Blockchain technology's rapid development has brought increasing Cross-Rollup interoperability needs between more and more chains. With more and more blockchain projects deployed on the Ethereum Virtual Machine (EVM), how to design an efficient and secure Cross-Rollup system has become a hot topic in the industry. This article will explore the core components, architecture and workflow of Cross-Rollup systems under the EVM environment combining Vitalik's design ideas, and discuss the latest optimization upgrade proposals.

The Need and Background of Cross-Rollup Systems

With the emergence of various Rollup-as-a-Service (RaaS) projects and technical frameworks such as OP Stack, Arbitrum Orbit, it is becoming increasingly simple to deploy a blockchain based on EVM. According to statistics from L2Beat, there are currently more than 70 layer-2 (L2) chains live, and more than 80 L2s are in the works. As the number of chains increases, the lack of interconnection between chains is becoming more and more serious, and the demand for Cross-Rollup interoperability is becoming more urgent.

In this context, how to build a comprehensive Cross-Rollup system has become a hot issue of concern in the current ecosystem. Vitalik proposed the design of Cross-Rollup systems to meet this demand and aims to build a unified Cross-Rollup system leveraging existing Ethereum Improvement Proposals (EIPs).

Cross-Rollup System Design: Core Components, Architecture, and Optimization

Core Components: Foundation for Building Cross-Rollup Systems

The three core EIPs play a key role in Vitalik's proposed Cross-Rollup system design:

• EIP-3770: Unified EVM address format
  • EIP-3770 has proposed the chain:address format to standardize all EVM addresses. This standardization measure helps prevent discrepancies in referencing the same address across different networks, thereby simplifying user experience and enhancing the efficiency of Cross-Rollup operations.
• EIP-3668: Off-chain data retrieval mechanism
  • EIP-3668 (CCIP-Read) represents a smart contract standard that establishes a secure off-chain data retrieval mechanism. This innovation enables smart contracts to access and validate off-chain data while upholding blockchain determinism. Such a mechanism stands as a critical element in Cross-Rollup systems, facilitating the acquisition of state proofs from the target chain, validation of Cross-Rollup transaction legitimacy, and retrieval of supplementary data essential for Cross-Rollup operations.
• EIP-7683: Standardised Cross-Rollup transaction intent
  • EIP-7683 introduces a standardized approach for describing Cross-Rollup transaction intents, thereby facilitating the establishment of a universal Solver network for Cross-Rollup transactions. Through the uniform format for Cross-Rollup operation descriptions, it streamlines the initiation and execution processes of Cross-Rollup transactions, consequently enhancing the overall usability of Cross-Rollup systems.

Architecture and Workflow: From Design to Practice

Based on the above core components, the overall architecture and workflow of the Cross-Rollup system is shown below:

1. Cross-Rollup Transaction Initiation: The user initiates a Cross-Rollup transaction on ChainA to purchase an NFT on ChainB. Given the user's assets reside solely on ChainA, a designated DApp is essential for this operation.
2. Standardized Address Mapping: The DApp supports EIP-3770, enabling the user to designate a ChainB:Address for receiving the NFT on ChainB.
3. Cross-Rollup Transaction Intent: The user creates an EIP-7683 transaction, delineating the target chain and intent specifics.
4. Off-chain Component Processing: Off-chain components translate the user's intent into traditional transactions, leveraging a Cross-Rollup liquidity provider akin to a Solver to facilitate Cross-Rollup transactions.
5. NFT Transfer and Proof Generation: The Solver procures the NFT on ChainB and dispatches it to the user's specified ChainB:Address. Subsequently, the proof system on ChainB generates the requisite proof.
6. Proof Verification and Asset Settlement: The Solver initiates a settlement process with the DApp, instigating a transaction based on EIP-3668 to acquire the proof generated on ChainB. Once verification is complete, the user's locked assets on ChainA are reimbursed to the Solver.

Optimization and Upgrading: Towards a More Efficient Cross-Rollup System

On top of the infrastructure, Vitalik's Phase 2 optimization upgrade further enhances the performance and security of the Cross-Rollup system.

• The introduction of Keystore Rollup:
  • Keystore Rollup serves as a public component designed to address the silo problem within multi-chain account systems. By deploying on L2 and utilizing Cross-Rollup proofs, it maintains a unified account state globally. This method significantly enhances key management efficiency within Cross-Rollup systems, optimizes the performance of Cross-Rollup account systems, and streamlines the key management process for users.
• Proof Aggregation Techniques:
  • The technology of proof aggregation enables the consolidation of multiple zero-knowledge proofs (ZKPs) into a singular proof. This not only markedly decreases the computational burden of verifying multiple ZKPs but also enhances the system's scalability and privacy protection capabilities. The incorporation of proof aggregation further provides a more efficient and secure solution for Cross-Rollup communication, enabling Cross-Rollup transactions to achieve heightened atomicity and independence from specific chain architectures.

Through these optimizations and upgrades, the Cross-Rollup system has experienced notable enhancements in performance, security, and user experience. By harmonizing standards and protocols across different layers from a top-level design, a consistent and coordinated Cross-Rollup ecosystem is established, with Ethereum L1 serving as the entity responsible for recording the ultimate state and safeguarding system security. Moreover, standardized inter-chain communication promotes consistency in Cross-Rollup operations and message transmission within the EVM environment, with inter-chain communication gradually evolving towards a more generalized zero-knowledge (zk) communication protocol.

Cross-Rollup Solutions in the Current Ecosystem

In addition to Vitalik's proposed "orthodox" Cross-Rollup system design, several components and solutions have emerged in the current EVM ecosystem that strive to offer similar functionalities. The following are some representative projects:

OP Stack Superchain

The Superchain represents a crucial concept within OP Stack, with the objective of linking multiple Rollups to establish a unified, large-scale decentralized network. In the design of the Superchain within the Cross-rollups system, significant improvements in interoperability and communication efficiency among different Rollups are achieved through a unified framework, shared security, atomic transaction mechanism, interoperability protocols, and modular architecture.

Therefore, how does the Superchain achieve interoperability? In essence, it accomplishes this through the implementation of the following four standards:

1. Message Passing Protocol: The Superchain's message passing protocol facilitates the creation and transmission of Cross-Rollup messages. Executing a Cross-Rollup message involves two events: the initiation transaction on the source chain and the execution transaction on the target chain. This messaging mechanism ensures the security and reliability of Cross-Rollup operations.
2. SuperchainERC20 Standard: To ensure the portability and standardization of Cross-Rollup assets, Superchain introduces the SuperchainERC20 standard. This extension, based on the ERC20 standard, enables seamless movement of assets across the various chains within Superchain. Grounded on the message passing protocol, this standard serves as a trust-minimized Cross-Rollup bridge solution.
3. Interoperability Fault Proof: This shared proof system is implemented to guarantee interconnected chains' security within Superchain. This mechanism supports asset management within the Superchain ecosystem.
4. Interoperable Chain Set: Chains within Superchain are structured as a set of interdependent chains that exchange data with each other. For instance, the data dependencies among OP Mainnet, Mode, and Base form an interconnected chain set.

Polygon AggLayer

Polygon AggLayer is dedicated to streamlining and enhancing interactions across Rollups by implementing a unified verification and communication mechanism. Its design objective is to elevate the efficiency and security of Cross-Rollup operations within a multi-chain ecosystem. This initiative introduces increased efficiency and security to asset management in Cross-Rollup applications and multi-chain ecosystems. The implications for the Cross-Rollups ecosystem include:

• Unified State Verification and Aggregation: AggLayer simplifies and enhances cross-Rollup interactions by aggregating the states of multiple Rollups, providing a unified verification layer. This streamlines Cross-Rollup operations, requiring only a single unified verification step and reducing redundancy across Rollups. Consequently, it improves efficiency and mitigates risks of errors during verification processes.
• Simplified Cross-Rollup Communication: Serving as a pivotal link in Cross-Rollup communication, AggLayer acts as a bridge facilitating message passing and data sharing between Rollups through a unified communication interface. This streamlined approach enhances the smoothness and efficiency of interactions among diverse rollups.
• Enhanced Interoperability and Scalability: AggLayer's architecture enables multiple Rollups to interoperate within a unified framework, thereby elevating the overall system's interoperability and scalability. This feature is crucial for future multi-chain ecosystems, as it supports concurrent operation of numerous Rollups while maintaining overall system performance.
• Reduced Complexity of Cross-Rollup Operations: The implementation of AggLayer significantly reduces the complexity associated with Cross-Rollup operations for both developers and users. By offering a unified layer, developers are relieved from managing specific differences across individual Rollups, while users can enjoy a simplified and seamless Cross-Rollup experience.

These characteristics are dependent on their design of Cross-Rollup message delivery methods, which encompass two types of Cross-Rollup message transmission.

1. Optimistic Confirmation: This method enables rapid Cross-Rollup message transmission based on batches and message queues submitted to the AggLayer without the need for proof generation. While it does not ensure transaction validity, it greatly reduces latency and enhances user experience.
2. Atomic Cross-Rollup Interaction: This approach ensures that transactions on multiple chains either succeed simultaneously or fail simultaneously, a critical feature for scenarios such as arbitrage between two chains. AggLayer maintains transaction security by freezing relevant states until verification is completed.

Avail

Avail, as a modular blockchain infrastructure, provides a robust solution for interoperability in the Cross-Rollup ecosystem through its core components (Avail DA, Avail Nexus, and Avail Fusion).

• Avail DA: Avail DA serves as the foundational layer of Avail, dedicated to resolving scalability challenges within Rollup and blockchain domains. By provisioning ample blobspace, it facilitates substantial scalability for Rollup and blockchain systems while upholding security standards. In the realm of Cross-Rollup, Avail DA guarantees that all involved Rollups receive and validate requisite data, thus mitigating transaction failures or delays stemming from data unavailability. Compliant with diverse execution environments such as EVM and WASM, Avail DA enables seamless integration of multiple Rollups, thereby propelling the advancement of the cross-Rollup ecosystem.
  • Implements Erasure coding to fortify data reliability and redundancy
  • Ensures data integrity through KZG polynomial commitments
  • Employs DAS technology to enable lightweight clients to verify the availability of the entire dataset by sampling a limited number of data blocks.
• Avail Nexus: Avail Nexus stands as a pivotal component within the Avail architecture, dedicated to facilitating interoperability among multiple blockchains and Rollups. By managing Cross-Rollup transactions, Avail Nexus empowers users to interact seamlessly across diverse Rollups without the need for manual bridging. Users may operate without explicit awareness that their transactions are mediated through Avail Nexus, thereby streamlining the intricacies of Cross-Rollup operations significantly.
  • Leveraging ZKP technology, Avail Nexus expeditiously verifies and processes Cross-Rollup transactions while upholding transaction privacy and security.
  • Employing staking mechanisms to acquire sequencer thresholds crucial for the selection of transaction order sequencers.
• Avail Fusion: Avail Fusion further fortifies the cohesive layer of the Avail network by implementing a multi-asset staking mechanism, enhancing security for operations spanning across Rollups and ecosystems. Fusion permits native assets from prominent ecosystems such as Bitcoin and Ethereum to be staked in conjunction with Avail's native assets, thereby enhancing the cryptographic economic security of the Cross-Rollup ecosystem. By incorporating these mature crypto assets, Avail not only reinforces its consensus mechanism but also establishes a more secure and dependable operational milieu for Cross-Rollup transactions.

Looking Ahead: The Way Forward for Cross-Rollup Systems

Looking towards the future, the development of Cross-Rollup will progress along two primary directions: interoperability of homogeneous chains and abstract handling of heterogeneous chains. These directions signify the distinct developmental pathways of Cross-Rollup technology in unified chain frameworks and heterogeneous chain interactions.

1. Interoperability between homogeneous chains Within homogeneous chains, interoperability is poised to become more straightforward and efficient. Given their shared or similiar technical frameworks, unified standards and protocols will facilitate seamless communication between chains. Platforms such as OP Stack Superchain, Cosmos SDK, and Polkadot Substrate offer standardized frameworks for blockchain construction and management. These frameworks not only streamline interoperability between chains but also mitigate the complexity of Cross-Rollup operations through standardized development tools and protocols.

2. Abstraction between heterogeneous chains Interoperability between heterogeneous chains presents greater challenges compared to homogeneous chains. These chains typically exhibit diverse technical architectures and consensus mechanisms, necessitating heightened levels of abstraction to facilitate Cross-Rollup interactions. The Shared Sequencer assumes a pivotal role in this context by ensuring equity and uniformity in Cross-Rollup transactions by treating all transactions impartially. Furthermore, with the maturation of technologies such as ZKP, transaction proofs from disparate chains can adhere to standardized protocols, enabling seamless integration of Cross-Rollup operations. This standardized composability will significantly enhance the efficiency and security of Cross-Rollup operations, potentially enabling the development of more intricate Cross-Rollup applications.

Summary

The establishment of a unified and harmonized Cross-Rollup ecosystem holds paramount importance for the future advancement of blockchain technology. By amalgamating Vitalik's foundational design principles with practical solutions from the EVM ecosystem, Cross-Rollup systems have witnessed notable enhancements in performance, security, and user experience. As technology progresses incessantly, we can anticipate the emergence of a more efficient, secure, and interoperable Cross-Rollup ecosystem.