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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.10.0 General
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.10.1 Terminal initiated session release
| 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.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.10.2 PSTN initiated session release
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.10.3 Network initiated session release
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.10.3.0 Removal of IP‑CAN bearer used to transport IMS SIP signalling
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.10.3.1 Network initiated session release - P‑CSCF initiated
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.10.3.1.0 General
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.10.3.1.2 Void
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.10.3.2 Network initiated session release - S‑CSCF Initiated
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11 Procedures to enable enhanced multimedia services
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.1 Session Hold and Resume Procedures
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.1.0 General
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.1.1 Mobile-to-Mobile Session Hold and Resume Procedures
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.1.2 Mobile-initiated Hold and Resume of a Mobile-PSTN Session
| 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.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.1.3 PSTN-initiated Hold and Resume of a Mobile-PSTN Session
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.2 Procedures for anonymous session establishment
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.2.0 General
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.2.1 Signalling requirements for anonymous session establishment
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.2.2 Bearer path requirements for anonymous session establishment
| Procedures for establishment of an anonymous bearer path are not standardised in this release.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.3 Procedures for codec and media characteristics flow negotiations
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.3.0 General
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.3.1 Codec and media characteristics flow negotiation during initial session establishment
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.3.2 Codec or media characteristics flow change within the existing reservation
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.3.3 Codec or media characteristics flow change requiring new resources and/or authorization
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.3.4 Sample MM session flow - addition of another media
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.4 Procedures for providing or blocking identity
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.4.0 General
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.4.1 Procedures for providing the authenticated identity of the originating party
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.4.2 Procedures for blocking the identity of the originating party
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.4.3 Procedures for providing the authenticated identity of the originating party (PSTN origination)
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.4.4 Procedures for providing the authenticated identity of the originating party (PSTN termination)
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.5 Session Redirection Procedures
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.5.0 General
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.5.1 Session Redirection initiated by S‑CSCF to IMS
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.5.2 Session Redirection to PSTN Termination (S‑CSCF #2 forwards INVITE)
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.5.3 Session Redirection initiated by S‑CSCF to general endpoint (REDIRECT to originating UE#1)
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.5.4 Session Redirection initiated by P‑CSCF
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.5.5 Session Redirection initiated by UE
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.5.6 Session Redirection initiated by originating UE#1 after Bearer Establishment (REDIRECT to originating UE#1)
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.6 Session Transfer Procedures
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.6.0 General
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.6.1 Refer operation
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.6.2 Application to Session Transfer Services
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.6.2.0 General
| This clause shows how the Refer primitive given above can be used to provide common session-transfer services.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.6.2.1 Blind Transfer and Assured Transfer
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.6.2.2 Consultative Transfer
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.11.6.2.3 Three-way Session
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.12 Mobile Terminating call procedures to unregistered Public User Identities
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.12.0 General
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.12.1 Mobile Terminating call procedures to unregistered Public User Identity that has services related to unregistered state
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.12.2 Mobile Terminating call procedures to unregistered Public User Identity that has no services related to unregistered state
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.13 IMS Emergency Sessions
| Emergency sessions via IMS are specified in TS 23.167 [58].
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.14 Interactions involving the MRFC/MRFP
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.14.0 General
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.14.1 Interactions between the UE and the MRFC
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.14.2 Service control based interactions between the MRFC and the AS
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.14.3 Interactions for services using both the Ut interface and MRFC capabilities
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.14.4 Transcoding services involving the MRFC/MRFP
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.15 Mobile Terminating session procedure for unknown user
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.15.0 General
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.15.1 Unknown user determined in the HSS.
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.15.2 Unknown user determined in the SLF
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16 IMS messaging concepts and procedures
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.0 General
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.1 Immediate Messaging
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.1.0 General
| This clause describes architectural concepts and procedures for fulfilling the requirements for Immediate Messaging described in TS 22.340 [29a].
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.1.1 Procedures to enable Immediate Messaging
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.1.1.0 General
| 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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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.1.1.1 Immediate messaging procedure to registered Public User Identity
|
Figure 5.47: Immediate Messaging procedure to registered Public User Identity
1. UE#1 generates the multimedia content intended to be sent to UE#2.
2. UE#1 sends the MESSAGE request to P‑CSCF#1 that includes the multimedia content in the message body.
3. P‑CSCF#1 forwards the MESSAGE request to S‑CSCF#1 along the path determined upon UE#1's most recent registration procedure. Before forwarding, if P-CSCF#1 has received the MPS for Messaging indication for the UE as set (enabled), P-CSCF#1 sets the Resource-Priority information in the MESSAGE to a value appropriate for MPS.
4. Based on operator policy S‑CSCF#1 may reject the MESSAGE request with an appropriate response, e.g. if content length or content type of the MESSAGE are not acceptable. S‑CSCF#1 invokes whatever service control logic is appropriate for this MESSAGE request. This may include routing the MESSAGE request to an Application Server, which processes the request further on.
5. S-CSC#1 forwards the MESSAGE 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 MESSAGE request to S‑CSCF#2.
8. Based on operator policy S‑CSCF#2 may reject the MESSAGE request with an appropriate response, e.g. if content length or content type of the MESSAGE are not acceptable. S‑CSCF#2 invokes whatever service control logic is appropriate for this MESSAGE request. This may include routing the MESSAGE request to an Application Server, which processes the request further on.
For example, the UE#2 may have a service activated that blocks the delivery of incoming messages that fulfil criteria set by the user. The AS may then respond to the MESSAGE request with an appropriate error response.
9. S‑CSCF#2 forwards the MESSAGE request to P‑CSCF#2 along the path determined upon UE#2's most recent registration procedure.
10. P‑CSCF#2 forwards the MESSAGE request to UE#2. After receiving the MESSAGE UE#2 renders the multimedia content to the user.
11–16. UE#2 acknowledges the MESSAGE request with a response that indicates that the destination entity has received the MESSAGE request. The response traverses the transaction path back to UE#1.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.1.1.2 Immediate messaging procedure to unregistered Public User Identity
|
Figure 5.48: Immediate messaging to unregistered Public User Identity, service control invoked
1-5. The same actions apply as for when the Public User Identity is registered, see step 1-5 in clause 5.16.1.1.1.
6. I‑CSCF#2 interacts with the HSS as per the terminating procedures defined for unregistered Public User Identities in clause 5.12.1. If the Public User Identity has no services related to unregistered state activated the interaction with HSS would be as per the procedure defined in clause 5.12.2.
7. I‑CSCF#2 forwards the MESSAGE request to S‑CSCF#2.
8. Based on operator policy S‑CSCF#2 may reject the MESSAGE request with an appropriate response, e.g. if content length or content type of the MESSAGE are not acceptable or the UE#2 does not have a service activated that temporarily hold the MESSAGE request in the network.
S‑CSCF#2 invokes whatever service control logic appropriate for this MESSAGE request. This may include routing the MESSAGE request to an Application Server, which processes the request further on.
For example, the UE#2 may have a service activated that allows delivery of any pending MESSAGE request. The AS may then hold the MESSAGE request and deliver the MESSAGE request when the UE#2 becomes reachable. In this case, depending on user settings UE#2 controls the delivery of the pending MESSAGEs.
9-12. The MESSAGE request is acknowledged with an appropriate acknowledgement response. The acknowledgement response traverses the transaction path back to UE#1.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.1.2 Immediate messages with multiple recipients
| IMS users shall be able to send a single immediate message to multiple recipients, as specified in TS 22.340 [29a]. The following means are supported to achieve this:
- A PSI identifying a new group is created in the appropriate Application Server and members are added to this group (e.g. by the user via the Ut interface or by the operator via O&M mechanisms). Immediate messages addressed to this PSI will be routed to the AS hosting the PSI and this AS shall create and send immediate messages addressed to a group member of the group identified by the PSI.
- The user can send an immediate message by indicating the individual addresses (Public User Identities for IMS recipients) of the intended recipients as part of the immediate message. The AS of the user shall then create and send immediate messages addressed to each one of the intended recipients.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.2 Session-based Messaging
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.2.0 General
| This clause describes architectural concepts and procedures for fulfilling the requirements for Session-based Messaging described in TS 22.340 [29a].
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.2.1 Architectural principles
| Session-based IMS messaging communications shall as much as possible use the same basic IMS session delivery mechanisms (e.g. routing, security, service control) as defined in clause 4 and 5 of this document. For session based messaging the session shall include a messaging media component, other media components may also be included.
As the messaging media component usually does not require QoS beyond best-effort, use of the preconditions mechanism as defined in IETF RFC 3312 [41] is not required for session based messaging establishment that only includes a messaging media component.
NOTE: Pre-conditions mechanism may still be required for session establishment with additional media components that require the establishment of additional IP‑CAN bearers.
Once the session containing a messaging media component is established, messages in the session are transported between the session participants as per the parameters defined in the messaging media component part of the session description (SDP).
The invited UE shall host the message session (accept a connection for the message session from the other endpoint). In order to host the message session the UE needs an appropriate IP‑CAN bearer, on which it can accept the connection for the message media component. This IP‑CAN bearer may be e.g. a general purpose bearer available prior to starting the session initiation or a dedicated bearer that is established during session establishment. Messages within a message session should be transported over a connection-oriented reliable transport protocol. Message sessions may be either established end to end between two UEs or may involve one or more intermediate nodes (e.g. a chat server for multi party chat or an Application Server to perform per message charging).
For addressing chat-group-type session based messaging the concept of Public Service Identities is used.
Session based messaging is available for users that are registered in the IMS.
The session based messaging shall be able to provide the following functionality:
- Per-message-based charging, as well as content- and size-based charging.
- Operator-controlled policy to be set on the size and content of the messages.
- Support for indication of maximum message content size that a UA will accept to be received.
- Support for a messaging media component as part of a session where other media components are also included.
- Support for messaging-only sessions.
If charging mechanisms like charging based on the message content, message type or number of sent and/or received messages (see TS 22.340 [29a]) are required, then an intermediate node (messaging AS) shall be involved, which is able to inspect the SIP signalling as well as the exchanged messages and their content. Such an intermediate node may also provide support for time- and/or volume based charging.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.2.2 Procedures to enable Session based Messaging
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.2.2.0 General
| IMS users shall be able to exchange session-based messages with each other by using the procedures described in this clause. These procedures 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 documents.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.2.2.1 Session based messaging procedure to registered Public User Identity
| The following procedure shows the establishment of a message session between two registered UEs where the UEs are able to exchange messages end-to-end. The signalling flow is based on the general flow shown in clause 5.7a of this specification.
Figure 5.48a: Message session establishment
1-30. These steps are identical to the steps 1 to 30 in the flow of clause 5.7a. After that the message session is established. For session based messaging the SDP offer in the first INVITE request may indicate the maximum message size UE#1 accepts to receive and the 200 OK (Offer response) to the INVITE request may indicate the maximum message size UE#2 accepts to receive.
For MPS for Messaging, the following exception at step 3 in the referenced flow applies: If P-CSCF#1 has received the MPS for Messaging indication for the UE as set (enabled), P-CSCF#1 sets the Resource-Priority information on the INVITE request to a value appropriate for MPS.
31. UE#1 establishes a reliable end-to-end connection with UE#2 to exchange the message media.
32. UE#1 generates the message content and sends it to UE#2 using the established message connection.
33. UE#2 acknowledges the message with a response that indicates that UE#2 has received the message. The response traverses back to UE#1. After receiving the message UE#2 renders the multimedia content to the user.
Further messages may be exchanged in either direction between UE#1 and UE#2 using the established connection. The size of the messages exchanged within the session shall be within the size limits indicated by UE#1 and UE#2 respectively.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.2.2.2 Session based messaging procedure using multiple UEs
| Session based messaging between more than two UEs require the establishment of a session based messaging conference.
Within session based messaging conferences including multiple UEs (e.g. multiparty chat conferences) an MRFC/MRFP or an IMS AS shall be used to control the media resources.
When MRFC/MRFP are used, then conferencing principles are used to provide the chat service:
- MRFP must be able to establish message connections with all involved parties.
- MRFC/MRFP must be able to receive messages from conference participants and to distribute messages to all or some of the participants.
- In order to enable the UE managing information related to the session based messaging conference the MRFC may be co-located with an IMS AS.
- MRFC/MRFP roles and interactions with an AS are described in more detail in clauses 4.7, 5.14.1 and 5.14.2.
- The interface for session based messaging between MRFC and MRFP is not standardised in this release. When an AS is used, then the IMS service control architecture is used to provide the chat service. Both signalling and user plane are then supported by the AS. For more details, see clause 4.2.
The following flow shows the originating session based messaging set up using an intermediate server for a chat service. In this case the intermediate chat server is addressed by the UE#1 using a PSI. It is assumed that UE#1 is the first UE entering the chat session.
NOTE: Interactions between MRFC and MRFP are not shown in the flows below since these interactions are not standardized. An optional ringing response from MRFC/AS to the UE is not shown in the following procedure.
Figure 5.48b: Session based messaging using a chat server
1. UE #1 sends the SIP INVITE request addressed to a conferencing or chat PSI to the P‑CSCF. The SDP offer indicates that UE#1 wants to establish a message session and contains all necessary information to do that. The SDP offer may indicate the maximum message size UE#1 accepts to receive.
2. P‑CSCF forwards the INVITE request to the S‑CSCF.
For MPS for Messaging, before forwarding, if the P-CSCF has received the MPS for Messaging indication for the UE as set (enabled), the P-CSCF sets the Resource-Priority information on the INVITE request to a value appropriate for MPS.
3. S‑CSCF may invoke service control logic for UE#1.
4. S‑CSCF forwards the INVITE request to the MRFC/AS.
5. 6. and 8. MRFC/AS acknowledges the INVITE with a 200 OK, which traverses back to UE#1. The 200 OK (Offer response) may indicate the maximum message size the host of the PSI accepts to receive.
7. Based on operator policy P‑CSCF may instruct PCRF/PCF to authorize the resources necessary for this session.
9.-11. UE#1 acknowledges the establishment of the messaging session with an ACK towards MRFC/AS.
12. UE#1 establishes a reliable end-to-end connection with MRFP/AS to exchange the message media.
13. UE#1 sends a message towards MRFP/AS.
14. MRFP/AS acknowledges the message.
A1. Another UE (UE#2) sends an INVITE request addressed to the same conferencing or chat PSI. The initial SDP indicates that the UE wants to establish a message session and contains all necessary information to do that.
A2. MRFC/AS acknowledges the INVITE request with a 200 OK.
A3. UE#2 acknowledges the 200 OK with an ACK.
A4. UE#2 establishes a reliable end-to-end connection with MRFP/AS to exchange the message media.
A5. MRFP/AS forwards the message to all recipients, e.g. all participants in the chat room.
A6. The recipients acknowledge the message towards MRFP/AS.
B1. and C1. Further INVITE requests from new possible participants may arrive at any time.
Further messages may be exchanged in either direction between the participating UEs using the established connection via the MRFC/MRFP or AS. The size of the messages exchanged within the session shall be within the size limits indicated by UE#1 and the host of the PSI respectively.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.2.2.3 Session based messaging procedure with an intermediate node
| The following procedure shows the originating session based messaging involving an intermediate node. An optional ringing response from AS to the UE or vice versa is not shown in the following procedure.
Figure 5.48c: Session based messaging with an intermediate node
1. UE#1 sends the SIP INVITE request addressed to UE#2, containing an initial SDP, to the P‑CSCF. The SDP offer may indicate the maximum message size UE#1 accepts to receive.
2. The P‑CSCF forwards the INVITE request to the S‑CSCF along the path determined upon UE#1's most recent registration procedure.
For MPS for Messaging, before forwarding, if the P-CSCF has received the MPS for Messaging indication for the UE as set (enabled), the P-CSCF sets the Resource-Priority information on the INVITE request to a value appropriate for MPS.
3. Based on operator policy the S‑CSCF may reject the INVITE request with an appropriate response. S‑CSCF may invoke whatever service control logic is appropriate for this INVITE request. In this case the Filter Criteria trigger the INVITE request to be routed to an Application Server that acts as an intermediate node for the message session.
4. The S‑CSCF forwards the INVITE request to the AS. The AS may modify the content of the SDP (such as IP address/port numbers). Based on operator policy the AS may either reject the session set-up or decrease the maximum message size indication.
5. The AS sends the INVITE request to the S‑CSCF.
6. The S‑CSCF forwards the INVITE request to the destination network. The destination network will perform the terminating procedure.
7–8. UE#2 or AS in the terminating network accepts the INVITE request with a 200 OK response. The 200 OK response is forwarded by the S‑CSCF to the AS. The 200 OK (Offer response) may indicate the maximum message size UE#2 accepts to receive, possibly decreased by the AS.
9‑10. The AS acknowledges the 200 OK response from the terminating network with an ACK, which traverses back to UE#2 or AS in the terminating network via the S‑CSCF.
11. The AS initiates the establishment of a reliable end-to-end connection with UE#2 or the AS in the terminating network to exchange the message media. This step can take place in parallel with step 12.
12, 13 and 15. The AS accepts the message session with a 200 OK response. The 200 OK response traverses back to UE#1.
14. Based on operator policy P‑CSCF may instruct PCRF/PCF to authorize the resources necessary for this session.
16‑18. UE#1 acknowledges the 200 OK with an ACK, which traverses back to the AS.
19. UE#1 establishes a reliable end-to-end connection with the AS to exchange the message media.
20. UE#1 generates the message content and sends it to the AS using the established message connection.
21. The AS forwards the message content using the established message connection with the terminating network.
22. UE#2 or AS in the terminating network acknowledges the message with a response that indicates the reception of the message. The response traverses back to the AS.
23. The AS forwards the message response back to UE#1.
Further messages may be exchanged in either direction between UE#1 and the terminating network using the established message connection via the AS. The size of the messages exchanged within the session shall be within the size limits indicated by UE#1 and UE#2 respectively, possibly decreased by the AS.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.2.2.4 Session based messaging release procedure
| The following procedure shows the release of a message session, which was established between two UEs. It is assumed that UE#1 is the session host.
Figure 5.48d: Message session release procedure
1–6. UE#1 indicates its intent to terminate the message session by sending a BYE request to UE#2. UE#1 stops sending messages and tears down the message connection on the transport level and destroy local state for the message session. The UE#1 may use the IP‑CAN bearer for some other services; hence it keeps the bearer activated.
7-8. UE#2 agrees to end the session and tear down the message connection on the transport level and destroy local state for the message session. The UE#2 may use the IP‑CAN bearer for some other services; hence it keeps the bearer activated.
9-13. UE#2 acknowledges the BYE request by sending a 200 OK to UE#1, which traverses back the signalling path.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.16.2.2.5 Session based messaging release procedure with an intermediate node
| The following procedure shows the release of a message session, which was established between two UEs via an intermediate node. It is assumed that UE#1 is the session host.
Figure 5.48e: Message session release procedure with intermediate node
1–4. UE#1 indicates its intent to terminate the message session by sending a BYE request to UE#2, via the AS. UE#1 stops sending messages and tears down the message connection on the transport level and destroy local state for the message session. The UE#1 may use the IP‑CAN bearer for some other services; hence it keeps the bearer activated.
5. The AS forwards the BYE request to the UE#2.
6-9. The AS tears down the message connection on the transport level and destroys local state for the message session. The AS acknowledges the BYE request by sending a 200 OK to UE#1, which traverses back the signalling path
10. The AS receives the acknowledgement from UE#2 to end the session.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.17 Refreshing sessions
| The active sessions in stateful network elements (e.g. CSCFs, ASs) may need to be refreshed periodically. This allows these stateful elements to detect and free resources that are being used by hanging sessions.
This SIP‑level session refreshing mechanism is to be used to allow removing session state from the stateful elements of the session path upon unexpected error situations (e.g. loss of radio coverage, crash of application in the UE, etc…). The refreshing period is typically in the range of several tens of minutes / hours. The mechanism is intended as a complementary mechanism for the "Network initiated session release" described in clause 5.10.3. Whether the session refresh mechanism is used for a particular session is negotiated between the endpoints of the session upon session initiation.
IMS entities acting as User Agents as defined in IETF RFC 3261 [12] should support the refresh mechanism of SIP sessions. This includes support for the negotiation of the session refresh details upon session initiation and the initiation of session refresh requests.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.18 Void
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.19 Support for Transit scenarios in IMS
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.19.1 General
| This clause presents some high level flows to describe the procedures for supporting IMS transit network scenarios.
The IMS Transit Functions perform an analysis of the destination address and determine where to route the session. The session may be routed directly to an MGCF, BGCF, or to another IMS entity in the same network, to another IMS network, or to a CS domain or PSTN. The address analysis may use public (e.g. DNS, ENUM) and/or private database lookups and/or locally configured data. As described in clause 4.15 there are various transit configurations possible that may be supported.
For the case where an operator is providing transit functions for other operators and/or enterprise networks, the configuration is as shown in Figure 5.49. The configuration in Figure 5.49 is also intended to cover scenarios where an operator routes traffic from other IMS- or SIP-networks to CS domain or PSTN customers through the IMS transit network. In this case the terminating network as shown in the figure indicates the operator's CS domain or PSTN.
Figure 5.49: IMS transit network
For the transit operator in Figure 5.49, ISUP messages that arrive at a configured MGCF, are translated into SIP and are passed to the IMS Transit Functions. SIP messages may arrive directly at the configured entity supporting the transit functions or first pass through an IBCF before arriving at the IMS Transit Functions. The IMS Transit Functions determine whether to route directly to an MGCF, BGCF, or to another IP entity on the path (e.g. an IBCF). In this transit operator configuration, the IMS Transit Functions might reside in a stand-alone entity or might be combined with the functionality of an MGCF, a BGCF, an I‑CSCF, an S‑CSCF or an IBCF. When residing in a stand-alone entity the IMS Transit Functions shall be able to generate CDRs.
For the case where the operator is the terminating network operator handling a terminating service for its own customers, the configuration and operation may be more complex as shown in figure 5.50.
NOTE 1: In the case of Fixed Broadband Access to IMS the term "CS domain" in the following text and in figure 5.50 may be replaced by the term "PSTN".
Figure 5.50: Terminating IMS network with transit support, Transit Functions first
For the operator in figure 5.50, ISUP messages arriving at an MGCF may be destined for an IMS or a CS domain customer (see clause 4.15). The ISUP messages are translated into SIP.
The operator can choose whether to route all traffic through the IMS Transit Functions, which subsequently route to the I‑CSCF for IMS terminating call scenarios or to an MGCF for the case of CS domain subscribers as described above. This is depicted in figure 5.50. In this case, there may be an additional delay for terminating sessions destined for IMS subscribers.
NOTE 2: In this case, the IMS Transit Functions perform selection of the appropriate domain to terminate the call to, followed by routing to the CS domain (for CS domain destined traffic).
As an alternative, the operator may choose to route all traffic to the I‑CSCF directly and then identify those sessions that are not destined to IMS subscribers based on an HSS query. Based on the response from the HSS, sessions are either routed to an S‑CSCF or to the CS domain (optionally via Transit Functions). In this case there may be an additional delay for terminating sessions destined for the CS domain subscribers.
NOTE 3: If in this case, the I‑CSCF becomes aware that the call is not destined to an IMS subscriber and forwards it to the Transit Function for further routing, then the Transit Functions only perform routing to the CS domain.
It is the operator's choice to determine which way to route the SIP messages, first through IMS Transit Functions or first to an I‑CSCF. This may depend on whether the majority of the sessions that enter the IMS network, are destined to IMS or CS domain subscribers.
NOTE 4: In either configuration of the terminating network scenario, once it is determined that the call is not destined for an IMS subscriber, it is necessary to verify that the call is destined for a CS domain subscriber rather than to a ported number or to a wrong number. At which stage of the session establishment this decision is made is FFS.
In the terminating network configuration shown in figure 5.50, the IMS Transit Functions might reside in a stand-alone entity or might be combined with the functionality of an MGCF, a BGCF, an I‑CSCF, an S‑CSCF, or an IBCF. When residing in a stand-alone entity the IMS Transit Functions shall be able to generate CDRs.
For the case where an IMS network serves as a transit network and as a terminating network (depending on the destination of the session), the configuration and operation resembles that of the previous case as shown in figure 5.50a and figure 5.50b.
Figure 5.50a: Terminating/Transit IMS network, Transit Functions first
Figure 5.50b: Terminating/Transit IMS network, I-CSCF first
For the operator in figure 5.50a and figure 5.50b, ISUP messages arriving at an MGCF may be destined for the own IMS network of for a succeeding network. The ISUP messages are translated into SIP. This is not depicted in figure 5.50a and figure 5.50b.
The operator can choose whether to route all traffic through the IMS Transit Functions, which subsequently route to the I‑CSCF for sessions destined for subscribers of the own IMS network, to the own CS domain for sessions destined to subscribers of the own CS domain, or to a succeeding network for sessions not destined for subscribers of the own IMS network or the own CS domain. This is depicted in figure 5.50a. In this case there may be an additional delay for sessions destined to subscribers of the own network.
NOTE 5: In this case, the Transit Functions perform selection of the appropriate domain to terminate the call to, for subscribers of the own network, followed by routing to another network (if the session is not destined to the own network).
As an alternative, the operator may choose to route all traffic to the I‑CSCF directly and then identify those sessions that are not destined to IMS subscribers of the own IMS network based on an HSS query. Based on the response from the HSS, sessions are either routed to an S‑CSCF of the own IMS network or to Transit Functions. The Transit Functions subsequently route the session to either the CS domain of the own network or to a succeeding network. This is depicted in figure 5.50b. In this case there may be an additional delay for sessions not destined to subscribers of the own IMS network.
It is the operator's choice to determine which way to route the SIP messages, first through IMS Transit Functions or first to an I‑CSCF. This may depend on whether the majority of the sessions that enter the IMS network, are destined to subscribers of the own IMS network or not. This operator's choice may be implemented as a functionality of an entry functions such as an IBCF.
In the terminating/transit network configuration shown in figure 5.50a and figure 5.50b, the IMS Transit Functions might reside in a stand-alone entity or might be combined with the functionality of an MGCF, a BGCF, an I‑CSCF, an S‑CSCF, or an IBCF. When residing in a stand-alone entity the IMS Transit Functions shall be able to generate CDRs.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.19.2 Providing IMS application services in transit network scenarios
| This clause provides an overview of how IMS application services in transit network scenarios are provided.
Figure 5.50c: IMS application services in transit network
The procedure for IMS application services in transit network is as follows:
1. The Transit function receives an incoming request from a preceding network.
2. Based on local configured Transit invocation criteria, the Transit function determines whether one or more services are to be performed.
If the preceding network is the served network, for which special services are invoked, the invocation criteria will trigger and invoke the related services based on the origination of the request.
If the succeeding network is the served network, for which special services are invoked, the invocation criteria will trigger and invoke the related services based on the termination point of the request.
The related service(s) are invoked.
3. The Transit function performs the transit routing according to clause 5.19.1 and forwards the Session Request towards the succeeding network.
NOTE: An AS that acts as a B2BUA can decide to not route back the call to the transit function. In this case, it will use the terminating UA mode of operation for request from the Transit function. It can apply originating UA procedures according to TS 23.218 [71].
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.20 Procedures for Assigning, Using and Processing GRUUs
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.20.1 UE
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.20.1.1 Obtaining a GRUU during registration
| A UE shall indicate its support for the GRUU mechanism in the registration request and retain the GRUU set (P‑GRUU and T‑GRUU) in the registration response. The UE may retain some or all of the previous T‑GRUUs obtained during the initial registration or previous re-registrations along with the new T‑GRUU or the UE may replace some or all of the previous T‑GRUUs with the new T‑GRUU. The UE shall generate an instance identifier that is a unique identifier for that UE. The UE shall include an instance identifier in all registration requests. Instance identifiers shall conform to the mandatory requirements for instance identifiers specified in RFC 5627 [49] and RFC 5626 [48].
If the registered Public User Identity is part of an implicit registration set, the UE shall obtain and retain the GRUU sets for each implicitly registered SIP URI sent by the S‑CSCF in accordance to RFC 5628 [50].
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.20.1.2 Using a GRUU
| When sending SIP requests from an explicitly or implicitly registered Public User Identity for which a UE obtained P‑GRUU and at least one T‑GRUU, the UE should use the corresponding retained P‑GRUU or a T‑GRUU as a Contact address.
When responding to SIP requests where the identification of the called party is a registered Public User Identity for which a UE obtained a GRUU, the UE shall use the corresponding retained P‑GRUU or T‑GRUU as the Contact address when addressing that UE.
If the UE has obtained GRUUs for its Public User Identity being used in a request or response and the user does not require privacy the UE should use the P‑GRUU as the Contact address.
A UE may learn a GRUU of another UE using mechanisms that are outside the scope of this specification, (e.g. a UE may learn a GRUU from the contact header of a request, from presence information, or by other mechanisms).
If a UE that receives a notification from the S‑CSCF indicating that an implicit registration has occurred for a contact the UE has registered, then the UE shall retain the GRUUs included in the notification for future use.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.20.1.3 Using a GRUU while requesting Privacy
| When a UE sends a request or response containing a GRUU and it wishes to block the delivery of its Public User Identity to an untrusted destination, the UE shall use a T-GRUU as the Contact address.
|
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.20.2 Serving‑CSCF
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.20.2.1 Allocating a GRUU during registration
| The S‑CSCF, when receiving a registration request from a UE that includes an instance id, shall allocate a GRUU set. If the UE indicates support of GRUU in the REGISTER request, then the S‑CSCF shall return the GRUU set in the registration response and associate that GRUU set with the registered contact information for that UE.
NOTE: As long as the instance id provided in the register request is the same, the resulting P‑GRUU in the GRUU set will always be the same for a given Public User Identity. The T‑GRUU will be different from those returned during previous re-registrations. All T‑GRUUs that are allocated continue to remain valid until that UE Instance ID and Public User Identity pair are deregistered.
If there are implicitly registered Public User Identities, the S‑CSCF shall generate a GRUU set for each implicitly registered Public User Identity and include the corresponding GRUU set with the notification of each implicitly registered Public User Identity
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.20.2.2 Using a GRUU
| The filter criteria in the service profile may check for the presence of a GRUU in the Request URI or related parameters of a request.
For originations, the S‑CSCF shall validate the GRUU conveyed in the contact header of the SIP request and pass the SIP request with the validated GRUU to Application Servers based on the filter criteria.
For terminations, the S‑CSCF may validate the GRUU conveyed in the Request URI header of the SIP request and pass the SIP request with the validated GRUU to Application Servers based on filter criteria.
Application servers may then apply services to the GRUU.
If the SIP message is destined to a GRUU, then the S‑CSCF shall associate the request with the corresponding Public User Identity. The S‑CSCF will not fork this request, but will direct the call to the identified instance.
S‑CSCF shall provide an indication to UE that the SIP request was targeted to a GRUU.
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.20.3 Interrogating‑CSCF
| When routing requests addressed to a GRUU to the terminating S‑CSCF, the I‑CSCF uses the contents of the Request URI when querying the HSS. Requests routed to the terminating S‑CSCF are addressed to the GRUU.
5.20.3a HSS
The HSS shall remove the P‑GRUU as part of the canonicalization process of SIP URIs, to obtain the Public User Identity for identity look-up as it is defined in TS 29.228 [30].
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235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.20.4 Elements other than UE acting as a UA
| |
235ff1c5a6dec6f988ef6afae94ebbde | 23.228 | 5.20.4.1 Using a GRUU
| It shall be possible for other IMS elements other than UEs, that act as UAs (e.g. MGCF, Application Server) to use a GRUU referring to itself when inserting a contact address in a SIP message. The MGCF and MRF are not required to store GRUUs beyond a session. If the incoming contact address that is being replaced by the B2BUA functionality contains a GRUU, then the replacement URI in the outgoing SIP message should also contain a GRUU.
If an element so uses a GRUU, it shall handle requests received outside of the session in which the contact was provided.
Routing procedures amongst IMS elements other than UEs that act as UAs are unchanged when GRUUs are in use.
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