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List:       sip-implementors
Subject:    Re: [Sip-implementors] change in tag field in To header
From:       Paul Kyzivat <pkyzivat () alum ! mit ! edu>
Date:       2013-02-28 22:02:06
Message-ID: 512FD3DE.6030908 () alum ! mit ! edu
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On 3/1/13 4:27 AM, Neel Neelakantan wrote:
> Ramesh,
> 
> See inline.
> 
> > -----Original Message-----
> > From: sip-implementors-bounces@lists.cs.columbia.edu [mailto:sip-
> > implementors-bounces@lists.cs.columbia.edu] On Behalf Of Ramesh Babu
> > Kuppili
> > Sent: Wednesday, February 27, 2013 12:42 AM
> > To: Paul Kyzivat; sip-implementors@lists.cs.columbia.edu
> > Subject: Re: [Sip-implementors] change in tag field in To header
> > 
> > Paul,
> > 
> > Sorry.  I mistyped UAC.  I meant to say UAC.  Not UAS.  Corrected text:
> > 
> > Thanks for the response.  Is there any way to know how many forks were
> > made by a proxy.  My application is VoIP.  I need to provide some kind of
> > error tone when the UAC determines that the call cannot be landed on any
> > of the forks.  Ideally, I would like to know the total number of forks made by
> > the proxy and provide error tone to the voice user when all the forks
> > responsed with error final responsed.
> > 
> [Neel]
> It is not possible to know how many forks the proxy (or subsequent downstream \
> proxy) made.  Typically a proxy when forks, it collects all the responses and send \
> only one (See section 16 of RFC 3261)).

There is no way to *know* how many forks there were.
You can use certain information to guess. This may work better in some 
deployments than others. Namely:

1) You can note the total number of to-tags you see in responses. You 
may get 1xx responses with a number of different to-tags, as you have 
seen, as well as a final response that may be from a to-tag already seen 
or a different one. This will fail to count forks that fail without 
sending a 1xx.

2) You may look for a History-Info header in the responses. If found, it 
may be analyzed to count the number of forks. This of course will only 
work if History-Info is used by the systems involved. (History-Info is 
specified by RFC4244, and there is a nearly complete revision called 
draft-ietf-sipcore-rfc4244bis-11.

	Thanks,
	Paul

> > Thanks again for your time.
> > 
> > - ramesh
> > 
> > -----Original Message-----
> > From: sip-implementors-bounces@lists.cs.columbia.edu [mailto:sip-
> > implementors-bounces@lists.cs.columbia.edu] On Behalf Of Ramesh Babu
> > Kuppili
> > Sent: Wednesday, February 27, 2013 10:51 AM
> > To: Paul Kyzivat; sip-implementors@lists.cs.columbia.edu
> > Subject: Re: [Sip-implementors] change in tag field in To header
> > 
> > Paul,
> > 
> > Thanks for the response.  Is there any way to know how many forks were
> > made by a proxy.  My application is VoIP.  I need to provide some kind of
> > error tone when the UAS determines that the call cannot be landed on any
> > of the forks.  Ideally, I would like to know the total number of forks made by
> > the proxy and provide error tone to the voice user when all the forks
> > responsed with error final responsed.
> > 
> > Thanks again for your time.
> > 
> > - ramesh
> > 
> > -----Original Message-----
> > From: sip-implementors-bounces@lists.cs.columbia.edu [mailto:sip-
> > implementors-bounces@lists.cs.columbia.edu] On Behalf Of Paul Kyzivat
> > Sent: Wednesday, February 27, 2013 3:50 AM
> > To: sip-implementors@lists.cs.columbia.edu
> > Subject: Re: [Sip-implementors] change in tag field in To header
> > 
> > On 2/27/13 12:38 AM, Ramesh Babu Kuppili wrote:
> > > Hello,
> > > 
> > > I am having a problem with the To tag in the incoming "603 Declined" from
> > UAS.
> > > 
> > > 1. Outgoing INVITE
> > > - no tag in the To header.
> > > 2. Incoming "183 Session Progress"
> > > - To tag -
> > tag=h7g4Esbg_p65547t1361882364m374065c90075s1_4069438959-13.
> > Incoming "603 Declined"
> > > - To tag -
> > > tag=h7g4Esbg_p65547t1361882364m374065c90075s1_4069388685-
> > 1940037509
> > > Can the To tag change from "183 Session Progress" to "603 Declined".  How
> > do the UAS handle this?
> > 
> > Yes.
> > 
> > This is consistent with the request having been forked by a proxy.
> > The 183 came from one fork. Then the 603 came from a different fork.
> > Once this has happened, you generally will get no further response from the
> > first fork.
> > 
> > 	Good Luck,
> > 	Paul
> > 
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