| 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. Efficient network implementation: Meanwhile, the Linux kernel makes available a well-structured implementation of the network functionality, which will be our main focus of discussion in the next 27 chapters of this book. The functions can be adapted to the special requirements of the desired system and meet the specifications of the Internet Engineering Task Force (IETF), IEE, and ISO better than many other systems. In the creation of a new kernel, its desired functionality can be individually configured. For instance, you can enable a large number of optimization options or add specific functionalities (e.g., multicast support and various protocols). While the system is running, you can also use the Proc file system (see Section 2.8) to change parameters For example, various timeout values for the TCP transport protocol or configuration parameters of other protocol instances. There is even a way to use the Proc file system to enable and disable certain functions at runtime, such as packet forwarding. IP Next Generation: Since Version 2.1.38, the Linux kernel provides a stable and relatively complete implementation of the new Internet Protocol IPv6. (See Chapter 23.) And, finally, the best argument: Linux is free of charge. Everyone can download it from the Internet or buy it on CD for a few dollars, usually with a few gigabyte applications (some of them being more useful, some less) and with extensive installation instructions and free support. This means that, for very little money, you can get a high-performing, extremely stable, and easily adaptable operating system that turns a Pentium PC into a high-performing workstation, a highly reliable server, or an individually configurable Internet router.
This chapter has been a brief introduction to Linux; the next chapter will introduce the internal structure of the Linux kernel. We will then discuss the basic structure of communication systems in general and the structure of the Linux network architecture in particular. |
