draft-ietf-httpbis-client-hints-03.txt   draft-ietf-httpbis-client-hints-latest.txt 
HTTP Working Group I. Grigorik HTTP Working Group I. Grigorik
Internet-Draft Google Internet-Draft Google
Intended status: Experimental December 2, 2016 Intended status: Experimental March 24, 2017
Expires: June 5, 2017 Expires: September 25, 2017
HTTP Client Hints HTTP Client Hints
draft-ietf-httpbis-client-hints-03 draft-ietf-httpbis-client-hints-latest
Abstract Abstract
An increasing diversity of Web-connected devices and software An increasing diversity of Web-connected devices and software
capabilities has created a need to deliver optimized content for each capabilities has created a need to deliver optimized content for each
device. device.
This specification defines a set of HTTP request header fields, This specification defines a set of HTTP request header fields,
colloquially known as Client Hints, to address this. They are colloquially known as Client Hints, to address this. They are
intended to be used as input to proactive content negotiation; just intended to be used as input to proactive content negotiation; just
as the Accept header field allows clients to indicate what formats as the Accept header field allows user agents to indicate what
they prefer, Client Hints allow clients to indicate a list of device formats they prefer, Client Hints allow user agents to indicate
and agent specific preferences. device and agent specific preferences.
Note to Readers Note to Readers
Discussion of this draft takes place on the HTTP working group Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at mailing list (ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/. https://lists.w3.org/Archives/Public/ietf-http-wg/ .
Working Group information can be found at http://httpwg.github.io/; Working Group information can be found at http://httpwg.github.io/ ;
source code and issues list for this draft can be found at source code and issues list for this draft can be found at
https://github.com/httpwg/http-extensions/labels/client-hints. https://github.com/httpwg/http-extensions/labels/client-hints .
Status of this Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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 http://datatracker.ietf.org/drafts/current/. Drafts is at http://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 June 5, 2017. This Internet-Draft will expire on September 25, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 4 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Client Hint Request Header Fields . . . . . . . . . . . . . . 5 2. Client Hint Request Header Fields . . . . . . . . . . . . . . 4
2.1. Sending Client Hints . . . . . . . . . . . . . . . . . . . 5 2.1. Sending Client Hints . . . . . . . . . . . . . . . . . . 4
2.2. Server Processing of Client Hints . . . . . . . . . . . . 5 2.2. Server Processing of Client Hints . . . . . . . . . . . . 4
2.2.1. Advertising Support for Client Hints . . . . . . . . . 5 2.2.1. Advertising Support for Client Hints . . . . . . . . 5
2.2.2. Interaction with Caches . . . . . . . . . . . . . . . 6 2.2.2. Interaction with Caches . . . . . . . . . . . . . . . 5
3. The DPR Client Hint . . . . . . . . . . . . . . . . . . . . . 7 3. Client Hints . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Confirming Selected DPR . . . . . . . . . . . . . . . . . 7 3.1. The DPR header field . . . . . . . . . . . . . . . . . . 6
4. The Width Client Hint . . . . . . . . . . . . . . . . . . . . 8 3.1.1. Confirming Selected DPR . . . . . . . . . . . . . . . 6
5. The Viewport-Width Client Hint . . . . . . . . . . . . . . . . 8 3.2. The Width header field . . . . . . . . . . . . . . . . . 7
6. The Downlink Client Hint . . . . . . . . . . . . . . . . . . . 8 3.3. The Viewport-Width header field . . . . . . . . . . . . . 7
7. The Save-Data Client Hint . . . . . . . . . . . . . . . . . . 8 3.4. The Downlink header field . . . . . . . . . . . . . . . . 8
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.5. The Save-Data header field . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 11 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10.2. Content-DPR . . . . . . . . . . . . . . . . . . . . . . . 11 6.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 10
10.3. Downlink . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.2. Content-DPR . . . . . . . . . . . . . . . . . . . . . . . 10
10.4. DPR . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.3. Downlink . . . . . . . . . . . . . . . . . . . . . . . . 10
10.5. Save-Data . . . . . . . . . . . . . . . . . . . . . . . . 11 6.4. DPR . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.6. Viewport-Width . . . . . . . . . . . . . . . . . . . . . . 12 6.5. Save-Data . . . . . . . . . . . . . . . . . . . . . . . . 11
10.7. Width . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6.6. Viewport-Width . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6.7. Width . . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . . 12 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . . 13 7.1. Normative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . . 13 7.2. Informative References . . . . . . . . . . . . . . . . . 12
A.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 13 Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 12
A.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 13 A.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 12
A.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 14 A.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 14 A.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14 A.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
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. well as dynamic user and client preferences.
One way to infer some of these capabilities is through User-Agent One way to infer some of these capabilities is through User-Agent
(UA; Section 5.5.3 of [RFC7231]) detection against an established (Section 5.5.3 of [RFC7231]) header field detection against an
database of client signatures. However, this technique requires established database of client signatures. However, this technique
acquiring such a database, integrating it into the serving path, and requires acquiring such a database, integrating it into the serving
keeping it up to date. However, even once this infrastructure is path, and keeping it up to date. However, even once this
deployed, UA sniffing has numerous limitations: infrastructure is deployed, user agent sniffing has numerous
limitations:
o UA detection cannot reliably identify all static variables o User agent detection cannot reliably identify all static variables
o UA detection cannot infer any dynamic client preferences o User agent detection cannot infer any dynamic client preferences
o UA detection requires an external device database o User agent detection requires an external device database
o UA detection is not cache friendly o User agent detection is not cache friendly
A popular alternative strategy is to use HTTP cookies ([RFC6265]) to A popular alternative strategy is to use HTTP cookies ([RFC6265]) to
communicate some information about the client. However, this communicate some information about the user agent. However, this
approach is also not cache friendly, bound by same origin policy, and approach is also not cache friendly, bound by same origin policy, and
imposes additional client-side latency by requiring JavaScript imposes additional client-side latency by requiring JavaScript
execution to create and manage HTTP cookies. execution to create and manage HTTP cookies.
This document defines a set of new request header fields that allow This document defines a set of new request header fields that allow
the client to perform proactive content negotiation (Section 3.4.1 of user agent to perform proactive content negotiation (Section 3.4.1 of
[RFC7231]) by indicating a list of device and agent specific [RFC7231]) by indicating device and agent specific preferences,
preferences, through a mechanism similar to the Accept header field through a mechanism similar to the Accept header field which is used
which is used to indicate preferred response formats. to indicate preferred response formats.
Client Hints does not supersede or replace the User-Agent header Client Hints does not supersede or replace the User-Agent header
field. Existing device detection mechanisms can continue to use both field. Existing device detection mechanisms can continue to use both
mechanisms if necessary. By advertising its capabilities within a mechanisms if necessary. By advertising its capabilities within a
request header field, Client Hints allows for cache friendly and request header field, Client Hints allows for cache friendly and
proactive content negotiation. proactive content negotiation.
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", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
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], Appendix [RFC5234] with the list rule extension defined in [RFC7230],
B. It includes by reference the DIGIT rule from [RFC5234] and the OWS Appendix B. It includes by reference the DIGIT rule from [RFC5234]
and field-name rules from [RFC7230]. 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 configuration data that can be used
by the server to select an appropriate response. Each one conveys a by the server to select an appropriate response. Each one conveys
list of client preferences that the server can use to adapt and client preferences that the server can use to adapt and optimize the
optimize the response. response.
2.1. Sending Client Hints 2.1. Sending Client Hints
Clients control which Client Hint headers and their respective header Clients control which Client Hints are sent in requests, based on
fields are communicated, based on their default settings, user their default settings, user configuration and/or preferences.
configuration and/or preferences. The user can be given the choice Implementers might provide user choice mechanisms so that users may
to enable, disable, or override specific hints. 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 The client and server, or an intermediate proxy, can use an opt-in
mechanism to negotiate which fields should be reported to allow for mechanism to negotiate which fields should be reported to allow for
efficient content adaption. efficient content adaption.
2.2. Server Processing of Client Hints 2.2. Server Processing of Client Hints
Servers respond with an optimized response based on one or more When presented with a request that contains one or more client hint
received hints from the client. When doing so, and if the resource headers, servers can optimise the response based upon the information
is cacheable, the server MUST also emit a Vary response header field in them. When doing so, and if the resource is cacheable, the server
(Section 7.1.4 of [RFC7231]), and optionally Key MUST also generate a Vary response header field (Section 7.1.4 of
([I-D.ietf-httpbis-key]), to indicate which hints can affect the [RFC7231]), and optionally Key ([I-D.ietf-httpbis-key]), to indicate
selected response and whether the selected response is appropriate which hints can affect the selected response and whether the selected
for a later request. response is appropriate for a later request.
Further, depending on the used hint, the server can emit additional Further, depending on the hint used, the server can generate
response header fields to confirm the property of the response, such additional response header fields to convey related values to aid
that the client can adjust its processing. For example, this client processing. For example, this specification defines "Content-
specification defines "Content-DPR" response header field that needs DPR" response header field that needs to be returned by the server
to be returned by the server when the "DPR" hint is used to select when the "DPR" hint is used to select the response.
the response.
2.2.1. Advertising Support for Client Hints 2.2.1. Advertising Support for Client Hints
Servers can advertise support for Client Hints using the Accept-CH Servers can advertise support for Client Hints using the Accept-CH
header field or an equivalent HTML meta element with http-equiv header field or an equivalent HTML meta element with http-equiv
attribute ([W3C.REC-html5-20141028]). attribute ([W3C.REC-html5-20141028]).
Accept-CH = #field-name Accept-CH = #field-name
For example: For example:
Accept-CH: DPR, Width, Viewport-Width, Downlink Accept-CH: DPR, Width, Viewport-Width, Downlink
When a client receives Accept-CH, or if it is capable of processing 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 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 treat it as a signal that the server is interested in receiving
Client-Hint header fields that match the advertised field-values; specified request header fields that match the advertised field-
subsequent requests initiated to the same server and, optionally any values; subsequent requests initiated to the same server and,
subresource requests initiated as a result of processing the response optionally any subresource requests initiated as a result of
from the server that includes the Accept-CH opt-in, can include the processing the response from the server that includes the Accept-CH
Client-Hint header fields that match the advertised field-values. 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 For example, based on Accept-CH example above, a user agent could
append DPR, Width, Viewport-Width, and Downlink header fields to all append DPR, Width, Viewport-Width, and Downlink header fields to all
subresource requests initiated by the page constructed from the subresource requests initiated by the page constructed from the
response. Alternatively, a client can treat advertised support as a response. Alternatively, a client can treat advertised support as a
persistent origin preference and append same header fields on all persistent origin preference and append same header fields on all
future requests initiated to and by the resources associated with future requests initiated to and by the resources associated with
that origin. that origin.
2.2.2. Interaction with Caches 2.2.2. Interaction with Caches
When selecting an optimized response based on one or more Client When selecting an optimized response based on one or more Client
Hints, and if the resource is cacheable, the server needs to emit a Hints, and if the resource is cacheable, the server needs to generate
Vary response header field ([RFC7234]) to indicate which hints can a Vary response header field ([RFC7234]) to indicate which hints can
affect the selected response and whether the selected response is affect the selected response and whether the selected response is
appropriate for a later request. appropriate for a later request.
Vary: DPR Vary: DPR
Above example indicates that the cache key needs to include the DPR Above example indicates that the cache key needs to include the DPR
header field. header field.
Vary: DPR, Width, Downlink Vary: DPR, Width, Downlink
skipping to change at page 7, line 16 skipping to change at page 6, line 29
of the Width header field and be partitioned into groups of 320: of the Width header field and be partitioned into groups of 320:
0-320, 320-640, and so on. 0-320, 320-640, and so on.
Key: Downlink;partition=0.5:1.0:3.0:5.0:10 Key: Downlink;partition=0.5:1.0:3.0:5.0:10
Above example indicates that the cache key needs to include the Above example indicates that the cache key needs to include the
(Mbps) value of the Downlink header field with six segments: less (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 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. than 5.0, 5.0 to less than 10; 10 or higher.
3. The DPR Client Hint 3. Client Hints
3.1. The DPR header field
The "DPR" request header field is a number that indicates the The "DPR" request header field is a number that indicates the
client's current Device Pixel Ratio (DPR), which is the ratio of client's current Device Pixel Ratio (DPR), which is the ratio of
physical pixels over CSS px (Section 5.2 of physical pixels over CSS px (Section 5.2 of
[W3C.CR-css-values-3-20160929]) of the layout viewport (Section 9.1.1 [W3C.CR-css-values-3-20160929]) of the layout viewport (Section 9.1.1
of [CSS2]) on the device. of [CSS2]) on the device.
DPR = 1*DIGIT [ "." 1*DIGIT ] DPR = 1*DIGIT [ "." 1*DIGIT ]
If DPR occurs in a message more than once, the last value overrides If DPR occurs in a message more than once, the last value overrides
all previous occurrences. all previous occurrences.
3.1. Confirming Selected DPR 3.1.1. Confirming Selected DPR
The "Content-DPR" response header field is a number that indicates The "Content-DPR" response header field is a number that indicates
the ratio between physical pixels over CSS px of the selected image the ratio between physical pixels over CSS px of the selected image
response. response.
Content-DPR = 1*DIGIT [ "." 1*DIGIT ] Content-DPR = 1*DIGIT [ "." 1*DIGIT ]
DPR ratio affects the calculation of intrinsic size of image DPR ratio affects the calculation of intrinsic size of image
resources on the client - i.e. typically, the client automatically resources on the client - i.e. typically, the client automatically
scales the natural size of the image by the DPR ratio to derive its scales the natural size of the image by the DPR ratio to derive its
skipping to change at page 8, line 8 skipping to change at page 7, line 23
returned value MUST take precedence. returned value MUST take precedence.
Note that DPR confirmation is only required for image responses, and Note that DPR confirmation is only required for image responses, and
the server does not need to confirm the resource width as this value 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 can be derived from the resource itself once it is decoded by the
client. client.
If Content-DPR occurs in a message more than once, the last value If Content-DPR occurs in a message more than once, the last value
overrides all previous occurrences. overrides all previous occurrences.
4. The Width Client Hint 3.2. The Width header field
The "Width" request header field is a number that indicates the The "Width" request header field is a number that indicates the
desired resource width in physical px (i.e. intrinsic size of an desired resource width in physical px (i.e. intrinsic size of an
image). The provided physical px value is a number rounded to the image). The provided physical px value is a number rounded to the
smallest following integer (i.e. ceiling value). smallest following integer (i.e. ceiling value).
Width = 1*DIGIT Width = 1*DIGIT
If the desired resource width is not known at the time of the request 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 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 can be omitted. If Width occurs in a message more than once, the
last value overrides all previous occurrences. last value overrides all previous occurrences.
5. The Viewport-Width Client Hint 3.3. The Viewport-Width header field
The "Viewport-Width" request header field is a number that indicates 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 the layout viewport width in CSS px. The provided CSS px value is a
number rounded to the smallest following integer (i.e. ceiling number rounded to the smallest following integer (i.e. ceiling
value). value).
Viewport-Width = 1*DIGIT Viewport-Width = 1*DIGIT
If Viewport-Width occurs in a message more than once, the last value If Viewport-Width occurs in a message more than once, the last value
overrides all previous occurrences. overrides all previous occurrences.
6. The Downlink Client Hint 3.4. The Downlink header field
The "Downlink" request header field is a number that indicates the The "Downlink" request header field is a number that indicates the
client's maximum downlink speed in megabits per second (Mbps), as client's maximum downlink speed in megabits per second (Mbps).
defined by the "downlinkMax" attribute in the W3C Network Information
API ([NETINFO]).
Downlink = 1*DIGIT [ "." 1*DIGIT ] Downlink = 1*DIGIT [ "." 1*DIGIT ]
If Downlink occurs in a message more than once, the minimum value If Downlink occurs in a message more than once, the minimum value
should be used to override other occurrences. should be used to override other occurrences.
7. The Save-Data Client Hint 3.5. The Save-Data header field
The "Save-Data" request header field consists of one or more tokens The "Save-Data" request header field consists of one or more tokens
that indicate client's preference for reduced data usage, due to high that indicate client's preference for reduced data usage, due to high
transfer costs, slow connection speeds, or other reasons. transfer costs, slow connection speeds, or other reasons.
Save-Data = sd-token *( OWS ";" OWS [sd-token] ) Save-Data = sd-token *( OWS ";" OWS [sd-token] )
sd-token = token sd-token = token
This document defines the "on" sd-token value, which is used as a 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 signal indicating explicit user opt-in into a reduced data usage mode
on the client, and when communicated to origins allows them to on the client, and when communicated to origins allows them to
deliver alternate content honoring such preference - e.g. smaller deliver alternate content honoring such preference - e.g. smaller
image and video resources, alternate markup, and so on. New token image and video resources, alternate markup, and so on. New token
and extension token values can only be defined by revisions of this and extension token values can be defined by updates to this
specification. specification.
8. Examples 4. Examples
For example, given the following request header fields: For example, given the following request header fields:
DPR: 2.0 DPR: 2.0
Width: 320 Width: 320
Viewport-Width: 320 Viewport-Width: 320
The server knows that the device pixel ratio is 2.0, that the 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 intended display width of the requested resource is 160 CSS px (320
physical pixels at 2x resolution), and that the viewport width is 320 physical pixels at 2x resolution), and that the viewport width is 320
skipping to change at page 10, line 10 skipping to change at page 9, line 22
Alternatively, the server could select an alternate resource based on Alternatively, the server could select an alternate resource based on
the maximum downlink speed advertised in the request header fields: the maximum downlink speed advertised in the request header fields:
Downlink: 0.384 Downlink: 0.384
The server knows that the client's maximum downlink speed is The server knows that the client's maximum downlink speed is
0.384Mbps (GPRS EDGE), and it can use this information to select an 0.384Mbps (GPRS EDGE), and it can use this information to select an
optimized resource - for example, an alternate image asset, optimized resource - for example, an alternate image asset,
stylesheet, HTML document, media stream, and so on. stylesheet, HTML document, media stream, and so on.
9. Security Considerations 5. Security Considerations
Client Hints defined in this specification do not expose new The request header fields defined in this specification expose
information about the user's environment beyond what is already information that is already available to Web applications in the
available to, and can be communicated by, the application at runtime browser runtime itself (e.g., using JavaScript and CSS). For
via JavaScript and CSS. For example, the application can obtain example, the application can obtain viewport width, image display
viewport width, image display width, and device pixel ratio via width, and device pixel ratio via JavaScript, or through the use of
JavaScript, or through the use of CSS media queries and unique CSS media queries and unique resource URLs even if JavaScript is
resource URLs even if JavaScript is disabled. However, implementors disabled. However, servers that gather this information through such
should consider the privacy implications of various methods to enable mechanisms are typically observable (e.g., you can see that they're
delivery of Client Hints - see "Sending Client Hints" section. 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 For example, sending Client Hints on all requests can make
information about the user's environment available to origins that information about the user's environment available to origins that
otherwise did not have access to this data, which may or may not be 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 the desired outcome - e.g. this may enable an image optimization
service to deliver a tailored asset, and it may reveal same service to deliver a tailored asset, and it may reveal same
information about the user to other origins that may not have had information about the user to other origins that may not have had
access to it before. Similarly, sending highly granular data, such access to it before. Similarly, sending highly granular data, such
as image and viewport width may help identify users across multiple as image and viewport width may help identify users across multiple
requests. Restricting such field values to an enumerated range, requests. Restricting such field values to an enumerated range,
where the user agent advertises a threshold value that is close but where the user agent advertises a threshold value that is close but
is not an exact representation of the current value, might reduce is not an exact representation of the current value, can help
such fingerprinting risks. mitigate the risk of such fingerprinting.
The implementers can provide mechanisms and policies to control how Implementers ought to provide mechanisms and policies to control how
and when such hints are advertised: require origin opt-in and and when such hints are advertised. For example, they could require
restrict delivery to same origin subrequests; limit delivery to origin opt-in; restrict delivery to same origin subrequests; limit
requests that already carry indentifying information (e.g. cookies); delivery to requests that already carry indentifying information
modify delivery policy when in an "incognito" or a similar privacy (e.g. cookies); modify delivery policy when in an "incognito" or a
mode; enable user configuration and opt in, and so on. similar privacy mode; enable user configuration and opt in, and so
on.
10. IANA Considerations 6. IANA Considerations
This document defines the "Accept-CH", "DPR", "Width", and "Downlink" This document defines the "Accept-CH", "DPR", "Width", and "Downlink"
HTTP request fields, "Content-DPR" HTTP response field, and registers HTTP request fields, "Content-DPR" HTTP response field, and registers
them in the Permanent Message Header Fields registry. them in the Permanent Message Header Fields registry.
10.1. 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 2.2.1 of this document o Specification document(s): Section 2.2.1 of this document
o Related information: for Client Hints o Related information: for Client Hints
10.2. Content-DPR 6.2. Content-DPR
o Header field name: Content-DPR o Header field name: Content-DPR
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 3.1.1 of this document
o Related information: for Client Hints o Related information: for Client Hints
10.3. Downlink 6.3. Downlink
o Header field name: Downlink o Header field name: Downlink
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 6 of this document o Specification document(s): Section 3.4 of this document
o Related information: for Client Hints o Related information: for Client Hints
10.4. DPR 6.4. DPR
o Header field name: DPR o Header field name: DPR
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 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
10.5. Save-Data 6.5. Save-Data
o Header field name: Save-Data o Header field name: Save-Data
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 7 of this document o Specification document(s): Section 3.5 of this document
o Related information: for Client Hints o Related information: for Client Hints
10.6. Viewport-Width 6.6. Viewport-Width
o Header field name: Viewport-Width o Header field name: Viewport-Width
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 5 of this document o Specification document(s): Section 3.3 of this document
o Related information: for Client Hints o Related information: for Client Hints
10.7. Width 6.7. Width
o Header field name: Width o Header field name: Width
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 4 of this document o Specification document(s): Section 3.2 of this document
o Related information: for Client Hints o Related information: for Client Hints
11. References 7. References
11.1. Normative References 7.1. Normative References
[CSS2] Bos, B., Celic, T., Hickson, I., and H. Lie, "Cascading [CSS2] Bos, B., Celic, T., Hickson, I., and H. Lie, "Cascading
Style Sheets Level 2 Revision 1 (CSS 2.1) Specification", Style Sheets Level 2 Revision 1 (CSS 2.1) Specification",
W3C Recommendation REC-CSS2-20110607, June 2011, W3C Recommendation REC-CSS2-20110607, June 2011,
<http://www.w3.org/TR/2011/REC-CSS2-20110607>. <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 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ Requirement Levels", BCP 14, RFC 2119,
RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/ Specifications: ABNF", STD 68, RFC 5234,
RFC5234, January 2008, DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>. <http://www.rfc-editor.org/info/rfc5234>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing", Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014, RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>. <http://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014, DOI 10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>. <http://www.rfc-editor.org/info/rfc7231>.
[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,
<http://www.rfc-editor.org/info/rfc7234>. <http://www.rfc-editor.org/info/rfc7234>.
[W3C.CR-css-values-3-20160929] [W3C.CR-css-values-3-20160929]
Atkins, T. and E. Etemad, "CSS Values and Units Module Atkins, T. and E. Etemad, "CSS Values and Units Module
Level 3", World Wide Web Consortium CR CR-css-values-3- Level 3", World Wide Web Consortium CR CR-css-values-
20160929, September 2016, 3-20160929, September 2016, <https://www.w3.org/TR/2016/
<https://www.w3.org/TR/2016/CR-css-values-3-20160929>. CR-css-values-3-20160929>.
[W3C.REC-html5-20141028] [W3C.REC-html5-20141028]
Hickson, I., Berjon, R., Faulkner, S., Leithead, T., Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
Navara, E., O&#039;Connor, T., and S. Pfeiffer, "HTML5", Navara, E., O&#039;Connor, T., and S. Pfeiffer, "HTML5",
World Wide Web Consortium Recommendation REC-html5- World Wide Web Consortium Recommendation REC-
20141028, October 2014, html5-20141028, October 2014,
<http://www.w3.org/TR/2014/REC-html5-20141028>. <http://www.w3.org/TR/2014/REC-html5-20141028>.
11.2. Informative References 7.2. Informative References
[I-D.ietf-httpbis-key] [I-D.ietf-httpbis-key]
Fielding, R. and M. Nottingham, "The Key HTTP Response Fielding, R. and M. Nottingham, "The Key HTTP Response
Header Field", draft-ietf-httpbis-key-01 (work in Header Field", draft-ietf-httpbis-key-01 (work in
progress), March 2016. 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,
<http://www.rfc-editor.org/info/rfc6265>. <http://www.rfc-editor.org/info/rfc6265>.
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