chore(design): create authority-discovery design doc (#4499)

Co-authored-by: Kirill <[email protected]>

docs/docs/design/authority-discovery.md

@@ -0,0 +1,39 @@
+---
+layout: default
+title: Authority Discovery Overview
+permalink: /design/authority-discovery/
+---
+
+# Authority Discovery
+
+## Questions
+
+### Is the authority discovery implemented on Kademlia for a custom peer discovery?
+
+No, it uses Kademlia capabilities however it adds a custom handlers for DHT Events, like: DhtEvent::ValueFound
+- DhtEvent::ValueNotFound
+- DhtEvent::ValuePut
+- DhtEvent::ValuePutFailed
+- DhtEvent::PutRecordRequest
+
+These events are produced by the Discovery layer (Kademlia) and propagated to the `authority-discovery` that handles each of them. Specifically for `DhtEvent::PutRecordRequest` it does a validation step and then store the record. 
+
+What happens is that the `authority-discovery` is the actual protocol implementation for discovery in substrate/polkadot given that it produces and store the records that are available in the DHT for other peers to query, validate and use.
+
+### What are the functionalities of the authority discovery, beyond discovery new peers based on its authority ids?
+
+- Publish the node external address, more specifically, create the authority discovery record, sign it and distributing through DHT allowing other nodes to query it.
+- Handle DHT events, validating incoming records and caching other authorities address.
+
+### What subsystems uses the authority discovery mechanism? Describe the usages.
+
+- Availability Distribution
+- Availability Recovery
+- Gossip Support
+
+### Check if we can currently extend the authority discovery protocol can to golang kademlia p2p library
+
+Yes, we pretty much can implement the protocol. 
+- We are able to encode the records using the same [proto schema used by substrate/polkadot](https://github.com/paritytech/polkadot-sdk/blob/master/substrate/client/authority-discovery/src/worker/schema/dht-v3.proto) - We can query for specific keys in DHT.
+- What can write a custom validator to validate incoming records.
+- golang-libp2p does not works in the same way rust-libp2p works by enabling us to receive those DHT events but we can achieve the same protocol implementation using what the library provide to us.

docs/mkdocs.yml

@@ -73,6 +73,7 @@ nav:
     - Availability Subsystems: ./design/availability-subsystems.md
     - Prospective Parachains: ./design/prospective-parachains.md
     - Prospective Parachains - Fragment Chain: ./design/fragment-chain.md
+    - Authority Discovery: ./design/authority-discovery.md
     - Statement Distribution: ./design/statement-distribution.md
     - Grid Topology: ./design/grid-topology.md
     - Gossip Support: ./design/gossip-support.md