Asynchronous Byzantine Approximate Consensus in Directed Networks

In this work, we study the approximate consensus problem in asynchronous message-passing networks where some nodes may become Byzantine faulty. We answer an open problem raised by Tseng and Vaidya, 2012, proposing the first algorithm of optimal resilience for directed networks. Interestingly, our results show that the tight condition on the underlying communication networks for asynchronous Byzantine approximate consensus coincides with the tight condition for synchronous Byzantine exact consensus. Our results can be viewed as a non-trivial generalization of the algorithm by Abraham et al., 2004, which applies to the special case of complete networks. The tight condition and techniques identified in the paper shed light on the fundamental properties for solving approximate consensus in asynchronous directed networks.Comment: 25 pages, 2 figure

Reliable broadcast with respect to topology knowledge

We study the Reliable Broadcast problem in incomplete networks against a Byzantine adversary. We examine the problem under the locally bounded adversary model of Koo (Proceedings of the 23rd annual ACM symposium on principles of distributed computing, PODC ’04, St. John’s, Newfoundland, Canada, 25–28 July 2004, ACM New York pp 275–282, 2004) and the general adversary model of Hirt and Maurer (Proceedings of the 16th annual ACM symposium on principles of distributed computing, PODC ’97, Santa Barbara, California, USA, August 21–24, 1997 ACM, New York pp 25–34, 1997) and explore the tradeoff between the level of topology knowledge and the solvability of the problem. In order to explore this tradeoff we introduce the partial knowledge model which captures the situation where each player has arbitrary topology knowledge. We refine the local pair-cut technique of Pelc and Peleg (Inf Process Lett 93(3):109–115, 2005) in order to obtain impossibility results for every level of topology knowledge and any type of corruption distribution. On the positive side we devise protocols that match the obtained bounds, and thus, exactly characterize the classes of graphs in which Reliable Broadcast is possible. Among others, we show that Koo’s Certified Propagation Algorithm (CPA) is unique, against locally bounded adversaries in ad hoc networks, among all safe algorithms, i.e., algorithms which never cause a node to decide on an incorrect value. This means that CPA can tolerate as many local corruptions as any other safe algorithm; this settles an open question posed by Pelc and Peleg. We also provide an adaptation of CPA achieving reliable broadcast against general adversaries and prove that this algorithm too is unique under this model. To the best of our knowledge this is the first optimal algorithm for Reliable Broadcast in generic topology ad hoc networks against general adversaries. © 2016, Springer-Verlag Berlin Heidelberg

A Topological Perspective on Distributed Network Algorithms

International audienceMore than two decades ago, combinatorial topology was shown to be useful for analyzing distributed fault-tolerant algorithms in shared memory systems and in message passing systems. In this work, we show that combinatorial topology can also be useful for analyzing distributed algorithms in networks of arbitrary structure. To illustrate this, we analyze consensus, set-agreement, and approximate agreement in networks, and derive lower bounds for these problems under classical computational settings, such as the LOCAL model and dynamic networks