Internet Core Protocols: The Definitive Guide

Application addressing with TCP ports

Applications communicate with TCP through the use of ports, which are practically identical to the ports found in UDP. Application are assigned 16-bit port numbers when they register with TCP, and TCP uses these port numbers for all incoming and outgoing traffic.

Conceptually, port numbers provide “extensions” for the individual applications in use on a system, with the IP address of the local system acting as the main phone number. Remote applications “call” the host system (using the IP address), and also provide the extension number (port number) of the destination application that they want to communicate with. TCP uses this information to identify the sending and receiving applications, and to deliver data to the correct application.

Technically, this procedure is a bit more complex than it is being described here. When an application wishes to communicate with another application, it will give the data to TCP through its assigned port number, telling TCP the port number and IP address of the destination application. TCP will then create the necessary TCP message (called a “segment”), marking the source and destination port numbers in the message headers, and storing whatever data is being sent in the payload portion of the message. A complete TCP segment will then get passed off to the local IP software for delivery to the remote system (which will create the necessary IP datagram and shoot it off).

Once the IP datagram is received by ...