To allow the 5G system to interwork with AFs related to existing services, e.g. IMS based services as described in TS 23.228, Mission Critical Push To Talk services as described in TS 23.179, the PCF shall support the corresponding IMS procedures defined in the main body of this TS via Rx interface. This facilitates the migration from EPC to 5GC without requiring these AFs to upgrade to support the Npcf_PolicyAuthorization services in Rel-16.

Session Binding applies for PDU Sessions of IP type only. The functionality described for Multimedia Priority Services (clause 6.1.3.11) and Mission Critical service (clause 6.1.3.19) applies via Rx interface. In order to support IMS Emergency services over Rx interface, in addition to the functional description in clause 6.1.3.10, the following applies: The PCF shall provide the IMEI and the subscriber identifiers (IMSI, MSISDN) (if available), received from the SMF at PDU Session establishment, if so requested by the P-CSCF. The PCF derives the IMEI from the PEI, the IMSI from the SUPI and the MSISDN from the GPSI. Any AF using Rx, such as P-CSCF, the BSF determines the selected PCF address according to the information included in the incoming Rx requests and the information stored at the BSF. The BSF is able to proxy or redirect Rx requests targeting an IP address of a UE to the selected PCF. The following event reporting is supported over Rx interface:

Event	Description	Availability for Rx Session
PLMN Identifier Notification	The PLMN identifier where the UE is currently located.	Yes
Change of Access Type	The Access Type and, if applicable, the RAT Type of the PDU Session has changed.	Yes
EPS fallback	EPS fallback is initiated	Yes
Signalling path status	The status of the resources related to the signalling traffic of the AF session.	Yes
Access Network Charging Correlation Information	The Access Network Charging Correlation Information of the resources allocated for the AF session.	Yes
Access Network Information Notification	The user location and/or timezone when the PDU Session has changed in relation to the AF session.	Yes
Reporting Usage for Sponsored Data Connectivity	The usage threshold provided by the AF has been reached; or the AF session is terminated.	Yes
Resource allocation status	The status of the resources related to the AF session (established/released).	Yes
QoS targets can no longer (or can again) be fulfilled	The QoS targets can no longer (or can again) be fulfilled by the network for (a part of) the AF session.	No
Out of credit	Credit is no longer available.	Yes