I have a default installation of Tomcat 6.0, and I have an app that stores information in the session variable. 

It seems that my session is invalidated after a short period of inactivity. So long as I'm navigating to another page every few seconds, the session data is there, but if I stop clicking for about half a minute, the session id that the browser stores changes. (I've confirmed this using Firebug, and see the same behavior from IE8 which is my target browser.)

Why might this be happening? The web.xml for my app specifies 30 minute sessions, and I've made a call to `session.setMaxInactiveInterval(60)` to see if that changed anything, but it doesn't seem to. 

Sanity check:

My web app web.xml starts like this:

```
<?xml version="1.0" encoding="utf-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/j2ee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_5.xsd"
    version="2.5">
  <session-config>
    <session-timeout>30</session-timeout>
  </session-config>

```

My server's web.xml contains these lines:

```
<session-config>
    <session-timeout>30</session-timeout>
</session-config>

```