working through attestation aggregation
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Danny Ryan
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@ -164,9 +164,9 @@ Unaggregated and aggregated attestations from all shards are sent to the `beacon
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#### Mainnet
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Shards are grouped into their own subnets (defined by a shard topic). The number of shard subnets is defined via `SHARD_SUBNET_COUNT` and the shard `shard_number % SHARD_SUBNET_COUNT` is assigned to the topic: `shard{shard_number % SHARD_SUBNET_COUNT}_beacon_attestation`. Unaggregated attestations are sent to the subnet topic. Aggregated attestations are sent to the `beacon_attestation` topic.
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Attestation broadcasting is grouped into subnets defined by a topic. The number of subnets is defined via `ATTESTATION_SUBNET_COUNT`. The `CommitteeIndex`, `index`, is assigned to the topic: `index{index % ATTESTATION_SUBNET_COUNT}_beacon_attestation`.
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TODO: [aggregation strategy](https://github.com/ethereum/eth2.0-specs/issues/1331)
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Unaggregated attestations are sent to the subnet topic, `index{attestation.data.index % ATTESTATION_SUBNET_COUNT}_beacon_attestation`. Aggregated attestations are sent to the `beacon_attestation` topic.
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### Messages
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@ -37,7 +37,7 @@
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- [Attestations](#attestations)
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- [Deposits](#deposits)
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- [Voluntary exits](#voluntary-exits)
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- [Attestations](#attestations-1)
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- [Attesting](#attesting)
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- [Attestation data](#attestation-data)
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- [LMD GHOST vote](#lmd-ghost-vote)
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- [FFG vote](#ffg-vote)
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@ -46,6 +46,15 @@
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- [Aggregation bits](#aggregation-bits)
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- [Custody bits](#custody-bits)
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- [Aggregate signature](#aggregate-signature)
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- [Broadcast attestation](#broadcast-attestation)
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- [Attestation aggregation](#attestation-aggregation)
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- [Construct aggregate](#construct-aggregate)
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- [Data](#data-1)
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- [Aggregation bits](#aggregation-bits-1)
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- [Custody bits](#custody-bits-1)
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- [Aggregate signature](#aggregate-signature-1)
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- [Broadcast aggregate](#broadcast-aggregate)
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- [How to avoid slashing](#how-to-avoid-slashing)
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- [Proposer slashing](#proposer-slashing)
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- [Attester slashing](#attester-slashing)
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@ -272,11 +281,11 @@ The `proof` for each deposit must be constructed against the deposit root contai
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Up to `MAX_VOLUNTARY_EXITS`, [`VoluntaryExit`](../core/0_beacon-chain.md#voluntaryexit) objects can be included in the `block`. The exits must satisfy the verification conditions found in [exits processing](../core/0_beacon-chain.md#voluntary-exits).
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### Attestations
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### Attesting
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A validator is expected to create, sign, and broadcast an attestation during each epoch. The `committee`, assigned `index`, and assigned `slot` for which the validator performs this role during an epoch are defined by `get_committee_assignment(state, epoch, validator_index)`.
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A validator should create and broadcast the attestation halfway through the `slot` during which the validator is assigned―that is, `SECONDS_PER_SLOT * 0.5` seconds after the start of `slot`.
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A validator should create and broadcast the `attestation` to the associated attestation subnet one-third of the way through the `slot` during which the validator is assigned―that is, `SECONDS_PER_SLOT / 3` seconds after the start of `slot`.
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#### Attestation data
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@ -314,7 +323,7 @@ Set `attestation.data = attestation_data` where `attestation_data` is the `Attes
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##### Aggregation bits
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- Let `attestation.aggregation_bits` be a `Bitlist[MAX_VALIDATORS_PER_COMMITTEE]` where the bits at the index in the aggregated validator's `committee` is set to `0b1`.
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- Let `attestation.aggregation_bits` be a `Bitlist[MAX_VALIDATORS_PER_COMMITTEE]` of length `len(committee)`, where the bit of the index of the validator in the `committee` is set to `0b1`.
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*Note*: Calling `get_attesting_indices(state, attestation.data, attestation.aggregation_bits)` should return a list of length equal to 1, containing `validator_index`.
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@ -339,6 +348,69 @@ def get_signed_attestation_data(state: BeaconState, attestation: IndexedAttestat
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return bls_sign(privkey, hash_tree_root(attestation_data_and_custody_bit), domain)
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```
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#### Broadcast attestation
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Finally, the validator broadcasts `attestation` to the associated attestation subnet -- the `index{attestation.data.index % ATTESTATION_SUBNET_COUNT}_beacon_attestation` pubsub topic.
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## Attestation aggregation
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The validator is expected to locally aggregate attestations with a similar `attestation_data` to their constructed `attestation` for the assigned `slot`.
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The validator then _might_ broadcast their best aggregate to a global attestation channel two-thirds of the way through the `slot`-that is, `SECONDS_PER_SLOT * 2 / 3` seconds after the start of `slot`.
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#### Construct aggregate
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Collect `attestations` seen via gossip during the `slot` that have an equivalent `attestation_data` to that constructed by the validator.
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The validator should create an `aggregate_attestation` with the following fields.
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##### Data
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Set `aggregate_attestation.data = attestation_data` where `attestation_data` is the `AttestationData` object that is the same for each individual attestation being aggregated.
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##### Aggregation bits
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Let `aggregate_attestation.aggregation_bits` be a `Bitlist[MAX_VALIDATORS_PER_COMMITTEE]` of length `len(committee)`, where each bit set from each individual attestation is set to `0b1`.
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##### Custody bits
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- Let `aggregate_attestation.custody_bits` be a `Bitlist[MAX_VALIDATORS_PER_COMMITTEE]` filled with zeros of length `len(committee)`.
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*Note*: This is a stub for Phase 0.
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##### Aggregate signature
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Set `aggregate_attestation.signature = aggregate_signature` where `aggregate_signature` is obtained from:
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```python
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def get_aggregate_signature(attestations: Attestation) -> BLSSignature:
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signatures = [attestation.signature for attestation in attestations]
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aggregate_signature = bls_aggregate_signatures(signatures)
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return aggregate_signature
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```
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#### Broadcast aggregate
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Starting two-thirds of the way into the slot, the validator begins running the following routine to decide if their best aggregate should be broadcast to the global attestation channel (`beacon_attestation`). `seen()` is defined as having seen an aggregate
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```python
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def should_broadcast_aggregate(aggregate_attestation: Attestation, index: ValidatorIndex) -> bool:
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if seen(index):
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return False
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if random.randrange(SIDED_DIE) == 0:
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return True
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time.sleep(WAIT // 1000)
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```
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Define `seen_better(aggregate_attestation, attestations)` as your aggregate
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having more attestations included than any in the attestations you've seen on
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the network so far. Could also make it better by _some_ amount.
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```python
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def have_better_aggregate(aggregate_attestation, attestations):
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```
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## How to avoid slashing
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"Slashing" is the burning of some amount of validator funds and immediate ejection from the active validator set. In Phase 0, there are two ways in which funds can be slashed: [proposer slashing](#proposer-slashing) and [attester slashing](#attester-slashing). Although being slashed has serious repercussions, it is simple enough to avoid being slashed all together by remaining _consistent_ with respect to the messages a validator has previously signed.
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