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KEP-5100: [RETROACTIVE] DSR and Overlay support in Windows kube-proxy

<!-- This is the title of your KEP. Keep it short, simple, and descriptive. A good title can help communicate what the KEP is and should be considered as part of any review. --> <!-- A table of contents is helpful for quickly jumping to sections of a KEP and for highlighting any additional information provided beyond the standard KEP template. Ensure the TOC is wrapped with <code>&lt;!-- toc --&rt;&lt;!-- /toc --&rt;</code> tags, and then generate with `hack/update-toc.sh`. --> <!-- toc -->

• Release Signoff Checklist
• Summary
• Motivation
  • Goals
  • Non-Goals
• Proposal
  • User Stories (Optional)
    • Story 1
    • Story 2
  • Notes/Constraints/Caveats (Optional)
  • Risks and Mitigations
• Design Details
  • DSR Enablement
  • Overlay support
  • Test Plan
    • Prerequisite testing updates
    • Unit tests
    • Integration tests
    • e2e tests
  • Graduation Criteria
    • Alpha
    • Beta
    • GA
  • Upgrade / Downgrade Strategy
  • Version Skew Strategy
• Production Readiness Review Questionnaire
  • Feature Enablement and Rollback
  • Rollout, Upgrade and Rollback Planning
  • Monitoring Requirements
  • Dependencies
  • Scalability
  • Troubleshooting
• Implementation History
• Drawbacks
• Alternatives
• Infrastructure Needed (Optional)

<!-- /toc -->

§ Release Signoff Checklist

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• (R) Design details are appropriately documented
• (R) Test plan is in place, giving consideration to SIG Architecture and SIG Testing input (including test refactors)
  • e2e Tests for all Beta API Operations (endpoints)
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• (R) Production readiness review completed
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• "Implementation History" section is up-to-date for milestone
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• Supporting documentation—e.g., additional design documents, links to mailing list discussions/SIG meetings, relevant PRs/issues, release notes

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§ Summary

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Add support for DSR (Direct Server Return) and Overlay networking mode support for Windows kube-proxy.

Support for both of these features was added in K8s v1.14 without a KEP. This KEP is to retroactively document the changes made to Windows kube-proxy to support these features and provide a path for promoting these features to GA.

§ Motivation

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DSR support was added to Windows Server 2019 as part of the May 2020 update. DSR provides performance optimizations by allowing the return traffic routed through load balancers to bypass the load balancer and respond directly to the client; reducing load on the load balancer and also reducing overall latency. More information on DSR on Windows can be found here.

Overlay networking mode is a common networking mode used in Kubernetes clusters and is required by some for some important scenarios like network policy support with Calico CNI. Adding support for overlay networking mode in Windows kube-proxy will allow users to use more CNI solutions with Windows nodes.

Goals

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Enable DSR and overlay networking on Windows nodes running kube-proxy in Kubernetes clusters.

Non-Goals

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§ Proposal

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DSR and Overlay networking mode support is already implemented in Windows kube-proxy and has been extensively tested in the Windows CI pipeline. This proposal is to promote the existing implementations to GA.

Additionally, DSR support on Windows is supported on both EKS and AKS. Both DSR and overlay networking support have been used in the Windows CI pipelines running release-informing jobs since K8s v1.20.

User Stories (Optional)

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Story 1

As a cluster administrator, I want to enable DSR functionality on Windows nodes in order to reduce load in the Host Network Service and reduce latency for client requests.

Story 2

As a cluster administrator, I want to be able to enable network policy on Windows nodes which requires overlay networking mode support in kube-proxy for some CNI solutions.

Notes/Constraints/Caveats (Optional)

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Overlay networking mode is not compatible with dualstack networking on Windows.

If kube-proxy is started with both overlay networking mode and dualstack networking enabled, a warning message will be added and ip address space with be downgraded to ipv4 only. This is existing behavior and has not caused any reported issues.

Risks and Mitigations

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Enabling DSR and overlay networking mode support in Windows kube-proxy both have very little risk.

For DSR, the Windows Host Network Service handles all of the logic for managing network traffic; kube-proxy only needs to specify if DSR should be used when creating/sycing load balancer rules. Additionally, DSR must be enabled with a kube-proxy command switch (--enable-dsr=true) disabling DSR is can be performed by redeploying kube-proxy on Windows nodes.

Overlay networking support in Windows has been used in the Windows CI pipelines running release-informing capz-windows-master jobs since K8s v1.20.

§ Design Details

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Since the functionality is already implemented, the design details section will cover the current implementation.

DSR Enablement

DSR is enabled by passing --enable-dsr=true as a command line switch to the Windows kube-proxy. Prior to GA, kube-proxy will ensure that WinDSR=true is specified in the feature-gates and will fail to start if DSR is enabled without that.

Checks for terminating and service endpoints handle DSR traffic differently than non-DSR traffic to adhere to behavior defined in KEP-1669: Proxy Terminating Endpoints

• Local endpoints will be skipped when determining if all endpoints for a service are terminated if DSR is enabled and service type is load balancer.
• Non-local endpoints will be skipped when considering if all endpoints for a service are non-serving if DSR is enabled and service type is load balancer.

Flags passed to HNS (Host Networking Service) calls used for the following operators will be updated to include a flag indicating if DSR is enabled for all get, create, and update loadbalancer HNS calls.

Overlay support

To enable overlay networking on Windows nodes, HNS network created on the node prior to starting kube-proxy and specified by $KUBE_NETWORK should be of type Overlay. Prior to GA WinOverlay=true must be specified in the kube-proxy feature gates. If the specified network is of type Overlay and the the feature gate is not set, kube-proxy will log an error and fail to start.

Addintionally, in overlay networking node, kube-proxy needs to know the source IP address of the traffic it is proxying by setting --source-vip=$sourceVIP on the kube-proxy command line.

Creating the endpoint varries by CNI implementation and here are two examples:

• For Flannel, the endpoint is created prior to starting kube-proxy like in this example
• For Calico, the endpoint is crated by the node agent and queried by name prior to starting kube-proxy like in this example

Once kube-proxy is running in overlay networking mode, the specified source VIP will sometimes be used on in load balancer policy rules based on the backend endpoints using the following logic:

a) Backend endpoints are any IP's outside the cluster ==> Choose Node's IP as the source VIP b) Backend endpoints are IP addresses of a remote node => Choose Node's IP as the source VIP c) Everything else (Local POD's, Remote POD's, Node IP of current Node) ==> Choose the specified source VIP

Everything else is handled by the Windows HNS.

Test Plan

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[x] I/we understand the owners of the involved components may require updates to existing tests to make this code solid enough prior to committing the changes necessary to implement this enhancement.

Prerequisite testing updates

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Unit tests

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Kube-proxy for Windows must run on Windows machines so coverage is not reported in ci-kubernetes-coverage-unit. This coverage data was run manually on a Windows Server 2022 machine:

• k8s.io/kubernetes/pkg/proxy/winkernel: 2025-02-11 - 58.8% of statements

Integration tests

<!-- Integration tests are contained in k8s.io/kubernetes/test/integration. Integration tests allow control of the configuration parameters used to start the binaries under test. This is different from e2e tests which do not allow configuration of parameters. Doing this allows testing non-default options and multiple different and potentially conflicting command line options. --> <!-- This question should be filled when targeting a release. For Alpha, describe what tests will be added to ensure proper quality of the enhancement. For Beta and GA, add links to added tests together with links to k8s-triage for those tests: https://storage.googleapis.com/k8s-triage/index.html -->

Functionality described in this KEP require Windows nodes and are primarily validated with unit and e2e tests. The Kubernetes project does not currently have support for running integration tests for Windows specific code-paths.

e2e tests

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All Windows nodes running kube-proxy in https://testgrid.k8s.io/sig-windows-master-release#capz-windows-master have DSR and overlay networking configured.

Graduation Criteria

<!-- **Note:** *Not requir