RFC2119
https://tools.ietf.org/html/rfc2119
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.