The Linux kernel supports many different network architectures (TCP/IP being just one of them), implements several alternative algorithms for scheduling the network packets, and includes programs that make it easy for system administrators to set up routers, gateways, firewalls, and even a simple World Wide Web server, directly at the kernel level.
The current code, inspired from the original Berkeley Unix implementation, is referred to as Net-4. As the name suggests, it is the fourth major version of Linux networking. Similar to VFS, the code uses objects to provide a common interface to the large number of available architectures. However, contrary to VFS, the networking code is organized into layers, each of which has a well-defined interface with the adjacent layers. Since data transmitted along the network is not reusable, there is no need to cache it. For the sake of efficiency, Linux avoids copying the data across layers; the original data is stored in a memory buffer, which is large enough to contain the control information requested by each layer.
Packing a detailed description of the Linux networking code in a single chapter of a book would be a truly impossible mission. In fact, nearly 20 percent of all kernel source code is devoted to networking. Therefore, we couldn't even succeed, within the space constraints of a single chapter, in mentioning the names of all the features, components, and data structures of the Linux network subsystem.
Our objective is more limited. We concentrate on the well-known TCP/IP stack of protocols and consider only the data link layer, the network layer, and the transport layer. Furthermore, for the sake of simplicity, we focus our attention on the UDP protocol and attempt to give a succinct description of how the kernel succeeds in sending or receiving a single datagram. Finally, we assume that our computer is connected to a local area network by means of an Ethernet card.
The first section of the chapter covers the main data structures used by Linux networking, while the second one illustrates the system calls needed to send or receive a single datagram and describes sketchily the corresponding service routines. The last two sections describe how the kernel interacts with the network card to send or receive a packet.
We assume that you already have some background in network protocols, layers, and applications. There are many good books on these topics, some of which are listed in the Bibliography at the end of this book.
One final remark: writing programs for the network subsystem is quite a hard task. While you have to stick to the documented standards, following them is not enough because they do not specify the smallest, most cumbersome details of the protocols. Thus you have to take into account the implementations of the already existing network programs, even those in other operating systems (bugs included). And, of course, you must write fast and efficient programs; otherwise your server will not keep up with the highest network loads.