Python in a Nutshell, 3rd Edition

Chapter 17. Networking Basics

Modern distributed computer systems make extensive use of networking. Understanding network communications is an essential part of building such systems, since such communications underlie the use of any network-based service. A protocol is an agreed “language” that two parties (often, nowadays, computer programs) use to communicate with each other. There are two basic flavors of networking, connection-oriented and connectionless, which we cover in “Networking Principles”, each with its own protocols. Both flavors can operate over a wide range of transport mechanisms thanks to the ubiquity of the TCP/IP stack and the socket interface, first devised to support networking in BSD Unix.

The overall task of communication splits networking into protocol layers, to separate different functions. Data can be carried between systems in many different ways—over an Ethernet, across a serial link, and so on—and it would needlessly complicate application code to have to handle all the differences. The ISO (International Standards Organization) defined a seven-layer model, but this has proved in practice to be unnecessarily complex; TCP/IP uses a four-layer model.

The application code implements the process or application layer, concerned with exchanging messages between two processes potentially running on different computers (although TCP/IP works just as well between local processes). This layer passes messages to the host-to-host or transport layer, concerned ...