Content for  TS 23.228  Word version:  18.3.0

The IMS Multimedia Priority Service provides Service Users access to IMS services in a prioritised manner. The P-CSCF shall control the priority of IMS based MPS sessions, using PCC procedures. The P-CSCF shall permit any authorised Service User to originate an IMS based MPS session. The detection of MPS sessions is handled by the P-CSCF at the originating network. PCC shall always be enabled in a network supporting IMS Multimedia Priority Services. HSS shall store IMS Priority Indication and Priority Level as part of the subscription information. The P-CSCF at the originating end shall determine whether the INVITE message requires priority handling based on user profile stored during the registration procedure and/or MPS code/identifier provided by the INVITE message. If the session is determined to require priority handling, then the P-CSCF inserts/replaces the MPS priority indication in the INVITE and, if the Service User's priority level is known, may include it and forwards the INVITE to the S-CSCF. If the Service User's priority level is not known, the P-CSCF includes the priority indication without the Service User's priority level. The S-CSCF routes (using initial Filter Criteria set for the MPS code/identifier) the INVITE to the AS for authentication/authorization for MPS (if needed), and the AS adds the Service User's priority level if it is not in the INVITE already. The AS or the S-CSCF may assert the authorization for priority in the INVITE. The AS then forwards the INVITE (with MPS priority indication and the Service User's priority level) to the next entity in the network via the S-CSCF as part of the normal IMS routing. All subsequent SIP messages carry both MPS priority indication and the Service User's priority level. When the P-CSCF at the originating end determines that priority handling is required, the P-CSCF shall derive session information and interact with the PCRF/PCF providing the session information. The derived session information shall indicate the priority of the MPS session which depends if the Service User's priority level is known at this stage. The PCC interaction between the P-CSCF and the PCRF/PCF is described in TS 23.203 and TS 23.503. The P-CSCF at the terminating end shall determine whether the INVITE message requires priority handling based on MPS priority indication and the originating Service User's priority level received from the originating network. If priority handling is required, P-CSCF shall derive the session information based on the Service User's priority level to indicate the priority of the MPS session and interact with the PCRF/PCF providing the session information. The P-CSCF shall adjust the priority treatment of transactions or dialogs according to the most recently received authorized MPS priority indication. When the terminating user is a Service User, while the session request is from a normal user, the IMS signalling bearer may be given priority treatment when operator policy and MPS (IMS) priority subscription indicates so. For a Service User originating a non-priority session, the IMS signalling bearer may be given priority treatment when operator policy and MPS (IMS) priority subscription indicates so. For IMS media, priority treatment is not required in these cases. If so configured by the operator, a P-CSCF or an IBCF shall prohibit the negotiation of ECN during SDP offer/answer exchanges and shall not invoke ECN (as described in clause 4.22) for IMS based MPS sessions. A conferencing AS, if enabled by local policy, shall permit an authorized host with an MPS (IMS) priority subscription (i.e. the Service User that established the conference) to request an upgrade of the host itself, specific participants, or all participants including the host in the conference, whether participants have an MPS subscription or not. Once the conference has been upgraded, the AS will upgrade new participants to MPS without explicit host invocation. For MPS conferencing sessions, upon request from a host to upgrade an existing conference, the AS shall first authenticate/authorize the host for MPS. The session upgrade of UE conference participants is based on existing IMS routing procedures, including interaction between the P-CSCF and PCRF/PCF for that purpose. For E-UTRAN access, priority support for EPS bearer is described in TS 23.401. For 5GS, support Multimedia Priority Service is described in TS 23.501.