Changeset 1284 for draft-ietf-httpbis/latest/p6-cache.xml
- Timestamp:
- 20/05/11 02:28:25 (10 years ago)
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draft-ietf-httpbis/latest/p6-cache.xml
r1281 r1284 1502 1502 <x:anchor-alias value="pragma-directive"/> 1503 1503 <t> 1504 The "Pragma" header field is used to include 1505 implementation-specific directives that might apply to any recipient along 1506 the request/response chain. All pragma directives specify optional behavior 1507 from the viewpoint of the protocol; however, some systems &MAY; require 1508 that behavior be consistent with the directives. 1504 The "Pragma" header field allows backwards compatibility with HTTP/1.0 1505 caches, so that clients can specify a "no-cache" request that they will 1506 understand (as Cache-Control was not defined until HTTP/1.1). When the 1507 Cache-Control header is also present and understood in a request, Pragma is 1508 ignored. 1509 </t> 1510 <t> 1511 In HTTP/1.0, Pragma was defined as an extensible field for 1512 implementation-specified directives for recipients. This specification 1513 deprecates such extensions to improve interoperability. 1509 1514 </t> 1510 1515 <figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/> … … 1514 1519 </artwork></figure> 1515 1520 <t> 1516 When the no-cache directive is present in a request message, a cache 1517 &SHOULD; forward the request toward the origin server even if it has a 1518 stored copy of what is being requested. This pragma directive has the same 1519 semantics as the no-cache request directive (see <xref 1520 target="cache-request-directive" />) and is defined here for backward 1521 compatibility with HTTP/1.0. A client &SHOULD; include both header fields 1522 when a no-cache request is sent to a server not known to be HTTP/1.1 1523 compliant. A cache &SHOULD; treat "Pragma: no-cache" as if the 1524 client had sent "Cache-Control: no-cache". 1521 When the Cache-Control header is not present in a request, the no-cache 1522 request pragma-directive &MUST; have the same effect on caches as if 1523 "Cache-Control: no-cache" were present (see <xref 1524 target="cache-request-directive" />). 1525 </t> 1526 <t> 1527 When sending a no-cache request, a client &SHOULD; include both pragma and 1528 cache-control directives unless Cache-Control: no-cache is purposefully 1529 omitted to target other Cache-Control response directives at HTTP/1.1 1530 caches. For example: 1531 </t> 1532 <figure> 1533 <artwork type="code"> 1534 GET / HTTP/1.1 1535 Host: www.example.com 1536 Cache-Control: max-age=30 1537 Pragma: no-cache 1538 </artwork> 1539 </figure> 1540 <t> 1541 will constrain HTTP/1.1 caches to serve a response no older than 30 1542 seconds, while precluding implementations that do not understand 1543 Cache-Control from serving a cached response. 1525 1544 </t> 1526 1545 <x:note> 1527 1546 <t> 1528 <x:h>Note:</x:h> Because the meaning of "Pragma: no-cache" as a1529 header field is not actually specified, it does not provide a1530 reliable replacement for "Cache-Control: no-cache" in a response.1531 1547 Note: Because the meaning of "Pragma: no-cache" in responses is not 1548 specified, it does not provide a reliable replacement for 1549 "Cache-Control: no-cache" in them. 1550 </t> 1532 1551 </x:note> 1533 <t>1534 This mechanism is deprecated; no new Pragma directives will be defined in1535 HTTP.1536 </t>1537 1552 </section> 1538 1553 … … 2684 2699 <section title="Since draft-ietf-httpbis-p6-cache-14" anchor="changes.since.14"> 2685 2700 <t> 2686 None yet. 2701 Closed issues: 2702 <list style="symbols"> 2703 <t> 2704 <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/292"/>: 2705 "Pragma" 2706 </t> 2707 </list> 2687 2708 </t> 2688 2709 </section>
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