This design evolved from Design #1 along with feedback from Parity at the XCM workshop.

Summary

• Unordered messaging
• All messages routed through AH
• Off-chain fee estimation
• P→E Fee Asset: WETH
• E→P Fee Asset: ETH
• Relayer rewards for both directions paid out on AH in WETH

Polkadot→Ethereum

Given source parachain $S$, with native token $S^{'}$ and the initial xcm $x_0$ to be executed on $S$.

Step 1: User agent constructs initial XCM

The user agent constructs an initial XCM message $x_0$ that will be executed on S.

The fee amounts in this message should be high enough to enable dry-running, after which they will be lowered.

Step 2: User agent estimates fees

• Given source parachain $S$, with native token $S^{'}$ and the initial xcm $x_0$ to be executed on $S$.
• The native currency $P^{'}$of the Polkadot relay chain, and $E^{'}$ of Ethereum.
• Suppose that the user agent chooses relayer reward $r$ in $E^{'}$.
• Suppose that the exchange rates $K_{P^{'}/S^{'}}$ and $K_{E^{'}/S^{'}}$. The user agent chooses a multiplier to $\beta$ to cover volatility in these rates.