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 ($2Δ$) 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 $2Δ+ \varepsilon$, 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.

翻译：随着拜占庭容错（BFT）协议开始被用于面向用户应用（如支付）的许可区块链中，其提供低延迟变得至关重要。为追求低延迟，一些近期提出的BFT共识协议采用了无领导的乐观快速路径，其中客户端将请求直接广播给副本，而无需先在领导者处对请求进行序列化，从而在无故障的同步期间实现端到端提交延迟为2个消息延迟（$2Δ$）。然而，此类快速路径仅在无竞争条件下有效：并发的竞争请求可能导致副本在接收冲突请求时出现顺序分歧，从而触发代价高昂的恢复过程。本文提出Aspen，一种无领导BFT协议，其实现了接近最优的延迟$2Δ+ \varepsilon$，其中$\varepsilon$表示短暂的等待延迟。Aspen通过利用基于松散同步时钟和网络延迟估计的尽力而为排序层，消除了无竞争条件。Aspen需要$n = 3f + 2p + 1$个副本来应对最多$f$个拜占庭节点。额外的$2p$个节点使得Aspen的快速路径即使因不可预测的网络延迟导致最多$p$个副本出现分歧时仍能继续执行。当乐观条件不满足时，Aspen回退至PBFT风格协议，在部分同步条件下保证安全性与活性。在广域分布式副本的实验中，Aspen在75毫秒内提交请求，相比先前协议实现了1.2至3.3$\times$的性能提升，同时支持每秒19,000个请求。