bionic (8) tc-red.8.gz

Provided by: iproute2_4.15.0-2ubuntu1.3_amd64 bug

NAME

       red - Random Early Detection

SYNOPSIS

       tc  qdisc  ...  red  limit  bytes  [  min  bytes  ] [ max bytes ] avpkt bytes [ burst packets ] [ ecn ] [
       harddrop] [ bandwidth rate ] [ probability chance ] [ adaptive ]

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  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

       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. 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

       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). 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.

       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. 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.

       harddrop
              If average flow queue size is above max bytes,  this  parameter  forces  a  drop  instead  of  ecn
              marking.

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

EXAMPLE

       # 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

       tc(8), tc-choke(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.

       o      Adaptive RED  : http://icir.org/floyd/papers/adaptiveRed.pdf

AUTHORS

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