The object of this tester is to allow users to confirm that a particular browser is correctly configured to use the University's web cache server. The University of Cambridge does not enforce use of the proxy, but using it is 'strongly recommended'. In the days of trans-Atlantic traffic charging getting this right was particularly important since failing to do this would affect the University's traffic bill. While the total bill is no longer traffic related, internal recharge arrangements are still based on external traffic volumes so not using the cache can still be expensive.
Experience has shown that it's quite common for a user to believe that his browser is configured to use the cache when it isn't, and this is a situation that is hard for the user to detect. Mis-typing the URL of our proxy auto-configuration file is a common reason (some browsers tend not to mention this), browsers with bugs in their proxy code are another common cause, as are users who turned off using the cache for some reason but forgot to turn it back on. A previous version of this tester proved useful in identifying such cases.
The tester is intended to work as follows:
The user is presented with a page containing 3 iframes (or ordinary links for browsers that don't support iframes). These three link to content from an 'external' site, an 'ac.uk' and a 'cam.ac.uk' site, and the user's browser fetches each one. Our cache is configured with access control lists which intercept requests for the 'external' and 'ac.uk' test URLs and cause content from the cache to be returned instead. So what the user sees will say 'Yes, you went via the cache' or 'No, you didn't go via the cache' as appropriate. There are variants of the 'Yes' and 'No' pages that go on to say if this behavior is expected or not - the main test page chooses which test pages to request based on the name/address of the machine being tested. [The cam.ac.uk test could work the same way, but since I control that one its simply a further dynamic page that adjusts it's output depending on the name/address of the machine being tested and whether it sees connections from one of out cache machines or not.]
You should be able to see the whole thing working if you try some of the test links on the demonstration page available (for the time being at least) at:
http://www.cam.ac.uk/cs/webcache/test/tests.html
The 'results' will be a bit misleading when used from outside the University, since they will be based on what your actual machine does - by and large you will 'fail' test 1 because your connections are (I hope!) not going via the University of Cambridge's cache.
We have yet to test version of the tester in production, but I believe it will work. Clearly it doesn't test everything that it could.