UE#1 sends the SIP INVITE request, containing an initial SDP, to the P-CSCF determined via the CSCF discovery mechanism. The initial SDP may represent one or more media for a multi-media session.
P-CSCF remembers (from the registration procedure) the next hop CSCF for this UE. In this case it forwards the INVITE to the S-CSCF in the home network.
P-CSCF determines whether the INVITE message requires priority handling based on user profile stored during the registration procedure and/or the priority requested by the user and/or MPS code/identifier included in the INVITE message. If the session is determined to require priority handling, then P-CSCF inserts/replaces the priority indication and forwards the INVITE to the S-CSCF.
S-CSCF validates the service profile, if a GRUU is received as the contact, ensures that the Public User Identity of the served user in the request and the Public User Identity associated with the GRUU belong to the same service profile, and invokes any origination service logic required for this user. This includes authorization of the requested SDP based on the user's subscription for multi-media services. If the Request-URI contains the SIP representation of an E.164 address then the procedure specified in clause 220.127.116.11
S-CSCF forwards the request, as specified by the S-S procedures. When the INVITE message includes priority indication, the S-CSCF forwards the INVITE, including the Service User's priority level if available.
The media stream capabilities of the destination are returned along the signalling path, per the S-S procedures.
S-CSCF forwards the Offer Response message to P-CSCF
P-CSCF instructs the PCRF/PCF to authorize the resources necessary for this session.
P-CSCF forwards the Offer Response message to the originating endpoint.
UE decides the offered set of media streams for this session, confirms receipt of the Offer Response and sends the Response Confirmation to P-CSCF. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response received in Step 8 or a subset. If new media are defined by this SDP, a new authorization (as in Step 7) will be done following Step 14. The originating UE is free to continue to offer new media on this operation or on subsequent exchanges using the Update method. Each offer/answer exchange will cause the P-CSCF to repeat the Step 7 again.
Depending on the bearer establishment mode selected for the IP-CAN session, resource reservation shall be initiated either by the UE or by the IP-CAN itself. The UE initiates resource reservation procedures for the offered media as shown in Figure 5.15
. Otherwise, the IP-CAN initiates the reservation of required resources after step 7.
P-CSCF forwards this message to S-CSCF
S-CSCF forwards this message to the terminating endpoint, as per the S-S procedure.
The terminating end point responds to the originating end with an acknowledgement. If Optional SDP is contained in the Response Confirmation, the Confirmation Acknowledge will also contain an SDP response. If the SDP has changed, the PCSCF authorizes the media.
PCSCF forwards the answered media towards the UE.
When the resource reservation is completed, UE sends the successful Resource Reservation message to the terminating endpoint, via the signalling path established by the INVITE message. The message is sent first to P-CSCF.
The terminating end point responds to the originating end when successful resource reservation has occurred. If the SDP has changed, the P-CSCF again authorizes that the resources are allowed to be used.
The destination UE may optionally perform alerting. If so, it signals this to the originating party by a provisional response indicating Ringing. This message is sent to S-CSCF per the S-S procedure. It is sent from there toward the originating end along the signalling path.
UE indicates to the originating user that the destination is ringing.
When the destination party answers, the terminating endpoint sends a SIP 200-OK final response along the signalling path to the originating end, as specified by the termination procedures and the S-S procedures, to S-CSCF.
P-CSCF indicates that the media flows authorized for this session should now be enabled.
P-CSCF passes the 200-OK response back to UE
UE starts the media flow(s) for this session.
UE responds to the 200 OK with an ACK message which is sent to P-CSCF and passed along the signalling path to the terminating end.