<!ELEMENT protocol (class+)>
<!ATTLIST protocol>

<!ELEMENT class (message+)>
<!ATTLIST class
  name CDATA #REQUIRED
>

<!ELEMENT message (field*)>
<!ATTLIST message
  name CDATA #REQUIRED
  id   CDATA #REQUIRED
  link CDATA #IMPLIED
>

<!ELEMENT field EMPTY>
<!ATTLIST field
  name CDATA #REQUIRED
  type CDATA #REQUIRED
  format CDATA #IMPLIED
  unit CDATA #IMPLIED
  values CDATA #IMPLIED
  alt_unit CDATA #IMPLIED
  alt_unit_coef CDATA #IMPLIED
>