@classmethod
def default_config(cls):
    """Returns a default configuration object of appropriate type.

        :returns: an instantiated configuration object
        :rtype:   :class:`VUrlConfig`

        Protocols require different configuration object
        structure. This method allows instantiating an object and
        manipulating e.g. link-level configuration objects which are
        common to all configuration classes.

        """
    raise NotImplementedError()
