Source: https://trac.ietf.org/trac/httpbis/browser/draft-ietf-httpbis/06/p6-cache.xml?rev=553
Timestamp: 2020-08-04 09:20:37
Document Index: 609326037

Matched Legal Cases: ['art5', 'art4', 'art 6', 'art 6', 'art\n1', 'art1', 'art3', 'art5', 'art1', 'art1', 'art1', 'art1', 'art1', 'art1']

p6-cache.xml in draft-ietf-httpbis/06 – Hypertext Transfer Protocol Wiki
source: draft-ietf-httpbis/06/p6-cache.xml @ 553
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<?xml-stylesheet type='text/xsl' href='rfc2629.xslt'?>
<!ENTITY ID-VERSION "06">
<!ENTITY ID-MONTH "March">
<!ENTITY ID-YEAR "2009">
<!ENTITY partial "<xref target='Part5' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
<!ENTITY header-last-modified "<xref target='Part4' x:rel='#header.last-modified' xmlns:x='http://purl.org/net/xml2rfc/ext'/>">
<rfc category="std" docName="draft-ietf-httpbis-p6-cache-&ID-VERSION;" ipr="pre5378Trust200902"
obsoletes="2616" x:maturity-level="draft" xmlns:x="http://purl.org/net/xml2rfc/ext">
<author fullname="Roy T. Fielding" initials="R." role="editor" surname="Fielding">
<author fullname="Jim Gettys" initials="J." surname="Gettys">
<author fullname="Jeffrey C. Mogul" initials="J." surname="Mogul">
<author fullname="Henrik Frystyk Nielsen" initials="H." surname="Frystyk">
<author fullname="Larry Masinter" initials="L." surname="Masinter">
<author fullname="Paul J. Leach" initials="P." surname="Leach">
<author fullname="Tim Berners-Lee" initials="T." surname="Berners-Lee">
<author fullname="Yves Lafon" initials="Y." role="editor" surname="Lafon">
<author fullname="Julian F. Reschke" initials="J. F." role="editor" surname="Reschke">
<date month="&ID-MONTH;" year="&ID-YEAR;" day="9"/>
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 "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.
Discussion of this draft should take place on the HTTPBIS working group mailing list
(ietf-http-wg@w3.org). The current issues list is at <eref
target="http://tools.ietf.org/wg/httpbis/trac/report/11" /> and related documents
(including fancy diffs) can be found at <eref target="http://tools.ietf.org/wg/httpbis/" />.
The changes in this draft are summarized in <xref target="changes.since.05" />.
<section anchor="caching" title="Introduction">
HTTP is typically used for distributed information systems, where performance can be
improved by the use of response caches. This document defines aspects of HTTP/1.1 related to
caching and reusing response messages.
<section anchor="intro.purpose" title="Purpose">
<iref item="cache" />
An HTTP <x:dfn>cache</x:dfn> is a local store of response messages and the subsystem that
controls its message storage, retrieval, and deletion. A cache stores cacheable responses
in order to reduce the response time and network bandwidth consumption on future,
equivalent requests. Any client or server may include a cache, though a cache cannot be
used by a server that is acting as a tunnel.
Caching would be useless if it did not significantly improve performance. The goal of
caching in HTTP/1.1 is to reuse a prior 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 <xref target="expiration.model" />). 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 <xref target="validation.model" />).
<section anchor="intro.terminology" title="Terminology">
>This specification uses a number of terms to refer to the roles played by participants
in, and objects of, HTTP caching.
<iref item="cacheable" />
<t>A response is cacheable if a cache is allowed to store a copy of the response message
for use in answering subsequent requests. Even when a response is cacheable, there may
be additional constraints on whether a cache can use the cached copy to satisfy a
particular request.</t>
<iref item="explicit expiration time" />
<t>The time at which the origin server intends that an entity should no longer be
returned by a cache without further validation.</t>
<iref item="heuristic expiration time" />
<t>An expiration time assigned by a cache when no explicit expiration time is
available.</t>
<iref item="age" />
<t>The age of a response is the time since it was sent by, or successfully validated
with, the origin server.</t>
<iref item="first-hand" />
<t>A response is first-hand if the freshness model is not in use; i.e., its age is
0.</t>
<iref item="freshness lifetime" />
<t>The length of time between the generation of a response and its expiration time. </t>
<iref item="fresh" />
<t>A response is fresh if its age has not yet exceeded its freshness lifetime.</t>
<iref item="stale" />
<t>A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).</t>
<iref item="validator" />
<t>A protocol element (e.g., an entity tag or a Last-Modified time) that is used to find
out whether a stored response is an equivalent copy of an entity.</t>
<t anchor="shared.and.non-shared.caches">
<x:dfn>shared cache</x:dfn>
<t>A cache that is accessible to more than one user. A non-shared cache is
dedicated to a single user.</t>
<section anchor="intro.requirements" title="Requirements">
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD
NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
described in <xref target="RFC2119" />.
An implementation is not compliant if it fails to satisfy one or more of the &MUST;
or &REQUIRED; level requirements for the protocols it implements. An implementation
that satisfies all the &MUST; or &REQUIRED; level and all the &SHOULD; level
requirements for its protocols is said to be "unconditionally compliant"; one that
satisfies all the &MUST; level requirements but not all the &SHOULD; level
requirements for its protocols is said to be "conditionally compliant."
<x:ref>port</x:ref> = &lt;port, defined in &uri;&gt;
<x:ref>uri-host</x:ref> = &lt;uri-host, defined in &uri;&gt;
<section anchor="caching.overview" title="Cache Operation">
<section anchor="response.cacheability" title="Response Cacheability">
A cache &MUST-NOT; store a response to any request, unless:
<t>The request method is defined as being cacheable, and</t>
<t>the "no-store" cache directive (see <xref target="header.cache-control" />) does not
appear in request or response headers, and</t>
<t>the "private" cache response directive (see <xref target="header.cache-control" />
does not appear in the response, if the cache is shared, and</t>
<t>the "Authorization" header (see &header-authorization;) does not appear in the request, if
the cache is shared (unless the "public" directive is present; see <xref
target="header.cache-control" />), and</t>
<t>the cache understands partial responses, if the response is partial or incomplete
(see <xref target="errors.or.incomplete.response.cache.behavior" />).</t>
Note that in normal operation, most caches will not store a response that has neither a
cache validator nor an explicit expiration time, as such responses are not usually
useful to store. However, caches are not prohibited from storing such responses.
<section anchor="errors.or.incomplete.response.cache.behavior" title="Storing Partial and Incomplete Responses">
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 &MUST;
treat it as a partial response &partial;. Partial responses
can be combined as described in &combining-byte-ranges;; the result might be a
full response or might still be partial. A cache &MUST-NOT; return a partial
response to a client without explicitly marking it as such using the 206 (Partial
Content) status code.
A cache that does not support the Range and Content-Range headers &MUST-NOT; store
incomplete or partial responses.
<section anchor="constructing.responses.from.caches" title="Constructing Responses from Caches">
For a presented request, a cache &MUST-NOT; return a stored response, unless:
<t>The presented Request-URI and that of the stored response match (see
<cref>TBD</cref>), and</t>
<t>the request method associated with the stored response allows it to be
used for the presented request, and</t>
<t>selecting request-headers nominated by the stored response (if any) match those presented (see <xref
target="caching.negotiated.responses" />), and</t>
<t>the presented request and stored response are free from directives that would prevent
its use (see <xref target="header.cache-control" /> and <xref target="header.pragma"/>),
and</t>
<t>the stored response is either:
<t>fresh (see <xref target="expiration.model" />), or</t>
<t>allowed to be served stale (see <xref target="serving.stale.responses" />), or</t>
<t>successfully validated (see <xref target="validation.model" />).</t>
<cref>TODO: define method cacheability for GET, HEAD and POST in p2-semantics.</cref>
When a stored response is used to satisfy a request, caches &MUST; include a
single Age header field <xref target="header.age" /> in the response with a value equal to the stored response's
current_age; see <xref target="age.calculations" />.
<cref>DISCUSS: this currently includes successfully validated responses.</cref>
Requests with methods that are unsafe (&safe-methods;) &MUST; 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.
Also, note that unsafe requests might invalidate already stored responses; see
<xref target="invalidation.after.updates.or.deletions" />.
Caches &MUST; 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
<cref>TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in p1</cref>
<section anchor="expiration.model" title="Freshness Model">
When a response is "fresh" in the cache, it can be used to satisfy subsequent
requests without contacting the origin server, thereby improving efficiency.
The primary mechanism for determining freshness is for an origin server to provide an
explicit expiration time in the future, using either the Expires header (<xref
target="header.expires" />) or the max-age response cache directive (<xref
target="cache-response-directive" />). Generally, origin servers will assign future
explicit expiration times to responses in the belief that the entity is not likely to
change in a semantically significant way before the expiration time is reached.
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.
<cref>This wording may cause confusion, because the response may still be served stale.</cref>
Since origin servers do not always provide explicit expiration times, HTTP caches may
also assign heuristic expiration times when they 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 algorithms, but does
impose worst-case constraints on their results.
<artwork type="code">
The freshness_lifetime is defined in <xref target="calculating.freshness.lifetime" />;
the current_age is defined in <xref target="age.calculations" />.
Additionally, clients may 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 <xref target="cache-request-directive" />.
<cref>ISSUE: there are not requirements directly applying to cache-request-directives and
freshness.</cref>
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 <xref target="history.lists" /> for an explanation of
the difference between caches and history mechanisms.
<section anchor="calculating.freshness.lifetime" title="Calculating Freshness Lifetime">
A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
response by using the first match of:
<t>If the cache is shared and the s-maxage response cache directive (<xref
target="cache-response-directive" />) is present, use its value, or</t>
<t>If the max-age response cache directive (<xref target="cache-response-directive"
/>) is present, use its value, or</t>
<t>If the Expires response header (<xref target="header.expires" />) is present, use
its value minus the value of the Date response header, or</t>
<t>Otherwise, no explicit expiration time is present in the response, but a heuristic
may be used; see <xref target="heuristic.freshness" />.</t>
Note that this calculation is not vulnerable to clock skew, since all of the
information comes from the origin server.
<section anchor="heuristic.freshness" title="Calculating Heuristic Freshness">
If no explicit expiration time is present in a stored response that has a status code
of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be
calculated. Heuristics &MUST-NOT; be used for other response status codes.
When a heuristic is used to calculate freshness lifetime, the cache &SHOULD;
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.
Also, if the response has a Last-Modified header (&header-last-modified;), the
heuristic expiration value &SHOULD; be no more than some fraction of the interval
<cref>REVIEW: took away HTTP/1.0 query string heuristic uncacheability.</cref>
<section anchor="age.calculations" title="Calculating Age">
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.
The term "age_value" denotes the value of the Age header, in a form appropriate for
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 (see &header-date;).
The term "date_value" denotes the value of the Date header, in a form appropriate for
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, &SHOULD; use NTP <xref target="RFC1305" /> or some similar protocol to
synchronize their clocks to a globally accurate time standard.
<t>A response's age can be calculated in two entirely independent ways:
<t>now minus date_value, if the local clock is reasonably well synchronized to the
origin server's clock. If the result is negative, the result is replaced by zero.</t>
<t>age_value, if all of the caches along the response path implement HTTP/1.1.</t>
<preamble>These are combined as</preamble>
When an Age value is received, it &MUST; be interpreted relative to the time the
request was initiated, not the time that the response was received.
where "request_time" is the time (according to the local clock) when the request that
elicited this response was sent.
The current_age of a stored response can then be calculated by adding the amount of
time (in seconds) since the stored response was last validated by the origin server to
the corrected_initial_age.
age_value - Age header field-value received with the response
date_value - Date header field-value received with the response
request_time - local time when the cache made the request
now - current local time
<section anchor="serving.stale.responses" title="Serving Stale Responses">
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
<xref target="expiration.model" />.
Caches &MUST-NOT; return a stale response if it is prohibited by an explicit
in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a
"must-revalidate" cache-response-directive, or an applicable "s-maxage" or
"proxy-revalidate" cache-response-directive; see <xref target="cache-response-directive"/>).
Caches &SHOULD-NOT; 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 <xref target="cache-request-directive" />).
Stale responses &SHOULD; have a Warning header with the 110 warn-code (see <xref
target="header.warning" />). Likewise, the 112 warn-code &SHOULD; be sent on stale responses if
the cache is disconnected.
If a cache receives a first-hand response (either an entire response, or a 304 (Not
Modified) response) that it would normally forward to the requesting client, and the
received response is no longer fresh, the cache &SHOULD; forward it to the
requesting client without adding a new Warning (but without removing any existing
Warning headers). A cache &SHOULD-NOT; attempt to validate a response simply because
that response became stale in transit.
<section anchor="validation.model" title="Validation Model">
Checking with the origin server to see if a stale or otherwise unusable cached response
can be reused is called "validating" or "revalidating." Doing so potentially avoids
the overhead of retransmitting the response body when the stored response is valid.
HTTP's conditional request mechanism &conditional; is used for this purpose. When a stored
response includes one or more validators, such as the field values of an ETag or
Last-Modified header field, then a validating request &SHOULD; be made conditional
to those field values.
A 304 (Not Modified) response status code indicates that the stored
response can be updated and reused; see <xref target="combining.headers"/>.
If instead the cache receives a full response (i.e., one with a response body), it is used to satisfy the
request and replace the stored response. <cref>Should there be a requirement here?</cref>
If a cache receives a 5xx response while attempting to validate a response, it &MAY;
either forward this response to the requesting client, or act as if the server failed to
respond. In the latter case, it &MAY; return a previously stored response (which &SHOULD; include the
111 warn-code; see <xref target="header.warning"/>) unless the
stored response includes the "must-revalidate" cache directive (see <xref
target="serving.stale.responses" />).
<section anchor="invalidation.after.updates.or.deletions" title="Request Methods that Invalidate">
Because unsafe methods (&safe-methods;) have the potential for changing state on the
origin server, intervening caches can use them to keep their contents
The following HTTP methods &MUST; cause a cache to invalidate the Request-URI as well
as the Location and Content-Location headers (if present):
An invalidation based on the URI in a Location or Content-Location header &MUST-NOT;
be performed if the host part of that URI differs from the host part in the Request-URI.
This helps prevent denial of service attacks.
<cref>TODO: "host part" needs to be specified better.</cref>
A cache that passes through requests for methods it does not understand &SHOULD;
invalidate the Request-URI.
Here, "invalidate" means that the cache will either remove all stored responses related
to the 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.
Note that this does not guarantee that all appropriate responses are invalidated. For
example, the request that caused the change at the origin server might not have gone
through the cache where a response is stored.
<cref>TODO: specify that only successful (2xx, 3xx?) responses invalidate.</cref>
<section anchor="caching.negotiated.responses" title="Caching Negotiated Responses">
Use of server-driven content negotiation (&server-driven-negotiation;) alters
the conditions under which a cache can use the response for subsequent
When a cache receives a request that can be satisfied by a stored response
that includes a Vary header field (<xref target="header.vary"/>), it &MUST-NOT; use that response unless
all of the selecting request-headers in the presented request match the corresponding
stored request-headers from the original request.
The selecting request-headers from two requests are defined to match if and only if the
selecting request-headers in the first request can be transformed to the selecting
request-headers in the second request by adding or removing linear white space
<cref>[ref]</cref> at places where this is allowed by the corresponding ABNF, and/or
combining multiple message-header fields with the same field name following the rules
about message headers in &message-headers;. <cref>DISCUSS: header-specific canonicalisation</cref>
A Vary header field-value of "*" always fails to match, and subsequent requests to that
resource can only be properly interpreted by the origin server.
If no stored response matches, the cache &MAY; forward the presented request to the origin
server in a conditional request, and &SHOULD; include all ETags stored with
potentially suitable responses in an If-None-Match request header. If the server responds with 304 (Not Modified) and
includes an entity tag or Content-Location that indicates the entity to be used, that
cached response &MUST; be used to satisfy the presented request, and &SHOULD;
be used to update the corresponding stored response; see <xref target="combining.headers"/>.
If any of the stored responses contains only partial content, its entity-tag &SHOULD-NOT;
be included in the If-None-Match header field unless the request is for a range that would
be fully satisfied by that stored response.
If a cache receives a successful response whose Content-Location field matches that of an
existing stored response for the same Request-URI, whose entity-tag differs from that of
the existing stored response, and whose Date is more recent than that of the existing
response, the existing response &SHOULD-NOT; be returned in response to future
requests and &SHOULD; be deleted from the cache.<cref>DISCUSS: Not sure if this is necessary.</cref>
<section anchor="combining.headers" title="Combining Responses">
When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response,
it needs to update the stored response with the new one, so that the updated response can
be sent to the client.
If the status code is 304 (Not Modified), the cache &SHOULD; use the stored entity-body as
the updated entity-body. If the status code is 206 (Partial Content) and the ETag or
Last-Modified headers match exactly, the cache &MAY; combine the stored entity-body in
the stored response with the updated entity-body received in the response and use the
result as the updated entity-body (see &combining-byte-ranges;).
The stored response headers are used for the updated response, except that
<t>any stored Warning headers with warn-code 1xx (see <xref target="header.warning" />)
&MUST; be deleted from the stored response and the forwarded response.</t>
<t>any stored Warning headers with warn-code 2xx &MUST; be retained in the stored
response and the forwarded response.</t>
<t>any headers provided in the 304 or 206 response &MUST; replace the corresponding
headers from the stored response.</t>
A cache &MUST; also replace any stored headers with corresponding headers received in the
incoming response, except for Warning headers as described immediately above. If a header
field-name in the incoming response matches more than one header in the stored response,
all such old headers &MUST; be replaced. It &MAY; store the combined
entity-body.
<cref>ISSUE: discuss how to handle HEAD updates</cref>
<section anchor="header.fields" title="Header Field Definitions">
<t>This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.</t>
<t>For entity-header fields, both sender and recipient refer to either the client or the
server, depending on who sends and who receives the entity.</t>
<section anchor="header.age" title="Age">
<iref item="Age header" primary="true" x:for-anchor="" />
<iref item="Headers" primary="true" subitem="Age" x:for-anchor="" />
<x:anchor-alias value="Age-v"/>
The response-header field "Age" conveys the sender's estimate of the amount of time since
the response (or its validation) was generated at the origin server. Age values are
calculated as specified in <xref target="age.calculations" />.
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Age"/><iref primary="true" item="Grammar" subitem="Age-v"/>
<x:ref>Age</x:ref> = "Age" ":" <x:ref>OWS</x:ref> <x:ref>Age-v</x:ref>
<x:ref>Age-v</x:ref> = <x:ref>delta-seconds</x:ref>
<x:anchor-alias value="delta-seconds" />
Age field-values are non-negative decimal integers, representing time in seconds.
<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="delta-seconds" />
If a cache receives a value larger than the largest positive integer it can represent, or
if any of its age calculations overflows, it &MUST; transmit an Age header with a
field-value of 2147483648 (2<x:sup>31</x:sup>). Caches &SHOULD; use an arithmetic type
of at least 31 bits of range.
The presence of an Age header field in a response implies that a response is not
first-hand. However, the converse is not true, since HTTP/1.0 caches may not implement the
Age header field.
<section anchor="header.cache-control" title="Cache-Control">
<iref item="Cache-Control header" primary="true" x:for-anchor="" />
<iref item="Headers" primary="true" subitem="Cache-Control" x:for-anchor="" />
<x:anchor-alias value="Cache-Control-v"/>
The general-header field "Cache-Control" is used to specify directives that &MUST; be
obeyed by all caches along the request/response chain. The directives specify behavior
intended to prevent caches from adversely interfering with the request or response. 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.
<t>Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
Pragma: no-cache (see <xref target="header.pragma" />).</t>
Cache directives &MUST; be passed through by a proxy or gateway application,
regardless of their significance to that application, since the directives might be
applicable to all recipients along the request/response chain. It is not possible to
target a directive to a specific cache.
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Cache-Control"/><iref primary="true" item="Grammar" subitem="Cache-Control-v"/><iref primary="true" item="Grammar" subitem="cache-extension"/>
<x:ref>Cache-Control</x:ref> = "Cache-Control" ":" <x:ref>OWS</x:ref> <x:ref>Cache-Control-v</x:ref>
<x:ref>Cache-Control-v</x:ref> = 1#<x:ref>cache-directive</x:ref>
/ <x:ref>cache-response-directive</x:ref>
<x:ref>cache-extension</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
<section anchor="cache-request-directive" title="Request Cache-Control Directives">
<x:anchor-alias value="cache-request-directive" />
<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-request-directive" />
/ "max-age" "=" <x:ref>delta-seconds</x:ref>
/ "max-stale" [ "=" <x:ref>delta-seconds</x:ref> ]
/ "min-fresh" "=" <x:ref>delta-seconds</x:ref>
/ <x:ref>cache-extension</x:ref>
<iref item="Cache Directives" primary="true" subitem="no-cache" />
<t>The no-cache request directive indicates that a stored response &MUST-NOT; be
used to satisfy the request without successful validation on the origin server.</t>
<iref item="Cache Directives" primary="true" subitem="no-store" />
<iref item="no-store" primary="true" subitem="Cache Directive" />
<t>The no-store request directive indicates that a cache &MUST-NOT; store any part
of either this request or any response to it. This directive applies to both
non-shared and shared caches. "&MUST-NOT; store" in this context means that the
cache &MUST-NOT; intentionally store the information in non-volatile storage,
and &MUST; make a best-effort attempt to remove the information from volatile
storage as promptly as possible after forwarding it.</t>
<t>This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In
particular, malicious or compromised caches might not recognize or obey this
directive, and communications networks may be vulnerable to eavesdropping.</t>
<iref item="Cache Directives" primary="true" subitem="max-age" />
<iref item="max-age" primary="true" subitem="Cache Directive" />
<t>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
max-stale directive is also included, the client is not willing to accept a stale
response.</t>
<iref item="Cache Directives" primary="true" subitem="max-stale" />
<iref item="max-stale" primary="true" subitem="Cache Directive" />
<t>The max-stale request directive indicates that the client is willing to accept a
response that has exceeded its expiration time. If max-stale is assigned a value,
then the client is willing to accept a response that has exceeded its expiration
time by no more than the specified number of seconds. If no value is assigned to
max-stale, then the client is willing to accept a stale response of any age. <cref source="mnot">of any staleness?</cref></t>
<iref item="Cache Directives" primary="true" subitem="min-fresh" />
<iref item="min-fresh" primary="true" subitem="Cache Directive" />
<t>The min-fresh request directive indicates that the client is willing to accept a
response whose freshness lifetime is no less than its current age plus the specified
time in seconds. That is, the client wants a response that will still be fresh for
at least the specified number of seconds.</t>
<iref item="Cache Directives" primary="true" subitem="no-transform" />
<iref item="no-transform" primary="true" subitem="Cache Directive" />
<t>The no-transform request directive indicates that an intermediate cache or proxy
&MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request
headers, nor the request entity-body.</t>
<iref item="Cache Directives" primary="true" subitem="only-if-cached" />
<iref item="only-if-cached" primary="true" subitem="Cache Directive" />
<t>The only-if-cached request directive indicates that the client only wishes to
return a stored response. If it receives this directive, a cache &SHOULD; either
respond using a stored response that is consistent with the other constraints of the
request, or respond with a 504 (Gateway Timeout) status. If a group of caches is
being operated as a unified system with good internal connectivity, such a request
&MAY; be forwarded within that group of caches.</t>
<section anchor="cache-response-directive" title="Response Cache-Control Directives">
<x:anchor-alias value="cache-response-directive" />
<figure><artwork type="abnf2616"><iref item="Grammar" primary="true" subitem="cache-response-directive" />
/ "private" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
/ "no-cache" [ "=" <x:ref>DQUOTE</x:ref> 1#<x:ref>field-name</x:ref> <x:ref>DQUOTE</x:ref> ]
/ "s-maxage" "=" <x:ref>delta-seconds</x:ref>
<iref item="Cache Directives" primary="true" subitem="public" />
<iref item="public" primary="true" subitem="Cache Directive" />
<t>The public response directive indicates that the response &MAY; be cached, even
if it would normally be non-cacheable or cacheable only within a non-shared cache.
(See also Authorization, &header-authorization;, for additional details.) </t>
<iref item="Cache Directives" primary="true" subitem="private" />
<iref item="private" primary="true" subitem="Cache Directive" />
<t>The private response directive indicates that the response message is intended for
a single user and &MUST-NOT; be stored by a shared cache. A private (non-shared)
cache &MAY; store the response.</t>
<t>If the private response directive specifies one or more field-names, this
requirement is limited to the field-values associated with the listed response
headers. That is, the specified field-names(s) &MUST-NOT; be stored by a shared
cache, whereas the remainder of the response message &MAY; be.</t>
<x:h>Note:</x:h> This usage of the word private only controls where the response may
be stored, and cannot ensure the privacy of the message content.</t>
<t>The no-cache response directive indicates that the response &MUST-NOT; be used to
satisfy a subsequent request without successful validation on the origin server.
This allows an origin server to prevent caching even by caches that have been
configured to return stale responses.</t>
<t>If the no-cache response directive specifies one or more field-names, this
requirement is limited to the field-values assosicated with the listed response
headers. That is, the specified field-name(s) &MUST-NOT; be sent in the response
to a subsequent request without successful validation on the origin server. This
allows an origin server to prevent the re-use of certain header fields in a
response, while still allowing caching of the rest of the response.</t>
<x:h>Note:</x:h> Most HTTP/1.0 caches will not recognize or obey this directive.
<t>The no-store response directive indicates that a cache &MUST-NOT; store any
part of either the immediate request or response. This directive applies to both
<iref item="Cache Directives" primary="true" subitem="must-revalidate" />
<iref item="must-revalidate" primary="true" subitem="Cache Directive" />
<t>The must-revalidate response directive indicates that once it has become stale, the response &MUST-NOT; be
used to satisfy subsequent requests without successful validation on the origin server.</t>
<t>The must-revalidate directive is necessary to support reliable operation for
certain protocol features. In all circumstances an HTTP/1.1 cache &MUST; obey
the must-revalidate directive; in particular, if the cache cannot reach the origin
server for any reason, it &MUST; generate a 504 (Gateway Timeout) response.</t>
<t>Servers &SHOULD; send the must-revalidate directive if and only if failure to
validate a request on the entity could result in incorrect operation, such as a
silently unexecuted financial transaction.</t>
<iref item="Cache Directives" primary="true" subitem="proxy-revalidate" />
<iref item="proxy-revalidate" primary="true" subitem="Cache Directive" />
<t>The proxy-revalidate response directive has the same meaning as the must-revalidate
response directive, except that it does not apply to non-shared caches.</t>
<t>The max-age response directive indicates that response is to be considered stale
after its age is greater than the specified number of seconds.</t>
<iref item="Cache Directives" primary="true" subitem="s-maxage" />
<iref item="s-maxage" primary="true" subitem="Cache Directive" />
<t>The s-maxage response directive indicates that, in shared caches, the maximum age
specified by this directive overrides the 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.</t>
<t>The no-transform response directive indicates that an intermediate cache or proxy
&MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response
headers, nor the response entity-body.</t>
<section anchor="cache.control.extensions" title="Cache Control Extensions">
The Cache-Control header field can be extended through the use of one or more
cache-extension tokens, each with an optional value. Informational extensions (those
that do not require a change in cache behavior) can be added without changing the
semantics of other directives. Behavioral extensions are designed to work by acting as
modifiers to the existing base of cache directives. Both the new directive and the
standard directive are supplied, such that applications that do not understand the new
directive will default to the behavior specified by the standard directive, and those
that understand the new directive will recognize it as modifying the requirements
associated with the standard directive. In this way, extensions to the cache-control
directives can be made without requiring changes to the base protocol.
This extension mechanism depends on an HTTP cache obeying all of the cache-control
directives defined for its native HTTP-version, obeying certain extensions, and ignoring
all directives that it does not understand.
For example, consider a hypothetical new response directive called "community" that
acts as a modifier to the private directive. 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
A cache seeing this header field will act correctly even if the cache does not
understand the community cache-extension, since it will also see and understand the
private directive and thus default to the safe behavior.
Unrecognized cache directives &MUST; be ignored; it is assumed that any cache
directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard
directives (or the response's default cacheability) such that the cache behavior will
remain minimally correct even if the cache does not understand the extension(s).
<section anchor="header.expires" title="Expires">
<iref item="Expires header" primary="true" x:for-anchor="" />
<iref item="Headers" primary="true" subitem="Expires" x:for-anchor="" />
<x:anchor-alias value="Expires-v"/>
The entity-header field "Expires" gives the date/time after which the response is
considered stale. See <xref target="expiration.model" /> for further discussion of the
freshness model.
The presence of an Expires field does not imply that the original resource will change or
cease to exist at, before, or after that time.
The field-value is an absolute date and time as defined by HTTP-date in &full-date;;
it &MUST; be sent in rfc1123-date format.
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Expires"/><iref primary="true" item="Grammar" subitem="Expires-v"/>
<x:ref>Expires</x:ref> = "Expires" ":" <x:ref>OWS</x:ref> <x:ref>Expires-v</x:ref>
<x:ref>Expires-v</x:ref> = <x:ref>HTTP-date</x:ref>
<preamble>For example</preamble>
directive (see <xref target="cache-response-directive" />), that directive overrides
the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.</t>
HTTP/1.1 servers &SHOULD-NOT; send Expires dates more than one year in the future.
HTTP/1.1 clients and caches &MUST; treat other invalid date formats, especially
including the value "0", as in the past (i.e., "already expired").
<section anchor="header.pragma" title="Pragma">
<iref item="Pragma header" primary="true" x:for-anchor="" />
<iref item="Headers" primary="true" subitem="Pragma" x:for-anchor="" />
<x:anchor-alias value="Pragma-v"/>
>The general-header field "Pragma" 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
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Pragma"/><iref primary="true" item="Grammar" subitem="Pragma-v"/><iref primary="true" item="Grammar" subitem="pragma-directive"/><iref primary="true" item="Grammar" subitem="extension-pragma"/>
<x:ref>Pragma</x:ref> = "Pragma" ":" <x:ref>OWS</x:ref> <x:ref>Pragma-v</x:ref>
<x:ref>Pragma-v</x:ref> = 1#<x:ref>pragma-directive</x:ref>
<x:ref>pragma-directive</x:ref> = "no-cache" / <x:ref>extension-pragma</x:ref>
<x:ref>extension-pragma</x:ref> = <x:ref>token</x:ref> [ "=" ( <x:ref>token</x:ref> / <x:ref>quoted-string</x:ref> ) ]
When the no-cache directive is present in a request message, an application &SHOULD;
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 <xref target="cache-response-directive" />) and is defined here for backward
compatibility with HTTP/1.0. Clients &SHOULD; 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
&SHOULD; treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".
<x:h>Note:</x:h> because the meaning of "Pragma: no-cache" as a response-header field
is not actually specified, it does not provide a reliable replacement for
"Cache-Control: no-cache" in a response.</t>
<section anchor="header.vary" title="Vary">
<iref item="Vary header" primary="true" x:for-anchor="" />
<iref item="Headers" primary="true" subitem="Vary" x:for-anchor="" />
<x:anchor-alias value="Vary-v"/>
The "Vary" response-header field's value indicates the set of request-header fields that
determines, while the response is fresh, whether a cache is permitted to use the
response to reply to a subsequent request without validation; see <xref
target="caching.negotiated.responses" />.
In uncacheable or stale responses, the Vary field value advises the user agent about
the criteria that were used to select the representation.
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Vary"/><iref primary="true" item="Grammar" subitem="Vary-v"/>
<x:ref>Vary</x:ref> = "Vary" ":" <x:ref>OWS</x:ref> <x:ref>Vary-v</x:ref>
<x:ref>Vary-v</x:ref> = "*" / 1#<x:ref>field-name</x:ref>
The set of header fields named by the Vary field value is known as the selecting
request-headers.
Servers &SHOULD; include a Vary header field with any cacheable response that is
subject to server-driven negotiation. Doing so allows a cache to properly interpret future
requests on that resource and informs the user agent about the presence of negotiation on
that resource. A server &MAY; include a Vary header field with a non-cacheable
response that is subject to server-driven negotiation, since this might provide the user
agent with useful information about the dimensions over which the response varies at the
time of the response.
A Vary field value of "*" signals that unspecified parameters not limited to the
request-headers (e.g., the network address of the client), play a role in the selection of
the response representation; therefore, a cache cannot determine whether this response is
appropriate. The "*" value &MUST-NOT; be generated by a proxy server;
it may only be generated by an origin server.
The field-names given are not limited to the set of standard request-header fields
defined by this specification. Field names are case-insensitive.
<section anchor="header.warning" title="Warning">
<iref item="Warning header" primary="true" x:for-anchor="" />
<iref item="Headers" primary="true" subitem="Warning" x:for-anchor="" />
<x:anchor-alias value="Warning-v"/>
The general-header field "Warning" 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 entity body of the message.
Warnings can be used for other purposes, both cache-related and otherwise. The use of a
warning, rather than an error status code, distinguish these responses from true failures.
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.
<figure><artwork type="abnf2616"><iref primary="true" item="Grammar" subitem="Warning"/><iref primary="true" item="Grammar" subitem="Warning-v"/><iref primary="true" item="Grammar" subitem="warning-value"/><iref primary="true" item="Grammar" subitem="warn-code"/><iref primary="true" item="Grammar" subitem="warn-agent"/><iref primary="true" item="Grammar" subitem="warn-text"/><iref primary="true" item="Grammar" subitem="warn-date"/>
<x:ref>Warning</x:ref> = "Warning" ":" <x:ref>OWS</x:ref> <x:ref>Warning-v</x:ref>
<x:ref>Warning-v</x:ref> = 1#<x:ref>warning-value</x:ref>
<x:ref>warn-agent</x:ref> = ( <x:ref>uri-host</x:ref> [ ":" <x:ref>port</x:ref> ] ) / <x:ref>pseudonym</x:ref>
Multiple warnings can be attached to a response (either by the origin server or by
a cache), including multiple warnings with the same code number. For example, a server
might provide the same warning with texts in both English and Basque.
When this occurs, the user agent &SHOULD; inform the user of as many of them as
possible, in the order that they appear in the response. If it is not possible to inform
the user of all of the warnings, the user agent &SHOULD; follow these heuristics:
<t>Warnings that appear early in the response take priority over those appearing later
in the response.</t>
<t>Warnings in the user's preferred character set take priority over warnings in other
character sets but with identical warn-codes and warn-agents.</t>
Systems that generate multiple Warning headers &SHOULD; order them with this user
agent behavior in mind. New Warning headers &SHOULD; be added after any existing
Warning headers.
Warnings are assigned three digit warn-codes. The first digit indicates whether the
Warning is required to be deleted from a stored response after validation:
<t>1xx Warnings that describe the freshness or validation status of the response, and so
&MUST; be deleted by caches after validation. They &MUST-NOT; be generated by a cache
except when validating a cached entry, and &MUST-NOT; be generated by clients.</t>
<t>2xx Warnings that describe some aspect of the entity body or entity headers that is
not rectified by a validation (for example, a lossy compression of the entity bodies)
and &MUST-NOT; be deleted by caches after validation, unless a full response is
returned, in which case they &MUST; be.</t>
The warn-text &SHOULD; be in a natural language and character set that is most likely
to be intelligible to the human user receiving the response. This decision can be based on
any available knowledge, such as the location of the cache or user, the Accept-Language
field in a request, the Content-Language field in a response, etc. The default language is
English and the default character set is ISO-8859-1 (<xref target="ISO-8859-1" />).
If a character set other than ISO-8859-1 is used, it &MUST; be encoded in the
warn-text using the method described in <xref target="RFC2047" />.
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 &MUST; include in each warning-value a
warn-date that matches the Date header in the message.
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 &MUST; be deleted from the message before storing, forwarding, or using
it. (preventing the consequences of naive caching of Warning header fields.) If all of the
warning-values are deleted for this reason, the Warning header &MUST; be deleted as
The following warn-codes are defined by this specification, each with a recommended
warn-text in English, and a description of its meaning.
<t><?rfc needLines="4"?>
<t>&SHOULD; be included whenever the returned response is stale.</t>
<t>&SHOULD; be included if a cache returns a stale response because an attempt to
validate the response failed, due to an inability to reach the server.</t>
<t>&SHOULD; be included if the cache is intentionally disconnected from the rest of
the network for a period of time.</t>
<t>&SHOULD; be included if the cache heuristically chose a freshness lifetime
greater than 24 hours and the response's age is greater than 24 hours.</t>
<t>The warning text can include arbitrary information to be presented to a human
user, or logged. A system receiving this warning &MUST-NOT; take any automated
action, besides presenting the warning to the user.</t>
<t>&MUST; be added by an intermediate cache or proxy if it applies any
transformation changing the content-coding (as specified in the Content-Encoding
header) or media-type (as specified in the Content-Type header) of the response, or
the entity-body of the response, unless this Warning code already appears in the
action.</t>
<section anchor="history.lists" title="History Lists">
User agents often have history mechanisms, such as "Back" buttons and history lists, that
can be used to redisplay an entity retrieved earlier in a session.
History mechanisms and caches are different. In particular history mechanisms
&SHOULD-NOT; try to show a correct view of the current state of a resource. Rather, a
history mechanism is meant to show exactly what the user saw at the time when the resource
was retrieved.
By default, an expiration time does not apply to history mechanisms. If the entity is still
in storage, a history mechanism &SHOULD; display it even if the entity has expired,
unless the user has specifically configured the agent to refresh expired history documents.
This is not to be construed to prohibit the history mechanism from telling the user that a
view might be stale.
<x:h>Note:</x:h> if history list mechanisms unnecessarily prevent users from viewing
stale resources, this will tend to force service authors to avoid using HTTP expiration
controls and cache controls when they would otherwise like to. Service authors may
consider it important that users not be presented with error messages or warning
messages when they use navigation controls (such as BACK) to view previously fetched
resources. Even though sometimes such resources ought not be cached, or ought to expire
quickly, user interface considerations may force service authors to resort to other
means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects
of improperly functioning history mechanisms.</t>
<section anchor="IANA.considerations" title="IANA Considerations">
<section anchor="message.header.registration" title="Message Header Registration">
The Message Header Registry located at <eref
target="http://www.iana.org/assignments/message-headers/message-header-index.html" />
should be updated with the permanent registrations below (see <xref target="RFC3864" />):
<texttable align="left" anchor="iana.header.registration.table" suppress-title="true">
<xref target="header.age" />
<xref target="header.cache-control" />
<xref target="header.expires" />
<xref target="header.pragma" />
<xref target="header.vary" />
<xref target="header.warning" />
<section anchor="security.considerations" title="Security Considerations">
Caches expose additional potential vulnerabilities, since the contents of the cache
represent an attractive target for malicious exploitation. Because cache contents persist
after an HTTP request is complete, an attack on the cache can reveal information long after
a user believes that the information has been removed from the network. Therefore, cache
contents should be protected as sensitive information.
<section anchor="ack" title="Acknowledgments">
Much of the content and presentation of the caching design is due to suggestions and
comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris,
and Larry Masinter.
<title> Information technology -- 8-bit single-byte coded graphic character sets -- Part
1: Latin alphabet No. 1 </title>
<date year="1998" />
<seriesInfo name="ISO/IEC" value="8859-1:1998" />
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p1-messaging-&ID-VERSION;" />
<x:source basename="p1-messaging" href="p1-messaging.xml" />
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p2-semantics-&ID-VERSION;" />
<x:source basename="p2-semantics" href="p2-semantics.xml" />
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p3-payload-&ID-VERSION;" />
<x:source basename="p3-payload" href="p3-payload.xml" />
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p4-conditional-&ID-VERSION;" />
<x:source basename="p4-conditional" href="p4-conditional.xml" />
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p5-range-&ID-VERSION;" />
<x:source basename="p5-range" href="p5-range.xml" />
<seriesInfo name="Internet-Draft" value="draft-ietf-httpbis-p7-auth-&ID-VERSION;" />
<x:source basename="p7-auth" href="p7-auth.xml" />
<title abbrev="Message Header Extensions">MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text</title>
<author fullname="Keith Moore" initials="K." surname="Moore">
<date month="November" year="1996" />
<seriesInfo name="RFC" value="2047" />
<author fullname="Scott Bradner" initials="S." surname="Bradner">
<date month="March" year="1997" />
<seriesInfo name="BCP" value="14" />
<seriesInfo name="RFC" value="2119" />
<author fullname="David L. Mills" initials="D." surname="Mills">
<date month="March" year="1992" />
<seriesInfo name="RFC" value="1305" />
<author fullname="R. Fielding" initials="R." surname="Fielding">
<author fullname="J. Gettys" initials="J." surname="Gettys">
<author fullname="J. Mogul" initials="J." surname="Mogul">
<author fullname="H. Frystyk" initials="H." surname="Frystyk">
<author fullname="L. Masinter" initials="L." surname="Masinter">
<author fullname="P. Leach" initials="P." surname="Leach">
<author fullname="T. Berners-Lee" initials="T." surname="Berners-Lee">
<date month="June" year="1999" />
<seriesInfo name="RFC" value="2616" />
<author fullname="G. Klyne" initials="G." surname="Klyne">
<author fullname="M. Nottingham" initials="M." surname="Nottingham">
<date month="September" year="2004" />
<seriesInfo name="BCP" value="90" />
<seriesInfo name="RFC" value="3864" />
<section anchor="compatibility" title="Compatibility with Previous Versions">
<section anchor="changes.from.rfc.2068" title="Changes from RFC 2068">
A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
this missing case.
(Sections <xref format="counter" target="response.cacheability" />, <xref format="counter" target="header.cache-control" />).
Transfer-coding and message lengths all interact in ways that required fixing exactly
when chunked encoding is used (to allow for transfer encoding that may not be self
delimiting); it was important to straighten out exactly how message lengths are computed.
(see also <xref target="Part1" />, <xref target="Part3" /> and <xref target="Part5" />)
<cref source="jre">This used to refer to the text about non-modifiable headers, and will have to be updated later on.</cref>
>Range request responses would become very verbose if all meta-data were always returned;
by allowing the server to only send needed headers in a 206 response, this problem can be
(<xref target="combining.headers" />)
The Cache-Control: max-age directive was not properly defined for responses.
(<xref target="cache-response-directive"/>)
Warnings could be cached incorrectly, or not updated appropriately. (Section <xref
format="counter" target="expiration.model" />, <xref format="counter"
target="combining.headers" />, <xref format="counter" target="header.cache-control" />,
and <xref format="counter" target="header.warning" />) Warning also needed to be a general
header, as PUT or other methods may have need for it in requests.
<section anchor="changes.from.rfc.2616" title="Changes from RFC 2616">
(<xref target="invalidation.after.updates.or.deletions" />)
<artwork type="abnf" name="p6-cache.parsed-abnf">
<x:ref>Age</x:ref> = "Age:" OWS Age-v
<x:ref>Age-v</x:ref> = delta-seconds
<x:ref>Cache-Control</x:ref> = "Cache-Control:" OWS Cache-Control-v
<x:ref>Cache-Control-v</x:ref> = *( "," OWS ) cache-directive *( OWS "," [ OWS
<x:ref>Expires</x:ref> = "Expires:" OWS Expires-v
<x:ref>Expires-v</x:ref> = HTTP-date
<x:ref>HTTP-date</x:ref> = &lt;HTTP-date, defined in [Part1], Section 3.2.1&gt;
<x:ref>Pragma</x:ref> = "Pragma:" OWS Pragma-v
<x:ref>Pragma-v</x:ref> = *( "," OWS ) pragma-directive *( OWS "," [ OWS
<x:ref>Vary</x:ref> = "Vary:" OWS Vary-v
<x:ref>Vary-v</x:ref> = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
<x:ref>Warning</x:ref> = "Warning:" OWS Warning-v
<x:ref>Warning-v</x:ref> = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
<x:ref>cache-directive</x:ref> = cache-request-directive / cache-response-directive
<x:ref>cache-extension</x:ref> = token [ "=" ( token / quoted-string ) ]
<x:ref>cache-request-directive</x:ref> = "no-cache" / "no-store" / ( "max-age="
<x:ref>cache-response-directive</x:ref> = "public" / ( "private" [ "=" DQUOTE *( ","
<x:ref>delta-seconds</x:ref> = 1*DIGIT
<x:ref>extension-pragma</x:ref> = token [ "=" ( token / quoted-string ) ]
<x:ref>field-name</x:ref> = &lt;field-name, defined in [Part1], Section 4.2&gt;
<x:ref>port</x:ref> = &lt;port, defined in [Part1], Section 2.1&gt;
<x:ref>pragma-directive</x:ref> = "no-cache" / extension-pragma
<x:ref>pseudonym</x:ref> = &lt;pseudonym, defined in [Part1], Section 8.9&gt;
<x:ref>quoted-string</x:ref> = &lt;quoted-string, defined in [Part1], Section 1.2.2&gt;
<x:ref>uri-host</x:ref> = &lt;uri-host, defined in [Part1], Section 2.1&gt;
<x:ref>warn-agent</x:ref> = ( uri-host [ ":" port ] ) / pseudonym
<x:ref>warn-code</x:ref> = 3DIGIT
<x:ref>warn-date</x:ref> = DQUOTE HTTP-date DQUOTE
<x:ref>warn-text</x:ref> = quoted-string
<x:ref>warning-value</x:ref> = warn-code SP warn-agent SP warn-text [ SP warn-date
<section anchor="change.log" title="Change Log (to be removed by RFC Editor before publication)">
<t>Extracted relevant partitions from <xref target="RFC2616" />.</t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/9" />: "Trailer" (<eref target="http://purl.org/NET/http-errata#trailer-hop" />)</t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/12" />: "Invalidation after Update or Delete" (<eref target="http://purl.org/NET/http-errata#invalidupd" />)</t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/35" />: "Normative and Informative references"</t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/48" />: "Date reference typo"</t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/49" />: "Connection header text"</t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/65" />: "Informative references"</t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/66" />: "ISO-8859-1 Reference"</t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/86" />: "Normative up-to-date references"</t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/87" />: "typo in 13.2.2"</t>
<t>Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on <eref
target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/82" />: "rel_path not used"</t>
<t>Get rid of duplicate BNF rule names ("host" -&gt; "uri-host") (work in progress
on <eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36" />)</t>
<t>Add explicit references to BNF syntax and rules imported from other parts of the
specification.</t>
<section anchor="changes.since.02" title="Since draft-ietf-httpbis-p6-cache-02">
Ongoing work on IANA Message Header Registration (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/40" />):
<t>Reference RFC 3984, and update header registrations for headers defined in this
document.</t>
<section anchor="changes.since.03" title="Since draft-ietf-httpbis-p6-cache-03">
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/106" />: "Vary header classification"</t>
<section anchor="changes.since.04" title="Since draft-ietf-httpbis-p6-cache-04">
<section anchor="changes.since.05" title="Since draft-ietf-httpbis-p6-cache-05">
<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/54" />: "Definition of 1xx Warn-Codes"</t>
<eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/60" />: "Placement of 13.5.1 and 13.5.2"</t>
<eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/138" />: "The role of Warning and Semantic Transparency in Caching"</t>
<eref target="http://trac.tools.ietf.org/wg/httpbis/trac/ticket/139" />: "Methods and Caching"</t>
In addition: Final work on ABNF conversion (<eref target="http://tools.ietf.org/wg/httpbis/trac/ticket/36"/>):