draft-ietf-httpbis-client-hints-15.txt   rfc8942.txt 
HTTP Working Group I. Grigorik Internet Engineering Task Force (IETF) I. Grigorik
Internet-Draft Y. Weiss Request for Comments: 8942 Y. Weiss
Intended status: Experimental Google Category: Experimental Google
Expires: January 4, 2021 July 3, 2020 ISSN: 2070-1721 February 2021
HTTP Client Hints HTTP Client Hints
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.
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
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/ [1].
Working Group information can be found at http://httpwg.github.io/
[2]; source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/client-hints [3].
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This document is not an Internet Standards Track specification; it is
provisions of BCP 78 and BCP 79. published for examination, experimental implementation, and
evaluation.
Internet-Drafts are working documents of the Internet Engineering This document defines an Experimental Protocol for the Internet
Task Force (IETF). Note that other groups may also distribute community. This document is a product of the Internet Engineering
working documents as Internet-Drafts. The list of current Internet- Task Force (IETF). It represents the consensus of the IETF
Drafts is at https://datatracker.ietf.org/drafts/current/. community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are candidates for any level of
Internet Standard; see Section 2 of RFC 7841.
Internet-Drafts are draft documents valid for a maximum of six months Information about the current status of this document, any errata,
and may be updated, replaced, or obsoleted by other documents at any and how to provide feedback on it may be obtained at
time. It is inappropriate to use Internet-Drafts as reference https://www.rfc-editor.org/info/rfc8942.
material or to cite them other than as "work in progress."
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) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4 1.1. Notational Conventions
2. Client Hint Request Header Fields . . . . . . . . . . . . . . 4 2. Client Hints Request Header Fields
2.1. Sending Client Hints . . . . . . . . . . . . . . . . . . 4 2.1. Sending Client Hints
2.2. Server Processing of Client Hints . . . . . . . . . . . . 5 2.2. Server Processing of Client Hints
3. Advertising Server Support . . . . . . . . . . . . . . . . . 5 3. Advertising Server Support
3.1. The Accept-CH Response Header Field . . . . . . . . . . . 5 3.1. The Accept-CH Response Header Field
3.2. Interaction with Caches . . . . . . . . . . . . . . . . . 6 3.2. Interaction with Caches
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7 4. Security Considerations
4.1. Information Exposure . . . . . . . . . . . . . . . . . . 7 4.1. Information Exposure
4.2. Deployment and Security Risks . . . . . . . . . . . . . . 9 4.2. Deployment and Security Risks
4.3. Abuse Detection . . . . . . . . . . . . . . . . . . . . . 9 4.3. Abuse Detection
5. Cost of Sending Hints . . . . . . . . . . . . . . . . . . . . 9 5. Cost of Sending Hints
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 6. IANA Considerations
6.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 10 6.1. Accept-CH
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. References
7.1. Normative References . . . . . . . . . . . . . . . . . . 10 7.1. Normative References
7.2. Informative References . . . . . . . . . . . . . . . . . 11 7.2. Informative References
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Acknowledgements
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses
A.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 11
A.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.8. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.9. Since -08 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.10. Since -09 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.11. Since -10 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.12. Since -11 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.13. Since -12 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.14. Since -13 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.15. Since -14 . . . . . . . . . . . . . . . . . . . . . . . . 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 user agent preferences. Historically, well as dynamic user and user agent preferences. Historically,
applications that wanted the server to optimize content delivery and applications that wanted the server to optimize content delivery and
user experience based on such capabilities had to rely on passive user experience based on such capabilities had to rely on passive
identification (e.g., by matching the User-Agent header field identification (e.g., by matching the User-Agent header field
(Section 5.5.3 of [RFC7231]) against an established database of user (Section 5.5.3 of [RFC7231]) against an established database of user
agent signatures), use HTTP cookies [RFC6265] and URL parameters, or agent signatures), use HTTP cookies [RFC6265] and URL parameters, or
use some combination of these and similar mechanisms to enable ad hoc use some combination of these and similar mechanisms 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 are in use during the negotiation:
o User agent detection cannot reliably identify all static * User agent detection cannot reliably identify all static
variables, cannot infer dynamic user agent preferences, requires variables, cannot infer dynamic user agent preferences, requires
an external device database, is not cache friendly, and is reliant an external device database, is not cache friendly, and is reliant
on a passive fingerprinting surface. on a passive fingerprinting surface.
o Cookie-based approaches are not portable across applications and * 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 * 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
to encode content negotiation data inside of the URL of each 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 based on servers can select (or formulate) an appropriate response based on
those request headers (or on other, implicit characteristics). 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
skipping to change at page 4, line 18 skipping to change at line 132
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 they mitigate privacy concerns of passive
requiring explicit opt-in and disclosure of required headers by the fingerprinting by requiring explicit opt-in and disclosure of
server through the use of the Accept-CH response header, turning required headers by the server through the use of the Accept-CH
passive fingerprinting vectors into active ones. 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
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in
14 [RFC2119] [RFC8174] when, and only when, they appear in all BCP 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 Hints Request Header Fields
A Client Hint request header field is a HTTP header field that is A Client Hints request header field is an HTTP header field that is
used by HTTP user agents to indicate data that can be used by the used by HTTP user agents to indicate data that can be used by the
server to select an appropriate response. Each one conveys user server to select an appropriate response. Each one conveys user-
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 they can
additional mechanisms (e.g., as outlined in optionally use additional mechanisms (e.g., as outlined in
[CLIENT-HINTS-INFRASTRUCTURE]) to negotiate delegation policies that [CLIENT-HINTS-INFRASTRUCTURE]) to negotiate delegation policies that
control access of third parties to those same header fields. User control access of third parties to those same header fields. User
agents SHOULD require an opt-in to send any hints that are not listed agents SHOULD require an opt-in to send any hints that are not
in the low-entropy hint table at [CLIENT-HINTS-INFRASTRUCTURE]. considered low-entropy. See the low-entropy hint table at
[CLIENT-HINTS-INFRASTRUCTURE] for examples of hints that expose low
amounts of entropy.
Implementers need to be aware of the fingerprinting implications when Implementers need to be aware of the fingerprinting implications when
implementing support for Client Hints, and follow the considerations implementing support for Client Hints and follow the considerations
outlined in the Security Considerations (Section 4) section of this outlined in the Security Considerations section of this document (see
document. Section 4).
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 Hints
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 to user agents that hints is no mechanism for servers to indicate to user agents that hints
were ignored. were ignored.
skipping to change at page 5, line 44 skipping to change at line 209
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 the Accept-CH response
to their responses as early as possible. 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 [RFC8941]. Its value MUST be an sf-
Its value MUST be an sf-list (Section 3.1 of list (Section 3.1 of [RFC8941]) whose members are Tokens
[I-D.ietf-httpbis-header-structure]) whose members are tokens (Section 3.3.4 of [RFC8941]). Its ABNF is:
(Section 3.3.4 of [I-D.ietf-httpbis-header-structure]). Its ABNF is:
Accept-CH = sf-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 it indicates that the origin opts-in to receive the indicated request
request header fields for subsequent same-origin requests. The opt- header fields for subsequent same-origin requests. The opt-in MUST
in MUST be ignored if delivered over non-secure transport (using a be ignored if delivered over non-secure transport (using a scheme
scheme different from HTTPS). It SHOULD be persisted and bound to different from HTTPS). It SHOULD be persisted and bound to the
the origin to enable delivery of Client Hints on subsequent requests origin to enable delivery of Client Hints on subsequent requests to
to the server's origin, for the duration of the user's session (as 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://site.example", and delivered to a user agent navigating to "https://site.example", and delivered
over a secure transport, persisted Accept-CH preferences will be over a secure transport, persisted Accept-CH preferences will be
bound to "https://site.example". It will then use it for navigations bound to "https://site.example". It will then use it for navigations
to e.g., "https://site.example/foobar.html", but not to e.g., to for example, "https://site.example/foobar.html", but not to, for
"https://foobar.site.example/". It will similarly use the preference example, "https://foobar.site.example/". It will similarly use the
for any same-origin resource requests (e.g., to preference for any same-origin resource requests (e.g., to
"https://site.example/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.example/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://site.example" from other origins (e.g., from navigations to "https://site.example" from other origins (e.g., from navigations to
"https://other.example/"). "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
The above example indicates that the cache key needs to include the The above example indicates that the cache key needs to include the
Sec-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
The above example indicates that the cache key needs to include the The above example indicates that the cache key needs to include the
Sec-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 privacy- expose information about the user's environment to enable privacy-
preserving proactive content negotiation, and avoid exposing passive preserving proactive content negotiation and avoid exposing passive
fingerprinting vectors. However, implementers need to bear in mind fingerprinting vectors. However, implementers need to bear in mind
that in the worst case, uncontrolled and unmonitored active that in the worst case, uncontrolled and unmonitored active
fingerprinting is not better than passive fingerprinting. In order fingerprinting is not better than passive fingerprinting. In order
to provide user privacy benefits, user agents need to apply further to provide user privacy benefits, user agents need to apply further
policies that prevent abuse of the information exposed by features policies that prevent abuse of the information exposed by features
using Client Hints. using Client Hints.
The information exposed by features might reveal new information 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 JavaScript 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, it means that
servers that want access to information about the user's environment servers wanting 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 removal of passive upon it. The header-based opt-in means that removal of passive
fingerprinting vectors is possible, such as the User-Agent string fingerprinting vectors is possible. As an example, the user agent
(enabling active access to that information through User-Agent Client can reduce the information exposed by the User-Agent string, while
Hints ([UA-CH]) or otherwise expose information already available enabling active access to that information through User-Agent Client
through script (e.g., the Save-Data Client Hint [4]), without Hints [UA-CH]. Otherwise, the user agent can expose information
increasing the passive fingerprinting surface. User agents already available through script (e.g., the Save-Data Client Hints
<https://wicg.github.io/savedata/#save-data-request-header-field>),
without increasing the passive fingerprinting surface. User agents
supporting Client Hints features which send certain information to supporting Client Hints features which send certain information to
opted-in servers SHOULD avoid sending the equivalent information opted-in servers SHOULD avoid sending the equivalent information
passively. 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 made headers MUST NOT provide new information that is otherwise not made
available to the application by the user agent, 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: exposed information:
o Entropy - Exposing highly granular data can be used to help Entropy: Exposing highly granular data can be used to help identify
identify users across multiple requests to different origins. 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 to
but is not an exact representation of the current value, can
improve privacy and reduce risk of linkability by ensuring that
the same value is sent by multiple users.
Reducing the set of header field values that can be expressed, or Sensitivity: The feature SHOULD NOT expose user-sensitive
restricting them to an enumerated range where the advertised value
is close to but is not an exact representation of the current
value, can improve privacy and reduce risk of linkability by
ensuring that the same value is sent by multiple users.
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
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 space between low-entropy, non-sensitive, and static information
(e.g., user agent information), and high-entropy, sensitive and (e.g., user agent information) and high-entropy, sensitive, and
dynamic information (e.g., geolocation). User agents need to dynamic information (e.g., geolocation). User agents need to
consider the value provided by a particular feature vs these consider the value provided by a particular feature vs. these
considerations, and may wish to have different policies regarding considerations and may wish to have different policies regarding that
that tradeoff on a per-feature or other fine-grained basis. 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 * 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
* Implementers that consider 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 the privacy
privacy implications involved, such as the risks of passive implications involved to users, such as the risks of passive
fingerprinting, may be challenging or even impractical. fingerprinting, may be challenging or even impractical.
o Implementations specific to certain use cases or threat models MAY
avoid transmitting some or all of Client Hints header fields. For * Implementations specific to certain use cases or threat models MAY
example, avoid transmission of header fields that can carry higher avoid transmitting some or all of the Client Hints header fields.
risks of linkability. For example, avoid transmission of header fields that can carry
higher risks of linkability.
User agents MUST clear persisted opt-in preferences when any one of User agents MUST clear persisted opt-in preferences when any one of
site data, browsing history, browsing cache, cookies, or similar, are site data, 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 * Potential conflicts due to existing use of a header field name
o Properties of the data communicated in header field value * Properties of the data communicated in a 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.,
scripts), or whether they need to be exclusively set by the user scripts) or whether the Client Hints need to be exclusively set by
agent. In the latter case, the Sec- prefix on the header field name the user agent. In the latter case, the Sec- prefix on the header
has the effect of preventing scripts and other application content field name has the effect of preventing scripts and other application
from setting them in user agents. Using the "Sec-" prefix signals to content from setting them in user agents. Using the "Sec-" prefix
servers that the user agent - and not application content - generated signals to servers that the user agent -- and not application content
the values. See [FETCH] for more information. -- generated 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 unrecognized hints from requests by privacy filters).
A Client Hints request header negotiated using the Accept-CH opt-in 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 mechanism MUST have a field name that matches sf-token (Section 3.3.4
of [I-D.ietf-httpbis-header-structure]). of [RFC8941]).
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 similar 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
to requests that contain Client Hints differ from those with to requests that contain Client Hints differ from those with
different values, and from those without. This might be used to different values and from those without values. This might be used
reveal which Client Hints are in use, allowing researchers to further to reveal which Client Hints are in use, allowing researchers to
analyze that use. further analyze that use.
5. Cost of Sending Hints 5. Cost of Sending Hints
Sending Client Hints to the server incurs an increase in request byte Sending Client Hints to the server incurs an increase in request byte
size. Some of this increase can be mitigated by HTTP header size. Some of this increase can be mitigated by HTTP header
compression schemes, but each new hint sent will still lead to some compression schemes, but each new hint sent will still lead to some
increased bandwidth usage. Servers SHOULD take that into account increased bandwidth usage. Servers SHOULD take that into account
when opting in to receive Client Hints, and SHOULD NOT opt-in to when opting in to receive Client Hints and SHOULD NOT opt-in to
receive hints unless they are to be used for content adaptation receive hints unless they are to be used for content adaptation
purposes. 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 Features relying on this document are expected to register added
request header fields in the Permanent Message Header Fields registry request header fields in the "Permanent Message Header Field Names"
([RFC3864]). registry [RFC3864].
This document defines the "Accept-CH" HTTP response header field, and This document defines the "Accept-CH" HTTP response header field;
registers it in the same registry. IANA has registered it in the same registry.
6.1. Accept-CH 6.1. Accept-CH
o Header field name: Accept-CH Header field name: Accept-CH
o Applicable protocol: HTTP
o Status: experimental
o Author/Change controller: IETF
o Specification document(s): Section 3.1 of this document
o Related information: for Client Hints
7. References Applicable protocol: HTTP
7.1. Normative References Status: experimental
[CLIENT-HINTS-INFRASTRUCTURE] Author/Change controller: IETF
Weiss, Y., "Client Hints Infrastructure", n.d.,
<https://wicg.github.io/client-hints-infrastructure/>.
[I-D.ietf-httpbis-header-structure] Specification document(s): Section 3.1 of this RFC
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", draft-ietf-httpbis-header-structure-19 (work in Related information: for Client Hints
progress), June 2020.
7. References
7.1. Normative References
[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 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864, Procedures for Message Header Fields", BCP 90, RFC 3864,
DOI 10.17487/RFC3864, September 2004, DOI 10.17487/RFC3864, September 2004,
<https://www.rfc-editor.org/info/rfc3864>. <https://www.rfc-editor.org/info/rfc3864>.
skipping to change at page 11, line 24 skipping to change at line 470
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, [RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
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>.
[RFC8941] Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/info/rfc8941>.
7.2. Informative References 7.2. Informative References
[FETCH] van Kesteren, A., "Fetch", n.d., [CLIENT-HINTS-INFRASTRUCTURE]
Weiss, Y., "Client Hints Infrastructure", July 2020,
<https://wicg.github.io/client-hints-infrastructure/>.
[FETCH] WHATWG, "Fetch - Living Standard",
<https://fetch.spec.whatwg.org/>. <https://fetch.spec.whatwg.org/>.
[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", August
<https://wicg.github.io/ua-client-hints/>. 2020, <https://wicg.github.io/ua-client-hints/>.
7.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] http://httpwg.github.io/
[3] https://github.com/httpwg/http-extensions/labels/client-hints
[4] https://wicg.github.io/savedata/#save-data-request-header-field
Appendix A. Changes
A.1. Since -00
o Issue 168 (make Save-Data extensible) updated ABNF.
o Issue 163 (CH review feedback) editorial feedback from httpwg
list.
o Issue 153 (NetInfo API citation) added normative reference.
A.2. Since -01
o Issue 200: Moved Key reference to informative.
o Issue 215: Extended passive fingerprinting and mitigation
considerations.
o Changed document status to experimental.
A.3. Since -02
o Issue 239: Updated reference to CR-css-values-3
o Issue 240: Updated reference for Network Information API
o Issue 241: Consistency in IANA considerations
o Issue 250: Clarified Accept-CH
A.4. Since -03
o Issue 284: Extended guidance for Accept-CH
o Issue 308: Editorial cleanup
o Issue 306: Define Accept-CH-Lifetime
A.5. Since -04
o Issue 361: Removed Downlink
o Issue 361: Moved Key to appendix, plus other editorial feedback
A.6. Since -05
o Issue 372: Scoped CH opt-in and delivery to secure transports
o Issue 373: Bind CH opt-in to origin
A.7. Since -06
o Issue 524: Save-Data is now defined by NetInfo spec, dropping
o PR 775: Removed specific features to be defined in other
specifications
A.8. Since -07
o Issue 761: Clarified that the defined headers are response
headers.
o Issue 730: Replaced Key reference with Variants.
o Issue 700: Replaced ABNF with structured headers.
o PR 878: Removed Accept-CH-Lifetime based on feedback at IETF 105
A.9. Since -08
o PR 985: Describe the bytesize cost of hints.
o PR 776: Add Sec- and CH- prefix considerations.
o PR 1001: Clear CH persistence when cookies are cleared.
A.10. Since -09
o PR 1064: Fix merge issues with "cost of sending hints".
A.11. Since -10
o PR 1072: LC feedback from Julian Reschke.
o PR 1080: Improve list style.
o PR 1082: Remove section mentioning Variants.
o PR 1097: Editorial feedback from mnot.
o PR 1131: Remove unused references.
o PR 1132: Remove nested list.
A.12. Since -11
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.
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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