Howto:SIP Reverse Negotiation - Call Transfer
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Applies To
This information applies to
- all innovaphone products
The need of Reverse-Negotiation.
Reverse-Negotiation means:
- INVITE without SDP
- 200/OK with SDP-Offer
- ACK with SDP-Answer
(RFC-3264 "An Offer/Answer Model Session Description Protocol" June 2002)
For the Interop with innovaphone the support of the Reverse-Negotiation is is very important!
- The Reverse-Negotiation is needed for Call-Transfer.
- The Reverse-Negotiation is needed for Call-Pickup.
- The Reverse-Negotiation is needed for Call-Forking (Broadcast Group and Multiple Registrations).
Example Call-Transfer with consultation (Attended Transfer):
- A is connenced with B through PBX.
- B is connenced with C through PBX.
- B initiates a Transfer (sends REFER with Replaces to PBX).
- A and C have to be connected by the PBX.
- Therefore A and C need to do a Media-Negotiation.
- Therefore one of the subsribers must provide an SDP-Offer, that is passed through to the other.
- Therefore the PBX starts an Reverse-Negotiation to one of the both partners, for instance A ..
- A gets an INVITE without SDP.
- A answers with 200/OK with SDP-Offer.
- This SDP-Offer will be forwarded by the PBX with an INVITE to C.
- C selects the coder and answers with 200/OK with SDP-Answer.
- This SDP-Answer will be forwarded with an ACK to A.
- C gets from the PBX an ACK without SDP.