| .TH RED 8 "13 December 2001" "iproute2" "Linux" |
| .SH NAME |
| red \- Random Early Detection |
| .SH SYNOPSIS |
| .B tc qdisc ... red |
| .B limit |
| bytes |
| .B min |
| bytes |
| .B max |
| bytes |
| .B avpkt |
| bytes |
| .B burst |
| packets |
| .B [ ecn ] [ bandwidth |
| rate |
| .B ] probability |
| chance |
| |
| .SH 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 |
| .B 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 if ECN is configured, marking either means dropping or |
| purely marking a packet as overlimit. |
| .SH 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 |
| .B min |
| bytes, no packet will ever be marked. When it exceeds |
| .B min, |
| the probability of doing so climbs linearly up |
| to |
| .B probability, |
| until the average queue size hits |
| .B max |
| bytes. Because |
| .B probability |
| is normally not set to 100%, the queue size might |
| conceivably rise above |
| .B max |
| bytes, so the |
| .B limit |
| parameter is provided to set a hard maximum for the size of the queue. |
| |
| .SH PARAMETERS |
| .TP |
| min |
| Average queue size at which marking becomes a possibility. |
| .TP |
| max |
| At this average queue size, the marking probability is maximal. Should be at |
| least twice |
| .B min |
| to prevent synchronous retransmits, higher for low |
| .B min. |
| .TP |
| 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). |
| .TP |
| 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 |
| .B max. |
| .TP |
| 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). |
| .TP |
| avpkt |
| Specified in bytes. Used with burst to determine the time constant for |
| average queue size calculations. 1000 is a good value. |
| .TP |
| 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. |
| .TP |
| 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 |
| .B limit |
| bytes. Needs a tc binary with RED support compiled in. Recommended. |
| |
| .SH SEE ALSO |
| .BR tc (8) |
| |
| .SH SOURCES |
| .TP |
| o |
| Floyd, S., and Jacobson, V., Random Early Detection gateways for |
| Congestion Avoidance. http://www.aciri.org/floyd/papers/red/red.html |
| .TP |
| o |
| Some changes to the algorithm by Alexey N. Kuznetsov. |
| |
| .SH AUTHORS |
| Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>, Alexey Makarenko |
| <makar@phoenix.kharkov.ua>, J Hadi Salim <hadi@nortelnetworks.com>. |
| This manpage maintained by bert hubert <ahu@ds9a.nl> |
| |
| |
