HTTP Working Group I. Grigorik
Internet-Draft Google
Intended status: Experimental April 18, 2017
Expires: October 20, 2017
HTTP Client Hints
draft-ietf-httpbis-client-hints-04
Abstract
An increasing diversity of Web-connected devices and software
capabilities has created a need to deliver optimized content for each
device.
This specification defines a set of HTTP request header fields,
colloquially known as Client Hints, to address this. They are
intended to be used as input to proactive content negotiation; just
as the Accept header field allows user agents to indicate what
formats they prefer, Client Hints allow user agents to indicate
device and agent specific preferences.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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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 October 20, 2017.
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Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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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 . . . . . . . . . . . . 4
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. Client Hints . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. The DPR header field . . . . . . . . . . . . . . . . . . 7
3.1.1. Confirming Selected DPR . . . . . . . . . . . . . . . 7
3.2. The Width header field . . . . . . . . . . . . . . . . . 8
3.3. The Viewport-Width header field . . . . . . . . . . . . . 8
3.4. The Downlink header field . . . . . . . . . . . . . . . . 8
3.5. The Save-Data header field . . . . . . . . . . . . . . . 8
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 10
6.2. Accept-CH-Lifetime . . . . . . . . . . . . . . . . . . . 11
6.3. Content-DPR . . . . . . . . . . . . . . . . . . . . . . . 11
6.4. Downlink . . . . . . . . . . . . . . . . . . . . . . . . 11
6.5. DPR . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.6. Save-Data . . . . . . . . . . . . . . . . . . . . . . . . 11
6.7. Viewport-Width . . . . . . . . . . . . . . . . . . . . . 12
6.8. Width . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 13
A.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 13
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A.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 14
A.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 14
A.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
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
imposes additional client-side latency by requiring JavaScript
execution to create and manage HTTP cookies.
This document defines a set of new request header fields that allow
user agent to perform proactive content negotiation (Section 3.4.1 of
[RFC7231]) by indicating device and agent specific preferences,
through a mechanism similar to the Accept header field which is used
to indicate preferred response formats.
Client Hints does not supersede or replace the User-Agent header
field. Existing device detection mechanisms can continue to use both
mechanisms if necessary. By advertising its capabilities within a
request header field, Client Hints allows for cache friendly and
proactive content negotiation.
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1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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/or preferences.
Implementers might provide user choice mechanisms so that users may
balance privacy concerns with bandwidth limitations. Implementations
specific to certain use cases or threat models might avoid
transmitting these headers altogether, or limit them to secure
contexts or authenticated sessions. Implementers should be aware
that explaining the privacy implications of passive fingerprinting or
network information disclosure may be challenging.
The client and server, or an intermediate proxy, can use an opt-in
mechanism to negotiate which fields should be reported to allow for
efficient content adaption.
2.2. Server Processing of Client Hints
When presented with a request that contains one or more client hint
headers, 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]), and optionally Key ([I-D.ietf-httpbis-key]), 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. For example, this specification defines "Content-
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DPR" response header field that needs to be returned by the server
when the "DPR" hint is used to select the response.
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 ([W3C.REC-html5-20141028]).
Accept-CH = #field-name
For example:
Accept-CH: DPR, Width, Viewport-Width
When a client receives Accept-CH, or if it is capable of processing
the HTML response and finds an equivalent HTML meta element, it can
treat it as a signal that the application is interested in receiving
specified request header fields that match the advertised field-
values; subresource requests initiated as a result of processing the
response from the server that includes the Accept-CH opt-in can
include the request header fields that match the advertised field-
values.
For example, based on Accept-CH example above, a user agent could
append DPR, Width, and Viewport-Width header fields to all
subresource 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 an origin-wide Accept-CH
preference for a specified period of time to enable delivery of
Client Hints on all subsequent requests to the origin, and on
subresource requests initiated as a result of processing a response
from the origin.
Accept-CH-Lifetime = #delta-seconds
The field-value indicates that the Accept-CH preference should be
considered stale after its age is greater than the specified number
of seconds.
Accept-CH: DPR, Viewport-Width
Accept-CH-Lifetime: 86400
For example, based on the Accept-CH and Accept-CH-Lifetime example
above, a user agent could persist an origin-wide Accept-CH preference
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for up to 86400 seconds (1 day). Then, if a request is initiated to
the same origin before the preference is stale (e.g. as a result of a
navigation to the origin, or fetching a subresource from the origin)
the client could append the requested header fields (DPR and
Viewport-Width in this example) to the request and any subresource
requests initiated as a result of processing a response from same
origin.
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: DPR
Above example indicates that the cache key needs to include the DPR
header field.
Vary: DPR, Width, Downlink
Above example indicates that the cache key needs to include the DPR,
Width, and Downlink header fields.
Client Hints MAY be combined with Key ([I-D.ietf-httpbis-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: DPR;partition=1.5:2.5:4.0
Above example indicates that the cache key needs to include the value
of the DPR header field with three segments: less than 1.5, 1.5 to
less than 2.5, and 4.0 or greater.
Key: Width;div=320
Above example indicates that the cache key needs to include the value
of the Width header field and be partitioned into groups of 320:
0-320, 320-640, and so on.
Key: Downlink;partition=0.5:1.0:3.0:5.0:10
Above example indicates that the cache key needs to include the
(Mbps) value of the Downlink header field with six segments: less
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than 0.5, 0.5 to less than 1.0, 1.0 to less than 3.0, 3.0 to less
than 5.0, 5.0 to less than 10; 10 or higher.
3. Client Hints
3.1. The DPR header field
The "DPR" request header field is a number that indicates the
client's current Device Pixel Ratio (DPR), which is the ratio of
physical pixels over CSS px (Section 5.2 of
[W3C.CR-css-values-3-20160929]) of the layout viewport (Section 9.1.1
of [CSS2]) on the device.
DPR = 1*DIGIT [ "." 1*DIGIT ]
If DPR occurs in a message more than once, the last value overrides
all previous occurrences.
3.1.1. Confirming Selected DPR
The "Content-DPR" response header field is a number that indicates
the ratio between physical pixels over CSS px of the selected image
response.
Content-DPR = 1*DIGIT [ "." 1*DIGIT ]
DPR ratio affects the calculation of intrinsic size of image
resources on the client - i.e. typically, the client automatically
scales the natural size of the image by the DPR ratio to derive its
display dimensions. As a result, the server MUST explicitly indicate
the DPR of the selected image response whenever the DPR hint is used,
and the client MUST use the DPR value returned by the server to
perform its calculations. In case the server returned Content-DPR
value contradicts previous client-side DPR indication, the server
returned value MUST take precedence.
Note that DPR confirmation is only required for image responses, and
the server does not need to confirm the resource width as this value
can be derived from the resource itself once it is decoded by the
client.
If Content-DPR occurs in a message more than once, the last value
overrides all previous occurrences.
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3.2. The Width header field
The "Width" request header field is a number that indicates the
desired resource width in physical px (i.e. intrinsic size of an
image). The provided physical px value is a number rounded to the
smallest following integer (i.e. ceiling value).
Width = 1*DIGIT
If the desired resource width is not known at the time of the request
or the resource does not have a display width, the Width header field
can be omitted. If Width occurs in a message more than once, the
last value overrides all previous occurrences.
3.3. The Viewport-Width header field
The "Viewport-Width" request header field is a number that indicates
the layout viewport width in CSS px. The provided CSS px value is a
number rounded to the smallest following integer (i.e. ceiling
value).
Viewport-Width = 1*DIGIT
If Viewport-Width occurs in a message more than once, the last value
overrides all previous occurrences.
3.4. The Downlink header field
The "Downlink" request header field is a number that indicates the
client's maximum downlink speed in megabits per second (Mbps).
Downlink = 1*DIGIT [ "." 1*DIGIT ]
If Downlink occurs in a message more than once, the minimum value
should be used to override other occurrences.
3.5. The Save-Data header field
The "Save-Data" request header field consists of one or more tokens
that indicate client's preference for reduced data usage, due to high
transfer costs, slow connection speeds, or other reasons.
Save-Data = sd-token *( OWS ";" OWS [sd-token] )
sd-token = token
This document defines the "on" sd-token value, which is used as a
signal indicating explicit user opt-in into a reduced data usage mode
on the client, and when communicated to origins allows them to
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deliver alternate content honoring such preference - e.g. smaller
image and video resources, alternate markup, and so on. New token
and extension token values can be defined by updates to this
specification.
4. Examples
For example, given the following request header fields:
DPR: 2.0
Width: 320
Viewport-Width: 320
The server knows that the device pixel ratio is 2.0, that the
intended display width of the requested resource is 160 CSS px (320
physical pixels at 2x resolution), and that the viewport width is 320
CSS px.
If the server uses above hints to perform resource selection for an
image asset, it must confirm its selection via the Content-DPR
response header to allow the client to calculate the appropriate
intrinsic size of the image response. The server does not need to
confirm resource width, only the ratio between physical pixels and
CSS px of the selected image resource:
Content-DPR: 1.0
The Content-DPR response header field indicates to the client that
the server has selected resource with DPR ratio of 1.0. The client
can use this information to perform additional processing on the
resource - for example, calculate the appropriate intrinsic size of
the image resource such that it is displayed at the correct
resolution.
Alternatively, the server could select an alternate resource based on
the maximum downlink speed advertised in the request header fields:
Downlink: 0.384
The server knows that the client's maximum downlink speed is
0.384Mbps (GPRS EDGE), and it can use this information to select an
optimized resource - for example, an alternate image asset,
stylesheet, HTML document, media stream, and so on.
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5. Security Considerations
The request header fields defined in this specification expose
information that is already available to Web applications in the
browser runtime itself (e.g., using JavaScript and CSS). For
example, the application can obtain viewport width, image display
width, and device pixel ratio via JavaScript, or through the use of
CSS media queries and unique resource URLs even if JavaScript is
disabled. However, servers that gather this information through such
mechanisms are typically observable (e.g., you can see that they're
using JavaScript to gather it), whereas servers' use of the header
fields introduced by this specification is not observable.
Section 2.1 discusses potential mitigations.
For example, sending Client Hints on all requests can make
information about the user's environment available to origins that
otherwise did not have access to this data, which may or may not be
the desired outcome - e.g. this may enable an image optimization
service to deliver a tailored asset, and it may reveal same
information about the user to other origins that may not have had
access to it before. Similarly, sending highly granular data, such
as image and viewport width may help identify users across multiple
requests. Restricting such field values to an enumerated range,
where the user agent advertises a threshold value that is close but
is not an exact representation of the current value, can help
mitigate the risk of such fingerprinting.
Implementers ought to provide mechanisms and policies to control how
and when such hints are advertised. For example, they could require
origin opt-in via Accept-CH; clear remembered opt-in, as set by
Accept-CH-Lifetime, when site data, browsing history, browsing cache,
or similar, are cleared; restrict delivery to same origin
subrequests; limit delivery to requests that already carry
identifying information (e.g. cookies); modify delivery policy when
in an "incognito" or a similar privacy mode; enable user
configuration and opt in, and so on.
6. IANA Considerations
This document defines the "Accept-CH", "DPR", "Width", and "Downlink"
HTTP request fields, "Content-DPR" HTTP response field, and registers
them in the Permanent Message Header Fields registry.
6.1. Accept-CH
o Header field name: Accept-CH
o Applicable protocol: HTTP
o Status: standard
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o Author/Change controller: IETF
o Specification document(s): Section 2.2.1 of this document
o Related information: for Client Hints
6.2. 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
6.3. Content-DPR
o Header field name: Content-DPR
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 3.1.1 of this document
o Related information: for Client Hints
6.4. Downlink
o Header field name: Downlink
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 3.4 of this document
o Related information: for Client Hints
6.5. DPR
o Header field name: DPR
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 3.1 of this document
o Related information: for Client Hints
6.6. Save-Data
o Header field name: Save-Data
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 3.5 of this document
o Related information: for Client Hints
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6.7. Viewport-Width
o Header field name: Viewport-Width
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 3.3 of this document
o Related information: for Client Hints
6.8. Width
o Header field name: Width
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 3.2 of this document
o Related information: for Client Hints
7. References
7.1. Normative References
[CSS2] Bos, B., Celic, T., Hickson, I., and H. Lie, "Cascading
Style Sheets Level 2 Revision 1 (CSS 2.1) Specification",
W3C Recommendation REC-CSS2-20110607, June 2011,
<http://www.w3.org/TR/2011/REC-CSS2-20110607>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://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,
<http://www.rfc-editor.org/info/rfc5234>.
[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,
<http://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,
<http://www.rfc-editor.org/info/rfc7231>.
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[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,
<http://www.rfc-editor.org/info/rfc7234>.
[W3C.CR-css-values-3-20160929]
Atkins, T. and E. Etemad, "CSS Values and Units Module
Level 3", World Wide Web Consortium CR CR-css-values-
3-20160929, September 2016, <https://www.w3.org/TR/2016/
CR-css-values-3-20160929>.
[W3C.REC-html5-20141028]
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>.
7.2. Informative References
[I-D.ietf-httpbis-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,
<http://www.rfc-editor.org/info/rfc6265>.
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.
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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 None
Author's Address
Ilya Grigorik
Google
Email: ilya@igvita.com
URI: https://www.igvita.com/
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