*Optimism Governance commissioned this report to provide an overview of current cross-chain interoperability solutions, their relevance to the Optimism Superchain, and recommendations for future development.

*This is an edited version of the "Cross-Chain Interoperability Report." Read the full report here.

Key Takeaways

When we talk about cross-chain interoperability, we are talking about the ability to transmit information between blockchains, and therefore also let actions in one blockchain trigger actions on another blockchain. We will often use the term "message" to refer to a generic piece of information.

For the most part, cross-chain interoperability is concerned with bridging messages and assets. We depart from some connotations of "bridging" by not confining ourselves to the app layer, as we also consider chain-level architecture possibilities — for instance, rollup sequencing.

While we talk about "cross-chain interoperability", the real goal of this report is to uncover possibilities for cross-rollup interoperability. In the process, we will however say a good deal that applies to any kind of chain. Similarly, we will use the classical architecture of EVM chains as a basis for conversation and nomenclature, though a lot of the discussion generalizes.

We note that this report is not meant to be a comparison of existing solutions, though some are mentionned. Instead, we try to abstract away from specific implementations and focus on the general solutions and their properties.

1. Intro: Superchain and Interoperability

1.1 Interoperability in Optimism Superchain

In considering the interoperability of two rollup systems, it is important to consider their relationship. For instance, is the interoperability possible permissionlessly, or does it require collaboration (or even software changes) from one or both sides?

The Optimism Superchain is "A horizontally scalable network of chains that share security, a communication layer, and an open source development stack." OP Labs has unveiled some of its plans for cross-rollup interoperability, but many avenues remain open in the future.

Source: Superchain Ecosystem | Chains

In our report, we will consider three models:

• The small model, where the inclusion of a chain in the Superchain is a permissioned process, that requires a number of guarantees from the candidate chain. This seems to be the current "social" model, embodied by The Law of Chains.
• The large model, where any chain can join the Superchain, with minimal oversight and overhead.