Uniform Resource Name

Description

Within the 3GPP architecture, a Uniform Resource Name (URN) is a specific type of Uniform Resource Identifier (URI) defined by IETF RFC 2141, used as a persistent and location-independent identifier for resources. Unlike a URL (Uniform Resource Locator), which specifies both identity and a network location/access mechanism, a URN is intended to provide a globally unique and persistent name that remains valid even if the resource it identifies moves or becomes unavailable. The syntax follows the structure 'urn:<namespace identifier>:<namespace-specific string>'. 3GPP defines and registers specific URN namespaces for its own use, such as 'urn:3gpp' and 'urn:oma', to avoid collisions with identifiers from other organizations.

In practical operation, URNs are extensively used in the IP Multimedia Subsystem (IMS) and other service delivery platforms. For instance, a Public Service Identity (PSI) used to identify an IMS application server (like a conferencing service) can be expressed as a URN (e.g., urn:service:sos.fire for an emergency service). When a User Equipment (UE) initiates a SIP session, the Request-URI in the SIP INVITE message may contain a URN representing the desired service. The IMS core, specifically the Serving-Call Session Control Function (S-CSCF), uses this URN to perform service triggering. It queries the Home Subscriber Server (HSS) or a dedicated Application Server (AS) to resolve the URN into routable contact information (like a SIP URI or tel URI) or to invoke the appropriate service logic.

The management and resolution of URNs are critical components. 3GPP specifications define procedures for URN comparison (ensuring case-insensitive handling where appropriate) and resolution mechanisms. Resolution often involves ENUM (Telephone Number Mapping) services or dedicated name resolution functions within the network. For example, a URN representing a telephone number in the 'tel' namespace (urn:tel:+1-555-123-4567) can be resolved via DNS/ENUM to a SIP URI for routing over IP. The use of URNs decouples service logic from network topology, enabling features like service portability, federation between different operator networks, and the creation of abstract service identities that are not tied to a specific server IP address or domain name.

Purpose & Motivation

The adoption of URNs within 3GPP, particularly from Release 6 onwards with the full deployment of IMS, was driven by the need for a standardized, flexible, and future-proof naming scheme for resources in an all-IP network. Prior circuit-switched networks relied primarily on E.164 telephone numbers (MSISDN) and IMSI for identification, which are number-space constrained and not inherently suitable for naming abstract services, multimedia content, or devices. As networks evolved to support multimedia services, service discovery, and machine-to-machine communication, a more versatile identifier was required.

URNs solve several key problems: they provide persistence (a service name doesn't change even if the hosting server changes), global uniqueness (managed through registered namespaces), and abstraction from location. This allows for service mobility—a user can access their 'video mailbox' service via the same URN regardless of their geographic location or the network operator they are roaming onto. It also facilitates interoperability between different vendor implementations and between 3GPP networks and other IP-based service platforms (like those defined by the Open Mobile Alliance - OMA). By using a standardized IETF mechanism, 3GPP leveraged an existing, well-understood technology, avoiding the creation of a proprietary and potentially incompatible identification system.