Ettore Benedetti schrieb:
My question is how must a SDP be constructed if a participant wants to join a conference. As the participant receives multicast he doesn't need to provide a unicast contact in his offer. He only needs the answer from the conference focus to know where to send his unicast stream and the mulicast announcement.
So, is should really the offer/answer model be used for that scenario? Or does the participant sends a blank SDP to the focus and the focus consucts a SDP with the multicast media description and the unicast media description for reception?
I couln't find any information in the RFCs and IETF drafts.
Hi Michael,
It's legal to send an INVITE without any SDP body. In this case the offer is made by the called UA and delivered on the 200 OK message (or in any 1xx response but the 200 has to confirm these "unreliable" offers). At this point, the caller fills out the ACK with his SDP response. Check sections 13.2.2.4 and 13.3.1.4 in RFC3261 for further details.
Ettore
Hi Ettore,
...but how about the concept of the "Conference-Unaware Participant".
in draft-ietf-sipping-conferencing-framework-03.txt it says:
[The notion of a conference-unaware participant is important in this framework. A conference-unaware participant does not even know that the UA it is communicating with happens to be a focus. As far as it's concerned, its a UA just like any other. The focus, of course, knows that its a focus, and it performs the tasks needed for the conference to operate.]
but to set up a session as described above the Participant (as you said) has to send an empty SDP. But if he is unaware that he is talking to a conference focus he does not know that he shouldn't send a SDP offer. And if he does send one, the focus can't switch to the single-source multicast model.
The only solution I can think of is to accept his SDP offer and afterwards send a reINVITE to switch to multicast.
is that the only possibility? Dosn't sound very elegant to me.
Michael...
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