Subject: [Sip] Negotiating audio mixing capabilities

From: "Mark Zeren" <[email-masked]>
Date: Sat, 17 Nov 2001 11:22:28 -0500

I have several questions regarding the negotiation of audio mixing
capabilities.

The recent "draft-rosenberg-mmusic-sdp-offer-answer-00.txt," states: "when
receiving multiple streams of the same type, the streams MUST be mixed
before playing them out."

If the answerer cannot mix multiple streams (for example because of a
hardware limitation) how should it indicate this information to the offerer?

If the offerer is optionally capable of mixing to one m line (e.g. by
acquiring an external conferencing resource), how should that capability be
communicated to and allocated by the answerer?

These questions also bear on the "Centralized Signaling, Distributed Media"
conferencing model recently described in
"draft-ietf-sipping-conferencing-models-00.txt". In that model a central
controller would send an invite with multiple m lines, one for each other
participant in the call. How can a controller indicate that it is also able
to allocate a conferencing resource to mix the media for a less capable
terminal? Additionally, once the controller has allocated a conferencing
resource it may want to offer the resource to the other participants in the
call. A re-invite could be used to force all other parties to use the
conferencing resource, but I see no way to offer the conferencing resource
as an "option" to the other participants.

I have read a reasonable number of the SIP related drafts and RFCs but I
have not yes found material that addresses this specific topic.

Mark Zeren - [email-masked]

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