HTTP framework for time-based access to resource states -- Memento
draft-vandesompel-memento-05

Abstract

The HTTP-based Memento framework bridges the present and past Web by interlinking current resources with resources that encapsulate their past. It facilitates accessing prior states of a resource, encapsulated in 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. It also introduces discovery mechanisms to support these capabilities.

Status of This Memo

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This Internet-Draft will expire on May 20, 2013.

Copyright Notice

Copyright (c) 2012 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.

Table of Contents

• 1. Introduction
• 1.1. Terminology
• 1.2. Purpose
• 1.3. Notational Conventions
• 2. The Memento Framework, Datetime Negotiation component: HTTP headers, HTTP Link Relation Types
• 2.1. HTTP Headers
• 2.1.1. Accept-Datetime, Memento-Datetime
• 2.1.1.1. Values for Accept-Datetime
• 2.1.1.2. Values for Memento-Datetime
• 2.1.2. Vary
• 2.1.3. Location
• 2.1.4. Link
• 2.2. Link Header Relation Types
• 2.2.1. Memento Framework Relation Types
• 2.2.1.1. Relation Type "original"
• 2.2.1.2. Relation Type "timegate"
• 2.2.1.3. Relation Type "timemap"
• 2.2.1.4. Relation Type "memento"
• 2.2.2. Other Relation Types
• 3. The Memento Framework, Datetime Negotiation component: HTTP Interactions
• 3.1. Interactions with an Original Resource
• 3.1.1. Step 1: User Agent Requests an Original Resource
• 3.1.2. Step 2: Server Responds to a Request for an Original Resource
• 3.1.2.1. Original Resource is an Appropriate Memento
• 3.1.2.2. Server Exists and Original Resource Used to Exist
• 3.1.2.3. Missing "timegate" Link in Original Server's Response
• 3.2. Interactions with a TimeGate
• 3.2.1. Step 3: User Agent Negotiates with a TimeGate
• 3.2.2. Step 4: Server Responds to Negotiation with TimeGate
• 3.2.2.1. Successful Scenario
• 3.2.2.2. Multiple Matching Mementos
• 3.2.2.3. Accept-Datetime and other Accept Headers Provided
• 3.2.2.4. Accept-Datetime Not Provided
• 3.2.2.5. Accept-Datetime Unparseable
• 3.2.2.6. TimeGate Does Not Exist
• 3.2.2.7. HTTP Methods other than HEAD/GET
• 3.2.3. Recognizing a TimeGate
• 3.3. Interactions with a Memento
• 3.3.1. Step 5: User Agent Requests a Memento
• 3.3.2. Step 6: Server Responds to a Request for a Memento
• 3.3.2.1. Common Scenario
• 3.3.2.2. Memento of a 3XX Response
• 3.3.2.3. Memento of Responses with Other HTTP Status Codes
• 3.3.2.4. Mementos Without a TimeGate
• 3.3.2.5. Memento Does not Exist
• 3.3.3. Recognizing a Memento
• 3.4. Interactions with a Redirecting Resource
• 3.5. Interactions with a TimeMap
• 3.5.1. User Agent Requests a TimeMap
• 3.5.2. Server Responds to a Request for a TimeMap
• 4. The Memento Framework, Discovery of TimeGates, TimeMaps, Mementos
• 5. IANA Considerations
• 6. Security Considerations
• 7. Changelog
• 8. Acknowledgements
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Appendix: A Sample, Successful Memento Request/Response cycle
• Authors' Addresses

1. Introduction

1.1. Terminology

This specification uses the terms "resource", "request", "response", "entity-body", "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:

• Original Resource: An Original Resource is a resource that exists or used to exist, and for which access to one of its prior states may be required.
• 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 the Original Resource had at time Tj.
• TimeGate: A TimeGate for an Original Resource is a resource that is capable of datetime negotiation to support access to prior states of the Original Resource.
• TimeMap: A TimeMap for an Original Resource is a resource from which a list of URIs of Mementos of the Original Resource is available.
• Redirecting Resource: A Redirecting Resource is a resource that issues a redirect to a TimeGate, to a Memento, or to another Redirecting Resource, and thus plays an active role in the Memento infrastructure.

1.2. Purpose

The state of an Original Resource may change over time. Dereferencing its URI at any specific moment in time yields a response that reflects the resource's state at that moment: a representation of the resource's state (e.g. "200 OK" HTTP status code), an indication of its non-existence (e.g. "404 Not Found" HTTP status code), a relation to another resource (e.g. "302 Found" HTTP status code), etc. However, resources may also exist that encapsulate prior states of an Original Resource. Each such resource, named a Memento, has its own URI that, when dereferenced, yields a response that reflects a prior state of the Original Resource: a representation of a prior state of the Original Resource, an indication that the Original Resource did not exist at some time in the past, a relation that the Original Resource had to another resource at some time in the past, etc. Mementos exist in Web archives, Content Management Systems, or Revision Control Systems, among others. For any given Original Resource several Mementos may exist, each one reflecting a frozen prior state of the Original Resource. In terms of the Ontology for Relating Generic and Specific Information Resources [W3C.gen-ont-20090420]), a Memento is a FixedResource.

Examples are:

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

• http://web.archive.org/web/19970107171109/http://www.ietf.org/
• http://webarchive.nationalarchives.gov.uk/20080906200044/http://www.ietf.org/

Mementos for Original Resource http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol :

• http://en.wikipedia.org/w/index.php?title=Hypertext_Transfer_Protocol&oldid=366806574
• http://en.wikipedia.org/w/index.php?title=Hypertext_Transfer_Protocol&oldid=33912
• http://web.archive.org/web/20071011153017/http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol

Mementos for Original Resource http://www.w3.org/TR/webarch/ :

• http://www.w3.org/TR/2004/PR-webarch-20041105/
• http://www.w3.org/TR/2002/WD-webarch-20020830/
• 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:

• 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;
• Uses the global notion of datetime as a resource version indicator and access key;
• Leverages the following primitives of [W3C.REC-aww-20041215]: resource, resource state, representation, content negotiation, and link.

The core components of Memento's mechanism to access resource versions are:

1. The abstract notion of the state of 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].

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

• An appropriately typed link from the 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;
• The ability to negotiate in the datetime dimension with the TimeGate identified by URI-G, as a means to access 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:

• URI-R is used to denote the URI of an Original Resource.
• URI-G is used to denote the URI of a TimeGate.
• URI-M is used to denote the URI of a Memento.
• URI-T is used to denote the URI of a TimeMap.
• 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.

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 access a Memento that encapsulates a past state of an Original Resource. To that end, the "Accept-Datetime" header is conveyed in an HTTP request issued against a TimeGate for an Original Resource, and its value indicates the datetime of the desired past state of the Original Resource.

The "Memento-Datetime" response header is used by a server to indicate that the response is 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]), a Memento is a FixedResource.

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

• The server that originally assigns the "Memento-Datetime" header and value MUST retain that header in all responses to HTTP requests (with or without "Accept-Datetime" header) that occur against the Memento after the time of the original assignment of the header, and it MUST NOT change its associated value.
• Applications that mirror Mementos at a different URI MUST retain the "Memento-Datetime" header and MUST NOT change its value unless mirroring involves a meaningful state change. This allows, for example, duplicating a Web archive at a new location while preserving the value of the "Memento-Datetime" header of the archived resources. In this example, the "Last-Modified" header will be updated to reflect the time of mirroring at the new URI, whereas the value for "Memento-Datetime" will be maintained.

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"

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

Vary: negotiate, accept-datetime

Vary: negotiate, accept-datetime, accept

1: UA --- HTTP HEAD/GET; Accept-Datetime: Tj ---------------> URI-R
2: UA <-- HTTP 200; Link: URI-G ----------------------------- URI-R
3: UA --- HTTP HEAD/GET; 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

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

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

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

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

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

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"
   ; from="Tue, 15 Sep 2000 11:28:26 GMT"
   ; until="Tue, 08 Jul 2008 09:34:33 GMT",
 <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

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

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"
   ; from="Tue, 15 Sep 2000 11:28:26 GMT"
   ; until="Tue, 08 Jul 2008 09:34:33 GMT", 
 <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

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

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

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

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

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

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

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

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

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="self";type="application/link-format"
   ; from="Tue, 20 Jun 2000 18:02:59 GMT"
   ; until="Wed, 09 Apr 2008 20:30:51 GMT",
 <http://arxiv.example.net/timegate/http://a.example.org>
   ; rel="timegate",
 <http://arxiv.example.net/web/20000620180259/http://a.example.org>
   ; rel="first memento";datetime="Tue, 20 Jun 2000 18:02:59 GMT"
   ; license="http://creativecommons.org/publicdomain/zero/1.0/", 
 <http://arxiv.example.net/web/20091027204954/http://a.example.org>
    ; rel="last memento";datetime="Tue, 27 Oct 2009 20:49:54 GMT"
    ; license="http://creativecommons.org/publicdomain/zero/1.0/",
 <http://arxiv.example.net/web/20000621011731/http://a.example.org>
   ; rel="memento";datetime="Wed, 21 Jun 2000 01:17:31 GMT"
   ; license="http://creativecommons.org/publicdomain/zero/1.0/",
 <http://arxiv.example.net/web/20000621044156/http://a.example.org>
   ; rel="memento";datetime="Wed, 21 Jun 2000 04:41:56 GMT"
   ; license="http://creativecommons.org/publicdomain/zero/1.0/",
 ...

<http://a.example.org>;rel="original",
 <http://arxiv.example.net/timegate/http://a.example.org>
   ; rel="timegate",
 <http://arxiv.example.net/timemap/http://a.example.org>
   ; rel="self";type="application/link-format",
 <http://arxiv1.example.net/timemap/http://a.example.org>
   ; rel="timemap";type="application/link-format"
   ; from="Wed, 21 Jun 2000 04:41:56 GMT"
   ; until="Wed, 09 Apr 2008 20:30:51 GMT",
 <http://arxiv2.example.net/timemap/http://a.example.org>
   ; rel="timemap";type="application/link-format"
   ; from="Thu, 10 Apr 2008 20:30:51 GMT"
   ; until="Tue, 27 Oct 2009 20:49:54 GMT",
 <http://arxiv3.example.net/timemap/http://a.example.org>
   ; rel="timemap";type="application/link-format"
   ; from="Thu, 29 Oct 2009 20:30:51 GMT"

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

<?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>

<?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}'
        from='Tue, 20 Jun 2000 18:02:59 GMT'
        until='Wed, 09 Apr 2008 20:30:51 GMT'>
  </Link>
 </XRD>

<?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='memento' 
   template='http://arxiv.example.net/2010/{uri}'
   license='http://creativecommons.org/publicdomain/zero/1.0/'>
  </Link>
  <Link rel='memento' 
   template='http://arxiv.example.net/2011/{uri}'
   license='http://creativecommons.org/publicdomain/zero/1.0/'>
  </Link>
 </XRD>

Step 1 : UA --- HTTP HEAD/GET; 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/timegate/http://a.example.org>
   ; rel="timegate"
Content-Length: 255
Connection: close
Content-Type: text/html; charset=iso-8859-1

Step 3 : UA --- HTTP HEAD/GET; 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
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