Understanding Linux Network Internals

Chapter 27. Neighboring Subsystem: Infrastructure

In Chapter 26, we saw the main problems that the neighboring protocols are asked to solve. You also learned that the Linux kernel abstracted out parts of the solution into a common infrastructure shared by various neighboring protocols. In this chapter, we will see how the infrastructure is designed. In particular, we will see how protocols interface to the common infrastructure, how caching and proxying are implemented, and how external subsystems such as higher-layer protocols notify the neighboring protocols about interesting events. We will conclude the chapter with a description of how L3 protocols such as IPv4 actually interface with their neighboring protocols, and how queuing is implemented for buffers awaiting address resolution.

Main Data Structures

To understand the code for the neighboring infrastructure, we first need to describe a few data structures used heavily in the neighboring subsystem, and see how they interact with each other.

Most of the definitions for these structures can be found in the file include/net/neighbour.h. Note that the Linux kernel code uses the British spelling neighbour for data structures and functions related to this subsystem. When speaking generically of neighbors, this book sticks to the American spelling, which is the spelling found in RFCs and other official documents.

struct neighbour

Stores information about a neighbor, such as the L2 and L3 addresses, the NUD state, the device through ...