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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 7240

Individual Submission                                           J. Snell
Internet-Draft                                          December 6, 2011
Intended status: Informational
Expires: June 8, 2012


                         Prefer Header for HTTP
                       draft-snell-http-prefer-05

Abstract

   This specification defines an HTTP header field that can be used by a
   user-agent to request that certain behaviors be implemented by a
   server while processing a request.

Status of this Memo

   This Internet-Draft is submitted to IETF 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 June 8, 2012.

Copyright Notice

   Copyright (c) 2011 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
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   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.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  The Prefer Request Header  . . . . . . . . . . . . . . . . . .  3
   3.  The "return-accepted" Preference . . . . . . . . . . . . . . .  4
   4.  The "return-representation" Preference . . . . . . . . . . . .  4
   5.  The "return-status" Preference . . . . . . . . . . . . . . . .  5
   6.  The "return-minimal" Preference  . . . . . . . . . . . . . . .  5
   7.  The "wait" Preference  . . . . . . . . . . . . . . . . . . . .  6
   8.  The "strict" and "lenient" Processing Preferences  . . . . . .  6
   9.  Registered Preferences . . . . . . . . . . . . . . . . . . . .  7
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
     10.1.  The Registry of Preferences . . . . . . . . . . . . . . .  7
       10.1.1.  Initial Registry Contents . . . . . . . . . . . . . .  8
   11. Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   12. Normative References . . . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11


































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1.  Introduction

   This specification defines a new HTTP request header field that may
   be used by user-agents to request optional behaviors be applied by a
   server during the processing the request.

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in [RFC2119].


2.  The Prefer Request Header

   The Prefer request-header field is used to indicate that particular
   server behaviors are preferred by the user-agent, but not required
   for successful completion of the request.  Prefer is similar in
   nature to the Expect header field defined by
   [I-D.ietf-httpbis-p2-semantics] Section 9.3 with the exception that
   servers are allowed to ignore stated preferences.

     Prefer           =  "Prefer" ":" 1#preference
     preference       = OWS preference-token OWS *prefer-params OWS
     preference-value = token / quoted-string
     preference-token = token OWS [ "=" OWS preference-value OWS ]
     prefer-params    =  ";" OWS preference-token

   This header field is defined with an extensible syntax to allow for
   future values included in the Registry of Preferences
   (Section 10.1)).  A server that does not recognize or is unable to
   comply with particular preference values in the Prefer header field
   of a request MUST ignore those values and MUST NOT stop processing or
   signal an error.

   A preference token MAY specify a preference-value.  Empty, or zero
   length preference-values on both the preference directive and
   parameters are equivalent to no value being specified at all.  The
   following, then, are equivalent:

     Prefer: foo; bar=""
     Prefer: foo=; bar
     Prefer: foo=""; bar=

   An optional, arbitrary collection of preference parameters MAY be
   specified for any preference directive.  The meaning and application
   of such parameters is dependent on the definition of each preference
   directive and the server's implementation thereof.

   If a particular preference directive or parameter is specified



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   multiple times, repeated occurrences MUST be ignored without
   signaling an error or otherwise altering the processing of the
   request.

   Comparison of preference token names is case-insensitive while values
   are case-sensitive regardless of whether token or quoted-string
   values are used.

   Note that the application of a preference by the server MAY affect
   the caching characteristics of the response.  Specifically, should
   the application of a preference result in a variance to the
   representation returned by a cacheable response, a Vary header field
   MUST be included listing the Prefer header field as one of the
   selecting header fields.

   The Prefer request header field MUST be forwarded by the proxy if the
   request is forwarded.  In various situations, A proxy may determine
   that it is capable of honoring a preference independently of the
   server to which the request is directed.  For instance, an
   intervening proxy may be capable of transparently providing
   asynchronous handling of a request using a 202 Accepted responses
   independently of the origin server.  Such proxies could choose to
   honor the "return-accepted" preference.  Individual preference
   directives MAY define their own requirements and restrictions as to
   whether and how proxies may apply the preference to a request
   independently of the origin server.


3.  The "return-accepted" Preference

   The "return-accepted" preference indicates that the user-agent
   prefers the server to respond with a 202 Accepted status in the case
   where the length of time it takes to generate a response will exceed
   some arbitrary threshold established by the server.

     return-accepted = "return-accepted"

   The key motivation for the "return-accepted" preference is to
   facilitate the operation of asynchronous request handling by allowing
   the user-agent to indicate to a server it's capability and preference
   for handling 202 Accepted responses.


4.  The "return-representation" Preference

   The "return-representation" preference indicates that the user-agent
   prefers that the server include an entity representing the current
   state of the resource in the response to a successful request.



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     return-representation = "return-representation"

   When honoring the "return-representation" preference, the server MUST
   include a Content-Location header field specifying the URI of the
   resource representation being returned.  Per section 6.1 of
   [I-D.ietf-httpbis-p2-semantics], the presence of the Content-Location
   header field in the response asserts that the payload is a
   representation of the resource identified by the Content-Location
   URI.

   The "return-representation" preference is intended primarily to
   provide a means of optimizing communication between the user-agent
   and server by eliminating the need for a subsequent GET request to
   retrieve the current representation of the resource following a
   modification.

   Currently, after successfully processing a modification request such
   as a POST or PUT, a server may choose to return either an entity
   describing the status of the operation or a representation of the
   modified resource itself.  While the selection of which type of
   entity to return, if any at all, is solely at the discretion of the
   server, the "return-representation" preference -- along with the
   "return-status" and "return-minimal" directives defined below --
   allow the server to take the user-agent's preferences into
   consideration while constructing the response.


5.  The "return-status" Preference

   The "return-status" preference indicates that the user-agent prefers
   the server to include an entity describing the status of the request
   in the response as opposed to returning a representation of the
   current state of the resource.

     return-status = "return-status"

   When honoring the "return-status" preference, the server SHOULD NOT
   include a Content-Location header field in the response.


6.  The "return-minimal" Preference

   The "return-minimal" preference indicates that the user-agent wishes
   the server to return a minimal response to a successful request.
   Typically, such responses would utilize the 204 No Content status,
   but other codes MAY be used as appropriate, such as a 200 status with
   a zero-length response entity.  The determination of what constitutes
   an appropriate minimal response is solely at the discretion of the



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   server.

     return-minimal = "return-minimal"

   The "return-minimal" preference is intended to provide a means of
   optimizing communication between the user-agent and server by
   reducing the amount of data the server is required to return to the
   user-agent following a request.  This can be particularly useful, for
   instance, when communicating with limited-bandwidth mobile devices or
   when the user-agent simply does not require any further information
   about the result of a request beyond knowing if it was successfully
   processed.


7.  The "wait" Preference

   The "wait" preference can be used to establish an upper bound on the
   length of time, in seconds, the user-agent is willing to wait for a
   response, after which the user-agent may choose to abandon the
   request.  In the case generating a response will take longer than the
   time specified, the server, or proxy, can choose to either return a
   202 Accepted response, cancel processing, or continue attempting to
   complete the request.

     wait = "wait" OWS "=" OWS delta-seconds

   User-Agents specifying the "wait" Preference SHOULD also use the Date
   header field, as specified in [I-D.ietf-httpbis-p2-semantics] Section
   9.2, within the request to establish the time at which the client
   began waiting for the completion of the request.


8.  The "strict" and "lenient" Processing Preferences

   ED NOTE: This preference directive is currently exploratory in
   nature.  I've added it to solicit feedback as to it's general
   utility.  It is possible that I may pull this back out.

   The "strict" and "lenient" preferences are mutually-exclusive
   directives indicating, at the servers discretion, how the user-agent
   wishes the server to handle potential error conditions that may arise
   in the processing of a request.  For instance, if the payload of a
   request contains various minor syntactical or semantic errors, but
   the server is still capable of comprehending and successfully
   processing the request, a decision must be made to either reject the
   request with an appropriate 4xx error response or to go ahead with
   processing.  The "strict" preference can be used by the user-agent to
   indicate that, in such conditions, it would prefer that the server



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   reject the request, while the "lenient" preference indicates that the
   user-agent would prefer the server to attempt to process the request.
   The specific meaning and application of the "strict" and "lenient"
   directives is specific to each type of resource, the request method
   and the operation of the server.

     handling = "strict" / "lenient"


9.  Registered Preferences

   Well-defined preferences can be registered for convenience and/or to
   promote reuse by other applications.  This specification establishes
   an IANA registry of such relation types see Section Section 10.1.

   Registered preference names MUST conform to the token rule, and MUST
   be compared character-by-character in a case-insensitive fashion.
   They SHOULD be appropriate to the specificity of the preference;
   i.e., if the semantics are highly specific to a particular
   application, the name should reflect that, so that more general names
   are available for less specific use.

   Registered preferences MUST NOT constrain servers, user-agents or any
   intermediaries involved in the exchange and processing of a request
   to any behavior required for successful processing.  The use and
   application of a preference within a given request MUST be optional
   on the part of all participants.


10.  IANA Considerations

   The 'Prefer' header field should be added to the permanent registry
   (see [RFC3864]).


       Header field name: Prefer
       Applicable Protocol: HTTP
       Status:
       Author/Change controller: IETF
       Specification document: this specification


10.1.  The Registry of Preferences

   Preferences are registered on the advice of a Designated Expert
   (appointed by the IESG or their delegate), with a Specification
   Required (using terminology from [RFC5226]).




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   The requirements for registered preferences are described in
   Section 9

   Registration requests consist of the completed registration template
   below, typically published in an RFC or Open Standard (in the sense
   described by [RFC2026], Section 7).  However, to allow for the
   allocation of values prior to publication, the Designated Expert may
   approve registration once they are satisfied that a specification
   will be published.

   Note that relation types can be registered by third parties, if the
   Designated Expert determines that an unregistered relation type is
   widely deployed and not likely to be registered in a timely manner.

   The registration template is:

   o  Preference: (A value for the Prefer request header field that
      conforms to the syntax rule given in Section 2)
   o  Description:
   o  Reference:
   o  Notes: [optional]
   o  Application Data: [optional]

   Registration requests should be sent to the preferences@ietf.org
   mailing list, marked clearly in the subject line (e.g., "NEW
   PREFERENCE - example" to register an "example" preference).

   Within at most 14 days of the request, the Designated Expert(s) will
   either approve or deny the registration request, communicating this
   decision to the review list and IANA.  Denials should include an
   explanation and, if applicable, suggestions as to how to make the
   request successful.

   Decisions (or lack thereof) made by the Designated Expert can be
   first appealed to Application Area Directors (contactable using
   app-ads@tools.ietf.org email address or directly by looking up their
   email addresses on http://www.iesg.org/ website) and, if the
   appellant is not satisfied with the response, to the full IESG (using
   the iesg@iesg.org mailing list).

   IANA should only accept registry updates from the Designated
   Expert(s), and should direct all requests for registration to the
   review mailing list.

10.1.1.  Initial Registry Contents

   The Preferences Registry's initial contents are:




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   o  Preference: return-accepted
   o  Description: Indicates that the user-agent prefers the server to
      respond with a 202 Accepted status as described by Section 3
   o  Reference: [this specification]

   o  Preference: return-minimal
   o  Description: Indicates that the user-agent prefers the server
      return a minimal response to a request as described by Section 6
   o  Reference: [this specification]

   o  Preference: return-representation
   o  Description: Indicates that the user-agent prefers the server to
      include a representation of the current state of the resource in
      response to a request as described by Section 4
   o  Reference: [this specification]

   o  Preference: return-status
   o  Description: Indicates that the user-agent prefers the server to
      return an entity describing the current state of a resource in
      response to a request as described by Section 5
   o  Reference: [this specification]

   o  Preference: wait
   o  Description: Indicates an upper bound to the lenght of time the
      user-agent is willing to wait for a response, after which the
      request may be aborted.
   o  Reference: [this specification]

   o  Preference: strict
   o  Description: Indicates that the client wishes the server to apply
      strict validation and error handling to the processing of a
      request.
   o  Reference: [this specification]

   o  Preference: lenient
   o  Description: Indicates that the client wishes the server to apply
      lenient validation and error handling to the processing of a
      request.
   o  Reference: [this specification]


11.  Security Considerations

   Specific preferences requested by a client can introduce security
   considerations and concerns beyond those discussed in HTTP/1.1 Parts
   1 [I-D.ietf-httpbis-p1-messaging], 2 [I-D.ietf-httpbis-p2-semantics],
   3 [I-D.ietf-httpbis-p3-payload], 4 [I-D.ietf-httpbis-p4-conditional],
   5 [I-D.ietf-httpbis-p5-range], 6 [I-D.ietf-httpbis-p6-cache], and 7



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   [I-D.ietf-httpbis-p7-auth].  Implementors must refer to the
   specifications and descriptions of each preference to determine the
   security considerations relevant to each.


12.  Normative References

   [I-D.ietf-httpbis-p1-messaging]
              Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
              J. Reschke, "HTTP/1.1, part 1: URIs, Connections, and
              Message Parsing", draft-ietf-httpbis-p1-messaging-17 (work
              in progress), October 2011.

   [I-D.ietf-httpbis-p2-semantics]
              Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
              J. Reschke, "HTTP/1.1, part 2: Message Semantics",
              draft-ietf-httpbis-p2-semantics-17 (work in progress),
              October 2011.

   [I-D.ietf-httpbis-p3-payload]
              Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
              J. Reschke, "HTTP/1.1, part 3: Message Payload and Content
              Negotiation", draft-ietf-httpbis-p3-payload-17 (work in
              progress), October 2011.

   [I-D.ietf-httpbis-p4-conditional]
              Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
              J. Reschke, "HTTP/1.1, part 4: Conditional Requests",
              draft-ietf-httpbis-p4-conditional-17 (work in progress),
              October 2011.

   [I-D.ietf-httpbis-p5-range]
              Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
              J. Reschke, "HTTP/1.1, part 5: Range Requests and Partial
              Responses", draft-ietf-httpbis-p5-range-17 (work in
              progress), October 2011.

   [I-D.ietf-httpbis-p6-cache]
              Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y.,
              Nottingham, M., and J. Reschke, "HTTP/1.1, part 6:
              Caching", draft-ietf-httpbis-p6-cache-17 (work in
              progress), October 2011.



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   [I-D.ietf-httpbis-p7-auth]
              Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
              J. Reschke, "HTTP/1.1, part 7: Authentication",
              draft-ietf-httpbis-p7-auth-17 (work in progress),
              October 2011.

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

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

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.


Author's Address

   James M Snell


   Phone:
   Email: jasnell@gmail.com
   URI:

















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