Subject: Re: RE: [Sip] Open Issue #58: Rejected mid-call SDP in 2xx

From: "Kiranmai Pavoori" <[email-masked]>
Date: Thu, 06 Dec 2001 16:24:56 -0600

This is for an ideal case.

How would this scenario be handled, when B sees that it had received SDP, (media not supported) in INVITE to C. B can always reject with not 200 OK message. But how would the transaction complete between C and A. C still needs to send ACK to complete the transaction, does this mean that ACK again carries SDP of the current existing session. ( reflecting session between C and A ? )

when the UA-A sends 200 OK ( with sdP ) in response to a INVITE from C with no SDP...And when C sends an INVITE to B with this 

>>> "Chris Hogg" <[email-masked]> 10/04 8:34 AM >>>
Hi Christer,

>I am not sure I agree on that. Isn't the 200 for a non-SDP re-INVITE
>supposed to return the current remote SDP, and if the call has been put
>on hold that should be a call-hold SDP (ie the SDP that was returned in
>the 200 for the call hold re-INVITE)?

I agree with this understanding.  I think this is also useful in 3pcc cases
whereby a controller (C) is already in session with a UA "A" but wishes to
establish a session between A and a new UA "B".  C sends a Re-INVITE to A
with no SDP causing A to send a 200 OK with its' SDP (which will be the
current remote SDP).  C may then initiate a new INVITE to B with the SDP it
has gleaned from A. The ACK from C to A will contain SDPb thus enabling the
call signalling to go from A to B via the controller but the media path to
be set up between A and B.  Thus in this case, the ACK does require to
modify the SDP from that of the original session but will not add any
additional media streams. I suppose if the SDP from B (sent to A from C in
the ACK) was inconsistent with that from A then the redirect would fail and
the session would revert to the call between A and C. 

Just wanted to understand the issues some more - comments welcome.

Thanks

Regards

Chris 

Chris Hogg
Nortel Networks (UK).

-----Original Message-----
From: Christer Holmberg [mailto:[email-masked]] 
Sent: 04 October 2001 06:11
To: [email-masked] 
Cc: [email-masked] 
Subject: Re: [Sip] Open Issue #58: Rejected mid-call SDP in 2xx

Hi,

>>However, my understanding was that the UAC
>>should be able to modify the existing streams
>>in the ACK SDP (but not add new streams).
>>Is that wrong?
>You are correct.

Good :)

>>At least I am :) But, again, my concern is
>>if there IS a scenario where
>>the UAC DOES add streams in the ACK SDP?
>Within the offer-answer model (both INVITE-200
>and 200-ACK), it is a protocol violation to
>add media streams in the answer.

I agree.

>>Yes, and that is related to my question:
>>what is 200/ACK SDP exchange
>>really needed for, if no party is allowed
>>to change anything in the SDPs?
>After a call has been established and placed
>on hold, a reinvite-without-sdp will likely
>trigger non-hold SDP in the 200 and ACK.
>Thus one or both SDP's have changed.

I am not sure I agree on that. Isn't the 200 for a non-SDP re-INVITE
supposed to return the current remote SDP, and if the call has been put
on hold that should be a call-hold SDP (ie the SDP that was returned in
the 200 for the call hold re-INVITE)?

Thanks for your comments!

Christer Holmberg
Ericsson Finland

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This list is for NEW development of the core SIP Protocol
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_______________________________________________
Sip mailing list  http://www1.ietf.org/mailman/listinfo/sip
This list is for NEW development of the core SIP Protocol
Use [email-masked] for questions on current sip
Use [email-masked] for new developments on the application of sip

---

From: "Chris Hogg" <[email-masked]>
Date: Fri, 7 Dec 2001 11:59:48 -0000

Hi Kiranmai,

A session has already been established between A and C so if the controller
C takes the SDP that it has received as part of a 200 OK OFFER from A and
passes it on to UA B and B subsequently rejects this SDP then I think you
have a point, there would be no SDP from B to go in the ACK sent from C to A
(which would constitute a protocol error so far as SDP offer answer goes).
I think under these circumstances the controller would need to realise that
the transfer (which is essentially what is being done within this 3pcc
scenario) is not able to complete and so the Re-INVITE would need to be
rejected by sending a CANCEL - i.e. no ACK would be sent from C to A.  This
is what I mean when I say that if the transfer fails it shold revert back to
the existing session.

As I said before, I'm just exploring the issues and welcome any
discussion/comments.

Best Regards,

Chris.

Chris Hogg
Nortel Networks (UK).

-----Original Message-----
From: Kiranmai Pavoori [mailto:[email-masked]]
Sent: 06 December 2001 22:25
To: brett; christer.holmberg; Hogg, Chris [MDN05:MA01:EXCH]
Cc: sip
Subject: Re: RE: [Sip] Open Issue #58: Rejected mid-call SDP in 2xx

How would this scenario be handled, when B sees that it had received SDP,
(media not supported) in INVITE to C. B can always reject with not 200 OK
message. But how would the transaction complete between C and A. C still
needs to send ACK to complete the transaction, does this mean that ACK again
carries SDP of the current existing session. ( reflecting session between C
and A ? )

when the UA-A sends 200 OK ( with sdP ) in response to a INVITE from C with
no SDP...And when C sends an INVITE to B with this 

Hi Christer,

Thanks

Regards

Chris 

Chris Hogg
Nortel Networks (UK).

-----Original Message-----
From: Christer Holmberg [mailto:[email-masked]] 
Sent: 04 October 2001 06:11
To: [email-masked] 
Cc: [email-masked] 
Subject: Re: [Sip] Open Issue #58: Rejected mid-call SDP in 2xx

Hi,

Good :)

I agree.

Christer Holmberg
Ericsson Finland