Process Algebra Research Articles

A general overview of formal languages for individual-based modelling of ecosystems

Various formal languages have been proposed in the literature for the individual-based modelling of ecological systems. These languages differ in their treatment of time and space. Each modelling language offers a distinct view and techniques for analyzing systems. Most of the languages are based on process calculi or P systems. In this article, we present a general overview of the existing modelling languages based on process calculi. We also discuss, briefly, other approaches such as P systems, cellular automata and Petri nets. Finally, we show advantages and disadvantages of these modelling languages and we propose some future research directions.

A formal approach to open multiparty interactions

We present a process algebra aimed at describing interactions that are multiparty, i.e. that may involve more than two processes and that are open, i.e. the number of the processes they involve is not fixed or known a priori. Here we focus on the theory of a core version of a process calculus, without message passing, called Core Network Algebra (CNA). In CNA communication actions are given not in terms of channels but in terms of chains of links that record the source and the target ends of each hop of interactions. The operational semantics of our calculus mildly extends the one of CCS. The abstract semantics is given in the style of bisimulation but requires some ingenuity. Remarkably, the abstract semantics is a congruence for all operators of CNA and also with respect to substitutions, which is not the case for strong bisimilarity in CCS. As a motivating and running example, we illustrate the model of a simple software defined network infrastructure.

Nested-unit Petri nets

Petri nets can express concurrency and nondeterminism but neither locality nor hierarchy. This article presents an extension of Petri nets, in which places can be grouped into so-called “units” expressing sequential components. Units can be recursively nested to reflect both the concurrent and hierarchical nature of complex systems. This model called NUPN (Nested-Unit Petri Nets) was originally developed for translating process calculi to Petri nets, but later found also useful beyond this setting. It allows significant savings in the memory representation of markings for both explicit-state and symbolic verification. Thirteen software tools already implement the NUPN model, which has also been adopted for the benchmarks of the Model Checking Contest (MCC) and the parallel problems of the Rigorous Examination of Reactive Systems (RERS) challenges.

Process calculi may reveal the equivalence lying at the heart of RNA and proteins

The successful use of process calculi to specify behavioural models allows us to compare RNA and protein folding processes from a new perspective. We model the folding processes as behaviours resulting from the interactions that nucleotides and amino acids (the elementary units that compose RNAs and proteins respectively) perform on their linear sequences. This approach is intended to provide new knowledge about the studied systems without strictly relying on empirical data. By applying Milner’s CCS process algebra to highlight the distinguishing features of the two folding processes, we discovered an abstraction level at which they show behavioural equivalences. We believe that this result could be interpreted as a clue in favour of the highly-debated RNA World theory, according to which, in the early stages of cell evolution, RNA molecules played most of the functional and structural roles carried out today by proteins.

Towards a categorical representation of reversible event structures

We study categories for reversible computing, focussing on reversible forms of event structures. Event structures are a well-established model of true concurrency. There exist a number of forms of event structures, including prime event structures, asymmetric event structures, and general event structures. More recently, reversible forms of these types of event structure have been defined. We formulate corresponding categories and functors between them. We show that products and coproducts exist in many cases.We define stable reversible general event structures and stable configuration systems, and we obtain an isomorphism between the subcategory of the former in normal form and the finitely enabled subcategory of the latter.In most work on reversible computing, including reversible process calculi, a causality condition is posited, meaning that the cause of an event may not be reversed before the event itself. Since reversible event structures are not assumed to be causal in general, we also define causal subcategories of these event structures.

: Specification and refinement of Safety-Critical Java programs

Safety-Critical Java (SCJ) is a version of Java for real-time, embedded, safety-critical applications. It supports certification via abstractions that enforce a particular program architecture, with controlled concurrency and memory models. SCJ is an Open Group standard, with a reference implementation, but little support for reasoning. Here, we present SCJ-Circus , a refinement notation for specification and verification of low-level models of SCJ programs. SCJ-Circus is part of the Circus family of state-rich process algebras: it includes the Circus constructs for modelling of sequential and concurrent behaviour based on Z and CSP, and the real-time and object-oriented extensions of Circus , in addition to the SCJ abstractions. We present the syntax of SCJ-Circus and its semantics, defined by mapping SCJ-Circus constructs to those of Circus . We also detail a refinement strategy that takes a Circus design that adheres to a multiprocessor cyclic executive pattern and produces an SCJ program design, described in SCJ-Circus . Finally, we show how this refinement strategy can be extended for more complex program architectures. A refinement notation for specification and verification of SCJ programs is proposed. Abstract patterns for SCJ specifications are identified. A refinement strategy for SCJ programs is described.

Categorical combinatorics of scheduling and synchronization in game semantics

Game semantics is the art of interpreting types as games and programs as strategies interacting in space and time with their environment. In order to reflect the interactive behavior of programs, strategies are required to follow specific scheduling policies. Typically, in the case of a purely sequential programming language, the program (Player) and its environment (Opponent) will play one after the other, in a strictly alternating way. On the other hand, in the case of a concurrent language, Player and Opponent will be allowed to play several moves in a row, in a non-alternating way. In both cases, the scheduling policy is designed very carefully in order to ensure that the strategies synchronize properly and compose well when plugged together. A longstanding conceptual problem has been to understand when and why a given scheduling policy works and is compositional in that sense. In this paper, we exhibit a number of simple and fundamental combinatorial structures which ensure that a given scheduling policy encoded as synchronization template defines a symmetric monoidal closed (and in fact star-autonomous) bicategory of games, strategies and simulations. To that purpose, we choose to work at a very general level, and illustrate our method by constructing two template game models of linear logic with different flavors (alternating and non-alternating) using the same categorical combinatorics, performed in the category of small categories. As a whole, the paper may be seen as a hymn in praise of synchronization, building on the notion of synchronization algebra in process calculi and adapting it smoothly to programming language semantics, using a combination of ideas at the converging point of game semantics and of categorical algebra.

Process Calculus for Modeling and Quantifying Location Privacy

In the mobile wireless Internet, location privacy is serious concerns. As a response to these concerns, many (formal) location-privacy protection mechanisms (LPPMs) and evaluation metrics for LPPMs have been proposed. It is necessary to integrate formal models into assessments, because this integration can deduce the gap between them: after designing a LPPM, we adopt this integration to formalize and measure it. In this paper, we propose a probabilistic process calculus to model the obfuscation-based schemes (OBS, one LPPM ) and use the relative entropy to measure the degree of location privacy OBS can leak. We integrate the two approaches into one unified model. Examples demonstrate the accuracy of our model. Our work decreases the gap between the formalization and the measurement for OBS.

The Structure-Behavior Coalescence Approach for Systems Modeling

The systems modeling technique is used for developing an abstract model to help analyze and understand the functionality of a system. In this paper, we introduce a new approach for general-purpose systems modeling called structure-behavior coalescence (SBC), which supports the integration of modeling both structural and behavioral aspects of a system in a single diagram. Process algebra is utilized to describe the interactions between the components of the system. SBC combines graphical notation and process algebra to provide formalism and an intuitive representation in the development of the conceptual model. The SBC model can clearly describe the interactions between system components and precisely specify the execution order of an entire system via algebraic operations.

A Process Algebra With Qualitative Calculus for Modeling Spatio-Temporal Behaviour

Distributive real-time autonomous systems like cyber-physical systems (CPS) and biology systems have become a hot topic nowadays. The high-level design of these systems shares a common characteristic that the behavior of agents is always space-related: i.e., agents are involved in an environment where the space relations between agents count. Agents are always sensitive to the discrete change of space relations among them at explicit time. We call such behavior the spatio-temporal behavior, which has become a critical research point recently. In this paper, we propose a process algebra called Communicating Sequential Process with Qualitative calculus (QCSP) as a formal language for modeling the spatio-temporal behavior. The key feature of our algebra is that it can reason about the space relations between agents by a finite set of binary relations that obey certain mathematical conditions - a qualitative calculus. In QCSP, each agent is confined in a space region. The space region (with an agent in it) can move as a whole and interact with other space regions. The topological structure of space regions forms the outer environment of agents, and the agents react timely according to their environment from time to time. We built our calculi by conservatively extending Timed CSP with qualitative calculus embedded in it, and illustrate how QCSP can be used to model the spatio-temporal behavior by analyzing three examples. We propose a special process algebra as a formal model for behavior that is sensitive to qualitative changing space and time. The key feature of our algebra is that it can reason about the relative location between agents by using qualitative calculus. Each agent is confined in a space region. The space region can move as a whole along with the agent confined in it and interact with other space regions. The behavior of agent depends on the interaction of space regions at an explicit time, shown by that certain actions can be triggered (or allowed) under certain space region relations and time condition. Such spatio-temporal behavior is a critical research point in nowadays distributive real-time autonomous systems like CPS or intelligent transportation systems (ITS). We propose our calculi by conservatively extending Timed CSP with qualitative calculus embedded in it to capture space region relations among agents.

State Equations in Stochastic Process Algebra Models

State equations are usually used for structural or qualitative analysis, such as deadlock checking, in P/T systems. In this paper, we instead consider timed state equations in stochastic process algebra models, to derive quantified dynamic information on the system modeled in the face of the state space explosion problem. The average of these state equations is demonstrated as the linear combination of the system transitions, with the combination coefficients specified by the bias term of the empirical transition rates to their steady state. The approaches of stochastic simulation and fluid approximation, straightforwardly generated from the quantified state equations, are studied, with the consistency being investigated both theoretically and experimentally.

A Secure Protocol for Remote-Code Integrity Attestation of Embedded Systems: The CSP Approach

No doubt, a person of modern society relying on Embedded Systems (ESs) has increased rapidly and the era of digital machines is gaining popularity among users and also systems providers. At the same time, such instruments face substantial security challenges because they usually operate in a physically unprotected environment, and thus attract the attackers to gain unauthorized access for utilizing the system functions. Accordingly, system integrity is important and hence there is a need to propose a technique/tool to verify that the original/pure systems codes have been used in those devices. In this research, our main objective is to design a system architecture with a secure communication for code integrity attestation of an ES. Indeed, the study presents the proposed system architecture for ESs integrity attestation which includes two main phases: fetching an ES code at a server site and examining the ES at a remote site (using a designed user application). Essentially, the hash function (SHA-2) with a random key to calculate a unique digest value for a targeted system have been utilized. Also, the study used timestamps and nonce values, two secure keys, and public key algorithm to design a secure protocol in-order to prevent potential attacks during data and the associated values transfer between the server and the remote user application. As many researchers state that the formal methods are very precise and accurate for presenting system specifications; this study modeled and analyzed the proposed attestation protocol using the Communicating Sequential Processes (CSP) formal method approach. Besides, the Compiler for the Analysis of Security Protocols (Casper) has been used to translate the protocol description into the corresponding process algebra CSP model. Then, the researcher used the Failures Divergences Refinement (FDR) to evaluate the proposed protocol. Those formal method tools are considered as a reliable verification measurement in-order to figure-out potential flaws and correct them. Overall, the final output of checking all the defined secrecy and authentication assertions using FDR 4.2.0, and thus all the secrecy and authentication specifications defined in the developed Casper script are passed.

Operation Chain Model of Reconfigurable Printing Manufacturing System Based on Stochastic Process Algebra

When printing orders change, the reconfigurable printing manufacturing system (RPMS) needs to be reconfigured, which produces many feasible reconfiguration schemes. To find the optimal reconfiguration scheme and verify its integrity, an operation chain model (OCM) of an RPMS is proposed to describe the operating process of an RPMS. First, a comprehensive evaluation algorithm considering both subjective and objective evaluation probabilities is designed by analyzing the characteristics of the printing production process. Then, the syntax of stochastic process algebra is conservatively extended to meet the operation chain's specifications. The syntax and transition rules of the OCM are defined and the semantic model of the operation chain is constructed. Finally, we carried out experiments on the RPMS of a printing enterprise and provide the experimental results. The results show that the optimal printing scheme that is selected by the OCM is in line with expectations. In addition, the measurement result I CP = true shows that the optimal reconfigurable printing scheme process has good integrity.

The QoS analysis of web services composition based on Jackson queuing network

With the popularity of distributed computing and mobile internet, services composition has become a hot research in the field of service computing. The QoS of web services composition organised by BPEL is determined by the internal structure, the probability distribution in the variables value of BPEL process and the external factors such as the hardware and software processing capabilities of web servers and the load of web servers. At present, the generalised Petri nets, queuing Petri nets, Markov processes and stochastic process algebra model which are widely used and cannot have a comprehensive analysis in obtaining the actual performance of web services composition in internet environment. In this paper, a set of mapping rules are established to transform the internal and external factors which have impacts on the QoS of web services composition into Jackson queuing network, meanwhile, a set of performance indexes and their formula are given. Based on these performance indexes, we comprehensively analyse the performance of web services composition and locate the bottleneck of web services composition when the internal and external factors are changed so that, the performance of web services composition can be predicted before its deployment.

Even Simple Processes of π-calculus are Hard for Analysis

Mathematical models of distributed computations, based on the calculus of mobile processes (π-calculus) are widely used for checking the information security properties of cryptographic protocols. Since π-calculus is Turing-complete, this problem is undecidable in general case. Therefore, the study is carried out only for some special classes of π-calculus processes with restricted computational capabilities, for example, for non-recursive processes, in which all runs have a bounded length, for processes with a bounded number of parallel components, etc. However, even in these cases, the proposed checking procedures are time consuming. We assume that this is due to the very nature of the π -calculus processes. The goal of this paper is to show that even for the weakest model of passive adversary and for relatively simple protocols that use only the basic π-calculus operations, the task of checking the information security properties of these protocols is co-NP-complete.

Measuring security in IoT communications

More smart objects and more applications on the Internet of Things (IoT) mean more security challenges. In IoT security is crucial but difficult to obtain. On the one hand the usual trade-off between highly secure and usable systems is more impelling than ever; on the other hand security is considered a feature that has a cost often unaffordable. Therefore, IoT designers not only need tools to assess possible risks and to study countermeasures, but also methodologies to estimate their costs. Here, we present a methodology, based on the process calculus IoT-LySa, to infer quantitative measures on evolution of systems. The derived quantitative evaluation is exploited to establish the cost of the possible security countermeasures, in terms of time and energy.

Checking business process evolution

Business processes support the design and implementation of software as workflows of local and inter-organisation activities. Tools provide the business process designer with modelling and execution facilities, but they barely provide formal analysis techniques. When one makes a process evolve, for example by refactoring it or by adding new features in it, it is important to be able to check whether, and how, this process has changed, and possibly correct evolution flaws. To reach this objective, we first present a model transformation from the BPMN standard notation to the LNT process algebra and LTS formal models. We then propose a set of relations for comparing business processes at the formal model level. With reference to related work, we propose a richer set of comparison primitives supporting renaming, refinement, property and context-awareness. We also support BPMN processes containing unbalanced structures among gateways. In order to make the checking of evolution convenient for business process designers, we have implemented tool support for our approach as a web application.

Tracking CSP computations

Tracing is one of the most important techniques for program understanding and debugging. A trace gives the user access to otherwise hidden information about a computation. In the context of concurrent languages, computations are particularly complex due to the non-deterministic execution order of processes and to the restrictions imposed on this order by synchronizations; hence, a tracer is a powerful tool to explore, understand and debug concurrent computations. In CSP, traces are sequences of events that define a particular execution. This notion of trace is completely different to the one used in other paradigms where traces are formed by those source code expressions evaluated during a particular execution. We refer to this second notion of traces as tracks. In this work, we introduce the theoretical basis for tracking concurrent and explicitly synchronized computations in process algebras such as CSP. Tracking computations in this kind of systems is a difficult task due to the subtleties of the underlying operational semantics which combines concurrency, non-determinism and non-termination. We define an instrumented operational semantics that generates as a side-effect an appropriate data structure (a track) which can be used to track computations. The formal definition of a tracking semantics improves the understanding of the tracking process, but also, it allows us to formally prove the correctness of the computed tracks.

Angelic processes for CSP via the UTP

Demonic and angelic nondeterminism play fundamental roles as abstraction mechanisms for formal modelling. In contrast with its demonic counterpart, in an angelic choice failure is avoided whenever possible. Although it has been extensively studied in refinement calculi, in the context of process algebras, and of the Communicating Sequential Processes (CSP) algebra for refinement, in particular, it has been elusive. We show here that a semantics for an extended version of CSP that includes both demonic and angelic choice can be provided using Hoare and He's Unifying Theories of Programming (UTP). Since CSP is given semantics in the UTP via reactive designs (pre and postcondition pairs) we have developed a theory of angelic designs and a conservative extension of the CSP theory using reactive angelic designs. To characterise angelic nondeterminism appropriately in an algebra of processes, however, a notion of divergence that can undo the history of events needs to be considered. Taking this view, we present a model for CSP where angelic choice completely avoids divergence just like in the refinement calculi for sequential programs.

Modeling and Verification of Reconfigurable Printing System Based on Process Algebra

With the print production process as the research object in this paper, the intelligent-context-reconfigurable printing system model is analyzed using process algebra (PA). First, combined with the printing production process, the overall framework of the system model, based on the agent-resource-worker (ARW) component model, is proposed. Abstract and formal verification of the system model are then carried out, and the verification process of the complex calibration process is discussed. Finally, the security and progress attributes of the model are validated by mCRL2. The results show that the system model has good reliability.