NAME

       CoDel - Controlled-Delay Active Queue Management algorithm

SYNOPSIS

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

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 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 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

   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.  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.

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
       Subramanian.  Please  reports  corrections  to   the   Linux   Networking   mailing   list
       <netdev@vger.kernel.org>.