ASP.NET Push Redirect on Session Timeout

I'm looking for a tutorial, blog entry, or some help on the technique behind websites that automatically push users (ie without a postback) when the session expires. Any help is appreciated

Usually, you set the session timeout, and you can additionally add a page header to automatically redirect the current page to a page where you clear the session right before the session timeout.

From http://aspalliance.com/1621_Implementing_a_Session_Timeout_Page_in_ASPNET.2

namespace SessionExpirePage
{
    public partial class Secure : System.Web.UI.MasterPage
    {
        public int SessionLengthMinutes
        {
            get { return Session.Timeout; }
        }
        public string SessionExpireDestinationUrl
        {
            get { return "/SessionExpired.aspx"; }
        }
        protected override void OnPreRender(EventArgs e)
        {
            base.OnPreRender(e);
            this.PageHead.Controls.Add(new LiteralControl(
                String.Format("<meta http-equiv='refresh' content='{0};url={1}'>", 
                SessionLengthMinutes*60, SessionExpireDestinationUrl)));
        }
    }
}

The SessionExpireDestinationUrl should link to a page where you clear the session and any other user data.

When the refresh header expires, it will automatically redirect them to that page.

You can't really "push" a client from your website. Your site will respond to requests from the client, but that's really it.

What this means is that you need to write something client-side (Javascript) that will determine when the user has timed out, probably by comparing the current time with the most recent time they have in a site cookie (which you update with the current time each time the user visits a page on your site), and then redirect if the difference is greater than a certain amount.

(I note that some people are advocating just creating a script that will forward the user after a certain amount of time on a page. This will work in the simple case, but if the user has two windows open on the site, and is using one window a lot, and the other window not-so-much, the not-so-much one will suddenly redirect the user to the forwarding page, even though the user has been on the site constantly. Additionally, it's not really in sync with any session keeping you're doing on the server side. On the other hand, it's certainly easier to code, and if that's good enough, then great!)