Shared Domain Name

Description

The Shared Domain Name is a concept introduced in 3GPP Release 11 within the IP Multimedia Subsystem (IMS) architecture. It refers to a domain name that appears in an IMS Network-Independent Public User Identity (INIPUI) and is shared among multiple IMS operators. An INIPUI is a public user identity that is not tied to a specific network operator, allowing a subscriber to use the same identity (e.g., sip:[email protected]) regardless of which operator's network they are attached to. This is particularly useful for enabling number portability, service roaming, and multi-operator environments.

In traditional IMS, public user identities are typically associated with a single operator's domain (e.g., sip:[email protected]). However, with the Shared Domain Name, multiple operators can agree to serve the same domain, meaning that a subscriber's identity remains constant even if they change operators or roam. This requires coordination among operators to ensure that routing, authentication, and authorization are handled correctly. The Shared Domain Name is resolved via DNS to the appropriate IMS core network elements, such as the Interrogating-CSCF (I-CSCF) and Serving-CSCF (S-CSCF), which may belong to different operators.

The architecture for Shared Domain Names involves several key components. The Home Subscriber Server (HSS) stores the INIPUI and associated service profiles, which may be replicated or shared among operators. The IMS core network uses the Shared Domain Name to route session initiation requests. When a UE registers with an INIPUI containing a Shared Domain Name, the I-CSCF queries the HSS to find the appropriate S-CSCF. Since multiple operators serve the domain, the HSS may be partitioned or federated. 3GPP specifications 22.228 and 22.894 define the requirements and procedures for INIPUI and Shared Domain Names, including security and privacy considerations.

One of the main challenges is ensuring that the Shared Domain Name is globally unique and that all participating operators have consistent DNS records. Additionally, charging and settlement between operators must be handled, as the serving operator may differ from the home operator. The Shared Domain Name concept supports the vision of a seamless IMS ecosystem where users can move between operators without changing their identity.

In later releases, the terminology evolved from 'INIPUI Operators' to 'IMS Operators', reflecting a broader applicability. The Shared Domain Name remains a niche but important feature for enabling operator-independent identities, particularly in markets with multiple IMS providers or for enterprise deployments.

Purpose & Motivation

The Shared Domain Name was created to address the need for operator-independent public user identities in IMS. In traditional IMS, a user's SIP URI is tied to a specific operator's domain, which creates lock-in and complicates roaming and number portability. When a user switches operators, they must change their identity, which is inconvenient and can disrupt services. The Shared Domain Name allows multiple operators to share the same domain, so users can keep their identity even when changing operators or when served by different operators in a multi-operator environment.

This concept also supports regulatory requirements for number portability and enhances competition by reducing switching costs. It enables new business models where third-party service providers can offer IMS services using a shared domain, independent of the underlying access network. The specifications 22.228 and 22.894 provide the framework for INIPUI and Shared Domain Names, ensuring interoperability and security.

While adoption has been limited, the Shared Domain Name represents an important step towards a more open and flexible IMS ecosystem. It anticipates future needs where identities are decoupled from network operators, aligning with trends like eSIM and multi-operator core networks.