Re: [Sip] Contact header in 1xx responses/updates remote target or not?

[Paul Kyzivat <pkyzivat at cisco.com>]

No, I don't think we have an answer, though I do have an opinion. :-)

Are you asking about whether the contact may be omitted? Or if it may be 
changed?

AFAIK there is nothing normative that *forbids* the contact from being 
changed in subsequent provisional responses. But doing so is *asking* 
for trouble, so you shouldn't do it. Perhaps you could get away with it 
in a *reliable* provisional, but then you would still have a bunch of 
messy cases. The likelihood of it working is slim. Doing it with 
*unreliable* provisionals is going to cause trouble because they can be 
lost. Even with reliable provisionals, you will have to be prepared to 
receive on both addresses for some period of time.

It seems clear that a contact may be omitted in a 100, since it is hop 
by hop, and presumably must be omitted in the 100 responses from 
proxies. Some other 1xx responses may be returned by proxies (notably 
181) and presumably they should also not contain a contact in those cases.

A 1xx that establishes an early dialog must contain a contact or else 
you really don't have a dialog. I think it is ambiguous whether:
- the first 1xx with a particular to-tag *does* establish an early
   dialog and so *must* contain a contact,

- OR a 1xx with a new to-tag, but without a contact, simply doesn't
   establish a dialog, but is legal.

I would split the difference and say that the UAS MUST include a contact 
in the first 1xx it returns with a particular to-tag, but a UAC should 
accept a 1xx with new to-tag and without a contact and simply treat it 
as not establishing an early dialog. (Don't establish the dialog until 
you get enough info to do so.)

	Paul

Rockson Li (zhengyli) wrote:
> Paul , Christer and all,
>  
> I am following the email thread as in subject above
> http://www.ietf.org/mail-archive/web/sip/current/msg19322.html
>  
> However, I do not find the definite answer to Christer's question,
> Do we have the answer now?
>  
> Thanks a lot
> 
> Regards,
> Rockson Li
> 
> Cisco R&D Center, Shanghai, PRC
> 
>  
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