Re: [Sip] Multiple SDP descriptions for conferencing

[Alan Johnston <[e-mail]>]

Hi Seonman,

You will find more updated work on SIP conferencing with the SIPPING 
Conferencing Design Team:

      http://www.softarmor.com/sipping/teams/conf/

The framework document includes a discussion of the this model:

      http://www.ietf.org/internet-drafts/draft-rosenberg-sipping-conferencing-framework-00.txt

Instead of multiple SDP message bodies, the re-INVITE would add an extra 
media line for each participant that enters the conference.  See below for 
an example.

For other valid offer/answer examples of SDP, see:

      http://www.ietf.org/internet-drafts/draft-johnston-mmusic-offer-answer-examples-00.txt

Thanks,
Alan Johnston
WorldCom
sip:[e-mail]


At 01:43 PM 1/22/2003 -0800, Seonman Kim wrote:
>Hi,
>
>I am trying to implement a SIP-based multi-point conferencing based on
>"Centralized Signaling, Distributed Media" model
>(draft-ietf-sipping-conferencing-models-01.txt). In this model, each
>participant  UA gets updated SDP descriptions using "re-INVITE" whenever a
>user enters to (or exits from) the conference. If the number of participants
>becomes more than two,
>then the re-INVITE message should contain two or more SDP descriptions.
>For example, if user A, B and C are in the conference, then a re-INVITE to
>user A should contain two SDP description, one for user B and the other for
>user C.
>
>My question here is: how to describe multiple SDP descriptions in the
>content body
>of INVITE (or 200 OK) message? Do we just concatenate them?
>For example, if the conference server sends re-INVITE to user A with SDPs of
>user B and C, the content body will look like this:
>
>v=0
>o=user-b 0 0 IN IP4 192.168.1.100
>s=session
>c=IN IP4 192.168.1.100
>b=CT:1000
>t=0 0
>m=audio 30001 RTP/AVP 0
>a=rtpmap:0 PCMU/8000
>v=0
>o=user-c 0 0 IN IP4 192.168.1.101
>s=session
>c=IN IP4 192.168.1.101
>b=CT:1000
>t=0 0
>m=audio 30002 RTP/AVP 0
>a=rtpmap:0 PCMU/8000
>
>Is this correct implementation for multiple SDP sessions?

No, I think it would look like this:

v=0
o=user-b 0 0 IN IP4 192.168.1.100
s=session
b=CT:1000
t=0 0
m=audio 30001 RTP/AVP 0
c=IN IP4 192.168.1.100
a=rtpmap:0 PCMU/8000
m=audio 30002 RTP/AVP 0
c=IN IP4 192.168.1.101
a=rtpmap:0 PCMU/8000

>I could not find any descriptions in RTF 3261 that say UA should handle
>multiple SDP sessions.
>
>Thanks,
>Seonman
>
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