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