Maybe things have changed, but, if one actually believes the
robustness principle, then, in the case Geoff cites, Exchange is
simply non-conforming -- not because the spec prohibits
rejecting on the basis of a fine distinction about IPv6 formats,
but because doing so is unnecessary, inconsistent with the
robustness principle, and, arguably, plain silly.
I'm afraid I'm going to have to disagree here. If you look at RFC 5321 and
are unaware of the history of how the text came about, it gives the definite
appearance of going out of its way to ban the use of :: to replace a single
0. A reasonable interpretation is therefore that such forms are disallowed
for a reason.
It's fine to be tolerant of stuff the relevant standard doesn't
allow but doesn't call out explicitly. But that's not the case here.
In any case, if you want to "fix" this, we could change RFC 5321 to accept this
form. But as Mark Andrews points out, you can't make it legal to send such
forms without breaking interoperability. I suppose we could make the change and
recycle at proposed, but that seems rather extreme to fix what is in fact a
nonissue.
I'll also point out that this has diddley-squat to do with formal verification
processes. Again as Mark Anrdrews points out, we deployed something with a
restriction that subsequently turned out to be unnecessary, and now we're stuck
with it. Indeed, had formal verification processes been properly used, they
would have flagged any attempt to change this as breaking interoperability.
Ned