OKD uses a software-defined networking (SDN) approach to provide a unified cluster network that enables communication between pods across the OKD cluster. This pod network is established and maintained by the OpenShift SDN, which configures an overlay network using Open vSwitch (OVS).
OpenShift SDN provides three SDN plug-ins for configuring the pod network:
The ovs-subnet plug-in is the original plug-in, which provides a "flat" pod network where every pod can communicate with every other pod and service.
The ovs-multitenant plug-in provides project-level isolation for pods and services. Each project receives a unique Virtual Network ID (VNID) that identifies traffic from pods assigned to the project. Pods from different projects cannot send packets to or receive packets from pods and services of a different project.
However, projects that receive VNID 0 are more privileged in that they are allowed to communicate with all other pods, and all other pods can communicate with them. In OKD clusters, the default project has VNID 0. This facilitates certain services, such as the load balancer, to communicate with all other pods in the cluster and vice versa.
The ovs-networkpolicy plug-in (currently in Tech Preview) allows project administrators to configure their own isolation policies using NetworkPolicy objects.
Information on configuring the SDN on masters and nodes is available in Configuring the SDN.
On an OKD master, OpenShift SDN maintains a registry of nodes, stored in etcd. When the system administrator registers a node, OpenShift SDN allocates an unused subnet from the cluster network and stores this subnet in the registry. When a node is deleted, OpenShift SDN deletes the subnet from the registry and considers the subnet available to be allocated again.
In the default configuration, the cluster network is the 10.128.0.0/14 network (i.e. 10.128.0.0 - 10.131.255.255), and nodes are allocated /23 subnets (i.e., 10.128.0.0/23, 10.128.2.0/23, 10.128.4.0/23, and so on). This means that the cluster network has 512 subnets available to assign to nodes, and a given node is allocated 510 addresses that it can assign to the containers running on it. The size and address range of the cluster network are configurable, as is the host subnet size.
Note that the OpenShift SDN on a master does not configure the local (master) host to have access to any cluster network. Consequently, a master host does not have access to pods via the cluster network, unless it is also running as a node.
When using the ovs-multitenant plug-in, the OpenShift SDN master also watches for the creation and deletion of projects, and assigns VXLAN VNIDs to them, which are later used by the nodes to isolate traffic correctly.
On a node, OpenShift SDN first registers the local host with the SDN master in the aforementioned registry so that the master allocates a subnet to the node.
Next, OpenShift SDN creates and configures three network devices:
br0: the OVS bridge device that pod containers will be attached to.
OpenShift SDN also configures a set of non-subnet-specific flow rules on this
tun0: an OVS internal port (port 2 on
br0). This gets assigned the
cluster subnet gateway address, and is used for external network
access. OpenShift SDN configures netfilter and routing rules to enable access
from the cluster subnet to the external network via NAT.
vxlan_sys_4789: The OVS VXLAN device (port 1 on
br0), which provides access to containers on remote nodes. Referred to as
vxlan0 in the OVS rules.
Each time a pod is started on the host, OpenShift SDN:
assigns the pod a free IP address from the node’s cluster subnet.
attaches the host side of the pod’s veth interface pair to the OVS bridge
adds OpenFlow rules to the OVS database to route traffic addressed to the new pod to the correct OVS port.
in the case of the ovs-multitenant plug-in, adds OpenFlow rules to tag traffic coming from the pod with the pod’s VNID, and to allow traffic into the pod if the traffic’s VNID matches the pod’s VNID (or is the privileged VNID 0). Non-matching traffic is filtered out by a generic rule.
OpenShift SDN nodes also watch for subnet updates from the SDN master. When a
new subnet is added, the node adds OpenFlow rules on
br0 so that packets with
a destination IP address in the remote subnet go to
vxlan0 (port 1 on
thus out onto the network. The ovs-subnet plug-in sends all packets across the
VXLAN with VNID 0, but the ovs-multitenant plug-in uses the appropriate VNID
for the source container.
Suppose you have two containers, A and B, where the peer virtual Ethernet device for container A’s eth0 is named vethA and the peer for container B’s eth0 is named vethB.
If the Docker service’s use of peer virtual Ethernet devices is not already familiar to you, see Docker’s advanced networking documentation.
Now suppose first that container A is on the local host and container B is also on the local host. Then the flow of packets from container A to container B is as follows:
eth0 (in A’s netns) → vethA → br0 → vethB → eth0 (in B’s netns)
Next, suppose instead that container A is on the local host and container B is on a remote host on the cluster network. Then the flow of packets from container A to container B is as follows:
eth0 (in A’s netns) → vethA → br0 → vxlan0 → network  → vxlan0 → br0 → vethB → eth0 (in B’s netns)
Finally, if container A connects to an external host, the traffic looks like:
eth0 (in A’s netns) → vethA → br0 → tun0 → (NAT) → eth0 (physical device) → Internet
Almost all packet delivery decisions are performed with OpenFlow rules in the OVS bridge br0, which simplifies the plug-in network architecture and provides flexible routing. In the case of the ovs-multitenant plug-in, this also provides enforceable network isolation.
You can use the ovs-multitenant plug-in to achieve network isolation. When a packet exits a pod assigned to a non-default project, the OVS bridge br0 tags that packet with the project’s assigned VNID. If the packet is directed to another IP address in the node’s cluster subnet, the OVS bridge only allows the packet to be delivered to the destination pod if the VNIDs match.
If a packet is received from another node via the VXLAN tunnel, the Tunnel ID is used as the VNID, and the OVS bridge only allows the packet to be delivered to a local pod if the tunnel ID matches the destination pod’s VNID.
Packets destined for other cluster subnets are tagged with their VNID and delivered to the VXLAN tunnel with a tunnel destination address of the node owning the cluster subnet.
As described before, VNID 0 is privileged in that traffic with any VNID is allowed to enter any pod assigned VNID 0, and traffic with VNID 0 is allowed to enter any pod. Only the default OKD project is assigned VNID 0; all other projects are assigned unique, isolation-enabled VNIDs. Cluster administrators can optionally control the pod network for the project using the administrator CLI.