Main question: my ISP compresses HTML and images, unless I press Ctrl + Shift + R on Chrome (and Firefox). So, how does the ISP know that I forced a refresh?

Full story:

I recently switched to Vodafone 4G router, from my good unnamed broadband. I realized that images looked blurry, and many websites looked strange. But when I connected back to broadband, things were back to normal. Turned out that Vodafone actually compresses data (very lossy compression!!). It made jpeg images very blurry, and removed whitespace from the HTML. It even removed the comments that I wrote inside HTML on my website. The URLs in the HTML are not changed; the blurry images appear to be hosted at same URL. The same effect when hitting Ctrl + R, or when clearing my browser's cache and just loading the page.

All this was expected, except, when I force reload using Ctrl + Shift + R, the uncompressed web page appears! This means that my own webpage's code had those comments and whitespace intact. If I press Ctrl + R after this, it showed the compressed version again. I tested this multiple times on Chrome and Firefox, and on 2 laptops.

Here is my question: how does Vodafone know that I pressed force reload?

  • Which version do you see when clearing your local cache and visiting the page? And does the compressed HTML also have different URLs in <img src=...>? – Arjan Apr 24 '16 at 7:05
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    I guess the weirdest thing is that pressing Ctrl+R makes the browser get new images (the blurry ones) while it already should have cached the good ones. For Ctrl+R, the Network tab in web developer tools will show you headers like If-Modified-Since or Etag for items that are already cached, which Vodafone seems to be ignoring...? – Arjan Apr 24 '16 at 8:05
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    There must be something in the request your browser sends that makes Vodafone see the difference. Your browser's debugging view or capturing your packets with i.e. Wireshark should show you what it could be. It's probably some cache control header. – Sander Steffann Apr 24 '16 at 9:58
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    And messing with content like that sounds like a huge violation of net neutrality. The company transporting your data shouldn't mess with its contents. – Sander Steffann Apr 24 '16 at 9:59
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    For images, pressing Ctrl+Shift+R gives new images, but pressing Ctrl+R after again does not give back the blurry ones, because chrome keeps the sharper once. But for webpage, there is no persistent cache perhaps. Strangest thing ever -> on chrome with developer tools -> network tab open, pressing Ctrl+Shift+R keeps giving the compressed version every time. But I found this on firefox, it has the tag "Cache-Control: no-cache" in firefox when Ctrl+Shift+R, and tag "Cache-Control: max-age=0" with Ctrl+R in request header! This is what vodafone is responding to. – user257838 Apr 24 '16 at 17:43

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