http://www.rfc-editor.org/rfc/rfc8246.txt
RFC 8246: HTTP Immutable Responses, P. McManus2017-09-01T23:00:00-00:00The immutable HTTP response Cache-Control extension allows servers to identify resources that will not be updated during their freshness lifetime. This ensures that a client never needs to revalidate a cached fresh resource to be certain it has not been modified.