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RED(8) Linux RED(8)

NAME
red - Random Early Detection

SYNOPSIS
tc qdisc ... red limit bytes [ min bytes ] [ max bytes ] avpkt bytes [
burst packets ] [ ecn ] [ harddrop] [ bandwidth rate ] [ probability
chance ] [ adaptive ]

DESCRIPTION
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 per-
forms 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 max average queue
length, although the queue might get bigger.

This has a host of benefits over simple taildrop, while not being pro-
cessor 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 of traffic.

Depending on if ECN is configured, marking either means dropping or
purely marking a packet as overlimit.

ALGORITHM
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 min bytes, no packet will ever be
marked. When it exceeds min, the probability of doing so climbs lin-
early up to probability, until the average queue size hits max bytes.
Because probability is normally not set to 100%, the queue size might
conceivably rise above max bytes, so the limit parameter is provided to
set a hard maximum for the size of the queue.

PARAMETERS
min Average queue size at which marking becomes a possibility. De-
faults to max /3

max At this average queue size, the marking probability is maximal.
Should be at least twice min to prevent synchronous retransmits,
higher for low min. Default to limit /4

probability
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

limit Hard limit on the real (not average) queue size in bytes. Fur-
ther packets are dropped. Should be set higher than max+burst.
It is advised to set this a few times higher than max.

burst Used for determining how fast the average queue size is influ-
enced 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).

avpkt Specified in bytes. Used with burst to determine the time con-
stant for average queue size calculations. 1000 is a good value.

bandwidth
This rate is used for calculating the average queue size after
some idle time. Should be set to the bandwidth of your inter-
face. Does not mean that RED will shape for you! Optional. De-
fault : 10Mbit

ecn 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 limit bytes. Recommended.

harddrop
If average flow queue size is above max bytes, this parameter
forces a drop instead of ecn marking.

adaptive
(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

EXAMPLE
# 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