Jiri Srba

Infinite Runs in Weighted Timed Automata with Energy Constraints

We study the problems of existence and construction of infinite schedules for finite weighted automata and one-clock weighted timed automata, subject to boundary constraints on the accumulated weight. More specifically, we consider automata equipped with positive and negative weights on transitions and locations, corresponding to the production and consumption of some resource (e.g.energy). We ask the question whether there exists an infinite path for which the accumulated weight for any finite prefix satisfies certain constraints (e.g.remains between 0 and some given upper-bound). We also consider a game version of the above, where certain transitions may be uncontrollable.

Strong Bisimilarity and Regularity of Basic Parallel Processes Is PSPACE-Hard

We show that the problem of checking whether two processes definable in the syntax of Basic Parallel Processes (BPP) are strongly bisimilar is PSPACE-hard.We also demonstrate that there is a polynomial time reduction from the strong bisimilarity checking problem of regular BPP to the strong regularity (finiteness) checking of BPP. This implies that strong regularity of BPP is also PSPACE-hard.

On determinism in modal transition systems

Modal transition system (MTS) is a formalism which extends the classical notion of labelled transition systems by introducing transitions of two types: must transitions that have to be present in any implementation of the MTS and may transitions that are allowed but not required. The MTS framework has proved to be useful as a specification formalism of component-based systems as it supports compositional verification and stepwise refinement. Nevertheless, there are some limitations of the theory, namely that the naturally defined notions of modal refinement and modal composition are incomplete with respect to the semantic view based on the sets of the implementations of a given MTS specification. Recent work indicates that some of these limitations might be overcome by considering deterministic systems, which seem to be more manageable but still interesting for several application areas. In the present article, we provide a comprehensive account of the MTS framework in the deterministic setting. We study a number of problems previously considered on MTS and point out to what extend we can expect better results under the restriction of determinism.

Strong Bisimilarity and Regularity of Basic Process Algebra Is PSPACE-Hard

Strong bisimilarity and regularity checking problems of Basic Process Algebra (BPA) are decidable, with the complexity upper bounds 2-EXPTIME. On the other hand, no lower bounds were known. In this paper we demonstrate PSPACE-hardness of these problems.

Parametric modal transition systems

Modal transition systems (MTS) is a well-studied specification formalism of reactive systems supporting a step-wise refinement methodology. Despite its many advantages, the formalism as well as its currently known extensions are incapable of expressing some practically needed aspects in the refinement process like exclusive, conditional and persistent choices. We introduce a new model called parametric modal transition systems (PMTS) together with a general modal refinement notion that overcome many of the limitations and we investigate the computational complexity of modal refinement checking.

TAPAAL 2.0: integrated development environment for timed-arc petri nets

TAPAAL 2.0 is a platform-independent modelling, simulation and verification tool for extended timed-arc Petri nets. The tool supports component-based modelling and offers an automated verification of the EF, AG, EG and AF fragments of TCTL via translations to Uppaal timed automata and via its own dedicated verification engine. After more than three years of active development with a main focus on usability aspects and on the efficiency of the verification algorithms, we present the new version of TAPAAL 2.0 that has by now reached its maturity and offers the first publicly available tool supporting the analysis and verification of timed-arc Petri nets.

Modal Transition Systems with Weight Intervals

We propose weighted modal transition systems, an extension to the well-studied specification formalism of modal transition systems that allows to express both required and optional behaviours of their intended implementations. In our extension we decorate each transition with a weight interval that indicates the range of concrete weight values available to the potential implementations. In this way resource constraints can be modelled using the modal approach. We focus on two problems. First, we study the question of existence/finding the largest common refinement for a number of finite deterministic specifications and we show PSPACE-completeness of this problem. By constructing the most general common refinement, we allow for a stepwise and iterative construction of a common implementation. Second, we study a logical characterisation of the formalism and show that a formula in a natural weight extension of the logic CTL is satisfied by a given modal specification if and only if it is satisfied by all its refinements. The weight extension is general enough to express different sorts of properties that we want our weights to satisfy.

Extending modal transition systems with structured labels

We introduce a novel formalism of label-structured modal transition systems that combines the classical may/must modalities on transitions with structured labels that represent quantitative aspects of the model. On the one hand, the specification formalism is general enough to include models like weighted modal transition systems and allows system developers to employ more complex label refinement than in previously studied theories. On the other hand, the formalism maintains the desirable properties required by any specification theory supporting compositional reasoning. In particular, we study modal and thorough refinement, determinisation, parallel composition, conjunction, quotient and logical characterisation of label-structured modal transition systems.

Verification of timed-arc Petri nets

Timed-Arc Petri Nets (TAPN) are an extension of the classical P/T nets with continuous time. Tokens in TAPN carry an age and arcs between places and transitions are labelled with time intervals restricting the age of tokens available for transition firing. The TAPN model posses a number of interesting theoretical properties distinguishing them from other time extensions of Petri nets. We shall give an overview of the recent theory developed in the verification of TAPN extended with features like read/transport arcs, timed inhibitor arcs and age invariants. We will examine in detail the boundaries of automatic verification and the connections between TAPN and the model of timed automata. Finally, we will mention the tool TAPAAL that supports modelling, simulation and verification of TAPN and discuss a small case study of alternating bit protocol.

A formal analysis of the web services atomic transaction protocol with UPPAAL

We present a formal analysis of the Web Services Atomic Transaction (WS-AT) protocol. WS-AT is a part of the WS-Coordination framework and describes an algorithm for reaching agreement on the outcome of a distributed transaction. The protocol is modelled and verified using the model checker Uppaal. Our model is based on an already available formalization using the mathematical language TLA+ where the protocol was verified using the model checker TLC. We discuss the key aspects of these two approaches, including the characteristics of the specification languages, the performances of the tools, and the robustness of the specifications with respect to extensions.