2 NSSF Supported Services

This chapter includes information about the services supported by NSSF.

Note:

The performance and capacity of the NSSF system may vary based on the call model, Feature or Interface configuration, and underlying CNE and hardware environment.

2.1 Network Slice Selection Service

The Network Slice Selection service is identified by the service operation name, Nnssf_NSSelection. This service supports the GET request during the following procedures by UE:

Initial Registration:

When NSSF is able to find authorized network slice information for the requested network slice, the response includes a payload containing at least the Allowed NSSAI, target AMF Set, or the list of candidate AMF(s).

Following diagram illustrates the procedure of initial registration:

Figure 2-1 Initial Registration

Initial Registration

  • The AMF sends a GET request to the NSSF.

    The AMF GET request must include:
    • Subscribed S-NSSAIs (with an indication if marked as default S-NSSAI)
    • Any Allowed NSSAI

    The query parameters may also contain:

    • Requested NSSAI
    • Mapping of requested NSSAI to configured NSSAI for the HPLMN
    • Mapping to the Configured NSSAI for the HPLMN
    • PLMN ID of the Subscription Permanent Identifier (SUPI)
    • UE's current Tracking Area
    • NF type of the NF service consumer
    • AMF ID
  • Based on the query parameters mentioned above, local configuration, and locally available information, including Radio Access Network (RAN) capabilities obtained by the current Tracking Area for the UE, NSSF does the following:
    • It selects the Network Slice instance(s) to serve the UE. When multiple Network Slice instances in the UE's Tracking Areas are able to serve a given S-NSSAI ( based on operator's configuration), NSSF selects one slice to serve the UE, or defer the selection of the Network Slice instance until a NF or service within the Network Slice instance needs to be selected.
    • It determines the target AMF set to be used to serve the UE or based on configuration, the list of candidate AMF(s), possibly after querying the NRF.
    • The AuthorizedNetworkSliceInfo response for UE-registration must mandatorily include both AllowedNSSAI and Candidate AMF list or target amfset. The AllowedNSSAI is computed by taking into account the input request and applying operator policies as specified in 3GPP Spec 29.531 Release 15.5.
    • NSSF calculates ConfiguredNSSAI by determining the intersection of S-NSSAI(s) in ConfiguredNSSAI for the PLMN (operator configured) and S-NSSAI(s) in SubscribedNSSAI (from the message indicating SubscribedNSSAI by the UE).
    • It determines the Allowed NSSAI(s) for the applicable Access Type(s), taking also into account the availability of the Network Slice instances that are able to serve the S-NSSAI(s) in the Allowed NSSAI and the current UE's tracking areas.
    • Based on operator configuration, the NSSF determines the NRF(s) to be used to select NFs or services within the selected Network Slice instance(s).
  • When the NSSF locates the authorized network slice information for the requested network, NSSF sends Discovery Request for AMF to NRF.
  • The NRF responds with the list of all candidate AMFs to NSSF.
  • The NSSF returns to the current AMF the Allowed NSSAI for the applicable Access Type(s), the target AMF Set, or the list of candidate AMF(s) based on configuration.
    • NSSF returns the NRF(s) to be used to select NFs/services within the selected Network Slice instance(s) and the NRF to be used to determine the list of candidate AMF(s) from the AMF Set.
    • NSSF returns NSI ID(s) to be associated to the Network Slice instance(s) corresponding to certain S-NSSAIs.
    • NSSF also returns the rejected S-NSSAI(s) and the Configured NSSAI for the Serving PLMN.
  • Candidate AMF selection by NSSF:
    • In response to the Initial Registration request, NSSF responds with AMFs that support the Authorized NSSAI in the specified TAI. NSSF prioritizes runtime data, and if a suitable match is not found, it falls back to operator-configured data. Runtime data consists of NsAvailabilityData sent by the AMF.
    • This approach is chosen to ensure that NSSF responds with consideration of dynamic data. In scenarios where sufficient data is not present (for example, AMFs have not sent an Availability Update), NSSF relies on operator-configured data.
    • This ensures that a response is provided when NSSF is just initialized and does not have Availability Data. The recommendation is that AMFs should have a configuration to update NsAvailability information with NSSF, so that NSSF can respond based on dynamic data.

PDU Session Establishment:

When the NSSF receives a PDU-Session establishment request from the NF consumer, it determines the network slice that can serve the requested S-NSSAI based on the user configured policies, and responds with the URL of the NRF that manages the Slice and Slice ID of the matching Network slice computed.

The PDU session establishment in a Network Slice to a Data Network (DN) allows data transmission in a Network Slice. A PDU Session is associated with a S-NSSAI and a Data Network Name (DNN). Following diagram illustrates the procedure of PDU Session Establishment:

Figure 2-2 PDU Session Establishment

PDU Session Establishment

The following is performed for PDU Session Establishment:

  • If the AMF is not able to determine the appropriate NRF to query for the S-NSSAI provided by the UE, the AMF sends a GET request to the NSSF. The AMF queries the NSSF with this specific S-NSSAI, the NF type of the NF service consumer, Requester ID, PLMN ID of the SUPI, and the location information.
  • The NSSF determines and returns the appropriate NRF to be used for selecting NFs or services within the selected Network Slice instance. The NSSF may also return an NSI ID identifying the Network Slice instance to use for this S-NSSAI.

    When a PDU Session for a given S-NSSAI is established using a specific Network Slice instance, the cloud native provides the RAN with S-NSSAI corresponding to this Network Slice instance, which enables the RAN to perform access specific functions.

UE-Config-Update:

When the UDM updates the Subscribed S-NSSAI(s) to the serving AMF, based on configuration in the AMF, the NSSF determines the mapping of the Configured NSSAI for the serving PLMN and Allowed NSSAI to the Subscribed S-NSSAI(s).

Following diagram illustrates the procedure of UE-Config-Update:

Figure 2-3 UE-Config-Update

UE-Config-Update

The following is performed for UE-Config-Update:

  • The AMF sends a UE-Config-Update (GET) request to NSSF. NSSF checks and validates the Subscribed S-NSSAI(s), Requested S-NSSAI(s), PLMN ID of the SUPI, TAI, NF type, and NF instance ID. If message is valid, NSSF searches for Allowed S-NSSAI list based on policy configuration and input parameters.
  • NSSF responds with "200 OK with AuthorizedNetworkSliceInfo" if it finds a match.
  • The AuthorizedNetworkSliceInfo response for UE-Config-Update must mandatorily include both AllowedNSSAI and ConfiguredNSSAI. The AllowedNSSAI is computed by taking into account the input request and applying operator policies as specified in 3GPP Spec 29.531 Release 15.5.
  • NSSF calculates ConfiguredNSSAI by determining the intersection of S-NSSAI(s) in ConfiguredNSSAI for the PLMN (operator configured) and S-NSSAI(s) in SubscribedNSSAI (from the message indicating SubscribedNSSAI by the UE).
  • NSSF responds with error code if it finds incorrect parameter validation.

2.2 NSSAI Availability Service

The NSSAI Availability service is identified by the service name, Nnssf_NSSAIAvailability. The following operations are defined for this service:
  • Update Service Operation
  • Subscribe Service Operation
  • Unsubscribe Service Operation
  • Notify Service Operation
  • Delete Service Operation
  1. Update Service Operation

    The AMF uses this operation to update the NSSF with the supported S-NSSAI(s) on a per TA basis and to get informed on the S-NSSAIs available per TA (unrestricted) and the restricted S-NSSAI(s) per PLMN in that TA in the serving PLMN of the UE.

    Figure 2-4 Update the S-NSSAIs the AMF supports per TA

    Update the S-NSSAIs the AMF supports per TA
    • The NF service consumer (for example, AMF) sends a PUT request to NSSF with NSSAI availability information, identified by {nfId}, using NssaiAvailabilityInfo.
      • The message contains a list of S-NSSAIs supported by the AMF on a per TA basis.
    • NSSF checks (from operator configuration) if the S-NSSAIs are allowed in the TAI and responds with the S-NSSAIs that are authorized by NSSF and supported by the AMF for each TAI.
    • If none of the S-NSSAIs in the request are authorized by NSSF (either not configured by the operator or restricted by the operator), NSSF responds with a 403 message.
    • In cases where ONSSAI is true and the AMF sends a TaiList or TaiRangeList additionally:
      • NSSF computes the authorized S-NSSAIs for the main TAI.
      • NSSF includes only those TaiList or TaiRangeList that support all the authorized S-NSSAIs from the request in the response.
    • NSSF supports HTTP PATCH for NSAvailability Update.
    • Upon receiving a PUT or PATCH message, NSSF stores or updates the list in the session database.
    • NSSF authorizes the list based on NSSAI authorization rules and responds with the list of allowed S-NSSAIs for that AMF on a per TAI basis as per the request.
  2. Subscribe Service Operation

    The Subscribe operation is used by NF Service Consumer (AMF) to get the notifications for any change in NSSAI availability information.

    Figure 2-5 Subscription creation

    Subscription creation
    • AMF sends a POST request to NSSF with notification URL and a list of TAIs as JSON body.
    • NSSF stores the subscription request and responds with the list of allowed S-NSSAI(s) per TAI in the request. NSSF also returns a subscription-id and expiry (duration up to which NSSF sends notifications for any change in the status of Grant of S-NSSAI for subscribed TAI(s)).
  3. Unsubscribe Service Operation

    The Unsubscribe service operation is used by AMF to unsubscribe to a notification of any previously subscribed changes to the NSSAI availability information.

    Figure 2-6 Unsubscribe a Subscription

    Unsubscribe a Subscription
    • AMF sends a Delete request to NSSF with subscription-id.
    • NSSF checks for active subscription with the id and if found, deletes the subscription and responds with the message 204.
  4. Notify Service Operation
    The Notify service operation is used by the NSSF to update the AMF with any change in status, on a per TA basis, of the S-NSSAIs available per TA (unrestricted) and the S-NSSAIs restricted per PLMN in that TA in the serving PLMN of the UE.

    Figure 2-7 Update the AMF with any S-NSSAI restricted per TA

    Update the AMF with any S-NSSAI restricted per TA
    • NSSF sends notification to subscribed AMF when one or more following conditions are true:
      • There is change at Grant rules on S-NSSAI corresponding to one or more of TAIs subscribed by AMF.
      • An S-NSSAI has been added or deleted for one or more of TAIs subscribed by AMF.
  5. Delete Service Operation

    The AMF uses this operation to delete the NSSAI Availability information stored for that AMF in the NSSF.

    Figure 2-8 Delete the NSSAI Availability Information at NSSF

    Delete the NSSAI Availability Information at NSSF
    • The NF service consumer (For example: AMF) sends a DELETE request to NSSF with nfId.
    • The NSSF searches in session database for the NSAvailability data corresponding to nfId and deletes them.