Chapter Fourteen

Abstract

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. 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 ker nel structur es, and it then " transmits " and " receives " blocks on request, by means of its request 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. Ther e ar e a few important differ ences between mounted disks and packet-delivery inter faces. To begin with, a disk exists as a special file in the /dev dir ectory, wher eas a network interface has no such entry point. The normal file operations (r ead, 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 differ ent set of operations. Although you may object that applications use the read and write system calls when using sockets, those calls act on a software object that is distinct from the inter face. Several hundred sockets can be multiplexed on the same physical interface. But the most important differ ence between the two is that block drivers operate only in response to requests from the kernel, whereas network drivers receive packets asynchronously from the outside. Thus, while a block driver is asked to send a buffer toward the kernel, the network device asks to push incoming packets toward the kernel. The kernel interface for network drivers is designed for this differ ent mode of operation.