Ambient Calculus Research Articles

Type Inference for Mobile Ambients in Prolog

The type system for the ambient calculus M3 [Mobility Types for Mobile Processes in Mobile Ambients, in: Electronic Notes in Theoretical Computer Science, vol. 78, Elsevier, Amsterdam, 2003] is presented in a new form that derives the type of a term with the minimal set of mobility assumptions, and is therefore more amenable than the original form to a translation into a type inference algorithm. From the new formulation a Prolog program is derived, which implements a type inference algorithm for M3 analogous to the one previously specified through formal rules. The implementation exploits in the standard way the peculiarities of the logic programming paradigm, and is therefore, in a sense, more abstract than the original algorithm's specification itself.

Access control for mobile agents

Boxed Ambients are a variant of Mobile Ambients that result from dropping the open capability and introducing new primitives for ambient communication. The new model of communication is faithful to the principles of distribution and location-awareness of Mobile Ambients, and complements the constructs in and out for mobility with finer-grained mechanisms for ambient interaction. We introduce the new calculus, study the impact of the new mechanisms for communication of typing and mobility, and show that they yield an effective framework for resource protection and access control in distributed systems.

On the expressiveness of pure safe ambients

We consider the Pure Safe Ambient Calculus, which is Levi and Sangiorgi's Safe Ambient Calculus (a variant of Cardelli and Gordon's Mobile Ambient Calculus) restricted to its mobility primitives – in particular, we focus on its expressive power. Since it has no form of communication or substitution, we show how these notions can be simulated by mobility and modifications in the hierarchical structure of ambients. As a main result, we use these techniques to design an encoding of the synchronous $\pi$-calculus into pure ambients, and we study its correctness, thus showing that pure ambients are as expressive as the $\pi$-calculus. In order to simplify the proof and give an intuitive understanding of the encoding, we design an intermediate language, the $\pi$-Calculus with Explicit Substitutions and Channels, which is an extension of the $\pi$-calculus in which communication and substitution are broken into simpler steps, and we show that is has the same expressive power as the $\pi$-calculus.

On abstract interpretation of Mobile Ambients

We introduce an abstract interpretation framework for Mobile Ambients, based on a new semantics called normal semantics. Then, we derive within this setting two analyses computing a safe approximation of the run-time topological structure of processes. Such a static information can be successfully used to establish interesting security properties.

Ambient Petri Nets

Both the Ambient Calculus by L. Cardelli and the Elementary Object Systems by R. Valk model the behaviour of mobile systems. The Ambient Calculus is based on the concept of ambient, which is an environment with a given name that is delimited by a boundary, where some internal processes are executed. The main property of these ambients is that they can be moved to a new location thus modeling mobility. Elementary Object Systems are two-level net systems composed of a system net and one or more object nets, which can be seen as high-level token objects of the system net modeling the execution of mobile processes. This paper intends to contribute to the relationship between both frameworks by defining a multilevel extension of Elementary Object Systems, which will be used to provide a denotational semantics of a new process algebra called APBC (Ambient Petri Box Calculus). Such process algebra is an extension of the Petri Box Calculus that includes both ambients and their mobility capabilities, which conversely can be also interpreted as an extension of the Ambient Calculus with the main operations from the PBC.

A calculus of Kells

This paper introduces the Kell calculus, a new process calculus that retains the original insights of the M-calculus (local actions, higher-order processes and programmable membranes) in a much simpler setting. The calculus is shown expressive enough to provide a direct encoding of several recent distributed process calculi such as Mobile Ambients and the Distributed Join calculus.

Equational properties of mobile ambients

The ambient calculus is a process calculus for describing mobile computation. We develop a theory of Morris-style contextual equivalence for proving properties of mobile ambients. We prove a context lemma that allows derivation of contextual equivalences by considering contexts of a particular limited form, rather than all arbitrary contexts. We give an activity lemma that characterises the possible interactions between a process and a context. We prove several examples of contextual equivalence. The proofs depend on characterising reductions in the ambient calculus in terms of a labelled transition system.

Information Leakage Detection in Boundary Ambients

A variant of Mobile Ambient Calculus is introduced, called Boundary Ambient, to model multilevel security policies. Ambients that may guarantee to properly protect their content are explicitly identified as boundaries: a boundary can be seen as a resource access manager for confidential data. In this setting, we define a notion of non-interference which captures the absence of any (both direct and indirect) information leakage. Then, we guarantee non-interference by extending a control flow analysis that computes an over approximation of all ambients and capabilities that may be affected by the actual values of high level data.

On the Expressiveness of Pure Mobile Ambients

We consider the Pure Ambient Calculus, which is Cardelli and Gordon's Ambient Calculus (or more precisely its safe version by Levi and Sangiorgi) restricted to its mobility primitives, and we focus on its expressive power. Since it has no form of communication or substitution, we show how these notions can be simulated by mobility and modifications in the hierarchical structure of ambients. As an example, we give an encoding of the synchronous π-calculus into pure ambients and we state an operational correspondence result. In order to simplify the proof and give an intuitive understanding of the encoding, we design an intermediate language: the π-Calculus with Explicit Substitutions and Channels, which is a syntactic extension of the π-calculus with a specific operational semantics.

On the Expressiveness of Pure Mobile Ambients

On the Expressiveness of Pure Mobile Ambients

Abstract interpretation of mobile ambients

We show how abstract interpretation can be expressed in a constraint-based formalism that is becoming increasingly popular for the analysis of functional and object-oriented languages. This is illustrated by developing analyses for the ambient calculus. The first step of the development constructs an analysis for counting occurrences of processes inside other processes; we show that the analysis is semantically correct and that the set of acceptable solutions constitutes a Moore family. The second step considers a previously developed control flow analysis and shows how to induce it from the counting analysis; we show that its properties can be derived from those of the counting analysis using general results about abstract interpretation for constraint-based analyses.

Bigraphs and transitions

A bigraphical reactive system (BRS) involves bigraphs , in which the nesting of nodes represents locality, independently of the edges connecting them. BRSs represent a wide variety of calculi for mobility, including λ-calculus and ambient calculus. A labelled transition system (LTS) for each BRS is here derived uniformly, adapting previous work of Leifer and Milner, so that under certain conditions the resulting bisimilarity is automatically a congruence. For an asynchronous λ-calculus, this LTS and its bisimilarity agree closely with the standard.

HOPLA--A Higher-Order Process Language

A small but powerful language for higher-order nondeterministic processes is introduced. Its roots in a linear domain theory for concurrency are sketched though for the most part it lends itself to a more operational account. The language can be viewed as an extension of the lambda calculus with a ``prefixed sum'', in which types express the form of computation path of which a process is capable. Its operational semantics, bisimulation, congruence properties and expressive power are explored; in particular, it is shown how it can directly encode process languages such as CCS, CCS with process passing, and mobile ambients with public names.

Model checking mobile ambients

We settle the complexity bounds of the model checking problem for the ambient calculus with public names against the ambient logic. We show that if either the calculus contains replication or the logic contains the guarantee operator, the problem is undecidable. In the case of the replication-free calculus and guarantee-free logic we prove that the problem is PSPACE-complete. For the complexity upper bound, we devise a new representation of processes that remains of polynomial size during process execution; this allows us to keep the model checking procedure in polynomial space. Moreover, we prove PSPACE-hardness of the problem for several quite simple fragments of the calculus and the logic; this suggests that there are no interesting fragments with polynomial-time model checking algorithms.

Ambient Calculus and its Logic in the Calculus of Inductive Constructions

The Ambient Calculus has been recently proposed as a model of mobility of agents in a dynamically changing hierarchy of domains. In this paper, we describe the implementation of the theory and metatheory of Ambient Calculus and its modal logic in the Calculus of Inductive Constructions. We take full advantage of Higher-Order Abstract Syntax, using the Theory of Contexts a fundamental tool for developing formally the metatheory of the object system. Among others, we have successfully proved a set of fresh renamings properties, and formalized the connection between the Theory of Contexts and Gabbay-Pitts' “new” quantifier. As a feedback, we introduce a new definition of satisfaction for the Ambients logic and derive some of the properties originally assumed as axioms in the Theory of Contexts.

Types for the Ambient Calculus

The ambient calculus is a concurrent calculus where the unifying notion of ambient is used to model many different constructs for distributed and mobile computation. We study a type system that describes several properties of ambient behavior. The type system allows ambients to be partitioned in disjoint sets ( groups ), according to the intended design of a system, in order to specify both the communication and the mobility behavior of ambients.

Types for the Ambient Calculus

The ambient calculus is a concurrent calculus where the unifying notion of ambient is used to model many different constructs for distributed and mobile computation. We study a type system that describes several properties of ambient behavior. The type system allows ambients to be partitioned in disjoint sets ( groups ), according to the intended design of a system, in order to specify both the communication and the mobility behavior of ambients.

On the Expressiveness of Movement in Pure Mobile Ambients

Pure Mobile Ambients (i.e., Mobile Ambients without communication) provides three mobility primitives: in and out for ambient movement, and open to dissolve ambient boundaries. In this paper we consider the expressiveness of the primitives in and out for ambient movement; more precisely, we concentrate on the interplay between ambient movement and the ability to create new names (exploiting the restriction operator). To this aim, we consider a version of Pure Mobile Ambients (with explicit recursive definitions instead of replication) and we concentrate on the three fragments of the calculus that can be obtained removing either one or both between movement and the ability to create new names. The unique mobility primitive that we retain in all of the considered calculi is open. Te three fragments are denoted as follows: MA−mv without ambient movement, MA−v without restriction, and MA−mv−v without both movement and restriction. We prove that both the fragments MA−mv and MA−v are Turing-complete, while this is not the case for MA−mv−v. Indeed, we prove that in this latter calculus the existence of an infinite computation turns to be a decidable property.

Barbs and Congruences for Safe Mobile Ambients

The Ambient Calculus offers many ways in which processes can interact and be observed. In the context of Levi and Sangiorgi's Safe Mobile Ambients (SA), the extra co-capabilities required for interaction complicate the fundamental observations. We show that different formulations of barbs lead to the same barbed congruence. We prove this by following Honda and Yoshida's approach for the π-calculus by defining the insensitive terms of SA.

Guardians for Ambient-Based Monitoring

In the Mobile Ambients of Cardelli and Gordon an ambient is a unit for mobility, which may contain processes (data) and sub-ambients. Since the seminal work of Cardelli and Gordon, several ambient-based calculi have been proposed (Seal, Box-π, Safe Ambients, Secure Safe Ambients, Boxed Ambients), mainly for supporting security. At the operational level these (box- and) ambient-based calculi differ only in the capabilities of processes. We propose a way of extending ambient-based calculi, which embodies two principles: an ambient is a unit for monitoring and coordination, the name of an ambient determines its (monitoring and coordination) policy. More specifically, to each ambient we attach a guardian, which monitors the activity of sub-components (i.e. processes and sub-ambients) and the interaction with the external environment. In our proposal, guardians and processes play a dual role: guardians are centralized entities monitoring and inhibiting actions, while processes are decentralized entities performing actions. We exemplify the use of guardians for enforcing security properties.