A formal approach to conformance testing

In order to assure successful communication between computer systems from different manufacturers, standardized communication protocols are being developed and specified. As a next step implementations of these protocols are needed that conform to these specifications. Testing is a way to check correctness of protocol implementations with respect to their specifications. This activity is known as protocol conformance testing. This thesis deals with a formal approach to protocol conformance testing. Testing is performed based on a formal specification of the protocol. The final aim is to obtain methods for the (automatic) derivation of useful sets of tests from formal specifications. The derived tests should be provably correct, which means that they should not detect errors in correct implementations. Moreover, the derived tests should be meaningful: erroneous implementations should be detected with a high probability. An important aspect is a formal definition of what constitutes correctness, i.e. when does a protocol implementation conform to a protocol specification. Starting points for this thesis are the current, informal approach to conformance testing as it is described in the international standard ISO IS-9646 ”OSI Conformance Testing Methodology and Framework”, and the specification formalisms for distributed systems based on labelled transition systems and process algebra. The most important concepts of the standard ISO IS-9646, and of the specification formalisms used are introduced in chapter 1. Chapter 2 presents a framework for conformance testing. It is derived by giving a formal interpretation to the most important concepts from the standard ISO IS-9646, such as conformance requirement, the meaning of conformance, test purpose, test method, and different kinds of tests. This interpretation is shown to lead naturally to a definition of conformance as a (preorder) relation on the specification formalism. Such a relation is called an implementation relation. In chapter 3 the framework is elaborated with existing implementation relations for labelled transition systems. The relations are introduced using the principle of observations: the behaviour of an implementation is correct if all observations made of the implementation by an environment, can be explained from the behaviour of the specification. In chapter 4 test derivation algorithms are developed for one particular implementation