Expires: vs. Supersedes: (was: Intent to revive "expires" header from draft-ietf-mailext-new-fields-15)

Keith Moore wrote:

>>> behavior for mail-to-NetNews gateways should also be specified.

>> Transparent, "Expires:" can be kept as is (modulo magic SP in news)
 
> my concern here is that (as best I recall) in NetNews, Expires
> pretty much means the message will get deleted after that date.

Yes, that is what *expiration* does, depending on the server and 
the group.  But servers are in now way forced to honour Expires:.
Expires: is only a recommendation roughly matching your proposed
semantics.  Quoting the MISSREF-blocked RFC (section 3.2.5):

 The Expires header field specifies a date and time when the poster
 deems the article to be no longer relevant and could usefully be
 removed ("expired").

    NOTE: This header field is useful when the poster desires an
    unusually long or an unusually short expiry time.


 expires         =  "Expires:" SP date-time CRLF

 See the remarks under Section 3.1.1 regarding the syntax of
 <date-time> and the requirements and recommendations to which it is
 subject.

    NOTE: The Expires header field is also sometimes used in Email
    with a similar meaning; see [RFC2156].

> I haven't looked at netnews in years but I seem to recall that
> expires used to be used mostly with periodic postings like FAQs
> that were reissued (and perhaps revised) on a regular basis.

Possibly.  But FAQs typically use "Supersedes:" for this purpose.
"Supersedes:" is a combination of a cancel-message for article A
posting a replacement article B.  And news servers are also free 
to ignore "Supersedes:" and cancel-messages => local policy. 

 Frank