What is a Cloud Native Network Function (CNF)?
In order to talk about CNFs, we need to define cloud native . Cloud native systems are, among other things, a set of loosely coupled services. These services, also known as microservices, are deployed onto immutable infrastructure while being managed by an orchestrator. This paper includes four links to other papers that detail the definitions of cloud native, microservices, immutable infrastructure, and CNFs from an OSI layer perspective.
How are cloud native systems loosely coupled?
Cloud native systems have a clear separation between their processes . They utilize the Unix philosophy of doing one thing and doing it well. These microservices usually use technologies such as containers and aim for one process per container . As such, cloud native applications should have all of their dependencies packaged in the container during the build phase and leveraged during deployment .
How is immutable infrastructure provisioned?
Immutable infrastructure (the orchestrator and all of the software and hardware that it depends on) is provisioned using baked and versioned templates  (e.g. server images produced from packer ) or a combination of templates and bootstrapping  (some repeatable and versioned process that is applied to the template e.g. Kubeadm). The underlying infrastructure is not changed after it is made ready for use. New changes to the infrastructure are rolled out as new instances of infrastructure.
How are cloud native systems deployed?
Cloud native applications are deployed onto immutable infrastructure (generic host servers that support orchestration ). Cloud native applications have not changed after deployment. New features for an application are rolled out as new artifacts and configuration (e.g., containers)
How are cloud native systems configured?
Cloud native systems are configured declaratively . This means that the system configuration declares "what" a loosely coupled system should look like, not "how" it should be created, updated, or deleted. The tooling determines the "how" of the application (e.g., the orchestrator, operators, and CRDs).
So what is a CNF, actually?
A CNF is network functionality delivered in software via cloud native development and delivery practices. This functionality lives within the layers of the OSI Model , which is used to define a network's stack. The lower layers (layers 1 and in some cases, layer 2) are provisioned for the higher layers (2-7) to provide transport. These higher layers in this instance act as applications that act upon a network payload (frames, packets datagrams etc). A physical layer 1 networking device should be "flashed" with a complete replacement of its artifacts for updates. The configuration for physical layer 1 is done with an atomic application of a versioned configuration file, which replaces the configuration on the device at once. Virtual Layer 1 (and some layer 2) is managed via templated images and bootstrapping. In contrast, layers 2 through 7 are managed by higher-level orchestration or an established control plane (an orchestrator pushing configuration versus a network protocol modifying a route table).
Why is this relevant to Service Providers?
Service providers currently find themselves at a unique transition point within the industry. Their push towards normalization within the world of NFV has finally begun to bear fruit, yet cloud native software approaches are already being pushed by a plethora of vendors, each with their unique approach. Providers now find themselves in a situation where they must find ways to achieve a return on their investment into NFV while also managing the industry's paradigm shift with regards to software development.
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LIST OF CONTRIBUTORS
If you would like credit for helping with these documents (for either this document or any of the other four documents linked above), please add your name to the list of contributors.
W Watson Vulk Coop
Taylor Carpenter Vulk Coop
Denver Williams Vulk Coop
Jeffrey Saelens Charter Communications
Bill Mulligan Loodse
Daniel Bernier Bell Canada
"CNCF Cloud Native Definition v1.0", TOC: 2018-06-11, https://github.com/cncf/toc/blob/master/DEFINITION.md, Cloud native technologies empower organizations to build and run scalable applications in modern, dynamic environments such as public, private, and hybrid clouds. Containers, service meshes, microservices, immutable infrastructure, and declarative APIs exemplify this approach. These techniques enable loosely coupled systems that are resilient, manageable, and observable. Combined with robust automation, they allow engineers to make high-impact changes frequently and predictably with minimal toil.
Stine, Matt. Migrating to Cloud-Native Application Architecture, O'reilly, 2015, pp. 10–11
“Codebase [...] Each deployable app is tracked as one codebase tracked in revision control. It may have many deployed instances across multiple environments.
Dependencies [...] An app explicitly declares and isolates dependencies via appropriate tooling (e.g., Maven, Bundler, NPM) rather than depending on implicitly realized dependencies in its deployment environment.
Config [...] Configuration, or anything that is likely to differ between deployment environments (e.g., development, staging, production) is injected via operating system-level environment variables.
Backing services [...] Backing services, such as databases or message brokers, are treated as attached resources and consumed identically across all environments.
Build, release, run [...] The stages of building a deployable app artifact, combining that artifact with configuration, and starting one or more processes from that artifact/configuration combination, are strictly separated.
Processes [...] The app executes as one or more stateless processes (e.g., master/workers) that share nothing. Any necessary state is externalized to backing services (cache, object store, etc.).
Port binding [...] The app is self-contained and exports any/all services via port binding (including HTTP).
Concurrency [...] Concurrency is usually accomplished by scaling out app processes horizontally (though processes may also multiplex work via internally managed threads if desired).
Disposability [...] Robustness is maximized via processes that start up quickly and shut down gracefully. These aspects allow for rapid elastic scaling, deployment of changes, and recovery from crashes.
Dev/prod parity [...] Continuous delivery and deployment are enabled by keeping development, staging, and production environments as similar as possible.
Logs Rather than managing logfiles, treat logs as event streams, allowing the execution environment to collect, aggregate, index, and analyze the events via centralized services.
Admin processes [...] Administrative or management tasks, such as database migrations, are executed as one-off processes in environments identical to the app’s long-running processes.”
The best way to think of a container is as a method to package a service, application, or job. It's an RPM on steroids, taking the application and adding in its dependencies, as well as providing a standard way for its host system to manage its runtime environment . Rather than a single container running multiple processes, aim for multiple containers, each running one process. These processes then become independent, loosely coupled entities. This makes containers a nice match for microservice application architectures. Morris, Kief. Infrastructure as Code: Managing Servers in the Cloud (Kindle Locations 1708-1711). O'Reilly Media. Kindle Edition.
The benefits of decoupling runtime requirements from the host system are particularly powerful for infrastructure management. It creates a clean separation of concerns between infrastructure and applications. The host system only needs to have the container runtime software installed, and then it can run nearly any container image. Applications, services, and jobs are packaged into containers along with all of their dependencies [...]. These dependencies can include operating system packages, language runtimes, libraries, and system files. Different containers may have different, even conflicting dependencies, but still run on the same host without issues. Changes to the dependencies can be made without any changes to the host system. Morris, Kief. Infrastructure as Code: Managing Servers in the Cloud (Kindle Locations 1652-1658). O'Reilly Media. Kindle Edition.
The immutable server pattern mentioned in "Server Change Management Models" doesn't make configuration updates to existing servers. Instead, changes are made by building a new server with the new configuration. With immutable servers, configuration is usually baked into the server template. When the configuration is updated, a new template is packaged. New instances of existing servers are built from the new template and used to replace the older servers. This approach treats server templates like software artifacts. Each build is versioned and tested before being deployed for production use. This creates a high level of confidence in the consistency of the server configuration between testing and production. Advocates of immutable servers view making a change to the configuration of a production server as bad practice, no better than modifying the source code of software directly on a production server. Immutable servers can also simplify configuration management, by reducing the area of the server that needs to be managed by definition files. Morris, Kief. Infrastructure as Code: Managing Servers in the Cloud (Kindle Locations 2239-2247). O'Reilly Media. Kindle Edition.
Bootstrap Configuration with Immutable Servers: The purest use of immutable servers is to bake everything onto the server template and change nothing, even when creating server instances from the template. But some teams have found that for certain types of changes, the turnaround time needed to build a new template is too slow. An emerging practice is to put almost everything into the server template, but add one or two elements when bootstrapping a new server. This might be a configuration setting that is only known when the server is created, or it might be a frequently changing element such as an application build for testing. A small development team using continuous integration (CI) or continuous delivery (CD) is likely to deploy dozens of builds of their application a day, so building a new server template for every build may be unacceptably slow. Having a standard server template image that can pull in and start a specified application build when it is started is particularly useful for microservices. This still follows the immutable server pattern, in that any change to the server’s configuration (such as a new version of the microservice) is carried out by building a new server instance. It shortens the turnaround time for changes to a microservice, because it doesn't require building a new server template. However, this practice arguably weakens the testing benefits from the pure immutable model. Ideally, a given combination of server template version and microservice version will have been tested through each stage of a change management pipeline. But there is some risk that the process of installing a microservice, or making other changes, when creating a server will behave slightly differently when done for different servers. This could cause unexpected behavior. So this practice trades some of the consistency benefits of baking everything into a template and using it unchanged in every instance in order to speed up turnaround times for changes made in this way. In many cases, such as those involving frequent changes, this trade-off works quite well. Morris, Kief. Infrastructure as Code: Managing Servers in the Cloud (Kindle Locations 3099-3116). O'Reilly Media. Kindle Edition.
Containerized services works by packaging applications and services in lightweight containers (as popularized by Docker). This reduces coupling between server configuration and the things that run on the servers. So host servers tend to be very simple, with a lower rate of change. One of the other change management models still needs to be applied to these hosts, but their implementation becomes much simpler and easier to maintain. Most effort and attention goes into packaging, testing, distributing, and orchestrating the services and applications, but this follows something similar to the immutable infrastructure model, which again is simpler than managing the configuration of full-blown virtual machines and servers. Morris, Kief. Infrastructure as Code: Managing Servers in the Cloud (Kindle Locations 1617-1621). O'Reilly Media. Kindle Edition.
Declarative configuration is different from imperative configuration , where you simply take a series of actions (e.g., apt-get install foo ) to modify the world. Years of production experience have taught us that maintaining a written record of the system's desired state leads to a more manageable, reliable system. Declarative configuration enables numerous advantages, including code review for configurations as well as documenting the current state of the world for distributed teams. Additionally, it is the basis for all of the self-healing behaviors in Kubernetes that keep applications running without user action." Hightower, Kelsey; Burns, Brendan; Beda, Joe. Kubernetes: Up and Running: Dive into the Future of Infrastructure (Kindle Locations 892-896). Kindle Edition.
[...] OSI reference model with up to seven layers, where each layer provides a different level of abstraction and performs a set of well-defined functions. These seven layers are as follows. 1. Physical layer: These protocols employ methods for bit transmission over physical media and include such typical functions as signal processing, timing, and encoding. 2. Data Link Control (DLC) layer: Its protocols establish point-to-point communication over a physical or logical link, performing such functions as organization of bits in data units (frames) organization, error detection, and flow control. 3. Network layer: These protocols deliver data units over a network composed of the links established through the DLC protocols of layer 2. Part of these protocols is identification of the route the data units will follow to reach their target. 4. Transport layer: Transport protocols establish end-to-end communication between end systems over the network defined by a layer 3 protocol. Often, transport layer protocols provide reliability, which refers to complete and correct data transfer between end systems. Reliability can be achieved through mechanisms for end-to-end error detection, retransmissions, and flow control. 5. Session layer: This layer enables and manages sessions for complete data exchange between end nodes. Sessions may consist of multiple transport layer connections. 6. Presentation layer: This layer is responsible for the presentation of exchanged data in formats that can be consumed by the application layer. 7. Application layer: The application layer includes protocols that implement or facilitate end-to-end distributed applications over the network. Serpanos, Dimitrios,Wolf, Tilman. Architecture of Network Systems (The Morgan Kaufmann Series in Computer Architecture and Design) (pp. 13-14). Elsevier Science. Kindle Edition.
https://www.packer.io/intro/, “Packer is an open source tool for creating identical machine images for multiple platforms from a single source configuration. Packer is lightweight, runs on every major operating system, and is highly performant, creating machine images for multiple platforms in parallel. Packer does not replace configuration management like Chef or Puppet. In fact, when building images, Packer is able to use tools like Chef or Puppet to install software onto the image. A machine image is a single static unit that contains a pre-configured operating system and installed software which is used to quickly create new running machines. Machine image formats change for each platform. Some examples include AMIs for EC2, VMDK/VMX files for VMware, OVF exports for VirtualBox, etc.."