The packet's history begins when a user on one system sends a message or issues a command that must access a remote system. 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 system, as shown in Figure 1–1. 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.