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 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 lin-
       early 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  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.

       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@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)