draft-dalal-deprecation-header-01.xml

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<rfc docName="draft-dalal-deprecation-header-01" category="std">

  <front>
    <title>The Deprecation HTTP Header Field</title>

    <author initials="S." surname="Dalal" fullname="Sanjay Dalal">
      <organization></organization>
      <address>
        <email>sanjay.dalal@cal.berkeley.edu</email>
        <uri>https://github.com/sdatspun2</uri>
      </address>
    </author>
    <author initials="E." surname="Wilde" fullname="Erik Wilde">
      <organization></organization>
      <address>
        <email>erik.wilde@dret.net</email>
        <uri>http://dret.net/netdret</uri>
      </address>
    </author>

    <date year="2019" month="June" day="17"/>

    
    
    

    <abstract>


<t>The HTTP Deprecation response header field can be used to signal to consumers of a URI-identified resource that the resource has been deprecated. Additionally, the deprecation link relation can be used to link to a resource that provides additional context for the deprecation, and possibly ways in which clients can find a replacement for the deprecated resource.</t>



    </abstract>


  </front>

  <middle>


<section anchor="introduction" title="Introduction">

<t>Deprecation of a URI-identified resource is a technique to communicate information about the lifecycle of a resource. It encourages applications to migrate away from the resource, discourages applications from forming new dependencies on the resource, and informs applications about the risk of continuing dependence upon the resource.</t>

<t>The act of deprecation does not change any behavior of the resource. It just informs client of the fact that a resource is deprecated. The Deprecation HTTP response header field MAY be used to convey this fact at runtime to clients. The header field can carry information indicating since when the deprecation is in effect.</t>

<t>In addition to the Deprecation header field the resource provider can use other header fields to convey additional information related to deprecation. For example, information such as where to find documentation related to the deprecation or what should be used as an alternate and when the deprecated resource would be unreachable, etc. Alternates of a resource can be similar resource(s) or a newer version of the same resource.</t>

<section anchor="notational-conventions" title="Notational Conventions">

<t>The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only when, they appear in all capitals, as shown here.</t>

<t>This specification uses the Augmented Backus-Naur Form (ABNF) notation of <xref target="RFC5234"/> and includes, by reference, the HTTP-date rule as defined within Sections 3.2.6 and 7 of <xref target="RFC7230"/> and Section 7.1.1 of <xref target="RFC7231"/>.</t>

</section>
</section>
<section anchor="the-deprecation-http-response-header-field" title="The Deprecation HTTP Response Header Field">

<t>The <spanx style="verb">Deprecation</spanx> HTTP response header field allows a server to communicate to a client that the URI-identified resource in context of the message is or will be deprecated.</t>

<section anchor="syntax" title="Syntax">

<t>The <spanx style="verb">Deprecation</spanx> response header field describes the deprecation. It either shows the deprecation date, which may be in the future (the resource context will be deprecated at that date) or in the past (the resource context has been deprecated at that date), or it simply flags the resource context as being deprecated:</t>

<figure><artwork><![CDATA[
Deprecation = HTTP-date / "true"
]]></artwork></figure>

<t>Servers MUST NOT include more than one <spanx style="verb">Deprecation</spanx> header field in the same response.</t>

<t>The date, if present, is the date when the resource context was or will be deprecated. It is in the form of an HTTP-date timestamp, as defined in Section 7.1.1.1 of <xref target="RFC7231"/>.</t>

<t>The following example shows that the resource context has been deprecated on Friday, November 11, 2018 at 23:59:59 GMT:</t>

<figure><artwork><![CDATA[
Deprecation: Sun, 11 Nov 2018 23:59:59 GMT
]]></artwork></figure>

<t>The deprecation date can be in the future. If the value of <spanx style="verb">date</spanx> is in the future, it means that the resource will be deprecated at the given date in future.</t>

<t>If the deprecation date is not known, the header field can carry the simple string “true”, indicating that the resource context is deprecated, without indicating when that happened:</t>

<figure><artwork><![CDATA[
    Deprecation: true
]]></artwork></figure>

</section>
</section>
<section anchor="the-deprecation-link-relation-type" title="The Deprecation Link Relation Type">

<t>In addition to the Deprecation HTTP header field, the server can use links with the “deprecation” link relation type to communicate to the client where to find more information about deprecation of the context. This can happen before the actual deprecation, to make a deprecation policy discoverable, or after deprecation, when there may be documentation about the deprecation, and possibly documentation of how to manage it.</t>

<t>This specification places no restrictions on the representation of the interlinked deprecation policy. In particular, the deprecation policy may be available as human-readable documentation or as machine-readable description.</t>

<section anchor="documentation" title="Documentation">

<t>For a URI-identified resource, deprecation could involve one or more parts of request, response or both. These parts could be one or more of the following.</t>

<t><list style="symbols">
  <t>URI - deprecation of one ore more query parameter(s) or path element(s)</t>
  <t>method - HTTP method for the resource is deprecated</t>
  <t>request header - one or more HTTP request header(s) is deprecated</t>
  <t>response header - HTTP response header(s) is deprecated</t>
  <t>request body - request body contains one or more deprecated element(s)</t>
  <t>response body - response body contains one or more deprecated element(s)</t>
</list></t>

<t>The purpose of the <spanx style="verb">Deprecation</spanx> header is to provide just enough “hints” about the deprecation to the client application developer. It is safe to assume that on reception of the <spanx style="verb">Deprecation</spanx> header, the client developer would look up the resource’s documentation in order to find deprecation related semantics. The resource developer could provide a link to the resource documentation using a <spanx style="verb">Link</spanx> header with relation type <spanx style="verb">deprecation</spanx> as shown below.</t>

<figure><artwork><![CDATA[
Link: <https://developer.example.com/deprecation>; rel="deprecation"; type="text/html"
]]></artwork></figure>

<t>In this example, the interlinked content provides additional information about the deprecation of the resource context. In this example, there is no Deprecation header field in the response, and thus the resource is not deprecated. However, the resource already exposes a link where information is available how deprecation is managed for the context. This may be documentation explaining the use of the Deprecation header field, and also explaining under which circumstances and with which policies (announcement before deprecation; continued operation after deprecation) deprecation might be happening.</t>

<t>The following example uses the same link header, but also announces a deprecation date using a Deprecation header field.</t>

<figure><artwork><![CDATA[
Deprecation: Sun, 11 Nov 2018 23:59:59 GMT
Link: <https://developer.example.com/deprecation>; rel="deprecation"; type="text/html"
]]></artwork></figure>

<t>Given that the deprecation date is in the past, the linked resource may have been updated to include information about the deprecation, allowing clients to discover information about the deprecation that happened.</t>

</section>
</section>
<section anchor="recommend-replacement" title="Recommend Replacement">

<t><spanx style="verb">Link</spanx> <xref target="RFC8288"/> header could be used in addition to the <spanx style="verb">Deprecation</spanx> header to recommend the client application about available alternates to the deprecated resource. Following relation types as defined in <xref target="RFC8288"/> are RECOMMENDED to use for the purpose.</t>

<t><list style="symbols">
  <t><spanx style="verb">successor-version</spanx>: Points to a resource containing the successor version. <xref target="RFC5829"/></t>
  <t><spanx style="verb">latest-version</spanx>: Points to a resource containing the latest (e.g., current) version. <xref target="RFC5829"/></t>
  <t><spanx style="verb">alternate</spanx>: Designates a substitute. [W3C.REC-html401-19991224]</t>
</list></t>

<t>The following example provides link to the successor version of the requested resource that is deprecated.</t>

<figure><artwork><![CDATA[
Deprecation: Sun, 11 Nov 2018 23:59:59 GMT
Link: <https://api.example.com/v2/customers>; rel="successor-version"
]]></artwork></figure>

<t>This example provides link to an alternate resource to the requested resource that is deprecated.</t>

<figure><artwork><![CDATA[
Deprecation: Sun, 11 Nov 2018 23:59:59 GMT
Link: <https://api.example.com/v1/clients>; rel="alternate"
]]></artwork></figure>

</section>
<section anchor="sunset" title="Sunset">

<t>In addition to the deprecation related information, if the resource provider wants to convey to the client application that the deprecated resource is expected to become unresponsive at a specific point in time, the <xref target="RFC8594"/> header could be used in addition to the <spanx style="verb">Deprecation</spanx> header.</t>

<t>The following example shows that the resource in context has been deprecated since Friday, November 11, 2018 at 23:59:59 GMT and its sunset date is Friday, November 11, 2020 at 23:59:59 GMT.</t>

<figure><artwork><![CDATA[
Deprecation: Sun, 11 Nov 2018 23:59:59 GMT
Sunset: Wed, 11 Nov 2020 23:59:59 GMT
]]></artwork></figure>

</section>
<section anchor="resource-behavior" title="Resource Behavior">

<t>The act of deprecation does not change any behavior of the resource. Deprecated resources SHOULD keep functioning as before, allowing consumers to still use the resources in the same way as they did before the resources were declared deprecated.</t>

</section>
<section anchor="iana-considerations" title="IANA Considerations">

<section anchor="the-deprecation-response-header-field" title="The Deprecation Response Header Field">

<t>The <spanx style="verb">Deprecation</spanx> response header should be added to the permanent registry of message header fields (see <xref target="RFC3864"/>), taking into account the guidelines given by HTTP/1.1 <xref target="RFC7231"/>.</t>

<figure><artwork><![CDATA[
Header Field Name: Deprecation

Applicable Protocol: Hypertext Transfer Protocol (HTTP)

Status: Standard

Author: Sanjay Dalal <sanjay.dalal@cal.berkeley.edu>, 
        Erik Wilde <erik.wilde@dret.net>

Change controller: IETF

Specification document: this specification, 
            Section 2 "The Deprecation HTTP Response Header Field"
]]></artwork></figure>

</section>
<section anchor="the-deprecation-link-relation-type-1" title="The Deprecation Link Relation Type">

<t>The <spanx style="verb">deprecation</spanx> link relation type should be added to the permanent registry of link relation types according to Section 4.2 of <xref target="RFC8288"/>:</t>

<figure><artwork><![CDATA[
Relation Type: deprecation

Applicable Protocol: Hypertext Transfer Protocol (HTTP)

Status: Standard

Author: Sanjay Dalal <sanjay.dalal@cal.berkeley.edu>, 
        Erik Wilde <erik.wilde@dret.net>

Change controller: IETF

Specification document: this specification, 
        Section 3 "The Deprecation Link Relation Type"
]]></artwork></figure>

</section>
</section>
<section anchor="implementation-status" title="Implementation Status">

<t>Note to RFC Editor: Please remove this section before publication.</t>

<t>This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in <xref target="RFC7942"/>.  The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs.  Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors.  This is not intended as, and must not be construed to be, a catalog of available implementations or their features.  Readers are advised to note that
other implementations may exist.</t>

<t>According to RFC 7942, “this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit”.</t>

<t>Organization: Zapier</t>

<t>Description: Zapier uses two custom HTTP headers named <spanx style="verb">X-API-Deprecation-Date</spanx> and <spanx style="verb">X-API-Deprecation-Info</spanx></t>

<t>Reference:  https://zapier.com/engineering/api-geriatrics/</t>

<t>Organization: IBM</t>

<t>IBM uses a custom HTTP header named <spanx style="verb">Deprecated</spanx></t>

<t>Reference: https://www.ibm.com/support/knowledgecenter/en/SS42VS_7.3.1/com.ibm.qradar.doc/c_rest_api_getting_started.html</t>

<t>Organization: Ultipro</t>

<t>Description: Ultipro uses the HTTP <spanx style="verb">Warning</spanx> header as described in Section 5.5 of <xref target="RFC7234"/> with code <spanx style="verb">299</spanx></t>

<t>Reference:  https://connect.ultipro.com/api-deprecation</t>

<t>Organization: Clearbit</t>

<t>Description: Clearbit uses a custom HTTP header named <spanx style="verb">X-API-Warn</spanx></t>

<t>Reference: https://blog.clearbit.com/dealing-with-deprecation/</t>

<t>Organization: PayPal</t>

<t>Description: PayPal uses a custom HTTP header named <spanx style="verb">PayPal-Deprecated</spanx></t>

<t>Reference: https://github.com/paypal/api-standards/blob/master/api-style-guide.md#runtime</t>

</section>
<section anchor="security-considerations" title="Security Considerations">

<t>The Deprecation header field SHOULD be treated as a hint, meaning that the resource is indicating (and not guaranteeing with certainty) that it is deprecated. Applications consuming the resource SHOULD check the resource documentation to verify authenticity and accuracy. Resource documentation SHOULD provide additional information about the deprecation including recommendation(s) for replacement.</t>

<t>In cases, where the Deprecation header field value is a date in future, it can lead to information that otherwise might not be available. Therefore, applications consuming the resource SHOULD verify the resource documentation and if possible, consult the resource developer to discuss potential impact due to deprecation and plan for possible transition to recommended resource.</t>

<t>In cases where <spanx style="verb">Link</spanx> header is used to provide more documentation and/or recommendation for replacement, one should assume that the content of the  <spanx style="verb">Link</spanx> header field may not be secure, private or integrity-guaranteed, and due caution should be exercised when using it. Applications consuming the resource SHOULD check the referred resource documentation to verify authenticity and accuracy.</t>

<t>The suggested <spanx style="verb">Link</spanx> header fields make extensive use of IRIs and URIs. See <xref target="RFC3987"/> for security considerations relating to IRIs. See <xref target="RFC3986"/> for security considerations relating to URIs. See <xref target="RFC7230"/> for security considerations relating to HTTP headers.</t>

<t>Applications that take advantage of typed links should consider the attack vectors opened by automatically following, trusting, or otherwise using links gathered from the HTTP headers. In particular, Link headers that use the <spanx style="verb">successor-version</spanx>, <spanx style="verb">latest-version</spanx> or <spanx style="verb">alternate</spanx> relation types should be treated with due caution. See <xref target="RFC5829"/> for security considerations relating to these link relation types.</t>

</section>
<section anchor="examples" title="Examples">

<t>The first example shows a deprecation header field without date information:</t>

<figure><artwork><![CDATA[
Deprecation: true
]]></artwork></figure>

<t>The second example shows a deprecation header with date information and a link to the successor version:</t>

<figure><artwork><![CDATA[
Deprecation: Sun, 11 Nov 2018 23:59:59 GMT
Link: <https://api.example.com/v2/customers>; rel="successor-version"
]]></artwork></figure>

<t>The third example shows a deprecation header field with links for the successor version and for the API’s deprecation policy. In addition, it shows the sunset date for the deprecated resource:</t>

<figure><artwork><![CDATA[
Deprecation: Sun, 11 Nov 2018 23:59:59 GMT
Sunset: Wed, 11 Nov 2020 23:59:59 GMT
Link: <https://api.example.com/v2/customers>; rel="successor-version", <https://developer.example.com/deprecation>; rel="deprecation"
]]></artwork></figure>

</section>


  </middle>

  <back>

    <references title='Normative References'>





<reference  anchor="RFC2119" target='https://www.rfc-editor.org/info/rfc2119'>
<front>
<title>Key words for use in RFCs to Indicate Requirement Levels</title>
<author initials='S.' surname='Bradner' fullname='S. Bradner'><organization /></author>
<date year='1997' month='March' />
<abstract><t>In many standards track documents several words are used to signify the requirements in the specification.  These words are often capitalized. This document defines these words as they should be interpreted in IETF documents.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t></abstract>
</front>
<seriesInfo name='BCP' value='14'/>
<seriesInfo name='RFC' value='2119'/>
<seriesInfo name='DOI' value='10.17487/RFC2119'/>
</reference>



<reference  anchor="RFC8174" target='https://www.rfc-editor.org/info/rfc8174'>
<front>
<title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title>
<author initials='B.' surname='Leiba' fullname='B. Leiba'><organization /></author>
<date year='2017' month='May' />
<abstract><t>RFC 2119 specifies common key words that may be used in protocol  specifications.  This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the  defined special meanings.</t></abstract>
</front>
<seriesInfo name='BCP' value='14'/>
<seriesInfo name='RFC' value='8174'/>
<seriesInfo name='DOI' value='10.17487/RFC8174'/>
</reference>



<reference  anchor="RFC5234" target='https://www.rfc-editor.org/info/rfc5234'>
<front>
<title>Augmented BNF for Syntax Specifications: ABNF</title>
<author initials='D.' surname='Crocker' fullname='D. Crocker' role='editor'><organization /></author>
<author initials='P.' surname='Overell' fullname='P. Overell'><organization /></author>
<date year='2008' month='January' />
<abstract><t>Internet technical specifications often need to define a formal syntax.  Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications.  The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power.  The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges.  This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='STD' value='68'/>
<seriesInfo name='RFC' value='5234'/>
<seriesInfo name='DOI' value='10.17487/RFC5234'/>
</reference>



<reference  anchor="RFC7230" target='https://www.rfc-editor.org/info/rfc7230'>
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</title>
<author initials='R.' surname='Fielding' fullname='R. Fielding' role='editor'><organization /></author>
<author initials='J.' surname='Reschke' fullname='J. Reschke' role='editor'><organization /></author>
<date year='2014' month='June' />
<abstract><t>The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems.  This document provides an overview of HTTP architecture and its associated terminology, defines the &quot;http&quot; and &quot;https&quot; Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes related security concerns for implementations.</t></abstract>
</front>
<seriesInfo name='RFC' value='7230'/>
<seriesInfo name='DOI' value='10.17487/RFC7230'/>
</reference>



<reference  anchor="RFC7231" target='https://www.rfc-editor.org/info/rfc7231'>
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</title>
<author initials='R.' surname='Fielding' fullname='R. Fielding' role='editor'><organization /></author>
<author initials='J.' surname='Reschke' fullname='J. Reschke' role='editor'><organization /></author>
<date year='2014' month='June' />
<abstract><t>The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems.  This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation.</t></abstract>
</front>
<seriesInfo name='RFC' value='7231'/>
<seriesInfo name='DOI' value='10.17487/RFC7231'/>
</reference>



<reference  anchor="RFC8288" target='https://www.rfc-editor.org/info/rfc8288'>
<front>
<title>Web Linking</title>
<author initials='M.' surname='Nottingham' fullname='M. Nottingham'><organization /></author>
<date year='2017' month='October' />
<abstract><t>This specification defines a model for the relationships between resources on the Web (&quot;links&quot;) and the type of those relationships (&quot;link relation types&quot;).</t><t>It also defines the serialisation of such links in HTTP headers with the Link header field.</t></abstract>
</front>
<seriesInfo name='RFC' value='8288'/>
<seriesInfo name='DOI' value='10.17487/RFC8288'/>
</reference>



<reference  anchor="RFC3864" target='https://www.rfc-editor.org/info/rfc3864'>
<front>
<title>Registration Procedures for Message Header Fields</title>
<author initials='G.' surname='Klyne' fullname='G. Klyne'><organization /></author>
<author initials='M.' surname='Nottingham' fullname='M. Nottingham'><organization /></author>
<author initials='J.' surname='Mogul' fullname='J. Mogul'><organization /></author>
<date year='2004' month='September' />
<abstract><t>This specification defines registration procedures for the message header fields used by Internet mail, HTTP, Netnews and other applications.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t></abstract>
</front>
<seriesInfo name='BCP' value='90'/>
<seriesInfo name='RFC' value='3864'/>
<seriesInfo name='DOI' value='10.17487/RFC3864'/>
</reference>



<reference  anchor="RFC7234" target='https://www.rfc-editor.org/info/rfc7234'>
<front>
<title>Hypertext Transfer Protocol (HTTP/1.1): Caching</title>
<author initials='R.' surname='Fielding' fullname='R. Fielding' role='editor'><organization /></author>
<author initials='M.' surname='Nottingham' fullname='M. Nottingham' role='editor'><organization /></author>
<author initials='J.' surname='Reschke' fullname='J. Reschke' role='editor'><organization /></author>
<date year='2014' month='June' />
<abstract><t>The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems.  This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.</t></abstract>
</front>
<seriesInfo name='RFC' value='7234'/>
<seriesInfo name='DOI' value='10.17487/RFC7234'/>
</reference>



<reference  anchor="RFC3987" target='https://www.rfc-editor.org/info/rfc3987'>
<front>
<title>Internationalized Resource Identifiers (IRIs)</title>
<author initials='M.' surname='Duerst' fullname='M. Duerst'><organization /></author>
<author initials='M.' surname='Suignard' fullname='M. Suignard'><organization /></author>
<date year='2005' month='January' />
<abstract><t>This document defines a new protocol element, the Internationalized  Resource Identifier (IRI), as a complement of the Uniform Resource  Identifier (URI). An IRI is a sequence of characters from the  Universal Character Set (Unicode/ISO 10646). A mapping from IRIs to   URIs is defined, which means that IRIs can be used instead of URIs,  where appropriate, to identify resources.</t><t> The approach of defining a new protocol element was chosen instead of extending or changing the definition of URIs.  This was done in order  to allow a clear distinction and to avoid incompatibilities with  existing software. Guidelines are provided for the use and deployment of IRIs in various protocols, formats, and software components that currently deal with URIs.</t></abstract>
</front>
<seriesInfo name='RFC' value='3987'/>
<seriesInfo name='DOI' value='10.17487/RFC3987'/>
</reference>



<reference  anchor="RFC3986" target='https://www.rfc-editor.org/info/rfc3986'>
<front>
<title>Uniform Resource Identifier (URI): Generic Syntax</title>
<author initials='T.' surname='Berners-Lee' fullname='T. Berners-Lee'><organization /></author>
<author initials='R.' surname='Fielding' fullname='R. Fielding'><organization /></author>
<author initials='L.' surname='Masinter' fullname='L. Masinter'><organization /></author>
<date year='2005' month='January' />
<abstract><t>A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource.  This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet.  The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier.  This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='STD' value='66'/>
<seriesInfo name='RFC' value='3986'/>
<seriesInfo name='DOI' value='10.17487/RFC3986'/>
</reference>




    </references>

    <references title='Informative References'>





<reference  anchor="RFC5829" target='https://www.rfc-editor.org/info/rfc5829'>
<front>
<title>Link Relation Types for Simple Version Navigation between Web Resources</title>
<author initials='A.' surname='Brown' fullname='A. Brown'><organization /></author>
<author initials='G.' surname='Clemm' fullname='G. Clemm'><organization /></author>
<author initials='J.' surname='Reschke' fullname='J. Reschke' role='editor'><organization /></author>
<date year='2010' month='April' />
<abstract><t>This specification defines a set of link relation types that may be used on Web resources for navigation between a resource and other resources related to version control, such as past versions and working copies.  This document is not an Internet Standards Track  specification; it is published for informational purposes.</t></abstract>
</front>
<seriesInfo name='RFC' value='5829'/>
<seriesInfo name='DOI' value='10.17487/RFC5829'/>
</reference>



<reference  anchor="RFC8594" target='https://www.rfc-editor.org/info/rfc8594'>
<front>
<title>The Sunset HTTP Header Field</title>
<author initials='E.' surname='Wilde' fullname='E. Wilde'><organization /></author>
<date year='2019' month='May' />
<abstract><t>This specification defines the Sunset HTTP response header field, which indicates that a URI is likely to become unresponsive at a specified point in the future.  It also defines a sunset link relation type that allows linking to resources providing information about an upcoming resource or service sunset.</t></abstract>
</front>
<seriesInfo name='RFC' value='8594'/>
<seriesInfo name='DOI' value='10.17487/RFC8594'/>
</reference>



<reference  anchor="RFC7942" target='https://www.rfc-editor.org/info/rfc7942'>
<front>
<title>Improving Awareness of Running Code: The Implementation Status Section</title>
<author initials='Y.' surname='Sheffer' fullname='Y. Sheffer'><organization /></author>
<author initials='A.' surname='Farrel' fullname='A. Farrel'><organization /></author>
<date year='2016' month='July' />
<abstract><t>This document describes a simple process that allows authors of Internet-Drafts to record the status of known implementations by including an Implementation Status section.  This will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature.</t><t>This process is not mandatory.  Authors of Internet-Drafts are encouraged to consider using the process for their documents, and working groups are invited to think about applying the process to all of their protocol specifications.  This document obsoletes RFC 6982, advancing it to a Best Current Practice.</t></abstract>
</front>
<seriesInfo name='BCP' value='205'/>
<seriesInfo name='RFC' value='7942'/>
<seriesInfo name='DOI' value='10.17487/RFC7942'/>
</reference>




    </references>


<section anchor="acknowledgments" title="Acknowledgments">

<t>The authors would like to thank Mark Nottingham and Nikhil Kolekar for reviewing this specification.</t>

<t>The authors take all responsibility for errors and omissions.</t>

</section>


  </back>

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