RFC2119 Keywords

I am working in a standards group and a disagreement has
arisen concerning RFC2119.

Is the 2119 keyword simply "must" or "MUST" or "Must" or
"MuSt" ... , or is the weight of the 2119 definition
reserved strictly for "MUST" (all caps)?  I assume the
answer would equally apply to the entire list of 2119
keywords.

Regards,

David Fischer
Drummond Group.