TYPO3 Core

When a page is fully cacheable ("client-cacheable"), TYPO3 either sends NO HTTP Response Headers, or - when set - config.sendCacheHeaders = 1 - headers so the client (= the browser) is allowed to cache a page.

Imagine this in combination with config.cache_timeout = 604.800 (60*60*24*7 = 7 days) - this would mean, that your clients would only see updated content if they
a) clear the browser cache
b) hit shift-enter

Otherwise, TYPO3 says "Hey, you can cache this for 7 days". and the HTTP Headers say so.

This is usually not what you want. Nowadays, you want the client to actually check again and have your server respond again with the cached variant (if available) or with fresh content.

However, we could leverage "Cache-Control: s-maxage=12345" which allows for shared caches (which one could have under control) to keep a cached version instead of asking the webserver.