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RED


Section: Linux (8)
Updated: 13 December 2001
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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 the 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.
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, <[email protected]>, Alexey Makarenko
<[email protected]>, J Hadi Salim <[email protected]>.
This manpage maintained by bert hubert <[email protected]>





 

Index



NAME

SYNOPSIS

DESCRIPTION

ALGORITHM

PARAMETERS

SEE ALSO

SOURCES

AUTHORS



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