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identity identity: add implementation of ChainID Jan 11, 2017
img clarification about nested index Nov 25, 2017
schema zstd: add constants to specs-go/v1 Aug 23, 2019
specs-go zstd: add constants to specs-go/v1 Aug 23, 2019
.gitignore docs: image base reference Mar 8, 2017
.header *: add license header checking and tweak Makefile May 19, 2016
.pullapprove.yml .pullapprove.yml: Switch to v2 and other project-template updates Mar 20, 2017
.travis.yml fix typo Mar 30, 2017 replace file system with filesystem to keep consistent May 12, 2017
Makefile Makefile: variable cleanup and creation Dec 18, 2018 return to one-sentence-per-line format Aug 21, 2019 Add governance and release process Jul 22, 2016 Fix a small typo Mar 21, 2019 conversion: add platform conversions Jul 16, 2017 descriptor: fix link Jan 15, 2018 Unknown descriptor mediatypes MUST be ignored Feb 19, 2019 clarification about nested index Nov 25, 2017 media-types: Define layer media types suffix '+zstd' Aug 21, 2019 media-types: Define layer media types suffix '+zstd' Aug 21, 2019 *: add common files from runtime-spec Apr 4, 2016

OCI Image Format Specification

The OCI Image Format project creates and maintains the software shipping container image format spec (OCI Image Format).

The specification can be found here.

This repository also provides Go types, intra-blob validation tooling, and JSON Schema. The Go types and validation should be compatible with the current Go release; earlier Go releases are not supported.

Additional documentation about how this group operates:

The optional and base layers of all OCI projects are tracked in the OCI Scope Table.

Running an OCI Image

The OCI Image Format partner project is the OCI Runtime Spec project. The Runtime Specification outlines how to run a "filesystem bundle" that is unpacked on disk. At a high-level an OCI implementation would download an OCI Image then unpack that image into an OCI Runtime filesystem bundle. At this point the OCI Runtime Bundle would be run by an OCI Runtime.

This entire workflow supports the UX that users have come to expect from container engines like Docker and rkt: primarily, the ability to run an image with no additional arguments:

  • docker run
  • rkt run,version=v1.0.0

To support this UX the OCI Image Format contains sufficient information to launch the application on the target platform (e.g. command, arguments, environment variables, etc).


Q: Why doesn't this project mention distribution?

A: Distribution, for example using HTTP as both Docker v2.2 and AppC do today, is currently out of scope on the OCI Scope Table. There has been some discussion on the TOB mailing list to make distribution an optional layer, but this topic is a work in progress.

Q: What happens to AppC or Docker Image Formats?

A: Existing formats can continue to be a proving ground for technologies, as needed. The OCI Image Format project strives to provide a dependable open specification that can be shared between different tools and be evolved for years or decades of compatibility; as the deb and rpm format have.

Find more FAQ on the OCI site.


The GitHub milestones lay out the path to the future improvements.


Development happens on GitHub for the spec. Issues are used for bugs and actionable items and longer discussions can happen on the mailing list.

The specification and code is licensed under the Apache 2.0 license found in the LICENSE file of this repository.

Discuss your design

The project welcomes submissions, but please let everyone know what you are working on.

Before undertaking a nontrivial change to this specification, send mail to the mailing list to discuss what you plan to do. This gives everyone a chance to validate the design, helps prevent duplication of effort, and ensures that the idea fits. It also guarantees that the design is sound before code is written; a GitHub pull-request is not the place for high-level discussions.

Typos and grammatical errors can go straight to a pull-request. When in doubt, start on the mailing-list.


Please see the OCI org repository README for the most up-to-date information on OCI contributor and maintainer meeting schedules. You can also find links to meeting agendas and minutes for all prior meetings.

Mailing List

You can subscribe and join the mailing list on Google Groups.


OCI discussion happens on #opencontainers on Freenode (logs).

Markdown style

To keep consistency throughout the Markdown files in the Open Container spec all files should be formatted one sentence per line. This fixes two things: it makes diffing easier with git and it resolves fights about line wrapping length. For example, this paragraph will span three lines in the Markdown source.

Git commit

Sign your work

The sign-off is a simple line at the end of the explanation for the patch, which certifies that you wrote it or otherwise have the right to pass it on as an open-source patch. The rules are pretty simple: if you can certify the below (from

Developer Certificate of Origin
Version 1.1

Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
660 York Street, Suite 102,
San Francisco, CA 94110 USA

Everyone is permitted to copy and distribute verbatim copies of this
license document, but changing it is not allowed.

Developer's Certificate of Origin 1.1

By making a contribution to this project, I certify that:

(a) The contribution was created in whole or in part by me and I
    have the right to submit it under the open source license
    indicated in the file; or

(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
    license and I have the right under that license to submit that
    work with modifications, whether created in whole or in part
    by me, under the same open source license (unless I am
    permitted to submit under a different license), as indicated
    in the file; or

(c) The contribution was provided directly to me by some other
    person who certified (a), (b) or (c) and I have not modified

(d) I understand and agree that this project and the contribution
    are public and that a record of the contribution (including all
    personal information I submit with it, including my sign-off) is
    maintained indefinitely and may be redistributed consistent with
    this project or the open source license(s) involved.

then you just add a line to every git commit message:

Signed-off-by: Joe Smith <>

using your real name (sorry, no pseudonyms or anonymous contributions.)

You can add the sign off when creating the git commit via git commit -s.

Commit Style

Simple house-keeping for clean git history. Read more on How to Write a Git Commit Message or the Discussion section of git-commit(1).

  1. Separate the subject from body with a blank line
  2. Limit the subject line to 50 characters
  3. Capitalize the subject line
  4. Do not end the subject line with a period
  5. Use the imperative mood in the subject line
  6. Wrap the body at 72 characters
  7. Use the body to explain what and why vs. how
  • If there was important/useful/essential conversation or information, copy or include a reference
  1. When possible, one keyword to scope the change in the subject (i.e. "README: ...", "runtime: ...")
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