HTTP Working Group I. Grigorik
Internet-Draft Google
Intended status: Experimental March 11, 2019
Expires: September 12, 2019
HTTP Client Hints
draft-ietf-httpbis-client-hints-07
Abstract
HTTP defines proactive content negotiation to allow servers to select
the appropriate response for a given request, based upon the user
agent's characteristics, as expressed in request headers. In
practice, clients are often unwilling to send those request headers,
because it is not clear whether they will be used, and sending them
impacts both performance and privacy.
This document defines two response headers, Accept-CH and Accept-CH-
Lifetime, that servers can use to advertise their use of request
headers for proactive content negotiation, along with a set of
guidelines for the creation of such headers, colloquially known as
"Client Hints."
It also defines an initial set of 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/.
Working Group information can be found at http://httpwg.github.io/;
source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/client-hints.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 12, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4
2. Client Hint Request Header Fields . . . . . . . . . . . . . . 4
2.1. Sending Client Hints . . . . . . . . . . . . . . . . . . 4
2.2. Server Processing of Client Hints . . . . . . . . . . . . 5
2.2.1. Advertising Support via Accept-CH Header Field . . . 5
2.2.2. The Accept-CH-Lifetime Header Field . . . . . . . . . 5
2.2.3. Interaction with Caches . . . . . . . . . . . . . . . 6
3. Security Considerations . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
4.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Accept-CH-Lifetime . . . . . . . . . . . . . . . . . . . 8
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Normative References . . . . . . . . . . . . . . . . . . 8
5.2. Informative References . . . . . . . . . . . . . . . . . 9
5.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Interaction with Key Response Header Field . . . . . 9
Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 10
B.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.8. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 11
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Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
There are thousands of different devices accessing the web, each with
different device capabilities and preference information. These
device capabilities include hardware and software characteristics, as
well as dynamic user and client preferences.
One way to infer some of these capabilities is through User-Agent
(Section 5.5.3 of [RFC7231]) header field detection against an
established database of client signatures. However, this technique
requires acquiring such a database, integrating it into the serving
path, and keeping it up to date. However, even once this
infrastructure is deployed, user agent sniffing has numerous
limitations:
o User agent detection cannot reliably identify all static variables
o User agent detection cannot infer any dynamic client preferences
o User agent detection requires an external device database
o User agent detection is not cache friendly
A popular alternative strategy is to use HTTP cookies ([RFC6265]) to
communicate some information about the user agent. However, this
approach is also not cache friendly, bound by same origin policy, and
often imposes additional client-side latency by requiring JavaScript
execution to create and manage HTTP cookies.
Proactive content negotiation (Section 3.4.1 of [RFC7231]) offers an
alternative approach; user agents use specified, well-defined request
headers to advertise their capabilities and characteristics, so that
servers can select (or formulate) an appropriate response.
However, proactive content negotiation requires clients to send these
request headers prolifically. This causes performance concerns
(because it creates "bloat" in requests), as well as privacy issues;
passively providing such information allows servers to silently
fingerprint the user agent.
This document defines a new response header, Accept-CH, that allows
an origin server to explicitly ask that clients send these headers in
requests, for a period of time bounded by the Accept-CH-Lifetime
response header. It also defines guidelines for content negotiation
mechanisms that use it, colloquially referred to as Client Hints.
Client Hints mitigate the performance concerns by assuring that
clients will only send the request headers when they're actually
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going to be used, and the privacy concerns of passive fingerprinting
by requiring explicit opt-in and disclosure of required headers by
the server through the use of the Accept-CH response header.
This document defines the Client Hints infrastructure, a framework
that enables servers to opt-in to specific proactive content
negotiation features, which will enable them to adapt their content
accordingly. However, it does not define any specific features that
will use that infrastructure. Those features will be defined in
their respective specifications.
This document does not supersede or replace the User-Agent header
field. Existing device detection mechanisms can continue to use both
mechanisms if necessary. By advertising user agent capabilities
within a request header field, Client Hints allow for cache friendly
and proactive content negotiation.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234] with the list rule extension defined in [RFC7230],
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
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
by the server to select an appropriate response. Each one conveys
client preferences that the server can use to adapt and optimize the
response.
2.1. Sending Client Hints
Clients control which Client Hints are sent in requests, based on
their default settings, user configuration, and server preferences.
The client and server can use an opt-in mechanism outlined below to
negotiate which fields should be sent to allow for efficient content
adaption, and optionally use additional mechanisms to negotiate
delegation policies that control access of third parties to same
fields.
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Implementers should be aware of the passive fingerprinting
implications when implementing support for Client Hints, and follow
the considerations outlined in "Security Considerations" section of
this document.
2.2. Server Processing of Client Hints
When presented with a request that contains one or more client hint
header fields, servers can optimize the response based upon the
information in them. When doing so, and if the resource is
cacheable, the server MUST also generate a Vary response header field
(Section 7.1.4 of [RFC7231]) to indicate which hints can affect the
selected response and whether the selected response is appropriate
for a later request.
Further, depending on the hint used, the server can generate
additional response header fields to convey related values to aid
client processing.
2.2.1. Advertising Support via Accept-CH Header Field
Servers can advertise support for Client Hints using the Accept-CH
header field or an equivalent HTML meta element with http-equiv
attribute ([HTML5]).
Accept-CH = #field-name
For example:
Accept-CH: Sec-CH-Example, Sec-CH-Example-2
When a client receives an HTTP response advertising support for
Client Hints, it should process it as origin ([RFC6454]) opt-in to
receive Client Hint header fields advertised in the field-value. The
opt-in MUST be delivered over a secure transport.
For example, based on Accept-CH example above, a user agent could
append the Sec-CH-Example and Sec-CH-Example-2 header fields to all
same-origin resource requests initiated by the page constructed from
the response.
2.2.2. The Accept-CH-Lifetime Header Field
Servers can ask the client to remember the set of Client Hints that
the server supports for a specified period of time, to enable
delivery of Client Hints on subsequent requests to the server's
origin ([RFC6454]).
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Accept-CH-Lifetime = #delta-seconds
When a client receives an HTTP response that contains Accept-CH-
Lifetime header field, the field-value indicates that the Accept-CH
preference SHOULD be persisted and bound to the origin, and be
considered stale after response's age ([RFC7234], section 4.2) is
greater than the specified number of seconds. The preference MUST be
delivered over a secure transport, 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-3
Accept-CH-Lifetime: 86400
For example, based on the Accept-CH and Accept-CH-Lifetime example
above, which is received in response to a user agent navigating to
"https://example.com", and delivered over a secure transport: a user
agent SHOULD persist an Accept-CH preference bound to
"https://example.com" for up to 86400 seconds (1 day), and use it for
user agent navigations to "https://example.com" and any same-origin
resource requests initiated by the page constructed from the
navigation's response. This preference SHOULD NOT extend to resource
requests initiated to "https://example.com" from other origins.
If Accept-CH-Lifetime occurs in a message more than once, the last
value overrides all previous occurrences.
2.2.3. Interaction with Caches
When selecting an optimized response based on one or more Client
Hints, and if the resource is cacheable, the server needs to generate
a Vary response header field ([RFC7234]) to indicate which hints can
affect the selected response and whether the selected response is
appropriate for a later request.
Vary: Sec-CH-Example
Above example indicates that the cache key needs to include the Sec-
CH-Example header field.
Vary: Sec-CH-Example, Sec-CH-Example-2
Above example indicates that the cache key needs to include the Sec-
CH-Example and Sec-CH-Example-2 header fields.
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3. Security Considerations
The request header fields defined in this document, and those that
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
information that is otherwise not available to the application via
other means, such as using HTML, CSS, or JavaScript. Further,
sending highly granular data, such as image and viewport width may
help identify users across multiple requests. 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. However, such precautions can still be insufficient
for some types of data, especially data that can change over time.
Implementers ought to consider both user and server controlled
mechanisms and policies to control which Client Hints header fields
are advertised:
o Implementers SHOULD restrict delivery of some or all Client Hints
header fields to the opt-in origin only, unless the opt-in origin
has explicitly delegated permission to another origin to request
Client Hints header fields.
o Implementers MAY provide user choice mechanisms so that users may
balance privacy concerns with bandwidth limitations. However,
implementers should also be aware that explaining the privacy
implications of passive fingerprinting to users may be
challenging.
o Implementations specific to certain use cases or threat models MAY
avoid transmitting some or all of Client Hints header fields. For
example, avoid transmission of header fields that can carry higher
risks of linkability.
Implementers SHOULD support Client Hints opt-in mechanisms and MUST
clear persisted opt-in preferences when any one of site data,
browsing history, browsing cache, or similar, are cleared.
4. IANA Considerations
This document defines the "Accept-CH" and "Accept-CH-Lifetime" HTTP
response fields, and registers them in the Permanent Message Header
Fields registry.
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4.1. Accept-CH
o Header field name: Accept-CH
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 2.2.1 of this document
o Related information: for Client Hints
4.2. Accept-CH-Lifetime
o Header field name: Accept-CH-Lifetime
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 2.2.2 of this document
o Related information: for Client Hints
5. References
5.1. Normative References
[HTML5] Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
Navara, E., O'Connor, T., and S. Pfeiffer, "HTML5",
World Wide Web Consortium Recommendation REC-
html5-20141028, October 2014,
<http://www.w3.org/TR/2014/REC-html5-20141028>.
[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>.
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[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>.
5.2. 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,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
Appendix A. Interaction with Key Response Header Field
Client Hints may be combined with Key response header field ([KEY])
to enable fine-grained control of the cache key for improved cache
efficiency. For example, the server can return the following set of
instructions:
Key: Sec-CH-Example;partition=1.5:2.5:4.0
Above example indicates that the cache key needs to include the value
of the Sec-CH-Example header field with three segments: less than
1.5, 1.5 to less than 2.5, and 4.0 or greater.
Key: Width;Sec-CH-Example=320
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Above example indicates that the cache key needs to include the value
of the Sec-CH-Example header field and be partitioned into groups of
320: 0-320, 320-640, and so on.
Appendix B. Changes
B.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.
B.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.
B.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
B.4. Since -03
o Issue 284: Extended guidance for Accept-CH
o Issue 308: Editorial cleanup
o Issue 306: Define Accept-CH-Lifetime
B.5. Since -04
o Issue 361: Removed Downlink
o Issue 361: Moved Key to appendix, plus other editorial feedback
B.6. Since -05
o Issue 372: Scoped CH opt-in and delivery to secure transports
o Issue 373: Bind CH opt-in to origin
B.7. Since -06
o Issue 524: Save-Data is now defined by NetInfo spec, dropping
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B.8. Since -07
o Removed specific features to be defined in other specifications
Acknowledgements
Thanks to Mark Nottingham, Julian Reschke, Chris Bentzel, Yoav Weiss,
Ben Greenstein, Tarun Bansal, Roy Fielding, Vasiliy Faronov, Ted
Hardie, Jonas Sicking, and numerous other members of the IETF HTTP
Working Group for invaluable help and feedback.
Author's Address
Ilya Grigorik
Google
Email: ilya@igvita.com
URI: https://www.igvita.com/
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