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Understanding the Linux Kernel, Second Edition by Marco Cesati, Daniel P. Bovet

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Chapter 18. Networking

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.

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