Section 4.2.3. Age = delta-seconds The Age field-value is a non-negative integer, representing time in seconds (see Section 1.2.1). The presence of an Age header field implies that the response was not generated or validated by the origin server for this request. However, lack of an Age header field does not imply the origin was contacted, since the response might have been received from an HTTP/1.0 cache that does not implement Age. Section 5.2.3 for information about how Cache-Control directives defined elsewhere are handled. Note: Some HTTP/1.0 caches might not implement Cache-Control. A proxy, whether or not it implements a cache, MUST pass cache directives through in forwarded messages, 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. Cache directives are identified by a token, to be compared case-insensitively, and have an optional argument, that can use both token and quoted-string syntax. For the directives defined below that define arguments, recipients ought to accept both forms, even if one is documented to be preferred. For any directive not defined by this specification, a recipient MUST accept both forms.
Cache-Control = 1#cache-directive cache-directive = token [ "=" ( token / quoted-string ) ] For the cache directives defined below, no argument is defined (nor allowed) unless stated otherwise. Section 1.2.1) The "max-age" request directive indicates that the client is unwilling to accept a response whose age is greater than the specified number of seconds. Unless the max-stale request directive is also present, the client is not willing to accept a stale response. This directive uses the token form of the argument syntax: e.g., 'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the quoted-string form. Section 1.2.1) The "max-stale" request directive indicates that the client is willing to accept a response that has exceeded its freshness lifetime. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its freshness lifetime 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. This directive uses the token form of the argument syntax: e.g., 'max-stale=10' not 'max-stale="10"'. A sender SHOULD NOT generate the quoted-string form. Section 1.2.1)
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. This directive uses the token form of the argument syntax: e.g., 'min-fresh=20' not 'min-fresh="20"'. A sender SHOULD NOT generate the quoted-string form. Section 5.7.2 of [RFC7230].
a 504 (Gateway Timeout) status code. If a group of caches is being operated as a unified system with good internal connectivity, a member cache MAY forward such a request within that group of caches.
This directive uses the quoted-string form of the argument syntax. A sender SHOULD NOT generate the token form (even if quoting appears not to be needed for single-entry lists). Note: Although it has been back-ported to many implementations, some HTTP/1.0 caches will not recognize or obey this directive. Also, no-cache response directives with field-names are often handled by caches as if an unqualified no-cache directive was received; i.e., the special handling for the qualified form is not widely implemented. Section 5.7.2 of [RFC7230]. Section 3.2 for additional details related to the use of public in response to a request containing Authorization, and Section 3 for details of how public affects responses that would normally not be stored, due to their status codes not being defined as cacheable by default; see Section 4.2.2.)
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 cache MAY store the response and reuse it for later requests, even if the response would normally be non-cacheable. If the private response directive specifies one or more field-names, this requirement is limited to the field-values associated with the listed response header fields. That is, a shared cache MUST NOT store the specified field-names(s), whereas it MAY store the remainder of the response message. The field-names given are not limited to the set of header fields defined by this specification. Field names are case-insensitive. This directive uses the quoted-string form of the argument syntax. A sender SHOULD NOT generate the token form (even if quoting appears not to be needed for single-entry lists). Note: This usage of the word "private" only controls where the response can be stored; it cannot ensure the privacy of the message content. Also, private response directives with field-names are often handled by caches as if an unqualified private directive was received; i.e., the special handling for the qualified form is not widely implemented. Section 1.2.1) The "max-age" response directive indicates that the response is to be considered stale after its age is greater than the specified number of seconds. This directive uses the token form of the argument syntax: e.g., 'max-age=5' not 'max-age="5"'. A sender SHOULD NOT generate the quoted-string form.
Section 1.2.1) 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 field. The s-maxage directive also implies the semantics of the proxy-revalidate response directive. This directive uses the token form of the argument syntax: e.g., 's-maxage=10' not 's-maxage="10"'. A sender SHOULD NOT generate the quoted-string form.
Section 4.2 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 Expires value is an HTTP-date timestamp, as defined in Section 18.104.22.168 of [RFC7231]. Expires = HTTP-date For example Expires: Thu, 01 Dec 1994 16:00:00 GMT A cache recipient MUST interpret invalid date formats, especially the value "0", as representing a time in the past (i.e., "already expired"). If a response includes a Cache-Control field with the max-age directive (Section 22.214.171.124), a recipient MUST ignore the Expires field. Likewise, if a response includes the s-maxage directive (Section 126.96.36.199), a shared cache recipient MUST ignore the Expires field. In both these cases, the value in Expires is only intended for recipients that have not yet implemented the Cache-Control field. An origin server without a clock MUST NOT generate an Expires field unless its value represents a fixed time in the past (always expired) or its value has been associated with the resource by a system or user with a reliable clock. Historically, HTTP required the Expires field-value to be no more than a year in the future. While longer freshness lifetimes are no longer prohibited, extremely large values have been demonstrated to cause problems (e.g., clock overflows due to use of 32-bit integers for time values), and many caches will evict a response far sooner than that.
Section 5.2.1). When sending a no-cache request, a client ought to include both the pragma and cache-control directives, unless Cache-Control: no-cache is purposefully omitted to target other Cache-Control response directives at HTTP/1.1 caches. For example: GET / HTTP/1.1 Host: www.example.com Cache-Control: max-age=30 Pragma: no-cache will constrain HTTP/1.1 caches to serve a response no older than 30 seconds, while precluding implementations that do not understand Cache-Control from serving a cached response. Note: Because the meaning of "Pragma: no-cache" in responses is not specified, it does not provide a reliable replacement for "Cache-Control: no-cache" in them.
Warnings can be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status code, distinguishes these responses from true failures. Warning header fields can in general be applied to any message, however some warn-codes are specific to caches and can only be applied to response messages. Warning = 1#warning-value warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date ] warn-code = 3DIGIT warn-agent = ( uri-host [ ":" port ] ) / pseudonym ; the name or pseudonym of the server adding ; the Warning header field, for use in debugging ; a single "-" is recommended when agent unknown warn-text = quoted-string warn-date = DQUOTE HTTP-date DQUOTE Multiple warnings can be generated in a response (either by the origin server or by a cache), including multiple warnings with the same warn-code number that only differ in warn-text. A user agent that receives one or more Warning header fields SHOULD inform the user of as many of them as possible, in the order that they appear in the response. Senders that generate multiple Warning header fields are encouraged to order them with this user agent behavior in mind. A sender that generates new Warning header fields MUST append them after any existing Warning header fields. 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: o 1xx warn-codes describe the freshness or validation status of the response, and so they MUST be deleted by a cache after validation. They can only be generated by a cache when validating a cached entry, and MUST NOT be generated in any other situation. o 2xx warn-codes describe some aspect of the representation that is not rectified by a validation (for example, a lossy compression of the representation) and they MUST NOT be deleted by a cache after validation, unless a full response is sent, in which case they MUST be.
If a sender generates one or more 1xx warn-codes in a message to be sent to a recipient known to implement only HTTP/1.0, the sender MUST include in each corresponding warning-value a warn-date that matches the Date header field in the message. For example: HTTP/1.1 200 OK Date: Sat, 25 Aug 2012 23:34:45 GMT Warning: 112 - "network down" "Sat, 25 Aug 2012 23:34:45 GMT" Warnings have accompanying warn-text that describes the error, e.g., for logging. It is advisory only, and its content does not affect interpretation of the warn-code. If a recipient that uses, evaluates, or displays Warning header fields receives a warn-date that is different from the Date value in the same message, the recipient MUST exclude the warning-value containing that warn-date before storing, forwarding, or using the message. This allows recipients to exclude warning-values that were improperly retained after a cache validation. If all of the warning-values are excluded, the recipient MUST exclude the Warning header field 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. The procedure for defining additional warn codes is described in Section 7.2.1.
Section 4.2) does not necessarily apply to history mechanisms. That is, a history mechanism can display a previous representation even if it has expired. This does not prohibit the history mechanism from telling the user that a view might be stale or from honoring cache directives (e.g., Cache-Control: no-store). http://www.iana.org/assignments/http-cache-directives>.
o Pointer to specification text Values to be added to this namespace require IETF Review (see [RFC5226], Section 4.1). Section 5.2.3. Section 188.8.131.52, Section 184.108.40.206 | | max-stale | Section 220.127.116.11 | | min-fresh | Section 18.104.22.168 | | must-revalidate | Section 22.214.171.124 | | no-cache | Section 126.96.36.199, Section 188.8.131.52 | | no-store | Section 184.108.40.206, Section 220.127.116.11 | | no-transform | Section 18.104.22.168, Section 22.214.171.124 | | only-if-cached | Section 126.96.36.199 | | private | Section 188.8.131.52 | | proxy-revalidate | Section 184.108.40.206 | | public | Section 220.127.116.11 | | s-maxage | Section 18.104.22.168 | | stale-if-error | [RFC5861], Section 4 | | stale-while-revalidate | [RFC5861], Section 3 | +------------------------+----------------------------------+
http://www.iana.org/assignments/http-warn-codes>. RFC5226], Section 4.1). Section 5.5.1 | | 111 | Revalidation Failed | Section 5.5.2 | | 112 | Disconnected Operation | Section 5.5.3 | | 113 | Heuristic Expiration | Section 5.5.4 | | 199 | Miscellaneous Warning | Section 5.5.5 | | 214 | Transformation Applied | Section 5.5.6 | | 299 | Miscellaneous Persistent Warning | Section 5.5.7 | +-----------+----------------------------------+---------------+ http://www.iana.org/assignments/message-headers/>.
This document defines the following HTTP header fields, so the "Permanent Message Header Field Names" registry has been updated accordingly (see [BCP90]). +-------------------+----------+----------+-------------+ | Header Field Name | Protocol | Status | Reference | +-------------------+----------+----------+-------------+ | Age | http | standard | Section 5.1 | | Cache-Control | http | standard | Section 5.2 | | Expires | http | standard | Section 5.3 | | Pragma | http | standard | Section 5.4 | | Warning | http | standard | Section 5.5 | +-------------------+----------+----------+-------------+ The change controller is: "IETF (email@example.com) - Internet Engineering Task Force". RFC7230] and semantics [RFC7231]. 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 need to be protected as sensitive information. In particular, various attacks might be amplified by being stored in a shared cache; such "cache poisoning" attacks use the cache to distribute a malicious payload to many clients, and are especially effective when an attacker can use implementation flaws, elevated privileges, or other techniques to insert such a response into a cache. One common attack vector for cache poisoning is to exploit differences in message parsing on proxies and in user agents; see Section 3.3.3 of [RFC7230] for the relevant requirements. Likewise, implementation flaws (as well as misunderstanding of cache operation) might lead to caching of sensitive information (e.g., authentication credentials) that is thought to be private, exposing it to unauthorized parties.
Furthermore, the very use of a cache can bring about privacy concerns. For example, if two users share a cache, and the first one browses to a site, the second may be able to detect that the other has been to that site, because the resources from it load more quickly, thanks to the cache. Note that the Set-Cookie response header field [RFC6265] does not inhibit caching; a cacheable response with a Set-Cookie header field can be (and often is) used to satisfy subsequent requests to caches. Servers who wish to control caching of these responses are encouraged to emit appropriate Cache-Control response header fields. Section 10 of [RFC7230]. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, June 2014. [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, June 2014. [RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests", RFC 7232, June 2014. [RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Range Requests", RFC 7233, June 2014. [RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014.
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, September 2004. [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5861] Nottingham, M., "HTTP Cache-Control Extensions for Stale Content", RFC 5861, April 2010. [RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010. [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, April 2011.
Section 3.2) New status codes can now define that caches are allowed to use heuristic freshness with them. Caches are now allowed to calculate heuristic freshness for URIs with query components. (Section 4.2.2) The algorithm for calculating age is now less conservative. Caches are now required to handle dates with time zones as if they're invalid, because it's not possible to accurately guess. (Section 4.2.3) The Content-Location response header field is no longer used to determine the appropriate response to use when validating. (Section 4.3) The algorithm for selecting a cached negotiated response to use has been clarified in several ways. In particular, it now explicitly allows header-specific canonicalization when processing selecting header fields. (Section 4.1) Requirements regarding denial-of-service attack avoidance when performing invalidation have been clarified. (Section 4.4) Cache invalidation only occurs when a successful response is received. (Section 4.4) Cache directives are explicitly defined to be case-insensitive. Handling of multiple instances of cache directives when only one is expected is now defined. (Section 5.2) The "no-store" request directive doesn't apply to responses; i.e., a cache can satisfy a request with no-store on it and does not invalidate it. (Section 22.214.171.124) The qualified forms of the private and no-cache cache directives are noted to not be widely implemented; for example, "private=foo" is interpreted by many caches as simply "private". Additionally, the meaning of the qualified form of no-cache has been clarified. (Section 5.2.2) The "no-cache" response directive's meaning has been clarified. (Section 126.96.36.199)
The one-year limit on Expires header field values has been removed; instead, the reasoning for using a sensible value is given. (Section 5.3) The Pragma header field is now only defined for backwards compatibility; future pragmas are deprecated. (Section 5.4) Some requirements regarding production and processing of the Warning header fields have been relaxed, as it is not widely implemented. Furthermore, the Warning header field no longer uses RFC 2047 encoding, nor does it allow multiple languages, as these aspects were not implemented. (Section 5.5) This specification introduces the Cache Directive and Warn Code Registries, and defines considerations for new cache directives. (Section 7.1 and Section 7.2) Appendix B.1 of [RFC5234]: 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), and VCHAR (any visible US-ASCII character). The rules below are defined in [RFC7230]: OWS = <OWS, see [RFC7230], Section 3.2.3> field-name = <field-name, see [RFC7230], Section 3.2> quoted-string = <quoted-string, see [RFC7230], Section 3.2.6> token = <token, see [RFC7230], Section 3.2.6> port = <port, see [RFC7230], Section 2.7> pseudonym = <pseudonym, see [RFC7230], Section 5.7.1> uri-host = <uri-host, see [RFC7230], Section 2.7> The rules below are defined in other parts: HTTP-date = <HTTP-date, see [RFC7231], Section 188.8.131.52>
Section 1.2 of [RFC7230]. Age = delta-seconds Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS cache-directive ] ) Expires = HTTP-date HTTP-date = <HTTP-date, see [RFC7231], Section 184.108.40.206> OWS = <OWS, see [RFC7230], Section 3.2.3> Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS pragma-directive ] ) Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ] ) cache-directive = token [ "=" ( token / quoted-string ) ] delta-seconds = 1*DIGIT extension-pragma = token [ "=" ( token / quoted-string ) ] field-name = <field-name, see [RFC7230], Section 3.2> port = <port, see [RFC7230], Section 2.7> pragma-directive = "no-cache" / extension-pragma pseudonym = <pseudonym, see [RFC7230], Section 5.7.1> quoted-string = <quoted-string, see [RFC7230], Section 3.2.6> token = <token, see [RFC7230], Section 3.2.6> uri-host = <uri-host, see [RFC7230], Section 2.7> warn-agent = ( uri-host [ ":" port ] ) / pseudonym warn-code = 3DIGIT warn-date = DQUOTE HTTP-date DQUOTE warn-text = quoted-string warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date ]
31 111 (warn-code) 31 112 (warn-code) 31 113 (warn-code) 31 199 (warn-code) 32 2 214 (warn-code) 32 299 (warn-code) 32 A age 11 Age header field 21 C cache 4 cache entry 5 cache key 5-6 Cache-Control header field 21 D Disconnected Operation (warn-text) 31 E Expires header field 28 explicit expiration time 11 F fresh 11 freshness lifetime 11 G Grammar Age 21 Cache-Control 22 cache-directive 22 delta-seconds 5 Expires 28 extension-pragma 29 Pragma 29 pragma-directive 29 warn-agent 29 warn-code 29 warn-date 29 warn-text 29
Warning 29 warning-value 29 H Heuristic Expiration (warn-text) 31 heuristic expiration time 11 M max-age (cache directive) 22, 26 max-stale (cache directive) 22 min-fresh (cache directive) 22 Miscellaneous Persistent Warning (warn-text) 32 Miscellaneous Warning (warn-text) 32 must-revalidate (cache directive) 24 N no-cache (cache directive) 23, 25 no-store (cache directive) 23, 24 no-transform (cache directive) 23, 25 O only-if-cached (cache directive) 23 P Pragma header field 29 private (cache directive) 25 private cache 4 proxy-revalidate (cache directive) 26 public (cache directive) 25 R Response is Stale (warn-text) 30 Revalidation Failed (warn-text) 31 S s-maxage (cache directive) 27 shared cache 4 stale 11 strong validator 18 T Transformation Applied (warn-text) 32 V validator 16 W Warning header field 29
http://roy.gbiv.com/ Mark Nottingham (editor) Akamai EMail: firstname.lastname@example.org URI: http://www.mnot.net/ Julian F. Reschke (editor) greenbytes GmbH Hafenweg 16 Muenster, NW 48155 Germany EMail: email@example.com URI: http://greenbytes.de/tech/webdav/