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2 Internet Engineering Task Force H. VandeSompel
3 Internet-Draft Los Alamos National Laboratory
4 Intended status: Informational M. Nelson
5 Expires: October 30, 2011 Old Dominion University
6 R. Sanderson
7 Los Alamos National Laboratory
8 April 28, 2011
10 HTTP framework for time-based access to resource states -- Memento
11 draft-vandesompel-memento-01
13 Abstract
15 The HTTP-based Memento framework bridges the present and past Web by
16 interlinking current resources with resources that encapsulate their
17 past. It facilitates obtaining representations of prior states of a
18 resource, available from archival resources in Web archives or
19 version resources in content management systems, by leveraging the
20 resource's URI and a preferred datetime. To this end, the framework
21 introduces datetime negotiation (a variation on content negotiation),
22 and new Relation Types for the HTTP Link header aimed at interlinking
23 resources with their archival/version resources. It also introduces
24 various discovery mechanisms that further support briding the present
25 and past Web.
27 Status of this Memo
29 This Internet-Draft is submitted in full conformance with the
30 provisions of BCP 78 and BCP 79.
32 Internet-Drafts are working documents of the Internet Engineering
33 Task Force (IETF). Note that other groups may also distribute
34 working documents as Internet-Drafts. The list of current Internet-
35 Drafts is at http://datatracker.ietf.org/drafts/current/.
37 Internet-Drafts are draft documents valid for a maximum of six months
38 and may be updated, replaced, or obsoleted by other documents at any
39 time. It is inappropriate to use Internet-Drafts as reference
40 material or to cite them other than as "work in progress."
42 This Internet-Draft will expire on October 30, 2011.
44 Copyright Notice
46 Copyright (c) 2011 IETF Trust and the persons identified as the
47 document authors. All rights reserved.
49 This document is subject to BCP 78 and the IETF Trust's Legal
50 Provisions Relating to IETF Documents
51 (http://trustee.ietf.org/license-info) in effect on the date of
52 publication of this document. Please review these documents
53 carefully, as they describe your rights and restrictions with respect
54 to this document. Code Components extracted from this document must
55 include Simplified BSD License text as described in Section 4.e of
56 the Trust Legal Provisions and are provided without warranty as
57 described in the Simplified BSD License.
59 Table of Contents
61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
62 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
63 1.2. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 4
64 1.3. Notational Conventions . . . . . . . . . . . . . . . . . . 6
65 2. The Memento Framework, Datetime Negotiation component:
66 HTTP headers, HTTP Link Relation Types . . . . . . . . . . . . 7
67 2.1. HTTP Headers . . . . . . . . . . . . . . . . . . . . . . . 7
68 2.1.1. Accept-Datetime, Memento-Datetime . . . . . . . . . . 7
69 2.1.1.1. Values for Accept-Datetime . . . . . . . . . . . . 8
70 2.1.1.2. Values for Memento-Datetime . . . . . . . . . . . 9
71 2.1.2. Vary . . . . . . . . . . . . . . . . . . . . . . . . . 9
72 2.1.3. Location . . . . . . . . . . . . . . . . . . . . . . . 10
73 2.1.4. Link . . . . . . . . . . . . . . . . . . . . . . . . . 10
74 2.2. Link Header Relation Types . . . . . . . . . . . . . . . . 10
75 2.2.1. Memento Framework Relation Types . . . . . . . . . . . 10
76 2.2.1.1. Relation Type "original" . . . . . . . . . . . . . 11
77 2.2.1.2. Relation Type "timegate" . . . . . . . . . . . . . 11
78 2.2.1.3. Relation Type "timemap" . . . . . . . . . . . . . 12
79 2.2.1.4. Relation Type "memento" . . . . . . . . . . . . . 12
80 2.2.2. Other Relation Types . . . . . . . . . . . . . . . . . 14
81 3. The Memento Framework, Datetime Negotiation component:
82 HTTP Interactions . . . . . . . . . . . . . . . . . . . . . . 15
83 3.1. Interactions with an Original Resource . . . . . . . . . . 16
84 3.1.1. Step 1: User Agent Requests an Original Resource . . . 16
85 3.1.2. Step 2: Server Responds to a Request for an
86 Original Resource . . . . . . . . . . . . . . . . . . 17
87 3.1.2.1. Original Resource is an Appropriate Memento . . . 18
88 3.1.2.2. Server Exists and Original Resource Used to
89 Exist . . . . . . . . . . . . . . . . . . . . . . 19
90 3.1.2.3. Missing or Inadequate "timegate" Link in
91 Original Server's Response . . . . . . . . . . . . 20
92 3.2. Interactions with a TimeGate . . . . . . . . . . . . . . . 20
93 3.2.1. Step 3: User Agent Negotiates with a TimeGate . . . . 20
94 3.2.2. Step 4: Server Responds to Negotiation with
95 TimeGate . . . . . . . . . . . . . . . . . . . . . . . 21
97 3.2.2.1. Successful Scenario . . . . . . . . . . . . . . . 21
98 3.2.2.2. Accept-Datetime and other Accept Headers
99 Provided . . . . . . . . . . . . . . . . . . . . . 23
100 3.2.2.3. Accept-Datetime Not Provided . . . . . . . . . . . 23
101 3.2.2.4. Multiple Matching Mementos . . . . . . . . . . . . 24
102 3.2.2.5. Datetime Out of the User Agent's Range . . . . . . 24
103 3.2.2.6. Accept-Datetime Unparseable . . . . . . . . . . . 26
104 3.2.2.7. TimeGate Does Not Exist . . . . . . . . . . . . . 26
105 3.2.2.8. HTTP Methods other than HEAD/GET . . . . . . . . . 26
106 3.2.3. Recognizing a TimeGate . . . . . . . . . . . . . . . . 27
107 3.3. Interactions with a Memento . . . . . . . . . . . . . . . 27
108 3.3.1. Step 5: User Agent Requests a Memento . . . . . . . . 27
109 3.3.2. Step 6: Server Responds to a Request for a Memento . . 28
110 3.3.2.1. Memento Does not Exist . . . . . . . . . . . . . . 29
111 3.3.2.2. Mementos Without a TimeGate . . . . . . . . . . . 29
112 3.3.3. Recognizing a Memento . . . . . . . . . . . . . . . . 30
113 3.4. Interactions with a TimeMap . . . . . . . . . . . . . . . 31
114 3.4.1. User Agent Requests a TimeMap . . . . . . . . . . . . 31
115 3.4.2. Server Responds to a Request for a TimeMap . . . . . . 32
116 4. The Memento Framework, Discovery Component . . . . . . . . . . 33
117 4.1. Discovery of TimeGates Via Robots Exclusion Protocol . . . 34
118 4.2. Discovery of TimeMaps Via TimeMap Feeds and Robots
119 Exclusion Protocol . . . . . . . . . . . . . . . . . . . . 35
120 4.2.1. TimeMap Feeds . . . . . . . . . . . . . . . . . . . . 35
121 4.2.1.1. TimeMap Feeds: Feed-level Elements . . . . . . . . 36
122 4.2.1.2. TimeMap Feeds: Entry-level Elements . . . . . . . 38
123 4.2.2. Discovering TimeMap Feeds via Robots Exclusion
124 Protocol . . . . . . . . . . . . . . . . . . . . . . . 41
125 4.3. Discovering Mementos via Robots Exclusion Protocol . . . . 41
126 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42
127 6. Security Considerations . . . . . . . . . . . . . . . . . . . 42
128 7. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 42
129 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 43
130 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
131 9.1. Normative References . . . . . . . . . . . . . . . . . . . 43
132 9.2. Informative References . . . . . . . . . . . . . . . . . . 44
133 Appendix A. Appendix B: A Sample, Successful Memento
134 Request/Response cycle . . . . . . . . . . . . . . . 44
135 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 46
137 1. Introduction
139 1.1. Terminology
141 This specification uses the terms "resource", "request", "response",
142 "entity", "entity-body", "entity-header", "content negotiation",
143 "client", "user agent", "server" as described in RFC 2616 [RFC2616],
144 and it uses the terms "representation" and "resource state" as
145 described in W3C.REC-aww-20041215 [W3C.REC-aww-20041215].
147 In addition, the following terms specific to the Memento framework
148 are introduced:
150 o Original Resource: An Original Resource is a resource that exists
151 or used to exist, and for which access to one of its prior states
152 is desired.
154 o Memento: A Memento for an Original Resource is a resource that
155 encapsulates a prior state of the Original Resource. A Memento
156 for an Original Resource as it existed at time Tj is a resource
157 that encapsulates the state that the Original Resource had at time
158 Tj.
160 o TimeGate: A TimeGate for an Original Resource is a resource that
161 supports negotiation to allow selective, datetime-based, access to
162 prior states of the Original Resource.
164 o TimeMap: A TimeMap for an Original Resource is a resource from
165 which a list of URIs of Mementos of the Original Resource is
166 available.
168 1.2. Purpose
170 The state of an Original Resource may change over time.
171 Dereferencing its URI at any specific moment in time during its
172 existence yields a representation of its then current state.
173 Dereferencing its URI at any time past its existence no longer yields
174 a meaningful representation, if any. Still, in both cases, resources
175 may exist that encapsulate prior states of the Original Resource.
176 Each such resource, named a Memento, has its own URI that, when
177 dereferenced, returns a representation of a prior state of the
178 Original Resource. Mementos may, for example, exist in Web archives,
179 Content Management Systems, or Revision Control Systems.
181 Examples are:
183 Mementos for Original Resource http://www.ietf.org/ :
185 o http://web.archive.org/web/19970107171109/http://www.ietf.org/
187 o http://webarchive.nationalarchives.gov.uk/20080906200044/http://
188 www.ietf.org/
190 Mementos for Original Resource
191 http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol :
193 o http://en.wikipedia.org/w/
194 index.php?title=Hypertext_Transfer_Protocol&oldid=366806574
196 o http://en.wikipedia.org/w/
197 index.php?title=Hypertext_Transfer_Protocol&oldid=33912
199 o http://web.archive.org/web/20071011153017/http://en.wikipedia.org/
200 wiki/Hypertext_Transfer_Protocol
202 Mementos for Original Resource http://www.w3.org/TR/webarch/ :
204 o http://www.w3.org/TR/2004/PR-webarch-20041105/
206 o http://www.w3.org/TR/2002/WD-webarch-20020830/
208 o http://webarchive.nationalarchives.gov.uk/20100304163140/http://
209 www.w3.org/TR/webarch/
211 In the abstract, Memento introduces a mechanism to access versions of
212 Web resources that:
214 o Is fully distributed in the sense that resource versions may
215 reside on multiple hosts, and that any such host is likely only
216 aware of the versions it holds;
218 o Uses the global notion of datetime as a resource version indicator
219 and access key;
221 o Leverages the following primitives of W3C.REC-aww-20041215
222 [W3C.REC-aww-20041215]: resource, resource state, representation,
223 content negotiation, and link.
225 The core components of Memento's mechanism to access resource
226 versions are:
228 1. The abstract notion of the state of a resource identified by
229 URI-R as it existed at some time Tj. Note the relationship with the
230 ability to identify a the state of a resource at some datetime Tj by
231 means of a URI as intended by the proposed Dated URI scheme
232 I-D.masinter-dated-uri [I-D.masinter-dated-uri].
234 2. A bridge from the present to the past, consisting of:
236 o An appropriately typed link from a resource identified by URI-R to
237 an associated TimeGate identified by URI-G, which is aware of (at
238 least part of the) version history of the resource identified by
239 URI-R;
241 o The ability to content negotiate in the datetime dimension with
242 the TimeGate identified by URI-G, as a means to obtain a
243 representation of the state that the resource identified by URI-R
244 had at some datetime Tj.
246 3. A bridge from the past to the present, consisting of an
247 appropriately typed link from a resource identified by URI-M, which
248 encapsulates the state a resource identified by URI-R had at some
249 datetime Tj, to the resource identified by URI-R.
251 Section 2 and Section 3 of this document are concerned with
252 specifying an instantiation of these abstractions for resources that
253 are identified by HTTP(S) URIs, whereas Section 4 details approaches
254 to discover TimeGates, TimeMaps, and Mementos on the HTTP(S) Web by
255 other means than typed links.
257 1.3. Notational Conventions
259 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
260 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
261 document are to be interpreted as described in RFC 2119 [RFC2119].
263 When needed for extra clarity, the following conventions are used:
265 o URI-R is used to denote the URI of an Original Resource.
267 o URI-G is used to denote the URI of a TimeGate.
269 o URI-M is used to denote the URI of a Memento.
271 o URI-T is used to denote the URI of a TimeMap.
273 o When scenarios are described that involve multiple Mementos,
274 URI-M0 denotes the URI of the first Memento known to the
275 responding server, URI-Mn denotes the URI of the most recent known
276 Memento, URI-Mj denotes the URI of the selected Memento, URI-Mi
277 denotes the URI of the Memento that is temporally previous to the
278 selected Memento, and URI-Mk denotes the URI of the Memento that
279 is temporally after the selected Memento. The respective
280 datetimes for these Mementos are T0, Tn, Tj, Ti, and Tk; it holds
281 that T0 <= Ti <= Tj <= Tk <= Tn.
283 2. The Memento Framework, Datetime Negotiation component: HTTP headers,
284 HTTP Link Relation Types
286 The Memento framework is concerned with Original Resources,
287 TimeGates, Mementos, and TimeMaps that are identified by HTTP or
288 HTTPS URIs. Details are only provided for resources identified by
289 HTTP URIs but apply similarly to those with HTTPS URIs.
291 2.1. HTTP Headers
293 The Memento framework operates at the level of HTTP request and
294 response headers. It introduces two new headers ("Accept-Datetime",
295 "Memento-Datetime"), introduces new values for two existing headers
296 ("Vary", "Link"), and uses an existing header ("Location") without
297 modification. All these headers are described below. Other HTTP
298 headers are present or absent in Memento response/request cycles as
299 specified by RFC 2616 [RFC2616].
301 2.1.1. Accept-Datetime, Memento-Datetime
303 The "Accept-Datetime" request header is used by a user agent to
304 indicate it wants to retrieve a representation of a Memento that
305 encapsulates a past state of an Original Resource. To that end, the
306 "Accept-Datetime" header is conveyed in an HTTP GET/HEAD request
307 issued against a TimeGate for an Original Resource, and its value
308 indicates the datetime of the desired past state of the Original
309 Resource. The "Accept-Datetime" request header has no defined
310 meaning for HTTP methods other than HEAD and GET.
312 The "Memento-Datetime" response header is used by a server to
313 indicate that the response contains a representation of a Memento,
314 and its value expresses the datetime of the state of an Original
315 Resource that is encapsulated in that Memento. The URI of that
316 Original Resource is provided in the response, as the Target IRI (see
317 RFC5988 [RFC5988]) of a link provided in the HTTP "Link" header that
318 has a Relation Type of "original" (see Section 2.2).
320 The presence of a "Memento-Datetime" header and associated value for
321 a given resource constitutes a promise that the resource is stable
322 and that its state will no longer change. This means that, in terms
323 of the Ontology for Relating Generic and Specific Information
324 Resources (see W3C.gen-ont-20090420 [W3C.gen-ont-20090420]), a
325 Memento is a FixedResource.
327 As a consequence, "Memento-Datetime" headers associated with a
328 Memento MUST be "sticky" in the following ways:
330 o The server that originally assigns the "Memento-Datetime" header
331 and value MUST retain that header in all responses to HTTP HEAD/
332 GET requests (with or without "Accept-Datetime" header) that occur
333 against the Memento after the time of the original assignment of
334 the header, and it MUST NOT change its associated value.
336 o Applications that mirror Mementos at a different URI MUST NOT
337 change the "Memento-Datetime" header and value of those Mementos
338 unless mirroring involves a meaningful state change. This allows,
339 for example, duplicating a Web archive at a new location while
340 preserving the value of the "Memento-Datetime" header of the
341 archived resources. In this example, the "Last-Modified" header
342 will be updated to reflect the time of mirroring at the new URI,
343 whereas the value for "Memento-Datetime" will be sticky.
345 2.1.1.1. Values for Accept-Datetime
347 Values for the "Accept-Datetime" header consist of a MANDATORY
348 datetime expressed according to the RFC 1123 [RFC1123] format, which
349 is formalized by the rfc1123-date construction rule of the BNF in
350 Figure 1, and an OPTIONAL interval indicator expressed according to
351 the iso8601-interval rule of the BNF in Figure 1. The datetime MUST
352 be represented in Greenwich Mean Time (GMT).
354 Examples of "Accept-Datetime" request headers with and without an
355 interval indicator:
357 Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT
358 Accept-Datetime: Thu, 31 May 2007 20:35:00 GMT; -P3DT5H;+P2DT6H
360 The user agent uses the MANDATORY datetime value to convey its
361 preferred datetime for a Memento; it uses the OPTIONAL interval
362 indicator to convey it is interested in retrieving Mementos that
363 reside within this interval around the preferred datetime, and not
364 interested in Mementos that reside outside of it. Not using an
365 interval indicator is equivalent to expressing an infinite interval
366 around the preferred datetime.
368 The interval mechanism can be regarded as an implementation of the
369 functionality intended by the q-value approach that is used in
370 regular content negotiation. The q-value approach is not supported
371 for Memento's datetime negotiation because it is well-suited for
372 negotiation over a discrete space of mostly predictable values, not
373 for negotiation over a continuum of unpredictable datetime values.
375 accept-dt-value = rfc1123-date *SP [ iso8601-interval ]
376 rfc1123-date = wkday "," SP date1 SP time SP "GMT"
377 date1 = 2DIGIT SP month SP 4DIGIT
378 ; day month year (e.g., 20 Mar 1957)
379 time = 2DIGIT ":" 2DIGIT ":" 2DIGIT
380 ; 00:00:00 - 23:59:59 (e.g., 14:33:22)
381 wkday = "Mon" | "Tue" | "Wed" | "Thu" | "Fri" | "Sat" |
382 "Sun"
383 month = "Jan" | "Feb" | "Mar" | "Apr" | "May" | "Jun" |
384 "Jul" | "Aug" | "Sep" | "Oct" | "Nov" | "Dec"
385 iso8601-interval = ";" *SP "-" duration *SP ";" *SP "+" duration
386 duration = "P" ( dur-date | dur-week )
387 dur-date = ( dur-day | dur-month | dur-year ) [ dur-time ]
388 dur-year = 1*DIGIT "Y" [ dur-month ] [ dur-day ]
389 dur-month = 1*DIGIT "M" [ dur-day ]
390 dur-day = 1*DIGIT "D"
391 dur-time = "T" ( dur-hour | dur-minute | dur-second )
392 dur-hour = 1*DIGIT "H" [ dur-minute ] [ dur-second ]
393 dur-minute = 1*DIGIT "M" [ dur-second ]
394 dur-second = 1*DIGIT "S"
395 dur-week = 1*DIGIT "W"
397 Figure 1: BNF for the datetime format
399 2.1.1.2. Values for Memento-Datetime
401 Values for the "Memento-Datetime" headers MUST be datetimes expressed
402 according to the rfc1123-date construction rule of the BNF in
403 Figure 1; they MUST be represented in Greenwich Mean Time (GMT).
405 An example "Memento-Datetime" response header:
407 Memento-Datetime: Wed, 30 May 2007 18:47:52 GMT
409 2.1.2. Vary
411 The "Vary" response header is used in responses to indicate the
412 dimensions in which content negotiation was successfully applied.
413 This header is used in the Memento framework to indicate both whether
414 datetime negotiation was applied or is supported by the responding
415 server.
417 For example, this use of the "Vary" header indicates that datetime is
418 the only dimension in which negotiation was applied:
420 Vary: negotiate, accept-datetime
421 The use of the "Vary" header in this example shows that both datetime
422 negotiation, and media type content negotiation were applied:
424 Vary: negotiate, accept-datetime, accept
426 2.1.3. Location
428 The "Location" header is used as defined in RFC 2616 [RFC2616].
429 Examples are given in Section 3 below.
431 2.1.4. Link
433 The "Link" response header is specified in RFC5988 [RFC5988]. The
434 Memento framework introduces new Relation Types to convey typed links
435 among Original Resources, TimeGates, Mementos, and TimeMaps. Already
436 existing Relation Types, among others, aimed at supporting navigation
437 among a series of ordered resources may also be used in the Memento
438 framework. This is detailed in Link Header Relation Types
439 (Section 2.2), below.
441 2.2. Link Header Relation Types
443 The "Link" header specified in RFC5988 [RFC5988] is semantically
444 equivalent to the "" element in HTML, as well as the "atom:
445 link" feed-level element in Atom RFC 4287 [RFC4287]. By default, the
446 origin of a link expressed by an entry in a "Link" header (named
447 Context IRI in RFC5988 [RFC5988]) is the IRI of the requested
448 resource. This default can be overwritten using the "anchor"
449 attribute in the entry.
451 2.2.1. Memento Framework Relation Types
453 The Relation Types used in the Memento framework are listed in the
454 remainder of this section, and their use is summarized in the below
455 table. Appendix A shows a Memento request/response cycle that uses
456 all the Relation Types that are introduced here.
458 +-----------+-------------------+-----------------+-----------------+
459 | Relation | Original Resource | TimeGate | Memento |
460 | Type | | | |
461 +-----------+-------------------+-----------------+-----------------+
462 | original | NA, except see | REQUIRED, 1 | REQUIRED, 1 |
463 | | Section 3.1.2.1 | | |
464 | timegate | RECOMMENDED, 0 or | NA | RECOMMENDED, 0 |
465 | | more | | or more |
466 | timemap | NA | RECOMMENDED, 0 | RECOMMENDED, 0 |
467 | | | or more | or more |
468 | memento | NA, except see | REQUIRED, 1 or | REQUIRED, 1 or |
469 | | Section 3.1.2.1 | more | more |
470 +-----------+-------------------+-----------------+-----------------+
472 Table 1: The use of Relation Types
474 2.2.1.1. Relation Type "original"
476 "original" -- A "Link" header entry with a Relation Type of
477 "original" is used to point from a TimeGate or a Memento to their
478 associated Original Resource. In both cases, an entry with the
479 "original" Relation Type MUST occur exactly once in a "Link" header.
480 Details for the entry are as follows:
482 o Context IRI: URI-G, URI-M
484 o Target IRI: URI-R
486 o Relation Type: "original"
488 o Use: REQUIRED
490 o Cardinality: 1
492 2.2.1.2. Relation Type "timegate"
494 "timegate" -- A "Link" header entry with a Relation Type of
495 "timegate" is used to point both from an Original Resource or a
496 Memento to a TimeGate for the Original Resource. In both cases, the
497 use of an entry with the "timegate" Relation Type is RECOMMENDED.
498 Since more than one TimeGate can exist for any Original Resource,
499 multiple entries with a "timegate" Relation Type MAY occur, each with
500 a distinct Target IRI. Since a TimeGate has no mime type, the "type"
501 attribute MUST NOT be used on Links with a "timegate" Relation Type.
502 Details for the entry are as follows:
504 o Context IRI: URI-R or URI-Mj
506 o Target IRI: URI-G
508 o Relation Type: "timegate"
510 o Use: RECOMMENDED
512 o Cardinality: 0 or more
514 2.2.1.3. Relation Type "timemap"
516 "timemap" -- A "Link" header entry with a Relation Type of "timemap"
517 is used to point from both a TimeGate or a Memento to a TimeMap
518 resource from which a list of Mementos known to the responding server
519 is available. Use of an entry with the "timemap" Relation Type is
520 RECOMMENDED, and, since multiple serializations of a TimeMap are
521 possible, multiple entries with a "timemap" Relation Type MAY occur,
522 each with a distinct Target IRI, and each with a MANDATORY "type"
523 attribute to convey the mime type of the TimeMap serialization.
524 Details for the entry are as follows:
526 o Context IRI: URI-G or URI-Mi
528 o Target IRI: URI-T
530 o Relation Type: "timemap"
532 o Target Attribute: "type"
534 o Use: RECOMMENDED
536 o Cardinality: 0 or more
538 Further details about TimeMap serializations are provided in
539 Section 3.4.
541 2.2.1.4. Relation Type "memento"
543 "memento" -- A "Link" header entry with a Relation Type of "memento"
544 is used to point from both a TimeGate and a Memento to various
545 Mementos for an Original Resource. This link MUST include a
546 "datetime" attribute with a value that matches the "Memento-Datetime"
547 of the Memento that is the target of the link; that is, the value of
548 the "Memento-Datetime" header that is returned when the URI of the
549 linked Memento is dereferenced. In addition, the link MAY include an
550 "embargo" attribute to convey the datetime until which the Memento
551 will remain inaccessible. The value for both the "datetime" and
552 "embargo" attributes MUST be a datetime expressed according to the
553 rfc1123-date construction rule of the BNF in Figure 1 and it MUST be
554 represented in Greenwich Mean Time (GMT). This link MAY also include
555 a "license" attribute to associate a license with the Memento; the
556 value for the "license" attribute SHOULD be a URI. The link SHOULD
557 also include a "type" attribute to convey the mime type of the
558 Memento that is the target of the link. Use of entries with the
559 "memento" Relation Type is REQUIRED and it MUST be as follows:
561 For all responses to HTTP HEAD/GET requests issued against a TimeGate
562 or a Memento in which a Memento is selected or served by the
563 responding server:
565 o One "memento" link MUST be included that has as Target IRI the URI
566 of the Memento that was selected or served;
568 o One "memento" link MUST be included that has as Target IRI the URI
569 of the temporally first Memento known to the responding server;
571 o One "memento" link MUST be included that has as Target IRI the URI
572 of the temporally most recent Memento known to the responding
573 server.
575 o One "memento" link SHOULD be included that has as Target IRI the
576 URI of the Memento that is previous to the selected Memento in the
577 temporal series of all Mementos (sorted by ascending "Memento-
578 Datetime" values) known to the server;
580 o One "memento" link SHOULD be included that has as Target IRI the
581 URI the Memento that is next to the selected Memento in the
582 temporal series of all Mementos (sorted by ascending "Memento-
583 Datetime" values) known to the server.
585 o Other "memento" links MAY only be included if both the
586 aforementioned previous and next links are provided. Each of
587 these OPTIONAL "memento" links MUST have as Target IRI the URI of
588 a Memento other than the ones listed above.
590 For all responses to HTTP HEAD/GET requests issued against an
591 existing TimeGate or Memento in which no Memento is selected or
592 served by the responding server:
594 o One "memento" link MUST be included that has as Target IRI the URI
595 of the temporally first Memento known to the responding server;
597 o One "memento" link MUST be included that has as Target IRI the URI
598 of the temporally most recent Memento known to the responding
599 server.
601 o Other "memento" links MAY be included, and each of these OPTIONAL
602 links MUST have as Target IRI the URI of a Memento other than the
603 two listed above.
605 Note that the Target IRI of some of these links may coincide. For
606 example, if the selected Memento actually is the first Memento known
607 to the server, only three distinct "memento" links may result. The
608 value for the "datetime" attribute of these links would be the
609 datetimes of the first (equal to selected), next, and most recent
610 Memento known to the responding server.
612 The summary is as follows:
614 o Context IRI: URI-G, URI-Mj
616 o Target IRI: URI-M
618 o Relation Type: "memento"
620 o Target Attributes: "datetime", "embargo", "license"
622 o Use: REQUIRED
624 o Cardinality: 1 or more
626 2.2.2. Other Relation Types
628 Web Linking RFC5988 [RFC5988] allows for the inclusion of links with
629 different Relation Types but the same Target IRI, and hence the
630 Relation Types introduced by the Memento framework MAY be combined
631 with others as deemed necessary. As the "memento" Relation Type
632 focuses on conveying the datetime of a linked Memento, Relation Types
633 that allow navigating among the temporally ordered series of Mementos
634 known to a server are of particular importance. With this regard,
635 the Relation Types listed in the below table SHOULD be considered for
636 combination with the "memento" Relation Type. A distinction is made
637 between responding servers that can be categorized as systems that
638 are the focus of RFC5829 [RFC5829] (such as version control systems)
639 and others that can not (such as Web archives). Note that, in terms
640 of RFC5829 [RFC5829], the last Memento (URI-Mn) is the version prior
641 to the latest (i.e. current) version.
643 +-----------------------------+---------------------+---------------+
644 | Memento Type | RFC5988 system | non RFC5988 |
645 | | | system |
646 +-----------------------------+---------------------+---------------+
647 | First Memento (URI-M0) | first | first |
648 | Last Memento (URI-Mn) | last | last |
649 | Selected Memento (URI-Mj) | NA | NA |
650 | Memento prior to selected | predecessor-version | prev |
651 | Memento (URI-Mi) | | |
652 | Memento next to selected | successor-version | next |
653 | Memento (URI-Mk) | | |
654 +-----------------------------+---------------------+---------------+
656 Table 2: The use of Relation Types
658 3. The Memento Framework, Datetime Negotiation component: HTTP
659 Interactions
661 This section describes the HTTP interactions of the Memento framework
662 for a variety of scenarios. First, Figure 2 provides a schematic
663 overview of a successful request/response chain that involves
664 datetime negotiation. Dashed lines depict HTTP transactions between
665 user agent and server. Appendix A shows these HTTP interactions in
666 detail for the case where the Original Resource resides on one
667 server, whereas both the TimeGate and the Mementos reside on another.
668 Scenarios also exist in which all these resources are on the same
669 server (for example, Content Management Systems) or on different
670 servers (for example, an aggregator of TimeGates). Note that, in
671 Step 2 and Step 6, the HTTP status code of the response is shown as
672 "200 OK", but a series of "206 Partial Content" responses could be
673 substituted without loss of generality.
675 1: UA --- HTTP GET/HEAD; Accept-Datetime: Tj ---------------> URI-R
676 2: UA <-- HTTP 200; Link: URI-G ----------------------------- URI-R
677 3: UA --- HTTP GET/HEAD; Accept-Datetime: Tj ---------------> URI-G
678 4: UA <-- HTTP 302; Location: URI-Mj; Vary; Link:
679 URI-R,URI-T,URI-M0,URI-Mn,URI-Mi,URI-Mj,URI-Mk -------- URI-G
680 5: UA --- HTTP GET URI-Mj; Accept-Datetime: Tj -------------> URI-Mj
681 6: UA <-- HTTP 200; Memento-Datetime: Tj; Link:
682 URI-R,URI-T,URI-G,URI-M0,URI-Mn,URI-Mi,URI-Mj,URI-Mk -- URI-Mj
684 Figure 2: Typical Memento request/response chain
686 o Step 1: In order to determine what the URI is of a TimeGate for an
687 Original Resource, the user agent issues an HTTP HEAD/GET request
688 against the URI of the Original Resource (URI-R).
690 o Step 2: The entity-header of the response from URI-R includes an
691 HTTP "Link" header with a Relation Type of "timegate" pointing at
692 a TimeGate (URI-G) for the Original Resource.
694 o Step 3: The user agent starts the datetime negotiation process
695 with the TimeGate by issuing an HTTP GET request against its URI-G
696 thereby including an "Accept-Datetime" HTTP header with a value of
697 the datetime of the desired prior state of the Original Resource.
699 o Step 4: The entity-header of the response from URI-G includes a
700 "Location" header pointing at the URI of a Memento (URI-Mj) for
701 the Original Resource. In addition, the entity-header contains an
702 HTTP "Link" header with a Relation Type of "original" pointing at
703 the Original Resource, and an HTTP "Link" header with a Relation
704 Type of "timemap" pointing at a TimeMap (URI-T). Also HTTP Links
705 pointing at various Mementos are provided using the "memento"
706 Relation Type, as specified in Section 2.2.1.4.
708 o Step 5: The user agent issues an HTTP GET request against the
709 URI-Mj of a Memento, obtained in Step 4.
711 o Step 6: The entity-header of the response from URI-Mj includes a
712 "Memento-Datetime" HTTP header with a value of the datetime of the
713 Memento. It also contains an HTTP "Link" header with a Relation
714 Type of "original" pointing at the Original Resource, with a
715 Relation Type of "timegate" pointing at a TimeGate associated with
716 the Original Resource, and with a Relation Type of "timemap"
717 pointing at a TimeMap. The state that is expressed by the
718 representation provided in the response is the state the Original
719 Resource had at the datetime expressed in the "Memento-Datetime"
720 header. This response also includes HTTP Links with a "memento"
721 Relation Type pointing at various Mementos, as specified in
722 Section 2.2.1.4.
724 The following sections detail the specifics of HTTP interactions with
725 Original Resources, TimeGates, Mementos, and TimeMaps under various
726 conditions.
728 3.1. Interactions with an Original Resource
730 This section details HTTP GET/HEAD requests targeted at an Original
731 Resource (URI-R).
733 3.1.1. Step 1: User Agent Requests an Original Resource
735 In order to try and discover a TimeGate for the Original Resource,
736 the user agent SHOULD issue an HTTP HEAD or GET request against the
737 Original Resource's URI. Use of the "Accept-Datetime" header in the
738 HTTP HEAD/GET request is OPTIONAL.
740 Figure 3 shows the use of HTTP HEAD indicating the user agent is not
741 interested in retrieving a representation of the Original Resource,
742 but only in determining a TimeGate for it. It also shows the use of
743 the "Accept-Datetime" header anticipating that the user agent will
744 set it for the entire duration of a Memento request/response cycle.
746 HEAD / HTTP/1.1
747 Host: a.example.org
748 Accept-Datetime: Tue, 11 Sep 2001 20:35:00 GMT
749 Connection: close
751 Figure 3: User Agent Requests Original Resource
753 3.1.2. Step 2: Server Responds to a Request for an Original Resource
755 The response of the Original Resource's server to the user agent's
756 HTTP HEAD/GET request of Step 1, for the case where the Original
757 Resource exists, is as it would be in a regular HTTP request/response
758 cycle, but in addition MAY include a HTTP "Link" header with a
759 Relation Type of "timegate" that conveys the URI of the Original
760 Resource's TimeGate as the Target IRI of the Link. Multiple HTTP
761 Links with a relation type of "timegate" MAY be provided to
762 accommodate situations in which the server is aware of multiple
763 TimeGates for an Original Resource. The actual Target IRI provided
764 in the "timegate" Link may depend on several factors including the
765 datetime provided in the "Accept-Datetime" header, and the IP address
766 of the user agent. A response for this case is illustrated in
767 Figure 4.
769 HTTP/1.1 200 OK
770 Date: Thu, 21 Jan 2010 00:02:12 GMT
771 Server: Apache
772 Link:
773 ; rel="timegate"
774 Content-Length: 255
775 Connection: close
776 Content-Type: text/html; charset=iso-8859-1
778 Figure 4: Server of Original Resource Responds
780 Servers that actively maintain archives of their resources SHOULD
781 include the "timegate" HTTP "Link" header because this link is an
782 important way for a user agent to discover TimeGates for those
783 resources. This includes servers such as Content Management Systems,
784 Control Version Systems, and Web servers with associated
785 transactional archives Fitch [Fitch]. Servers that do not actively
786 maintain archives of their resources MAY include the "timegate" HTTP
787 "Link" header as a way to convey a preference for TimeGates for their
788 resources exposed by a third party archive. This includes servers
789 that rely on Web archives such as the Internet Archive to archive
790 their resources.
792 The server of the Original Resource MUST treat requests with and
793 without an "Accept-Datetime" header in the same way:
795 o The response MUST either always or never include a HTTP "Link"
796 header with an entry that has a "timegate" Relation Type and the
797 URI of a TimeGate as the Target IRI.
799 o The entity-body of the response MUST be the same, for user agent
800 requests with or without a "Accept-Datetime" header.
802 3.1.2.1. Original Resource is an Appropriate Memento
804 The "Memento-Datetime" header MAY be applied to an Original Resource
805 directly to indicate it is a FixedResource (see W3C.gen-ont-20090420
806 [W3C.gen-ont-20090420]), meaning that the state of the Original
807 Resource has not changed since the datetime conveyed in the "Memento-
808 Datetime" header, and as a promise that it will not change anymore
809 beyond it. This may occur, for example, for certain stable media
810 resources on news sites. In case the user agent's preferred datetime
811 is equal to or more recent than the datetime conveyed as the value of
812 "Memento-Datetime" in the server's response in Step 2, the user agent
813 SHOULD conclude it has located an appropriate Memento, and it SHOULD
814 NOT continue to Step 3.
816 Figure 5 illustrates such a response to a request for the resource
817 with URI http://a.example.org/pic that has been stable since it was
818 created. Note the use of both the "memento" and "original" Relation
819 Types for links that have as Target IRI the URI of the Original
820 Resource.
822 HTTP/1.1 200 OK
823 Date: Thu, 21 Jan 2010 00:02:12 GMT
824 Server: Apache
825 Link:
826
827 ; rel="original memento"
828 ; datetime="Fri, 20 Mar 2009 11:00:00 GMT"
829 Memento-Datetime: Fri, 20 Mar 2009 11:00:00 GMT
830 Content-Length: 255
831 Connection: close
832 Content-Type: text/html; charset=iso-8909-1
834 Figure 5: Response to a request for an Original Resource that was
835 created as a FixedResource
837 Cases may also exist in which a resource becomes stable at a certain
838 point in its existence, but changed previously. In such cases, the
839 Original Resource may know about a TimeGate that is aware of its
840 prior history and hence MAY also include a link with a "timegate"
841 Relation Type. This is illustrated in Figure 6, where the "memento"
842 and "original" Relation Types are used as in Figure 5, and the
843 existence of a TimeGate to negotiate for Mementos with datetimes
844 prior to Fri, 20 Mar 2009 11:00:00 GMT is indicated.
846 HTTP/1.1 200 OK
847 Date: Thu, 21 Jan 2010 00:02:12 GMT
848 Server: Apache
849 Link:
850
851 ; rel="original memento"
852 ; datetime="Fri, 20 Mar 2009 11:00:00 GMT",
853
854 ; rel="timegate"
855 Memento-Datetime: Fri, 20 Mar 2009 11:00:00 GMT
856 Content-Length: 255
857 Connection: close
858 Content-Type: text/html; charset=iso-8909-1
860 Figure 6: Response to a request for an Original Resource that became
861 a FixedResource
863 3.1.2.2. Server Exists and Original Resource Used to Exist
865 Servers SHOULD also provide a "timegate" HTTP "Link" header in
866 responses to requests for an Original Resource that the server knows
867 used to exist, but no longer does. This allows the use of an
868 Original Resource's URI as an entry point to representations of its
869 prior states even if the resource itself no longer exists. A
870 server's response for this case is illustrated in Figure 7.
872 HTTP/1.1 404 Not Found
873 Date: Thu, 21 Jan 2010 00:02:12 GMT
874 Server: Apache
875 Link:
876
877 ; rel="timegate"
878 Content-Length: 255
879 Connection: close
880 Content-Type: text/html; charset=iso-8909-1
882 Figure 7: Response to a request for an Original Resource that not
883 longer exists
885 In case the server is not aware of the prior existence of the
886 Original Resource, its response SHOULD NOT include a "timegate" HTTP
887 Link. Section 3.1.2.3 details what the user agent's behavior should
888 be in such cases.
890 3.1.2.3. Missing or Inadequate "timegate" Link in Original Server's
891 Response
893 A user agent MAY ignore the TimeGate returned in Step 2. However,
894 when engaging in a Memento request/response cycle, a user agent
895 SHOULD NOT proceed immediately to Step 3 by using a TimeGate of its
896 own preference but rather SHOULD always start the cycle by issuing an
897 HTTP GET/HEAD against the Original Resource (Step 1, Figure 3) as it
898 is an important way to learn about dedicated or preferred TimeGates
899 for the Original Resource. Also, cases exist in which the response
900 in Step 2 will not provide a "timegate" link, including:
902 o The Original Resource's server does not support the Memento
903 framework;
905 o The Original Resource no longer exists and the responding server
906 is not aware of its prior existence;
908 o The server that hosted the Original Resource no longer exists;
910 In all these cases, the user agent SHOULD attempt to determine an
911 appropriate TimeGate for the Original Resource, either automatically
912 or interactively supported by the user. The discovery mechanisms
913 described in Section 4 can support the user agent with this regard.
915 3.2. Interactions with a TimeGate
917 This section details HTTP GET/HEAD requests targeted at a TimeGate
918 (URI-G).
920 3.2.1. Step 3: User Agent Negotiates with a TimeGate
922 In order to negotiate with a TimeGate, the user agent MUST issue a
923 HTTP HEAD or GET against its URI, its request MUST include the
924 "Accept-Datetime" header to express its datetime preference, and the
925 use of that header MUST be as described in Section 2.1.1.1. The URI
926 of the TimeGate may have been provided as the Target IRI of a
927 "timegate" HTTP "Link" header in the response from the Original
928 Resource (Step 2, Figure 4), or may have resulted from another
929 discovery mechanism (see Section 4) or user interaction. Such a
930 request is illustrated in Figure 8.
932 GET /timegate/http://a.example.org HTTP/1.1
933 Host: arxiv.example.net
934 Accept-Datetime: Tue, 11 Sep 2001 20:35:00 GMT
935 Connection: close
937 Figure 8: User agent negotiates with TimeGate
939 3.2.2. Step 4: Server Responds to Negotiation with TimeGate
941 In order to respond to a datetime negotiation request (Step 3,
942 Section 3.2.1), the server uses an internal algorithm to select the
943 Memento that best meets the user agent's datetime preference, and
944 redirects to it. The exact nature of the selection algorithm is at
945 the server's discretion but SHOULD be consistent. A variety of
946 approaches can be used including selecting the Memento that is
947 nearest in time (either past or future) or nearest in the past
948 relative to the requested datetime. Special cases for datetime
949 negotiation with a TimeGate exist, and they are addressed in
950 Section 3.2.2.3 through Section 3.2.2.7.
952 3.2.2.1. Successful Scenario
954 In cases where the TimeGate exists, and the datetime provided in the
955 user agent's "Accept-Datetime" header can be parsed and is not out of
956 the user agent's range (see Section 3.2.2.5), the server selects a
957 Memento based on the user agent's datetime preference. The response
958 MUST have a "302 Found" HTTP status code, and the "Location" header
959 MUST be used to convey the URI of the selected Memento. The "Vary"
960 header MUST be provided and it MUST include the "negotiate" and
961 "accept-datetime" values to indicate that datetime negotiation has
962 taken place. The "Link" header MUST be provided and contain links
963 with Relation Types subject to the considerations described in
964 Section 2.2. Such a response is illustrated in Figure 9.
966 HTTP/1.1 302 Found
967 Date: Thu, 21 Jan 2010 00:06:50 GMT
968 Server: Apache
969 Vary: negotiate, accept-datetime
970 Location:
971 http://arxiv.example.net/web/20010911203610/http://a.example.org
972 Link: ; rel="original",
973
974 ; rel="timemap"; type="application/link-format",
975
976 ; rel="first memento"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
977
978 ; rel="last memento"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
979
980 ; rel="memento"; datetime="Tue, 11 Sep 2001 20:36:10 GMT",
981
982 ; rel="prev memento"; datetime="Tue, 11 Sep 2001 20:30:51 GMT",
983
984 ; rel="next memento"; datetime="Tue, 11 Sep 2001 20:47:33 GMT"
985 Content-Length: 0
986 Content-Type: text/plain; charset=UTF-8
987 Connection: close
989 Figure 9: Server of TimeGate responds
991 Note that if a user agent's "Accept-Datetime" header does not convey
992 an interval indicator, and conveys a datetime that is either earlier
993 than the datetime of the first Memento or later than the datetime of
994 the most recent Memento known to the server, the server's response is
995 as just described yet entails the selection of the first or most
996 recent Memento, respectively. This approach is consistent with
997 interpreting the abscence of an interval indicator in the user
998 agent's request as an indication of an infinite interval around its
999 preferred datetime (see Section 2.1.1.1).
1001 This is illustrated in Figure 10 that shows the response from a
1002 TimeGate exposed by a MediaWiki server to a request by a user agent
1003 that has an "Accept-Datetime: Mon, 31 May 1999 00:00:00 GMT" header.
1004 Note that a link is provided with a "successor-version" Relation Type
1005 but not with a "predecessor-version" Relation Type.
1007 HTTP/1.1 302 Found
1008 Server: Apache
1009 Content-Length: 709
1010 Content-Type: text/html; charset=utf-8
1011 Date: Thu, 21 Jan 2010 00:09:40 GMT
1012 Location:
1013 http://a.example.org/w/index.php?title=Clock&oldid=1493688
1014 Vary: negotiate, accept-datetime
1015 Link: ; rel="original",
1016
1017 ; rel="timemap",
1018
1019 ; rel="first memento"; datetime="Sun, 28 Sep 2003 01:42:00 GMT",
1020
1021 ; rel="successor-version memento"
1022 ; datetime="Tue, 30 Sep 2003 14:28:00 GMT",
1023
1024 ; rel="last memento"; datetime="Tue, 12 Jan 2010 19:55:00 GMT"
1025 Connection: close
1027 Figure 10: A TimeGate's response to a request for a Memento with a
1028 datetime earlier than that of the first Memento
1030 3.2.2.2. Accept-Datetime and other Accept Headers Provided
1032 When interacting with a TimeGate, the regular content negotiation
1033 dimensions (media type, character encoding, language, and
1034 compression) remain available. It is the TimeGate server's
1035 responsibility to honor (or not) such content negotiation, and in
1036 doing so it MUST always first select a Memento that meets the user
1037 agent's datetime preference, and then consider honoring regular
1038 content negotiation for it. As a result of this approach, the
1039 returned Memento will not necessarily meet the user agent's regular
1040 content negotiation preferences. Therefore, it is RECOMMENDED that
1041 the server provides HTTP Links with a "memento" Relation Type
1042 pointing at Mementos that do meet the user agent's regular content
1043 negotiation requests and that have a Memento-Datetime value in the
1044 temporal vicinity of the user agent's preferred datetime value.
1046 3.2.2.3. Accept-Datetime Not Provided
1048 In case, in Step 3, a user agent issues a request to a TimeGate and
1049 fails to include an "Accept-Datetime" request header, the response
1050 MUST be handled as in Section 3.2.2.1, with a selection of the most
1051 recent Memento known to the responding server.
1053 3.2.2.4. Multiple Matching Mementos
1055 Because the finest datetime granularity expressable using the RFC
1056 1123 [RFC1123] format used in HTTP is seconds level, cases may occur
1057 in which a TimeGate server is aware of multiple Mementos that meet
1058 the user agent's datetime preference. This may occur in Content
1059 Management Systems with very high update rates. The response in this
1060 case MUST be handled as in Section 3.2.2.1, with the selection of one
1061 of the matching Mementos.
1063 As an example, Figure 11 shows a hypothetical response from a
1064 TimeGate on a MediaWiki server to a request for a Memento for the
1065 Original Resource http://a.example.org/w/Clock for which two Mementos
1066 exist for the user agent's preferred datetime.
1068 HTTP/1.1 302 Found
1069 Server: Apache
1070 Content-Length: 705
1071 Content-Type: text/html; charset=utf-8
1072 Date: Thu, 21 Jan 2010 00:09:40 GMT
1073 Vary: negotiate, accept-datetime
1074 Location:
1075 http://a.example.org/w/index.php?title=Clock&oldid=322586071
1076 Link: ; rel="original",
1077
1078 ; rel="timemap";type="application/link-format",
1079
1080 ; rel="first memento"; datetime="Sun, 28 Sep 2003 01:42:00 GMT",
1081
1082 ; rel="last memento"; datetime="Tue, 12 Jan 2010 19:55:00 GMT",
1083
1084 ; rel="memento"; datetime="Sun, 31 May 2009 15:43:00 GMT",
1085
1086 ; rel="memento successor-version"
1087 ; datetime="Sun, 31 May 2009 15:43:00 GMT"
1088
1089 ; rel="memento predecessor-version"
1090 ; datetime="Sun, 31 May 2009 15:41:24 GMT"
1091 Connection: close
1093 Figure 11: A TimeGate's response to a request that has multiple
1094 Mementos with a matching datetime
1096 3.2.2.5. Datetime Out of the User Agent's Range
1098 In case, in Step 3, a user agent conveys an interval indicator, and
1099 the responding server is not aware of any Mementos with datetimes
1100 within the expressed interval, the server's response MUST have a "406
1101 Not Acceptable" HTTP status code. The use of the "Vary" header MUST
1102 be as described in Section 3.2.2.1. The use of the "Link" header
1103 MUST be as described in Section 2.2. Specifically, the use of links
1104 with a "memento" Relation Type MUST follow the rules for the case
1105 where no Memento is selected by the responding server
1106 (Section 2.2.1.4).
1108 Figure 12 shows a user agent using an "Accept-Datetime" header
1109 conveying an interval of interest starting 5 hours before and ending
1110 6 hours after Tue, 11 Sep 2001 20:35:00 GMT. Figure 13 shows the
1111 "406 Not Acceptable" response from the TimeGate that has links to the
1112 first and last Memento, as well to a Memento outside of the user
1113 agent's interval yet in the temporal vicinity of its preferred
1114 datetime.
1116 GET /timegate/http://a.example.org HTTP/1.1
1117 Host: arxiv.example.net
1118 Accept-Datetime: Tue, 11 Sep 2001 20:35:00 GMT; -P5H;+P6H
1119 Connection: close
1121 Figure 12: User agent expresses interval of interest in Accept-
1122 Datetime header
1124 HTTP/1.1 406 Not Acceptable
1125 Date: Thu, 21 Jan 2010 00:06:50 GMT
1126 Server: Apache
1127 Vary: negotiate, accept-datetime
1128 Link: ; rel="original",
1129
1130 ; rel="timemap";type="application/link-format",
1131
1132 ; rel="memento first"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
1133
1134 ; rel="memento last"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
1135
1136 ; rel="memento"; datetime="Mon, 10 Sep 2001 08:22:00 GMT"
1137 Content-Length: 1732
1138 Connection: close
1139 Content-Type: text/plain; charset=UTF-8
1141 Figure 13: A TimeGate's response indicating it has no Mementos within
1142 the interval of interest
1144 3.2.2.6. Accept-Datetime Unparseable
1146 In case, in Step 3, a user agent conveys a value for the "Accept-
1147 Datetime" request header that does not conform to the accept-dt-value
1148 construction rule of the BNF in Figure 1, the TimeGate server's
1149 response MUST have a "400 Bad Request" HTTP status code. With all
1150 other respects, responses in this case MUST be handled as described
1151 in Section 3.2.2.5
1153 3.2.2.7. TimeGate Does Not Exist
1155 Cases may occur in which a user agent issues a request against a
1156 TimeGate that does not exist. This may, for example, occur when a
1157 user agent uses internal knowledge to construct the URI of an
1158 assumed, yet non-existent TimeGate. In these cases, the response
1159 from the target server MUST have a "404 Not Found" HTTP status code,
1160 and SHOULD include a "Vary" header that includes the "negotiate" and
1161 "accept-datetime" values as an indication that, generally, the server
1162 is capable of datetime negotiation. The response MUST NOT include a
1163 "Link" header with any of the Relation Types introduced in
1164 Section 2.2.1.
1166 3.2.2.8. HTTP Methods other than HEAD/GET
1168 In the above, the safe HTTP methods GET and HEAD are described for
1169 TimeGates. TimeGates MAY support the safe HTTP methods OPTIONS and
1170 TRACE in the way described in RFC 2616 [RFC2616]. Unsafe HTTP
1171 methods (i.e. PUT, POST, DELETE) MUST NOT be supported by a
1172 TimeGate. Such requests MUST yield a response with a "405 Method Not
1173 Allowed" HTTP status code, and MUST include an "Allow" header to
1174 convey that only the HEAD and GET (and OPTIONALLY the OPTIONS and
1175 TRACE) methods are supported. In addition, the response MUST have a
1176 "Vary" header that includes the "negotiate" and "accept-datetime"
1177 values to indicate the TimeGate supports datetime negotiation.
1178 Figure 14 shows such a response.
1180 HTTP/1.1 405 Method Not Allowed
1181 Date: Thu, 21 Jan 2010 00:02:12 GMT
1182 Server: Apache
1183 Vary: negotiate, accept-datetime
1184 Allow: HEAD, GET
1185 Content-Length: 255
1186 Connection: close
1187 Content-Type: text/html; charset=iso-8909-1
1189 Figure 14: Response from a TimeGate accessed with HTTP method other
1190 than HEAD/GET
1192 3.2.3. Recognizing a TimeGate
1194 When a user agent issues a HTTP HEAD/GET request against a resource
1195 of which it found the URI as the Target IRI of an entry in the "Link"
1196 header with a "timegate" Relation Type, it SHOULD NOT assume that the
1197 targeted resource effectively is a TimeGate and hence will behave as
1198 described in Section 3.2.2.
1200 A user agent MUST decide it has reached a TimeGate if the response to
1201 a HTTP HEAD/GET request against the resource's URI contains a "Vary"
1202 header that includes the "negotiate" and "accept-datetime" values.
1203 If the response does not, the user agent MUST decide it has not
1204 reached a TimeGate and proceed as follows:
1206 o If the response contains a redirection, the user agent SHOULD
1207 follow it. Note that even a chain of redirections is possible,
1208 e.g. URI-R -> URI-1 -> URI-2 -> ... -> URI-G
1210 o If the response does not contain a redirection, or if the
1211 redirection (chain) does not lead to a TimeGate, the user agent
1212 SHOULD attempt to determine an appropriate TimeGate for the
1213 Original Resource, either automatically or interactively supported
1214 by the user. The discovery mechanisms described in Section 4 can
1215 support the user agent with this regard.
1217 Resources that are not TimeGates (i.e. do not behave as described in
1218 Section 3.2.2) MUST NOT use a "Vary" header that includes the
1219 "accept-datetime" value.
1221 3.3. Interactions with a Memento
1223 This section details HTTP GET/HEAD requests targeted at a Memento
1224 (URI-M).
1226 3.3.1. Step 5: User Agent Requests a Memento
1228 In Step 5, the user agent issues a HTTP GET request against the URI
1229 of a Memento. The user agent MAY include an "Accept-Datetime" header
1230 in this request, but the existence or absence of this header MUST NOT
1231 affect the server's response. The URI of the Memento may have
1232 resulted from a response in Step 4, or the user agent may simply have
1233 happened upon it. Such a request is illustrated in Figure 15.
1235 GET /web/20010911203610/http://a.example.org HTTP/1.1
1236 Host: arxiv.example.net
1237 Accept-Datetime: Tue, 11 Sep 2001 20:35:00 GMT
1238 Connection: close
1239 Figure 15: User agent requests Memento
1241 3.3.2. Step 6: Server Responds to a Request for a Memento
1243 If the Memento requested by the user agent in Step 5 exists, the
1244 server's response MUST have a "200 OK" HTTP status code (or "206
1245 Partial Content", where appropriate), and it MUST include a "Memento-
1246 Datetime" header with a value equal to the archival datetime of the
1247 Memento, that is, the datetime of the state of the Original Resource
1248 that is encapsulated in the Memento. The "Link" header MUST be
1249 provided and contain links subject to the considerations described in
1250 Section 2.2. The Target IRI and, when applicable, the datetime
1251 values in the "Link" header associated with the "memento" Relation
1252 Type SHOULD be the same as conveyed in Step 4, in case the TimeGate
1253 and the selected Memento reside on the same server. However, they
1254 MAY be different in case the TimeGate and the selected Memento reside
1255 on different servers.
1257 Figure 16 illustrates the server's response to the request issued
1258 against a Memento in Step 5 (Figure 15).
1260 HTTP/1.1 200 OK
1261 Date: Thu, 21 Jan 2010 00:09:40 GMT
1262 Server: Apache-Coyote/1.1
1263 Memento-Datetime: Tue, 11 Sep 2001 20:36:10 GMT
1264 Link: ; rel="original",
1265
1266 ; rel="timemap"; type="application/link-format",
1267
1268 ; rel="timegate",
1269
1270 ; rel="first memento"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
1271
1272 ; rel="last memento"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
1273
1274 ; rel="memento"; datetime="Tue, 11 Sep 2001 20:36:10 GMT",
1275
1276 ; rel="prev memento"; datetime="Tue, 11 Sep 2001 20:30:51 GMT",
1277
1278 ; rel="next memento"; datetime="Tue, 11 Sep 2001 20:47:33 GMT"
1279 Content-Length: 23364
1280 Content-Type: text/html;charset=utf-8
1281 Connection: close
1283 Figure 16: Server of Memento responds
1285 The server's response MUST include the "Memento-Datetime" header
1286 regardless whether the user agent's request contained an "Accept-
1287 Datetime" header or not. This is the way by which resources make
1288 explicit that they are Mementos. Due to the sparseness of Mementos
1289 in most archives, the value of the "Memento-Datetime" header returned
1290 by a server may differ (significantly) from the value conveyed by the
1291 user agent in "Accept-Datetime".
1293 Although a Memento encapsulates a prior state of an Original
1294 Resource, the entity-body returned in response to an HTTP GET request
1295 issued against a Memento may very well not be byte-to-byte the same
1296 as an entity-body that was previously returned by that Original
1297 Resource. Various reasons exist why there are significant chances
1298 these would be different yet do convey substantially the same
1299 information. These include format migrations as part of a digital
1300 preservation strategy, URI-rewriting as applied by some Web archives,
1301 and the addition of banners as a means to brand Web archives.
1303 3.3.2.1. Memento Does not Exist
1305 Cases may occur in which a TimeGate's response (Step 4) points at a
1306 Memento that actually does not exist, resulting in a user agent's
1307 request (Step 5) for a non-existent Memento. In this case, the
1308 server's response MUST have the expected "404 Not Found" HTTP Status
1309 Code and it MUST NOT contain a "Memento-Datetime" header.
1311 3.3.2.2. Mementos Without a TimeGate
1313 Cases may occur in which a server that hosts Mementos does not expose
1314 a TimeGate for those Mementos. This can, for example, be the case if
1315 the server's Mementos result from taking a snapshot of the state of a
1316 set of Original Resources from another server at the time this other
1317 server is being retired. As a result, only a single Memento per
1318 Original Resource is hosted, making the introduction of a TimeGate
1319 unnecessary. But it may also be the case for servers that hosts
1320 multiple Mementos for an Original Resource but consider exposing
1321 TimeGates too expensive.
1323 In cases of Mementos without associated TimeGates, responses to a
1324 request for a Memento by a user agent MUST be as described in
1325 Section 3.3.2 with the exception that it will not contain a HTTP Link
1326 with a "timegate" Relation Type pointing at a TimeGate exposed by the
1327 responding server. It MAY still contain such a Link pointing at a
1328 TimeGate exposed elsewhere. Depending on whether one or more
1329 Mementos are hosted for an Original Resource, the response may or may
1330 not have a HTTP Link with a "timemap" Relation Type. However, the
1331 response MUST still contain a "Memento-Datetime" response header with
1332 a value that corresponds to archival datetime of the Memento.
1334 Figure 17 illustrates the server's response to the request issued
1335 against a Memento in Step 5 (Figure 15) for the case that Memento has
1336 no associated TimeGate. In this example, it is also assumed there is
1337 only one Memento for the Original Resource, and hence the Links with
1338 Relation Types "memento", "first", "last" all point at the same -
1339 responding - Memento.
1341 HTTP/1.1 200 OK
1342 Date: Thu, 21 Jan 2010 00:09:40 GMT
1343 Server: Apache-Coyote/1.1
1344 Memento-Datetime: Tue, 11 Sep 2001 20:36:10 GMT
1345 Link: ; rel="original",
1346
1347 ; rel="first last memento"
1348 ; datetime="Tue, 15 Sep 2000 11:28:26 GMT"
1349 Content-Length: 23364
1350 Content-Type: text/html;charset=utf-8
1351 Connection: close
1353 Figure 17: Server of Memento without TimeGate responds
1355 Note that a server issuing a response similar to that of Figure 17
1356 does not imply that there is no server whatsoever that exposes a
1357 TimeGate; it merely means that the responding server neither provides
1358 nor is aware of the location of a TimeGate.
1360 3.3.3. Recognizing a Memento
1362 When following the redirection provided by a confirmed TimeGate (see
1363 Section 3.2.3), a user agent SHOULD NOT assume that the targeted
1364 resource effectively is a Memento and hence will behave as described
1365 in Section 3.3.2.
1367 A user agent MUST decide it has reached a Memento if the response to
1368 a HTTP HEAD/GET request against the resource's URI contains a
1369 "Memento-Datetime" header with a legitimate value. If the response
1370 does not, the following applies:
1372 o If the response contains a redirection, the user agent SHOULD
1373 follow it. Even a chain of redirections is possible, e.g. URI-G
1374 -> URI-X -> URI-Y -> ... -> URI-M.
1376 o If the response by a confirmed TimeGate does not contain a
1377 redirection, or if the redirection (chain) that started at a
1378 confirmed TimeGate does not lead to a resource that provides a
1379 "Memento-Datetime" header, the user agent MAY still conclude that
1380 it has likely arrived at a Memento. That is because cases exist
1381 in which Web archives and CMS are made compliant with the Memento
1382 framework "by proxy". In these cases TimeGates will redirect to
1383 Mementos in such systems, but the responses from these Mementos
1384 will not (yet) include a "Memento-Datetime" header.
1386 3.4. Interactions with a TimeMap
1388 A TimeMap is introduced to support retrieving a comprehensive list of
1389 all Mementos for a specific Original Resource, known to a responding
1390 server. The entity-body of a response to an HTTP GET request issued
1391 against a TimeMap's URI:
1393 o MUST list the URI of the Original Resource that the response lists
1394 Mementos for;
1396 o MUST list the URI and datetime of each Memento for the Original
1397 Resource known to the responding server;
1399 o MUST list the URI of one or more TimeGates for the Original
1400 Resource except when no TimeGate exists (see Section 3.3.2.2);
1402 o SHOULD, for self-containment, list the URI of the TimeMap itself;
1404 o MUST unambiguously type listed resources as being Original
1405 Resource, TimeGate, Memento, or TimeMap.
1407 The entity-body of a response from a TimeMap MAY be serialized in
1408 various ways, but the link-value format serialization MUST be
1409 supported. In this serialization, the entity-body MUST be formatted
1410 in the same way as the value of a HTTP "Link" header, and hence MUST
1411 comply to the "link-value" construction rule of "Section 5. The Link
1412 Header Field" of RFC5988 [RFC5988], and the media type of the entity-
1413 body MUST be "application/link-format" as introduced in I-D.ietf-
1414 core-link-format [I-D.ietf-core-link-format]. All links conveyed in
1415 this serialization MUST be interpreted as having the URI of the
1416 Original Resource as their Context IRI. The URI of the Original
1417 Resource is provided in the entity-body as the Target IRI of the link
1418 with an "original" Relation Type.
1420 3.4.1. User Agent Requests a TimeMap
1422 In order to retrieve the link-value serialization of a TimeMap, a
1423 user agent SHOULD use an "Accept" request header with a value set to
1424 "application/link-format". This is shown in Figure 18.
1426 GET /timemap/http://a.example.org HTTP/1.1
1427 Host: arxiv.example.net
1428 Accept: application/link-format;q=1.0
1429 Connection: close
1430 Figure 18: Request for a TimeMap
1432 3.4.2. Server Responds to a Request for a TimeMap
1434 If the TimeMap requested by the user agent exists, the server's
1435 response MUST have a "200 OK" HTTP status code (or "206 Partial
1436 Content", where appropriate). Note that a TimeMap is itself an
1437 Original Resource for which Mementos may exist. For example, a
1438 response from a TimeMap could provide a "timegate" Link to a TimeGate
1439 via which prior TimeMap versions are available. In this case, the
1440 use of the "Link" header is subject to all considerations described
1441 in Section 2.2, with the TimeMap acting as the Original Resource.
1443 However, in case a TimeMap wants to explicitly indicate in its
1444 response headers for which Original Resource it is a TimeMap, it MUST
1445 do so by including a HTTP "Link" header with the following
1446 characteristics:
1448 o The Context IRI for the HTTP Link is the URI of the Original
1449 Resource;
1451 o The Relation Type is "timemap";
1453 o The Target IRI for the HTTP Link is the URI of the TimeMap.
1455 Because the Context IRI of this HTTP Link is not the URI of the
1456 TimeMap, as per RFC5988 [RFC5988], the default Context IRI must be
1457 overwritten by using the "anchor" attribute with a value of the URI
1458 of the Original Resource.
1460 The response from the TimeMap to the request of Figure 18 is shown in
1461 Figure 19. The response header shows the TimeMap explicitly
1462 conveying the URI of the Original Resource for which it is a TimeMap;
1463 for practical reasons the entity-body in the example has been
1464 abbreviated. Notice also the use of the "license" and "embargo"
1465 attributes introduced in Section 2.2.1.4 on the "memento" links in
1466 the TimeMap.
1468 HTTP/1.1 200 OK
1469 Date: Thu, 21 Jan 2010 00:06:50 GMT
1470 Server: Apache
1471 Link:
1472 ; anchor="http://a.example.org"; rel="timemap"
1473 ; type="application/link-format"
1474 Content-Length: 4883
1475 Content-Type: application/link-format
1476 Connection: close
1478 ;rel="original",
1479
1480 ; rel="timemap";type="application/link-format",
1481
1482 ; rel="timegate",
1483
1484 ; rel="first memento";datetime="Tue, 20 Jun 2000 18:02:59 GMT"
1485 ; license="http://creativecommons.org/publicdomain/zero/1.0/",
1486
1487 ; rel="last memento";datetime="Tue, 27 Oct 2009 20:49:54 GMT"
1488 ; license="http://creativecommons.org/publicdomain/zero/1.0/"
1489 ; embargo="Tue, 19 Apr 2011 00:00:00 GMT",
1490
1491 ; rel="memento";datetime="Wed, 21 Jun 2000 01:17:31 GMT"
1492 ; license="http://creativecommons.org/publicdomain/zero/1.0/",
1493
1494 ; rel="memento";datetime="Wed, 21 Jun 2000 04:41:56 GMT"
1495 ; license="http://creativecommons.org/publicdomain/zero/1.0/",
1496 ...
1498 Figure 19: Response from a TimeMap
1500 4. The Memento Framework, Discovery Component
1502 Section 3 describes how TimeGates, Mementos, Original Resources, and
1503 TimeMaps can be discovered by following HTTP Links with Relation
1504 Types "timegate", "memento", "original", and "timemap", respectively.
1506 Naturally, some of these links can also be embedded into
1507 representations of resources that have a media type that allows
1508 embedding of typed links. For example, an Original Resource that has
1509 an HTML representation can include a "timegate" link by using HTML's
1510 LINK element, e.g. . The use of such embedded links is also subject to
1513 the considerations of Section 2.2.
1515 In this section additional approaches are introduced that support
1516 batch discovery of TimeGates, TimeMaps, and Mementos. The approaches
1517 leverage the Robots Exclusion Protocol and a special-purpose profile
1518 of Atom Feeds named TimeMap Feeds.
1520 4.1. Discovery of TimeGates Via Robots Exclusion Protocol
1522 The Robots Exclusion Protocol's robots.txt file [robotstxt.org] is
1523 commonly used by Web site owners to give instructions about their
1524 site to Web robots. It is used both to protect resources hosted by a
1525 server from crawling and to facilitate discovering them. This
1526 document introduces the "TimeGate" and "Archived" directives for
1527 robots.txt to provide a server-wide mechanism to support TimeGate
1528 discovery that SHOULD be used by:
1530 o Servers of Original Resources;
1532 o Servers that provide access to Mementos of Original Resources by
1533 exposing TimeGates.
1535 A robots.txt file MAY contain zero or more occurrences of the
1536 "TimeGate" directive, and each occurrence MUST be followed by one or
1537 more associated "Archived" directives. The meaning of the directives
1538 is as follows:
1540 o TimeGate: Conveys the base URL (that is URI scheme, host and path
1541 component) that is shared by all URIs of TimeGates of a set of
1542 Original Resources.
1544 o Archived: Indicates - by means of mandatory host and optional path
1545 parts of a URI - for which set of Original Resources actual
1546 TimeGates are available that have the base URL conveyed in the
1547 associated TimeGate directive.
1549 For example, consider a wiki at http://a.example.org/w/ that supports
1550 the Memento framework and exposes TimeGates to access the wiki's
1551 history pages at base URL
1552 http://a.example.org/w/index.php/Special:TimeGate/. An actual
1553 TimeGate for the wiki's http://a.example.org/w/My_Title page would
1554 then be at http://a.example.org/w/index.php/Special:TimeGate/http://
1555 a.example.org/w/My_Title. This wiki SHOULD make its TimeGates
1556 discoverable by using the directives shown in Figure 20 in its
1557 robots.txt file.
1559 TimeGate: http://a.example.org/w/index.php/Special:TimeGate/
1560 Archived: a.example.org/w/
1562 Figure 20: robots.txt for a wiki, host of Original Resources,
1563 TimeGates, and Mementos
1565 As another example, consider a server of Original Resources at
1566 http://a.example.org/ and http://www.a.example.org/ that is aware
1567 that its resources are regularly crawled by a Web archive that
1568 generally exposes TimeGates at base URL
1569 http://arxiv.example.net/timegate/ and hence has TimeGate
1570 http://arxiv.example.net/timegate/http://a.example.org/ to access
1571 Mementos for http://a.example.org/. This server SHOULD make the
1572 remote TimeGates discoverable by including the directives shown in
1573 Figure 21 in its robots.txt file:
1575 TimeGate: http://arxiv.example.net/timegate/
1576 Archived: a.example.org/
1577 Archived: www.a.example.org/
1579 Figure 21: robots.txt for a server of Original Resources aware of
1580 remote TimeGates
1582 And, consider a Web archive that crawls a wide range of Original
1583 Resources, and exposes TimeGates to access the resulting Mementos at
1584 base URL http://arxiv.example.net/timegate/. In order to make its
1585 TimeGates discoverable, this Web archive SHOULD include the
1586 directives shown in Figure 22 in its robots.txt file:
1588 TimeGate: http://arxiv.example.net/timegate/
1589 Archived: *
1591 Figure 22: robots.txt for a Web Archive that hosts Mementos for a
1592 wide range of Original Resources
1594 4.2. Discovery of TimeMaps Via TimeMap Feeds and Robots Exclusion
1595 Protocol
1597 Atom Feeds [RFC4287] are commonly used to support discovery of news
1598 items by humans and are also frequently used for automated discovery
1599 by a variety of applications. This section introduces a profile of
1600 Atom Feeds named TimeMap Feeds intended to support batch discovery of
1601 TimeMaps. The discovery of TimeMap Feeds is in its turn supported by
1602 the new "TimeMapFeed" directive for robots.txt.
1604 4.2.1. TimeMap Feeds
1606 TimeMap Feeds are special-purpose Atom Feeds that SHOULD be published
1607 by servers to support batch discovery of their Mementos. The
1608 following are the essential characteristics of a TimeMap Feed:
1610 o The feed has one entry per Original Resource for which the servers
1611 hosts Mementos;
1613 o Each entry conveys a TimeMap that lists the Mementos the server
1614 hosts for the associated Original Resource;
1616 o An entry can convey a TimeMap by-value (as inline content) or by-
1617 reference (as linked content);
1619 o Entries in the feed are provided in reverse chronological order:
1620 the most recently updated entry MUST be listed first;
1622 o Any change to a TimeMap conveyed by an entry MUST result in a
1623 change of the updated time of the entry, irrespective of the
1624 TimeMap being conveyed by-value or by-reference;
1626 o Depending on the size of the collection of Mementos, a server may
1627 need to publish multiple TimeMap Feeds. In this case, the server
1628 SHOULD also publish an Index of TimeMaps Feed that has an entry
1629 per published TimeMap Feed.
1631 Further details about the use of feed-level and entry-level elements
1632 in a TimeMap Feed are provided in Section 4.2.1.1 and
1633 Section 4.2.1.2, respectively.
1635 4.2.1.1. TimeMap Feeds: Feed-level Elements
1637 This section discusses the use of feed-level Atom elements in TimeMap
1638 Feeds. All elements are as specified in Atom [RFC4287], yet
1639 additional constraints or guidelines apply to some when used in
1640 TimeMap Feeds.
1642 4.2.1.1.1. Feed-level element: atom:id
1644 As the content of the atom:id element, a tag URI [RFC4151] or an HTTP
1645 URI equal to the one provided as the value of the "href" attribute of
1646 the MANDATORY feed-level "atom:link" element with a "rel" attribute
1647 equal to "self" is RECOMMENDED.
1649 4.2.1.1.2. Feed-level element: atom:author
1651 The atom:author element MUST occur exactly once, and is constructed
1652 as follows:
1654 o It MUST contain exactly one "atom:name" element, which contains
1655 the name of the server that publishes the TimeMap Feed.
1657 o It MUST contain exactly one "atom:uri" element, which contains the
1658 URI of the server that publishes the TimeMap Feed.
1660 o It SHOULD contain exactly one "atom:email" element, which contains
1661 the email address of a technical contact for the server that
1662 publishes the TimeMap Feed.
1664 4.2.1.1.3. Feed-level element: atom:category
1666 The atom:category element MUST occur at least once, and its use is as
1667 follows:
1669 o One atom:category element MUST be included to assert that the feed
1670 is a TimeMap Feed. It MUST have
1671 "http://purl.org/memento/categories/feedtype" as the value of the
1672 "scheme" attribute and it MUST have "TimeMapFeed" as the value of
1673 the "term" attribute.
1675 o In case the feed is an Index of TimeMap Feeds, an atom:category
1676 element SHOULD be included that has
1677 "http://purl.org/memento/categories/feedtype" as the value of the
1678 "scheme" attribute and "IndexFeed" as the value of the "term"
1679 attribute.
1681 o One or more atom:category elements SHOULD be included to coarsely
1682 convey information about the Original Resources for which the feed
1683 provides TimeMaps. It MUST have
1684 "http://purl.org/memento/categories/archived" as the value of the
1685 "scheme" attribute and the value of the "term" attribute MUST be
1686 as specified for the "Archived" directive of the Memento extension
1687 for robots.txt (see Section 4.1).
1689 o One atom:category element SHOULD be included to convey the type of
1690 archival system for which the TimeMap Feed provides an inventory.
1691 It MUST have "http://purl.org/memento/categories/class" as the
1692 value of the "scheme" attribute. Values of the "term" attribute
1693 MUST be taken from the vocabulary at
1694 http://purl.org/memento/categories/class.
1696 4.2.1.1.4. Feed-level elements: Example
1698 Figure 23 shows the use of feed-level elements for a TimeMap Feed
1699 published by the server http://arxiv.example.net/.
1701
1702 http://arxiv.example.net/timemapfeeds/feed1
1703 Feed 1 of arxiv.example.net TimeMaps
1704 2011-05-01 12:34:00 GMT
1705
1706 Example Web Archive
1707 http://arxiv.example.net/
1708 admin@arxiv.example.net
1709
1710 Content of this feed is public domain.
1711 http://arxiv.example.net/images/icon.png
1712
1714
1716
1718
1720
1724 ... entries go here ...
1726
1728 Figure 23: Feed-level elements for a TimeMap Feed
1730 4.2.1.2. TimeMap Feeds: Entry-level Elements
1732 This section discusses the use of entry-level Atom elements in
1733 TimeMap Feeds. All elements are as specified in Atom [RFC4287], yet
1734 additional constraints or guidelines apply to some when used in
1735 TimeMap Feeds.
1737 4.2.1.2.1. Entry-level element: atom:id
1739 The content of the atom:id element MUST be a tag URI [RFC4151] as
1740 specified by the "timemap-tagURI" construction rule of Figure 24, and
1741 it MUST have the URI of the Original Resource as the value for the
1742 "or-uri" component of that rule. If the feed is moved or copied, the
1743 tag URI that is provided as the value of the atom:id element MUST
1744 remain the same.
1746 timemap-tagURI = "tag:" taggingEntity ":" or-uri
1747 taggingEntity = DNSname "," "2011"
1748 DNSname = DNScomp *( "." DNScomp ) ; see RFC 1035
1749 DNScomp = alphaNum [*(alphaNum /"-") alphaNum]
1750 alphaNum = DIGIT / ALPHA
1751 or-uri = scheme ":" hier-part [ "?" query ] ; see RFC 3986
1752 scheme = "http" | "https"
1754 Figure 24: BNF for a tag URI for entries in TimeMap feeds
1756 4.2.1.2.2. Entry-level element: atom:author
1758 The atom:author element MUST NOT be used. Authorship information for
1759 an entry is inherited as follows:
1761 o From the atom:source child element of the entry, if it exists and
1762 contains authorship information;
1764 o From the feed-level atom:author element otherwise.
1766 4.2.1.2.3. Entry-level element: atom:updated
1768 The atom:updated element MUST be used and its value MUST change
1769 whenever the entry changes, including when the TimeMap conveyed by
1770 the entry (by-value or by-reference) changes.
1772 4.2.1.2.4. Entry-level element: atom:link
1774 The atom:link element MUST occur at least once and its use is as
1775 follows:
1777 o One atom:link element MUST be included to convey the URI of the
1778 Original Resource for which the entry conveys a TimeMap. This
1779 atom:link MUST have "original" as the value of the "rel"
1780 attribute, and the URI of the Original Resource as the value of
1781 the "href" attribute. This atom:link MAY have a "type" attribute
1782 to convey a mime type.
1784 o If the TimeMap is not embedded within the atom:content child
1785 element of the entry (see below), there MUST be an atom:link
1786 element that has both "alternate" and "timemap" as values for the
1787 "rel" attribute, and that has the URI of a TimeMap serialized
1788 according to the link-value format (see Section 3.4) as the value
1789 of the "href" attribute. The value of the "type" attribute for
1790 this link MUST be "application/link-format".
1792 o Additional atom:link elements MAY be provided, each linking to
1793 another TimeMap serialization (such as RDF/XML). These atom:link
1794 elements MUST have "timemap" as the value for the "rel" attribute
1795 and MUST have a "type" attribute that conveys the mime type of the
1796 serialization.
1798 o An atom:link element MAY be provided that links to a TimeGate for
1799 the Original Resource. This atom:link MUST have "timegate" as the
1800 value for the "rel" attribute, and it MUST NOT have a "type"
1801 attribute.
1803 4.2.1.2.5. Entry-level element: atom:content
1805 If the entry does not contain an atom:link element pointing to a
1806 TimeMap serialized according to the link-value format, then the atom:
1807 content element MUST be used to directly contain such a TimeMap
1808 wrapped in a CDATA section. The "type" attribute of this atom:
1809 content element MUST be used and it MUST have the value "application/
1810 link-format" (see Section 3.4).
1812 4.2.1.2.6. Entry-level elements: Example
1814 Figure 25 shows the use of entry-level elements for a TimeMap Feed
1815 published by the server http://arxiv.example.net/.
1817
1819 ... feed information ...
1821
1822 tag:arxiv.example.net,2011:http://a.example.org/
1823
1824 2011-05-01 12:34:00 GMT
1825
1828
1829
1831
1833 ... more entries ...
1835
1837 Figure 25: Entry-level elements for a TimeMap Feed
1839 4.2.2. Discovering TimeMap Feeds via Robots Exclusion Protocol
1841 Servers that publish TimeMap Feeds SHOULD make them discoverable by
1842 using the "TimeMapFeed" directive for robots.txt that is introduced
1843 here.
1845 A robots.txt file MAY contain zero or more occurrences of the
1846 "TimeMapFeed" directive, and its meaning is as follows:
1848 o TimeMapFeed: Conveys the HTTP(S) URI of a TimeMap Feed or of an
1849 Index of TimeMap Feeds.
1851 Figure 26 shows an excerpt of the robots.txt file of the server at
1852 http://arxiv.example.net/ that hosts two TimeMap Feeds to make its
1853 Mementos discoverable.
1855 TimeMapFeed: http://arxiv.example.net/timemapfeeds/feed1
1856 TimeMapFeed: http://arxiv.example.net/timemapfeeds/feed2
1858 Figure 26: robots.txt to support discovery of TimeMap Feeds
1860 4.3. Discovering Mementos via Robots Exclusion Protocol
1862 Servers can support discovery of their Mementos by crawlers through
1863 the use of the Robots Exclusion Protocol, but SHOULD do so in a
1864 manner that conveys to crawlers and mirroring applications that the
1865 sticky Memento-Datetime behavior (see Section 2.1.1) MUST be
1866 respected. To that end, servers SHOULD use the "User-agent" and
1867 "Allow" directives of the Robots Exclusion Protocol in the following
1868 manner:
1870 o User-agent: Has "memento" as its value;
1872 o Allow: Lists the path that contains Mementos that can be crawled,
1873 and for which content can be mirrored subject to the sticky
1874 Memento-Datetime behavior.
1876 Figure 27 shows the robots.txt for a server that generally disallows
1877 crawling, yet allows agents that respect the sticky Memento-Datetime
1878 behavior to crawl Mementos in the /web/ path.
1880 User-agent: *
1881 Disallow: /
1882 User-agent: memento
1883 Allow: /web/
1885 Figure 27: Restricting crawling to agents that respect sticky
1886 Memento-Datetime behavior
1888 5. IANA Considerations
1890 This memo requires IANA to register the Accept-Datetime and Memento-
1891 Datetime HTTP headers defined in Section 2.1.1 in the appropriate
1892 IANA registry.
1894 This memo requires IANA to register the "Link" header Relation Types
1895 "original", "timegate", "timemap", and "memento" defined in
1896 Section 2.2.1 in the appropriate IANA registry.
1898 This memo requires IANA to register the "datetime", "license", and
1899 "embargo" attributes for Link headers with a "memento" Relation Type,
1900 as defined in Section 2.2.1.4 in the appropriate IANA registry.
1902 6. Security Considerations
1904 Provision of a "timegate" HTTP "Link" header in responses to requests
1905 for an Original Resource that is protected (e.g., 401 or 403 HTTP
1906 response codes) is OPTIONAL. The inclusion of this Link when
1907 requesting authentication is at the server's discretion; cases may
1908 exist in which a server protects the current state of a resource, but
1909 supports open access to prior states and thus chooses to supply a
1910 "timegate" HTTP "Link" header. Conversely, the server may choose to
1911 not advertise the TimeGate URIs (e.g., they exist in an intranet
1912 archive) for unauthenticated requests.
1914 Authentication, encryption and other security related issues are
1915 otherwise orthogonal to Memento.
1917 7. Changelog
1919 v02 2011-05-11 HVDS MLN RS draft-vandesompel-memento-01
1921 o Introduced wording and reference to indicate a Memento is a
1922 FixedResource.
1924 o Introduced "Sticky Memento-Datetime" notion and clarified wording
1925 about retaining "Memento-Datetime" headers and values when a
1926 Memento is mirrored at different URI.
1928 o Introduced section about handling both datetime and regular
1929 negotiation.
1931 o Introduced section about Mementos Without TimeGate.
1933 o Made various changes in the section Relation Type "memento",
1934 including addition of "license" and "embargo" attributes, and
1935 clarification of rules regarding the use of "memento" links.
1937 o Moved section about TimeMaps inside the Datetime Negotiation
1938 section, and updated it.
1940 o Restarted the Discovery section from scratch.
1942 v01 2010-11-11 HVDS MLN RS First public version
1943 draft-vandesompel-memento-00
1945 v00 2010-10-19 HVDS MLN RS Limited circulation version
1947 2010-07-22 HVDS MLN First internal version
1949 8. Acknowledgements
1951 The Memento effort is funded by the Library of Congress. Many thanks
1952 to Kris Carpenter Negulescu, Michael Hausenblas, Erik Hetzner, Larry
1953 Masinter, Gordon Mohr, Mark Nottingham, David Rosenthal, Ed Summers
1954 for early feedback. Many thanks to Samuel Adams, Scott Ainsworth,
1955 Lyudmilla Balakireva, Frank McCown, Harihar Shankar, Brad Tofel for
1956 early implementations.
1958 9. References
1960 9.1. Normative References
1962 [I-D.ietf-core-link-format]
1963 Shelby, Z., "CoRE Link Format",
1964 draft-ietf-core-link-format-03 (work in progress),
1965 March 2011.
1967 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
1968 Requirement Levels", BCP 14, RFC 2119, March 1997.
1970 [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
1971 Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
1972 Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
1974 [RFC4151] Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
1975 RFC 4151, October 2005.
1977 [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
1978 Syndication Format", RFC 4287, December 2005.
1980 [RFC5829] Brown, A., Clemm, G., and J. Reschke, "Link Relation Types
1981 for Simple Version Navigation between Web Resources",
1982 RFC 5829, April 2010.
1984 [RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
1986 9.2. Informative References
1988 [Fitch] Fitch, "Web site archiving - an approach to recording
1989 every materially different response produced by a
1990 website", July 2003,
1991 .
1993 [I-D.masinter-dated-uri]
1994 Masinter, L., "The 'tdb' and 'duri' URI schemes, based on
1995 dated URIs", draft-masinter-dated-uri-08 (work in
1996 progress), January 2011.
1998 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application
1999 and Support", STD 3, RFC 1123, October 1989.
2001 [W3C.REC-aww-20041215]
2002 Jacobs and Walsh, "Architecture of the World Wide Web",
2003 December 2004, .
2005 [W3C.gen-ont-20090420]
2006 Berners-Lee, "Architecture of the World Wide Web",
2007 April 2009, .
2009 [robotstxt.org]
2010 "Robots Exclusion Protocol", August 2010,
2011 .
2013 Appendix A. Appendix B: A Sample, Successful Memento Request/Response
2014 cycle
2016 Step 1 : UA --- HTTP GET/HEAD; Accept-Datetime: Tj ---------> URI-R
2018 HEAD / HTTP/1.1
2019 Host: a.example.org
2020 Accept-Datetime: Tue, 11 Sep 2001 20:35:00 GMT
2021 Connection: close
2023 Step 2 : UA <-- HTTP 200; Link: URI-G ----------------------- URI-R
2025 HTTP/1.1 200 OK
2026 Date: Thu, 21 Jan 2010 00:02:12 GMT
2027 Server: Apache
2028 Link:
2029 ; rel="timegate"
2030 Content-Length: 255
2031 Connection: close
2032 Content-Type: text/html; charset=iso-8859-1
2034 Step 3 : UA --- HTTP GET/HEAD; Accept-Datetime: Tj ---------> URI-G
2036 GET /timegate/http://a.example.org
2037 HTTP/1.1
2038 Host: arxiv.example.net
2039 Accept-Datetime: Tue, 11 Sep 2001 20:35:00 GMT
2040 Connection: close
2042 Step 4 : UA <-- HTTP 302; Location: URI-Mj; Vary; Link:
2043 URI-R, URI-T, URI-M0, URI-Mn, URI-Mi, URI-Mj, URI-Mk ---- URI-G
2045 HTTP/1.1 302 Found
2046 Date: Thu, 21 Jan 2010 00:06:50 GMT
2047 Server: Apache
2048 Vary: negotiate, accept-datetime
2049 Location:
2050 http://arxiv.example.net/web/20010911203610/http://a.example.org
2051 Link: ; rel="original",
2052
2053 ; rel="first memento"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
2054
2055 ; rel="last memento"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
2056
2057 ; rel="timemap"; type="application/link-format",
2058
2059 ; rel="memento"; datetime="Tue, 11 Sep 2001 20:36:10 GMT",
2060
2061 ; rel="prev memento"; datetime="Tue, 11 Sep 2001 20:30:51 GMT",
2062
2063 ; rel="next memento"; datetime="Tue, 11 Sep 2001 20:47:33 GMT"
2064 Content-Length: 0
2065 Content-Type: text/plain; charset=UTF-8
2066 Connection: close
2068 Step 5 : UA --- HTTP GET URI-Mj; Accept-Datetime: Tj -------> URI-Mj
2070 GET /web/20010911203610/http://a.example.org
2071 HTTP/1.1
2072 Host: arxiv.example.net
2073 Accept-Datetime: Tue, 11 Sep 2001 20:35:00 GMT
2074 Connection: close
2075 Step 6 : UA <-- HTTP 200; Memento-Datetime: Tj; Link: URI-R,
2076 URI-T, URI-G, URI-M0, URI-Mn, URI-Mi, URI-Mj, URI-Mk ---- URI-Mj
2078 HTTP/1.1 200 OK
2079 Date: Thu, 21 Jan 2010 00:09:40 GMT
2080 Server: Apache-Coyote/1.1
2081 Memento-Datetime: Tue, 11 Sep 2001 20:36:10 GMT
2082 Link: ; rel="original",
2083
2084 ; rel="first memento"; datetime="Tue, 15 Sep 2000 11:28:26 GMT",
2085
2086 ; rel="last memento"; datetime="Tue, 08 Jul 2008 09:34:33 GMT",
2087
2088 ; rel="timemap"; type="application/link-format",
2089
2090 ; rel="timegate",
2091
2092 ; rel="memento"; datetime="Tue, 11 Sep 2001 20:36:10 GMT",
2093
2094 ; rel="prev memento"; datetime="Tue, 11 Sep 2001 20:30:51 GMT",
2095
2096 ; rel="next memento"; datetime="Tue, 11 Sep 2001 20:47:33 GMT"
2097 Content-Length: 23364
2098 Content-Type: text/html;charset=utf-8
2099 Connection: close
2101 A successful flow with TimeGate and Mementos on the same server
2103 Authors' Addresses
2105 Herbert VandeSompel
2106 Los Alamos National Laboratory
2107 PO Box 1663
2108 Los Alamos, New Mexico 87545
2109 USA
2111 Phone: +1 505 667 1267
2112 Email: hvdsomp@gmail.com
2113 URI: http://public.lanl.gov/herbertv/
2114 Michael Nelson
2115 Old Dominion University
2116 Norfolk, Virginia 23529
2117 USA
2119 Phone: +1 757 683 6393
2120 Email: mln@cs.odu.edu
2121 URI: http://www.cs.odu.edu/~mln/
2123 Robert Sanderson
2124 Los Alamos National Laboratory
2125 PO Box 1663
2126 Los Alamos, New Mexico 87545
2127 USA
2129 Phone: +1 505 665 5804
2130 Email: azaroth42@gmail.com