Network Function Service Set

Description

The NF Service Set is a concept introduced in 3GPP Release 19 as part of the ongoing evolution of the 5G Service Based Architecture (SBA). In the SBA, a Network Function (NF) is composed of one or more NF service instances, each providing a specific service (e.g., the Npcf_PolicyAuthorization service of the PCF). An NF Service Set is a logical grouping of interchangeable NF service instances of the same service type within a single NF instance. These instances are considered functionally equivalent and have access to the same context data, meaning they can serve any request for that service without requiring state synchronization at the service level.

The primary role of an NF Service Set is to enable scalability and redundancy at the granularity of individual services. Unlike an NF Set, which groups entire NF instances, an NF Service Set allows a single NF instance to scale a particular service independently. For example, a PCF may have multiple instances of its policy authorization service, grouped into an NF Service Set, while its other services remain single-instance. This fine-grained scalability is particularly useful for services with varying load patterns.

From an architectural perspective, the NF Service Set is defined in 3GPP TS 23.501 and TS 29.500. When an NF consumer discovers a service via the Network Repository Function (NRF), it may receive a set of service instance identifiers belonging to the same NF Service Set. The consumer can then select any instance within the set to send its request, based on local policies such as load balancing or proximity. The selected instance processes the request and accesses the shared context data, ensuring continuity even if the request is retried on a different instance within the set.

The shared context data is a critical aspect: it means that any instance in the set can handle any request for that service without needing to retrieve state from another instance. This is achieved through a shared data layer or by synchronizing context across instances. The exact mechanism is implementation-specific but must ensure consistency. This design simplifies failover and load balancing because the consumer does not need to track which instance holds which context.

NF Service Sets are identified by an NF Service Set Identifier (NF Service Set ID), which is a globally unique string. This identifier is used in service discovery and in routing decisions. The NRF stores the mapping of NF Service Set IDs to the individual service instances and their addresses. When a consumer requests a service, the NRF can return the NF Service Set ID, and the consumer can then use it to select an instance. The concept is part of the broader 5G SBA framework, which aims to provide a modular, scalable, and resilient core network.

In summary, the NF Service Set is a Rel-19 enhancement that brings service-level granularity to the concepts of NF Set and NF instance, enabling more efficient resource utilization and improved fault tolerance in the 5G core network.

Purpose & Motivation

The NF Service Set was introduced to address the need for finer-grained scalability and redundancy in the 5G Service Based Architecture. Prior to Rel-19, the concepts of NF Instance and NF Set provided grouping at the NF level, but individual services within an NF could not be scaled or made redundant independently. This limitation meant that if a particular service within an NF experienced high load, the entire NF instance had to be scaled, potentially wasting resources for other services. The NF Service Set solves this by allowing a group of interchangeable service instances of the same type to share context data, enabling load balancing and failover at the service level.

The motivation also stems from the increasing complexity of 5G networks, where NFs often provide multiple services with different performance and availability requirements. For example, a SMF may have a session management service that requires high throughput and a policy service that requires low latency. By grouping instances of each service into separate NF Service Sets, operators can scale and manage each service independently, optimizing resource usage and meeting diverse QoS requirements. This flexibility is essential for network slicing and edge computing scenarios, where services may be deployed in a distributed manner.

Furthermore, the NF Service Set enhances resilience by allowing seamless failover between service instances without impacting other services of the same NF. If one instance fails, the consumer can retry the request on another instance in the set, which has access to the same context data, ensuring service continuity. This is a significant improvement over previous approaches where a failure of a service instance might require state transfer or re-establishment. The concept aligns with the 3GPP vision of a cloud-native, microservices-based core network, where each service can be independently scaled and managed.