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Versions: 00 01 02 03 04 05 06 07 08 09 10 11              Informational
Network Working Group                                           E. Wilde
Internet-Draft                                         November 29, 2018
Intended status: Informational
Expires: June 2, 2019

                         The Sunset HTTP Header


   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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on June 2, 2019.

Copyright Notice

   Copyright (c) 2018 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
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   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 . . . . . . . . . . . . . . .   4
   4.  Sunset and Caching  . . . . . . . . . . . . . . . . . . . . .   5
   5.  Sunset Scope  . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  The Sunset Link Relation Type . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  The Sunset Response Header  . . . . . . . . . . . . . . .   6
     7.2.  The Sunset Link Relation Type . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

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 link relation type "sunset" that
   allows to provide information about a resource's or a service's
   sunset policy, and/or upcoming sunsets, and/or 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 at humans, at machines, or a combination of 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 order represented by a resource may already list
   all the details of the order, but may only exist for a limited time
   unless it is confirmed and only then becomes permanent.  In such a
   case, the service managing the pending 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 also is 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.  If this is planned in advance, then
   for the time before the actual deprecation is rolled out, the
   resources that will be affected by the deprecation can make the date
   of their deprecation known.  This allows consumers of the API to be
   notified of the upcoming deprecation.

   In this scenario, the announced sunset date typically affects the
   whole API or parts of it (i.e., sets of resources), and not just a
   single resource.  In this case, it makes sense for the API to define
   rules 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 Section 5 discusses how
   the scope of the Sunset header field may change because of how a
   resource is using it.

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  The Sunset HTTP Response Header

   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 which
   they can use to control their usage of the resource.

   The Sunset header contains a single timestamp which 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 of [RFC7231], and SHOULD be a timestamp in the

   Timestamps in the past SHOULD be considered to mean the present time,
   meaning that the resource is expected to become unavailable at any

   Sunset = HTTP-date

   For example

   Sunset: Sat, 31 Dec 2018 23:59:59 GMT

   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 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 getting any notification about it beforehand.

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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 more efficiently used.
   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

5.  Sunset Scope

   The Sunset header field applies to the resource that returns it,
   meaning that it announces the upcoming sunset of that specific
   resources.  However, as discussed in Section Section 1.4, there may
   be scenarios where the scope of the announced Sunset information it
   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 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, and thus 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.

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6.  The Sunset Link Relation Type

   The Sunset HTTP header field indicates the upcoming retirement of a
   resource or a service.  In addition, 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 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.

      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 at
   humans, at machines, or a combination of 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

   The Sunset response header should be added to the permanent registry
   of message header fields (see [RFC3864]), taking into account the
   guidelines given by HTTP/1.1 [RFC7231].

      Header Field Name: Sunset

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      Applicable Protocol: Hypertext Transfer Protocol (HTTP)

      Status: Informational

      Author/Change controller: IETF

      Specification document(s): RFC XXXX

7.2.  The Sunset Link Relation Type

   The sunset link relation type should be added to the permanent
   registry of link relation types according to Section 4.2 of RFC 8288

      Relation Name: sunset

      Description: Identifies a resource that provides information about
      the context's retirement policy.

      Reference: RFC XXXX

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 resource expire
   after one year) should be treated in the same way as if this
   information would be made available directly in the resource's

   The Sunset header field should be treated as a resource hint, meaning
   that the resource is indicating its potential retirement.  The
   definitive test whether or not the resource in fact is available or
   not will be to attempt to interact with it.  Applications should
   never treat an advertised Sunset date as a definitive prediction that
   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

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   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
   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

   Assuming that a resource has been created in an archive that for
   management or compliance reasons stores resources for ten years, and
   permanently deletes them afterwards, then 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: Fri, 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 information 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 even appears for the first time (it may not
   appear fro 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.  If
   communicated as an HTTP header field, it might look as following:

   Link: <http://example.net/sunset>;rel="sunset";type="text/html"

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   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
   headers will be served as soon as the sunset date is 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", RFC 2119, March 1997.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

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

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,

10.2.  Informative References

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982, July

   [RFC7234]  Fielding, R., Nottingham, M., and J. Reschke, "Hypertext
              Transfer Protocol (HTTP/1.1): Caching", RFC 7234, June

Appendix A.  Acknowledgements

   Thanks for comments and suggestions provided by Phil Sturgeon and
   Asbjoern Ulsberg.

Author's Address

   Erik Wilde

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

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