Section: 4.11.1.3.4 EPS to 5GS Mobility Registration Procedure (Idle) using N26 interface with AMF reallocation
Content: During Idle state mobility registration procedure from EPS to 5GS, the initial AMF may select a new AMF based on S-NSSAIs associated with the established PDU Session, received in NAS Registration Request. Figure 4.11.1.3.4-1: EPS to 5GS mobility with AMF re-allocation for single-registration mode and N26 interface configuration 1. Step 1 to 7 of clause 4.11.1.3.3 in EPS to 5GS mobility using N26 interface is performed. 2. Step 3 to 4 of clause 4.2.2.2.3 in Registration with AMF reallocation procedure is performed with the difference that before the Network Slice selection the AMF needs to derive the S-NSSAI for the serving PLMN as described in clause 5.15.5.2.1 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. 3. [Conditional] Initial AMF to MME: Context Acknowledge (failure cause) to MME according to TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [13]. The initial AMF decides a new AMF needs to be reselected. The initial AMF sends a Context Acknowledge message with cause code indicating that the procedure is not successful. The MME shall continue as if Context Request was never received. 4. Step 6 to 7 of clause 4.2.2.2.3 in Registration with AMF reallocation procedure are performed. 5. After receiving the Registration Request message, the new AMF continues the registration from step 5 until step 18 of figure 4.11.1.3.3 (EPS to 5GS mobility using N26 procedure), which includes the UE context retrieved from old AMF. If the 5G security context is received from the initial AMF, the new AMF continues using that one instead of the mapped 5G security context retrieved from MME.