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Versions: 00 01 02 03 04 RFC 7089

Internet Engineering Task Force                           H. VandeSompel
Internet-Draft                            Los Alamos National Laboratory
Intended status: Informational                                 M. Nelson
Expires: May 16, 2011                            Old Dominion University
                                                            R. Sanderson
                                          Los Alamos National Laboratory
                                                       November 12, 2010


   HTTP framework for time-based access to resource states -- Memento
                      draft-vandesompel-memento-00

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
   an approach to discover and serialize a list of resources known to a
   server, each of which provides access to a representation of a prior
   state of a same resource.

Status of this Memo

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

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

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

   This Internet-Draft will expire on May 16, 2011.






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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 . . . . . . . . . . . . . . . . . . . . . . . 10
       2.1.4.  Link . . . . . . . . . . . . . . . . . . . . . . . . . 10
     2.2.  Link Header Relation Types . . . . . . . . . . . . . . . . 10
       2.2.1.  Memento Framework Relation Types . . . . . . . . . . . 10
         2.2.1.1.  Relation Type "original" . . . . . . . . . . . . . 11
         2.2.1.2.  Relation Type "timegate" . . . . . . . . . . . . . 11
         2.2.1.3.  Relation Type "timemap"  . . . . . . . . . . . . . 12
         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
         3.2.2.1.  Successful Scenario  . . . . . . . . . . . . . . . 20
         3.2.2.2.  Datetime Out of the Server's Range . . . . . . . . 21
         3.2.2.3.  Accept-Datetime Not Provided . . . . . . . . . . . 22
         3.2.2.4.  Multiple Matching Mementos . . . . . . . . . . . . 22
         3.2.2.5.  Datetime Out of the User Agent's Range . . . . . . 23
         3.2.2.6.  Accept-Datetime Unparseable  . . . . . . . . . . . 24
         3.2.2.7.  TimeGate Does Not Exist  . . . . . . . . . . . . . 24
         3.2.2.8.  HTTP Methods other than HEAD/GET . . . . . . . . . 25
       3.2.3.  Recognizing a TimeGate . . . . . . . . . . . . . . . . 25
     3.3.  Interactions with a Memento  . . . . . . . . . . . . . . . 26
       3.3.1.  Step 5: User Agent Requests a Memento  . . . . . . . . 26



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       3.3.2.  Step 6: Server Responds to a Request for a Memento . . 26
         3.3.2.1.  Memento Does not Exist . . . . . . . . . . . . . . 27
       3.3.3.  Recognizing a Memento  . . . . . . . . . . . . . . . . 28
   4.  The Memento Framework, Discovery Component . . . . . . . . . . 28
     4.1.  TimeMaps . . . . . . . . . . . . . . . . . . . . . . . . . 28
     4.2.  Discovery of TimeMaps, TimeGates . . . . . . . . . . . . . 30
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 30
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 31
   7.  Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 31
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 31
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 31
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 32
   Appendix A.  Appendix B: A Sample, Successful Memento
                Request/Response cycle  . . . . . . . . . . . . . . . 32
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34
   Intellectual Property and Copyright Statements . . . . . . . . . . 36


































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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 RFC 2616 [RFC2616],
   and it uses the terms "representation" and "resource state" as
   described in W3C.REC-aww-20041215 [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
      supports 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 [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
   appropriately typed link from a resource identified by URI-M, which
   encapsulates the state a resource identified by URI-R had at some
   dateimte Tj, to the resource identified by URI-R.

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

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 RFC 2119 [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 is 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 HTTPS resources.

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 RFC 2616 [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 [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.  Therefore, the server that
   originally assigns the header and value, MUST retain the Memento-
   Datetime header in all responses to HTTP HEAD/GET requests (with or
   without "Accept-Datetime" header) that occur against the resource
   after the time of the original assignment of the header, and it MUST
   NOT change its associated value.  Similarly, if an application is
   mirroring the resource at a different URI, it SHOULD retain the
   resource's Memento-Datetime header and value if mirroring the
   resource does not include a meaningful change to the resource's



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   state.  For example, this behavior allows duplicating a Web archive
   at a new location while preserving the Memento-Datetime values of the
   archived resources.

2.1.1.1.  Values for Accept-Datetime

   Values for the "Accept-Datetime" header consist of a MANDATORY
   datetime expressed according to the RFC 1123 [RFC1123] format, which
   is formalized by the rfc1123-date construction rule of the BNF in
   Figure 1, and an OPTIONAL interval indicator expressed according to
   the iso8601-interval rule of the BNF in Figure 1.  The datetime MUST
   be represented in Greenwich Mean Time (GMT).

   Examples of "Accept-Datetime" request headers with and without an
   interval indicator:

   Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT
   Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT; -P3DT5H;+P2DT6H

   The user agent uses the MANDATORY datetime value to convey its
   preferred datetime for a Memento; it uses the OPTIONAL interval
   indicator to convey it is interested in retrieving Mementos that
   reside within this interval around the preferred datetime, and not
   interested in Mementos that reside outside of it.  Not using an
   interval indicator is equivalent with expressing an infinite interval
   around the preferred datetime.

   The interval mechanism can be regarded as an implementation of the
   functionality intended by the q-value approach that is used in
   regular content negotiation.  The q-value approach is not supported
   for Memento's datetime negotiation because it is well-suited for
   negotiation over a discrete space of mostly predictable values, not
   for negotiation over a continuum of unpredictable datetime values.


















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   accept-dt-value = rfc1123-date *SP [ iso8601-interval ]
   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"
   iso8601-interval = ";" *SP "-" duration *SP ";" *SP "+" duration
   duration = "P" ( dur-date | dur-week )
   dur-date = ( dur-day | dur-month | dur-year ) [ dur-time ]
   dur-year = 1*DIGIT "Y" [ dur-month ] [ dur-day ]
   dur-month = 1*DIGIT "M" [ dur-day ]
   dur-day = 1*DIGIT "D"
   dur-time = "T" ( dur-hour | dur-minute | dur-second )
   dur-hour = 1*DIGIT "H" [ dur-minute ] [ dur-second ]
   dur-minute = 1*DIGIT "M" [ dur-second ]
   dur-second = 1*DIGIT "S"
   dur-week = 1*DIGIT "W"


                   Figure 1: BNF for the datetime format

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




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

   Vary: negotiate, accept-datetime, accept

2.1.3.  Location

   The "Location" header is used as defined in RFC 2616 [RFC2616].
   Examples are given in Section 3 below.

2.1.4.  Link

   The "Link" response header is specified in RFC5988 [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 [RFC5988] is semantically
   equivalent to the "<LINK>" element in HTML, as well as the "atom:
   link" feed-level element in Atom RFC 4287 [RFC4287].  By default, the
   origin of a link expressed by an entry in a "Link" header (named
   Context IRI in RFC5988 [RFC5988]) is the IRI of the requested
   resource.

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  |       NA      |  RECOMMENDED, 0 |
   |           |         more        |               |     or more     |
   |  timemap  |          NA         |  RECOMMENDED, |  RECOMMENDED, 0 |
   |           |                     |     0 or 1    |     or more     |





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   |  memento  |    NA, except see   |  REQUIRED, 1  |  REQUIRED, 1 or |
   |           |   Section 3.1.2.1   |    or more    |       more      |
   +-----------+---------------------+---------------+-----------------+

                    Table 1: The use of Relation Types

   For several of the Relation Types introduced in the Memento
   framework, the use of a "datetime" attribute is REQUIRED.  The value
   for this attribute MUST be a datetime expressed according to the RFC
   1123 [RFC1123] format, which is formalized by the rfc1123-date
   construction rule of the BNF in Figure 1; it MUST be represented in
   Greenwich Mean Time (GMT).

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 all 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-Mj

   o  Target IRI: URI-R

   o  Relation Type: "original"

   o  Use: REQUIRED

   o  Cardinality: 1

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.  Details for the entry are as follows:

   o  Context IRI: URI-R or URI-Mj

   o  Target IRI: URI-G

   o  Relation Type: "timegate"





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   o  Use: RECOMMENDED

   o  Cardinality: 0 or more

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 if used it MUST occur exactly once.  This link MUST
   include a "type" attribute and its value MUST be "application/
   link-format", referring to the MIME type introduced in I-D.ietf-core-
   link-format [I-D.ietf-core-link-format].  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"

   o  Use: RECOMMENDED

   o  Cardinality: 0 or more

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.  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 an
   existing TimeGate or Memento:

   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.



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   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 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.

   o  Other "memento" links MAY only be included if both the 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.

   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 Attribute: "datetime"

   o  Use: REQUIRED

   o  Cardinality: 1 or more

2.2.2.  Other Relation Types

   Web Linking RFC5988 [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



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   with others as deemed necessary.  As the "memento" Relation Type
   focuses on conveying the datetime of a linked Memento, Relation Types
   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 [RFC5829] (such as version contol systems)
   and others that can not (such as Web archives).  Note that, in terms
   of RFC5829 [RFC5829], the last Memento (URI-Mn) is the version prior
   to the latest (i.e. current) version.

   +-----------------------------+---------------------+---------------+
   |         Memento Type        |    RFC5988 system   |  non RFC5988  |
   |                             |                     |     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" 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 and Mementos 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 MAY 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
   accomodate 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/web/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 as both an indication 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
   with URI http://a.example.org/pic that has been stable since it was



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   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 stable

   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/web/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
                                  stable






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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/web/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 does no longer exist 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.

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, for example, based on the aggregation of
   TimeMaps (Section 4.1) or user interaction.  Such a request is
   illustrated in Figure 8.

   GET /web/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.  Special cases for datetime
   negotiation with a TimeGate exist, and they are addressed in
   Section 3.2.2.3 through Section 3.2.2.7.

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 and is not out of
   range, the server selects a Memento based on the user agent's
   datetime preference.  The response MUST have a "302 Found" HTTP



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   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.  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/web/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

   Note that 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.

3.2.2.2.  Datetime Out of the Server's Range

   In case, in Step 3, a user agent's "Accept-Datetime" header does not
   convey an interval indicator, and 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 MUST be as described in Section 3.2.2.1, with a selection of
   the first or most recent Memento, respectively.



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   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.

   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

3.2.2.3.  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.

3.2.2.4.  Multiple Matching Mementos

   Because the finest datetime granularity epxressable using the RFC
   1123 [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 CMS 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



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   Original Resource http://a.example.org/w/Clock for which two Mementos
   exist for the user agent's preferred datetime.

   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

3.2.2.5.  Datetime Out of the User Agent's Range

   In case, in Step 3, a user agent conveys an interval indicator, and
   the responding server is not aware of any Mementos with datetimes
   within the expressed interval, the server's response MUST have a "406
   Not Acceptable" 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).

   Figure 12 shows a user agent using an "Accept-Datetime" header
   conveying an interval of interest starting 5 hours before and ending
   6 hours after Tue, 11 Sep 2001 20:35:00 GMT.  Figure 13 shows the
   response from the TimeGate.



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   GET /web/timegate/http://a.example.org HTTP/1.1
   Host: arxiv.example.net
   Accept-Datetime: Tue, 11 Sep 2001 20:35:00 GMT; -P5H;+P6H
   Connection: close

      Figure 12: User agent expresses interval of interest in Accept-
                              Datetime header


   HTTP/1.1 406 Not Acceptable
   Date: Thu, 21 Jan 2010 00:06:50 GMT
   Server: Apache
   Vary: negotiate, accept-datetime
   Link: <http://an.example.org>; rel="original",
    <http://arxiv.example.net/web/timemap/http://a.example.org>
      ; rel="timemap";type="application/link-format",
    <http://arxiv.example.net/web/20000915112826/http://a.example.org>
      ; rel="memento first"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
    <http://arxiv.example.net/web/20080708093433/http://a.example.org>
      ; rel="memento last"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
   Content-Length: 1732
   Connection: close
   Content-Type: text/plain; charset=UTF-8

   Figure 13: A TimeGate's response indicating it has no Mementos within
                         the interval of interest

3.2.2.6.  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.  With all
   other respects, responses in this case MUST be handled as described
   in Section 3.2.2.5

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.



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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 RFC 2616 [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 14 shows such a response.

   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 14: 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 a resource
   of which it found the URI as the Target IRI of an entry in the "Link"
   header with a "timegate" Relation Type, it SHOULD NOT assume 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 even 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



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      by the user.

   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.

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 15.

   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 15: User agent requests Memento

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

   If the Memento requested by the user agent in Step 5 exists, the
   server's response MUST have a "200 OK" HTTP status code (or "206
   Partial Content", where appropriate), 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, 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 16 illustrates the server's response to the request issued
   against a Memento in Step 5 (Figure 15).






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   HTTP/1.1 200 OK
   Date: Thu, 21 Jan 2010 00:09:40 GMT
   Server: Apache-Coyote/1.1
   Memento-Datetime: Tue, 11 Sep 2001 20:36:10 GMT
   Link: <http://a.example.org>; rel="original",
    <http://arxiv.example.net/web/timemap/http://a.example.org>
      ; rel="timemap"; type="application/link-format",
    <http://arxiv.example.net/web/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 16: 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,
   and the addition of banners as a means to brand Web archives.

3.3.2.1.  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



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   server's response MUST have the expected "404 Not Found" HTTP Status
   Code and it MUST NOT contain a "Memento-Datetime" header.

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 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.


4.  The Memento Framework, Discovery Component

4.1.  TimeMaps

   A TimeMap resource is introduced to support retrieving a
   comprehensive list of all Mementos 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 response lists
      Mementos for;

   o  MUST list the URI of one or more TimeGates for the Original
      Resource;

   o  MUST list the URI and datetime of each Memento known to the
      responding server;




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   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.

   TimeMaps 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 [RFC5988].  The
   media type of the entity-body MUST be "application/link-format", and
   the use of the Relation Types is subject to the considerations in
   Section 2.2 with the following execptions:

   o  Instead of a Memento selected by the responding server, all
      Mementos known to the server MUST be listed;

   o  Since no Memento was selected by the server, the entity-body MUST
      NOT contain links with "prev", "next", "predecessor-version",
      "successor-version" Relation Types.

   In order to retrieve the link-value serialization of a TimeMap, a
   user agent SHOULD use an "Accept: application/link-format" header.
   This is shown in Figure 17.  The response from the TimeMap is shown
   in Figure 18; for practical reasons the entity-body in the example
   has been abbreviated.

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

                     Figure 17: Request for a TimeMap


















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   HTTP/1.1 200 OK
   Date: Thu, 21 Jan 2010 00:06:50 GMT
   Server: Apache
   Connection: close
   Content-Type: application/link-format

    <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/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 18: Response from a TimeMap

4.2.  Discovery of TimeMaps, TimeGates

   As described in Section 3, TimeMaps and TimeGates can be discovered
   via HTTP Links with the "timemap" and "timegate" Relation Type,
   respectively.  Additional discovery mechanisms are RECOMMENDED,
   including:

   o  The inclusion of HTML LINK elements with "timegate" and "timemap"
      rel types in Original Resources that provide an HTML response,
      e.g. <link
      href="http://arxiv.example.net/timegate/http://a.example.org"
      rel="timegate"> ;

   o  The implementation of batch discovery mechanisms for TimeMaps
      using SiteMaps or feed technology.


5.  IANA Considerations

   This memo requires IANA to register the "Link" header Relation Types
   defined in Section 2.2.1 in the appropriate IANA registry.

   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.



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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

   o  v01 2010-11-11 HVDS MLN RS First public version

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

   o  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-01 (work in progress),
              October 2010.

   [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.,



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              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC5829]  Brown, A., Clemm, G., and J. Reschke, "Link Relation Types
              for Simple Version Navigation between Web Resources",
              RFC 5829, April 2010.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

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-07 (work in
              progress), October 2010.

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

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

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


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

   HTTP/1.1 200 OK
   Date: Thu, 21 Jan 2010 00:02:12 GMT
   Server: Apache
   Link: <http://arxiv.example.net/web/timegate/http://a.example.org>



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

   Step 6 : UA <-- HTTP 200; Memento-Datetime: Tj; Link: URI-R,



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       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/web/timemap/http://a.example.org>
      ; rel="timemap"; type="application/link-format",
    <http://arxiv.example.net/web/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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Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.











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