RFC 1521 does not include any concept equivalent to HTTP/1.0's Content-Encoding header field. Since this acts as a modifier on the media type, proxies and gateways from HTTP to MIME-compliant protocols must either change the value of the Content-Type header field or decode the Entity-Body before forwarding the message. (Some experimental applications of Content-Type for Internet mail have used a media-type parameter of ";conversions=<content-coding>" to perform an equivalent function as Content-Encoding. However, this parameter is not part of RFC 1521.)