Ethereum Outbound Queue

Sends messages from an origin in the Polkadot ecosystem to Ethereum.

Release

Polkadot SDK Stable 2412

lib.rs:

Pallet for committing outbound messages for delivery to Ethereum

Overview

Messages come either from sibling parachains via XCM, or BridgeHub itself via the snowbridge-pallet-system:

1. snowbridge_router_primitives::outbound::EthereumBlobExporter::deliver
2. snowbridge_pallet_system::Pallet::send

The message submission pipeline works like this:

1. The message is first validated via the implementation for snowbridge_core::outbound::SendMessage::validate
2. The message is then enqueued for later processing via the implementation for snowbridge_core::outbound::SendMessage::deliver
3. The underlying message queue is implemented by Config::MessageQueue
4. The message queue delivers messages back to this pallet via the implementation for frame_support::traits::ProcessMessage::process_message
5. The message is processed in Pallet::do_process_message: a. Assigned a nonce b. ABI-encoded, hashed, and stored in the MessageLeaves vector
6. At the end of the block, a merkle root is constructed from all the leaves in MessageLeaves.
7. This merkle root is inserted into the parachain header as a digest item
8. Offchain relayers are able to relay the message to Ethereum after: a. Generating a merkle proof for the committed message using the prove_message runtime API b. Reading the actual message content from the Messages vector in storage

On the Ethereum side, the message root is ultimately the thing being verified by the Polkadot light client.

Message Priorities

The processing of governance commands can never be halted. This effectively allows us to pause processing of normal user messages while still allowing governance commands to be sent to Ethereum.

Fees

An upfront fee must be paid for delivering a message. This fee covers several components:

1. The weight of processing the message locally
2. The gas refund paid out to relayers for message submission
3. An additional reward paid out to relayers for message submission

Messages are weighed to determine the maximum amount of gas they could consume on Ethereum. Using this upper bound, a final fee can be calculated.

The fee calculation also requires the following parameters:

• Average ETH/DOT exchange rate over some period
• Max fee per unit of gas that bridge is willing to refund relayers for

By design, it is expected that governance should manually update these parameters every few weeks using the set_pricing_parameters extrinsic in the system pallet.

This is an interim measure. Once ETH/DOT liquidity pools are available in the Polkadot network, we'll use them as a source of pricing info, subject to certain safeguards.

Fee Computation Function

LocalFee(Message) = WeightToFee(ProcessMessageWeight(Message))
RemoteFee(Message) = MaxGasRequired(Message) * Params.MaxFeePerGas + Params.Reward
RemoteFeeAdjusted(Message) = Params.Multiplier * (RemoteFee(Message) / Params.Ratio("ETH/DOT"))
Fee(Message) = LocalFee(Message) + RemoteFeeAdjusted(Message)

By design, the computed fee includes a safety factor (the Multiplier) to cover unfavourable fluctuations in the ETH/DOT exchange rate.

Fee Settlement

On the remote side, in the gateway contract, the relayer accrues

Min(GasPrice, Message.MaxFeePerGas) * GasUsed() + Message.Reward

Or in plain english, relayers are refunded for gas consumption, using a price that is a minimum of the actual gas price, or Message.MaxFeePerGas.

Extrinsics

• Call::set_operating_mode: Set the operating mode

Runtime API

• prove_message: Generate a merkle proof for a committed message
• calculate_fee: Calculate the delivery fee for a message