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 associated with the command or message formats the packet so that it can be handled by the appropriate transport layer protocol, TCP or UDP.

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 containing 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 containing the command must be formatted in a manner that UDP expects. This type of packet is referred to as a message.