Amend holocene spec to specify eip-1559 parameters should be in extra…

…Data instead of nonce.

specs/protocol/holocene/exec-engine.md

@@ -14,14 +14,13 @@
   - [Rationale](#rationale)
   - [`eip1559Params` in Header](#eip1559params-in-header)
     - [Header Validity Rules](#header-validity-rules)
-    - [Encoding](#encoding)
     - [Rationale](#rationale-1)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
 ## Overview
 
-The EIP-1559 parameters are encoded in the block header's `nonce` field and can be
+The EIP-1559 parameters are encoded in the block header's `extraData` field and can be
 configured dynamically through the `SystemConfig`.
 
 ## Timestamp Activation
@@ -46,13 +45,13 @@ PayloadAttributesV3: {
     transactions: array of DATA
     noTxPool: bool
     gasLimit: QUANTITY or null
-    eip1559Params: DATA (8 bytes)
+    eip1559Params: DATA (8 bytes) or null
 }
 ```
 
 #### PayloadID computation
 
-If `eip1559Params != null`, the `eip1559Params` is included in the `PayloadID` hasher directly after the `gasLimit`
+If `eip1559Params` != null`, the `eip1559Params` is included in the `PayloadID` hasher directly after the `gasLimit`
 field.
 
 #### `eip1559Params` encoding
@@ -64,19 +63,26 @@ field.
 
 ### Execution
 
-During execution, the EIP-1559 parameters used to calculate the next block base fee should come from the
-parent header's `nonce` field rather than the previous protocol constants, if it is non-zero.
+During execution, the EIP-1559 parameters used to calculate the next block base fee should come from the parent header's
+`extraData` field rather than the previous protocol constants, if it is non-empty.
+
+- Prior to Holocene activation, `eip1559Parameters` in the `PayloadAttributesV3` must be null and is otherwise
+  considered invalid.
 
-- If, before Holocene activation, `eip1559Parameters` in the `PayloadAttributesV3` is non-null, the attributes are to
-  be considered invalid by the engine.
 - At and after Holocene activation:
-  - if `eip1559Params` in the `PayloadAttributesV3` is null, the attributes are to be considered invalid by the
-    engine.
-  - if `eip1559Params`' `denominator` is `0`, the attributes are to be considered invalid by the engine.
-  - if `parent_header.nonce` is zero, the [canyon base fee parameter constants](../exec-engine.md#1559-parameters) are
-    used for the block's base fee parameters.
-  - if `parent_header.nonce` is non-zero, the EIP-1559 parameters encoded within the parent header's `nonce` field are
-    used for the block's base fee parameters.
+  - if `eip1559Params` in the `PayloadAttributesV3` is null, the attributes are considered invalid.
+  - if `eip1559Params`' `denominator` is `0`, the attributes are considered invalid.
+  - if Holocene is not active in `parent_header.timestamp`, the [canyon base fee parameter
+	constants](../exec-engine.md#1559-parameters) or prior EIP-1559 constants are used for the block's base fee
+	parameters. While `parent_header.extraData` is typically empty prior to Holocene, there are some legacy cases where
+	it may be set arbitrarily, so it must not be assumed to be empty.
+  - if Holocene is active in `parent_header.timestamp`, then `parent_header.extraData` is exactly 9 bytes with the
+    following format:
+    - the first byte is set to `0`, used as version byte for the content.
+	- the next 8 bytes are used to encode the 1559 params following the [`eip1559Params` encoding](#eip1559params-encoding).
+
+Note that `extraData` has a maximum capacity of 32 bytes (to fit in the L1 beacon-chain `extraData` data-type) and may
+be modified/extended in future versions.
 
 ### Rationale
 
@@ -87,27 +93,23 @@ how it must reference the `SystemConfig` for the `gasLimit` field.
 
 ### `eip1559Params` in Header
 
-Upon Holocene activation, the L2 block header's `nonce` field will consist of the 8-byte `eip1559Params` value from
-the `PayloadAttributesV3`, or the canyon EIP-1559 constants if `eip1559Params` is equal to zero.
+Upon Holocene activation, the L2 block header's `extraData` field will consist of a version byte with value
+byte(0), followed by an 8-byte `eip1559Params` value from the `PayloadAttributesV3`, or the canyon EIP-1559
+constants if `eip1559Params` is equal to zero.
 
 #### Header Validity Rules
 
-Prior to Holocene activation, the L2 block header's `nonce` field is valid iff it is equal to `u64(0)`.
-
-At and after Holocene activation, The L2 block header's `nonce` field is valid iff it is non-zero.
+Prior to Holocene activation, the L2 block header's `extraData` field is valid iff it is empty.
 
-#### Encoding
-
-The encoding of the `eip1559Params` value is described in [`eip1559Params` encoding](#eip1559params-encoding).
+At and after Holocene activation, The L2 block header's `extraData` field is valid iff it consists of a leading version
+byte with value 0 followed by the eip-1559 parameters encoded as described in [`eip1559Params`
+encoding](#eip1559params-encoding).
 
 #### Rationale
 
-By chosing to put the `eip1559Params` in the `PayloadAttributes` rather than in the L1 block info transaction,
-the EIP-1559 parameters for the chain are not available within history. This would place a burden on performing
-historical execution, as L1 would have to be consulted for fetching the values from the `SystemConfig` contract.
-Instead, we re-use an unused field in the L1 block header as to make these parameters available, retaining the
-purity of the function that computes the next block's base fee from the chain configuration, parent block header,
-and next block timestamp.
-
-[l2-to-l1-mp]: ../../protocol/predeploys.md#L2ToL1MessagePasser
-[output-root]: ../../glossary.md#l2-output-root
+By choosing to put the `eip1559Params` in the `PayloadAttributes` rather than in the L1 block info transaction, the
+EIP-1559 parameters for the chain are not available within history. This would place a burden on performing historical
+execution, as L1 would have to be consulted for fetching the values from the `SystemConfig` contract.  Instead, we
+re-use an unused field in the L2 block header as to make these parameters available, retaining the purity of the
+function that computes the next block's base fee from the chain configuration, parent block header, and next block
+timestamp.

specs/protocol/holocene/system-config.md

@@ -27,10 +27,12 @@ The following `ConfigUpdate` event is defined where the `CONFIG_VERSION` is `uin
 | Name | Value | Definition | Usage |
 | ---- | ----- | --- | -- |
 | `BATCHER` | `uint8(0)` | `abi.encode(address)` | Modifies the account that is authorized to progress the safe chain |
-| `FEE_SCALARS` | `uint8(1)` | `(uint256(0x01) << 248) \| (uint256(_blobbasefeeScalar) << 32) \| _basefeeScalar` | Modifies the fee scalars |
+| `FEE_SCALARS` | `uint8(1)` | `abi.encode(uint256(0), (uint256(0x01) << 248) \| (uint256(_blobbasefeeScalar) << 32) \| _basefeeScalar)` | Modifies the fee scalars |
 | `GAS_LIMIT` | `uint8(2)` | `abi.encode(uint64 _gasLimit)` | Modifies the L2 gas limit |
 | `UNSAFE_BLOCK_SIGNER` | `uint8(3)` | `abi.encode(address)` | Modifies the account that is authorized to progress the unsafe chain |
-| `EIP_1559_PARAMS` | `uint8(4)` | `(uint256(_denominator) << 32) \| _elasticity` | Modifies the EIP-1559 denominator and elasticity |
+| `EIP_1559_PARAMS` | `uint8(4)` | `abi.encode((uint256(_denominator) << 32) \| _elasticity)` | Modifies the EIP-1559 denominator and elasticity |
+
+Note that the above encoding is the format emitted by the SystemConfig event, which differs from the format in extraData from the block header.
 
 ### Initialization