eth2.0-specs/specs/merge/beacon-chain.md

The Merge -- The Beacon Chain

Notice: This document is a work-in-progress for researchers and implementers.

Table of contents

• Introduction
• Custom types
• Constants
  • Execution
• Configuration
  • Transition settings
• Containers
  • Extended containers
    • BeaconBlockBody
    • BeaconState
  • New containers
    • ExecutionPayload
    • ExecutionPayloadHeader
• Helper functions
  • Predicates
    • is_merge_complete
    • is_merge_block
    • is_execution_enabled
  • Misc
    • compute_timestamp_at_slot
• Beacon chain state transition function
  • Execution engine
    • execute_payload
  • Block processing
  • Execution payload processing
    • is_valid_gas_limit
    • process_execution_payload
• Testing

Introduction

This patch adds transaction execution to the beacon chain as part of the Merge fork.

Custom types

Note: The Transaction type is a stub which is not final.

Name	SSZ equivalent	Description
Transaction	ByteList[MAX_BYTES_PER_TRANSACTION]	either a typed transaction envelope or a legacy transaction
ExecutionAddress	Bytes20	Address of account on the execution layer

Constants

Execution

Name	Value
MAX_BYTES_PER_TRANSACTION	uint64(2**20) (= 1,048,576)
MAX_TRANSACTIONS_PER_PAYLOAD	uint64(2**14) (= 16,384)
BYTES_PER_LOGS_BLOOM	uint64(2**8) (= 256)
GAS_LIMIT_DENOMINATOR	uint64(2**10) (= 1,024)
MIN_GAS_LIMIT	uint64(5000) (= 5,000)
MAX_EXTRA_DATA_BYTES	2**5 (= 32)

Configuration

Transition settings

Name	Value
TERMINAL_TOTAL_DIFFICULTY	TBD
TERMINAL_BLOCK_HASH	Hash32()
TERMINAL_BLOCK_HASH_ACTIVATION_EPOCH	FAR_FUTURE_EPOCH

Containers

Extended containers

BeaconBlockBody

class BeaconBlockBody(Container):
    randao_reveal: BLSSignature
    eth1_data: Eth1Data  # Eth1 data vote
    graffiti: Bytes32  # Arbitrary data
    # Operations
    proposer_slashings: List[ProposerSlashing, MAX_PROPOSER_SLASHINGS]
    attester_slashings: List[AttesterSlashing, MAX_ATTESTER_SLASHINGS]
    attestations: List[Attestation, MAX_ATTESTATIONS]
    deposits: List[Deposit, MAX_DEPOSITS]
    voluntary_exits: List[SignedVoluntaryExit, MAX_VOLUNTARY_EXITS]
    sync_aggregate: SyncAggregate
    # Execution
    execution_payload: ExecutionPayload  # [New in Merge]

BeaconState

class BeaconState(Container):
    # Versioning
    genesis_time: uint64
    genesis_validators_root: Root
    slot: Slot
    fork: Fork
    # History
    latest_block_header: BeaconBlockHeader
    block_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
    state_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
    historical_roots: List[Root, HISTORICAL_ROOTS_LIMIT]
    # Eth1
    eth1_data: Eth1Data
    eth1_data_votes: List[Eth1Data, EPOCHS_PER_ETH1_VOTING_PERIOD * SLOTS_PER_EPOCH]
    eth1_deposit_index: uint64
    # Registry
    validators: List[Validator, VALIDATOR_REGISTRY_LIMIT]
    balances: List[Gwei, VALIDATOR_REGISTRY_LIMIT]
    # Randomness
    randao_mixes: Vector[Bytes32, EPOCHS_PER_HISTORICAL_VECTOR]
    # Slashings
    slashings: Vector[Gwei, EPOCHS_PER_SLASHINGS_VECTOR]  # Per-epoch sums of slashed effective balances
    # Participation
    previous_epoch_participation: List[ParticipationFlags, VALIDATOR_REGISTRY_LIMIT]
    current_epoch_participation: List[ParticipationFlags, VALIDATOR_REGISTRY_LIMIT]
    # Finality
    justification_bits: Bitvector[JUSTIFICATION_BITS_LENGTH]  # Bit set for every recent justified epoch
    previous_justified_checkpoint: Checkpoint
    current_justified_checkpoint: Checkpoint
    finalized_checkpoint: Checkpoint
    # Inactivity
    inactivity_scores: List[uint64, VALIDATOR_REGISTRY_LIMIT]
    # Sync
    current_sync_committee: SyncCommittee
    next_sync_committee: SyncCommittee
    # Execution
    latest_execution_payload_header: ExecutionPayloadHeader  # [New in Merge]

New containers

ExecutionPayload

class ExecutionPayload(Container):
    # Execution block header fields
    parent_hash: Hash32
    coinbase: ExecutionAddress  # 'beneficiary' in the yellow paper
    state_root: Bytes32
    receipt_root: Bytes32  # 'receipts root' in the yellow paper
    logs_bloom: ByteVector[BYTES_PER_LOGS_BLOOM]
    random: Bytes32  # 'difficulty' in the yellow paper
    block_number: uint64  # 'number' in the yellow paper
    gas_limit: uint64
    gas_used: uint64
    timestamp: uint64
    extra_data: ByteList[MAX_EXTRA_DATA_BYTES]
    base_fee_per_gas: uint256
    # Extra payload fields
    block_hash: Hash32  # Hash of execution block
    transactions: List[Transaction, MAX_TRANSACTIONS_PER_PAYLOAD]

ExecutionPayloadHeader

class ExecutionPayloadHeader(Container):
    # Execution block header fields
    parent_hash: Hash32
    coinbase: ExecutionAddress
    state_root: Bytes32
    receipt_root: Bytes32
    logs_bloom: ByteVector[BYTES_PER_LOGS_BLOOM]
    random: Bytes32
    block_number: uint64
    gas_limit: uint64
    gas_used: uint64
    timestamp: uint64
    extra_data: ByteList[MAX_EXTRA_DATA_BYTES]
    base_fee_per_gas: uint256
    # Extra payload fields
    block_hash: Hash32  # Hash of execution block
    transactions_root: Root

Helper functions

Predicates

is_merge_complete

def is_merge_complete(state: BeaconState) -> bool:
    return state.latest_execution_payload_header != ExecutionPayloadHeader()

is_merge_block

def is_merge_block(state: BeaconState, body: BeaconBlockBody) -> bool:
    return not is_merge_complete(state) and body.execution_payload != ExecutionPayload()

is_execution_enabled

def is_execution_enabled(state: BeaconState, body: BeaconBlockBody) -> bool:
    return is_merge_block(state, body) or is_merge_complete(state)

Misc

compute_timestamp_at_slot

Note: This function is unsafe with respect to overflows and underflows.

def compute_timestamp_at_slot(state: BeaconState, slot: Slot) -> uint64:
    slots_since_genesis = slot - GENESIS_SLOT
    return uint64(state.genesis_time + slots_since_genesis * SECONDS_PER_SLOT)

Beacon chain state transition function

Execution engine

The implementation-dependent ExecutionEngine protocol encapsulates the execution sub-system logic via:

• a state object self.execution_state of type ExecutionState
• a state transition function self.execute_payload which applies changes to the self.execution_state

Note: execute_payload is a function accessed through the EXECUTION_ENGINE module which instantiates the ExecutionEngine protocol.

The body of this function is implementation dependent. The Engine API may be used to implement this and similarly defined functions via an external execution engine.

execute_payload

def execute_payload(self: ExecutionEngine, execution_payload: ExecutionPayload) -> bool:
    """
    Return ``True`` if and only if ``execution_payload`` is valid with respect to ``self.execution_state``.
    """
    ...

Block processing

Note: The call to the process_execution_payload must happen before the call to the process_randao as the former depends on the randao_mix computed with the reveal of the previous block.

def process_block(state: BeaconState, block: BeaconBlock) -> None:
    process_block_header(state, block)
    if is_execution_enabled(state, block.body):
        process_execution_payload(state, block.body.execution_payload, EXECUTION_ENGINE)  # [New in Merge]
    process_randao(state, block.body)
    process_eth1_data(state, block.body)
    process_operations(state, block.body)
    process_sync_aggregate(state, block.body.sync_aggregate)

Execution payload processing

is_valid_gas_limit

def is_valid_gas_limit(payload: ExecutionPayload, parent: ExecutionPayloadHeader) -> bool:
    parent_gas_limit = parent.gas_limit

    # Check if the payload used too much gas
    if payload.gas_used > payload.gas_limit:
        return False

    # Check if the payload changed the gas limit too much
    if payload.gas_limit >= parent_gas_limit + parent_gas_limit // GAS_LIMIT_DENOMINATOR:
        return False
    if payload.gas_limit <= parent_gas_limit - parent_gas_limit // GAS_LIMIT_DENOMINATOR:
        return False

    # Check if the gas limit is at least the minimum gas limit
    if payload.gas_limit < MIN_GAS_LIMIT:
        return False

    return True

process_execution_payload

def process_execution_payload(state: BeaconState, payload: ExecutionPayload, execution_engine: ExecutionEngine) -> None:
    # Verify consistency of the parent hash, block number, base fee per gas and gas limit
    # with respect to the previous execution payload header
    if is_merge_complete(state):
        assert payload.parent_hash == state.latest_execution_payload_header.block_hash
        assert payload.block_number == state.latest_execution_payload_header.block_number + uint64(1)
        assert is_valid_gas_limit(payload, state.latest_execution_payload_header)
    # Verify random
    assert payload.random == get_randao_mix(state, get_current_epoch(state))
    # Verify timestamp
    assert payload.timestamp == compute_timestamp_at_slot(state, state.slot)
    # Verify the execution payload is valid
    assert execution_engine.execute_payload(payload)
    # Cache execution payload header
    state.latest_execution_payload_header = ExecutionPayloadHeader(
        parent_hash=payload.parent_hash,
        coinbase=payload.coinbase,
        state_root=payload.state_root,
        receipt_root=payload.receipt_root,
        logs_bloom=payload.logs_bloom,
        random=payload.random,
        block_number=payload.block_number,
        gas_limit=payload.gas_limit,
        gas_used=payload.gas_used,
        timestamp=payload.timestamp,
        extra_data=payload.extra_data,
        base_fee_per_gas=payload.base_fee_per_gas,
        block_hash=payload.block_hash,
        transactions_root=hash_tree_root(payload.transactions),
    )

Testing

Note: The function initialize_beacon_state_from_eth1 is modified for pure Merge testing only. Modifications include:

1. Use MERGE_FORK_VERSION as the current fork version.
2. Utilize the Merge BeaconBlockBody when constructing the initial latest_block_header.
3. Initialize latest_execution_payload_header. If execution_payload_header == ExecutionPayloadHeader(), then the Merge has not yet occurred. Else, the Merge starts from genesis and the transition is incomplete.

def initialize_beacon_state_from_eth1(eth1_block_hash: Bytes32,
                                      eth1_timestamp: uint64,
                                      deposits: Sequence[Deposit],
                                      execution_payload_header: ExecutionPayloadHeader=ExecutionPayloadHeader()
                                      ) -> BeaconState:
    fork = Fork(
        previous_version=MERGE_FORK_VERSION,  # [Modified in Merge] for testing only
        current_version=MERGE_FORK_VERSION,  # [Modified in Merge]
        epoch=GENESIS_EPOCH,
    )
    state = BeaconState(
        genesis_time=eth1_timestamp + GENESIS_DELAY,
        fork=fork,
        eth1_data=Eth1Data(block_hash=eth1_block_hash, deposit_count=uint64(len(deposits))),
        latest_block_header=BeaconBlockHeader(body_root=hash_tree_root(BeaconBlockBody())),
        randao_mixes=[eth1_block_hash] * EPOCHS_PER_HISTORICAL_VECTOR,  # Seed RANDAO with Eth1 entropy
    )

    # Process deposits
    leaves = list(map(lambda deposit: deposit.data, deposits))
    for index, deposit in enumerate(deposits):
        deposit_data_list = List[DepositData, 2**DEPOSIT_CONTRACT_TREE_DEPTH](*leaves[:index + 1])
        state.eth1_data.deposit_root = hash_tree_root(deposit_data_list)
        process_deposit(state, deposit)

    # Process activations
    for index, validator in enumerate(state.validators):
        balance = state.balances[index]
        validator.effective_balance = min(balance - balance % EFFECTIVE_BALANCE_INCREMENT, MAX_EFFECTIVE_BALANCE)
        if validator.effective_balance == MAX_EFFECTIVE_BALANCE:
            validator.activation_eligibility_epoch = GENESIS_EPOCH
            validator.activation_epoch = GENESIS_EPOCH

    # Set genesis validators root for domain separation and chain versioning
    state.genesis_validators_root = hash_tree_root(state.validators)

    # Fill in sync committees
    # Note: A duplicate committee is assigned for the current and next committee at genesis
    state.current_sync_committee = get_next_sync_committee(state)
    state.next_sync_committee = get_next_sync_committee(state)

    # [New in Merge] Initialize the execution payload header
    # If empty, will initialize a chain that has not yet gone through the Merge transition
    state.latest_execution_payload_header = execution_payload_header

    return state