Equational Reasonings in Wireless Network Gossip Protocols

Gossip protocols have been proposed as a robust and efficient method for disseminating information throughout large-scale networks. In this paper, we propose a compositional analysis technique to study formal probabilistic models of gossip protocols expressed in a simple probabilistic timed process calculus for wireless sensor networks. We equip the calculus with a simulation theory to compare probabilistic protocols that have similar behaviour up to a certain tolerance. The theory is used to prove a number of algebraic laws which revealed to be very effective to estimate the performances of gossip networks, with and without communication collisions, and randomised gossip networks. Our simulation theory is an asymmetric variant of the weak bisimulation metric that maintains most of the properties of the original definition. However, our asymmetric version is particularly suitable to reason on protocols in which the systems under consideration are not approximately equivalent, as in the case of gossip protocols

Passive scalar intermittency in compressible flow

A compressible generalization of the Kraichnan model (Phys. Rev. Lett. 72, 1016 (1994)) of passive scalar advection is considered. The dynamical role of compressibility on the intermittency of the scalar statistics is investigated for the direct cascade regime. Simple physical arguments suggest that an enhanced intermittency should appear for increasing compressibility, due to the slowing down of Lagrangian trajectory separations. This is confirmed by a numerical study of the dependence of intermittency exponents on the degree of compressibility, by a Lagrangian method for calculating simultaneous N-point tracer correlations.Comment: 4 pages, 3 figures Revised version, accepted for publication in PRE - Rapid communication

An economic history of the Champagne contracts, lessons for regional development

This paper highlights the success factors of the governance of the Champagne supply chain. Scholars on economic organisation stress the role of the contractual enforcement to explain the stability of the economic exchanges and the ability of the economic and political actors to foster their own development (NORTH 1999). Our contribution detailed explicit and implicit mechanisms related to the vinegrower-merchant relations in the regional system. The Champagne region had the particularity to posses a double-head organisation, regrouping all the farms and firms involved in the agronomic, and commercial process of the regional wines. This private board is supported by an institutional environment, common market organisation, French rural acts, and national and international legislations on geographical indication. These legislatives and administrative components define precisely the productive and market rules. Rely on a longitudinal approach we reinterpret the way the interprofessionnal (general) agreement, essential part of the governance of the regional market, evolved during decades (BARRERE 2003). This rereading illustrates the interdependency between explicit and implicit enforcement mechanisms which foster the cooperation. We argue that asymmetric investments in advertising play a major role in the stability of the regional cooperation. The achievement of the reputation of the AOC Champagne by massive advertising and commercial investments mainly realised by the negociants is central to understand the convergence of both party strategies on a long term. These investments step in as catalyst of a negotiated environment and award the self-enforcing character of the contracts. It makes efficient the set of private arrangements and regulatory mechanisms designed to eradicate opportunistic behaviours. During all the second part of the 20th century, the form of the contractual agreements evolved. Governance tools were added and suppressed. However these forced or desired adaptations slightly alter the nature of the cooperative process. The flexibility of the private arrangement, as well as the comprehensive economic policy, ensures the durability of the general agreement in spite of crisis. These results backup the hypothesis of the new institutional economics on the necessity of complementary institutions to make the market efficient (AOKI 2001).

A Formal Approach to Cyber-Physical Attacks

We apply formal methods to lay and streamline theoretical foundations to reason about Cyber-Physical Systems (CPSs) and cyber-physical attacks. We focus on %a formal treatment of both integrity and DoS attacks to sensors and actuators of CPSs, and on the timing aspects of these attacks. Our contributions are threefold: (1) we define a hybrid process calculus to model both CPSs and cyber-physical attacks; (2) we define a threat model of cyber-physical attacks and provide the means to assess attack tolerance/vulnerability with respect to a given attack; (3) we formalise how to estimate the impact of a successful attack on a CPS and investigate possible quantifications of the success chances of an attack. We illustrate definitions and results by means of a non-trivial engineering application

Internal stresses and breakup of rigid isostatic aggregates in homogeneous and isotropic turbulence

By characterising the hydrodynamic stresses generated by statistically homogeneous and isotropic turbulence in rigid aggregates, we estimate theoretically the rate of turbulent breakup of colloidal aggregates and the size distribution of the formed fragments. The adopted method combines Direct Numerical Simulation of the turbulent field with a Discrete Element Method based on Stokesian dynamics. In this way, not only the mechanics of the aggregate is modelled in detail, but the internal stresses are evaluated while the aggregate is moving in the turbulent flow. We examine doublets and cluster-cluster isostatic aggregates, where the failure of a single contact leads to the rupture of the aggregate and breakup occurs when the tensile force at a contact exceeds the cohesive strength of the bond. Due to the different role of the internal stresses, the functional relationship between breakup frequency and turbulence dissipation rate is very different in the two cases. In the limit of very small and very large values, the frequency of breakup scales exponentially with the turbulence dissipation rate for doublets, while it follows a power law for cluster-cluster aggregates. For the case of large isostatic aggregates it is confirmed that the proper scaling length for maximum stress and breakup is the radius of gyration. The cumulative fragment distribution function is nearly independent of the mean turbulence dissipation rate and can be approximated by the sum of a small erosive component and a term that is quadratic with respect to fragment size.Comment: 31 pages, 19 figure