Update Location Request

Description

The Update Location Request (ULR) is a critical Diameter-based command message defined in 3GPP TS 29.272 and TS 33.501 for the 5G System (5GS). It is used on the N8 interface (in 5G) between the Access and Mobility Management Function (AMF) and the Unified Data Management (UDM). The primary function of the ULR command is to inform the UDM of the UE's current serving network node (AMF) and to request the UE's subscription profile and authentication data. This procedure is central to the initial registration, mobility registration updates, and periodic registration processes in 5G.

Architecturally, the AMF acts as the Diameter client, initiating the ULR command towards the UDM, which acts as the Diameter server. The message contains a wealth of information in its Attribute-Value Pairs (AVPs). Key AVPs include the User-Name (containing the SUPI or SUCI), the Visited-PLMN-Id, the RAT-Type (e.g., NR, E-UTRA), and the AMF's identity. Upon receiving a ULR, the UDM validates the request, checks the subscriber's status, and may trigger authentication procedures with the Authentication Server Function (AUSF).

How it works is integrated into the UE registration flow. When a UE attempts to register with the 5G network, the AMF, after initial interaction with the UE, sends a ULR command to the UDM. The UDM processes this request: it authenticates the subscriber, authorizes the registration for the given PLMN and access type, and retrieves the subscriber's profile from its database. The UDM then responds with an Update Location Answer (ULA) command. The ULA carries essential data back to the AMF, including the subscriber's Access and Mobility Subscription data, Session Management Subscription data, and potentially authentication vectors.

Its role is foundational for mobility, security, and policy enforcement. The ULR/ULA exchange ensures the UDM, as the central repository of subscriber data, is always aware of the UE's serving AMF. This enables features like lawful interception, subscription-based service restrictions, and mobility event notifications to other Network Functions (NFs). It replaces the Update Location Request procedure used in the Diameter-based S6a/S6d interface between MME and HSS in 4G EPS, adapted for the service-based architecture of 5GC.

Purpose & Motivation

The ULR command exists to provide a standardized, secure, and efficient mechanism for location update and subscription data retrieval in the service-based 5G Core network. It addresses the need for a decoupled, scalable procedure in the new cloud-native architecture, moving away from the point-to-point GTP and Diameter interfaces of previous generations. In 4G EPS, a similar Update Location Request existed on the S6a interface between MME and HSS, but the 5G ULR is adapted for the new network functions (AMF/UDM) and supports enhanced security features like subscription concealment (SUCI).

The problem it solves is maintaining accurate and timely knowledge of a subscriber's point of attachment within the network for mobility management, service delivery, and regulatory compliance. Without this procedure, the core network would not know which AMF is serving a UE, preventing the routing of mobile-terminated sessions, applying correct mobility policies, or ensuring the subscriber is authorized to use the network. It is the primary trigger for the UDM to provision the AMF with the subscriber's service profile.

Furthermore, its creation was motivated by the 5G design principles of network slicing, stateless NFs, and service-based interactions. The ULR procedure, using HTTP/2 with JSON or Diameter over SBA, allows the AMF to subscribe to changes in the UDM data, enabling more dynamic and efficient updates compared to the static pull model, thereby supporting advanced use cases like network slicing and edge computing.