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

[Matt Corallo <lf-lists@mattcorallo.com>]

This sounds like a great idea!

Bitcoin is no longer a homogeneous network of one client - it is many, with different features implemented in each. The 
Bitcoin protocol hasn't (fully) evolved to capture that reality. Initially the Bitcoin protocol had a simple numerical 
version field, but that is wholly impractical for any diverse network - some clients may not wish to implement every 
possible new relay mechanic, and why should they have to in order to use other new features?

Bitcoin protocol changes have, many times in recent history, been made via new dummy "negotiation" messages, which take 
advantage of the fact that the Bitcoin protocol has always expected clients to ignore unknown messages. Given that 
pattern, it makes sense to have an explicit negotiation phase - after version and before verack, just send the list of 
features that you support to negotiate what the connection will be capable of. The exact way we do that doesn't matter 
much, and sending it as a stream of messages which each indicate support for a given protocol feature perfectly captures 
the pattern that has been used in several recent network upgrades, keeping consistency.

Matt

On 8/14/20 3:28 PM, Suhas Daftuar via bitcoin-dev wrote:
> 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 
> <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 
> <https://github.com/sdaftuar/bips/blob/2020-08-generalized-feature-negotiation/bip-p2p-feature-negotiation.mediawiki>
> 
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