Jovian: Execution Engine

Table of Contents

• Minimum Base Fee
  • Minimum Base Fee in Block Header
  • Minimum Base Fee in PayloadAttributesV3
  • Rationale
• DA Footprint Block Limit
  • Scalar loading
  • Rationale
• Operator Fee
  • Fee Formula Update
  • Maximum value

Minimum Base Fee

Jovian introduces a configurable minimum base fee to reduce the duration of priority-fee auctions on OP Stack chains.

The minimum base fee is configured via SystemConfig (see ./system-config.md) and enforced by the execution engine via the block header extraData encoding and the Engine API PayloadAttributesV3 parameters.

Minimum Base Fee in Block Header

Like Holocene's dynamic EIP-1559 parameters, Jovian encodes fee parameters in the extraData field of each L2 block header. The format is extended to include an additional u64 field for the minimum base fee in wei.

Constraints:

• version MUST be 1 (incremented from Holocene's 0).
• There MUST NOT be any data beyond these 17 bytes.

The minBaseFee field is an absolute minimum expressed in wei. During base fee computation, if the computed baseFee is less than minBaseFee, it MUST be clamped to minBaseFee.

if (baseFee < minBaseFee) {
  baseFee = minBaseFee
}

Note: extraData has a maximum capacity of 32 bytes (to fit the L1 beacon-chain extraData type) and may be extended by future upgrades.

Minimum Base Fee in PayloadAttributesV3

The Engine API PayloadAttributesV3 is extended with a new field minBaseFee. The existing eip1559Params remains 8 bytes (Holocene format).

PayloadAttributesV3: {
    timestamp: QUANTITY
    prevRandao: DATA (32 bytes)
    suggestedFeeRecipient: DATA (20 bytes)
    withdrawals: array of WithdrawalV1
    parentBeaconBlockRoot: DATA (32 bytes)
    transactions: array of DATA
    noTxPool: bool
    gasLimit: QUANTITY or null
    eip1559Params: DATA (8 bytes) or null
    minBaseFee: QUANTITY or null
}

The minBaseFee MUST be null prior to the Jovian fork, and MUST be non-null after the Jovian fork.

Rationale

As with Holocene's dynamic EIP-1559 parameters, placing the minimum base fee in the block header allows us to avoid reaching into the state during block sealing. This retains the purity of the function that computes the next block's base fee from its parent block header, while still allowing them to be dynamically configured. Dynamic configuration is handled similarly to gasLimit, with the derivation pipeline providing the appropriate SystemConfig contract values to the block builder via PayloadAttributesV3 parameters.

DA Footprint Block Limit

A DA footprint block limit is introduced to limit the total amount of estimated compressed transaction data that can fit into a block. For each transaction, a new resource called DA footprint is tracked, next to its gas usage. It is scaled to the gas dimension so that its block total can also be limited by the block gas limit, like a block's total gas usage.

Let a block's daFootprint be defined as follows:

def daFootprint(block: Block) -> int:
  daFootprint = 0

  for tx in block.transactions:
      if tx.type == DEPOSIT_TX_TYPE:
          continue

      daUsageEstimate = max(
          minTransactionSize,
          (intercept + fastlzCoef * tx.fastlzSize) // 1e6
      )
      daFootprint += daUsageEstimate * daFootprintGasScalar

  return daFootprint 

where intercept, minTransactionSize, fastlzCoef and fastlzSize are defined in the Fjord specs, DEPOSIT_TX_TYPE is 0x7E, and // represents integer floor division.

From Jovian, the blobGasUsed property of each block header is set to that block's daFootprint. Note that pre-Jovian, since Ecotone, it was set to 0, as OP Stack chains don't support blobs. It is now repurposed to store the DA footprint.

During block building and header validation, it must be guaranteed and checked, respectively, that the block's daFootprint stays below the gasLimit, just like the gasUsed property. Note that this implies that blocks may have no more than gasLimit/daFootprintGasScalar total estimated DA usage bytes.

Furthermore, from Jovian, the base fee update calculation now uses gasMetered := max(gasUsed, blobGasUsed) in place of the gasUsed value used before. As a result, blocks with high DA usage may cause the base fee to increase in subsequent blocks.

Scalar loading

The daFootprintGasScalar is loaded in a similar way to the operatorFeeScalar and operatorFeeConstant included in the Isthmus fork. It can be read in two interchangable ways:

• read from the deposited L1 attributes (daFootprintGasScalar) of the current L2 block (decoded according to the jovian schema)
• read from the L1 Block Info contract (0x4200000000000000000000000000000000000015)
  • using the solidity getter function daFootprintGasScalar
  • using a direct storage-read: big-endian uint16 in slot 8 at offset 12.

It takes on a default value as described in the section on L1 Attributes.

Rationale

While the current L1 fee mechanism charges for DA usage based on an estimate of the DA footprint of a transaction, no protocol mechanism currently reflects the limited available DA throughput on L1. E.g. on Ethereum L1 with Pectra enabled, the available blob throughput is ~96 kB/s (with a target of ~64 kB/s), but the calldata floor gas price of 40 for calldata-heavy L2 transactions allows for more incompressible transaction data to be included on most OP Stack chains than the Ethereum blob space could handle. This is currently mitigated at the policy level by batcher-sequencer throttling: a mechanism which artificially constricts block building. This can cause base fees to fall, which implies unnecessary losses for chain operators and a negative user experience (transaction inclusion delays, priority fee auctions). So hard-limiting a block's DA footprint in a way that also influences the base fee mitigates the aforementioned problems of policy-based solutions.

Operator Fee

Fee Formula Update

Jovian updates the operator fee calculation so that higher fees may be charged. Starting at the Jovian activation, the operator fee MUST be computed as:

$$\text{operatorFee}=(\text{gas}\times \text{operatorFeeScalar}\times 100)+\text{operatorFeeConstant}$$

The effective per-gas scalar applied is therefore 100 * operatorFeeScalar. Otherwise, the data types and operator fee semantics described in the Isthmus spec continue to apply.

Maximum value

With the new formula, the operator fee's maximum value has 103 bits:

$${\text{operatorFee}}_{\text{max}}=({\text{uint64}}_{\text{max}}\times {\text{uint32}}_{\text{max}}\times 100)+{\text{uint64}}_{\text{max}}\approx 7.924660923989131\times 10^{30}$$

Implementations that use uint256 for intermediate arithmetic do not need additional overflow checks.