HTTP Usage Reporting for Cached Resources

This document defines a mechanism by which an HTTP origin can advertise an endpoint for downstream usage reporting and by which an authenticated client operator can report usage of cached representations that were used without issuing additional requests to the origin. The mechanism complements access-time response metadata by allowing a client operator to report downstream usage associated with a prior origin-served response context. The protocol defines discovery of a usage reporting endpoint, submission semantics, and a minimal record format suitable for reconciliation and billing. This specification does not define payment, settlement, or enforcement mechanisms. It defines a good-faith accounting protocol for reporting downstream usage.