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Deneb -- Honest Validator

Table of contents

Introduction

This document represents the changes to be made in the code of an "honest validator" to implement Deneb.

Prerequisites

This document is an extension of the Capella -- Honest Validator guide. All behaviors and definitions defined in this document, and documents it extends, carry over unless explicitly noted or overridden.

All terminology, constants, functions, and protocol mechanics defined in the updated Beacon Chain doc of Deneb are requisite for this document and used throughout. Please see related Beacon Chain doc before continuing and use them as a reference throughout.

Helpers

BlobsBundle

[New in Deneb:EIP4844]

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@dataclass
class BlobsBundle(object):
    commitments: Sequence[KZGCommitment]
    proofs: Sequence[KZGProof]
    blobs: Sequence[Blob]

Modified GetPayloadResponse

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@dataclass
class GetPayloadResponse(object):
    execution_payload: ExecutionPayload
    block_value: uint256
    blobs_bundle: BlobsBundle  # [New in Deneb:EIP4844]

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def compute_signed_block_header(signed_block: SignedBeaconBlock) -> SignedBeaconBlockHeader:
    block = signed_block.message
    block_header = BeaconBlockHeader(
        slot=block.slot,
        proposer_index=block.proposer_index,
        parent_root=block.parent_root,
        state_root=block.state_root,
        body_root=hash_tree_root(block.body),
    )
    return SignedBeaconBlockHeader(message=block_header, signature=signed_block.signature)

Protocol

ExecutionEngine

Modified get_payload

Given the payload_id, get_payload returns the most recent version of the execution payload that has been built since the corresponding call to notify_forkchoice_updated method.

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def get_payload(self: ExecutionEngine, payload_id: PayloadId) -> GetPayloadResponse:
    """
    Return ExecutionPayload, uint256, BlobsBundle objects.
    """
    # pylint: disable=unused-argument
    ...

Beacon chain responsibilities

All validator responsibilities remain unchanged other than those noted below.

Block and sidecar proposal

Constructing the BeaconBlockBody

ExecutionPayload

prepare_execution_payload is updated from the Capella specs to provide the parent beacon block root.

Note: In this section, state is the state of the slot for the block proposal without the block yet applied. That is, state is the previous_state processed through any empty slots up to the assigned slot using process_slots(previous_state, slot).

Note: The only change made to prepare_execution_payload is to add the parent beacon block root as an additional parameter to the PayloadAttributes.

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def prepare_execution_payload(state: BeaconState,
                              safe_block_hash: Hash32,
                              finalized_block_hash: Hash32,
                              suggested_fee_recipient: ExecutionAddress,
                              execution_engine: ExecutionEngine) -> Optional[PayloadId]:
    # Verify consistency of the parent hash with respect to the previous execution payload header
    parent_hash = state.latest_execution_payload_header.block_hash

    # Set the forkchoice head and initiate the payload build process
    payload_attributes = PayloadAttributes(
        timestamp=compute_timestamp_at_slot(state, state.slot),
        prev_randao=get_randao_mix(state, get_current_epoch(state)),
        suggested_fee_recipient=suggested_fee_recipient,
        withdrawals=get_expected_withdrawals(state),
        parent_beacon_block_root=hash_tree_root(state.latest_block_header),  # [New in Deneb:EIP4788]
    )
    return execution_engine.notify_forkchoice_updated(
        head_block_hash=parent_hash,
        safe_block_hash=safe_block_hash,
        finalized_block_hash=finalized_block_hash,
        payload_attributes=payload_attributes,
    )
Blob KZG commitments

[New in Deneb:EIP4844]

  1. The execution payload is obtained from the execution engine as defined above using payload_id. The response also includes a blobs_bundle entry containing the corresponding blobs, commitments, and proofs.
  2. Set block.body.blob_kzg_commitments = commitments.

Constructing the BlobSidecars

[New in Deneb:EIP4844]

To construct a BlobSidecar, a blob_sidecar is defined with the necessary context for block and sidecar proposal.

Sidecar

Blobs associated with a block are packaged into sidecar objects for distribution to the associated sidecar topic, the blob_sidecar_{subnet_id} pubsub topic.

Each sidecar is obtained from: 

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def get_blob_sidecars(signed_block: SignedBeaconBlock,
                      blobs: Sequence[Blob],
                      blob_kzg_proofs: Sequence[KZGProof]) -> Sequence[BlobSidecar]:
    block = signed_block.message
    signed_block_header = compute_signed_block_header(signed_block)
    return [
        BlobSidecar(
            index=index,
            blob=blob,
            kzg_commitment=block.body.blob_kzg_commitments[index],
            kzg_proof=blob_kzg_proofs[index],
            signed_block_header=signed_block_header,
            kzg_commitment_inclusion_proof=compute_merkle_proof(
                block.body,
                get_generalized_index(BeaconBlockBody, 'blob_kzg_commitments', index),
            ),
        )
        for index, blob in enumerate(blobs)
    ]

The subnet_id for the blob_sidecar is calculated with: - Let blob_index = blob_sidecar.index. - Let subnet_id = compute_subnet_for_blob_sidecar(blob_index).

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def compute_subnet_for_blob_sidecar(blob_index: BlobIndex) -> SubnetID:
    return SubnetID(blob_index % BLOB_SIDECAR_SUBNET_COUNT)

After publishing the peers on the network may request the sidecar through sync-requests, or a local user may be interested.

The validator MUST hold on to sidecars for MIN_EPOCHS_FOR_BLOB_SIDECARS_REQUESTS epochs and serve when capable, to ensure the data-availability of these blobs throughout the network.

After MIN_EPOCHS_FOR_BLOB_SIDECARS_REQUESTS nodes MAY prune the sidecars and/or stop serving them.