draft-ietf-httpbis-client-hints-04.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 April 18, 2017 Intended status: Experimental July 22, 2017
Expires: October 20, 2017 Expires: January 23, 2018
HTTP Client Hints HTTP Client Hints
draft-ietf-httpbis-client-hints-04 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
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at 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 October 20, 2017. This Internet-Draft will expire on January 23, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 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.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4
2. Client Hint Request Header Fields . . . . . . . . . . . . . . 4 2. Client Hint Request Header Fields . . . . . . . . . . . . . . 4
2.1. Sending Client Hints . . . . . . . . . . . . . . . . . . 4 2.1. Sending Client Hints . . . . . . . . . . . . . . . . . . 4
2.2. Server Processing of Client Hints . . . . . . . . . . . . 4 2.2. Server Processing of Client Hints . . . . . . . . . . . . 4
2.2.1. Advertising Support via Accept-CH header field . . . 5 2.2.1. Advertising Support via Accept-CH Header Field . . . 5
2.2.2. The Accept-CH-Lifetime header field . . . . . . . . . 5 2.2.2. The Accept-CH-Lifetime Header Field . . . . . . . . . 5
2.2.3. Interaction with Caches . . . . . . . . . . . . . . . 6 2.2.3. Interaction with Caches . . . . . . . . . . . . . . . 6
3. Client Hints . . . . . . . . . . . . . . . . . . . . . . . . 7 3. Client Hints . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. The DPR header field . . . . . . . . . . . . . . . . . . 7 3.1. The DPR Header Field . . . . . . . . . . . . . . . . . . 6
3.1.1. Confirming Selected DPR . . . . . . . . . . . . . . . 7 3.1.1. Confirming Selected DPR . . . . . . . . . . . . . . . 7
3.2. The Width header field . . . . . . . . . . . . . . . . . 8 3.2. The Width Header Field . . . . . . . . . . . . . . . . . 7
3.3. The Viewport-Width header field . . . . . . . . . . . . . 8 3.3. The Viewport-Width Header Field . . . . . . . . . . . . . 7
3.4. The Downlink header field . . . . . . . . . . . . . . . . 8 3.4. The Save-Data Header Field . . . . . . . . . . . . . . . 8
3.5. The Save-Data header field . . . . . . . . . . . . . . . 8 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 10 6.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 10
6.2. Accept-CH-Lifetime . . . . . . . . . . . . . . . . . . . 11 6.2. Accept-CH-Lifetime . . . . . . . . . . . . . . . . . . . 10
6.3. Content-DPR . . . . . . . . . . . . . . . . . . . . . . . 11 6.3. Content-DPR . . . . . . . . . . . . . . . . . . . . . . . 10
6.4. Downlink . . . . . . . . . . . . . . . . . . . . . . . . 11 6.4. DPR . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.5. DPR . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.5. Save-Data . . . . . . . . . . . . . . . . . . . . . . . . 10
6.6. Save-Data . . . . . . . . . . . . . . . . . . . . . . . . 11 6.6. Viewport-Width . . . . . . . . . . . . . . . . . . . . . 11
6.7. Viewport-Width . . . . . . . . . . . . . . . . . . . . . 12 6.7. Width . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.8. Width . . . . . . . . . . . . . . . . . . . . . . . . . . 12 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . 12 8.1. Normative References . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . 13 8.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . 13 Appendix A. Interaction with Key Response Header Field . . . . . 12
A.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 13 Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 13
A.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 13 B.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 14 B.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 14 B.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 13
A.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 14 B.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 13
B.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 13
B.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
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
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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", "NOT RECOMMENDED", "MAY", and
document are to be interpreted as described in [RFC2119]. "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 This document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234] with the list rule extension defined in [RFC7230], [RFC5234] with the list rule extension defined in [RFC7230],
Appendix B. It includes by reference the DIGIT rule from [RFC5234] Appendix B. It includes by reference the DIGIT rule from [RFC5234]
and the OWS 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
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client preferences that the server can use to adapt and optimize the client preferences that the server can use to adapt and optimize the
response. response.
2.1. Sending Client Hints 2.1. Sending Client Hints
Clients control which Client Hints are sent in requests, based on Clients control which Client Hints are sent in requests, based on
their default settings, user configuration and/or preferences. their default settings, user configuration and/or preferences.
Implementers might provide user choice mechanisms so that users may Implementers might provide user choice mechanisms so that users may
balance privacy concerns with bandwidth limitations. Implementations balance privacy concerns with bandwidth limitations. Implementations
specific to certain use cases or threat models might avoid specific to certain use cases or threat models might avoid
transmitting these headers altogether, or limit them to secure transmitting these header fields altogether, or limit them to secure
contexts or authenticated sessions. Implementers should be aware contexts or authenticated sessions. Implementers should be aware
that explaining the privacy implications of passive fingerprinting or that explaining the privacy implications of passive fingerprinting or
network information disclosure may be challenging. 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
When presented with a request that contains one or more client hint When presented with a request that contains one or more client hint
headers, servers can optimize the response based upon the information header fields, servers can optimize the response based upon the
in them. When doing so, and if the resource is cacheable, the server information in them. When doing so, and if the resource is
MUST also generate a Vary response header field (Section 7.1.4 of cacheable, the server MUST also generate a Vary response header field
[RFC7231]), and optionally Key ([I-D.ietf-httpbis-key]), to indicate (Section 7.1.4 of [RFC7231]) to indicate which hints can affect the
which hints can affect the selected response and whether the selected selected response and whether the selected response is appropriate
response is appropriate for a later request. for a later request.
Further, depending on the hint used, the server can generate Further, depending on the hint used, the server can generate
additional response header fields to convey related values to aid additional response header fields to convey related values to aid
client processing. For example, this specification defines "Content- client processing. For example, this specification defines the
DPR" response header field that needs to be returned by the server "Content-DPR" response header field that needs to be returned by the
when the "DPR" hint is used to select the response. server when the "DPR" hint is used to select the response.
2.2.1. Advertising Support via Accept-CH header field 2.2.1. Advertising Support via Accept-CH Header Field
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 ([HTML5]).
Accept-CH = #field-name Accept-CH = #field-name
For example: For example:
Accept-CH: DPR, Width, Viewport-Width Accept-CH: DPR, Width, Viewport-Width
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 application is interested in receiving treat it as a signal that the application is interested in receiving
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values; subresource requests initiated as a result of processing the values; subresource requests initiated as a result of processing the
response from the server that includes the Accept-CH opt-in can response from the server that includes the Accept-CH opt-in can
include the request header fields that match the advertised field- include the request header fields that match the advertised field-
values. 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, and Viewport-Width header fields to all append DPR, Width, and Viewport-Width header fields to all
subresource requests initiated by the page constructed from the subresource requests initiated by the page constructed from the
response. response.
2.2.2. The Accept-CH-Lifetime header field 2.2.2. The Accept-CH-Lifetime Header Field
Servers can ask the client to remember an origin-wide Accept-CH Servers can ask the client to remember an origin-wide Accept-CH
preference for a specified period of time to enable delivery of preference for a specified period of time to enable delivery of
Client Hints on all subsequent requests to the origin, and on Client Hints on all subsequent requests to the origin ([RFC6454]),
subresource requests initiated as a result of processing a response and on any requests initiated as a result of processing a response
from the origin. from the origin.
Accept-CH-Lifetime = #delta-seconds Accept-CH-Lifetime = #delta-seconds
The field-value indicates that the Accept-CH preference should be The field-value indicates that the Accept-CH preference SHOULD be
considered stale after its age is greater than the specified number considered stale after its age is greater than the specified number
of seconds. of seconds.
Accept-CH: DPR, Viewport-Width Accept-CH: DPR, Width
Accept-CH: Viewport-Width
Accept-CH-Lifetime: 86400 Accept-CH-Lifetime: 86400
For example, based on the Accept-CH and Accept-CH-Lifetime example For example, based on the Accept-CH and Accept-CH-Lifetime example
above, a user agent could persist an origin-wide Accept-CH preference above, a user agent could persist an origin-wide Accept-CH preference
for up to 86400 seconds (1 day). Then, if a request is initiated to 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 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) navigation to the origin, or fetching a subresource from the origin)
the client could append the requested header fields (DPR and the client could append the requested header fields (DPR, Width, and
Viewport-Width in this example) to the request and any subresource Viewport-Width in this example) to the request and any subresource
requests initiated as a result of processing a response from same requests initiated as a result of processing a response from same
origin. origin.
If Accept-CH-Lifetime occurs in a message more than once, the last
value overrides all previous occurrences.
2.2.3. Interaction with Caches 2.2.3. 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 generate Hints, and if the resource is cacheable, the server needs to generate
a 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
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 Above example indicates that the cache key needs to include the DPR
(Mbps) value of the Downlink header field with six segments: less and Width header fields.
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. Client Hints
3.1. The DPR header field 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 [CSSVAL]) of the layout
[W3C.CR-css-values-3-20160929]) of the layout viewport (Section 9.1.1 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.
3.1.1. 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
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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.
3.2. The Width header field 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.
3.3. The Viewport-Width header field 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.
3.4. The Downlink header field 3.4. The Save-Data 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 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 be defined by updates to this and extension token values can be defined by updates to this
specification. specification.
If Save-Data occurs in a message more than once, the last value
overrides all previous occurrences.
4. 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
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Content-DPR: 1.0 Content-DPR: 1.0
The Content-DPR response header field indicates to the client that The Content-DPR response header field indicates to the client that
the server has selected resource with DPR ratio of 1.0. The client the server has selected resource with DPR ratio of 1.0. The client
can use this information to perform additional processing on the can use this information to perform additional processing on the
resource - for example, calculate the appropriate intrinsic size of resource - for example, calculate the appropriate intrinsic size of
the image resource such that it is displayed at the correct the image resource such that it is displayed at the correct
resolution. 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.
5. Security Considerations 5. Security Considerations
The request header fields defined in this specification expose The request header fields defined in this specification expose
information that is already available to Web applications in the information that is already available to Web applications in the
browser runtime itself (e.g., using JavaScript and CSS). For browser runtime itself (e.g., using JavaScript and CSS). For
example, the application can obtain viewport width, image display example, the application can obtain viewport width, image display
width, and device pixel ratio via JavaScript, or through the use of width, and device pixel ratio via JavaScript, or through the use of
CSS media queries and unique resource URLs even if JavaScript is CSS media queries and unique resource URLs even if JavaScript is
disabled. However, servers that gather this information through such disabled. However, servers that gather this information through such
mechanisms are typically observable (e.g., you can see that they're mechanisms are typically observable (e.g., you can see that they're
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origin opt-in via Accept-CH; clear remembered opt-in, as set by origin opt-in via Accept-CH; clear remembered opt-in, as set by
Accept-CH-Lifetime, when site data, browsing history, browsing cache, Accept-CH-Lifetime, when site data, browsing history, browsing cache,
or similar, are cleared; restrict delivery to same origin or similar, are cleared; restrict delivery to same origin
subrequests; limit delivery to requests that already carry subrequests; limit delivery to requests that already carry
identifying information (e.g. cookies); modify delivery policy when identifying information (e.g. cookies); modify delivery policy when
in an "incognito" or a similar privacy mode; enable user in an "incognito" or a similar privacy mode; enable user
configuration and opt in, and so on. configuration and opt in, and so on.
6. IANA Considerations 6. IANA Considerations
This document defines the "Accept-CH", "DPR", "Width", and "Downlink" This document defines the "Accept-CH", "DPR", "Save-Data", "Viewport-
HTTP request fields, "Content-DPR" HTTP response field, and registers Width", and "Width" HTTP request fields, "Accept-CH", "Accept-CH-
them in the Permanent Message Header Fields registry. Lifetime", and "Content-DPR" HTTP response field, and registers them
in the Permanent Message Header Fields registry.
6.1. 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
skipping to change at page 11, line 26 skipping to change at page 10, line 32
6.3. Content-DPR 6.3. 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 3.1.1 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
6.4. Downlink 6.4. DPR
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 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 3.1 of this document o Specification document(s): Section 3.1 of this document
o Related information: for Client Hints o Related information: for Client Hints
6.6. 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 3.5 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
6.7. 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 3.3 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
6.8. 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 3.2 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
7. References 7. Acknowledgements
7.1. Normative References 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.
8. References
8.1. 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>.
[CSSVAL] 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>.
[HTML5] Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
Navara, E., O&#039;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 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<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, Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008, DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>. <http://www.rfc-editor.org/info/rfc5234>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<http://www.rfc-editor.org/info/rfc6454>.
[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] 8.2. Informative References
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&#039;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] [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>.
Appendix A. Changes Appendix A. Interaction with Key Response Header Field
A.1. Since -00 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: 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.
Appendix B. Changes
B.1. Since -00
o Issue 168 (make Save-Data extensible) updated ABNF. o Issue 168 (make Save-Data extensible) updated ABNF.
o Issue 163 (CH review feedback) editorial feedback from httpwg o Issue 163 (CH review feedback) editorial feedback from httpwg
list. list.
o Issue 153 (NetInfo API citation) added normative reference. o Issue 153 (NetInfo API citation) added normative reference.
A.2. Since -01 B.2. Since -01
o Issue 200: Moved Key reference to informative. o Issue 200: Moved Key reference to informative.
o Issue 215: Extended passive fingerprinting and mitigation o Issue 215: Extended passive fingerprinting and mitigation
considerations. considerations.
o Changed document status to experimental. o Changed document status to experimental.
A.3. Since -02 B.3. Since -02
o Issue 239: Updated reference to CR-css-values-3 o Issue 239: Updated reference to CR-css-values-3
o Issue 240: Updated reference for Network Information API o Issue 240: Updated reference for Network Information API
o Issue 241: Consistency in IANA considerations o Issue 241: Consistency in IANA considerations
o Issue 250: Clarified Accept-CH o Issue 250: Clarified Accept-CH
A.4. Since -03 B.4. Since -03
o Issue 284: Extended guidance for Accept-CH o Issue 284: Extended guidance for Accept-CH
o Issue 308: Editorial cleanup o Issue 308: Editorial cleanup
o Issue 306: Define Accept-CH-Lifetime o Issue 306: Define Accept-CH-Lifetime
A.5. Since -04 B.5. Since -04
o None yet. o Issue 361: Removed Downlink
o Issue 361: Moved Key to appendix, plus other editorial feedback.
B.6. Since -05
o None
Author's Address Author's Address
Ilya Grigorik Ilya Grigorik
Google Google
Email: ilya@igvita.com Email: ilya@igvita.com
URI: https://www.igvita.com/ URI: https://www.igvita.com/
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