HTTP Browser Hints
draft-nottingham-http-browser-hints-04
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| Author | Mark Nottingham | ||
| Last updated | 2012-10-14 | ||
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draft-nottingham-http-browser-hints-04
Network Working Group M. Nottingham
Internet-Draft October 15, 2012
Intended status: Informational
Expires: April 18, 2013
HTTP Browser Hints
draft-nottingham-http-browser-hints-04
Abstract
Over time, Web browsers have adapted how they use HTTP based upon
common server configurations and behaviours. While this is necessary
in the common case, it can be detrimental for performance and
interoperability.
This document establishes a mechanism whereby origin servers can make
available hints for browsers about their preferences and
capabilities, without imposing overhead on their interactions or
requiring support for them.
This is intended to allow browsers to safely optimise connections to
servers.
Note to Readers
Feedback for this draft should take place on the
apps-discuss@ietf.org mailing list
<https://www.ietf.org/mailman/listinfo/apps-discuss>.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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 April 18, 2013.
Copyright Notice
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Copyright (c) 2012 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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. A file format for Browser Hints . . . . . . . . . . . . . . . 3
3.1. The 'prefixlist' Value Type . . . . . . . . . . . . . . . 4
4. Discovering Browser Hints for a Web site . . . . . . . . . . . 4
4.1. Notifying Clients with the BH Response Header Field . . . 5
4.2. Interaction with HTTP Proxies . . . . . . . . . . . . . . 6
5. Pre-defined Browser Hints . . . . . . . . . . . . . . . . . . 6
5.1. max-conns . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. pconn-ip . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.3. ip-balance . . . . . . . . . . . . . . . . . . . . . . . . 7
5.4. connect-timeout . . . . . . . . . . . . . . . . . . . . . 7
5.5. read-timeout . . . . . . . . . . . . . . . . . . . . . . . 8
5.6. max-pipeline-depth . . . . . . . . . . . . . . . . . . . . 8
5.7. small-hdrs . . . . . . . . . . . . . . . . . . . . . . . . 8
5.8. relative-referer . . . . . . . . . . . . . . . . . . . . . 8
5.9. chunk-req-bodies . . . . . . . . . . . . . . . . . . . . . 9
5.10. omit-cookies . . . . . . . . . . . . . . . . . . . . . . . 9
5.11. cookie-whitelist . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7.1. The 'browser-hints' Well-Known URI . . . . . . . . . . . . 9
7.2. The BH HTTP Response Header Field . . . . . . . . . . . . 10
7.3. The HTTP Browser Hints Registry . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
HTTP [RFC2616] clients -- especially browsers -- typically use
hardcoded values or heuristics to determine how many TCP connections
to use to a server, based on common-case server behaviours and
limitations.
Likewise, they often send voluminous request headers (e.g., in User-
Agent and Allow) because they fear that changing those headers'
values will break some sites that depend upon specific values.
These are just two examples of common, conservative behaviour by
browsers that is good for interoperability, but potentially bad for
performance in certain circumstances.
This document specifies a mechanism whereby a HTTP server can
advertise hints for browsers (and other clients), so that
communication with them can be optimised.
It does so by defining a file format for such Browser Hints
Section 3, and defining how clients can discover it for a given Web
site Section 4. Finally, an extensible vocabulary of hints is
defined Section 5.
2. Requirements
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].
3. A file format for Browser Hints
Browser hints are conveyed in a JSON [RFC4627] formatted file,
containing a single object whose member's names are browser hints, as
defined by the registry Section 7.3.
For example;
{
"max-conns": 5,
"small-hdrs": true
}
By their nature, all browser hints are optional; i.e., browsers are
free to ignore them.
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3.1. The 'prefixlist' Value Type
Each browser hint is defined to have a JSON-derived value type; e.g.,
'string' or 'array'. This section defines a special value type,
'prefixlist' that is an array containing one or more arrays, each
containing a path prefix followed by either 'true' or 'false' to
indicate whether the hint applies to that path.
Prefixlists are evaluated in order, with the first case-sensitive,
character-by-character prefix match for a given URI's path
determining whether the hint applies.
For example, given the following hint document:
{
"omit-cookies": [
["/images/users/", false],
["/images/", true]
]
}
We can tell that "omit-cookies" applies to resources under the
"/images/" path (such as "/images/123.jpg"), except for those under
"/images/users/" (such as "/images/users/bob.jpg").
If a value specified to be a prefixlist is either 'true' or 'false'
that indicates that the hint applies to the whole site, or does not
apply to the whole site, respectively.
For example,
{
"omit-cookies": true
}
Indicates that the "omit-cookies" hint applies to the entire site.
Prefixlists can only be used when the browser hint's registration
nominates their use.
4. Discovering Browser Hints for a Web site
The hints relevant to a given site can be determined by fetching the
URI path "/.well-known/browser-hints" for that site.
Typically, clients (especially browsers) will not block other
requests to a site while fetching the browser hints (because they're
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optional); instead, it will usually be done concurrently with other
requests, or on idle connections for future use.
In this specification, "site" is scoped by the URI scheme and
authority. As such, all of the following are considered to be
different sites, and therefore have different browser hints:
o http://foo.com/
o https://foo.com/
o http://foo.com:8000/
o http://www.foo.com/
Clients SHOULD follow HTTP 3xx redirects when retrieving hints.
A successful response is valid for its associated site for as long as
it can be cached in HTTP.
If the response has a 200 status code but no explicit freshness
(e.g., a Cache-Control: max-age or Expires: header), clients SHOULD
cache the response heuristically for a generous fixed period (e.g.,
14 days).
If the response has a 404 status code but no explicit freshness,
clients SHOULD cache the response heuristically for a generous fixed
period (e.g., 14 days).
4.1. Notifying Clients with the BH Response Header Field
It is anticipated that Browser Hints will be used by some, but not
all, Web sites. Because clients might be reluctant to optimistically
request the well-known URI, this document defines a new HTTP response
header field, BH, to indicate that hints are available on a site.
For example,
HTTP/1.1 200 OK
Content-Type: text/html
Content-Length: 324
BH: 1
The presence of the BH header field in a response indicates that the
origin associated with the effective request URI has a Browser Hints
resource available at the well-known URI.
The header field value MAY be "0" or "1".
Origin servers that wish to indicate to clients that Browser Hints
are available SHOULD include a BH header in all responses with a
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value of "1".
Proxy servers that wish to suppress the use of certain Browser Hints
MAY set (or reset) the BH header's value to "0".
4.2. Interaction with HTTP Proxies
Browser Hints are intended to optimise the connection between a
client and the origin server. However, HTTP allows proxies to be
interposed between browsers and origin servers, meaning that careless
use of some hints -- especially those that are connection-oriented --
might not be applicable, and might even be harmful to the proxy.
To mitigate these risks, some hints identify additional requirements
for clients consuming browser hints when there is evidence of a proxy
in use.
A proxy is considered to be in use if:
o A proxy is explicitly configured by the client, or
o The BH response header field has a value of "0".
Note that the presence of the Via header is not considered, because
it can also be generated by intermediaries working on behalf of the
origin server ("reverse proxies").
Proxies MAY modify the value of the BH header field to be "0", or
insert a BH header field with the value "0" if it is not present.
Proxies MUST NOT modify a response so that the BH header field is "1"
where it was previously not.
5. Pre-defined Browser Hints
5.1. max-conns
o Browser Hint Name: max-conns
o Description: When present, this hint indicates the maximum number
of concurrent persistent connections that the site would like
clients to use.
o Value Type: number
o Contact: mnot@mnot.net
o Notes: Not to be used when there is evidence of a proxy.
5.2. pconn-ip
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o Browser Hint Name: pconn-ip
o Description: When true, this hint indicates that the site allows
clients to reuse persistent connections keyed by IP address,
rather than by hostname. Note that all sites that are sharing the
connection MUST declare this hint for it to be used, and if a
transport-layer certificate is in use (e.g., for TLS [RFC5246]),
it MUST be valid for all sites.
o Value Type: true | false
o Contact: mnot@mnot.net
o Specification: [this document]
o Notes: Not to be used when there is evidence of a proxy.
In other words, if both www.example.com and foo.example.org resolve
to the address 192.0.2.5, and indicate this hint, then clients can
send a request to www.example.com and then a request to
foo.example.org on the same TCP connection to that address.
If any of the sites grouped together for the purposes of pconn-ip
declare a max-conns hint, the max-conns value for that address is
considered to be the maximum of the declared max-conn hints present.
5.3. ip-balance
o Browser Hint Name: ip-balance
o Description: When present, this hint indicates a preferred policy
for clients to handle a DNS lookup that return multiple IPv4
addresses for the site.
o Value Type: string
o Contact: mnot@mnot.net
o Notes: Not to be used when there is evidence of a proxy.
Defined values include:
o round-robin - Use each IP address in succession, using the next
address each time a new connection is opened.
o random - Use a random IP address from the list for each new
connection.
o failover - Use the first IP address, falling back to the following
address upon failure, and so forth.
o fastest - Attempt to connect to all IP addresses, using the
fastest for this and subsequent connections.
5.4. connect-timeout
o Browser Hint Name: connect-timeout
o Description: When present, this hint indicates how long the site
wishes browsers to wait for a connection to be established, in
seconds, before considering that connection unresponsive.
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o Value Type: integer
o Contact: mnot@mnot.net
o Notes: Not to be used when there is evidence of a proxy.
5.5. read-timeout
o Browser Hint Name: read-timeout
o Description: When present, this hint indicates how long the site
wishes browsers to wait before considering a connection
unresponsive, when data is outstanding (either a response or part
thereof), in seconds.
o Value Type: integer
o Contact: mnot@mnot.net
Note that requests on timed-out connections can be retried, subject
to the constraints of HTTP.
5.6. max-pipeline-depth
o Browser Hint Name: max-pipeline-depth
o Description: When present, this hint indicates the maximum number
of pipelined requests per connection that the site would like
clients to use.
o Value Type: number
o Contact: mnot@mnot.net
o Notes: Not to be used when there is evidence of a proxy.
5.7. small-hdrs
o Browser Hint Name: small-hdrs
o Description: When true, this hint indicates that clients can omit
the Accept and Accept-Charset request headers when communicating
with the resource, and that they can use a shortened version of
the User-Agent header.
o Value Type: prefixlist
o Contact: mnot@mnot.net
5.8. relative-referer
o Browser Hint Name: relative-referer
o Description: When true, this hint indicates that servers prefer a
relative URI in the Referer request header.
o Value Type: true | false
o Contact: mnot@mnot.net
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5.9. chunk-req-bodies
o Browser Hint Name: chunk-req-bodies
o Description: When true, this hint indicates that the server can
successfully process a request with a chunk-encoded body; i.e.,
that it should not return a 411 Length Required status. Note that
clients may still encounter a 411 response status, even in when
this hint is present (e.g., a proxy). When false, the server may
or may not require a Content-Length on requests with bodies.
o Value Type: true | false
o Contact: mnot@mnot.net
5.10. omit-cookies
o Browser Hint Name: omit-cookies
o Description: When true, this hint indicates that cookies [RFC6265]
can be omitted in requests.
o Value Type: prefixlist
o Contact: mnot@mnot.net
5.11. cookie-whitelist
o Browser Hint Name: cookie-whitelist
o Description: When present, indicates that the browser can omit any
cookie [RFC6265] whose cookie-name is not a member of the value
array.
o Value Type: array
o Contact: mnot@mnot.net
6. Security Considerations
TBD
7. IANA Considerations
7.1. The 'browser-hints' Well-Known URI
This document defines the "browser-hints" Well-Known URI [RFC5785].
o URI suffix: browser-hints
o Change controller: mnot@mnot.net
o Specification document(s): [this document]
o Related information:
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7.2. The BH HTTP Response Header Field
This document defines the "BH" HTTP header field, and registers it in
the Permanent Message Headers registry.
o Header field name: BH
o Applicable protocol: HTTP
o Status: Informational
o Author/Change controller: Mark Nottingham, mnot@mnot.net
o Specification document(s): [this document]
o Related information:
7.3. The HTTP Browser Hints Registry
This document establishes the HTTP Browser Hints Registry.
New hints are registered First Come First Served (see [RFC5226]), by
sending e-mail to <mailto:iana@iana.org> (or using other mechanisms,
as established by IANA).
Registration requests MUST use the following template:
o Browser Hint Name: [name of hint]
o Description: [description of hint]
o Value Type: [JSON value type]
o Contact: [e-mail address(es)]
o Specification: [optional; reference or URI to more info]
o Notes: [optional]
New hints MUST be optional; they cannot place requirements upon
implementations.
Likewise, new hints MUST be relevant to browser use cases; other non-
browsing hints and metadata would make the hints response undesirably
large. However, note that non-browser clients MAY use them.
Finally, 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. For example, if a hint indicates that
browsers can compress request headers using GZIP, intermediaries that
are interposed are likely to fail.
The initial contents of the registry are defined in Section 5.
8. References
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8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
8.2. Informative References
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
April 2010.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011.
Appendix A. Acknowledgements
Thanks to Mike Belshe, Artur Bergman, Jason Duell, Poul-Henning Kamp,
Anirban Kundu, Patrick McManus, Steve Souders, and Martin Thompson
for their suggestions and feedback.
The author takes all responsibility for errors and omissions.
Author's Address
Mark Nottingham
Email: mnot@mnot.net
URI: http://www.mnot.net/
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