OmniSMF Operations Guide

Overview

OmniSMF implements the Session Management Function (SMF) of the 5G Core, responsible for the full lifecycle of PDU sessions. It anchors the N4 (PFCP) interface toward the UPF, the N7 interface toward the PCF, and the N10/N11 interfaces toward the UDM and AMF respectively.

Each PDU session is managed by a dedicated SessionWorker GenServer under a SessionSupervisor DynamicSupervisor (process-per-session architecture). All session state (QoS flows, PFCP context, UE IP, UP connection state) is owned by the worker process. One session crash does not affect others. All outbound SBI calls are made over plain HTTP using static URIs configured at startup; NRF-based dynamic discovery is not active in the current release.

Architecture

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3GPP Role and Specification References

Specification	Relevance
TS 23.501	System architecture — SMF role, PDU session concept
TS 23.502	Procedures — PDU Session Establishment (4.3.2), Modification (4.3.3), Release (4.3.4)
TS 29.502	Nsmf_PDUSession API (HTTP/2 SBI)
TS 29.244	N4 interface — PFCP protocol between SMF and UPF
TS 29.503	Nudm_SubscriberDataManagement and Nudm_UEContextManagement
TS 29.512	Npcf_SMPolicyControl — SM policy association toward PCF
TS 29.518	Namf_Communication — N1N2MessageTransfer toward AMF

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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
POST	/nsmf-pdusession/v1/sm-contexts	Nsmf_PDUSession	Create SM Context (PDU session establishment)	TS 29.502 6.1.6.2.2
POST	/nsmf-pdusession/v1/sm-contexts/{smContextRef}/modify	Nsmf_PDUSession	Update SM Context (N2 info, UP state change, handover, release flag)	TS 29.502 6.1.6.4.2
POST	/nsmf-pdusession/v1/sm-contexts/{smContextRef}/release	Nsmf_PDUSession	Release SM Context (session teardown)	TS 29.502 6.1.6.5.2

Request / Response Summary

Create SM Context — mandatory request fields: supi, sNssai, servingNetwork, dnn, smContextStatusUri, pduSessionId. Returns 201 Created with Location: /nsmf-pdusession/v1/sm-contexts/{ref}.

Update SM Context — dispatches on the first recognized key in the body:

Body key	Scenario
n2SmInfo + n2SmInfoType	N2 SM information from gNB (PDU_RES_SETUP_RSP / PDU_RES_REL_RSP)
upCnxState: DEACTIVATED	AN release / UE idle entry
upCnxState: ACTIVATING	Service request — returns N2 PDU_RES_SETUP_REQ
release: true	AMF-initiated release
servingNfId	AMF instance change during mobility

Release SM Context — body may include ueLocation for final location recording. Returns 204 No Content.

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Configuration Reference

Configuration is read from the application environment key :omnismf. The standard deployment mechanism is config/runtime.exs or OS environment variables mapped at startup.

config :omnismf,
  sbi_scheme: "http",
  sbi_addr:   "127.0.0.4",
  sbi_port:   7777,
  nrf_uri:    "http://127.0.0.10:7777",
  udm_uri:    "http://127.0.0.12:7777",
  pcf_uri:    "http://127.0.0.13:7777",
  amf_uri:    "http://127.0.0.5:7777",
  upf_addr:   "127.0.0.7",
  upf_pfcp_port: 8805,
  mcc: "999",
  mnc: "70",
  heartbeat_interval: 10_000,
  pfcp_addr: "127.0.0.4",
  ue_ip_pool: "10.45.0.0/16",
  dns:  ["8.8.8.8", "8.8.4.4"],
  dns6: ["2001:4860:4860::8888"],
  mtu:  1400

Parameter Table

Parameter	Type	Default	Description
sbi_scheme	string	"http"	HTTP scheme for the SBI listening socket (http or https)
sbi_addr	string	"127.0.0.4"	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 (NRF-based discovery is not active)
udm_uri	string	"http://127.0.0.12:7777"	Base URI of the UDM. Used for Nudm-SDM (sm-data) and Nudm-UECM (smf-registrations) calls
pcf_uri	string	"http://127.0.0.13:7777"	Base URI of the PCF. Used for Npcf-SMPolicyControl (sm-policies) calls
amf_uri	string	"http://127.0.0.5:7777"	Base URI of the AMF. Used for N1N2MessageTransfer (namf-comm). Overridden per-session by the smContextStatusUri host if present
upf_addr	string	"127.0.0.7"	IP address of the UPF PFCP endpoint (N4 interface)
upf_pfcp_port	integer	8805	UDP port of the UPF PFCP endpoint. Standard PFCP port per TS 29.244
mcc	string	"999"	Mobile Country Code of the serving PLMN
mnc	string	"70"	Mobile Network Code of the serving PLMN
heartbeat_interval	integer (ms)	10000	Interval in milliseconds between NRF heartbeat requests. Also used to derive the URR time threshold (heartbeat_interval / 1000 * 6 seconds)
dns	list of strings	["8.8.8.8", "8.8.4.4"]	IPv4 DNS server addresses delivered to the UE in PDU Session Establishment Accept
dns6	list of strings	["2001:4860:4860::8888"]	IPv6 DNS server addresses delivered to the UE in PDU Session Establishment Accept
pfcp_addr	string	"127.0.0.4"	Source IP address for PFCP (N4) messages sent to the UPF. Must be routable from the UPF
ue_ip_pool	string	"10.45.0.0/16"	CIDR subnet from which UE IPv4 addresses are allocated. Managed by UeIpPool GenServer with deduplication and release-on-teardown
mtu	integer	1400	MTU value advertised to the UE for the PDU session

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Key Procedures

PDU Session Establishment (TS 23.502 Section 4.3.2)

PDU Session Release (TS 23.502 Section 4.3.4)

UE Idle / Service Request (UP Connection State)

N4 PFCP Session Structure

Each PDU session results in the following PFCP IEs being installed on the UPF:

IE	ID	Direction	Purpose
PDR (uplink)	1	Access -> Core	Match GTP-U traffic from gNB on UPF N3 F-TEID
PDR (downlink)	2	Core -> Access	Match traffic from N6 by UE IP address
FAR (uplink)	1	Core	Forward to N6 (no outer header)
FAR (downlink)	2	Access	Initially BUFFER; updated to GTP-U FORWARD after PDU_RES_SETUP_RSP
QER	1	Both	Enforce session AMBR (uplink and downlink MBR)
URR	1	Both	Time-based usage reporting (threshold = heartbeat_interval * 6 s)

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Prometheus Metrics

SBI / PDU Session Metrics

Metric	Type	Tags	Description
omni_smf.pdu_session.create.count	counter	result	PDU session create operations
omni_smf.pdu_session.modify.count	counter	result	PDU session modify operations
omni_smf.pdu_session.release.count	counter	result	PDU session release operations
omni_smf.pdu_session_creates.total	counter	result, dnn	Total PDU session creates by DNN
omni_smf.pdu_session_releases.total	counter	result	Total PDU session releases
omni_smf.active_pdu_sessions.count	gauge	dnn	Number of active PDU sessions by DNN
omni_smf.session.duration_seconds	distribution	--	PDU session duration in seconds (buckets: 1, 5, 15, 30, 60, 300, 900, 3600, 86400)

PFCP / UPF Metrics

Metric	Type	Tags	Description
omni_smf.pfcp_sessions.total	counter	operation	Total PFCP session operations
omni_smf.upf.health	gauge	--	UPF PFCP association health (1=up, 0=down)

NRF Metrics

Metric	Type	Tags	Description
omni_smf.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

The following gaps were identified during an operational audit. Reference IDs correspond to internal tracking.

ID	Area	Description
SMF-H5	N4 Reporting	Session Report Request messages received from the UPF are not processed. Usage reports from URR triggers are silently discarded.
SMF-H7	NAS	The N1 SM container in SmContextCreateData is not decoded. NAS PDU Session Establishment Request content (5GSM capability, extended protocol configuration options) is not inspected or forwarded.
SMF-M1	Handover	Handover state handling is partial. Xn-based handover and inter-AMF N2 handover path switches are not fully supported; the servingNfId update path handles AMF change but the full handover procedure (TS 23.502 4.9) is not implemented.
SMF-M3	UE IP	UE IPv4 addresses are allocated randomly from 10.45.0.0/16 on each session establishment. There is no address pool management, no overlap detection, and no IPv6 prefix delegation.
SMF-M4	Notifications	SM context status notifications to the AMF callback URI (smContextStatusUri) are not sent. The AMF is not informed of SMF-initiated state changes (e.g., network-triggered release).
SMF-M5	NRF Discovery	An NRF registration module exists and the SMF registers with the NRF. However, all outbound SBI calls (to UDM, PCF, AMF) use static URIs from configuration. NRF-based discovery is not performed.
SMF-M7	PFCP Reporting	SessionReportResponse is not sent to the UPF in response to Session Report Requests.
SMF-L1	SM Context Retrieval	There is no GET /nsmf-pdusession/v1/sm-contexts/{ref} endpoint. SM context retrieval by external consumers is not supported.
SMF-L3	QoS Modification	QoS flow modification after session establishment is not supported. Adding, modifying, or deleting QoS flows (TS 23.502 4.3.3) cannot be triggered post-establishment.

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Troubleshooting

PDU Session Establishment Fails with 404 DNN_DENIED

The UDM returned session management subscription data that does not include the requested DNN. Verify that the UDM's subscriber data for the SUPI contains a dnnConfigurations entry matching the requested DNN (case-insensitive). Check the SMF log for the line SM context creation failed: dnn_not_found.

PDU Session Establishment Fails with 503 NF_DISCOVERY_FAILURE

The SMF could not reach the UDM at udm_uri. Verify connectivity from the SMF host to the configured udm_uri. Check firewall rules and that the UDM SBI port is reachable. The SMF log will show [SBI->UDM] GET sm-data failed with the underlying reason.

PFCP Session Establishment Rejected

The UPF returned a non-"Request accepted" cause in the PFCP Session Establishment Response. Common causes:

• upf_addr or upf_pfcp_port is incorrect — verify with ss -ulnp | grep 8805 on the UPF host.
• The UPF does not support the PDR/FAR IEs sent by the SMF — check UPF capability logs.
• PFCP heartbeat (association) is not established — the SMF must complete PFCP Association Setup before sending session requests. Check logs for [PFCP] Association messages at startup.

The SMF log will contain [PFCP] Session establishment rejected: <cause> or [PFCP] Session establishment failed: <reason>.

N1N2MessageTransfer to AMF Fails

After the SMF returns 201 to the AMF for context creation, it asynchronously sends the N1N2MessageTransfer. If this call fails, the PDU session is created in the SMF and UPF but the UE will not receive the Establishment Accept. The SMF logs [SBI->AMF] N1N2 transfer failed. This failure is currently non-fatal from the SMF's perspective — no retry is attempted. To recover, release the SM context and retry the PDU session establishment.

UE Receives No IP Address

IP allocation is performed at PFCP session establishment time using a random selection from 10.45.0.0/16. If the UPF returns a different IP in its Created PDR (UPF-allocated addressing), the SMF will use the UPF-returned F-TEID but will still record its own randomly generated IP. Ensure the UPF is configured to accept SMF-specified UE IP addresses or to allocate and return them via the Created PDR. See limitation SMF-M3.

High Session Count / Memory Growth

Session contexts are stored in an in-process Agent. They are only removed on explicit release_sm_context. If AMF or UE-initiated releases are missed (e.g., due to AMF restart), orphaned contexts will accumulate. Monitor with the /statistics internal API endpoint. There is currently no session timeout or garbage collection.

Log Correlation

Each log line from the session path is prefixed [{supi}:{pdu_session_id}]. Filter by SUPI to trace a subscriber's session lifecycle end-to-end. PFCP-path log lines are prefixed [PFCP] and SBI client calls are prefixed [SBI->UDM], [SBI->PCF], or [SBI->AMF].