Description
The Service Location Protocol (SLP) is an IETF-defined protocol (RFC 2608) adopted and profiled by 3GPP for use within its architecture, primarily to facilitate service discovery. In the 3GPP context, SLP is a key enabler for location services (LCS), allowing a client, such as User Equipment (UE) or a network entity, to discover the address of a location server, most notably the Secure User Plane Location (SUPL) Location Platform (SLP). The protocol operates in a client-server model where User Agents (UAs) issue service requests and Service Agents (SAs) advertise services. Directory Agents (DAs) can optionally be used as centralized repositories for service advertisements, improving scalability in large networks. The protocol uses service URLs and attributes to describe services, and discovery can be performed via multicast or unicast. For 3GPP, SLP is often used in the SUPL architecture, where a SUPL Enabled Terminal (SET) uses SLP to discover the address of its Home SLP (H-SLP) or a Discovered SLP (D-SLP) to initiate a positioning session. This discovery is crucial for establishing a secure user-plane connection for delivering location information. The protocol supports authentication and provides a flexible framework for finding various types of services beyond just location, though its primary 3GPP application is within the SUPL ecosystem. Its integration allows for decentralized service discovery without requiring pre-configuration of server addresses in every device, enhancing flexibility and manageability for network operators.
Purpose & Motivation
SLP was introduced to solve the problem of dynamic service discovery in IP-based networks, which became increasingly relevant with the rise of mobile data services and location-based applications. Prior to standardized service discovery, devices often required manual configuration of server addresses (like a home location server), which was inflexible, difficult to manage at scale, and hindered service portability and roaming. The protocol provides a standardized, automated method for clients to find necessary network services. Within 3GPP, its adoption, particularly starting in Release 7 with SUPL 1.0, was motivated by the need for an efficient user-plane location solution. SLP enables a SUPL Enabled Terminal to dynamically discover its appropriate SLP, whether it's the home network's SLP or a visited network's SLP during roaming. This capability is essential for emergency services (e.g., E911), commercial location services, and regulatory compliance, as it ensures a device can always find a positioning server without user intervention. It decouples the client from a static network configuration, allowing operators to deploy and relocate servers more freely and support a growing ecosystem of location-based services.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (15 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-7, normative work from Rel-15.
In Release 15, the SLP function was updated to include the provision of High Accuracy location estimates and to allow for the distinguishing of the Location Source specifically when a sensor method is used. Furthermore, the associated OMA SUPL information was updated to support these enhancements. These changes improved the precision and source transparency of location services delivered via the SLP.
In Release 16, the SLP function, via SUPL, received corrections to enhance its support for user-plane positioning and for specific NR positioning methods. These updates focused on refining the protocol's elementary procedures and its application interface to better handle location information. The corrections also addressed the provision of sensor-based location information elements within the service.
In Release 17, the SLP function was enhanced to support the inclusion of high accuracy GNSS location estimates in the NMEA GGA sentence format. This update specifically introduced the capability to convey "NMEA GGA sentence info" as part of the location information exchanged via the relevant application protocol. The change allowed for the distribution of more precise positioning data within the specified service framework.
- NMEA GGA sentence info in high accuracy GNSS location estimates [HA-GNSS-NMEA] TS 37.355CR0349
In Release 18, the key new development for the SLP function was the introduction of network verification of UE location, based on the LPP protocol, along with corrections and OAM requirements for this verification feature. Additionally, support was added for LPP to enable location information reporting with a periodicity of less than one second. The release also introduced the capability for providing Mobile TRP (Transmission Reception Point) location information.
- Introduction of network verification of UE location in TS 37.355 TS 37.355CR0482
- LPP support for sub 1s location information reporting periodicity [Sub_1s_periodicity] TS 37.355CR0501
- Introduction of network verification of UE location TS 38.305CR0154
- Correction on network verification of UE location TS 37.355CR0511
- Introduction of Mobile TRP location info TS 38.305CR0160
- Correction to OAM requirement for UE location verification TS 38.305CR0171
In Release 19, the Service Location Protocol (SLP) function was enhanced to support location services involving WAB-Nodes. This introduces new capabilities for location determination within the network architecture, extending the functional scope of location-based protocols.
- Support of Location Service Involving WAB-Nodes TS 38.305CR0196
Explore further
Broader topics and technologies where SLP plays a role.
Defining Specifications
3GPP specifications that define or reference SLP, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 23.167 vj11 | IMS Emergency Sessions | Rel-19 |
| TS 23.271 vj00 | LCS Stage 2 Specification | Rel-19 |
| TS 23.303 vj00 | Proximity Services (ProSe) Stage 2 | Rel-19 |
| TS 23.700 vk00 | XR Services Application Enablement Layer | Rel-20 |
| TR 23.730 ve00 | Study on extended CIoT architecture | Rel-14 |
| TS 23.868 v900 | Study on IMS Emergency Calls | Rel-9 |
| TS 29.078 vj00 | CAMEL Phase 4 CAP Specification | Rel-19 |
| TS 29.171 vj00 | LCS Application Protocol (LCS-AP) Specification | Rel-19 |
| TS 29.278 vj00 | CAMEL Application Part (CAP) for IMS Phase 4 | Rel-19 |
| TS 29.819 vd00 | Diameter Base Protocol Update Analysis | Rel-13 |
| TS 33.533 vj00 | Security for 5G Ranging & Sidelink Positioning | Rel-19 |
| TS 36.305 vj00 | UE Positioning in E-UTRAN Stage 2 | Rel-19 |
| TS 36.355 vj00 | LTE Positioning Protocol (LPP) | Rel-19 |
| TS 36.809 vc00 | Study on RF Pattern Matching for LTE Positioning | Rel-12 |
| TS 36.840 vc10 | LTE 450 MHz Band Technical Requirements for Brazil | Rel-12 |
| TS 37.355 vj20 | LTE Positioning Protocol (LPP) | Rel-19 |
| TS 38.305 vj00 | NG-RAN UE Positioning Stage 2 | Rel-19 |