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specs/altair/sync-protocol.md
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@ -52,7 +52,8 @@ uses sync committees introduced in [this beacon chain extension](./beacon-chain.
| Name | Value | Notes |
| - | - | - |
| `MIN_SYNC_COMMITTEE_PARTICIPANTS` | `1` | |
| `SAFETY_THRESHOLD_CALCULATION_PERIOD` | `4096` | ~13.6 hours |
| `SAFETY_THRESHOLD_PERIOD` | `SLOTS_PER_EPOCH * EPOCHS_PER_SYNC_COMMITTEE_PERIOD` | ~13.6 hours |
| `UPDATE_TIMEOUT` | `SLOTS_PER_EPOCH * EPOCHS_PER_SYNC_COMMITTEE_PERIOD` | ~27.3 hours |
## Containers
@ -88,9 +89,9 @@ class LightClientStore(object):
best_valid_update: Optional[LightClientUpdate]
# Most recent available reasonably-safe header
optimistic_header: BeaconBlockHeader
# Max number of participants in a sync committee (used to calculate safety threshold)
previous_period_max_attendance: uint64
current_period_max_attendance: uint64
# Max number of active participants in a sync committee (used to calculate safety threshold)
previous_max_active_participants: uint64
current_max_active_participants: uint64
```
## Helper functions
@ -106,7 +107,9 @@ def get_subtree_index(generalized_index: GeneralizedIndex) -> uint64:
```python
def get_active_header(update: LightClientUpdate) -> BeaconBlockHeader:
# Is the update trying to convince us of a finalized header or an optimistic header?
# The "active header" is the header that the update is trying to convince us
# to accept. If a finalized header is present, it's the finalized header,
# otherwise it's the attested header
if update.finalized_header != BeaconBlockHeader():
return update.finalized_header
else:
@ -118,8 +121,8 @@ def get_active_header(update: LightClientUpdate) -> BeaconBlockHeader:
```python
def get_safety_threshold(store: LightClientStore):
return max(
store.previous_period_max_attendance,
store.current_period_max_attendance
store.previous_max_active_participants,
store.current_max_active_participants
) // 2
```
@ -131,9 +134,9 @@ A light client maintains its state in a `store` object of type `LightClientStore
```python
def process_slot(store: LightClientStore, current_slot: Slot):
if current_slot % SAFETY_THRESHOLD_CALCULATION_PERIOD == 0:
store.previous_period_max_attendance = store.current_period_max_attendance
store.current_period_max_attendance = 0
if current_slot % SAFETY_THRESHOLD_PERIOD == 0:
store.previous_max_active_participants = store.current_max_active_participants
store.current_max_active_participants = 0
```
#### `validate_light_client_update`
@ -141,11 +144,12 @@ def process_slot(store: LightClientStore, current_slot: Slot):
```python
def validate_light_client_update(store: LightClientStore,
update: LightClientUpdate,
current_slot: Slot,
genesis_validators_root: Root) -> None:
# Verify update slot is larger than slot of current best finalized header
active_header = get_active_header(update)
assert active_header.slot > store.finalized_header.slot
assert current_slot >= active_header.slot > store.finalized_header.slot
# Verify update does not skip a sync committee period
finalized_period = compute_epoch_at_slot(store.finalized_header.slot) // EPOCHS_PER_SYNC_COMMITTEE_PERIOD
@ -209,15 +213,15 @@ def process_light_client_update(store: LightClientStore,
current_slot: Slot,
genesis_validators_root: Root) -> None:
validate_light_client_update(store, update, genesis_validators_root)
validate_light_client_update(store, update, current_slot, genesis_validators_root)
# Update the best update in case we have to force-update to it if the timeout elapses
if sum(update.sync_committee_bits) > sum(store.best_finalization_update.sync_committee_bits):
store.best_finalization_update = update
if sum(update.sync_committee_bits) > sum(store.best_valid_update.sync_committee_bits):
store.best_valid_update = update
# Track the maximum attendance in the committee signatures
store.current_period_max_attendance = max(
store.current_period_max_attendance,
# Track the maximum numebr of active participants in the committee signatures
store.current_max_active_participants = max(
store.current_max_active_participants,
update.sync_committee_bits.count(1)
)
@ -236,7 +240,7 @@ def process_light_client_update(store: LightClientStore,
# Normal update through 2/3 threshold
apply_light_client_update(store, update)
store.best_valid_update = None
elif current_slot > store.finalized_header.slot + update_timeout:
elif current_slot > store.finalized_header.slot + UPDATE_TIMEOUT:
# Forced best update when the update timeout has elapsed
apply_light_client_update(store, store.best_valid_update)
store.best_valid_update = None