Merge pull request #10 from dunxen/2204-notes

Add notes for PR2204

Author: dunxen

_posts/2023-04-27-#2204.md

@@ -10,14 +10,22 @@ status: upcoming
 commit:
 ---
 
-_Notes and questions to follow soon!_
-
-<!-- TODO: Before meeting, add notes and questions
 ## Notes
+* There are various locations in LN structures where support for certain functionality is advertised and made possible, as enumerated in [bLIP-0002](https://github.com/lightning/blips/blob/master/blip-0002.md):
+	* [Feature bits](https://github.com/lightning/bolts/blob/master/09-features.md) allows a node to publically advertise support for or requirement of a specific feature. Bits within the range `0-255` are reserved for BOLTs and so can not be used for custom features.
+		* These feature bits can appear in the following contexts
+			* `init` message (minimally sized `feature` field bitmask)
+			* `node_announcement` message (minimally sized `feature` field bitmask)
+			* `channel_announcement` message (minimally sized `feature` field bitmask)
+			* BOLT 11 invoices (bLIPs only in range `256-5114` clarified by [bLIPs#24](https://github.com/lightning/blips/pull/24)
+			* `allowed_features` field of a blinded path
+	* LN message format: new message types can be used for custom features. Message types within the range `0-32767` are reserved for BOLTs. Custom features (bLIPs) must use types in the range `32768-65535`.
+	* TLV fields in BOLT messages: included in the trailing TLV stream of a BOLT message. bLIPs must use types greater than `65536`.
+* PR2204 adds the methods `provided_node_festures()` and `provided_init_features()` to the `CustomMessageHandler` trait. Implementers are then able to set and provide their own features.
+* The PR also adds functionality for setting feature bits that are unknown to LDK with `Features::set_custom_bit()`, but the bit must be in the custom range defined by bLIP 2 of course.
 
 ## Questions
-1. Did you review the PR? [Concept ACK, approach ACK, tested ACK, or NACK](https://github.com/lightningdevkit/rust-lightning/blob/master/CONTRIBUTING.md#peer-review)?
--->
-
-
--->
+1. Did you review the PR? Concept ACK, approach ACK, tested ACK, or NACK?
+1. What is the `CustomMessageHandler` trait and where do we delegate message handling to an implementer of this trait?
+1. Why are custom feature bits in the invoice (I) context limited to the range `256-5114`? Why don't other feature contexts, such as `init`, `node_announcement`, etc, have this specific restriction.
+1. Other than specified in the notes, there is another restriction that applies to `Features::set_custom_bit()` for bits in the custom range. What is it and how is it identified?