tc-red(8) - phpMan

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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 performs tail drop, but does so in a more  grad-
       ual 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 big-
       ger.

       This has a host of benefits over simple taildrop, while not being processor  inten-
       sive.  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  sup-
              port the following guideline: (min+min+max)/(3*avpkt).

       avpkt  Specified in bytes. Used with burst to determine the time constant for aver-
              age 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 AT ms2.ru>,      Alexey     Makarenko
       <makar AT phoenix.ua>, J Hadi Salim <hadi AT nortelnetworks.com>.   This  manpage
       maintained by bert hubert <ahu AT ds9a.nl>





iproute2                       13 December 2001                         RED(8)

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