R. Moats. <a href="http://www.ietf.org/rfc/rfc2141.txt"><cite>URN Syntax.</cite></a> IETF RFC 2141. May 1997.  URL: <a href="http://www.ietf.org/rfc/rfc2141.txt">http://www.ietf.org/rfc/rfc2141.txt</a>