The Container Runtime Interface (CRI) defines the protocol for the communication between:

CRI defines the gRPC protocol for communication between the kubelet, which runs on each node, and the container runtime (e.g., containerd, CRI-O) responsible for managing containers on that node. This interface allows Kubernetes to use various container runtimes without being tightly coupled to a specific one.

CRI does not define communication between the container runtime and etcd; etcd stores cluster state, accessed by the API server.

Communication between the kube-apiserver and the kubelet is typically over HTTP/HTTPS, not specifically defined by CRI.

The container runtime interacts with the image registry to pull images, but CRI specifically defines the interface between the kubelet and the runtime itself.