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Optimizing networking

The OpenShift SDN uses OpenvSwitch, virtual extensible LAN (VXLAN) tunnels, OpenFlow rules, and iptables. This network can be tuned by using jumbo frames, multi-queue, and ethtool settings.

OVN-Kubernetes uses Generic Network Virtualization Encapsulation (Geneve) instead of VXLAN as the tunnel protocol. This network can be tuned by using network interface controller (NIC) offloads.

VXLAN provides benefits over VLANs, such as an increase in networks from 4096 to over 16 million, and layer 2 connectivity across physical networks. This allows for all pods behind a service to communicate with each other, even if they are running on different systems.

VXLAN encapsulates all tunneled traffic in user datagram protocol (UDP) packets. However, this leads to increased CPU utilization. Both these outer- and inner-packets are subject to normal checksumming rules to guarantee data is not corrupted during transit. Depending on CPU performance, this additional processing overhead can cause a reduction in throughput and increased latency when compared to traditional, non-overlay networks.

Cloud, VM, and bare metal CPU performance can be capable of handling much more than one Gbps network throughput. When using higher bandwidth links such as 10 or 40 Gbps, reduced performance can occur. This is a known issue in VXLAN-based environments and is not specific to containers or OKD. Any network that relies on VXLAN tunnels will perform similarly because of the VXLAN implementation.

If you are looking to push beyond one Gbps, you can:

  • Evaluate network plugins that implement different routing techniques, such as border gateway protocol (BGP).

  • Use VXLAN-offload capable network adapters. VXLAN-offload moves the packet checksum calculation and associated CPU overhead off of the system CPU and onto dedicated hardware on the network adapter. This frees up CPU cycles for use by pods and applications, and allows users to utilize the full bandwidth of their network infrastructure.

VXLAN-offload does not reduce latency. However, CPU utilization is reduced even in latency tests.

Optimizing the MTU for your network

There are two important maximum transmission units (MTUs): the network interface controller (NIC) MTU and the cluster network MTU.

The NIC MTU is configured at the time of OKD installation, and you can also change the MTU of a cluster as a postinstallation task. For more information, see "Changing cluster network MTU".

For a cluster that uses the OVN-Kubernetes plugin, the MTU must be less than 100 bytes to the maximum supported value of the NIC of your network. If you are optimizing for throughput, choose the largest possible value, such as 8900. If you are optimizing for lowest latency, choose a lower value.

If your cluster uses the OVN-Kubernetes plugin and the network uses a NIC to send and receive unfragmented jumbo frame packets over the network, you must specify 9000 bytes as the MTU value for the NIC so that pods do not fail.

The OpenShift SDN network plugin overlay MTU must be less than the NIC MTU by 50 bytes at a minimum. This accounts for the SDN overlay header. So, on a normal ethernet network, this should be set to 1450. On a jumbo frame ethernet network, this should be set to 8950. These values should be set automatically by the Cluster Network Operator based on the NIC’s configured MTU. Therefore, cluster administrators do not typically update these values. Amazon Web Services (AWS) and bare-metal environments support jumbo frame ethernet networks. This setting will help throughput, especially with transmission control protocol (TCP).

OpenShift SDN CNI is deprecated as of OKD 4.14. As of OKD 4.15, the network plugin is not an option for new installations. In a subsequent future release, the OpenShift SDN network plugin is planned to be removed and no longer supported. Red Hat will provide bug fixes and support for this feature until it is removed, but this feature will no longer receive enhancements. As an alternative to OpenShift SDN CNI, you can use OVN Kubernetes CNI instead. For more information, see OpenShift SDN CNI removal.

For OVN and Geneve, the MTU must be less than the NIC MTU by 100 bytes at a minimum.

This 50 byte overlay header is relevant to the OpenShift SDN network plugin. Other SDN solutions might require the value to be more or less.
Additional resources

When installing large clusters or scaling the cluster to larger node counts, set the cluster network cidr accordingly in your install-config.yaml file before you install the cluster.

Example install-config.yaml file with a network configuration for a cluster with a large node count
networking:
  clusterNetwork:
  - cidr: 10.128.0.0/14
    hostPrefix: 23
  machineNetwork:
  - cidr: 10.0.0.0/16
  networkType: OVNKubernetes
  serviceNetwork:
  - 172.30.0.0/16

The default cluster network cidr 10.128.0.0/14 cannot be used if the cluster size is more than 500 nodes. The cidr must be set to 10.128.0.0/12 or 10.128.0.0/10 to get to larger node counts beyond 500 nodes.

Impact of IPsec

Because encrypting and decrypting node hosts uses CPU power, performance is affected both in throughput and CPU usage on the nodes when encryption is enabled, regardless of the IP security system being used.

IPSec encrypts traffic at the IP payload level, before it hits the NIC, protecting fields that would otherwise be used for NIC offloading. This means that some NIC acceleration features might not be usable when IPSec is enabled and leads to decreased throughput and increased CPU usage.

Additional resources