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 ] [ 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 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. 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>

iproute2							 13 December 2001							    RED(8)