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rfc:rfc7240

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.

Snell Standards Track [Page 1] RFC 7240 HTTP Prefer June 2014

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

Snell Standards Track [Page 2] RFC 7240 HTTP Prefer June 2014

 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].

Snell Standards Track [Page 3] RFC 7240 HTTP Prefer June 2014

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.

Snell Standards Track [Page 4] RFC 7240 HTTP Prefer June 2014

 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

Snell Standards Track [Page 5] RFC 7240 HTTP Prefer June 2014

 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
   {...}

Snell Standards Track [Page 6] RFC 7240 HTTP Prefer June 2014

 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.

Snell Standards Track [Page 7] RFC 7240 HTTP Prefer June 2014

 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.

Snell Standards Track [Page 8] RFC 7240 HTTP Prefer June 2014

 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")

Snell Standards Track [Page 9] RFC 7240 HTTP Prefer June 2014

 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.

Snell Standards Track [Page 10] RFC 7240 HTTP Prefer June 2014

 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.

Snell Standards Track [Page 11] RFC 7240 HTTP Prefer June 2014

 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.

Snell Standards Track [Page 12] RFC 7240 HTTP Prefer June 2014

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.

Snell Standards Track [Page 13] RFC 7240 HTTP Prefer June 2014

 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

Snell Standards Track [Page 14] RFC 7240 HTTP Prefer June 2014

 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

Snell Standards Track [Page 15] RFC 7240 HTTP Prefer June 2014

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.

Snell Standards Track [Page 16] RFC 7240 HTTP Prefer June 2014

Author's Address

 James M Snell
 EMail: jasnell@gmail.com

Snell Standards Track [Page 17]

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