Unbeatable consensus

The unbeatability of a consensus protocol, introduced by Halpern et al. (SIAM J Comput 31:838–865, 2001), is a stronger notion of optimality than the accepted notion of early stopping protocols. Using a novel knowledge-based analysis, this paper derives the first explicit unbeatable consensus protocols in the literature, for the standard synchronous message-passing model with crash failures. These protocols strictly dominate the best kno-wn protocols for uniform and for Nonuniform Consensus, in some cases improving on them by a large margin. The analysis provides a new understanding of the logical structure of consensus, and of the distinction between uniform and nonuniform Consensus. All protocols presented in this paper have very concise descriptions, and are shown to be efficiently implementable.