public function allowClientIncoming($clientName) {

        // clientName must be a non-zero length alphanumeric string
        if (preg_match('/\W/', $clientName)) {
            throw new \InvalidArgumentException(
                'Only alphanumeric characters allowed in client name.');
        }

        if (strlen($clientName) == 0) {
            throw new \InvalidArgumentException(
                'Client name must not be a zero length string.');
        }

        $this->clientName = $clientName;
        $this->allow('client', 'incoming',
            array('clientName' => $clientName));
    }