CoDel − Controlled-Delay Active Queue Management algorithm
tc qdisc ... codel [ limit PACKETS ] [ target TIME ] [ interval TIME ] [ ecn | noecn ]
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.
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.
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.
# 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
tc(8), tc-red(8)
o Kathleen Nichols and Van Jacobson, "Controlling Queue Delay", ACM Queue, http://queue.acm.org/detail.cfm?id=2209336
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>.