Revisiting Speculative Leaderless Protocols for Low-Latency BFT Replication

TL;DR

Aspen is a leaderless BFT protocol that achieves near-optimal low latency and high throughput in permissioned blockchains by effectively handling contention and network delays, outperforming previous protocols.

Contribution

Aspen introduces a best-effort sequencing layer with loosely synchronized clocks to remove contention restrictions in leaderless BFT protocols, enabling low latency even under network delays.

Findings

Aspen commits requests in less than 75 ms in wide-area settings.

Achieves 19,000 requests per second throughput.

Provides a 1.2 to 3.3× performance improvement over prior protocols.

Abstract

As Byzantine Fault Tolerant (BFT) protocols begin to be used in permissioned blockchains for user-facing applications such as payments, it is crucial that they provide low latency. In pursuit of low latency, some recently proposed BFT consensus protocols employ a leaderless optimistic fast path, in which clients broadcast their requests directly to replicas without first serializing requests at a leader, resulting in an end-to-end commit latency of 2 message delays ($2\Delta$) during fault-free, synchronous periods. However, such a fast path only works if there is no contention: concurrent contending requests can cause replicas to diverge if they receive conflicting requests in different orders, triggering costly recovery procedures. In this work, we present Aspen, a leaderless BFT protocol that achieves a near-optimal latency of $2\Delta + \varepsilon$, where $\varepsilon$ indicates a short waiting delay. Aspen removes the no-contention condition by utilizing a best-effort sequencing layer based on loosely synchronized clocks and network delay estimates. Aspen requires $n = 3f + 2p + 1$ replicas to cope with up to $f$ Byzantine nodes. The $2p$ extra nodes allow Aspen's fast path to proceed even if up to $p$ replicas diverge due to unpredictable network delays. When its optimistic conditions do not hold, Aspen falls back to PBFT-style protocol, guaranteeing safety and liveness under partial synchrony. In experiments with wide-area distributed replicas, Aspen commits requests in less than 75 ms, a 1.2 to 3.3$\times$ improvement compared to previous protocols, while supporting 19,000 requests per second.