NAME

       red - Random Early Detection

SYNOPSIS

       tc  qdisc  ... red limit bytes [ min bytes ] [ max bytes ] avpkt bytes [ burst packets ] [
       ecn ] [ harddrop ] [ nodrop ] [ bandwidth rate ] [ probability chance ]  [  adaptive  ]  [
       qevent early_drop block index ] [ qevent mark block index ]

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.

       nodrop With this parameter, traffic that should be marked,  but  is  not  ECN-capable,  is
              enqueued. Without the parameter it is early-dropped.

       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

QEVENTS

       See tc (8) for some general notes about qevents. The  RED  qdisc  supports  the  following
       qevents:

       early_drop
              The associated block is executed when packets are early-dropped. This includes non-
              ECT packets in ECN mode.

       mark   The associated block is executed when packets are marked in ECN mode.

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>