Relay Service Code

Description

The Relay Service Code (RSC) is a critical identifier in the 5G Proximity Services (ProSe) architecture, specifically within the framework of relay communications. It serves two primary scenarios: UE-to-Network Relay and UE-to-UE Relay. In the UE-to-Network Relay case, as defined in TS 23.303, the RSC identifies the connectivity service that a relay UE provides to remote UEs, enabling them to access the network via the relay. For UE-to-UE Relay, introduced in Release 19, the RSC takes on an expanded role: it identifies both the connectivity service offered by the 5G ProSe UE-to-UE Relay and the set of authorized users that the relay is permitted to serve. This dual function ensures that only UEs with the appropriate authorization can leverage the relay's services, thereby enforcing access control and service differentiation.

Architecturally, the RSC is used during the discovery and association procedures. When a remote UE seeks to connect via a relay, it discovers available relays by monitoring for broadcast or groupcast messages that include the RSC. These messages are typically transmitted using the PC5 interface, the direct sidelink communication link between UEs. The remote UE compares the received RSCs with its own provisioned list of authorized RSCs. If a match is found, the remote UE can initiate a connection to the relay. This process ensures that the remote UE only connects to relays that are authorized to provide the desired service, preventing unauthorized access and enabling service-specific relaying.

The authorization aspect is particularly important for UE-to-UE Relay. The 5G ProSe UE-to-UE Relay may be configured with a set of RSCs that it is allowed to serve, and a remote UE may be provisioned with a set of RSCs that it is allowed to use. The relay will only offer service to remote UEs that present an RSC that is both allowed by the relay and authorized for the remote UE. This mutual authorization helps prevent misuse of relay resources and ensures that only legitimate UEs can communicate through the relay. The values of RSCs for UE-to-UE Relay are not specified in 3GPP specifications; they are typically assigned by the service provider or operator, allowing flexibility for different deployments.

The RSC is provisioned in the UE by the 5G ProSe layer, often through policies configured by the network (e.g., via the PCF). The UE may receive RSC information during registration or via dedicated signaling. In the case of UE-to-Network Relay, the RSC may also be used in the relay selection process defined in TS 23.303, where the remote UE uses the RSC to determine which relay can provide the desired connectivity service, such as a specific APN or network slice. Overall, the RSC is a key enabler for authorized and differentiated relay services in 5G ProSe, ensuring that relayed connections are secure and appropriate for the intended service.

Purpose & Motivation

The Relay Service Code was introduced to address the need for a standardized identifier that enables service differentiation and authorization in relay-based communication scenarios. In earlier releases, relay selection was based on simpler criteria such as signal strength or operator policy, but as relay use cases expanded—especially with the advent of 5G ProSe and the introduction of UE-to-UE relaying in Release 19—there arose a requirement to distinguish between different connectivity services and to control which UEs could access them. The RSC fulfills this by acting as a service identifier that can be provisioned in UEs and used during discovery and association.

In UE-to-Network Relay, the RSC allows a remote UE to select a relay that offers the specific service it requires, such as access to a particular network slice or APN. Without such an identifier, remote UEs would have to rely on other means, potentially leading to inefficient or unauthorized relay usage. The RSC thus enhances the efficiency and security of relay selection.

The introduction of UE-to-UE Relay in Release 19 brought additional challenges: not only must the relay service be identified, but also the set of users authorized to use that service must be controlled. The RSC addresses this by serving as both a service identifier and an authorization token. The relay only serves remote UEs that present an authorized RSC, and remote UEs only connect to relays that advertise RSCs they are allowed to use. This mutual authorization prevents unauthorized access and ensures that relay resources are used as intended. By leaving the actual values of RSCs out of scope, 3GPP provides flexibility for operators and service providers to define RSCs according to their own service portfolios.