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...

By using existing information in packet headers, routers can provide system administrators a facility to manage network connections between computers. Host address, network number, interface, direction, protocol, and port number are parameters that may be used to implement an access control policy. We present experiences developing the packet filtering facility in the NetBlazer dial-up IP router. We address the sometimes conflicting design goals of efficient performance and ease of administration by choosing internal data structures that simplify per packet lookup and then devoting 90 per cent of our code to implementing commands that maintain these tables in manner that is easy for system administrators. Introduction Wide area networks provide remote sites convenient access to local networks. With this increased convenience comes the often complex problem of unauthorized access to network resources. Packet filtering in an IP router can be used to manage this complexity by co...

this paper.