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Capella -- Networking

This document contains the networking specification for Capella.

The specification of these changes continues in the same format as the network specifications of previous upgrades, and assumes them as pre-requisite.

Modifications in Capella
- The gossip domain: gossipsub
  - Topics and messages
    - Global topics
      - beacon_block
      - bls_to_execution_change
  - Transitioning the gossip
- The Req/Resp domain
  - Messages
    - BeaconBlocksByRange v2
    - BeaconBlocksByRoot v2

Modifications in Capella

The gossip domain: gossipsub

A new topic is added to support the gossip of withdrawal credential change messages. And an existing topic is upgraded for updated types in Capella.

Topics and messages

Topics follow the same specification as in prior upgrades. All existing topics remain stable except the beacon block topic which is updated with the modified type.

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)
`bls_to_execution_change`	`SignedBLSToExecutionChange`

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

Global topics

Capella changes the type of the global beacon block topic and adds one global topic to propagate withdrawal credential change messages to all potential proposers of beacon blocks.

`beacon_block`

The type of the payload of this topic changes to the (modified) SignedBeaconBlock found in Capella. Specifically, this type changes with the addition of bls_to_execution_changes to the inner BeaconBlockBody. See Capella state transition document for further details.

`bls_to_execution_change`

This topic is used to propagate signed bls to execution change messages to be included in future blocks.

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

[IGNORE] current_epoch >= CAPELLA_FORK_EPOCH, where current_epoch is defined by the current wall-clock time.
[IGNORE] The signed_bls_to_execution_change is the first valid signed bls to execution change received for the validator with index signed_bls_to_execution_change.message.validator_index.
[REJECT] All of the conditions within process_bls_to_execution_change pass validation.

Transitioning the gossip

See gossip transition details found in the Altair document for details on how to handle transitioning gossip topics for Capella.

The Req/Resp domain

Messages

BeaconBlocksByRange v2

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

The Capella fork-digest is introduced to the context enum to specify Capella block 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`
`BELLATRIX_FORK_VERSION`	`bellatrix.SignedBeaconBlock`
`CAPELLA_FORK_VERSION`	`capella.SignedBeaconBlock`

BeaconBlocksByRoot v2

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