Abstract. Service Level Agreements are a key issue in Service Oriented Computing. SLA contracts specify client requirements and service guarantees, with emphasis on Quality of Service (cost, performance, availability, etc.). In this work we propose a simple model of contracts for QoS and SLAs that also allows to study mechanisms for resource allocation and for joining different SLA requirements. Our language combines two basic programming paradigms: name-passing calculi and concurrent constraint programming (cc programming). Specifically, we extend cc programming by adding synchronous communication and by providing a treatment of names in terms of restriction and structural axioms closer to nominal calculi than to variables with existential quantification. In the resulting framework, SLA requirements are constraints that can be generated either by a single party or by the synchronisation of two agents. Moreover, restricting the scope of names allows for local stores of constraints, which may become global as a consequence of synchronisations. Our approach relies on a system of named constraints that equip classical constraints with a suitable algebraic structure providing a richer mechanism of constraint combination. We give reductionpreserving translations of both cc programming and the calculus of explicit fusions.

Abstract. Properties of security protocols such as authentication and secrecy are often verified by explictly generating an operational model of the protocol and then seeking for insecure states. However, message exchange between the intruder and the honest participants induces a form of state explosion that makes the model infinite in principle. Building on previous work on symbolic semantics, we propose a general framework for automatic analysis of security protocols that make use of a variety of crypto-functions. We start from a base language akin to the spi-calculus, equipped with a set of generic cryptographic primitives. We propose a symbolic operational semantics that relies on unification and provides finite and effective protocol models. Next, we give a method to carry out trace analysis directly on the symbolic model. Under certain conditions on the given cryptographic primitives, our method is proven complete for the considered class of properties.

Abstract. We study the expressiveness of the join calculus by comparison with (generalised, coloured) Petri nets and using tools from type theory. More precisely, we consider four classes of nets of increasing expressiveness, Π i, introduce a hierarchy of type systems of decreasing strictness, ∆i, i = 0, . . . , 3, and we prove that a join process is typeable according to ∆i if and only if it is (strictly equivalent to) a net of class Πi. In the details, Π0 and Π1 contain, resp., usual place/transition and coloured Petri nets, while Π2 and Π3 propose two natural notions of high-level net accounting for dynamic reconfiguration and process creation and called reconfigurable and dynamic Petri nets, respectively.

In security protocols, message exchange between the intruder and honest participants induces a form of state explosion which makes protocol models infinite. We propose a general method for automatic analysis of security protocols based on the notion of frame, essentially a rewrite system plus a set of distinguished terms called messages. Frames are intended to model generic crypto-systems. Based on frames, we introduce a process language akin to Abadi and Fournet's applied pi. For this language, we define a symbolic operational semantics that relies on unification and provides finite and effective protocol models. Next, we give a method to carry out trace analysis directly on the symbolic model. We spell out a regularity condition on the underlying frame, which guarantees completeness of our method for the considered class of properties, including secrecy and various forms of authentication. We show how to instantiate our method to some of the most common crypto-systems, including shared-and public-key encryption, hashing and Diffie-Hellman key exchange.

Abstract. The concurrent constraint pi-calculus (cc-pi-calculus) has been introduced as a model for concluding Service Level Agreements. The cc-pi calculus combines the synchronous communication paradigm of process calculi with the constraint handling mechanism of concurrent constraint programming. While in the original presentation of the calculus a reduction semantics has been proposed, in this work we investigate the abstract semantics of cc-pi processes. First, we define a labelled transition system of the calculus and a notion of open bisimilaritỳ a la pi-calculus that is proved to be a congruence. Next, we give a symbolic characterisation of bisimulation and we prove that the two semantics coincide. Essentially, two processes are open bisimilar if they have the same stores of constraints -this can be statically checked -and if their moves can be mutually simulated. A key idea of the symbolic transition system is to have 'contextual' labels, i.e. labels specifying that a process can evolve only in presence of certain constraints. Finally, we show that the polyadic Explicit Fusions calculus introduced by Gardner and Wischik can be translated into monadic cc-pi and that such a transition preserves open bisimilarity. The mapping exploits fusions and tuple unifications as constraints.

Service composition concerns both integration of heterogeneous distributed applications and dynamic selection of services. QoS-aware selection enables a service requester with certain QoS requirements to classify services according to their QoS guarantees. In this paper we present a method that allows for a fuzzy-valued description of QoS parameters. Fuzzy sets are suited to specify both the QoS preferences raised by a service requester such as 'response time must be as lower as possible and cannot be more that 1000ms' and approximate estimates a provider can make on the QoS capabilities of its services like 'availability is roughly between 95% and 99%'. We propose a matchmaking procedure based on a fuzzy-valued similarity measure that, given the specifications of QoS parameters of the requester and the providers, selects the most appropriate service among several functionally-equivalent ones. We also devise a method for dynamical update of service offers by means of runtime monitoring of the actual QoS performance

Abstract. Several models based on process calculi have addressed the definition of linguistic primitives for handling long running transactions and Service Level Agreement (SLA) in service oriented applications. Nevertheless, the approaches appeared in the literature deal with these aspects as independent features. We claim that transactional mechanisms are relevant for programming multi-step SLA negotiations and, hence, it is worth investigating the interplay among such formal approaches. In this paper we propose a process calculus, the committed cc-pi, that combines two proposals: (i) cc-pi calculus accounting for SLA negotiation and (ii) cJoin as a model of long running transactions. We provide both a small-and a big-step operational semantics of committed cc-pi as labelled transition systems, and we prove a correspondence result.

Abstract. Fusion calculus is commonly regarded as a generalisation of picalculus. Actually, we prove that there is no uniform fully abstract embedding of pi-calculus into Fusion. This fact motivates the introduction of a new calculus, D-Fusion, with two binders, λ and ν. We show that D-Fusion is strictly more expressive than both pi-calculus and Fusion. The expressiveness gap is further clarified by the existence of a fully abstract encoding of mixed guarded choice into the choice-free fragment of D-Fusion.