Internet Core Protocols: The Definitive Guide

TCP in Action

Since TCP offers such a wide variety of services, it is easy to understand why so many applications choose to use it as a transport. Among the core services that TCP provides are application and network management, flow-control and reliability, all of which are implemented using TCP’s virtual circuit architecture.

The thing to remember is that TCP’s virtual circuit design is what makes all of the services offered by TCP possible. Without virtual circuits, TCP’s buffering, flow control and reliability services would be much more difficult to implement. But by using this design, these services come for free, at least to the applications that use them.

In order to illustrate how these services work, a variety of diagrams and captures are shown throughout the remainder of this section. Figure 7.35 shows what happens during a TCP session, from the moment that an application is loaded until a circuit is terminated, while Figure 7.38 shows what happens on the wire when TCP opens and closes virtual circuits. Figure 7.39 shows what happens when many small segments are sent using an interactive application such as Echo, while Section 7.3.5 shows what happens when large blocks of data are sent using applications such as Chargen.