Described herein are systems, devices, and methods for content delivery on the Internet. In certain non-limiting embodiments, a caching model is provided that can support caching for indefinite time periods, potentially with infinite or relatively long time-to-live values, yet provide prompt updates when the underlying origin content changes. In one approach, an origin server can annotate its responses to content requests with tokens, e.g., placing them in an appended HTTP header or otherwise. The tokens can drive the process of caching, and can be used as handles for later invalidating the responses within caching proxy servers delivering the content. This caching and invalidation model can be extended out to clients, such that clients may be notified of invalid data and obtain timely updates.