phpman > man > tc-codel(8)

CoDel(8)                                        Linux                                       CoDel(8)



NAME
       CoDel - Controlled-Delay Active Queue Management algorithm

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


DESCRIPTION
       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 devel‐
       oped 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
       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 mea‐
       sured 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
   limit
       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.


   target
       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.


   interval
       is used to ensure that the measured minimum delay does not become too stale. The minimum  de‐
       lay  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.  De‐
       fault 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.


   ce_threshold
       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 thresholds.



EXAMPLES
        # 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
       tc(8), tc-red(8)


SOURCES
       o    Kathleen  Nichols  and   Van   Jacobson,   "Controlling   Queue   Delay",   ACM   Queue,
       http://queue.acm.org/detail.cfm?id=2209336


AUTHORS
       CoDel  was implemented by Eric Dumazet and David Taht. This manpage was written by Vijay Sub‐
       ramanian. Please reports corrections to the Linux Networking mailing  list  <netdev AT vger.ker‐
       nel.org>.



iproute2                                     23 May 2012                                    CoDel(8)