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The Chrome data reduction feature relies on HTTP headers to 
work correctly and efficiently with the data reduction 
proxy. This include both standard HTTP headers (like Via) 
and custom headers (like Chrome-Proxy). Tampering on these 
headers could lead to miserable user experience, taking 10s 
to load some pages, for example.

In the past, we have seen such headers being stripped by 
middle box proxies (for example, the WWW-Authenticate header 
was stripped by some carrier). It has been known that mobile 
carriers are doing HTTP traffic optimizations. We also want 
to know whether mobile carriers are trying to "optimize" the 
already optimized data reduction proxy response body, which 
might lead to higher cost to users.


We propose a mechanism in Chromium to enable us to learn the 
scale and the types of such tampers. In short, the mechanism 
will check whether a predefined set of HTTP response headers 
and the response body have been changed in a way that could 
affect the data reduction proxy. It will detect such changes 
by using pre-calculated header (and probably content) hashes 
sent by the server. Chromium will report through UMA the 
count of each tamper types has happened. This will only be 
enabled for a fraction of the data reduction proxy users.

BUG=381907

Review URL: https://codereview.chromium.org/338483002

Cr-Commit-Position: refs/heads/master@{#288492}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288492 0039d316-1c4b-4281-b951-d872f2087c98