tc-red - phpMan

RED(8) Linux RED(8)

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 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 grad-
ual 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 big-
ger.

This has a host of benefits over simple taildrop, while not being processor inten-
sive. 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 if 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
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.

PARAMETERS
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.

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 sup-
port the following guideline: (min+min+max)/(3*avpkt).

avpkt Specified in bytes. Used with burst to determine the time constant for aver-
age 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.

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.