A.2 Consultative transfer

24.6293GPPExplicit Communication Transfer (ECT) using IP Multimedia (IM) Core Network (CN) subsystemProtocol specificationRelease 18TS

Figure A.2 signalling flow shows a consultative transfer scenario.

Figure A.2: Consultative transfer

1. Multimedia sessions exist between A-B and between B-C. B initiates transfer of A to C, by sending a REFER request including a To header field set to UE-A GRUU a Refer-To header field set to UE-C?Replaces=dialog2&Require=replaces and a Referred-By header field set to UE-B. The REFER request reuses the dialog that exists from A-B.

1.1 Upon reception of the REFER request AS-B must check whether there is no outgoing call barring active from B to C. Because B is charged for the call from B-C when A is referred to C, when outgoing call barring is active from B-C the REFER is rejected.

AS-B checks whether B is allowed to transfer calls, if it is allowed to transfer the call then AS-B generates an ECT session identifier URI, addressed to itself, with the new destination information and billing information that will be needed for the new session. It replaces the Refer-To header field value with the ECT session identifier URI. This ensures that AS-B will remain in the loop.

2. The REFER request is forwarded to AS-A.

2.1 AS-A checks whether it is allowed to transfer A.

3. The REFER is forwarded to A by AS-A.

4.1, 7.1, 31.1 AS-A can use result messages and notifications caused by the REFER request to track success of the REFER request and take appropriate actions. The AS-A can ensure that header fields that where replaced with other content are recreated with the original content on the way back.

5.1, 8.1, 32.1 AS-B can use this to track success of the REFER request and take appropriate actions. The AS-B can ensure that header fields that where replaced with other content are recreated with the original content on the way back.

13. UE-A initiates a new session by sending an INVITE to AS-B’s ECT session identifier URI (which represents UE-C).

13.1 AS-A checks whether A is allowed to use the Replaces extension and routes the INVITE request to AS-B using the AS-B’s ECT session identifier URI using normal SIP routing procedures. Normal charging from A to B applies.

14.1 Upon receiving the INVITE request to the ECT session identifier URI that was inserted by AS-B, AS-B replaces the Request URI and creates an INVITE targeted towards UE-C. Further AS-B inserts a Replaces header field with the value of the Replaces parameter of the stored transfer target URI if it is available, which will allow the new session to replace the existing session between B and C.
In this scenario it can be assumed that there is no active outgoing call barring towards UE-C, because UE-B was able to setup a call to UE-C in the first place. However when there was no consultation call to UE-C, there is an issue but this is solved at the initial reception of the REFER from UE-C and not at this stage.
The INVITE request is forwarded to UE-C using normal routing procedures.

15.1, 17.1 Normal terminating services apply for UE-C. The call will be treated as a call from A-C regarding call policies. AS-C checks whether the Replaces mechanism is used.

19.1 AS-A. Normal response handling applies.

21.1 AS-A. Normal ACK handling applies.

22.1 AS-B replaces all codified values and the ECT session identifier URI with stored values.

25. UE-C terminates dialog 2 as consequence of normal Replaces procedures according to RFC 3891 [4].