Routing Indicator

Description

The Routing Indicator is an identifier assigned by the UE's home network operator to be used together with the Home Network Identifier (HNI) for routing network signalling. It is provisioned in the USIM (Universal Subscriber Identity Module) and is used in the 5G System to route authentication and other signalling messages to the appropriate AUSF (Authentication Server Function) and UDM (Unified Data Management) instances that can serve the subscriber. The Routing Indicator is part of the Subscription Concealed Identifier (SUCI) and is used by the serving network to route the initial registration or authentication request to the correct home network function.

In 5G, when a UE registers with a serving network, it may need to send a SUCI to the home network for authentication. The SUCI contains the Home Network Identifier (which includes the Mobile Country Code and Mobile Network Code) and the Routing Indicator. The serving network uses the Routing Indicator to select the appropriate AUSF and UDM instances in the home network. This is particularly important in roaming scenarios, where the serving network needs to route the request to the correct home network entity.

The Routing Indicator is provisioned in the USIM by the home network operator. It is typically a string of digits, and its format and allocation are defined in 3GPP TS 23.501 and TS 24.501. The Routing Indicator is used in conjunction with the Home Network Identifier to form the SUCI. The SUCI is a privacy-preserving identifier that conceals the permanent subscriber identifier (SUPI) using public key cryptography.

The Routing Indicator allows the home network to deploy multiple AUSF and UDM instances, possibly for different subscriber groups or for load balancing. By assigning different Routing Indicators to different groups of subscribers, the home network can direct signalling to specific instances. This enables scalability and flexibility in the 5G core network.

The term 'Routing Indicator' corresponds to the term 'Routing ID' used in 3GPP TS 23.501. The definition has evolved over releases. In Release 15, it was defined as an identifier assigned by the home network operator to be used together with the Home Network Identifier for routing network signalling, and it is provisioned in the USIM. In Release 16, the definition was changed to 'Indicator that allows together with SUCI/SUPI Home Network Identifier to route network signalling to AUSF and UDM instances capable to serve the subscriber.' This wording emphasizes the role of the Routing Indicator in routing to specific AUSF and UDM instances. Subsequent releases have retained this definition.

The Routing Indicator is a critical element for 5G security and privacy, as it enables the concealment of the SUPI while still allowing the home network to route the authentication request correctly. It is part of the 5G security architecture specified in 3GPP TS 33.501.

Purpose & Motivation

The Routing Indicator was introduced in Release 15 as part of the 5G System architecture to address the need for routing network signalling in a flexible and scalable manner. In previous generations, the routing of authentication signalling was often based on the IMSI (International Mobile Subscriber Identity), which contained the MCC and MNC but did not provide a way to route to specific network function instances. With the introduction of the Service Based Architecture in 5G, the home network can deploy multiple AUSF and UDM instances, and there was a need for a mechanism to route signalling to the correct instance. The Routing Indicator provides this mechanism by allowing the home network operator to assign a value that, together with the HNI, identifies the target AUSF/UDM instance or set.

Another motivation was privacy. The SUCI, which contains the Routing Indicator, is used to conceal the SUPI. The Routing Indicator is not considered sensitive information, so it can be sent in clear text over the air. This allows the serving network to route the request without needing to decrypt the SUCI. The home network can then decrypt the SUCI to obtain the SUPI and the Routing Indicator, and use the Routing Indicator to route the request to the appropriate AUSF/UDM.

The Routing Indicator also supports load balancing and failover. By assigning different Routing Indicators to different subscriber groups, the home network can distribute the authentication load across multiple AUSF/UDM instances. If an instance fails, the home network can update the Routing Indicator assignments to redirect traffic to other instances. This improves the resilience and scalability of the 5G core network.

Overall, the Routing Indicator is a key enabler for the 5G Service Based Architecture, providing a standardized way to route authentication signalling to the correct home network functions while preserving subscriber privacy.