Understanding Linux Network Internals

Chapter 13. Protocol Handlers

Protocols are the framework for all communication: they indicate to each correspondent how to understand the other side of a conversation. In Linux, communication is understood through a protocol handler at each networking layer. This chapter explains how these handlers are installed, chosen at runtime, and invoked.

To understand the relationship among communication layers and protocols, imagine a possible situation in real life where I have to talk to a stranger. What language should I use? If I’m in Italy I’ll begin with Italian, and if I’m in the United States I’ll try English. If these don’t work, there may be ways to negotiate the use of a different language.

On top of that basic protocol, there are others. When writing a letter, for instance, my relationship with the correspondent determines whether I begin “Dear Madam” or “Hi, gal!” These sorts of choices take place at many layers of real-life communication. Networks have layers too, and the choice of protocols becomes formalized in network code.

Overview of Network Stack

Readers of this book are expected to be familiar with the basic TCP/IP protocols, but there are some other protocols in common use—such as Logical Link Control (LLC) and Subnetwork Access Protocol (SNAP)—that you may not know. This section introduces key protocols and shows their relationships.

The two best-known models for network protocols are the seven-layer OSI model and the five-layer TCP/IP model, shown in Figure 13-1. The ...