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rfc:rfc7089

Independent Submission H. Van de Sompel Request for Comments: 7089 Los Alamos National Laboratory Category: Informational M. Nelson ISSN: 2070-1721 Old Dominion University

                                                          R. Sanderson
                                        Los Alamos National Laboratory
                                                         December 2013
 HTTP Framework for Time-Based Access to Resource States -- Memento

Abstract

 The HTTP-based Memento framework bridges the present and past Web.
 It facilitates obtaining representations of prior states of a given
 resource by introducing datetime negotiation and TimeMaps.  Datetime
 negotiation is a variation on content negotiation that leverages the
 given resource's URI and a user agent's preferred datetime.  TimeMaps
 are lists that enumerate URIs of resources that encapsulate prior
 states of the given resource.  The framework also facilitates
 recognizing a resource that encapsulates a frozen prior state of
 another resource.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This is a contribution to the RFC Series, independently of any other
 RFC stream.  The RFC Editor has chosen to publish this document at
 its discretion and makes no statement about its value for
 implementation or deployment.  Documents approved for publication by
 the RFC Editor are not a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7089.

Van de Sompel, et al. Informational [Page 1] RFC 7089 HTTP Memento December 2013

Copyright Notice

 Copyright (c) 2013 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
 (http://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.

Table of Contents

 1. Introduction ....................................................4
    1.1. Terminology ................................................4
    1.2. Notational Conventions .....................................4
    1.3. Purpose ....................................................5
 2. HTTP Headers, Link Relation Types ...............................7
    2.1. HTTP Headers ...............................................7
         2.1.1. Accept-Datetime and Memento-Datetime ................7
         2.1.2. Vary ................................................8
         2.1.3. Link ................................................8
    2.2. Link Relation Types ........................................9
         2.2.1. Link Relation Type "original" .......................9
         2.2.2. Link Relation Type "timegate" .......................9
         2.2.3. Link Relation Type "timemap" ........................9
         2.2.4. Link Relation Type "memento" .......................10
 3. Overview of the Memento Framework ..............................11
    3.1. Datetime Negotiation ......................................11
    3.2. TimeMaps ..................................................13
 4. Datetime Negotiation: HTTP Interactions ........................14
    4.1. Pattern 1 - The Original Resource Acts as Its Own
         TimeGate ..................................................15
         4.1.1. Pattern 1.1 - URI-R=URI-G; 302-Style
                Negotiation; Distinct URI-M  for Mementos ..........16
         4.1.2. Pattern 1.2 - URI-R=URI-G; 200-Style
                Negotiation; Distinct URI-M for Mementos ...........18
         4.1.3. Pattern 1.3 - URI-R=URI-G; 200-Style
                Negotiation; No Distinct URI-M for Mementos ........19
    4.2. Pattern 2 - A Remote Resource Acts as a TimeGate
         for the Original Resource .................................20
         4.2.1. Pattern 2.1 - URI-R<>URI-G; 302-Style
                Negotiation; Distinct URI-M for Mementos ...........22
         4.2.2. Pattern 2.2 - URI-R<>URI-G; 200-Style
                Negotiation; Distinct URI-M for Mementos ...........24
         4.2.3. Pattern 2.3 - URI-R<>URI-G; 200-Style
                Negotiation; No Distinct URI-M for Mementos ........25

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    4.3. Pattern 3 - The Original Resource is a Fixed Resource .....26
    4.4. Pattern 4 - Mementos without a TimeGate ...................27
    4.5. Special Cases .............................................29
         4.5.1. Original Resource Provides No "timegate" Link ......29
         4.5.2. Server Exists but Original Resource No
                Longer Does ........................................29
         4.5.3. Issues with Accept-Datetime ........................30
         4.5.4. Memento of a 3XX Response ..........................30
         4.5.5. Memento of Responses with 4XX or 5XX HTTP
                Status Codes .......................................32
         4.5.6. Sticky "Memento-Datetime" and "original"
                Link for Mementos ..................................33
         4.5.7. Intermediate Resources .............................34
         4.5.8. Resources Excluded from Datetime Negotiation .......35
 5. TimeMaps: Content and Serialization ............................36
    5.1. Special Cases .............................................38
         5.1.1. Index and Paging TimeMaps ..........................38
         5.1.2. Mementos for TimeMaps ..............................39
 6. IANA Considerations ............................................40
    6.1. HTTP Headers ..............................................40
    6.2. Link Relation Types .......................................40
 7. Security Considerations ........................................41
 8. Acknowledgements ...............................................42
 9. References .....................................................42
    9.1. Normative References ......................................42
    9.2. Informative References ....................................42
 Appendix A. Use of Headers and Relation Types per Pattern .........43

Van de Sompel, et al. Informational [Page 3] RFC 7089 HTTP Memento December 2013

1. Introduction

1.1. Terminology

 This specification uses the terms "resource", "request", "response",
 "entity-body", "content negotiation", "user agent", and "server" as
 described in [RFC2616], and it uses the terms "representation" and
 "resource state" as described in [W3C.REC-aww-20041215].
 In addition, the following terms specific to the Memento framework
 are introduced:
 o  Original Resource: An Original Resource is a resource that exists
    or used to exist, and for which access to one of its prior states
    may be required.
 o  Memento: A Memento for an Original Resource is a resource that
    encapsulates a prior state of the Original Resource.  A Memento
    for an Original Resource as it existed at time T is a resource
    that encapsulates the state the Original Resource had at time T.
 o  TimeGate: A TimeGate for an Original Resource is a resource that
    is capable of datetime negotiation to support access to prior
    states of the Original Resource.
 o  TimeMap: A TimeMap for an Original Resource is a resource from
    which a list of URIs of Mementos of the Original Resource is
    available.

1.2. Notational Conventions

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].
 When needed for extra clarity, the following conventions are used:
 o  URI-R is used to denote the URI of an Original Resource.
 o  URI-G is used to denote the URI of a TimeGate.
 o  URI-M is used to denote the URI of a Memento.
 o  URI-T is used to denote the URI of a TimeMap.

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1.3. Purpose

 The state of an Original Resource may change over time.
 Dereferencing its URI at any specific moment yields a response that
 reflects the resource's state at that moment: a representation of the
 resource's state (e.g., "200 OK" HTTP status code), an indication of
 its nonexistence (e.g., "404 Not Found" HTTP status code), a relation
 to another resource (e.g., "302 Found" HTTP status code), etc.
 However, responses may also exist that reflect prior states of an
 Original Resource: a representation of a prior state of the Original
 Resource, an indication that the Original Resource did not exist at
 some time in the past, a relation that the Original Resource had to
 another resource at some time in the past, etc.  Mementos that
 provide such responses exist in Web archives, content management
 systems, or revision control systems, among others.  For any given
 Original Resource several Mementos may exist, each one reflecting a
 frozen prior state of the Original Resource.
 Examples are:
 Mementos for Original Resource http://www.ietf.org/ are as follows:
 o  http://web.archive.org/web/19970107171109/http://www.ietf.org/
 o  http://webarchive.nationalarchives.gov.uk/20080906200044/http://
    www.ietf.org/
 Mementos for Original Resource
 http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol are as
 follows:
 o  http://en.wikipedia.org/w/
    index.php?title=Hypertext_Transfer_Protocol&oldid=366806574
 o  http://en.wikipedia.org/w/
    index.php?title=Hypertext_Transfer_Protocol&oldid=33912
 o  http://web.archive.org/web/20071011153017/http://en.wikipedia.org/
    wiki/Hypertext_Transfer_Protocol
 Mementos for Original Resource http://www.w3.org/TR/webarch/ are as
 follows:
 o  http://www.w3.org/TR/2004/PR-webarch-20041105/
 o  http://www.w3.org/TR/2002/WD-webarch-20020830/
 o  http://archive.is/20120527002537/http://www.w3.org/TR/webarch/

Van de Sompel, et al. Informational [Page 5] RFC 7089 HTTP Memento December 2013

 In the abstract, the Memento framework introduces a mechanism to
 access versions of Web resources that:
 o  Is fully distributed in the sense that resource versions may
    reside on multiple servers, and that any such server is likely
    only aware of the versions it holds;
 o  Uses the global notion of datetime as a resource version indicator
    and access key;
 o  Leverages the following primitives of [W3C.REC-aww-20041215]:
    resource, resource state, representation, content negotiation, and
    link.
 The core components of Memento's mechanism to access resource
 versions are:
 1.  The abstract notion of the state of an Original Resource (URI-R)
     as it existed at datetime T.  Note the relationship with the
     ability to identify the state of a resource at datetime T by
     means of a URI as intended by the proposed Dated URI scheme
     [DATED-URI].
 2.  A "bridge" from the present to the past, consisting of:
     o  The existence of a TimeGate (URI-G), which is aware of (at
        least part of the) version history of the Original Resource
        (URI-R);
     o  The ability to negotiate in the datetime dimension with that
        TimeGate (URI-G), as a means to access the state that the
        Original Resource (URI-R) had at datetime T.
 3.  A "bridge" from the past to the present, consisting of an
     appropriately typed link from a Memento (URI-M), which
     encapsulates the state the Original Resource (URI-R) had at
     datetime T, to the Original Resource (URI-R).
 4.  The existence of a TimeMap (URI-T) from which a list of all
     Mementos that encapsulate a prior state of the Original Resource
     (URI-R) can be obtained.
 This document is concerned with specifying an instantiation of these
 abstractions for resources that are identified by HTTP(S) URIs.

Van de Sompel, et al. Informational [Page 6] RFC 7089 HTTP Memento December 2013

2. HTTP Headers, Link Relation Types

 The Memento framework is concerned with HEAD and GET interactions
 with Original Resources, TimeGates, Mementos, and TimeMaps that are
 identified by HTTP or HTTPS URIs.  Details are only provided for
 resources identified by HTTP URIs but apply similarly to those with
 HTTPS URIs.

2.1. HTTP Headers

 The Memento framework operates at the level of HTTP request and
 response headers.  It introduces two new headers ("Accept-Datetime"
 and "Memento-Datetime") and introduces new values for two existing
 headers ("Vary" and "Link").  Other HTTP headers are present or
 absent in Memento response/request cycles as specified by [RFC2616].

2.1.1. Accept-Datetime and Memento-Datetime

 The "Accept-Datetime" request header is transmitted by a user agent
 to indicate it wants to access a past state of an Original Resource.
 To that end, the "Accept-Datetime" header is conveyed in an HTTP
 request issued against a TimeGate for an Original Resource, and its
 value indicates the datetime of the desired past state of the
 Original Resource.
 Example of an "Accept-Datetime" request header:
 Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT
 The "Memento-Datetime" response header is used by a server to
 indicate that a response reflects a prior state of an Original
 Resource.  Its value expresses the datetime of that state.  The URI
 of the Original Resource for which the response reflects a prior
 state is provided as the Target IRI of a link provided in the HTTP
 "Link" header that has a Relation Type of "original" (see
 Section 2.2).
 The presence of a "Memento-Datetime" header and associated value for
 a given response constitutes a promise that the resource state
 reflected in the response will no longer change (see Section 4.5.6).
 Example of a "Memento-Datetime" response header:
 Memento-Datetime: Wed, 30 May 2007 18:47:52 GMT
 Values for the "Accept-Datetime" and "Memento-Datetime" headers
 consist of a MANDATORY datetime expressed according to the [RFC1123]
 format, which is formalized by the rfc1123-date construction rule of

Van de Sompel, et al. Informational [Page 7] RFC 7089 HTTP Memento December 2013

 the BNF in Figure 1.  This BNF is derived from the HTTP-date
 construction of the BNF for Full Dates provided in [RFC2616].  The
 datetime is case sensitive with names for days and months exactly as
 shown in the wkday and month construction rules of the BNF,
 respectively.  The datetime MUST be represented in Greenwich Mean
 Time (GMT).
 accept-dt-value = rfc1123-date
 rfc1123-date = wkday "," SP date1 SP time SP "GMT"
 date1        = 2DIGIT SP month SP 4DIGIT
                   ; day month year (e.g., 20 Mar 1957)
 time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                   ; 00:00:00 - 23:59:59 (e.g., 14:33:22)
 wkday        = "Mon" | "Tue" | "Wed" | "Thu" | "Fri" | "Sat" |
                "Sun"
 month        = "Jan" | "Feb" | "Mar" | "Apr" | "May" | "Jun" |
                "Jul" | "Aug" | "Sep" | "Oct" | "Nov" | "Dec"
                 Figure 1: BNF for the Datetime Format

2.1.2. Vary

 Generally, the "Vary" header is used in HTTP responses to indicate
 the dimensions in which content negotiation is possible.  In the
 Memento framework, a TimeGate uses the "Vary" header with a value
 that includes "accept-datetime" to convey that datetime negotiation
 is possible.
 For example, this use of the "Vary" header indicates that datetime is
 the only dimension in which negotiation is possible:
 Vary: accept-datetime
 The use of the "Vary" header in this example shows that both datetime
 negotiation and media type content negotiation are possible:
 Vary: accept-datetime, accept

2.1.3. Link

 The Memento framework defines the "original", "timegate", "timemap",
 and "memento" Relation Types to convey typed links among Original
 Resources, TimeGates, Mementos, and TimeMaps.  They are defined in
 Section 2.2, below.  In addition, existing Relation Types may be
 used, for example, to support navigating among Mementos.  Examples
 are "first", "last", "prev", "next", "predecessor-version", and
 "successor-version" as detailed in [RFC5988] and [RFC5829].

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2.2. Link Relation Types

 This section introduces the Relation Types used in the Memento
 framework.  They are defined in a general way, and their use in HTTP
 "Link" headers [RFC5988] is described in detail.  The use of these
 Relation Types in TimeMaps is described in Section 5.

2.2.1. Link Relation Type "original"

 "original" -- A link with an "original" Relation Type is used to
 point from a TimeGate or a Memento to its associated Original
 Resource.
 Use in HTTP "Link" headers: Responses to HTTP HEAD/GET requests
 issued against a TimeGate or a Memento MUST include exactly one link
 with an "original" Relation Type in their HTTP "Link" header.

2.2.2. Link Relation Type "timegate"

 "timegate" -- A link with a "timegate" Relation Type is used to point
 from the Original Resource, as well as from a Memento associated with
 the Original Resource, to a TimeGate for the Original Resource.
 Use in HTTP "Link" headers: If there is a TimeGate associated with an
 Original Resource or Memento that is preferred for use, then
 responses to HTTP HEAD/GET requests issued against these latter
 resources MUST include a link with a "timegate" Relation Type in
 their HTTP "Link" header.  Since multiple TimeGates can exist for any
 Original Resource, multiple "timegate" links MAY occur, each with a
 distinct Target IRI.

2.2.3. Link Relation Type "timemap"

 "timemap" -- A link with a "timemap" Relation Type is used to point
 from a TimeGate or a Memento associated with an Original Resource, as
 well as from the Original Resource itself, to a TimeMap for the
 Original Resource.
 Attributes: A link with a "timemap" Relation Type SHOULD use the
 "type" attribute to convey the MIME type of the TimeMap
 serialization.  The "from" and "until" attributes may be used to
 express the start and end of the temporal interval covered by
 Mementos listed in the TimeMap.  That is, the linked TimeMap will not
 contain Mementos with archival datetimes outside of the expressed
 temporal interval.  Attempts SHOULD be made to convey this interval
 as accurately as possible.  The value for the these attributes MUST

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 be a datetime expressed according to the rfc1123-date construction
 rule of the BNF in Figure 1, and it MUST be represented in Greenwich
 Mean Time (GMT).
 Use in HTTP "Link" headers: If there is a TimeMap associated with an
 Original Resource, a TimeGate, or a Memento that is preferred for
 use, then responses to HTTP HEAD/GET requests issued against these
 latter resources MUST include a link with a "timemap" Relation Type
 in their HTTP "Link" header.  Multiple such links, each with a
 distinct Target IRI, MAY be expressed as a means to point to
 different TimeMaps or to different serializations of the same
 TimeMap.  In all cases, use of the "from" and "until" attributes is
 OPTIONAL.

2.2.4. Link Relation Type "memento"

 "memento" -- A link with a "memento" Relation Type is used to point
 from a TimeGate or a Memento for an Original Resource, as well as
 from the Original Resource itself, to a Memento for the Original
 Resource.
 Attributes: A link with a "memento" Relation Type MUST include a
 "datetime" attribute with a value that matches the "Memento-Datetime"
 of the Memento that is the target of the link; that is, the value of
 the "Memento-Datetime" header that is returned when the URI of the
 linked Memento is dereferenced.  The value for the "datetime"
 attribute MUST be a datetime expressed according to the rfc1123-date
 construction rule of the BNF in Figure 1, and it MUST be represented
 in Greenwich Mean Time (GMT).  This link MAY include a "license"
 attribute to associate a license with the Memento; the value for the
 "license" attribute MUST be a URI.
 Use in HTTP "Link" headers: Responses to HTTP HEAD/GET requests
 issued against an Original Resource, a TimeGate, and a Memento MAY
 include links in their HTTP "Link" headers with a "memento" Relation
 Type.  For responses in which a Memento is selected, the provision of
 navigational links that lead to Mementos other than the selected one
 can be beneficial to the user agent.  Of special importance are links
 that lead to the temporally first and last Memento known to the
 responding server, as well as links leading to Mementos that are
 temporally adjacent to the selected one.

Van de Sompel, et al. Informational [Page 10] RFC 7089 HTTP Memento December 2013

3. Overview of the Memento Framework

 The Memento framework defines two complementary approaches to support
 obtaining representations of prior states of an Original Resource:
 o  Datetime Negotiation: Datetime negotiation is a variation on
    content negotiation by which a user agent expresses a datetime
    preference pertaining to the representation of an Original
    Resource, instead of, for example, a media type preference.  Based
    on the responding server's knowledge of the past of the Original
    Resource, it selects a Memento of the Original Resource that best
    meets the user agent's datetime preference.  An overview is
    provided in Section 3.1; details are in Section 4.
 o  TimeMaps: A TimeMap is a resource from which a list can be
    obtained that provides a comprehensive overview of the past of an
    Original Resource.  A server makes a TimeMap available that
    enumerates all Mementos that the server is aware of, along with
    their archival datetime.  A user agent can obtain the TimeMap and
    select Mementos from it.  An overview is provided in Section 3.2;
    details are in Section 5.

3.1. Datetime Negotiation

 Figure 2 provides a schematic overview of a successful request/
 response chain that involves datetime negotiation.  Dashed lines
 depict HTTP transactions between user agent and server.  The
 interactions are for a scenario where the Original Resource resides
 on one server, whereas both its TimeGate and Mementos reside on
 another (Pattern 2.1 (Section 4.2.1) in Section 4).  Scenarios also
 exist in which all these resources are on the same server (for
 example, content management systems) or all are on different servers
 (for example, an aggregator of TimeGates).
 1: UA --- HTTP HEAD/GET; Accept-Datetime: T ----------------> URI-R
 2: UA <-- HTTP 200; Link: URI-G ----------------------------- URI-R
 3: UA --- HTTP HEAD/GET; Accept-Datetime: T ----------------> URI-G
 4: UA <-- HTTP 302; Location: URI-M; Vary; Link:
       URI-R,URI-T ------------------------------------------> URI-G
 5: UA --- HTTP GET URI-M; Accept-Datetime: T ---------------> URI-M
 6: UA <-- HTTP 200; Memento-Datetime: T; Link:
       URI-R,URI-T,URI-G ------------------------------------- URI-M
        Figure 2: A Datetime Negotiation Request/Response Chain

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 Step 1:  The user agent that wants to access a prior state of the
          Original Resource issues an HTTP HEAD/GET against URI-R that
          has an "Accept-Datetime" HTTP header with a value of the
          datetime of the desired state.
 Step 2:  The response from URI-R includes an HTTP "Link" header with
          a Relation Type of "timegate" pointing at a TimeGate (URI-G)
          for the Original Resource.
 Step 3:  The user agent starts the datetime negotiation process with
          the TimeGate by issuing an HTTP GET request against URI-G
          that has an "Accept-Datetime" HTTP header with a value of
          the datetime of the desired prior state of the Original
          Resource.
 Step 4:  The response from URI-G includes a "Location" header
          pointing at a Memento (URI-M) for the Original Resource.  In
          addition, the response contains an HTTP "Link" header with a
          Relation Type of "original" pointing at the Original
          Resource (URI-R), and an HTTP "Link" header with a Relation
          Type of "timemap" pointing at a TimeMap (URI-T).
 Step 5:  The user agent issues an HTTP GET request against URI-M.
 Step 6:  The response from URI-M includes a "Memento-Datetime" HTTP
          header with a value of the archival datetime of the Memento.
          It also contains an HTTP "Link" header with a Relation Type
          of "original" pointing at the Original Resource (URI-R),
          with a Relation Type of "timegate" pointing at a TimeGate
          (URI-G) for the Original Resource, and with a Relation Type
          of "timemap" pointing at a TimeMap (URI-T) for the Original
          Resource.  The state that is expressed by the response is
          the state the Original Resource had at the archival datetime
          expressed in the "Memento-Datetime" header.
 In order to respond to a datetime negotiation request, the server
 uses an internal algorithm to select the Memento that best meets the
 user agent's datetime preference.  The exact nature of the selection
 algorithm is at the server's discretion but is intended to be
 consistent, for example, always selecting the Memento that is nearest
 in time relative to the requested datetime, always selecting the
 Memento that is nearest in the past relative to the requested
 datetime, etc.
 Due to the sparseness of Mementos in most systems, the value of the
 "Memento-Datetime" header returned by a server may differ
 (significantly) from the value conveyed by the user agent in "Accept-
 Datetime".

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 Although a Memento encapsulates a prior state of an Original
 Resource, the entity-body returned in response to an HTTP GET request
 issued against a Memento may very well not be byte-to-byte the same
 as an entity-body that was previously returned by that Original
 Resource.  Various reasons exist why there are significant chances
 these would be different yet do convey substantially the same
 information.  These include format migrations as part of a digital
 preservation strategy, URI-rewriting as applied by some Web archives,
 and the addition of banners as a means to brand Web archives.
 When negotiating in the datetime dimension, the regular content
 negotiation dimensions (media type, character encoding, language, and
 compression) remain available.  It is the TimeGate server's
 responsibility to honor (or not) such content negotiation, and in
 doing so it MUST always first select a Memento that meets the user
 agent's datetime preference, and then consider honoring regular
 content negotiation for it.  As a result of this approach, the
 returned Memento will not necessarily meet the user agent's regular
 content negotiation preferences.  Therefore, it is RECOMMENDED that
 the server provides "memento" links in the HTTP "Link" header
 pointing at Mementos that do meet the user agent's regular content
 negotiation requests and that have a value for the "Memento-Datetime"
 header in the temporal vicinity of the user agent's preferred
 datetime value.
 A user agent that engages in datetime negotiation with a resource
 typically starts by issuing an HTTP HEAD, not GET, request with an
 "Accept-Datetime" header in order to determine how to proceed.  This
 strategy is related to the existence of various server implementation
 patterns as will become clear in Section 4.
 Details about the HTTP interactions involved in datetime negotiation
 are provided in Section 4.

3.2. TimeMaps

 Figure 3 provides a schematic overview of a successful request/
 response chain that shows a user agent obtaining a TimeMap.  The
 pictorial conventions are the same as the ones used in Figure 2, as
 is the scenario.  Note that, in addition to a TimeGate, an Original
 Resource and a Memento can also provide a link to a TimeMap.

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 1: UA --- HTTP HEAD/GET ------------------------------------> URI-R
 2: UA <-- HTTP 200; Link: URI-G ----------------------------- URI-R
 3: UA --- HTTP HEAD/GET ------------------------------------> URI-G
 4: UA <-- HTTP 302; Location: URI-M; Vary; Link:
       URI-R,URI-T ------------------------------------------> URI-G
 5: UA --- HTTP GET URI-T -----------------------------------> URI-T
 6: UA <-- HTTP 200 ------------------------------------------ URI-T
        Figure 3: A Request/Response Chain to Obtain a TimeMap
 Step 1:  The user agent that wants to access a TimeMap for the
          Original Resource issues an HTTP HEAD/GET against URI-R.
          This can be done with or without an "Accept-Datetime" HTTP
          header.
 Step 2:  Irrespective of the use of an "Accept-Datetime" HTTP header
          in Step 1, the response from URI-R includes an HTTP "Link"
          header with a Relation Type of "timegate" pointing at a
          TimeGate (URI-G) for the Original Resource.
 Step 3:  The user agent issues an HTTP GET request against URI-G.
          This can be done with or without an "Accept-Datetime" HTTP
          header.
 Step 4:  Irrespective of the use of an "Accept-Datetime" HTTP header
          in Step 1, the response contains an HTTP "Link" header with
          a Relation Type of "timemap" pointing at a TimeMap (URI-T).
 Step 5:  The user agent issues an HTTP GET request against URI-T.
 Step 6:  The response from URI-T has an entity-body that lists all
          Mementos for the Original Resource known to the responding
          server, as well as their archival datetimes.
 Details about the content and serialization of TimeMaps are provided
 in Section 5.

4. Datetime Negotiation: HTTP Interactions

 Figure 2 depicts a specific pattern to implement the Memento
 framework.  Multiple patterns exist, and they can be grouped as
 follows:
 o  Pattern 1 (Section 4.1) - The Original Resource acts as its own
    TimeGate
 o  Pattern 2 (Section 4.2) - A remote resource acts as a TimeGate for
    the Original Resource

Van de Sompel, et al. Informational [Page 14] RFC 7089 HTTP Memento December 2013

 o  Pattern 3 (Section 4.3) - The Original Resource is a Fixed
    Resource
 o  Pattern 4 (Section 4.4) - Mementos without a TimeGate
 Details of the HTTP interactions for common cases for each of those
 patterns are provided in Sections 4.1 through 4.4.  Appendix A
 summarizes the use of the "Vary", "Memento-Datetime", and "Link"
 headers in responses from Original Resources, TimeGates, and Mementos
 for the various patterns.  Special cases are described in
 Section 4.5.  Note that in the following sections, the HTTP status
 code of the responses with an entity-body is shown as "200 OK", but a
 series of "206 Partial Content" responses could be substituted.
 Figure 4 shows a user agent that attempts to datetime negotiate with
 the Original Resource http://a.example.org/ by including an "Accept-
 Datetime" header in its HTTP HEAD request.  This initiating request
 is the same for Pattern 1 (Section 4.1) through Pattern 3
 (Section 4.3).
 HEAD / HTTP/1.1
 Host: a.example.org
 Accept-Datetime: Tue, 20 Mar 2001 20:35:00 GMT
 Connection: close
          Figure 4: User Agent Attempts Datetime Negotiation
                        with Original Resource

4.1. Pattern 1 - The Original Resource Acts as Its Own TimeGate

 In this implementation pattern, the Original Resource acts as its own
 TimeGate, which means that URI-R and URI-G coincide.  Content
 management systems and revision control systems can support datetime
 negotiation in this way as they are commonly aware of the version
 history of their own resources.
 The response to this request when datetime negotiation for this
 resource is supported depends on the negotiation style it uses (200-
 style or 302-style) and on the existence or absence of a URI-M for
 Mementos that is distinct from the URI-R of the associated Original
 Resource.  The various cases are summarized in the below table, and
 the server responses for each are detailed in the remainder of this
 section.

Van de Sompel, et al. Informational [Page 15] RFC 7089 HTTP Memento December 2013

 +-------------------+------------+----------+---------+-------------+
 |      Pattern      |  Original  | TimeGate | Memento | Negotiation |
 |                   |  Resource  |          |         |    Style    |
 +-------------------+------------+----------+---------+-------------+
 |    Pattern 1.1    |    URI-R   |   URI-R  |  URI-M  |     302     |
 |  (Section 4.1.1)  |            |          |         |             |
 |    Pattern 1.2    |    URI-R   |   URI-R  |  URI-M  |     200     |
 |  (Section 4.1.2)  |            |          |         |             |
 |    Pattern 1.3    |    URI-R   |   URI-R  |  URI-R  |     200     |
 |  (Section 4.1.3)  |            |          |         |             |
 +-------------------+------------+----------+---------+-------------+
                          Table 1: Pattern 1

4.1.1. Pattern 1.1 - URI-R=URI-G; 302-Style Negotiation; Distinct URI-M

 In this case, the response to the user agent's request of Figure 4
 has a "302 Found" HTTP status code, and the "Location" header conveys
 the URI-M of the selected Memento.  The use of Memento response
 headers and links in the response from URI-R=URI-G is as follows:
 o  The "Vary" header MUST be provided, and it MUST include the
    "accept-datetime" value.
 o  The response MUST NOT contain a "Memento-Datetime" header.
 o  The "Link" header MUST be provided, and it MUST contain at least a
    link with the "original" Relation Type that has the URI-R of the
    Original Resource as Target IRI.  The provision of other links is
    encouraged and is subject to the considerations described in
    Section 2.2.
 The server's response to the request of Figure 4 is shown in
 Figure 5.  Note the inclusion of the recommended link to the TimeGate
 that, in this case, has a Target IRI that is the URI-R of the
 Original Resource.

Van de Sompel, et al. Informational [Page 16] RFC 7089 HTTP Memento December 2013

 HTTP/1.1 302 Found
 Date: Thu, 21 Jan 2010 00:06:50 GMT
 Server: Apache
 Vary: accept-datetime
 Location:
  http://a.example.org/?version=20010320133610
 Link: <http://a.example.org/>; rel="original timegate"
 Content-Length: 0
 Content-Type: text/plain; charset=UTF-8
 Connection: close
          Figure 5: Response from URI-R=URI-G for Pattern 1.1
 In a subsequent request, shown in Figure 6, the user agent can obtain
 the selected Memento by issuing an HTTP GET request against the URI-M
 that was provided in the "Location" header.  The inclusion of the
 "Accept-Datetime" header in this request is not needed but will
 typically occur as the user agent is in datetime negotiation mode.
 GET /?version=20010320133610 HTTP/1.1
 Host: a.example.org
 Accept-Datetime: Tue, 20 Mar 2001 20:35:00 GMT
 Connection: close
            Figure 6: User Agent Requests Selected Memento
 The response has a "200 OK" HTTP status code, and the entity-body of
 the response contains the representation of the selected Memento.
 The use of Memento response headers and links in the response from
 URI-M is as follows:
 o  A "Vary" header that includes an "accept-datetime" value MUST NOT
    be provided.
 o  The response MUST include a "Memento-Datetime" header.  Its value
    expresses the archival datetime of the Memento.
 o  The "Link" header MUST be provided, and it MUST contain at least a
    link with the "original" Relation Type that has the URI-R of the
    Original Resource as Target IRI.  The provision of other links is
    encouraged and is subject to the considerations described in
    Section 2.2.
 The server's response to the request of Figure 6 is shown in
 Figure 7.  Note the provision of the required "original", and the
 recommended "timegate" and "timemap" links.  The former two point to

Van de Sompel, et al. Informational [Page 17] RFC 7089 HTTP Memento December 2013

 the Original Resource, which acts as its own TimeGate.  The latter
 has "from" and "until" attributes to indicate the temporal interval
 covered by Mementos listed in the linked TimeMap.
 HTTP/1.1 200 OK
 Date: Thu, 21 Jan 2010 00:06:51 GMT
 Server: Apache-Coyote/1.1
 Memento-Datetime: Tue, 20 Mar 2001 13:36:10 GMT
 Link: <http://a.example.org/>; rel="original timegate",
  <http://a.example.org/?version=all&style=timemap>
    ; rel="timemap"; type="application/link-format"
    ; from="Tue, 15 Sep 2000 11:28:26 GMT"
    ; until="Wed, 20 Jan 2010 09:34:33 GMT"
 Content-Length: 23364
 Content-Type: text/html;charset=utf-8
 Connection: close
             Figure 7: Response from URI-M for Pattern 1.1

4.1.2. Pattern 1.2 - URI-R=URI-G; 200-Style Negotiation; Distinct URI-M

 In this case, the response to the user agent's request of Figure 4
 has a "200 OK" HTTP status code, and the "Content-Location" header
 conveys the URI-M of the selected Memento.  The use of Memento
 response headers and links in the response from URI-R=URI-G is as
 follows:
 o  The "Vary" header MUST be provided, and it MUST include the
    "accept-datetime" value.
 o  The response MUST include a "Memento-Datetime" header.  Its value
    expresses the archival datetime of the selected Memento.
 o  The "Link" header MUST be provided, and it MUST contain at least a
    link with the "original" Relation Type that has the URI-R of the
    Original Resource as Target IRI.  The provision of other links is
    encouraged and is subject to the considerations described in
    Section 2.2.
 The server's response to the request of Figure 4 is shown in
 Figure 8.  Note the provision of optional "memento" links pointing at
 the oldest and most recent Memento for the Original Resource known to
 the responding server.

Van de Sompel, et al. Informational [Page 18] RFC 7089 HTTP Memento December 2013

 HTTP/1.1 200 OK
 Date: Thu, 21 Jan 2010 00:06:50 GMT
 Server: Apache
 Vary: accept-datetime
 Content-Location:
  http://a.example.org/?version=20010320133610
 Memento-Datetime: Tue, 20 Mar 2001 13:36:10 GMT
 Link: <http://a.example.org/>; rel="original timegate",
  <http://a.example.org/?version=20000915112826>
  ; rel="memento first"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
  <http://a.example.org/?version=20100120093433>
  ; rel="memento last"; datetime="Wed, 20 Jan 2010 09:34:33 GMT",
  <http://a.example.org/?version=all&style=timemap>
  ; rel="timemap"; type="application/link-format"
 Content-Length: 23364
 Content-Type: text/html;charset=utf-8
 Connection: close
          Figure 8: Response from URI-R=URI-G for Pattern 1.2
 In a subsequent request, which is the same as Figure 4 but with HTTP
 GET instead of HEAD, the user agent can obtain the representation of
 the selected Memento.  It will be provided as the entity-body of a
 response that has the same Memento headers as in Figure 8.

4.1.3. Pattern 1.3 - URI-R=URI-G; 200-Style Negotiation; No Distinct

      URI-M
 In this case, the response to the user agent's request of Figure 4
 has a "200 OK" HTTP status code, and it does not contain a "Content-
 Location" nor a "Location" header as there is no URI-M of the
 selected Memento to convey.  The use of Memento response headers and
 links in the response from URI-R=URI-G is as follows:
 o  The "Vary" header MUST be provided, and it MUST include the
    "accept-datetime" value.
 o  The response MUST include a "Memento-Datetime" header.  Its value
    expresses the archival datetime of the selected Memento.
 o  The "Link" header MUST be provided, and it MUST contain at least a
    link with the "original" Relation Type that has the URI-R of the
    Original Resource as Target IRI.  The provision of other links is
    encouraged and is subject to the considerations described in
    Section 2.2.

Van de Sompel, et al. Informational [Page 19] RFC 7089 HTTP Memento December 2013

 The server's response to the request of Figure 4 is shown in
 Figure 9.  The recommended "timemap" and "timegate" links are
 included in addition to the mandatory "original" link.
 HTTP/1.1 200 OK
 Date: Thu, 21 Jan 2010 00:06:50 GMT
 Server: Apache
 Vary: accept-datetime
 Memento-Datetime: Tue, 20 Mar 2001 13:36:10 GMT
 Link: <http://a.example.org/>; rel="original timegate",
  <http://a.example.org/?version=all&style=timemap>
    ; rel="timemap"; type="application/link-format"
 Content-Length: 23364
 Content-Type: text/html;charset=utf-8
 Connection: close
          Figure 9: Response from URI-R=URI-G for Pattern 1.3
 In a subsequent request, which is the same as Figure 4 but with HTTP
 GET instead of HEAD, the user agent can obtain the representation of
 the selected Memento.  It will be provided as the entity-body of a
 response that has the same Memento headers as in Figure 9.

4.2. Pattern 2 - A Remote Resource Acts as a TimeGate for the Original

    Resource
 In this implementation pattern, the Original Resource does not act as
 its own TimeGate, which means that URI-R and URI-G are different.
 This pattern is typically implemented by servers for which the
 history of their resources is recorded in remote systems such as Web
 archives and transactional archives [Fitch].  But servers that
 maintain their own history, such as content management systems and
 version control systems, may also implement this pattern, for
 example, to distribute the load involved in responding to requests
 for current and prior representations of resources between different
 servers.
 This pattern is summarized in the below table and is detailed in the
 remainder of this section.  Three cases exist that differ regarding
 the negotiation style that is used by the remote TimeGate and
 regarding the existence of a URI-M for Mementos that is distinct from
 the URI-G of the TimeGate.

Van de Sompel, et al. Informational [Page 20] RFC 7089 HTTP Memento December 2013

 +-------------------+------------+----------+---------+-------------+
 |      Pattern      |  Original  | TimeGate | Memento | Negotiation |
 |                   |  Resource  |          |         |    Style    |
 +-------------------+------------+----------+---------+-------------+
 |    Pattern 2.1    |    URI-R   |   URI-G  |  URI-M  |     302     |
 |  (Section 4.2.1)  |            |          |         |             |
 |    Pattern 2.2    |    URI-R   |   URI-G  |  URI-M  |     200     |
 |  (Section 4.2.2)  |            |          |         |             |
 |    Pattern 2.3    |    URI-R   |   URI-G  |  URI-G  |     200     |
 |  (Section 4.2.3)  |            |          |         |             |
 +-------------------+------------+----------+---------+-------------+
                          Table 2: Pattern 2
 The response by the Original Resource to the request shown in
 Figure 4 is the same for all three cases.  The use of headers and
 links in the response from URI-R is as follows:
 o  A "Vary" header that includes an "accept-datetime" value MUST NOT
    be provided.
 o  The response MUST NOT contain a "Memento-Datetime" header.
 o  The "Link" header SHOULD be provided.  It MUST NOT include a link
    with an "original" Relation Type.  If a preferred TimeGate is
    associated with the Original Resource, then it MUST include a link
    with a "timegate" Relation Type that has the URI-G of the TimeGate
    as Target IRI.  If a preferred TimeMap is associated with the
    Original Resource, then it SHOULD include a link with a "timemap"
    Relation Type that has the URI-T of the TimeGate as Target IRI.
    Multiple "timegate" and "timemap" links can be provided to
    accommodate situations in which the server is aware of multiple
    TimeGates or TimeMaps for the Original Resource.
 Figure 10 shows such a response.  Note the absence of an "original"
 link as the responding resource is neither a TimeGate nor a Memento.
 HTTP/1.1 200 OK
 Date: Thu, 21 Jan 2010 00:02:12 GMT
 Server: Apache
 Link: <http://arxiv.example.net/timegate/http://a.example.org/>
    ; rel="timegate"
 Content-Length: 255
 Connection: close
 Content-Type: text/html; charset=iso-8859-1
          Figure 10: Response from URI-R<>URI-G for Pattern 2

Van de Sompel, et al. Informational [Page 21] RFC 7089 HTTP Memento December 2013

 Once a user agent has obtained the URI-G of a remote TimeGate for the
 Original Resource, it can engage in datetime negotiation with that
 TimeGate.  Figure 11 shows the request issued against the TimeGate,
 whereas Sections 4.2.1 through 4.2.3 detail the responses for various
 TimeGate implementation patterns.
 HEAD /timegate/http://a.example.org/ HTTP/1.1
 Host: arxiv.example.net
 Accept-Datetime: Tue, 20 Mar 2001 20:35:00 GMT
 Connection: close
         Figure 11: User Agent Engages in Datetime Negotiation
                         with Remote TimeGate

4.2.1. Pattern 2.1 - URI-R<>URI-G; 302-Style Negotiation; Distinct

      URI-M
 In case the TimeGate uses a 302 negotiation style, the response to
 the user agent's request of Figure 11 has a "302 Found" HTTP status
 code, and the "Location" header conveys the URI-M of the selected
 Memento.  The use of Memento response headers and links in the
 response from URI-G is as follows:
 o  The "Vary" header MUST be provided, and it MUST include the
    "accept-datetime" value.
 o  The response MUST NOT contain a "Memento-Datetime" header.
 o  The "Link" header MUST be provided, and it MUST contain at least a
    link with the "original" Relation Type that has the URI-R of the
    Original Resource as Target IRI.  The provision of other links is
    encouraged and is subject to the considerations described in
    Section 2.2.
 The server's response to the request of Figure 11 is shown in
 Figure 12.  It contains the mandatory "original" link that points
 back to the Original Resource associated with this TimeGate, and it
 shows the recommended "timemap" link that includes "from" and "until"
 attributes.

Van de Sompel, et al. Informational [Page 22] RFC 7089 HTTP Memento December 2013

 HTTP/1.1 302 Found
 Date: Thu, 21 Jan 2010 00:02:14 GMT
 Server: Apache
 Vary: accept-datetime
 Location:
  http://arxiv.example.net/web/20010321203610/http://a.example.org/
 Link: <http://a.example.org/>; rel="original",
  <http://arxiv.example.net/timemap/http://a.example.org/>
    ; rel="timemap"; type="application/link-format"
    ; from="Tue, 15 Sep 2000 11:28:26 GMT"
    ; until="Wed, 20 Jan 2010 09:34:33 GMT"
 Content-Length: 0
 Content-Type: text/plain; charset=UTF-8
 Connection: close
         Figure 12: Response from URI-G<>URI-R for Pattern 2.1
 In a subsequent HTTP GET request, shown in Figure 13, the user agent
 can obtain the selected Memento by issuing an HTTP GET request
 against the URI-M that was provided in the "Location" header.  The
 inclusion of the "Accept-Datetime" header in this request is not
 needed but will typically occur as the user agent is in datetime
 negotiation mode.
 GET /web/20010321203610/http://a.example.org/ HTTP/1.1
 Host: arxiv.example.net/
 Accept-Datetime: Tue, 20 Mar 2001 20:35:00 GMT
 Connection: close
            Figure 13: User Agent Requests Selected Memento
 The response has a "200 OK" HTTP status code.  The use of Memento
 response headers and links in the response from URI-M is as follows:
 o  A "Vary" header that includes an "accept-datetime" value MUST NOT
    be provided.
 o  The response MUST include a "Memento-Datetime" header.  Its value
    expresses the archival datetime of the Memento.
 o  The "Link" header MUST be provided, and it MUST contain at least a
    link with the "original" Relation Type that has the URI-R of the
    Original Resource as Target IRI.  The provision of other links is
    encouraged and is subject to the considerations described in
    Section 2.2.

Van de Sompel, et al. Informational [Page 23] RFC 7089 HTTP Memento December 2013

 The server's response to the request of Figure 13 is shown in
 Figure 14.  Note the provision of the recommended "timegate" and
 "timemap" links.
 HTTP/1.1 200 OK
 Date: Thu, 21 Jan 2010 00:02:15 GMT
 Server: Apache-Coyote/1.1
 Memento-Datetime: Wed, 21 Mar 2001 20:36:10 GMT
 Link: <http://a.example.org/>; rel="original",
  <http://arxiv.example.net/timemap/http://a.example.org/>
    ; rel="timemap"; type="application/link-format",
  <http://arxiv.example.net/timegate/http://a.example.org/>
    ; rel="timegate"
 Content-Length: 25532
 Content-Type: text/html;charset=utf-8
 Connection: close
            Figure 14: Response from URI-M for Pattern 2.1

4.2.2. Pattern 2.2 - URI-R<>URI-G; 200-Style Negotiation; Distinct

      URI-M
 In case the TimeGate uses a 200 negotiation style, and each Memento
 has a distinct URI-M, the response to the user agent's request of
 Figure 11 has a "200 OK" HTTP status code, and the "Content-Location"
 header conveys the URI-M of the selected Memento.  The use of Memento
 response headers and links in the response from URI-G is as follows:
 o  The "Vary" header MUST be provided, and it MUST include the
    "accept-datetime" value.
 o  The response MUST include a "Memento-Datetime" header.  Its value
    expresses the archival datetime of the Memento.
 o  The "Link" header MUST be provided, and it MUST contain at least a
    link with the "original" Relation Type that has the URI-R of the
    Original Resource as Target IRI.  The provision of other links is
    encouraged and is subject to the considerations described in
    Section 2.2.
 The server's response to the request of Figure 11 is shown in
 Figure 15.

Van de Sompel, et al. Informational [Page 24] RFC 7089 HTTP Memento December 2013

 HTTP/1.1 200 OK
 Date: Thu, 21 Jan 2010 00:09:40 GMT
 Server: Apache-Coyote/1.1
 Vary: accept-datetime
 Content-Location:
  http://arxiv.example.net/web/20010321203610/http://a.example.org/
 Memento-Datetime: Wed, 21 Mar 2001 20:36:10 GMT
 Link: <http://a.example.org/>; rel="original",
  <http://arxiv.example.net/timemap/http://a.example.org/>
    ; rel="timemap"; type="application/link-format",
  <http://arxiv.example.net/timegate/http://a.example.org/>
    ; rel="timegate"
 Content-Length: 25532
 Content-Type: text/html;charset=utf-8
 Connection: close
         Figure 15: Response from URI-G<>URI-R for Pattern 2.2
 In a subsequent request, which is the same as Figure 11 but with HTTP
 GET instead of HEAD, the user agent can obtain the representation of
 the selected Memento.  It will be provided as the entity-body of a
 response that has the same Memento headers as Figure 15.

4.2.3. Pattern 2.3 - URI-R<>URI-G; 200-Style Negotiation; No Distinct

      URI-M
 In case the TimeGate uses a 200 negotiation style, but Mementos have
 no distinct URIs, the response to the user agent's request of
 Figure 11 has a "200 OK" HTTP status code, and it does not contain a
 "Content-Location" nor "Location" header as there is no URI-M of the
 selected Memento to convey.  The use of Memento response headers and
 links in the response from URI-G is as follows:
 o  The "Vary" header MUST be provided, and it MUST include the
    "accept-datetime" value.
 o  The response MUST include a "Memento-Datetime" header.  Its value
    expresses the archival datetime of the Memento.
 o  The "Link" header MUST be provided, and it MUST contain at least a
    link with the "original" Relation Type that has the URI-R of the
    Original Resource as Target IRI.  The provision of other links is
    encouraged and is subject to the considerations described in
    Section 2.2.

Van de Sompel, et al. Informational [Page 25] RFC 7089 HTTP Memento December 2013

 The server's response to the request of Figure 11 is shown in
 Figure 16.
 HTTP/1.1 200 OK
 Date: Thu, 21 Jan 2010 00:09:40 GMT
 Server: Apache-Coyote/1.1
 Vary: accept-datetime
 Memento-Datetime: Wed, 21 Mar 2001 20:36:10 GMT
 Link: <http://a.example.org/>; rel="original",
  <http://arxiv.example.net/timemap/http://a.example.org/>
    ; rel="timemap"; type="application/link-format",
  <http://arxiv.example.net/timegate/http://a.example.org/>
    ; rel="timegate"
 Content-Length: 25532
 Content-Type: text/html;charset=utf-8
 Connection: close
         Figure 16: Response from URI-G<>URI-R for Pattern 2.3
 In a subsequent request, which is the same as Figure 11 but with HTTP
 GET instead of HEAD, the user agent can obtain the representation of
 the selected Memento.  It will be provided as the entity-body of a
 response that has the same Memento headers as Figure 16.

4.3. Pattern 3 - The Original Resource is a Fixed Resource

 This pattern does not involve datetime negotiation with a TimeGate,
 but it can be implemented for Original Resources that never change
 state or do not change anymore past a certain point in their
 existence, meaning that URI-R and URI-M coincide either from the
 outset or starting at some point in time.  This pattern is summarized
 in the below table.  Examples are tweets or stable media resources on
 news sites.
 +----------+----------------+----------+---------+------------------+
 |  Pattern |    Original    | TimeGate | Memento |    Negotiation   |
 |          |    Resource    |          |         |       Style      |
 +----------+----------------+----------+---------+------------------+
 |  Pattern |      URI-R     |     -    |  URI-R  |         -        |
 |     3    |                |          |         |                  |
 +----------+----------------+----------+---------+------------------+
                          Table 3: Pattern 3

Van de Sompel, et al. Informational [Page 26] RFC 7089 HTTP Memento December 2013

 Servers that host such resources can support the Memento framework by
 treating the stable resource (FixedResource as per
 [W3C.gen-ont-20090420]) as a Memento.  The use of Memento response
 headers and links in responses from such a stable resource is as
 follows:
 o  A "Vary" header that includes an "accept-datetime" value MUST NOT
    be provided.
 o  The response MUST include a "Memento-Datetime" header.  Its value
    expresses the datetime at which the resource became stable.
    Providing this value includes a promise that the resource has not
    changed since this datetime and will not change anymore beyond it.
 o  The "Link" header MUST be provided and MUST have a link with the
    "original" Relation Type that has the URI-R of the stable resource
    itself as Target IRI.
 Figure 17 shows a response to an HTTP HEAD request for the resource
 with URI-R http://a.example.org/ that has been stable since March 20,
 2009.
 HTTP/1.1 200 OK
 Date: Thu, 21 Jan 2010 00:09:40 GMT
 Server: Apache-Coyote/1.1
 Memento-Datetime: Fri, 20 Mar 2009 11:00:00 GMT
 Link: <http://a.example.org/>; rel="original"
 Content-Length: 875
 Content-Type: text/html;charset=utf-8
 Connection: close
          Figure 17: Response from URI-R=URI-M for Pattern 3

4.4. Pattern 4 - Mementos without a TimeGate

 Cases may occur in which a server hosts Mementos but does not expose
 a TimeGate for them.  This can, for example, be the case if the
 server's Mementos result from taking a snapshot of the state of a set
 of Original Resources from another server as it is being retired.  As
 a result, only a single Memento per Original Resource is hosted,
 making the introduction of a TimeGate unnecessary.  But it may also
 be the case for servers that host multiple Mementos for an Original
 Resource but consider exposing TimeGates too expensive.  In this
 case, URI-R and URI-M are distinct, but a TimeGate is absent.  This
 case is summarized in the below table.

Van de Sompel, et al. Informational [Page 27] RFC 7089 HTTP Memento December 2013

 +----------+----------------+----------+---------+------------------+
 |  Pattern |    Original    | TimeGate | Memento |    Negotiation   |
 |          |    Resource    |          |         |       Style      |
 +----------+----------------+----------+---------+------------------+
 |  Pattern |      URI-R     |     -    |  URI-M  |         -        |
 |     4    |                |          |         |                  |
 +----------+----------------+----------+---------+------------------+
                          Table 4: Pattern 4
 Servers that host such Mementos without TimeGates can still support
 the Memento framework by providing the appropriate Memento headers
 and links.  Their use is as follows for a response from URI-M:
 o  A "Vary" header that includes an "accept-datetime" value MUST NOT
    be provided.
 o  The response MUST include a "Memento-Datetime" header.  Its value
    expresses the archival datetime of the Memento.
 o  The "Link" header MUST be provided, and it MUST have a link with
    the "original" Relation Type that has the URI-R of the associated
    Original Resource as Target IRI.  The provision of other links is
    encouraged and is subject to the considerations described in
    Section 2.2.
 Figure 18 shows a response to an HTTP HEAD request for the Memento
 with URI-M
 http://arxiv.example.net/web/20010321203610/http://a.example.org/.
 Note the use of links: three links have the URI-M of the Memento as
 Target IRI and have respective Relation Types "memento", "first", and
 "last".  This combination indicates that this is the only Memento for
 the Original Resource with Target IRI provided by the "original" link
 (http://a.example.org/) of which the server is aware.  Note also that
 such a response does not imply that there is no server whatsoever
 that exposes a TimeGate; it merely means that the responding server
 neither provides nor is aware of the location of a TimeGate.

Van de Sompel, et al. Informational [Page 28] RFC 7089 HTTP Memento December 2013

 HTTP/1.1 200 OK
 Date: Thu, 21 Jan 2010 00:09:40 GMT
 Server: Apache-Coyote/1.1
 Memento-Datetime: Wed, 21 Mar 2001 20:36:10 GMT
 Link: <http://a.example.org/>; rel="original",
  <http://arxiv.example.net/web/20010321203610/http://a.example.org/>
    ; rel="first last memento"
    ; datetime="Wed, 21 Mar 2001 20:36:10 GMT"
 Content-Length: 25532
 Content-Type: text/html;charset=utf-8
 Connection: close
          Figure 18: Response from URI-M<>URI-R for Pattern 4

4.5. Special Cases

4.5.1. Original Resource Provides No "timegate" Link

 Cases exist in which the response from the Original Resource does not
 contain a "timegate" link, including:
 o  The Original Resource's server does not support the Memento
    framework;
 o  The Original Resource no longer exists, and the responding server
    is not aware of its prior existence;
 o  The server that hosted the Original Resource no longer exists.
 In all these cases, the user agent should attempt to determine an
 appropriate TimeGate for the Original Resource, either automatically
 or interactively supported by the user.

4.5.2. Server Exists but Original Resource No Longer Does

 Cases exist in which the server knows that an Original Resource used
 to exist, but no longer provides a current representation.  If there
 is a preferred TimeGate for such a discontinued Original Resource,
 then the server MUST include a "timegate" link in responses to
 requests for it.  This may allow access to Mementos for the Original
 Resource even if it no longer exists.  A server's response to a
 request for the discontinued resource http://a.example.org/pic is
 illustrated in Figure 19.

Van de Sompel, et al. Informational [Page 29] RFC 7089 HTTP Memento December 2013

 HTTP/1.1 404 Not Found
 Date: Thu, 21 Jan 2010 00:02:12 GMT
 Server: Apache
 Link:
  <http://arxiv.example.net/timegate/http://a.example.org/pic>
   ; rel="timegate"
 Content-Length: 255
 Content-Type: text/html; charset=iso-8909-1
 Connection: close
  Figure 19: Response from an Original Resource That No Longer Exists

4.5.3. Issues with Accept-Datetime

 The following special cases may occur regarding the "Accept-Datetime"
 header when a user agent issues a request against a TimeGate:
 o  If the value of the "Accept-Datetime" is either earlier than the
    datetime of the first Memento or later than the datetime of the
    most recent Memento known to the TimeGate, the first or most
    recent Memento MUST be selected, respectively.
 o  If the value of the "Accept-Datetime" does not conform to the
    rfc1123-date construction rule of the BNF in Figure 1, the
    response MUST have a "400 Bad Request" HTTP status code.
 o  If a user agent issues a request against a TimeGate and fails to
    include an "Accept-Datetime" request header, the most recent
    Memento SHOULD be selected.
 In all cases, the use of headers and links in responses is as
 described for TimeGates in the respective scenarios.

4.5.4. Memento of a 3XX Response

 Cases exist in which HTTP responses with 3XX status codes are
 archived.  For example, crawl-based Web archives commonly archive
 responses with HTTP status codes "301 Moved Permanently" and "302
 Found", whereas Linked Data archives hold on to "303 See Other"
 responses.
 If the Memento requested by the user agent is an archived version of
 an HTTP response with a 3XX status code, the server's response MUST
 have the same 3XX HTTP status code.  The use of other Memento headers
 is as described for Mementos in the respective scenarios.

Van de Sompel, et al. Informational [Page 30] RFC 7089 HTTP Memento December 2013

 The user agent's handling of an HTTP response with a 3XX status code
 is not affected by the presence of a "Memento-Datetime" header.  The
 user agent MUST behave in the same manner as it does with HTTP
 responses with a 3XX status code that do not have a "Memento-
 Datetime" header.
 However, the user agent MUST be aware that the URI that was selected
 from the "Location" header of an HTTP response with a 3XX status code
 might not be that of a Memento but rather of an Original Resource.
 In the latter case, it SHOULD proceed by looking for a Memento of the
 selected Original Resource.
 For example, Figure 20 shows the response to an HTTP GET request for
 http://a.example.org issued on April 11, 2008.  This response is
 archived as a Memento of http://a.example.org that has as URI-M
 http://arxiv.example.net/web/20080411000650/http://a.example.org.
 The response to an HTTP GET on this URI-M is shown in Figure 21.  It
 is a replay of the original response with "Memento-Datetime" and
 "Link" headers added, to allow a user agent to understand the
 response is a Memento.  In Figure 21, the value of the "Location"
 header is the same as in the original response; it identifies an
 Original Resource.  The user agent proceeds with finding a Memento
 for this Original Resource.  Web archives sometimes overwrite the
 value that was originally provided in the "Location" header in order
 to point at a Memento they hold of the resource to which the redirect
 originally led.  This is shown in Figure 22.  In this case, the user
 agent may decide it found an appropriate Memento.
 HTTP/1.1 301 Moved Permanently
 Date: Fri, 11 Apr 2008 00:06:50 GMT
 Server: Apache
 Location: http://b.example.org
 Content-Length: 0
 Content-Type: text/plain; charset=UTF-8
 Connection: close
                   Figure 20: Response Is a Redirect

Van de Sompel, et al. Informational [Page 31] RFC 7089 HTTP Memento December 2013

 HTTP/1.1 301 Moved Permanently
 Date: Thu, 21 Jan 2010 00:09:40 GMT
 Server: Apache-Coyote/1.1
 Memento-Datetime: Fri, 11 Apr 2008 00:06:50 GMT
 Location: http://b.example.org
 Link: <http://a.example.org>; rel="original",
  <http://arxiv.example.net/timemap/http://a.example.org>
    ; rel="timemap"; type="application/link-format",
  <http://arxiv.example.net/timegate/http://a.example.org>
    ; rel="timegate"
 Content-Length: 0
 Content-Type: text/plain; charset=UTF-8
 Connection: close
         Figure 21: Response is a Memento of a Redirect; Leads
                        to an Original Resource
 HTTP/1.1 301 Moved Permanently
 Date: Thu, 21 Jan 2010 00:09:40 GMT
 Server: Apache-Coyote/1.1
 Memento-Datetime: Fri, 11 Apr 2008 00:06:50 GMT
 Location:
  http://arxiv.example.net/web/20080411000655/http://b.example.org
 Link: <http://a.example.org>; rel="original",
  <http://arxiv.example.net/timemap/http://a.example.org>
    ; rel="timemap"; type="application/link-format",
  <http://arxiv.example.net/timegate/http://a.example.org>
    ; rel="timegate"
 Content-Length: 0
 Content-Type: text/plain; charset=UTF-8
 Connection: close
  Figure 22: Response is a Memento of a Redirect; Leads to a Memento

4.5.5. Memento of Responses with 4XX or 5XX HTTP Status Codes

 Cases exist in which responses with 4XX and 5XX HTTP status codes are
 archived.  If the Memento requested by the user agent is an archived
 version of such an HTTP response, the server's response MUST have the
 same 4XX or 5XX HTTP status code.  The use of headers and links in
 responses is as described for Mementos in the respective scenarios.
 For example, Figure 23 shows the 404 response to an HTTP GET request
 for http://a.example.org issued on April 11, 2008.  This response is
 archived as a Memento of http://a.example.org that has as URI-M
 http://arxiv.example.net/web/20080411000650/http://a.example.org.
 The response to an HTTP HEAD on this URI-M is shown in Figure 24.  It

Van de Sompel, et al. Informational [Page 32] RFC 7089 HTTP Memento December 2013

 is a replay of the original response with "Memento-Datetime" and
 "Link" headers added, to allow a user agent to understand the
 response is a Memento.
 HTTP/1.1 404 Not Found
 Date: Fri, 11 Apr 2008 00:06:50 GMT
 Server: Apache
 Content-Length: 255
 Content-Type: text/plain; charset=UTF-8
 Connection: close
                     Figure 23: Response Is a 404
 HTTP/1.1 404 Not Found
 Date: Thu, 21 Jan 2010 00:09:40 GMT
 Server: Apache-Coyote/1.1
 Memento-Datetime: Fri, 11 Apr 2008 00:06:50 GMT
 Link: <http://a.example.org>; rel="original",
  <http://arxiv.example.net/timemap/http://a.example.org>
    ; rel="timemap"; type="application/link-format",
  <http://arxiv.example.net/timegate/http://a.example.org>
    ; rel="timegate"
 Content-Length: 255
 Content-Type: text/plain; charset=UTF-8
 Connection: close
               Figure 24: Response Is a Memento of a 404

4.5.6. Sticky "Memento-Datetime" and "original" Link for Mementos

 A response to an HTTP HEAD/GET request issued against a Memento:
 o  Includes a "Memento-Datetime" header that entails a promise that
    the response is archived, frozen in time.  The value of the header
    expresses the archival datetime of the Memento.
 o  Includes a link in the HTTP "Link" header with an "original"
    Relation Type that unambiguously points to the Original Resource
    associated with the Memento.  The Target IRI of the link is the
    URI-R of that Original Resource.
 Both the "Memento-Datetime" header and the "original" link MUST be
 "sticky" in the following ways:
 o  The server that originally assigns them MUST retain them in all
    responses to HTTP requests (with or without an "Accept-Datetime"
    request header) that occur against the Memento after the time of

Van de Sompel, et al. Informational [Page 33] RFC 7089 HTTP Memento December 2013

    their original assignment, and the server MUST NOT change the
    value of the "Memento-Datetime" header nor the Target IRI of the
    "original" link.
 o  Applications that mirror Mementos at a different URI MUST retain
    them and MUST NOT change them unless mirroring involves a
    meaningful state change.  This allows, among others, duplicating a
    Web archive at a new location while preserving the value of the
    "Memento-Datetime" header and the link with the "original"
    Relation Type for the archived resources.  For example, when
    mirroring, the "Last-Modified" header will be updated to reflect
    the time of mirroring at the new URI, whereas the value for
    "Memento-Datetime" will be maintained.

4.5.7. Intermediate Resources

 An intermediate resource is a resource that issues a redirect to a
 TimeGate, to a Memento, or to another intermediate resource, and thus
 plays an active role in the Memento infrastructure.  Intermediate
 resources commonly exist in Web archives on the path from a TimeGate
 to an appropriate Memento.
 A response of an intermediate resource has an HTTP status code
 indicative of HTTP redirection (e.g., 302) and uses Memento headers
 and links that allow user agents to recognize that the resource plays
 a role in the Memento framework:
 o  A "Vary" header that includes an "accept-datetime" value MUST NOT
    be provided.
 o  The response MUST NOT include a "Memento-Datetime" header.
 o  The "Link" header MUST be provided, and it MUST have a link with
    the "original" Relation Type that has the URI-R of the associated
    Original Resource as Target IRI.  Links with "timegate",
    "timemap", and "memento" Relation Types are OPTIONAL and, if
    provided, MUST pertain to the Original Resource for which the user
    agent is trying to obtain a Memento.
 A user agent MUST follow a redirection provided by an intermediate
 resource; multiple such redirections can be chained.
 Consider the case where a user agent follows the "timegate" link
 provided in Figure 10 and engages in datetime negotiation with the
 assumed TimeGate in the manner shown in Figure 11.  But instead of
 receiving a response as shown in Figure 12, it receives the one shown
 below in Figure 25.  Such a response is unambiguously recognizable as
 coming from an intermediate resource.

Van de Sompel, et al. Informational [Page 34] RFC 7089 HTTP Memento December 2013

 HTTP/1.1 302 Found
 Date: Thu, 21 Jan 2010 00:06:50 GMT
 Server: Apache
 Location:
  http://arxiv.example.net/new-timegate/http://a.example.org/
 Link: <http://a.example.org>; rel="original"
 Content-Length: 0
 Content-Type: text/plain; charset=UTF-8
 Connection: close
        Figure 25: Redirecting Resource Redirects to a TimeGate

4.5.8. Resources Excluded from Datetime Negotiation

 When delivering a Memento to a user agent, a Web archive commonly
 enhances that Memento's archived content, for example, by including a
 banner that provides branding and highlights the archival status of
 the Memento.  The resources that are involved in providing such
 system-specific functionality, many times JavaScript or images, must
 be used in their current state.
 A server that generally supports datetime negotiation should make
 resources that need to be excluded from datetime negotiation
 recognizable.  Doing so allows a user agent to refrain from
 attempting to access a Memento for them.  In order to achieve this,
 the server SHOULD include a special-purpose link in the HTTP "Link"
 header when responding to an HTTP HEAD/GET request to a resource
 excluded from datetime negotiation.  This link has
 "http://mementoweb.org/terms/donotnegotiate" as Target IRI and
 "type", defined in [RFC6903], as the value of the "rel" attribute.
 Other Memento headers as defined in Section 2.1 SHOULD NOT be
 provided.
 Figure 26 shows the response to an HTTP HEAD request from a resource
 excluded from datetime negotiation.
 HTTP/1.1 200 OK
 Date: Thu, 21 Jan 2010 00:09:40 GMT
 Server: Apache-Coyote/1.1
 Link: <http://mementoweb.org/terms/donotnegotiate>; rel="type"
 Content-Length: 238
 Content-Type: application/javascript; charset=UTF-8
 Connection: close
 Figure 26: Response to an HTTP HEAD Request from a Resource Excluded
                       from Datetime Negotiation

Van de Sompel, et al. Informational [Page 35] RFC 7089 HTTP Memento December 2013

5. TimeMaps: Content and Serialization

 A TimeMap is introduced to support retrieving a comprehensive list of
 all Mementos for a specific Original Resource known to a server.  The
 entity-body of a response to an HTTP GET request issued against a
 TimeMap's URI-T:
 o  MUST list the URI-R of the Original Resource that the TimeMap is
    about;
 o  MUST list the URI-M and archival datetime of each Memento for the
    Original Resource known to the server, preferably in a single
    document, or, alternatively in multiple documents that can be
    gathered by following contained links with a "timemap" Relation
    Type;
 o  SHOULD list the URI-G of one or more TimeGates for the Original
    Resource known to the responding server;
 o  SHOULD, for self-containment, list the URI-T of the TimeMap
    itself;
 o  MUST unambiguously type listed resources as being Original
    Resource, TimeGate, Memento, or TimeMap.
 The entity-body of a response from a TimeMap MAY be serialized in
 various ways, but the link-value format serialization described here
 MUST be supported.  In this serialization, the entity-body MUST be
 formatted in the same way as the value of an HTTP "Link" header, and
 hence MUST comply to the "link-value" construction rule of Section 5.
 The Link header field of [RFC5988], and the media type of the entity-
 body MUST be "application/link-format" as introduced in [RFC6690].
 Links contained in the entity-body MUST be interpreted as follows:
 o  The Context IRI is set to the anchor parameter, when specified;
 o  The Context IRI of links with the "self" Relation Types is the
    URI-T of the TimeMap, i.e., the URI of the resource from which the
    TimeMap was requested;
 o  The Context IRI of all other links is the URI-R of the Original
    Resource, which is provided as the Target IRI of the link with an
    "original" Relation Type.
 In order to retrieve the link-value serialization of a TimeMap, a
 user agent uses an "Accept" request header with a value set to
 "application/link-format".  This is shown in Figure 27.

Van de Sompel, et al. Informational [Page 36] RFC 7089 HTTP Memento December 2013

 GET /timemap/http://a.example.org/ HTTP/1.1
 Host: arxiv.example.net
 Accept: application/link-format;q=1.0
 Connection: close
                   Figure 27: Request for a TimeMap
 If the TimeMap requested by the user agent exists, the server's
 response has a "200 OK" HTTP status code and the list of Mementos is
 provided in the entity-body of the response.  Such a response is
 shown in Figure 28.
 HTTP/1.1 200 OK
 Date: Thu, 21 Jan 2010 00:06:50 GMT
 Server: Apache
 Content-Length: 4883
 Content-Type: application/link-format
 Connection: close
  <http://a.example.org>;rel="original",
  <http://arxiv.example.net/timemap/http://a.example.org>
    ; rel="self";type="application/link-format"
    ; from="Tue, 20 Jun 2000 18:02:59 GMT"
    ; until="Wed, 09 Apr 2008 20:30:51 GMT",
  <http://arxiv.example.net/timegate/http://a.example.org>
    ; rel="timegate",
  <http://arxiv.example.net/web/20000620180259/http://a.example.org>
    ; rel="first memento";datetime="Tue, 20 Jun 2000 18:02:59 GMT"
    ; license="http://creativecommons.org/publicdomain/zero/1.0/",
  <http://arxiv.example.net/web/20091027204954/http://a.example.org>
     ; rel="last memento";datetime="Tue, 27 Oct 2009 20:49:54 GMT"
     ; license="http://creativecommons.org/publicdomain/zero/1.0/",
  <http://arxiv.example.net/web/20000621011731/http://a.example.org>
    ; rel="memento";datetime="Wed, 21 Jun 2000 01:17:31 GMT"
    ; license="http://creativecommons.org/publicdomain/zero/1.0/",
  <http://arxiv.example.net/web/20000621044156/http://a.example.org>
    ; rel="memento";datetime="Wed, 21 Jun 2000 04:41:56 GMT"
    ; license="http://creativecommons.org/publicdomain/zero/1.0/",
  ...
                  Figure 28: Response from a TimeMap

Van de Sompel, et al. Informational [Page 37] RFC 7089 HTTP Memento December 2013

5.1. Special Cases

5.1.1. Index and Paging TimeMaps

 Cases exist in which a TimeMap points at one or more other TimeMaps:
 o  Index TimeMap - A TimeMap can merely point at other TimeMaps and
    not list any Mementos itself.  This can happen when Mementos are
    spread across several archives that share a front-end.  An example
    is shown in Figure 29.
 o  Paging TimeMap - The number of available Mementos can require
    introducing multiple TimeMaps that can be paged.  An example is
    shown in Figure 30.  Note that a Paging TimeMap contains links to
    other TimeMaps but actually also lists Mementos.
 In both cases, including the "from" and "until" attributes for
 "timemap" links is RECOMMENDED as a means to express the temporal
 span of Mementos listed in each TimeMap.  Note that TimeMaps obtained
 by following a "timemap" link can contain links to further TimeMaps.
 <http://a.example.org>;rel="original",
  <http://arxiv.example.net/timegate/http://a.example.org>
    ; rel="timegate",
  <http://arxiv.example.net/timemap/http://a.example.org>
    ; rel="self";type="application/link-format",
  <http://arxiv1.example.net/timemap/http://a.example.org>
    ; rel="timemap";type="application/link-format"
    ; from="Wed, 21 Jun 2000 04:41:56 GMT"
    ; until="Wed, 09 Apr 2008 20:30:51 GMT",
  <http://arxiv2.example.net/timemap/http://a.example.org>
    ; rel="timemap";type="application/link-format"
    ; from="Thu, 10 Apr 2008 20:30:51 GMT"
    ; until="Tue, 27 Oct 2009 20:49:54 GMT",
  <http://arxiv3.example.net/timemap/http://a.example.org>
    ; rel="timemap";type="application/link-format"
    ; from="Thu, 29 Oct 2009 20:30:51 GMT"
                       Figure 29: Index TimeMap

Van de Sompel, et al. Informational [Page 38] RFC 7089 HTTP Memento December 2013

 <http://a.example.org>;rel="original",
  <http://arxiv.example.net/timegate/http://a.example.org>
    ; rel="timegate",
  <http://arxiv.example.net/timemap/1/http://a.example.org>
    ; rel="self";type="application/link-format"
    ; from="Tue, 20 Jun 2000 18:02:59 GMT"
    ; until="Wed, 09 Apr 2008 20:30:51 GMT",
  <http://arxiv.example.net/timemap/2/http://a.example.org>
    ; rel="timemap";type="application/link-format"
    ; from="Thu, 10 Apr 2008 20:30:51 GMT"
    ; until="Tue, 27 Oct 2009 20:49:54 GMT",
  <http://arxiv.example.net/timemap/3/http://a.example.org>
    ; rel="timemap";type="application/link-format"
    ; from="Thu, 29 Oct 2009 20:30:51 GMT"
    ; until="Fri, 31 Aug 2012 12:22:34 GMT"
  <http://arxiv.example.net/web/20000620180259/http://a.example.org>
    ; rel="memento";datetime="Tue, 20 Jun 2000 18:02:59 GMT",
  <http://arxiv.example.net/web/20000621011731/http://a.example.org>
    ; rel="memento";datetime="Wed, 21 Jun 2000 01:17:31 GMT",
  <http://arxiv.example.net/web/20000621044156/http://a.example.org>
    ; rel="memento";datetime="Wed, 21 Jun 2000 04:41:56 GMT",
  ...
                       Figure 30: Paging TimeMap

5.1.2. Mementos for TimeMaps

 A TimeMap itself can act as an Original Resource for which a TimeGate
 and Mementos may exist.  Hence, the response from a TimeMap could
 include a "timegate" link to a TimeGate via which prior TimeMap
 versions are available.  And, in cases where URI-T=URI-R=URI-G (a
 TimeMap is an Original Resource that acts as its own TimeGate), an
 "original" link pointing at the TimeMap URI-T would be included.
 Therefore, caution is required in cases where a TimeMap for an
 Original Resource wants to explicitly express in a "Link" header for
 which Original Resource it is a TimeMap.  It can do so by including a
 "timemap" link that has the URI-R of the Original Resource as Context
 IRI and the URI-T of the TimeMap as Target IRI.
 Figure 31 shows the response to an HTTP HEAD request against a
 TimeMap that has
 http://arxiv.example.net/timemap/http://a.example.org as URI-T.  This
 TimeMap provides information about Mementos for the Original Resource
 that has http://a.example.org as URI-R.  The response includes an
 "original" link pointing to the Original Resource that this TimeMap
 is about.  Note the use of the "anchor" attribute in this link to
 convey the URI-R of that Original Resource.

Van de Sompel, et al. Informational [Page 39] RFC 7089 HTTP Memento December 2013

 HTTP/1.1 200 OK
 Date: Thu, 21 Jan 2010 00:06:50 GMT
 Server: Apache
 Link: <http://arxiv.example.net/timemap/http://a.example.org>
           ; anchor="http://a.example.org"; rel="timemap"
           ; type="application/link-format"
 Content-Length: 0
 Content-Type: application/link-format; charset=UTF-8
 Connection: close
     Figure 31: TimeMap Links to the Original Resource It Is about

6. IANA Considerations

6.1. HTTP Headers

 IANA has registered the "Accept-Datetime" and "Memento-Datetime" HTTP
 headers (defined in Section 2.1.1) in the "Permanent Message Header
 Field Names" registry:
 o  Header field name: Accept-Datetime
 o  Applicable protocol: "http" (RFC 2616)
 o  Status: informational
 o  Author/Change controller: Herbert Van de Sompel, Los Alamos
    National Laboratory, hvdsomp@gmail.com
 o  Specification document(s): this document
 o  Header field name: Memento-Datetime
 o  Applicable protocol: "http" (RFC 2616)
 o  Status: informational
 o  Author/Change controller: Herbert Van de Sompel, Los Alamos
    National Laboratory, hvdsomp@gmail.com
 o  Specification document(s): this document

6.2. Link Relation Types

 IANA has registered the Relation Types "original", "timegate",
 "timemap", and "memento" (defined in Section 2.2) in the "Link
 Relation Types" registry:
 o  Relation Name: original
 o  Description: The Target IRI points to an Original Resource.
 o  Reference: this document
 o  Notes: An Original Resource is a resource that exists or used to
    exist, and for which access to one of its prior states may be
    required.

Van de Sompel, et al. Informational [Page 40] RFC 7089 HTTP Memento December 2013

 o  Relation Name: timegate
 o  Description: The Target IRI points to a TimeGate for an Original
    Resource.
 o  Reference: this document
 o  Notes: A TimeGate for an Original Resource is a resource that is
    capable of datetime negotiation to support access to prior states
    of the Original Resource.
 o  Relation Name: timemap
 o  Description: The Target IRI points to a TimeMap for an Original
    Resource.
 o  Reference: this document
 o  Notes: A TimeMap for an Original Resource is a resource from which
    a list of URIs of Mementos of the Original Resource is available.
 o  Relation Name: memento
 o  Description: The Target IRI points to a Memento, a fixed resource
    that will not change state anymore.
 o  Reference: this document
 o  Notes: A Memento for an Original Resource is a resource that
    encapsulates a prior state of the Original Resource.

7. Security Considerations

 Provision of a "timegate" HTTP "Link" header in responses to requests
 for an Original Resource that is protected (e.g., 401 or 403 HTTP
 response codes) is OPTIONAL.  The inclusion of this Link when
 requesting authentication is at the server's discretion; cases may
 exist in which a server protects the current state of a resource, but
 supports open access to prior states and thus chooses to supply this
 HTTP "Link" header.  Conversely, the server may choose to not
 advertise the TimeGate URIs (e.g., they exist in an intranet archive)
 for unauthenticated requests.
 The veracity of archives and the relationships between Original
 Resources and Mementos is beyond the scope of this document.  Even in
 the absence of malice, it is possible for separate archives to have
 different Mementos for the same Original Resource at the same
 datetime if the state of the Original Resource was dependent on the
 requesting archive's user agent IP address, specific HTTP request
 headers, and possibly other factors.
 Further authentication, encryption, and other security-related issues
 are otherwise orthogonal to Memento.

Van de Sompel, et al. Informational [Page 41] RFC 7089 HTTP Memento December 2013

8. Acknowledgements

 The Memento effort is funded by the Library of Congress.  Many thanks
 to Kris Carpenter Negulescu, Michael Hausenblas, Erik Hetzner, Larry
 Masinter, Gordon Mohr, David Rosenthal, Ed Summers, James Anderson,
 Tim Starling, Martin Klein, and Mark Nottingham for feedback.  Many
 thanks to Samuel Adams, Scott Ainsworth, Lyudmilla Balakireva, Frank
 McCown, Harihar Shankar, Brad Tofel, Andrew Jackson, Ahmed Alsum, Mat
 Kelly, and Ilya Kreymer for implementations that informed the
 specification.

9. References

9.1. Normative References

 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2616]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
             Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
             Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
 [RFC5829]   Brown, A., Clemm, G., and J. Reschke, "Link Relation
             Types for Simple Version Navigation between Web
             Resources", RFC 5829, April 2010.
 [RFC5988]   Nottingham, M., "Web Linking", RFC 5988, October 2010.
 [RFC6690]   Shelby, Z., "Constrained RESTful Environments (CoRE) Link
             Format", RFC 6690, August 2012.
 [RFC6903]   Snell, J., "Additional Link Relation Types", RFC 6903,
             March 2013.

9.2. Informative References

 [DATED-URI] Masinter, L., "The 'tdb' and 'duri' URI schemes, based on
             dated URIs", Work in Progress, January 2012.
 [Fitch]     Fitch, K., "Web site archiving - an approach to recording
             every materially different response produced by a
             website", July 2003,
             <http://ausweb.scu.edu.au/aw03/papers/fitch/paper.html>.
 [RFC1123]   Braden, R., "Requirements for Internet Hosts -
             Application and Support", STD 3, RFC 1123, October 1989.

Van de Sompel, et al. Informational [Page 42] RFC 7089 HTTP Memento December 2013

 [W3C.REC-aww-20041215]
             Jacobs, I. and N. Walsh, "Architecture of the World Wide
             Web", December 2004, <http://www.w3.org/TR/webarch/>.
 [W3C.gen-ont-20090420]
             Berners-Lee, T., "Architecture of the World Wide Web",
             April 2009, <http://www.w3.org/2006/gen/ont>.

Van de Sompel, et al. Informational [Page 43] RFC 7089 HTTP Memento December 2013

Appendix A. Use of Headers and Relation Types per Pattern

 +-----------------+-----------------+----------+----------+---------+
 | Response Header |     Pattern     | Original | TimeGate | Memento |
 |                 |                 | Resource |          |         |
 +-----------------+-----------------+----------+----------+---------+
 |      Vary:      |   Pattern 1.1   |     1    |     1    |    0    |
 | accept-datetime | (Section 4.1.1) |          |          |         |
 |                 |         &       |          |          |         |
 |                 |    Pattern 1.2  |          |          |         |
 |                 | (Section 4.1.2) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 1.3   |     1    |     1    |    1    |
 |                 | (Section 4.1.3) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 2.1   |     0    |     1    |    0    |
 |                 | (Section 4.2.1) |          |          |         |
 |                 |         &       |          |          |         |
 |                 |    Pattern 2.2  |          |          |         |
 |                 | (Section 4.2.2) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 2.3   |     0    |     1    |    1    |
 |                 | (Section 4.2.3) |          |          |         |
 |                 |                 |          |          |         |
 |                 |    Pattern 3    |     1    |    NA    |    1    |
 |                 |  (Section 4.3)  |          |          |         |
 |                 |                 |          |          |         |
 |                 |    Pattern 4    |     0    |    NA    |    1    |
 |                 |  (Section 4.4)  |          |          |         |
 +-----------------+-----------------+----------+----------+---------+

(cont.)

Van de Sompel, et al. Informational [Page 44] RFC 7089 HTTP Memento December 2013

 +-----------------+-----------------+----------+----------+---------+
 | Response Header |     Pattern     | Original | TimeGate | Memento |
 |                 |                 | Resource |          |         |
 +-----------------+-----------------+----------+----------+---------+
 |     Vary:       |                 |          |          |         |
 | Memento-        |   Pattern 1.1   |     0    |     0    |    1    |
 |    Datetime     | (Section 4.1.1) |          |          |         |
 |                 |         &       |          |          |         |
 |                 |    Pattern 1.2  |          |          |         |
 |                 | (Section 4.1.2) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 1.3   |     1    |     1    |    1    |
 |                 | (Section 4.1.3) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 2.1   |     0    |     0    |    1    |
 |                 | (Section 4.2.1) |          |          |         |
 |                 |         &       |          |          |         |
 |                 |    Pattern 2.2  |          |          |         |
 |                 | (Section 4.2.2) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 2.3   |     0    |     1    |    1    |
 |                 | (Section 4.2.3) |          |          |         |
 |                 |                 |          |          |         |
 |                 |    Pattern 3    |     1    |    NA    |    1    |
 |                 |  (Section 4.3)  |          |          |         |
 |                 |                 |          |          |         |
 |                 |    Pattern 4    |     0    |    NA    |    1    |
 |                 |  (Section 4.4)  |          |          |         |
 +-----------------+-----------------+----------+----------+---------+

(cont.)

Van de Sompel, et al. Informational [Page 45] RFC 7089 HTTP Memento December 2013

 +-----------------+-----------------+----------+----------+---------+
 | Response Header |     Pattern     | Original | TimeGate | Memento |
 |                 |                 | Resource |          |         |
 +-----------------+-----------------+----------+----------+---------+
 |       Link:     |                 |          |          |         |
 |  rel="original" |   Pattern 1.1   |     0    |     1    |    1    |
 |                 | (Section 4.1.1) |          |          |         |
 |                 |         &       |          |          |         |
 |                 |    Pattern 1.2  |          |          |         |
 |                 | (Section 4.1.2) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 1.3   |     1    |     1    |    1    |
 |                 | (Section 4.1.3) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 2.1   |     0    |     1    |    1    |
 |                 | (Section 4.2.1) |          |          |         |
 |                 |         &       |          |          |         |
 |                 |    Pattern 2.2  |          |          |         |
 |                 | (Section 4.2.2) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 2.3   |     0    |     1    |    1    |
 |                 | (Section 4.2.3) |          |          |         |
 |                 |                 |          |          |         |
 |                 |    Pattern 3    |     1    |    NA    |    1    |
 |                 |  (Section 4.3)  |          |          |         |
 |                 |                 |          |          |         |
 |                 |    Pattern 4    |     0    |    NA    |    1    |
 |                 |  (Section 4.4)  |          |          |         |
 +-----------------+-----------------+----------+----------+---------+

(cont.)

Van de Sompel, et al. Informational [Page 46] RFC 7089 HTTP Memento December 2013

 +-----------------+-----------------+----------+----------+---------+
 | Response Header |     Pattern     | Original | TimeGate | Memento |
 |                 |                 | Resource |          |         |
 +-----------------+-----------------+----------+----------+---------+
 |      Link:      |                 |          |          |         |
 |  rel="timegate" |   Pattern 1.1   |    >=0   |    >=0   |   >=0   |
 |                 | (Section 4.1.1) |          |          |         |
 |                 |         &       |          |          |         |
 |                 |    Pattern 1.2  |          |          |         |
 |                 | (Section 4.1.2) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 1.3   |    >=0   |    >=0   |   >=0   |
 |                 | (Section 4.1.3) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 2.1   |    >=0   |     0    |   >=0   |
 |                 | (Section 4.2.1) |          |          |         |
 |                 |         &       |          |          |         |
 |                 |    Pattern 2.2  |          |          |         |
 |                 | (Section 4.2.2) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 2.3   |    >=0   |    >=0   |   >=0   |
 |                 | (Section 4.2.3) |          |          |         |
 |                 |                 |          |          |         |
 |                 |    Pattern 3    |    NA    |    NA    |    NA   |
 |                 |  (Section 4.3)  |          |          |         |
 |                 |                 |          |          |         |
 |                 |    Pattern 4    |    NA    |    NA    |    NA   |
 |                 |  (Section 4.4)  |          |          |         |
 +-----------------+-----------------+----------+----------+---------+

(cont.)

Van de Sompel, et al. Informational [Page 47] RFC 7089 HTTP Memento December 2013

 +-----------------+-----------------+----------+----------+---------+
 | Response Header |     Pattern     | Original | TimeGate | Memento |
 |                 |                 | Resource |          |         |
 +-----------------+-----------------+----------+----------+---------+
 |     Link:       |                 |          |          |         |
 |  rel="timemap"  |   Pattern 1.1   |    >=0   |    >=0   |   >=0   |
 |                 | (Section 4.1.1) |          |          |         |
 |                 |         &       |          |          |         |
 |                 |    Pattern 1.2  |          |          |         |
 |                 | (Section 4.1.2) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 1.3   |    >=0   |    >=0   |   >=0   |
 |                 | (Section 4.1.3) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 2.1   |    >=0   |    >=0   |   >=0   |
 |                 | (Section 4.2.1) |          |          |         |
 |                 |         &       |          |          |         |
 |                 |    Pattern 2.2  |          |          |         |
 |                 | (Section 4.2.2) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 2.3   |    >=0   |    >=0   |   >=0   |
 |                 | (Section 4.2.3) |          |          |         |
 |                 |                 |          |          |         |
 |                 |    Pattern 3    |    >=0   |    NA    |   >=0   |
 |                 |  (Section 4.3)  |          |          |         |
 |                 |                 |          |          |         |
 |                 |    Pattern 4    |    >=0   |    NA    |   >=0   |
 |                 |  (Section 4.4)  |          |          |         |
 |                 |                 |          |          |         |
 +-----------------+-----------------+----------+----------+---------+

(cont.)

Van de Sompel, et al. Informational [Page 48] RFC 7089 HTTP Memento December 2013

 +-----------------+-----------------+----------+----------+---------+
 | Response Header |     Pattern     | Original | TimeGate | Memento |
 |                 |                 | Resource |          |         |
 +-----------------+-----------------+----------+----------+---------+
 |      Link:      |                 |          |          |         |
 |  rel="memento"  |   Pattern 1.1   |    >=0   |    >=0   |   >=0   |
 |                 | (Section 4.1.1) |          |          |         |
 |                 |         &       |          |          |         |
 |                 |    Pattern 1.2  |          |          |         |
 |                 | (Section 4.1.2) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 1.3   |    >=0   |    >=0   |   >=0   |
 |                 | (Section 4.1.3) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 2.1   |    >=0   |    >=0   |   >=0   |
 |                 | (Section 4.2.1) |          |          |         |
 |                 |         &       |          |          |         |
 |                 |    Pattern 2.2  |          |          |         |
 |                 | (Section 4.2.2) |          |          |         |
 |                 |                 |          |          |         |
 |                 |   Pattern 2.3   |    >=0   |    >=0   |   >=0   |
 |                 | (Section 4.2.3) |          |          |         |
 |                 |                 |          |          |         |
 |                 |    Pattern 3    |    >=0   |    NA    |   >=0   |
 |                 |  (Section 4.3)  |          |          |         |
 |                 |                 |          |          |         |
 |                 |    Pattern 4    |    >=0   |    NA    |   >=0   |
 |                 |  (Section 4.4)  |          |          |         |
 +-----------------+-----------------+----------+----------+---------+
                       Table 5: Memento Headers

Van de Sompel, et al. Informational [Page 49] RFC 7089 HTTP Memento December 2013

Authors' Addresses

 Herbert Van de Sompel
 Los Alamos National Laboratory
 PO Box 1663
 Los Alamos, New Mexico  87545
 USA
 Phone: +1 505 667 1267
 EMail: hvdsomp@gmail.com
 URI:   http://public.lanl.gov/herbertv/
 Michael Nelson
 Old Dominion University
 Norfolk, Virginia  23529
 USA
 Phone: +1 757 683 6393
 EMail: mln@cs.odu.edu
 URI:   http://www.cs.odu.edu/~mln/
 Robert Sanderson
 Los Alamos National Laboratory
 PO Box 1663
 Los Alamos, New Mexico  87545
 USA
 Phone: +1 505 665 5804
 EMail: azaroth42@gmail.com
 URI:   http://public.lanl.gov/rsanderson/

Van de Sompel, et al. Informational [Page 50]

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