CDN and “Cache-Control: max-age”

If we ever use “max-age” for content that is partially dynamic in the sense that it may or may not change in “X” minutes and we use a CDN to cache that and we also have a “max-age” header then we need to remember to do this:

  1. Ensure that the max-age is counted down by the CDN each and every time it is accessed
  2. Or, if it is not counted down then ensure that the HTTP “Date:” header also should not change as well

The reasoning is that the clients (browsers etc.) would use these two values to decide on refreshing the cache.

Consider an OCSP response which is served through a CDN. We also know that the OCSP response has a “nextUpdate” temporal value and the RFC 5019¬†clearly states that the “max-age” should be less (or equal to since it seems that the clients allow for equality as a positive case as well) than the “nextUpdate“.

The issue is when the “max-age” is not counted down by the CDN and the value in¬†the “Date:” header is – we get an issue where the OCSP response might be stale but it is not timed out of the cache either for validation or a fetch.

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