def METHOD_NAME(self):
    """ Establishes the actual connection to the referred RSE.
        :param credentials: Provide all necessary information to establish a connection
            to the referred storage system. Some is loaded from the repository inside the
            RSE class and some must be provided specific for the SFTP protocol like
            username, password, private_key, private_key_pass, port.
            For details about possible additional parameters and details about their usage
            see the pysftp.Connection() documentation.
            NOTE: the host parametrer is overwritten with the value provided by the repository
        :raise RSEAccessDenied
    """
    raise NotImplementedError