function keepalive(){
    var img = document.getElementById("pinger_img");
    if(img == null){
        img = document.createElement("img");
        img.id ="pinger_img";
        img.style.visibility = "hidden";
        document.body.appendChild(img);
    }

    img.src = "/ajax/service/ping?t="+(new Date()).getTime();
    setTimeout(keepalive,DELAY_MS);
}

var DELAY_MS=2*60000;// in minutes
// start loop after delay
setTimeout(keepalive,DELAY_MS);