Avoid XMLHttpRequest caching

var url = "../index.html?randomKey=" + Math.random() * Date.parse(new Date());

The task is to poll server periodly for availability with a scheduled XMLHttpRequest (using JS's setInterval("heartbeat()", 15*1000); However, in IE the response is cached even though those meta tags such as 'cache-control', 'expired' have been set to no-cache. And I have tried to set these in request header and not helpful either. The ajax calls are still cached and always return status 200 even though server is down and 404 should be returned. The IE caching is based on url, therefore, append a random key will resolve this issue.