Internet DRAFT - draft-grigorik-http-client-hints
draft-grigorik-http-client-hints
HTTP Working Group I. Grigorik
Internet-Draft Google
Intended status: Informational August 24, 2015
Expires: February 25, 2016
HTTP Client Hints
draft-grigorik-http-client-hints-03
Abstract
An increasing diversity of Web-connected devices and software
capabilities has created a need to deliver optimized content for each
device.
This specification defines a set of HTTP request header fields,
colloquially known as Client Hints, to address this. They are
intended to be used as input to proactive content negotiation; just
as the Accept header allows clients to indicate what formats they
prefer, Client Hints allow clients to indicate a list of device and
agent specific preferences.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 25, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3
2. Client Hint Request Header Fields . . . . . . . . . . . . . . 4
2.1. Sending Client Hints . . . . . . . . . . . . . . . . . . . 4
2.2. Server Processing of Client Hints . . . . . . . . . . . . 4
2.2.1. Advertising Support for Client Hints . . . . . . . . . 4
2.2.2. Interaction with Caches . . . . . . . . . . . . . . . 5
3. The DPR Client Hint . . . . . . . . . . . . . . . . . . . . . 6
3.1. Confirming Selected DPR . . . . . . . . . . . . . . . . . 6
4. The Width Client Hint . . . . . . . . . . . . . . . . . . . . 6
5. The Viewport-Width Client Hint . . . . . . . . . . . . . . . . 7
6. The Downlink Client Hint . . . . . . . . . . . . . . . . . . . 7
7. The Save-Data Hint . . . . . . . . . . . . . . . . . . . . . . 7
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11. Normative References . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
There are thousands of different devices accessing the web, each with
different device capabilities and preference information. These
device capabilities include hardware and software characteristics, as
well as dynamic user and client preferences.
One way to infer some of these capabilities is through User-Agent
(UA) detection against an established database of client signatures.
However, this technique requires acquiring such a database,
integrating it into the serving path, and keeping it up to date.
However, even once this infrastructure is deployed, UA sniffing has
numerous limitations:
o UA detection cannot reliably identify all static variables
o UA detection cannot infer any dynamic client preferences
o UA detection requires an external device database
o UA detection is not cache friendly
A popular alternative strategy is to use HTTP cookies to communicate
some information about the client. However, this approach is also
not cache friendly, bound by same origin policy, and imposes
additional client-side latency by requiring JavaScript execution to
create and manage HTTP cookies.
This document defines a set of new request header fields that allow
the client to perform proactive content negotiation [RFC7231] by
indicating a list of device and agent specific preferences, through a
mechanism similar to the Accept header which is used to indicate
preferred response formats.
Client Hints does not supersede or replace the User-Agent header
field. Existing device detection mechanisms can continue to use both
mechanisms if necessary. By advertising its capabilities within a
request header field, Client Hints allows for cache friendly and
proactive content negotiation.
1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC5234] with the list rule extension defined in [RFC7230], Appendix
B. It includes by reference the DIGIT rule from [RFC5234]; the OWS,
field-name and quoted-string rules from [RFC7230]; and the parameter
rule from [RFC7231].
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2. Client Hint Request Header Fields
A Client Hint request header field is a HTTP header field that is
used by HTTP clients to indicate configuration data that can be used
by the server to select an appropriate response. Each one conveys a
list of client preferences that the server can use to adapt and
optimize the response.
This document defines a selection of Client Hint request header
fields, and can be referenced by other specifications wishing to use
the same syntax and processing model.
2.1. Sending Client Hints
Clients control which Client Hint headers and their respective header
fields are communicated, based on their default settings, user
configuration and/or preferences. The user may be given the choice
to enable, disable, or override specific hints.
The client and server, or an intermediate proxy, may use an opt-in
mechanism to negotiate which fields should be reported to allow for
efficient content adaption.
2.2. Server Processing of Client Hints
Servers MAY respond with an optimized response based on one or more
received hints from the client. When doing so, and if the resource
is cacheable, the server MUST also emit a Vary response header field
([RFC7234]), and optionally Key ([I-D.fielding-http-key]), to
indicate which hints were used and whether the selected response is
appropriate for a later request.
Further, depending on the used hint, the server MAY also need to emit
additional response header fields to confirm the property of the
response, such that the client can adjust its processing. For
example, this specification defines "Content-DPR" response header
field that MUST be returned by the server when the "DPR" hint is used
to select the response.
2.2.1. Advertising Support for Client Hints
Servers can advertise support for Client Hints using the Accept-CH
header or an equivalent HTML meta element with http-equiv attribute.
Accept-CH = #token
For example:
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Accept-CH: DPR, Width, Viewport-Width, Downlink
When a client receives Accept-CH, it SHOULD append the Client Hint
headers that match the advertised field-values. For example, based
on Accept-CH example above, the client would append DPR, Width,
Viewport-Width, and Downlink headers to all subsequent requests.
2.2.2. Interaction with Caches
When selecting an optimized response based on one or more Client
Hints, and if the resource is cacheable, the server MUST also emit a
Vary response header field ([RFC7234]) to indicate which hints were
used and whether the selected response is appropriate for a later
request.
Vary: DPR
Above example indicates that the cache key should be based on the DPR
header.
Vary: DPR, Width, Downlink
Above example indicates that the cache key should be based on the
DPR, Width, and Downlink headers.
Client Hints MAY be combined with Key ([I-D.fielding-http-key]) to
enable fine-grained control of the cache key for improved cache
efficiency. For example, the server MAY return the following set of
instructions:
Key: DPR;r=[1.5:]
Above examples indicates that the cache key should be based on the
DPR header, and the resource should be cached and made available for
any client whose device pixel ratio is 1.5, or higher.
Key: Width;r=[320:640]
Above example indicates that the cache key should be based on the
Width header, and the resource should be cached and made available
for any request whose display width falls between 320 and 640px.
Key: Downlink;r=[0.384:]
Above example indicates that the cache key should be based on the
Downlink header, and the resource should be cached and made available
for any request whose advertised maxim downlink speed is 0.384Mbps
(GPRS EDGE), or higher.
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3. The DPR Client Hint
The "DPR" header field is a number that, in requests, indicates the
client's current Device Pixel Ratio (DPR), which is the ratio of
physical pixels over CSS px of the layout viewport on the device.
DPR = 1*DIGIT [ "." 1*DIGIT ]
If DPR occurs in a message more than once, the last value overrides
all previous occurrences.
3.1. Confirming Selected DPR
The "Content-DPR" 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.
4. The Width Client Hint
The "Width" header field is a number that, in requests, indicates the
resource width in CSS px (e.g. display width of an image). The
provided CSS px value is a number rounded to the largest smallest
following integer (i.e. ceiling value).
Width = 1*DIGIT
If the resource width is not known at the time of the request or the
resource does not have a display width, the Width header field may be
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omitted. If Width occurs in a message more than once, the last value
overrides all previous occurrences.
5. The Viewport-Width Client Hint
The "Viewport-Width" header field is a number that, in requests,
indicates the layout viewport width in CSS px. The provided CSS px
value is a number rounded to the largest smallest following integer
(i.e. ceiling value).
Viewport-Width = 1*DIGIT
If Viewport-Width occurs in a message more than once, the last value
overrides all previous occurrences.
6. The Downlink Client Hint
The "Downlink" header field is a number that, in requests, indicates
the client's maximum downlink speed in megabits per second (Mbps), as
defined by the "downlinkMax" attribute in the W3C Network Information
API.
Downlink = 1*DIGIT [ "." 1*DIGIT ]
If Downlink occurs in a message more than once, the minimum value
should be used to override other occurrences.
7. The Save-Data Hint
The "Save-Data" header field is a boolean that, in requests,
indicates client's preference for reduced data usage, due to high
transfer costs, slow connection speeds, or other reasons.
Save-Data = 1
The boolean is a signal indicating explicit user opt-in into a
reduced data usage mode on the client, and when communicated to
origins allows them to deliver alternate content honoring such
preference - e.g. smaller image and video resources, alternate
markup, and so on.
8. Examples
For example, given the following request headers:
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DPR: 2.0
Width: 160
Viewport-Width: 320
The server knows that the device pixel ratio is 2.0, that the
intended display width of requested resource is 160 CSS px, and that
the viewport width is 320 CSS px.
If the server uses above hints to perform resource selection for an
image asset, it must confirm its selection via the Content-DPR
response header to allow the client to calculate the appropriate
intrinsic size of the image response. The server does not need to
confirm resource width, only the ratio between physical pixels and
CSS px of the selected image resource:
Content-DPR: 1.0
The Content-DPR response header indicates to the client that the
server has selected resource with DPR ratio of 1.0. The client may
use this information to perform additional processing on the resource
- for example, calculate the appropriate intrinsic size of the image
resource such that it is displayed at the correct resolution.
Alternatively, the server could select an alternate resource based on
the maximum downlink speed advertised in the request headers:
Downlink: 0.384
The server knows that the client's maximum downlink speed is
0.384Mbps (GPRS EDGE), and it may use this information to select an
optimized resource - for example, an alternate image asset,
stylesheet, HTML document, media stream, and so on.
9. Security Considerations
Client Hints defined in this specification do not expose any new
information about the user's environment beyond what is already
available to, and may be communicated by, the application at runtime
via JavaScript - e.g. viewport and image display width, device pixel
ratio, and so on.
However, implementors should consider the privacy implications of
various methods to enable delivery of Client Hints - see "Sending
Client Hints" section. For example, sending Client Hints on all
requests may make information about the user's environment available
to origins that otherwise did not have access to this data (e.g.
origins hosting non-script resources), which may or not be the
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desired outcome. The implementors may want to provide mechanisms to
control such behavior via explicit opt-in, or other mechanisms.
Similarly, the implementors should consider how and whether delivery
of Client Hints is affected when the user is in "incognito" or
similar privacy mode.
10. IANA Considerations
This document defines the "Accept-CH", "DPR", "Width", and "Downlink"
HTTP request fields, "Content-DPR" HTTP response field, and registers
them in the Permanent Message Header Fields registry.
o Header field name: DPR
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: Ilya Grigorik, ilya@igvita.com
o Specification document(s): [this document]
o Related information: for Client Hints
o Header field name: Width
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: Ilya Grigorik, ilya@igvita.com
o Specification document(s): [this document]
o Related information: for Client Hints
o Header field name: Viewport-Width
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: Ilya Grigorik, ilya@igvita.com
o Specification document(s): [this document]
o Related information: for Client Hints
o Header field name: Downlink
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: Ilya Grigorik, ilya@igvita.com
o Specification document(s): [this document]
o Related information: for Client Hints
o Header field name: Content-DPR
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: Ilya Grigorik, ilya@igvita.com
o Specification document(s): [this document]
o Related information: for Client Hints
o Header field name: Accept-CH
o Applicable protocol: HTTP
o Status: standard
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o Author/Change controller: Ilya Grigorik, ilya@igvita.com
o Specification document(s): [this document]
o Related information: for Client Hints
o Header field name: Save-Data
o Applicable protocol: HTTP
o Status: standard
o Author/Change controller: Ilya Grigorik, ilya@igvita.com
o Specification document(s): [this document]
o Related information: for Client Hints
11. Normative References
[I-D.fielding-http-key]
Fielding, R. and M. Nottingham, "The Key HTTP Response
Header Field", draft-fielding-http-key-02 (work in
progress), February 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<http://www.rfc-editor.org/info/rfc7234>.
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Author's Address
Ilya Grigorik
Google
Email: ilya@igvita.com
URI: https://www.igvita.com/
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