| 1 | <!DOCTYPE html |
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| 2 | PUBLIC "-//W3C//DTD HTML 4.01//EN"> |
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| 3 | <html lang="en"> |
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| 4 | <head profile="http://www.w3.org/2006/03/hcard http://dublincore.org/documents/2008/08/04/dc-html/"> |
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| 5 | <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> |
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| 6 | <title>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</title><script> |
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| 7 | var buttonsAdded = false; |
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| 8 | |
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| 9 | function init() { |
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| 10 | var fb = document.createElement("div"); |
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| 11 | fb.className = "feedback noprint"; |
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| 12 | fb.setAttribute("onclick", "feedback();"); |
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| 13 | fb.appendChild(document.createTextNode("feedback")); |
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| 14 | |
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| 15 | var bodyl = document.getElementsByTagName("body"); |
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| 16 | bodyl.item(0).appendChild(fb); |
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| 17 | } |
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| 18 | |
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| 19 | function feedback() { |
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| 20 | toggleButtonsToElementsByName("h1"); |
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| 21 | toggleButtonsToElementsByName("h2"); |
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| 22 | toggleButtonsToElementsByName("h3"); |
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| 23 | toggleButtonsToElementsByName("h4"); |
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| 24 | |
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| 25 | buttonsAdded = !buttonsAdded; |
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| 26 | } |
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| 27 | |
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| 28 | function toggleButtonsToElementsByName(name) { |
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| 29 | var list = document.getElementsByTagName(name); |
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| 30 | for (var i = 0; i < list.length; i++) { |
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| 31 | toggleButton(list.item(i)); |
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| 32 | } |
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| 33 | } |
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| 34 | |
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| 35 | function toggleButton(node) { |
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| 36 | if (! buttonsAdded) { |
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| 37 | |
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| 38 | // docname |
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| 39 | var template = "mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&body=<{ref}>:"; |
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| 40 | |
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| 41 | var id = node.getAttribute("id"); |
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| 42 | // better id available? |
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| 43 | var titlelinks = node.getElementsByTagName("a"); |
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| 44 | for (var i = 0; i < titlelinks.length; i++) { |
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| 45 | var tl = titlelinks.item(i); |
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| 46 | if (tl.getAttribute("id")) { |
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| 47 | id = tl.getAttribute("id"); |
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| 48 | } |
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| 49 | } |
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| 50 | |
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| 51 | // ref |
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| 52 | var ref = window.location.toString(); |
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| 53 | var hash = ref.indexOf("#"); |
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| 54 | if (hash != -1) { |
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| 55 | ref = ref.substring(0, hash); |
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| 56 | } |
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| 57 | if (id != "") { |
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| 58 | ref += "#" + id; |
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| 59 | } |
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| 60 | |
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| 61 | // docname |
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| 62 | var docname = "draft-ietf-httpbis-p4-conditional-latest"; |
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| 63 | |
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| 64 | // section |
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| 65 | var section = node.textContent; |
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| 66 | section = section.replace("\u00a0", " "); |
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| 67 | |
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| 68 | // build URI from template |
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| 69 | var uri = template.replace("{docname}", encodeURIComponent(docname)); |
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| 70 | uri = uri.replace("{section}", encodeURIComponent(section)); |
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| 71 | uri = uri.replace("{ref}", encodeURIComponent(ref)); |
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| 72 | |
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| 73 | var button = document.createElement("a"); |
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| 74 | button.className = "fbbutton noprint"; |
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| 75 | button.setAttribute("href", uri); |
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| 76 | button.appendChild(document.createTextNode("send feedback")); |
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| 77 | node.appendChild(button); |
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| 78 | } |
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| 79 | else { |
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| 80 | var buttons = node.getElementsByTagName("a"); |
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| 81 | for (var i = 0; i < buttons.length; i++) { |
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| 82 | var b = buttons.item(i); |
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| 83 | if (b.className == "fbbutton noprint") { |
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| 84 | node.removeChild(b); |
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| 85 | } |
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| 86 | } |
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| 87 | } |
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| 88 | }</script><style type="text/css" title="Xml2Rfc (sans serif)"> |
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| 89 | a { |
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| 90 | text-decoration: none; |
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| 91 | } |
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| 92 | a.smpl { |
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| 93 | color: black; |
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| 94 | } |
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| 95 | a:hover { |
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| 96 | text-decoration: underline; |
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| 97 | } |
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| 98 | a:active { |
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| 99 | text-decoration: underline; |
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| 100 | } |
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| 101 | address { |
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| 102 | margin-top: 1em; |
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| 103 | margin-left: 2em; |
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| 104 | font-style: normal; |
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| 105 | } |
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| 106 | body { |
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| 107 | color: black; |
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| 108 | font-family: verdana, helvetica, arial, sans-serif; |
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| 109 | font-size: 10pt; |
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| 110 | margin-right: 2em; |
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| 111 | } |
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| 112 | cite { |
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| 113 | font-style: normal; |
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| 114 | } |
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| 115 | div.note { |
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| 116 | margin-left: 2em; |
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| 117 | } |
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| 118 | dl { |
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| 119 | margin-left: 2em; |
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| 120 | } |
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| 121 | ul.empty { |
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| 122 | list-style-type: none; |
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| 123 | } |
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| 124 | ul.empty li { |
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| 125 | margin-top: .5em; |
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| 126 | } |
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| 127 | dl p { |
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| 128 | margin-left: 0em; |
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| 129 | } |
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| 130 | dt { |
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| 131 | margin-top: .5em; |
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| 132 | } |
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| 133 | h1 { |
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| 134 | font-size: 14pt; |
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| 135 | line-height: 21pt; |
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| 136 | page-break-after: avoid; |
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| 137 | } |
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| 138 | h1.np { |
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| 139 | page-break-before: always; |
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| 140 | } |
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| 141 | h1 a { |
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| 142 | color: #333333; |
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| 143 | } |
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| 144 | h2 { |
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| 145 | font-size: 12pt; |
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| 146 | line-height: 15pt; |
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| 147 | page-break-after: avoid; |
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| 148 | } |
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| 149 | h3, h4, h5, h6 { |
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| 150 | font-size: 10pt; |
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| 151 | page-break-after: avoid; |
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| 152 | } |
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| 153 | h2 a, h3 a, h4 a, h5 a, h6 a { |
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| 154 | color: black; |
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| 155 | } |
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| 156 | img { |
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| 157 | margin-left: 3em; |
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| 158 | } |
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| 159 | li { |
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| 160 | margin-left: 2em; |
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| 161 | } |
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| 162 | ol { |
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| 163 | margin-left: 2em; |
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| 164 | } |
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| 165 | ol.la { |
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| 166 | list-style-type: lower-alpha; |
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| 167 | } |
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| 168 | ol.ua { |
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| 169 | list-style-type: upper-alpha; |
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| 170 | } |
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| 171 | ol p { |
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| 172 | margin-left: 0em; |
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| 173 | } |
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| 174 | p { |
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| 175 | margin-left: 2em; |
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| 176 | } |
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| 177 | pre { |
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| 178 | margin-left: 3em; |
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| 179 | background-color: lightyellow; |
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| 180 | padding: .25em; |
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| 181 | page-break-inside: avoid; |
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| 182 | } |
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| 183 | pre.text2 { |
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| 184 | border-style: dotted; |
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| 185 | border-width: 1px; |
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| 186 | background-color: #f0f0f0; |
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| 187 | width: 69em; |
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| 188 | } |
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| 189 | pre.inline { |
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| 190 | background-color: white; |
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| 191 | padding: 0em; |
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| 192 | } |
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| 193 | pre.text { |
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| 194 | border-style: dotted; |
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| 195 | border-width: 1px; |
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| 196 | background-color: #f8f8f8; |
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| 197 | width: 69em; |
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| 198 | } |
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| 199 | pre.drawing { |
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| 200 | border-style: solid; |
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| 201 | border-width: 1px; |
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| 202 | background-color: #f8f8f8; |
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| 203 | padding: 2em; |
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| 204 | } |
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| 205 | table { |
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| 206 | margin-left: 2em; |
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| 207 | } |
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| 208 | table.tt { |
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| 209 | vertical-align: top; |
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| 210 | } |
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| 211 | table.full { |
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| 212 | border-style: outset; |
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| 213 | border-width: 1px; |
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| 214 | } |
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| 215 | table.headers { |
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| 216 | border-style: outset; |
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| 217 | border-width: 1px; |
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| 218 | } |
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| 219 | table.tt td { |
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| 220 | vertical-align: top; |
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| 221 | } |
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| 222 | table.full td { |
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| 223 | border-style: inset; |
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| 224 | border-width: 1px; |
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| 225 | } |
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| 226 | table.tt th { |
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| 227 | vertical-align: top; |
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| 228 | } |
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| 229 | table.full th { |
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| 230 | border-style: inset; |
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| 231 | border-width: 1px; |
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| 232 | } |
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| 233 | table.headers th { |
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| 234 | border-style: none none inset none; |
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| 235 | border-width: 1px; |
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| 236 | } |
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| 237 | table.left { |
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| 238 | margin-right: auto; |
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| 239 | } |
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| 240 | table.right { |
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| 241 | margin-left: auto; |
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| 242 | } |
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| 243 | table.center { |
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| 244 | margin-left: auto; |
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| 245 | margin-right: auto; |
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| 246 | } |
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| 247 | caption { |
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| 248 | caption-side: bottom; |
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| 249 | font-weight: bold; |
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| 250 | font-size: 9pt; |
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| 251 | margin-top: .5em; |
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| 252 | } |
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| 253 | |
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| 254 | table.header { |
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| 255 | border-spacing: 1px; |
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| 256 | width: 95%; |
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| 257 | font-size: 10pt; |
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| 258 | color: white; |
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| 259 | } |
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| 260 | td.top { |
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| 261 | vertical-align: top; |
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| 262 | } |
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| 263 | td.topnowrap { |
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| 264 | vertical-align: top; |
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| 265 | white-space: nowrap; |
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| 266 | } |
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| 267 | table.header td { |
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| 268 | background-color: gray; |
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| 269 | width: 50%; |
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| 270 | } |
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| 271 | table.header a { |
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| 272 | color: white; |
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| 273 | } |
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| 274 | td.reference { |
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| 275 | vertical-align: top; |
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| 276 | white-space: nowrap; |
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| 277 | padding-right: 1em; |
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| 278 | } |
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| 279 | thead { |
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| 280 | display:table-header-group; |
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| 281 | } |
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| 282 | ul.toc, ul.toc ul { |
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| 283 | list-style: none; |
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| 284 | margin-left: 1.5em; |
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| 285 | padding-left: 0em; |
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| 286 | } |
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| 287 | ul.toc li { |
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| 288 | line-height: 150%; |
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| 289 | font-weight: bold; |
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| 290 | font-size: 10pt; |
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| 291 | margin-left: 0em; |
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| 292 | } |
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| 293 | ul.toc li li { |
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| 294 | line-height: normal; |
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| 295 | font-weight: normal; |
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| 296 | font-size: 9pt; |
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| 297 | margin-left: 0em; |
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| 298 | } |
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| 299 | li.excluded { |
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| 300 | font-size: 0pt; |
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| 301 | } |
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| 302 | ul p { |
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| 303 | margin-left: 0em; |
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| 304 | } |
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| 305 | ul.ind, ul.ind ul { |
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| 306 | list-style: none; |
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| 307 | margin-left: 1.5em; |
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| 308 | padding-left: 0em; |
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| 309 | page-break-before: avoid; |
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| 310 | } |
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| 311 | ul.ind li { |
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| 312 | font-weight: bold; |
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| 313 | line-height: 200%; |
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| 314 | margin-left: 0em; |
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| 315 | } |
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| 316 | ul.ind li li { |
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| 317 | font-weight: normal; |
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| 318 | line-height: 150%; |
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| 319 | margin-left: 0em; |
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| 320 | } |
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| 321 | .avoidbreak { |
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| 322 | page-break-inside: avoid; |
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| 323 | } |
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| 324 | .bcp14 { |
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| 325 | font-style: normal; |
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| 326 | text-transform: lowercase; |
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| 327 | font-variant: small-caps; |
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| 328 | } |
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| 329 | .comment { |
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| 330 | background-color: yellow; |
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| 331 | } |
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| 332 | .center { |
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| 333 | text-align: center; |
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| 334 | } |
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| 335 | .error { |
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| 336 | color: red; |
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| 337 | font-style: italic; |
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| 338 | font-weight: bold; |
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| 339 | } |
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| 340 | .figure { |
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| 341 | font-weight: bold; |
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| 342 | text-align: center; |
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| 343 | font-size: 9pt; |
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| 344 | } |
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| 345 | .filename { |
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| 346 | color: #333333; |
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| 347 | font-weight: bold; |
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| 348 | font-size: 12pt; |
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| 349 | line-height: 21pt; |
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| 350 | text-align: center; |
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| 351 | } |
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| 352 | .fn { |
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| 353 | font-weight: bold; |
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| 354 | } |
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| 355 | .hidden { |
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| 356 | display: none; |
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| 357 | } |
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| 358 | .left { |
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| 359 | text-align: left; |
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| 360 | } |
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| 361 | .right { |
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| 362 | text-align: right; |
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| 363 | } |
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| 364 | .title { |
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| 365 | color: #990000; |
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| 366 | font-size: 18pt; |
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| 367 | line-height: 18pt; |
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| 368 | font-weight: bold; |
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| 369 | text-align: center; |
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| 370 | margin-top: 36pt; |
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| 371 | } |
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| 372 | .vcardline { |
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| 373 | display: block; |
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| 374 | } |
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| 375 | .warning { |
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| 376 | font-size: 14pt; |
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| 377 | background-color: yellow; |
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| 378 | } |
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| 379 | .feedback { |
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| 380 | position: fixed; |
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| 381 | bottom: 1%; |
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| 382 | right: 1%; |
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| 383 | padding: 3px 5px; |
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| 384 | color: white; |
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| 385 | border-radius: 5px; |
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| 386 | background: #a00000; |
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| 387 | border: 1px solid silver; |
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| 388 | } |
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| 389 | .fbbutton { |
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| 390 | margin-left: 1em; |
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| 391 | color: #303030; |
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| 392 | font-size: small; |
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| 393 | font-weight: normal; |
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| 394 | background: #d0d000; |
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| 395 | padding: 1px 4px; |
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| 396 | border: 1px solid silver; |
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| 397 | border-radius: 5px; |
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| 398 | } |
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| 399 | |
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| 400 | @media print { |
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| 401 | .noprint { |
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| 402 | display: none; |
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| 403 | } |
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| 404 | |
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| 405 | a { |
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| 406 | color: black; |
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| 407 | text-decoration: none; |
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| 408 | } |
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| 409 | |
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| 410 | table.header { |
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| 411 | width: 90%; |
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| 412 | } |
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| 413 | |
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| 414 | td.header { |
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| 415 | width: 50%; |
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| 416 | color: black; |
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| 417 | background-color: white; |
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| 418 | vertical-align: top; |
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| 419 | font-size: 12pt; |
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| 420 | } |
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| 421 | |
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| 422 | ul.toc a:nth-child(2)::after { |
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| 423 | content: leader('.') target-counter(attr(href), page); |
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| 424 | } |
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| 425 | |
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| 426 | ul.ind li li a { |
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| 427 | content: target-counter(attr(href), page); |
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| 428 | } |
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| 429 | |
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| 430 | .print2col { |
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| 431 | column-count: 2; |
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| 432 | -moz-column-count: 2; |
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| 433 | column-fill: auto; |
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| 434 | } |
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| 435 | } |
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| 436 | |
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| 437 | @page { |
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| 438 | @top-left { |
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| 439 | content: "Internet-Draft"; |
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| 440 | } |
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| 441 | @top-right { |
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| 442 | content: "June 2013"; |
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| 443 | } |
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| 444 | @top-center { |
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| 445 | content: "HTTP/1.1 Conditional Requests"; |
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| 446 | } |
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| 447 | @bottom-left { |
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| 448 | content: "Fielding & Reschke"; |
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| 449 | } |
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| 450 | @bottom-center { |
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| 451 | content: "Expires December 5, 2013"; |
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| 452 | } |
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| 453 | @bottom-right { |
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| 454 | content: "[Page " counter(page) "]"; |
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| 455 | } |
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| 456 | } |
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| 457 | |
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| 458 | @page:first { |
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| 459 | @top-left { |
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| 460 | content: normal; |
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| 461 | } |
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| 462 | @top-right { |
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| 463 | content: normal; |
|---|
| 464 | } |
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| 465 | @top-center { |
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| 466 | content: normal; |
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| 467 | } |
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| 468 | } |
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| 469 | </style><link rel="Contents" href="#rfc.toc"> |
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| 470 | <link rel="Author" href="#rfc.authors"> |
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| 471 | <link rel="Copyright" href="#rfc.copyrightnotice"> |
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| 472 | <link rel="Index" href="#rfc.index"> |
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| 473 | <link rel="Chapter" title="1 Introduction" href="#rfc.section.1"> |
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| 474 | <link rel="Chapter" title="2 Validators" href="#rfc.section.2"> |
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| 475 | <link rel="Chapter" title="3 Precondition Header Fields" href="#rfc.section.3"> |
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| 476 | <link rel="Chapter" title="4 Status Code Definitions" href="#rfc.section.4"> |
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| 477 | <link rel="Chapter" title="5 Evaluation and Precedence" href="#rfc.section.5"> |
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| 478 | <link rel="Chapter" title="6 IANA Considerations" href="#rfc.section.6"> |
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| 479 | <link rel="Chapter" title="7 Security Considerations" href="#rfc.section.7"> |
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| 480 | <link rel="Chapter" title="8 Acknowledgments" href="#rfc.section.8"> |
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| 481 | <link rel="Chapter" href="#rfc.section.9" title="9 References"> |
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| 482 | <link rel="Appendix" title="A Changes from RFC 2616" href="#rfc.section.A"> |
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| 483 | <link rel="Appendix" title="B Imported ABNF" href="#rfc.section.B"> |
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| 484 | <link rel="Appendix" title="C Collected ABNF" href="#rfc.section.C"> |
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| 485 | <link rel="Appendix" title="D Change Log (to be removed by RFC Editor before publication)" href="#rfc.section.D"> |
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| 486 | <link href="p2-semantics.html" rel="prev"> |
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| 487 | <link href="p5-range.html" rel="next"> |
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| 488 | <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.597, 2013/05/27 06:39:42, XSLT vendor: SAXON 8.9 from Saxonica http://www.saxonica.com/"> |
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| 489 | <link rel="schema.dct" href="http://purl.org/dc/terms/"> |
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| 490 | <meta name="dct.creator" content="Fielding, R."> |
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| 491 | <meta name="dct.creator" content="Reschke, J. F."> |
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| 492 | <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p4-conditional-latest"> |
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| 493 | <meta name="dct.issued" scheme="ISO8601" content="2013-06-03"> |
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| 494 | <meta name="dct.replaces" content="urn:ietf:rfc:2616"> |
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| 495 | <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."> |
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| 496 | <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."> |
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| 497 | </head> |
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| 498 | <body onload="init();"> |
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| 499 | <table class="header"> |
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| 500 | <tbody> |
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| 501 | <tr> |
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| 502 | <td class="left">HTTPbis Working Group</td> |
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| 503 | <td class="right">R. Fielding, Editor</td> |
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| 504 | </tr> |
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| 505 | <tr> |
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| 506 | <td class="left">Internet-Draft</td> |
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| 507 | <td class="right">Adobe</td> |
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| 508 | </tr> |
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| 509 | <tr> |
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| 510 | <td class="left">Obsoletes: <a href="http://tools.ietf.org/html/rfc2616">2616</a> (if approved) |
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| 511 | </td> |
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| 512 | <td class="right">J. Reschke, Editor</td> |
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| 513 | </tr> |
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| 514 | <tr> |
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| 515 | <td class="left">Intended status: Standards Track</td> |
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| 516 | <td class="right">greenbytes</td> |
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| 517 | </tr> |
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| 518 | <tr> |
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| 519 | <td class="left">Expires: December 5, 2013</td> |
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| 520 | <td class="right">June 3, 2013</td> |
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| 521 | </tr> |
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| 522 | </tbody> |
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| 523 | </table> |
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| 524 | <p class="title">Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests<br><span class="filename">draft-ietf-httpbis-p4-conditional-latest</span></p> |
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| 525 | <h1 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h1> |
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| 526 | <p>The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information |
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| 527 | systems. This document defines HTTP/1.1 conditional requests, including metadata header fields for indicating state changes, |
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| 528 | request header fields for making preconditions on such state, and rules for constructing the responses to a conditional request |
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| 529 | when one or more preconditions evaluate to false. |
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| 530 | </p> |
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| 531 | <h1 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h1> |
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| 532 | <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>>. |
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| 533 | </p> |
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| 534 | <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>>. |
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| 535 | </p> |
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| 536 | <p>The changes in this draft are summarized in <a href="#changes.since.22" title="Since draft-ietf-httpbis-p4-conditional-22">Appendix D.4</a>. |
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| 537 | </p> |
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| 538 | <h1><a id="rfc.status" href="#rfc.status">Status of This Memo</a></h1> |
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| 539 | <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.</p> |
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| 540 | <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute |
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| 541 | 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>. |
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| 542 | </p> |
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| 543 | <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other |
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| 544 | documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work |
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| 545 | in progress”. |
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| 546 | </p> |
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| 547 | <p>This Internet-Draft will expire on December 5, 2013.</p> |
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| 548 | <h1><a id="rfc.copyrightnotice" href="#rfc.copyrightnotice">Copyright Notice</a></h1> |
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| 549 | <p>Copyright © 2013 IETF Trust and the persons identified as the document authors. All rights reserved.</p> |
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| 550 | <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 |
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| 551 | and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License |
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| 552 | text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified |
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| 553 | BSD License. |
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| 554 | </p> |
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| 555 | <p>This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November |
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| 556 | 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to |
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| 557 | allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) |
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| 558 | controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative |
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| 559 | works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate |
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| 560 | it into languages other than English. |
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| 561 | </p> |
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| 562 | <hr class="noprint"> |
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| 563 | <h1 class="np" id="rfc.toc"><a href="#rfc.toc">Table of Contents</a></h1> |
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| 564 | <ul class="toc"> |
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| 565 | <li><a href="#rfc.section.1">1.</a> <a href="#introduction">Introduction</a><ul> |
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| 566 | <li><a href="#rfc.section.1.1">1.1</a> <a href="#conformance">Conformance and Error Handling</a></li> |
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| 567 | <li><a href="#rfc.section.1.2">1.2</a> <a href="#notation">Syntax Notation</a></li> |
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| 568 | </ul> |
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| 569 | </li> |
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| 570 | <li><a href="#rfc.section.2">2.</a> <a href="#validators">Validators</a><ul> |
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| 571 | <li><a href="#rfc.section.2.1">2.1</a> <a href="#weak.and.strong.validators">Weak versus Strong</a></li> |
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| 572 | <li><a href="#rfc.section.2.2">2.2</a> <a href="#header.last-modified">Last-Modified</a><ul> |
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| 573 | <li><a href="#rfc.section.2.2.1">2.2.1</a> <a href="#lastmod.generation">Generation</a></li> |
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| 574 | <li><a href="#rfc.section.2.2.2">2.2.2</a> <a href="#lastmod.comparison">Comparison</a></li> |
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| 575 | </ul> |
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| 576 | </li> |
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| 577 | <li><a href="#rfc.section.2.3">2.3</a> <a href="#header.etag">ETag</a><ul> |
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| 578 | <li><a href="#rfc.section.2.3.1">2.3.1</a> <a href="#entity.tag.generation">Generation</a></li> |
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| 579 | <li><a href="#rfc.section.2.3.2">2.3.2</a> <a href="#entity.tag.comparison">Comparison</a></li> |
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| 580 | <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> |
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| 581 | </ul> |
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| 582 | </li> |
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| 583 | <li><a href="#rfc.section.2.4">2.4</a> <a href="#when.to.use.entity.tags.and.last-modified.dates">When to Use Entity-tags and Last-Modified Dates</a></li> |
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| 584 | </ul> |
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| 585 | </li> |
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| 586 | <li><a href="#rfc.section.3">3.</a> <a href="#header.field.definitions">Precondition Header Fields</a><ul> |
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| 587 | <li><a href="#rfc.section.3.1">3.1</a> <a href="#header.if-match">If-Match</a></li> |
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| 588 | <li><a href="#rfc.section.3.2">3.2</a> <a href="#header.if-none-match">If-None-Match</a></li> |
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| 589 | <li><a href="#rfc.section.3.3">3.3</a> <a href="#header.if-modified-since">If-Modified-Since</a></li> |
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| 590 | <li><a href="#rfc.section.3.4">3.4</a> <a href="#header.if-unmodified-since">If-Unmodified-Since</a></li> |
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| 591 | <li><a href="#rfc.section.3.5">3.5</a> <a href="#header.if-range">If-Range</a></li> |
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| 592 | </ul> |
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| 593 | </li> |
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| 594 | <li><a href="#rfc.section.4">4.</a> <a href="#status.code.definitions">Status Code Definitions</a><ul> |
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| 595 | <li><a href="#rfc.section.4.1">4.1</a> <a href="#status.304">304 Not Modified</a></li> |
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| 596 | <li><a href="#rfc.section.4.2">4.2</a> <a href="#status.412">412 Precondition Failed</a></li> |
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| 597 | </ul> |
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| 598 | </li> |
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| 599 | <li><a href="#rfc.section.5">5.</a> <a href="#precedence">Evaluation and Precedence</a></li> |
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| 600 | <li><a href="#rfc.section.6">6.</a> <a href="#IANA.considerations">IANA Considerations</a><ul> |
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| 601 | <li><a href="#rfc.section.6.1">6.1</a> <a href="#status.code.registration">Status Code Registration</a></li> |
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| 602 | <li><a href="#rfc.section.6.2">6.2</a> <a href="#header.field.registration">Header Field Registration</a></li> |
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| 603 | </ul> |
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| 604 | </li> |
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| 605 | <li><a href="#rfc.section.7">7.</a> <a href="#security.considerations">Security Considerations</a></li> |
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| 606 | <li><a href="#rfc.section.8">8.</a> <a href="#acks">Acknowledgments</a></li> |
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| 607 | <li><a href="#rfc.section.9">9.</a> <a href="#rfc.references">References</a><ul> |
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| 608 | <li><a href="#rfc.section.9.1">9.1</a> <a href="#rfc.references.1">Normative References</a></li> |
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| 609 | <li><a href="#rfc.section.9.2">9.2</a> <a href="#rfc.references.2">Informative References</a></li> |
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| 610 | </ul> |
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| 611 | </li> |
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| 612 | <li><a href="#rfc.authors">Authors' Addresses</a></li> |
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| 613 | <li><a href="#rfc.section.A">A.</a> <a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li> |
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| 614 | <li><a href="#rfc.section.B">B.</a> <a href="#imported.abnf">Imported ABNF</a></li> |
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| 615 | <li><a href="#rfc.section.C">C.</a> <a href="#collected.abnf">Collected ABNF</a></li> |
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| 616 | <li><a href="#rfc.section.D">D.</a> <a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul> |
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| 617 | <li><a href="#rfc.section.D.1">D.1</a> <a href="#changes.since.19">Since draft-ietf-httpbis-p4-conditional-19</a></li> |
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| 618 | <li><a href="#rfc.section.D.2">D.2</a> <a href="#changes.since.20">Since draft-ietf-httpbis-p4-conditional-20</a></li> |
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| 619 | <li><a href="#rfc.section.D.3">D.3</a> <a href="#changes.since.21">Since draft-ietf-httpbis-p4-conditional-21</a></li> |
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| 620 | <li><a href="#rfc.section.D.4">D.4</a> <a href="#changes.since.22">Since draft-ietf-httpbis-p4-conditional-22</a></li> |
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| 621 | </ul> |
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| 622 | </li> |
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| 623 | <li><a href="#rfc.index">Index</a></li> |
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| 624 | </ul> |
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| 625 | <h1 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a> <a id="introduction" href="#introduction">Introduction</a></h1> |
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| 626 | <p id="rfc.section.1.p.1">Conditional requests are HTTP requests <a href="#Part2" id="rfc.xref.Part2.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a> that include one or more header fields indicating a precondition to be tested before applying the method semantics to the |
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| 627 | target resource. This document defines the HTTP/1.1 conditional request mechanisms in terms of the architecture, syntax notation, |
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| 628 | and conformance criteria defined in <a href="#Part1" id="rfc.xref.Part1.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>. |
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| 629 | </p> |
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| 630 | <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="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, to prevent the "lost update" problem: |
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| 631 | one client accidentally overwriting the work of another client that has been acting in parallel. |
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| 632 | </p> |
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| 633 | <p id="rfc.section.1.p.3"><span id="rfc.iref.s.1"></span> Conditional request preconditions are based on the state of the target resource as a whole (its current value set) or the |
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| 634 | state as observed in a previously obtained representation (one value in that set). A resource might have multiple current |
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| 635 | representations, each with its own observable state. The conditional request mechanisms assume that the mapping of requests |
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| 636 | to a "selected representation" (<a href="p2-semantics.html#representations" title="Representations">Section 3</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>) will be consistent over time if the server intends to take advantage of conditionals. Regardless, if the mapping is inconsistent |
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| 637 | and the server is unable to select the appropriate representation, then no harm will result when the precondition evaluates |
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| 638 | to false. |
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| 639 | </p> |
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| 640 | <p id="rfc.section.1.p.4">The conditional request preconditions defined by this specification are evaluated by comparing the validators provided in |
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| 641 | the conditional request header fields to the current validators for the selected representation in the order defined by <a href="#precedence" title="Evaluation and Precedence">Section 5</a>. |
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| 642 | </p> |
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| 643 | <h2 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a> <a id="conformance" href="#conformance">Conformance and Error Handling</a></h2> |
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| 644 | <p id="rfc.section.1.1.p.1">The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" |
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| 645 | in this document are to be interpreted as described in <a href="#RFC2119" id="rfc.xref.RFC2119.1"><cite title="Key words for use in RFCs to Indicate Requirement Levels">[RFC2119]</cite></a>. |
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| 646 | </p> |
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| 647 | <p id="rfc.section.1.1.p.2">Conformance criteria and considerations regarding error handling are defined in <a href="p1-messaging.html#conformance" title="Conformance and Error Handling">Section 2.5</a> of <a href="#Part1" id="rfc.xref.Part1.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>. |
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| 648 | </p> |
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| 649 | <h2 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a> <a id="notation" href="#notation">Syntax Notation</a></h2> |
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| 650 | <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="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[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. |
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| 651 | </p> |
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| 652 | <div id="rfc.iref.m.1"></div> |
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| 653 | <div id="rfc.iref.v.1"></div> |
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| 654 | <h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a> <a id="validators" href="#validators">Validators</a></h1> |
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| 655 | <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: |
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| 656 | 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. |
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| 657 | </p> |
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| 658 | <div id="rfc.iref.v.2"></div> |
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| 659 | <div id="rfc.iref.v.3"></div> |
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| 660 | <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> |
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| 661 | <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. |
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| 662 | Strong validators are ideal for comparisons but can be very difficult (and occasionally impossible) to generate efficiently. |
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| 663 | Rather than impose that all forms of resource adhere to the same strength of validator, HTTP exposes the type of validator |
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| 664 | in use and imposes restrictions on when weak validators can be used as preconditions. |
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| 665 | </p> |
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| 666 | <p id="rfc.section.2.1.p.2">A "strong validator" is representation metadata that changes value whenever a change occurs to the representation data that |
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| 667 | would be observable in the payload body of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to GET. |
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| 668 | </p> |
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| 669 | <p id="rfc.section.2.1.p.3">A strong validator might change for other reasons, such as when a semantically significant part of the representation metadata |
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| 670 | 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 |
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| 671 | responses held by remote caches and authoring tools. A strong validator is unique across all representations of a given resource, |
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| 672 | such that no two representations of that resource can share the same validator unless their representation data is identical. |
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| 673 | </p> |
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| 674 | <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 |
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| 675 | an entry using a validator that it obtained in the distant past. A strong validator is unique across all versions of all representations |
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| 676 | associated with a particular resource over time. However, there is no implication of uniqueness across representations of |
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| 677 | different resources (i.e., the same strong validator might be in use for representations of multiple resources at the same |
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| 678 | time and does not imply that those representations are equivalent). |
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| 679 | </p> |
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| 680 | <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 |
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| 681 | to a representation always results in a unique node name and revision identifier being assigned before the representation |
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| 682 | is made accessible to GET. A collision-resistant hash function applied to the representation data is also sufficient if the |
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| 683 | data is available prior to the response header fields being sent and the digest does not need to be recalculated every time |
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| 684 | a validation request is received. However, if a resource has distinct representations that differ only in their metadata, |
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| 685 | 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. |
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| 686 | </p> |
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| 687 | <p id="rfc.section.2.1.p.6">In contrast, a "weak validator" is representation metadata that might not change for every change to the representation data. |
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| 688 | This weakness might be due to limitations in how the value is calculated, such as clock resolution or an inability to ensure |
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| 689 | uniqueness for all possible representations of the resource, or due to a desire by the resource owner to group representations |
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| 690 | 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. |
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| 691 | In other words, a weak entity-tag ought to change whenever the origin server wants caches to invalidate old responses. |
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| 692 | </p> |
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| 693 | <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 |
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| 694 | be grouped into sets of equivalent representations (from the origin server's perspective) with the same weak validator in |
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| 695 | order to allow cached representations to be valid for a reasonable period of time (perhaps adjusted dynamically based on server |
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| 696 | load or weather quality). Likewise, a representation's modification time, if defined with only one-second resolution, might |
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| 697 | be a weak validator if it is possible for the representation to be modified twice during a single second and retrieved between |
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| 698 | those modifications. |
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| 699 | </p> |
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| 700 | <p id="rfc.section.2.1.p.8">Likewise, a validator is weak if it is shared by two or more representations of a given resource at the same time, unless |
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| 701 | those representations have identical representation data. For example, if the origin server sends the same validator for a |
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| 702 | representation with a gzip content coding applied as it does for a representation with no content coding, then that validator |
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| 703 | is weak. However, two simultaneous representations might share the same strong validator if they differ only in the representation |
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| 704 | metadata, such as when two different media types are available for the same representation data. |
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| 705 | </p> |
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| 706 | <p id="rfc.section.2.1.p.9">A "use" of a validator occurs when either a client generates a request and includes the validator in a precondition or when |
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| 707 | a server compares two validators. Weak validators are only usable in contexts that do not depend on exact equality of the |
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| 708 | representation data. Strong validators are usable and preferred for all conditional requests, including cache validation, |
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| 709 | partial content ranges, and "lost update" avoidance. |
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| 710 | </p> |
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| 711 | <div id="rfc.iref.l.1"></div> |
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| 712 | <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> |
|---|
| 713 | <p id="rfc.section.2.2.p.1">The "Last-Modified" header field in a response provides a timestamp indicating the date and time at which the origin server |
|---|
| 714 | believes the selected representation was last modified, as determined at the conclusion of handling the request. |
|---|
| 715 | </p> |
|---|
| 716 | <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> |
|---|
| 717 | </pre><p id="rfc.section.2.2.p.3">An example of its use is</p> |
|---|
| 718 | <div id="rfc.figure.u.2"></div><pre class="text"> Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT |
|---|
| 719 | </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> |
|---|
| 720 | <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, |
|---|
| 721 | since its use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>) results in a substantial reduction of HTTP traffic on the Internet and can be a significant factor in improving service |
|---|
| 722 | scalability and reliability. |
|---|
| 723 | </p> |
|---|
| 724 | <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 |
|---|
| 725 | the most recent time that any of those parts were changed. How that value is determined for any given resource is an implementation |
|---|
| 726 | detail beyond the scope of this specification. What matters to HTTP is how recipients of the Last-Modified header field can |
|---|
| 727 | use its value to make conditional requests and test the validity of locally cached responses. |
|---|
| 728 | </p> |
|---|
| 729 | <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 |
|---|
| 730 | time, especially if the representation changes near the time that the response is generated. |
|---|
| 731 | </p> |
|---|
| 732 | <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, |
|---|
| 733 | 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 |
|---|
| 734 | on cache validation. |
|---|
| 735 | </p> |
|---|
| 736 | <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 |
|---|
| 737 | with a reliable clock. |
|---|
| 738 | </p> |
|---|
| 739 | <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> |
|---|
| 740 | <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 |
|---|
| 741 | strong, using the following rules: |
|---|
| 742 | </p> |
|---|
| 743 | <ul> |
|---|
| 744 | <li>The validator is being compared by an origin server to the actual current validator for the representation and,</li> |
|---|
| 745 | <li>That origin server reliably knows that the associated representation did not change twice during the second covered by the |
|---|
| 746 | presented validator. |
|---|
| 747 | </li> |
|---|
| 748 | </ul> |
|---|
| 749 | <p id="rfc.section.2.2.2.p.2">or </p> |
|---|
| 750 | <ul> |
|---|
| 751 | <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 |
|---|
| 752 | </li> |
|---|
| 753 | <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 |
|---|
| 754 | </li> |
|---|
| 755 | <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li> |
|---|
| 756 | </ul> |
|---|
| 757 | <p id="rfc.section.2.2.2.p.3">or </p> |
|---|
| 758 | <ul> |
|---|
| 759 | <li>The validator is being compared by an intermediate cache to the validator stored in its cache entry for the representation, |
|---|
| 760 | and |
|---|
| 761 | </li> |
|---|
| 762 | <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 |
|---|
| 763 | </li> |
|---|
| 764 | <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li> |
|---|
| 765 | </ul> |
|---|
| 766 | <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 |
|---|
| 767 | 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 |
|---|
| 768 | 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. |
|---|
| 769 | </p> |
|---|
| 770 | <div id="rfc.iref.e.1"></div> |
|---|
| 771 | <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> |
|---|
| 772 | <p id="rfc.section.2.3.p.1">The "ETag" header field in a response provides the current entity-tag for the selected representation, as determined at the |
|---|
| 773 | conclusion of handling the request. An entity-tag is an opaque validator for differentiating between multiple representations |
|---|
| 774 | of the same resource, regardless of whether those multiple representations are due to resource state changes over time, content |
|---|
| 775 | negotiation resulting in multiple representations being valid at the same time, or both. An entity-tag consists of an opaque |
|---|
| 776 | quoted string, possibly prefixed by a weakness indicator. |
|---|
| 777 | </p> |
|---|
| 778 | <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> |
|---|
| 779 | |
|---|
| 780 | <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> |
|---|
| 781 | <a href="#header.etag" class="smpl">weak</a> = %x57.2F ; "W/", case-sensitive |
|---|
| 782 | <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> |
|---|
| 783 | <a href="#header.etag" class="smpl">etagc</a> = %x21 / %x23-7E / <a href="#imported.abnf" class="smpl">obs-text</a> |
|---|
| 784 | ; <a href="#imported.abnf" class="smpl">VCHAR</a> except double quotes, plus obs-text |
|---|
| 785 | </pre><div class="note" id="rfc.section.2.3.p.3"> |
|---|
| 786 | <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 |
|---|
| 787 | tags. |
|---|
| 788 | </p> |
|---|
| 789 | </div> |
|---|
| 790 | <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 |
|---|
| 791 | modification dates, where the one-second resolution of HTTP date values is not sufficient, or where modification dates are |
|---|
| 792 | not consistently maintained. |
|---|
| 793 | </p> |
|---|
| 794 | <div id="rfc.figure.u.4"></div> |
|---|
| 795 | <p>Examples:</p><pre class="text"> ETag: "xyzzy" |
|---|
| 796 | ETag: W/"xyzzy" |
|---|
| 797 | ETag: "" |
|---|
| 798 | </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 |
|---|
| 799 | for a representation and the generation of that entity-tag does not satisfy all of the characteristics of a strong validator |
|---|
| 800 | (<a href="#weak.and.strong.validators" title="Weak versus Strong">Section 2.1</a>), then the origin server <em class="bcp14">MUST</em> mark the entity-tag as weak by prefixing its opaque value with "W/" (case-sensitive). |
|---|
| 801 | </p> |
|---|
| 802 | <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> |
|---|
| 803 | <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 |
|---|
| 804 | the most accurate and efficient validation mechanism for that resource, and that any such mechanism can be mapped to a simple |
|---|
| 805 | sequence of octets for easy comparison. Since the value is opaque, there is no need for the client to be aware of how each |
|---|
| 806 | entity-tag is constructed. |
|---|
| 807 | </p> |
|---|
| 808 | <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 |
|---|
| 809 | number, perhaps combined with a variance identifier for content negotiation, to accurately differentiate between representations. |
|---|
| 810 | Other implementations might use a collision-resistant hash of representation content, a combination of various filesystem |
|---|
| 811 | attributes, or a modification timestamp that has sub-second resolution. |
|---|
| 812 | </p> |
|---|
| 813 | <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 |
|---|
| 814 | the entity-tag's use in conditional requests and evaluating cache freshness (<a href="#Part6" id="rfc.xref.Part6.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>) can result in a substantial reduction of HTTP network traffic and can be a significant factor in improving service scalability |
|---|
| 815 | and reliability. |
|---|
| 816 | </p> |
|---|
| 817 | <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> |
|---|
| 818 | <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 |
|---|
| 819 | or not: |
|---|
| 820 | </p> |
|---|
| 821 | <ul> |
|---|
| 822 | <li><dfn>Strong comparison</dfn>: two entity-tags are equivalent if both are not weak and their opaque-tags match character-by-character. |
|---|
| 823 | </li> |
|---|
| 824 | <li><dfn>Weak comparison</dfn>: two entity-tags are equivalent if their opaque-tags match character-by-character, regardless of either or both being tagged |
|---|
| 825 | as "weak". |
|---|
| 826 | </li> |
|---|
| 827 | </ul> |
|---|
| 828 | <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> |
|---|
| 829 | <div id="rfc.table.u.1"> |
|---|
| 830 | <table class="tt full left" cellpadding="3" cellspacing="0"> |
|---|
| 831 | <thead> |
|---|
| 832 | <tr> |
|---|
| 833 | <th>ETag 1</th> |
|---|
| 834 | <th>ETag 2</th> |
|---|
| 835 | <th>Strong Comparison</th> |
|---|
| 836 | <th>Weak Comparison</th> |
|---|
| 837 | </tr> |
|---|
| 838 | </thead> |
|---|
| 839 | <tbody> |
|---|
| 840 | <tr> |
|---|
| 841 | <td class="left">W/"1"</td> |
|---|
| 842 | <td class="left">W/"1"</td> |
|---|
| 843 | <td class="left">no match</td> |
|---|
| 844 | <td class="left">match</td> |
|---|
| 845 | </tr> |
|---|
| 846 | <tr> |
|---|
| 847 | <td class="left">W/"1"</td> |
|---|
| 848 | <td class="left">W/"2"</td> |
|---|
| 849 | <td class="left">no match</td> |
|---|
| 850 | <td class="left">no match</td> |
|---|
| 851 | </tr> |
|---|
| 852 | <tr> |
|---|
| 853 | <td class="left">W/"1"</td> |
|---|
| 854 | <td class="left">"1"</td> |
|---|
| 855 | <td class="left">no match</td> |
|---|
| 856 | <td class="left">match</td> |
|---|
| 857 | </tr> |
|---|
| 858 | <tr> |
|---|
| 859 | <td class="left">"1"</td> |
|---|
| 860 | <td class="left">"1"</td> |
|---|
| 861 | <td class="left">match</td> |
|---|
| 862 | <td class="left">match</td> |
|---|
| 863 | </tr> |
|---|
| 864 | </tbody> |
|---|
| 865 | </table> |
|---|
| 866 | </div> |
|---|
| 867 | <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> |
|---|
| 868 | <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 3.4</a> of <a href="#Part2" id="rfc.xref.Part2.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>), and where the representations sent in response to 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 5.3.4</a> of <a href="#Part2" id="rfc.xref.Part2.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>): |
|---|
| 869 | </p> |
|---|
| 870 | <div id="rfc.figure.u.5"></div> |
|---|
| 871 | <p>>> Request:</p><pre class="text2">GET /index HTTP/1.1 |
|---|
| 872 | Host: www.example.com |
|---|
| 873 | Accept-Encoding: gzip |
|---|
| 874 | |
|---|
| 875 | </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> |
|---|
| 876 | <div id="rfc.figure.u.6"></div> |
|---|
| 877 | <p>>> Response:</p><pre class="text">HTTP/1.1 200 OK |
|---|
| 878 | Date: Fri, 26 Mar 2010 00:05:00 GMT |
|---|
| 879 | ETag: "123-a" |
|---|
| 880 | Content-Length: 70 |
|---|
| 881 | Vary: Accept-Encoding |
|---|
| 882 | Content-Type: text/plain |
|---|
| 883 | |
|---|
| 884 | <span id="exbody">Hello World! |
|---|
| 885 | Hello World! |
|---|
| 886 | Hello World! |
|---|
| 887 | Hello World! |
|---|
| 888 | Hello World! |
|---|
| 889 | </span></pre><p id="rfc.section.2.3.3.p.5">An alternative representation that does use gzip content coding would be:</p> |
|---|
| 890 | <div id="rfc.figure.u.7"></div> |
|---|
| 891 | <p>>> Response:</p><pre class="text">HTTP/1.1 200 OK |
|---|
| 892 | Date: Fri, 26 Mar 2010 00:05:00 GMT |
|---|
| 893 | ETag: "123-b" |
|---|
| 894 | Content-Length: 43 |
|---|
| 895 | Vary: Accept-Encoding |
|---|
| 896 | Content-Type: text/plain |
|---|
| 897 | Content-Encoding: gzip |
|---|
| 898 | |
|---|
| 899 | <em>...binary data...</em></pre><div class="note" id="rfc.section.2.3.3.p.7"> |
|---|
| 900 | <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 |
|---|
| 901 | from an unencoded representation to prevent conflicts during cache updates and range requests. In contrast, transfer codings |
|---|
| 902 | (<a href="p1-messaging.html#transfer.codings" title="Transfer Codings">Section 4</a> of <a href="#Part1" id="rfc.xref.Part1.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>) apply only during message transfer and do not require distinct entity-tags. |
|---|
| 903 | </p> |
|---|
| 904 | </div> |
|---|
| 905 | <h2 id="rfc.section.2.4"><a href="#rfc.section.2.4">2.4</a> <a id="when.to.use.entity.tags.and.last-modified.dates" href="#when.to.use.entity.tags.and.last-modified.dates">When to Use Entity-tags and Last-Modified Dates</a></h2> |
|---|
| 906 | <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 |
|---|
| 907 | ought to be used, and for what purposes. |
|---|
| 908 | </p> |
|---|
| 909 | <p id="rfc.section.2.4.p.2">In <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> responses to GET or HEAD, an origin server: |
|---|
| 910 | </p> |
|---|
| 911 | <ul> |
|---|
| 912 | <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one. |
|---|
| 913 | </li> |
|---|
| 914 | <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, |
|---|
| 915 | or if it is unfeasible to send a strong entity-tag. |
|---|
| 916 | </li> |
|---|
| 917 | <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. |
|---|
| 918 | </li> |
|---|
| 919 | </ul> |
|---|
| 920 | <p id="rfc.section.2.4.p.3">In other words, the preferred behavior for an origin server is to send both a strong entity-tag and a <a href="#header.last-modified" class="smpl">Last-Modified</a> value in successful responses to a retrieval request. |
|---|
| 921 | </p> |
|---|
| 922 | <p id="rfc.section.2.4.p.4">A client: </p> |
|---|
| 923 | <ul> |
|---|
| 924 | <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. |
|---|
| 925 | </li> |
|---|
| 926 | <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. |
|---|
| 927 | </li> |
|---|
| 928 | <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. |
|---|
| 929 | </li> |
|---|
| 930 | <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. |
|---|
| 931 | </li> |
|---|
| 932 | </ul> |
|---|
| 933 | <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> |
|---|
| 934 | <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="Evaluation and Precedence">Section 5</a> defines when the preconditions are applied and the order of evaluation when more than one precondition is present. |
|---|
| 935 | </p> |
|---|
| 936 | <div id="rfc.iref.i.1"></div> |
|---|
| 937 | <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> |
|---|
| 938 | <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 |
|---|
| 939 | for one or more representations of the target resource. |
|---|
| 940 | </p> |
|---|
| 941 | <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 |
|---|
| 942 | overwrites when multiple clients are acting in parallel on the same resource (i.e., the "lost update" problem). An If-Match |
|---|
| 943 | field-value of "*" places the precondition on the existence of any current representation for the target resource. |
|---|
| 944 | </p> |
|---|
| 945 | <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> |
|---|
| 946 | </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 |
|---|
| 947 | of the selected representation using the weak comparison function (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. |
|---|
| 948 | </p> |
|---|
| 949 | <p id="rfc.section.3.1.p.5">If the condition is met, the server <em class="bcp14">MAY</em> perform the request method. |
|---|
| 950 | </p> |
|---|
| 951 | <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 |
|---|
| 952 | Failed)</a> status code. |
|---|
| 953 | </p> |
|---|
| 954 | <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. |
|---|
| 955 | </p> |
|---|
| 956 | <p id="rfc.section.3.1.p.8">Examples:</p> |
|---|
| 957 | <div id="rfc.figure.u.9"></div><pre class="text"> If-Match: "xyzzy" |
|---|
| 958 | If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" |
|---|
| 959 | If-Match: * |
|---|
| 960 | </pre><div id="rfc.iref.i.2"></div> |
|---|
| 961 | <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> |
|---|
| 962 | <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 |
|---|
| 963 | values for representations of the target resource. |
|---|
| 964 | </p> |
|---|
| 965 | <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 |
|---|
| 966 | amount of transaction overhead. A client that has one or more representations previously obtained from the target resource |
|---|
| 967 | 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 |
|---|
| 968 | Modified)</a> response if at least one of those representations matches the selected representation. |
|---|
| 969 | </p> |
|---|
| 970 | <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 |
|---|
| 971 | an existing representation of the target resource when the client believes that the resource does not have a current representation |
|---|
| 972 | (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 4.2.1</a> of <a href="#Part2" id="rfc.xref.Part2.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>). This is a variation on the "lost update" problem that might arise if more than one client attempts to create an initial |
|---|
| 973 | representation for the target resource. |
|---|
| 974 | </p> |
|---|
| 975 | <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> |
|---|
| 976 | </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 |
|---|
| 977 | entity-tag of the selected representation using the weak comparison function (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. |
|---|
| 978 | </p> |
|---|
| 979 | <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 |
|---|
| 980 | Failed)</a> status code when the condition is not met. |
|---|
| 981 | </p> |
|---|
| 982 | <p id="rfc.section.3.2.p.7">If the condition is met, the server <em class="bcp14">MAY</em> perform the requested method and <em class="bcp14">MUST</em> 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> send a <a href="#status.304" class="smpl">304 (Not Modified)</a> response. |
|---|
| 983 | </p> |
|---|
| 984 | <p id="rfc.section.3.2.p.8">Examples:</p> |
|---|
| 985 | <div id="rfc.figure.u.11"></div><pre class="text"> If-None-Match: "xyzzy" |
|---|
| 986 | If-None-Match: W/"xyzzy" |
|---|
| 987 | If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" |
|---|
| 988 | If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" |
|---|
| 989 | If-None-Match: * |
|---|
| 990 | </pre><div id="rfc.iref.i.3"></div> |
|---|
| 991 | <h2 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a> <a id="header.if-modified-since" href="#header.if-modified-since">If-Modified-Since</a></h2> |
|---|
| 992 | <p id="rfc.section.3.3.p.1">The "If-Modified-Since" header field can be used with GET or HEAD to make the method conditional by modification date: if |
|---|
| 993 | the selected representation has not been modified since the time specified in this field, then do not perform the request |
|---|
| 994 | method; instead, respond as detailed below. |
|---|
| 995 | </p> |
|---|
| 996 | <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> |
|---|
| 997 | </pre><p id="rfc.section.3.3.p.3">An example of the field is:</p> |
|---|
| 998 | <div id="rfc.figure.u.13"></div><pre class="text"> If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT |
|---|
| 999 | </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#header.range" class="smpl">Range</a> header field requests that the selected representation be transferred only if it has been modified since the date given by |
|---|
| 1000 | the If-Modified-Since header field. The algorithm for determining this includes the following cases: |
|---|
| 1001 | </p> |
|---|
| 1002 | <ol> |
|---|
| 1003 | <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 |
|---|
| 1004 | that is later than the server's current time is invalid. |
|---|
| 1005 | </li> |
|---|
| 1006 | <li>If the selected representation has been modified since the If-Modified-Since date, the response is exactly the same as for |
|---|
| 1007 | a normal GET. |
|---|
| 1008 | </li> |
|---|
| 1009 | <li>If the selected representation has not been modified since a valid If-Modified-Since date, the server <em class="bcp14">SHOULD</em> send a <a href="#status.304" class="smpl">304 (Not Modified)</a> response. |
|---|
| 1010 | </li> |
|---|
| 1011 | </ol> |
|---|
| 1012 | <p id="rfc.section.3.3.p.6">The two purposes of this feature are to allow efficient updates of cached information, with a minimum amount of transaction |
|---|
| 1013 | overhead, and to limit the scope of a web traversal to resources that have recently changed. |
|---|
| 1014 | </p> |
|---|
| 1015 | <p id="rfc.section.3.3.p.7">When used for cache updates, a cache will typically use the value of the cached message's <a href="#header.last-modified" class="smpl">Last-Modified</a> field to generate the field value of If-Modified-Since. This behavior is most interoperable for cases where clocks are poorly |
|---|
| 1016 | synchronized or when the server has chosen to only honor exact timestamp matches (due to a problem with Last-Modified dates |
|---|
| 1017 | that appear to go "back in time" when the origin server's clock is corrected or a representation is restored from an archived |
|---|
| 1018 | backup). However, caches occasionally generate the field value based on other data, such as the <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field of the cached message or the local clock time that the message was received, particularly when the cached message |
|---|
| 1019 | does not contain a <a href="#header.last-modified" class="smpl">Last-Modified</a> field. |
|---|
| 1020 | </p> |
|---|
| 1021 | <p id="rfc.section.3.3.p.8">When used for limiting the scope of retrieval to a recent time window, a user agent will generate an If-Modified-Since field |
|---|
| 1022 | value based on either its own local clock or a <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field received from the server during a past run. Origin servers that choose an exact timestamp match based on the |
|---|
| 1023 | selected representation's <a href="#header.last-modified" class="smpl">Last-Modified</a> field will not be able to help the user agent limit its data transfers to only those changed during the specified window. |
|---|
| 1024 | </p> |
|---|
| 1025 | <div class="note" id="rfc.section.3.3.p.9"> |
|---|
| 1026 | <p> <b>Note:</b> If a client uses an arbitrary date in the If-Modified-Since header field instead of a date taken from a <a href="#header.last-modified" class="smpl">Last-Modified</a> or <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field from the origin server, the client ought to be aware that its date will be interpreted according to the server's |
|---|
| 1027 | understanding of time. |
|---|
| 1028 | </p> |
|---|
| 1029 | </div> |
|---|
| 1030 | <div id="rfc.iref.i.4"></div> |
|---|
| 1031 | <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> |
|---|
| 1032 | <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 |
|---|
| 1033 | 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">MAY</em> perform the request. |
|---|
| 1034 | </p> |
|---|
| 1035 | <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> |
|---|
| 1036 | </pre><p id="rfc.section.3.4.p.3">An example of the field is:</p> |
|---|
| 1037 | <div id="rfc.figure.u.15"></div><pre class="text"> If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT |
|---|
| 1038 | </pre><p id="rfc.section.3.4.p.5">A server <em class="bcp14">MUST</em> ignore the If-Unmodified-Since header field if the received value is not a valid HTTP-date. |
|---|
| 1039 | </p> |
|---|
| 1040 | <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> |
|---|
| 1041 | <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 range requests. If-Range is defined in <a href="p5-range.html#header.if-range" title="If-Range">Section 3.2</a> of <a href="#Part5" id="rfc.xref.Part5.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a>. |
|---|
| 1042 | </p> |
|---|
| 1043 | <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> |
|---|
| 1044 | <div id="rfc.iref.21"></div> |
|---|
| 1045 | <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> |
|---|
| 1046 | <p id="rfc.section.4.1.p.1">The <dfn>304 (Not Modified)</dfn> status code indicates that a conditional GET or HEAD 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 |
|---|
| 1047 | to transfer a representation of the target resource because the request indicates that the client, which made the request |
|---|
| 1048 | conditional, already has a valid representation; the server is therefore redirecting the client to make use of that stored |
|---|
| 1049 | representation as if it were the payload of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response. |
|---|
| 1050 | </p> |
|---|
| 1051 | <p id="rfc.section.4.1.p.2">The server generating a 304 response <em class="bcp14">MUST</em> generate any of the following header fields that would have been sent in a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to the same request: <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>, and <a href="p2-semantics.html#header.vary" class="smpl">Vary</a>. |
|---|
| 1052 | </p> |
|---|
| 1053 | <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, |
|---|
| 1054 | a sender <em class="bcp14">SHOULD NOT</em> generate representation metadata other than the above listed fields unless said metadata exists for the purpose of guiding |
|---|
| 1055 | cache updates (e.g., <a href="#header.last-modified" class="smpl">Last-Modified</a> might be useful if the response does not have an <a href="#header.etag" class="smpl">ETag</a> field). |
|---|
| 1056 | </p> |
|---|
| 1057 | <p id="rfc.section.4.1.p.4">Requirements on a cache that receives a 304 response are defined in <a href="p6-cache.html#freshening.responses" title="Freshening Stored Responses upon Validation">Section 4.2.1</a> of <a href="#Part6" id="rfc.xref.Part6.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[Part6]</cite></a>. If the conditional request originated with an outbound client, such as a user agent with its own cache sending a conditional |
|---|
| 1058 | GET to a shared proxy, then the proxy <em class="bcp14">SHOULD</em> forward the 304 response to that client. |
|---|
| 1059 | </p> |
|---|
| 1060 | <p id="rfc.section.4.1.p.5">A 304 response cannot contain a message-body; it is always terminated by the first empty line after the header fields.</p> |
|---|
| 1061 | <div id="rfc.iref.21"></div> |
|---|
| 1062 | <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> |
|---|
| 1063 | <p id="rfc.section.4.2.p.1">The <dfn>412 (Precondition Failed)</dfn> status code indicates that one or more preconditions given in the request header fields evaluated to false when tested on |
|---|
| 1064 | the server. This response code allows the client to place preconditions on the current resource state (its current representations |
|---|
| 1065 | and metadata) and thus prevent the request method from being applied if the target resource is in an unexpected state. |
|---|
| 1066 | </p> |
|---|
| 1067 | <h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a> <a id="precedence" href="#precedence">Evaluation and Precedence</a></h1> |
|---|
| 1068 | <p id="rfc.section.5.p.1">For each conditional request, a server <em class="bcp14">MUST</em> evaluate the request preconditions after it has successfully performed its normal request checks (i.e., just before it would |
|---|
| 1069 | perform the action associated with the request method). Preconditions are ignored if the server determines that an error or |
|---|
| 1070 | redirect response applies before they are evaluated. Otherwise, the evaluation depends on both the method semantics and the |
|---|
| 1071 | choice of conditional. |
|---|
| 1072 | </p> |
|---|
| 1073 | <p id="rfc.section.5.p.2">A conditional request header field that is designed specifically for cache validation, which includes <a href="#header.if-none-match" class="smpl">If-None-Match</a> and <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> when used in a GET or HEAD request, allows cached representations to be refreshed without repeatedly transferring data already |
|---|
| 1074 | held by the client. Evaluating to false is thus an indication that the client can continue to use its local copy of the selected |
|---|
| 1075 | representation, as indicated by the server generating a <a href="#status.304" class="smpl">304 (Not Modified)</a> response that includes only those header fields useful for refreshing the cached representation. |
|---|
| 1076 | </p> |
|---|
| 1077 | <p id="rfc.section.5.p.3">All other conditionals are intended to signal failure when the precondition evaluates to false. For example, an <a href="#header.if-match" class="smpl">If-Match</a> conditional sent with a state-changing method (e.g., POST, PUT, DELETE) is intended to prevent the request from taking effect |
|---|
| 1078 | on the target resource if the resource state does not match the expected state. In other words, evaluating the condition to |
|---|
| 1079 | false means that the resource has been changed by some other client, perhaps by another user attempting to edit the same resource, |
|---|
| 1080 | and thus preventing the request from being applied saves the client from overwriting some other client's work. This result |
|---|
| 1081 | is indicated by the server generating a <a href="#status.412" class="smpl">412 (Precondition Failed)</a> response. |
|---|
| 1082 | </p> |
|---|
| 1083 | <p id="rfc.section.5.p.4">The conditional request header fields defined by this specification are ignored for request methods that never involve the |
|---|
| 1084 | selection or modification of a <a href="p2-semantics.html#representations" class="smpl">selected representation</a> (e.g., CONNECT, OPTIONS, and TRACE). Other conditional request header fields, defined by extensions to HTTP, might place conditions |
|---|
| 1085 | on the state of the target resource in general, or on a group of resources. For instance, the If header field in WebDAV can |
|---|
| 1086 | make a request conditional on various aspects (such as locks) of multiple resources (<a href="#RFC4918" id="rfc.xref.RFC4918.2"><cite title="HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)">[RFC4918]</cite></a>, <a href="http://tools.ietf.org/html/rfc4918#section-10.4">Section 10.4</a>). |
|---|
| 1087 | </p> |
|---|
| 1088 | <p id="rfc.section.5.p.5">When more than one conditional request header field is present in a request, the order in which the fields are evaluated becomes |
|---|
| 1089 | important. In practice, the fields defined in this document are consistently implemented in a single, logical order, due to |
|---|
| 1090 | 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. |
|---|
| 1091 | </p> |
|---|
| 1092 | <p id="rfc.section.5.p.6">The general rule of conditional precedence is that exact match conditions are evaluated before cache-validating conditions |
|---|
| 1093 | and, within that order, last-modified conditions are only evaluated if the corresponding entity tag condition is not present |
|---|
| 1094 | (or not applicable because the selected representation does not have an entity tag). |
|---|
| 1095 | </p> |
|---|
| 1096 | <p id="rfc.section.5.p.7">Specifically, the fields defined by this specification are evaluated as follows: </p> |
|---|
| 1097 | <ol> |
|---|
| 1098 | <li id="precedence1">When <a href="#header.if-match" class="smpl">If-Match</a> is present, evaluate it: |
|---|
| 1099 | <ul> |
|---|
| 1100 | <li>if true, continue to step <a href="#precedence3">3</a></li> |
|---|
| 1101 | <li>if false, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a></li> |
|---|
| 1102 | </ul> |
|---|
| 1103 | </li> |
|---|
| 1104 | <li id="precedence2">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: |
|---|
| 1105 | <ul> |
|---|
| 1106 | <li>if true, continue to step <a href="#precedence3">3</a></li> |
|---|
| 1107 | <li>if false, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a></li> |
|---|
| 1108 | </ul> |
|---|
| 1109 | </li> |
|---|
| 1110 | <li id="precedence3">When <a href="#header.if-none-match" class="smpl">If-None-Match</a> is present, evaluate it: |
|---|
| 1111 | <ul> |
|---|
| 1112 | <li>if true, continue to step <a href="#precedence5">5</a></li> |
|---|
| 1113 | <li>if false for GET/HEAD, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li> |
|---|
| 1114 | <li>if false for other methods, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a></li> |
|---|
| 1115 | </ul> |
|---|
| 1116 | </li> |
|---|
| 1117 | <li id="precedence4">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: |
|---|
| 1118 | <ul> |
|---|
| 1119 | <li>if true, continue to step <a href="#precedence5">5</a></li> |
|---|
| 1120 | <li>if false, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li> |
|---|
| 1121 | </ul> |
|---|
| 1122 | </li> |
|---|
| 1123 | <li id="precedence5">When the method is GET and both <a href="p5-range.html#header.range" class="smpl">Range</a> and <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> are present, evaluate If-Range: |
|---|
| 1124 | <ul> |
|---|
| 1125 | <li>if the validator matches and the Range specification is applicable to the selected representation, respond <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a> <a href="#Part5" id="rfc.xref.Part5.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[Part5]</cite></a></li> |
|---|
| 1126 | </ul> |
|---|
| 1127 | </li> |
|---|
| 1128 | <li id="precedencelast">Otherwise, |
|---|
| 1129 | <ul> |
|---|
| 1130 | <li>all conditions are met, so perform the requested action and respond according to its success or failure.</li> |
|---|
| 1131 | </ul> |
|---|
| 1132 | </li> |
|---|
| 1133 | </ol> |
|---|
| 1134 | <p id="rfc.section.5.p.8">Any extension to HTTP/1.1 that defines additional conditional request header fields ought to define its own expectations regarding |
|---|
| 1135 | the order for evaluating such fields in relation to those defined in this document and other conditionals that might be found |
|---|
| 1136 | in practice. |
|---|
| 1137 | </p> |
|---|
| 1138 | <h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a> <a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1> |
|---|
| 1139 | <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> |
|---|
| 1140 | <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: |
|---|
| 1141 | </p> |
|---|
| 1142 | <div id="rfc.table.1"> |
|---|
| 1143 | <div id="iana.status.code.registration.table"></div> |
|---|
| 1144 | <table class="tt full left" cellpadding="3" cellspacing="0"> |
|---|
| 1145 | <thead> |
|---|
| 1146 | <tr> |
|---|
| 1147 | <th>Value</th> |
|---|
| 1148 | <th>Description</th> |
|---|
| 1149 | <th>Reference</th> |
|---|
| 1150 | </tr> |
|---|
| 1151 | </thead> |
|---|
| 1152 | <tbody> |
|---|
| 1153 | <tr> |
|---|
| 1154 | <td class="left">304</td> |
|---|
| 1155 | <td class="left">Not Modified</td> |
|---|
| 1156 | <td class="left"> <a href="#status.304" id="rfc.xref.status.304.1" title="304 Not Modified">Section 4.1</a> |
|---|
| 1157 | </td> |
|---|
| 1158 | </tr> |
|---|
| 1159 | <tr> |
|---|
| 1160 | <td class="left">412</td> |
|---|
| 1161 | <td class="left">Precondition Failed</td> |
|---|
| 1162 | <td class="left"> <a href="#status.412" id="rfc.xref.status.412.1" title="412 Precondition Failed">Section 4.2</a> |
|---|
| 1163 | </td> |
|---|
| 1164 | </tr> |
|---|
| 1165 | </tbody> |
|---|
| 1166 | </table> |
|---|
| 1167 | </div> |
|---|
| 1168 | <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> |
|---|
| 1169 | <p id="rfc.section.6.2.p.1">HTTP header fields are registered within the Message Header Field Registry maintained 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>>. |
|---|
| 1170 | </p> |
|---|
| 1171 | <p id="rfc.section.6.2.p.2">This document defines the following HTTP header fields, so their associated registry entries shall be updated according to |
|---|
| 1172 | the permanent registrations below (see <a href="#BCP90" id="rfc.xref.BCP90.1"><cite title="Registration Procedures for Message Header Fields">[BCP90]</cite></a>): |
|---|
| 1173 | </p> |
|---|
| 1174 | <div id="rfc.table.2"> |
|---|
| 1175 | <div id="iana.header.registration.table"></div> |
|---|
| 1176 | <table class="tt full left" cellpadding="3" cellspacing="0"> |
|---|
| 1177 | <thead> |
|---|
| 1178 | <tr> |
|---|
| 1179 | <th>Header Field Name</th> |
|---|
| 1180 | <th>Protocol</th> |
|---|
| 1181 | <th>Status</th> |
|---|
| 1182 | <th>Reference</th> |
|---|
| 1183 | </tr> |
|---|
| 1184 | </thead> |
|---|
| 1185 | <tbody> |
|---|
| 1186 | <tr> |
|---|
| 1187 | <td class="left">ETag</td> |
|---|
| 1188 | <td class="left">http</td> |
|---|
| 1189 | <td class="left">standard</td> |
|---|
| 1190 | <td class="left"> <a href="#header.etag" id="rfc.xref.header.etag.2" title="ETag">Section 2.3</a> |
|---|
| 1191 | </td> |
|---|
| 1192 | </tr> |
|---|
| 1193 | <tr> |
|---|
| 1194 | <td class="left">If-Match</td> |
|---|
| 1195 | <td class="left">http</td> |
|---|
| 1196 | <td class="left">standard</td> |
|---|
| 1197 | <td class="left"> <a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section 3.1</a> |
|---|
| 1198 | </td> |
|---|
| 1199 | </tr> |
|---|
| 1200 | <tr> |
|---|
| 1201 | <td class="left">If-Modified-Since</td> |
|---|
| 1202 | <td class="left">http</td> |
|---|
| 1203 | <td class="left">standard</td> |
|---|
| 1204 | <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> |
|---|
| 1205 | </td> |
|---|
| 1206 | </tr> |
|---|
| 1207 | <tr> |
|---|
| 1208 | <td class="left">If-None-Match</td> |
|---|
| 1209 | <td class="left">http</td> |
|---|
| 1210 | <td class="left">standard</td> |
|---|
| 1211 | <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> |
|---|
| 1212 | </td> |
|---|
| 1213 | </tr> |
|---|
| 1214 | <tr> |
|---|
| 1215 | <td class="left">If-Unmodified-Since</td> |
|---|
| 1216 | <td class="left">http</td> |
|---|
| 1217 | <td class="left">standard</td> |
|---|
| 1218 | <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> |
|---|
| 1219 | </td> |
|---|
| 1220 | </tr> |
|---|
| 1221 | <tr> |
|---|
| 1222 | <td class="left">Last-Modified</td> |
|---|
| 1223 | <td class="left">http</td> |
|---|
| 1224 | <td class="left">standard</td> |
|---|
| 1225 | <td class="left"> <a href="#header.last-modified" id="rfc.xref.header.last-modified.2" title="Last-Modified">Section 2.2</a> |
|---|
| 1226 | </td> |
|---|
| 1227 | </tr> |
|---|
| 1228 | </tbody> |
|---|
| 1229 | </table> |
|---|
| 1230 | </div> |
|---|
| 1231 | <p id="rfc.section.6.2.p.3">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p> |
|---|
| 1232 | <h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a> <a id="security.considerations" href="#security.considerations">Security Considerations</a></h1> |
|---|
| 1233 | <p id="rfc.section.7.p.1">This section is meant to inform developers, information providers, and users of known security concerns specific to the HTTP/1.1 |
|---|
| 1234 | conditional request mechanisms. More general security considerations are addressed in HTTP messaging <a href="#Part1" id="rfc.xref.Part1.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a> and semantics <a href="#Part2" id="rfc.xref.Part2.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>. |
|---|
| 1235 | </p> |
|---|
| 1236 | <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 |
|---|
| 1237 | changes, or detect man-in-the-middle attacks. At best, they enable more efficient cache updates and optimistic concurrent |
|---|
| 1238 | writes when all participants are behaving nicely. At worst, the conditions will fail and the client will receive a response |
|---|
| 1239 | that is no more harmful than an HTTP exchange without conditional requests. |
|---|
| 1240 | </p> |
|---|
| 1241 | <p id="rfc.section.7.p.3">An entity-tag can be abused in ways that create privacy risks. For example, a site might deliberately construct a semantically |
|---|
| 1242 | invalid entity-tag that is unique to the user or user agent, send it in a cacheable response with a long freshness time, and |
|---|
| 1243 | then read that entity-tag in later conditional requests as a means of re-identifying that user or user agent. Such an identifying |
|---|
| 1244 | tag would become a persistent identifier for as long as the user agent retained the original cache entry. User agents that |
|---|
| 1245 | cache representations ought to ensure that the cache is cleared or replaced whenever the user performs privacy-maintaining |
|---|
| 1246 | actions, such as clearing stored cookies or changing to a private browsing mode. |
|---|
| 1247 | </p> |
|---|
| 1248 | <h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a> <a id="acks" href="#acks">Acknowledgments</a></h1> |
|---|
| 1249 | <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="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>. |
|---|
| 1250 | </p> |
|---|
| 1251 | <h1 id="rfc.references"><a id="rfc.section.9" href="#rfc.section.9">9.</a> References |
|---|
| 1252 | </h1> |
|---|
| 1253 | <h2 id="rfc.references.1"><a href="#rfc.section.9.1" id="rfc.section.9.1">9.1</a> Normative References |
|---|
| 1254 | </h2> |
|---|
| 1255 | <table> |
|---|
| 1256 | <tr> |
|---|
| 1257 | <td class="reference"><b id="Part1">[Part1]</b></td> |
|---|
| 1258 | <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., 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">Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing</a>”, Internet-Draft draft-ietf-httpbis-p1-messaging-latest (work in progress), June 2013. |
|---|
| 1259 | </td> |
|---|
| 1260 | </tr> |
|---|
| 1261 | <tr> |
|---|
| 1262 | <td class="reference"><b id="Part2">[Part2]</b></td> |
|---|
| 1263 | <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., 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">Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content</a>”, Internet-Draft draft-ietf-httpbis-p2-semantics-latest (work in progress), June 2013. |
|---|
| 1264 | </td> |
|---|
| 1265 | </tr> |
|---|
| 1266 | <tr> |
|---|
| 1267 | <td class="reference"><b id="Part5">[Part5]</b></td> |
|---|
| 1268 | <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">Hypertext Transfer Protocol (HTTP/1.1): Range Requests</a>”, Internet-Draft draft-ietf-httpbis-p5-range-latest (work in progress), June 2013. |
|---|
| 1269 | </td> |
|---|
| 1270 | </tr> |
|---|
| 1271 | <tr> |
|---|
| 1272 | <td class="reference"><b id="Part6">[Part6]</b></td> |
|---|
| 1273 | <td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:mnot@mnot.net" title="Akamai">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">Hypertext Transfer Protocol (HTTP/1.1): Caching</a>”, Internet-Draft draft-ietf-httpbis-p6-cache-latest (work in progress), June 2013. |
|---|
| 1274 | </td> |
|---|
| 1275 | </tr> |
|---|
| 1276 | <tr> |
|---|
| 1277 | <td class="reference"><b id="RFC2119">[RFC2119]</b></td> |
|---|
| 1278 | <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. |
|---|
| 1279 | </td> |
|---|
| 1280 | </tr> |
|---|
| 1281 | <tr> |
|---|
| 1282 | <td class="reference"><b id="RFC5234">[RFC5234]</b></td> |
|---|
| 1283 | <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. |
|---|
| 1284 | </td> |
|---|
| 1285 | </tr> |
|---|
| 1286 | </table> |
|---|
| 1287 | <h2 id="rfc.references.2"><a href="#rfc.section.9.2" id="rfc.section.9.2">9.2</a> Informative References |
|---|
| 1288 | </h2> |
|---|
| 1289 | <table> |
|---|
| 1290 | <tr> |
|---|
| 1291 | <td class="reference"><b id="BCP90">[BCP90]</b></td> |
|---|
| 1292 | <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. |
|---|
| 1293 | </td> |
|---|
| 1294 | </tr> |
|---|
| 1295 | <tr> |
|---|
| 1296 | <td class="reference"><b id="RFC2616">[RFC2616]</b></td> |
|---|
| 1297 | <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. |
|---|
| 1298 | </td> |
|---|
| 1299 | </tr> |
|---|
| 1300 | <tr> |
|---|
| 1301 | <td class="reference"><b id="RFC4918">[RFC4918]</b></td> |
|---|
| 1302 | <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. |
|---|
| 1303 | </td> |
|---|
| 1304 | </tr> |
|---|
| 1305 | </table> |
|---|
| 1306 | <div class="avoidbreak"> |
|---|
| 1307 | <h1 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h1> |
|---|
| 1308 | <address class="vcard"><span class="vcardline"><span class="fn">Roy T. Fielding</span> |
|---|
| 1309 | (editor) |
|---|
| 1310 | <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> |
|---|
| 1311 | <address class="vcard"><span class="vcardline"><span class="fn">Julian F. Reschke</span> |
|---|
| 1312 | (editor) |
|---|
| 1313 | <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> |
|---|
| 1314 | </div> |
|---|
| 1315 | <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> |
|---|
| 1316 | <p id="rfc.section.A.p.1">The definition of validator weakness has been expanded and clarified. (<a href="#weak.and.strong.validators" title="Weak versus Strong">Section 2.1</a>) |
|---|
| 1317 | </p> |
|---|
| 1318 | <p id="rfc.section.A.p.2">Weak entity-tags are now allowed 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>). |
|---|
| 1319 | </p> |
|---|
| 1320 | <p id="rfc.section.A.p.3">The <a href="#header.etag" class="smpl">ETag</a> header field ABNF has been changed to not use quoted-string, thus avoiding escaping issues. (<a href="#header.etag" id="rfc.xref.header.etag.3" title="ETag">Section 2.3</a>) |
|---|
| 1321 | </p> |
|---|
| 1322 | <p id="rfc.section.A.p.4">ETag is defined to provide an entity tag for the selected representation, thereby clarifying what it applies to in various |
|---|
| 1323 | situations (such as a PUT response). (<a href="#header.etag" id="rfc.xref.header.etag.4" title="ETag">Section 2.3</a>) |
|---|
| 1324 | </p> |
|---|
| 1325 | <p id="rfc.section.A.p.5">The precedence for evaluation of conditional requests has been defined. (<a href="#precedence" title="Evaluation and Precedence">Section 5</a>) |
|---|
| 1326 | </p> |
|---|
| 1327 | <h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a> <a id="imported.abnf" href="#imported.abnf">Imported ABNF</a></h1> |
|---|
| 1328 | <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 |
|---|
| 1329 | (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 |
|---|
| 1330 | character). |
|---|
| 1331 | </p> |
|---|
| 1332 | <p id="rfc.section.B.p.2">The rules below are defined in <a href="#Part1" id="rfc.xref.Part1.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>: |
|---|
| 1333 | </p> |
|---|
| 1334 | <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="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>, <a href="p1-messaging.html#whitespace" title="Whitespace">Section 3.2.3</a>> |
|---|
| 1335 | <a href="#imported.abnf" class="smpl">obs-text</a> = <obs-text, defined in <a href="#Part1" id="rfc.xref.Part1.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>, <a href="p1-messaging.html#field.components" title="Field value components">Section 3.2.6</a>> |
|---|
| 1336 | </pre><p id="rfc.section.B.p.4">The rules below are defined in other parts:</p> |
|---|
| 1337 | <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.Part2.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[Part2]</cite></a>, <a href="p2-semantics.html#http.date" title="Date/Time Formats">Section 7.1.1.1</a>> |
|---|
| 1338 | </pre><h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a> <a id="collected.abnf" href="#collected.abnf">Collected ABNF</a></h1> |
|---|
| 1339 | <p id="rfc.section.C.p.1">In the collected ABNF below, list rules are expanded as per <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</a> of <a href="#Part1" id="rfc.xref.Part1.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[Part1]</cite></a>. |
|---|
| 1340 | </p> |
|---|
| 1341 | <div id="rfc.figure.u.18"></div><pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = entity-tag |
|---|
| 1342 | |
|---|
| 1343 | <a href="#imported.abnf" class="smpl">HTTP-date</a> = <HTTP-date, defined in [Part2], Section 7.1.1.1> |
|---|
| 1344 | |
|---|
| 1345 | <a href="#header.if-match" class="smpl">If-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS |
|---|
| 1346 | entity-tag ] ) ) |
|---|
| 1347 | <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = HTTP-date |
|---|
| 1348 | <a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS |
|---|
| 1349 | entity-tag ] ) ) |
|---|
| 1350 | <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = HTTP-date |
|---|
| 1351 | |
|---|
| 1352 | <a href="#header.last-modified" class="smpl">Last-Modified</a> = HTTP-date |
|---|
| 1353 | |
|---|
| 1354 | <a href="#imported.abnf" class="smpl">OWS</a> = <OWS, defined in [Part1], Section 3.2.3> |
|---|
| 1355 | |
|---|
| 1356 | <a href="#header.etag" class="smpl">entity-tag</a> = [ weak ] opaque-tag |
|---|
| 1357 | <a href="#header.etag" class="smpl">etagc</a> = "!" / %x23-7E ; '#'-'~' |
|---|
| 1358 | / obs-text |
|---|
| 1359 | |
|---|
| 1360 | <a href="#imported.abnf" class="smpl">obs-text</a> = <obs-text, defined in [Part1], Section 3.2.6> |
|---|
| 1361 | <a href="#header.etag" class="smpl">opaque-tag</a> = DQUOTE *etagc DQUOTE |
|---|
| 1362 | |
|---|
| 1363 | <a href="#header.etag" class="smpl">weak</a> = %x57.2F ; W/ |
|---|
| 1364 | </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> |
|---|
| 1365 | <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>>. |
|---|
| 1366 | </p> |
|---|
| 1367 | <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> |
|---|
| 1368 | <p id="rfc.section.D.1.p.1">Closed issues: </p> |
|---|
| 1369 | <ul> |
|---|
| 1370 | <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" |
|---|
| 1371 | </li> |
|---|
| 1372 | <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" |
|---|
| 1373 | </li> |
|---|
| 1374 | <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" |
|---|
| 1375 | </li> |
|---|
| 1376 | <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" |
|---|
| 1377 | </li> |
|---|
| 1378 | <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" |
|---|
| 1379 | </li> |
|---|
| 1380 | <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/363">http://tools.ietf.org/wg/httpbis/trac/ticket/363</a>>: "Rare cases" |
|---|
| 1381 | </li> |
|---|
| 1382 | <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" |
|---|
| 1383 | </li> |
|---|
| 1384 | <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" |
|---|
| 1385 | </li> |
|---|
| 1386 | <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" |
|---|
| 1387 | </li> |
|---|
| 1388 | </ul> |
|---|
| 1389 | <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> |
|---|
| 1390 | <p id="rfc.section.D.2.p.1"> </p> |
|---|
| 1391 | <ul> |
|---|
| 1392 | <li>Conformance criteria and considerations regarding error handling are now defined in Part 1.</li> |
|---|
| 1393 | </ul> |
|---|
| 1394 | <h2 id="rfc.section.D.3"><a href="#rfc.section.D.3">D.3</a> <a id="changes.since.21" href="#changes.since.21">Since draft-ietf-httpbis-p4-conditional-21</a></h2> |
|---|
| 1395 | <p id="rfc.section.D.3.p.1">Closed issues: </p> |
|---|
| 1396 | <ul> |
|---|
| 1397 | <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/96">http://tools.ietf.org/wg/httpbis/trac/ticket/96</a>>: "Conditional GET text" |
|---|
| 1398 | </li> |
|---|
| 1399 | <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" |
|---|
| 1400 | </li> |
|---|
| 1401 | <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/384">http://tools.ietf.org/wg/httpbis/trac/ticket/384</a>>: "unclear prose in definition of 304" |
|---|
| 1402 | </li> |
|---|
| 1403 | <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/401">http://tools.ietf.org/wg/httpbis/trac/ticket/401</a>>: "ETags and Conneg" |
|---|
| 1404 | </li> |
|---|
| 1405 | <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/402">http://tools.ietf.org/wg/httpbis/trac/ticket/402</a>>: "Comparison function for If-Match and If-None-Match" |
|---|
| 1406 | </li> |
|---|
| 1407 | <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/406">http://tools.ietf.org/wg/httpbis/trac/ticket/406</a>>: "304 without validator" |
|---|
| 1408 | </li> |
|---|
| 1409 | <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/427">http://tools.ietf.org/wg/httpbis/trac/ticket/427</a>>: "If-Match and 428" |
|---|
| 1410 | </li> |
|---|
| 1411 | </ul> |
|---|
| 1412 | <h2 id="rfc.section.D.4"><a href="#rfc.section.D.4">D.4</a> <a id="changes.since.22" href="#changes.since.22">Since draft-ietf-httpbis-p4-conditional-22</a></h2> |
|---|
| 1413 | <p id="rfc.section.D.4.p.1">Closed issues: </p> |
|---|
| 1414 | <ul> |
|---|
| 1415 | <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/436">http://tools.ietf.org/wg/httpbis/trac/ticket/436</a>>: "explain list expansion in ABNF appendices" |
|---|
| 1416 | </li> |
|---|
| 1417 | <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/437">http://tools.ietf.org/wg/httpbis/trac/ticket/437</a>>: "incorrect example dates" |
|---|
| 1418 | </li> |
|---|
| 1419 | </ul> |
|---|
| 1420 | <p id="rfc.section.D.4.p.2">Partly resolved issues: </p> |
|---|
| 1421 | <ul> |
|---|
| 1422 | <li> <<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/461">http://tools.ietf.org/wg/httpbis/trac/ticket/461</a>>: "Editorial suggestions" |
|---|
| 1423 | </li> |
|---|
| 1424 | </ul> |
|---|
| 1425 | <h1 id="rfc.index"><a href="#rfc.index">Index</a></h1> |
|---|
| 1426 | <p class="noprint"><a href="#rfc.index.3">3</a> <a href="#rfc.index.4">4</a> <a href="#rfc.index.B">B</a> <a href="#rfc.index.E">E</a> <a href="#rfc.index.G">G</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> |
|---|
| 1427 | </p> |
|---|
| 1428 | <div class="print2col"> |
|---|
| 1429 | <ul class="ind"> |
|---|
| 1430 | <li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul> |
|---|
| 1431 | <li>304 Not Modified (status code) <a href="#rfc.iref.21"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">6.1</a></li> |
|---|
| 1432 | </ul> |
|---|
| 1433 | </li> |
|---|
| 1434 | <li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul> |
|---|
| 1435 | <li>412 Precondition Failed (status code) <a href="#rfc.iref.21"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">6.1</a></li> |
|---|
| 1436 | </ul> |
|---|
| 1437 | </li> |
|---|
| 1438 | <li><a id="rfc.index.B" href="#rfc.index.B"><b>B</b></a><ul> |
|---|
| 1439 | <li><em>BCP90</em> <a href="#rfc.xref.BCP90.1">6.2</a>, <a href="#BCP90"><b>9.2</b></a></li> |
|---|
| 1440 | </ul> |
|---|
| 1441 | </li> |
|---|
| 1442 | <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul> |
|---|
| 1443 | <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>, <a href="#rfc.xref.header.etag.4">A</a></li> |
|---|
| 1444 | </ul> |
|---|
| 1445 | </li> |
|---|
| 1446 | <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul> |
|---|
| 1447 | <li><tt>Grammar</tt> |
|---|
| 1448 | <ul> |
|---|
| 1449 | <li><tt>entity-tag</tt> <a href="#rfc.iref.g.3"><b>2.3</b></a></li> |
|---|
| 1450 | <li><tt>ETag</tt> <a href="#rfc.iref.g.2"><b>2.3</b></a></li> |
|---|
| 1451 | <li><tt>etagc</tt> <a href="#rfc.iref.g.6"><b>2.3</b></a></li> |
|---|
| 1452 | <li><tt>If-Match</tt> <a href="#rfc.iref.g.7"><b>3.1</b></a></li> |
|---|
| 1453 | <li><tt>If-Modified-Since</tt> <a href="#rfc.iref.g.9"><b>3.3</b></a></li> |
|---|
| 1454 | <li><tt>If-None-Match</tt> <a href="#rfc.iref.g.8"><b>3.2</b></a></li> |
|---|
| 1455 | <li><tt>If-Unmodified-Since</tt> <a href="#rfc.iref.g.10"><b>3.4</b></a></li> |
|---|
| 1456 | <li><tt>Last-Modified</tt> <a href="#rfc.iref.g.1"><b>2.2</b></a></li> |
|---|
| 1457 | <li><tt>opaque-tag</tt> <a href="#rfc.iref.g.5"><b>2.3</b></a></li> |
|---|
| 1458 | <li><tt>weak</tt> <a href="#rfc.iref.g.4"><b>2.3</b></a></li> |
|---|
| 1459 | </ul> |
|---|
| 1460 | </li> |
|---|
| 1461 | </ul> |
|---|
| 1462 | </li> |
|---|
| 1463 | <li><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul> |
|---|
| 1464 | <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> |
|---|
| 1465 | <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> |
|---|
| 1466 | <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> |
|---|
| 1467 | <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> |
|---|
| 1468 | </ul> |
|---|
| 1469 | </li> |
|---|
| 1470 | <li><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul> |
|---|
| 1471 | <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> |
|---|
| 1472 | </ul> |
|---|
| 1473 | </li> |
|---|
| 1474 | <li><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul> |
|---|
| 1475 | <li>metadata <a href="#rfc.iref.m.1"><b>2</b></a></li> |
|---|
| 1476 | </ul> |
|---|
| 1477 | </li> |
|---|
| 1478 | <li><a id="rfc.index.P" href="#rfc.index.P"><b>P</b></a><ul> |
|---|
| 1479 | <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>, <a href="#rfc.xref.Part1.10">C</a><ul> |
|---|
| 1480 | <li><em>Section 1.2</em> <a href="#rfc.xref.Part1.3">1.2</a>, <a href="#rfc.xref.Part1.10">C</a></li> |
|---|
| 1481 | <li><em>Section 2.5</em> <a href="#rfc.xref.Part1.2">1.1</a></li> |
|---|
| 1482 | <li><em>Section 3.2.3</em> <a href="#rfc.xref.Part1.8">B</a></li> |
|---|
| 1483 | <li><em>Section 3.2.6</em> <a href="#rfc.xref.Part1.9">B</a></li> |
|---|
| 1484 | <li><em>Section 4</em> <a href="#rfc.xref.Part1.4">2.3.3</a></li> |
|---|
| 1485 | <li><em>Section 9</em> <a href="#rfc.xref.Part1.6">8</a></li> |
|---|
| 1486 | </ul> |
|---|
| 1487 | </li> |
|---|
| 1488 | <li><em>Part2</em> <a href="#rfc.xref.Part2.1">1</a>, <a href="#rfc.xref.Part2.2">1</a>, <a href="#rfc.xref.Part2.3">2.3.3</a>, <a href="#rfc.xref.Part2.4">2.3.3</a>, <a href="#rfc.xref.Part2.5">3.2</a>, <a href="#rfc.xref.Part2.6">7</a>, <a href="#Part2"><b>9.1</b></a>, <a href="#rfc.xref.Part2.7">B</a><ul> |
|---|
| 1489 | <li><em>Section 3</em> <a href="#rfc.xref.Part2.2">1</a></li> |
|---|
| 1490 | <li><em>Section 3.4</em> <a href="#rfc.xref.Part2.3">2.3.3</a></li> |
|---|
| 1491 | <li><em>Section 4.2.1</em> <a href="#rfc.xref.Part2.5">3.2</a></li> |
|---|
| 1492 | <li><em>Section 5.3.4</em> <a href="#rfc.xref.Part2.4">2.3.3</a></li> |
|---|
| 1493 | <li><em>Section 7.1.1.1</em> <a href="#rfc.xref.Part2.7">B</a></li> |
|---|
| 1494 | </ul> |
|---|
| 1495 | </li> |
|---|
| 1496 | <li><em>Part5</em> <a href="#rfc.xref.Part5.1">3.5</a>, <a href="#rfc.xref.Part5.2">5</a>, <a href="#Part5"><b>9.1</b></a><ul> |
|---|
| 1497 | <li><em>Section 3.2</em> <a href="#rfc.xref.Part5.1">3.5</a></li> |
|---|
| 1498 | </ul> |
|---|
| 1499 | </li> |
|---|
| 1500 | <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="#rfc.xref.Part6.4">4.1</a>, <a href="#Part6"><b>9.1</b></a><ul> |
|---|
| 1501 | <li><em>Section 4.2.1</em> <a href="#rfc.xref.Part6.4">4.1</a></li> |
|---|
| 1502 | </ul> |
|---|
| 1503 | </li> |
|---|
| 1504 | </ul> |
|---|
| 1505 | </li> |
|---|
| 1506 | <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul> |
|---|
| 1507 | <li><em>RFC2119</em> <a href="#rfc.xref.RFC2119.1">1.1</a>, <a href="#RFC2119"><b>9.1</b></a></li> |
|---|
| 1508 | <li><em>RFC2616</em> <a href="#rfc.xref.RFC2616.1">2.3</a>, <a href="#RFC2616"><b>9.2</b></a><ul> |
|---|
| 1509 | <li><em>Section 3.11</em> <a href="#rfc.xref.RFC2616.1">2.3</a></li> |
|---|
| 1510 | </ul> |
|---|
| 1511 | </li> |
|---|
| 1512 | <li><em>RFC4918</em> <a href="#rfc.xref.RFC4918.1">2</a>, <a href="#rfc.xref.RFC4918.2">5</a>, <a href="#RFC4918"><b>9.2</b></a><ul> |
|---|
| 1513 | <li><em>Section 10.4</em> <a href="#rfc.xref.RFC4918.2">5</a></li> |
|---|
| 1514 | </ul> |
|---|
| 1515 | </li> |
|---|
| 1516 | <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> |
|---|
| 1517 | <li><em>Appendix B.1</em> <a href="#rfc.xref.RFC5234.2">B</a></li> |
|---|
| 1518 | </ul> |
|---|
| 1519 | </li> |
|---|
| 1520 | </ul> |
|---|
| 1521 | </li> |
|---|
| 1522 | <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul> |
|---|
| 1523 | <li>selected representation <a href="#rfc.iref.s.1"><b>1</b></a></li> |
|---|
| 1524 | </ul> |
|---|
| 1525 | </li> |
|---|
| 1526 | <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul> |
|---|
| 1527 | <li>validator <a href="#rfc.iref.v.1"><b>2</b></a><ul> |
|---|
| 1528 | <li>strong <a href="#rfc.iref.v.3"><b>2.1</b></a></li> |
|---|
| 1529 | <li>weak <a href="#rfc.iref.v.2"><b>2.1</b></a></li> |
|---|
| 1530 | </ul> |
|---|
| 1531 | </li> |
|---|
| 1532 | </ul> |
|---|
| 1533 | </li> |
|---|
| 1534 | </ul> |
|---|
| 1535 | </div> |
|---|
| 1536 | </body> |
|---|
| 1537 | </html> |
|---|
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