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Network Working Group                                      M. Nottingham
Internet-Draft                                             June 20, 2012
Intended status: Informational
Expires: December 22, 2012


                           HTTP Browser Hints
                 draft-nottingham-http-browser-hints-03

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.

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 December 22, 2012.

Copyright Notice

   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



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   (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
   5.  Pre-defined Browser Hints  . . . . . . . . . . . . . . . . . .  5
     5.1.  max-conns  . . . . . . . . . . . . . . . . . . . . . . . .  5
     5.2.  pconn-ip . . . . . . . . . . . . . . . . . . . . . . . . .  5
     5.3.  ip-balance . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.4.  connect-timeout  . . . . . . . . . . . . . . . . . . . . .  6
     5.5.  read-timeout . . . . . . . . . . . . . . . . . . . . . . .  6
     5.6.  max-pipeline-depth . . . . . . . . . . . . . . . . . . . .  7
     5.7.  small-hdrs . . . . . . . . . . . . . . . . . . . . . . . .  7
     5.8.  relative-referer . . . . . . . . . . . . . . . . . . . . .  7
     5.9.  chunk-req-bodies . . . . . . . . . . . . . . . . . . . . .  7
     5.10. omit-cookies . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     7.1.  The 'browser-hints' Well-Known URI . . . . . . . . . . . .  8
     7.2.  The HTTP Browser Hints Registry  . . . . . . . . . . . . .  8
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10















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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 memo proposes 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.

   Feedback for this draft should take place on the
   apps-discuss@ietf.org mailing list
   <https://www.ietf.org/mailman/listinfo/apps-discuss>.


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 indicating using a JSON [RFC4627] formatted file,
   containing a single object whose member's names are browser hints, as
   defined by the registry Section 7.2.

   For example;

             {
             "max-conns": 5,
             "small-hdrs": true
             }



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   By their nature, all browser hints are optional; i.e., browsers are
   free to ignore them.

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.



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   Typically, clients (especially browsers) will not block other
   requests to a site while fetching the browser hints (because they're
   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).


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

5.2.  pconn-ip

   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]),



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      it MUST be valid for all sites.
   o  Value Type: true | false
   o  Contact: mnot@mnot.net
   o  Specification: [this document]

   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

   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.
   o  Value Type: integer
   o  Contact: mnot@mnot.net

5.5.  read-timeout

   o  Browser Hint Name: read-timeout





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   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, subjects
   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

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

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 require a 411 response status, even in when this
      hint is present.  When false, the server may or may not require a
      Content-Length on requests with bodies.





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   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


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:

7.2.  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]

   New hints MUST be optional; they cannot place requirements upon
   implementations.

   Likewise, new hints MUST be relevant to browser use cases; other non-



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   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

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, Poul-Henning Kamp, Anirban
   Kundu, Patrick McManus, Steve Souders, and Martin Thompson for their
   suggestions and feedback.



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   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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