t the Network Layer, con- gestion control. Congestion Control At its most fundamental level, congestion control is a mechanism for reducing the volume of traffic on a particular route through some form of load balancing. No matter how large, diverse, or capable a network is, Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Protocols Chapter 2 some degree of congestion is inevitable. It can result from sudden, unex- pectedly high utilization levels in one area of the network, from failures of network components, or from poor engineering. In the telephone net- work, for example, the busiest calling day of the year in the United States is Mother's Day. To reduce the probability that a caller will not be able to complete a call to Mom, network traffic engineers take extraordi- nary steps to load balance the network. For example, when subscribers on the east coast are making long-distance calls at 9:00 in the morning, west coast subscribers haven't even turned on their latte machines yet. Network resources in the west are underutilized during that period, SO engineers route east coast traffic westward and then hairpin it back to its destination, to spread the load across the entire network. As the day gets later, they reduce the volume of westward-bound traffic to ensure that California has adequate network resources for its own calls. There are two terms that are important in this discussion. One is con- gestion; the other is delay. The terms are often used interchangeably, but they are not the same thing, nor are they always related to one another. Years ago I lived in the San Francisco Bay area, where traffic conges- tion is a way of life. I often had to drive across the many bridges that crisscross San Francisco Bay, the Suisun Straits, or the Sacramento River. Many of those bridges require drivers to stop and pay a toll, result- ing in locali