Good-case Latency of Byzantine Broadcast: A Complete Categorization

TL;DR

This paper provides a comprehensive analysis of the good-case latency in Byzantine fault-tolerant broadcast protocols, establishing tight bounds across different network synchrony models and revealing surprising results about protocol efficiency.

Contribution

It offers a complete characterization of tight bounds on good-case latency in various settings, including new results on PBFT-style protocols and latency bounds under different fault thresholds.

Findings

2-round PBFT-style broadcast possible if and only if n≥5f-1

Tight bounds on good-case latency under synchrony and asynchrony

Some bounds are not integer multiples of delay bound

Abstract

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 $n\geq 5f-1$, 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.