Linux Device Drivers

Chapter 14. Network Drivers

We are now through discussing char and block drivers and are ready to move on to the fascinating world of networking. Network interfaces are the third standard class of Linux devices, and this chapter describes how they interact with the rest of the kernel.

A network interface doesn’t exist in the filesystem the way char and block devices do. Instead, it deals with packet transmission and reception at the kernel level, without being bound to an open file in a process.

The role of a network interface within the system is similar to that of a mounted block device. A block device registers its features in the blk_dev array and other kernel structures, and it then ``transmits'' and ``receives'' blocks on request, by means of its request_fn function. Similarly, a network interface must register itself in specific data structures in order to be invoked when packets are exchanged with the outside world.

There are a few important differences between mounted disks and packet-delivery interfaces. To begin with, a disk exists as a node in the /dev directory, while a network interface doesn’t appear in the filesystem. But the most important difference between the two is that while the disk is asked to send a buffer towards the kernel, the net device asks to push incoming packets towards the kernel.

The network subsystem of the Linux kernel is designed to be completely protocol-independent. This applies to both networking protocols (IP vs. IPX or other protocols) ...