tc-codel

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 man-
       agement algorithm (AQM) scheme that was developed to address the short-
       comings 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
       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 stand-
       ing  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  de-
       lay)  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 con-
       trol  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 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.  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  algo-
       rithms 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 in-
       terval 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 Subramanian. Please reports corrections to  the  Linux
       Networking mailing list <netdev@vger.kernel.org>.

iproute2                          23 May 2012                         CoDel(8)