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115 lines
3.9 KiB
Groff
115 lines
3.9 KiB
Groff
.TH CoDel 8 "23 May 2012" "iproute2" "Linux"
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.SH NAME
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CoDel \- Controlled-Delay Active Queue Management algorithm
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.SH SYNOPSIS
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.B tc qdisc ... codel
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[
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.B limit
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PACKETS ] [
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.B target
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TIME ] [
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.B interval
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TIME ] [
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.B ecn
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.B noecn
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]
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.SH DESCRIPTION
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CoDel (pronounced "coddle") is an adaptive "no-knobs" active queue management
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algorithm (AQM) scheme that was developed to address the shortcomings of
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RED and its variants. It was developed with the following goals
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in mind:
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o It should be parameterless.
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o It should keep delays low while permitting bursts of traffic.
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o It should control delay.
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o It should adapt dynamically to changing link rates with no impact on
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utilization.
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o It should be simple and efficient and should scale from simple to
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complex routers.
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.SH ALGORITHM
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CoDel comes with three major innovations. Instead of using queue size or queue
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average, it uses the local minimum queue as a measure of the standing/persistent queue.
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Second, it uses a single state-tracking variable of the minimum delay to see where it
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is relative to the standing queue delay. Third, instead of measuring queue size
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in bytes or packets, it is measured in packet-sojourn time in the queue.
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CoDel measures the minimum local queue delay (i.e. standing queue delay) and
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compares it to the value of the given acceptable queue delay
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.B target.
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As long as the minimum queue delay is less than
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.B target
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or the buffer contains fewer than MTU worth of bytes, packets are not dropped.
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Codel enters a dropping mode when the minimum queue delay has exceeded
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.B target
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for a time greater than
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.B interval.
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In this mode, packets are dropped at different drop times which is set by a
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control law. The control law ensures that the packet drops cause a linear change
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in the throughput. Once the minimum delay goes below
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.B target,
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packets are no longer dropped.
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Additional details can be found in the paper cited below.
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.SH PARAMETERS
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.SS limit
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hard limit on the real queue size. When this limit is reached, incoming packets
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are dropped. If the value is lowered, packets are dropped so that the new limit is
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met. Default is 1000 packets.
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.SS target
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is the acceptable minimum standing/persistent queue delay. This minimum delay
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is identified by tracking the local minimum queue delay that packets experience.
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Default and recommended value is 5ms.
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.SS interval
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is used to ensure that the measured minimum delay does not become too stale. The
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minimum delay must be experienced in the last epoch of length
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.B interval.
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It should be set on the order of the worst-case RTT through the bottleneck to
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give endpoints sufficient time to react. Default value is 100ms.
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.SS ecn | noecn
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can be used to mark packets instead of dropping them. If
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.B ecn
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has been enabled,
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.B noecn
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can be used to turn it off and vice-a-versa. By default,
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.B ecn
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is turned off.
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.SH EXAMPLES
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# tc qdisc add dev eth0 root codel
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# tc -s qdisc show
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qdisc codel 801b: dev eth0 root refcnt 2 limit 1000p target 5.0ms
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interval 100.0ms
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Sent 245801662 bytes 275853 pkt (dropped 0, overlimits 0 requeues 24)
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backlog 0b 0p requeues 24
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count 0 lastcount 0 ldelay 2us drop_next 0us
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maxpacket 7306 ecn_mark 0 drop_overlimit 0
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# tc qdisc add dev eth0 root codel limit 100 target 4ms interval 30ms ecn
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# tc -s qdisc show
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qdisc codel 801c: dev eth0 root refcnt 2 limit 100p target 4.0ms
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interval 30.0ms ecn
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Sent 237573074 bytes 268561 pkt (dropped 0, overlimits 0 requeues 5)
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backlog 0b 0p requeues 5
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count 0 lastcount 0 ldelay 76us drop_next 0us
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maxpacket 2962 ecn_mark 0 drop_overlimit 0
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.SH SEE ALSO
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.BR tc (8),
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.BR tc-red (8)
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.SH SOURCES
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o Kathleen Nichols and Van Jacobson, "Controlling Queue Delay", ACM Queue,
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http://queue.acm.org/detail.cfm?id=2209336
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.SH AUTHORS
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CoDel was implemented by Eric Dumazet and David Taht. This manpage was written
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by Vijay Subramanian. Please reports corrections to the Linux Networking
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mailing list <netdev@vger.kernel.org>.
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