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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
       ] [ 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 be‐
       haviour. 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  ex‐
       ceeds  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 max‐
       imum 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 en‐
              queued. 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 AT ms2.ru>,  Alexey Makarenko <makar AT phoenix.ua>,  J
       Hadi  Salim  <hadi AT nortelnetworks.com>,  Eric Dumazet <eric.dumazet AT gmail.com>.  This manpage
       maintained by bert hubert <ahu AT ds9a.nl>



iproute2                                  13 December 2001                                    RED(8)