Wred - ZyXEL Communications MGS3600-24F User Manual

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7.14.12 WRED

Use the WRED sub-menu to configure Weighted Random Early Detection.
Classic Random Early Detection (RED) monitors the average queue size and drops or marks packets based on statistical probabilities. If the buffer
is almost empty all incoming packets are accepted. As the queue grows, the probability for dropping an incoming packet grows too. When the buf-
fer is full the probability has reached 1 and all incoming packets are dropped.
RED is more fair than tail drop, in the sense that it does not possess a bias against burst traffic that uses only a small portion of the bandwidth. The
more a host transmits, the more likely it is that its packets are dropped as the probability of a host's packet being dropped is proportional to the
amount of data it has in a queue. Early detection helps avoid TCP global synchronization.
There were two bugs in classic RED, so improvements to the algorithm were developed. A draft paper was prepared but never published, so the
improvements were not widely implemented. Pure RED does not accommodate quality of service (QoS) differentiation. WRED and RED with In
and Out (RIO) provide early detection with QoS considerations.
The following illustration shows the drop probability function with associated parameters.
Max. DP 1-3 is the drop probability when the average queue filling level is 100%. Frames marked with Drop Precedence Level 0 are never
dropped. Min. Threshold is the average queue filling level where the queues randomly start dropping frames. The drop probability for frames
marked with Drop Precedence Level n increases linearly from zero (at Min. Threshold average queue filling level) to Max. DP n (at 100% average
queue filling level).
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