Rare Event Simulation for non-Markovian repairable Fault Trees

Dynamic Fault Trees (DFT) are widely adopted in industry to assess the dependability of safety-critical equipment. Since many systems are too large to be studied numerically, DFTs dependability is often analysed using Monte Carlo simulation. A bottleneck here is that many simulation samples are required in the case of rare events, e.g. in highly reliable systems where components fail seldomly. Rare Event Simulation (RES) provides techniques to reduce the number of samples in the case of rare events. We present a RES technique based on importance splitting, to study failures in highly reliable DFTs. Whereas RES usually requires meta-information from an expert, our method is fully automatic: by cleverly exploiting the fault tree structure we extract the so-called importance function. We handle DFTs with Markovian and non-Markovian failure and repair distributions (for which no numerical methods exist) and show the efficiency of our approach on several case studies

Characterising Probabilistic Processes Logically

In this paper we work on (bi)simulation semantics of processes that exhibit both nondeterministic and probabilistic behaviour. We propose a probabilistic extension of the modal mu-calculus and show how to derive characteristic formulae for various simulation-like preorders over finite-state processes without divergence. In addition, we show that even without the fixpoint operators this probabilistic mu-calculus can be used to characterise these behavioural relations in the sense that two states are equivalent if and only if they satisfy the same set of formulae.Comment: 18 page

Location-Aware Quality of Service Measurements for Service-Level Agreements

We add specifications of location-aware measurements to performance models in a compositional fashion, promoting precision in performance measurement design. Using immediate actions to send control signals between measurement components we are able to obtain more accurate measurements from our stochastic models without disturbing their structure. A software tool processes both the model and the measurement specifications to give response time distributions and quantiles, an essential calculation in determining satisfaction of service-level agreements (SLAs)

Model-Based Verification, Optimization, Synthesis and Performance Evaluation of Real-Time Systems

International audienceThis article aims at providing a concise and precise Travellers Guide, Phrase Book or Reference Manual to the timed automata modeling formalism introduced by Alur and Dill [8, 9]. The paper gives comprehensive definitions of timed automata, priced (or weighted) timed automata, and timed games and highlights a number of results on associated decision problems related to model checking, equivalence checking, optimal scheduling, the existence of winning strategies, and then statistical model checking

Least upper bounds for probability measures and their applications to abstractions

Abstraction is a key technique to combat the state space explosion problem in model checking probabilistic systems. In this paper we present new ways to abstract Discrete Time Markov Chains (DTMCs), Markov Decision Processes (MDPs), and Continuous Time Markov Chains (CTMCs). The main advantage of our abstractions is that they result in abstract models that are purely probabilistic, which maybe more amenable to automatic analysis than models with both nondeterministic and probabilistic steps that typically arise from previously known abstraction techniques. A key technical tool, developed in this paper, is the construction of least upper bounds for any collection of probability measures. This upper bound construction may be of independent interest that could be useful in the abstract interpretation and static analysis of probabilistic programs

A Proof System for Timed Automata

A proof system for timed automata is presented, based on a CCS-style language for describing timed automata. It consists of the standard monoid laws for bisimulation and a set of inference rules. The judgements of the proof system are conditional equations of the form phirhd t=u where phi is a clock constraint and t, u are terms denoting timed automata. It is proved that the proof system is complete over the recursion-free subset of the language. The completeness proof relies on the notion of symbolic timed bisimulation. Two variations of the axiomatisation are also discussed, one on timed automata by associating an invariant constraint to each node and the other on bisimulation by abstracting away delay transitions.Note: To be included in the proceedings of FOSSACS'0

Formal security verification of transport protocols for wireless sensor networks

In this paper, we address the problem of formal security verification of transport protocols for wireless sensor networks (WSN) that perform cryptographic operations. Analyzing this class of protocols is a difficult task because they typically consist of complex behavioral characteristics, such as launching timers, performing probabilistic behavior, and cryptographic operations. Some of the recently published WSN transport protocols are DTSN, which does not include cryptographic security mechanism, and two of its secured versions, SDTP and STWSN. In our previous work, we formally analyzed the security of Distributed Transport for Sensor Networks (DTSN) and Distributed Transport Protocol for Wireless Sensor Networks (SDTP), and showed that they are vulnerable against packet modification attacks. In another work we proposed a new Secure Transport Protocol for WSNs (STWSN), with the goal of eliminating the vulnerability of DTSN and SDTP, however, its security properties have only been informally argued. In this paper, we apply formal method to analyze the security of STWSN

Compositional Metric Reasoning with Probabilistic Process Calculi

Abstract. We study which standard operators of probabilistic process calculi al-low for compositional reasoning with respect to bisimulation metric semantics. We argue that uniform continuity (generalizing the earlier proposed property of non-expansiveness) captures the essential nature of compositional reasoning and allows now also to reason compositionally about recursive processes. We charac-terize the distance between probabilistic processes composed by standard process algebra operators. Combining these results, we demonstrate how compositional reasoning about systems specified by continuous process algebra operators allows for metric assume-guarantee like performance validation.