This book describes the design and implementation of the 4.4BSD operating system, the latest release of what previously was known as the "Berkeley" version of UNIX. Because the 4.4BSD operating system is freely available in source and binary form, it is the system of choice for researchers, developers, and programmers. From its years of use in commercial environments, the robustness of the 4.4BSD operating system has made it the most common platform for providing network services on the Internet including routing, firewall protection, WWW services, and electronic mail handling. As key participants in the development of 4.4BSD, the authors provide comprehensive and up-to-date technical information needed by system programmers and application programmers.