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. > > Please do not print this email unless it is absolutely necessary. > > The information contained in this electronic message and any attachments to > this message are intended for the exclusive use of the addressee(s) and may > contain proprietary, confidential or privileged information. If you are not > the intended recipient, you should not disseminate, distribute or copy this > e-mail. Please notify the sender immediately and destroy all copies of this > message and any attachments. > > WARNING: Computer viruses can be transmitted via email. The recipient should > check this email and any attachments for the presence of viruses. The company > accepts no liability for any damage caused by any virus transmitted by this > email. > > www.wipro.com > > > _______________________________________________ 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
