2008-11-17 ■ RFC 標準化 RFC 2119 “Key words for use in RFCs to Indicate Requirement Levels” "MUST" "MUST NOT" "REQUIRED" "SHALL" "SHALL NOT" "SHOULD" "SHOULD NOT" "RECOMMENDED" "MAY" "OPTIONAL" 和訳→http://www.cam.hi-ho.ne.jp/mendoxi/rfc/rfc2119j.html