IP Multimedia Public User Identity

Description

The IP Multimedia Public User Identity (IMPU) is the public-facing identifier used to route sessions and messages to a user within the IMS. It takes the form of a SIP Uniform Resource Identifier (SIP URI), like sip:user@domain.com, or a TEL URI for telephone numbers, like tel:+1234567890. Unlike the private IMPI, the IMPU is used for all public communications, such as being placed in the From and To headers of SIP INVITE requests. A single user subscription, anchored by one IMPI, can be associated with multiple IMPUs, allowing for different aliases, service types (e.g., business, personal), or group identities. During IMS registration, the UE registers one or more IMPUs by associating them with the authenticated IMPI. This registration is stored in the S-CSCF, which downloads the user's service profile from the HSS, including the list of allowed IMPUs and their associated filter criteria for triggering application servers. When a session is initiated to an IMPU, the I-CSCF queries the HSS to find the serving S-CSCF for that identity, which then executes the user's service logic. IMPUs can be in different states: registered, unregistered, or barred. They are also the key enabler for features like implicit registration sets, where registering one IMPU automatically registers others, and for identity hiding services. The management of IMPUs, including their barring and service profile linkage, is central to IMS service delivery and personalization.

Purpose & Motivation

The IMPU was created to provide a flexible, user-friendly addressing scheme for the new generation of IP-based multimedia services in IMS, moving beyond the circuit-switched era's reliance solely on E.164 telephone numbers. It addresses the need for a rich set of identities that can represent a user in various contexts (personal, work, device-specific) and for different service types (voice, video, messaging, presence). The separation of public identity (IMPU) from private identity (IMPI) is a core IMS design principle that enhances security and privacy by ensuring the permanent authentication key is never used in routing. This model solves the problem of how to support both traditional telephony (via TEL URI) and native Internet communication (via SIP URI) under a unified architecture. It was motivated by the vision of convergence, allowing users to be reachable via multiple identifiers while maintaining a single subscription and consistent service experience, thereby enabling advanced services like multimedia telephony, instant messaging, and presence over any IP access network.