D. Crocker and P. Overell. <a href='http://www.ietf.org/rfc/rfc5234.txt'><cite>Augmented BNF for Syntax Specifications: ABNF.</cite></a> January 2008. Internet RFC 5234. URL: <a href='http://www.ietf.org/rfc/rfc5234.txt'>http://www.ietf.org/rfc/rfc5234.txt</a>