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>.