Cisco IOS Cookbook, 2nd Edition

Dropping Packets and Congestion Avoidance

Imagine a queue that holds packets as they enter a network bottleneck. These packets carry data for many different applications to many different destinations. If the amount of traffic arriving is less than the available bandwidth in the bottleneck, then the queue just holds the packets long enough to transmit them downstream. Queues become much more important if there is not enough bandwidth in the bottleneck to carry all of the incoming traffic.

If the excess is a short burst, the queue will attempt to smooth the flow rate, delivering the first packets as they are received and delaying the later ones briefly before transmitting them. However, if the burst is longer, or if it is actually more like a continuous stream, the queue will have to stop accepting new packets while it deals with the backlog. The queue simply discards the overflowing inbound packets. This is called a tail drop.

Some applications and some protocols deal with dropped packets more gracefully than others. For example, if an application doesn’t have the ability to re-send the lost information, then a dropped packet could be devastating. On the other hand, some real-time applications don’t want their packets delayed. For these applications, it is better to drop the data than to delay it.

From the network’s point of view, some protocols are better behaved than others. Applications that use TCP are able to adapt to dropping an occasional packet by backing off and sending data ...