S8 Home Routing

Description

S8 Home Routing (S8HR) is a 5G System (5GS) roaming architecture defined by 3GPP. It describes how a User Equipment (UE) roaming in a Visited Public Land Mobile Network (VPLMN) connects to the 5G Core Network (5GC) of its Home Public Land Mobile Network (HPLMN). The model is named after the S8 interface, which is the inter-PLMN reference point between the VPLMN and HPLMN. S8HR is characterized by the home-routing of user plane traffic.

In this architecture, the control plane signaling (specifically the N16 interface between the VPLMN's SMF and the HPLMN's SMF, which corresponds to the S8-C reference point) is routed back to the home network. This allows the HPLMN to maintain control over policy, charging, and subscription data. Crucially, the user plane path is also home-routed. The UE establishes a PDU Session anchored at a UPF (User Plane Function) in the VPLMN. This VPLMN UPF then connects via the S8-U interface (using the N9 reference point protocol) to a UPF in the HPLMN. All user data traffic is funneled through this home UPF before reaching the external Data Network (DN).

This is in contrast to the Local Breakout (LBO) roaming model, where the user plane breaks out to the internet directly from the VPLMN. S8HR ensures that the home operator can apply consistent service policies, perform deep packet inspection, implement charging rules, and provide value-added services (like firewalls or parental controls) from the home network, regardless of the UE's location. The architecture relies on security mechanisms like IPsec or TLS on the S8 interface and requires inter-operator agreements for the N16 (S8-C) and N9 (S8-U) interconnections.

Purpose & Motivation

S8 Home Routing was introduced to provide a standardized, secure, and policy-enforced roaming architecture for 5G networks. Prior to its formalization in 5G, similar concepts existed in 4G/EPC roaming (S8 Home Routing). The evolution to 5G required a new definition aligned with the Service-Based Architecture (SBA) and the separation of control plane (SMF) and user plane (UPF).

It solves the problem of how to allow a roaming subscriber to access services while ensuring the home operator retains control over the user's service experience, charging, and security policies. In a globally connected ecosystem, operators need a reliable method to offer consistent services to their subscribers abroad and to ensure revenue from roaming traffic is correctly accounted for. S8HR addresses the limitations of less controlled breakout models by guaranteeing that traffic passes through the home network's policy enforcement points.