draft-ietf-httpbis-client-hints-13.txt   draft-ietf-httpbis-client-hints-14.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: October 26, 2020 April 24, 2020 Expires: November 19, 2020 May 18, 2020
HTTP Client Hints HTTP Client Hints
draft-ietf-httpbis-client-hints-13 draft-ietf-httpbis-client-hints-14
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, clients are often unwilling to send those request headers, practice, user agents are often unwilling to send those request
because it is not clear whether they will be used, and sending them headers, because it is not clear whether they will be used, and
impacts both performance and privacy. sending them impacts both performance and privacy.
This document defines an Accept-CH response header that servers can This document defines an Accept-CH response header that servers can
use to advertise their use of request headers for proactive content use to advertise their use of request headers for proactive content
negotiation, along with a set of guidelines for the creation of such negotiation, along with a set of guidelines for the creation of such
headers, colloquially known as "Client Hints." headers, colloquially known as "Client Hints."
Note to Readers Note to Readers
Discussion of this draft takes place on the HTTP working group Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at mailing list (ietf-http-wg@w3.org), which is archived at
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
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 October 26, 2020. This Internet-Draft will expire on November 19, 2020.
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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Table of Contents Table of Contents
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 . . . . . . . . . . . . . . . . . . . 6 4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
4.1. Information Exposure . . . . . . . . . . . . . . . . . . 6 4.1. Information Exposure . . . . . . . . . . . . . . . . . . 7
4.2. Deployment and Security Risks . . . . . . . . . . . . . . 8 4.2. Deployment and Security Risks . . . . . . . . . . . . . . 8
4.3. Abuse Detection . . . . . . . . . . . . . . . . . . . . . 8 4.3. Abuse Detection . . . . . . . . . . . . . . . . . . . . . 9
5. Cost of Sending Hints . . . . . . . . . . . . . . . . . . . . 9 5. Cost of Sending Hints . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 9 6.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 9 7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . 10
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 10 Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 11
A.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 10 A.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 11
A.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 11 A.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 11
A.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 11 A.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 11
A.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 11 A.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 11
A.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 11 A.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 11
A.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 11 A.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 11 A.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.8. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 11 A.8. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.9. Since -08 . . . . . . . . . . . . . . . . . . . . . . . . 12 A.9. Since -08 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.10. Since -09 . . . . . . . . . . . . . . . . . . . . . . . . 12 A.10. Since -09 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.11. Since -10 . . . . . . . . . . . . . . . . . . . . . . . . 12 A.11. Since -10 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.12. Since -11 . . . . . . . . . . . . . . . . . . . . . . . . 12 A.12. Since -11 . . . . . . . . . . . . . . . . . . . . . . . . 12
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12 A.13. Since -12 . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 A.14. Since -13 . . . . . . . . . . . . . . . . . . . . . . . . 13
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 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 client preferences. Historically, well as dynamic user and user agent preferences. Historically,
applications that wanted to allow the server to optimize content applications that wanted to allow the server to optimize content
delivery and user experience based on such capabilities had to rely delivery and user experience based on such capabilities had to rely
on passive identification (e.g., by matching the User-Agent header on passive identification (e.g., by matching the User-Agent header
field (Section 5.5.3 of [RFC7231]) against an established database of field (Section 5.5.3 of [RFC7231]) against an established database of
client signatures), use HTTP cookies [RFC6265] and URL parameters, or user agent signatures), use HTTP cookies [RFC6265] and URL
use some combination of these and similar mechanisms to enable ad hoc parameters, or use some combination of these and similar mechanisms
content negotiation. to enable ad hoc content negotiation.
Such techniques are expensive to setup 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 client and server to understand which data is required hard for both user agent and server to understand which data are
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 client preferences, requires variables, cannot infer dynamic user agent preferences, requires
external device database, is not cache friendly, and is reliant on external device database, is not cache friendly, and is reliant on
a passive fingerprinting surface. 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.
However, traditional proactive content negotiation techniques often However, traditional proactive content negotiation techniques often
mean that clients 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 agent. allows servers to silently fingerprint the user.
This document defines a new response header, Accept-CH, that allows
an origin server to explicitly ask that clients send these headers in
requests. It also defines guidelines for content negotiation
mechanisms that use it, colloquially referred to as Client Hints.
Client Hints mitigate performance concerns by assuring that clients
will only send the request headers when they're actually going to be
used, and privacy concerns of passive fingerprinting by requiring
explicit opt-in and disclosure of required headers by the server
through the use of the Accept-CH response header.
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. However, it does not define any adapting their content accordingly, as well as guidelines for content
specific features that will use that infrastructure. Those features negotiation mechanisms that use the framework. This document also
will be defined in their respective specifications. defines a new response header, Accept-CH, that allows an origin
server to explicitly ask that user agents send these headers in
requests.
One example of such a feature is the User Agent Client Hints feature Client Hints mitigate performance concerns by assuring that user
[UA-CH]. agents will only send the request headers when they're actually going
to be used, and privacy concerns of passive fingerprinting by
requiring explicit opt-in and disclosure of required headers by the
server through the use of the Accept-CH response header.
The document does not define specific usages of Client Hints. Such
usages need to be defined in their respective specifications.
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
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
This document uses the Augmented Backus-Naur Form (ABNF) notation of This document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234]. [RFC5234].
2. Client Hint Request Header Fields 2. Client Hint Request Header Fields
A Client Hint request header field is a HTTP header field that is A Client Hint request header field is a HTTP header field that is
used by HTTP clients to indicate data that can be used by the server used by HTTP user agents to indicate data that can be used by the
to select an appropriate response. Each one conveys client server to select an appropriate response. Each one conveys user
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
Clients choose what Client Hints to send in a request based on their User agents choose what Client Hints to send in a request based on
default settings, user configuration, and server preferences their default settings, user configuration, and server preferences
expressed in "Accept-CH". The client and server can use an opt-in expressed in "Accept-CH". The user agent and server can use an opt-
mechanism outlined below to negotiate which header fields need to be in mechanism outlined below to negotiate which header fields need to
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 to negotiate delegation policies that control
access of third parties to same header fields. 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].
Implementers SHOULD be aware of the passive fingerprinting Implementers need to be aware of the passive fingerprinting
implications when implementing support for Client Hints, and follow implications when implementing support for Client Hints, and follow
the considerations outlined in the Security Considerations the considerations outlined in the Security Considerations
(Section 4) section of this document. (Section 4) section of this 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
is no mechanism for servers to indicate user agents that hints 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. hints indicated in its value. Servers wishing to receive user agent
information through Client Hints SHOULD add Accept-CH response header
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 sh-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 = sh-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 client receives an HTTP response containing "Accept-CH", that When a user agent receives an HTTP response containing "Accept-CH",
indicates that the origin opts-in to receive the indicated request that indicates that the origin opts-in to receive the indicated
header fields for subsequent same-origin requests. The opt-in MUST request header fields for subsequent same-origin requests. The opt-
be ignored if delivered over non-secure transport (using a scheme in MUST be ignored if delivered over non-secure transport (using a
different from HTTPS). It SHOULD be persisted and bound to the scheme different from HTTPS). It SHOULD be persisted and bound to
origin to enable delivery of Client Hints on subsequent requests to the origin to enable delivery of Client Hints on subsequent requests
the server's origin. 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
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://example.com", and delivered
over a secure transport, a user agent will have to persist an Accept- 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 CH preference bound to "https://example.com". It will then use it
for user agent navigations to e.g. "https://example.com/foobar.html", for navigations to e.g. "https://example.com/foobar.html", but not to
but not to e.g. "https://foobar.example.com/". It will similarly use e.g. "https://foobar.example.com/". It will similarly use the
the preference for any same-origin resource requests (e.g. to preference for any same-origin resource requests (e.g. to
"https://example.com/image.jpg") initiated by the page constructed "https://example.com/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.com/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://example.com" from other origins (e.g. from navigations to
"https://other-example.com/"). "https://other-example.com/").
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
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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 proactive
content negotiation. Such information might reveal new information content negotiation. Such information 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 JavsScript 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 user agents and privacy need to actively ask for it, enabling clients and privacy researchers
researchers to keep track of which origins collect that data, and to keep track of which origins collect that data, and potentially act
potentially act upon it. The header-based opt-in means that we can upon it. The header-based opt-in means that we can remove passive
remove passive fingerprinting vectors, such as the User-Agent string fingerprinting vectors, such as the User-Agent string (enabling
(enabling active access to that information through User-Agent Client active access to that information through User-Agent Client Hints
Hints [4]), or otherwise expose information already available through [4]), or otherwise expose information already available through
script (e.g. the Save-Data Client Hint [5]), without increasing the script (e.g. the Save-Data Client Hint [5]), without increasing the
passive fingerprinting surface. passive fingerprinting surface.
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
available to the application via other means, such as existing available to the application via other means, such as existing
request headers, HTML, CSS, or JavaScript. request headers, HTML, CSS, or JavaScript.
Such features SHOULD 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 but is not an exact representation of the current value,
can improve privacy and reduce risk of linkability by ensuring can improve privacy and reduce risk of linkability by ensuring
that the same value is sent by multiple users. 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
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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 but is not an exact representation of the current value,
can improve privacy and reduce risk of linkability by ensuring can improve privacy and reduce risk of linkability by ensuring
that the same value is sent by multiple users. 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 (e.g.
user agent information), and high-entropy, sensitive and dynamic user agent information), and high-entropy, sensitive and dynamic
information (e.g. geolocation). User agents SHOULD consider the information (e.g. geolocation). User agents need to consider the
value provided by a particular feature vs these considerations, and value provided by a particular feature vs these considerations, and
MAY have different policies regarding that tradeoff on a per-feature MAY have different policies regarding that tradeoff on a per-feature
basis. 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
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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.,
scripts), or whether they need to be exclusively set by the user scripts), or whether they need to be exclusively set by the user
agent. In the latter case, the Sec- prefix on the header field name agent. In the latter case, the Sec- prefix on the header field name
has the effect of preventing scripts and other application content has the effect of preventing scripts and other application content
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).
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.
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6. IANA Considerations 6. IANA Considerations
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 Permanent Message Header Fields 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: standard 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., [FETCH] van Kesteren, A., "Fetch", n.d.,
<https://fetch.spec.whatwg.org/>. <https://fetch.spec.whatwg.org/>.
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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]
Weiss, Y., "Client Hints Infrastructure", n.d.,
<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/
skipping to change at page 12, line 28 skipping to change at page 12, line 47
o PR 1080: Improve list style. o PR 1080: Improve list style.
o PR 1082: Remove section mentioning Variants. o PR 1082: Remove section mentioning Variants.
o PR 1097: Editorial feedback from mnot. o PR 1097: Editorial feedback from mnot.
o PR 1131: Remove unused references. o PR 1131: Remove unused references.
o PR 1132: Remove nested list. o PR 1132: Remove nested list.
A.12. Since -11 A.12. Since -11
o PR 1134: Re-insert back section. o PR 1134: Re-insert back section.
A.13. Since -12
o PR 1160: AD review.
A.14. Since -13
o PR 1171: Genart 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
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