Attila, It is my interpretation that the directionality of the stream is reversed in case of multicast. So for example, in the unicast case, if a participant lets say party1 generates an SDP with
m=audio 30000 RTP/AVP 0 c=10.0.0.10 a=recvonly It means party 1 will *only* receive audio at port 30000 and address 10.0.0.10. Where in case of multicast m=audio 30000 RTP/AVP 0 c=239.199.0.10 a=sendonly Indicates that party 1 can only transmit video at address 239.199.0.10 and port 30000. That is the *direction* of offer in SDP is from transmit perspective rather than receive perspective ( in terms of port numbers and multicast IP address). Although arguably you could receive on the same port/address in multicast. Typically, if a multicast offer is made the answer would make sense in multicast only. However, it is allowed that a unicast offer get a multicast reply. ( internet radio, would be an example, where client could send a unicast request/offer, and the server could reply back either with a unicast or multicast answer ). All the participants in a call who indicate capability to send on a given multicast address can send on that multicast. So for a chat session, all the participants could use a single multicast address with every participant using that for a group chat. ( Every party ignoring its own multicast out). To re-iterate, all participants who indicate that they can send on a given multicast address are allowed to send RTP on that multicast address. thanks Arun -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Attila Sipos Sent: Friday, May 18, 2007 8:42 AM To: Sip-implementors@cs.columbia.edu Subject: [Sip-implementors] Does anyone know multicast? (was RE: RFC 3264 offer/answer - multicast) Does anyone have a clue about multicast and the questions below? Any help would be greatly appreciated. Regards, Attila -----Original Message----- From: Attila Sipos Sent: 17 May 2007 17:30 To: Sip-implementors@cs.columbia.edu Subject: [Sip-implementors] RFC 3264 offer/answer - multicast 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 _______________________________________________ Sip-implementors mailing list Sip-implementors@cs.columbia.edu https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors _______________________________________________ Sip-implementors mailing list Sip-implementors@cs.columbia.edu https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
