Congestion Avoidance - 3Com 5500-EI PWR Install Manual

Figure 1-8 Diagram for WRR queuing
WRR queue-scheduling algorithm schedules all the queues in turn and every queue can be
assured of a certain service time.
In a typical Switch 5500-EI there are eight output queues on each port. WRR configures a
weight value for each queue, for example: w7, w6, w5, w4, w3, w2, w1, and w0 respectively
for queue 7 through queue 0. A weight value indicates the proportion of resources available
for a queue. On a 100-Mbps port, configure the weight value of WRR queue-scheduling
algorithm to 5, 5, 3, 3, 1, 1, 1, and 1 (corresponding to w7, w6, w5, w4, w3, w2, w1, and w0
in order). In this way, the queue with the lowest priority can get 5 Mbps (100 Mbps ×
1/(5+5+3+3+1+1+1+1)) bandwidth at least, and the disadvantage of SP queue-scheduling
that the packets in queues with lower priority may not get service for a long time is avoided.
Another advantage of WRR queue is that: though the queues are scheduled in order, the
service time for each queue is not fixed; that is to say, if a queue is empty, the next queue
will be scheduled. In this way, the bandwidth resources are made full use.

Congestion Avoidance

Congestion may cause network resource unavailable and thus need to be prevented. As a
type of flow control mechanism, congestion avoidance aims to relieve network load through
traffic adjusting. With congestion avoidance configuration performed, packets are dropped
in advance when the utilization of certain network resources (such as output queues or
buffer created in the memory) reaches certain degree.
Traditional packet dropping policy
Tail drop is adopted in traditional packet drop policies. It drops all the newly arrived packets
when the current queue length reaches a specific value.
Such a policy will result in global TCP connection synchronization. If a queue drops
packets of multiple TCP connections simultaneously, the TCP connections will turn to the
state of congestion avoidance and slow startup for the traffics to be regulated. The traffic
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