Subject: [SIP] Tag in provisional response

From: Benny Prijono <[email-masked]>
Date: Mon, 04 Sep 2000 17:21:50 +0100

Hi all,

Issue regarding to TAG in provisional responses:

Current bis-01 draft doesn't mandate servers to add tag in provisional
response for the initial INVITE request, as written in Section 6.43
(last paragraph, page 56):

 "These servers MAY add the tag for informational responses during 
  the initial INVITE transaction, but MUST add a tag to informational 
  responses for all subsequent transactions."

Shouldn't it be changed to MUST, so that the client can distinguish
which response goes to which server? I imagine that a GUI client will
display two different feedback messages if it gets two provisional
responses with different tags, so that the client/user awares that
his/her call has reached two different destinations. For example, the
first line is "call 1: queued, 1 person ahead" and second line is
"call 2: ringing". And this is only possible if the tag is present in
the provisional response.

Comments?

cheers,
Bennylp

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From: Jonathan Rosenberg <[email-masked]>
Date: Mon, 04 Sep 2000 23:00:44 -0400

Best we can really do is SHOULD. MUST doesn't help you since its not
compatible with rfc2543 in that case.

-Jonathan R.

-- 
Jonathan D. Rosenberg                       72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (973) 952-5050
http://www.cs.columbia.edu/~jdrosen         PHONE: (732) 741-7244
http://www.dynamicsoft.com

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From: Benny Prijono <[email-masked]>
Date: Tue, 05 Sep 2000 11:56:59 +0100

> compatible with rfc2543 in that case.

Yes, I think that's the best we can do.

There's another scenario actually, where I think the presence of tag
is important. Consider a client receives two provisional responses
(from two servers), and both of them have SDP body. Both SDPs have
different codecs. Without a tag, the client will happily (or
unhappily) change its codec each time it receives SDP from each 
server, because it assumes that it has received SDP from one server 
only, and that server is changing its codecs.

With a tag in the provisional response, the client will know that it
has received SDP from two different servers, thus it may disable
processing of SDP (and RTP) from one server if it wish. Or it can 
prepare two codecs and a mixing device if it can.

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From: "Jo Hornsby" <[email-masked]>
Date: Tue, 5 Sep 2000 12:05:48 +0100

> prepare two codecs and a mixing device if it can.

This probably isn't a massive problem, since if the SDP is
important, the Server involved will probably be using Reliable
Provisional Responses; the Reliable Provisional Response Draft
mandates the use of a tag.

HTH,

 - Jo.

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From: Benny Prijono <[email-masked]>
Date: Tue, 05 Sep 2000 16:21:37 +0100

> mandates the use of a tag.

Thanks for pointing this out.
So now it's clear to me that Reliable Provisional Response is
the only solution for the client to know which provisional responses
came from which server.

cheers,
Bennylp