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
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (546 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the Intermediate Session Management Function (I-SMF) was newly introduced as an SMF inserted to support a PDU Session when the UE moves to an area where the original SMF cannot control the session because the required User Plane Functions belong to a different SMF service area. This enables session management continuity and local breakout by allowing a new SMF to take over control within its specific service area. The introduction of the I-SMF facilitates key Release 15 features like user plane management for EPS interworking and handling of non-3GPP access.
- Addition of PDU Session type IPv4v6 TS 23.501CR0181
- Configuration information the UE may exchange with the SMF during the lifetime of a PDU Session TS 23.501CR0003
- Homogeneous support for IMS voice over PS Session supported indication TS 23.501CR0046
- UE support for Multi-homed IPv6 PDU Session TS 23.501CR0105
- User Plane management to support interworking with EPS TS 23.501CR0122
- DL signalling handling for non-3GPP PDU Session TS 23.501CR0135
+ 63 more changes
In Release 16, the I-SMF's role was significantly expanded to control specific UPF functions like the Uplink Classifier (UL CL) and Branching Point (BP) for traffic steering, and to manage local traffic switching via the transfer of N4 information. These enhancements were introduced to better support Multi-Access (MA) PDU Sessions, including their establishment, update, and release procedures over a single access. Furthermore, new service operations were defined for the I-SMF's insertion, change, and for the retrieval and release of the SM context during I-SMF changes.
- 5GS Logical TSN bridge management TS 23.501CR1002
- QoS for Multi-Access PDU Session TS 23.501CR0770
- UL CL/BP controlled by I-SMF TS 23.501CR0848
- Clarification on MA PDU session TS 23.501CR1033
- Transfer of N4 information for local traffic switching from SMF to I-SMF TS 23.501CR1050
- Further detailing of 5G LAN group management TS 23.501CR1052
+ 186 more changes
In Release 17, the I-SMF function was enhanced with new procedures for selection per DNAI and for its own removal when targeting a specific DNAI. The release also introduced a specific I-SMF/V-SMF restoration procedure and clarified the inclusion of Downlink Tunnel Info of NG-RAN in the I-SMF selection process. Furthermore, support was added for resolving Edge Nodes for future PDU Sessions and for managing inter-PLMN mobility of PDU sessions involving the I-SMF.
- MA PDU sessions with connectivity over E-UTRAN/EPC and non-3GPP access to 5GC TS 23.501CR2527
- Informative guideline on supporting session/service continuity between SNPN and PLMN when using N3IWF TS 23.501CR2563
- KI #1-1, I-SMF selection TS 23.501CR2634
- Adding the usage of Session Management Congestion Control Experience analytics TS 23.501CR2708
- Enabling restricted PDU Session for remote provisioning of UE via User Plane TS 23.501CR2709
- Clarification on UE provides PDU Session Pair ID based on URSP rules TS 23.501CR2736
+ 104 more changes
In Release 18, the I-SMF's role was enhanced to support the Home Routed PDU Session with Session Breakout in the VPLMN (HR-SBO) scenario, requiring specific PFCP extensions for managing such sessions. Furthermore, the N4 session modification procedure was updated to enable the network slice replacement feature, allowing the I-SMF to facilitate the change of a network slice instance for a PDU session.
- UPF event exposure service for TSC management TS 23.501CR3720
- The support of Home Routed PDU Session supporting Session Breakout in VPLMN (HR-SBO) TS 23.501CR3830
- Change of Network Slice instance for PDU sessions TS 23.501CR3867
- Service area provisioning and LADN aspects for enhanced group management TS 23.501CR3914
- Considering ML model management capability during ADRF discovery and selection TS 23.501CR3929
- Support non-3GPP access leg of MA-PDU Session with PDN connection in EPC TS 23.501CR3937
+ 95 more changes
In Release 19, the I-SMF function was enhanced primarily to support Local Offloading Management, introducing new procedures and interface enhancements for this purpose. Key additions include specific architectural support (KI#1 Architecture) and new capabilities for the PCF to become aware of I-SMF insertion for managing local offloading. Furthermore, the release defined enhancements to the N11, N16a, and N38 interfaces specifically for I-SMF based Local Offloading Management.
- I-SMF selection/insertion based on local offloading allowed indication TS 23.501CR5604
- HSBO_roaming traffic offloading via session breakout in HPLMN TS 23.501CR5748
- MWAB BH PDU session clarification TS 23.501CR5828
- NAT functionality in the UPF of BH PDU Session TS 23.501CR5650
- Local Offloading handling at I-SMF TS 23.501CR5744
- KI#1 Architecture for Local Offloading Management TS 23.501CR5752
+ 66 more changes
In Release 20, the I-SMF function was enhanced to support Ethernet type PDU Sessions for Mobile Wireless Access Backhaul (MWAB), expanding its role in managing sessions beyond traditional IP types. Additionally, corrections were introduced for the Energy Consumption calculation methodology, specifically addressing scenarios involving redundant transmission or PDU Sessions with multiple PDU Session Anchors. These updates refined the I-SMF's operational and reporting accuracy within the 5G architecture.
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 |