Reliable Communication in a Dynamic Network in the Presence of Byzantine Faults

We consider the following problem: two nodes want to reliably communicate in a dynamic multihop network where some nodes have been compromised, and may have a totally arbitrary and unpredictable behavior. These nodes are called Byzantine. We consider the two cases where cryptography is available and not available. We prove the necessary and sufficient condition (that is, the weakest possible condition) to ensure reliable communication in this context. Our proof is constructive, as we provide Byzantine-resilient algorithms for reliable communication that are optimal with respect to our impossibility results. In a second part, we investigate the impact of our conditions in three case studies: participants interacting in a conference, robots moving on a grid and agents in the subway. Our simulations indicate a clear benefit of using our algorithms for reliable communication in those contexts

La qualité pour recourir des collectivités publiques dans le contentieux de la fonction publique: une clarification : en attendant un arrêt de principe concernant le recours constitutionnel subsidiaire

mars 19961996/03-1996/05

A framework for proving the self-organization of dynamic systems

This paper aims at providing a rigorous definition of self- organization, one of the most desired properties for dynamic systems (e.g., peer-to-peer systems, sensor networks, cooperative robotics, or ad-hoc networks). We characterize different classes of self-organization through liveness and safety properties that both capture information re- garding the system entropy. We illustrate these classes through study cases. The first ones are two representative P2P overlays (CAN and Pas- try) and the others are specific implementations of \Omega (the leader oracle) and one-shot query abstractions for dynamic settings. Our study aims at understanding the limits and respective power of existing self-organized protocols and lays the basis of designing robust algorithm for dynamic systems

Emergent velocity agreement in robot networks

In this paper we propose and prove correct a new self-stabilizing velocity agreement (flocking) algorithm for oblivious and asynchronous robot networks. Our algorithm allows a flock of uniform robots to follow a flock head emergent during the computation whatever its direction in plane. Robots are asynchronous, oblivious and do not share a common coordinate system. Our solution includes three modules architectured as follows: creation of a common coordinate system that also allows the emergence of a flock-head, setting up the flock pattern and moving the flock. The novelty of our approach steams in identifying the necessary conditions on the flock pattern placement and the velocity of the flock-head (rotation, translation or speed) that allow the flock to both follow the exact same head and to preserve the flock pattern. Additionally, our system is self-healing and self-stabilizing. In the event of the head leave (the leading robot disappears or is damaged and cannot be recognized by the other robots) the flock agrees on another head and follows the trajectory of the new head. Also, robots are oblivious (they do not recall the result of their previous computations) and we make no assumption on their initial position. The step complexity of our solution is O(n)

Population structure of plant-pathogenic Fusarium species in overwintered stalk residues from Bt-transformed and non-transformed maize crops

Bt-transformed maize contains genes from Bacillus thuringiensis encoding for insecticidal crystal proteins. Less insect damage on Bt maize stalks can cause a reduced infection by Fusarium species through plant injuries. This could affect the presence of plant-pathogenic Fusarium species on maize residues which serve as an inoculum source for subsequent crops. We collected overwintered maize stalks of four different Bt maize hybrids and their corresponding non-Bt lines in two consecutive years in a field trial in Germany. Fusarium spp. were isolated from 67% of 648 collected maize stalks. Identification with new multiplex PCR assays showed that F. graminearum, F. avenaceum, and F. proliferatum were the most abundant Fusarium species, isolated from 42%, 26%, and 15% of the stalks, respectively. Species abundances varied between varieties and collection years. No consistent difference was found between Bt and non-Bt stalks. Fusariumgraminearum isolates were subject to a population genetic structure analysis with eight newly developed microsatellites. Significant association of loci and overrepresentation of repeated multilocus haplotypes indicated a substantial asexual component of reproduction, supporting selection of haplotypes. The data suggested selection of particular F. graminearum haplotypes by collection years but not by maize Bt transformation. Haplotypic changes between years caused no divergence in the distribution of alleles, suggesting that gene flow beyond the field scale prevented substructuring. We present evidence for gene flow between our saprophytic F.graminearum population on maize residues and a wheat-pathogenic population from a field 100km distan

Inheritance of Naphthazarin Production and Pathogenicity to Pea in Nectria haematococca

A naphthazarin-producing (fusarubin, novarubin, javanicin, norjavanicin), highly pathogenic strain of Nectria haematococca (sexual state of Fusarium solani var. martii) was crossed with a non-naphthazarin-producing, slightly pathogenic mutant strain. This cross and several backcrosses were studied by unordered tetrad analysis. In all tetrads a 4: 4 ratio of naphthazarin-producing ascospore strains to non-naphthazarin-producing ones resulted. By two-factor analysis for this character and the markers determining mating type, sexual type, colonial growth, and perithecial colour respectively, parental-ditype, non-parental-ditype, and tetratype tetrads resulted. This indicates that the loss of the capacity to produce naphthazarins is due to a mutation at a single locus, which probably blocks the biosynthesis of a polyketide precursor. When the same tetrads were tested for pathogenicity, the strains could not be classified into a group with high pathogenicity and one with low pathogenicity, as they showed intermediate degrees of pathogenicity. It is therefore concluded that the degree of pathogenicity is polygenically inherited. The difference in the inheritance of the capacity to produce naphthazarins and of pathogenicity, as well as the appearance of highly pathogenic though non-naphthazarin-producing segregants, indicates that the capacity to produce novarubin, fusarubin, javanicin, and norjavanicin is not a major factor in the determination of a high degree of pathogenicit

Non-uniform circle formation algorithm for oblivious mobile robots with convergence toward uniformity

AbstractThis paper presents a distributed algorithm whereby a group of mobile robots self-organize and position themselves into forming a circle in a loosely synchronized environment. In spite of its apparent simplicity, the difficulty of the problem comes from the weak assumptions made on the system. In particular, robots are anonymous, oblivious (i.e., stateless), unable to communicate directly, and disoriented in the sense that they share no knowledge of a common coordinate system. Furthermore, robots’ activations are not synchronized. More specifically, the proposed algorithm ensures that robots deterministically form a non-uniform circle in a finite number of steps and converges to a situation in which all robots are located evenly on the boundary of the circle