tc-red(8)

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>

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