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161 lines
4.6 KiB
Groff
161 lines
4.6 KiB
Groff
.TH RED 8 "13 December 2001" "iproute2" "Linux"
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.SH NAME
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red \- Random Early Detection
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.SH SYNOPSIS
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.B tc qdisc ... red
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.B limit
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bytes
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.B [ min
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bytes
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.B ] [ max
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bytes
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.B ] avpkt
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bytes
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.B [ burst
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packets
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.B ] [ ecn ] [ harddrop] [ bandwidth
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rate
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.B ] [ probability
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chance
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.B ] [ adaptive ]
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.SH DESCRIPTION
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Random Early Detection is a classless qdisc which manages its queue size
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smartly. Regular queues simply drop packets from the tail when they are
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full, which may not be the optimal behaviour. RED also performs tail drop,
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but does so in a more gradual way.
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Once the queue hits a certain average length, packets enqueued have a
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configurable chance of being marked (which may mean dropped). This chance
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increases linearly up to a point called the
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.B max
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average queue length, although the queue might get bigger.
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This has a host of benefits over simple taildrop, while not being processor
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intensive. It prevents synchronous retransmits after a burst in traffic,
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which cause further retransmits, etc.
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The goal is to have a small queue size, which is good for interactivity
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while not disturbing TCP/IP traffic with too many sudden drops after a burst
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of traffic.
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Depending on if ECN is configured, marking either means dropping or
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purely marking a packet as overlimit.
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.SH ALGORITHM
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The average queue size is used for determining the marking
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probability. This is calculated using an Exponential Weighted Moving
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Average, which can be more or less sensitive to bursts.
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When the average queue size is below
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.B min
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bytes, no packet will ever be marked. When it exceeds
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.B min,
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the probability of doing so climbs linearly up
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to
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.B probability,
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until the average queue size hits
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.B max
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bytes. Because
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.B probability
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is normally not set to 100%, the queue size might
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conceivably rise above
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.B max
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bytes, so the
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.B limit
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parameter is provided to set a hard maximum for the size of the queue.
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.SH PARAMETERS
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.TP
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min
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Average queue size at which marking becomes a possibility. Defaults to
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.B max
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/3
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.TP
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max
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At this average queue size, the marking probability is maximal. Should be at
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least twice
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.B min
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to prevent synchronous retransmits, higher for low
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.B min.
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Default to
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.B limit
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/4
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.TP
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probability
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Maximum probability for marking, specified as a floating point
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number from 0.0 to 1.0. Suggested values are 0.01 or 0.02 (1 or 2%,
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respectively). Default : 0.02
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.TP
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limit
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Hard limit on the real (not average) queue size in bytes. Further packets
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are dropped. Should be set higher than max+burst. It is advised to set this
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a few times higher than
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.B max.
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.TP
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burst
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Used for determining how fast the average queue size is influenced by the
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real queue size. Larger values make the calculation more sluggish, allowing
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longer bursts of traffic before marking starts. Real life experiments
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support the following guideline: (min+min+max)/(3*avpkt).
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.TP
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avpkt
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Specified in bytes. Used with burst to determine the time constant for
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average queue size calculations. 1000 is a good value.
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.TP
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bandwidth
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This rate is used for calculating the average queue size after some
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idle time. Should be set to the bandwidth of your interface. Does not mean
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that RED will shape for you! Optional. Default : 10Mbit
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.TP
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ecn
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As mentioned before, RED can either 'mark' or 'drop'. Explicit Congestion
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Notification allows RED to notify remote hosts that their rate exceeds the
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amount of bandwidth available. Non-ECN capable hosts can only be notified by
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dropping a packet. If this parameter is specified, packets which indicate
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that their hosts honor ECN will only be marked and not dropped, unless the
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queue size hits
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.B limit
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bytes. Recommended.
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.TP
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harddrop
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If average flow queue size is above
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.B max
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bytes, this parameter forces a drop instead of ecn marking.
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.TP
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adaptive
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(Added in linux-3.3) Sets RED in adaptive mode as described in http://icir.org/floyd/papers/adaptiveRed.pdf
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.nf
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Goal of Adaptive RED is to make 'probability' dynamic value between 1% and 50% to reach the target average queue :
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.B (max - min) / 2
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.fi
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.SH EXAMPLE
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.P
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# tc qdisc add dev eth0 parent 1:1 handle 10: red
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limit 400000 min 30000 max 90000 avpkt 1000
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burst 55 ecn adaptive bandwidth 10Mbit
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.SH SEE ALSO
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.BR tc (8),
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.BR tc-choke (8)
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.SH SOURCES
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.TP
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o
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Floyd, S., and Jacobson, V., Random Early Detection gateways for
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Congestion Avoidance. http://www.aciri.org/floyd/papers/red/red.html
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.TP
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o
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Some changes to the algorithm by Alexey N. Kuznetsov.
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.TP
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o
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Adaptive RED : http://icir.org/floyd/papers/adaptiveRed.pdf
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.SH AUTHORS
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Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>, Alexey Makarenko
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<makar@phoenix.kharkov.ua>, J Hadi Salim <hadi@nortelnetworks.com>,
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Eric Dumazet <eric.dumazet@gmail.com>.
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This manpage maintained by bert hubert <ahu@ds9a.nl>
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