Mastering Kubernetes - Second Edition

Container runtime

CNI defines a plugin spec for networking application containers, but the plugin must be plugged into a container runtime that provides some services. In the context of CNI, an application container is a network-addressable entity (has its own IP address). For Docker, each container has its own IP address. For Kubernetes, each pod has its own IP address and the pod is the CNI container and not the containers within the pod.

Likewise, rkt's app containers are similar to Kubernetes pods in that they may contain multiple Linux containers. If in doubt, just remember that a CNI container must have its own IP address. The runtime's job is to configure a network and then execute one or more CNI plugins, passing them the network ...