306 lines
10 KiB
Markdown
306 lines
10 KiB
Markdown
# Capella -- The Beacon Chain
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## Table of contents
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## Introduction
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Capella is a consensus-layer upgrade containing a number of features related
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to validator withdrawals. Including:
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* Automatic withdrawals of `withdrawable` validators
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* Partial withdrawals during block proposal
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* Operation to change from `BLS_WITHDRAWAL_PREFIX` to
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`ETH1_ADDRESS_WITHDRAWAL_PREFIX` versioned withdrawal credentials to enable withdrawals for a validator
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## Custom types
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## Constants
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We define the following Python custom types for type hinting and readability:
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| Name | SSZ equivalent | Description |
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| - | - | - |
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| `TransactionType` | `Bytes1` | an EIP-2718 type |
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## Preset
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### State list lengths
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| Name | Value | Unit | Duration |
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| - | - | :-: | :-: |
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| `WITHDRAWAL_TRANSACTION_LIMIT` | `uint64(2**40)` (= 1,099,511,627,776) | withdrawal transactions enqueued in state|
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### Execution
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| Name | Value | Description |
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| - | - | - |
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| `TX_TYPE_WITHDRAWAL` | `TransactionType('0x05')` | EIP-2718 TX Type |
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| `MAX_WITHDRAWAL_TRANSACTIONS_PER_PAYLOAD` | `uint64(2**4)` (= 16) | Maximum amount of withdrawal transactions allowed in each payload |
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## Configuration
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## Containers
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### Extended Containers
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#### `Validator`
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```python
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class Validator(Container):
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pubkey: BLSPubkey
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withdrawal_credentials: Bytes32 # Commitment to pubkey for withdrawals
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effective_balance: Gwei # Balance at stake
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slashed: boolean
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# Status epochs
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activation_eligibility_epoch: Epoch # When criteria for activation were met
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activation_epoch: Epoch
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exit_epoch: Epoch
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withdrawable_epoch: Epoch # When validator can withdraw funds
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withdrawn_epoch: Epoch # [New in Capella]
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```
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#### `BeaconState`
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```python
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class BeaconState(Container):
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# Versioning
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genesis_time: uint64
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genesis_validators_root: Root
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slot: Slot
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fork: Fork
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# History
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latest_block_header: BeaconBlockHeader
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block_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
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state_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
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historical_roots: List[Root, HISTORICAL_ROOTS_LIMIT]
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# Eth1
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eth1_data: Eth1Data
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eth1_data_votes: List[Eth1Data, EPOCHS_PER_ETH1_VOTING_PERIOD * SLOTS_PER_EPOCH]
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eth1_deposit_index: uint64
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# Registry
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validators: List[Validator, VALIDATOR_REGISTRY_LIMIT]
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balances: List[Gwei, VALIDATOR_REGISTRY_LIMIT]
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# Randomness
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randao_mixes: Vector[Bytes32, EPOCHS_PER_HISTORICAL_VECTOR]
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# Slashings
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slashings: Vector[Gwei, EPOCHS_PER_SLASHINGS_VECTOR] # Per-epoch sums of slashed effective balances
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# Participation
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previous_epoch_participation: List[ParticipationFlags, VALIDATOR_REGISTRY_LIMIT]
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current_epoch_participation: List[ParticipationFlags, VALIDATOR_REGISTRY_LIMIT]
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# Finality
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justification_bits: Bitvector[JUSTIFICATION_BITS_LENGTH] # Bit set for every recent justified epoch
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previous_justified_checkpoint: Checkpoint
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current_justified_checkpoint: Checkpoint
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finalized_checkpoint: Checkpoint
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# Inactivity
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inactivity_scores: List[uint64, VALIDATOR_REGISTRY_LIMIT]
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# Sync
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current_sync_committee: SyncCommittee
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next_sync_committee: SyncCommittee
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# Execution
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latest_execution_payload_header: ExecutionPayloadHeader
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# Withdrawals
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withdrawal_receipts: List[WithdrawalTransaction, WITHDRAWAL_TRANSACTION_LIMIT] # [New in Capella]
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```
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#### `ExecutionPayload`
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```python
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class ExecutionPayload(Container):
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# Execution block header fields
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parent_hash: Hash32
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fee_recipient: ExecutionAddress # 'beneficiary' in the yellow paper
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state_root: Bytes32
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receipts_root: Bytes32
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logs_bloom: ByteVector[BYTES_PER_LOGS_BLOOM]
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prev_randao: Bytes32 # 'difficulty' in the yellow paper
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block_number: uint64 # 'number' in the yellow paper
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gas_limit: uint64
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gas_used: uint64
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timestamp: uint64
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extra_data: ByteList[MAX_EXTRA_DATA_BYTES]
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base_fee_per_gas: uint256
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# Extra payload fields
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block_hash: Hash32 # Hash of execution block
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transactions: List[Transaction, MAX_TRANSACTIONS_PER_PAYLOAD]
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withdrawal_transactions: List[WithdrawalTransaction, MAX_WITHDRAWAL_TRANSACTIONS_PER_PAYLOAD] # [New in Capella]
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```
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#### `ExecutionPayloadHeader`
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```python
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class ExecutionPayloadHeader(Container):
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# Execution block header fields
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parent_hash: Hash32
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fee_recipient: ExecutionAddress
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state_root: Bytes32
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receipts_root: Bytes32
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logs_bloom: ByteVector[BYTES_PER_LOGS_BLOOM]
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prev_randao: Bytes32
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block_number: uint64
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gas_limit: uint64
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gas_used: uint64
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timestamp: uint64
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extra_data: ByteList[MAX_EXTRA_DATA_BYTES]
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base_fee_per_gas: uint256
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# Extra payload fields
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block_hash: Hash32 # Hash of execution block
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transactions_root: Root
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withdrawal_transactions_root: Root # [New in Capella]
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```
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### New containers
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#### `WithdrawalTransaction`
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New EIP-2718 transaction type, with the format being the single byte `TX_TYPE_WITHDRAWAL`
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followed by an SSZ encoding of the `WithdrawalTransaction` container comprising the transaction contents.
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*Note*: This container is used for both a special TX that goes into an `ExecutionPayload`
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as well as in the `BeaconState`'s `withdrawal_receipts` queue.
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```python
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class WithdrawalTransaction(Container):
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address: ExecutionAddress
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amount: Gwei
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```
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## Helpers
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### Beacon state mutators
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#### `withdraw`
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```python
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def withdraw(state: BeaconState, index: ValidatorIndex, amount: Gwei) -> None:
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# Decrease the validator's balance
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decrease_balance(state, index, amount)
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# Create a corresponding withdrawal receipt
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receipt = WithdrawalTransaction(
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address=state.validators[index].withdrawal_credentials[12:],
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amount=amount,
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)
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state.withdrawal_receipts.append(receipt)
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```
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### Predicates
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#### `is_fully_withdrawable_validator`
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```python
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def is_fully_withdrawable_validator(validator: Validator, epoch: Epoch) -> bool:
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"""
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Check if ``validator`` is fully withdrawable.
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"""
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is_eth1_withdrawal_prefix = validator.withdrawal_credentials[0:1] == ETH1_ADDRESS_WITHDRAWAL_PREFIX
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return is_eth1_withdrawal_prefix and validator.withdrawable_epoch <= epoch < validator.withdrawn_epoch
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```
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## Beacon chain state transition function
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### Epoch processing
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```python
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def process_epoch(state: BeaconState) -> None:
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process_justification_and_finalization(state)
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process_inactivity_updates(state)
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process_rewards_and_penalties(state)
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process_registry_updates(state)
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process_slashings(state)
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process_eth1_data_reset(state)
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process_effective_balance_updates(state)
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process_slashings_reset(state)
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process_randao_mixes_reset(state)
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process_historical_roots_update(state)
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process_participation_flag_updates(state)
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process_sync_committee_updates(state)
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process_full_withdrawals(state) # [New in Capella]
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```
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#### Withdrawals
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*Note*: The function `process_full_withdrawals` is new.
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```python
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def process_full_withdrawals(state: BeaconState) -> None:
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current_epoch = get_current_epoch(state)
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for index, validator in enumerate(state.validators):
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if is_fully_withdrawable_validator(validator, current_epoch):
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# TODO, consider the zero-balance case
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withdraw(state, ValidatorIndex(index), state.balances[index])
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validator.withdrawn_epoch = current_epoch
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```
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### Block processing
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```python
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def process_block(state: BeaconState, block: BeaconBlock) -> None:
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process_block_header(state, block)
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if is_execution_enabled(state, block.body):
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process_withdrawal_transactions(state, block.body.execution_payload) # [New in Capella]
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process_execution_payload(state, block.body.execution_payload, EXECUTION_ENGINE) # [Modified in Capella]
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process_randao(state, block.body)
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process_eth1_data(state, block.body)
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process_operations(state, block.body)
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process_sync_aggregate(state, block.body.sync_aggregate)
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```
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#### New `process_withdrawal_transactions`
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```python
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def process_withdrawal_transactions(state: BeaconState, payload: ExecutionPayload) -> None:
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num_withdrawal_transactions = min(MAX_WITHDRAWAL_TRANSACTIONS_PER_PAYLOAD, len(state.withdrawal_receipts))
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dequeued_withdrawal_receipts = state.withdrawal_receipts[:num_withdrawal_transactions]
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assert len(dequeued_withdrawal_receipts) == len(payload.withdrawal_transactions)
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for dequeued_receipt, withdrawal_transaction in zip(dequeued_withdrawal_receipts, payload.withdrawal_transactions):
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assert dequeued_receipt == withdrawal_transaction
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# Ensure no withdrawal type transactions in the normal payload transactions
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# assert no_withdrawal_type_transactions_in(payload.transactions)
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# Remove dequeued receipts from state
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state.withdrawal_receipts = state.withdrawal_receipts[num_withdrawal_transactions:]
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```
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#### Modified `process_execution_payload`
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*Note*: The function `process_execution_payload` is modified to use the new `ExecutionPayloadHeader` type.
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```python
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def process_execution_payload(state: BeaconState, payload: ExecutionPayload, execution_engine: ExecutionEngine) -> None:
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# Verify consistency of the parent hash with respect to the previous execution payload header
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if is_merge_transition_complete(state):
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assert payload.parent_hash == state.latest_execution_payload_header.block_hash
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# Verify prev_randao
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assert payload.prev_randao == get_randao_mix(state, get_current_epoch(state))
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# Verify timestamp
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assert payload.timestamp == compute_timestamp_at_slot(state, state.slot)
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# Verify the execution payload is valid
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assert execution_engine.notify_new_payload(payload)
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# Cache execution payload header
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state.latest_execution_payload_header = ExecutionPayloadHeader(
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parent_hash=payload.parent_hash,
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fee_recipient=payload.fee_recipient,
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state_root=payload.state_root,
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receipts_root=payload.receipts_root,
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logs_bloom=payload.logs_bloom,
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prev_randao=payload.prev_randao,
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block_number=payload.block_number,
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gas_limit=payload.gas_limit,
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gas_used=payload.gas_used,
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timestamp=payload.timestamp,
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extra_data=payload.extra_data,
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base_fee_per_gas=payload.base_fee_per_gas,
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block_hash=payload.block_hash,
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transactions_root=hash_tree_root(payload.transactions),
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withdrawal_transactions_root=hash_tree_root(payload.withdrawal_transactions),
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)
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```
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