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Fee scheme in Boba Network

Fee Scheme

Fees on Boba are, for the most part, significantly lower than on the Ethereum mainnet. The cost of every transaction is the sum of two values:

  1. Your L2 (execution) fee, and
  2. Your L1 (security) fee.

At a high level, the L2 fee is the cost to execute your transaction in L2 and the L1 fee is the estimated cost to submit your transaction to L1 (in a rollup batch).

  1. L2 execution fee is charged as tx.gasPrice * l2GasUsed (up to tx.gasLimit). The L2 gas price will vary depending on network congestion.

  2. L1 security fee is charged as l1GasPrice * l1GasUsed . This is the cost of storing the transaction's data on L1.

    • l1GasPrice is the same as the normal gas price in L1 Ethereum
    • l1GasUsed is calculated as 1.2*(overhead + calldataGas). Thus, more calldata your transaction includes, the more expensive your L1 fee will be. For example, an ETH transfer has no calldata, so it will have the cheapest L1 fee, whereas large contract deployments can have over 25kb of calldata and will result in a high L1 fee. We currently just the overhead value to the L1 fee to ensure the fee paid covers the actual L1 costs.

  3. Total cost

    We accept BOBA and ETH as the fee token. The total cost for ETH is calculated as tx.gasPrice * (l2GasUsed + l1GasPrice * l1GasUsed / tx.gasPrice). The total cost for BOBA is calculated as tx.gasPrice * (l2GasUsed + l1GasPrice * l1GasUsed / tx.gasPrice) * priceRatio where the price ratio is ETH price / BOBA price * discount percentage. The L2 gas Used is higher than l1, because we add the L1 security fee.

    • The gas uage of transferring ETH is 26730 on Boba Network. It includes 21000 (l2GasUsed) and 5370 (l1SecurityFee).

To obtain ETH and BOBA on Boba Network you can deposit ETH via https://gateway.boba.network on both Rinkeby or Mainnet. Soon you will be able to also deposit ETH for slightly cheaper via Anyswap.

For backend developers

  • You must send your transaction with a tx.gasPrice that is greater than or equal to the sequencer's l2 gas price. You can read this value from the Sequencer by querying the OVM_GasPriceOracle contract (OVM_GasPriceOracle.gasPrice) or by simply making an RPC query to eth_gasPrice. If you don't specify your gasPrice as an override when sending a transaction , ethers by default queries eth_gasPrice which will return the lowest acceptable L2 gas price.
  • You can set your tx.gasLimit however you might normally set it (e.g. via eth_estimateGas). The gas usage for transactions on Boba Network will be larger than the gas usage on Ethereum, becuase the l1SecurityFee is included in the gas usage.
  • We recommend building error handling around the Fee too Low error detailed below, to allow users to re-calculate their tx.gasLimit and resend their transaction if L1 gas price spikes.

For Frontend and Wallet developers

  • We recommend displaying an estimated fee to users using eth_estimateGas
import { ethers } from 'ethers'
const WETH = new Contract(...) //Contract with no signer
const fee = WETH.estimateGas.transfer(to, amount)

  • You should not allow users to change their tx.gasLimit to any number that is smaller than the value from eth_estimateGas.

    • If they lower it, their transaction will revert and tx fee will be charged

  • You should not allow users to change their tx.gasPrice

    • If they lower it, their transaction will revert
    • If they increase it, they will still have their tx immediately included, but will have overpaid.

  • Users are welcome to change their tx.gasLimit as it functions exactly like on L1. You can show the math :

    ETH Fee: .00098 ETH ($3.94)
Boba Fee: 1.96 BOBA ($3.94)

    We recommend displaying the right fee token and transaction fee to users by calling Boba_GasPriceOracle to calculate total fee and get the fee token

    import { ethers } from 'ethers'
const BobaGasPriceOracleInterface = new utils.Interface([
  'function useBobaAsFeeToken()',
  'function useETHAsFeeToken()',
  'function bobaFeeTokenUsers(address) view returns (bool)',
  'function priceRatio() view returns (uint256)'
])
const Proxy__Boba_GasPriceOracle = new Contract(
	Proxy__Boba_GasPriceOracleAddress,
	BobaGasPriceOracleInterface
	l2Provider
) //Contract with no signer
const isBobaAsFeeToken = await Boba_GasPriceOracle.bobaFeeTokenUsers(walletAddress)
const priceRatio = await Boba_GasPriceOracle.priceRatio()

    If isBobaAsFeeToken is true, then the user picks BOBA as the fee token and the total Boba transaction fee is tx.gasLimit * tx.GasPrice * priceRatio.

  • Might need to regularly refresh the L2 Fee estimate to ensure it is accurate at the time the user sends it (e.g. they get the fee quote and leave for 12 hours then come back)

  • Ideas: If the L2 fee quoted is > X minutes old, could display a warning next to it

Common RPC Errors

There are three common errors that would cause your transaction to be rejected

  1. Insufficient funds

    ETH as the fee token

    • If you are trying to send a transaction and you do not have enough ETH to pay for that L2 fee charged, your transaction will be rejected.
    • Error code: -32000
    • Error message: invalid transaction: insufficient funds for l1Fee + l2Fee + value

    BOBA as the fee token

    • If you are trying to send a transaction and you do not have enough BOBA to pay for that L2 fee charged, your transaction will be rejected.
    • Error code: -32000
    • Error message: invalid transaction: insufficient Boba funds for l1Fee + l2Fee + value

  2. Gas Price to low

    • Error code: -32000
    • Error message: gas price too low: 1000 wei, use at least tx.gasPrice = X wei where x is l2GasPrice.
      • Note: values in this error message vary based on the tx sent and current L2 gas prices
    • It is recommended to build in error handling for this. If a user's transaction is rejected at this level, just set a new tx.gasPrice via RPC query at eth_gasPrice or by calling OVM_GasPriceOracle.gasPrice

  3. Fee too large

    • Error code: -32000
    • Error message: gas price too high: 1000000000000000 wei, use at most tx.gasPrice = Y wei where x is 3*l2GasPrice.
    • When the tx.gasPrice provided is ≥3x the expected tx.gasPrice, you will get this error^, note this is a runtime config option and is subject to change