Description
The Session and Service Continuity Mode Selection Policy (SSCMSP) is a network control function within the 5G Core (5GC) architecture, standardized in 3GPP TS 23.501. It governs the selection of the Session and Service Continuity (SSC) mode for a Protocol Data Unit (PDU) Session established by a User Equipment (UE). The SSC mode defines the continuity behavior of the session's User Plane Function (UPF) anchor point—the point where the session connects to the Data Network (DN)—during UE mobility. The SSCMSP is a crucial policy applied by the Session Management Function (SMF) during PDU Session Establishment or Modification procedures.
The policy logic resides within the SMF, which acts as the policy enforcement point. The SMF evaluates multiple inputs to make the SSC mode selection decision. Key inputs include: the subscriber's SSC mode subscription data retrieved from the Unified Data Management (UDM), network operator policies configured in the Policy Control Function (PCF), and the UE's reported capabilities and preferences. Based on these inputs, the SMF selects one of the three defined SSC modes. SSC Mode 1 maintains the same PDU Session anchor (UPF) even when the UE moves, offering the highest level of session continuity. SSC Mode 2 allows the network to temporarily disconnect the old PDU Session and immediately establish a new one with a new anchor UPF, providing a brief break in connectivity. SSC Mode 3 allows the old and new PDU Sessions (with different anchors) to coexist for a period, enabling the UE to switch data flow seamlessly.
Once selected, the SMF enforces the SSC mode by configuring the appropriate UPF(s) and N4 session rules, and by providing the selected mode to the UE via the NAS SM signaling. The SSCMSP enables the 5G system to tailor session continuity to the needs of diverse services. For example, a mission-critical IoT sensor might use SSC Mode 1 to maintain a constant IP address, while a background file sync application might efficiently use SSC Mode 2 or 3. This policy-driven approach is a key enabler for network slicing, as different slices can apply different SSCMSPs to meet their specific service level agreements for mobility and continuity.
Purpose & Motivation
The SSCMSP was created to address the complex trade-off between seamless service continuity and optimal routing/localization in a highly flexible, service-based 5G architecture. In previous mobile generations, IP address continuity was often tied to a specific network anchor (e.g., the PGW in 4G), which could lead to suboptimal, 'tromboning' data paths when the UE moved far from its anchor. 5G's distributed UPF architecture allows the anchor point to be relocated, but this can break the session's IP address.
The purpose of the SSCMSP is to provide a policy framework that intelligently manages this trade-off on a per-session basis. It solves the problem of 'one-size-fits-all' mobility management. Different services have vastly different requirements: real-time communication needs uninterrupted sessions (SSC Mode 1), while many data services can tolerate a brief interruption for the benefit of a more efficient data path (SSC Mode 2/3). The SSCMSP allows the network operator to define policies based on service type, subscriber tier, network slice, and current network conditions. This enables efficient network resource utilization, reduces latency by allowing anchor relocation closer to the UE, and still guarantees the required level of continuity for premium services. It is a foundational element for delivering the diverse performance promises of 5G, from enhanced Mobile Broadband (eMBB) to Ultra-Reliable Low-Latency Communications (URLLC).
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (404 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the SSCMSP (Session and Service Continuity Mode Selection Policy) function was newly introduced to govern the selection of Session and Service Continuity (SSC) modes for PDU Sessions. This policy, provided by the PCF, enables the network to control the trade-off between session continuity, which preserves the IP address, and service continuity, which maintains the user experience even if the IP address or anchoring point changes. The SSC Mode Selection mechanism is a key part of the overall Policy and Charging Control framework for managing PDU Session mobility and continuity.
- Addition of PDU Session type IPv4v6 TS 23.501CR0181
- Architectural solution for User Plane (UP) Security policy and User Plane Integrity Protection TS 23.501CR0066
- Selection of NAS procedures for E-UTRA connected to both EPC and 5GC TS 23.501CR0147
- Configuration information the UE may exchange with the SMF during the lifetime of a PDU Session TS 23.501CR0003
- Paging Policy Differentiation correction TS 23.501CR0013
- Clarification on PCF selection TS 23.501CR0015
+ 66 more changes
In Release 16, the SSCMSP function was enhanced to support the selection and steering of traffic for Multi-Access (MA) PDU Sessions, enabling service continuity between 3GPP and non-3GPP accesses. It also introduced the use of analytics, via the NWDAF, to influence the discovery and selection of network functions like the SMF and UPF. Furthermore, the policy was extended to handle subscription information influencing PDU Session rate control and to clarify procedures for Access Traffic Steering, Switching, and Splitting (ATSSS).
- CIoT Introduction of CN Selection and Steering TS 23.501CR0896
- QoS for Multi-Access PDU Session TS 23.501CR0770
- ATSSS-SMF and UPF selection TS 23.501CR0761
- Trusted non-3GPP Access Network Selection TS 23.501CR0783
- eSBA communication schemas related to general discovery and selection TS 23.501CR0799
- eSBA communication schemas related to UDM and UDR discovery and selection TS 23.501CR0800
+ 125 more changes
In Release 17, the SSCMSP function was enhanced to support session and service continuity for MA PDU sessions with connectivity over E-UTRAN/EPC and non-3GPP access to 5GC. It also received updates to provide informative guidelines for supporting continuity between SNPN and PLMN when using the N3IWF. Furthermore, the selection of network functions like the I-SMF and N3IWF was refined to support new scenarios such as N3SLICE and network selection for NR satellite access.
- 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
- NWDAF discovery and selection TS 23.501CR2575
- NWDAF discovery and selection based on provided ML models TS 23.501CR2585
- UP path selection enhancement based on analytics info provided by NWDAF TS 23.501CR2586
- NWDAF discovery and selection for model sharing TS 23.501CR2614
+ 55 more changes
In Release 18, the SSCMSP function was enhanced to support policy control for multi-modal flows and to allow a UE to simultaneously send data to different groups with different QoS policies. Furthermore, enhancements were made to support the graceful termination of PDU sessions during network slice decommissioning and to optimize time validity policies for a network slice.
- N3IWF selection enhancement for support of S-NSSAI needed by UE TS 23.501CR3707
- Discovery and Selection of the NWDAF Supporting Federated Learning in 5GC TS 23.501CR3772
- KI#4 23.501 AF traffic influence for common EAS, DNAI selection TS 23.501CR3788
- Common EAS/DNAI selection by AF TS 23.501CR3789
- The support of Home Routed PDU Session supporting Session Breakout in VPLMN (HR-SBO) TS 23.501CR3830
- Policy control enhancements to support multi-modal flows TS 23.501CR3864
+ 76 more changes
In Release 19, the SSCMSP function was enhanced to support I-SMF selection or insertion based on a local offloading allowed indication, enabling more granular control for traffic breakout. Furthermore, the release introduced support for UPF selection considering N6 delay and refined the parameters for network function discovery and selection, including for the NWDAF and EIF. These updates provide the PCF with more sophisticated criteria for managing session and service continuity policies.
- RVAS with target NF selection enhancement. TS 23.501CR5364
- NF discovery and selection by target PLMN TS 23.501CR5399
- Support of UPF selection according to the conclusion in FS_UPEAS_Ph2 TS 23.501CR5441
- Local Offloading Policy provisioning TS 23.501CR5463
- Exposure enhancements for static UE IP address assignment and 5G VN group's User Plane Security Policy TS 23.501CR5492
- I-SMF selection/insertion based on local offloading allowed indication TS 23.501CR5604
+ 47 more changes
In Release 20, the SSCMSP function was enhanced to support policy control for network energy saving, involving updates to the architecture and the Exposure and Integration Function (EIF). Furthermore, the release introduced support for Ethernet type PDU Sessions in specific mobility contexts and refined procedures for SMF and PCF selection.
- Support of Ethernet type of BH PDU Session for MWAB TS 23.501CR6330
- Energy Consumption information exposure and policy control TS 23.501CR6508
- Update on architecture and EIF function to support policy control for network energy saving TS 23.501CR6521
- SMF and PCF Selection for UIA_ARC TS 23.501CR6504
- Correction on Energy Consumption calculation for redundant transmission or PDU Session with multiple PDU Session Anchors TS 23.501CR6522
Explore further
Broader topics and technologies where SSCMSP plays a role.
Defining Specifications
3GPP specifications that define or reference SSCMSP, 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 |