Host Based Mobility

Description

Host Based Mobility (HBM) is a fundamental mobility management paradigm defined within the 3GPP Evolved Packet Core (EPC) architecture, specifically for non-3GPP access networks like Wi-Fi. In HBM, the intelligence and signaling required for maintaining IP session continuity as a User Equipment (UE) moves are primarily hosted within the network infrastructure, not on the UE itself. The key network entity enabling HBM is the Packet Data Network Gateway (PGW) in EPC, which acts as the mobility anchor and Home Agent (HA). The UE is assigned a stable, persistent IP address known as the Home Address (HoA) from its home network. When the UE attaches via a non-3GPP access network (e.g., a trusted WLAN), the network assigns it a local topologically correct IP address called the Care-of Address (CoA).

The core protocol implementing HBM for 3GPP interworking is Proxy Mobile IPv6 (PMIPv6), specified in IETF RFC 5213 and adopted by 3GPP. In PMIPv6, the mobility management is proxied by network entities called the Mobile Access Gateway (MAG) and the Local Mobility Anchor (LMA). In the 3GPP context, the MAG function resides in the Trusted WLAN Access Gateway (TWAG) for trusted WLAN access, and the LMA function resides in the PGW. The MAG detects the UE's attachment and movement, and on behalf of the UE, it registers the UE's CoA with the LMA. The LMA then maintains a binding between the UE's HoA (or its network prefix) and its current CoA. All data packets destined for the UE's HoA are intercepted by the LMA, encapsulated in an IP-in-IP tunnel (using the CoA as the destination), and forwarded to the MAG, which decapsulates and delivers them to the UE. Uplink traffic from the UE is typically routed directly or via the MAG to the LMA.

This architecture completely shields the UE from mobility signaling. The UE is not required to implement a mobility client; it simply uses its HoA as its IP address, and the network ensures this address remains reachable regardless of attachment point changes within the trusted non-3GPP domain. HBM via PMIPv6 is a cornerstone for seamless mobility and traffic offloading between 3GPP (LTE) and trusted non-3GPP accesses, forming part of the S2a and S2b reference points in the EPC. It provides network-controlled mobility with reduced complexity and power consumption on the UE compared to client-based solutions like Dual-Stack MIPv6 (DSMIPv6).

Purpose & Motivation

HBM was developed to provide seamless IP mobility management for devices that may lack sophisticated mobility client software or where optimizing UE battery life and complexity is a priority. Client-based mobility protocols like MIPv6 require the UE to actively participate in signaling—detecting movement, acquiring a CoA, and sending binding updates to its Home Agent. This increases UE processing overhead, power consumption, and implementation complexity, which is undesirable for simple IoT devices or mass-market handsets.

3GPP adopted HBM, specifically PMIPv6, to enable the integration of non-3GPP access networks (primarily WLAN) into the EPC in a way that is transparent to the UE. The primary problem it solves is providing session continuity and consistent policy enforcement for a UE moving between LTE and a trusted WLAN without requiring changes to the UE's IP stack. The network operator maintains full control over the mobility process and IP address allocation, allowing for centralized policy enforcement, charging, and quality of service (QoS) application. This approach facilitated the 3GPP-WLAN interworking (IFOM, MAPCON) and later access traffic steering, switching, and splitting (ATSSS) features defined in later releases.