3Com MSR 50 Series Configuration Manual page 1684

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HAPTER ONGESTION VOIDANCE
Unlike RED, the random number of WRED generated is based on priority. It uses IP
precedence to determine the dropping policy thus the dropping probability of
packets with high priority will relatively decrease.
RED and WRED employ the random packet dropping policy to avoid TCP
synchronization - when packet of one TCP connection is dropped with a decreased
sending rate, other TCP connections will still keep higher sending rate. So there
are always some TCP connections, which have a higher sending rate to realize
more efficient network bandwidth utilization.
Average queue length
To drop the packets through comparing the length of the queue with the
maximum/minimum limitations will treat the bursting data stream unfairly and
influence the transmission of data stream. WRED uses the average queue and
maximum/minimum limitations comparison to determine the dropping probability.
The average queue length is the result of low pass filtering of queue length. The
average queue length reflects the changing of queue and is insensitive to bursting
change of queue length, preventing the unfair treatment for the bursting data
stream.
When WFQ is adopted, you can set index, maximum limitation, minimum
limitation, and packet-dropping probability when calculating average queue
length for different queues that has different priorities. So packet with different
priority will have different packet dropping characters.
When FIFO, PQ and CQ are adopted, you can set index, maximum limitation,
minimum limitation, and packet-dropping probability when calculating average
queue length for each queue. So packet with different priority will have different
packet dropping characters.
Relationship between WRED and queue mechanism
The relation between WRED and Queue mechanism is shown as in the following
figure.