red - Random Early Detection
tc qdisc ... red limit
bytes [ min
bytes ] [ max
bytes ] avpkt
bytes [ burst
packets ] [ ecn ] [ harddrop] [
rate ] [ probability
chance ] [ adaptive ]
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
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
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.
- Average queue size at which marking becomes a possibility.
Defaults to max /3
- 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
- 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.
- 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:
- Specified in bytes. Used with burst to determine the time
constant for average queue size calculations. 1000 is a good value.
- 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
- 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.
- 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
Goal of Adaptive RED is to make 'probability' dynamic value between 1% and 50% to reach the target average queue :
(max - min) / 2
# 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
- Floyd, S., and Jacobson, V., Random Early Detection
gateways for Congestion Avoidance.
- Some changes to the algorithm by Alexey N. Kuznetsov.
- Adaptive RED : http://icir.org/floyd/papers/adaptiveRed.pdf
Alexey N. Kuznetsov, <firstname.lastname@example.org>, Alexey Makarenko
<email@example.com>, J Hadi Salim
<firstname.lastname@example.org>, Eric Dumazet <email@example.com>.
This manpage maintained by bert hubert <firstname.lastname@example.org>