Byzantine fault-tolerant (BFT) state machine replication (SMR) has been
studied for over 30 years. Recently it has received more attention due to its
application in permissioned blockchain systems. A sequence of research efforts
focuses on improving the commit latency of the SMR protocol in the common good
case, including PBFT with $3$-round latency and $ngeq 3f+1$ and FaB with
$2$-round latency and $ngeq 5f+1$. In this paper, we abstract a single-shot
BFT SMR with a new broadcast formulation named partially synchronous validated
Byzantine broadcast (psync-VBB), and propose a $2$-round psync-VBB protocol
under the optimal resilience $ngeq 5f-1$ with a matching lower bound. Our
protocol solves $2$-round BFT SMR with only $ngeq 5f-1$ replicas, which
refutes the optimal resiliency claim made in FaB for needing $n geq 5f+1$ for
$2$-round PBFT-style BFT protocols. For the special case when $f=1$, our
protocol needs only $4$ replicas, and strictly improves PBFT by reducing the
latency by one round (even when one backup is faulty).

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Author Of this post: <a href="">Ittai Abraham</a>, <a href="">Kartik Nayak</a>, <a href="">Ling Ren</a>, <a href="">Zhuolun Xiang</a>

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