MAY"> MUST"> MUST NOT"> OPTIONAL"> RECOMMENDED"> REQUIRED"> SHALL"> SHALL NOT"> SHOULD"> SHOULD NOT"> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> ]> Day Software

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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; employ 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 might 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 a representation 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 has an equivalent copy of a representation. 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 ¬ation; (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 : 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 &basic-rules;:

quoted-string = <quoted-string, defined in &basic-rules;> token = <token, defined in &basic-rules;> OWS = <OWS, defined in &basic-rules;>

The ABNF rules below are defined in other parts:

field-name = <field-name, defined in &header-fields;> HTTP-date = <HTTP-date, defined in &full-date;> port = <port, defined in &uri;> pseudonym = <pseudonym, defined in &header-via;> uri-host = <uri-host, defined in &uri;>

A cache &MUST-NOT; store a response to any request, unless: The request method is understood by the cache and defined as being cacheable, and the response status code is understood by the cache, and the "no-store" cache directive (see ) does not appear in request or response headers, and the "private" cache response directive (see does not appear in the response, if the cache is shared, and the "Authorization" header (see &header-authorization;) does not appear in the request, if the cache is shared, unless the response explicitly allows it (see ), and the response either: contains an Expires header (see ), or contains a max-age response cache directive (see ), or contains a s-maxage response cache directive and the cache is shared, or contains a Cache Control Extension (see ) that allows it to be cached, or has a status code that can be served with heuristic freshness (see ). In this context, a cache has "understood" a request method or a response status code if it recognises it and implements any cache-specific behaviour. In particular, 206 Partial Content responses cannot be cached by an implementation that does not handle partial content (see ). 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;. 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.

For a presented request, a cache &MUST-NOT; return a stored response, unless: The presented effective request URI (&effective-request-uri;) and that of the stored response match, and the request method associated with the stored response allows it to be used for the presented request, and selecting request-headers nominated by the stored response (if any) match those presented (see ), and the presented request and stored response are free from directives that would prevent its use (see and ), and the stored response is either: fresh (see ), or allowed to be served stale (see ), or successfully validated (see ). When a stored response is used to satisfy a request without validation, caches &MUST; include a single Age header field () in the response with a value equal to the stored response's current_age; see . 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 . 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.

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 representation 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 to indicate that the response is already stale. Compliant caches will validate the cached response before reusing it for subsequent requests. Since origin servers do not always provide explicit expiration times, HTTP caches &MAY; assign heuristic expiration times when explicit times are not specified, employing algorithms that use other header values (such as the Last-Modified time) to estimate a plausible expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does impose worst-case constraints on their results.

The calculation to determine if a response is fresh is: response_is_fresh = (freshness_lifetime > current_age)

The freshness_lifetime is defined in ; the current_age is defined in . Additionally, clients might need to influence freshness calculation. They can do this using several request cache directives, with the effect of either increasing or loosening constraints on freshness. See . 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, or If the max-age response cache directive () is present, use its value, or If the Expires response header () is present, use its value minus the value of the Date response header, or Otherwise, no explicit expiration time is present in the response. A heuristic freshness lifetime might be applicable; 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 whose definition allows heuristic freshness to be used (including the following in &status-codes;: 200, 203, 206, 300, 301 and 410), a heuristic expiration time &MAY; be calculated. Heuristics &MUST-NOT; be used for response status codes that do not explicitly allow it. 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 since that time. A typical setting of this fraction might be 10%. Note: RFC 2616 () required that caches do not calculate heuristic freshness for URLs with query components (i.e., those containing '?'). In practice, this has not been widely implemented. Therefore, servers are encouraged to send explicit directives (e.g., Cache-Control: no-cache) if they wish to preclude caching.

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 following data is used for the age calculation: age_value The term "age_value" denotes the value of the Age header (), in a form appropriate for arithmetic operation; or 0, if not available. date_value HTTP/1.1 requires origin servers to send a Date header, if possible, with every response, giving the time at which the response was generated. The term "date_value" denotes the value of the Date header, in a form appropriate for arithmetic operations. See &header-date; for the definition of the Date header, and for requirements regarding responses without a Date response header. now 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. request_time The current value of the clock at the host at the time the request resulting in the stored response was made. response_time The current value of the clock at the host at the time the response was received. A response's age can be calculated in two entirely independent ways: the "apparent_age": response_time 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. the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1; note this value &MUST; be interpreted relative to the time the request was initiated, not the time that the response was received.

apparent_age = max(0, response_time - date_value); response_delay = response_time - request_time; corrected_age_value = age_value + response_delay;

These are combined as corrected_initial_age = max(apparent_age, corrected_age_value);

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.

resident_time = now - response_time; current_age = corrected_initial_age + resident_time;

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.

When a cache has one or more stored responses for a requested URI, but cannot serve any of them (e.g., because they are not fresh, or one cannot be selected; see ), it can use the conditional request mechanism &conditional; in the forwarded request to give the origin server an opportunity to both select a valid stored response to be used, and to update it. This process is known as "validating" or "revalidating" the stored response. When sending such a conditional request, the cache &SHOULD; add an If-Modified-Since header whose value is that of the Last-Modified header from the selected (see ) stored response, if available. Additionally, the cache &SHOULD; add an If-None-Match header whose value is that of the ETag header(s) from all responses stored for the requested URI, if present. However, if any of the stored responses contains only partial content, its entity-tag &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. A 304 (Not Modified) response status code indicates that the stored response can be updated and reused; see . A full response (i.e., one with a response body) indicates that none of the stored responses nominated in the conditional request is suitable. Instead, the full response &SHOULD; be used to satisfy the request and &MAY; replace the stored response. 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 (see ).

Because unsafe methods (&safe-methods;) 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 effective Request URI (&effective-request-uri;) as well as the URI(s) in the Location and Content-Location headers (if present): PUT DELETE POST An invalidation based on a URI from a Location or Content-Location header &MUST-NOT; be performed if the host part of that URI differs from the host part in the effective request URI (&effective-request-uri;). This helps prevent denial of service attacks. A cache that passes through requests for methods it does not understand &SHOULD; invalidate the effective request URI (&effective-request-uri;). Here, "invalidate" means that the cache will either remove all stored responses related to the effective request URI, or will mark these as "invalid" and in need of a mandatory validation before they can be returned in response to a subsequent request. 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.

Shared caches &MUST-NOT; use a cached response to a request with an Authorization header (&header-authorization;) to satisfy any subsequent request unless a cache directive that allows such responses to be stored is present in the response. In this specification, the following Cache-Control response directives () have such an effect: must-revalidate, public, s-maxage. Note that cached responses that contain the "must-revalidate" and/or "s-maxage" response directives are not allowed to be served stale () by shared caches. In particular, a response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to satisfy a subsequent request without revalidating it on the origin server.

When a cache receives a request that can be satisfied by a stored response that has a Vary header field (), it &MUST-NOT; use that response unless all of the selecting request-headers nominated by the Vary header match in both the original request (i.e., that associated with the stored response), and the presented request. The selecting request-headers from two requests are defined to match if and only if those in the first request can be transformed to those in the second request by applying any of the following: adding or removing whitespace, where allowed in the header's syntax combining multiple message-header fields with the same field name (see &header-fields;) normalizing both header values in a way that is known to have identical semantics, according to the header's specification (e.g., re-ordering field values when order is not significant; case-normalization, where values are defined to be case-insensitive) If (after any normalization that might take place) a header field is absent from a request, it can only match another request if it is also absent there. 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. The stored response with matching selecting request-headers is known as the selected response. If no selected response is available, the cache &MAY; forward the presented request to the origin server in a conditional request; see .

When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response (in this section, the "new" response"), it needs to created an updated response by combining the stored response with the new one, so that the updated response can be used to satisfy the request, and potentially update the cached response. If the new response contains an ETag, it identifies the stored response to use. might need language about Content-Location hereShouldn't this be the selected response? If the new response's status code is 206 (partial content), both the stored and new responses &MUST; have validators, and those validators &MUST; match using the strong comparison function (see &weak-and-strong;). Otherwise, the responses &MUST-NOT; be combined. The stored response headers are used as those of the updated response, except that any stored Warning headers with warn-code 1xx (see ) &MUST; be deleted. any stored Warning headers with warn-code 2xx &MUST; be retained. any other headers provided in the new response &MUST; replace all instances of the corresponding headers from the stored response. The updated response headers &MUST; be used to replace those of the stored response in cache (unless the stored response is removed from cache). In the case of a 206 response, the combined representation &MAY; be stored.

This section defines the syntax and semantics of HTTP/1.1 header fields related to caching.

The "Age" response-header field conveys the sender's estimate of the amount of time since the response was generated or successfully validated at the origin server. Age values are calculated as specified in .

Age = "Age" ":" OWS Age-v Age-v = delta-seconds

Age field-values are non-negative integers, representing time in seconds.

delta-seconds = 1*DIGIT

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 (231). 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 might not implement the Age header field.

The "Cache-Control" general-header field is used to specify directives for caches along the request/response chain. Such cache directives are unidirectional in that the presence of a directive in a request does not imply that the same directive is to be given in the response. HTTP/1.1 caches &MUST; obey the requirements of the Cache-Control directives defined in this section. See for information about how Cache-Control directives defined elsewhere are handled. Note: 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.

Cache-Control = "Cache-Control" ":" OWS Cache-Control-v Cache-Control-v = 1#cache-directive cache-directive = cache-request-directive / cache-response-directive cache-extension = token [ "=" ( token / quoted-string ) ]

cache-request-directive = "no-cache" / "no-store" / "max-age" "=" delta-seconds / "max-stale" [ "=" delta-seconds ] / "min-fresh" "=" delta-seconds / "no-transform" / "only-if-cached" / cache-extension

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 might 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 the max-stale request directive is also present, 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. 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 representation. 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 code. 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.

cache-response-directive = "public" / "private" [ "=" DQUOTE 1#field-name DQUOTE ] / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ] / "no-store" / "no-transform" / "must-revalidate" / "proxy-revalidate" / "max-age" "=" delta-seconds / "s-maxage" "=" delta-seconds / cache-extension

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, &header-authorization;, 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 can be stored; it cannot ensure the privacy of the message content. Also, private response directives with field-names are often handled by implementations as if an unqualified private directive was received; i.e., the special handling for the qualified form is not widely implemented. 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 a cache from using it to satisfy a request without contacting it, 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 associated 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. Also, no-cache response directives with field-names are often handled by implementations as if an unqualified no-cache directive was received; i.e., the special handling for the qualified form is not widely implemented. 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 might 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 representation 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 representation.

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

Cache-Control: private, community="UCI"

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 HTTP Cache Directive Registry defines the name space for the cache directives. Registrations &MUST; include the following fields: Cache Directive Name Pointer to specification text Values to be added to this name space are subject to IETF review (). The registry itself is maintained at .

The "Expires" header field 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 &full-date;; it &MUST; be sent in rfc1123-date format.

Expires = "Expires" ":" OWS Expires-v Expires-v = HTTP-date

For example Expires: Thu, 01 Dec 1994 16:00:00 GMT

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 "Pragma" general-header field is used to include implementation-specific directives that might apply to any recipient along the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however, some systems &MAY; require that behavior be consistent with the directives.

Pragma = "Pragma" ":" OWS Pragma-v Pragma-v = 1#pragma-directive pragma-directive = "no-cache" / extension-pragma extension-pragma = token [ "=" ( token / quoted-string ) ]

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 conveys the set of request-header fields that were used to select the representation. Caches use this information, in part, to determine whether a stored response can be used to satisfy a given request; see . 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.

Vary = "Vary" ":" OWS Vary-v Vary-v = "*" / 1#field-name

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. 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 "Warning" general-header field is used to carry additional information about the status or transformation of a message that might not be reflected in the message. This information is typically used to warn about possible incorrectness introduced by caching operations or transformations applied to the payload of the message. 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 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.

Warning = "Warning" ":" OWS Warning-v Warning-v = 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, for use in debugging warn-text = quoted-string warn-date = DQUOTE HTTP-date DQUOTE

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, only differing in warn-text. 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. 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 describe the freshness or validation status of the response, and so &MUST; be deleted by caches after validation. They can only be generated by a cache when validating a cached entry, and &MUST-NOT; be generated in any other situation. 2xx Warnings describe some aspect of the representation that is not rectified by a validation (for example, a lossy compression of the representation) and &MUST-NOT; be deleted by caches after validation, unless a full response is returned, in which case they &MUST; be. 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 proxy if it applies any transformation to the representation, such as changing the content-coding, media-type, or modifying the representation data, unless this Warning code already appears in the response. 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 a representation retrieved earlier in a session. The freshness model () does not necessarily apply to history mechanisms. I.e., 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).

The registration procedure for HTTP Cache Directives is defined by of this document. The HTTP Cache Directive Registry shall be created at and be populated with the registrations below: Cache Directive Reference max-age , max-stale min-fresh must-revalidate no-cache , no-store , no-transform , only-if-cached private proxy-revalidate public s-maxage stale-if-error stale-while-revalidate

The Message Header Field Registry located at shall be updated with the permanent registrations below (see ): Header Field Name Protocol Status Reference Age http standard Cache-Control http standard Expires http standard Pragma http standard Vary http standard Warning http standard 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 need to 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.

Day Software

fielding@gbiv.com

Alcatel-Lucent Bell Labs

jg@freedesktop.org

Hewlett-Packard Company

JeffMogul@acm.org

Microsoft Corporation

henrikn@microsoft.com

Adobe Systems, Incorporated

LMM@acm.org

Microsoft Corporation

paulle@microsoft.com

World Wide Web Consortium

timbl@w3.org

World Wide Web Consortium

ylafon@w3.org

greenbytes GmbH

julian.reschke@greenbytes.de

Day Software

fielding@gbiv.com

Alcatel-Lucent Bell Labs

jg@freedesktop.org

Hewlett-Packard Company

JeffMogul@acm.org

Microsoft Corporation

henrikn@microsoft.com

Adobe Systems, Incorporated

LMM@acm.org

Microsoft Corporation

paulle@microsoft.com

World Wide Web Consortium

timbl@w3.org

World Wide Web Consortium

ylafon@w3.org

greenbytes GmbH

julian.reschke@greenbytes.de

Day Software

fielding@gbiv.com

Alcatel-Lucent Bell Labs

jg@freedesktop.org

Hewlett-Packard Company

JeffMogul@acm.org

Microsoft Corporation

henrikn@microsoft.com

Adobe Systems, Incorporated

LMM@acm.org

Microsoft Corporation

paulle@microsoft.com

World Wide Web Consortium

timbl@w3.org

World Wide Web Consortium

ylafon@w3.org

greenbytes GmbH

julian.reschke@greenbytes.de

Day Software

fielding@gbiv.com

Alcatel-Lucent Bell Labs

jg@freedesktop.org

Hewlett-Packard Company

JeffMogul@acm.org

Microsoft Corporation

henrikn@microsoft.com

Adobe Systems, Incorporated

LMM@acm.org

Microsoft Corporation

paulle@microsoft.com

World Wide Web Consortium

timbl@w3.org

World Wide Web Consortium

ylafon@w3.org

greenbytes GmbH

julian.reschke@greenbytes.de

Day Software

fielding@gbiv.com

Alcatel-Lucent Bell Labs

jg@freedesktop.org

Hewlett-Packard Company

JeffMogul@acm.org

Microsoft Corporation

henrikn@microsoft.com

Adobe Systems, Incorporated

LMM@acm.org

Microsoft Corporation

paulle@microsoft.com

World Wide Web Consortium

timbl@w3.org

World Wide Web Consortium

ylafon@w3.org

greenbytes GmbH

julian.reschke@greenbytes.de

Harvard University

sob@harvard.edu

Brandenburg InternetWorking

dcrocker@bbiw.net

THUS plc.

paul.overell@thus.net

University of Delaware, Electrical Engineering Department

mills@udel.edu

University of California, Irvine

fielding@ics.uci.edu

W3C

jg@w3.org

Compaq Computer Corporation

mogul@wrl.dec.com

MIT Laboratory for Computer Science

frystyk@w3.org

Xerox Corporation

masinter@parc.xerox.com

Microsoft Corporation

paulle@microsoft.com

W3C

timbl@w3.org

Nine by Nine

GK-IETF@ninebynine.org

BEA Systems

mnot@pobox.com

HP Labs

JeffMogul@acm.org

IBM

narten@us.ibm.com

Google

Harald@Alvestrand.no

Yahoo! Inc.

mnot@yahoo-inc.com

Make the specified age calculation algorithm less conservative. () Remove requirement to consider Content-Location in successful responses in order to determine the appropriate response to use. () Clarify denial of service attack avoidance requirement. () Do not mention RFC 2047 encoding and multiple languages in Warning headers anymore, as these aspects never were implemented. ()

Age = "Age:" OWS Age-v Age-v = delta-seconds Cache-Control = "Cache-Control:" OWS Cache-Control-v Cache-Control-v = *( "," OWS ) cache-directive *( OWS "," [ OWS cache-directive ] ) Expires = "Expires:" OWS Expires-v Expires-v = HTTP-date HTTP-date = <HTTP-date, defined in [Part1], Section 6.1> OWS = <OWS, defined in [Part1], Section 1.2.2> Pragma = "Pragma:" OWS Pragma-v Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS pragma-directive ] ) Vary = "Vary:" OWS Vary-v Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ] ) ) Warning = "Warning:" OWS Warning-v Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ] ) cache-directive = cache-request-directive / cache-response-directive cache-extension = token [ "=" ( token / quoted-string ) ] cache-request-directive = "no-cache" / "no-store" / ( "max-age=" delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / ( "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" / cache-extension cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / ( "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" / "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds ) / ( "s-maxage=" delta-seconds ) / cache-extension delta-seconds = 1*DIGIT extension-pragma = token [ "=" ( token / quoted-string ) ] field-name = <field-name, defined in [Part1], Section 3.2> port = <port, defined in [Part1], Section 2.6> pragma-directive = "no-cache" / extension-pragma pseudonym = <pseudonym, defined in [Part1], Section 9.9> quoted-string = <quoted-string, defined in [Part1], Section 1.2.2> token = <token, defined in [Part1], Section 1.2.2> uri-host = <uri-host, defined in [Part1], Section 2.6> 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 ]

ABNF diagnostics: ; Age defined but not used ; Cache-Control defined but not used ; Expires defined but not used ; Pragma defined but not used ; Vary defined but not used ; Warning defined but not used

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.

Closed issues: : "base for numeric protocol elements" Affected issues: : Vary and non-existant headers

Closed issues: : "Definition of 1xx Warn-Codes" : "Content-Location on 304 responses" : "private and no-cache CC directives with headers" : "RFC2047 and warn-text"

Closed issues: : "serving negotiated responses from cache: header-specific canonicalization" : "Effect of CC directives on history lists" Affected issues: : Status codes and caching Partly resolved issues: : "Placement of 13.5.1 and 13.5.2"

Closed issues: : "Age calculation" : "Clarify differences between / requirements for request and response CC directives" : "Caching authenticated responses" : "IANA registry for cache-control directives" : "Heuristic caching of URLs with query components" Partly resolved issues: : "Term for the requested resource's URI"

Closed issues: : "Clarify entity / representation / variant terminology" : "consider removing the 'changes from 2068' sections" : "Allowing heuristic caching for new status codes" : "Allowing heuristic caching for new status codes" Clean up TODOs and prose in "Combining Responses."