Process Algebraic Framework Research Articles

On the Modeling and Verification of Collective and Cooperative Systems.

The formal description and verification of networks of cooperative and interacting agents is made difficult by the interplay of several different behavioral patterns, models of communication, scalability issues. In this paper, we will explore the functionalities and the expressiveness of a general-purpose process algebraic framework for the specification and model checking based analysis of collective and cooperative systems. The proposed syntactic and semantic schemes are general enough to be adapted with small modifications to heterogeneous application domains, like, e.g., crowdsourcing systems, trustworthy networks, and distributed ledger technologies.

A process algebraic mutation framework with application to a vehicle charging protocol

Modern day renewable and smart energy systems are increasingly playing an important role in our societies. Electric vehicle charging services infrastructures are becoming ever more popular and accessible, although at a pace that we argue is not well understood yet in terms of the reliability and security of this new paradigm. Often solutions are rolled out to the market with little or no analysis of their resilience and even at the standards level, specifications of systems and protocol often omit or skim over the question of reliability. In this paper, we propose a new formal approach for analysing the effects of single failures on process algebraic specifications used often in specifying communication protocols. We apply this new approach to one part of an international standard protocol, the Open Charge Point Protocol, for electric vehicle charging, and demonstrate how the effects of specification faults can impact the safety and security of the protocol.

Probabilistic Process Algebra to Unifying Quantum and Classical Computing in Closed Systems

We have unified quantum and classical computing in open quantum systems called qACP which is a quantum generalization of process algebra ACP. But, an axiomatization for quantum and classical processes with an assumption of closed quantum systems is still missing. For closed quantum systems, unitary operator, quantum measurement and quantum entanglement are three basic components for quantum computing. This leads to probability unavoidable. Along the solution of qACP to unify quantum and classical computing in open quantum systems, we unify quantum and classical computing with an assumption of closed systems under the framework of ACP-like probabilistic process algebra. This unification make it can be used widely in verification for quantum and classical computing mixed systems, such as most quantum communication protocols.

Replicated Computations Results (RCR) Report for “Design and Verification of Trusted Collective Adaptive Systems”

The article “Design and Verification of Trusted Collective Adaptive Systems” by Aldini proposes a process-algebraic framework for modeling and verifying trusted collective adaptive systems. To favor reuse, the system and trust models can be specified separately, only to be integrated at the semantic level. Through a combination of behavioral equivalence checking and model checking against branching-time temporal logic with trust predicates, the framework allows comparative analyses of different trust models as well as analyses of the effects of attacks to the trust models. The applicability of the formal framework is illustrated by means of two representative use cases: the security analysis of a trust-incentive service management system and a comparison of two different reputation systems. This replicated computations results report focuses on the reproducibility of the experiments performed in the aforementioned article, i.e. on the automatic verification of properties against models of these use cases encoded in the well-known NuSMV model checker. It was straightforward to reproduce all results from the article in reasonable time using a standard laptop machine.

A Routing Calculus with Distance Vector Routing Updates

We propose a routing calculus in a process algebraic framework to implement dynamic updates of routing table using distance vector routing. This calculus is an extension of an existing routing calculus DRωπ where routing tables are fixed except when new nodes are created in which case the routing tables are appended with relevant entries. The main objective of implementing dynamic routing updates is to demonstrate the formal modeling of distributed networks which is closer to the networks in practice. We justify our calculus by showing its reduction equivalence with its specification Dπ (distributed π-calculus) after abstracting away the unnecessary details from our calculus which in fact is one of the implementations of Dπ. We nomenclate our calculus with routing table updates as DRϕπ .

A Formal Framework for Modeling Trust and Reputation in Collective Adaptive Systems

Trust and reputation models for distributed, collaborative systems have been studied and applied in several domains, in order to stimulate cooperation while preventing selfish and malicious behaviors. Nonetheless, such models have received less attention in the process of specifying and analyzing formally the functionalities of the systems mentioned above. The objective of this paper is to define a process algebraic framework for the modeling of systems that use (i) trust and reputation to govern the interactions among nodes, and (ii) communication models characterized by a high level of adaptiveness and flexibility. Hence, we propose a formalism for verifying, through model checking techniques, the robustness of these systems with respect to the typical attacks conducted against webs of trust.

Performance Verification for Behavior-Based Robot Missions

Certain robot missions need to perform predictably in a physical environment that may have significant uncertainty. One approach is to leverage automatic software verification techniques to establish a performance guarantee. The addition of an environment model and uncertainty in both program and environment, however, means that the state space of a model-checking solution to the problem can be prohibitively large. An approach based on behavior-based controllers in a process-algebra framework that avoids state-space combinatorics is presented here. In this approach, verification of the robot program in the uncertain environment is reduced to a filtering problem for a Bayesian network. Validation results are presented for the verification of a multiple-waypoint and an autonomous exploration robot mission.

Enabling Synchronous and Asynchronous Communications in CSP for SOC

Service Oriented Computing (SOC) is based on service composition, that is, loosely coupled autonomous heterogeneous services, which are collectively composed to implement a particular task. We develop a new calculus for SOC within the framework of CSP process algebra, aiming to improve the verification techniques and to enhance the expressiveness of SOC calculi. This paper presents the part of the calculus that extends CSP with built-in buffers to facilitate direct asynchronous communications. We provide the operational semantics of the calculus and extend the FDR (a CSP model checker) by implementing functions for asynchronous communications.

Performance analysis of concurrent systems in algebra dtsiPBC

Petri box calculus PBC is a well-known algebra of concurrent processes with a Petri net semantics. In the paper, an extension of PBC with discrete stochastic time and immediate multiactions, which is referred to as discrete time stochastic and immediate PBC (dtsiPBC), is considered. Performance analysis methods for concurrent and distributed systems with random time delays are investigated in the framework of the new stochastic process algebra. It is demonstrated that the performance evaluation is possible not only via the underlying semi-Markov chains of the dtsiPBC expressions but also with the use of the underlying discrete time Markov chains, and the latter analysis technique is more optimal.

Sound and mechanised compositional verification of input‐output conformance

SUMMARYThis paper mechanises conformance verification in the setting of the CSP process algebra. The verification strategy is captured by a theorem stated as a process refinement expression, which can be verified by a model checker such as FDR. The conformance relation, cspio, distinguishes input and output events. The process algebraic framework of CSP is used to address compositional conformance verification by establishing compositionality properties for cspio with respect to the CSP operators. Although cspio has been defined in the standard CSP traces model, one can address quiescence situations using a special output event, in which case it is formally established that cspio is equivalent to Tretmans ioco. All the results have been mechanically proved using the CSP‐Prover. The proposed testing theory has been adopted in an industrial context involving collaboration with Motorola, on testing mobile applications. Several examples and a case study are presented to illustrate the overall approach. Copyright © 2013 John Wiley & Sons, Ltd.

A process algebra framework for multi-scale modelling of biological systems

We introduce a novel process algebra for modelling biological systems at multiple scales, called process algebra with hooks (PAH). Processes represent biological entities, such as molecules, cells and tissues, while two algebraic operators, both symmetric, define composition of processes within and between scales. Composed actions allow for biological events to interact within and between scales at the same time. The algebra has a stochastic semantics based on functional rates of reactions. Two bisimulations are defined on PAH processes. The first bisimulation is used to aid model development by checking that two biological scales can interact correctly. The second bisimulation is a congruence that relates models, or part of models, that can perform the same timed events at a specified scale. Finally, we provide a PAH model of pattern formation in a tissue and illustrate reasoning about its behaviour using the PAH framework.

Composition and abstraction of logical regulatory modules: application to multicellular systems

Logical (Boolean or multi-valued) modelling is widely used to study regulatory or signalling networks. Even though these discrete models constitute a coarse, yet useful, abstraction of reality, the analysis of large networks faces a classical combinatorial problem. Here, we propose to take advantage of the intrinsic modularity of inter-cellular networks to set up a compositional procedure that enables a significant reduction of the dynamics, yet preserving the reachability of stable states. To that end, we rely on process algebras, a well-established computational technique for the specification and verification of interacting systems. We develop a novel compositional approach to support the logical modelling of interconnected cellular networks. First, we formalize the concept of logical regulatory modules and their composition. Then, we make this framework operational by transposing the composition of logical modules into a process algebra framework. Importantly, the combination of incremental composition, abstraction and minimization using an appropriate equivalence relation (here the safety equivalence) yields huge reductions of the dynamics. We illustrate the potential of this approach with two case-studies: the Segment-Polarity and the Delta-Notch modules.

PADS: An approach to modeling resource demand and supply for the formal analysis of hierarchical scheduling

As real-time embedded systems become more complex, resource partitioning is increasingly used to guarantee real-time performance. Recently, several compositional frameworks of resource partitioning have been proposed using real-time scheduling theory with various notions of real-time tasks running under restricted resource supply environments. However, these real-time scheduling-based approaches are limited in their expressiveness in that, although capable of describing resource-demand tasks, they are unable to model resource supply. This paper describes a process algebraic framework PADSfor reasoning about resource demand and resource supply inspired by the timed process algebra ACSR. In ACSR, real-time tasks are specified by enunciating their consumption needs for resources. To also accommodate resource-supply processes in PADS, given a resource c p u , we write c p u ¯ to denote the availability of c p u for a requesting task process. Using PADS, we define a supply-demand relation where a pair ( T , S ) belongs to the relation if the demand process T can be scheduled under supply S . We develop a theory of compositional schedulability analysis as well as a technique for synthesizing an optimal supply process for a set of tasks. Furthermore, we define ordering relations between supplies which describe when a supply offers more resource capacity than another. With this notion it is possible to formally represent hierarchical scheduling approaches that assign more “generous” resource allocations to tasks in exchange for a simple representation. We illustrate our techniques via a number of examples.

Approximating Markovian testing equivalence

Several approaches have been proposed to relax behavioral equivalences for fine-grain models including probabilities and time. All of them face two problems behind the notion of approximation, i.e., the lack of transitivity and the efficiency of the verification algorithm. While the typical equivalence under approximation is bisimulation, we present a relaxation of Markovian testing equivalence in a process algebraic framework. In this coarser setting, we show that it is particularly intuitive to manage separately three different dimensions of the approximation–execution time, event probability, and observed behavior–by illustrating in each case, results concerning the two problems mentioned above. Finally, a unified definition combining the three orthogonal aspects is provided in order to favor trade-off analyses.

Process algebraic support for web service composition

Web services technology provides a platform on which we can develop distributed services. The interoperability among these services is achieved by various standard protocols. In recent years, several researches suggested that process algebras provide a satisfactory assistance to the whole process of web services development. Business transactions, on the other hand, involve the coordination and interaction between multiple partners. With the emergence of web services, business transactions are conducted using these services. The coordination among the business processes is crucial, so is the handling of faults that can arise at any stage of a transaction. Compensation is an error recovery mechanism to handle faults in business transactions. Compensating CSP (cCSP) is a language defined to model business transactions within the framework of standard CSP process algebra. The language defines a transaction as a process and has the constructs for orchestration of compensations. In this short note, we report a case study showing how cCSP can be used to model business transactions. We also show a mechanical verification technique to verify theoretical properties of the language

Making random choices invisible to the scheduler

When dealing with process calculi and automata which express both nondeterministic and probabilistic behavior, it is customary to introduce the notion of scheduler to resolve the nondeterminism. It has been observed that for certain applications, notably those in security, the scheduler needs to be restricted so not to reveal the outcome of the protocol’s random choices, or otherwise the model of adversary would be too strong even for “obviously correct” protocols. We propose a process-algebraic framework in which the control on the scheduler can be specified in syntactic terms, and we show how to apply it to solve the problem mentioned above. We also consider the definition of (probabilistic) may and must preorders, and we show that they are precongruences with respect to the restricted schedulers. Furthermore, we show that all the operators of the language, except replication, distribute over probabilistic summation, which is a useful property for verification.

Formal Specification of Correlation in WS Orchestrations Using BP-calculus

Correlation is an important mechanism used in orchestration languages for Web Services. It expresses means by which many instances of the same service can be carried on at the same time. In this paper we extend the BP-calculus, a language based on the π-calculus and dedicated to the specification of web service orchestrations, with a message algebra, a mechanism of function evaluation and a mechanism of correlation. The mechanism of function evaluation allows message handling while the mechanism of correlation guarantees uniqueness of service instances by preventing reception of messages inducing the same assignments of a correlation set. We also show how it can be used to express the semantics of the BPEL constructs coping with correlation. As an illustration of the usefulness of this process algebraic framework, we terminate with the presentation of a motivating example, the Trade Market example.

A process algebraic framework for specification and validation of real-time systems

Abstract Following the trend to combine techniques to cover several facets of the development of modern systems, an integration of Z and CSP, called Circus , has been proposed as a refinement language; its relational model, based on the unifying theories of programming (UTP), justifies refinement in the context of both Z and CSP. In this paper, we introduce Circus Time , a timed extension of Circus , and present a new UTP time theory, which we use to give semantics to Circus Time and to validate some of its laws. In addition, we provide a framework for validation of timed programs based on FDR, the CSP model-checker. In this technique, a syntactic transformation strategy is used to split a timed program into two parallel components: an untimed program that uses timer events, and a collection of timers. We show that, with the timer events, it is possible to reason about time properties in the untimed language, and so, using FDR. Soundness is established using a Galois connection between the untimed UTP theory of Circus (and CSP) and our time theory.

Atomicity Analysis of Service Composition across Organizations

Atomicity is a highly desirable property for achieving application consistency in service compositions. To achieve atomicity, a service composition should satisfy the atomicity sphere, a structural criterion for the backend processes of involved services. Existing analysis techniques for atomicity sphere generally assume complete knowledge of all involved backend processes. Such an assumption is invalid when some service providers do not release all details of their backend processes to service consumers outside the organizations. To address this problem, we propose a process algebraic framework to publish atomicity-equivalent public views from the backend processes. These public views extract relevant task properties and reveal only partial process details that service providers need to expose. Our framework enables the analysis of atomicity sphere for service compositions using these public views instead of their backend processes. This allows service consumers to choose suitable services such that their composition satisfies the atomicity sphere without disclosing the details of their backend processes. Based on the theoretical result, we present algorithms to construct atomicity-equivalent public views and to analyze the atomicity sphere for a service composition. Two case studies from supply chain and insurance domains are given to evaluate our proposal and demonstrate the applicability of our approach.

Performance Evaluation of Distributed Systems Based on a Discrete Real- and Stochastic-Time Process Algebra

We present a process-algebraic framework for performance evaluation of discrete-time discrete-event systems. The modeling of the system builds on a process algebra with conditionallydistributed discrete-time delays and generally-distributed stochastic delays. In the general case, the performance analysis is done with the toolset of the modeling language χ by means of discrete-event simulation. The process-algebraic setting allows for expansion laws for the parallel composition and the maximal progress operator, so one can directly manipulate the process terms and transform the specification in a required form. This approach is illustrated by specifying and solving the recursive specification of the G/G/1/∞ queue, as well as by specifying a variant of the concurrent alternating bit protocol with generally-distributed unreliable channels. In a specific situation when all delays are assumed deterministic, we turn to performance analysis of probabilistic timed systems. This work employs discrete-time probabilistic reward graphs, which comprise deterministic delays and immediate probabilistic choices. Here, we extend previous investigations on the topic, which only touched long-run analysis, to tackle transient analysis as well. The theoretical results obtained allow us to extend the χ-toolset. For illustrative purposes, we analyze the concurrent alternating bit protocol in the extended environment of the χ-toolset using discrete-event simulation for generallydistributed channels, the developed analytical method for deterministic channels, and Markovian analysis for exponentially-distributed delays.