Sync HotStuff: Synchronous SMR with 2∆ Latency and Optimistic Responsiveness

Synchronous solutions for building Byzantine Fault Tolerance (BFT) replication can be safe when < 1/2 of the replicas fail. Assuming ∆ is an upper bound on the time for messages to arrive, these solutions must incur at least ∆ latency for consensus on a single value. In this work, we show a consensus protocol named Sync HotStuff designed to achieve consensus on a sequence of values with a latency of 2∆ in the common mode when less than half of the replicas are Byzantine. Thus, in the common mode, Sync HotStuff is within a factor of 2 of the optimal latency. Moreover, Sync HotStuff has responsiveness, i.e., it advances at network speed, when < 1/4 of the replicas are not responding, a small sacrifice in availability compared with optimal asynchronous solutions. Borrowing from practical BFT solutions in the asynchronous arena, Sync HotStuff has an extremely simple, two-phase leader-based structure, that easily fits in one frame of pseudo-code.