S Suzana Andova

Discrete-Time Rewards Model-Checked

This paper presents a model-checking approach for analyzing discrete-time Markov reward models. For this purpose, the temporal logic probabilistic CTL is extended with reward constraints. This allows to formulate complex measures – involving expected as well as accumulated rewards – in a precise and succinct way. Algorithms to efficiently analyze such formulae are introduced. The approach is illustrated by model-checking a probabilistic cost model of the IPv4 zeroconf protocol for distributed address assignment in ad-hoc networks.

Process Algebra with Probabilistic Choice

Published results show that various models may be obtained by combining parallel composition with probability and with or without non-determinism. In this paper we treat this problem in the setting of process algebra in the form of ACP. First, probabilities are introduced by an operator for the internal probabilistic choice. In this way we obtain the Basic Process Algebra with probabilistic choice prBPA. Afterwards, prBPA is extended with parallel composition to ACP+π. We give the axiom system for ACP+π and a complete operational semantics that preserves the interleaving model for the dynamic concurrent processes. Considering the PAR protocol, a communication protocol that can be used in the case of unreliable channels, we investigate the applicability of ACP+π. Using in addition only the priority operator and the preabstraction operator we obtain a recursive specifications of the behaviour of the protocol that can be viewed as a Markov chain.

Abstraction in Probabilistic Process Algebra

Process algebras with abstraction have been widely used for the specification and verification of non-probabilistic concurrent systems. The main strategy in these algebras is introducing a constant, denoting an internal action, and a set of fairness rules. Following the same approach, in this paper we propose a fully probabilistic process algebra with abstraction which contains a set of verification rules as counterparts of the fairness rules in standard ACP-like process algebras with abstraction. Having probabilities present and employing the results from Markov chain analysis, these rules are expressible in a very intuitive way. In addition to this algebraic approach, we introduce a new version of probabilistic branching bisimulation for the alternating model of probabilistic systems. Different from other approaches, this bisimulation relation requires the same probability measure only for specific related processes called entries. We claim this definition corresponds better with intuition. Moreover, the fairness rules are sound in the model based on this bisimulation. Finally, we present an algorithm to decide our branching bisimulation with a polynomial-time complexity in the number of the states of the probabilistic graph.

Branching bisimulation for probabilistic systems: characteristics and decidability

We address the concept of abstraction in the setting of probabilistic reactive systems, and study its formal underpinnings for the strictly alternating model of Hansson. In particular, we define the notion of branching bisimilarity and study its properties by studying two other equivalence relations, viz. coloured trace equivalence and branching bisimilarity using maximal probabilities. We show that both alternatives coincide with branching bisimilarity. The alternative characterisations have their own merits and focus on different aspects of branching bisimilarity. Coloured trace equivalence can be understood without knowledge of probability theory and is independent of the notion of a scheduler. Branching bisimilarity, rephrased in terms of maximal probabilities gives rise to an algorithm of polynomial complexity for deciding the equivalence. Together they give a better understanding of branching bisimilarity. Furthermore, we show that the notions of branching bisimilarity in the alternating model of Hansson and in the nonalternating model of Segala differ: branching bisimilarity in the latter setting turns out to discriminate between systems that are intuitively branching bisimilar.

Retaining the probabilities in probabilistic testing theory

This paper considers the probabilistic may/must testing theory for processes having external, internal, and probabilistic choices. We observe that the underlying testing equivalence is too strong and distinguishes between processes that are observationally equivalent. The problem arises from the observation that the classical compose-and-schedule approach yields unrealistic overestimation of the probabilities, a phenomenon that has been recently well studied from the point of view of compositionality (de Alfaro/Henzinger/Jhala 2001, Cheung/Lynch/Segala/Vaandrager 2006), in the context of randomized protocols (Chatzikokolakis/Palamidessi 2007), and in probabilistic model checking (Giro/D’Argenio/Ferrer Fioriti 2009). To that end, we propose a new testing theory, aiming at preserving the probability information in a parallel context. The resulting testing equivalence is insensitive to the exact moment the internal and the probabilistic choices occur. We also give an alternative characterization of the testing preorder as a probabilistic ready-trace preorder.

Dynamic Consistency in Process Algebra: From Paradigm to ACP

The coordination modelling language Paradigm addresses collaboration between components in terms of dynamic constraints. Within a Paradigm model, component dynamics are consistently specified at various levels of abstraction. To enable automated verification of Paradigm models, a translation of Paradigm into process algebra is provided. Examples are given and guidelines for a systematic translation into the process algebra ACP are discussed. Verification results building on the mCRL2 toolset are presented as well.

Formalizing a domain specific language using SOS: an industrial case study

This paper describes the process of formalizing an existing, industrial domain specific language (DSL) that is based on the task-resource paradigm. Initially, the semantics of this DSL is defined informally and implicitly through an interpreter. The formalization starts by projecting the existing concrete syntax onto a formal abstract syntax that defines the language operators and process terms. Next, we define the dynamic operational semantics at the level of individual syntactical notions, using structural operational semantics (SOS) as a formal meta-language. Here, the impact of the formalization process on the DSL is considered in terms of disambiguation of underlying (semantic) language design decisions.

Formalizing Adaptation On-the-Fly

Paradigm models specify coordination of collaborating components via constraint control. Component McPal allows for later addition of new constraints and new control in view of unforeseen adaptation. After addition McPal starts coordinating migration accordingly, adapting the system towards to-be collaboration. Once done, McPal removes obsolete control and constraints. All coordination remains ongoing while migrating on-the-fly, being deflected without any quiescence. Through translation into process algebra, supporting formal analysis is arranged carefully, showing that as-is and to-be processes are proper abstractions of the migrating process. A canonical critical section problem illustrates the approach.

Probabilistic may/must testing: retaining probabilities by restricted schedulers

This paper considers the probabilistic may/must testing theory for processes having external, internal, and probabilistic choices. We observe that the underlying testing equivalence is too strong and distinguishes between processes that are observationally equivalent. The problem arises from the observation that the classical compose-and-schedule approach yields unrealistic overestimation of the probabilities, a phenomenon that has been recently well studied from the point of view of compositionality, in the context of randomized protocols and in probabilistic model checking. To that end, we propose a new testing theory, aiming at preserving the probability information in a parallel context. The resulting testing equivalence is insensitive to the exact moment the internal and the probabilistic choices occur. We also give an alternative characterization of the testing preorder as a probabilistic ready-trace preorder.

Dynamic consistency in process algebra: From Paradigm to ACP

The coordination modelling language Paradigm addresses collaboration between components in terms of dynamic constraints. Within a Paradigm model, component dynamics are consistently specified at various levels of abstraction. The operational semantics of Paradigm is given. For a large, general subclass of Paradigm models a translation into process algebra is provided. Once expressed in process algebra, relying on a correctness result, Paradigm models are amenable to process algebraic reasoning and to verification via the mCRL2 toolset. Examples of a scheduling problem illustrate the approach.