Session and Service Continuity Mode Selection Policy

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).