# Builder-API post ePBS for Staked Builders

This follows from the work on defining the Builder-API post ePBS for Off-Protocol Builders. Details on the work can be found at: https://hackmd.io/SecxqKCKTTCQE0NA9AMBnA

## Introduction

As the name suggests, ePBS (enshrined Proposer-Builder Separation) integrates a builder entity directly into the Ethereum L1 protocol. It separates execution block validation from consensus block validation, creating a cleaner architectural division.

Under ePBS, the proposer receives bids from both enshrined builders and external entities like relays, then commits to a specific `ExecutionPayloadBid`. The proposer releases a `BeaconBlock` containing the committed `ExecutionPayloadBid`, which is subsequently attested by validators.

The `ExecutionPayloadEnvelope` is broadcast to the PTC (Payload Timeliness Committee) by the builder/off-protocol relays. Simultaneously, blobs from the `BlobsBundle` are split into data columns and distributed to the network.

The PTC observes the `ExecutionPayloadEnvelope` and sends a `PayloadAttestation` which indicates whether the payload has been seen or not. It ensures that the payload has been released in a timely fashion by the builder.

With ePBS, Builders can be staked in protocol by staking a minimum of 1ETH and using a 0x03 withdraw credential. Builder entities are different from Validator entities. Builders are staked but they do not engage in validator duties

The ePBS spec defines a p2p gossip topic called [`execution_payload_bid` ](https://github.com/ethereum/consensus-specs/blob/master/specs/gloas/p2p-interface.md?plain=1#L270). Validators can listen to bids for the execution payloads from builders via this gossip topic.

The P2P gossip topic might not always be ideal to transmit bids to the proposer. The proposer might want to directly connect to the builder via a TCP connection to receive the `SignedExecutionPayloadBid`.

This doc aims to specify how such an API could look like.

## Builder-API definition

The Builder-API for staked builders should be fairly simple as it just involves the proposer requesting the execution payload bid from the builder.

### High Level Flows

The flow would go like the following:
1. The proposer sends POST HTTP requests to builders querying the `SignedExecutionPayloadBid`.
2. The builder returns a `SignedExecutionPayloadBid` indicating the value of the bid via either the `bid.value` field or the `bid.execution_payment` field.
3. The proposer embeds the chosen `SignedExecutionPayloadBid` into a `SignedBeaconBlock` and returns it to the builder. Both the proposer and the builder broadcast the `SignedBeaconBlock` to the network.
4. The builder will then construct and broadcast the `SignedExecutionPayloadEnvelope` corresponding to the `SignedExecutionPayloadBid` to the PTC commitee.

### Validator Registrations

The proposer can send `ValidatorRegistrations` to the connected builders to communicate their `fee_recipient`, `gas_limit` and whether they are willing to accept a trusted payment from the connected builder.

We can define a new `ValidatorRegistrationV2` like the below:

class ValidatorRegistrationV2(Container):
    builder_index: BuilderIndex 
    validator_index: ValidatorIndex
    fee_recipient: ExecutionAddress
    proposal_slot: Slot
    gas_limit: uint64
    builder_preferences: BuilderPreferences

We can define a `BuilderPreferences` object like the below:

class BuilderPreferences(Container):
    execution_payment_accepted: boolean

The `builder_index` is the index of the staked builder to which the validator is sending this registration. It is the index of the builder in the `builders` list in the `BeaconState`.
The `validator_index` is the index of the validator in the `validators` list in the `BeaconState`.
The `proposal_slot` is a slot at which the validator will be a proposer.

Validators send the registrations only if they are proposing in a slot in the upcoming epoch unlike today where all validators send their registrations every epoch.

## API definitions

### Execution Payload Bid retrival

The proposer queries the staked builder for a `SignedExecutionPayloadBid`.

GET `/eth/v1/builder/execution_payload_bid`

### Execution Payload Bid commitment

If the proposer chooses the `SignedExecutionPayloadBid`, it embeds it in its `SignedBeaconBlock` and sends it to the builder. The builder is responsible for constructing and broadcasting the `SignedExecutionPayloadEnvelope` to the PTC commitee. Both the proposer and builder can broadcast the `SignedBeaconBlock` for maximal desimination.

POST `/eth/v1/builder/beacon_block`

### Send Validator Registration

Called by a validator every epoch if they will be proposing in the upcoming epoch.

POST `eth/v2/builder/validators`

## Notes

1. The builder can pay the proposer either via the execution payload or via the trustless payment mechanism. The proposer can know which payment method is being used by looking at the `bid.value` and `bid.execution_payment` fields are set.
2. The proposer will have to manually store the urls of the staked builder. They could pass it in as a cmd line flag or a config file.
3. We still will have validator registrations for staked builders. As builders would require the fee recipient in advance if they choose to pay the validator via the execution payload rather than trustless payments.
4. We can just have SSZ support for all these APIs.

## Questions

1. What happens if `bid.value` and `bid.execution_payment` both are set?