[bitcoin-dev] Generalizing feature negotiation when new p2p connections are setup

[Matt Corallo]

This seems to be pretty overengineered. Do you have a specific use-case in mind for anything more than simply continuing 
the pattern we've been using of sending a message indicating support for a given feature? If we find some in the future, 
we could deploy something like this, though the current proposal makes it possible to do it on a per-feature case.

The great thing about Suhas' proposal is the diff is about -1/+1 (not including tests), while still getting all the 
flexibility we need. Even better, the code already exists.

Matt

On 8/21/20 3:50 PM, Jeremy wrote:
> I have a proposal:
> 
> Protocol >= 70016 cease to send or process VERACK, and instead use HANDSHAKEACK, which is completed after feature 
> negotiation.
> 
> This should make everyone happy/unhappy, as in a new protocol number it's fair game to change these semantics to be 
> clear that we're acking more than version.
> 
> I don't care about when or where these messages are sequenced overall, it seems to have minimal impact. If I had free 
> choice, I slightly agree with Eric that verack should come before feature negotiation, as we want to divorce the idea 
> that protocol number and feature support are tied.
> 
> But once this is done, we can supplant Verack with HANDSHAKENACK or HANDSHAKEACK to signal success or failure to agree 
> on a connection. A NACK reason (version too high/low or an important feature missing) could be optional. Implicit NACK 
> would be disconnecting, but is discouraged because a peer doesn't know if it should reconnect or the failure was 
> intentional.
> 
> ------
> 
> AJ: I think I generally do prefer to have a FEATURE wrapper as you suggested, or a rule that all messages in this period 
> are interpreted as features (and may be redundant with p2p message types -- so you can literally just use the p2p 
> message name w/o any data).
> 
> I think we would want a semantic (which could be based just on message names, but first-class support would be nice) for 
> ACKing that a feature is enabled. This is because a transcript of:
> 
> NODE0:
> FEATURE A
> FEATURE B
> VERACK
> 
> NODE1:
> FEATURE A
> VERACK
> 
> It remains unclear if Node 1 ignored B because it's an unknown feature, or because it is disabled. A transcript like:
> 
> NODE0:
> FEATURE A
> FEATURE B
> FEATURE C
> ACK A
> VERACK
> 
> NODE1:
> FEATURE A
> ACK A
> NACK B
> VERACK
> 
> would make it clear that A and B are known, B is disabled, and C is unknown. C has 0 support, B Node 0 should support 
> inbound messages but knows not to send to Node 1, and A has full bilateral support. Maybe instead it could a message 
> FEATURE SEND A and FEATURE RECV A, so we can make the split explicit rather than inferred from ACK/NACK.
> 
> 
> ------
> 
> I'd also propose that we add a message which is SYNC, which indicates the end of a list of FEATURES and a request to 
> send ACKS or NACKS back (which are followed by a SYNC). This allows multi-round negotiation where based on the presence 
> of other features, I may expand the set of features I am offering. I think you could do without SYNC, but there are more 
> edge cases and the explicitness is nice given that this already introduces future complexity.
> 
> This multi-round makes it an actual negotiation rather than a pure announcement system. I don't think it would be used 
> much in the near term, but it makes sense to define it correctly now. Build for the future and all...
> 
> 
> 
> --
> @JeremyRubin <https://twitter.com/JeremyRubin><https://twitter.com/JeremyRubin>