Ed Brinksma

Introduction to the ISO specification language LOTOS

LOTOS is a specification language that has been specifically developed for the formal description of the OSI (Open Systems Interconnection) architecture, although it is applicable to distributed, concurrent systems in general. In LOTOS a system is seen as a set of processes which interact and exchange data with each other and with their environment. LOTOS is expected to become an ISO international standard by 1988.

Specification styles in distributed systems design and verification

Substantial experience with the use of formal specification languages in the design of distributed systems has shown that finding appropriate structures for formal specifications presents a serious, and often underestimated problem. Its solutions are of great importance for ensuring the quality of the various designs that need to be developed at different levels of abstraction along the design trajectory of a system. This paper introduces four specification styles that allow us to structure formal specifications in different ways: the monolithic, the constraint-oriented, the state-oriented, and the resource-oriented style. These styles have been selected on the basis of their suitability to express design concerns by structuring specifications and their suitability to pursue qualitative design principles such as generality, orthogonality, and open-endedness. By giving a running example, a query-answer service, in the ISO specification language LOTOS, these styles are discussed in detail. The support of verification and correctness preserving transformation by these styles is shown by verifying designs, expressed in different styles, with respect to each other. This verification is based on equational laws for (weak) bisimulation equivalence.

As Cheap as Possible: Efficient Cost-Optimal Reachability for Priced Timed Automata

In this paper we present an algorithm for efficiently computing optimal cost of reaching a goal state in the model of Linearly Priced Timed Automata (LPTA). The central contribution of this paper is a priced extension of so-called zones. This, together with a notion of facets of a zone, allows the entire machinery for symbolic reachability for timed automata in terms of zones to be lifted to cost-optimal reachability using priced zones. We report on experiments with a cost-optimizing extension of Uppaal on a number of examples.

A test generation framework for quiescent real-time systems

A composition and method for the oxidative coloring of hair are disclosed, on the basis of a developer substance-coupler substance combination, whereby as coupler substance at least one 3,5-diaminopyridine derivative of the general Formula (I) in which R1 and R2 independent of each other are CH3, C2H5 or C2H4OH and R3 is hydrogen, C1-C4-alkyl or C1-C4-hydroxyalkyl, also in the form of the physiologically compatible salt, is used. The coupler substance, 3,5-diamino-2,6-dimethoxypyridine being preferred, should be present in the hair coloring composition in a concentration from 0.01 to 3.0% by weight, preferably 0.1 to 2.0% by weight. The coupler substances according to Formual I are storage-stable, well soluble in water, and have toxicologically as well as dermatologically favorable characteristics. The coupler substances according to Formula I produce in combination with 1,4-diaminobenzene or its derivatives very intensive blue-black tones without red portions, and in combination with 4-aminophenol, strongly lustrous gold-orange tones.

Lectures on Formal Methods and Performance Analysis

Formal Methods for Performance Evaluation.- Markovian Models for Performance and Dependability Evaluation.- to Stochastic Petri Nets.- Non-Markovian Analysis.- Process Algebra and Markov Chains.- Verification of Randomized Distributed Algorithms.- Constructing Automata from Temporal Logic Formulas: A Tutorial?.- Exploiting Structure in Solution: Decomposing Compositional Models.- Stochastic Activity Networks: Formal Definitions and Concepts?.- Distributed and Structured Analysis Approaches to Study Large and Complex Systems?.- General Distributions in Process Algebra.Traditionally, models and methods for the analysis of the functional correctness of reactive systems and those for the analysis of their performance (and dependability) aspects have been studied by different research communities. However, in modern systems the difference between their functional features and their performance properties has blurred. During the last decade, this trend has motivated and increased interest in combining insights from the field of formal methods, traditionally focused on functionality, with techniques from performance modeling and analysis. Examples of this cross-fertilization are extensions of process algebra and Petri nets allowing for automatic generation of performance models, the use of formal proof techniques to assess the correctness of randomized algorithms, and extensions of models checking techniques to analysis requirements automatically.

Optimal infinite scheduling for multi-priced timed automata

Abstract This paper is concerned with the derivation of infinite schedules for timed automata that are in some sense optimal. To cover a wide class of optimality criteria we start out by introducing an extension of the (priced) timed automata model that includes both costs and rewards as separate modelling features. A precise definition is then given of what constitutes optimal infinite behaviours for this class of models. We subsequently show that the derivation of optimal non-terminating schedules for such double-priced timed automata is computable. This is done by a reduction of the problem to the determination of optimal mean-cycles in finite graphs with weighted edges. This reduction is obtained by introducing the so-called corner-point abstraction, a powerful abstraction technique of which we show that it preserves optimal schedules.

Process algebra and Markov chains

This paper surveys and relates the basic concepts of process algebra and the modelling of continuous time Markov chains. It provides basic introductions to both fields, where we also study the Markov chains from an algebraic perspective, viz. that of Markov chain algebra. We then proceed to study the interrelation of reactive processes and Markov chains in this setting, and introduce the algebra of Interactive Markov Chains as an orthogonal extension of both process and Markov chain algebra. We conclude with comparing this approach to related (Markovian) stochastic process algebras by analysing the algebraic principles that they support.

A Stochastic Causality-Based Process Algebra

This paper discusses stochastic extensions of a simple process algebra in a causality-based setting. Atomic actions are supposed to happen after a delay that is determined by a stochastic variable with a certain distribution. A simple stochastic type of event structures is discussed, restricting the distribution functions to be exponential. A corresponding operational semantics of this model is given and compared to existing (interleaved) approaches. Secondly, a stochastic variant of event structures is discussed where distributions are of a much more general nature, viz. of phase-type. This includes exponential, Erlang, Coxian and mixtures of exponential distributions.

Verification and Optimization of a PLC Control Schedule

We report on the use of the SPIN model checker for both the verification of a process control program and the derivation of optimal control schedules. This work was carried out as part of a case study for the EC VHS project (Verification of Hybrid Systems), in which the program for a Programmable Logic Controller (PLC) of an experimental chemical plant had to be designed and verified. The intention of our approach was to see how much could be achieved here using the standard model checking environment of SPIN/Promela. As the symbolic calculations of real-time model checkers can be quite expensive it is interesting to try and exploit the efficiency of established non-real-time model checkers like SPIN in those cases where promising work-arounds seem to exist. In our case we handled the relevant real-time properties of the PLC controller using a time-abstraction technique; for the scheduling we implemented in Promela a so-called variable time advance procedure. For this case study these techniques proved sufficient to verify the design of the controller and derive (time-)optimal schedules with reasonable time and space requirements.

Refining Interfaces of Communicating Systems

There are now several theories for describing and reasoning about the behavior of communicating systems, where the behavior of a communicating system is described in terms of its capabilities to perform communication actions in cooperation with its environment. In such theories, preorders or equivalences are defined as criteria for when one system is an acceptable substitute or implementation of another. Existing theories of communicating systems define preorders or equivalence relations only between systems with identical sets of communication actions. In many practical design situations, however, it may be desirable to refine a system by changing its set of communication actions. We present a simple method for carrying out such refinements. The method is first formulated in a general setting, and then elaborated in more detail in the trace model and a simple version of the failure model. We illustrate the usefulness of our method by an application to I.451, an ISDN access protocol.