Understanding Linux Network Internals

Chapter 18. Internet Protocol Version 4 (IPv4): Concepts

This chapter explains what the IP protocol is responsible for, and provides a discussion of the IP header fields that support these activities and the impact of these responsibilities on possible implementations. While the chapter discusses some of the choices made in Linux, implementation details are covered in subsequent chapters.

It would be interesting to show how the protocols of the IPsec security suite have been integrated with the IP protocol, but I could not include this topic for lack of space. However, we will sometimes see how the presence of IPsec transformations influences the implementation of core routines.

IP Protocol: The Big Picture

Figure 18-1 shows the important relationships among the components of Linux that handle IPv4. The flow of traffic between major functions is represented by arrows. We will analyze all of these functions in the next few chapters. The figure shows the placement of two subsystems described elsewhere—the Neighboring subsystem and the Traffic Control subsystem—as well as the many hooks where the Netfilter firewalling system can be invoked.^[*]

Figure 18-1 is a useful reference when you’re examining networking code and wondering whether a particular function is used for input or output, whether it is called during forwarding, and who calls it.

Since the IP layer does not interact directly with the Traffic Control subsystem, that subsystem is left to Part VI. However, in the section "Interface ...