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Sharding -- Networking

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

Table of contents

Introduction

The specification of these changes continues in the same format as the network specifications of previous upgrades, and assumes them as pre-requisite. The adjustments and additions for Shards are outlined in this document.

Constants

Misc

Name Value Description
SHARD_BLOB_SUBNET_COUNT 64 The number of shard_blob_{subnet_id} subnets used in the gossipsub protocol.
SHARD_TX_PROPAGATION_GRACE_SLOTS 4 The number of slots for a late transaction to propagate
SHARD_TX_PROPAGATION_BUFFER_SLOTS 8 The number of slots for an early transaction to propagate

Gossip domain

Topics and messages

Following the same scheme as the Phase0 gossip topics, names and payload types are:

Name Message Type
shard_blob_{subnet_id} SignedShardBlob
shard_blob_header SignedShardBlobHeader
shard_blob_tx SignedShardBlobHeader
shard_proposer_slashing ShardProposerSlashing

The DAS network specification defines additional topics.

Shard blob subnets

Shard blob subnets are used by builders to make their blobs available after selection by shard proposers.

shard_blob_{subnet_id}

Shard blob data, in the form of a SignedShardBlob is published to the shard_blob_{subnet_id} subnets.

def compute_subnet_for_shard_blob(state: BeaconState, slot: Slot, shard: Shard) -> uint64:
    """
    Compute the correct subnet for a shard blob publication.
    Note, this mimics compute_subnet_for_attestation().
    """
    committee_index = compute_committee_index_from_shard(state, slot, shard)
    committees_per_slot = get_committee_count_per_slot(state, compute_epoch_at_slot(slot))
    slots_since_epoch_start = Slot(slot % SLOTS_PER_EPOCH)
    committees_since_epoch_start = committees_per_slot * slots_since_epoch_start

    return uint64((committees_since_epoch_start + committee_index) % SHARD_BLOB_SUBNET_COUNT)

The following validations MUST pass before forwarding the signed_blob, on the horizontal subnet or creating samples for it. Alias blob = signed_blob.message.

  • [IGNORE] The blob is published 1 slot early or later (with a MAXIMUM_GOSSIP_CLOCK_DISPARITY allowance) -- i.e. validate that blob.slot <= current_slot + 1 (a client MAY queue future blobs for propagation at the appropriate slot).
  • [IGNORE] The blob is new enough to still be processed -- i.e. validate that compute_epoch_at_slot(blob.slot) >= get_previous_epoch(state)
  • [REJECT] The shard blob is for an active shard -- i.e. blob.shard < get_active_shard_count(state, compute_epoch_at_slot(blob.slot))
  • [REJECT] The blob.shard MUST have a committee at the blob.slot -- i.e. validate that compute_committee_index_from_shard(state, blob.slot, blob.shard) doesn't raise an error
  • [REJECT] The shard blob is for the correct subnet -- i.e. compute_subnet_for_shard_blob(state, blob.slot, blob.shard) == subnet_id
  • [IGNORE] The blob is the first blob with valid signature received for the (blob.proposer_index, blob.slot, blob.shard) combination.
  • [REJECT] The blob is not too large -- the data MUST NOT be larger than the SSZ list-limit, and a client MAY apply stricter bounds.
  • [REJECT] The blob.body.data MUST NOT contain any point p >= MODULUS. Although it is a uint256, not the full 256 bit range is valid.
  • [REJECT] The blob builder defined by blob.builder_index exists and has sufficient balance to back the fee payment.
  • [REJECT] The blob signature, signed_blob.signature, is valid for the aggregate of proposer and builder -- i.e. bls.FastAggregateVerify([builder_pubkey, proposer_pubkey], blob_signing_root, signed_blob.signature).
  • [REJECT] The blob is proposed by the expected proposer_index for the blob's slot and shard, in the context of the current shuffling (defined by the current node head state and blob.slot). If the proposer_index cannot immediately be verified against the expected shuffling, the blob MAY be queued for later processing while proposers for the blob's branch are calculated -- in such a case do not REJECT, instead IGNORE this message.

Global topics

There are three additional global topics for Sharding.

  • shard_blob_header: co-signed headers to be included on-chain and to serve as a signal to the builder to publish full data.
  • shard_blob_tx: builder-signed headers, also known as "data transaction".
  • shard_proposer_slashing: slashings of duplicate shard proposals.
shard_blob_header

Shard header data, in the form of a SignedShardBlobHeader is published to the global shard_blob_header subnet. Shard blob headers select shard blob bids by builders and should be timely to ensure builders can publish the full shard blob before subsequent attestations.

The following validations MUST pass before forwarding the signed_blob_header on the network. Alias header = signed_blob_header.message.

  • [IGNORE] The header is published 1 slot early or later (with a MAXIMUM_GOSSIP_CLOCK_DISPARITY allowance) -- i.e. validate that header.slot <= current_slot + 1 (a client MAY queue future headers for propagation at the appropriate slot).
  • [IGNORE] The header is new enough to still be processed -- i.e. validate that compute_epoch_at_slot(header.slot) >= get_previous_epoch(state)
  • [REJECT] The shard header is for an active shard -- i.e. header.shard < get_active_shard_count(state, compute_epoch_at_slot(header.slot))
  • [REJECT] The header.shard MUST have a committee at the header.slot -- i.e. validate that compute_committee_index_from_shard(state, header.slot, header.shard) doesn't raise an error.
  • [IGNORE] The header is the first header with valid signature received for the (header.proposer_index, header.slot, header.shard) combination.
  • [REJECT] The blob builder defined by blob.builder_index exists and has sufficient balance to back the fee payment.
  • [REJECT] The header signature, signed_blob_header.signature, is valid for the aggregate of proposer and builder -- i.e. bls.FastAggregateVerify([builder_pubkey, proposer_pubkey], blob_signing_root, signed_blob_header.signature).
  • [REJECT] The header is proposed by the expected proposer_index for the blob's header.slot and header.shard in the context of the current shuffling (defined by the current node head state and header.slot). If the proposer_index cannot immediately be verified against the expected shuffling, the blob MAY be queued for later processing while proposers for the blob's branch are calculated -- in such a case do not REJECT, instead IGNORE this message.
shard_blob_tx

Shard data-transactions in the form of a SignedShardBlobHeader are published to the global shard_blob_tx subnet. These shard blob headers are signed solely by the blob-builder.

The following validations MUST pass before forwarding the signed_blob_header on the network. Alias header = signed_blob_header.message.

  • [IGNORE] The header is not propagating more than SHARD_TX_PROPAGATION_BUFFER_SLOTS slots ahead of time -- i.e. validate that header.slot <= current_slot + SHARD_TX_PROPAGATION_BUFFER_SLOTS.
  • [IGNORE] The header is not propagating later than SHARD_TX_PROPAGATION_GRACE_SLOTS slots too late -- i.e. validate that header.slot + SHARD_TX_PROPAGATION_GRACE_SLOTS >= current_slot
  • [REJECT] The shard header is for an active shard -- i.e. header.shard < get_active_shard_count(state, compute_epoch_at_slot(header.slot))
  • [REJECT] The header.shard MUST have a committee at the header.slot -- i.e. validate that compute_committee_index_from_shard(state, header.slot, header.shard) doesn't raise an error.
  • [IGNORE] The header is not stale -- i.e. the corresponding shard proposer has not already selected a header for (header.slot, header.shard).
  • [IGNORE] The header is the first header with valid signature received for the (header.builder_index, header.slot, header.shard) combination.
  • [REJECT] The blob builder, define by header.builder_index, exists and has sufficient balance to back the fee payment.
  • [IGNORE] The header fee SHOULD be higher than previously seen headers for (header.slot, header.shard), from any builder. Propagating nodes MAY increase fee increments in case of spam.
  • [REJECT] The header signature, signed_blob_header.signature, is valid for ONLY the builder -- i.e. bls.Verify(builder_pubkey, blob_signing_root, signed_blob_header.signature). The signature is not an aggregate with the proposer.
  • [REJECT] The header is designated for proposal by the expected proposer_index for the blob's header.slot and header.shard in the context of the current shuffling (defined by the current node head state and header.slot). If the proposer_index cannot immediately be verified against the expected shuffling, the blob MAY be queued for later processing while proposers for the blob's branch are calculated -- in such a case do not REJECT, instead IGNORE this message.
shard_proposer_slashing

Shard proposer slashings, in the form of ShardProposerSlashing, are published to the global shard_proposer_slashing topic.

The following validations MUST pass before forwarding the shard_proposer_slashing on to the network.

  • [IGNORE] The shard proposer slashing is the first valid shard proposer slashing received for the proposer with index proposer_slashing.proposer_index. The proposer_slashing.slot and proposer_slashing.shard are ignored, there are no repeated or per-shard slashings.
  • [REJECT] All of the conditions within process_shard_proposer_slashing pass validation.