public function getUnits()
    {
        $headers = $this->__getLastResponseHeaders();

        if (!empty($headers) && preg_match('@^Units: (\d+)/(\d+)/(\d+)@m', $headers, $m)) {
            return new Units($m[1], $m[2], $m[3]);
        }

        return new Units(-1, -1, -1);
    }