A Petri Net Semantics of a Simple Process Algebra for Mobility

In this paper, we propose a structural translation of terms from a simple variant of the Klaim process algebra into behaviourally equivalent finite high level Petri nets. This yields a formal semantics for mobility allowing one to deal directly with concurrency and causality.     

Propositional dynamic logic with recursive programs

We extend the propositional dynamic logic PDL of Fischer and Ladner with a restricted kind of recursive programs using the formalism of visibly pushdown automata [R. Alur, P. Madhusudan, Visibly pushdown languages, in: Procceings of the 36th Annual ACM Symposium on Theory of Computing (STOC 2004), 2004, ACM, pp. 202–211]. We show that the satisfiability problem for this extension remains decidable, generalising known decidability results for extensions of PDL by non-regular programs. Our decision procedure establishes a 2-ExpTime upper complexity bound, and we prove a matching lower bound that applies already to rather weak extensions of PDL with non-regular programs. Thus, we also show that such extensions tend to be more complex than standard PDL.     

Algebraic–coalgebraic specification in CoCasl

We introduce CoCasl as a light-weight but expressive coalgebraic extension of the algebraic specification language Casl. CoCasl allows the nested combination of algebraic datatypes and coalgebraic process types. Moreover, it provides syntactic sugar for an observer-indexed modal logic that allows e.g. expressing fairness properties. This logic includes a generic definition of modal operators for observers with structured equational result types. We prove existence of final models for specifications in a format that allows the use of equationally specified initial datatypes as observations, as well as modal axioms. The use of CoCasl is illustrated by specifications of the process algebras CSP and CCS.     

MMC: the Mono Model Checker

The Mono Model Checker (mmc) is a software model checker for cil bytecode programs. mmc has been developed on the Mono platform. mmc is able to detect deadlocks and assertion violations in cil programs. The design of mmc is inspired by the Java PathFinder (jpf), a model checker for Java programs. The performance of mmc is comparable to jpf. This paper introduces mmc and presents its main architectural characteristics.     

synERJY An Object-oriented Synchronous Language

The programming language synERJY is presented. It integrates object-orientation and synchronous formalisms in the spirit of Esterel, Lustre, and Statecharts.     

CSP-CASL-Prover: A Generic Tool for Process and Data Refinement

The specification language Csp-Casl allows one to model processes as well as data of distributed systems within one framework. In our paper, we describe how a combination of the existing tools Hets and Csp-Prover can solve the challenges that Csp-Casl raises on integrated theorem proving for processes and data. For building this new tool, the automated generation of theorems and their proofs in Isabelle/HOL plays a fundamental role. A case study of industrial strength demonstrates that our approach scales up to complex problems.     

Hard constraint satisfaction problems have hard gaps at location 1

An instance of the maximum constraint satisfaction problem (Max CSP) is a finite collection of constraints on a set of variables, and the goal is to assign values to the variables that maximises the number of satisfied constraints. Max CSP captures many well-known problems (such as Maxk-SAT and Max Cut) and is consequently NP-hard. Thus, it is natural to study how restrictions on the allowed constraint types (or constraint language) affect the complexity and approximability of Max CSP. The PCP theorem is equivalent to the existence of a constraint language for which Max CSP has a hard gap at location 1; i.e. it is NP-hard to distinguish between satisfiable instances and instances where at most some constant fraction of the constraints are satisfiable. All constraint languages, for which the CSP problem (i.e., the problem of deciding whether all constraints can be satisfied) is currently known to be NP-hard, have a certain algebraic property. We prove that any constraint language with this algebraic property makes Max CSP have a hard gap at location 1 which, in particular, implies that such problems cannot have a PTAS unless P=NP. We then apply this result to Max CSP restricted to a single constraint type; this class of problems contains, for instance, Max Cut and Max DiCut. Assuming P≠NP, we show that such problems do not admit PTAS except in some trivial cases. Our results hold even if the number of occurrences of each variable is bounded by a constant. Finally, we give some applications of our results.     

An efficient abstract machine for Safe Ambients

Safe Ambients (SA) are a variant of the Ambient Calculus (AC) in which types can be used to avoid certain forms of interferences among processes called grave interferences.An abstract machine, called GcPan, for a distributed implementation of typed SA is presented and studied. Our machine improves over previous proposals for executing AC, or variants of it, mainly through a better management of special agents (the forwarders), created upon code migration to transmit messages to the target location of the migration. Well-known methods (such as reference counting and union-find) are applied in order to garbage collect forwarders, thus avoiding long – possibly distributed – chains of forwarders, as well as avoiding useless persistent forwarders.We present the proof of correctness of GcPan w.r.t. typed SA processes. We describe a distributed implementation of the abstract machine in OCaml.More broadly, this study is a contribution towards understanding issues of correctness and optimisations in implementations of distributed languages encompassing mobility.     

PMaude: Rewrite-based Specification Language for Probabilistic Object Systems

We introduce a rewrite-based specification language for modelling probabilistic concurrent and distributed systems. The language, based on PMaude, has both a rigorous formal basis and the characteristics of a high-level rule-based programming language. Furthermore, we provide tool support for performing discrete-event simulations of models written in PMaude, and for statistically analyzing various quantitative aspects of such models based on the samples that are generated through discrete-event simulation. Because distributed and concurrent communication protocols can be modelled using actors (concurrent objects with asynchronous message passing), we provide an actor PMaude module. The module aids writing specifications in a probabilistic actor formalism. This allows us to easily write specifications that are purely probabilistic – and not just non-deterministic. The absence of such (un-quantified) non-determinism in a probabilistic system is necessary for a form of statistical analysis that we also discuss. Specifically, we introduce a query language called Quantitative Temporal Expressions (or QuaTEx in short), to query various quantitative aspects of a probabilistic model. We also describe a statistical technique to evaluate QuaTEx expressions for a probabilistic model.     

Enantioselective, potentiometric membrane electrodes based on cyclodextrins for the determination of l-histidine

Four enantioselective, potentiometric membrane electrodes based on carbon paste impregnated with α-, β-, 2-hydroxyl-3-trimethylammoniopropyl-β-(as chloride salt) and γ-cyclodextrins (γ-CDs) are proposed for the assay of l-histidine (l-his). The proposed electrodes showed near-Nernstian response over l-his but not over d-histidine (d-his). The recovery of l-his in the presence of d-his was higher than 99.10% with R.S.D. lower than 0.1%. The surfaces of the electrodes are easily renewable by simply polishing on an alumina paper.     

Stochastic Ambient Calculus

Mobile Ambients (MA) have acquired a fundamental role in modelling mobility in systems with mobile code and mobile devices, and in computation over administrative domains. We present the stochastic version of Mobile Ambients, called Stochastic Mobile Ambients (SMA), where we extend MA with time and probabilities. Inspired by previous models, PEPA and Sπ, we enhance the prefix of the capabilities with a rate and the ambient with a linear function that operates on the rates of processes executing inside it. The linear functions associated with ambients represent the delays that govern particular administrative domains. We derive performance measures from the labelled transition semantics as in standard models. We also define a strong Markov bisimulation in the style of reduction semantics known as barbed bisimulation. We argue that performance measures are of vital importance in designing any kind of distributed system, and that SMA can be useful in the design of the complicated mobile systems.     

Model Checking Quantitative Linear Time Logic

This paper considers QLtl, a quantitative analagon of Ltl and presents algorithms for model checking QLtl over quantitative versions of Kripke structures and Markov chains.     

Model checking mobile stochastic logic

The Temporal Mobile Stochastic Logic (MoSL) has been introduced in previous work by the authors for formulating properties of systems specified in StoKlaim, a Markovian extension of Klaim. The main purpose of MoSL is to address key functional aspects of global computing such as distribution awareness, mobility, and security and their integration with performance and dependability guarantees. In this paper, we present MoSL⁺${\text{<small-caps>MoSL</small-caps>}}^{+}$, an extension of MoSL, which incorporates some basic features of the Modal Logic for MObility (MoMo), a logic specifically designed for dealing with resource management and mobility aspects of concurrent behaviours. We also show how MoSL⁺${\text{<small-caps>MoSL</small-caps>}}^{+}$ formulae can be model-checked against StoKlaim specifications. For this purpose, we show how existing state-based stochastic model-checkers, like e.g. the Markov Reward Model Checker (MRMC), can be exploited by using a front-end for StoKlaim that performs appropriate pre-processing of MoSL⁺${\text{<small-caps>MoSL</small-caps>}}^{+}$ formulae. The proposed approach is illustrated by modelling and verifying a sample system.     

Reactive systems, (semi-)saturated semantics and coalgebras on presheaves

The semantics of process calculi has traditionally been specified by labelled transition systems (ltss), but, with the development of name calculi, it turned out that reaction rules (i.e., unlabelled transition rules) are often more natural. This leads to the question of how behavioral equivalences (bisimilarity, trace equivalence, etc.) defined for lts can be transferred to unlabelled transition systems. Recently, in order to answer this question, several proposals have been made with the aim of automatically deriving an lts from reaction rules in such a way that the resulting equivalences are congruences. Furthermore, these equivalences should agree with the standard semantics, whenever one exists.In this paper, we propose saturated semantics, based on a weaker notion of observation and orthogonal to all the previous proposals, and we demonstrate the appropriateness of our semantics by means of two examples: logic programming and open Petri nets. We also show that saturated semantics can be efficiently characterized through the so called semi-saturated games. Finally, we provide coalgebraic models relying on presheaves.     

A new version of the Improved Primal Simplex for degenerate linear programs

The Improved Primal Simplex (IPS) algorithm [Elhallaoui I, Metrane A, Desaulniers G, Soumis F. An Improved Primal Simplex algorithm for degenerate linear programs. SIAM Journal of Optimization, submitted for publication] is a dynamic constraint reduction method particularly effective on degenerate linear programs. It is able to achieve a reduction in CPU time of over a factor of three on some problems compared to the commercial implementation of the simplex method CPLEX. We present a number of further improvements and effective parameter choices for IPS. On certain types of degenerate problems, our improvements yield CPU times lower than those of CPLEX by a factor of 12.     

The Stable Revivals Model in CSP-Prover

The stable revivals model provides a new semantic framework for the process algebra Csp. The model has recently been added to the realm of established Csp models. Within the Csp context, it enhances the analysis of systems with regards to properties such as responsiveness and stuckness. These properties are essential in component based system design. In this paper we report on the implementation of different variants of the model within Csp-Prover. Based on Isabelle/HOL, Csp-Prover is an interactive proof tool for Csp refinement, which is generic in the underlying Csp model. On the practical side, our encoding of the model provides semi-automatic proof support for reasoning on responsiveness and stuckness. On the theoretical side, our implementation also yields a machine verification of the model 's soundness as well as of its expected properties.     

On the Comparison of Proof Planning Systems: , Ωmega and IsaPlanner

We present a framework for describing proof planners. This framework is based around a decomposition of proof planners into planning states, proof language, proof plans, proof methods, proof revision, proof control and planning algorithms.We use this framework to motivate the comparison of three recent proof planning systems, λCLaM, Ωmega and IsaPlanner, and demonstrate how the framework allows us to discuss and illustrate both their similarities and differences in a consistent fashion. This analysis reveals that proof control and the use of contextual information in planning states are key areas in need of further investigation.     

MOBILE AGENT SECURITY AGAINST MALICIOUS PLATFORMS

This article presents a review of security mechanisms that have been developed for mobile agent security against malicious platforms. It has been almost 10 years since a prominent review of security in agent systems was presented by Jansen (2000 Jansen , W. 2000 . Countermeasures for mobile agent security . Computer Communications 23 ( 17 ): 1667 – 1676 . [Google Scholar]). We present new developments that have been suggested over the years, evaluate limitations of these schemes, and highlight possible areas of improvement. This article further presents possible threats to the mobile agent paradigm and distinguishes between detection and prevention security mechanisms.