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[Sip] Re: [Simple] -05 MESSAGE and Expires header

This has been an interesting discussion with more people weighing
in the 'leave it alone' category.  However, here is an additional
point to consider: Expires is one of the headers that can be
encrypted.  An S/MIME encrypted MESSAGE request containing 'Expires: 0'
arriving at a relay UAS will get the same processing as will
an unencrypted MESSAGE with 'Expires: 3600' -- probably not good.  The
Priority header, on the other hand, is not permitted to be encrypted
(Table 3, RFC 3261); thus relays can make use of it.

Jonathan Rosenberg gave a concise explanation on why Priority and
'Expires: 0' to mean immediate delivery are different:

 > I believe that priority is quite different. A message can be for
 > immediate delivery, and be either urgent ("my house is on fire") or
 > not ("leaving now for lunch"). A message can be OK for storage (some
 > non-zero expires) and be urgent ("I need that business presentation
 > by 10pm!") or non-urgent ("Did you see that movie"). The fact that two
 > things can be used orthogonally is a sign that they are, in fact, two
 > different things.

In this context, maybe Priority is not the right header, but it is
the closest we have in base SIP to impart immediate delivery.

The whole point of 'Expires: 0' in the MESSAGE I-D is for the relay UAS
and the destination UAS to act on this request immediately.  Since the
Expire header can be encrypted, the directive of immediate delivery is
lost at the relay UAS.  Priority preserves it, and in fact, to avoid any
further ambiguity,  the MESSAGE I-D can further restrict the presence of
the Priority header in only those IM messages that denote urgency of
some sort, and thus need special processing by the relays.

Regards,

- vijay
-- 
Vijay K. Gurbani  vkg@{lucent.com,research.bell-labs.com,acm.org}
Wireless Networks Group/Internet Software and Services
Lucent Technologies/Bell Labs Innovations, 2000 Lucent Lane, Rm 6G-440
Naperville, Illinois 60566     Voice: +1 630 224 0216



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