Subject: [MMUSIC] RFC 3264 offer/answer - multicast questions

From: "Attila Sipos" <[email-masked]>
Date: Fri, 18 May 2007 07:27:04 +0100

(SIP-Implementors didn't come up with an answer.
 Maybe MMUSIC will...)

I have some questions on section 5.2:

5 Generating the Initial Offer
5.2 Multicast Streams

   If a session description contains a multicast media stream which is
   listed as receive (send) only, it means that the participants,
   including the offerer and answerer, can only receive (send) on that
   stream.  This differs from the unicast view, where the directionality
   refers to the flow of media between offerer and answerer.

I am confused about this.  It seems to say that if the offer has recvonly
then both the "offerer and answerer" can only receive on that port.

I am sure my interpretation is wrong -
what is the correct interpretation?

              =========================

Also regarding, "6.2 Multicast Streams", generating the answer, it seems
that if the offer was a multicast SDP, the answer has to be an almost identical
mulicast SDP.  Fine.  But what happens if the offer was unicast and the
desired answer is multicast?

For example, if you sent an INVITE to a server with unicast
but the server wanted to use multicast to send the same RTP to
everyone, can this happen?
Would there need to be anything special in the SDP?

              =========================

And who is allowed to send (say RTP) to a multicast IP address?
Surely it only makes sense that the server sends the multicasts
while the clients keep quiet.  ??

Regards,

	Attila

_______________________________________________
mmusic mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/mmusic

---

From: Jonathan Rosenberg <[email-masked]>
Date: Mon, 21 May 2007 13:16:38 -0400

inline.

> what is the correct interpretation?

You are correct.

The usage of multicast with SIP was trying (for right or wrong), to 
maintain the original, original, way-back-when mode of operation of SIP, 
whereby you were inviting someone else to an mbone conference. In those 
original models there was only an SDP in the INVITE and both 
participants just joined the multicast group. So if the mbone session 
was recv-only, it was recv-only for everyone.

Not permitted at this time.

This might be fixable with the new capability negotiation framework.

> everyone, can this happen?

No.

> Would there need to be anything special in the SDP?

Depends on the session and who owns it.

-Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.                   600 Lanidex Plaza
Cisco Fellow                                   Parsippany, NJ 07054-2711
Cisco Systems
[email-masked]                              FAX:   (973) 952-5050
http://www.jdrosen.net                         PHONE: (973) 952-5000
http://www.cisco.com