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Content for  TR 23.700-10  Word version:  17.1.0

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6  Solutionsp. 13

6.1  Solution 1: 5GC network slice selection for IMS traffic route via dedicated slice(s)p. 13

6.1.1  Descriptionp. 13

6.1.1.1  Generalp. 13

This solution addresses scenario 2 of key issue#1 (Network Slicing and IMS) to ensure IMS service uses specific slice(s) in the 5GC.
The solution describes how the UE selects S-NSSAI(s) for IMS service including SIP signalling and media.
It is proposed that all IMS signalling related information is conveyed to the IMS network via a dedicated 5GC network slice and that media associated to a particular IMS session can be routed via different 5G network slice(s) based on the application's requirements.
When establishing a 5GS PDU session for IMS service, the UE selects an S-NSSAI from list of S-NSSAIs included in Allowed NSSAI based on URSP rules.
A URSP rule can e.g. contain an S-NSSAI for IMS service with associated registration area. If the current registration area associated to an S-NSSAI in Allowed NSSAI is available in the registration area associated to the S-NSSAI for IMS service in URSP rules, then UE selects the S-NSSAI for IMS service at current registration area. At IMS session setup, URSP rule(s) can also be used to establish dedicated 5GS PDU session(s) over the same or new S-NSSAI(s).
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6.1.1.2  Procedurep. 13

The following flow describes the procedure for the UE selecting an S-NSSAI for IMS service.
Copy of original 3GPP image for 3GPP TS 23.700-10, Fig. 6.1.1.2: Procedure to select an S-NSSAI for IMS service
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The steps required are as follows:
Step 1.
The UE performs registration procedure as described in clause 4.2.2 of TS 23.502. Here, the registration accept message includes for example S-NSSAI #1.
Step 2.
The AMF and PCF performs UE Policy Association Establishment as described in clause 4.16.11 of TS 23.502.
Step 3.
Along with step 2, the PCF performs UE Configuration Update procedure as described in clause 4.2.4.3 of TS 23.502. The MANAGE UE POLICY COMMAND message contains a URSP and the URSP contains for example S-NSSAI #1 for IMS service with available registration area and S-NSSAI #2 for other traffic.
Step 4.
When UE requests to establish a 5GS PDU session for IMS service, if the UE determines S-NSSAI #1 in Allowed NSSAI is usable in the current registration area based on the URSP in which S-NSSAI #1 is associated to available registration area, then the UE selects S-NSSAI #1 for IMS service.
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6.1.2  Impacts on existing nodes and functionsp. 14

6.1.3  Solution Evaluationp. 14

The solution does not clarify how the IMS network can provide IMS services to a UE registered with multiple IP addresses. Therefore, the solution does not qualify as a basis for the normative work.

6.2  Solution 2: P-CSCF discovery support with one slice connecting to multiple IMS networkp. 15

6.2.1  Descriptionp. 15

This solution addresses scenario 3 of Key Issue #1 Network Slicing and IMS to support P-CSCF discovery for one slice connecting to different IMS network.
In scenario 3 as discussed in clause 5.1, different UEs may be connected to their home IMS networks via the same single slice instance using the same DNN, e.g. well-known IMS DNN, which means the P-CSCF discovery mechanism needs the same SMF to return different P-CSCFs addresses to UEs belonging to different home IMS network using information other than DNN and S-NSSAI.
To fulfil the requirement above, the IMS network domain information of the UE home IMS network or IMS service additional to DNN and S-NSSAI is provided to SMF and is taken into account during P-CSCF discovery. The IMS network domain information indicates what IMS domain the UE is connecting to. It can be the IMS home domain name used for IMS registration, e.g. domain name derived from UICC as specified in TS 23.003, or other type of IMS network domain information which is configured in the UE and can be recognized by serving 5GC. If the domain name is derived from UICC, it is assumed that UEs belongs to different HPLMNs.
When the UE initiates the PDU session establishment for IMS after the slice for IMS is selected, the UE includes IMS network domain information in PDU SESSION ESTABLISHMENT REQUEST message, along with S-NSSAI and DNN. After receiving network domain information from the UE, the SMF returns P-CSCF addresses to the UE accordingly, taking into account the information.
The following flow describes the procedure for P-CSCF discovery taking into account IMS network domain information provided by the UE.
Copy of original 3GPP image for 3GPP TS 23.700-10, Fig. 6.2.1-1: P-CSCF discovery with IMS network domain information provided by the UE
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Step 0.
The UE derives or is configured with IMS network domain information.
Step 1.
The UE initiates PDU session establishment procedure to create a PDU session for IMS by sending PDU SESSION ESTABLISHMENT REQUEST, including IMS network domain information.
Step 2.
The AMF selects SMF.
Step 3.
The AMF invokes Nsmf_PDUSession_CreateSMContext service operation to create a PDU session for IMS, including IMS network domain information received from the UE.
Step 4.
The SMF discovers P-CSCF taking IMS network domain information into account, as specified in solution #6 of TR 23.700-12.
Step 5.
The SMF returns the discovered P-CSCF address to the UE following existing procedure.
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6.2.2  Impacts on existing nodes and functionsp. 16

The UE initiated PDU session establishment procedure in TS 24.501 is impacted as follows:
  • The UE derives IMS network domain information from UICC or from local configuration; and
  • The UE includes IMS network domain information in PDU SESSION ESTABLISHMENT REQUEST.
The P-CSCF selection functionality in the SMF is impacted. The SMF supports receiving new parameter of IMS network domain information and taking it into account when discovering P-CSCF address.
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6.2.3  Solution Evaluationp. 16

6.3  Solution 3: Selecting an appropriate slice by IMS subscriptionp. 16

6.3.1  Descriptionp. 16

This solution addresses Key Issue #1 for scenario 1.
In scenario 1, the IMS traffic of the UE to the separate IMS networks is routed to different 5GC network slices. For the separate IMS networks, the UE has information of IMS subscriptions (i.e. public and private user identities, home network domain name, used for IMS registration) retrieved from ISIM, derived from USIM, configured in the ME, provided by third party IMS applications or the 5GC network. This scenario has 3 cases:
Case #1:
The third party application uses its own IMS network.
Case #2:
The IMS network and services are provided by operator.
Case #3:
The third party application uses the IMS network provided by operator.
In order for each of IMS clients in the UE to access an IMS network via a desired 5GC network slice, a PDU session associated with the desired 5GC network slice needs to be established. TS 24.526 describes how PDU session attributes (e.g., S-NSSAI) is determined based on information provided by an application in the UE (i.e., an IMS client in this case).
According to the encoding of traffic descriptor in URSP, the information which can be provided by an application in the UE is defined as follows:
  • OS Id + OS App Id type
  • IPv4 remote address type
  • IPv6 remote address/prefix length type
  • Protocol identifier/next header type
  • Single remote port type
  • Remote port range type
  • IP 3 tuple type
  • Security parameter index type
  • Type of service/traffic class type
  • Flow label type
  • Destination MAC address type
  • 802.1Q C-TAG VID type
  • 802.1Q S-TAG VID type
  • 802.1Q C-TAG PCP/DEI type
  • 802.1Q S-TAG PCP/DEI type
  • Ethertype type
  • DNN type
  • Connection capabilities type
  • Destination FQDN
  • Regular expression
  • OS App Id type
  • Destination MAC address range type
In some cases, IMS clients in a UE:
  • cannot provide distinguished information regarding the parameters listed above; and
  • can provide different IMS home network domain names;
where a new parameter "IMS home network domain name" should be introduced in traffic descriptor.
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6.3.2  Impacts on existing nodes and functionsp. 17

The PCF and UE need to be updated to accommodate the newly added parameter in the traffic descriptor.

6.3.3  Solution Evaluationp. 17

The solution allows a UE to assign appropriate S-NSSAIs to PDU sessions providing different IMS networks for IMS clients in the UE if the IMS clients in the UE provide the same existing traffic descriptor parameters and different IMS home network domain names.

6.4  Solution 4: IMS DNN including information identifying the corresponding IMS networkp. 17

6.4.1  Descriptionp. 17

This solution addresses Scenarios 1 and 3 of Key Issue 1.
According to clause AB.1.2.3 of TS 23.228, in order for an SNPN to enable IMS services to be provided via multiple independent ISHs, it is required that for every SNPN subscriber, the IMS DNN stored in UDM of the SNPN contains the information, which is used to identify the corresponding ISH, e.g. ISH domain name is encoded as part of IMS DNN.
The same requirement (IMS DNN includes information identifying the corresponding IMS network) can be applied in a PLMN if the PLMN allows multiple IMS networks.
Then, based on the existing mechanisms, using the DNN, the UE can populate PDU session attributes and the SMF can discover an appropriate P-CSCF.
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6.4.2  Impacts on existing nodes and functionsp. 18

The IMS DNN stored in the UDM includes the information identifying the corresponding IMS network.

6.4.3  Solution evaluationp. 18

The solution clarifies how a DNN can be used to distinguish different IMS networks. Using DNNs including information identifying the corresponding IMS network:
  • in Scenario 1 of Key Issue 1, there is no confusion in distinguishing IMS clients using DNNs where the IMS clients in the UE are for different IMS networks and hence the existing mechanisms, e.g. specified in Case #1 of Solution 3, can be used; and
  • in Scenario 3 of Key Issue 1, there is no confusion in selecting P-CSCFs in different IMS networks using DNNs.
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