Local Mobility Anchor

Description

The Local Mobility Anchor (LMA) is a critical network entity specified within 3GPP's architecture for non-3GPP access interworking, primarily defined in the context of Proxy Mobile IPv6 (PMIPv6). It resides in the home operator's core network (EPC) and serves as the topological anchor point for a User Equipment's (UE) IP session(s). When a UE attaches via a trusted non-3GPP access network, such as a carrier-managed Wi-Fi network, the LMA assigns one or more Home Network Prefixes (HNPs) to the UE. The UE configures its IP address(es) from these prefixes, which remain constant regardless of its point of attachment within the PMIPv6 domain. This provides the UE with seamless IP session continuity, a form of network-based mobility where the UE itself is not required to participate in the mobility signaling.

The LMA works in conjunction with a Mobility Access Gateway (MAG), which is the function in the access network (e.g., within an evolved Packet Data Gateway (ePDG) or a Trusted WLAN Access Gateway (TWAG)) that detects the UE's attachment and movement. The MAG acts as a proxy, performing mobility signaling on behalf of the UE. Upon initial attachment, the MAG sends a Proxy Binding Update (PBU) message to the LMA. The LMA then creates a Binding Cache Entry (BCE), which binds the UE's assigned HNP(s) and its network access identifier (NAI) to the care-of address represented by the MAG's Proxy Care-of Address (Proxy-CoA). All IP packets destined for the UE are routed to the LMA, which then tunnels them (using IP-in-IP, GRE, or PMIPv6 tunneling) to the current MAG based on the BCE. Uplink traffic from the UE is similarly tunneled from the MAG to the LMA, which then de-capsulates and routes it towards the external packet data network (PDN).

Key architectural roles of the LMA include being the endpoint of the IP tunnel with the MAG, managing the BCEs for all attached UEs, and interfacing with the PCRF (Policy and Charging Rules Function) for policy enforcement via the Gx interface. It also plays a role in IPv4 address allocation if the UE is using IPv4. The LMA's centralized anchoring function simplifies mobility for the UE but centralizes traffic, which can lead to sub-optimal routing (tromboning). To address this, later enhancements like LMA relocation and selective traffic offload were studied. The LMA is a cornerstone of the S2a and S2b reference points for trusted and untrusted non-3GPP access, respectively, enabling tight integration of Wi-Fi into the mobile core network.

Purpose & Motivation

The LMA was created to enable seamless, network-based mobility management for UEs connecting via non-3GPP IP access networks, most notably Wi-Fi, into the 3GPP Evolved Packet Core (EPC). Prior to its standardization, mobility between 3GPP and non-3GPP accesses was often break-before-make, requiring the UE to obtain a new IP address and breaking ongoing IP sessions. The problem was how to provide IP session continuity and consistent policy enforcement across heterogeneous access technologies without requiring changes to the UE's IP stack.

Proxy Mobile IPv6 (PMIPv6), and by extension the LMA, was adopted by 3GPP to solve this. It provides a network-based solution where the network entities (MAG and LMA) handle all mobility signaling, making the mobility transparent to the UE. This is particularly advantageous for legacy devices and for simplifying UE implementation. The LMA serves as the stable anchor point in the operator's network, ensuring the UE's IP address remains unchanged, which is crucial for many applications and security associations. Its creation in Rel-8 was part of the broader System Architecture Evolution (SAE) effort to define a unified, IP-based core network (EPC) that could integrate a multitude of access technologies beyond just 3GPP radio, thereby fulfilling the vision of 'access agnostic' core services.