def _do_logoff(self):
        """
        Log off, unconditionally.

        Raises:

          :exc:`~zhmcclient.ServerAuthError`
          :exc:`~zhmcclient.ConnectionError`
          :exc:`~zhmcclient.ParseError`
          :exc:`~zhmcclient.HTTPError`
        """
        session_uri = '/api/sessions/this-session'
        self.delete(session_uri, logon_required=False)
        self._session_id = None
        self._session = None
        self._headers.pop('X-API-Session', None)