Description
The Intermediate Session Management Function (I-SMF) is a core network function within the 5G System (5GS) defined from 3GPP Release 16 onwards. It is a specialized instance of the SMF designed to operate in conjunction with an Anchor SMF (A-SMF). The primary architectural role of the I-SMF is to provide localized session management when a User Equipment (UE) connects via an access network that is different from the one served by its A-SMF, or when the UE moves into a data network locality that requires a separate user plane path. In such scenarios, the I-SMF is inserted into the control plane path between the Access and Mobility Management Function (AMF) and the A-SMF.
Operationally, the I-SMF is responsible for managing the Protocol Data Unit (PDU) Session for the UE within its local domain. This includes interacting with the local User Plane Function (UPF) – often an Intermediate UPF (I-UPF) – for user plane management, handling local policy enforcement, and performing session-related signaling with the AMF. Crucially, the I-SMF relays certain session management messages to and from the A-SMF, which retains overall responsibility for the PDU Session, including interaction with the Unified Data Management (UDM) for subscription data and the Policy Control Function (PCF) for policy decisions. The I-SMF essentially acts as a proxy, allowing the A-SMF to remain the session anchor point while delegating local management tasks.
Key components of the I-SMF's functionality include its N11 interface to the AMF, its N16 interface to the A-SMF (using the Nsmf_PDUSession service), and its N4 interface to the locally deployed UPF. It performs local UPF selection, establishes, modifies, and releases N4 sessions, and enforces QoS rules provided by the PCF via the A-SMF. The I-SMF's role is vital in enabling efficient routing for scenarios like access to local area data networks (LADNs), non-3GPP interworking (e.g., with Wireline Access), and mobility events where inserting a local breakout point reduces latency and backhaul traffic. It decouples local access topology from the core anchor point, providing significant flexibility in 5G network design.
Purpose & Motivation
The I-SMF was introduced in Release 16 to address specific architectural challenges arising from the 5G Core's service-based architecture and the need for efficient support of edge computing, non-3GPP access, and complex mobility. Prior to its introduction, the SMF was a monolithic entity managing the entire PDU Session from a single logical point. This model became inefficient when a UE accessed services via a non-3GPP network (like WLAN) or moved into a localized service area (like a factory campus). Traffic would need to be backhauled to the anchor UPF controlled by the A-SMF, introducing unnecessary latency and load on the core network transport.
The I-SMF solves this by enabling a split SMF architecture. It allows network operators to deploy a lightweight SMF instance locally, close to the UE's point of attachment. This local I-SMF can manage the local user plane path (via an I-UPF) for low-latency breakout, while the A-SMF in the central cloud maintains the session's anchoring and overall policy context. This separation is particularly critical for supporting Ultra-Reliable Low-Latency Communications (URLLC) and efficient Mobile Edge Computing (MEC), where user plane functions must be deployed at the network edge. The I-SMF concept directly addresses the limitations of a single, centralized SMF by providing topological flexibility and enabling efficient data routing tailored to the UE's current location and access type.
Classification
Release Timeline
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (98 CRs across 4 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 16, the I-SMF function was formally introduced to manage a PDU session when a UE moves outside its original SMF's service area, establishing a new N16a interface between the SMF and I-SMF. The release specified procedures for I-SMF insertion, change, and removal, including updates to service operations like Create, Update, Retrieve, and Release SM Context. It also detailed the I-SMF's new responsibilities, such as controlling UPF settings for traffic offload, inserting/removing UL CL/BP and PSA nodes in the data path, and handling area restrictions for the S-NSSAI.
- UL CL/BP controlled by I-SMF TS 23.501CR0848
- Transfer of N4 information for local traffic switching from SMF to I-SMF TS 23.501CR1050
- Traffic offload by UPF controlled by the I-SMF TS 23.501CR1179
- Update SM context service operation for additional PDU Session Anchor and Branching Point or UL CL controlled by I-SMF TS 29.502CR0124
- Update Retrieve SM Context service operation from Source I-SMF or SMF TS 29.502CR0123
- Update Release SM Context service operation for I-SMF change and removal TS 29.502CR0122
+ 48 more changes
In Release 17, the I-SMF function was enhanced with new procedures for its selection, restoration, and removal, particularly based on Data Network Access Identifier (DNAI) changes. Specific updates included defining the handover of a PDU session with an I-SMF from 3GPP to non-3GPP access and clarifying the management of N9 forwarding tunnels between UPFs controlled by the I-SMF. Furthermore, the release introduced protocol support for restoration procedures and specified the SMF's behavior for I-SMF insertion and removal, including interactions over the N16a interface.
- KI #1-1, I-SMF selection TS 23.501CR2634
- I-SMF/V-SMF Restoration procedure TS 29.502CR0444
- Downlink Tunnel Info of NG-RAN in I-SMF selection per DNAI TS 29.502CR0462
- I-SMF removal for target DNAI TS 29.502CR0491
- Void I-SMF/V-SMF Restoration procedure from normal specification clauses TS 29.502CR0494
- Update on I-SMF selection per DNAI TS 23.501CR2914
+ 12 more changes
In Release 18, the I-SMF function was enhanced to support the population of a full Data Network Access Identifier list, the exchange of specific PDU session parameters, and the support of Explicit Congestion Notification marking for Low Latency, Low Loss, Scalable throughput between the V/I-SMF and the (H-)SMF. Furthermore, the release introduced capabilities for conveying periodicity and N6 jitter information and defined a specific error code for exceeded slice data rate during PDU session establishment when a V/I-SMF is involved.
- Populating a fullDnaiList to (new) I-SMF TS 29.502CR0577
- PDU set Parameters between the V/I-SMF and the (H-)SMF TS 29.502CR0711
- Support of ECN marking for L4S between the V/I-SMF and the (H-)SMF TS 29.502CR0723
- Periodicity and N6 Jitter Information associated with Periodicity between the V/I-SMF and the (H-)SMF TS 29.502CR0727
- EXCEEDED_SLICE_DATA_RATE error for PDU session establishment with V/I-SMF TS 29.502CR0753
In Release 19, the I-SMF function was significantly enhanced to support Local Offloading Management, with new capabilities for selection and insertion based on a local offloading allowed indication. Key enhancements were made to the N11 and N16a interfaces to support this management, and the PCF was made explicitly aware of I-SMF insertion for these offloading procedures. Furthermore, the release introduced support for the I-SMF to manage Edge Application Server Deployment (EAS Deployment) information and clarified its role in handling area restrictions for always-on PDU Sessions.
- I-SMF selection/insertion based on local offloading allowed indication TS 23.501CR5604
- Local Offloading handling at I-SMF TS 23.501CR5744
- Local Offloading handling at I-SMF TS 23.501CR5825
- PCF's awareness of I-SMF insertion for Local Offloading Management TS 23.501CR5833
- I-SMF selection/insertion based on local offloading allowed indication TS 23.501CR5871
- N11 and N16a enhancements for I-SMF based Local Offloading Management TS 29.502CR0810
+ 15 more changes
Explore further
Broader topics and technologies where I-SMF plays a role.
Defining Specifications
3GPP specifications that define or reference I-SMF, 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 |
| TR 23.726 vg00 | SMF/UPF Topology Enhancements in 5G | Rel-16 |
| TS 29.244 vj40 | PFCP Specification for Control/User Plane Separation | Rel-19 |
| TS 29.502 vj50 | 5G System; Nsmf Service Based Interface; Stage 3 | Rel-19 |
| TS 29.508 vj40 | 5G Session Management Event Exposure Service | Rel-19 |
| TS 29.512 vj40 | 5G Session Management Policy Control Service | Rel-19 |
| TS 29.542 vj30 | SMF NIDD Service Based Interface Stage 3 | Rel-19 |
| TS 29.561 vj30 | 5G Interworking with External Data Networks | Rel-19 |
| TS 29.591 vj40 | 5G NEF Southbound Services Stage 3 | Rel-19 |
| TR 29.820 vh00 | Study on PFCP Best Practice | Rel-17 |
| TS 29.892 vg00 | Study on User Plane Protocol in 5GC | Rel-16 |
| TS 32.291 vj40 | Charging Management: Service-Based Interface Protocol | Rel-19 |
