How Does WRED Work?
RED works on the basis of active queue management, and addresses the shortcomings
of tail drop.A RED-enabled router signals congestion to TCP senders
by dropping packets before the router is actually out of buffer space. Compliant
TCP senders detecting the dropped packets will throttle back the send rate using
the TCP slow start algorithm. RED drops arriving packets randomly so the probability
of a particular flow having packets dropped is in proportion to the flow’s
share of the bandwidth.Thus, flows using more bandwidth have a greater chance
of dropped packets than flows using small amounts of the overall bandwidth.
RED operates by monitoring the buffer level and discarding packets probabilistically
(see Figure 8.3) based on minimum and maximum threshold values.
www.syngress.com
248 Chapter 8 • Advanced QoS for AVVID Environments
Below the minimum threshold, no packets are dropped; above the maximum
threshold, all packets are dropped.When the buffer is between these two thresholds,
the drop rate is calculated as a function of the average queue size.The
average queue size is a running average over time. How responsive this average is
to changes is reflected in the configurable weighting average (discussed later).
Because of the randomness in which packets are dropped, packets across all flows
are dropped at different times, thus preventing the phenomenon of global synchronization
commonly associated with tail drop.