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

[Ben Campbell <bcampbell@dynamicsoft.com>]

It seems to me that if the UAC cares about intermediaries seeing the 
expires header, it would put an expires header in the outer message.

Vijay K. Gurbani wrote:
> 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




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