Description
The Session Management Client (SMC) is a crucial protocol-level entity within the User Equipment (UE) that handles all aspects of Packet Data Unit (PDU) session management. In the 3GPP system architecture, particularly from EPS (4G) onwards and central to 5G, the SMC is the endpoint in the UE for the session management signaling protocol (e.g., the Non-Access Stratum (NAS) protocol's Session Management messages). Its primary counterpart in the network is the Session Management Function (SMF) in the core network. The SMC is responsible for the end-to-end signaling flow that establishes, modifies, and releases PDU sessions, which are the logical connections providing IP connectivity or other types of data connectivity to a Data Network (DN).
Operationally, the SMC works by exchanging specific Session Management (SM) messages with the SMF over the NAS signaling connection. When an application on the UE requires data connectivity, the SMC formulates a PDU Session Establishment Request. This request includes critical parameters such as the requested Data Network Name (DNN), Session and Service Continuity (SSC) mode, and PDU session type (e.g., IPv4, IPv6, IPv4v6, Ethernet, Unstructured). The SMC sends this request via the Access Stratum (through the gNB/NG-RAN in 5G or eNB in 4G) to the SMF. The SMF processes the request, interacts with other network functions like the PCF and UPF, and sends a response back to the SMC. The SMC then configures the UE's internal packet processing (the UE's PDU Session Anchor) based on the information received, such as the assigned IP address and QoS rules.
Key internal components of the SMC logic include the protocol state machine for session management, handlers for different SM message types (Establishment, Modification, Release), a local policy engine to interpret network-provided QoS and charging rules, and interfaces to the UE's application layer and IP stack. Throughout the lifetime of a PDU session, the SMC handles modification procedures triggered by network policies (e.g., QoS change, handover) or UE requests. It also manages the release of sessions, either initiated locally or by the network. The SMC's correct operation is fundamental to providing seamless, policy-controlled, and QoS-aware data connectivity to the user, making it a core component of the UE's protocol stack.
Purpose & Motivation
The SMC exists to provide a standardized, UE-resident control point for managing data sessions in packet-switched mobile networks. Before standardized session management (in early 3GPP releases), connectivity was simpler but less flexible. The evolution towards all-IP networks and rich multimedia services demanded a sophisticated mechanism to establish sessions with specific quality, charging, and continuity characteristics. The SMC, as part of the NAS protocol, was developed to meet this need, separating session management control from mobility management and radio resource control.
It solves the problem of how a UE can dynamically request and negotiate the complex parameters of a data connection in a network that supports diverse services—from best-effort web browsing to low-latency gaming or high-reliability IoT. The SMC protocol allows the network to exert precise control over the UE's session parameters, enabling advanced features like network slicing, differentiated QoS, and efficient resource utilization. Its creation was motivated by the shift from circuit-switched data and simple PDP contexts in GPRS to the fully IP-based, service-driven architecture of EPS and 5GS. The SMC provides the necessary intelligence in the UE to participate in this negotiated, policy-driven session establishment process, which is fundamental to modern mobile broadband and critical communication services.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (22 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-5, normative work from Rel-15.
In Release 15, the Session Management Client (SMC) function saw updates to its NAS SMC procedure, including the introduction of a KAMF change flag and clarified handling for concurrent authentication. The procedure was also aligned with SA2 and RAN3 specifications, and updates were made to AS SMC handling and failure scenarios, such as AMF confirming UE SUPI when NAS SMC fails.
- Rules on concurrent running of authentication and NAS SMC procedure TS 33.501CR0004
- CR-slice-management-security TS 33.501CR0290
- Deletion of EN in Caluse 10.2.1 Authenticated IMS Emergency Sessions TS 33.501CR0262
- AS SMC Handling Update TS 33.501CR0272
- Align AS SMC procedure with SA2 and RAN3 TS 33.501CR0284
- CR on adding KAMF change flag in NAS SMC TS 33.501CR0363
+ 5 more changes
In Release 16, the SMC (Session Management Client) function was not updated with new procedures or capabilities related to remote interference management or cross link interference management, as those changes pertained to RAN-level protocols. The grounding context for Release 16 continues to define the SMC strictly as a client that interfaces to the Session Management and Mobility Management (SM MM) entity using an intermediate protocol to signal session setup and release requirements for PDP contexts.
In Release 17, clarifications were provided for the Session Management Client (SMC) function, specifically regarding the handling of multicast security contexts during the session creation procedure. Additionally, updates were made to clarify aspects of the secondary authentication process for PDU Sessions initiated by the SMC. These refinements ensured clearer implementation of security and session management protocols between the SMC and the Session Management and Mobility Management (SM MM) entity.
- Clairfication on AS key generation after runing NAS SMC TS 33.501CR1158
- Clarifications on the multicast security context handling in session creation procedure TS 33.501CR1379
- Clarification of SNI usage for NF clients and servers TS 33.501CR1447
- Clarifications to secondary authentication PDU Session Container TS 33.501CR1359
In Release 18, enhancements for the Session Management Client (SMC) function included a correction to the AUN3 device procedure for the SMC and introduced validation for parameters sent by an OAuth 2.0 client, such as an NF Service Consumer, within the access token request. Additionally, a correction was made pertaining to the handling of unauthenticated IMS emergency sessions, which involve the SIP Client terminating IMS signalling in the user equipment. These updates refine the intermediate protocol between the SMC and the Session Management and Mobility Management (SM MM) entity for session setup and release requirements.
In Release 19, the Session Management Client (SMC) function was enhanced to support the recovery of N3mb path failure specifically for unicast transport of multicast sessions. Additionally, the release introduced support for Continuous Management-based MDT (MDT) for the SMC, expanding its session management capabilities. These updates allow the SMC, which interfaces with the Session Management and Mobility Management entity to signal session requirements, to handle new procedures for transport path resilience and managed device tracing.
Explore further
Broader topics and technologies where SMC plays a role.
Defining Specifications
3GPP specifications that define or reference SMC, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 22.944 vj00 | UE Functionality Split Scenarios and Requirements | Rel-19 |
| TS 24.801 v810 | CT1 SAE NAS Aspects for EPC | Rel-8 |
| TS 33.401 vj10 | EPS Security Architecture | Rel-19 |
| TS 33.501 vk00 | 5G Security Architecture and Procedures | Rel-20 |
| TS 33.700 | 3GPP TR 33.700 | Rel-5 |
| TS 33.701 vj00 | Study on mitigations against bidding down attacks | Rel-19 |
| TS 33.821 v900 | LTE/SAE Security Threat Analysis and Countermeasures | Rel-9 |
| TR 33.853 vh00 | Study on User Plane Integrity Protection | Rel-17 |
| TS 33.859 vb10 | UTRAN Key Hierarchy Enhancement Study | Rel-11 |
| TS 36.331 vj00 | LTE RRC Protocol Specification | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |