Authentication cookie holds old session id

[briand]

I have the following code in my base page to redirect to a session timeout
page.
override protected void OnInit(EventArgs e)
{
base.OnInit(e);
//It appears from testing that the Request and Response both share the
// same cookie collection. If I set a cookie myself in the Reponse, it is
// also immediately visible to the Request collection. This just means
that
// since the ASP.Net_SessionID is set in the Session HTTPModule (which
// has already run), thatwe can't use our own code to see if the cookie
was
// actually sent by the agent with the request using the collection.
Check if
// the given page supports session or not (this tested as reliable
indicator
// if EnableSessionState is true), should not care about a page that does
// not need session
if (Context.Session != null)
{
//Tested and the IsNewSession is more advanced then simply checking if
// a cookie is present, it does take into account a session timeout,
because
// I tested a timeout and it did show as a new session
if (Session.IsNewSession)
{
// If it says it is a new session, but an existing cookie exists, then
it must
// have timed out (can't use the cookie collection because even on first
// request it already contains the cookie (request and response
// seem to share the collection)
string szCookieHeader = Request.Headers["Cookie"];
if ((null != szCookieHeader) &&
(szCookieHeader.IndexOf("ASP.NET_SessionId") >= 0))
{
Response.Redirect("sessionTimeout.htm");
}
}
}
}

The session timeout page then has a link to the login page. This all works
fine if the session timesout. However, if the user open the browser, logs in,
then closes the browser, then re-opens the browser to again to the previous
location you get rediected to the session timeout page straight away. This is
because the cookie still has the previous session id.
The cookie lifespan is 7 days.

How do I force the session info in the cookie to be cleared when the browser
is closed?

--
Thanks Brian