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INFORMATIONAL

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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