Emmanuel Godard

Election in partially anonymous networks with arbitrary knowledge in message passing systems

This paper attempts to find an answer to an open question of Angluin in her seminal paper (1980) about the election problem for families of graphs (Section 4, page 87). More precisely, we characterize families of (labelled) graphs which admit an election algorithm in the message passing model by using the notion of quasi-coverings which captures “the existence of large enough area of one graph that looks locally like another graph”.

A Characterization of Dynamic Networks Where Consensus Is Solvable

We consider the Consensus problem in arbitrary dynamic networks. A dynamic network is a communication network whose topology evolves from round to round. We make no assumptions on the possible topologies. We give the first complete necessary and sufficient condition for dynamic networks where it is possible to solve Consensus. n nWe show that we can complement the necessary condition for solvability of Consensus given, in the context of omission faults, in [GP11] in the context of dynamic networks. We prove that this condition is actually sufficient by presenting a new Consensus algorithm. This algorithm is based upon reconstructing a partial, but significant, view of the actual communications that occurred during the execution.

A Self-stabilizing Algorithm for the Median Problem in Partial Rectangular Grids and Their Relatives

Given a graph G=(V,E), a vertex v of G is a median vertex if it minimizes the sum of the distances to all other vertices of G. The median problem consists of finding the set of all median vertices of G. In this note, we present self-stabilizing algorithms for the median problem in partial rectangular grids and relatives. Our algorithms are based on the fact that partial rectangular grids can be isometrically embedded into the Cartesian product of two trees, to which we apply the algorithm proposed by Antonoiu and Srimani (J. Comput. Syst. Sci. 58:215–221, 1999) and Bruell et al. (SIAM J. Comput. 29:600–614, 1999) for computing the medians in trees. Then we extend our approach from partial rectangular grids to a more general class of plane quadrangulations. We also show that the characterization of medians of trees given by Gerstel and Zaks (Networks 24:23–29, 1994) extends to cube-free median graphs, a class of graphs which includes these quadrangulations.

Computing the Dynamic Diameter of Non-Deterministic Dynamic Networks is Hard

A dynamic network is a communication network whose communication structure can evolve over time. The dynamic diameter is the counterpart of the classical static diameter, it is the maximum time needed for a node to causally influence any other node in the network. We consider the problem of computing the dynamic diameter of a given dynamic network. If the evolution is known a priori, that is if the network is deterministic, it is known it is quite easy to compute this dynamic diameter. If the evolution is not known a priori, that is if the network is non-deterministic, we show that the problem is hard to solve or approximate. In some cases, this hardness holds also when there is a static connected subgraph for the dynamic network.

Expressivity of time-varying graphs

Time-varying graphs model in a natural way infrastructure-less highly dynamic systems, such as wireless ad-hoc mobile networks, robotic swarms, vehicular networks, etc. In these systems, a path from a node to another might still exist over time, rendering computing possible, even though at no time the path exists in its entirety. Some of these systems allow waiting (i.e., provide the nodes with store-carry-forward-like mechanisms such as local buffering) while others do not. n nIn this paper, we focus on the structure of the time-varying graphs modelling these highly dynamical environments. We examine the complexity of these graphs, with respect to waiting, in terms of their expressivity; that is in terms of the language generated by the feasible journeys (i.e., the paths over time). n nWe prove that the set of languages

A characterization of oblivious message adversaries for which Consensus is solvable

{cal L}_{nowait}

What Do We Need to Know to Elect in Networks with Unknown Participants

when no waiting is allowed contains all computable languages. On the other end, using algebraic properties of quasi-orders, we prove that

Anonymous Graph Exploration with Binoculars

{cal L}_{wait}

Minimal Obstructions for the Coordinated Attack Problem and Beyond

is just the family of regular languages, even if the presence of edges is controlled by some arbitrary function of the time. In other words, we prove that, when waiting is allowed, the power of the accepting automaton drops drastically from being as powerful as a Turing machine, to becoming that of a Finite-State machine. This large gap provides a measure of the impact of waiting. n nWe also study bounded waiting; that is when waiting is allowed at a node for at most d time units. We prove that

A self-stabilizing algorithm for the median problem in partial rectangular grids and their relatives

{cal L}_{wait[d]} = {cal L}_{nowait}