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

That is a somewhat different question. The easy answer to this (and many
other seemingly obscure parts of SIP) is related to forking. Without
to-tags it is hard to distinguish retransmissions from responses to
forked requests. Most of the time it doesn't matter, but when debugging
a network, it can be useful. Perhaps someone else can offer more examples...

Section 17.1.3 discusses much the same problem in relation to multicast,
if you are interested.

Michael

[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.
>
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