Internet Core Protocols: The Definitive Guide

Congestion avoidance

The congestion avoidance routines are called whenever a system needs to use a slower, more sensitive form of congestion avoidance than the exponential mechanisms offered by the slow start procedure. A slow congestion avoidance mechanism may be required when a system detects congestion from the presence of multiple duplicate acknowledgments, or as part of the recovery mechanisms that are utilized when an acknowledgment timer reaches zero.

Although duplicate acknowledgments are not uncommon (and are allowed for by TCP’s error-recovery mechanisms), the presence of many such acknowledgments tends to indicate that an IP datagram has been lost somewhere, most likely due to congestion occurring on the network. As such, RFC 1122 states that if three or more duplicate acknowledgments are received, then the size of the congestion window should be cut in half, and the congestion avoidance technique is to be used in an effort to return the network to full throttle.

Another scenario where congestion avoidance is used is if the sender’s acknowledgment timer has expired, which means that no acknowledgments are coming back from the other end. This signifies that there are serious congestion problems, or that the other system has left the network. In an effort to recover from this event, the congestion window is shrunk so small that only one segment can be sent. Then the slow start mechanism is called upon and used until the congestion window is half of its original size. Congestion ...