16 Inter UE Session Transfer

26.2373GPPIP Multimedia Subsystem (IMS) based Packet Switch Streaming (PSS) and Multimedia Broadcast/Multicast Service (MBMS) User ServiceProtocolsRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

16.0 Introduction

Inter UE Session transfer (IUT) is used for the transfer or replication of an ongoing PSS session from a transferor UE (UE-1) to a transferee UE (UE-2). IUT includes the transfer of the session control as well as the media flows.

UE-1 and UE-2 are under the same user subscription and are served by the same SCF.

Inter UE Session transfer follows the general Inter UE session transfer procedures as defined in [40] and [41].

NOTE 1: Functionality of SCC AS [40] for Inter UE transfer can be implemented by the SCF.

In the push mode, the session transfer is initiated by UE-1. In the pull mode, the session transfer is initiated by UE-2.

16.1 Push mode

UE-1 is involved in an IMS session with the PSS server. The information flow in Figure 34 shows the transfer of the session from UE-1 to UE-2. The grey boxes refer to the session transfer procedures as defined in clause 15 of [41].

Figure 34: Inter UE session transfer (push mode)

Note: This sequence is simplified and does not e.g. show session progress messages

1.-2. UE-1 is in a IMS controlled PSS session with PSS server receiving a media stream.

3. UE-1 may initiate nBookmark procedure according to clause 12.1.

NOTE: When a media session is replicated from UE-1 to UE-2, it may be needed that the media presentation on the two UEs is synchronized for good user quality of experience. In order to obtain synchronized playout with UE-2, UE-1 may include "suggestedPresentationOffset" in the bookmark. "suggestedPresentationOffset" specifies an offset in time from the bookmark created time to the bookmark presentation time at UE1.

4.-5. SIP REFER request initiating the inter UE transfer to UE-2 is sent from UE-1 to SCC

The SIP REFER request shall follow the procedures in [41] section 15.

Additionally, the Refer-To header shall be extended by a To header field which includes the original content identifier copied from the Request URI of the original SIP INVITE request initiated from the transferor.

Further a body header that contains the SDP body to be included in the PSS session initiation request initiated from the transferee UE. The SDP body shall contain the same number of media lines as the SDP used in the original PSS session from the transferor. Each media line shall indicate the same media type as its corresponding media component in the SDP used in the original session by the transferor UE.

The body of the REFER request shall include the bookmark as defined in Annex J.

If step 3 is carried out, the bookmark may be not present. In this case, an identifier of the stored bookmark is included.

NOTE: Before initiation of IUT, UE 1 may discover UE-2, e.g. by presence service, which is out of scope of TS 26.237.

6. The SCC authorizes the request and if authorization is passed successfully, the SCC forwards the SIP REFER request further.

7.-8. SIP REFER request (SCC to UE-2).

9.-12. SIP 202 response to the SIP REFER request (UE-2 to UE-1).

13. PSS session initiation using the bookmark according to clause 12.2.2.

NOTE: In order to obtain synchronized media playout with UE-1, when the media session is replicated on UE-2, at time T (in the wall-clock time), UE-2 may present the media unit with an offset equal to T – bookmarkcreated time + bookmark offset – suggestedPresentationOffset from the beginning of the program.

14. UE-2 is in an IMS controlled PSS session with PSS server.

The media path is now established between UE-2 and PSS server and the IMS service control between UE-2 and SCC AS.

15.-18. SIP NOTIFY request (UE-2 to UE-1 over intermediate IM CN subsystem entities and SCC).

The UE-2 generates the SIP NOTIFY request carrying the message/sipfrag body and sends it towards UE-1.

19.-22. SIP 200 OK response to the SIP NOTIFY request (UE-1 to UE-2 over intermediate IM CN subsystem entities and SCC).

23. In case of session transfer, a PSS session teardown between UE-1 and PSS server according to clause 8.2.6.1.1 is executed. In case of session replication, no PSS session teardown between UE-1 and PSS server is executed. The streaming session between UE-1 and PSS server should be still ongoing.

16.2 Pull mode

UE-1 is involved in an IMS session with the PSS server. The information flow in Figure X shows the transfer of the session from UE-1 to UE-2. The grey boxes refer to the session transfer procedures as defined in sub-clause 27.2 of [41].

Figure 35: Inter UE session transfer (pull mode).

NOTE: This sequence is simplified and does not e.g. show session progress messages

1.-2. UE-1 is in a IMS controlled PSS session with PSS server receiving a media stream.

3. UE-2 discovers the sessions of UE-1 as defined in clause 21 of [41].

4.-7. SIP REFER request (UE-2 to UE-1)

The UE-2 sends SIP REFER request to UE-1 to request a bookmark.

8.-11. SIP 202 (Accepted) response for the SIP REFER request (UE-1 to UE-2)

12. UE-1 may initiate nBookmark procedure according to clause 12.1.

13.-16. SIP MESSAGE request (UE-1 to UE-2)

Based on the received SIP REFER request, the UE-1 generates a SIP MESSAGE request.

The body of the MESSAGE request shall include the bookmark as defined in Annex J.

If step 3 is carried out, the bookmark may be not present. In this case, an identifier of the stored bookmark is included.

17.-20. SIP (200) OK response to SIP MESSAGE request (UE-2 to UE-1)

21. PSS session initiation using the bookmark according to clause 12.2.2. The timestamp information in the bookmark may be ignored in case of live session initiation.

22.-23. UE-2 is in a IMS controlled PSS session with PSS server receiving a media stream.

24.-27. SIP NOTIFY request (UE-1 to UE-2)

The UE-1 informs the UE-2 that the action triggered by SIP REFER request was successfully completed.

28.-31. SIP 200 (OK) response to SIP NOTIFY request (UE-2 to UE-1)

32. In case of session transfer, a PSS session teardown between UE-1 and PSS server according to clause 8.2.6.1.1 is executed. In case of session replication, no PSS session teardown between UE-1 and PSS server is executed. The streaming session between UE-1 and PSS server should be still ongoing.