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Versions: 00 01 02 03 04 05 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: November 25, 2012                       Old Dominion University
                                                            R. Sanderson
                                          Los Alamos National Laboratory
                                                            May 24, 2012


   HTTP framework for time-based access to resource states -- Memento
                      draft-vandesompel-memento-04

Abstract

   The HTTP-based Memento framework bridges the present and past Web by
   interlinking current resources with resources that encapsulate their
   past.  It facilitates obtaining representations of prior states of a
   resource, available from archival resources in Web archives or
   version resources in content management systems, by leveraging the
   resource's URI and a preferred datetime.  To this end, the framework
   introduces datetime negotiation (a variation on content negotiation),
   and new Relation Types for the HTTP "Link" header aimed at
   interlinking resources with their archival/version resources.  It
   also introduces various discovery mechanisms that further support
   bridging the present and past Web.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 November 25, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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.  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.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Notational Conventions . . . . . . . . . . . . . . . . . .  6
   2.  The Memento Framework, Datetime Negotiation component:
       HTTP headers, HTTP Link Relation Types . . . . . . . . . . . .  7
     2.1.  HTTP Headers . . . . . . . . . . . . . . . . . . . . . . .  7
       2.1.1.  Accept-Datetime, Memento-Datetime  . . . . . . . . . .  7
         2.1.1.1.  Values for Accept-Datetime . . . . . . . . . . . .  8
         2.1.1.2.  Values for Memento-Datetime  . . . . . . . . . . .  9
       2.1.2.  Vary . . . . . . . . . . . . . . . . . . . . . . . . .  9
       2.1.3.  Location . . . . . . . . . . . . . . . . . . . . . . .  9
       2.1.4.  Link . . . . . . . . . . . . . . . . . . . . . . . . .  9
     2.2.  Link Header Relation Types . . . . . . . . . . . . . . . .  9
       2.2.1.  Memento Framework Relation Types . . . . . . . . . . . 10
         2.2.1.1.  Relation Type "original" . . . . . . . . . . . . . 10
         2.2.1.2.  Relation Type "timegate" . . . . . . . . . . . . . 11
         2.2.1.3.  Relation Type "timemap"  . . . . . . . . . . . . . 11
         2.2.1.4.  Relation Type "memento"  . . . . . . . . . . . . . 12
       2.2.2.  Other Relation Types . . . . . . . . . . . . . . . . . 13
   3.  The Memento Framework, Datetime Negotiation component:
       HTTP Interactions  . . . . . . . . . . . . . . . . . . . . . . 14
     3.1.  Interactions with an Original Resource . . . . . . . . . . 16
       3.1.1.  Step 1: User Agent Requests an Original Resource . . . 16
       3.1.2.  Step 2: Server Responds to a Request for an
               Original Resource  . . . . . . . . . . . . . . . . . . 16
         3.1.2.1.  Original Resource is an Appropriate Memento  . . . 17
         3.1.2.2.  Server Exists and Original Resource Used to
                   Exist  . . . . . . . . . . . . . . . . . . . . . . 19
         3.1.2.3.  Missing or Inadequate "timegate" Link in
                   Original Server's Response . . . . . . . . . . . . 19
     3.2.  Interactions with a TimeGate . . . . . . . . . . . . . . . 20
       3.2.1.  Step 3: User Agent Negotiates with a TimeGate  . . . . 20
       3.2.2.  Step 4: Server Responds to Negotiation with
               TimeGate . . . . . . . . . . . . . . . . . . . . . . . 20



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         3.2.2.1.  Successful Scenario  . . . . . . . . . . . . . . . 20
         3.2.2.2.  Multiple Matching Mementos . . . . . . . . . . . . 22
         3.2.2.3.  TimeGate Redirects to another TimeGate . . . . . . 23
         3.2.2.4.  Accept-Datetime and other Accept Headers
                   Provided . . . . . . . . . . . . . . . . . . . . . 24
         3.2.2.5.  Accept-Datetime Unparseable  . . . . . . . . . . . 25
         3.2.2.6.  Accept-Datetime Not Provided . . . . . . . . . . . 25
         3.2.2.7.  TimeGate Does Not Exist  . . . . . . . . . . . . . 25
         3.2.2.8.  HTTP Methods other than HEAD/GET . . . . . . . . . 25
       3.2.3.  Recognizing a TimeGate . . . . . . . . . . . . . . . . 26
     3.3.  Interactions with a Memento  . . . . . . . . . . . . . . . 27
       3.3.1.  Step 5: User Agent Requests a Memento  . . . . . . . . 27
       3.3.2.  Step 6: Server Responds to a Request for a Memento . . 27
         3.3.2.1.  Common Scenario  . . . . . . . . . . . . . . . . . 27
         3.3.2.2.  Memento of a 3XX Response  . . . . . . . . . . . . 29
         3.3.2.3.  Memento of Responses with Other HTTP Status
                   Codes  . . . . . . . . . . . . . . . . . . . . . . 31
         3.3.2.4.  Mementos Without a TimeGate  . . . . . . . . . . . 32
         3.3.2.5.  Memento Does not Exist . . . . . . . . . . . . . . 33
       3.3.3.  Recognizing a Memento  . . . . . . . . . . . . . . . . 34
     3.4.  Interactions with a TimeMap  . . . . . . . . . . . . . . . 34
       3.4.1.  User Agent Requests a TimeMap  . . . . . . . . . . . . 35
       3.4.2.  Server Responds to a Request for a TimeMap . . . . . . 35
   4.  The Memento Framework, Discovery of TimeGates and TimeMaps . . 38
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 41
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 41
   7.  Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 41
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 43
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 43
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 44
   Appendix A.  Appendix B: A Sample, Successful Memento
                Request/Response cycle  . . . . . . . . . . . . . . . 44
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 46

















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1.  Introduction

1.1.  Terminology

   This specification uses the terms "resource", "request", "response",
   "entity", "entity-body", "entity-header", "content negotiation",
   "client", "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
      is desired.

   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 Tj is a resource
      that encapsulates the state that the Original Resource had at time
      Tj.

   o  TimeGate: A TimeGate for an Original Resource is a resource that
      is capable of negotiation to allow selective, datetime-based,
      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.  Purpose

   The state of an Original Resource may change over time.
   Dereferencing its URI at any specific moment in time during its
   existence yields a representation of its then current state.
   Dereferencing its URI at any time past its existence no longer yields
   a meaningful representation, if any.  Still, in both cases, resources
   may exist that encapsulate prior states of the Original Resource.
   Each such resource, named a Memento, has its own URI that, when
   dereferenced, returns a representation of a prior state of the
   Original Resource.  Mementos may, for example, exist in Web archives,
   Content Management Systems, or Revision Control Systems.

   Examples are:

   Mementos for Original Resource http://www.ietf.org/ :




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   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/

   In the abstract, Memento introduces a mechanism to access versions of
   Web resources that:

   o  Is fully distributed in the sense that resource versions may
      reside on multiple hosts, and that any such host 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
      [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 a resource identified by
   URI-R as it existed at some time Tj.  Note the relationship with the
   ability to identify a the state of a resource at some datetime Tj by
   means of a URI as intended by the proposed Dated URI scheme
   [I-D.masinter-dated-uri].



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   2.  A bridge from the present to the past, consisting of:

   o  An appropriately typed link from a resource identified by URI-R to
      an associated TimeGate identified by URI-G, which is aware of (at
      least part of the) version history of the resource identified by
      URI-R;

   o  The ability to content negotiate in the datetime dimension with
      the TimeGate identified by URI-G, as a means to obtain a
      representation of the state that the resource identified by URI-R
      had at some datetime Tj.

   3.  A bridge from the past to the present, consisting of an
   appropriately typed link from a resource identified by URI-M, which
   encapsulates the state a resource identified by URI-R had at some
   datetime Tj, to the resource identified by URI-R.

   Section 2 and Section 3 of this document are concerned with
   specifying an instantiation of these abstractions for resources that
   are identified by HTTP(S) URIs, whereas Section 4 details an approach
   to support batch discovery of TimeGates and TimeMaps that is based on
   well-known URI [RFC5785] and host-meta [RFC6415].

1.3.  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.

   o  When scenarios are described that involve multiple Mementos,
      URI-M0 denotes the URI of the first Memento known to the
      responding server, URI-Mn denotes the URI of the most recent known
      Memento, URI-Mj denotes the URI of the selected Memento, URI-Mi
      denotes the URI of the Memento that is temporally previous to the
      selected Memento, and URI-Mk denotes the URI of the Memento that
      is temporally after the selected Memento.  The respective
      datetimes for these Mementos are T0, Tn, Tj, Ti, and Tk; it holds
      that T0 <= Ti <= Tj <= Tk <= Tn.



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2.  The Memento Framework, Datetime Negotiation component: HTTP headers,
    HTTP Link Relation Types

   The Memento framework is concerned 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",
   "Memento-Datetime"), introduces new values for two existing headers
   ("Vary", "Link"), and uses an existing header ("Location") without
   modification.  All these headers are described below.  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 used by a user agent to
   indicate it wants to retrieve a representation of a Memento that
   encapsulates a past state of an Original Resource.  To that end, the
   "Accept-Datetime" header is conveyed in an HTTP GET/HEAD request
   issued against a TimeGate for an Original Resource, and its value
   indicates the datetime of the desired past state of the Original
   Resource.  The "Accept-Datetime" request header has no defined
   meaning for HTTP methods other than HEAD and GET.

   The "Memento-Datetime" response header is used by a server to
   indicate that the response contains a representation of a Memento,
   and its value expresses the datetime of the state of an Original
   Resource that is encapsulated in that Memento.  The URI of that
   Original Resource is provided in the response, as the Target IRI (see
   [RFC5988]) 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 resource constitutes a promise that the resource is stable
   and that its state will no longer change.  This means that, in terms
   of the Ontology for Relating Generic and Specific Information
   Resources (see W3C.gen-ont-20090420 [W3C.gen-ont-20090420]), a
   Memento is a FixedResource.

   As a consequence, "Memento-Datetime" headers associated with a
   Memento MUST be "sticky" in the following ways:





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   o  The server that originally assigns the "Memento-Datetime" header
      and value MUST retain that header in all responses to HTTP HEAD/
      GET 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 NOT
      change the "Memento-Datetime" header and value of those Mementos
      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 sticky.

2.1.1.1.  Values for Accept-Datetime

   Values for the "Accept-Datetime" header 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).

   Example of an "Accept-Datetime" request header:

   Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT

   The user agent uses the MANDATORY datetime value to convey its
   preferred datetime for a Memento.


   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







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2.1.1.2.  Values for Memento-Datetime

   Values for the "Memento-Datetime" headers MUST be datetimes expressed
   according to the rfc1123-date construction rule of the BNF in
   Figure 1; they MUST be represented in Greenwich Mean Time (GMT).

   An example "Memento-Datetime" response header:

   Memento-Datetime: Wed, 30 May 2007 18:47:52 GMT

2.1.2.  Vary

   The "Vary" response header is used in responses to indicate the
   dimensions in which content negotiation was successfully applied.
   This header is used in the Memento framework to indicate both whether
   datetime negotiation was applied or is supported by the responding
   server.

   For example, this use of the "Vary" header indicates that datetime is
   the only dimension in which negotiation was applied:

   Vary: negotiate, accept-datetime

   The use of the "Vary" header in this example shows that both datetime
   negotiation, and media type content negotiation were applied:

   Vary: negotiate, accept-datetime, accept

2.1.3.  Location

   The "Location" header is used as defined in [RFC2616].  Examples are
   given in Section 3 below.

2.1.4.  Link

   The "Link" response header is specified in [RFC5988].  The Memento
   framework introduces new Relation Types to convey typed links among
   Original Resources, TimeGates, Mementos, and TimeMaps.  Already
   existing Relation Types, among others, aimed at supporting navigation
   among a series of ordered resources may also be used in the Memento
   framework.  This is detailed in Link Header Relation Types
   (Section 2.2), below.

2.2.  Link Header Relation Types

   The "Link" header specified in [RFC5988] is semantically equivalent
   to the "<LINK>" element in HTML, as well as the "atom:link" feed-
   level element in Atom [RFC4287].  By default, the origin of a link



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   expressed by an entry in a "Link" header (named Context IRI in
   [RFC5988]) is the IRI of the requested resource.  This default can be
   overwritten using the "anchor" attribute in the entry.

2.2.1.  Memento Framework Relation Types

   The Relation Types used in the Memento framework are listed in the
   remainder of this section, and their use is summarized in the below
   table.  Appendix A shows a Memento request/response cycle that uses
   all the Relation Types that are introduced here.

   +----------+-------------------+---------------------+--------------+
   | Relation | Original Resource |       TimeGate      |    Memento   |
   |   Type   |                   |                     |              |
   +----------+-------------------+---------------------+--------------+
   | original |   NA, except see  |     REQUIRED, 1     |  REQUIRED, 1 |
   |          |  Section 3.1.2.1  |                     |              |
   | timegate | RECOMMENDED, 0 or | REQUIRED, 1 in case | RECOMMENDED, |
   |          |        more       |  of Section 3.2.2.3 |   0 or more  |
   |  timemap |         NA        |  RECOMMENDED, 0 or  | RECOMMENDED, |
   |          |                   |         more        |   0 or more  |
   |  memento |   NA, except see  | REQUIRED, 1 or more |  REQUIRED, 1 |
   |          |  Section 3.1.2.1  |                     |    or more   |
   +----------+-------------------+---------------------+--------------+

                    Table 1: The use of Relation Types

2.2.1.1.  Relation Type "original"

   "original" -- A "Link" header entry with a Relation Type of
   "original" is used to point from a TimeGate or a Memento to their
   associated Original Resource.  In both cases, an entry with the
   "original" Relation Type MUST occur exactly once in a "Link" header.
   Details for the entry are as follows:

   o  Context IRI: URI-G, URI-M

   o  Target IRI: URI-R

   o  Relation Type: "original"

   o  Use: REQUIRED

   o  Cardinality: 1







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2.2.1.2.  Relation Type "timegate"

   "timegate" -- A "Link" header entry with a Relation Type of
   "timegate" is used to point both from an Original Resource or a
   Memento to a TimeGate for the Original Resource.  In both cases, the
   use of an entry with the "timegate" Relation Type is RECOMMENDED.
   Since more than one TimeGate can exist for any Original Resource,
   multiple entries with a "timegate" Relation Type MAY occur, each with
   a distinct Target IRI.  Since a TimeGate has no mime type, the "type"
   attribute MUST NOT be used on Links with a "timegate" Relation Type.
   Details for the entry are as follows:

   o  Context IRI: URI-R or URI-Mj

   o  Target IRI: URI-G

   o  Relation Type: "timegate"

   o  Use: RECOMMENDED

   o  Cardinality: 0 or more

   In the special case (see Section 3.2.2.3) where a TimeGate redirects
   to another TimeGate for the Original Resource, a "Link" header entry
   with a Relation Type of "timegate" MUST be used to point from the
   former to the latter.

2.2.1.3.  Relation Type "timemap"

   "timemap" -- A "Link" header entry with a Relation Type of "timemap"
   is used to point from both a TimeGate or a Memento to a TimeMap
   resource from which a list of Mementos known to the responding server
   is available.  Use of an entry with the "timemap" Relation Type is
   RECOMMENDED, and, since multiple serializations of a TimeMap are
   possible, multiple entries with a "timemap" Relation Type MAY occur,
   each with a distinct Target IRI, and each with a MANDATORY "type"
   attribute to convey the mime type of the TimeMap serialization.
   Details for the entry are as follows:

   o  Context IRI: URI-G or URI-Mi

   o  Target IRI: URI-T

   o  Relation Type: "timemap"

   o  Target Attribute: "type"





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   o  Use: RECOMMENDED

   o  Cardinality: 0 or more

   Further details about TimeMap serializations are provided in
   Section 3.4.

2.2.1.4.  Relation Type "memento"

   "memento" -- A "Link" header entry with a Relation Type of "memento"
   is used to point from both a TimeGate and a Memento to various
   Mementos for an Original Resource.  This link 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).  The link SHOULD also include a "type"
   attribute to convey the mime type of the Memento that is the target
   of the link.  Use of entries with the "memento" Relation Type is
   REQUIRED and it MUST be as follows:

   For all responses to HTTP HEAD/GET requests issued against a TimeGate
   or a Memento in which a Memento is selected or served by the
   responding server:

   o  One "memento" link MUST be included that has as Target IRI the URI
      of the Memento that was selected or served;

   o  One "memento" link MUST be included that has as Target IRI the URI
      of the temporally first Memento known to the responding server;

   o  One "memento" link MUST be included that has as Target IRI the URI
      of the temporally most recent Memento known to the responding
      server.

   o  One "memento" link SHOULD be included that has as Target IRI the
      URI of the Memento that is previous to the selected Memento in the
      temporal series of all Mementos (sorted by ascending "Memento-
      Datetime" values) known to the server;

   o  One "memento" link SHOULD be included that has as Target IRI the
      URI the Memento that is next to the selected Memento in the
      temporal series of all Mementos (sorted by ascending "Memento-
      Datetime" values) known to the server.





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   o  Other "memento" links MAY only be included if both the
      aforementioned previous and next links are provided.  Each of
      these OPTIONAL "memento" links MUST have as Target IRI the URI of
      a Memento other than the ones listed above.

   For all responses to HTTP HEAD/GET requests issued against an
   existing TimeGate or Memento in which no Memento is selected or
   served by the responding server:

   o  One "memento" link MUST be included that has as Target IRI the URI
      of the temporally first Memento known to the responding server;

   o  One "memento" link MUST be included that has as Target IRI the URI
      of the temporally most recent Memento known to the responding
      server.

   o  Other "memento" links MAY be included, and each of these OPTIONAL
      links MUST have as Target IRI the URI of a Memento other than the
      two listed above.

   Note that the Target IRI of some of these links may coincide.  For
   example, if the selected Memento actually is the first Memento known
   to the server, only three distinct "memento" links may result.  The
   value for the "datetime" attribute of these links would be the
   datetimes of the first (equal to selected), next, and most recent
   Memento known to the responding server.

   The summary is as follows:

   o  Context IRI: URI-G, URI-Mj

   o  Target IRI: URI-M

   o  Relation Type: "memento"

   o  Target Attributes: "datetime"

   o  Use: REQUIRED

   o  Cardinality: 1 or more

2.2.2.  Other Relation Types

   Web Linking [RFC5988] allows for the inclusion of links with
   different Relation Types but the same Target IRI, and hence the
   Relation Types introduced by the Memento framework MAY be combined
   with others as deemed necessary.  As the "memento" Relation Type
   focuses on conveying the datetime of a linked Memento, Relation Types



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   that allow navigating among the temporally ordered series of Mementos
   known to a server are of particular importance.  With this regard,
   the Relation Types listed in the below table SHOULD be considered for
   combination with the "memento" Relation Type.  A distinction is made
   between responding servers that can be categorized as systems that
   are the focus of [RFC5829] (such as version control systems) and
   others that can not (such as Web archives).  Note that, in terms of
   [RFC5829], the last Memento (URI-Mn) is the version prior to the
   latest (i.e. current) version.

   +-----------------------------+---------------------+---------------+
   |         Memento Type        |    RFC5829 system   |  non RFC5829  |
   |                             |                     |     system    |
   +-----------------------------+---------------------+---------------+
   |    First Memento (URI-M0)   |        first        |     first     |
   |    Last Memento (URI-Mn)    |         last        |      last     |
   |  Selected Memento (URI-Mj)  |          NA         |       NA      |
   |  Memento prior to selected  | predecessor-version |      prev     |
   |       Memento (URI-Mi)      |                     |               |
   |   Memento next to selected  |  successor-version  |      next     |
   |       Memento (URI-Mk)      |                     |               |
   +-----------------------------+---------------------+---------------+

                    Table 2: The use of Relation Types


3.  The Memento Framework, Datetime Negotiation component: HTTP
    Interactions

   This section describes the HTTP interactions of the Memento framework
   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.  Appendix A shows these HTTP interactions in
   detail for the case where the Original Resource resides on one
   server, whereas both the TimeGate and the Mementos reside on another.
   Scenarios also exist in which all these resources are on the same
   server (for example, Content Management Systems) or on different
   servers (for example, an aggregator of TimeGates).  Note that, in
   Step 2 and Step 6, the HTTP status code of the response is shown as
   "200 OK", but a series of "206 Partial Content" responses could be
   substituted without loss of generality.









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   1: UA --- HTTP GET/HEAD; Accept-Datetime: Tj ---------------> URI-R
   2: UA <-- HTTP 200; Link: URI-G ----------------------------- URI-R
   3: UA --- HTTP GET/HEAD; Accept-Datetime: Tj ---------------> URI-G
   4: UA <-- HTTP 302; Location: URI-Mj; Vary; Link:
         URI-R,URI-T,URI-M0,URI-Mn,URI-Mi,URI-Mj,URI-Mk -------- URI-G
   5: UA --- HTTP GET URI-Mj; Accept-Datetime: Tj -------------> URI-Mj
   6: UA <-- HTTP 200; Memento-Datetime: Tj; Link:
         URI-R,URI-T,URI-G,URI-M0,URI-Mn,URI-Mi,URI-Mj,URI-Mk -- URI-Mj

             Figure 2: Typical Memento request/response chain

   o  Step 1: In order to determine what the URI is of a TimeGate for an
      Original Resource, the user agent issues an HTTP HEAD/GET request
      against the URI of the Original Resource (URI-R).

   o  Step 2: The entity-header of 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 its URI-G
      thereby including an "Accept-Datetime" HTTP header with a value of
      the datetime of the desired prior state of the Original Resource.

   o  Step 4: The entity-header of the response from URI-G includes a
      "Location" header pointing at the URI of a Memento (URI-Mj) for
      the Original Resource.  In addition, the entity-header contains an
      HTTP "Link" header with a Relation Type of "original" pointing at
      the Original Resource, and an HTTP "Link" header with a Relation
      Type of "timemap" pointing at a TimeMap (URI-T).  Also HTTP Links
      pointing at various Mementos are provided using the "memento"
      Relation Type, as specified in Section 2.2.1.4.

   o  Step 5: The user agent issues an HTTP GET request against the
      URI-Mj of a Memento, obtained in Step 4.

   o  Step 6: The entity-header of the response from URI-Mj includes a
      "Memento-Datetime" HTTP header with a value of the datetime of the
      Memento.  It also contains an HTTP "Link" header with a Relation
      Type of "original" pointing at the Original Resource, with a
      Relation Type of "timegate" pointing at a TimeGate associated with
      the Original Resource, and with a Relation Type of "timemap"
      pointing at a TimeMap.  The state that is expressed by the
      representation provided in the response is the state the Original
      Resource had at the datetime expressed in the "Memento-Datetime"
      header.  This response also includes HTTP Links with a "memento"
      Relation Type pointing at various Mementos, as specified in
      Section 2.2.1.4.



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   The following sections detail the specifics of HTTP interactions with
   Original Resources, TimeGates, Mementos, and TimeMaps under various
   conditions.

3.1.  Interactions with an Original Resource

   This section details HTTP GET/HEAD requests targeted at an Original
   Resource (URI-R).

3.1.1.  Step 1: User Agent Requests an Original Resource

   In order to try and discover a TimeGate for the Original Resource,
   the user agent SHOULD issue an HTTP HEAD or GET request against the
   Original Resource's URI.  Use of the "Accept-Datetime" header in the
   HTTP HEAD/GET request is OPTIONAL.

   Figure 3 shows the use of HTTP HEAD indicating the user agent is not
   interested in retrieving a representation of the Original Resource,
   but only in determining a TimeGate for it.  It also shows the use of
   the "Accept-Datetime" header anticipating that the user agent will
   set it for the entire duration of a Memento request/response cycle.

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

              Figure 3: User Agent Requests Original Resource

3.1.2.  Step 2: Server Responds to a Request for an Original Resource

   The response of the Original Resource's server to the user agent's
   HTTP HEAD/GET request of Step 1, for the case where the Original
   Resource exists, is as it would be in a regular HTTP request/response
   cycle, but in addition MAY include a HTTP "Link" header with a
   Relation Type of "timegate" that conveys the URI of the Original
   Resource's TimeGate as the Target IRI of the Link.  Multiple HTTP
   Links with a relation type of "timegate" MAY be provided to
   accommodate situations in which the server is aware of multiple
   TimeGates for an Original Resource.  The actual Target IRI provided
   in the "timegate" Link may depend on several factors including the
   datetime provided in the "Accept-Datetime" header, and the IP address
   of the user agent.  A response for this case is illustrated in
   Figure 4.







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   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 4: Server of Original Resource Responds

   Servers that actively maintain archives of their resources SHOULD
   include the "timegate" HTTP "Link" header because this link is an
   important way for a user agent to discover TimeGates for those
   resources.  This includes servers such as Content Management Systems,
   Control Version Systems, and Web servers with associated
   transactional archives Fitch [Fitch].  Servers that do not actively
   maintain archives of their resources MAY include the "timegate" HTTP
   "Link" header as a way to convey a preference for TimeGates for their
   resources exposed by a third party archive.  This includes servers
   that rely on Web archives such as the Internet Archive to archive
   their resources.

   The server of the Original Resource MUST treat requests with and
   without an "Accept-Datetime" header in the same way:

   o  The response MUST either always or never include a HTTP "Link"
      header with an entry that has a "timegate" Relation Type and the
      URI of a TimeGate as the Target IRI.

   o  The entity-body of the response MUST be the same, for user agent
      requests with or without a "Accept-Datetime" header.

3.1.2.1.  Original Resource is an Appropriate Memento

   The "Memento-Datetime" header MAY be applied to an Original Resource
   directly to indicate it is a FixedResource (see W3C.gen-ont-20090420
   [W3C.gen-ont-20090420]), meaning that the state of the Original
   Resource has not changed since the datetime conveyed in the "Memento-
   Datetime" header, and as a promise that it will not change anymore
   beyond it.  This may occur, for example, for certain stable media
   resources on news sites.  In case the user agent's preferred datetime
   is equal to or more recent than the datetime conveyed as the value of
   "Memento-Datetime" in the server's response in Step 2, the user agent
   SHOULD conclude it has located an appropriate Memento, and it SHOULD
   NOT continue to Step 3.

   Figure 5 illustrates such a response to a request for the resource



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   with URI http://a.example.org/pic that has been stable since it was
   created.  Note the use of both the "memento" and "original" Relation
   Types for links that have as Target IRI the URI of the Original
   Resource.

   HTTP/1.1 200 OK
   Date: Thu, 21 Jan 2010 00:02:12 GMT
   Server: Apache
   Link:
    <http://a.example.org/pic>
     ; rel="original memento"
     ; datetime="Fri, 20 Mar 2009 11:00:00 GMT"
   Memento-Datetime: Fri, 20 Mar 2009 11:00:00 GMT
   Content-Length: 255
   Connection: close
   Content-Type: text/html; charset=iso-8909-1


     Figure 5: Response to a request for an Original Resource that was
                        created as a FixedResource

   Cases may also exist in which a resource becomes stable at a certain
   point in its existence, but changed previously.  In such cases, the
   Original Resource may know about a TimeGate that is aware of its
   prior history and hence MAY also include a link with a "timegate"
   Relation Type.  This is illustrated in Figure 6, where the "memento"
   and "original" Relation Types are used as in Figure 5, and the
   existence of a TimeGate to negotiate for Mementos with datetimes
   prior to Fri, 20 Mar 2009 11:00:00 GMT is indicated.

   HTTP/1.1 200 OK
   Date: Thu, 21 Jan 2010 00:02:12 GMT
   Server: Apache
   Link:
    <http://a.example.org/pic>
     ; rel="original memento"
     ; datetime="Fri, 20 Mar 2009 11:00:00 GMT",
    <http://arxiv.example.net/timegate/http://a.example.org/pic>
     ; rel="timegate"
   Memento-Datetime: Fri, 20 Mar 2009 11:00:00 GMT
   Content-Length: 255
   Connection: close
   Content-Type: text/html; charset=iso-8909-1

   Figure 6: Response to a request for an Original Resource that became
                              a FixedResource





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3.1.2.2.  Server Exists and Original Resource Used to Exist

   Servers SHOULD also provide a "timegate" HTTP "Link" header in
   responses to requests for an Original Resource that the server knows
   used to exist, but no longer does.  This allows the use of an
   Original Resource's URI as an entry point to representations of its
   prior states even if the resource itself no longer exists.  A
   server's response for this case is illustrated in Figure 7.

   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/gone>
     ; rel="timegate"
   Content-Length: 255
   Connection: close
   Content-Type: text/html; charset=iso-8909-1


     Figure 7: Response to a request for an Original Resource that not
                               longer exists

   In case the server is not aware of the prior existence of the
   Original Resource, its response SHOULD NOT include a "timegate" HTTP
   Link.  Section 3.1.2.3 details what the user agent's behavior should
   be in such cases.

3.1.2.3.  Missing or Inadequate "timegate" Link in Original Server's
          Response

   A user agent MAY ignore the TimeGate returned in Step 2.  However,
   when engaging in a Memento request/response cycle, a user agent
   SHOULD NOT proceed immediately to Step 3 by using a TimeGate of its
   own preference but rather SHOULD always start the cycle by issuing an
   HTTP GET/HEAD against the Original Resource (Step 1, Figure 3) as it
   is an important way to learn about dedicated or preferred TimeGates
   for the Original Resource.  Also, cases exist in which the response
   in Step 2 will not provide 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;




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   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.  The discovery mechanisms
   described in Section 4 can support the user agent with this regard.

3.2.  Interactions with a TimeGate

   This section details HTTP GET/HEAD requests targeted at a TimeGate
   (URI-G).

3.2.1.  Step 3: User Agent Negotiates with a TimeGate

   In order to negotiate with a TimeGate, the user agent MUST issue a
   HTTP HEAD or GET against its URI, its request MUST include the
   "Accept-Datetime" header to express its datetime preference, and the
   use of that header MUST be as described in Section 2.1.1.1.  The URI
   of the TimeGate may have been provided as the Target IRI of a
   "timegate" HTTP "Link" header in the response from the Original
   Resource (Step 2, Figure 4), or may have resulted from another
   discovery mechanism (see Section 4) or user interaction.  Such a
   request is illustrated in Figure 8.

   GET /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 8: User agent negotiates with TimeGate

3.2.2.  Step 4: Server Responds to Negotiation with TimeGate

   In order to respond to a datetime negotiation request (Step 3,
   Section 3.2.1), the server uses an internal algorithm to select the
   Memento that best meets the user agent's datetime preference, and
   redirects to it.  The exact nature of the selection algorithm is at
   the server's discretion but SHOULD be consistent.  A variety of
   approaches can be used including selecting the Memento that is
   nearest in time (either past or future) or nearest in the past
   relative to the requested datetime.  The commons scenario for
   datetime negotiation with a TimeGate is described in Section 3.2.2.1
   but special cases exist, and they are addressed in Section 3.2.2.2
   through Section 3.2.2.8.

3.2.2.1.  Successful Scenario

   In cases where the TimeGate exists, and the datetime provided in the
   user agent's "Accept-Datetime" header can be parsed, the server
   selects a Memento based on the user agent's datetime preference.  The



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   response MUST have a "302 Found" HTTP status code, and the "Location"
   header MUST be used to convey the URI of the selected Memento.  The
   "Vary" header MUST be provided and it MUST include the "negotiate"
   and "accept-datetime" values to indicate that datetime negotiation
   has taken place.  The "Link" header MUST be provided and contain
   links with Relation Types subject to the considerations described in
   Section 2.2.  The response MUST NOT contain a "Memento-Datetime"
   header.  Such a response is illustrated in Figure 9.

   HTTP/1.1 302 Found
   Date: Thu, 21 Jan 2010 00:06:50 GMT
   Server: Apache
   Vary: negotiate, 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",
    <http://arxiv.example.net/web/20000915112826/http://a.example.org>
      ; rel="first memento"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
    <http://arxiv.example.net/web/20080708093433/http://a.example.org>
      ; rel="last memento"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="memento"; datetime="Tue, 11 Sep 2001 20:36:10 GMT",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="prev memento"; datetime="Tue, 11 Sep 2001 20:30:51 GMT",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="next memento"; datetime="Tue, 11 Sep 2001 20:47:33 GMT"
   Content-Length: 0
   Content-Type: text/plain; charset=UTF-8
   Connection: close

                   Figure 9: Server of TimeGate responds

   If a user agent's "Accept-Datetime" header conveys a datetime that is
   either earlier than the datetime of the first Memento or later than
   the datetime of the most recent Memento known to the server, the
   server's response is as just described yet entails the selection of
   the first or most recent Memento, respectively.

   This is illustrated in Figure 10 that shows the response from a
   TimeGate exposed by a MediaWiki server to a request by a user agent
   that has an "Accept-Datetime: Mon, 31 May 1999 00:00:00 GMT" header.
   Note that a link is provided with a "successor-version" Relation Type
   but not with a "predecessor-version" Relation Type.






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   HTTP/1.1 302 Found
   Server: Apache
   Content-Length: 709
   Content-Type: text/html; charset=utf-8
   Date: Thu, 21 Jan 2010 00:09:40 GMT
   Location:
    http://a.example.org/w/index.php?title=Clock&oldid=1493688
   Vary: negotiate, accept-datetime
   Link: <http://a.example.org/w/Clock>; rel="original",
    <http://a.example.org/Special:TimeMap/http://a.example.org/w/Clock>
      ; rel="timemap",
    <http://a.example.org/w/index.php?title=Clock&oldid=1493688>
      ; rel="first memento"; datetime="Sun, 28 Sep 2003 01:42:00 GMT",
       <http://a.example.org/w/index.php?title=Clock&oldid=1493854>
      ; rel="successor-version memento"
      ; datetime="Tue, 30 Sep 2003 14:28:00 GMT",
    <http://a.example.org/w/index.php?title=Clock&oldid=337446696>
      ; rel="last memento"; datetime="Tue, 12 Jan 2010 19:55:00 GMT"
   Connection: close

    Figure 10: A TimeGate's response to a request for a Memento with a
              datetime earlier than that of the first Memento

3.2.2.2.  Multiple Matching Mementos

   Because the finest datetime granularity expressible using the
   [RFC1123] format used in HTTP is seconds level, cases may occur in
   which a TimeGate server is aware of multiple Mementos that meet the
   user agent's datetime preference.  This may occur in Content
   Management Systems with very high update rates.  The response in this
   case MUST be handled as in Section 3.2.2.1, with the selection of one
   of the matching Mementos.

   As an example, Figure 11 shows a hypothetical response from a
   TimeGate on a MediaWiki server to a request for a Memento for the
   Original Resource http://a.example.org/w/Clock for which two Mementos
   exist for the user agent's preferred datetime.














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   HTTP/1.1 302 Found
   Server: Apache
   Content-Length: 705
   Content-Type: text/html; charset=utf-8
   Date: Thu, 21 Jan 2010 00:09:40 GMT
   Vary: negotiate, accept-datetime
   Location:
    http://a.example.org/w/index.php?title=Clock&oldid=322586071
   Link: <http://a.example.org/w/Clock>; rel="original",
    <http://a.example.org/Special:TimeMap/http://a.example.org/w/Clock>
      ; rel="timemap";type="application/link-format",
    <http://a.example.org/w/index.php?title=Clock&oldid=1493688>
      ; rel="first memento"; datetime="Sun, 28 Sep 2003 01:42:00 GMT",
    <http://a.example.org/w/index.php?title=Clock&oldid=337446696>
      ; rel="last memento"; datetime="Tue, 12 Jan 2010 19:55:00 GMT",
    <http://a.example.org/w/index.php?title=Clock&oldid=322586071>
      ; rel="memento"; datetime="Sun, 31 May 2009 15:43:00 GMT",
    <http://a.example.org/w/index.php?title=Clock&oldid=326164283>
      ; rel="memento successor-version"
      ; datetime="Sun, 31 May 2009 15:43:00 GMT"
    <http://a.example.org/w/index.php?title=Clock&oldid=326164283>
      ; rel="memento predecessor-version"
      ; datetime="Sun, 31 May 2009 15:41:24 GMT"
   Connection: close

      Figure 11: A TimeGate's response to a request that has multiple
                     Mementos with a matching datetime

3.2.2.3.  TimeGate Redirects to another TimeGate

   Cases may exist in which a TimeGate's response entails a redirects to
   another TimeGate, for example, because the responding TimeGate is
   aware that the other TimeGate is able to more precisely respond to a
   client's datetime preference.  In such cases, the TimeGate's response
   MUST have a "302 Found" HTTP status code, and the "Location" header
   MUST be used to convey the URI of the other TimeGate.  The "Vary"
   header MUST be provided and it MUST include the "negotiate" and
   "accept-datetime" values to indicate that, although datetime
   negotiation has not taken place, the responding TimeGate is capable
   of it.  The "Link" header MUST be provided and contain links with
   Relation Types subject to the considerations described in
   Section 2.2.  Specifically, the use of links with a "memento"
   Relation Type MUST follow the rules for the case where no Memento is
   selected by the responding server (Section 2.2.1.4).  Also, a link
   with a "timegate" Relation Type MUST be provided that has as Target
   IRI the URI of the TimeGate to which the current TimeGate is
   redirecting the client.  The response MUST NOT contain a "Memento-
   Datetime" header.



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   A response in which the client is redirected by TimeGate
   http://arxiv.example.net/timegate/http://a.example.org to TimeGate
   http://otherarxiv.example.com/timegate/http://a.example.org for the
   Original Resource http://a.example.org is illustrated in Figure 12.
   Note the URI of the latter TimeGate in both the "Location" and "Link"
   header, in the latter case as the Target IRI of a "timegate" link.
   Note also that the "memento" and "timemap" links in this response
   reflect the knowledge of the responding TimeGate, not of the remote
   TimeGate.

   HTTP/1.1 302 Found
   Date: Thu, 21 Jan 2010 00:06:50 GMT
   Server: Apache
   Vary: negotiate, accept-datetime
   Location:
    http://otherarxiv.example.com/timegate/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",
    <http://arxiv.example.net/web/20000915112826/http://a.example.org>
      ; rel="first memento"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
    <http://arxiv.example.net/web/20080708093433/http://a.example.org>
      ; rel="last memento"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
    <http://otherarxiv.example.com/timegate/http://a.example.org>
      ; rel="timegate"
   Content-Length: 0
   Content-Type: text/plain; charset=UTF-8
   Connection: close

             Figure 12: TimeGate redirects to another TimeGate

3.2.2.4.  Accept-Datetime and other Accept Headers Provided

   When interacting with a TimeGate, 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 HTTP Links with a "memento" Relation Type
   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.




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3.2.2.5.  Accept-Datetime Unparseable

   In case, in Step 3, a user agent conveys a value for the "Accept-
   Datetime" request header that does not conform to the accept-dt-value
   construction rule of the BNF in Figure 1, the TimeGate server's
   response MUST have a "400 Bad Request" HTTP status code.  The use of
   the "Vary" header MUST be as described in Section 3.2.2.1.  The use
   of the "Link" header MUST be as described in Section 2.2.
   Specifically, the use of links with a "memento" Relation Type MUST
   follow the rules for the case where no Memento is selected by the
   responding server, i.e. only "memento" links to the first and most
   recent Mementos MUST be provided (Section 2.2.1.4).

3.2.2.6.  Accept-Datetime Not Provided

   In case, in Step 3, a user agent issues a request to a TimeGate and
   fails to include an "Accept-Datetime" request header, the response
   MUST be handled as in Section 3.2.2.1, with a selection of the most
   recent Memento known to the responding server.

3.2.2.7.  TimeGate Does Not Exist

   Cases may occur in which a user agent issues a request against a
   TimeGate that does not exist.  This may, for example, occur when a
   user agent uses internal knowledge to construct the URI of an
   assumed, yet non-existent TimeGate.  In these cases, the response
   from the target server MUST have a "404 Not Found" HTTP status code,
   and SHOULD include a "Vary" header that includes the "negotiate" and
   "accept-datetime" values as an indication that, generally, the server
   is capable of datetime negotiation.  The response MUST NOT include a
   "Link" header with any of the Relation Types introduced in
   Section 2.2.1, and it MUST NOT contain a "Memento-Datetime" header.

3.2.2.8.  HTTP Methods other than HEAD/GET

   In the above, the safe HTTP methods GET and HEAD are described for
   TimeGates.  TimeGates MAY support the safe HTTP methods OPTIONS and
   TRACE in the way described in [RFC2616].  Unsafe HTTP methods (i.e.
   PUT, POST, DELETE) MUST NOT be supported by a TimeGate.  Such
   requests MUST yield a response with a "405 Method Not Allowed" HTTP
   status code, and MUST include an "Allow" header to convey that only
   the HEAD and GET (and OPTIONALLY the OPTIONS and TRACE) methods are
   supported.  In addition, the response MUST have a "Vary" header that
   includes the "negotiate" and "accept-datetime" values to indicate the
   TimeGate supports datetime negotiation.  Figure 13 shows such a
   response.





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   HTTP/1.1 405 Method Not Allowed
   Date: Thu, 21 Jan 2010 00:02:12 GMT
   Server: Apache
   Vary: negotiate, accept-datetime
   Allow: HEAD, GET
   Content-Length: 255
   Connection: close
   Content-Type: text/html; charset=iso-8909-1

    Figure 13: Response from a TimeGate accessed with HTTP method other
                               than HEAD/GET

3.2.3.  Recognizing a TimeGate

   When a user agent issues a HTTP HEAD/GET request against an assumed
   TimeGate URI (e.g.  URI is Target IRI of a link with a "timegate"
   Relation Type, URI is discovered as described in Section 4, etc.), it
   SHOULD NOT conclude that the targeted resource effectively is a
   TimeGate and hence will behave as described in Section 3.2.2.

   A user agent MUST decide it has reached a TimeGate if the response to
   a HTTP HEAD/GET request against the resource's URI contains a "Vary"
   header that includes the "negotiate" and "accept-datetime" values.
   If the response does not, the user agent MUST decide it has not
   reached a TimeGate and proceed as follows:

   o  If the response contains a redirection, the user agent SHOULD
      follow it.  Note that a chain of redirections is possible, e.g.
      URI-R -> URI-1 -> URI-2 -> ... -> URI-G

   o  If the response does not contain a redirection, or if the
      redirection (chain) does not lead to a TimeGate, the user agent
      SHOULD attempt to determine an appropriate TimeGate for the
      Original Resource, either automatically or interactively supported
      by the user.  The discovery mechanisms described in Section 4 can
      support the user agent with this regard.

   Resources that are not TimeGates (i.e. do not behave as described in
   Section 3.2.2) MUST NOT use a "Vary" header that includes the
   "accept-datetime" value.

   In certain cases, it is possible to implement Memento support in such
   a manner that an Original Resource coincides with its TimeGate, i.e.
   URI-R and URI-G are the same.  This implementation pattern is NOT
   RECOMMENDED.  It can make determining whether a resource is a
   TimeGate more challenging, and, more importantly, it may cause
   problems with caches.  Observed caching problems, which
   implementations must take care to avoid, include:



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   o  Cache invalidation when switching between a request for the
      Original Resource and a negotiation with the TimeGate.

   o  Delivering a (cached) Original Resource response when a TimeGate
      response was requested, and vice versa.

3.3.  Interactions with a Memento

   This section details HTTP GET/HEAD requests targeted at a Memento
   (URI-M).

3.3.1.  Step 5: User Agent Requests a Memento

   In Step 5, the user agent issues a HTTP GET request against the URI
   of a Memento.  The user agent MAY include an "Accept-Datetime" header
   in this request, but the existence or absence of this header MUST NOT
   affect the server's response.  The URI of the Memento may have
   resulted from a response in Step 4, or the user agent may simply have
   happened upon it.  Such a request is illustrated in Figure 14.

   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 14: User agent requests Memento

3.3.2.  Step 6: Server Responds to a Request for a Memento

   This section describes possible responses to a request for a Memento.
   Section 3.3.2.1 discusses the common scenario, whereas
   Section 3.3.2.2 and Section 3.3.2.3 detail special cases whereby
   Mementos are archived copies of HTTP responses with 3xx, 4xx and 5xx
   status codes.

3.3.2.1.  Common Scenario

   If the Memento requested by the user agent in Step 5 exists, and is
   not a special Memento as described in Section 3.3.2.2 and
   Section 3.3.2.2, the server's response MUST have a "200 OK" HTTP
   status code or, where appropriate "206 Partial Content", and it MUST
   include a "Memento-Datetime" header with a value equal to the
   archival datetime of the Memento, that is, the datetime of the state
   of the Original Resource that is encapsulated in the Memento.  The
   "Link" header MUST be provided and contain links subject to the
   considerations described in Section 2.2.  The Target IRI and, when
   applicable, the datetime values in the "Link" header associated with
   the "memento" Relation Type SHOULD be the same as conveyed in Step 4,



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   in case the TimeGate and the selected Memento reside on the same
   server.  However, they MAY be different in case the TimeGate and the
   selected Memento reside on different servers.

   Figure 15 illustrates the server's response to the request issued
   against a Memento in Step 5 (Figure 14).

   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/timemap/http://a.example.org>
      ; rel="timemap"; type="application/link-format",
    <http://arxiv.example.net/timegate/http://a.example.org>
      ; rel="timegate",
    <http://arxiv.example.net/web/20000915112826/http://a.example.org>
      ; rel="first memento"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
    <http://arxiv.example.net/web/20080708093433/http://a.example.org>
      ; rel="last memento"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="memento"; datetime="Tue, 11 Sep 2001 20:36:10 GMT",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="prev memento"; datetime="Tue, 11 Sep 2001 20:30:51 GMT",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="next memento"; datetime="Tue, 11 Sep 2001 20:47:33 GMT"
   Content-Length: 23364
   Content-Type: text/html;charset=utf-8
   Connection: close

                   Figure 15: Server of Memento responds

   The server's response MUST include the "Memento-Datetime" header
   regardless whether the user agent's request contained an "Accept-
   Datetime" header or not.  This is the way by which resources make
   explicit that they are Mementos.  Due to the sparseness of Mementos
   in most archives, 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,



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   and the addition of banners as a means to brand Web archives.

3.3.2.2.  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.  But also other 3XX responses may be archived.

   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, and it MUST include a "Memento-
   Datetime" header with a value equal to the archival datetime of the
   original 3XX response.  All other considerations, e.g. pertaining to
   the use of "Link" header, expressed in Section 3.3.2.1 apply.

   The client's handling of a HTTP response with a 3XX status code is
   not affected by the presence of a "Memento-Datetime" header.  The
   client 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.  For example:

   o  For a response from a Memento that has a 3XX status code and
      contains a "Location" header, the client SHOULD continue on to the
      URI specified in that header.

   o  For a response from a Memento that has a "300 Multiple Choices"
      status code, the response body SHOULD be presented to the user to
      allow selection of a URI.

   However, the client MUST be aware that the URI that was selected from
   the HTTP response with a 3XX status code might not be that of a
   Memento but rather of an Original Resource.  In that case it SHOULD
   proceed by looking for a Memento of the selected Original Resource.

   For example, on April 11 2008 Figure 16 is the response to an HTTP
   GET request for http://a.example.org.  This response is archived as a
   Memento of http://a.example.org, and this Memento's URI is
   http://arxiv.example.net/web/20080411000650/http://a.example.org.
   The response to a HTTP HEAD/GET on this Memento is shown in
   Figure 17.  In essence, it is a replay of the original response with
   "Memento-Datetime" and "Link" headers added, to allow a client to
   understand the response is a Memento.  In Figure 17, the value of the
   "Location" header is the same as in the original response; it
   identifies an Original Resource.  The client proceeds with finding a
   Memento for this Original Resource.  Web archives sometimes overwrite
   the value that was originally provided in the "Location" header in



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   order to point at a Memento they hold of the resource to which the
   redirect originally led.  This is shown in Figure 18.  In this case,
   the client 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 16: Response to the User Agent Request 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",
    <http://arxiv.example.net/web/20000915112826/http://a.example.org>
      ; rel="first memento"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
    <http://arxiv.example.net/web/20080708093433/http://a.example.org>
      ; rel="last memento"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
    <http://arxiv.example.net/web/20080411000650/http://a.example.org>
      ; rel="memento"; datetime="Fri, 11 Apr 2008 00:06:50 GMT",
    <http://arxiv.example.net/web/20080410203051/http://a.example.org>
      ; rel="prev memento"; datetime="Thu, 10 Apr 2008 20:30:51 GMT",
    <http://arxiv.example.net/web/20080412204733/http://a.example.org>
      ; rel="next memento"; datetime="Sat, 12 Apr 2008 20:47:33 GMT"
   Content-Length: 0
   Content-Type: text/plain; charset=UTF-8
   Connection: close

      Figure 17: Response to a User Agent Request for 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",
    <http://arxiv.example.net/web/20000915112826/http://a.example.org>
      ; rel="first memento"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
    <http://arxiv.example.net/web/20080708093433/http://a.example.org>
      ; rel="last memento"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
    <http://arxiv.example.net/web/20080411000650/http://a.example.org>
      ; rel="memento"; datetime="Fri, 11 Apr 2008 00:06:50 GMT",
    <http://arxiv.example.net/web/20080410203051/http://a.example.org>
      ; rel="prev memento"; datetime="Thu, 10 Apr 2008 20:30:51 GMT",
    <http://arxiv.example.net/web/20080412204733/http://a.example.org>
      ; rel="next memento"; datetime="Sat, 12 Apr 2008 20:47:33 GMT"
   Content-Length: 0
   Content-Type: text/plain; charset=UTF-8
   Connection: close

      Figure 18: Response to a User Agent Request for a Memento of a
                       Redirect; leads to a Memento

3.3.2.3.  Memento of Responses with Other 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, and it MUST include a "Memento-
   Datetime" header with a value equal to the archival datetime of the
   original response.  All other considerations, e.g. pertaining to the
   use of "Link" header, expressed in Section 3.3.2.1 apply.

   For example, on April 11 2008, Figure 19 is the 404 response to an
   HTTP GET request for http://a.example.org.  This response is archived
   as a Memento of http://a.example.org, and this Memento's URI is
   http://arxiv.example.net/web/20080411000650/http://a.example.org.
   The response to a HTTP HEAD/GET on this Memento is shown in
   Figure 20.  It is a replay of the original response with "Memento-
   Datetime" and "Link" headers added, to allow a client to understand
   the response is a Memento.





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   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 19: Response to the User Agent Request 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",
    <http://arxiv.example.net/web/20000915112826/http://a.example.org>
      ; rel="first memento"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
    <http://arxiv.example.net/web/20080708093433/http://a.example.org>
      ; rel="last memento"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
    <http://arxiv.example.net/web/20080411000650/http://a.example.org>
      ; rel="memento"; datetime="Fri, 11 Apr 2008 00:06:50 GMT",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="prev memento"; datetime="Thu, 10 Apr 2008 20:30:51 GMT",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="next memento"; datetime="Sat, 12 Apr 2008 20:47:33 GMT"
   Content-Length: 0
   Content-Type: text/plain; charset=UTF-8
   Connection: close

    Figure 20: Response to a User Agent Request for a Memento of a 404
                                 Response

3.3.2.4.  Mementos Without a TimeGate

   Cases may occur in which a server that hosts Mementos does not expose
   a TimeGate for those Mementos.  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 at the time this other
   server 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 hosts
   multiple Mementos for an Original Resource but consider exposing
   TimeGates too expensive.




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   In cases of Mementos without associated TimeGates, responses to a
   request for a Memento by a user agent MUST be as described in
   Section 3.3.2 with the exception that it will not contain a HTTP
   "Link" with a "timegate" Relation Type pointing at a TimeGate exposed
   by the responding server.  It MAY still contain such a Link pointing
   at a TimeGate exposed elsewhere.  Depending on whether one or more
   Mementos are hosted for an Original Resource, the response may or may
   not have a HTTP Link with a "timemap" Relation Type.  However, the
   response MUST still contain a "Memento-Datetime" response header with
   a value that corresponds to archival datetime of the Memento.

   Figure 21 illustrates the server's response to the request issued
   against a Memento in Step 5 (Figure 14) for the case that Memento has
   no associated TimeGate.  In this example, it is also assumed there is
   only one Memento for the Original Resource, and hence the Links with
   Relation Types "memento", "first", "last" all point at the same -
   responding - Memento.

   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, 15 Sep 2000 11:28:26 GMT"
   Content-Length: 23364
   Content-Type: text/html;charset=utf-8
   Connection: close

          Figure 21: Server of Memento without TimeGate responds

   Note that a server issuing a response similar to that of Figure 21
   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.

3.3.2.5.  Memento Does not Exist

   Cases may occur in which a TimeGate's response (Step 4) points at a
   Memento that actually does not exist, resulting in a user agent's
   request (Step 5) for a non-existent Memento.  In this case, the
   server's response MUST have the expected "404 Not Found" HTTP Status
   Code and it MUST NOT contain a "Memento-Datetime" header.  Note that
   the absence of a Memento in an archive is distinct from the case of
   an archived response with a "404 Not Found" HTTP status code as is
   described in Section 3.3.2.3




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3.3.3.  Recognizing a Memento

   When following the redirection provided by a confirmed TimeGate (see
   Section 3.2.3), a user agent SHOULD NOT assume that the targeted
   resource effectively is a Memento and hence will behave as described
   in Section 3.3.2.

   A user agent MUST decide it has reached a Memento if the response to
   a HTTP HEAD/GET request against the resource's URI contains a
   "Memento-Datetime" header with a legitimate value.  If the response
   does not, the following applies:

   o  If the response contains a redirection, the user agent SHOULD
      follow it.  Even a chain of redirections is possible, e.g.  URI-G
      -> URI-X -> URI-Y -> ... -> URI-M.

   o  If the response by a confirmed TimeGate does not contain a
      redirection, or if the redirection (chain) that started at a
      confirmed TimeGate does not lead to a resource that provides a
      "Memento-Datetime" header, the user agent MAY still conclude that
      it has likely arrived at a Memento.  That is because cases exist
      in which Web archives and CMS are made compliant with the Memento
      framework "by proxy".  In these cases TimeGates will redirect to
      Mementos in such systems, but the responses from these Mementos
      will not (yet) include a "Memento-Datetime" header.

3.4.  Interactions with a TimeMap

   A TimeMap is introduced to support retrieving a comprehensive list of
   all Mementos for a specific Original Resource, known to a responding
   server.  The entity-body of a response to an HTTP GET request issued
   against a TimeMap's URI:

   o  MUST list the URI of the Original Resource that the TimeMap is
      about;

   o  MUST preferably list the URI and datetime of each Memento for the
      Original Resource known to the responding server, or,
      alternatively support assembling such a list by following links
      with a "timemap" Relation Type provided in the TimeMap;

   o  MUST list the URI of one or more TimeGates for the Original
      Resource known to the responding server;

   o  SHOULD, for self-containment, list the URI of the TimeMap itself;

   o  MUST unambiguously type listed resources as being Original
      Resource, TimeGate, Memento, or TimeMap.



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   The entity-body of a response from a TimeMap MAY be serialized in
   various ways, but the link-value format serialization MUST be
   supported.  In this serialization, the entity-body MUST be formatted
   in the same way as the value of a 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
   [I-D.ietf-core-link-format].  All links conveyed in this
   serialization MUST be interpreted as having the URI of the Original
   Resource as their Context IRI.  The URI of the Original Resource is
   provided in the entity-body as the Target IRI of the link with an
   "original" Relation Type.

3.4.1.  User Agent Requests a TimeMap

   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 22.

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

                     Figure 22: Request for a TimeMap

3.4.2.  Server Responds to a Request for a TimeMap

   If the TimeMap requested by the user agent exists, the server's
   response MUST have a "200 OK" HTTP status code (or "206 Partial
   Content", where appropriate).  Note that a TimeMap is itself an
   Original Resource for which Mementos may exist.  For example, a
   response from a TimeMap could provide a "timegate" Link to a TimeGate
   via which prior TimeMap versions are available.  In this case, the
   use of the "Link" header is subject to all considerations described
   in Section 2.2, with the TimeMap acting as the Original Resource.

   However, in case a TimeMap wants to explicitly indicate in its
   response headers for which Original Resource it is a TimeMap, it MUST
   do so by including a HTTP "Link" header with the following
   characteristics:

   o  The Context IRI for the HTTP Link is the URI of the Original
      Resource;

   o  The Relation Type is "timemap";





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   o  The Target IRI for the HTTP Link is the URI of the TimeMap.

   Because the Context IRI of this HTTP Link is not the URI of the
   TimeMap, as per [RFC5988], the default Context IRI must be
   overwritten by using the "anchor" attribute with a value of the URI
   of the Original Resource.

   The response from the TimeMap to the request of Figure 22 is shown in
   Figure 23.  The response header shows the TimeMap explicitly
   conveying the URI of the Original Resource for which it is a TimeMap;
   for practical reasons the entity-body in the example has been
   abbreviated.  Notice also the use of the "self" Relation Type on the
   "timemap" link in the TimeMap.

   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: 4883
   Content-Type: application/link-format
   Connection: close

    <http://a.example.org>;rel="original",
    <http://arxiv.example.net/timemap/http://a.example.org>
      ; rel="timemap self";type="application/link-format",
    <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",
    <http://arxiv.example.net/web/20091027204954/http://a.example.org>
       ; rel="last memento";datetime="Tue, 27 Oct 2009 20:49:54 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 23: Response from a TimeMap

   Cases exist in which a TimeMap points at several other TimeMaps.  In
   one such case, a TimeMap could 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.  Such a TimeMap can
   be considered an index of TimeMaps and is shown in Figure 24.  Note
   the use of the Relation Types "self" and index" when pointing to the
   index TimeMap itself, and "first" and "last" to point at remote



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   TimeMaps that contain information on the earliest and most recent
   Mementos, respectively.  Another case is when the number of available
   Mementos requires introducing multiple TimeMaps that can be paged.
   Such a TimeMap is shown in Figure 25; Note the use of the "self"
   Relation Type when pointing at the current TimeMap itself, and the
   use of "first", "last", "prev", and "next" to point at others.  Note
   that this TimeMap also lists actual Mementos.

   <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="timemap self index";type="application/link-format",
    <http://arxiv1.example.net/timemap/http://a.example.org>
      ; rel="timemap first";type="application/link-format",
    <http://arxiv2.example.net/timemap/http://a.example.org>
      ; rel="timemap";type="application/link-format"
       <http://arxiv3.example.net/timemap/http://a.example.org>
      ; rel="timemap last";type="application/link-format"

                        Figure 24: An index TimeMap


   <http://a.example.org>;rel="original",
    <http://arxiv.example.net/timegate/http://a.example.org>
      ; rel="timegate",
    <http://arxiv.example.net/timemap/3/http://a.example.org>
      ; rel="timemap self";type="application/link-format",
    <http://arxiv.example.net/timemap/1/http://a.example.org>
      ; rel="timemap first";type="application/link-format",
    <http://arxiv.example.net/timemap/6/http://a.example.org>
      ; rel="timemap last";type="application/link-format",
     <http://arxiv.example.net/timemap/2/http://a.example.org>
      ; rel="timemap prev";type="application/link-format",
     <http://arxiv.example.net/timemap/4/http://a.example.org>
      ; rel="timemap next";type="application/link-format",
    <http://arxiv.example.net/timemap/5/http://a.example.org>
      ; rel="timemap";type="application/link-format",
    <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/20091027204954/http://a.example.org>
       ; rel="memento";datetime="Tue, 27 Oct 2009 20:49:54 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",
    ...




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                         Figure 25: TimeMap Paging


4.  The Memento Framework, Discovery of TimeGates and TimeMaps

   Section 3 describes how TimeGates, Mementos, Original Resources, and
   TimeMaps can be discovered by following HTTP Links with Relation
   Types "timegate", "memento", "original", and "timemap", respectively.

   Naturally, some of these links can also be included in
   representations of resources that have a media type that allows
   embedding typed links.  For example, an Original Resource that has an
   HTML representation can include a "timegate" link by using HTML's
   LINK element, e.g. <link
   href="http://arxiv.example.net/timegate/http://a.example.org"
   rel="timegate">.  The use of such embedded links is also subject to
   the considerations of Section 2.2.

   In this section an additional approach is introduced to support batch
   discovery of TimeGates and TimeMaps.  The approach leverages the
   well-known URI [RFC5785] and host-meta [RFC6415] specifications.  It
   can be used in cases where the URI of a TimeGate or TimeMap has as
   part of its path-component the URI of the associated Original
   Resource.  This is commonly the case for web archives and CMS that
   support the Memento protocol.  The approach uses the following
   variables for URI templates used in host-meta files:

   o  The "uri" variable, as reserved by [RFC6415], which stands for any
      URI in the server's document scope.

   o  The "anyuri" variable, introduced here, which stands for any URI,
      i.e. not just URIs in the server's document scope.

   o  The "path" variable, introduced here, which stands for any path
      and/or query component that can be added to the URI that precedes
      the variable in the URI template.

   A server SHOULD make TimeGates discoverable in the following way:

   o  As per [RFC5785] and [RFC6415], the server publishes a document
      with the name "host-meta" in its /.well-known/ path.

   o  As per [RFC6415], the host-meta document uses the XRD 1.0 document
      format.

   o  The host-meta document includes one or more "Link" elements to
      support discovery of TimeGates for the server's Original
      Resources.  Each such Link element has a value of "timegate" for



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      its "rel" attribute, and it has a "template" attribute that has a
      URI template as its value.  The URI template MAY either use the
      "uri", "anyuri", or "path" variable.

   o  The URI template MUST be such that, when applying the URI of an
      Original Resource to the "uri" or "anyuri" variable, or when
      applying a path and/or query component of an Original Resource's
      URI to the "path" variable, the URI of a potential TimeGate for
      that Original Resource results.

   o  Because Mementos do not exist for all possible URIs, there are no
      guarantees that these resulting TimeGates effectively exist.  But
      URI templates should be created to try and maximize chances that
      the TimeGate effectively exists.

   The considerations described for TimeGate discovery also apply to
   TimeMap discovery.  However, for TimeMaps, each Link element has a
   value of "timemap" for its "rel" attribute.

   Figure 26 and Figure 27 illustrate how a server makes TimeGates
   associated with its own Original Resources discoverable, whereas
   Figure 28 illustrates how a web archive that hosts Mementos for
   Original Resources originating from various domains makes its
   TimeGates and TimeMaps discoverable.

   Figure 26 shows a host-meta document published at
   http://a.example.org/.well-known/host-meta by a wiki that has
   http://a.example.org as its base URI.  The document supports
   discovery of TimeGates that allow accessing prior versions of the
   wiki's Original Resources.  The URI template uses the "uri" variable
   to stand for a document in the wiki's document range.  Assuming
   http://a.example.org/w/Clock is the URI of an Original Resource on
   the wiki, applying the template to that URI yields
   http://a.example.org/Special:TimeGate/http://a.example.org/w/Clock as
   its corresponding TimeGate.

   <?xml version='1.0' encoding='UTF-8'?>
    <XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'
          xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>
     <Link rel='timegate'
           template='http://a.example.org/Special:TimeGate/{uri}'>
     </Link>
    </XRD>

    Figure 26: A host-meta Document Supporting TimeGate Discovery for a
                         Wiki's Original Resources

   Figure 27 shows a host-meta document published at



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   http://a.example.org/.well-known/host-meta by the server with base
   URI http://a.example.org to support discovery of TimeGates for its
   Original Resources.  The example shows that the URI of TimeGates
   differs depending on the server path.  Assuming
   http://a.example.org/b/Clock is the URI of an Original Resource on
   the wiki, applying that URI to the appropriate template yields
   http://a2.example.net/tg/http://a.example.org/b/Clock as its
   corresponding TimeGate.

   <?xml version='1.0' encoding='UTF-8'?>
    <XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'
         xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>
     <Link rel='timegate'
      template='http://a1.example.net/tg/http://a.example.org/a{path}'>
     </Link>
     <Link rel='timegate'
      template='http://a2.example.net/tg/http://a.example.org/b{path}'>
     </Link>
    </XRD>

    Figure 27: A host-meta Document Supporting TimeGate Discovery for a
                        Server's Original Resources

   Figure 28 shows a host-meta document published at
   http://arxiv.example.net/.well-known/host-meta by a web archive that
   has http://arxiv.example.net/ as its base URI.  The document supports
   discovery of TimeGates and TimeMaps that the archive exposes, and
   that are associated with Original Resources for which the archive
   holds Mementos.  The URI templates use the "anyuri" variable, which
   is an indication that the web archive holds Mementos for a wide
   variety of domains.

   <?xml version='1.0' encoding='UTF-8'?>
    <XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'
         xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'>
     <Link rel='timegate'
           template='http://arxiv.example.net/timegate/{anyuri}'>
     </Link>
     <Link rel='timemap'
           template='http://arxiv.example.net/timemap/{anyuri}'>
     </Link>
    </XRD>

   Figure 28: A host-meta Document Supporting Discovery of TimeGates and
                     TimeMaps Exposed by a Web Archive






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5.  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 "Link" header Relation Types
   "original", "timegate", "timemap", and "memento" defined in
   Section 2.2.1 in the appropriate IANA registry.

   This memo requires IANA to register the "datetime" attribute for
   "Link" headers with a "memento" Relation Type, as defined in
   Section 2.2.1.4 in the appropriate IANA registry.


6.  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.

   Authentication, encryption and other security related issues are
   otherwise orthogonal to Memento.


7.  Changelog

   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.



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   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.

   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 client, 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.





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   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


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, Mark Nottingham, David Rosenthal, Ed Summers
   for early feedback.  Many thanks to Samuel Adams, Scott Ainsworth,
   Lyudmilla Balakireva, Frank McCown, Harihar Shankar, Brad Tofel for
   early implementations.


9.  References

9.1.  Normative References

   [I-D.ietf-core-link-format]
              Shelby, Z., "CoRE Link Format",
              draft-ietf-core-link-format-12 (work in progress),
              May 2012.

   [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.



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   [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.

9.2.  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
              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>.

   [robotstxt.org]
              "Robots Exclusion Protocol", August 2010,
              <http://www.robotstxt.org/robotstxt.html>.


Appendix A.  Appendix B: A Sample, Successful Memento Request/Response
             cycle

   Step 1 : UA --- HTTP GET/HEAD; Accept-Datetime: Tj ---------> URI-R

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

   Step 2 : UA <-- HTTP 200; Link: URI-G ----------------------- URI-R



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   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

   Step 3 : UA --- HTTP GET/HEAD; Accept-Datetime: Tj ---------> URI-G

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

   Step 4 : UA <-- HTTP 302; Location: URI-Mj; Vary; Link:
       URI-R, URI-T, URI-M0, URI-Mn, URI-Mi, URI-Mj, URI-Mk ---- URI-G

   HTTP/1.1 302 Found
   Date: Thu, 21 Jan 2010 00:06:50 GMT
   Server: Apache
   Vary: negotiate, accept-datetime
   Location:
    http://arxiv.example.net/web/20010911203610/http://a.example.org
   Link: <http://a.example.org>; rel="original",
    <http://arxiv.example.net/web/20000915112826/http://a.example.org>
      ; rel="first memento"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
    <http://arxiv.example.net/web/20080708093433/http://a.example.org>
      ; rel="last memento"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
    <http://arxiv.example.net/timemap/http://a.example.org>
      ; rel="timemap"; type="application/link-format",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="memento"; datetime="Tue, 11 Sep 2001 20:36:10 GMT",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="prev memento"; datetime="Tue, 11 Sep 2001 20:30:51 GMT",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="next memento"; datetime="Tue, 11 Sep 2001 20:47:33 GMT"
   Content-Length: 0
   Content-Type: text/plain; charset=UTF-8
   Connection: close

   Step 5 : UA --- HTTP GET URI-Mj; Accept-Datetime: Tj -------> URI-Mj

   GET /web/20010911203610/http://a.example.org
    HTTP/1.1
   Host: arxiv.example.net



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   Accept-Datetime: Tue, 11 Sep 2001 20:35:00 GMT
   Connection: close

   Step 6 : UA <-- HTTP 200; Memento-Datetime: Tj; Link: URI-R,
       URI-T, URI-G, URI-M0, URI-Mn, URI-Mi, URI-Mj, URI-Mk ---- URI-Mj

   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/20000915112826/http://a.example.org>
      ; rel="first memento"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
    <http://arxiv.example.net/web/20080708093433/http://a.example.org>
      ; rel="last memento"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
    <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",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="memento"; datetime="Tue, 11 Sep 2001 20:36:10 GMT",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="prev memento"; datetime="Tue, 11 Sep 2001 20:30:51 GMT",
    <http://arxiv.example.net/web/20010911203610/http://a.example.org>
      ; rel="next memento"; datetime="Tue, 11 Sep 2001 20:47:33 GMT"
   Content-Length: 23364
   Content-Type: text/html;charset=utf-8
   Connection: close

      A successful flow with TimeGate and Mementos on the same server


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/








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   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

































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