Altair -- Networking

Table of contents

• Introduction
• Modifications in Altair
• MetaData
• The gossip domain: gossipsub
  • Topics and messages
  • Global topics
    • beacon_block
    • sync_committee_contribution_and_proof
  • Sync committee subnets
    • sync_committee_{subnet_id}
  • Sync committees and aggregation
  • Transitioning the gossip
• The Req/Resp domain
  • Req-Resp interaction
  • ForkDigest-context
  • Messages
  • BeaconBlocksByRange v2
  • BeaconBlocksByRoot v2
  • GetMetaData v2
  • Transitioning from v1 to v2
• The discovery domain: discv5
  • ENR structure
  • Sync committee bitfield

Introduction

This document contains the networking specification for Altair. This document should be viewed as additive to the document from Phase 0 and will be referred to as the "Phase 0 document" hereafter. Readers should understand the Phase 0 document and use it as a basis to understand the changes outlined in this document.

Altair adds new messages, topics and data to the Req-Resp, Gossip and Discovery domain. Some Phase 0 features will be deprecated, but not removed immediately.

Modifications in Altair

MetaData

The MetaData stored locally by clients is updated with an additional field to communicate the sync committee subnet subscriptions:

(
  seq_number: uint64
  attnets: Bitvector[ATTESTATION_SUBNET_COUNT]
  syncnets: Bitvector[SYNC_COMMITTEE_SUBNET_COUNT]
)

Where

• seq_number and attnets have the same meaning defined in the Phase 0 document.
• syncnets is a Bitvector representing the node's sync committee subnet subscriptions. This field should mirror the data in the node's ENR as outlined in the validator guide.

The gossip domain: gossipsub

Gossip meshes are added in Altair to support the consensus activities of the sync committees. Validators use an aggregation scheme to balance the processing and networking load across all of the relevant actors.

Topics and messages

Topics follow the same specification as in the Phase 0 document. New topics are added in Altair to support the sync committees and the beacon block topic is updated with the modified type.

The specification around the creation, validation, and dissemination of messages has not changed from the Phase 0 document.

The derivation of the message-id has changed starting with Altair to incorporate the message topic along with the message data. These are fields of the Message Protobuf, and interpreted as empty byte strings if missing. The message-id MUST be the following 20 byte value computed from the message: * If message.data has a valid snappy decompression, set message-id to the first 20 bytes of the SHA256 hash of the concatenation of the following data: MESSAGE_DOMAIN_VALID_SNAPPY, the length of the topic byte string (encoded as little-endian uint64), the topic byte string, and the snappy decompressed message data: i.e. SHA256(MESSAGE_DOMAIN_VALID_SNAPPY + uint_to_bytes(uint64(len(message.topic))) + message.topic + snappy_decompress(message.data))[:20]. * Otherwise, set message-id to the first 20 bytes of the SHA256 hash of the concatenation of the following data: MESSAGE_DOMAIN_INVALID_SNAPPY, the length of the topic byte string (encoded as little-endian uint64), the topic byte string, and the raw message data: i.e. SHA256(MESSAGE_DOMAIN_INVALID_SNAPPY + uint_to_bytes(uint64(len(message.topic))) + message.topic + message.data)[:20].

Implementations may need to carefully handle the function that computes the message-id. In particular, messages on topics with the Phase 0 fork digest should use the message-id procedure specified in the Phase 0 document. Messages on topics with the Altair fork digest should use the message-id procedure defined here. If an implementation only supports a single message-id function, it can define a switch inline; for example, if topic in phase0_topics: return phase0_msg_id_fn(message) else return altair_msg_id_fn(message).

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)
sync_committee_contribution_and_proof	SignedContributionAndProof
sync_committee_{subnet_id}	SyncCommitteeMessage

Definitions of these new types can be found in the Altair validator guide.

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

Global topics

Altair changes the type of the global beacon block topic and adds one global topic to propagate partially aggregated sync committee messages to all potential proposers of beacon blocks.

beacon_block

The existing specification for this topic does not change from the Phase 0 document, but the type of the payload does change to the (modified) SignedBeaconBlock. This type changes due to the inclusion of the inner BeaconBlockBody that is modified in Altair.

See the state transition document for Altair for further details.

sync_committee_contribution_and_proof

This topic is used to propagate partially aggregated sync committee messages to be included in future blocks.

The following validations MUST pass before forwarding the signed_contribution_and_proof on the network; define contribution_and_proof = signed_contribution_and_proof.message, contribution = contribution_and_proof.contribution, and the following function get_sync_subcommittee_pubkeys for convenience:

def get_sync_subcommittee_pubkeys(state: BeaconState, subcommittee_index: uint64) -> Sequence[BLSPubkey]:
    # Committees assigned to `slot` sign for `slot - 1`
    # This creates the exceptional logic below when transitioning between sync committee periods
    next_slot_epoch = compute_epoch_at_slot(Slot(state.slot + 1))
    if compute_sync_committee_period(get_current_epoch(state)) == compute_sync_committee_period(next_slot_epoch):
        sync_committee = state.current_sync_committee
    else:
        sync_committee = state.next_sync_committee

    # Return pubkeys for the subcommittee index
    sync_subcommittee_size = SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT
    i = subcommittee_index * sync_subcommittee_size
    return sync_committee.pubkeys[i:i + sync_subcommittee_size]

• [IGNORE] The contribution's slot is for the current slot (with a MAXIMUM_GOSSIP_CLOCK_DISPARITY allowance), i.e. contribution.slot == current_slot.
• [REJECT] The subcommittee index is in the allowed range, i.e. contribution.subcommittee_index < SYNC_COMMITTEE_SUBNET_COUNT.
• [REJECT] The contribution has participants -- that is, any(contribution.aggregation_bits).
• [REJECT] contribution_and_proof.selection_proof selects the validator as an aggregator for the slot -- i.e. is_sync_committee_aggregator(contribution_and_proof.selection_proof) returns True.
• [REJECT] The aggregator's validator index is in the declared subcommittee of the current sync committee -- i.e. state.validators[contribution_and_proof.aggregator_index].pubkey in get_sync_subcommittee_pubkeys(state, contribution.subcommittee_index).
• [IGNORE] A valid sync committee contribution with equal slot, beacon_block_root and subcommittee_index whose aggregation_bits is non-strict superset has not already been seen.
• [IGNORE] The sync committee contribution is the first valid contribution received for the aggregator with index contribution_and_proof.aggregator_index for the slot contribution.slot and subcommittee index contribution.subcommittee_index (this requires maintaining a cache of size SYNC_COMMITTEE_SIZE for this topic that can be flushed after each slot).
• [REJECT] The contribution_and_proof.selection_proof is a valid signature of the SyncAggregatorSelectionData derived from the contribution by the validator with index contribution_and_proof.aggregator_index.
• [REJECT] The aggregator signature, signed_contribution_and_proof.signature, is valid.
• [REJECT] The aggregate signature is valid for the message beacon_block_root and aggregate pubkey derived from the participation info in aggregation_bits for the subcommittee specified by the contribution.subcommittee_index.

Sync committee subnets

Sync committee subnets are used to propagate unaggregated sync committee messages to subsections of the network.

sync_committee_{subnet_id}

The sync_committee_{subnet_id} topics are used to propagate unaggregated sync committee messages to the subnet subnet_id to be aggregated before being gossiped to the global sync_committee_contribution_and_proof topic.

The following validations MUST pass before forwarding the sync_committee_message on the network:

• [IGNORE] The message's slot is for the current slot (with a MAXIMUM_GOSSIP_CLOCK_DISPARITY allowance), i.e. sync_committee_message.slot == current_slot.
• [REJECT] The subnet_id is valid for the given validator, i.e. subnet_id in compute_subnets_for_sync_committee(state, sync_committee_message.validator_index). Note this validation implies the validator is part of the broader current sync committee along with the correct subcommittee.
• [IGNORE] There has been no other valid sync committee message for the declared slot for the validator referenced by sync_committee_message.validator_index (this requires maintaining a cache of size SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT for each subnet that can be flushed after each slot). Note this validation is per topic so that for a given slot, multiple messages could be forwarded with the same validator_index as long as the subnet_ids are distinct.
• [REJECT] The signature is valid for the message beacon_block_root for the validator referenced by validator_index.

Sync committees and aggregation

The aggregation scheme closely follows the design of the attestation aggregation scheme. Sync committee messages are broadcast into "subnets" defined by a topic. The number of subnets is defined by SYNC_COMMITTEE_SUBNET_COUNT in the Altair validator guide. Sync committee members are divided into "subcommittees" which are then assigned to a subnet for the duration of tenure in the sync committee. Individual validators can be duplicated in the broader sync committee such that they are included multiple times in a given subcommittee or across multiple subcommittees.

Unaggregated messages (along with metadata) are sent as SyncCommitteeMessages on the sync_committee_{subnet_id} topics.

Aggregated sync committee messages are packaged into (signed) SyncCommitteeContribution along with proofs and gossiped to the sync_committee_contribution_and_proof topic.

Transitioning the gossip

With any fork, the fork version, and thus the ForkDigestValue, change. Message types are unique per topic, and so for a smooth transition a node must temporarily subscribe to both the old and new topics.

The topics that are not removed in a fork are updated with a new ForkDigestValue. In advance of the fork, a node SHOULD subscribe to the post-fork variants of the topics.

Subscriptions are expected to be well-received, all updated nodes should subscribe as well. Topic-meshes can be grafted quickly as the nodes are already connected and exchanging gossip control messages.

Messages SHOULD NOT be re-broadcast from one fork to the other. A node's behavior before the fork and after the fork are as follows: Pre-fork: - Peers who propagate messages on the post-fork topics MAY be scored negatively proportionally to time till fork, to account for clock discrepancy. - Messages can be IGNORED on the post-fork topics, with a MAXIMUM_GOSSIP_CLOCK_DISPARITY margin.

Post-fork: - Peers who propagate messages on the pre-fork topics MUST NOT be scored negatively. Lagging IWANT may force them to. - Messages on pre and post-fork variants of topics share application-level caches. E.g. an attestation on the both the old and new topic is ignored like any duplicate. - Two epochs after the fork, pre-fork topics SHOULD be unsubscribed from. This is well after the configured seen_ttl.

The Req/Resp domain

Req-Resp interaction

An additional <context-bytes> field is introduced to the response_chunk as defined in the Phase 0 document:

response_chunk  ::= <result> | <context-bytes> | <encoding-dependent-header> | <encoded-payload>

All Phase 0 methods are compatible: <context-bytes> is empty by default. On a non-zero <result> with ErrorMessage payload, the <context-bytes> is also empty.

In Altair and later forks, <context-bytes> functions as a short meta-data, defined per req-resp method, and can parametrize the payload decoder.

ForkDigest-context

Starting with Altair, and in future forks, SSZ type definitions may change. For this common case, we define the ForkDigest-context:

A fixed-width 4 byte <context-bytes>, set to the ForkDigest matching the chunk: compute_fork_digest(fork_version, genesis_validators_root).

Messages

BeaconBlocksByRange v2

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

Request and Response remain unchanged. A ForkDigest-context is used to select the fork namespace of the Response 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

BeaconBlocksByRoot v2

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

Request and Response remain unchanged. A ForkDigest-context is used to select the fork namespace of the Response 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

GetMetaData v2

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

No Request Content.

Response Content:

(
  MetaData
)

Requests the MetaData of a peer, using the new MetaData definition given above that is extended from phase 0 in Altair. Other conditions for the GetMetaData protocol are unchanged from the phase 0 p2p networking document.

Transitioning from v1 to v2

In advance of the fork, implementations can opt in to both run the v1 and v2 for a smooth transition. This is non-breaking, and is recommended as soon as the fork specification is stable.

The v1 variants will be deprecated, and implementations should use v2 when available (as negotiated with peers via LibP2P multistream-select).

The v1 method MAY be unregistered at the fork boundary. In the event of a request on v1 for an Altair specific payload, the responder MUST return the InvalidRequest response code.

The discovery domain: discv5

ENR structure

Sync committee bitfield

An additional bitfield is added to the ENR under the key syncnets to facilitate sync committee subnet discovery. The length of this bitfield is SYNC_COMMITTEE_SUBNET_COUNT where each bit corresponds to a distinct subnet_id for a specific sync committee subnet. The ith bit is set in this bitfield if the validator is currently subscribed to the sync_committee_{i} topic.

Key	Value
syncnets	SSZ Bitvector[SYNC_COMMITTEE_SUBNET_COUNT]

See the validator document for further details on how the new bits are used.