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Network Working Group                                           J. Snell
Internet-Draft                                          October 12, 2012
Intended status: Standards Track
Expires: April 15, 2013


                         Prefer Header for HTTP
                       draft-snell-http-prefer-15

Abstract

   This specification defines an HTTP header field that can be used by a
   client to request that certain behaviors be employed 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 April 15, 2013.

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
   (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  . . . . . . . . . . . . . . . . . . . . .  4
   2.  The Prefer Request Header Field  . . . . . . . . . . . . . . .  4
     2.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  The Preference-Applied Response Header Field . . . . . . . . .  7
   4.  Preference Definitions . . . . . . . . . . . . . . . . . . . .  8
     4.1.  The "return-asynch" Preference . . . . . . . . . . . . . .  8
     4.2.  The "return-representation" and "return-minimal"
           Preferences  . . . . . . . . . . . . . . . . . . . . . . .  9
     4.3.  The "wait" Preference  . . . . . . . . . . . . . . . . . . 11
     4.4.  The "strict" and "lenient" Processing Preferences  . . . . 12
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
     5.1.  The Registry of Preferences  . . . . . . . . . . . . . . . 13
     5.2.  Initial Registry Contents  . . . . . . . . . . . . . . . . 14
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 15
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16






























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1.  Introduction

   Within the course of processing an HTTP request there are typically a
   range of required and optional behaviors that a server or
   intermediary can employ.  These often manifest in a variety of subtle
   and not-so-subtle ways within the response.

   For example, when using the HTTP PUT method to modify a resource --
   similar to that defined for the Atom Publishing Protocol [RFC5023] --
   the server is given the option of returning either a complete
   representation of a modified resource or a minimal response that
   indicates only the successful completion of the operation.  The
   selection of which type of response to return to the client generally
   has no bearing on the successful processing of the request but could,
   for instance, have an impact on what actions the client must take
   after receiving the response.  That is, returning a representation of
   the modified resource within the response can allow the client to
   avoid sending an additional subsequent GET request.

   Similarly, servers that process requests are often faced with
   decisions about how to process requests that may be technically
   invalid or incorrect but are still understandable.  It might be the
   case that the server is able to overlook the technical errors in the
   request but still successfully process the request.  Depending on the
   specific requirements of the application and the nature of the
   request being made, the client might or might not consider such
   lenient processing of its request to be appropriate.

   While the decision of exactly which behaviors to apply in these cases
   lies with the server processing the request, the server might wish to
   defer to the client to specify which optional behavior is preferred.

   Currently, HTTP offers no explicitly defined means of expressing the
   client's preferences regarding the optional aspects of handling of a
   given request.  While HTTP does provide the Expect header -- which
   can be used to identify mandatory expectations for the processing of
   a request -- use of the field to communicate optional preferences is
   problematic:
   1.  The semantics of the Expect header field are such that
       intermediaries and servers are required to reject any request
       that states unrecognized or unsupported expectations.
   2.  While the Expect header field is end-to-end, the HTTP
       specification requires that the header be processed hop-by-hop.
       That is, every interceding intermediary that handles a request
       between the client and the origin server is required to process
       an expectation and determine whether it is capable of
       appropriately handling it.




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   The rigid, must-understand semantics of the Expect header, therefore,
   make it a poor choice for the general expression of optional
   preferences that may be specific to an individual application and are
   therefore unknown to an intermediary or are otherwise irrelevant to
   the intermediaries successful handling of the request and response.

   Another option available to clients is to utilize Request URI query-
   string parameters to express preferences.  Doing so, however, results
   in a variety of issues affecting the cacheability of responses.

   As an alternative, this specification defines a new HTTP request
   header field that can be used by clients to request that optional
   behaviors be applied by a server during the processing the request.
   Additionally, a handful of initial preference tokens for use with the
   new header are defined.

   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",
   "word", "OWS", "BWS" rules and the #rule extension as defined within
   Sections 3.2.1 and 3.2.4 of [I-D.ietf-httpbis-p1-messaging].


2.  The Prefer Request Header Field

   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 Section 6.1.2 of
   [I-D.ietf-httpbis-p2-semantics] with the exception that servers are
   allowed to ignore stated preferences.

   ABNF:

     Prefer     = "Prefer" ":" 1#preference
     preference = token [ BWS "=" BWS word ]
                  *( OWS ";" [ OWS parameter ] )
     parameter  = token [ BWS "=" BWS word ]

   This header field is defined with an extensible syntax to allow for
   future values included in the Registry of Preferences (Section 5.1).
   A server that does not recognize or is unable to comply with
   particular preference tokens in the Prefer header field of a request



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   MUST ignore those tokens and continue processing instead of
   signalling an error.

   A preference token can contain 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

   An optional set of parameters can 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.

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

   The Prefer header field is end-to-end and SHOULD be forwarded by a
   proxy if the request is forwarded unless Prefer is explicitly
   identified as being hop-by-hop using the Connection header field
   defined by [I-D.ietf-httpbis-p1-messaging], Section 6.1.

   In various situations, a proxy might determine that it is capable of
   honoring a preference independently of the server to which the
   request has been directed.  For instance, an intervening proxy might
   be capable of providing asynchronous handling of a request using 202
   Accepted responses independently of the origin server.  Such proxies
   can choose to honor the "return-asynch" preference on their own
   despite whether the origin is capable or willing to do so.

   Individual preference tokens MAY define their own requirements and
   restrictions as to whether and how intermediaries can apply the
   preference to a request independently of the origin server.

   Implementations MUST support 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 field.  The following
   examples are equivalent:









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   Multiple Prefer Header Fields:

     POST /foo HTTP/1.1
     Host: example.org
     Prefer: return-asynch
     Prefer: wait=100
     Date: Tue, 20 Dec 2011 12:34:56 GMT

   Single Prefer Header Field:

     POST /foo HTTP/1.1
     Host: example.org
     Prefer: wait=100, return-asynch
     Date: Tue, 20 Dec 2011 12:34:56 GMT

   To avoid possible ambiguity, individual preference tokens SHOULD NOT
   appear multiple times within a single request.  If any preference is
   specified more than once, only the first instance is to be
   considered.  All subsequent occurrences SHOULD be ignored without
   signaling an error or otherwise altering the processing of the
   request.  This is the only case in which the ordering of preferences
   within a request is considered to be significant.

   Due to the inherent complexities involved with properly implementing
   server-driven content negotiation, effective caching, and the
   application of optional preferences, implementors are urged to
   exercise caution when using preferences in a way that impacts the
   caching of a response and SHOULD NOT use the Prefer header mechanism
   for content negotiation.  If a server supports the optional
   application of a preference that might result in a variance to a
   cache's handling of a response entity, a Vary header field MUST be
   included with the response listing the Prefer header field regardless
   of whether the client actually used Prefer in the request.

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:

   1.  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-asynch, wait=10;
     Prefer: priority=5;





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   2.  Use lenient processing:

     Prefer: Lenient

   3.  Use of an optional, undefined parameter on the return-minimal
   preference:

     Prefer: return-minimal; foo="some parameter"


3.  The Preference-Applied Response Header Field

   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 a request.

   ABNF:

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

   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 only necessary when it is not
   readily and obviously apparent that a server applied a given
   preference and such ambiguity might have an impact on the client's
   handling of the response.  For instance, when using either the
   "return-representation" or "return-minimal" preferences, a client
   application might not be capable of reliably determining that the
   preference was applied simply by examining the payload of the
   response.  In such case the Preference-Applied header field can be
   used.

   Request:

     PATCH /my-document HTTP/1.1
     Host: example.org
     Content-Type: application/json-patch
     Prefer: return-representation

     [{"op": "add", "path": "/a", "value": 1}]










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   Response:

     HTTP/1.1 200 OK
     Content-Type: application/json
     Preference-Applied: return-representation
     Content-Location: /my-document

     {"a": 1}


4.  Preference Definitions

   The following subsections define an initial set of preferences.
   Additional 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 5.1).

   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
   remain 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.

4.1.  The "return-asynch" Preference

   The "return-asynch" preference indicates that the client prefers the
   server to respond asynchronously to a response.  For instance, in the
   case when the length of time it takes to generate a response will
   exceed some arbitrary threshold established by the server, the server
   can honor the return-asynch preference by returning a "202 Accepted"
   response.

   ABNF:

     return-asynch = "return-asynch"

   The key motivation for the "return-asynch" preference is to
   facilitate the operation of asynchronous request handling by allowing
   the client to indicate to a server its capability and preference for
   handling asynchronous responses.




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   An example request specifying the "return-asynch" preference:

     POST /collection HTTP/1.1
     Host: example.org
     Content-Type: text/plain
     Prefer: return-asynch

     {Data}

   An example asynchronous response using "202 Accepted":

     HTTP/1.1 202 Accepted
     Location: http://example.org/collection/123

   While the "202 Accepted" response status is defined by
   [I-D.ietf-httpbis-p2-semantics], little guidance is given on how and
   when to use the response code and the process for determining the
   subsequent final result of the operation is left entirely undefined.
   Therefore, whether and how any given server supports asynchronous
   responses is an implementation specific detail that is considered to
   be out of the scope of this specification.

4.2.  The "return-representation" and "return-minimal" Preferences

   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.

   The "return-minimal" preference, on the other hand, indicates that
   the client wishes the server to return only 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.

   ABNF:

     return-representation = "return-representation"
     return-minimal        = "return-minimal"

   When honoring the "return-representation" preference, the returned
   representation might not be a representation of the effective request
   URI when the request is affecting another resource.  In such cases,
   the Content-Location header can be used to identify the URI of the
   returned representation.

   The "return-representation" preference is intended to provide a means



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   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 can 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-minimal" preference defined below -- allow the server to take
   the client's preferences into consideration while constructing the
   response.

   An example request specifying the "return-representation" preference:

     PATCH /item/123 HTTP/1.1
     Host: example.org
     Content-Type: text/patch
     Prefer: return-representation

     1c1
     < ABCDEFGHIJKLMNOPQRSTUVWXYZ
     ---
     > BCDFGHJKLMNPQRSTVWXYZ

   An example response containing the resource representation:

     HTTP/1.1 200 OK
     Content-Location: http://example.org/item/123
     Preference-Applied: return-representation
     Content-Type: text/plain
     ETag: "d3b07384d113edec49eaa6238ad5ff00"

     BCDFGHJKLMNPQRSTVWXYZ

   In contrast, the "return-minimal" preference can reduce 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.









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   An example request specifying the "return-minimal" preference:

     POST /collection HTTP/1.1
     Host: example.org
     Content-Type: text/plain
     Prefer: return-minimal

     {Data}

   An example minimal response:

     HTTP/1.1 201 Created
     Location: http://example.org/collection/123

   The "return-minimal" and "return-representation" preferences are
   mutually exclusive directives.  A request that contains both
   preferences can be treated as though neither were specified.

4.3.  The "wait" Preference

   The "wait" preference can be used to establish an upper bound on the
   length of time, in seconds, the client expects it will take the
   server to process the request once it has been received.  In the case
   that generating a response will take longer than the time specified,
   the server, or proxy, can choose to utilize an asynchronous
   processing model by returning -- for example -- a "202 Accepted"
   response.

   ABNF:

     wait = "wait" BWS "=" BWS delta-seconds

   It is important to consider that HTTP messages spend some time
   traversing the network and being processed by intermediaries.  This
   increases the length of time that a client will wait for a response
   in addition to the time the server takes to process the request.  A
   client that has strict timing requirements can estimate these factors
   and adjust the wait value accordingly.

   As with other preferences, the "wait" preference could be ignored.
   Clients can abandon requests that take longer than they are prepared
   to wait.









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   For example, a server receiving the following request might choose to
   respond asynchronously if processing the request will take longer
   than 10 seconds:

     POST /collection HTTP/1.1
     Host: example.org
     Content-Type: text/plain
     Prefer: return-asynch, wait=10

     {Data}

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

   The "strict" and "lenient" preferences are mutually-exclusive
   directives indicating, at the server's discretion, how the client
   wishes the server to handle potential error conditions that can 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 go ahead with
   processing.  The "strict" preference can be used to indicate that,
   while any particular error may be recoverable, the client would
   prefer that the server reject the request.  The "lenient" preference,
   on the other hand, indicates that the client wishes the server to
   attempt to process the request.

   ABNF:

     handling = "strict" / "lenient"

   An example request specifying the "strict" preference:

     POST /collection HTTP/1.1
     Host: example.org
     Content-Type: text/plain
     Prefer: strict


5.  IANA Considerations

   The 'Prefer' and 'Preference-Applied' header fields should be added
   to the Permanent Message Header Fields registry defined in [RFC3864]
   (http://www.iana.org/assignments/message-headers/perm-headers.html).

      Header field name: Prefer





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      Applicable Protocol: HTTP
      Status: Standard
      Author: James M Snell <jasnell@gmail.com>
      Change controller: IETF
      Specification document: this specification

      Header field name: Preference-Applied
      Applicable Protocol: HTTP
      Status: Standard
      Author: James M Snell <jasnell@gmail.com>
      Change controller: IETF
      Specification document: this specification

5.1.  The Registry of Preferences

   IANA is asked to create a new registry, "HTTP Preferences", under the
   Hypertext Transfer Protocol (HTTP) Parameters group.  New
   registrations will use the Specification Required policy [RFC5226].
   The requirements for registered preferences are described in
   Section 4.

   Registration requests consist of the completed registration template
   below, typically published in the required specification.  However,
   to allow for the allocation of values prior to publication, the
   Designated Expert can approve registration based on a separately
   submitted template once they are satisfied that a specification will
   be published.  Preferences can be registered by third parties if the
   Designated Expert determines that an unregistered preference 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]

   Registration requests should be sent to the ietf-http-wg@w3.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.





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5.2.  Initial Registry Contents

   The Preferences Registry's initial contents are:

   o  Preference: return-asynch
   o  Description: Indicates that the client prefers the server to
      respond asynchronously to a request.
   o  Reference: [this specification], Section 4.1

   o  Preference: return-minimal
   o  Description: Indicates that the client prefers the server return a
      minimal response to a request.
   o  Reference: [this specification], Section 4.2

   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.
   o  Reference: [this specification], Section 4.2

   o  Preference: wait
   o  Description: Indicates an upper bound to the length of time the
      client expects it will take the server to process the request once
      it has been received.
   o  Reference: [this specification], Section 4.3

   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], Section 4.4

   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], Section 4.4


6.  Security Considerations

   Specific preferences requested by a client can introduce security
   considerations and concerns beyond those discussed within HTTP/1.1
   [I-D.ietf-httpbis-p1-messaging] and it's additional associated
   specification documents.  Implementers need to refer to the
   specifications and descriptions of each preference to determine the
   security considerations relevant to each.




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   A server could incur greater costs in attempting to comply with a
   particular preference (for instance, the cost of providing a
   representation in a response that would not ordinarily contain one;
   or the commitment of resources necessary to track state for an
   asynchronous response).  Unconditional compliance from a server could
   allow the use of preferences for denial of service.  A server can
   ignore an expressed preference to avoid expending resources that it
   does not wish to commit.


7.  References

7.1.  Normative References

   [I-D.ietf-httpbis-p1-messaging]
              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing",
              draft-ietf-httpbis-p1-messaging-21 (work in progress),
              October 2012.

   [I-D.ietf-httpbis-p2-semantics]
              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content",
              draft-ietf-httpbis-p2-semantics-21 (work in progress),
              October 2012.

   [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.

7.2.  Informative References

   [RFC5023]  Gregorio, J. and B. de hOra, "The Atom Publishing
              Protocol", RFC 5023, October 2007.




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Author's Address

   James M Snell

   Email: jasnell@gmail.com














































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