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tc-red



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 08 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 sensi­
       tive 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  pos­
              sibility.

       max    At this average queue size, the marking probability
              is maximal. Should be at least twice min to prevent
              values make the calculation more sluggish, allowing
              longer  bursts  of  traffic  before marking starts.
              Real life experiments support the following  guide­
              line: (min+min+max)/(3*avpkt).

       avpkt  Specified  in  bytes.  Used with burst to determine
              the time constant for average queue  size  calcula­
              tions. 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 indi­
              cate 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>

iproute2                 13 December 2001                  RED(8)
  




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