Minimal Synchrony for Byzantine Consensus

Solving the consensus problem requires in one way or another that the underlying system satisfies some synchrony assumption. Considering an asynchronous message-passing system of n processes where (a) up to t< n/3 may commit Byzantine failures, and (b) each pair of processes is connected by two uni-directional channels (with possibly different timing properties), this paper investigates the synchrony assumption required to solve consensus, and presents a signature-free consensus algorithm whose synchrony requirement is the existence of a process that is an eventual {t+1}bisource. Such a process p is a correct process that eventually has (a) timely input channels from t correct processes and (b) timely output channels to t correct processes (these input and output channels can connect p to different subsets of processes). As this synchrony condition was shown to be necessary and sufficient in the stronger asynchronous system model (a) enriched with message authentication, and (b) where the channels are bidirectional and have the same timing properties in both directions, it follows that it is also necessary and sufficient in the weaker system model considered in the paper. In addition to the fact that it closes a long-lasting problem related to Byzantine agreement, a noteworthy feature of the proposed algorithm lies in its design simplicity, which is a first-class property.