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Quadratic worst-case message complexity for State Machine Replication in the partial synchrony model

We consider the message complexity of State Machine Replication protocols dealing with Byzantine failures in the partial synchrony model. A result of Dolev and Reischuk gives a quadratic lower bound for the message complexity, but it was unknown whether this lower bound is tight, with the most efficient known protocols giving worst-case message complexity $O(n^3)$. We describe a protocol which meets Dolev and Reischuk's quadratic lower bound, while also satisfying other desirable properties. To specify these properties, suppose that we have $n$ replicas, $f$ of which display Byzantine faults (with $n\geq 3f+1$). Suppose that $Δ$ is an upper bound on message delay, i.e. if a message is sent at time $t$, then it is received by time $ \text{max} \{ t, GST \} +Δ$. We describe a deterministic protocol that simultaneously achieves $O(n^2)$ worst-case message complexity, optimistic responsiveness, $O(fΔ)$ time to first confirmation after $GST$ and $O(n)$ mean message complexity.