Clase base para manejar la DTD.
A continuacion podemos ver un estracto de la DTD que definirias las reglas de formacion del documento XML mostrado como ejemplo en Parser.
<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT xvc (broadcast, ethernet, real, unreal)>
<!ATTLIST xvc HeartBeat CDATA "2000">
<!-- broadcast -->
<!ELEMENT broadcast (INetAddress+)>
<!ELEMENT INetAddress EMPTY>
<!ATTLIST INetAddress Address CDATA #REQUIRED>
<!ATTLIST INetAddress Port CDATA #REQUIRED>
<!-- ethernet -->
<!ELEMENT ethernet (Input, Output)>
<!ATTLIST ethernet Mode (raw|ethernet) #REQUIRED>
<!ATTLIST ethernet VirtualAddress CDATA #REQUIRED>
<!ELEMENT Input EMPTY>
<!ATTLIST Input Device CDATA #REQUIRED>
<!ATTLIST Input PhysicalAccessPoint CDATA #REQUIRED>
<!ATTLIST Input MAC CDATA #REQUIRED>
<!ELEMENT Output EMPTY>
<!ATTLIST Output Device CDATA #REQUIRED>
<!ATTLIST Output PhysicalAccessPoint CDATA #REQUIRED>
<!ATTLIST Output MAC CDATA #REQUIRED>