int-to-point circuit, then there is very little that could happen to the data during the transmission, since the data would consist of an uninterrupted, "single-hop" stream-there are no switches along the way that could cause pieces of the message to go awry. The Transport Layer, therefore, would have little to do to guar- antee the delivery of the message. However, what if the architecture of the network is not as robust as a private line circuit? What if this is a packet network, in which case the message is broken into segments by the Transport Layer and these seg- ments are independently routed through the fabric of the network? Fur- thermore, what if there is no guarantee that all of the packets will take the same route through the network wilderness? In that case, the "route" actually consists of a series of routes between the switches, like a string of sausage links. In this situation, there is no guarantee that the compo- nents of the message will arrive in sequence; in fact, there is no guaran- tee that they will arrive at all! The Transport Layer, therefore, has a major responsibility to ensure that all of the message components arrive and that they carry enough additional information in the form of yet another header-this time on each packet-to allow them to be properly resequenced at the destination. The header, for example, contains Figure 2-27 Virtual private network (VPN) 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 Protocols sequence numbers that the receiving Transport Layer can use to reassemble the original message from the stream of random packets. Consider the following scenario: A transmitter fires a message into the network, where it passes through each of the upper layers until it reaches the originating Transport Layer, which segments the message into a series of five packets labeled one of five, two of 