深入解析Linux内核网络协议设计

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"《Linux网络体系结构：Linux内核中网络协议的设计与实现》是由Klaus Wehrle、Frank Pählke、Hartmut Ritter、Daniel Müller和Marc Bechler合著的一本专业书籍，由Prentice Hall出版社于2004年出版。本书共648页，全面深入地探讨了Linux操作系统下的网络架构，是学习和理解Linux网络内部运作的权威指南。书中涵盖了从Linux操作系统的基础到内核结构，再到网络协议的具体设计和实现。" 在深入探讨Linux网络体系结构的过程中，作者首先介绍了Linux操作系统及其本质，解释了为何选择Linux作为网络协议实现的平台。Linux作为一种开放源码的操作系统，因其可定制性、稳定性和广泛的社区支持而被广泛采用。接着，书中详细讲解了Linux内核的结构，对比了单体架构和微内核的不同，并描述了内核在处理各种活动时的角色。内核中的锁定机制，如原子操作，对于确保多线程环境中的数据一致性至关重要。这部分内容有助于读者理解内核如何在并发环境下管理网络流量。此外，书中还详细讨论了网络协议栈的各个层次，包括物理层、数据链路层、网络层、传输层以及应用层。这些层分别对应了OSI模型中的不同部分，如以太网、IP、TCP、UDP等协议的实现。通过这一部分，读者可以了解到Linux内核如何处理网络数据包，从硬件接口接收，经过网络层的路由决策，到传输层的错误控制和流控，直至最终交付给应用程序。书中的内容不仅限于理论介绍，还包括了大量的实践案例和调试技巧，使得读者能够更好地理解和解决实际问题。同时，书中引用了其他的信息来源，为读者提供了进一步学习和研究的路径。《Linux网络体系结构》是一本适合Linux系统管理员、软件开发者以及任何对操作系统和网络技术感兴趣的读者的深度读物。通过阅读，读者可以掌握Linux内核中网络协议的设计原理，了解其工作流程，并具备诊断和优化网络性能的能力。

1.1 The Linux Operating System

Linux is a freely available multiuser, multitasking, multiprocessor, and multiplatform UNIX

operating system. Its popularity and the number of users increase continually, making Linux an

increasingly serious factor in the operating-systems market. Thanks to the freely available source

code that everybody can obtain over the Internet and to the fact that everybody can participate in

and contribute to the further development of the Linux system, many developers, all over the

world, are constantly busy further developing this system, removing existing errors, and

optimizing the system's performance.

The fact that most developers do this very time-consuming work for free in their spare time is a

sign of the great fun working with Linux and mainly with the Linux kernel can be. As we progress

in this book, we will try to pass some of this enthusiasm on to our readers. The large number of

research projects at the University of Karlsruhe that have used, enhanced, or modified the Linux

network architecture experienced a high motivation of all participating students. The reason was

mainly that this offered them a way to participate in the "Linux movement."

The development of Linux was initiated by a student by the name of Linus B. Torvalds, in 1991.

At that time, he worked five months on his idea of a new PC-based UNIX-like operating system,

which he eventually made available for free on the Internet. It was intended to offer more

functions than the Minix system designed by Andrew S. Tanenbaum, which was developed for

teaching purposes only [Tane95]. With his message in the Minix newsgroup (see page 1), he set

a movement in motion, the current result of which is one of the most stable and widely developed

UNIX operating systems. Back then, Linus Torvalds planned only the development of a purely

experimental system, but his idea further developed during the following years, so that Linux is

now used successfully by many private people, corporations, and scientists alike. Mainly, the

interoperability with other systems (Apple, MS-Windows) and the ability to run on many different

platforms (Intel x86, MIPS, PA-RISC, IA64, Alpha, ARM, Sparc, PowerPC, M68, S390) make Linux

one of the most popular operating systems.

Not only the extensive functionality of Linux, but also the freely accessible source code of this

operating system, have convinced many private people and companies to use Linux. In addition,

the German government, with its program for the support of open-source software, promotes the

use of freely available programs with freely available source code. The main reason for this is

seen not in the low procurement cost, but in the transparency of the software used. In fact,

anyone can view the source code and investigate its functionality. Above all, anyone can check

what—perhaps security-relevant—functionalities or errors are contained in an application or

operating system. Especially with commercial systems and applications, there are often

speculations that they could convey information about the user or the installed applications to the

manufacturer.

You do not have such fears with freely developed software, where such a behavior would be

noticed and published quickly. Normally, several developers work concurrently on an open-

source project in a distributed way over the Internet, monitoring themselves implicitly. After all,

free software is not aimed at maximizing the profit of a company or its shareholders. Its goal is to

develop high-quality software for everybody. Linux is a very good example showing that freely

developed software is not just the hobby of a handful of freaks, but leads to serious and

extremely stable applications.

The authors of this book use Linux mainly for research work in the network area. The freely

available source texts allow us to implement and evaluate new theories and protocols in real-

world networks. For example, Linux was used to study various modifications of the TCP transport

protocol [WeRW01, Ritt01], to develop a framework for the KIDS QoS support [Wehr01b], and to

develop the high-resolution UKA-APIC timer [WeRi00].

1.2 What is Linux?

Originally, the term Linux described only the operating-system kernel that abstracts from the

hardware of a system, offering applications a uniform interface. Over time, the term Linux has

often come to mean the kernel (the actual Linux) together with the entire system environment,

including the following components:

the operating-system kernel (currently version 2.0, 2.2, or 2.4);

the system programs (compiler, libraries, tools, etc.);

the graphical user interface (e.g., XFree) and a window manager or an application

environment (KDE, Gnome, FVWM, etc.);

a large number of applications from all areas (editors, browsers, office applications, games,

etc.).

Different components not forming part of the kernel originate largely from the GNU project of

Free Software Foundation, which explains why the complete system environment is often called

"GNU/Linux system." A characteristic common to the Linux kernel and GNU programs is that they

may all be freely distributed under the GNU Public License (GPL), provided that the source text is

made publicly available. To the extent that enhancements or modifications have been effected to

the programs, then these are automatically governed by the GNU license (i.e., their source text

must also be made freely available). Since the advent of Linux, this has had the effect that the

system has been further developed free from corporate policy interests and that it has been more

strongly oriented to word its users' needs than are other, commercial operating systems. Anyone

can participate in the development and implement new capabilities, ones based on the freely

available source texts. This means that Linux is always involved in the support of international

standards, and no attempt is made to enforce corporate or proprietary standards to secure a

market position.

Errors made during the development of a piece of software are normally removed quickly. In

addition, there is a continual effort to keep the system performing as well as possible. This has

become very clear in the example of the network implementation in the last kernel version: After it

had become known that the performance of Linux in the area of protocol handling on

multiprocessor systems suffers from a few flaws, the network part was extensively rewritten to

remove these faults. This means that Linux is an example that clearly shows the benefits of open-

source projects:

stability,

performance, and

security.

1.3 Reasons for Using Linux

The previous section introduced the important properties and objectives of Linux as a free

software project. This section will discuss a number of general properties of the Linux kernel,

offering more reasons for its use:

Linux supports preemptive multitasking: All processes run independently in different

protected memory spaces, so that the failure of one process does not in any way impair the

other processes. When a process claims too much computing time, its processor can be

taken and allocated to another waiting application. Preemptive multitasking is a fundamental

requirement for stable systems.

Multiprocessor: Linux is one of the few operating systems supporting several processors in

SMP (Symmetric MultiProcessing) operation. This means that several processes can be

handled concurrently by different CPUs. Since kernel version 2.0, multiprocessor systems

with Intel and Sparc processors are supported. Version 2.2 and the current Version 2.4

additionally improved the performance and parallelism in the Linux kernel.

Multiuser: Several users can work concurrently in one system, when they are logged in over

different consoles. In addition, users can work easily on several graphical user interfaces.

Multiplatform: properties of: Linux was originally developed only for the personal computer

(Intel 80386), but it runs on more than ten processor architectures today. The bandwidth of

supported platforms extends from small digital personal assistants over the standard

personal computer to mainframe architectures: Intel x86, MIPS, PA-RISC, IA64, Alpha, ARM,

Sparc, PowerPC, M68, and so on.

Linux is a UNIX system: It is compatible with the POSIX-1300.1 standard

[2]

and includes

large parts of the functionality of UNIX System V and BSD. This means that you can use

UNIX standard software under Linux.

[2]

Portable Operating System Interface based on UniX—POSIX 1300.1 defines a

minimum interface that each UNIX-like operating system must offer.

Rich network functionality: The Linux network architecture makes available an extensive

choice of network protocols and functionalities in the networking area. The development of

the Internet and its services is inseparably linked to UNIX systems. This is why the

properties of the TCP/IP protocol family and its behavior can best be studied and controlled

in a UNIX system. Other PC operating systems would be unsuitable for this, especially those

with source code not publicly available.

Open source: The source code of the entire Linux kernel is freely available and can be used

according to the GNU Public License. A large number of programmers work on the further

development of the Linux kernel all over the world, continually enhancing and improving it.

Linux is distributed over the Internet so that each user can test the kernel and make

improvements or enhancements. The development of Linux in this dimension would not

have been possible without the Internet.

Formerly, users had to put up with defects in software they purchased; Linux now allows

everyone to remove such defects. And it really works. An often heard criticism has been that

the driver support for Linux is one of its major problems. This situation has changed

dramatically during the past years. For instance, all actually available network cards are

supported by Linux. In fact, we can rely to the Linux community to such an extent that there

will soon be a matching driver for each new device.

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