CoDel - Controlled-Delay Active Queue Management algorithm at Linux.org

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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 ]

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 min‐
imum 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 mini‐
mum 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 lin‐
ear 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 experi‐
enced 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.

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