Modern blockchains face a scalability challenge due to the intrinsic throughput limitations of consensus protocols. Layer 2 optimistic rollups (L2) are a faster alternative that offer the same interface in terms of smart contract development and user interaction. Optimistic rollups perform most computations offchain and make light use of an underlying blockchain (L1) to guarantee correct behavior, implementing a cheaper blockchain on a blockchain solution. With optimistic rollups, a sequencer calculates offchain batches of L2 transactions and commits batches (compressed or hashed) to the L1 blockchain. The use of hashes requires a data service to translate hashes into their corresponding batches. Current L2 implementations consist of a centralized sequencer (central authority) and an optional data availability committee (DAC). In this paper, we propose a decentralized L2 optimistic rollup based on Setchain, a decentralized Byzantine-tolerant implementation of sets. The main contribution is a fully decentralized "arranger" where arrangers are a formal definition combining sequencers and DACs. We prove our implementation correct and show empirical evidence that our solution scales. A final contribution is a system of incentives (payments) for servers that implement the sequencer and data availability committee protocols correctly, and a fraud-proof mechanism to detect violations of the protocol.

翻译：现代区块链因共识协议固有的吞吐量限制而面临可扩展性挑战。第二层乐观汇总（L2）是一种更快速的替代方案，其在智能合约开发和用户交互方面提供相同的接口。乐观汇总将大部分计算移至链下执行，并轻量级利用底层区块链（L1）来保证行为正确性，从而实现了“区块链之上的区块链”这一成本更低的解决方案。在乐观汇总机制中，定序器在链下计算L2交易批次，并将批次数据（压缩或哈希处理后）提交至L1区块链。哈希值的使用需要数据服务将哈希值转换回对应的批次数据。当前L2实施方案包含中心化定序器（中央权威机构）及可选的数据可用性委员会（DAC）。本文提出一种基于Setchain的去中心化L2乐观汇总方案——Setchain是一种具备拜占庭容错能力的去中心化集合实现。核心贡献在于构建了完全去中心化的“编排器”架构，其中编排器是统一定序器与数据可用性委员会功能的正式定义。我们证明了实施方案的正确性，并通过实验证据表明该方案具备可扩展性。最终贡献在于建立了一套激励系统（支付机制），用于奖励正确执行定序器和数据可用性委员会协议的服务节点，并设计了可检测协议违规行为的欺诈证明机制。