I'm not sure what the RG Chair(s) think, I'm not sure what host stack implementers think, and I'm not sure what the IETF Powers-that-Be think while wearing their official hat(s).
Well, this co-chair thinks (and my co-chair is free to disagree ;-) that host changes are permissible. They certainly come with some concurrent amount of deployment pain that we do want to take into account in our considerations, but there are no limitations within our charter that forbid us from going that direction if we so choose.
As a purely practical matter, at least one host OS implementer has a de facto veto on this question. If that host OS is not going to adopt a proposed host stack modification (or not adopt it in a timely way), then the percentage of deployed host OSs with the upgraded stack capabilities is very unlikely to become commercially interesting/viable.
If we're thinking of the same OS implementer, the situation is not quite so bleak. In a private meeting, this viewpoint was moderated to being one of making no 'unnecessary' host stack changes.
This leaves us with the overriding prerogative to do what we feel is best for the long-term architecture of the network, taking into account that whatever we deliver must be deployable and should minimize any associated pain. Obviously, this is going to be a judgement call that we should reach rough consensus on.
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