< draft-ietf-httpbis-client-hints-06.txt   draft-ietf-httpbis-client-hints-07.txt >
HTTP Working Group I. Grigorik HTTP Working Group I. Grigorik
Internet-Draft Google Internet-Draft Google
Intended status: Experimental July 16, 2018 Intended status: Experimental March 11, 2019
Expires: January 17, 2019 Expires: September 12, 2019
HTTP Client Hints HTTP Client Hints
draft-ietf-httpbis-client-hints-06 draft-ietf-httpbis-client-hints-07
Abstract Abstract
An increasing diversity of Web-connected devices and software HTTP defines proactive content negotiation to allow servers to select
capabilities has created a need to deliver optimized content for each the appropriate response for a given request, based upon the user
device. agent's characteristics, as expressed in request headers. In
practice, clients are often unwilling to send those request headers,
because it is not clear whether they will be used, and sending them
impacts both performance and privacy.
This specification defines an extensible and configurable set of HTTP This document defines two response headers, Accept-CH and Accept-CH-
request header fields, colloquially known as Client Hints, to address Lifetime, that servers can use to advertise their use of request
this. They are intended to be used as input to proactive content headers for proactive content negotiation, along with a set of
negotiation; just as the Accept header field allows user agents to guidelines for the creation of such headers, colloquially known as
indicate what formats they prefer, Client Hints allow user agents to "Client Hints."
indicate device and agent specific preferences.
It also defines an initial set of Client Hints.
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/ [1]. 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/;
[2]; 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 [3]. 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 https://datatracker.ietf.org/drafts/current/. Drafts is at https://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 January 17, 2019. This Internet-Draft will expire on September 12, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2019 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 . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Security Considerations . . . . . . . . . . . . . . . . . . . 7
3.1. The DPR Header Field . . . . . . . . . . . . . . . . . . 6 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
3.1.1. Confirming Selected DPR . . . . . . . . . . . . . . . 7 4.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. The Width Header Field . . . . . . . . . . . . . . . . . 7 4.2. Accept-CH-Lifetime . . . . . . . . . . . . . . . . . . . 8
3.3. The Viewport-Width Header Field . . . . . . . . . . . . . 7 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.1. Normative References . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8 5.2. Informative References . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 5.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 9 Appendix A. Interaction with Key Response Header Field . . . . . 9
6.2. Accept-CH-Lifetime . . . . . . . . . . . . . . . . . . . 9 Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 10
6.3. Content-DPR . . . . . . . . . . . . . . . . . . . . . . . 10 B.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 10
6.4. DPR . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 B.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 10
6.5. Viewport-Width . . . . . . . . . . . . . . . . . . . . . 10 B.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 10
6.6. Width . . . . . . . . . . . . . . . . . . . . . . . . . . 10 B.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 B.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10 B.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 12 B.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 10
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 12 B.8. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Interaction with Key Response Header Field . . . . . 12
Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 12 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
B.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 12 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
B.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 13
B.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 13
B.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 13
B.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 13
B.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 13
B.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 13
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 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
(Section 5.5.3 of [RFC7231]) header field detection against an (Section 5.5.3 of [RFC7231]) header field detection against an
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limitations: limitations:
o User agent detection cannot reliably identify all static variables o User agent detection cannot reliably identify all static variables
o User agent detection cannot infer any dynamic client preferences o User agent detection cannot infer any dynamic client preferences
o User agent detection requires an external device database o User agent detection requires an external device database
o User agent 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 user agent. 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 often 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 Proactive content negotiation (Section 3.4.1 of [RFC7231]) offers an
user agent to perform proactive content negotiation (Section 3.4.1 of alternative approach; user agents use specified, well-defined request
[RFC7231]) by indicating device and agent specific preferences, headers to advertise their capabilities and characteristics, so that
through a mechanism similar to the Accept header field which is used servers can select (or formulate) an appropriate response.
to indicate preferred response formats.
Client Hints does not supersede or replace the User-Agent header However, proactive content negotiation requires clients to send these
request headers prolifically. This causes performance concerns
(because it creates "bloat" in requests), as well as privacy issues;
passively providing such information allows servers to silently
fingerprint the user agent.
This document defines a new response header, Accept-CH, that allows
an origin server to explicitly ask that clients send these headers in
requests, for a period of time bounded by the Accept-CH-Lifetime
response header. It also defines guidelines for content negotiation
mechanisms that use it, colloquially referred to as Client Hints.
Client Hints mitigate the performance concerns by assuring that
clients will only send the request headers when they're actually
going to be used, and the privacy concerns of passive fingerprinting
by requiring explicit opt-in and disclosure of required headers by
the server through the use of the Accept-CH response header.
This document defines the Client Hints infrastructure, a framework
that enables servers to opt-in to specific proactive content
negotiation features, which will enable them to adapt their content
accordingly. However, it does not define any specific features that
will use that infrastructure. Those features will be defined in
their respective specifications.
This document does not supersede or replace the User-Agent header
field. Existing device detection mechanisms can continue to use both field. Existing device detection mechanisms can continue to use both
mechanisms if necessary. By advertising its capabilities within a mechanisms if necessary. By advertising user agent capabilities
request header field, Client Hints allows for cache friendly and within a request header field, Client Hints allow for cache friendly
proactive content negotiation. and 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", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. 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
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When presented with a request that contains one or more client hint When presented with a request that contains one or more client hint
header fields, servers can optimize the response based upon the header fields, servers can optimize the response based upon the
information in them. When doing so, and if the resource is information in them. When doing so, and if the resource is
cacheable, the server MUST also generate a Vary response header field cacheable, the server MUST also generate a Vary response header field
(Section 7.1.4 of [RFC7231]) to indicate which hints can affect the (Section 7.1.4 of [RFC7231]) to indicate which hints can affect the
selected response and whether the selected response is appropriate selected response and whether the selected 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 the client processing.
"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 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 ([HTML5]). attribute ([HTML5]).
Accept-CH = #field-name Accept-CH = #field-name
For example: For example:
Accept-CH: DPR, Width, Viewport-Width Accept-CH: Sec-CH-Example, Sec-CH-Example-2
When a client receives an HTTP response advertising support for When a client receives an HTTP response advertising support for
Client Hints, it should process it as origin ([RFC6454]) opt-in to Client Hints, it should process it as origin ([RFC6454]) opt-in to
receive Client Hint header fields advertised in the field-value. The receive Client Hint header fields advertised in the field-value. The
opt-in MUST be delivered over a secure transport. opt-in MUST be delivered over a secure transport.
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 same- append the Sec-CH-Example and Sec-CH-Example-2 header fields to all
origin resource requests initiated by the page constructed from the same-origin resource requests initiated by the page constructed from
response. the 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 the set of Client Hints that Servers can ask the client to remember the set of Client Hints that
the server supports for a specified period of time, to enable the server supports for a specified period of time, to enable
delivery of Client Hints on subsequent requests to the server's delivery of Client Hints on subsequent requests to the server's
origin ([RFC6454]). origin ([RFC6454]).
Accept-CH-Lifetime = #delta-seconds Accept-CH-Lifetime = #delta-seconds
When a client receives an HTTP response that contains Accept-CH- When a client receives an HTTP response that contains Accept-CH-
Lifetime header field, the field-value indicates that the Accept-CH Lifetime header field, the field-value indicates that the Accept-CH
preference SHOULD be persisted and bound to the origin, and be preference SHOULD be persisted and bound to the origin, and be
considered stale after response's age ([RFC7234], section 4.2) is considered stale after response's age ([RFC7234], section 4.2) is
greater than the specified number of seconds. The preference MUST be greater than the specified number of seconds. The preference MUST be
delivered over a secure transport, and MUST NOT be persisted for an delivered over a secure transport, and MUST NOT be persisted for an
origin that isn't HTTPS. origin that isn't HTTPS.
Accept-CH: DPR, Width Accept-CH: Sec-CH-Example, Sec-CH-Example-2
Accept-CH: Viewport-Width Accept-CH: Sec-CH-Example-3
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, which is received in response to a user agent navigating to above, which is received in response to a user agent navigating to
"https://example.com", and delivered over a secure transport: a user "https://example.com", and delivered over a secure transport: a user
agent SHOULD persist an Accept-CH preference bound to agent SHOULD persist an Accept-CH preference bound to
"https://example.com" for up to 86400 seconds (1 day), and use it for "https://example.com" for up to 86400 seconds (1 day), and use it for
user agent navigations to "https://example.com" and any same-origin user agent navigations to "https://example.com" and any same-origin
resource requests initiated by the page constructed from the resource requests initiated by the page constructed from the
navigation's response. This preference SHOULD NOT extend to resource navigation's response. This preference SHOULD NOT extend to resource
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value overrides all previous occurrences. 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: Sec-CH-Example
Above example indicates that the cache key needs to include the DPR
header field.
Vary: DPR, Width
Above example indicates that the cache key needs to include the DPR
and Width header fields.
3. Client Hints
3.1. The DPR Header Field
The "DPR" request header field is a number that indicates the
client's current Device Pixel Ratio (DPR), which is the ratio of
physical pixels over CSS px (Section 5.2 of [CSSVAL]) of the layout
viewport (Section 9.1.1 of [CSS2]) on the device.
DPR = 1*DIGIT [ "." 1*DIGIT ]
If DPR occurs in a message more than once, the last value overrides
all previous occurrences.
3.1.1. Confirming Selected DPR
The "Content-DPR" response header field is a number that indicates
the ratio between physical pixels over CSS px of the selected image
response.
Content-DPR = 1*DIGIT [ "." 1*DIGIT ]
DPR ratio affects the calculation of intrinsic size of image
resources on the client - i.e. typically, the client automatically
scales the natural size of the image by the DPR ratio to derive its
display dimensions. As a result, the server MUST explicitly indicate
the DPR of the selected image response whenever the DPR hint is used,
and the client MUST use the DPR value returned by the server to
perform its calculations. In case the server returned Content-DPR
value contradicts previous client-side DPR indication, the server
returned value MUST take precedence.
Note that DPR confirmation is only required for image responses, and
the server does not need to confirm the resource width as this value
can be derived from the resource itself once it is decoded by the
client.
If Content-DPR occurs in a message more than once, the last value
overrides all previous occurrences.
3.2. The Width Header Field
The "Width" request header field is a number that indicates the
desired resource width in physical px (i.e. intrinsic size of an
image). The provided physical px value is a number rounded to the
smallest following integer (i.e. ceiling value).
Width = 1*DIGIT
If the desired resource width is not known at the time of the request
or the resource does not have a display width, the Width header field
can be omitted. If Width occurs in a message more than once, the
last value overrides all previous occurrences.
3.3. The Viewport-Width Header Field
The "Viewport-Width" request header field is a number that indicates
the layout viewport width in CSS px. The provided CSS px value is a
number rounded to the smallest following integer (i.e. ceiling
value).
Viewport-Width = 1*DIGIT
If Viewport-Width occurs in a message more than once, the last value
overrides all previous occurrences.
4. Examples
For example, given the following request header fields:
DPR: 2.0
Width: 320
Viewport-Width: 320
The server knows that the device pixel ratio is 2.0, that the
intended display width of the requested resource is 160 CSS px (320
physical pixels at 2x resolution), and that the viewport width is 320
CSS px.
If the server uses above hints to perform resource selection for an Above example indicates that the cache key needs to include the Sec-
image asset, it must confirm its selection via the Content-DPR CH-Example header field.
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 Vary: Sec-CH-Example, Sec-CH-Example-2
The Content-DPR response header field indicates to the client that Above example indicates that the cache key needs to include the Sec-
the server has selected resource with DPR ratio of 1.0. The client CH-Example and Sec-CH-Example-2 header fields.
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.
5. Security Considerations 3. Security Considerations
The request header fields defined in this specification, and those The request header fields defined in this document, and those that
that extend it, expose information about the user's environment to extend it, expose information about the user's environment to enable
enable proactive content negotiation. Such information may reveal proactive content negotiation. Such information may reveal new
new information about the user and implementers ought to consider the information about the user and implementers ought to consider the
following considerations, recommendations, and best practices. following considerations, recommendations, and best practices.
Transmitted Client Hints header fields SHOULD NOT provide new Transmitted Client Hints header fields SHOULD NOT provide new
information that is otherwise not available to the application via information that is otherwise not available to the application via
other means, such as using HTML, CSS, or JavaScript. Further, other means, such as using HTML, CSS, or JavaScript. Further,
sending highly granular data, such as image and viewport width may sending highly granular data, such as image and viewport width may
help identify users across multiple requests. Reducing the set of help identify users across multiple requests. Reducing the set of
field values that can be expressed, or restricting them to an field values that can be expressed, or restricting them to an
enumerated range where the advertised value is close but is not an enumerated range where the advertised value is close but is not an
exact representation of the current value, can improve privacy and exact representation of the current value, can improve privacy and
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balance privacy concerns with bandwidth limitations. However, balance privacy concerns with bandwidth limitations. However,
implementers should also be aware that explaining the privacy implementers should also be aware that explaining the privacy
implications of passive fingerprinting to users may be implications of passive fingerprinting to users may be
challenging. challenging.
o Implementations specific to certain use cases or threat models MAY o Implementations specific to certain use cases or threat models MAY
avoid transmitting some or all of Client Hints header fields. For avoid transmitting some or all of Client Hints header fields. For
example, avoid transmission of header fields that can carry higher example, avoid transmission of header fields that can carry higher
risks of linkability. risks of linkability.
Implementers SHOULD support Client Hints opt-in mechanisms and MUST Implementers SHOULD support Client Hints opt-in mechanisms and MUST
clear persisted opt-in preferences when site data, browsing history, clear persisted opt-in preferences when any one of site data,
browsing cache, or similar, are cleared. browsing history, browsing cache, or similar, are cleared.
6. IANA Considerations 4. IANA Considerations
This document defines the "Accept-CH", "DPR", "Viewport-Width", and This document defines the "Accept-CH" and "Accept-CH-Lifetime" HTTP
"Width" HTTP request fields, "Accept-CH", "Accept-CH-Lifetime", and response fields, and registers them in the Permanent Message Header
"Content-DPR" HTTP response field, and registers them in the Fields registry.
Permanent Message Header Fields registry.
6.1. Accept-CH 4.1. 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
6.2. Accept-CH-Lifetime 4.2. Accept-CH-Lifetime
o Header field name: Accept-CH-Lifetime o Header field name: Accept-CH-Lifetime
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 2.2.2 of this document o Specification document(s): Section 2.2.2 of this document
o Related information: for Client Hints o Related information: for Client Hints
6.3. Content-DPR 5. References
o Header field name: Content-DPR
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 3.1.1 of this document
o Related information: for Client Hints
6.4. DPR
o Header field name: DPR
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 3.1 of this document
o Related information: for Client Hints
6.5. Viewport-Width
o Header field name: Viewport-Width
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 3.3 of this document
o Related information: for Client Hints
6.6. Width
o Header field name: Width
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 3.2 of this document
o Related information: for Client Hints
7. References
7.1. Normative References
[CSS2] Bos, B., Celic, T., Hickson, I., and H. Lie, "Cascading
Style Sheets Level 2 Revision 1 (CSS 2.1) Specification",
W3C Recommendation REC-CSS2-20110607, June 2011,
<http://www.w3.org/TR/2011/REC-CSS2-20110607>.
[CSSVAL] Atkins, T. and E. Etemad, "CSS Values and Units Module 5.1. Normative References
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., [HTML5] 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- World Wide Web Consortium Recommendation REC-
html5-20141028, October 2014, html5-20141028, October 2014,
<http://www.w3.org/TR/2014/REC-html5-20141028>. <http://www.w3.org/TR/2014/REC-html5-20141028>.
[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,
skipping to change at page 12, line 5 skipping to change at page 9, line 19
[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,
<https://www.rfc-editor.org/info/rfc7234>. <https://www.rfc-editor.org/info/rfc7234>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References 5.2. Informative References
[KEY] Fielding, R. and M. Nottingham, "The Key HTTP Response [KEY] 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,
<https://www.rfc-editor.org/info/rfc6265>. <https://www.rfc-editor.org/info/rfc6265>.
7.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] http://httpwg.github.io/
[3] https://github.com/httpwg/http-extensions/labels/client-hints
Appendix A. Interaction with Key Response Header Field Appendix A. Interaction with Key Response Header Field
Client Hints may be combined with Key response header field ([KEY]) Client Hints may be combined with Key response header field ([KEY])
to enable fine-grained control of the cache key for improved cache to enable fine-grained control of the cache key for improved cache
efficiency. For example, the server can return the following set of efficiency. For example, the server can return the following set of
instructions: instructions:
Key: DPR;partition=1.5:2.5:4.0 Key: Sec-CH-Example;partition=1.5:2.5:4.0
Above example indicates that the cache key needs to include the value 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 of the Sec-CH-Example header field with three segments: less than
less than 2.5, and 4.0 or greater. 1.5, 1.5 to less than 2.5, and 4.0 or greater.
Key: Width;div=320 Key: Width;Sec-CH-Example=320
Above example indicates that the cache key needs to include the value Above example indicates that the cache key needs to include the value
of the Width header field and be partitioned into groups of 320: of the Sec-CH-Example header field and be partitioned into groups of
0-320, 320-640, and so on. 320: 0-320, 320-640, and so on.
Appendix B. Changes Appendix B. Changes
B.1. Since -00 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.
skipping to change at page 13, line 39 skipping to change at page 11, line 5
B.6. Since -05 B.6. Since -05
o Issue 372: Scoped CH opt-in and delivery to secure transports o Issue 372: Scoped CH opt-in and delivery to secure transports
o Issue 373: Bind CH opt-in to origin o Issue 373: Bind CH opt-in to origin
B.7. Since -06 B.7. Since -06
o Issue 524: Save-Data is now defined by NetInfo spec, dropping o Issue 524: Save-Data is now defined by NetInfo spec, dropping
B.8. Since -07
o Removed specific features to be defined in other specifications
Acknowledgements Acknowledgements
Thanks to Mark Nottingham, Julian Reschke, Chris Bentzel, Yoav Weiss, Thanks to Mark Nottingham, Julian Reschke, Chris Bentzel, Yoav Weiss,
Ben Greenstein, Tarun Bansal, Roy Fielding, Vasiliy Faronov, Ted Ben Greenstein, Tarun Bansal, Roy Fielding, Vasiliy Faronov, Ted
Hardie, Jonas Sicking, and numerous other members of the IETF HTTP Hardie, Jonas Sicking, and numerous other members of the IETF HTTP
Working Group for invaluable help and feedback. Working Group for invaluable help and feedback.
Author's Address Author's Address
Ilya Grigorik Ilya Grigorik
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