HotStuff-1: Linear Consensus with One-Phase Speculation (2408.04728v3)

Abstract: This paper introduces HotStuff-1, a BFT consensus protocol that improves the latency of HotStuff-2 by two network hops while maintaining linear communication complexity against faults. Furthermore, HotStuff-1 incorporates an incentive-compatible leader rotation design that motivates leaders to propose transactions promptly. HotStuff-1 achieves a reduction of two network hops by speculatively sending clients early confirmations, after one phase of the protocol. Introducing speculation into streamlined protocols is challenging because, unlike stable-leader protocols, these protocols cannot stop the consensus and recover from failures. Thus, we identify prefix speculation dilemma in the context of streamlined protocols; HotStuff-1 is the first streamlined protocol to resolve it. HotStuff-1 embodies an additional mechanism, slotting, that thwarts delays caused by (1) rationally-incentivized leaders and (2) malicious leaders inclined to sabotage other's progress. The slotting mechanism allows leaders to dynamically drive as many decisions as allowed by network transmission delays before view timers expire, thus mitigating both threats.

• The paper introduces speculative execution for early finality, reducing client confirmation from seven to three half-phases.
• It presents an adaptive slotting mechanism that mitigates leader slowness and tail-forking, enhancing throughput and resilience.
• Experimental results indicate up to 26.1% lower latency and robust performance under adversarial conditions.

Overview of "HotStuff-1: Linear Consensus with One-Phase Speculation"

This paper introduces HotStuff-1, a Byzantine Fault-Tolerant (BFT) consensus protocol designed to reduce transaction latency significantly while maintaining linear scalability. The key contributions of the paper lie in two major areas: leveraging speculative execution to provide early finality confirmations and introducing an adaptive slotting mechanism to mitigate leader-related attacks such as leader slowness and tail-forking.

Key Contributions

1. Speculative Execution for Early Finality: HotStuff-1 allows replicas to speculatively execute transactions and provide early finality confirmations to clients. This reduces the delay from seven half-phases in HotStuff-2 to three half-phases for the client to receive a finality confirmation. The protocol ensures safety by enforcing a prefix speculation rule which necessitates that the block being speculatively executed extends a committed block.
2. Adaptive Slotting Mechanism: To address leader-related inefficiencies like leader slowness and tail-forking attacks, HotStuff-1 introduces slotting. Each leader is granted multiple slots within its view to propose transactions. Slotting incentivizes rational leaders to propose promptly and helps in evenly distributing the proposal load, ensuring higher throughput and lower latency.

Detailed Design

Speculative Core

The speculative execution mechanism in HotStuff-1 allows replicas to respond with early finality confirmations when a prepare-certificate is generated. This mechanism maintains safety by ensuring that the client collects responses from at least $n-f$ replicas before concluding a transaction as final. The speculative execution is governed by the prefix speculation rule, which specifies that a transaction can be speculatively executed only if it extends a previously committed block, thus avoiding scenarios where speculative execution could violate consensus safety.

Streamlined Design

The streamlined variant of HotStuff-1 integrates the benefits of speculative execution within a single-phase consensus process, simplifying the protocol and potentially increasing throughput by allowing frequent leader proposals. The core principles from the basic variant extend here, with adjustments for the protocol's streamlined nature, ensuring that only the current view's block is speculated upon, and earlier views are already committed.

Adaptive Slotting

Slotting allows leaders to propose multiple consecutive blocks within their view, dynamically adapting to network conditions and rational behaviors of leaders. This mitigates slowness attacks by incentivizing leaders to propose promptly and multiple times, distributing the proposal load more evenly. If a leader attempts to delay, it loses opportunities to propose additional blocks, reducing its potential gains from MEVs (Maximum Extractable Value).

Implications of Research

The practical implications of HotStuff-1 are significant for blockchain and distributed ledger systems employing BFT consensus protocols. By reducing latency and enhancing scalability through linear communication complexity, HotStuff-1 makes BFT consensus more viable for high-performance applications. The introduction of slotting also ensures that leader slowness and tail-forking attacks are mitigated, leading to more robust and resilient systems.

Experimental Results

The experimental evaluation conducted on Apache ResilientDB demonstrates that HotStuff-1 can achieve up to $26.1\%$ lower latency compared to HotStuff-2, and with slotting, it is resilient to leader slowness, showing up to $14.57 \times$ lower latency under specific attack scenarios. The throughput results show that HotStuff-1 maintains comparable throughput to HotStuff and HotStuff-2, with a marginal decrease when slotting is employed due to the additional computational load on the leader.

Future Directions

Future directions for research could explore further optimizations in the slotting mechanism to fully accommodate (optimistic) responsiveness, improve the computational efficiency of prepare-certificates, and investigate the applicability of the prefix speculation rule to other BFT protocols. Additionally, real-world deployment and testing would provide deeper insights into the practical benefits and limitations of HotStuff-1 under diverse network conditions and failure scenarios.

In summary, this paper's contributions to the field lie in reducing latency through speculative execution and enhancing robustness via adaptive slotting, making HotStuff-1 a substantial step forward in the design of efficient and resilient BFT consensus protocols.