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Jolteon and Ditto: Network-Adaptive Efficient Consensus with Asynchronous Fallback (2106.10362v4)

Published 18 Jun 2021 in cs.DC and cs.CR

Abstract: Existing committee-based Byzantine state machine replication (SMR) protocols, typically deployed in production blockchains, face a clear trade-off: (1) they either achieve linear communication cost in the happy path, but sacrifice liveness during periods of asynchrony, or (2) they are robust (progress with probability one) but pay quadratic communication cost. We believe this trade-off is unwarranted since existing linear protocols still have asymptotic quadratic cost in the worst case. We design Ditto, a Byzantine SMR protocol that enjoys the best of both worlds: optimal communication on and off the happy path (linear and quadratic, respectively) and progress guarantee under asynchrony and DDoS attacks. We achieve this by replacing the view-synchronization of partially synchronous protocols with an asynchronous fallback mechanism at no extra asymptotic cost. Specifically, we start from HotStuff, a state-of-the-art linear protocol, and gradually build Ditto. As a separate contribution and an intermediate step, we design a 2-chain version of HotStuff, Jolteon, which leverages a quadratic view-change mechanism to reduce the latency of the standard 3-chain HotStuff. We implement and experimentally evaluate all our systems. Notably, Jolteon's commit latency outperforms HotStuff by 200-300ms with varying system size. Additionally, Ditto adapts to the network and provides better performance than Jolteon under faulty conditions and better performance than VABA (a state-of-the-art asynchronous protocol) under faultless conditions. This proves our case that breaking the robustness-efficiency trade-off is in the realm of practicality.

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Citations (107)
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Summary

  • The paper introduces innovative protocols, Jolteon and Ditto, that significantly reduce commit latency in consensus mechanisms.
  • The protocols use a 2-chain commit for Jolteon and an asynchronous fallback for Ditto to balance efficiency and network robustness.
  • Experimental results demonstrate a 200-300ms latency improvement and enhanced throughput under network faults compared to HotStuff and VABA.

Jolteon and Ditto: Network-Adaptive Efficient Consensus with Asynchronous Fallback

The paper "Jolteon and Ditto: Network-Adaptive Efficient Consensus with Asynchronous Fallback" presents two innovative Byzantine fault-tolerant (BFT) protocols aimed at improving the performance of consensus mechanisms in blockchain and distributed systems. These protocols, Jolteon and Ditto, address the existing trade-offs in committee-based Byzantine state machine replication (SMR) protocols, particularly focusing on communication complexity and robustness against network asynchrony.

Core Contributions

  1. Jolteon Protocol: This protocol modifies the existing HotStuff protocol by introducing a 2-chain commit rule, thereby reducing the block-commit latency by approximately 30% compared to the original HotStuff's 3-chain commit rule. Jolteon specifically preserves HotStuff's linear communication pattern during good network conditions but accepts a quadratic view-change during network disruptions. This adjustment allows for faster commitment of transactions without sacrificing communication efficiency under optimal conditions.
  2. Ditto Protocol: Building upon Jolteon, Ditto combines the linear communication advantage of HotStuff while ensuring robustness during asynchronous network states. Unlike Jolteon, which relies on a partially synchronous network model, Ditto employs an asynchronous fallback mechanism that takes over when network conditions are non-ideal. The fallback mechanism guarantees progress under asynchrony with quadratic communication cost, aligning with the theoretical cost of acknowledged asynchronous protocols like VABA.

Numerical Outcomes and Evaluation

  • Commit Latency: Jolteon achieves a commit latency improvement of 200-300 milliseconds over HotStuff across different system sizes (10, 20, and 50 nodes) in a wide area network (WAN) deployment.
  • Comparison with VABA: Ditto shows superior performance in faultless conditions compared to VABA, a state-of-the-art asynchronous protocol, demonstrating the practical viability of breaking robustness-efficiency trade-offs.
  • Resilience Under Faulty Conditions: Ditto displays better throughput under faulty conditions and network instability compared to Jolteon and HotStuff, proving its adaptability and resilience.

Theoretical Implications

The paper advances the paper of consensus protocols by proving that it is feasible to coalesce the efficiency of linear protocols with the resilience of quadratic protocols under adverse conditions. Jolteon's design acknowledges the inevitable quadratic cost of worst-case network scenarios and optimizes for latency under normal conditions, thereby lifting a significant limitation in previous work. Ditto's introduction of a seamless switch between synchronous and asynchronous paths without cost overhead reinforces the theoretical possibility of versatile BFT solutions.

Practical Implications

The practical implications of this research are considerable, particularly for blockchain systems requiring both high throughput and robustness against asynchrony and DDoS attacks. The ability to gracefully transition between communication paradigms ensures that systems using Jolteon or Ditto can maintain operational continuity across diverse and unpredictable network conditions. This adaptability benefits critical infrastructures that rely on blockchain technologies, enhancing their reliability and efficiency.

Future Directions

Future research could explore incorporating optimistic 1-chain commits within the Jolteon and Ditto frameworks, potentially further reducing latency in faultless environments. Additionally, it would be worthwhile to investigate the scalability of these protocols across larger and more heterogeneous network architectures, ensuring their applicability in a broader range of real-world scenarios.

In summary, the paper proposes significant advancements in BFT consensus protocols by developing Jolteon and Ditto. These protocols rectify existing inefficiencies and vulnerabilities, paving the way for more robust and efficient blockchain systems suited to handle both synchronous and asynchronous network conditions adeptly.

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