The spec talks about Resumable Push but it is not clear how this should be performed. Docker API v2 spec defines that when sending a chunk in a session, if the range does not match, then the response should be of the following form

416 Requested Range Not Satisfiable
Location: /v2/<name>/blobs/uploads/<uuid>
Range: 0-<last valid range>
Content-Length: 0
Docker-Upload-UUID: <uuid>

the key here being that there is Range: 0-<last valid range> that would tell the client about the last position written.