The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "may",
and "OPTIONAL" in this document are to be interpreted as
described in RFC 2119 (see http://www.ietf.org/rfc/rfc2119.txt).
However, for readability,
these words do not appear in all uppercase letters in this
specification.