A proxy that caches some responses and not others, for whatever
   reason, may choose to implement the Meter header as a caching proxy
   for the responses that it caches, and as a non-caching proxy for the
   responses that it does not cache, as long as its external behavior
   with respect to any particularly response is fully consistent with
   this specification.








 
RFC 2227            Hit-Metering and Usage-Limiting         October 1997


5.6 Implementation by cooperating caches

   Several HTTP cache implementations, most notably the Harvest/Squid
   cache [2], create cooperative arrangements between several caches.
   If such caches use a protocol other than HTTP to communicate between
   themselves, such as the Internet Cache Protocol (ICP) [12], and if
   they implement the Meter header, then they MUST act to ensure that
   their cooperation does not violate the intention of this
   specification.

   In particular, if one member of a group of cooperating caches agrees
   with a server to hit-meter a particular response, and then passes
   this response via a non-HTTP protocol to a second cache in the group,
   the caches MUST ensure that the server which requested the metering
   receives reports that appropriately account for any uses or resues
   made by the second cache.  Similarly, if the first cache agreed to
   usage-limit the response, the total number of uses by the group of
   caches MUST be limited to the agreed-upon number.

6 Examples

6.1 Example of a complete set of exchanges

   This example shows how the protocol is intended to be used most of
   the time: for hit-metering without usage-limiting.  Entity bodies are
   omitted.

   A client sends request to a proxy:

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

   The proxy forwards request to the origin server:

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

   thus offering (implicitly) "will-report-and-limit".

   The server responds to the proxy:

       HTTP/1.1 200 OK
       Date: Fri, 06 Dec 1996 18:44:29 GMT
       Cache-control: max-age=3600
       Connection: meter
       Etag: "abcde"






 
RFC 2227            Hit-Metering and Usage-Limiting         October 1997


   thus (implicitly) requiring "do-report" (but not requiring
   usage-limiting).

   The proxy responds to the client:

       HTTP/1.1 200 OK
       Date: Fri, 06 Dec 1996 18:44:29 GMT
       Etag: "abcde"
       Cache-control: max-age=3600, proxy-mustcheck
       Age: 1

   Since the proxy does not know if its client is an end-system, or a
   proxy that doesn't do metering, it adds the "proxy-mustcheck"
   directive.

   Another client soon asks for the resource:

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

   and the proxy sends the same response as it sent to the other client,
   except (perhaps) for the Age value.

   After an hour has passed, a third client asks for the response:

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