code: def client_id_is_known(provider, authentication_request):
      
      if authentication_request['client_id'] not in provider.clients:
          logger.error('Unknown client_id \'{}\''.format(authentication_request['client_id']))
          raise InvalidAuthenticationRequest('Unknown client_id',
                                             authentication_request,
                                             oauth_error='unauthorized_client')
docs: Verifies the client identifier is known.
      :param provider: provider instance
      :param authentication_request: the authentication request to verify
      :raise InvalidAuthenticationRequest: if the client_id is unknown