Content for TR 23.700-89 Word version: 18.0.0

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7.1 Evaluation on Key Issue #1: RFSP Index consistency when UE moves from 5GC to EPC 7.1.1 For deployment without N26 interface interworking 7.1.2 For deployment with N26-based interworking

7 Overall Evaluation p. 30

7.1 Evaluation on Key Issue #1: RFSP Index consistency when UE moves from 5GC to EPC p. 30

7.1.1 For deployment without N26 interface interworking p. 30

According to clause 5.3.4.3.1 of TS 23.501, when receiving the authorized RFSP Index from the PCF, for non-roaming subscribers, the AMF chooses the RFSP Index in use according to one of the following procedures, depending on operator's configuration:

the RFSP Index in use is identical to the authorized RFSP Index; or
the AMF chooses the RFSP Index in use based on the authorized RFSP Index, the locally configured operator's policies, the Allowed NSSAI and the UE related context information available at the AMF, including UE's usage setting, if received during Registration procedures.

The above would mean that the "RFSP Index in use" chosen by AMF may be different from the authorized RFSP Index from PCF, therefore it is the RFSP Index in use (but not the authorized RFSP Index) that should be sent to the EPC. Besides, when the PCF provides modified/authorized RFSP Index (e.g. based on network congestion analytics), PCF has the knowledge how long the congestion may last, therefore it should be the PCF (but not the EPC/MME) that determines the validity time, as the EPC/MME does not have the necessary information.

There are following solutions addressing the deployment scenario without N26 interface:

Sol#2, Sol#4, Sol#5, Sol#7, Sol#8, Sol#9.

Among the above solutions, except Sol#8, the other solutions propose to involve UDM+HSS either by modifying the existing Subscribed RFSP Index in HSS or add new parameter (either "Authorized RFSP Index" or "RFSP Index in use" for MME to use.

Table 7.1.1-1 below list the comparison in the following aspects:

PCF or AMF notifies or provisions UDM of the RFSP Index chosen by the AMF?

Which NF determines the validity time?

Is a new parameter introduced in the subscription data provided by HSS? If yes, what parameter name?

Table 7.1.1-1: Evaluation on principles of KI#1 candidate solutions on without N26 interface scenario

Sol#	PCF or AMF notifies/provisions UDM?	Which NF determines validity time?	New parameter in subscription data in HSS	UDM+HSS maintaining UE's 5G registration as in legacy?	Evaluation
#2	AMF notifies UDM of new parameter "Authorized RFSP Index"	MME (when receiving "Authorized RFSP Index" from HSS)	"Authorized RFSP Index"	Yes	Conceptually incorrect as "Authorized RFSP Index" may be different from "RFSP Index in use". Data entry to be managed by UDM, better to use PP instead of AMF Notify. unclear Service Operation.
#5	AMF notifies UDM, with validity time	PCF	"Authorized RFSP Index" + validity time	Yes
#4	AMF use PP service, new para "RFSP Index in use" with validity time	PCF	"RFSP Index in use" + "validity time"	Yes
#7	Similar as Sol#2	Similar as Sol#2	Similar as Sol#2	No, UDM+HSS maintain UE's 5G state temporarily for some duration.	UDM+HSS behaviour not BW compatible, see NOTE 1.
#8	NA	NA	NA	Yes	Changes also the handling of 5GS to EPS mobility due to other reasons than KI#1, see NOTE 2.
#9	PCF notifies UDM of "Authorized RFSP Index" PCF does not release AM Policy association for the UE	No timer mentioned	Use "subscribed RFSP Index" Prerequisite: MME must use "Subscribed RFSP Index"	Yes	Unclear how PCF not releasing AM policy association. Limitation that MME must use subscribed RFSP Index.
NOTE 1: Sol#7 proposes that at 5GS to EPS mobility without N26, the MME its address in UDM+HSS and then UDM+HSS shall maintain UE's 5G registration state temporarily for some duration. Such proposal is not (backward) compatible with the existing system behaviour that the MME indicates to the UDM+HSS not to cancel the AMF as specified in clause 4.11.2.4.1 of TS 23.502. Note that no addition address of MME needs to be sent to HSS+UDM. NOTE 2: As RFSP Index change is not the only reason for UE mobility from 5GS to EPS, the MME will not be able to tell, therefore Sol#8 (see below) changes also the handling of 5GS to EPS mobility due to other reasons than KI#1 and implication needs further analysis and evaluation. "…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…" If the operator can accept the implication that MME sets a timer for 5GS to EPS mobility regardless of the reason, this can be done without standardization.

7.1.2 For deployment with N26-based interworking p. 32

There are 4 solutions proposed for Key Issue #1, targeting to provide RFSP Index consistency when UE moves from 5GC to EPC for N26-based interworking scenario.

Table 7.1.2-1 compares the principles between the proposed solutions.

Table 7.1.2-1: Evaluation on principles of KI#1 candidate solutions on N26-based interworking scenario

Sol#	Need to keep AM Policy Association after UE Moves from 5GC to EPC	Impacts to Existing Nodes and Proposed Enhancements				Use of Validity Condition(s)
Sol#		AMF	MME	HSS+UDM	PCF for a UE	Use of Validity Condition(s)
1	X	X	X	X (AMF be aware of the change of serving MME from HSS/UDM)	-	New timer for AM association maintenance at AMF and equivalent new timer for MME to use RFSP Index from AMF.
3	-	X	X	-	X (PCF for a UE generates a timer along with the authorized RFSP Index)	MME re-evaluates the RFSP Index when the timer expires.
6	X	X	X	X (maintain the UE's 5G registration state temporarily for some duration and provide the MME address to AMF)	-	HSS+UDM maintain UE's 5G registration state for AM Policy Association maintenance timer and provides it to AMF. During the validity timer, if PCF for a UE updates RFSP index, AMF queries the MME address from HSS+UDM and send the updated RFSP index to MME.
10	-	X (AMF replaces "subscribed RFSP Index" with "authorized RFSP Index" from PCF in UE context and send to MME)	X	-	-	When the configured duration is reached, the MME may remove the "5GC Authorized RFSP Index" from its context for the UE and start using the "subscribed RFSP Index" that was received from the HSS.

All these solutions are trying to answer the following 3 questions for N26-based interworking in KI#1:

Q1:

Whether the current interworking procedure supports MME received the RFSP Index in use from 5GC? If no, what enhancements is needed.

For N26-based interworking: (Solution #1, #3, #6 and #10) the current specification already supports the RFSP Index in use transferred to MME in UE context during handover/TAU.

If there is any enhanced information, it should also be transferred via N26.

Q2:

If the MME get the RFSP Index in used in handover procedure or idle mode mobility procedure, how and when it resumes to the subscription RFSP Index.

(Solution #1 and #6) The PCF for a UE maintains the AM Policy Association when UE move from 5G to 4G and updates the authorized RFSP Index according to triggers and conditions defined in current specification. The MME re-selects the RFSP Index in use considering the dynamic input from PCF at the cost of network overhead and extra procedures.
When MME realizes a UE is moving from 5GC, it starts a timer within which value of the RFSP Index in use will not be changed. When the timer expires, the MME will be free to re-select the RFSP Index in use according to current method. The value of this timer may be set to:
(Solution #10) A local value preconfigured in MME, which is same to all UE and not adjustable.
(Solution #3) A value selected by PCF when it generates an authorized RFSP Index value indicating the UE to move from 5G to 4G. So the timer is per UE from 5G and not adjustable after the UE in 4G. PCF can decide that timer based on information, e.g. from UDR or NWDAF. If the AMF modifies the authorized RFSP Index from PCF, i.e. the AMF selects a different value of RFSP Index in use, whether the timer value from PCF should still be sent to MME is clarified in S2-2205525 (ongoing discussion).

Considering the trade-off of system impact and flexibility, the PCF-selected timer seems to solve the problem in a more agreeable way. Preconfigured timer in MME can implementation specific without standardization as such timer will apply to EPS to 5GS mobility scenario not targeted in the KI.

Q3:

When UE is under EPC, should MME receive any update of RFSP Index from 5GC and how.

(Solution #1 and #6) Yes. The PCF is able to update the RFSP Index In Use or the timer according to, e.g. AF's influence request. And MME is able to change the RFSP Index in use in a timely manner.
(Solution #3 and #10) No. The MME is able to re-select the RFSP Index only after the validity time provided by the AMF (decided by PCF) expires.

The evaluation of Q2 also applies to Q3.