Subject: [Sip] Open Issue #20: Mixing unicast/multicast in offer/answer

From: Jonathan Rosenberg <[email-masked]>
Date: Wed, 22 Aug 2001 18:31:51 -0400

Issue:

Appendix B is silent on whether its allowed to send an SDP offer with a c
line thats unicast, and answer with multicast. Same with the inverse: can
you answer  a multicast offer with unicast?

Points for discussion:

In general, it is my view that the answer SDP should always be something
that the offerer is guaranteed to be able to use. That is a consequence of
the two way offer/answer handshake. If an offer is unicast, answering
multicast seems a bad thing, since you don't know whether the offerer can do
unicast. The argument is a bit weaker for the inverse case, but there could
be cases where one is being invited to an mbone session that is multicast,
where responding unicast makes little sense.

Furthermore, the semantics of mixed unicast/multicast in offer answer aren't
entirely clear. 

Proposal:

If the offer is unicast, so must be the answer. If the offer is multicast,
the answer should mirror that address (i.e., use the same multicast group).
If you want a particular stream to change from unicast to multicast,
re-invite to update it.

There was consensus on this proposal at IETF 51. Any list objections?

Thanks,
Jonathan R.

---
Jonathan D. Rosenberg, Ph.D.                72 Eagle Rock Ave.
Chief Scientist                             First Floor
dynamicsoft                                 East Hanover, NJ 07936
[email-masked]                     FAX:   (973) 952-5050
http://www.jdrosen.net                      PHONE: (973) 952-5000
http://www.dynamicsoft.com

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