XCMP (Cross-Chain Message Passing)

Overview

In a scenario where ParaA wants to send a message to ParaB (ParaA -> ParaB), various steps and data structures come into play.

Goals with this Repo

• Better understand an efficient implementations of different XCMP approaches(proof sizes, overall onchain benchmarking, code complexity)
• Achieve consensus on an XCMP design and approach to put forward to the community (By showing pros and cons of a few different approaches)

Approach

The proofs which are constructed via a Relayer involve a 4 tiered authenticated datastructure using 2 Binary Merkle Trees and 2 MMRs.

ChannelMessageMmr

• This is one of many MMRs which XCMP messages are stored into. There is a single XCMP message per block so each index can be thought of as the message contents for a particular block.

XCMPChannelBinaryMerkleTree Contents

• MMRab root, MMRac root, etc.: Each receipient which a sender has an open channel with has a dedicated ChannelMessageMmr. This each one of these ChannelMessageMmrRoots are bundled up into a Binary Merkle Tree whos root is put into the Digest of each Parachains BlockHeader

Beefy MMR

• Every Leaf of the Beefy Mmr contains a committment every parachains block header in the form of a BinaryMerkleRoot

Combining

By combining these 4 tiers of Merkle structures together we have a proof which from a single message commitment on some parachain sender X we can compare it against another parachain receiver Y. This requires a latest Beefy Root (Which lives on the Polkadot Relaychain) to be inserted into each parachains state for proof verification.

Insert Diagram of 4 tiered merkle structure

Flow of Messages

ParaA(Sender):

• Sends an XCM message as usual.
• Adds the message as a leaf into one of its message XcmpChannelMessageMmrs.
• The message MMR root gets stored inside the XCMPChannelBinaryMerkleTree.

Relayer (Having full nodes of ParaA, ParaB, Relaychain):

1. Detection of Destination:

   • Relayer module detects a message from ParaA to ParaB is sent.

2. Proof Construction:

   • Constructs 4 tiered MerkleProof

3. Proof Submission:

   • Submits proof to ParaB's XCMP extrinsic labeled submit_xcmp_proof(proof, channel_id).

ParaB(Receiver):

Proof Verification:

• Because receipient has the Beefy Mmr Root(The top of the Merkle Path) it can unravel the proof and compare against this root.

Incentivization of Relayer:

1. Any parachain that opens a channel with another parachain could potentially run relayer nodes.
2. The XCM message sender provides a percentage fee to the relayer.

Building:

Clone

git clone [email protected]:w3f/xcmp_prototype_playground.git && cd xcmp_prototype_playground

Run Build script

chmod +x build.sh && ./build.sh

Clone and compile current XCMP supported Polkadot

cd ../ && \
git clone https://github.com/coax1d/polkadot-sdk/tree/xcmp_customized_sdk && \
cd polkadot-sdk && \
cargo build --release -p polkadot

Move all necessary Polkadot binaries to bin directory with other collator binaries

cd ../xcmp_prototype_playground && \
cp ../polkadot-sdk/target/release/polkadot bin/polkadot && \
cp ../polkadot-sdk/target/release/polkadot-execute-worker bin/polkadot-execute-worker && \
cp ../polkadot-sdk/target/release/polkadot-prepare-worker bin/polkadot-prepare-worker

Running Entire setup E2E:

Open Terminal

cd xcmp_prototype_playground

Run zombienet

zombienet-macos spawn -p native zombienet/config.toml

Open new terminal wait ~ 1 minute for collators to be onboarded to Relaychain

./target/release/xcmp_relayer