The Network Repository Function is essentially a database used to store/query information about NFs on the network.

To do this, the NRF has 3 services: the Nnrf_NFManagement Service is used to insert/update/remove records into the database of NFs on the network, the Nrf_NFDiscovery service is used to query this database, and the OAuth2 Authorization service is used to authenticate / authorize the transactions of the first two securely.

The Nnrf_NFManagement Service

The Nnrf_NFManagement Service is the service that handles the registration, deregistration and updating the database of service-producing NFs.

When a NF offering a service becomes ready for service, it registers with the NRF to advise the NRF of the services it offers and so service-consumers (Clients) are able to find and then request services from this NF.

Let’s take a look at the NFRegister process, which is the service operation used to register an NF in the NRF.

At its heart, the NFRegister process is a HTTP2 PUT sent from the NF to the NRF to the path <nrf_ip>/nnrf-nfm/v1/nf-instances/{nrf-instance-id} with a JSON payload containing all the specifics of the services being offered by the NF (NFProfile), which is sent to the NRF.

The NRF parses through the message body (That JSON Payload containing the NFProfile) and adds all this info to the internal database in the NRF as to which service producers it has available.

If this is all OK from the NRF’s perspective, then a 200 OK is sent back by the NRF.

The NFProfile JSON body

The NFProfile is a JSON encoded message body that contains all the info about the NF that the NRF takes and adds to its internal database of services offered by that NF.

nfInstanceID – The Network Function Instance ID is the unique identity if this NF. (It’s also included in the Request URI) It’s a unique ID and typically generated randomly by the NF itself.

nfType – The type of the NF itself, there’s a long list of supported NF Types, NRF, UDM, AMF, SMF, AUSF, etc, etc.

nfStatus – Is the status of the NF, either REGISTERED (Discoverable by other NFs), SUSPENDED (Registered in the NRF but not able to be discovered / invisible, NFs in this state are usually here because they’ve had a high failure rate or haven’t been responding to heartbeats), UNDISCOVERABLE (Used when an NF wants to stop offering services to new consumers, but is still able to offer services to existing consumers, the idea being this is taking it out of the discovery pool, but still able to serve in-process requests).

plmnList – This is a list of PLMNs that this NF serves.

fqdn – The Fully Qualified Domain name of the NF

IPv4Address / IPv6Address – The IPv4 and/or IPv6 address of the NF

nfService – This is a list of JSON objects, detailing the services offered by the NF, such as the service names it is offering, version of the service, IDs of the service instances and IP Endpoints for requests to be sent to.

priority / capacity / load – Used for selection of this NF from the pool and the order of results returned

The Nnrf_NFManagement Service also includes a Subscription service, to allow NFs to subscribe to changes in state, for example getting a notification if a NF that was registered becomes deregistered.