But now the response's max-age has been exceeded, so the proxy
   revalidates the response with the origin server:

       GET /bar.html HTTP/1.1
       If-None-Match: "abcde"
       Host: foo.com
       Connection: Meter
       Meter: count=1/0

   thus simultaneously fulfilling its duties to validate the response
   and to report the one "use" that wasn't forwarded.

   The origin server responds:

       HTTP/1.1 304 Not Modified
       Date: Fri, 06 Dec 1996 19:44:29 GMT
       Cache-control: max-age=3600
       Etag: "abcde"

   so the proxy can use the original response to reply to the new
   client; the proxy also zeros the use-count it associates with that
   response.




 
RFC 2227            Hit-Metering and Usage-Limiting         October 1997


   Another client soon asks for the resource:

       GET http://foo.com/bar.html HTTP/1.1

   and the proxy sends the appropriate response.

   After another few hours, the proxy decides to remove the cache entry.
   When it does so, it sends to the origin server:

       HEAD /bar.html HTTP/1.1
       If-None-Match: "abcde"
       Host: foo.com
       Connection: Meter
       Meter: count=1/0

   reporting that one more use of the response was satisfied from the
   cache.

6.2 Protecting against HTTP/1.0 proxies

   An origin server that does not want HTTP/1.0 caches to store the
   response at all, and is willing to have HTTP/1.0 end-system clients
   generate excess GETs (which will be forwarded by HTTP/1.0 proxies)
   could send this for its reply:

       HTTP/1.1 200 OK
       Cache-control: max-age=3600
       Connection: meter
       Etag: "abcde"
       Expires: Sun, 06 Nov 1994 08:49:37 GMT

   HTTP/1.0 caches will see the ancient Expires header, but HTTP/1.1
   caches will see the max-age directive and will ignore Expires.

      Note: although most major HTTP/1.0 proxy implementations observe
      the Expires header, it is possible that some are in use that do
      not.  Use of the Expires header to prevent caching by HTTP/1.0
      proxies might not be entirely reliable.

6.3 More elaborate examples

   Here is a request from a proxy that is willing to hit-meter but is
   not willing to usage-limit:

       GET /bar.html HTTP/1.1
       Host: foo.com
       Connection: Meter
       Meter: wont-limit




 
RFC 2227            Hit-Metering and Usage-Limiting         October 1997


   Here is a response from an origin server that does not want hit
   counting, but does want "uses" limited to 3, and "reuses" limited to
   6:

       HTTP/1.1 200 OK
       Cache-control: max-age=3600
       Connection: meter
       Etag: "abcde"
       Expires: Sun, 06 Nov 1994 08:49:37 GMT
       Meter: max-uses=3, max-reuses=6, dont-report

   Here is the same example with abbreviated Meter directive names:

       HTTP/1.1 200 OK