AJAX request and IfModified help jQuery

I have to request many xml files from different sources, then, I have to check if there is an update from a feed. This is what I tried to do:

function requeteRSS() {
setTimeout(requeteRSS, 30000)
function RSS(url, sujet)
    $.ajax(url, {
            method: "GET",
            type: 'xml',
            success: function(content){
                useContent(content, sujet)

Right now, my function load every 30 seconds all the content from the xml file, even if the last modified date is still the same as when it was loaded. Also, it loads all the element, not just the new one. How can I make it so that it only works when the last modified date changes, and that it loads only the new element?

Thanks for your help.