This paper describes a new operating system kernel, called the x-kernel, that provides an explicit architecture for constructing and composing network protocols. Our experience implementing and evaluating several protocols in the x-kernel shows that this architecture is both general enough to accommodate a wide range of protocols, yet efficient enough to perform competitively with less structured operating systems. 1 Introduction Network software is at the heart of any distributed system. It manages the communication hardware that connects the processors in the system and it defines abstractions through which processes running on those processors exchange messages. Network software is extremely complex: it must hide the details of the underlying hardware, recover from transmission failures, ensure that messages are delivered to the application processes in the appropriate order, and manage the encoding and decoding of data. To help manage this complexity, network software is divi...

Twelve years ago, the Computer Science Department established the Xinu laboratory, which is currently used for education as well as operating systems research. During the past year, facilities in the lab were replaced by Intel Pentium computers, and the support system was redesigned to accommodate the new hardware. This paper describes the new hardware and software used in the lab. It shows how computers are connected to networks, and explains how the support software permits users to cooperate in using the new hardware. 1 Introduction Xinu [2, 3] is a small, multi-threaded operating system that follows a hierarchical structure. Twelve years ago, the Computer Science Department established the Xinu laboratory, which is currently used for education as well as operating systems research. The laboratory includes two sets of computers. Computers in one set, called front-ends, are conventional UNIX workstations that students use to create, compile, and link operating system images. Comput...

this paper.