Content for  TS 23.228  Word version:  18.1.0

At IP-CAN bearer activation the user shall have access to either IP-CAN services without Policy and Charging Control, or IP-CAN services with Policy and Charging Control. It is operator choice whether to offer both or only one of these alternatives for accessing the IM Subsystem. When using IP-CAN without Policy and Charging Control, IP-CAN bearers are established according to the user's subscription, local operator's IP bearer resource based policy, local operator's admission control function and roaming agreements. When using IP-CAN with Policy and Charging Control, PCC decisions (e.g. authorization and control) are also applied to the IP-CAN bearer. The description in this clause and the following clauses (clauses 5.4.7.1 - 5.4.7.7) is applicable for the case when Policy and Charging Control is employed. The IP-Connectivity Access Network contains a Policy and Charging Enforcement Function (PCEF, in 5GS corresponding to the combination of SMF and UPF) that has the capability of policing packet flow into the IP network, and restricting the set of IP destinations that may be reached from/through an IP-CAN bearer according to a packet classifier. This policy 'gate' function has an external control interface that allows it to be selectively 'opened' or 'closed' on the basis of IP destination address and port. When open, the gate allows packets to pass through (to the destination specified in the classifier) and when closed, no packets are allowed to pass through. The control is performed by a PCRF/PCF (the interface between the PCRF/PCF and the P-CSCF is the Rx interface standardised in TS 23.203 or the N5 interface (using the Npcf_PolicyAutorization service), standardised in TS 23.503 and TS 29.514). There are eight interactions defined for Policy and Charging Control:

1. Authorize QoS Resources.
2. Resource Reservation.
3. Enabling of media flows authorized in (1), e.g. 'open' the 'gate'.
4. Disabling of media flows authorized in (1), e.g. 'close' the 'gate'.
5. Revoke Authorization for IP-CAN and IP resources.
6. Indication of IP-CAN bearer release from the PCEF in the IP-Connectivity Access Network to the PCRF/PCF.
7. Authorization of IP-CAN bearer modification.
8. Indication of IP-CAN bearer modification from the PCEF in the IP-Connectivity Access Network to the PCRF/PCF.

These requirements and functional description of these interactions are explained further in the following clauses. The complete specification of the interface between the PCRF/PCF and the PCEF is contained in TS 23.203 and TS 23.503. The Policy and Charging Control can also be used to enable the P-CSCF to retrieve the user location and/or UE Time Zone information from the access network as specified in TS 23.203 and TS 23.503.

The Authorize QoS Resources procedure is used during an establishment and a modification of a SIP session. The P-CSCF shall use the SDP contained in the SIP signalling to derive the session information that is relevant for Policy and Charging Control and forwards it to the PCRF/PCF. The PCRF/PCF shall use the received information to calculate the proper authorization. This enables the PCRF/PCF to authorize the required QoS resources. The authorization shall be expressed in terms of the IP resources to be authorized and shall include limits on media flows, and may include restrictions on IP destination address and port. The PCC shall authorize each SIP session independently (including additional parallel sessions, e.g. Call Waiting) and shall take into consideration the amount of IP resources the user's subscription allows.

The IP-CAN provides the Policy and Charging Enforcement Point that implements the policy decisions for performing admission control and authorising the IP-CAN and IP BS QoS Resource request, and policing media flows entering the external IP network. Authorization of IP-CAN and IP QoS Resources shall be required for access to the IP Multimedia Subsystem. The IP-CAN shall determine the need for authorization, possibly based on provisioning and/or based on requested parameters, which may be IP-CAN specific. Resource Reservation is initiated either by the UE or the IP-CAN depending on the bearer establishment mode selected for the IP-CAN session, see TS 23.203 and TS 23.503:

• Resource reservation requests initiated from the UE shall (if possible for the used IP-CAN) contain the traffic mapping information which enables the IP-CAN to correctly match the reservation request to the corresponding authorization. The authorization is normally 'Pulled' from the PCRF/PCF by the PCEF within the IP-CAN when the reservation request is received from the UE.

With a request for IP-CAN QoS resources, the PCEF within the IP-CAN shall verify the request is less than the sum of the authorized IP resources (within the error tolerance of the conversion mechanism) for all of the combined media flows.

• Resource reservation requests initiated by the IP-CAN take place after successful authorization of QoS resources. The PCRF/PCF "Pushes" the authorization for IP-CAN bearer resources to the PCEF within the IP-CAN, which then enforces the authorization by either modifying the characteristics of one existing IP-CAN bearer or requesting the establishment of a new one.
• Resource reservation requests initiated by the IP-CAN shall (if possible for the used IP-CAN) contain the traffic mapping information which enables the UE to correctly match the reservation request to the corresponding media of the SIP session.

The PCRF/PCF makes policy decisions and provides an indication to the PCEF within the IP-CAN that the user is now allowed to use the allocated QoS resources for per-session authorizations unless this was done based on Policy and Charging Control at the time of the Resource Reservation procedure. If there is more than one response for the same session, indicating that the session has been forked in the network, the PCRF/PCF may authorize the "logical OR" of the resources requested in the responses. When the session established indication has been received, if the PCRF/PCF earlier have authorized the "logical OR" of the resources then the PCRF/PCF will modify the authorization and enable the corresponding media flows according to the session established indication. The PCEF within the IP-CAN enforces the policy decisions. The IP-CAN shall restrict any use of the IP resources prior to this indication from the PCRF/PCF, e.g. by keeping the gate closed and disabling the use of resources for the media flow. Based on local policy, IP-CAN and/or IP resources may be allowed to be used by the user at the time they are authorized by the PCRF/PCF.

The PCRF/PCF makes policy decisions and provides an indication to the PCEF within the IP-CAN about revoking the user's capacity to use the allocated QoS resources for per-session authorizations. The indication for disabling media flows shall be sent as a separate decision to the PCEF within the IP-CAN corresponding to the previous request to enable media flows. The PCEF within the IP-CAN enforces the policy decisions. The IP-CAN shall restrict any use of the IP resources after this indication from the PCRF/PCF, e.g. by closing the gate and blocking the media flow.

At IP multimedia session release, the UE should deactivate the IP-CAN bearer(s) used for the IP multimedia session. In various cases the UE will be unable to perform this release itself. The PCRF/PCF provides indication to the PCEF within the IP-CAN when the resources previous authorized, and possibly allocated by the UE, are to be released. The IP-CAN shall deactivate the IP-CAN bearer used for the IP multimedia session.

Any release of IP-CAN bearer(s) that were established based on authorization from the PCRF/PCF shall be reported to the PCRF/PCF by the PCEF within the IP-CAN. This indication is forwarded to the P-CSCF and may be used by the P-CSCF to initiate a session release towards the remote endpoint e.g. if all IP-CAN bearer(s) associated with the session were released, the procedures in clause 5.10.3.1 can be executed.

When an IP-CAN bearer is modified by the UE, such that the requested QoS falls outside of the limits that were authorized at IP-CAN bearer activation (or last modification) or such that new binding information is received, then the PCEF within the IP-CAN shall verify the authorization of this IP-CAN bearer modification. If the PCEF within the IP-CAN does not have sufficient information to authorize the IP-CAN bearer modification request, the PCEF within the IP-CAN shall send an authorization request to the PCRF. The PCRF authorizes the modified IP-CAN bearer based on the current session information. Note that the P-CSCF sends an update of the session information in the case of a modification of a SIP session which results in an update of the authorization as described in clause 5.4.7.1. When the P-CSCF sends an update of the session information and the bearer establishment is controlled by the IP-CAN, the PCRF/PCF shall send an updated authorization to the PCEF. The PCEF within the IP-CAN enforces the policy decision accordingly (e.g. by requesting the reservation of new IP-CAN bearer resources in the case of the addition of a new media component to the session or release of previously reserved resources if a media component has been removed from the IP Multimedia session).

When an IP-CAN bearer is modified such that the maximum bit rate (downlink and uplink) is downgraded to 0 kbit/s or changed from 0 kbit/s to a value that falls within the limits that were authorized at IP-CAN bearer activation (or last modification) then the PCEF within the IP-CAN shall report this to the PCRF/PCF. This indication is forwarded to the P-CSCF and may be used by the P-CSCF to initiate a session release towards the remote endpoint.

The P-CSCF may indicate to the PCRF/PCF that the resource allocation for a media flow is allowed to use the same priority as other media flows of the same media type for the UE engaged in multiple sessions by providing a priority sharing indicator and an optional pre-emption control information in addition to the application identifier and the service priority. For MCPTT, the service priority and the priority sharing indicator are defined in TS 23.179. The pre-emption control information is used to indicate to the PCRF/PCF how to perform pre-emption in accordance to TS 23.203 and TS 23.503. The following scenario is subject to priority sharing:

• The P-CSCF receives a priority sharing indicator associated with the media flow from the Application Server.

Upon detection by the P-CSCF that a session includes a priority sharing indicator for a media flow and may optionally include pre-emption control information, the P-CSCF shall convey to the PCRF/PCF the priority sharing indicator and, when available, the pre-emption control information as described in TS 23.203 and TS 23.503.