22 Roles for collaborative session handling upon loss of collaborative session control

24.3373GPPIP Multimedia (IM) Core Network (CN) subsystem IP Multimedia Subsystem (IMS) inter-UE transferRelease 17Stage 3TS

22.1 Introduction

This clause specifies the roles of controller UE, controller capable UE and the SCC AS when collaborative session control is lost and collaborative session control is transferred to another UE.

22.2 SC UE

22.2.1 Controller UE

The user may send or modify controller loss preference at any time during a session using XCAP over Ut interface. Candidate UEs in controller loss preference should be configured in a priority order.

NOTE: The controller UE can support the session timer mechanism as specified in IETF RFC 4028 [37] and 3GPP TS 24.229 [7] in order to aid the SCC AS detection of loss of collaborative session control.

22.2.2 Controller capable UE

There are no additional procedures for the controller capable UE over those specified in subclause 18.2.2.

22.3 SCC AS

22.3.1 Session handling upon controller loss

The following means may be used by to the SCC AS to detect of loss of collaborative session control:

1) the SCC AS receives a BYE message with a reason header 503 (Service Unavailable) response code;

NOTE: Since the SCC AS inserted itself in record-route header in IUT scenario, when radio/bearer resources are no longer available or the signalling bearer is lost to the UE for a session (e.g. abort session request from PCRF) (as specified in 18.229 [9] subclause 5.2.8.1.2), the SCC AS will receive a BYE message with a reason header 503 (Service Unavailable) response code.

2) the SCC AS detects a session timeout using the mechanism specified in IETF RFC 4028 [37] and 3GPP TS 24.229 [7].

3) the SCC AS receives a 5xx response for a subsequent request sent to the controller UE.

Upon detection of a loss of collaborative session control using one of the above mechanisms the SCC AS checks controller loss preference in the IUT user preferences (see annex C.3). If the <controller loss preferences> element is present, the SCC AS obtains the list of controller capable UEs in the <controller> elements to assume the controller UE role.

NOTE: The SCC AS can use subscription information to determine whether to enable selection of a new controller UE based on controller loss preference.

The SCC AS invites in turn according to the priority order of the <controller> elements in the <controller loss preference> each controller capable UE to become a controller of the collaborative session using the following procedure:

1) The SCC AS shall send a SIP INVITE request or SIP re-INVITE request to the UE identified by the SIP URI in the <controller> element, containing:

a) Request-URI with SIP URI from the <controller> element;

b) the P-Asserted-Identity header field containing the identity of the remote UE as received in the P-Asserted-Identity header field from the remote UE at the original session establishment; and

c) the Content-Type header field set to the value of "multipart/mixed"; and

f) a "multipart/mixed" MIME body containing the following MIME parts:

A) the SDP information for the media component to be transferred with the Content-Type set to "application/sdp" along with SDP for the media type for each of the media (m=) lines in the session set as follows:

– media lines for media on other UEs other than UE that control is being transferred to with the port number set to zero

– media line(s) for media on the UE that is requested to become a controller of the collaborative session containing the port numbers of the remote UE;

B) containing the Content-Type set to " application/vnd.3gpp.iut+xml " along with the handling parameter set to optional and the XML specified in annex C.2 containing a <controlTransfer> element containing a <targetController> element containing the SIP URI from the <controller> element;

2) If the SIP final response was a non 2xx response or if the Contact header field from the received SIP 2xx response does not contain the media feature tag g.3gpp current-iut-controller set to "active" then the SCC AS shall repeats the above procedure for the next controller capable UE in the list of <controller> elements in the <controller loss preference>.

3) Upon receiving a final response to the SIP INVITE request or SIP re-INVITE request which was sent towards the controller capable UE, the SCC AS shall:

a) send a SIP ACK request to the controller UE that sent the final response;

b) if the Contact header field from the received SIP 2xx response contains the media feature tag g.3gpp current-iut-controller set to "active" then consider the UE that sent the SIP 2xx response as the controller of the collaborative session.

4) If no suitable candidate controller capable UEs are left in the list of <controller> elements in the <controller loss preference>, then the SCC AS shall send SIP BYE request to all UEs participating in the session.