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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 ] [ bandwidth rate ] probability chance


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 the have a small queue size, which is good for  interactiv-
       ity  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.

       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.

       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).

       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.

       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. Needs a tc binary with RED support
              compiled in. Recommended.


SEE ALSO
       tc(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.


AUTHORS
       Alexey  N.   Kuznetsov,   <kuznetATms2.ru>,    Alexey   Makarenko
       <makarATphoenix.ua>,  J  Hadi  Salim  <hadiATnortelnetworks.com>.
       This manpage maintained by bert hubert <ahuATds9a.nl>





iproute2                       13 December 2001                         RED(8)