Market-Based Adjusting Cross-Chain Fee System

Wang Pengfei

Content Outline

Overview of the cross-chain fee mechanism

​

Introduce the current common cross-chain fee model and explain their importance.

Market-based �fee adjusting

​

�Describes the cross-chain fee design used by the Darwinia team, and the specific implementation strategy.

​

Combine the �cross-chain model with fee market

�The changes brought about by combining the fee-market mechanism with the cross-chain model.

​

Cross-Chain Message Model

Cross-chain �message process:

• The user sends cross-chain transactions to the source chain �and pays a cross-chain fee. The fee �is calculated by the fee-calculator module of the source chain.
• The bridge monitors the cross-chain message and sends a delivery transaction to the target chain with the cross-chain information.
• Once the cross-chain message has been executed in the target chain, �the bridge needs to send a confirmation transaction to the source chain indicating that the transaction has crossed the chain successfully.

Fee �Calculation

The fees paid by the user must include the bridger expenses:

• The cost of delivering message at the target chain
• The cost of confirming message at the target chain
• Extra reward to incentive bridger at the source chain

Cross-chain fee contain 4 parts:

The cost of sending message

The cost of delivering message

The cost of confirming message

An extra reward to the bridger work

The Challenges

The fee-calculator should take into account the �token conversion between the source and target chains when calculating the cross-chain fees.

The fee-calculator needs to be able to quickly customize the way fees are calculated, since different chains calculate fees differently.

Using a simplified or general method to calculate cross-chain costs. Simplicity and general minimizes the chances of errors.

Design more motivating incentive schemes for bridges. �The better the incentive, the more bridgers will be running, and the more reliable the delivery service �will be.

​

Market-based �fee adjusting

​

Fee �Calculation

• The fee-market module encapsulates the cross-chain fee calculation, and the user only needs to pay the fee generated by the fee-market module.

Cross-chain fee only 1 part:

The cost of sending message

The cost of delivering message

The cost of confirming message

An extra reward to the bridger work

The fee provided by the fee-market module

Fee �Calculation

Fee Market generate �its fees:

• First, the bridgers lock its asset �and submit its own delivery message price(fees).
• Fee market collects bridgers' delivery message prices(fees), then picks one based on specific rules.

Darwinia�Tiered Quotation Market Rule

• Collect fees from bridgers, at least 3, otherwise no market fee can be provided and wait for more bridger to participate.

​

• Select the third one as the market fee.

​

• Selected relayers are responsible for delivering message and will be penalized if failed.

Bridger1 ⇒ Price1

Bridger2 ⇒ Price2

Bridger3 ⇒ Price3

Bridger4 ⇒ Price4

That’s the current market fee

P4 < p3 < p2 < P1

Darwinia fee market rule

Choose the first 3 value

Sort

Combine the cross-chain model with �fee market

• Replace fee-calculator with �Fee market.
• The user pays the cross-chain �fees generated by the fee market.

​

USER

BRIDGER

FEE CALCULATOR

​

MESSAGE QUOTE

EXECUTIVE ENGINE

Pay cross-chain fee

1. Send cross-chain message tx

delivery message

confirm message

FEE MARKET

Cross-chain fee = market fee

Chain A

Chain B

The advantage

The cross-chain fees paid by users are regulated by the market. The bridgers and users constitute a secondary supply-and-demand market, where prices rise when supply is low and fall when supply is abundant.

More scalable, FeeMarket can be customized with more strategies to generate market fees.

​

Changing complex cross-chain cost calculations from on-chain to market behavior, without token conversion.

​

Thank you.