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UAs should have more than a single UA entry #81

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Mar 16, 2020
Merged

UAs should have more than a single UA entry #81

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c386e4d
UA should have more than a single UA entry
yoavweiss Mar 9, 2020
a769f4d
UA should have more than a single UA entry
yoavweiss Mar 9, 2020
28c54a7
Some review comments
yoavweiss Mar 15, 2020
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Review comments
yoavweiss Mar 15, 2020
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Merge branch 'master' into ua_set_explanation
yoavweiss Mar 16, 2020
1acae72
Adapt sec-ch-ua value to specific instance of BrandVersion
yoavweiss Mar 16, 2020
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Merge branch 'ua_set_explanation' of github.com:yoavweiss/ua-client-h…
yoavweiss Mar 16, 2020
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Typo!!
yoavweiss Mar 16, 2020
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78 changes: 34 additions & 44 deletions index.bs
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Expand Up @@ -266,34 +266,23 @@ are fairly clearly sniffable by "examining the structure of other headers and by
availability and semantics of the features introduced or modified between releases of a particular
browser" [[Janc2014]]).

To <dfn abstract-op local-lt="set-ua">return the `Sec-CH-UA` value for a request</dfn>, given a
<a href="https://wicg.github.io/client-hints-infrastructure/#environment-settings-object-client-hints-set">client hints set</a> (|set|),
[=user agents=] MUST:
To <dfn abstract-op local-lt="set-ua">return the `Sec-CH-UA` value for a request</dfn>, [=user agents=] MUST:

1. Let |value| be a [=Structured Header=] object whose value is a [=structured header/list=].
1. Let |list| be an [=/list=], initially empty.
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2. Let |version| be the [=user agent=]'s [=user agent/full version=] if |set|
[=list/contains=] `UA`, and the [=user agent=]'s [=user agent/significant version=] otherwise.
2. For each |brandVersion| in [=user agent/UA list=]:

3. Let |ua| be a string whose value is the [=string/concatenation=] of the [=user agent=]'s
[=user agent/brand=], the string ";v=", and |version|.
1. Let |parameter| be a [=dictionary=], initially empty.

4. [=list/Append=] |ua| to |value|.
2. Set |parameter|["param_name"] to "v".

5. The [=user agent=] MAY execute the following steps:
3. Set |parameter|["param_value"] to |brandVersion|'s {{NavigatorUABrandVersion/version}}.

1. [=list/Append=] additional items to |value| containing arbitrary brand and version
combinations.
2. Let |pair| be a tuple comprised of |brandVersion|'s {{NavigatorUABrandVersion/brand}} and |parameter|.

2. Randomize the order of the items in |value|.
3. Append |pair| to |list|.
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Note: See [[#grease]] for more details on why these steps might be appropriate.

6. Return |value|.

Issue: Fix the processing model here so that it will use Structured Headers
serialization, and make sure that the value set for headers if the same as the
one returned from the JS API.
3. Return the output of running <a href="https://httpwg.org/http-extensions/draft-ietf-httpbis-header-structure.html#ser-list">serializing a list</a> with |list| as input.

The 'Sec-CH-UA-Mobile' Header Field {#sec-ch-mobile}
--------------------------------
Expand All @@ -308,7 +297,6 @@ The header's ABNF is:
Sec-CH-UA-Mobile = sh-boolean
```


Integration with Fetch {#fetch-integration}
----------------------
Fetch integration of this specification is defined as part of the <a href="https://wicg.github.io/client-hints-infrastructure/#fetch">Client Hints infrastructure</a> specification.
Expand Down Expand Up @@ -353,7 +341,7 @@ Processing model {#processing}

<h3 id="monkeypatch-html-windoworworkerglobalscope"><code>WindowOrWorkerGlobalScope</code></h3>

Each [=user agent=] has an associated <dfn>UA list</dfn>, which is a [=/list=] created by running [=create UA list=].
Each [=user agent=] has an associated <dfn for="user agent">UA list</dfn>, which is a [=/list=] created by running [=create UA list=].

Every {{WindowOrWorkerGlobalScope}} object has an associated <dfn for="WindowOrWorkerGlobalScope">UA frozen array</dfn>, which is a <code><a interface>FrozenArray</a>&lt;<a dictionary>NavigatorUABrandVersion</a>></code>. It is initially the result of [=create a frozen array|creating a frozen array=] from the [=user agent=]'s [=UA list=].

Expand All @@ -372,7 +360,7 @@ When asked to run the <dfn>create UA list</dfn> algorithm, the [=user agent=] MU

2. Append |dict| to |list|.

4. The [=user agent=] MAY execute the following steps:
4. The [=user agent=] SHOULD execute the following steps:

1. [=list/Append=] additional items to |list| containing {{NavigatorUABrandVersion}} objects,
initialized with arbitrary {{NavigatorUABrandVersion/brand}} and {{NavigatorUABrandVersion/version}} combinations.
Expand Down Expand Up @@ -465,31 +453,33 @@ order to reduce the fingerprint the header provides.
GREASE-like UA Strings {#grease}
----------------------

History has shown us that there are real incentives for [=user agents=] to lie
about their branding in order to thread the needle of sites' sniffing scripts,
and prevent their users from being blocked by UA-based allow/block lists.
History has shown us that there are real incentives for [=user agents=] to lie about their branding
in order to thread the needle of sites' sniffing scripts, and prevent their users from being blocked
by UA-based allow/block lists.

Reseting expectations may help to prevent abuse of the UA string's brand in the
short term, but probably won't help in the long run. The world of network
protocol introduced the notion of <abbr title="Generate Random Extensions And
Sustain Extensibility">GREASE</abbr> [[I-D.ietf-tls-grease]]. We could borrow
from that concept to tackle this problem.
Reseting expectations may help to prevent abuse of the UA string's brand in the short term, but
probably won't help in the long run. The world of network protocol introduced the notion of <abbr
title="Generate Random Extensions And Sustain Extensibility">GREASE</abbr> [[I-D.ietf-tls-grease]].
We could borrow from that concept to tackle this problem.

[=User agents=] SHOULD model `UA` as a set, rather than a single entry. They then
could encourage standardized processing of the `UA` string, by randomly
including additional, intentionally incorrect, comma-separated entries with
arbitrary ordering. That would reduce the chance that we ossify on a few
required strings.
[=User agents=]' [=user agent/UA list=] containing more than a single entry could encourage
standardized processing of the `UA` string. By randomly including additional, intentionally
incorrect, comma-separated entries with arbitrary ordering, they would reduce the chance that we
ossify on a few required strings.

Let's examine a few examples:
* In order to avoid sites from barring unknown browsers from their allow lists, Chrome could send a UA set that includes an non-existent browser, and which varies once in a while.
- `"Chrome"; v="73", "NotBrowser"; v="12"`
* In order to enable equivalence classes based on Chromium versions, Chrome could add the rendering engine and its version to that.
- `"Chrome"; v="73", "NotBrowser"; v="12", "Chromium"; v="73"`
* In order to encourage sites to rely on equivalence classes based on Chromium versions rather than exact UA sniffing, Chrome might remove itself from the set entirely.
- `"Chromium"; v="73", "NotBrowser"; v="12"`
* Browsers based on Chromium may use a similar UA string, but use their own brand as part of the set, enabling sites to count them.
- `"Chrome"; v="73", "Awesome Browser"; v="60", "Chromium"; v="73"`
* In order to avoid sites from barring unknown browsers from their allow lists, Chrome could send a
UA set that includes an non-existent browser, and which varies once in a while.
- `"Chrome"; v="73", "NotBrowser"; v="12"`
* In order to enable equivalence classes based on Chromium versions, Chrome could add the rendering
engine and its version to that.
- `"Chrome"; v="73", "NotBrowser"; v="12", "Chromium"; v="73"`
* In order to encourage sites to rely on equivalence classes based on Chromium versions rather than
exact UA sniffing, Chrome might remove itself from the set entirely.
- `"Chromium"; v="73", "NotBrowser"; v="12"`
* Browsers based on Chromium may use a similar UA string, but use their own brand as part of the
set, enabling sites to count them.
- `"Chrome"; v="73", "Awesome Browser"; v="60", "Chromium"; v="73"`

When choosing GREASE strategies, [=user agents=] SHOULD keep caching variance in mind and minimize variance among identical [=user agent=] versions.

Expand Down