「Understanding Darwinia Bridge 1–4」The New Message Protocol for the Ethereum-Darwinia Bridge

In the layered structure of a cross-chain bridge, an ideal message protocol should be as consensus-agnostic and application-agnostic as possible. In this article, we will introduce our new version of the design of Ethereum-Darwinia bridge in this direction.

Concepts

Source chain and target chain

The source chain is the network from which messages are generated, while the target chain refers to the network where messages are delivered.

Message

An application in the source chain uses a message to call a function with parameters in the target chain. Like a remote procedure call (RPC), a message contains data that identifies the source application, the target application, and a valid encoded that the target application expects to understand.

Channel

A channel is responsible for the unidirectional delivery of messages from the source chain to the target chain, comprising an outbound lane in the source chain and an inbound lane in the target chain. Users or applications wanting to transfer messages to another blockchain must submit the messages to a particular channel. The message layer consists of multiple channels and provides a variety of protections, including replay protection and ordered or unordered delivery of messages.

Relayer

A relayer is an off-chain actor who connects to both the source chain and the target chain and faithfully transfers the data between the source chain and the target chain.

Message Workflow

The message protocol can be implemented with smart contracts or a module in the runtime. It is at the discretion of developers. Since our DVM(Darwinia Virtual Machine) is Ethereum compatible, we have implemented the protocol in smart contracts on both sides of the bridge. We will explain the complete lifecycle of a cross-chain message in conjunction with a time sequence diagram and the data structure of message.

The data structure for message is as follows:

It contains two structs in it, key(MessageKey) and data(MessageData). From its members, we can see that the struct MessageKey uniquely identifies a message in storage. The struct MessageData represents an asynchronous call that a submitter(a user or an application) wants to make on a smart contract in another blockchain.

A message is generated by a smart contract(in the source chain) and then delivered to a smart contract (in the target chain) and eventually processed by this remote smart contract. Theoretically, encoded in the struct MessageData can be anything because the message protocol does not apply any syntactic or format restriction on it. In our design, it encapsulates the data understandable to the smart contract on the receiving end. The receiving smart contract will decode it into the data type it needs. We apply ABI encoding for encoded because it is convenient for EVM to generate and send and the gas fee is low.

Following is the sequence diagram of a complete process of a message delivery from Ethereum to Darwinia.

In the first phase, the message is delivered from Ethereum(the source chain) to Darwinia(the target chain).

  • First, a user or an application in Ethereum(source chain) calls send_message() in the outbound lane to initiate a message to call a Darwinia function. The submitter specifies the preferred channel, the message itself and the fee that they are willing to pay for the message delivery and dispatch;
  • The outbound lane in Ethereum checks and saves the message and emits an event of MessgeAccepted. If the message passes all checks, the nonce is assigned and the message is stored in the contrac storage. The message is now in the accepteded state.
  • Once the relayer notices an accepteded message, it requests Ethereum for the receipt_proofor storage_proof;
  • Upon receiving the message proof, the relayer calls receive_messages_proof() in the inbound lane in Darwinia to append the message. There is a verification of the message proof verify_message_proof() involved here;
  • After the Ethereum light client verifies the message, the inbound lane in Darwinia dispatches it and emits an event of MessageDispatched, and hence the state of the message turns into delivered .

Up till now, the message has been transferred and dispatched successfully. However, the source chain does not know about this. Therefore in the second phase (confirmation), the relayer needs to notify the outbound lane on the Ethereum side.

The confirmation phase is necessary for two reasons:

  1. With it, the number of messages delivered yet not confirmed can be used for flow control;
  2. If there is an incentivization scheme for the relayer’s work, the relayer needs to prove they have actually delivered the message and the proof needs to be accepted by the source chain.

The steps are similar to those in the first phase. The Darwinia light client in Ethereum verifies the proof of message delivery submitted by the relayer, and then the inbound lane emits an event of MessageDelivered. The state of the message is now confirmed.

If the relayer gets any reward for their work, there will be an extra step, payment of the relayer fee. Since the message does not exist any more, rewarded is not a real state of the message. It is just for the convenience of explanation.

But the story does not stop here. The relayer should notify the target chain(Darwinia) that the message delivery is confirmed in the source chain(Ethereum) for the delivery iteration to continue. Otherwise, messages will pile up in the inbound lane, and it will stop accepting new messages. The relayer includes a nonce of the latest confirmed message in next receive_messages_proof() call to achieve this goal.

Above is a message delivery from Ethereum to Darwinia. For a message delivery in the opposite direction (Darwinia to Ethereum), the process is symmetric.

The steps of relay header in the diagram is represented with dashed line because this is not part of the message protocol in a strict sense. However, it is indispensable for the verification of messages.

Discussion

The protocol has no assumptions about relayers incentivization scheme. We are running the relayer nodes on our own at present. However, we have devised a scheme of Relayer Fee Market, which was introduced in another article, to attract more builders to participate in the ecosystem.

Although we take the Ethereum-Darwinia bridge as an example to explain the new protocol, developers can apply this design to implement other cross-chain bridges. As a matter of fact, we use the same scheme in our Substrate-to-Substrate bridges.

The inbound lane can filter received messages with a Cross-Chain Filter interface implemented in the smart contracts on the application level in the target chain. Thus the smart contracts can decide on receiving messages from which chains or which applications.

Related repos

Series Articles

「Understanding Darwinia Bridge 1–1」Darwinia Relayer Incentive Scheme — Fee Market

「Understanding Darwinia Bridge 1–2」Mapping Token Factory

「Understanding Darwinia Bridge 1–3」The Token Bridge Solution

About Darwinia Network

Darwinia Network is a decentralized cross-chain bridge network building on Substrate, which is the “cross-chain bridge hub” of the Web 3.0 Metaverse. It provides a safe and general bridging solution, connects to Polkadot, Ethereum, TRON, and other heterogeneous chains by cross-chain transfer of assets and remote chain calls.

Darwinia Network has gained high reputation and recognition along the way to build the decentralized cross-chain bridge protocol. In 2020, Darwinia was written in Polkadot light-paper as one of the friends of Polkadot and Substrate. And Darwinia was selected to join Substrate Builder Program and Web3.0 Bootcamp, and for the outstanding work in Substrate Builder Program, Darwinia Network was officially awarded the Level 2 badge by Parity. The products and tools developed by Darwinia have been rewarded three W3F Grants.

Darwinia has been contributing to the compatibility and interoperability of the Metaverse.

The application areas of Darwinia Network include DeFi, cross-chain NFT trading, games, etc. Darwinia also develops the Metaverse game Evolution Land.

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As an open cross-chain bridge protocol based on Substrate, Darwinia focuses on the construction of future Internet of Tokens. TG: http://t.me/DarwiniaNetwork