David Streader

CONFLICTS AND FAIR TESTING

This paper studies conflicts from a process-algebraic point of view and shows how they are related to the testing theory of fair testing. Conflicts have been introduced in the context of discrete event systems, where two concurrent systems are said to be in conflict if they can get trapped in a situation where they are waiting or running endlessly, forever unable to complete their common task. In order to analyse complex discrete event systems, conflict-preserving notions of refinement and equivalence are needed. This paper characterises an appropriate refinement, called the conflict preorder, and provides a denotational semantics for it. Its relationship to other known process preorders is explored, and it is shown to generalise the fair testing preorder in process-algebra for reasoning about conflicts in discrete event systems.

Modular Synthesis of Discrete Controllers

This paper presents supervisory control theory in a process-algebraic setting, and proposes a way of synthesising modular supervisors that guarantee nonblocking. The framework used includes the possibility of hiding actions which results in nondeterminism. As modularity crucially depends on the process equivalence used, the paper studies possible equivalences and points out that, in order to be consistent with respect to the nonblocking property and to supervisor synthesis, a conflict-preserving equivalence must be used. It applies the results to synthesise nonblocking modular supervisors for a manufacturing system.

Fair testing revisited: A process-algebraic characterisation of conflicts

This paper studies conflicts from a process-algebraic point of view and shows how they are related to the testing theory of fair testing. Conflicts have been introduced in the context of discrete event systems, where two concurrent systems are said to be in conflict if they can get trapped in a situation where they are waiting or running endlessly, forever unable to complete their common task. In order to analyse complex discrete event systems, conflict-preserving notions of refinement and equivalence are needed. This paper characterises an appropriate refinement, called the conflict preorder, and provides a denotational semantics for it. Its relationship to other known process preorders is explored, and it is shown to generalise the fair testing preorder in process-algebra for reasoning about conflicts in discrete event systems.

Comparison of Data and Process Refinement

From what point of view is it reasonable, or possible, to refine a one place buffer into a two place buffer? In order to answer this question we characterise refinement based on substitution in restricted contexts. We see that data refinement (specifically in Z) and process refinement give differing answers to the original question, and we compare the precise circumstances which give rise to this difference by translating programs and processes into labelled transition systems, so providing a common basis upon which to make the comparison. We also look at the closely related area of sub-typing of objects. Along the way we see how all these sorts of computational construct are related as far as refinement is concerned, and discover and characterise some (as far as we can tell) new sorts of refinement.

Data refinement and singleton failures refinement are not equivalent

In this paper, we give simple example abstract data types, with atomic operations, that are related by data refinement under a definition used widely in the literature, but these same abstract data types are not related by singleton failure refinement. This contradicts results found in the literature.

Atomic components

The operational definition of observational congruence in CCS and ACP can be split into two parts: one, the definition of an observational semantics (ie abstraction); and two, the definition of a strong congruence. In both cases this “separation of concerns” has been applied with abstraction that is implicitly “fair”. We define a novel (if obvious) observational semantics with no implicit “fairness”. When combining this observational semantics with failure equality the resulting observational semantics is shown to be equal, other than for minor details, to NDFD semantics. We also combine our observational semantics with singleton failure semantics and we establish congruence results for this new observational equality.

General Refinement, Part One: Interfaces, Determinism and Special Refinement

We introduce a general model of refinement. This is defined in terms of what contexts an entity can appear in, and what observations can be made of it in those contexts. We show explicitly how five refinement relations, taken from the refinement literature, are instances of our general model. Henceforth, since they are specialisations of a general model, we call these instances special models. We show these theories of refinement are special models simply by fixing the sets of contexts and observations involved in appropriate ways.

General Refinement, Part Two: Flexible Refinement

In the previous, companion, paper [Reeves, S. and D. Streader, General refinement, part one: interfaces, determinism and special refinement, Proceedings of Refine 2008, Electronic Notes in Theoretical Computer Science (2008).] to this paper we introduced our general model of refinement, discussed ideas around determinism and interfaces that the general definition raised, and gave several examples showing how the general definition could be specialised to the sorts of refinement we see in the literature. In this paper we continue the story and we define vertical refinement on our general model. Vertical refinement can be seen as a generalisation of what, in the literature, has been called action refinement or non-atomic refinement. Alternatively, by viewing a special model (from the previous paper) as a logical theory, vertical refinement can be seen as a theory morphism, formalised as a Galois connection. We give an example of the utility of this definition by constructing a vertical refinement between broadcast processes and interactive branching programs, and we see how interactive branching programs can be implemented on a platform which provides broadcast communication. We also show how developments that fall outside the usual, special theories of refinement can be brought into the refinement world by giving examples of development which were thought not to be possible using refinement. Throughout, the central, simple idea of refinement as a development process that moves from abstract to concrete while preserving certain valuable guarantees will guide us.

Global and local knowledge in SDN

It appears to be SDN dogma that middleboxes are bad. However middleboxes form near half the elements that comprise todays networks and at least three active research fields are exploring how to improve their functionality. Fifteen recent SDN survey papers were examined for evidence to support the SDN dogma and its assertion that all network functionality belongs in the control plane, without success. In fact many chose to ignore the subject. Consequently this paper critically examines what is almost an SDN article of faith and finds this dogma creates a number of interesting and avoidable problems. Three common middlebox types are examined: firewall, NAT and load balancer to highlight the preferred SDN architecture and generate critical thinking. Finally the concept of global and local knowledge is used in a framework to determine whether an application should reside in the control plane or data plane.

Anonymous mobile service collaboration: quality of service

Mobile services depend on user context and preferences, and a mobile users context is constantly changing. Many services are only available locally. The most appropriate service for a users context is not known in advance and a user may enter or leave a services range at any time. For a seamless user experience, services need to collaborate. These complex collaborations should be instantaneous yet anonymous - without disclosing user information. The paper proposes a new service collaboration model using event-based interaction. A prototypical implementation is used to demonstrate functionality, inter-operability, and generality of our solution. The solution guarantees ad-hoc service collaboration while protecting user information.