Home Routed with Session Breakout in VPLMN

Description

HR-SBO (Home Routed with Session Breakout in VPLMN) is a 5G System (5GS) roaming architecture defined in 3GPP Release 18. It represents an evolution of the traditional Home Routed (HR) model, where all user plane traffic is tunneled back to the Home Public Land Mobile Network (HPLMN). In HR-SBO, the Protocol Data Unit (PDU) Session is still anchored at the Home UPF (User Plane Function), preserving the home network's control and policy enforcement for the primary session. However, the architecture introduces a key enhancement: the ability to create a local breakout path for specific, authorized traffic flows directly within the Visited PLMN (VPLMN). This is achieved through the establishment of a secondary, localized PDU Session Anchor (PSA) or a dedicated traffic offload function within the VPLMN's user plane.

The architectural implementation relies on enhancements to the 5G Core's Session Management Function (SMF) and Policy Control Function (PCF). The home SMF (H-SMF), residing in the HPLMN, remains the primary session manager. It collaborates with a Visited SMF (V-SMF) in the VPLMN to orchestrate the local breakout. The decision to apply SBO is driven by policies from the home PCF, which can dictate that certain Application Function (AF) requests or Data Network Names (DNNs) are better served locally. For instance, a policy might specify that traffic destined for a local edge computing application or a content delivery network (CDN) node in the visited country should use the breakout path.

From a signaling perspective, the H-SMF selects a V-SMF in the visited network to handle the local breakout leg. The V-SMF then instantiates a local UPF (L-UPS) in the VPLMN to serve as the breakout point. User plane traffic matching the breakout policy is steered from the (R)AN to this L-UPS, which then forwards it directly to the local Data Network (DN), bypassing the long haul to the HPLMN. The H-SMF maintains the overall session context and can apply charging and policy control for both the home-routed and locally broken-out traffic via interaction with the Charging Function (CHF) and PCF. This model requires secure N16 and N9 interfaces between the V-SMF/H-SMF and L-UPS/Home UPF, respectively.

HR-SBO's role is to enable efficient 5G roaming that supports both global service consistency and localized performance optimization. It is a foundational enabler for roaming scenarios requiring ultra-low latency, such as mobile gaming, augmented reality, or access to localized regulatory services, while ensuring the home operator retains billing, policy, and security oversight. It represents a middle ground between the full control of Home Routed roaming and the full localization of Local Breakout (LBO) roaming.

Purpose & Motivation

HR-SBO was created to solve the inherent latency and inefficiency problems of traditional Home Routed (HR) roaming in the 5G era, particularly for latency-sensitive and locally-relevant services. In classic HR roaming, all user traffic is tunneled back to the home network, even if the content or application server is geographically close to the user's current location in the visited country. This 'tromboning' effect introduces significant round-trip delay, increases transport costs on inter-operator links, and degrades the quality of experience for applications like cloud gaming, industrial IoT, and real-time video analytics. The primary motivation was to enhance 5G roaming to meet the stringent performance requirements of new verticals without sacrificing the home operator's control over the subscriber session.

Previous approaches presented a binary choice: HR for full control or Local Breakout (LBO) for optimal routing. LBO routes all traffic directly in the visited network, which minimizes latency but cedes significant control (e.g., detailed charging, real-time policy) to the visited operator and can complicate access to home-network services. HR-SBO addresses the limitations of both by introducing selectivity. It allows operators to define policies that determine which traffic flows benefit from local breakout, thereby solving the performance problem for specific services, while keeping the primary session and other traffic home-routed for control and consistency.

The creation of HR-SBO was driven by 3GPP's work on enhanced 5G roaming architectures in Release 18, focusing on enabling edge computing and low-latency service delivery across borders. It provides a standardized, policy-driven mechanism for 'service-based roaming,' where the breakout decision is not all-or-nothing but tied to the application's needs. This facilitates new roaming business models between operators, allowing them to offer tiered roaming packages with guaranteed low latency for premium services, all within a secure and billable framework defined by the home network.