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EIP-4844 -- Networking

This document contains the consensus-layer networking specification for EIP-4844.

The specification of these changes continues in the same format as the network specifications of previous upgrades, and assumes them as pre-requisite.

Table of contents

Preset

Name Value
MAX_BLOBS_PER_BLOCK uint64(2**4) (= 16)

Configuration

Name Value Description
MAX_REQUEST_BLOBS_SIDECARS 2**7 (= 128) Maximum number of blobs sidecars in a single request
MIN_EPOCHS_FOR_BLOBS_SIDECARS_REQUESTS 2**13 (= 8192, ~1.2 months) The minimum epoch range over which a node must serve blobs sidecars

Containers

BlobsSidecar

class BlobsSidecar(Container):
    beacon_block_root: Root
    beacon_block_slot: Slot
    blobs: List[Blob, MAX_BLOBS_PER_BLOCK]

SignedBlobsSidecar

class SignedBlobsSidecar(Container):
    message: BlobsSidecar
    signature: BLSSignature

The gossip domain: gossipsub

Some gossip meshes are upgraded in the fork of EIP4844 to support upgraded types.

Topics and messages

Topics follow the same specification as in prior upgrades. All topics remain stable except the beacon block topic which is updated with the modified type.

The specification around the creation, validation, and dissemination of messages has not changed from the Bellatrix document unless explicitly noted here.

The derivation of the message-id remains stable.

The new topics along with the type of the data field of a gossipsub message are given in this table:

Name Message Type
beacon_block SignedBeaconBlock (modified)
blobs_sidecar SignedBlobsSidecar (new)

Note that the ForkDigestValue path segment of the topic separates the old and the new beacon_block topics.

Global topics

EIP4844 changes the type of the global beacon block topic and introduces a new global topic for blobs-sidecars.

beacon_block

The type of the payload of this topic changes to the (modified) SignedBeaconBlock found in EIP4844.

In addition to the gossip validations for this topic from prior specifications, the following validations MUST pass before forwarding the signed_beacon_block on the network. Alias block = signed_beacon_block.message, execution_payload = block.body.execution_payload.

  • [REJECT] The KZG commitments of the blobs are all correctly encoded compressed BLS G1 Points. -- i.e. all(bls.KeyValidate(commitment) for commitment in block.body.blob_kzgs)
  • [REJECT] The KZG commitments correspond to the versioned hashes in the transactions list. -- i.e. verify_kzgs_against_transactions(block.body.execution_payload.transactions, block.body.blob_kzgs)
blobs_sidecar

This topic is used to propagate data blobs included in any given beacon block.

The following validations MUST pass before forwarding the signed_blobs_sidecar on the network; Alias sidecar = signed_blobs_sidecar.message.

  • [IGNORE] the sidecar.beacon_block_slot is for the current slot (with a MAXIMUM_GOSSIP_CLOCK_DISPARITY allowance) -- i.e. blobs_sidecar.beacon_block_slot == current_slot.
  • [REJECT] the sidecar.blobs are all well formatted, i.e. the BLSFieldElement in valid range (x < BLS_MODULUS).
  • [REJECT] the beacon proposer signature, signed_blobs_sidecar.signature, is valid -- i.e.
domain = get_domain(state, DOMAIN_BLOBS_SIDECAR, blobs_sidecar.beacon_block_slot // SLOTS_PER_EPOCH)
signing_root = compute_signing_root(blobs_sidecar, domain)
assert bls.Verify(proposer_pubkey, signing_root, signed_blob_header.signature)

where proposer_pubkey is the pubkey of the beacon block proposer of blobs_sidecar.beacon_block_slot

  • [IGNORE] The sidecar is the first sidecar with valid signature received for the (proposer_index, sidecar.beacon_block_slot) combination, where proposer_index is the validator index of the beacon block proposer of blobs_sidecar.beacon_block_slot

Note that a sidecar may be propagated before or after the corresponding beacon block.

Once both sidecar and beacon block are received, verify_blobs_sidecar can unlock the data-availability fork-choice dependency.

Transitioning the gossip

See gossip transition details found in the Altair document for details on how to handle transitioning gossip topics for this upgrade.

The Req/Resp domain

Messages

BeaconBlocksByRange v2

Protocol ID: /eth2/beacon_chain/req/beacon_blocks_by_range/2/

The EIP-4844 fork-digest is introduced to the context enum to specify EIP-4844 beacon block type.

Per context = compute_fork_digest(fork_version, genesis_validators_root):

fork_version Chunk SSZ type
GENESIS_FORK_VERSION phase0.SignedBeaconBlock
ALTAIR_FORK_VERSION altair.SignedBeaconBlock
BELLATRIX_FORK_VERSION bellatrix.SignedBeaconBlock
EIP4844_FORK_VERSION eip4844.SignedBeaconBlock

BeaconBlocksByRoot v2

Protocol ID: /eth2/beacon_chain/req/beacon_blocks_by_root/2/

The EIP-4844 fork-digest is introduced to the context enum to specify EIP-4844 beacon block type.

Per context = compute_fork_digest(fork_version, genesis_validators_root):

fork_version Chunk SSZ type
GENESIS_FORK_VERSION phase0.SignedBeaconBlock
ALTAIR_FORK_VERSION altair.SignedBeaconBlock
BELLATRIX_FORK_VERSION bellatrix.SignedBeaconBlock
EIP4844_FORK_VERSION eip4844.SignedBeaconBlock

BlobsSidecarsByRange v1

Protocol ID: /eth2/beacon_chain/req/blobs_sidecars_by_range/1/

Request Content:

(
  start_slot: Slot
  count: uint64
)

Response Content:

(
  List[BlobsSidecar, MAX_REQUEST_BLOBS_SIDECARS]
)

Requests blobs sidecars in the slot range [start_slot, start_slot + count), leading up to the current head block as selected by fork choice.

The response is unsigned, i.e. BlobsSidecarsByRange, as the signature of the beacon block proposer may not be available beyond the initial distribution via gossip.

Before consuming the next response chunk, the response reader SHOULD verify the blobs sidecar is well-formatted and correct w.r.t. the expected KZG commitments through verify_blobs_sidecar.

BlobsSidecarsByRange is primarily used to sync blobs that may have been missed on gossip.

The request MUST be encoded as an SSZ-container.

The response MUST consist of zero or more response_chunk. Each successful response_chunk MUST contain a single BlobsSidecar payload.

Clients MUST keep a record of signed blobs sidecars seen on the epoch range [max(GENESIS_EPOCH, current_epoch - MIN_EPOCHS_FOR_BLOBS_SIDECARS_REQUESTS), current_epoch] where current_epoch is defined by the current wall-clock time, and clients MUST support serving requests of blocks on this range.

Peers that are unable to reply to block requests within the MIN_EPOCHS_FOR_BLOBS_SIDECARS_REQUESTS epoch range SHOULD respond with error code 3: ResourceUnavailable. Such peers that are unable to successfully reply to this range of requests MAY get descored or disconnected at any time.

Note: The above requirement implies that nodes that start from a recent weak subjectivity checkpoint MUST backfill the local blobs database to at least epoch current_epoch - MIN_EPOCHS_FOR_BLOBS_SIDECARS_REQUESTS to be fully compliant with BlobsSidecarsByRange requests. To safely perform such a backfill of blocks to the recent state, the node MUST validate both (1) the proposer signatures and (2) that the blocks form a valid chain up to the most recent block referenced in the weak subjectivity state.

Note: Although clients that bootstrap from a weak subjectivity checkpoint can begin participating in the networking immediately, other peers MAY disconnect and/or temporarily ban such an un-synced or semi-synced client.

Clients MUST respond with at least the first blobs sidecar that exists in the range, if they have it, and no more than MAX_REQUEST_BLOBS_SIDECARS sidecars.

The following blobs sidecars, where they exist, MUST be sent in consecutive order.

Clients MAY limit the number of blobs sidecars in the response.

The response MUST contain no more than count blobs sidecars.

Clients MUST respond with blobs sidecars from their view of the current fork choice -- that is, blobs sidecars as included by blocks from the single chain defined by the current head. Of note, blocks from slots before the finalization MUST lead to the finalized block reported in the Status handshake.

Clients MUST respond with blobs sidecars that are consistent from a single chain within the context of the request.

After the initial blobs sidecar, clients MAY stop in the process of responding if their fork choice changes the view of the chain in the context of the request.

Design decision rationale

Why are blobs relayed as a sidecar, separate from beacon blocks?

This "sidecar" design provides forward compatibility for further data increases by black-boxing is_data_available(): with full sharding is_data_available() can be replaced by data-availability-sampling (DAS) thus avoiding all blobs being downloaded by all beacon nodes on the network.

Such sharding design may introduce an updated BlobsSidecar to identify the shard, but does not affect the BeaconBlock structure.