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Network Working Group                                      M. Nottingham
Internet-Draft                                         February 12, 2013
Intended status: Informational
Expires: August 16, 2013


                       HTTP Origin and Hop Hints
                 draft-nottingham-http-browser-hints-05

Abstract

   Over time, HTTP clients -- especially Web browsers -- have adapted
   how they use the protocol 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 both origin servers and
   intermediaries can make hints available to clients about their
   preferences and capabilities, without imposing undue overhead on
   their interactions or requiring support for them.

   This is intended to allow clients 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 August 16, 2013.

Copyright Notice



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   Copyright (c) 2013 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.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  3
   3.  Origin Hints . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Hop Hints  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Hint Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  Pre-defined Hints  . . . . . . . . . . . . . . . . . . . . . .  6
     6.1.  Small Request Headers  . . . . . . . . . . . . . . . . . .  6
     6.2.  Relative Referers  . . . . . . . . . . . . . . . . . . . .  6
     6.3.  Omitting Cookies . . . . . . . . . . . . . . . . . . . . .  6
     6.4.  Sharing Connections  . . . . . . . . . . . . . . . . . . .  7
     6.5.  Pipeline Depth . . . . . . . . . . . . . . . . . . . . . .  7
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     8.1.  The OH HTTP Response Header Field  . . . . . . . . . . . .  8
     8.2.  The HH HTTP Response Header Field  . . . . . . . . . . . .  8
     8.3.  The HTTP Hints Registry  . . . . . . . . . . . . . . . . .  8
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     9.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     9.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . .  9
   Appendix B.  Open Issue: Hint Syntax . . . . . . . . . . . . . . . 10
   Appendix C.  Open Issue: Hint Value Types  . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10












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1.  Introduction

   HTTP [HTTP-p1] clients -- especially browsers -- typically use
   hardcoded values or heuristics to determine how they use TCP, based
   on common-case server behaviours and limitations.

   For example, 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 conservative behaviours are 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 two headers; "OH" (Origin Hints) for end-to-
   end hints from the Origin Server, and "HH" (Hop Hints) for hop-by-hop
   hints from the upstream server (origin or proxy).  A selection of
   hints are also defined in this document, and a registry is defined to
   allow future such hints.


2.  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], and explicitly includes the following rules from it:
   ALPHA, DIGIT.  Additionally, it uses the list rule extension defined
   in [HTTP-p1], Appendix B.


3.  Origin Hints

   Origin Hints are applicable to all future requests to the origin
   [RFC6454] associated with the response, until they are overridden.
   They are carried in the "OH" header field's value.

   OH = #hint

   A hint is considered overridden when an OH header field is seen from
   the same origin that does not contain the hint, or contains a
   different value for the hint.




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   Practically speaking, this means that clients MAY cache origin hint
   values indefinitely, updating them when new OH header fields are seen
   from an origin.

   For example, an origin may send an OH header on every response, in
   which case the latest one seen would be the value used, or it could
   send one sporadically (e.g., upon connection), in which case the
   origin hints in effect would be the last one seen by the client.

   Practically speaking, origins that wish to have their hints available
   to clients as soon as possible will send them on every response;
   those that wish to limit the sizes of responses might use some other
   strategy, knowing that clients will eventually cache their hints.

   For example:

   HTTP/1.1 200 OK
   Content-Type: text/html
   OH: a,b=6,c
   Content-Length: 1234
   Cache-Control: max-age=60

   ...

   Here, the origin hints "a", "b", and "c" are indicated; the "b" hint
   has a value of "6".  If a subsequent message from the same origin to
   the same client were:

   HTTP/1.1 303 See Other
   Content-Type: text/html
   OH: a,b=2
   Content-Length: 1234
   Cache-Control: max-age=60

   ...

   The client would now consider "a" to still be true, whereas the value
   of "b" would be 2, and "c" would be false.

   Origin hints SHOULD NOT be generated by proxy servers.


4.  Hop Hints

   Hop Hints are applicable to all future requests on the TCP connection
   they occur upon, until they are overridden.  The are carried in the
   "HH" header field's value.




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   HH = #hint

   A hint is considered overridden when an HH header field is seen on
   the same connection that does not contain the hint, or contains a
   different value for the hint.

   Typically, a server (whether origin or proxy) will send the HH header
   field on the first response, omitting it from subsequent responses
   unless it wishes to change a value.

   When it occurs in a message, the HH header field MUST be listed in
   the Connection header's field-value.

   For example:

   HTTP/1.1 404 Not Found
   Content-Type: text/plain
   HH: x,ya
   Content-Length: 5678
   Connection: HH
   Cache-Control: max-age=60

   ...

   Here, the hop hints "x" and "ya" are indicated.


5.  Hint Syntax

   Both origin hints and hop hints share a common syntax, consisting of
   a string of alphanumeric characters.  This form is designed to be
   compact without sacrificing readability.

   Every hint has a case-sensitive hint identifier.

   hint = 1*ALPHA [ "=" 1*DIGIT ]

   Hints are allowed to have a numeric argument.  However, wherever
   possible, they are encouraged to be defined as flags (i.e., as a hint
   identifier only), so that the hints don't consume too much space in
   responses.

   Hints can be defined as one of two types:
   o  Boolean - indicated by the presence of the hint identifier.  If
      the hint identifier is absent in the last message containing the
      relevant hint header field, it is considered false.





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   o  Numeric - value indicated by the digits after "=", up to the first
      non-digit character.

   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 of a given type (origin or hop) are collected from all
   instances of that header on the message in question before being
   considered complete.


6.  Pre-defined Hints

6.1.  Small Request Headers

   o  Hint Name: s
   o  Hint Type: origin
   o  Description: When true, this hint indicates that clients can omit
      the Accept and Accept-Charset request headers when communicating
      with the origin, and that they can use a shortened version of the
      User-Agent header.
   o  Value Type: boolean
   o  Contact: mnot@mnot.net

   This hint can help reduce request sizes.

6.2.  Relative Referers

   o  Hint Name: r
   o  Hint Type: origin
   o  Description: When true, this hint indicates that the origin
      prefers a relative URI in the Referer request header.
   o  Value Type: boolean
   o  Contact: mnot@mnot.net

   This hint can help reduce request sizes.

6.3.  Omitting Cookies

   o  Hint Name: c
   o  Hint Type: origin
   o  Description: When true, this hint indicates that all cookies
      [RFC6265] can be omitted in requests to the origin.
   o  Value Type: boolean
   o  Contact: mnot@mnot.net

   This hint can help reduce request sizes.





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6.4.  Sharing Connections

   o  Hint Name: sc
   o  Hint Type: hop
   o  Description: When true, this hint indicates that the server allows
      clients to reuse persistent connections keyed by IP address,
      rather than by hostname.  Note that all origins 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 origins.
   o  Value Type: boolean
   o  Contact: mnot@mnot.net
   o  Specification: [this document]
   o  Notes: Although it is a Hop Hint, this MUST NOT be set by servers
      other than origins.

   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.

6.5.  Pipeline Depth

   o  Hint Name: p
   o  Hint Type: hop
   o  Description: When present, this hint indicates the maximum number
      of pipelined requests per connection that the server would like
      clients to use.
   o  Value Type: numeric
   o  Contact: mnot@mnot.net


7.  Security Considerations

   By their nature, hints are both optional and advisory; clients ought
   to exercise judgement when applying them, as an attacker might use a
   naive implementation to trick the client into generating abnormal
   traffic.  For example, the "p" hint should not be the only input into
   determining how deep to pipeline requests.

   Hints are also only as secure as the channel they are transmitted
   upon; if HTTP is used in the clear, then hints might be observed
   (which typically is not a great risk), and modified (which could be,
   for a naive client implementation).

   The Hop Hints mechanism uses the "Connection" header to scope the
   hint to a single HTTP hop.  A few old implementations have been
   observed to not properly strip headers indicated by "Connection".



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8.  IANA Considerations

8.1.  The OH HTTP Response Header Field

   This document defines the "OH" HTTP header field, and registers it in
   the Permanent Message Headers registry.

   o  Header field name: OH
   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: for Origin Hints

8.2.  The HH HTTP Response Header Field

   This document defines the "HH" HTTP header field, and registers it in
   the Permanent Message Headers registry.

   o  Header field name: HH
   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: for Hop Hints

8.3.  The HTTP Hints Registry

   This document establishes the HTTP Hints Registry.

   New hints are registered using Expert Review (see [RFC5226]), by
   sending e-mail to <mailto: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.  For example, if a hint indicates that
   browsers can compress request headers using GZIP, intermediaries that
   are interposed are likely to fail.

   Registration requests MUST use the following template:



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   o  Hint Name: [name of hint]
   o  Hint Type: ["origin" or "hop"]
   o  Description: [description of hint]
   o  Value Type: ["boolean" or "numeric"]
   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 6.


9.  References

9.1.  Normative References

   [HTTP-p1]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              draft-ietf-httpbis-p1-messaging-21; (work in progress),
              Feb 2013.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              December 2011.

9.2.  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.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.


Appendix A.  Acknowledgements

   Thanks to Mike Belshe, Artur Bergman, William Chan, Jason Duell,
   Poul-Henning Kamp, Anirban Kundu, Patrick McManus, Ryan Sleevi, Steve
   Souders, and Martin Thompson for their suggestions and feedback.




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   The author takes all responsibility for errors and omissions.


Appendix B.  Open Issue: Hint Syntax

   This revision defines the syntax of hints as a comma-delimited list.
   This is convenient (especially for delimiting hints with values), but
   if many hints need to be conveyed, it'll be inefficient.

   An alternate syntax could remove the commas, but we'd likely be
   constrained in the number of hints we'd be able to define.

   Yet another approach would be to define a bitfield, and an ASCII
   representation of that field.  However, this would be cumbersome.

   Feedback appreciated.


Appendix C.  Open Issue: Hint Value Types

   Are the defined hint value types sufficient?  Feedback appreciated.


Author's Address

   Mark Nottingham

   Email: mnot@mnot.net
   URI:   http://www.mnot.net/






















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