Practical Low Latency Proof of Work Consensus (1909.11261v3)

Abstract: Bitcoin is the first fully-decentralized permissionless blockchain protocol to achieve a high level of security, but at the expense of poor throughput and latency. Scaling the performance of Bitcoin has a been a major recent direction of research. One successful direction of work has involved replacing proof of work (PoW) by proof of stake (PoS). Proposals to scale the performance in the PoW setting itself have focused mostly on parallelizing the mining process, scaling throughput; the few proposals to improve latency have either sacrificed throughput or the latency guarantees involve large constants rendering it practically useless. Our first contribution is to design a new PoW blockchain Prism++ that has provably low latency and high throughput; the design retains the parallel-chain approach espoused in Prism but invents a new confirmation rule to infer the permanency of a block by combining information across the parallel chains. We show security at the level of Bitcoin with very small confirmation latency (a small constant factor of block interarrival time). A key aspect to scaling the performance is to use a large number of parallel chains, which puts significant strain on the system. Our second contribution is the design and evaluation of a practical system to efficiently manage the memory, computation, and I/O imperatives of a large number of parallel chains. Our implementation of Prism++ achieves a throughput of over 80,000 transactions per second and confirmation latency of tens of seconds on networks of up to 900 EC2 Virtual Machines.

• The paper introduces a novel PoW protocol that decouples block roles to improve throughput and reduce confirmation latency.
• It employs multiple voter chains and a unique confirmation rule to achieve over 80,000 transactions per second with confirmation delays of only tens of seconds.
• The security model refines traditional PoW guarantees by aggregating decentralized votes, matching Bitcoin’s security while significantly boosting performance.

Practical Low Latency Proof of Work Consensus: An Overview

This essay discusses the paper "Practical Low Latency Proof of Work Consensus," which introduces Prism++, a blockchain protocol designed to achieve both high throughput and low latency in a proof-of-work (PoW) setting, maintaining similar security levels to Bitcoin. The paper outlines two significant contributions: a novel consensus protocol that fundamentally improves upon the performance limitations of existing PoW systems and a practical system implementation capable of achieving these improvements in contemporary network environments.

Key Contributions and Methodology

The authors identify the latency and throughput bottlenecks in conventional PoW-based consensus mechanisms utilized by protocols such as Bitcoin. The primary limitations stem from the inherent coupling between a single chain's voting process and the consensus on transaction ordering, which mandates relatively slow and deep confirmations to thwart adversarial attacks. Prism++ uses design insights from Prism, a previous protocol, and further innovates through its confirmation rule and system architecture.

Prism++ Design:

1. Block Types and Parallel Chains:
   • Prism++ decouples the various roles of blocks in the Nakamoto consensus by assigning three distinct types: proposer blocks, transaction blocks, and multiple voter chains.
   • This decoupling allows for a significantly higher transaction rate by using separate transaction blocks while employing proposer and voter blocks for maintaining consensus.
2. Novel Confirmation Rule:
   • A key innovation is the Prism++ confirmation rule that ensures low confirmation latency. This rule infers the permanence of a block by consolidating information across multiple voter chains.
   • Security analysis demonstrates that with this rule, blocks can be confirmed much faster (in terms of network delay) compared to traditional confirmation depths required by single-chain methods, without compromising security.
3. Security Model:
   • The paper leverages advancements in security models for blockchains, allowing for a more accurate portrayal of security guarantees in a continuous-time framework over an infinite horizon.
4. System Performance:
   • The protocol achieves a throughput of over 80,000 transactions per second with confirmation latencies of tens of seconds under simulations with up to 900 EC2 instances.

Theoretical and Practical Implications

Theoretical Implications:

• Prism++ contributes to the theoretical understanding of blockchain consensus by explicitly designing its confirmation logic to account for worst-case adversarial strategies. The analysis provides a refined understanding of how multiple voter chains can collectively enhance security.

Practical Implications:

• For practitioners, Prism++ offers an approach adaptable into existing PoW frameworks, potentially transforming scalability prospects for decentralized networks without altering trust assumptions radically.
• The system implementation highlights practical considerations for blockchain client design, emphasizing the role of efficient transaction processing and state management to utilize the protocol's theoretical benefits.

Comparative Performance and Future Directions

Prism++ is tested against other blockchain protocols like Algorand and Bitcoin-NG, showing superior performance in combined throughput and latency. While other protocols achieve improvements by different mechanisms, Prism++ demonstrates that substantial performance gains are achievable within the PoW framework by thoughtful redesign of consensus processing and parallelization.

Future Directions:

• Further research could explore optimizing transaction execution and database operations to push the throughput limits beyond the current state.
• Expanding Prism++'s conceptual framework to other consensus mechanisms like PoS could reveal additional areas for latency and throughput improvements.

In conclusion, Prism++ presents a significant step forward in PoW consensus design, addressing longstanding challenges in blockchain scalability and paving the way for more performant permissionless systems.