User Datagram Protocol (UDP), which will be discussed a bit later. TP4 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 is the most common ISO Transport Layer protocol and is equivalent in capability to the IETF's Transmission Control Protocol (TCP). It provides an ironclad transport function and operates under the assumption that the network is wholly unreliable and must therefore take extraordinary steps to safeguard the user's data. Before we descend into the wilds of the Network Layer, let's introduce the concepts of switching and routing. Modern networks are often represented as a cloud filled with boxes representing switches or routers. Depending upon such factors as con- gestion, cost, number of hops between routers, and other considerations, the network selects the optimal end-to-end path for the stream of pack- ets created by the Transport Layer. Depending upon the nature of the Network Layer protocol that is in use, the network will take one of two actions. It will either establish a single path over which all the packets will travel, in sequence, or, the network will simply be handed the pack- ets and told to deliver them as it sees fit. The first technique, which establishes a seemingly dedicated path, is called connection-oriented ser- vice; the other technique, which does not dedicate a path, is called con- nectionless service. We will discuss each of these in turn. Before we do, however, let's discuss the evolution of switched networks. Switched Networks Modern switched networks typically fall into one of two major categories: circuit switched, in which the network preestablishes a path for trans- port of traffic from a source to a destination, as is done in the traditional telephone network; and store-and-forward networks, in which the traffic is handed from one switch to the next as it makes its way across 