Having discussed char and block drivers, we are now ready to move on to the 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.
The role of a network interface within the system is similar to that of a mounted block device. A block device registers its disks and methods with the kernel, and then "transmits" and "receives" blocks on request, by means of its request function. Similarly, a network interface must register itself within specific kernel 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 special file in the /dev directory, whereas a network interface has no such entry point. The normal file operations (read, write, and so on) do not make sense when applied to network interfaces, so it is not possible to apply the Unix "everything is a file" approach to them. Thus, network interfaces exist in their own namespace and export a different set of operations.