Translation-covariant Markovian master equation for a test particle in a quantum fluid

A recently proposed master equation in the Lindblad form is studied with respect to covariance properties and existence of a stationary solution. The master equation describes the interaction of a test particle with a quantum fluid, the so-called Rayleigh gas, and is characterized by the appearance of a two-point correlation function known as dynamic structure factor, which reflects symmetry and statistical mechanics properties of the fluid. In the case of a free gas all relevant physical parameters, such as fugacity, ratio between the masses, momentum transfer and energy transfer are put into evidence, giving an exact expansion of the dynamic structure factor. The limit in which these quantities are small is then considered. In particular in the Brownian limit a Fokker-Planck equation is obtained in which the corrections due to quantum statistics can be explicitly evaluated and are given in terms of the Bose function $g_0 (z)$ and the Fermi function $f_0 (z)$.Comment: 18 pages, revtex, no figures, to appear in J. Math. Phy

A Trust Overlay Architecture and Protocol for Enhanced Protection against Spam

The effectiveness of current anti-spam systems is limited by the ability of spammers to adapt to new filtering techniques and the lack of incentive for mail domains to filter outgoing spam. This paper describes a new approach to spam protection based on distributed trust management. This is motivated by the fact that the SMTP mail infrastructure is managed in a distributed way by a community of mail domain administrators. A trust overlay architecture and a new protocol are presented. The TOPAS protocol specifies how experiences and recommendations are communicated between a spam filter at each mail domain and its associated trust manager, and between trust managers of different mail servers. A technique for improving mail filtering using these trust measures is also described. Initial simulations indicate the potential of this approach to improve rates of false positives and false negatives in anti-spam systems

Integrating Players, Reputation and Ranking to Manage Cheating in MMOGs

In this paper, we propose an approach that uses in-game reputation as a solution to the problem of cheating in massively multiplayer online games. What constitutes cheating is however quite context-specific and subjective, and there is no universal view. Thus our approach aims to adjust to the particular forms of cheating to which players object rather than deciding a priori which forms of cheating should be controlled. The main feature of our approach is an architecture and model for maintaining player-based and context-appropriate trust and reputation measures, with the integration of these into the game’s ranking system. When an avatar loses reputation, our approach intervenes to reduce its ranking. It is envisaged that players will come to attach value to reputation in its own right. We also present the results of relatively large-scale simulations of various scenarios involving sequences of encounters between players, with an initial implementation of our reputation and ranking model in place, to observe the impact on cheaters (and non-cheaters)

On fluid approximation for stable networks

Projet MEVALRésumé disponible dans le fichier PD

An experimental testbed to predict the performance of XACML Policy Decision Points

The performance and scalability of access control systems is a growing concern as organisations deploy ever more complex communications and content management systems. This paper describes how an (offline) experimental testbed may be used to address performance concerns. To begin, timing measurements are collected from a server component incorporating the Policy Decision Point (PDP) under test, using representative policies and corresponding requests. Our experiments with two XACML PDP implementations show that measured request service times are typically clustered by request type; thus an algorithm for request cluster identification is presented. Cluster characterisations are used as inputs to a PDP performance model for a given policy/request mix and an analytic (queueing) model is used to estimate the equilibrium server load for different mixes of request clusters. The analytic performance prediction model is validated and extended by discrete event simulation of a PDP subject to additional load. These predictive models enable network administrators to explore the capacity of the PDP for different overall loadings (requests per unit time) and profiles (relative frequencies) of requests

Ergodicity of conservative communication networks

Projet MEVALWe analyze a communication network with several types of calls. For a wide class of conservative service disciplines, we give ergodicity criteria. Exponentially fast convergence to steady state is also proved