This document contains the networking specification for Ethereum 2.0 clients added during the Altair deployment.
This document should be viewed as additive to the [document from Phase 0](../phase0/p2p-interface.md) 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.
Altair changes the type of the global beacon block topic and adds one global topic to propagate partially aggregated sync committee signatures to all potential proposers of beacon 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:
- _[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`.
- _[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)`.
- _[REJECT]_ The `contribution_and_proof.selection_proof` is a valid signature of the `SyncCommitteeSigningData` derived from the `contribution` by the validator with index `contribution_and_proof.aggregator_index`.
- _[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 are used to propagate unaggregated sync committee signatures to subsections of the network.
##### `sync_committee_{subnet_id}`
The `sync_committee_{subnet_id}` topics are used to propagate unaggregated sync committee signatures 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_signature` on the network:
- _[IGNORE]_ The signature's slot is for the current slot, i.e. `sync_committee_signature.slot == current_slot`.
- _[IGNORE]_ The block being signed over (`sync_committee_signature.beacon_block_root`) has been seen (via both gossip and non-gossip sources).
- _[IGNORE]_ There has been no other valid sync committee signature for the declared `slot` for the validator referenced by `sync_committee_signature.validator_index`.
- _[REJECT]_ The `subnet_id` is valid for the given validator, i.e. `subnet_id in compute_subnets_for_sync_committee(state, sync_committee_signature.validator_index)`.
- _[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 signatures are broadcast into "subnets" defined by a topic.
The number of subnets is defined by `SYNC_COMMITTEE_SUBNET_COUNT` in the [Altair validator guide](./validator.md#constants).
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 signatures (along with metadata) are sent as `SyncCommitteeSignature`s on the `sync_committee_{subnet_id}` topics.
Aggregated sync committee signatures are packaged into (signed) `SyncCommitteeContribution` along with proofs and gossiped to the `sync_committee_contribution_and_proof` topic.
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`.
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 `i`th bit is set in this bitfield if the validator is currently subscribed to the `sync_committee_{i}` topic.
See the [validator document](./validator.md#sync-committee-subnet-stability) for further details on how the new bits are used.
