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

       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

              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 early-

              (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 TCP/IP
       networking and traffic) project.  Information about the project can
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iproute2                      13 December 2001                        RED(8)

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