Inter-System Routing Policies

Description

Inter-System Routing Policies (ISRP) are a component of the Access Network Discovery and Selection Function (ANDSF) framework defined by 3GPP. ANDSF is a network element that provides policies to User Equipment (UE) to assist in the discovery and selection of available access networks (like LTE, WLAN, WiMAX) and to manage traffic routing across them. ISRP specifically governs the routing decisions for individual IP flows or groups of flows (based on factors like destination IP address, port, or protocol). It enables a UE to establish and maintain simultaneous connections to multiple access networks (e.g., a PDN connection over LTE and an IPsec tunnel over Wi-Fi) and to steer specific application traffic over the most appropriate access according to the policy rules.

The architecture involves the ANDSF server in the operator's network, which hosts the ISRP policies, and the ANDSF client within the UE. Communication occurs over the S14 interface (or via OMA DM) using the OMA Device Management protocol or SOAP/XML. An ISRP policy is composed of one or more rules. Each rule contains a set of selection criteria (defining which IP flows it applies to) and a routing descriptor (specifying the preferred or forbidden access for those flows). Routing descriptors can specify a single access (e.g., "route over WLAN"), an ordered list of accesses, or even define a multi-access PDN connection (MAPCON) or IP Flow Mobility (IFOM) scenario. The UE evaluates these rules in a defined priority order, matching its active IP flows against the criteria to determine the optimal routing path.

How ISRP works is policy-driven. When a UE connects to an operator network that supports ANDSF, it can request or receive ISRP policies. The UE's IP stack and connection manager then use these policies to make real-time decisions. For example, a rule might dictate that all video streaming traffic (identified by a specific APN or server IP range) should be routed over a WLAN connection if available and if the WLAN signal strength is above a threshold, otherwise fall back to 3GPP access. This allows for seamless traffic offloading, load balancing, and service-based access selection. ISRP's role is to provide the operator with granular control over network resource utilization and to enhance user experience by dynamically selecting the best available access for each service, potentially aggregating bandwidth or choosing the most cost-effective path.

Purpose & Motivation

ISRP was created to address the growing complexity of heterogeneous network environments where a UE has simultaneous access to multiple radio technologies (3GPP LTE/5G NR and non-3GPP like Wi-Fi). The core problem was the lack of intelligent, policy-based control over how traffic from different applications was distributed across these available paths. Early solutions were binary (connect to either cellular or Wi-Fi) or relied on simple device-side heuristics (like Wi-Fi signal strength), which often led to suboptimal performance, poor user experience (e.g., sticky Wi-Fi connections), and inefficient use of operator resources.

Introduced in 3GPP Release 10, ISRP was motivated by the need for operator-managed traffic steering and offloading. It gave operators a tool to implement business rules (e.g., offload best-effort traffic to Wi-Fi, keep voice over IMS on secure cellular), optimize network load, and enable advanced services like seamless flow mobility. It addressed the limitations of previous non-integrated selection methods by providing a standardized, extensible policy framework that could consider a wide range of dynamic conditions (network discovery information, time of day, location, user preferences) to make sophisticated routing decisions. This was a key step towards realizing the vision of Always Best Connected and later, integrated 5G access.

Key Features

• Policy-based routing of individual IP flows across multiple accesses
• Integration with the ANDSF architecture and S14 reference point
• Support for routing methods: Multi-Access PDN Connectivity (MAPCON), IP Flow Mobility (IFOM), and non-seamless WLAN offload
• Rule-based system with prioritization, validity conditions, and update triggers
• Enables simultaneous multi-access connectivity and intelligent traffic steering
• Allows operator control while potentially incorporating user preferences

Evolution Across Releases

Defining Specifications