Transaction fees on L2

Understanding the basics

Transaction fees on Rollux work a lot like fees on Syscoin NEVM or Ethereum. However, Layer 2 introduces some new paradigms that means it can never be exactly like Ethereum. Luckily, Rollux's EVM equivalence makes these differences easy to understand and even easier to handle within your app. Let's take a look at the two sources of cost in a transaction on Rollux: the L2 execution fee and the L1 data/security fee.

The L2 execution fee

Just like on Syscoin NEVM, transactions on Rollux have to pay gas for the amount of computation and storage that they use. Every L2 transaction will pay some execution fee, equal to the amount of gas used by the transaction multiplied by the gas price attached to the transaction. This is exactly how fees work on Ethereum and Syscoin with the added bonus that gas prices on Rollux are seriously low.

Here's the (simple) math:

l2_execution_fee = transaction_gas_price * l2_gas_used

The amount of L2 gas used depends on the particular transaction that you're trying to send. Thanks to EVM equivalence, transactions typically use approximately the same amount of gas on Rollux as they do on Syscoin or Ethereum.

The L1 data fee

Rollux differs from Syscoin NEVM and Ethereum because all transactions on Rollux are also published to Syscoin. This step is crucial to the security properties of Rollux because it means that all of the data you need to sync a Rollux node is always publicly available on Syscoin. It's what makes Rollux an L2.

Users on Rollux have to pay for the cost of submitting their transactions to Syscoin. We call this the L1 data fee, and it's the primary discrepancy between Rollux and Syscoin. Because of BitcoinDA Protocol, the L1 data fee with Rollux is typically very low, in fact comparable to what is paid on Optimism today with Proto-Danksharding (EIP-4844), except secured by Bitcoin's Proof-of-Work.

This fee is based on four factors:

1. The current gas price on Syscoin.
2. The gas cost to publish the transaction through BitcoinDA to the Syscoin UTXO chain.
3. A fixed overhead cost denominated in gas. This is currently set to 2100.
4. A dynamic overhead cost which scales the L1 fee paid by a fixed number. This is currently set to 1.0.

Here's the math:

l1_data_fee = l1_gas_price * (tx_data_gas + fixed_overhead) * dynamic_overhead

Where tx_data_gas is:

tx_data_gas = count_zero_bytes(tx_data) * 4 + count_non_zero_bytes(tx_data) * 16

You can read the parameter values from the gas oracle contract (opens new window).

NOTE

Syscoin has limited support for adding custom transaction types. As a result, unlike the L2 execution fee, users are not able to set limits for the L1 data fee that they may be charged. The L1 gas price used to charge the data fee is automatically updated when new data is received from Syscoin. Changes in gas costs on the Syscoin L1 can mean users pay a higher or lower-than-estimated L1 data fee, by up to 25%. This difference is capped at 25%.

Stuff to keep in mind

Sending transactions

The process of sending a transaction on Rollux is identical to the process of sending a transaction on Syscoin NEVM or Ethereum. When sending a transaction, you should provide a gas price greater than or equal to the current L2 gas price. Like on Ethereum, you can query this gas price with the eth_gasPrice RPC method. Similarly, you should set your transaction gas limit in the same way that you would set your transaction gas limit on Ethereum (e.g. via eth_estimateGas).

Responding to gas price updates

Gas prices on L2 default to 0.001 Gwei but can increase dynamically if the network is congested. When this happens, the lowest fee that the network will accept increases.

Displaying fees to users

Many Ethereum applications display estimated fees to users by multiplying the gas price by the gas limit. However, as discussed earlier, users on Rollux are charged both an L2 execution fee and an L1 data fee. As a result, you should display the sum of both of these fees to give users the most accurate estimate of the total cost of a transaction.

See here for a code sample using the JavaScript SDK (opens new window)

Estimating the L2 execution fee

You can estimate the L2 execution fee by multiplying the gas price by the gas limit, just like on Ethereum.

Estimating the L1 data fee

You can use the SDK (see here) (opens new window). Alternatively, you can estimate the L1 data fee using the GasPriceOracle predeployed smart contract located at 0x420000000000000000000000000000000000000F (opens new window). The GasPriceOracle contract (opens new window) is located at the same address on every Rollux network (mainnet and Tanenbaum testnet). To do so, call GasPriceOracle.getL1Fee(<unsigned RLP encoded transaction>).

Estimating the total fee

You can estimate the total fee by combining your estimates for the L2 execution fee and L1 data fee.

Sending max SYS

Sending the maximum amount of SYS that a user has in their wallet is a relatively common use case. When doing this, you will need to subtract the estimated L2 execution fee and the estimated L1 data fee from the amount of SYS the user wants to send. Use the logic described above for estimating the total fee.

Common RPC Errors

Insufficient funds

• Error code: -32000
• Error message: invalid transaction: insufficient funds for l1Fee + l2Fee + value

You'll get this error when attempting to send a transaction and you don't have enough SYS to pay for the value of the transaction, the L2 execution fee, and the L1 data fee. You might get this error when attempting to send max SYS if you aren't properly accounting for both the L2 execution fee and the L1 data fee.

Gas price too low

• Error code: -32000
• Error message: gas price too low: X wei, use at least tx.gasPrice = Y wei

This is a custom RPC error that Rollux returns when a transaction is rejected because the gas price is too low. See the section on Responding to gas price updates for more information.

Gas price too high

• Error code: -32000
• Error message: gas price too high: X wei, use at most tx.gasPrice = Y wei

This is a custom RPC error that Rollux returns when a transaction is rejected because the gas price is too high. We include this as a safety measure to prevent users from accidentally sending a transaction with an extremely high L2 gas price. See the section on Responding to gas price updates for more information.