Add spec for data propagation

specs/networking/block-propagation.md

@@ -0,0 +1,59 @@
+ETH 2.0 Networking Spec - Data Propagation
+===
+
+# Abstract
+
+This specification describes how data will be propagated ("gossiped") on the network among Ethereum 2.0 nodes.
+
+The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL”, NOT", “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119.
+
+# Dependencies
+
+This specification assumes familiarity with the [RPC Interface](./rpc-interface.md) and [Beacon Chain](../core/0_beacon-chain.md) specifications.
+
+# Specification
+
+## Pubsub
+
+Ethereum 2.0 uses a publish/subscribe ("pubsub") construction to propagate information throughout the network. In pubsub applications, nodes tag the messages they send with a topic. Only the nodes interested in that topic will process the message. This allows nodes to decide for themselves which information they are interested in.
+
+There are a variety of network-based pubsub implementations available. `libp2p` includes three: `floodsub`, `gossipsub`, and most recently `EpiSub`. `gossipsub` is currently the most likely implementation to be adopted as the official standard, however this may change as a result of performance testing. Given these conditions, **this specification makes no assumptions about the underlying pubsub implementation.**
+
+## Shard Subnets
+
+Topics involving single shards are mapped to "subnets." A shard's subnet is calculating by calculating `shard_number % SHARD_SUBNET_COUNT`. Grouping shards into subnets as described confers the following benefits:
+
+- Shards using smaller amounts network traffic are grouped with shards with more network traffic, thus increasing the stability of the network by reducing the likelihood that less-used shareds will be eclipsed.
+- The existence of the `SHARD_SUBNET_COUNT` creates a quality-of-service parameter that we can tweak to ensure the health over the network.
+
+We expect that over time `SHARD_SUBNET_COUNT` will be increased to equal the total number of shards.
+
+## Encoding
+
+Messages are serialized using SSZ unless otherwise noted.
+
+## 
+
+## Propagated Data
+
+### Beacon Blocks
+
+**Topic:** `beacon/blocks`
+
+**Body:**
+
+```
+(
+	block_root: HashTreeRoot
+)
+```
+
+Notifies interested nodes that a new beacon block has been created. Nodes are expected to use one of the RPC APIs (Method ID `12`, for example) to fill out the block header and block body.
+
+### Attestations
+
+**Topic:** `attestations/${subnet}`
+
+**Body:** SSZ-serialized `Attestation` object. Note that the `Attestation` is not included in a container, and is passed over the wire directly.
+
+Notifies interested nodes that a new Attestation has been created.