如何检查另一台服务器是否在javascript中？

In my webapp I need to check if another server (on a different ip address) is up. I know I can't use direct ajax requests for this because of cross domain restrictions. I cannot use JSONP because I don't have any control over the other server. I also need to repeatedly check for the server being up as it may be unavailable some of the time. All these don't make it easy.In my webapp I need to check if another server