Attnet revamp: Subnet backbone structure based on beacon nodes

specs/phase0/validator.md

@@ -88,10 +88,12 @@ All terminology, constants, functions, and protocol mechanics defined in the [Ph
 
 | Name | Value | Unit | Duration |
 | - | - | :-: | :-: |
-| `TARGET_AGGREGATORS_PER_COMMITTEE` | `2**4` (= 16) | validators | |
-| `RANDOM_SUBNETS_PER_VALIDATOR` | `2**0` (= 1) | subnets | |
-| `EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION` | `2**8` (= 256) | epochs | ~27 hours |
+| `TARGET_AGGREGATORS_PER_COMMITTEE` | `2**4` (= 16) | validators |
+| `EPOCHS_PER_SUBNET_SUBSCRIPTION` | `2**8` (= 256) | epochs | ~27 hours |
 | `ATTESTATION_SUBNET_COUNT` | `64` | The number of attestation subnets used in the gossipsub protocol. |
+| `ATTESTATION_SUBNET_EXTRA_BITS` | `0` | The number of extra bits of a NodeId to use when mapping to a subscribed subnet |
+| `SUBNETS_PER_NODE` | `2` | The number of long-lived subnets a beacon node should be subscribed to. |
+| `ATTESTATION_SUBNET_PREFIX_BITS` | `(ceillog2(ATTESTATION_SUBNET_COUNT) + ATTESTATION_SUBNET_EXTRA_BITS)` | |
 
 ## Containers
 
@@ -606,15 +608,31 @@ def get_aggregate_and_proof_signature(state: BeaconState,
 
 ## Phase 0 attestation subnet stability
 
-Because Phase 0 does not have shards and thus does not have Shard Committees, there is no stable backbone to the attestation subnets (`beacon_attestation_{subnet_id}`). To provide this stability, each validator must:
+Because Phase 0 does not have shards and thus does not have Shard Committees, there is no stable backbone to the attestation subnets (`beacon_attestation_{subnet_id}`). To provide this stability, each beacon node should:
 
-* Randomly select and remain subscribed to `RANDOM_SUBNETS_PER_VALIDATOR` attestation subnets
-* Maintain advertisement of the randomly selected subnets in their node's ENR `attnets` entry by setting the randomly selected `subnet_id` bits to `True` (e.g. `ENR["attnets"][subnet_id] = True`) for all persistent attestation subnets
-* Set the lifetime of each random subscription to a random number of epochs between `EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION` and `2 * EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION]`. At the end of life for a subscription, select a new random subnet, update subnet subscriptions, and publish an updated ENR
+* Remain subscribed to `SUBNETS_PER_NODE` for `EPOCHS_PER_SUBNET_SUBSCRIPTION` epochs.
+* Maintain advertisement of the selected subnets in their node's ENR `attnets` entry by setting the selected `subnet_id` bits to `True` (e.g. `ENR["attnets"][subnet_id] = True`) for all persistent attestation subnets.
+* Select these subnets based on their node-id as specified by the following `compute_subscribed_subnets(node_id,epoch)` function.
 
-*Note*: Short lived beacon committee assignments should not be added in into the ENR `attnets` entry.
+```python
+def compute_subscribed_subnet(node_id: int, epoch: Epoch, index: int) -> int:
+    node_id_prefix = node_id >> (256 - int(ATTESTATION_SUBNET_PREFIX_BITS))
+    node_offset = node_id % EPOCHS_PER_SUBNET_SUBSCRIPTION
+    permutation_seed = hash(uint_to_bytes(uint64((epoch + node_offset) // EPOCHS_PER_SUBNET_SUBSCRIPTION)))
+    permutated_prefix = compute_shuffled_index(
+        node_id_prefix,
+        1 << int(ATTESTATION_SUBNET_PREFIX_BITS),
+        permutation_seed,
+    )
+    return (permutated_prefix + index) % ATTESTATION_SUBNET_COUNT
+```
+
+```python
+def compute_subscribed_subnets(node_id: int, epoch: Epoch) -> Sequence[int]:
+    return [compute_subscribed_subnet(node_id, epoch, idx) for idx in range(SUBNETS_PER_NODE)]
+```
 
-*Note*: When preparing for a hard fork, a validator must select and subscribe to random subnets of the future fork versioning at least `EPOCHS_PER_RANDOM_SUBNET_SUBSCRIPTION` epochs in advance of the fork. These new subnets for the fork are maintained in addition to those for the current fork until the fork occurs. After the fork occurs, let the subnets from the previous fork reach the end of life with no replacements.
+*Note*: When preparing for a hard fork, a validator must select and subscribe to subnets of the future fork versioning at least `EPOCHS_PER_SUBNET_SUBSCRIPTION` epochs in advance of the fork. These new subnets for the fork are maintained in addition to those for the current fork until the fork occurs. After the fork occurs, let the subnets from the previous fork reach the end of life with no replacements.
 
 ## How to avoid slashing
 

tests/core/pyspec/eth2spec/test/phase0/unittests/test_config_invariants.py

@@ -75,7 +75,7 @@ def test_time(spec, state):
 @with_all_phases
 @spec_state_test
 def test_networking(spec, state):
-    assert spec.RANDOM_SUBNETS_PER_VALIDATOR <= spec.ATTESTATION_SUBNET_COUNT
+    assert spec.SUBNETS_PER_NODE <= spec.ATTESTATION_SUBNET_COUNT
 
 
 @with_all_phases

tests/core/pyspec/eth2spec/test/phase0/unittests/validator/test_validator_unittest.py

@@ -1,6 +1,12 @@
+import random
+
 from eth2spec.test.context import (
+    single_phase,
     spec_state_test,
-    always_bls, with_phases, with_all_phases,
+    spec_test,
+    always_bls,
+    with_phases,
+    with_all_phases,
 )
 from eth2spec.test.helpers.constants import PHASE0
 from eth2spec.test.helpers.attestations import build_attestation_data, get_valid_attestation
@@ -476,3 +482,34 @@ def test_get_aggregate_and_proof_signature(spec, state):
         privkey=privkey,
         pubkey=pubkey,
     )
+
+
+def run_compute_subscribed_subnets_arguments(spec, rng=random.Random(1111)):
+    node_id = rng.randint(0, 2**40 - 1)  # try VALIDATOR_REGISTRY_LIMIT
+    epoch = rng.randint(0, 2**64 - 1)
+    subnets = spec.compute_subscribed_subnets(node_id, epoch)
+    assert len(subnets) == spec.SUBNETS_PER_NODE
+
+
+@with_all_phases
+@spec_test
+@single_phase
+def test_compute_subscribed_subnets_random_1(spec):
+    rng = random.Random(1111)
+    run_compute_subscribed_subnets_arguments(spec, rng)
+
+
+@with_all_phases
+@spec_test
+@single_phase
+def test_compute_subscribed_subnets_random_2(spec):
+    rng = random.Random(2222)
+    run_compute_subscribed_subnets_arguments(spec, rng)
+
+
+@with_all_phases
+@spec_test
+@single_phase
+def test_compute_subscribed_subnets_random_3(spec):
+    rng = random.Random(3333)
+    run_compute_subscribed_subnets_arguments(spec, rng)