Chapter 17. Network Drivers

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.

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 interface. Several hundred sockets can be multiplexed on the same physical interface.

But the most important difference 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 different mode of operation.

Network drivers also have to be prepared to support a number of administrative tasks, such as setting addresses, modifying transmission parameters, and maintaining traffic and error statistics. The API for network drivers reflects this need and, therefore, looks somewhat different from the interfaces we have seen so far.

The network subsystem of the Linux kernel is designed to be completely protocol-independent. This applies to both networking protocols ( Internet protocol [IP] versus IPX or other protocols) and hardware protocols (Ethernet versus token ring, etc.). Interaction between a network driver and the kernel properly deals with one network packet at a time; this allows protocol issues to be hidden neatly from the driver and the physical transmission to be hidden from the protocol.

This chapter describes how the network interfaces fit in with the rest of the Linux kernel and provides examples in the form of a memory-based modularized network interface, which is called (you guessed it) snull. To simplify the discussion, the interface uses the Ethernet hardware protocol and transmits IP packets. The knowledge you acquire from examining snull can be readily applied to protocols other than IP, and writing a non-Ethernet driver is different only in tiny details related to the actual network protocol.

This chapter doesn't talk about IP numbering schemes, network protocols, or other general networking concepts. Such topics are not (usually) of concern to the driver writer, and it's impossible to offer a satisfactory overview of networking technology in less than a few hundred pages. The interested reader is urged to refer to other books describing networking issues.

One note on terminology is called for before getting into network devices. The networking world uses the term octet to refer to a group of eight bits, which is generally the smallest unit understood by networking devices and protocols. The term byte is almost never encountered in this context. In keeping with standard usage, we will use octet when talking about networking devices.

The term "header" also merits a quick mention. A header is a set of bytes (err, octets) prepended to a packet as it is passed through the various layers of the networking subsystem. When an application sends a block of data through a TCP socket, the networking subsystem breaks that data up into packets and puts a TCP header, describing where each packet fits within the stream, at the beginning. The lower levels then put an IP header, used to route the packet to its destination, in front of the TCP header. If the packet moves over an Ethernet-like medium, an Ethernet header, interpreted by the hardware, goes in front of the rest. Network drivers need not concern themselves with higher-level headers (usually), but they often must be involved in the creation of the hardware-level header.