Specifying Gas & Fee Info
When users submit a TxExecutionRequest on the Aztec Network, they provide a TxContext, which holds GasSettings for the transaction.
An abridged version of the class diagram is shown below:
All fees are denominated in Fee Juice.
Gas Dimensions and Max Inclusion Fee
Transactions are metered for their gas consumption across two dimensions:
- Data Availability (DA) Gas: This dimension measures data usage by the transaction, e.g. creating/spending notes, emitting logs, etc.
- Layer 2 (L2) Gas: This dimension measures computation usage of the public VM.
This is similar to the gas model in Ethereum, where transaction consume gas to perform operations, and may also consume blob gas for storing data.
Separately, every transaction has overhead costs associated with it, e.g. verifying its encompassing rollup proof on L1, which are captured in the maxInclusionFee, which is not tied to gas consumption on the transaction, but is specified in FPA.
See the Fee Schedule for a detailed breakdown of costs associated with different actions.
gasLimits and teardownGasLimits
Transactions can optionally have a "teardown" phase as part of their public execution, during which the "transaction fee" is available to public functions. This is useful to transactions/contracts that need to compute a "refund", e.g. contracts that facilitate fee abstraction.
Because the transaction fee must be known at the time teardown is executed, transactions must effectively "prepay" for the teardown phase. Thus, the teardownGasLimits are portions of the gasLimits that are reserved for the teardown phase.
For example, if a transaction has gasLimits of 1000 DA gas and 2000 L2 gas, and teardownGasLimits of 100 DA gas and 200 L2 gas, then the transaction will be able to consume 900 DA gas and 1800 L2 gas during the main execution phase, but 100 DA gas and 200 L2 gas will be consumed to cover the teardown phase: even if teardown does not consume that much gas, the transaction will still be charged for it; even if the transaction does not have a teardown phase, the gas will still be consumed.
maxFeesPerGas and feePerGas
The maxFeesPerGas field specifies the maximum fees that the user is willing to pay per gas unit consumed in each dimension.
Separately, the protocol specifies the current feePerGas for each dimension, which is used to calculate the transaction fee.
These are held in the L2 blocks Header
A transaction cannot be executed if the maxFeesPerGas is less than the feePerGas for any dimension.
The feePerGas is presently held constant at 1 for both dimensions, but may be updated in future protocol versions.
totalFees is the total fees collected in the block in FPA.
coinbase is the L1 address that receives the fees.
Transaction Fee
The transaction fee is calculated as:
transactionFee = maxInclusionFee + (DA gas consumed * feePerDaGas) + (L2 gas consumed * feePerL2Gas)
Why is the "max" inclusion fee charged? We're working on a mechanism that will allow users to specify a maximum fee they are willing to pay, and the network will only charge them the actual fee. This is not yet implemented, so the "max" fee is always charged.
See more on how the "gas consumed" values are calculated in the Fee Schedule.
Maximum Transaction Fee
The final transaction fee cannot be calculated until all public function execution is complete. However, a maximum theoretical fee can be calculated as:
maxTransactionFee = maxInclusionFee + (gasLimits.daGas * maxFeesPerDaGas) + (gasLimits.l2Gas * maxFeesPerL2Gas)
This is useful for imposing validity conditions.
fee_payer
The fee_payer is the entity that pays the transaction fee.
It is effectively set in private by the contract that calls context.set_as_fee_payer().
This manifests as a boolean flag is_fee_payer in the PrivateCircuitPublicInputs. The private kernel circuits will check this flag for every call stack item.
When a call stack item is found with is_fee_payer set, the kernel circuit will set fee_payer in its PrivateKernelCircuitPublicInputs to be the callContext.contractAddress.
This is subsequently passed through the PublicKernelCircuitPublicInputs to the KernelCircuitPublicInputs.
If the fee_payer is not set, the transaction will be considered invalid.
If a transaction attempts to set fee_payer multiple times, the transaction will be considered invalid.