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Linux 2.6 - man page for tc-red (linux section 8)

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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 ] [ band-
       width 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 retrans-
       mits, etc.

       The goal is to have a small queue size, which is good for interactivity while not disturb-
       ing 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  follow-
	      ing 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 Notifica-
	      tion  allows RED to notify remote hosts that their rate exceeds the amount of band-
	      width 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 Avoid-
	      ance. 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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