Internet Core Protocols: The Definitive Guide

How Application Protocols Communicate Over IP

Almost all IP applications follow the same basic model: a client sends a request of some kind to a server running on another system, and the server examines the request, acts upon it in some form, and then possibly returns some form of data back to the client. This is not always the case (many UDP-based “servers” do not return any data, but simply monitor network activity), but it holds true for most applications.

Server-based applications (like an email server or web server) are generally loaded by the operating system when the computer is started. The servers then go into a “listen” state, watching for incoming connections. Conversely, client applications will only establish a connection when some sort of action is required (like “get new messages”).

Applications communicate with the transport protocols through the use of “ports,” which are unique I/O identifiers used by the transport protocols and the specific instance of the application protocol. “Ports” are conceptually similar to the mailboxes used at your local post office. When a letter comes in for a recipient, it is placed into a known mailbox reserved for that specific recipient. Whenever the recipient comes by, he will pick up any messages in that mailbox and process the data at his convenience.

Similarly, ports provide TCP and UDP with a way to deliver data to higher-layer application protocols. Every time an application protocol opens a connection to one of the transport ...