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Optimistic Sync

../specs/bellatrix/fork-choice.md#validate_merge_block

Introduction

In order to provide a syncing execution engine with a partial view of the head of the chain, it may be desirable for a consensus engine to import beacon blocks without verifying the execution payloads. This partial sync is called an optimistic sync.

Optimistic sync is designed to be opt-in and backwards compatible (i.e., non-optimistic nodes can tolerate optimistic nodes on the network and vice versa). Optimistic sync is not a fundamental requirement for consensus nodes. Rather, it's a stop-gap measure to allow execution nodes to sync via established methods until future Ethereum roadmap items are implemented (e.g., statelessness).

Constants

Name Value Unit
SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY 128 slots

Note: the SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY must be user-configurable. See Fork Choice Poisoning.

Helpers

Let head: BeaconBlock be the result of calling of the fork choice algorithm at the time of block production. Let head_block_root: Root be the root of that block.

Let blocks: Dict[Root, BeaconBlock] and block_states: Dict[Root, BeaconState] be the blocks (and accompanying states) that have been verified either completely or optimistically.

Let optimistic_roots: Set[Root] be the set of hash_tree_root(block) for all optimistically imported blocks which have only received a SYNCING designation from an execution engine (i.e., they are not known to be INVALID or VALID).

Let current_slot: Slot be (time - genesis_time) // SECONDS_PER_SLOT where time is the UNIX time according to the local system clock.

@dataclass
class OptimisticStore(object):
    optimistic_roots: Set[Root]
    head_block_root: Root
    blocks: Dict[Root, BeaconBlock]
    block_states: Dict[Root, BeaconState]
def is_optimistic(opt_store: OptimisticStore, block: BeaconBlock) -> bool:
    return hash_tree_root(block) in opt_store.optimistic_roots
def latest_verified_ancestor(opt_store: OptimisticStore, block: BeaconBlock) -> BeaconBlock:
    # It is assumed that the `block` parameter is never an INVALID block.
    while True:
        if not is_optimistic(opt_store, block) or block.parent_root == Root():
            return block
        block = opt_store.blocks[block.parent_root]
def is_execution_block(block: BeaconBlock) -> bool:
    return block.body.execution_payload != ExecutionPayload()
def is_optimistic_candidate_block(opt_store: OptimisticStore, current_slot: Slot, block: BeaconBlock) -> bool:
    justified_root = opt_store.block_states[opt_store.head_block_root].current_justified_checkpoint.root
    justifed_is_execution_block = is_execution_block(opt_store.blocks[justified_root])
    block_is_deep = block.slot + SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY <= current_slot
    return justifed_is_execution_block or block_is_deep

Let only a node which returns is_optimistic(opt_store, head) is True be an optimistic node. Let only a validator on an optimistic node be an optimistic validator.

When this specification only defines behaviour for an optimistic node/validator, but not for the non-optimistic case, assume default behaviours without regard for optimistic sync.

Mechanisms

When to optimistically import blocks

A block MAY be optimistically imported when is_optimistic_candidate_block(opt_store, current_slot, block) returns True. This ensures that blocks are only optimistically imported if either:

  1. The justified checkpoint has execution enabled.
  2. The current slot (as per the system clock) is at least SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY ahead of the slot of the block being imported.

See Fork Choice Poisoning for the motivations behind these conditions.

How to optimistically import blocks

To optimistically import a block:

  • The execute_payload function MUST return True if the execution engine returns SYNCING or VALID. An INVALID response MUST return False.
  • The validate_merge_block function MUST NOT raise an assertion if both the pow_block and pow_parent are unknown to the execution engine.
    • All other assertions in validate_merge_block (e.g., TERMINAL_BLOCK_HASH) MUST prevent an optimistic import.
  • The parent of the block MUST NOT have an INVALID execution payload.

In addition to this change in validation, the consensus engine MUST track which blocks returned SYNCING and which returned VALID for subsequent processing.

Optimistically imported blocks MUST pass all verifications included in process_block (withstanding the modifications to execute_payload).

A consensus engine MUST be able to retrospectively (i.e., after import) modify the status of SYNCING blocks to be either VALID or INVALID based upon responses from an execution engine. I.e., perform the following transitions:

  • SYNCING -> VALID
  • SYNCING -> INVALID

When a block transitions from SYNCING -> VALID, all ancestors of the block MUST also transition from SYNCING -> VALID. Such a block and any previously SYNCING ancestors are no longer considered "optimistically imported".

When a block transitions from SYNCING -> INVALID, all descendants of the block MUST also transition from SYNCING -> INVALID.

When a block transitions from the SYNCING state, it is removed from the set of opt_store.optimistic_roots.

When a "merge block" (i.e. the first block which enables execution in a chain) is declared to be VALID by an execution engine (either directly or indirectly), the full validate_merge_block MUST be run against the merge block. If the block fails validate_merge_block, the merge block MUST be treated the same as an INVALID block (i.e., it and all its descendants are invalidated and removed from the block tree).

Execution Engine Errors

When an execution engine returns an error or fails to respond to a payload validity request for some block, a consensus engine:

  • MUST NOT optimistically import the block.
  • MUST NOT apply the block to the fork choice store.
  • MAY queue the block for later processing.

Assumptions about Execution Engine Behaviour

This specification assumes execution engines will only return SYNCING when there is insufficient information available to make a VALID or INVALID determination on the given ExecutionPayload (e.g., the parent payload is unknown). Specifically, SYNCING responses should be fork-specific, in that the search for a block on one chain MUST NOT trigger a SYNCING response for another chain.

Re-Orgs

The consensus engine MUST support any chain reorganisation which does not affect the justified checkpoint.

If the justified checkpoint transitions from SYNCING -> INVALID, a consensus engine MAY choose to alert the user and force the application to exit.

Fork Choice

Consensus engines MUST support removing blocks from fork choice that transition from SYNCING to INVALID. Specifically, a block deemed INVALID at any point MUST NOT be included in the canonical chain and the weights from those INVALID blocks MUST NOT be applied to any VALID or SYNCING ancestors.

Fork Choice Poisoning

During the merge transition it is possible for an attacker to craft a BeaconBlock with an execution payload that references an eternally-unavailable body.execution_payload.parent_hash (i.e., the parent hash is random bytes). In rare circumstances, it is possible that an attacker can build atop such a block to trigger justification. If an optimistic node imports this malicious chain, that node will have a "poisoned" fork choice store, such that the node is unable to produce a block that descends from the head (due to the invalid chain of payloads) and the node is unable to produce a block that forks around the head (due to the justification of the malicious chain).

If an honest chain exists which justifies a higher epoch than the malicious chain, that chain will take precedence and revive any poisoned store. Such a chain, if imported before the malicious chain, will prevent the store from being poisoned. Therefore, the poisoning attack is temporary if >= 2/3rds of the network is honest and non-faulty.

The SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY parameter assumes that the network will justify a honest chain within some number of slots. With this assumption, it is acceptable to optimistically import transition blocks during the sync process. Since there is an assumption that an honest chain with a higher justified checkpoint exists, any fork choice poisoning will be short-lived and resolved before that node is required to produce a block.

However, the assumption that the honest, canonical chain will always justify within SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY slots is dubious. Therefore, clients MUST provide the following command line flag to assist with manual disaster recovery:

  • --safe-slots-to-import-optimistically: modifies the SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY.

Checkpoint Sync (Weak Subjectivity Sync)

A consensus engine MAY assume that the ExecutionPayload of a block used as an anchor for checkpoint sync is VALID without necessarily providing that payload to an execution engine.

Validator assignments

An optimistic node is not a full node. It is unable to produce blocks, since an execution engine cannot produce a payload upon an unknown parent. It cannot faithfully attest to the head block of the chain, since it has not fully verified that block.

Block Production

An optimistic validator MUST NOT produce a block (i.e., sign across the DOMAIN_BEACON_PROPOSER domain).

Attesting

An optimistic validator MUST NOT participate in attestation (i.e., sign across the DOMAIN_BEACON_ATTESTER, DOMAIN_SELECTION_PROOF or DOMAIN_AGGREGATE_AND_PROOF domains).

Participating in Sync Committees

An optimistic validator MUST NOT participate in sync committees (i.e., sign across the DOMAIN_SYNC_COMMITTEE, DOMAIN_SYNC_COMMITTEE_SELECTION_PROOF or DOMAIN_CONTRIBUTION_AND_PROOF domains).

Ethereum Beacon APIs

Consensus engines which provide an implementation of the Ethereum Beacon APIs must take care to avoid presenting optimistic blocks as fully-verified blocks.

Helpers

Let the following response types be defined as any response with the corresponding HTTP status code:

  • "Success" Response: Status Codes 200-299.
  • "Not Found" Response: Status Code 404.
  • "Syncing" Response: Status Code 503.

Requests for Optimistic Blocks

When information about an optimistic block is requested, the consensus engine:

  • MUST NOT respond with success.
  • MAY respond with not found.
  • MAY respond with syncing.

Requests for an Optimistic Head

When is_optimistic(opt_store, head) is True, the consensus engine:

  • MUST NOT return an optimistic head.
  • MAY substitute the head block with latest_verified_ancestor(block).
  • MAY return syncing.

Requests to Validators Endpoints

When is_optimistic(opt_store, head) is True, the consensus engine MUST return syncing to all endpoints which match the following pattern:

  • eth/*/validator/*

Design Decision Rationale

Why SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY?

Nodes can only import an optimistic block if their justified checkpoint is verified or the block is older than SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY.

These restraints are applied in order to mitigate an attack where a block which enables execution (a transition block) can reference a junk parent hash. This makes it impossible for honest nodes to build atop that block. If an attacker exploits a nuance in fork choice filter_block_tree, they can, in some rare cases, produce a junk block that out-competes all locally produced blocks for the head. This prevents a node from producing a chain of blocks, therefore breaking liveness.

Thankfully, if 2/3rds of validators are not poisoned, they can justify an honest chain which will un-poison all other nodes.

Notably, this attack only exists for optimistic nodes. Nodes which fully verify the transition block will reject a block with a junk parent hash. Therefore, liveness is unaffected if a vast majority of nodes have fully synced execution and consensus clients before and during the transition.

Given all of this, we can say two things:

  1. BNs which are following the head during the transition shouldn't optimistically import the transition block. If 1/3rd of validators optimistically import the poison block, there will be no remaining nodes to justify an honest chain.
  2. BNs which are syncing can optimistically import transition blocks. In this case a justified chain already exists blocks. The poison block would be quickly reverted and would have no affect on liveness.

Astute readers will notice that (2) contains a glaring assumption about network liveness. This is necessary because a node cannot feasibly ascertain that the transition block is justified without importing that block and risking poisoning. Therefore, we use SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY to say something along the lines of: "if the transition block is sufficiently old enough, then we can just assume that block is honest or there exists an honest justified chain to out-compete it."

Note the use of "feasibly" in the previous paragraph. One can imagine mechanisms to check that a block is justified before importing it. For example, just keep processing blocks without adding them to fork choice. However, there are still edge-cases here (e.g., when to halt and declare there was no justification?) and how to mitigate implementation complexity. At this point, it's important to reflect on the attack and how likely it is to happen. It requires some rather contrived circumstances and it seems very unlikely to occur. Therefore, we need to consider if adding complexity to avoid an unlikely attack increases or decreases our total risk. Presently, it appears that SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY sits in a sweet spot for this trade-off.

Transitioning from VALID -> INVALID or INVALID -> VALID

These operations are purposefully omitted. It is outside of the scope of the specification since it's only possible with a faulty EE.

Such a scenario requires manual intervention.

What about Light Clients?

An alternative to optimistic sync is to run a light client inside/alongside beacon nodes that mitigates the need for optimistic sync by providing tip-of-chain blocks to the execution engine. However, light clients comes with their own set of complexities. Relying on light clients may also restrict nodes from syncing from genesis, if they so desire.

A notable thing about optimistic sync is that it's optional. Should an implementation decide to go the light-client route, then they can just ignore optimistic sync all together.

What if TERMINAL_BLOCK_HASH is used?

If the terminal block hash override is used (i.e., TERMINAL_BLOCK_HASH != Hash32()), the validate_merge_block function will deterministically return True or False. Whilst it's not technically required retrospectively call validate_merge_block on a transition block that matches TERMINAL_BLOCK_HASH after an optimistic sync, doing so will have no effect. For simplicity, the optimistic sync specification does not define edge-case behaviour for when TERMINAL_BLOCK_HASH is used.