Section: 4.9.1.2 Xn based inter NG-RAN handover 4.9.1.2.1 General
Content: Clause 4.9.1.2 includes details regarding the Xn based inter NG-RAN handover with and without UPF re-allocation. Xn handovers are only supported for intra-AMF mobility. New AMF can be selected by the target NG-RAN node as specified in clause 5.21.2 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. The handover preparation and execution phases are performed as specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [9], in the case of handover to a shared network, source NG-RAN determines a PLMN or an SNPN to be used in the target network as specified by TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. If the serving PLMN or SNPN changes during Xn-based handover, the source NG-RAN node shall indicate to the target NG-RAN node (in the Mobility Restriction List) the selected PLMN ID or SNPN ID to be used in the target network. During Xn based handover into a shared NG-RAN node the source NG RAN node shall include the serving NID (if available) in the Mobility Restriction List to be used by the target NG-RAN node. If the AMF generates the N2 downlink signalling during the ongoing handover and receives a rejection to a N2 interface procedure (e.g. Location Reporting Control; DL NAS message transfer; etc.) from the NG-RAN with an indication that a Xn based handover procedure is in progress, the AMF may reattempt the same N2 interface procedure either when the handover is complete or the handover is deemed to have failed, when possible. The failure is known by expiry of the timer guarding the N2 interface procedure. Upon reception for an SMF initiated N1 and/or N2 request(s) with an indication that the request has been temporarily rejected due to handover procedure in progress, the SMF starts a locally configured guard timer. Any NF (e.g. the SMF) should hold any signalling messages targeted towards AMF for a given UE during the handover preparation phase unless it detects that the handover execution is completed or handover has failed/cancelled. The NF (e.g. the SMF) may re-attempt, up to a pre-configured number of times, when either it detects that the handover is completed or has failed using message reception or at expiry of the guard timer.