Merge pull request #3346 from ppopth/sidecar-subnets

Specify the number of sidecar subnets

specs/deneb/p2p-interface.md

@@ -22,7 +22,7 @@ The specification of these changes continues in the same format as the network s
     - [Topics and messages](#topics-and-messages)
       - [Global topics](#global-topics)
         - [`beacon_block`](#beacon_block)
-        - [`blob_sidecar_{index}`](#blob_sidecar_index)
+        - [`blob_sidecar_{subnet_id}`](#blob_sidecar_subnet_id)
     - [Transitioning the gossip](#transitioning-the-gossip)
   - [The Req/Resp domain](#the-reqresp-domain)
     - [Messages](#messages)
@@ -107,7 +107,7 @@ The new topics along with the type of the `data` field of a gossipsub message ar
 
 | Name | Message Type |
 | - | - |
-| `blob_sidecar_{index}` | `SignedBlobSidecar` (new) |
+| `blob_sidecar_{subnet_id}` | `SignedBlobSidecar` (new) |
 
 ##### Global topics
 
@@ -117,13 +117,13 @@ Deneb introduces new global topics for blob sidecars.
 
 The *type* of the payload of this topic changes to the (modified) `SignedBeaconBlock` found in deneb.
 
-###### `blob_sidecar_{index}`
+###### `blob_sidecar_{subnet_id}`
 
 This topic is used to propagate signed blob sidecars, one for each sidecar index. The number of indices is defined by `MAX_BLOBS_PER_BLOCK`.
 
 The following validations MUST pass before forwarding the `signed_blob_sidecar` on the network, assuming the alias `sidecar = signed_blob_sidecar.message`:
 
-- _[REJECT]_ The sidecar is for the correct topic -- i.e. `sidecar.index` matches the topic `{index}`.
+- _[REJECT]_ The sidecar is for the correct subnet -- i.e. `compute_subnet_for_blob_sidecar(sidecar.index) == subnet_id`.
 - _[IGNORE]_ The sidecar is not from a future slot (with a `MAXIMUM_GOSSIP_CLOCK_DISPARITY` allowance) -- i.e. validate that `sidecar.slot <= current_slot` (a client MAY queue future sidecars for processing at the appropriate slot).
 - _[IGNORE]_ The sidecar is from a slot greater than the latest finalized slot -- i.e. validate that `sidecar.slot > compute_start_slot_at_epoch(state.finalized_checkpoint.epoch)`
 - _[IGNORE]_ The sidecar's block's parent (defined by `sidecar.block_parent_root`) has been seen (via both gossip and non-gossip sources) (a client MAY queue sidecars for processing once the parent block is retrieved).

specs/deneb/validator.md

@@ -10,6 +10,8 @@
 
 - [Introduction](#introduction)
 - [Prerequisites](#prerequisites)
+- [Constants](#constants)
+  - [Misc](#misc)
 - [Helpers](#helpers)
   - [`BlobsBundle`](#blobsbundle)
   - [Modified `GetPayloadResponse`](#modified-getpayloadresponse)
@@ -38,6 +40,14 @@ All behaviors and definitions defined in this document, and documents it extends
 All terminology, constants, functions, and protocol mechanics defined in the updated [Beacon Chain doc of Deneb](./beacon-chain.md) are requisite for this document and used throughout.
 Please see related Beacon Chain doc before continuing and use them as a reference throughout.
 
+## Constants
+
+### Misc
+
+| Name | Value | Unit |
+| - | - | :-: |
+| `BLOB_SIDECAR_SUBNET_COUNT` | `4` | The number of blob sidecar subnets used in the gossipsub protocol. |
+
 ## Helpers
 
 ### `BlobsBundle`
@@ -136,7 +146,7 @@ def get_blob_sidecars(block: BeaconBlock,
 
 ```
 
-Then for each sidecar, `signed_sidecar = SignedBlobSidecar(message=sidecar, signature=signature)` is constructed and published to the `blob_sidecar_{index}` topics according to its index.
+Then for each sidecar, `signed_sidecar = SignedBlobSidecar(message=sidecar, signature=signature)` is constructed and published to the associated sidecar topic, the `blob_sidecar_{subnet_id}` pubsub topic.
 
 `signature` is obtained from:
 
@@ -149,6 +159,15 @@ def get_blob_sidecar_signature(state: BeaconState,
     return bls.Sign(privkey, signing_root)
 ```
 
+The `subnet_id` for the `signed_sidecar` is calculated with:
+- Let `blob_index = signed_sidecar.message.index`.
+- Let `subnet_id = compute_subnet_for_blob_sidecar(blob_index)`.
+
+```python
+def compute_subnet_for_blob_sidecar(blob_index: BlobIndex) -> SubnetID:
+    return SubnetID(blob_index % BLOB_SIDECAR_SUBNET_COUNT)
+```
+
 After publishing the peers on the network may request the sidecar through sync-requests, or a local user may be interested.
 
 The validator MUST hold on to sidecars for `MIN_EPOCHS_FOR_BLOB_SIDECARS_REQUESTS` epochs and serve when capable,