Tech-invite
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Content for  TS 23.228  Word version:  16.6.0

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5.8  Procedures related to routing information interrogationWord‑p. 143

5.8.0  General |R6|

When a mobile terminated session set-up arrives at an I-CSCF that is authorized to route sessions, the I-CSCF interrogates the HSS for routing information. The mobile terminated sessions for a user shall be routed to a S-CSCF.
The Cx reference point shall support retrieval of routing information from HSS to I-CSCF. The resulting routing information is the contact information of S-CSCF.

5.8.1  User identity to HSS resolution

This clause describes the resolution mechanism, which enables the I-CSCF, the S-CSCF and the AS to find the address of the HSS, that holds the subscriber data for a given user identity when multiple and separately addressable HSSs have been deployed by the network operator. This resolution mechanism is implemented using a Subscription Locator Function (SLF) or a Diameter Proxy Agent that proxies the request to the HSS. This resolution mechanism is not required in networks that utilise a single HSS e.g. optionally, it could be switched off on the I-CSCF and on the S-CSCF and/or on the AS using O&M mechanisms. An example for a single HSS solution is a server farm architecture. By default, the resolution mechanism shall be supported.
On REGISTER and on MT INVITEs, the I-CSCF queries the HSS for user's subscription specific data, e. g. the actual location or authentication parameters. This also has to be accomplished by the S-CSCF on REGISTER. In the case when more than one independently addressable HSS is utilized by a network operator, the HSS where user information for a given subscriber is available has to be found. To get the HSS name the I-CSCF and the S-CSCF query the SLF entity or the I-CSCF and the S-CSCF send the query to the HSS via a Diameter Proxy Agent.
The SLF is accessed via the Dx interface or via the Dh interface. The Dx interface is the standard interface between the CSCF and the SLF and the Dh interface is the standard interface between the AS and the SLF. The synchronisation between the SLF and the different HSSs is an O&M issue.
A way to use the SLF is described in the following.
The Dx interface provides:
  • an operation to query the SLF from the I-CSCF or from the S-CSCF, respectively.
  • a response to provide the HSS name towards the I-CSCF or towards the S-CSCF, respectively.
By sending the Dx-operation DX_SLF_QUERY the I-CSCF or the S-CSCF indicates a user identity of which it is looking for an HSS. By the Dx-operation DX_SLF_RESP the SLF responds with the HSS name. The I-CSCF or the S-CSCF, respectively, continues by querying the selected HSS. The I-CSCF may forward the HSS name towards the S-CSCF. The S-CSCF may use this name to find the subscriber's HSS.
Clause 5.8.2 presents the session flows on REGISTER and clause 5.8.3 on INVITE messages.
The Dh interface provides:
  • an operation to query the SLF from the AS.
  • a response to provide the HSS name towards the AS.
By sending the Dh-operation DH_SLF_QUERY the AS indicates a Public User Identity of which it is looking for an HSS. By the Dh-operation DH_SLF_RESP the SLF responds with the HSS name. The AS continues by querying the selected HSS. The AS may store the HSS name for the subsequent Sh-operations.
Clause 5.8.4 presents the message flow on the Dh interface.
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5.8.2  SLF on registerWord‑p. 144

(not reproduced yet)
Figure 5.20: SLF on register (1st case)
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Step 1.
I-CSCF receives a REGISTER request and now has to query for the location of the user's subscription data.
Step 2.
The I-CSCF sends a DX_SLF_QUERY to the SLF and includes as parameter the user identity which is stated in the REGISTER request.
Step 3.
The SLF looks up its database for the queried user identity.
Step 4.
The SLF answers with the HSS name in which the user's subscription data can be found.
Step 5.
The I-CSCF can proceed by querying the appropriate HSS.
(not reproduced yet)
Figure 5.20a: SLF on register (2nd case)
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Step 1.
I-CSCF sends a REGISTER request to the S-CSCF. This now has to query for the location of the user's subscription data.
Step 2.
The S-CSCF sends a DX_SLF_QUERY to the SLF and includes as parameter the user identity which is stated in the REGISTER request.
Step 3.
The SLF looks up its database for the queried user identity.
Step 4.
The SLF answers with the HSS name in which the user's subscription data can be found.

5.8.3  SLF on UE inviteWord‑p. 145

(not reproduced yet)
Figure 5.21: SLF on UE invite
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Step 1.
I-CSCF receives an INVITE request and now has to query for the location of the user's subscription data.
Step 2.
The I-CSCF sends a DX_SLF_QUERY to the SLF and includes as parameter the user identity which is stated in the INVITE request. If the user identity is an E.164 number in the SIP URI with user=phone parameter format the I-CSCF shall first translate it into the Tel: URI format per RFC 3966 prior to sending to the SLF the DX_SLF_QUERY.
Step 3.
The SLF looks up its database for the queried user identity.
Step 4.
The SLF answers with the HSS name in which the user's subscription data can be found.
To prevent an SLF service failure e.g. in the event of a server outage, the SLF could be distributed over multiple servers. Several approaches could be employed to discover these servers. An example is the use of the DNS mechanism in combination with a new DNS SRV record. The specific algorithm for this however does not affect the basic SLF concept and is outside the scope of this document.
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5.8.4  SLF on AS access to HSS |R6|Word‑p. 146

The flow shown below is where the AS queries the SLF to identify the HSS to access.
(not reproduced yet)
Figure 5.21a: SLF on AS access to HSS
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Step 1.
An AS sends a DH_SLF_QUERY to the SLF and includes as a parameter the Public User Identity.
Step 2.
The SLF looks up its database for the queried Public User Identity.
Step 3.
The SLF answers with the HSS name in which the user's subscription data can be found.
Step 4.
The AS sends the Sh message towards the correct HSS.

5.9  Routing of mid-session signalling

During the signalling exchanges that occur to establish an IM Session, the following elements must ensure future signalling messages related to this session are routed through them:
  • P-CSCF serving the originating UE, in order to generate the CDR record in the roaming case, and to force release of the resources used for the session.
  • S-CSCF serving the originating UE, in order to invoke any service logic required at session setup completion, and to generate the CDR record at session termination.
  • S-CSCF serving the terminating UE, in order to invoke any service logic required at session setup completion, and to generate the CDR record at session termination.
  • P-CSCF serving the terminating UE, in order to generate the CDR record in the roaming case, and to force release of the resources used for the session.
Other CSCFs (e.g. I-CSCFs) may optionally request this as well, for example if they perform some function needed in handling mid-session changes or session clearing operations.
All signalling message from the UE related to IMS sessions shall be sent to the P-CSCF.
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5.10  Session release proceduresWord‑p. 147

5.10.0  General |R6|

This clause provides scenarios showing SIP application session release. Note that these flows have avoided the strict use of specific SIP protocol message names. This is in an attempt to focus on the architectural aspects rather than the protocol. SIP is assumed to be the protocol used in these flows.
The session release procedures are necessary to ensure that the appropriate billing information is captured and to reduce the opportunity for theft of service by confirming that the bearers associated with a particular SIP session are deleted at the same time as the SIP control signalling and vice versa. Session release is specified for the following situations:
  • Normal session termination resulting from an end user requesting termination of the session using session control signalling or deletion of the IP bearers associated with a session;
  • Session termination resulting from network operator intervention;
  • Loss of the session control bearer or IP bearer for the transport of the IMS signalling; and
  • Loss of one or more radio connections which are used to transport the IMS signalling.
As a design principle the session release procedures shall have a high degree of commonality in all situations to avoid complicating the implementation.
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5.10.1  Terminal initiated session release

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.
(not reproduced yet)
Figure 5.22: Terminal initiated session release
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Step 1.
One party hangs up, which generates a message (Bye message in SIP) from the UE to the P-CSCF.
Step 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.
Step 3.
Void.
Step 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.
Step 5.
The P-CSCF sends a Hangup to the S-CSCF of the releasing party.
Step 6.
The S-CSCF invokes whatever service logic procedures are appropriate for this ending session.
Step 7.
The S-CSCF of the releasing party forwards the Hangup to the S-CSCF of the other party.
Step 8.
The S-CSCF invokes whatever service logic procedures are appropriate for this ending session.
Step 9.
The S-CSCF of the other party forwards the Hangup on to the P-CSCF.
Step 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.
Step 11.
The P-CSCF forwards the Hangup on to the UE.
Step 12.
The terminal responds with an acknowledgement, the SIP OK message (number 200), that is sent back to the P-CSCF.
Step 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.
Step 14.
Void.
Step 15.
The SIP OK message is sent to the S-CSCF.
Step 16.
The S-CSCF of the other party forwards the OK to the S-CSCF of the releasing.
Step 17.
The S-CSCF of the releasing party forwards the OK to the P-CSCF of the releasing.
Step 18.
The P-CSCF of the releasing party forwards the OK to the UE.
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5.10.2  PSTN initiated session releaseWord‑p. 149

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.
(not reproduced yet)
Figure 5.23: PSTN initiated session release
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Step 1.
PSTN party hangs up, which generates an ISUP REL message to the MGCF.
Step 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.
Step 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.
Step 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.
Step 5.
The MGW sends an acknowledgement to the MGCF upon completion of step 4.
Step 6.
The S-CSCF invokes whatever service logic procedures are appropriate for this ending session.
Step 7.
The S-CSCF forwards the Hangup to the P-CSCF.
Step 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.
Step 9.
The P-CSCF forwards the Hangup to the UE.
Step 10.
The terminal responds with an acknowledgement, the SIP OK message (number 200), which is sent back to the P-CSCF.
Step 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 11 and 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.
Step 13.
The SIP OK message is sent to the S-CSCF.
Step 14.
The S-CSCF forwards the message to the MGCF.
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5.10.3  Network initiated session releaseWord‑p. 150

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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5.10.3.1  Network initiated session release - P-CSCF initiated

5.10.3.1.0  General |R6|
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 and TS 23.503). The flow for this case is shown in Figure 5.26.
Other network initiated session release scenarios are of course possible.
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5.10.3.1.1  Network initiated session release - P-CSCF initiated - after removal of IP-Connectivity Access Network bearerWord‑p. 151
(not reproduced yet)
Figure 5.26: Network initiated session release - P-CSCF initiated - after removal of IP-CAN bearer
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Step 1.
A bearer related to the session is terminated. The P-CSCF receives an indication via PCRF/PCF of IP-CAN bearer release.
Step 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 and TS 23.503). 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).
Step 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 and TS 23.503).
The following steps are only performed if the P-CSCF has decided to terminate the session.
Step 4.
The P-CSCF generates a Hangup (Bye message in SIP) to the S-CSCF of the releasing party.
Step 5.
The S-CSCF invokes whatever service logic procedures are appropriate for this ending session.
Step 6.
The S-CSCF of the releasing party forwards the Hangup to the S-CSCF of the other party.
Step 7.
The S-CSCF invokes whatever service logic procedures are appropriate for this ending session.
Step 8.
The S-CSCF of the other party forwards the Hangup on to the P-CSCF.
Step 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.
Step 10.
The P-CSCF forwards the Hangup on to the UE.
Step 11.
The UE responds with an acknowledgement, the SIP OK message (number 200), which is sent back to the P-CSCF.
Step 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.
Step 14.
The SIP OK message is sent to the S-CSCF.
Step 15.
The S-CSCF of the other party forwards the OK to the S-CSCF of the releasing party.
Step 16.
The S-CSCF of the releasing party forwards the OK to the P-CSCF of the releasing party.
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5.10.3.1.2Void

5.10.3.2  Network initiated session release - S-CSCF InitiatedWord‑p. 152

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.
(not reproduced yet)
Figure 5.27: Network initiated session release - S-CSCF initiated
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Information flow procedures are as follows:
Step 1.
S-CSCF#1 decides the session should be terminated, due to administrative reasons or due to service expiration.
Step 2.
S-CSCF#1 sends a Hangup message to P-CSCF#1
Step 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.
Step 4.
P-CSCF#1 forwards the Hangup message to UE#1.
Step 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.
Step 6.
UE#1 responds with a SIP-OK message to its proxy, P-CSCF#1.
Step 7.
P-CSCF#1 forwards the SIP-OK message to S-CSCF#1.
Step 8.
S-CSCF#1 sends a Hangup message to S-CSCF#2. This is done at the same time as flow#2
Step 9.
S-CSCF#2 invokes whatever service logic procedures are appropriate for this ending session.
Step 10.
S-CSCF#2 forwards the Hangup message to P-CSCF#2.
Step 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.
Step 12.
P-CSCF#2 forwards the Hangup message to UE#2.
Step 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.
Step 14.
UE#2 acknowledges receipt of the Hangup message with a SIP-OK final response, send to P-CSCF#2.
Step 15.
P-CSCF#2 forwards the SIP-OK final response to S-CSCF#2.
Step 16.
S-CSCF#2 forwards the SIP-OK final response to S-CSCF#1.
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