OmniNSSF Operations Guide

Overview

OmniNSSF implements the Network Slice Selection Function (NSSF) of the 5G Core. It provides two SBI services: Nnssf_NSSelection (TS 29.531) which answers AMF queries about which network slices a UE may use, and Nnssf_NSSAIAvailability (TS 29.531) which accepts NSSAI availability reports from AMFs and manages subscriptions to availability change notifications.

Slice configuration — the mapping of S-NSSAIs to NRF URIs, allowed NSSAI per PLMN, and AMF set assignments — is loaded from application configuration at startup. NSSAI availability state reported by AMFs is held in-process in an Agent. Availability change notifications are delivered to subscribers as fire-and-forget HTTP POST calls.

Architecture

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3GPP Role and Specification References

Specification	Relevance
TS 23.501	System architecture — NSSF role, network slice concept, S-NSSAI definition (Section 5.15)
TS 23.502	Procedures — slice selection during registration (4.2.3.2), PDU session establishment (4.3.2)
TS 29.531	Nnssf_NSSelection and Nnssf_NSSAIAvailability APIs
TS 23.003	S-NSSAI structure (SST, SD), NSI definition

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SBI Endpoints

All endpoints are served under the base URL {sbi_scheme}://{sbi_addr}:{sbi_port}.

Method	Path	Service	Description	Spec
GET	/nnssf-nsselection/v2/network-slice-information	Nnssf_NSSelection	Retrieve authorized network slice info for a UE	TS 29.531 5.2.1
PUT	/nnssf-nssaiavailability/v1/nssai-availability/{nfId}	Nnssf_NSSAIAvailability	Report or replace NSSAI availability for an AMF	TS 29.531 5.2.2.2
DELETE	/nnssf-nssaiavailability/v1/nssai-availability/{nfId}	Nnssf_NSSAIAvailability	Remove NSSAI availability record for an AMF	TS 29.531 5.2.2.4
POST	/nnssf-nssaiavailability/v1/nssai-availability/subscriptions	Nnssf_NSSAIAvailability	Subscribe to NSSAI availability change notifications	TS 29.531 5.2.2.3
DELETE	/nnssf-nssaiavailability/v1/nssai-availability/subscriptions/{subscriptionId}	Nnssf_NSSAIAvailability	Cancel a NSSAI availability subscription	TS 29.531 5.2.2.5

Request / Response Summary

GET network-slice-information — mandatory query parameters: nf-id, nf-type. Optional: slice-info-request-for-registration, slice-info-request-for-pdu-session, snssai, tai, home-plmn-id, supported-features. Returns 200 OK with AuthorizedNetworkSliceInfo.

PUT nssai-availability/{nfId} — body: NssaiAvailabilityInfo (must contain supportedSnssaiList or supportedNssaiAvailabilityData). Returns 200 OK with AuthorizedNssaiAvailabilityInfo. Triggers notification to all current subscribers.

POST subscriptions — body: NssaiAvailabilitySubscription (must contain nfNssaiAvailabilityUri). Returns 201 Created with subscription object and Location header.

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Configuration Reference

Configuration is read from the application environment key :omnissf.

config :omnissf,
  sbi_scheme: "http",
  sbi_addr:   "127.0.0.14",
  sbi_port:   7777,
  nrf_uri:    "http://127.0.0.10:7777",
  mcc: "999",
  mnc: "70",
  heartbeat_interval: 10_000,

  nsi_list: [
    %{
      s_nssai: %{sst: 1, sd: "0x000001"},
      nrf_uri: "http://127.0.0.10:7777",
      nsi_id:  "1"
    }
  ],

  allowed_nssai: %{
    "999-70" => [
      %{sst: 1, sd: "0x000001"}
    ]
  },

  amf_set_mapping: %{
    "1-0x000001" => ["1"]
  },

  configured_nssai: %{}

Base Parameter Table

Parameter	Type	Default	Description
sbi_scheme	string	"http"	HTTP scheme for the SBI listening socket (http or https)
sbi_addr	string	"127.0.0.14"	IP address the SBI HTTP server binds to
sbi_port	integer	7777	TCP port the SBI HTTP server listens on
nrf_uri	string	"http://127.0.0.10:7777"	Base URI of the NRF. Used for NF registration and heartbeat only
mcc	string	"999"	Mobile Country Code of the home PLMN. Used as the fallback PLMN key when home-plmn-id is absent from a selection query
mnc	string	"70"	Mobile Network Code of the home PLMN
heartbeat_interval	integer (ms)	10000	Interval in milliseconds between NRF heartbeat requests

Slice Configuration Parameters

nsi_list

A list of Network Slice Instance (NSI) entries. Each entry binds an S-NSSAI to an NRF URI. When the NSSF receives a slice selection query, it looks up the requested S-NSSAI in this list and returns the associated NRF URI in the nsiInformation field of the response.

Field	Type	Description
s_nssai.sst	integer	Slice/Service Type (1–255). Standard values: 1=eMBB, 2=URLLC, 3=MIoT (TS 23.501 Table 5.15.2.2-1)
s_nssai.sd	string or nil	Slice Differentiator as a hex string (e.g., "0x000001"). nil or absent matches any SD for the given SST (see NSSF-L5)
nrf_uri	string	Base URI of the NRF responsible for NF discovery within this slice instance
nsi_id	string	Opaque identifier for this NSI, included in the nsiInformation response

If no nsi_list entry matches the requested S-NSSAI, the NSSF returns 403 Forbidden with cause SNSSAI_NOT_FOUND.

allowed_nssai

A map from PLMN key ("{mcc}-{mnc}") to a list of S-NSSAI structs. Controls which slices are included in the allowedNssaiList field of the selection response. If the requesting PLMN has no entry, the home PLMN's allowed NSSAI is used as a fallback.

Each S-NSSAI in the list:

Field	Type	Description
sst	integer	Slice/Service Type
sd	string or nil	Slice Differentiator

amf_set_mapping

A map from S-NSSAI key ("{sst}-{sd}" or "{sst}" when no SD) to a list of AMF set ID strings. When populated, the NSSF includes the first entry as targetAmfSet in the selection response (see limitation NSSF-L3 — candidateAmfList is not populated).

configured_nssai

A map from S-NSSAI key to a configured NSSAI structure for roaming scenarios. When present for a given S-NSSAI, the NSSF includes configuredNssai in the selection response. Empty by default.

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Key Procedures

Network Slice Selection — Registration (TS 23.502 Section 4.2.3.2)

Network Slice Selection — PDU Session (TS 23.502 Section 4.3.2)

NSSAI Availability Reporting (TS 29.531 Section 5.2.2)

NSSAI Availability Subscription / Notification

S-NSSAI Lookup Logic

The NSSF matches a requested S-NSSAI against nsi_list using the following rules:

1. SST must match exactly.
2. SD matching: if the nsi_list entry has sd: nil, it matches any requested SD for that SST (wildcard). If the entry has a specific SD, it is compared case-insensitively after hex normalization.

This means an nsi_list entry with sd: nil acts as a catch-all for its SST value. See limitation NSSF-L5 for the operational implications of this behavior.

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Prometheus Metrics

NSSF Metrics

Metric	Type	Tags	Description
omni_nssf.nsselection.requests.count	counter	result, nf_type	NS selection requests
omni_nssf.nssai_availability.update.count	counter	nf_id	NSSAI availability updates
omni_nssf.nssai_availability.delete.count	counter	nf_id	NSSAI availability deletions
omni_nssf.nssai_availability.subscribe.count	counter	--	Availability subscriptions
omni_nssf.nssai_availability.unsubscribe.count	counter	--	Availability unsubscriptions
omni_nssf.ns_selection_requests.total	counter	result	Total network slice selection requests
omni_nssf.nssai_availability_updates.total	counter	--	Total NSSAI availability update notifications
omni_nssf.nrf.registration.status	gauge	nf_type	NRF registration status (1=registered, 0=not)

BEAM VM Metrics

Metric	Type	Description
beam.memory.total	gauge	Total BEAM memory in bytes
beam.memory.processes	gauge	Memory used by Erlang processes
beam.memory.processes_used	gauge	Memory actually used by processes
beam.memory.system	gauge	System memory
beam.memory.atom	gauge	Total atom memory
beam.memory.atom_used	gauge	Used atom memory
beam.memory.binary	gauge	Binary memory
beam.memory.code	gauge	Code memory
beam.memory.ets	gauge	ETS table memory
beam.processes.count	gauge	Number of Erlang processes
beam.ports.count	gauge	Number of Erlang ports
beam.atom.count	gauge	Number of atoms
beam.vm.uptime	gauge	VM uptime in seconds

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Known Limitations

ID	Area	Description
NSSF-M2	NSSAI Availability PATCH	There is no PATCH /nnssf-nssaiavailability/v1/nssai-availability/{nfId} endpoint. Partial updates to an AMF's availability data are not supported; the full record must be replaced with PUT.
NSSF-M3	Availability Authorization	The NSSF's authorization of reported NSSAI availability is pass-through. The authorizedNssaiAvailabilityData in the PUT response is identical to the submitted supportedNssaiAvailabilityData. The NSSF does not cross-check reported S-NSSAIs against the configured nsi_list.
NSSF-L1	Supported Features	The supported-features query parameter is accepted but not processed. No capability negotiation is performed between the AMF and NSSF.
NSSF-L2	Rejected NSSAI	The AuthorizedNetworkSliceInfo response does not include rejectedNssaiInRa (rejected in registration area) or rejectedNssaiInTa (rejected in tracking area) fields. UEs whose requested S-NSSAIs are not available in a specific TA will not receive explicit rejection information.
NSSF-L3	Candidate AMF List	The candidateAmfList field is not populated in the selection response. Only targetAmfSet (the first entry from amf_set_mapping) is included when applicable. Full AMF selection assistance per TS 29.531 is not provided.
NSSF-L4	Subscription PATCH	There is no PATCH /nnssf-nssaiavailability/v1/nssai-availability/subscriptions/{subscriptionId} endpoint. Subscription parameters cannot be updated after creation; the subscription must be deleted and re-created.
NSSF-L5	SD Wildcard Matching	An nsi_list entry with s_nssai.sd: nil matches any SD for the given SST, not just requests without an SD. This means a wildcard entry will also match explicit SD requests. If multiple slices share an SST but differ by SD, each must have its own explicit SD entry in nsi_list ordered before any wildcard entry (the first match wins).

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Troubleshooting

Slice Selection Returns 403 SNSSAI_NOT_FOUND

No entry in nsi_list matches the requested S-NSSAI. Verify:

1. The SST in the request matches an nsi_list entry exactly (integer comparison).
2. The SD in the request matches the entry's SD, or the entry has sd: nil (wildcard).
3. The nsi_list configuration was reloaded after changes — OmniNSSF reads nsi_list from application env at startup only. A restart is required to pick up changes.

The NSSF log will show NSSelection: Cannot find NSI for S-NSSAI [SST:{sst} SD:{sd}].

Slice Selection Returns 400 MANDATORY_QUERY_PARAM_MISSING

The nf-id or nf-type query parameter is absent from the request. Both are mandatory per TS 29.531. Verify the AMF is including these parameters in its selection request.

Allowed NSSAI Missing from Selection Response

The allowedNssaiList field is only included in the response when a PLMN key can be determined from the request. The PLMN key is derived from the home-plmn-id query parameter, or falls back to the NSSF's own MCC/MNC (mcc/mnc config). If neither can be parsed, the field is omitted. Verify:

1. mcc and mnc are correctly configured for the home PLMN.
2. The allowed_nssai map contains an entry for the key "{mcc}-{mnc}".

TAI-Based Filtering Removes All Allowed NSSAIs

When a tai parameter is provided and no AMF has reported availability data via PUT, the NSSF defaults to allowing all configured NSSAIs (empty availability map = no restriction). Once any AMF reports availability, only S-NSSAIs included in a matching TAI entry in that availability data are returned. If the TAI in the query does not appear in any AMF's reported supportedNssaiAvailabilityData, all NSSAIs will be filtered out. To diagnose, check whether AMFs have submitted PUT requests and whether the TAC in those reports matches the TAC in the query.

SD Wildcard Causes Wrong NSI to Be Selected

If an nsi_list entry with sd: nil is listed before more specific entries, it will match first for any SST request regardless of the SD. Ensure specific SD entries appear before wildcard entries in the nsi_list. See limitation NSSF-L5.

Availability Notifications Not Received by Subscriber

Notifications are sent asynchronously (fire-and-forget) in a spawned Task. Delivery failures are logged as warnings but are not retried. Verify:

1. The nfNssaiAvailabilityUri in the subscription is reachable from the NSSF host.
2. The subscriber NF is accepting POST requests at that URI and returning a 2xx response.

Failed notification attempts are logged as NSSAIAvailability: notification to {uri} failed: {reason}.

Stale Availability or Subscription State After AMF Restart

NSSAI availability records and subscriptions are stored in-process and are not persistent. They survive OmniNSSF restarts only for the lifetime of the Erlang VM. If the AMF restarts and does not re-register its availability via PUT, the NSSF will continue to serve stale (or absent) availability data for that AMF's NF ID. The AMF should re-submit its NSSAI availability on reconnection. Similarly, subscriptions created before an NSSF restart must be re-created by the subscriber.

Log Correlation

NSSAI selection log lines are prefixed NSSelection: and include the requesting nf-id. NSSAI availability log lines are prefixed NSSAIAvailability: and include the nfId. Subscription notification attempts log the callback URI. Use nf-id values to correlate selection requests with availability updates from the same AMF.