Keith's TCP software, which is typically just a part of the OS, is in charge of delivering the data to the web server. TCP provides several services, so it needs a place to record some information about those services. Therefore, TCP defines a header. As mentioned in Chapter 5, "Rules of the Road: How to Use the Local (Network) Roadway," a header is a bunch of overhead bits that a networking protocol adds to some data. The protocol that adds the headerTCP in this caseuses those bits to record things it needs to know to do its job. Many of the upcoming features covered in this chapter record information in the TCP header to perform some useful function.
The process of adding the TCP header to the application data is called encapsulation. Refer to Chapter 5 for a review of encapsulation.
Imagine that Paul drives a truck for the shipping company. He gets to work each morning, and someone has loaded the truck. He gets in, starts driving, and stops to drop off and pick up packages.
Paul doesn't care what's inside the packages. He might care a little about the size of the packages, particularly if a package is too large or heavy to move. Really, he just cares about what's on the shipping label and making sure he delivers the package to the right place.
Similarly, TCP doesn't really care what data it's delivering to another computer. Figure 9-3 shows TCP's perspective on what really happens in Figure 9-2.
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