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.
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.
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
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
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
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.
If average flow queue size is above max bytes, this parameter
forces a drop instead of ecn marking.
(Added in linux-3.3) Sets RED in adaptive mode as described in
Goal of Adaptive RED is to make 'probability' dynamic value between 1% and 50% to reach the target average queue :
(max - min) / 2
Alexey N. Kuznetsov, <email@example.com>, Alexey Makarenko
<firstname.lastname@example.org>, J Hadi Salim <email@example.com>,
Eric Dumazet <firstname.lastname@example.org>. This manpage maintained by
bert hubert <email@example.com>
This page is part of the iproute2 (utilities for controlling TCP/IP
networking and traffic) project. Information about the project can
be found at
If you have a bug report for this manual page, send it to
firstname.lastname@example.org, email@example.com. This page was obtained
from the project's upstream Git repository
on 2017-03-13. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-to-
date source for the page, or you have corrections or improvements to
the information in this COLOPHON (which is not part of the original
manual page), send a mail to firstname.lastname@example.org
iproute2 13 December 2001 RED(8)