Section: New Results

Distributed algorithms for dynamic networks

Participants : Luciana Bezerra Arantes [correspondent] , Rudyar Cortes, Guthemberg Da Silva Silvestre, Raluca Diaconu, Ruijing Hu, Anissa Lamani, Jonathan Lejeune, Olivier Marin, Sébastien Monnet, Franck Petit [correspondent] , Karine Pires, Maria Potop-Butucaru, Pierre Sens, Véronique Simon, Julien Sopena.

This objective aims to design distributed algorithms adapted to new large scale or dynamic distributed systems, such as mobile networks, sensor networks, P2P systems, Grids, Cloud environments, and robot networks. Efficiency in such demanding environments requires specialised protocols, providing features such as fault or heterogeneity tolerance, scalability, quality of service, and self-stabilization. Our approach covers the whole spectrum from theory to experimentation. We design algorithms, prove them correct, implement them, and evaluate them in simulation, using OMNeT++ or PeerSim, and on large-scale real platforms such as Grid'5000. The theory ensures that our solutions are correct and whenever possible optimal; experimental evidence is necessary to show that they are relevant and practical.

Within this thread, we have considered a number of specific applications, including massively multi-player on-line games (MMOGs) and peer certification.

Since 2008, we have obtained results both on fundamental aspects of distributed algorithms and on specific emerging large-scale applications.

We study various key topics of distributed algorithms: mutual exclusion, failure detection, data dissemination and data finding in large scale systems, self-stabilization and self-* services.

Mutual Exclusion and Failure Detection.

Mutual Exclusion and Fault Tolerance are two major basic building blocks in the design of distributed systems. Most of the current mutual exclusion algorithms are not suitable for modern distributed architectures because they are not scalable, they ignore the network topology, and they do not consider application quality of service constraints. Under the ANR Project MyCloud and the FSE Nu@age, we study locking algorithms fulfilling some QoS constraints often found in Cloud Computing [46] , [38] .

A classical way for a distributed system to tolerate failures is to detect them and then recover. It is now well recognized that the dominant factor in system unavailability lies in the failure detection phase. Regal has worked for many years on practical and theoretical aspects of failure detections and pioneered hierarchical scalable failure detectors. (Recent work by Leners et al published in SOSP 2011 uses our DSN 2003 paper as basis for performance comparison) Since 2008, we have studied the adaptation of failure detectors to dynamic networks. In 2013, we studied $\Omega$, the eventual leader election failure detector. $\Omega$ ensures that, eventually, each process in the system will be provided by an unique leader, elected among the set of correct processes in spite of crashes and uncertainties. It is known to be weakest failure detector to solve agreement protocols such as Paxos. Then, a number of eventual leader election protocols were suggested. Nonetheless, as far as we are aware of, no one of these protocols tolerates a free pattern of node mobility. In [27] we propose a new protocol for this scenario of dynamic and mobile unknown networks.

Self-Stabilization and Self-* Services.

We have also approached fault tolerance through self-stabilization. Self-stabilization is a versatile technique to design distributed algorithms that withstand transient faults. In particular, we have worked on the unison problem, (C. Boulinier, F. Petit, and V. Villain. Synchronous vs. asynchronous unison. Algorithmica, 51(1):61-80, 2008) i.e., the design of self-stabilizing algorithms to synchronize a distributed clock. As part of the ANR project SPADES, we have proposed several snap-stabilizing algorithms for the message forwarding problem that are optimal in terms of number of required buffers. A snap-stabilizing algorithm is a self-stabilizing algorithm that stabilizes in 0 steps; in other words, such an algorithm always behaves according to its specification.

Finally, we have applied our expertise in distributed algorithms for dynamic and self-* systems in domains that at first glance seem quite far from the core expertise of the team, namely ad-hoc systems and swarms of mobile robots. In the latter, as part of ANR project R-Discover, we have studied various problems such as exploration and gathering.

Dissemination and Data Finding in Large Scale Systems.

In the area of large-scale P2P networks, we have studied the problems of data dissemination and overlay maintenance, i.e., maintenance of a logical network built over the a P2P network. In 2013, we have proposed a new distributed algorithm suitable for scale-free random topologies which model some complex real world networks [37] , [52] .

Peer certification.

In a distributed system, the certification of transactions makes it possible to circumscribe malicious behaviors. Certification requires the use of a trusted third party which must be centralized to guarantee safety. At a large scale, however, centralized certification represents a bottleneck and a single point of attack or failure.

We proposed two decentralized approaches towards certifying transactions with a high probability of success. The first approach replicates transactions over multiple peers and retains identical results from a qualified majority to certify that a service has been carried out for a given client at a given time [30] . The second approach uses distributed reputations to identify trusted nodes and use them as game referees to detect and prevent cheating [57] .