| 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://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 type="application/javascript"> |
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| 7 | var buttonsAdded = false; |
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| 8 | |
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| 9 | function initFeedback() { |
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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 | document.body.appendChild(fb); |
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| 16 | } |
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| 17 | |
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| 18 | function feedback() { |
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| 19 | toggleButtonsToElementsByName("h2"); |
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| 20 | toggleButtonsToElementsByName("h3"); |
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| 21 | toggleButtonsToElementsByName("h4"); |
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| 22 | toggleButtonsToElementsByName("h5"); |
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| 23 | |
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| 24 | buttonsAdded = !buttonsAdded; |
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| 25 | } |
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| 26 | |
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| 27 | function toggleButtonsToElementsByName(name) { |
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| 28 | var list = document.getElementsByTagName(name); |
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| 29 | for (var i = 0; i < list.length; i++) { |
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| 30 | toggleButton(list.item(i)); |
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| 31 | } |
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| 32 | } |
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| 33 | |
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| 34 | function toggleButton(node) { |
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| 35 | if (! buttonsAdded) { |
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| 36 | |
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| 37 | // docname |
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| 38 | var template = "mailto:ietf-http-wg@w3.org?subject={docname},%20%22{section}%22&body=<{ref}>:"; |
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| 39 | |
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| 40 | var id = node.getAttribute("id"); |
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| 41 | // better id available? |
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| 42 | var titlelinks = node.getElementsByTagName("a"); |
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| 43 | for (var i = 0; i < titlelinks.length; i++) { |
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| 44 | var tl = titlelinks.item(i); |
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| 45 | if (tl.getAttribute("id")) { |
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| 46 | id = tl.getAttribute("id"); |
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| 47 | } |
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| 48 | } |
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| 49 | |
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| 50 | // ref |
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| 51 | var ref = window.location.toString(); |
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| 52 | var hash = ref.indexOf("#"); |
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| 53 | if (hash != -1) { |
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| 54 | ref = ref.substring(0, hash); |
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| 55 | } |
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| 56 | if (id != "") { |
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| 57 | ref += "#" + id; |
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| 58 | } |
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| 59 | |
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| 60 | // docname |
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| 61 | var docname = "draft-ietf-httpbis-p4-conditional-latest"; |
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| 62 | |
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| 63 | // section |
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| 64 | var section = node.textContent; |
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| 65 | section = section.replace("\u00a0", " "); |
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| 66 | |
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| 67 | // build URI from template |
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| 68 | var uri = template.replace("{docname}", encodeURIComponent(docname)); |
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| 69 | uri = uri.replace("{section}", encodeURIComponent(section)); |
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| 70 | uri = uri.replace("{ref}", encodeURIComponent(ref)); |
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| 71 | |
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| 72 | var button = document.createElement("a"); |
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| 73 | button.className = "fbbutton noprint"; |
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| 74 | button.setAttribute("href", uri); |
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| 75 | button.appendChild(document.createTextNode("send feedback")); |
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| 76 | node.appendChild(button); |
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| 77 | } |
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| 78 | else { |
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| 79 | var buttons = node.getElementsByTagName("a"); |
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| 80 | for (var i = 0; i < buttons.length; i++) { |
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| 81 | var b = buttons.item(i); |
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| 82 | if (b.className == "fbbutton noprint") { |
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| 83 | node.removeChild(b); |
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| 84 | } |
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| 85 | } |
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| 86 | } |
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| 87 | }</script><style type="text/css" title="Xml2Rfc (sans serif)"> |
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| 88 | a { |
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| 89 | text-decoration: none; |
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| 90 | } |
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| 91 | a.smpl { |
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| 92 | color: black; |
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| 93 | } |
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| 94 | a:hover { |
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| 95 | text-decoration: underline; |
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| 96 | } |
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| 97 | a:active { |
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| 98 | text-decoration: underline; |
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| 99 | } |
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| 100 | address { |
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| 101 | margin-top: 1em; |
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| 102 | margin-left: 2em; |
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| 103 | font-style: normal; |
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| 104 | } |
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| 105 | body { |
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| 106 | color: black; |
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| 107 | font-family: cambria, georgia, serif; |
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| 108 | font-size: 12pt; |
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| 109 | margin: 2em auto; |
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| 110 | max-width: 1000px; |
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| 111 | } |
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| 112 | samp, tt, code, pre { |
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| 113 | font-family: consolas, monaco, monospace; |
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| 114 | } |
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| 115 | cite { |
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| 116 | font-style: normal; |
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| 117 | } |
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| 118 | div.note { |
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| 119 | margin-left: 2em; |
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| 120 | } |
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| 121 | dl { |
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| 122 | margin-left: 2em; |
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| 123 | } |
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| 124 | dl > dt { |
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| 125 | float: left; |
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| 126 | margin-right: 1em; |
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| 127 | } |
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| 128 | dl.nohang > dt { |
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| 129 | float: none; |
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| 130 | } |
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| 131 | dl > dd { |
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| 132 | margin-bottom: .5em; |
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| 133 | } |
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| 134 | dl.compact > dd { |
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| 135 | margin-bottom: .0em; |
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| 136 | } |
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| 137 | dl > dd > dl { |
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| 138 | margin-top: 0.5em; |
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| 139 | } |
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| 140 | ul.empty { |
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| 141 | list-style-type: none; |
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| 142 | } |
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| 143 | ul.empty li { |
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| 144 | margin-top: .5em; |
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| 145 | } |
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| 146 | dl p { |
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| 147 | margin-left: 0em; |
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| 148 | } |
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| 149 | dl.reference > dt { |
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| 150 | font-weight: bold; |
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| 151 | } |
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| 152 | dl.reference > dd { |
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| 153 | margin-left: 6em; |
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| 154 | } |
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| 155 | h1 { |
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| 156 | color: green; |
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| 157 | font-size: 150%; |
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| 158 | line-height: 18pt; |
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| 159 | font-weight: bold; |
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| 160 | text-align: center; |
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| 161 | margin-top: 36pt; |
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| 162 | margin-bottom: 0pt; |
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| 163 | } |
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| 164 | h2 { |
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| 165 | font-size: 130%; |
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| 166 | line-height: 21pt; |
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| 167 | page-break-after: avoid; |
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| 168 | } |
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| 169 | h2.np { |
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| 170 | page-break-before: always; |
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| 171 | } |
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| 172 | h3 { |
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| 173 | font-size: 120%; |
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| 174 | line-height: 15pt; |
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| 175 | page-break-after: avoid; |
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| 176 | } |
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| 177 | h4 { |
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| 178 | font-size: 110%; |
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| 179 | page-break-after: avoid; |
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| 180 | } |
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| 181 | h5, h6 { |
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| 182 | page-break-after: avoid; |
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| 183 | } |
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| 184 | h1 a, h2 a, h3 a, h4 a, h5 a, h6 a { |
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| 185 | color: black; |
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| 186 | } |
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| 187 | img { |
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| 188 | margin-left: 3em; |
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| 189 | } |
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| 190 | li { |
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| 191 | margin-left: 2em; |
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| 192 | } |
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| 193 | ol { |
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| 194 | margin-left: 2em; |
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| 195 | } |
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| 196 | ol.la { |
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| 197 | list-style-type: lower-alpha; |
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| 198 | } |
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| 199 | ol.ua { |
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| 200 | list-style-type: upper-alpha; |
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| 201 | } |
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| 202 | ol p { |
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| 203 | margin-left: 0em; |
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| 204 | } |
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| 205 | p { |
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| 206 | margin-left: 2em; |
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| 207 | } |
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| 208 | pre { |
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| 209 | font-size: 11pt; |
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| 210 | margin-left: 3em; |
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| 211 | background-color: lightyellow; |
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| 212 | padding: .25em; |
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| 213 | page-break-inside: avoid; |
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| 214 | } |
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| 215 | pre.text2 { |
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| 216 | border-style: dotted; |
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| 217 | border-width: 1px; |
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| 218 | background-color: #f0f0f0; |
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| 219 | } |
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| 220 | pre.inline { |
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| 221 | background-color: white; |
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| 222 | padding: 0em; |
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| 223 | page-break-inside: auto; |
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| 224 | } |
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| 225 | pre.text { |
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| 226 | border-style: dotted; |
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| 227 | border-width: 1px; |
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| 228 | background-color: #f8f8f8; |
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| 229 | } |
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| 230 | pre.drawing { |
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| 231 | border-style: solid; |
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| 232 | border-width: 1px; |
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| 233 | background-color: #f8f8f8; |
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| 234 | padding: 2em; |
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| 235 | } |
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| 236 | table { |
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| 237 | margin-left: 2em; |
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| 238 | } |
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| 239 | table.tt { |
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| 240 | vertical-align: top; |
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| 241 | border-color: gray; |
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| 242 | } |
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| 243 | table.tt th { |
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| 244 | border-color: gray; |
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| 245 | } |
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| 246 | table.tt td { |
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| 247 | border-color: gray; |
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| 248 | } |
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| 249 | table.all { |
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| 250 | border-style: solid; |
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| 251 | border-width: 2px; |
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| 252 | } |
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| 253 | table.full { |
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| 254 | border-style: solid; |
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| 255 | border-width: 2px; |
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| 256 | } |
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| 257 | table.tt td { |
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| 258 | vertical-align: top; |
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| 259 | } |
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| 260 | table.all td { |
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| 261 | border-style: solid; |
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| 262 | border-width: 1px; |
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| 263 | } |
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| 264 | table.full td { |
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| 265 | border-style: none solid; |
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| 266 | border-width: 1px; |
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| 267 | } |
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| 268 | table.tt th { |
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| 269 | vertical-align: top; |
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| 270 | } |
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| 271 | table.all th { |
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| 272 | border-style: solid; |
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| 273 | border-width: 1px; |
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| 274 | } |
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| 275 | table.full th { |
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| 276 | border-style: solid; |
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| 277 | border-width: 1px 1px 2px 1px; |
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| 278 | } |
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| 279 | table.headers th { |
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| 280 | border-style: none none solid none; |
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| 281 | border-width: 2px; |
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| 282 | } |
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| 283 | table.tleft { |
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| 284 | margin-right: auto; |
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| 285 | } |
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| 286 | table.tright { |
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| 287 | margin-left: auto; |
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| 288 | } |
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| 289 | table.tcenter { |
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| 290 | margin-left: auto; |
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| 291 | margin-right: auto; |
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| 292 | } |
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| 293 | caption { |
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| 294 | caption-side: bottom; |
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| 295 | font-weight: bold; |
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| 296 | font-size: 10pt; |
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| 297 | margin-top: .5em; |
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| 298 | } |
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| 299 | |
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| 300 | table.header { |
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| 301 | border-spacing: 1px; |
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| 302 | width: 95%; |
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| 303 | font-size: 11pt; |
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| 304 | color: white; |
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| 305 | } |
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| 306 | td.top { |
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| 307 | vertical-align: top; |
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| 308 | } |
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| 309 | td.topnowrap { |
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| 310 | vertical-align: top; |
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| 311 | white-space: nowrap; |
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| 312 | } |
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| 313 | table.header td { |
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| 314 | background-color: gray; |
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| 315 | width: 50%; |
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| 316 | } |
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| 317 | table.header a { |
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| 318 | color: white; |
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| 319 | } |
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| 320 | thead { |
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| 321 | display:table-header-group; |
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| 322 | } |
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| 323 | ul.toc, ul.toc ul { |
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| 324 | list-style: none; |
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| 325 | margin-left: 1.5em; |
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| 326 | padding-left: 0em; |
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| 327 | } |
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| 328 | ul.toc li { |
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| 329 | line-height: 150%; |
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| 330 | font-weight: bold; |
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| 331 | margin-left: 0em; |
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| 332 | } |
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| 333 | ul.toc li li { |
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| 334 | line-height: normal; |
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| 335 | font-weight: normal; |
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| 336 | font-size: 11pt; |
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| 337 | margin-left: 0em; |
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| 338 | } |
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| 339 | li.excluded { |
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| 340 | font-size: 0pt; |
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| 341 | } |
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| 342 | ul p { |
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| 343 | margin-left: 0em; |
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| 344 | } |
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| 345 | .filename, h1, h2, h3, h4 { |
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| 346 | font-family: candara, calibri, segoe, optima, arial, sans-serif; |
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| 347 | } |
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| 348 | ul.ind, ul.ind ul { |
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| 349 | list-style: none; |
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| 350 | margin-left: 1.5em; |
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| 351 | padding-left: 0em; |
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| 352 | page-break-before: avoid; |
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| 353 | } |
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| 354 | ul.ind li { |
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| 355 | font-weight: bold; |
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| 356 | line-height: 200%; |
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| 357 | margin-left: 0em; |
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| 358 | } |
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| 359 | ul.ind li li { |
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| 360 | font-weight: normal; |
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| 361 | line-height: 150%; |
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| 362 | margin-left: 0em; |
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| 363 | } |
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| 364 | .avoidbreakinside { |
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| 365 | page-break-inside: avoid; |
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| 366 | } |
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| 367 | .avoidbreakafter { |
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| 368 | page-break-after: avoid; |
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| 369 | } |
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| 370 | .bcp14 { |
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| 371 | font-style: normal; |
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| 372 | text-transform: lowercase; |
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| 373 | font-variant: small-caps; |
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| 374 | } |
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| 375 | .comment { |
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| 376 | background-color: yellow; |
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| 377 | } |
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| 378 | .center { |
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| 379 | text-align: center; |
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| 380 | } |
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| 381 | .error { |
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| 382 | color: red; |
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| 383 | font-style: italic; |
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| 384 | font-weight: bold; |
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| 385 | } |
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| 386 | .figure { |
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| 387 | font-weight: bold; |
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| 388 | text-align: center; |
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| 389 | font-size: 10pt; |
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| 390 | } |
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| 391 | .filename { |
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| 392 | color: #333333; |
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| 393 | font-size: 112%; |
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| 394 | font-weight: bold; |
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| 395 | line-height: 21pt; |
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| 396 | text-align: center; |
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| 397 | margin-top: 0pt; |
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| 398 | } |
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| 399 | .fn { |
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| 400 | font-weight: bold; |
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| 401 | } |
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| 402 | .left { |
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| 403 | text-align: left; |
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| 404 | } |
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| 405 | .right { |
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| 406 | text-align: right; |
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| 407 | } |
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| 408 | .warning { |
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| 409 | font-size: 130%; |
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| 410 | background-color: yellow; |
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| 411 | }.feedback { |
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| 412 | position: fixed; |
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| 413 | bottom: 1%; |
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| 414 | right: 1%; |
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| 415 | padding: 3px 5px; |
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| 416 | color: white; |
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| 417 | border-radius: 5px; |
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| 418 | background: #006400; |
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| 419 | border: 1px solid silver; |
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| 420 | -webkit-user-select: none; |
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| 421 | -moz-user-select: none; |
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| 422 | -ms-user-select: none; |
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| 423 | } |
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| 424 | .fbbutton { |
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| 425 | margin-left: 1em; |
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| 426 | color: #303030; |
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| 427 | font-size: small; |
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| 428 | font-weight: normal; |
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| 429 | background: #d0d000; |
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| 430 | padding: 1px 4px; |
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| 431 | border: 1px solid silver; |
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| 432 | border-radius: 5px; |
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| 433 | } |
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| 434 | .publishedasrfc { |
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| 435 | background-color: yellow; |
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| 436 | color: green; |
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| 437 | font-size: 14pt; |
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| 438 | text-align: center; |
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| 439 | } |
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| 440 | |
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| 441 | @media screen { |
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| 442 | pre.text, pre.text2 { |
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| 443 | width: 69em; |
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| 444 | } |
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| 445 | } |
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| 446 | |
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| 447 | @media print { |
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| 448 | .noprint { |
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| 449 | display: none; |
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| 450 | } |
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| 451 | |
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| 452 | a { |
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| 453 | color: black; |
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| 454 | text-decoration: none; |
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| 455 | } |
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| 456 | |
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| 457 | table.header { |
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| 458 | width: 90%; |
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| 459 | } |
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| 460 | |
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| 461 | td.header { |
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| 462 | width: 50%; |
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| 463 | color: black; |
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| 464 | background-color: white; |
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| 465 | vertical-align: top; |
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| 466 | font-size: 110%; |
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| 467 | } |
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| 468 | |
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| 469 | ul.toc a:last-child::after { |
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| 470 | content: leader('.') target-counter(attr(href), page); |
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| 471 | } |
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| 472 | |
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| 473 | ul.ind li li a { |
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| 474 | content: target-counter(attr(href), page); |
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| 475 | } |
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| 476 | |
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| 477 | pre { |
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| 478 | font-size: 10pt; |
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| 479 | } |
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| 480 | |
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| 481 | .print2col { |
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| 482 | column-count: 2; |
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| 483 | -moz-column-count: 2; |
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| 484 | column-fill: auto; |
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| 485 | } |
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| 486 | } |
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| 487 | |
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| 488 | @page { |
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| 489 | @top-left { |
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| 490 | content: "Internet-Draft"; |
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| 491 | } |
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| 492 | @top-right { |
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| 493 | content: "June 2014"; |
|---|
| 494 | } |
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| 495 | @top-center { |
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| 496 | content: "HTTP/1.1 Conditional Requests"; |
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| 497 | } |
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| 498 | @bottom-left { |
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| 499 | content: "Fielding & Reschke"; |
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| 500 | } |
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| 501 | @bottom-center { |
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| 502 | content: "Expires December 2014"; |
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| 503 | } |
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| 504 | @bottom-right { |
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| 505 | content: "[Page " counter(page) "]"; |
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| 506 | } |
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| 507 | } |
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| 508 | |
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| 509 | @page:first { |
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| 510 | @top-left { |
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| 511 | content: normal; |
|---|
| 512 | } |
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| 513 | @top-right { |
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| 514 | content: normal; |
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| 515 | } |
|---|
| 516 | @top-center { |
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| 517 | content: normal; |
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| 518 | } |
|---|
| 519 | } |
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| 520 | </style><link rel="Contents" href="#rfc.toc"> |
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| 521 | <link rel="Author" href="#rfc.authors"> |
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| 522 | <link rel="Copyright" href="#rfc.copyrightnotice"> |
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| 523 | <link rel="Index" href="#rfc.index"> |
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| 524 | <link rel="Chapter" title="1 Introduction" href="#rfc.section.1"> |
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| 525 | <link rel="Chapter" title="2 Validators" href="#rfc.section.2"> |
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| 526 | <link rel="Chapter" title="3 Precondition Header Fields" href="#rfc.section.3"> |
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| 527 | <link rel="Chapter" title="4 Status Code Definitions" href="#rfc.section.4"> |
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| 528 | <link rel="Chapter" title="5 Evaluation" href="#rfc.section.5"> |
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| 529 | <link rel="Chapter" title="6 Precedence" href="#rfc.section.6"> |
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| 530 | <link rel="Chapter" title="7 IANA Considerations" href="#rfc.section.7"> |
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| 531 | <link rel="Chapter" title="8 Security Considerations" href="#rfc.section.8"> |
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| 532 | <link rel="Chapter" title="9 Acknowledgments" href="#rfc.section.9"> |
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| 533 | <link rel="Chapter" href="#rfc.section.10" title="10 References"> |
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| 534 | <link rel="Appendix" title="A Changes from RFC 2616" href="#rfc.section.A"> |
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| 535 | <link rel="Appendix" title="B Imported ABNF" href="#rfc.section.B"> |
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| 536 | <link rel="Appendix" title="C Collected ABNF" href="#rfc.section.C"> |
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| 537 | <link rel="Alternate" title="RFC7232" href="http://svn.tools.ietf.org/svn/wg/httpbis/specs/rfc7232.html"> |
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| 538 | <link href="p2-semantics.html" rel="prev"> |
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| 539 | <link href="p5-range.html" rel="next"> |
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| 540 | <meta name="generator" content="http://greenbytes.de/tech/webdav/rfc2629.xslt, Revision 1.756, 2015/12/31 17:59:21, XSLT vendor: Saxonica http://www.saxonica.com/"> |
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| 541 | <meta name="keywords" content="Hypertext Transfer Protocol, HTTP, HTTP conditional requests"> |
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| 542 | <link rel="schema.dct" href="http://purl.org/dc/terms/"> |
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| 543 | <meta name="dct.creator" content="Fielding, R."> |
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| 544 | <meta name="dct.creator" content="Reschke, J. F."> |
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| 545 | <meta name="dct.identifier" content="urn:ietf:id:draft-ietf-httpbis-p4-conditional-latest"> |
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| 546 | <meta name="dct.issued" scheme="ISO8601" content="2014-06"> |
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| 547 | <meta name="dct.replaces" content="urn:ietf:rfc:2616"> |
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| 548 | <meta name="dct.abstract" content="The Hypertext Transfer Protocol (HTTP) is a stateless 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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| 549 | <meta name="description" content="The Hypertext Transfer Protocol (HTTP) is a stateless 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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| 550 | </head> |
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| 551 | <body onload="initFeedback();"> |
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| 552 | <table class="header" id="rfc.headerblock"> |
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| 553 | <tbody> |
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| 554 | <tr> |
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| 555 | <td class="left">HTTPbis Working Group</td> |
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| 556 | <td class="right">R. Fielding, Editor</td> |
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| 557 | </tr> |
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| 558 | <tr> |
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| 559 | <td class="left">Internet-Draft</td> |
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| 560 | <td class="right">Adobe</td> |
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| 561 | </tr> |
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| 562 | <tr> |
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| 563 | <td class="left">Obsoletes: <a href="https://tools.ietf.org/html/rfc2616">2616</a> (if approved) |
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| 564 | </td> |
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| 565 | <td class="right">J. Reschke, Editor</td> |
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| 566 | </tr> |
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| 567 | <tr> |
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| 568 | <td class="left">Intended status: Standards Track</td> |
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| 569 | <td class="right">greenbytes</td> |
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| 570 | </tr> |
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| 571 | <tr> |
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| 572 | <td class="left">Expires: December 2014</td> |
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| 573 | <td class="right">June 2014</td> |
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| 574 | </tr> |
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| 575 | </tbody> |
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| 576 | </table> |
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| 577 | <div id="rfc.title"> |
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| 578 | <h1>Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests</h1> |
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| 579 | <div class="filename">draft-ietf-httpbis-p4-conditional-latest</div> |
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| 580 | </div> |
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| 581 | <p class="publishedasrfc"><b>Note:</b> a later version of this document has been published as <a href="http://svn.tools.ietf.org/svn/wg/httpbis/specs/rfc7232.html">RFC7232</a>. |
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| 582 | |
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| 583 | </p> |
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| 584 | <h2 id="rfc.abstract"><a href="#rfc.abstract">Abstract</a></h2> |
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| 585 | <p>The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for |
|---|
| 586 | distributed, collaborative, hypertext information systems. This document defines HTTP/1.1 |
|---|
| 587 | conditional requests, including metadata header fields for indicating state changes, |
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| 588 | request header fields for making preconditions on such state, and rules for constructing |
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| 589 | the responses to a conditional request when one or more preconditions evaluate to |
|---|
| 590 | false. |
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| 591 | </p> |
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| 592 | <h2 id="rfc.note.1"><a href="#rfc.note.1">Editorial Note (To be removed by RFC Editor)</a></h2> |
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| 593 | <p>Discussion of this draft takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), |
|---|
| 594 | 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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| 595 | </p> |
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| 596 | <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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| 597 | </p> |
|---|
| 598 | <p><em>This is a temporary document for the purpose of tracking the editorial changes made |
|---|
| 599 | during the AUTH48 (RFC publication) phase.</em> |
|---|
| 600 | </p> |
|---|
| 601 | <div id="rfc.status"> |
|---|
| 602 | <h2><a href="#rfc.status">Status of This Memo</a></h2> |
|---|
| 603 | <p>This Internet-Draft is submitted in full conformance with the provisions of BCP 78 |
|---|
| 604 | and BCP 79. |
|---|
| 605 | </p> |
|---|
| 606 | <p>Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). |
|---|
| 607 | Note that other groups may also distribute working documents as Internet-Drafts. The |
|---|
| 608 | list of current Internet-Drafts is at <a href="http://datatracker.ietf.org/drafts/current/">http://datatracker.ietf.org/drafts/current/</a>. |
|---|
| 609 | </p> |
|---|
| 610 | <p>Internet-Drafts are draft documents valid for a maximum of six months and may be updated, |
|---|
| 611 | replaced, or obsoleted by other documents at any time. It is inappropriate to use |
|---|
| 612 | Internet-Drafts as reference material or to cite them other than as “work in progress”. |
|---|
| 613 | </p> |
|---|
| 614 | <p>This Internet-Draft will expire in December 2014.</p> |
|---|
| 615 | </div> |
|---|
| 616 | <div id="rfc.copyrightnotice"> |
|---|
| 617 | <h2><a href="#rfc.copyrightnotice">Copyright Notice</a></h2> |
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| 618 | <p>Copyright © 2014 IETF Trust and the persons identified as the document authors. All |
|---|
| 619 | rights reserved. |
|---|
| 620 | </p> |
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| 621 | <p>This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating |
|---|
| 622 | 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 |
|---|
| 623 | carefully, as they describe your rights and restrictions with respect to this document. |
|---|
| 624 | Code Components extracted from this document must include Simplified BSD License text |
|---|
| 625 | as described in Section 4.e of the Trust Legal Provisions and are provided without |
|---|
| 626 | warranty as described in the Simplified BSD License. |
|---|
| 627 | </p> |
|---|
| 628 | <p>This document may contain material from IETF Documents or IETF Contributions published |
|---|
| 629 | or made publicly available before November 10, 2008. The person(s) controlling the |
|---|
| 630 | copyright in some of this material may not have granted the IETF Trust the right to |
|---|
| 631 | allow modifications of such material outside the IETF Standards Process. Without obtaining |
|---|
| 632 | an adequate license from the person(s) controlling the copyright in such materials, |
|---|
| 633 | this document may not be modified outside the IETF Standards Process, and derivative |
|---|
| 634 | works of it may not be created outside the IETF Standards Process, except to format |
|---|
| 635 | it for publication as an RFC or to translate it into languages other than English. |
|---|
| 636 | </p> |
|---|
| 637 | </div> |
|---|
| 638 | <hr class="noprint"> |
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| 639 | <div id="rfc.toc"> |
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| 640 | <h2 class="np"><a href="#rfc.toc">Table of Contents</a></h2> |
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| 641 | <ul class="toc"> |
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| 642 | <li><a href="#rfc.section.1">1.</a> <a href="#introduction">Introduction</a><ul> |
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| 643 | <li><a href="#rfc.section.1.1">1.1</a> <a href="#conformance">Conformance and Error Handling</a></li> |
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| 644 | <li><a href="#rfc.section.1.2">1.2</a> <a href="#notation">Syntax Notation</a></li> |
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| 645 | </ul> |
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| 646 | </li> |
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| 647 | <li><a href="#rfc.section.2">2.</a> <a href="#validators">Validators</a><ul> |
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| 648 | <li><a href="#rfc.section.2.1">2.1</a> <a href="#weak.and.strong.validators">Weak versus Strong</a></li> |
|---|
| 649 | <li><a href="#rfc.section.2.2">2.2</a> <a href="#header.last-modified">Last-Modified</a><ul> |
|---|
| 650 | <li><a href="#rfc.section.2.2.1">2.2.1</a> <a href="#lastmod.generation">Generation</a></li> |
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| 651 | <li><a href="#rfc.section.2.2.2">2.2.2</a> <a href="#lastmod.comparison">Comparison</a></li> |
|---|
| 652 | </ul> |
|---|
| 653 | </li> |
|---|
| 654 | <li><a href="#rfc.section.2.3">2.3</a> <a href="#header.etag">ETag</a><ul> |
|---|
| 655 | <li><a href="#rfc.section.2.3.1">2.3.1</a> <a href="#entity.tag.generation">Generation</a></li> |
|---|
| 656 | <li><a href="#rfc.section.2.3.2">2.3.2</a> <a href="#entity.tag.comparison">Comparison</a></li> |
|---|
| 657 | <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> |
|---|
| 658 | </ul> |
|---|
| 659 | </li> |
|---|
| 660 | <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> |
|---|
| 661 | </ul> |
|---|
| 662 | </li> |
|---|
| 663 | <li><a href="#rfc.section.3">3.</a> <a href="#preconditions">Precondition Header Fields</a><ul> |
|---|
| 664 | <li><a href="#rfc.section.3.1">3.1</a> <a href="#header.if-match">If-Match</a></li> |
|---|
| 665 | <li><a href="#rfc.section.3.2">3.2</a> <a href="#header.if-none-match">If-None-Match</a></li> |
|---|
| 666 | <li><a href="#rfc.section.3.3">3.3</a> <a href="#header.if-modified-since">If-Modified-Since</a></li> |
|---|
| 667 | <li><a href="#rfc.section.3.4">3.4</a> <a href="#header.if-unmodified-since">If-Unmodified-Since</a></li> |
|---|
| 668 | <li><a href="#rfc.section.3.5">3.5</a> <a href="#header.if-range">If-Range</a></li> |
|---|
| 669 | </ul> |
|---|
| 670 | </li> |
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| 671 | <li><a href="#rfc.section.4">4.</a> <a href="#status.code.definitions">Status Code Definitions</a><ul> |
|---|
| 672 | <li><a href="#rfc.section.4.1">4.1</a> <a href="#status.304">304 Not Modified</a></li> |
|---|
| 673 | <li><a href="#rfc.section.4.2">4.2</a> <a href="#status.412">412 Precondition Failed</a></li> |
|---|
| 674 | </ul> |
|---|
| 675 | </li> |
|---|
| 676 | <li><a href="#rfc.section.5">5.</a> <a href="#evaluation">Evaluation</a></li> |
|---|
| 677 | <li><a href="#rfc.section.6">6.</a> <a href="#precedence">Precedence</a></li> |
|---|
| 678 | <li><a href="#rfc.section.7">7.</a> <a href="#IANA.considerations">IANA Considerations</a><ul> |
|---|
| 679 | <li><a href="#rfc.section.7.1">7.1</a> <a href="#status.code.registration">Status Code Registration</a></li> |
|---|
| 680 | <li><a href="#rfc.section.7.2">7.2</a> <a href="#header.field.registration">Header Field Registration</a></li> |
|---|
| 681 | </ul> |
|---|
| 682 | </li> |
|---|
| 683 | <li><a href="#rfc.section.8">8.</a> <a href="#security.considerations">Security Considerations</a></li> |
|---|
| 684 | <li><a href="#rfc.section.9">9.</a> <a href="#acks">Acknowledgments</a></li> |
|---|
| 685 | <li><a href="#rfc.section.10">10.</a> <a href="#rfc.references">References</a><ul> |
|---|
| 686 | <li><a href="#rfc.section.10.1">10.1</a> <a href="#rfc.references.1">Normative References</a></li> |
|---|
| 687 | <li><a href="#rfc.section.10.2">10.2</a> <a href="#rfc.references.2">Informative References</a></li> |
|---|
| 688 | </ul> |
|---|
| 689 | </li> |
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| 690 | <li><a href="#rfc.section.A">A.</a> <a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li> |
|---|
| 691 | <li><a href="#rfc.section.B">B.</a> <a href="#imported.abnf">Imported ABNF</a></li> |
|---|
| 692 | <li><a href="#rfc.section.C">C.</a> <a href="#collected.abnf">Collected ABNF</a></li> |
|---|
| 693 | <li><a href="#rfc.index">Index</a></li> |
|---|
| 694 | <li><a href="#rfc.authors">Authors' Addresses</a></li> |
|---|
| 695 | </ul> |
|---|
| 696 | </div> |
|---|
| 697 | <div id="introduction"> |
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| 698 | <h2 id="rfc.section.1" class="np"><a href="#rfc.section.1">1.</a> <a href="#introduction">Introduction</a></h2> |
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| 699 | <div id="rfc.section.1.p.1"> |
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| 700 | <p>Conditional requests are HTTP requests <a href="#RFC7231" id="rfc.xref.RFC7231.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a> that include one or more header fields indicating a precondition to be tested before |
|---|
| 701 | applying the method semantics to the target resource. This document defines the HTTP/1.1 |
|---|
| 702 | conditional request mechanisms in terms of the architecture, syntax notation, and |
|---|
| 703 | conformance criteria defined in <a href="#RFC7230" id="rfc.xref.RFC7230.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>. |
|---|
| 704 | </p> |
|---|
| 705 | </div> |
|---|
| 706 | <div id="rfc.section.1.p.2"> |
|---|
| 707 | <p>Conditional GET requests are the most efficient mechanism for HTTP cache updates <a href="#RFC7234" id="rfc.xref.RFC7234.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>. Conditionals can also be applied to state-changing methods, such as PUT and DELETE, |
|---|
| 708 | to prevent the "lost update" problem: one client accidentally overwriting the work |
|---|
| 709 | of another client that has been acting in parallel. |
|---|
| 710 | </p> |
|---|
| 711 | </div> |
|---|
| 712 | <div id="rfc.section.1.p.3"> |
|---|
| 713 | <p><span id="rfc.iref.s.1"></span> Conditional request preconditions are based on the state of the target resource as |
|---|
| 714 | a whole (its current value set) or the state as observed in a previously obtained |
|---|
| 715 | representation (one value in that set). A resource might have multiple current representations, |
|---|
| 716 | each with its own observable state. The conditional request mechanisms assume that |
|---|
| 717 | the mapping of requests to a "selected representation" (<a href="p2-semantics.html#representations" title="Representations">Section 3</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>) will be consistent over time if the server intends to take advantage of conditionals. |
|---|
| 718 | Regardless, if the mapping is inconsistent and the server is unable to select the |
|---|
| 719 | appropriate representation, then no harm will result when the precondition evaluates |
|---|
| 720 | to false. |
|---|
| 721 | </p> |
|---|
| 722 | </div> |
|---|
| 723 | <div id="rfc.section.1.p.4"> |
|---|
| 724 | <p>The conditional request preconditions defined by this specification (<a href="#preconditions" title="Precondition Header Fields">Section 3</a>) are evaluated when applicable to the recipient (<a href="#evaluation" title="Evaluation">Section 5</a>) according to their order of precedence (<a href="#precedence" title="Precedence">Section 6</a>). |
|---|
| 725 | </p> |
|---|
| 726 | </div> |
|---|
| 727 | <div id="conformance"> |
|---|
| 728 | <h3 id="rfc.section.1.1"><a href="#rfc.section.1.1">1.1</a> <a href="#conformance">Conformance and Error Handling</a></h3> |
|---|
| 729 | <div id="rfc.section.1.1.p.1"> |
|---|
| 730 | <p>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD |
|---|
| 731 | NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted |
|---|
| 732 | 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>. |
|---|
| 733 | </p> |
|---|
| 734 | </div> |
|---|
| 735 | <div id="rfc.section.1.1.p.2"> |
|---|
| 736 | <p>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="#RFC7230" id="rfc.xref.RFC7230.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>. |
|---|
| 737 | </p> |
|---|
| 738 | </div> |
|---|
| 739 | </div> |
|---|
| 740 | <div id="notation"> |
|---|
| 741 | <h3 id="rfc.section.1.2"><a href="#rfc.section.1.2">1.2</a> <a href="#notation">Syntax Notation</a></h3> |
|---|
| 742 | <div id="rfc.section.1.2.p.1"> |
|---|
| 743 | <p>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 a list extension, defined in <a href="p1-messaging.html#abnf.extension" title="ABNF List Extension: #rule">Section 7</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, that allows for compact definition of comma-separated lists using a '#' operator |
|---|
| 744 | (similar to how the '*' operator indicates repetition). <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 grammar with all list operators expanded to standard ABNF notation. |
|---|
| 745 | </p> |
|---|
| 746 | </div> |
|---|
| 747 | </div> |
|---|
| 748 | </div> |
|---|
| 749 | <div id="validators"> |
|---|
| 750 | <h2 id="rfc.section.2"><a href="#rfc.section.2">2.</a> <a href="#validators">Validators</a></h2> |
|---|
| 751 | <div id="rfc.section.2.p.1"> |
|---|
| 752 | <p>This specification defines two forms of metadata that are commonly used to observe |
|---|
| 753 | resource state and test for preconditions: 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 |
|---|
| 754 | of HTTP, such as Web Distributed Authoring and Versioning (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 |
|---|
| 755 | to as a "<dfn>validator</dfn>" when it is used within a precondition. |
|---|
| 756 | </p> |
|---|
| 757 | </div> |
|---|
| 758 | <div id="weak.and.strong.validators"> |
|---|
| 759 | <h3 id="rfc.section.2.1"><a href="#rfc.section.2.1">2.1</a> <a href="#weak.and.strong.validators">Weak versus Strong</a></h3> |
|---|
| 760 | <div id="rfc.section.2.1.p.1"> |
|---|
| 761 | <p>Validators come in two flavors: strong or weak. Weak validators are easy to generate |
|---|
| 762 | but are far less useful for comparisons. Strong validators are ideal for comparisons |
|---|
| 763 | but can be very difficult (and occasionally impossible) to generate efficiently. Rather |
|---|
| 764 | than impose that all forms of resource adhere to the same strength of validator, HTTP |
|---|
| 765 | exposes the type of validator in use and imposes restrictions on when weak validators |
|---|
| 766 | can be used as preconditions. |
|---|
| 767 | </p> |
|---|
| 768 | </div> |
|---|
| 769 | <div id="rfc.section.2.1.p.2"> |
|---|
| 770 | <p>A "strong validator" is representation metadata that changes value whenever a change |
|---|
| 771 | occurs to the representation data that would be observable in the payload body of |
|---|
| 772 | a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response to GET. |
|---|
| 773 | </p> |
|---|
| 774 | </div> |
|---|
| 775 | <div id="rfc.section.2.1.p.3"> |
|---|
| 776 | <p>A strong validator might change for reasons other than a change to the representation |
|---|
| 777 | data, such as when a semantically significant part of the representation metadata |
|---|
| 778 | 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 |
|---|
| 779 | it is necessary to invalidate the stored responses held by remote caches and authoring |
|---|
| 780 | tools. |
|---|
| 781 | </p> |
|---|
| 782 | </div> |
|---|
| 783 | <div id="rfc.section.2.1.p.4"> |
|---|
| 784 | <p>Cache entries might persist for arbitrarily long periods, regardless of expiration |
|---|
| 785 | times. Thus, a cache might attempt to validate an entry using a validator that it |
|---|
| 786 | obtained in the distant past. A strong validator is unique across all versions of |
|---|
| 787 | all representations associated with a particular resource over time. However, there |
|---|
| 788 | is no implication of uniqueness across representations of different resources (i.e., |
|---|
| 789 | the same strong validator might be in use for representations of multiple resources |
|---|
| 790 | at the same time and does not imply that those representations are equivalent). |
|---|
| 791 | </p> |
|---|
| 792 | </div> |
|---|
| 793 | <div id="rfc.section.2.1.p.5"> |
|---|
| 794 | <p>There are a variety of strong validators used in practice. The best are based on strict |
|---|
| 795 | revision control, wherein each change to a representation always results in a unique |
|---|
| 796 | node name and revision identifier being assigned before the representation is made |
|---|
| 797 | accessible to GET. A collision-resistant hash function applied to the representation |
|---|
| 798 | data is also sufficient if the data is available prior to the response header fields |
|---|
| 799 | being sent and the digest does not need to be recalculated every time a validation |
|---|
| 800 | request is received. However, if a resource has distinct representations that differ |
|---|
| 801 | only in their metadata, such as might occur with content negotiation over media types |
|---|
| 802 | that happen to share the same data format, then the origin server needs to incorporate |
|---|
| 803 | additional information in the validator to distinguish those representations. |
|---|
| 804 | </p> |
|---|
| 805 | </div> |
|---|
| 806 | <div id="rfc.section.2.1.p.6"> |
|---|
| 807 | <p>In contrast, a "weak validator" is representation metadata that might not change for |
|---|
| 808 | every change to the representation data. This weakness might be due to limitations |
|---|
| 809 | in how the value is calculated, such as clock resolution, an inability to ensure uniqueness |
|---|
| 810 | for all possible representations of the resource, or a desire of the resource owner |
|---|
| 811 | to group representations by some self-determined set of equivalency rather than unique |
|---|
| 812 | sequences of data. An origin server <em class="bcp14">SHOULD</em> change a weak entity-tag whenever it considers prior representations to be unacceptable |
|---|
| 813 | as a substitute for the current representation. In other words, a weak entity-tag |
|---|
| 814 | ought to change whenever the origin server wants caches to invalidate old responses. |
|---|
| 815 | </p> |
|---|
| 816 | </div> |
|---|
| 817 | <div id="rfc.section.2.1.p.7"> |
|---|
| 818 | <p>For example, the representation of a weather report that changes in content every |
|---|
| 819 | second, based on dynamic measurements, might be grouped into sets of equivalent representations |
|---|
| 820 | (from the origin server's perspective) with the same weak validator in order to allow |
|---|
| 821 | cached representations to be valid for a reasonable period of time (perhaps adjusted |
|---|
| 822 | dynamically based on server load or weather quality). Likewise, a representation's |
|---|
| 823 | modification time, if defined with only one-second resolution, might be a weak validator |
|---|
| 824 | if it is possible for the representation to be modified twice during a single second |
|---|
| 825 | and retrieved between those modifications. |
|---|
| 826 | </p> |
|---|
| 827 | </div> |
|---|
| 828 | <div id="rfc.section.2.1.p.8"> |
|---|
| 829 | <p>Likewise, a validator is weak if it is shared by two or more representations of a |
|---|
| 830 | given resource at the same time, unless those representations have identical representation |
|---|
| 831 | data. For example, if the origin server sends the same validator for a representation |
|---|
| 832 | with a gzip content coding applied as it does for a representation with no content |
|---|
| 833 | coding, then that validator is weak. However, two simultaneous representations might |
|---|
| 834 | share the same strong validator if they differ only in the representation metadata, |
|---|
| 835 | such as when two different media types are available for the same representation data. |
|---|
| 836 | </p> |
|---|
| 837 | </div> |
|---|
| 838 | <div id="rfc.section.2.1.p.9"> |
|---|
| 839 | <p>Strong validators are usable for all conditional requests, including cache validation, |
|---|
| 840 | partial content ranges, and "lost update" avoidance. Weak validators are only usable |
|---|
| 841 | when the client does not require exact equality with previously obtained representation |
|---|
| 842 | data, such as when validating a cache entry or limiting a web traversal to recent |
|---|
| 843 | changes. |
|---|
| 844 | </p> |
|---|
| 845 | </div> |
|---|
| 846 | </div> |
|---|
| 847 | <div id="header.last-modified"> |
|---|
| 848 | <h3 id="rfc.section.2.2"><a href="#rfc.section.2.2">2.2</a> <a href="#header.last-modified">Last-Modified</a></h3> |
|---|
| 849 | <div id="rfc.section.2.2.p.1"> |
|---|
| 850 | <p>The "Last-Modified" header field in a response provides a timestamp indicating the |
|---|
| 851 | date and time at which the origin server believes the selected representation was |
|---|
| 852 | last modified, as determined at the conclusion of handling the request. |
|---|
| 853 | </p> |
|---|
| 854 | </div> |
|---|
| 855 | <div id="rfc.figure.u.1"><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> |
|---|
| 856 | </pre></div> |
|---|
| 857 | <div id="rfc.section.2.2.p.2"> |
|---|
| 858 | <p>An example of its use is</p> |
|---|
| 859 | </div> |
|---|
| 860 | <div id="rfc.figure.u.2"><pre class="text"> Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT |
|---|
| 861 | </pre></div> |
|---|
| 862 | <div id="lastmod.generation"> |
|---|
| 863 | <h4 id="rfc.section.2.2.1"><a href="#rfc.section.2.2.1">2.2.1</a> <a href="#lastmod.generation">Generation</a></h4> |
|---|
| 864 | <div id="rfc.section.2.2.1.p.1"> |
|---|
| 865 | <p>An origin server <em class="bcp14">SHOULD</em> send Last-Modified for any selected representation for which a last modification date |
|---|
| 866 | can be reasonably and consistently determined, since its use in conditional requests |
|---|
| 867 | and evaluating cache freshness (<a href="#RFC7234" id="rfc.xref.RFC7234.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>) results in a substantial reduction of HTTP traffic on the Internet and can be a |
|---|
| 868 | significant factor in improving service scalability and reliability. |
|---|
| 869 | </p> |
|---|
| 870 | </div> |
|---|
| 871 | <div id="rfc.section.2.2.1.p.2"> |
|---|
| 872 | <p>A representation is typically the sum of many parts behind the resource interface. |
|---|
| 873 | The last-modified time would usually be the most recent time that any of those parts |
|---|
| 874 | were changed. How that value is determined for any given resource is an implementation |
|---|
| 875 | detail beyond the scope of this specification. What matters to HTTP is how recipients |
|---|
| 876 | of the Last-Modified header field can use its value to make conditional requests and |
|---|
| 877 | test the validity of locally cached responses. |
|---|
| 878 | </p> |
|---|
| 879 | </div> |
|---|
| 880 | <div id="rfc.section.2.2.1.p.3"> |
|---|
| 881 | <p>An origin server <em class="bcp14">SHOULD</em> obtain the Last-Modified value of the representation as close as possible to the time |
|---|
| 882 | 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 |
|---|
| 883 | of the representation's modification time, especially if the representation changes |
|---|
| 884 | near the time that the response is generated. |
|---|
| 885 | </p> |
|---|
| 886 | </div> |
|---|
| 887 | <div id="rfc.section.2.2.1.p.4"> |
|---|
| 888 | <p>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 |
|---|
| 889 | (<a href="p2-semantics.html#header.date" class="smpl">Date</a>). If the last modification time is derived from implementation-specific metadata |
|---|
| 890 | that evaluates to some time in the future, according to the origin server's clock, |
|---|
| 891 | then the origin server <em class="bcp14">MUST</em> replace that value with the message origination date. This prevents a future modification |
|---|
| 892 | date from having an adverse impact on cache validation. |
|---|
| 893 | </p> |
|---|
| 894 | </div> |
|---|
| 895 | <div id="rfc.section.2.2.1.p.5"> |
|---|
| 896 | <p>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 |
|---|
| 897 | the resource by some other system or user with a reliable clock. |
|---|
| 898 | </p> |
|---|
| 899 | </div> |
|---|
| 900 | </div> |
|---|
| 901 | <div id="lastmod.comparison"> |
|---|
| 902 | <h4 id="rfc.section.2.2.2"><a href="#rfc.section.2.2.2">2.2.2</a> <a href="#lastmod.comparison">Comparison</a></h4> |
|---|
| 903 | <div id="rfc.section.2.2.2.p.1"> |
|---|
| 904 | <p>A Last-Modified time, when used as a validator in a request, is implicitly weak unless |
|---|
| 905 | it is possible to deduce that it is strong, using the following rules: |
|---|
| 906 | </p> |
|---|
| 907 | <ul> |
|---|
| 908 | <li>The validator is being compared by an origin server to the actual current validator |
|---|
| 909 | for the representation and, |
|---|
| 910 | </li> |
|---|
| 911 | <li>That origin server reliably knows that the associated representation did not change |
|---|
| 912 | twice during the second covered by the presented validator. |
|---|
| 913 | </li> |
|---|
| 914 | </ul> |
|---|
| 915 | </div> |
|---|
| 916 | <div id="rfc.section.2.2.2.p.2"> |
|---|
| 917 | <p>or </p> |
|---|
| 918 | <ul> |
|---|
| 919 | <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>, or <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> header field, because the client has a cache entry for the associated representation, |
|---|
| 920 | and |
|---|
| 921 | </li> |
|---|
| 922 | <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 |
|---|
| 923 | </li> |
|---|
| 924 | <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li> |
|---|
| 925 | </ul> |
|---|
| 926 | </div> |
|---|
| 927 | <div id="rfc.section.2.2.2.p.3"> |
|---|
| 928 | <p>or </p> |
|---|
| 929 | <ul> |
|---|
| 930 | <li>The validator is being compared by an intermediate cache to the validator stored in |
|---|
| 931 | its cache entry for the representation, and |
|---|
| 932 | </li> |
|---|
| 933 | <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 |
|---|
| 934 | </li> |
|---|
| 935 | <li>The presented Last-Modified time is at least 60 seconds before the Date value.</li> |
|---|
| 936 | </ul> |
|---|
| 937 | </div> |
|---|
| 938 | <div id="rfc.section.2.2.2.p.4"> |
|---|
| 939 | <p>This method relies on the fact that if two different responses were sent by the origin |
|---|
| 940 | server during the same second, but both had the same Last-Modified time, then at least |
|---|
| 941 | 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 |
|---|
| 942 | the possibility that the Date and Last-Modified values are generated from different |
|---|
| 943 | 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. |
|---|
| 944 | </p> |
|---|
| 945 | </div> |
|---|
| 946 | </div> |
|---|
| 947 | </div> |
|---|
| 948 | <div id="header.etag"> |
|---|
| 949 | <h3 id="rfc.section.2.3"><a href="#rfc.section.2.3">2.3</a> <a href="#header.etag">ETag</a></h3> |
|---|
| 950 | <div id="rfc.section.2.3.p.1"> |
|---|
| 951 | <p>The "ETag" header field in a response provides the current entity-tag for the selected |
|---|
| 952 | representation, as determined at the conclusion of handling the request. An entity-tag |
|---|
| 953 | is an opaque validator for differentiating between multiple representations of the |
|---|
| 954 | same resource, regardless of whether those multiple representations are due to resource |
|---|
| 955 | state changes over time, content negotiation resulting in multiple representations |
|---|
| 956 | being valid at the same time, or both. An entity-tag consists of an opaque quoted |
|---|
| 957 | string, possibly prefixed by a weakness indicator. |
|---|
| 958 | </p> |
|---|
| 959 | </div> |
|---|
| 960 | <div id="rfc.figure.u.3"><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> |
|---|
| 961 | |
|---|
| 962 | <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> |
|---|
| 963 | <a href="#header.etag" class="smpl">weak</a> = %x57.2F ; "W/", case-sensitive |
|---|
| 964 | <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> |
|---|
| 965 | <a href="#header.etag" class="smpl">etagc</a> = %x21 / %x23-7E / <a href="#imported.abnf" class="smpl">obs-text</a> |
|---|
| 966 | ; <a href="#imported.abnf" class="smpl">VCHAR</a> except double quotes, plus obs-text |
|---|
| 967 | </pre></div> |
|---|
| 968 | <div class="note"> |
|---|
| 969 | <div id="rfc.section.2.3.p.2"> |
|---|
| 970 | <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="https://tools.ietf.org/html/rfc2616#section-3.11">Section 3.11</a>); thus, some recipients might perform backslash unescaping. Servers therefore ought |
|---|
| 971 | to avoid backslash characters in entity tags. |
|---|
| 972 | </p> |
|---|
| 973 | </div> |
|---|
| 974 | </div> |
|---|
| 975 | <div id="rfc.section.2.3.p.3"> |
|---|
| 976 | <p>An entity-tag can be more reliable for validation than a modification date in situations |
|---|
| 977 | where it is inconvenient to store modification dates, where the one-second resolution |
|---|
| 978 | of HTTP date values is not sufficient, or where modification dates are not consistently |
|---|
| 979 | maintained. |
|---|
| 980 | </p> |
|---|
| 981 | </div> |
|---|
| 982 | <div id="rfc.figure.u.4"> |
|---|
| 983 | <p>Examples:</p><pre class="text"> ETag: "xyzzy" |
|---|
| 984 | ETag: W/"xyzzy" |
|---|
| 985 | ETag: "" |
|---|
| 986 | </pre></div> |
|---|
| 987 | <div id="rfc.section.2.3.p.4"> |
|---|
| 988 | <p>An entity-tag can be either a weak or strong validator, with strong being the default. |
|---|
| 989 | If an origin server provides an entity-tag for a representation and the generation |
|---|
| 990 | of that entity-tag does not satisfy all of the characteristics of a strong validator |
|---|
| 991 | (<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). |
|---|
| 992 | </p> |
|---|
| 993 | </div> |
|---|
| 994 | <div id="entity.tag.generation"> |
|---|
| 995 | <h4 id="rfc.section.2.3.1"><a href="#rfc.section.2.3.1">2.3.1</a> <a href="#entity.tag.generation">Generation</a></h4> |
|---|
| 996 | <div id="rfc.section.2.3.1.p.1"> |
|---|
| 997 | <p>The principle behind entity-tags is that only the service author knows the implementation |
|---|
| 998 | of a resource well enough to select the most accurate and efficient validation mechanism |
|---|
| 999 | for that resource, and that any such mechanism can be mapped to a simple sequence |
|---|
| 1000 | of octets for easy comparison. Since the value is opaque, there is no need for the |
|---|
| 1001 | client to be aware of how each entity-tag is constructed. |
|---|
| 1002 | </p> |
|---|
| 1003 | </div> |
|---|
| 1004 | <div id="rfc.section.2.3.1.p.2"> |
|---|
| 1005 | <p>For example, a resource that has implementation-specific versioning applied to all |
|---|
| 1006 | changes might use an internal revision number, perhaps combined with a variance identifier |
|---|
| 1007 | for content negotiation, to accurately differentiate between representations. Other |
|---|
| 1008 | implementations might use a collision-resistant hash of representation content, a |
|---|
| 1009 | combination of various file attributes, or a modification timestamp that has sub-second |
|---|
| 1010 | resolution. |
|---|
| 1011 | </p> |
|---|
| 1012 | </div> |
|---|
| 1013 | <div id="rfc.section.2.3.1.p.3"> |
|---|
| 1014 | <p>An origin server <em class="bcp14">SHOULD</em> send an ETag for any selected representation for which detection of changes can be |
|---|
| 1015 | reasonably and consistently determined, since the entity-tag's use in conditional |
|---|
| 1016 | requests and evaluating cache freshness (<a href="#RFC7234" id="rfc.xref.RFC7234.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>) can result in a substantial reduction of HTTP network traffic and can be a significant |
|---|
| 1017 | factor in improving service scalability and reliability. |
|---|
| 1018 | </p> |
|---|
| 1019 | </div> |
|---|
| 1020 | </div> |
|---|
| 1021 | <div id="entity.tag.comparison"> |
|---|
| 1022 | <h4 id="rfc.section.2.3.2"><a href="#rfc.section.2.3.2">2.3.2</a> <a href="#entity.tag.comparison">Comparison</a></h4> |
|---|
| 1023 | <div id="rfc.section.2.3.2.p.1"> |
|---|
| 1024 | <p>There are two entity-tag comparison functions, depending on whether or not the comparison |
|---|
| 1025 | context allows the use of weak validators: |
|---|
| 1026 | </p> |
|---|
| 1027 | <ul> |
|---|
| 1028 | <li><dfn>Strong comparison</dfn>: two entity-tags are equivalent if both are not weak and their opaque-tags match |
|---|
| 1029 | character-by-character. |
|---|
| 1030 | </li> |
|---|
| 1031 | <li><dfn>Weak comparison</dfn>: two entity-tags are equivalent if their opaque-tags match character-by-character, |
|---|
| 1032 | regardless of either or both being tagged as "weak". |
|---|
| 1033 | </li> |
|---|
| 1034 | </ul> |
|---|
| 1035 | </div> |
|---|
| 1036 | <div id="rfc.section.2.3.2.p.2" class="avoidbreakafter"> |
|---|
| 1037 | <p>The example below shows the results for a set of entity-tag pairs and both the weak |
|---|
| 1038 | and strong comparison function results: |
|---|
| 1039 | </p> |
|---|
| 1040 | </div> |
|---|
| 1041 | <div id="rfc.table.u.1"> |
|---|
| 1042 | <table class="tt full tleft" cellpadding="3" cellspacing="0"> |
|---|
| 1043 | <thead> |
|---|
| 1044 | <tr> |
|---|
| 1045 | <th>ETag 1</th> |
|---|
| 1046 | <th>ETag 2</th> |
|---|
| 1047 | <th>Strong Comparison</th> |
|---|
| 1048 | <th>Weak Comparison</th> |
|---|
| 1049 | </tr> |
|---|
| 1050 | </thead> |
|---|
| 1051 | <tbody> |
|---|
| 1052 | <tr> |
|---|
| 1053 | <td class="left">W/"1"</td> |
|---|
| 1054 | <td class="left">W/"1"</td> |
|---|
| 1055 | <td class="left">no match</td> |
|---|
| 1056 | <td class="left">match</td> |
|---|
| 1057 | </tr> |
|---|
| 1058 | <tr> |
|---|
| 1059 | <td class="left">W/"1"</td> |
|---|
| 1060 | <td class="left">W/"2"</td> |
|---|
| 1061 | <td class="left">no match</td> |
|---|
| 1062 | <td class="left">no match</td> |
|---|
| 1063 | </tr> |
|---|
| 1064 | <tr> |
|---|
| 1065 | <td class="left">W/"1"</td> |
|---|
| 1066 | <td class="left">"1"</td> |
|---|
| 1067 | <td class="left">no match</td> |
|---|
| 1068 | <td class="left">match</td> |
|---|
| 1069 | </tr> |
|---|
| 1070 | <tr> |
|---|
| 1071 | <td class="left">"1"</td> |
|---|
| 1072 | <td class="left">"1"</td> |
|---|
| 1073 | <td class="left">match</td> |
|---|
| 1074 | <td class="left">match</td> |
|---|
| 1075 | </tr> |
|---|
| 1076 | </tbody> |
|---|
| 1077 | </table> |
|---|
| 1078 | </div> |
|---|
| 1079 | </div> |
|---|
| 1080 | <div id="example.entity.tag.vs.conneg"> |
|---|
| 1081 | <h4 id="rfc.section.2.3.3"><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></h4> |
|---|
| 1082 | <div id="rfc.section.2.3.3.p.1" class="avoidbreakafter"> |
|---|
| 1083 | <p>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="#RFC7231" id="rfc.xref.RFC7231.3"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</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="#RFC7231" id="rfc.xref.RFC7231.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>): |
|---|
| 1084 | </p> |
|---|
| 1085 | </div> |
|---|
| 1086 | <div id="rfc.figure.u.5"> |
|---|
| 1087 | <p>>> Request:</p><pre class="text2">GET /index HTTP/1.1 |
|---|
| 1088 | Host: www.example.com |
|---|
| 1089 | Accept-Encoding: gzip |
|---|
| 1090 | |
|---|
| 1091 | </pre></div> |
|---|
| 1092 | <div id="rfc.section.2.3.3.p.2" class="avoidbreakafter"> |
|---|
| 1093 | <p>In this case, the response might or might not use the gzip content coding. If it does |
|---|
| 1094 | not, the response might look like: |
|---|
| 1095 | </p> |
|---|
| 1096 | </div> |
|---|
| 1097 | <div id="rfc.figure.u.6"> |
|---|
| 1098 | <p>>> Response:</p><pre class="text">HTTP/1.1 200 OK |
|---|
| 1099 | Date: Fri, 26 Mar 2010 00:05:00 GMT |
|---|
| 1100 | ETag: "123-a" |
|---|
| 1101 | Content-Length: 70 |
|---|
| 1102 | Vary: Accept-Encoding |
|---|
| 1103 | Content-Type: text/plain |
|---|
| 1104 | |
|---|
| 1105 | <span id="exbody">Hello World! |
|---|
| 1106 | Hello World! |
|---|
| 1107 | Hello World! |
|---|
| 1108 | Hello World! |
|---|
| 1109 | Hello World! |
|---|
| 1110 | </span></pre></div> |
|---|
| 1111 | <div id="rfc.section.2.3.3.p.3" class="avoidbreakafter"> |
|---|
| 1112 | <p>An alternative representation that does use gzip content coding would be:</p> |
|---|
| 1113 | </div> |
|---|
| 1114 | <div id="rfc.figure.u.7"> |
|---|
| 1115 | <p>>> Response:</p><pre class="text">HTTP/1.1 200 OK |
|---|
| 1116 | Date: Fri, 26 Mar 2010 00:05:00 GMT |
|---|
| 1117 | ETag: "123-b" |
|---|
| 1118 | Content-Length: 43 |
|---|
| 1119 | Vary: Accept-Encoding |
|---|
| 1120 | Content-Type: text/plain |
|---|
| 1121 | Content-Encoding: gzip |
|---|
| 1122 | |
|---|
| 1123 | <em>...binary data...</em></pre></div> |
|---|
| 1124 | <div class="note"> |
|---|
| 1125 | <div id="rfc.section.2.3.3.p.4"> |
|---|
| 1126 | <p><b>Note:</b> Content codings are a property of the representation data, so a strong entity-tag |
|---|
| 1127 | for a content-encoded representation has to be distinct from the entity tag of an |
|---|
| 1128 | unencoded representation to prevent potential conflicts during cache updates and range |
|---|
| 1129 | requests. In contrast, transfer codings (<a href="p1-messaging.html#transfer.codings" title="Transfer Codings">Section 4</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>) apply only during message transfer and do not result in distinct entity-tags. |
|---|
| 1130 | </p> |
|---|
| 1131 | </div> |
|---|
| 1132 | </div> |
|---|
| 1133 | </div> |
|---|
| 1134 | </div> |
|---|
| 1135 | <div id="when.to.use.entity.tags.and.last-modified.dates"> |
|---|
| 1136 | <h3 id="rfc.section.2.4"><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></h3> |
|---|
| 1137 | <div id="rfc.section.2.4.p.1"> |
|---|
| 1138 | <p>In <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> responses to GET or HEAD, an origin server: |
|---|
| 1139 | </p> |
|---|
| 1140 | <ul> |
|---|
| 1141 | <li><em class="bcp14">SHOULD</em> send an entity-tag validator unless it is not feasible to generate one. |
|---|
| 1142 | </li> |
|---|
| 1143 | <li><em class="bcp14">MAY</em> send a weak entity-tag instead of a strong entity-tag, if performance considerations |
|---|
| 1144 | support the use of weak entity-tags, or if it is unfeasible to send a strong entity-tag. |
|---|
| 1145 | </li> |
|---|
| 1146 | <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. |
|---|
| 1147 | </li> |
|---|
| 1148 | </ul> |
|---|
| 1149 | </div> |
|---|
| 1150 | <div id="rfc.section.2.4.p.2"> |
|---|
| 1151 | <p>In other words, the preferred behavior for an origin server is to send both a strong |
|---|
| 1152 | entity-tag and a <a href="#header.last-modified" class="smpl">Last-Modified</a> value in successful responses to a retrieval request. |
|---|
| 1153 | </p> |
|---|
| 1154 | </div> |
|---|
| 1155 | <div id="rfc.section.2.4.p.3"> |
|---|
| 1156 | <p>A client: </p> |
|---|
| 1157 | <ul> |
|---|
| 1158 | <li><em class="bcp14">MUST</em> send that entity-tag in any cache validation 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. |
|---|
| 1159 | </li> |
|---|
| 1160 | <li><em class="bcp14">SHOULD</em> send the <a href="#header.last-modified" class="smpl">Last-Modified</a> value in non-subrange cache validation 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. |
|---|
| 1161 | </li> |
|---|
| 1162 | <li><em class="bcp14">MAY</em> send the <a href="#header.last-modified" class="smpl">Last-Modified</a> value in subrange cache validation 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 |
|---|
| 1163 | user agent <em class="bcp14">SHOULD</em> provide a way to disable this, in case of difficulty. |
|---|
| 1164 | </li> |
|---|
| 1165 | <li><em class="bcp14">SHOULD</em> send both validators in cache validation 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 |
|---|
| 1166 | caches to respond appropriately. |
|---|
| 1167 | </li> |
|---|
| 1168 | </ul> |
|---|
| 1169 | </div> |
|---|
| 1170 | </div> |
|---|
| 1171 | </div> |
|---|
| 1172 | <div id="preconditions"> |
|---|
| 1173 | <h2 id="rfc.section.3"><a href="#rfc.section.3">3.</a> <a href="#preconditions">Precondition Header Fields</a></h2> |
|---|
| 1174 | <div id="rfc.section.3.p.1"> |
|---|
| 1175 | <p>This section defines the syntax and semantics of HTTP/1.1 header fields for applying |
|---|
| 1176 | preconditions on requests. <a href="#evaluation" title="Evaluation">Section 5</a> defines when the preconditions are applied. <a href="#precedence" title="Precedence">Section 6</a> defines the order of evaluation when more than one precondition is present. |
|---|
| 1177 | </p> |
|---|
| 1178 | </div> |
|---|
| 1179 | <div id="header.if-match"> |
|---|
| 1180 | <h3 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a> <a href="#header.if-match">If-Match</a></h3> |
|---|
| 1181 | <div id="rfc.section.3.1.p.1"> |
|---|
| 1182 | <p>The "If-Match" header field makes the request method conditional on the recipient |
|---|
| 1183 | origin server either having at least one current representation of the target resource, |
|---|
| 1184 | when the field-value is "*", or having a current representation of the target resource |
|---|
| 1185 | that has an entity-tag matching a member of the list of entity-tags provided in the |
|---|
| 1186 | field-value. |
|---|
| 1187 | </p> |
|---|
| 1188 | </div> |
|---|
| 1189 | <div id="rfc.section.3.1.p.2"> |
|---|
| 1190 | <p>An origin server <em class="bcp14">MUST</em> use the strong comparison function when comparing entity-tags for If-Match (<a href="#entity.tag.comparison" title="Comparison">Section 2.3.2</a>), since the client intends this precondition to prevent the method from being applied |
|---|
| 1191 | if there have been any changes to the representation data. |
|---|
| 1192 | </p> |
|---|
| 1193 | </div> |
|---|
| 1194 | <div id="rfc.figure.u.8"><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> |
|---|
| 1195 | </pre></div> |
|---|
| 1196 | <div id="rfc.section.3.1.p.3" class="avoidbreakafter"> |
|---|
| 1197 | <p>Examples:</p> |
|---|
| 1198 | </div> |
|---|
| 1199 | <div id="rfc.figure.u.9"><pre class="text"> If-Match: "xyzzy" |
|---|
| 1200 | If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" |
|---|
| 1201 | If-Match: * |
|---|
| 1202 | </pre></div> |
|---|
| 1203 | <div id="rfc.section.3.1.p.4"> |
|---|
| 1204 | <p>If-Match is most often used with state-changing methods (e.g., POST, PUT, DELETE) |
|---|
| 1205 | to prevent accidental overwrites when multiple user agents might be acting in parallel |
|---|
| 1206 | on the same resource (i.e., to prevent the "lost update" problem). It can also be |
|---|
| 1207 | used with safe methods to abort a request if the <a href="p2-semantics.html#representations" class="smpl">selected representation</a> does not match one already stored (or partially stored) from a prior request. |
|---|
| 1208 | </p> |
|---|
| 1209 | </div> |
|---|
| 1210 | <div id="rfc.section.3.1.p.5"> |
|---|
| 1211 | <p>An origin server that receives an If-Match header field <em class="bcp14">MUST</em> evaluate the condition prior to performing the method (<a href="#evaluation" title="Evaluation">Section 5</a>). If the field-value is "*", the condition is false if the origin server does not |
|---|
| 1212 | have a current representation for the target resource. If the field-value is a list |
|---|
| 1213 | of entity-tags, the condition is false if none of the listed tags match the entity-tag |
|---|
| 1214 | of the selected representation. |
|---|
| 1215 | </p> |
|---|
| 1216 | </div> |
|---|
| 1217 | <div id="rfc.section.3.1.p.6"> |
|---|
| 1218 | <p>An origin server <em class="bcp14">MUST NOT</em> perform the requested method if a received If-Match condition evaluates to false; |
|---|
| 1219 | instead, the origin server <em class="bcp14">MUST</em> respond with either a) the <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code or b) one of the <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> status codes if the origin server has verified that a state change is being requested |
|---|
| 1220 | and the final state is already reflected in the current state of the target resource |
|---|
| 1221 | (i.e., the change requested by the user agent has already succeeded, but the user |
|---|
| 1222 | agent might not be aware of it, perhaps because the prior response was lost or a compatible |
|---|
| 1223 | change was made by some other user agent). In the latter case, the origin server <em class="bcp14">MUST NOT</em> send a validator header field in the response unless it can verify that the request |
|---|
| 1224 | is a duplicate of an immediately prior change made by the same user agent. |
|---|
| 1225 | </p> |
|---|
| 1226 | </div> |
|---|
| 1227 | <div id="rfc.section.3.1.p.7"> |
|---|
| 1228 | <p>The If-Match header field can be ignored by caches and intermediaries because it is |
|---|
| 1229 | not applicable to a stored response. |
|---|
| 1230 | </p> |
|---|
| 1231 | </div> |
|---|
| 1232 | </div> |
|---|
| 1233 | <div id="header.if-none-match"> |
|---|
| 1234 | <h3 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a> <a href="#header.if-none-match">If-None-Match</a></h3> |
|---|
| 1235 | <div id="rfc.section.3.2.p.1"> |
|---|
| 1236 | <p>The "If-None-Match" header field makes the request method conditional on a recipient |
|---|
| 1237 | cache or origin server either not having any current representation of the target |
|---|
| 1238 | resource, when the field-value is "*", or having a selected representation with an |
|---|
| 1239 | entity-tag that does not match any of those listed in the field-value. |
|---|
| 1240 | </p> |
|---|
| 1241 | </div> |
|---|
| 1242 | <div id="rfc.section.3.2.p.2"> |
|---|
| 1243 | <p>A recipient <em class="bcp14">MUST</em> use the weak comparison function when comparing entity-tags for If-None-Match (<a href="#entity.tag.comparison" title="Comparison">Section 2.3.2</a>), since weak entity-tags can be used for cache validation even if there have been |
|---|
| 1244 | changes to the representation data. |
|---|
| 1245 | </p> |
|---|
| 1246 | </div> |
|---|
| 1247 | <div id="rfc.figure.u.10"><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> |
|---|
| 1248 | </pre></div> |
|---|
| 1249 | <div id="rfc.section.3.2.p.3" class="avoidbreakafter"> |
|---|
| 1250 | <p>Examples:</p> |
|---|
| 1251 | </div> |
|---|
| 1252 | <div id="rfc.figure.u.11"><pre class="text"> If-None-Match: "xyzzy" |
|---|
| 1253 | If-None-Match: W/"xyzzy" |
|---|
| 1254 | If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" |
|---|
| 1255 | If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz" |
|---|
| 1256 | If-None-Match: * |
|---|
| 1257 | </pre></div> |
|---|
| 1258 | <div id="rfc.section.3.2.p.4"> |
|---|
| 1259 | <p>If-None-Match is primarily used in conditional GET requests to enable efficient updates |
|---|
| 1260 | of cached information with a minimum amount of transaction overhead. When a client |
|---|
| 1261 | desires to update one or more stored responses that have entity-tags, the client <em class="bcp14">SHOULD</em> generate an If-None-Match header field containing a list of those entity-tags when |
|---|
| 1262 | making a GET request; this allows recipient servers to send a <a href="#status.304" class="smpl">304 (Not Modified)</a> response to indicate when one of those stored responses matches the selected representation. |
|---|
| 1263 | </p> |
|---|
| 1264 | </div> |
|---|
| 1265 | <div id="rfc.section.3.2.p.5"> |
|---|
| 1266 | <p>If-None-Match can also be used with a value of "*" to prevent an unsafe request method |
|---|
| 1267 | (e.g., PUT) from inadvertently modifying an existing representation of the target |
|---|
| 1268 | resource when the client believes that the resource does not have a current representation |
|---|
| 1269 | (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 4.2.1</a> of <a href="#RFC7231" id="rfc.xref.RFC7231.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>). This is a variation on the "lost update" problem that might arise if more than |
|---|
| 1270 | one client attempts to create an initial representation for the target resource. |
|---|
| 1271 | </p> |
|---|
| 1272 | </div> |
|---|
| 1273 | <div id="rfc.section.3.2.p.6"> |
|---|
| 1274 | <p>An origin server that receives an If-None-Match header field <em class="bcp14">MUST</em> evaluate the condition prior to performing the method (<a href="#evaluation" title="Evaluation">Section 5</a>). If the field-value is "*", the condition is false if the origin server has a current |
|---|
| 1275 | representation for the target resource. If the field-value is a list of entity-tags, |
|---|
| 1276 | the condition is false if one of the listed tags match the entity-tag of the selected |
|---|
| 1277 | representation. |
|---|
| 1278 | </p> |
|---|
| 1279 | </div> |
|---|
| 1280 | <div id="rfc.section.3.2.p.7"> |
|---|
| 1281 | <p>An origin server <em class="bcp14">MUST NOT</em> perform the requested method if the condition evaluates to false; instead, the origin |
|---|
| 1282 | server <em class="bcp14">MUST</em> respond with either a) the <a href="#status.304" class="smpl">304 (Not Modified)</a> status code if the request method is GET or HEAD or b) the <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code for all other request methods. |
|---|
| 1283 | </p> |
|---|
| 1284 | </div> |
|---|
| 1285 | <div id="rfc.section.3.2.p.8"> |
|---|
| 1286 | <p>Requirements on cache handling of a received If-None-Match header field are defined |
|---|
| 1287 | in <a href="p6-cache.html#validation.received" title="Handling a Received Validation Request">Section 4.3.2</a> of <a href="#RFC7234" id="rfc.xref.RFC7234.4"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>. |
|---|
| 1288 | </p> |
|---|
| 1289 | </div> |
|---|
| 1290 | </div> |
|---|
| 1291 | <div id="header.if-modified-since"> |
|---|
| 1292 | <h3 id="rfc.section.3.3"><a href="#rfc.section.3.3">3.3</a> <a href="#header.if-modified-since">If-Modified-Since</a></h3> |
|---|
| 1293 | <div id="rfc.section.3.3.p.1"> |
|---|
| 1294 | <p>The "If-Modified-Since" header field makes a GET or HEAD request method conditional |
|---|
| 1295 | on the selected representation's modification date being more recent than the date |
|---|
| 1296 | provided in the field-value. Transfer of the selected representation's data is avoided |
|---|
| 1297 | if that data has not changed. |
|---|
| 1298 | </p> |
|---|
| 1299 | </div> |
|---|
| 1300 | <div id="rfc.figure.u.12"><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> |
|---|
| 1301 | </pre></div> |
|---|
| 1302 | <div id="rfc.section.3.3.p.2" class="avoidbreakafter"> |
|---|
| 1303 | <p>An example of the field is:</p> |
|---|
| 1304 | </div> |
|---|
| 1305 | <div id="rfc.figure.u.13"><pre class="text"> If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT |
|---|
| 1306 | </pre></div> |
|---|
| 1307 | <div id="rfc.section.3.3.p.3"> |
|---|
| 1308 | <p>A recipient <em class="bcp14">MUST</em> ignore If-Modified-Since if the request contains an <a href="#header.if-none-match" class="smpl">If-None-Match</a> header field; the condition in <a href="#header.if-none-match" class="smpl">If-None-Match</a> is considered to be a more accurate replacement for the condition in If-Modified-Since, |
|---|
| 1309 | and the two are only combined for the sake of interoperating with older intermediaries |
|---|
| 1310 | that might not implement <a href="#header.if-none-match" class="smpl">If-None-Match</a>. |
|---|
| 1311 | </p> |
|---|
| 1312 | </div> |
|---|
| 1313 | <div id="rfc.section.3.3.p.4"> |
|---|
| 1314 | <p>A recipient <em class="bcp14">MUST</em> ignore the If-Modified-Since header field if the received field-value is not a valid |
|---|
| 1315 | HTTP-date, or if the request method is neither GET nor HEAD. |
|---|
| 1316 | </p> |
|---|
| 1317 | </div> |
|---|
| 1318 | <div id="rfc.section.3.3.p.5"> |
|---|
| 1319 | <p>A recipient <em class="bcp14">MUST</em> interpret an If-Modified-Since field-value's timestamp in terms of the origin server's |
|---|
| 1320 | clock. |
|---|
| 1321 | </p> |
|---|
| 1322 | </div> |
|---|
| 1323 | <div id="rfc.section.3.3.p.6"> |
|---|
| 1324 | <p>If-Modified-Since is typically used for two distinct purposes: 1) to allow efficient |
|---|
| 1325 | updates of a cached representation that does not have an entity-tag and 2) to limit |
|---|
| 1326 | the scope of a web traversal to resources that have recently changed. |
|---|
| 1327 | </p> |
|---|
| 1328 | </div> |
|---|
| 1329 | <div id="rfc.section.3.3.p.7"> |
|---|
| 1330 | <p>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 |
|---|
| 1331 | for cases where clocks are poorly synchronized or when the server has chosen to only |
|---|
| 1332 | honor exact timestamp matches (due to a problem with Last-Modified dates that appear |
|---|
| 1333 | to go "back in time" when the origin server's clock is corrected or a representation |
|---|
| 1334 | is restored from an archived backup). However, caches occasionally generate the field |
|---|
| 1335 | 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, |
|---|
| 1336 | particularly when the cached message does not contain a <a href="#header.last-modified" class="smpl">Last-Modified</a> field. |
|---|
| 1337 | </p> |
|---|
| 1338 | </div> |
|---|
| 1339 | <div id="rfc.section.3.3.p.8"> |
|---|
| 1340 | <p>When used for limiting the scope of retrieval to a recent time window, a user agent |
|---|
| 1341 | will generate an If-Modified-Since field value based on either its own local clock |
|---|
| 1342 | or a <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field received from the server in a prior response. Origin servers that choose |
|---|
| 1343 | an exact timestamp match based on the 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 |
|---|
| 1344 | changed during the specified window. |
|---|
| 1345 | </p> |
|---|
| 1346 | </div> |
|---|
| 1347 | <div id="rfc.section.3.3.p.9"> |
|---|
| 1348 | <p>An origin server that receives an If-Modified-Since header field <em class="bcp14">SHOULD</em> evaluate the condition prior to performing the method (<a href="#evaluation" title="Evaluation">Section 5</a>). The origin server <em class="bcp14">SHOULD NOT</em> perform the requested method if the selected representation's last modification date |
|---|
| 1349 | is earlier than or equal to the date provided in the field-value; instead, the origin |
|---|
| 1350 | server <em class="bcp14">SHOULD</em> generate a <a href="#status.304" class="smpl">304 (Not Modified)</a> response, including only those metadata that are useful for identifying or updating |
|---|
| 1351 | a previously cached response. |
|---|
| 1352 | </p> |
|---|
| 1353 | </div> |
|---|
| 1354 | <div id="rfc.section.3.3.p.10"> |
|---|
| 1355 | <p>Requirements on cache handling of a received If-Modified-Since header field are defined |
|---|
| 1356 | in <a href="p6-cache.html#validation.received" title="Handling a Received Validation Request">Section 4.3.2</a> of <a href="#RFC7234" id="rfc.xref.RFC7234.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>. |
|---|
| 1357 | </p> |
|---|
| 1358 | </div> |
|---|
| 1359 | </div> |
|---|
| 1360 | <div id="header.if-unmodified-since"> |
|---|
| 1361 | <h3 id="rfc.section.3.4"><a href="#rfc.section.3.4">3.4</a> <a href="#header.if-unmodified-since">If-Unmodified-Since</a></h3> |
|---|
| 1362 | <div id="rfc.section.3.4.p.1"> |
|---|
| 1363 | <p>The "If-Unmodified-Since" header field makes the request method conditional on the |
|---|
| 1364 | selected representation's last modification date being earlier than or equal to the |
|---|
| 1365 | date provided in the field-value. This field accomplishes the same purpose as <a href="#header.if-match" class="smpl">If-Match</a> for cases where the user agent does not have an entity-tag for the representation. |
|---|
| 1366 | </p> |
|---|
| 1367 | </div> |
|---|
| 1368 | <div id="rfc.figure.u.14"><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> |
|---|
| 1369 | </pre></div> |
|---|
| 1370 | <div id="rfc.section.3.4.p.2" class="avoidbreakafter"> |
|---|
| 1371 | <p>An example of the field is:</p> |
|---|
| 1372 | </div> |
|---|
| 1373 | <div id="rfc.figure.u.15"><pre class="text"> If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT |
|---|
| 1374 | </pre></div> |
|---|
| 1375 | <div id="rfc.section.3.4.p.3"> |
|---|
| 1376 | <p>A recipient <em class="bcp14">MUST</em> ignore If-Unmodified-Since if the request contains an <a href="#header.if-match" class="smpl">If-Match</a> header field; the condition in <a href="#header.if-match" class="smpl">If-Match</a> is considered to be a more accurate replacement for the condition in If-Unmodified-Since, |
|---|
| 1377 | and the two are only combined for the sake of interoperating with older intermediaries |
|---|
| 1378 | that might not implement <a href="#header.if-match" class="smpl">If-Match</a>. |
|---|
| 1379 | </p> |
|---|
| 1380 | </div> |
|---|
| 1381 | <div id="rfc.section.3.4.p.4"> |
|---|
| 1382 | <p>A recipient <em class="bcp14">MUST</em> ignore the If-Unmodified-Since header field if the received field-value is not a valid |
|---|
| 1383 | HTTP-date. |
|---|
| 1384 | </p> |
|---|
| 1385 | </div> |
|---|
| 1386 | <div id="rfc.section.3.4.p.5"> |
|---|
| 1387 | <p>A recipient <em class="bcp14">MUST</em> interpret an If-Unmodified-Since field-value's timestamp in terms of the origin server's |
|---|
| 1388 | clock. |
|---|
| 1389 | </p> |
|---|
| 1390 | </div> |
|---|
| 1391 | <div id="rfc.section.3.4.p.6"> |
|---|
| 1392 | <p>If-Unmodified-Since is most often used with state-changing methods (e.g., POST, PUT, |
|---|
| 1393 | DELETE) to prevent accidental overwrites when multiple user agents might be acting |
|---|
| 1394 | in parallel on a resource that does not supply entity-tags with its representations |
|---|
| 1395 | (i.e., to prevent the "lost update" problem). It can also be used with safe methods |
|---|
| 1396 | to abort a request if the <a href="p2-semantics.html#representations" class="smpl">selected representation</a> does not match one already stored (or partially stored) from a prior request. |
|---|
| 1397 | </p> |
|---|
| 1398 | </div> |
|---|
| 1399 | <div id="rfc.section.3.4.p.7"> |
|---|
| 1400 | <p>An origin server that receives an If-Unmodified-Since header field <em class="bcp14">MUST</em> evaluate the condition prior to performing the method (<a href="#evaluation" title="Evaluation">Section 5</a>). The origin server <em class="bcp14">MUST NOT</em> perform the requested method if the selected representation's last modification date |
|---|
| 1401 | is more recent than the date provided in the field-value; instead the origin server <em class="bcp14">MUST</em> respond with either a) the <a href="#status.412" class="smpl">412 (Precondition Failed)</a> status code or b) one of the <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> status codes if the origin server has verified that a state change is being requested |
|---|
| 1402 | and the final state is already reflected in the current state of the target resource |
|---|
| 1403 | (i.e., the change requested by the user agent has already succeeded, but the user |
|---|
| 1404 | agent might not be aware of that because the prior response message was lost or a |
|---|
| 1405 | compatible change was made by some other user agent). In the latter case, the origin |
|---|
| 1406 | server <em class="bcp14">MUST NOT</em> send a validator header field in the response unless it can verify that the request |
|---|
| 1407 | is a duplicate of an immediately prior change made by the same user agent. |
|---|
| 1408 | </p> |
|---|
| 1409 | </div> |
|---|
| 1410 | <div id="rfc.section.3.4.p.8"> |
|---|
| 1411 | <p>The If-Unmodified-Since header field can be ignored by caches and intermediaries because |
|---|
| 1412 | it is not applicable to a stored response. |
|---|
| 1413 | </p> |
|---|
| 1414 | </div> |
|---|
| 1415 | </div> |
|---|
| 1416 | <div id="header.if-range"> |
|---|
| 1417 | <h3 id="rfc.section.3.5"><a href="#rfc.section.3.5">3.5</a> <a href="#header.if-range">If-Range</a></h3> |
|---|
| 1418 | <div id="rfc.section.3.5.p.1"> |
|---|
| 1419 | <p>The "If-Range" header field provides a special conditional request mechanism that |
|---|
| 1420 | is similar to the <a href="#header.if-match" class="smpl">If-Match</a> and <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> header fields but that instructs the recipient to ignore the <a href="p5-range.html#header.range" class="smpl">Range</a> header field if the validator doesn't match, resulting in transfer of the new selected |
|---|
| 1421 | representation instead of a <a href="#status.412" class="smpl">412 (Precondition Failed)</a> response. If-Range is defined in <a href="p5-range.html#header.if-range" title="If-Range">Section 3.2</a> of <a href="#RFC7233" id="rfc.xref.RFC7233.1"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[RFC7233]</cite></a>. |
|---|
| 1422 | </p> |
|---|
| 1423 | </div> |
|---|
| 1424 | </div> |
|---|
| 1425 | </div> |
|---|
| 1426 | <div id="status.code.definitions"> |
|---|
| 1427 | <h2 id="rfc.section.4"><a href="#rfc.section.4">4.</a> <a href="#status.code.definitions">Status Code Definitions</a></h2> |
|---|
| 1428 | <div id="status.304"> |
|---|
| 1429 | <h3 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a> <a href="#status.304">304 Not Modified</a></h3> |
|---|
| 1430 | <div id="rfc.section.4.1.p.1"> |
|---|
| 1431 | <p>The <dfn>304 (Not Modified)</dfn> status code indicates that a conditional GET or HEAD request has been received and |
|---|
| 1432 | 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 evaluated to false. In other |
|---|
| 1433 | words, there is no need for the server to transfer a representation of the target |
|---|
| 1434 | resource because the request indicates that the client, which made the request conditional, |
|---|
| 1435 | already has a valid representation; the server is therefore redirecting the client |
|---|
| 1436 | to make use of that stored representation as if it were the payload of a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response. |
|---|
| 1437 | </p> |
|---|
| 1438 | </div> |
|---|
| 1439 | <div id="rfc.section.4.1.p.2"> |
|---|
| 1440 | <p>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="p2-semantics.html#header.date" class="smpl">Date</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>. |
|---|
| 1441 | </p> |
|---|
| 1442 | </div> |
|---|
| 1443 | <div id="rfc.section.4.1.p.3"> |
|---|
| 1444 | <p>Since the goal of a 304 response is to minimize information transfer when the recipient |
|---|
| 1445 | already has one or more cached representations, a sender <em class="bcp14">SHOULD NOT</em> generate representation metadata other than the above listed fields unless said metadata |
|---|
| 1446 | exists for the purpose of guiding 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). |
|---|
| 1447 | </p> |
|---|
| 1448 | </div> |
|---|
| 1449 | <div id="rfc.section.4.1.p.4"> |
|---|
| 1450 | <p>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 4.3.4</a> of <a href="#RFC7234" id="rfc.xref.RFC7234.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Caching">[RFC7234]</cite></a>. If the conditional request originated with an outbound client, such as a user agent |
|---|
| 1451 | with its own cache sending a conditional GET to a shared proxy, then the proxy <em class="bcp14">SHOULD</em> forward the 304 response to that client. |
|---|
| 1452 | </p> |
|---|
| 1453 | </div> |
|---|
| 1454 | <div id="rfc.section.4.1.p.5"> |
|---|
| 1455 | <p>A 304 response cannot contain a message-body; it is always terminated by the first |
|---|
| 1456 | empty line after the header fields. |
|---|
| 1457 | </p> |
|---|
| 1458 | </div> |
|---|
| 1459 | </div> |
|---|
| 1460 | <div id="status.412"> |
|---|
| 1461 | <h3 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a> <a href="#status.412">412 Precondition Failed</a></h3> |
|---|
| 1462 | <div id="rfc.section.4.2.p.1"> |
|---|
| 1463 | <p>The <dfn>412 (Precondition Failed)</dfn> status code indicates that one or more conditions given in the request header fields |
|---|
| 1464 | evaluated to false when tested on the server. This response code allows the client |
|---|
| 1465 | to place preconditions on the current resource state (its current representations |
|---|
| 1466 | and metadata) and, thus, prevent the request method from being applied if the target |
|---|
| 1467 | resource is in an unexpected state. |
|---|
| 1468 | </p> |
|---|
| 1469 | </div> |
|---|
| 1470 | </div> |
|---|
| 1471 | </div> |
|---|
| 1472 | <div id="evaluation"> |
|---|
| 1473 | <h2 id="rfc.section.5"><a href="#rfc.section.5">5.</a> <a href="#evaluation">Evaluation</a></h2> |
|---|
| 1474 | <div id="rfc.section.5.p.1"> |
|---|
| 1475 | <p>Except when excluded below, a recipient cache or origin server <em class="bcp14">MUST</em> evaluate received request preconditions after it has successfully performed its normal |
|---|
| 1476 | request checks and just before it would perform the action associated with the request |
|---|
| 1477 | method. A server <em class="bcp14">MUST</em> ignore all received preconditions if its response to the same request without those |
|---|
| 1478 | conditions would have been a status code other than a <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> or <a href="#status.412" class="smpl">412 (Precondition Failed)</a>. In other words, redirects and failures take precedence over the evaluation of preconditions |
|---|
| 1479 | in conditional requests. |
|---|
| 1480 | </p> |
|---|
| 1481 | </div> |
|---|
| 1482 | <div id="rfc.section.5.p.2"> |
|---|
| 1483 | <p>A server that is not the origin server for the target resource and cannot act as a |
|---|
| 1484 | cache for requests on the target resource <em class="bcp14">MUST NOT</em> evaluate the conditional request header fields defined by this specification, and |
|---|
| 1485 | it <em class="bcp14">MUST</em> forward them if the request is forwarded, since the generating client intends that |
|---|
| 1486 | they be evaluated by a server that can provide a current representation. Likewise, |
|---|
| 1487 | a server <em class="bcp14">MUST</em> ignore the conditional request header fields defined by this specification when received |
|---|
| 1488 | with a request method that does not involve the selection or modification of a <a href="p2-semantics.html#representations" class="smpl">selected representation</a>, such as CONNECT, OPTIONS, or TRACE. |
|---|
| 1489 | </p> |
|---|
| 1490 | </div> |
|---|
| 1491 | <div id="rfc.section.5.p.3"> |
|---|
| 1492 | <p>Conditional request header fields that are defined by extensions to HTTP might place |
|---|
| 1493 | conditions on all recipients, on the state of the target resource in general, or on |
|---|
| 1494 | a group of resources. For instance, the "If" header field in WebDAV can make a request |
|---|
| 1495 | conditional on various aspects of multiple resources, such as locks, if the recipient |
|---|
| 1496 | understands and implements that field (<a href="#RFC4918" id="rfc.xref.RFC4918.2"><cite title="HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)">[RFC4918]</cite></a>, <a href="https://tools.ietf.org/html/rfc4918#section-10.4">Section 10.4</a>). |
|---|
| 1497 | </p> |
|---|
| 1498 | </div> |
|---|
| 1499 | <div id="rfc.section.5.p.4"> |
|---|
| 1500 | <p>Although conditional request header fields are defined as being usable with the HEAD |
|---|
| 1501 | method (to keep HEAD's semantics consistent with those of GET), there is no point |
|---|
| 1502 | in sending a conditional HEAD because a successful response is around the same size |
|---|
| 1503 | as a <a href="#status.304" class="smpl">304 (Not Modified)</a> response and more useful than a <a href="#status.412" class="smpl">412 (Precondition Failed)</a> response. |
|---|
| 1504 | </p> |
|---|
| 1505 | </div> |
|---|
| 1506 | </div> |
|---|
| 1507 | <div id="precedence"> |
|---|
| 1508 | <h2 id="rfc.section.6"><a href="#rfc.section.6">6.</a> <a href="#precedence">Precedence</a></h2> |
|---|
| 1509 | <div id="rfc.section.6.p.1"> |
|---|
| 1510 | <p>When more than one conditional request header field is present in a request, the order |
|---|
| 1511 | in which the fields are evaluated becomes important. In practice, the fields defined |
|---|
| 1512 | in this document are consistently implemented in a single, logical order, since "lost |
|---|
| 1513 | update" preconditions have more strict requirements than cache validation, a validated |
|---|
| 1514 | cache is more efficient than a partial response, and entity tags are presumed to be |
|---|
| 1515 | more accurate than date validators. |
|---|
| 1516 | </p> |
|---|
| 1517 | </div> |
|---|
| 1518 | <div id="rfc.section.6.p.2"> |
|---|
| 1519 | <p>A recipient cache or origin server <em class="bcp14">MUST</em> evaluate the request preconditions defined by this specification in the following |
|---|
| 1520 | order: |
|---|
| 1521 | </p> |
|---|
| 1522 | <ol> |
|---|
| 1523 | <li id="precedence1">When recipient is the origin server and <a href="#header.if-match" class="smpl">If-Match</a> is present, evaluate the <a href="#header.if-match" class="smpl">If-Match</a> precondition: |
|---|
| 1524 | <ul> |
|---|
| 1525 | <li>if true, continue to step <a href="#precedence3">3</a></li> |
|---|
| 1526 | <li>if false, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a> unless it can be determined that the state-changing request has already succeeded |
|---|
| 1527 | (see <a href="#header.if-match" id="rfc.xref.header.if-match.1" title="If-Match">Section 3.1</a>) |
|---|
| 1528 | </li> |
|---|
| 1529 | </ul> |
|---|
| 1530 | </li> |
|---|
| 1531 | <li id="precedence2">When recipient is the origin server, <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 the <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> precondition: |
|---|
| 1532 | <ul> |
|---|
| 1533 | <li>if true, continue to step <a href="#precedence3">3</a></li> |
|---|
| 1534 | <li>if false, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a> unless it can be determined that the state-changing request has already succeeded |
|---|
| 1535 | (see <a href="#header.if-unmodified-since" id="rfc.xref.header.if-unmodified-since.1" title="If-Unmodified-Since">Section 3.4</a>) |
|---|
| 1536 | </li> |
|---|
| 1537 | </ul> |
|---|
| 1538 | </li> |
|---|
| 1539 | <li id="precedence3">When <a href="#header.if-none-match" class="smpl">If-None-Match</a> is present, evaluate the <a href="#header.if-none-match" class="smpl">If-None-Match</a> precondition: |
|---|
| 1540 | <ul> |
|---|
| 1541 | <li>if true, continue to step <a href="#precedence5">5</a></li> |
|---|
| 1542 | <li>if false for GET/HEAD, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li> |
|---|
| 1543 | <li>if false for other methods, respond <a href="#status.412" class="smpl">412 (Precondition Failed)</a></li> |
|---|
| 1544 | </ul> |
|---|
| 1545 | </li> |
|---|
| 1546 | <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 the <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> precondition: |
|---|
| 1547 | <ul> |
|---|
| 1548 | <li>if true, continue to step <a href="#precedence5">5</a></li> |
|---|
| 1549 | <li>if false, respond <a href="#status.304" class="smpl">304 (Not Modified)</a></li> |
|---|
| 1550 | </ul> |
|---|
| 1551 | </li> |
|---|
| 1552 | <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 the <a href="p5-range.html#header.if-range" class="smpl">If-Range</a> precondition: |
|---|
| 1553 | <ul> |
|---|
| 1554 | <li>if the validator matches and the Range specification is applicable to the selected |
|---|
| 1555 | representation, respond <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a> <a href="#RFC7233" id="rfc.xref.RFC7233.2"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Range Requests">[RFC7233]</cite></a></li> |
|---|
| 1556 | </ul> |
|---|
| 1557 | </li> |
|---|
| 1558 | <li id="precedencelast">Otherwise, |
|---|
| 1559 | <ul> |
|---|
| 1560 | <li>all conditions are met, so perform the requested action and respond according to its |
|---|
| 1561 | success or failure. |
|---|
| 1562 | </li> |
|---|
| 1563 | </ul> |
|---|
| 1564 | </li> |
|---|
| 1565 | </ol> |
|---|
| 1566 | </div> |
|---|
| 1567 | <div id="rfc.section.6.p.3"> |
|---|
| 1568 | <p>Any extension to HTTP/1.1 that defines additional conditional request header fields |
|---|
| 1569 | ought to define its own expectations regarding the order for evaluating such fields |
|---|
| 1570 | in relation to those defined in this document and other conditionals that might be |
|---|
| 1571 | found in practice. |
|---|
| 1572 | </p> |
|---|
| 1573 | </div> |
|---|
| 1574 | </div> |
|---|
| 1575 | <div id="IANA.considerations"> |
|---|
| 1576 | <h2 id="rfc.section.7"><a href="#rfc.section.7">7.</a> <a href="#IANA.considerations">IANA Considerations</a></h2> |
|---|
| 1577 | <div id="status.code.registration"> |
|---|
| 1578 | <h3 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a> <a href="#status.code.registration">Status Code Registration</a></h3> |
|---|
| 1579 | <div id="rfc.section.7.1.p.1" class="avoidbreakafter"> |
|---|
| 1580 | <p>The "Hypertext Transfer Protocol (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>> has been updated with the registrations below: |
|---|
| 1581 | </p> |
|---|
| 1582 | </div> |
|---|
| 1583 | <div id="rfc.table.1"> |
|---|
| 1584 | <div id="iana.status.code.registration.table"></div> |
|---|
| 1585 | <table class="tt full tleft" cellpadding="3" cellspacing="0"> |
|---|
| 1586 | <thead> |
|---|
| 1587 | <tr> |
|---|
| 1588 | <th>Value</th> |
|---|
| 1589 | <th>Description</th> |
|---|
| 1590 | <th>Reference</th> |
|---|
| 1591 | </tr> |
|---|
| 1592 | </thead> |
|---|
| 1593 | <tbody> |
|---|
| 1594 | <tr> |
|---|
| 1595 | <td class="left">304</td> |
|---|
| 1596 | <td class="left">Not Modified</td> |
|---|
| 1597 | <td class="left"><a href="#status.304" id="rfc.xref.status.304.1" title="304 Not Modified">Section 4.1</a> |
|---|
| 1598 | </td> |
|---|
| 1599 | </tr> |
|---|
| 1600 | <tr> |
|---|
| 1601 | <td class="left">412</td> |
|---|
| 1602 | <td class="left">Precondition Failed</td> |
|---|
| 1603 | <td class="left"><a href="#status.412" id="rfc.xref.status.412.1" title="412 Precondition Failed">Section 4.2</a> |
|---|
| 1604 | </td> |
|---|
| 1605 | </tr> |
|---|
| 1606 | </tbody> |
|---|
| 1607 | </table> |
|---|
| 1608 | </div> |
|---|
| 1609 | </div> |
|---|
| 1610 | <div id="header.field.registration"> |
|---|
| 1611 | <h3 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a> <a href="#header.field.registration">Header Field Registration</a></h3> |
|---|
| 1612 | <div id="rfc.section.7.2.p.1"> |
|---|
| 1613 | <p>HTTP header fields are registered within the "Message Headers" registry maintained |
|---|
| 1614 | at <<a href="http://www.iana.org/assignments/message-headers/">http://www.iana.org/assignments/message-headers/</a>>. |
|---|
| 1615 | </p> |
|---|
| 1616 | </div> |
|---|
| 1617 | <div id="rfc.section.7.2.p.2" class="avoidbreakafter"> |
|---|
| 1618 | <p>This document defines the following HTTP header fields, so their associated registry |
|---|
| 1619 | entries have been updated according to the permanent registrations below (see <a href="#BCP90" id="rfc.xref.BCP90.1"><cite title="Registration Procedures for Message Header Fields">[BCP90]</cite></a>): |
|---|
| 1620 | </p> |
|---|
| 1621 | </div> |
|---|
| 1622 | <div id="rfc.table.2"> |
|---|
| 1623 | <div id="iana.header.registration.table"></div> |
|---|
| 1624 | <table class="tt full tleft" cellpadding="3" cellspacing="0"> |
|---|
| 1625 | <thead> |
|---|
| 1626 | <tr> |
|---|
| 1627 | <th>Header Field Name</th> |
|---|
| 1628 | <th>Protocol</th> |
|---|
| 1629 | <th>Status</th> |
|---|
| 1630 | <th>Reference</th> |
|---|
| 1631 | </tr> |
|---|
| 1632 | </thead> |
|---|
| 1633 | <tbody> |
|---|
| 1634 | <tr> |
|---|
| 1635 | <td class="left">ETag</td> |
|---|
| 1636 | <td class="left">http</td> |
|---|
| 1637 | <td class="left">standard</td> |
|---|
| 1638 | <td class="left"><a href="#header.etag" id="rfc.xref.header.etag.2" title="ETag">Section 2.3</a> |
|---|
| 1639 | </td> |
|---|
| 1640 | </tr> |
|---|
| 1641 | <tr> |
|---|
| 1642 | <td class="left">If-Match</td> |
|---|
| 1643 | <td class="left">http</td> |
|---|
| 1644 | <td class="left">standard</td> |
|---|
| 1645 | <td class="left"><a href="#header.if-match" id="rfc.xref.header.if-match.2" title="If-Match">Section 3.1</a> |
|---|
| 1646 | </td> |
|---|
| 1647 | </tr> |
|---|
| 1648 | <tr> |
|---|
| 1649 | <td class="left">If-Modified-Since</td> |
|---|
| 1650 | <td class="left">http</td> |
|---|
| 1651 | <td class="left">standard</td> |
|---|
| 1652 | <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> |
|---|
| 1653 | </td> |
|---|
| 1654 | </tr> |
|---|
| 1655 | <tr> |
|---|
| 1656 | <td class="left">If-None-Match</td> |
|---|
| 1657 | <td class="left">http</td> |
|---|
| 1658 | <td class="left">standard</td> |
|---|
| 1659 | <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> |
|---|
| 1660 | </td> |
|---|
| 1661 | </tr> |
|---|
| 1662 | <tr> |
|---|
| 1663 | <td class="left">If-Unmodified-Since</td> |
|---|
| 1664 | <td class="left">http</td> |
|---|
| 1665 | <td class="left">standard</td> |
|---|
| 1666 | <td class="left"><a href="#header.if-unmodified-since" id="rfc.xref.header.if-unmodified-since.2" title="If-Unmodified-Since">Section 3.4</a> |
|---|
| 1667 | </td> |
|---|
| 1668 | </tr> |
|---|
| 1669 | <tr> |
|---|
| 1670 | <td class="left">Last-Modified</td> |
|---|
| 1671 | <td class="left">http</td> |
|---|
| 1672 | <td class="left">standard</td> |
|---|
| 1673 | <td class="left"><a href="#header.last-modified" id="rfc.xref.header.last-modified.2" title="Last-Modified">Section 2.2</a> |
|---|
| 1674 | </td> |
|---|
| 1675 | </tr> |
|---|
| 1676 | </tbody> |
|---|
| 1677 | </table> |
|---|
| 1678 | </div> |
|---|
| 1679 | <div id="rfc.section.7.2.p.3"> |
|---|
| 1680 | <p>The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p> |
|---|
| 1681 | </div> |
|---|
| 1682 | </div> |
|---|
| 1683 | </div> |
|---|
| 1684 | <div id="security.considerations"> |
|---|
| 1685 | <h2 id="rfc.section.8"><a href="#rfc.section.8">8.</a> <a href="#security.considerations">Security Considerations</a></h2> |
|---|
| 1686 | <div id="rfc.section.8.p.1"> |
|---|
| 1687 | <p>This section is meant to inform developers, information providers, and users of known |
|---|
| 1688 | security concerns specific to the HTTP conditional request mechanisms. More general |
|---|
| 1689 | security considerations are addressed in HTTP "Message Syntax and Routing" <a href="#RFC7230" id="rfc.xref.RFC7230.5"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a> and "Semantics and Content" <a href="#RFC7231" id="rfc.xref.RFC7231.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>. |
|---|
| 1690 | </p> |
|---|
| 1691 | </div> |
|---|
| 1692 | <div id="rfc.section.8.p.2"> |
|---|
| 1693 | <p>The validators defined by this specification are not intended to ensure the validity |
|---|
| 1694 | of a representation, guard against malicious changes, or detect man-in-the-middle |
|---|
| 1695 | attacks. At best, they enable more efficient cache updates and optimistic concurrent |
|---|
| 1696 | writes when all participants are behaving nicely. At worst, the conditions will fail |
|---|
| 1697 | and the client will receive a response that is no more harmful than an HTTP exchange |
|---|
| 1698 | without conditional requests. |
|---|
| 1699 | </p> |
|---|
| 1700 | </div> |
|---|
| 1701 | <div id="rfc.section.8.p.3"> |
|---|
| 1702 | <p>An entity-tag can be abused in ways that create privacy risks. For example, a site |
|---|
| 1703 | might deliberately construct a semantically invalid entity-tag that is unique to the |
|---|
| 1704 | user or user agent, send it in a cacheable response with a long freshness time, and |
|---|
| 1705 | then read that entity-tag in later conditional requests as a means of re-identifying |
|---|
| 1706 | that user or user agent. Such an identifying tag would become a persistent identifier |
|---|
| 1707 | for as long as the user agent retained the original cache entry. User agents that |
|---|
| 1708 | cache representations ought to ensure that the cache is cleared or replaced whenever |
|---|
| 1709 | the user performs privacy-maintaining actions, such as clearing stored cookies or |
|---|
| 1710 | changing to a private browsing mode. |
|---|
| 1711 | </p> |
|---|
| 1712 | </div> |
|---|
| 1713 | </div> |
|---|
| 1714 | <div id="acks"> |
|---|
| 1715 | <h2 id="rfc.section.9"><a href="#rfc.section.9">9.</a> <a href="#acks">Acknowledgments</a></h2> |
|---|
| 1716 | <div id="rfc.section.9.p.1"> |
|---|
| 1717 | <p>See <a href="p1-messaging.html#acks" title="Acknowledgments">Section 10</a> of <a href="#RFC7230" id="rfc.xref.RFC7230.6"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>. |
|---|
| 1718 | </p> |
|---|
| 1719 | </div> |
|---|
| 1720 | </div> |
|---|
| 1721 | <h2 id="rfc.references"><a id="rfc.section.10" href="#rfc.section.10">10.</a> References |
|---|
| 1722 | </h2> |
|---|
| 1723 | <h3 id="rfc.references.1"><a href="#rfc.section.10.1" id="rfc.section.10.1">10.1</a> Normative References |
|---|
| 1724 | </h3> |
|---|
| 1725 | <dl class="reference"> |
|---|
| 1726 | <dt id="RFC2119">[RFC2119]</dt> |
|---|
| 1727 | <dd>Bradner, S., “<a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>”, BCP 14, RFC 2119, March 1997. |
|---|
| 1728 | </dd> |
|---|
| 1729 | <dt id="RFC5234">[RFC5234]</dt> |
|---|
| 1730 | <dd>Crocker, D., Ed. and P. Overell, “<a href="https://tools.ietf.org/html/rfc5234">Augmented BNF for Syntax Specifications: ABNF</a>”, STD 68, RFC 5234, January 2008. |
|---|
| 1731 | </dd> |
|---|
| 1732 | <dt id="RFC7230">[RFC7230]</dt> |
|---|
| 1733 | <dd>Fielding, R., Ed. and J. Reschke, Ed., “<a href="https://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 2014. |
|---|
| 1734 | </dd> |
|---|
| 1735 | <dt id="RFC7231">[RFC7231]</dt> |
|---|
| 1736 | <dd>Fielding, R., Ed. and J. Reschke, Ed., “<a href="https://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 2014. |
|---|
| 1737 | </dd> |
|---|
| 1738 | <dt id="RFC7233">[RFC7233]</dt> |
|---|
| 1739 | <dd>Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “<a href="https://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 2014. |
|---|
| 1740 | </dd> |
|---|
| 1741 | <dt id="RFC7234">[RFC7234]</dt> |
|---|
| 1742 | <dd>Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “<a href="https://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 2014. |
|---|
| 1743 | </dd> |
|---|
| 1744 | </dl> |
|---|
| 1745 | <h3 id="rfc.references.2"><a href="#rfc.section.10.2" id="rfc.section.10.2">10.2</a> Informative References |
|---|
| 1746 | </h3> |
|---|
| 1747 | <dl class="reference"> |
|---|
| 1748 | <dt id="BCP90">[BCP90]</dt> |
|---|
| 1749 | <dd>Klyne, G., Nottingham, M., and J. Mogul, “<a href="https://tools.ietf.org/html/rfc3864">Registration Procedures for Message Header Fields</a>”, BCP 90, RFC 3864, September 2004. |
|---|
| 1750 | </dd> |
|---|
| 1751 | <dt id="RFC2616">[RFC2616]</dt> |
|---|
| 1752 | <dd>Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “<a href="https://tools.ietf.org/html/rfc2616">Hypertext Transfer Protocol -- HTTP/1.1</a>”, RFC 2616, June 1999. |
|---|
| 1753 | </dd> |
|---|
| 1754 | <dt id="RFC4918">[RFC4918]</dt> |
|---|
| 1755 | <dd>Dusseault, L., Ed., “<a href="https://tools.ietf.org/html/rfc4918">HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)</a>”, RFC 4918, June 2007. |
|---|
| 1756 | </dd> |
|---|
| 1757 | </dl> |
|---|
| 1758 | <div id="changes.from.rfc.2616"> |
|---|
| 1759 | <h2 id="rfc.section.A" class="np"><a href="#rfc.section.A">A.</a> <a href="#changes.from.rfc.2616">Changes from RFC 2616</a></h2> |
|---|
| 1760 | <div id="rfc.section.A.p.1"> |
|---|
| 1761 | <p>The definition of validator weakness has been expanded and clarified. (<a href="#weak.and.strong.validators" title="Weak versus Strong">Section 2.1</a>) |
|---|
| 1762 | </p> |
|---|
| 1763 | </div> |
|---|
| 1764 | <div id="rfc.section.A.p.2"> |
|---|
| 1765 | <p>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>) |
|---|
| 1766 | </p> |
|---|
| 1767 | </div> |
|---|
| 1768 | <div id="rfc.section.A.p.3"> |
|---|
| 1769 | <p>The <a href="#header.etag" class="smpl">ETag</a> header field ABNF has been changed to not use quoted-string, thus avoiding escaping |
|---|
| 1770 | issues. (<a href="#header.etag" id="rfc.xref.header.etag.3" title="ETag">Section 2.3</a>) |
|---|
| 1771 | </p> |
|---|
| 1772 | </div> |
|---|
| 1773 | <div id="rfc.section.A.p.4"> |
|---|
| 1774 | <p>ETag is defined to provide an entity tag for the selected representation, thereby |
|---|
| 1775 | clarifying what it applies to in various situations (such as a PUT response). (<a href="#header.etag" id="rfc.xref.header.etag.4" title="ETag">Section 2.3</a>) |
|---|
| 1776 | </p> |
|---|
| 1777 | </div> |
|---|
| 1778 | <div id="rfc.section.A.p.5"> |
|---|
| 1779 | <p>The precedence for evaluation of conditional requests has been defined. (<a href="#precedence" title="Precedence">Section 6</a>) |
|---|
| 1780 | </p> |
|---|
| 1781 | </div> |
|---|
| 1782 | </div> |
|---|
| 1783 | <div id="imported.abnf"> |
|---|
| 1784 | <h2 id="rfc.section.B"><a href="#rfc.section.B">B.</a> <a href="#imported.abnf">Imported ABNF</a></h2> |
|---|
| 1785 | <div id="rfc.section.B.p.1"> |
|---|
| 1786 | <p>The following core rules are included by reference, as defined in <a href="https://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 |
|---|
| 1787 | 0-9), DQUOTE (double quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET |
|---|
| 1788 | (any 8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII character). |
|---|
| 1789 | </p> |
|---|
| 1790 | </div> |
|---|
| 1791 | <div id="rfc.section.B.p.2" class="avoidbreakafter"> |
|---|
| 1792 | <p>The rules below are defined in <a href="#RFC7230" id="rfc.xref.RFC7230.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>: |
|---|
| 1793 | </p> |
|---|
| 1794 | </div> |
|---|
| 1795 | <div id="rfc.figure.u.16"><pre class="inline"> <a href="#imported.abnf" class="smpl">OWS</a> = <OWS, see <a href="#RFC7230" id="rfc.xref.RFC7230.8"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, <a href="p1-messaging.html#whitespace" title="Whitespace">Section 3.2.3</a>> |
|---|
| 1796 | <a href="#imported.abnf" class="smpl">obs-text</a> = <obs-text, see <a href="#RFC7230" id="rfc.xref.RFC7230.9"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>, <a href="p1-messaging.html#field.components" title="Field Value Components">Section 3.2.6</a>> |
|---|
| 1797 | </pre></div> |
|---|
| 1798 | <div id="rfc.section.B.p.3" class="avoidbreakafter"> |
|---|
| 1799 | <p>The rules below are defined in other parts:</p> |
|---|
| 1800 | </div> |
|---|
| 1801 | <div id="rfc.figure.u.17"><pre class="inline"> <a href="#imported.abnf" class="smpl">HTTP-date</a> = <HTTP-date, see <a href="#RFC7231" id="rfc.xref.RFC7231.7"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content">[RFC7231]</cite></a>, <a href="p2-semantics.html#http.date" title="Date/Time Formats">Section 7.1.1.1</a>> |
|---|
| 1802 | </pre></div> |
|---|
| 1803 | </div> |
|---|
| 1804 | <div id="collected.abnf"> |
|---|
| 1805 | <h2 id="rfc.section.C"><a href="#rfc.section.C">C.</a> <a href="#collected.abnf">Collected ABNF</a></h2> |
|---|
| 1806 | <div id="rfc.section.C.p.1"> |
|---|
| 1807 | <p>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="#RFC7230" id="rfc.xref.RFC7230.10"><cite title="Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing">[RFC7230]</cite></a>. |
|---|
| 1808 | </p> |
|---|
| 1809 | </div> |
|---|
| 1810 | <div id="rfc.figure.u.18"><pre class="inline"><a href="#header.etag" class="smpl">ETag</a> = entity-tag |
|---|
| 1811 | |
|---|
| 1812 | <a href="#imported.abnf" class="smpl">HTTP-date</a> = <HTTP-date, see [RFC7231], Section 7.1.1.1> |
|---|
| 1813 | |
|---|
| 1814 | <a href="#header.if-match" class="smpl">If-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS |
|---|
| 1815 | entity-tag ] ) ) |
|---|
| 1816 | <a href="#header.if-modified-since" class="smpl">If-Modified-Since</a> = HTTP-date |
|---|
| 1817 | <a href="#header.if-none-match" class="smpl">If-None-Match</a> = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS |
|---|
| 1818 | entity-tag ] ) ) |
|---|
| 1819 | <a href="#header.if-unmodified-since" class="smpl">If-Unmodified-Since</a> = HTTP-date |
|---|
| 1820 | |
|---|
| 1821 | <a href="#header.last-modified" class="smpl">Last-Modified</a> = HTTP-date |
|---|
| 1822 | |
|---|
| 1823 | <a href="#imported.abnf" class="smpl">OWS</a> = <OWS, see [RFC7230], Section 3.2.3> |
|---|
| 1824 | |
|---|
| 1825 | <a href="#header.etag" class="smpl">entity-tag</a> = [ weak ] opaque-tag |
|---|
| 1826 | <a href="#header.etag" class="smpl">etagc</a> = "!" / %x23-7E ; '#'-'~' |
|---|
| 1827 | / obs-text |
|---|
| 1828 | |
|---|
| 1829 | <a href="#imported.abnf" class="smpl">obs-text</a> = <obs-text, see [RFC7230], Section 3.2.6> |
|---|
| 1830 | <a href="#header.etag" class="smpl">opaque-tag</a> = DQUOTE *etagc DQUOTE |
|---|
| 1831 | |
|---|
| 1832 | <a href="#header.etag" class="smpl">weak</a> = %x57.2F ; W/ |
|---|
| 1833 | </pre></div> |
|---|
| 1834 | </div> |
|---|
| 1835 | <h2 id="rfc.index"><a href="#rfc.index">Index</a></h2> |
|---|
| 1836 | <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.R">R</a> <a href="#rfc.index.S">S</a> <a href="#rfc.index.V">V</a> |
|---|
| 1837 | </p> |
|---|
| 1838 | <div class="print2col"> |
|---|
| 1839 | <ul class="ind"> |
|---|
| 1840 | <li><a id="rfc.index.3" href="#rfc.index.3"><b>3</b></a><ul> |
|---|
| 1841 | <li>304 Not Modified (status code) <a href="#rfc.section.4.1"><b>4.1</b></a>, <a href="#rfc.xref.status.304.1">7.1</a></li> |
|---|
| 1842 | </ul> |
|---|
| 1843 | </li> |
|---|
| 1844 | <li><a id="rfc.index.4" href="#rfc.index.4"><b>4</b></a><ul> |
|---|
| 1845 | <li>412 Precondition Failed (status code) <a href="#rfc.section.4.2"><b>4.2</b></a>, <a href="#rfc.xref.status.412.1">7.1</a></li> |
|---|
| 1846 | </ul> |
|---|
| 1847 | </li> |
|---|
| 1848 | <li><a id="rfc.index.B" href="#rfc.index.B"><b>B</b></a><ul> |
|---|
| 1849 | <li><em>BCP90</em> <a href="#rfc.xref.BCP90.1">7.2</a>, <a href="#BCP90"><b>10.2</b></a></li> |
|---|
| 1850 | </ul> |
|---|
| 1851 | </li> |
|---|
| 1852 | <li><a id="rfc.index.E" href="#rfc.index.E"><b>E</b></a><ul> |
|---|
| 1853 | <li>ETag header field <a href="#rfc.xref.header.etag.1">2</a>, <a href="#rfc.section.2.3"><b>2.3</b></a>, <a href="#rfc.xref.header.etag.2">7.2</a>, <a href="#rfc.xref.header.etag.3">A</a>, <a href="#rfc.xref.header.etag.4">A</a></li> |
|---|
| 1854 | </ul> |
|---|
| 1855 | </li> |
|---|
| 1856 | <li><a id="rfc.index.G" href="#rfc.index.G"><b>G</b></a><ul> |
|---|
| 1857 | <li><tt>Grammar</tt> |
|---|
| 1858 | <ul> |
|---|
| 1859 | <li><tt>entity-tag</tt> <a href="#rfc.iref.g.3"><b>2.3</b></a></li> |
|---|
| 1860 | <li><tt>ETag</tt> <a href="#rfc.iref.g.2"><b>2.3</b></a></li> |
|---|
| 1861 | <li><tt>etagc</tt> <a href="#rfc.iref.g.6"><b>2.3</b></a></li> |
|---|
| 1862 | <li><tt>If-Match</tt> <a href="#rfc.iref.g.7"><b>3.1</b></a></li> |
|---|
| 1863 | <li><tt>If-Modified-Since</tt> <a href="#rfc.iref.g.9"><b>3.3</b></a></li> |
|---|
| 1864 | <li><tt>If-None-Match</tt> <a href="#rfc.iref.g.8"><b>3.2</b></a></li> |
|---|
| 1865 | <li><tt>If-Unmodified-Since</tt> <a href="#rfc.iref.g.10"><b>3.4</b></a></li> |
|---|
| 1866 | <li><tt>Last-Modified</tt> <a href="#rfc.iref.g.1"><b>2.2</b></a></li> |
|---|
| 1867 | <li><tt>opaque-tag</tt> <a href="#rfc.iref.g.5"><b>2.3</b></a></li> |
|---|
| 1868 | <li><tt>weak</tt> <a href="#rfc.iref.g.4"><b>2.3</b></a></li> |
|---|
| 1869 | </ul> |
|---|
| 1870 | </li> |
|---|
| 1871 | </ul> |
|---|
| 1872 | </li> |
|---|
| 1873 | <li><a id="rfc.index.I" href="#rfc.index.I"><b>I</b></a><ul> |
|---|
| 1874 | <li>If-Match header field <a href="#rfc.section.3.1"><b>3.1</b></a>, <a href="#rfc.xref.header.if-match.1">6</a>, <a href="#rfc.xref.header.if-match.2">7.2</a></li> |
|---|
| 1875 | <li>If-Modified-Since header field <a href="#rfc.section.3.3"><b>3.3</b></a>, <a href="#rfc.xref.header.if-modified-since.1">7.2</a></li> |
|---|
| 1876 | <li>If-None-Match header field <a href="#rfc.section.3.2"><b>3.2</b></a>, <a href="#rfc.xref.header.if-none-match.1">7.2</a>, <a href="#rfc.xref.header.if-none-match.2">A</a></li> |
|---|
| 1877 | <li>If-Unmodified-Since header field <a href="#rfc.section.3.4"><b>3.4</b></a>, <a href="#rfc.xref.header.if-unmodified-since.1">6</a>, <a href="#rfc.xref.header.if-unmodified-since.2">7.2</a></li> |
|---|
| 1878 | </ul> |
|---|
| 1879 | </li> |
|---|
| 1880 | <li><a id="rfc.index.L" href="#rfc.index.L"><b>L</b></a><ul> |
|---|
| 1881 | <li>Last-Modified header field <a href="#rfc.xref.header.last-modified.1">2</a>, <a href="#rfc.section.2.2"><b>2.2</b></a>, <a href="#rfc.xref.header.last-modified.2">7.2</a></li> |
|---|
| 1882 | </ul> |
|---|
| 1883 | </li> |
|---|
| 1884 | <li><a id="rfc.index.M" href="#rfc.index.M"><b>M</b></a><ul> |
|---|
| 1885 | <li>metadata <a href="#rfc.section.2"><b>2</b></a></li> |
|---|
| 1886 | </ul> |
|---|
| 1887 | </li> |
|---|
| 1888 | <li><a id="rfc.index.R" href="#rfc.index.R"><b>R</b></a><ul> |
|---|
| 1889 | <li><em>RFC2119</em> <a href="#rfc.xref.RFC2119.1">1.1</a>, <a href="#RFC2119"><b>10.1</b></a></li> |
|---|
| 1890 | <li><em>RFC2616</em> <a href="#rfc.xref.RFC2616.1">2.3</a>, <a href="#RFC2616"><b>10.2</b></a><ul> |
|---|
| 1891 | <li><em>Section 3.11</em> <a href="#rfc.xref.RFC2616.1">2.3</a></li> |
|---|
| 1892 | </ul> |
|---|
| 1893 | </li> |
|---|
| 1894 | <li><em>RFC4918</em> <a href="#rfc.xref.RFC4918.1">2</a>, <a href="#rfc.xref.RFC4918.2">5</a>, <a href="#RFC4918"><b>10.2</b></a><ul> |
|---|
| 1895 | <li><em>Section 10.4</em> <a href="#rfc.xref.RFC4918.2">5</a></li> |
|---|
| 1896 | </ul> |
|---|
| 1897 | </li> |
|---|
| 1898 | <li><em>RFC5234</em> <a href="#rfc.xref.RFC5234.1">1.2</a>, <a href="#RFC5234"><b>10.1</b></a>, <a href="#rfc.xref.RFC5234.2">B</a><ul> |
|---|
| 1899 | <li><em>Appendix B.1</em> <a href="#rfc.xref.RFC5234.2">B</a></li> |
|---|
| 1900 | </ul> |
|---|
| 1901 | </li> |
|---|
| 1902 | <li><em>RFC7230</em> <a href="#rfc.xref.RFC7230.1">1</a>, <a href="#rfc.xref.RFC7230.2">1.1</a>, <a href="#rfc.xref.RFC7230.3">1.2</a>, <a href="#rfc.xref.RFC7230.4">2.3.3</a>, <a href="#rfc.xref.RFC7230.5">8</a>, <a href="#rfc.xref.RFC7230.6">9</a>, <a href="#RFC7230"><b>10.1</b></a>, <a href="#rfc.xref.RFC7230.7">B</a>, <a href="#rfc.xref.RFC7230.8">B</a>, <a href="#rfc.xref.RFC7230.9">B</a>, <a href="#rfc.xref.RFC7230.10">C</a><ul> |
|---|
| 1903 | <li><em>Section 1.2</em> <a href="#rfc.xref.RFC7230.10">C</a></li> |
|---|
| 1904 | <li><em>Section 2.5</em> <a href="#rfc.xref.RFC7230.2">1.1</a></li> |
|---|
| 1905 | <li><em>Section 3.2.3</em> <a href="#rfc.xref.RFC7230.8">B</a></li> |
|---|
| 1906 | <li><em>Section 3.2.6</em> <a href="#rfc.xref.RFC7230.9">B</a></li> |
|---|
| 1907 | <li><em>Section 4</em> <a href="#rfc.xref.RFC7230.4">2.3.3</a></li> |
|---|
| 1908 | <li><em>Section 7</em> <a href="#rfc.xref.RFC7230.3">1.2</a></li> |
|---|
| 1909 | <li><em>Section 10</em> <a href="#rfc.xref.RFC7230.6">9</a></li> |
|---|
| 1910 | </ul> |
|---|
| 1911 | </li> |
|---|
| 1912 | <li><em>RFC7231</em> <a href="#rfc.xref.RFC7231.1">1</a>, <a href="#rfc.xref.RFC7231.2">1</a>, <a href="#rfc.xref.RFC7231.3">2.3.3</a>, <a href="#rfc.xref.RFC7231.4">2.3.3</a>, <a href="#rfc.xref.RFC7231.5">3.2</a>, <a href="#rfc.xref.RFC7231.6">8</a>, <a href="#RFC7231"><b>10.1</b></a>, <a href="#rfc.xref.RFC7231.7">B</a><ul> |
|---|
| 1913 | <li><em>Section 3</em> <a href="#rfc.xref.RFC7231.2">1</a></li> |
|---|
| 1914 | <li><em>Section 3.4</em> <a href="#rfc.xref.RFC7231.3">2.3.3</a></li> |
|---|
| 1915 | <li><em>Section 4.2.1</em> <a href="#rfc.xref.RFC7231.5">3.2</a></li> |
|---|
| 1916 | <li><em>Section 5.3.4</em> <a href="#rfc.xref.RFC7231.4">2.3.3</a></li> |
|---|
| 1917 | <li><em>Section 7.1.1.1</em> <a href="#rfc.xref.RFC7231.7">B</a></li> |
|---|
| 1918 | </ul> |
|---|
| 1919 | </li> |
|---|
| 1920 | <li><em>RFC7233</em> <a href="#rfc.xref.RFC7233.1">3.5</a>, <a href="#rfc.xref.RFC7233.2">6</a>, <a href="#RFC7233"><b>10.1</b></a><ul> |
|---|
| 1921 | <li><em>Section 3.2</em> <a href="#rfc.xref.RFC7233.1">3.5</a></li> |
|---|
| 1922 | </ul> |
|---|
| 1923 | </li> |
|---|
| 1924 | <li><em>RFC7234</em> <a href="#rfc.xref.RFC7234.1">1</a>, <a href="#rfc.xref.RFC7234.2">2.2.1</a>, <a href="#rfc.xref.RFC7234.3">2.3.1</a>, <a href="#rfc.xref.RFC7234.4">3.2</a>, <a href="#rfc.xref.RFC7234.5">3.3</a>, <a href="#rfc.xref.RFC7234.6">4.1</a>, <a href="#RFC7234"><b>10.1</b></a><ul> |
|---|
| 1925 | <li><em>Section 4.3.2</em> <a href="#rfc.xref.RFC7234.4">3.2</a>, <a href="#rfc.xref.RFC7234.5">3.3</a></li> |
|---|
| 1926 | <li><em>Section 4.3.4</em> <a href="#rfc.xref.RFC7234.6">4.1</a></li> |
|---|
| 1927 | </ul> |
|---|
| 1928 | </li> |
|---|
| 1929 | </ul> |
|---|
| 1930 | </li> |
|---|
| 1931 | <li><a id="rfc.index.S" href="#rfc.index.S"><b>S</b></a><ul> |
|---|
| 1932 | <li>selected representation <a href="#rfc.iref.s.1"><b>1</b></a></li> |
|---|
| 1933 | </ul> |
|---|
| 1934 | </li> |
|---|
| 1935 | <li><a id="rfc.index.V" href="#rfc.index.V"><b>V</b></a><ul> |
|---|
| 1936 | <li>validator <a href="#rfc.section.2"><b>2</b></a><ul> |
|---|
| 1937 | <li>strong <a href="#rfc.section.2.1"><b>2.1</b></a></li> |
|---|
| 1938 | <li>weak <a href="#rfc.section.2.1"><b>2.1</b></a></li> |
|---|
| 1939 | </ul> |
|---|
| 1940 | </li> |
|---|
| 1941 | </ul> |
|---|
| 1942 | </li> |
|---|
| 1943 | </ul> |
|---|
| 1944 | </div> |
|---|
| 1945 | <div class="avoidbreakinside"> |
|---|
| 1946 | <h2 id="rfc.authors"><a href="#rfc.authors">Authors' Addresses</a></h2> |
|---|
| 1947 | <p><b>Roy T. Fielding</b> |
|---|
| 1948 | (editor) |
|---|
| 1949 | <br>Adobe Systems Incorporated<br>345 Park Ave<br>San Jose, CA 95110<br>USA<br>Email: fielding@gbiv.com<br>URI: <a href="http://roy.gbiv.com/">http://roy.gbiv.com/</a></p> |
|---|
| 1950 | <p><b>Julian F. Reschke</b> |
|---|
| 1951 | (editor) |
|---|
| 1952 | <br>greenbytes GmbH<br>Hafenweg 16<br>Muenster, NW 48155<br>Germany<br>Email: julian.reschke@greenbytes.de<br>URI: <a href="http://greenbytes.de/tech/webdav/">http://greenbytes.de/tech/webdav/</a></p> |
|---|
| 1953 | </div> |
|---|
| 1954 | </body> |
|---|
| 1955 | </html> |
|---|
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