Optimistic Reliable Broadcast in Three Steps

2026-10-04

AI Summary

This study investigates the trade-off between optimistic delivery and total latency in signature-free asynchronous reliable broadcast. To address the latency bottleneck of existing optimistic two-phase protocols, this work proposes a three-phase protocol that matches the theoretical lower bound. By integrating asynchronous consensus, adaptive fault tolerance, and quadratic message complexity techniques, the proposed protocol achieves optimal total latency while maximizing the optimistic budget. Specifically, it preserves a two-step optimistic delay yet reduces the total latency to three steps under certain conditions, thereby attaining theoretical optimality. This research provides an efficient reliable broadcast solution for asynchronous distributed systems.

Abstract

We study the latency tradeoffs of signature-free asynchronous reliable broadcast with optimistic two-step delivery. We consider $n$ parties, of which at most $f$ are Byzantine. An optimistic fault budget $b\le f$ guarantees two-step delivery with an honest broadcaster and at most $b$ faults, while correctness tolerates $f$ faults. We write $(d_o,d_s,d_t)$ for optimistic latency, honest-broadcaster latency with up to $f$ faults, and totality delay-the elapsed time from first to last honest delivery. For $3f+1\le n<4f$, our first construction improves Opt-RBC's $(2,4,4)$ guarantees to $(2,3,4)$, preserving its maximal optimistic budget $b_*=\lfloor n/2\rfloor-f$. Thus maximal optimism is compatible with the optimal three-step good case. We prove a complementary lower bound: for $3f+1\le n<4f$, every deterministic reliable broadcast protocol in our asynchronous model with two-step optimistic budget $b\ge n-3f$ has totality delay at least three, even when the broadcaster is honest. Our second construction achieves $(2,3,3)$, matching this lower bound at maximal optimism when $f\ge2$ and $\lceil(10f-3)/3\rceil\le n<4f$. Both constructions use quadratic messages and tolerate adaptive corruption. In the matching range, maximal optimism therefore costs exactly one totality step relative to Bracha's $(3,3,2)$, without increasing good-case latency.

Problem

• reliable broadcast
• asynchronous systems
• latency tradeoffs
• Byzantine faults
• optimistic delivery

Innovation

• Optimistic Reliable Broadcast
• Asynchronous Byzantine Fault Tolerance
• Latency Tradeoffs
• Two-step Delivery
• Adaptive Corruption