[webkit-dev] Request for position: Critical-CH response header, part of Client Hints Reliability proposal

Aaron Tagliaboschi aarontag at chromium.org
Fri Jan 29 04:45:20 PST 2021

By default, the browser only sends the brand, major/significant version,
and whether or not the browser has signalled it wants a "mobile"
experience. Other than that, the browser only sends hints that the site has
signalled it wants through the Accept-CH response header.

If, for example, the browser has never visited the site before, then the
browser has never received an Accept-CH response header from that site and
therefore doesn't know that the site wants any extra headers. This also
happens when information about the site is clear, which also clears the
client hint preferences

It could also be that the site has changed the hints it wants to receive
since you last visited, in which case you might not have sent all of the
headers it actually wants.

In both cases, it's not that the browser is blocking the headers, but that
the preferences stored in the browser are out of sync with the preferences
of the site.

On Thu, Jan 28, 2021 at 10:53 PM Ryosuke Niwa <rniwa at webkit.org> wrote:

> I'm still confused here. In what scenario would a browser decide to not
> send client hints but later decide it's okay to send them?
> On Thu, Jan 28, 2021 at 7:13 PM Aaron Tagliaboschi <aarontag at chromium.org>
> wrote:
>> The Critical-CH header can trigger a request re-try. It's for situations
>> where the browser could be unaware of the site's CH preferences (like the
>> first navigation request to a site before the browser has received and
>> stored CH preferences) or if a site has changed those references, and the
>> site would rather drop the request and retry over getting a potentially
>> "incomplete" request
>> This would *not* override potential mitigations or reductions in
>> fingerprinting surfaces imposed by the browser. Any headers that would be
>> blocked would still be silently dropped.
>> (cc davidben, mjs who I forgot to CC the first time)
>> On Thu, Jan 28, 2021 at 9:35 PM Ryosuke Niwa <rniwa at webkit.org> wrote:
>>> What's the point of specifying Critical-CH as opposed to relying on CH
>>> provided by the browser?
>>> Is the idea that some browsers may decide to hide some client hints to
>>> reduce the fingerprinting surface?
>>> If so, then this new header seems to just defeat that because a website
>>> can specify all the client hints as critical.
>>> - R. Niwa
>>> On Wed, Jan 27, 2021 at 4:40 AM Aaron Tagliaboschi via webkit-dev <
>>> webkit-dev at lists.webkit.org> wrote:
>>>> Explainer:
>>>> https://github.com/WICG/client-hints-infrastructure/blob/master/reliability.md#critical-ch
>>>> Draft Spec:
>>>> https://tools.ietf.org/html/draft-davidben-http-client-hint-reliability-02#section-3
>>>> The Client Hint Reliability proposal is a set of features aimed at
>>>> making Client Hints
>>>> <https://tools.ietf.org/html/draft-ietf-httpbis-client-hints-15> more
>>>> reliably available and mitigating
>>>> mis-matches between a site's preferences and the preferences stored in
>>>> the browser. The idea
>>>> behind the Critical-CH response header is to signal to browsers that
>>>> there are hints the server
>>>> would rather pay a round trip than not have not the first request. The
>>>> basic algorithm is as follows:
>>>> If, after receiving a request with Critical-CH and Accept-CH headers,
>>>> there is a hint indicated in
>>>> the Critical-CH header that the browser did not send but would not
>>>> block sending, the browser
>>>> should store the new CH preferences, drop the request, and start a new
>>>> one with the new
>>>> headers included.
>>>> Aaron Tagliaboschi | Software Engineer, Chrome Trust & Safety
>>>> _______________________________________________
>>>> webkit-dev mailing list
>>>> webkit-dev at lists.webkit.org
>>>> https://lists.webkit.org/mailman/listinfo/webkit-dev
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