Section: 10.7.2 MR-DC with 5GC
Content: Inter-MN handover with/without MN initiated SN change is used to transfer UE context data from a source MN to a target MN while the UE context at the SN is kept or moved to another SN. During an Inter-Master Node handover, the target MN decides whether to keep or change the SN (or release the SN, as described in clause 10.8). Only intra-RAT Inter-Master node handover with/without SN change is supported (e.g. no transition from NGEN-DC to NR-DC). Figure 10.7.2-1: Inter-MN handover with/without MN initiated SN change procedure Figure 10.7.2-1 shows an example signalling flow for inter-MN handover with or without MN initiated SN change: NOTE 1: For an Inter-Master Node handover without Secondary Node change, the source SN and the target SN shown in Figure 10.7.2-1 are the same node. 1. The source MN starts the handover procedure by initiating the Xn Handover Preparation procedure including both MCG and SCG configuration. The source MN includes the source SN UE XnAP ID, SN ID and the UE context in the source SN in the Handover Request message. NOTE 2: The source MN may trigger the MN-initiated SN Modification procedure (to the source SN) to retrieve the current SCG configuration and SN-associated QMC configuration information and to allow provision of data forwarding related information before step 1. 2. If the target MN decides to keep the UE context in source SN, the target MN sends SN Addition Request to the SN including the SN UE XnAP ID as a reference to the UE context in the SN that was established by the source MN. If the target MN decides to change the SN allowing delta configuration, the target MN sends the SN Addition Request to the target SN including the UE context in the source SN that was established by the source MN. Otherwise, the target MN may send the SN Addition Request to the target SN including neither the SN UE XnAP ID nor the UE context in the source SN that was established by the source MN. 3. The (target) SN replies with SN Addition Request Acknowledge. The (target) SN may include the indication of the full or delta RRC configuration. NOTE 2a0: Void. 3a. For SN terminated bearers using MCG resources, the target MN provides Xn-U DL TNL address information in the Xn-U Address Indication message. 4. The target MN includes within the Handover Request Acknowledge message the MN RRC reconfiguration message to be sent to the UE in order to perform the handover, and may also provide forwarding addresses to the source MN. If PDU session split is performed in the target side during handover procedure, more than one data forwarding addresses corresponding to each node are included in the Handover Request Acknowledge message. The target MN indicates to the source MN that the UE context in the SN is kept if the target MN and the SN decided to keep the UE context in the SN in step 2 and step 3. 5a/5b. The source MN sends SN Release Request message to the (source) SN including a Cause indicating MCG mobility. The (source) SN acknowledges the release request. The source MN indicates to the (source) SN that the UE context in SN is kept, if it receives the indication from the target MN. If the indication as the UE context kept in SN is included, the SN keeps the UE context. 5c. The source MN sends XN-U Address Indication message to the (source) SN to transfer data forwarding information. More than one data forwarding addresses may be provided if the PDU session is split in the target side. 6. The source MN triggers the UE to perform handover and apply the new configuration. 7/8. The UE synchronizes to the target MN and replies with MN RRC reconfiguration complete message. 9. If configured with bearers requiring SCG radio resources, the UE synchronizes to the (target) SN. NOTE 2a1: The order the UE performs Random Access towards the MN (step 7) and performs the Random Access procedure towards the SN (step 9) is not defined. 10. If the RRC connection reconfiguration procedure was successful, the target MN informs the (target) SN via SN Reconfiguration Complete message. 11a. The source SN sends the Secondary RAT Data Usage Report message to the source MN and includes the data volumes delivered to and received from the UE over the NR/E-UTRA radio as described in clause 10.11.2. NOTE 2a2: The order the source SN sends the Secondary RAT Data Usage Report message and performs data forwarding with MN/target SN is not defined. The SN may send the report when the transmission of the related QoS is stopped. 11b. The source MN sends the Secondary RAT Report message to AMF to provide information on the used NR/E-UTRA resource. 12. For bearers using RLC AM, the source MN sends the SN Status Transfer message to the target MN, including, if needed, SN Status received from the source SN. The target forwards the SN Status to the target SN, if needed. 13. If applicable, data forwarding takes place from the source side. If the SN is kept, data forwarding may be omitted for SN terminated bearers or QoS flows kept in the SN. 14-17. The target MN initiates the Path Switch procedure. If the target MN includes multiple DL TEIDs for one PDU session in the Path Switch Request message, multiple UL TEID of the UPF for the PDU session should be included in the Path Switch Ack message in case there is TEID update in UPF. NOTE 3: If new UL TEIDs of the UPF for SN are included, the target MN performs MN initiated SN Modification procedure to provide them to the SN. 18. The target MN initiates the UE Context Release procedure towards the source MN. 19. Upon reception of the UE Context Release message from source MN, the (source) SN releases C-plane related resources associated to the UE context towards the source MN. Any ongoing data forwarding may continue. The SN shall not release the UE context associated with the target MN if the UE contest kept indication was included in the SN Release Request message in step 5.