              
    unfragmentable.                                                        
                                                                           
15.  Forward Compatibility                                                 
                                                                           
    Future versions of DCCP may add new options and features.  A few       
    simple guidelines will let extended DCCPs interoperate with normal     
    DCCPs.                                                                 
                                                                           
    o  DCCP processors MUST NOT act punitively towards options and         
       features they do not understand.  For example, DCCP processors      
       MUST NOT reset the connection if some field marked Reserved in      
       this specification is non-zero; if some unknown option is           
       present; or if some feature negotiation option mentions an          
       unknown feature.  Instead, DCCP processors MUST ignore these        
       events.  The Mandatory option is the single exception: if           
       Mandatory precedes some unknown option or feature, the connection   
       MUST be reset.                                                      
                                                                           
    o  DCCP processors MUST anticipate the possibility of unknown          
       feature values, which might occur as part of a negotiation for a    
       known feature.  For server-priority features, unknown values are    
       handled as a matter of course: since the non-extended DCCP's        
       priority list will not contain unknown values, the result of the    
       negotiation cannot be an unknown value.  A DCCP SHOULD respond      
       with an empty Confirm option if it is assigned an unacceptable      
       value for some non-negotiable feature.                              
                                                                           
    o  Each DCCP extension SHOULD be controlled by some feature.  The      
       default value of this feature should correspond to "extension not   
       available".  If an extended DCCP wants to use the extension, it     
       SHOULD attempt to change the feature's value using a Change L or    
       Change R option.  Any non-extended DCCP will ignore the option,     
       thus leaving the feature value at its default, "extension not       
       available".                                                         
                                                                           
    Section 19 lists DCCP assigned numbers reserved for experimental and   
    testing purposes.                                                      
                                                                           
                                                                           
                                                                           
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16.  Middlebox Considerations                                              
                                                                           
    This section describes properties of DCCP that firewalls, network      
    address translators, and other middleboxes should consider,            
    including parts of the packet that middleboxes should not change.      
    The intent is to draw attention to aspects of DCCP that may be         
    useful, or dangerous, for middleboxes, or that differ significantly    
    from TCP.                                                              
                                                                           
    The Service Code field in DCCP-Request packets provides information    
    The Service Code field in DCCP-Request packets provide information  
    that may be useful for stateful middleboxes.  With Service Code, a     
    middlebox can tell what protocol a connection will use without         
    relying on port numbers.  Middleboxes can disallow connections that    
    relying on port numbers.  Middleboxes can disallow attempted        
    attempt to access unexpected services by sending a DCCP-Reset with     
    connections accessing unexpected services by sending a DCCP-Reset   
    Reset Code 8, "Bad Service Code".  Middleboxes should not modify the   
    with Reset Code 8, "Bad Service Code".  Middleboxes probably        
    Service Code unless they are really changing the service a             
    shouldn't modify the Service Code, unless they are really changing  
    connection is accessing.                                               
    the service a connection is accessing.                              
                                                                           
    The Source and Destination Port fields are in the same packet          
    locations as the corresponding fields in TCP and UDP, which may        
    simplify some middlebox implementations.                               
                                                                           
    The forward compatibility considerations in Section 15 apply to        
    middleboxes as well.  In particular, middleboxes generally shouldn't   
    act punitively towards options and features they do not understand.    
                                                                           
    Modifying DCCP Sequence Numbers and Acknowledgement Numbers is more    
    tedious and dangerous than modifying TCP sequence numbers.  A          
    middlebox that added packets to, or removed packets from, a DCCP       
    connection would have to modify acknowledgement options, such as Ack   
    Vector, and CCID-specific options, such as TFRC's Loss Intervals, at   
    minimum.  On ECN-capable connections, the middlebox would have to      
    keep track of ECN Nonce information for packets it introduced or       
    removed, so that the relevant acknowledgement options continued to     
    have correct ECN Nonce Echoes, or risk the connection being reset      
    for "Aggression Penalty".  We therefore recommend that middleboxes     
    not modify packet streams by adding or removing packets.               
                                                                           
    Note that there is less need to modify DCCP's per-packet sequence      
    numbers than TCP's per-byte sequence numbers; for example, a           
    middlebox can change the contents of a packet without changing its     
    sequence number.  (In TCP, sequence number modification is required    
    to support protocols like FTP that carry variable-length addresses     
    in the data stream.  If such an application were deployed over DCCP,   
    middleboxes would simply grow or shrink the relevant packets as        
    necessary, without changing their sequence numbers.  This might        
    involve fragmenting the packet.)                                       
                                                                           
                                                                           
                                                                           
                                                                           
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    Middleboxes may, of course, reset connections in progress.  Clearly    
    this requires inserting a packet into one or both packet streams,      
    but the difficult issues do not arise.                                 
                                                                           
    DCCP is somewhat unfriendly to "connection splicing" [SHHP00], in      
    which clients' connection attempts are intercepted, but possibly       
    later "spliced in" to external server connections via sequence         
    number manipulations.  A connection splicer at minimum would have to   
    ensure that the spliced connections agreed on all relevant feature     
    values, which might take some renegotiation.                           
                                                                           
    The contents of this section should not be interpreted as a            
    wholesale endorsement of stateful middleboxes.                         
                                                                           
17.  Relations to Other Specifications                                     
                                                                           
17.1.  RTP                                                                 
17.1.  DCCP and RTP                                                     
                                                                           
    The Real-Time Transport Protocol, RTP [RFC 3550], is currently used    
    over UDP by many of DCCP's target applications (for instance,          
    streaming media).  Therefore, it is important to examine the           
    relationship between DCCP and RTP, and in particular, the question     
    of whether any changes in RTP are necessary or desirable when it is    
    layered over DCCP instead of UDP.                                      
                                                                           
    There are two potential sources of overhead in the RTP-over-DCCP       
    combination, duplicated acknowledgement information and duplicated     
    sequence numbers.  Together, these sources of overhead add slightly    
    more than 4 bytes per packet relative to RTP-over-UDP, and that        
    eliminating the redundancy would not reduce the overhead.              
                                                                           
    First, consider acknowledgements.  Both RTP and DCCP report feedback   
    about loss rates to data senders, via RTP Control Protocol Sender      
    about loss rates to data senders, via Real-Time Control Protocol    
    and Receiver Reports (RTCP SR/RR packets) and via DCCP                 
    Sender and Receiver Reports (RTCP SR/RR packets) and via DCCP       
    acknowledgement options.  These feedback mechanisms are potentially    
    redundant.  However, RTCP SR/RR packets contain information not        
    present in DCCP acknowledgements, such as "interarrival jitter", and   
    DCCP's acknowledgements contain information not transmitted by RTCP,   
    such as the ECN Nonce Echo.  Neither feedback mechanism makes the      
    other redundant.                                                       
                                                                           
    Sending both types of feedback need not be particularly costly         
    either.  RTCP reports may be sent relatively infrequently: once        
    every 5 seconds on average, for low-bandwidth flows.  In DCCP, some    
    every 5 seconds, for low-bandwidth flows.  In DCCP, some feedback   
    feedback mechanisms are expensive -- Ack Vector, for example, is       
    mechanisms are expensive -- Ack Vector, for example, is frequent and
    frequent and verbose -- but others are relatively cheap: CCID 3        
    verbose -- but others are relatively cheap: CCID 3 (TFRC)           
    (TFRC) acknowledgements take between 16 and 32 bytes of options sent   
    acknowledgements take between 16 and 32 bytes of options sent once  
    once per round-trip time.  (Reporting less frequently than once per    
    per round trip time.  (Reporting less frequently than once per RTT  
                                                                           
    would make congestion control less responsive to loss.)  We         
                                                                           
                                                                           
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    RTT would make congestion control less responsive to loss.)  We        
    therefore conclude that acknowledgement overhead in RTP-over-DCCP      
    need not be significantly higher than for RTP-over-UDP, at least for   
    CCID 3.                                                                
                                                                           
    One clear redundancy can be addressed at the application level.  The   
    verbose packet-by-packet loss reports sent in RTCP Extended Reports    
    Loss RLE Blocks [RFC 3611] can be derived from DCCP's Ack Vector       
    options.  (The converse is not true, since Loss RLE Blocks contain     
    no ECN information.)  Since DCCP implementations should provide an     
    API for application access to Ack Vector information, RTP-over-DCCP    
    applications might request either DCCP Ack Vectors or RTCP Extended    
    Report Loss RLE Blocks, but not both.                                  
                                                                           
    Now consider sequence number redundancy on data packets.  The          
    embedded RTP header contains a 16-bit RTP sequence number.  Most       
    data packets will use the DCCP-Data type; DCCP-DataAck and DCCP-Ack    
    packets need not usually be sent.  The DCCP-Data header is 12 bytes    
    long without options, including a 24-bit sequence number.  This is 4   
    bytes more than a UDP header.  Any options required on data packets    
    would add further overhead, although many CCIDs (for instance, CCID    
    3, TFRC) don't require options on most data packets.                   
                                                                           
    The DCCP sequence number cannot be inferred from the RTP sequence      
    number since it increments on non-data packets as well as data         
    packets.  The RTP sequence number cannot be inferred from the DCCP     
    sequence number either [RFC 3550].  Furthermore, removing RTP's        
    sequence number either; for instance, RTP sequence numbers might be 
    sequence number would not save any header space because of alignment   
    sent out of order.  Furthermore, removing RTP's sequence number     
    issues.  We therefore recommend that RTP transmitted over DCCP use     
    would not save any header space because of alignment issues.  We    
    the same headers currently defined.  The 4 byte header cost is a       
    therefore recommend that RTP transmitted over DCCP use the same     
    reasonable tradeoff for DCCP's congestion control features and         
    headers currently defined.  The 4 byte header cost is a reasonable  
    access to ECN.  Truly bandwidth-starved endpoints should use some      
    tradeoff for DCCP's congestion control features and access to ECN.  
    future header compression scheme.                                      
    Truly bandwidth-starved endpoints should use header compression.    
                                                                           
17.2.  Multiplexing Issues                                              
17.2.  Congestion Manager and Multiplexing                                 
                                                                           
    Since DCCP doesn't provide reliable, ordered delivery, multiple        
    application sub-flows may be multiplexed over a single DCCP            
    connection with no inherent performance penalty.  Thus, there is no    
    need for DCCP to provide built-in, SCTP-style support for multiple     
    sub-flows.                                                             
                                                                           
    Some applications might want to share congestion control state among   
    multiple DCCP flows that share the same source and destination         
    addresses.  This functionality could be provided by the Congestion     
    Manager [RFC 3124], a generic multiplexing facility.  However, the     
    CM would not fully support DCCP without change; it does not            
    gracefully handle multiple congestion control mechanisms, for          
                                                                           
                                                                           
                                                                           
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    example.                                                               
                                                                           
18.  Security Considerations                                               
                                                                           
    DCCP does not provide cryptographic security guarantees.               
    Applications desiring hard security should use IPsec or end-to-end     
    security of some kind.                                                 
                                                                           
    Nevertheless, DCCP is intended to protect against some classes of      
    attackers: Attackers cannot hijack a DCCP connection (close the        
    connection unexpectedly, or cause attacker data to be accepted by an   
    endpoint as if it came from the sender) unless they can guess valid    
    sequence numbers.  Thus, as long as endpoints choose initial           
    sequence numbers well, a DCCP attacker must snoop on data packets to   
    get any reasonable probability of success.  Sequence number validity   
    checks provide this guarantee.  Section 7.5.5 describes sequence       
    number security further.                                               
                                                                           
    This security property only holds assuming that DCCP's random          
    numbers are chosen according to the guidelines in [RFC 1750].          
                                                                           
    DCCP provides no protection against attackers that can snoop on data   
    packets.                                                               
                                                                           
18.1.  Security Considerations for Partial Checksums                       
                                                                           
    The partial checksum facility has a separate security impact,          
    particularly in its interaction with authentication and encryption     
    mechanisms.  The impact is the same in DCCP as in the UDP-Lite         
    protocol, and what follows was adapted from the corresponding text     
    in the UDP-Lite specification [RFC 3828].                              
                                                                           
    When a DCCP packet's Checksum Coverage field is not zero, the          
    uncovered portion of a packet may change in transit.  This is          
    contrary to the idea behind most authentication mechanisms:            
    authentication succeeds if the packet has not changed in transit.      
    Unless authentication mechanisms that operate only on the sensitive    
    part of packets are developed and used, authentication will always     
    fail for partially-checksummed DCCP packets whose uncovered part has   
    been damaged.                                                          
                                                                           
    The IPsec integrity check (Encapsulation Security Protocol, ESP, or    
    Authentication Header, AH) is applied (at least) to the entire IP      
    packet payload.  Corruption of any bit within that area will then      
    result in the IP receiver discarding a DCCP packet, even if the        
    corruption happened in an uncovered part of the DCCP application       
    data.                                                                  
                                                                           
                                                                           
                                                                           
                                                                           
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    When IPsec is used with ESP payload encryption, a link can not         
    determine the specific transport protocol of a packet being            
    forwarded by inspecting the IP packet payload.  In this case, the      
    link MUST provide a standard integrity check covering the entire IP    
    packet and payload.  DCCP partial checksums provide no benefit in      
    this case.                                                             
                                                                           
    Encryption (e.g., at the transport or application levels) may be       
    used.  Note that omitting an integrity check can, under certain        
    circumstances, compromise confidentiality [BEL98].                     
                                                                           
    If a few bits of an encrypted packet are damaged, the decryption       
    transform will typically spread errors so that the packet becomes      
    too damaged to be of use.  Many encryption transforms today exhibit    
    this behavior.  There exist encryption transforms, stream ciphers,     
    which do not cause error propagation.  Proper use of stream ciphers    
    can be quite difficult, especially when authentication-checking is     
    omitted [BB01].  In particular, an attacker can cause predictable      
    changes to the ultimate plaintext, even without being able to          
    decrypt the ciphertext.                                                
                                                                           
19.  IANA Considerations                                                   
                                                                           
    DCCP introduces eight sets of numbers whose values should be           
    DCCP introduces several sets of numbers whose values should be      
    allocated by IANA.  We refer to allocation policies, such as           
    allocated by IANA.  Following the policies outlined in [RFC 2434],  
    Standards Action, outlined in [RFC 2434], and most registries          
    the following sets of numbers are allocated through an IETF         
    reserve some values for experimental and