OIC…now i understand.
This is just SDP methods not Megaco, theoretically it can be used on any signalling that uses SDP.
This is used for the example in rfc3108. Bearers are not always initiated from the originating end. If i have announcements from the termi8ing end that need to come through prior to the call connecting, a backwards sdp is viable. I think the idea here is flexibility. I will verify with one of the authors though if there is a burning need, post your email and I can talk to some folks…youve piqued my interest. 🙂