G. Klyne; L. McIntyre. <a href="http://www.ietf.org/rfc/rfc2531.txt"><cite>Content Feature Schema for Internet Fax.</cite></a> March 1999. Internet RFC 2531. URL: <a href="http://www.ietf.org/rfc/rfc2531.txt">http://www.ietf.org/rfc/rfc2531.txt</a>