Description
The User Authorization Request (UAR) is a critical Diameter command defined in the Cx interface specification (TS 29.229) between the I-CSCF (Interrogating Call Session Control Function) and the HSS (Home Subscriber Server) in the IP Multimedia Subsystem (IMS). It is a fundamental part of the IMS registration and session initiation procedures. When a User Equipment (UE) initiates contact with the IMS network, the initial SIP REGISTER request is routed to an I-CSCF. The I-CSCF, which does not have subscriber data, uses the UAR command to query the HSS for information necessary to route the request to the appropriate S-CSCF (Serving-CSCF) that will serve the user.
The UAR command carries specific Attribute-Value Pairs (AVPs) in its request, including the user's public identity (IMPU), private identity (IMPI), and the visited network identifier. Upon receiving the UAR, the HSS performs subscriber authentication and authorization checks. It verifies the user's subscription status, service profile, and determines whether the user is allowed to register in the requesting network. Based on this, the HSS responds with a User Authorization Answer (UAA) command. The UAA contains crucial routing information, such as the name (or capabilities) of the S-CSCF assigned to serve the user, or an indication that the I-CSCF should select an S-CSCF based on specific capabilities.
This mechanism ensures that IMS sessions are securely and efficiently routed. It enables load balancing across multiple S-CSCFs, supports roaming scenarios by authorizing access from visited networks, and forms the basis for subsequent Diameter transactions like the Location Information Request (LIR) and Server-Assignment Request (SAR). The UAR/UAA exchange is thus the first step in establishing a trusted dialog between the user and the IMS core, gatekeeping access to multimedia services like VoLTE, VoNR, and RCS.
Purpose & Motivation
The UAR command was created to address the fundamental need for a centralized, secure, and standardized authorization mechanism in the IMS architecture, which was introduced to deliver IP-based multimedia services over packet-switched networks. Before IMS and its defined Diameter interfaces, traditional telephony relied on SS7 signaling and HLRs for mobility, but there was no unified method for authorizing and routing SIP-based multimedia session requests in a scalable, carrier-grade manner.
The primary problem UAR solves is the stateless nature of the I-CSCF. The I-CSCF, often located at the network edge, acts as a gateway and does not maintain subscriber data. It needs a dynamic way to discover which S-CSCF should handle a specific user's registration or session. The UAR protocol provides this by querying the central subscriber database (HSS). This design separates routing logic from subscriber data, enhancing scalability, security, and flexibility. It allows for network architectures where S-CSCF pools can be dynamically selected based on load or capability, and it provides a single point of policy enforcement for user access at the HSS. Its creation was motivated by the need for a robust, extensible authentication and authorization framework that could support complex service delivery, roaming agreements, and interoperability between different vendors' network elements in a multi-service IP environment.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (13 CRs across 4 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, the UAR function was enhanced to support S-CSCF restoration after a failure, introducing a new reselection indication procedure. Specifically, when an I-CSCF cannot contact the assigned S-CSCF, it reselects a new one and forwards the request with this indication, prompting the new S-CSCF to retrieve S-CSCF restoration information from the HSS using a SAR with Server Assignment Type set to NO_ASSIGNMENT. This mechanism allows the HSS to overwrite the current S-CSCF assignment and provide stored restoration data, enabling faster service recovery with minimal user impact.
In Release 16, the UAR function was enhanced to support IMS restoration procedures, specifically enabling S-CSCF reselection during failures. The I-CSCF, upon being unable to contact the assigned S-CSCF, now reselects a new S-CSCF and includes a reselection indication in the forwarded request. This indication prompts the HSS to allow the new S-CSCF to overwrite the previous assignment and, when applicable, to provide stored S-CSCF restoration information.
- 5G P-CSCF restoration using Nudm TS 23.380CR0107
- P-CSCF restoration TS 23.380CR0109
- Correction to P-CSCF Restoration Procedures TS 23.380CR0105
- Add P-CSCF subscription info to Restoration information TS 23.380CR0106
- S-CSCF restoration after registration timer expiry TS 23.380CR0110
- P-CSCF restoration alignment TS 23.380CR0113
+ 1 more changes
In Release 17, the enhancements for the UAR function specifically introduced support for P-CSCF restoration procedures, including both HSS-based and PCRF-based mechanisms. This involved defining new behaviors for network elements to handle service interruption scenarios, ensuring the S-CSCF could retrieve restoration information from the HSS using specific Server Assignment Types. The updates provided a more resilient framework for session continuity by speeding up restoration when core network functions fail.
In Release 19, the UAR function was enhanced to support IMS restoration procedures, specifically enabling the I-CSCF to explicitly request S-CSCF capabilities from the HSS via a Location Information Request (LIR) when a previously assigned S-CSCF fails during a terminating procedure. This allows for the reselection of a new S-CSCF based on those capabilities, to which the terminating request is then forwarded. Furthermore, the procedures were clarified for scenarios where the HSS must send stored S-CSCF restoration information to a newly assigned S-CSCF to recover user session data after a failure.
Explore further
Broader topics and technologies where UAR plays a role.
Defining Specifications
3GPP specifications that define or reference UAR, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.380 vj10 | IMS Restoration Procedures | Rel-19 |