Network-Based IP Flow Mobility

Description

NBIFOM (Network-Based IP Flow Mobility) is a Release 13 feature that extends the concept of IP flow mobility to networks that rely on network-based mobility protocols, specifically GPRS Tunnelling Protocol (GTP) and Proxy Mobile IPv6 (PMIP). IP flow mobility allows an operator to route individual IP flows — distinguished by their IP 5-tuple (source and destination IP address, source and destination port, and protocol) — over different access networks simultaneously, while preserving session continuity. Unlike host-based mobility protocols such as DSMIPv6, where the UE manages mobility signaling, NBIFOM places the mobility logic in the network, making it transparent to the device and applicable to UEs that do not support client-based mobility.

Architecturally, NBIFOM operates within the Evolved Packet Core (EPC) and builds on the multi-access PDN connectivity framework. A UE may be connected simultaneously over 3GPP access (E-UTRAN) and non-3GPP access (e.g., Wi-Fi or trusted/untrusted WLAN). The PDN Gateway (P-GW) anchors the IP session, and the mobility management entity (MME) or the evolved Packet Data Gateway (ePDG) manages access-specific signaling. Routing rules are installed in the UE by the network, typically via ANDSF (Access Network Discovery and Selection Function) or via PCO (Protocol Configuration Options) during PDN connection establishment, and in the P-GW via PCRF/PCF signaling using Gx and Sd interfaces.

How it works: When a UE establishes a multi-access PDN connection, the network creates a single IP address and set of bearers spanning both accesses. NBIFOM then uses routing rules to determine which IP flows are routed over which access. The UE's routing rules are matched against flow descriptions, and the P-GW enforces the corresponding forwarding decisions. If a flow is moved from one access to another, the P-GW updates its routing state and the UE updates its local routing table, all without changing the IP address or breaking the flow. This is achieved via GTP or PMIP signaling extensions defined in 3GPP TS 23.161 and TS 24.161.

Key protocol interactions include the Gx interface between the PCRF and P-GW/BBERF for policy provisioning, the Sd interface for application detection and control, and the S11/S5/S8 interfaces for bearer management. The UE communicates its capabilities and routing rule preferences via NAS and PCO signaling. The ANDSF may provide inter-system routing policies that complement NBIFOM rules. Management of routing rules also involves the S14 interface for ANDSF and the S6a/S6b interfaces for subscription data.

NBIFOM is important because it enables operators to offload selected traffic to Wi-Fi or other non-3GPP accesses while keeping latency-sensitive or premium flows on cellular, all under network control. This is particularly valuable in environments with heterogeneous access, high cellular load, or fixed-mobile convergence scenarios. It also provides a standards-based alternative to proprietary flow mobility solutions and complements host-based IP flow mobility (IFOM) for devices that support DSMIPv6.

Purpose & Motivation

NBIFOM was created to address a gap in 3GPP's mobility toolkit: while host-based IP flow mobility (IFOM) using DSMIPv6 allowed selective flow routing, it required UE support and placed significant signaling and policy complexity on the device. Many IoT devices and legacy UEs do not support DSMIPv6, and operators wanted a network-controlled alternative that could work with the widely deployed GTP and PMIP protocols. NBIFOM fills this gap by enabling flow-level mobility decisions to be made and enforced by the core network.

The feature also responds to the growing need for intelligent traffic steering in multi-access networks. As Wi-Fi became ubiquitous, operators sought ways to offload traffic without degrading user experience. Traditional access network discovery and selection (ANDSF) provided access-level decisions but could not move individual flows while preserving IP continuity. NBIFOM enables finer granularity, allowing, for example, video streaming to be offloaded to Wi-Fi while VoLTE remains on cellular.

Finally, NBIFOM supports fixed-mobile convergence and multi-access edge computing scenarios by allowing flows to be routed to the most appropriate access based on policy, network load, or application requirements. Its network-based nature ensures backward compatibility with devices that lack advanced mobility clients, broadening the applicability of flow mobility across the device ecosystem.