3Com Switch 8800 Advanced Software V5 Configuration Guide

CONGESTION AVOIDANCE

When configuring traffic classification and traffic shaping, go to these sections for

information you are interested in:

■ “Congestion Avoidance Overview” on page 859

■ “Configuring WRED” on page 861

■ “Displaying and Maintaining WRED” on page 862

■ “WRED Configuration Examples” on page 862

Congestion Avoidance

Overview

Excessive congestion can endanger network resources greatly, so some congestion

avoidance measures must be taken. Congestion avoidance refers to a traffic

control mechanism that can monitor the occupancy status of network resources

(such as the queues or buffer). As congestion becomes worse, the system actively

drops packets and tries to avoid the network overload through adjusting the

network traffics.

Comparing with the end-to-end traffic control, this traffic control herein is of

broader significance, which affects more loads of application streams through a

device. Of course, while dropping packets, the device may cooperate with traffic

control actions (such as TCP traffic control) on the source end to adjust the

network’s traffic to a reasonable load level. A good combination of packet-drop

policies with traffic control mechanisms can maximize the throughput and

utilization of network and minimize the packet drop and delay.

Traditional packet-drop policy

The traditional packet-drop policy is tail-drop, that is, when the amount of packets

in a queue reaches the maximum value, all newly arrived packets are dropped.

This drop policy leads to global TCP synchronization, that is, when queues drop

packets of several TCP links at the same time, these TCP links enter congestion

avoidance and slow start status to adjust traffics simultaneously, and then reach

traffic peak simultaneously. In this way, network traffic keeps in frequent rises and

decreases.

RED and WRED

To avoid global TCP synchronization, random early detection (RED) or weighted

random early detection (WRED) can be used.

In RED algorithm, a maximum threshold and a minimum threshold are set for each

queue. The packets in the queue are processed as follows:

■ When the queue length is smaller than the minimum threshold, no packet is

dropped.