OCI has

Tag 2: the server asks the client whether it holds an image, by digest; the client answers with exactly one channel response, the byte 0, or the byte 1 followed by a repository path, and the channel response finish; a finish that no response precedes states that the client does not hold it.

The server opens this channel when it would obtain the image of the request from the client, before it asks for anything of the image.

• When. Between the request and the deploy.
• The request. The tag byte 2 followed by the digest as JSON. Request states its form.
• The response. Exactly one channel response, the byte 0, or the byte 1 followed by the repository path the client holds the image under, and the channel response finish. Response states the sequence.
• Whether to ask. The server asks only when it would obtain the image from the client. A server that obtains the image from a source of its own does not open this channel, and a client is not told where the image came from.