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RED -- Click element;
drops packets according to RED
RED(MIN_THRESH, MAX_THRESH, MAX_P [, KEYWORDS])
Ports: 1 input, 1-2 outputs
Processing: agnostic, but output 1 is push
Package: aqm (core)
Implements the Random Early Detection packet dropping
algorithm.
A RED element is associated with one or more Storage elements (usually
Queues). It maintains a running average of the sum of the queue lengths, and
marks packets with a probability proportional to that sum. By default, the
Queues are found with flow-based router context. If the RED is a push element,
it uses the nearest downstream Queues; if it is a pull element, it uses the
nearest upstream Queues.
Marked packets are dropped, or emitted on output 1 if RED has two output
ports.
Arguments are:
- MIN_THRESH
- Integer. When the average queue length less than or equal to MIN_THRESH,
input packets are never marked.
- MAX_THRESH
- Integer; must be greater than or equal to MIN_THRESH. When the average queue
length equals MAX_THRESH, input packets are marked with probability MAX_P.
When the average queue length is between MIN_THRESH and MAX_THRESH, input
packets are marked with probability linearly varying from 0 to MAX_P. For
behavior above MAX_THRESH, see the SIMPLE argument.
- MAX_P
- Real number between 0 and 1. The probability of dropping a packet if the
average queue length equals MAX_THRESH.
- QUEUES
- This argument is a space-separated list of Storage element names. RED will use
those elements' queue lengths, rather than any elements found via flow-based
router context.
- STABILITY
- Unsigned. This number determines how stable the average queue size is -- that
is, how quickly it changes due to fluctuations in the instantaneous queue
size. Higher numbers mean more stability. The corresponding conventional RED
parameter is w_q; STABILITY equals -log_2(w_q).
STABILITY should equal
log_2 (1 - e^(-1/K)),
where K is the link bandwidth in packets per second. Default STABILITY is 4.
This is very low (unstable) for most purposes; it corresponds to a link
bandwidth of roughly 15 packets per second, or a w_q of 0.25. The NS default
setting for w_q is 0.002, corresponding to a STABILITY of roughly 9.
A STABILITY of 0 means always use the instantaneous queue length.
- GENTLE
- Boolean. If true (the default), implement the Gentle RED variant first
proposed by Sally Floyd in October 1997. In this variant, when the average
queue length is between MAX_THRESH and 2*MAX_THRESH, input packets are dropped
with probability linearly varying from MAX_P to 100%; at lengths above
2*MAX_THRESH all packets are dropped. If GENTLE is false, then at lengths
above MAX_THRESH all packets are dropped.
... -> RED(5, 50, 0.02) -> Queue(200) -> ...
- min_thresh (read/write)
- Returns or sets the MIN_THRESH configuration parameter.
- max_thresh (read/write)
- Returns or sets the MAX_THRESH configuration parameter.
- max_p (read/write)
- Returns or sets the MAX_P configuration parameter.
- drops (read-only)
- Returns the number of packets dropped so far.
- queues (read-only)
- Returns the Queues associated with this RED element, listed one per line.
- avg_queue_size (read-only)
- Returns the current average queue size.
- stats (read-only)
- Returns some human-readable statistics.
AdaptiveRED, Queue
Sally Floyd and Van Jacobson. Random Early Detection Gateways for
Congestion Avoidance. ACM Transactions on Networking, 1(4), August
1993, pp 397-413.
Sally Floyd. "Optimum functions for computing the drop
probability", October 1997. http://www.icir.org/floyd/REDfunc.txt.
Generated by 'click-elem2man' from '../elements/aqm/red.hh' on 28/Feb/2010.