Besides the above mentioned headers, the following headers are mandatory.
* every answer to a GET reqest should always include <code>[http://en.wikipedia.org/wiki/HTTP_ETag ETag]</code> and <code>Last-Modified</code> header. This allows a proxy to cache requests and a client to revalidate already fetched data.
* the service must recognize <code>ETag</code>, <code>Last-Modified</code> and <code>Cache-Control: none</code> provided by the client and act accordingly.
* every answer to a GET request must include proper <code>Cache-Control</code> headers