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5.5.2 (S-S#2) Single network operator performing origination and termination
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The Serving‑CSCF handling session origination performs an analysis of the destination address and determines that it belongs to a subscriber of the same operator. The request is therefore forwarded to a local I‑CSCF. The I‑CSCF queries the HSS for current location information and finds the user either located in the home service area, or roaming. The I‑CSCF therefore forwards the request to the S‑CSCF serving the destination user.
Refer to table 5.2 in clause 5.4a to see which origination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Originating Network".
MO#1 Mobile origination, roaming. The "Originating Network" of S-S#2 is therefore a visited network.
MO#2 Mobile origination, home. The "Originating Network" of S-S#2 is therefore the home network.
AS-O Application Server origination. The" Originating Network" of S-S#1 is the home network. The element labelled S‑CSCF#1 corresponds to the S‑CSCF in figure 5.16b.
Refer to table 5.2 in clause 5.4a to see which termination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Terminating Network".
MT#1 Mobile termination, roaming,. The "Terminating Network" of S-S#2 is a visited network.
MT#2 Mobile termination, home. The "Terminating Network" of S-S#2 is the home network.
MT#3 Mobile termination, CS Domain roaming. The "Terminating Network" of S-S#2 is a CS domain network.
AS-T#1,2,3,4 Application Server termination. The elements of the corresponding AS-T termination figure (5.7.5, 5.7.6, 5.7.7 and 5.7.8) replace all elements of the Terminating Home Network and Terminating Network off figure 5.11.
Figure 5.11: Serving to serving procedure - same operator
Procedure S-S#2 is as follows:
1. The SIP INVITE request is sent from the UE to S‑CSCF#1 by the procedures of the originating flow. This message should contain the initial media description offer in the SDP.
2. S‑CSCF#1 invokes whatever service logic is appropriate for this session setup attempt
3. S‑CSCF#1 performs an analysis of the destination address and determines the network operator to whom the subscriber belongs. Since it is local, the request is passed to a local I‑CSCF.
4. I‑CSCF shall query the HSS for current location information.
5. HSS responds with the address of the current Serving‑CSCF for the terminating user.
6. I‑CSCF forwards the INVITE request to the S‑CSCF (S‑CSCF#2) that will handle the session termination.
7. S‑CSCF#2 invokes whatever service logic is appropriate for this session setup attempt
8. The sequence continues with the message flows determined by the termination procedure.
9-12. The terminating end point responds with an answer to the offered SDP and this message is passed along the established session path.
13-16. The originator decides on the offered set of media streams, confirms receipt of the Offer Response with a Response Confirmation and forwards this information to S‑CSCF#1 by the origination procedures. This message is forwarded via the established session path to the terminating end point. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response received in Step 12 or a subset.
17-20. Terminating end point responds to the offered SDP and the response if forwarded to the originating end point via the established session path.
21-24. Originating end point sends successful resource reservation information towards the terminating end point via the established session path.
25-28. Terminating end point sends successful resource reservation acknowledgement towards the originating end point via the established session path
29-32. Terminating end point sends ringing message toward the originating end point via the established session path.
33-36. The SIP final response, 200-OK, is sent by the terminating endpoint over the signalling path. This is typically generated when the user has accepted the incoming session setup attempt. The message is sent to S‑CSCF#2 per the termination procedure.
37-40. The originating endpoint sends the final acknowledgement to S‑CSCF#1 by the origination procedures and it is then sent over the signalling path to the terminating end point.
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5.5.3 (S-S#3) Session origination with PSTN termination in the same network as the S‑CSCF.
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The Serving‑CSCF handling session origination performs an analysis of the destination address and determines, with support of applications or other databases, that the session is destined to the PSTN. The request is therefore forwarded to a local BGCF. The BGCF determines that the MGCF should be in the same network and selects a MGCF in that network. The request is then forwarded to the MGCF.
Refer to table 5.2 in clause 5.4a to see which origination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Originating Network".
MO#1 Mobile origination, roaming. The "Originating Network" of S-S#3 is therefore a visited network.
MO#2 Mobile origination, located in home service area. The "Originating Network" of S-S#3 is therefore the home network.
AS- O Application Server origination. The" Originating Network" of S-S#1 is the home network. The element labelled S‑CSCF corresponds to the S‑CSCF in figure 5.16b.
Refer to table 5.2 in clause 5.4a to see which termination sequences share this common S-S procedure. In addition the text below clarifies the role of the "Terminating Network".
PSTN-T PSTN termination. This occurs when the MGCF is selected to be in the same network as the S‑CSCF.
Figure 5.12: Serving to PSTN procedure - same operator
Procedure S-S#3 is as follows:
1. The SIP INVITE request is sent from the UE to S‑CSCF#1 by the procedures of the originating flow. This message should contain the initial media description offer in the SDP.
2. S‑CSCF#1 invokes whatever service logic is appropriate for this session setup attempt
3. S‑CSCF#1 performs an analysis of the destination address. From the analysis of the destination address, S‑CSCF#1 determines that this is for the PSTN and passes the request to the BGCF.
4. The BGCF determines that the MGCF shall be in the same network and hence proceeds to select an appropriate MGCF. The SIP INVITE request is forwarded to the MGCF. The PSTN terminating information flows are then followed.
5-7. The media stream capabilities of the destination are returned along the signalling path, as per the PSTN termination procedure.
8. The originator decides the offered set of media streams, confirms receipt of the Offer Response with a Response Confirmation and forwards this information to S‑CSCF#1 by the origination procedures. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response received in Step 7 or a subset.
9-10. S‑CSCF#1 forwards the offered SDP to the terminating endpoint as per the PSTN terminating procedures via the established session path.
11-13. The terminating end point answers to the offered SDP and the message is passed through the established session path to the originating end point.
14-16. When the originating endpoint has completed the resource reservation procedures, it sends the successful resource reservation message to S‑CSCF#1 by the origination procedures and it is passed to the terminating end point through the session path.
17-19. The terminating endpoint acknowledges the result and the message is passed onto the originating end point via the session path.
20-22. Terminating end point generates ringing message and forwards it to BGCF which in turn forwards the message to SCSCF#1. S‑CSCF#1 forwards the ringing message to the originator, per the origination procedure.
23. When the destination party answers, the termination procedure results in a SIP 200-OK final response to the BGCF.
24-25. The BGCF forwards this information to the S‑CSCF#1 and then it is forwarded to the originating end point.
26. The 200-OK is returned to the originating endpoint, by the origination procedure from terminating end point.
27. The originating endpoint sends the final acknowledgement to S‑CSCF#1 by the origination procedures.
28. S‑CSCF#1 forwards this message to the terminating endpoint as per the PSTN terminating procedures.
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5.5.4 (S-S#4) Session origination with PSTN termination in a different network from the S‑CSCF.
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The Serving‑CSCF handling session origination performs an analysis of the destination address and determines, with support of applications or other databases, that the session is destined to the PSTN. The request is therefore forwarded to a local BGCF. The BGCF determines that the PSTN interworking should occur in another network and forwards this to a BGCF in the interworking network. The BGCF then selects a MGCF in that network. The request is then forwarded to the MGCF.
Refer to table 5.2 in clause 5.4a to see which origination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Terminating Network".
MO#1 Mobile origination, roaming. The "Originating Network" of S-S#4 is therefore a visited network.
MO#2 Mobile origination, located in home service area. The "Originating Network" of S-S#4 is therefore the home network.
AS- O Application Server origination. The" Originating Network" of S-S#1 is the home network. The element labelled S‑CSCF#1 corresponds to the S‑CSCF in figure 5.16b.
Refer to table 5.2 in clause 5.4a to see which termination sequences share this common S-S procedure. In addition the text below clarifies the role of the" Terminating Network".
PSTN-T PSTN termination. This occurs when the MGCF is selected to be in a different network than the S‑CSCF.
Figure 5.13: Serving to PSTN procedure - different operator
Procedure S-S#4 is as follows:
1. The SIP INVITE request is sent from the UE to S‑CSCF#1 by the procedures of the originating flow. This message should contain the initial media description offer in the SDP.
2. S‑CSCF#1 invokes whatever service logic is appropriate for this session setup attempt
3. S‑CSCF#1 performs an analysis of the destination address. From the analysis of the destination address, S‑CSCF#1 determines that this is for the PSTN and passes the request to the BGCF#1.
4. The BGCF#1 determines that the PSTN interworking should occur in interworking network and forwards the request on to BGCF#2.
5. BGCF#2 determines that the MGCF shall be in the same network and hence proceeds to select an appropriate MGCF. The SIP INVITE request is forwarded to the MGCF. The PSTN terminating information flows are then followed.
6-8. The media stream capabilities of the destination are returned along the signalling path, as per the PSTN termination procedure.
9. S‑CSCF#1 forwards the SDP to the originator, as per the originating procedure.
10. The originator decides the offered set of media streams, confirms receipt of the Offer Response with a Response Confirmation and forwards this information to S‑CSCF#1 by the origination procedures. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response received in Step 12 or a subset.
11-13. S‑CSCF#1 forwards the offered SDP to the terminating endpoint, as per the PSTN terminating procedure.
14-17. Terminating end point responds to the offer via the established session path towards the originating end point.
18-21. When the originating endpoint has completed the resource reservation procedures, it sends the successful resource reservation message to S‑CSCF#1 by the origination procedures and it is forwarded to the terminating end point via established session path.
22-25. The terminating end point responds to the message towards the originating end point.
26-29. Terminating end point generates ringing message towards the originating end point.
30-33. Terminating end point sends 200 OK when the destination party answers the session.
34-37. Originating end point acknowledges the establishment of the session.
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5.6 Origination procedures
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5.6.0 General
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This clause presents the detailed application level flows to define the Procedures for session originations.
The flows presented in the clause assume the use of Policy and Charging Control for the establishment of QoS-Assured Sessions.
The session origination procedures specify the signalling path between the UE initiating a session setup attempt and the Serving‑CSCF that is assigned to perform the session origination service. This signalling path is determined at the time of UE registration and remains fixed for the life of the registration.
A UE always has a proxy (P‑CSCF) associated with it. This P‑CSCF performs resource authorization and may have additional functions in handling of emergency and priority sessions. The P‑CSCF is determined by the CSCF discovery process, described in clause 5.1.1 (Local CSCF Discovery).
As a result of the registration procedure, the P‑CSCF determines the next hop toward the Serving‑CSCF. This next hop is to the S‑CSCF in the home network (MO#1). These next-hop addresses could be IP addresses, or could be names that are translated via DNS to an IP address.
Sessions originated in the PSTN to a destination in an IMS network are a special case of the Origination procedures. The MGCF uses H.248 [18] to control a Media Gateway and communicates with the SS7 network. The MGCF initiates the SIP request and subsequent nodes consider the signalling as if it came from a S‑CSCF.
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5.6.1 (MO#1) Mobile origination, roaming
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This origination procedure applies to roaming users.
The UE is located in a visited network and determines the P‑CSCF via the CSCF discovery procedure described in clause 5.1.1. The home network advertises the S‑CSCF as the entry point from the visited network.
When registration is complete, P‑CSCF knows the name/address of the next hop in the signalling path toward the serving‑CSCF.
Figure 5.14: Mobile origination procedure - roaming
Procedure MO#1 is as follows:
1. UE 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.
2. P‑CSCF remembers (from the registration procedure) the next hop CSCF for this UE.
This next hop is either the S‑CSCF that is serving the visiting UE.
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.
3. 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 belongs 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 URI representation of an E.164 address then the procedure specified in clause 4.3.5.3 applies.
4. 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.
5. The media stream capabilities of the destination are returned along the signalling path, per the S-S procedures.
6. S‑CSCF forwards the Offer Response message to P‑CSCF.
7. P‑CSCF instructs the PCRF/PCF to authorize the resources necessary for this session.
8. P‑CSCF forwards the Offer Response message to the originating endpoint.
9. UE decides the offered set of media streams for this session, confirms receipt of the Offer Response and sends the Response Confirmation to the 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 instruct the PCRF/PCF to repeat the Authorization step (Step 7) again.
10. 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 the reservation procedures for the resources needed for this session after determining the needed resources in step 8 as shown in Figure 5.14. Otherwise, the IP‑CAN initiates the reservation of required resources after step 7.
11. P‑CSCF forwards the Response Confirmation to S‑CSCF.
12. S‑CSCF forwards this message to the terminating endpoint, as per the S-S procedure.
13-15. 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 P‑CSCF validates that the resources are allowed to be used.
16-18. 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.
19-21. The terminating end point responds to the originating end when successful resource reservation has occurred. If the SDP has changed, the P‑CSCF authorizes that the resources are allowed to be used.
22-24. Terminating end point may generate ringing and it is then forwarded via the session path to the UE.
25. UE indicates to the originating user that the destination is ringing.
26. When the destination party answers, the terminating endpoint sends a SIP 200-OK final response, as specified by the termination procedures and the S-S procedures, to S‑CSCF.
27. S‑CSCF sends a SIP 200-OK final response along the signalling path back to P‑CSCF.
28. P‑CSCF indicates that the media flows authorized for this session should now be enabled.
29. P‑CSCF sends a SIP 200-OK final response to the session originator.
30. UE starts the media flow(s) for this session.
31-33. UE responds to the 200 OK with a SIP ACK message sent along the signalling path.
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5.6.2 (MO#2) Mobile origination, home
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This origination procedure applies to users located in their home service area.
The UE is located in the home network and determines the P‑CSCF via the CSCF discovery procedure described in clause 5.1.1. During registration, the home network allocates an S‑CSCF in the home network.
When registration is complete, P‑CSCF knows the name/address of S‑CSCF.
Figure 5.15: Mobile origination procedure - home
Procedure MO#2 is as follows:
1. 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.
2. 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.
3. 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 4.3.5.3 applies.
4. 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.
5. The media stream capabilities of the destination are returned along the signalling path, per the S-S procedures.
6. S‑CSCF forwards the Offer Response message to P‑CSCF.
7. P‑CSCF instructs the PCRF/PCF to authorize the resources necessary for this session.
8. P‑CSCF forwards the Offer Response message to the originating endpoint.
9. 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.
10. 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.
11. P‑CSCF forwards this message to S‑CSCF.
12. S‑CSCF forwards this message to the terminating endpoint, as per the S-S procedure.
13-14. 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.
15. PCSCF forwards the answered media towards the UE.
16-18. 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.
19-21. 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.
22-24. 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.
25. UE indicates to the originating user that the destination is ringing.
26-27. 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.
28. P‑CSCF indicates that the media flows authorized for this session should now be enabled.
29. P‑CSCF passes the 200-OK response back to UE.
30. UE starts the media flow(s) for this session.
31-33. 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.
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5.6.3 (PSTN-O) PSTN origination
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The MGCF in the IM CN subsystem is a SIP endpoint that initiates requests on behalf of the PSTN and Media Gateway. The subsequent nodes consider the signalling as if it came from a S‑CSCF. The MGCF incorporates the network security functionality of the S‑CSCF. This MGCF does not invoke Service Control, as this may be carried out in the GSTN or at the terminating S‑CSCF.
Due to routing of sessions within the PSTN, this origination procedure will only occur in the home network of the destination subscriber. However, due to cases of session forwarding and electronic surveillance, the destination of the session through the IM CN subsystem may actually be another PSTN termination.
Figure 5.16: PSTN origination procedure
The PSTN Origination procedure is as follows:
1. The PSTN establishes a bearer path to the MGW and signals to the MGCF with a IAM message, giving the trunk identity and destination information.
2. The MGCF initiates a H.248 command, to seize the trunk and an IP port.
3. The MGCF initiates a SIP INVITE request addressed to a tel URI or, if directed by operator's local policy, to a SIP URI (using an E.164 address in the user portion and the setting user=phone), includes an initial SDP in the INVITE request and forwards the request to a configured I‑CSCF, as per the proper S-S procedure. If configured through policies, the MGCF adds to the SIP INVITE attestation information based on the trunk identity or other sources of the request.
4. The media stream capabilities of the destination are returned along the signalling path, per the S-S procedures.
5. MGCF initiates a H.248 command to modify the connection parameters and instruct the MGW to reserve the resources needed for the session.
6. MGCF decides the offered set of media streams for this session, confirms receipt of the Offer Response and sends the Response Confirmation per the S-S procedures.
7. Terminating end point responds to the Response Confirmation. If Optional SDP is contained in the Response Confirmation, the Confirmation Acknowledge will also contain an SDP response.
8. MGW reserves the resources needed for the session.
9. When the resource reservation is completed, MGCF sends the successful Resource Reservation message to the terminating endpoint, per the S-S procedures.
10. Terminating end point responds to the successful media resource reservation.
11. The destination endpoint may optionally perform alerting. If so, it signals this to the originating party by a provisional response indicating Ringing. This message is sent to MGCF per the S-S procedure.
12. If alerting is being performed, the MGCF forwards an ACM message to PSTN.
13. When the destination party answers, the terminating and S-S procedures result in a SIP 200-OK final response being sent to MGCF.
14. MGCF forwards an ANM message to the PSTN.
15. MGCF initiates a H.248 command to alter the connection at MGW to make it bi-directional.
16. MGCF acknowledges the SIP final response with a SIP ACK message.
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5.6.4 (NI-O) Non-IMS Origination procedure from an external SIP client
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This clause describes the session setup procedures when originating from an external SIP client that doesn't support the required IMS SIP extensions, towards an IMS UE.
An incoming SIP request may arrive, where the UE detects that the originating party does not support the IMS SIP extensions described in TS 24.229 [10a]. If the external SIP client does not support the Precondition extension of SIP, the UE continues to setup the session without activating media transfer until the session has been accepted. Figure 5.16a shows an example of an end-to-end session setup in such a case.
For illustration purposes these session flows show the case of a non-roaming termination. This flow is a variant of MT#2 defined in clause 5.7.2. The same principles apply in roaming cases, i.e. analogous variants of MT#1 defined in clause 5.7.1 are also supported for interworking with SIP clients that do not support the required IMS procedures.
Figure 5.16a: Originating session from external SIP client
1-2. A session request arrives at the UE in the IMS network with media information but without requiring precondition capability.
3. P‑CSCF examines the media parameters. If P‑CSCF finds media parameters not allowed to be used within an IMS session (based on P‑CSCF local policies, or if available bandwidth authorization limitation information coming from the PCRF/PCF), it rejects the session initiation attempt.
NOTE 1: Whether the P‑CSCF should interact with PCRF/PCF in this step is based on operator policy.
4. P‑CSCF forwards the INVITE request to the UE.
5. 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 begins the resource reservation according to the session and media parameters as shown in Figure 5.16a. Otherwise, the IP‑CAN initiates the reservation of required resources after step 10.
6-8. Ringing information is sent end to end towards the originating party. These steps may proceed in parallel with step 5.
9. The UE accepts the session with a 200 OK response.
10. Based on operator policy the P‑CSCF may instruct the PCRF/PCF to authorize the resources necessary for this session.
11-12. The 200 OK response is forwarded to the originating party.
13-15. The originating party acknowledges the session.
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5.6.5 Application Server Origination Procedure
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5.6.5.1 (AS-O) Origination at Application Server
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This origination procedure applies to an Application Server that initiates a session on behalf of a user or a Public Service Identity. If the AS initiates the session on behalf of a user, the user may be an IMS user (i.e. referred to by a Public User Identity) or a non-IMS user (i.e. with no profile in the HSS, e.g. a PSTN user). It will be referred as a non-IMS user. If the AS initiates the session on behalf of a user, the identity-related fields of the initial request are populated the same way as if the request was originated by the user himself.
In the case of originating unregistered procedures, the handling of the S‑CSCF in the HSS will follow the same principle as terminating unregistered user handling.
In the case of originating unregistered procedures, the S‑CSCF shall execute any unregistered origination service logic before forwarding requests from an AS on behalf of an IMS user (i.e. referred to by a Public User Identity) or a Public Service Identity, as specified by the S-S procedures. In order to allow an AS to retrieve the S‑CSCF name via Sh interface the S‑CSCF may keep its name in the HSS for Public User Identities that have services related to the unregistered state.
AS shall contact the S‑CSCF only in the case that it has the knowledge of the serving S‑CSCF based, e.g. on Sh query or third party registration. Otherwise, AS shall contact an I‑CSCF to continue the session initiation.
The procedure described below assumes that the Application Server takes care of the user plane connection.
Figure 5.16b: Application Server origination procedure
Procedure for Application Server origination is as follows:
1. The AS may proceed in either of the following ways:
- If the session requires the use of a S‑CSCF and:
- If the AS has acquired the address of the S‑CSCF (if not available already) for the Public User Identity or the Public Service Identity on whose behalf the AS intends to originate the session, e.g. through Sh interface or based on third party registration, the AS sends the session initiation request to the S‑CSCF (see step 2a).
- If the AS could not acquire a S‑CSCF address for the Public User Identity or the Public Service Identity, the AS sends the session initiation request to an I‑CSCF (see step 2d).
- If the Public Service Identity on whose behalf the AS intends to generate the session does not require the use of a S‑CSCF or if the user on whose behalf the AS intends to generate the session is a non-IMS user:
- If the AS supports routing capabilities (e.g. ENUM support, etc.), the AS sends the session initiation request directly towards the terminating network. In this case the AS may use the principles defined in IETF RFC 3263 [44] (see step 2b) to route the session initiation request.
- If the AS doesn't support routing capabilities, the AS shall send the session initiation request to the IMS Transit Functions (see step 2c). The IMS Transit Functions routes the session initiation request to the destination as described in clause 5.19.
2a. The AS sends the SIP INVITE request, containing an initial SDP, to the S‑CSCF.
The initial SDP may represent one or more media for a multi-media session.
2b. The AS sends the SIP INVITE request, containing an initial SDP, to the terminating network.
The subsequent steps assume that the session initiation procedure involves the S‑CSCF, i.e. they show the continuation of step 2a.
2c. The AS sends the SIP INVITE request, containing an initial SDP, to the IMS Transit Functions.
2d. The AS sends the SIP INVITE request, containing an initial SDP, to an I‑CSCF indicating that it is an originating request. The I‑CSCF selects the S‑CSCF and forwards the SIP INVITE to that S‑CSCF for further process. If the request is sent on behalf of the unregistered user, the procedure is described in clause 5.6.5.3.
3. S‑CSCF identifies the incoming request as an originating request and invokes any origination service logic required for this Public User Identity / Public Service Identity. The S‑CSCF handles the incoming request as an authenticated and authorized request, as it was originated by a trusted entity within the network. If the Request URI contains the SIP representation of a telephone number then the procedure specified in clause 4.3.5.3 applies.
4. S‑CSCF forwards the request, as specified by the S-S procedures.
5-6. The media stream capabilities of the destination are returned along the signalling path.
7-8. The AS decides the offered set of media streams for this session, confirms receipt of the Offer Response and sends the Response Confirmation along the signalling path towards the destination network. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response or a subset. The AS is free to continue to offer new media on this operation or on subsequent exchanges using the Update method.
9-10. The terminating end point responds to the originating end with an acknowledgement, which is forwarded along the session signalling path. If Optional SDP is contained in the Response Confirmation, the Confirmation Acknowledge will also contain an SDP response.
11-12. The terminating endpoint responds to the originating end when successful resource reservation has occurred.
13-14. 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 the AS along the signalling path.
15-16. When the destination party answers, the terminating endpoint sends a SIP 200-OK final response along the signalling path to the originating end.
17-18. The AS responds to the 200 OK with an ACK message which is passed along the signalling path to the terminating end.
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5.6.5.2 Void
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5.6.5.3 S‑CSCF selection by I‑CSCF for AS Originating call procedures
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In figure 5.16c below the AS has no information of the serving S‑CSCF and therefore the AS sends the request to an I‑CSCF as the entry point of the home network of the Public User Identity or the Public Service Identity. The AS finds an I‑CSCF by using the same mechanism as the S‑CSCF uses to find an I‑CSCF of the terminating network (see clauses 5.5.1 and 5.5.2). The request shall indicate that it is an originating request sent on behalf of the Public User Identity or the Public Service Identity.
NOTE 1: If border control concepts are applied, the contact point within an operator's network may be different, see clause 4.14 and Annex I for details.
NOTE 2: The procedure described below can be used by an external AS that cannot access HSS data using the Sh interface.
The procedure described below assumes that the Application Server takes care of the user plane connection.
This is shown by the information flow in figure 5.16c:
Figure 5.16c: S‑CSCF selection by I‑CSCF for AS Originating call procedure
1. The I‑CSCF receives an INVITE message indicating that it is an AS originating procedure.
2. The I‑CSCF queries the HSS for current location information of the Public User Identity/Public Service Identity on whose behalf the request is sent.
3. The HSS either responds with the required S‑CSCF capabilities which the I‑CSCF should use as an input to select a S‑CSCF or provides the I‑CSCF with the previously allocated S‑CSCF name for that user or service.
NOTE 3: The HSS sends back the capabilities even if the Public User Identity/Public Service Identity is not registered and has no initial filter criteria related to the unregistered state.
4. If the I‑CSCF has not been provided with the location of the S‑CSCF, the I‑CSCF selects a S‑CSCF.
5. The I‑CSCF forwards the INVITE request to the S‑CSCF. The I‑CSCF must indicate that it is an originating request sent on behalf of the Public User Identity/Public Service Identity.
6. The S‑CSCF sends Cx-Put/Cx-Pull (Public User Identity/Public Service Identity, S‑CSCF name) to the HSS. When multiple and separately addressable HSSs have been deployed by the network operator, then the S‑CSCF needs to query the SLF to resolve the HSS. The HSS stores the S‑CSCF name for Public Service Identity or Public User Identities of that user. This will result in all traffic related to the Public Service Identity or the Public User Identities of that user being routed to this particular S‑CSCF until the registration period expires or the user attaches the Public User Identity to the network.
NOTE 4: Optionally the S‑CSCF can omit the Cx-Put/Cx-Pull request if it has the relevant information from the user profile.
7. The HSS shall store the S‑CSCF name for that user or service and return the information flow Cx-Put Resp/Cx-Pull Resp (user information) to the S‑CSCF. The S‑CSCF shall store it.
8. The S‑CSCF invokes whatever service logic is appropriate for this call attempt, if required.
NOTE 5: If the Public User Identity/Public Service Identity is not registered and has no initial filter criteria related to the unregistered state, the S‑CSCF just routes the request further without invoking any service logic for this request.
9. The session setup continues as for normal origination procedures.
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5.7 Termination procedures
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5.7.0 General
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This clause presents the detailed application level flows to define the Procedures for session terminations.
The flows presented in the clause assume the use of Policy and Charging Control for the establishment of QoS-Assured Sessions.
The session termination procedures specify the signalling path between the Serving‑CSCF assigned to perform the session termination service and the UE. This signalling path is determined at the time of UE registration and remains fixed for the life of the registration.
A UE always has a proxy (P‑CSCF) associated with it. This P‑CSCF performs resource authorization for the sessions to the UE and may have additional functions in handling of priority sessions. The P‑CSCF is determined by the CSCF discovery process, described in clause 5.1.1 (Local CSCF Discovery).
As a result of the registration procedure, the P‑CSCF knows the address of the UE. The assigned S‑CSCF, knows the name/address of the P‑CSCF (procedure MT#3 and MT#4, depending on the location of S‑CSCF and P‑CSCF).
Sessions destined to the PSTN are a special case of the Termination procedures. The MGCF uses H.248 to control a Media Gateway and communicates with the SS7 network. The MGCF receives and processes SIP requests and subsequent nodes consider the signalling as if it came from a S‑CSCF.
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5.7.1 (MT#1) Mobile termination, roaming
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This termination procedure applies to roaming users.
The UE is located in a visited network and determines the P‑CSCF via the CSCF discovery procedure described in clause 5.1.1. The home network advertises the S‑CSCF as the entry point from the visited network.
When registration is complete, S‑CSCF knows the name/address of its next hop in the signalling path, the P‑CSCF and P‑CSCF knows the name/address of the UE.
Figure 5.17: Mobile termination procedure - roaming
Procedure MT#1 is as follows:
1. The originating party sends the SIP INVITE request, containing an initial SDP, via one of the origination procedures and via one of the Inter-Serving procedures, to the Serving‑CSCF for the terminating users.
2. S‑CSCF validates the service profile and invokes any termination service logic required for this user. This includes authorization of the requested SDP based on the user's subscription for multi-media services.
3. S‑CSCF remembers (from the registration procedure) the next hop CSCF for this UE. It forwards the INVITE to the P‑CSCF in the visited network.
4. If the P-CSCF determines that the termination is for an MPS session, the P-CSCF derives the session information and invokes dynamic policy sending the derived session information to the PCRF/PCF. The P‑CSCF remembers (from the registration procedure) the UE address and forwards the INVITE to the UE.
5. UE determines the subset of the media flows proposed by the originating endpoint that it supports and responds with an Offer Response message back to the originator. The SDP may represent one or more media for a multi-media session. This response is sent to P‑CSCF.
6. P‑CSCF instructs the PCRF/PCF to authorize the resources necessary for this session.
NOTE: P‑CSCF can additionally authorize the resources in step 4 in scenarios where request indicates requirements for resource reservation or that the required resources are already available on the originating side, as in such cases no SDP answer is received before the PCRF/PCF is requested to authorize the required QoS resources.
7. P‑CSCF forwards the Offer Response message to S‑CSCF.
8. S‑CSCF forwards the Offer Response message to the originator, per the S-S procedure.
9. The originating endpoint sends a Response Confirmation via the S-S procedure, to S‑CSCF. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response sent in Step 8 or a subset. If new media are defined by this SDP, a new authorization (as in Step 6) will be done following Step 12. 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 6 again.
10. S‑CSCF forwards the Response Confirmation to P‑CSCF. This may possibly be routed through the I‑CSCF depending on operator configuration of the I‑CSCF.
11. P‑CSCF forwards the Response Confirmation to UE.
12. UE responds to the Response Confirmation with an acknowledgement. If Optional SDP is contained in the Response Confirmation, the Confirmation Ack will also contain an SDP response. If the SDP has changed, the P‑CSCF authorizes that the resources are allowed to be used.
13. 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 the reservation procedures for the resources needed for this session as shown in Figure 5.17. Otherwise, the IP‑CAN initiates the reservation of required resources after step 6.
14-15. PCSCF forwards the Confirmation Ack to the S‑CSCF and then to the originating end point via session path. Step 14 may be similar to Step 7 depending on whether or not configuration hiding is used.
16-18. When the originating endpoint has completed its resource reservation, it sends the successful Resource Reservation message to S‑CSCF, via the S-S procedures. The S‑CSCF forwards the message toward the terminating endpoint along the signalling path.
19. UE#2 alerts the destination user of an incoming session setup attempt.
20-22. UE#2 responds to the successful resource reservation towards the originating end point.
23-25. UE may alert the user and wait for an indication from the user before completing the session setup. If so, it indicates this to the originating party by a provisional response indicating Ringing. This message is sent to P‑CSCF and along the signalling path to the originating end.
26. When the destination party answers, the UE sends a SIP 200-OK final response to P‑CSCF.
27. P‑CSCF indicates to PCRF/PCF that the media flows for this session should now be enabled.
28. UE starts the media flow(s) for this session.
29-30. P‑CSCF sends a SIP 200-OK final response along the signalling path back to the S‑CSCF.
31-33. The originating party responds to the 200-OK final response with a SIP ACK message that is sent to S‑CSCF via the S-S procedure and forwarded to the terminating end along the signalling path.
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5.7.2 (MT#2) Mobile termination, home
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This termination procedure applies to users located in their home service area.
The UE is located in the home network and determines the P‑CSCF via the CSCF discovery procedures described in clause 5.1.1.
When registration is complete, S‑CSCF knows the name/address of P‑CSCF and P‑CSCF knows the name/address of the UE.
Figure 5.18: Mobile termination procedure - home
Procedure MT#2 is as follows:
1. UE#1 sends the SIP INVITE request, containing an initial SDP, via one of the origination procedures and via one of the Serving to Serving‑CSCF procedures, to the Serving‑CSCF for the terminating user.
2. S‑CSCF validates the service profile and invokes any termination service logic required for this user. This includes authorization of the requested SDP based on the user's subscription for multi-media services.
3. S‑CSCF remembers (from the registration procedure) the next hop CSCF for this UE. It forwards the INVITE to the P‑CSCF in the home network.
4. If the P-CSCF determines that the termination is for an MPS session, the P-CSCF derives the session information and invokes dynamic policy sending the derived session information to the PCRF/PCF. The P‑CSCF remembers (from the registration procedure) the UE address and forwards the INVITE to the UE.
5. UE determines the subset of the media flows proposed by the originating endpoint that it supports and responds with an Offer Response message back to the originator. The SDP may represent one or more media for a multi-media session. This response is sent to P‑CSCF.
6. P‑CSCF instructs PCRF/PCF to authorize the resources necessary for this session.
NOTE: P‑CSCF can additionally authorize the resources in step 4 in scenarios where request indicates no requirements for resource reservation or that the required resources are already available on the originating side, as in such cases no SDP answer is received before the PCRF/PCF is requested to authorize the required QoS resources.
7. P‑CSCF forwards the Offer Response message to S‑CSCF.
8. S‑CSCF forwards the Offer Response message to the originator, per the S-S procedure.
9. The originating endpoint sends a Response Confirmation via the S-S procedure, to S‑CSCF. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response sent in Step 8 or a subset. If new media are defined by this SDP, a new authorization (as in Step 6) will be done following Step 12. 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 6 again.
10. S‑CSCF forwards the Response Confirmation to P‑CSCF.
11. P‑CSCF forwards the Response Confirmation to UE.
12. UE responds to the Response Confirmation with an acknowledgement. If Optional SDP is contained in the Response Confirmation, the Confirmation Ack will also contain an SDP response. If the SDP has changed, the P‑CSCF authorizes that the resources are allowed to be used.
13. 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 the reservation procedures for the resources needed for this session as shown in Figure 5.18. Otherwise, the IP‑CAN initiates the reservation of required resources after step 6.
14-15. The response is forwarded to the originating end point.
16-18. When the originating endpoint has completed its resource reservation, it sends the successful Resource Reservation message to S‑CSCF, via the S-S procedures. The S‑CSCF forwards the message toward the terminating endpoint along the signalling path.
19. UE#2 alerts the destination user of an incoming session setup attempt.
20-22. UE#2 responds to the successful resource reservation and the message is forwarded to the originating end.
23-25. UE may alert the user and wait for an indication from the user before completing the session. If so, it indicates this to the originating party by a provisional response indicating Ringing. This message is sent to P‑CSCF and along the signalling path to the originating end.
26. When the destination party answers, UE sends a SIP 200-OK final response to P‑CSCF.
27. P‑CSCF indicates that the authorized media flows for this session should now be enabled.
28. UE starts the media flow(s) for this session.
29-30. P‑CSCF forwards the 200-OK to S‑CSCF, following the signalling path.
31-33. The session originator responds to the 200-OK by sending the ACK message to S‑CSCF via the S-S procedure and it is forwarded to the terminating end along the signalling path.
5.7.2a (MT#3) Mobile termination, CS Domain roaming
This termination procedure applies to a user registered for CS services, either in the home network or in a visited network. The user has both IMS and CS subscriptions but is unregistered for IMS services
Figure 5.18a: Mobile Terminating procedures to a user that is unregistered for IMS services but is registered for CS services
1. If the terminating user does not have an S‑CSCF allocated, the session attempt is routed according to the clause 5.12.1 (Mobile Terminating procedures to unregistered IMS user that has services related to unregistered state).
2. S‑CSCF invokes service control appropriate for this session setup attempt, which may result in e.g. re-routing the session to a messaging service, or continued routing towards the user's CS domain termination address (e.g. E.164).
3. S‑CSCF performs whatever further actions are appropriate for this session setup attempt. In the case of routing towards the user's CS domain termination address, the S‑CSCF performs an analysis of this address. From the analysis of the destination address, S‑CSCF determines that this is for the CS domain and passes the request to the BGCF.
4. The BGCF forwards the SIP INVITE message to the appropriate MGCF in the home network, or to a BGCF in another network. This depends on the PSTN interworking configuration of the IMS network. Eventually, the session initiation arrives to an MGCF.
5. Normal session setup continues according to PSTN-T flow as described in clause 5.7.3.
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5.7.3 (PSTN-T) PSTN termination
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The MGCF in the IM CN subsystem is a SIP endpoint that initiates and receives requests on behalf of the PSTN and Media Gateway (MGW).Other nodes consider the signalling as if it came from a S‑CSCF. The MGCF incorporates the network security functionality of the S‑CSCF.
PSTN termination may be done in the same operator's network as the S‑CSCF of the session originator. Therefore, the location of the MGCF/MGW are given only as "Terminating Network" rather than "Home Network" or "Visited Network".
Further, agreements between network operators may allow PSTN termination in a network other than the originator's visited network or home network. This may be done, for example, to avoid long distance or international tariffs.
Figure 5.19: PSTN termination procedure
The PSTN termination procedure is as follows:
1. MGCF receives an INVITE request, containing an initial SDP, through one of the origination procedures and via one of the inter-serving procedures.
2. MGCF initiates a H.248 interaction to pick an outgoing channel and determine media capabilities of the MGW.
3. MGCF determines the subset of the media flows proposed by the originating endpoint that it supports and responds with an Offer Response message back to the originator. This response is sent via the S-S procedure.
4. The originating endpoint sends a Response Confirmation. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response sent in Step 3 or a subset. The originating UE is free to continue to offer new media on this operation or on subsequent exchanges using the Update method.
5. MGCF initiates a H.248 interaction to modify the connection established in step #2 and instruct MGW to reserve the resources necessary for the media streams.
6. MGCF responds to the offered media towards the originating party.
7. GW reserved the resources necessary for the media streams.
8. When the originating endpoint has completed its resource reservation, it sends the successful Resource Reservation message to MGCF, via the S-S procedures.
9. MGCF sends an IAM message to the PSTN.
10. MGCF sends response to the successful resource reservation towards originating end.
11. The PSTN establishes the path to the destination. It may optionally alert the destination user before completing the session. If so, it responds with an ACM message.
12. If the PSTN is alerting the destination user, MGCF indicates this to the originating party by a provisional response indicating Ringing. This message is sent via the S-S procedures.
13. When the destination party answers, the PSTN sends an ANM message to MGCF.
14. MGCF initiates a H.248 interaction to make the connection in the MGW bi-directional.
15. MGCF sends a SIP 200-OK final response along the signalling path back to the session originator.
16. The Originating party acknowledges the final response with a SIP ACK message.
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5.7.4 (NI-T) Non-IMS Termination to an external SIP client
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This clause describes the IMS session setup procedures towards external SIP clients that don't support the required IMS SIP extensions.
In this scenario, the UE originates an IMS session without requiring the support for precondition capabilities, towards an external SIP entity that does not support those capabilities. Since required resources are not yet available at the UE, all the media components are set to inactive. In this example the external SIP client does not support the Precondition extension of SIP so the related precondition information within SIP/SDP is ignored.
When both parties have agreed to the session and media parameters and the UE has established resources for the media, the UE initiates session modification setting the status of the media components to active and is thus enabling the media transfer to start. Figures 5.19b and 5.19c together illustrate session flows for one possible originating session establishment towards a non-IMS client in an external network with QoS authorization and Policy and Charging Control support.
For illustration purposes these session flows show the case of a non-roaming origination. This flow is a variant of MO#2 defined in clause 5.6.2. The same principles apply in roaming cases, i.e. analogous variants of MO#1 defined in clause 5.6.1 are also supported for interworking with SIP clients that do not support the required IMS procedures.
Figure 5.19a: Void
Figure 5.19b: Terminating session towards external SIP client, initiate session set up not requiring precondition capabilities and with inactive media
The UE initiates an INVITE message, which indicates the support of the precondition capability Since required resources are not yet available, the UE sets all media components to inactive state, as shown in figure 5.19b.
1‑3. UE initiates a new IMS session indicating the support of the precondition capability and setting all media components to inactive state.
4‑5. Acknowledgement of the session and media parameters are sent from the terminating side to the P‑CSCF.
6. The P‑CSCF may at this point instruct PCRF/PCF to authorize the resources being negotiated.
7. The acknowledgement of the session and media parameters forwarded towards the originating UE.
8‑10. The session is established, but media transfer is not allowed yet.
11. 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 starts the resource reservation for the media as shown in Figure 5.19b. Otherwise, the IP‑CAN initiates the reservation of required resources after step 6.
Figure 5.19c: Continuation of terminating session towards external SIP client, session set up with active media
Once the session parameters have been agreed and the UE has successfully reserved resources for the media components, the session set-up continues by setting the media components to active, as shown in session flow 5.19c.
12‑14. UE initiates activation of media by initiating an INVITE procedure towards the terminating party.
15‑16. The terminating party accepts media activation and corresponding signalling is passed back towards the originating party along the session path.
17. The P‑CSCF receives the acceptance of media activation. At this point, the P‑CSCF may instruct the PCRF/PCF to enable the media flows that have been authorized for the session.
18. The P‑CSCF forwards the signalling message to the originating UE indicating that the session setup can continue and activation of media is performed.
19‑21. The Session establishment is then acknowledged through the session path.
At this point in time, the session is established between the two parties.
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5.7.5 (AS-T#1) PSI based Application Server termination – direct
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This clause depicts a routing example for incoming session where the session request is routed directly to the AS hosting the PSI.
Figure 5.19d: Incoming session, direct route towards the AS
1. I‑CSCF receives a request destined to the PSI.
2-3. I‑CSCF queries the HSS in order to determine the next hop in the routing path for the PSI.
4. HSS determines the routing information, i.e. the address of the AS hosting the PSI.
5. HSS returns the AS address to the I‑CSCF.
6-7. I‑CSCF forwards the request to the address received from the query.
8-9. Session setup continues as per existing procedures.
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5.7.6 (AS-T#2) PSI based Application Server termination – indirect
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This clause depicts an example routing scenario where the basic IMS routing via S‑CSCF is used to route the session.
Figure 5.19e: Incoming session, indirect route to AS via S‑CSCF
1. I‑CSCF receives a request destined to the PSI.
2-3. I‑CSCF queries HSS in order to determine the next hop in the routing path for the PSI.
4. HSS determines the routing information, which is the S‑CSCF defined for the "PSI user".
5. HSS returns the S‑CSCF address/capabilities to the I‑CSCF.
6-7. I‑CSCF, as per existing procedures, forwards the request towards the entity (i.e. S‑CSCF) received from the query, or the I‑CSCF selects a new S‑CSCF if required.
8. S‑CSCF evaluates the filter criteria and gets the AS address where to forward the request.
9. The request is then routed towards the AS identified by the filter criteria.
10-12. Session setup continues as per existing procedures.
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5.7.7 (AS-T#3) PSI based Application Server termination – DNS routing
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This clause shows an example of DNS based routing of an incoming session from an external network. The routing from the external network leads to the entry point of the IMS subsystem hosting the subdomain of the PSI.
Figure 5.19f: Incoming session, direct route to AS using DNS
1. I‑CSCF receives a request that is destined to the PSI.
2. I‑CSCF has been configured with the list of supported domains/network names, or it may have been configured to directly query a local DNS server.
3. In this case the I‑CSCF checks the list and finds a match.
4. I‑CSCF sends DNS query to find the route.
5. DNS server returns the IP address of the AS hosting the PSI.
6-7. I‑CSCF forwards the request towards the IP address received from the query.
8-9. Session setup continues as per existing procedures.
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5.7.8 (AST#4) Termination at Application Server based on service logic
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This termination procedure applies to an Application Server that terminates a session. In this case the addressed user is a UE and is not hosted by the AS. Based on the invoked service logic at the Application Server the session is terminated at the AS.
The procedure described below assumes that the Application Server takes care of the user plane connection.
Figure 5.19g: Application Server termination
1. I‑CSCF receives a request destined to the user.
2-3. I‑CSCF queries HSS in order to determine the next hop in the routing path for the user.
4. HSS determines the routing information, which is the S‑CSCF defined for the user.
5. HSS returns the S‑CSCF address/capabilities to the I‑CSCF.
6-7. I‑CSCF, as per existing procedures, forwards the request to S‑CSCF that will handle the session termination.
8. S‑CSCF evaluates the filter criteria and gets the AS address where to forward the request.
9. The request is then routed towards the AS identified by the filter criteria. The AS terminates the session instead of allowing it to continue on to the address end user.
10-12. Session setup continues as per existing procedures.
5.7a Procedures for the establishment of sessions without preconditions
5.7a.1 General
These clauses present the general end-to-end session flow procedures without preconditions. The flow in clause 5.7a.2 is applicable to services without real-time QoS requirements before session becomes active and thus do not need to set-up dedicated IP‑CAN bearers but can use existing IP‑CAN bearers and to services which do not require that the terminating endpoint obtains a SIP‑level notification when the originating endpoint's IP‑CAN bearer becomes available.
NOTE: The flows in clauses 5.6 and 5.7 apply for services with real-time QoS requirements before session becomes active.
Note that the flows in these clauses do not show the use of a THIG. If a THIG is used, the use is completely analogous to the use in clauses 5.5, 5.6 and 5.7.
5.7a.2 Procedures for the establishment of sessions without preconditions - no resource reservation required before session becomes active
Figure 5.19h: End-to-end session flow procedure without preconditions - no resource reservation required before session becomes active
1. UE#1 sends the SIP INVITE request, containing an initial SDP, to the P‑CSCF#1 determined via the P‑CSCF discovery mechanism. The initial SDP may represent one or more media for a multi-media session. It should be noted that a media offer without preconditions in general implies that the offering entity might expect to receive incoming media for any of the offered media as soon as the offer is received by the other endpoint. Therefore either an existing IP‑CAN bearer is assumed to be available for use or the application is implemented such that incoming media is not expected until some later point in time.
2. P‑CSCF#1 examines the media parameters. If P‑CSCF#1 finds media parameters not allowed to be used within an IMS session (based on P‑CSCF local policies, or if available bandwidth authorization limitation information coming from the PCRF/PCF), it rejects the session initiation attempt.
NOTE 0a: Whether the P‑CSCF should interact with PCRF/PCF in this step is based on operator policy.
3. P‑CSCF#1 forwards the INVITE request to S‑CSCF#1 along the path determined upon UE#1's most recent registration procedure.
4. Based on operator policy S‑CSCF#1 validates the user's service profile and may invoke whatever service control logic is appropriate for this INVITE request. This may include routing the INVITE request to an Application Server, which processes the request further on.
5. S‑CSCF#1 forwards INVITE request to I‑CSCF#2.
6. I‑CSCF#2 performs Location Query procedure with the HSS to acquire the S‑CSCF address of the destination user (S‑CSCF#2).
7. I‑CSCF#2 forwards the INVITE request to S‑CSCF#2.
8. Based on operator policy S‑CSCF#2 validates the user's service profile and may invoke whatever service control logic is appropriate for this INVITE request. This may include routing the INVITE request to an Application Server, which processes the request further on.
9. S‑CSCF#2 forwards the INVITE request to P‑CSCF#2 along the path determined upon UE#2's most recent registration procedure.
10. P‑CSCF#2 examines the media parameters. If P‑CSCF#2 finds media parameters not allowed to be used within an IMS session (based on P‑CSCF local policies, or if available bandwidth authorization limitation information coming from the PCRF/PCF), it rejects the session initiation attempt.
NOTE 0b: Whether the P‑CSCF should interact with PCRF/PCF in this step is based on operator policy.
11. P‑CSCF#2 forwards the INVITE request to UE#2.
12. - 17. UE#2 may optionally generate a ringing message towards UE#1.
18. 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. UE#2 may reserve a dedicated IP‑CAN bearer for media based on the media parameters received in the SDP offer as shown in Figure 5.19h. Otherwise, the IP‑CAN#2 initiates the reservation of required resources after step 20 instead.
NOTE 1: The sequential ordering of 18 and 19 does not indicate that these steps are necessarily performed one after the other. If step 19 is performed before step 18 is finished, UE#2 shall use an existing IP‑CAN bearer to send and receive media unless the application is such that a new bearer is not needed until some later point in time. If step 18 is performed successfully, media are sent and received by UE#2 on the dedicated IP‑CAN bearer.
19. UE#2 accepts the session with a 200 OK response. The 200 OK response is sent to P‑CSCF#2.
20. Based on operator policy P‑CSCF#2 may instruct PCRF/PCF to authorize the resources necessary for this session.
21. - 24. The 200 OK response traverses back to UE#1.
25. Based on operator policy P‑CSCF#1 may instruct the PCRF/PCF to authorize the resources necessary for this session.
26. P‑CSCF#1 forwards the 200 OK response to UE#1.
27. - 31. UE#1 acknowledges the 200 OK with an ACK, which traverses back to UE#2.
32. 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. UE#1 may reserve a dedicated IP‑CAN bearer for media based on the media parameters received in the SDP answer as shown in Figure 5.19h. Otherwise, the IP‑CAN#1 initiates the reservation of required resources after step 25.
NOTE 2: The sequential ordering of 27 and 32 does not indicate that these steps are necessarily performed one after the other. If step 32 is performed successfully, media are sent and received by UE#1 on the reserved dedicated IP‑CAN bearer. UE#1 may also use an existing IP‑CAN bearer to send and receive media.
5.7a.3 Void
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5.8 Procedures related to routing information interrogation
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5.8.0 General
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When a mobile terminated session set-up arrives at an I‑CSCF that is authorized to route sessions, the I‑CSCF interrogates the HSS for routing information. The mobile terminated sessions for a user shall be routed to a S‑CSCF.
The Cx reference point shall support retrieval of routing information from HSS to I‑CSCF. The resulting routing information is the contact information of S‑CSCF.
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5.8.1 User identity to HSS resolution
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This clause describes the resolution mechanism, which enables the I‑CSCF, the S‑CSCF and the AS to find the address of the HSS, that holds the subscriber data for a given user identity when multiple and separately addressable HSSs have been deployed by the network operator. This resolution mechanism is implemented using a Subscription Locator Function (SLF) or a Diameter Proxy Agent that proxies the request to the HSS. This resolution mechanism is not required in networks that utilise a single HSS e.g. optionally, it could be switched off on the I‑CSCF and on the S‑CSCF and/or on the AS using O&M mechanisms. An example for a single HSS solution is a server farm architecture. By default, the resolution mechanism shall be supported.
On REGISTER and on MT INVITEs, the I‑CSCF queries the HSS for user's subscription specific data, e. g. the actual location or authentication parameters. This also has to be accomplished by the S‑CSCF on REGISTER. In the case when more than one independently addressable HSS is utilized by a network operator, the HSS where user information for a given subscriber is available has to be found. To get the HSS name the I‑CSCF and the S‑CSCF query the SLF entity or the I‑CSCF and the S‑CSCF send the query to the HSS via a Diameter Proxy Agent.
The SLF is accessed via the Dx interface or via the Dh interface. The Dx interface is the standard interface between the CSCF and the SLF and the Dh interface is the standard interface between the AS and the SLF. The synchronisation between the SLF and the different HSSs is an O&M issue.
A way to use the SLF is described in the following.
The Dx interface provides:
- an operation to query the SLF from the I‑CSCF or from the S‑CSCF, respectively.
- a response to provide the HSS name towards the I‑CSCF or towards the S‑CSCF, respectively.
By sending the Dx-operation DX_SLF_QUERY the I‑CSCF or the S‑CSCF indicates a user identity of which it is looking for an HSS. By the Dx-operation DX_SLF_RESP the SLF responds with the HSS name. The I‑CSCF or the S‑CSCF, respectively, continues by querying the selected HSS. The I‑CSCF may forward the HSS name towards the S‑CSCF. The S‑CSCF may use this name to find the subscriber's HSS.
Clause 5.8.2 presents the session flows on REGISTER and clause 5.8.3 on INVITE messages.
The Dh interface provides:
- an operation to query the SLF from the AS.
- a response to provide the HSS name towards the AS.
By sending the Dh-operation DH_SLF_QUERY the AS indicates a Public User Identity of which it is looking for an HSS. By the Dh-operation DH_SLF_RESP the SLF responds with the HSS name. The AS continues by querying the selected HSS. The AS may store the HSS name for the subsequent Sh-operations.
Clause 5.8.4 presents the message flow on the Dh interface.
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5.8.2 SLF on register
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Figure 5.20: SLF on register (1st case)
1. I‑CSCF receives a REGISTER request and now has to query for the location of the user's subscription data.
2. The I‑CSCF sends a DX_SLF_QUERY to the SLF and includes as parameter the user identity which is stated in the REGISTER request.
3. The SLF looks up its database for the queried user identity.
4. The SLF answers with the HSS name in which the user's subscription data can be found.
5. The I‑CSCF can proceed by querying the appropriate HSS.
Figure 5.20a: SLF on register (2nd case)
1. I‑CSCF sends a REGISTER request to the S‑CSCF. This now has to query for the location of the user's subscription data.
2. The S‑CSCF sends a DX_SLF_QUERY to the SLF and includes as parameter the user identity which is stated in the REGISTER request.
3. The SLF looks up its database for the queried user identity.
4. The SLF answers with the HSS name in which the user's subscription data can be found.
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5.8.3 SLF on UE invite
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Figure 5.21: SLF on UE invite
1. I‑CSCF receives an INVITE request and now has to query for the location of the user's subscription data.
2. The I‑CSCF sends a DX_SLF_QUERY to the SLF and includes as parameter the user identity which is stated in the INVITE request. If the user identity is an E.164 number in the SIP URI with user=phone parameter format the I‑CSCF shall first translate it into the Tel: URI format per IETF RFC 3966 [15] prior to sending to the SLF the DX_SLF_QUERY.
3. The SLF looks up its database for the queried user identity.
4. The SLF answers with the HSS name in which the user's subscription data can be found.
To prevent an SLF service failure e.g. in the event of a server outage, the SLF could be distributed over multiple servers. Several approaches could be employed to discover these servers. An example is the use of the DNS mechanism in combination with a new DNS SRV record. The specific algorithm for this however does not affect the basic SLF concept and is outside the scope of this document.
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5.8.4 SLF on AS access to HSS
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The flow shown below is where the AS queries the SLF to identify the HSS to access.
Figure 5.21a: SLF on AS access to HSS
1. An AS sends a DH_SLF_QUERY to the SLF and includes as a parameter the Public User Identity.
2. The SLF looks up its database for the queried Public User Identity.
3. The SLF answers with the HSS name in which the user's subscription data can be found.
4. The AS sends the Sh message towards the correct HSS.
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5.9 Routing of mid-session signalling
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During the signalling exchanges that occur to establish an IM Session, the following elements must ensure future signalling messages related to this session are routed through them:
- P‑CSCF serving the originating UE, in order to generate the CDR record in the roaming case and to force release of the resources used for the session.
- S‑CSCF serving the originating UE, in order to invoke any service logic required at session setup completion and to generate the CDR record at session termination.
- S‑CSCF serving the terminating UE, in order to invoke any service logic required at session setup completion and to generate the CDR record at session termination.
- P‑CSCF serving the terminating UE, in order to generate the CDR record in the roaming case and to force release of the resources used for the session.
Other CSCFs (e.g. I‑CSCFs) may optionally request this as well, for example if they perform some function needed in handling mid-session changes or session clearing operations.
All signalling message from the UE related to IMS sessions shall be sent to the P‑CSCF.
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5.10 Session release procedures
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5.10.0 General
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This clause provides scenarios showing SIP application session release. Note that these flows have avoided the strict use of specific SIP protocol message names. This is in an attempt to focus on the architectural aspects rather than the protocol. SIP is assumed to be the protocol used in these flows.
The session release procedures are necessary to ensure that the appropriate billing information is captured and to reduce the opportunity for theft of service by confirming that the bearers associated with a particular SIP session are deleted at the same time as the SIP control signalling and vice versa. Session release is specified for the following situations:
- Normal session termination resulting from an end user requesting termination of the session using session control signalling or deletion of the IP bearers associated with a session;
- Session termination resulting from network operator intervention;
- Loss of the session control bearer or IP bearer for the transport of the IMS signalling; and
- Loss of one or more radio connections which are used to transport the IMS signalling.
As a design principle the session release procedures shall have a high degree of commonality in all situations to avoid complicating the implementation.
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5.10.1 Terminal initiated session release
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The following flow shows a terminal initiated IM CN subsystem application (SIP) session release. It is assumed that the session is active and that the bearer was established directly between the two visited networks (the visited networks could be the Home network in either or both cases). Furthermore, the flow also assumes that Policy and Charging Control is in use.
Figure 5.22: Terminal initiated session release
1. One party hangs up, which generates a message (Bye message in SIP) from the UE to the P‑CSCF.
2. Depending on the bearer establishment mode selected for the IP‑CAN session, release resource(s) shall be initiated either by the UE or by the IP‑CAN itself. The UE initiates the release procedures for the resources used for this session as shown in Figure 5.22. Otherwise, the IP‑CAN initiates the release of used resources after step 4.
3. Void.
4. The P‑CSCF instruct the PCRF/PCF to remove the authorization for resources that had previously been issued for this endpoint for this session. This step will also result in a release indication to the IP‑CAN to confirm that the IP bearers associated with the session have been deleted.
5. The P‑CSCF sends a Hangup to the S‑CSCF of the releasing party.
6. The S‑CSCF invokes whatever service logic procedures are appropriate for this ending session.
7. The S‑CSCF of the releasing party forwards the Hangup to the S‑CSCF of the other party.
8. The S‑CSCF invokes whatever service logic procedures are appropriate for this ending session.
9. The S‑CSCF of the other party forwards the Hangup on to the P‑CSCF.
10. The P‑CSCF instructs the PCRF/PCF to remove the authorization for resources that had previously been issued for this endpoint for this session. This step also results in a release indication to the IP‑CAN to confirm that the IP bearers associated with the UE#2 session have been deleted.
11. The P‑CSCF forwards the Hangup on to the UE.
12. The terminal responds with an acknowledgement, the SIP OK message (number 200), that is sent back to the P‑CSCF.
13. Depending on the bearer establishment mode selected for the IP‑CAN session, release resource(s) shall be initiated either by the UE or by the IP‑CAN itself. The UE initiates the release procedures for the resources used for this session as shown in Figure 5.22. Otherwise, the IP‑CAN initiates the release of used resources after step 11.
14 Void.
15. The SIP OK message is sent to the S‑CSCF.
16. The S‑CSCF of the other party forwards the OK to the S‑CSCF of the releasing.
17. The S‑CSCF of the releasing party forwards the OK to the P‑CSCF of the releasing.
18. The P‑CSCF of the releasing party forwards the OK to the UE.
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5.10.2 PSTN initiated session release
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The following flow shows a PSTN terminal initiated IM CN subsystem application (SIP) session release. It is assumed that the session is active and that the bearer was established to the PSTN from the Home Network (the visited network could be the Home network in this case). Furthermore, this flow assumes that Policy and Charging Control is used.
Figure 5.23: PSTN initiated session release
1. PSTN party hangs up, which generates an ISUP REL message to the MGCF.
2. The MGCF sends a Hangup (Bye message in SIP) to the S‑CSCF to notify the terminal that the far end party has disconnected.
3. Step 3 may be done in parallel with Step 2. Depending on the GSTN network type Step 3 may need to wait until after step 14. The MGCF notes the reception of the REL and acknowledges it with an RLC. This is consistent with the ISUP protocol.
4. The MGCF requests the MGW to release the vocoder and ISUP trunk using the H.248/MEGACO Transaction Request (subtract). This also results in disconnecting the two parties in the H.248 context. The IP network resources that were reserved for the message receive path to the PSTN for this session are now released. This is initiated from the MGW. If RSVP was used to allocated resources, then the appropriate release messages for that protocol would be invoked here.
5. The MGW sends an acknowledgement to the MGCF upon completion of step 4.
6. The S‑CSCF invokes whatever service logic procedures are appropriate for this ending session.
7. The S‑CSCF forwards the Hangup to the P‑CSCF.
8. The P‑CSCF instructs the PCRF/PCF to remove the authorization for resources that had previously been issued for this endpoint for this session. This step also results in a release indication to the IP‑CAN to confirm that the IP bearers associated with the UE#2 session have been deleted.
9. The P‑CSCF forwards the Hangup to the UE.
10. The terminal responds with an acknowledgement, the SIP OK message (number 200), which is sent back to the P‑CSCF.
11-12. The IP network resources that had been reserved for the message receive path to the endpoint for this session are released, taking into account the bearer establishment mode used for the IP‑CAN session. Steps 11and 12 may be done in parallel with step 10. If RSVP was used to allocated resources, then the appropriate release messages for that protocol would be invoked here.
13. The SIP OK message is sent to the S‑CSCF.
14. The S‑CSCF forwards the message to the MGCF.
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5.10.3 Network initiated session release
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5.10.3.0 Removal of IP‑CAN bearer used to transport IMS SIP signalling
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It is possible that the IP‑CAN removes the IP‑CAN bearer used to transport IMS SIP signalling (e.g. due to overload situations).
In this case the UE or network shall initiate a procedure to re-establish (or modify where possible) an IP‑CAN bearer to transport IMS SIP signalling. After the re-establishment of an IP‑CAN bearer the UE should perform a re-registration to the IMS.
If the re-establishment (or the modification) fails then the UE or network shall de-activate all other IMS related IP‑CAN bearer(s).
The deactivation of the IP‑CAN bearer(s) results in the P‑CSCF being informed via PCRF/PCF of the IP-CAN bearer release P-CSCF may, depending on policy, initiate a network initiated session release as described in clause 5.10.3.1.
The failure in re-establishing the ability to communicate towards the UE results also in the P‑CSCF/PCRF/PCF being informed that the IMS SIP signalling transport to the UE is no longer possible which shall lead to a network initiated session release (initiated by the P‑CSCF) as described in clause 5.10.3.1 if any IMS related session is still ongoing for that UE. Additionally, the P‑CSCF shall reject subsequent incoming session requests towards the remote endpoint indicating that the user is not reachable, until either:
- the registration timer expires in P‑CSCF and the user is de-registered from IMS.
- a new Register message from the UE is received providing an indication to the P‑CSCF that the IMS SIP signalling transport for that user has become available again and session requests can be handled again.
The P‑CSCF shall not assume that the IMS SIP signalling transport is lost unless the P‑CSCF receives a notification of loss of signalling connectivity from the PCRF/PCF as defined in this clause. The P‑CSCF shall not reject subsequent incoming session requests towards the remote endpoint based upon notification of other events e.g. upon PCRF/PCF notification of loss of a media bearer or upon the failure to deliver an INVITE message to the UE.
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5.10.3.1 Network initiated session release - P‑CSCF initiated
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5.10.3.1.0 General
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This clause assumes that Policy and Charging Control is applied
The following flows show a Network initiated IM CN subsystem application (SIP) session release. It is assumed that the session is active and that the bearer was established directly between the two visited networks (the visited networks could be the Home network in either or both cases).
A bearer is removed e.g. triggered by a UE power down, due to a previous loss of coverage, or accidental/malicious removal, etc. In this case an IP‑CAN session modification procedure (GW initiated) will be performed (see TS 23.203 [54] and TS 23.503 [95]). The flow for this case is shown in Figure 5.26.
Other network initiated session release scenarios are of course possible.
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5.10.3.1.1 Network initiated session release - P‑CSCF initiated – after removal of IP-Connectivity Access Network bearer
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Figure 5.26: Network initiated session release - P‑CSCF initiated – after removal of IP‑CAN bearer
1. A bearer related to the session is terminated. The P‑CSCF receives an indication via PCRF/PCF of IP‑CAN bearer release.
2. The P‑CSCF instructs PCRF/PCF to remove the authorization for resources related to the released bearer that had previously been issued for this endpoint for this session (see TS 23.203 [54] and TS 23.503 [95]). It is optional for the P‑CSCF to instruct PCRF/PCF to deactivate additional IP‑CAN bearers (e.g. an IP‑CAN bearer for chat could still be allowed).
3. The P‑CSCF decides on the termination of the session. For example, the P‑CSCF may decide to terminate the session if all IP‑CAN bearers related to the same IMS session are deleted. In the event of the notification that the signalling transport to the UE is no longer possible, the P‑CSCF shall terminate any ongoing session with that specific UE.
If the P‑CSCF decides to terminate the session, then the P‑CSCF instructs the PCRF/PCF to remove the authorization for resources that has previously been issued for this endpoint for this session (see TS 23.203 [54] and TS 23.503 [95]).
The following steps are only performed if the P‑CSCF has decided to terminate the session.
4. The P‑CSCF generates a Hangup (Bye message in SIP) to the S‑CSCF of the releasing party.
5. The S‑CSCF invokes whatever service logic procedures are appropriate for this ending session.
6. The S‑CSCF of the releasing party forwards the Hangup to the S‑CSCF of the other party.
7. The S‑CSCF invokes whatever service logic procedures are appropriate for this ending session.
8. The S‑CSCF of the other party forwards the Hangup on to the P‑CSCF.
9. The P‑CSCF instructs the PCRF/PCF to remove the authorization for resources that had previously been issued for this endpoint for this session. This step also results in a release indication to the IP‑CAN to confirm that the IP bearers associated with the session have been deleted for UE#2.
10. The P‑CSCF forwards the Hangup on to the UE.
11. The UE responds with an acknowledgement, the SIP OK message (number 200), which is sent back to the P‑CSCF.
12-13. Steps 12 and 13 may be done in parallel with step 11. The IP network resources that had been reserved for the UE for this session are released, taking into account the bearer establishment mode used for the IP‑CAN session. If RSVP was used to allocated resources, then the appropriate release messages for that protocol would be invoked here.
14. The SIP OK message is sent to the S‑CSCF.
15. The S‑CSCF of the other party forwards the OK to the S‑CSCF of the releasing party.
16. The S‑CSCF of the releasing party forwards the OK to the P‑CSCF of the releasing party.
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5.10.3.2 Network initiated session release - S‑CSCF Initiated
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The following flow shows a network-initiated IM CN subsystem application session release, where the release is initiated by the S‑CSCF. This can occur in various service scenarios, e.g. administrative, or prepaid.
The procedures for clearing a session, when initiated by an S‑CSCF, are as shown in the following information flow. The flow assumes that Policy and Charging Control is in use.
Figure 5.27: Network initiated session release - S‑CSCF initiated
Information flow procedures are as follows:
1. S‑CSCF#1 decides the session should be terminated, due to administrative reasons or due to service expiration.
2. S‑CSCF#1 sends a Hangup message to P‑CSCF#1.
3. P‑CSCF#1 removes the authorization for resources that had previously been issued for this endpoint for this session. This step also results in a release indication to the IP‑CAN to confirm that the IP bearers associated with the session have been deleted for UE#1.
4. P‑CSCF#1 forwards the Hangup message to UE#1.
5. UE#1 stops sending the media stream to the remote endpoint and the resources used for the session are released taking into account the bearer establishment mode used for the IP‑CAN session.
6. UE#1 responds with a SIP‑OK message to its proxy, P‑CSCF#1.
7. P‑CSCF#1 forwards the SIP‑OK message to S‑CSCF#1.
8. S‑CSCF#1 sends a Hangup message to S‑CSCF#2. This is done at the same time as flow#2.
9. S‑CSCF#2 invokes whatever service logic procedures are appropriate for this ending session.
10. S‑CSCF#2 forwards the Hangup message to P‑CSCF#2.
11. P‑CSCF#2 removes the authorization for resources that had previously been issued for this endpoint for this session. This step also results in a release indication to the IP‑CAN to confirm that the IP bearers associated with the session have been deleted for UE#2.
12. P‑CSCF#2 forwards the Hangup message to UE#2.
13. UE#2 stops sending the media stream to the remote end point and the resources used for the session are released taking into account the bearer establishment mode used for the IP‑CAN session.
14. UE#2 acknowledges receipt of the Hangup message with a SIP‑OK final response, send to P‑CSCF#2.
15. P‑CSCF#2 forwards the SIP‑OK final response to S‑CSCF#2.
16. S‑CSCF#2 forwards the SIP‑OK final response to S‑CSCF#1.
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5.11 Procedures to enable enhanced multimedia services
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5.11.1 Session Hold and Resume Procedures
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5.11.1.0 General
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This clause gives information flows for the procedures for placing sessions on hold that were previously established by the mechanisms of clauses 5.4, 5.5, 5.6 and 5.7 and resuming the session afterwards. Two cases are presented: mobile-to-mobile (UE-UE) and a UE-initiated hold of a UE-PSTN session.
For a multi-media session, it shall be possible to place a subset of the media streams on hold while maintaining the others.
These procedures do not show the use of optional I‑CSCFs. If an I‑CSCF was included in the signalling path during the session establishment procedure, it would continue to be used in any subsequent flows such as the ones described in this clause.
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5.11.1.1 Mobile-to-Mobile Session Hold and Resume Procedures
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An IMS session was previously established between an initiating UE and a terminating UE. Each of these UEs has an associated P‑CSCF and a S‑CSCF assigned in their home network. The procedures are independent of whether the P‑CSCFs are located in the home or visited networks. Therefore there is no distinction in this clause of home network vs. visited network.
The hold and resume procedures are identical whether the UE that initiated the session also initiates the session-hold, or whether the UE that terminated the session initiates the session-hold.
When a media stream has been placed on hold, it shall not be resumed by any endpoint other than the one that placed it on hold.
The procedures for placing a media stream on hold and later resuming the media stream, are as shown in the following information flow:
Figure 5.28: Mobile to Mobile session hold and resume
Information flow procedures are as follows:
1. UE#1 detects a request from the user to place a media stream on hold. UE#1 stops sending the media stream to the remote endpoint, but keeps the resources for the session reserved.
2. UE#1 sends a Hold message to its proxy, P‑CSCF#1.
3. P‑CSCF#1 forwards the Hold message to S‑CSCF#1.
4. S‑CSCF#1 forwards the Hold message to S‑CSCF#2.
5. S‑CSCF#2 forwards the Hold message to P‑CSCF#2.
6. P‑CSCF#2 forwards the Hold message to UE#2.
7. UE#2 stops sending the media stream to the remote endpoint, but keeps the resources for the session reserved.
8. UE#2 acknowledges receipt of the Hold message with a 200-OK final response, send to P‑CSCF#2.
9. P‑CSCF#2 forwards the 200 OK final response to S‑CSCF#2.
10. S‑CSCF#2 forwards the 200 OK final response to S‑CSCF#1.
11. S‑CSCF#1 forwards the 200 OK final response to P‑CSCF#1.
12. P‑CSCF#1 forwards the 200 OK final response to UE#1.
13. UE#1 detects a request from the user to resume the media stream previously placed on hold. UE#1 sends a Resume message to its proxy, P‑CSCF#1.
14. P‑CSCF#1 forwards the Resume message to S‑CSCF#1.
15. S‑CSCF#1 forwards the Resume message to S‑CSCF#2.
16. S‑CSCF#2 forwards the Resume message to P‑CSCF#2.
17. P‑CSCF#2 forwards the Resume message to UE#2.
18. UE#2 resumes sending the media stream to the remote endpoint.
19. UE#2 acknowledges receipt of the Resume message with a 200-OK final response, sent to P‑CSCF#2.
20. P‑CSCF#2 forwards the 200 OK final response to S‑CSCF#2.
21. S‑CSCF#2 forwards the 200 OK final response to S‑CSCF#1.
22. S‑CSCF#1 forwards the 200 OK final response to P‑CSCF#1.
23. P‑CSCF#1 forwards the 200 OK final response to UE#1.
24. UE#1 resumes sending the media stream to the remote endpoint.
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5.11.1.2 Mobile-initiated Hold and Resume of a Mobile-PSTN Session
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An IMS session was previously established between an initiating UE and a MGCF acting as a gateway for a session terminating on the PSTN, or between an initiating MGCF acting as a gateway for a session originating on the PSTN to a terminating UE. The UE has an associated P‑CSCF, an S‑CSCF assigned in its home network and a BGCF that chooses the MGCF. The procedures are independent of whether the P‑CSCF is located in the subscriber's home or visited network. Therefore there is no distinction in this clause of home network vs. visited network.
The session hold and resume procedure is similar whether the UE initiated the session to the PSTN, or if the PSTN initiated the session to the UE. The only difference is the optional presence of the BGCF in the case of a session initiated by the UE. Note that the BGCF might or might not be present in the signalling path after the first INVITE is routed.
The procedures for placing a media stream on hold and later resuming the media stream, are as shown in the following information flow:
Figure 5.29: Mobile-initiated Hold and Resume of a Mobile-PSTN Session
Information flow procedures are as follows:
1. UE detects a request from the user to place a media stream on hold. UE#1 stops sending the media stream to the remote endpoint, but keeps the resources for the session reserved.
2. UE sends a Hold message to its proxy, P‑CSCF.
3. P‑CSCF forwards the Hold message to S‑CSCF.
4. S‑CSCF forwards the Hold message to BGCF.
5. BGCF forwards the Hold message to MGCF.
5a MGCF sends a CPG(hold) in order to express that the call has been placed on hold.
6. MGCF initiates a H.248 interaction with MGW instructing it to stop sending the media stream, but to keep the resources for the session reserved.
7. MGCF acknowledges receipt of the Hold message with a 200-OK final response, send to BGCF.
8. BGCF forwards the 200-OK to the S‑CSCF.
9. S‑CSCF forwards the 200 OK final response to P‑CSCF.
10. P‑CSCF forwards the 200 OK final response to UE.
11. UE detects a request from the user to resume the media stream previously placed on hold. UE sends a Resume message to its proxy, P‑CSCF.
12. P‑CSCF forwards the Resume message to S‑CSCF.
13. S‑CSCF forwards the Resume message to BGCF.
14. BGCF forwards the Resume message to MGCF.
14a. MGCF sends a CPG(resume) in order to resume the call.
15. MGCF initiates a H.248 interaction with MGW instructing it to resume sending the media stream.
16. MGCF acknowledges receipt of the Resume message with a 200-OK final response, sent to BGCF.
17. BGCF forwards the 200 OK final response to the S‑CSCF.
18. S‑CSCF forwards the 200 OK final response to P‑CSCF.
19. P‑CSCF forwards the 200 OK final response to UE.
20. UE resumes sending the media stream to the remote endpoint.
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5.11.1.3 PSTN-initiated Hold and Resume of a Mobile-PSTN Session
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An IMS session was previously established between an initiating UE and a MGCF acting as a gateway for a session terminating on the PSTN, or between an initiating MGCF acting as a gateway for a session originating on the PSTN to a terminating UE. The UE has an associated P‑CSCF, an S‑CSCF assigned in its home network and a BGCF that chooses the MGCF. The procedures are independent of whether the P‑CSCF is located in the subscriber's home or visited network. Therefore there is no distinction in this clause of home network vs. visited network.
The session hold and resume procedure is similar whether the UE initiated the session to the PSTN, or if the PSTN initiated the session to the UE. The only difference is the optional presence of the BGCF in the case of a session initiated by the UE. Note that the BGCF might or might not be present in the signalling path after the first INVITE is routed.
The following information flow shows the procedures, where the session is set on hold from the PSTN side:
Figure 5.29a: PSTN-initiated Hold and Resume of a Mobile-PSTN Session
Information flow procedures are as follows:
1. The call is placed on hold in the PSTN.
2. The MGCF receives a CPG (hold) from the PSTN, which indicates that the call has been placed on hold.
3. MGCF sends a Hold message to BGCF.
4. BGCF forwards the Hold message to S‑CSCF.
5. S‑CSCF forwards the Hold message to P‑CSCF.
6. P‑CSCF forwards the Hold message to the UE.
7. UE stops sending the media stream to the remote endpoint, but keeps the resources for the session reserved.
8. The UE acknowledges receipt of the Hold message with a 200-OK final response, send to P‑CSCF.
9. P‑CSCF forwards the 200-OK final response to S‑CSCF.
10. S‑CSCF forwards the 200 OK final response to BGCF.
11. BGCF forwards the 200 OK final response to MGCF.
12. The call is resumed in the PSTN.
13. MGCF receives a CPG (resume) request from the PSTN, which indicates that the call is resumed.
14. MGCF sends a resume message to BGCF.
15. BGCF forwards the Resume message to S‑CSCF.
16. S‑CSCF forwards the Resume message to P‑CSCF.
17. P‑CSCF forwards the Resume message to UE.
18. UE resumes sending the media stream to the remote endpoint.
19. UE acknowledges receipt of the Resume message with a 200-OK final response, sent to P‑CSCF.
20. P‑CSCF forwards the 200 OK final response to the S‑CSCF.
21. S‑CSCF forwards the 200 OK final response to BGCF.
22. BGCF forwards the 200 OK final response to MGCF.
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5.11.2 Procedures for anonymous session establishment
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5.11.2.0 General
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This clause gives information flows for the procedures for an anonymous session. However, sessions are not intended to be anonymous to the originating or terminating network operators.
The purpose of the mechanism is to give an IMS user the possibility to withhold certain identity information as specified in IETF RFC 3323 [39] and IETF RFC 3325 [40].
The privacy mechanism for IMS networks shall not create states in the CSCFs other than the normal SIP states.
IMS entities shall determine whether they are communicating with an element of the same Trust Domain for Asserted Identity or not as described in IETF RFC 3325 [40].
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5.11.2.1 Signalling requirements for anonymous session establishment
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The user shall be able to request that her identity information is not revealed to the terminating party.
If the originating user requests the session to be anonymous, the terminating side must not reveal any identity or signalling routing information to the destination endpoint. The terminating network should distinguish at least two cases, first where the originator intended the session to be anonymous and second where the originator's identity was deleted by a transit network.
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5.11.2.2 Bearer path requirements for anonymous session establishment
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Procedures for establishment of an anonymous bearer path are not standardised in this release.
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5.11.3 Procedures for codec and media characteristics flow negotiations
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5.11.3.0 General
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This clause gives information flows for:
- the procedures for determining the set of negotiated characteristics between the endpoints of a multi-media session, determining the initial media characteristics (including common codecs) to be used for the multi-media session and
- the procedures for modifying a session within the existing resources reservation or with a new resources reservation (adding/deleting a media flow, changing media characteristics including codecs, changing bandwidth requirements) when the session is already established.
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5.11.3.1 Codec and media characteristics flow negotiation during initial session establishment
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Initial session establishment in the IM CN subsystem must determine a negotiated set of media characteristics (including a common codec or set of common codecs for multi-media sessions) that will be used for the session. This is done through an end-to-end message exchange to determine the complete set of media characteristics, then the decision is made by the session initiator as to the initial set of media flows.
The session initiator includes an SDP in the SIP INVITE message that lists every media characteristics (including codecs) that the originator is willing to support for this session. When the message arrives at the destination endpoint, it responds with the media characteristics (e.g. common subset of codecs) that it is also willing to support for the session. Media authorization is performed for these media characteristics. The session initiator, upon receiving the common subset, determines the media characteristics (including codecs) to be used initially.
The negotiation may take multiple media offered and answered between the end points until the media set is agreed upon.
Once the session is established, the procedures of clause 5.11.3.2 may be used by either endpoint to change to a different media characteristic (e.g. codec) that was included in the initial session description and for which no additional resources are required for media transport. The procedures of clause 5.11.3.3 may be used by either endpoint to change the session, which requires resources beyond those allocated to the existing session.
The flow presented here assumes that Policy and Charging Control is in use.
Figure 5.30: Codec negotiation during initial session establishment
The detailed procedure is as follows:
1. UE#1 inserts the codec(s) to a SDP payload. The inserted codec(s) shall reflect the UE#1's terminal capabilities and user preferences for the session capable of supporting for this session. It builds a SDP containing bandwidth requirements and characteristics of each and assigns local port numbers for each possible media flow. Multiple media flows may be offered and for each media flow (m= line in SDP), there may be multiple codec choices offered.
2. UE#1 sends the initial INVITE message to P‑CSCF#1 containing this SDP
3. P‑CSCF#1 examines the media parameters. If P‑CSCF#1 finds media parameters not allowed to be used within an IMS session (based on P‑CSCF local policies, or if available bandwidth authorization limitation information coming from the PCRF/PCF), it rejects the session initiation attempt. This rejection shall contain sufficient information for the originating UE to re-attempt session initiation with media parameters that are allowed by local policy of P‑CSCF#1's network according to the procedures specified in IETF RFC 3261 [12].
In this flow described in Figure 5.30 above the P‑CSCF#1 allows the initial session initiation attempt to continue.
NOTE 1: Whether the P‑CSCF should interact with PCRF/PCF in this step is based on operator policy.
4. P‑CSCF#1 forwards the INVITE message to S‑CSCF#1
5. S‑CSCF#1 examines the media parameters. If S‑CSCF#1 finds media parameters that local policy or the originating user's subscriber profile does not allow to be used within an IMS session, it rejects the session initiation attempt. This rejection shall contain sufficient information for the originating UE to re-attempt session initiation with media parameters that are allowed by the originating user's subscriber profile and by local policy of S‑CSCF#1's network according to the procedures specified in IETF RFC 3261 [12].
In this flow described in Figure 5.30 above the S‑CSCF#1 allows the initial session initiation attempt to continue.
6. S‑CSCF#1 forwards the INVITE, through the S-S Session Flow Procedures, to S‑CSCF#2.
7. S‑CSCF#2 examines the media parameters. If S‑CSCF#2 finds media parameters that local policy or the terminating user's subscriber profile does not allow to be used within an IMS session, it rejects the session initiation attempt. This rejection shall contain sufficient information for the originating UE to re-attempt session initiation with media parameters that are allowed by the terminating user's subscriber profile and by local policy of S‑CSCF#2's network according to the procedures specified in IETF RFC 3261 [12].
In this flow described in Figure 5.30 above the S‑CSCF#2 allows the initial session initiation attempt to continue.
8. S‑CSCF#2 forwards the INVITE message to P‑CSCF#2.
9. P‑CSCF#2 examines the media parameters. If P‑CSCF#2 finds media parameters not allowed to be used within an IMS session (based on P‑CSCF local policies, or if available bandwidth authorization limitation information coming from the PCRF/PCF), it rejects the session initiation attempt. This rejection shall contain sufficient information for the originating UE to re-attempt session initiation with media parameters that are allowed by local policy of P‑CSCF#2's network according to the procedures specified in IETF RFC 3261 [12].
In this flow described in Figure 5.30 above the P‑CSCF#2 allows the initial session initiation attempt to continue.
NOTE 2: Whether the P‑CSCF should interact with PCRF/PCF in this step is based on operator policy.
10. P‑CSCF#2 forwards the INVITE message to UE#2.
11. UE#2 determines the complete set of codecs that it is capable of supporting for this session. It determines the intersection with those appearing in the SDP in the INVITE message. For each media flow that is not supported, UE#2 inserts a SDP entry for media (m= line) with port=0. For each media flow that is supported, UE#2 inserts a SDP entry with an assigned port and with the codecs in common with those in the SDP from UE#1.
12. UE#2 returns the SDP listing common media flows and codecs to P‑CSCF#2
13. P‑CSCF#2 authorizes the QoS resources for the remaining media flows and codec choices.
14. P‑CSCF#2 forwards the SDP response to S‑CSCF#2.
15. S‑CSCF#2 forwards the SDP response to S‑CSCF#1.
16. S‑CSCF#1 forwards the SDP response to P‑CSCF#1.
17. P‑CSCF#1 authorizes the QoS resources for the remaining media flows and codec choices.
18. P‑CSCF#1 forwards the SDP response to UE#1.
19. UE#1 determines which media flows should be used for this session and which codecs should be used for each of those media flows. If there was more than one media flow, or if there was more than one choice of codec for a media flow, then UE#1 need to renegotiate the codecs by sending another offer to reduce codec to one with the UE#2.
20-24. UE#1 sends the "Offered SDP" message to UE#2, along the signalling path established by the INVITE request
The remainder of the multi-media session completes identically to a single media/single codec session, if the negotiation results in a single codec per media.
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5.11.3.2 Codec or media characteristics flow change within the existing reservation
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After the multi-media session is established, it is possible for either endpoint to change the set of media flows or media characteristics (e.g. codecs) for media flows. If the change is within the resources already reserved, then it is only necessary to synchronise the change with the other endpoint. Note that an admission control decision will not fail if the new resource request is within the existing reservation.
The flow presented here assumes that Policy and Charging Control is in use.
Figure 5.31: Codec or media flow change - same reservation
The detailed procedure is as follows:
1. UE#1 determines that a new media stream is desired, or that a change is needed in the codec in use for an existing media stream. UE#1 evaluates the impact of this change and determines the existing resources reserved for the session are adequate. UE#1 builds a revised SDP that includes all the common media flows determined by the initial negotiation, but assigns a codec and port number only to those to be used onward. UE#1 stops transmitting media streams on those to be dropped from the session.
2-6. UE#1 sends an INVITE message through the signalling path to UE#2. At each step along the way, the CSCFs recognise the SDP is a proper subset of that previously authorized and take no further action.
7. UE#2 receives the INVITE message and agrees that it is a change within the previous resource reservation. (If not, it would respond with a SDP message, following the procedures of 5.11.3.1). UE#2 stops sending the media streams to be deleted and initialises its media receivers for the new codec.
8-12. UE#2 forwards a 200-OK final response to the INVITE message along the signalling path back to UE#1.
13. UE#1 starts sending media using the new codecs. UE#1 also releases any excess resources no longer needed.
14-18. UE#1 sends the SIP final acknowledgement, ACK, to UE#2.
19. UE#2 starts sending media using the new codecs. UE#2 also releases any excess resources no longer needed
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5.11.3.3 Codec or media characteristics flow change requiring new resources and/or authorization
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After the multi-media session is established, it is possible for either endpoint to change the set of media flows or media characteristics (e.g. codecs) for media flow(s). If the change requires different resources beyond those previously reserved, then it is necessary to perform the resource reservation and bearer establishment procedures. If the reservation request fails for whatever reason, the original multi-media session remains in progress.
The flow presented here assumes that Policy and Charging Control is in use.
Figure 5.32: Codec or media flow change - new reservation
The detailed procedure is as follows:
1. UE#1 inserts the revised set of codecs to a SDP payload. The inserted codec(s) shall reflect the UE#1's terminal capabilities and user preferences for the session. It builds a SDP containing bandwidth requirements and characteristics of each and assigns local port numbers for each possible media flow. Multiple media flows may be offered and for each media flow (m= line in SDP), there may be multiple codec choices offered.
2. UE#1 sends an INVITE message to P‑CSCF#1 containing this SDP.
3. P‑CSCF#1 examines the media parameters. If P‑CSCF#1 finds media parameters not allowed to be used within an IMS session (based on P‑CSCF local policies, or if available bandwidth authorization limitation information coming from the PCRF/PCF), it rejects the session modification attempt. This rejection shall contain sufficient information for the originating UE to re-attempt session modification with media parameters that are allowed by local policy of P‑CSCF#1's network according to the procedures specified in IETF RFC 3261 [12].
In this flow described in Figure 5.32 above the P‑CSCF#1 allows the initial session modification attempt to continue.
NOTE 1: Whether the P‑CSCF interacts with PCRF/PCF in this step is based on operator policy.
4. P‑CSCF#1 forwards the INVITE message to S‑CSCF#1.
5. S‑CSCF#1 examines the media parameters. If S‑CSCF#1 finds media parameters that local policy or the originating user's subscriber profile does not allow to be used within an IMS session, it rejects the session modification attempt. This rejection shall contain sufficient information for the originating UE to re-attempt session modification with media parameters that are allowed by the originating user's subscriber profile and by local policy of S‑CSCF#1's network according to the procedures specified in IETF RFC 3261 [12].
In this flow described in Figure 5.32 above the S‑CSCF#1 allows the initial session modification attempt to continue.
6. S‑CSCF#1 forwards the INVITE, through the S-S Session Flow Procedures, to S‑CSCF#2.
7. S‑CSCF#2 examines the media parameters. If S‑CSCF#2 finds media parameters that local policy or the terminating user's subscriber profile does not allow to be used within an IMS session, it rejects the session modification attempt. This rejection shall contain sufficient information for the originating UE to re-attempt session modification with media parameters that are allowed by the terminating user's subscriber profile and by local policy of S‑CSCF#2's network according to the procedures specified in IETF RFC 3261 [12].
In this flow described in Figure 5.32 above the S‑CSCF#2 allows the initial session modification attempt to continue.
8. S‑CSCF#3 forwards the INVITE message to P‑CSCF#2.
9. P‑CSCF#2 examines the media parameters. If P‑CSCF#2 finds media parameters not allowed to be used within an IMS session (based on P‑CSCF local policies, or if available bandwidth authorization limitation information coming from the PCRF/PCF), it rejects the session modification attempt. This rejection shall contain sufficient information for the originating UE to re-attempt session modification with media parameters that are allowed by local policy of P‑CSCF#2's network according to the procedures specified in IETF RFC 3261 [12].
In this flow described in Figure 5.32 above the P‑CSCF#2 allows the initial session modification attempt to continue.
NOTE 2: If session modification request indicates no requirements for resource reservation or that the required resources are already available on the originating side, the P‑CSCF#2 can send updated session information to PCRF/PCF whenever SDP offer is contained in the session establishment request, as in such cases no SDP answer is received before the PCRF/PCF is requested to authorize the required QoS resources. Otherwise, whether the P‑CSCF interacts with PCRF/PCF in this step is based on operator policy.
10. P‑CSCF#2 forwards the INVITE message to UE#2.
11. UE#2 determines the complete set of codecs that it is capable of supporting for this session. It determines the intersection with those appearing in the SDP in the INVITE message. For each media flow that is not supported, UE#2 inserts a SDP entry for media (m= line) with port=0. For each media flow that is supported, UE#2 inserts a SDP entry with an assigned port and with the codecs in common with those in the SDP from UE#1.
12. UE#2 returns the SDP listing common media flows and codecs to P‑CSCF#2. It may additionally provide more codecs than originally offered and then the offered set need to be renegotiated.
13. P‑CSCF#2 increases the authorization for the QoS resources, if needed, for the remaining media flows and codec choices.
NOTE 3: P‑CSCF can additionally authorize the resources in step 9.
14. P‑CSCF#2 forwards the SDP response to S‑CSCF#2 toward the originating end along the signalling path.
15. P‑CSCF#1 increases the authorization for the QoS resources, if needed, for the remaining media flows and codec choices.
16. P‑CSCF#1 forwards the SDP response to UE#1.
17. UE#1 determines which media flows should be used for this session and which codecs should be used for each of those media flows. If there was more than one media flow, or if there was more than one choice of codec for a media flow, then UE#1 must include an SDP in the response message by including SDP to UE#2.
18. UE#1 sends the offered SDP message to UE#2, including the SDP from step #17 if needed.
19. UE#1 and UE#2 reserve the resources needed for the added or changed media flows. If the reservation is successfully completed by UE#1, it stops transmitting any deleted media streams. If UE#1 has sent a new media offer in step 18, it would for example wait for the response in step 20 prior to reserving resources.
20. If UE#1 has sent an updated offer of SDP in step 18, then UE#2 responds to the offer and P‑CSCF#1 authorizes the offered SDP sent by UE#2.
21. UE#1 sends the Resource Reservation Successful message with final SDP to UE#2, via the signalling path through the CSCFs.
22. UE#2 stops sending the media streams to be deleted and initialises its media receivers for the new codec.
23. UE#2 sends the 200-OK final response to UE#1, along the signalling path.
24. UE#1 starts sending media using the new codecs. UE#1 also releases any excess resources no longer needed.
25. UE#1 sends the SIP final acknowledgement, ACK, to UE#2 along the signalling path.
26. UE#2 starts sending media using the new codecs. UE#2 also releases any excess resources no longer needed.
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5.11.3.4 Sample MM session flow - addition of another media
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For this end-to-end session flow, we assume the originator is a UE located within the service area of the network operator to whom the UE is subscribed. The UE has already established an IM CN session and is generating an invite to add another media (e.g. video to a voice call) to the already established session. Note that the invite to add media to an existing session could be originated by either end. The invite and subsequent flows, are assumed to follow the path determined when the initial session was established. Any I‑CSCFs that were included in the initial session would be included in this session.
The originating party addresses a destination that is a subscriber of the same network operator.
The destination party is a UE located within the service area of the network operator to which it is subscribed.
The flow presented here assumes that Policy and Charging Control is in use.
Figure 5.33: Multimedia session flow - addition of another media
Step-by-step processing of this end-to-end session flow is as follows:
1. UE#1 sends a SIP INVITE request, containing new SDP for the new media and including the original SDP, to P‑CSCF#1, which was obtained from the CSCF discovery procedures.
2. P‑CSCF#1 forwards the INVITE to the next hop name/address, as determined from the registration procedures. In this case the next hop is S‑CSCF#1 within the same operator's network.
3. S‑CSCF#1 validates the service profile and invokes whatever service logic is appropriate for this session attempt.
4. S‑CSCF#1 recognises that this invite applies to an existing session. It therefore forwards the INVITE along the existing path to S‑CSCF#2.
5. S‑CSCF#2 validates the service profile and invokes whatever service logic is appropriate for this session attempt.
6. S‑CSCF#2 remembers (from the registration procedure) the next hop CSCF for this UE. It forwards the INVITE to P‑CSCF#2 in the home network.
7. P‑CSCF#2 remembers (from the registration procedure) the address of UE#2 and forwards the INVITE to UE#2.
NOTE 1: If session modification request indicates no requirements for resource reservation or that the required resources are already available on the originating side, the P‑CSCF#2 can send updated session information to PCRF/PCF whenever SDP offer is contained in the session establishment request, as in such cases no SDP answer is received before the PCRF/PCF is requested to authorize the required QoS resources. Otherwise, whether the P‑CSCF interacts with PCRF/PCF in this step is based on operator policy.
8. UE#2 returns the media stream capabilities of the destination to the session originator, along the signalling path established by the INVITE message.
9. P‑CSCF#2 authorizes the QoS resources required for this additional media.
NOTE 2: P‑CSCF can additionally authorize the resources in step 7.
10. P‑CSCF#2 forwards the SDP to S‑CSCF#2.
11. S‑CSCF#2 forwards the SDP to S‑CSCF#1.
12. S‑CSCF#1 forwards the SDP message to P‑CSCF#1.
13. P‑CSCF#1 authorizes the additional resources necessary for this new media.
14. P‑CSCF#1 forwards the SDP message to the originating endpoint, UE#1.
15-19. The originator decides the offered set of media streams for this media addition and sends the offered SDP to P‑CSCF#1.
20. 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. UE#2 initiates the resource reservation procedures for the resources necessary for this additional media as shown in figure 5.33. Otherwise, the IP‑CAN initiates the reservation of required resources after step 9.
21. 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. After determining the offered set of media streams for this additional media, in step #15 above, UE#1 initiates the reservation procedures for the additional resources needed for this new media as shown in figure 5.33. Otherwise, the IP‑CAN#1 initiates the reservation of required resources after step 13.
22-25. When the terminating side has successfully reserved the needed resources, it sends the "reservation successful" message to UE#1 along the signalling path established by the INVITE message. The message is sent first to P‑CSCF#1.
25a. P‑CSCF#1 authorizes any additional media for the proposed SDP.
26. P‑CSCF#1 forwards the message to UE#1.
27-31. UE#1 sends the final agreed SDP to UE#2 via the established path.
32-35. UE#2 responds to the offered final media.
35a. P‑CSCF#1 authorizes the media agreed.
36. The response is forwarded to UE#1.
37. UE#2 may optionally delay the session establishment in order to alert the user to the incoming additional media.
38. If UE#2 performs alerting, it sends a ringing indication to the originator via the signalling path. The message is sent first to P‑CSCF#2.
39. P‑CSCF#2 forwards the ringing message to S‑CSCF#2.S‑CSCF#2 invokes whatever service logic is appropriate for this ringing flow.
40. S‑CSCF#2 forwards the message to S‑CSCF#1.
41. S‑CSCF#1 forwards the message to P‑CSCF#1.
42. P‑CSCF#1 forwards the message to UE#1.
43. UE#1 indicates to the originator that the media addition is being delayed due to alerting. Typically this involves playing a ringback sequence.
44. When the destination party accepts the additional media, UE#2 sends a SIP 200-OK final response along the signalling path back to the originator. The message is sent first to P‑CSCF#2.
44a. After sending the 200-OK, UE#2 may initiate the new media flow(s).
45. P‑CSCF#2 enables the media flows authorized for this additional media.
46. P‑CSCF#2 forwards the final response to S‑CSCF#2.
47. S‑CSCF#2 forwards the final response to S‑CSCF#1.
48. S‑CSCF#1 forwards the final response to P‑CSCF#1.
49. P‑CSCF#1 enables the media flows authorized for this additional media.
50. P‑CSCF#1 forwards the final response to UE#1.
51. UE#1 starts the media flow(s) for this additional media.
52. UE#1 responds to the final response with a SIP ACK message, which is passed to the destination via the signalling path. The message is sent first to P‑CSCF#1.
53. P‑CSCF#1 forwards the ACK to S‑CSCF#1
54. S‑CSCF#1 forwards the ACK to S‑CSCF#2.
55. S‑CSCF#2 forwards the ACK to P‑CSCF#2.
56. P‑CSCF#2 forwards the ACK to UE#2.
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5.11.4 Procedures for providing or blocking identity
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5.11.4.0 General
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Identity is composed of a Public User Identity and an optional display name:
- The Public User Identity is used by any user for requesting communications to other users (see clause 4.3.3.2).
- The display name is the user's name if available, an indication of privacy or unavailability otherwise. The display name is a text string which may identify the subscriber, the user or the terminal.
This clause gives information flows for the procedures for providing the authenticated Public User Identity and the optional display Name information of the originating party to the terminating party. It also describes the mechanisms for blocking the display of Public User Identity and optional display name if requested by the originating party.
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5.11.4.1 Procedures for providing the authenticated identity of the originating party
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Authentication of the subscriber is performed during the registration procedures, as described in clause 5.2.2.3. As a result of the registration procedures, one or several Public User Identity(ies) of the originating party is/are stored in P‑CSCF#1. As part of this procedure, the display name associated with each Public User Identity, if provided by the HSS, is also returned via the S‑CSCF and stored in the P‑CSCF#1. This is shown in the sub-procedure represented in the following information flow in step 1.
When UE#1 attempts to initiate a new session, the UE shall include one of the Public User Identities the UE received during the SIP registration in the INVITE request. The P‑CSCF#1 ensures that the INVITE request includes an authenticated Public User Identity, including the associated display name if provided by the S‑CSCF during the registration procedures, before forwarding the INVITE request to the S‑CSCF#1.
In the following call flow, it is assumed that no privacy has been required by UE#1.If the Public User Identity supplied by UE#1 in the INVITE request is incorrect, or if the UE did not provide a public identify, then the P‑CSCF may reject the request, or may overwrite with the correct URI, including the associated display name if provided by the S‑CSCF during the registration procedures.
Figure 5.34: Providing the authenticated Identity of the originating party
The detailed procedure is as follows:
1. Registration and authentication of UE#1 is performed. One or more authenticated identities for UE#1, including display names if provided, are stored in the P‑CSCF#1 and the UE.
2. UE#1 initiates a new multi-media session, by sending an INVITE request to P‑CSCF#1. This INVITE request includes a Public User Identity and may include a display name that may identify the specific person using the UE.
3. P‑CSCF#1 checks the Public User Identity of the originating party and replaces it (or rejects the request) if it is incorrect. If provided during registration procedures via the S‑CSCF, the P‑CSCF#1 ensures that the display name associated with the verified Public User Identity is present before forwarding the INVITE request.
4. P‑CSCF#1 forwards the INVITE request, with the verified Public User Identity and display name of the originating party if present, to S‑CSCF#1.
5. S‑CSCF#1 invokes whatever service logic is appropriate for this session set up attempt to check in particular that no identity restriction is active.
6. S‑CSCF#1 forwards the INVITE request, with verified Public User Identity and display name of the originating party if present, to S‑CSCF#2.
7. S‑CSCF#2 stores the Public User Identity and associated information.
8. S‑CSCF#2 forwards the INVITE request to P‑CSCF#2.
9. P‑CSCF#2 forwards the INVITE request to UE#2.
10. UE#2 displays the Public User Identity and the display name information (i.e. user-name if available, indication of privacy or unavailability otherwise) to the terminating party.
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5.11.4.2 Procedures for blocking the identity of the originating party
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Regulatory agencies, as well as subscribers, may require the ability of an originating party to block the display of their identity either permanently or on a session by session basis. This is a function performed by the destination P‑CSCF. In this way, the terminating party is still able to do a session-return, session-trace, transfer, or any other supplementary service.
In this call flow, it is assumed that privacy has been required by UE#1 on Public User Identity (i.e. 'id' privacy).
Figure 5.35: Blocking the identity of the originating party
The detailed procedure is as follows:
1. UE#1 initiates a new multi-media session, by sending an INVITE request to P‑CSCF#1. This INVITE request includes Public User Identity and may include a display name that may identify the specific person using the UE. Also included in this INVITE message is an indication that the identity of the originating party shall not be revealed to the destination.
2. P‑CSCF#1 checks the Public User Identity of the originating party and replaces it (or rejects the request) if it is incorrect. If provided during registration procedures, the P‑CSCF#1 ensures that the display name associated with the Public User Identity is present before forwarding the INVITE request.
3. P‑CSCF#1 forwards the INVITE request, with the verified Public User Identity and display name, to S‑CSCF#1.
4. S‑CSCF#1 invokes whatever service logic is appropriate for this session set up attempt. Based on the subscriber's profile, S‑CSCF#1 may insert an indication in the INVITE message that the identity of the originating party shall not be revealed to the terminating party. S‑CSCF#1 may insert an indication to block the IP address of UE#1 too and may remove other information from the messaging which may identify the caller to the terminating party.
5. S‑CSCF#1 forwards the INVITE request, with verified Public User Identity and with user-name of the originating party if present, to S‑CSCF#2.
6. If the terminating party has an override functionality in S‑CSCF#2/Application Server in the terminating network the S‑CSCF#2/Application Server removes the indication of privacy from the message.
7. S‑CSCF#2 forwards the INVITE request to P‑CSCF#2.
8. If privacy of the user identity is required, P‑CSCF#2 removes the Public User Identity, including the display name if present, from the message.
9. P‑CSCF#2 forwards the INVITE request to UE#2.
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5.11.4.3 Procedures for providing the authenticated identity of the originating party (PSTN origination)
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For calls originating from the PSTN, the MGCF extracts information received from the PSTN and inserts an asserted identity into the SIP message. If the incoming information includes the calling name, or the MGCF can obtain the calling name, the MGCF may insert the information into the display name portion of the asserted identity.
The MGCF must propagate the privacy indicators received from the PSTN in the SIP message.
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5.11.4.4 Procedures for providing the authenticated identity of the originating party (PSTN termination)
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For calls terminating to the PSTN, the MGCF extracts information received in the SIP message and inserts the information into the PSTN signalling. This information must include the privacy setting and may include the display name.
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5.11.5 Session Redirection Procedures
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5.11.5.0 General
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This clause gives information flows for the procedures for performing session redirection. The decision to redirect a session to a different destination may be made for different reasons by a number of different functional elements and at different points in the establishment of the session.
Three cases of session redirection prior to bearer establishment are presented and one case of session redirection after bearer establishment.
These cases enable the typical services of "Session Forward Unconditional", "Session Forward Busy", "Session Forward Variable", "Selective Session Forwarding" and "Session Forward No Answer", though it is important to recognise that the implementation is significantly different from the counterparts in the CS domain.
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5.11.5.1 Session Redirection initiated by S‑CSCF to IMS
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One of the functional elements in a basic session flow that may initiate a redirection is the S‑CSCF of the destination user. The user profile information obtained from the HSS by the 'Cx-pull' during registration may contain complex logic and triggers causing session redirection. S‑CSCF#2 sends the SIP INVITE request to the I‑CSCF for the new destination (I‑CSCF#F in the diagram), who forwards it to S‑CSCF#F, who forwards it to the new destination.
In cases when the destination user is not currently registered in the IM CN subsystem, the I‑CSCF may assign a temporary S‑CSCF to invoke the service logic on behalf of the intended destination. This temporary S‑CSCF takes the role of S‑CSCF#2 in the following information flow.
The service implemented by this information flow is typically "Session Forward Unconditional", "Session Forward Variable" or "Selective Session Forwarding". S‑CSCF#2 may also make use of knowledge of current sessions in progress at the UE and implement "Session Forwarding Busy" in this way.
This is shown in the following information flow:
Figure 5.36: Session redirection initiated by S‑CSCF to IMS
Step-by-step processing is as follows:
1. The SIP INVITE request is sent from the UE to S‑CSCF#1 by the procedures of the originating flow.
2. S‑CSCF#1 invokes whatever service logic is appropriate for this session setup attempt.
3. S‑CSCF#1 performs an analysis of the destination address and determines the network operator to whom the destination subscriber belongs. The INVITE message is sent to an I‑CSCF for that operator.
4. I‑CSCF queries the HSS for current location information of the destination user.
5. HSS responds with the address of the current Serving CSCF (S‑CSCF#2) for the terminating user.
6. I‑CSCF forwards the INVITE request to S‑CSCF#2, who will handle the session termination.
7. S‑CSCF#2 invokes whatever service logic is appropriate for this session setup attempt. As a result of this service control logic, S‑CSCF#2 determines that the session should be redirected to a new destination URI within the IP Multimedia Subsystem. Based on operator policy and the user profile, S‑CSCF#2 may restrict the media streams allowed in the redirected session.
8. S‑CSCF#2 sends a SIP INVITE request to an I‑CSCF (I‑CSCF#F) for the network operator to whom the forwarded destination subscribes.
9. I‑CSCF#F queries the HSS (HSS#F) for current location information of the destination user.
10. HSS#F responds with the address of the current Serving CSCF (S‑CSCF#F) for the terminating user.
11. I‑CSCF forwards the INVITE request to S‑CSCF#F, who will handle the session termination.
12. S‑CSCF#F invokes whatever service logic is appropriate for this session setup attempt
13. S‑CSCF#F forwards the INVITE toward the destination UE, according to the procedures of the terminating flow.
14-19. The destination UE responds with the SDP message and the session establishment proceeds normally.
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5.11.5.2 Session Redirection to PSTN Termination (S‑CSCF #2 forwards INVITE)
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The S‑CSCF of the destination user (S‑CSCF#2) may determine that the session is to be redirected to a PSTN Termination; e.g. CS-domain endpoint, or to the PSTN. For session redirection to PSTN termination where the S‑CSCF of the called party (S‑CSCF#2) wishes to remain in the path of SIP signalling, the S‑CSCF forwards the INVITE to a BGCF. Then the BGCF (in the local network or in another network) will forward the INVITE to a MGCF, which will forward towards the destination according to the termination flow.
In cases when the destination user is not currently registered in the IM CN subsystem, the I‑CSCF may assign a temporary S‑CSCF to invoke the service logic on behalf of the intended destination. This temporary S‑CSCF takes the role of S‑CSCF#2 in the following information flow.
Handling of redirection to a PSTN Termination where the S‑CSCF#2 forwards the INVITE is shown in the figure 5.37:
Figure 5.37: Session redirection to PSTN Termination (S‑CSCF #2 forwards INVITE)
Step-by-step processing is as follows:
1. The SIP INVITE request is sent from the UE #1 to S‑CSCF#1 by the procedures of the originating flow.
2. S‑CSCF#1 performs whatever service control logic is appropriate for this session setup attempt.
3. S‑CSCF#1 performs an analysis of the destination address and determines the network operator to whom the subscriber belongs. The INVITE message is sent to an I‑CSCF for that operator.
4. I‑CSCF queries the HSS for current location information of the destination user.
5. HSS responds with the address of the current Serving CSCF (S‑CSCF#2) for the terminating user.
6. I‑CSCF forwards the INVITE request to S‑CSCF#2, who will handle the session termination.
7. S‑CSCF#2 invokes whatever service logic is appropriate for this session setup attempt. As a result of this service control logic, S‑CSCF#2 determines that the session should be redirected to a PSTN termination. S‑CSCF#2 determines that it wishes to remain in the path of the SIP signalling.
8. S‑CSCF#2 forwards the INVITE using the Serving to Serving procedures S-S#3 or S-S#4. The PSTN terminating flows are then followed.
9-12. The destination responds with the SDP message and the session establishment proceeds normally.
5.11.5.2a Session Redirection to PSTN Termination (REDIRECT to originating UE#1)
The S‑CSCF of the destination user (S‑CSCF#2) may determine that the session is to be redirected to a PSTN Termination; e.g. CS-domain endpoint, or to the PSTN. For session redirection to PSTN termination where the S‑CSCF of the called party (S‑CSCF#2) wishes to use the SIP REDIRECT method, the S‑CSCF#2 will pass the new destination information (the PSTN Termination information) to the originator. The originator can then initiate a new session to the redirected to destination denoted by S‑CSCF#2. The originator may be a UE as shown in the example flow in figure 5.37a, or it may be any other type of originating entity as defined in clause 5.4a. The endpoint to which the session is redirected may be the PSTN as shown in figure 5.37a, or it may be any other type of terminating entity as defined in clause 5.4a. The originator may alternately receive a redirect from a non-IMS network SIP client. Only the scenario in which a call from a UE is redirected by S‑CSCF service logic to a PSTN endpoint is shown.
Handling of redirection to a PSTN Termination where the S‑CSCF#2 REDIRECTS to the originating UE#1 is shown in the figure 5.37a:
Figure 5.37a: Session redirection to PSTN Termination (REDIRECT to originating UE#1)
Step-by-step processing is as follows:
1. The SIP INVITE request is sent from the UE#1 to S‑CSCF#1 by the procedures of the originating flow.
2. S‑CSCF#1 invokes whatever service logic is appropriate for this session setup attempt.
3. S‑CSCF#1 performs an analysis of the destination address and determines the network operator to whom the subscriber belongs. The INVITE message is sent to an I‑CSCF for that operator.
4. I‑CSCF queries the HSS for current location information of the destination user.
5. HSS responds with the address of the current Serving CSCF (S‑CSCF#2) for the terminating user.
6. I‑CSCF forwards the INVITE request to S‑CSCF#2, who will handle the session termination.
7. S‑CSCF#2 invokes whatever service logic is appropriate for this session setup attempt. As a result of this service control logic, S‑CSCF#2 determines that the session should be redirected to a PSTN termination.
S‑CSCF#2 determines that it wishes to use the SIP REDIRECT method to pass the redirection destination information (the 'redirected-to PSTN Termination' information) to the originator (UE#1).
8. S‑CSCF#2 sends a SIP Redirect response to I‑CSCF with the redirection destination.
9. I‑CSCF sends a Redirect response to S‑CSCF#1, containing the redirection destination.
10. S‑CSCF#2 forwards the Redirect response to UE#1, containing the redirection destination
11. UE#1 initiates a session to the 'redirected-to PSTN Termination' according to the mobile origination procedures supported in the UE (e.g. CS, IMS).
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5.11.5.3 Session Redirection initiated by S‑CSCF to general endpoint (REDIRECT to originating UE#1)
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The S‑CSCF in the scenario above may determine that the session is to be redirected to an endpoint outside the IP MultiMedia System and outside the CS-domain. Examples of these destinations include web pages, email addresses, etc. It recognizes this situation by the redirected URI being other than a sip: URI or tel: URL.
In cases when the destination subscriber is not currently registered in the IM CN subsystem, the I‑CSCF may assign a temporary S‑CSCF to invoke the service logic on behalf of the intended destination. This temporary S‑CSCF takes the role of S‑CSCF#2 in the following information flow. For session redirection to a general endpoint where the S‑CSCF of the called party (S‑CSCF#2) wishes to use the SIP REDIRECT method, the S‑CSCF#2 will pass the new destination information to the originator. As a result the originator should initiate a new session to the redirected-to destination provided by S‑CSCF#2. The originator may be a UE as shown in the example flow in figure 5.38, an Application Server or a non-IMS network SIP client. The originator may also receive a redirect from a non-IMS network SIP client. Only the scenario in which the originating UE receives a redirect based on S‑CSCF service logic is shown.
Handling of redirection to a general URI is shown in the following information flow:
Figure 5.38: Session redirection initiated by S‑CSCF to general endpoint
Step-by-step processing is as follows:
1. The SIP INVITE request is sent from the UE to S‑CSCF#1 by the procedures of the originating flow.
2. S‑CSCF#1 invokes whatever service logic is appropriate for this session setup attempt.
3. S‑CSCF#1 performs an analysis of the destination address and determines the network operator to whom the subscriber belongs. The INVITE message is sent to an I‑CSCF for that operator.
4. I‑CSCF queries the HSS for current location information of the destination user.
5. HSS responds with the address of the current Serving CSCF (S‑CSCF#2) for the terminating user.
6. I‑CSCF forwards the INVITE request to S‑CSCF#2, who will handle the session termination.
7. S‑CSCF#2 invokes whatever service logic is appropriate for this session setup attempt. As a result of this service control logic, S‑CSCF#2 determines that the session should be redirected to a new destination URI outside the IMS and outside the CS domain, i.e. other than a sip: URI or tel: URL.
8. S‑CSCF#2 sends a SIP Redirect response back to I‑CSCF, with redirection destination being the general URI.
9. I‑CSCF sends a Redirect response back to S‑CSCF#1, containing the redirection destination.
10. S‑CSCF#1 forwards the Redirect response back to UE#1.
11. UE#1 initiates the session to the indicated destination.
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5.11.5.4 Session Redirection initiated by P‑CSCF
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One of the functional elements in a basic session flow that may initiate a redirection is the P‑CSCF of the destination user. In handling of an incoming session setup attempt, the P‑CSCF normally sends the INVITE request to the destination UE and retransmits it as necessary until obtaining an acknowledgement indicating reception by the UE.
In cases when the destination user is not currently reachable in the IM CN subsystem (due to such factors as roaming outside the service area or loss of battery, but the registration has not yet expired), the P‑CSCF may initiate a redirection of the session. The P‑CSCF informs the S‑CSCF of this redirection, without specifying the new location; S‑CSCF determines the new destination and performs according to clauses 5.11.5.1, 5.11.5.2, or 5.11.5.3 above, based on the type of destination.
This is shown in the following information flow:
Figure 5.39: Session redirection initiated by P‑CSCF
Step-by-step processing is as follows:
1. The SIP INVITE request is sent from the UE to S‑CSCF#1 by the procedures of the originating flow.
2. S‑CSCF#1 invokes whatever service logic is appropriate for this session setup attempt.
3. S‑CSCF#1 performs an analysis of the destination address and determines the network operator to whom the subscriber belongs. The INVITE message is sent to an I‑CSCF for that operator.
4. I‑CSCF queries the HSS for current location information of the destination user.
5. HSS responds with the address of the current Serving CSCF (S‑CSCF#2) for the terminating user.
6. I‑CSCF forwards the INVITE request to S‑CSCF#2, who will handle the session termination.
7. S‑CSCF#2 invokes whatever service logic is appropriate for this session setup attempt.
8. S‑CSCF#2 forwards the INVITE request to P‑CSCF#2.
9. P‑CSCF#2 forwards the INVITE request to UE#2.
10. Timeout expires in P‑CSCF waiting for a response from UE#2. P‑CSCF therefore assumes UE#2 is unreachable.
11. P‑CSCF#2 generates an Unavailable response, without including a new destination and sends the message to S‑CSCF#2.
12. S‑CSCF#2 invokes whatever service logic is appropriate for this session redirection. If the user does not subscribe to session redirection service, or did not supply a forwarding destination, S‑CSCF#2 may terminate the session setup attempt with a failure response. Otherwise, S‑CSCF#2 supplies a new destination URI, which may be a phone number, an email address, a web page, or anything else that can be expressed as a URI. Processing continues according to clauses 5.11.5.1, 5.11.5.2, or 5.11.5.3 above, based on the type of destination URI.
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5.11.5.5 Session Redirection initiated by UE
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The next functional element in a basic session flow that may initiate a redirection is the UE of the destination user. The UE may implement customer-specific feature processing and base its decision to redirect this session on such things as identity of caller, current sessions in progress, other applications currently being accessed, etc. UE sends the SIP Redirect response to its P‑CSCF, who forwards back along the signalling path to S‑CSCF#1, who initiates a session to the new destination.
The service implemented by this information flow is typically "Session Forward Busy", "Session Forward Variable" or "Selective Session Forwarding".
This is shown in the following information flow:
Figure 5.40: Session redirection initiated by UE
Step-by-step processing is as follows:
1. The SIP INVITE request is sent from the UE to S‑CSCF#1 by the procedures of the originating flow.
2. S‑CSCF#1 invokes whatever service logic is appropriate for this session setup attempt.
3. S‑CSCF#1 performs an analysis of the destination address and determines the network operator to whom the subscriber belongs. The INVITE message is sent to an I‑CSCF for that operator.
4. I‑CSCF queries the HSS for current location information of the destination user.
5. HSS responds with the address of the current Serving CSCF (S‑CSCF#2) for the terminating user.
6. I‑CSCF forwards the INVITE request to S‑CSCF#2, who will handle the session termination.
7. S‑CSCF#2 invokes whatever service logic is appropriate for this session setup attempt.
8. S‑CSCF#2 forwards the INVITE request to P‑CSCF#2.
9. P‑CSCF#2 forwards the INVITE request to UE#2.
10. UE#2 determines that this session should be redirected and optionally supplies the new destination URI. This new destination URI may be a phone number, an email address, a web page, or anything else that can be expressed as a URI. The Redirect response is sent to P‑CSCF#2.
11. P‑CSCF#2 forwards the Redirect response to S‑CSCF#2.
12. S‑CSCF#2 invokes whatever service logic is appropriate for this session redirection. If UE#2 does not subscribe to session redirection service, or did not supply a new destination URI, S‑CSCF#2 may supply one or may terminate the session setup attempt with a failure response. The new destination URI may be a phone number, an email address, a web page, or anything else that can be expressed as a URI. The procedures of clause 5.11.5.1, 5.11.5.2, or 5.11.5.3 given above are followed, based on the type of URI.
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5.11.5.6 Session Redirection initiated by originating UE#1 after Bearer Establishment (REDIRECT to originating UE#1)
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The UE of the destination user may request the session be redirected after a customer-specified ringing interval. The UE may also implement customer-specific feature processing and base its decision to redirect this session on such things as identity of caller, current sessions in progress, other applications currently being accessed, etc. UE sends the SIP Redirect response to its P‑CSCF, who forwards back along the signalling path to the originating endpoint, who initiates a session to the new destination.
The service implemented by this information flow is typically "Session Forward No Answer".
The originating end point may be a UE as shown in the example flow in figure 5.41 or it may be any other type of originating entity as defined in clause 5.4a. Redirect to another IMS endpoint (e.g. a sip: URI) is shown in the figure. The redirecting endpoint may be a UE as shown or an Application Server or a non-IMS network SIP client. Further, the endpoint to which the session is redirected may be a UE as shown in figure 5.41, or it may be any other type of terminating entity as defined in clause 5.4a. Only the scenario in which a call from the first UE is redirected by a second UE to a third UE is shown.
The flow presented here assumes that Policy and Charging Control is in use.
Figure 5.41: Session redirection after bearer establishment
Step-by-step processing is as follows:
1-10. Normal handling of a basic session establishment, up through establishment of the bearer channel and alerting of the destination user or by a previous session redirection after bearer establishment procedure.
11. Based on a timeout or other indications, UE#2 decides the current session should be redirected to a new destination URI. This new destination URI may be a phone number, an email address, a web page, or anything else that can be expressed as a URI. The Redirect response is sent to P‑CSCF#2.
12. P‑CSCF#2 shall revoke any authorization for QoS for the current session.
13. P‑CSCF#2 forwards the Redirect response to S‑CSCF#2.
14. S‑CSCF#2 invokes whatever service logic is appropriate for this session redirection. If UE#2 does not subscribe to session redirection service, or did not supply a new destination URI, S‑CSCF#2 service logic may supply one or may terminate the session setup attempt with a failure response. The new destination URI may be a phone number, an email address, a web page, or anything else that can be expressed as a URI. If S‑CSCF#2 service logic requires that it remain on the path for the redirected request, the service logic generates a private URI, addressed to itself, as the new destination.
15. S‑CSCF#2 sends a SIP Redirect response back to I‑CSCF, containing the new destination URI.
16. I‑CSCF sends a Redirect response back to S‑CSCF#1, containing the new destination.
17. S‑CSCF#1 service logic may check the number of redirections that have occurred for this session setup attempt and if excessive, abort the session. If S‑CSCF#1 service logic requires that UE#1 not know the new destination URI, the service logic stores the new destination information, generates a private URI addressed to itself pointing to the stored information and generates a modified Redirect response with the private URI.
18. S‑CSCF#1 sends the Redirect response to P‑CSCF#1.
19. P‑CSCF#1 revokes any authorization for QoS for the current session and sends the Redirect response to UE#1.
20. UE#1 initiates a new INVITE request to the address provided in the Redirect response. The new INVITE request is sent to P‑CSCF#1.
21. P‑CSCF#1 forwards the INVITE request to S‑CSCF#1.
22. S‑CSCF#1 invokes whatever service logic is appropriate for this new session setup attempt. The service logic may retrieve destination information if saved in step #17.
23. S‑CSCF#1 determines the network operator of the new destination address. If the service logic in step #14 did not provide its private URI as a new destination, the procedure continues with step #26, bypassing steps #24 and #25. If the service logic in step #14 did provide a private URI as a new destination, the INVITE message is sent to I‑CSCF#2, the I‑CSCF for S‑CSCF#2.
24. I‑CSCF forwards the INVITE to S‑CSCF#2.
25. S‑CSCF#2 decodes the private URI, determines the network operator of the new destination and sends the INVITE request to the I‑CSCF for that network operator.
26-30. The remainder of this session completes as normal.
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5.11.6 Session Transfer Procedures
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5.11.6.0 General
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This clause gives information flows for the procedures for performing session transfers. This is presented in two steps: first a basic primitive that can be used by endpoints to cause a multi-media session to be transferred and second the procedures by which this primitive can be used to implement some well-known session-transfer services.
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5.11.6.1 Refer operation
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The refer primitive is an information flow indicating a "Refer" operation, which includes a component element "Refer-To" and a component element "Referred-By". The end point receiving a referral may be UE#1 as shown in the example flow in figure 5.42 or it may be any other type of originating entity as defined in clause 5.4a. The referring endpoint may be either UE#2 as shown, an Application Server or a non-IMS network SIP client. The referred-to destination may be UE#F as shown in figure 5.42 or it may be any other type of terminating entity as defined in clause 5.4a. Only the scenario in which a call from the first UE is referred by a second UE to a third UE is shown.
An information flow illustrating this is as follows:
Figure 5.42: Refer operation
Step-by-step description of the information flow:
1. A multi-media session is assumed to already exist between UE#1 and UE#2, established either as a basic session or by one of the supplemental services described in this clause.
2. UE#2 sends the Refer command to P‑CSCF#2, containing "Refer-To" UE#F and "Referred-By" UE#2. If UE#2 knows the GRUU of UE#F and desires to reach a particular instance of UE#F, the "Refer-To" contains the GRUU of UE#F otherwise the "Refer-To" contains the Public User Identity of UE#F.
3. P‑CSCF#2 forwards the message to S‑CSCF#2.
4. S‑CSCF#2 invokes whatever service logic is appropriate for this request. If UE#2 does not subscribe to a transfer service, service logic may reject the request. If S‑CSCF#2 service logic requires that it remain on the path for the subsequent request, the service logic generates a private URI, addressed to itself, the "Refer-To" value in the request with the private URI.
5. S‑CSCF#2 forwards the message to S‑CSCF#1.
6. S‑CSCF#1 invokes whatever service logic is appropriate for this request. To hide the identities of UE#2 and UE#F, S‑CSCF#1 service logic stores the "Refer-To" and "Referred-By" information and replaces them with private URIs.
7. S‑CSCF#1 forwards the message to P‑CSCF#1.
8. P‑CSCF#1 forwards the message to UE#1.
9. UE#1 initiates a new multi-media session to the destination given by the "Refer-To", which may either be a URI for UE#F, a private URI pointing to S‑CSCF#2, or a private URI pointing to S‑CSCF#1.
10. P‑CSCF#1 forwards the INVITE request to S‑CSCF#1.
11. S‑CSCF#1 retrieves the destination information for the new session and invokes whatever service logic is appropriate for this new session.
12. S‑CSCF#1 determines the network operator addressed by the destination URI and forwards the INVITE to either S‑CSCF#F or S‑CSCF#2 (actually I‑CSCF#F or I‑CSCF#2, the public entry points for S‑CSCF#F and S‑CSCF#2, respectively). If S‑CSCF#1 forwards the INVITE to S‑CSCF#F, the procedure continues with step #14, bypassing step #13.
13. S‑CSCF#2 decodes the private URI destination and determines the final destination of the new session. It determines the network operator addressed by the destination URI. The request is then forwarded onward to S‑CSCF#F as in a normal session establishment.
14. S‑CSCF#F invokes whatever service logic is appropriate for this new session and forwards the request to P‑CSCF#F.
15. P‑CSCF#F forwards the request to UE#F.
16-21. The normal session establishment continues through bearer establishment, optional alerting and reaches the point when the new session is accepted by UE#F. UE#F then sends the 200-OK final response to P‑CSCF#F, which is forwarded through S‑CSCF#F, S‑CSCF#2 (optionally), S‑CSCF#1, P‑CSCF#1, to UE#1. At this point a new session is successfully established between UE#1 and UE#F.
22-26. The Refer request was successful and UE#1 sends a 200-OK final response to UE#2. This response is sent through P‑CSCF#1, S‑CSCF#1, S‑CSCF#2, P‑CSCF#2 and to UE#2.
27-31. UE#2 clears the original session with UE#1 by sending the BYE message. This message is routed through P‑CSCF#2, S‑CSCF#2, S‑CSCF#1, P‑CSCF#1, to UE#1.
32-36. UE#1 acknowledges the BYE and terminates the original session. It responds with the 200-OK response, routed through P‑CSCF#1, S‑CSCF#1, S‑CSCF#2, P‑CSCF#2, to UE#2.
NOTE: The last BYE message to clear the original session can be issued either by UE#1 or by UE#2.
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5.11.6.2 Application to Session Transfer Services
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5.11.6.2.0 General
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This clause shows how the Refer primitive given above can be used to provide common session-transfer services.
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5.11.6.2.1 Blind Transfer and Assured Transfer
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A Blind Transfer starts with an existing session, established between the Initiator (I) and the Recipient (R). In a typical case, this session was actually initiated by R. In the end it is desired that the Recipient has a session with the Target (T).
From the starting configuration, shown in the leftmost diagram, I sends a Refer message to R, who then initiates a session with the Target (T), as shown in the middle diagram. Immediately after sending the Refer message to R, I issues the BYE message to terminate its connection with R. The end configuration is shown in the rightmost diagram.
An Assured Transfer is identical to the above, except that I waits until the Refer successfully completes before issuing the BYE message to terminate its connection with R. If the new session from R to T were to fail, R would still have a session with I.
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5.11.6.2.2 Consultative Transfer
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A Consultative Transfer again starts with an existing session, established from the Initiator (I) to the Recipient (R). The Initiator first consults with the Target (T), then decides to transfer the original session to T.
From the starting configuration, as shown in the leftmost diagram in the previous clause, I places the session with R on hold and establishes a new session with T. This is shown in the leftmost diagram below. I then sends a Refer message to T, causing T to establish a session with R. This is shown in the second diagram. When the Refer operation completes, I clears its two active sessions, first with R (leaving the configuration as shown in the third diagram) then with T. The end configuration is shown in the rightmost diagram.
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5.11.6.2.3 Three-way Session
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A three-way session starts with an existing session, between the Initiator (I) and party (A). The initiator places this session on hold and establishes a second session with party (B). The initiator then decides to create an ad-hoc conference of all three parties.
From the point where the initiator decides to create the ad-hoc conference, shown in the leftmost diagram below, the initiator establishes another session with a third-party conference bridge service. This is shown in the centre diagram. The initiator then transfers both of the existing sessions, I->A and I->B, to the bridge, ending in the configuration shown in the rightmost diagram.
The conference bridge service is in control of the termination sequence. On termination of one of the three sessions, it may either terminate the other two sessions by use of the session clearing procedures of clause 5.11, or may utilize the procedures of clause 5.11.6.2.1 above to transfer one of the remaining endpoints to the other, resulting in a simple two-party session.
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5.12 Mobile Terminating call procedures to unregistered Public User Identities
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5.12.0 General
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This clause describes information flows for the procedures of Mobile Terminating call flows for unregistered IMS Public User Identities. The detection of an unregistered Public User Identity is done in HSS and if this Public User Identity has services related to unregistered state, a S‑CSCF is selected for the unregistered Public User Identity. S‑CSCF performs whatever further actions are appropriate for the call attempt to the unregistered IMS Public User Identity.
Two basic examples for "services related to unregistered" are call redirection to CS domain and voice mailbox service. Call redirection to CS domain is supported to cover the cases when the UE is not registered in IMS but can be reached via the CS domain. Then, a temporary S‑CSCF is selected and performs whatever further actions are appropriate for the call attempt.
The principle established in clause 4.3.3.4, where the Public User Identities for the same profile are allocated to the same S‑CSCF, is followed.
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5.12.1 Mobile Terminating call procedures to unregistered Public User Identity that has services related to unregistered state
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In Figure 5.43 below the Public User Identity is unregistered for IMS and the Public User Identity has services related to unregistered state. In this case, the HSS responds back to I‑CSCF with an indication that I‑CSCF should select S‑CSCF for this MT call to the unregistered Public User Identity of the user or provide the I‑CSCF with the previously allocated S‑CSCF name. Before S‑CSCF selection, I‑CSCF shall query HSS for the information related to the required S‑CSCF capabilities. I‑CSCF selects a S‑CSCF to invoke service logic and I‑CSCF routes the call further to the selected destination. If the S‑CSCF does not have the relevant information from the user profile then the S‑CSCF shall download the relevant information from HSS before it invokes service logic and any further actions in the call attempt. The service implemented by this information flow could be e.g. "Call Forward Unconditional".
This is shown by the information flow in Figure 5.43:
Figure 5.43: Mobile Terminating call procedures to unregistered IMS Public User Identity that has services related to unregistered state
1. I‑CSCF receives an INVITE message.
2. I‑CSCF queries the HSS for current location information.
3. HSS either responds with the required S‑CSCF capabilities which I‑CSCF should use as an input to select a S‑CSCF for the unregistered Public User Identity of the user or provides the I‑CSCF with the previously allocated S‑CSCF name for that user.
4. If the I‑CSCF has not been provided with the location of the S‑CSCF, the I‑CSCF selects an S‑CSCF for the unregistered Public User Identity of the user.
5. I‑CSCF forwards the INVITE request to the S‑CSCF.
6. The S‑CSCF sends Cx-Put/Cx-Pull (Public User Identity, S‑CSCF name) to the HSS. When multiple and separately addressable HSSs have been deployed by the network operator, then the S‑CSCF needs to query the SLF to resolve the HSS. The HSS stores the S‑CSCF name for unregistered Public User Identities of that user. This will result in all terminating traffic for unregistered Public User Identities of that user being routed to this particular S‑CSCF until the registration period expires or the user attaches the Public User Identity to the network. Note: Optionally the S‑CSCF can omit the Cx-Put/Cx-Pull request if it has the relevant information from the user profile.
7. The HSS shall stores the S‑CSCF name for that user and return the information flow Cx-Put Resp/Cx-Pull Resp (user information) to the S‑CSCF. The S‑CSCF shall store it for that indicated Public User Identity.
8. S‑CSCF invokes whatever service logic is appropriate for this call attempt.
9. S‑CSCF performs whatever further actions are appropriate for this call attempt (in the case where the S‑CSCF decides to redirect the session towards CS domain, the Mobile Termination Procedure MT#3 (clause 5.7.2a) applies).
The S‑CSCF may deregister the Public User Identity at any time (e.g. according to operator network engineering requirements) by issuing a Cx-Put2 (Public User Identity, clear S‑CSCF name) clearing the S‑CSCF name stored in the HSS. If S‑CSCF name stored by the HSS does not match the name of the S‑CSCF that originated the Cx-Put2 then the HSS will acknowledge the clearing request but take no further action.
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5.12.2 Mobile Terminating call procedures to unregistered Public User Identity that has no services related to unregistered state
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In the example information flow the Public User Identity of the user is unregistered and the Public User Identity has no services related to unregistered state.
This is shown in the following information flow (figure 5.44):
Figure 5.44: Mobile Terminating call procedures to unregistered Public User Identity that has no services related to unregistered state
1. I‑CSCF receives an INVITE message.
2. I‑CSCF queries the HSS for current location information.
3. HSS responds with an indication that the Public User Identity is unregistered, but no services are related to unregistered state.
4. I‑CSCF responds to the origin of the request that the user is not reachable at the moment.
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5.13 IMS Emergency Sessions
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Emergency sessions via IMS are specified in TS 23.167 [58].
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5.14 Interactions involving the MRFC/MRFP
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5.14.0 General
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The MRFC/MRFP are resources of the IMS that provide support for bearer related services such as for example multi-party sessions, announcements to a user or bearer transcoding. This clause describes how the resources of the MRFC/MRFP are used.
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5.14.1 Interactions between the UE and the MRFC
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In some cases an operator may wish to make an MRFC available directly to a UE, for example to support ad-hoc multi-party sessions to be initiated by the UE. In this case, the operator advertises the name of one or more MRFCs and a UE will invite an MRFC to a session. The session invitation would need to contain additional information indicating the specific capabilities (e.g. multi-party) desired. A conference ID would be assigned by the MRFC and returned to the UE. This would then be used by the UE in subsequent interactions with the MRFC and other UEs participating in the session.
There are two approaches to invite new participants to the multiparty session. In the first, a UE directs other UEs to join the multiparty session based on the use of the SIP REFER method. This allows session invitations with consultation. In the second method, the MRFC uses information received from a UE e.g. within a list of session participants to invite other UEs to the multiparty session. This allows session invitations without consultation.
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5.14.2 Service control based interactions between the MRFC and the AS
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The MRFC/MRFP resources may also be used, based on service control in an IMS, for services such as multiparty sessions, announcements or transcoding. In this case an Application Server interacts with an MRFC. Session control messages are exchanged between the AS and the MRFC.
There are two approaches for the AS to control the sessions. In the first, the AS uses 3rd party call control. The second approach uses the SIP REFER method.
In either case, the appropriate service in the AS would be triggered by a UE initiated SIP message containing information indicating the specific capabilities desired. This session invitation would also carry additional information indicating the specific capabilities (e.g. multi-party). A conference ID would be assigned by the MRFC and would be used by the AS in subsequent interactions with the MRFC in INVITE messages connecting other endpoints.
3rd party call control can also be used to invoke announcement and transcoding services. That is, the AS will send an INVITE to the MRFC with an indication of the capability being requested and with additional information related to the specific service such as identification of the announcement to be played or identification of the specific transcoding requirements.
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5.14.3 Interactions for services using both the Ut interface and MRFC capabilities
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Network services hosted on an AS and configurable by the user via the Ut interface may also use the capabilities provided by the MRFC. For this case, the AS either supports MRFC capabilities, or communicates with an MRFC.
Communications across the Ut interface between the UE and the AS allow the UE to securely manage and configure data for such services (e.g. conference type services). Means for the AS to propagate this management and configuration information to the MRFC is not standardized in this Release.
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5.14.4 Transcoding services involving the MRFC/MRFP
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Network services involving MRFC and MRFP are not limited to conferencing and announcements, but also involve transcoding support for interworking between IMSs or inter-domain sessions and intra-domain sessions between access technologies supported in an IMS (e.g. wireline wireless interworking, or interworking with non-3GPP wireless technologies).
The MRFC and MRFP act as transcoding entity in an IMS solving media encoding mismatches due to codec selection between operator networks, as well as to deal with encoding formats in a converged service environment. Service requests sent to the MRFC shall contain sufficient information to associate the systems that require media transcoding and also for reservation of resources required at the MRFP. The MRFC shall always grant the requests from the control plane, unless there is a lack of resources. Media transcoding support based on MRFC/MRFP shall support the offer/answer procedure as defined in IETF RFC 3264 [72].
Additional description of transcoding support involving the MRFC/MRFP is provided in Annex P.
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5.15 Mobile Terminating session procedure for unknown user
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5.15.0 General
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This clause describes information flows Mobile Terminating procedure for an unknown user. The unknown user cases include those where session requests are made towards Public User Identities that are incorrect, un-issued or have been cancelled/deleted. The determination of unknown user is carried out in the HSS and/or the SLF (for networks that require SLF functionality). The information flows of figures 5.45 and 5.46 illustrate how SIP messages can be used to inform the requesting party that the requested user is not known within the network.
In the case where the destination Public User Identity is an E.164 number in the SIP URI with user=phone parameter format, the I‑CSCF shall first translate it into the Tel: URI format per IETF RFC 3966 [15] prior to sending to the HSS a Cx_LocQuery (or to the SLF a DX_SLF_QUERY). If a failure occurs under these circumstances, the Mobile Terminating user is not an IMS user of this network. In this case, the I‑CSCF may invoke the portion of transit functionality that translates the E.164 address contained in the Request‑URI of the Tel: URI format to a routable SIP URI, or BGCF for further routing as described in clause 5.19.
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5.15.1 Unknown user determined in the HSS.
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In Figure 5.45 the unknown status of the requested party is determined in the HSS. The I‑CSCF requests information on the user to be reached and the HSS responds back to the I‑CSCF with an indication that the user is unknown. The I‑CSCF uses the indication that the user is unknown returned from the HSS to formulate the correct SIP message back towards the originating party to inform them that the user is unknown. The case where the SLF determines unknown status is in clause 5.15.2. The flows of figure 5.45 could include SLF determination of the HSS, however these are not shown for clarity.
Figure 5.45: HSS determination of unknown user
1) I‑CSCF receives an INVITE.
2) I‑CSCF queries the HSS for current location information.
3) HSS responds with an indication that the user is unknown.
4) The I‑CSCF responds to the origin of the request that the user is unknown.
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5.15.2 Unknown user determined in the SLF
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In Figure 5.46 the unknown status of the requested party is determined in the SLF. The I‑CSCF requests information on the user to be reached and the SLF responds back to the I‑CSCF with an indication that the user is unknown. The I‑CSCF uses the indication that the user is unknown returned from the SLF to formulate the correct SIP message back towards the originating party to inform them that the user is unknown.
Figure 5.46: SLF determination of unknown user
1) The ICSCF receives an INVITE request and now has to query for the location of the user's subscription data.
2) The I‑CSCF sends a DX_SLF_QUERY to the SLF and includes as parameter the user identity which is stated in the INVITE request.
3) The SLF looks up its database for the queried user identity.
4) The SLF answers with an indication that the user is unknown.
5) The I‑CSCF responds to the origin of the request that the user is unknown.
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5.16 IMS messaging concepts and procedures
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5.16.0 General
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This clause describes architectural concepts and procedures for providing Messaging in the IM CN Subsystem. The service enablers for Messaging and possible reuse of IMS service enablers within this context as well security and charging expectations, addressing, privacy, content handling and limitations, filtering, media types and message lengths, etc. are to be further studied.
Any ISIM or, for UEs supporting only non-3GPP accesses and containing IMC, any IMC related architectural requirements would be studied as part of overall IMS Messaging.
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5.16.1 Immediate Messaging
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5.16.1.0 General
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This clause describes architectural concepts and procedures for fulfilling the requirements for Immediate Messaging described in TS 22.340 [29a].
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5.16.1.1 Procedures to enable Immediate Messaging
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5.16.1.1.0 General
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IMS users shall be able to exchange immediate messages with each other by using the procedure described in this clause. This procedure shall allow the exchange of any type of multimedia content (subject to possible restrictions based on operator policy and user preferences/intent), for example but not limited to:
- Pictures, video clips, sound clips with a format defined in the respective access specific annex.
If the message size exceeds the size limit for MESSAGE requests, the UE shall use alternative means to deliver the content of the Immediate Message. Session based messaging specified in clause 5.16.2 provides such means.
IETF RFC 3428 [43] presents guidelines for the selection of transport mechanism for an Immediate Message. The message size limitations described above are meant to be applicable for Immediate Messages sent over end-to-end congestion safe transport, i.e. are not necessarily equal to the limitations specified for MESSAGE over congestion-unsafe transport by IETF RFC 3428 [43].
NOTE: The actual size limit is part of stage-3 design.
If the size limit for a terminating MESSAGE request is exceeded, the network may refuse the request or respond to the sender with an indication that the size of the message is too large.
The sender UE can include an indication in the message regarding the length of time the message will be considered valid.
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