Add a CRI doc for developers

docs/devel/container-runtime-interface.md

+# CRI: the Container Runtime Interface
+
+## What is CRI?
+
+CRI (_Container Runtime Interface_) consists of a
+[protobuf API](../../pkg/kubelet/api/v1alpha1/runtime/api.proto),
+specifications/requirements (to-be-added),
+and [libraries] (https://github.com/kubernetes/kubernetes/tree/master/pkg/kubelet/server/streaming)
+for container runtimes to integrate with kubelet on a node. CRI is currently in Alpha.
+
+In the future, we plan to add more developer tools such as the CRI validation
+tests.
+
+
+## Why develop CRI?
+
+Prior to the existence of CRI, container runtimes (e.g., `docker`, `rkt`) were
+integrated with kubelet through implementing an internal, high-level interface
+in kubelet. The entrance barrier for runtimes was high because the integration
+required understanding the internals of kubelet and contributing to the main
+Kubernetes repository. More importantly, this would not scale because every new
+addition incurs a significant maintenance overhead in the main kubernetes
+repository.
+
+Kubernetes aims to be extensible. CRI is one small, yet important step to enable
+pluggable container runtimes and build a healthier ecosystem.
+
+## How to use CRI?
+
+1. Start the image and runtime services on your node. You can have a single
+   service acting as both image and runtime services.
+2. Set the kubelet flags
+   - Pass the unix socket(s) to which your services listen to kubelet:
+     `--container-runtime-endpoint` and `--image-service-endpoint`.
+   - Enable CRI in kubelet by`--experimental-cri=true`).
+   - Use the "remote" runtime by `--container-runtime=remote`.
+
+Please see the [Status Update](#status-update) section for known issues for
+each release.
+
+Note that CRI is still in its early stages. We are actively incorporating
+feedback from early developers to improve the API. Developers should expect
+occasional API breaking changes.
+
+## Does Kubelet use CRI today?
+
+No, but we are working on it.
+
+The first step is to switch kubelet to integrate with Docker via CRI by
+default. The current [Docker CRI implementation](https://github.com/kubernetes/kubernetes/blob/release-1.5/pkg/kubelet/dockershim)
+already passes most end-to-end tests, and has mandatory PR builders to prevent
+regressions. While we are expanding the test coverage gradually, it is
+difficult to test on all combinations of OS distributions, platforms, and
+plugins. There are also many experimental or even undocumented features relied
+upon by some users. We would like to **encourage the community to help test
+this Docker-CRI integration and report bugs and/or missing features** to
+smooth the transition in the near future. Please file a Github issue and
+include @kubernetes/sig-node for any CRI problem.
+
+### How to test the new Docker CRI integration?
+
+Start kubelet with the following flags:
+  - Use the Docker container runtime by `--container-runtime=docker`(the default).
+  - Enable CRI in kubelet by`--experimental-cri=true`.
+
+Please also see the [known issues](#docker-cri-1.5-known-issues) before trying
+out.
+
+
+## Design docs and proposals
+
+We plan to add CRI specifications/requirements in the near future. For now,
+these proposals and design docs are the best sources to understand CRI
+besides discussions on Github issues.
+
+  - [Original proposal](https://github.com/kubernetes/kubernetes/blob/release-1.5/docs/proposals/container-runtime-interface-v1.md)
+  - [Exec/attach/port-forward streaming requests](https://docs.google.com/document/d/1OE_QoInPlVCK9rMAx9aybRmgFiVjHpJCHI9LrfdNM_s/edit?usp=sharing)
+  - [Container stdout/stderr logs](https://github.com/kubernetes/kubernetes/blob/release-1.5/docs/proposals/kubelet-cri-logging.md)
+  - Networking: The CRI runtime handles network plugins and the
+    setup/teardown of the pod sandbox.
+
+
+## Work-In-Progress CRI runtimes
+
+ - [cri-o](https://github.com/kubernetes-incubator/cri-o)
+ - [rktlet](https://github.com/kubernetes-incubator/rktlet)
+ - [frakti](https://github.com/kubernetes/frakti)
+
+
+## [Status update](#status-update)
+
+### Kubernetes v1.5 release (CRI v1alpha1)
+
+  - [v1alpha1 version](https://github.com/kubernetes/kubernetes/blob/release-1.5/pkg/kubelet/api/v1alpha1/runtime/api.proto) of CRI is released.
+
+
+#### [CRI known issues](#cri-1.5-known-issues):
+
+  - Container metrics are not defined yet in CRI ([#27097](https://github.com/kubernetes/kubernetes/issues/27097)).
+  - CRI may not be compatible with other experimental features (e.g., Seccomp)
+  - Streaming server needs to be further productionized:
+     - Authentication: [#36666](https://github.com/kubernetes/kubernetes/issues/36666)
+     - Avoid including user data in the redirect URL: [#36187](https://github.com/kubernetes/kubernetes/issues/36187)
+
+
+#### [Docker CRI integration known issues](#docker-cri-1.5-known-issues)
+
+  - Docker compatibility: Support only Docker v1.11 and v1.12.
+  - Network: Does not support host port and bandwidth shaping
+    [#35457](https://github.com/kubernetes/kubernetes/issues/35457)
+  - Exec/attach/port-forward (streaming requests): Does not support `nsenter`
+    as the exec handler (`--exec-handler=nsenter`). Also see
+    (#cri-1.5-known-issues) for limitations on CRI streaming.
+
+## Contacts
+
+  - Email: sig-node (kubernetes-sig-node@googlegroups.com)
+  - Slack: https://kubernetes.slack.com/messages/sig-node
+
+
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