Congestion Avoidance - Random Early Detection

Random Early Detection (RED) is a mechanism used to prevent tail-drop.

RED drops packets at random before a queue is full, the rate of drops increases as the queue grows. RED is not flow orientated and will drop packets at random without preference. Statistically packets in more aggressive flows will be dropped more frequently.

RED is only effective when the majority of traffic is TCP flows. Because non-TCP flows do not use window sizes they will not benefit from RED.

Three parameters are used to configure RED: low threshold, high threshold and Mark Probability Denominator (MPD). RED will drop one packet of MPD value, for example if MPD was 40, 1 in 40 packets would be dropped.

If the minimum threshold is too low then packets may be dropped too soon, when not necessary. If the maximum threshold is too high when RED will not be able to prevent global syncronisation.