specification.md

Engine API

This document specifies the Engine API methods that the Consensus Layer uses to interact with the Execution Layer.

Table of contents

• Underlying protocol
• Versioning
• Message ordering
• Load-balancing and advanced configurations
• Errors
• Structures
  • ExecutionPayloadV1
  • ForkchoiceStateV1
  • PayloadAttributesV1
  • PayloadStatusV1
  • TransitionConfigurationV1
• Routines
  • Payload validation
  • Sync
  • Payload building
• Core
  • engine_newPayloadV1
    • Request
    • Response
    • Specification
  • engine_forkchoiceUpdatedV1
    • Request
    • Response
    • Specification
  • engine_getPayloadV1
    • Request
    • Response
    • Specification
  • engine_exchangeTransitionConfigurationV1
    • Request
    • Response
    • Specification

Underlying protocol

This specification is based on Ethereum JSON-RPC API and inherits the following properties of this standard:

• Supported communication protocols (HTTP and WebSocket)
• Message format and encoding notation
• Error codes improvement proposal

Client software MUST expose Engine API at a port independent from JSON-RPC API. The default port for the Engine API is 8550. The Engine API is exposed under the engine namespace.

To facilitate an Engine API consumer to access state and logs (e.g. proof-of-stake deposits) through the same connection, the client MUST also expose the eth namespace.

Versioning

The versioning of the Engine API is defined as follows:

• The version of each method and structure is independent from versions of other methods and structures.
• The VX, where the X is the number of the version, is suffixed to the name of each method and structure.
• The version of a method or a structure MUST be incremented by one if any of the following is changed:
  • a set of method parameters
  • a method response value
  • a method behavior
  • a set of structure fields
• The specification MAY reference a method or a structure without the version suffix e.g. engine_newPayload. These statements should be read as related to all versions of the referenced method or structure.

Message ordering

Consensus Layer client software MUST respect the order of the corresponding fork choice update events when making calls to the engine_forkchoiceUpdated method.

Execution Layer client software MUST process engine_forkchoiceUpdated method calls in the same order as they have been received.

Load-balancing and advanced configurations

The Engine API supports a one-to-many Consensus Layer to Execution Layer configuration. Intuitively this is because the Consensus Layer drives the Execution Layer and thus can drive many of them independently.

On the other hand, generic many-to-one Consensus Layer to Execution Layer configurations are not supported out-of-the-box. The Execution Layer, by default, only supports one chain head at a time and thus has undefined behavior when multiple Consensus Layers simultaneously control the head. The Engine API does work properly, if in such a many-to-one configuration, only one Consensus Layer instantiation is able to write to the Execution Layer's chain head and initiate the payload build process (i.e. call engine_forkchoiceUpdated ), while other Consensus Layers can only safely insert payloads (i.e. engine_newPayload) and read from the Execution Layer.

Errors

The list of error codes introduced by this specification can be found below.

Code	Message	Meaning
-32700	Parse error	Invalid JSON was received by the server.
-32600	Invalid Request	The JSON sent is not a valid Request object.
-32601	Method not found	The method does not exist / is not available.
-32602	Invalid params	Invalid method parameter(s).
-32603	Internal error	Internal JSON-RPC error.
-32000	Server error	Generic client error while processing request.
-32001	Unknown payload	Payload does not exist / is not available.

Each error returns a null data value, except -32000 which returns the data object with a err member that explains the error encountered.

For example:

$ curl https://localhost:8550 \
    -X POST \
    -H "Content-Type: application/json" \
    -d '{"jsonrpc":"2.0","method":"engine_getPayloadV1","params": ["0x1"],"id":1}'
{
  "jsonrpc": "2.0",
  "id": 1,
  "error": {
    "code": -32000,
    "message": "Server error",
    "data": {
        "err": "Database corrupted"
    }
  }
}

Structures

Fields having DATA and QUANTITY types MUST be encoded according to the HEX value encoding section of Ethereum JSON-RPC API.

Note: Byte order of encoded value having QUANTITY type is big-endian.

ExecutionPayloadV1

This structure maps on the ExecutionPayload structure of the beacon chain spec. The fields are encoded as follows:

• parentHash: DATA, 32 Bytes
• feeRecipient: DATA, 20 Bytes
• stateRoot: DATA, 32 Bytes
• receiptsRoot: DATA, 32 Bytes
• logsBloom: DATA, 256 Bytes
• prevRandao: DATA, 32 Bytes
• blockNumber: QUANTITY, 64 Bits
• gasLimit: QUANTITY, 64 Bits
• gasUsed: QUANTITY, 64 Bits
• timestamp: QUANTITY, 64 Bits
• extraData: DATA, 0 to 32 Bytes
• baseFeePerGas: QUANTITY, 256 Bits
• blockHash: DATA, 32 Bytes
• transactions: Array of DATA - Array of transaction objects, each object is a byte list (DATA) representing TransactionType || TransactionPayload or LegacyTransaction as defined in EIP-2718

ForkchoiceStateV1

This structure encapsulates the fork choice state. The fields are encoded as follows:

• headBlockHash: DATA, 32 Bytes - block hash of the head of the canonical chain
• safeBlockHash: DATA, 32 Bytes - the "safe" block hash of the canonical chain under certain synchrony and honesty assumptions. This value MUST be either equal to or an ancestor of headBlockHash
• finalizedBlockHash: DATA, 32 Bytes - block hash of the most recent finalized block

PayloadAttributesV1

This structure contains the attributes required to initiate a payload build process in the context of an engine_forkchoiceUpdated call. The fields are encoded as follows:

• timestamp: QUANTITY, 64 Bits - value for the timestamp field of the new payload
• prevRandao: DATA, 32 Bytes - value for the prevRandao field of the new payload
• suggestedFeeRecipient: DATA, 20 Bytes - suggested value for the feeRecipient field of the new payload

PayloadStatusV1

This structure contains the result of processing a payload. The fields are encoded as follows:

• status: enum - "VALID" | "INVALID" | "SYNCING" | "ACCEPTED" | "INVALID_BLOCK_HASH" | "INVALID_TERMINAL_BLOCK"
• latestValidHash: DATA|null, 32 Bytes - the hash of the most recent valid block in the branch defined by payload and its ancestors
• validationError: String|null - a message providing additional details on the validation error if the payload is classified as INVALID, INVALID_BLOCK_HASH or INVALID_TERMINAL_BLOCK.

TransitionConfigurationV1

This structure contains configurable settings of the transition process. The fields are encoded as follows:

• terminalTotalDifficulty: QUANTITY, 256 Bits - maps on the TERMINAL_TOTAL_DIFFICULTY parameter of EIP-3675
• terminalBlockHash: DATA, 32 Bytes - maps on TERMINAL_BLOCK_HASH parameter of EIP-3675
• terminalBlockNumber: QUANTITY, 64 Bits - maps on TERMINAL_BLOCK_NUMBER parameter of EIP-3675

Routines

Payload validation

Payload validation process consists of validating a payload with respect to the block header and execution environment rule sets. The process is specified as follows:

1. Client software MAY obtain a parent state by executing ancestors of a payload as a part of the validation process. In this case each ancestor MUST also pass payload validation process.

2. Client software MUST validate that the most recent PoW block in the chain of a payload ancestors satisfies terminal block conditions according to EIP-3675. This check maps to the transition block validity section of the EIP. If this validation fails, the response MUST contain {status: INVALID_TERMINAL_BLOCK, latestValidHash: null}. Additionally, each block in a tree of descendants of an invalid terminal block MUST be deemed INVALID.

3. Client software MUST validate a payload according to the block header and execution environment rule set with modifications to these rule sets defined in the Block Validity section of EIP-3675:

• If validation succeeds, the response MUST contain {status: VALID, latestValidHash: payload.blockHash}
• If validation fails, the response MUST contain {status: INVALID, latestValidHash: validHash} where validHash is the block hash of the most recent valid ancestor of the invalid payload. That is, the valid ancestor of the payload with the highest blockNumber
• Client software MUST NOT surface an INVALID payload over any API endpoint and p2p interface.

4. Client software MAY provide additional details on the validation error if a payload is deemed INVALID by assigning the corresponding message to the validationError field.

5. The process of validating a payload on the canonical chain MUST NOT be affected by an active sync process on a side branch of the block tree. For example, if side branch B is SYNCING but the requisite data for validating a payload from canonical branch A is available, client software MUST run full validation of the payload and respond accordingly.

Sync

In the context of this specification, the sync is understood as the process of obtaining data required to validate a payload. The sync process may consist of the following stages:

1. Pulling data from remote peers in the network.
2. Passing ancestors of a payload through the Payload validation and obtaining a parent state.

Note: Each of these stages is optional. Exact behavior of client software during the sync process is implementation dependent.

Payload building

The payload build process is specified as follows:

1. Client software MUST set the payload field values according to the set of parameters passed into this method with exception of the suggestedFeeRecipient. The built ExecutionPayload MAY deviate the feeRecipient field value from what is specified by the suggestedFeeRecipient parameter.

2. Client software SHOULD build the initial version of the payload which has an empty transaction set.

3. Client software SHOULD start the process of updating the payload. The strategy of this process is implementation dependent. The default strategy is to keep the transaction set up-to-date with the state of local mempool.

4. Client software SHOULD stop the updating process when either a call to engine_getPayload with the build process's payloadId is made or SECONDS_PER_SLOT (12s in the Mainnet configuration) have passed since the point in time identified by the timestamp parameter.

Core

engine_newPayloadV1

Request

• method: engine_newPayloadV1
• params:
  1. ExecutionPayloadV1

Response

• result: PayloadStatusV1
• error: code and message set in case an exception happens while processing the payload.

Specification

1. Client software MUST validate blockHash value as being equivalent to Keccak256(RLP(ExecutionBlockHeader)), where ExecutionBlockHeader is the execution layer block header (the former PoW block header structure). Fields of this object are set to the corresponding payload values and constant values according to the Block structure section of EIP-3675, extended with the corresponding section of EIP-4399. Client software MUST run this validation in all cases even if this branch or any other branches of the block tree are in an active sync process.

2. Client software MAY initiate a sync process if requisite data for payload validation is missing. Sync process is specified in the Sync section.

3. Client software MUST validate the payload if it extends the canonical chain and requisite data for the validation is locally available. The validation process is specified in the Payload validation section.

4. Client software MAY NOT validate the payload if the payload doesn't belong to the canonical chain.

5. Client software MUST respond to this method call in the following way:

• {status: INVALID_BLOCK_HASH, latestValidHash: null, validationError: errorMessage | null} if the blockHash validation has failed
• {status: INVALID_TERMINAL_BLOCK, latestValidHash: null, validationError: errorMessage | null} if terminal block conditions are not satisfied
• {status: SYNCING, latestValidHash: null, validationError: null} if the payload extends the canonical chain and requisite data for its validation is missing
• with the payload status obtained from the Payload validation process if the payload has been fully validated while processing the call
• {status: ACCEPTED, latestValidHash: null, validationError: null} if the following conditions are met:
  • the blockHash of the payload is valid
  • the payload doesn't extend the canonical chain
  • the payload hasn't been fully validated.

6. If any of the above fails due to errors unrelated to the normal processing flow of the method, client software MUST respond with an error object.

engine_forkchoiceUpdatedV1

Request

• method: "engine_forkchoiceUpdatedV1"
• params:
  1. forkchoiceState: Object - instance of ForkchoiceStateV1
  2. payloadAttributes: Object|null - instance of PayloadAttributesV1 or null

Response

• result: object
  • payloadStatus: PayloadStatusV1; values of the status field in the context of this method are restricted to the following subset:
    • "VALID"
    • "INVALID"
    • "SYNCING"
    • "INVALID_TERMINAL_BLOCK"
  • payloadId: DATA|null, 8 Bytes - identifier of the payload build process or null
• error: code and message set in case an exception happens while the validating payload, updating the forkchoice or initiating the payload build process.

Specification

1. Client software MAY initiate a sync process if forkchoiceState.headBlockHash references an unknown payload or a payload that can't be validated because data that are requisite for the validation is missing. The sync process is specified in the Sync section.

2. Client software MAY skip an update of the forkchoice state and MUST NOT begin a payload build process if forkchoiceState.headBlockHash doesn't reference a leaf of the block tree. That is, the block referenced by forkchoiceState.headBlockHash is neither the head of the canonical chain nor a block at the tip of any other chain.

3. If forkchoiceState.headBlockHash references a PoW block, client software MUST validate this block with respect to terminal block conditions according to EIP-3675. This check maps to the transition block validity section of the EIP. Additionally, if this validation fails, client software MUST NOT update the forkchoice state and MUST NOT begin a payload build process.

4. Before updating the forkchoice state, client software MUST ensure the validity of the payload referenced by forkchoiceState.headBlockHash, and MAY validate the payload while processing the call. The validation process is specified in the Payload validation section.

5. Client software MUST update its forkchoice state if payloads referenced by forkchoiceState.headBlockHash and forkchoiceState.finalizedBlockHash are VALID. The update is specified as follows:

• The values (forkchoiceState.headBlockHash, forkchoiceState.finalizedBlockHash) of this method call map on the POS_FORKCHOICE_UPDATED event of EIP-3675 and MUST be processed according to the specification defined in the EIP
• All updates to the forkchoice state resulting from this call MUST be made atomically.

6. Client software MUST begin a payload build process building on top of forkchoiceState.headBlockHash and identified via buildProcessId value if payloadAttributes is not null and the forkchoice state has been updated successfully. The build process is specified in the Payload building section.

7. Client software MUST respond to this method call in the following way:

• {payloadStatus: {status: SYNCING, latestValidHash: null, validationError: null}, payloadId: null} if forkchoiceState.headBlockHash references an unknown payload or a payload that can't be validated because requisite data for the validation is missing
• {payloadStatus: {status: INVALID, latestValidHash: validHash, validationError: errorMessage | null}, payloadId: null} obtained from the Payload validation process if the payload is deemed INVALID
• {payloadStatus: {status: INVALID_TERMINAL_BLOCK, latestValidHash: null, validationError: errorMessage | null}, payloadId: null} obtained either from the Payload validation process or as a result of validating a PoW block referenced by forkchoiceState.headBlockHash
• {payloadStatus: {status: VALID, latestValidHash: forkchoiceState.headBlockHash, validationError: null}, payloadId: null} if the payload is deemed VALID and a build process hasn't been started
• {payloadStatus: {status: VALID, latestValidHash: forkchoiceState.headBlockHash, validationError: null}, payloadId: buildProcessId} if the payload is deemed VALID and the build process has begun.

8. If any of the above fails due to errors unrelated to the normal processing flow of the method, client software MUST respond with an error object.

engine_getPayloadV1

Request

• method: engine_getPayloadV1
• params:
  1. payloadId: DATA, 8 Bytes - Identifier of the payload build process

Response

• result: ExecutionPayloadV1
• error: code and message set in case an exception happens while getting the payload.

Specification

1. Given the payloadId client software MUST return the most recent version of the payload that is available in the corresponding build process at the time of receiving the call.

2. The call MUST return -32001: Unknown payload error if the build process identified by the payloadId does not exist.

3. Client software MAY stop the corresponding build process after serving this call.

engine_exchangeTransitionConfigurationV1

Request

• method: engine_exchangeTransitionConfigurationV1
• params:
  1. transitionConfiguration: Object - instance of TransitionConfigurationV1; terminalBlockNumber MUST be set to 0

Response

• result: TransitionConfigurationV1
• error: code and message set in case an exception happens while getting a transition configuration.

Specification

1. Execution Layer client software MUST respond with configurable setting values that are set according to the Client software configuration section of EIP-3675.

2. Execution Layer client software SHOULD surface an error to the user if local configuration settings mismatch corresponding values received in the call of this method, with exception for terminalBlockNumber value.

3. Consensus Layer client software SHOULD surface an error to the user if local configuration settings mismatch corresponding values obtained from the response to the call of this method.