ClusterIP

When people say that Kubernetes networking is difficult, they very often refer to this type of service. One of the reasons for this perception is that all of its complexity is hidden behind a very minimalistic API. A common way of defining a Service only takes 5 lines of configuration (plus the standard metadata):

apiVersion: v1
kind: Service
metadata:
  name: clusterIP-example
spec:
  ports:
  - name: http
    port: 80
  selector:
    app: my-backend-app

Quite unexpectedly, these 5 lines can generate a large amount of state inside the cluster as each Service has to be implemented on all Nodes and its state grows proportionally to the number of backend Endpoints. In order to better understand the networking behind it, the remainder of this chapter will be broken down into the following sections:

• Control Plane will examine the mechanics of interaction between the user input, the API server processing it and a distributed set of node-local agents ultimately consuming it.
• Data Plane will cover some of the standard implementations including iptables, ipvs and eBPF.