Section: 6.8.2.2.2 RAN-based notification area update to a new gNB/ng-eNB
Content: When the UE decides to initiate the RANU procedure the UE may initiate the procedure with a new gNB/ng-eNB. In this case, the UE, the target gNB/ng-eNB and the source gNB/ng-eNB follow the detailed procedure as described in clause 6.8.2.1.3 with the following deviations: The target gNB/ng-eNB shall check if it supports the ciphering and integrity algorithms the UE used with the last source cell. If the target gNB/ng-eNB does not support the ciphering and integrity algorithms used in the last source cell or if the target gNB/ng-eNB prefers to use different algorithms than the source gNB/ng-eNB, then the target gNB/ng-eNB shall send an RRCSetup message on SRB0 to the UE in order to proceed with RRC connection establishment as if the UE was in RRC_IDLE (i.e., fallback procedure). If the target gNB/ng-eNB selects the ciphering and integrity protection algorithms which the UE used with the last source cell and the target gNB/ng-eNB decides to send the UE directly back to RRC_INACTIVE state without bringing the UE to RRC_CONNECTED state, the target gNB/ng-eNB shall perform a Path Switch procedure with the AMF to get a fresh {NCC, NH} pair before sending the RRCRelease message to the UE. After the target gNB/ng-eNB receives a fresh {NCC, NH} pair in the Path Switch Acknowledgement message from the AMF, the target gNB/ng-eNB shall set the value of NCC in the RRCRelease message to the NCC value of the received fresh {NCC, NH} pair. After the source gNB/ng-eNB (old gNB/ng-eNB) validates the ResumeMAC-I/shortResumeMAC-I received from the target gNB/ng-eNB (new gNB/ng-eNB) in the RETRIEVE UE CONTEXT REQUEST message, the old gNB/ng-eNB may decide not to relocate the UE context to the new gNB/ng-eNB. In this case, the old gNB/ng-eNB builds the RRCRelease message (MSG4) with a fresh I-RNTI, integrity protect it and encrypt it using the RRC keys that were derived from the new KgNB* similar to RRCResume message (MSG4) protection as specified in clause 6.8.2.1.3. Then, the old gNB/ng-eNB sends the integrity protected and encrypted RRCRelease message to the new gNB/ng-eNB in the RETRIEVE UE CONTEXT FAILURE message.