"Hello, I'm at Your Service"
"Hello, this is Fred at Fred's Shipping Company. How can I help you today?" Yep, Fred, the owner, is answering the phones at his small shipping company. He's waiting for the phone to ring so that he can send his one driver, Barney, in his one truck to pick up a package and deliver it across town. To be ready for that first phone call, Fred bought a truck; hired Barney; got an office, phone lines, and Internet website; and did some advertising. Finally, the phone rang, giving Fred his first opportunity to ship the goods and make some money.
Like Fred, transport layer protocols must be prepared for when an application layer protocol needs its services. If you use TCP/IP applications, the application layer protocols use TCP/IP transport layer protocols to send their messages across the network. Whereas the application layer protocols are typically part of the application software, the transport layer protocols are typically part of the operating system (OS) of the computer. So, the transport layer software hangs around, waiting on the application programspecifically the part of the application program that implements the application layer protocolsto ask it to do something. Figure 9-1 depicts the general idea.