Estimate Fees

Estimate Fees in Mantle v2 Tectonic

Concerning the selection of the gas fee token on layer 2 (L2), Mantle v2 Tectonic departs from the conventional ETH solution adopted by most rollups, and employs MNT instead. This introduces a variable called tokenRatio for dynamic adjustment of gas fees. The overall gas fee calculation is as follows:

$$L2ExecutionFee = L2 gasPrice * L2 gasUsed * tokenRatio$$ $$L1 Rollup Fee = (rollupDataGas + overhead) * L1 gasPrice * tokenRatio * scalar$$

Let's break down the variables involved. Both scalar and overhead are retrieved from the BVM_GasPriceOracle contract, while L1gasPrice can be obtained from layer 1 (L1). tokenRatio is a variable updated through an external Oracle contract, which you can fetch from the BVM_GasPriceOracle contract. Therefore, obtaining L2gasPrice and rollupDataGas allows the calculation of a suitable gasLimit.

info

Why is a suitable gasLimit necessary?

Determining an appropriate gasLimit is crucial. If the gasLimit is set too low, the transaction may fail to execute. If set too high, the computed upfront transaction fee might exceed the user's balance, leading to insufficient funds. Additionally, setting a high gasLimit may discourage validators from including the transaction in a block, as validators tend to prefer including more transactions with lower gasLimit within the fixed block gasLimit. Therefore, estimating the fee is a pivotal step.

How to Obtain Parameters for BVM_GasPriceOracle

overhead

cast call --rpc-url https://rpc.mantle.xyz 0x420000000000000000000000000000000000000F 'overhead()(uint256)'

scalar

cast call --rpc-url https://rpc.mantle.xyz 0x420000000000000000000000000000000000000F 'scalar()(uint256)'

You need to divide the value you get by 1,000,000 to get the true scalar value. For example, if the value you get is 10,000, you need to divide it by 1,000,000 to get 0.01.

tokenRatio

cast call --rpc-url https://rpc.mantle.xyz 0x420000000000000000000000000000000000000F 'tokenRatio()(uint256)'

L1gasPrice

Also known as L1BaseFee.

cast call --rpc-url https://rpc.mantle.xyz 0x420000000000000000000000000000000000000F 'l1BaseFee()(uint256)'

How to Obtain L2 gasPrice

Different transaction types, such as Legacy and EIP-1559 transactions, require distinct handling. For detailed information, please refer to this.

How to Obtain rollupDataGas

It's essential to note that rollupDataGas is not constructed during the estimateGas process in the OP Stack codebase. Thus, an interface is needed to estimate the cost. Mantle addresses this by constructing rollupDataGas and introducing a constant to cover V, R, S signatures, and other data, ensuring that the estimateGas's gasLimit covers L1 costs.

We provide interfaces that return L1GasUsed data directly, i.e., the sum of (rollupDataGas + overhead)

# _data is rlp-encoded data of the signed tx

cast call --rpc-url https://rpc.mantle.xyz 0x420000000000000000000000000000000000000F 'getL1GasUsed(_data)(uint256)'

If you want to learn more, please refer to this.

Ensuring transaction success

estimateGas provides only an estimate. To prevent transaction failure due to insufficient gas fees, Mantle Network internally magnifies the estimate by a factor, ensuring the correct execution of transactions.

Why We Need to Use estimateGas

The cost of fees on L1 is difficult to predict accurately due to the influence of L1 gasPrice, therefore, in Mantle v1, the estimateGas function only returns an estimate of the L2 gas fee. However, in Mantle v2 Tectonic, we have addressed this issue by providing an optimized solution. The estimateGas interface now returns an estimate of the total gas fee, including both L1 and L2 costs. Consequently, when constructing transactions, adjustments need to be made to accommodate this optimization. We encourage all transactions to first call estimateGas to obtain an estimate of the gas fee before construction.

Example

Here is an example illustrating the difference in gasLimit settings for transaction construction between Mantle v1 and Mantle v2 Tectonic.

In Mantle v1, the gasUsed on a typical native token transfer on L2 is a fixed value (21,000), so developers might opt to skip calling estimateGas and set it to a fixed value.

However, in Mantle v2 Tectonic, the gasLimit that developers need to set when constructing transactions will include both L1 and L2 components, so without calling estimateGas and setting it to a fixed value may result in transaction failures.

To avoid transaction failures, we recommend calling estimateGas when constructing transactions to obtain an estimate of the gas fee.