distribution: Add in-scope and out-of-scope wording

Docker's use of Bearer requires information beyond what's covered in
RFC 6749 and 6750 [1].  Folks writing a client that will interact with
a Docker registry that uses that auth approach will need a "Docker
registry's 'Bearer' additions" spec to follow, but Derek believes the
situation is salvageable with external work [2].

Also pin the docker/registry links to a specific version so the links
will survive future docker/registry changes (including removing the
docs after the OCI picks them up).  As long as the TOB-selected
version isn't far behind (how far will the spec move during a week of
voting?), it should be easy for the new maintainets to catch up on any
subsequent drift.

The signing scope is from Stephen in [3].  The discovery scope is from
Derek [4].  The content-management scope is from Stephen [5] and Liang
[6].

This commit also add's Mike's interoperability statement [7], which
mentions one reason for the OCI inclusion is the
implementation-agnostic location where implementers can collaborating
on a common specification.  The project scoping is open to drift with
the limits imposed by the TOB's scope table.  If the TOB thinks
subsequent drift is excessive, they are free to make further
scope-table adjustments in follow-up proposals.

I've also added Mike's library/busybox scoping example [8].

[1]: xiekeyang/oci-discovery#64 (comment)
[2]: opencontainers#37 (comment)
[3]: opencontainers#35 (comment)
[4]: opencontainers#34 (comment)
[5]: opencontainers#35 (comment)
[6]: opencontainers#37 (comment)
[7]: opencontainers#35 (comment)
[8]: opencontainers#37 (comment)

Signed-off-by: W. Trevor King <wking@tremily.us>

proposals/distribution.md

@@ -1,16 +1,17 @@
 # Abstract
 
-The Docker registry protocol has become the defacto standard across the container registry world ([https://github.com/docker/distribution/blob/master/docs/spec/api.md](https://github.com/docker/distribution/blob/master/docs/spec/api.md)).
+The Docker registry protocol has become the defacto standard across the container registry world.
 
 In the OCI, having a solid, common distribution specification with conformance testing will ensure long lasting security and interoperability throughout the container ecosystem.
 
 ## Proposal
 
-TL;DR; Move [https://github.com/docker/distribution/tree/master/docs/spec](https://github.com/docker/distribution/tree/master/docs/spec) to [https://github.com/opencontainers/distribution-spec](https://github.com/opencontainers/distribution-spec)
+TL;DR; Move [`api.md`][api.md] to a new [distribution-spec project](https://github.com/opencontainers/distribution-spec).
 
 This proposal covers the distribution API spec, and while it does not cover the code for the docker-registry, that implementation is considered the reference implementation. There are other implementations of this protocol, not all are open-source though (Google gcr.io, Amazon ECR, CoreOS Quay, Gitlab registry, JFrog Artifactory registry, Huawei Dockyard, etc).
 
 In the past when the topic of having an OCI specification around the distribution of container images was discussed, it was deferred as "let’s get the image format defined, meanwhile the industry will settle on a distribution standard". Fast forward, OCI image format is out and adopted, and the Registry v2 is the defacto standard. There is and will be use-cases for alternate methods and the future will likely hold creative ways to push, fetch and share container images, but right now this promotion serves to acknowledge by the OCI the current industry standard of distributing container images.
+This proposal also provides the container ecosystem with a means to discuss and schedule extensions to the distribution specification.
 
 There is polish that is needed e.g. broken links to storage-driver docs, as well as making sections more generic regarding the OCI descriptors and media-types, but on the whole this is a lateral move.
 
@@ -57,10 +58,92 @@ The API spec is currently considered v2 and we will start the specification at v
 
 ## Frequently Asked Questions (FAQ)
 
-* Does this include the code of the docker-registry?
-    * No. This is an API specification discussion.
+**Q: Does this include the code of the docker-registry?**
+
+A: No. This is an API specification discussion.
+
+**Q: Does this change the OCI Charter or Scope Table?**
+
+A: Not the charter, but it does change the scope table.
+This project is scoped to specifying per-image client ↔ registry interaction with an [HTTP][rfc7230]-based protocol.
+The following scope entries should be removed from the [scope table][scope]:
+
+* “Use of Hash as Content Addressable name for immutable containers”.
+    This entry is in scope for this project, and a more detailed entry will be added as described below.
+* “Creating Reference spec for optional DNS based naming & distribution”.
+    This entry conflates naming and distribution, which will be separated by this proposal.
+* “Standardizing on a particular Distribution method”.
+    This proposal will provide one (of possibly many) distribution specifications, so the old “There is no current agreement on how to distribute content” no longer applies.
+
+The following entries should be added to the [scope table][scope]:
+
+* “Specifying authentication and authorization schemes”.
+    Docker's current registry uses an [extension][token] of the [`Bearer`][rfc6750] [auth scheme][rfc7235-s2.1].
+    Work on specifying Docker's scheme will continue independently, and is orthogonal to the registry API.
+
+    * What: Specifying authentication and authorization schemes
+    * In/Out/Future: Out of scope
+    * Status: N/A
+    * Description: Defining protocols for authenticating and authorizing distribution access.
+    * Why: As a HTTP-based protocol, clients and servers can negotiate authentication via HTTP's [challenge-response authentication framework][rfc7235-s2.1].
+        There is no need for the distribution specification to choose a particular authentication scheme, because clients receiving 401 and 407 responses can use IANA's [HTTP Authentication Scheme Registry][iana-auth] to look up referenced schemes and take appropriate action.
+        It is reasonable to provide a standardized way to use DNS based distribution in conjunction with OCI without requiring its use.
+
+* “Creating a reference spec for optional DNS based naming and discovery”.
+    Discovery and registry protocols are completely separate and do not need to be added together.
+    This entry replaces part of the previous “Creating Reference spec for optional DNS based naming & distribution” entry.
+
+    * What: Creating a reference spec for optional DNS based naming and discovery
+    * In/Out/Future: In scope for future specification
+    * Status: Not currently being worked
+    * Description: Define a protocol for resolving an image name to retrieval information.
+        When we address this, we will also allow for alternative, parallel name-to-image discovery protocols in parallel with the OCI-specified protocol.
+    * Why: It is reasonable to provide a standardized way to use DNS based distribution in conjunction with OCI without requiring its use.
+        There are many good use cases for DNS based distribution, but not all use cases support this.
+        Furthermore, encoding the location of a bundle into the bundle can cause issues with downloads from alternate locations other than the origin specified in the name.
+
+* “Specifying a distribution method”.
+    This entry replaces part of the previous “Creating Reference spec for optional DNS based naming & distribution” and “Standardizing on a particular Distribution method” entries.
+
+    Retrieving images covers the current “tag listing” (e.g. “what named manifests are in `library/busybox`?”), because tags are entries in the image format's [`manifests` array][manifests].
+    Other tag-listing endpoints needed for backwards-compatibility are therefore in scope as well.
+
+    Repository actions and listing images within a repository are considered part of distribution policy or content management and are out of scope for this entry's per-image action.
+    For example, “what images are under `library/`?” is out of scope for this project.
+
+    * What: Specifying a distribution method
+    * In/Out/Future: In scope
+    * Status: In progress (see opencontainers/distribution-spec)
+    * Description: Define a protocol for image, manifest, config, and blob creation, retrieval, and deletion.
+        Listing, creating, and deleting repositories and listing images within repositories are both out of scope for this entry.
+    * Why: This specification will provide one (of possibly many) distribution specifications.
+        Alternative distribution specifications may be developed for uses cases not covered by this specification, but defining them is out of scope for the OCI.
+
+* “Retrieving images by their content-addressable hash”.
+    Docker's registery API does not currently provide endpoints for fetching image or manifest objects by digest, but this is the project where that will happen.
+
+    * What: Retrieving images by their content-addressable hash
+    * In/Out/Future: In scope
+    * Status: In progress (see opencontainers/distribution-spec)
+    * Description: Specify a protocol for retrieving an image from a distribution engine by the image's content-addressable hash.
+    * Why: Using a hash as a name is a way to ensure a unique and container name without relying on a particular naming authority/or system.
+        Using hashing for name is an acceptable addition as it does not encode any centralized namespace.
+
+The following entries should remain in the [scope table][scope] but not be addressed by this project:
+
+* “Specifying way to attach signatures”.
+    We don't need to address this as part of distribution, because all resources are content-addressable and can be signed in external systems.
 
 ## Related GitHub Issues
 
 * Simplifies tag listing: docker/distribution#2169
 * Allows listing of manifests: docker/distribution#2199
+
+[api.md]: https://github.com/docker/distribution/blob/cb406d511b7b9163bff9b6439072e4892e5ae3b/docs/spec/api.md
+[iana-auth]: http://www.iana.org/assignments/http-authschemes/http-authschemes.xhtml
+[manifests]: https://github.com/opencontainers/image-spec/blame/v1.0.1/image-index.md#L23
+[rfc6750]: https://tools.ietf.org/html/rfc6750
+[rfc7230]: https://tools.ietf.org/html/rfc7230
+[rfc7235-s2.1]: https://tools.ietf.org/html/rfc7235#section-2.1
+[scope]: https://www.opencontainers.org/about/oci-scope-table
+[token]: https://github.com/docker/distribution/blob/5cb406d511b7b9163bff9b6439072e4892e5ae3b/docs/spec/auth/token.md