HTTP/1.1, part 6: CachingDay Software23 Corporate Plaza DR, Suite 280Newport BeachCA92660USA+1-949-706-5300+1-949-706-5305fielding@gbiv.comhttp://roy.gbiv.com/One Laptop per Child21 Oak Knoll RoadCarlisleMA01741USAjg@laptop.orghttp://www.laptop.org/Hewlett-Packard CompanyHP Labs, Large Scale Systems Group1501 Page Mill Road, MS 1177Palo AltoCA94304USAJeffMogul@acm.orgMicrosoft Corporation1 Microsoft WayRedmondWA98052USAhenrikn@microsoft.comAdobe Systems, Incorporated345 Park AveSan JoseCA95110USALMM@acm.orghttp://larry.masinter.net/Microsoft Corporation1 Microsoft WayRedmondWA98052paulle@microsoft.comWorld Wide Web ConsortiumMIT Computer Science and Artificial Intelligence LaboratoryThe Stata Center, Building 3232 Vassar StreetCambridgeMA02139USAtimbl@w3.orghttp://www.w3.org/People/Berners-Lee/World Wide Web ConsortiumW3C / ERCIM2004, rte des LuciolesSophia-AntipolisAM06902Franceylafon@w3.orghttp://www.raubacapeu.net/people/yves/greenbytes GmbHHafenweg 16MuensterNW48155Germany+49 251 2807760+49 251 2807761julian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/HTTPbis Working Group
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 and related documents
(including fancy diffs) can be found at .
The changes in this draft are summarized in .
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.
An HTTP cache 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 ). 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 ).
This specification uses a number of terms to refer to the roles played by participants
in, and objects of, HTTP caching.
cacheable
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.
explicit expiration time
The time at which the origin server intends that an entity should no longer be
returned by a cache without further validation.
heuristic expiration time
An expiration time assigned by a cache when no explicit expiration time is
available.
age
The age of a response is the time since it was sent by, or successfully validated
with, the origin server.
first-hand
A response is first-hand if the freshness model is not in use; i.e., its age is
0.
freshness lifetime
The length of time between the generation of a response and its expiration time.
fresh
A response is fresh if its age has not yet exceeded its freshness lifetime.
stale
A response is stale if its age has passed its freshness lifetime (either explicit or heuristic).
validator
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.
shared cache
A cache that is accessible to more than one user. A non-shared cache is
dedicated to a single user.
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 .
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."
This specification uses the ABNF syntax defined in Section 1.2 of (which
extends the syntax defined in with a list rule).
shows the collected ABNF, with the list
rule expanded.
The following core rules are included by
reference, as defined in , Appendix B.1:
ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls),
DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed),
OCTET (any 8-bit sequence of data), SP (space),
VCHAR (any visible USASCII character),
and WSP (whitespace).
The core rules below are defined in Section 1.2.2 of :
The ABNF rules below are defined in other parts:
A cache MUST NOT store a response to any request, unless:
The request method is defined as being cacheable, andthe "no-store" cache directive (see ) does not
appear in request or response headers, andthe "private" cache response directive (see
does not appear in the response, if the cache is shared, andthe "Authorization" header (see Section 3.1 of ) does not appear in the request, if
the cache is shared (unless the "public" directive is present; see ), andthe cache understands partial responses, if the response is partial or incomplete
(see ).
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.
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 responses
can be combined as described in Section 4 of ; 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.
For a presented request, a cache MUST NOT return a stored response, unless:
The presented Request-URI and that of the stored response match (see
TBD), andthe request method associated with the stored response allows it to be
used for the presented request, andselecting request-headers nominated by the stored response (if any) match those presented (see ), andthe presented request and stored response are free from directives that would prevent
its use (see and ),
andthe stored response is either:
fresh (see ), orallowed to be served stale (see ), orsuccessfully validated (see ).TODO: define method cacheability for GET, HEAD and POST in p2-semantics.
When a stored response is used to satisfy a request, caches MUST include a
single Age header field in the response with a value equal to the stored response's
current_age; see .
DISCUSS: this currently includes successfully validated responses.
Requests with methods that are unsafe (Section 7.1.1 of ) 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
.
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
use.
TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in p1
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 () or the max-age response cache 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.
This wording may cause confusion, because the response may still be served stale.
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.
The freshness_lifetime is defined in ;
the current_age is defined in .
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 .
ISSUE: there are not requirements directly applying to cache-request-directives and
freshness.
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 for an explanation of
the difference between caches and history mechanisms.
A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a
response by using the first match of:
If the cache is shared and the s-maxage response cache directive () is present, use its value, orIf the max-age response cache directive () is present, use its value, orIf the Expires response header () is present, use
its value minus the value of the Date response header, orOtherwise, no explicit expiration time is present in the response, but a heuristic
may be used; see .
Note that this calculation is not vulnerable to clock skew, since all of the
information comes from the origin server.
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 (Section 6.6 of ), the
heuristic expiration value SHOULD be no more than some fraction of the interval
since that time. A typical setting of this fraction might be 10%.
REVIEW: took away HTTP/1.0 query string heuristic uncacheability.
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
arithmetic operations.
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 Section 8.3 of ).
The term "date_value" denotes the value of the Date header, in a form appropriate for
arithmetic operations.
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 or some similar protocol to
synchronize their clocks to a globally accurate time standard.
A response's age can be calculated in two entirely independent ways:
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.age_value, if all of the caches along the response path implement HTTP/1.1.
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.
In summary:
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
.
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 ).
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 ).
Stale responses SHOULD have a Warning header with the 110 warn-code (see ). 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.
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 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 .
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. Should there be a requirement here?
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 ) unless the
stored response includes the "must-revalidate" cache directive (see ).
Because unsafe methods (Section 7.1.1 of ) have the potential for changing state on the
origin server, intervening caches can use them to keep their contents
up-to-date.
The following HTTP methods MUST cause a cache to invalidate the Request-URI as well
as the Location and Content-Location headers (if present):
PUTDELETEPOST
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.
TODO: "host part" needs to be specified better.
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.
TODO: specify that only successful (2xx, 3xx?) responses invalidate.
Use of server-driven content negotiation (Section 4.1 of ) alters
the conditions under which a cache can use the response for subsequent
requests.
When a cache receives a request that can be satisfied by a stored response
that includes a Vary header field (), 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
[ref] 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 Section 4.2 of . DISCUSS: header-specific canonicalisation
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 .
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.DISCUSS: Not sure if this is necessary.
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 Section 4 of ).
The stored response headers are used for the updated response, except that
any stored Warning headers with warn-code 1xx (see )
MUST be deleted from the stored response and the forwarded response.any stored Warning headers with warn-code 2xx MUST be retained in the stored
response and the forwarded response.any headers provided in the 304 or 206 response MUST replace the corresponding
headers from the stored response.
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.
ISSUE: discuss how to handle HEAD updatesThis section defines the syntax and semantics of HTTP/1.1 header fields related to caching.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.
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 .
Age field-values are non-negative decimal integers, representing time in 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 (2^31). 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.
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.
Note that HTTP/1.0 caches might not implement Cache-Control and might only implement
Pragma: no-cache (see ).
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.
no-cache
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.
no-store
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.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.
max-age
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.
max-stale
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. of any staleness?
min-fresh
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.
no-transform
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.
only-if-cached
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.
public
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, Section 3.1 of , for additional details.)
private
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.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.
Note: This usage of the word private only controls where the response may
be stored, and cannot ensure the privacy of the message content.
no-cache
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.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.
Note: Most HTTP/1.0 caches will not recognize or obey this directive.
no-store
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
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.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.
must-revalidate
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.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.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.
proxy-revalidate
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.
max-age
The max-age response directive indicates that response is to be considered stale
after its age is greater than the specified number of seconds.
s-maxage
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.
no-transform
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.
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).
The entity-header field "Expires" gives the date/time after which the response is
considered stale. See 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 Section 3.2.1 of ;
it MUST be sent in rfc1123-date format.
Note: if a response includes a Cache-Control field with the max-age
directive (see ), that directive overrides
the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.
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").
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
MAY require that behavior be consistent with the directives.
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 ) 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".
Note: 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.
This mechanism is deprecated; no new Pragma directives will be defined in HTTP.
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 .
In uncacheable or stale responses, the Vary field value advises the user agent about
the criteria that were used to select the representation.
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.
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.
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:
Warnings that appear early in the response take priority over those appearing later
in the response.Warnings in the user's preferred character set take priority over warnings in other
character sets but with identical warn-codes and warn-agents.
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:
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.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.
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 ().
If a character set other than ISO-8859-1 is used, it MUST be encoded in the
warn-text using the method described in .
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
well.
The following warn-codes are defined by this specification, each with a recommended
warn-text in English, and a description of its meaning.
110 Response is stale
SHOULD be included whenever the returned response is stale.
111 Revalidation failed
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.
112 Disconnected operation
SHOULD be included if the cache is intentionally disconnected from the rest of
the network for a period of time.
113 Heuristic expiration
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.
199 Miscellaneous warning
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.
214 Transformation applied
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
response.
299 Miscellaneous persistent warning
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.
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.
Note: 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.
The Message Header Registry located at
should be updated with the permanent registrations below (see ):
Header Field NameProtocolStatusReferenceAgehttpstandardCache-ControlhttpstandardExpireshttpstandardPragmahttpstandardVaryhttpstandardWarninghttpstandard
The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".
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.
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.
Information technology -- 8-bit single-byte coded graphic character sets -- Part
1: Latin alphabet No. 1 International Organization for StandardizationHTTP/1.1, part 1: URIs, Connections, and Message ParsingDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHTTP/1.1, part 2: Message SemanticsDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHTTP/1.1, part 3: Message Payload and Content NegotiationDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHTTP/1.1, part 4: Conditional RequestsDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHTTP/1.1, part 5: Range Requests and Partial ResponsesDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deHTTP/1.1, part 7: AuthenticationDay Softwarefielding@gbiv.comOne Laptop per Childjg@laptop.orgHewlett-Packard CompanyJeffMogul@acm.orgMicrosoft Corporationhenrikn@microsoft.comAdobe Systems, IncorporatedLMM@acm.orgMicrosoft Corporationpaulle@microsoft.comWorld Wide Web Consortiumtimbl@w3.orgWorld Wide Web Consortiumylafon@w3.orggreenbytes GmbHjulian.reschke@greenbytes.deMIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII TextUniversity of Tennesseemoore@cs.utk.eduKey words for use in RFCs to Indicate Requirement LevelsHarvard Universitysob@harvard.eduAugmented BNF for Syntax Specifications: ABNFBrandenburg InternetWorking675 Spruce Dr.SunnyvaleCA94086US+1.408.246.8253dcrocker@bbiw.netTHUS plc.1/2 Berkeley Square99 Berkely StreetGlasgowG3 7HRUKpaul.overell@thus.netNetwork Time Protocol (Version 3) Specification, ImplementationUniversity of Delaware, Electrical Engineering Departmentmills@udel.eduHypertext Transfer Protocol -- HTTP/1.1University of California, Irvinefielding@ics.uci.eduW3Cjg@w3.orgCompaq Computer Corporationmogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgXerox Corporationmasinter@parc.xerox.comMicrosoft Corporationpaulle@microsoft.comW3Ctimbl@w3.orgRegistration Procedures for Message Header FieldsNine by NineGK-IETF@ninebynine.orgBEA Systemsmnot@pobox.comHP LabsJeffMogul@acm.org
A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add
this missing case.
(Sections , ).
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 , and )
This used to refer to the text about non-modifiable headers, and will have to be updated later on.
Proxies should be able to add Content-Length when appropriate.
This used to refer to the text about non-modifiable headers, and will have to be updated later on.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
avoided.
()
The Cache-Control: max-age directive was not properly defined for responses.
()
Warnings could be cached incorrectly, or not updated appropriately. (Section , , ,
and ) Warning also needed to be a general
header, as PUT or other methods may have need for it in requests.
Clarify denial of service attack avoidance requirement.
()
Extracted relevant partitions from .
Closed issues:
: "Trailer" (): "Invalidation after Update or Delete" (): "Normative and Informative references": "Date reference typo": "Connection header text": "Informative references": "ISO-8859-1 Reference": "Normative up-to-date references": "typo in 13.2.2"
Other changes:
Use names of RFC4234 core rules DQUOTE and HTAB (work in progress on )
Closed issues:
: "rel_path not used"
Other changes:
Get rid of duplicate BNF rule names ("host" -> "uri-host") (work in progress
on )Add explicit references to BNF syntax and rules imported from other parts of the
specification.
Ongoing work on IANA Message Header Registration ():
Reference RFC 3984, and update header registrations for headers defined in this
document.
Closed issues:
: "Vary header classification"
Ongoing work on ABNF conversion ():
Use "/" instead of "|" for alternatives.
Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
whitespace ("OWS") and required whitespace ("RWS").
Rewrite ABNFs to spell out whitespace rules, factor out
header value format definitions.
This is a total rewrite of this part of the specification.
Affected issues:
: "Definition of 1xx Warn-Codes": "Placement of 13.5.1 and 13.5.2": "The role of Warning and Semantic Transparency in Caching": "Methods and Caching"
In addition: Final work on ABNF conversion ():
Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.