For clarity purposes the words like MUST should be defined by the spec. For example:

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.. link: http://tools.ietf.org/html/rfc2119