The protocols shown in Table 8-1 happen to be application layer protocols according to the TCP/IP model. Interestingly, there is an important distinction here between the application itself and the application layer protocol. Figure 8-4 provides a good backdrop to discuss the concept.
Figure 8-4. E-Mail Client Application and Its Use of Application Layer Protocols
A user uses e-mail client software to generate, send, receive, and read e-mails. The e-mail client must do work that has nothing to do with communication. It must have a user interface, accept text typed from the keyboard, understand what a user clicks on the screen, store e-mails on the computer hard disk, and so on. None of those tasks requires a network.
The e-mail client also needs to use the network. To do so, the people who wrote the e-mail client software had to read, understand, and implement e-mail TCP/IP protocols in the e-mail client software. So, the e-mail client application is not the same thing as the application layer protocol; rather, to communicate, the e-mail client must implement the correct application layer e-mail protocols.