fork-choice.md

Deneb -- Fork Choice

Table of contents

• Introduction
• Containers
• Helpers
  • Extended PayloadAttributes
  • is_data_available
• Updated fork-choice handlers
  • on_block

Introduction

This is the modification of the fork choice accompanying the Deneb upgrade.

Containers

Helpers

Extended PayloadAttributes

PayloadAttributes is extended with the parent beacon block root for EIP-4788.

@dataclass
class PayloadAttributes(object):
    timestamp: uint64
    prev_randao: Bytes32
    suggested_fee_recipient: ExecutionAddress
    withdrawals: Sequence[Withdrawal]
    parent_beacon_block_root: Root  # [New in Deneb:EIP4788]

is_data_available

[New in Deneb:EIP4844]

The implementation of is_data_available will become more sophisticated during later scaling upgrades. Initially, verification requires every verifying actor to retrieve all matching Blobs and KZGProofs, and validate them with verify_blob_kzg_proof_batch.

The block MUST NOT be considered valid until all valid Blobs have been downloaded. Blocks that have been previously validated as available SHOULD be considered available even if the associated Blobs have subsequently been pruned.

Note: Extraneous or invalid Blobs (in addition to KZG expected/referenced valid blobs) received on the p2p network MUST NOT invalidate a block that is otherwise valid and available.

def is_data_available(beacon_block_root: Root, blob_kzg_commitments: Sequence[KZGCommitment]) -> bool:
    # `retrieve_blobs_and_proofs` is implementation and context dependent
    # It returns all the blobs for the given block root, and raises an exception if not available
    # Note: the p2p network does not guarantee sidecar retrieval outside of
    # `MIN_EPOCHS_FOR_BLOB_SIDECARS_REQUESTS`
    blobs, proofs = retrieve_blobs_and_proofs(beacon_block_root)

    return verify_blob_kzg_proof_batch(blobs, blob_kzg_commitments, proofs)

Updated fork-choice handlers

on_block

Note: The only modification is the addition of the blob data availability check.

def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
    """
    Run ``on_block`` upon receiving a new block.
    """
    block = signed_block.message
    # Parent block must be known
    assert block.parent_root in store.block_states
    # Make a copy of the state to avoid mutability issues
    pre_state = copy(store.block_states[block.parent_root])
    # Blocks cannot be in the future. If they are, their consideration must be delayed until they are in the past.
    assert get_current_slot(store) >= block.slot

    # Check that block is later than the finalized epoch slot (optimization to reduce calls to get_ancestor)
    finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
    assert block.slot > finalized_slot
    # Check block is a descendant of the finalized block at the checkpoint finalized slot
    finalized_checkpoint_block = get_checkpoint_block(
        store,
        block.parent_root,
        store.finalized_checkpoint.epoch,
    )
    assert store.finalized_checkpoint.root == finalized_checkpoint_block

    # [New in Deneb:EIP4844]
    # Check if blob data is available
    # If not, this block MAY be queued and subsequently considered when blob data becomes available
    # *Note*: Extraneous or invalid Blobs (in addition to the expected/referenced valid blobs)
    # received on the p2p network MUST NOT invalidate a block that is otherwise valid and available
    assert is_data_available(hash_tree_root(block), block.body.blob_kzg_commitments)

    # Check the block is valid and compute the post-state
    state = pre_state.copy()
    block_root = hash_tree_root(block)
    state_transition(state, signed_block, True)

    # Add new block to the store
    store.blocks[block_root] = block
    # Add new state for this block to the store
    store.block_states[block_root] = state

    # Add block timeliness to the store
    time_into_slot = (store.time - store.genesis_time) % SECONDS_PER_SLOT
    is_before_attesting_interval = time_into_slot < SECONDS_PER_SLOT // INTERVALS_PER_SLOT
    is_timely = get_current_slot(store) == block.slot and is_before_attesting_interval
    store.block_timeliness[hash_tree_root(block)] = is_timely

    # Add proposer score boost if the block is timely and not conflicting with an existing block
    is_first_block = store.proposer_boost_root == Root()
    if is_timely and is_first_block:
        store.proposer_boost_root = hash_tree_root(block)

    # Update checkpoints in store if necessary
    update_checkpoints(store, state.current_justified_checkpoint, state.finalized_checkpoint)

    # Eagerly compute unrealized justification and finality.
    compute_pulled_up_tip(store, block_root)