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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 RFC 7089

Internet Engineering Task Force                           H. VandeSompel
Internet-Draft                            Los Alamos National Laboratory
Intended status: Informational                                 M. Nelson
Expires: August 29, 2013                         Old Dominion University
                                                            R. Sanderson
                                          Los Alamos National Laboratory
                                                       February 25, 2013


   HTTP framework for time-based access to resource states -- Memento
                      draft-vandesompel-memento-06

Abstract

   The HTTP-based Memento framework bridges the present and past Web. It
   facilitates obtaining representations of prior states of a resource
   by leveraging the resource's URI and a preferred datetime.  To this
   end, the framework introduces datetime negotiation, a variation on
   content negotiation.  It also facilitates recongizing a resource that
   encapsulates a frozen prior state of another resource by expressing
   that state's datetime and by providing an appropriately typed link to
   that other resource.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 29, 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



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Notational Conventions . . . . . . . . . . . . . . . . . .  4
     1.3.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  HTTP headers, Relation Types . . . . . . . . . . . . . . . . .  6
     2.1.  HTTP Headers . . . . . . . . . . . . . . . . . . . . . . .  7
       2.1.1.  Accept-Datetime, Memento-Datetime  . . . . . . . . . .  7
       2.1.2.  Vary . . . . . . . . . . . . . . . . . . . . . . . . .  8
       2.1.3.  Link . . . . . . . . . . . . . . . . . . . . . . . . .  8
     2.2.  Relation Types . . . . . . . . . . . . . . . . . . . . . .  8
       2.2.1.  Relation Type "original" . . . . . . . . . . . . . . .  9
       2.2.2.  Relation Type "timegate" . . . . . . . . . . . . . . .  9
       2.2.3.  Relation Type "timemap"  . . . . . . . . . . . . . . .  9
       2.2.4.  Relation Type "memento"  . . . . . . . . . . . . . . . 10
     2.3.  Memento response headers: Summary  . . . . . . . . . . . . 10
   3.  General considerations regarding Datetime Negotiation  . . . . 11
   4.  HTTP Interactions for Datetime Negotiation . . . . . . . . . . 12
     4.1.  Pattern 1 - The Original Resource acts as its own
           TimeGate . . . . . . . . . . . . . . . . . . . . . . . . . 14
       4.1.1.  Pattern 1.1 - URI-R=URI-G ; 302-style negotiation
               ; distinct URI-M for Mementos  . . . . . . . . . . . . 15
       4.1.2.  Pattern 1.2 - URI-R=URI-G ; 200-style negotiation
               ; distinct URI-M for Mementos  . . . . . . . . . . . . 17
       4.1.3.  Pattern 1.3 - URI-R=URI-G ; 200-style negotiation
               ; no distinct URI-M for Mementos . . . . . . . . . . . 18
     4.2.  Pattern 2 - A remote resource acts as a TimeGate for
           the Original Resource  . . . . . . . . . . . . . . . . . . 19
       4.2.1.  Pattern 2.1 - URI-R<>URI-G ; 302-style negotiation
               ; distinct URI-M for Mementos  . . . . . . . . . . . . 21
       4.2.2.  Pattern 2.2 - URI-R<>URI-G ; 200-style negotiation
               ; distinct URI-M for Mementos  . . . . . . . . . . . . 23
       4.2.3.  Pattern 2.3 - URI-R<>URI-G ; 200-style negotiation
               ; no distinct URI-M for Mementos . . . . . . . . . . . 24
     4.3.  Pattern 3 - The Original Resource is a Fixed Resource  . . 25
     4.4.  Pattern 4 - Mementos without a TimeGate  . . . . . . . . . 26
     4.5.  Special Cases  . . . . . . . . . . . . . . . . . . . . . . 28
       4.5.1.  Original Resource provides no "timegate" link  . . . . 28
       4.5.2.  Server exists but Original Resource no longer does . . 28



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       4.5.3.  Issues with Accept-Datetime  . . . . . . . . . . . . . 28
       4.5.4.  Memento of a 3XX response  . . . . . . . . . . . . . . 29
       4.5.5.  Memento of responses with 4XX or 5XX HTTP status
               codes  . . . . . . . . . . . . . . . . . . . . . . . . 31
       4.5.6.  Sticky "Memento-Datetime" value for Mementos . . . . . 32
       4.5.7.  Intermediate Resources . . . . . . . . . . . . . . . . 32
   5.  TimeMaps . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     5.1.  Index and Paging TimeMaps  . . . . . . . . . . . . . . . . 35
     5.2.  Mementos for TimeMaps  . . . . . . . . . . . . . . . . . . 37
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 37
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 38
   8.  Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 38
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 40
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 41
     10.2. Informative References . . . . . . . . . . . . . . . . . . 41
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 42


































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1.  Introduction

1.1.  Terminology

   This specification uses the terms "resource", "request", "response",
   "entity-body", "content negotiation", "user agent", "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 non-existence (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/ :

   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 :

   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/ :

   o  http://www.w3.org/TR/2004/PR-webarch-20041105/

   o  http://www.w3.org/TR/2002/WD-webarch-20020830/

   o  http://webarchive.nationalarchives.gov.uk/20100304163140/http://
      www.w3.org/TR/webarch/




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   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
   [I-D.masinter-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).

   This document is concerned with specifying an instantiation of these
   abstractions for resources that are identified by HTTP(S) URIs.


2.  HTTP headers, 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.



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2.1.  HTTP Headers

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

2.1.1.  Accept-Datetime, Memento-Datetime

   The "Accept-Datetime" request header is trasnmitted 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
   the BNF in Figure 1.  The datetime MUST be represented in Greenwich
   Mean Time (GMT).









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   accept-dt-value = rfc1123-date *SP
   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 negotation 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.  The 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",
   "successor-version" as detailed in [RFC5988] and [RFC5829].

2.2.  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.



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2.2.1.  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: A TimeGate and a Memento MUST include
   exactly one link with an "original" Relation Type in their HTTP
   "Link" header.

2.2.2.  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: An Original Resource and a Memento SHOULD
   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.  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
   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: An Original Resource, a TimeGate and a
   Memento SHOULD 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.




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2.2.4.  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 SHOULD be a URI.

   Use in HTTP "Link" headers: 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.

2.3.  Memento response headers: Summary

   The below table summarizes the use of the Vary, Memento-Datetime, and
   Link headers in responses from Original Resources, TimeGates, and
   Mementos.  The value of the Vary header MUST contain the "accept-
   datetime" value, whereas the value of the Memento-Datetime header
   MUST convey the datetime of the state of the Original Resource that
   is encapsulated in the Memento selected by the response.  From now
   on, this datetime will be referred to as the archival datetime of the
   Memento.

   The Link header can contain links with the "original", "timegate",
   "timemap", and "memento" Relation Types and their use for the various
   resource types is also detailed in the table.

   The inclusion of Memento headers and links in responses from the
   respective resources is as shown in the table irrespective of whether
   an HTTP HEAD/GET request against their URI includes an "Accept-
   Datetime" request header: requests with and without an "Accept-
   Datetime" header yield the inclusion of the same response headers.
   Values for those response headers depend on the request.



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   The use of response headers and links for specific scenarios can be
   derived from this table.  For example:

   o  URI-R<>URI-G<>URI-M - If the TimeGate is distinct from the
      associated Original Resource and Memento, then their use is as
      shown in the respective columns.

   o  URI-R=URI-G - If the TimeGate coincides with the Original
      Resource, then their use is obtained by taking the union of the
      headers for a TimeGate and for an Original Resource and by
      selecting REQUIRED over RECOMMENDED and REQUIRED over PROHIBITED.
      For example, this case must have a "Vary" header that includes an
      "accept-datetime" value.

   o  URI-R=URI-M - If the Original Resource and its Memento coincide
      (the Original Resource is a FixedResource as per
      [W3C.gen-ont-20090420]), their use is obtained by taking the union
      of the headers for an Original Resource and for a Memento and by
      selecting REQUIRED over RECOMMENDED and REQUIRED over PROHIBITED.
      For example, this case must have a "Memento-Datetime" header.

   +------------------+---------------+---------------+----------------+
   |  Response Header |    Original   |    TimeGate   |     Memento    |
   |                  |    Resource   |               |                |
   +------------------+---------------+---------------+----------------+
   |       Vary:      |   PROHIBITED  |  REQUIRED, 1  |   PROHIBITED   |
   |  accept-datetime |               |               |                |
   | Memento-Datetime |   PROHIBITED  |   PROHIBITED  |   REQUIRED, 1  |
   |       Link       |  RECOMMENDED, |  REQUIRED, 1  |   REQUIRED, 1  |
   |                  |     0 or 1    |               |                |
   |  "original" link |       NA      |  REQUIRED, 1  |   REQUIRED, 1  |
   |  "timegate" link |  RECOMMENDED, |       NA      | RECOMMENDED, 0 |
   |                  |   0 or more   |               |     or more    |
   |  "timemap" link  |  RECOMMENDED, |  RECOMMENDED, | RECOMMENDED, 0 |
   |                  |   0 or more   |   0 or more   |     or more    |
   |  "memento" link  |  OPTIONAL, 0  |  OPTIONAL, 0  | OPTIONAL, 0 or |
   |                  |    or more    |    or more    |      more      |
   +------------------+---------------+---------------+----------------+

                         Table 1: Memento Headers


3.  General considerations regarding Datetime Negotiation

   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 SHOULD be consistent, for



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   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".

   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 the below.

   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.


4.  HTTP Interactions for Datetime Negotiation

   This section describes the HTTP interactions of the Memento framework



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   for a variety of scenarios.  First, 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.  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

   o  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.

   o  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.

   o  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.

   o  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).

   o  Step 5: The user agent issues an HTTP GET request against URI-M.

   o  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



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      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.

   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

   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 Section 4.1 through Section 4.4.  Special
   cases are described in Section 4.5.

   Figure 3 shows a user agent that attemtps 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, 11 Sep 2001 20:35:00 GMT
   Connection: close

     Figure 3: 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.




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   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.

   +-------------------+------------+----------+---------+-------------+
   |      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 2: Pattern 1

4.1.1.  Pattern 1.1 - URI-R=URI-G ; 302-style negotiation ; distinct
        URI-M for Mementos

   In this case, the response to the user agent's request of Figure 3
   has a "302 Found" HTTP status code, and the "Location" header conveys
   the URI-M of the selected Memento.  As per Section 2.3, 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 3 is shown in
   Figure 4.  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.





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   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=20010911203610
   Link: <http://a.example.org/>; rel="original timegate"
   Content-Length: 0
   Content-Type: text/plain; charset=UTF-8
   Connection: close

            Figure 4: Response from URI-R=URI-G for Pattern 1.1

   In a subsequent request, shown in Figure 5, 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=20010911203610 HTTP/1.1
   Host: a.example.org
   Accept-Datetime: Tue, 11 Sep 2001 20:35:00 GMT
   Connection: close

              Figure 5: 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.  As
   per Section 2.3, 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 5 is shown in
   Figure 6.  Note the provision of the required "original", and the
   recommended "timegate" and "timemap" links.  The former two point to
   the Original Resource, which acts as its own TimeGate.  The latter
   has "from" and "until" attributes to indicate the temporal interval



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   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, 11 Sep 2001 20:36:10 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 6: Response from URI-M for Pattern 1.1

4.1.2.  Pattern 1.2 - URI-R=URI-G ; 200-style negotiation ; distinct
        URI-M for Mementos

   In this case, the response to the user agent's request of Figure 3
   has a "200 OK" HTTP status code, and the "Content-Location" header
   conveys the URI-M of the selected Memento.  As per Section 2.3, 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 3 is shown in
   Figure 7.  Note the provision of optional "memento" links pointing at
   the oldest and most recent Memento for the Original Resource known to
   the responding server.









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   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=20010911203610
   Memento-Datetime: Tue, 11 Sep 2001 20:36:10 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"
   Content-Length: 2312
   Content-Type: text/html; charset=UTF-8
   Connection: close

            Figure 7: Response from URI-R=URI-G for Pattern 1.2

   In a subsequent request, which is the same as Figure 3 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 7.

4.1.3.  Pattern 1.3 - URI-R=URI-G ; 200-style negotiation ; no distinct
        URI-M for Mementos

   In this case, the response to the user agent's request of Figure 3
   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.  As per Section 2.3, 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 3 is shown in
   Figure 8.  The recommended "timemap" and "timegate" links are
   included in addition to the mandatory "original" link.




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   HTTP/1.1 200 OK
   Date: Thu, 21 Jan 2010 00:06:50 GMT
   Server: Apache
   Vary: accept-datetime
   Memento-Datetime: Tue, 11 Sep 2001 20:36:10 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: 0
   Content-Type: text/plain; charset=UTF-8
   Connection: close

            Figure 8: Response from URI-R=URI-G for Pattern 1.3

   In a subsequent request, which is the same as Figure 3 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.

4.2.  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.

   +-------------------+------------+----------+---------+-------------+
   |      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)  |            |          |         |             |




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   |    Pattern 2.3    |    URI-R   |   URI-G  |  URI-G  |     200     |
   |  (Section 4.2.3)  |            |          |         |             |
   +-------------------+------------+----------+---------+-------------+

                            Table 3: Pattern 2

   The response by the Original Resource to the request shown in
   Figure 3 is the same for all three cases.  As per Section 2.3, 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.  It SHOULD include a link with a
      "timegate" Relation Type that has the URI-G of the TimeGate as
      Target IRI.  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 9 shows such a response.  Note the absence of an "original"
   link as the responding resource is neither a TimeGate or 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 9: Response from URI-R<>URI-G for Pattern 2

   Once a user agent has obtained the URI-G of a remote TimeGate for the
   Original Resource it can engage in datetime negotation with that
   TimeGate.  Figure 10 shows the request issued against the TimeGate
   whereas Section 4.2.1 through Section 4.2.3 detail the responses for
   various TimeGate implementation patterns.








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   HEAD /timegate/http://a.example.org/ HTTP/1.1
   Host: arxiv.example.net
   Accept-Datetime: Tue, 11 Sep 2001 20:35:00 GMT
   Connection: close

     Figure 10: 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 for Mementos

   In case the TimeGate uses a 302 negotiation style, the response to
   the user agent's request of Figure 10 has a "302 Found" HTTP status
   code, and the "Location" header conveys the URI-M of the selected
   Memento.  As per Section 2.3, 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 10 is shown in
   Figure 11.  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.


















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   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/20010911203610/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 11: Response from URI-G<>URI-R for Pattern 2.1

   In a subsequent HTTP GET request, shown in Figure 12, 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/20010911203610/http://a.example.org/ HTTP/1.1
   Host: arxiv.example.net/
   Accept-Datetime: Tue, 11 Sep 2001 20:35:00 GMT
   Connection: close

              Figure 12: User Agent Requests Selected Memento

   The response has a "200 OK" HTTP status code.  As per Section 2.3,
   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 12 is shown in



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   Figure 13.  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: Tue, 11 Sep 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: 23364
   Content-Type: text/html;charset=utf-8
   Connection: close

              Figure 13: Response from URI-M for Pattern 2.1

4.2.2.  Pattern 2.2 - URI-R<>URI-G ; 200-style negotiation ; distinct
        URI-M for Mementos

   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 10 has a "200 OK" HTTP status code, and the "Content-Location"
   header conveys the URI-M of the selected Memento.  As per
   Section 2.3, 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 10 is shown in
   Figure 14.









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   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/20010911203610/http://a.example.org/
   Memento-Datetime: Tue, 11 Sep 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: 23364
   Content-Type: text/html; charset=UTF-8
   Connection: close

           Figure 14: Response from URI-G<>URI-R for Pattern 2.2

   In a subsequent request, which is the same as Figure 10 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 14.

4.2.3.  Pattern 2.3 - URI-R<>URI-G ; 200-style negotiation ; no distinct
        URI-M for Mementos

   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 10 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.  As per Section 2.3, 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 10 is shown in
   Figure 15.




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   HTTP/1.1 200 OK
   Date: Thu, 21 Jan 2010 00:09:40 GMT
   Server: Apache-Coyote/1.1
   Vary: accept-datetime
   Memento-Datetime: Tue, 11 Sep 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: 23364
   Content-Type: text/html; charset=UTF-8
   Connection: close

           Figure 15: Response from URI-G<>URI-R for Pattern 2.3

   In a subsequent request, which is the same as Figure 10 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.

4.3.  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 4: Pattern 3

   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.  As per Section 2.3, the use of
   Memento response headers and links in responses from such a stable
   resource is as follows:





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   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 16 shows a response to an HTTP HEAD request for the resource
   with URI-R http://a.example.org/ that has been stable since March
   20th 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: 0
   Content-Type: text/plain; charset=UTF-8
   Connection: close

            Figure 16: 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.

   +----------+----------------+----------+---------+------------------+
   |  Pattern |    Original    | TimeGate | Memento |    Negotiation   |
   |          |    Resource    |          |         |       Style      |
   +----------+----------------+----------+---------+------------------+
   |  Pattern |      URI-R     |     -    |  URI-M  |         -        |
   |     4    |                |          |         |                  |
   +----------+----------------+----------+---------+------------------+




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                            Table 5: Pattern 4

   Servers that host such Mementos without TimeGates can still support
   the Memento framework by providing the appropriate Memento headers
   and links.  As per Section 2.3, these are 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 17 shows a response to an HTTP HEAD request for the Memento
   with URI-M
   http://arxiv.example.net/web/20010911203610/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/) that the server is aware of.  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.

   HTTP/1.1 200 OK
   Date: Thu, 21 Jan 2010 00:09:40 GMT
   Server: Apache-Coyote/1.1
   Memento-Datetime: Tue, 11 Sep 2001 20:36:10 GMT
   Link: <http://a.example.org/>; rel="original",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org/>
      ; rel="first last memento"
      ; datetime="Tue, 11 Sep 2001 20:36:10 GMT"
   Content-Length: 0
   Content-Type: text/plain; charset=UTF-8
   Connection: close

            Figure 17: Response from URI-M<>URI-R for Pattern 4







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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

   If possible, a server SHOULD also provide a "timegate" link in
   responses to requests for an Original Resource that the server knows
   used to exist, but no longer provides a current representation.  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 18.

   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
   Connection: close
   Content-Type: text/html; charset=iso-8909-1


   Figure 18: Response from an Original Resource that not 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:






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   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 SHOULD 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.  The use
      of Memento headers is as per Section 2.3.

   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.

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 in the Memento column of Section 2.3.

   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 SHOULD 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 19 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 20.  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 20, the value of the "Location"
   header is the same as in the original response; it identifies an



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   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 21.  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 19: Response is a redirect


   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 20: Response is a Memento of a redirect; leads to an Original
                                 Resource

















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   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 21: 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 other Memento headers
   is as described in the Memento column of Section 2.3.

   For example, Figure 22 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 23.  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.

   HTTP/1.1 404 Not Found
   Date: Fri, 11 Apr 2008 00:06:50 GMT
   Server: Apache
   Content-Length: 0
   Content-Type: text/plain; charset=UTF-8
   Connection: close

                       Figure 22: Response is a 404









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   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: 0
   Content-Type: text/plain; charset=UTF-8
   Connection: close

                 Figure 23: Response is a Memento of a 404

4.5.6.  Sticky "Memento-Datetime" value for Mementos

   The provision of a "Memento-Datetime" in a response entails a promise
   that the response is frozen in time.  As a consequence, the "Memento-
   Datetime" header associated with a Memento MUST be "sticky" in the
   following ways:

   o  The server that originally assigns the "Memento-Datetime" header
      and value to a specific response MUST retain that header in all
      future responses to HTTP requests (with or without "Accept-
      Datetime" header) that occur against the Memento after the time of
      the original assignment of the header, and it MUST NOT change its
      associated value.

   o  Applications that mirror Mementos at a different URI MUST retain
      the "Memento-Datetime" header and MUST NOT change its value unless
      mirroring involves a meaningful state change.  This allows, for
      example, duplicating a web archive at a new location while
      preserving the value of the "Memento-Datetime" header of the
      archived resources.  In this example, 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



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   and links that allow 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 SHOULD 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 9 and engages in datetime negotiation with the
   assumed TimeGate in the manner shown in Figure 10.  But instead of
   receiving a response as shown in Figure 11, it receives the one shown
   below in Figure 24.  Such a response is umabiguosuly recognizable as
   coming from an intermediate resource.

   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 24: Redirecting Resource redirects to a TimeGate


5.  TimeMaps

   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;




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   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 SHOULD use an "Accept" request header with a value set to
   "application/link-format".  This is shown in Figure 25.

   GET /timemap/http://a.example.org/ HTTP/1.1
   Host: arxiv.example.net
   Accept: application/link-format;q=1.0
   Connection: close

                     Figure 25: 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



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   provided in the entity-body of the response.  Such a response is
   shown in Figure 26

   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 26: Response from a TimeMap

5.1.  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 27.

   o  Paging Timemap - The number of available Mementos can require
      introducing multiple TimeMaps that can be paged.  An example is
      shown in Figure 28.  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



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   "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 27: Index TimeMap


   <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 28: Paging TimeMap



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5.2.  Mementos for TimeMaps

   A TimeMap can itself act as an Original Resource for which a TimeGate
   and Mementos may exist.  Hence, as per Section 2.3, 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 29 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.

   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 29: TimeMap links to the Original Resource it is about


6.  IANA Considerations

   This memo requires IANA to register the Accept-Datetime and Memento-
   Datetime HTTP headers defined in Section 2.1.1 in the appropriate
   IANA registry.

   This memo requires IANA to register the Relation Types "original",
   "timegate", "timemap", and "memento" defined in Section 2.2 in the
   appropriate IANA registry.




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   This memo requires IANA to register the "datetime" and "license"
   attributes for the "memento" Relation Type, as defined in
   Section 2.2.4, in the appropriate IANA registry.

   This memo requires IANA to register the "from" and "until" attributes
   for the "timemap" Relation Type, as defined in Section 2.2.4, in the
   appropriate IANA registry.


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 a
   "timegate" 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.


8.  Changelog

   v06 2013-02-14 HVDS MLN RS draft-vandesompel-memento-06

   o  Major overhaul of the presentation of the specification.

   o  Specification of patterns whereby URI-R=URI-G and with both 200
      and 302 negotation style.

   o  Removal of Discovery section to increase focus on datetime
      negotation aspects.

   v05 2012-09-01 HVDS MLN RS draft-vandesompel-memento-05





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   o  Clarified the section on Memento Relation Types.

   o  Re-introduced "license" attribute for "memento" Relation Type as
      it will become essential for IIPC.

   o  Introduced from and until attributes for "timemap" links to
      accomodate paged TimeMap cases.

   o  Introduced the notion of Redirecting Resource and inserted related
      information in various sections.

   o  Added discovery of Mementos via host-meta.

   o  Corrected ambiguous uses of the term "representation".

   v04 2012-05-18 HVDS MLN RS draft-vandesompel-memento-04

   o  Removed the possibility to use an interval indicator in an Accept-
      Datetime header as no one is implementing it.

   o  Corrected typo in Other Relation Types table.

   o  Added TimeMap examples to illustrate index of TimeMaps and TimeMap
      paging.

   o  Changed Discovery component from using robots.txt with Memento-
      specific add-ons to well-known URI and host-meta.

   o  Removed "embargo" and "license" attributes for links with a
      "memento" Relation Type because no one is using them.

   v04 2011-12-20 HVDS MLN RS draft-vandesompel-memento-03

   o  Added description of Mementos of HTTP responses with 3XX, 4XX and
      5XX status code.

   o  Clarified that a TimeGate must not use the "Memento-Datetime"
      header.

   o  Added wording to warn for possible cache problems with Memento
      implementations that choose to have an Original Resource and and
      its TimeGate coincide.

   v03 2011-05-11 HVDS MLN RS draft-vandesompel-memento-02

   o  Added scenario in which a TimeGate redirects to another TimeGate.





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   o  Reorganized TimeGate section to better reflect the difference
      between requests with and without interval indicator.

   o  Added recommendation to provide "memento" links to Mementos in the
      vicinity of the preferred interval provided by the user agent, in
      case of a 406 response.

   o  Removed TimeMap Feed material from the Discovery section as a
      result of discussions regarding (lack of) scalability of the
      approach with representatives of the International Internet
      Preservation Consortium.  An alternative approach to support batch
      discovery of Mementos will be specified.

   v02 2011-04-28 HVDS MLN RS draft-vandesompel-memento-01

   o  Introduced wording and reference to indicate a Memento is a
      FixedResource.

   o  Introduced "Sticky Memento-Datetime" notion and clarified wording
      about retaining "Memento-Datetime" headers and values when a
      Memento is mirrored at different URI.

   o  Introduced section about handling both datetime and regular
      negotiation.

   o  Introduced section about Mementos Without TimeGate.

   o  Made various changes in the section Relation Type "memento",
      including addition of "license" and "embargo" attributes, and
      clarification of rules regarding the use of "memento" links.

   o  Moved section about TimeMaps inside the Datetime Negotiation
      section, and updated it.

   o  Restarted the Discovery section from scratch.

   v01 2010-11-11 HVDS MLN RS First public version
   draft-vandesompel-memento-00

   v00 2010-10-19 HVDS MLN RS Limited circulation version

   2010-07-22 HVDS MLN First internal version


9.  Acknowledgements

   The Memento effort is funded by the Library of Congress.  Many thanks
   to Kris Carpenter Negulescu, Michael Hausenblas, Erik Hetzner, Larry



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   Masinter, Gordon Mohr, Mark Nottingham, David Rosenthal, Ed Summers,
   James Anderson, Tim Starling, Martin Klein for feedback.  Many thanks
   to Samuel Adams, Scott Ainsworth, Lyudmilla Balakireva, Frank McCown,
   Harihar Shankar, Brad Tofel for early implementations.


10.  References

10.1.  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.

   [RFC4151]  Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
              RFC 4151, October 2005.

   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, December 2005.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              April 2010.

   [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.

   [RFC6415]  Hammer-Lahav, E. and B. Cook, "Web Host Metadata",
              RFC 6415, October 2011.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, August 2012.

10.2.  Informative References

   [Fitch]    Fitch, "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>.

   [I-D.masinter-dated-uri]
              Masinter, L., "The 'tdb' and 'duri' URI schemes, based on



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              dated URIs", draft-masinter-dated-uri-10 (work in
              progress), January 2012.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [W3C.REC-aww-20041215]
              Jacobs and Walsh, "Architecture of the World Wide Web",
              December 2004, <http://www.w3.org/TR/webarch/>.

   [W3C.gen-ont-20090420]
              Berners-Lee, "Architecture of the World Wide Web",
              April 2009, <http://www.w3.org/2006/gen/ont>.


Authors' Addresses

   Herbert VandeSompel
   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/




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