draft-ietf-httpbis-client-hints-08.txt   draft-ietf-httpbis-client-hints-latest.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: May 21, 2020 November 18, 2019 Expires: July 23, 2020 January 20, 2020
HTTP Client Hints HTTP Client Hints
draft-ietf-httpbis-client-hints-08 draft-ietf-httpbis-client-hints-08
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,
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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 May 21, 2020. This Internet-Draft will expire on July 23, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 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
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4
2. Client Hint Request Header Fields . . . . . . . . . . . . . . 4 2. Client Hint Request Header Fields . . . . . . . . . . . . . . 4
2.1. Sending Client Hints . . . . . . . . . . . . . . . . . . 4 2.1. Sending Client Hints . . . . . . . . . . . . . . . . . . 4
2.2. Server Processing of Client Hints . . . . . . . . . . . . 5 2.2. Server Processing of Client Hints . . . . . . . . . . . . 5
3. Advertising Server Support . . . . . . . . . . . . . . . . . 5 3. Advertising Server Support . . . . . . . . . . . . . . . . . 5
3.1. The Accept-CH Response Header Field . . . . . . . . . . . 5 3.1. The Accept-CH Response Header Field . . . . . . . . . . . 5
3.1.1. Interaction with Caches . . . . . . . . . . . . . . . 6 3.1.1. Interaction with Caches . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6 4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 4.1. Information Exposure . . . . . . . . . . . . . . . . . . 6
5.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 7 5. Cost of Sending Hints . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 5.1. Deployment and Security Risks . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 7 5.2. Abuse Detection . . . . . . . . . . . . . . . . . . . . . 9
6.2. Informative References . . . . . . . . . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9 6.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Interaction with Variants Response Header Field . . 9 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 9 7.1. Normative References . . . . . . . . . . . . . . . . . . 9
B.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 9 7.2. Informative References . . . . . . . . . . . . . . . . . 10
B.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 9 7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 10
B.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 9 Appendix A. Interaction with Variants Response Header Field . . 11
B.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 9 Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 11
B.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 10 B.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 11
B.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 10 B.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 11
B.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 10 B.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 11
B.8. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 10 B.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 11
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10 B.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 B.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 11
B.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 12
B.8. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 12
B.9. Since -08 . . . . . . . . . . . . . . . . . . . . . . . . 12
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 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. Applications that want
to allow the server to optimize content delivery and user experience to allow the server to optimize content delivery and user experience
based on such capabilities have, historically, had to rely on passive based on such capabilities have, historically, had to rely on passive
identification (e.g., by matching User-Agent (Section 5.5.3 of identification (e.g., by matching User-Agent (Section 5.5.3 of
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client processing. 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 mechnisms
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 or the equivalent HTML meta
element with http-equiv attribute ([HTML5]) indicate server support element with http-equiv attribute ([HTML]) indicate server support
for particular 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 3.7 of [I-D.ietf-httpbis-header-structure]). Its ABNF is:
Accept-CH = sh-list Accept-CH = sh-list
For example: For example:
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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
The request header fields defined in this document, and those that 4.1. Information Exposure
extend it, expose information about the user's environment to enable
proactive content negotiation. Such information may reveal new
information about the user and implementers ought to consider the
following considerations, recommendations, and best practices.
Transmitted Client Hints header fields SHOULD NOT provide new Request header fields used in features relying on this document
information that is otherwise not available to the application via expose information about the user's environment to enable proactive
other means, such as using HTML, CSS, or JavaScript. Further, content negotiation. Such information may reveal new information
sending highly granular data, such as image and viewport width may about the user and implementers ought to consider the following
help identify users across multiple requests. Reducing the set of considerations, recommendations, and best practices.
field values that can be expressed, or restricting them to an
enumerated range where the advertised value is close but is not an The underlying assumption is that exposing information about the user
exact representation of the current value, can improve privacy and as a request header is equivalent to the capability of that request's
reduce risk of linkability by ensuring that the same value is sent by origin to access that information by other means and transmit it to
multiple users. However, such precautions can still be insufficient itself.
for some types of data, especially data that can change over time.
Therefore, features relying on this document to define Client Hint
headers MUST NOT provide new information that is otherwise not
available to the application via other means, such as existing
request headers, HTML, CSS, or JavaScript.
Such features SHOULD take into account the following aspects of the
information exposed:
o Entropy
* Exposing highly granular data may help identify users across
multiple requests to different origins. Reducing the set of
field values that can be expressed, or restricting them to an
enumerated range where the advertised value is close but is not
an exact representation of the current value, 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 application, but gated
behind specific user actions (e.g. a permission prompt or user
activation) SHOULD NOT be exposed as a Client Hint.
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
space between low-entropy, non-sensitive and static information (e.g.
user agent information), and high-entropy, sensitive and dynamic
information (e.g. geolocation). User agents SHOULD consider the
value provided by a particular feature vs these considerations, and
MAY have different policies regarding that tradeoff on a per-feature
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 may
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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 may 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, or similar, are cleared. browsing history, browsing cache, cookies, or similar, are cleared.
5. IANA Considerations 5. Cost of Sending Hints
While HTTP header compression schemes reduce the cost of adding HTTP
header fields, sending Client Hints to the server incurs an increase
in request byte size. Servers SHOULD take that into account when
opting in to receive Client Hints, and SHOULD NOT opt-in to receive
hints unless they are to be used for content adaptation purposes.
Due to request byte size increase, features relying on this document
to define Client Hints MAY consider restricting sending those hints
to certain request destinations [FETCH], where they are more likely
to be useful.
5.1. Deployment and Security Risks
Deployment of new request headers requires several considerations:
o Potential conflicts due to existing use of field name
o Properties of the data communicated in field value
Authors of new Client Hints are advised to carefully consider whether
they should be able to be added by client-side content (e.g.,
scripts), or whether they should be exclusively set by the user
agent. In the latter case, the Sec- prefix on the header field name
has the effect of preventing scripts and other application content
from setting them in user agents. Using the "Sec-" prefix signals to
servers that the user agent - and not application content - generated
the values. See [FETCH] for more information.
By convention, request headers that are client hints are encouraged
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.
Doing so makes them easier to identify programmatically (e.g., for
stripping unrecognised hints from requests by privacy filters).
5.2. Abuse Detection
A user agent that tracks access to active fingerprinting information
SHOULD consider emission of Client Hints headers similarly to the way
it would consider access to the equivalent API.
Research into abuse of Client Hints might look at how HTTP responses
that contain Client Hints differ from those with different values,
and from those without. This might be used to reveal which Client
Hints are in use, allowing researchers to further analyze that use.
6. IANA Considerations
This document defines the "Accept-CH" HTTP response field, and This document defines the "Accept-CH" HTTP response field, and
registers it in the Permanent Message Header Fields registry. registers it in the Permanent Message Header Fields registry.
5.1. 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
6. References 7. References
6.1. Normative References 7.1. Normative References
[HTML5] Hickson, I., Berjon, R., Faulkner, S., Leithead, T., [FETCH] van Kesteren, A., "Fetch", n.d.,
Navara, E., O&#039;Connor, T., and S. Pfeiffer, "HTML5", <https://fetch.spec.whatwg.org/>.
World Wide Web Consortium Recommendation REC-
html5-20141028, October 2014, [HTML] Hickson, I., Pieters, S., van Kesteren, A., Jaegenstedt,
<http://www.w3.org/TR/2014/REC-html5-20141028>. P., and D. Denicola, "HTML", n.d.,
<https://html.spec.whatwg.org/>.
[I-D.ietf-httpbis-header-structure] [I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP", Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-14 (work in progress), draft-ietf-httpbis-header-structure-14 (work in progress),
October 2019. October 2019.
[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>.
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[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>.
6.2. Informative References 7.2. Informative References
[KEY] Fielding, R. and M. Nottingham, "The Key HTTP Response
Header Field", draft-ietf-httpbis-key-01 (work in
progress), March 2016.
[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>.
[VARIANTS] [VARIANTS]
Nottingham, M., "HTTP Representation Variants", draft- Nottingham, M., "HTTP Representation Variants", draft-
ietf-httpbis-variants-05 (work in progress), March 2019. ietf-httpbis-variants-06 (work in progress), November
2019.
6.3. URIs 7.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/ [1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] http://httpwg.github.io/ [2] http://httpwg.github.io/
[3] https://github.com/httpwg/http-extensions/labels/client-hints [3] https://github.com/httpwg/http-extensions/labels/client-hints
Appendix A. Interaction with Variants Response Header Field Appendix A. Interaction with Variants Response Header Field
Client Hints may be combined with Variants response header field Client Hints may be combined with Variants response header field
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specifications specifications
B.8. Since -07 B.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
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.
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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