Subject: [Sipping] how to change of SSRC in the media stream?

From: "Sreedhar Pampati" <[email-masked]>
Date: Wed, 14 Dec 2005 17:19:28 -0800

I have the following scenario:

1. SIP Phone1 calls an App Server (AS) and the AS connects the call to
play prompts. This sets up the stream from the AS to the phone with
SSRC=1000 in the RTP header.

2. After collecting the digits (and the destination number) the AS
(which is acting in B2BUA mode) places an outgoing call to SIP Phone 2.
This sets up the stream from the Phone 2 to the AS with SSRC=9999.

3. The AS is in the media path and in order to synchronize the SSRC and
timestamps on the RTP (streams) between the Phone 1 and Phone 2, the AS
needs to let the Phone1 to expect a new SSRC (namely 9999). 

My question is, can the AS send a reINVITE to the SIP Phone1 with a
version number increment in the o= line in the SDP? Is that sufficient
for Phone 1 to expect a new SSRC on the incoming RTP packets?

I am pasting relevant sections from the RFC 3264 below. Please provide
your feedback.

rfc3264 -- SDP Offer-Answer model
---------------------------------------------

8 Modifying the Session

   Nearly all aspects of the session can be modified.  New streams can
   be added, existing streams can be deleted, and parameters of existing
   streams can change.  When issuing an offer that modifies the session,
   the "o=" line of the new SDP MUST be identical to that in the
   previous SDP, except that the version in the origin field MUST
   increment by one from the previous SDP.  If the version in the origin
   line does not increment, the SDP MUST be identical to the SDP with
   that version number.  The answerer MUST be prepared to receive an
   offer that contains SDP with a version that has not changed; this is
   effectively a no-op.  However, the answerer MUST generate a valid
   answer (which MAY be the same as the previous SDP from the answerer,
   or MAY be different), according to the procedures defined in Section
   6.

8.3.4 Changing Attributes

   Any other attributes in a media description MAY be updated in an
   offer or answer.  Generally, an agent MUST send media (if the
   directionality of the stream allows) using the new parameters once
   the SDP with the change is received.

Thanks
Sreedhar

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---

From: Colin Perkins <[email-masked]>
Date: Fri, 16 Dec 2005 00:31:55 +0000

> for Phone 1 to expect a new SSRC on the incoming RTP packets?

The SSRC can change within a session, without explicit signalling.  
Send an RTCP BYE for the old SSRC and start using the new SSRC.  
Receivers will recognise the RTCP CNAME as being constant and  
associate it with the original sender. See RFC 3550, in particular  
sections 7 and 8.

Colin