L2 Execution Engine

Table of Contents

Overview
Timestamp Activation
Dynamic EIP-1559 Parameters

Overview

The EIP-1559 parameters are encoded in the block header's extraData field and can be configured dynamically through the SystemConfig.

Timestamp Activation

Holocene, like other network upgrades, is activated at a timestamp. Changes to the L2 Block execution rules are applied when the L2 Timestamp >= activation time.

Dynamic EIP-1559 Parameters

EIP-1559 Parameters in Block Header

With the Holocene upgrade, the extraData header field of each block must have the following format:

Name	Type	Byte Offset
`version`	`u8`	`[0, 1)`
`denominator`	`u32 (big-endian)`	`[1, 5)`
`elasticity`	`u32 (big-endian)`	`[5, 9)`

Additionally,

version must be 0
denominator must be non-zero
there is no additional data beyond these 9 bytes

Note that extraData has a maximum capacity of 32 bytes (to fit in the L1 beacon-chain extraData data-type) and its format may be modified/extended by future upgrades.

Note also that if the chain had Holocene genesis, the genesis block must have an above-formatted extraData representing the initial parameters to be used by the chain.

EIP-1559 Parameters in `PayloadAttributesV3`

The PayloadAttributesV3 type is extended with an additional value, eip1559Params:

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
}

Encoding

At and after Holocene activation, eip1559Parameters in PayloadAttributeV3 must be exactly 8 bytes with the following format:

Name	Type	Byte Offset
`denominator`	`u32 (big-endian)`	`[0, 4)`
`elasticity`	`u32 (big-endian)`	`[4, 8)`

PayloadID computation

If eip1559Params != null, the eip1559Params is included in the PayloadID hasher directly after the gasLimit field.

Execution

Payload Attributes Processing

Prior to Holocene activation, eip1559Parameters in PayloadAttributesV3 must be null and is otherwise considered invalid.

At and after Holocene activation, any ExecutionPayload corresponding to some PayloadAttributesV3 must contain extraData formatted as the header value. The denominator and elasticity values within this extraData must correspond to those in eip1559Parameters, unless both are 0. When both are 0, the prior EIP-1559 constants must be used to populate extraData instead.

Base Fee Computation

Prior to the Holocene upgrade, the EIP-1559 denominator and elasticity parameters used to compute the block base fee were constants.

With the Holocene upgrade, these parameters are instead determined as follows:

if Holocene is not active in parent_header.timestamp, the prior EIP-1559 constants are used. Note that parent_header.extraData is empty prior to Holocene, except possibly for the genesis block.
if Holocene is active at parent_header.timestamp, then the parameters from parent_header.extraData are used.

Rationale

Placing the EIP-1559 parameters within the L2 block header allows us to retain 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.

OP Stack Specification