Mysticeti: Reaching the Limits of Latency with Uncertified DAGs (2310.14821v4)

Abstract: We introduce Mysticeti-C, the first DAG-based Byzantine consensus protocol to achieve the lower bounds of latency of 3 message rounds. Since Mysticeti-C is built over DAGs it also achieves high resource efficiency and censorship resistance. Mysticeti-C achieves this latency improvement by avoiding explicit certification of the DAG blocks and by proposing a novel commit rule such that every block can be committed without delays, resulting in optimal latency in the steady state and under crash failures. We further extend Mysticeti-C to Mysticeti-FPC, which incorporates a fast commit path that achieves even lower latency for transferring assets. Unlike prior fast commit path protocols, Mysticeti-FPC minimizes the number of signatures and messages by weaving the fast path transactions into the DAG. This frees up resources, which subsequently result in better performance. We prove the safety and liveness in a Byzantine context. We evaluate both Mysticeti protocols and compare them with state-of-the-art consensus and fast path protocols to demonstrate their low latency and resource efficiency, as well as their more graceful degradation under crash failures. Mysticeti-C is the first Byzantine consensus protocol to achieve WAN latency of 0.5s for consensus commit while simultaneously maintaining state-of-the-art throughput of over 200k TPS. Finally, we report on integrating Mysticeti-C as the consensus protocol into the Sui blockchain, resulting in over 4x latency reduction.

• The paper introduces Mysticeti’s novel DAG-based consensus that achieves sub-second commit times and over 50k TPS in routine operation.
• The paper demonstrates robust fault tolerance using multiple leaders and pipelining, maintaining performance even during node failures.
• The paper presents Mysticeti-FPC, a fast commit variant that processes up to 175k TPS while reducing cryptographic overhead.

Mysticeti: Low-Latency DAG Consensus with Fast Commit Path

The paper "Mysticeti: Low-Latency DAG Consensus with Fast Commit Path" introduces Mysticeti, a family of Byzantine consensus protocols aimed at achieving low-latency and high resource efficiency. This paper is authored by Kushal Babel, Andrey Chursin, George Danezis, Lefteris Kokoris-Kogias, and Alberto Sonnino and discusses the design, implementation, and evaluation of mysticeti in comparison with other state-of-the-art consensus protocols.

Overview

Mysticeti represents a significant advancement in consensus algorithms by leveraging Directed Acyclic Graphs (DAGs) and integrating innovations in pipelining and multiple leaders to reduce latency even under crash failures. There are two main variants of Mysticeti discussed: Mysticeti-C, which is focused on consensus, and Mysticeti-FPC, which incorporates a fast commit path for transactions that can bypass the need for consensus.

Key Innovations

1. DAG-Based Structure: Mysticeti-C utilizes a DAG based on Threshold Clocks to structure the consensus process. This structure ensures scalability in throughput while maintaining low-latency commits.
2. Universal Commit Rule: The universal commit rule in Mysticeti-C extends the commitment functionality to every block rather than only leader blocks, thereby reducing the latency further by permitting immediate commitments.
3. Multiple Leaders and Pipelining: By utilizing multiple leaders and pipelining, Mysticeti-C can maintain low latency and high throughput even when some nodes fail.
4. Mysticeti-FPC: This variant integrates the fast-path protocols to enable low-latency commits (comparable to Zef and FastPay) without the need for extensive cryptographic operations that typically bog down system performance.

Results and Implications

The experimental evaluation demonstrates that Mysticeti achieves substantial performance gains:

• Latency and Throughput: Mysticeti-C consistently achieves sub-second latency under steady-state operation, reaching 0.5 seconds latency for consensus commit at a throughput of over 50k TPS. This surpasses traditional certified DAG-based designs (e.g., Narwhal-HotStuff and Bullshark) in terms of both latency and throughput.
• Fault Tolerance: Under crash failures, Mysticeti-C maintains sub-second latency and significant throughput, contrasting with HotStuff's substantial performance degradation under similar conditions.
• Efficiency of Fast Path: Mysticeti-FPC achieves a throughput of 175k TPS at 0.5s latency, significantly higher than Zef, making it highly resource-efficient.

Practical and Theoretical Implications

Practical:

• Blockchain Integration: Mysticeti's protocols could integrate seamlessly into existing and new blockchain systems. Its efficient resource utilization and low latency make it ideal for high-frequency transaction environments.
• Reduced Validator Overheads: By minimizing cryptographic operations, Mysticeti reduces CPU overhead, making it feasible to run single-worker nodes without compromising performance.

Theoretical:

• Late Commit Rule and Optimal Latency: The universal commit rule and the three-round latency without relying on optimistic paths indicate a departure from traditional latencies seen in protocols like PBFT and Zyzzyva.
• Fast-Unlock Design: The introduction of a simplified and efficient unlock protocol for fast-path transactions offers a novel mechanism for handling transaction conflicts, preserving liveness without imposing extensive computational requirements.

Future Directions

Future research could focus on:

• Adding Worker-Primary Structures: Enhancing Mysticeti with worker-primary structures similar to Narwhal-based designs to explore unlimited scalability in throughput.
• Further Optimizing the Fast Path: Investigating additional optimizations in the fast path for better privacy guarantees and reduced latency.
• Leader Reputation Mechanisms: Adopting leader reputation mechanisms could further stabilize and enhance Mysticeti’s performance by mitigating the effects of unreliable leaders.

Conclusion

Mysticeti offers a comprehensive solution for achieving low-latency and high-throughput consensus in Byzantine settings. Its innovations in DAG-based structures, universal commit rules, and integrated fast-path protocols demonstrate superior performance over state-of-the-art designs. This makes Mysticeti a valuable contribution for practical blockchain systems and theoretical consensus protocol development.