A virtual network function (or VNF), as commonly known today, is a software implementation of a network function that runs on one or more virtual machines (VMs) on top of the hardware networking infrastructure — routers, switches, etc. Individual virtual network functions can be connected or combined together as building blocks to offer a full-scale networking communication service. A VNF may be implemented as standalone entity using existing networking and orchestration paradigms - for example being managed through CLI, SNMP or Netconf. Alternatively, an NFV may be a part of an SDN architecture, where the control plane resides in an SDN controller and the data plane is implemented in the VNF.

A cloud-native VNF is a VNF designed for the emerging cloud environment - it runs in a container rather than a VM, its lifecycle is orchestrated by a container orchestration system, such as Kubernetes, and it's using cloud-native orchestration paradigms. In other words, its control/management plane looks just like any other container based 12-factor app. to orchestrator or external clients it exposes REST or gRPC APIs, data stored in centralized KV data stores, communicate over message bus, cloud-friendly logging and config, cloud friendly build & deployment process, etc., Depending on the desired functionality, scale and performance, a cloud- native VNF may provide a high-performance data plane, such as the VPP.

Ligato (https://github.com/ligato) is a Golang platform for building cloud-native microservices. Although it was originally intended for development/implementation of custom management/control plane agents for cloud-native Virtual Network Functions (VNFs), it can be used to develop any microservice. Each management/control plane app built on top of the CN-Infra platform is basically a set of modules called "plugins", where each plugin provides a very specific/focused functionality. Some plugins are provided by the platform itself, some are written by the app's implementors. In other words, the Ligato platform itself is implemented as a set of plugins that together provide the platform's functionality, such as logging, health checks, messaging (e.g. Kafka), a common front-end API and back-end connectivity to various KV data stores (Etcd, Cassandra, Redis, ...), and REST and gRPC APIs.

The platform is modular and extensible. Plugins supporting new functionality (e.g. another KV store or another message bus) can be easily added to the existing set of Ligato platform plugins. Moreover, Ligato based apps can be built in layers: a set of app plugins together with platform plugins can form a new platform providing APIs/services to higher layer apps. This approach was used to create a management/control plane agent for VPP (https://FD.io). The VPP Agent is basically a set of VPP-specific plugins that use the core Ligato platform to interact with other services / microservices in the cloud (e.g. a KV data store, messaging, log warehouse, etc.). The VPP Agent exposes VPP functionality to client apps via a VPP-specific model-driven API. Clients that consume this API may be either external (connecting to the VPP Agent via REST, gRPC API, Etcd or message bus transport), or local Apps and/or Extension plugins running on the same CN-Infra platform in the same Linux process.