tc qdisc ... red limit bytes min bytes max bytes avpkt bytes burst
packets [ ecn ] [ bandwidth rate ] probability chance
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 per-
forms 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 pro-
cessor 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 lin-
early 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.
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.
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
limit Hard limit on the real (not average) queue size in bytes. Fur-
This rate is used for calculating the average queue size after
some idle time. Should be set to the bandwidth of your inter-
face. 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.
o Floyd, S., and Jacobson, V., Random Early Detection gateways for
o Some changes to the algorithm by Alexey N. Kuznetsov.
Alexey N. Kuznetsov, <firstname.lastname@example.org>, Alexey Makarenko
<email@example.com>, J Hadi Salim <firstname.lastname@example.org>.
This manpage maintained by bert hubert <email@example.com>
iproute2 13 December 2001 RED(8)
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