The x-Kernel: An architecture for implementing network protocols - IDA

The x-Kernel: An Architecture for Implementing Network
Protocols
Norman C. Hutchinson and Larry L. Peterson
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 divided into multiple layers—commonly called protocols—
each of which is responsible for some aspect of the communication. Typically, a system’s protocols are
implemented in the operating system kernel on each processor.
This paper describes a new operating system kernel—called the x-kernel—that is designed to facilitate
the implementation of efficient communication protocols. The x-kernel runs on Sun3 workstations, is
configurable, supports multiple address spaces and light-weight processes, and provides an architecture for
implementing and composing network protocols. We have used the x-kernel as a vehicle for experimenting
with the decomposition of large protocols into primitive building block pieces [15 ], as a workbench for
designing and evaluating new protocols [22], and as a platform for accessing heterogeneous collections
of network services [23].
Many operating systems support abstractions for encapsulating protocols; examples include Berkeley
Unix sockets [17] and System V Unix streams [28]. Such abstractions are useful because they provide
a common interface to a collection of dissimilar protocols, thereby simplifying the task of composing
protocols. Defining truly general abstractions is difficult because protocols range from connection-oriented
Authors’ Address: Department of Computer Science, University of Arizona, Tucson, AZ 85721. This work supported in
part by National Science Foundation Grant CCR-8811423.
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