Provided by: iproute_20041019-4ubuntu5_i386 bug

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
       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  the  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.

       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.
              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.

       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,   <kuznet@ms2.inr.ac.ru>,    Alexey  Makarenko
       <makar@phoenix.kharkov.ua>,  J  Hadi  Salim  <hadi@nortelnetworks.com>.
       This manpage maintained by bert hubert <ahu@ds9a.nl>