Hermes: Low Tail-Latency Via Prefix Consensus

2026-07-28

AI Summary

This work addresses the inefficiencies of traditional leader-based Byzantine fault-tolerant protocols, which suffer from wasted views and struggle to balance tail latency with liveness when the leader fails. The authors propose a two-round rotating-leader protocol that introduces prefix consensus and a comparability quorum mechanism, eliminating empty votes and ensuring that even timed-out views can achieve finality on the heaviest common prefix. By leveraging techniques such as prefix-ordered values, single voting, parent-relative Delta tipcuts, explicit skipping, and erasure coding with sender-index amplification, the protocol guarantees that under a partial synchrony model with \( n = 5f + 1 \) nodes, an honest leader can finalize a full proposal within \( 2\delta \). Otherwise, any \( n - f \) heterogeneous votes suffice to finalize the common prefix, achieving communication complexity of \( O(n^2 \hat{m} + \lambda n^3) \) bits and \( O(n^2) \) messages per view.

Abstract

Leader-based BFT protocols finalize through their leaders: a view whose leader is crashed or slow finalizes nothing, and the timeout that ends it admits no good setting. A conservative timeout turns every crashed leader into a long stall; an aggressive one voids the views of leaders that are merely slow. Either way the expired view is wasted, and this trade-off, not the good case, governs tail latency. Hermes makes expired views finalize. Hermes is a two-round rotating-leader protocol for $n=5f+1$ processes under partial synchrony built on prefix consensus: votes carry values ordered by a prefix relation, and quorums require comparability rather than equality. Every process broadcasts a justified proposal at view start and casts a single vote, for the leader's proposal upon delivery or for a fallback proposal at the timeout; there are no nullify votes. A timely honest leader finalizes its full proposal in $2δ$. Otherwise, any $n-f$ votes, which need not match, finalize the heaviest common prefix. We instantiate Hermes as a finality gadget over an available chain and over Autobahn-style multi-lane dissemination, where parent-relative delta tipcuts with explicit skips keep independent proposals comparable and sender-indexed erasure-coded amplification keeps complexity at $O(n^2\hat{m}+λn^3)$ bits and $O(n^2)$ messages per view, where $\hat{m}$ is the number of concurrent dissemination lanes in Autobahn and $λ$ the security parameter.

Problem

• tail latency
• leader-based BFT
• view expiration
• consensus
• partial synchrony

Innovation

• prefix consensus
• low tail-latency
• rotating-leader BFT
• finality gadget
• comparability-based quorum