On Modular Algebraic Protocol Specification
Jacob Brunekreef

Abstract:
The information exchange between computers is governed by a set of
rules that is usually called a `communication protocol'. Such a
protocol contains a detailed description of how to act and, equally
important, of how to react in a communication session with one or more
other computers. This thesis is about communication protocols.

Process algebra is a new part of computer science. For about fifteen
years the behaviour of systems is being studied within this particular
mathematical framework. Behaviour is described (specified) in process
equations that look like the well--known algebraic equations.
Furthermore, like in ordinary algebra, the elements of these equations
can be subject to symbolic manipulations. This thesis is about the
specification of communication protocols in terms of process algebra
equations.

A modular approach is obvious when large problems have to be solved.
The decomposition of a large problem into a set of smaller
sub--problems (modules) is a well--known technique in computer
science. On the other hand, a `library' of modules can be helpful in
the composition of a solution out of previously solved sub--problems.
This thesis is about a modular approach to algebraic protocol
specification.

Process algebra is one of the many Formal Description Techniques that
have been defined during the last decades. These Formal Description
Techniques play an increasing role in the battle against erroneous
software that is delivered too late at a price that is is much too
high. In Chapter 1 a general introduction to the application of Formal
Methods in the field of software development is given. The algebraic
specification formalism PSF, applied throughout this thesis, is
introduced.

If formal specifications ever are to play a significant role, they not
only have to be correct, but also have to be comprehensible for human
readers. In Chapter 2 the construction of (large) algebraic
specifications is discussed. Topics such as specification styles,
modular specification and a well--considered application of data
structures are investigated. In this chapter a modular specification
style is developed that will be applied in the case studies in the
last three chapters.

The world of communication protocols is briefly introduced in Chapter
3. In this chapter also some general aspects of algebraic protocol
specification are treated, such as channel specification and the
modelling of timeouts.

The remainder of this thesis consists of three case studies in modular
algebraic protocol specification.

Sliding Window Protocols are used to obtain an error--free two--way
communication link for data exchange between two adjacent computers in
a network. In Chapter 4 three Sliding Window Protocols of increasing
complexity are specified. The protocols are taken from the literature
on computer networks.

The Ethernet Protocol is used in Local Area Networks in which a
single communication medium (a `bus') is used by all connected network
stations. In this protocol the problem that has to be solved is the
use of the medium by two or more network stations at the same time. In
Chapter 5 the Ethernet Protocol is specified. This protocol is one of
the growing number of communication protocols that has been officially
standardised. The IEEE/ANSI standard 802.3 has served as the basis for
the formal specifications in this chapter. As the complete standard
contains many details, in this chapter first an `abstract' version is
specified which focusses on the central issues of the Ethernet
Protocol.

Leader Election Protocols are used when the election of a single
network process out of a set of processes is needed. Usually the
elected `leader' holds certain rights with respect to the offering of
certain services or with respect to network management functions. In
Chapter 6 three related Leader Election Protocols for a broadcast
network (in which each transmitted message is received by all other
network components) are specified. The final protocol is
fault--tolerant: network components may crash and revive at any moment
during the election.

At the end of each case study some concluding remarks are made. These
remarks are concerned with the protocols as well as with the
limitations of the specification formalism PSF.