CNI provides plugins that handle tasks such as assigning IP addresses, configuring network interfaces, and enabling traffic routing for containers. It is widely used in cloud-native environments to achieve seamless and dynamic networking for containers.

Key Components of CNI:

  1. Specification:
    A set of rules and guidelines for implementing container networking solutions. It ensures compatibility between container runtimes and network plugins.
  2. Plugins:
    Modular tools that perform networking tasks such as IP address management, DNS configuration, and traffic routing. Common CNI plugins include:
    • Calico: For secure networking and policy enforcement.
    • Flannel: A simple overlay network for Kubernetes.
    • Weave: For seamless networking across clusters.
    • Cilium: Focused on security and observability with eBPF.
  3. Container Runtime Support:
    CNI is supported by popular container runtimes like Docker, CRI-O, and containerd, making it versatile and widely adopted.
  4. Execution Model:
    When a container is created, the runtime calls a CNI plugin to:
    • Assign an IP address.
    • Create and configure network interfaces.
    • Apply any network policies.

How CNI Works:

  1. Container Creation:
    The container runtime requests network configuration when starting a container.
  2. CNI Plugin Execution:
    The CNI plugin is invoked to set up the container’s network interface and assign it an IP address.
  3. Networking Established:
    The container is connected to the network, allowing communication with other containers, nodes, or external services.
  4. Container Termination:
    When a container stops, the CNI plugin is called again to clean up the network configuration.

Why is CNI Important?

  • Standardization:
    CNI ensures that network configuration is consistent across various container runtimes and platforms.
  • Modularity:
    Developers can choose from a wide range of CNI plugins, each catering to different use cases, such as security, performance, or scalability.
  • Scalability:
    CNI makes it easy to scale containerized applications while maintaining robust network connectivity.
  • Interoperability:
    Its support across multiple container runtimes and orchestrators like Kubernetes promotes ecosystem-wide adoption.

Use Cases for CNI:

  1. Kubernetes Networking:
    CNI plugins are integral to setting up and managing pod networking in Kubernetes clusters.
  2. Service Mesh Integration:
    Works alongside service meshes like Istio to manage container networking and enhance observability.
  3. Custom Networking Policies:
    Tools like Calico use CNI to enforce advanced network security policies.
  4. Edge and Multi-Cloud Deployments:
    Enables seamless networking across geographically distributed clusters or hybrid cloud environments.

Further Reading and Resources

  1. Documentation
  2. Kubernetes Networking Overview
  3. Popular CNI Plugins: