MAY"> MUST"> MUST NOT"> OPTIONAL"> RECOMMENDED"> REQUIRED"> SHALL"> SHALL NOT"> SHOULD"> SHOULD NOT"> Note:"> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> "> ]> Adobe Systems Incorporated

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Applications HTTPbis Hypertext Transfer Protocol HTTP HTTP Caching The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.

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 storage, retrieval, and deletion of messages in it. 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. A shared cache is a cache that stores responses to be reused by more than one user; shared caches are usually (but not always) deployed as a part of an intermediary. A private cache, in contrast, is dedicated to a single user; often, they are deployed as a component of a user agent. The goal of caching in HTTP/1.1 is to significantly improve performance by reusing a prior response message to satisfy a current request. A stored response is considered "fresh", as defined in , if the response can be reused without "validation" (checking with the origin server to see if the cached response remains valid for this request). A fresh response can therefore reduce both latency and network overhead each time it is reused. When a cached response is not fresh, it might still be reusable if it can be freshened by validation () or if the origin is unavailable ().

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 . Conformance criteria and considerations regarding error handling are defined in &conformance;.

This specification uses the Augmented Backus-Naur Form (ABNF) notation of with a list extension, defined in &abnf-extension;, that allows for compact definition of comma-separated lists using a '#' operator (similar to how the '*' operator indicates repetition). describes rules imported from other documents. shows the collected grammar with all list operators expanded to standard ABNF notation.

The delta-seconds rule specifies a non-negative integer, representing time in seconds.

delta-seconds = 1*DIGIT

A recipient parsing a delta-seconds value and converting it to binary form ought to use an arithmetic type of at least 31 bits of non-negative integer range. If a cache receives a delta-seconds value greater than the greatest integer it can represent, or if any of its subsequent calculations overflows, the cache &MUST; consider the value to be either 2147483648 (231) or the greatest positive integer it can conveniently represent. &Note; The value 2147483648 is here for historical reasons, effectively represents infinity (over 68 years), and does not need to be stored in binary form; an implementation could produce it as a canned string if any overflow occurs, even if the calculations are performed with an arithmetic type incapable of directly representing that number. What matters here is that an overflow be detected and not treated as a negative value in later calculations.

Proper cache operation preserves the semantics of HTTP transfers (&semantics;) while eliminating the transfer of information already held in the cache. Although caching is an entirely &OPTIONAL; feature of HTTP, it can be assumed that reusing a cached response is desirable and that such reuse is the default behavior when no requirement or local configuration prevents it. Therefore, HTTP cache requirements are focused on preventing a cache from either storing a non-reusable response or reusing a stored response inappropriately, rather than mandating that caches always store and reuse particular responses. Each cache entry consists of a cache key and one or more HTTP responses corresponding to prior requests that used the same key. The most common form of cache entry is a successful result of a retrieval request: i.e., a 200 (OK) response to a GET request, which contains a representation of the resource identified by the request target (&GET;). However, it is also possible to cache permanent redirects, negative results (e.g., 404 (Not Found)), incomplete results (e.g., 206 (Partial Content)), and responses to methods other than GET if the method's definition allows such caching and defines something suitable for use as a cache key. The primary cache key consists of the request method and target URI. However, since HTTP caches in common use today are typically limited to caching responses to GET, many caches simply decline other methods and use only the URI as the primary cache key. If a request target is subject to content negotiation, its cache entry might consist of multiple stored responses, each differentiated by a secondary key for the values of the original request's selecting header fields ().

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 header fields, and the "private" response directive (see ) does not appear in the response, if the cache is shared, and the Authorization header field (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 field (see ), or contains a max-age response directive (see ), or contains a s-maxage response directive (see ) and the cache is shared, or contains a Cache Control Extension (see ) that allows it to be cached, or has a status code that is defined as cacheable by default (see ), or contains a public response directive (see ). Note that any of the requirements listed above can be overridden by a cache-control extension; see . In this context, a cache has "understood" a request method or a response status code if it recognizes it and implements all specified caching-related behavior. Note that, in normal operation, some 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 response message is considered complete when all of the octets indicated by the message framing (&messaging;) are received prior to the connection being closed. If the request method is GET, the response status code is 200 (OK), and the entire response header section has been received, a cache &MAY; store an incomplete response message body if the cache entry is recorded as incomplete. Likewise, a 206 (Partial Content) response &MAY; be stored as if it were an incomplete 200 (OK) cache entry. However, a cache &MUST-NOT; store incomplete or partial-content responses if it does not support the Range and Content-Range header fields or if it does not understand the range units used in those fields. A cache &MAY; complete a stored incomplete response by making a subsequent range request (&partial;) and combining the successful response with the stored entry, as defined in . A cache &MUST-NOT; use an incomplete response to answer requests unless the response has been made complete or the request is partial and specifies a range that is wholly within the incomplete response. A cache &MUST-NOT; send a partial response to a client without explicitly marking it as such using the 206 (Partial Content) status code.

A shared cache &MUST-NOT; use a cached response to a request with an Authorization header field (&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, and 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.

A response might transfer only a partial representation if the connection closed prematurely or if the request used one or more Range specifiers (&partial;). After several such transfers, a cache might have received several ranges of the same representation. A cache &MAY; combine these ranges into a single stored response, and reuse that response to satisfy later requests, if they all share the same strong validator and the cache complies with the client requirements in &combining-byte-ranges;. When combining the new response with one or more stored responses, a cache &MUST;: delete any Warning header fields in the stored response with warn-code 1xx (see ); retain any Warning header fields in the stored response with warn-code 2xx; and, use other header fields provided in the new response, aside from Content-Range, to replace all instances of the corresponding header fields in the stored response.

When presented with a request, a cache &MUST-NOT; reuse 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 header fields nominated by the stored response (if any) match those presented (see ), and the presented request does not contain the no-cache pragma (), nor the no-cache cache directive (), unless the stored response is successfully validated (), and the stored response does not contain the no-cache cache directive (), unless it is successfully validated (), and the stored response is either: fresh (see ), or allowed to be served stale (see ), or successfully validated (see ). Note that any of the requirements listed above can be overridden by a cache-control extension; see . When a stored response is used to satisfy a request without validation, a cache &MUST; generate an Age header field (), replacing any present in the response with a value equal to the stored response's current_age; see . A cache &MUST; write through requests with methods that are unsafe (&safe-methods;) to the origin server; i.e., a cache is not allowed to generate a 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 . When more than one suitable response is stored, a cache &MUST; use the most recent response (as determined by the Date header field). It can also forward the request with "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to use. A cache that does not have a clock available &MUST-NOT; use stored responses without revalidating them upon every use.

When a cache receives a request that can be satisfied by a stored response that has a Vary header field (&header-vary;), it &MUST-NOT; use that response unless all of the selecting header fields nominated by the Vary header field match in both the original request (i.e., that associated with the stored response), and the presented request. The selecting header fields 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 field's syntax combining multiple header fields with the same field name (see &header-fields;) normalizing both header field values in a way that is known to have identical semantics, according to the header field's specification (e.g., reordering 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. The stored response with matching selecting header fields is known as the selected response. If multiple selected responses are available (potentially including responses without a Vary header field), the cache will need to choose one to use. When a selecting header field has a known mechanism for doing so (e.g., qvalues on Accept and similar request header fields), that mechanism &MAY; be used to select preferred responses; of the remainder, the most recent response (as determined by the Date header field) is used, as per . If no selected response is available, the cache cannot satisfy the presented request. Typically, it is forwarded to the origin server in a (possibly conditional; see ) request.

A fresh response is one whose age has not yet exceeded its freshness lifetime. Conversely, a stale response is one where it has. A response's freshness lifetime is the length of time between its generation by the origin server and its expiration time. An explicit expiration time is the time at which the origin server intends that a stored response can no longer be used by a cache without further validation, whereas a heuristic expiration time is assigned by a cache when no explicit expiration time is available. A response's age is the time that has passed since it was generated by, or successfully validated with, the origin server. 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 field () or the max-age response 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 normally validate a stale cached response before reusing it for subsequent requests (see ). Since origin servers do not always provide explicit expiration times, caches are also allowed to use a heuristic to determine an expiration time under certain circumstances (see ).

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

freshness_lifetime is defined in ; current_age is defined in . Clients can send the max-age or min-fresh cache directives in a request to constrain or relax freshness calculations for the corresponding response (). When calculating freshness, to avoid common problems in date parsing: Although all date formats are specified to be case-sensitive, a cache recipient &SHOULD; match day, week, and time-zone names case-insensitively. If a cache recipient's internal implementation of time has less resolution than the value of an HTTP-date, the recipient &MUST; internally represent a parsed Expires date as the nearest time equal to or earlier than the received value. A cache recipient &MUST-NOT; allow local time zones to influence the calculation or comparison of an age or expiration time. A cache recipient &SHOULD; consider a date with a zone abbreviation other than GMT or UTC to be invalid for calculating expiration. 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 the following: If the cache is shared and the s-maxage response directive () is present, use its value, or If the max-age response directive () is present, use its value, or If the Expires response header field () is present, use its value minus the value of the Date response header field, 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. When there is more than one value present for a given directive (e.g., two Expires header fields, multiple Cache-Control: max-age directives), the directive's value is considered invalid. Caches are encouraged to consider responses that have invalid freshness information to be stale.

Since origin servers do not always provide explicit expiration times, a cache &MAY; assign a heuristic expiration time when an explicit time is not specified, employing algorithms that use other header field values (such as the Last-Modified time) to estimate a plausible expiration time. This specification does not provide specific algorithms, but does impose worst-case constraints on their results. A cache &MUST-NOT; use heuristics to determine freshness when an explicit expiration time is present in the stored response. Because of the requirements in , this means that, effectively, heuristics can only be used on responses without explicit freshness whose status codes are defined as cacheable by default (see &status-overview;), and those responses without explicit freshness that have been marked as explicitly cacheable (e.g., with a "public" response directive). If the response has a Last-Modified header field (&header-last-modified;), caches are encouraged to use a heuristic expiration value that is no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%. When a heuristic is used to calculate freshness lifetime, a cache &SHOULD; generate a Warning header field with a 113 warn-code (see ) in the response if its current_age is more than 24 hours and such a warning is not already present. &Note; prohibited caches from calculating heuristic freshness for URIs with query components (i.e., those containing '?'). In practice, this has not been widely implemented. Therefore, origin servers are encouraged to send explicit directives (e.g., Cache-Control: no-cache) if they wish to preclude caching.

The Age header field is used to convey an estimated age of the response message when obtained from a cache. The Age field value is the cache's estimate of the number of seconds 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 field (), in a form appropriate for arithmetic operation; or 0, if not available. date_value The term "date_value" denotes the value of the Date header field, in a form appropriate for arithmetic operations. See &header-date; for the definition of the Date header field, and for requirements regarding responses without it. now The term "now" means "the current value of the clock at the host performing the calculation". A host ought to use NTP () or some similar protocol to synchronize its clocks to Coordinated Universal Time. 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. A cache &MUST; interpret this value 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);

unless the cache is confident in the value of the Age header field (e.g., because there are no HTTP/1.0 hops in the Via header field), in which case the corrected_age_value &MAY; be used as the corrected_initial_age. 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 . A cache &MUST-NOT; generate 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 ). A cache &MUST-NOT; send stale responses unless it is disconnected (i.e., it cannot contact the origin server or otherwise find a forward path) or doing so is explicitly allowed (e.g., by the max-stale request directive; see ). A cache &SHOULD; generate a Warning header field with the 110 warn-code (see ) in stale responses. Likewise, a cache &SHOULD; generate a 112 warn-code (see ) in stale responses if the cache is disconnected. A cache &SHOULD-NOT; generate a new Warning header field when forwarding a response that does not have an Age header field, even if the response is already stale. A cache need not validate a response that merely 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 next inbound server an opportunity to select a valid stored response to use, updating the stored metadata in the process, or to replace the stored response(s) with a new response. This process is known as "validating" or "revalidating" the stored response.

When sending a conditional request for cache validation, a cache sends one or more precondition header fields containing validator metadata from its stored response(s), which is then compared by recipients to determine whether a stored response is equivalent to a current representation of the resource. One such validator is the timestamp given in a Last-Modified header field (&header-last-modified;), which can be used in an If-Modified-Since header field for response validation, or in an If-Unmodified-Since or If-Range header field for representation selection (i.e., the client is referring specifically to a previously obtained representation with that timestamp). Another validator is the entity-tag given in an ETag header field (&header-etag;). One or more entity-tags, indicating one or more stored responses, can be used in an If-None-Match header field for response validation, or in an If-Match or If-Range header field for representation selection (i.e., the client is referring specifically to one or more previously obtained representations with the listed entity-tags).

Each client in the request chain may have its own cache, so it is common for a cache at an intermediary to receive conditional requests from other (outbound) caches. Likewise, some user agents make use of conditional requests to limit data transfers to recently modified representations or to complete the transfer of a partially retrieved representation. If a cache receives a request that can be satisfied by reusing one of its stored 200 (OK) or 206 (Partial Content) responses, the cache &SHOULD; evaluate any applicable conditional header field preconditions received in that request with respect to the corresponding validators contained within the selected response. A cache &MUST-NOT; evaluate conditional header fields that are only applicable to an origin server, found in a request with semantics that cannot be satisfied with a cached response, or applied to a target resource for which it has no stored responses; such preconditions are likely intended for some other (inbound) server. The proper evaluation of conditional requests by a cache depends on the received precondition header fields and their precedence, as defined in &conditional-precedence;. The If-Match and If-Unmodified-Since conditional header fields are not applicable to a cache. A request containing an If-None-Match header field (&header-if-none-match;) indicates that the client wants to validate one or more of its own stored responses in comparison to whichever stored response is selected by the cache. If the field-value is "*", or if the field-value is a list of entity-tags and at least one of them matches the entity-tag of the selected stored response, a cache recipient &SHOULD; generate a 304 (Not Modified) response (using the metadata of the selected stored response) instead of sending that stored response. When a cache decides to revalidate its own stored responses for a request that contains an If-None-Match list of entity-tags, the cache &MAY; combine the received list with a list of entity-tags from its own stored set of responses (fresh or stale) and send the union of the two lists as a replacement If-None-Match header field value in the forwarded request. If a stored response contains only partial content, the cache &MUST-NOT; include its entity-tag in the union unless the request is for a range that would be fully satisfied by that partial stored response. If the response to the forwarded request is 304 (Not Modified) and has an ETag header field value with an entity-tag that is not in the client's list, the cache &MUST; generate a 200 (OK) response for the client by reusing its corresponding stored response, as updated by the 304 response metadata (). If an If-None-Match header field is not present, a request containing an If-Modified-Since header field (&header-if-modified-since;) indicates that the client wants to validate one or more of its own stored responses by modification date. A cache recipient &SHOULD; generate a 304 (Not Modified) response (using the metadata of the selected stored response) if one of the following cases is true: 1) the selected stored response has a Last-Modified field-value that is earlier than or equal to the conditional timestamp; 2) no Last-Modified field is present in the selected stored response, but it has a Date field-value that is earlier than or equal to the conditional timestamp; or, 3) neither Last-Modified nor Date is present in the selected stored response, but the cache recorded it as having been received at a time earlier than or equal to the conditional timestamp. A cache that implements partial responses to range requests, as defined in &partial;, also needs to evaluate a received If-Range header field (&header-if-range;) with respect to its selected stored response.

Cache handling of a response to a conditional request is dependent upon its status code: 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 payload body) indicates that none of the stored responses nominated in the conditional request is suitable. Instead, the cache &MUST; use the full response to satisfy the request and &MAY; replace the stored response(s). However, if a cache receives a 5xx (Server Error) response while attempting to validate a response, it can either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, the cache &MAY; send a previously stored response (see ).

When a cache receives a 304 (Not Modified) response and already has one or more stored 200 (OK) responses for the same cache key, the cache needs to identify which of the stored responses are updated by this new response and then update the stored response(s) with the new information provided in the 304 response. The stored response to update is identified by using the first match (if any) of the following: If the new response contains a strong validator (see &weak-and-strong;), then that strong validator identifies the selected representation for update. All of the stored responses with the same strong validator are selected. If none of the stored responses contain the same strong validator, then the cache &MUST-NOT; use the new response to update any stored responses. If the new response contains a weak validator and that validator corresponds to one of the cache's stored responses, then the most recent of those matching stored responses is selected for update. If the new response does not include any form of validator (such as in the case where a client generates an If-Modified-Since request from a source other than the Last-Modified response header field), and there is only one stored response, and that stored response also lacks a validator, then that stored response is selected for update. If a stored response is selected for update, the cache &MUST;: delete any Warning header fields in the stored response with warn-code 1xx (see ); retain any Warning header fields in the stored response with warn-code 2xx; and, use other header fields provided in the 304 (Not Modified) response to replace all instances of the corresponding header fields in the stored response.

A response to the HEAD method is identical to what an equivalent request made with a GET would have been, except it lacks a body. This property of HEAD responses can be used to invalidate or update a cached GET response if the more efficient conditional GET request mechanism is not available (due to no validators being present in the stored response) or if transmission of the representation body is not desired even if it has changed. When a cache makes an inbound HEAD request for a given request target and receives a 200 (OK) response, the cache &SHOULD; update or invalidate each of its stored GET responses that could have been selected for that request (see ). For each of the stored responses that could have been selected, if the stored response and HEAD response have matching values for any received validator fields (ETag and Last-Modified) and, if the HEAD response has a Content-Length header field, the value of Content-Length matches that of the stored response, the cache &SHOULD; update the stored response as described below; otherwise, the cache &SHOULD; consider the stored response to be stale. If a cache updates a stored response with the metadata provided in a HEAD response, the cache &MUST;: delete any Warning header fields in the stored response with warn-code 1xx (see ); retain any Warning header fields in the stored response with warn-code 2xx; and, use other header fields provided in the HEAD response to replace all instances of the corresponding header fields in the stored response and append new header fields to the stored response's header section unless otherwise restricted by the Cache-Control header field.

Because unsafe request methods (&safe-methods;) such as PUT, POST or DELETE have the potential for changing state on the origin server, intervening caches can use them to keep their contents up to date. A cache &MUST; invalidate the effective Request URI (&effective-request-uri;) as well as the URI(s) in the Location and Content-Location response header fields (if present) when a non-error status code is received in response to an unsafe request method. However, a cache &MUST-NOT; invalidate a URI from a Location or Content-Location response header field 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 &MUST; invalidate the effective request URI (&effective-request-uri;) when it receives a non-error response to a request with a method whose safety is unknown. Here, a "non-error response" is one with a 2xx (Successful) or 3xx (Redirection) status code. "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 sent in response to a subsequent request. Note that this does not guarantee that all appropriate responses are invalidated. For example, a state-changing request might invalidate responses in the caches it travels through, but relevant responses still might be stored in other caches that it has not.

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

The "Age" 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 = delta-seconds

The Age field-value is a non-negative integer, representing time in seconds (see ). 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.

The "Cache-Control" 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. A cache &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; 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.

Argument syntax: delta-seconds (see ) 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.

Argument syntax: delta-seconds (see ) 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.

Argument syntax: delta-seconds (see ) 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.

The "no-cache" request directive indicates that a cache &MUST-NOT; use a stored response to satisfy the request without successful validation on the origin server.

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 private 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. Note that if a request containing this directive is satisfied from a cache, the no-store request directive does not apply to the already stored response.

The "no-transform" request directive indicates that an intermediary (whether or not it implements a cache) &MUST-NOT; transform the payload, as defined in &transformations;.

The "only-if-cached" request directive indicates that the client only wishes to obtain 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, a member cache &MAY; forward such a request within that group of caches.

The "must-revalidate" response directive indicates that once it has become stale, a cache &MUST-NOT; use the response 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 a cache &MUST; obey the must-revalidate directive; in particular, if a cache cannot reach the origin server for any reason, it &MUST; generate a 504 (Gateway Timeout) response. The must-revalidate directive ought to be used by servers if and only if failure to validate a request on the representation could result in incorrect operation, such as a silently unexecuted financial transaction.

Argument syntax: #field-name 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 send stale responses. If the no-cache response directive specifies one or more field-names, then a cache &MAY; use the response to satisfy a subsequent request, subject to any other restrictions on caching. However, any header fields in the response that have the field-name(s) listed &MUST-NOT; be sent in the response to a subsequent request without successful revalidation with 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. 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; 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.

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 private 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.

The "no-transform" response directive indicates that an intermediary (regardless of whether it implements a cache) &MUST-NOT; transform the payload, as defined in &transformations;.

The "public" response directive indicates that any cache &MAY; store the response, even if the response would normally be non-cacheable or cacheable only within a private cache. (See for additional details related to the use of public in response to a request containing Authorization, and 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 .)

Argument syntax: #field-name 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.

The "proxy-revalidate" response directive has the same meaning as the must-revalidate response directive, except that it does not apply to private caches.

Argument syntax: delta-seconds (see ) 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.

Argument syntax: delta-seconds (see ) 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.

The Cache-Control header field can be extended through the use of one or more cache-extension tokens, each with an optional value. A cache &MUST; ignore unrecognized cache directives. 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 old directive are supplied, such that applications that do not understand the new directive will default to the behavior specified by the old directive, and those that understand the new directive will recognize it as modifying the requirements associated with the old directive. In this way, extensions to the existing cache-control directives can be made without breaking deployed caches. For example, consider a hypothetical new response directive called "community" that acts as a modifier to the private directive: in addition to private caches, any cache that is shared only by members of the named community is allowed to 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 that recognizes such a community cache-extension could broaden its behavior in accordance with that extension. A cache that does not recognize the community cache-extension would ignore it and adhere to the private directive.

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 Expires value is an HTTP-date timestamp, as defined in &http-date;.

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 (), a recipient &MUST; ignore the Expires field. Likewise, if a response includes the s-maxage directive (), 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.

The "Pragma" header field allows backwards compatibility with HTTP/1.0 caches, so that clients can specify a "no-cache" request that they will understand (as Cache-Control was not defined until HTTP/1.1). When the Cache-Control header field is also present and understood in a request, Pragma is ignored. In HTTP/1.0, Pragma was defined as an extensible field for implementation-specified directives for recipients. This specification deprecates such extensions to improve interoperability.

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

When the Cache-Control header field is not present in a request, caches &MUST; consider the no-cache request pragma-directive as having the same effect as if "Cache-Control: no-cache" were present (see ). 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.

The "Warning" header field is used to carry additional information about the status or transformation of a message that might not be reflected in the status code. 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 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: 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. 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 .

A cache &SHOULD; generate this whenever the sent response is stale.

A cache &SHOULD; generate this when sending a stale response because an attempt to validate the response failed, due to an inability to reach the server.

A cache &SHOULD; generate this if it is intentionally disconnected from the rest of the network for a period of time.

A cache &SHOULD; generate this if it heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than 24 hours.

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.

This Warning code &MUST; be added by a 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.

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. 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).

The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry" defines the namespace for the cache directives. It has been created and is now maintained at .

A registration &MUST; include the following fields: Cache Directive Name Pointer to specification text Values to be added to this namespace require IETF Review (see ).

New extension directives ought to consider defining: What it means for a directive to be specified multiple times, When the directive does not take an argument, what it means when an argument is present, When the directive requires an argument, what it means when it is missing, Whether the directive is specific to requests, responses, or able to be used in either. See also .

The registry has been 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 "Hypertext Transfer Protocol (HTTP) Warn Codes" registry defines the namespace for warn codes. It has been created and is now maintained at .

A registration &MUST; include the following fields: Warn Code (3 digits) Short Description Pointer to specification text Values to be added to this namespace require IETF Review (see ).

The registry has been populated with the registrations below: Warn Code Short Description Reference 110 Response is Stale 111 Revalidation Failed 112 Disconnected Operation 113 Heuristic Expiration 199 Miscellaneous Warning 214 Transformation Applied 299 Miscellaneous Persistent Warning

HTTP header fields are registered within the "Message Headers" registry maintained at . This document defines the following HTTP header fields, so the "Permanent Message Header Field Names" registry has been updated accordingly (see ). Header Field Name Protocol Status Reference Age http standard Cache-Control http standard Expires http standard Pragma http standard Warning http standard The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".

This section is meant to inform developers, information providers, and users of known security concerns specific to HTTP caching. More general security considerations are addressed in HTTP messaging &messaging; and semantics &semantics;. 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 &body.length; 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 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.

See &acks;.

Adobe Systems Incorporated

fielding@gbiv.com

greenbytes GmbH

julian.reschke@greenbytes.de

Content-Length Via Adobe Systems Incorporated

fielding@gbiv.com

greenbytes GmbH

julian.reschke@greenbytes.de

2xx (Successful) 200 (OK) 203 (Non-Authoritative Information) 3xx (Redirection) 300 (Multiple Choices) 301 (Moved Permanently) 404 (Not Found) 410 (Gone) 5xx (Server Error) 504 (Gateway Timeout) Accept Content-Encoding Content-Location Content-Type Date Location Vary Adobe Systems Incorporated

fielding@gbiv.com

greenbytes GmbH

julian.reschke@greenbytes.de

304 304 (Not Modified) 412 (Precondition Failed) ETag If-Modified-Since If-Unmodified-Since If-Match If-None-Match Last-Modified Adobe Systems Incorporated

fielding@gbiv.com

World Wide Web Consortium

ylafon@w3.org

greenbytes GmbH

julian.reschke@greenbytes.de

206 (Partial Content) Content-Range If-Range Range Adobe Systems Incorporated

fielding@gbiv.com

greenbytes GmbH

julian.reschke@greenbytes.de

Authorization Harvard University

sob@harvard.edu

Brandenburg InternetWorking

dcrocker@bbiw.net

THUS plc.

paul.overell@thus.net

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

University of California, Berkeley

abarth@eecs.berkeley.edu

The specification has been substantially rewritten for clarity. The conditions under which an authenticated response can be cached have been clarified. () 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. () 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. () The Content-Location response header field is no longer used to determine the appropriate response to use when validating. () 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. () Requirements regarding denial-of-service attack avoidance when performing invalidation have been clarified. () Cache invalidation only occurs when a successful response is received. () Cache directives are explicitly defined to be case-insensitive. Handling of multiple instances of cache directives when only one is expected is now defined. () 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. () 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. () The "no-cache" response directive's meaning has been clarified. () The one-year limit on Expires header field values has been removed; instead, the reasoning for using a sensible value is given. () The Pragma header field is now only defined for backwards compatibility; future pragmas are deprecated. () 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. () This specification introduces the Cache Directive and Warn Code Registries, and defines considerations for new cache directives. ( and )

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), and VCHAR (any visible US-ASCII character). The rules below are defined in :

OWS = <OWS, see &whitespace;> field-name = <field-name, see &header-fields;> quoted-string = <quoted-string, see &field-components;> token = <token, see &field-components;> port = <port, see &uri;> pseudonym = <pseudonym, see &header-via;> uri-host = <uri-host, see &uri;>

The rules below are defined in other parts:

HTTP-date = <HTTP-date, see &http-date;>

In the collected ABNF below, list rules are expanded as per .

Age = delta-seconds Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS cache-directive ] ) Expires = HTTP-date HTTP-date = <HTTP-date, see [RFC7231], Section 7.1.1.1> 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 ]