The π-calculus is a process algebra which originates from CCS and permits a natural modelling of mobility (i.e., dynamic reconfigurations of the process linkage) using communication of names. Previous research has shown that the π-calculus has much greater expressiveness than CCS, but also a much more complex mathematical theory. The primary goal of this work is to understand the reasons of this gap. Another goal is to compare the expressiveness of name-passing calculi, i.e., calculi like π-calculus where mobility is achieved via exchange of names, and that of agent-passing calculi, i.e., calculi where mobility is achieved via exchange of agents. We separate the mobility mechanisms of the π-calculus into two, respectively called internal mobility and external mobility. The study of the subcalculus which only uses internal mobility, called I, suggests that internal mobility is responsible for much of the expressiveness of the π-calculus, whereas external mobility is responsible for many of...

The most popular method for establishing bisimilarities among processes is to exhibit bisimulation relations. By definition, R is a bisimulation relation if R progresses to R itself, i.e., pairs of processes in R can match each other's actions and their derivatives are again in R. We study generalisations of the method aimed at reducing the size of the relations to exhibit and hence relieving the proof work needed to establish bisimilarity results. We allow a relation R to progress to a different relation F(R), where F is a function on relations. Functions which can be safely used in this way (i.e., such that if R progresses to F(R), then R only includes pairs of bisimilar processes) are sound. We give a simple condition which ensures soundness. We show that the class of sound functions contains non-trivial functions and we study the closure properties of the class w.r.t. various important function constructors, like composition, union and iteration. These properties allow us to cons...

This paper studies causality in ß-calculus. Our notion of causality combines the dependencies given by the syntactic structure of processes with those originated by passing names. Our studies show that two transitions not causally related may however occur in a fixed ordering in any computation, i.e., ß-calculus may implicitly express a precedence between actions. Our causality relation still induces the same partial order of transitions for all the computations that are obtained by shuffling transitions that are concurrent (i.e. related neither by causality nor by precedence). Other non-interleaving semantics are investigated and compared. The presentation takes advantage from a parametric definition of process behaviour given in an SOS style. All the results on bisimulationbased equivalences, congruences, axiomatizations and logics are taken (almost) without modifications from the interleaving theory. Finally, we extend our approach to higher-order ß-calculus, enriched with a spawn ...

Abstract. We study causality in the ß-calculus. Our notion of causality combines the dependencies given by the syntactic structure of processes with those originated by passing names. It turns out that two transitions not causally related may although occur in a fixed ordering in any computation, i.e., ß-calculus may express implicitly a priority between actions. Our causality relation still induces the same partial order of transitions for all the computations that are obtained by shuffling transitions that are concurrent (= related neither by causality nor by priority). The presentation takes advantage from a parametric definition of process behaviour that highlights the essence of the topic. All the results on bisimulation based equivalences, congruences, axiomatizations and logics are taken (almost) for free from the interleaving theory. 1 Introduction The study of the behaviour of a distributed system may benefit from knowledge on the causal relation between its events. For examp...

In this paper we present history-dependent automata (HD-automata in brief). They are an extension of ordinary automata that overcomes their limitations in dealing with history-dependent formalisms. In a history-dependent formalism the actions that a system can perform carry information generated in the past history of the system. The most interesting example is -calculus: channel names can be created by some actions and they can then be referenced by successive actions. Other examples are CCS with localities and the history-preserving semantics of Petri nets. Ordinary

We study syntax-free models for name-passing processes. For interleaving semantics, we identify the indexing structure required of an early labelled transition system to support the usual pi-calculus operations, defining Indexed Labelled Transition Systems. For noninterleaving causal semantics we define Indexed Labelled Asynchronous Transition Systems, smoothly generalizing both our interleaving model and the standard Asynchronous Transition Systems model for CCS-like calculi. In each case we relate a denotational semantics to an operational view, for bisimulation and causal bisimulation respectively. We establish completeness properties of, and adjunctions between, categories of the two models. Alternative indexing structures and possible applications are also discussed. These are first steps towards a uniform understanding of the semantics and operations of name-passing calculi.

) y Nobuko Yoshida ? Abstract. In [18, 19], we presented a theory of concurrent combinators for the asynchronous monadic ß-calculus without match or summation operator [7, 16]. The system of concurrent combinators is based on a finite number of atoms and fixed interaction rules, but is as expressive as the original calculus, so that it can represent diverse interaction structures, including polyadic synchronous name passing [23] and input guarded summations [26]. The present paper shows that each of the five basic combinators introduced in [18] is indispensable to represent the whole computation, i.e. if one of the combinators is missing, we can no longer express the original calculus up to weak bisimilarity. Expressive power of several interesting subsystems of the asynchronous ß-calculus is also measured by using appropriate subsets of the combinators and their variants. Finally as an application of the main result, we show there is no semantically sound encoding of the calculus in...

) (Appeared in the Proceedings of the Fourth International Conference on Algebraic Methodology and Software Technology, July 1995 Lecture Notes in Computer Science, Volume 936) Lalita Jategaonkar Jagadeesan Software Production Research Dept. AT&T Bell Laboratories Naperville, IL 60566 (USA) lalita@research.att.com Radha Jagadeesan ? Math. Sciences Loyola University Chicago, IL 60626 (USA) radha@math.luc.edu 1 Introduction The pi--calculus [18, 17] is a process algebra for describing networks of processes with dynamically evolving communication structure. The key idea underlying the pi--calculus is the notion of naming: names are used to refer to channels --- the links between processes, and can be dynamically created or hidden. Names together with a rich algebra of process combinators that includes parallel composition, allow the pi--calculus to encode asynchronous networks of processes that evolve dynamically. In turn, mobility --- this ability to change the network configuratio...

We present CCS-R, a reversible variant of Milner's CCS. This simple process algebra o#ers a backtracking mechanism which is shown to be sound. We then discuss how biological systems satisfying a "perfect mix" assumption can be formalized within CCS-R.

. A distributed semantics for ß-calculus, based on P/T Petri nets with inhibitor arcs, is presented. This net semantics is sound w.r.t. the original early transition system semantics: the interleaving semantics of an agent p is retrievable from the net by considering the interleaving marking graph for the associated marking dec(p). Furthermore, ß-calculus is equipped with a multistep and a causal semantics. The latter is compared with recent proposals appeared in the literature. 1 Introduction Distributed semantics for CCS and related languages based on (different classes of) Petri Nets have received a lot of attention in recent years (see, e.g., [5, 8, 10, 19], just to mention a few). The Place/Transition net semantics proposed in the literature can be classified into two main groups. The first one, we call location-oriented, exploits the syntactical structure of the process terms (notably, the parallel operator) to define their associated sets of places [5, 19]. The second one, we ...