Changeset 2726 for draft-ietf-httpbis/15/p4-conditional.html
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- 14/06/14 11:20:37 (8 years ago)
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draft-ietf-httpbis/15/p4-conditional.html
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… … 327 330 background-color: white; 328 331 vertical-align: top; 329 font-size: 1 2pt;332 font-size: 110%; 330 333 } 331 334 332 ul.toc a: :after {335 ul.toc a:nth-child(2)::after { 333 336 content: leader('.') target-counter(attr(href), page); 334 337 } 335 338 336 339 ul.ind li li a { 337 340 content: target-counter(attr(href), page); 338 341 } 339 342 340 343 .print2col { 341 344 column-count: 2; … … 347 350 @page { 348 351 @top-left { 349 content: "Internet-Draft"; 350 } 352 content: "Internet-Draft"; 353 } 351 354 @top-right { 352 content: "July 2011"; 353 } 355 content: "July 2011"; 356 } 354 357 @top-center { 355 content: "HTTP/1.1, Part 4"; 356 } 358 content: "HTTP/1.1, Part 4"; 359 } 357 360 @bottom-left { 358 content: "Fielding, et al."; 359 } 361 content: "Fielding, et al."; 362 } 360 363 @bottom-center { 361 content: "Expires January 12, 2012"; 362 } 364 content: "Expires January 12, 2012"; 365 } 363 366 @bottom-right { 364 content: "[Page " counter(page) "]"; 365 } 366 } 367 368 @page:first { 367 content: "[Page " counter(page) "]"; 368 } 369 } 370 371 @page:first { 369 372 @top-left { 370 373 content: normal; … … 392 395 <link rel="Appendix" title="B Collected ABNF" href="#rfc.section.B"> 393 396 <link rel="Appendix" title="C Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.C"> 394 <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1. 550, 2011-05-30 14:02:12, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/">397 <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.640, 2014/06/13 12:42:58, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/"> 395 398 <link rel="schema.dct" href="http://purl.org/dc/terms/"> 396 399 <meta name="dct.creator" content="Fielding, R."> … … 421 424 </tr> 422 425 <tr> 423 <td class="left">Obsoletes: <a href="http ://tools.ietf.org/html/rfc2616">2616</a> (if approved)426 <td class="left">Obsoletes: <a href="https://tools.ietf.org/html/rfc2616">2616</a> (if approved) 424 427 </td> 425 428 <td class="right">J. Gettys</td> … … 492 495 </table> 493 496 <p class="title">HTTP/1.1, part 4: Conditional Requests<br><span class="filename">draft-ietf-httpbis-p4-conditional-15</span></p> 494 <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> 497 <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> 495 498 <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information 496 499 systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the 497 500 seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 498 501 4 defines request header fields for indicating conditional requests and the rules for constructing responses to those requests. 499 </p> 500 <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> 502 </p> 503 <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> 501 504 <p>Discussion of this draft should take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived 502 505 at <<a href="http://lists.w3.org/Archives/Public/ietf-http-wg/">http://lists.w3.org/Archives/Public/ietf-http-wg/</a>>. 503 </p> 506 </p> 504 507 <p>The current issues list is at <<a href="http://tools.ietf.org/wg/httpbis/trac/report/3">http://tools.ietf.org/wg/httpbis/trac/report/3</a>> and related documents (including fancy diffs) can be found at <<a href="http://tools.ietf.org/wg/httpbis/">http://tools.ietf.org/wg/httpbis/</a>>. 505 </p> 508 </p> 506 509 <p>The changes in this draft are summarized in <a href="#changes.since.14" title="Since draft-ietf-httpbis-p4-conditional-14">Appendix C.16</a>. 507 </p>508 <h1><a id="rfc.status" href="#rfc.status">Status of This Memo</a></h1>509 <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>510 <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute511 working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>.512 510 </p> 513 <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other 514 documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work 515 in progress”. 516 </p> 517 <p>This Internet-Draft will expire on January 12, 2012.</p> 518 <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1> 519 <p>Copyright © 2011 IETF Trust and the persons identified as the document authors. All rights reserved.</p> 520 <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights 521 and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License 522 text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified 523 BSD License. 524 </p> 525 <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 526 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to 527 allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) 528 controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative 529 works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate 530 it into languages other than English. 531 </p> 511 <div id="rfc.status"> 512 <h1><a href="#rfc.status">Status of This Memo</a></h1> 513 <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p> 514 <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute 515 working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>. 516 </p> 517 <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other 518 documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work 519 in progress”. 520 </p> 521 <p>This Internet-Draft will expire on January 12, 2012.</p> 522 </div> 523 <div id="rfc.copyrightnotice"> 524 <h1><a href="#rfc.copyrightnotice">Copyright Notice</a></h1> 525 <p>Copyright © 2011 IETF Trust and the persons identified as the document authors. All rights reserved.</p> 526 <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights 527 and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License 528 text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified 529 BSD License. 530 </p> 531 <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 532 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to 533 allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) 534 controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative 535 works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate 536 it into languages other than English. 537 </p> 538 </div> 532 539 <hr class="noprint"> 533 540 <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1> 534 541 <ul class="toc"> 535 <li> 1. <a href="#introduction">Introduction</a><ul>536 <li> 1.1 <a href="#intro.requirements">Requirements</a></li>537 <li> 1.2 <a href="#notation">Syntax Notation</a></li>542 <li><a href="#rfc.section.1">1.</a> <a href="#introduction">Introduction</a><ul> 543 <li><a href="#rfc.section.1.1">1.1</a> <a href="#intro.requirements">Requirements</a></li> 544 <li><a href="#rfc.section.1.2">1.2</a> <a href="#notation">Syntax Notation</a></li> 538 545 </ul> 539 546 </li> 540 <li> 2. <a href="#resource.metadata">Resource State Metadata (Validators)</a><ul>541 <li> 2.1 <a href="#header.last-modified">Last-Modified</a><ul>542 <li> 2.1.1 <a href="#lastmod.generation">Generation</a></li>543 <li> 2.1.2 <a href="#lastmod.comparison">Comparison</a></li>547 <li><a href="#rfc.section.2">2.</a> <a href="#resource.metadata">Resource State Metadata (Validators)</a><ul> 548 <li><a href="#rfc.section.2.1">2.1</a> <a href="#header.last-modified">Last-Modified</a><ul> 549 <li><a href="#rfc.section.2.1.1">2.1.1</a> <a href="#lastmod.generation">Generation</a></li> 550 <li><a href="#rfc.section.2.1.2">2.1.2</a> <a href="#lastmod.comparison">Comparison</a></li> 544 551 </ul> 545 552 </li> 546 <li> 2.2 <a href="#header.etag">ETag</a><ul>547 <li> 2.2.1 <a href="#entity.tag.generation">Generation</a></li>548 <li> 2.2.2 <a href="#weak.and.strong.validators">Weak versus Strong</a></li>549 <li> 2.2.3 <a href="#entity.tag.comparison">Comparison</a></li>550 <li> 2.2.4 <a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity-tags and Last-Modified Dates</a></li>551 <li> 2.2.5 <a href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></li>553 <li><a href="#rfc.section.2.2">2.2</a> <a href="#header.etag">ETag</a><ul> 554 <li><a href="#rfc.section.2.2.1">2.2.1</a> <a href="#entity.tag.generation">Generation</a></li> 555 <li><a href="#rfc.section.2.2.2">2.2.2</a> <a href="#weak.and.strong.validators">Weak versus Strong</a></li> 556 <li><a href="#rfc.section.2.2.3">2.2.3</a> <a href="#entity.tag.comparison">Comparison</a></li> 557 <li><a href="#rfc.section.2.2.4">2.2.4</a> <a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity-tags and Last-Modified Dates</a></li> 558 <li><a href="#rfc.section.2.2.5">2.2.5</a> <a href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></li> 552 559 </ul> 553 560 </li> 554 561 </ul> 555 562 </li> 556 <li> 3. <a href="#header.fields">Precondition Header Fields</a><ul>557 <li> 3.1 <a href="#header.if-match">If-Match</a></li>558 <li> 3.2 <a href="#header.if-none-match">If-None-Match</a></li>559 <li> 3.3 <a href="#header.if-modified-since">If-Modified-Since</a></li>560 <li> 3.4 <a href="#header.if-unmodified-since">If-Unmodified-Since</a></li>561 <li> 3.5 <a href="#header.if-range">If-Range</a></li>563 <li><a href="#rfc.section.3">3.</a> <a href="#header.fields">Precondition Header Fields</a><ul> 564 <li><a href="#rfc.section.3.1">3.1</a> <a href="#header.if-match">If-Match</a></li> 565 <li><a href="#rfc.section.3.2">3.2</a> <a href="#header.if-none-match">If-None-Match</a></li> 566 <li><a href="#rfc.section.3.3">3.3</a> <a href="#header.if-modified-since">If-Modified-Since</a></li> 567 <li><a href="#rfc.section.3.4">3.4</a> <a href="#header.if-unmodified-since">If-Unmodified-Since</a></li> 568 <li><a href="#rfc.section.3.5">3.5</a> <a href="#header.if-range">If-Range</a></li> 562 569 </ul> 563 570 </li> 564 <li> 4. <a href="#status.code.definitions">Status Code Definitions</a><ul>565 <li> 4.1 <a href="#status.304">304 Not Modified</a></li>566 <li> 4.2 <a href="#status.412">412 Precondition Failed</a></li>571 <li><a href="#rfc.section.4">4.</a> <a href="#status.code.definitions">Status Code Definitions</a><ul> 572 <li><a href="#rfc.section.4.1">4.1</a> <a href="#status.304">304 Not Modified</a></li> 573 <li><a href="#rfc.section.4.2">4.2</a> <a href="#status.412">412 Precondition Failed</a></li> 567 574 </ul> 568 575 </li> 569 <li> 5. <a href="#IANA.considerations">IANA Considerations</a><ul>570 <li> 5.1 <a href="#status.code.registration">Status Code Registration</a></li>571 <li> 5.2 <a href="#header.field.registration">Header Field Registration</a></li>576 <li><a href="#rfc.section.5">5.</a> <a href="#IANA.considerations">IANA Considerations</a><ul> 577 <li><a href="#rfc.section.5.1">5.1</a> <a href="#status.code.registration">Status Code Registration</a></li> 578 <li><a href="#rfc.section.5.2">5.2</a> <a href="#header.field.registration">Header Field Registration</a></li> 572 579 </ul> 573 580 </li> 574 <li> 6. <a href="#security.considerations">Security Considerations</a></li>575 <li> 7. <a href="#ack">Acknowledgments</a></li>576 <li> 8. <a href="#rfc.references">References</a><ul>577 <li> 8.1 <a href="#rfc.references.1">Normative References</a></li>578 <li> 8.2 <a href="#rfc.references.2">Informative References</a></li>581 <li><a href="#rfc.section.6">6.</a> <a href="#security.considerations">Security Considerations</a></li> 582 <li><a href="#rfc.section.7">7.</a> <a href="#ack">Acknowledgments</a></li> 583 <li><a href="#rfc.section.8">8.</a> <a href="#rfc.references">References</a><ul> 584 <li><a href="#rfc.section.8.1">8.1</a> <a href="#rfc.references.1">Normative References</a></li> 585 <li><a href="#rfc.section.8.2">8.2</a> <a href="#rfc.references.2">Informative References</a></li> 579 586 </ul> 580 587 </li> 581 <li><a href="#rfc.authors">Authors' Addresses</a></li> 582 <li>A. <a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li> 583 <li>B. <a href="#collected.abnf">Collected ABNF</a></li> 584 <li>C. <a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul> 585 <li>C.1 <a href="#rfc.section.C.1">Since RFC 2616</a></li> 586 <li>C.2 <a href="#rfc.section.C.2">Since draft-ietf-httpbis-p4-conditional-00</a></li> 587 <li>C.3 <a href="#rfc.section.C.3">Since draft-ietf-httpbis-p4-conditional-01</a></li> 588 <li>C.4 <a href="#changes.since.02">Since draft-ietf-httpbis-p4-conditional-02</a></li> 589 <li>C.5 <a href="#changes.since.03">Since draft-ietf-httpbis-p4-conditional-03</a></li> 590 <li>C.6 <a href="#changes.since.04">Since draft-ietf-httpbis-p4-conditional-04</a></li> 591 <li>C.7 <a href="#changes.since.05">Since draft-ietf-httpbis-p4-conditional-05</a></li> 592 <li>C.8 <a href="#changes.since.06">Since draft-ietf-httpbis-p4-conditional-06</a></li> 593 <li>C.9 <a href="#changes.since.07">Since draft-ietf-httpbis-p4-conditional-07</a></li> 594 <li>C.10 <a href="#changes.since.08">Since draft-ietf-httpbis-p4-conditional-08</a></li> 595 <li>C.11 <a href="#changes.since.09">Since draft-ietf-httpbis-p4-conditional-09</a></li> 596 <li>C.12 <a href="#changes.since.10">Since draft-ietf-httpbis-p4-conditional-10</a></li> 597 <li>C.13 <a href="#changes.since.11">Since draft-ietf-httpbis-p4-conditional-11</a></li> 598 <li>C.14 <a href="#changes.since.12">Since draft-ietf-httpbis-p4-conditional-12</a></li> 599 <li>C.15 <a href="#changes.since.13">Since draft-ietf-httpbis-p4-conditional-13</a></li> 600 <li>C.16 <a href="#changes.since.14">Since draft-ietf-httpbis-p4-conditional-14</a></li> 588 <li><a href="#rfc.section.A">A.</a> <a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li> 589 <li><a href="#rfc.section.B">B.</a> <a href="#collected.abnf">Collected ABNF</a></li> 590 <li><a href="#rfc.section.C">C.</a> <a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul> 591 <li><a href="#rfc.section.C.1">C.1</a> <a href="#rfc.section.C.1">Since RFC 2616</a></li> 592 <li><a href="#rfc.section.C.2">C.2</a> <a href="#rfc.section.C.2">Since draft-ietf-httpbis-p4-conditional-00</a></li> 593 <li><a href="#rfc.section.C.3">C.3</a> <a href="#rfc.section.C.3">Since draft-ietf-httpbis-p4-conditional-01</a></li> 594 <li><a href="#rfc.section.C.4">C.4</a> <a href="#changes.since.02">Since draft-ietf-httpbis-p4-conditional-02</a></li> 595 <li><a href="#rfc.section.C.5">C.5</a> <a href="#changes.since.03">Since draft-ietf-httpbis-p4-conditional-03</a></li> 596 <li><a href="#rfc.section.C.6">C.6</a> <a href="#changes.since.04">Since draft-ietf-httpbis-p4-conditional-04</a></li> 597 <li><a href="#rfc.section.C.7">C.7</a> <a href="#changes.since.05">Since draft-ietf-httpbis-p4-conditional-05</a></li> 598 <li><a href="#rfc.section.C.8">C.8</a> <a href="#changes.since.06">Since draft-ietf-httpbis-p4-conditional-06</a></li> 599 <li><a href="#rfc.section.C.9">C.9</a> <a href="#changes.since.07">Since draft-ietf-httpbis-p4-conditional-07</a></li> 600 <li><a href="#rfc.section.C.10">C.10</a> <a href="#changes.since.08">Since draft-ietf-httpbis-p4-conditional-08</a></li> 601 <li><a href="#rfc.section.C.11">C.11</a> <a href="#changes.since.09">Since draft-ietf-httpbis-p4-conditional-09</a></li> 602 <li><a href="#rfc.section.C.12">C.12</a> <a href="#changes.since.10">Since draft-ietf-httpbis-p4-conditional-10</a></li> 603 <li><a href="#rfc.section.C.13">C.13</a> <a href="#changes.since.11">Since draft-ietf-httpbis-p4-conditional-11</a></li> 604 <li><a href="#rfc.section.C.14">C.14</a> <a href="#changes.since.12">Since draft-ietf-httpbis-p4-conditional-12</a></li> 605 <li><a href="#rfc.section.C.15">C.15</a> <a href="#changes.since.13">Since draft-ietf-httpbis-p4-conditional-13</a></li> 606 <li><a href="#rfc.section.C.16">C.16</a> <a href="#changes.since.14">Since draft-ietf-httpbis-p4-conditional-14</a></li> 601 607 </ul> 602 608 </li> 603 609 <li><a href="#rfc.index">Index</a></li> 610 <li><a href="#rfc.authors">Authors' Addresses</a></li> 604 611 </ul> 605 <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a> <a id="introduction" href="#introduction">Introduction</a></h1> 606 <p id="rfc.section.1.p.1">This document defines the HTTP/1.1 conditional request mechanisms, including both response metadata that can be used to indicate 607 or observe changes to resource state and request header fields that specify preconditions to be checked before performing 608 the action given by the request method. Conditional GET requests are the most efficient mechanism for HTTP cache updates <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, to prevent the "lost update" problem: 609 one client accidentally overwriting the work of another client that has been acting in parallel. 610 </p> 611 <p id="rfc.section.1.p.2">Conditional request preconditions are based on the state of the target resource as a whole (its current value set) or the 612 state as observed in a previously obtained representation (one value in that set). A resource might have multiple current 613 representations, each with its own observable state. The conditional request mechanisms assume that the mapping of requests 614 to corresponding representations will be consistent over time if the server intends to take advantage of conditionals. Regardless, 615 if the mapping is inconsistent and the server is unable to select the appropriate representation, then no harm will result 616 when the precondition evaluates to false. 617 </p> 618 <p id="rfc.section.1.p.3"><span id="rfc.iref.s.1"></span> We use the term "<dfn>selected representation</dfn>" to refer to the current representation of the target resource that would have been selected in a successful response if 619 the same request had used the method GET and had excluded all of the conditional request header fields. The conditional request 620 preconditions are evaluated by comparing the values provided in the request header fields to the current metadata for the 621 selected representation. 622 </p> 623 <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a> <a id="intro.requirements" href="#intro.requirements">Requirements</a></h2> 624 <p id="rfc.section.1.1.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" 625 in this document are to be interpreted as described in <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</cite></a>. 626 </p> 627 <p id="rfc.section.1.1.p.2">An implementation is not compliant if it fails to satisfy one or more of the "MUST" or "REQUIRED" level requirements for the 628 protocols it implements. An implementation that satisfies all the "MUST" or "REQUIRED" level and all the "SHOULD" level requirements 629 for its protocols is said to be "unconditionally compliant"; one that satisfies all the "MUST" level requirements but not 630 all the "SHOULD" level requirements for its protocols is said to be "conditionally compliant". 631 </p> 632 <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a> <a id="notation" href="#notation">Syntax Notation</a></h2> 633 <p id="rfc.section.1.2.p.1">This specification uses the ABNF syntax defined in <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</a> of <a href="#Part1" id="rfc.xref.Part1.1"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a> (which extends the syntax defined in <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with a list rule). <a href="#collected.abnf" title="Collected ABNF">Appendix B</a> shows the collected ABNF, with the list rule expanded. 634 </p> 635 <p id="rfc.section.1.2.p.2">The following core rules are included by reference, as defined in <a href="#RFC5234" id="rfc.xref.RFC5234.2"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a>, <a href="http://tools.ietf.org/html/rfc5234#appendix-B.1">Appendix B.1</a>: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG 636 (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), VCHAR (any visible USASCII character), 637 and WSP (whitespace). 638 </p> 639 <p id="rfc.section.1.2.p.3">The ABNF rules below are defined in other parts:</p> 640 <div id="rfc.figure.u.1"></div><pre class="inline"> <a href="#notation" class="smpl">quoted-string</a> = <quoted-string, defined in <a href="#Part1" id="rfc.xref.Part1.2"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>> 612 <div id="introduction"> 613 <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a> <a href="#introduction">Introduction</a></h1> 614 <p id="rfc.section.1.p.1">This document defines the HTTP/1.1 conditional request mechanisms, including both response metadata that can be used to indicate 615 or observe changes to resource state and request header fields that specify preconditions to be checked before performing 616 the action given by the request method. Conditional GET requests are the most efficient mechanism for HTTP cache updates <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, to prevent the "lost update" problem: 617 one client accidentally overwriting the work of another client that has been acting in parallel. 618 </p> 619 <p id="rfc.section.1.p.2">Conditional request preconditions are based on the state of the target resource as a whole (its current value set) or the 620 state as observed in a previously obtained representation (one value in that set). A resource might have multiple current 621 representations, each with its own observable state. The conditional request mechanisms assume that the mapping of requests 622 to corresponding representations will be consistent over time if the server intends to take advantage of conditionals. Regardless, 623 if the mapping is inconsistent and the server is unable to select the appropriate representation, then no harm will result 624 when the precondition evaluates to false. 625 </p> 626 <p id="rfc.section.1.p.3"><span id="rfc.iref.s.1"></span> We use the term "<dfn>selected representation</dfn>" to refer to the current representation of the target resource that would have been selected in a successful response if 627 the same request had used the method GET and had excluded all of the conditional request header fields. The conditional request 628 preconditions are evaluated by comparing the values provided in the request header fields to the current metadata for the 629 selected representation. 630 </p> 631 <div id="intro.requirements"> 632 <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a> <a href="#intro.requirements">Requirements</a></h2> 633 <p id="rfc.section.1.1.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" 634 in this document are to be interpreted as described in <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</cite></a>. 635 </p> 636 <p id="rfc.section.1.1.p.2">An implementation is not compliant if it fails to satisfy one or more of the "MUST" or "REQUIRED" level requirements for the 637 protocols it implements. An implementation that satisfies all the "MUST" or "REQUIRED" level and all the "SHOULD" level requirements 638 for its protocols is said to be "unconditionally compliant"; one that satisfies all the "MUST" level requirements but not 639 all the "SHOULD" level requirements for its protocols is said to be "conditionally compliant". 640 </p> 641 </div> 642 <div id="notation"> 643 <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a> <a href="#notation">Syntax Notation</a></h2> 644 <p id="rfc.section.1.2.p.1">This specification uses the ABNF syntax defined in <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</a> of <a href="#Part1" id="rfc.xref.Part1.1"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a> (which extends the syntax defined in <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with a list rule). <a href="#collected.abnf" title="Collected ABNF">Appendix B</a> shows the collected ABNF, with the list rule expanded. 645 </p> 646 <p id="rfc.section.1.2.p.2">The following core rules are included by reference, as defined in <a href="#RFC5234" id="rfc.xref.RFC5234.2"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a>, <a href="https://tools.ietf.org/html/rfc5234#appendix-B.1">Appendix B.1</a>: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG 647 (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), VCHAR (any visible USASCII character), 648 and WSP (whitespace). 649 </p> 650 <p id="rfc.section.1.2.p.3">The ABNF rules below are defined in other parts:</p> 651 <div id="rfc.figure.u.1"></div><pre class="inline"> <a href="#notation" class="smpl">quoted-string</a> = <quoted-string, defined in <a href="#Part1" id="rfc.xref.Part1.2"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>> 641 652 <a href="#notation" class="smpl">OWS</a> = <OWS, defined in <a href="#Part1" id="rfc.xref.Part1.3"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a>> 642 653 <a href="#notation" class="smpl">HTTP-date</a> = <HTTP-date, defined in <a href="#Part1" id="rfc.xref.Part1.4"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#date.time.formats.full.date" title="Date/Time Formats: Full Date">Section 6.1</a>> 643 </pre><div id="rfc.iref.m.1"></div> 644 <div id="rfc.iref.v.1"></div> 645 <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a> <a id="resource.metadata" href="#resource.metadata">Resource State Metadata (Validators)</a></h1> 646 <p id="rfc.section.2.p.1">This specification defines two forms of metadata that are commonly used to observe resource state and test for preconditions: 647 modification dates and opaque entity tags. Additional metadata that reflects resource state has been defined by various extensions 648 of HTTP, such as WebDAV <a href="#RFC4918" id="rfc.xref.RFC4918.1"><cite title="HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)">[RFC4918]</cite></a>, that are beyond the scope of this specification. A resource metadata value is referred to as a "<dfn>validator</dfn>" when it is used within a precondition. 649 </p> 650 <div id="rfc.iref.l.1"></div> 651 <div id="rfc.iref.h.1"></div> 652 <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a> <a id="header.last-modified" href="#header.last-modified">Last-Modified</a></h2> 653 <p id="rfc.section.2.1.p.1">The "Last-Modified" header field indicates the date and time at which the origin server believes the selected representation 654 was last modified. 655 </p> 656 <div id="rfc.figure.u.2"></div><pre class="inline"><span id="rfc.iref.g.1"></span> <a href="#header.last-modified" class="smpl">Last-Modified</a> = <a href="#notation" class="smpl">HTTP-date</a> 654 </pre></div> 655 </div> 656 <div id="resource.metadata"> 657 <div id="rfc.iref.m.1"></div> 658 <div id="rfc.iref.v.1"></div> 659 <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a> <a href="#resource.metadata">Resource State Metadata (Validators)</a></h1> 660 <p id="rfc.section.2.p.1">This specification defines two forms of metadata that are commonly used to observe resource state and test for preconditions: 661 modification dates and opaque entity tags. Additional metadata that reflects resource state has been defined by various extensions 662 of HTTP, such as WebDAV <a href="#RFC4918" id="rfc.xref.RFC4918.1"><cite title="HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)">[RFC4918]</cite></a>, that are beyond the scope of this specification. A resource metadata value is referred to as a "<dfn>validator</dfn>" when it is used within a precondition. 663 </p> 664 <div id="header.last-modified"> 665 <div id="rfc.iref.l.1"></div> 666 <div id="rfc.iref.h.1"></div> 667 <h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a> <a href="#header.last-modified">Last-Modified</a></h2> 668 <p id="rfc.section.2.1.p.1">The "Last-Modified" header field indicates the date and time at which the origin server believes the selected representation 669 was last modified. 670 </p> 671 <div id="rfc.figure.u.2"></div><pre class="inline"><span id="rfc.iref.g.1"></span> <a href="#header.last-modified" class="smpl">Last-Modified</a> = <a href="#notation" class="smpl">HTTP-date</a> 657 672 </pre><p id="rfc.section.2.1.p.3">An example of its use is</p> 658 <div id="rfc.figure.u.3"></div><pre class="text"> Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT 659 </pre><h3 id="rfc.section.2.1.1"><a href="#rfc.section.2.1.1">2.1.1</a> <a id="lastmod.generation" href="#lastmod.generation">Generation</a></h3> 660 <p id="rfc.section.2.1.1.p.1">Origin servers <em class="bcp14">SHOULD</em> send Last-Modified for any selected representation for which a last modification date can be reasonably and consistently determined, 661 since its use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) results in a substantial reduction of HTTP traffic on the Internet and can be a significant factor in improving service 662 scalability and reliability. 663 </p> 664 <p id="rfc.section.2.1.1.p.2">A representation is typically the sum of many parts behind the resource interface. The last-modified time would usually be 665 the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation 666 detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can 667 use its value to make conditional requests and test the validity of locally cached responses. 668 </p> 669 <p id="rfc.section.2.1.1.p.3">An origin server <em class="bcp14">SHOULD</em> obtain the Last-Modified value of the representation as close as possible to the time that it generates the Date field-value 670 for its response. This allows a recipient to make an accurate assessment of the representation's modification time, especially 671 if the representation changes near the time that the response is generated. 672 </p> 673 <p id="rfc.section.2.1.1.p.4">An origin server with a clock <em class="bcp14">MUST NOT</em> send a Last-Modified date that is later than the server's time of message origination (Date). If the last modification time 674 is derived from implementation-specific metadata that evaluates to some time in the future, according to the origin server's 675 clock, then the origin server <em class="bcp14">MUST</em> replace that value with the message origination date. This prevents a future modification date from having an adverse impact 676 on cache validation. 677 </p> 678 <h3 id="rfc.section.2.1.2"><a href="#rfc.section.2.1.2">2.1.2</a> <a id="lastmod.comparison" href="#lastmod.comparison">Comparison</a></h3> 679 <p id="rfc.section.2.1.2.p.1">A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is 680 strong, using the following rules: 681 </p> 682 <ul> 683 <li>The validator is being compared by an origin server to the actual current validator for the representation and,</li> 684 <li>That origin server reliably knows that the associated representation did not change twice during the second covered by the 685 presented validator. 686 </li> 687 </ul> 688 <p id="rfc.section.2.1.2.p.2">or </p> 689 <ul> 690 <li>The validator is about to be used by a client in an If-Modified-Since or If-Unmodified-Since header field, because the client 691 has a cache entry for the associated representation, and 692 </li> 693 <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li> 694 <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li> 695 </ul> 696 <p id="rfc.section.2.1.2.p.3">or </p> 697 <ul> 698 <li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the representation, 699 and 700 </li> 701 <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li> 702 <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li> 703 </ul> 704 <p id="rfc.section.2.1.2.p.4">This method relies on the fact that if two different responses were sent by the origin server during the same second, but 705 both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified 706 time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from 707 different clocks, or at somewhat different times during the preparation of the response. An implementation <em class="bcp14">MAY</em> use a value larger than 60 seconds, if it is believed that 60 seconds is too short. 708 </p> 709 <div id="rfc.iref.e.1"></div> 710 <div id="rfc.iref.h.2"></div> 711 <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a> <a id="header.etag" href="#header.etag">ETag</a></h2> 712 <p id="rfc.section.2.2.p.1">The ETag header field provides the current entity-tag for the selected representation. An entity-tag is an opaque validator 713 for differentiating between multiple representations of the same resource, regardless of whether those multiple representations 714 are due to resource state changes over time, content negotiation resulting in multiple representations being valid at the 715 same time, or both. An entity-tag consists of an opaque quoted string, possibly prefixed by a weakness indicator. 716 </p> 717 <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span> <a href="#header.etag" class="smpl">ETag</a> = <a href="#header.etag" class="smpl">entity-tag</a> 673 <div id="rfc.figure.u.3"></div><pre class="text"> Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT 674 </pre><div id="lastmod.generation"> 675 <h3 id="rfc.section.2.1.1"><a href="#rfc.section.2.1.1">2.1.1</a> <a href="#lastmod.generation">Generation</a></h3> 676 <p id="rfc.section.2.1.1.p.1">Origin servers <em class="bcp14">SHOULD</em> send Last-Modified for any selected representation for which a last modification date can be reasonably and consistently determined, 677 since its use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) results in a substantial reduction of HTTP traffic on the Internet and can be a significant factor in improving service 678 scalability and reliability. 679 </p> 680 <p id="rfc.section.2.1.1.p.2">A representation is typically the sum of many parts behind the resource interface. The last-modified time would usually be 681 the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation 682 detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can 683 use its value to make conditional requests and test the validity of locally cached responses. 684 </p> 685 <p id="rfc.section.2.1.1.p.3">An origin server <em class="bcp14">SHOULD</em> obtain the Last-Modified value of the representation as close as possible to the time that it generates the Date field-value 686 for its response. This allows a recipient to make an accurate assessment of the representation's modification time, especially 687 if the representation changes near the time that the response is generated. 688 </p> 689 <p id="rfc.section.2.1.1.p.4">An origin server with a clock <em class="bcp14">MUST NOT</em> send a Last-Modified date that is later than the server's time of message origination (Date). If the last modification time 690 is derived from implementation-specific metadata that evaluates to some time in the future, according to the origin server's 691 clock, then the origin server <em class="bcp14">MUST</em> replace that value with the message origination date. This prevents a future modification date from having an adverse impact 692 on cache validation. 693 </p> 694 </div> 695 <div id="lastmod.comparison"> 696 <h3 id="rfc.section.2.1.2"><a href="#rfc.section.2.1.2">2.1.2</a> <a href="#lastmod.comparison">Comparison</a></h3> 697 <p id="rfc.section.2.1.2.p.1">A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is 698 strong, using the following rules: 699 </p> 700 <ul> 701 <li>The validator is being compared by an origin server to the actual current validator for the representation and,</li> 702 <li>That origin server reliably knows that the associated representation did not change twice during the second covered by the 703 presented validator. 704 </li> 705 </ul> 706 <p id="rfc.section.2.1.2.p.2">or </p> 707 <ul> 708 <li>The validator is about to be used by a client in an If-Modified-Since or If-Unmodified-Since header field, because the client 709 has a cache entry for the associated representation, and 710 </li> 711 <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li> 712 <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li> 713 </ul> 714 <p id="rfc.section.2.1.2.p.3">or </p> 715 <ul> 716 <li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the representation, 717 and 718 </li> 719 <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li> 720 <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li> 721 </ul> 722 <p id="rfc.section.2.1.2.p.4">This method relies on the fact that if two different responses were sent by the origin server during the same second, but 723 both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified 724 time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from 725 different clocks, or at somewhat different times during the preparation of the response. An implementation <em class="bcp14">MAY</em> use a value larger than 60 seconds, if it is believed that 60 seconds is too short. 726 </p> 727 </div> 728 </div> 729 <div id="header.etag"> 730 <div id="rfc.iref.e.1"></div> 731 <div id="rfc.iref.h.2"></div> 732 <h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a> <a href="#header.etag">ETag</a></h2> 733 <p id="rfc.section.2.2.p.1">The ETag header field provides the current entity-tag for the selected representation. An entity-tag is an opaque validator 734 for differentiating between multiple representations of the same resource, regardless of whether those multiple representations 735 are due to resource state changes over time, content negotiation resulting in multiple representations being valid at the 736 same time, or both. An entity-tag consists of an opaque quoted string, possibly prefixed by a weakness indicator. 737 </p> 738 <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span> <a href="#header.etag" class="smpl">ETag</a> = <a href="#header.etag" class="smpl">entity-tag</a> 718 739 719 740 <a href="#header.etag" class="smpl">entity-tag</a> = [ <a href="#header.etag" class="smpl">weak</a> ] <a href="#header.etag" class="smpl">opaque-tag</a> … … 721 742 <a href="#header.etag" class="smpl">opaque-tag</a> = <a href="#notation" class="smpl">quoted-string</a> 722 743 </pre><p id="rfc.section.2.2.p.3">An entity-tag can be more reliable for validation than a modification date in situations where it is inconvenient to store 723 modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are724 not consistently maintained.725 </p>726 <div id="rfc.figure.u.5"></div>727 <p>Examples:</p><pre class="text"> ETag: "xyzzy"744 modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are 745 not consistently maintained. 746 </p> 747 <div id="rfc.figure.u.5"></div> 748 <p>Examples:</p><pre class="text"> ETag: "xyzzy" 728 749 ETag: W/"xyzzy" 729 750 ETag: "" 730 </pre><h3 id="rfc.section.2.2.1"><a href="#rfc.section.2.2.1">2.2.1</a> <a id="entity.tag.generation" href="#entity.tag.generation">Generation</a></h3> 731 <p id="rfc.section.2.2.1.p.1">The principle behind entity-tags is that only the service author knows the implementation of a resource well enough to select 732 the most accurate and efficient validation mechanism for that resource, and that any such mechanism can be mapped to a simple 733 sequence of octets for easy comparison. Since the value is opaque, there is no need for the client to be aware of how each 734 entity-tag is constructed. 735 </p> 736 <p id="rfc.section.2.2.1.p.2">For example, a resource that has implementation-specific versioning applied to all changes might use an internal revision 737 number, perhaps combined with a variance identifier for content negotiation, to accurately differentiate between representations. 738 Other implementations might use a stored hash of representation content, a combination of various filesystem attributes, or 739 a modification timestamp that has sub-second resolution. 740 </p> 741 <p id="rfc.section.2.2.1.p.3">Origin servers <em class="bcp14">SHOULD</em> send ETag for any selected representation for which detection of changes can be reasonably and consistently determined, since 742 the entity-tag's use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) can result in a substantial reduction of HTTP network traffic and can be a significant factor in improving service scalability 743 and reliability. 744 </p> 745 <h3 id="rfc.section.2.2.2"><a href="#rfc.section.2.2.2">2.2.2</a> <a id="weak.and.strong.validators" href="#weak.and.strong.validators">Weak versus Strong</a></h3> 746 <p id="rfc.section.2.2.2.p.1">Since both origin servers and caches will compare two validators to decide if they indicate the same or different representations, 747 one normally would expect that if the representation (including both representation header fields and representation body) 748 changes in any way, then the associated validator would change as well. If this is true, then we call that validator a "strong 749 validator". One example of a strong validator is an integer that is incremented in stable storage every time a representation 750 is changed. 751 </p> 752 <p id="rfc.section.2.2.2.p.2">However, there might be cases when a server prefers to change the validator only when it desires cached representations to 753 be invalidated. For example, the representation of a weather report that changes in content every second, based on dynamic 754 measurements, might be grouped into sets of equivalent representations (from the origin server's perspective) in order to 755 allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server load 756 or weather quality). A validator that does not always change when the representation changes is a "weak validator". 757 </p> 758 <p id="rfc.section.2.2.2.p.3">One can think of a strong validator as part of an identifier for a specific representation, whereas a weak validator is part 759 of an identifier for a set of equivalent representations (where this notion of equivalence is entirely governed by the origin 760 server and beyond the scope of this specification). 761 </p> 762 <p id="rfc.section.2.2.2.p.4">An entity-tag is normally a strong validator, but the protocol provides a mechanism to tag an entity-tag as "weak". </p> 763 <ul class="empty"> 764 <li>A representation's modification time, if defined with only one-second resolution, could be a weak validator, since it is possible 765 that the representation might be modified twice during a single second. 766 </li> 767 <li>Support for weak validators is optional. However, weak validators allow for more efficient caching of equivalent objects; 768 for example, a hit counter on a site is probably good enough if it is updated every few days or weeks, and any value during 769 that period is likely "good enough" to be equivalent. 770 </li> 771 </ul> 772 <p id="rfc.section.2.2.2.p.5">A strong entity-tag <em class="bcp14">MUST</em> change whenever the associated representation changes in any way. A weak entity-tag <em class="bcp14">SHOULD</em> change whenever the origin server considers prior representations to be unacceptable as a substitute for the current representation. 773 In other words, a weak entity tag <em class="bcp14">SHOULD</em> change whenever the origin server wants caches to invalidate old responses. 774 </p> 775 <p id="rfc.section.2.2.2.p.6">A "strong entity-tag" <em class="bcp14">MAY</em> be shared by two representations of a resource only if they are equivalent by octet equality. 776 </p> 777 <p id="rfc.section.2.2.2.p.7">A "weak entity-tag", indicated by the "W/" prefix, <em class="bcp14">MAY</em> be shared by two representations of a resource. A weak entity-tag can only be used for weak comparison. 778 </p> 779 <p id="rfc.section.2.2.2.p.8">Cache entries might persist for arbitrarily long periods, regardless of expiration times. Thus, a cache might attempt to validate 780 an entry using a validator that it obtained in the distant past. A strong entity-tag <em class="bcp14">MUST</em> be unique across all versions of all representations associated with a particular resource over time. However, there is no 781 implication of uniqueness across entity-tags of different resources (i.e., the same entity-tag value might be in use for representations 782 of multiple resources at the same time and does not imply that those representations are equivalent). 783 </p> 784 <h3 id="rfc.section.2.2.3"><a href="#rfc.section.2.2.3">2.2.3</a> <a id="entity.tag.comparison" href="#entity.tag.comparison">Comparison</a></h3> 785 <p id="rfc.section.2.2.3.p.1">There are two entity-tag comparison functions, depending on whether the comparison context allows the use of weak validators 786 or not: 787 </p> 788 <ul> 789 <li>The strong comparison function: in order to be considered equal, both opaque-tags <em class="bcp14">MUST</em> be identical character-by-character, and both <em class="bcp14">MUST NOT</em> be weak. 790 </li> 791 <li>The weak comparison function: in order to be considered equal, both opaque-tags <em class="bcp14">MUST</em> be identical character-by-character, but either or both of them <em class="bcp14">MAY</em> be tagged as "weak" without affecting the result. 792 </li> 793 </ul> 794 <p id="rfc.section.2.2.3.p.2">A "use" of a validator is either when a client generates a request and includes the validator in a precondition, or when a 795 server compares two validators. 796 </p> 797 <p id="rfc.section.2.2.3.p.3">Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality 798 of a representation. For example, either kind is usable for a normal conditional GET. 799 </p> 800 <p id="rfc.section.2.2.3.p.4">The example below shows the results for a set of entity-tag pairs, and both the weak and strong comparison function results:</p> 801 <div id="rfc.table.u.1"> 802 <table class="tt full left" cellpadding="3" cellspacing="0"> 803 <thead> 804 <tr> 805 <th>ETag 1</th> 806 <th>ETag 2</th> 807 <th>Strong Comparison</th> 808 <th>Weak Comparison</th> 809 </tr> 810 </thead> 811 <tbody> 812 <tr> 813 <td class="left">W/"1"</td> 814 <td class="left">W/"1"</td> 815 <td class="left">no match</td> 816 <td class="left">match</td> 817 </tr> 818 <tr> 819 <td class="left">W/"1"</td> 820 <td class="left">W/"2"</td> 821 <td class="left">no match</td> 822 <td class="left">no match</td> 823 </tr> 824 <tr> 825 <td class="left">W/"1"</td> 826 <td class="left">"1"</td> 827 <td class="left">no match</td> 828 <td class="left">match</td> 829 </tr> 830 <tr> 831 <td class="left">"1"</td> 832 <td class="left">"1"</td> 833 <td class="left">match</td> 834 <td class="left">match</td> 835 </tr> 836 </tbody> 837 </table> 838 </div> 839 <p id="rfc.section.2.2.3.p.5">An entity-tag is strong unless it is explicitly tagged as weak.</p> 840 <h3 id="rfc.section.2.2.4"><a href="#rfc.section.2.2.4">2.2.4</a> <a id="rules.for.when.to.use.entity.tags.and.last-modified.dates" href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity-tags and Last-Modified Dates</a></h3> 841 <p id="rfc.section.2.2.4.p.1">We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types 842 ought to be used, and for what purposes. 843 </p> 844 <p id="rfc.section.2.2.4.p.2">HTTP/1.1 origin servers: </p> 845 <ul> 846 <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one. 847 </li> 848 <li><em class="bcp14">MAY</em> send a weak entity-tag instead of a strong entity-tag, if performance considerations support the use of weak entity-tags, 849 or if it is unfeasible to send a strong entity-tag. 850 </li> 851 <li><em class="bcp14">SHOULD</em> send a Last-Modified value if it is feasible to send one. 852 </li> 853 </ul> 854 <p id="rfc.section.2.2.4.p.3">In other words, the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity-tag and a Last-Modified 855 value. 856 </p> 857 <p id="rfc.section.2.2.4.p.4">HTTP/1.1 clients: </p> 858 <ul> 859 <li><em class="bcp14">MUST</em> use that entity-tag in any cache-conditional request (using If-Match or If-None-Match) if an entity-tag has been provided 860 by the origin server. 861 </li> 862 <li><em class="bcp14">SHOULD</em> use the Last-Modified value in non-subrange cache-conditional requests (using If-Modified-Since) if only a Last-Modified value 863 has been provided by the origin server. 864 </li> 865 <li><em class="bcp14">MAY</em> use the Last-Modified value in subrange cache-conditional requests (using If-Unmodified-Since) if only a Last-Modified value 866 has been provided by an HTTP/1.0 origin server. The user agent <em class="bcp14">SHOULD</em> provide a way to disable this, in case of difficulty. 867 </li> 868 <li><em class="bcp14">SHOULD</em> use both validators in cache-conditional requests if both an entity-tag and a Last-Modified value have been provided by the 869 origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately. 870 </li> 871 </ul> 872 <p id="rfc.section.2.2.4.p.5">An HTTP/1.1 origin server, upon receiving a conditional request that includes both a Last-Modified date (e.g., in an If-Modified-Since 873 or If-Unmodified-Since header field) and one or more entity-tags (e.g., in an If-Match, If-None-Match, or If-Range header 874 field) as cache validators, <em class="bcp14">MUST NOT</em> return a response status code of 304 (Not Modified) unless doing so is consistent with all of the conditional header fields 875 in the request. 876 </p> 877 <p id="rfc.section.2.2.4.p.6">An HTTP/1.1 caching proxy, upon receiving a conditional request that includes both a Last-Modified date and one or more entity-tags 878 as cache validators, <em class="bcp14">MUST NOT</em> return a locally cached response to the client unless that cached response is consistent with all of the conditional header 879 fields in the request. 880 </p> 881 <ul class="empty"> 882 <li> <b>Note:</b> The general principle behind these rules is that HTTP/1.1 servers and clients ought to transmit as much non-redundant information 883 as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative 884 assumptions about the validators they receive. 885 </li> 886 <li>HTTP/1.0 clients and caches might ignore entity-tags. Generally, last-modified values received or used by these systems will 887 support transparent and efficient caching, and so HTTP/1.1 origin servers should provide Last-Modified values. In those rare 888 cases where the use of a Last-Modified value as a validator by an HTTP/1.0 system could result in a serious problem, then 889 HTTP/1.1 origin servers should not provide one. 890 </li> 891 </ul> 892 <h3 id="rfc.section.2.2.5"><a href="#rfc.section.2.2.5">2.2.5</a> <a id="example.entity.tag.vs.conneg" href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></h3> 893 <p id="rfc.section.2.2.5.p.1">Consider a resource that is subject to content negotiation (<a href="p3-payload.html#content.negotiation" title="Content Negotiation">Section 5</a> of <a href="#Part3" id="rfc.xref.Part3.1"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>), and where the representations returned upon a GET request vary based on the Accept-Encoding request header field (<a href="p3-payload.html#header.accept-encoding" title="Accept-Encoding">Section 6.3</a> of <a href="#Part3" id="rfc.xref.Part3.2"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>): 894 </p> 895 <div id="rfc.figure.u.6"></div> 896 <p>>> Request:</p><pre class="text2">GET /index HTTP/1.1 751 </pre><div id="entity.tag.generation"> 752 <h3 id="rfc.section.2.2.1"><a href="#rfc.section.2.2.1">2.2.1</a> <a href="#entity.tag.generation">Generation</a></h3> 753 <p id="rfc.section.2.2.1.p.1">The principle behind entity-tags is that only the service author knows the implementation of a resource well enough to select 754 the most accurate and efficient validation mechanism for that resource, and that any such mechanism can be mapped to a simple 755 sequence of octets for easy comparison. Since the value is opaque, there is no need for the client to be aware of how each 756 entity-tag is constructed. 757 </p> 758 <p id="rfc.section.2.2.1.p.2">For example, a resource that has implementation-specific versioning applied to all changes might use an internal revision 759 number, perhaps combined with a variance identifier for content negotiation, to accurately differentiate between representations. 760 Other implementations might use a stored hash of representation content, a combination of various filesystem attributes, or 761 a modification timestamp that has sub-second resolution. 762 </p> 763 <p id="rfc.section.2.2.1.p.3">Origin servers <em class="bcp14">SHOULD</em> send ETag for any selected representation for which detection of changes can be reasonably and consistently determined, since 764 the entity-tag's use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) can result in a substantial reduction of HTTP network traffic and can be a significant factor in improving service scalability 765 and reliability. 766 </p> 767 </div> 768 <div id="weak.and.strong.validators"> 769 <h3 id="rfc.section.2.2.2"><a href="#rfc.section.2.2.2">2.2.2</a> <a href="#weak.and.strong.validators">Weak versus Strong</a></h3> 770 <p id="rfc.section.2.2.2.p.1">Since both origin servers and caches will compare two validators to decide if they indicate the same or different representations, 771 one normally would expect that if the representation (including both representation header fields and representation body) 772 changes in any way, then the associated validator would change as well. If this is true, then we call that validator a "strong 773 validator". One example of a strong validator is an integer that is incremented in stable storage every time a representation 774 is changed. 775 </p> 776 <p id="rfc.section.2.2.2.p.2">However, there might be cases when a server prefers to change the validator only when it desires cached representations to 777 be invalidated. For example, the representation of a weather report that changes in content every second, based on dynamic 778 measurements, might be grouped into sets of equivalent representations (from the origin server's perspective) in order to 779 allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server load 780 or weather quality). A validator that does not always change when the representation changes is a "weak validator". 781 </p> 782 <p id="rfc.section.2.2.2.p.3">One can think of a strong validator as part of an identifier for a specific representation, whereas a weak validator is part 783 of an identifier for a set of equivalent representations (where this notion of equivalence is entirely governed by the origin 784 server and beyond the scope of this specification). 785 </p> 786 <p id="rfc.section.2.2.2.p.4">An entity-tag is normally a strong validator, but the protocol provides a mechanism to tag an entity-tag as "weak". </p> 787 <ul class="empty"> 788 <li>A representation's modification time, if defined with only one-second resolution, could be a weak validator, since it is possible 789 that the representation might be modified twice during a single second. 790 </li> 791 <li>Support for weak validators is optional. However, weak validators allow for more efficient caching of equivalent objects; 792 for example, a hit counter on a site is probably good enough if it is updated every few days or weeks, and any value during 793 that period is likely "good enough" to be equivalent. 794 </li> 795 </ul> 796 <p id="rfc.section.2.2.2.p.5">A strong entity-tag <em class="bcp14">MUST</em> change whenever the associated representation changes in any way. A weak entity-tag <em class="bcp14">SHOULD</em> change whenever the origin server considers prior representations to be unacceptable as a substitute for the current representation. 797 In other words, a weak entity tag <em class="bcp14">SHOULD</em> change whenever the origin server wants caches to invalidate old responses. 798 </p> 799 <p id="rfc.section.2.2.2.p.6">A "strong entity-tag" <em class="bcp14">MAY</em> be shared by two representations of a resource only if they are equivalent by octet equality. 800 </p> 801 <p id="rfc.section.2.2.2.p.7">A "weak entity-tag", indicated by the "W/" prefix, <em class="bcp14">MAY</em> be shared by two representations of a resource. A weak entity-tag can only be used for weak comparison. 802 </p> 803 <p id="rfc.section.2.2.2.p.8">Cache entries might persist for arbitrarily long periods, regardless of expiration times. Thus, a cache might attempt to validate 804 an entry using a validator that it obtained in the distant past. A strong entity-tag <em class="bcp14">MUST</em> be unique across all versions of all representations associated with a particular resource over time. However, there is no 805 implication of uniqueness across entity-tags of different resources (i.e., the same entity-tag value might be in use for representations 806 of multiple resources at the same time and does not imply that those representations are equivalent). 807 </p> 808 </div> 809 <div id="entity.tag.comparison"> 810 <h3 id="rfc.section.2.2.3"><a href="#rfc.section.2.2.3">2.2.3</a> <a href="#entity.tag.comparison">Comparison</a></h3> 811 <p id="rfc.section.2.2.3.p.1">There are two entity-tag comparison functions, depending on whether the comparison context allows the use of weak validators 812 or not: 813 </p> 814 <ul> 815 <li>The strong comparison function: in order to be considered equal, both opaque-tags <em class="bcp14">MUST</em> be identical character-by-character, and both <em class="bcp14">MUST NOT</em> be weak. 816 </li> 817 <li>The weak comparison function: in order to be considered equal, both opaque-tags <em class="bcp14">MUST</em> be identical character-by-character, but either or both of them <em class="bcp14">MAY</em> be tagged as "weak" without affecting the result. 818 </li> 819 </ul> 820 <p id="rfc.section.2.2.3.p.2">A "use" of a validator is either when a client generates a request and includes the validator in a precondition, or when a 821 server compares two validators. 822 </p> 823 <p id="rfc.section.2.2.3.p.3">Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality 824 of a representation. For example, either kind is usable for a normal conditional GET. 825 </p> 826 <p id="rfc.section.2.2.3.p.4">The example below shows the results for a set of entity-tag pairs, and both the weak and strong comparison function results:</p> 827 <div id="rfc.table.u.1"> 828 <table class="tt full left" cellpadding="3" cellspacing="0"> 829 <thead> 830 <tr> 831 <th>ETag 1</th> 832 <th>ETag 2</th> 833 <th>Strong Comparison</th> 834 <th>Weak Comparison</th> 835 </tr> 836 </thead> 837 <tbody> 838 <tr> 839 <td class="left">W/"1"</td> 840 <td class="left">W/"1"</td> 841 <td class="left">no match</td> 842 <td class="left">match</td> 843 </tr> 844 <tr> 845 <td class="left">W/"1"</td> 846 <td class="left">W/"2"</td> 847 <td class="left">no match</td> 848 <td class="left">no match</td> 849 </tr> 850 <tr> 851 <td class="left">W/"1"</td> 852 <td class="left">"1"</td> 853 <td class="left">no match</td> 854 <td class="left">match</td> 855 </tr> 856 <tr> 857 <td class="left">"1"</td> 858 <td class="left">"1"</td> 859 <td class="left">match</td> 860 <td class="left">match</td> 861 </tr> 862 </tbody> 863 </table> 864 </div> 865 <p id="rfc.section.2.2.3.p.5">An entity-tag is strong unless it is explicitly tagged as weak.</p> 866 </div> 867 <div id="rules.for.when.to.use.entity.tags.and.last-modified.dates"> 868 <h3 id="rfc.section.2.2.4"><a href="#rfc.section.2.2.4">2.2.4</a> <a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity-tags and Last-Modified Dates</a></h3> 869 <p id="rfc.section.2.2.4.p.1">We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types 870 ought to be used, and for what purposes. 871 </p> 872 <p id="rfc.section.2.2.4.p.2">HTTP/1.1 origin servers: </p> 873 <ul> 874 <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one. 875 </li> 876 <li><em class="bcp14">MAY</em> send a weak entity-tag instead of a strong entity-tag, if performance considerations support the use of weak entity-tags, 877 or if it is unfeasible to send a strong entity-tag. 878 </li> 879 <li><em class="bcp14">SHOULD</em> send a Last-Modified value if it is feasible to send one. 880 </li> 881 </ul> 882 <p id="rfc.section.2.2.4.p.3">In other words, the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity-tag and a Last-Modified 883 value. 884 </p> 885 <p id="rfc.section.2.2.4.p.4">HTTP/1.1 clients: </p> 886 <ul> 887 <li><em class="bcp14">MUST</em> use that entity-tag in any cache-conditional request (using If-Match or If-None-Match) if an entity-tag has been provided 888 by the origin server. 889 </li> 890 <li><em class="bcp14">SHOULD</em> use the Last-Modified value in non-subrange cache-conditional requests (using If-Modified-Since) if only a Last-Modified value 891 has been provided by the origin server. 892 </li> 893 <li><em class="bcp14">MAY</em> use the Last-Modified value in subrange cache-conditional requests (using If-Unmodified-Since) if only a Last-Modified value 894 has been provided by an HTTP/1.0 origin server. The user agent <em class="bcp14">SHOULD</em> provide a way to disable this, in case of difficulty. 895 </li> 896 <li><em class="bcp14">SHOULD</em> use both validators in cache-conditional requests if both an entity-tag and a Last-Modified value have been provided by the 897 origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately. 898 </li> 899 </ul> 900 <p id="rfc.section.2.2.4.p.5">An HTTP/1.1 origin server, upon receiving a conditional request that includes both a Last-Modified date (e.g., in an If-Modified-Since 901 or If-Unmodified-Since header field) and one or more entity-tags (e.g., in an If-Match, If-None-Match, or If-Range header 902 field) as cache validators, <em class="bcp14">MUST NOT</em> return a response status code of 304 (Not Modified) unless doing so is consistent with all of the conditional header fields 903 in the request. 904 </p> 905 <p id="rfc.section.2.2.4.p.6">An HTTP/1.1 caching proxy, upon receiving a conditional request that includes both a Last-Modified date and one or more entity-tags 906 as cache validators, <em class="bcp14">MUST NOT</em> return a locally cached response to the client unless that cached response is consistent with all of the conditional header 907 fields in the request. 908 </p> 909 <ul class="empty"> 910 <li><b>Note:</b> The general principle behind these rules is that HTTP/1.1 servers and clients ought to transmit as much non-redundant information 911 as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative 912 assumptions about the validators they receive. 913 </li> 914 <li>HTTP/1.0 clients and caches might ignore entity-tags. Generally, last-modified values received or used by these systems will 915 support transparent and efficient caching, and so HTTP/1.1 origin servers should provide Last-Modified values. In those rare 916 cases where the use of a Last-Modified value as a validator by an HTTP/1.0 system could result in a serious problem, then 917 HTTP/1.1 origin servers should not provide one. 918 </li> 919 </ul> 920 </div> 921 <div id="example.entity.tag.vs.conneg"> 922 <h3 id="rfc.section.2.2.5"><a href="#rfc.section.2.2.5">2.2.5</a> <a href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></h3> 923 <p id="rfc.section.2.2.5.p.1">Consider a resource that is subject to content negotiation (<a href="p3-payload.html#content.negotiation" title="Content Negotiation">Section 5</a> of <a href="#Part3" id="rfc.xref.Part3.1"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>), and where the representations returned upon a GET request vary based on the Accept-Encoding request header field (<a href="p3-payload.html#header.accept-encoding" title="Accept-Encoding">Section 6.3</a> of <a href="#Part3" id="rfc.xref.Part3.2"><cite title="HTTP/1.1, part 3: Message Payload and Content Negotiation">[Part3]</cite></a>): 924 </p> 925 <div id="rfc.figure.u.6"></div> 926 <p>>> Request:</p><pre class="text2">GET /index HTTP/1.1 897 927 Host: www.example.com 898 928 Accept-Encoding: gzip 899 929 900 930 </pre><p id="rfc.section.2.2.5.p.3">In this case, the response might or might not use the gzip content coding. If it does not, the response might look like:</p> 901 <div id="rfc.figure.u.7"></div>902 <p>>> Response:</p><pre class="text">HTTP/1.1 200 OK931 <div id="rfc.figure.u.7"></div> 932 <p>>> Response:</p><pre class="text">HTTP/1.1 200 OK 903 933 Date: Thu, 26 Mar 2010 00:05:00 GMT 904 934 ETag: "123-a" … … 913 943 Hello World! 914 944 </span></pre><p id="rfc.section.2.2.5.p.5">An alternative representation that does use gzip content coding would be:</p> 915 <div id="rfc.figure.u.8"></div>916 <p>>> Response:</p><pre class="text">HTTP/1.1 200 OK945 <div id="rfc.figure.u.8"></div> 946 <p>>> Response:</p><pre class="text">HTTP/1.1 200 OK 917 947 Date: Thu, 26 Mar 2010 00:05:00 GMT 918 948 ETag: "123-b" … … 922 952 Content-Encoding: gzip 923 953 924 <em>...binary data...</em></pre><div class="note" id="rfc.section.2.2.5.p.7"> 925 <p> <b>Note:</b> Content codings are a property of the representation, so therefore an entity-tag of an encoded representation must be distinct 926 from an unencoded representation to prevent conflicts during cache updates and range requests. In contrast, transfer codings 927 (<a href="p1-messaging.html#transfer.codings" title="Transfer Codings">Section 6.2</a> of <a href="#Part1" id="rfc.xref.Part1.5"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>) apply only during message transfer and do not require distinct entity-tags. 928 </p> 954 <em>...binary data...</em></pre><div class="note" id="rfc.section.2.2.5.p.7"> 955 <p><b>Note:</b> Content codings are a property of the representation, so therefore an entity-tag of an encoded representation must be distinct 956 from an unencoded representation to prevent conflicts during cache updates and range requests. In contrast, transfer codings 957 (<a href="p1-messaging.html#transfer.codings" title="Transfer Codings">Section 6.2</a> of <a href="#Part1" id="rfc.xref.Part1.5"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>) apply only during message transfer and do not require distinct entity-tags. 958 </p> 959 </div> 960 </div> 961 </div> 929 962 </div> 930 <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a> <a id="header.fields" href="#header.fields">Precondition Header Fields</a></h1> 931 <p id="rfc.section.3.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields for applying preconditions on requests.</p> 932 <div id="rfc.iref.i.1"></div> 933 <div id="rfc.iref.h.3"></div> 934 <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a> <a id="header.if-match" href="#header.if-match">If-Match</a></h2> 935 <p id="rfc.section.3.1.p.1">The "If-Match" header field <em class="bcp14">MAY</em> be used to make a request method conditional on the current existence or value of an entity-tag for one or more representations 936 of the target resource. If-Match is generally useful for resource update requests, such as PUT requests, as a means for protecting 937 against accidental overwrites when multiple clients are acting in parallel on the same resource (i.e., the "lost update" problem). 938 An If-Match field-value of "*" places the precondition on the existence of any current representation for the target resource. 939 </p> 940 <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.6"></span> <a href="#header.if-match" class="smpl">If-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a> 963 <div id="header.fields"> 964 <h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a> <a href="#header.fields">Precondition Header Fields</a></h1> 965 <p id="rfc.section.3.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields for applying preconditions on requests.</p> 966 <div id="header.if-match"> 967 <div id="rfc.iref.i.1"></div> 968 <div id="rfc.iref.h.3"></div> 969 <h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a> <a href="#header.if-match">If-Match</a></h2> 970 <p id="rfc.section.3.1.p.1">The "If-Match" header field <em class="bcp14">MAY</em> be used to make a request method conditional on the current existence or value of an entity-tag for one or more representations 971 of the target resource. If-Match is generally useful for resource update requests, such as PUT requests, as a means for protecting 972 against accidental overwrites when multiple clients are acting in parallel on the same resource (i.e., the "lost update" problem). 973 An If-Match field-value of "*" places the precondition on the existence of any current representation for the target resource. 974 </p> 975 <div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.6"></span> <a href="#header.if-match" class="smpl">If-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a> 941 976 </pre><p id="rfc.section.3.1.p.3">If any of the entity-tags listed in the If-Match field value match (as per <a href="#entity.tag.comparison" title="Comparison">Section 2.2.3</a>) the entity-tag of the selected representation for the target resource, or if "*" is given and any current representation 942 exists for the target resource, then the server <em class="bcp14">MAY</em> perform the request method as if the If-Match header field was not present.943 </p>944 <p id="rfc.section.3.1.p.4">If none of the entity-tags match, or if "*" is given and no current representation exists, the server <em class="bcp14">MUST NOT</em> perform the requested method. Instead, the server <em class="bcp14">MUST</em> respond with the 412 (Precondition Failed) status code.945 </p>946 <p id="rfc.section.3.1.p.5">If the request would, without the If-Match header field, result in anything other than a 2xx or 412 status code, then the947 If-Match header field <em class="bcp14">MUST</em> be ignored.948 </p>949 <p id="rfc.section.3.1.p.6">Examples:</p>950 <div id="rfc.figure.u.10"></div><pre class="text"> If-Match: "xyzzy"977 exists for the target resource, then the server <em class="bcp14">MAY</em> perform the request method as if the If-Match header field was not present. 978 </p> 979 <p id="rfc.section.3.1.p.4">If none of the entity-tags match, or if "*" is given and no current representation exists, the server <em class="bcp14">MUST NOT</em> perform the requested method. Instead, the server <em class="bcp14">MUST</em> respond with the 412 (Precondition Failed) status code. 980 </p> 981 <p id="rfc.section.3.1.p.5">If the request would, without the If-Match header field, result in anything other than a 2xx or 412 status code, then the 982 If-Match header field <em class="bcp14">MUST</em> be ignored. 983 </p> 984 <p id="rfc.section.3.1.p.6">Examples:</p> 985 <div id="rfc.figure.u.10"></div><pre class="text"> If-Match: "xyzzy" 951 986 If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" 952 987 If-Match: * 953 988 </pre><p id="rfc.section.3.1.p.8">The result of a request having both an If-Match header field and either an If-None-Match or an If-Modified-Since header fields 954 is undefined by this specification. 955 </p> 956 <div id="rfc.iref.i.2"></div> 957 <div id="rfc.iref.h.4"></div> 958 <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a> <a id="header.if-none-match" href="#header.if-none-match">If-None-Match</a></h2> 959 <p id="rfc.section.3.2.p.1">The "If-None-Match" header field <em class="bcp14">MAY</em> be used to make a request method conditional on not matching any of the current entity-tag values for representations of the 960 target resource. If-None-Match is primarily used in conditional GET requests to enable efficient updates of cached information 961 with a minimum amount of transaction overhead. A client that has one or more representations previously obtained from the 962 target resource can send If-None-Match with a list of the associated entity-tags in the hope of receiving a 304 response if 963 at least one of those representations matches the selected representation. 964 </p> 965 <p id="rfc.section.3.2.p.2">If-None-Match MAY also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying 966 an existing representation of the target resource when the client believes that the resource does not have a current representation. 967 This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial representation 968 for the target resource. 969 </p> 970 <div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.7"></span> <a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a> 989 is undefined by this specification. 990 </p> 991 </div> 992 <div id="header.if-none-match"> 993 <div id="rfc.iref.i.2"></div> 994 <div id="rfc.iref.h.4"></div> 995 <h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a> <a href="#header.if-none-match">If-None-Match</a></h2> 996 <p id="rfc.section.3.2.p.1">The "If-None-Match" header field <em class="bcp14">MAY</em> be used to make a request method conditional on not matching any of the current entity-tag values for representations of the 997 target resource. If-None-Match is primarily used in conditional GET requests to enable efficient updates of cached information 998 with a minimum amount of transaction overhead. A client that has one or more representations previously obtained from the 999 target resource can send If-None-Match with a list of the associated entity-tags in the hope of receiving a 304 response if 1000 at least one of those representations matches the selected representation. 1001 </p> 1002 <p id="rfc.section.3.2.p.2">If-None-Match MAY also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying 1003 an existing representation of the target resource when the client believes that the resource does not have a current representation. 1004 This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial representation 1005 for the target resource. 1006 </p> 1007 <div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.7"></span> <a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a> 971 1008 </pre><p id="rfc.section.3.2.p.4">If any of the entity-tags listed in the If-None-Match field-value match (as per <a href="#entity.tag.comparison" title="Comparison">Section 2.2.3</a>) the entity-tag of the selected representation, or if "*" is given and any current representation exists for that resource, 972 then the server <em class="bcp14">MUST NOT</em> perform the requested method. Instead, if the request method was GET or HEAD, the server <em class="bcp14">SHOULD</em> respond with a 304 (Not Modified) status code, including the cache-related header fields (particularly ETag) of the selected973 representation that has a matching entity-tag. For all other request methods, the server <em class="bcp14">MUST</em> respond with a 412 (Precondition Failed) status code.974 </p>975 <p id="rfc.section.3.2.p.5">If none of the entity-tags match, then the server <em class="bcp14">MAY</em> perform the requested method as if the If-None-Match header field did not exist, but <em class="bcp14">MUST</em> also ignore any If-Modified-Since header field(s) in the request. That is, if no entity-tags match, then the server <em class="bcp14">MUST NOT</em> return a 304 (Not Modified) response.976 </p>977 <p id="rfc.section.3.2.p.6">If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status code, then978 the If-None-Match header field <em class="bcp14">MUST</em> be ignored. (See <a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates" title="Rules for When to Use Entity-tags and Last-Modified Dates">Section 2.2.4</a> for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.)979 </p>980 <p id="rfc.section.3.2.p.7">Examples:</p>981 <div id="rfc.figure.u.12"></div><pre class="text"> If-None-Match: "xyzzy"1009 then the server <em class="bcp14">MUST NOT</em> perform the requested method. Instead, if the request method was GET or HEAD, the server <em class="bcp14">SHOULD</em> respond with a 304 (Not Modified) status code, including the cache-related header fields (particularly ETag) of the selected 1010 representation that has a matching entity-tag. For all other request methods, the server <em class="bcp14">MUST</em> respond with a 412 (Precondition Failed) status code. 1011 </p> 1012 <p id="rfc.section.3.2.p.5">If none of the entity-tags match, then the server <em class="bcp14">MAY</em> perform the requested method as if the If-None-Match header field did not exist, but <em class="bcp14">MUST</em> also ignore any If-Modified-Since header field(s) in the request. That is, if no entity-tags match, then the server <em class="bcp14">MUST NOT</em> return a 304 (Not Modified) response. 1013 </p> 1014 <p id="rfc.section.3.2.p.6">If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status code, then 1015 the If-None-Match header field <em class="bcp14">MUST</em> be ignored. (See <a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates" title="Rules for When to Use Entity-tags and Last-Modified Dates">Section 2.2.4</a> for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.) 1016 </p> 1017 <p id="rfc.section.3.2.p.7">Examples:</p> 1018 <div id="rfc.figure.u.12"></div><pre class="text"> If-None-Match: "xyzzy" 982 1019 If-None-Match: W/"xyzzy" 983 1020 If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" … … 985 1022 If-None-Match: * 986 1023 </pre><p id="rfc.section.3.2.p.9">The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since header 987 fields is undefined by this specification. 988 </p> 989 <div id="rfc.iref.i.3"></div> 990 <div id="rfc.iref.h.5"></div> 991 <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a> <a id="header.if-modified-since" href="#header.if-modified-since">If-Modified-Since</a></h2> 992 <p id="rfc.section.3.3.p.1">The "If-Modified-Since" header field <em class="bcp14">MAY</em> be used to make a request method conditional by modification date: if the selected representation has not been modified since 993 the time specified in this field, then do not perform the request method; instead, respond as detailed below. 994 </p> 995 <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.8"></span> <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = <a href="#notation" class="smpl">HTTP-date</a> 1024 fields is undefined by this specification. 1025 </p> 1026 </div> 1027 <div id="header.if-modified-since"> 1028 <div id="rfc.iref.i.3"></div> 1029 <div id="rfc.iref.h.5"></div> 1030 <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a> <a href="#header.if-modified-since">If-Modified-Since</a></h2> 1031 <p id="rfc.section.3.3.p.1">The "If-Modified-Since" header field <em class="bcp14">MAY</em> be used to make a request method conditional by modification date: if the selected representation has not been modified since 1032 the time specified in this field, then do not perform the request method; instead, respond as detailed below. 1033 </p> 1034 <div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.8"></span> <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = <a href="#notation" class="smpl">HTTP-date</a> 996 1035 </pre><p id="rfc.section.3.3.p.3">An example of the field is:</p> 997 <div id="rfc.figure.u.14"></div><pre class="text"> If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT1036 <div id="rfc.figure.u.14"></div><pre class="text"> If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT 998 1037 </pre><p id="rfc.section.3.3.p.5">A GET method with an If-Modified-Since header field and no Range header field requests that the selected representation be 999 transferred only if it has been modified since the date given by the If-Modified-Since header field. The algorithm for determining 1000 this includes the following cases: 1001 </p> 1002 <ol> 1003 <li>If the request would normally result in anything other than a 200 (OK) status code, or if the passed If-Modified-Since date 1004 is invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is 1005 invalid. 1006 </li> 1007 <li>If the selected representation has been modified since the If-Modified-Since date, the response is exactly the same as for 1008 a normal GET. 1009 </li> 1010 <li>If the selected representation has not been modified since a valid If-Modified-Since date, the server <em class="bcp14">SHOULD</em> return a 304 (Not Modified) response. 1011 </li> 1012 </ol> 1013 <p id="rfc.section.3.3.p.6">The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. </p> 1014 <ul class="empty"> 1015 <li> <b>Note:</b> The Range header field modifies the meaning of If-Modified-Since; see <a href="p5-range.html#header.range" title="Range">Section 5.4</a> of <a href="#Part5" id="rfc.xref.Part5.1"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a> for full details. 1016 </li> 1017 <li> <b>Note:</b> If-Modified-Since times are interpreted by the server, whose clock might not be synchronized with the client. 1018 </li> 1019 <li> <b>Note:</b> When handling an If-Modified-Since header field, some servers will use an exact date comparison function, rather than a less-than 1020 function, for deciding whether to send a 304 (Not Modified) response. To get best results when sending an If-Modified-Since 1021 header field for cache validation, clients are advised to use the exact date string received in a previous Last-Modified header 1022 field whenever possible. 1023 </li> 1024 <li> <b>Note:</b> If a client uses an arbitrary date in the If-Modified-Since header field instead of a date taken from the Last-Modified header 1025 field for the same request, the client needs to be aware that this date is interpreted in the server's understanding of time. 1026 Unsynchronized clocks and rounding problems, due to the different encodings of time between the client and server, are concerns. 1027 This includes the possibility of race conditions if the document has changed between the time it was first requested and the 1028 If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the If-Modified-Since 1029 date is derived from the client's clock without correction to the server's clock. Corrections for different time bases between 1030 client and server are at best approximate due to network latency. 1031 </li> 1032 </ul> 1033 <p id="rfc.section.3.3.p.7">The result of a request having both an If-Modified-Since header field and either an If-Match or an If-Unmodified-Since header 1034 fields is undefined by this specification. 1035 </p> 1036 <div id="rfc.iref.i.4"></div> 1037 <div id="rfc.iref.h.6"></div> 1038 <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a> <a id="header.if-unmodified-since" href="#header.if-unmodified-since">If-Unmodified-Since</a></h2> 1039 <p id="rfc.section.3.4.p.1">The "If-Unmodified-Since" header field <em class="bcp14">MAY</em> be used to make a request method conditional by modification date: if the selected representation has been modified since 1040 the time specified in this field, then the server <em class="bcp14">MUST NOT</em> perform the requested operation and <em class="bcp14">MUST</em> instead respond with the 412 (Precondition Failed) status code. If the selected representation has not been modified since 1041 the time specified in this field, the server <em class="bcp14">SHOULD</em> perform the request method as if the If-Unmodified-Since header field were not present. 1042 </p> 1043 <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.9"></span> <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = <a href="#notation" class="smpl">HTTP-date</a> 1038 transferred only if it has been modified since the date given by the If-Modified-Since header field. The algorithm for determining 1039 this includes the following cases: 1040 </p> 1041 <ol> 1042 <li>If the request would normally result in anything other than a 200 (OK) status code, or if the passed If-Modified-Since date 1043 is invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is 1044 invalid. 1045 </li> 1046 <li>If the selected representation has been modified since the If-Modified-Since date, the response is exactly the same as for 1047 a normal GET. 1048 </li> 1049 <li>If the selected representation has not been modified since a valid If-Modified-Since date, the server <em class="bcp14">SHOULD</em> return a 304 (Not Modified) response. 1050 </li> 1051 </ol> 1052 <p id="rfc.section.3.3.p.6">The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. </p> 1053 <ul class="empty"> 1054 <li><b>Note:</b> The Range header field modifies the meaning of If-Modified-Since; see <a href="p5-range.html#header.range" title="Range">Section 5.4</a> of <a href="#Part5" id="rfc.xref.Part5.1"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a> for full details. 1055 </li> 1056 <li><b>Note:</b> If-Modified-Since times are interpreted by the server, whose clock might not be synchronized with the client. 1057 </li> 1058 <li><b>Note:</b> When handling an If-Modified-Since header field, some servers will use an exact date comparison function, rather than a less-than 1059 function, for deciding whether to send a 304 (Not Modified) response. To get best results when sending an If-Modified-Since 1060 header field for cache validation, clients are advised to use the exact date string received in a previous Last-Modified header 1061 field whenever possible. 1062 </li> 1063 <li><b>Note:</b> If a client uses an arbitrary date in the If-Modified-Since header field instead of a date taken from the Last-Modified header 1064 field for the same request, the client needs to be aware that this date is interpreted in the server's understanding of time. 1065 Unsynchronized clocks and rounding problems, due to the different encodings of time between the client and server, are concerns. 1066 This includes the possibility of race conditions if the document has changed between the time it was first requested and the 1067 If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the If-Modified-Since 1068 date is derived from the client's clock without correction to the server's clock. Corrections for different time bases between 1069 client and server are at best approximate due to network latency. 1070 </li> 1071 </ul> 1072 <p id="rfc.section.3.3.p.7">The result of a request having both an If-Modified-Since header field and either an If-Match or an If-Unmodified-Since header 1073 fields is undefined by this specification. 1074 </p> 1075 </div> 1076 <div id="header.if-unmodified-since"> 1077 <div id="rfc.iref.i.4"></div> 1078 <div id="rfc.iref.h.6"></div> 1079 <h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a> <a href="#header.if-unmodified-since">If-Unmodified-Since</a></h2> 1080 <p id="rfc.section.3.4.p.1">The "If-Unmodified-Since" header field <em class="bcp14">MAY</em> be used to make a request method conditional by modification date: if the selected representation has been modified since 1081 the time specified in this field, then the server <em class="bcp14">MUST NOT</em> perform the requested operation and <em class="bcp14">MUST</em> instead respond with the 412 (Precondition Failed) status code. If the selected representation has not been modified since 1082 the time specified in this field, the server <em class="bcp14">SHOULD</em> perform the request method as if the If-Unmodified-Since header field were not present. 1083 </p> 1084 <div id="rfc.figure.u.15"></div><pre class="inline"><span id="rfc.iref.g.9"></span> <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = <a href="#notation" class="smpl">HTTP-date</a> 1044 1085 </pre><p id="rfc.section.3.4.p.3">An example of the field is:</p> 1045 <div id="rfc.figure.u.16"></div><pre class="text"> If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT1086 <div id="rfc.figure.u.16"></div><pre class="text"> If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT 1046 1087 </pre><p id="rfc.section.3.4.p.5">If the request normally (i.e., without the If-Unmodified-Since header field) would result in anything other than a 2xx or 1047 412 status code, the If-Unmodified-Since header field <em class="bcp14">SHOULD</em> be ignored. 1048 </p> 1049 <p id="rfc.section.3.4.p.6">If the specified date is invalid, the header field <em class="bcp14">MUST</em> be ignored. 1050 </p> 1051 <p id="rfc.section.3.4.p.7">The result of a request having both an If-Unmodified-Since header field and either an If-None-Match or an If-Modified-Since 1052 header fields is undefined by this specification. 1053 </p> 1054 <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a> <a id="header.if-range" href="#header.if-range">If-Range</a></h2> 1055 <p id="rfc.section.3.5.p.1">The If-Range header field provides a special conditional request mechanism that is similar to If-Match and If-Unmodified-Since 1056 but specific to HTTP range requests. If-Range is defined in <a href="p5-range.html#header.if-range" title="If-Range">Section 5.3</a> of <a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>. 1057 </p> 1058 <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a> <a id="status.code.definitions" href="#status.code.definitions">Status Code Definitions</a></h1> 1059 <div id="rfc.iref.24"></div> 1060 <div id="rfc.iref.s.2"></div> 1061 <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a> <a id="status.304" href="#status.304">304 Not Modified</a></h2> 1062 <p id="rfc.section.4.1.p.1">The 304 status code indicates that a conditional GET request has been received and would have resulted in a 200 (OK) response 1063 if it were not for the fact that the condition has evaluated to false. In other words, there is no need for the server to 1064 transfer a representation of the target resource because the client's request indicates that it already has a valid representation, 1065 as indicated by the 304 response header fields, and is therefore redirecting the client to make use of that stored representation 1066 as if it were the payload of a 200 response. The 304 response <em class="bcp14">MUST NOT</em> contain a message-body, and thus is always terminated by the first empty line after the header fields. 1067 </p> 1068 <p id="rfc.section.4.1.p.2">A 304 response <em class="bcp14">MUST</em> include a Date header field (<a href="p1-messaging.html#header.date" title="Date">Section 9.3</a> of <a href="#Part1" id="rfc.xref.Part1.6"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>) unless its omission is required by <a href="p1-messaging.html#clockless.origin.server.operation" title="Clockless Origin Server Operation">Section 9.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>. If a 200 response to the same request would have included any of the header fields Cache-Control, Content-Location, ETag, 1069 Expires, Last-Modified, or Vary, then those same header fields <em class="bcp14">MUST</em> be sent in a 304 response. 1070 </p> 1071 <p id="rfc.section.4.1.p.3">Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations, 1072 the response <em class="bcp14">SHOULD NOT</em> include representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding 1073 cache updates (e.g., future HTTP extensions). 1074 </p> 1075 <p id="rfc.section.4.1.p.4">If the recipient of a 304 response does not have a cached representation corresponding to the entity-tag indicated by the 1076 304 response, then the recipient <em class="bcp14">MUST NOT</em> use the 304 to update its own cache. If this conditional request originated with an outbound client, such as a user agent 1077 with its own cache sending a conditional GET to a shared proxy, then the 304 response <em class="bcp14">MAY</em> be forwarded to the outbound client. Otherwise, the recipient <em class="bcp14">MUST</em> disregard the 304 response and repeat the request without any preconditions. 1078 </p> 1079 <p id="rfc.section.4.1.p.5">If a cache uses a received 304 response to update a cache entry, the cache <em class="bcp14">MUST</em> update the entry to reflect any new field values given in the response. 1080 </p> 1081 <div id="rfc.iref.25"></div> 1082 <div id="rfc.iref.s.3"></div> 1083 <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a> <a id="status.412" href="#status.412">412 Precondition Failed</a></h2> 1084 <p id="rfc.section.4.2.p.1">The 412 status code indicates that one or more preconditions given in the request header fields evaluated to false when tested 1085 on the server. This response code allows the client to place preconditions on the current resource state (its current representations 1086 and metadata) and thus prevent the request method from being applied if the target resource is in an unexpected state. 1087 </p> 1088 <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a> <a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1> 1089 <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a> <a id="status.code.registration" href="#status.code.registration">Status Code Registration</a></h2> 1090 <p id="rfc.section.5.1.p.1">The HTTP Status Code Registry located at <<a href="http://www.iana.org/assignments/http-status-codes">http://www.iana.org/assignments/http-status-codes</a>> shall be updated with the registrations below: 1091 </p> 1092 <div id="rfc.table.1"> 1093 <div id="iana.status.code.registration.table"></div> 1094 <table class="tt full left" cellpadding="3" cellspacing="0"> 1095 <thead> 1096 <tr> 1097 <th>Value</th> 1098 <th>Description</th> 1099 <th>Reference</th> 1100 </tr> 1101 </thead> 1102 <tbody> 1103 <tr> 1104 <td class="left">304</td> 1105 <td class="left">Not Modified</td> 1106 <td class="left"> <a href="#status.304" id="rfc.xref.status.304.1" title="304 Not Modified">Section 4.1</a> 1107 </td> 1108 </tr> 1109 <tr> 1110 <td class="left">412</td> 1111 <td class="left">Precondition Failed</td> 1112 <td class="left"> <a href="#status.412" id="rfc.xref.status.412.1" title="412 Precondition Failed">Section 4.2</a> 1113 </td> 1114 </tr> 1115 </tbody> 1116 </table> 1088 412 status code, the If-Unmodified-Since header field <em class="bcp14">SHOULD</em> be ignored. 1089 </p> 1090 <p id="rfc.section.3.4.p.6">If the specified date is invalid, the header field <em class="bcp14">MUST</em> be ignored. 1091 </p> 1092 <p id="rfc.section.3.4.p.7">The result of a request having both an If-Unmodified-Since header field and either an If-None-Match or an If-Modified-Since 1093 header fields is undefined by this specification. 1094 </p> 1095 </div> 1096 <div id="header.if-range"> 1097 <h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a> <a href="#header.if-range">If-Range</a></h2> 1098 <p id="rfc.section.3.5.p.1">The If-Range header field provides a special conditional request mechanism that is similar to If-Match and If-Unmodified-Since 1099 but specific to HTTP range requests. If-Range is defined in <a href="p5-range.html#header.if-range" title="If-Range">Section 5.3</a> of <a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>. 1100 </p> 1101 </div> 1117 1102 </div> 1118 <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a> <a id="header.field.registration" href="#header.field.registration">Header Field Registration</a></h2> 1119 <p id="rfc.section.5.2.p.1">The Message Header Field Registry located at <<a href="http://www.iana.org/assignments/message-headers/message-header-index.html">http://www.iana.org/assignments/message-headers/message-header-index.html</a>> shall be updated with the permanent registrations below (see <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a>): 1120 </p> 1121 <div id="rfc.table.2"> 1122 <div id="iana.header.registration.table"></div> 1123 <table class="tt full left" cellpadding="3" cellspacing="0"> 1124 <thead> 1125 <tr> 1126 <th>Header Field Name</th> 1127 <th>Protocol</th> 1128 <th>Status</th> 1129 <th>Reference</th> 1130 </tr> 1131 </thead> 1132 <tbody> 1133 <tr> 1134 <td class="left">ETag</td> 1135 <td class="left">http</td> 1136 <td class="left">standard</td> 1137 <td class="left"> <a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section 2.2</a> 1138 </td> 1139 </tr> 1140 <tr> 1141 <td class="left">If-Match</td> 1142 <td class="left">http</td> 1143 <td class="left">standard</td> 1144 <td class="left"> <a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section 3.1</a> 1145 </td> 1146 </tr> 1147 <tr> 1148 <td class="left">If-Modified-Since</td> 1149 <td class="left">http</td> 1150 <td class="left">standard</td> 1151 <td class="left"> <a href="#header.if-modified-since" id="rfc.xref.header.if-modified-since.1" title="If-Modified-Since">Section 3.3</a> 1152 </td> 1153 </tr> 1154 <tr> 1155 <td class="left">If-None-Match</td> 1156 <td class="left">http</td> 1157 <td class="left">standard</td> 1158 <td class="left"> <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.1" title="If-None-Match">Section 3.2</a> 1159 </td> 1160 </tr> 1161 <tr> 1162 <td class="left">If-Unmodified-Since</td> 1163 <td class="left">http</td> 1164 <td class="left">standard</td> 1165 <td class="left"> <a href="#header.if-unmodified-since" id="rfc.xref.header.if-unmodified-since.1" title="If-Unmodified-Since">Section 3.4</a> 1166 </td> 1167 </tr> 1168 <tr> 1169 <td class="left">Last-Modified</td> 1170 <td class="left">http</td> 1171 <td class="left">standard</td> 1172 <td class="left"> <a href="#header.last-modified" id="rfc.xref.header.last-modified.1" title="Last-Modified">Section 2.1</a> 1173 </td> 1174 </tr> 1175 </tbody> 1176 </table> 1103 <div id="status.code.definitions"> 1104 <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a> <a href="#status.code.definitions">Status Code Definitions</a></h1> 1105 <div id="status.304"> 1106 <div id="rfc.iref.3.1"></div> 1107 <div id="rfc.iref.s.2"></div> 1108 <h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a> <a href="#status.304">304 Not Modified</a></h2> 1109 <p id="rfc.section.4.1.p.1">The 304 status code indicates that a conditional GET request has been received and would have resulted in a 200 (OK) response 1110 if it were not for the fact that the condition has evaluated to false. In other words, there is no need for the server to 1111 transfer a representation of the target resource because the client's request indicates that it already has a valid representation, 1112 as indicated by the 304 response header fields, and is therefore redirecting the client to make use of that stored representation 1113 as if it were the payload of a 200 response. The 304 response <em class="bcp14">MUST NOT</em> contain a message-body, and thus is always terminated by the first empty line after the header fields. 1114 </p> 1115 <p id="rfc.section.4.1.p.2">A 304 response <em class="bcp14">MUST</em> include a Date header field (<a href="p1-messaging.html#header.date" title="Date">Section 9.3</a> of <a href="#Part1" id="rfc.xref.Part1.6"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>) unless its omission is required by <a href="p1-messaging.html#clockless.origin.server.operation" title="Clockless Origin Server Operation">Section 9.3.1</a> of <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>. If a 200 response to the same request would have included any of the header fields Cache-Control, Content-Location, ETag, 1116 Expires, Last-Modified, or Vary, then those same header fields <em class="bcp14">MUST</em> be sent in a 304 response. 1117 </p> 1118 <p id="rfc.section.4.1.p.3">Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations, 1119 the response <em class="bcp14">SHOULD NOT</em> include representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding 1120 cache updates (e.g., future HTTP extensions). 1121 </p> 1122 <p id="rfc.section.4.1.p.4">If the recipient of a 304 response does not have a cached representation corresponding to the entity-tag indicated by the 1123 304 response, then the recipient <em class="bcp14">MUST NOT</em> use the 304 to update its own cache. If this conditional request originated with an outbound client, such as a user agent 1124 with its own cache sending a conditional GET to a shared proxy, then the 304 response <em class="bcp14">MAY</em> be forwarded to the outbound client. Otherwise, the recipient <em class="bcp14">MUST</em> disregard the 304 response and repeat the request without any preconditions. 1125 </p> 1126 <p id="rfc.section.4.1.p.5">If a cache uses a received 304 response to update a cache entry, the cache <em class="bcp14">MUST</em> update the entry to reflect any new field values given in the response. 1127 </p> 1128 </div> 1129 <div id="status.412"> 1130 <div id="rfc.iref.4.1"></div> 1131 <div id="rfc.iref.s.3"></div> 1132 <h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a> <a href="#status.412">412 Precondition Failed</a></h2> 1133 <p id="rfc.section.4.2.p.1">The 412 status code indicates that one or more preconditions given in the request header fields evaluated to false when tested 1134 on the server. This response code allows the client to place preconditions on the current resource state (its current representations 1135 and metadata) and thus prevent the request method from being applied if the target resource is in an unexpected state. 1136 </p> 1137 </div> 1177 1138 </div> 1178 <p id="rfc.section.5.2.p.2">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p> 1179 <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a> <a id="security.considerations" href="#security.considerations">Security Considerations</a></h1> 1180 <p id="rfc.section.6.p.1">No additional security considerations have been identified beyond those applicable to HTTP in general <a href="#Part1" id="rfc.xref.Part1.8"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>. 1181 </p> 1182 <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a> <a id="ack" href="#ack">Acknowledgments</a></h1> 1139 <div id="IANA.considerations"> 1140 <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a> <a href="#IANA.considerations">IANA Considerations</a></h1> 1141 <div id="status.code.registration"> 1142 <h2 id="rfc.section.5.1"><a href="#rfc.section.5.1">5.1</a> <a href="#status.code.registration">Status Code Registration</a></h2> 1143 <p id="rfc.section.5.1.p.1">The HTTP Status Code Registry located at <<a href="http://www.iana.org/assignments/http-status-codes">http://www.iana.org/assignments/http-status-codes</a>> shall be updated with the registrations below: 1144 </p> 1145 <div id="rfc.table.1"> 1146 <div id="iana.status.code.registration.table"></div> 1147 <table class="tt full left" cellpadding="3" cellspacing="0"> 1148 <thead> 1149 <tr> 1150 <th>Value</th> 1151 <th>Description</th> 1152 <th>Reference</th> 1153 </tr> 1154 </thead> 1155 <tbody> 1156 <tr> 1157 <td class="left">304</td> 1158 <td class="left">Not Modified</td> 1159 <td class="left"><a href="#status.304" id="rfc.xref.status.304.1" title="304 Not Modified">Section 4.1</a> 1160 </td> 1161 </tr> 1162 <tr> 1163 <td class="left">412</td> 1164 <td class="left">Precondition Failed</td> 1165 <td class="left"><a href="#status.412" id="rfc.xref.status.412.1" title="412 Precondition Failed">Section 4.2</a> 1166 </td> 1167 </tr> 1168 </tbody> 1169 </table> 1170 </div> 1171 </div> 1172 <div id="header.field.registration"> 1173 <h2 id="rfc.section.5.2"><a href="#rfc.section.5.2">5.2</a> <a href="#header.field.registration">Header Field Registration</a></h2> 1174 <p id="rfc.section.5.2.p.1">The Message Header Field Registry located at <<a href="http://www.iana.org/assignments/message-headers/message-header-index.html">http://www.iana.org/assignments/message-headers/message-header-index.html</a>> shall be updated with the permanent registrations below (see <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a>): 1175 </p> 1176 <div id="rfc.table.2"> 1177 <div id="iana.header.registration.table"></div> 1178 <table class="tt full left" cellpadding="3" cellspacing="0"> 1179 <thead> 1180 <tr> 1181 <th>Header Field Name</th> 1182 <th>Protocol</th> 1183 <th>Status</th> 1184 <th>Reference</th> 1185 </tr> 1186 </thead> 1187 <tbody> 1188 <tr> 1189 <td class="left">ETag</td> 1190 <td class="left">http</td> 1191 <td class="left">standard</td> 1192 <td class="left"><a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section 2.2</a> 1193 </td> 1194 </tr> 1195 <tr> 1196 <td class="left">If-Match</td> 1197 <td class="left">http</td> 1198 <td class="left">standard</td> 1199 <td class="left"><a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section 3.1</a> 1200 </td> 1201 </tr> 1202 <tr> 1203 <td class="left">If-Modified-Since</td> 1204 <td class="left">http</td> 1205 <td class="left">standard</td> 1206 <td class="left"><a href="#header.if-modified-since" id="rfc.xref.header.if-modified-since.1" title="If-Modified-Since">Section 3.3</a> 1207 </td> 1208 </tr> 1209 <tr> 1210 <td class="left">If-None-Match</td> 1211 <td class="left">http</td> 1212 <td class="left">standard</td> 1213 <td class="left"><a href="#header.if-none-match" id="rfc.xref.header.if-none-match.1" title="If-None-Match">Section 3.2</a> 1214 </td> 1215 </tr> 1216 <tr> 1217 <td class="left">If-Unmodified-Since</td> 1218 <td class="left">http</td> 1219 <td class="left">standard</td> 1220 <td class="left"><a href="#header.if-unmodified-since" id="rfc.xref.header.if-unmodified-since.1" title="If-Unmodified-Since">Section 3.4</a> 1221 </td> 1222 </tr> 1223 <tr> 1224 <td class="left">Last-Modified</td> 1225 <td class="left">http</td> 1226 <td class="left">standard</td> 1227 <td class="left"><a href="#header.last-modified" id="rfc.xref.header.last-modified.1" title="Last-Modified">Section 2.1</a> 1228 </td> 1229 </tr> 1230 </tbody> 1231 </table> 1232 </div> 1233 <p id="rfc.section.5.2.p.2">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p> 1234 </div> 1235 </div> 1236 <div id="security.considerations"> 1237 <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a> <a href="#security.considerations">Security Considerations</a></h1> 1238 <p id="rfc.section.6.p.1">No additional security considerations have been identified beyond those applicable to HTTP in general <a href="#Part1" id="rfc.xref.Part1.8"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>. 1239 </p> 1240 </div> 1241 <div id="ack"> 1242 <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a> <a href="#ack">Acknowledgments</a></h1> 1243 </div> 1183 1244 <h1 id="rfc.references"><a id="rfc.section.8" href="#rfc.section.8">8.</a> References 1184 1245 </h1> 1185 1246 <h2 id="rfc.references.1"><a href="#rfc.section.8.1" id="rfc.section.8.1">8.1</a> Normative References 1186 1247 </h2> 1187 <table> 1248 <table> 1188 1249 <tr> 1189 1250 <td class="reference"><b id="Part1">[Part1]</b></td> 1190 <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http ://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-15">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</a>”, Internet-Draft draft-ietf-httpbis-p1-messaging-15 (work in progress), July 2011.1251 <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-15">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</a>”, Internet-Draft draft-ietf-httpbis-p1-messaging-15 (work in progress), July 2011. 1191 1252 </td> 1192 1253 </tr> 1193 1254 <tr> 1194 1255 <td class="reference"><b id="Part3">[Part3]</b></td> 1195 <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http ://tools.ietf.org/html/draft-ietf-httpbis-p3-payload-15">HTTP/1.1, part 3: Message Payload and Content Negotiation</a>”, Internet-Draft draft-ietf-httpbis-p3-payload-15 (work in progress), July 2011.1256 <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p3-payload-15">HTTP/1.1, part 3: Message Payload and Content Negotiation</a>”, Internet-Draft draft-ietf-httpbis-p3-payload-15 (work in progress), July 2011. 1196 1257 </td> 1197 1258 </tr> 1198 1259 <tr> 1199 1260 <td class="reference"><b id="Part5">[Part5]</b></td> 1200 <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http ://tools.ietf.org/html/draft-ietf-httpbis-p5-range-15">HTTP/1.1, part 5: Range Requests and Partial Responses</a>”, Internet-Draft draft-ietf-httpbis-p5-range-15 (work in progress), July 2011.1261 <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p5-range-15">HTTP/1.1, part 5: Range Requests and Partial Responses</a>”, Internet-Draft draft-ietf-httpbis-p5-range-15 (work in progress), July 2011. 1201 1262 </td> 1202 1263 </tr> 1203 1264 <tr> 1204 1265 <td class="reference"><b id="Part6">[Part6]</b></td> 1205 <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, <a href="mailto:mnot@mnot.net">Nottingham, M., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http ://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-15">HTTP/1.1, part 6: Caching</a>”, Internet-Draft draft-ietf-httpbis-p6-cache-15 (work in progress), July 2011.1266 <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:jg@freedesktop.org" title="Alcatel-Lucent Bell Labs">Gettys, J.</a>, <a href="mailto:JeffMogul@acm.org" title="Hewlett-Packard Company">Mogul, J.</a>, <a href="mailto:henrikn@microsoft.com" title="Microsoft Corporation">Frystyk, H.</a>, <a href="mailto:LMM@acm.org" title="Adobe Systems Incorporated">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, <a href="mailto:timbl@w3.org" title="World Wide Web Consortium">Berners-Lee, T.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, <a href="mailto:mnot@mnot.net">Nottingham, M., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="https://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-15">HTTP/1.1, part 6: Caching</a>”, Internet-Draft draft-ietf-httpbis-p6-cache-15 (work in progress), July 2011. 1206 1267 </td> 1207 1268 </tr> 1208 1269 <tr> 1209 1270 <td class="reference"><b id="RFC2119">[RFC2119]</b></td> 1210 <td class="top"><a href="mailto:sob@harvard.edu" title="Harvard University">Bradner, S.</a>, “<a href="http ://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP 14, RFC 2119, March 1997.1271 <td class="top"><a href="mailto:sob@harvard.edu" title="Harvard University">Bradner, S.</a>, “<a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP 14, RFC 2119, March 1997. 1211 1272 </td> 1212 1273 </tr> 1213 1274 <tr> 1214 1275 <td class="reference"><b id="RFC5234">[RFC5234]</b></td> 1215 <td class="top"><a href="mailto:dcrocker@bbiw.net" title="Brandenburg InternetWorking">Crocker, D., Ed.</a> and <a href="mailto:paul.overell@thus.net" title="THUS plc.">P. Overell</a>, “<a href="http ://tools.ietf.org/html/rfc5234">Augmented BNF for Syntax Specifications: ABNF</a>”, STD 68, RFC 5234, January 2008.1276 <td class="top"><a href="mailto:dcrocker@bbiw.net" title="Brandenburg InternetWorking">Crocker, D., Ed.</a> and <a href="mailto:paul.overell@thus.net" title="THUS plc.">P. Overell</a>, “<a href="https://tools.ietf.org/html/rfc5234">Augmented BNF for Syntax Specifications: ABNF</a>”, STD 68, RFC 5234, January 2008. 1216 1277 </td> 1217 1278 </tr> … … 1219 1280 <h2 id="rfc.references.2"><a href="#rfc.section.8.2" id="rfc.section.8.2">8.2</a> Informative References 1220 1281 </h2> 1221 <table> 1282 <table> 1222 1283 <tr> 1223 1284 <td class="reference"><b id="RFC2616">[RFC2616]</b></td> 1224 <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="W3C">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, and <a href="mailto:timbl@w3.org" title="W3C">T. Berners-Lee</a>, “<a href="http ://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC 2616, June 1999.1285 <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="W3C">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Compaq Computer Corporation">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Frystyk, H.</a>, <a href="mailto:masinter@parc.xerox.com" title="Xerox Corporation">Masinter, L.</a>, <a href="mailto:paulle@microsoft.com" title="Microsoft Corporation">Leach, P.</a>, and <a href="mailto:timbl@w3.org" title="W3C">T. Berners-Lee</a>, “<a href="https://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC 2616, June 1999. 1225 1286 </td> 1226 1287 </tr> 1227 1288 <tr> 1228 1289 <td class="reference"><b id="RFC3864">[RFC3864]</b></td> 1229 <td class="top"><a href="mailto:GK-IETF@ninebynine.org" title="Nine by Nine">Klyne, G.</a>, <a href="mailto:mnot@pobox.com" title="BEA Systems">Nottingham, M.</a>, and <a href="mailto:JeffMogul@acm.org" title="HP Labs">J. Mogul</a>, “<a href="http ://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>”, BCP 90, RFC 3864, September 2004.1290 <td class="top"><a href="mailto:GK-IETF@ninebynine.org" title="Nine by Nine">Klyne, G.</a>, <a href="mailto:mnot@pobox.com" title="BEA Systems">Nottingham, M.</a>, and <a href="mailto:JeffMogul@acm.org" title="HP Labs">J. Mogul</a>, “<a href="https://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>”, BCP 90, RFC 3864, September 2004. 1230 1291 </td> 1231 1292 </tr> 1232 1293 <tr> 1233 1294 <td class="reference"><b id="RFC4918">[RFC4918]</b></td> 1234 <td class="top"><a href="mailto:ldusseault@commerce.net" title="CommerceNet">Dusseault, L., Ed.</a>, “<a href="http ://tools.ietf.org/html/rfc4918">HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</a>”, RFC 4918, June 2007.1295 <td class="top"><a href="mailto:ldusseault@commerce.net" title="CommerceNet">Dusseault, L., Ed.</a>, “<a href="https://tools.ietf.org/html/rfc4918">HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</a>”, RFC 4918, June 2007. 1235 1296 </td> 1236 1297 </tr> 1237 1298 </table> 1238 <div class="avoidbreak"> 1239 <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1> 1240 <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span> 1241 (editor) 1242 <span class="n hidden"><span class="family-name">Fielding</span><span class="given-name">Roy T.</span></span></span><span class="org vcardline">Adobe Systems Incorporated</span><span class="adr"><span class="street-address vcardline">345 Park Ave</span><span class="vcardline"><span class="locality">San Jose</span>, <span class="region">CA</span> <span class="postal-code">95110</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:fielding@gbiv.com"><span class="email">fielding@gbiv.com</span></a></span><span class="vcardline">URI: <a href="http://roy.gbiv.com/" class="url">http://roy.gbiv.com/</a></span></address> 1243 <address class="vcard"><span class="vcardline"><span class="fn">Jim Gettys</span><span class="n hidden"><span class="family-name">Gettys</span><span class="given-name">Jim</span></span></span><span class="org vcardline">Alcatel-Lucent Bell Labs</span><span class="adr"><span class="street-address vcardline">21 Oak Knoll Road</span><span class="vcardline"><span class="locality">Carlisle</span>, <span class="region">MA</span> <span class="postal-code">01741</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:jg@freedesktop.org"><span class="email">jg@freedesktop.org</span></a></span><span class="vcardline">URI: <a href="http://gettys.wordpress.com/" class="url">http://gettys.wordpress.com/</a></span></address> 1244 <address class="vcard"><span class="vcardline"><span class="fn">Jeffrey C. Mogul</span><span class="n hidden"><span class="family-name">Mogul</span><span class="given-name">Jeffrey C.</span></span></span><span class="org vcardline">Hewlett-Packard Company</span><span class="adr"><span class="street-address vcardline">HP Labs, Large Scale Systems Group</span><span class="street-address vcardline">1501 Page Mill Road, MS 1177</span><span class="vcardline"><span class="locality">Palo Alto</span>, <span class="region">CA</span> <span class="postal-code">94304</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:JeffMogul@acm.org"><span class="email">JeffMogul@acm.org</span></a></span></address> 1245 <address class="vcard"><span class="vcardline"><span class="fn">Henrik Frystyk Nielsen</span><span class="n hidden"><span class="family-name">Frystyk</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span> <span class="postal-code">98052</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:henrikn@microsoft.com"><span class="email">henrikn@microsoft.com</span></a></span></address> 1246 <address class="vcard"><span class="vcardline"><span class="fn">Larry Masinter</span><span class="n hidden"><span class="family-name">Masinter</span><span class="given-name">Larry</span></span></span><span class="org vcardline">Adobe Systems Incorporated</span><span class="adr"><span class="street-address vcardline">345 Park Ave</span><span class="vcardline"><span class="locality">San Jose</span>, <span class="region">CA</span> <span class="postal-code">95110</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:LMM@acm.org"><span class="email">LMM@acm.org</span></a></span><span class="vcardline">URI: <a href="http://larry.masinter.net/" class="url">http://larry.masinter.net/</a></span></address> 1247 <address class="vcard"><span class="vcardline"><span class="fn">Paul J. Leach</span><span class="n hidden"><span class="family-name">Leach</span><span class="given-name">Paul J.</span></span></span><span class="org vcardline">Microsoft Corporation</span><span class="adr"><span class="street-address vcardline">1 Microsoft Way</span><span class="vcardline"><span class="locality">Redmond</span>, <span class="region">WA</span> <span class="postal-code">98052</span></span></span><span class="vcardline">Email: <a href="mailto:paulle@microsoft.com"><span class="email">paulle@microsoft.com</span></a></span></address> 1248 <address class="vcard"><span class="vcardline"><span class="fn">Tim Berners-Lee</span><span class="n hidden"><span class="family-name">Berners-Lee</span><span class="given-name">Tim</span></span></span><span class="org vcardline">World Wide Web Consortium</span><span class="adr"><span class="street-address vcardline">MIT Computer Science and Artificial Intelligence Laboratory</span><span class="street-address vcardline">The Stata Center, Building 32</span><span class="street-address vcardline">32 Vassar Street</span><span class="vcardline"><span class="locality">Cambridge</span>, <span class="region">MA</span> <span class="postal-code">02139</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:timbl@w3.org"><span class="email">timbl@w3.org</span></a></span><span class="vcardline">URI: <a href="http://www.w3.org/People/Berners-Lee/" class="url">http://www.w3.org/People/Berners-Lee/</a></span></address> 1249 <address class="vcard"><span class="vcardline"><span class="fn">Yves Lafon</span> 1250 (editor) 1251 <span class="n hidden"><span class="family-name">Lafon</span><span class="given-name">Yves</span></span></span><span class="org vcardline">World Wide Web Consortium</span><span class="adr"><span class="street-address vcardline">W3C / ERCIM</span><span class="street-address vcardline">2004, rte des Lucioles</span><span class="vcardline"><span class="locality">Sophia-Antipolis</span>, <span class="region">AM</span> <span class="postal-code">06902</span></span><span class="country-name vcardline">France</span></span><span class="vcardline">Email: <a href="mailto:ylafon@w3.org"><span class="email">ylafon@w3.org</span></a></span><span class="vcardline">URI: <a href="http://www.raubacapeu.net/people/yves/" class="url">http://www.raubacapeu.net/people/yves/</a></span></address> 1252 <address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span> 1253 (editor) 1254 <span class="n hidden"><span class="family-name">Reschke</span><span class="given-name">Julian F.</span></span></span><span class="org vcardline">greenbytes GmbH</span><span class="adr"><span class="street-address vcardline">Hafenweg 16</span><span class="vcardline"><span class="locality">Muenster</span>, <span class="region">NW</span> <span class="postal-code">48155</span></span><span class="country-name vcardline">Germany</span></span><span class="vcardline tel">Phone: <a href="tel:+492512807760"><span class="value">+49 251 2807760</span></a></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+492512807761"><span class="value">+49 251 2807761</span></a></span><span class="vcardline">Email: <a href="mailto:julian.reschke@greenbytes.de"><span class="email">julian.reschke@greenbytes.de</span></a></span><span class="vcardline">URI: <a href="http://greenbytes.de/tech/webdav/" class="url">http://greenbytes.de/tech/webdav/</a></span></address> 1299 <div id="changes.from.rfc.2616"> 1300 <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a> <a href="#changes.from.rfc.2616">Changes from RFC 2616</a></h1> 1301 <p id="rfc.section.A.p.1">Allow weak entity-tags in all requests except range requests (Sections <a href="#weak.and.strong.validators" title="Weak versus Strong">2.2.2</a> and <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.2" title="If-None-Match">3.2</a>). 1302 </p> 1303 <p id="rfc.section.A.p.2">Change ABNF productions for header fields to only define the field value. (<a href="#header.fields" title="Precondition Header Fields">Section 3</a>) 1304 </p> 1255 1305 </div> 1256 <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a> <a id="changes.from.rfc.2616" href="#changes.from.rfc.2616">Changes from RFC 2616</a></h1> 1257 <p id="rfc.section.A.p.1">Allow weak entity-tags in all requests except range requests (Sections <a href="#weak.and.strong.validators" title="Weak versus Strong">2.2.2</a> and <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.2" title="If-None-Match">3.2</a>). 1258 </p> 1259 <p id="rfc.section.A.p.2">Change ABNF productions for header fields to only define the field value. (<a href="#header.fields" title="Precondition Header Fields">Section 3</a>) 1260 </p> 1261 <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a> <a id="collected.abnf" href="#collected.abnf">Collected ABNF</a></h1> 1262 <div id="rfc.figure.u.17"></div> <pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = entity-tag 1306 <div id="collected.abnf"> 1307 <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a> <a href="#collected.abnf">Collected ABNF</a></h1> 1308 <div id="rfc.figure.u.17"></div><pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = entity-tag 1263 1309 1264 1310 <a href="#notation" class="smpl">HTTP-date</a> = <HTTP-date, defined in [Part1], Section 6.1> … … 1282 1328 1283 1329 <a href="#header.etag" class="smpl">weak</a> = %x57.2F ; W/ 1284 </pre> 1285 <p>ABNF diagnostics:</p><pre class="inline">; ETag defined but not used1330 </pre><div id="rfc.figure.u.18"></div> 1331 <p>ABNF diagnostics:</p><pre class="inline">; ETag defined but not used 1286 1332 ; If-Match defined but not used 1287 1333 ; If-Modified-Since defined but not used … … 1289 1335 ; If-Unmodified-Since defined but not used 1290 1336 ; Last-Modified defined but not used 1291 </pre><h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a> <a id="change.log" href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1> 1292 <h2 id="rfc.section.C.1"><a href="#rfc.section.C.1">C.1</a> Since RFC 2616 1293 </h2> 1294 <p id="rfc.section.C.1.p.1">Extracted relevant partitions from <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>. 1295 </p> 1296 <h2 id="rfc.section.C.2"><a href="#rfc.section.C.2">C.2</a> Since draft-ietf-httpbis-p4-conditional-00 1297 </h2> 1298 <p id="rfc.section.C.2.p.1">Closed issues: </p> 1299 <ul> 1300 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/35">http://tools.ietf.org/wg/httpbis/trac/ticket/35</a>>: "Normative and Informative references" 1301 </li> 1302 </ul> 1303 <p id="rfc.section.C.2.p.2">Other changes: </p> 1304 <ul> 1305 <li>Move definitions of 304 and 412 condition codes from Part2.</li> 1306 </ul> 1307 <h2 id="rfc.section.C.3"><a href="#rfc.section.C.3">C.3</a> Since draft-ietf-httpbis-p4-conditional-01 1308 </h2> 1309 <p id="rfc.section.C.3.p.1">Ongoing work on ABNF conversion (<<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>>): 1310 </p> 1311 <ul> 1312 <li>Add explicit references to BNF syntax and rules imported from other parts of the specification.</li> 1313 </ul> 1314 <h2 id="rfc.section.C.4"><a href="#rfc.section.C.4">C.4</a> <a id="changes.since.02" href="#changes.since.02">Since draft-ietf-httpbis-p4-conditional-02</a></h2> 1315 <p id="rfc.section.C.4.p.1">Closed issues: </p> 1316 <ul> 1317 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/116">http://tools.ietf.org/wg/httpbis/trac/ticket/116</a>>: "Weak ETags on non-GET requests" 1318 </li> 1319 </ul> 1320 <p id="rfc.section.C.4.p.2">Ongoing work on IANA Message Header Field Registration (<<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/40">http://tools.ietf.org/wg/httpbis/trac/ticket/40</a>>): 1321 </p> 1322 <ul> 1323 <li>Reference RFC 3984, and update header field registrations for header fields defined in this document.</li> 1324 </ul> 1325 <h2 id="rfc.section.C.5"><a href="#rfc.section.C.5">C.5</a> <a id="changes.since.03" href="#changes.since.03">Since draft-ietf-httpbis-p4-conditional-03</a></h2> 1326 <p id="rfc.section.C.5.p.1">Closed issues: </p> 1327 <ul> 1328 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/71">http://tools.ietf.org/wg/httpbis/trac/ticket/71</a>>: "Examples for ETag matching" 1329 </li> 1330 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/124">http://tools.ietf.org/wg/httpbis/trac/ticket/124</a>>: "'entity value' undefined" 1331 </li> 1332 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/126">http://tools.ietf.org/wg/httpbis/trac/ticket/126</a>>: "bogus 2068 Date header reference" 1333 </li> 1334 </ul> 1335 <h2 id="rfc.section.C.6"><a href="#rfc.section.C.6">C.6</a> <a id="changes.since.04" href="#changes.since.04">Since draft-ietf-httpbis-p4-conditional-04</a></h2> 1336 <p id="rfc.section.C.6.p.1">Ongoing work on ABNF conversion (<<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>>): 1337 </p> 1338 <ul> 1339 <li>Use "/" instead of "|" for alternatives.</li> 1340 <li>Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").</li> 1341 <li>Rewrite ABNFs to spell out whitespace rules, factor out header field value format definitions.</li> 1342 </ul> 1343 <h2 id="rfc.section.C.7"><a href="#rfc.section.C.7">C.7</a> <a id="changes.since.05" href="#changes.since.05">Since draft-ietf-httpbis-p4-conditional-05</a></h2> 1344 <p id="rfc.section.C.7.p.1">Final work on ABNF conversion (<<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>>): 1345 </p> 1346 <ul> 1347 <li>Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.</li> 1348 </ul> 1349 <h2 id="rfc.section.C.8"><a href="#rfc.section.C.8">C.8</a> <a id="changes.since.06" href="#changes.since.06">Since draft-ietf-httpbis-p4-conditional-06</a></h2> 1350 <p id="rfc.section.C.8.p.1">Closed issues: </p> 1351 <ul> 1352 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/153">http://tools.ietf.org/wg/httpbis/trac/ticket/153</a>>: "case-sensitivity of etag weakness indicator" 1353 </li> 1354 </ul> 1355 <h2 id="rfc.section.C.9"><a href="#rfc.section.C.9">C.9</a> <a id="changes.since.07" href="#changes.since.07">Since draft-ietf-httpbis-p4-conditional-07</a></h2> 1356 <p id="rfc.section.C.9.p.1">Closed issues: </p> 1357 <ul> 1358 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/116">http://tools.ietf.org/wg/httpbis/trac/ticket/116</a>>: "Weak ETags on non-GET requests" (If-Match still was defined to require strong matching) 1359 </li> 1360 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/198">http://tools.ietf.org/wg/httpbis/trac/ticket/198</a>>: "move IANA registrations for optional status codes" 1361 </li> 1362 </ul> 1363 <h2 id="rfc.section.C.10"><a href="#rfc.section.C.10">C.10</a> <a id="changes.since.08" href="#changes.since.08">Since draft-ietf-httpbis-p4-conditional-08</a></h2> 1364 <p id="rfc.section.C.10.p.1">No significant changes.</p> 1365 <h2 id="rfc.section.C.11"><a href="#rfc.section.C.11">C.11</a> <a id="changes.since.09" href="#changes.since.09">Since draft-ietf-httpbis-p4-conditional-09</a></h2> 1366 <p id="rfc.section.C.11.p.1">No significant changes.</p> 1367 <h2 id="rfc.section.C.12"><a href="#rfc.section.C.12">C.12</a> <a id="changes.since.10" href="#changes.since.10">Since draft-ietf-httpbis-p4-conditional-10</a></h2> 1368 <p id="rfc.section.C.12.p.1">Closed issues: </p> 1369 <ul> 1370 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/69">http://tools.ietf.org/wg/httpbis/trac/ticket/69</a>>: "Clarify 'Requested Variant'" 1371 </li> 1372 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/109">http://tools.ietf.org/wg/httpbis/trac/ticket/109</a>>: "Clarify entity / representation / variant terminology" 1373 </li> 1374 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/220">http://tools.ietf.org/wg/httpbis/trac/ticket/220</a>>: "consider removing the 'changes from 2068' sections" 1375 </li> 1376 </ul> 1377 <h2 id="rfc.section.C.13"><a href="#rfc.section.C.13">C.13</a> <a id="changes.since.11" href="#changes.since.11">Since draft-ietf-httpbis-p4-conditional-11</a></h2> 1378 <p id="rfc.section.C.13.p.1">None.</p> 1379 <h2 id="rfc.section.C.14"><a href="#rfc.section.C.14">C.14</a> <a id="changes.since.12" href="#changes.since.12">Since draft-ietf-httpbis-p4-conditional-12</a></h2> 1380 <p id="rfc.section.C.14.p.1">Closed issues: </p> 1381 <ul> 1382 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/224">http://tools.ietf.org/wg/httpbis/trac/ticket/224</a>>: "Header Classification" 1383 </li> 1384 </ul> 1385 <h2 id="rfc.section.C.15"><a href="#rfc.section.C.15">C.15</a> <a id="changes.since.13" href="#changes.since.13">Since draft-ietf-httpbis-p4-conditional-13</a></h2> 1386 <p id="rfc.section.C.15.p.1">Closed issues: </p> 1387 <ul> 1388 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/89">http://tools.ietf.org/wg/httpbis/trac/ticket/89</a>>: "If-* and entities" 1389 </li> 1390 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/101">http://tools.ietf.org/wg/httpbis/trac/ticket/101</a>>: "Definition of validator weakness" 1391 </li> 1392 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/276">http://tools.ietf.org/wg/httpbis/trac/ticket/276</a>>: "untangle ABNFs for header fields" 1393 </li> 1394 <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/269">http://tools.ietf.org/wg/httpbis/trac/ticket/269</a>>: "ETags and Quotes" 1395 </li> 1396 </ul> 1397 <h2 id="rfc.section.C.16"><a href="#rfc.section.C.16">C.16</a> <a id="changes.since.14" href="#changes.since.14">Since draft-ietf-httpbis-p4-conditional-14</a></h2> 1398 <p id="rfc.section.C.16.p.1">None.</p> 1337 </pre></div> 1338 <div id="change.log"> 1339 <h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a> <a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1> 1340 <div> 1341 <h2 id="rfc.section.C.1"><a href="#rfc.section.C.1">C.1</a> Since RFC 2616 1342 </h2> 1343 <p id="rfc.section.C.1.p.1">Extracted relevant partitions from <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>. 1344 </p> 1345 </div> 1346 <div> 1347 <h2 id="rfc.section.C.2"><a href="#rfc.section.C.2">C.2</a> Since draft-ietf-httpbis-p4-conditional-00 1348 </h2> 1349 <p id="rfc.section.C.2.p.1">Closed issues: </p> 1350 <ul> 1351 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/35">http://tools.ietf.org/wg/httpbis/trac/ticket/35</a>>: "Normative and Informative references" 1352 </li> 1353 </ul> 1354 <p id="rfc.section.C.2.p.2">Other changes: </p> 1355 <ul> 1356 <li>Move definitions of 304 and 412 condition codes from Part2.</li> 1357 </ul> 1358 </div> 1359 <div> 1360 <h2 id="rfc.section.C.3"><a href="#rfc.section.C.3">C.3</a> Since draft-ietf-httpbis-p4-conditional-01 1361 </h2> 1362 <p id="rfc.section.C.3.p.1">Ongoing work on ABNF conversion (<<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>>): 1363 </p> 1364 <ul> 1365 <li>Add explicit references to BNF syntax and rules imported from other parts of the specification.</li> 1366 </ul> 1367 </div> 1368 <div id="changes.since.02"> 1369 <h2 id="rfc.section.C.4"><a href="#rfc.section.C.4">C.4</a> <a href="#changes.since.02">Since draft-ietf-httpbis-p4-conditional-02</a></h2> 1370 <p id="rfc.section.C.4.p.1">Closed issues: </p> 1371 <ul> 1372 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/116">http://tools.ietf.org/wg/httpbis/trac/ticket/116</a>>: "Weak ETags on non-GET requests" 1373 </li> 1374 </ul> 1375 <p id="rfc.section.C.4.p.2">Ongoing work on IANA Message Header Field Registration (<<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/40">http://tools.ietf.org/wg/httpbis/trac/ticket/40</a>>): 1376 </p> 1377 <ul> 1378 <li>Reference RFC 3984, and update header field registrations for header fields defined in this document.</li> 1379 </ul> 1380 </div> 1381 <div id="changes.since.03"> 1382 <h2 id="rfc.section.C.5"><a href="#rfc.section.C.5">C.5</a> <a href="#changes.since.03">Since draft-ietf-httpbis-p4-conditional-03</a></h2> 1383 <p id="rfc.section.C.5.p.1">Closed issues: </p> 1384 <ul> 1385 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/71">http://tools.ietf.org/wg/httpbis/trac/ticket/71</a>>: "Examples for ETag matching" 1386 </li> 1387 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/124">http://tools.ietf.org/wg/httpbis/trac/ticket/124</a>>: "'entity value' undefined" 1388 </li> 1389 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/126">http://tools.ietf.org/wg/httpbis/trac/ticket/126</a>>: "bogus 2068 Date header reference" 1390 </li> 1391 </ul> 1392 </div> 1393 <div id="changes.since.04"> 1394 <h2 id="rfc.section.C.6"><a href="#rfc.section.C.6">C.6</a> <a href="#changes.since.04">Since draft-ietf-httpbis-p4-conditional-04</a></h2> 1395 <p id="rfc.section.C.6.p.1">Ongoing work on ABNF conversion (<<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>>): 1396 </p> 1397 <ul> 1398 <li>Use "/" instead of "|" for alternatives.</li> 1399 <li>Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").</li> 1400 <li>Rewrite ABNFs to spell out whitespace rules, factor out header field value format definitions.</li> 1401 </ul> 1402 </div> 1403 <div id="changes.since.05"> 1404 <h2 id="rfc.section.C.7"><a href="#rfc.section.C.7">C.7</a> <a href="#changes.since.05">Since draft-ietf-httpbis-p4-conditional-05</a></h2> 1405 <p id="rfc.section.C.7.p.1">Final work on ABNF conversion (<<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>>): 1406 </p> 1407 <ul> 1408 <li>Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.</li> 1409 </ul> 1410 </div> 1411 <div id="changes.since.06"> 1412 <h2 id="rfc.section.C.8"><a href="#rfc.section.C.8">C.8</a> <a href="#changes.since.06">Since draft-ietf-httpbis-p4-conditional-06</a></h2> 1413 <p id="rfc.section.C.8.p.1">Closed issues: </p> 1414 <ul> 1415 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/153">http://tools.ietf.org/wg/httpbis/trac/ticket/153</a>>: "case-sensitivity of etag weakness indicator" 1416 </li> 1417 </ul> 1418 </div> 1419 <div id="changes.since.07"> 1420 <h2 id="rfc.section.C.9"><a href="#rfc.section.C.9">C.9</a> <a href="#changes.since.07">Since draft-ietf-httpbis-p4-conditional-07</a></h2> 1421 <p id="rfc.section.C.9.p.1">Closed issues: </p> 1422 <ul> 1423 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/116">http://tools.ietf.org/wg/httpbis/trac/ticket/116</a>>: "Weak ETags on non-GET requests" (If-Match still was defined to require strong matching) 1424 </li> 1425 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/198">http://tools.ietf.org/wg/httpbis/trac/ticket/198</a>>: "move IANA registrations for optional status codes" 1426 </li> 1427 </ul> 1428 </div> 1429 <div id="changes.since.08"> 1430 <h2 id="rfc.section.C.10"><a href="#rfc.section.C.10">C.10</a> <a href="#changes.since.08">Since draft-ietf-httpbis-p4-conditional-08</a></h2> 1431 <p id="rfc.section.C.10.p.1">No significant changes.</p> 1432 </div> 1433 <div id="changes.since.09"> 1434 <h2 id="rfc.section.C.11"><a href="#rfc.section.C.11">C.11</a> <a href="#changes.since.09">Since draft-ietf-httpbis-p4-conditional-09</a></h2> 1435 <p id="rfc.section.C.11.p.1">No significant changes.</p> 1436 </div> 1437 <div id="changes.since.10"> 1438 <h2 id="rfc.section.C.12"><a href="#rfc.section.C.12">C.12</a> <a href="#changes.since.10">Since draft-ietf-httpbis-p4-conditional-10</a></h2> 1439 <p id="rfc.section.C.12.p.1">Closed issues: </p> 1440 <ul> 1441 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/69">http://tools.ietf.org/wg/httpbis/trac/ticket/69</a>>: "Clarify 'Requested Variant'" 1442 </li> 1443 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/109">http://tools.ietf.org/wg/httpbis/trac/ticket/109</a>>: "Clarify entity / representation / variant terminology" 1444 </li> 1445 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/220">http://tools.ietf.org/wg/httpbis/trac/ticket/220</a>>: "consider removing the 'changes from 2068' sections" 1446 </li> 1447 </ul> 1448 </div> 1449 <div id="changes.since.11"> 1450 <h2 id="rfc.section.C.13"><a href="#rfc.section.C.13">C.13</a> <a href="#changes.since.11">Since draft-ietf-httpbis-p4-conditional-11</a></h2> 1451 <p id="rfc.section.C.13.p.1">None.</p> 1452 </div> 1453 <div id="changes.since.12"> 1454 <h2 id="rfc.section.C.14"><a href="#rfc.section.C.14">C.14</a> <a href="#changes.since.12">Since draft-ietf-httpbis-p4-conditional-12</a></h2> 1455 <p id="rfc.section.C.14.p.1">Closed issues: </p> 1456 <ul> 1457 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/224">http://tools.ietf.org/wg/httpbis/trac/ticket/224</a>>: "Header Classification" 1458 </li> 1459 </ul> 1460 </div> 1461 <div id="changes.since.13"> 1462 <h2 id="rfc.section.C.15"><a href="#rfc.section.C.15">C.15</a> <a href="#changes.since.13">Since draft-ietf-httpbis-p4-conditional-13</a></h2> 1463 <p id="rfc.section.C.15.p.1">Closed issues: </p> 1464 <ul> 1465 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/89">http://tools.ietf.org/wg/httpbis/trac/ticket/89</a>>: "If-* and entities" 1466 </li> 1467 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/101">http://tools.ietf.org/wg/httpbis/trac/ticket/101</a>>: "Definition of validator weakness" 1468 </li> 1469 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/276">http://tools.ietf.org/wg/httpbis/trac/ticket/276</a>>: "untangle ABNFs for header fields" 1470 </li> 1471 <li><<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/269">http://tools.ietf.org/wg/httpbis/trac/ticket/269</a>>: "ETags and Quotes" 1472 </li> 1473 </ul> 1474 </div> 1475 <div id="changes.since.14"> 1476 <h2 id="rfc.section.C.16"><a href="#rfc.section.C.16">C.16</a> <a href="#changes.since.14">Since draft-ietf-httpbis-p4-conditional-14</a></h2> 1477 <p id="rfc.section.C.16.p.1">None.</p> 1478 </div> 1479 </div> 1399 1480 <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1> 1400 1481 <p class="noprint"><a href="#rfc.index.3">3</a> <a href="#rfc.index.4">4</a> <a href="#rfc.index.E">E</a> <a href="#rfc.index.G">G</a> <a href="#rfc.index.H">H</a> <a href="#rfc.index.I">I</a> <a href="#rfc.index.L">L</a> <a href="#rfc.index.M">M</a> <a href="#rfc.index.P">P</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> <a href="#rfc.index.V">V</a> … … 1403 1484 <ul class="ind"> 1404 1485 <li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul> 1405 <li>304 Not Modified (status code) <a href="#rfc.iref. 24"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">5.1</a></li>1486 <li>304 Not Modified (status code) <a href="#rfc.iref.3.1"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">5.1</a></li> 1406 1487 </ul> 1407 1488 </li> 1408 1489 <li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul> 1409 <li>412 Precondition Failed (status code) <a href="#rfc.iref. 25"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">5.1</a></li>1490 <li>412 Precondition Failed (status code) <a href="#rfc.iref.4.1"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">5.1</a></li> 1410 1491 </ul> 1411 1492 </li> … … 1508 1589 </ul> 1509 1590 </div> 1591 <div class="avoidbreak"> 1592 <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1> 1593 <p><b>Roy T. Fielding</b> 1594 (editor) 1595 <br>Adobe Systems Incorporated<br>345 Park Ave<br>San Jose, CA 95110<br>USA<br>Email: <a href="mailto:fielding@gbiv.com">fielding@gbiv.com</a><br>URI: <a href="http://roy.gbiv.com/">http://roy.gbiv.com/</a></p> 1596 <p><b>Jim Gettys</b><br>Alcatel-Lucent Bell Labs<br>21 Oak Knoll Road<br>Carlisle, MA 01741<br>USA<br>Email: <a href="mailto:jg@freedesktop.org">jg@freedesktop.org</a><br>URI: <a href="http://gettys.wordpress.com/">http://gettys.wordpress.com/</a></p> 1597 <p><b>Jeffrey C. Mogul</b><br>Hewlett-Packard Company<br>HP Labs, Large Scale Systems Group<br>1501 Page Mill Road, MS 1177<br>Palo Alto, CA 94304<br>USA<br>Email: <a href="mailto:JeffMogul@acm.org">JeffMogul@acm.org</a></p> 1598 <p><b>Henrik Frystyk Nielsen</b><br>Microsoft Corporation<br>1 Microsoft Way<br>Redmond, WA 98052<br>USA<br>Email: <a href="mailto:henrikn@microsoft.com">henrikn@microsoft.com</a></p> 1599 <p><b>Larry Masinter</b><br>Adobe Systems Incorporated<br>345 Park Ave<br>San Jose, CA 95110<br>USA<br>Email: <a href="mailto:LMM@acm.org">LMM@acm.org</a><br>URI: <a href="http://larry.masinter.net/">http://larry.masinter.net/</a></p> 1600 <p><b>Paul J. Leach</b><br>Microsoft Corporation<br>1 Microsoft Way<br>Redmond, WA 98052<br>Email: <a href="mailto:paulle@microsoft.com">paulle@microsoft.com</a></p> 1601 <p><b>Tim Berners-Lee</b><br>World Wide Web Consortium<br>MIT Computer Science and Artificial Intelligence Laboratory<br>The Stata Center, Building 32<br>32 Vassar Street<br>Cambridge, MA 02139<br>USA<br>Email: <a href="mailto:timbl@w3.org">timbl@w3.org</a><br>URI: <a href="http://www.w3.org/People/Berners-Lee/">http://www.w3.org/People/Berners-Lee/</a></p> 1602 <p><b>Yves Lafon</b> 1603 (editor) 1604 <br>World Wide Web Consortium<br>W3C / ERCIM<br>2004, rte des Lucioles<br>Sophia-Antipolis, AM 06902<br>France<br>Email: <a href="mailto:ylafon@w3.org">ylafon@w3.org</a><br>URI: <a href="http://www.raubacapeu.net/people/yves/">http://www.raubacapeu.net/people/yves/</a></p> 1605 <p><b>Julian F. Reschke</b> 1606 (editor) 1607 <br>greenbytes GmbH<br>Hafenweg 16<br>Muenster, NW 48155<br>Germany<br>Phone: <a href="tel:+492512807760">+49 251 2807760</a><br>Fax: <a href="fax:+492512807761">+49 251 2807761</a><br>Email: <a href="mailto:julian.reschke@greenbytes.de">julian.reschke@greenbytes.de</a><br>URI: <a href="http://greenbytes.de/tech/webdav/">http://greenbytes.de/tech/webdav/</a></p> 1608 </div> 1510 1609 </body> 1511 1610 </html>
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