draft-ietf-httpbis-client-hints-10.txt   draft-ietf-httpbis-client-hints-11.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: August 21, 2020 February 18, 2020 Expires: September 12, 2020 March 11, 2020
HTTP Client Hints HTTP Client Hints
draft-ietf-httpbis-client-hints-10 draft-ietf-httpbis-client-hints-11
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, clients are often unwilling to send those request headers,
because it is not clear whether they will be used, and sending them because it is not clear whether they will be used, and sending them
impacts both performance and privacy. impacts both performance and privacy.
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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 August 21, 2020. This Internet-Draft will expire on September 12, 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
3.1.1. Interaction with Caches . . . . . . . . . . . . . . . 6 3.2. Interaction with Caches . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6 4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
4.1. Information Exposure . . . . . . . . . . . . . . . . . . 6 4.1. Information Exposure . . . . . . . . . . . . . . . . . . 6
4.2. Deployment and Security Risks . . . . . . . . . . . . . . 8 4.2. Deployment and Security Risks . . . . . . . . . . . . . . 8
4.3. Abuse Detection . . . . . . . . . . . . . . . . . . . . . 8 4.3. Abuse Detection . . . . . . . . . . . . . . . . . . . . . 8
5. Cost of Sending Hints . . . . . . . . . . . . . . . . . . . . 9 5. Cost of Sending Hints . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 9 6.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 7. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9 7.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 10 7.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 9
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Interaction with Variants Response Header Field . . 11 7.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 11 7.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 11 7.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 11 7.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 11 7.8. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 11 7.9. Since -08 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 11 7.10. Since -09 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 11 7.11. Since -10 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 12 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
B.8. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 12 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
B.9. Since -08 . . . . . . . . . . . . . . . . . . . . . . . . 12 9.1. Normative References . . . . . . . . . . . . . . . . . . 11
B.10. Since -09 . . . . . . . . . . . . . . . . . . . . . . . . 12 9.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 12 9.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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. Applications that want well as dynamic user and client preferences. Historically,
to allow the server to optimize content delivery and user experience applications that wanted to allow the server to optimize content
based on such capabilities have, historically, had to rely on passive delivery and user experience based on such capabilities had to rely
identification (e.g., by matching User-Agent (Section 5.5.3 of on passive identification (e.g., by matching User-Agent
[RFC7231]) header field against an established database of client (Section 5.5.3 of [RFC7231]) header field against an established
signatures), used HTTP cookies and URL parameters, or use some database of client signatures), used HTTP cookies [RFC6265] and URL
combination of these and similar mechanisms to enable ad hoc content parameters, or use some combination of these and similar mechanisms
negotiation. to enable ad hoc content negotiation.
Such techniques are expensive to setup and maintain, are not portable Such techniques are expensive to setup and maintain, and are not
across both applications and servers, and make it hard to reason for portable across both applications and servers. They also make it
both client and server about which data is required and is in use hard for both client and server to reason about which data is
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 client 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, proactive content negotiation requires clients to send these However, traditional proactive content negotiation techniques often
request headers prolifically. This causes performance concerns mean that clients send these request headers prolifically. This
(because it creates "bloat" in requests), as well as privacy issues; causes performance concerns (because it creates "bloat" in requests),
passively providing such information allows servers to silently as well as privacy issues; passively providing such information
fingerprint the user agent. allows servers to silently fingerprint the user agent.
This document defines a new response header, Accept-CH, that allows This document defines a new response header, Accept-CH, that allows
an origin server to explicitly ask that clients send these headers in an origin server to explicitly ask that clients send these headers in
requests. It also defines guidelines for content negotiation requests. It also defines guidelines for content negotiation
mechanisms that use it, colloquially referred to as Client Hints. mechanisms that use it, colloquially referred to as Client Hints.
Client Hints mitigate the performance concerns by assuring that Client Hints mitigate performance concerns by assuring that clients
clients will only send the request headers when they're actually will only send the request headers when they're actually going to be
going to be used, and the privacy concerns of passive fingerprinting used, and privacy concerns of passive fingerprinting by requiring
by requiring explicit opt-in and disclosure of required headers by explicit opt-in and disclosure of required headers by the server
the server through the use of the Accept-CH response header. through the use of the Accept-CH response header.
This document defines the Client Hints infrastructure, a framework This document defines Client Hints, a framework that enables servers
that enables servers to opt-in to specific proactive content to opt-in to specific proactive content negotiation features,
negotiation features, which will enable them to adapt their content adapting their content accordingly. However, it does not define any
accordingly. However, it does not define any specific features that specific features that will use that infrastructure. Those features
will use that infrastructure. Those features will be defined in will be defined in their respective specifications.
their respective specifications.
One example of such a feature is the User Agent Client Hints feature
[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] with the list rule extension defined in [RFC7230], [RFC5234].
Appendix B. It includes by reference the DIGIT rule from [RFC5234]
and the OWS and field-name rules from [RFC7230].
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 configuration data that can be used used by HTTP clients to indicate data that can be used by the server
by the server to select an appropriate response. Each one conveys to select an appropriate response. Each one conveys client
client preferences that the server can use to adapt and optimize the preferences that the server can use to adapt and optimize the
response. response.
2.1. Sending Client Hints 2.1. Sending Client Hints
Clients control which Client Hints are sent in requests, based on Clients choose what Client Hints to send in a request based on their
their default settings, user configuration, and server preferences. default settings, user configuration, and server preferences
The client and server can use an opt-in mechanism outlined below to expressed in "Accept-CH". The client and server can use an opt-in
negotiate which fields should be sent to allow for efficient content mechanism outlined below to negotiate which header fields need to be
adaption, and optionally use additional mechanisms to negotiate sent to allow for efficient content adaption, and optionally use
delegation policies that control access of third parties to same additional mechanisms to negotiate delegation policies that control
fields. access of third parties to same header fields.
Implementers should be aware of the passive fingerprinting Implementers SHOULD 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 "Security Considerations" section of the considerations outlined in the Security Considerations
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.
Further, depending on the hint used, the server can generate Furthermore, the server can generate additional response header
additional response header fields to convey related values to aid fields (as specified by the hint or hints in use) that convey related
client processing. values to aid client processing.
3. Advertising Server Support 3. Advertising Server Support
Servers can advertise support for Client Hints using the mechnisms 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 or the equivalent HTML meta The Accept-CH response header field indicates server support for the
element with http-equiv attribute ([HTML]) indicate server support hints indicated in its value.
for particular hints indicated in its value.
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 3.7 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 advertising support for When a client receives an HTTP response containing "Accept-CH", it
provided list of Clients Hints, it SHOULD process it as origin indicates that the origin opts-in to receive the indicated request
([RFC6454]) opt-in to receive Client Hint header fields advertised in header fields for subsequent same-origin requests. The opt-in MUST
the field-value, for subsequent same-origin requests. be ignored if delivered over non-secure transport or for an origin
with a scheme different from HTTPS. It SHOULD be persisted and bound
o The opt-in MUST be delivered over a secure transport. to the origin to enable delivery of Client Hints on subsequent
requests to the server's origin.
o The opt-in SHOULD be persisted and bound to the origin to enable For example:
delivery of Client Hints on subsequent requests to the server's
origin, and MUST NOT be persisted for an origin that isn't HTTPS.
Accept-CH: Sec-CH-Example, Sec-CH-Example-2 Accept-CH: Sec-CH-Example, Sec-CH-Example-2
Accept-CH: Sec-CH-Example-3 Accept-CH: Sec-CH-Example-3
For example, based on the Accept-CH example above, which is received Based on the Accept-CH example above, which is received in response
in response to a user agent navigating to "https://example.com", and to a user agent navigating to "https://example.com", and delivered
delivered over a secure transport: a user agent SHOULD persist an over a secure transport: a user agent will have to persist an Accept-
Accept-CH preference bound to "https://example.com" and use it for CH preference bound to "https://example.com" and use it for user
user agent navigations to "https://example.com" and any same-origin agent navigations to "https://example.com" and any same-origin
resource requests initiated by the page constructed from the resource requests initiated by the page constructed from the
navigation's response. This preference SHOULD NOT extend to resource navigation's response. This preference will not extend to resource
requests initiated to "https://example.com" from other origins. requests initiated to "https://example.com" from other origins.
3.1.1. Interaction with Caches 3.2. Interaction with Caches
When selecting an optimized response based on one or more Client When selecting a response based on one or more Client Hints, and if
Hints, and if the resource is cacheable, the server needs to generate the resource is cacheable, the server needs to generate a Vary
a Vary response header field ([RFC7234]) to indicate which hints can response header field ([RFC7234]) to indicate which hints can affect
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- Above example indicates that the cache key needs to include the Sec-
CH-Example header field. 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- Above example indicates that the cache key needs to include the Sec-
CH-Example and Sec-CH-Example-2 header fields. 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 proactive
content negotiation. Such information may 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 to the capability of that request's as a request header is equivalent to the capability of that request's
origin to access that information by other means and transmit it to origin to access that information by other means and transmit it to
itself. itself.
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 SHOULD take into account the following aspects of the
information exposed: information exposed:
o Entropy o Entropy - Exposing highly granular data can be used to help
identify users across multiple requests to different origins.
* Exposing highly granular data may help identify users across Reducing the set of header field values that can be expressed, or
multiple requests to different origins. Reducing the set of restricting them to an enumerated range where the advertised value
field values that can be expressed, or restricting them to an is close but is not an exact representation of the current value,
enumerated range where the advertised value is close but is not can improve privacy and reduce risk of linkability by ensuring
an exact representation of the current value, can improve that the same value is sent by multiple users.
privacy and reduce risk of linkability by ensuring that the o Sensitivity - The feature SHOULD NOT expose user sensitive
same value is sent by multiple users. information. To that end, information available to the
o Sensitivity application, but gated behind specific user actions (e.g. a
permission prompt or user activation) SHOULD NOT be exposed as a
* The feature SHOULD NOT expose user sensitive information. To Client Hint.
that end, information available to the application, but gated o Change over time - The feature SHOULD NOT expose user information
behind specific user actions (e.g. a permission prompt or user that changes over time, unless the state change itself is also
activation) SHOULD NOT be exposed as a Client Hint. exposed (e.g. through JavaScript callbacks).
o Change over time
* The feature SHOULD NOT expose user information that changes
over time, unless the state change itself is also 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 SHOULD 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
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 MAY provide user choice mechanisms so that users may o Implementers MAY provide user choice mechanisms so that users can
balance privacy concerns with bandwidth limitations. However, balance privacy concerns with bandwidth limitations. However,
implementers should also be aware that explaining the privacy implementers SHOULD also be aware that explaining the privacy
implications of passive fingerprinting to users may be implications of passive fingerprinting to users can be
challenging. challenging.
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 Implementers SHOULD support Client Hints opt-in mechanisms and MUST
clear persisted opt-in preferences when any one of site data, clear persisted opt-in preferences when any one of site data,
browsing history, browsing cache, cookies, or similar, are cleared. browsing history, browsing cache, cookies, or similar, are 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 field name o Potential conflicts due to existing use of header field name
o Properties of the data communicated in 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 should 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 should 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
skipping to change at page 9, line 20 skipping to change at page 9, line 12
opting in to receive Client Hints, and SHOULD NOT opt-in to receive opting in to receive Client Hints, and SHOULD NOT opt-in to receive
hints unless they are to be used for content adaptation purposes. 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
This document defines the "Accept-CH" HTTP response 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: standard
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. Changes
7.1. Normative References
[FETCH] van Kesteren, A., "Fetch", n.d.,
<https://fetch.spec.whatwg.org/>.
[HTML] Hickson, I., Pieters, S., van Kesteren, A., Jaegenstedt,
P., and D. Denicola, "HTML", n.d.,
<https://html.spec.whatwg.org/>.
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-15 (work in progress),
January 2020.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[VARIANTS]
Nottingham, M., "HTTP Representation Variants", draft-
ietf-httpbis-variants-06 (work in progress), November
2019.
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
Appendix A. Interaction with Variants Response Header Field
Client Hints may be combined with Variants response header field
[VARIANTS] to enable fine-grained control of the cache key for
improved cache efficiency. Features that define Client Hints will
need to specify the related variants algorithms as described in
Section 6 of [VARIANTS].
Appendix B. Changes
B.1. Since -00 7.1. 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.
B.2. Since -01 7.2. 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.
B.3. Since -02 7.3. Since -02
o Issue 239: Updated reference to CR-css-values-3 o Issue 239: Updated reference to CR-css-values-3
o Issue 240: Updated reference for Network Information API o Issue 240: Updated reference for Network Information API
o Issue 241: Consistency in IANA considerations o Issue 241: Consistency in IANA considerations
o Issue 250: Clarified Accept-CH o Issue 250: Clarified Accept-CH
B.4. Since -03 7.4. Since -03
o Issue 284: Extended guidance for Accept-CH o Issue 284: Extended guidance for Accept-CH
o Issue 308: Editorial cleanup o Issue 308: Editorial cleanup
o Issue 306: Define Accept-CH-Lifetime o Issue 306: Define Accept-CH-Lifetime
B.5. Since -04 7.5. Since -04
o Issue 361: Removed Downlink o Issue 361: Removed Downlink
o Issue 361: Moved Key to appendix, plus other editorial feedback o Issue 361: Moved Key to appendix, plus other editorial feedback
B.6. Since -05 7.6. Since -05
o Issue 372: Scoped CH opt-in and delivery to secure transports o Issue 372: Scoped CH opt-in and delivery to secure transports
o Issue 373: Bind CH opt-in to origin o Issue 373: Bind CH opt-in to origin
B.7. Since -06 7.7. Since -06
o Issue 524: Save-Data is now defined by NetInfo spec, dropping o Issue 524: Save-Data is now defined by NetInfo spec, dropping
o PR 775: Removed specific features to be defined in other o PR 775: Removed specific features to be defined in other
specifications specifications
B.8. Since -07 7.8. Since -07
o Issue 761: Clarified that the defined headers are response o Issue 761: Clarified that the defined headers are response
headers. headers.
o Issue 730: Replaced Key reference with Variants. o Issue 730: Replaced Key reference with Variants.
o Issue 700: Replaced ABNF with structured headers. o Issue 700: Replaced ABNF with structured headers.
o PR 878: Removed Accept-CH-Lifetime based on feedback at IETF 105 o PR 878: Removed Accept-CH-Lifetime based on feedback at IETF 105
B.9. Since -08 7.9. Since -08
o PR 985: Describe the bytesize cost of hints. o PR 985: Describe the bytesize cost of hints.
o PR 776: Add Sec- and CH- prefix considerations. o PR 776: Add Sec- and CH- prefix considerations.
o PR 1001: Clear CH persistence when cookies are cleared. o PR 1001: Clear CH persistence when cookies are cleared.
B.10. Since -09 7.10. Since -09
o PR 1064: Fix merge issues with "cost of sending hints". o PR 1064: Fix merge issues with "cost of sending hints".
7.11. 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.
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.
9. References
9.1. Normative References
[FETCH] van Kesteren, A., "Fetch", n.d.,
<https://fetch.spec.whatwg.org/>.
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Field Values for
HTTP", draft-ietf-httpbis-header-structure-16 (work in
progress), March 2020.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[UA-CH] West, M. and Y. Weiss, "User Agent Client Hints", n.d.,
<https://wicg.github.io/ua-client-hints/>.
9.3. 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
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
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