Jean-Michel Ilié

Design and Evaluation of a Symbolic and Abstraction-Based Model Checker

Symbolic model-checking usually includes two steps: the building of a compact representation of a state graph and the evaluation of the properties of the system upon this data structure. In case of properties expressed with a linear time logic, it appears that the second step is often more time consuming than the first one. In this work, we present a mixed solution which builds an observation graph represented in a non symbolic way but where the nodes are essentially symbolic set of states. Due to the small number of events to be observed in a typical formula, this graph has a very moderate size and thus the complexity time of verification is neglectible w.r.t. the time to build the observation graph. Thus we propose different symbolic implementations for the construction of the nodes of this graph. The evaluations we have done on standard examples show that our method outperforms the pure symbolic methods which makes it attractive.

A Symbolic Symbolic State Space Representation

Symmetry based approaches are known to attack the state space explosion problem encountered during the analysis of distributed systems. In another way, BDD-like encodings enable the management of huge data sets. In this paper, we show how to benefit from both approaches automatically. Hence, a quotient set is built from a coloured Petri net description modeling the system. The reachability set is managed under some explicit symbolic operations. Also, data representations are managed symbolically based on a recently introduced data structure, called Data Decisions Diagrams, that allow flexible definition of application specific operators. Performances yielded by our prototype are reported in the paper.

Exploiting Partial Symmetries in Well-formed nets for the Reachability and the Linear Time Model Checking Problems

Abstract Taking advantage of the symmetries of a system is an efficient way to cope with the combinatory explosion involved by the verification process. Whereas numerous algorithms and tools efficiently deal with the verification of a symmetrical formula on a symmetrical model, the management of partial symmetries is still an open research topic. In this work, We present the design and the evaluation of two methods applicable on coloured Petri nets. These two methods are extensions of the symbolic reachability graph construction for the well-formed Petri nets. The first algorithm, called the extended symbolic reachability graph construction, tackles the reachability problem. The second one called the symbolic synchronized product, checks a partially symmetric linear time formula on a net. The evaluations show that these two methods outperform the previous approaches dealing with partial symmetries. Furthermore they are complementary ones since the former while being less general gives better results than the latter when applied to the reachability problem.

Exploiting Partial Symmetries for Markov Chain Aggregation

Abstract The technique presented in this paper allows the automatic construction of a lumped Markov chain for almost symmetrical Stochastic Well-formed Net (SWN) models. The starting point is the Extended Symbolic Reachability Graph (ESRG), which is a reduced representation of a SWN model reachability graph (RG), based on the aggregation of states into classes. These classes may be used as aggregates for lumping the Continuous Time Markov Chain (CTMC) isomorphic to the model RG: however it is not always true that the lumpability condition is verified by this partition of states. In the paper we propose an algorithm that progressively refines the ESRG classes until a lumped Markov chain is obtained.

A Higher-Order Agent Model with Contextual Planning Management for Ambient Systems

This paper presents a concrete software architecture dedicated to ambient intelligence (AmI) features and requirements. The proposed behavioral model, called Higher-order Agent (HoA) captures the evolution of the mental representation of the agent and the one of its plan simultaneously. Plan expressions are written and composed using a formal algebraic language, namely AgLOTOS, so that plans are built automatically and on the fly, as a system of concurrent processes. Based on a specific semantics, a guidance service is also proposed to assist the agent in its execution. Moreover due to the specific structure of AgLOTOS expressions, the update of sub-plans is realized automatically accordingly to the revising of intentions, hence maintaining the consistency of the agent.

On the use of exact lumpability in partially symmetrical well-formed nets

Well-formed nets (WNs) have proved an efficient model for building quotient reachability graphs that can be used either for qualitative or performance analysis. However, local asymmetries often break any possibility of grouping states into classes, thus drastically reducing the interest of the approach. An efficient solution has been proposed for qualitative analysis, which relies on a separate representation of the asymmetries in a so called control automaton. The quotient graph is then obtained by synchronizing the transitions of the WN model with the transitions of the control automaton. In this paper, we improve this approach to quantitative analysis. We show that it can be used to build an aggregated graph that is isomorphic to a Markov chain which verifies exact lumpability. Theoretical considerations and practical experiments show that our method outperforms previous approaches.

Complementary Formal Approaches for Dependability Analysis

Evaluating the robustness of digital circuits with respect to soft errors has become an important part of the design flow for many applications. The identification of the most or less critical registers is often necessary, in order to reach the lowest overheads while achieving a given application-level robustness. The goal here is to identify those soft errors actually harmful for the system, not to compute the Soft Error Rate. In this context, we investigate new approaches based on formal techniques to improve design-time robustness evaluations at least for the most critical blocks in a circuit. Preliminary results are shown, focusing on the evaluation of self-healing (or self-repairing) capabilities.

An incremental verification technique using decomposition of Petri nets

We propose a modular verification technique for bounded Petri nets which efficiency relies on both behavioral and structural features. By focusing on linear evenemential temporal logic formula, we demonstrate how to choose a subnet on which it is enough to perform the model checking.

Maximality semantics based stochastic process algebra for performance evaluation

We introduce a true concurrency stochastic process algebra called S-LOTOS, such that stochastic durations of actions relate to general distributions. A structural operational semantics is defined so that the underlying model called MSLTS, extends the Maximality-based Labeled Transition Systems [1]. S-LOTOS is also viewed as a high level representation to specify Generalized Semi-Markov Processes (GSMP) in order to evaluate the system performances.

Modular verification of petri nets properties: a structure-based approach

In this paper, we address the modular verification problem for a Petri net obtained by composition of two subnets. At first, we show how to transform an asynchronous composition into a synchronous one where the new subnets are augmented from the original ones by means of linear invariants. Then we introduce a non-constraining rela- tion between subnets based on their behaviour. Whenever this relation is satisfied, standard properties like the liveness and the boundedness and generic properties specified by a linear time logic may be checked by examination of the augmented subnets in isolation. Finally, we give a sufficient condition for this relation which can be detected modularly using an efficient algorithm.