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Bellatrix -- Networking

This document contains the networking specification for the Bellatrix.

The specification of these changes continues in the same format as the network specifications of previous upgrades, and assumes them as pre-requisite. This document should be viewed as additive to the documents from Phase 0 and from Altair and will be referred to as the "Phase 0 document" and "Altair document" respectively, hereafter. Readers should understand the Phase 0 and Altair documents and use them as a basis to understand the changes outlined in this document.

Modifications in Bellatrix
- The gossip domain: gossipsub
  - Topics and messages
    - Global topics
      - beacon_block
  - Transitioning the gossip
- The Req/Resp domain
  - Messages
    - BeaconBlocksByRange v2
    - BeaconBlocksByRoot v2
Design decision rationale
- Gossipsub
  - Why was the max gossip message size increased at Bellatrix?
- Req/Resp
  - Why was the max chunk response size increased at Bellatrix?
  - Why allow invalid payloads on the P2P network?

Modifications in Bellatrix

The gossip domain: gossipsub

Some gossip meshes are upgraded in Bellatrix to support upgraded types.

Topics and messages

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

The specification around the creation, validation, and dissemination of messages has not changed from the Phase 0 and Altair documents unless explicitly noted here.

The derivation of the message-id remains stable.

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)

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

Global topics

Bellatrix changes the type of the global beacon block topic.

`beacon_block`

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

Blocks with execution enabled will be permitted to propagate regardless of the validity of the execution payload. This prevents network segregation between optimistic and non-optimistic nodes.

In addition to the gossip validations for this topic from prior specifications, the following validations MUST pass before forwarding the signed_beacon_block on the network. Alias block = signed_beacon_block.message, execution_payload = block.body.execution_payload.

If the execution is enabled for the block -- i.e. is_execution_enabled(state, block.body) then validate the following:
- [REJECT] The block's execution payload timestamp is correct with respect to the slot -- i.e. execution_payload.timestamp == compute_timestamp_at_slot(state, block.slot).
- If execution_payload verification of block's parent by an execution node is not complete:
  - [REJECT] The block's parent (defined by block.parent_root) passes all validation (excluding execution node verification of the block.body.execution_payload).
- otherwise:
  - [IGNORE] The block's parent (defined by block.parent_root) passes all validation (including execution node verification of the block.body.execution_payload).

The following gossip validation from prior specifications MUST NOT be applied if the execution is enabled for the block -- i.e. is_execution_enabled(state, block.body):

[REJECT] The block's parent (defined by block.parent_root) passes validation.

Transitioning the gossip

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

The Req/Resp domain

Non-faulty, optimistic nodes may send blocks which result in an INVALID response from an execution engine. To prevent network segregation between optimistic and non-optimistic nodes, transmission of an INVALID execution payload via the Req/Resp domain SHOULD NOT cause a node to be down-scored or disconnected. Transmission of a block which is invalid due to any consensus layer rules (i.e., not execution layer rules) MAY result in down-scoring or disconnection.

Messages

BeaconBlocksByRange v2

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

Request and Response remain unchanged unless explicitly noted here.

Bellatrix fork-digest is introduced to the context enum to specify Bellatrix 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`

BeaconBlocksByRoot v2

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

Request and Response remain unchanged. Bellatrix fork-digest is introduced to the context enum to specify Bellatrix 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`

Design decision rationale

Gossipsub

Why was the max gossip message size increased at Bellatrix?

With the addition of ExecutionPayload to BeaconBlocks, there is a dynamic field -- transactions -- which can validly exceed the GOSSIP_MAX_SIZE limit (1 MiB) put in place at Phase 0, so GOSSIP_MAX_SIZE has increased to 10 Mib on the network. At the GAS_LIMIT (~30M) currently seen on mainnet in 2021, a single transaction filled entirely with data at a cost of 16 gas per byte can create a valid ExecutionPayload of ~2 MiB. Thus we need a size limit to at least account for current mainnet conditions.

Note, that due to additional size induced by the BeaconBlock contents (e.g. proposer signature, operations lists, etc) this does reduce the theoretical max valid ExecutionPayload (and transactions list) size as slightly lower than 10 MiB. Considering that BeaconBlock max size is on the order of 128 KiB in the worst case and the current gas limit (~30M) bounds max blocksize to less than 2 MiB today, this marginal difference in theoretical bounds will have zero impact on network functionality and security.

Req/Resp

Why was the max chunk response size increased at Bellatrix?

Similar to the discussion about the maximum gossip size increase, the ExecutionPayload type can cause BeaconBlocks to exceed the 1 MiB bounds put in place during Phase 0.

As with the gossip limit, 10 MiB is selected because this is firmly above any valid block sizes in the range of gas limits expected in the medium term.

As with both gossip and req/rsp maximum values, type-specific limits should always by simultaneously respected.

Why allow invalid payloads on the P2P network?

The specification allows blocks with invalid execution payloads to propagate across gossip and via RPC calls. The reasoning for this is as follows:

Optimistic nodes must listen to block gossip to obtain a view of the head of the chain.
Therefore, optimistic nodes must propagate gossip blocks. Otherwise, they'd be censoring.
If optimistic nodes will propagate blocks via gossip, then they must respond to requests for the parent via RPC.
Therefore, optimistic nodes must send optimistic blocks via RPC.

So, to prevent network segregation from optimistic nodes inadvertently sending invalid execution payloads, nodes should never downscore/disconnect nodes due to such invalid payloads. This does open the network to some DoS attacks from invalid execution payloads, but the scope of actors is limited to validators who can put those payloads in valid (and slashable) beacon blocks. Therefore, it is argued that the DoS risk introduced in tolerable.

More complicated schemes are possible that could restrict invalid payloads from RPC. However, it's not clear that complexity is warranted.

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