Public Identity

Description

In 3GPP identity management, a Public Identity is an umbrella term that refers to either a Public User Identity (IMPU) or a Public Service Identity (IMPSI). These identities are used for routing and communication within the IP Multimedia Subsystem (IMS) and are publicly known, unlike private identities which are used for authentication. The concept was introduced in Release 8 and is defined in specifications such as TS 29.228 and TS 29.562. A Public User Identity is typically a SIP URI or a Tel URI (e.g., E.164 number) that identifies a user for communication. A user may have multiple public identities, which can be shared across services or devices. A Public Service Identity is used to identify a service, such as a conferencing server or an application server, and is used for routing service requests.

The architecture separates public and private identities to enhance security and flexibility. Public identities are used in SIP signaling, such as in the From, To, and Contact headers, to route messages. They are stored in the HSS and can be associated with one or more private identities. During registration, a UE provides a public identity (or implies it) along with a private identity for authentication. The IMS core (e.g., P-CSCF, I-CSCF, S-CSCF) uses the public identity to route messages and apply services. The HSS maintains the mapping between public and private identities and enforces service profiles.

For Public Service Identities, they are used to represent services themselves, not users. They can be used for service invocation, routing, and authorization. For instance, a conferencing service might have a public identity (e.g., sip:[email protected]) that users can dial to join a conference. The network uses this identity to route the request to the appropriate application server. Public Service Identities are typically provisioned in the HSS and can be shared across users.

The definition of Public Identity has remained stable from Rel-8 through Rel-20, with no substantive changes. This consistency reflects the maturity of the IMS identity framework. Minor editorial changes in some releases (e.g., Rel-17 and Rel-20) do not alter the technical semantics. The Public Identity concept is critical for interoperability and user experience in IMS-based services, including VoLTE and VoWiFi. It enables users to have multiple addresses and services to be reachable via well-known identifiers. The separation from private identities ensures that authentication is based on secure credentials while communication can use public, routable identities.

Purpose & Motivation

The Public Identity concept exists to provide a clear and flexible way to address users and services in IMS. Before its introduction, networks primarily used phone numbers (MSISDN) for addressing, but IMS introduced SIP URIs to support multimedia services. The Public Identity allows a user to have multiple addresses (e.g., different SIP URIs for personal and business use) while sharing a single private identity for authentication. This separation enhances privacy and enables service personalization. It also allows services to be addressed directly, which is essential for applications like conferencing, messaging, and presence.

For services, the Public Service Identity enables service providers to offer services that can be invoked by users via a public address. This is similar to a phone number for a service. The network can route requests to the appropriate service based on the public identity. This simplifies service discovery and invocation. The Public Identity is also used in charging and policy control, as it identifies the user or service for which resources are consumed.

The stability of the Public Identity definition across releases indicates that the foundational principles of IMS addressing have not required fundamental changes. The introduction of 5G and network slicing has not altered the core concept, though new identities may be introduced for specific 5G services. The Public Identity remains a cornerstone of IMS service delivery.