Re: Benjamin Kaduk's No Objection on draft-ietf-httpbis-client-hints-14: (with COMMENT)

Thanks for reviewing and apologies for the delayed reply :/

Comments addressed below and incorporated into
https://github.com/httpwg/http-extensions/pull/1220
Your review would be appreciated :)

On Tue, May 19, 2020 at 10:56 PM Benjamin Kaduk via Datatracker <
noreply@ietf.org> wrote:

> Benjamin Kaduk has entered the following ballot position for
> draft-ietf-httpbis-client-hints-14: No Objection
>
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> Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
> for more information about IESG DISCUSS and COMMENT positions.
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>
> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-httpbis-client-hints/
>
>
>
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
>
> Section 1
>
>    There are thousands of different devices accessing the web, each with
>    different device capabilities and preference information.  These
>    device capabilities include hardware and software characteristics, as
>    well as dynamic user and user agent preferences.  Historically,
>
> nit: should "user-agent" be hyphenated?
>

In web specifications it typically isn't
<https://infra.spec.whatwg.org/#user-agent>. RFC 7231
<https://tools.ietf.org/html/rfc7231> also doesn't seem to hyphen it.


>    applications that wanted to allow the server to optimize content
>    delivery and user experience based on such capabilities had to rely
>    on passive identification (e.g., by matching the User-Agent header
>
> nit: it feels like "allow the server" would be something that involves
> granting permission or the client sending an active signal (as proposed
> by this document), as opposed to just the apaplication that "wanted the
> server to optimize" and had to make do with such limited signal as was
> already available.
>

OK. Removing "allow the".


>
>    field (Section 5.5.3 of [RFC7231]) against an established database of
>    user agent signatures), use HTTP cookies [RFC6265] and URL
>
> nit: hyphenate user-agent again, used as an adjective.
>

TIL: compound adjective
<https://www.grammarbook.com/punctuation/hyphens.asp#:~:text=Rule%201.,is%20called%20a%20compound%20adjective.&text=When%20a%20compound%20adjective%20follows,hyphen%20is%20usually%20not%20necessary.>
Done!

>
>    o  User agent detection cannot reliably identify all static
>       variables, cannot infer dynamic user agent preferences, requires
>       external device database, is not cache friendly, and is reliant on
>
> nit: singular/plural mismatch ("an external device database" or
> "external device databases")
>

Done

>
>    o  Cookie-based approaches are not portable across applications and
>       servers, impose additional client-side latency by requiring
>       JavaScript execution, and are not cache friendly.
>
> (I think I missed a step in why a cookie-based approach inherently
> requires javascript execution, though maybe it doesn't matter.)
>

Essentially, if you want to dynamically set your cookies based on
client-side information, you need javascript to do that.


>    Proactive content negotiation (Section 3.4.1 of [RFC7231]) offers an
>    alternative approach; user agents use specified, well-defined request
>    headers to advertise their capabilities and characteristics, so that
>
> Chasing the reference, it's not clear that it supports quite this strong
> of a statement: in addition to the explicit negotiation fields, it also
> allows using implicit characteristics such as client IP address and
> User-Agent.
>

Would ending that section with the following work?
", so that servers can select (or formulate) an appropriate response, based
on those request headers (or on other, implicit characteristics)."


> Section 2.1
>
>    access of third parties to those same header fields.  Without such an
>    opt-in, user agents SHOULD NOT send high-entropy hints, but MAY send
>    low-entropy ones [CLIENT-HINTS-INFRASTRUCTURE].
>
> It looks like the reference only defines a registry for low-entropy
> hints, and we are inferring that any hints not listed in that table are
> to be treated as "high-entropy".  Perhaps we could reword both
> directions of this directive to refer only to the registry of
> low-entropy hints (e.g., "SHOULD NOT send hints that are not listed in
> [registry]")?
>

Makes sense.


>
>    Implementers need to be aware of the passive fingerprinting
>    implications when implementing support for Client Hints, and follow
>    the considerations outlined in the Security Considerations
>    (Section 4) section of this document.
>
> side note: in some sense the Accept-CH mechanism transforms it from a
> passive to an active fingerprinting mechanism.
>

Good point! Removed "passive" here.


>
> Section 2.2
>
>    information in them.  When doing so, and if the resource is
>    cacheable, the server MUST also generate a Vary response header field
>    (Section 7.1.4 of [RFC7231]) to indicate which hints can affect the
>    selected response and whether the selected response is appropriate
>    for a later request.
>
> side note: I suspect the answer I want is already present with a
> detailed reading of RFC 7231, but I wonder if it's worth saying
> something here about whether the Vary response header could/should
> include registered client hint header field names that were not present
> in the request in question.
>

https://tools.ietf.org/html/rfc7231#section-7.1.4 implies that Vary can be
set to header names that are missing from the request. ("or lack thereof")
I'm not sure we should mention that explicitly here.


> Section 3.1
>
>    Based on the Accept-CH example above, which is received in response
>    to a user agent navigating to "https://example.com", and delivered
>    over a secure transport, a user agent will have to persist an Accept-
>    CH preference bound to "https://example.com".  It will then use it
>
> What level of requirement is implied by "will have to" here?  IIUC, it's
> just that "if anything is persisted, it must be keyed on" but with no
> obligation to do any persistence.  If so, perhaps a wording like "any
> persisted Accept-CH preference will be bound to" would be better?
>

The normative requirement in the paragraph above it is SHOULD.
I'll modify the wording to your suggested one.


>
>    for navigations to e.g. "https://example.com/foobar.html", but not to
>    e.g. "https://foobar.example.com/".  It will similarly use the
>    preference for any same-origin resource requests (e.g. to
>
> nit: comma after "e.g." (throughout).
>

OK


>
>    "https://example.com/image.jpg") initiated by the page constructed
>    from the navigation's response, but not to cross-origin resource
>    requests (e.g. "https://thirdparty.com/resource.js").  This
>    preference will not extend to resource requests initiated to
>    "https://example.com" from other origins (e.g. from navigations to
>    "https://other-example.com/").
>
> Perhaps thirdparty.example and other.example, to stay within the BCP32
> space?
>

Done


>
> Section 3.2
>
>    When selecting a response based on one or more Client Hints, and if
>    the resource is cacheable, the server needs to generate a Vary
>    response header field ([RFC7234]) to indicate which hints can affect
>    the selected response and whether the selected response is
>    appropriate for a later request.
>
> Is BCP 14 language approprite here?
>

Indeed. Changed to SHOULD.


>    Above example indicates that the cache key needs to include the Sec-
>    CH-Example header field.
>
> nit: please add the article "the" to make this a complete sentence.
>

Yup


>
> Section 4
>
> While I don't expect that I can tell the major browser vendors anything
> new about the privacy considerations to client hints, I do think that we
> should give some guidance to implementors of other HTTP clients, who may
> not have such extensive depth of knowlege, on the general landscape in
> which this mechanism is set.  The subsections hereof do a great job
> covering a lot of relevant details and specific factors to consider;
> thank you!  I think it may also be appropriate to have some more generic
> lead-in text, noting that in the worst case, merely converting a passive
> fingerprinting mechanism to an active fingerprinting mechanism with
> server opt-in does not actually provide any privacy benefit (the worst
> case being when all servers ask for all the data and clients accede)!
> While we might hope that the need to jump through an extra hoop to
> access fingerprinting information might dissuade some servers from
> asking for it, it seems imprudent to assume that it will happen, so in
> order to obtain real privacy benefit there needs to be some additional
> policy controls in the client and in what hints are defined/implemented.
> As I mentioned already, we already have a lot of the details for how to
> apply such policy controls, and limitations to only define hints that
> expose information already available in other means; what I'd like to
> see is the high-level picture that ties them together.
>
>
OK. Added something. I'd appreciate your review to see if it matches what
you had in mind.


> Section 4.1
>
>    upon it.  The header-based opt-in means that we can remove passive
>    fingerprinting vectors, such as the User-Agent string (enabling
>    active access to that information through User-Agent Client Hints
>    [4]), or otherwise expose information already available through
>
> I think this [4] is the same as [UA-CH].
>

It's pointing to a specific section of UA-CH. I'm not sure if this is
critical.


>
> Also, use of the first person ("we") is somewhat unusual in RFC style.
>

Changed.


>
>    Therefore, features relying on this document to define Client Hint
>    headers MUST NOT provide new information that is otherwise not
>    available to the application via other means, such as existing
>    request headers, HTML, CSS, or JavaScript.
>
> As written, this is a fairly weird condition.  What constitutes
> "available to the application via other means"?  Does "put up an
> interstitial until the user provides the information in question" count?
>

Changed to "not made available to the application by the user agent"


>
>    o  Entropy - Exposing highly granular data can be used to help
>       identify users across multiple requests to different origins.
>       Reducing the set of header field values that can be expressed, or
>       restricting them to an enumerated range where the advertised value
>       is close but is not an exact representation of the current value,
>
> nit: "close to" seems like it would scan better.
>

Yup


>
>    Different features will be positioned in different points in the
>    space between low-entropy, non-sensitive and static information (e.g.
>    user agent information), and high-entropy, sensitive and dynamic
>    information (e.g. geolocation).  User agents need to consider the
>    value provided by a particular feature vs these considerations, and
>    MAY have different policies regarding that tradeoff on a per-feature
>    basis.
>
> How about on a per-origin basis (and, e.g., domain reputation)?  An
> "entropy budget" where an origin that asks for too many distinct hints
> won't get all of them?
>

Those are definitely policies that user agents can apply (e.g. one concrete
proposal that looks a lot like your "entropy budget" is
https://github.com/bslassey/privacy-budget)


> (I also wonder if a descriptive "may wish to have" is better than the
> normative "MAY", here.)
>

Sure.

>
>    o  Implementers SHOULD restrict delivery of some or all Client Hints
>       header fields to the opt-in origin only, unless the opt-in origin
>       has explicitly delegated permission to another origin to request
>       Client Hints header fields.
>
> Am I reading things right that this document does not define any such
> delegation mechanisms but is just admitting the possibility of such
> mechanisms being defined in the future?  I'd suggest clarifying up in
> §2.1 with a parenthetical (akin to the "outlined below" note about the
> opt-in mechanism).
>

Added an "(as outlined in {{CLIENT-HINTS-INFRASTRUCTURE}})" clarification
to 2.1


>    Implementers SHOULD support Client Hints opt-in mechanisms and MUST
>    clear persisted opt-in preferences when any one of site data,
>    browsing history, browsing cache, cookies, or similar, are cleared.
>
> Who is the target audience for this SHOULD?  If it's just "people
> implementing this document", it seems ineffectual, and if it's any
> broader scope it seems unenforcable.
>

Removed the SHOULD here as it's already defined elsewhere that high entropy
hints require an opt-in.
Also changed "implementers" to "user agents".


> Section 4.3
>
>    Research into abuse of Client Hints might look at how HTTP responses
>    that contain Client Hints differ from those with different values,
>
> nit: what are "responses that contain Client Hints"?  We have discussed
> Accept-CH header fields in responses, and client hints in requests, but
> the only mention I recall of hints in responses was in the Vary header
> field, and it's not clear that that is what was intended.
>

Good catch! Changed to "responses to requests that contain Client Hints".


> Section 5
>
>    While HTTP header compression schemes reduce the cost of adding HTTP
>    header fields, sending Client Hints to the server incurs an increase
>    in request byte size.  Servers SHOULD take that into account when
>
> nit: I wonder if this would be more clear as:
>
> % Sending Client Hints to the server incurs an increase in request byte
> % size.  Some of this increase can be mitigated by HTTP header
> % compression schemes, but each new hint will still lead to some
> % increased bandwidth usage.  Servers SHOULD [...]
>

Changed.

>
> Section 7.1
>
> I'm not sure I understand why [FETCH] is listed as a normative
> reference.
>

Moved it to be informative.


>
> I find it amusing that we reference both 7231 and 7234 for Vary, though
> to my untrained eye the current references both seem appropriate in
> their respective locations.
>
> Section 7.2
>
> If [CLIENT-HINTS-INFRASTRUCTURE] is to be the source of truth for
> low-entropy (and, by deduction) high-entropy hints, it seems like it
> should be normative.
>

Moved.