protected function createCleanupTimer()
    {
        if (!isset($this->clientTtl)) {
            return;
        }
        $this->_clientsCleanupTimer = $this->loop->addPeriodicTimer(3, function($timer) {
            $expireTime = time() - $this->clientTtl;
            foreach ($this->_clients as $client) {
                if ($client->createdAt < $expireTime) {
                    $client->clientClose();
                }
            }
        });
    }