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| 276 | |
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| 280 | |
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| [1099] | 281 | ul.ind li li a { |
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| 284 | |
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| 289 | } |
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| 290 | } |
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| 291 | |
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| 292 | @page { |
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| 293 | @top-left { |
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| [1099] | 294 | content: "Internet-Draft"; |
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| [231] | 295 | } |
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| 296 | @top-right { |
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| 297 | content: "February 2008"; |
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| 298 | } |
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| 299 | @top-center { |
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| 300 | content: "HTTP/1.1, Part 4"; |
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| 301 | } |
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| 302 | @bottom-left { |
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| 303 | content: "Fielding, et al."; |
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| 304 | } |
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| 305 | @bottom-center { |
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| 306 | content: "Standards Track"; |
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| 307 | } |
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| 308 | @bottom-right { |
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| 311 | } |
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| 312 | |
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| 313 | @page:first { |
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| 314 | @top-left { |
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| 315 | content: normal; |
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| 316 | } |
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| 317 | @top-right { |
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| 318 | content: normal; |
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| 319 | } |
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| 320 | @top-center { |
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| 321 | content: normal; |
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| 322 | } |
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| 323 | } |
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| 324 | </style><link rel="Contents" href="#rfc.toc"> |
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| 325 | <link rel="Author" href="#rfc.authors"> |
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| 326 | <link rel="Copyright" href="#rfc.copyright"> |
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| 327 | <link rel="Index" href="#rfc.index"> |
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| 328 | <link rel="Chapter" title="1 Introduction" href="#rfc.section.1"> |
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| 329 | <link rel="Chapter" title="2 Notational Conventions and Generic Grammar" href="#rfc.section.2"> |
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| 330 | <link rel="Chapter" title="3 Entity Tags" href="#rfc.section.3"> |
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| 331 | <link rel="Chapter" title="4 Status Code Definitions" href="#rfc.section.4"> |
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| 332 | <link rel="Chapter" title="5 Weak and Strong Validators" href="#rfc.section.5"> |
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| 333 | <link rel="Chapter" title="6 Rules for When to Use Entity Tags and Last-Modified Dates" href="#rfc.section.6"> |
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| 334 | <link rel="Chapter" title="7 Header Field Definitions" href="#rfc.section.7"> |
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| 335 | <link rel="Chapter" title="8 IANA Considerations" href="#rfc.section.8"> |
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| 336 | <link rel="Chapter" title="9 Security Considerations" href="#rfc.section.9"> |
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| 337 | <link rel="Chapter" title="10 Acknowledgments" href="#rfc.section.10"> |
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| 338 | <link rel="Chapter" href="#rfc.section.11" title="11 References"> |
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| 339 | <link rel="Appendix" title="A Compatibility with Previous Versions" href="#rfc.section.A"> |
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| 340 | <link rel="Appendix" title="B Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.B"> |
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| [1099] | 341 | <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.537, 2010-12-30 14:21:59, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/"> |
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| 342 | <link rel="schema.dct" href="http://purl.org/dc/terms/"> |
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| 343 | <meta name="dct.creator" content="Fielding, R."> |
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| 344 | <meta name="dct.creator" content="Gettys, J."> |
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| 345 | <meta name="dct.creator" content="Mogul, J."> |
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| 346 | <meta name="dct.creator" content="Frystyk, H."> |
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| 347 | <meta name="dct.creator" content="Masinter, L."> |
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| 348 | <meta name="dct.creator" content="Leach, P."> |
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| 349 | <meta name="dct.creator" content="Berners-Lee, T."> |
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| 350 | <meta name="dct.creator" content="Lafon, Y."> |
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| 351 | <meta name="dct.creator" content="Reschke, J. F."> |
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| 352 | <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p4-conditional-02"> |
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| 353 | <meta name="dct.issued" scheme="ISO8601" content="2008-02-24"> |
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| 354 | <meta name="dct.replaces" content="urn:ietf:rfc:2616"> |
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| 355 | <meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 4 defines request header fields for indicating conditional requests and the rules for constructing responses to those requests."> |
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| 356 | <meta name="description" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 4 defines request header fields for indicating conditional requests and the rules for constructing responses to those requests."> |
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| [231] | 357 | </head> |
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| 358 | <body> |
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| [1099] | 359 | <table class="header"> |
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| 360 | <tbody> |
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| 361 | <tr> |
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| 362 | <td class="left">Network Working Group</td> |
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| 363 | <td class="right">R. Fielding, Editor</td> |
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| 364 | </tr> |
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| 365 | <tr> |
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| 366 | <td class="left">Internet-Draft</td> |
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| 367 | <td class="right">Day Software</td> |
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| 368 | </tr> |
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| 369 | <tr> |
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| 370 | <td class="left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2616">2616</a> (if approved) |
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| 371 | </td> |
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| 372 | <td class="right">J. Gettys</td> |
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| 373 | </tr> |
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| 374 | <tr> |
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| 375 | <td class="left">Intended status: Standards Track</td> |
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| 376 | <td class="right">One Laptop per Child</td> |
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| 377 | </tr> |
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| 378 | <tr> |
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| 379 | <td class="left">Expires: August 27, 2008</td> |
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| 380 | <td class="right">J. Mogul</td> |
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| 381 | </tr> |
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| 382 | <tr> |
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| 383 | <td class="left"></td> |
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| 384 | <td class="right">HP</td> |
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| 385 | </tr> |
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| 386 | <tr> |
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| 387 | <td class="left"></td> |
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| 388 | <td class="right">H. Frystyk</td> |
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| 389 | </tr> |
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| 390 | <tr> |
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| 391 | <td class="left"></td> |
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| 392 | <td class="right">Microsoft</td> |
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| 393 | </tr> |
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| 394 | <tr> |
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| 395 | <td class="left"></td> |
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| 396 | <td class="right">L. Masinter</td> |
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| 397 | </tr> |
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| 398 | <tr> |
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| 399 | <td class="left"></td> |
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| 400 | <td class="right">Adobe Systems</td> |
|---|
| 401 | </tr> |
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| 402 | <tr> |
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| 403 | <td class="left"></td> |
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| 404 | <td class="right">P. Leach</td> |
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| 405 | </tr> |
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| 406 | <tr> |
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| 407 | <td class="left"></td> |
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| 408 | <td class="right">Microsoft</td> |
|---|
| 409 | </tr> |
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| 410 | <tr> |
|---|
| 411 | <td class="left"></td> |
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| 412 | <td class="right">T. Berners-Lee</td> |
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| 413 | </tr> |
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| 414 | <tr> |
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| 415 | <td class="left"></td> |
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| 416 | <td class="right">W3C/MIT</td> |
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| 417 | </tr> |
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| 418 | <tr> |
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| 419 | <td class="left"></td> |
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| 420 | <td class="right">Y. Lafon, Editor</td> |
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| 421 | </tr> |
|---|
| 422 | <tr> |
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| 423 | <td class="left"></td> |
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| 424 | <td class="right">W3C</td> |
|---|
| 425 | </tr> |
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| 426 | <tr> |
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| 427 | <td class="left"></td> |
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| 428 | <td class="right">J. Reschke, Editor</td> |
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| 429 | </tr> |
|---|
| 430 | <tr> |
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| 431 | <td class="left"></td> |
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| 432 | <td class="right">greenbytes</td> |
|---|
| 433 | </tr> |
|---|
| 434 | <tr> |
|---|
| 435 | <td class="left"></td> |
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| 436 | <td class="right">February 24, 2008</td> |
|---|
| 437 | </tr> |
|---|
| 438 | </tbody> |
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| [231] | 439 | </table> |
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| 440 | <p class="title">HTTP/1.1, part 4: Conditional Requests<br><span class="filename">draft-ietf-httpbis-p4-conditional-02</span></p> |
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| 441 | <h1><a id="rfc.status" href="#rfc.status">Status of this Memo</a></h1> |
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| 442 | <p>By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she |
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| 443 | is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section |
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| 444 | 6 of BCP 79. |
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| 445 | </p> |
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| 446 | <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note |
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| 447 | that other groups may also distribute working documents as Internet-Drafts. |
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| 448 | </p> |
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| 449 | <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other |
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| 450 | documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work |
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| 451 | in progress”. |
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| 452 | </p> |
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| [1099] | 453 | <p>The list of current Internet-Drafts can be accessed at <a href="http://www.ietf.org/ietf/1id-abstracts.txt">http://www.ietf.org/ietf/1id-abstracts.txt</a>. |
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| [231] | 454 | </p> |
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| [1099] | 455 | <p>The list of Internet-Draft Shadow Directories can be accessed at <a href="http://www.ietf.org/shadow.html">http://www.ietf.org/shadow.html</a>. |
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| [231] | 456 | </p> |
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| [1099] | 457 | <p>This Internet-Draft will expire on August 27, 2008.</p> |
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| [231] | 458 | <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> |
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| 459 | <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information |
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| 460 | systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document is Part 4 of the |
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| 461 | seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part |
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| 462 | 4 defines request header fields for indicating conditional requests and the rules for constructing responses to those requests. |
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| 463 | </p> |
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| 464 | <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> |
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| 465 | <p>Discussion of this draft should take place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org). The current issues |
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| 466 | list is at <<a href="http://www.tools.ietf.org/wg/httpbis/trac/report/11">http://www.tools.ietf.org/wg/httpbis/trac/report/11</a>> and related documents (including fancy diffs) can be found at <<a href="http://www.tools.ietf.org/wg/httpbis/">http://www.tools.ietf.org/wg/httpbis/</a>>. |
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| 467 | </p> |
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| 468 | <p>This draft incorporates those issue resolutions that were either collected in the original RFC2616 errata list (<<a href="http://purl.org/NET/http-errata">http://purl.org/NET/http-errata</a>>), or which were agreed upon on the mailing list between October 2006 and November 2007 (as published in "draft-lafon-rfc2616bis-03"). |
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| 469 | </p> |
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| 470 | <hr class="noprint"> |
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| 471 | <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1> |
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| 472 | <ul class="toc"> |
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| [1099] | 473 | <li>1. <a href="#introduction">Introduction</a><ul> |
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| 474 | <li>1.1 <a href="#intro.requirements">Requirements</a></li> |
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| [231] | 475 | </ul> |
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| 476 | </li> |
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| [1099] | 477 | <li>2. <a href="#notation">Notational Conventions and Generic Grammar</a></li> |
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| 478 | <li>3. <a href="#entity.tags">Entity Tags</a></li> |
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| 479 | <li>4. <a href="#rfc.section.4">Status Code Definitions</a><ul> |
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| 480 | <li>4.1 <a href="#status.304">304 Not Modified</a></li> |
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| 481 | <li>4.2 <a href="#status.412">412 Precondition Failed</a></li> |
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| [231] | 482 | </ul> |
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| 483 | </li> |
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| [1099] | 484 | <li>5. <a href="#weak.and.strong.validators">Weak and Strong Validators</a></li> |
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| 485 | <li>6. <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> |
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| 486 | <li>7. <a href="#header.fields">Header Field Definitions</a><ul> |
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| 487 | <li>7.1 <a href="#header.etag">ETag</a></li> |
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| 488 | <li>7.2 <a href="#header.if-match">If-Match</a></li> |
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| 489 | <li>7.3 <a href="#header.if-modified-since">If-Modified-Since</a></li> |
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| 490 | <li>7.4 <a href="#header.if-none-match">If-None-Match</a></li> |
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| 491 | <li>7.5 <a href="#header.if-unmodified-since">If-Unmodified-Since</a></li> |
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| 492 | <li>7.6 <a href="#header.last-modified">Last-Modified</a></li> |
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| [231] | 493 | </ul> |
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| 494 | </li> |
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| [1099] | 495 | <li>8. <a href="#IANA.considerations">IANA Considerations</a></li> |
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| 496 | <li>9. <a href="#security.considerations">Security Considerations</a></li> |
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| 497 | <li>10. <a href="#ack">Acknowledgments</a></li> |
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| 498 | <li>11. <a href="#rfc.references">References</a><ul> |
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| 499 | <li>11.1 <a href="#rfc.references.1">Normative References</a></li> |
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| 500 | <li>11.2 <a href="#rfc.references.2">Informative References</a></li> |
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| [231] | 501 | </ul> |
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| 502 | </li> |
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| [1099] | 503 | <li><a href="#rfc.authors">Authors' Addresses</a></li> |
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| 504 | <li>A. <a href="#compatibility">Compatibility with Previous Versions</a><ul> |
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| 505 | <li>A.1 <a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li> |
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| [231] | 506 | </ul> |
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| 507 | </li> |
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| [1099] | 508 | <li>B. <a href="#rfc.section.B">Change Log (to be removed by RFC Editor before publication)</a><ul> |
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| 509 | <li>B.1 <a href="#rfc.section.B.1">Since RFC2616</a></li> |
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| 510 | <li>B.2 <a href="#rfc.section.B.2">Since draft-ietf-httpbis-p4-conditional-00</a></li> |
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| 511 | <li>B.3 <a href="#rfc.section.B.3">Since draft-ietf-httpbis-p4-conditional-01</a></li> |
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| [231] | 512 | </ul> |
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| 513 | </li> |
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| [1099] | 514 | <li><a href="#rfc.index">Index</a></li> |
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| 515 | <li><a href="#rfc.ipr">Intellectual Property and Copyright Statements</a></li> |
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| [231] | 516 | </ul> |
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| 517 | <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a> <a id="introduction" href="#introduction">Introduction</a></h1> |
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| 518 | <p id="rfc.section.1.p.1">This document defines HTTP/1.1 response metadata for indicating potential changes to payload content, including modification |
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| 519 | time stamps and opaque entity-tags, and the HTTP conditional request mechanisms that allow preconditions to be placed on a |
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| 520 | request method. Conditional GET requests allow for efficient cache updates. Other conditional request methods are used to |
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| 521 | protect against overwriting or misunderstanding the state of a resource that has been changed unbeknownst to the requesting |
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| 522 | client. |
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| 523 | </p> |
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| 524 | <p id="rfc.section.1.p.2">This document is currently disorganized in order to minimize the changes between drafts and enable reviewers to see the smaller |
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| 525 | errata changes. The next draft will reorganize the sections to better reflect the content. In particular, the sections on |
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| 526 | resource metadata will be discussed first and then followed by each conditional request-header, concluding with a definition |
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| 527 | of precedence and the expectation of ordering strong validator checks before weak validator checks. It is likely that more |
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| 528 | content from <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a> will migrate to this part, where appropriate. The current mess reflects how widely dispersed these topics and associated requirements |
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| 529 | had become in <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>. |
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| 530 | </p> |
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| 531 | <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> |
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| 532 | <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" |
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| 533 | 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>. |
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| 534 | </p> |
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| 535 | <p id="rfc.section.1.1.p.2">An implementation is not compliant if it fails to satisfy one or more of the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level requirements for the protocols it implements. An implementation that satisfies all the <em class="bcp14">MUST</em> or <em class="bcp14">REQUIRED</em> level and all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the <em class="bcp14">MUST</em> level requirements but not all the <em class="bcp14">SHOULD</em> level requirements for its protocols is said to be "conditionally compliant." |
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| 536 | </p> |
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| 537 | <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a> <a id="notation" href="#notation">Notational Conventions and Generic Grammar</a></h1> |
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| [1099] | 538 | <p id="rfc.section.2.p.1">This specification uses the ABNF syntax defined in <a href="p1-messaging.html#notation.abnf" title="Augmented BNF">Section 2.1</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> and the core rules defined in <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 2.2</a> of <a href="#Part1" id="rfc.xref.Part1.2"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>: <span class="comment" id="abnf.dep">[<a href="#abnf.dep" class="smpl">abnf.dep</a>: ABNF syntax and basic rules will be adopted from RFC 5234, see <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/36">http://tools.ietf.org/wg/httpbis/trac/ticket/36</a>>.]</span> |
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| [231] | 539 | </p> |
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| 540 | <div id="rfc.figure.u.1"></div><pre class="inline"> quoted-string = <quoted-string, 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 2.2</a>> |
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| 541 | </pre><div id="abnf.dependencies"> |
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| 542 | <p id="rfc.section.2.p.3">The ABNF rules below are defined in other parts:</p> |
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| 543 | </div> |
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| 544 | <div id="rfc.figure.u.2"></div><pre class="inline"> HTTP-date = <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#full.date" title="Full Date">Section 3.3.1</a>> |
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| 545 | </pre><h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a> <a id="entity.tags" href="#entity.tags">Entity Tags</a></h1> |
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| 546 | <p id="rfc.section.3.p.1">Entity tags are used for comparing two or more entities from the same requested resource. HTTP/1.1 uses entity tags in the |
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| 547 | ETag (<a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section 7.1</a>), If-Match (<a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section 7.2</a>), If-None-Match (<a href="#header.if-none-match" id="rfc.xref.header.if-none-match.1" title="If-None-Match">Section 7.4</a>), and If-Range (<a href="p5-range.html#header.if-range" title="If-Range">Section 6.3</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>) header fields. The definition of how they are used and compared as cache validators is in <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section 5</a>. An entity tag consists of an opaque quoted string, possibly prefixed by a weakness indicator. |
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| 548 | </p> |
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| 549 | <div id="rfc.figure.u.3"></div><pre class="inline"><span id="rfc.iref.g.1"></span><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span> entity-tag = [ weak ] opaque-tag |
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| 550 | weak = "W/" |
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| 551 | opaque-tag = quoted-string |
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| 552 | </pre><p id="rfc.section.3.p.3">A "strong entity tag" <em class="bcp14">MAY</em> be shared by two entities of a resource only if they are equivalent by octet equality. |
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| 553 | </p> |
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| 554 | <p id="rfc.section.3.p.4">A "weak entity tag," indicated by the "W/" prefix, <em class="bcp14">MAY</em> be shared by two entities of a resource only if the entities are equivalent and could be substituted for each other with no |
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| 555 | significant change in semantics. A weak entity tag can only be used for weak comparison. |
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| 556 | </p> |
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| 557 | <p id="rfc.section.3.p.5">An entity tag <em class="bcp14">MUST</em> be unique across all versions of all entities associated with a particular resource. A given entity tag value <em class="bcp14">MAY</em> be used for entities obtained by requests on different URIs. The use of the same entity tag value in conjunction with entities |
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| 558 | obtained by requests on different URIs does not imply the equivalence of those entities. |
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| 559 | </p> |
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| 560 | <h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a> Status Code Definitions |
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| 561 | </h1> |
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| 562 | <div id="rfc.iref.3"></div> |
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| 563 | <div id="rfc.iref.s.1"></div> |
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| 564 | <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> |
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| 565 | <p id="rfc.section.4.1.p.1">If the client has performed a conditional GET request and access is allowed, but the document has not been modified, the server <em class="bcp14">SHOULD</em> respond with this status code. 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. |
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| 566 | </p> |
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| 567 | <p id="rfc.section.4.1.p.2">The response <em class="bcp14">MUST</em> include the following header fields: |
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| 568 | </p> |
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| 569 | <ul> |
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| 570 | <li>Date, unless its omission is required by <a href="p1-messaging.html#clockless.origin.server.operation" title="Clockless Origin Server Operation">Section 8.3.1</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></li> |
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| 571 | </ul> |
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| 572 | <p id="rfc.section.4.1.p.3">If a clockless origin server obeys these rules, and proxies and clients add their own Date to any response received without |
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| 573 | one (as already specified by <a href="#RFC2068" id="rfc.xref.RFC2068.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2068]</cite></a>, <a href="http://tools.ietf.org/html/rfc2068#section-14.19">Section 14.19</a>), caches will operate correctly. |
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| 574 | </p> |
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| 575 | <ul> |
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| 576 | <li>ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request</li> |
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| 577 | <li>Expires, Cache-Control, and/or Vary, if the field-value might differ from that sent in any previous response for the same |
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| 578 | variant |
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| 579 | </li> |
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| 580 | </ul> |
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| 581 | <p id="rfc.section.4.1.p.4">If the conditional GET used a strong cache validator (see <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section 5</a>), the response <em class="bcp14">SHOULD NOT</em> include other entity-headers. Otherwise (i.e., the conditional GET used a weak validator), the response <em class="bcp14">MUST NOT</em> include other entity-headers; this prevents inconsistencies between cached entity-bodies and updated headers. |
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| 582 | </p> |
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| 583 | <p id="rfc.section.4.1.p.5">If a 304 response indicates an entity not currently cached, then the cache <em class="bcp14">MUST</em> disregard the response and repeat the request without the conditional. |
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| 584 | </p> |
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| 585 | <p id="rfc.section.4.1.p.6">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. |
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| 586 | </p> |
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| 587 | <div id="rfc.iref.4"></div> |
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| 588 | <div id="rfc.iref.s.2"></div> |
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| 589 | <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> |
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| 590 | <p id="rfc.section.4.2.p.1">The precondition given in one or more of the request-header fields evaluated to false when it was tested on the server. This |
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| 591 | response code allows the client to place preconditions on the current resource metainformation (header field data) and thus |
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| 592 | prevent the requested method from being applied to a resource other than the one intended. |
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| 593 | </p> |
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| 594 | <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a> <a id="weak.and.strong.validators" href="#weak.and.strong.validators">Weak and Strong Validators</a></h1> |
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| 595 | <p id="rfc.section.5.p.1">Since both origin servers and caches will compare two validators to decide if they represent the same or different entities, |
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| 596 | one normally would expect that if the entity (the entity-body or any entity-headers) changes in any way, then the associated |
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| 597 | validator would change as well. If this is true, then we call this validator a "strong validator." |
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| 598 | </p> |
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| 599 | <p id="rfc.section.5.p.2">However, there might be cases when a server prefers to change the validator only on semantically significant changes, and |
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| 600 | not when insignificant aspects of the entity change. A validator that does not always change when the resource changes is |
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| 601 | a "weak validator." |
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| 602 | </p> |
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| 603 | <p id="rfc.section.5.p.3">Entity tags are normally "strong validators," but the protocol provides a mechanism to tag an entity tag as "weak." One can |
|---|
| 604 | think of a strong validator as one that changes whenever the bits of an entity changes, while a weak value changes whenever |
|---|
| 605 | the meaning of an entity changes. Alternatively, one can think of a strong validator as part of an identifier for a specific |
|---|
| 606 | entity, while a weak validator is part of an identifier for a set of semantically equivalent entities. |
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| 607 | </p> |
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| [1099] | 608 | <ul class="empty"> |
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| 609 | <li> <b>Note:</b> One example of a strong validator is an integer that is incremented in stable storage every time an entity is changed. |
|---|
| 610 | </li> |
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| 611 | <li>An entity's modification time, if represented with one-second resolution, could be a weak validator, since it is possible |
|---|
| [231] | 612 | that the resource might be modified twice during a single second. |
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| [1099] | 613 | </li> |
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| 614 | <li>Support for weak validators is optional. However, weak validators allow for more efficient caching of equivalent objects; |
|---|
| [231] | 615 | 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 |
|---|
| 616 | that period is likely "good enough" to be equivalent. |
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| [1099] | 617 | </li> |
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| 618 | </ul> |
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| [231] | 619 | <p id="rfc.section.5.p.4">A "use" of a validator is either when a client generates a request and includes the validator in a validating header field, |
|---|
| 620 | or when a server compares two validators. |
|---|
| 621 | </p> |
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| 622 | <p id="rfc.section.5.p.5">Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality |
|---|
| 623 | of an entity. For example, either kind is usable for a conditional GET of a full entity. However, only a strong validator |
|---|
| 624 | is usable for a sub-range retrieval, since otherwise the client might end up with an internally inconsistent entity. |
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| 625 | </p> |
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| 626 | <p id="rfc.section.5.p.6">Clients <em class="bcp14">MAY</em> issue simple (non-subrange) GET requests with either weak validators or strong validators. Clients <em class="bcp14">MUST NOT</em> use weak validators in other forms of request. |
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| 627 | </p> |
|---|
| 628 | <p id="rfc.section.5.p.7">The only function that HTTP/1.1 defines on validators is comparison. There are two validator comparison functions, depending |
|---|
| 629 | on whether the comparison context allows the use of weak validators or not: |
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| 630 | </p> |
|---|
| 631 | <ul> |
|---|
| 632 | <li>The strong comparison function: in order to be considered equal, both validators <em class="bcp14">MUST</em> be identical in every way, and both <em class="bcp14">MUST NOT</em> be weak. |
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| 633 | </li> |
|---|
| 634 | <li>The weak comparison function: in order to be considered equal, both validators <em class="bcp14">MUST</em> be identical in every way, but either or both of them <em class="bcp14">MAY</em> be tagged as "weak" without affecting the result. |
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| 635 | </li> |
|---|
| 636 | </ul> |
|---|
| 637 | <p id="rfc.section.5.p.8">An entity tag is strong unless it is explicitly tagged as weak. <a href="#entity.tags" title="Entity Tags">Section 3</a> gives the syntax for entity tags. |
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| 638 | </p> |
|---|
| 639 | <p id="rfc.section.5.p.9">A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is |
|---|
| 640 | strong, using the following rules: |
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| 641 | </p> |
|---|
| 642 | <ul> |
|---|
| 643 | <li>The validator is being compared by an origin server to the actual current validator for the entity and,</li> |
|---|
| 644 | <li>That origin server reliably knows that the associated entity did not change twice during the second covered by the presented |
|---|
| 645 | validator. |
|---|
| 646 | </li> |
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| 647 | </ul> |
|---|
| 648 | <p id="rfc.section.5.p.10">or </p> |
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| 649 | <ul> |
|---|
| 650 | <li>The validator is about to be used by a client in an If-Modified-Since or If-Unmodified-Since header, because the client has |
|---|
| 651 | a cache entry for the associated entity, and |
|---|
| 652 | </li> |
|---|
| 653 | <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li> |
|---|
| 654 | <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li> |
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| 655 | </ul> |
|---|
| 656 | <p id="rfc.section.5.p.11">or </p> |
|---|
| 657 | <ul> |
|---|
| 658 | <li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the entity, and</li> |
|---|
| 659 | <li>That cache entry includes a Date value, which gives the time when the origin server sent the original response, and</li> |
|---|
| 660 | <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li> |
|---|
| 661 | </ul> |
|---|
| 662 | <p id="rfc.section.5.p.12">This method relies on the fact that if two different responses were sent by the origin server during the same second, but |
|---|
| 663 | both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified |
|---|
| 664 | time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from |
|---|
| 665 | 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. |
|---|
| 666 | </p> |
|---|
| 667 | <p id="rfc.section.5.p.13">If a client wishes to perform a sub-range retrieval on a value for which it has only a Last-Modified time and no opaque validator, |
|---|
| 668 | it <em class="bcp14">MAY</em> do this only if the Last-Modified time is strong in the sense described here. |
|---|
| 669 | </p> |
|---|
| 670 | <p id="rfc.section.5.p.14">A cache or origin server receiving a conditional request, other than a full-body GET request, <em class="bcp14">MUST</em> use the strong comparison function to evaluate the condition. |
|---|
| 671 | </p> |
|---|
| 672 | <p id="rfc.section.5.p.15">These rules allow HTTP/1.1 caches and clients to safely perform sub-range retrievals on values that have been obtained from |
|---|
| 673 | HTTP/1.0 servers. |
|---|
| 674 | </p> |
|---|
| 675 | <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</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></h1> |
|---|
| 676 | <p id="rfc.section.6.p.1">We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types |
|---|
| 677 | ought to be used, and for what purposes. |
|---|
| 678 | </p> |
|---|
| 679 | <p id="rfc.section.6.p.2">HTTP/1.1 origin servers: </p> |
|---|
| 680 | <ul> |
|---|
| 681 | <li><em class="bcp14">SHOULD</em> send an entity tag validator unless it is not feasible to generate one. |
|---|
| 682 | </li> |
|---|
| 683 | <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, |
|---|
| 684 | or if it is unfeasible to send a strong entity tag. |
|---|
| 685 | </li> |
|---|
| 686 | <li><em class="bcp14">SHOULD</em> send a Last-Modified value if it is feasible to send one, unless the risk of a breakdown in semantic transparency that could |
|---|
| 687 | result from using this date in an If-Modified-Since header would lead to serious problems. |
|---|
| 688 | </li> |
|---|
| 689 | </ul> |
|---|
| 690 | <p id="rfc.section.6.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 |
|---|
| 691 | value. |
|---|
| 692 | </p> |
|---|
| 693 | <p id="rfc.section.6.p.4">In order to be legal, a strong entity tag <em class="bcp14">MUST</em> change whenever the associated entity value changes in any way. A weak entity tag <em class="bcp14">SHOULD</em> change whenever the associated entity changes in a semantically significant way. |
|---|
| 694 | </p> |
|---|
| [1099] | 695 | <ul class="empty"> |
|---|
| 696 | <li> <b>Note:</b> in order to provide semantically transparent caching, an origin server must avoid reusing a specific strong entity tag value |
|---|
| [231] | 697 | for two different entities, or reusing a specific weak entity tag value for two semantically different entities. Cache entries |
|---|
| 698 | might persist for arbitrarily long periods, regardless of expiration times, so it might be inappropriate to expect that a |
|---|
| 699 | cache will never again attempt to validate an entry using a validator that it obtained at some point in the past. |
|---|
| [1099] | 700 | </li> |
|---|
| 701 | </ul> |
|---|
| [231] | 702 | <p id="rfc.section.6.p.5">HTTP/1.1 clients: </p> |
|---|
| 703 | <ul> |
|---|
| 704 | <li>If an entity tag has been provided by the origin server, <em class="bcp14">MUST</em> use that entity tag in any cache-conditional request (using If-Match or If-None-Match). |
|---|
| 705 | </li> |
|---|
| 706 | <li>If only a Last-Modified value has been provided by the origin server, <em class="bcp14">SHOULD</em> use that value in non-subrange cache-conditional requests (using If-Modified-Since). |
|---|
| 707 | </li> |
|---|
| 708 | <li>If only a Last-Modified value has been provided by an HTTP/1.0 origin server, <em class="bcp14">MAY</em> use that value in subrange cache-conditional requests (using If-Unmodified-Since:). The user agent <em class="bcp14">SHOULD</em> provide a way to disable this, in case of difficulty. |
|---|
| 709 | </li> |
|---|
| 710 | <li>If both an entity tag and a Last-Modified value have been provided by the origin server, <em class="bcp14">SHOULD</em> use both validators in cache-conditional requests. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately. |
|---|
| 711 | </li> |
|---|
| 712 | </ul> |
|---|
| 713 | <p id="rfc.section.6.p.6">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 |
|---|
| 714 | 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 |
|---|
| 715 | field) as cache validators, <em class="bcp14">MUST NOT</em> return a response status of 304 (Not Modified) unless doing so is consistent with all of the conditional header fields in |
|---|
| 716 | the request. |
|---|
| 717 | </p> |
|---|
| 718 | <p id="rfc.section.6.p.7">An HTTP/1.1 caching proxy, upon receiving a conditional request that includes both a Last-Modified date and one or more entity |
|---|
| 719 | tags 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 |
|---|
| 720 | fields in the request. |
|---|
| 721 | </p> |
|---|
| [1099] | 722 | <ul class="empty"> |
|---|
| 723 | <li> <b>Note:</b> The general principle behind these rules is that HTTP/1.1 servers and clients should transmit as much non-redundant information |
|---|
| [231] | 724 | as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative |
|---|
| 725 | assumptions about the validators they receive. |
|---|
| [1099] | 726 | </li> |
|---|
| 727 | <li>HTTP/1.0 clients and caches will ignore entity tags. Generally, last-modified values received or used by these systems will |
|---|
| [231] | 728 | support transparent and efficient caching, and so HTTP/1.1 origin servers should provide Last-Modified values. In those rare |
|---|
| 729 | 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 |
|---|
| 730 | HTTP/1.1 origin servers should not provide one. |
|---|
| [1099] | 731 | </li> |
|---|
| 732 | </ul> |
|---|
| [231] | 733 | <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a> <a id="header.fields" href="#header.fields">Header Field Definitions</a></h1> |
|---|
| 734 | <p id="rfc.section.7.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to conditional requests.</p> |
|---|
| 735 | <p id="rfc.section.7.p.2">For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who |
|---|
| 736 | receives the entity. |
|---|
| 737 | </p> |
|---|
| 738 | <div id="rfc.iref.e.1"></div> |
|---|
| 739 | <div id="rfc.iref.h.1"></div> |
|---|
| 740 | <h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a> <a id="header.etag" href="#header.etag">ETag</a></h2> |
|---|
| 741 | <p id="rfc.section.7.1.p.1">The ETag response-header field provides the current value of the entity tag for the requested variant. The headers used with |
|---|
| 742 | entity tags are described in Sections <a href="#header.if-match" id="rfc.xref.header.if-match.2" title="If-Match">7.2</a> and <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.2" title="If-None-Match">7.4</a> of this document, and in <a href="p5-range.html#header.if-range" title="If-Range">Section 6.3</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>. The entity tag <em class="bcp14">MAY</em> be used for comparison with other entities from the same resource (see <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section 5</a>). |
|---|
| 743 | </p> |
|---|
| 744 | <div id="rfc.figure.u.4"></div><pre class="inline"><span id="rfc.iref.g.4"></span> ETag = "ETag" ":" entity-tag |
|---|
| 745 | </pre><div id="rfc.figure.u.5"></div> |
|---|
| 746 | <p>Examples:</p> <pre class="text"> ETag: "xyzzy" |
|---|
| 747 | ETag: W/"xyzzy" |
|---|
| 748 | ETag: "" |
|---|
| 749 | </pre><p id="rfc.section.7.1.p.4">The ETag response-header field value, an entity tag, provides for an "opaque" cache validator. This might allow more reliable |
|---|
| 750 | validation in situations where it is inconvenient to store modification dates, where the one-second resolution of HTTP date |
|---|
| 751 | values is not sufficient, or where the origin server wishes to avoid certain paradoxes that might arise from the use of modification |
|---|
| 752 | dates. |
|---|
| 753 | </p> |
|---|
| 754 | <p id="rfc.section.7.1.p.5">The principle behind entity tags is that only the service author knows the semantics of a resource well enough to select an |
|---|
| 755 | appropriate cache validation mechanism, and the specification of any validator comparison function more complex than byte-equality |
|---|
| 756 | would open up a can of worms. Thus, comparisons of any other headers (except Last-Modified, for compatibility with HTTP/1.0) |
|---|
| 757 | are never used for purposes of validating a cache entry. |
|---|
| 758 | </p> |
|---|
| 759 | <div id="rfc.iref.i.1"></div> |
|---|
| 760 | <div id="rfc.iref.h.2"></div> |
|---|
| 761 | <h2 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a> <a id="header.if-match" href="#header.if-match">If-Match</a></h2> |
|---|
| 762 | <p id="rfc.section.7.2.p.1">The If-Match request-header field is used with a method to make it conditional. A client that has one or more entities previously |
|---|
| 763 | obtained from the resource can verify that one of those entities is current by including a list of their associated entity |
|---|
| 764 | tags in the If-Match header field. Entity tags are defined in <a href="#entity.tags" title="Entity Tags">Section 3</a>. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. |
|---|
| 765 | It is also used, on updating requests, to prevent inadvertent modification of the wrong version of a resource. As a special |
|---|
| 766 | case, the value "*" matches any current entity of the resource. |
|---|
| 767 | </p> |
|---|
| 768 | <div id="rfc.figure.u.6"></div><pre class="inline"><span id="rfc.iref.g.5"></span> If-Match = "If-Match" ":" ( "*" | 1#entity-tag ) |
|---|
| 769 | </pre><p id="rfc.section.7.2.p.3">If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET |
|---|
| 770 | request (without the If-Match header) on that resource, or if "*" is given and any current entity exists for that resource, |
|---|
| 771 | then the server <em class="bcp14">MAY</em> perform the requested method as if the If-Match header field did not exist. |
|---|
| 772 | </p> |
|---|
| 773 | <p id="rfc.section.7.2.p.4">A server <em class="bcp14">MUST</em> use the strong comparison function (see <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section 5</a>) to compare the entity tags in If-Match. |
|---|
| 774 | </p> |
|---|
| 775 | <p id="rfc.section.7.2.p.5">If none of the entity tags match, or if "*" is given and no current entity exists, the server <em class="bcp14">MUST NOT</em> perform the requested method, and <em class="bcp14">MUST</em> return a 412 (Precondition Failed) response. This behavior is most useful when the client wants to prevent an updating method, |
|---|
| 776 | such as PUT, from modifying a resource that has changed since the client last retrieved it. |
|---|
| 777 | </p> |
|---|
| 778 | <p id="rfc.section.7.2.p.6">If the request would, without the If-Match header field, result in anything other than a 2xx or 412 status, then the If-Match |
|---|
| 779 | header <em class="bcp14">MUST</em> be ignored. |
|---|
| 780 | </p> |
|---|
| 781 | <p id="rfc.section.7.2.p.7">The meaning of "If-Match: *" is that the method <em class="bcp14">SHOULD</em> be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see <a href="p6-cache.html#header.vary" title="Vary">Section 16.5</a> of <a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) exists, and <em class="bcp14">MUST NOT</em> be performed if the representation does not exist. |
|---|
| 782 | </p> |
|---|
| 783 | <p id="rfc.section.7.2.p.8">A request intended to update a resource (e.g., a PUT) <em class="bcp14">MAY</em> include an If-Match header field to signal that the request method <em class="bcp14">MUST NOT</em> be applied if the entity corresponding to the If-Match value (a single entity tag) is no longer a representation of that resource. |
|---|
| 784 | This allows the user to indicate that they do not wish the request to be successful if the resource has been changed without |
|---|
| 785 | their knowledge. Examples: |
|---|
| 786 | </p> |
|---|
| 787 | <div id="rfc.figure.u.7"></div><pre class="text"> If-Match: "xyzzy" |
|---|
| 788 | If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" |
|---|
| 789 | If-Match: * |
|---|
| 790 | </pre><p id="rfc.section.7.2.p.10">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 |
|---|
| 791 | is undefined by this specification. |
|---|
| 792 | </p> |
|---|
| 793 | <div id="rfc.iref.i.2"></div> |
|---|
| 794 | <div id="rfc.iref.h.3"></div> |
|---|
| 795 | <h2 id="rfc.section.7.3"><a href="#rfc.section.7.3">7.3</a> <a id="header.if-modified-since" href="#header.if-modified-since">If-Modified-Since</a></h2> |
|---|
| 796 | <p id="rfc.section.7.3.p.1">The If-Modified-Since request-header field is used with a method to make it conditional: if the requested variant has not |
|---|
| 797 | been modified since the time specified in this field, an entity will not be returned from the server; instead, a 304 (Not |
|---|
| 798 | Modified) response will be returned without any message-body. |
|---|
| 799 | </p> |
|---|
| 800 | <div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.6"></span> If-Modified-Since = "If-Modified-Since" ":" HTTP-date |
|---|
| 801 | </pre><p id="rfc.section.7.3.p.3">An example of the field is:</p> |
|---|
| 802 | <div id="rfc.figure.u.9"></div><pre class="text"> If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT |
|---|
| 803 | </pre><p id="rfc.section.7.3.p.5">A GET method with an If-Modified-Since header and no Range header requests that the identified entity be transferred only |
|---|
| 804 | if it has been modified since the date given by the If-Modified-Since header. The algorithm for determining this includes |
|---|
| 805 | the following cases: |
|---|
| 806 | </p> |
|---|
| 807 | <ol> |
|---|
| 808 | <li>If the request would normally result in anything other than a 200 (OK) status, or if the passed If-Modified-Since date is |
|---|
| 809 | invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is invalid. |
|---|
| 810 | </li> |
|---|
| 811 | <li>If the variant has been modified since the If-Modified-Since date, the response is exactly the same as for a normal GET.</li> |
|---|
| 812 | <li>If the variant has not been modified since a valid If-Modified-Since date, the server <em class="bcp14">SHOULD</em> return a 304 (Not Modified) response. |
|---|
| 813 | </li> |
|---|
| 814 | </ol> |
|---|
| 815 | <p id="rfc.section.7.3.p.6">The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. </p> |
|---|
| [1099] | 816 | <ul class="empty"> |
|---|
| 817 | <li> <b>Note:</b> The Range request-header field modifies the meaning of If-Modified-Since; see <a href="p5-range.html#header.range" title="Range">Section 6.4</a> of <a href="#Part5" id="rfc.xref.Part5.3"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a> for full details. |
|---|
| 818 | </li> |
|---|
| 819 | <li> <b>Note:</b> If-Modified-Since times are interpreted by the server, whose clock might not be synchronized with the client. |
|---|
| 820 | </li> |
|---|
| 821 | <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 |
|---|
| [231] | 822 | function, for deciding whether to send a 304 (Not Modified) response. To get best results when sending an If-Modified-Since |
|---|
| 823 | header field for cache validation, clients are advised to use the exact date string received in a previous Last-Modified header |
|---|
| 824 | field whenever possible. |
|---|
| [1099] | 825 | </li> |
|---|
| 826 | <li> <b>Note:</b> If a client uses an arbitrary date in the If-Modified-Since header instead of a date taken from the Last-Modified header for |
|---|
| [231] | 827 | the same request, the client should be aware of the fact that this date is interpreted in the server's understanding of time. |
|---|
| 828 | The client should consider unsynchronized clocks and rounding problems due to the different encodings of time between the |
|---|
| 829 | client and server. This includes the possibility of race conditions if the document has changed between the time it was first |
|---|
| 830 | requested and the If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the |
|---|
| 831 | If-Modified-Since date is derived from the client's clock without correction to the server's clock. Corrections for different |
|---|
| 832 | time bases between client and server are at best approximate due to network latency. |
|---|
| [1099] | 833 | </li> |
|---|
| 834 | </ul> |
|---|
| [231] | 835 | <p id="rfc.section.7.3.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 |
|---|
| 836 | fields is undefined by this specification. |
|---|
| 837 | </p> |
|---|
| 838 | <div id="rfc.iref.i.3"></div> |
|---|
| 839 | <div id="rfc.iref.h.4"></div> |
|---|
| 840 | <h2 id="rfc.section.7.4"><a href="#rfc.section.7.4">7.4</a> <a id="header.if-none-match" href="#header.if-none-match">If-None-Match</a></h2> |
|---|
| 841 | <p id="rfc.section.7.4.p.1">The If-None-Match request-header field is used with a method to make it conditional. A client that has one or more entities |
|---|
| 842 | previously obtained from the resource can verify that none of those entities is current by including a list of their associated |
|---|
| 843 | entity tags in the If-None-Match header field. The purpose of this feature is to allow efficient updates of cached information |
|---|
| 844 | with a minimum amount of transaction overhead. It is also used to prevent a method (e.g. PUT) from inadvertently modifying |
|---|
| 845 | an existing resource when the client believes that the resource does not exist. |
|---|
| 846 | </p> |
|---|
| 847 | <p id="rfc.section.7.4.p.2">As a special case, the value "*" matches any current entity of the resource.</p> |
|---|
| 848 | <div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.7"></span> If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag ) |
|---|
| 849 | </pre><p id="rfc.section.7.4.p.4">If any of the entity tags match the entity tag of the entity that would have been returned in the response to a similar GET |
|---|
| 850 | request (without the If-None-Match header) on that resource, or if "*" is given and any current entity exists for that resource, |
|---|
| 851 | then the server <em class="bcp14">MUST NOT</em> perform the requested method, unless required to do so because the resource's modification date fails to match that supplied |
|---|
| 852 | in an If-Modified-Since header field in the request. Instead, if the request method was GET or HEAD, the server <em class="bcp14">SHOULD</em> respond with a 304 (Not Modified) response, including the cache-related header fields (particularly ETag) of one of the entities |
|---|
| 853 | that matched. For all other request methods, the server <em class="bcp14">MUST</em> respond with a status of 412 (Precondition Failed). |
|---|
| 854 | </p> |
|---|
| 855 | <p id="rfc.section.7.4.p.5">See <a href="#weak.and.strong.validators" title="Weak and Strong Validators">Section 5</a> for rules on how to determine if two entities tags match. The weak comparison function can only be used with GET or HEAD requests. |
|---|
| 856 | </p> |
|---|
| 857 | <p id="rfc.section.7.4.p.6">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. |
|---|
| 858 | </p> |
|---|
| 859 | <p id="rfc.section.7.4.p.7">If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status, then the |
|---|
| 860 | If-None-Match header <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 6</a> for a discussion of server behavior when both If-Modified-Since and If-None-Match appear in the same request.) |
|---|
| 861 | </p> |
|---|
| 862 | <p id="rfc.section.7.4.p.8">The meaning of "If-None-Match: *" is that the method <em class="bcp14">MUST NOT</em> be performed if the representation selected by the origin server (or by a cache, possibly using the Vary mechanism, see <a href="p6-cache.html#header.vary" title="Vary">Section 16.5</a> of <a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) exists, and <em class="bcp14">SHOULD</em> be performed if the representation does not exist. This feature is intended to be useful in preventing races between PUT operations. |
|---|
| 863 | </p> |
|---|
| 864 | <p id="rfc.section.7.4.p.9">Examples:</p> |
|---|
| 865 | <div id="rfc.figure.u.11"></div><pre class="text"> If-None-Match: "xyzzy" |
|---|
| 866 | If-None-Match: W/"xyzzy" |
|---|
| 867 | If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" |
|---|
| 868 | If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" |
|---|
| 869 | If-None-Match: * |
|---|
| 870 | </pre><p id="rfc.section.7.4.p.11">The result of a request having both an If-None-Match header field and either an If-Match or an If-Unmodified-Since header |
|---|
| 871 | fields is undefined by this specification. |
|---|
| 872 | </p> |
|---|
| 873 | <div id="rfc.iref.i.4"></div> |
|---|
| 874 | <div id="rfc.iref.h.5"></div> |
|---|
| 875 | <h2 id="rfc.section.7.5"><a href="#rfc.section.7.5">7.5</a> <a id="header.if-unmodified-since" href="#header.if-unmodified-since">If-Unmodified-Since</a></h2> |
|---|
| 876 | <p id="rfc.section.7.5.p.1">The If-Unmodified-Since request-header field is used with a method to make it conditional. If the requested resource has not |
|---|
| 877 | been modified since the time specified in this field, the server <em class="bcp14">SHOULD</em> perform the requested operation as if the If-Unmodified-Since header were not present. |
|---|
| 878 | </p> |
|---|
| 879 | <p id="rfc.section.7.5.p.2">If the requested variant has been modified since the specified time, the server <em class="bcp14">MUST NOT</em> perform the requested operation, and <em class="bcp14">MUST</em> return a 412 (Precondition Failed). |
|---|
| 880 | </p> |
|---|
| 881 | <div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.8"></span> If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date |
|---|
| 882 | </pre><p id="rfc.section.7.5.p.4">An example of the field is:</p> |
|---|
| 883 | <div id="rfc.figure.u.13"></div><pre class="text"> If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT |
|---|
| 884 | </pre><p id="rfc.section.7.5.p.6">If the request normally (i.e., without the If-Unmodified-Since header) would result in anything other than a 2xx or 412 status, |
|---|
| 885 | the If-Unmodified-Since header <em class="bcp14">SHOULD</em> be ignored. |
|---|
| 886 | </p> |
|---|
| 887 | <p id="rfc.section.7.5.p.7">If the specified date is invalid, the header is ignored.</p> |
|---|
| 888 | <p id="rfc.section.7.5.p.8">The result of a request having both an If-Unmodified-Since header field and either an If-None-Match or an If-Modified-Since |
|---|
| 889 | header fields is undefined by this specification. |
|---|
| 890 | </p> |
|---|
| 891 | <div id="rfc.iref.l.1"></div> |
|---|
| 892 | <div id="rfc.iref.h.6"></div> |
|---|
| 893 | <h2 id="rfc.section.7.6"><a href="#rfc.section.7.6">7.6</a> <a id="header.last-modified" href="#header.last-modified">Last-Modified</a></h2> |
|---|
| 894 | <p id="rfc.section.7.6.p.1">The Last-Modified entity-header field indicates the date and time at which the origin server believes the variant was last |
|---|
| 895 | modified. |
|---|
| 896 | </p> |
|---|
| 897 | <div id="rfc.figure.u.14"></div><pre class="inline"><span id="rfc.iref.g.9"></span> Last-Modified = "Last-Modified" ":" HTTP-date |
|---|
| 898 | </pre><p id="rfc.section.7.6.p.3">An example of its use is</p> |
|---|
| 899 | <div id="rfc.figure.u.15"></div><pre class="text"> Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT |
|---|
| 900 | </pre><p id="rfc.section.7.6.p.5">The exact meaning of this header field depends on the implementation of the origin server and the nature of the original resource. |
|---|
| 901 | For files, it may be just the file system last-modified time. For entities with dynamically included parts, it may be the |
|---|
| 902 | most recent of the set of last-modify times for its component parts. For database gateways, it may be the last-update time |
|---|
| 903 | stamp of the record. For virtual objects, it may be the last time the internal state changed. |
|---|
| 904 | </p> |
|---|
| 905 | <p id="rfc.section.7.6.p.6">An origin server <em class="bcp14">MUST NOT</em> send a Last-Modified date which is later than the server's time of message origination. In such cases, where the resource's |
|---|
| 906 | last modification would indicate some time in the future, the server <em class="bcp14">MUST</em> replace that date with the message origination date. |
|---|
| 907 | </p> |
|---|
| 908 | <p id="rfc.section.7.6.p.7">An origin server <em class="bcp14">SHOULD</em> obtain the Last-Modified value of the entity as close as possible to the time that it generates the Date value of its response. |
|---|
| 909 | This allows a recipient to make an accurate assessment of the entity's modification time, especially if the entity changes |
|---|
| 910 | near the time that the response is generated. |
|---|
| 911 | </p> |
|---|
| 912 | <p id="rfc.section.7.6.p.8">HTTP/1.1 servers <em class="bcp14">SHOULD</em> send Last-Modified whenever feasible. |
|---|
| 913 | </p> |
|---|
| 914 | <p id="rfc.section.7.6.p.9">The Last-Modified entity-header field value is often used as a cache validator. In simple terms, a cache entry is considered |
|---|
| 915 | to be valid if the entity has not been modified since the Last-Modified value. |
|---|
| 916 | </p> |
|---|
| 917 | <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a> <a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1> |
|---|
| [1099] | 918 | <p id="rfc.section.8.p.1"> <span class="comment" id="rfc.comment.1">[<a href="#rfc.comment.1" class="smpl">rfc.comment.1</a>: TBD.]</span> |
|---|
| [231] | 919 | </p> |
|---|
| 920 | <h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a> <a id="security.considerations" href="#security.considerations">Security Considerations</a></h1> |
|---|
| 921 | <p id="rfc.section.9.p.1">No additional security considerations have been identified beyond those applicable to HTTP in general <a href="#Part1" id="rfc.xref.Part1.6"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>. |
|---|
| 922 | </p> |
|---|
| 923 | <h1 id="rfc.section.10"><a href="#rfc.section.10">10.</a> <a id="ack" href="#ack">Acknowledgments</a></h1> |
|---|
| 924 | <h1 id="rfc.references"><a id="rfc.section.11" href="#rfc.section.11">11.</a> References |
|---|
| 925 | </h1> |
|---|
| 926 | <h2 id="rfc.references.1"><a href="#rfc.section.11.1" id="rfc.section.11.1">11.1</a> Normative References |
|---|
| 927 | </h2> |
|---|
| [1099] | 928 | <table> |
|---|
| [231] | 929 | <tr> |
|---|
| 930 | <td class="reference"><b id="Part1">[Part1]</b></td> |
|---|
| [1099] | 931 | <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">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-02">HTTP/1.1, part 1: URIs, Connections, and Message Parsing</a>”, Internet-Draft draft-ietf-httpbis-p1-messaging-02 (work in progress), February 2008. |
|---|
| [231] | 932 | </td> |
|---|
| 933 | </tr> |
|---|
| 934 | <tr> |
|---|
| 935 | <td class="reference"><b id="Part5">[Part5]</b></td> |
|---|
| [1099] | 936 | <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">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-02">HTTP/1.1, part 5: Range Requests and Partial Responses</a>”, Internet-Draft draft-ietf-httpbis-p5-range-02 (work in progress), February 2008. |
|---|
| [231] | 937 | </td> |
|---|
| 938 | </tr> |
|---|
| 939 | <tr> |
|---|
| 940 | <td class="reference"><b id="Part6">[Part6]</b></td> |
|---|
| [1099] | 941 | <td class="top"><a href="mailto:fielding@gbiv.com" title="Day Software">Fielding, R., Ed.</a>, <a href="mailto:jg@laptop.org" title="One Laptop per Child">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-p6-cache-02">HTTP/1.1, part 6: Caching</a>”, Internet-Draft draft-ietf-httpbis-p6-cache-02 (work in progress), February 2008. |
|---|
| [231] | 942 | </td> |
|---|
| 943 | </tr> |
|---|
| 944 | <tr> |
|---|
| 945 | <td class="reference"><b id="RFC2119">[RFC2119]</b></td> |
|---|
| [1099] | 946 | <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. |
|---|
| [231] | 947 | </td> |
|---|
| 948 | </tr> |
|---|
| 949 | </table> |
|---|
| 950 | <h2 id="rfc.references.2"><a href="#rfc.section.11.2" id="rfc.section.11.2">11.2</a> Informative References |
|---|
| 951 | </h2> |
|---|
| [1099] | 952 | <table> |
|---|
| [231] | 953 | <tr> |
|---|
| 954 | <td class="reference"><b id="RFC2068">[RFC2068]</b></td> |
|---|
| [1099] | 955 | <td class="top"><a href="mailto:fielding@ics.uci.edu" title="University of California, Irvine, Department of Information and Computer Science">Fielding, R.</a>, <a href="mailto:jg@w3.org" title="MIT Laboratory for Computer Science">Gettys, J.</a>, <a href="mailto:mogul@wrl.dec.com" title="Digital Equipment Corporation, Western Research Laboratory">Mogul, J.</a>, <a href="mailto:frystyk@w3.org" title="MIT Laboratory for Computer Science">Nielsen, H.</a>, and <a href="mailto:timbl@w3.org" title="MIT Laboratory for Computer Science">T. Berners-Lee</a>, “<a href="http://tools.ietf.org/html/rfc2068">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC 2068, January 1997. |
|---|
| [231] | 956 | </td> |
|---|
| 957 | </tr> |
|---|
| 958 | <tr> |
|---|
| 959 | <td class="reference"><b id="RFC2616">[RFC2616]</b></td> |
|---|
| [1099] | 960 | <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. |
|---|
| [231] | 961 | </td> |
|---|
| 962 | </tr> |
|---|
| 963 | </table> |
|---|
| [1099] | 964 | <div class="avoidbreak"> |
|---|
| 965 | <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1> |
|---|
| 966 | <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span> |
|---|
| 967 | (editor) |
|---|
| 968 | <span class="n hidden"><span class="family-name">Fielding</span><span class="given-name">Roy T.</span></span></span><span class="org vcardline">Day Software</span><span class="adr"><span class="street-address vcardline">23 Corporate Plaza DR, Suite 280</span><span class="vcardline"><span class="locality">Newport Beach</span>, <span class="region">CA</span> <span class="postal-code">92660</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline tel">Phone: <a href="tel:+1-949-706-5300"><span class="value">+1-949-706-5300</span></a></span><span class="vcardline tel"><span class="type">Fax</span>: <a href="fax:+1-949-706-5305"><span class="value">+1-949-706-5305</span></a></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> |
|---|
| 969 | <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">One Laptop per Child</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@laptop.org"><span class="email">jg@laptop.org</span></a></span><span class="vcardline">URI: <a href="http://www.laptop.org/" class="url">http://www.laptop.org/</a></span></address> |
|---|
| 970 | <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> |
|---|
| 971 | <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> |
|---|
| 972 | <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> |
|---|
| 973 | <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> |
|---|
| 974 | <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> |
|---|
| 975 | <address class="vcard"><span class="vcardline"><span class="fn">Yves Lafon</span> |
|---|
| 976 | (editor) |
|---|
| 977 | <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> |
|---|
| 978 | <address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span> |
|---|
| 979 | (editor) |
|---|
| 980 | <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> |
|---|
| 981 | </div> |
|---|
| 982 | <h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a> <a id="compatibility" href="#compatibility">Compatibility with Previous Versions</a></h1> |
|---|
| [231] | 983 | <h2 id="rfc.section.A.1"><a href="#rfc.section.A.1">A.1</a> <a id="changes.from.rfc.2616" href="#changes.from.rfc.2616">Changes from RFC 2616</a></h2> |
|---|
| 984 | <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a> Change Log (to be removed by RFC Editor before publication) |
|---|
| 985 | </h1> |
|---|
| 986 | <h2 id="rfc.section.B.1"><a href="#rfc.section.B.1">B.1</a> Since RFC2616 |
|---|
| 987 | </h2> |
|---|
| 988 | <p id="rfc.section.B.1.p.1">Extracted relevant partitions from <a href="#RFC2616" id="rfc.xref.RFC2616.2"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>. |
|---|
| 989 | </p> |
|---|
| 990 | <h2 id="rfc.section.B.2"><a href="#rfc.section.B.2">B.2</a> Since draft-ietf-httpbis-p4-conditional-00 |
|---|
| 991 | </h2> |
|---|
| 992 | <p id="rfc.section.B.2.p.1">Closed issues: </p> |
|---|
| 993 | <ul> |
|---|
| 994 | <li> <<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/35</a>>: "Normative and Informative references" |
|---|
| 995 | </li> |
|---|
| 996 | </ul> |
|---|
| 997 | <p id="rfc.section.B.2.p.2">Other changes: </p> |
|---|
| 998 | <ul> |
|---|
| 999 | <li>Move definitions of 304 and 412 condition codes from Part2.</li> |
|---|
| 1000 | </ul> |
|---|
| 1001 | <h2 id="rfc.section.B.3"><a href="#rfc.section.B.3">B.3</a> Since draft-ietf-httpbis-p4-conditional-01 |
|---|
| 1002 | </h2> |
|---|
| 1003 | <p id="rfc.section.B.3.p.1">Ongoing work on ABNF conversion (<<a href="http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36">http://www3.tools.ietf.org/wg/httpbis/trac/ticket/36</a>>): |
|---|
| 1004 | </p> |
|---|
| 1005 | <ul> |
|---|
| 1006 | <li>Add explicit references to BNF syntax and rules imported from other parts of the specification.</li> |
|---|
| 1007 | </ul> |
|---|
| 1008 | <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1> |
|---|
| 1009 | <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.P">P</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> |
|---|
| 1010 | </p> |
|---|
| 1011 | <div class="print2col"> |
|---|
| 1012 | <ul class="ind"> |
|---|
| [1099] | 1013 | <li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul> |
|---|
| 1014 | <li>304 Not Modified (status code) <a href="#rfc.iref.3"><b>4.1</b></a></li> |
|---|
| [231] | 1015 | </ul> |
|---|
| 1016 | </li> |
|---|
| [1099] | 1017 | <li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul> |
|---|
| 1018 | <li>412 Precondition Failed (status code) <a href="#rfc.iref.4"><b>4.2</b></a></li> |
|---|
| [231] | 1019 | </ul> |
|---|
| 1020 | </li> |
|---|
| [1099] | 1021 | <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul> |
|---|
| 1022 | <li>ETag header <a href="#rfc.xref.header.etag.1">3</a>, <a href="#rfc.iref.e.1"><b>7.1</b></a></li> |
|---|
| [231] | 1023 | </ul> |
|---|
| 1024 | </li> |
|---|
| [1099] | 1025 | <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul> |
|---|
| 1026 | <li><tt>Grammar</tt> |
|---|
| 1027 | <ul> |
|---|
| 1028 | <li><tt>entity-tag</tt> <a href="#rfc.iref.g.1"><b>3</b></a></li> |
|---|
| 1029 | <li><tt>ETag</tt> <a href="#rfc.iref.g.4"><b>7.1</b></a></li> |
|---|
| 1030 | <li><tt>If-Match</tt> <a href="#rfc.iref.g.5"><b>7.2</b></a></li> |
|---|
| 1031 | <li><tt>If-Modified-Since</tt> <a href="#rfc.iref.g.6"><b>7.3</b></a></li> |
|---|
| 1032 | <li><tt>If-None-Match</tt> <a href="#rfc.iref.g.7"><b>7.4</b></a></li> |
|---|
| 1033 | <li><tt>If-Unmodified-Since</tt> <a href="#rfc.iref.g.8"><b>7.5</b></a></li> |
|---|
| 1034 | <li><tt>Last-Modified</tt> <a href="#rfc.iref.g.9"><b>7.6</b></a></li> |
|---|
| 1035 | <li><tt>opaque-tag</tt> <a href="#rfc.iref.g.3"><b>3</b></a></li> |
|---|
| 1036 | <li><tt>weak</tt> <a href="#rfc.iref.g.2"><b>3</b></a></li> |
|---|
| [231] | 1037 | </ul> |
|---|
| 1038 | </li> |
|---|
| 1039 | </ul> |
|---|
| 1040 | </li> |
|---|
| [1099] | 1041 | <li><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul> |
|---|
| 1042 | <li>Headers |
|---|
| 1043 | <ul> |
|---|
| 1044 | <li>ETag <a href="#rfc.xref.header.etag.1">3</a>, <a href="#rfc.iref.h.1"><b>7.1</b></a></li> |
|---|
| 1045 | <li>If-Match <a href="#rfc.xref.header.if-match.1">3</a>, <a href="#rfc.xref.header.if-match.2">7.1</a>, <a href="#rfc.iref.h.2"><b>7.2</b></a></li> |
|---|
| 1046 | <li>If-Modified-Since <a href="#rfc.iref.h.3"><b>7.3</b></a></li> |
|---|
| 1047 | <li>If-None-Match <a href="#rfc.xref.header.if-none-match.1">3</a>, <a href="#rfc.xref.header.if-none-match.2">7.1</a>, <a href="#rfc.iref.h.4"><b>7.4</b></a></li> |
|---|
| 1048 | <li>If-Unmodified-Since <a href="#rfc.iref.h.5"><b>7.5</b></a></li> |
|---|
| 1049 | <li>Last-Modified <a href="#rfc.iref.h.6"><b>7.6</b></a></li> |
|---|
| [231] | 1050 | </ul> |
|---|
| 1051 | </li> |
|---|
| 1052 | </ul> |
|---|
| 1053 | </li> |
|---|
| [1099] | 1054 | <li><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul> |
|---|
| 1055 | <li>If-Match header <a href="#rfc.xref.header.if-match.1">3</a>, <a href="#rfc.xref.header.if-match.2">7.1</a>, <a href="#rfc.iref.i.1"><b>7.2</b></a></li> |
|---|
| 1056 | <li>If-Modified-Since header <a href="#rfc.iref.i.2"><b>7.3</b></a></li> |
|---|
| 1057 | <li>If-None-Match header <a href="#rfc.xref.header.if-none-match.1">3</a>, <a href="#rfc.xref.header.if-none-match.2">7.1</a>, <a href="#rfc.iref.i.3"><b>7.4</b></a></li> |
|---|
| 1058 | <li>If-Unmodified-Since header <a href="#rfc.iref.i.4"><b>7.5</b></a></li> |
|---|
| [231] | 1059 | </ul> |
|---|
| 1060 | </li> |
|---|
| [1099] | 1061 | <li><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul> |
|---|
| 1062 | <li>Last-Modified header <a href="#rfc.iref.l.1"><b>7.6</b></a></li> |
|---|
| [231] | 1063 | </ul> |
|---|
| 1064 | </li> |
|---|
| [1099] | 1065 | <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul> |
|---|
| 1066 | <li><em>Part1</em> <a href="#rfc.xref.Part1.1">2</a>, <a href="#rfc.xref.Part1.2">2</a>, <a href="#rfc.xref.Part1.3">2</a>, <a href="#rfc.xref.Part1.4">2</a>, <a href="#rfc.xref.Part1.5">4.1</a>, <a href="#rfc.xref.Part1.6">9</a>, <a href="#Part1"><b>11.1</b></a><ul> |
|---|
| 1067 | <li><em>Section 2.1</em> <a href="#rfc.xref.Part1.1">2</a></li> |
|---|
| 1068 | <li><em>Section 2.2</em> <a href="#rfc.xref.Part1.2">2</a>, <a href="#rfc.xref.Part1.3">2</a></li> |
|---|
| 1069 | <li><em>Section 3.3.1</em> <a href="#rfc.xref.Part1.4">2</a></li> |
|---|
| 1070 | <li><em>Section 8.3.1</em> <a href="#rfc.xref.Part1.5">4.1</a></li> |
|---|
| [231] | 1071 | </ul> |
|---|
| 1072 | </li> |
|---|
| [1099] | 1073 | <li><em>Part5</em> <a href="#rfc.xref.Part5.1">3</a>, <a href="#rfc.xref.Part5.2">7.1</a>, <a href="#rfc.xref.Part5.3">7.3</a>, <a href="#Part5"><b>11.1</b></a><ul> |
|---|
| 1074 | <li><em>Section 6.3</em> <a href="#rfc.xref.Part5.1">3</a>, <a href="#rfc.xref.Part5.2">7.1</a></li> |
|---|
| 1075 | <li><em>Section 6.4</em> <a href="#rfc.xref.Part5.3">7.3</a></li> |
|---|
| [231] | 1076 | </ul> |
|---|
| 1077 | </li> |
|---|
| [1099] | 1078 | <li><em>Part6</em> <a href="#rfc.xref.Part6.1">1</a>, <a href="#rfc.xref.Part6.2">7.2</a>, <a href="#rfc.xref.Part6.3">7.4</a>, <a href="#Part6"><b>11.1</b></a><ul> |
|---|
| 1079 | <li><em>Section 16.5</em> <a href="#rfc.xref.Part6.2">7.2</a>, <a href="#rfc.xref.Part6.3">7.4</a></li> |
|---|
| [231] | 1080 | </ul> |
|---|
| 1081 | </li> |
|---|
| 1082 | </ul> |
|---|
| 1083 | </li> |
|---|
| [1099] | 1084 | <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul> |
|---|
| 1085 | <li><em>RFC2068</em> <a href="#rfc.xref.RFC2068.1">4.1</a>, <a href="#RFC2068"><b>11.2</b></a><ul> |
|---|
| 1086 | <li><em>Section 14.19</em> <a href="#rfc.xref.RFC2068.1">4.1</a></li> |
|---|
| [231] | 1087 | </ul> |
|---|
| 1088 | </li> |
|---|
| [1099] | 1089 | <li><em>RFC2119</em> <a href="#rfc.xref.RFC2119.1">1.1</a>, <a href="#RFC2119"><b>11.1</b></a></li> |
|---|
| 1090 | <li><em>RFC2616</em> <a href="#rfc.xref.RFC2616.1">1</a>, <a href="#RFC2616"><b>11.2</b></a>, <a href="#rfc.xref.RFC2616.2">B.1</a></li> |
|---|
| [231] | 1091 | </ul> |
|---|
| 1092 | </li> |
|---|
| [1099] | 1093 | <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul> |
|---|
| 1094 | <li>Status Codes |
|---|
| 1095 | <ul> |
|---|
| 1096 | <li>304 Not Modified <a href="#rfc.iref.s.1"><b>4.1</b></a></li> |
|---|
| 1097 | <li>412 Precondition Failed <a href="#rfc.iref.s.2"><b>4.2</b></a></li> |
|---|
| [231] | 1098 | </ul> |
|---|
| 1099 | </li> |
|---|
| 1100 | </ul> |
|---|
| 1101 | </li> |
|---|
| 1102 | </ul> |
|---|
| 1103 | </div> |
|---|
| [1099] | 1104 | <h1><a id="rfc.copyright" href="#rfc.copyright">Full Copyright Statement</a></h1> |
|---|
| 1105 | <p>Copyright © The IETF Trust (2008).</p> |
|---|
| 1106 | <p>This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the |
|---|
| 1107 | authors retain all their rights. |
|---|
| 1108 | </p> |
|---|
| 1109 | <p>This document and the information contained herein are provided on an “AS IS” basis and THE CONTRIBUTOR, THE ORGANIZATION |
|---|
| 1110 | HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE |
|---|
| 1111 | DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN |
|---|
| 1112 | WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. |
|---|
| 1113 | </p> |
|---|
| 1114 | <h1><a id="rfc.ipr" href="#rfc.ipr">Intellectual Property</a></h1> |
|---|
| 1115 | <p>The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might |
|---|
| 1116 | be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any |
|---|
| 1117 | license under such rights might or might not be available; nor does it represent that it has made any independent effort to |
|---|
| 1118 | identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and |
|---|
| 1119 | BCP 79. |
|---|
| 1120 | </p> |
|---|
| 1121 | <p>Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result |
|---|
| 1122 | of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users |
|---|
| 1123 | of this specification can be obtained from the IETF on-line IPR repository at <a href="http://www.ietf.org/ipr">http://www.ietf.org/ipr</a>. |
|---|
| 1124 | </p> |
|---|
| 1125 | <p>The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary |
|---|
| 1126 | rights that may cover technology that may be required to implement this standard. Please address the information to the IETF |
|---|
| 1127 | at <a href="mailto:ietf-ipr@ietf.org">ietf-ipr@ietf.org</a>. |
|---|
| 1128 | </p> |
|---|
| [231] | 1129 | </body> |
|---|
| 1130 | </html> |
|---|
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