Add implementation recommendation for metadata (#168)

Based on recent learnings.

---------

Co-authored-by: Hanno Cornelius <68783915+jm-clius@users.noreply.github.com>

waku/standards/core/66/metadata.md

@@ -3,9 +3,10 @@ slug: 66
 title: 66/WAKU2-METADATA
 name: Waku Metadata Protocol
 status: draft
-editor: Alvaro Revuelta <alrevuelta@status.im>
+editor: Franck Royer <franck@status.im>
 contributors:
- - Filip Dimitrijevic <filip@status.im>
+  - Filip Dimitrijevic <filip@status.im>
+  - Alvaro Revuelta <alrevuelta@status.im>
 ---
 
 ## Abstract
@@ -15,16 +16,19 @@ that can be associated with a [10/WAKU2](/waku/standards/core/10/waku2.md) node.
 
 ## Metadata Protocol
 
-Waku specifies a req/resp protocol that provides information about the node's medatadata.
-Such metadata is meant to be used by the node to decide if a peer is worth connecting
-or not.
+The keywords “MUST”, // List style “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”,
+“NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC 2119](https://www.ietf.org/rfc/rfc2119.txt).
+
+Waku specifies a req/resp protocol that provides information about the node's capabilities.
+Such metadata MAY be used by other peers for subsequent actions such as light protocol requests or disconnection.
+
 The node that makes the request,
 includes its metadata so that the receiver is aware of it,
-without requiring an extra interaction.
+without requiring another round trip.
 The parameters are the following:
 
 * `clusterId`: Unique identifier of the cluster that the node is running in.
-* `shards`: Shard indexes that the node is subscribed to.
+* `shards`: Shard indexes that the node is subscribed to via [`11/WAKU2-RELAY`](/waku/standards/core/11/relay.md).
 
 ***Protocol Identifier***
 
@@ -48,6 +52,51 @@ message WakuMetadataResponse {
 }
 ```
 
+## Implementation Suggestions
+
+### Triggering Metadata Request
+
+A node SHOULD proceed with metadata request upon first connection to a remote node.
+A node SHOULD use the remote node's libp2p peer id as identifier for this heuristic.
+
+A node MAY proceed with metadata request upon reconnection to a remote peer.
+
+A node SHOULD store the remote peer's metadata information for future reference.
+A node MAY implement a TTL regarding a remote peer's metadata, and refresh it upon expiry by initiating another metadata request.
+It is RECOMMENDED to set the TTL to 6 hours.
+
+A node MAY trigger a metadata request after receiving an error response from a remote note
+stating they do not support a specific cluster or shard.
+For example, when using a request-response service such as [`19/WAKU2-LIGHTPUSH`](/waku/standards/core/19/lightpush.md).
+
+### Providing Cluster Id
+
+A node MUST include their cluster id into their metadata payload.
+It is RECOMMENDED for a node to operate on a single cluster id.
+
+### Providing Shard Information
+
+* Nodes that mount [`11/WAKU2-RELAY`](/waku/standards/core/11/relay.md) MAY include the shards they are subscribed to in their metadata payload.
+* Shard-relevant services are message related services,
+  such as [`13/WAKU2-STORE`](/waku/standards/core/13/store.md), [12/WAKU2-FILTER](/waku/standards/core/12/filter.md)
+  and [`19/WAKU2-LIGHTPUSH`](/waku/standards/core/19/lightpush.md)
+  but not [`34/WAKU2-PEER-EXCHANGE`](/waku/standards/core/34/peer-exchange.md)
+* Nodes that mount [`11/WAKU2-RELAY`](/waku/standards/core/11/relay.md) and a shard-relevant service SHOULD include the shards they are subscribed to in their metadata payload.
+* Nodes that do not mount [`11/WAKU2-RELAY`](/waku/standards/core/11/relay.md) SHOULD NOT include any shard information
+
+### Using Cluster Id
+
+When reading the cluster id of a remote peer, the local node MAY disconnect if their cluster id is different from the remote peer.
+
+### Using Shard Information
+
+It is NOT RECOMMENDED to disconnect from a peer based on the fact that their shard information is different from the local node.
+
+Ahead of doing a shard-relevant request,
+a node MAY use the previously received metadata shard information to select a peer that support the targeted shard.
+
+For non-shard-relevant requests, a node SHOULD NOT discriminate a peer based on medata shard information.
+
 ## Copyright
 
 Copyright and related rights waived via