We present an actor language which is an extension of a simple functional language, and provide a precise operational semantics for this extension. Actor configurations represent open distributed systems, by which we mean that the specification of an actor system explicitly takes into account the interface with external components. We study the composability of such systems. We define and study various notions of testing equivalence on actor expressions and configurations. The model we develop provides fairness. An important result is that the three forms of equivalence, namely, convex, must, and may equivalences, collapse to two in the presence of fairness. We further develop methods for proving laws of equivalence and provide example proofs to illustrate our methodology.

The agent expressions of the π-calculus can be translated into a theory of linear logic in such a way that the reflective and transitive closure of π-calculus (unlabeled) reduction is identified with “entailed-by”. Under this translation, parallel composition is mapped to the multiplicative disjunct (“par”) and restriction is mapped to universal quantification. Prefixing, non-deterministic choice (+), replication (!), and the match guard are all represented using non-logical constants, which are specified using a simple form of axiom, called here a process clause. These process clauses resemble Horn clauses except that they may have multiple conclusions; that is, their heads may be the par of atomic formulas. Such multiple conclusion clauses are used to axiomatize communications among agents. Given this translation, it is nature to ask to what extent proof theory can be used to understand the meta-theory of the π-calculus. We present some preliminary results along this line for π0, the “propositional ” fragment of the π-calculus, which lacks restriction and value passing (π0 is a subset of CCS). Using ideas from proof-theory, we introduce co-agents and show that they can specify some testing equivalences for π0. If negation-as-failure-to-prove is permitted as a co-agent combinator, then testing equivalence based on co-agents yields observational equivalence for π0. This latter result follows from observing that co-agents directly represent formulas in the Hennessy-Milner modal logic. 1

We develop a model for timed, reactive computation by extending the asynchronous, untimed concurrent constraint programming model in a simple and uniform way. In the spirit of process algebras, we develop some combinators expressible in this model, and reconcile their operational, logical and denotational character. We show how programs may be compiled into finite-state machines with loop-free computations at each state, thus guaranteeing bounded response time. 1 Introduction and Motivation Reactive systems [12,3,9] are those that react continuously with their environment at a rate controlled by the environment. Execution in a reactive system proceeds in bursts of activity. In each phase, the environment stimulates the system with an input, obtains a response in bounded time, and may then be inactive (with respect to the system) for an arbitrary period of time before initiating the next burst. Examples of reactive systems are controllers and signal-processing systems. The primary issu...

Active Networks is a network infrastructure which is programmable on a per-user or even per-packet basis. Increasing the flexibility of such network infrastructures invites new security risks. Coping with these security risks represents the most fundamental contribution of Active Network research. The security concerns can be divided into those which affect the network as a whole and those which affect individual elements. It is clear that the element problems must be solved first, as the integrity of networklevel solutions will be based on trust of the network elements. In this

) Davide Sangiorgi LFCS - Department of Computer Science Edinburgh University Edinburgh - EH9 3JZ - UK Abstract The use of lambda calculus in richer settings, possibly involving parallelism, is examined in terms of its effect on the equivalence between lambda terms. We concentrate here on Abramsky's lazy lambda calculus and we follow two directions. First, the lambda calculus is studied within a process calculus by examining the equivalence $ induced by Milner's encoding into the -calculus. We give exact operational and denotational characterizations for $. Secondly, we examine Abramsky's applicative bisimulation when the lambda calculus is augmented with (well-formed) operators, i.e. symbols equipped with reduction rules describing their behaviour. Then, maximal discrimination is obtained when all operators are considered; we show that this discrimination coincides with the one given by $ and that the adoption of certain non-deterministic operators is sufficient and necessary...

Abstract. This paper introduces occam-π, an efficient and safe binding of key elements from Hoare’s CSP and Milner’s π-calculus into a programming language of industrial strength. A brief overview of classical occam is presented, before focussing on the extensions providing data, channel and process mobility. Some implementation details are given, along with current benchmark results. Application techniques exploiting mobile processes for the direct modelling of large-scale natural systems are outlined, including the modelling of locality (so that free-ranging processes can locate each other). Run-time overheads are sufficiently low so that systems comprising millions of dynamically assembling and communicating processes are practical on modest processor resources. The ideas and technology will scale further to address larger systems of arbitrary complexity, distributed over multiple processors with no semantic discontinuity. Semantic design, comprehension and analysis are made possible through a natural structuring of systems into multiple levels of network and the compositionality of the underlying algebra. 1

We propose a new framework called ACL for concurrent computation based on linear logic. ACL is a kind of linear logic programming framework, where its operational semantics is described in terms of proof construction in linear logic. We also give a model-theoretic semantics as a natural extension of phase semantics, a model of linear logic. Our framework well captures concurrent computation based on asynchronous communication. It will, therefore, provide us with a new insight into other models of concurrent computation from a logical point of view. We also expect ACL to become a formal framework for verification, reasoning, and transformation of concurrent programs by the use of techniques for traditional logic programming. ACL's attractive features for concurrent programming paradigms are also discussed. 1 Introduction For future massively parallel processing environments, concurrent programming languages based on asynchronous communication would become more and more important. Due ...

. In a mobile communication network some nodes change locations, and are therefore connected to different other nodes at different points in time. We show how some important aspects of such a network can be formally defined and verified using the \pi-calculus, which is a development of CCS (Calculus of Communicating Systems) allowing port names to be sent as parameters in communication events. As an example of a mobile network we consider the Public Land Mobile Network currently being developed by the European Telecommunication Standards Institute and concentrate on the handover procedure which controls the dynamic topology of the network. 1. Introduction The need for mathematically rigorous definitions of communication protocol standards is today widely acknowledged. Such definitions are needed to specify protocols and services, and to verify that the protocols fulfil their services. Unfortunately most protocols still rely on informal definitions. One reason for this is that many aspec...

This paper explores Lhc expressive power of Lhc tcc paradigm. The origin of Lhc work in Lhc inLcgraLion of synchronous and consLrainL programming is described. The basic conceptual and maLhcmaLical framework developed in Lhc spirk of Lhc model-based approach characLcrisLic of LhcorcLical compuLcr science is reviewed. Wc show LhaL a range of consLrucLs for expressing LimcouLs, prccmpLion and oLhcr complicaLcd paLLcrns of Lcmporal acLivky arc expressible in the basic model and language-framework. Indeed, we present a single construct on processes, definable in the language, that can simulate the effect of other preemption constructs

This document consists of a slightly revised and corrected version of a dissertation