# Extensibility

Implementations that are reading/processing [manifests](manifest.md) or [image indexes](image-index.md) MUST NOT generate an error if they encounter an unknown property.
Instead they MUST ignore unknown properties.

# Canonicalization

OCI Images [are](descriptor.md) [content-addressable](image-layout.md).
One benefit of content-addressable storage is easy deduplication.
Many images might depend on a particular [layer](layer.md), but there will only be one blob in the [store](image-layout.md).
With a different serialization, that same semantic layer would have a different hash, and if both versions of the layer are referenced there will be two blobs with the same semantic content.
To allow efficient storage, implementations serializing content for blobs SHOULD use a canonical serialization.
This increases the chance that different implementations can push the same semantic content to the store without creating redundant blobs.

## JSON

[JSON][] content SHOULD be serialized as [canonical JSON][canonical-json].
Of the [OCI Image Format Specification media types](media-types.md), all the types ending in `+json` contain JSON content.
Implementations:

* [Go][]: [github.com/docker/go][], which claims to implement [canonical JSON][canonical-json] except for Unicode normalization.

[canonical-json]: http://wiki.laptop.org/go/Canonical_JSON
[github.com/docker/go]: https://github.com/docker/go/
[Go]: https://golang.org/
[JSON]: http://json.org/
