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This book constitutes the refereed proceedings of the 13th IFIP WG 6.1 International Conference on Formal Methods for Open Object-Based Distributed Systems, FMOODS 2011, and the 31st IFIP WG 6.1 Formal Techniques for Networked and Distributed Systems, FORTE 2011, held in Reykjavik, Island, in June 2011, as one of the DisCoTec 2011 events. The 21 revised full papers presented together with one invited talk were carefully reviewed and selected from 65 submissions. The topics covered are formal verification, formal modeling and specification, run-time monitoring, and testing to address challenges in many different application areas, including dynamic and ad hoc networks, mobile and adaptive computation, reactive and timed systems, business processes, and distributed and concurrent systems and algorithms.

Inhaltsverzeichnis

Frontmatter

On Global Types and Multi-party Sessions

Abstract
We present a new, streamlined language of global types equipped with a trace-based semantics and whose features and restrictions are semantically justified. The multi-party sessions obtained projecting our global types enjoy a liveness property in addition to the traditional progress and are shown to be sound and complete with respect to the set of traces of the originating global type. Our notion of completeness is less demanding than the classical ones, allowing a multi-party session to leave out redundant traces from an underspecified global type.
Giuseppe Castagna, Mariangiola Dezani-Ciancaglini, Luca Padovani

Linear-Time and May-Testing in a Probabilistic Reactive Setting

Abstract
We consider reactive probabilistic labelled transition systems (rplts), a model where internal choices are refined by probabilistic choices. In this setting, we study the relationship between linear-time and may-testing semantics, where an angelic view of nondeterminism is taken. Building on the model of d-trees of Cleaveland et al., we first introduce a clean model of probabilistic may-testing, based on simple concepts from measure theory. In particular, we define a probability space where statements of the form “p may pass test o” naturally correspond to measurable events. We then obtain an observer-independent characterization of the may-testing preorder, based on comparing the probability of sets of traces, rather than of individual traces. This entails that may-testing is strictly finer than linear-time semantics. Next, we characterize the may-testing preorder in terms of the probability of satisfying safety properties, expressed as languages of infinite trees rather than traces. We then identify a significative subclass of rplts where linear and may-testing semantics do coincide: these are the separated rplts, where actions are partitioned into probabilistic and nondeterministic ones, and at each state only one type is available.
Lucia Acciai, Michele Boreale, Rocco De Nicola

A Model-Checking Tool for Families of Services

Abstract
We propose a model-checking tool for on-the-fly verification of properties expressed in a branching-time temporal logic based on a deontic interpretation of classical modal and temporal operators over modal transition systems. We apply this tool to the analysis of variability in behavioural descriptions of families of services.
Patrizia Asirelli, Maurice H. ter Beek, Alessandro Fantechi, Stefania Gnesi

Partial Order Methods for Statistical Model Checking and Simulation

Abstract
Statistical model checking has become a promising technique to circumvent the state space explosion problem in model-based verification. It trades time for memory, via a probabilistic simulation and exploration of the model behaviour—often combined with effective a posteriori hypothesis testing. However, as a simulation-based approach, it can only provide sound verification results if the underlying model is a stochastic process. This drastically limits its applicability in verification, where most models are indeed variations of nondeterministic transition systems. In this paper, we describe a sound extension of statistical model checking to scenarios where nondeterminism is present. We focus on probabilistic automata, and discuss how partial order reduction can be twisted such as to apply statistical model checking to models with spurious nondeterminism. We report on an implementation of this technique and on promising results in the context of verification and dependability analysis of distributed systems.
Jonathan Bogdoll, Luis María Ferrer Fioriti, Arnd Hartmanns, Holger Hermanns

Counterexample Generation for Markov Chains Using SMT-Based Bounded Model Checking

Abstract
Generation of counterexamples is a highly important task in the model checking process. In contrast to, e.,g., digital circuits where counterexamples typically consist of a single path leading to a critical state of the system, in the probabilistic setting counterexamples may consist of a large number of paths. In order to be able to handle large systems and to use the capabilities of modern SAT-solvers, bounded model checking (BMC) for discrete-time Markov chains was established.
In this paper we introduce the usage of SMT-solving over linear real arithmetic for the BMC procedure. SMT-solving, extending SAT with theories in this context on the one hand leads to a convenient way to express conditions on the probability of certain paths and on the other hand allows to handle Markov reward models. We use the former to find paths with high probability first. This leads to more compact counterexamples. We report on some experiments, which show promising results.
Bettina Braitling, Ralf Wimmer, Bernd Becker, Nils Jansen, Erika Ábrahám

Adaptable Processes (Extended Abstract)

Abstract
We propose the concept of adaptable processes as a way of overcoming the limitations that process calculi have for describing patterns of dynamic process evolution. Such patterns rely on direct ways of controlling the behavior and location of running processes, and so they are at the heart of the adaptation capabilities present in many modern concurrent systems. Adaptable processes have a location and are sensible to actions of dynamic update at runtime. This allows to express a wide range of evolvability patterns for processes. We introduce a core calculus of adaptable processes and propose two verification problems for them: bounded and eventual adaptation. While the former ensures that at most k consecutive errors will arise in future states, the latter ensures that if the system enters into an error state then it will eventually reach a correct state. We study the (un)decidability of these two problems in different fragments of the calculus. Rather than a specification language, our calculus intends to be a basis for investigating the fundamental properties of evolvable processes and for developing richer languages with evolvability capabilities.
Mario Bravetti, Cinzia Di Giusto, Jorge A. Pérez, Gianluigi Zavattaro

A Framework for Verifying Data-Centric Protocols

Abstract
Data centric languages, such as recursive rule based languages, have been proposed to program distributed applications over networks. They simplify greatly the code, while still admitting efficient distributed execution. We show that they also provide a promising approach to the verification of distributed protocols, thanks to their data centric orientation, which allows us to explicitly handle global structures such as the topology of the network. We consider a framework using an original formalization in the Coq proof assistant of a distributed computation model based on message passing with either synchronous or asynchronous behavior. The declarative rules of the Netlog language for specifying distributed protocols and the virtual machines for evaluating these rules are encoded in Coq as well. We consider as a case study tree protocols, and show how this framework enables us to formally verify them in both the asynchronous and synchronous setting.
Yuxin Deng, Stéphane Grumbach, Jean-François Monin

Relational Concurrent Refinement: Timed Refinement

Abstract
Data refinement in a state-based language such as Z is defined using a relational model in terms of the behaviour of abstract programs. Downward and upward simulation conditions form a sound and jointly complete methodology to verify relational data refinements, which can be checked on an event-by-event basis rather than per trace. In models of concurrency, refinement is often defined in terms of sets of observations, which can include the events a system is prepared to accept or refuse, or depend on explicit properties of states and transitions. By embedding such concurrent semantics into a relational one, eventwise verification methods for such refinement relations can be derived. In this paper we continue our program of deriving simulation conditions for process algebraic refinement by considering how notions of time should be embedded into a relational model, and thereby deriving relational notions of timed refinement.
John Derrick, Eerke Boiten

Galois Connections for Flow Algebras

Abstract
We generalise Galois connections from complete lattices to flow algebras. Flow algebras are algebraic structures that are less restrictive than idempotent semirings in that they replace distributivity with monotonicity and dispense with the annihilation property; therefore they are closer to the approach taken by Monotone Frameworks and other classical analyses. We present a generic framework for static analysis based on flow algebras and program graphs. Program graphs are often used in Model Checking to model concurrent and distributed systems. The framework allows to induce new flow algebras using Galois connections such that correctness of the analyses is preserved. The approach is illustrated for a mutual exclusion algorithm.
Piotr Filipiuk, Michał Terepeta, Hanne Riis Nielson, Flemming Nielson

An Accurate Type System for Information Flow in Presence of Arrays

Abstract
Secure information flow analysis aims to check that the execution of a program does not reveal information about secret data manipulated by this program. In this paper, we consider programs dealing with arrays; unlike most of existing works, we will not assume that arrays are homogeneous in terms of security levels. Some part of an array can be declared as secret whereas another part is public. Based on a pre-computed approximation of integer variables (serving as indices for arrays), we devise a type system such that typed programs do not leak unauthorized information. Soundness of our type system is proved by a non-interference theorem.
Séverine Fratani, Jean-Marc Talbot

Analysis of Deadlocks in Object Groups

Abstract
Object groups are collections of objects that perform collective work. We study a calculus with object groups and develop a technique for the deadlock analysis of such systems based on abstract descriptions of method’s behaviours.
Elena Giachino, Cosimo Laneve

Monitoring Distributed Systems Using Knowledge

Abstract
In this paper, we use knowledge-based control theory to monitor global properties in a distributed system. We control the system to enforce that if a given global property is violated, at least one process knows this fact, and therefore may report it. Our approach uses knowledge properties that are precalculated based on model checking. As local knowledge is not always sufficient to monitor a global property in a concurrent system, we allow adding temporary synchronizations between two or more processes to achieve sufficient knowledge. Since synchronizations are expensive, we aim at minimizing their number using the knowledge analysis.
Susanne Graf, Doron Peled, Sophie Quinton

Global State Estimates for Distributed Systems

Abstract
We consider distributed systems modeled as communicating finite state machines with reliable unbounded FIFO channels. As an essential sub-routine for control, monitoring and diagnosis applications, we provide an algorithm that computes, during the execution of the system, an estimate of the current global state of the distributed system for each local subsystem. This algorithm does not change the behavior of the system; each subsystem only computes and records a symbolic representation of the state estimates, and piggybacks some extra information to the messages sent to the other subsystems in order to refine their estimates. Our algorithm relies on the computation of reachable states. Since the reachability problem is undecidable in our model, we use abstract interpretation techniques to obtain regular overapproximations of the possible FIFO channel contents, and hence of the possible current global states. An implementation of this algorithm provides an empirical evaluation of our method.
Gabriel Kalyon, Tristan Le Gall, Hervé Marchand, Thierry Massart

A Process Calculus for Dynamic Networks

Abstract
In this paper we propose a process calculus framework for dynamic networks in which the network topology may change as computation proceeds. The proposed calculus allows one to abstract away from neighborhood-discovery computations and it contains features for broadcasting at multiple transmission ranges and for viewing networks at different levels of abstraction. We develop a theory of confluence for the calculus and we use the machinery developed towards the verification of a leader-election algorithm for mobile ad hoc networks.
Dimitrios Kouzapas, Anna Philippou

On Asynchronous Session Semantics

Abstract
This paper studies a behavioural theory of the π-calculus with session types under the fundamental principles of the practice of distributed computing — asynchronous communication which is order-preserving inside each connection (session), augmented with asynchronous inspection of events (message arrivals). A new theory of bisimulations is introduced, distinct from either standard asynchronous or synchronous bisimilarity, accurately capturing the semantic nature of session-based asynchronously communicating processes augmented with event primitives. The bisimilarity coincides with the reduction-closed barbed congruence. We examine its properties and compare them with existing semantics. Using the behavioural theory, we verify that the program transformation of multithreaded into event-driven session based processes, using Lauer-Needham duality, is type and semantic preserving.
Dimitrios Kouzapas, Nobuko Yoshida, Kohei Honda

Towards Verification of the Pastry Protocol Using TLA + 

Abstract
Pastry is an algorithm that provides a scalable distributed hash table over an underlying P2P network. Several implementations of Pastry are available and have been applied in practice, but no attempt has so far been made to formally describe the algorithm or to verify its properties. Since Pastry combines rather complex data structures, asynchronous communication, concurrency, resilience to churn and fault tolerance, it makes an interesting target for verification. We have modeled Pastry’s core routing algorithms and communication protocol in the specification language TLA + . In order to validate the model and to search for bugs we employed the TLA +  model checker tlc to analyze several qualitative properties. We obtained non-trivial insights in the behavior of Pastry through the model checking analysis. Furthermore, we started to verify Pastry using the very same model and the interactive theorem prover tlaps for TLA + . A first result is the reduction of global Pastry correctness properties to invariants of the underlying data structures.
Tianxiang Lu, Stephan Merz, Christoph Weidenbach

Dynamic Soundness in Resource-Constrained Workflow Nets

Abstract
Workflow Petri nets (wf-nets) are an important formalism for the modeling of business processes. For them we are typically interested in the soundness problem, that intuitively consists in deciding whether several concurrent executions can always terminate properly. Resource-Constrained Workflow Nets (rcfw-nets) are wf-nets enriched with static places, that model global resources. In this paper we prove the undecidability of soundness for rcwf-nets when there may be several static places and in which instances are allowed to terminate having created or consumed resources. In order to have a clearer presentation of the proof, we define an asynchronous version of a class of Petri nets with dynamic name creation. Then, we prove that reachability is undecidable for them, and reduce it to dynamic soundness in rcwf-nets. Finally, we prove that if we restrict our class of rcwf-nets, assuming in particular that a single instance is sound when it is given infinitely many global resources, then dynamic soundness is decidable by reducing it to the home space problem in P/T nets for a linear set of markings.
María Martos-Salgado, Fernando Rosa-Velardo

SimGrid MC: Verification Support for a Multi-API Simulation Platform

Abstract
SimGrid MC is a stateless model checker for distributed systems that is part of the SimGrid Simulation Framework. It verifies implementations of distributed algorithms, written in C and using any of several communication APIs provided by the simulator. Because the model checker is fully integrated in the simulator that programmers use to validate their implementations, they gain powerful verification capabilities without having to adapt their code. We describe the architecture of SimGrid MC, and show how it copes with the state space explosion problem. In particular, we argue that a generic Dynamic Partial Order Reductions algorithm is effective for handling the different communication APIs that are provided by SimGrid. As a case study, we verify an implementation of Chord, where SimGrid MC helped us discover an intricate bug in a matter of seconds.
Stephan Merz, Martin Quinson, Cristian Rosa

Ownership Types for the Join Calculus

Abstract
This paper investigates ownership types in a concurrent setting using the Join calculus as the model of processes. Ownership types have the effect of statically preventing certain communication, and can block the accidental or malicious leakage of secrets. Intuitively, a channel defines a boundary and forbids access to its inside from outer channels, thus preserving the secrecy of the inner names from malicious outsiders. Secrecy is also preserved in the context of an untyped opponent.
Marco Patrignani, Dave Clarke, Davide Sangiorgi

Contracts for Multi-instance UML Activities

Abstract
We present a novel way of encapsulating UML activities using interface contracts, which allows to verify functional properties that depend on the synchronization of parallel instances of software components. Encapsulated UML activities can be reused together with their verification results in SPACE, a model-driven engineering method for reactive systems. Such compositional verification significantly improves the scalability of the method. Employing a small example of a load balancing system, we explain the semantics of the contracts using the temporal logic TLA. Thereafter, we propose a more easily comprehensible graphical notation and clarify that the contracts are able to express the variants of multiplicity that we can encounter using UML activities. Finally, we give the results of verifying some properties of the example system using the TLC model checker.
Vidar Slåtten, Peter Herrmann

Annotation Inference for Separation Logic Based Verifiers

Abstract
With the years, program complexity has increased dramatically: ensuring program correctness has become considerably more difficult with the advent of multithreading, security has grown more prominent during the last decade, etc. As a result, static verification has become more important than ever.
Automated verification tools exist, but they are only able to prove a limited set of properties, such as memory safety. If we want to prove full functional correctness of a program, other more powerful tools are available, but they generally require a lot more input from the programmer: they often need the code to be verified to be heavily annotated.
In this paper, we attempt to combine the best of both worlds by starting off with a manual verification tool based on separation logic for which we develop techniques to automatically generate part of the required annotations. This approach provides more flexibility: for instance, it makes it possible to automatically check as large a part of the program as possible for memory errors and then manually add extra annotations only to those parts of the code where automated tools failed and/or full correctness is actually needed.
Frédéric Vogels, Bart Jacobs, Frank Piessens, Jan Smans

Analyzing BGP Instances in Maude

Abstract
Analyzing Border Gateway Protocol (BGP) instances is a crucial step in the design and implementation of safe BGP systems. Today, the analysis is a manual and tedious process. Researchers study the instances by manually constructing execution sequences, hoping to either identify an oscillation or show that the instance is safe by exhaustively examining all possible sequences. We propose to automate the analysis by using Maude, a tool based on rewriting logic. We have developed a library specifying a generalized path vector protocol, and methods to instantiate the library with customized routing policies. Protocols can be analyzed automatically by Maude, once users provide specifications of the network topology and routing policies. Using our Maude library, protocols or policies can be easily specified and checked for problems. To validate our approach, we performed safety analysis of well-known BGP instances and actual routing configurations.
Anduo Wang, Carolyn Talcott, Limin Jia, Boon Thau Loo, Andre Scedrov

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