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arXiv · 2607.25916

Hermes: Low Tail-Latency Via Prefix Consensus

Alejandro Ranchal-Pedrosa·Dakai Kang·Neil Giridharan·Dahlia Malkhi·Mohammad Sadoghi·Ben Marsh
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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 $2\delta$. 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 $O(n^2\hat{m}+\lambda n^3)$ bits and $O(n^2)$ messages per view, where $\hat{m}$ is the number of concurrent dissemination lanes in Autobahn and $\lambda$ the security parameter.

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Alejandro Ranchal-Pedrosa, Dakai Kang, Neil Giridharan, Dahlia Malkhi, Mohammad Sadoghi, Ben Marsh. 2026-07-28. Hermes: Low Tail-Latency Via Prefix Consensus. https://arxiv.org/abs/2607.25916

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