public function getLtiSession()
    {
        $session = common_session_SessionManager::getSession();
        if (!$session instanceof TaoLtiSession) {
            throw new LtiException(__FUNCTION__ . ' called on a non LTI session', LtiErrorMessage::ERROR_SYSTEM_ERROR);
        }
        $this->getServiceLocator()->propagate($session);

        return $session;
    }