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Sharding -- Networking

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

Introduction
Constants
- Misc
Gossip domain
- Topics and messages
  - Builder block bid
    - builder_block_bid
  - Shard sample subnets
    - shard_row_{subnet_id}

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_ROW_SUBNET_COUNT`	`512`	The number of `shard_row_{subnet_id}` subnets used in the gossipsub protocol.
`SHARD_COLUMN_SUBNET_COUNT`	`512`	The number of `shard_column_{subnet_id}` subnets used in the gossipsub protocol.

Gossip domain

Topics and messages

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

Name	Message Type
`shard_row_{subnet_id}`	`SignedShardSample`
`shard_column_{subnet_id}`	`SignedShardSample`
`builder_block_bid`	`BuilderBlockBid`

The DAS network specification defines additional topics.

Builder block bid

`builder_block_bid`

[IGNORE] The bid is published 1 slot early or later (with a MAXIMUM_GOSSIP_CLOCK_DISPARITY allowance) -- i.e. validate that bid.slot <= current_slot + 1 (a client MAY queue future samples for propagation at the appropriate slot).
[IGNORE] The bid is for the current or next block i.e. validate that bid.slot >= current_slot
[IGNORE] The bid is the first bid valid bid for bid.slot, or the bid is at least 1% higher than the previous known bid
[REJECT] The validator defined by bid.validator_index exists and is slashable.
[REJECT] The bid signature, which is an Eth1 signature, needs to be valid and the address needs to contain enough Ether to cover the bid and the data gas base fee.

Shard sample subnets

Shard sample (row/column) subnets are used by builders to make their samples available as part of their intermediate block release after selection by beacon block proposers.

`shard_row_{subnet_id}`

Shard sample data, in the form of a SignedShardSample is published to the shard_row_{subnet_id} and shard_column_{subnet_id} subnets.

The following validations MUST pass before forwarding the sample.

[IGNORE] The sample is published 1 slot early or later (with a MAXIMUM_GOSSIP_CLOCK_DISPARITY allowance) -- i.e. validate that sample.slot <= current_slot + 1 (a client MAY queue future samples for propagation at the appropriate slot).
[IGNORE] The sample is new enough to still be processed -- i.e. validate that compute_epoch_at_slot(sample.slot) >= get_previous_epoch(state)
[REJECT] The shard sample is for the correct subnet -- i.e. sample.row == subnet_id for shard_row_{subnet_id} and sample.column == subnet_id for shard_column_{subnet_id}
[IGNORE] The sample is the first sample with valid signature received for the (sample.builder, sample.slot, sample.row, sample.column) combination.
[REJECT] The sample.data MUST NOT contain any point x >= BLS_MODULUS. Although it is a uint256, not the full 256 bit range is valid.
[REJECT] The validator defined by sample.builder exists and is slashable.
[REJECT] The sample is proposed by the expected builder for the sample's slot. i.e., the beacon block at sample.slot - 1 according to the node's fork choice contains an IntermediateBlockBid with intermediate_block_bid.validator_index == sample.builder
[REJECT] The sample signature, sample.signature, is valid for the builder -- i.e. bls.Verify(builder_pubkey, sample_signing_root, sample.signature) OR sample.signature == Bytes96(b"\0" * 96) AND the sample verification verify_sample passes

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