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+<!--
+**Note:** When your KEP is complete, all of these comment blocks should be removed.
+
+To get started with this template:
+
+- [X] **Pick a hosting SIG.**
+  Make sure that the problem space is something the SIG is interested in taking
+  up. KEPs should not be checked in without a sponsoring SIG.
+- [X] **Create an issue in kubernetes/enhancements**  (Issue #4006).
+  When filing an enhancement tracking issue, please make sure to complete all
+  fields in that template. One of the fields asks for a link to the KEP. You
+  can leave that blank until this KEP is filed, and then go back to the
+  enhancement and add the link.
+- [X] **Make a copy of this template directory.**
+  Copy this template into the owning SIG's directory and name it
+  `NNNN-short-descriptive-title`, where `NNNN` is the issue number (with no
+  leading-zero padding) assigned to your enhancement above.
+- [X] **Fill out as much of the kep.yaml file as you can.**
+  At minimum, you should fill in the "Title", "Authors", "Owning-sig",
+  "Status", and date-related fields.
+- [X] **Fill out this file as best you can.**
+  At minimum, you should fill in the "Summary" and "Motivation" sections.
+  These should be easy if you've preflighted the idea of the KEP with the
+  appropriate SIG(s).
+- [X] **Create a PR for this KEP.**
+  Assign it to people in the SIG who are sponsoring this process.
+- [ ] **Merge early and iterate.**
+  Avoid getting hung up on specific details and instead aim to get the goals of
+  the KEP clarified and merged quickly. The best way to do this is to just
+  start with the high-level sections and fill out details incrementally in
+  subsequent PRs.
+
+Just because a KEP is merged does not mean it is complete or approved. Any KEP
+marked as `provisional` is a working document and subject to change. You can
+denote sections that are under active debate as follows:
+
+```
+<<[UNRESOLVED optional short context or usernames ]>>
+Stuff that is being argued.
+<<[/UNRESOLVED]>>
+```
+
+When editing KEPS, aim for tightly-scoped, single-topic PRs to keep discussions
+focused. If you disagree with what is already in a document, open a new PR
+with suggested changes.
+
+One KEP corresponds to one "feature" or "enhancement" for its whole lifecycle.
+You do not need a new KEP to move from beta to GA, for example. If
+new details emerge that belong in the KEP, edit the KEP. Once a feature has become
+"implemented", major changes should get new KEPs.
+
+The canonical place for the latest set of instructions (and the likely source
+of this file) is [here](/keps/NNNN-kep-template/README.md).
+
+**Note:** Any PRs to move a KEP to `implementable`, or significant changes once
+it is marked `implementable`, must be approved by each of the KEP approvers.
+If none of those approvers are still appropriate, then changes to that list
+should be approved by the remaining approvers and/or the owning SIG (or
+SIG Architecture for cross-cutting KEPs).
+-->
+# KEP-4006: Transition from SPDY to WebSockets
+
+<!--
+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](#release-signoff-checklist)
+- [Summary](#summary)
+- [Motivation](#motivation)
+  - [Goals](#goals)
+  - [Non-Goals](#non-goals)
+- [Proposal](#proposal)
+  - [User Stories (Optional)](#user-stories-optional)
+  - [Notes/Constraints/Caveats (Optional)](#notesconstraintscaveats-optional)
+  - [Risks and Mitigations](#risks-and-mitigations)
+- [Design Details](#design-details)
+  - [Background: Streaming Protocol Basics](#background-streaming-protocol-basics)
+  - [Background: <code>RemoteCommand</code> Subprotocol](#background--subprotocol)
+  - [Background: API Server and Kubelet <code>UpgradeAwareProxy</code>](#background-api-server-and-kubelet-)
+  - [Proposal: <code>kubectl</code> WebSocket Executor and Fallback Executor](#proposal--websocket-executor-and-fallback-executor)
+  - [Proposal: <code>v5.channel.k8s.io</code> subprotocol version](#proposal--subprotocol-version)
+  - [Proposal: API Server <code>StreamTranslatorProxy</code>](#proposal-api-server-)
+  - [Future: Kubelet <code>StreamTranslatorProxy</code>](#future-kubelet-)
+  - [Test Plan](#test-plan)
+      - [Prerequisite testing updates](#prerequisite-testing-updates)
+      - [Unit tests](#unit-tests)
+      - [Integration tests](#integration-tests)
+      - [e2e tests](#e2e-tests)
+  - [Graduation Criteria](#graduation-criteria)
+    - [Alpha](#alpha)
+  - [Upgrade / Downgrade Strategy](#upgrade--downgrade-strategy)
+  - [Version Skew Strategy](#version-skew-strategy)
+- [Production Readiness Review Questionnaire](#production-readiness-review-questionnaire)
+  - [Feature Enablement and Rollback](#feature-enablement-and-rollback)
+  - [Rollout, Upgrade and Rollback Planning](#rollout-upgrade-and-rollback-planning)
+  - [Monitoring Requirements](#monitoring-requirements)
+  - [Dependencies](#dependencies)
+  - [Scalability](#scalability)
+  - [Troubleshooting](#troubleshooting)
+- [Implementation History](#implementation-history)
+- [Drawbacks](#drawbacks)
+- [Alternatives](#alternatives)
+- [Infrastructure Needed (Optional)](#infrastructure-needed-optional)
+<!-- /toc -->
+
+## Release Signoff Checklist
+
+<!--
+**ACTION REQUIRED:** In order to merge code into a release, there must be an
+issue in [kubernetes/enhancements] referencing this KEP and targeting a release
+milestone **before the [Enhancement Freeze](https://git.k8s.io/sig-release/releases)
+of the targeted release**.
+
+For enhancements that make changes to code or processes/procedures in core
+Kubernetes—i.e., [kubernetes/kubernetes], we require the following Release
+Signoff checklist to be completed.
+
+Check these off as they are completed for the Release Team to track. These
+checklist items _must_ be updated for the enhancement to be released.
+-->
+
+Items marked with (R) are required *prior to targeting to a milestone / release*.
+
+- [ ] (R) Enhancement issue in release milestone, which links to KEP dir in [kubernetes/enhancements] (not the initial KEP PR)
+- [ ] (R) KEP approvers have approved the KEP status as `implementable`
+- [ ] (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)
+  - [ ] (R) Ensure GA e2e tests meet requirements for [Conformance Tests](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-architecture/conformance-tests.md) 
+  - [ ] (R) Minimum Two Week Window for GA e2e tests to prove flake free
+- [ ] (R) Graduation criteria is in place
+  - [ ] (R) [all GA Endpoints](https://github.com/kubernetes/community/pull/1806) must be hit by [Conformance Tests](https://github.com/kubernetes/community/blob/master/contributors/devel/sig-architecture/conformance-tests.md) 
+- [ ] (R) Production readiness review completed
+- [ ] (R) Production readiness review approved
+- [ ] "Implementation History" section is up-to-date for milestone
+- [ ] User-facing documentation has been created in [kubernetes/website], for publication to [kubernetes.io]
+- [ ] Supporting documentation—e.g., additional design documents, links to mailing list discussions/SIG meetings, relevant PRs/issues, release notes
+
+<!--
+**Note:** This checklist is iterative and should be reviewed and updated every time this enhancement is being considered for a milestone.
+-->
+
+[kubernetes.io]: https://kubernetes.io/
+[kubernetes/enhancements]: https://git.k8s.io/enhancements
+[kubernetes/kubernetes]: https://git.k8s.io/kubernetes
+[kubernetes/website]: https://git.k8s.io/website
+
+## Summary
+
+<!--
+This section is incredibly important for producing high-quality, user-focused
+documentation such as release notes or a development roadmap. It should be
+possible to collect this information before implementation begins, in order to
+avoid requiring implementors to split their attention between writing release
+notes and implementing the feature itself. KEP editors and SIG Docs
+should help to ensure that the tone and content of the `Summary` section is
+useful for a wide audience.
+
+A good summary is probably at least a paragraph in length.
+
+Both in this section and below, follow the guidelines of the [documentation
+style guide]. In particular, wrap lines to a reasonable length, to make it
+easier for reviewers to cite specific portions, and to minimize diff churn on
+updates.
+
+[documentation style guide]: https://github.com/kubernetes/community/blob/master/contributors/guide/style-guide.md
+-->
+
+Some Kubernetes clients need to communicate with the API Server using a bi-directional
+streaming protocol, instead of the standard HTTP request/response mechanism. A streaming
+protocol provides the ability read and write arbitrary data messages between the
+client and server, instead of providing a single response to a client request.
+For example, the commands `kubectl exec`, `kubectl attach`, `kubectl port-forward`,
+and `kubectl cp` all benefit from a bi-directional streaming protocol. Currently,
+the bi-directional streaming solution for these `kubectl` commands is SPDY/3.1. For
+the communication leg between `kubectl` and the API Server, this enhancement transitions
+the bi-directional streaming protocol to WebSockets from SPDY/3.1 for three `kubectl`
+commands: `exec`, `attach`, and `cp`.
+
+## Motivation
+
+<!--
+This section is for explicitly listing the motivation, goals, and non-goals of
+this KEP.  Describe why the change is important and the benefits to users. The
+motivation section can optionally provide links to [experience reports] to
+demonstrate the interest in a KEP within the wider Kubernetes community.
+
+[experience reports]: https://github.com/golang/go/wiki/ExperienceReports
+-->
+
+The SPDY streaming protocol has been deprecated for around eight years, and by now
+many proxies, gateways, and load-balancers do not support SPDY. Our effort to modernize
+the streaming protocol between Kubernetes clients and the API Server using WebSockets
+is necessary to enable the aforementioned intermediaries. WebSockets is a currently
+supported standardized protocol (https://www.rfc-editor.org/rfc/rfc6455) that guarantees
+compatibility and interoperability with the different components and programming
+languages. Finally, WebSockets is preferrable to HTTP/2.0 because the updated HTTP
+standard does not support streaming well. The decision to forego HTTP/2.0 is discussed
+at greater length in the [Alternatives Section](##Alternatives).
+
+### Goals
+
+<!--
+List the specific goals of the KEP. What is it trying to achieve? How will we
+know that this has succeeded?
+-->
+
+1. Transition the bi-directional streaming protocol from SPDY/3.1 to WebSockets for
+`kubectl exec`, `kubectl attach`, and `kubectl cp` for the communication leg
+between `kubectl` and the API Server.
+
+### Non-Goals
+
+<!--
+What is out of scope for this KEP? Listing non-goals helps to focus discussion
+and make progress.
+-->
+
+1. We do not intend to initially transition the current SPDY bi-directional streaming
+protocol for `kubectl port-forward`. This command requires a different subprotocol
+than the previously stated three `kubectl` commands (which use the `RemoteCommand`
+subprotocol). We intend to transition `kubectl port-forward` in a future release.
+
+2. We do not initially intend to modify *any* of the SPDY communication paths between
+other cluster components other than `kubectl` and the API Server. In other words,
+the current SPDY communication paths between the API Server and Kubelet, or Kubelet
+and the Container Runtime will not change.
+
+3. We will not make *any* changes to current WebSocket based browser/javascript clients.
+
+## Proposal
+
+<!--
+This is where we get down to the specifics of what the proposal actually is.
+This should have enough detail that reviewers can understand exactly what
+you're proposing, but should not include things like API designs or
+implementation. What is the desired outcome and how do we measure success?.
+The "Design Details" section below is for the real
+nitty-gritty.
+-->
+
+Currently, the bi-directional streaming protocols (either SPDY or WebSockets) are 
+initiated from clients, proxied by the API Server and Kubelet, and terminated at
+the Container Runtime (e.g. containerd or CRI-O). This enhancement proposes to 1)
+modify `kubectl` to request a WebSocket based streaming connection, and to 2) modify
+the current API Server proxy to translate the `kubectl` WebSockets data stream to
+a SPDY upstream connection. In this way, the cluster components upstream from the
+API Server will not need to be changed.
+
+### User Stories (Optional)
+
+<!--
+Detail the things that people will be able to do if this KEP is implemented.
+Include as much detail as possible so that people can understand the "how" of
+the system. The goal here is to make this feel real for users without getting
+bogged down.
+-->
+
+<!--
+User stories are not added, since this functionality should be transparent
+to users.
+-->
+
+N/A
+
+### Notes/Constraints/Caveats (Optional)
+
+<!--
+What are the caveats to the proposal?
+What are some important details that didn't come across above?
+Go in to as much detail as necessary here.
+This might be a good place to talk about core concepts and how they relate.
+-->
+
+N/A
+
+### Risks and Mitigations
+
+<!--
+What are the risks of this proposal, and how do we mitigate? Think broadly.
+For example, consider both security and how this will impact the larger
+Kubernetes ecosystem.
+
+How will security be reviewed, and by whom?
+
+How will UX be reviewed, and by whom?
+
+Consider including folks who also work outside the SIG or subproject.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+## Design Details
+
+<!--
+This section should contain enough information that the specifics of your
+change are understandable. This may include API specs (though not always
+required) or even code snippets. If there's any ambiguity about HOW your
+proposal will be implemented, this is the place to discuss them.
+-->
+
+**Current SPDY Streaming Architectural Diagram**
+
+![Current SPDY Streaming Architectural Diagram](./spdy-architechtural-diagram.png)
+
+### Background: Streaming Protocol Basics
+
+`kubectl` bi-directional streaming connections are by created by upgrading an
+initial HTTP/1.1 request. By adding two headers (`Connection: Upgrade`, `Upgrade: SPDY/3.1`),
+the request can initiate the streaming upgrade. And when the response returns status
+`101 Switching Protocols` signalling success, the connection can then be kept open
+for subsequent streaming. An example of an upgraded HTTP Request/Response for `kubectl exec`
+could look like:
+
+**HTTP Request**
+```
+POST /api/v1/…/pods/nginx/exec?command=<CMD>... HTTP/1.1
+Connection: Upgrade
+Upgrade: SPDY/3.1
+X-Stream-Protocol-Version: v4.channel.k8s.io
+X-Stream-Protocol-Version: v3.channel.k8s.io
+X-Stream-Protocol-Version: v2.channel.k8s.io
+X-Stream-Protocol-Version: v1.channel.k8s.io
+```
+
+**HTTP Response**
+```
+HTTP/1.1 101 Switching Protocols
+Connection: Upgrade
+Upgrade: SPDY/3.1
+X-Stream-Protocol-Version: v4.channel.k8s.io
+```
+
+If the upgrade is successful, one of the requested subprotocol versions is chosen
+and returned in the response. In this instance, the chosen version of the subprotocol
+is: `v4.channel.k8s.io`.
+
+### Background: `RemoteCommand` Subprotocol
+
+![Remote Command Subprotocol](./remote-command-subprotocol.png)
+
+Once the connection is upgraded to a bi-directional streaming connection, the
+client and server can exchange data messages. These messages are interpreted with
+agreed upon standards which are called subprotocols. The three `kubectl` commands
+(`exec`, `attach`, and `cp`) communicate using the `RemoteCommand` subprotocol. Basically,
+this subprotocol provides command line functionality from the client to a running
+container in the cluster. By multiplexing `stdin`, `stdout`, `stderr`, and `tty`
+resizing over a streaming connection, this subprotocol supports clients executing
+and interacting with commands executed on a container in the cluster. An example of
+`kubectl exec` running the `date` command on an `nginx` pod/container is:
+
+```
+$ kubectl exec nginx -- date
+Tue May 16 03:34:04 PM PDT 2023
+```
+
+### Background: API Server and Kubelet `UpgradeAwareProxy`
+
+In order to route the data streamed between the client and the container, both the
+API Server and Kubelet must proxy these data messages. Both the API Server and the
+Kubelet provide this functionality with the `UpgradeAwareProxy`, which is a reverse
+proxy that knows how to deal with the connection upgrade handshake.
+
+### Proposal: `kubectl` WebSocket Executor and Fallback Executor
+
+This enhancement proposes adding a `WebSocketExecutor` to `kubectl`, implementing
+the WebSocket client using a new subprotocol version (`v5.channel.k8s.io`). Additionally,
+we propose creating a `FallbackExecutor` to address client/server version skew. The
+`FallbackExecutor`  first attempts to upgrade the connection to WebSockets
+version five (`v5.channel.k8s.io`) with the `WebSocketExecutor`, then falls back
+to the legacy `SPDYExecutor` version four (`v4.channel.k8s.io`), if the upgrade is
+unsuccessful. Note that this mechanism can require two request/response trips instead
+of one. While the fallback mechanism may require an extra request/response if the
+initial upgrade is not successful, we believe this possible extra roundtrip is justified
+for the following reasons:
+
+1. The upgrade handshake is implemented in low-level SPDY and WebSocket libraries,
+and it is not easily exposed by these libraries. If it is even possible to modify
+the upgrade handshake, the added complexity would not be worth the effort.
+2. The streaming is already IO heavy, so another roundtrip will not substantially
+affect the perceived performance.
+3. As releases increment, the probablity of a WebSocket enabled `kubectl` communicating
+with an older non-WebSocket enabled API Server decreases.
+
+### Proposal: `v5.channel.k8s.io` subprotocol version
+
+As previously mentioned, we propose incrementing the subprotocol version from four
+to five: `v5.channel.k8s.io`. This version will indicate to upstream components
+that `kubectl` is interested in using WebSockets between the client and the API Server
+if it is supported.
+
+### Proposal: API Server `StreamTranslatorProxy`
+
+![Stream Translator Proxy](./stream-translator-proxy-2.png)
+
+Currently, the API Server role within client/container streaming is to proxy the
+data stream using the `UpgradeAwareProxy`. This enhancement proposes to modify the
+SPDY data stream between `kubectl` and the API Server by conditionally adding a
+`StreamTranslatorProxy` at the API Server. If the request is for a WebSocket upgrade
+of version `v5.channel.k8s.io`, the `UpgradeAwareProxy` will delegate to the
+`StreamTranslatorProxy`. This translation proxy terminates the WebSocket connection,
+and de-multiplexes the various streams in order to pass the data on to a SPDY connection,
+which continues upstream (to Kubelet and eventually the container runtime).
+
+### Future: Kubelet `StreamTranslatorProxy`
+
+The eventual plan is to incrementally transition all SPDY communication legs to WebSockets.
+After the WebSocket communication leg from `kubectl` to the API Server is proven
+to work, the next communication leg to transition is the one from the API Server
+to the Kubelet. Both the API Server and the Kubelet stream data messages using the
+`UpgradeAwareProxy`. Since the initial plan is to modify the `UpgradeAwareProxy`
+to include the `StreamTranslatorProxy`, it will be straighforward to transition
+this next communication leg by integrating the `StreamTranslatorProxy` the same as
+in the API Server.
+
+The final communication leg to transition from SPDY to WebSockets will be the one
+from Kubelet to the Container Runtimes. This effort will be more work, since it will
+require modifying not just Kubelet, but **all** Container Runtimes.
+
+### Test Plan
+
+<!--
+**Note:** *Not required until targeted at a release.*
+The goal is to ensure that we don't accept enhancements with inadequate testing.
+
+All code is expected to have adequate tests (eventually with coverage
+expectations). Please adhere to the [Kubernetes testing guidelines][testing-guidelines]
+when drafting this test plan.
+
+[testing-guidelines]: https://git.k8s.io/community/contributors/devel/sig-testing/testing.md
+-->
+
+[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
+
+<!--
+Based on reviewers feedback describe what additional tests need to be added prior
+implementing this enhancement to ensure the enhancements have also solid foundations.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+##### Unit tests
+
+<!--
+In principle every added code should have complete unit test coverage, so providing
+the exact set of tests will not bring additional value.
+However, if complete unit test coverage is not possible, explain the reason of it
+together with explanation why this is acceptable.
+-->
+
+<!--
+Additionally, for Alpha try to enumerate the core package you will be touching
+to implement this enhancement and provide the current unit coverage for those
+in the form of:
+- <package>: <date> - <current test coverage>
+The data can be easily read from:
+https://testgrid.k8s.io/sig-testing-canaries#ci-kubernetes-coverage-unit
+
+This can inform certain test coverage improvements that we want to do before
+extending the production code to implement this enhancement.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+- `<package>`: `<date>` - `<test coverage>`
+
+##### 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
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+- `<test>: <link to test coverage>`
+
+##### e2e tests
+
+<!--
+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
+
+We expect no non-infra related flakes in the last month as a GA graduation criteria.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+- `<test>: <link to test coverage>`
+
+### Graduation Criteria
+
+<!--
+**Note:** *Not required until targeted at a release.*
+
+Define graduation milestones.
+
+These may be defined in terms of API maturity, [feature gate] graduations, or as
+something else. The KEP should keep this high-level with a focus on what
+signals will be looked at to determine graduation.
+
+Consider the following in developing the graduation criteria for this enhancement:
+- [Maturity levels (`alpha`, `beta`, `stable`)][maturity-levels]
+- [Feature gate][feature gate] lifecycle
+- [Deprecation policy][deprecation-policy]
+
+Clearly define what graduation means by either linking to the [API doc
+definition](https://kubernetes.io/docs/concepts/overview/kubernetes-api/#api-versioning)
+or by redefining what graduation means.
+
+In general we try to use the same stages (alpha, beta, GA), regardless of how the
+functionality is accessed.
+
+[feature gate]: https://git.k8s.io/community/contributors/devel/sig-architecture/feature-gates.md
+[maturity-levels]: https://git.k8s.io/community/contributors/devel/sig-architecture/api_changes.md#alpha-beta-and-stable-versions
+[deprecation-policy]: https://kubernetes.io/docs/reference/using-api/deprecation-policy/
+
+Below are some examples to consider, in addition to the aforementioned [maturity levels][maturity-levels].
+
+#### Alpha
+
+- Feature implemented behind a feature flag
+- Initial e2e tests completed and enabled
+
+#### Beta
+
+- Gather feedback from developers and surveys
+- Complete features A, B, C
+- Additional tests are in Testgrid and linked in KEP
+
+#### GA
+
+- N examples of real-world usage
+- N installs
+- More rigorous forms of testing—e.g., downgrade tests and scalability tests
+- Allowing time for feedback
+
+**Note:** Generally we also wait at least two releases between beta and
+GA/stable, because there's no opportunity for user feedback, or even bug reports,
+in back-to-back releases.
+
+**For non-optional features moving to GA, the graduation criteria must include
+[conformance tests].**
+
+[conformance tests]: https://git.k8s.io/community/contributors/devel/sig-architecture/conformance-tests.md
+
+#### Deprecation
+
+- Announce deprecation and support policy of the existing flag
+- Two versions passed since introducing the functionality that deprecates the flag (to address version skew)
+- Address feedback on usage/changed behavior, provided on GitHub issues
+- Deprecate the flag
+-->
+
+#### Alpha
+
+```
+<<[UNRESOLVED @seans3]>>
+TBD - currently not finished
+<<[/UNRESOLVED]>>
+```
+
+- `WebSocketExecutor` and `FallbackExecutor` completed and functional behind a `kubectl` feature flag.
+- `StreamTranslatorProxy` successfully integrated into the `UpgradeAwareProxy` behind an API Server feature flag.
+- Initial unit tests completed and enabled
+- Initial integration tests completed and enabled
+- Initial e2e tests completed and enabled
+- Additional SPDY integration and e2e tests for components other than `kubectl` to
+ensure this current functionality is not adversely affected.
+
+### Upgrade / Downgrade Strategy
+
+<!--
+If applicable, how will the component be upgraded and downgraded? Make sure
+this is in the test plan.
+
+Consider the following in developing an upgrade/downgrade strategy for this
+enhancement:
+- What changes (in invocations, configurations, API use, etc.) is an existing
+  cluster required to make on upgrade, in order to maintain previous behavior?
+- What changes (in invocations, configurations, API use, etc.) is an existing
+  cluster required to make on upgrade, in order to make use of the enhancement?
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+### Version Skew Strategy
+
+<!--
+If applicable, how will the component handle version skew with other
+components? What are the guarantees? Make sure this is in the test plan.
+
+Consider the following in developing a version skew strategy for this
+enhancement:
+- Does this enhancement involve coordinating behavior in the control plane and
+  in the kubelet? How does an n-2 kubelet without this feature available behave
+  when this feature is used?
+- Will any other components on the node change? For example, changes to CSI,
+  CRI or CNI may require updating that component before the kubelet.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+<!-- DISCUSS the FallbackExecutor and v5.channel.k8s.io to address version skew -->
+
+## Production Readiness Review Questionnaire
+
+<!--
+
+Production readiness reviews are intended to ensure that features merging into
+Kubernetes are observable, scalable and supportable; can be safely operated in
+production environments, and can be disabled or rolled back in the event they
+cause increased failures in production. See more in the PRR KEP at
+https://git.k8s.io/enhancements/keps/sig-architecture/1194-prod-readiness.
+
+The production readiness review questionnaire must be completed and approved
+for the KEP to move to `implementable` status and be included in the release.
+
+In some cases, the questions below should also have answers in `kep.yaml`. This
+is to enable automation to verify the presence of the review, and to reduce review
+burden and latency.
+
+The KEP must have a approver from the
+[`prod-readiness-approvers`](http://git.k8s.io/enhancements/OWNERS_ALIASES)
+team. Please reach out on the
+[#prod-readiness](https://kubernetes.slack.com/archives/CPNHUMN74) channel if
+you need any help or guidance.
+-->
+
+### Feature Enablement and Rollback
+
+<!--
+This section must be completed when targeting alpha to a release.
+-->
+
+###### How can this feature be enabled / disabled in a live cluster?
+
+<!--
+Pick one of these and delete the rest.
+
+Documentation is available on [feature gate lifecycle] and expectations, as
+well as the [existing list] of feature gates.
+
+[feature gate lifecycle]: https://git.k8s.io/community/contributors/devel/sig-architecture/feature-gates.md
+[existing list]: https://kubernetes.io/docs/reference/command-line-tools-reference/feature-gates/
+-->
+
+```
+<<[UNRESOLVED @seans3]>>
+TBD - currently not finished
+<<[/UNRESOLVED]>>
+```
+
+- [X] Feature gate (also fill in values in `kep.yaml`)
+  - Feature gate name: ClientRemoteCommandWebsockets
+  - Components depending on the feature gate: kubectl, API Server
+- [ ] Other
+  - Describe the mechanism: TODO
+  - Will enabling / disabling the feature require downtime of the control
+    plane? Yes. The API Server would have to be restarted to change the value of
+	the feature flag. `kubectl`, however, would only need an environment variable
+	to change the value of the feature flag, so it would not affect the control
+	plane.
+  - Will enabling / disabling the feature require downtime or reprovisioning
+    of a node? No.
+
+###### Does enabling the feature change any default behavior?
+
+<!--
+Any change of default behavior may be surprising to users or break existing
+automations, so be extremely careful here.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### Can the feature be disabled once it has been enabled (i.e. can we roll back the enablement)?
+
+<!--
+Describe the consequences on existing workloads (e.g., if this is a runtime
+feature, can it break the existing applications?).
+
+Feature gates are typically disabled by setting the flag to `false` and
+restarting the component. No other changes should be necessary to disable the
+feature.
+
+NOTE: Also set `disable-supported` to `true` or `false` in `kep.yaml`.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### What happens if we reenable the feature if it was previously rolled back?
+
+###### Are there any tests for feature enablement/disablement?
+
+<!--
+The e2e framework does not currently support enabling or disabling feature
+gates. However, unit tests in each component dealing with managing data, created
+with and without the feature, are necessary. At the very least, think about
+conversion tests if API types are being modified.
+
+Additionally, for features that are introducing a new API field, unit tests that
+are exercising the `switch` of feature gate itself (what happens if I disable a
+feature gate after having objects written with the new field) are also critical.
+You can take a look at one potential example of such test in:
+https://github.com/kubernetes/kubernetes/pull/97058/files#diff-7826f7adbc1996a05ab52e3f5f02429e94b68ce6bce0dc534d1be636154fded3R246-R282
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+### Rollout, Upgrade and Rollback Planning
+
+<!--
+This section must be completed when targeting beta to a release.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### How can a rollout or rollback fail? Can it impact already running workloads?
+
+<!--
+Try to be as paranoid as possible - e.g., what if some components will restart
+mid-rollout?
+
+Be sure to consider highly-available clusters, where, for example,
+feature flags will be enabled on some API servers and not others during the
+rollout. Similarly, consider large clusters and how enablement/disablement
+will rollout across nodes.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### What specific metrics should inform a rollback?
+
+<!--
+What signals should users be paying attention to when the feature is young
+that might indicate a serious problem?
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### Were upgrade and rollback tested? Was the upgrade->downgrade->upgrade path tested?
+
+<!--
+Describe manual testing that was done and the outcomes.
+Longer term, we may want to require automated upgrade/rollback tests, but we
+are missing a bunch of machinery and tooling and can't do that now.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### Is the rollout accompanied by any deprecations and/or removals of features, APIs, fields of API types, flags, etc.?
+
+<!--
+Even if applying deprecation policies, they may still surprise some users.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+### Monitoring Requirements
+
+<!--
+This section must be completed when targeting beta to a release.
+
+For GA, this section is required: approvers should be able to confirm the
+previous answers based on experience in the field.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### How can an operator determine if the feature is in use by workloads?
+
+<!--
+Ideally, this should be a metric. Operations against the Kubernetes API (e.g.,
+checking if there are objects with field X set) may be a last resort. Avoid
+logs or events for this purpose.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### How can someone using this feature know that it is working for their instance?
+
+<!--
+For instance, if this is a pod-related feature, it should be possible to determine if the feature is functioning properly
+for each individual pod.
+Pick one more of these and delete the rest.
+Please describe all items visible to end users below with sufficient detail so that they can verify correct enablement
+and operation of this feature.
+Recall that end users cannot usually observe component logs or access metrics.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+- [ ] Events
+  - Event Reason: 
+- [ ] API .status
+  - Condition name: 
+  - Other field: 
+- [ ] Other (treat as last resort)
+  - Details:
+
+###### What are the reasonable SLOs (Service Level Objectives) for the enhancement?
+
+<!--
+This is your opportunity to define what "normal" quality of service looks like
+for a feature.
+
+It's impossible to provide comprehensive guidance, but at the very
+high level (needs more precise definitions) those may be things like:
+  - per-day percentage of API calls finishing with 5XX errors <= 1%
+  - 99% percentile over day of absolute value from (job creation time minus expected
+    job creation time) for cron job <= 10%
+  - 99.9% of /health requests per day finish with 200 code
+
+These goals will help you determine what you need to measure (SLIs) in the next
+question.
+-->
+
+###### What are the SLIs (Service Level Indicators) an operator can use to determine the health of the service?
+
+<!--
+Pick one more of these and delete the rest.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+- [ ] Metrics
+  - Metric name:
+  - [Optional] Aggregation method:
+  - Components exposing the metric:
+- [ ] Other (treat as last resort)
+  - Details:
+
+###### Are there any missing metrics that would be useful to have to improve observability of this feature?
+
+<!--
+Describe the metrics themselves and the reasons why they weren't added (e.g., cost,
+implementation difficulties, etc.).
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+### Dependencies
+
+<!--
+This section must be completed when targeting beta to a release.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### Does this feature depend on any specific services running in the cluster?
+
+<!--
+Think about both cluster-level services (e.g. metrics-server) as well
+as node-level agents (e.g. specific version of CRI). Focus on external or
+optional services that are needed. For example, if this feature depends on
+a cloud provider API, or upon an external software-defined storage or network
+control plane.
+
+For each of these, fill in the following—thinking about running existing user workloads
+and creating new ones, as well as about cluster-level services (e.g. DNS):
+  - [Dependency name]
+    - Usage description:
+      - Impact of its outage on the feature:
+      - Impact of its degraded performance or high-error rates on the feature:
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+### Scalability
+
+<!--
+For alpha, this section is encouraged: reviewers should consider these questions
+and attempt to answer them.
+
+For beta, this section is required: reviewers must answer these questions.
+
+For GA, this section is required: approvers should be able to confirm the
+previous answers based on experience in the field.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### Will enabling / using this feature result in any new API calls?
+
+<!--
+Describe them, providing:
+  - API call type (e.g. PATCH pods)
+  - estimated throughput
+  - originating component(s) (e.g. Kubelet, Feature-X-controller)
+Focusing mostly on:
+  - components listing and/or watching resources they didn't before
+  - API calls that may be triggered by changes of some Kubernetes resources
+    (e.g. update of object X triggers new updates of object Y)
+  - periodic API calls to reconcile state (e.g. periodic fetching state,
+    heartbeats, leader election, etc.)
+-->
+
+The proposed design envisions a fallback mechanism when a new `kubectl` communicates
+with an older API Server. The client will initially request an upgrade to WebSockets,
+but it will fallback to the legacy SPDY if it is not supported. In this version
+skew scenario where the client implements the new functionality but the server does
+not, there is an extra request/response. Since bi-directional streaming already is
+very IO intensive, this extra request/response should not be significant. Additionally,
+as releases are incremented, the probability of the version skew will continually
+decrease.
+
+<!-- DISCUSS SPDY heartbeat versus WebSocket heartbeat; should be the same load -->
+
+###### Will enabling / using this feature result in introducing new API types?
+
+<!--
+Describe them, providing:
+  - API type
+  - Supported number of objects per cluster
+  - Supported number of objects per namespace (for namespace-scoped objects)
+-->
+
+No
+
+###### Will enabling / using this feature result in any new calls to the cloud provider?
+
+<!--
+Describe them, providing:
+  - Which API(s):
+  - Estimated increase:
+-->
+
+No
+
+###### Will enabling / using this feature result in increasing size or count of the existing API objects?
+
+<!--
+Describe them, providing:
+  - API type(s):
+  - Estimated increase in size: (e.g., new annotation of size 32B)
+  - Estimated amount of new objects: (e.g., new Object X for every existing Pod)
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently not finished
+<<[/UNRESOLVED]>>
+```
+
+<!-- DESCRIBE the data framing for WebSocket (which is less than) SPDY. -->
+
+###### Will enabling / using this feature result in increasing time taken by any operations covered by existing SLIs/SLOs?
+
+<!--
+Look at the [existing SLIs/SLOs].
+
+Think about adding additional work or introducing new steps in between
+(e.g. need to do X to start a container), etc. Please describe the details.
+
+[existing SLIs/SLOs]: https://git.k8s.io/community/sig-scalability/slos/slos.md#kubernetes-slisslos
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### Will enabling / using this feature result in non-negligible increase of resource usage (CPU, RAM, disk, IO, ...) in any components?
+
+<!--
+Things to keep in mind include: additional in-memory state, additional
+non-trivial computations, excessive access to disks (including increased log
+volume), significant amount of data sent and/or received over network, etc.
+This through this both in small and large cases, again with respect to the
+[supported limits].
+
+[supported limits]: https://git.k8s.io/community//sig-scalability/configs-and-limits/thresholds.md
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### Can enabling / using this feature result in resource exhaustion of some node resources (PIDs, sockets, inodes, etc.)?
+
+<!--
+Focus not just on happy cases, but primarily on more pathological cases
+(e.g. probes taking a minute instead of milliseconds, failed pods consuming resources, etc.).
+If any of the resources can be exhausted, how this is mitigated with the existing limits
+(e.g. pods per node) or new limits added by this KEP?
+
+Are there any tests that were run/should be run to understand performance characteristics better
+and validate the declared limits?
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+### Troubleshooting
+
+<!--
+This section must be completed when targeting beta to a release.
+
+For GA, this section is required: approvers should be able to confirm the
+previous answers based on experience in the field.
+
+The Troubleshooting section currently serves the `Playbook` role. We may consider
+splitting it into a dedicated `Playbook` document (potentially with some monitoring
+details). For now, we leave it here.
+-->
+
+###### How does this feature react if the API server and/or etcd is unavailable?
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### What are other known failure modes?
+
+<!--
+For each of them, fill in the following information by copying the below template:
+  - [Failure mode brief description]
+    - Detection: How can it be detected via metrics? Stated another way:
+      how can an operator troubleshoot without logging into a master or worker node?
+    - Mitigations: What can be done to stop the bleeding, especially for already
+      running user workloads?
+    - Diagnostics: What are the useful log messages and their required logging
+      levels that could help debug the issue?
+      Not required until feature graduated to beta.
+    - Testing: Are there any tests for failure mode? If not, describe why.
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+###### What steps should be taken if SLOs are not being met to determine the problem?
+
+## Implementation History
+
+<!--
+Major milestones in the lifecycle of a KEP should be tracked in this section.
+Major milestones might include:
+- the `Summary` and `Motivation` sections being merged, signaling SIG acceptance
+- the `Proposal` section being merged, signaling agreement on a proposed design
+- the date implementation started
+- the first Kubernetes release where an initial version of the KEP was available
+- the version of Kubernetes where the KEP graduated to general availability
+- when the KEP was retired or superseded
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+## Drawbacks
+
+<!--
+Why should this KEP _not_ be implemented?
+-->
+
+```
+<<[UNRESOLVED]>>
+TBD - currently unimplemented
+<<[/UNRESOLVED]>>
+```
+
+## Alternatives
+
+<!--
+What other approaches did you consider, and why did you rule them out? These do
+not need to be as detailed as the proposal, but should include enough
+information to express the idea and why it was not acceptable.
+-->
+
+```
+<<[UNRESOLVED @seans3]>>
+TODO - currently not complete
+<<[/UNRESOLVED]>>
+```
+
+<!-- @seans3 to flesh this out -->
+Ironically, HTTP/2.0 is based on SPDY. But the upgraded HTTP standard did not
+surface streaming functionality.
+
+Reasons why HTTP/2.0 is not adequate:
+
+1. HTTP/2.0 does not support upgrading connections for streaming.
+2. HTTP/2.0 does not surface streaming functionality.
+
+## Infrastructure Needed (Optional)
+
+<!--
+Use this section if you need things from the project/SIG. Examples include a
+new subproject, repos requested, or GitHub details. Listing these here allows a
+SIG to get the process for these resources started right away.
+-->
+
+N/A
diff --git a/keps/sig-api-machinery/4006-transition-spdy-to-websockets/kep.yaml b/keps/sig-api-machinery/4006-transition-spdy-to-websockets/kep.yaml
new file mode 100644
index 000000000000..aa4543f76850
--- /dev/null
+++ b/keps/sig-api-machinery/4006-transition-spdy-to-websockets/kep.yaml
@@ -0,0 +1,43 @@
+title: Transition from SPDY to Websockets
+kep-number: 4006
+authors:
+  - "@seans3"
+owning-sig: sig-api-machinery
+participating-sigs:
+  - sig-cli
+status: provisional
+# status other states:  |implementable|implemented|deferred|rejected|withdrawn|replaced
+creation-date: 2023-05-15
+reviewers:
+  - "@aojea"
+  - "@ardaguclu"
+approvers:
+  - "@deads2k"
+  - "@jpbetz"
+
+# The target maturity stage in the current dev cycle for this KEP.
+stage: alpha
+
+# The most recent milestone for which work toward delivery of this KEP has been
+# done. This can be the current (upcoming) milestone, if it is being actively
+# worked on.
+latest-milestone: "v1.28"
+
+# The milestone at which this feature was, or is targeted to be, at each stage.
+milestone:
+  alpha: "v1.28"
+  beta: "v1.29"
+  stable: "v1.30"
+
+# The following PRR answers are required at alpha release
+# List the feature gate name and the components for which it must be enabled
+feature-gates:
+  - name: ClientRemoteCommandWebsockets
+    components:
+      - kube-apiserver
+      - kubectl
+disable-supported: true
+
+# The following PRR answers are required at beta release
+metrics:
+  - websocket_fallback_count
diff --git a/keps/sig-api-machinery/4006-transition-spdy-to-websockets/remote-command-subprotocol.png b/keps/sig-api-machinery/4006-transition-spdy-to-websockets/remote-command-subprotocol.png
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