Counter Subscription/Retrieval (N28 Session Creation)

This section describes the Counter Subscription/Retrieval (N28 Session Creation) call flow.

The decision to subscribe to N28 counters is determined based on the presence of the SpendingLimitRequest service configuration. If this service configuration is present in the policy, then the Policy Engine triggers the N28 session creation.

N28 Subscribe (N7 Create) Call Flow
Note
  • Counter retrieval:

    • Generating the list of counters to subscribe is based on the SpendingLimitSubscription and RequestPolicyCounters service configuration: SpendingLimitSubscription also includes a list of counters to subscribe to. The RequestPolicyCounters service also satisfies the same role (providing a list of counters to subscribe to) and is expected to be used in cases where counters can come from different sources. For example, specific counters per LDAP attribute.

    • Policy Engine sends the subscribe request to PCF REST EP. The REST EP in turn attempts to lookup a CHF (based either on local configuration or via NRF discovery).

      • If no endpoint is available, error response is generated towards the engine.

      • If local endpoint is available, the REST EP invokes the Nchf_SpendingLimitControl_Subscribe operation towards the CHF.

        • If no response/error response is received, an error response is generated towards the engine for further action.

        • On success response, the counter information is forwarded to engine for further action.

      • If discovery is performed and endpoint is available, the REST EP invokes the Nchf_SpendingLimitControl_Subscribe operation towards the CHF which is handled as mentioned above.

        • The discovered CHF EP is also cached locally (in a TTL cache) so that it can be used for subsequent N28 operations.

        • For subsequent operations, the one of the locally cached EPs is randomly selected (that matches the selection criteria). Currently, only the NFType is supported as selected criteria.

        • The cached NFs are expired from local cache based on the ValidityPeriod provided by NRF in discovery response. Any subsequent operation that requires the EP will then result in a fresh discovery.

        • The discovered NF profile caching is generic and currently applicable for both CHF and UDR.

N28 Subscribe (N7 Create) Call Flow Description

Step

Description

1

The SMF sends a N7 Create request to the PCF.

2

Based on the routing configuration, PCF configures the lookup or caches the CHF.

3

If CHF is not found locally, then the PCF sends a Discover CH request to NRF.

4

The NRF responds with the CHF profiles with the PCF.

5

The PCF sends a N28 Subscribe request to the CHF.

6

The CHF responds with the N28 Subscribe result to the PCF.

7

The PCF performs the policy calculation.

8

The PCF sends the N7 Create response after calculating the policy to the SMF.