draft-ietf-httpbis-client-hints-14.txt   draft-ietf-httpbis-client-hints-15.txt 
HTTP Working Group I. Grigorik HTTP Working Group I. Grigorik
Internet-Draft Y. Weiss Internet-Draft Y. Weiss
Intended status: Experimental Google Intended status: Experimental Google
Expires: November 19, 2020 May 18, 2020 Expires: January 4, 2021 July 3, 2020
HTTP Client Hints HTTP Client Hints
draft-ietf-httpbis-client-hints-14 draft-ietf-httpbis-client-hints-15
Abstract Abstract
HTTP defines proactive content negotiation to allow servers to select HTTP defines proactive content negotiation to allow servers to select
the appropriate response for a given request, based upon the user the appropriate response for a given request, based upon the user
agent's characteristics, as expressed in request headers. In agent's characteristics, as expressed in request headers. In
practice, user agents are often unwilling to send those request practice, user agents are often unwilling to send those request
headers, because it is not clear whether they will be used, and headers, because it is not clear whether they will be used, and
sending them impacts both performance and privacy. sending them impacts both performance and privacy.
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 19, 2020. This Internet-Draft will expire on January 4, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4
2. Client Hint Request Header Fields . . . . . . . . . . . . . . 4 2. Client Hint Request Header Fields . . . . . . . . . . . . . . 4
2.1. Sending Client Hints . . . . . . . . . . . . . . . . . . 4 2.1. Sending Client Hints . . . . . . . . . . . . . . . . . . 4
2.2. Server Processing of Client Hints . . . . . . . . . . . . 5 2.2. Server Processing of Client Hints . . . . . . . . . . . . 5
3. Advertising Server Support . . . . . . . . . . . . . . . . . 5 3. Advertising Server Support . . . . . . . . . . . . . . . . . 5
3.1. The Accept-CH Response Header Field . . . . . . . . . . . 5 3.1. The Accept-CH Response Header Field . . . . . . . . . . . 5
3.2. Interaction with Caches . . . . . . . . . . . . . . . . . 6 3.2. Interaction with Caches . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
4.1. Information Exposure . . . . . . . . . . . . . . . . . . 7 4.1. Information Exposure . . . . . . . . . . . . . . . . . . 7
4.2. Deployment and Security Risks . . . . . . . . . . . . . . 8 4.2. Deployment and Security Risks . . . . . . . . . . . . . . 9
4.3. Abuse Detection . . . . . . . . . . . . . . . . . . . . . 9 4.3. Abuse Detection . . . . . . . . . . . . . . . . . . . . . 9
5. Cost of Sending Hints . . . . . . . . . . . . . . . . . . . . 9 5. Cost of Sending Hints . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 9 6.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10 7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . 11
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 11 7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 11 Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 11
A.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 11 A.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 11
A.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 11 A.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 11 A.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 11 A.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 11 A.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 12 A.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 12 A.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.8. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 12 A.8. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.9. Since -08 . . . . . . . . . . . . . . . . . . . . . . . . 12 A.9. Since -08 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.10. Since -09 . . . . . . . . . . . . . . . . . . . . . . . . 12 A.10. Since -09 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.11. Since -10 . . . . . . . . . . . . . . . . . . . . . . . . 12 A.11. Since -10 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.12. Since -11 . . . . . . . . . . . . . . . . . . . . . . . . 12 A.12. Since -11 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.13. Since -12 . . . . . . . . . . . . . . . . . . . . . . . . 12 A.13. Since -12 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.14. Since -13 . . . . . . . . . . . . . . . . . . . . . . . . 13 A.14. Since -13 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.15. Since -14 . . . . . . . . . . . . . . . . . . . . . . . . 13
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction 1. Introduction
There are thousands of different devices accessing the web, each with There are thousands of different devices accessing the web, each with
different device capabilities and preference information. These different device capabilities and preference information. These
device capabilities include hardware and software characteristics, as device capabilities include hardware and software characteristics, as
well as dynamic user and user agent preferences. Historically, well as dynamic user and user agent preferences. Historically,
applications that wanted to allow the server to optimize content applications that wanted the server to optimize content delivery and
delivery and user experience based on such capabilities had to rely user experience based on such capabilities had to rely on passive
on passive identification (e.g., by matching the User-Agent header identification (e.g., by matching the User-Agent header field
field (Section 5.5.3 of [RFC7231]) against an established database of (Section 5.5.3 of [RFC7231]) against an established database of user
user agent signatures), use HTTP cookies [RFC6265] and URL agent signatures), use HTTP cookies [RFC6265] and URL parameters, or
parameters, or use some combination of these and similar mechanisms use some combination of these and similar mechanisms to enable ad hoc
to enable ad hoc content negotiation. content negotiation.
Such techniques are expensive to set up and maintain, and are not Such techniques are expensive to set up and maintain, and are not
portable across both applications and servers. They also make it portable across both applications and servers. They also make it
hard for both user agent and server to understand which data are hard for both user agent and server to understand which data are
required and is in use during the negotiation: required and is in use during the negotiation:
o User agent detection cannot reliably identify all static o User agent detection cannot reliably identify all static
variables, cannot infer dynamic user agent preferences, requires variables, cannot infer dynamic user agent preferences, requires
external device database, is not cache friendly, and is reliant on an external device database, is not cache friendly, and is reliant
a passive fingerprinting surface. on a passive fingerprinting surface.
o Cookie-based approaches are not portable across applications and o Cookie-based approaches are not portable across applications and
servers, impose additional client-side latency by requiring servers, impose additional client-side latency by requiring
JavaScript execution, and are not cache friendly. JavaScript execution, and are not cache friendly.
o URL parameters, similar to cookie-based approaches, suffer from o URL parameters, similar to cookie-based approaches, suffer from
lack of portability, and are hard to deploy due to a requirement lack of portability, and are hard to deploy due to a requirement
to encode content negotiation data inside of the URL of each to encode content negotiation data inside of the URL of each
resource. resource.
Proactive content negotiation (Section 3.4.1 of [RFC7231]) offers an Proactive content negotiation (Section 3.4.1 of [RFC7231]) offers an
alternative approach; user agents use specified, well-defined request alternative approach; user agents use specified, well-defined request
headers to advertise their capabilities and characteristics, so that headers to advertise their capabilities and characteristics, so that
servers can select (or formulate) an appropriate response. servers can select (or formulate) an appropriate response based on
those request headers (or on other, implicit characteristics).
However, traditional proactive content negotiation techniques often However, traditional proactive content negotiation techniques often
mean that user agents send these request headers prolifically. This mean that user agents send these request headers prolifically. This
causes performance concerns (because it creates "bloat" in requests), causes performance concerns (because it creates "bloat" in requests),
as well as privacy issues; passively providing such information as well as privacy issues; passively providing such information
allows servers to silently fingerprint the user. allows servers to silently fingerprint the user.
This document defines Client Hints, a framework that enables servers This document defines Client Hints, a framework that enables servers
to opt-in to specific proactive content negotiation features, to opt-in to specific proactive content negotiation features,
adapting their content accordingly, as well as guidelines for content adapting their content accordingly, as well as guidelines for content
negotiation mechanisms that use the framework. This document also negotiation mechanisms that use the framework. This document also
defines a new response header, Accept-CH, that allows an origin defines a new response header, Accept-CH, that allows an origin
server to explicitly ask that user agents send these headers in server to explicitly ask that user agents send these headers in
requests. requests.
Client Hints mitigate performance concerns by assuring that user Client Hints mitigate performance concerns by assuring that user
agents will only send the request headers when they're actually going agents will only send the request headers when they're actually going
to be used, and privacy concerns of passive fingerprinting by to be used, and privacy concerns of passive fingerprinting by
requiring explicit opt-in and disclosure of required headers by the requiring explicit opt-in and disclosure of required headers by the
server through the use of the Accept-CH response header. server through the use of the Accept-CH response header, turning
passive fingerprinting vectors into active ones.
The document does not define specific usages of Client Hints. Such The document does not define specific usages of Client Hints. Such
usages need to be defined in their respective specifications. usages need to be defined in their respective specifications.
One example of such usage is the User Agent Client Hints [UA-CH]. One example of such usage is the User Agent Client Hints [UA-CH].
1.1. Notational Conventions 1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
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agent preferences that the server can use to adapt and optimize the agent preferences that the server can use to adapt and optimize the
response. response.
2.1. Sending Client Hints 2.1. Sending Client Hints
User agents choose what Client Hints to send in a request based on User agents choose what Client Hints to send in a request based on
their default settings, user configuration, and server preferences their default settings, user configuration, and server preferences
expressed in "Accept-CH". The user agent and server can use an opt- expressed in "Accept-CH". The user agent and server can use an opt-
in mechanism outlined below to negotiate which header fields need to in mechanism outlined below to negotiate which header fields need to
be sent to allow for efficient content adaption, and optionally use be sent to allow for efficient content adaption, and optionally use
additional mechanisms to negotiate delegation policies that control additional mechanisms (e.g., as outlined in
access of third parties to those same header fields. Without such an [CLIENT-HINTS-INFRASTRUCTURE]) to negotiate delegation policies that
opt-in, user agents SHOULD NOT send high-entropy hints, but MAY send control access of third parties to those same header fields. User
low-entropy ones [CLIENT-HINTS-INFRASTRUCTURE]. agents SHOULD require an opt-in to send any hints that are not listed
in the low-entropy hint table at [CLIENT-HINTS-INFRASTRUCTURE].
Implementers need to be aware of the passive fingerprinting Implementers need to be aware of the fingerprinting implications when
implications when implementing support for Client Hints, and follow implementing support for Client Hints, and follow the considerations
the considerations outlined in the Security Considerations outlined in the Security Considerations (Section 4) section of this
(Section 4) section of this document. document.
2.2. Server Processing of Client Hints 2.2. Server Processing of Client Hints
When presented with a request that contains one or more Client Hint When presented with a request that contains one or more Client Hint
header fields, servers can optimize the response based upon the header fields, servers can optimize the response based upon the
information in them. When doing so, and if the resource is information in them. When doing so, and if the resource is
cacheable, the server MUST also generate a Vary response header field cacheable, the server MUST also generate a Vary response header field
(Section 7.1.4 of [RFC7231]) to indicate which hints can affect the (Section 7.1.4 of [RFC7231]) to indicate which hints can affect the
selected response and whether the selected response is appropriate selected response and whether the selected response is appropriate
for a later request. for a later request.
Servers MUST ignore hints they do not understand nor support. There Servers MUST ignore hints they do not understand nor support. There
is no mechanism for servers to indicate user agents that hints were is no mechanism for servers to indicate to user agents that hints
ignored. were ignored.
Furthermore, the server can generate additional response header Furthermore, the server can generate additional response header
fields (as specified by the hint or hints in use) that convey related fields (as specified by the hint or hints in use) that convey related
values to aid client processing. values to aid client processing.
3. Advertising Server Support 3. Advertising Server Support
Servers can advertise support for Client Hints using the mechanism Servers can advertise support for Client Hints using the mechanism
described below. described below.
3.1. The Accept-CH Response Header Field 3.1. The Accept-CH Response Header Field
The Accept-CH response header field indicates server support for the The Accept-CH response header field indicates server support for the
hints indicated in its value. Servers wishing to receive user agent hints indicated in its value. Servers wishing to receive user agent
information through Client Hints SHOULD add Accept-CH response header information through Client Hints SHOULD add Accept-CH response header
to their responses as early as possible. to their responses as early as possible.
Accept-CH is a Structured Header [I-D.ietf-httpbis-header-structure]. Accept-CH is a Structured Header [I-D.ietf-httpbis-header-structure].
Its value MUST be an sh-list (Section 3.1 of Its value MUST be an sf-list (Section 3.1 of
[I-D.ietf-httpbis-header-structure]) whose members are tokens [I-D.ietf-httpbis-header-structure]) whose members are tokens
(Section 3.3.4 of [I-D.ietf-httpbis-header-structure]). Its ABNF is: (Section 3.3.4 of [I-D.ietf-httpbis-header-structure]). Its ABNF is:
Accept-CH = sh-list Accept-CH = sf-list
For example: For example:
Accept-CH: Sec-CH-Example, Sec-CH-Example-2 Accept-CH: Sec-CH-Example, Sec-CH-Example-2
When a user agent receives an HTTP response containing "Accept-CH", When a user agent receives an HTTP response containing "Accept-CH",
that indicates that the origin opts-in to receive the indicated that indicates that the origin opts-in to receive the indicated
request header fields for subsequent same-origin requests. The opt- request header fields for subsequent same-origin requests. The opt-
in MUST be ignored if delivered over non-secure transport (using a in MUST be ignored if delivered over non-secure transport (using a
scheme different from HTTPS). It SHOULD be persisted and bound to scheme different from HTTPS). It SHOULD be persisted and bound to
the origin to enable delivery of Client Hints on subsequent requests the origin to enable delivery of Client Hints on subsequent requests
to the server's origin, for the duration of the user's session (as to the server's origin, for the duration of the user's session (as
defined by the user agent). An opt-in overrides previous persisted defined by the user agent). An opt-in overrides previous persisted
opt-in values and SHOULD be persisted in its stead. opt-in values and SHOULD be persisted in its stead.
Based on the Accept-CH example above, which is received in response Based on the Accept-CH example above, which is received in response
to a user agent navigating to "https://example.com", and delivered to a user agent navigating to "https://site.example", and delivered
over a secure transport, a user agent will have to persist an Accept- over a secure transport, persisted Accept-CH preferences will be
CH preference bound to "https://example.com". It will then use it bound to "https://site.example". It will then use it for navigations
for navigations to e.g. "https://example.com/foobar.html", but not to to e.g., "https://site.example/foobar.html", but not to e.g.,
e.g. "https://foobar.example.com/". It will similarly use the "https://foobar.site.example/". It will similarly use the preference
preference for any same-origin resource requests (e.g. to for any same-origin resource requests (e.g., to
"https://example.com/image.jpg") initiated by the page constructed "https://site.example/image.jpg") initiated by the page constructed
from the navigation's response, but not to cross-origin resource from the navigation's response, but not to cross-origin resource
requests (e.g. "https://thirdparty.com/resource.js"). This requests (e.g., "https://thirdparty.example/resource.js"). This
preference will not extend to resource requests initiated to preference will not extend to resource requests initiated to
"https://example.com" from other origins (e.g. from navigations to "https://site.example" from other origins (e.g., from navigations to
"https://other-example.com/"). "https://other.example/").
3.2. Interaction with Caches 3.2. Interaction with Caches
When selecting a response based on one or more Client Hints, and if When selecting a response based on one or more Client Hints, and if
the resource is cacheable, the server needs to generate a Vary the resource is cacheable, the server needs to generate a Vary
response header field ([RFC7234]) to indicate which hints can affect response header field ([RFC7234]) to indicate which hints can affect
the selected response and whether the selected response is the selected response and whether the selected response is
appropriate for a later request. appropriate for a later request.
Vary: Sec-CH-Example Vary: Sec-CH-Example
Above example indicates that the cache key needs to include the Sec- The above example indicates that the cache key needs to include the
CH-Example header field. Sec-CH-Example header field.
Vary: Sec-CH-Example, Sec-CH-Example-2 Vary: Sec-CH-Example, Sec-CH-Example-2
Above example indicates that the cache key needs to include the Sec- The above example indicates that the cache key needs to include the
CH-Example and Sec-CH-Example-2 header fields. Sec-CH-Example and Sec-CH-Example-2 header fields.
4. Security Considerations 4. Security Considerations
4.1. Information Exposure 4.1. Information Exposure
Request header fields used in features relying on this document Request header fields used in features relying on this document
expose information about the user's environment to enable proactive expose information about the user's environment to enable privacy-
content negotiation. Such information might reveal new information preserving proactive content negotiation, and avoid exposing passive
fingerprinting vectors. However, implementers need to bear in mind
that in the worst case, uncontrolled and unmonitored active
fingerprinting is not better than passive fingerprinting. In order
to provide user privacy benefits, user agents need to apply further
policies that prevent abuse of the information exposed by features
using Client Hints.
The information exposed by features might reveal new information
about the user and implementers ought to consider the following about the user and implementers ought to consider the following
considerations, recommendations, and best practices. considerations, recommendations, and best practices.
The underlying assumption is that exposing information about the user The underlying assumption is that exposing information about the user
as a request header is equivalent (from a security perspective) to as a request header is equivalent (from a security perspective) to
exposing this information by other means. (For example, if the exposing this information by other means. (For example, if the
request's origin can access that information using JavsScript APIs, request's origin can access that information using JavaScript APIs,
and transmit it to its servers). and transmit it to its servers).
Because Client Hints is an explicit opt-in mechanism, that means that Because Client Hints is an explicit opt-in mechanism, that means that
servers that want access to information about the user's environment servers that want access to information about the user's environment
need to actively ask for it, enabling clients and privacy researchers need to actively ask for it, enabling clients and privacy researchers
to keep track of which origins collect that data, and potentially act to keep track of which origins collect that data, and potentially act
upon it. The header-based opt-in means that we can remove passive upon it. The header-based opt-in means that removal of passive
fingerprinting vectors, such as the User-Agent string (enabling fingerprinting vectors is possible, such as the User-Agent string
active access to that information through User-Agent Client Hints (enabling active access to that information through User-Agent Client
[4]), or otherwise expose information already available through Hints ([UA-CH]) or otherwise expose information already available
script (e.g. the Save-Data Client Hint [5]), without increasing the through script (e.g., the Save-Data Client Hint [4]), without
passive fingerprinting surface. increasing the passive fingerprinting surface. User agents
supporting Client Hints features which send certain information to
opted-in servers SHOULD avoid sending the equivalent information
passively.
Therefore, features relying on this document to define Client Hint Therefore, features relying on this document to define Client Hint
headers MUST NOT provide new information that is otherwise not headers MUST NOT provide new information that is otherwise not made
available to the application via other means, such as existing available to the application by the user agent, such as existing
request headers, HTML, CSS, or JavaScript. request headers, HTML, CSS, or JavaScript.
Such features need to take into account the following aspects of the Such features need to take into account the following aspects of the
information exposed: information exposed:
o Entropy - Exposing highly granular data can be used to help o Entropy - Exposing highly granular data can be used to help
identify users across multiple requests to different origins. identify users across multiple requests to different origins.
Reducing the set of header field values that can be expressed, or Reducing the set of header field values that can be expressed, or
restricting them to an enumerated range where the advertised value restricting them to an enumerated range where the advertised value
is close but is not an exact representation of the current value, is close to but is not an exact representation of the current
can improve privacy and reduce risk of linkability by ensuring value, can improve privacy and reduce risk of linkability by
that the same value is sent by multiple users. ensuring that the same value is sent by multiple users.
o Sensitivity - The feature SHOULD NOT expose user-sensitive o Sensitivity - The feature SHOULD NOT expose user-sensitive
information. To that end, information available to the information. To that end, information available to the
application, but gated behind specific user actions (e.g. a application, but gated behind specific user actions (e.g., a
permission prompt or user activation) SHOULD NOT be exposed as a permission prompt or user activation) SHOULD NOT be exposed as a
Client Hint. Client Hint.
o Change over time - The feature SHOULD NOT expose user information o Change over time - The feature SHOULD NOT expose user information
that changes over time, unless the state change itself is also that changes over time, unless the state change itself is also
exposed (e.g. through JavaScript callbacks). exposed (e.g., through JavaScript callbacks).
Different features will be positioned in different points in the Different features will be positioned in different points in the
space between low-entropy, non-sensitive and static information (e.g. space between low-entropy, non-sensitive and static information
user agent information), and high-entropy, sensitive and dynamic (e.g., user agent information), and high-entropy, sensitive and
information (e.g. geolocation). User agents need to consider the dynamic information (e.g., geolocation). User agents need to
value provided by a particular feature vs these considerations, and consider the value provided by a particular feature vs these
MAY have different policies regarding that tradeoff on a per-feature considerations, and may wish to have different policies regarding
basis. that tradeoff on a per-feature or other fine-grained basis.
Implementers ought to consider both user- and server- controlled Implementers ought to consider both user- and server- controlled
mechanisms and policies to control which Client Hints header fields mechanisms and policies to control which Client Hints header fields
are advertised: are advertised:
o Implementers SHOULD restrict delivery of some or all Client Hints o Implementers SHOULD restrict delivery of some or all Client Hints
header fields to the opt-in origin only, unless the opt-in origin header fields to the opt-in origin only, unless the opt-in origin
has explicitly delegated permission to another origin to request has explicitly delegated permission to another origin to request
Client Hints header fields. Client Hints header fields.
o Implementers considering providing user choice mechanisms that o Implementers considering providing user choice mechanisms that
allow users to balance privacy concerns against bandwidth allow users to balance privacy concerns against bandwidth
limitations need to also consider that explaining to users the limitations need to also consider that explaining to users the
privacy implications involved, such as the risks of passive privacy implications involved, such as the risks of passive
fingerprinting, is challenging and likely impractical. fingerprinting, may be challenging or even impractical.
o Implementations specific to certain use cases or threat models MAY o Implementations specific to certain use cases or threat models MAY
avoid transmitting some or all of Client Hints header fields. For avoid transmitting some or all of Client Hints header fields. For
example, avoid transmission of header fields that can carry higher example, avoid transmission of header fields that can carry higher
risks of linkability. risks of linkability.
Implementers SHOULD support Client Hints opt-in mechanisms and MUST User agents MUST clear persisted opt-in preferences when any one of
clear persisted opt-in preferences when any one of site data, site data, browsing history, browsing cache, cookies, or similar, are
browsing history, browsing cache, cookies, or similar, are cleared. cleared.
4.2. Deployment and Security Risks 4.2. Deployment and Security Risks
Deployment of new request headers requires several considerations: Deployment of new request headers requires several considerations:
o Potential conflicts due to existing use of header field name o Potential conflicts due to existing use of header field name
o Properties of the data communicated in header field value o Properties of the data communicated in header field value
Authors of new Client Hints are advised to carefully consider whether Authors of new Client Hints are advised to carefully consider whether
they need to be able to be added by client-side content (e.g., they need to be able to be added by client-side content (e.g.,
skipping to change at page 9, line 13 skipping to change at page 9, line 27
from setting them in user agents. Using the "Sec-" prefix signals to from setting them in user agents. Using the "Sec-" prefix signals to
servers that the user agent - and not application content - generated servers that the user agent - and not application content - generated
the values. See [FETCH] for more information. the values. See [FETCH] for more information.
By convention, request headers that are Client Hints are encouraged By convention, request headers that are Client Hints are encouraged
to use a CH- prefix, to make them easier to identify as using this to use a CH- prefix, to make them easier to identify as using this
framework; for example, CH-Foo or, with a "Sec-" prefix, Sec-CH-Foo. framework; for example, CH-Foo or, with a "Sec-" prefix, Sec-CH-Foo.
Doing so makes them easier to identify programmatically (e.g., for Doing so makes them easier to identify programmatically (e.g., for
stripping unrecognised hints from requests by privacy filters). stripping unrecognised hints from requests by privacy filters).
A Client Hints request header negotiated using the Accept-CH opt-in
mechanism MUST have a field name that matches sf-token (Section 3.3.4
of [I-D.ietf-httpbis-header-structure]).
4.3. Abuse Detection 4.3. Abuse Detection
A user agent that tracks access to active fingerprinting information A user agent that tracks access to active fingerprinting information
SHOULD consider emission of Client Hints headers similarly to the way SHOULD consider emission of Client Hints headers similarly to the way
it would consider access to the equivalent API. it would consider access to the equivalent API.
Research into abuse of Client Hints might look at how HTTP responses Research into abuse of Client Hints might look at how HTTP responses
that contain Client Hints differ from those with different values, to requests that contain Client Hints differ from those with
and from those without. This might be used to reveal which Client different values, and from those without. This might be used to
Hints are in use, allowing researchers to further analyze that use. reveal which Client Hints are in use, allowing researchers to further
analyze that use.
5. Cost of Sending Hints 5. Cost of Sending Hints
While HTTP header compression schemes reduce the cost of adding HTTP Sending Client Hints to the server incurs an increase in request byte
header fields, sending Client Hints to the server incurs an increase size. Some of this increase can be mitigated by HTTP header
in request byte size. Servers SHOULD take that into account when compression schemes, but each new hint sent will still lead to some
opting in to receive Client Hints, and SHOULD NOT opt-in to receive increased bandwidth usage. Servers SHOULD take that into account
hints unless they are to be used for content adaptation purposes. when opting in to receive Client Hints, and SHOULD NOT opt-in to
receive hints unless they are to be used for content adaptation
purposes.
Due to request byte size increase, features relying on this document Due to request byte size increase, features relying on this document
to define Client Hints MAY consider restricting sending those hints to define Client Hints MAY consider restricting sending those hints
to certain request destinations [FETCH], where they are more likely to certain request destinations [FETCH], where they are more likely
to be useful. to be useful.
6. IANA Considerations 6. IANA Considerations
Features relying on this document are expected to register added
request header fields in the Permanent Message Header Fields registry
([RFC3864]).
This document defines the "Accept-CH" HTTP response header field, and This document defines the "Accept-CH" HTTP response header field, and
registers it in the Permanent Message Header Fields registry. registers it in the same registry.
6.1. Accept-CH 6.1. Accept-CH
o Header field name: Accept-CH o Header field name: Accept-CH
o Applicable protocol: HTTP o Applicable protocol: HTTP
o Status: experimental o Status: experimental
o Author/Change controller: IETF o Author/Change controller: IETF
o Specification document(s): Section 3.1 of this document o Specification document(s): Section 3.1 of this document
o Related information: for Client Hints o Related information: for Client Hints
7. References 7. References
7.1. Normative References 7.1. Normative References
[FETCH] van Kesteren, A., "Fetch", n.d., [CLIENT-HINTS-INFRASTRUCTURE]
<https://fetch.spec.whatwg.org/>. Weiss, Y., "Client Hints Infrastructure", n.d.,
<https://wicg.github.io/client-hints-infrastructure/>.
[I-D.ietf-httpbis-header-structure] [I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Field Values for Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", draft-ietf-httpbis-header-structure-18 (work in HTTP", draft-ietf-httpbis-header-structure-19 (work in
progress), April 2020. progress), June 2020.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008, DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>. <https://www.rfc-editor.org/info/rfc5234>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014, DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>. <https://www.rfc-editor.org/info/rfc7231>.
skipping to change at page 10, line 43 skipping to change at page 11, line 26
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014, RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>. <https://www.rfc-editor.org/info/rfc7234>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References 7.2. Informative References
[CLIENT-HINTS-INFRASTRUCTURE] [FETCH] van Kesteren, A., "Fetch", n.d.,
Weiss, Y., "Client Hints Infrastructure", n.d., <https://fetch.spec.whatwg.org/>.
<https://wicg.github.io/client-hints-infrastructure/>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011, DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>. <https://www.rfc-editor.org/info/rfc6265>.
[UA-CH] West, M. and Y. Weiss, "User Agent Client Hints", n.d., [UA-CH] West, M. and Y. Weiss, "User Agent Client Hints", n.d.,
<https://wicg.github.io/ua-client-hints/>. <https://wicg.github.io/ua-client-hints/>.
7.3. URIs 7.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/ [1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] http://httpwg.github.io/ [2] http://httpwg.github.io/
[3] https://github.com/httpwg/http-extensions/labels/client-hints [3] https://github.com/httpwg/http-extensions/labels/client-hints
[4] https://wicg.github.io/ua-client-hints/#http-ua-hints [4] https://wicg.github.io/savedata/#save-data-request-header-field
[5] https://wicg.github.io/savedata/#save-data-request-header-field
Appendix A. Changes Appendix A. Changes
A.1. Since -00 A.1. Since -00
o Issue 168 (make Save-Data extensible) updated ABNF. o Issue 168 (make Save-Data extensible) updated ABNF.
o Issue 163 (CH review feedback) editorial feedback from httpwg o Issue 163 (CH review feedback) editorial feedback from httpwg
list. list.
o Issue 153 (NetInfo API citation) added normative reference. o Issue 153 (NetInfo API citation) added normative reference.
A.2. Since -01 A.2. Since -01
o Issue 200: Moved Key reference to informative. o Issue 200: Moved Key reference to informative.
o Issue 215: Extended passive fingerprinting and mitigation o Issue 215: Extended passive fingerprinting and mitigation
considerations. considerations.
o Changed document status to experimental. o Changed document status to experimental.
A.3. Since -02 A.3. Since -02
skipping to change at page 13, line 9 skipping to change at page 13, line 36
o PR 1134: Re-insert back section. o PR 1134: Re-insert back section.
A.13. Since -12 A.13. Since -12
o PR 1160: AD review. o PR 1160: AD review.
A.14. Since -13 A.14. Since -13
o PR 1171: Genart review. o PR 1171: Genart review.
A.15. Since -14
o PR 1220: AD review.
Acknowledgements Acknowledgements
Thanks to Mark Nottingham, Julian Reschke, Chris Bentzel, Ben Thanks to Mark Nottingham, Julian Reschke, Chris Bentzel, Ben
Greenstein, Tarun Bansal, Roy Fielding, Vasiliy Faronov, Ted Hardie, Greenstein, Tarun Bansal, Roy Fielding, Vasiliy Faronov, Ted Hardie,
Jonas Sicking, Martin Thomson, and numerous other members of the IETF Jonas Sicking, Martin Thomson, and numerous other members of the IETF
HTTP Working Group for invaluable help and feedback. HTTP Working Group for invaluable help and feedback.
Authors' Addresses Authors' Addresses
Ilya Grigorik Ilya Grigorik
Google Google
Email: ilya@igvita.com Email: ilya@igvita.com
URI: https://www.igvita.com/ URI: https://www.igvita.com/
Yoav Weiss Yoav Weiss
Google Google
Email: yoav@yoav.ws Email: yoav@yoav.ws
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