International Professional University of Technology in Nagoya Repository
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Byzantine-tolerant Uniform Node Sampling Service
No full textWe consider the problem of achieving uniform node sampling in large scale systems in presence of Byzantine nodes. The uniform node sampling service offers to applications using it a single simple primitive that returns, upon invocation, the identifier of a random node that belongs to the system. We first propose an omniscient strategy that processes on the fly an unbounded and arbitrarily biased input stream made of node identifiers exchanged within the system, and outputs a stream that preserves the uniformity property. Informally, uniformity states that any node in the system should have the same probability to appear in the sample of any correct node of the system. We show through a Markov chain analysis that this property holds despite any arbitrary bias introduced by the adversary. We then propose a strategy based on a sketch data structure that is capable of approximating the omniscient strategy without requiring any prior knowledge on the composition of the input stream. We show through both theoretical analysis and extensive simulations that this " knowledge-free " strategy accurately approximates the omniscient one. We evaluate the resilience of the knowledge-free strategy by studying two representative attacks (flooding and targeted attacks). We quantify the minimum number of identifiers that Byzantine nodes must insert in the input stream to prevent uniformity. Finally, we propose a new construction that processes each input stream with sketches put in series that allows to both increase the accuracy of a single sketch and decrease the time to converge to a uniform output stream. To our knowledge, such a work has never been proposed before
Computing the distance between two finite element solutions defined on different 3D meshes on a GPU
No full textThis article introduces a new method to efficiently compute the distance (i.e., L p norm of the difference) between two functions supported by two different meshes of the same 3D domain. The functions that we consider are typically finite element solutions discretized in different function spaces supported by meshes that are potentially completely unrelated. Our method computes an approximation of the distance by resampling both fields over a set of parallel 2D regular grids. By lever-aging the parallel horse power of computer graphics hardware (GPU), our method can efficiently compute distances between meshes with multi-million elements in seconds. We demonstrate our method applied to different problems (distance between known functions, Poisson solutions, linear elasticity solutions) using different function spaces (Lagrange polynomials from order one to seven) and different meshes (tetrahedral, hexahedral, with linear or quadratic geometry)
Amorphous YBaCuO semiconducting thin films for radiation thermal detectors: migrating from IR to THz sensor optimal structure
No full textInternational audienc
Measurable lower limit of thin film conductivity with parallel plate waveguide terahertz time domain spectroscopy
No full textInternational audienc
Cartographie du photo-courant sur des jonctions à contact passivant en silicium poly-cristallin sur oxyde
No full textInternational audienc
SnO2 nanoparticles as catalyst precursors for plasma-assisted VLS growth with controlled surface density
No full textInternational audienc
Polaritonic figure of merit of plane structures
No full textInternational audienceBased on the ability of plane structures to simultaneously optimize the propagation, confinement, and energy of surface plasmon-polaritons or surface phonon-polaritons, we develop the polaritonic figure of merit Z = βRΛ2/δ, where βR, Λ and δ are the longitudinal wave vector, propagation length, and penetration depth, respectively. Explicit and analytical expressions of Z are derived for a single interface and a suspended thin film, as functions of the material permittivities and the film thickness. Higher Z are obtained for thinner films and smaller energy losses. The application of the obtained results for a SiC-air interface and a SiC thin film suspended in air shows that both structures are able to maximize the presence of polaritons at a frequency near to, but different than that at which the real part of the SiC permittivity exhibits a dip. Furthermore, using the temperature change of this dip, we show that the propagation length, confinement and energy of polaritons increases with its deepness, which provides an effective way to enhance the overall Z of polaritonic structures
Indices de Sobol généralisés aux variables dépendantes : tests de performance de l’algorithme HOGS couplé à plusieurs estimateurs paramétriques
No full textInternational audienceThe algorithm " Hierarchically Orthogonal Gram-Schmidt " (HOGS) (Chastaing et al., 2015) estimates generalized Sobol indices dedicated to models with dependent inputs, quantifying explicitly the model sensitivity due to correlations. HOGS constructs a meta-model for each variable of interest by projection on a functional basis suited to indices calculation. Regression coefficients are obtained with the ordinary least-square estimator (OLS) or penalized regression methods Lasso, Ridge and Elastic Net (EN). Four study cases are proposed: three toy models allowing to investigate HOGS functioning and numerical properties, and the LNAS (Log-Normal Allocation and Senescence) model dedicated to the complex dynamics of plant growth. Several HOGS configurations and meta-model accuracy are tested by means of a consistency index. An interpretation of Sobol indices is given for LNAS. It appears that HOGS-OLS is the most efficient method when simulation resources are not limited. Otherwise, considering the issue of parameter estimation with sparsity highlights that: i) EN is more robust but more costly than Lasso, ii) the basis constructed by HOGS is too large which creates artificial sparsity. A modification of HOGS has been proposed to reduce the dimension of the basis.L'algorithme " Hierarchically Orthogonal Gram–Schmidt " (HOGS) (Chastaing et al., 2015) estime des indices de Sobol généralisés aux modèles à entrées dépendantes, quantifiant explicitement la sensibilité du modèle due aux corrélations. HOGS construit un méta-modèle pour chaque variable d'intérêt par projection sur une base fonctionnelle bien choisie pour le calcul des indices. Les coefficients de projection sont obtenus par l'estimateur des moindres carrés (OLS) ou les régressions pénalisées lasso, ridge et Elastic Net (EN). Quatre cas d'étude sont proposés : trois modèles simples permettent d'appréhender le fonctionnement de HOGS, et le modèle LNAS (Log-Normal Allocation and Senescence) dédié à la dynamique complexe de la croissance des plantes. Plusieurs configurations de HOGS et la pertinence du méta-modèle sont étudiées grâce à un indice de consistance. L'interprétation des indices de Sobol est illustrée grâce à LNAS. En conclusion, HOGS-OLS est la méthode la plus performante lorsque les ressources informatiques ne sont pas limitantes. Dans le cas contraire, la question de l'estimation paramétrique avec hypothèse de sparsité met en évidence que : i) EN est plus robuste mais plus coûteux numériquement que le Lasso, ii) HOGS génère une base trop grande, créant de la sparsité artificielle. Un amendement de HOGS a été proposé pour réduire la dimension de la base
Automated Verification of Security Chains in Software-Defined Networks with Synaptic
No full textInternational audienceSoftware-defined networks provide new facilities for deploying security mechanisms dynamically. In particular, it is possible to build and adjust security chains to protect the infrastructures, by combining different security functions, such as firewalls, intrusion detection systems and services for preventing data leakage. It is important to ensure that these security chains, in view of their complexity and dynamics, are consistent and do not include security violations. We propose in this paper an automated strategy for supporting the verification of security chains in software-defined networks. It relies on an architecture integrating formal verification methods for checking both the control and data planes of these chains, before their deployment. We describe algorithms for translating specifications of security chains into formal models that can then be verified by Satisfiability Modulo Theories (SMT) solving or model checking. Our solution is prototyped as a package, named Synaptic, built as an extension of the Frenetic family of SDN programming languages. The performances of our approach are evaluated through extensive experimentations based on the CVC4, veriT, and nuXmv checkers