Procedure: Network-Triggered Additional PDU Session (URSP-based)

Mirrored documentation

Mirrored read-only from claudia-5gc @ v2.3.1. Edit upstream, not here.

Spec: TS 23.502 §4.3.2.2.1 (UE-requested PDU Session Establishment) + §4.2.4.3 (UE Policy delivery), TS 23.503 §6.6.2 (URSP), TS 24.526 (URSP encoding), TS 24.501 §6.4.1 (UE-requested PDU session procedures) Status: 🟢 Implemented Primary NFs: PCF, AMF, SMF Other NFs involved: UDR, UPF, UDM Trigger surface: Management Portal /qos → “NW-Triggered PDU Session” panel · POST /api/v1/qos/nw-sessions

1. 3GPP Analysis — why there is no “NW-initiated PDU Session Establishment”

In 5GS, PDU Session Establishment is always UE-requested (TS 23.502 §4.3.2.2.1). Unlike EPS (NW-initiated dedicated bearer activation, TS 23.401 §5.4.1), there is no NAS message by which the core can directly create a new PDU session at the UE. The standard-conformant mechanism for the network to cause an additional PDU session when the subscriber starts using a new application/service is URSP steering:

1. Application detection — the network learns the UE is using a new app: UPF traffic detection reported via PCC (TS 23.503 §6.2.2.2) or an AF request to the PCF (Npcf_PolicyAuthorization, TS 23.503 §6.1.3.18). In this project the detection event is simulated by an operator action in the management portal (acting as the AF).
2. UE Policy update — the PCF builds an updated URSP rule whose Traffic Descriptor matches the new application’s traffic (DNN and/or destination FQDN/IP) and whose Route Selection Descriptor points at a DNN/S-NSSAI combination not matching any established PDU session. The PCF delivers it through the AMF via the UE Policy delivery procedure (TS 23.502 §4.2.4.3): DL NAS TRANSPORT, Payload container type 0x05, carrying a MANAGE UE POLICY COMMAND (TS 24.501 Annex D).
3. URSP evaluation at the UE (TS 23.503 §6.6.2.3) — application traffic matches the new rule; no existing PDU session satisfies the Route Selection Descriptor; the UE therefore initiates a UE-requested PDU Session Establishment for an additional session with a new PSI (TS 24.501 §6.4.1.2; up to 15 concurrent PSIs).
4. QoS binding — at establishment the SMF performs Npcf_SMPolicyControl_Create (N7, TS 29.512 §5.2.2.2); the PCF returns the 5QI/ARP/Session-AMBR configured for this trigger, so the additional session comes up with the requested QoS profile end-to-end (N1 QoS rules, N2 QoS flow profile, N4 QER).

Testbed note: stock UERANSIM v3.2.8 does not implement URSP (it logs Unhandled payload container type [5] and never ACKs the MANAGE UE POLICY COMMAND), so step 3 had to be simulated by driving a hand-built nr-cli ps-establish with explicit S-NSSAI/DNN. The modified UERANSIM in this repo implements URSP delivery + evaluation: step 3 is now a real UE-side decision via nr-cli <ue> -e "ursp-establish <dnn|app>", which picks the S-NSSAI/DNN/SSC mode from the matched URSP rule. See tools/ueransim/FEATURES.md (tools/ueransim/patches/0010,0020).

2. Sequence

sequenceDiagram
  participant OP as Portal (AF / app-detection)
  participant PCF
  participant UDR
  participant AMF
  participant UE as UE (UERANSIM)
  participant SMF
  participant UPF

  Note over OP: Operator/AF signals: "UE started app X"<br/>POST /api/v1/qos/nw-sessions
  OP->>PCF: PUT /pcf-internal/v1/subscribers/{supi}/sm-policy-override<br/>{5qi, dnn, ambr} (DNN-scoped QoS for the upcoming session)
  OP->>UDR: upsert subscription_policy URSP rule<br/>(TD: dnn[, fqdn] → RSD: dnn + S-NSSAI)
  OP->>AMF: POST /amf/v1/ue-contexts/{supi}/push-policies
  AMF->>PCF: Npcf_UEPolicyControl_Create (N15, TS 29.525)
  PCF->>UDR: Nudr_DR policy-data query (N36)
  UDR-->>PCF: per-subscriber URSP rules
  PCF-->>AMF: UE Policy Container (TS 24.526 §4.2, base64)
  AMF->>UE: DL NAS TRANSPORT (payload container 0x05,<br/>MANAGE UE POLICY COMMAND — TS 24.501 Annex D)
  Note over UE: URSP evaluation (TS 23.503 §6.6.2.3)<br/>SIMULATED: nr-cli ps-establish IPv4 --sst --sd --dnn
  UE->>AMF: UL NAS TRANSPORT (PDU Session Establishment Request,<br/>new PSI — TS 24.501 §6.4.1.2)
  AMF->>SMF: Nsmf_PDUSession_CreateSMContext (TS 29.502)
  SMF->>PCF: Npcf_SMPolicyControl_Create (N7, TS 29.512 §5.2.2.2)
  PCF-->>SMF: qosDecs {5qi, arp} + sessRules {sessAmbr}<br/>x5gcQosSource=PCF_OVERRIDE
  SMF->>UPF: PFCP Session Establishment (N4, QER with new QFI — TS 29.244 §7.5.2.5)
  SMF-->>AMF: N1N2MessageTransfer (N2 PDU Session Resource Setup Request + N1 Accept)
  AMF->>UE: PDU Session Establishment Accept (QoS rules, new PSI)
  Note over OP: verify via SMF GET /nsmf-management/v1/sessions:<br/>additional PSI ACTIVE with requested 5QI

3. Trigger API (portal orchestration — not a 3GPP SBI)

POST /api/v1/qos/nw-sessions

{
  "supi": "imsi-001010000000001",
  "app": "cloud-gaming",
  "dnn": "internet",
  "sst": 1,
  "sd": "000001",
  "5qi": 3,
  "ambr_uplink": "50 Mbps",
  "ambr_downlink": "200 Mbps"
}

Response: ordered steps[] (pcf_qos_override, ursp_rule_store, ursp_push, ue_establish, verify) each with success, duration_ms, detail — plus the resulting session (pdu_session_id, ue_ip, 5qi, qos_source).

4. Key IEs / data

Element	Where	Spec
Traffic Descriptor (DNN, optional FQDN)	URSP rule	TS 24.526 §5.2
Route Selection Descriptor (SSC mode, S-NSSAI, DNN, PDU session type)	URSP rule	TS 24.526 §5.3
Payload container type 0x05 (UE policy)	DL NAS TRANSPORT	TS 24.501 §9.11.3.40
PDU Session ID (new PSI, UE-assigned)	UL NAS TRANSPORT	TS 24.501 §9.4
qosDecs / sessRules (5QI, ARP, AMBR)	Npcf_SMPolicyControl_Create response	TS 29.512 §5.2.2.2
QER (gate + MBR per QFI)	PFCP Session Establishment	TS 29.244 §7.5.2.5

5. Error cases

Case	Behaviour
UE not registered	ursp_push step fails (AMF 404) → orchestration aborts; policy + override remain stored for next registration
UE CM-IDLE	AMF returns 409; URSP delivery deferred; UE establishment still attempted (nr-cli triggers Service Request implicitly)
Session for same DNN/S-NSSAI already exists	UERANSIM establishes an additional PSI anyway (allowed by TS 24.501 — multiple sessions per DNN are legal, e.g. different SSC modes); the verify step reports the newest PSI
PCF override rejected (invalid 5QI)	pcf_qos_override fails → abort, nothing pushed
UERANSIM container not running	ue_establish fails with actionable error (start scenario via portal)
Max 15 PSIs reached	UERANSIM rejects locally; verify finds no new PSI → step fails

6. Validation

See docs/validation-commands.md §15 → NW-Triggered Additional PDU Session, and the E2E checklist embedded in the portal panel (/qos).