Revisiting Speculative Leaderless Protocols for Low-Latency BFT Replication

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.