tc-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

       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

       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

       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 24)
           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 5)
           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 Queue,

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 
       If you have a bug report for this manual page, send it to,  This page was
       obtained from the project's upstream Git repository
       ⟨⟩ on
       2023-12-22.  (At that time, the date of the most recent commit
       that was found in the repository was 2023-12-20.)  If you
       discover any rendering problems in this HTML version of the page,
       or you believe there is a better or more up-to-date source for
       the page, or you have corrections or improvements to the
       information in this COLOPHON (which is not part of the original
       manual page), send a mail to

iproute2                       23 May 2012                      CoDel(8)

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