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+# Optimistic Sync
+
+## Introduction
+
+In order to provide a syncing execution engine with a (partially-verified) 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*.
+
+## Mechanisms
+
+To perform an optimistic sync:
+
+- 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.
+
+In addition to these changes to validation, the consensus engine MUST be able
+to ascertain, after import, which blocks returned SYNCING and which returned
+VALID. This document will assume consensus engines store the following sets:
+
+- `valid_roots: Set[Root]`: `hash_tree_root(block)` where
+	`block.body.execution_payload` is known to be VALID.
+- `optimistic_roots: Set[Root]`: `hash_tree_root(block)` where
+	`block.body.execution_payload` is known to be SYNCING.
+
+Notably, `optimistic_roots` only includes blocks which have execution enabled
+whilst `valid_roots` contains blocks *with or without* execution enabled (i.e.,
+all blocks).
+
+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.
+
+When a block transitions from SYNCING -> INVALID, all *descendants* of the
+block MUST also transition from SYNCING -> INVALID.
+
+## Fork Choice
+
+Consensus engines MUST support removing blocks that transition from SYNCING to
+INVALID. Specifically, an INVALID block MUST NOT be included in the canonical
+chain and the weights from INVALID blocks MUST NOT be applied to any VALID or
+SYNCING ancestors.
+
+## Validator assignments
+
+An entirely optimistically synced 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.
+
+Let `head_block: BeaconBlock` be the result of calling of the fork choice
+algorithm at the time of block production. Let `justified_block: BeaconBlock`
+be the latest current justified ancestor ancestor of the `head_block`.
+
+```python
+def is_optimistic(block: BeaconBlock) -> bool:
+	hash_tree_root(block) in optimistic_roots
+```
+
+```python
+def latest_valid_ancestor(block: BeaconBlock) -> Optional[BeaconBlock]:
+	while block.parent_root != Root():
+		if not is_optimistic(block):
+			return block
+		block = get_block(block.parent_root)
+
+	return None
+```
+
+Let a node which returns `is_optimistic(head) == True` be an *optimistic
+node*. Let a validator on an optimistic node be an *optimistic validator*.
+
+### Block production
+
+A optimistic validator MUST NOT produce a block (i.e., sign across the
+`DOMAIN_BEACON_PROPOSER` domain), unless one of the follow exceptions are met:
+
+#### Exception 1.
+
+If the justified block is fully verified (i.e., `not
+is_optimistic(justified_block)`, the validator MUST produce a block upon
+`latest_valid_ancestor(head)`.
+
+### 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), unless one of the follow exceptions are
+met:
+
+#### Exception
+
+#### Exception 1.
+
+If a validator *does not* have an optimistic head (i.e., `not
+is_optimistic(head_block)`), the node is *fully synced*.
+The validator may produce an attestation.