Description
The Address Of Record (AOR) is a fundamental identifier within the 3GPP IP Multimedia Subsystem (IMS) architecture, defined as a Session Initiation Protocol (SIP) Uniform Resource Identifier (URI) that represents a user's public identity. Unlike temporary contact addresses that change with network attachment points, the AOR serves as a stable, globally routable identifier that remains constant regardless of the user's current location or device. This permanent identifier is stored in the Home Subscriber Server (HSS) as part of the user's service profile and is used by various IMS network elements to route signaling messages, manage registrations, and invoke services.
Architecturally, the AOR functions as the primary key in the IMS registration process. When a user registers with the IMS network, the Serving-Call Session Control Function (S-CSCF) receives the AOR from the user equipment and validates it against the HSS database. Upon successful authentication, the S-CSCF creates a binding between the AOR and the user's current contact address (typically a SIP URI containing the IP address of the user equipment). This binding enables the IMS network to route incoming sessions to the correct destination even as the user moves between networks or changes devices.
The AOR follows specific formatting rules defined in 3GPP specifications and RFC standards. It typically takes the form of 'sip:user@domain' or 'sips:user@domain' for secure connections, where the domain portion identifies the user's home network. The user portion can contain various identifiers including telephone numbers (in E.164 format with 'tel:' URI conversion), email-style addresses, or other unique identifiers. The AOR's domain component is crucial for routing decisions, as it determines which IMS network should handle the initial request through DNS-based procedures.
In operation, the AOR enables multiple critical IMS functions beyond basic registration. It serves as the key for service profile retrieval from the HSS, allowing the S-CSCF to apply appropriate service logic through interaction with Application Servers. The AOR also supports identity management features such as implicit registration sets, where registering one public identity automatically registers other associated identities. Furthermore, it forms the basis for privacy mechanisms, as users can choose to reveal or conceal their AOR during communications based on privacy preferences configured in their service profile.
The AOR's role extends throughout the session lifecycle. During session establishment, the AOR appears in SIP headers such as From, To, and Request-URI, enabling proper routing and service invocation. It also supports forking scenarios where a single AOR can be associated with multiple endpoints, allowing simultaneous ringing on different devices. The separation between the permanent AOR and temporary contact addresses provides the foundation for IMS mobility management, enabling seamless service continuity as users move across access networks.
Purpose & Motivation
The Address Of Record was created to solve fundamental identification and routing challenges in IP-based multimedia networks. Prior to IMS, telecommunications networks relied on circuit-switched identifiers like MSISDN (Mobile Station International Subscriber Directory Number) that were tightly coupled to specific network technologies and lacked the flexibility needed for converged IP services. These legacy identifiers couldn't adequately support the rich multimedia services, multi-device scenarios, and internet-style applications that 3GPP envisioned for future networks.
The AOR addresses several key limitations of traditional telecommunication identifiers. First, it provides technology independence by using SIP URIs that work across any IP-based access network, enabling true convergence between cellular, fixed, and WiFi networks. Second, it separates the permanent user identity from temporary network attachment points, solving the mobility challenge where users frequently change IP addresses. Third, it enables rich service invocation through its association with user profiles in the HSS, allowing personalized services regardless of access method or device type.
Historically, the AOR concept originated from the IETF's SIP standards, where it was defined as a permanent address for a user. 3GPP adopted and extended this concept within the IMS architecture to create a unified identification framework that could support both traditional telephony and innovative multimedia services. The AOR's design specifically addresses the requirements of all-IP networks where users might have multiple devices, use different access technologies, and require consistent service experiences across all scenarios. By providing a stable anchor point in a dynamic IP environment, the AOR enables the service continuity, personal mobility, and flexible service delivery that define modern IMS-based communications.
Key Features
- Permanent SIP URI identifier for IMS users
- Globally routable across IMS networks
- Decouples user identity from network attachment points
- Enables registration and binding management in S-CSCF
- Supports implicit registration of multiple identities
- Forms basis for service profile retrieval and invocation
Evolution Across Releases
Introduced AOR as part of the enhanced IMS architecture for service continuity and identity management. Defined the AOR as a SIP URI for permanent user identification with standardized formatting rules. Established the binding mechanism between AOR and contact addresses in the S-CSCF for mobility support.
Defining Specifications
| Specification | Title |
|---|---|
| TS 24.525 | 3GPP TS 24.525 |