User Authorization Answer

Description

The User Authorization Answer (UAA) is a Diameter answer command defined in the Cx/Dx interface specifications (TS 29.229, referenced via TS 23.380) for the IP Multimedia Subsystem (IMS). It is the direct response to a User Authorization Request (UAR) command. The UAA is sent from the Home Subscriber Server (HSS), which is the central user database, to the Interrogating Call Session Control Function (I-CSCF), which is the initial contact point within a home IMS network. The primary function of the UAA is to convey the HSS's decision regarding a user's request to register with the IMS network and to provide the necessary routing information.

Upon receiving a UAR from the I-CSCF, the HSS performs authentication and authorization checks on the user's identity (Private User Identity and Public User Identity). The HSS then formulates the UAA, which contains a Result-Code AVP indicating success or failure. In the case of a successful authorization, the UAA carries one or more critical Attribute-Value Pairs (AVPs). The most important of these is the Server-Capabilities AVP, which describes the capabilities required by the S-CSCF that should serve this user (e.g., supported media types, mandatory capabilities). Alternatively, if a specific S-CSCF has already been assigned for the user, the UAA may contain the Server-Name AVP with the direct address (FQDN) of that S-CSCF.

The I-CSCF uses the information in the UAA to select an appropriate S-CSCF from its network's pool that matches the capabilities indicated by the HSS. If a Server-Name is provided, the I-CSCF simply routes the registration request to that specific S-CSCF. The UAA is therefore the key message that enables dynamic S-CSCF assignment, a core principle of IMS that allows for load balancing and service-based assignment. This command is exclusively part of the IMS registration and initial filter criteria retrieval procedures, making it a cornerstone for establishing a user's presence and service profile within the IMS core.

Purpose & Motivation

The UAA command exists to centralize user authorization and S-CSCF assignment logic within the IMS architecture. It solves the problem of how an I-CSCF, which has no prior knowledge of a user, can determine where to route a registration request. Before IMS, traditional telephony switches often had static configurations mapping users to serving nodes, which was inflexible for IP-based multimedia services. The UAA/UAR dialogue allows for dynamic, policy-driven assignment based on user profile and network conditions.

The creation of the UAA was motivated by the need for a standardized, secure, and scalable method to authenticate users and assign them to a serving node (S-CSCF) capable of handling their subscribed services. It separates the concerns of user data management (HSS) from session control routing (I-CSCF). This design allows network operators to scale and manage HSS and CSCF pools independently. The UAA carries the authoritative decision from the HSS, ensuring that only authorized users can register and that they are assigned to an S-CSCF with the necessary capabilities to execute their service logic, such as telephony application server (TAS) interactions.

As a Diameter-based command, it also represents the evolution from the SS7-based MAP protocol used in circuit-switched cores to an IP-based protocol suitable for all-IP networks. Its introduction in Rel-8 with the EPS/IMS alignment was crucial for enabling fixed-mobile convergence and rich communication services over LTE and subsequent 5G cores.