1 .TH CoDel 8 "23 May 2012" "iproute2" "Linux"
3 CoDel \- Controlled-Delay Active Queue Management algorithm
19 CoDel (pronounced "coddle") is an adaptive "no-knobs" active queue management
20 algorithm (AQM) scheme that was developed to address the shortcomings of
21 RED and its variants. It was developed with the following goals
23 o It should be parameterless.
24 o It should keep delays low while permitting bursts of traffic.
25 o It should control delay.
26 o It should adapt dynamically to changing link rates with no impact on
28 o It should be simple and efficient and should scale from simple to
32 CoDel comes with three major innovations. Instead of using queue size or queue
33 average, it uses the local minimum queue as a measure of the standing/persistent queue.
34 Second, it uses a single state-tracking variable of the minimum delay to see where it
35 is relative to the standing queue delay. Third, instead of measuring queue size
36 in bytes or packets, it is measured in packet-sojourn time in the queue.
38 CoDel measures the minimum local queue delay (i.e. standing queue delay) and
39 compares it to the value of the given acceptable queue delay
41 As long as the minimum queue delay is less than
43 or the buffer contains fewer than MTU worth of bytes, packets are not dropped.
44 Codel enters a dropping mode when the minimum queue delay has exceeded
46 for a time greater than
48 In this mode, packets are dropped at different drop times which is set by a
49 control law. The control law ensures that the packet drops cause a linear change
50 in the throughput. Once the minimum delay goes below
52 packets are no longer dropped.
54 Additional details can be found in the paper cited below.
58 hard limit on the real queue size. When this limit is reached, incoming packets
59 are dropped. If the value is lowered, packets are dropped so that the new limit is
60 met. Default is 1000 packets.
63 is the acceptable minimum standing/persistent queue delay. This minimum delay
64 is identified by tracking the local minimum queue delay that packets experience.
65 Default and recommended value is 5ms.
68 is used to ensure that the measured minimum delay does not become too stale. The
69 minimum delay must be experienced in the last epoch of length
71 It should be set on the order of the worst-case RTT through the bottleneck to
72 give endpoints sufficient time to react. Default value is 100ms.
75 can be used to mark packets instead of dropping them. If
79 can be used to turn it off and vice-a-versa. By default,
84 # tc qdisc add dev eth0 root codel
86 qdisc codel 801b: dev eth0 root refcnt 2 limit 1000p target 5.0ms
88 Sent 245801662 bytes 275853 pkt (dropped 0, overlimits 0 requeues 24)
89 backlog 0b 0p requeues 24
90 count 0 lastcount 0 ldelay 2us drop_next 0us
91 maxpacket 7306 ecn_mark 0 drop_overlimit 0
93 # tc qdisc add dev eth0 root codel limit 100 target 4ms interval 30ms ecn
95 qdisc codel 801c: dev eth0 root refcnt 2 limit 100p target 4.0ms
97 Sent 237573074 bytes 268561 pkt (dropped 0, overlimits 0 requeues 5)
98 backlog 0b 0p requeues 5
99 count 0 lastcount 0 ldelay 76us drop_next 0us
100 maxpacket 2962 ecn_mark 0 drop_overlimit 0
108 o Kathleen Nicols and Van Jaconson, "Controlling Queue Delay", ACM Queue,
109 http://queue.acm.org/detail.cfm?id=2209336
112 CoDel was implemented by Eric Dumazet and David Taht. This manpage was written
113 by Vijay Subramanian. Please reports corrections to the Linux Networking
114 mailing list <netdev@vger.kernel.org>.