Routing filter

Description

A Routing filter is a set of packet flow IP header parameter values or ranges that are used to identify one or more IP flows for routing purposes. It is a fundamental concept in 3GPP specifications for policy and charging control (PCC) and for traffic steering in mobile networks. Routing filters are used to match IP packets based on their header fields, such as source and destination IP addresses, port numbers, protocol type, and other parameters. By defining a routing filter, an operator can specify which IP flows should be routed through a particular path or treated with a specific policy.

In the context of 3GPP, routing filters are often used in conjunction with traffic detection and routing rules. For example, in the Policy and Charging Control (PCC) architecture, a Policy and Charging Rules Function (PCRF) or Policy Control Function (PCF) can provide a Routing Rule that includes a routing filter to identify the IP flows that should be routed via a specific access network or through a specific gateway. This allows for differentiated treatment of traffic, such as routing certain flows via a local breakout or a specific slice.

Routing filters are also used in the Access Network Discovery and Selection Function (ANDSF) for inter-system mobility, where they help determine which traffic should be routed over which access technology. In 5G, routing filters are used in the User Plane Function (UPF) for uplink classifier (ULCL) and multi-homed PDU sessions, where traffic can be routed to different data networks based on filter rules.

The concept of a routing filter was introduced in Release 9 and has been refined over subsequent releases. The definition has remained largely consistent, describing a set of packet flow IP header parameter values/ranges used to identify one or more IP flows for routing purposes. The syntax and usage of routing filters are specified in various 3GPP technical specifications, including TS 23.161, TS 23.261, and TS 23.861.

Routing filters are typically expressed using a syntax similar to IP filter rules, with fields for source and destination addresses, ports, and protocol. They can be used in both uplink and downlink directions. The matching is performed on the IP 5-tuple (source IP, destination IP, source port, destination port, protocol) or a subset thereof. Routing filters can also include other parameters such as DSCP values or flow labels.

In summary, a Routing filter is a powerful tool for traffic steering and policy enforcement in mobile networks. It allows operators to define granular rules for routing IP flows based on header information, enabling differentiated services and efficient use of network resources.

Purpose & Motivation

The Routing filter was introduced to address the need for granular traffic steering and policy-based routing in mobile networks. In earlier releases, routing decisions were often based on coarse-grained parameters such as APN or bearer, which limited the ability to differentiate traffic within the same bearer. As networks evolved to support multiple access technologies and services, there was a need to identify specific IP flows and route them differently based on operator policies. The Routing filter provides a standardized way to define such flows using IP header parameters.

One of the key motivations was the desire to support offloading of specific traffic to local networks or to route traffic through specific gateways. For example, an operator might want to route video streaming traffic through a specific path to optimize quality of experience, while routing other traffic through a different path. The Routing filter allows the operator to define rules that match the video traffic based on IP addresses and ports, and then apply a routing policy to that flow.

Another motivation was the need to support multi-access connectivity, where a user equipment (UE) can be connected to multiple access networks simultaneously. Routing filters enable the network to decide which IP flows should be routed over which access network, based on policies. This is particularly important in heterogeneous networks where different access technologies have different characteristics.

Over time, the concept of Routing filter has been extended to support new features such as network slicing and edge computing, where traffic needs to be routed to specific slice instances or edge applications. The Routing filter remains a fundamental building block for these advanced routing capabilities.