# tc-red

## 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
    


## 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
    


* * *