CoDel(8) — Linux manual page


CoDel(8)                            Linux                           CoDel(8)

NAME         top

       CoDel - Controlled-Delay Active Queue Management algorithm

SYNOPSIS         top

       tc qdisc ... codel [ limit PACKETS ] [ target TIME ] [ interval TIME
       ] [ ecn | noecn ] [ ce_threshold TIME ]

DESCRIPTION         top

       CoDel (pronounced "coddle") is an adaptive "no-knobs" active queue
       management algorithm (AQM) scheme that was developed to address the
       shortcomings of RED and its variants. It was developed with the
       following goals in mind:
        o It should be parameterless.
        o It should keep delays low while permitting bursts of traffic.
        o It should control delay.
        o It should adapt dynamically to changing link rates with no impact
       on utilization.
        o It should be simple and efficient and should scale from simple to
       complex routers.

ALGORITHM         top

       CoDel comes with three major innovations. Instead of using queue size
       or queue average, it uses the local minimum queue as a measure of the
       standing/persistent queue.  Second, it uses a single state-tracking
       variable of the minimum delay to see where it is relative to the
       standing queue delay. Third, instead of measuring queue size in bytes
       or packets, it is measured in packet-sojourn time in the queue.

       CoDel measures the minimum local queue delay (i.e. standing queue
       delay) and compares it to the value of the given acceptable queue
       delay target.  As long as the minimum queue delay is less than target
       or the buffer contains fewer than MTU worth of bytes, packets are not
       dropped.  Codel enters a dropping mode when the minimum queue delay
       has exceeded target for a time greater than interval.  In this mode,
       packets are dropped at different drop times which is set by a control
       law. The control law ensures that the packet drops cause a linear
       change in the throughput. Once the minimum delay goes below target,
       packets are no longer dropped.

       Additional details can be found in the paper cited below.

PARAMETERS         top

       hard limit on the real queue size. When this limit is reached,
       incoming packets are dropped. If the value is lowered, packets are
       dropped so that the new limit is met. Default is 1000 packets.

       is the acceptable minimum standing/persistent queue delay. This
       minimum delay is identified by tracking the local minimum queue delay
       that packets experience.  Default and recommended value is 5ms.

       is used to ensure that the measured minimum delay does not become too
       stale. The minimum delay must be experienced in the last epoch of
       length interval.  It should be set on the order of the worst-case RTT
       through the bottleneck to give endpoints sufficient time to react.
       Default value is 100ms.

   ecn | noecn
       can be used to mark packets instead of dropping them. If ecn has been
       enabled, noecn can be used to turn it off and vice-a-versa. By
       default, ecn is turned off.

       sets a threshold above which all packets are marked with ECN
       Congestion Experienced. This is useful for DCTCP-style congestion
       control algorithms that require marking at very shallow queueing

EXAMPLES         top

        # tc qdisc add dev eth0 root codel
        # tc -s qdisc show
          qdisc codel 801b: dev eth0 root refcnt 2 limit 1000p target 5.0ms
       interval 100.0ms
           Sent 245801662 bytes 275853 pkt (dropped 0, overlimits 0 requeues
           backlog 0b 0p requeues 24
            count 0 lastcount 0 ldelay 2us drop_next 0us
            maxpacket 7306 ecn_mark 0 drop_overlimit 0

        # tc qdisc add dev eth0 root codel limit 100 target 4ms interval
       30ms ecn
        # tc -s qdisc show
          qdisc codel 801c: dev eth0 root refcnt 2 limit 100p target 4.0ms
       interval 30.0ms ecn
           Sent 237573074 bytes 268561 pkt (dropped 0, overlimits 0 requeues
           backlog 0b 0p requeues 5
            count 0 lastcount 0 ldelay 76us drop_next 0us
            maxpacket 2962 ecn_mark 0 drop_overlimit 0

SEE ALSO         top

       tc(8), tc-red(8)

SOURCES         top

       o   Kathleen Nichols and Van Jacobson, "Controlling Queue Delay", ACM

AUTHORS         top

       CoDel was implemented by Eric Dumazet and David Taht. This manpage
       was written by Vijay Subramanian. Please reports corrections to the
       Linux Networking mailing list <>.

COLOPHON         top

       This page is part of the iproute2 (utilities for controlling TCP/IP
       networking and traffic) project.  Information about the project can
       be found at 
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       from the project's upstream Git repository
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       2020-11-01.  (At that time, the date of the most recent commit that
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       (which is not part of the original manual page), send a mail to

iproute2                         23 May 2012                        CoDel(8)