What happens when there is network request time out during AJAX request to ASP.NET MVC Action

Regardless of whether the timeout happens only for a couple of ms or more, the request will fail. The success callback function of your AJAX request will not be executed and the request will end with the execution of complete callback function. By default, all AJAX requests will have a timeout of 0ms (unlimited), but it will hit the default timeout of the browser.

When an AJAX request times out, the error callback function will be invoked. The second argument of this function is a string describing the type of error and in this case, it will have the value timeout. You can handle request timeouts by handling this callback function and by optionally specifying a timeout value (if not specified, works on the default value) in the AJAX request body:

$.ajax({    ...    timeout: 5000, //specify the timeout value in milliseconds    error: function(jqXHR, textStatus, errorThrown) {        if(textStatus==="timeout") {           //code to execute when timeout occurs        }     }});​

Additionally, you can also check if the request has timed out in the complete callback function (in a similar way as shown above) by checking the second argument which is a string and it will have the value timeout if the request was timed out.

Also, note this:

The timeout period starts at the point the $.ajax call is made; if several other requests are in progress and the browser has no connections available, it is possible for a request to time out before it can be sent.

Request timeouts are usually either left at their default or set as a global default using $.ajaxSetup() rather than being overridden for specific requests with the timeout option.

I would suggest you to use an alternative HTTP/s traffic monitoring tool like fiddler to find the mystery behind the second request.

More info: jQuery ajax documentation

The request will "fail", meaning it will enter the onError state of your AJAX request. The status code will then be 0, since there is no response from the server to determine the real status code (eg. 200 OK or 500 Internal Server Error).

In case of time-out your success callback wont execute so you have to write an error callback at the client side to handle such issues.

You have to raise an exception from server side in case of time-out so that it will get back to the client as an error that is the one way you can handle the time-out.