private static function fetchProtocolVersion(string $version): string
    {
        $v = substr($version, 5);

        if ($v === '2.0') {
            return '2';
        }

        // Fallback for values outside of valid protocol versions
        if (!in_array($v, static::$allowedVersions, true)) {
            return '1.1';
        }

        return $v;
    }