Source: https://trac.ietf.org/trac/httpbis/browser/draft-ietf-httpbis/latest/p6-cache.html?rev=1807
Timestamp: 2019-08-18 23:29:35
Document Index: 128207196

Matched Legal Cases: ['art 6', 'art4', 'art4', 'art 4', 'art4', 'art4', 'art4', 'art 4', 'art4', 'art4', 'art4', 'art 4', 'art4', 'art1', 'art1', 'art 1', 'art1', 'art1', 'art1', 'art 1', 'art1', 'art2', 'art2', 'art 2', 'art2', 'art7', 'art7', 'art 7', 'art7', 'art1', 'art1', 'art 1', 'art1', 'art5', 'art5', 'art 5', 'art5', 'art7', 'art7', 'art 7', 'art7', 'art1', 'art1', 'art 1', 'art1', 'art2', 'art2', 'art 2', 'art2', 'art2', 'art2', 'art 2', 'art2', 'art4', 'art4', 'art 4', 'art4', 'art2', 'art2', 'art 2', 'art2', 'art4', 'art4', 'art 4', 'art4', 'art1', 'art1', 'art 1', 'art1', 'art5', 'art5', 'art 5', 'art5', 'art5', 'art5', 'art 5', 'art5', 'art2', 'art2', 'art 2', 'art2', 'art1', 'art1', 'art 1', 'art1', 'art1', 'art1', 'art 1', 'art1', 'art1', 'art1', 'art 1', 'art1', 'art2', 'art2', 'art 2', 'art2', 'art1', 'art1', 'art 1', 'art1', 'art 1', 'art 2', 'art 5', 'art 7', 'art1', 'art1', 'art 1', 'art1', 'art1', 'art1', 'art 1', 'art1', 'art1', 'art1', 'art 1', 'art1', 'art1', 'art1', 'art 1', 'art1', 'art1', 'art1', 'art 1', 'art1', 'art1', 'art1', 'art 1', 'art1', 'art1', 'art1', 'art 1', 'art1', 'art1', 'art1', 'art 1', 'art1', 'art2', 'art2', 'art 2', 'art2', 'art2', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1', 'art2', 'art2', 'art2', 'art2', 'art2', 'art2', 'art2', 'art2', 'art2', 'art2', 'art2', 'art2', 'art2', 'art2', 'art2', 'art4', 'art4', 'art4', 'art4', 'art4', 'art4', 'art4', 'art4', 'art5', 'art5', 'art5', 'art5', 'art5', 'art5', 'art7', 'art7', 'art7', 'art7', 'art7', 'art7']

source: draft-ietf-httpbis/latest/p6-cache.html @ 1807
Last change on this file since 1807 was 1807, checked in by julian.reschke@…, 7 years ago
Prepare release of -20 drafts
<title>HTTP/1.1, part 6: Caching</title><script>
content: "Expires January 17, 2013";
<link rel="Chapter" title="5 Updating Caches with HEAD Responses" href="#rfc.section.5">
<link rel="Chapter" title="6 Request Methods that Invalidate" href="#rfc.section.6">
<link rel="Chapter" title="8 History Lists" href="#rfc.section.8">
<link rel="Chapter" title="9 IANA Considerations" href="#rfc.section.9">
<link rel="Chapter" title="10 Security Considerations" href="#rfc.section.10">
<link rel="Chapter" title="11 Acknowledgments" href="#rfc.section.11">
<meta name="dct.issued" scheme="ISO8601" content="2012-07-16">
<td class="left">Expires: January 17, 2013</td>
<td class="right">July 16, 2012</td>
<p>The changes in this draft are summarized in <a href="#changes.since.20" title="Since draft-ietf-httpbis-p6-cache-20">Appendix&nbsp;D.2</a>.
<p>This Internet-Draft will expire on January 17, 2013.</p>
<li>1.4.1&nbsp;&nbsp;&nbsp;<a href="#delta-seconds">Delta Seconds</a></li>
<li>2.&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Overview of Cache Operation</a></li>
<li>3.&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Storing Responses in Caches</a><ul>
<li>3.1&nbsp;&nbsp;&nbsp;<a href="#incomplete.responses">Storing Incomplete Responses</a></li>
<li>3.2&nbsp;&nbsp;&nbsp;<a href="#caching.authenticated.responses">Storing Responses to Authenticated Requests</a></li>
<li>4.&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses from Caches</a><ul>
<li>4.1&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Freshness Model</a><ul>
<li>4.1.1&nbsp;&nbsp;&nbsp;<a href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a></li>
<li>4.1.2&nbsp;&nbsp;&nbsp;<a href="#heuristic.freshness">Calculating Heuristic Freshness</a></li>
<li>4.1.3&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Calculating Age</a></li>
<li>4.1.4&nbsp;&nbsp;&nbsp;<a href="#serving.stale.responses">Serving Stale Responses</a></li>
<li>4.2&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a><ul>
<li>4.2.1&nbsp;&nbsp;&nbsp;<a href="#freshening.responses">Freshening Responses with 304 Not Modified</a></li>
<li>4.3&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Using Negotiated Responses</a></li>
<li>4.4&nbsp;&nbsp;&nbsp;<a href="#combining.responses">Combining Partial Content</a></li>
<li>5.&nbsp;&nbsp;&nbsp;<a href="#head.effects">Updating Caches with HEAD Responses</a></li>
<li>6.&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></li>
<li>7.&nbsp;&nbsp;&nbsp;<a href="#header.field.definitions">Header Field Definitions</a><ul>
<li>7.1&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
<li>7.2&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul>
<li>7.2.1&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive">Request Cache-Control Directives</a><ul>
<li>7.2.1.1&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive.no-cache">no-cache</a></li>
<li>7.2.1.2&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive.no-store">no-store</a></li>
<li>7.2.1.3&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive.max-age">max-age</a></li>
<li>7.2.1.4&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive.max-stale">max-stale</a></li>
<li>7.2.1.5&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive.min-fresh">min-fresh</a></li>
<li>7.2.1.6&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive.no-transform">no-transform</a></li>
<li>7.2.1.7&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive.only-if-cached">only-if-cached</a></li>
<li>7.2.2&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive">Response Cache-Control Directives</a><ul>
<li>7.2.2.1&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.only-if-cached">public</a></li>
<li>7.2.2.2&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.private">private</a></li>
<li>7.2.2.3&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.no-cache">no-cache</a></li>
<li>7.2.2.4&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.no-store">no-store</a></li>
<li>7.2.2.5&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.must-revalidate">must-revalidate</a></li>
<li>7.2.2.6&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.proxy-revalidate">proxy-revalidate</a></li>
<li>7.2.2.7&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.max-age">max-age</a></li>
<li>7.2.2.8&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.s-maxage">s-maxage</a></li>
<li>7.2.2.9&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive.no-transform">no-transform</a></li>
<li>7.2.3&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
<li>7.3&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
<li>7.4&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
<li>7.5&nbsp;&nbsp;&nbsp;<a href="#header.vary">Vary</a></li>
<li>7.6&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a><ul>
<li>7.6.1&nbsp;&nbsp;&nbsp;<a href="#warn.110">110 Response is Stale</a></li>
<li>7.6.2&nbsp;&nbsp;&nbsp;<a href="#warn.111">111 Revalidation Failed</a></li>
<li>7.6.3&nbsp;&nbsp;&nbsp;<a href="#warn.112">112 Disconnected Operation</a></li>
<li>7.6.4&nbsp;&nbsp;&nbsp;<a href="#warn.113">113 Heuristic Expiration</a></li>
<li>7.6.5&nbsp;&nbsp;&nbsp;<a href="#warn.199">199 Miscellaneous Warning</a></li>
<li>7.6.6&nbsp;&nbsp;&nbsp;<a href="#warn.214">214 Transformation Applied</a></li>
<li>7.6.7&nbsp;&nbsp;&nbsp;<a href="#warn.299">299 Miscellaneous Persistent Warning</a></li>
<li>7.6.8&nbsp;&nbsp;&nbsp;<a href="#warn.code.extensions">Warn Code Extensions</a></li>
<li>8.&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
<li>9.&nbsp;&nbsp;&nbsp;<a href="#IANA.considerations">IANA Considerations</a><ul>
<li>9.1&nbsp;&nbsp;&nbsp;<a href="#cache.directive.registration">Cache Directive Registry</a></li>
<li>9.2&nbsp;&nbsp;&nbsp;<a href="#warn.code.registration">Warn Code Registry</a></li>
<li>9.3&nbsp;&nbsp;&nbsp;<a href="#header.field.registration">Header Field Registration</a></li>
<li>10.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
<li>11.&nbsp;&nbsp;&nbsp;<a href="#acks">Acknowledgments</a></li>
<li>12.&nbsp;&nbsp;&nbsp;<a href="#rfc.references">References</a><ul>
<li>12.1&nbsp;&nbsp;&nbsp;<a href="#rfc.references.1">Normative References</a></li>
<li>12.2&nbsp;&nbsp;&nbsp;<a href="#rfc.references.2">Informative References</a></li>
<li>B.&nbsp;&nbsp;&nbsp;<a href="#imported.abnf">Imported ABNF</a></li>
<li>D.1&nbsp;&nbsp;&nbsp;<a href="#changes.since.19">Since draft-ietf-httpbis-p6-cache-19</a></li>
<li>D.2&nbsp;&nbsp;&nbsp;<a href="#changes.since.20">Since draft-ietf-httpbis-p6-cache-20</a></li>
request. A stored response is considered "fresh", as defined in <a href="#expiration.model" title="Freshness Model">Section&nbsp;4.1</a>, if the response can be reused without "validation" (checking with the origin server to see if the cached response remains
When a cached response is not fresh, it might still be reusable if it can be freshened by validation (<a href="#validation.model" title="Validation Model">Section&nbsp;4.2</a>) or if the origin is unavailable.
<li>A protocol element (e.g., an entity-tag or a <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> time) that is used to find out whether a stored response is an equivalent copy of a representation. See <a href="p4-conditional.html#weak.and.strong.validators" title="Weak versus Strong">Section 2.1</a> of <a href="#Part4" id="rfc.xref.Part4.1"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>.
i.e., an entity-tag that is not marked as weak (<a href="p4-conditional.html#header.etag" title="ETag">Section 2.3</a> of <a href="#Part4" id="rfc.xref.Part4.2"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>) or, if no entity-tag is provided, a <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> value that is strong in the sense defined by <a href="p4-conditional.html#lastmod.comparison" title="Comparison">Section 2.2.2</a> of <a href="#Part4" id="rfc.xref.Part4.3"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>.
<p id="rfc.section.1.3.p.2">This specification targets conformance criteria according to the role of a participant in HTTP communication. Hence, HTTP
requirements are placed on senders, recipients, clients, servers, user agents, intermediaries, origin servers, proxies, gateways,
or caches, depending on what behavior is being constrained by the requirement. See <a href="p1-messaging.html#architecture" title="Architecture">Section 2</a> of <a href="#Part1" id="rfc.xref.Part1.1"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a> for definitions of these terms.
<p id="rfc.section.1.3.p.3">The verb "generate" is used instead of "send" where a requirement differentiates between creating a protocol element and merely
forwarding a received element downstream.
<p id="rfc.section.1.3.p.4">An implementation is considered conformant if it complies with all of the requirements associated with the roles it partakes
in HTTP. Note that SHOULD-level requirements are relevant here, unless one of the documented exceptions is applicable.
<p id="rfc.section.1.3.p.5">This document also uses ABNF to define valid protocol elements (<a href="#notation" title="Syntax Notation">Section&nbsp;1.4</a>). In addition to the prose requirements placed upon them, senders <em class="bcp14">MUST NOT</em> generate protocol elements that do not match the grammar defined by the ABNF rules for those protocol elements that are applicable
to the sender's role. If a received protocol element is processed, the recipient <em class="bcp14">MUST</em> be able to parse any value that would match the ABNF rules for that protocol element, excluding only those rules not applicable
to the recipient's role.
<p id="rfc.section.1.3.p.6">Unless noted otherwise, a recipient <em class="bcp14">MAY</em> attempt to recover a usable protocol element from an invalid construct. HTTP does not define specific error handling mechanisms
except when they have a direct impact on security, since different applications of the protocol require different error handling
strategies. For example, a Web browser might wish to transparently recover from a response where the <a href="p2-semantics.html#header.location" class="smpl">Location</a> header field doesn't parse according to the ABNF, whereas a systems control client might consider any form of error recovery
<p id="rfc.section.1.4.p.1">This specification uses the Augmented Backus-Naur Form (ABNF) notation of <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with the list rule extension defined in <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</a> of <a href="#Part1" id="rfc.xref.Part1.2"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>. <a href="#imported.abnf" title="Imported ABNF">Appendix&nbsp;B</a> describes rules imported from other documents. <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;C</a> shows the collected ABNF with the list rule expanded.
<h3 id="rfc.section.1.4.1"><a href="#rfc.section.1.4.1">1.4.1</a>&nbsp;<a id="delta-seconds" href="#delta-seconds">Delta Seconds</a></h3>
<p id="rfc.section.1.4.1.p.1">The delta-seconds rule specifies a non-negative integer, representing time in seconds.</p>
</pre><p id="rfc.section.1.4.1.p.3">If an implementation receives a delta-seconds value larger than the largest positive integer it can represent, or if any of
<h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="caching.overview" href="#caching.overview">Overview of Cache Operation</a></h1>
<p id="rfc.section.2.p.1">Proper cache operation preserves the semantics of HTTP transfers (<a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Semantics and Payloads">[Part2]</cite></a>) while eliminating the transfer of information already held in the cache. Although caching is an entirely <em class="bcp14">OPTIONAL</em> feature of HTTP, we assume that reusing the cached response is desirable and that such reuse is the default behavior when
form of cache entry is a successful result of a retrieval request: i.e., a <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> response containing a representation of the resource identified by the request target. However, it is also possible to cache
negative results (e.g., <a href="p2-semantics.html#status.404" class="smpl">404 (Not
Found)</a>, incomplete results (e.g., <a href="p5-range.html#status.206" class="smpl">206 (Partial
Content)</a>), and responses to methods other than GET if the method's definition allows such caching and defines something suitable for
responses to GET, many implementations simply decline other methods and use only the URI as the key.
by a secondary key for the values of the original request's selecting header fields (<a href="#caching.negotiated.responses" title="Using Negotiated Responses">Section&nbsp;4.3</a>).
<h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="response.cacheability" href="#response.cacheability">Storing Responses in Caches</a></h1>
<li>the "no-store" cache directive (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.1" title="Cache-Control">Section&nbsp;7.2</a>) does not appear in request or response header fields, and
<li>the "private" cache response directive (see <a href="#cache-response-directive.private" title="private">Section&nbsp;7.2.2.2</a>) does not appear in the response, if the cache is shared, and
<li>the <a href="p7-auth.html#header.authorization" class="smpl">Authorization</a> header field (see <a href="p7-auth.html#header.authorization" title="Authorization">Section 4.1</a> of <a href="#Part7" id="rfc.xref.Part7.1"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a>) does not appear in the request, if the cache is shared, unless the response explicitly allows it (see <a href="#caching.authenticated.responses" title="Storing Responses to Authenticated Requests">Section&nbsp;3.2</a>), and
<li>contains an <a href="#header.expires" class="smpl">Expires</a> header field (see <a href="#header.expires" id="rfc.xref.header.expires.1" title="Expires">Section&nbsp;7.3</a>), or
<li>contains a max-age response cache directive (see <a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;7.2.2.7</a>), or
<li>contains a Cache Control Extension (see <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;7.2.3</a>) that allows it to be cached, or
<li>has a status code that can be served with heuristic freshness (see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;4.1.2</a>).
<p id="rfc.section.3.p.2">Note that any of the requirements listed above can be overridden by a cache-control extension; see <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;7.2.3</a>.
<p id="rfc.section.3.p.3">In this context, a cache has "understood" a request method or a response status code if it recognizes it and implements any
<p id="rfc.section.3.p.4">Note that, in normal operation, many caches will not store a response that has neither a cache validator nor an explicit expiration
<h2 id="rfc.section.3.1"><a href="#rfc.section.3.1">3.1</a>&nbsp;<a id="incomplete.responses" href="#incomplete.responses">Storing Incomplete Responses</a></h2>
<p id="rfc.section.3.1.p.1">A response message is considered complete when all of the octets indicated by the message framing (<a href="#Part1" id="rfc.xref.Part1.3"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>) are received prior to the connection being closed. If the request is GET, the response status is <a href="p2-semantics.html#status.200" class="smpl">200
(OK)</a>, and the entire response header block has been received, a cache <em class="bcp14">MAY</em> store an incomplete response message body if the cache entry is recorded as incomplete. Likewise, a <a href="p5-range.html#status.206" class="smpl">206 (Partial Content)</a> response <em class="bcp14">MAY</em> be stored as if it were an incomplete <a href="p2-semantics.html#status.200" class="smpl">200
(OK)</a> cache entry. However, a cache <em class="bcp14">MUST NOT</em> store incomplete or partial content responses if it does not support the <a href="p5-range.html#range.retrieval.requests" class="smpl">Range</a> and <a href="p5-range.html#header.content-range" class="smpl">Content-Range</a> header fields or if it does not understand the range units used in those fields.
<p id="rfc.section.3.1.p.2">A cache <em class="bcp14">MAY</em> complete a stored incomplete response by making a subsequent range request (<a href="#Part5" id="rfc.xref.Part5.1"><cite title="HTTP/1.1, part 5: Range Requests">[Part5]</cite></a>) and combining the successful response with the stored entry, as defined in <a href="#combining.responses" title="Combining Partial Content">Section&nbsp;4.4</a>. A cache <em class="bcp14">MUST NOT</em> use an incomplete response to answer requests unless the response has been made complete or the request is partial and specifies
<h2 id="rfc.section.3.2"><a href="#rfc.section.3.2">3.2</a>&nbsp;<a id="caching.authenticated.responses" href="#caching.authenticated.responses">Storing Responses to Authenticated Requests</a></h2>
<p id="rfc.section.3.2.p.1">A shared cache <em class="bcp14">MUST NOT</em> use a cached response to a request with an <a href="p7-auth.html#header.authorization" class="smpl">Authorization</a> header field (<a href="p7-auth.html#header.authorization" title="Authorization">Section 4.1</a> of <a href="#Part7" id="rfc.xref.Part7.2"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a>) to satisfy any subsequent request unless a cache directive that allows such responses to be stored is present in the response.
<p id="rfc.section.3.2.p.2">In this specification, the following <a href="#header.cache-control" class="smpl">Cache-Control</a> response directives (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;7.2.2</a>) have such an effect: must-revalidate, public, s-maxage.
served stale (<a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.1.4</a>) by shared caches. In particular, a response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to satisfy
<h1 id="rfc.section.4"><a href="#rfc.section.4">4.</a>&nbsp;<a id="constructing.responses.from.caches" href="#constructing.responses.from.caches">Constructing Responses from Caches</a></h1>
<p id="rfc.section.4.p.1">For a presented request, a cache <em class="bcp14">MUST NOT</em> return a stored response, unless:
<li>The presented effective request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.4"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>) and that of the stored response match, and
<li>selecting header fields nominated by the stored response (if any) match those presented (see <a href="#caching.negotiated.responses" title="Using Negotiated Responses">Section&nbsp;4.3</a>), and
<li>the presented request does not contain the no-cache pragma (<a href="#header.pragma" id="rfc.xref.header.pragma.1" title="Pragma">Section&nbsp;7.4</a>), nor the no-cache cache directive (<a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;7.2.1</a>), unless the stored response is successfully validated (<a href="#validation.model" title="Validation Model">Section&nbsp;4.2</a>), and
<li>the stored response does not contain the no-cache cache directive (<a href="#cache-response-directive.no-cache" title="no-cache">Section&nbsp;7.2.2.3</a>), unless it is successfully validated (<a href="#validation.model" title="Validation Model">Section&nbsp;4.2</a>), and
<li>fresh (see <a href="#expiration.model" title="Freshness Model">Section&nbsp;4.1</a>), or
<li>allowed to be served stale (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.1.4</a>), or
<li>successfully validated (see <a href="#validation.model" title="Validation Model">Section&nbsp;4.2</a>).
<p id="rfc.section.4.p.2">Note that any of the requirements listed above can be overridden by a cache-control extension; see <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;7.2.3</a>.
<p id="rfc.section.4.p.3">When a stored response is used to satisfy a request without validation, a cache <em class="bcp14">MUST</em> include a single <a href="#header.age" class="smpl">Age</a> header field (<a href="#header.age" id="rfc.xref.header.age.1" title="Age">Section&nbsp;7.1</a>) in the response with a value equal to the stored response's current_age; see <a href="#age.calculations" title="Calculating Age">Section&nbsp;4.1.3</a>.
<p id="rfc.section.4.p.4">A cache <em class="bcp14">MUST</em> write through requests with methods that are unsafe (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 2.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="HTTP/1.1, part 2: Semantics and Payloads">[Part2]</cite></a>) to the origin server; i.e., a cache is not allowed to generate a reply to such a request before having forwarded the request
<p id="rfc.section.4.p.5">Also, note that unsafe requests might invalidate already stored responses; see <a href="#invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">Section&nbsp;6</a>.
<p id="rfc.section.4.p.6">When more than one suitable response is stored, a cache <em class="bcp14">MUST</em> use the most recent response (as determined by the <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field). It can also forward a request with "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate
<p id="rfc.section.4.p.7">A cache that does not have a clock available <em class="bcp14">MUST NOT</em> use stored responses without revalidating them on every use. A cache, especially a shared cache, <em class="bcp14">SHOULD</em> use a mechanism, such as NTP <a href="#RFC1305" id="rfc.xref.RFC1305.1"><cite title="Network Time Protocol (Version 3) Specification, Implementation">[RFC1305]</cite></a>, to synchronize its clock with a reliable external standard.
<h2 id="rfc.section.4.1"><a href="#rfc.section.4.1">4.1</a>&nbsp;<a id="expiration.model" href="#expiration.model">Freshness Model</a></h2>
<p id="rfc.section.4.1.p.1">When a response is "fresh" in the cache, it can be used to satisfy subsequent requests without contacting the origin server,
<p id="rfc.section.4.1.p.2">The primary mechanism for determining freshness is for an origin server to provide an explicit expiration time in the future,
using either the <a href="#header.expires" class="smpl">Expires</a> header field (<a href="#header.expires" id="rfc.xref.header.expires.2" title="Expires">Section&nbsp;7.3</a>) or the max-age response cache directive (<a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;7.2.2.7</a>). Generally, origin servers will assign future explicit expiration times to responses in the belief that the representation
<p id="rfc.section.4.1.p.3">If an origin server wishes to force a cache to validate every request, it can assign an explicit expiration time in the past
it for subsequent requests (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.1.4</a>).
<p id="rfc.section.4.1.p.4">Since origin servers do not always provide explicit expiration times, a cache <em class="bcp14">MAY</em> assign a heuristic expiration time when an explicit time is not specified, employing algorithms that use other header field
</pre> <p id="rfc.section.4.1.p.6">The freshness_lifetime is defined in <a href="#calculating.freshness.lifetime" title="Calculating Freshness Lifetime">Section&nbsp;4.1.1</a>; the current_age is defined in <a href="#age.calculations" title="Calculating Age">Section&nbsp;4.1.3</a>.
<p id="rfc.section.4.1.p.7">Additionally, clients can influence freshness calculation — either constraining it relaxing it — by using the max-age and
min-fresh request cache directives. See <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;7.2.1</a> for details.
<p id="rfc.section.4.1.p.8">Note that freshness applies only to cache operation; it cannot be used to force a user agent to refresh its display or reload
a resource. See <a href="#history.lists" title="History Lists">Section&nbsp;8</a> for an explanation of the difference between caches and history mechanisms.
<h3 id="rfc.section.4.1.1"><a href="#rfc.section.4.1.1">4.1.1</a>&nbsp;<a id="calculating.freshness.lifetime" href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a></h3>
<p id="rfc.section.4.1.1.p.1">A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a response by using the first match of: </p>
<li>If the cache is shared and the s-maxage response cache directive (<a href="#cache-response-directive.s-maxage" title="s-maxage">Section&nbsp;7.2.2.8</a>) is present, use its value, or
<li>If the max-age response cache directive (<a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;7.2.2.7</a>) is present, use its value, or
<li>If the <a href="#header.expires" class="smpl">Expires</a> response header field (<a href="#header.expires" id="rfc.xref.header.expires.3" title="Expires">Section&nbsp;7.3</a>) is present, use its value minus the value of the <a href="p2-semantics.html#header.date" class="smpl">Date</a> response header field, or
<li>Otherwise, no explicit expiration time is present in the response. A heuristic freshness lifetime might be applicable; see <a href="#heuristic.freshness" title="Calculating Heuristic Freshness">Section&nbsp;4.1.2</a>.
<p id="rfc.section.4.1.1.p.2">Note that this calculation is not vulnerable to clock skew, since all of the information comes from the origin server.</p>
<p id="rfc.section.4.1.1.p.3">When there is more than one value present for a given directive (e.g., two <a href="#header.expires" class="smpl">Expires</a> header fields, multiple Cache-Control: max-age directives), it is considered invalid. Caches are encouraged to consider responses
that have invalid freshness information to be stale.
<h3 id="rfc.section.4.1.2"><a href="#rfc.section.4.1.2">4.1.2</a>&nbsp;<a id="heuristic.freshness" href="#heuristic.freshness">Calculating Heuristic Freshness</a></h3>
<p id="rfc.section.4.1.2.p.1">If no explicit expiration time is present in a stored response that has a status code whose definition allows heuristic freshness
to be used (including the following in <a href="p2-semantics.html#status.codes" title="Status Codes">Section 4</a> of <a href="#Part2" id="rfc.xref.Part2.3"><cite title="HTTP/1.1, part 2: Semantics and Payloads">[Part2]</cite></a>: <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a>, <a href="p2-semantics.html#status.203" class="smpl">203 (Non-Authoritative Information)</a>, <a href="p5-range.html#status.206" class="smpl">206 (Partial
Content)</a>, <a href="p2-semantics.html#status.300" class="smpl">300 (Multiple Choices)</a>, <a href="p2-semantics.html#status.301" class="smpl">301 (Moved
Permanently)</a> and <a href="p2-semantics.html#status.410" class="smpl">410 (Gone)</a>), a cache <em class="bcp14">MAY</em> calculate a heuristic expiration time. A cache <em class="bcp14">MUST NOT</em> use heuristics to determine freshness for responses with status codes that do not explicitly allow it.
<p id="rfc.section.4.1.2.p.2">When a heuristic is used to calculate freshness lifetime, a cache <em class="bcp14">SHOULD</em> attach a <a href="#header.warning" class="smpl">Warning</a> header field with a 113 warn-code to the response if its current_age is more than 24 hours and such a warning is not already
<p id="rfc.section.4.1.2.p.3">Also, if the response has a <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> header field (<a href="p4-conditional.html#header.last-modified" title="Last-Modified">Section 2.2</a> of <a href="#Part4" id="rfc.xref.Part4.4"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>), caches are encouraged to use a heuristic expiration value that is no more than some fraction of the interval since that
<div class="note" id="rfc.section.4.1.2.p.4">
<p> <b>Note:</b> <a href="http://tools.ietf.org/html/rfc2616#section-13.9">Section 13.9</a> of <a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a> prohibited caches from calculating heuristic freshness for URIs with query components (i.e., those containing '?'). In practice,
this has not been widely implemented. Therefore, servers are encouraged to send explicit directives (e.g., Cache-Control:
<h3 id="rfc.section.4.1.3"><a href="#rfc.section.4.1.3">4.1.3</a>&nbsp;<a id="age.calculations" href="#age.calculations">Calculating Age</a></h3>
<p id="rfc.section.4.1.3.p.1">HTTP/1.1 uses the <a href="#header.age" class="smpl">Age</a> header field to convey the estimated age of the response message when obtained from a cache. The Age field value is the cache's
estimate of the amount of time since the response was generated or validated by the origin server. In essence, the Age value
is the sum of the time that the response has been resident in each of the caches along the path from the origin server, plus
the amount of time it has been in transit along network paths.
<p id="rfc.section.4.1.3.p.2">The following data is used for the age calculation:</p>
<p id="rfc.section.4.1.3.p.3"> <dfn>age_value</dfn>
<li>The term "age_value" denotes the value of the <a href="#header.age" class="smpl">Age</a> header field (<a href="#header.age" id="rfc.xref.header.age.2" title="Age">Section&nbsp;7.1</a>), in a form appropriate for arithmetic operation; or 0, if not available.
<p id="rfc.section.4.1.3.p.4"> <dfn>date_value</dfn>
<li>HTTP/1.1 requires origin servers to send a <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field, if possible, with every response, giving the time at which the response was generated. The term "date_value"
denotes the value of the Date header field, in a form appropriate for arithmetic operations. See <a href="p2-semantics.html#header.date" title="Date">Section 9.10</a> of <a href="#Part2" id="rfc.xref.Part2.4"><cite title="HTTP/1.1, part 2: Semantics and Payloads">[Part2]</cite></a> for the definition of the Date header field, and for requirements regarding responses without it.
<p id="rfc.section.4.1.3.p.5"> <dfn>now</dfn>
<p id="rfc.section.4.1.3.p.6"> <dfn>request_time</dfn>
<p id="rfc.section.4.1.3.p.7"> <dfn>response_time</dfn>
<p id="rfc.section.4.1.3.p.8">A response's age can be calculated in two entirely independent ways: </p>
<div id="rfc.figure.u.3"></div> <pre class="text"> apparent_age = max(0, response_time - date_value);
</pre> <div id="rfc.figure.u.4"></div>
</pre><p id="rfc.section.4.1.3.p.11">unless the cache is confident in the value of the <a href="#header.age" class="smpl">Age</a> header field (e.g., because there are no HTTP/1.0 hops in the <a href="p1-messaging.html#header.via" class="smpl">Via</a> header field), in which case the corrected_age_value <em class="bcp14">MAY</em> be used as the corrected_initial_age.
<p id="rfc.section.4.1.3.p.12">The current_age of a stored response can then be calculated by adding the amount of time (in seconds) since the stored response
</pre><p id="rfc.section.4.1.3.p.14">Additionally, to avoid common problems in date parsing:</p>
<p id="rfc.section.4.1.3.p.15"> </p>
<li>An HTTP/1.1 implementation <em class="bcp14">MAY</em> internally represent a parsed <a href="#header.expires" class="smpl">Expires</a> date as earlier than the proper value, but <em class="bcp14">MUST NOT</em> internally represent a parsed Expires date as later than the proper value.
<h3 id="rfc.section.4.1.4"><a href="#rfc.section.4.1.4">4.1.4</a>&nbsp;<a id="serving.stale.responses" href="#serving.stale.responses">Serving Stale Responses</a></h3>
<p id="rfc.section.4.1.4.p.1">A "stale" response is one that either has explicit expiry information or is allowed to have heuristic expiry calculated, but
is not fresh according to the calculations in <a href="#expiration.model" title="Freshness Model">Section&nbsp;4.1</a>.
<p id="rfc.section.4.1.4.p.2">A cache <em class="bcp14">MUST NOT</em> return a stale response if it is prohibited by an explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache
see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;7.2.2</a>).
<p id="rfc.section.4.1.4.p.3">A cache <em class="bcp14">MUST NOT</em> return stale responses unless it is disconnected (i.e., it cannot contact the origin server or otherwise find a forward path)
or doing so is explicitly allowed (e.g., by the max-stale request directive; see <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;7.2.1</a>).
<p id="rfc.section.4.1.4.p.4">A cache <em class="bcp14">SHOULD</em> append a <a href="#header.warning" class="smpl">Warning</a> header field with the 110 warn-code (see <a href="#header.warning" id="rfc.xref.header.warning.1" title="Warning">Section&nbsp;7.6</a>) to stale responses. Likewise, a cache <em class="bcp14">SHOULD</em> add the 112 warn-code to stale responses if the cache is disconnected.
<p id="rfc.section.4.1.4.p.5">If a cache receives a first-hand response (either an entire response, or a <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> response) that it would normally forward to the requesting client, and the received response is no longer fresh, the cache
can forward it to the requesting client without adding a new <a href="#header.warning" class="smpl">Warning</a> (but without removing any existing Warning header fields). A cache shouldn't attempt to validate a response simply because
<h2 id="rfc.section.4.2"><a href="#rfc.section.4.2">4.2</a>&nbsp;<a id="validation.model" href="#validation.model">Validation Model</a></h2>
<p id="rfc.section.4.2.p.1">When a cache has one or more stored responses for a requested URI, but cannot serve any of them (e.g., because they are not
fresh, or one cannot be selected; see <a href="#caching.negotiated.responses" title="Using Negotiated Responses">Section&nbsp;4.3</a>), it can use the conditional request mechanism <a href="#Part4" id="rfc.xref.Part4.5"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a> in the forwarded request to give the origin server an opportunity to both select a valid stored response to be used, and to
<p id="rfc.section.4.2.p.2">When sending such a conditional request, a cache adds an <a href="p4-conditional.html#header.if-modified-since" class="smpl">If-Modified-Since</a> header field whose value is that of the <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> header field from the selected (see <a href="#caching.negotiated.responses" title="Using Negotiated Responses">Section&nbsp;4.3</a>) stored response, if available.
<p id="rfc.section.4.2.p.3">Additionally, a cache can add an <a href="p4-conditional.html#header.if-none-match" class="smpl">If-None-Match</a> header field whose value is that of the <a href="p4-conditional.html#header.etag" class="smpl">ETag</a> header field(s) from all responses stored for the requested URI, if present. However, if any of the stored responses contains
only partial content, the cache shouldn't include its entity-tag in the If-None-Match header field unless the request is for
a range that would be fully satisfied by that stored response.
<p id="rfc.section.4.2.p.4">Cache handling of a response to a conditional request is dependent upon its status code:</p>
<p id="rfc.section.4.2.p.5"> </p>
<li>A <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> response status code indicates that the stored response can be updated and reused; see <a href="#freshening.responses" title="Freshening Responses with 304 Not Modified">Section&nbsp;4.2.1</a>.
if the server failed to respond. In the latter case, it can return a previously stored response (see <a href="#serving.stale.responses" title="Serving Stale Responses">Section&nbsp;4.1.4</a>).
<h3 id="rfc.section.4.2.1"><a href="#rfc.section.4.2.1">4.2.1</a>&nbsp;<a id="freshening.responses" href="#freshening.responses">Freshening Responses with 304 Not Modified</a></h3>
<p id="rfc.section.4.2.1.p.1">When a cache receives a <a href="p4-conditional.html#status.304" class="smpl">304 (Not Modified)</a> response and already has one or more stored <a href="p2-semantics.html#status.200" class="smpl">200 (OK)</a> responses for the same cache key, the cache needs to identify which of the stored responses are updated by this new response
<p id="rfc.section.4.2.1.p.2">If a stored response is selected for update, the cache <em class="bcp14">MUST</em>:
<li>delete any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.2" title="Warning">Section&nbsp;7.6</a>);
<h2 id="rfc.section.4.3"><a href="#rfc.section.4.3">4.3</a>&nbsp;<a id="caching.negotiated.responses" href="#caching.negotiated.responses">Using Negotiated Responses</a></h2>
<p id="rfc.section.4.3.p.1">When a cache receives a request that can be satisfied by a stored response that has a <a href="#header.vary" class="smpl">Vary</a> header field (<a href="#header.vary" id="rfc.xref.header.vary.1" title="Vary">Section&nbsp;7.5</a>), it <em class="bcp14">MUST NOT</em> use that response unless all of the selecting header fields nominated by the Vary header field match in both the original
<p id="rfc.section.4.3.p.2">The selecting header fields from two requests are defined to match if and only if those in the first request can be transformed
<li>combining multiple header fields with the same field name (see <a href="p1-messaging.html#header.fields" title="Header Fields">Section 3.2</a> of <a href="#Part1" id="rfc.xref.Part1.5"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>)
<p id="rfc.section.4.3.p.3">If (after any normalization that might take place) a header field is absent from a request, it can only match another request
<p id="rfc.section.4.3.p.4">A <a href="#header.vary" class="smpl">Vary</a> header field-value of "*" always fails to match, and subsequent requests to that resource can only be properly interpreted
<p id="rfc.section.4.3.p.5">The stored response with matching selecting header fields is known as the selected response.</p>
<p id="rfc.section.4.3.p.6">If multiple selected responses are available, the most recent response (as determined by the <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field) is used; see <a href="#constructing.responses.from.caches" title="Constructing Responses from Caches">Section&nbsp;4</a>.
<p id="rfc.section.4.3.p.7">If no selected response is available, the cache can forward the presented request to the origin server in a conditional request;
see <a href="#validation.model" title="Validation Model">Section&nbsp;4.2</a>.
<h2 id="rfc.section.4.4"><a href="#rfc.section.4.4">4.4</a>&nbsp;<a id="combining.responses" href="#combining.responses">Combining Partial Content</a></h2>
<p id="rfc.section.4.4.p.1">A response might transfer only a partial representation if the connection closed prematurely or if the request used one or
more Range specifiers (<a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests">[Part5]</cite></a>). After several such transfers, a cache might have received several ranges of the same representation. A cache <em class="bcp14">MAY</em> combine these ranges into a single stored response, and reuse that response to satisfy later requests, if they all share the
same strong validator and the cache complies with the client requirements in <a href="p5-range.html#combining.byte.ranges" title="Combining Ranges">Section 4.2</a> of <a href="#Part5" id="rfc.xref.Part5.3"><cite title="HTTP/1.1, part 5: Range Requests">[Part5]</cite></a>.
<p id="rfc.section.4.4.p.2">When combining the new response with one or more stored responses, a cache <em class="bcp14">MUST</em>:
<li>delete any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.3" title="Warning">Section&nbsp;7.6</a>);
<h1 id="rfc.section.5"><a href="#rfc.section.5">5.</a>&nbsp;<a id="head.effects" href="#head.effects">Updating Caches with HEAD Responses</a></h1>
<p id="rfc.section.5.p.1">A response to the HEAD method is identical to what an equivalent request made with a GET would have been, except it lacks
a body. This property of HEAD responses is used to both invalidate and update cached GET responses.
<p id="rfc.section.5.p.2">If one or more stored GET responses can be selected (as per <a href="#caching.negotiated.responses" title="Using Negotiated Responses">Section&nbsp;4.3</a>) for a HEAD request, and the <a href="p1-messaging.html#header.content-length" class="smpl">Content-Length</a>, <a href="p4-conditional.html#header.etag" class="smpl">ETag</a> or <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> value of a HEAD response differs from that in a selected GET response, the cache <em class="bcp14">MUST</em> consider that selected response to be stale.
<p id="rfc.section.5.p.3">If the <a href="p1-messaging.html#header.content-length" class="smpl">Content-Length</a>, <a href="p4-conditional.html#header.etag" class="smpl">ETag</a> and <a href="p4-conditional.html#header.last-modified" class="smpl">Last-Modified</a> values of a HEAD response (when present) are the same as that in a selected GET response (as per <a href="#caching.negotiated.responses" title="Using Negotiated Responses">Section&nbsp;4.3</a>), the cache <em class="bcp14">SHOULD</em> update the remaining header fields in the stored response using the following rules:
<li>delete any <a href="#header.warning" class="smpl">Warning</a> header fields in the stored response with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.4" title="Warning">Section&nbsp;7.6</a>);
<li>use other header fields provided in the response to replace all instances of the corresponding header fields in the stored
<h1 id="rfc.section.6"><a href="#rfc.section.6">6.</a>&nbsp;<a id="invalidation.after.updates.or.deletions" href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></h1>
<p id="rfc.section.6.p.1">Because unsafe request methods (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 2.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.5"><cite title="HTTP/1.1, part 2: Semantics and Payloads">[Part2]</cite></a>) such as PUT, POST or DELETE have the potential for changing state on the origin server, intervening caches can use them
<p id="rfc.section.6.p.2">A cache <em class="bcp14">MUST</em> invalidate the effective Request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.6"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>) as well as the URI(s) in the <a href="p2-semantics.html#header.location" class="smpl">Location</a> and <a href="p2-semantics.html#header.content-location" class="smpl">Content-Location</a> response header fields (if present) when a non-error response to a request with an unsafe method is received.
<p id="rfc.section.6.p.3">However, a cache <em class="bcp14">MUST NOT</em> invalidate a URI from a <a href="p2-semantics.html#header.location" class="smpl">Location</a> or <a href="p2-semantics.html#header.content-location" class="smpl">Content-Location</a> response header field if the host part of that URI differs from the host part in the effective request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>). This helps prevent denial of service attacks.
<p id="rfc.section.6.p.4">A cache <em class="bcp14">MUST</em> invalidate the effective request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 5.5</a> of <a href="#Part1" id="rfc.xref.Part1.8"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>) when it receives a non-error response to a request with a method whose safety is unknown.
<p id="rfc.section.6.p.5">Here, a "non-error response" is one with a <a href="p2-semantics.html#status.2xx" class="smpl">2xx (Successful)</a> or <a href="p2-semantics.html#status.3xx" class="smpl">3xx (Redirection)</a> status code. "Invalidate" means that the cache will either remove all stored responses related to the effective request URI,
or will mark these as "invalid" and in need of a mandatory validation before they can be returned in response to a subsequent
<p id="rfc.section.6.p.6">Note that this does not guarantee that all appropriate responses are invalidated. For example, the request that caused the
<h1 id="rfc.section.7"><a href="#rfc.section.7">7.</a>&nbsp;<a id="header.field.definitions" href="#header.field.definitions">Header Field Definitions</a></h1>
<p id="rfc.section.7.p.1">This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</p>
<h2 id="rfc.section.7.1"><a href="#rfc.section.7.1">7.1</a>&nbsp;<a id="header.age" href="#header.age">Age</a></h2>
<p id="rfc.section.7.1.p.1">The "Age" header field conveys the sender's estimate of the amount of time since the response was generated or successfully
validated at the origin server. Age values are calculated as specified in <a href="#age.calculations" title="Calculating Age">Section&nbsp;4.1.3</a>.
</pre><p id="rfc.section.7.1.p.3">Age field-values are non-negative integers, representing time in seconds (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.4.1</a>).
<p id="rfc.section.7.1.p.4">The presence of an Age header field in a response implies that a response is not first-hand. However, the converse is not
<h2 id="rfc.section.7.2"><a href="#rfc.section.7.2">7.2</a>&nbsp;<a id="header.cache-control" href="#header.cache-control">Cache-Control</a></h2>
<p id="rfc.section.7.2.p.1">The "Cache-Control" header field is used to specify directives for caches along the request/response chain. Such cache directives
<p id="rfc.section.7.2.p.2">A cache <em class="bcp14">MUST</em> obey the requirements of the Cache-Control directives defined in this section. See <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;7.2.3</a> for information about how Cache-Control directives defined elsewhere are handled.
<div class="note" id="rfc.section.7.2.p.3">
<p> <b>Note:</b> HTTP/1.0 caches might not implement Cache-Control and might only implement Pragma: no-cache (see <a href="#header.pragma" id="rfc.xref.header.pragma.2" title="Pragma">Section&nbsp;7.4</a>).
<p id="rfc.section.7.2.p.4">A proxy, whether or not it implements a cache, <em class="bcp14">MUST</em> pass cache directives through in forwarded messages, regardless of their significance to that application, since the directives
<p id="rfc.section.7.2.p.5">Cache directives are identified by a token, to be compared case-insensitively, and have an optional argument, that can use
forms, even if one is documented to be preferred. For any directive not defined by this specification, recipients <em class="bcp14">MUST</em> accept both forms.
</pre><p id="rfc.section.7.2.p.7">For the cache directives defined below, no argument is defined (nor allowed) otherwise stated otherwise.</p>
<h3 id="rfc.section.7.2.1"><a href="#rfc.section.7.2.1">7.2.1</a>&nbsp;<a id="cache-request-directive" href="#cache-request-directive">Request Cache-Control Directives</a></h3>
<h4 id="rfc.section.7.2.1.1"><a href="#rfc.section.7.2.1.1">7.2.1.1</a>&nbsp;<a id="cache-request-directive.no-cache" href="#cache-request-directive.no-cache">no-cache</a></h4>
<p id="rfc.section.7.2.1.1.p.1">The "no-cache" request directive indicates that a cache <em class="bcp14">MUST NOT</em> use a stored response to satisfy the request without successful validation on the origin server.
<h4 id="rfc.section.7.2.1.2"><a href="#rfc.section.7.2.1.2">7.2.1.2</a>&nbsp;<a id="cache-request-directive.no-store" href="#cache-request-directive.no-store">no-store</a></h4>
<p id="rfc.section.7.2.1.2.p.1">The "no-store" request directive indicates that a cache <em class="bcp14">MUST NOT</em> store any part of either this request or any response to it. This directive applies to both private and shared caches. "<em class="bcp14">MUST NOT</em> store" in this context means that the cache <em class="bcp14">MUST NOT</em> intentionally store the information in non-volatile storage, and <em class="bcp14">MUST</em> make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it.
<p id="rfc.section.7.2.1.2.p.2">This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
<p id="rfc.section.7.2.1.2.p.3">Note that if a request containing this directive is satisfied from a cache, the no-store request directive does not apply
<h4 id="rfc.section.7.2.1.3"><a href="#rfc.section.7.2.1.3">7.2.1.3</a>&nbsp;<a id="cache-request-directive.max-age" href="#cache-request-directive.max-age">max-age</a></h4>
<p id="rfc.section.7.2.1.3.p.1">Argument syntax: </p>
<li> <a href="#delta-seconds" class="smpl">delta-seconds</a> (see <a href="#delta-seconds" title="Delta Seconds">Section&nbsp;1.4.1</a>)
<p id="rfc.section.7.2.1.3.p.2">The "max-age" request directive indicates that the client is unwilling to accept a response whose age is greater than the
<p id="rfc.section.7.2.1.3.p.3"> <b>Note:</b> This directive uses the token form of the argument syntax; e.g., 'max-age=5', not 'max-age="5"'. Senders <em class="bcp14">SHOULD NOT</em> use the quoted-string form.
<h4 id="rfc.section.7.2.1.4"><a href="#rfc.section.7.2.1.4">7.2.1.4</a>&nbsp;<a id="cache-request-directive.max-stale" href="#cache-request-directive.max-stale">max-stale</a></h4>
<p id="rfc.section.7.2.1.4.p.1">Argument syntax: </p>
<p id="rfc.section.7.2.1.4.p.2">The "max-stale" request directive indicates that the client is willing to accept a response that has exceeded its expiration
<p id="rfc.section.7.2.1.4.p.3"> <b>Note:</b> This directive uses the token form of the argument syntax; e.g., 'max-stale=10', not 'max-stale="10"'. Senders <em class="bcp14">SHOULD NOT</em> use the quoted-string form.
<div id="rfc.iref.c.11"></div>
<h4 id="rfc.section.7.2.1.5"><a href="#rfc.section.7.2.1.5">7.2.1.5</a>&nbsp;<a id="cache-request-directive.min-fresh" href="#cache-request-directive.min-fresh">min-fresh</a></h4>
<p id="rfc.section.7.2.1.5.p.1">Argument syntax: </p>
<p id="rfc.section.7.2.1.5.p.2">The "min-fresh" request directive indicates that the client is willing to accept a response whose freshness lifetime is no
<p id="rfc.section.7.2.1.5.p.3"> <b>Note:</b> This directive uses the token form of the argument syntax; e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders <em class="bcp14">SHOULD NOT</em> use the quoted-string form.
<div id="rfc.iref.c.12"></div>
<h4 id="rfc.section.7.2.1.6"><a href="#rfc.section.7.2.1.6">7.2.1.6</a>&nbsp;<a id="cache-request-directive.no-transform" href="#cache-request-directive.no-transform">no-transform</a></h4>
<p id="rfc.section.7.2.1.6.p.1">The "no-transform" request directive indicates that an intermediary (whether or not it implements a cache) <em class="bcp14">MUST NOT</em> change the <a href="p2-semantics.html#header.content-encoding" class="smpl">Content-Encoding</a>, <a href="p5-range.html#header.content-range" class="smpl">Content-Range</a> or <a href="p2-semantics.html#header.content-type" class="smpl">Content-Type</a> request header fields, nor the request representation.
<div id="rfc.iref.c.13"></div>
<h4 id="rfc.section.7.2.1.7"><a href="#rfc.section.7.2.1.7">7.2.1.7</a>&nbsp;<a id="cache-request-directive.only-if-cached" href="#cache-request-directive.only-if-cached">only-if-cached</a></h4>
<p id="rfc.section.7.2.1.7.p.1">The "only-if-cached" request directive indicates that the client only wishes to obtain a stored response. If it receives this
<h3 id="rfc.section.7.2.2"><a href="#rfc.section.7.2.2">7.2.2</a>&nbsp;<a id="cache-response-directive" href="#cache-response-directive">Response Cache-Control Directives</a></h3>
<div id="rfc.iref.c.14"></div>
<h4 id="rfc.section.7.2.2.1"><a href="#rfc.section.7.2.2.1">7.2.2.1</a>&nbsp;<a id="cache-response-directive.only-if-cached" href="#cache-response-directive.only-if-cached">public</a></h4>
<p id="rfc.section.7.2.2.1.p.1">The "public" response directive indicates that a response whose associated request contains an 'Authentication' header <em class="bcp14">MAY</em> be stored (see <a href="#caching.authenticated.responses" title="Storing Responses to Authenticated Requests">Section&nbsp;3.2</a>).
<div id="rfc.iref.c.15"></div>
<h4 id="rfc.section.7.2.2.2"><a href="#rfc.section.7.2.2.2">7.2.2.2</a>&nbsp;<a id="cache-response-directive.private" href="#cache-response-directive.private">private</a></h4>
<p id="rfc.section.7.2.2.2.p.1">Argument syntax: </p>
<p id="rfc.section.7.2.2.2.p.2">The "private" response directive indicates that the response message is intended for a single user and <em class="bcp14">MUST NOT</em> be stored by a shared cache. A private cache <em class="bcp14">MAY</em> store the response.
<p id="rfc.section.7.2.2.2.p.3">If the private response directive specifies one or more field-names, this requirement is limited to the field-values associated
<p id="rfc.section.7.2.2.2.p.4">The field-names given are not limited to the set of standard header fields defined by this specification. Field names are
<p id="rfc.section.7.2.2.2.p.5"> <b>Note:</b> This usage of the word "private" only controls where the response can be stored; it cannot ensure the privacy of the message
<p id="rfc.section.7.2.2.2.p.6"> <b>Note:</b> This directive uses the quoted-string form of the argument syntax. Senders <em class="bcp14">SHOULD NOT</em> use the token form (even if quoting appears not to be needed for single-entry lists).
<div id="rfc.iref.c.16"></div>
<h4 id="rfc.section.7.2.2.3"><a href="#rfc.section.7.2.2.3">7.2.2.3</a>&nbsp;<a id="cache-response-directive.no-cache" href="#cache-response-directive.no-cache">no-cache</a></h4>
<p id="rfc.section.7.2.2.3.p.1">Argument syntax: </p>
<p id="rfc.section.7.2.2.3.p.2">The "no-cache" response directive indicates that the response <em class="bcp14">MUST NOT</em> be used to satisfy a subsequent request without successful validation on the origin server. This allows an origin server to
<p id="rfc.section.7.2.2.3.p.3">If the no-cache response directive specifies one or more field-names, then a cache <em class="bcp14">MAY</em> use the response to satisfy a subsequent request, subject to any other restrictions on caching. However, any header fields
<p id="rfc.section.7.2.2.3.p.4">The field-names given are not limited to the set of standard header fields defined by this specification. Field names are
<p id="rfc.section.7.2.2.3.p.5"> <b>Note:</b> Many HTTP/1.0 caches will not recognize or obey this directive. Also, no-cache response directives with field-names are often
<p id="rfc.section.7.2.2.3.p.6"> <b>Note:</b> This directive uses the quoted-string form of the argument syntax. Senders <em class="bcp14">SHOULD NOT</em> use the token form (even if quoting appears not to be needed for single-entry lists).
<div id="rfc.iref.c.17"></div>
<h4 id="rfc.section.7.2.2.4"><a href="#rfc.section.7.2.2.4">7.2.2.4</a>&nbsp;<a id="cache-response-directive.no-store" href="#cache-response-directive.no-store">no-store</a></h4>
<p id="rfc.section.7.2.2.4.p.1">The "no-store" response directive indicates that a cache <em class="bcp14">MUST NOT</em> store any part of either the immediate request or response. This directive applies to both private and shared caches. "<em class="bcp14">MUST NOT</em> store" in this context means that the cache <em class="bcp14">MUST NOT</em> intentionally store the information in non-volatile storage, and <em class="bcp14">MUST</em> make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it.
<p id="rfc.section.7.2.2.4.p.2">This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches
<div id="rfc.iref.c.18"></div>
<h4 id="rfc.section.7.2.2.5"><a href="#rfc.section.7.2.2.5">7.2.2.5</a>&nbsp;<a id="cache-response-directive.must-revalidate" href="#cache-response-directive.must-revalidate">must-revalidate</a></h4>
<p id="rfc.section.7.2.2.5.p.1">The "must-revalidate" response directive indicates that once it has become stale, a cache <em class="bcp14">MUST NOT</em> use the response to satisfy subsequent requests without successful validation on the origin server.
<p id="rfc.section.7.2.2.5.p.2">The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances
<p id="rfc.section.7.2.2.5.p.3">The must-revalidate directive ought to be used by servers if and only if failure to validate a request on the representation
<div id="rfc.iref.c.19"></div>
<h4 id="rfc.section.7.2.2.6"><a href="#rfc.section.7.2.2.6">7.2.2.6</a>&nbsp;<a id="cache-response-directive.proxy-revalidate" href="#cache-response-directive.proxy-revalidate">proxy-revalidate</a></h4>
<p id="rfc.section.7.2.2.6.p.1">The "proxy-revalidate" response directive has the same meaning as the must-revalidate response directive, except that it does
<div id="rfc.iref.c.20"></div>
<h4 id="rfc.section.7.2.2.7"><a href="#rfc.section.7.2.2.7">7.2.2.7</a>&nbsp;<a id="cache-response-directive.max-age" href="#cache-response-directive.max-age">max-age</a></h4>
<p id="rfc.section.7.2.2.7.p.1">Argument syntax: </p>
<p id="rfc.section.7.2.2.7.p.2">The "max-age" response directive indicates that the response is to be considered stale after its age is greater than the specified
<p id="rfc.section.7.2.2.7.p.3"> <b>Note:</b> This directive uses the token form of the argument syntax; e.g., 'max-age=5', not 'max-age="5"'. Senders <em class="bcp14">SHOULD NOT</em> use the quoted-string form.
<div id="rfc.iref.c.21"></div>
<h4 id="rfc.section.7.2.2.8"><a href="#rfc.section.7.2.2.8">7.2.2.8</a>&nbsp;<a id="cache-response-directive.s-maxage" href="#cache-response-directive.s-maxage">s-maxage</a></h4>
<p id="rfc.section.7.2.2.8.p.1">Argument syntax: </p>
<p id="rfc.section.7.2.2.8.p.2">The "s-maxage" response directive indicates that, in shared caches, the maximum age specified by this directive overrides
<p id="rfc.section.7.2.2.8.p.3"> <b>Note:</b> This directive uses the token form of the argument syntax; e.g., 's-maxage=10', not 's-maxage="10"'. Senders <em class="bcp14">SHOULD NOT</em> use the quoted-string form.
<div id="rfc.iref.c.22"></div>
<h4 id="rfc.section.7.2.2.9"><a href="#rfc.section.7.2.2.9">7.2.2.9</a>&nbsp;<a id="cache-response-directive.no-transform" href="#cache-response-directive.no-transform">no-transform</a></h4>
<p id="rfc.section.7.2.2.9.p.1">The "no-transform" response directive indicates that an intermediary (regardless of whether it implements a cache) <em class="bcp14">MUST NOT</em> change the <a href="p2-semantics.html#header.content-encoding" class="smpl">Content-Encoding</a>, <a href="p5-range.html#header.content-range" class="smpl">Content-Range</a> or <a href="p2-semantics.html#header.content-type" class="smpl">Content-Type</a> response header fields, nor the response representation.
<h3 id="rfc.section.7.2.3"><a href="#rfc.section.7.2.3">7.2.3</a>&nbsp;<a id="cache.control.extensions" href="#cache.control.extensions">Cache Control Extensions</a></h3>
<p id="rfc.section.7.2.3.p.1">The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional
<p id="rfc.section.7.2.3.p.2">This extension mechanism depends on an HTTP cache obeying all of the cache-control directives defined for its native HTTP-version,
<p id="rfc.section.7.2.3.p.3">For example, consider a hypothetical new response directive called "community" that acts as a modifier to the private directive.
community named within its value is allowed to cache the response. An origin server wishing to allow the UCI community to
use an otherwise private response in their shared cache(s) could do so by including
</pre><p id="rfc.section.7.2.3.p.5">A cache seeing this header field will act correctly even if the cache does not understand the community cache-extension, since
<p id="rfc.section.7.2.3.p.6">A cache <em class="bcp14">MUST</em> ignore unrecognized cache directives; it is assumed that any cache directive likely to be unrecognized by an HTTP/1.1 cache
<p id="rfc.section.7.2.3.p.7">New extension directives ought to consider defining:</p>
<p id="rfc.section.7.2.3.p.8"> </p>
<li>When the directive requires an argument, what it means when it is missing.</li>
<p id="rfc.section.7.2.3.p.9">The HTTP Cache Directive Registry defines the name space for the cache directives.</p>
<p id="rfc.section.7.2.3.p.10">A registration <em class="bcp14">MUST</em> include the following fields:
<p id="rfc.section.7.2.3.p.11">Values to be added to this name space require IETF Review (see <a href="#RFC5226" id="rfc.xref.RFC5226.1"><cite title="Guidelines for Writing an IANA Considerations Section in RFCs">[RFC5226]</cite></a>, <a href="http://tools.ietf.org/html/rfc5226#section-4.1">Section 4.1</a>).
<p id="rfc.section.7.2.3.p.12">The registry itself is maintained at &lt;<a href="http://www.iana.org/assignments/http-cache-directives">http://www.iana.org/assignments/http-cache-directives</a>&gt;.
<h2 id="rfc.section.7.3"><a href="#rfc.section.7.3">7.3</a>&nbsp;<a id="header.expires" href="#header.expires">Expires</a></h2>
<p id="rfc.section.7.3.p.1">The "Expires" header field gives the date/time after which the response is considered stale. See <a href="#expiration.model" title="Freshness Model">Section&nbsp;4.1</a> for further discussion of the freshness model.
<p id="rfc.section.7.3.p.2">The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after
<p id="rfc.section.7.3.p.3">The field-value is an absolute date and time as defined by HTTP-date in <a href="p2-semantics.html#http.date" title="Date/Time Formats">Section 5.1</a> of <a href="#Part2" id="rfc.xref.Part2.6"><cite title="HTTP/1.1, part 2: Semantics and Payloads">[Part2]</cite></a>; a sender <em class="bcp14">MUST</em> use the rfc1123-date format.
</pre><p id="rfc.section.7.3.p.6">A cache <em class="bcp14">MUST</em> treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").
<div class="note" id="rfc.section.7.3.p.7">
<p> <b>Note:</b> If a response includes a <a href="#header.cache-control" class="smpl">Cache-Control</a> field with the max-age directive (see <a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;7.2.2.7</a>), that directive overrides the Expires field. Likewise, the s-maxage directive (<a href="#cache-response-directive.s-maxage" title="s-maxage">Section&nbsp;7.2.2.8</a>) overrides the <a href="#header.expires" class="smpl">Expires</a> header fieldin shared caches.
<p id="rfc.section.7.3.p.8">Historically, HTTP required the Expires field-value to be no more than a year in the future. While longer freshness lifetimes
of 32-bit integers for time values), and many caches will evict a response far sooner than that. Therefore, senders ought
<p id="rfc.section.7.3.p.9">An origin server without a clock <em class="bcp14">MUST NOT</em> assign Expires values to a response unless these values were associated with the resource by a system or user with a reliable
<h2 id="rfc.section.7.4"><a href="#rfc.section.7.4">7.4</a>&nbsp;<a id="header.pragma" href="#header.pragma">Pragma</a></h2>
<p id="rfc.section.7.4.p.1">The "Pragma" header field allows backwards compatibility with HTTP/1.0 caches, so that clients can specify a "no-cache" request
<p id="rfc.section.7.4.p.2">In HTTP/1.0, Pragma was defined as an extensible field for implementation-specified directives for recipients. This specification
</pre><p id="rfc.section.7.4.p.4">When the <a href="#header.cache-control" class="smpl">Cache-Control</a> header field is not present in a request, the no-cache request pragma-directive <em class="bcp14">MUST</em> have the same effect on caches as if "Cache-Control: no-cache" were present (see <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;7.2.1</a>).
<p id="rfc.section.7.4.p.5">When sending a no-cache request, a client ought to include both the pragma and cache-control directives, unless Cache-Control:
<div id="rfc.figure.u.12"></div> <pre class="text">GET / HTTP/1.1
</pre> <p id="rfc.section.7.4.p.7">will constrain HTTP/1.1 caches to serve a response no older than 30 seconds, while precluding implementations that do not
<div class="note" id="rfc.section.7.4.p.8">
<h2 id="rfc.section.7.5"><a href="#rfc.section.7.5">7.5</a>&nbsp;<a id="header.vary" href="#header.vary">Vary</a></h2>
<p id="rfc.section.7.5.p.1">The "Vary" header field conveys the set of header fields that were used to select the representation.</p>
<p id="rfc.section.7.5.p.2">Caches use this information, in part, to determine whether a stored response can be used to satisfy a given request; see <a href="#caching.negotiated.responses" title="Using Negotiated Responses">Section&nbsp;4.3</a>.
<p id="rfc.section.7.5.p.3">In uncacheable or stale responses, the Vary field value advises the user agent about the criteria that were used to select
<div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.9"></span> <a href="#header.vary" class="smpl">Vary</a> = "*" / 1#<a href="#imported.abnf" class="smpl">field-name</a>
</pre><p id="rfc.section.7.5.p.5">The set of header fields named by the Vary field value is known as the selecting header fields.</p>
<p id="rfc.section.7.5.p.6">A server <em class="bcp14">SHOULD</em> include a Vary header field with any cacheable response that is subject to server-driven negotiation. Doing so allows a cache
<p id="rfc.section.7.5.p.7">A Vary field value of "*" signals that unspecified parameters not limited to the header fields (e.g., the network address
<p id="rfc.section.7.5.p.8">The field-names given are not limited to the set of standard header fields defined by this specification. Field names are
<h2 id="rfc.section.7.6"><a href="#rfc.section.7.6">7.6</a>&nbsp;<a id="header.warning" href="#header.warning">Warning</a></h2>
<p id="rfc.section.7.6.p.1">The "Warning" header field is used to carry additional information about the status or transformation of a message that might
<p id="rfc.section.7.6.p.2">Warnings can be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status
<p id="rfc.section.7.6.p.3">Warning header fields can in general be applied to any message, however some warn-codes are specific to caches and can only
<div id="rfc.figure.u.14"></div><pre class="inline"><span id="rfc.iref.g.10"></span><span id="rfc.iref.g.11"></span><span id="rfc.iref.g.12"></span><span id="rfc.iref.g.13"></span><span id="rfc.iref.g.14"></span><span id="rfc.iref.g.15"></span> <a href="#header.warning" class="smpl">Warning</a> = 1#<a href="#header.warning" class="smpl">warning-value</a>
[<a href="#imported.abnf" class="smpl">SP</a> <a href="#header.warning" class="smpl">warn-date</a>]
</pre><p id="rfc.section.7.6.p.5">Multiple warnings can be attached to a response (either by the origin server or by a cache), including multiple warnings with
<p id="rfc.section.7.6.p.6">When this occurs, the user agent <em class="bcp14">SHOULD</em> inform the user of as many of them as possible, in the order that they appear in the response.
<p id="rfc.section.7.6.p.7">Systems that generate multiple Warning header fields are encouraged to order them with this user agent behavior in mind. New
Warning header fields are added after any existing Warning header fields.
<p id="rfc.section.7.6.p.8">Warnings are assigned three digit warn-codes. The first digit indicates whether the Warning is required to be deleted from
<p id="rfc.section.7.6.p.9">If an implementation sends a message with one or more Warning header fields to a receiver whose version is HTTP/1.0 or lower,
then the sender <em class="bcp14">MUST</em> include in each warning-value a warn-date that matches the <a href="p2-semantics.html#header.date" class="smpl">Date</a> header field in the message.
<p id="rfc.section.7.6.p.10">If a system receives a message with a warning-value that includes a warn-date, and that warn-date is different from the <a href="p2-semantics.html#header.date" class="smpl">Date</a> value in the response, then that warning-value <em class="bcp14">MUST</em> be deleted from the message before storing, forwarding, or using it. (preventing the consequences of naive caching of Warning
<p id="rfc.section.7.6.p.11">The following warn-codes are defined by this specification, each with a recommended warn-text in English, and a description
<div id="rfc.iref.76"></div>
<h3 id="rfc.section.7.6.1"><a href="#rfc.section.7.6.1">7.6.1</a>&nbsp;<a id="warn.110" href="#warn.110">110 Response is Stale</a></h3>
<p id="rfc.section.7.6.1.p.1">A cache <em class="bcp14">SHOULD</em> include this whenever the returned response is stale.
<div id="rfc.iref.77"></div>
<h3 id="rfc.section.7.6.2"><a href="#rfc.section.7.6.2">7.6.2</a>&nbsp;<a id="warn.111" href="#warn.111">111 Revalidation Failed</a></h3>
<p id="rfc.section.7.6.2.p.1">A cache <em class="bcp14">SHOULD</em> include this when returning a stale response because an attempt to validate the response failed, due to an inability to reach
<h3 id="rfc.section.7.6.3"><a href="#rfc.section.7.6.3">7.6.3</a>&nbsp;<a id="warn.112" href="#warn.112">112 Disconnected Operation</a></h3>
<p id="rfc.section.7.6.3.p.1">A cache <em class="bcp14">SHOULD</em> include this if it is intentionally disconnected from the rest of the network for a period of time.
<h3 id="rfc.section.7.6.4"><a href="#rfc.section.7.6.4">7.6.4</a>&nbsp;<a id="warn.113" href="#warn.113">113 Heuristic Expiration</a></h3>
<p id="rfc.section.7.6.4.p.1">A cache <em class="bcp14">SHOULD</em> include this if it heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than 24
<h3 id="rfc.section.7.6.5"><a href="#rfc.section.7.6.5">7.6.5</a>&nbsp;<a id="warn.199" href="#warn.199">199 Miscellaneous Warning</a></h3>
<p id="rfc.section.7.6.5.p.1">The warning text can include arbitrary information to be presented to a human user, or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action, besides presenting the warning to the user.
<h3 id="rfc.section.7.6.6"><a href="#rfc.section.7.6.6">7.6.6</a>&nbsp;<a id="warn.214" href="#warn.214">214 Transformation Applied</a></h3>
<p id="rfc.section.7.6.6.p.1"> <em class="bcp14">MUST</em> be added by a proxy if it applies any transformation to the representation, such as changing the content-coding, media-type,
<h3 id="rfc.section.7.6.7"><a href="#rfc.section.7.6.7">7.6.7</a>&nbsp;<a id="warn.299" href="#warn.299">299 Miscellaneous Persistent Warning</a></h3>
<p id="rfc.section.7.6.7.p.1">The warning text can include arbitrary information to be presented to a human user, or logged. A system receiving this warning <em class="bcp14">MUST NOT</em> take any automated action.
<h3 id="rfc.section.7.6.8"><a href="#rfc.section.7.6.8">7.6.8</a>&nbsp;<a id="warn.code.extensions" href="#warn.code.extensions">Warn Code Extensions</a></h3>
<p id="rfc.section.7.6.8.p.1">The HTTP Warn Code Registry defines the name space for warn codes.</p>
<p id="rfc.section.7.6.8.p.2">A registration <em class="bcp14">MUST</em> include the following fields:
<p id="rfc.section.7.6.8.p.3">Values to be added to this name space require IETF Review (see <a href="#RFC5226" id="rfc.xref.RFC5226.2"><cite title="Guidelines for Writing an IANA Considerations Section in RFCs">[RFC5226]</cite></a>, <a href="http://tools.ietf.org/html/rfc5226#section-4.1">Section 4.1</a>).
<p id="rfc.section.7.6.8.p.4">The registry itself is maintained at &lt;<a href="http://www.iana.org/assignments/http-warn-codes">http://www.iana.org/assignments/http-warn-codes</a>&gt;.
<h1 id="rfc.section.8"><a href="#rfc.section.8">8.</a>&nbsp;<a id="history.lists" href="#history.lists">History Lists</a></h1>
<p id="rfc.section.8.p.1">User agents often have history mechanisms, such as "Back" buttons and history lists, that can be used to redisplay a representation
<p id="rfc.section.8.p.2">The freshness model (<a href="#expiration.model" title="Freshness Model">Section&nbsp;4.1</a>) does not necessarily apply to history mechanisms. I.e., a history mechanism can display a previous representation even if
<p id="rfc.section.8.p.3">This does not prohibit the history mechanism from telling the user that a view might be stale, or from honoring cache directives
<h1 id="rfc.section.9"><a href="#rfc.section.9">9.</a>&nbsp;<a id="IANA.considerations" href="#IANA.considerations">IANA Considerations</a></h1>
<h2 id="rfc.section.9.1"><a href="#rfc.section.9.1">9.1</a>&nbsp;<a id="cache.directive.registration" href="#cache.directive.registration">Cache Directive Registry</a></h2>
<p id="rfc.section.9.1.p.1">The registration procedure for HTTP Cache Directives is defined by <a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;7.2.3</a> of this document.
<p id="rfc.section.9.1.p.2">The HTTP Cache Directive Registry shall be created at &lt;<a href="http://www.iana.org/assignments/http-cache-directives">http://www.iana.org/assignments/http-cache-directives</a>&gt; and be populated with the registrations below:
<td class="left"> <a href="#cache-request-directive.max-age" title="max-age">Section&nbsp;7.2.1.3</a>, <a href="#cache-response-directive.max-age" title="max-age">Section&nbsp;7.2.2.7</a>
<td class="left"> <a href="#cache-request-directive.max-stale" title="max-stale">Section&nbsp;7.2.1.4</a>
<td class="left"> <a href="#cache-request-directive.min-fresh" title="min-fresh">Section&nbsp;7.2.1.5</a>
<td class="left"> <a href="#cache-response-directive.must-revalidate" title="must-revalidate">Section&nbsp;7.2.2.5</a>
<td class="left"> <a href="#cache-request-directive.no-cache" title="no-cache">Section&nbsp;7.2.1.1</a>, <a href="#cache-response-directive.no-cache" title="no-cache">Section&nbsp;7.2.2.3</a>
<td class="left"> <a href="#cache-request-directive.no-store" title="no-store">Section&nbsp;7.2.1.2</a>, <a href="#cache-response-directive.no-store" title="no-store">Section&nbsp;7.2.2.4</a>
<td class="left"> <a href="#cache-request-directive.no-transform" title="no-transform">Section&nbsp;7.2.1.6</a>, <a href="#cache-response-directive.no-transform" title="no-transform">Section&nbsp;7.2.2.9</a>
<td class="left"> <a href="#cache-request-directive.only-if-cached" title="only-if-cached">Section&nbsp;7.2.1.7</a>
<td class="left"> <a href="#cache-response-directive.private" title="private">Section&nbsp;7.2.2.2</a>
<td class="left"> <a href="#cache-response-directive.proxy-revalidate" title="proxy-revalidate">Section&nbsp;7.2.2.6</a>
<td class="left"> <a href="#cache-response-directive.only-if-cached" title="public">Section&nbsp;7.2.2.1</a>
<td class="left"> <a href="#cache-response-directive.s-maxage" title="s-maxage">Section&nbsp;7.2.2.8</a>
<h2 id="rfc.section.9.2"><a href="#rfc.section.9.2">9.2</a>&nbsp;<a id="warn.code.registration" href="#warn.code.registration">Warn Code Registry</a></h2>
<p id="rfc.section.9.2.p.1">The registration procedure for HTTP Warn Codes is defined by <a href="#warn.code.extensions" title="Warn Code Extensions">Section&nbsp;7.6.8</a> of this document.
<p id="rfc.section.9.2.p.2">The HTTP Warn Code Registry shall be created at &lt;<a href="http://www.iana.org/assignments/http-cache-directives">http://www.iana.org/assignments/http-cache-directives</a>&gt; and be populated with the registrations below:
<td class="left"> <a href="#warn.110" id="rfc.xref.warn.110.1" title="110 Response is Stale">Section&nbsp;7.6.1</a>
<td class="left"> <a href="#warn.111" id="rfc.xref.warn.111.1" title="111 Revalidation Failed">Section&nbsp;7.6.2</a>
<td class="left"> <a href="#warn.112" id="rfc.xref.warn.112.1" title="112 Disconnected Operation">Section&nbsp;7.6.3</a>
<td class="left"> <a href="#warn.113" id="rfc.xref.warn.113.1" title="113 Heuristic Expiration">Section&nbsp;7.6.4</a>
<td class="left"> <a href="#warn.199" id="rfc.xref.warn.199.1" title="199 Miscellaneous Warning">Section&nbsp;7.6.5</a>
<td class="left"> <a href="#warn.214" id="rfc.xref.warn.214.1" title="214 Transformation Applied">Section&nbsp;7.6.6</a>
<td class="left"> <a href="#warn.299" id="rfc.xref.warn.299.1" title="299 Miscellaneous Persistent Warning">Section&nbsp;7.6.7</a>
<h2 id="rfc.section.9.3"><a href="#rfc.section.9.3">9.3</a>&nbsp;<a id="header.field.registration" href="#header.field.registration">Header Field Registration</a></h2>
<p id="rfc.section.9.3.p.1">The Message Header Field Registry located at &lt;<a href="http://www.iana.org/assignments/message-headers/message-header-index.html">http://www.iana.org/assignments/message-headers/message-header-index.html</a>&gt; shall be updated with the permanent registrations below (see <a href="#RFC3864" id="rfc.xref.RFC3864.1"><cite title="Registration Procedures for Message Header Fields">[RFC3864]</cite></a>):
<td class="left"> <a href="#header.age" id="rfc.xref.header.age.3" title="Age">Section&nbsp;7.1</a>
<td class="left"> <a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;7.2</a>
<td class="left"> <a href="#header.expires" id="rfc.xref.header.expires.4" title="Expires">Section&nbsp;7.3</a>
<td class="left"> <a href="#header.pragma" id="rfc.xref.header.pragma.3" title="Pragma">Section&nbsp;7.4</a>
<td class="left"> <a href="#header.vary" id="rfc.xref.header.vary.2" title="Vary">Section&nbsp;7.5</a>
<td class="left"> <a href="#header.warning" id="rfc.xref.header.warning.5" title="Warning">Section&nbsp;7.6</a>
<p id="rfc.section.9.3.p.2">The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".</p>
<h1 id="rfc.section.10"><a href="#rfc.section.10">10.</a>&nbsp;<a id="security.considerations" href="#security.considerations">Security Considerations</a></h1>
<p id="rfc.section.10.p.1">Caches expose additional potential vulnerabilities, since the contents of the cache represent an attractive target for malicious
<h1 id="rfc.section.11"><a href="#rfc.section.11">11.</a>&nbsp;<a id="acks" href="#acks">Acknowledgments</a></h1>
<p id="rfc.section.11.p.1">See <a href="p1-messaging.html#acks" title="Acknowledgments">Section 9</a> of <a href="#Part1" id="rfc.xref.Part1.9"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>.
<h1 id="rfc.references"><a id="rfc.section.12" href="#rfc.section.12">12.</a> References
<h2 id="rfc.references.1"><a href="#rfc.section.12.1" id="rfc.section.12.1">12.1</a> Normative References
<td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-latest">HTTP/1.1, part 1: Message Routing and Syntax"</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p1-messaging-latest (work in progress), July&nbsp;2012.
<td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p2-semantics-latest">HTTP/1.1, part 2: Semantics and Payloads</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p2-semantics-latest (work in progress), July&nbsp;2012.
<td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p5-range-latest">HTTP/1.1, part 5: Range Requests</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p5-range-latest (work in progress), July&nbsp;2012.
<td class="top"><a href="mailto:fielding@gbiv.com" title="Adobe Systems Incorporated">Fielding, R., Ed.</a>, <a href="mailto:ylafon@w3.org" title="World Wide Web Consortium">Lafon, Y., Ed.</a>, and <a href="mailto:julian.reschke@greenbytes.de" title="greenbytes GmbH">J. Reschke, Ed.</a>, “<a href="http://tools.ietf.org/html/draft-ietf-httpbis-p7-auth-latest">HTTP/1.1, part 7: Authentication</a>”, Internet-Draft&nbsp;draft-ietf-httpbis-p7-auth-latest (work in progress), July&nbsp;2012.
<h2 id="rfc.references.2"><a href="#rfc.section.12.2" id="rfc.section.12.2">12.2</a> Informative References
<p id="rfc.section.A.p.1">Make the specified age calculation algorithm less conservative. (<a href="#age.calculations" title="Calculating Age">Section&nbsp;4.1.3</a>)
<p id="rfc.section.A.p.2">Remove requirement to consider <a href="p2-semantics.html#header.content-location" class="smpl">Content-Location</a> in successful responses in order to determine the appropriate response to use. (<a href="#validation.model" title="Validation Model">Section&nbsp;4.2</a>)
<p id="rfc.section.A.p.3">Clarify denial of service attack avoidance requirement. (<a href="#invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">Section&nbsp;6</a>)
<p id="rfc.section.A.p.4">Change ABNF productions for header fields to only define the field value. (<a href="#header.field.definitions" title="Header Field Definitions">Section&nbsp;7</a>)
<p id="rfc.section.A.p.5">Do not mention RFC 2047 encoding and multiple languages in <a href="#header.warning" class="smpl">Warning</a> header fields anymore, as these aspects never were implemented. (<a href="#header.warning" id="rfc.xref.header.warning.6" title="Warning">Section&nbsp;7.6</a>)
<p id="rfc.section.A.p.6">Introduce Cache Directive and Warn Code Registries. (<a href="#cache.control.extensions" title="Cache Control Extensions">Section&nbsp;7.2.3</a> and <a href="#warn.code.extensions" title="Warn Code Extensions">Section&nbsp;7.6.8</a>)
<h1 id="rfc.section.B"><a href="#rfc.section.B">B.</a>&nbsp;<a id="imported.abnf" href="#imported.abnf">Imported ABNF</a></h1>
<p id="rfc.section.B.p.2">The rules below are defined in <a href="#Part1" id="rfc.xref.Part1.10"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>:
<div id="rfc.figure.u.15"></div><pre class="inline"> <a href="#imported.abnf" class="smpl">OWS</a> = &lt;OWS, defined in <a href="#Part1" id="rfc.xref.Part1.11"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>, <a href="p1-messaging.html#whitespace" title="Whitespace">Section 3.2.1</a>&gt;
<a href="#imported.abnf" class="smpl">field-name</a> = &lt;field-name, defined in <a href="#Part1" id="rfc.xref.Part1.12"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>, <a href="p1-messaging.html#header.fields" title="Header Fields">Section 3.2</a>&gt;
<a href="#imported.abnf" class="smpl">quoted-string</a> = &lt;quoted-string, defined in <a href="#Part1" id="rfc.xref.Part1.13"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>, <a href="p1-messaging.html#field.components" title="Field value components">Section 3.2.4</a>&gt;
<a href="#imported.abnf" class="smpl">token</a> = &lt;token, defined in <a href="#Part1" id="rfc.xref.Part1.14"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>, <a href="p1-messaging.html#field.components" title="Field value components">Section 3.2.4</a>&gt;
<a href="#imported.abnf" class="smpl">port</a> = &lt;port, defined in <a href="#Part1" id="rfc.xref.Part1.15"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>, <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.8</a>&gt;
<a href="#imported.abnf" class="smpl">pseudonym</a> = &lt;pseudonym, defined in <a href="#Part1" id="rfc.xref.Part1.16"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>, <a href="p1-messaging.html#header.via" title="Via">Section 6.2</a>&gt;
<a href="#imported.abnf" class="smpl">uri-host</a> = &lt;uri-host, defined in <a href="#Part1" id="rfc.xref.Part1.17"><cite title="HTTP/1.1, part 1: Message Routing and Syntax&#34;">[Part1]</cite></a>, <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.8</a>&gt;
<div id="rfc.figure.u.16"></div><pre class="inline"> <a href="#imported.abnf" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in <a href="#Part2" id="rfc.xref.Part2.7"><cite title="HTTP/1.1, part 2: Semantics and Payloads">[Part2]</cite></a>, <a href="p2-semantics.html#http.date" title="Date/Time Formats">Section 5.1</a>&gt;
</pre><h1 id="rfc.section.C"><a href="#rfc.section.C">C.</a>&nbsp;<a id="collected.abnf" href="#collected.abnf">Collected ABNF</a></h1>
<div id="rfc.figure.u.17"></div> <pre class="inline"><a href="#header.age" class="smpl">Age</a> = delta-seconds
<a href="#imported.abnf" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in [Part2], Section 5.1&gt;
<a href="#imported.abnf" class="smpl">field-name</a> = &lt;field-name, defined in [Part1], Section 3.2&gt;
<a href="#imported.abnf" class="smpl">port</a> = &lt;port, defined in [Part1], Section 2.8&gt;
<a href="#imported.abnf" class="smpl">pseudonym</a> = &lt;pseudonym, defined in [Part1], Section 6.2&gt;
<a href="#imported.abnf" class="smpl">quoted-string</a> = &lt;quoted-string, defined in [Part1], Section 3.2.4&gt;
<a href="#imported.abnf" class="smpl">token</a> = &lt;token, defined in [Part1], Section 3.2.4&gt;
<a href="#imported.abnf" class="smpl">uri-host</a> = &lt;uri-host, defined in [Part1], Section 2.8&gt;
</pre> <h1 id="rfc.section.D"><a href="#rfc.section.D">D.</a>&nbsp;<a id="change.log" href="#change.log">Change Log (to be removed by RFC Editor before publication)</a></h1>
<p id="rfc.section.D.p.1">Changes up to the first Working Group Last Call draft are summarized in &lt;<a href="http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C">http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-19#appendix-C</a>&gt;.
<h2 id="rfc.section.D.1"><a href="#rfc.section.D.1">D.1</a>&nbsp;<a id="changes.since.19" href="#changes.since.19">Since draft-ietf-httpbis-p6-cache-19</a></h2>
<li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/307">http://tools.ietf.org/wg/httpbis/trac/ticket/307</a>&gt;: "untangle Cache-Control ABNF"
<li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/353">http://tools.ietf.org/wg/httpbis/trac/ticket/353</a>&gt;: "Multiple values in Cache-Control header fields"
<li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/355">http://tools.ietf.org/wg/httpbis/trac/ticket/355</a>&gt;: "Case sensitivity of header fields in CC values"
<li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/356">http://tools.ietf.org/wg/httpbis/trac/ticket/356</a>&gt;: "Spurious 'MAYs'"
<li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/360">http://tools.ietf.org/wg/httpbis/trac/ticket/360</a>&gt;: "enhance considerations for new cache control directives"
<li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/368">http://tools.ietf.org/wg/httpbis/trac/ticket/368</a>&gt;: "note introduction of new IANA registries as normative changes"
<h2 id="rfc.section.D.2"><a href="#rfc.section.D.2">D.2</a>&nbsp;<a id="changes.since.20" href="#changes.since.20">Since draft-ietf-httpbis-p6-cache-20</a></h2>
<li>110 Response is Stale (warn code)&nbsp;&nbsp;<a href="#rfc.iref.76"><b>7.6.1</b></a>, <a href="#rfc.xref.warn.110.1">9.2</a></li>
<li>111 Revalidation Failed (warn code)&nbsp;&nbsp;<a href="#rfc.iref.77"><b>7.6.2</b></a>, <a href="#rfc.xref.warn.111.1">9.2</a></li>
<li>112 Disconnected Operation (warn code)&nbsp;&nbsp;<a href="#rfc.iref.78"><b>7.6.3</b></a>, <a href="#rfc.xref.warn.112.1">9.2</a></li>
<li>113 Heuristic Expiration (warn code)&nbsp;&nbsp;<a href="#rfc.iref.79"><b>7.6.4</b></a>, <a href="#rfc.xref.warn.113.1">9.2</a></li>
<li>199 Miscellaneous Warning (warn code)&nbsp;&nbsp;<a href="#rfc.iref.80"><b>7.6.5</b></a>, <a href="#rfc.xref.warn.199.1">9.2</a></li>
<li>214 Transformation Applied (warn code)&nbsp;&nbsp;<a href="#rfc.iref.81"><b>7.6.6</b></a>, <a href="#rfc.xref.warn.214.1">9.2</a></li>
<li>299 Miscellaneous Persistent Warning (warn code)&nbsp;&nbsp;<a href="#rfc.iref.82"><b>7.6.7</b></a>, <a href="#rfc.xref.warn.299.1">9.2</a></li>
<li>Age header field&nbsp;&nbsp;<a href="#rfc.xref.header.age.1">4</a>, <a href="#rfc.xref.header.age.2">4.1.3</a>, <a href="#rfc.iref.a.2"><b>7.1</b></a>, <a href="#rfc.xref.header.age.3">9.3</a></li>
<li>max-age&nbsp;&nbsp;<a href="#rfc.iref.c.9"><b>7.2.1.3</b></a>, <a href="#rfc.iref.c.20"><b>7.2.2.7</b></a></li>
<li>max-stale&nbsp;&nbsp;<a href="#rfc.iref.c.10"><b>7.2.1.4</b></a></li>
<li>min-fresh&nbsp;&nbsp;<a href="#rfc.iref.c.11"><b>7.2.1.5</b></a></li>
<li>must-revalidate&nbsp;&nbsp;<a href="#rfc.iref.c.18"><b>7.2.2.5</b></a></li>
<li>no-cache&nbsp;&nbsp;<a href="#rfc.iref.c.7"><b>7.2.1.1</b></a>, <a href="#rfc.iref.c.16"><b>7.2.2.3</b></a></li>
<li>no-store&nbsp;&nbsp;<a href="#rfc.iref.c.8"><b>7.2.1.2</b></a>, <a href="#rfc.iref.c.17"><b>7.2.2.4</b></a></li>
<li>no-transform&nbsp;&nbsp;<a href="#rfc.iref.c.12"><b>7.2.1.6</b></a>, <a href="#rfc.iref.c.22"><b>7.2.2.9</b></a></li>
<li>only-if-cached&nbsp;&nbsp;<a href="#rfc.iref.c.13"><b>7.2.1.7</b></a></li>
<li>private&nbsp;&nbsp;<a href="#rfc.iref.c.15"><b>7.2.2.2</b></a></li>
<li>proxy-revalidate&nbsp;&nbsp;<a href="#rfc.iref.c.19"><b>7.2.2.6</b></a></li>
<li>public&nbsp;&nbsp;<a href="#rfc.iref.c.14"><b>7.2.2.1</b></a></li>
<li>s-maxage&nbsp;&nbsp;<a href="#rfc.iref.c.21"><b>7.2.2.8</b></a></li>
<li>Cache-Control header field&nbsp;&nbsp;<a href="#rfc.xref.header.cache-control.1">3</a>, <a href="#rfc.iref.c.6"><b>7.2</b></a>, <a href="#rfc.xref.header.cache-control.2">9.3</a></li>
<li>Expires header field&nbsp;&nbsp;<a href="#rfc.xref.header.expires.1">3</a>, <a href="#rfc.xref.header.expires.2">4.1</a>, <a href="#rfc.xref.header.expires.3">4.1.1</a>, <a href="#rfc.iref.e.2"><b>7.3</b></a>, <a href="#rfc.xref.header.expires.4">9.3</a></li>
<li><tt>Age</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.2"><b>7.1</b></a></li>
<li><tt>Cache-Control</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.3"><b>7.2</b></a></li>
<li><tt>cache-directive</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.4"><b>7.2</b></a></li>
<li><tt>delta-seconds</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.1"><b>1.4.1</b></a></li>
<li><tt>Expires</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.5"><b>7.3</b></a></li>
<li><tt>extension-pragma</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.8"><b>7.4</b></a></li>
<li><tt>Pragma</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.6"><b>7.4</b></a></li>
<li><tt>pragma-directive</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.7"><b>7.4</b></a></li>
<li><tt>Vary</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.9"><b>7.5</b></a></li>
<li><tt>warn-agent</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.13"><b>7.6</b></a></li>
<li><tt>warn-code</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.12"><b>7.6</b></a></li>
<li><tt>warn-date</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.15"><b>7.6</b></a></li>
<li><tt>warn-text</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.14"><b>7.6</b></a></li>
<li><tt>Warning</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.10"><b>7.6</b></a></li>
<li><tt>warning-value</tt>&nbsp;&nbsp;<a href="#rfc.iref.g.11"><b>7.6</b></a></li>
<li>Age&nbsp;&nbsp;<a href="#rfc.xref.header.age.1">4</a>, <a href="#rfc.xref.header.age.2">4.1.3</a>, <a href="#rfc.iref.h.2"><b>7.1</b></a>, <a href="#rfc.xref.header.age.3">9.3</a></li>
<li>Cache-Control&nbsp;&nbsp;<a href="#rfc.xref.header.cache-control.1">3</a>, <a href="#rfc.iref.h.3"><b>7.2</b></a>, <a href="#rfc.xref.header.cache-control.2">9.3</a></li>
<li>Expires&nbsp;&nbsp;<a href="#rfc.xref.header.expires.1">3</a>, <a href="#rfc.xref.header.expires.2">4.1</a>, <a href="#rfc.xref.header.expires.3">4.1.1</a>, <a href="#rfc.iref.h.4"><b>7.3</b></a>, <a href="#rfc.xref.header.expires.4">9.3</a></li>
<li>Pragma&nbsp;&nbsp;<a href="#rfc.xref.header.pragma.1">4</a>, <a href="#rfc.xref.header.pragma.2">7.2</a>, <a href="#rfc.iref.h.5"><b>7.4</b></a>, <a href="#rfc.xref.header.pragma.3">9.3</a></li>
<li>Vary&nbsp;&nbsp;<a href="#rfc.xref.header.vary.1">4.3</a>, <a href="#rfc.iref.h.6"><b>7.5</b></a>, <a href="#rfc.xref.header.vary.2">9.3</a></li>
<li>Warning&nbsp;&nbsp;<a href="#rfc.xref.header.warning.1">4.1.4</a>, <a href="#rfc.xref.header.warning.2">4.2.1</a>, <a href="#rfc.xref.header.warning.3">4.4</a>, <a href="#rfc.xref.header.warning.4">5</a>, <a href="#rfc.iref.h.7"><b>7.6</b></a>, <a href="#rfc.xref.header.warning.5">9.3</a>, <a href="#rfc.xref.header.warning.6">A</a></li>
<li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.m.1"><b>7.2.1.3</b></a>, <a href="#rfc.iref.m.5"><b>7.2.2.7</b></a></li>
<li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.m.2"><b>7.2.1.4</b></a></li>
<li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.m.3"><b>7.2.1.5</b></a></li>
<li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.m.4"><b>7.2.2.5</b></a></li>
<li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.n.1"><b>7.2.1.1</b></a>, <a href="#rfc.iref.n.4"><b>7.2.2.3</b></a></li>
<li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.n.2"><b>7.2.1.2</b></a>, <a href="#rfc.iref.n.5"><b>7.2.2.4</b></a></li>
<li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.n.3"><b>7.2.1.6</b></a>, <a href="#rfc.iref.n.6"><b>7.2.2.9</b></a></li>
<li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.o.1"><b>7.2.1.7</b></a></li>
<li><em>Part1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.1">1.3</a>, <a href="#rfc.xref.Part1.2">1.4</a>, <a href="#rfc.xref.Part1.3">3.1</a>, <a href="#rfc.xref.Part1.4">4</a>, <a href="#rfc.xref.Part1.5">4.3</a>, <a href="#rfc.xref.Part1.6">6</a>, <a href="#rfc.xref.Part1.7">6</a>, <a href="#rfc.xref.Part1.8">6</a>, <a href="#rfc.xref.Part1.9">11</a>, <a href="#Part1"><b>12.1</b></a>, <a href="#rfc.xref.Part1.10">B</a>, <a href="#rfc.xref.Part1.11">B</a>, <a href="#rfc.xref.Part1.12">B</a>, <a href="#rfc.xref.Part1.13">B</a>, <a href="#rfc.xref.Part1.14">B</a>, <a href="#rfc.xref.Part1.15">B</a>, <a href="#rfc.xref.Part1.16">B</a>, <a href="#rfc.xref.Part1.17">B</a><ul>
<li><em>Section 2.8</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.15">B</a>, <a href="#rfc.xref.Part1.17">B</a></li>
<li><em>Section 3.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.5">4.3</a>, <a href="#rfc.xref.Part1.12">B</a></li>
<li><em>Section 3.2.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.11">B</a></li>
<li><em>Section 3.2.4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.13">B</a>, <a href="#rfc.xref.Part1.14">B</a></li>
<li><em>Section 5.5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.4">4</a>, <a href="#rfc.xref.Part1.6">6</a>, <a href="#rfc.xref.Part1.7">6</a>, <a href="#rfc.xref.Part1.8">6</a></li>
<li><em>Section 6.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.16">B</a></li>
<li><em>Section 9</em>&nbsp;&nbsp;<a href="#rfc.xref.Part1.9">11</a></li>
<li><em>Part2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.1">2</a>, <a href="#rfc.xref.Part2.2">4</a>, <a href="#rfc.xref.Part2.3">4.1.2</a>, <a href="#rfc.xref.Part2.4">4.1.3</a>, <a href="#rfc.xref.Part2.5">6</a>, <a href="#rfc.xref.Part2.6">7.3</a>, <a href="#Part2"><b>12.1</b></a>, <a href="#rfc.xref.Part2.7">B</a><ul>
<li><em>Section 2.1.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.2">4</a>, <a href="#rfc.xref.Part2.5">6</a></li>
<li><em>Section 4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.3">4.1.2</a></li>
<li><em>Section 5.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.6">7.3</a>, <a href="#rfc.xref.Part2.7">B</a></li>
<li><em>Section 9.10</em>&nbsp;&nbsp;<a href="#rfc.xref.Part2.4">4.1.3</a></li>
<li><em>Part4</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.1">1.2</a>, <a href="#rfc.xref.Part4.2">1.2</a>, <a href="#rfc.xref.Part4.3">1.2</a>, <a href="#rfc.xref.Part4.4">4.1.2</a>, <a href="#rfc.xref.Part4.5">4.2</a>, <a href="#Part4"><b>12.1</b></a><ul>
<li><em>Section 2.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part4.4">4.1.2</a></li>
<li><em>Part5</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.1">3.1</a>, <a href="#rfc.xref.Part5.2">4.4</a>, <a href="#rfc.xref.Part5.3">4.4</a>, <a href="#Part5"><b>12.1</b></a><ul>
<li><em>Section 4.2</em>&nbsp;&nbsp;<a href="#rfc.xref.Part5.3">4.4</a></li>
<li><em>Part7</em>&nbsp;&nbsp;<a href="#rfc.xref.Part7.1">3</a>, <a href="#rfc.xref.Part7.2">3.2</a>, <a href="#Part7"><b>12.1</b></a><ul>
<li><em>Section 4.1</em>&nbsp;&nbsp;<a href="#rfc.xref.Part7.1">3</a>, <a href="#rfc.xref.Part7.2">3.2</a></li>
<li>Pragma header field&nbsp;&nbsp;<a href="#rfc.xref.header.pragma.1">4</a>, <a href="#rfc.xref.header.pragma.2">7.2</a>, <a href="#rfc.iref.p.5"><b>7.4</b></a>, <a href="#rfc.xref.header.pragma.3">9.3</a></li>
<li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.p.3"><b>7.2.2.2</b></a></li>
<li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.p.4"><b>7.2.2.6</b></a></li>
<li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.p.2"><b>7.2.2.1</b></a></li>
<li><em>RFC1305</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC1305.1">4</a>, <a href="#rfc.xref.RFC1305.2">4.1.3</a>, <a href="#RFC1305"><b>12.2</b></a></li>
<li><em>RFC2119</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2119.1">1.3</a>, <a href="#RFC2119"><b>12.1</b></a></li>
<li><em>RFC2616</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">4.1.2</a>, <a href="#RFC2616"><b>12.2</b></a><ul>
<li><em>Section 13.9</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC2616.1">4.1.2</a></li>
<li><em>RFC3864</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC3864.1">9.3</a>, <a href="#RFC3864"><b>12.2</b></a></li>
<li><em>RFC5226</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5226.1">7.2.3</a>, <a href="#rfc.xref.RFC5226.2">7.6.8</a>, <a href="#RFC5226"><b>12.2</b></a><ul>
<li><em>Section 4.1</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5226.1">7.2.3</a>, <a href="#rfc.xref.RFC5226.2">7.6.8</a></li>
<li><em>RFC5234</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5234.1">1.4</a>, <a href="#RFC5234"><b>12.1</b></a>, <a href="#rfc.xref.RFC5234.2">B</a><ul>
<li><em>RFC5861</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.1">9.1</a>, <a href="#rfc.xref.RFC5861.2">9.1</a>, <a href="#RFC5861"><b>12.2</b></a><ul>
<li><em>Section 3</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.2">9.1</a></li>
<li><em>Section 4</em>&nbsp;&nbsp;<a href="#rfc.xref.RFC5861.1">9.1</a></li>
<li>Cache Directive&nbsp;&nbsp;<a href="#rfc.iref.s.4"><b>7.2.2.8</b></a></li>
<li>Vary header field&nbsp;&nbsp;<a href="#rfc.xref.header.vary.1">4.3</a>, <a href="#rfc.iref.v.3"><b>7.5</b></a>, <a href="#rfc.xref.header.vary.2">9.3</a></li>
<li>110 Response is Stale&nbsp;&nbsp;<a href="#rfc.iref.w.2"><b>7.6.1</b></a>, <a href="#rfc.xref.warn.110.1">9.2</a></li>
<li>111 Revalidation Failed&nbsp;&nbsp;<a href="#rfc.iref.w.3"><b>7.6.2</b></a>, <a href="#rfc.xref.warn.111.1">9.2</a></li>
<li>112 Disconnected Operation&nbsp;&nbsp;<a href="#rfc.iref.w.4"><b>7.6.3</b></a>, <a href="#rfc.xref.warn.112.1">9.2</a></li>
<li>113 Heuristic Expiration&nbsp;&nbsp;<a href="#rfc.iref.w.5"><b>7.6.4</b></a>, <a href="#rfc.xref.warn.113.1">9.2</a></li>
<li>199 Miscellaneous Warning&nbsp;&nbsp;<a href="#rfc.iref.w.6"><b>7.6.5</b></a>, <a href="#rfc.xref.warn.199.1">9.2</a></li>
<li>214 Transformation Applied&nbsp;&nbsp;<a href="#rfc.iref.w.7"><b>7.6.6</b></a>, <a href="#rfc.xref.warn.214.1">9.2</a></li>
<li>299 Miscellaneous Persistent Warning&nbsp;&nbsp;<a href="#rfc.iref.w.8"><b>7.6.7</b></a>, <a href="#rfc.xref.warn.299.1">9.2</a></li>
<li>Warning header field&nbsp;&nbsp;<a href="#rfc.xref.header.warning.1">4.1.4</a>, <a href="#rfc.xref.header.warning.2">4.2.1</a>, <a href="#rfc.xref.header.warning.3">4.4</a>, <a href="#rfc.xref.header.warning.4">5</a>, <a href="#rfc.iref.w.1"><b>7.6</b></a>, <a href="#rfc.xref.header.warning.5">9.3</a>, <a href="#rfc.xref.header.warning.6">A</a></li>