Network Working Group I. Grigorik
Internet-Draft March 15, 2013
Intended status: Informational
Expires: September 16, 2013
HTTP Client Hints
draft-grigorik-http-client-hints-00
Abstract
An increasing diversity of connected device form factors and software
capabilities has created a need to deliver varying, or optimized
content for each device.
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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 16, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . . 3
2. The "CH" Request Header Field . . . . . . . . . . . . . . . . . 3
2.1. Hint Syntax . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Pre-defined Hints . . . . . . . . . . . . . . . . . . . . . 4
2.2.1. dh . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2.2. dw . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.3. dpr . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Server opt-in with Hop and Origin Hints . . . . . . . . . . 5
2.5. Interaction with Caches . . . . . . . . . . . . . . . . . . 6
2.6. Relationship to the User-Agent Request Header . . . . . . . 6
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
3.1. The CH Request Header Field . . . . . . . . . . . . . . . . 6
3.2. The HTTP Hints . . . . . . . . . . . . . . . . . . . . . . 7
3.3. The HTTP Client Hints Registry . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
5. 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
the following limitations:
o UA detection requires an external device database
o UA detection cannot reliably identify all static variables
o UA detection cannot infer any dynamic client preferences
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 new request Client Hint header field, "CH",
that allows 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 prefered 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. The "CH" Request Header Field
The "CH" request header field describes an example list of client
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preferences that the server can use to adapt and optimize the
resource to satisfy a given request. The CH field-value is a comma-
delimited list of header fields, and the field-name values are case
insensitive.
CH = #client-hint
client-hint = parameter
2.1. Hint Syntax
Hints are allowed to have a numeric value. However, where possible,
they can can be defined as flags (i.e., as a hint name only), so that
the hints don't consume too much space in client requests.
Hints can be defined as one of two types:
o Boolean - indicated by the presence of the hint name. If the hint
name is absent in the last message containing the client hint
header field, it is considered false.
o Numeric - value indicated by the digits after "=", 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/1.1 allows headers with comma-separated values to be
conveyed using multiple instances of the same header; as a result,
the hints are collected from all instances of the CH header on the
message in question before being considered complete.
2.2. Pre-defined Hints
The client controls which header fields are communicated within the
CH header, based on its default settings, or based on user
configuration and preferences. The user may be given the choice to
enable, disable, or override specific hints. For example, to allow
the request for low-resolution images or other content type's while
roaming on a foreign network, even while on a high-bandwidth link.
The client and server, or an intermediate proxy, may use an
additional mechanism to negotiate which fields should be reported to
allow for efficient content adaption.
This document defines the following hint names:
2.2.1. dh
o Description: device-width in secondary orientation, in density
independent pixels.
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o Value Type: number
2.2.2. dw
o Description: device-width in primary orientation, in density
independent pixels.
o Value Type: number
2.2.3. dpr
o Description: Device Pixel Ratio (dpr), is the ratio between
physical pixels and density independent pixels on the device.
o Value Type: number
Other client hints may be communicated by the client. The decision
as to which specific hints will be sent is made by the client.
2.3. Examples
For example, given the following request header:
CH: dh=598, dw=384, dpr=2.0
The server knows that the client's screen height is 598px, width is
384px, as measured by density independent pixels on the device, and
that the device pixel ratio is 2.0.
2.4. Server opt-in with Hop and Origin Hints
CH is an optional header which may be sent by the client when making
a request to the server. The client may decide to always send the
header, or use an optional opt-in mechanism, such as a predefined
list of origins, user specified list of origins, or any other forms
of opt-in.
For example, the server may advertise its support for Client Hints
via Hop and/or Origin Hint ([I-D.nottingham-http-browser-hints]):
OH: ch
When a client receives the Hop or Origin Hint header indicating
support for Client Hint adaptation, it should append the CH header to
subsequent requests to the same origin server. Further, the client
may remember this hint and automatically append the CH header for all
future requests to the same origin.
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2.5. 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;pr=dw[320:640]
Above example indicates that the cache key should be based on the CH
header, and the asset should be cached and made available for any
client whose device width (dw) falls between 320 and 640 px.
Key: CH;pr=dpr[1.5:]
Above examples indicates that the cache key should be based on the CH
header, and the asset should be cached and made available for any
client whose device pixel ratio (dpr) is 1.5, or higher.
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: CH
2.6. 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.
3. IANA Considerations
3.1. The CH Request Header Field
This document defines the "CH" HTTP request field, and registers it
in the Permanent Message Headers registry.
o Header field name: CH
o Applicable protocol: HTTP
o Status: Informational
o Author/Change controller: Ilya Grigorik, ilya@igvita.com
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o Specification document(s): [this document]
o Related information: for Client Hints
3.2. The HTTP Hints
This document registers the "ch" HTTP Hint
([I-D.nottingham-http-browser-hints]), as defined in section 2.1:
o Hint Name: ch
o Hint Type: origin, hop
o Description: When present, this hint indicates support for Client-
Hints adaptation.
o Value Type: numeric
o Contact: ilya@igvita.com
o Specification: this document
3.3. The HTTP Client Hints Registry
This document establishes the HTTP Client Hints Registry.
New hints are registered using Expert Review (see [RFC5226]), by
sending e-mail to iana@iana.org (or using other mechanisms, as
established by IANA).
New hints are expected to be implemented in at least one client in
common use. The Expert MAY use their judgement in determining what
"common" is, and when something is considered to be implemented.
New hints MUST be optional; they cannot place requirements upon
implementations. Specifically, new hints MUST NOT make communication
non-conformant with HTTP itself; i.e., this is not a mechanism for
changing the HTTP protocol in incompatible ways.
See section 2.1 for constraints on the syntax of hint names and hint
values.
Registration requests MUST use the following template:
o Hint Name: [name of hint]
o Hint Value: ["boolean" or "numeric"]
o Description: [description of hint]
o Contact: [e-mail address(es)]
o Specification: [optional; reference or URI to more info]
o Notes: [optional]
The initial contents of the registry are defined in section 2.2.
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4. 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.
5. 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-22 (work in progress),
February 2013.
[I-D.ietf-httpbis-p2-semantics]
Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p2-semantics-22 (work in progress),
February 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.
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Author's Address
Ilya Grigorik
Email: ilya@igvita.com
URI: http://www.igvita.com/
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