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Tighten restriction on a "seen" attestation

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Declares that only a verified block can stop an attestation from being propagated.

This achieves two things:

1. Ensures that clients don't need to scan invalid blocks for attestations and then modify their state based upon them.
1. Disallows "muting" attestations by sending around a junk block with that attestation in it.
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Paul Hauner 2020-03-30 10:44:46 +11:00 committed by GitHub
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specs/phase0/p2p-interface.md
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@ -286,7 +286,7 @@ There are two primary global topics used to propagate beacon blocks and aggregat
- The block is proposed by the expected `proposer_index` for the block's slot in the context of the current shuffling (defined by `parent_root`/`slot`). If the `proposer_index` cannot immediately be verified against the expected shuffling, the block MAY be queued for later processing while proposers for the block's branch are calculated.
- `beacon_aggregate_and_proof` - This topic is used to propagate aggregated attestations (as `SignedAggregateAndProof`s) to subscribing nodes (typically validators) to be included in future blocks. The following validations MUST pass before forwarding the `signed_aggregate_and_proof` on the network. (We define the following for convenience -- `aggregate_and_proof = signed_aggregate_and_proof.message` and `aggregate = aggregate_and_proof.aggregate`)
- `aggregate.data.slot` is within the last `ATTESTATION_PROPAGATION_SLOT_RANGE` slots (with a `MAXIMUM_GOSSIP_CLOCK_DISPARITY` allowance) -- i.e. `aggregate.data.slot + ATTESTATION_PROPAGATION_SLOT_RANGE >= current_slot >= aggregate.data.slot` (a client MAY queue future aggregates for processing at the appropriate slot).
- The aggregate attestation defined by `hash_tree_root(aggregate)` has _not_ already been seen (via aggregate gossip, within a block, or through the creation of an equivalent aggregate locally).
- The aggregate attestation defined by `hash_tree_root(aggregate)` has _not_ already been seen (via aggregate gossip, within a verified block, or through the creation of an equivalent aggregate locally).
- The `aggregate` is the first valid aggregate received for the aggregator with index `aggregate_and_proof.aggregator_index` for the slot `aggregate.data.slot`.
- The block being voted for (`aggregate.data.beacon_block_root`) passes validation.
- `aggregate_and_proof.selection_proof` selects the validator as an aggregator for the slot -- i.e. `is_aggregator(state, aggregate.data.slot, aggregate.data.index, aggregate_and_proof.selection_proof)` returns `True`.