Section: Linux (8)
Updated: 13 December 2001
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red - Random Early Detection
tc qdisc ... red
] [ max
] [ ecn ] [ harddrop] [ bandwidth
] [ probability
] [ adaptive ]
Random Early Detection is a classless qdisc which manages its queue size
smartly. Regular queues simply drop packets from the tail when they are
full, which may not be the optimal behaviour. RED also performs tail drop,
but does so in a more gradual way.
Once the queue hits a certain average length, packets enqueued have a
configurable chance of being marked (which may mean dropped). This chance
increases linearly up to a point called the
average queue length, although the queue might get bigger.
This has a host of benefits over simple taildrop, while not being processor
intensive. It prevents synchronous retransmits after a burst in traffic,
which cause further retransmits, etc.
The goal is to have a small queue size, which is good for interactivity
while not disturbing TCP/IP traffic with too many sudden drops after a burst
Depending on if ECN is configured, marking either means dropping or
purely marking a packet as overlimit.
The average queue size is used for determining the marking
probability. This is calculated using an Exponential Weighted Moving
Average, which can be more or less sensitive to bursts.
When the average queue size is below
bytes, no packet will ever be marked. When it exceeds
the probability of doing so climbs linearly up
until the average queue size hits
is normally not set to 100%, the queue size might
conceivably rise above
bytes, so the
parameter is provided to set a hard maximum for the size of the queue.
Average queue size at which marking becomes a possibility. Defaults to
At this average queue size, the marking probability is maximal. Should be at
to prevent synchronous retransmits, higher for low
Maximum probability for marking, specified as a floating point
number from 0.0 to 1.0. Suggested values are 0.01 or 0.02 (1 or 2%,
respectively). Default : 0.02
Hard limit on the real (not average) queue size in bytes. Further packets
are dropped. Should be set higher than max+burst. It is advised to set this
a few times higher than
Used for determining how fast the average queue size is influenced by the
real queue size. Larger values make the calculation more sluggish, allowing
longer bursts of traffic before marking starts. Real life experiments
support the following guideline: (min+min+max)/(3*avpkt).
Specified in bytes. Used with burst to determine the time constant for
average queue size calculations. 1000 is a good value.
This rate is used for calculating the average queue size after some
idle time. Should be set to the bandwidth of your interface. Does not mean
that RED will shape for you! Optional. Default : 10Mbit
As mentioned before, RED can either 'mark' or 'drop'. Explicit Congestion
Notification allows RED to notify remote hosts that their rate exceeds the
amount of bandwidth available. Non-ECN capable hosts can only be notified by
dropping a packet. If this parameter is specified, packets which indicate
that their hosts honor ECN will only be marked and not dropped, unless the
queue size hits
If average flow queue size is above
bytes, this parameter forces a drop instead of ecn marking.
(Added in linux-3.3) Sets RED in adaptive mode as described in http://icir.org/floyd/papers/adaptiveRed.pdf
Goal of Adaptive RED is to make 'probability' dynamic value between 1% and 50% to reach the target average queue :
(max - min) / 2
# tc qdisc add dev eth0 parent 1:1 handle 10: red
limit 400000 min 30000 max 90000 avpkt 1000
burst 55 ecn adaptive bandwidth 10Mbit
Floyd, S., and Jacobson, V., Random Early Detection gateways for
Congestion Avoidance. http://www.aciri.org/floyd/papers/red/red.html
Some changes to the algorithm by Alexey N. Kuznetsov.
Adaptive RED : http://icir.org/floyd/papers/adaptiveRed.pdf
Alexey N. Kuznetsov, <firstname.lastname@example.org
>, Alexey Makarenko
>, J Hadi Salim <email@example.com
Eric Dumazet <firstname.lastname@example.org
This manpage maintained by bert hubert <email@example.com