RFC 7089
HTTP Framework for Time-Based Access to Resource States -- Memento
Pages: 50
Informational
Informational
Part 2 of 3 – Pages 14 to 35
4. Datetime Negotiation: HTTP Interactions
Figure 2 depicts a specific pattern to implement the Memento framework. Multiple patterns exist, and they can be grouped as follows: o Pattern 1 (Section 4.1) - The Original Resource acts as its own TimeGate o Pattern 2 (Section 4.2) - A remote resource acts as a TimeGate for the Original Resource
o Pattern 3 (Section 4.3) - The Original Resource is a Fixed Resource o Pattern 4 (Section 4.4) - Mementos without a TimeGate Details of the HTTP interactions for common cases for each of those patterns are provided in Sections 4.1 through 4.4. Appendix A summarizes the use of the "Vary", "Memento-Datetime", and "Link" headers in responses from Original Resources, TimeGates, and Mementos for the various patterns. Special cases are described in Section 4.5. Note that in the following sections, the HTTP status code of the responses with an entity-body is shown as "200 OK", but a series of "206 Partial Content" responses could be substituted. Figure 4 shows a user agent that attempts to datetime negotiate with the Original Resource http://a.example.org/ by including an "Accept- Datetime" header in its HTTP HEAD request. This initiating request is the same for Pattern 1 (Section 4.1) through Pattern 3 (Section 4.3). HEAD / HTTP/1.1 Host: a.example.org Accept-Datetime: Tue, 20 Mar 2001 20:35:00 GMT Connection: close Figure 4: User Agent Attempts Datetime Negotiation with Original Resource4.1. Pattern 1 - The Original Resource Acts as Its Own TimeGate
In this implementation pattern, the Original Resource acts as its own TimeGate, which means that URI-R and URI-G coincide. Content management systems and revision control systems can support datetime negotiation in this way as they are commonly aware of the version history of their own resources. The response to this request when datetime negotiation for this resource is supported depends on the negotiation style it uses (200- style or 302-style) and on the existence or absence of a URI-M for Mementos that is distinct from the URI-R of the associated Original Resource. The various cases are summarized in the below table, and the server responses for each are detailed in the remainder of this section.
+-------------------+------------+----------+---------+-------------+ | Pattern | Original | TimeGate | Memento | Negotiation | | | Resource | | | Style | +-------------------+------------+----------+---------+-------------+ | Pattern 1.1 | URI-R | URI-R | URI-M | 302 | | (Section 4.1.1) | | | | | | Pattern 1.2 | URI-R | URI-R | URI-M | 200 | | (Section 4.1.2) | | | | | | Pattern 1.3 | URI-R | URI-R | URI-R | 200 | | (Section 4.1.3) | | | | | +-------------------+------------+----------+---------+-------------+ Table 1: Pattern 14.1.1. Pattern 1.1 - URI-R=URI-G; 302-Style Negotiation; Distinct URI-M
In this case, the response to the user agent's request of Figure 4 has a "302 Found" HTTP status code, and the "Location" header conveys the URI-M of the selected Memento. The use of Memento response headers and links in the response from URI-R=URI-G is as follows: o The "Vary" header MUST be provided, and it MUST include the "accept-datetime" value. o The response MUST NOT contain a "Memento-Datetime" header. o The "Link" header MUST be provided, and it MUST contain at least a link with the "original" Relation Type that has the URI-R of the Original Resource as Target IRI. The provision of other links is encouraged and is subject to the considerations described in Section 2.2. The server's response to the request of Figure 4 is shown in Figure 5. Note the inclusion of the recommended link to the TimeGate that, in this case, has a Target IRI that is the URI-R of the Original Resource.
HTTP/1.1 302 Found Date: Thu, 21 Jan 2010 00:06:50 GMT Server: Apache Vary: accept-datetime Location: http://a.example.org/?version=20010320133610 Link: <http://a.example.org/>; rel="original timegate" Content-Length: 0 Content-Type: text/plain; charset=UTF-8 Connection: close Figure 5: Response from URI-R=URI-G for Pattern 1.1 In a subsequent request, shown in Figure 6, the user agent can obtain the selected Memento by issuing an HTTP GET request against the URI-M that was provided in the "Location" header. The inclusion of the "Accept-Datetime" header in this request is not needed but will typically occur as the user agent is in datetime negotiation mode. GET /?version=20010320133610 HTTP/1.1 Host: a.example.org Accept-Datetime: Tue, 20 Mar 2001 20:35:00 GMT Connection: close Figure 6: User Agent Requests Selected Memento The response has a "200 OK" HTTP status code, and the entity-body of the response contains the representation of the selected Memento. The use of Memento response headers and links in the response from URI-M is as follows: o A "Vary" header that includes an "accept-datetime" value MUST NOT be provided. o The response MUST include a "Memento-Datetime" header. Its value expresses the archival datetime of the Memento. o The "Link" header MUST be provided, and it MUST contain at least a link with the "original" Relation Type that has the URI-R of the Original Resource as Target IRI. The provision of other links is encouraged and is subject to the considerations described in Section 2.2. The server's response to the request of Figure 6 is shown in Figure 7. Note the provision of the required "original", and the recommended "timegate" and "timemap" links. The former two point to
the Original Resource, which acts as its own TimeGate. The latter has "from" and "until" attributes to indicate the temporal interval covered by Mementos listed in the linked TimeMap. HTTP/1.1 200 OK Date: Thu, 21 Jan 2010 00:06:51 GMT Server: Apache-Coyote/1.1 Memento-Datetime: Tue, 20 Mar 2001 13:36:10 GMT Link: <http://a.example.org/>; rel="original timegate", <http://a.example.org/?version=all&style=timemap> ; rel="timemap"; type="application/link-format" ; from="Tue, 15 Sep 2000 11:28:26 GMT" ; until="Wed, 20 Jan 2010 09:34:33 GMT" Content-Length: 23364 Content-Type: text/html;charset=utf-8 Connection: close Figure 7: Response from URI-M for Pattern 1.14.1.2. Pattern 1.2 - URI-R=URI-G; 200-Style Negotiation; Distinct URI-M
In this case, the response to the user agent's request of Figure 4 has a "200 OK" HTTP status code, and the "Content-Location" header conveys the URI-M of the selected Memento. The use of Memento response headers and links in the response from URI-R=URI-G is as follows: o The "Vary" header MUST be provided, and it MUST include the "accept-datetime" value. o The response MUST include a "Memento-Datetime" header. Its value expresses the archival datetime of the selected Memento. o The "Link" header MUST be provided, and it MUST contain at least a link with the "original" Relation Type that has the URI-R of the Original Resource as Target IRI. The provision of other links is encouraged and is subject to the considerations described in Section 2.2. The server's response to the request of Figure 4 is shown in Figure 8. Note the provision of optional "memento" links pointing at the oldest and most recent Memento for the Original Resource known to the responding server.
HTTP/1.1 200 OK Date: Thu, 21 Jan 2010 00:06:50 GMT Server: Apache Vary: accept-datetime Content-Location: http://a.example.org/?version=20010320133610 Memento-Datetime: Tue, 20 Mar 2001 13:36:10 GMT Link: <http://a.example.org/>; rel="original timegate", <http://a.example.org/?version=20000915112826> ; rel="memento first"; datetime="Tue, 15 Sep 2000 11:28:26 GMT", <http://a.example.org/?version=20100120093433> ; rel="memento last"; datetime="Wed, 20 Jan 2010 09:34:33 GMT", <http://a.example.org/?version=all&style=timemap> ; rel="timemap"; type="application/link-format" Content-Length: 23364 Content-Type: text/html;charset=utf-8 Connection: close Figure 8: Response from URI-R=URI-G for Pattern 1.2 In a subsequent request, which is the same as Figure 4 but with HTTP GET instead of HEAD, the user agent can obtain the representation of the selected Memento. It will be provided as the entity-body of a response that has the same Memento headers as in Figure 8.4.1.3. Pattern 1.3 - URI-R=URI-G; 200-Style Negotiation; No Distinct URI-M
In this case, the response to the user agent's request of Figure 4 has a "200 OK" HTTP status code, and it does not contain a "Content- Location" nor a "Location" header as there is no URI-M of the selected Memento to convey. The use of Memento response headers and links in the response from URI-R=URI-G is as follows: o The "Vary" header MUST be provided, and it MUST include the "accept-datetime" value. o The response MUST include a "Memento-Datetime" header. Its value expresses the archival datetime of the selected Memento. o The "Link" header MUST be provided, and it MUST contain at least a link with the "original" Relation Type that has the URI-R of the Original Resource as Target IRI. The provision of other links is encouraged and is subject to the considerations described in Section 2.2.
The server's response to the request of Figure 4 is shown in Figure 9. The recommended "timemap" and "timegate" links are included in addition to the mandatory "original" link. HTTP/1.1 200 OK Date: Thu, 21 Jan 2010 00:06:50 GMT Server: Apache Vary: accept-datetime Memento-Datetime: Tue, 20 Mar 2001 13:36:10 GMT Link: <http://a.example.org/>; rel="original timegate", <http://a.example.org/?version=all&style=timemap> ; rel="timemap"; type="application/link-format" Content-Length: 23364 Content-Type: text/html;charset=utf-8 Connection: close Figure 9: Response from URI-R=URI-G for Pattern 1.3 In a subsequent request, which is the same as Figure 4 but with HTTP GET instead of HEAD, the user agent can obtain the representation of the selected Memento. It will be provided as the entity-body of a response that has the same Memento headers as in Figure 9.4.2. Pattern 2 - A Remote Resource Acts as a TimeGate for the Original Resource
In this implementation pattern, the Original Resource does not act as its own TimeGate, which means that URI-R and URI-G are different. This pattern is typically implemented by servers for which the history of their resources is recorded in remote systems such as Web archives and transactional archives [Fitch]. But servers that maintain their own history, such as content management systems and version control systems, may also implement this pattern, for example, to distribute the load involved in responding to requests for current and prior representations of resources between different servers. This pattern is summarized in the below table and is detailed in the remainder of this section. Three cases exist that differ regarding the negotiation style that is used by the remote TimeGate and regarding the existence of a URI-M for Mementos that is distinct from the URI-G of the TimeGate.
+-------------------+------------+----------+---------+-------------+ | Pattern | Original | TimeGate | Memento | Negotiation | | | Resource | | | Style | +-------------------+------------+----------+---------+-------------+ | Pattern 2.1 | URI-R | URI-G | URI-M | 302 | | (Section 4.2.1) | | | | | | Pattern 2.2 | URI-R | URI-G | URI-M | 200 | | (Section 4.2.2) | | | | | | Pattern 2.3 | URI-R | URI-G | URI-G | 200 | | (Section 4.2.3) | | | | | +-------------------+------------+----------+---------+-------------+ Table 2: Pattern 2 The response by the Original Resource to the request shown in Figure 4 is the same for all three cases. The use of headers and links in the response from URI-R is as follows: o A "Vary" header that includes an "accept-datetime" value MUST NOT be provided. o The response MUST NOT contain a "Memento-Datetime" header. o The "Link" header SHOULD be provided. It MUST NOT include a link with an "original" Relation Type. If a preferred TimeGate is associated with the Original Resource, then it MUST include a link with a "timegate" Relation Type that has the URI-G of the TimeGate as Target IRI. If a preferred TimeMap is associated with the Original Resource, then it SHOULD include a link with a "timemap" Relation Type that has the URI-T of the TimeGate as Target IRI. Multiple "timegate" and "timemap" links can be provided to accommodate situations in which the server is aware of multiple TimeGates or TimeMaps for the Original Resource. Figure 10 shows such a response. Note the absence of an "original" link as the responding resource is neither a TimeGate nor a Memento. HTTP/1.1 200 OK Date: Thu, 21 Jan 2010 00:02:12 GMT Server: Apache Link: <http://arxiv.example.net/timegate/http://a.example.org/> ; rel="timegate" Content-Length: 255 Connection: close Content-Type: text/html; charset=iso-8859-1 Figure 10: Response from URI-R<>URI-G for Pattern 2
Once a user agent has obtained the URI-G of a remote TimeGate for the Original Resource, it can engage in datetime negotiation with that TimeGate. Figure 11 shows the request issued against the TimeGate, whereas Sections 4.2.1 through 4.2.3 detail the responses for various TimeGate implementation patterns. HEAD /timegate/http://a.example.org/ HTTP/1.1 Host: arxiv.example.net Accept-Datetime: Tue, 20 Mar 2001 20:35:00 GMT Connection: close Figure 11: User Agent Engages in Datetime Negotiation with Remote TimeGate4.2.1. Pattern 2.1 - URI-R<>URI-G; 302-Style Negotiation; Distinct URI-M
In case the TimeGate uses a 302 negotiation style, the response to the user agent's request of Figure 11 has a "302 Found" HTTP status code, and the "Location" header conveys the URI-M of the selected Memento. The use of Memento response headers and links in the response from URI-G is as follows: o The "Vary" header MUST be provided, and it MUST include the "accept-datetime" value. o The response MUST NOT contain a "Memento-Datetime" header. o The "Link" header MUST be provided, and it MUST contain at least a link with the "original" Relation Type that has the URI-R of the Original Resource as Target IRI. The provision of other links is encouraged and is subject to the considerations described in Section 2.2. The server's response to the request of Figure 11 is shown in Figure 12. It contains the mandatory "original" link that points back to the Original Resource associated with this TimeGate, and it shows the recommended "timemap" link that includes "from" and "until" attributes.
HTTP/1.1 302 Found Date: Thu, 21 Jan 2010 00:02:14 GMT Server: Apache Vary: accept-datetime Location: http://arxiv.example.net/web/20010321203610/http://a.example.org/ Link: <http://a.example.org/>; rel="original", <http://arxiv.example.net/timemap/http://a.example.org/> ; rel="timemap"; type="application/link-format" ; from="Tue, 15 Sep 2000 11:28:26 GMT" ; until="Wed, 20 Jan 2010 09:34:33 GMT" Content-Length: 0 Content-Type: text/plain; charset=UTF-8 Connection: close Figure 12: Response from URI-G<>URI-R for Pattern 2.1 In a subsequent HTTP GET request, shown in Figure 13, the user agent can obtain the selected Memento by issuing an HTTP GET request against the URI-M that was provided in the "Location" header. The inclusion of the "Accept-Datetime" header in this request is not needed but will typically occur as the user agent is in datetime negotiation mode. GET /web/20010321203610/http://a.example.org/ HTTP/1.1 Host: arxiv.example.net/ Accept-Datetime: Tue, 20 Mar 2001 20:35:00 GMT Connection: close Figure 13: User Agent Requests Selected Memento The response has a "200 OK" HTTP status code. The use of Memento response headers and links in the response from URI-M is as follows: o A "Vary" header that includes an "accept-datetime" value MUST NOT be provided. o The response MUST include a "Memento-Datetime" header. Its value expresses the archival datetime of the Memento. o The "Link" header MUST be provided, and it MUST contain at least a link with the "original" Relation Type that has the URI-R of the Original Resource as Target IRI. The provision of other links is encouraged and is subject to the considerations described in Section 2.2.
The server's response to the request of Figure 13 is shown in Figure 14. Note the provision of the recommended "timegate" and "timemap" links. HTTP/1.1 200 OK Date: Thu, 21 Jan 2010 00:02:15 GMT Server: Apache-Coyote/1.1 Memento-Datetime: Wed, 21 Mar 2001 20:36:10 GMT Link: <http://a.example.org/>; rel="original", <http://arxiv.example.net/timemap/http://a.example.org/> ; rel="timemap"; type="application/link-format", <http://arxiv.example.net/timegate/http://a.example.org/> ; rel="timegate" Content-Length: 25532 Content-Type: text/html;charset=utf-8 Connection: close Figure 14: Response from URI-M for Pattern 2.14.2.2. Pattern 2.2 - URI-R<>URI-G; 200-Style Negotiation; Distinct URI-M
In case the TimeGate uses a 200 negotiation style, and each Memento has a distinct URI-M, the response to the user agent's request of Figure 11 has a "200 OK" HTTP status code, and the "Content-Location" header conveys the URI-M of the selected Memento. The use of Memento response headers and links in the response from URI-G is as follows: o The "Vary" header MUST be provided, and it MUST include the "accept-datetime" value. o The response MUST include a "Memento-Datetime" header. Its value expresses the archival datetime of the Memento. o The "Link" header MUST be provided, and it MUST contain at least a link with the "original" Relation Type that has the URI-R of the Original Resource as Target IRI. The provision of other links is encouraged and is subject to the considerations described in Section 2.2. The server's response to the request of Figure 11 is shown in Figure 15.
HTTP/1.1 200 OK Date: Thu, 21 Jan 2010 00:09:40 GMT Server: Apache-Coyote/1.1 Vary: accept-datetime Content-Location: http://arxiv.example.net/web/20010321203610/http://a.example.org/ Memento-Datetime: Wed, 21 Mar 2001 20:36:10 GMT Link: <http://a.example.org/>; rel="original", <http://arxiv.example.net/timemap/http://a.example.org/> ; rel="timemap"; type="application/link-format", <http://arxiv.example.net/timegate/http://a.example.org/> ; rel="timegate" Content-Length: 25532 Content-Type: text/html;charset=utf-8 Connection: close Figure 15: Response from URI-G<>URI-R for Pattern 2.2 In a subsequent request, which is the same as Figure 11 but with HTTP GET instead of HEAD, the user agent can obtain the representation of the selected Memento. It will be provided as the entity-body of a response that has the same Memento headers as Figure 15.4.2.3. Pattern 2.3 - URI-R<>URI-G; 200-Style Negotiation; No Distinct URI-M
In case the TimeGate uses a 200 negotiation style, but Mementos have no distinct URIs, the response to the user agent's request of Figure 11 has a "200 OK" HTTP status code, and it does not contain a "Content-Location" nor "Location" header as there is no URI-M of the selected Memento to convey. The use of Memento response headers and links in the response from URI-G is as follows: o The "Vary" header MUST be provided, and it MUST include the "accept-datetime" value. o The response MUST include a "Memento-Datetime" header. Its value expresses the archival datetime of the Memento. o The "Link" header MUST be provided, and it MUST contain at least a link with the "original" Relation Type that has the URI-R of the Original Resource as Target IRI. The provision of other links is encouraged and is subject to the considerations described in Section 2.2.
The server's response to the request of Figure 11 is shown in Figure 16. HTTP/1.1 200 OK Date: Thu, 21 Jan 2010 00:09:40 GMT Server: Apache-Coyote/1.1 Vary: accept-datetime Memento-Datetime: Wed, 21 Mar 2001 20:36:10 GMT Link: <http://a.example.org/>; rel="original", <http://arxiv.example.net/timemap/http://a.example.org/> ; rel="timemap"; type="application/link-format", <http://arxiv.example.net/timegate/http://a.example.org/> ; rel="timegate" Content-Length: 25532 Content-Type: text/html;charset=utf-8 Connection: close Figure 16: Response from URI-G<>URI-R for Pattern 2.3 In a subsequent request, which is the same as Figure 11 but with HTTP GET instead of HEAD, the user agent can obtain the representation of the selected Memento. It will be provided as the entity-body of a response that has the same Memento headers as Figure 16.4.3. Pattern 3 - The Original Resource is a Fixed Resource
This pattern does not involve datetime negotiation with a TimeGate, but it can be implemented for Original Resources that never change state or do not change anymore past a certain point in their existence, meaning that URI-R and URI-M coincide either from the outset or starting at some point in time. This pattern is summarized in the below table. Examples are tweets or stable media resources on news sites. +----------+----------------+----------+---------+------------------+ | Pattern | Original | TimeGate | Memento | Negotiation | | | Resource | | | Style | +----------+----------------+----------+---------+------------------+ | Pattern | URI-R | - | URI-R | - | | 3 | | | | | +----------+----------------+----------+---------+------------------+ Table 3: Pattern 3
Servers that host such resources can support the Memento framework by treating the stable resource (FixedResource as per [W3C.gen-ont-20090420]) as a Memento. The use of Memento response headers and links in responses from such a stable resource is as follows: o A "Vary" header that includes an "accept-datetime" value MUST NOT be provided. o The response MUST include a "Memento-Datetime" header. Its value expresses the datetime at which the resource became stable. Providing this value includes a promise that the resource has not changed since this datetime and will not change anymore beyond it. o The "Link" header MUST be provided and MUST have a link with the "original" Relation Type that has the URI-R of the stable resource itself as Target IRI. Figure 17 shows a response to an HTTP HEAD request for the resource with URI-R http://a.example.org/ that has been stable since March 20, 2009. HTTP/1.1 200 OK Date: Thu, 21 Jan 2010 00:09:40 GMT Server: Apache-Coyote/1.1 Memento-Datetime: Fri, 20 Mar 2009 11:00:00 GMT Link: <http://a.example.org/>; rel="original" Content-Length: 875 Content-Type: text/html;charset=utf-8 Connection: close Figure 17: Response from URI-R=URI-M for Pattern 34.4. Pattern 4 - Mementos without a TimeGate
Cases may occur in which a server hosts Mementos but does not expose a TimeGate for them. This can, for example, be the case if the server's Mementos result from taking a snapshot of the state of a set of Original Resources from another server as it is being retired. As a result, only a single Memento per Original Resource is hosted, making the introduction of a TimeGate unnecessary. But it may also be the case for servers that host multiple Mementos for an Original Resource but consider exposing TimeGates too expensive. In this case, URI-R and URI-M are distinct, but a TimeGate is absent. This case is summarized in the below table.
+----------+----------------+----------+---------+------------------+ | Pattern | Original | TimeGate | Memento | Negotiation | | | Resource | | | Style | +----------+----------------+----------+---------+------------------+ | Pattern | URI-R | - | URI-M | - | | 4 | | | | | +----------+----------------+----------+---------+------------------+ Table 4: Pattern 4 Servers that host such Mementos without TimeGates can still support the Memento framework by providing the appropriate Memento headers and links. Their use is as follows for a response from URI-M: o A "Vary" header that includes an "accept-datetime" value MUST NOT be provided. o The response MUST include a "Memento-Datetime" header. Its value expresses the archival datetime of the Memento. o The "Link" header MUST be provided, and it MUST have a link with the "original" Relation Type that has the URI-R of the associated Original Resource as Target IRI. The provision of other links is encouraged and is subject to the considerations described in Section 2.2. Figure 18 shows a response to an HTTP HEAD request for the Memento with URI-M http://arxiv.example.net/web/20010321203610/http://a.example.org/. Note the use of links: three links have the URI-M of the Memento as Target IRI and have respective Relation Types "memento", "first", and "last". This combination indicates that this is the only Memento for the Original Resource with Target IRI provided by the "original" link (http://a.example.org/) of which the server is aware. Note also that such a response does not imply that there is no server whatsoever that exposes a TimeGate; it merely means that the responding server neither provides nor is aware of the location of a TimeGate.
HTTP/1.1 200 OK Date: Thu, 21 Jan 2010 00:09:40 GMT Server: Apache-Coyote/1.1 Memento-Datetime: Wed, 21 Mar 2001 20:36:10 GMT Link: <http://a.example.org/>; rel="original", <http://arxiv.example.net/web/20010321203610/http://a.example.org/> ; rel="first last memento" ; datetime="Wed, 21 Mar 2001 20:36:10 GMT" Content-Length: 25532 Content-Type: text/html;charset=utf-8 Connection: close Figure 18: Response from URI-M<>URI-R for Pattern 44.5. Special Cases
4.5.1. Original Resource Provides No "timegate" Link
Cases exist in which the response from the Original Resource does not contain a "timegate" link, including: o The Original Resource's server does not support the Memento framework; o The Original Resource no longer exists, and the responding server is not aware of its prior existence; o The server that hosted the Original Resource no longer exists. In all these cases, the user agent should attempt to determine an appropriate TimeGate for the Original Resource, either automatically or interactively supported by the user.4.5.2. Server Exists but Original Resource No Longer Does
Cases exist in which the server knows that an Original Resource used to exist, but no longer provides a current representation. If there is a preferred TimeGate for such a discontinued Original Resource, then the server MUST include a "timegate" link in responses to requests for it. This may allow access to Mementos for the Original Resource even if it no longer exists. A server's response to a request for the discontinued resource http://a.example.org/pic is illustrated in Figure 19.
HTTP/1.1 404 Not Found Date: Thu, 21 Jan 2010 00:02:12 GMT Server: Apache Link: <http://arxiv.example.net/timegate/http://a.example.org/pic> ; rel="timegate" Content-Length: 255 Content-Type: text/html; charset=iso-8909-1 Connection: close Figure 19: Response from an Original Resource That No Longer Exists4.5.3. Issues with Accept-Datetime
The following special cases may occur regarding the "Accept-Datetime" header when a user agent issues a request against a TimeGate: o If the value of the "Accept-Datetime" is either earlier than the datetime of the first Memento or later than the datetime of the most recent Memento known to the TimeGate, the first or most recent Memento MUST be selected, respectively. o If the value of the "Accept-Datetime" does not conform to the rfc1123-date construction rule of the BNF in Figure 1, the response MUST have a "400 Bad Request" HTTP status code. o If a user agent issues a request against a TimeGate and fails to include an "Accept-Datetime" request header, the most recent Memento SHOULD be selected. In all cases, the use of headers and links in responses is as described for TimeGates in the respective scenarios.4.5.4. Memento of a 3XX Response
Cases exist in which HTTP responses with 3XX status codes are archived. For example, crawl-based Web archives commonly archive responses with HTTP status codes "301 Moved Permanently" and "302 Found", whereas Linked Data archives hold on to "303 See Other" responses. If the Memento requested by the user agent is an archived version of an HTTP response with a 3XX status code, the server's response MUST have the same 3XX HTTP status code. The use of other Memento headers is as described for Mementos in the respective scenarios.
The user agent's handling of an HTTP response with a 3XX status code is not affected by the presence of a "Memento-Datetime" header. The user agent MUST behave in the same manner as it does with HTTP responses with a 3XX status code that do not have a "Memento- Datetime" header. However, the user agent MUST be aware that the URI that was selected from the "Location" header of an HTTP response with a 3XX status code might not be that of a Memento but rather of an Original Resource. In the latter case, it SHOULD proceed by looking for a Memento of the selected Original Resource. For example, Figure 20 shows the response to an HTTP GET request for http://a.example.org issued on April 11, 2008. This response is archived as a Memento of http://a.example.org that has as URI-M http://arxiv.example.net/web/20080411000650/http://a.example.org. The response to an HTTP GET on this URI-M is shown in Figure 21. It is a replay of the original response with "Memento-Datetime" and "Link" headers added, to allow a user agent to understand the response is a Memento. In Figure 21, the value of the "Location" header is the same as in the original response; it identifies an Original Resource. The user agent proceeds with finding a Memento for this Original Resource. Web archives sometimes overwrite the value that was originally provided in the "Location" header in order to point at a Memento they hold of the resource to which the redirect originally led. This is shown in Figure 22. In this case, the user agent may decide it found an appropriate Memento. HTTP/1.1 301 Moved Permanently Date: Fri, 11 Apr 2008 00:06:50 GMT Server: Apache Location: http://b.example.org Content-Length: 0 Content-Type: text/plain; charset=UTF-8 Connection: close Figure 20: Response Is a Redirect
HTTP/1.1 301 Moved Permanently Date: Thu, 21 Jan 2010 00:09:40 GMT Server: Apache-Coyote/1.1 Memento-Datetime: Fri, 11 Apr 2008 00:06:50 GMT Location: http://b.example.org Link: <http://a.example.org>; rel="original", <http://arxiv.example.net/timemap/http://a.example.org> ; rel="timemap"; type="application/link-format", <http://arxiv.example.net/timegate/http://a.example.org> ; rel="timegate" Content-Length: 0 Content-Type: text/plain; charset=UTF-8 Connection: close Figure 21: Response is a Memento of a Redirect; Leads to an Original Resource HTTP/1.1 301 Moved Permanently Date: Thu, 21 Jan 2010 00:09:40 GMT Server: Apache-Coyote/1.1 Memento-Datetime: Fri, 11 Apr 2008 00:06:50 GMT Location: http://arxiv.example.net/web/20080411000655/http://b.example.org Link: <http://a.example.org>; rel="original", <http://arxiv.example.net/timemap/http://a.example.org> ; rel="timemap"; type="application/link-format", <http://arxiv.example.net/timegate/http://a.example.org> ; rel="timegate" Content-Length: 0 Content-Type: text/plain; charset=UTF-8 Connection: close Figure 22: Response is a Memento of a Redirect; Leads to a Memento4.5.5. Memento of Responses with 4XX or 5XX HTTP Status Codes
Cases exist in which responses with 4XX and 5XX HTTP status codes are archived. If the Memento requested by the user agent is an archived version of such an HTTP response, the server's response MUST have the same 4XX or 5XX HTTP status code. The use of headers and links in responses is as described for Mementos in the respective scenarios. For example, Figure 23 shows the 404 response to an HTTP GET request for http://a.example.org issued on April 11, 2008. This response is archived as a Memento of http://a.example.org that has as URI-M http://arxiv.example.net/web/20080411000650/http://a.example.org. The response to an HTTP HEAD on this URI-M is shown in Figure 24. It
is a replay of the original response with "Memento-Datetime" and "Link" headers added, to allow a user agent to understand the response is a Memento. HTTP/1.1 404 Not Found Date: Fri, 11 Apr 2008 00:06:50 GMT Server: Apache Content-Length: 255 Content-Type: text/plain; charset=UTF-8 Connection: close Figure 23: Response Is a 404 HTTP/1.1 404 Not Found Date: Thu, 21 Jan 2010 00:09:40 GMT Server: Apache-Coyote/1.1 Memento-Datetime: Fri, 11 Apr 2008 00:06:50 GMT Link: <http://a.example.org>; rel="original", <http://arxiv.example.net/timemap/http://a.example.org> ; rel="timemap"; type="application/link-format", <http://arxiv.example.net/timegate/http://a.example.org> ; rel="timegate" Content-Length: 255 Content-Type: text/plain; charset=UTF-8 Connection: close Figure 24: Response Is a Memento of a 4044.5.6. Sticky "Memento-Datetime" and "original" Link for Mementos
A response to an HTTP HEAD/GET request issued against a Memento: o Includes a "Memento-Datetime" header that entails a promise that the response is archived, frozen in time. The value of the header expresses the archival datetime of the Memento. o Includes a link in the HTTP "Link" header with an "original" Relation Type that unambiguously points to the Original Resource associated with the Memento. The Target IRI of the link is the URI-R of that Original Resource. Both the "Memento-Datetime" header and the "original" link MUST be "sticky" in the following ways: o The server that originally assigns them MUST retain them in all responses to HTTP requests (with or without an "Accept-Datetime" request header) that occur against the Memento after the time of
their original assignment, and the server MUST NOT change the
value of the "Memento-Datetime" header nor the Target IRI of the
"original" link.
o Applications that mirror Mementos at a different URI MUST retain
them and MUST NOT change them unless mirroring involves a
meaningful state change. This allows, among others, duplicating a
Web archive at a new location while preserving the value of the
"Memento-Datetime" header and the link with the "original"
Relation Type for the archived resources. For example, when
mirroring, the "Last-Modified" header will be updated to reflect
the time of mirroring at the new URI, whereas the value for
"Memento-Datetime" will be maintained.
4.5.7. Intermediate Resources
An intermediate resource is a resource that issues a redirect to a
TimeGate, to a Memento, or to another intermediate resource, and thus
plays an active role in the Memento infrastructure. Intermediate
resources commonly exist in Web archives on the path from a TimeGate
to an appropriate Memento.
A response of an intermediate resource has an HTTP status code
indicative of HTTP redirection (e.g., 302) and uses Memento headers
and links that allow user agents to recognize that the resource plays
a role in the Memento framework:
o A "Vary" header that includes an "accept-datetime" value MUST NOT
be provided.
o The response MUST NOT include a "Memento-Datetime" header.
o The "Link" header MUST be provided, and it MUST have a link with
the "original" Relation Type that has the URI-R of the associated
Original Resource as Target IRI. Links with "timegate",
"timemap", and "memento" Relation Types are OPTIONAL and, if
provided, MUST pertain to the Original Resource for which the user
agent is trying to obtain a Memento.
A user agent MUST follow a redirection provided by an intermediate
resource; multiple such redirections can be chained.
Consider the case where a user agent follows the "timegate" link
provided in Figure 10 and engages in datetime negotiation with the
assumed TimeGate in the manner shown in Figure 11. But instead of
receiving a response as shown in Figure 12, it receives the one shown
below in Figure 25. Such a response is unambiguously recognizable as
coming from an intermediate resource.
HTTP/1.1 302 Found Date: Thu, 21 Jan 2010 00:06:50 GMT Server: Apache Location: http://arxiv.example.net/new-timegate/http://a.example.org/ Link: <http://a.example.org>; rel="original" Content-Length: 0 Content-Type: text/plain; charset=UTF-8 Connection: close Figure 25: Redirecting Resource Redirects to a TimeGate4.5.8. Resources Excluded from Datetime Negotiation
When delivering a Memento to a user agent, a Web archive commonly enhances that Memento's archived content, for example, by including a banner that provides branding and highlights the archival status of the Memento. The resources that are involved in providing such system-specific functionality, many times JavaScript or images, must be used in their current state. A server that generally supports datetime negotiation should make resources that need to be excluded from datetime negotiation recognizable. Doing so allows a user agent to refrain from attempting to access a Memento for them. In order to achieve this, the server SHOULD include a special-purpose link in the HTTP "Link" header when responding to an HTTP HEAD/GET request to a resource excluded from datetime negotiation. This link has "http://mementoweb.org/terms/donotnegotiate" as Target IRI and "type", defined in [RFC6903], as the value of the "rel" attribute. Other Memento headers as defined in Section 2.1 SHOULD NOT be provided. Figure 26 shows the response to an HTTP HEAD request from a resource excluded from datetime negotiation. HTTP/1.1 200 OK Date: Thu, 21 Jan 2010 00:09:40 GMT Server: Apache-Coyote/1.1 Link: <http://mementoweb.org/terms/donotnegotiate>; rel="type" Content-Length: 238 Content-Type: application/javascript; charset=UTF-8 Connection: close Figure 26: Response to an HTTP HEAD Request from a Resource Excluded from Datetime Negotiation
(next page on part 3)