consensus-specs/specs/altair/light-client/p2p-interface.md

Altair Light Client -- Networking

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

Table of contents

• Networking
  • Configuration
  • The gossip domain: gossipsub
    • Topics and messages
      • Global topics
        • light_client_finality_update
        • light_client_optimistic_update
  • The Req/Resp domain
    • Messages
      • GetLightClientBootstrap
      • LightClientUpdatesByRange
      • GetLightClientFinalityUpdate
      • GetLightClientOptimisticUpdate
• Light clients
• Validator assignments
  • Beacon chain responsibilities
  • Sync committee

Networking

This section extends the networking specification for Altair with additional messages, topics and data to the Req-Resp and Gossip domains.

Configuration

Name	Value	Description
MAX_REQUEST_LIGHT_CLIENT_UPDATES	2**7 (= 128)	Maximum number of LightClientUpdate instances in a single request

The gossip domain: gossipsub

Gossip meshes are added to allow light clients to stay in sync with the network.

Topics and messages

New global topics are added to provide light clients with the latest updates.

name	Message Type
light_client_finality_update	LightClientFinalityUpdate
light_client_optimistic_update	LightClientOptimisticUpdate

Global topics

light_client_finality_update

This topic is used to propagate the latest LightClientFinalityUpdate to light clients, allowing them to keep track of the latest finalized_header.

The following validations MUST pass before forwarding the finality_update on the network.

• [IGNORE] The finalized_header.beacon.slot is greater than that of all previously forwarded finality_updates
• [IGNORE] The finality_update is received after the block at signature_slot was given enough time to propagate through the network -- i.e. validate that one-third of finality_update.signature_slot has transpired (SECONDS_PER_SLOT / INTERVALS_PER_SLOT seconds after the start of the slot, with a MAXIMUM_GOSSIP_CLOCK_DISPARITY allowance)

For full nodes, the following validations MUST additionally pass before forwarding the finality_update on the network.

• [IGNORE] The received finality_update matches the locally computed one exactly (as defined in create_light_client_finality_update)

For light clients, the following validations MUST additionally pass before forwarding the finality_update on the network.

• [REJECT] The finality_update is valid -- i.e. validate that process_light_client_finality_update does not indicate errors
• [IGNORE] The finality_update advances the finalized_header of the local LightClientStore -- i.e. validate that processing finality_update increases store.finalized_header.beacon.slot

Light clients SHOULD call process_light_client_finality_update even if the message is ignored.

The gossip ForkDigest-context is determined based on compute_fork_version(compute_epoch_at_slot(finality_update.attested_header.beacon.slot)).

Per context = compute_fork_digest(fork_version, genesis_validators_root):

fork_version	Message SSZ type
GENESIS_FORK_VERSION	n/a
ALTAIR_FORK_VERSION and later	altair.LightClientFinalityUpdate

light_client_optimistic_update

This topic is used to propagate the latest LightClientOptimisticUpdate to light clients, allowing them to keep track of the latest optimistic_header.

The following validations MUST pass before forwarding the optimistic_update on the network.

• [IGNORE] The attested_header.beacon.slot is greater than that of all previously forwarded optimistic_updates
• [IGNORE] The optimistic_update is received after the block at signature_slot was given enough time to propagate through the network -- i.e. validate that one-third of optimistic_update.signature_slot has transpired (SECONDS_PER_SLOT / INTERVALS_PER_SLOT seconds after the start of the slot, with a MAXIMUM_GOSSIP_CLOCK_DISPARITY allowance)

For full nodes, the following validations MUST additionally pass before forwarding the optimistic_update on the network.

• [IGNORE] The received optimistic_update matches the locally computed one exactly (as defined in create_light_client_optimistic_update)

For light clients, the following validations MUST additionally pass before forwarding the optimistic_update on the network.

• [REJECT] The optimistic_update is valid -- i.e. validate that process_light_client_optimistic_update does not indicate errors
• [IGNORE] The optimistic_update either matches corresponding fields of the most recently forwarded LightClientFinalityUpdate (if any), or it advances the optimistic_header of the local LightClientStore -- i.e. validate that processing optimistic_update increases store.optimistic_header.beacon.slot

Light clients SHOULD call process_light_client_optimistic_update even if the message is ignored.

The gossip ForkDigest-context is determined based on compute_fork_version(compute_epoch_at_slot(optimistic_update.attested_header.beacon.slot)).

Per context = compute_fork_digest(fork_version, genesis_validators_root):

fork_version	Message SSZ type
GENESIS_FORK_VERSION	n/a
ALTAIR_FORK_VERSION and later	altair.LightClientOptimisticUpdate

The Req/Resp domain

Messages

GetLightClientBootstrap

Protocol ID: /eth2/beacon_chain/req/light_client_bootstrap/1/

Request Content:

(
  Root
)

Response Content:

(
  LightClientBootstrap
)

Requests the LightClientBootstrap structure corresponding to a given post-Altair beacon block root.

The request MUST be encoded as an SSZ-field.

Peers SHOULD provide results as defined in create_light_client_bootstrap. To fulfill a request, the requested block and its post state need to be known.

When a LightClientBootstrap instance cannot be produced for a given block root, peers SHOULD respond with error code 3: ResourceUnavailable.

A ForkDigest-context based on compute_fork_version(compute_epoch_at_slot(bootstrap.header.beacon.slot)) is used to select the fork namespace of the Response type.

Per context = compute_fork_digest(fork_version, genesis_validators_root):

fork_version	Response SSZ type
GENESIS_FORK_VERSION	n/a
ALTAIR_FORK_VERSION and later	altair.LightClientBootstrap

LightClientUpdatesByRange

Protocol ID: /eth2/beacon_chain/req/light_client_updates_by_range/1/

Request Content:

(
  start_period: uint64
  count: uint64
)

Response Content:

(
  List[LightClientUpdate, MAX_REQUEST_LIGHT_CLIENT_UPDATES]
)

Requests the LightClientUpdate instances in the sync committee period range [start_period, start_period + count), leading up to the current head sync committee period as selected by fork choice.

The request MUST be encoded as an SSZ-container.

The response MUST consist of zero or more response_chunk. Each successful response_chunk MUST contain a single LightClientUpdate payload.

Peers SHOULD provide results as defined in create_light_client_update. They MUST respond with at least the earliest known result within the requested range, and MUST send results in consecutive order (by period). The response MUST NOT contain more than min(MAX_REQUEST_LIGHT_CLIENT_UPDATES, count) results.

For each response_chunk, a ForkDigest-context based on compute_fork_version(compute_epoch_at_slot(update.attested_header.beacon.slot)) is used to select the fork namespace of the Response type. Note that this fork_version may be different from the one used to verify the update.sync_aggregate, which is based on update.signature_slot.

Per context = compute_fork_digest(fork_version, genesis_validators_root):

fork_version	Response chunk SSZ type
GENESIS_FORK_VERSION	n/a
ALTAIR_FORK_VERSION and later	altair.LightClientUpdate

GetLightClientFinalityUpdate

Protocol ID: /eth2/beacon_chain/req/light_client_finality_update/1/

No Request Content.

Response Content:

(
  LightClientFinalityUpdate
)

Requests the latest LightClientFinalityUpdate known by a peer.

Peers SHOULD provide results as defined in create_light_client_finality_update.

When no LightClientFinalityUpdate is available, peers SHOULD respond with error code 3: ResourceUnavailable.

A ForkDigest-context based on compute_fork_version(compute_epoch_at_slot(finality_update.attested_header.beacon.slot)) is used to select the fork namespace of the Response type. Note that this fork_version may be different from the one used to verify the finality_update.sync_aggregate, which is based on finality_update.signature_slot.

Per context = compute_fork_digest(fork_version, genesis_validators_root):

fork_version	Response SSZ type
GENESIS_FORK_VERSION	n/a
ALTAIR_FORK_VERSION and later	altair.LightClientFinalityUpdate

GetLightClientOptimisticUpdate

Protocol ID: /eth2/beacon_chain/req/light_client_optimistic_update/1/

No Request Content.

Response Content:

(
  LightClientOptimisticUpdate
)

Requests the latest LightClientOptimisticUpdate known by a peer.

Peers SHOULD provide results as defined in create_light_client_optimistic_update.

When no LightClientOptimisticUpdate is available, peers SHOULD respond with error code 3: ResourceUnavailable.

A ForkDigest-context based on compute_fork_version(compute_epoch_at_slot(optimistic_update.attested_header.beacon.slot)) is used to select the fork namespace of the Response type. Note that this fork_version may be different from the one used to verify the optimistic_update.sync_aggregate, which is based on optimistic_update.signature_slot.

Per context = compute_fork_digest(fork_version, genesis_validators_root):

fork_version	Response SSZ type
GENESIS_FORK_VERSION	n/a
ALTAIR_FORK_VERSION and later	altair.LightClientOptimisticUpdate

Light clients

Light clients using libp2p to stay in sync with the network SHOULD subscribe to the light_client_finality_update and light_client_optimistic_update pubsub topics and validate all received messages while the light client sync process supports processing LightClientFinalityUpdate and LightClientOptimisticUpdate structures.

Light clients MAY also collect historic light client data and make it available to other peers. If they do, they SHOULD advertise supported message endpoints in the Req/Resp domain, and MAY also update the contents of their Status message to reflect the locally available light client data.

If only limited light client data is locally available, the light client SHOULD use data based on genesis_block and GENESIS_SLOT in its Status message. Hybrid peers that also implement full node functionality MUST only incorporate data based on their full node sync progress into their Status message.

Validator assignments

This section extends the honest validator specification with additional responsibilities to enable light clients to sync with the network.

Beacon chain responsibilities

All full nodes SHOULD subscribe to and provide stability on the light_client_finality_update and light_client_optimistic_update pubsub topics by validating all received messages.

Sync committee

Whenever fork choice selects a new head block with a sync aggregate participation >= MIN_SYNC_COMMITTEE_PARTICIPANTS and a post-Altair parent block, full nodes with at least one validator assigned to the current sync committee at the block's slot SHOULD broadcast derived light client data as follows:

• If finalized_header.beacon.slot increased, a LightClientFinalityUpdate SHOULD be broadcasted to the pubsub topic light_client_finality_update if no matching message has not yet been forwarded as part of gossip validation.
• If attested_header.beacon.slot increased, a LightClientOptimisticUpdate SHOULD be broadcasted to the pubsub topic light_client_optimistic_update if no matching message has not yet been forwarded as part of gossip validation.

These messages SHOULD be broadcasted after one-third of slot has transpired (SECONDS_PER_SLOT / INTERVALS_PER_SLOT seconds after the start of the slot). To ensure that the corresponding block was given enough time to propagate through the network, they SHOULD NOT be sent earlier. Note that this is different from how other messages are handled, e.g., attestations, which may be sent early.