What is a web cache?

A web cache stores temporary copies of web pages to improve performance and reduce bandwidth consumption.

Do I have a web cache?

You probably have several. Every web browser has a built in cache. Internet accelerators that you can buy in the store and install on your PC are usually web caches. Many internet service providers offer an internet accelerator as part of their service.

Some schools, organizations and internet service providers operate their own web caches that do not require software to be installed on your PC. These are used not only to improve performance, but sometimes to filter out objectionable material, or to ensure privacy.

At any given time, you may be browsing the web through several layers of caches.

Are your web caches properly configured?

Caches are supposed to perform their function behind the scenes, offering their benefits without changing how web pages work. This is known as semantic transparency.

The HTTP 1.1 specification gives a set of rules regarding transparency "because non-transparent operation may confuse non-expert users, and might be incompatible with certain server applications (such as those for ordering merchandise)"

Sometimes web caches break these rules in order to offer improved performance. Sometimes web caches can be improperly configured to break these rules. Breaking the semantic transparency rules of the HTTP specification can also break the expected behavior of web applications.

The following tests check to see if the web caches you are using have semantic transparency.

Test Status
No Validator, No Expiration Perform Test
Pragma No-Cache Perform Test
Query Perform Test
Expires Perform Test
Cache-Control No-Cache Perform Test
Cache-Control Max-Age Perform Test
POST Request-URI Perform Test
POST Location Perform Test
POST Self Perform Test
Reset all tests

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About these tests

These tests work by examining the IP address of a sequence of requests to determine if your browser has cached a page in the sequence.

If your web cache issues requests using multiple IP addresses, these tests will not work.

This may be the case for AOL users and other internet service providers.