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Fulu -- The Beacon Chain

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

Introduction

Note: This specification is built upon Electra and is under active development.

Configuration

Blob schedule

[New in Fulu:EIP7892] This schedule defines the maximum blobs per block limit for a given epoch.

There MUST NOT exist multiple blob schedule entries with the same epoch value. The epoch value in each entry MUST be greater than or equal to FULU_FORK_EPOCH. The maximum blobs per block limit in each entry MUST be less than or equal to MAX_BLOB_COMMITMENTS_PER_BLOCK. The blob schedule entries SHOULD be sorted by epoch in ascending order. The blob schedule MAY be empty.

Note: The blob schedule is to be determined.

Epoch Max Blobs Per Block Description

Beacon chain state transition function

Block processing

Execution payload

Modified process_execution_payload
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def process_execution_payload(
    state: BeaconState, body: BeaconBlockBody, execution_engine: ExecutionEngine
) -> None:
    payload = body.execution_payload

    # Verify consistency of the parent hash with respect to the previous execution payload header
    assert payload.parent_hash == state.latest_execution_payload_header.block_hash
    # Verify prev_randao
    assert payload.prev_randao == get_randao_mix(state, get_current_epoch(state))
    # Verify timestamp
    assert payload.timestamp == compute_time_at_slot(state, state.slot)
    # [Modified in Fulu:EIP7892]
    # Verify commitments are under limit
    assert (
        len(body.blob_kzg_commitments)
        <= get_blob_parameters(get_current_epoch(state)).max_blobs_per_block
    )
    # Verify the execution payload is valid
    versioned_hashes = [
        kzg_commitment_to_versioned_hash(commitment) for commitment in body.blob_kzg_commitments
    ]
    assert execution_engine.verify_and_notify_new_payload(
        NewPayloadRequest(
            execution_payload=payload,
            versioned_hashes=versioned_hashes,
            parent_beacon_block_root=state.latest_block_header.parent_root,
            execution_requests=body.execution_requests,
        )
    )
    # Cache execution payload header
    state.latest_execution_payload_header = ExecutionPayloadHeader(
        parent_hash=payload.parent_hash,
        fee_recipient=payload.fee_recipient,
        state_root=payload.state_root,
        receipts_root=payload.receipts_root,
        logs_bloom=payload.logs_bloom,
        prev_randao=payload.prev_randao,
        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),
        withdrawals_root=hash_tree_root(payload.withdrawals),
        blob_gas_used=payload.blob_gas_used,
        excess_blob_gas=payload.excess_blob_gas,
    )

Containers

Extended Containers

BeaconState

Note: The BeaconState container is extended with the proposer_lookahead field, which is a list of validator indices covering the full lookahead period, starting from the beginning of the current epoch. For example, proposer_lookahead[0] is the validator index for the first proposer in the current epoch, proposer_lookahead[1] is the validator index for the next proposer in the current epoch, and so forth. The length of the proposer_lookahead list is (MIN_SEED_LOOKAHEAD + 1) * SLOTS_PER_EPOCH, reflecting how far ahead proposer indices are computed based on the MIN_SEED_LOOKAHEAD parameter.

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class BeaconState(Container):
    genesis_time: uint64
    genesis_validators_root: Root
    slot: Slot
    fork: Fork
    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_data: Eth1Data
    eth1_data_votes: List[Eth1Data, EPOCHS_PER_ETH1_VOTING_PERIOD * SLOTS_PER_EPOCH]
    eth1_deposit_index: uint64
    validators: List[Validator, VALIDATOR_REGISTRY_LIMIT]
    balances: List[Gwei, VALIDATOR_REGISTRY_LIMIT]
    randao_mixes: Vector[Bytes32, EPOCHS_PER_HISTORICAL_VECTOR]
    slashings: Vector[Gwei, EPOCHS_PER_SLASHINGS_VECTOR]
    previous_epoch_participation: List[ParticipationFlags, VALIDATOR_REGISTRY_LIMIT]
    current_epoch_participation: List[ParticipationFlags, VALIDATOR_REGISTRY_LIMIT]
    justification_bits: Bitvector[JUSTIFICATION_BITS_LENGTH]
    previous_justified_checkpoint: Checkpoint
    current_justified_checkpoint: Checkpoint
    finalized_checkpoint: Checkpoint
    inactivity_scores: List[uint64, VALIDATOR_REGISTRY_LIMIT]
    current_sync_committee: SyncCommittee
    next_sync_committee: SyncCommittee
    latest_execution_payload_header: ExecutionPayloadHeader
    next_withdrawal_index: WithdrawalIndex
    next_withdrawal_validator_index: ValidatorIndex
    historical_summaries: List[HistoricalSummary, HISTORICAL_ROOTS_LIMIT]
    deposit_requests_start_index: uint64
    deposit_balance_to_consume: Gwei
    exit_balance_to_consume: Gwei
    earliest_exit_epoch: Epoch
    consolidation_balance_to_consume: Gwei
    earliest_consolidation_epoch: Epoch
    pending_deposits: List[PendingDeposit, PENDING_DEPOSITS_LIMIT]
    pending_partial_withdrawals: List[PendingPartialWithdrawal, PENDING_PARTIAL_WITHDRAWALS_LIMIT]
    pending_consolidations: List[PendingConsolidation, PENDING_CONSOLIDATIONS_LIMIT]
    # [New in Fulu:EIP7917]
    proposer_lookahead: Vector[ValidatorIndex, (MIN_SEED_LOOKAHEAD + 1) * SLOTS_PER_EPOCH]

Helper functions

Misc

New BlobParameters

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@dataclass
class BlobParameters:
    epoch: Epoch
    max_blobs_per_block: uint64

New get_blob_parameters

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def get_blob_parameters(epoch: Epoch) -> BlobParameters:
    """
    Return the blob parameters at a given epoch.
    """
    for entry in sorted(BLOB_SCHEDULE, key=lambda e: e["EPOCH"], reverse=True):
        if epoch >= entry["EPOCH"]:
            return BlobParameters(entry["EPOCH"], entry["MAX_BLOBS_PER_BLOCK"])
    return BlobParameters(ELECTRA_FORK_EPOCH, MAX_BLOBS_PER_BLOCK_ELECTRA)

Modified compute_fork_digest

Note: The compute_fork_digest helper is updated to account for Blob-Parameters-Only forks.

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def compute_fork_digest(
    genesis_validators_root: Root,
    epoch: Epoch,
) -> ForkDigest:
    """
    Return the 4-byte fork digest for the ``genesis_validators_root`` at a given ``epoch``.

    This is a digest primarily used for domain separation on the p2p layer.
    4-bytes suffices for practical separation of forks/chains.
    """
    fork_version = compute_fork_version(epoch)
    base_digest = compute_fork_data_root(fork_version, genesis_validators_root)

    # [Modified in Fulu:EIP7892]
    # Bitmask digest with hash of blob parameters
    blob_parameters = get_blob_parameters(epoch)
    return ForkDigest(
        bytes(
            xor(
                base_digest,
                hash(
                    uint_to_bytes(uint64(blob_parameters.epoch))
                    + uint_to_bytes(uint64(blob_parameters.max_blobs_per_block))
                ),
            )
        )[:4]
    )

New compute_proposer_indices

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def compute_proposer_indices(
    state: BeaconState, epoch: Epoch, seed: Bytes32, indices: Sequence[ValidatorIndex]
) -> Vector[ValidatorIndex, SLOTS_PER_EPOCH]:
    """
    Return the proposer indices for the given ``epoch``.
    """
    start_slot = compute_start_slot_at_epoch(epoch)
    seeds = [hash(seed + uint_to_bytes(Slot(start_slot + i))) for i in range(SLOTS_PER_EPOCH)]
    return [compute_proposer_index(state, indices, seed) for seed in seeds]

Beacon state accessors

Modified get_beacon_proposer_index

Note: The function get_beacon_proposer_index is modified to use the pre-calculated current_proposer_lookahead instead of calculating it on-demand.

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def get_beacon_proposer_index(state: BeaconState) -> ValidatorIndex:
    """
    Return the beacon proposer index at the current slot.
    """
    return state.proposer_lookahead[state.slot % SLOTS_PER_EPOCH]

New get_beacon_proposer_indices

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def get_beacon_proposer_indices(
    state: BeaconState, epoch: Epoch
) -> Vector[ValidatorIndex, SLOTS_PER_EPOCH]:
    """
    Return the proposer indices for the given ``epoch``.
    """
    indices = get_active_validator_indices(state, epoch)
    seed = get_seed(state, epoch, DOMAIN_BEACON_PROPOSER)
    return compute_proposer_indices(state, epoch, seed, indices)

Epoch processing

Modified process_epoch

Note: The function process_epoch is modified in Fulu to call process_proposer_lookahead to update the proposer_lookahead in the beacon state.

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def process_epoch(state: BeaconState) -> None:
    process_justification_and_finalization(state)
    process_inactivity_updates(state)
    process_rewards_and_penalties(state)
    process_registry_updates(state)
    process_slashings(state)
    process_eth1_data_reset(state)
    process_pending_deposits(state)
    process_pending_consolidations(state)
    process_effective_balance_updates(state)
    process_slashings_reset(state)
    process_randao_mixes_reset(state)
    process_historical_summaries_update(state)
    process_participation_flag_updates(state)
    process_sync_committee_updates(state)
    # [New in Fulu:EIP7917]
    process_proposer_lookahead(state)

New process_proposer_lookahead

Note: This function updates the proposer_lookahead field in the beacon state by shifting out proposer indices from the earliest epoch and appending new proposer indices for the latest epoch. With MIN_SEED_LOOKAHEAD set to 1, this means that at the start of epoch N, the proposer lookahead for epoch N+1 will be computed and included in the beacon state's lookahead.

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def process_proposer_lookahead(state: BeaconState) -> None:
    last_epoch_start = len(state.proposer_lookahead) - SLOTS_PER_EPOCH
    # Shift out proposers in the first epoch
    state.proposer_lookahead[:last_epoch_start] = state.proposer_lookahead[SLOTS_PER_EPOCH:]
    # Fill in the last epoch with new proposer indices
    last_epoch_proposers = get_beacon_proposer_indices(
        state, Epoch(get_current_epoch(state) + MIN_SEED_LOOKAHEAD + 1)
    )
    state.proposer_lookahead[last_epoch_start:] = last_epoch_proposers