Polymer’s Multi-Hop IBC Spec Merges into IBC Repository

Polymer announces multi-hop IBC specification is now merged into the IBC repository.
Polymer Labs
Jan 8, 2024
5 Min.
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TLDR; With the addition of  Polymer's multi-hop specification to the IBC repo, the IBC protocol is armed for expansion to all ecosystems and for aggressive scalability while optimizing network topology.

The multi-hop upgrade simplifies the way IBC channels communicate by significantly reducing the total number of connections required. This allows for more efficient cross-chain connectivity without the need for numerous individual links. As a result, any chain choosing to opt into the IBC network can easily interoperate with all other enabled counterparties via a single plug-in.

The Polymer OP Stack rollup will leverage multi-hop to serve as a middle hub that aggregates these IBC connections, creating more efficient and secure message paths between the expanding Ethereum rollup ecosystem.

Overcoming Direct Chain to Chain Connection Limitations

Before the multi-hop upgrade, the IBC network was designed to grow organically with Cosmos-SDK appchains launching direct 1:1 connections with each other to enable interoperability. The result: a dense network topology with multiple redundant connections (i.e., multiple paths between any two chains) and suboptimal network scalability. 

Source: Map of Zones

A snapshot from Map of Zones shows that any chain currently joining the IBC network is forced to establish a connection with every other chain of interest one by one. Consequently, IBC as an interoperability network has been continuously limited to exponential time complexity.

The idea behind multi-hop is to reduce the number of redundant connections by redesigning IBC topology as a sparse network. The upgrade substantially increases the efficiency of connectivity through a middle hub where any chain can establish a single IBC connection and indirectly access and submit state update requests to any other chain in the network.

Source: The Multi-Hop IBC Upgrade Will Take IBC to Ethereum and Beyond

The sparse IBC network enabled by the multi-hop upgrade reduces interoperability to linear complexity, meaning that the total required connections for the entire network to be interoperable scales linearly with every new chain.

How Multi-Hop Routing Works

The technical approach behind multi-hop IBC leverages the inherent properties of an IBC connection, specifically the ability to store the consensus state of a counterparty chain via light clients. By forming a linked list of connections pointed to a specific chain, relayers can prove the entire series of IBC connections using Merkle inclusion proofs, without requiring additional writes to intermediate hops.

These components are key in understanding how Polymer can serve as the de-facto interoperability rollup for the Ethereum rollup ecosystem.

Relayers in Multi-Hop IBC

Relayers act as the messengers of information along the route from the source to the destination chain. With the multi-hop upgrade to IBC, relayers can use a linked list of an IBC channel’s pre-existing connections to authenticate all intermediate steps. Merkle inclusion proofs verify correctness of the list and eliminate the need for strictly direct chain-to-chain messaging.

In simpler words, multi-hop enables IBC connections to be stored more efficiently, which allows a channel to operate across multiple paths to different chains instead of being stuck to one. As a result, there can be any number of connections in the middle steps between the source and destination chain, and the proving logic will remain the same. In the future, dynamic path aggregation for the most efficient message route is the potential next step.

Streamlining Intermediary Processes

One of the significant advantages of multi-hop is the elimination for the need to maintain additional record-keeping or changes at the intermediate chain hops. By avoiding the complexity of modifying each chain in the hop sequence, the Polymer rollup hub ensures a smoother and more efficient relay process from the source to destination chain.

Maintaining Compatibility and Minimizing Changes

Furthermore, the multi-hop upgrade is meticulously designed to ensure backward compatibility. In other words, multi-hop does not render existing connections and protocols obsolete; instead, it seamlessly integrates with them. The need for minimal changes to the existing IBC specification ensures that the transition to multi-hop is as smooth and disruption-free as possible for all stakeholders.

History Rhymes

The current state of Ethereum rollup interoperability mirrors the dense connectivity and suboptimal scaling strategies used in 1:1 connections. The current process for achieving interoperability between two rollups requires deploying smart contract endpoints of a general message passing (GMP) protocol on both chains. The route from source to destination assumes that the GMP protocol will handle the complete verification of a cross-chain message correctly.

The process is suboptimal for two reasons:

  1. Non-Standardized Transport Layer: as new rollups emerge with different VM types and execution engines, deploying contract endpoints becomes increasingly difficult.
  2. Slow Interoperability Scaling: the current mechanism still requires rollups to use 1:1 connections with each other in order to enable interoperability.

If Ethereum rollups are to form a cohesive ecosystem, a similar redesign of the topology will be required.

Evolving Ethereum Interoperability

As crypto applications improve and develop cost-effective ways to enhance on-chain user experiences, their corresponding infrastructure stacks will trend more and more towards complete modularity. Early evidence can be observed in the quantity of Layer 2 services popping up in the Ethereum ecosystem to provide abstracted execution environments, data availability layers, and ZK-proving systems. And as more Layer 2 rollups enter the market, the average user transaction fee in EVM world will asymptomatically trend towards zero.

Scalability roadmaps are on their way to a big green check mark, but the job isn’t finished until the thousands of Layer 2s that are likely to exist in the future are able to inherit an enshrined interoperability standard. Protocol fragmentation and multi-sig bridging techniques fail to meet the standards for a robust user environment, leaving the Ethereum rollup market ripe for a secure and neutral alternative: IBC.

The Polymer rollup serves as an IBC hub and multi-hop router that makes the entire Ethereum rollup ecosystem look like a tightly coupled mesh network of execution environments, data availability layers, oracle providers, and so forth. Any two Ethereum rollups can establish secure IBC connections by connecting to Polymer, greatly reducing total costs of interoperability. Each IBC connection can also have different security properties. 

Rollups Together Strong

Polymer plays a key role in facilitating the multi-hop routing of IBC packet data, laying the foundation for a potential mesh network of interoperable Ethereum rollups.

In mesh design, a network of securely interconnected rollups is centered around a middle hub that enables any new rollup added to the network to immediately start transmitting messages to existing participants through a single IBC connection to the Polymer hub. The design shifts scalability from an exponential to a linear model, enhancing interoperability in a more manageable and efficient way.

In addition, novel rollup designs combining modular components will benefit from being able to use Polymer to completely abstract away the transport layer. Rollups with modular components, such as an SVM engine in the case of Eclipse, can focus on building fast execution environments while delegating cross-chain processes to a standardized IBC network facilitated by Polymer.

Source: Polymer: Ethereum's Interoperability Hub

The combination of these two factors will significantly improve the scalability of interoperability by enshrining interoperability into the Ethereum ecosystem. 

The multi-hop IBC breakthrough not only enhances the efficiency of data packet transmission across various networks but also paves the way for a seamless, unified rollup landscape. These advancements promise to revolutionize the way we approach rollup interoperability, setting a novel standard for the Ethereum ecosystem.

Polymer Labs is a team of experienced distributed systems / infrastructure engineers, crypto OGs, and skilled business operators building the Ethereum Interoperability Hub, the first blockchain interoperability solution providing Ethereum rollups cross chain connectivity powered by Ethereum security. Polymer is also a contributor to Inter-Blockchain Communication and an active steward of the IBC community, including spearheading IBC Summit and OpenIBC.

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