Snowman for partial synchrony

TL;DR

This paper extends the Snowman consensus framework to partial synchrony by introducing Snowman$^{\diamond}$ and its Binary counterpart Snowflake$^{\diamond}$, removing the previous lockstep assumption and allowing rounds to progress at local speeds. It provides probabilistic agreement and consistency proofs under the standard Byzantine model with $f< n/5$, adapting the Snowflake$^{+}$ analysis to partial synchrony via interval-based invariants and locking dynamics. The work also explores practical accelerations to finality through clock-synchronization assumptions (\Delta^*) and a concept of temporary finality, offering latency reductions while bounding revert risk. Collectively, these results improve scalability and robustness of blockchain consensus in real-world networks with asynchronous or partially synchronous communication. The approach combines hash-chain-based block finalization with multi-instance Snowflake$^{\diamond}$ SMR, delivering probabilistic guarantees suitable for practical deployment.

Abstract

Snowman is the consensus protocol run by blockchains on Avalanche. Recent work established a rigorous proof of probabilistic consistency for Snowman in the \emph{synchronous} setting, under the simplifying assumption that correct processes execute sampling rounds in `lockstep'. In this paper, we describe a modification of the protocol that ensures consistency in the \emph{partially synchronous} setting, and when correct processes carry out successive sampling rounds at their own speed, with the time between sampling rounds determined by local message delays.