Here's an excerpt from the preface:
This book deals with the architecture of the network subsystem in the Linux kernel. The idea for this book was born at the Institute of Telematics at the University of Karlsruhe, Germany, where the Linux kernel has been used in many research projects and its network functionality is modified or enhanced, respectively, in a targeted way. For instance, new services and protocols were developed for the next-generation Internet, and their behavior was studied. In addition, existing protocols, such as the TCP transport protocol, were modified to improve their behavior and adapt them to the new situation in the Internet.
In the course of these research projects, it has been found that the Linux kernel is very suitable for studying new network functionalities, because it features a stable and extensive implementation of the TCP/IP protocol family. The freely available source code allows us to modify and enhance the functionality of protocol instances easily. In addition, the enhancement of the kernel functionality is very elegantly supported by the principle of the kernel modules. However, many studies and theses in this field showed that familiarization with the Linux network architecture, which is required before you can modify the behavior of a protocol instance, demands considerable work and time. Unfortunately, this is mainly due to the facts that the network subsystem of the Linux kernel is poorly documented and that there is no material that would explain and summarize the basic concepts.
Although there are a few books that deal with the Linux kernel architecture and introduce its basic concepts, none of these books includes a full discussion of the network implementation. This situation may be due to the following two reasons:
The network subsystem in the Linux kernel is very complex. As mentioned above, it implements a large number of protocols, which is probably one good reason for the enormous success of Linux. Both [BoCe00] and [BBDK+01] mention that the description of all these protocols and their concepts would actually fill an entire book. Well, you are reading such a book now, and, as you can see, it has eventually turned out to be quite a large volume, although it describes only part of the network functionality, in addition to the basic concepts of the Linux network architecture.
Operating-system developers normally deal with the classical topics of system architecture—for example, the management of memories, processes, and devices, or the synchronization of parallel activities in a system—rather than with the handling of network packets. As you go along in this book, you will surely notice that it has been written not by system developers, but by computer-science specialists and communication engineers.
While considering the facts that there was little documentation covering the Linux network architecture and that students had to familiarize themselves with it over and over again, we had the idea of creating a simple documentation of the Linux network architecture ourselves. Another wish that eventually led to the more extensive concept of this book was a stronger discussion of important communication issues: design and implementation of network protocols in real-world systems. Networking courses teach students the most important concepts and standards in the field of telecommunication, but the design and implementation of network functionality (mainly of network protocols) by use of computer-science concepts has enjoyed little attention in teaching efforts, despite the fact that this knowledge could have been used often within the scope of studies and theses. The authors consider the description of the implementation of the Linux network architecture and its structure, interfaces, and applied concepts a step towards strengthening the informatics component in networking classes.
The authors hope that this book will help to make the processes and structures of the Linux network architecture easier to understand, and, above all, that our readers will have fun dealing with it and perhaps learn a few things about the networking concept and its practical implementation.
The content of this book corresponds to our knowledge of the Linux network architecture. This knowledge is neither comprehensive nor exhaustive. Nevertheless, we have tried to represent the processes and structures of the Linux network architecture in a fashion as easily understandable and detailed as possible. We are thankful for all hints, suggestions for improvement, ideas, and comments, and we will try to consider them in later editions. Updated information about the Linux network architecture and this book is available online at http://www.Linux-netzwerkarchitektur.de.
~blacky