eth2.0-specs/specs/_features/eip7594/das-core.md

EIP-7594 -- Data Availability Sampling Core

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

Table of contents

• Custom types
• Configuration
  • Data size
  • Custody setting
  • Helper functions
    • get_custody_lines
    • compute_extended_data
    • compute_extended_matrix
    • get_data_column_sidecars
• Custody
  • Custody requirement
  • Public, deterministic selection
• Peer discovery
• Extended data
• Column gossip
  • Parameters
  • Reconstruction and cross-seeding
• Peer sampling
• Peer scoring
• DAS providers
• A note on fork choice
• FAQs
  • Row (blob) custody
  • Subnet stability

Custom types

We define the following Python custom types for type hinting and readability:

Name	SSZ equivalent	Description
DataColumn	List[Cell, MAX_BLOBS_PER_BLOCK]	The data of each column in EIP7594
ExtendedMatrix	List[Cell, MAX_BLOBS_PER_BLOCK * NUMBER_OF_COLUMNS]	The full data with blobs and one-dimensional erasure coding extension
FlatExtendedMatrix	List[BLSFieldElement, MAX_BLOBS_PER_BLOCK * FIELD_ELEMENTS_PER_BLOB * NUMBER_OF_COLUMNS]	The flattened format of ExtendedMatrix

Configuration

Data size

Name	Value	Description
NUMBER_OF_COLUMNS	uint64((FIELD_ELEMENTS_PER_BLOB * 2) // FIELD_ELEMENTS_PER_CELL) (= 128)	Number of columns in the extended data matrix.

Custody setting

Name	Value	Description
SAMPLES_PER_SLOT	8	Number of random samples a node queries per slot
CUSTODY_REQUIREMENT	2	Minimum number of columns an honest node custodies and serves samples from
TARGET_NUMBER_OF_PEERS	70	Suggested minimum peer count

Helper functions

get_custody_lines

def get_custody_lines(node_id: NodeID, custody_size: uint64) -> Sequence[ColumnIndex]:
    assert custody_size <= NUMBER_OF_COLUMNS
    column_index = node_id % NUMBER_OF_COLUMNS
    return [ColumnIndex((column_index + i) % NUMBER_OF_COLUMNS) for i in range(custody_size)]

compute_extended_data

def compute_extended_data(data: Sequence[BLSFieldElement]) -> Sequence[BLSFieldElement]:
    # TODO
    # pylint: disable=unused-argument
    ...

compute_extended_matrix

def compute_extended_matrix(blobs: Sequence[Blob]) -> FlatExtendedMatrix:
    matrix = [compute_extended_data(blob) for blob in blobs]
    return FlatExtendedMatrix(matrix)

get_data_column_sidecars

def get_data_column_sidecars(signed_block: SignedBeaconBlock,
                             blobs: Sequence[Blob]) -> Sequence[DataColumnSidecar]:
    signed_block_header = compute_signed_block_header(signed_block)
    block = signed_block.message
    kzg_commitments_inclusion_proof = compute_merkle_proof(
        block.body,
        get_generalized_index(BeaconBlockBody, 'blob_kzg_commitments'),
    )
    cells_and_proofs = [compute_cells_and_proofs(blob) for blob in blobs]
    blob_count = len(blobs)
    cells = [cells_and_proofs[i][0] for i in range(blob_count)]
    proofs = [cells_and_proofs[i][1] for i in range(blob_count)]
    sidecars = []
    for column_index in range(NUMBER_OF_COLUMNS):
        column = DataColumn([cells[row_index][column_index]
                             for row_index in range(blob_count)])
        kzg_proof_of_column = [proofs[row_index][column_index]
                               for row_index in range(blob_count)]
        sidecars.append(DataColumnSidecar(
            index=column_index,
            column=column,
            kzg_commitments=block.body.blob_kzg_commitments,
            kzg_proofs=kzg_proof_of_column,
            signed_block_header=signed_block_header,
            kzg_commitments_inclusion_proof=kzg_commitments_inclusion_proof,
        ))
    return sidecars

Custody

Custody requirement

Each node downloads and custodies a minimum of CUSTODY_REQUIREMENT columns per slot. The particular columns that the node is required to custody are selected pseudo-randomly (more on this below).

A node may choose to custody and serve more than the minimum honesty requirement. Such a node explicitly advertises a number greater than CUSTODY_REQUIREMENT via the peer discovery mechanism -- for example, in their ENR (e.g. custody_lines: 8 if the node custodies 8 columns each slot) -- up to a NUMBER_OF_COLUMNS (i.e. a super-full node).

A node stores the custodied columns for the duration of the pruning period and responds to peer requests for samples on those columns.

Public, deterministic selection

The particular columns that a node custodies are selected pseudo-randomly as a function (get_custody_lines) of the node-id and custody size -- importantly this function can be run by any party as the inputs are all public.

Note: increasing the custody_size parameter for a given node_id extends the returned list (rather than being an entirely new shuffle) such that if custody_size is unknown, the default CUSTODY_REQUIREMENT will be correct for a subset of the node's custody.

Peer discovery

At each slot, a node needs to be able to readily sample from any set of columns. To this end, a node should find and maintain a set of diverse and reliable peers that can regularly satisfy their sampling demands.

A node runs a background peer discovery process, maintaining at least TARGET_NUMBER_OF_PEERS of various custody distributions (both custody_size and column assignments). The combination of advertised custody_size size and public node-id make this readily and publicly accessible.

TARGET_NUMBER_OF_PEERS should be tuned upward in the event of failed sampling.

Note: while high-capacity and super-full nodes are high value with respect to satisfying sampling requirements, a node should maintain a distribution across node capacities as to not centralize the p2p graph too much (in the extreme becomes hub/spoke) and to distribute sampling load better across all nodes.

Note: A DHT-based peer discovery mechanism is expected to be utilized in the above. The beacon-chain network currently utilizes discv5 in a similar method as described for finding peers of particular distributions of attestation subnets. Additional peer discovery methods are valuable to integrate (e.g., latent peer discovery via libp2p gossipsub) to add a defense in breadth against one of the discovery methods being attacked.

Extended data

In this construction, we extend the blobs using a one-dimensional erasure coding extension. The matrix comprises maximum MAX_BLOBS_PER_BLOCK rows and fixed NUMBER_OF_COLUMNS columns, with each row containing a Blob and its corresponding extension.

Column gossip

Parameters

For each column -- use data_column_sidecar_{subnet_id} subnets, where each column index maps to the subnet_id. The sidecars can be computed with get_data_column_sidecars(signed_block: SignedBeaconBlock, blobs: Sequence[Blob]) helper.

To custody a particular column, a node joins the respective gossip subnet. Verifiable samples from their respective column are gossiped on the assigned subnet.

Reconstruction and cross-seeding

If the node obtains 50%+ of all the columns, they can reconstruct the full data matrix via recover_samples_impl helper.

If a node fails to sample a peer or fails to get a column on the column subnet, a node can utilize the Req/Resp message to query the missing column from other peers.

Once the node obtain the column, the node should send the missing columns to the column subnets.

Note: A node always maintains a matrix view of the rows and columns they are following, able to cross-reference and cross-seed in either direction.

Note: There are timing considerations to analyze -- at what point does a node consider samples missing and choose to reconstruct and cross-seed.

Note: There may be anti-DoS and quality-of-service considerations around how to send samples and consider samples -- is each individual sample a message or are they sent in aggregate forms.

Peer sampling

At each slot, a node makes (locally randomly determined) SAMPLES_PER_SLOT queries for samples from their peers via DataColumnSidecarByRoot request. A node utilizes get_custody_lines helper to determine which peer(s) to request from. If a node has enough good/honest peers across all rows and columns, this has a high chance of success.

Peer scoring

Due to the deterministic custody functions, a node knows exactly what a peer should be able to respond to. In the event that a peer does not respond to samples of their custodied rows/columns, a node may downscore or disconnect from a peer.

DAS providers

A DAS provider is a consistently-available-for-DAS-queries, super-full (or high capacity) node. To the p2p, these look just like other nodes but with high advertised capacity, and they should generally be able to be latently found via normal discovery.

DAS providers can also be found out-of-band and configured into a node to connect to directly and prioritize. Nodes can add some set of these to their local configuration for persistent connection to bolster their DAS quality of service.

Such direct peering utilizes a feature supported out of the box today on all nodes and can complement (and reduce attackability and increase quality-of-service) alternative peer discovery mechanisms.

A note on fork choice

Fork choice spec TBD, but it will just be a replacement of is_data_available() call in Deneb with column sampling instead of full download. Note the is_data_available(slot_N) will likely do a -1 follow distance so that you just need to check the availability of slot N-1 for slot N (starting with the block proposer of N).

The fork choice rule (essentially a DA filter) is orthogonal to a given DAS design, other than the efficiency of a particular design impacting it.

In any DAS design, there are probably a few degrees of freedom around timing, acceptability of short-term re-orgs, etc.

For example, the fork choice rule might require validators to do successful DAS on slot N to be able to include block of slot N in its fork choice. That's the tightest DA filter. But trailing filters are also probably acceptable, knowing that there might be some failures/short re-orgs but that they don't hurt the aggregate security. For example, the rule could be — DAS must be completed for slot N-1 for a child block in N to be included in the fork choice.

Such trailing techniques and their analysis will be valuable for any DAS construction. The question is — can you relax how quickly you need to do DA and in the worst case not confirm unavailable data via attestations/finality, and what impact does it have on short-term re-orgs and fast confirmation rules.

FAQs

Row (blob) custody

In the one-dimension construction, a node samples the peers by requesting the whole DataColumn. In reconstruction, a node can reconstruct all the blobs by 50% of the columns. Note that nodes can still download the row via blob_sidecar_{subnet_id} subnets.

The potential benefits of having row custody could include:

1. Allow for more "natural" distribution of data to consumers -- e.g., roll-ups -- but honestly, they won't know a priori which row their blob is going to be included in in the block, so they would either need to listen to all rows or download a particular row after seeing the block. The former looks just like listening to column [0, N) and the latter is req/resp instead of gossiping.
2. Help with some sort of distributed reconstruction. Those with full rows can compute extensions and seed missing samples to the network. This would either need to be able to send individual points on the gossip or would need some sort of req/resp faculty, potentially similar to an IHAVEPOINTBITFIELD and IWANTSAMPLE.

However, for simplicity, we don't assign row custody assignments to nodes in the current design.

Subnet stability

To start with a simple, stable backbone, for now, we don't shuffle the subnet assignments via the deterministic custody selection helper get_custody_lines. However, staggered rotation likely needs to happen on the order of the pruning period to ensure subnets can be utilized for recovery. For example, introducing an epoch argument allows the function to maintain stability over many epochs.