State-of-the art UNIX operating systems are powerful in performance, robust, rich in features, and come with a lot of ancillary tools and packages out of the box. Due to their performance criteria and robustness, UNIX operating systems are the choice for many critical applications in enterprise and service provider environments, data centers, and machine clusters. GNU packages, especially compilers, form the foundation of most UNIX operating systems, with GNU/Linux being the most prominent open-source representative. Because of high-performance network stacks and a complete set of supported protocols, they are also a good choice for gateways, routers, and firewall systems.
This chapter offers a quick overview of the UNIX flavors used in this book, where they come from, some historical background, and a few words about their strengths and peculiarities. Aspects such as user space, kernel space, and the modular, monolithic, and microkernel approaches are discussed as well; UNIX, UNIX-like, and commercial UNIX derivatives are put into perspective. The embedded Cisco IOS architecture is discussed as a contrasting example of an operating system running on dedicated hardware, optimized and tuned for high-performance packet forwarding and manipulation enabled by specialized application-specific integrated circuits (ASICs) and bus architectures.