[SIPForum-discussion] Early Media

Fri Apr 20 10:08:42 UTC 2007

Siva,

The AS may need to utilize the UPDATE method in this case. As Xu Rui
explained the RE-INVITE is the only mechanism outlined in RFC 3261. The
UPDATE mechanism provides a solution here.

1) the UAC A receives the 183 from the Media Server (via the B2BUA AS) and
the 'early media' is established between A and the media server.

2) At some point C sends a 200 OK with a new SDP. The AS then sends a CANCEL
to the media server and the media server responds with a 487.

3) The AS can then send an UPDATE to A containing the SDP descriptor from
the 200 OK (from C). The UAC responds with 200 OK to the UPDATE.

4) The session is then modified and media from A to C is established.

5) The 200 OK ( from C ) can then be relayed to A with the SDP descriptor
already passed via the UPDATE. The session is unchanged (already established
in 4) and the dialog is confirmed.

Rob.

-----Original Message-----
From: Siva M-Q16748 [mailto:sivam at motorola.com] 
Sent: 20 April 2007 10:36
To: Robert Mansfield; discussion at sipforum.org
Subject: RE: [SIPForum-discussion] Early Media

Hi Rob

Thanks for your explanation

In my case the the AS which is handling the call will cancel the media as
soon as User C answers .

But I agree User A will be confused whether he has to listen to Media port
of Media Server or Media port of C . 

Hence a re-INVITE would be required to inform A to listen to C and not to
Media Server as soon an C sends 200 OK . Does that make sense ??

Siva M 

-----Original Message-----
From: Robert Mansfield [mailto:RJM at Redwoodtech.com] 
Sent: Friday, April 20, 2007 2:28 PM
To: Siva M-Q16748; Robert Mansfield; discussion at sipforum.org
Subject: RE: [SIPForum-discussion] Early Media

Siva,

In the case you describe the two SDP descriptors will be different and
that's ok. The RFC's outline some issues for the UAC receiving the RTP
stream and how the users experience of transition from one audio source to
another.

In the example you describe both the media resource (playing ringing) and
the C party have the SDP address of A and will send packets to the A. How
does A deal with receiving two audio streams?

The same problem exists when the AS performs forking and receives multiple
early media responses. If the UAC/AS chooses one stream to listen to there
is no guarantee it will be the same stream when the session is established
(similar to your example).

So the bottom line is that YES you can have a different SDP descriptor in
the 200 OK and the 183. 

In the simple GATEWAY model (no forking or call diverting) this typically
doesn't happen as the audio stream originates in the PSTN. The audio stream
is constant and doesn't change from the early to the established session.
The UAC hears one constant audio input (I've also seen the 200 OK with no
SDP descriptor in this case).

In the AS model there is more scope for different SDP descriptors in the
early and establised stages so yes the 200 OK and 183 can have different SDP
descriptors. 

Rob.

-----Original Message-----
From: Siva M-Q16748 [mailto:sivam at motorola.com]
Sent: 20 April 2007 06:06
To: Robert Mansfield; discussion at sipforum.org
Subject: RE: [SIPForum-discussion] Early Media

Hi 

Would anybody be able to point out any Logical Reason why a 183 and 200
should not have same SDP ??

May be some scenario in which it creates issues ..

I hope there should be a Logical reason why RFC says so ..

Siva M 

-----Original Message-----
From: Robert Mansfield [mailto:RJM at Redwoodtech.com] 
Sent: Thursday, April 19, 2007 2:55 PM
To: Siva M-Q16748
Subject: RE: [SIPForum-discussion] Early Media

Siva,

If the use of early media is in the 'Gateway' model then I believe the SDP
descriptor in the 183 and 200 will be the same.

The scenario you describe is more akin to the Application Server model (both
models described in RFC 3960).

"Application Server sends the SDP info he got from Media Server to A in the
183 message A gets a Music played by media"-In this case the B2BUA AS could
make use of the 'early-session' extension tag in the 'Content-Disposition'
header. This allows for the establishment of an early session with a
descriptor that is different from the SDP returned in the 200 OK.

"Server Now Application server forwards the call to C If C answers , A
should get the SDP info of C ( I thought it would be sent in 200 OK )" Ok so
now A needs to talk to C. The SDP descriptor is different and represents the
normal session represented by the INVITE/200/ACK.

I only have experience of this RFC in terms of the GATEWAY model so what I'm
not staking my life on the accuracy of this explanation however I would be
interested in your thoughts from RFC 3960 Application Server model.

Hope this helps,

Rob.

-----Original Message-----
From: Siva M-Q16748 [mailto:sivam at motorola.com]
Sent: 19 April 2007 09:56
To: Robert Mansfield; discussion at sipforum.org
Subject: RE: [SIPForum-discussion] Early Media

Hi

If the 183 and 200 OK should have same SDP . How does the following scenario
work ?? 

A calls ( throgh a B2BUA AS) B
B doesnot answer
Application Server sends INVITE to Media Server
Application Server sends the SDP info he got from Media Server to A in the
183 message A gets a Music played by media Server Now Application server
forwards the call to C If C answers , A should get the SDP info of C ( I
thought it would be sent in 200 OK )

If 200 OK sent to A should have the same SDP as that of 183 , How can A and
C talk in this case ..

Thanks
Siva M 

-----Original Message-----
From: Robert Mansfield [mailto:RJM at Redwoodtech.com] 
Sent: Thursday, April 19, 2007 1:54 PM
To: Siva M-Q16748
Subject: RE: [SIPForum-discussion] Early Media

Siva,

I'm not 100% sure but my understanding is that the early media WILL contain
the same SDP descriptor as the 200 OK. There is a 'early-session' header
(look at RFC 3959 and 3960 for some guidance) that is used in an Application
Server model for early media. This allows for different SDP descriptors
during session establishment.

>From my experience with PSTN-SIP Gateways (Gateway model) the descriptor
will be the same.

Rob.

-----Original Message-----
From: Siva M-Q16748 [mailto:sivam at motorola.com]
Sent: 19 April 2007 06:49
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Early Media

Hi

In case of Early Media for announcements

Can 183 and 200OK have different SDP info 

Thanks
Siva M 

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