Chapter 1. Operating System Issues and Features—The Big Picture

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.

Why UNIX Is Viable

When it comes to performance and throughput, a well-tuned combination of low-level software and hardware always has advantages over general-purpose systems. Although modern UNIX platforms employ more powerful CPUs and 64-bit mainboard architectures, ASIC-centered dedicated-purpose hardware still has a slight overall-performance advantage. Nonetheless, the striking advantage of general-purpose UNIX operating systems running on off-the-shelf servers certainly is the price/performance ratio and the intrinsic scripting and customizing capabilities of the UNIX OS and shells.

UNIX gateways successfully started to adopt hardware aspects by incorporating hardware crypto accelerator cards for speeding up Triple Data Encryption Standard (3DES) and Advanced Encryption Standard (AES) crypto processing and network adapters with enhanced I/O buffers and intelligence. Excellent serial WAN cards, Fibre Channel support, and EtherChannel-capable network interface cards (NICs) are available for UNIX as well. Asynchronous cards have been deployed successfully to build cheap terminal servers with high port density.

With regards to port density, UNIX gateways have closed the gap with commercial dedicated appliances by using affordable multiport NICs. In addition, UNIX can now use modern server bus architectures, which are capable of driving Gigabit Ethernet full-duplex NICs and ATM adapters.

UNIX operating systems are not as stripped-down or dedicated as embedded systems software; it is designed for carrying out a lot of tasks besides packet forwarding and processing. Nevertheless, the UNIX IP stacks are mature, perform well, and have been the test bed for a lot of infant technologies long before showing up in commercial hardware appliances. Good examples for this statement are multicasting, quality of service (QoS), and IPv6.