In any event, 822 is an existence-proof that decades-long upgrades are entirely
possible without the scorched-earth approach of versioning. ...
Nope, see the PS. But anyway.
I don't understand this scorched earth stuff. This is not IPv6, which
is often incompatible for the sake of being incompatible, and is
intended to make IPv4 go away sometime in the fourth millenium.
The idea with DKIM v=2 is that there are things that you cannot say in
a v=1 signature, no matter how many new tags you add, so you need some
way to tell verifiers what they need to understand. How about this?
We rebrand the v= tag to be a feature list so the syntax is now roughly
v= word (, word)*
where each word describes a semantic feature. Feature tag "1" is all
the stuff in RFC6376. My feature is mandatory to understand tags,
feature name "mandatory", so the signatures start
DKIM-Signature: v=1,mandatory ...
PS: The reason you haven't noticed the versions in RFC822 is that we
put the version flags into SMTP. An 8BITMIME or EAI mail message is
not backward compatible with RFC822.
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