Network Working Group I. Grigorik
Internet-Draft December 6, 2013
Intended status: Informational
Expires: June 9, 2014
HTTP Client Hints
draft-grigorik-http-client-hints-01
Abstract
An increasing diversity of Web-connected device form factors and
software capabilities has created a need to deliver varying, or
optimized content for each device.
HTTP Client Hints can be used as input to proactive content
negotiation; just as the Accept header allowed 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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This Internet-Draft will expire on June 9, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3
2. Client Hint Request Header Fields . . . . . . . . . . . . . . . 3
2.1. Hint Values . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Sending Client Hints . . . . . . . . . . . . . . . . . . . 4
2.3. Server Processing of Client Hints . . . . . . . . . . . . . 5
2.3.1. Advertising Support for Client Hints . . . . . . . . . 5
2.3.2. Interaction with Caches . . . . . . . . . . . . . . . . 5
3. The CH-DPR Client Hint . . . . . . . . . . . . . . . . . . . . 6
4. The CH-RW Client Hint . . . . . . . . . . . . . . . . . . . . . 6
4.1. Confirming Selected DPR . . . . . . . . . . . . . . . . . . 6
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Relationship to the User-Agent Request Header . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
8. Normative References . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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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
[I-D.ietf-httpbis-p2-semantics] 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.
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
[I-D.ietf-httpbis-p1-messaging], Appendix B. It includes by
reference the OWS, field-name and quoted-string rules from that
document, and the parameter rule from
[I-D.ietf-httpbis-p2-semantics].
2. Client Hint Request Header Fields
A Client Hint request header field is a HTTP header field that is
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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.
Client Hint request headers share a common syntax. As a convention,
those defined in this specification have names prefixed with "CH-",
but this is only a convenience.
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. Hint Values
Client-Hint field-values consist of either a token or a comma-
delimited list of parameters.
client-hint-value = token | 1#parameter
When the value is a token, it can either be boolean or a numeric
value. Where possible, this form SHOULD be used, so that the hints
don't consume too much space in requests.
Boolean values are indicated by the presence of the hint field-name
in the request headers. If the hint name is absent in the last
message containing the client hint header field, it is considered
false.
Numeric values are indicated by the full field-value contents (single
value), or by the digits after "=" of the hint name (parameter
value), up to the first non-digit character. If the hint does not
have an argument, its value is assumed to be 0.
Note that HTTP allows headers with comma-separated values to be
conveyed using multiple instances of the same header field; as a
result, the hints are collected from all instances of the same header
on the message in question before being considered complete. If the
same hint is used more than once, then the last hint overrides all
previous occurrences, and the final ordering of unique hints is not
significant.
2.2. 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.
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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.
2.3. 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 "CH-DPR" request header field.
2.3.1. 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 CH-DPR and CH-RW
headers to subsequent requests.
2.3.2. 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: CH-DPR;r=[1.5:]
Above examples indicates that the cache key should be based on the
CH-DPR header, and the resource should be cached and made available
for any client whose device pixel ratio is 1.5, or higher.
Key: CH-RW;r=[320:640]
Above example indicates that the cache key should be based on the
CH-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
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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: CH-DPR
Above example indicates that the cache key should be based on the CH-
DPR header.
Vary: CH-DPR, CH-RW
Above example indicates that the cache key should be based on the CH-
DPR and CH-RW headers.
3. The CH-DPR Client Hint
The "CH-DPR" header field indicates the client's current Device Pixel
Ratio (DPR), the ratio between physical pixels and density
independent pixels on the device.
CH-DPR = 1*DIGIT [ "." 1*DIGIT ]
4. The CH-RW Client Hint
The "CH-RW" header field indicates the client's current Resource
Width (RW), the display width of the requested resource in density
independent pixels on the device.
CH-RW = 1*DIGIT
4.1. Confirming Selected DPR
The "DPR" header field indicates the ratio between physical pixels
and density independent pixels of the selected response.
DPR = 1*DIGIT [ "." 1*DIGIT ]
DPR ratio affects the calculation of intrinsic size of the image 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 resource whenever CH-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 DPR value contradicts previous client-
side DPR indication, the server returned value must take precedence.
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The server does not need to confirm resource width (RW) selection as
this value can be derived from the resource itself once it is decoded
by the client.
5. Examples
For example, given the following request header:
CH-DPR: 2.0
CH-RW: 160
The server knows that the device pixel ratio is 2.0, and that the
intended display width of requested resource is 160px, as measured by
density independent pixels on the device.
If the server uses above hints to perform resource selection, it must
confirm its selection via the DPR response header to allow the client
to calculate the appropriate intrinsic size of the image resource.
The server does not need to confirm resource width, only the ratio
between physical pixels and density independent pixels of the
selected image resource:
DPR: 1.0
The 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.
5.1. Relationship to the User-Agent Request Header
Client Hints does not supersede or replace User-Agent. Existing
device detection mechanisms can continue to use both mechanisms if
necessary. By advertising its capabilities within a request header,
Client Hints allows for cache friendly and proactive content
negotiation.
6. IANA Considerations
This document defines the "Accept-CH", "CH-DPR", "CH-RW", and "DPR"
HTTP request fields, and registers them in the Permanent Message
Headers registry.
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o Header field name: CH-DPR
o Applicable protocol: HTTP
o Status: Informational
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: CH-RW
o Applicable protocol: HTTP
o Status: Informational
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: DPR
o Applicable protocol: HTTP
o Status: Informational
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: Informational
o Author/Change controller: Ilya Grigorik, ilya@igvita.com
o Specification document(s): [this document]
o Related information: for Client Hints
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.
[I-D.ietf-httpbis-p1-messaging]
Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing",
draft-ietf-httpbis-p1-messaging-25 (work in progress),
November 2013.
[I-D.ietf-httpbis-p2-semantics]
Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
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(HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p2-semantics-25 (work in progress),
November 2013.
[I-D.nottingham-http-browser-hints]
Nottingham, M., "HTTP Origin and Hop Hints",
draft-nottingham-http-browser-hints-05 (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.
Author's Address
Ilya Grigorik
Email: ilya@igvita.com
URI: http://www.igvita.com/
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