Prefer Header for HTTP

Internet Engineering Task Force (IETF)                          J. Snell
Request for Comments: 7240                                     June 2014
Category: Standards Track
ISSN: 2070-1721


                         Prefer Header for HTTP

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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7240.

Copyright Notice

   Copyright (c) 2014 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 ....................................................2
      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 "respond-async" Preference .............................8
      4.2. The "return=representation" and "return=minimal"
           Preferences ................................................9
      4.3. The "wait" Preference .....................................11
      4.4. The "handling=strict" and "handling=lenient" Processing ...12
   5. IANA Considerations ............................................13
      5.1. The Registry of Preferences ...............................13
      5.2. Initial Registry Contents .................................15
   6. Security Considerations ........................................16
   7. References .....................................................16
      7.1. Normative References ......................................16
      7.2. Informative References ....................................16

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.

   The must-understand semantics of the Expect header make it a poor
   choice for the expression of optional preferences.

   Another option available to clients is to utilize Request URI
   query-string parameters to express preferences.  However, any
   mechanism that alters the URI can have undesirable effects, such as
   when caches record the altered URI.

   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", and "BWS" rules and the #rule extension as defined
   within Sections 3.2.1 and 3.2.4 of [RFC7230]; as well as the
   "delta-seconds" rule defined in Section 8.1.3 of [RFC7231].

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 are 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 [RFC7231] 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
   MUST ignore those tokens and continue processing instead of signaling
   an error.

   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 examples of a "foo" preference with a
   single "bar" parameter.

     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.  There is no significance given to the
   ordering of parameters on any given preference.

   For both preference token names and parameter names, comparison 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 MUST 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 [RFC7230], 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 "respond-async" preference on their
   own regardless of whether or not 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.

   A client MAY use multiple instances of the Prefer header field in a
   single message, or it MAY use a single Prefer header field with
   multiple comma-separated preference tokens.  If multiple Prefer
   header fields are used, it is equivalent to a single Prefer header
   field with the comma-separated concatenation of all of the tokens.
   For example, the following are equivalent:

   Multiple Prefer header fields defining three distinct preference
   tokens:

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

   A single Prefer header field defining the same three preference
   tokens:

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

   To avoid any 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, implementers 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 in the response listing the Prefer header field regardless
   of whether the client actually used Prefer in the request.
   Alternatively, the server MAY include a Vary header with the special
   value "*" as defined by [RFC7231], Section 8.2.1.  Note, however,
   that use of the "Vary: *" header will make it impossible for a proxy
   to cache the response.

   Note that while Preference tokens are similar in structure to HTTP
   Expect tokens, the Prefer and Expect header fields serve very
   distinct purposes and preferences cannot be used as expectations.

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 request cannot be processed within 10 seconds.  An undefined
   "priority" preference is also specified:

     POST /some-resource HTTP/1.1
     Host: example.org
     Content-Type: text/plain
     Prefer: respond-async, wait=10
     Prefer: priority=5

     {...}
   2.  Use lenient processing:

     POST /some-resource HTTP/1.1
     Host: example.org
     Content-Type: text/plain
     Prefer: Lenient

     {...}

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

     POST /some-resource HTTP/1.1
     Host: example.org
     Content-Type: text/plain
     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#applied-pref
     applied-pref = token [ BWS "=" BWS word ]

   The syntax of the Preference-Applied header differs from that of the
   Prefer header in that 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 if the
   preference was (or was not) applied simply by examining the payload
   of the response.  In such a case, the Preference-Applied header field
   can be used.

   Request:

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

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

   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 preferences (see Section 5.1).

4.1.  The "respond-async" Preference

   The "respond-async" 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 "respond-async" preference by returning a 202
   (Accepted) response.

   ABNF:

     respond-async = "respond-async"

   The key motivation for the "respond-async" 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.

   An example request specifying the "respond-async" preference:

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

     {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 [RFC7231],
   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 (OK) 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 = "return" BWS "=" BWS ("representation" / "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
   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.

   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: application/example-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
     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.

   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.  It is anticipated that there will
   never be a situation where it will make sense for a single request to
   include both preferences.  Any such requests will likely be the
   result of a coding error within the client.  As such, a request
   containing 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.

   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: respond-async, wait=10

     {Data}

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

   The "handling=strict" and "handling=lenient" preferences indicate, 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 "handling=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 "handling=lenient" preference, on the other hand,
   indicates that the client wishes the server to attempt to process the
   request.

   ABNF:

     handling = "handling" BWS "=" BWS ("strict" / "lenient")

   An example request specifying the "strict" preference:

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

   The "handling=strict" and "handling=lenient" preferences are mutually
   exclusive directives.  It is anticipated that there will never be a
   situation where it will make sense for a single request to include
   both preferences.  Any such requests will likely be the result of a
   coding error within the client.  As such, a request containing both
   preferences can be treated as though neither were specified.

5.  IANA Considerations

   The 'Prefer' and 'Preference-Applied' header fields have been added
   to the "Permanent Message Header Field Names" registry defined in
   [RFC3864] (http://www.iana.org/assignments/message-headers).

      Header field name: Prefer

      Applicable Protocol: HTTP

      Status: Standard

      Author: James M Snell <jasnell@gmail.com>

      Change controller: IETF

      Specification document: this specification, Section 2

      Header field name: Preference-Applied

      Applicable Protocol: HTTP

      Status: Standard

      Author: James M Snell <jasnell@gmail.com>

      Change controller: IETF

      Specification document: this specification, Section 3

5.1.  The Registry of Preferences

   IANA has created a new registry, "HTTP Preferences", under the
   "Hypertext Transfer Protocol (HTTP) Parameters" registry.  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  Value: (An enumeration or description of possible values for the
      preference token).

   o  Optional Parameters: (An enumeration of optional parameters, and
      their values, associated with the preference token).

   o  Description:

   o  Reference:

   o  Notes: [optional]

   The "Value" and "Optional Parameters" fields MAY be omitted from the
   registration template if the specific preference token definition
   does not define either.

   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.

   The Expert Reviewer shall ensure:

   o  That the requested preference name conforms to the token rule in
      Section 2 and that it is not identical to any other registered
      preference name;

   o  That any associated value, parameter names, and values conform to
      the relevant ABNF grammar specifications in Section 2;

   o  That the name is 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 uses.

   o  That requested preferences do not constrain servers, clients, or
      any intermediaries to any behavior required for successful
      processing; and
   o  That the specification document defining the preference includes a
      proper and complete discussion of any security considerations
      relevant to the use of the preference.

5.2.  Initial Registry Contents

   The "HTTP Preferences" registry's initial contents are:

   o  Preference: respond-async

   o  Description: Indicates that the client prefers that the server
      respond asynchronously to a request.

   o  Reference: [this specification], Section 4.1

   o  Preference: return

   o  Value: One of either "minimal" or "representation"

   o  Description: When the value is "minimal", it indicates that the
      client prefers that the server return a minimal response to a
      request.  When the value is "representation", it indicates that
      the client prefers that the server 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 for the server to process the request
      once it has been received.

   o  Reference: [this specification], Section 4.3

   o  Preference: handling

   o  Value: One of either "strict" or "lenient"

   o  Description: When value is "strict", it indicates that the client
      wishes the server to apply strict validation and error handling to
      the processing of a request.  When the value is "lenient", it
      indicates that the client wishes the server to apply lenient
      validation and error handling to the processing of the 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
   [RFC7230] and its associated specification documents (see [RFC7230]
   for the list of associated works).  Implementers need to refer to the
   specifications and descriptions of each preference to determine the
   security considerations relevant to each.

   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

   [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.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, June 2014.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              June 2014.

7.2.  Informative References

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

Author's Address

   James M Snell

   EMail: jasnell@gmail.com