Hi Jan:
Perhaps this could help you.
Best regards,
Leonardo Balliache
Pd: haven't never implemented one of this monsters.
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 limits
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 wether 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 sensi-
tive 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 pos-
sibility.
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 guide-
line: (min+min+max)/(3*avpkt).
avpkt Specified in bytes. Used with burst to determine
the time constant for average queue size calcula-
tions. 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 indi-
cate 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.
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.
SEE ALSO
tc-cbq(8), tc-htb(8), tc-red(8), tc-tbf(8), tc-pfifo(8),
tc-bfifo(8), tc-pfifo_fast(8), tc-filters(8)
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