Context Navigation

source: draft-ietf-httpbis/latest/p4-conditional.html @ 1860

Last change on this file since 1860 was 1860, checked in by fielding@…, 8 years ago
(editorial) move Vary to p2 because it is generated by origin servers and not limited to caching
Property svn:eol-style set to `native` Property svn:mime-type set to `text/html;charset=utf-8`
File size: 107.3 KB

Rev	Line
[52]	1	<!DOCTYPE html
	2	PUBLIC "-//W3C//DTD HTML 4.01//EN">
	3	<html lang="en">
[573]	4	<head profile="http://www.w3.org/2006/03/hcard http://dublincore.org/documents/2008/08/04/dc-html/">
[52]	5	<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
[1522]	6	<title>HTTP/1.1, part 4: Conditional Requests</title><script>
	7	var buttonsAdded = false;
	8
	9	function init() {
	10	var fb = document.createElement("div");
[1528]	11	fb.className = "feedback noprint";
[1522]	12	fb.setAttribute("onclick", "feedback();");
	13	fb.appendChild(document.createTextNode("feedback"));
	14
	15	var bodyl = document.getElementsByTagName("body");
	16	bodyl.item(0).appendChild(fb);
	17	}
	18
	19	function feedback() {
	20	toggleButtonsToElementsByName("h1");
	21	toggleButtonsToElementsByName("h2");
	22	toggleButtonsToElementsByName("h3");
	23	toggleButtonsToElementsByName("h4");
	24
	25	buttonsAdded = !buttonsAdded;
	26	}
	27
	28	function toggleButtonsToElementsByName(name) {
	29	var list = document.getElementsByTagName(name);
	30	for (var i = 0; i < list.length; i++) {
	31	toggleButton(list.item(i));
	32	}
	33	}
	34
	35	function toggleButton(node) {
	36	if (! buttonsAdded) {
	37
	38	// docname
	39	var template = "mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&body=<{ref}>:";
	40
	41	var id = node.getAttribute("id");
	42	// better id available?
	43	var titlelinks = node.getElementsByTagName("a");
	44	for (var i = 0; i < titlelinks.length; i++) {
	45	var tl = titlelinks.item(i);
	46	if (tl.getAttribute("id")) {
	47	id = tl.getAttribute("id");
	48	}
	49	}
	50
	51	// ref
	52	var ref = window.location.toString();
	53	var hash = ref.indexOf("#");
	54	if (hash != -1) {
	55	ref = ref.substring(0, hash);
	56	}
	57	if (id != "") {
	58	ref += "#" + id;
	59	}
	60
	61	// docname
	62	var docname = "draft-ietf-httpbis-p4-conditional-latest";
	63
	64	// section
	65	var section = node.textContent;
[1528]	66	section = section.replace("\u00a0", " ");
[1522]	67
	68	// build URI from template
	69	var uri = template.replace("{docname}", encodeURIComponent(docname));
	70	uri = uri.replace("{section}", encodeURIComponent(section));
	71	uri = uri.replace("{ref}", encodeURIComponent(ref));
	72
	73	var button = document.createElement("a");
[1528]	74	button.className = "fbbutton noprint";
[1522]	75	button.setAttribute("href", uri);
	76	button.appendChild(document.createTextNode("send feedback"));
	77	node.appendChild(button);
	78	}
	79	else {
	80	var buttons = node.getElementsByTagName("a");
	81	for (var i = 0; i < buttons.length; i++) {
	82	var b = buttons.item(i);
[1528]	83	if (b.className == "fbbutton noprint") {
[1522]	84	node.removeChild(b);
	85	}
	86	}
	87	}
	88	}</script><style type="text/css" title="Xml2Rfc (sans serif)">
[52]	89	a {
	90	text-decoration: none;
	91	}
	92	a.smpl {
	93	color: black;
	94	}
	95	a:hover {
	96	text-decoration: underline;
	97	}
	98	a:active {
	99	text-decoration: underline;
	100	}
	101	address {
	102	margin-top: 1em;
	103	margin-left: 2em;
	104	font-style: normal;
	105	}
	106	body {
	107	color: black;
	108	font-family: verdana, helvetica, arial, sans-serif;
	109	font-size: 10pt;
[1667]	110	margin-right: 2em;
[52]	111	}
	112	cite {
	113	font-style: normal;
	114	}
[563]	115	div.note {
	116	margin-left: 2em;
	117	}
[52]	118	dl {
	119	margin-left: 2em;
	120	}
[729]	121	ul.empty {
	122	list-style-type: none;
	123	}
	124	ul.empty li {
[52]	125	margin-top: .5em;
	126	}
	127	dl p {
	128	margin-left: 0em;
	129	}
	130	dt {
	131	margin-top: .5em;
	132	}
	133	h1 {
	134	font-size: 14pt;
	135	line-height: 21pt;
	136	page-break-after: avoid;
	137	}
	138	h1.np {
	139	page-break-before: always;
	140	}
	141	h1 a {
	142	color: #333333;
	143	}
	144	h2 {
	145	font-size: 12pt;
	146	line-height: 15pt;
	147	page-break-after: avoid;
	148	}
[446]	149	h3, h4, h5, h6 {
[52]	150	font-size: 10pt;
	151	page-break-after: avoid;
	152	}
[446]	153	h2 a, h3 a, h4 a, h5 a, h6 a {
[52]	154	color: black;
	155	}
	156	img {
	157	margin-left: 3em;
	158	}
	159	li {
	160	margin-left: 2em;
	161	}
	162	ol {
	163	margin-left: 2em;
	164	}
[1145]	165	ol.la {
	166	list-style-type: lower-alpha;
	167	}
	168	ol.ua {
	169	list-style-type: upper-alpha;
	170	}
[52]	171	ol p {
	172	margin-left: 0em;
	173	}
	174	p {
	175	margin-left: 2em;
	176	}
	177	pre {
	178	margin-left: 3em;
	179	background-color: lightyellow;
	180	padding: .25em;
[1667]	181	page-break-inside: avoid;
[52]	182	}
	183	pre.text2 {
	184	border-style: dotted;
	185	border-width: 1px;
	186	background-color: #f0f0f0;
	187	width: 69em;
	188	}
	189	pre.inline {
	190	background-color: white;
	191	padding: 0em;
	192	}
	193	pre.text {
	194	border-style: dotted;
	195	border-width: 1px;
	196	background-color: #f8f8f8;
	197	width: 69em;
	198	}
	199	pre.drawing {
	200	border-style: solid;
	201	border-width: 1px;
	202	background-color: #f8f8f8;
	203	padding: 2em;
	204	}
	205	table {
	206	margin-left: 2em;
	207	}
[253]	208	table.tt {
	209	vertical-align: top;
	210	}
	211	table.full {
	212	border-style: outset;
	213	border-width: 1px;
	214	}
	215	table.headers {
	216	border-style: outset;
	217	border-width: 1px;
	218	}
	219	table.tt td {
	220	vertical-align: top;
	221	}
	222	table.full td {
	223	border-style: inset;
	224	border-width: 1px;
	225	}
	226	table.tt th {
	227	vertical-align: top;
	228	}
	229	table.full th {
	230	border-style: inset;
	231	border-width: 1px;
	232	}
	233	table.headers th {
	234	border-style: none none inset none;
	235	border-width: 1px;
	236	}
[675]	237	table.left {
	238	margin-right: auto;
	239	}
	240	table.right {
	241	margin-left: auto;
	242	}
	243	table.center {
	244	margin-left: auto;
	245	margin-right: auto;
	246	}
[454]	247	caption {
	248	caption-side: bottom;
	249	font-weight: bold;
	250	font-size: 9pt;
	251	margin-top: .5em;
	252	}
	253
[52]	254	table.header {
[729]	255	border-spacing: 1px;
[52]	256	width: 95%;
	257	font-size: 10pt;
	258	color: white;
	259	}
	260	td.top {
	261	vertical-align: top;
	262	}
	263	td.topnowrap {
	264	vertical-align: top;
	265	white-space: nowrap;
	266	}
[729]	267	table.header td {
[52]	268	background-color: gray;
	269	width: 50%;
	270	}
[729]	271	table.header a {
[227]	272	color: white;
	273	}
[52]	274	td.reference {
	275	vertical-align: top;
	276	white-space: nowrap;
	277	padding-right: 1em;
	278	}
	279	thead {
	280	display:table-header-group;
	281	}
[1072]	282	ul.toc, ul.toc ul {
[52]	283	list-style: none;
	284	margin-left: 1.5em;
	285	padding-left: 0em;
	286	}
[1072]	287	ul.toc li {
[52]	288	line-height: 150%;
	289	font-weight: bold;
	290	font-size: 10pt;
	291	margin-left: 0em;
	292	}
[1072]	293	ul.toc li li {
[52]	294	line-height: normal;
	295	font-weight: normal;
	296	font-size: 9pt;
	297	margin-left: 0em;
	298	}
[1072]	299	li.excluded {
[52]	300	font-size: 0pt;
	301	}
	302	ul p {
	303	margin-left: 0em;
	304	}
[1095]	305	ul.ind, ul.ind ul {
[52]	306	list-style: none;
	307	margin-left: 1.5em;
	308	padding-left: 0em;
[440]	309	page-break-before: avoid;
[52]	310	}
[1072]	311	ul.ind li {
[52]	312	font-weight: bold;
	313	line-height: 200%;
	314	margin-left: 0em;
	315	}
[1072]	316	ul.ind li li {
[52]	317	font-weight: normal;
	318	line-height: 150%;
	319	margin-left: 0em;
	320	}
[662]	321	.avoidbreak {
	322	page-break-inside: avoid;
	323	}
[52]	324	.bcp14 {
	325	font-style: normal;
	326	text-transform: lowercase;
	327	font-variant: small-caps;
	328	}
	329	.comment {
	330	background-color: yellow;
	331	}
	332	.center {
	333	text-align: center;
	334	}
	335	.error {
	336	color: red;
	337	font-style: italic;
	338	font-weight: bold;
	339	}
	340	.figure {
	341	font-weight: bold;
	342	text-align: center;
	343	font-size: 9pt;
	344	}
	345	.filename {
	346	color: #333333;
	347	font-weight: bold;
	348	font-size: 12pt;
	349	line-height: 21pt;
	350	text-align: center;
	351	}
	352	.fn {
	353	font-weight: bold;
	354	}
	355	.hidden {
	356	display: none;
	357	}
	358	.left {
	359	text-align: left;
	360	}
	361	.right {
	362	text-align: right;
	363	}
	364	.title {
	365	color: #990000;
	366	font-size: 18pt;
	367	line-height: 18pt;
	368	font-weight: bold;
	369	text-align: center;
	370	margin-top: 36pt;
	371	}
	372	.vcardline {
	373	display: block;
	374	}
	375	.warning {
	376	font-size: 14pt;
	377	background-color: yellow;
	378	}
[1522]	379	.feedback {
	380	position: fixed;
	381	bottom: 1%;
	382	right: 1%;
	383	padding: 3px 5px;
	384	color: white;
	385	border-radius: 5px;
	386	background: #a00000;
	387	border: 1px solid silver;
	388	}
	389	.fbbutton {
	390	margin-left: 1em;
	391	color: #303030;
	392	font-size: small;
	393	font-weight: normal;
	394	background: #d0d000;
	395	padding: 1px 4px;
	396	border: 1px solid silver;
	397	border-radius: 5px;
	398	}
[52]	399
	400	@media print {
	401	.noprint {
	402	display: none;
	403	}
	404
	405	a {
	406	color: black;
	407	text-decoration: none;
	408	}
	409
	410	table.header {
	411	width: 90%;
	412	}
	413
	414	td.header {
	415	width: 50%;
	416	color: black;
	417	background-color: white;
	418	vertical-align: top;
	419	font-size: 12pt;
	420	}
	421
[1825]	422	ul.toc a:nth-child(2)::after {
[52]	423	content: leader('.') target-counter(attr(href), page);
	424	}
	425
[1072]	426	ul.ind li li a {
[52]	427	content: target-counter(attr(href), page);
	428	}
	429
	430	.print2col {
	431	column-count: 2;
[1528]	432	-moz-column-count: 2;
[52]	433	column-fill: auto;
	434	}
	435	}
	436
	437	@page {
	438	@top-left {
[754]	439	content: "Internet-Draft";
[52]	440	}
	441	@top-right {
[1845]	442	content: "September 2012";
[52]	443	}
	444	@top-center {
[120]	445	content: "HTTP/1.1, Part 4";
[52]	446	}
	447	@bottom-left {
	448	content: "Fielding, et al.";
	449	}
	450	@bottom-center {
[1860]	451	content: "Expires March 7, 2013";
[52]	452	}
	453	@bottom-right {
	454	content: "[Page " counter(page) "]";
	455	}
	456	}
	457
	458	@page:first {
	459	@top-left {
	460	content: normal;
	461	}
	462	@top-right {
	463	content: normal;
	464	}
	465	@top-center {
	466	content: normal;
	467	}
	468	}
	469	</style><link rel="Contents" href="#rfc.toc">
	470	<link rel="Author" href="#rfc.authors">
[754]	471	<link rel="Copyright" href="#rfc.copyrightnotice">
[52]	472	<link rel="Index" href="#rfc.index">
	473	<link rel="Chapter" title="1 Introduction" href="#rfc.section.1">
[1381]	474	<link rel="Chapter" title="2 Validators" href="#rfc.section.2">
[1253]	475	<link rel="Chapter" title="3 Precondition Header Fields" href="#rfc.section.3">
	476	<link rel="Chapter" title="4 Status Code Definitions" href="#rfc.section.4">
[1799]	477	<link rel="Chapter" title="5 Precedence" href="#rfc.section.5">
	478	<link rel="Chapter" title="6 IANA Considerations" href="#rfc.section.6">
	479	<link rel="Chapter" title="7 Security Considerations" href="#rfc.section.7">
	480	<link rel="Chapter" title="8 Acknowledgments" href="#rfc.section.8">
	481	<link rel="Chapter" href="#rfc.section.9" title="9 References">
[912]	482	<link rel="Appendix" title="A Changes from RFC 2616" href="#rfc.section.A">
[1805]	483	<link rel="Appendix" title="B Imported ABNF" href="#rfc.section.B">
	484	<link rel="Appendix" title="C Collected ABNF" href="#rfc.section.C">
	485	<link rel="Appendix" title="D Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.D">
[1810]	486	<link href="p2-semantics.html" rel="prev">
[1472]	487	<link href="p5-range.html" rel="next">
[1845]	488	<meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.588, 2012-08-25 12:28:24, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/">
[754]	489	<link rel="schema.dct" href="http://purl.org/dc/terms/">
	490	<meta name="dct.creator" content="Fielding, R.">
	491	<meta name="dct.creator" content="Lafon, Y.">
	492	<meta name="dct.creator" content="Reschke, J. F.">
	493	<meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p4-conditional-latest">
[1860]	494	<meta name="dct.issued" scheme="ISO8601" content="2012-09-03">
[754]	495	<meta name="dct.replaces" content="urn:ietf:rfc:2616">
[1783]	496	<meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating state changes, request header fields for making preconditions on such state, and rules for constructing the responses to a conditional request when one or more preconditions evaluate to false.">
	497	<meta name="description" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating state changes, request header fields for making preconditions on such state, and rules for constructing the responses to a conditional request when one or more preconditions evaluate to false.">
[52]	498	</head>
[1522]	499	<body onload="init();">
[729]	500	<table class="header">
	501	<tbody>
	502	<tr>
	503	<td class="left">HTTPbis Working Group</td>
	504	<td class="right">R. Fielding, Editor</td>
	505	</tr>
	506	<tr>
[754]	507	<td class="left">Internet-Draft</td>
[1106]	508	<td class="right">Adobe</td>
[729]	509	</tr>
	510	<tr>
[741]	511	<td class="left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2616">2616</a> (if approved)
[729]	512	</td>
[1586]	513	<td class="right">Y. Lafon, Editor</td>
[729]	514	</tr>
	515	<tr>
[741]	516	<td class="left">Intended status: Standards Track</td>
[729]	517	<td class="right">W3C</td>
	518	</tr>
	519	<tr>
[1860]	520	<td class="left">Expires: March 7, 2013</td>
[832]	521	<td class="right">J. Reschke, Editor</td>
[729]	522	</tr>
	523	<tr>
	524	<td class="left"></td>
	525	<td class="right">greenbytes</td>
	526	</tr>
	527	<tr>
	528	<td class="left"></td>
[1860]	529	<td class="right">September 3, 2012</td>
[729]	530	</tr>
	531	</tbody>
[52]	532	</table>
	533	<p class="title">HTTP/1.1, part 4: Conditional Requests<br><span class="filename">draft-ietf-httpbis-p4-conditional-latest</span></p>
[723]	534	<h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1>
[1373]	535	<p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information
[1783]	536	systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating state changes,
	537	request header fields for making preconditions on such state, and rules for constructing the responses to a conditional request
	538	when one or more preconditions evaluate to false.
[723]	539	</p>
	540	<h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1>
[1764]	541	<p>Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at <<a href="http://lists.w3.org/Archives/Public/ietf-http-wg/">http://lists.w3.org/Archives/Public/ietf-http-wg/</a>>.
[723]	542	</p>
[1268]	543	<p>The current issues list is at <<a href="http://tools.ietf.org/wg/httpbis/trac/report/3">http://tools.ietf.org/wg/httpbis/trac/report/3</a>> and related documents (including fancy diffs) can be found at <<a href="http://tools.ietf.org/wg/httpbis/">http://tools.ietf.org/wg/httpbis/</a>>.
	544	</p>
[1807]	545	<p>The changes in this draft are summarized in <a href="#changes.since.20" title="Since draft-ietf-httpbis-p4-conditional-20">Appendix D.2</a>.
[723]	546	</p>
[799]	547	<h1><a id="rfc.status" href="#rfc.status">Status of This Memo</a></h1>
	548	<p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p>
	549	<p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute
	550	working documents as Internet-Drafts. The list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>.
[52]	551	</p>
	552	<p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other
	553	documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work
	554	in progress”.
	555	</p>
[1860]	556	<p>This Internet-Draft will expire on March 7, 2013.</p>
[446]	557	<h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1>
[1497]	558	<p>Copyright © 2012 IETF Trust and the persons identified as the document authors. All rights reserved.</p>
[723]	559	<p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights
	560	and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License
[799]	561	text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified
	562	BSD License.
[446]	563	</p>
[723]	564	<p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November
	565	10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to
	566	allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s)
	567	controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative
	568	works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate
	569	it into languages other than English.
	570	</p>
[52]	571	<hr class="noprint">
	572	<h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1>
	573	<ul class="toc">
[1825]	574	<li><a href="#rfc.section.1">1.</a>   <a href="#introduction">Introduction</a><ul>
	575	<li><a href="#rfc.section.1.1">1.1</a>   <a href="#intro.conformance.and.error.handling">Conformance and Error Handling</a></li>
	576	<li><a href="#rfc.section.1.2">1.2</a>   <a href="#notation">Syntax Notation</a></li>
[1796]	577	</ul>
	578	</li>
[1825]	579	<li><a href="#rfc.section.2">2.</a>   <a href="#validators">Validators</a><ul>
	580	<li><a href="#rfc.section.2.1">2.1</a>   <a href="#weak.and.strong.validators">Weak versus Strong</a></li>
	581	<li><a href="#rfc.section.2.2">2.2</a>   <a href="#header.last-modified">Last-Modified</a><ul>
	582	<li><a href="#rfc.section.2.2.1">2.2.1</a>   <a href="#lastmod.generation">Generation</a></li>
	583	<li><a href="#rfc.section.2.2.2">2.2.2</a>   <a href="#lastmod.comparison">Comparison</a></li>
[424]	584	</ul>
	585	</li>
[1825]	586	<li><a href="#rfc.section.2.3">2.3</a>   <a href="#header.etag">ETag</a><ul>
	587	<li><a href="#rfc.section.2.3.1">2.3.1</a>   <a href="#entity.tag.generation">Generation</a></li>
	588	<li><a href="#rfc.section.2.3.2">2.3.2</a>   <a href="#entity.tag.comparison">Comparison</a></li>
	589	<li><a href="#rfc.section.2.3.3">2.3.3</a>   <a href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></li>
[1260]	590	</ul>
	591	</li>
[1825]	592	<li><a href="#rfc.section.2.4">2.4</a>   <a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity-tags and Last-Modified Dates</a></li>
[96]	593	</ul>
	594	</li>
[1825]	595	<li><a href="#rfc.section.3">3.</a>   <a href="#header.field.definitions">Precondition Header Fields</a><ul>
	596	<li><a href="#rfc.section.3.1">3.1</a>   <a href="#header.if-match">If-Match</a></li>
	597	<li><a href="#rfc.section.3.2">3.2</a>   <a href="#header.if-none-match">If-None-Match</a></li>
	598	<li><a href="#rfc.section.3.3">3.3</a>   <a href="#header.if-modified-since">If-Modified-Since</a></li>
	599	<li><a href="#rfc.section.3.4">3.4</a>   <a href="#header.if-unmodified-since">If-Unmodified-Since</a></li>
	600	<li><a href="#rfc.section.3.5">3.5</a>   <a href="#header.if-range">If-Range</a></li>
[52]	601	</ul>
	602	</li>
[1825]	603	<li><a href="#rfc.section.4">4.</a>   <a href="#status.code.definitions">Status Code Definitions</a><ul>
	604	<li><a href="#rfc.section.4.1">4.1</a>   <a href="#status.304">304 Not Modified</a></li>
	605	<li><a href="#rfc.section.4.2">4.2</a>   <a href="#status.412">412 Precondition Failed</a></li>
[52]	606	</ul>
	607	</li>
[1825]	608	<li><a href="#rfc.section.5">5.</a>   <a href="#precedence">Precedence</a></li>
	609	<li><a href="#rfc.section.6">6.</a>   <a href="#IANA.considerations">IANA Considerations</a><ul>
	610	<li><a href="#rfc.section.6.1">6.1</a>   <a href="#status.code.registration">Status Code Registration</a></li>
	611	<li><a href="#rfc.section.6.2">6.2</a>   <a href="#header.field.registration">Header Field Registration</a></li>
[253]	612	</ul>
	613	</li>
[1825]	614	<li><a href="#rfc.section.7">7.</a>   <a href="#security.considerations">Security Considerations</a></li>
	615	<li><a href="#rfc.section.8">8.</a>   <a href="#acks">Acknowledgments</a></li>
	616	<li><a href="#rfc.section.9">9.</a>   <a href="#rfc.references">References</a><ul>
	617	<li><a href="#rfc.section.9.1">9.1</a>   <a href="#rfc.references.1">Normative References</a></li>
	618	<li><a href="#rfc.section.9.2">9.2</a>   <a href="#rfc.references.2">Informative References</a></li>
[119]	619	</ul>
	620	</li>
[1072]	621	<li><a href="#rfc.authors">Authors' Addresses</a></li>
[1825]	622	<li><a href="#rfc.section.A">A.</a>   <a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
	623	<li><a href="#rfc.section.B">B.</a>   <a href="#imported.abnf">Imported ABNF</a></li>
	624	<li><a href="#rfc.section.C">C.</a>   <a href="#collected.abnf">Collected ABNF</a></li>
	625	<li><a href="#rfc.section.D">D.</a>   <a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul>
	626	<li><a href="#rfc.section.D.1">D.1</a>   <a href="#changes.since.19">Since draft-ietf-httpbis-p4-conditional-19</a></li>
	627	<li><a href="#rfc.section.D.2">D.2</a>   <a href="#changes.since.20">Since draft-ietf-httpbis-p4-conditional-20</a></li>
[115]	628	</ul>
	629	</li>
[1072]	630	<li><a href="#rfc.index">Index</a></li>
[52]	631	</ul>
	632	<h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a> <a id="introduction" href="#introduction">Introduction</a></h1>
[1803]	633	<p id="rfc.section.1.p.1">Conditional requests are HTTP requests <a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Semantics and Payloads">[Part2]</cite></a> that include one or more header fields indicating a precondition to be tested before applying the method semantics to the
[1783]	634	target resource. Each precondition is based on metadata that is expected to change if the selected representation of the target
	635	resource is changed. This document defines the HTTP/1.1 conditional request mechanisms in terms of the architecture, syntax
[1803]	636	notation, and conformance criteria defined in <a href="#Part1" id="rfc.xref.Part1.1"><cite title="HTTP/1.1, part 1: Message Routing and Syntax"">[Part1]</cite></a>.
[1783]	637	</p>
	638	<p id="rfc.section.1.p.2">Conditional GET requests are the most efficient mechanism for HTTP cache updates <a href="#Part6" id="rfc.xref.Part6.1"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, to prevent the "lost update" problem:
[1260]	639	one client accidentally overwriting the work of another client that has been acting in parallel.
[52]	640	</p>
[1783]	641	<p id="rfc.section.1.p.3">Conditional request preconditions are based on the state of the target resource as a whole (its current value set) or the
[1251]	642	state as observed in a previously obtained representation (one value in that set). A resource might have multiple current
	643	representations, each with its own observable state. The conditional request mechanisms assume that the mapping of requests
	644	to corresponding representations will be consistent over time if the server intends to take advantage of conditionals. Regardless,
	645	if the mapping is inconsistent and the server is unable to select the appropriate representation, then no harm will result
	646	when the precondition evaluates to false.
[1223]	647	</p>
[1783]	648	<p id="rfc.section.1.p.4"><span id="rfc.iref.s.1"></span> We use the term "<dfn>selected representation</dfn>" to refer to the current representation of the target resource that would have been selected in a successful response if
[1251]	649	the same request had used the method GET and had excluded all of the conditional request header fields. The conditional request
	650	preconditions are evaluated by comparing the values provided in the request header fields to the current metadata for the
	651	selected representation.
[163]	652	</p>
[1796]	653	<h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a> <a id="intro.conformance.and.error.handling" href="#intro.conformance.and.error.handling">Conformance and Error Handling</a></h2>
	654	<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"
	655	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>.
	656	</p>
	657	<p id="rfc.section.1.1.p.2">This specification targets conformance criteria according to the role of a participant in HTTP communication. Hence, HTTP
	658	requirements are placed on senders, recipients, clients, servers, user agents, intermediaries, origin servers, proxies, gateways,
[1803]	659	or caches, depending on what behavior is being constrained by the requirement. See <a href="p1-messaging.html#architecture" title="Architecture">Section 2</a> of <a href="#Part1" id="rfc.xref.Part1.2"><cite title="HTTP/1.1, part 1: Message Routing and Syntax"">[Part1]</cite></a> for definitions of these terms.
[1796]	660	</p>
	661	<p id="rfc.section.1.1.p.3">The verb "generate" is used instead of "send" where a requirement differentiates between creating a protocol element and merely
	662	forwarding a received element downstream.
	663	</p>
	664	<p id="rfc.section.1.1.p.4">An implementation is considered conformant if it complies with all of the requirements associated with the roles it partakes
	665	in HTTP. Note that SHOULD-level requirements are relevant here, unless one of the documented exceptions is applicable.
	666	</p>
	667	<p id="rfc.section.1.1.p.5">This document also uses ABNF to define valid protocol elements (<a href="#notation" title="Syntax Notation">Section 1.2</a>). In addition to the prose requirements placed upon them, senders <em class="bcp14">MUST NOT</em> generate protocol elements that do not match the grammar defined by the ABNF rules for those protocol elements that are applicable
	668	to the sender's role. If a received protocol element is processed, the recipient <em class="bcp14">MUST</em> be able to parse any value that would match the ABNF rules for that protocol element, excluding only those rules not applicable
	669	to the recipient's role.
	670	</p>
	671	<p id="rfc.section.1.1.p.6">Unless noted otherwise, a recipient <em class="bcp14">MAY</em> attempt to recover a usable protocol element from an invalid construct. HTTP does not define specific error handling mechanisms
	672	except when they have a direct impact on security, since different applications of the protocol require different error handling
	673	strategies. For example, a Web browser might wish to transparently recover from a response where the <a href="p2-semantics.html#header.location" class="smpl">Location</a> header field doesn't parse according to the ABNF, whereas a systems control client might consider any form of error recovery
	674	to be dangerous.
	675	</p>
	676	<h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a> <a id="notation" href="#notation">Syntax Notation</a></h2>
[1805]	677	<p id="rfc.section.1.2.p.1">This specification uses the Augmented Backus-Naur Form (ABNF) notation of <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with the list rule extension defined in <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</a> of <a href="#Part1" id="rfc.xref.Part1.3"><cite title="HTTP/1.1, part 1: Message Routing and Syntax"">[Part1]</cite></a>. <a href="#imported.abnf" title="Imported ABNF">Appendix B</a> describes rules imported from other documents. <a href="#collected.abnf" title="Collected ABNF">Appendix C</a> shows the collected ABNF with the list rule expanded.
[1796]	678	</p>
[1805]	679	<div id="rfc.iref.m.1"></div>
[1253]	680	<div id="rfc.iref.v.1"></div>
[1381]	681	<h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a> <a id="validators" href="#validators">Validators</a></h1>
[1251]	682	<p id="rfc.section.2.p.1">This specification defines two forms of metadata that are commonly used to observe resource state and test for preconditions:
[1733]	683	modification dates (<a href="#header.last-modified" id="rfc.xref.header.last-modified.1" title="Last-Modified">Section 2.2</a>) and opaque entity tags (<a href="#header.etag" id="rfc.xref.header.etag.1" title="ETag">Section 2.3</a>). Additional metadata that reflects resource state has been defined by various extensions of HTTP, such as WebDAV <a href="#RFC4918" id="rfc.xref.RFC4918.1"><cite title="HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)">[RFC4918]</cite></a>, that are beyond the scope of this specification. A resource metadata value is referred to as a "<dfn>validator</dfn>" when it is used within a precondition.
[424]	684	</p>
[1381]	685	<div id="rfc.iref.v.2"></div>
	686	<div id="rfc.iref.v.3"></div>
	687	<h2 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a> <a id="weak.and.strong.validators" href="#weak.and.strong.validators">Weak versus Strong</a></h2>
	688	<p id="rfc.section.2.1.p.1">Validators come in two flavors: strong or weak. Weak validators are easy to generate but are far less useful for comparisons.
	689	Strong validators are ideal for comparisons but can be very difficult (and occasionally impossible) to generate efficiently.
	690	Rather than impose that all forms of resource adhere to the same strength of validator, HTTP exposes the type of validator
	691	in use and imposes restrictions on when weak validators can be used as preconditions.
	692	</p>
	693	<p id="rfc.section.2.1.p.2">A "strong validator" is a representation metadata value that <em class="bcp14">MUST</em> be changed to a new, previously unused or guaranteed unique, value whenever a change occurs to the representation data such
[1740]	694	that a change would be observable in the payload body of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to GET.
	695	</p>
	696	<p id="rfc.section.2.1.p.3">A strong validator <em class="bcp14">MAY</em> be changed for other reasons, such as when a semantically significant part of the representation metadata is changed (e.g., <a href="p2-semantics.html#header.content-type" class="smpl">Content-Type</a>), but it is in the best interests of the origin server to only change the value when it is necessary to invalidate the stored
	697	responses held by remote caches and authoring tools. A strong validator <em class="bcp14">MUST</em> be unique across all representations of a given resource, such that no two representations of that resource share the same
[1381]	698	validator unless their payload body would be identical.
	699	</p>
[1740]	700	<p id="rfc.section.2.1.p.4">Cache entries might persist for arbitrarily long periods, regardless of expiration times. Thus, a cache might attempt to validate
[1381]	701	an entry using a validator that it obtained in the distant past. A strong validator <em class="bcp14">MUST</em> be unique across all versions of all representations associated with a particular resource over time. However, there is no
	702	implication of uniqueness across representations of different resources (i.e., the same strong validator might be in use for
	703	representations of multiple resources at the same time and does not imply that those representations are equivalent).
	704	</p>
[1740]	705	<p id="rfc.section.2.1.p.5">There are a variety of strong validators used in practice. The best are based on strict revision control, wherein each change
[1381]	706	to a representation always results in a unique node name and revision identifier being assigned before the representation
[1801]	707	is made accessible to GET. A collision-resistant hash function applied to the representation data is also sufficient if the
	708	data is available prior to the response header fields being sent and the digest does not need to be recalculated every time
	709	a validation request is received. However, if a resource has distinct representations that differ only in their metadata,
	710	such as might occur with content negotiation over media types that happen to share the same data format, then the origin server <em class="bcp14">SHOULD</em> incorporate additional information in the validator to distinguish those representations and avoid confusing cache behavior.
[1381]	711	</p>
[1740]	712	<p id="rfc.section.2.1.p.6">In contrast, a "weak validator" is a representation metadata value that might not be changed for every change to the representation
[1381]	713	data. This weakness might be due to limitations in how the value is calculated, such as clock resolution or an inability to
	714	ensure uniqueness for all possible representations of the resource, or due to a desire by the resource owner to group representations
[1691]	715	by some self-determined set of equivalency rather than unique sequences of data. An origin server <em class="bcp14">SHOULD</em> change a weak entity-tag whenever it considers prior representations to be unacceptable as a substitute for the current representation.
	716	In other words, a weak entity-tag ought to change whenever the origin server wants caches to invalidate old responses.
[1381]	717	</p>
[1740]	718	<p id="rfc.section.2.1.p.7">For example, the representation of a weather report that changes in content every second, based on dynamic measurements, might
[1381]	719	be grouped into sets of equivalent representations (from the origin server's perspective) with the same weak validator in
	720	order to allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server
	721	load or weather quality). Likewise, a representation's modification time, if defined with only one-second resolution, might
	722	be a weak validator if it is possible for the representation to be modified twice during a single second and retrieved between
	723	those modifications.
	724	</p>
[1740]	725	<p id="rfc.section.2.1.p.8">A "use" of a validator occurs when either a client generates a request and includes the validator in a precondition or when
[1381]	726	a server compares two validators. Weak validators are only usable in contexts that do not depend on exact equality of a representation's
	727	payload body. Strong validators are usable and preferred for all conditional requests, including cache validation, partial
	728	content ranges, and "lost update" avoidance.
	729	</p>
[1253]	730	<div id="rfc.iref.l.1"></div>
	731	<div id="rfc.iref.h.1"></div>
[1381]	732	<h2 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a> <a id="header.last-modified" href="#header.last-modified">Last-Modified</a></h2>
	733	<p id="rfc.section.2.2.p.1">The "Last-Modified" header field indicates the date and time at which the origin server believes the selected representation
[1260]	734	was last modified.
[205]	735	</p>
[1805]	736	<div id="rfc.figure.u.1"></div><pre class="inline"><span id="rfc.iref.g.1"></span> <a href="#header.last-modified" class="smpl">Last-Modified</a> = <a href="#imported.abnf" class="smpl">HTTP-date</a>
[1381]	737	</pre><p id="rfc.section.2.2.p.3">An example of its use is</p>
[1805]	738	<div id="rfc.figure.u.2"></div><pre class="text"> Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
[1381]	739	</pre><h3 id="rfc.section.2.2.1"><a href="#rfc.section.2.2.1">2.2.1</a> <a id="lastmod.generation" href="#lastmod.generation">Generation</a></h3>
	740	<p id="rfc.section.2.2.1.p.1">Origin servers <em class="bcp14">SHOULD</em> send Last-Modified for any selected representation for which a last modification date can be reasonably and consistently determined,
[1260]	741	since its use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.2"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) results in a substantial reduction of HTTP traffic on the Internet and can be a significant factor in improving service
	742	scalability and reliability.
	743	</p>
[1381]	744	<p id="rfc.section.2.2.1.p.2">A representation is typically the sum of many parts behind the resource interface. The last-modified time would usually be
[1253]	745	the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation
	746	detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can
	747	use its value to make conditional requests and test the validity of locally cached responses.
	748	</p>
[1740]	749	<p id="rfc.section.2.2.1.p.3">An origin server <em class="bcp14">SHOULD</em> obtain the Last-Modified value of the representation as close as possible to the time that it generates the <a href="p2-semantics.html#header.date" class="smpl">Date</a> field value for its response. This allows a recipient to make an accurate assessment of the representation's modification
	750	time, especially if the representation changes near the time that the response is generated.
[1253]	751	</p>
[1740]	752	<p id="rfc.section.2.2.1.p.4">An origin server with a clock <em class="bcp14">MUST NOT</em> send a Last-Modified date that is later than the server's time of message origination (<a href="p2-semantics.html#header.date" class="smpl">Date</a>). If the last modification time is derived from implementation-specific metadata that evaluates to some time in the future,
	753	according to the origin server's clock, then the origin server <em class="bcp14">MUST</em> replace that value with the message origination date. This prevents a future modification date from having an adverse impact
[1260]	754	on cache validation.
[1253]	755	</p>
[1436]	756	<p id="rfc.section.2.2.1.p.5">An origin server without a clock <em class="bcp14">MUST NOT</em> assign Last-Modified values to a response unless these values were associated with the resource by some other system or user
	757	with a reliable clock.
	758	</p>
[1381]	759	<h3 id="rfc.section.2.2.2"><a href="#rfc.section.2.2.2">2.2.2</a> <a id="lastmod.comparison" href="#lastmod.comparison">Comparison</a></h3>
	760	<p id="rfc.section.2.2.2.p.1">A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is
[1260]	761	strong, using the following rules:
[1253]	762	</p>
[1260]	763	<ul>
	764	<li>The validator is being compared by an origin server to the actual current validator for the representation and,</li>
	765	<li>That origin server reliably knows that the associated representation did not change twice during the second covered by the
	766	presented validator.
	767	</li>
	768	</ul>
[1381]	769	<p id="rfc.section.2.2.2.p.2">or </p>
[1260]	770	<ul>
[1739]	771	<li>The validator is about to be used by a client in an <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a>, <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> header field, because the client has a cache entry, or <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> for the associated representation, and
[1260]	772	</li>
[1740]	773	<li>That cache entry includes a <a href="p2-semantics.html#header.date" class="smpl">Date</a> value, which gives the time when the origin server sent the original response, and
	774	</li>
[1260]	775	<li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
	776	</ul>
[1381]	777	<p id="rfc.section.2.2.2.p.3">or </p>
[1260]	778	<ul>
	779	<li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the representation,
	780	and
	781	</li>
[1740]	782	<li>That cache entry includes a <a href="p2-semantics.html#header.date" class="smpl">Date</a> value, which gives the time when the origin server sent the original response, and
	783	</li>
[1260]	784	<li>The presented Last-Modified time is at least 60 seconds before the Date value.</li>
	785	</ul>
[1381]	786	<p id="rfc.section.2.2.2.p.4">This method relies on the fact that if two different responses were sent by the origin server during the same second, but
[1740]	787	both had the same Last-Modified time, then at least one of those responses would have a <a href="p2-semantics.html#header.date" class="smpl">Date</a> value equal to its Last-Modified time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified
	788	values are generated from 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.
[1253]	789	</p>
	790	<div id="rfc.iref.e.1"></div>
	791	<div id="rfc.iref.h.2"></div>
[1381]	792	<h2 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a> <a id="header.etag" href="#header.etag">ETag</a></h2>
[1772]	793	<p id="rfc.section.2.3.p.1">The "ETag" header field provides the current entity-tag for the selected representation. An entity-tag is an opaque validator
[1260]	794	for differentiating between multiple representations of the same resource, regardless of whether those multiple representations
	795	are due to resource state changes over time, content negotiation resulting in multiple representations being valid at the
	796	same time, or both. An entity-tag consists of an opaque quoted string, possibly prefixed by a weakness indicator.
[800]	797	</p>
[1805]	798	<div id="rfc.figure.u.3"></div><pre class="inline"><span id="rfc.iref.g.2"></span><span id="rfc.iref.g.3"></span><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span> <a href="#header.etag" class="smpl">ETag</a> = <a href="#header.etag" class="smpl">entity-tag</a>
[1260]	799
	800	<a href="#header.etag" class="smpl">entity-tag</a> = [ <a href="#header.etag" class="smpl">weak</a> ] <a href="#header.etag" class="smpl">opaque-tag</a>
	801	<a href="#header.etag" class="smpl">weak</a> = %x57.2F ; "W/", case-sensitive
[1805]	802	<a href="#header.etag" class="smpl">opaque-tag</a> = <a href="#imported.abnf" class="smpl">DQUOTE</a> *<a href="#header.etag" class="smpl">etagc</a> <a href="#imported.abnf" class="smpl">DQUOTE</a>
	803	<a href="#header.etag" class="smpl">etagc</a> = %x21 / %x23-7E / <a href="#imported.abnf" class="smpl">obs-text</a>
	804	; <a href="#imported.abnf" class="smpl">VCHAR</a> except double quotes, plus obs-text
[1470]	805	</pre><div class="note" id="rfc.section.2.3.p.3">
	806	<p> <b>Note:</b> Previously, opaque-tag was defined to be a quoted-string (<a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>, <a href="http://tools.ietf.org/html/rfc2616#section-3.11">Section 3.11</a>), thus some recipients might perform backslash unescaping. Servers therefore ought to avoid backslash characters in entity
	807	tags.
	808	</p>
	809	</div>
	810	<p id="rfc.section.2.3.p.4">An entity-tag can be more reliable for validation than a modification date in situations where it is inconvenient to store
[1260]	811	modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are
	812	not consistently maintained.
	813	</p>
[1805]	814	<div id="rfc.figure.u.4"></div>
[1253]	815	<p>Examples:</p> <pre class="text"> ETag: "xyzzy"
	816	ETag: W/"xyzzy"
	817	ETag: ""
[1470]	818	</pre><p id="rfc.section.2.3.p.6">An entity-tag can be either a weak or strong validator, with strong being the default. If an origin server provides an entity-tag
[1381]	819	for a representation and the generation of that entity-tag does not satisfy the requirements for a strong validator (<a href="#weak.and.strong.validators" title="Weak versus Strong">Section 2.1</a>), then that entity-tag <em class="bcp14">MUST</em> be marked as weak by prefixing its opaque value with "W/" (case-sensitive).
	820	</p>
	821	<h3 id="rfc.section.2.3.1"><a href="#rfc.section.2.3.1">2.3.1</a> <a id="entity.tag.generation" href="#entity.tag.generation">Generation</a></h3>
	822	<p id="rfc.section.2.3.1.p.1">The principle behind entity-tags is that only the service author knows the implementation of a resource well enough to select
[1260]	823	the most accurate and efficient validation mechanism for that resource, and that any such mechanism can be mapped to a simple
	824	sequence of octets for easy comparison. Since the value is opaque, there is no need for the client to be aware of how each
	825	entity-tag is constructed.
[1253]	826	</p>
[1381]	827	<p id="rfc.section.2.3.1.p.2">For example, a resource that has implementation-specific versioning applied to all changes might use an internal revision
[1260]	828	number, perhaps combined with a variance identifier for content negotiation, to accurately differentiate between representations.
[1801]	829	Other implementations might use a collision-resistant hash of representation content, a combination of various filesystem
	830	attributes, or a modification timestamp that has sub-second resolution.
[1253]	831	</p>
[1381]	832	<p id="rfc.section.2.3.1.p.3">Origin servers <em class="bcp14">SHOULD</em> send ETag for any selected representation for which detection of changes can be reasonably and consistently determined, since
[1260]	833	the entity-tag's use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.3"><cite title="HTTP/1.1, part 6: Caching">[Part6]</cite></a>) can result in a substantial reduction of HTTP network traffic and can be a significant factor in improving service scalability
	834	and reliability.
[1253]	835	</p>
[1381]	836	<h3 id="rfc.section.2.3.2"><a href="#rfc.section.2.3.2">2.3.2</a> <a id="entity.tag.comparison" href="#entity.tag.comparison">Comparison</a></h3>
	837	<p id="rfc.section.2.3.2.p.1">There are two entity-tag comparison functions, depending on whether the comparison context allows the use of weak validators
[1260]	838	or not:
	839	</p>
[52]	840	<ul>
[298]	841	<li>The strong comparison function: in order to be considered equal, both opaque-tags <em class="bcp14">MUST</em> be identical character-by-character, and both <em class="bcp14">MUST NOT</em> be weak.
[52]	842	</li>
[610]	843	<li>The weak comparison function: in order to be considered equal, both opaque-tags <em class="bcp14">MUST</em> be identical character-by-character, but either or both of them <em class="bcp14">MAY</em> be tagged as "weak" without affecting the result.
[52]	844	</li>
	845	</ul>
[1381]	846	<p id="rfc.section.2.3.2.p.2">The example below shows the results for a set of entity-tag pairs, and both the weak and strong comparison function results:</p>
[298]	847	<div id="rfc.table.u.1">
[675]	848	<table class="tt full left" cellpadding="3" cellspacing="0">
[298]	849	<thead>
	850	<tr>
	851	<th>ETag 1</th>
	852	<th>ETag 2</th>
	853	<th>Strong Comparison</th>
	854	<th>Weak Comparison</th>
	855	</tr>
	856	</thead>
	857	<tbody>
	858	<tr>
[704]	859	<td class="left">W/"1"</td>
	860	<td class="left">W/"1"</td>
	861	<td class="left">no match</td>
	862	<td class="left">match</td>
[298]	863	</tr>
	864	<tr>
[704]	865	<td class="left">W/"1"</td>
	866	<td class="left">W/"2"</td>
	867	<td class="left">no match</td>
	868	<td class="left">no match</td>
[298]	869	</tr>
	870	<tr>
[704]	871	<td class="left">W/"1"</td>
	872	<td class="left">"1"</td>
	873	<td class="left">no match</td>
	874	<td class="left">match</td>
[298]	875	</tr>
	876	<tr>
[704]	877	<td class="left">"1"</td>
	878	<td class="left">"1"</td>
	879	<td class="left">match</td>
	880	<td class="left">match</td>
[298]	881	</tr>
	882	</tbody>
	883	</table>
	884	</div>
[1381]	885	<h3 id="rfc.section.2.3.3"><a href="#rfc.section.2.3.3">2.3.3</a> <a id="example.entity.tag.vs.conneg" href="#example.entity.tag.vs.conneg">Example: Entity-tags varying on Content-Negotiated Resources</a></h3>
[1860]	886	<p id="rfc.section.2.3.3.p.1">Consider a resource that is subject to content negotiation (<a href="p2-semantics.html#content.negotiation" title="Content Negotiation">Section 3.5</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="HTTP/1.1, part 2: Semantics and Payloads">[Part2]</cite></a>), and where the representations returned upon a GET request vary based on the <a href="p2-semantics.html#header.accept-encoding" class="smpl">Accept-Encoding</a> request header field (<a href="p2-semantics.html#header.accept-encoding" title="Accept-Encoding">Section 6.3.4</a> of <a href="#Part2" id="rfc.xref.Part2.3"><cite title="HTTP/1.1, part 2: Semantics and Payloads">[Part2]</cite></a>):
[1381]	887	</p>
[1805]	888	<div id="rfc.figure.u.5"></div>
[1381]	889	<p>>> Request:</p><pre class="text2">GET /index HTTP/1.1
	890	Host: www.example.com
	891	Accept-Encoding: gzip
	892
	893	</pre><p id="rfc.section.2.3.3.p.3">In this case, the response might or might not use the gzip content coding. If it does not, the response might look like:</p>
[1805]	894	<div id="rfc.figure.u.6"></div>
[1381]	895	<p>>> Response:</p><pre class="text">HTTP/1.1 200 OK
	896	Date: Thu, 26 Mar 2010 00:05:00 GMT
	897	ETag: "123-a"
	898	Content-Length: 70
	899	Vary: Accept-Encoding
	900	Content-Type: text/plain
	901
	902	<span id="exbody">Hello World!
	903	Hello World!
	904	Hello World!
	905	Hello World!
	906	Hello World!
	907	</span></pre><p id="rfc.section.2.3.3.p.5">An alternative representation that does use gzip content coding would be:</p>
[1805]	908	<div id="rfc.figure.u.7"></div>
[1381]	909	<p>>> Response:</p><pre class="text">HTTP/1.1 200 OK
	910	Date: Thu, 26 Mar 2010 00:05:00 GMT
	911	ETag: "123-b"
	912	Content-Length: 43
	913	Vary: Accept-Encoding
	914	Content-Type: text/plain
	915	Content-Encoding: gzip
	916
	917	<em>...binary data...</em></pre><div class="note" id="rfc.section.2.3.3.p.7">
[1604]	918	<p> <b>Note:</b> Content codings are a property of the representation, so therefore an entity-tag of an encoded representation has to be distinct
[1381]	919	from an unencoded representation to prevent conflicts during cache updates and range requests. In contrast, transfer codings
[1805]	920	(<a href="p1-messaging.html#transfer.codings" title="Transfer Codings">Section 4</a> of <a href="#Part1" id="rfc.xref.Part1.4"><cite title="HTTP/1.1, part 1: Message Routing and Syntax"">[Part1]</cite></a>) apply only during message transfer and do not require distinct entity-tags.
[1381]	921	</p>
	922	</div>
	923	<h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a> <a id="rules.for.when.to.use.entity.tags.and.last-modified.dates" href="#rules.for.when.to.use.entity.tags.and.last-modified.dates">Rules for When to Use Entity-tags and Last-Modified Dates</a></h2>
	924	<p id="rfc.section.2.4.p.1">We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types
[52]	925	ought to be used, and for what purposes.
	926	</p>
[1381]	927	<p id="rfc.section.2.4.p.2">HTTP/1.1 origin servers: </p>
[52]	928	<ul>
[875]	929	<li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one.
[52]	930	</li>
[875]	931	<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,
	932	or if it is unfeasible to send a strong entity-tag.
[52]	933	</li>
[1739]	934	<li><em class="bcp14">SHOULD</em> send a <a href="#header.last-modified" class="smpl">Last-Modified</a> value if it is feasible to send one.
[52]	935	</li>
	936	</ul>
[1739]	937	<p id="rfc.section.2.4.p.3">In other words, the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity-tag and a <a href="#header.last-modified" class="smpl">Last-Modified</a> value.
[52]	938	</p>
[1381]	939	<p id="rfc.section.2.4.p.4">HTTP/1.1 clients: </p>
[52]	940	<ul>
[1739]	941	<li><em class="bcp14">MUST</em> use that entity-tag in any cache-conditional request (using <a href="#header.if-match" class="smpl">If-Match</a> or <a href="#header.if-none-match" class="smpl">If-None-Match</a>) if an entity-tag has been provided by the origin server.
[52]	942	</li>
[1739]	943	<li><em class="bcp14">SHOULD</em> use the <a href="#header.last-modified" class="smpl">Last-Modified</a> value in non-subrange cache-conditional requests (using <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a>) if only a Last-Modified value has been provided by the origin server.
[52]	944	</li>
[1739]	945	<li><em class="bcp14">MAY</em> use the <a href="#header.last-modified" class="smpl">Last-Modified</a> value in subrange cache-conditional requests (using <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a>) if only a Last-Modified value has been provided by an HTTP/1.0 origin server. The user agent <em class="bcp14">SHOULD</em> provide a way to disable this, in case of difficulty.
[52]	946	</li>
[1739]	947	<li><em class="bcp14">SHOULD</em> use both validators in cache-conditional requests if both an entity-tag and a <a href="#header.last-modified" class="smpl">Last-Modified</a> value have been provided by the origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately.
[52]	948	</li>
	949	</ul>
[1739]	950	<p id="rfc.section.2.4.p.5">An HTTP/1.1 origin server, upon receiving a conditional request that includes both a Last-Modified date (e.g., in an <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> or <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> header field) and one or more entity-tags (e.g., in an <a href="#header.if-match" class="smpl">If-Match</a>, <a href="#header.if-none-match" class="smpl">If-None-Match</a>, or <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> header field) as cache validators, <em class="bcp14">MUST NOT</em> return a response status code of <a href="#status.304" class="smpl">304 (Not Modified)</a> unless doing so is consistent with all of the conditional header fields in the request.
[52]	951	</p>
[1381]	952	<p id="rfc.section.2.4.p.6">An HTTP/1.1 caching proxy, upon receiving a conditional request that includes both a Last-Modified date and one or more entity-tags
[875]	953	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
[52]	954	fields in the request.
	955	</p>
[729]	956	<ul class="empty">
[970]	957	<li> <b>Note:</b> The general principle behind these rules is that HTTP/1.1 servers and clients ought to transmit as much non-redundant information
[52]	958	as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative
	959	assumptions about the validators they receive.
[729]	960	</li>
[1109]	961	<li>HTTP/1.0 clients and caches might ignore entity-tags. Generally, last-modified values received or used by these systems will
[1703]	962	support transparent and efficient caching, and so HTTP/1.1 origin servers still ought to provide Last-Modified values.
[729]	963	</li>
	964	</ul>
[1415]	965	<h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a> <a id="header.field.definitions" href="#header.field.definitions">Precondition Header Fields</a></h1>
[1799]	966	<p id="rfc.section.3.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields for applying preconditions on requests. <a href="#precedence" title="Precedence">Section 5</a> defines the order of evaluation when more than one precondition is present in a request.
	967	</p>
[183]	968	<div id="rfc.iref.i.1"></div>
[1253]	969	<div id="rfc.iref.h.3"></div>
	970	<h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a> <a id="header.if-match" href="#header.if-match">If-Match</a></h2>
[1691]	971	<p id="rfc.section.3.1.p.1">The "If-Match" header field can be used to make a request method conditional on the current existence or value of an entity-tag
[1757]	972	for one or more representations of the target resource.
[52]	973	</p>
[1757]	974	<p id="rfc.section.3.1.p.2">If-Match is generally useful for resource update requests, such as PUT requests, as a means for protecting against accidental
	975	overwrites when multiple clients are acting in parallel on the same resource (i.e., the "lost update" problem). An If-Match
	976	field-value of "*" places the precondition on the existence of any current representation for the target resource.
	977	</p>
[1805]	978	<div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.7"></span> <a href="#header.if-match" class="smpl">If-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a>
[1761]	979	</pre><p id="rfc.section.3.1.p.4">The If-Match condition is met if and only if any of the entity-tags listed in the If-Match field value match the entity-tag
	980	of the selected representation for the target resource (as per <a href="#entity.tag.comparison" title="Comparison">Section 2.3.2</a>), or if "*" is given and any current representation exists for the target resource.
[52]	981	</p>
[1761]	982	<p id="rfc.section.3.1.p.5">If the condition is met, the server <em class="bcp14">MAY</em> perform the request method as if the If-Match header field was not present.
[52]	983	</p>
[1761]	984	<p id="rfc.section.3.1.p.6">Origin servers <em class="bcp14">MUST NOT</em> perform the requested method if the condition is not met; instead they <em class="bcp14">MUST</em> respond with the <a href="#status.412" class="smpl">412 (Precondition
	985	Failed)</a> status code.
[1696]	986	</p>
[1761]	987	<p id="rfc.section.3.1.p.7">Proxy servers using a cached response as the selected representation <em class="bcp14">MUST NOT</em> perform the requested method if the condition is not met; instead, they <em class="bcp14">MUST</em> forward the request towards the origin server.
[52]	988	</p>
[1761]	989	<p id="rfc.section.3.1.p.8">If the request would, without the If-Match header field, result in anything other than a <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> or <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code, then the If-Match header field <em class="bcp14">MUST</em> be ignored.
	990	</p>
	991	<p id="rfc.section.3.1.p.9">Examples:</p>
[1805]	992	<div id="rfc.figure.u.9"></div><pre class="text"> If-Match: "xyzzy"
[362]	993	If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
	994	If-Match: *
[1799]	995	</pre><div id="rfc.iref.i.2"></div>
[1253]	996	<div id="rfc.iref.h.4"></div>
	997	<h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a> <a id="header.if-none-match" href="#header.if-none-match">If-None-Match</a></h2>
[1691]	998	<p id="rfc.section.3.2.p.1">The "If-None-Match" header field can be used to make a request method conditional on not matching any of the current entity-tag
[1757]	999	values for representations of the target resource.
[1253]	1000	</p>
[1757]	1001	<p id="rfc.section.3.2.p.2">If-None-Match is primarily used in conditional GET requests to enable efficient updates of cached information with a minimum
	1002	amount of transaction overhead. A client that has one or more representations previously obtained from the target resource
	1003	can send If-None-Match with a list of the associated entity-tags in the hope of receiving a <a href="#status.304" class="smpl">304 (Not
	1004	Modified)</a> response if at least one of those representations matches the selected representation.
	1005	</p>
	1006	<p id="rfc.section.3.2.p.3">If-None-Match can also be used with a value of "*" to prevent an unsafe request method (e.g., PUT) from inadvertently modifying
[1691]	1007	an existing representation of the target resource when the client believes that the resource does not have a current representation.
	1008	This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial representation
[1253]	1009	for the target resource.
	1010	</p>
[1805]	1011	<div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.8"></span> <a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / 1#<a href="#header.etag" class="smpl">entity-tag</a>
[1761]	1012	</pre><p id="rfc.section.3.2.p.5">The If-None-Match condition is met if and only if none of the entity-tags listed in the If-None-Match field value match the
	1013	entity-tag of the selected representation for the target resource (as per <a href="#entity.tag.comparison" title="Comparison">Section 2.3.2</a>), or if "*" is given and no current representation exists for that resource.
[1253]	1014	</p>
[1761]	1015	<p id="rfc.section.3.2.p.6">If the condition is not met, the server <em class="bcp14">MUST NOT</em> perform the requested method. Instead, if the request method was GET or HEAD, the server <em class="bcp14">SHOULD</em> respond with a <a href="#status.304" class="smpl">304 (Not Modified)</a> status code, including the cache-related header fields (particularly <a href="#header.etag" class="smpl">ETag</a>) of the selected representation that has a matching entity-tag. For all other request methods, the server <em class="bcp14">MUST</em> respond with a <a href="#status.412" class="smpl">412 (Precondition
	1016	Failed)</a> status code.
[1253]	1017	</p>
[1761]	1018	<p id="rfc.section.3.2.p.7">If the condition is met, 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 <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> header field(s) in the request. That is, if no entity-tags match, then the server <em class="bcp14">MUST NOT</em> return a <a href="#status.304" class="smpl">304
	1019	(Not Modified)</a> response.
[1253]	1020	</p>
[1761]	1021	<p id="rfc.section.3.2.p.8">If the request would, without the If-None-Match header field, result in anything other than a <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> or <a href="#status.304" class="smpl">304 (Not Modified)</a> status code, then the If-None-Match header field <em class="bcp14">MUST</em> be ignored. (See <a href="#rules.for.when.to.use.entity.tags.and.last-modified.dates" title="Rules for When to Use Entity-tags and Last-Modified Dates">Section 2.4</a> for a discussion of server behavior when both <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> and If-None-Match appear in the same request.)
	1022	</p>
	1023	<p id="rfc.section.3.2.p.9">Examples:</p>
[1805]	1024	<div id="rfc.figure.u.11"></div><pre class="text"> If-None-Match: "xyzzy"
[1253]	1025	If-None-Match: W/"xyzzy"
	1026	If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
	1027	If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
	1028	If-None-Match: *
[1799]	1029	</pre><div id="rfc.iref.i.3"></div>
[1253]	1030	<div id="rfc.iref.h.5"></div>
	1031	<h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a> <a id="header.if-modified-since" href="#header.if-modified-since">If-Modified-Since</a></h2>
[1799]	1032	<p id="rfc.section.3.3.p.1">The "If-Modified-Since" header field can be used with GET or HEAD to make the method conditional by modification date: if
	1033	the selected representation has not been modified since the time specified in this field, then do not perform the request
	1034	method; instead, respond as detailed below.
[52]	1035	</p>
[1805]	1036	<div id="rfc.figure.u.12"></div><pre class="inline"><span id="rfc.iref.g.9"></span> <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = <a href="#imported.abnf" class="smpl">HTTP-date</a>
[1253]	1037	</pre><p id="rfc.section.3.3.p.3">An example of the field is:</p>
[1805]	1038	<div id="rfc.figure.u.13"></div><pre class="text"> If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
[1738]	1039	</pre><p id="rfc.section.3.3.p.5">A GET method with an If-Modified-Since header field and no <a href="p5-range.html#range.retrieval.requests" class="smpl">Range</a> header field requests that the selected representation be transferred only if it has been modified since the date given by
	1040	the If-Modified-Since header field. The algorithm for determining this includes the following cases:
[52]	1041	</p>
	1042	<ol>
[1715]	1043	<li>If the request would normally result in anything other than a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> status code, or if the passed If-Modified-Since date is invalid, the response is exactly the same as for a normal GET. A date
	1044	which is later than the server's current time is invalid.
[52]	1045	</li>
[1223]	1046	<li>If the selected representation has been modified since the If-Modified-Since date, the response is exactly the same as for
	1047	a normal GET.
[52]	1048	</li>
[1708]	1049	<li>If the selected representation has not been modified since a valid If-Modified-Since date, the server <em class="bcp14">SHOULD</em> return a <a href="#status.304" class="smpl">304 (Not Modified)</a> response.
[861]	1050	</li>
[52]	1051	</ol>
[1253]	1052	<p id="rfc.section.3.3.p.6">The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. </p>
[729]	1053	<ul class="empty">
[1803]	1054	<li> <b>Note:</b> The <a href="p5-range.html#range.retrieval.requests" class="smpl">Range</a> header field modifies the meaning of If-Modified-Since; see <a href="p5-range.html#header.range" title="Range">Section 5.4</a> of <a href="#Part5" id="rfc.xref.Part5.1"><cite title="HTTP/1.1, part 5: Range Requests">[Part5]</cite></a> for full details.
[729]	1055	</li>
	1056	<li> <b>Note:</b> If-Modified-Since times are interpreted by the server, whose clock might not be synchronized with the client.
	1057	</li>
	1058	<li> <b>Note:</b> When handling an If-Modified-Since header field, some servers will use an exact date comparison function, rather than a less-than
[1708]	1059	function, for deciding whether to send a <a href="#status.304" class="smpl">304 (Not Modified)</a> response. To get best results when sending an If-Modified-Since header field for cache validation, clients are advised to
[1739]	1060	use the exact date string received in a previous <a href="#header.last-modified" class="smpl">Last-Modified</a> header field whenever possible.
[729]	1061	</li>
[1739]	1062	<li> <b>Note:</b> If a client uses an arbitrary date in the If-Modified-Since header field instead of a date taken from the <a href="#header.last-modified" class="smpl">Last-Modified</a> header field for the same request, the client needs to be aware that this date is interpreted in the server's understanding
	1063	of time. Unsynchronized clocks and rounding problems, due to the different encodings of time between the client and server,
	1064	are concerns. This includes the possibility of race conditions if the document has changed between the time it was first requested
	1065	and the If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the If-Modified-Since
[994]	1066	date is derived from the client's clock without correction to the server's clock. Corrections for different time bases between
	1067	client and server are at best approximate due to network latency.
[729]	1068	</li>
	1069	</ul>
[52]	1070	<div id="rfc.iref.i.4"></div>
[1253]	1071	<div id="rfc.iref.h.6"></div>
	1072	<h2 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a> <a id="header.if-unmodified-since" href="#header.if-unmodified-since">If-Unmodified-Since</a></h2>
[1691]	1073	<p id="rfc.section.3.4.p.1">The "If-Unmodified-Since" header field can be used to make a request method conditional by modification date: if the selected
[1708]	1074	representation has been modified since the time specified in this field, then the server <em class="bcp14">MUST NOT</em> perform the requested operation and <em class="bcp14">MUST</em> instead respond with the <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code. If the selected representation has not been modified since the time specified in this field, the server <em class="bcp14">SHOULD</em> perform the request method as if the If-Unmodified-Since header field were not present.
[52]	1075	</p>
[1805]	1076	<div id="rfc.figure.u.14"></div><pre class="inline"><span id="rfc.iref.g.10"></span> <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = <a href="#imported.abnf" class="smpl">HTTP-date</a>
[1253]	1077	</pre><p id="rfc.section.3.4.p.3">An example of the field is:</p>
[1805]	1078	<div id="rfc.figure.u.15"></div><pre class="text"> If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
[1734]	1079	</pre><p id="rfc.section.3.4.p.5">If a request normally (i.e., in absence of the If-Unmodified-Since header field) would result in anything other than a <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> or <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code, the If-Unmodified-Since header field <em class="bcp14">SHOULD</em> be ignored.
[52]	1080	</p>
[1253]	1081	<p id="rfc.section.3.4.p.6">If the specified date is invalid, the header field <em class="bcp14">MUST</em> be ignored.
[1223]	1082	</p>
[1260]	1083	<h2 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a> <a id="header.if-range" href="#header.if-range">If-Range</a></h2>
[1803]	1084	<p id="rfc.section.3.5.p.1">The "If-Range" header field provides a special conditional request mechanism that is similar to <a href="#header.if-match" class="smpl">If-Match</a> and <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> but specific to HTTP range requests. If-Range is defined in <a href="p5-range.html#header.if-range" title="If-Range">Section 5.3</a> of <a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests">[Part5]</cite></a>.
[1260]	1085	</p>
[1253]	1086	<h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a> <a id="status.code.definitions" href="#status.code.definitions">Status Code Definitions</a></h1>
[1470]	1087	<div id="rfc.iref.27"></div>
[1253]	1088	<div id="rfc.iref.s.2"></div>
	1089	<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>
[1715]	1090	<p id="rfc.section.4.1.p.1">The 304 status code indicates that a conditional GET request has been received and would have resulted in a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response if it were not for the fact that the condition has evaluated to false. In other words, there is no need for the server
	1091	to transfer a representation of the target resource because the client's request indicates that it already has a valid representation,
[1253]	1092	as indicated by the 304 response header fields, and is therefore redirecting the client to make use of that stored representation
	1093	as if it were the payload of a 200 response. The 304 response <em class="bcp14">MUST NOT</em> contain a message-body, and thus is always terminated by the first empty line after the header fields.
[52]	1094	</p>
[1860]	1095	<p id="rfc.section.4.1.p.2">A 304 response <em class="bcp14">MUST</em> include a <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field (<a href="p2-semantics.html#header.date" title="Date">Section 8.2.1</a> of <a href="#Part2" id="rfc.xref.Part2.4"><cite title="HTTP/1.1, part 2: Semantics and Payloads">[Part2]</cite></a>) unless the origin server does not have a clock that can provide a reasonable approximation of the current time. If a <a href="p2-semantics.html#status.200" class="smpl">200
	1096	(OK)</a> response to the same request would have included any of the header fields <a href="p6-cache.html#header.cache-control" class="smpl">Cache-Control</a>, <a href="p2-semantics.html#header.content-location" class="smpl">Content-Location</a>, <a href="#header.etag" class="smpl">ETag</a>, <a href="p6-cache.html#header.expires" class="smpl">Expires</a>, or <a href="p2-semantics.html#header.vary" class="smpl">Vary</a>, then those same header fields <em class="bcp14">MUST</em> be sent in a 304 response.
[52]	1097	</p>
[1253]	1098	<p id="rfc.section.4.1.p.3">Since the goal of a 304 response is to minimize information transfer when the recipient already has one or more cached representations,
	1099	the response <em class="bcp14">SHOULD NOT</em> include representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding
	1100	cache updates (e.g., future HTTP extensions).
[52]	1101	</p>
[1253]	1102	<p id="rfc.section.4.1.p.4">If the recipient of a 304 response does not have a cached representation corresponding to the entity-tag indicated by the
	1103	304 response, then the recipient <em class="bcp14">MUST NOT</em> use the 304 to update its own cache. If this conditional request originated with an outbound client, such as a user agent
[1758]	1104	with its own cache sending a conditional GET to a shared proxy, then the 304 response <em class="bcp14">MAY</em> be forwarded to that client. Otherwise, the recipient <em class="bcp14">MUST</em> disregard the 304 response and repeat the request without any preconditions.
[52]	1105	</p>
[1253]	1106	<p id="rfc.section.4.1.p.5">If a cache uses a received 304 response to update a cache entry, the cache <em class="bcp14">MUST</em> update the entry to reflect any new field values given in the response.
[52]	1107	</p>
[1470]	1108	<div id="rfc.iref.28"></div>
[1253]	1109	<div id="rfc.iref.s.3"></div>
	1110	<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>
	1111	<p id="rfc.section.4.2.p.1">The 412 status code indicates that one or more preconditions given in the request header fields evaluated to false when tested
	1112	on the server. This response code allows the client to place preconditions on the current resource state (its current representations
	1113	and metadata) and thus prevent the request method from being applied if the target resource is in an unexpected state.
[183]	1114	</p>
[1799]	1115	<h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a> <a id="precedence" href="#precedence">Precedence</a></h1>
	1116	<p id="rfc.section.5.p.1">When more than one conditional request header field is present in a request, the order in which the fields are evaluated becomes
	1117	important. In practice, the fields defined in this document are consistently implemented in a single, logical order, due to
	1118	the fact that entity tags are presumed to be more accurate than date validators. For example, the only reason to send both <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> and <a href="#header.if-none-match" class="smpl">If-None-Match</a> in the same GET request is to support intermediary caches that might not have implemented <a href="#header.if-none-match" class="smpl">If-None-Match</a>, so it makes sense to ignore the <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> when entity tags are understood and available for the selected representation.
[203]	1119	</p>
[1799]	1120	<p id="rfc.section.5.p.2">The general rule of conditional precedence is that exact match conditions are evaluated before cache-validating conditions
	1121	and, within that order, last-modified conditions are only evaluated if the corresponding entity tag condition is not present
	1122	(or not applicable because the selected representation does not have an entity tag).
	1123	</p>
	1124	<p id="rfc.section.5.p.3">Specifically, the fields defined by this specification are evaluated as follows: </p>
	1125	<ol>
	1126	<li>When <a href="#header.if-match" class="smpl">If-Match</a> is present, evaluate it:
	1127	<ul>
	1128	<li>if true, continue to step 3</li>
	1129	<li>if false, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a></li>
	1130	</ul>
	1131	</li>
	1132	<li>When <a href="#header.if-match" class="smpl">If-Match</a> is not present and <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> is present, evaluate it:
	1133	<ul>
	1134	<li>if true, continue to step 3</li>
	1135	<li>if false, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a></li>
	1136	</ul>
	1137	</li>
	1138	<li>When the method is GET and both <a href="p5-range.html#range.retrieval.requests" class="smpl">Range</a> and <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> are present, evaluate it:
	1139	<ul>
	1140	<li>if the validator matches, respond 206 (Partial Content)</li>
	1141	<li>if the validator does not match, respond <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a></li>
	1142	</ul>
	1143	</li>
	1144	<li>When <a href="#header.if-none-match" class="smpl">If-None-Match</a> is present, evaluate it:
	1145	<ul>
	1146	<li>if true, all conditions are met</li>
	1147	<li>if false for GET/HEAD, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li>
	1148	<li>if false for other methods, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a></li>
	1149	</ul>
	1150	</li>
	1151	<li>When the method is GET or HEAD, <a href="#header.if-none-match" class="smpl">If-None-Match</a> is not present, and <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> is present, evaluate it:
	1152	<ul>
	1153	<li>if true, all conditions are met</li>
	1154	<li>if false, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li>
	1155	</ul>
	1156	</li>
	1157	</ol>
	1158	<p id="rfc.section.5.p.4">Any extension to HTTP/1.1 that defines additional conditional request header fields ought to define its own expectations regarding
	1159	the order for evaluating such fields in relation to those defined in this document and other conditionals that might be found
	1160	in practice.
	1161	</p>
	1162	<h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a> <a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
	1163	<h2 id="rfc.section.6.1"><a href="#rfc.section.6.1">6.1</a> <a id="status.code.registration" href="#status.code.registration">Status Code Registration</a></h2>
	1164	<p id="rfc.section.6.1.p.1">The HTTP Status Code Registry located at <<a href="http://www.iana.org/assignments/http-status-codes">http://www.iana.org/assignments/http-status-codes</a>> shall be updated with the registrations below:
	1165	</p>
[290]	1166	<div id="rfc.table.1">
[700]	1167	<div id="iana.status.code.registration.table"></div>
	1168	<table class="tt full left" cellpadding="3" cellspacing="0">
	1169	<thead>
	1170	<tr>
	1171	<th>Value</th>
	1172	<th>Description</th>
	1173	<th>Reference</th>
	1174	</tr>
	1175	</thead>
	1176	<tbody>
	1177	<tr>
[704]	1178	<td class="left">304</td>
	1179	<td class="left">Not Modified</td>
[1253]	1180	<td class="left"> <a href="#status.304" id="rfc.xref.status.304.1" title="304 Not Modified">Section 4.1</a>
[700]	1181	</td>
	1182	</tr>
	1183	<tr>
[704]	1184	<td class="left">412</td>
	1185	<td class="left">Precondition Failed</td>
[1253]	1186	<td class="left"> <a href="#status.412" id="rfc.xref.status.412.1" title="412 Precondition Failed">Section 4.2</a>
[700]	1187	</td>
	1188	</tr>
	1189	</tbody>
	1190	</table>
	1191	</div>
[1799]	1192	<h2 id="rfc.section.6.2"><a href="#rfc.section.6.2">6.2</a> <a id="header.field.registration" href="#header.field.registration">Header Field Registration</a></h2>
	1193	<p id="rfc.section.6.2.p.1">The Message Header Field Registry located at <<a href="http://www.iana.org/assignments/message-headers/message-header-index.html">http://www.iana.org/assignments/message-headers/message-header-index.html</a>> shall be updated with the permanent registrations below (see <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a>):
[700]	1194	</p>
	1195	<div id="rfc.table.2">
[290]	1196	<div id="iana.header.registration.table"></div>
[675]	1197	<table class="tt full left" cellpadding="3" cellspacing="0">
[253]	1198	<thead>
	1199	<tr>
	1200	<th>Header Field Name</th>
	1201	<th>Protocol</th>
	1202	<th>Status</th>
	1203	<th>Reference</th>
	1204	</tr>
	1205	</thead>
	1206	<tbody>
	1207	<tr>
[704]	1208	<td class="left">ETag</td>
	1209	<td class="left">http</td>
	1210	<td class="left">standard</td>
[1733]	1211	<td class="left"> <a href="#header.etag" id="rfc.xref.header.etag.2" title="ETag">Section 2.3</a>
[253]	1212	</td>
	1213	</tr>
	1214	<tr>
[704]	1215	<td class="left">If-Match</td>
	1216	<td class="left">http</td>
	1217	<td class="left">standard</td>
[1260]	1218	<td class="left"> <a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section 3.1</a>
[253]	1219	</td>
	1220	</tr>
	1221	<tr>
[704]	1222	<td class="left">If-Modified-Since</td>
	1223	<td class="left">http</td>
	1224	<td class="left">standard</td>
[1253]	1225	<td class="left"> <a href="#header.if-modified-since" id="rfc.xref.header.if-modified-since.1" title="If-Modified-Since">Section 3.3</a>
[253]	1226	</td>
	1227	</tr>
	1228	<tr>
[704]	1229	<td class="left">If-None-Match</td>
	1230	<td class="left">http</td>
	1231	<td class="left">standard</td>
[1260]	1232	<td class="left"> <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.1" title="If-None-Match">Section 3.2</a>
[253]	1233	</td>
	1234	</tr>
	1235	<tr>
[704]	1236	<td class="left">If-Unmodified-Since</td>
	1237	<td class="left">http</td>
	1238	<td class="left">standard</td>
[1253]	1239	<td class="left"> <a href="#header.if-unmodified-since" id="rfc.xref.header.if-unmodified-since.1" title="If-Unmodified-Since">Section 3.4</a>
[253]	1240	</td>
	1241	</tr>
	1242	<tr>
[704]	1243	<td class="left">Last-Modified</td>
	1244	<td class="left">http</td>
	1245	<td class="left">standard</td>
[1733]	1246	<td class="left"> <a href="#header.last-modified" id="rfc.xref.header.last-modified.2" title="Last-Modified">Section 2.2</a>
[253]	1247	</td>
	1248	</tr>
	1249	</tbody>
	1250	</table>
	1251	</div>
[1799]	1252	<p id="rfc.section.6.2.p.2">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
	1253	<h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a> <a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
[1805]	1254	<p id="rfc.section.7.p.1">No additional security considerations have been identified beyond those applicable to HTTP in general <a href="#Part1" id="rfc.xref.Part1.5"><cite title="HTTP/1.1, part 1: Message Routing and Syntax"">[Part1]</cite></a>.
[52]	1255	</p>
[1801]	1256	<p id="rfc.section.7.p.2">The validators defined by this specification are not intended to ensure the validity of a representation, guard against malicious
	1257	changes, or detect man-in-the-middle attacks. At best, they enable more efficient cache updates and optimistic concurrent
	1258	writes when all participants are behaving nicely. At worst, the conditions will fail and the client will receive a response
	1259	that is no more harmful than an HTTP exchange without conditional requests.
	1260	</p>
[1799]	1261	<h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a> <a id="acks" href="#acks">Acknowledgments</a></h1>
[1805]	1262	<p id="rfc.section.8.p.1">See <a href="p1-messaging.html#acks" title="Acknowledgments">Section 9</a> of <a href="#Part1" id="rfc.xref.Part1.6"><cite title="HTTP/1.1, part 1: Message Routing and Syntax"">[Part1]</cite></a>.
[1364]	1263	</p>
[1799]	1264	<h1 id="rfc.references"><a id="rfc.section.9" href="#rfc.section.9">9.</a> References
[52]	1265	</h1>
[1799]	1266	<h2 id="rfc.references.1"><a href="#rfc.section.9.1" id="rfc.section.9.1">9.1</a> Normative References
[119]	1267	</h2>
[1643]	1268	<table>
[52]	1269	<tr>
	1270	<td class="reference"><b id="Part1">[Part1]</b></td>
[1845]	1271	<td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</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-latest">HTTP/1.1, part 1: Message Routing and Syntax"</a>”, Internet-Draft draft-ietf-httpbis-p1-messaging-latest (work in progress), September 2012.
[52]	1272	</td>
	1273	</tr>
	1274	<tr>
[1436]	1275	<td class="reference"><b id="Part2">[Part2]</b></td>
[1845]	1276	<td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</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-p2-semantics-latest">HTTP/1.1, part 2: Semantics and Payloads</a>”, Internet-Draft draft-ietf-httpbis-p2-semantics-latest (work in progress), September 2012.
[1436]	1277	</td>
	1278	</tr>
	1279	<tr>
[52]	1280	<td class="reference"><b id="Part5">[Part5]</b></td>
[1845]	1281	<td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</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-latest">HTTP/1.1, part 5: Range Requests</a>”, Internet-Draft draft-ietf-httpbis-p5-range-latest (work in progress), September 2012.
[52]	1282	</td>
	1283	</tr>
	1284	<tr>
	1285	<td class="reference"><b id="Part6">[Part6]</b></td>
[1845]	1286	<td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, <a href="mailto:mnot@mnot.net" title="Rackspace">Nottingham, M., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p6-cache-latest">HTTP/1.1, part 6: Caching</a>”, Internet-Draft draft-ietf-httpbis-p6-cache-latest (work in progress), September 2012.
[52]	1287	</td>
	1288	</tr>
	1289	<tr>
[119]	1290	<td class="reference"><b id="RFC2119">[RFC2119]</b></td>
[704]	1291	<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.
[52]	1292	</td>
	1293	</tr>
[425]	1294	<tr>
	1295	<td class="reference"><b id="RFC5234">[RFC5234]</b></td>
[704]	1296	<td class="top"><a href="mailto:dcrocker@bbiw.net" title="Brandenburg InternetWorking">Crocker, D., Ed.</a> and <a href="mailto:paul.overell@thus.net" title="THUS plc.">P. Overell</a>, “<a href="http://tools.ietf.org/html/rfc5234">Augmented BNF for Syntax Specifications: ABNF</a>”, STD 68, RFC 5234, January 2008.
[425]	1297	</td>
	1298	</tr>
[119]	1299	</table>
[1799]	1300	<h2 id="rfc.references.2"><a href="#rfc.section.9.2" id="rfc.section.9.2">9.2</a> Informative References
[119]	1301	</h2>
[1251]	1302	<table>
[52]	1303	<tr>
	1304	<td class="reference"><b id="RFC2616">[RFC2616]</b></td>
[704]	1305	<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.
[52]	1306	</td>
	1307	</tr>
[253]	1308	<tr>
	1309	<td class="reference"><b id="RFC3864">[RFC3864]</b></td>
[704]	1310	<td class="top"><a href="mailto:GK-IETF@ninebynine.org" title="Nine by Nine">Klyne, G.</a>, <a href="mailto:mnot@pobox.com" title="BEA Systems">Nottingham, M.</a>, and <a href="mailto:JeffMogul@acm.org" title="HP Labs">J. Mogul</a>, “<a href="http://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>”, BCP 90, RFC 3864, September 2004.
[253]	1311	</td>
	1312	</tr>
[1251]	1313	<tr>
	1314	<td class="reference"><b id="RFC4918">[RFC4918]</b></td>
	1315	<td class="top"><a href="mailto:ldusseault@commerce.net" title="CommerceNet">Dusseault, L., Ed.</a>, “<a href="http://tools.ietf.org/html/rfc4918">HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</a>”, RFC 4918, June 2007.
	1316	</td>
	1317	</tr>
[52]	1318	</table>
[662]	1319	<div class="avoidbreak">
	1320	<h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1>
	1321	<address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span>
	1322	(editor)
[1106]	1323	<span class="n hidden"><span class="family-name">Fielding</span><span class="given-name">Roy T.</span></span></span><span class="org vcardline">Adobe Systems Incorporated</span><span class="adr"><span class="street-address vcardline">345 Park Ave</span><span class="vcardline"><span class="locality">San Jose</span>, <span class="region">CA</span> <span class="postal-code">95110</span></span><span class="country-name vcardline">USA</span></span><span class="vcardline">Email: <a href="mailto:fielding@gbiv.com"><span class="email">fielding@gbiv.com</span></a></span><span class="vcardline">URI: <a href="http://roy.gbiv.com/" class="url">http://roy.gbiv.com/</a></span></address>
[662]	1324	<address class="vcard"><span class="vcardline"><span class="fn">Yves Lafon</span>
	1325	(editor)
[799]	1326	<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>
[662]	1327	<address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span>
	1328	(editor)
[1689]	1329	<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">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>
[662]	1330	</div>
[912]	1331	<h1 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a> <a id="changes.from.rfc.2616" href="#changes.from.rfc.2616">Changes from RFC 2616</a></h1>
[1381]	1332	<p id="rfc.section.A.p.1">Allow weak entity-tags in all requests except range requests (Sections <a href="#weak.and.strong.validators" title="Weak versus Strong">2.1</a> and <a href="#header.if-none-match" id="rfc.xref.header.if-none-match.2" title="If-None-Match">3.2</a>).
[245]	1333	</p>
[1739]	1334	<p id="rfc.section.A.p.2">Change <a href="#header.etag" class="smpl">ETag</a> header field ABNF not to use quoted-string, thus avoiding escaping issues. (<a href="#header.etag" id="rfc.xref.header.etag.3" title="ETag">Section 2.3</a>)
[1233]	1335	</p>
[1470]	1336	<p id="rfc.section.A.p.3">Change ABNF productions for header fields to only define the field value. (<a href="#header.field.definitions" title="Precondition Header Fields">Section 3</a>)
	1337	</p>
[1805]	1338	<h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a> <a id="imported.abnf" href="#imported.abnf">Imported ABNF</a></h1>
[1806]	1339	<p id="rfc.section.B.p.1">The following core rules are included by reference, as defined in <a href="http://tools.ietf.org/html/rfc5234#appendix-B.1">Appendix B.1</a> of <a href="#RFC5234" id="rfc.xref.RFC5234.2"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a>: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG
[1805]	1340	(hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
	1341	character).
	1342	</p>
	1343	<p id="rfc.section.B.p.2">The rules below are defined in <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: Message Routing and Syntax"">[Part1]</cite></a>:
	1344	</p>
	1345	<div id="rfc.figure.u.16"></div><pre class="inline"> <a href="#imported.abnf" class="smpl">OWS</a> = <OWS, defined in <a href="#Part1" id="rfc.xref.Part1.8"><cite title="HTTP/1.1, part 1: Message Routing and Syntax"">[Part1]</cite></a>, <a href="p1-messaging.html#whitespace" title="Whitespace">Section 3.2.1</a>>
	1346	<a href="#imported.abnf" class="smpl">obs-text</a> = <obs-text, defined in <a href="#Part1" id="rfc.xref.Part1.9"><cite title="HTTP/1.1, part 1: Message Routing and Syntax"">[Part1]</cite></a>, <a href="p1-messaging.html#field.components" title="Field value components">Section 3.2.4</a>>
	1347	</pre><p id="rfc.section.B.p.4">The rules below are defined in other parts:</p>
[1860]	1348	<div id="rfc.figure.u.17"></div><pre class="inline"> <a href="#imported.abnf" class="smpl">HTTP-date</a> = <HTTP-date, defined in <a href="#Part2" id="rfc.xref.Part2.5"><cite title="HTTP/1.1, part 2: Semantics and Payloads">[Part2]</cite></a>, <a href="p2-semantics.html#http.date" title="Date/Time Formats">Section 9.1</a>>
[1805]	1349	</pre><h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a> <a id="collected.abnf" href="#collected.abnf">Collected ABNF</a></h1>
	1350	<div id="rfc.figure.u.18"></div> <pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = entity-tag
[427]	1351
[1860]	1352	<a href="#imported.abnf" class="smpl">HTTP-date</a> = <HTTP-date, defined in [Part2], Section 9.1>
[427]	1353
[1233]	1354	<a href="#header.if-match" class="smpl">If-Match</a> = "" / ( ( "," OWS ) entity-tag *( OWS "," [ OWS
[421]	1355	entity-tag ] ) )
[1233]	1356	<a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = HTTP-date
	1357	<a href="#header.if-none-match" class="smpl">If-None-Match</a> = "" / ( ( "," OWS ) entity-tag *( OWS "," [ OWS
[421]	1358	entity-tag ] ) )
[1233]	1359	<a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = HTTP-date
[427]	1360
[1233]	1361	<a href="#header.last-modified" class="smpl">Last-Modified</a> = HTTP-date
[427]	1362
[1805]	1363	<a href="#imported.abnf" class="smpl">OWS</a> = <OWS, defined in [Part1], Section 3.2.1>
[427]	1364
[1260]	1365	<a href="#header.etag" class="smpl">entity-tag</a> = [ weak ] opaque-tag
[1470]	1366	<a href="#header.etag" class="smpl">etagc</a> = "!" / %x23-7E ; '#'-'~'
	1367	/ obs-text
[427]	1368
[1805]	1369	<a href="#imported.abnf" class="smpl">obs-text</a> = <obs-text, defined in [Part1], Section 3.2.4>
[1470]	1370	<a href="#header.etag" class="smpl">opaque-tag</a> = DQUOTE *etagc DQUOTE
[427]	1371
[1260]	1372	<a href="#header.etag" class="smpl">weak</a> = %x57.2F ; W/
[1805]	1373	</pre> <h1 id="rfc.section.D"><a href="#rfc.section.D">D.</a> <a id="change.log" href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1>
	1374	<p id="rfc.section.D.p.1">Changes up to the first Working Group Last Call draft are summarized in <<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-19#appendix-C">http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-19#appendix-C</a>>.
[115]	1375	</p>
[1805]	1376	<h2 id="rfc.section.D.1"><a href="#rfc.section.D.1">D.1</a> <a id="changes.since.19" href="#changes.since.19">Since draft-ietf-httpbis-p4-conditional-19</a></h2>
	1377	<p id="rfc.section.D.1.p.1">Closed issues: </p>
[1682]	1378	<ul>
[1800]	1379	<li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/241">http://tools.ietf.org/wg/httpbis/trac/ticket/241</a>>: "Need to clarify eval order/interaction of conditional headers"
	1380	</li>
[1819]	1381	<li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/345">http://tools.ietf.org/wg/httpbis/trac/ticket/345</a>>: "Required headers on 304 and 206"
	1382	</li>
	1383	<li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/350">http://tools.ietf.org/wg/httpbis/trac/ticket/350</a>>: "Optionality of Conditional Request Support"
	1384	</li>
[1696]	1385	<li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/354">http://tools.ietf.org/wg/httpbis/trac/ticket/354</a>>: "ETags and Conditional Requests"
	1386	</li>
[1682]	1387	<li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/361">http://tools.ietf.org/wg/httpbis/trac/ticket/361</a>>: "ABNF requirements for recipients"
	1388	</li>
[1703]	1389	<li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/363">http://tools.ietf.org/wg/httpbis/trac/ticket/363</a>>: "Rare cases"
	1390	</li>
[1802]	1391	<li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/365">http://tools.ietf.org/wg/httpbis/trac/ticket/365</a>>: "Conditional Request Security Considerations"
	1392	</li>
[1800]	1393	<li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/371">http://tools.ietf.org/wg/httpbis/trac/ticket/371</a>>: "If-Modified-Since lacks definition for method != GET"
	1394	</li>
[1761]	1395	<li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/372">http://tools.ietf.org/wg/httpbis/trac/ticket/372</a>>: "refactor conditional header field descriptions"
	1396	</li>
[1682]	1397	</ul>
[1807]	1398	<h2 id="rfc.section.D.2"><a href="#rfc.section.D.2">D.2</a> <a id="changes.since.20" href="#changes.since.20">Since draft-ietf-httpbis-p4-conditional-20</a></h2>
	1399	<p id="rfc.section.D.2.p.1">None yet.</p>
[52]	1400	<h1 id="rfc.index"><a href="#rfc.index">Index</a></h1>
[1253]	1401	<p class="noprint"><a href="#rfc.index.3">3</a> <a href="#rfc.index.4">4</a> <a href="#rfc.index.E">E</a> <a href="#rfc.index.G">G</a> <a href="#rfc.index.H">H</a> <a href="#rfc.index.I">I</a> <a href="#rfc.index.L">L</a> <a href="#rfc.index.M">M</a> <a href="#rfc.index.P">P</a> <a href="#rfc.index.R">R</a> <a href="#rfc.index.S">S</a> <a href="#rfc.index.V">V</a>
[52]	1402	</p>
	1403	<div class="print2col">
	1404	<ul class="ind">
[1072]	1405	<li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul>
[1799]	1406	<li>304 Not Modified (status code)  <a href="#rfc.iref.27"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">6.1</a></li>
[52]	1407	</ul>
	1408	</li>
[1072]	1409	<li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul>
[1799]	1410	<li>412 Precondition Failed (status code)  <a href="#rfc.iref.28"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">6.1</a></li>
[52]	1411	</ul>
	1412	</li>
[1072]	1413	<li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul>
[1799]	1414	<li>ETag header field  <a href="#rfc.xref.header.etag.1">2</a>, <a href="#rfc.iref.e.1"><b>2.3</b></a>, <a href="#rfc.xref.header.etag.2">6.2</a>, <a href="#rfc.xref.header.etag.3">A</a></li>
[52]	1415	</ul>
	1416	</li>
[1072]	1417	<li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul>
	1418	<li><tt>Grammar</tt>
	1419	<ul>
[1381]	1420	<li><tt>entity-tag</tt>  <a href="#rfc.iref.g.3"><b>2.3</b></a></li>
	1421	<li><tt>ETag</tt>  <a href="#rfc.iref.g.2"><b>2.3</b></a></li>
[1470]	1422	<li><tt>etagc</tt>  <a href="#rfc.iref.g.6"><b>2.3</b></a></li>
	1423	<li><tt>If-Match</tt>  <a href="#rfc.iref.g.7"><b>3.1</b></a></li>
	1424	<li><tt>If-Modified-Since</tt>  <a href="#rfc.iref.g.9"><b>3.3</b></a></li>
	1425	<li><tt>If-None-Match</tt>  <a href="#rfc.iref.g.8"><b>3.2</b></a></li>
	1426	<li><tt>If-Unmodified-Since</tt>  <a href="#rfc.iref.g.10"><b>3.4</b></a></li>
[1381]	1427	<li><tt>Last-Modified</tt>  <a href="#rfc.iref.g.1"><b>2.2</b></a></li>
	1428	<li><tt>opaque-tag</tt>  <a href="#rfc.iref.g.5"><b>2.3</b></a></li>
	1429	<li><tt>weak</tt>  <a href="#rfc.iref.g.4"><b>2.3</b></a></li>
[52]	1430	</ul>
	1431	</li>
	1432	</ul>
	1433	</li>
[1072]	1434	<li><a id="rfc.index.H" href="#rfc.index.H"><b>H</b></a><ul>
[1120]	1435	<li>Header Fields
[1072]	1436	<ul>
[1799]	1437	<li>ETag  <a href="#rfc.xref.header.etag.1">2</a>, <a href="#rfc.iref.h.2"><b>2.3</b></a>, <a href="#rfc.xref.header.etag.2">6.2</a>, <a href="#rfc.xref.header.etag.3">A</a></li>
	1438	<li>If-Match  <a href="#rfc.iref.h.3"><b>3.1</b></a>, <a href="#rfc.xref.header.if-match.1">6.2</a></li>
	1439	<li>If-Modified-Since  <a href="#rfc.iref.h.5"><b>3.3</b></a>, <a href="#rfc.xref.header.if-modified-since.1">6.2</a></li>
	1440	<li>If-None-Match  <a href="#rfc.iref.h.4"><b>3.2</b></a>, <a href="#rfc.xref.header.if-none-match.1">6.2</a>, <a href="#rfc.xref.header.if-none-match.2">A</a></li>
	1441	<li>If-Unmodified-Since  <a href="#rfc.iref.h.6"><b>3.4</b></a>, <a href="#rfc.xref.header.if-unmodified-since.1">6.2</a></li>
	1442	<li>Last-Modified  <a href="#rfc.xref.header.last-modified.1">2</a>, <a href="#rfc.iref.h.1"><b>2.2</b></a>, <a href="#rfc.xref.header.last-modified.2">6.2</a></li>
[52]	1443	</ul>
	1444	</li>
	1445	</ul>
	1446	</li>
[1072]	1447	<li><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul>
[1799]	1448	<li>If-Match header field  <a href="#rfc.iref.i.1"><b>3.1</b></a>, <a href="#rfc.xref.header.if-match.1">6.2</a></li>
	1449	<li>If-Modified-Since header field  <a href="#rfc.iref.i.3"><b>3.3</b></a>, <a href="#rfc.xref.header.if-modified-since.1">6.2</a></li>
	1450	<li>If-None-Match header field  <a href="#rfc.iref.i.2"><b>3.2</b></a>, <a href="#rfc.xref.header.if-none-match.1">6.2</a>, <a href="#rfc.xref.header.if-none-match.2">A</a></li>
	1451	<li>If-Unmodified-Since header field  <a href="#rfc.iref.i.4"><b>3.4</b></a>, <a href="#rfc.xref.header.if-unmodified-since.1">6.2</a></li>
[52]	1452	</ul>
	1453	</li>
[1072]	1454	<li><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul>
[1799]	1455	<li>Last-Modified header field  <a href="#rfc.xref.header.last-modified.1">2</a>, <a href="#rfc.iref.l.1"><b>2.2</b></a>, <a href="#rfc.xref.header.last-modified.2">6.2</a></li>
[52]	1456	</ul>
	1457	</li>
[1253]	1458	<li><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul>
	1459	<li>metadata  <a href="#rfc.iref.m.1"><b>2</b></a></li>
	1460	</ul>
	1461	</li>
[1072]	1462	<li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul>
[1805]	1463	<li><em>Part1</em>  <a href="#rfc.xref.Part1.1">1</a>, <a href="#rfc.xref.Part1.2">1.1</a>, <a href="#rfc.xref.Part1.3">1.2</a>, <a href="#rfc.xref.Part1.4">2.3.3</a>, <a href="#rfc.xref.Part1.5">7</a>, <a href="#rfc.xref.Part1.6">8</a>, <a href="#Part1"><b>9.1</b></a>, <a href="#rfc.xref.Part1.7">B</a>, <a href="#rfc.xref.Part1.8">B</a>, <a href="#rfc.xref.Part1.9">B</a><ul>
[1796]	1464	<li><em>Section 1.2</em>  <a href="#rfc.xref.Part1.3">1.2</a></li>
	1465	<li><em>Section 2</em>  <a href="#rfc.xref.Part1.2">1.1</a></li>
[1805]	1466	<li><em>Section 3.2.1</em>  <a href="#rfc.xref.Part1.8">B</a></li>
	1467	<li><em>Section 3.2.4</em>  <a href="#rfc.xref.Part1.9">B</a></li>
	1468	<li><em>Section 4</em>  <a href="#rfc.xref.Part1.4">2.3.3</a></li>
	1469	<li><em>Section 9</em>  <a href="#rfc.xref.Part1.6">8</a></li>
[52]	1470	</ul>
	1471	</li>
[1805]	1472	<li><em>Part2</em>  <a href="#rfc.xref.Part2.1">1</a>, <a href="#rfc.xref.Part2.2">2.3.3</a>, <a href="#rfc.xref.Part2.3">2.3.3</a>, <a href="#rfc.xref.Part2.4">4.1</a>, <a href="#Part2"><b>9.1</b></a>, <a href="#rfc.xref.Part2.5">B</a><ul>
[1860]	1473	<li><em>Section 3.5</em>  <a href="#rfc.xref.Part2.2">2.3.3</a></li>
	1474	<li><em>Section 6.3.4</em>  <a href="#rfc.xref.Part2.3">2.3.3</a></li>
	1475	<li><em>Section 8.2.1</em>  <a href="#rfc.xref.Part2.4">4.1</a></li>
	1476	<li><em>Section 9.1</em>  <a href="#rfc.xref.Part2.5">B</a></li>
[1436]	1477	</ul>
	1478	</li>
[1799]	1479	<li><em>Part5</em>  <a href="#rfc.xref.Part5.1">3.3</a>, <a href="#rfc.xref.Part5.2">3.5</a>, <a href="#Part5"><b>9.1</b></a><ul>
[1260]	1480	<li><em>Section 5.3</em>  <a href="#rfc.xref.Part5.2">3.5</a></li>
	1481	<li><em>Section 5.4</em>  <a href="#rfc.xref.Part5.1">3.3</a></li>
[52]	1482	</ul>
	1483	</li>
[1799]	1484	<li><em>Part6</em>  <a href="#rfc.xref.Part6.1">1</a>, <a href="#rfc.xref.Part6.2">2.2.1</a>, <a href="#rfc.xref.Part6.3">2.3.1</a>, <a href="#Part6"><b>9.1</b></a></li>
[52]	1485	</ul>
	1486	</li>
[1072]	1487	<li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul>
[1799]	1488	<li><em>RFC2119</em>  <a href="#rfc.xref.RFC2119.1">1.1</a>, <a href="#RFC2119"><b>9.1</b></a></li>
	1489	<li><em>RFC2616</em>  <a href="#rfc.xref.RFC2616.1">2.3</a>, <a href="#RFC2616"><b>9.2</b></a><ul>
[1470]	1490	<li><em>Section 3.11</em>  <a href="#rfc.xref.RFC2616.1">2.3</a></li>
	1491	</ul>
	1492	</li>
[1799]	1493	<li><em>RFC3864</em>  <a href="#rfc.xref.RFC3864.1">6.2</a>, <a href="#RFC3864"><b>9.2</b></a></li>
	1494	<li><em>RFC4918</em>  <a href="#rfc.xref.RFC4918.1">2</a>, <a href="#RFC4918"><b>9.2</b></a></li>
[1805]	1495	<li><em>RFC5234</em>  <a href="#rfc.xref.RFC5234.1">1.2</a>, <a href="#RFC5234"><b>9.1</b></a>, <a href="#rfc.xref.RFC5234.2">B</a><ul>
	1496	<li><em>Appendix B.1</em>  <a href="#rfc.xref.RFC5234.2">B</a></li>
[425]	1497	</ul>
	1498	</li>
[52]	1499	</ul>
	1500	</li>
[1072]	1501	<li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul>
[1224]	1502	<li>selected representation  <a href="#rfc.iref.s.1"><b>1</b></a></li>
[1072]	1503	<li>Status Codes
	1504	<ul>
[1799]	1505	<li>304 Not Modified  <a href="#rfc.iref.s.2"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">6.1</a></li>
	1506	<li>412 Precondition Failed  <a href="#rfc.iref.s.3"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">6.1</a></li>
[52]	1507	</ul>
	1508	</li>
	1509	</ul>
	1510	</li>
[1253]	1511	<li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul>
[1381]	1512	<li>validator  <a href="#rfc.iref.v.1"><b>2</b></a><ul>
	1513	<li>strong  <a href="#rfc.iref.v.3"><b>2.1</b></a></li>
	1514	<li>weak  <a href="#rfc.iref.v.2"><b>2.1</b></a></li>
	1515	</ul>
	1516	</li>
[1253]	1517	</ul>
	1518	</li>
[52]	1519	</ul>
	1520	</div>
	1521	</body>
	1522	</html>

Note: See TracBrowser for help on using the repository browser.

Download in other formats: