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 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.

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