title: N18 — NF ⇔ UDSF (Nudsf: the reference point for unstructured data storage) description: The N18 interface connecting any NF and the UDSF. Unlike N2/N4 it is an SBI (Service Based Interface), and its substance is the Nudsf service provided by the UDSF, running over HTTP/2 + JSON over TLS. The UDSF's own role and background are covered on the UDSF page; this page focuses on what is specific to N18 as an interface. keywords: - N18 - Nudsf - UDSF - unstructured data - stateless NF - HTTP2 - SBI - TS 29.598 tags: - Interface - Data - Rel-15 category: Interface

N18 — NF ⇔ UDSF (Nudsf: the reference point for unstructured data storage)¶

Learning objectives¶

After reading this page, you will be able to:

• Explain that N18 is an SBI (Service Based Interface) and that its Protocol/Transport is fundamentally different from N2 (NGAP/SCTP) and N4 (PFCP/UDP).
• Explain that the substance of N18 is the service Nudsf (TS 29.598) provided by the UDSF, and that "reference point N18" and "service Nudsf" are two views of the same thing.
• Explain that messages on N18 are RESTful operations over HTTP/2 and are observed on capture as HTTP/2.

Prerequisites¶

• The positioning of SBA / SBI and HTTP/2·TLS → Protocol dictionary
• The peer NF of N18 and the background of unstructured-data storage (required reading) → UDSF
• Contrast with the structured-data layer → UDR
• The idea of interfaces and reference points → Interface dictionary

Overview¶

N18 is the reference point connecting any NF ⇔ UDSF. However, N18 is an SBI (Service Based Interface), which is decisively different from N2 (NGAP/SCTP) and N4 (PFCP/UDP). SBI is a mechanism by which NFs call each other's services over a Web API (HTTP), using HTTP/2 + JSON (RESTful) over TLS as the common transport.

The substance of N18 is the service Nudsf (TS 29.598) provided by the UDSF. A consuming NF uses Nudsf to store, retrieve, update, delete, and subscribe/notify unstructured data. The contents and purpose of that data, and the point of statelessness—"what is deposited and why"—are explained on the UDSF page and are not repeated here.

Protocol / Transport¶

Architecture¶

The diagram shows how N18 connects any NF to the UDSF and stores unstructured data, in contrast with the structured-data side (the UDR: N35/N36/N37).

flowchart LR
  NF1["NF (e.g. AMF/SMF, etc.)"]
  NF2["another instance / another NF"]
  UDSF["UDSF"]
  UDR["UDR"]

  NF1 == "N18 (Nudsf / HTTP2)" ==> UDSF
  NF2 == "N18 (Nudsf / HTTP2)" ==> UDSF
  UDM["UDM/PCF/NEF"] -- "N35/N36/N37 (Nudr)" --> UDR

  classDef store fill:#efe,stroke:#3a3,stroke-width:2px;
  class UDSF,UDR store;

How to read the diagram: The thick lines (==>) are N18, the interaction between any NF (AMF/SMF, etc.) and the UDSF (SBI/HTTP2). Shown below for contrast is the structured-data side, where UDM/PCF/NEF access the UDR over N35/N36/N37 (Nudr). Note the division: N18 = unstructured (UDSF) / N35–37 = structured (UDR). The significance of externalizing data itself is covered on the UDSF page.

Main messages¶

Because N18 is an SBI, messages take the form of HTTP methods + resource operations (RESTful operations, not a procedureCode like NGAP). Representative operations are as follows.

The HTTP-operation mapping is a conceptual arrangement: The "HTTP operation" in the table above is a mapping for understanding; the exact method, resource URI, and call direction of each operation follow the OpenAPI definitions of TS 29.598. The strict method assignment is to be confirmed. Which NF deposits which unstructured data is implementation/deployment dependent (see UDSF).

Packet Analysis (Wireshark)¶

Because N18 is an SBI, it is observed on capture as HTTP/2 (L4-dedicated protocols like NGAP/SCTP or PFCP/UDP do not appear).

Decode highlights:

• N18 is encrypted with TLS. To see the contents (HTTP/2 headers or the JSON body) you need a decryption key (TLS session key, etc.). Without the key it appears only as the encrypted payload of tls.
• Once decrypted, you can identify operations by the HTTP/2 headers (:method = GET/PUT/POST/DELETE, etc.; a resource path whose :path contains .../nudsf/...).
• The body is carried as JSON, but note that the unstructured data inside is specific to each NF and 3GPP does not standardize its structure (interpretation depends on the depositing NF).
• Unlike N2's NGAP (binary, over SCTP), the key to observing N18 is that it is Web-like (HTTP/JSON). Do not use SCTP-specific filters (sctp.port, etc.).

3GPP Specification¶

• 3GPP TS 29.598 — the Nudsf (Unstructured Data Storage) service (the substance of N18). The specific section number is to be confirmed
• 3GPP TS 29.500 / TS 29.501 — the SBI framework (technical rules / design principles·OpenAPI). The specific section numbers are to be confirmed
• 3GPP TS 23.501 §4.2 — the reference-point representation of the system architecture (the definition of N18 = NF⇔UDSF; the Figure/sub-number is to be confirmed). Note: the functional definition of the UDSF itself is in the §6.2 series (see UDSF)
• 3GPP TS 33.501 — SBI security (TLS, etc.). The specific section number is to be confirmed

Note (to be confirmed): That N18 is the UDSF's reference point, that its substance is Nudsf (TS 29.598), and that the common SBI framework is the TS 29.500/29.501 series are general arrangements of the 3GPP architecture. The detailed section numbers, exact resource URIs, and HTTP method assignments of each operation differ by Release and are individually to be confirmed.

Summary¶

• N18 is the reference point connecting any NF ⇔ UDSF, but the fact that it is an SBI (Service Based Interface) makes it fundamentally different from N2 (NGAP/SCTP) and N4 (PFCP/UDP).
• Its Protocol is HTTP/2 + JSON (RESTful) over TLS, and its substance is the UDSF's service Nudsf (TS 29.598).
• Reference point N18 = service Nudsf are two views of the same NF⇔UDSF interaction (the reference-point view / the service view).
• The division is N18 = unstructured data (UDSF) / N35–37 = structured data (UDR).
• Background such as the contents of unstructured data, the significance of statelessness, the role split with the UDR, and the EPC comparison is covered on the UDSF page.

Next Step¶

• UDSF — the peer side of N18. The role and background of the unstructured-data storage NF are here (required reading)
• UDR — the structured-data storage layer (contrast with the N35/N36/N37 side)
• N35 — UDM⇔UDR (a reference point on the structured-data side)
• Interface dictionary — check the reference points