1. Network Slice Selection Function (NSSF) Cloud Native User's Guide
  2. NSSF Supported Services

2 NSSF Supported Services

This section includes information about the service supported by NSSF.

Network Slice Selection Service

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

  • Initial Register: When the NSSF is able to find authorized network slice information for the requested network slice selection information, the response body includes a payload body containing at least the Allowed NSSAI, target AMF Set or the list of candidate AMF(s).
  • PDU Session Establishment: When NSSF receives PDU-Session establishment request from NF consumer, NSSF determines network slice which can serve the requested SNSSAI, based on user configured policies, and responds with URL of NRF which manages to the Slice and/or Slice ID of the matching Network Slice computed.
  • UE-Config-Update: When the UDM updates the Subscribed S-NSSAI(s) to the serving AMF, based on configuration in this AMF, the NSSF determines the mapping of the Configured NSSAI for the serving PLMN and Allowed NSSAI to the Subscribed S-NSSAI(s).
  1. Initial Register

    Following diagram illustrates the procedure of Initial Register:

    Figure 2-1 Initial Register

    img/initial-register.png

    The following is performed for Initial Register:

    • The AMF sends a GET request to the NSSF.

      The AMF request must include:
      • Requested NSSAI
      • the Mapping of Requested NSSAI to Configured NSSAI for the HPLMN
      • the Subscribed S-NSSAIs (with an indication if marked as default S-NSSAI)
      • any Allowed NSSAI

      The query parameters may also contain:

      • 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 this information, local configuration and other locally available information including RAN capabilities in the current Tracking Area for the UE, the 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, the NSSF may select one of them to serve the UE, or the NSSF may 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.
      • 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 in the current UE's tracking areas.
      • Based on operator configuration, the NSSF may determine the NRF(s) to be used to select NFs or services within the selected Network Slice instance(s).
    • When the NSSF is able to find authorized network slice information for the requested network, NSSF sends Discovery Request for AMF to NRF.
    • The NRF responds with list of 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, based on configuration, the list of candidate AMF(s). The 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. The 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.
  2. PDU Session Establishment

    The PDU Session Establishment in a Network Slice to a DN allows data transmission in a Network Slice. A PDU Session is associated to an S-NSSAI and a DNN. Following diagram illustrates the procedure of PDU Session Establishment:

    Figure 2-2 PDU Session Establishment

    img/pdu-session.png

    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 location information.
    • The NSSF determines and returns the appropriate NRF to be used to select NFs/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 CN provides to the (R)AN the S-NSSAI corresponding to this Network Slice instance to enable the R(AN) to perform access specific functions.

  3. UE-Config-Update

    When the UDM updates the Subscribed S-NSSAI(s) to the serving AMF, based on configuration in this 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

    img/ue-config-update.png

    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 in case NSSF finds a match.
    • NSSF responds with 200 OK with empty AuthorizedNetworkSliceInfo in case there is no match found.
    • NSSF responds with error code in case of incorrect parameter validation.

NSSAI Availability Service

The NSSAI Availability service is identified by the service operation name, Nnssf_NSSAIAvailability. For the Nnssf_NSSAIAvailability service the following service operations are defined:
  • 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 of 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

    img/update.png
    • The NF service consumer (Example: AMF) sends a HTTP PUT message to NSSF with NSSAI availability information, identified by {nfId}, with NssaiAvalabilityInfo as body. Body of message contains a list of S-NSSAIs supported by AMF on a per TA basis.
    • On receiving a PUT /PATCH message, NSSF stores/updates the list in the session database.
    • Supports HTTP PATCH for NS-Availability Update
    • The NSSF authorizes the list based on NSSAI Auth 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 AMF to subscribe to a notification of any changes in status of the NSSAI availability information (example: S-NSSAIs available per TA and the restricted S-NSSAI(s) per PLMN in that TA in the serving PLMN of the UE) upon this is updated by another AMF.

    Figure 2-5 Create a Subscription

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

    The Unsubscribe 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

    img/unsubscribe.png
    • AMF sends a Delete request to NSSF with subscription-id.
    • NSSF checks for active subscription with the id and if found, deletes the subscription. NSSF responds with 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

    img/notify.png
    • 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

    img/delete.png
    • The NF service consumer (example: AMF) sends a DELETE request to NSSF with {nfId}.
    • The NSSF searches in session database for the NS-Availability data corresponding to nfId and deletes.