st hop count, the route beyond the port is severely congested. As a result, the packet is routed out port 13, which results in a longer path but avoids the congestion. And since there is no preordained rout through the network, the packet will simply have to get directions when it arrives at the next switch. Now, the second packet of the message arrives. Because this is a con- nectionless environment, however, the switch does not realize that the packet is related to the packet that preceded it. The switch examines the destination address on the second packet, then proceeds to route the packet as it did with the preceding one. This time, however, upon exam- ination of the network, the switch finds that port 7, the shortest path from the source to the destination, is no longer congested. It therefore transmits the packet on port 7, thus ensuring that packet two will, in all likelihood, arrive before packet one! Clearly, this poses a problem for message integrity and illustrates the criticality of the Transport Layer, which, you will recall, provides end-to-end message integrity by reassem- bling the message from a collection of out-of-order packets that arrive with varying degrees of delay because of the vagaries of connectionless networks. Connectionless service is often called unreliable because it fails to guarantee delay minimums, sequential delivery, or for that matter, any kind of delivery. This causes many people to question why network designers would rely on a technology that guarantees SO little. The 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 answer lies within the layered protocol model. While connectionless net- works do not guarantee sequential delivery or limits on delay, they will ultimately deliver the packets. Because they are not required to transmit along a fixed path, the switches in a connectionle