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OpenShift SDN

This is openshift-sdn, the default network plugin for OpenShift (both OKD and OCP). It uses Open vSwitch to connect pods locally, with VXLAN tunnels to connect different nodes.

OpenShift SDN is designed to be installed by the OpenShift Network Operator, and certain components of it (such as the Deployment and DaemonSet objects) are found there.

This module defines two images: the sdn image, which contains OpenShift SDN (both controller and node components) and the kube-proxy image, which is deployed by the Network Operator for third-party network plugins that need it. (Kube-proxy is built from here rather than from the origin repo so that we only have to maintain kube-proxy bugfix/security backports in one place.)

OpenShift SDN Types

For historical reasons, OpenShift SDN's types are defined in the network.openshift.io namespace and are part of the openshift/api module, despite being used only when OpenShift SDN is the configured network plugin.

Because the OpenShift aggregated apiserver runs in the pod network, not on the host network, OpenShift SDN cannot depend on it. Therefore, although the types are defined in openshift/api, they are actually implemented as CustomResourceDefinitions in the main apiserver. The Network Operator creates the CRD definitions.

The OpenShift SDN Controller

The network controller is run on the masters to handle cluster-level processing:

  • Creating NetNamespace objects corresponding to Namespaces
  • Creating HostSubnet objects corresponding to Nodes
  • Implementing high availability for egress IPs

In older releases, the controller was also responsible for reading the cluster master configuration and creating the ClusterNetwork object containing configuration information to be used by the nodes. As of OpenShift 4.2, the ClusterNetwork is created by the Network Operator.

OpenShift SDN Nodes

The openshift-sdn daemon runs on every node, reads the ClusterNetwork object and the HostSubnet object for the node it is running on, and uses that information to configure the node as part of the cluster. This includes:

  • Providing networking to Pods, as requested by the openshift-sdn CNI plugin (which is a small shim that just talks to the daemon).

  • Setting up the OVS bridge, and managing OVS flows as needed for Pods, Services, NetworkPolicy, and EgressNetworkPolicy; and adding and removing flows as needed for communicating with other nodes.

  • Setting up iptables rules for masquerading outbound traffic, and ensure that OpenShift's own traffic does not get firewalled.

  • Updating OVS flows and iptables rules for static egress IPs.

  • Implementing the Service proxy via a built-in copy of kube-proxy, in either the "userspace" mode, "iptables" mode, or the hybrid "unidling" mode.

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