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 ] [ nodrop ] [ bandwidth rate ] [
       probability chance ] [ adaptive ] [ qevent early_drop block index ] [
       qevent mark block index ]


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
       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 max 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 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
       linearly 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.
	      Defaults 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.
	      Further 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
	      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).

       avpkt  Specified in bytes. Used with burst to determine the time
	      constant 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
	      interface. Does not mean that RED will shape for you! Optional.
	      Default : 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.

       nodrop With this parameter, traffic that should be marked, but is not
	      ECN-capable, is enqueued. Without the parameter it is early-
	      dropped.

       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


QEVENTS
       See tc (8) for some general notes about qevents. The RED qdisc supports
       the following qevents:


       early_drop
	      The associated block is executed when packets are early-dropped.
	      This includes non-ECT packets in ECN mode.

       mark   The associated block is executed when packets are marked in ECN
	      mode.


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


SEE ALSO
       tc(8), tc-choke(8)


SOURCES
       o      Floyd, S., and Jacobson, V., Random Early Detection gateways for
	      Congestion Avoidance.
	      http://www.aciri.org/floyd/papers/red/red.html

       o      Some changes to the algorithm by Alexey N. Kuznetsov.

       o      Adaptive RED  : http://icir.org/floyd/papers/adaptiveRed.pdf


AUTHORS
       Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>,  Alexey Makarenko
       <makar@phoenix.kharkov.ua>, J Hadi Salim <hadi@nortelnetworks.com>,
       Eric Dumazet <eric.dumazet@gmail.com>.  This manpage maintained by bert
       hubert <ahu@ds9a.nl>

iproute2		       13 December 2001				RED(8)