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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 7, 2011
Intended status: Informational
Expires: June 9, 2012


                         Prefer Header for HTTP
                       draft-snell-http-prefer-06

Abstract

   This specification defines an HTTP header field that can be used by a
   client 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 9, 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
   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.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  3
   2.  The Prefer Request Header  . . . . . . . . . . . . . . . . . .  3
     2.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  The Preference-Applied Response Header . . . . . . . . . . . .  5
   4.  The "return-accepted" Preference . . . . . . . . . . . . . . .  7
   5.  The "return-representation" Preference . . . . . . . . . . . .  7
   6.  The "return-status" Preference . . . . . . . . . . . . . . . .  8
   7.  The "return-minimal" Preference  . . . . . . . . . . . . . . .  8
   8.  The "wait" Preference  . . . . . . . . . . . . . . . . . . . .  8
   9.  The "strict" and "lenient" Processing Preferences  . . . . . .  9
   10. The "detail" Preference  . . . . . . . . . . . . . . . . . . .  9
   11. Registered Preferences . . . . . . . . . . . . . . . . . . . . 10
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
     12.1. The Registry of Preferences  . . . . . . . . . . . . . . . 10
       12.1.1.  Initial Registry Contents . . . . . . . . . . . . . . 11
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   14. Normative References . . . . . . . . . . . . . . . . . . . . . 13
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14






























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1.  Introduction

   This specification defines a new HTTP request header field that may
   be used by clients 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].

1.1.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234] and includes, by reference, the "token",
   "quoted-string", "OWS", "BWS" rules and the #rule extension as
   defined within [I-D.ietf-httpbis-p1-messaging] Section 1.2.


2.  The Prefer Request Header

   The Prefer request-header field is used to indicate that particular
   server behaviors are preferred by the client, 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 = token [ BWS "=" BWS value ]
                  *( OWS ";" [ OWS parameter ] )
     parameter  = token [ BWS "=" BWS value ]
     value      = token / quoted-string

   This header field is defined with an extensible syntax to allow for
   future values included in the Registry of Preferences
   (Section 12.1)).  A server that does not recognize or is unable to
   comply with particular preference tokens in the Prefer header field
   of a request MUST ignore those tokens and MUST NOT stop processing or
   signal an error.

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

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



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

   If a particular preference token or parameter is specified 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 tokens
   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.

   As per [I-D.ietf-httpbis-p1-messaging], Section 3.2, Implementations
   MUST be capable of supporting either multiple instances of the Prefer
   header field in a single message as well as multiple preference
   tokens separated by commas in a single Prefer header, for instance,
   the following examples are equivalent:

     # Multiple Prefer Header Fields
     POST /foo HTTP/1.1
     Host: example.org
     Prefer: return-accepted
     Prefer: wait=100

     # Single Prefer Header Field
     POST /foo HTTP/1.1
     Host: example.org
     Prefer: return-accepted, wait=100



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2.1.  Examples

   The following examples illustrate the use of various Preferences
   defined by this specification, as well as undefined extensions for
   strictly illustrative purposes:

     # Return a 202 Accepted response for asynchronous processing
     # if the response cannot be processed within 10 seconds.
     # An undefined "priority" preference is also specified.

     Prefer: return-accepted;
     Prefer: wait=10;
     Prefer: priority=5;

     # Use lenient processing, a reporting detail level of 10,
     # and return an entity describing the status of the request
     # rather than a representation of the resource

     Prefer: Lenient, Detail=10, Return-Status

     # Use of an optional, undefined parameter on the
     # return-minimal preference requesting a response
     # status code of 204 for a successful response.

     Prefer: return-minimal; status=204


3.  The Preference-Applied Response Header

   The Preference-Applied response header MAY be included within a
   response message as an indication as to which Prefer tokens were
   honored by the server and applied to the processing of the request.

     Preference-Applied  =  "Preference-Applied" ":" 1#token

   Note that the syntax of the Preference-Applied header differs from
   that of the Prefer header in that token values and parameters are not
   included.

   Use of the Preference-Applied header is not required in all cases.
   For instance, when using the return-accepted, return-minimal or wait
   preferences, as defined below, the application of the preference will
   be apparent based on the context and nature of the response.
   However, there are cases in which the application of a preference
   cannot be easily determined by the client.  For instance, when
   posting a resource to a server using either the return-representation
   or return-status preferences, a client cannot be certain in all cases
   whether the returned entity is a representation of the modified



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   resource or a representation of the request status.  In such
   situations, where the nature of the response is ambiguous and a clear
   preference has been stated by the client using the Prefer request
   header field, the Preference-Applied response header field SHOULD be
   used.

   For example, here a request is sent to the server requesting the
   return of the resource representation.

     #Request
     POST /collection HTTP/1.1
     Host: example.org
     Content-Type: application/json
     Prefer: return-representation

     {...}

     # Response
     HTTP/1.1 201 Created
     Content-Type: application/json
     Preference-Applied: return-representation
     Location: /collection/1

     {...}

   Whereas in this example, the request is sent asking for the return of
   a status representation.

     #Request
     POST /collection HTTP/1.1
     Host: example.org
     Content-Type: application/json
     Prefer: return-status

     {...}

     # Response
     HTTP/1.1 201 Created
     Content-Type: application/json
     Preference-Applied: return-status
     Location: /collection/1

     {...}

   Use of the Preference-Applied response header allows the client to
   unambiguously determine whether the requested preference was applied.





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4.  The "return-accepted" Preference

   The "return-accepted" preference indicates that the client 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 client to indicate to a server it's capability and preference for
   handling 202 Accepted responses.


5.  The "return-representation" Preference

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

     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 client 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 client's preferences into consideration
   while constructing the response.





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6.  The "return-status" Preference

   The "return-status" preference indicates that the client 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.


7.  The "return-minimal" Preference

   The "return-minimal" preference indicates that the client 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
   server.

     return-minimal = "return-minimal"

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


8.  The "wait" Preference

   The "wait" preference can be used to establish an upper bound on the
   length of time, in seconds, the client is willing to wait for a
   response, after which the client 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" BWS "=" BWS delta-seconds

   Clients specifying the "wait" Preference SHOULD also use the Date
   header field, as specified in [I-D.ietf-httpbis-p2-semantics] Section



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   9.2, within the request to establish the time at which the client
   began waiting for the completion of the request.  Failing to include
   a Date header field in the request would require the server to use
   the instant it received and began processing the request as the
   baseline for determining how long the client has been waiting which
   could yield unintended results depending on how out of synch the
   client and server clocks are.


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

   The "strict" and "lenient" preferences are mutually-exclusive
   directives indicating, at the servers discretion, how the client
   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 client to
   indicate that, in such conditions, it would prefer that the server
   reject the request, while the "lenient" preference indicates that the
   client 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"


10.  The "detail" Preference

   The "detail" preference specifies the level of detail the client
   wishes the server to provide within a response to an operation.  This
   preference is akin to specifying the level of verbose output an
   operation should generate or to specifying the trace level within a
   debug log.  The detail level is specified as a non-negative integer
   in the range 0-100, where the value 0 indicates a server-determined
   default detail level and all other integer values specify a strictly
   decreasing level of detail as the integer value increases.

     detail = "detail" BWS "=" BWS ("100" / 1*2DIGIT )

   Implementations are free to apply additional constraints on the range
   of acceptable values for this directive but MUST NOT signal an error
   or fail to process the request should the client provide a value
   outside the acceptable range.  In such cases, the server SHOULD
   either ignore the preference or apply a reasonable default value.



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   One example of a potential use for the application of the "detail"
   preference would be in deciding the amount of detailed error
   information a server includes in the payload of a 4xx or 5xx
   response.  Solely at the discretion of the server, an error response
   to a request specifying a higher detail level (e.g., detail=1) may
   included significantly more detailed information about the error
   condition than an error response specifying a much lower detail level
   (e.g., detail=10).


11.  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 12.1.

   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, clients 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.


12.  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


12.1.  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 11

   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.

12.1.1.  Initial Registry Contents

   The Preferences Registry's initial contents are:




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   o  Preference: return-accepted
   o  Description: Indicates that the client prefers the server to
      respond with a 202 Accepted status as described by Section 4
   o  Reference: [this specification]

   o  Preference: return-minimal
   o  Description: Indicates that the client prefers the server return a
      minimal response to a request as described by Section 7
   o  Reference: [this specification]

   o  Preference: return-representation
   o  Description: Indicates that the client prefers the server to
      include a representation of the current state of the resource in
      response to a request as described by Section 5
   o  Reference: [this specification]

   o  Preference: return-status
   o  Description: Indicates that the client prefers the server to
      return an entity describing the current state of a resource in
      response to a request as described by Section 6
   o  Reference: [this specification]

   o  Preference: wait
   o  Description: Indicates an upper bound to the lenght of time the
      client 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]

   o  Preference: detail
   o  Description: Indicates the client's preference as to the amount of
      detail the server should include in responses to a request.
   o  Reference: [this specification]







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13.  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
   [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.


14.  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.



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   [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.

   [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.

   [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.

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


Author's Address

   James M Snell


   Phone:
   Email: jasnell@gmail.com
   URI:











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