случайное раннее обнаружение (Random Early Detection)
Имя (Name)
red - Random Early Detection
Синопсис (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 ]
Описание (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.
Алгоритм (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.
Параметры (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
Q-события (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.
Примеры (Examples)
# 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
Смотри также (See also)
tc(8), tc-choke(8)
Источники (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
