Application Layer—User Initiates Communication

The packet's history begins when a user on one host sends a message or issues a command that must access a remote host. The application protocol formats the packet so that the appropriate transport layer protocol, TCP or UDP, can handle the packet.

Suppose the user issues an rlogin command to log in to the remote host, as shown in Figure 2–1. The rlogin command uses the TCP transport layer protocol. TCP expects to receive data in the form of a stream of bytes that contain the information in the command. Therefore, rlogin sends this data as a TCP stream.

Not all application layer protocols use TCP, however. Suppose a user wants to mount a file system on a remote host, thus initiating the NIS+ application layer protocol. NIS+ uses the UDP transport layer protocol. Therefore, the packet that contains the command must be formatted in a manner that UDP expects. This type of packet is referred to as a message.