tc-red(8) — Linux manual page


RED(8)                            Linux                           RED(8)

NAME         top

       red - Random Early Detection

SYNOPSIS         top

       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         top

       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         top

       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         top

       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

              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.

              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.

              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

              (Added in linux-3.3) Sets RED in adaptive mode as
              described in
              Goal of Adaptive RED is to make 'probability' dynamic value between 1% and 50% to reach the target average queue :
              (max - min) / 2

QEVENTS         top

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

              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         top

       # 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         top

       tc(8), tc-choke(8)

SOURCES         top

       o      Floyd, S., and Jacobson, V., Random Early Detection
              gateways for Congestion Avoidance.

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

       o      Adaptive RED  :

AUTHORS         top

       Alexey N. Kuznetsov, <>,  Alexey Makarenko
       <>, J Hadi Salim
       <>, Eric Dumazet <>.
       This manpage maintained by bert hubert <>

COLOPHON         top

       This page is part of the iproute2 (utilities for controlling
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iproute2                    13 December 2001                      RED(8)

Pages that refer to this page: tc(8)tc-choke(8)tc-codel(8)tc-fq_codel(8)tc-pie(8)tc-sfb(8)tc-sfq(8)