NRF — Network Repository Function¶

Learning objectives¶

• Explain that the NRF is the "repository of NFs (a phone book / DNS-like entity)" in the 5GC SBA.
• Distinguish and explain the three core functions: NF Registration, NF Discovery, and NF selection support.
• Explain which NFs the NRF works with and over which interface (SBI / Nnrf).
• Explain, from the perspective of dynamic discovery in the SBA, why 4G/EPC has no NF that directly corresponds to the NRF.

Prerequisites¶

• Basics of SBA (Service Based Architecture) / NF (Network Function) / SBI → Curriculum
• Overview of interfaces → Interface dictionary
• Overview of protocols (HTTP/2 + JSON over TLS, etc.) → Protocol dictionary

What you'll learn in this chapter¶

First, as the Why, we confirm the problem that in an SBA environment where NFs are numerous and scale dynamically, the location of a peer cannot be configured statically. Next, as the What, we organize how the NRF takes on the repository functions of NF Registration, NF Discovery, and selection support. Finally, as the How, together with the Nnrf services, we understand the flow in which each NF registers its profile with the NRF at startup, and an NF that wants to use another NF queries the NRF (Discovery) to find an appropriate instance.

Why — why it's needed¶

In the SBA, many NFs such as AMF, SMF, and AUSF provide their functions as services, and the number of instances increases or decreases dynamically according to demand. In such an environment, statically configuring the FQDN or IP address of the peer every time one NF calls another is impractical. A mechanism is needed to dynamically "register" and "discover" who provides which service, and under which PLMN and S-NSSAI. The NRF is what plays this role.

Analogy: The NRF is like a company phone book, the DNS of the internet, or a reception roster. Without the NRF, NFs would lose the means to know each other's location and would be unable to find where to call for a service.

Overview¶

The NRF provides, as the central repository of the SBA, three functions: (1) NF Registration, where each NF registers its own NF Profile at startup, (2) NF Discovery, where an NF queries for candidate peer NFs it wants to use, and (3) selection support, which provides information (load, capacity, etc.) for choosing an appropriate instance from the discovery results. These are realized through the Nnrf service group.

Basic Concept — explanation for beginners¶

When a new employee (a newly started NF) joins the company, they first submit an arrival notice (NF Profile) to general affairs (the NRF). The notice states their department (NF type), employee number (NF instance ID), the tasks they can handle (supported NF services), contact details (FQDN/IP endpoint), and so on. Later, when you want to find someone in another department (NF Discovery), general affairs looks up the roster it manages and tells you who matches your conditions. Because general affairs also holds information such as who is busy right now (load/capacity), it helps you pick the appropriate peer from multiple candidates.

Network Function details¶

Architecture¶

flowchart TB
  AMF["AMF"] -->|"register (Nnrf_NFManagement)"| NRF["NRF (repository)"]
  SMF["SMF"] -->|"register (Nnrf_NFManagement)"| NRF
  AUSF["AUSF"] -->|"register (Nnrf_NFManagement)"| NRF
  AMF -->|"discover SMF (Nnrf_NFDiscovery)"| NRF
  NRF -->|"respond with NF instance candidates"| AMF

How to read the diagram: Each NF such as AMF, SMF, and AUSF registers its own NF Profile with the NRF at startup using Nnrf_NFManagement. When an NF (AMF in the diagram) wants to use another NF (SMF), it queries the NRF with Nnrf_NFDiscovery and receives candidate NF instances matching its conditions. The AMF then selects an appropriate instance from those candidates and calls it.

Interface¶

For details, see the Interface dictionary.

Appearance in procedures¶

The NRF first appears in NF registration at each NF's startup. In addition, it also appears in Discovery just before an NF uses another NF (for example, in the context of the AMF selecting an SMF, or the SMF selecting a UPF, discovering peer candidates beforehand). In each of the Registration and PDU Session procedures, scenes appear where each NF discovers its collaborating peer via the NRF. The specific procedural details (message ordering and conditional branches) in each individual procedure are to be confirmed.

Comparison with EPC¶

4G/EPC has no NF that directly corresponds to the NRF. In the EPC, collaborating peers between NFs were configured relatively statically, and there was no dedicated function responsible for dynamic registration and discovery. Diameter's DRA (Diameter Routing Agent) is partially similar in that it supports routing, but its role is limited and not equivalent to the NRF (to be confirmed). Dynamic NF discovery in the SBA is a concept newly introduced in 5G.

Release differences¶

• Rel-15: The basic functions of the NRF (NF Registration / NF Discovery / selection support) were defined.
• Rel-16: The relationship with the SCP (Service Communication Proxy) (in indirect communication Model C/D, the SCP may be delegated Discovery to the NRF) and extensions such as hierarchical NRF are said to have been introduced (details to be confirmed).
• Rel-17: Further extensions are said to have been made (details to be confirmed).

3GPP Specification¶

• TS 23.501 §6.2.6: Functional definition of the NRF (its role in the SBA)
• TS 29.510: Definition of the Nnrf services (Nnrf_NFManagement / Nnrf_NFDiscovery) (individual section numbers and sub-clauses to be confirmed)
• TS 23.502: Behavior of the NRF in each procedure (details of Registration/Discovery procedures to be confirmed)

Summary¶

• The NRF is the repository that forms the foundation of the SBA, responsible for registration, discovery, and selection support of all NFs (a phone book / DNS-like entity).
• Its core functions are three: NF Registration, NF Discovery, and selection support (providing load/capacity, etc.).
• Each NF registers its NF Profile with the NRF at startup, and an NF that wants to use another NF queries the NRF to discover its peer.
• The services it provides are Nnrf_NFManagement / Nnrf_NFDiscovery (TS 29.510), and its interface is SBI (Nnrf).
• 4G/EPC has no directly corresponding NF; dynamic discovery in the SBA is a concept new to 5G.

Next Step¶

• NSSF — the NF responsible for slice selection support
• AMF — the central NF for access and mobility management
• SMF — the NF responsible for session management
• NF dictionary
• Interface dictionary