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rfc:rfc8594

Internet Engineering Task Force (IETF) E. Wilde Request for Comments: 8594 May 2019 Category: Informational ISSN: 2070-1721

                    The Sunset HTTP Header Field

Abstract

 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.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 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).  Not all documents
 approved by the IESG are candidates for any level of Internet
 Standard; see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8594.

Copyright Notice

 Copyright (c) 2019 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
 (https://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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   1.1.  Temporary Resources . . . . . . . . . . . . . . . . . . .   3
   1.2.  Migration . . . . . . . . . . . . . . . . . . . . . . . .   3
   1.3.  Retention . . . . . . . . . . . . . . . . . . . . . . . .   3
   1.4.  Deprecation . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
 3.  The Sunset HTTP Response Header Field . . . . . . . . . . . .   4
 4.  Sunset and Caching  . . . . . . . . . . . . . . . . . . . . .   5
 5.  Sunset Scope  . . . . . . . . . . . . . . . . . . . . . . . .   6
 6.  The Sunset Link Relation Type . . . . . . . . . . . . . . . .   6
 7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.1.  The Sunset Response Header Field  . . . . . . . . . . . .   7
   7.2.  The Sunset Link Relation Type . . . . . . . . . . . . . .   8
 8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
 9.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
 10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
   10.2.  Informative References . . . . . . . . . . . . . . . . .  10
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  10
 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1. Introduction

 As a general rule, URIs should be stable and persistent so that
 applications can use them as stable and persistent identifiers for
 resources.  However, there are many scenarios where, for a variety of
 reasons, URIs have a limited lifetime.  In some of these scenarios,
 this limited lifetime is known in advance.  In this case, it can be
 useful for clients if resources make this information about their
 limited lifetime known.  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.
 This specification also defines a sunset link relation type that
 allows information to be provided about 1) the sunset policy of a
 resource or a service, and/or 2) upcoming sunsets, and/or 3) possible
 mitigation scenarios for resource/service users.  This specification
 does not place any constraints on the nature of the linked resource,
 which can be targeted to humans, machines, or both.
 Possible scenarios for known lifetimes of resources include, but are
 not limited to, the following scenarios.

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1.1. Temporary Resources

 Some resources may have a limited lifetime by definition.  For
 example, a pending shopping order represented by a resource may
 already list all order details, but it may only exist for a limited
 time unless it is confirmed and only then becomes an acknowledged
 shopping order.  In such a case, the service managing the pending
 shopping order can make this limited lifetime explicit, allowing
 clients to understand that the pending order, unless confirmed, will
 disappear at some point in time.

1.2. Migration

 If resources are changing identity because a service migrates them,
 then this may be known in advance.  While it may not yet be
 appropriate to use HTTP redirect status codes (3xx), it may be
 interesting for clients to learn about the service's plan to take
 down the original resource.

1.3. Retention

 There are many cases where regulation or legislation require that
 resources are kept available for a certain amount of time.  However,
 in many cases there is also a requirement for those resources to be
 permanently deleted after some period of time.  Since the deletion of
 the resource in this scenario is governed by well-defined rules, it
 could be made explicit for clients interacting with the resource.

1.4. Deprecation

 For Web APIs one standard scenario is that an API or specific subsets
 of an API may get deprecated.  Deprecation often happens in two
 stages: the first stage being that the API is not the preferred or
 recommended version anymore and the second stage being that the API
 or a specific version of the API gets decommissioned.
 For the first stage (the API is not the preferred or recommended
 version anymore), the Sunset header field is not appropriate: at this
 stage, the API remains operational and can still be used.  Other
 mechanisms can be used for signaling that first stage that might help
 with more visible deprecation management, but the Sunset header field
 does not aim to represent that information.
 For the second stage (the API or a specific version of the API gets
 decommissioned), the Sunset header field is appropriate: that is when
 the API or a version does become unresponsive.  From the Sunset
 header field's point of view, it does not matter that the API may not

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 have been the preferred or recommended version anymore.  The only
 thing that matters is that it will become unresponsive and that this
 time can be advertised using the Sunset header field.
 In this scenario, the announced sunset date typically affects all of
 the deprecated API or parts of it (i.e., just deprecated sets of
 resources), and not just a single resource.  In this case, it makes
 sense for the API to define rules about how an announced sunset on a
 specific resource (such as the API's home/start resource) implies the
 sunsetting of the whole API or parts of it (i.e., sets of resources),
 and not just the resource returning the sunset header field.
 Section 5 discusses how the scope of the Sunset header field may
 change because of how a resource is using it.

2. Terminology

 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 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

3. The Sunset HTTP Response Header Field

 The Sunset HTTP response header field allows a server to communicate
 the fact that a resource is expected to become unresponsive at a
 specific point in time.  It provides information for clients that
 they can use to control their usage of the resource.
 The Sunset header field contains a single timestamp that advertises
 the point in time when the resource is expected to become
 unresponsive.  The Sunset value is an HTTP-date timestamp, as defined
 in Section 7.1.1.1 of [RFC7231], and SHOULD be a timestamp in the
 future.
 It is safest to consider timestamps in the past mean the present
 time, meaning that the resource is expected to become unavailable at
 any time.
 Sunset = HTTP-date
 For example:
 Sunset: Sat, 31 Dec 2018 23:59:59 GMT

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 Clients SHOULD treat Sunset timestamps as hints: it is not guaranteed
 that the resource will, in fact, be available until that time and
 will not be available after that time.  However, since this
 information is provided by the resource itself, it does have some
 credibility.
 After the Sunset time has arrived, it is likely that interactions
 with the resource will result in client-side errors (HTTP 4xx status
 codes), redirect responses (HTTP 3xx status codes), or the client
 might not be able to interact with the resource at all.  The Sunset
 header field does not expose any information about which of those
 behaviors can be expected.
 Clients not interpreting an existing Sunset header field can operate
 as usual and simply may experience the resource becoming unavailable
 without recognizing any notification about it beforehand.

4. Sunset and Caching

 It should be noted that the Sunset HTTP response header field serves
 a different purpose than HTTP caching [RFC7234].  HTTP caching is
 concerned with making resource representations (i.e., represented
 resource state) reusable so that they can be used more efficiently.
 This is achieved by using header fields that allow clients and
 intermediaries to better understand when a resource representation
 can be reused or when resource state (and, thus, the representation)
 may have changed.
 The Sunset header field is not concerned with resource state at all.
 It only signals that a resource is expected to become unavailable at
 a specific point in time.  There are no assumptions about if, when,
 or how often a resource may change state in the meantime.
 For these reasons, the Sunset header field and HTTP caching should be
 seen as complementary and not as overlapping in scope and
 functionality.
 This also means that applications acting as intermediaries, such as
 search engines or archives that make resources discoverable, should
 treat Sunset information differently from caching information.  These
 applications may use Sunset information for signaling to users that a
 resource may become unavailable.  But they still have to account for
 the fact that resource state can change in the meantime and that
 Sunset information is a hint and, thus, future resource availability
 may differ from the advertised timestamp.

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5. Sunset Scope

 The Sunset header field applies to the resource that returns it,
 meaning that it announces the upcoming sunset of that specific
 resource.  However, as discussed in Section 1.4, there may be
 scenarios where the scope of the announced Sunset information is
 larger than just the single resource where it appears.
 Resources are free to define such an increased scope, and usually
 this scope will be documented by the resource so that consumers of
 the resource know about the increased scope and can behave
 accordingly.  However, it is important to take into account that such
 increased scoping is invisible for consumers who are unaware of the
 increased scoping rules.  This means that these consumers will not be
 aware of the increased scope, and they will not interpret Sunset
 information different from its standard meaning (i.e., it applies to
 the resource only).
 Using such an increased scope still may make sense, as Sunset
 information is only a hint anyway; thus, it is optional information
 that cannot be depended on, and clients should always be implemented
 in ways that allow them to function without Sunset information.
 Increased scope information may help clients to glean additional
 hints from resources (e.g., concluding that an API is being
 deprecated because its home/start resource announces a Sunset) and,
 thus, might allow them to implement behavior that allows them to make
 educated guesses about resources becoming unavailable.

6. The Sunset Link Relation Type

 The Sunset HTTP header field indicates the upcoming retirement of a
 resource or a service.  In addition, a resource may want to make
 information available that provides additional information about how
 retirement will be handled for resources or services.  This
 information can be broadly described by the following three topics:
 Sunset policy:  The policy for which resources and in which way
       sunsets may occur may be published as part of service's
       description.  Sunsets may only/mostly affect a subset of a
       service's resources, and they may be exposed according to a
       certain policy (e.g., one week in advance).
 Upcoming sunset:  There may be additional information about an
       upcoming sunset, which can be published as a resource that can
       be consumed by those looking for this additional information.

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 Sunset mitigation:  There may be information about possible
       mitigation/migration strategies, such as possible ways how
       resource users can switch to alternative resources/services.
 Any information regarding the above issues (and possibly additional
 ones) can be made available through a URI that then can be linked to
 using the sunset link relation type.  This specification places no
 constraints on the scope or the type of the linked resource.  The
 scope can be for a resource or for a service.  The type is determined
 by the media type of the linked resource and can be targeted to
 humans, machines, or both.
 If the linked resource does provide machine-readable information,
 consumers should be careful before acting on this information.  Such
 information may, for example, instruct consumers to use a migration
 rule so that sunset resources can be accessed at new URIs.  However,
 this kind of information amounts to a possibly large-scale identity
 migration of resources, so it is crucial that the migration
 information is authentic and accurate.

7. IANA Considerations

7.1. The Sunset Response Header Field

 The Sunset response header field has been added to the "Permanent
 Message Header Field Names" registry (see [RFC3864]), taking into
 account the guidelines given by HTTP/1.1 [RFC7231].
    Header Field Name: Sunset
    Protocol: http
    Status: informational
    Author/Change controller: IETF
    Reference: RFC 8594

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7.2. The Sunset Link Relation Type

 The sunset link relation type has been added to the permanent "Link
 Relation Types" registry according to Section 4.2 of [RFC8288]:
    Relation Name: sunset
    Description: Identifies a resource that provides information about
    the context's retirement policy.
    Reference: RFC 8594

8. Security Considerations

 Generally speaking, information about upcoming sunsets can leak
 information that otherwise might not be available.  For example, a
 resource representing a registration can leak information about the
 expiration date when it exposes sunset information.  For this reason,
 any use of sunset information where the sunset represents an
 expiration or allows the calculation of another date (such as
 calculating a creation date because it is known that resources expire
 after one year) should be treated in the same way as if this
 information would be made available directly in the resource's
 representation.
 The Sunset header field SHOULD be treated as a resource hint, meaning
 that the resource is indicating (and not guaranteeing with certainty)
 its potential retirement.  The definitive test whether or not the
 resource in fact is available will be to attempt to interact with it.
 Applications should never treat an advertised Sunset date as a
 definitive prediction of what is going to happen at the specified
 point in time: the Sunset indication may have been inserted by an
 intermediary or the advertised date may get changed or withdrawn by
 the resource owner.
 The main purpose of the Sunset header field is to signal intent so
 that applications using resources may get a warning ahead of time and
 can react accordingly.  What an appropriate reaction is (such as
 switching to a different resource or service), what it will be based
 on (such as machine-readable formats that allow the switching to be
 done automatically), and when it will happen (such as ahead of the
 advertised date or only when the resource in fact becomes
 unavailable) is outside the scope of this specification.
 In cases where a sunset policy is linked by using the sunset link
 relation type, clients SHOULD be careful about taking any actions
 based on this information.  It SHOULD be verified that the
 information is authentic and accurate.  Furthermore, it SHOULD be

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 tested that this information is only applied to resources that are
 within the scope of the policy, making sure that sunset policies
 cannot "hijack" resources by for example providing migration
 information for them.

9. Example

 If a resource has been created in an archive that, for management or
 compliance reasons, stores resources for ten years and permanently
 deletes them afterwards, the Sunset header field can be used to
 expose this information.  If such a resource has been created on
 November 11, 2016, then the following header field can be included in
 responses:
 Sunset: Wed, 11 Nov 2026 11:11:11 GMT
 This allows clients that are aware of the Sunset header field to
 understand that the resource likely will become unavailable at the
 specified point in time.  Clients can decide to ignore this
 information, adjust their own behavior accordingly, or alert
 applications or users about this timestamp.
 Even though the Sunset header field is made available by the resource
 itself, there is no guarantee that the resource indeed will become
 unavailable, and if so, how the response will look like for requests
 made after that timestamp.  In case of the archive used as an example
 here, the resource indeed may be permanently deleted, and requests
 for the URI after the Sunset timestamp may receive a "410 Gone" HTTP
 response.  (This is assuming that the archive keeps track of the URIs
 that it had previously assigned; if not, the response may be a more
 generic "404 Not Found".)
 Before the Sunset header field even appears for the first time (it
 may not appear from the very beginning), it is possible that the
 resource (or possibly just the "home" resource of the service
 context) communicates its sunset policy by using the sunset link
 relation type.  If communicated as an HTTP header field, it might
 look as follows:
 Link: <http://example.net/sunset>;rel="sunset";type="text/html"
 In this case, the linked resource provides sunset policy information
 about the service context.  It may be documentation aimed at
 developers, for example, informing them that the lifetime of a
 certain class of resources is ten years after creation and that
 Sunset header fields will be served as soon as the sunset date is

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 less than some given period of time.  It may also inform developers
 whether the service will respond with 410 or 404 after the sunset
 time, as discussed above.

10. References

10.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.
 [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
            Procedures for Message Header Fields", BCP 90, RFC 3864,
            DOI 10.17487/RFC3864, September 2004,
            <https://www.rfc-editor.org/info/rfc3864>.
 [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
            Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
            DOI 10.17487/RFC7231, June 2014,
            <https://www.rfc-editor.org/info/rfc7231>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.
 [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
            DOI 10.17487/RFC8288, October 2017,
            <https://www.rfc-editor.org/info/rfc8288>.

10.2. Informative References

 [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
            Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
            RFC 7234, DOI 10.17487/RFC7234, June 2014,
            <https://www.rfc-editor.org/info/rfc7234>.

Acknowledgements

 Thanks for comments and suggestions provided by Ben Campbell, Alissa
 Cooper, Benjamin Kaduk, Mirja Kuhlewind, Adam Roach, Phil Sturgeon,
 and Asbjorn Ulsberg.

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Author's Address

 Erik Wilde
 Email: erik.wilde@dret.net
 URI:   http://dret.net/netdret/

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