# ePBS -- Fork Choice ## Table of contents - [Introduction](#introduction) - [Constants](#constants) - [Containers](#containers) - [New `ChildNode`](#new-childnode) - [Helpers](#helpers) - [Modified `LatestMessage`](#modified-latestmessage) - [Modified `update_latest_messages`](#modified-update_latest_messages) - [Modified `Store`](#modified-store) - [`notify_ptc_messages`](#notify_ptc_messages) - [`is_payload_present`](#is_payload_present) - [`is_parent_node_full`](#is_parent_node_full) - [Modified `get_ancestor`](#modified-get_ancestor) - [Modified `get_checkpoint_block`](#modified-get_checkpoint_block) - [`is_supporting_vote`](#is_supporting_vote) - [New `compute_proposer_boost`](#new-compute_proposer_boost) - [New `compute_withhold_boost`](#new-compute_withhold_boost) - [New `compute_reveal_boost`](#new-compute_reveal_boost) - [Modified `get_weight`](#modified-get_weight) - [Modified `get_head`](#modified-get_head) - [Updated fork-choice handlers](#updated-fork-choice-handlers) - [Modified `on_block`](#modified-on_block) - [New fork-choice handlers](#new-fork-choice-handlers) - [New `on_execution_payload`](#new-on_execution_payload) - [`seconds_into_slot`](#seconds_into_slot) - [Modified `on_tick_per_slot`](#modified-on_tick_per_slot) - [`on_payload_attestation_message`](#on_payload_attestation_message) ## Introduction This is the modification of the fork choice accompanying the ePBS upgrade. ## Constants | Name | Value | | -------------------- | ----------- | | `PAYLOAD_TIMELY_THRESHOLD` | `PTC_SIZE/2` (=`uint64(256)`) | | `PROPOSER_SCORE_BOOST` | `20` # [modified in EIP-XXXX] | | `PAYLOAD_WITHHOLD_BOOST` | `40` | | `PAYLOAD_REVEAL_BOOST` | `40` | ## Containers ### New `ChildNode` Auxiliary class to consider `(block, slot, bool)` LMD voting ```python class ChildNode(Container): root: Root slot: Slot is_payload_present: bool ``` ## Helpers ### Modified `LatestMessage` **Note:** The class is modified to keep track of the slot instead of the epoch. ```python @dataclass(eq=True, frozen=True) class LatestMessage(object): slot: Slot root: Root ``` ### Modified `update_latest_messages` **Note:** the function `update_latest_messages` is updated to use the attestation slot instead of target. Notice that this function is only called on validated attestations and validators cannot attest twice in the same epoch without equivocating. Notice also that target epoch number and slot number are validated on `validate_on_attestation`. ```python def update_latest_messages(store: Store, attesting_indices: Sequence[ValidatorIndex], attestation: Attestation) -> None: slot = attestation.data.slot beacon_block_root = attestation.data.beacon_block_root non_equivocating_attesting_indices = [i for i in attesting_indices if i not in store.equivocating_indices] for i in non_equivocating_attesting_indices: if i not in store.latest_messages or slot > store.latest_messages[i].slot: store.latest_messages[i] = LatestMessage(slot=slot, root=beacon_block_root) ``` ### Modified `Store` **Note:** `Store` is modified to track the intermediate states of "empty" consensus blocks, that is, those consensus blocks for which the corresponding execution payload has not been revealed or has not been included on chain. ```python @dataclass class Store(object): time: uint64 genesis_time: uint64 justified_checkpoint: Checkpoint finalized_checkpoint: Checkpoint unrealized_justified_checkpoint: Checkpoint unrealized_finalized_checkpoint: Checkpoint proposer_boost_root: Root equivocating_indices: Set[ValidatorIndex] blocks: Dict[Root, BeaconBlock] = field(default_factory=dict) block_states: Dict[Root, BeaconState] = field(default_factory=dict) checkpoint_states: Dict[Checkpoint, BeaconState] = field(default_factory=dict) latest_messages: Dict[ValidatorIndex, LatestMessage] = field(default_factory=dict) unrealized_justifications: Dict[Root, Checkpoint] = field(default_factory=dict) execution_payload_states: Dict[Root, BeaconState] = field(default_factory=dict) # [New in EIP-XXXX] ptc_vote: Dict[Root, Vector[uint8, PTC_SIZE]] = field(default_factory=dict) # [New in EIP-XXXX] payload_withhold_boost_root: Root # [New in EIP-XXXX] payload_withhold_boost_full: bool # [New in EIP-XXXX] payload_reveal_boost_root: Root # [New in EIP-XXXX] ``` ### `notify_ptc_messages` ```python def notify_ptc_messages(store: Store, state: BeaconState, payload_attestations: Sequence[PayloadAttestation]) -> None: """ Extracts a list of ``PayloadAttestationMessage`` from ``payload_attestations`` and updates the store with them These Payload attestations are assumed to be in the beacon block hence signature verification is not needed """ if state.slot == 0: return for payload_attestation in payload_attestations: indexed_payload_attestation = get_indexed_payload_attestation(state, state.slot - 1, payload_attestation) for idx in indexed_payload_attestation.attesting_indices: store.on_payload_attestation_message(PayloadAttestationMessage(validator_index=idx, data=payload_attestation.data, signature= BLSSignature(), is_from_block=true)) ``` ### `is_payload_present` ```python def is_payload_present(store: Store, beacon_block_root: Root) -> bool: """ Return whether the execution payload for the beacon block with root ``beacon_block_root`` was voted as present by the PTC """ # The beacon block root must be known assert beacon_block_root in store.ptc_vote return store.ptc_vote[beacon_block_root].count(PAYLOAD_PRESENT) > PAYLOAD_TIMELY_THRESHOLD ``` ### `is_parent_node_full` ```python def is_parent_node_full(store: Store, block: BeaconBlock) -> bool: parent = store.blocks[block.parent_root] return block.body.signed_execution_payload_header.message.parent_block_hash == parent.body.signed_execution_payload_header.message.block_hash ``` ### Modified `get_ancestor` **Note:** `get_ancestor` is modified to return whether the chain is based on an *empty* or *full* block. ```python def get_ancestor(store: Store, root: Root, slot: Slot) -> ChildNode: """ Returns the beacon block root, the slot and the payload status of the ancestor of the beacon block with ``root`` at ``slot``. If the beacon block with ``root`` is already at ``slot`` or we are requesting an ancestor "in the future" it returns its PTC status instead of the actual payload content. """ block = store.blocks[root] if block.slot <= slot: return ChildNode(root=root, slot=slot, is_payload_present=is_payload_present(store, root)) parent = store.blocks[block.parent_root] if parent.slot > slot: return get_ancestor(store, block.parent_root, slot) return ChildNode(root=block.parent_root, slot=parent.slot, is_payload_present=is_parent_node_full(block)) ``` ### Modified `get_checkpoint_block` **Note:** `get_checkpoint_block` is modified to use the new `get_ancestor` ```python def get_checkpoint_block(store: Store, root: Root, epoch: Epoch) -> Root: """ Compute the checkpoint block for epoch ``epoch`` in the chain of block ``root`` """ epoch_first_slot = compute_start_slot_at_epoch(epoch) return get_ancestor(store, root, epoch_first_slot).root ``` ### `is_supporting_vote` ```python def is_supporting_vote(store: Store, node: ChildNode, message: LatestMessage) -> bool: """ Returns whether a vote for ``message.root`` supports the chain containing the beacon block ``node.root`` with the payload contents indicated by ``node.is_payload_present`` as head during slot ``node.slot``. """ if node.root == message.root: # an attestation for a given root always counts for that root regardless if full or empty # as long as the attestation happened after the requested slot. return node.slot <= message.slot message_block = store.blocks[message.root] if node.slot >= message_block.slot: return False ancestor = get_ancestor(store, message.root, node.slot) return (node.root == ancestor.root) and (node.is_payload_present == ancestor.is_payload_present) ``` ### New `compute_proposer_boost` This is a helper to compute the proposer boost. It applies the proposer boost to any ancestor of the proposer boost root taking into account the payload presence. There is one exception: if the requested node has the same root and slot as the block with the proposer boost root, then the proposer boost is applied to both empty and full versions of the node. ```python def compute_proposer_boost(store: Store, state: State, node: ChildNode) -> Gwei: if store.proposer_boost_root == Root(): return Gwei(0) ancestor = get_ancestor(store, store.proposer_boost_root, node.slot) if ancestor.root != node.root: return Gwei(0) proposer_boost_slot = store.blocks[store.proposer_boost_root].slot # Proposer boost is not applied after skipped slots if node.slot > proposer_boost_slot: return Gwei(0) if (node.slot < proposer_boost_slot) and (ancestor.is_payload_present != node.is_payload_present): return Gwei(0) committee_weight = get_total_active_balance(state) // SLOTS_PER_EPOCH return (committee_weight * PROPOSER_SCORE_BOOST) // 100 ``` ### New `compute_withhold_boost` This is a similar helper that applies for the withhold boost. In this case this always takes into account the reveal status. ```python def compute_withhold_boost(store: Store, state: State, node: ChildNode) -> Gwei: if store.payload_withhold_boost_root == Root(): return Gwei(0) ancestor = get_ancestor(store, store.payload_withold_boost_root, node.slot) if ancestor.root != node.root: return Gwei(0) if node.slot >= store.blocks[store.payload_withhold_boost_root].slot: ancestor.is_payload_present = store.payload_withhold_boost_full if ancestor.is_payload_present != node.is_payload_present: return Gwei(0) committee_weight = get_total_active_balance(state) // SLOTS_PER_EPOCH return (committee_weight * PAYLOAD_WITHHOLD_BOOST) // 100 ``` ### New `compute_reveal_boost` This is a similar helper to the last two, the only difference is that the reveal boost is only applied to the full version of the node when querying for the same slot as the revealed payload. ```python def compute_reveal_boost(store: Store, state: State, node: ChildNode) -> Gwei: if store.payload_reveal_boost_root == Root(): return Gwei(0) ancestor = get_ancestor(store, store.payload_reveal_boost_root, node.slot) if ancestor.root != node.root: return Gwei(0) if node.slot >= store.blocks[store.payload_reveal_boost_root].slot: ancestor.is_payload_present = True if is_ancestor_full != node.is_payload_present: return Gwei(0) committee_weight = get_total_active_balance(state) // SLOTS_PER_EPOCH return (committee_weight * PAYLOAD_REVEAL_BOOST) // 100 ``` ### Modified `get_weight` **Note:** `get_weight` is modified to only count votes for descending chains that support the status of a triple `Root, Slot, bool`, where the `bool` indicates if the block was full or not. `Slot` is needed for a correct implementation of `(Block, Slot)` voting. ```python def get_weight(store: Store, node: ChildNode) -> Gwei: state = store.checkpoint_states[store.justified_checkpoint] unslashed_and_active_indices = [ i for i in get_active_validator_indices(state, get_current_epoch(state)) if not is_slashed_attester(state.validators[i]) ] attestation_score = Gwei(sum( state.validators[i].effective_balance for i in unslashed_and_active_indices if (i in store.latest_messages and i not in store.equivocating_indices and is_supporting_vote(store, node, store.latest_messages[i])) )) # Compute boosts proposer_score = compute_boost(store, state, node) builder_reveal_score = compute_reveal_boost(store, state, node) builder_withhold_score = compute_withhold_boost(store, state, node) return attestation_score + proposer_score + builder_reveal_score + builder_withhold_score ``` ### Modified `get_head` **Note:** `get_head` is a modified to use the new `get_weight` function. It returns the `ChildNode` object corresponidng to the head block. ```python def get_head(store: Store) -> ChildNode: # Get filtered block tree that only includes viable branches blocks = get_filtered_block_tree(store) # Execute the LMD-GHOST fork choice justified_root = store.justified_checkpoint.root justified_block = store.blocks[justified_root] justified_slot = justified_block.slot justified_full = is_payload_present(store, justified_root) best_child = ChildNode(root=head_root, slot=head_slot, is_payload_present=head_full) while True: children = [ ChildNode(root=root, slot=block.slot, is_payload_present=present) for (root, block) in blocks.items() if block.parent_root == best_child.root and is_parent_node_full(store, block) == best_child.is_payload_present if root != store.justified_root for present in (True, False) if root in store.execution_payload_states or present == False ] if len(children) == 0: return best_child # if we have children we consider the current head advanced as a possible head children += [ChildNode(root=best_child.root, slot=best_child.slot + 1, best_child.is_payload_present)] # Sort by latest attesting balance with ties broken lexicographically # Ties broken by favoring full blocks according to the PTC vote # Ties are then broken by favoring full blocks # Ties broken then by favoring higher slot numbers # Ties then broken by favoring block with lexicographically higher root new_best_child = max(children, key=lambda child: (get_weight(store, child), is_payload_present(store, child.root), child.is_payload_present, child.slot, child.root)) if new_best_child.root == best_child.root: return new_best_child best_child = new_best_child ``` ## Updated fork-choice handlers ### Modified `on_block` *Note*: The handler `on_block` is modified to consider the pre `state` of the given consensus beacon block depending not only on the parent block root, but also on the parent blockhash. In addition we delay the checking of blob data availability until the processing of the execution payload. ```python 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 # Check if this blocks builds on empty or full parent block parent_block = store.blocks[block.parent_root] header = block.body.signed_execution_payload_header.message parent_header = parent_block.body.signed_execution_payload_header.message # Make a copy of the state to avoid mutability issues if is_parent_node_full(store, block): assert block.parent_root in store.execution_payload_states state = copy(store.execution_payload_states[block.parent_root]) else: assert header.parent_block_hash == parent_header.parent_block_hash 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. current_slot = get_current_slot(store) assert current_slot >= 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 # Check the block is valid and compute the post-state 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 a new PTC voting for this block to the store store.ptc_vote[block_root] = [PAYLOAD_ABSENT]*PTC_SIZE # Notify the store about the payload_attestations in the block store.notify_ptc_messages(state, block.body.payload_attestations) # Add proposer score boost if the block is timely time_into_slot = (store.time - store.genesis_time) % SECONDS_PER_SLOT is_before_attesting_interval = time_into_slot < SECONDS_PER_SLOT // INTERVALS_PER_SLOT if get_current_slot(store) == block.slot and is_before_attesting_interval: 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) ``` ## New fork-choice handlers ### New `on_execution_payload` The handler `on_execution_payload` is called when the node receives a `SignedExecutionPayloadEnvelope` to sync. ```python def on_execution_payload(store: Store, signed_envelope: SignedExecutionPayloadEnvelope) -> None: """ Run ``on_execution_payload`` upon receiving a new execution payload. """ envelope = signed_envelope.message # The corresponding beacon block root needs to be known assert envelope.beacon_block_root in store.block_states # Check if blob data is available # If not, this payload MAY be queued and subsequently considered when blob data becomes available assert is_data_available(envelope.beacon_block_root, envelope.blob_kzg_commitments) # Make a copy of the state to avoid mutability issues state = copy(store.block_states[envelope.beacon_block_root]) # Process the execution payload process_execution_payload(state, signed_envelope, EXECUTION_ENGINE) # Add new state for this payload to the store store.execution_payload_states[envelope.beacon_block_root] = state ``` ### `seconds_into_slot` ```python def seconds_into_slot(store: Store) -> uint64: return (store.time - store.genesis_time) % SECONDS_PER_SLOT ``` ### Modified `on_tick_per_slot` Modified to reset the payload boost roots ```python def on_tick_per_slot(store: Store, time: uint64) -> None: previous_slot = get_current_slot(store) # Update store time store.time = time current_slot = get_current_slot(store) # If this is a new slot, reset store.proposer_boost_root if current_slot > previous_slot: store.proposer_boost_root = Root() else: # Reset the payload boost if this is the attestation time if seconds_into_slot(store) >= SECONDS_PER_SLOT // INTERVALS_PER_SLOT: store.payload_withhold_boost_root = Root() store.payload_withhold_boost_full = False store.payload_reveal_boost_root = Root() # If a new epoch, pull-up justification and finalization from previous epoch if current_slot > previous_slot and compute_slots_since_epoch_start(current_slot) == 0: update_checkpoints(store, store.unrealized_justified_checkpoint, store.unrealized_finalized_checkpoint) ``` ### `on_payload_attestation_message` ```python def on_payload_attestation_message(store: Store, ptc_message: PayloadAttestationMessage, is_from_block: bool=False) -> None: """ Run ``on_payload_attestation_message`` upon receiving a new ``ptc_message`` directly on the wire. """ # The beacon block root must be known data = ptc_message.data # PTC attestation must be for a known block. If block is unknown, delay consideration until the block is found state = store.block_states[data.beacon_block_root] ptc = get_ptc(state, data.slot) # PTC votes can only change the vote for their assigned beacon block, return early otherwise if data.slot != state.slot: return # Check that the attester is from the PTC assert ptc_message.validator_index in ptc # Verify the signature and check that its for the current slot if it is coming from the wire if not is_from_block: # Check that the attestation is for the current slot assert data.slot == get_current_slot(store) # Verify the signature assert is_valid_indexed_payload_attestation(state, IndexedPayloadAttestation(attesting_indices = [ptc_message.validator_index], data = data, signature = ptc_message.signature)) # Update the ptc vote for the block ptc_index = ptc.index(ptc_message.validator_index) ptc_vote = store.ptc_vote[data.beacon_block_root] ptc_vote[ptc_index] = data.payload_status # Only update payload boosts with attestations from a block if the block is for the current slot and it's early if is_from_block && data.slot + 1 != get_current_slot(store): return time_into_slot = (store.time - store.genesis_time) % SECONDS_PER_SLOT if is_from_block and time_into_slot >= SECONDS_PER_SLOT // INTERVALS_PER_SLOT: return # Update the payload boosts if threshold has been achieved if ptc_vote.count(PAYLOAD_PRESENT) > PAYLOAD_TIMELY_THRESHOLD: store.payload_reveal_boost_root = data.beacon_block_root if ptc_vote.count(PAYLOAD_WITHHELD) > PAYLOAD_TIMELY_THRESHOLD: block = store.blocks[data.beacon_block_root] store.payload_withhold_boost_root = block.parent_root store.payload_withhold_boost_full = is_parent_node_full(block) ```