Understanding Linux Network Internals

Common Coding Patterns

Each networking feature, like any other kernel feature, is just one of the citizens inside the kernel. As such, it must make proper and fair use of memory, CPU, and all other shared resources. Most features are not written as standalone pieces of kernel code, but interact with other kernel components more or less heavily depending on the feature. They therefore try, as much as possible, to follow similar mechanisms to implement similar functionalities (there is no need to reinvent the wheel every time).

Some requirements are common to several kernel components, such as the need to allocate several instances of the same data structure type, the need to keep track of references to an instance of a data structure to avoid unsafe memory deallocations, etc. In the following subsections, we will view common ways in Linux to handle such requirements. I will also talk about common coding tricks that you may come across while browsing the kernel's code.

This book uses subsystem as a loose term to describe a collection of files that implement a major set of features—such as IP or routing—and that tend to be maintained by the same people and to change in lockstep. In the rest of the chapter, I'll also use the term kernel component to refer to these subsystems, because the conventions discussed here apply to most parts of the kernel, not just those involved in networking.

Memory Caches

The kernel uses the kmalloc and kfree functions to allocate and free a memory block, respectively. ...