This paper explores the problem good-case latency of Byzantine fault-tolerant
broadcast, motivated by the real-world latency and performance of practical
state machine replication protocols. The good-case latency measures the time it
takes for all non-faulty parties to commit when the designated broadcaster is
non-faulty. We provide a complete characterization of tight bounds on good-case
latency, in the authenticated setting under synchrony, partial synchrony and
asynchrony. Some of our new results may be surprising, e.g., 2-round PBFT-style
partially synchronous Byzantine broadcast is possible if and only if $ngeq
5f-1$, and a tight bound for good-case latency under $n/3<f<n/2$ under
synchrony is not an integer multiple of the delay bound.

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