Re: Is To-tag in non 2xx response mandatory

The simple answer to your question is: you must include the to-tag to 
conform to 3261.

I think the question you are really asking is: "will the UAC and 
intervening proxies do the right thing if the to-tag is not present in 
the response?"

The answer to that is "probably". RFC 2543 didn't have to/from tags. So 
if the UAC calls a UAS that only conforms to 2543 then it will get this 
situation. So any element that attempts to be 2543 compatible will deal 
with this. And, as you seem to have figured out, the tag doesn't 
contribute any information that is essential to properly processing the 
response. So, if you want to take the risk, you can try omitting the 
to-tag in this case. But you may encounter problems, or be branded as 
non-conforming. If it comes to finger pointing on this, you will lose. 
So why not just put the to-tag in? You need the logic in  most other 
cases anyway, so it seems like extra work to leave it out in this case.

	Thanks,
	Paul

[email protected] wrote:
> Thanks Michael,
> 
> I was searching for similar questions and I found the following query by
> Christer, [email protected]:
> 
> ---------------------------------------
> Hi,
> 
> Chapter of 3261 says:
> 
> "However, if the To header field in the request did not contain a tag,
> the URI in the To
> header field in the response MUST equal the URI in the To header field;
> additionally, the UAS MUST add a tag to the To header field in the
> response (with the exception of the 100 (Trying) response, in which a
> tag MAY be present).  This serves to identify the UAS that is
> responding, possibly resulting in a component of a dialog ID."
> 
> My question is: is there a reason why a final error response to an
> initial INVITE, for which NO provisional responses have been sent (ie no
> dialog has been established), must contain a To tag?
> 
> Regards,
> 
> Christer
> ---------------------------------------
> 
> I was not able to find answers to this query.
> 
> 
> Regards,
> Sunil
> 
> 
> -----Original Message-----
> From: Michael Procter [mailto:[email protected]] 
> Sent: Thursday, November 13, 2008 7:32 PM
> To: Sunil Bhagat (WT01 - Telecom Equipment)
> Cc: [email protected]
> Subject: Re: [Sip] Is To-tag in non 2xx response mandatory
> 
> [email protected] wrote:
>> Hi,
>>  
>> Should a to-tag be added to a 487 response to an initial INVITE? Even
> if 180 Ringing has not been sent?
>>  
>> Regards,
>>
>> Sunil
>>
> Yes.  RFC 3261 Section 8.2.6.2:
> 
>    additionally, the UAS MUST add a tag to the To header field in
>    the response (with the exception of the 100 (Trying) response, in
>    which a tag MAY be present)
> 
> 
> Regards,
> 
> Michael
> 
> PS Looking in the footer for this list shows:
>> Sip mailing list  https://www.ietf.org/mailman/listinfo/sip
>> This list is for NEW development of the core SIP Protocol
>> Use [email protected] for questions on current sip
>> Use [email protected] for new developments on the application of sip
> 
> Maybe you should consider sip-implementors for further questions like
> this.
> _______________________________________________
> Sip mailing list  https://www.ietf.org/mailman/listinfo/sip
> This list is for NEW development of the core SIP Protocol
> Use [email protected] for questions on current sip
> Use [email protected] for new developments on the application of sip
> 
_______________________________________________
Sip mailing list  https://www.ietf.org/mailman/listinfo/sip
This list is for NEW development of the core SIP Protocol
Use [email protected] for questions on current sip
Use [email protected] for new developments on the application of sip