We present a new version of the Seal Calculus, a calculus of mobile computation. We study observational congruence and bisimulation theory, and show how they are related.

We introduce the Xdπ calculus, a peer-to-peer model for reasoning about dynamic web data. Web data is not just stored statically. Rather it is referenced indirectly, for example using hyperlinks, service calls, or scripts for dynamically accessing data, which require the complex coordination of data and processes between sites. The Xdπ calculus models this coordination, by integrating the XML data structure with process orchestration techniques associated with the distributed pi-calculus. We study behavioural equivalences for Xdπ, to analyze the various possible patterns of data and process interaction.

We study the behavioural theory of Cardelli and Gordon's Mobile Ambients. We give an LTS based operational semantics, and a labelled bisimulation based equivalence that coincides with reduction barbed congruence. We also provide two up-to proof techniques that we use to prove a set of algebraic laws, including the perfect firewall equation.

The Seal Calculus is a process language for describing mobile computation. Threads and resources are tree structured; the nodes thereof correspond to agents, the units of mobility. The Calculus extends a �-calculus core with synchronous, objective mobility of agents over channels. This paper systematically compares all previous variants of Seal Calculus. We study their operational behaviour with labelled transition systems and bisimulations; by comparing the resulting algebraic theories we highlight the differences between these apparently similar approaches. This leads us to identify the dialect of Seal that is most amenable to operational reasoning and can form the basis of a distributed programming language. We propose type systems for characterising the communications in which an agent can engage. The type systems thus enforce a discipline of agent mobility, since the latter is coded in terms of higher-order communication.

Groupoidal relative pushouts (GRPOs) have recently been proposed by the authors as a new foundation for Leifer and Milner's approach to deriving labelled bisimulation congruences from reduction systems. In this paper, we develop the theory of GRPOs further, proving that well-known equivalences, other than bisimulation, are congruences. To demonstrate the type of category theoretic arguments which are inherent in the 2-categorical approach, we construct GRPOs in a category of `bunches and wirings.' Finally, we prove that the 2-categorical theory of GRPOs is a generalisation of the approaches based on Milner's precategories and Leifer's functorial reactive systems.

We study a behavioural theory of Mobile Ambients, a process calculus for modelling mobile agents in wide-area networks, focussing on reduction barbed congruence. Our contribution is threefold. (1) We prove a context lemma which shows that only parallel and nesting contexts need be examined to recover this congruence. (2) We characterise this congruence using a labelled bisimilarity: this requires novel techniques to deal with asynchronous movements of agents and with the invisibility of migrations of secret locations. (3) We develop refined proof methods involving up-to proof techniques, which allow us to verify a set of algebraic laws and the correctness of more complex examples.

We discuss a basic process calculus useful for modelling applications over global computing systems and present the associated semantic theories as determined by some basic notions of observation. The main features of the calculus are explicit distribution, remote operations, process mobility and asynchronous communication through distributed data spaces. We introduce some natural notions of extensional observations and study their closure under operational reductions and/or language contexts to obtain barbed congruence and may testing. For these equivalences, we provide alternative tractable characterizations as labelled bisimulation and trace equivalence. We discuss some of the induced equational laws and relate them to design choices of the calculus. In particular, we show that some of these laws do not hold any longer if the language is rendered less abstract by introducing (asynchronous and undetectable) failures or by implementing remote communications via process migrations and local exchanges. In both

Resource control has attracted increasing interest in foundational research on distributed systems. This paper focuses on space control and develops an analysis of space usage in the context of an ambient-like calculus with bounded capacities and weighed processes, where migration and activation require space.

A tutorial introduction to the key concepts of ambient calculi and their type disciplines, illustrated through a number of systems proposed in the last few years, such as Mobile Ambients, Safe Ambients, Boxed Ambients, and other related calculi with types.

There are many calculi for reasoning about concurrent communicating processes which have locations and are mobile. Examples include the original Ambient Calculus and its many variants, the Seal Calculus, the MR-calculus, the M-calculus, etc. It is desirable to use such calculi to describe the behavior of mobile agents. It seems reasonable that mobile agents should be able to follow non-predetermined paths and to carry non-predetermined types of data from location to location, collecting and delivering this data using communication primitives. Previous type systems for ambient calculi make this di#cult or impossible to express, because these systems (if they handle communication at all) have always globally mapped each ambient name to a type governing the type of values that can be communicated locally or with adjacent locations, and this type can not depend on where the ambient has traveled. We present a new type system PolyA where there are no global assignments of types to ambient names. Instead, the type of an ambient process P not only indicates what can be locally communicated but also gives an upper bound on the possible ambient nesting shapes of any process P # to which P can evolve, as well as the possible capabilities and names that can be exhibited or communicated at each location. Because these shapes can depend on which capabilities and names are actually communicated, the types support this with explicit dependencies on communication. PolyA is thus the first type system for an ambient calculus which provides type polymorphism of the kind that is usually present in polymorphic type systems for the #-calculus. 1