Chapter 4 Congestion Avoidance; Introduction To Congestion Avoidance - 3Com 3C13636 Configuration Manual

3Com Router 3000 Ethernet Family
Configuration Guide

Chapter 4 Congestion Avoidance

4.1 Introduction to Congestion Avoidance

Excessive congestion can endanger network resources greatly, so some avoidance
measures must be taken. The 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 has broader
significance, which affects more loads of application streams through router. Of course,
while dropping packets, the router may cooperate with traffic control action on the
source end, such as TCP traffic control to adjust the network's traffic to a reasonable
load level. A good combination of packet-dropping policy with traffic control mechanism
at the source end will always try to maximize the throughput and utilization of network
and minimize the packet dropping and delay.
I. Traditional packet-dropping policy
Traditional policy of dropping packets adopts the Tail-Drop method. When the amount
of packets in a queue reaches a certain maximum value, all new arrived packets will be
dropped.
This kind of dropping policy will lead to phenomenon of TCP synchronization - when
queues drop packets of several TCP connections at the same time, it will lead these
TCP connections to enter congestion avoidance and slow start status to adjust traffics
simultaneously, then reach a high peak of traffics simultaneously. In this way, Network
traffic keeps saw-teeth pattern. This causes frequent sudden rises and decreases of
bandwidth utilization ratio, lowering the bandwidth utilization efficiency.
II. RED and WRED
To avoid the phenomenon of TCP synchronization, RED (Random Early Detection) or
WRED (Weighted Random Early Detection) can be used.
In RED algorithm, it sets minimum and maximum thresholds for each queue.
When the length of queue is less than the minimum limitation, no packet will be
dropped.
When the length of queue exceeds the maximum limitation, all the incoming
packets will be dropped.
When the length of queue between low and high limitations, the packet will be
dropped randomly. The method is to give a random number to every new packet,
and then compare it with packet dropping probability of the current queue. If the
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Chapter 4 Congestion Avoidance