UNIX was born in the early 1960s at AT&T Research as a robust timesharing operating system to overcome some of the restrictions of Multics (Multiplexed Information and Computing System). Over time, myriad flavors and derivatives have emerged and evolved, while the original MILNET/ARPANET evolved into NFSNET and later into the Internet as we know it today.
With the advent of GNU/Linux, UNIX started to attract unprecedented public attention and widespread acceptance. This also led to more popularity of Berkeley Software Distribution (BSD)-like Unices and finally to Apple Inc.'s decision to shift the Macintosh OS toward an operating system based on FreeBSD. In addition, UNIX started to expand its undisputed reign in the server arena into workstations, notebooks, and even PDAs and cell phones. A new momentum was added to the mix with the introduction of distributions as a container for operating systems such as Gentoo, Debian, or RedHat/Fedora in the Linux arena.
I do not have a preference when it comes to UNIX flavors. This book is bias-free and stays away from the religious wars a lot of UNIX aficionados and evangelists engage in nowadays. Open-source as well as commercial and proprietary approaches all have their merits. I see the world as a blend of the AT&T System V (SVR4) UNIX and 4.4-lite BSD derivatives. In a world of POSIX standards, the difference is not such a big deal anymore. Most UNIX or UNIX-like systems offer the same look and feel and directory layout. The differences among these systems are most prevalent when dealing with disk organization and file systems.
Throughout this book, I stick to the three open-source operating systems?OpenBSD, FreeBSD, and Linux?for the discussion of the IP stack and user-space architectures. However, most of the conclusions and concepts are valid on other platforms as well, such as SUN Solaris, Mac OS X/Darwin, and NetBSD.
The brand UNIX is, after a long and glorious journey, now a registered trademark of the Open Group (http://www.opengroup.org). Hence, several open-source projects call their architectures UNIX-like to avoid copyright issues. I do not make this differentiation throughout this book.