How to determine latency of a remote server through the browser

Most applet technology, including Javascript, enforces a same-origin policy. It may be possible to dynamically add DOM elements, such as images, and collect timing information using the onload event handler.

Psuedo-code

for (server in servers) {
  var img = document.createElement('IMG');
  server.startTime = getCurrentTimeInMS();
  img.onload=function() { server.endTime = getcurrentTimeInMS(); }
  img.src = server.imgUrl;
}

Then wait an appropriate time and check the timing for each server object. Repeat as needed and compute averages if you want. I'm not sure what kind of accuracy you can expect.

Disadvantages:

• You are probably using the wrong tool for the job. A browser is not equipped for this sort of application.
• It's probably quite inaccurate.
• If the resource you request is cached it won't give you the results you want, but you can work around that by changing the url each time.
• This is bandwidth-intensive compared to a normal ping. Make the image tiny, such as a spacer.gif file.
• The timing depends not only on the latency of the remote server but the bandwidth of that server. This may be a more or less useful measure but it's important to note that it is not simply the latency.
• You need to be able to serve HTTP requests from the various servers and, crucially, each server should serve the exact same resource (or a resource of the same length). Conditions on the server can affect the response time, such as if one server is compressing the data and another isn't.

All you really need is the time from the connection start, to the time of the first readystate change...

function getPing() {
  var start;
  var client = getClient(); // xmlhttprequest object
  client.onreadystatechange = function() {
    if (client.readyState > 0) {
      pingDone(start); //handle ping
      client.onreadystatechange = null; //remove handler
    } 
  }

  start = new Date();
  client.open("HEAD", "/ping.txt"); //static file
  client.send();
}

function pingDone(start) {
  done = new Date();
  ms = done.valueOf() - start.valueOf();
  alert(ms + "ms ping time");
}

function getClient() {
  if (window.XMLHttpRequest)
    return new XMLHttpRequest();

  if (window.ActiveXObject)
    return new ActiveXObject('MSXML2.XMLHTTP.3.0');

  throw("No XMLHttpRequest Object Available.");
}

Before the call to the server, record the Javascript time:

var startTime = new Date();

Load an image from the server:

var img = new Image()
img.onload = function() {
    // record end time
}
img.src = "http://server1.domain.com/ping.jpg";

As soon as the request is finished, record the time again. (Given of course that the request didn't time out.)

var endTime = new Date();

Your ping in milliseconds is:

var ping = endTime. getTime() - startTime.getTime();