Delivering the Package to the Right Person, Not Just the Right Address

Delivering the Package to the Right Person, Not Just the Right Address

When you ship a package, you not only put the address on the shipping label, but you put the name of the person to whom the package is intended. The reason for that is obvious: You want to make sure the right person gets the package you are sending.

Transport layer protocols do the same kind of thing. Imagine, for instance, that Keith has two browsers open: one to look at http://www.fredsco.com, and one to look at http://www.espn.com. Keith also has his e-mail client software up all the time, and he has an FTP client working, downloading some files.

What happens when a new TCP segment arrives at Keith's computer? Well, the segment gets to the right computer, but it might have data for one of the two browsers, the e-mail client, or the FTP client, as shown in Figure 9-10.