Source: https://trac.ietf.org/trac/httpbis/browser/draft-ietf-httpbis/latest/p6-cache.html?rev=916&order=name
Timestamp: 2020-08-07 04:52:25
Document Index: 507031017

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

source: draft-ietf-httpbis/latest/p6-cache.html @ 916
Last change on this file since 916 was 916, checked in by julian.reschke@…, 10 years ago
Note change in [914] relating to issue 223 (see #223)
<meta name="dct.issued" scheme="ISO8601" content="2010-07-24">
<meta name="description" content="The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. This document is Part 6 of the seven-part specification that defines the protocol referred to as &#34;HTTP/1.1&#34; and, taken together, obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.">
<td class="left">Expires: January 25, 2011</td>
<td class="right">July 24, 2010</td>
systems. This document is Part 6 of the seven-part specification that defines the protocol referred to as "HTTP/1.1" and,
taken together, obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header fields that control
cache behavior or indicate cacheable response messages.
<p>The changes in this draft are summarized in <a href="#changes.since.10" title="Since draft-ietf-httpbis-p6-cache-10">Appendix&nbsp;C.12</a>.
<p>This Internet-Draft will expire in January 25, 2011.</p>
<li class="tocline1">1.4&nbsp;&nbsp;&nbsp;<a href="#notation">Syntax Notation</a><ul class="toc">
<li class="tocline1">1.4.1&nbsp;&nbsp;&nbsp;<a href="#core.rules">Core Rules</a></li>
<li class="tocline1">1.4.2&nbsp;&nbsp;&nbsp;<a href="#abnf.dependencies">ABNF Rules defined in other Parts of the Specification</a></li>
<li class="tocline0">2.&nbsp;&nbsp;&nbsp;<a href="#caching.overview">Cache Operation</a><ul class="toc">
<li class="tocline1">2.1&nbsp;&nbsp;&nbsp;<a href="#response.cacheability">Response Cacheability</a><ul class="toc">
<li class="tocline1">2.1.1&nbsp;&nbsp;&nbsp;<a href="#errors.or.incomplete.response.cache.behavior">Storing Partial and Incomplete Responses</a></li>
<li class="tocline1">2.2&nbsp;&nbsp;&nbsp;<a href="#constructing.responses.from.caches">Constructing Responses from Caches</a></li>
<li class="tocline1">2.3&nbsp;&nbsp;&nbsp;<a href="#expiration.model">Freshness Model</a><ul class="toc">
<li class="tocline1">2.3.1&nbsp;&nbsp;&nbsp;<a href="#calculating.freshness.lifetime">Calculating Freshness Lifetime</a><ul class="toc">
<li class="tocline1">2.3.1.1&nbsp;&nbsp;&nbsp;<a href="#heuristic.freshness">Calculating Heuristic Freshness</a></li>
<li class="tocline1">2.3.2&nbsp;&nbsp;&nbsp;<a href="#age.calculations">Calculating Age</a></li>
<li class="tocline1">2.3.3&nbsp;&nbsp;&nbsp;<a href="#serving.stale.responses">Serving Stale Responses</a></li>
<li class="tocline1">2.4&nbsp;&nbsp;&nbsp;<a href="#validation.model">Validation Model</a></li>
<li class="tocline1">2.5&nbsp;&nbsp;&nbsp;<a href="#invalidation.after.updates.or.deletions">Request Methods that Invalidate</a></li>
<li class="tocline1">2.6&nbsp;&nbsp;&nbsp;<a href="#caching.authenticated.responses">Shared Caching of Authenticated Responses</a></li>
<li class="tocline1">2.7&nbsp;&nbsp;&nbsp;<a href="#caching.negotiated.responses">Caching Negotiated Responses</a></li>
<li class="tocline1">2.8&nbsp;&nbsp;&nbsp;<a href="#combining.headers">Combining Responses</a></li>
<li class="tocline0">3.&nbsp;&nbsp;&nbsp;<a href="#header.fields">Header Field Definitions</a><ul class="toc">
<li class="tocline1">3.1&nbsp;&nbsp;&nbsp;<a href="#header.age">Age</a></li>
<li class="tocline1">3.2&nbsp;&nbsp;&nbsp;<a href="#header.cache-control">Cache-Control</a><ul class="toc">
<li class="tocline1">3.2.1&nbsp;&nbsp;&nbsp;<a href="#cache-request-directive">Request Cache-Control Directives</a></li>
<li class="tocline1">3.2.2&nbsp;&nbsp;&nbsp;<a href="#cache-response-directive">Response Cache-Control Directives</a></li>
<li class="tocline1">3.2.3&nbsp;&nbsp;&nbsp;<a href="#cache.control.extensions">Cache Control Extensions</a></li>
<li class="tocline1">3.3&nbsp;&nbsp;&nbsp;<a href="#header.expires">Expires</a></li>
<li class="tocline1">3.4&nbsp;&nbsp;&nbsp;<a href="#header.pragma">Pragma</a></li>
<li class="tocline1">3.5&nbsp;&nbsp;&nbsp;<a href="#header.vary">Vary</a></li>
<li class="tocline1">3.6&nbsp;&nbsp;&nbsp;<a href="#header.warning">Warning</a></li>
<li class="tocline0">4.&nbsp;&nbsp;&nbsp;<a href="#history.lists">History Lists</a></li>
<li class="tocline0">6.&nbsp;&nbsp;&nbsp;<a href="#security.considerations">Security Considerations</a></li>
<li class="tocline0">A.&nbsp;&nbsp;&nbsp;<a href="#changes.from.rfc.2616">Changes from RFC 2616</a></li>
<li class="tocline0">B.&nbsp;&nbsp;&nbsp;<a href="#collected.abnf">Collected ABNF</a></li>
<li class="tocline0">C.&nbsp;&nbsp;&nbsp;<a href="#change.log">Change Log (to be removed by RFC Editor before publication)</a><ul class="toc">
<li class="tocline1">C.1&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.1">Since RFC2616</a></li>
<li class="tocline1">C.2&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.2">Since draft-ietf-httpbis-p6-cache-00</a></li>
<li class="tocline1">C.3&nbsp;&nbsp;&nbsp;<a href="#rfc.section.C.3">Since draft-ietf-httpbis-p6-cache-01</a></li>
<li class="tocline1">C.4&nbsp;&nbsp;&nbsp;<a href="#changes.since.02">Since draft-ietf-httpbis-p6-cache-02</a></li>
<li class="tocline1">C.5&nbsp;&nbsp;&nbsp;<a href="#changes.since.03">Since draft-ietf-httpbis-p6-cache-03</a></li>
<li class="tocline1">C.6&nbsp;&nbsp;&nbsp;<a href="#changes.since.04">Since draft-ietf-httpbis-p6-cache-04</a></li>
<li class="tocline1">C.7&nbsp;&nbsp;&nbsp;<a href="#changes.since.05">Since draft-ietf-httpbis-p6-cache-05</a></li>
<li class="tocline1">C.8&nbsp;&nbsp;&nbsp;<a href="#changes.since.06">Since draft-ietf-httpbis-p6-cache-06</a></li>
<li class="tocline1">C.9&nbsp;&nbsp;&nbsp;<a href="#changes.since.07">Since draft-ietf-httpbis-p6-cache-07</a></li>
<li class="tocline1">C.10&nbsp;&nbsp;&nbsp;<a href="#changes.since.08">Since draft-ietf-httpbis-p6-cache-08</a></li>
<li class="tocline1">C.11&nbsp;&nbsp;&nbsp;<a href="#changes.since.09">Since draft-ietf-httpbis-p6-cache-09</a></li>
<li class="tocline1">C.12&nbsp;&nbsp;&nbsp;<a href="#changes.since.10">Since draft-ietf-httpbis-p6-cache-10</a></li>
response message to satisfy a current request. In some cases, a stored response can be reused without the need for a network
request, reducing latency and network round-trips; a "freshness" mechanism is used for this purpose (see <a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a>). Even when a new request is required, it is often possible to reuse all or parts of the payload of a prior response to satisfy
the request, thereby reducing network bandwidth usage; a "validation" mechanism is used for this purpose (see <a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>).
Even when a response is cacheable, there might be additional constraints on whether a cache can use the cached copy to satisfy
<p id="rfc.section.1.2.p.3"> <span id="rfc.iref.e.1"></span> <dfn>explicit expiration time</dfn>
<li>The time at which the origin server intends that a representation should no longer be returned by a cache without further
<p id="rfc.section.1.2.p.4"> <span id="rfc.iref.h.1"></span> <dfn>heuristic expiration time</dfn>
<p id="rfc.section.1.2.p.5"> <span id="rfc.iref.a.1"></span> <dfn>age</dfn>
<p id="rfc.section.1.2.p.6"> <span id="rfc.iref.f.1"></span> <dfn>first-hand</dfn>
<p id="rfc.section.1.2.p.9"> <span id="rfc.iref.s.1"></span> <dfn>stale</dfn>
<li>A protocol element (e.g., an entity-tag or a Last-Modified time) that is used to find out whether a stored response has an
equivalent copy of a representation.
<p id="rfc.section.1.2.p.11"> <span id="rfc.iref.v.2"></span> <dfn>shared cache</dfn>
<li>A cache that is accessible to more than one user. A non-shared cache is dedicated to a single user.</li>
<p id="rfc.section.1.3.p.2">An implementation is not compliant if it fails to satisfy one or more of the "MUST" or "REQUIRED" level requirements for the
<p id="rfc.section.1.4.p.1">This specification uses the ABNF syntax defined in <a href="p1-messaging.html#notation" title="Syntax Notation">Section 1.2</a> of <a href="#Part1" id="rfc.xref.Part1.1"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a> (which extends the syntax defined in <a href="#RFC5234" id="rfc.xref.RFC5234.1"><cite title="Augmented BNF for Syntax Specifications: ABNF">[RFC5234]</cite></a> with a list rule). <a href="#collected.abnf" title="Collected ABNF">Appendix&nbsp;B</a> shows the collected ABNF, with the list rule expanded.
<p id="rfc.section.1.4.1.p.1">The core rules below are defined in <a href="p1-messaging.html#basic.rules" title="Basic Rules">Section 1.2.2</a> of <a href="#Part1" id="rfc.xref.Part1.2"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>:
<div id="rfc.figure.u.2"></div><pre class="inline"> <a href="#abnf.dependencies" class="smpl">field-name</a> = &lt;field-name, defined in <a href="#Part1" id="rfc.xref.Part1.6"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#header.fields" title="Header Fields">Section 3.2</a>&gt;
<a href="#abnf.dependencies" class="smpl">HTTP-date</a> = &lt;HTTP-date, defined in <a href="#Part1" id="rfc.xref.Part1.7"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#date.time.formats.full.date" title="Date/Time Formats: Full Date">Section 6.1</a>&gt;
<a href="#abnf.dependencies" class="smpl">port</a> = &lt;port, defined in <a href="#Part1" id="rfc.xref.Part1.8"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.6</a>&gt;
<a href="#abnf.dependencies" class="smpl">pseudonym</a> = &lt;pseudonym, defined in <a href="#Part1" id="rfc.xref.Part1.9"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#header.via" title="Via">Section 9.9</a>&gt;
<a href="#abnf.dependencies" class="smpl">uri-host</a> = &lt;uri-host, defined in <a href="#Part1" id="rfc.xref.Part1.10"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>, <a href="p1-messaging.html#uri" title="Uniform Resource Identifiers">Section 2.6</a>&gt;
</pre><h1 id="rfc.section.2"><a href="#rfc.section.2">2.</a>&nbsp;<a id="caching.overview" href="#caching.overview">Cache Operation</a></h1>
<li>the "private" cache response directive (see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a> does not appear in the response, if the cache is shared, and
<li>the "Authorization" header (see <a href="p7-auth.html#header.authorization" title="Authorization">Section 3.1</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="Shared Caching of Authenticated Responses">Section&nbsp;2.6</a>), and
<li>contains an Expires header (see <a href="#header.expires" id="rfc.xref.header.expires.1" title="Expires">Section&nbsp;3.3</a>), or
<p id="rfc.section.2.1.p.2">In this context, a cache has "understood" a request method or a response status code if it recognises it and implements any
cache-specific behaviour. In particular, 206 Partial Content responses cannot be cached by an implementation that does not
handle partial content (see <a href="#errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">Section&nbsp;2.1.1</a>).
<p id="rfc.section.2.1.p.3">Note that in normal operation, most caches will not store a response that has neither a cache validator nor an explicit expiration
<h3 id="rfc.section.2.1.1"><a href="#rfc.section.2.1.1">2.1.1</a>&nbsp;<a id="errors.or.incomplete.response.cache.behavior" href="#errors.or.incomplete.response.cache.behavior">Storing Partial and Incomplete Responses</a></h3>
<p id="rfc.section.2.1.1.p.1">A cache that receives an incomplete response (for example, with fewer bytes of data than specified in a Content-Length header)
can store the response, but <em class="bcp14">MUST</em> treat it as a partial response <a href="#Part5" id="rfc.xref.Part5.1"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>. Partial responses can be combined as described in <a href="p5-range.html#combining.byte.ranges" title="Combining Ranges">Section 4</a> of <a href="#Part5" id="rfc.xref.Part5.2"><cite title="HTTP/1.1, part 5: Range Requests and Partial Responses">[Part5]</cite></a>; the result might be a full response or might still be partial. A cache <em class="bcp14">MUST NOT</em> return a partial response to a client without explicitly marking it as such using the 206 (Partial Content) status code.
<p id="rfc.section.2.1.1.p.2">A cache that does not support the Range and Content-Range headers <em class="bcp14">MUST NOT</em> store incomplete or partial responses.
<li>The presented Effective Request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 4.3</a> of <a href="#Part1" id="rfc.xref.Part1.11"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>) and that of the stored response match, and
<li>selecting request-headers nominated by the stored response (if any) match those presented (see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.7</a>), and
<li>the presented request and stored response are free from directives that would prevent its use (see <a href="#header.cache-control" id="rfc.xref.header.cache-control.2" title="Cache-Control">Section&nbsp;3.2</a> and <a href="#header.pragma" id="rfc.xref.header.pragma.1" title="Pragma">Section&nbsp;3.4</a>), and
<p id="rfc.section.2.2.p.2">When a stored response is used to satisfy a request, caches <em class="bcp14">MUST</em> include a single Age header field (<a href="#header.age" id="rfc.xref.header.age.1" title="Age">Section&nbsp;3.1</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;2.3.2</a>. <span class="comment" id="DISCUSS-includes-validated">[<a href="#DISCUSS-includes-validated" class="smpl">DISCUSS-includes-validated</a>: this currently includes successfully validated responses.]</span>
<p id="rfc.section.2.2.p.3">Requests with methods that are unsafe (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 7.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.1"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) <em class="bcp14">MUST</em> be written through the cache to the origin server; i.e., a cache must not reply to such a request before having forwarded
the request and having received a corresponding response.
<p id="rfc.section.2.2.p.4">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;2.5</a>.
<p id="rfc.section.2.2.p.5">Caches <em class="bcp14">MUST</em> use the most recent response (as determined by the Date header) when more than one suitable response is stored. They can also
forward a request with "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to use.
using either the Expires header (<a href="#header.expires" id="rfc.xref.header.expires.2" title="Expires">Section&nbsp;3.3</a>) or the max-age response cache directive (<a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>). Generally, origin servers will assign future explicit expiration times to responses in the belief that the representation
<p id="rfc.section.2.3.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.
This means that the response is always stale, so that caches should validate it before using it for subsequent requests. <span class="comment" id="TODO-response-stale">[<a href="#TODO-response-stale" class="smpl">TODO-response-stale</a>: This wording might cause confusion, because the response might still be served stale.]</span>
<p id="rfc.section.2.3.p.4">Since origin servers do not always provide explicit expiration times, HTTP caches <em class="bcp14">MAY</em> assign heuristic expiration times when explicit times are not specified, employing algorithms that use other header values
(such as the Last-Modified time) to estimate a plausible expiration time. The HTTP/1.1 specification does not provide specific
<div id="rfc.figure.u.3"></div>
<p id="rfc.section.2.3.p.7">Additionally, clients might need to influence freshness calculation. They can do this using several request cache directives,
with the effect of either increasing or loosening constraints on freshness. See <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>.
<p id="rfc.section.2.3.p.8"> <span class="comment" id="ISSUE-no-req-for-directives">[<a href="#ISSUE-no-req-for-directives" class="smpl">ISSUE-no-req-for-directives</a>: there are not requirements directly applying to cache-request-directives and freshness.]</span>
<p id="rfc.section.2.3.p.9">Note that freshness applies only to cache operation; it cannot be used to force a user agent to refresh its display or reload
<li>If the Expires response header (<a href="#header.expires" id="rfc.xref.header.expires.3" title="Expires">Section&nbsp;3.3</a>) is present, use its value minus the value of the Date response header, or
to be used (including the following in <a href="p2-semantics.html#status.codes" title="Status Code Definitions">Section 8</a> of <a href="#Part2" id="rfc.xref.Part2.2"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>: 200, 203, 206, 300, 301 and 410), a heuristic expiration time <em class="bcp14">MAY</em> be calculated. Heuristics <em class="bcp14">MUST NOT</em> be used for response status codes that do not explicitly allow it.
<p id="rfc.section.2.3.1.1.p.2">When a heuristic is used to calculate freshness lifetime, the cache <em class="bcp14">SHOULD</em> attach a Warning header with a 113 warn-code to the response if its current_age is more than 24 hours and such a warning is
not already present.
<p id="rfc.section.2.3.1.1.p.3">Also, if the response has a Last-Modified header (<a href="p4-conditional.html#header.last-modified" title="Last-Modified">Section 6.6</a> of <a href="#Part4" id="rfc.xref.Part4.1"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>), the heuristic expiration value <em class="bcp14">SHOULD</em> be no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.
<p> <b>Note:</b> RFC 2616 (<a href="#RFC2616" id="rfc.xref.RFC2616.1"><cite title="Hypertext Transfer Protocol -- HTTP/1.1">[RFC2616]</cite></a>, <a href="http://tools.ietf.org/html/rfc2616#section-13.9">Section 13.9</a>) required that caches do not calculate heuristic freshness for URLs with query components (i.e., those containing '?'). In
<p id="rfc.section.2.3.2.p.1">HTTP/1.1 uses the Age response-header 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.
<li>The term "age_value" denotes the value of the Age header (<a href="#header.age" id="rfc.xref.header.age.2" title="Age">Section&nbsp;3.1</a>), in a form appropriate for arithmetic operation; or 0, if not available.
<li>HTTP/1.1 requires origin servers to send a Date header, 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, in a form appropriate for arithmetic operations.
See <a href="p1-messaging.html#header.date" title="Date">Section 9.3</a> of <a href="#Part1" id="rfc.xref.Part1.12"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a> for the definition of the Date header, and for requirements regarding responses without a Date response header.
<li>The term "now" means "the current value of the clock at the host performing the calculation". Hosts that use HTTP, but especially
hosts running origin servers and caches, <em class="bcp14">SHOULD</em> use NTP (<a href="#RFC1305" id="rfc.xref.RFC1305.1"><cite title="Network Time Protocol (Version 3) Specification, Implementation">[RFC1305]</cite></a>) or some similar protocol to synchronize their clocks to a globally accurate time standard.
<li>the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1; note this value <em class="bcp14">MUST</em> be interpreted relative to the time the request was initiated, not the time that the response was received.
<div id="rfc.figure.u.4"></div> <pre class="text"> apparent_age = max(0, response_time - date_value);
</pre> <div id="rfc.figure.u.5"></div>
<p>These are combined as</p> <pre class="text"> corrected_initial_age = max(apparent_age, corrected_age_value);
</pre><p id="rfc.section.2.3.2.p.11">The current_age of a stored response can then be calculated by adding the amount of time (in seconds) since the stored response
<div id="rfc.figure.u.6"></div><pre class="text"> resident_time = now - response_time;
</pre><h3 id="rfc.section.2.3.3"><a href="#rfc.section.2.3.3">2.3.3</a>&nbsp;<a id="serving.stale.responses" href="#serving.stale.responses">Serving Stale Responses</a></h3>
<p id="rfc.section.2.3.3.p.2">Caches <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
<p id="rfc.section.2.3.3.p.3">Caches <em class="bcp14">SHOULD NOT</em> return stale responses unless they are disconnected (i.e., it cannot contact the origin server or otherwise find a forward
path) or otherwise explicitly allowed (e.g., the max-stale request directive; see <a href="#cache-request-directive" title="Request Cache-Control Directives">Section&nbsp;3.2.1</a>).
<p id="rfc.section.2.3.3.p.4">Stale responses <em class="bcp14">SHOULD</em> have a Warning header with the 110 warn-code (see <a href="#header.warning" id="rfc.xref.header.warning.1" title="Warning">Section&nbsp;3.6</a>). Likewise, the 112 warn-code <em class="bcp14">SHOULD</em> be sent on stale responses if the cache is disconnected.
forward to the requesting client, and the received response is no longer fresh, the cache <em class="bcp14">SHOULD</em> forward it to the requesting client without adding a new Warning (but without removing any existing Warning headers). A cache <em class="bcp14">SHOULD NOT</em> attempt to validate a response simply because that response became stale in transit.
fresh, or one cannot be selected; see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.7</a>), it can use the conditional request mechanism <a href="#Part4" id="rfc.xref.Part4.2"><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.2.4.p.2">When sending such a conditional request, the cache <em class="bcp14">SHOULD</em> add an If-Modified-Since header whose value is that of the Last-Modified header from the selected (see <a href="#caching.negotiated.responses" title="Caching Negotiated Responses">Section&nbsp;2.7</a>) stored response, if available.
<p id="rfc.section.2.4.p.3">Additionally, the cache <em class="bcp14">SHOULD</em> add an If-None-Match header whose value is that of the ETag header(s) from all responses stored for the requested URI, if
present. However, if any of the stored responses contains only partial content, its entity-tag <em class="bcp14">SHOULD NOT</em> be included in the If-None-Match header field unless the request is for a range that would be fully satisfied by that stored
<p id="rfc.section.2.4.p.4">A 304 (Not Modified) response status code indicates that the stored response can be updated and reused; see <a href="#combining.headers" title="Combining Responses">Section&nbsp;2.8</a>.
<p id="rfc.section.2.5.p.1">Because unsafe methods (<a href="p2-semantics.html#safe.methods" title="Safe Methods">Section 7.1.1</a> of <a href="#Part2" id="rfc.xref.Part2.3"><cite title="HTTP/1.1, part 2: Message Semantics">[Part2]</cite></a>) have the potential for changing state on the origin server, intervening caches can use them to keep their contents up-to-date.
<p id="rfc.section.2.5.p.2">The following HTTP methods <em class="bcp14">MUST</em> cause a cache to invalidate the Effective Request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 4.3</a> of <a href="#Part1" id="rfc.xref.Part1.13"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>) as well as the URI(s) in the Location and Content-Location headers (if present):
<p id="rfc.section.2.5.p.3">An invalidation based on a URI from a Location or Content-Location header <em class="bcp14">MUST NOT</em> be performed 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 4.3</a> of <a href="#Part1" id="rfc.xref.Part1.14"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>). This helps prevent denial of service attacks.
<p id="rfc.section.2.5.p.4"> <span class="comment" id="TODO-def-host-part">[<a href="#TODO-def-host-part" class="smpl">TODO-def-host-part</a>: "host part" needs to be specified better.]</span>
<p id="rfc.section.2.5.p.5">A cache that passes through requests for methods it does not understand <em class="bcp14">SHOULD</em> invalidate the Effective Request URI (<a href="p1-messaging.html#effective.request.uri" title="Effective Request URI">Section 4.3</a> of <a href="#Part1" id="rfc.xref.Part1.15"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>).
<p id="rfc.section.2.5.p.6">Here, "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 request.
<p id="rfc.section.2.5.p.7">Note that this does not guarantee that all appropriate responses are invalidated. For example, the request that caused the
<p id="rfc.section.2.5.p.8"> <span class="comment" id="TODO-spec-success-invalidate">[<a href="#TODO-spec-success-invalidate" class="smpl">TODO-spec-success-invalidate</a>: specify that only successful (2xx, 3xx?) responses invalidate.]</span>
<p id="rfc.section.2.6.p.1">Shared caches <em class="bcp14">MUST NOT</em> use a cached response to a request with an Authorization header (<a href="p7-auth.html#header.authorization" title="Authorization">Section 3.1</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.2.7.p.1">When a cache receives a request that can be satisfied by a stored response that has a Vary header field (<a href="#header.vary" id="rfc.xref.header.vary.1" title="Vary">Section&nbsp;3.5</a>), it <em class="bcp14">MUST NOT</em> use that response unless all of the selecting request-headers nominated by the Vary header match in both the original request
(i.e., that associated with the stored response), and the presented request.
<p id="rfc.section.2.7.p.2">The selecting request-headers from two requests are defined to match if and only if those in the first request can be transformed
<li>adding or removing whitespace, where allowed in the header's syntax</li>
<li>combining multiple message-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.Part1.16"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>)
<li>normalizing both header values in a way that is known to have identical semantics, according to the header's specification
<p id="rfc.section.2.7.p.5">The stored response with matching selecting request-headers is known as the selected response.</p>
<p id="rfc.section.2.7.p.6">If no selected response is available, the cache <em class="bcp14">MAY</em> forward the presented request to the origin server in a conditional request; see <a href="#validation.model" title="Validation Model">Section&nbsp;2.4</a>.
<h2 id="rfc.section.2.8"><a href="#rfc.section.2.8">2.8</a>&nbsp;<a id="combining.headers" href="#combining.headers">Combining Responses</a></h2>
<p id="rfc.section.2.8.p.1">When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response (in this section, the "new" response"),
it needs to created an updated response by combining the stored response with the new one, so that the updated response can
be used to satisfy the request.
<p id="rfc.section.2.8.p.2">If the new response contains an ETag, it identifies the stored response to use. <span class="comment" id="TODO-mention-CL">[<a href="#TODO-mention-CL" class="smpl">TODO-mention-CL</a>: might need language about Content-Location here]</span><span class="comment" id="TODO-inm-mult-etags">[<a href="#TODO-inm-mult-etags" class="smpl">TODO-inm-mult-etags</a>: cover case where INM with multiple etags was sent]</span>
<p id="rfc.section.2.8.p.3">If the status code is 206 (partial content), both the stored and new responses <em class="bcp14">MUST</em> have validators, and those validators <em class="bcp14">MUST</em> match using the strong comparison function (see <a href="p4-conditional.html#weak.and.strong.validators" title="Weak and Strong Validators">Section 4</a> of <a href="#Part4" id="rfc.xref.Part4.3"><cite title="HTTP/1.1, part 4: Conditional Requests">[Part4]</cite></a>). Otherwise, the responses <em class="bcp14">MUST NOT</em> be combined.
<p id="rfc.section.2.8.p.4">The stored response headers are used as those of the updated response, except that </p>
<li>any stored Warning headers with warn-code 1xx (see <a href="#header.warning" id="rfc.xref.header.warning.2" title="Warning">Section&nbsp;3.6</a>) <em class="bcp14">MUST</em> be deleted from the stored response and the updated response.
<li>any stored Warning headers with warn-code 2xx <em class="bcp14">MUST</em> be retained in the stored response and the updated response.
<li>any headers provided in the new response <em class="bcp14">MUST</em> replace the corresponding headers from the stored response.
<p id="rfc.section.2.8.p.5">If a header field-name in the new response matches more than one header in the stored response, all such stored headers <em class="bcp14">MUST</em> be replaced.
<p id="rfc.section.2.8.p.6">The updated response can <span class="comment" id="TODO-is-req">[<a href="#TODO-is-req" class="smpl">TODO-is-req</a>: requirement?]</span> be used to replace the stored response in cache. In the case of a 206 response, the combined representation <em class="bcp14">MAY</em> be stored.
<h1 id="rfc.section.3"><a href="#rfc.section.3">3.</a>&nbsp;<a id="header.fields" href="#header.fields">Header Field Definitions</a></h1>
<p id="rfc.section.3.p.2">For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who
receives the message.
<p id="rfc.section.3.1.p.1">The "Age" response-header field conveys the sender's estimate of the amount of time since the response was generated or successfully
<div id="rfc.figure.u.7"></div><pre class="inline"><span id="rfc.iref.g.1"></span><span id="rfc.iref.g.2"></span> <a href="#header.age" class="smpl">Age</a> = "Age" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.age" class="smpl">Age-v</a>
<a href="#header.age" class="smpl">Age-v</a> = <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
<p id="rfc.section.3.1.p.3"> Age field-values are non-negative integers, representing time in seconds.</p>
<div id="rfc.figure.u.8"></div><pre class="inline"><span id="rfc.iref.g.3"></span> <a href="#rule.delta-seconds" class="smpl">delta-seconds</a> = 1*<a href="#notation" class="smpl">DIGIT</a>
</pre><p id="rfc.section.3.1.p.5">If a cache receives a value larger than the largest positive integer it can represent, or if any of its age calculations overflows,
it <em class="bcp14">MUST</em> transmit an Age header with a field-value of 2147483648 (2<sup>31</sup>). Caches <em class="bcp14">SHOULD</em> use an arithmetic type of at least 31 bits of range.
<p id="rfc.section.3.1.p.6">The presence of an Age header field in a response implies that a response is not first-hand. However, the converse is not
<p id="rfc.section.3.2.p.1">The "Cache-Control" general-header field is used to specify directives for caches along the request/response chain. Such cache
directives are unidirectional in that the presence of a directive in a request does not imply that the same directive is to
be given in the response.
<p id="rfc.section.3.2.p.2">HTTP/1.1 caches <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;3.2.3</a> for information about how Cache-Control directives defined elsewhere are handled.
<p id="rfc.section.3.2.p.4">Cache directives <em class="bcp14">MUST</em> be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives
<div id="rfc.figure.u.9"></div><pre class="inline"><span id="rfc.iref.g.4"></span><span id="rfc.iref.g.5"></span><span id="rfc.iref.g.6"></span> <a href="#header.cache-control" class="smpl">Cache-Control</a> = "Cache-Control" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.cache-control" class="smpl">Cache-Control-v</a>
<a href="#header.cache-control" class="smpl">Cache-Control-v</a> = 1#<a href="#header.cache-control" class="smpl">cache-directive</a>
<div id="rfc.figure.u.10"></div><pre class="inline"><span id="rfc.iref.g.7"></span> <a href="#header.cache-control" class="smpl">cache-request-directive</a> =
/ "max-age" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
/ "max-stale" [ "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a> ]
/ "min-fresh" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
</pre><p id="rfc.section.3.2.1.p.2"> <dfn>no-cache</dfn> <span id="rfc.iref.c.4"></span> <span id="rfc.iref.n.1"></span>
<li>The no-cache request directive indicates that a stored response <em class="bcp14">MUST NOT</em> be used to satisfy the request without successful validation on the origin server.
<p id="rfc.section.3.2.1.p.3"> <dfn>no-store</dfn> <span id="rfc.iref.c.5"></span> <span id="rfc.iref.n.2"></span>
<li>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 non-shared 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.3.2.1.p.4"> <dfn>max-age</dfn> <span id="rfc.iref.c.6"></span> <span id="rfc.iref.m.1"></span>
<li>The max-age request directive indicates that the client is willing to accept a response whose age is no greater than the specified
time in seconds. Unless the max-stale request directive is also present, the client is not willing to accept a stale response.
<p id="rfc.section.3.2.1.p.5"> <dfn>max-stale</dfn> <span id="rfc.iref.c.7"></span> <span id="rfc.iref.m.2"></span>
a stale response of any age. <span class="comment" id="TODO-staleness">[<a href="#TODO-staleness" class="smpl">TODO-staleness</a>: of any staleness? --mnot]</span></li>
<p id="rfc.section.3.2.1.p.6"> <dfn>min-fresh</dfn> <span id="rfc.iref.c.8"></span> <span id="rfc.iref.m.3"></span>
<p id="rfc.section.3.2.1.p.7"> <dfn>no-transform</dfn> <span id="rfc.iref.c.9"></span> <span id="rfc.iref.n.3"></span>
<li>The no-transform request directive indicates that an intermediate cache or proxy <em class="bcp14">MUST NOT</em> change the Content-Encoding, Content-Range or Content-Type request headers, nor the request representation.
<p id="rfc.section.3.2.1.p.8"> <dfn>only-if-cached</dfn> <span id="rfc.iref.c.10"></span> <span id="rfc.iref.o.1"></span>
<li>The only-if-cached request directive indicates that the client only wishes to return a stored response. If it receives this
(Gateway Timeout) status. If a group of caches is being operated as a unified system with good internal connectivity, such
<div id="rfc.figure.u.11"></div><pre class="inline"><span id="rfc.iref.g.8"></span> <a href="#header.cache-control" class="smpl">cache-response-directive</a> =
/ "s-maxage" "=" <a href="#rule.delta-seconds" class="smpl">delta-seconds</a>
</pre><p id="rfc.section.3.2.2.p.2"> <dfn>public</dfn> <span id="rfc.iref.c.11"></span> <span id="rfc.iref.p.1"></span>
<li>The public response directive indicates that the response <em class="bcp14">MAY</em> be cached, even if it would normally be non-cacheable or cacheable only within a non-shared cache. (See also Authorization, <a href="p7-auth.html#header.authorization" title="Authorization">Section 3.1</a> of <a href="#Part7" id="rfc.xref.Part7.3"><cite title="HTTP/1.1, part 7: Authentication">[Part7]</cite></a>, for additional details.)
<p id="rfc.section.3.2.2.p.3"> <dfn>private</dfn> <span id="rfc.iref.c.12"></span> <span id="rfc.iref.p.2"></span>
<li>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 (non-shared) cache <em class="bcp14">MAY</em> store the response.
with the listed response headers. That is, the specified field-names(s) <em class="bcp14">MUST NOT</em> be stored by a shared cache, whereas the remainder of the response message <em class="bcp14">MAY</em> be.
<p id="rfc.section.3.2.2.p.4"> <dfn>no-cache</dfn> <span id="rfc.iref.c.13"></span> <span id="rfc.iref.n.4"></span>
with the listed response headers. That is, the specified field-name(s) <em class="bcp14">MUST NOT</em> be sent in the response to a subsequent request without successful validation on the origin server. This allows an origin
<p id="rfc.section.3.2.2.p.5"> <dfn>no-store</dfn> <span id="rfc.iref.c.14"></span> <span id="rfc.iref.n.5"></span>
<li>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 non-shared 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.3.2.2.p.6"> <dfn>must-revalidate</dfn> <span id="rfc.iref.c.15"></span> <span id="rfc.iref.m.4"></span>
<li>The must-revalidate response directive indicates that once it has become stale, the response <em class="bcp14">MUST NOT</em> be used to satisfy subsequent requests without successful validation on the origin server.
<li>Servers <em class="bcp14">SHOULD</em> send the must-revalidate directive if and only if failure to validate a request on the representation could result in incorrect
operation, such as a silently unexecuted financial transaction.
<p id="rfc.section.3.2.2.p.7"> <dfn>proxy-revalidate</dfn> <span id="rfc.iref.c.16"></span> <span id="rfc.iref.p.3"></span>
not apply to non-shared caches.
<p id="rfc.section.3.2.2.p.8"> <dfn>max-age</dfn> <span id="rfc.iref.c.17"></span> <span id="rfc.iref.m.5"></span>
<li>The max-age response directive indicates that response is to be considered stale after its age is greater than the specified
<p id="rfc.section.3.2.2.p.9"> <dfn>s-maxage</dfn> <span id="rfc.iref.c.18"></span> <span id="rfc.iref.s.2"></span>
maximum age specified by either the max-age directive or the Expires header. The s-maxage directive also implies the semantics
of the proxy-revalidate response directive.
<p id="rfc.section.3.2.2.p.10"> <dfn>no-transform</dfn> <span id="rfc.iref.c.19"></span> <span id="rfc.iref.n.6"></span>
<li>The no-transform response directive indicates that an intermediate cache or proxy <em class="bcp14">MUST NOT</em> change the Content-Encoding, Content-Range or Content-Type response headers, nor the response representation.
We define this new directive to mean that, in addition to any non-shared cache, any cache that is shared only by members of
the community named within its value may 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
<p id="rfc.section.3.2.3.p.6">Unrecognized cache directives <em class="bcp14">MUST</em> be ignored; it is assumed that any cache directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
<p id="rfc.section.3.2.3.p.8">Registrations <em class="bcp14">MUST</em> include the following fields:
<p id="rfc.section.3.3.p.1">The "Expires" entity-header field gives the date/time after which the response is considered stale. See <a href="#expiration.model" title="Freshness Model">Section&nbsp;2.3</a> for further discussion of the freshness model.
<p id="rfc.section.3.3.p.3">The field-value is an absolute date and time as defined by HTTP-date in <a href="p1-messaging.html#date.time.formats.full.date" title="Date/Time Formats: Full Date">Section 6.1</a> of <a href="#Part1" id="rfc.xref.Part1.17"><cite title="HTTP/1.1, part 1: URIs, Connections, and Message Parsing">[Part1]</cite></a>; it <em class="bcp14">MUST</em> be sent in rfc1123-date format.
<div id="rfc.figure.u.13"></div><pre class="inline"><span id="rfc.iref.g.9"></span><span id="rfc.iref.g.10"></span> <a href="#header.expires" class="smpl">Expires</a> = "Expires" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.expires" class="smpl">Expires-v</a>
<a href="#header.expires" class="smpl">Expires-v</a> = <a href="#abnf.dependencies" class="smpl">HTTP-date</a>
</pre><div class="note" id="rfc.section.3.3.p.6">
<p id="rfc.section.3.3.p.7">HTTP/1.1 servers <em class="bcp14">SHOULD NOT</em> send Expires dates more than one year in the future.
<p id="rfc.section.3.3.p.8">HTTP/1.1 clients and caches <em class="bcp14">MUST</em> treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").
<p id="rfc.section.3.4.p.1">The "Pragma" general-header field is used to include implementation-specific directives that might apply to any recipient
along the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however,
some systems <em class="bcp14">MAY</em> require that behavior be consistent with the directives.
<div id="rfc.figure.u.15"></div><pre class="inline"><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> <a href="#header.pragma" class="smpl">Pragma</a> = "Pragma" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.pragma" class="smpl">Pragma-v</a>
<a href="#header.pragma" class="smpl">Pragma-v</a> = 1#<a href="#header.pragma" class="smpl">pragma-directive</a>
</pre><p id="rfc.section.3.4.p.3">When the no-cache directive is present in a request message, an application <em class="bcp14">SHOULD</em> forward the request toward the origin server even if it has a cached copy of what is being requested. This pragma directive
has the same semantics as the no-cache response directive (see <a href="#cache-response-directive" title="Response Cache-Control Directives">Section&nbsp;3.2.2</a>) and is defined here for backward compatibility with HTTP/1.0. Clients <em class="bcp14">SHOULD</em> include both header fields when a no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches <em class="bcp14">SHOULD</em> treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
<div class="note" id="rfc.section.3.4.p.4">
<p> <b>Note:</b> Because the meaning of "Pragma: no-cache" as a response-header field is not actually specified, it does not provide a reliable
<p id="rfc.section.3.4.p.5">This mechanism is deprecated; no new Pragma directives will be defined in HTTP.</p>
<p id="rfc.section.3.5.p.1">The "Vary" response-header field conveys the set of request-header fields that were used to select the representation.</p>
<div id="rfc.figure.u.16"></div><pre class="inline"><span id="rfc.iref.g.15"></span><span id="rfc.iref.g.16"></span> <a href="#header.vary" class="smpl">Vary</a> = "Vary" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.vary" class="smpl">Vary-v</a>
<a href="#header.vary" class="smpl">Vary-v</a> = "*" / 1#<a href="#abnf.dependencies" class="smpl">field-name</a>
</pre><p id="rfc.section.3.5.p.5">The set of header fields named by the Vary field value is known as the selecting request-headers.</p>
<p id="rfc.section.3.5.p.6">Servers <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.3.5.p.7">A Vary field value of "*" signals that unspecified parameters not limited to the request-headers (e.g., the network address
response is appropriate. The "*" value <em class="bcp14">MUST NOT</em> be generated by a proxy server.
<p id="rfc.section.3.5.p.8">The field-names given are not limited to the set of standard request-header fields defined by this specification. Field names
<p id="rfc.section.3.6.p.1">The "Warning" general-header field is used to carry additional information about the status or transformation of a message
that might not be reflected in the message. This information is typically used to warn about possible incorrectness introduced
by caching operations or transformations applied to the payload of the message.
<p id="rfc.section.3.6.p.3">Warning headers can in general be applied to any message, however some warn-codes are specific to caches and can only be applied
to response messages.
<div id="rfc.figure.u.17"></div><pre class="inline"><span id="rfc.iref.g.17"></span><span id="rfc.iref.g.18"></span><span id="rfc.iref.g.19"></span><span id="rfc.iref.g.20"></span><span id="rfc.iref.g.21"></span><span id="rfc.iref.g.22"></span><span id="rfc.iref.g.23"></span> <a href="#header.warning" class="smpl">Warning</a> = "Warning" ":" <a href="#core.rules" class="smpl">OWS</a> <a href="#header.warning" class="smpl">Warning-v</a>
<a href="#header.warning" class="smpl">Warning-v</a> = 1#<a href="#header.warning" class="smpl">warning-value</a>
<p id="rfc.section.3.6.p.7">Systems that generate multiple Warning headers <em class="bcp14">SHOULD</em> order them with this user agent behavior in mind. New Warning headers <em class="bcp14">SHOULD</em> be added after any existing Warning headers.
<li>1xx Warnings describe the freshness or validation status of the response, and so <em class="bcp14">MUST</em> be deleted by caches after validation. They can only be generated by a cache when validating a cached entry, and <em class="bcp14">MUST NOT</em> be generated in any other situation.
of the representation) and <em class="bcp14">MUST NOT</em> be deleted by caches after validation, unless a full response is returned, in which case they <em class="bcp14">MUST</em> be.
<p id="rfc.section.3.6.p.9">If an implementation sends a message with one or more Warning headers 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 Date header in the message.
<p id="rfc.section.3.6.p.10">If an implementation receives a message with a warning-value that includes a warn-date, and that warn-date is different from
the Date 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.3.6.p.12"> 110 Response is stale </p>
<li><em class="bcp14">SHOULD</em> be included whenever the returned response is stale.
<p id="rfc.section.3.6.p.13"> 111 Revalidation failed </p>
<li><em class="bcp14">SHOULD</em> be included if a cache returns a stale response because an attempt to validate the response failed, due to an inability to
<p id="rfc.section.3.6.p.14"> 112 Disconnected operation </p>
<li><em class="bcp14">SHOULD</em> be included if the cache is intentionally disconnected from the rest of the network for a period of time.
<p id="rfc.section.3.6.p.15"> 113 Heuristic expiration </p>
<li><em class="bcp14">SHOULD</em> be included if the cache heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater
<p id="rfc.section.3.6.p.16"> 199 Miscellaneous warning </p>
<li>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.
<p id="rfc.section.3.6.p.17"> 214 Transformation applied </p>
<li><em class="bcp14">MUST</em> be added by an intermediate proxy if it applies any transformation to the representation, such as changing the content-coding,
media-type, or modifying the representation data, unless this Warning code already appears in the response.
<p id="rfc.section.3.6.p.18"> 299 Miscellaneous persistent warning </p>
<li>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.
<p id="rfc.section.5.1.p.2">The HTTP Cache Directive Registry should 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="#header.cache-control" id="rfc.xref.header.cache-control.3" title="Cache-Control">Section&nbsp;3.2</a>
<td class="left"> <a href="#header.warning" id="rfc.xref.header.warning.3" title="Warning">Section&nbsp;3.6</a>
long after a user believes that the information has been removed from the network. Therefore, cache contents should be protected
<p id="rfc.section.7.p.1">Much of the content and presentation of the caching design is due to suggestions and comments from individuals including:
<span class="n hidden"><span class="family-name">Nottingham</span><span class="given-name">Mark</span></span></span><span class="vcardline">Email: <a href="mailto:mnot@mnot.net"><span class="email">mnot@mnot.net</span></a></span><span class="vcardline">URI: <a href="http://www.mnot.net/" class="url">http://www.mnot.net/</a></span></address>
<p id="rfc.section.A.p.4">Do not mention RFC 2047 encoding and multiple languages in Warning headers anymore, as these aspects never were implemented.
(<a href="#header.warning" id="rfc.xref.header.warning.4" title="Warning">Section&nbsp;3.6</a>)
<div id="rfc.figure.u.18"></div> <pre class="inline"><a href="#header.age" class="smpl">Age</a> = "Age:" OWS Age-v
<a href="#header.age" class="smpl">Age-v</a> = delta-seconds
<a href="#header.cache-control" class="smpl">Cache-Control</a> = "Cache-Control:" OWS Cache-Control-v
<a href="#header.cache-control" class="smpl">Cache-Control-v</a> = *( "," OWS ) cache-directive *( OWS "," [ OWS
<a href="#header.expires" class="smpl">Expires</a> = "Expires:" OWS Expires-v
<a href="#header.expires" class="smpl">Expires-v</a> = HTTP-date
<a href="#header.pragma" class="smpl">Pragma</a> = "Pragma:" OWS Pragma-v
<a href="#header.pragma" class="smpl">Pragma-v</a> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
<a href="#header.vary" class="smpl">Vary</a> = "Vary:" OWS Vary-v
<a href="#header.vary" class="smpl">Vary-v</a> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
<a href="#header.warning" class="smpl">Warning</a> = "Warning:" OWS Warning-v
<a href="#header.warning" class="smpl">Warning-v</a> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
<a href="#rule.delta-seconds" class="smpl">delta-seconds</a> = 1*DIGIT
<a href="#abnf.dependencies" class="smpl">port</a> = &lt;port, defined in [Part1], Section 2.6&gt;
<a href="#abnf.dependencies" class="smpl">pseudonym</a> = &lt;pseudonym, defined in [Part1], Section 9.9&gt;
<a href="#abnf.dependencies" class="smpl">uri-host</a> = &lt;uri-host, defined in [Part1], Section 2.6&gt;
</pre> <div id="rfc.figure.u.19"></div>
<p id="rfc.section.C.4.p.1">Ongoing work on IANA Message Header Registration (&lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/40">http://tools.ietf.org/wg/httpbis/trac/ticket/40</a>&gt;):
<li> &lt;<a href="http://tools.ietf.org/wg/httpbis/trac/ticket/37">http://tools.ietf.org/wg/httpbis/trac/ticket/37</a>&gt;: Vary and non-existant headers
<li class="indline1">Age header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.age.1">2.2</a>, <a class="iref" href="#rfc.xref.header.age.2">2.3.2</a>, <a class="iref" href="#rfc.iref.a.2"><b>3.1</b></a>, <a class="iref" href="#rfc.xref.header.age.3">5.2</a></li>
<li class="indline1">max-age&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.6"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.c.17"><b>3.2.2</b></a></li>
<li class="indline1">max-stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.7"><b>3.2.1</b></a></li>
<li class="indline1">min-fresh&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.8"><b>3.2.1</b></a></li>
<li class="indline1">must-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.15"><b>3.2.2</b></a></li>
<li class="indline1">no-cache&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.4"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.c.13"><b>3.2.2</b></a></li>
<li class="indline1">no-store&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.5"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.c.14"><b>3.2.2</b></a></li>
<li class="indline1">no-transform&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.9"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.c.19"><b>3.2.2</b></a></li>
<li class="indline1">only-if-cached&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.10"><b>3.2.1</b></a></li>
<li class="indline1">private&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.12"><b>3.2.2</b></a></li>
<li class="indline1">proxy-revalidate&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.16"><b>3.2.2</b></a></li>
<li class="indline1">public&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.11"><b>3.2.2</b></a></li>
<li class="indline1">s-maxage&nbsp;&nbsp;<a class="iref" href="#rfc.iref.c.18"><b>3.2.2</b></a></li>
<li class="indline1">Cache-Control header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">2.2</a>, <a class="iref" href="#rfc.iref.c.3"><b>3.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.3">5.2</a></li>
<li class="indline1">Expires header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">2.1</a>, <a class="iref" href="#rfc.xref.header.expires.2">2.3</a>, <a class="iref" href="#rfc.xref.header.expires.3">2.3.1</a>, <a class="iref" href="#rfc.iref.e.2"><b>3.3</b></a>, <a class="iref" href="#rfc.xref.header.expires.4">5.2</a></li>
<li class="indline1"><tt>Age</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.1"><b>3.1</b></a></li>
<li class="indline1"><tt>Age-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.2"><b>3.1</b></a></li>
<li class="indline1"><tt>Cache-Control</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.4"><b>3.2</b></a></li>
<li class="indline1"><tt>Cache-Control-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.5"><b>3.2</b></a></li>
<li class="indline1"><tt>cache-extension</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.6"><b>3.2</b></a></li>
<li class="indline1"><tt>cache-request-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.7"><b>3.2.1</b></a></li>
<li class="indline1"><tt>cache-response-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.8"><b>3.2.2</b></a></li>
<li class="indline1"><tt>delta-seconds</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.3"><b>3.1</b></a></li>
<li class="indline1"><tt>Expires</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.9"><b>3.3</b></a></li>
<li class="indline1"><tt>Expires-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.10"><b>3.3</b></a></li>
<li class="indline1"><tt>extension-pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.14"><b>3.4</b></a></li>
<li class="indline1"><tt>Pragma</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.11"><b>3.4</b></a></li>
<li class="indline1"><tt>pragma-directive</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.13"><b>3.4</b></a></li>
<li class="indline1"><tt>Pragma-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.12"><b>3.4</b></a></li>
<li class="indline1"><tt>Vary</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.15"><b>3.5</b></a></li>
<li class="indline1"><tt>Vary-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.16"><b>3.5</b></a></li>
<li class="indline1"><tt>warn-agent</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.21"><b>3.6</b></a></li>
<li class="indline1"><tt>warn-code</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.20"><b>3.6</b></a></li>
<li class="indline1"><tt>warn-date</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.23"><b>3.6</b></a></li>
<li class="indline1"><tt>warn-text</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.22"><b>3.6</b></a></li>
<li class="indline1"><tt>Warning</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.17"><b>3.6</b></a></li>
<li class="indline1"><tt>Warning-v</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.18"><b>3.6</b></a></li>
<li class="indline1"><tt>warning-value</tt>&nbsp;&nbsp;<a class="iref" href="#rfc.iref.g.19"><b>3.6</b></a></li>
<li class="indline1">Age&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.age.1">2.2</a>, <a class="iref" href="#rfc.xref.header.age.2">2.3.2</a>, <a class="iref" href="#rfc.iref.h.2"><b>3.1</b></a>, <a class="iref" href="#rfc.xref.header.age.3">5.2</a></li>
<li class="indline1">Cache-Control&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.cache-control.1">2.1</a>, <a class="iref" href="#rfc.xref.header.cache-control.2">2.2</a>, <a class="iref" href="#rfc.iref.h.3"><b>3.2</b></a>, <a class="iref" href="#rfc.xref.header.cache-control.3">5.2</a></li>
<li class="indline1">Expires&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.expires.1">2.1</a>, <a class="iref" href="#rfc.xref.header.expires.2">2.3</a>, <a class="iref" href="#rfc.xref.header.expires.3">2.3.1</a>, <a class="iref" href="#rfc.iref.h.4"><b>3.3</b></a>, <a class="iref" href="#rfc.xref.header.expires.4">5.2</a></li>
<li class="indline1">Pragma&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">2.2</a>, <a class="iref" href="#rfc.xref.header.pragma.2">3.2</a>, <a class="iref" href="#rfc.iref.h.5"><b>3.4</b></a>, <a class="iref" href="#rfc.xref.header.pragma.3">5.2</a></li>
<li class="indline1">Vary&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">2.7</a>, <a class="iref" href="#rfc.iref.h.6"><b>3.5</b></a>, <a class="iref" href="#rfc.xref.header.vary.2">5.2</a></li>
<li class="indline1">Warning&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">2.3.3</a>, <a class="iref" href="#rfc.xref.header.warning.2">2.8</a>, <a class="iref" href="#rfc.iref.h.7"><b>3.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.3">5.2</a>, <a class="iref" href="#rfc.xref.header.warning.4">A</a></li>
<li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.1"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.m.5"><b>3.2.2</b></a></li>
<li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.2"><b>3.2.1</b></a></li>
<li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.3"><b>3.2.1</b></a></li>
<li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.m.4"><b>3.2.2</b></a></li>
<li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.1"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.n.4"><b>3.2.2</b></a></li>
<li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.2"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.n.5"><b>3.2.2</b></a></li>
<li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.n.3"><b>3.2.1</b></a>, <a class="iref" href="#rfc.iref.n.6"><b>3.2.2</b></a></li>
<li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.o.1"><b>3.2.1</b></a></li>
<li class="indline1"><em>Part1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.1">1.4</a>, <a class="iref" href="#rfc.xref.Part1.2">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.3">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.4">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.5">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.6">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.7">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.8">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.9">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.10">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.11">2.2</a>, <a class="iref" href="#rfc.xref.Part1.12">2.3.2</a>, <a class="iref" href="#rfc.xref.Part1.13">2.5</a>, <a class="iref" href="#rfc.xref.Part1.14">2.5</a>, <a class="iref" href="#rfc.xref.Part1.15">2.5</a>, <a class="iref" href="#rfc.xref.Part1.16">2.7</a>, <a class="iref" href="#rfc.xref.Part1.17">3.3</a>, <a class="iref" href="#Part1"><b>8.1</b></a><ul class="ind">
<li class="indline1"><em>Section 1.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.1">1.4</a></li>
<li class="indline1"><em>Section 1.2.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.2">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.3">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.4">1.4.1</a>, <a class="iref" href="#rfc.xref.Part1.5">1.4.1</a></li>
<li class="indline1"><em>Section 2.6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.8">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.10">1.4.2</a></li>
<li class="indline1"><em>Section 3.2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.6">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.16">2.7</a></li>
<li class="indline1"><em>Section 4.3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.11">2.2</a>, <a class="iref" href="#rfc.xref.Part1.13">2.5</a>, <a class="iref" href="#rfc.xref.Part1.14">2.5</a>, <a class="iref" href="#rfc.xref.Part1.15">2.5</a></li>
<li class="indline1"><em>Section 6.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.7">1.4.2</a>, <a class="iref" href="#rfc.xref.Part1.17">3.3</a></li>
<li class="indline1"><em>Section 9.3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.12">2.3.2</a></li>
<li class="indline1"><em>Section 9.9</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part1.9">1.4.2</a></li>
<li class="indline1"><em>Part2</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">2.2</a>, <a class="iref" href="#rfc.xref.Part2.2">2.3.1.1</a>, <a class="iref" href="#rfc.xref.Part2.3">2.5</a>, <a class="iref" href="#Part2"><b>8.1</b></a><ul class="ind">
<li class="indline1"><em>Section 7.1.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.1">2.2</a>, <a class="iref" href="#rfc.xref.Part2.3">2.5</a></li>
<li class="indline1"><em>Section 8</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part2.2">2.3.1.1</a></li>
<li class="indline1"><em>Part4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.1">2.3.1.1</a>, <a class="iref" href="#rfc.xref.Part4.2">2.4</a>, <a class="iref" href="#rfc.xref.Part4.3">2.8</a>, <a class="iref" href="#Part4"><b>8.1</b></a><ul class="ind">
<li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.3">2.8</a></li>
<li class="indline1"><em>Section 6.6</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part4.1">2.3.1.1</a></li>
<li class="indline1"><em>Part5</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.1">2.1.1</a>, <a class="iref" href="#rfc.xref.Part5.2">2.1.1</a>, <a class="iref" href="#Part5"><b>8.1</b></a><ul class="ind">
<li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part5.2">2.1.1</a></li>
<li class="indline1"><em>Part7</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">2.1</a>, <a class="iref" href="#rfc.xref.Part7.2">2.6</a>, <a class="iref" href="#rfc.xref.Part7.3">3.2.2</a>, <a class="iref" href="#Part7"><b>8.1</b></a><ul class="ind">
<li class="indline1"><em>Section 3.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.Part7.1">2.1</a>, <a class="iref" href="#rfc.xref.Part7.2">2.6</a>, <a class="iref" href="#rfc.xref.Part7.3">3.2.2</a></li>
<li class="indline1">Pragma header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.pragma.1">2.2</a>, <a class="iref" href="#rfc.xref.header.pragma.2">3.2</a>, <a class="iref" href="#rfc.iref.p.4"><b>3.4</b></a>, <a class="iref" href="#rfc.xref.header.pragma.3">5.2</a></li>
<li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.2"><b>3.2.2</b></a></li>
<li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.3"><b>3.2.2</b></a></li>
<li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.p.1"><b>3.2.2</b></a></li>
<li class="indline1"><em>RFC1305</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC1305.1">2.3.2</a>, <a class="iref" href="#RFC1305"><b>8.2</b></a></li>
<li class="indline1"><em>RFC2119</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2119.1">1.3</a>, <a class="iref" href="#RFC2119"><b>8.1</b></a></li>
<li class="indline1"><em>RFC2616</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.1">2.3.1.1</a>, <a class="iref" href="#RFC2616"><b>8.2</b></a>, <a class="iref" href="#rfc.xref.RFC2616.2">C.1</a><ul class="ind">
<li class="indline1"><em>Section 13.9</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC2616.1">2.3.1.1</a></li>
<li class="indline1"><em>RFC3864</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC3864.1">5.2</a>, <a class="iref" href="#RFC3864"><b>8.2</b></a></li>
<li class="indline1"><em>RFC5226</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC5226.1">3.2.3</a>, <a class="iref" href="#RFC5226"><b>8.2</b></a><ul class="ind">
<li class="indline1"><em>Section 4.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC5226.1">3.2.3</a></li>
<li class="indline1"><em>RFC5234</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC5234.1">1.4</a>, <a class="iref" href="#rfc.xref.RFC5234.2">1.4</a>, <a class="iref" href="#RFC5234"><b>8.1</b></a><ul class="ind">
<li class="indline1"><em>Appendix B.1</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC5234.2">1.4</a></li>
<li class="indline1"><em>RFC5861</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC5861.1">5.1</a>, <a class="iref" href="#rfc.xref.RFC5861.2">5.1</a>, <a class="iref" href="#RFC5861"><b>8.2</b></a><ul class="ind">
<li class="indline1"><em>Section 3</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC5861.2">5.1</a></li>
<li class="indline1"><em>Section 4</em>&nbsp;&nbsp;<a class="iref" href="#rfc.xref.RFC5861.1">5.1</a></li>
<li class="indline1">Cache Directive&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.2"><b>3.2.2</b></a></li>
<li class="indline1">stale&nbsp;&nbsp;<a class="iref" href="#rfc.iref.s.1">1.2</a></li>
<li class="indline1">validator&nbsp;&nbsp;<a class="iref" href="#rfc.iref.v.1">1.2</a>, <a class="iref" href="#rfc.iref.v.2">1.2</a></li>
<li class="indline1">Vary header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.vary.1">2.7</a>, <a class="iref" href="#rfc.iref.v.3"><b>3.5</b></a>, <a class="iref" href="#rfc.xref.header.vary.2">5.2</a></li>
<li class="indline1">Warning header&nbsp;&nbsp;<a class="iref" href="#rfc.xref.header.warning.1">2.3.3</a>, <a class="iref" href="#rfc.xref.header.warning.2">2.8</a>, <a class="iref" href="#rfc.iref.w.1"><b>3.6</b></a>, <a class="iref" href="#rfc.xref.header.warning.3">5.2</a>, <a class="iref" href="#rfc.xref.header.warning.4">A</a></li>