Routing Rule

Description

A routing rule is a fundamental concept in 3GPP's IP flow mobility and selective traffic routing framework, defined primarily in TS 23.261 (IP flow mobility and seamless WLAN offload) and TS 23.861 (multi-access PDN connectivity). It is the mechanism by which a UE determines which IP flows should be sent over which access network or to which destination address. The rule consists of two essential components: a routing filter and a routing address (or, in certain releases, a routing access type).

The routing filter is an IP packet filter — typically expressed as an IP 5-tuple (source/destination IP address, source/destination port, and protocol) or a more complex packet filter specification — that identifies a specific set of IP flows. The routing address is the destination address (such as a care-of address or a local gateway address) to which matching packets should be routed. When the routing rule uses a routing access type instead, it indicates which radio access technology (e.g., 3GPP access or WLAN access) the matching flows should use, rather than a specific IP address.

In the network architecture, routing rules are managed by the core network — specifically by the PDN Gateway (P-GW) or, in 5G, the User Plane Function (UPF) and Session Management Function (SMF) — and are delivered to the UE via control-plane signaling. For example, in an NB-IFOM (Network-Based IP Flow Mobility) scenario, the P-GW installs routing rules in the UE using the ANDSF (Access Network Discovery and Selection Function) or via dedicated signaling such as the WLAN Offload Control (WOC) protocol. The UE then applies these rules to its outbound IP traffic, ensuring that each flow is sent over the correct access network or to the correct destination.

Routing rules are essential for enabling seamless service continuity across heterogeneous access networks. They allow an operator to steer specific traffic (e.g., video streaming, VoLTE, or enterprise traffic) over a preferred access while other traffic uses a different access. This is particularly important in multi-access scenarios where a UE is simultaneously connected to both 3GPP (LTE/NR) and non-3GPP (WLAN) accesses. The rules also support IP flow mobility, where individual flows can be moved between accesses without tearing down the entire PDN connection.

The concept was introduced in Release 9 as part of the work on IP flow mobility and seamless WLAN offload. Over subsequent releases, the definition was refined, and the scope expanded to include routing access type in Release 12, reflecting the growing importance of access-type-based steering. Although later releases reverted to the simpler routing address formulation, the underlying functionality remains a cornerstone of multi-access and traffic steering in both 4G and 5G networks.

Purpose & Motivation

Routing rules exist to solve the problem of how to selectively route IP traffic across multiple access networks in a way that is transparent to the user and efficient for the operator. Before their introduction, a UE with multiple active accesses (e.g., LTE and WLAN) had limited ability to direct specific flows over specific accesses; traffic was typically bound to a single PDN connection and access. This led to suboptimal resource utilization, poor user experience when one access was congested, and inability to enforce operator policies on a per-flow basis.

The introduction of routing rules in Release 9 as part of the IP flow mobility (IFOM) and seamless WLAN offload (ISWO) work provided a standardized mechanism for the network to instruct the UE on how to distribute its IP flows. This allowed operators to offload traffic to WLAN without breaking service continuity, to route enterprise traffic through a specific gateway, and to apply differentiated treatment to different applications. It also enabled the UE to make intelligent decisions about which access to use for which flow, based on network-provided policies.

Over time, the concept evolved to include routing access type, which simplified policy enforcement when the exact destination address was not known in advance or when the operator wanted to steer traffic based on the access technology rather than a specific IP address. This flexibility is crucial in dynamic network environments where addresses may change or where multiple gateways exist. The routing rule thus remains a key enabler for multi-access connectivity, traffic steering, and policy-based management in 3GPP networks.