H3C S7500E Series Configuration Manual

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H3C S7500E Series Configuration Manual Manual pdf 71 page image

7-17 Congestion AvoidanceWhen configuring congestion avoidance, go to these sections for information you are interested in:z Congestion Avoidance Overviewz Introduction to WRED Configurationz Configuring WRED on an Interfacez Displaying and Maintaining WREDCongestion Avoidance OverviewAvoiding congestion before it occurs to deteriorate network performance is a proactive approach toimproving network performance. As a flow control mechanism, congestion avoidance actively dropspackets when congestion is expected to occur or deteriorate by monitoring the utilization of networkresources (such as queues or memory buffers) to alleviate the load on the network.Compared with end-to-end flow control, this flow control mechanism controls the load of more flows ina device. When dropping packets from a source end, it cooperates with the flow control mechanism(such as TCP flow control) at the source end to regulate the network traffic size. The combination ofthe local packet drop policy and the source-end flow control mechanism helps maximize throughputand network use efficiency and minimize packet loss and delay.Traditional packet drop policyTail drop is the traditional approach to congestion avoidance. In this approach, when the size of aqueue reaches the maximum threshold, all the subsequent packets are dropped.This results in global TCP synchronization. That is, if packets from multiple TCP connections aredropped, these TCP connections go into the state of congestion avoidance and slow start to reducetraffic, but traffic peak occurs later. Consequently, the network traffic jitters all the time.RED and WREDYou can use random early detection (RED) or weighted random early detection (WRED) to avoidglobal TCP synchronization.Both RED and WRED avoid global TCP synchronization by randomly dropping packets. Thus, whilethe sending rates of some TCP sessions slow down after their packets are dropped, other TCPsessions remain at high sending rates. As there are always TCP sessions at high sending rates, linkbandwidth is efficiently utilized.The RED or WRED algorithm sets an upper threshold and lower threshold for each queue, andprocesses the packets in a queue as follows:z When the queue size is shorter than the lower threshold, no packet is dropped;z When the queue size reaches the upper threshold, all subsequent packets are dropped;z When the queue size is between the lower threshold and the upper threshold, the receivedpackets are dropped at random. The longer a queue is, the higher the drop probability is. However,a maximum drop probability exists.