Section: 6.9.2.3.1 Intra-gNB-CU handover and intra-ng-eNB handover
Content: The gNB shall have a policy deciding at which intra-gNB -CU handovers the KgNB can be retained and at which a new KgNB needs to be derived. At an intra-gNB-CU handover, the gNB shall indicate to the UE whether to change or retain the current KgNB in the HO Command message. Retaining the current KgNB shall only be done during intra-gNB-CU handover. NOTE: The option of retaining the KeNB at intra-ng-eNB handover is not supported in ng-eNB. If the current KgNB is to be changed, the gNB/ng-eNB and the UE shall derive a KNG-RAN* as in Annex A.11/A.12 using target PCI, its frequency ARFCN-DL/EARFCN-DL, and either NH or the current KgNB depending on the following criteria: the gNB shall use the NH for deriving KNG-RAN* if an unused {NH, NCC} pair is available in the gNB (this is referred to as a vertical key derivation), otherwise if no unused {NH, NCC} pair is available in the gNB, the gNB shall derive KNG-RAN* from the current KgNB (this is referred to as a horizontal key derivation). The gNB shall send the NCC used for the KNG-RAN*derivation to UE in HO Command message. The gNB/ng-eNB and the UE shall use the KNG-RAN* as the KgNB, after handover. If the current KgNB is to be retained, the gNB and the UE shall continue using the current KgNB, after handover. NOTE 1: This clause is also applicable when gNB is implemented as a single unit, i.e., when the gNB is not split into CU and DU. NOTE 2: The key derivation mechanism described in this clause is also applicable to CHO defined in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ] [52].