Network Slice Selection Policy

Description

The Network Slice Selection Policy (NSSP) is a UE-local configuration that contains rules mapping application traffic to specific Network Slice Selection Assistance Information (S-NSSAI). It is a key enabler for device-driven slice selection, allowing the UE to intelligently request network slices that are optimal for its running applications without requiring explicit user intervention for each service. The NSSP is provided to the UE by the network, typically via the Unified Data Management (UDM) or Policy Control Function (PCF), and can be updated dynamically by the network operator.

An NSSP rule consists of several components: a traffic descriptor and a corresponding S-NSSAI. The traffic descriptor can be defined using various identifiers such as an Application Identifier (e.g., OSId + OSAppId), a Fully Qualified Domain Name (FQDN), or IP 3-tuple (destination IP address, protocol, port number). When an application on the UE generates traffic, the UE's operating system or modem software evaluates the NSSP rules. If the traffic matches a descriptor in an NSSP rule, the UE will include the associated S-NSSAI in its Requested NSSAI during registration or PDU Session Establishment procedures. This signals to the network (specifically to the NSSF and AMF) the UE's desire to use that particular network slice for the matched traffic.

The NSSP operates in conjunction with the UE Route Selection Policy (URSP), which is a broader policy that includes rules for traffic routing to specific Data Network Names (DNNs), SSC modes, and network slices. The NSSP can be considered a subset or a component focused specifically on slice selection. The presence of an NSSP allows for sophisticated use cases, such as a smartphone simultaneously using a default eMBB slice for general internet browsing while automatically requesting a low-latency slice for a cloud gaming application and a high-reliability slice for a mission-critical push-to-talk service, all based on pre-configured operator policies.

Purpose & Motivation

The NSSP was created to solve the problem of how a user device can intelligently and automatically select from multiple available network slices without relying on manual user configuration or simplistic network defaults. In early 5G deployments without NSSP, a UE might register with a single default slice for all its traffic, negating the benefits of having specialized slices. Alternatively, requiring users or application developers to manually select a slice for each app would be impractical and lead to a poor user experience.

The NSSP addresses this by putting policy intelligence directly in the UE, under the control of the network operator. This allows the operator to steer specific, identifiable application traffic to the slice that offers the most appropriate performance characteristics (e.g., low latency for gaming, high bandwidth for 4K video). It enables efficient network resource utilization by ensuring that premium slices are only used when needed by authorized applications. Furthermore, it future-proofs the device for new services; an operator can deploy a new slice for an emerging application (like AR glasses) and simply push a new NSSP rule to subscribed UEs to enable automatic use of that slice, without requiring a device OS update. The NSSP, therefore, is essential for realizing the user-facing benefits of network slicing in a scalable and automated manner.