Content for  TR 23.700-89  Word version:  18.0.0

For 5GS to EPS mobility procedure without N26 interface, i.e. no UE context on RFSP Index in use could be transferred from the source AMF to the target MME. If the MME immediately select the RFSP Index following the current specifications, it is likely that, it will select a value as "5G prioritized" that conflict with the one selected by PCF in 5GC as "4G prioritized" which drives the UE moving from 5GS to EPS. This may cause the ping-pong issue. So it is propose that, when the MME realizes that the UE is moving from 5GC with the indication provided by the UE, and not receiving UE context containing RFSP Index in use information as no N26 interface applies, after successful attachment, the target MME should set the value of RFSP Index as "4G prioritized" for a pre-configured timer at the MME. When the timer runs out, the MME can re-select the RFSP Index value following the current specifications. This can make sure that the UE won't be sent back to the 5GS immediately to trigger the ping-pong issue because of conflict on RFSP index between 5GC and EPC, and allow the MME to select the RFSP Index considering local configuration and subscribed RFSP Index from HSS.

This solution has no impact on current procedures.

The solution has the following impacts: MME:

• When a MME aware that a UE is moved from 5GS and without UE context of RFSP Index in use as no N26 interface applies, the MME locally sets the RFSP Index in use as "4G prioritized" for a certain duration of time pre-configured in the MME. When the validity condition expires, the MME selects the RFSP Index value following the current specifications.