IP Flow Mobility

Description

IP Flow Mobility (IFOM) is a sophisticated mobility management solution defined within the 3GPP Evolved Packet Core (EPC) architecture, specifically for scenarios involving trusted non-3GPP access. Its primary function is to enable a User Equipment (UE) to establish and maintain multiple IP flows concurrently over different access networks, such as LTE and Wi-Fi, with the ability to move individual flows between these accesses without service interruption. This is a granular, flow-level mobility mechanism, a significant evolution from simpler access selection methods that treat all of a UE's traffic as a single aggregate bearer.

Architecturally, IFOM relies on extensions to the core network nodes defined for non-3GPP access interworking, namely the evolved Packet Data Gateway (ePDG) for untrusted access and the Trusted WLAN Access Gateway (TWAG) for trusted access, though IFOM is primarily specified for trusted non-3GPP access. The central control point is the Policy and Charging Rules Function (PCRF), which provides the network-based policies dictating how IP flows are mapped to specific accesses. These policies, known as Access Network Discovery and Selection Function (ANDSF) policies or PCRF-provided rules, are communicated to the UE. The UE, equipped with dual radios, is responsible for implementing the flow binding decisions, managing multiple IP addresses (one per access), and performing the necessary routing of packets for each flow to the correct interface.

The technical operation involves the establishment of multiple Packet Data Network (PDN) connections or bearers, one per access type, to the same PDN (e.g., the internet). Each flow is identified by a set of IP 5-tuple parameters (source/destination IP, source/destination port, protocol). The UE and the network maintain a binding of each active flow to a specific access. When a policy dictates a flow handover—for instance, moving a video stream from cellular to Wi-Fi due to congestion or cost policies—the UE updates the flow binding. This update is signaled to the network core, ensuring that the Packet Data Network Gateway (PGW) correctly routes downlink packets for that flow to the corresponding access gateway. This process is seamless to the application, preserving the IP session continuity for each individual flow.

Purpose & Motivation

IFOM was created to address the growing heterogeneity of access networks and the need for intelligent traffic steering beyond simple network selection. Prior to IFOM, solutions like Mobile IP offered network-based mobility but treated all of a UE's traffic as a single entity, forcing an all-or-nothing handover between accesses. Similarly, early Wi-Fi offloading mechanisms often relied on breaking the IP session or were limited to specific applications. The proliferation of smartphones with multiple radios and the operator desire to leverage Wi-Fi not just for offload but as a managed, integrated access technology created the demand for more granular control.

The core problem IFOM solves is the efficient utilization of multiple available radio paths to optimize capacity, user experience, and cost. It allows an operator to implement sophisticated policies: for example, keeping latency-sensitive voice or gaming traffic on the managed LTE network for guaranteed quality, while offloading high-bandwidth, delay-tolerant downloads (like software updates or video streaming) to Wi-Fi. This granularity prevents the 'bad Wi-Fi' problem from degrading all services and allows traffic steering based on real-time network conditions, subscription plans, and application requirements. Its creation was motivated by the vision of Always Best Connected (ABC) services, providing users with the best possible connectivity experience by dynamically selecting the optimal path for each service flow.