Network Working Group I. Grigorik
Internet-Draft Google
Intended status: Informational September 4, 2014
Expires: March 8, 2015
HTTP Client Hints
draft-grigorik-http-client-hints-02
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 8, 2015.
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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 . . . . . . . . . . . . . . . . . . . . . . 5
4. The RW Client Hint . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Confirming Selected DPR . . . . . . . . . . . . . . . . . . 6
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
8. Normative References . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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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 can modify the response sent based upon Client Hints. When
doing so, it MUST confirm the selection for certain hints and
indicate the value of selected resource via corresponding response
header. For example, this specification defines "DPR" that
corresponds to the "DPR" request header field.
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:
Accept-CH: DPR, RW
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 and RW
headers to all subsequent requests.
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2.2.2. Interaction with Caches
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: RW;r=[320:640]
Above example indicates that the cache key should be based on the RW
header, and the resource should be cached and made available for any
request whose display width falls between 320 and 640px.
In absence of support for fine-grained control of the cache key via
the Key header field, Vary response header can be used to indicate
that served resource has been adapted based on specified Client Hint
preferences.
Vary: DPR
Above example indicates that the cache key should be based on the DPR
header.
Vary: DPR, RW
Above example indicates that the cache key should be based on the DPR
and RW headers.
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.
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4. The RW Client Hint
The "RW" header field is a number that, in requests, indicates the
Resource Width (RW) in CSS px, which is either the display width of
the requested resource (e.g. display width of an image), or the
layout viewport width if the resource does not have a display width
(e.g. a non-image asset).
RW = 1*DIGIT
If RW occurs in a message more than once, the last value overrides
all previous occurrences.
4.1. 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 (RW) 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.
5. Example
For example, given the following request headers:
DPR: 2.0
RW: 160
The server knows that the device pixel ratio is 2.0, and that the
intended display width of requested resource is 160 CSS px.
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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.
6. IANA Considerations
This document defines the "Accept-CH", "DPR", and "RW" 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: RW
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
o Author/Change controller: Ilya Grigorik, ilya@igvita.com
o Specification document(s): [this document]
o Related information: for Client Hints
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7. Security Considerations
The client controls which header fields are communicated and when.
In cases such as incognito or anonymous profile browsing, the header
can be omitted if necessary.
8. 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, March 1997.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230,
June 2014.
[RFC7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics and Content", RFC 7231, June 2014.
Author's Address
Ilya Grigorik
Google
Email: ilya@igvita.com
URI: https://www.igvita.com/
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