Description
The Trusted Non-3GPP Gateway Function (TNGF) is a critical component within the 5G Core Network (5GC) architecture, specifically defined for the Non-3GPP InterWorking Function (N3IWF) in the context of trusted non-3GPP access. Its primary role is to facilitate secure and controlled connectivity for UEs that utilize non-3GPP radio access technologies, most notably trusted Wi-Fi networks. Architecturally, the TNGF resides in the user plane and control plane, interfacing with other core network functions. On the N1 reference point towards the UE, it terminates the N1 interface over the non-3GPP access, managing the signaling connection. It establishes IPsec Security Associations (SAs) with the UE to create secure tunnels for both control plane (N1) and user plane (N3) traffic. The TNGF connects to the Access and Mobility Management Function (AMF) via the N2 interface for control plane procedures and to the User Plane Function (UPF) via the N3 interface for data forwarding.
Operationally, when a UE initiates access via a trusted non-3GPP network, it discovers and selects a TNGF. The UE and TNGF perform mutual authentication and establish IPsec tunnels. The TNGF then acts as a proxy, relaying the UE's registration and session management signaling to the 5G Core via the AMF. It is responsible for encapsulating and decapsulating user plane packets between the IPsec tunnel and the N3 GTP-U tunnel towards the UPF. The TNGF also interacts with the Authentication Server Function (AUSF) and Unified Data Management (UDM) for credential-based authentication (e.g., using 5G-AKA or EAP-AKA').
A key aspect of the TNGF is its 'trusted' designation, which implies that the 5G Core network operator has a trust relationship with the non-3GPP access network provider. This trust can be based on a roaming agreement or direct ownership, allowing the core network to rely on the access network's security to a certain degree, though the TNGF still enforces its own security at the IPsec layer. The TNGF supports mobility and session continuity procedures, enabling handovers between 3GPP (e.g., NG-RAN) and trusted non-3GPP access without dropping the PDU Session. It is a fundamental enabler for the 5G convergence goal, providing a unified core network experience regardless of the underlying access technology.
Purpose & Motivation
The TNGF was introduced in 3GPP Release 16 to formally define and standardize the gateway function for trusted non-3GPP access within the 5G System (5GS). Prior to 5G, non-3GPP interworking (e.g., via ePDG in EPS) was often treated as an untrusted access, requiring heavy security termination at the gateway. The creation of the TNGF addresses the growing importance of high-quality, carrier-grade Wi-Fi and other fixed wireless accesses as integral parts of the mobile operator's service offering. It solves the problem of providing seamless, secure, and policy-coherent access to 5G core services over these alternative networks.
The motivation stems from the need for true access-agnostic service delivery. Operators sought to leverage their Wi-Fi deployments, or partnerships with Wi-Fi providers, as a trusted extension of their 5G radio coverage, especially for indoor environments and fixed wireless access scenarios. The TNGF provides a standardized architecture that ensures security (through mandatory IPsec), supports 5G-specific features like network slicing and QoS over the non-3GPP link, and enables smooth mobility. It addresses limitations of previous non-3GPP interworking solutions by being natively integrated into the 5G Service-Based Architecture (SBA), using the same authentication frameworks and policy control (via the PCF) as 3GPP access, thereby eliminating functional silos.
Architecture
In the Network Map
- Mobile Network → 5G Core → TNGF (Access Networks)
Classification
Release Timeline
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (29 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 16, the TNGF (Trusted Non-3GPP Gateway Function) was formally introduced as the standardized gateway for connecting trusted non-3GPP access networks to the 5G Core via the N2 and N3 interfaces. The release specified that a UE establishes an IPsec tunnel with the TNGF for registration and defined the NWt reference point between the UE and TNGF for secure connection and NAS transport. Furthermore, Release 16 enabled N3 terminations at the TNGF for user plane handling and extended network congestion notification mechanisms to include TNGF overload scenarios.
In Release 17, the TNGF was formally added as a supported Non-3GPP Access type alongside the N3IWF and TWIF. The specification introduced technical clarifications for trusted access using TNGF and defined a requirement for a layer below IPsec to enable NAT traversal for TNGF access. However, the Ta interface between the TNAP and TNGF and the Tn interface for inter-TNGF mobility were noted as not being specified in this release.
In Release 18, key enhancements for the TNGF focused on enabling slice-based selection, where the network can select a TNGF compatible with the specific network slice (S-NSSAI) required by the UE. The release also introduced procedures to abort a registration if the selected TNGF is incompatible with the UE's allowed NSSAI and defined protections for TNGF identifier information in REGISTRATION REJECT messages to prevent registration loops.
- TNGF selection enhancement for support of S-NSSAI needed by UE TS 23.501CR3953
- UE to indicate its support for Slice-based TNGF selection to the network TS 24.501CR5121
- Aborting registration procedure when the selected TNGF is not compatible with the allowed NSSAI TS 24.501CR5122
- Protecting the N3IWF/TNGF identifier information in the REGISTRATION REJECT message TS 24.501CR5932
- Support of TNGF selection for S-NSSAI TS 29.525CR0255
- Slice based TNGF and N3IWF selection TS 29.525CR0323
+ 11 more changes
In Release 19, the specification introduced support for mobility of a UE connected to a Trusted Non-3GPP Access Point (TNAP) to another TNAP that is connected to the same TNGF, enabling seamless handovers within a single gateway's domain. This release also specified the handling of specific unprotected REGISTRATION REJECT messages from the core network, ensuring the TNGF can properly process causes indicating an incompatible NSSAI with the selected gateway. Furthermore, corrections were made to refine the procedures for this intra-TNGF TNAP mobility feature.
- Mobility of the UE connected to a TNAP to another TNAP connected to the same TNGF TS 24.502CR0313
- Handling of unprotected REGISTRATION REJECT message with causes #81 and #82 (Selected N3IWF/TNGF is not compatible with the allowed NSSAI) TS 24.501CR6795
- Correction to Mobility of the UE connected to a TNAP to another TNAP connected to the same TNGF TS 24.502CR0316
In Release 20, the new feature for the TNGF is N3IWF/TNGF reselection considering energy related information. The specification clarifies that a UE establishes an IPsec tunnel with a TNGF to register over trusted non-3GPP access, but the new reselection procedure now incorporates energy efficiency criteria. This enhancement builds upon the existing architecture where the TNGF connects a trusted non-3GPP access network to the 5G Core via N2 and N3 interfaces.
- N3IWF/TNGF reselection considering energy related information. TS 23.501CR6493
Explore further
Broader topics and technologies where TNGF plays a role.
Defining Specifications
3GPP specifications that define or reference TNGF, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 24.502 vj20 | 5G Core Access via Non-3GPP Networks; Stage 3 | Rel-19 |
| TS 24.526 vj30 | UE Policies for 5GS; Stage 3 | Rel-19 |
| TS 29.214 vj20 | Policy and Charging Control over Rx | Rel-19 |
| TS 29.413 vj00 | NGAP for Non-3GPP Access | Rel-19 |
| TS 29.502 vj50 | 5G System; Nsmf Service Based Interface; Stage 3 | Rel-19 |
| TS 29.510 vj50 | NRF Service Based Interface Protocol | Rel-19 |
| TS 29.525 vj40 | 5G UE Policy Control Service Stage 3 | Rel-19 |
| TS 33.127 vj50 | Lawful Interception Architecture and Functions | Rel-19 |
| TS 33.128 vj50 | 3GPP TS 33.128: Lawful Interception Protocols | Rel-19 |
| TS 33.501 vk00 | 5G Security Architecture and Procedures | Rel-20 |
| TS 33.807 vg01 | 5G Wireline-Wireless Convergence Security Study | Rel-16 |
| TS 38.413 vj10 | NG Application Protocol (NGAP) | Rel-19 |
