Optimal Good-case Latency for Byzantine Broadcast and State Machine Replication

This paper investigates Byzantine broadcast (BB) protocols with optimal good-case latency under synchrony and weaker variants of synchrony. One of them most important applications of BB is to implement Byzantine fault-tolerant (BFT) state machine replication (SMR), also known as blockchains recently. The traditional latency metric of BB focuses on the number of lock-step rounds needed in the worst case or expected case. We observe that the traditional latency metric fails to capture what's important in practice for two reasons. First, practical synchronous BFT SMR do not run in lock-step rounds. Second, practical SMR protocols make progress only when an honest leader is in charge. Thus, motivated by recent progress in synchronous BFT SMR, we study the \emph{good-case latency} of BB, i.e., the precise latency to commit measured in time (as opposed rounds) when the sender is honest. We propose the first synchronous BB protocol with optimal good-case latency. This closes the gap between the upper/lower bounds on good-case latency left open in the previous work by Abraham et al. \cite{synchotstuff}. To make the synchronous model more practical, we extend our protocol to handle two weaker network models, named mobile link failures and mobile sluggish faults. By providing a new lower bound in the mobile link failure model, we show that our protocols in these weak models also achieve optimal good-case latency. Finally, to demonstrate the applicability to the target application, we turn all our BB protocols into BFT SMR protocols with minimum modifications and guarantee the same good-case latency and tolerance to weaker synchrony variants.