Process Algebra Research Articles

Integrated Information in Process-Algebraic Compositions.

Integrated Information Theory (IIT) is most typically applied to Boolean Nets, a state transition model in which system parts cooperate by sharing state variables. By contrast, in Process Algebra, whose semantics can also be formulated in terms of (labeled) state transitions, system parts—“processes”—cooperate by sharing transitions with matching labels, according to interaction patterns expressed by suitable composition operators. Despite this substantial difference, questioning how much additional information is provided by the integration of the interacting partners above and beyond the sum of their independent contributions appears perfectly legitimate with both types of cooperation. In fact, we collect statistical data about —integrated information—relative to pairs of boolean nets that cooperate by three alternative mechanisms: shared variables—the standard choice for boolean nets—and two forms of shared transition, inspired by two process algebras. We name these mechanisms , and . Quantitative characterizations of all of them are obtained by considering three alternative execution modes, namely synchronous, asynchronous and “hybrid”, by exploring the full range of possible coupling degrees in all three cases, and by considering two possible definitions of based on two alternative notions of distribution distance.

Entanglement in Quantum Process Algebra

We explicitly model entanglement in quantum processes by treating entanglement as a kind of parallelism. We introduce a shadow constant quantum operation and a so-called entanglement merge into quantum process algebra qACP. The transition rules of the shadow constant quantum operation and entanglement merge are designed. We also do a sound and complete axiomatization modulo the so-called quantum bisimilarity for the shadow constant quantum operation and entanglement merge. Then, this new type entanglement merge is extended into the full qACP. The new qACP has wide use in verification for quantum protocols, since most quantum protocols have mixtures with classical and quantum information, and also there are many quantum protocols adopting entanglement.

Formal Verification for KMB09 Protocol

The unifying of qACP and classical ACP under the framework of quantum process configuration 〈p, ρ〉 makes verification for quantum protocols possible, not only the pure quantum protocol, but also protocol that mixes quantum information and classical information. In this paper, we verify the KMB09 protocol by use of quantum process algebra qACP.

Dynamic Epistemic Logic with Communication Actions

This work proposes a Dynamic Epistemic Logic with Communication Actions that can be performed concurrently. Unlike Concurrent Epistemic Action Logic introduced by Ditmarsch, Hoek and Kooi [van Ditmarsch, H., W. van der Hoek and B. Kooi, “Concurrent Dynamic Epistemic Logic,” Kluwer, Ed. V.F. Hendricks et al., vol. 322, 2003], where the concurrency mechanism is the so called true concurrency, here we use an approach based on process calculus, like CCS and CSP, and Action Models Logic. Our approach makes possible the proof of soundness, completeness and decidability, different from the others approaches. We present an axiomatisation and show that the proof of soundness, completeness and decidability can be done using a reduction method.

Ensuring liveness properties of distributed systems: Open problems

Often fairness assumptions need to be made in order to establish liveness properties of distributed systems, but in many situations they lead to false conclusions.This document presents a research agenda aiming at laying the foundations of a theory of concurrency that is equipped to ensure liveness properties of distributed systems without making fairness assumptions. This theory will encompass process algebra, temporal logic and semantic models. The agenda also includes the development of a methodology and tools that allow successful application of this theory to the specification, analysis and verification of realistic distributed systems.Contemporary process algebras and temporal logics fail to make distinctions between systems of which one has a crucial liveness property and the other does not, at least when assuming justness, a strong progress property, but not assuming fairness. Setting up an alternative framework involves giving up on identifying strongly bisimilar systems, inventing new induction principles, developing new axiomatic bases for process algebras and new congruence formats for operational semantics, and creating matching treatments of time and probability.Even simple systems like fair schedulers or mutual exclusion protocols cannot be accurately specified in standard process algebras (or Petri nets) in the absence of fairness assumptions. Hence the work involves the study of adequate language or model extensions, and their expressive power.

An Axiomatization for Quantum Processes to Unifying Quantum and Classical Computing

We establish an axiomatization for quantum processes, which is a quantum generalization of process algebra ACP (Algebra of Communicating Processes). We use the framework of a quantum process configuration 〈p, ϱ〉, but we treat it as two relative independent part: the structural part p and the quantum part ϱ, because the establishment of a sound and complete theory is dependent on the structural properties of the structural part p. We let the quantum part ϱ be the outcomes of execution of p to examine and observe the function of the basic theory of quantum mechanics. We establish not only a strong bisimilarity for quantum processes, but also a weak bisimilarity to model the silent step and abstract internal computations in quantum processes. The relationship between quantum bisimilarity and classical bisimilarity is established, which makes an axiomatization of quantum processes possible. An axiomatization for quantum processes called qACP is designed, which involves not only quantum information, but also classical information and unifies quantum computing and classical computing. qACP can be used easily and widely for verification of most quantum communication protocols.

A calculus of branching processes

CCS-like calculi can be viewed as an extension of classical automata with communication primitives. We are interested here to follow this principle, applied to tree-automata. It naturally yields a calculus of branching processes (CBP), where the continuations of communications are allowed to branch according to the arity of the communication channel. After introducing the calculus with a reduction semantics we show that CBP can be “implemented” by a fully compositional LTS semantics. We argue that CBP offers an interesting tradeoff between calculi with a fixed communication topology à la CCS and calculi with dynamic connectivity such as the π-calculus.

Discovering and correcting a deadlock in a channel implementation

We investigate the cause of a deadlock in the implementation of a channel in a message-passing concurrency API. We model the channel implementation using the process algebra CSP, and then use the model checker FDR to find the cause of the deadlock. The bug is rather subtle, and arguably infeasible to spot by hand. We then propose a straightforward fix to the bug, and use CSP and FDR to verify this fix.

On the relative expressiveness of higher-order session processes

By integrating constructs from the λ-calculus and the π-calculus, in higher-order process calculi exchanged values may contain processes. This paper studies the relative expressiveness of HOπ, the higher-order π-calculus in which communications are governed by session types. Our main discovery is that HO, a subcalculus of HOπ which lacks name-passing and recursion, can serve as a new core calculus for session-typed higher-order concurrency. We show that HO can encode HOπ fully abstractly (up to typed contextual equivalence) more precisely and efficiently than the first-order session π-calculus (π). Overall, under the discipline of session types, HOπ, HO, and π are equally expressive; however, we show that HOπ is more tightly related to HO than to π.

Model Checking Based Approach for Compliance Checking


 
 
 Process mining is the set of techniques to retrieve a process model starting from available logging data. The discovered process model has to be analyzed to verify it respects the defined properties, i.e., the so-called compliance checking. Our aim is to use a model checking based approach to verify compliance. First, we propose an integrated-tool approach using existing tools as ProM (a framework supporting process mining techniques) and CADP (a formal verification environment). More precisely, the execution traces from a software system are extracted. Then, using the “Mine Transition System” plugin in ProM, we obtain a labelled transition system, that can be easily used to verify formal properties trough CADP. However, this choice presents the “state explosion” problem, i.e., models discovered through the classical process mining techniques tend to be large and complex. In order to solve this problem, another custom-made approach is shown, which accomplishes a pre- processing on the traces to obtain abstract traces, where abstraction is based on the set of temporal logic formulae specifying the system properties. Then, from the set of abstracted traces, we discover a system described in Lotos, a process algebra specification language; in this way we do not build an operational model for the system, but we produce only a language description from which a model checking environment will automatically obtain the reduced corresponding transition system. Real systems have been used as case studies to evaluate the proposed methodologies.
 
 

A calculus for modeling floating authorizations

Controlling resource usage in distributed systems is a challenging task given the dynamics involved in access granting. Consider, e.g., the setting of floating licenses where access can be granted if the request originates in a licensed domain and if the number of active users is within the license limits. Access granting in such scenarios is given in terms of floating authorizations, addressed in this paper as first class entities of a process calculus model, encompassing the notions of domain, accounting and delegation. We present the operational semantics of the model in two equivalent alternative ways, and report on a preliminary investigation of the behavioral semantics, addressing fundamental properties and informing on the specific nature of our authorizations. We also introduce a typing discipline to single out systems that never get stuck due to lacking authorizations, addressing configurations where authorization assignment is not statically prescribed in the system specification. Finally, we present a refinement of the type system which paves the way for obtaining a more efficient type checking procedure.

Trypanosome-host interaction revealed through the zebrafish looking glass

As a first approximation of immune-mediated within-host parasite dynamics we can consider the immune response as a predator, with the parasite as its prey. In the ecological literature of predator–prey interactions there are a number of different functional responses used to describe how a predator reproduces in response to consuming prey. Until recently most of the models of the immune system that have taken a predator–prey approach have used simple mass action dynamics to capture the interaction between the immune response and the parasite. More recently Fenton and Perkins (2010) employed three of the most commonly used prey-dependent functional response terms from the ecological literature.In this paper we make use of a technique from computing science, process algebra, to develop mathematical models. The novelty of the process algebra approach is to allow stochastic models of the population (parasite and immune cells) to be developed from rules of individual cell behaviour. By using this approach in which individual cellular behaviour is captured we have derived a ratio-dependent response similar to that seen in the previous models of immune-mediated parasite dynamics, confirming that, whilst this type of term is controversial in ecological predator–prey models, it is appropriate for models of the immune system.

A comprehensive study of process calculi with routing tables

A comprehensive study of process calculi with routing tables

Cloud Service Security Adaptive Target Detection Algorithm Based on Bio-Inspired Performance Evaluation Process Algebra

Combining the principle of antibody concentration with the idea of biological evolution, this paper proposes an adaptive target detection algorithm for cloud service security based on Bio-Inspired Performance Evaluation Process Algebra (Bio-PEPA). The formal modelling of cloud services is formally modded by Bio-PEPA and the modules are transformed between cloud service internal structures and various components. Then, the security adaptive target detection algorithm of cloud service is divided into two processes, the short-term optimal action selection process which selects the current optimal detective action through the iterative operation of the expected function and the adaptive function, and the long-term detective strategy realized through the updates and eliminations of action planning table. The combination of the two processes reflects the self-adaptability of cloud service system to target detection. The simulating test detects three different kinds of security risks and then analyzes the relationship between the numbers of components with time in the service process. The performance of this method is compared with random detection method and three anomaly detection methods by the cloud service detection experiment. The detection time of this method is 50.1% of three kinds of detection methods and 86.3% of the random detection method. The service success rate is about 15% higher than that of random detection methods. The experimental results show that the algorithm has good time performance and high detection hit rate.

Verification of hypertorus communication grids by infinite petri nets and process algebra

A model of a hypertorus communication grid has been constructed in the form of an infinite Petri net. A grid cell represents either a packet switching device or a bioplast cell. A parametric expression is obtained to allow a finite specification of an infinite Petri net. To prove properties of an ideal communication protocol, we derive an infinite Diophantine system of equations from it, which is subsequently solved. Then we present the programs htgen and ht-mcrl2-gen, developed in the C language, which generate Petri net and process algebra models of a hypertorus with a given number of dimensions and grid size. These are the inputs for the respective modeling tools Tina and mCRL2, which provide model visualization, step simulation, state space generation and reduction, and structural analysis techniques. Benchmarks to compare the two approaches are obtained. An ad-hoc induction-like technique on invariants, obtained for a series of generated models, allows the calculation of a solution of the Diophantine system in a parametric form. It is proven that the basic solutions of the infinite system have been found and that the infinite Petri net is bounded and conservative. Some remarks regarding liveness and liveness enforcing techniques are also presented.

Symmetry reduction in CSP model checking

We present an extension of FDR, the model checker for the process algebra CSP, that exploits symmetry to reduce the size of the state space searched. We define what it means for a process to be symmetric with respect to a group of permutations on the transition labels. We factor the state space of the search by symmetry equivalence, mapping each state to a representative of its equivalence class, thereby considering all symmetric states together. We prove a powerful syntactic result, identifying conditions under which a process will be symmetric in a particular type. We show how to implement such a search using the powerful technique of supercombinators used in the implementation of FDR: we identify conditions on a supercombinator for it to be symmetric and explain how to apply a permutation to a state. Finally, we present a novel efficient technique for calculating representatives of equivalence classes, which normally finds unique representatives; our experiments suggest that this technique typically works faster than other techniques and in particular scales better.

Correctness checking of a quantum protocol for reliable communications via feedback

In this paper, a quantum version of classical alternating bit protocol is proposed. Thisprotocol provides a reliable method to transmit the secret quantum data via a noisyquantum channel while the entanglement between particles is not broken. Our protocol is based on quantum teleportation and superdense coding. By assuming that the participantscan distinguish the alternating qubit from other messages and also the assumption that data canbe resent unlimited times, an abstraction of this protocol can be derived. Using the quantum process algebra fullqACP, we show that the proposed protocol is correct, so the desired external behaviour of the protocol is guaranteed.

Fault-based refinement-testing for CSP

The process algebra CSP has been studied as a notation for model-based testing. Theoretical and practical work has been developed using its trace and failure semantics, and their refinement notions as conformance relations. Two sets of tests have been defined and proved to be exhaustive, in the sense that they can identify any SUT that is non-conforming with respect to the relevant refinement relation. However, these sets are usually infinite, and in this case, it is obviously not possible to apply them to verify the conformity of an SUT. Some classical selection criteria based on models have been studied. In this paper, we propose a procedure for online test generation for selection of finite test sets for traces refinement from CSP models. It is based on the notion of fault domains, focusing on the set of faulty implementations of interest. We investigate scenarios where the verdict of a test campaign can be reached after a finite number of test executions. We illustrate the usage of the procedure with some case studies.

Symbolic computation of differential equivalences

Ordinary differential equations (ODEs) are widespread in many natural sciences including chemistry, ecology, and systems biology, and in disciplines such as control theory and electrical engineering. Building on the celebrated molecules-as-processes paradigm, they have become increasingly popular in computer science, with high-level languages and formal methods such as Petri nets, process algebra, and rule-based systems that are interpreted as ODEs.We consider the problem of comparing and minimizing ODEs automatically. Influenced by traditional approaches in the theory of programming, we propose differential equivalence relations. We study them for a basic intermediate language, for which we have decidability results, that can be targeted by a class of high-level specifications. An ODE implicitly represents an uncountable state space, hence reasoning techniques cannot be borrowed from established domains such as probabilistic programs with finite-state Markov chain semantics. We provide novel symbolic procedures to check an equivalence and compute the largest one via partition refinement algorithms that use satisfiability modulo theories.We illustrate the generality of our framework by showing that differential equivalences include (i) well-known notions for the minimization of continuous-time Markov chains (lumpability), (ii) bisimulations for chemical reaction networks recently proposed by Cardelli et al., and (iii) behavioral relations for process algebra with ODE semantics. Using ERODE, the tool that implements our techniques, we are able to detect equivalences in biochemical models from the literature that cannot be reduced using competing automatic techniques.

A formal approach to AADL model-based software engineering

Formal methods have become a recommended practice in safety-critical software engineering. To be formally verified, a system should be specified with a specific formalism such as Petri nets, automata and process algebras, which requires a formal expertise and may become complex especially with large systems. In this paper, we report our experience in the formal verification of safety-critical real-time systems. We propose a formal mapping for a real-time task model using the LNT language, and we describe how it is used for the integration of a formal verification phase in an AADL model-based development process. We focus on real-time systems with event-driven tasks, asynchronous communication and preemptive fixed-priority scheduling. We provide a complete tool-chain for the automatic model transformation and formal verification of AADL models. Experimentation illustrates our results with the Flight control system and Line follower robot case studies.