Blockchains were originally designed as closed execution environments and lack the ability to communicate directly with external systems. To overcome this limitation, many blockchains employ relayers, external applications capable of transporting data between different blockchains. Typically, the process of relaying data is permissionless and multiple independent relayers work concurrently to transport the same information between two blockchains. While this model increases the reliability of data delivery by providing redundancy, it also introduces challenges that have not been previously discussed. In this work, we bridge this gap by discussing the shortcomings of permissionless cross-chain relaying systems and identifying three issues that adversely impact their performance, scalability and security. We take the first step towards addressing issues that hinder performance and scalability by proposing a novel protocol to enable coordination among independent relayers. Additionally, we provide an in-depth discussion about the trade-offs associated with the design of relayer coordination protocols for permissionless settings. Through this work we provide a foundation for improving cross-chain relaying services.

翻译：区块链最初被设计为封闭的执行环境，缺乏与外部系统直接通信的能力。为克服这一局限，许多区块链采用中继器——一种能够在不同区块链之间传输数据的外部应用。通常情况下，数据中继过程无需许可，多个独立中继器并行工作，在两条区块链间传输相同信息。尽管这种模型通过冗余机制提升了数据传输的可靠性，但也引入了此前未被探讨的挑战。本研究弥补了这一空白，系统阐述了无需许可的跨链中继系统的缺陷，并识别出三个对其性能、可扩展性及安全性产生负面影响的问题。我们首次提出一种新颖协议，通过协调独立中继器来解决阻碍性能与可扩展性的难题。此外，我们深入讨论了无需许可环境中中继器协调协议设计相关的权衡取舍。此项工作为改进跨链中继服务奠定了理论基础。