The current proposal to remove x= has normative text which
requires verifiers to hard code a value that approximates the
maximum transport time of SMTP. There are many problems
with this:
1) These transport times are only informally known, but putting
an absolute value in the DKIM spec, we will make it to brittle
to any changes for that assumption
2) DKIM has hooks for other services other than SMTP, and it
is certain the that new services will not share a common transport
3) By designating an absolute value in the spec, this proposal retains
the same failing that it claims as a motivation for removing x=. That
is, clock skew will still need to be considered, etc.
For these reasons, I propose that we retain the current semantics of
x=. Adding informative text to aid the signer in choosing a value
of the expiration time, including some discussion of the clock skew
issue would be helpful.
Mike
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