Application Program and Application Layer Asking the Transport Layer for Help

Figure 9-1. Application Program and Application Layer Asking the Transport Layer for Help

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As shown in Figure 8-4 in the previous chapter, the application program includes the user interface and the application layer protocols. In Figure 8-6, you saw some of the SMTP messages that were used to send e-mail. That figure and several others in Chapter 8 showed the messages going back and forth between two PCs. In reality, when an e-mail client like the one in Figure 9-1 sends these types of messages, it asks for a transport layer protocol to help.

Generally speaking, when Those Who Came Before Us created networking models like TCP/IP, they defined a bunch of protocols. As it turns out, the protocols at one layer provide services to the protocols at one layer above. Conversely, protocols at one layer expect services from protocols that are one layer below. In this chapter, you'll read about how the application layer protocols such as SMTP expect the transport layer protocols, such as TCP, to provide the service of transporting the data across the network.