@abc.abstractmethod<EOL><INDENT>def get_data_length(self):<DEDENT>
return<EOL>
Gets the length of the data represented by this content in bytes.

        :return: the length of the data stream
        :rtype: ``cardinal``
        :raise: ``IllegalState`` -- ``has_data_length()`` is ``false``

        *compliance: mandatory -- This method must be implemented.*