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 developed with the
       following goals in mind:

       *   It should be parameterless.

       *   It should keep delays low while permitting bursts of traffic.

       *   It should control delay.

       *   It should adapt dynamically to changing link rates with no impact
	   on utilization.

       *   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
       is the 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
       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)