HTTP Working Group I. Grigorik
Internet-Draft Google
Intended status: Experimental December 2, 2016
Expires: June 5, 2017
HTTP Client Hints
draft-ietf-httpbis-client-hints-03
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 clients to indicate what formats
they prefer, Client Hints allow clients to indicate a list of 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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://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
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 June 5, 2017.
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Copyright Notice
Copyright (c) 2016 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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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
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 for Client Hints . . . . . . . . 4
2.2.2. Interaction with Caches . . . . . . . . . . . . . . . 5
3. The DPR Client Hint . . . . . . . . . . . . . . . . . . . . . 6
3.1. Confirming Selected DPR . . . . . . . . . . . . . . . . . 6
4. The Width Client Hint . . . . . . . . . . . . . . . . . . . . 7
5. The Viewport-Width Client Hint . . . . . . . . . . . . . . . 7
6. The Downlink Client Hint . . . . . . . . . . . . . . . . . . 7
7. The Save-Data Client Hint . . . . . . . . . . . . . . . . . . 8
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . 10
10.2. Content-DPR . . . . . . . . . . . . . . . . . . . . . . 10
10.3. Downlink . . . . . . . . . . . . . . . . . . . . . . . . 10
10.4. DPR . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.5. Save-Data . . . . . . . . . . . . . . . . . . . . . . . 11
10.6. Viewport-Width . . . . . . . . . . . . . . . . . . . . . 11
10.7. Width . . . . . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 12
A.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 13
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
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
(UA; Section 5.5.3 of [RFC7231]) 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, UA sniffing has numerous limitations:
o UA detection cannot reliably identify all static variables
o UA detection cannot infer any dynamic client preferences
o UA detection requires an external device database
o UA detection is not cache friendly
A popular alternative strategy is to use HTTP cookies ([RFC6265]) to
communicate some information about the client. 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
the client to perform proactive content negotiation (Section 3.4.1 of
[RFC7231]) by indicating a list of 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.
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],
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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 a
list of client preferences that the server can use to adapt and
optimize the response.
2.1. Sending Client Hints
Clients control which Client Hint headers and their respective header
fields are communicated, based on their default settings, user
configuration and/or preferences. The user can be given the choice
to enable, disable, or override specific hints.
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
Servers respond with an optimized response based on one or more
received hints from the client. When doing so, and if the resource
is cacheable, the server MUST also emit 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 used hint, the server can emit additional
response header fields to confirm the property of the response, such
that the client can adjust its processing. For example, this
specification defines "Content-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 for Client Hints
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
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For example:
Accept-CH: DPR, Width, Viewport-Width, Downlink
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 server is interested in receiving the
Client-Hint header fields that match the advertised field-values;
subsequent requests initiated to the same server and, optionally any
subresource requests initiated as a result of processing the response
from the server that includes the Accept-CH opt-in, can include the
Client-Hint header fields that match the advertised field-values.
For example, based on Accept-CH example above, a user agent could
append DPR, Width, Viewport-Width, and Downlink header fields to all
subresource requests initiated by the page constructed from the
response. Alternatively, a client can treat advertised support as a
persistent origin preference and append same header fields on all
future requests initiated to and by the resources associated with
that origin.
2.2.2. 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 emit 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:
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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
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. The DPR Client Hint
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. 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
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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.
4. The Width Client Hint
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.
5. The Viewport-Width Client Hint
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.
6. The Downlink Client Hint
The "Downlink" request header field is a number that indicates the
client's maximum downlink speed in megabits per second (Mbps), as
defined by the "downlinkMax" attribute in the W3C Network Information
API ([NETINFO]).
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Downlink = 1*DIGIT [ "." 1*DIGIT ]
If Downlink occurs in a message more than once, the minimum value
should be used to override other occurrences.
7. The Save-Data Client Hint
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
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 only be defined by revisions of this
specification.
8. 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
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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.
9. Security Considerations
Client Hints defined in this specification do not expose new
information about the user's environment beyond what is already
available to, and can be communicated by, the application at runtime
via 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, implementors
should consider the privacy implications of various methods to enable
delivery of Client Hints - see "Sending Client Hints" section.
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, might reduce
such fingerprinting risks.
The implementers can provide mechanisms and policies to control how
and when such hints are advertised: require origin opt-in and
restrict delivery to same origin subrequests; limit delivery to
requests that already carry indentifying information (e.g. cookies);
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modify delivery policy when in an "incognito" or a similar privacy
mode; enable user configuration and opt in, and so on.
10. 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.
10.1. 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
10.2. 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 of this document
o Related information: for Client Hints
10.3. Downlink
o Header field name: Downlink
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 6 of this document
o Related information: for Client Hints
10.4. DPR
o Header field name: DPR
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 3 of this document
o Related information: for Client Hints
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10.5. 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 7 of this document
o Related information: for Client Hints
10.6. 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 5 of this document
o Related information: for Client Hints
10.7. Width
o Header field name: Width
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 4 of this document
o Related information: for Client Hints
11. References
11.1. 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>.
[NETINFO] Caceres, M., Moreno, F., and I. Grigorik, "Network
Information API", n.d., <https://w3c.github.io/netinfo/>.
[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>.
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[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>.
[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>.
11.2. 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.
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A.2. Since -01
o Issue 200: Moved Key reference to informative.
o Issue 215: Extended passive fingerprinting and mitigation
considerations.
o Changed document status to experimental.
A.3. Since -02
o Issue 239: Updated reference to CR-css-values-3
o Issue 240: Updated reference for Network Information API
o Issue 241: Consistency in IANA considerations
o Issue 250: Clarified Accept-CH
A.4. Since -03
None yet.
Author's Address
Ilya Grigorik
Google
Email: ilya@igvita.com
URI: https://www.igvita.com/
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