draft-ietf-httpbis-client-hints-07.txt   draft-ietf-httpbis-client-hints-latest.txt 
HTTP Working Group I. Grigorik HTTP Working Group I. Grigorik
Internet-Draft Google Internet-Draft Y. Weiss
Intended status: Experimental March 11, 2019 Intended status: Experimental Google
Expires: September 12, 2019 Expires: January 30, 2020 July 29, 2019
HTTP Client Hints HTTP Client Hints
draft-ietf-httpbis-client-hints-07 draft-ietf-httpbis-client-hints-07
Abstract Abstract
HTTP defines proactive content negotiation to allow servers to select HTTP defines proactive content negotiation to allow servers to select
the appropriate response for a given request, based upon the user the appropriate response for a given request, based upon the user
agent's characteristics, as expressed in request headers. In agent's characteristics, as expressed in request headers. In
practice, clients are often unwilling to send those request headers, practice, clients are often unwilling to send those request headers,
because it is not clear whether they will be used, and sending them because it is not clear whether they will be used, and sending them
impacts both performance and privacy. impacts both performance and privacy.
This document defines two response headers, Accept-CH and Accept-CH- This document defines an Accept-CH response header that servers can
Lifetime, that servers can use to advertise their use of request use to advertise their use of request headers for proactive content
headers for proactive content negotiation, along with a set of negotiation, along with a set of guidelines for the creation of such
guidelines for the creation of such headers, colloquially known as headers, colloquially known as "Client Hints."
"Client Hints."
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/ [1].
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 [2]; 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 [3].
skipping to change at page 2, line 6 skipping to change at page 2, line 4
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 30, 2020.
This Internet-Draft will expire on September 12, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2019 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.
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
(https://trustee.ietf.org/license-info) in effect on the date of (https://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 . . . . . . . . . . . . 5 2.2. Server Processing of Client Hints . . . . . . . . . . . . 5
2.2.1. Advertising Support via Accept-CH Header Field . . . 5 3. Advertising Server Support . . . . . . . . . . . . . . . . . 5
2.2.2. The Accept-CH-Lifetime Header Field . . . . . . . . . 5 3.1. The Accept-CH Response Header Field . . . . . . . . . . . 5
2.2.3. Interaction with Caches . . . . . . . . . . . . . . . 6 3.1.1. Interaction with Caches . . . . . . . . . . . . . . . 6
3. Security Considerations . . . . . . . . . . . . . . . . . . . 7 4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
4.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 8 5.1. Accept-CH . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Accept-CH-Lifetime . . . . . . . . . . . . . . . . . . . 8 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 6.1. Normative References . . . . . . . . . . . . . . . . . . 7
5.1. Normative References . . . . . . . . . . . . . . . . . . 8 6.2. Informative References . . . . . . . . . . . . . . . . . 8
5.2. Informative References . . . . . . . . . . . . . . . . . 9 6.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Appendix A. Interaction with Variants Response Header Field . . 9
Appendix A. Interaction with Key Response Header Field . . . . . 9 Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 9
Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 10 B.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 9
B.1. Since -00 . . . . . . . . . . . . . . . . . . . . . . . . 10 B.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 9
B.2. Since -01 . . . . . . . . . . . . . . . . . . . . . . . . 10 B.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 9
B.3. Since -02 . . . . . . . . . . . . . . . . . . . . . . . . 10 B.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 9
B.4. Since -03 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 10 B.5. Since -04 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 10 B.6. Since -05 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 10 B.7. Since -06 . . . . . . . . . . . . . . . . . . . . . . . . 10
B.8. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 11 B.8. Since -07 . . . . . . . . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
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. Applications that want
to allow the server to optimize content delivery and user experience
One way to infer some of these capabilities is through User-Agent based on such capabilities have, historically, had to rely on passive
(Section 5.5.3 of [RFC7231]) header field detection against an identification (e.g., by matching User-Agent (Section 5.5.3 of
established database of client signatures. However, this technique [RFC7231]) header field against an established database of client
requires acquiring such a database, integrating it into the serving signatures), used HTTP cookies and URL parameters, or use some
path, and keeping it up to date. However, even once this combination of these and similar mechanisms to enable ad hoc content
infrastructure is deployed, user agent sniffing has numerous negotiation.
limitations:
o User agent detection cannot reliably identify all static variables Such techniques are expensive to setup and maintain, are not portable
o User agent detection cannot infer any dynamic client preferences across both applications and servers, and make it hard to reason for
o User agent detection requires an external device database both client and server about which data is required and is in use
o User agent detection is not cache friendly during the negotiation:
A popular alternative strategy is to use HTTP cookies ([RFC6265]) to o User agent detection cannot reliably identify all static
communicate some information about the user agent. However, this variables, cannot infer dynamic client preferences, requires
approach is also not cache friendly, bound by same origin policy, and external device database, is not cache friendly, and is reliant on
often imposes additional client-side latency by requiring JavaScript a passive fingerprinting surface.
execution to create and manage HTTP cookies. o Cookie based approaches are not portable across applications and
servers, impose additional client-side latency by requiring
JavaScript execution, and are not cache friendly.
o URL parameters, similar to cookie based approaches, suffer from
lack of portability, and are hard to deploy due to a requirement
to encode content negotiation data inside of the URL of each
resource.
Proactive content negotiation (Section 3.4.1 of [RFC7231]) offers an Proactive content negotiation (Section 3.4.1 of [RFC7231]) offers an
alternative approach; user agents use specified, well-defined request alternative approach; user agents use specified, well-defined request
headers to advertise their capabilities and characteristics, so that headers to advertise their capabilities and characteristics, so that
servers can select (or formulate) an appropriate response. servers can select (or formulate) an appropriate response.
However, proactive content negotiation requires clients to send these However, proactive content negotiation requires clients to send these
request headers prolifically. This causes performance concerns request headers prolifically. This causes performance concerns
(because it creates "bloat" in requests), as well as privacy issues; (because it creates "bloat" in requests), as well as privacy issues;
passively providing such information allows servers to silently passively providing such information allows servers to silently
fingerprint the user agent. fingerprint the user agent.
This document defines a new response header, Accept-CH, that allows This document defines a new response header, Accept-CH, that allows
an origin server to explicitly ask that clients send these headers in an origin server to explicitly ask that clients send these headers in
requests, for a period of time bounded by the Accept-CH-Lifetime requests. It also defines guidelines for content negotiation
response header. It also defines guidelines for content negotiation
mechanisms that use it, colloquially referred to as Client Hints. mechanisms that use it, colloquially referred to as Client Hints.
Client Hints mitigate the performance concerns by assuring that Client Hints mitigate the performance concerns by assuring that
clients will only send the request headers when they're actually clients will only send the request headers when they're actually
going to be used, and the privacy concerns of passive fingerprinting going to be used, and the privacy concerns of passive fingerprinting
by requiring explicit opt-in and disclosure of required headers by by requiring explicit opt-in and disclosure of required headers by
the server through the use of the Accept-CH response header. the server through the use of the Accept-CH response header.
This document defines the Client Hints infrastructure, a framework This document defines the Client Hints infrastructure, a framework
that enables servers to opt-in to specific proactive content that enables servers to opt-in to specific proactive content
negotiation features, which will enable them to adapt their content negotiation features, which will enable them to adapt their content
accordingly. However, it does not define any specific features that accordingly. However, it does not define any specific features that
will use that infrastructure. Those features will be defined in will use that infrastructure. Those features will be defined in
their respective specifications. their respective specifications.
This document does not supersede or replace the User-Agent header
field. Existing device detection mechanisms can continue to use both
mechanisms if necessary. By advertising user agent capabilities
within a request header field, Client Hints allow for cache friendly
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
[RFC5234] with the list rule extension defined in [RFC7230], [RFC5234] with the list rule extension defined in [RFC7230],
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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. client processing.
2.2.1. Advertising Support via Accept-CH Header Field 3. Advertising Server Support
Servers can advertise support for Client Hints using the Accept-CH
header field or an equivalent HTML meta element with http-equiv
attribute ([HTML5]).
Accept-CH = #field-name Servers can advertise support for Client Hints using the mechnisms
described below.
For example: 3.1. The Accept-CH Response Header Field
Accept-CH: Sec-CH-Example, Sec-CH-Example-2 The Accept-CH response header field or the equivalent HTML meta
element with http-equiv attribute ([HTML5]) indicate server support
for particular hints indicated in its value.
When a client receives an HTTP response advertising support for Accept-CH is a Structured Header [I-D.ietf-httpbis-header-structure].
Client Hints, it should process it as origin ([RFC6454]) opt-in to Its value MUST be an sh-list (Section 3.1 of
receive Client Hint header fields advertised in the field-value. The [I-D.ietf-httpbis-header-structure]) whose members are tokens
opt-in MUST be delivered over a secure transport. (Section 3.7 of [I-D.ietf-httpbis-header-structure]). Its ABNF is:
For example, based on Accept-CH example above, a user agent could Accept-CH = sh-list
append the Sec-CH-Example and Sec-CH-Example-2 header fields to all
same-origin resource requests initiated by the page constructed from
the response.
2.2.2. The Accept-CH-Lifetime Header Field For example:
Servers can ask the client to remember the set of Client Hints that Accept-CH: Sec-CH-Example, Sec-CH-Example-2
the server supports for a specified period of time, to enable
delivery of Client Hints on subsequent requests to the server's
origin ([RFC6454]).
Accept-CH-Lifetime = #delta-seconds When a client receives an HTTP response advertising support for
provided list of Clients Hints, it SHOULD process it as origin
([RFC6454]) opt-in to receive Client Hint header fields advertised in
the field-value, for subsequent same-origin requests.
When a client receives an HTTP response that contains Accept-CH- o The opt-in MUST be delivered over a secure transport.
Lifetime header field, the field-value indicates that the Accept-CH o The opt-in SHOULD be persisted and bound to the origin to enable
preference SHOULD be persisted and bound to the origin, and be delivery of Client Hints on subsequent requests to the server's
considered stale after response's age ([RFC7234], section 4.2) is origin, and MUST NOT be persisted for an origin that isn't HTTPS.
greater than the specified number of seconds. The preference MUST be
delivered over a secure transport, and MUST NOT be persisted for an
origin that isn't HTTPS.
Accept-CH: Sec-CH-Example, Sec-CH-Example-2 Accept-CH: Sec-CH-Example, Sec-CH-Example-2
Accept-CH: Sec-CH-Example-3 Accept-CH: Sec-CH-Example-3
Accept-CH-Lifetime: 86400
For example, based on the Accept-CH and Accept-CH-Lifetime example For example, based on the Accept-CH example above, which is received
above, which is received in response to a user agent navigating to in response to a user agent navigating to "https://example.com", and
"https://example.com", and delivered over a secure transport: a user delivered over a secure transport: a user agent SHOULD persist an
agent SHOULD persist an Accept-CH preference bound to Accept-CH preference bound to "https://example.com" 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
requests initiated to "https://example.com" from other origins. requests initiated to "https://example.com" from other origins.
If Accept-CH-Lifetime occurs in a message more than once, the last 3.1.1. Interaction with Caches
value overrides all previous occurrences.
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: Sec-CH-Example Vary: Sec-CH-Example
Above example indicates that the cache key needs to include the Sec- Above example indicates that the cache key needs to include the Sec-
CH-Example header field. CH-Example header field.
Vary: Sec-CH-Example, Sec-CH-Example-2 Vary: Sec-CH-Example, Sec-CH-Example-2
Above example indicates that the cache key needs to include the Sec- Above example indicates that the cache key needs to include the Sec-
CH-Example and Sec-CH-Example-2 header fields. CH-Example and Sec-CH-Example-2 header fields.
3. Security Considerations 4. Security Considerations
The request header fields defined in this document, and those that The request header fields defined in this document, and those that
extend it, expose information about the user's environment to enable extend it, expose information about the user's environment to enable
proactive content negotiation. Such information may reveal new proactive content negotiation. Such information may reveal 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,
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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 any one of site data, clear persisted opt-in preferences when any one of site data,
browsing history, browsing cache, or similar, are cleared. browsing history, browsing cache, or similar, are cleared.
4. IANA Considerations 5. IANA Considerations
This document defines the "Accept-CH" and "Accept-CH-Lifetime" HTTP This document defines the "Accept-CH" HTTP response field, and
response fields, and registers them in the Permanent Message Header registers it in the Permanent Message Header Fields registry.
Fields registry.
4.1. Accept-CH 5.1. 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 3.1 of this document
o Related information: for Client Hints
4.2. Accept-CH-Lifetime
o Header field name: Accept-CH-Lifetime
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: IETF
o Specification document(s): Section 2.2.2 of this document
o Related information: for Client Hints o Related information: for Client Hints
5. References 6. References
5.1. Normative References 6.1. Normative References
[HTML5] Hickson, I., Berjon, R., Faulkner, S., Leithead, T., [HTML5] Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
Navara, E., O'Connor, T., and S. Pfeiffer, "HTML5", Navara, E., O'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>.
[I-D.ietf-httpbis-header-structure]
Nottingham, M. and P. Kamp, "Structured Headers for HTTP",
draft-ietf-httpbis-header-structure-11 (work in progress),
July 2019.
[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,
<https://www.rfc-editor.org/info/rfc2119>. <https://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,
<https://www.rfc-editor.org/info/rfc5234>. <https://www.rfc-editor.org/info/rfc5234>.
skipping to change at page 9, line 19 skipping to change at page 8, line 43
[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>.
5.2. Informative References 6.2. 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>.
5.3. URIs [VARIANTS]
Nottingham, M., "HTTP Representation Variants", draft-
ietf-httpbis-variants-05 (work in progress), March 2019.
6.3. URIs
[1] https://lists.w3.org/Archives/Public/ietf-http-wg/ [1] https://lists.w3.org/Archives/Public/ietf-http-wg/
[2] http://httpwg.github.io/ [2] http://httpwg.github.io/
[3] https://github.com/httpwg/http-extensions/labels/client-hints [3] https://github.com/httpwg/http-extensions/labels/client-hints
Appendix A. Interaction with Key Response Header Field Appendix A. Interaction with Variants Response Header Field
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: Sec-CH-Example;partition=1.5:2.5:4.0
Above example indicates that the cache key needs to include the value
of the Sec-CH-Example header field with three segments: less than
1.5, 1.5 to less than 2.5, and 4.0 or greater.
Key: Width;Sec-CH-Example=320
Above example indicates that the cache key needs to include the value Client Hints may be combined with Variants response header field
of the Sec-CH-Example header field and be partitioned into groups of [VARIANTS] to enable fine-grained control of the cache key for
320: 0-320, 320-640, and so on. improved cache efficiency. Features that define Client Hints will
need to specify the related variants algorithms as described in
Section 6 of [VARIANTS].
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 11, line 8 skipping to change at page 10, line 22
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 B.8. Since -07
o Removed specific features to be defined in other specifications o Removed specific features to be defined in other specifications
o Removed Accept-CH-Lifetime based on feedback at IETF 105
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, Martin Thomson, and numerous other members of
Working Group for invaluable help and feedback. the IETF HTTP Working Group for invaluable help and feedback.
Author's Address Authors' Addresses
Ilya Grigorik Ilya Grigorik
Google Google
Email: ilya@igvita.com Email: ilya@igvita.com
URI: https://www.igvita.com/ URI: https://www.igvita.com/
Yoav Weiss
Google
Email: yoav@yoav.ws
URI: https://blog.yoav.ws/
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