Hi,

Back in February I posted a proposal for WTXID-based transaction relay[1]
(now known as BIP 339), which included a proposal for feature negotiation
to take place prior to the VERACK message being received by each side.  In
my email to this list, I had asked for feedback as to whether that proposal
was problematic, and didn't receive any responses.

Since then, the implementation of BIP 339 has been merged into Bitcoin
Core, though it has not yet been released.

In thinking about the mechanism used there, I thought it would be helpful
to codify in a BIP the idea that Bitcoin network clients should ignore
unknown messages received before a VERACK.  A draft of my proposal is
available here[2].

I presume that software upgrading past protocol version 70016 was already
planning to either implement BIP 339, or ignore the wtxidrelay message
proposed in BIP 339 (if not, then this would create network split concerns
in the future -- so I hope that someone would speak up if this were a
problem).  When we propose future protocol upgrades that would benefit from
feature negotiation at the time of connection, I think it would be nice to
be able to use the same method as proposed in BIP 339, without even needing
to bump the protocol version.  So having an understanding that this is the
standard of how other network clients operate would be helpful.

If, on the other hand, this is problematic for some reason, I look forward
to hearing that as well, so that we can be careful about how we deploy
future p2p changes to avoid disruption.

Thanks,
Suhas Daftuar

[1]
https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020-February/017648.html

[2]
https://github.com/sdaftuar/bips/blob/2020-08-generalized-feature-negotiation/bip-p2p-feature-negotiation.mediawiki