Read and write arbitrary bits using HTTP server-side caching

The "sender" makes requests (or doesn't make them, to send 0 vs. 1) on a pre-determined schedule to an unpopular resource on a popular, unaffiliated third-party server. The "receiver" can request the same resource a few seconds later, and can determine based on the cache-related HTTP headers returned whether the page was generated recently or just now, with the current request. This of course requires the server to expose information about server-side caching via the headers; the example given is that the Expires header may be used to infer the original generation time.

The above quote stood out in light of: "Python 2.7 Retirement Countdown" ~ https://news.ycombinator.com/item?id=11793857