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Internet Engineering Task Force (IETF)                          E. WildeRequest for Comments: 8594                                      May 2019Category: InformationalISSN: 2070-1721The Sunset HTTP Header FieldAbstract   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; seeSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://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 toBCP 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.Wilde                         Informational                     [Page 1]

RFC 8594                      Sunset Header                     May 2019Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .21.1.  Temporary Resources . . . . . . . . . . . . . . . . . . .31.2.  Migration . . . . . . . . . . . . . . . . . . . . . . . .31.3.  Retention . . . . . . . . . . . . . . . . . . . . . . . .31.4.  Deprecation . . . . . . . . . . . . . . . . . . . . . . .32.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .43.  The Sunset HTTP Response Header Field . . . . . . . . . . . .44.  Sunset and Caching  . . . . . . . . . . . . . . . . . . . . .55.  Sunset Scope  . . . . . . . . . . . . . . . . . . . . . . . .66.  The Sunset Link Relation Type . . . . . . . . . . . . . . . .67.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .77.1.  The Sunset Response Header Field  . . . . . . . . . . . .77.2.  The Sunset Link Relation Type . . . . . . . . . . . . . .88.  Security Considerations . . . . . . . . . . . . . . . . . . .89.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .910. References  . . . . . . . . . . . . . . . . . . . . . . . . .1010.1.  Normative References . . . . . . . . . . . . . . . . . .1010.2.  Informative References . . . . . . . . . . . . . . . . .10   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .10   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .111.  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.Wilde                         Informational                     [Page 2]

RFC 8594                      Sunset Header                     May 20191.1.  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 notWilde                         Informational                     [Page 3]

RFC 8594                      Sunset Header                     May 2019   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 inBCP 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   inSection 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 GMTWilde                         Informational                     [Page 4]

RFC 8594                      Sunset Header                     May 2019   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.Wilde                         Informational                     [Page 5]

RFC 8594                      Sunset Header                     May 20195.  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 inSection 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.Wilde                         Informational                     [Page 6]

RFC 8594                      Sunset Header                     May 2019   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 Considerations7.1.  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 8594Wilde                         Informational                     [Page 7]

RFC 8594                      Sunset Header                     May 20197.2.  The Sunset Link Relation Type   The sunset link relation type has been added to the permanent "Link   Relation Types" registry according toSection 4.2 of [RFC8288]:      Relation Name: sunset      Description: Identifies a resource that provides information about      the context's retirement policy.      Reference:RFC 85948.  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 beWilde                         Informational                     [Page 8]

RFC 8594                      Sunset Header                     May 2019   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 isWilde                         Informational                     [Page 9]

RFC 8594                      Sunset Header                     May 2019   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.  References10.1.  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 inRFC2119 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.Wilde                         Informational                    [Page 10]

RFC 8594                      Sunset Header                     May 2019Author's Address   Erik Wilde   Email: erik.wilde@dret.net   URI:http://dret.net/netdret/Wilde                         Informational                    [Page 11]