How is a write-back cache possible in an HTTP proxy? I understand write-through caching for PUT requests (The proxy doesn't have to repeat future identical PUTs because PUT is idempotent), but how does the proxy know what the response to the original PUT request is unless it performs the request against the origin server immediately?

Edit: I just noticed that RFC 2616 mandates write-through behavior: http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13...