Kubernetes 1.25: PodHasNetwork Condition for Pods

Author: Deep Debroy (Apple)

Kubernetes 1.25 introduces Alpha support for a new kubelet-managed pod condition in the status field of a pod: PodHasNetwork. The kubelet, for a worker node, will use the PodHasNetwork condition to accurately surface the initialization state of a pod from the perspective of pod sandbox creation and network configuration by a container runtime (typically in coordination with CNI plugins). The kubelet starts to pull container images and start individual containers (including init containers) after the status of the PodHasNetwork condition is set to "True". Metrics collection services that report latency of pod initialization from a cluster infrastructural perspective (i.e. agnostic of per container characteristics like image size or payload) can utilize the PodHasNetwork condition to accurately generate Service Level Indicators (SLIs). Certain operators or controllers that manage underlying pods may utilize the PodHasNetwork condition to optimize the set of actions performed when pods repeatedly fail to come up.

Updates for Kubernetes 1.28

The PodHasNetwork condition has been renamed to PodReadyToStartContainers. Alongside that change, the feature gate PodHasNetworkCondition has been replaced by PodReadyToStartContainersCondition. You need to set PodReadyToStartContainersCondition to true in order to use the new feature in v1.28.0 and later.

How is this different from the existing Initialized condition reported for pods?

The kubelet sets the status of the existing Initialized condition reported in the status field of a pod depending on the presence of init containers in a pod.

If a pod specifies init containers, the status of the Initialized condition in the pod status will not be set to "True" until all init containers for the pod have succeeded. However, init containers, configured by users, may have errors (payload crashing, invalid image, etc) and the number of init containers configured in a pod may vary across different workloads. Therefore, cluster-wide, infrastructural SLIs around pod initialization cannot depend on the Initialized condition of pods.

If a pod does not specify init containers, the status of the Initialized condition in the pod status is set to "True" very early in the lifecycle of the pod. This occurs before the kubelet initiates any pod runtime sandbox creation and network configuration steps. As a result, a pod without init containers will report the status of the Initialized condition as "True" even if the container runtime is not able to successfully initialize the pod sandbox environment.

Relative to either situation above, the PodHasNetwork condition surfaces more accurate data around when the pod runtime sandbox was initialized with networking configured so that the kubelet can proceed to launch user-configured containers (including init containers) in the pod.

Special Cases

If a pod specifies hostNetwork as "True", the PodHasNetwork condition is set to "True" based on successful creation of the pod sandbox while the network configuration state of the pod sandbox is ignored. This is because the CRI implementation typically skips any pod sandbox network configuration when hostNetwork is set to "True" for a pod.

A node agent may dynamically re-configure network interface(s) for a pod by watching changes in pod annotations that specify additional networking configuration (e.g. k8s.v1.cni.cncf.io/networks). Dynamic updates of pod networking configuration after the pod sandbox is initialized by Kubelet (in coordination with a container runtime) are not reflected by the PodHasNetwork condition.

Try out the PodHasNetwork condition for pods

In order to have the kubelet report the PodHasNetwork condition in the status field of a pod, please enable the PodHasNetworkCondition feature gate on the kubelet.

For a pod whose runtime sandbox has been successfully created and has networking configured, the kubelet will report the PodHasNetwork condition with status set to "True":

$ kubectl describe pod nginx1
Name:             nginx1
Namespace:        default
...
Conditions:
  Type              Status
  PodHasNetwork     True
  Initialized       True
  Ready             True
  ContainersReady   True
  PodScheduled      True

For a pod whose runtime sandbox has not been created yet (and networking not configured either), the kubelet will report the PodHasNetwork condition with status set to "False":

$ kubectl describe pod nginx2
Name:             nginx2
Namespace:        default
...
Conditions:
  Type              Status
  PodHasNetwork     False
  Initialized       True
  Ready             False
  ContainersReady   False
  PodScheduled      True

What’s next?

Depending on feedback and adoption, the Kubernetes team plans to push the reporting of the PodHasNetwork condition to Beta in 1.26 or 1.27.

How can I learn more?

Please check out the documentation for the PodHasNetwork condition to learn more about it and how it fits in relation to other pod conditions.

How to get involved?

This feature is driven by the SIG Node community. Please join us to connect with the community and share your ideas and feedback around the above feature and beyond. We look forward to hearing from you!

Acknowledgements

We want to thank the following people for their insightful and helpful reviews of the KEP and PRs around this feature: Derek Carr (@derekwaynecarr), Mrunal Patel (@mrunalp), Dawn Chen (@dchen1107), Qiutong Song (@qiutongs), Ruiwen Zhao (@ruiwen-zhao), Tim Bannister (@sftim), Danielle Lancashire (@endocrimes) and Agam Dua (@agamdua).

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