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

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HTTPbis Working Group The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. 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), which is archived at . 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. 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 cache response can therefore reduce both latency and network transfers 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.

This specification uses a number of terms to refer to the roles played by participants in, and objects of, HTTP caching. cache A conformant implementation of a HTTP cache. Note that this implies an HTTP/1.1 cache; this specification does not define conformance for HTTP/1.0 caches. shared cache A cache that stores responses to be reused by more than one user; usually (but not always) deployed as part of an intermediary. private cache A cache that is dedicated to a single user. 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 stored 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 is an equivalent copy of a representation. See &weak-and-strong;. strong validator A validator that is defined by the origin server such that its current value will change if the representation body changes; i.e., an entity-tag that is not marked as weak (&entity-tags;) or, if no entity-tag is provided, a Last-Modified value that is strong in the sense defined by &lastmod-comparison;.

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 . This document defines conformance criteria for several roles in HTTP communication, including Senders, Recipients, Clients, Servers, User-Agents, Origin Servers, Intermediaries, Proxies and Gateways. See &architecture; for definitions of these terms. An implementation is considered conformant if it complies with all of the requirements associated with its role(s). Note that SHOULD-level requirements are relevant here, unless one of the documented exceptions is applicable. This document also uses ABNF to define valid protocol elements (). In addition to the prose requirements placed upon them, Senders &MUST-NOT; generate protocol elements that are invalid. Unless noted otherwise, Recipients &MAY; take steps to recover a usable protocol element from an invalid construct. However, HTTP does not define specific error handling mechanisms, except in cases where it has direct impact on security. This is because different uses of the protocol require different error handling strategies; for example, a Web browser may wish to transparently recover from a response where the Location header field doesn't parse according to the ABNF, whereby in a systems control protocol using HTTP, this type of error recovery could lead to dangerous consequences.

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), and VCHAR (any visible US-ASCII character).

The core rules below are defined in :

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

The ABNF rules below are defined in other parts:

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

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

delta-seconds = 1*DIGIT

If an implementation receives a delta-seconds value larger than the largest positive integer it can represent, or if any of its subsequent calculations overflows, it &MUST; consider the value to be 2147483648 (231). Recipients parsing a delta-seconds value &MUST; use an arithmetic type of at least 31 bits of range, and senders &MUST-NOT; send delta-seconds with a value greater than 2147483648.

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, we assume that reusing the cached response is desirable and that such reuse is the default behavior when no requirement or locally-desired 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. 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 containing a representation of the resource identified by the request target. However, it is also possible to cache negative results (e.g., 404 not found), incomplete results (e.g., 206 partial content), and responses to safe methods other than GET if the method's definition allows such caching and defines something suitable for use as a cache key. The default 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, most implementations simply decline other methods and use only the URI as the 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" cache 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 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 ). 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 any cache-specific behavior. 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 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 is GET, the response status is 200 (OK), and the entire response header block 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.

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 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; include a single Age header field () 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 must not 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 a 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 on every use. A cache, especially a shared cache, &SHOULD; use a mechanism, such as NTP , to synchronize its clock with a reliable external standard.

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 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 normally validate the cached response before reusing it for subsequent requests (see ). 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.

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 can influence freshness calculation -- either constraining it relaxing it -- by using the max-age and min-fresh request cache directives. See for details. 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 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.

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 cache &MAY; calculate a heuristic expiration time. A cache &MUST-NOT; use heuristics to determine freshness for responses with status codes that do not explicitly allow it. When a heuristic is used to calculate freshness lifetime, a cache &SHOULD; attach a Warning header field 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 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%. Note: RFC 2616 () required that caches do not calculate heuristic freshness for URIs 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 header field 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 field (), 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 field, 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 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 cache &SHOULD; use NTP () or some similar protocol to synchronize its 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. 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 &SHOULD; be combined as corrected_initial_age = max(apparent_age, corrected_age_value);

unless the cache is confident in the value of the Age header (e.g., because there are no HTTP/1.0 hops in the Via header), 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;

Additionally, to avoid common problems in date parsing: HTTP/1.1 clients and caches &SHOULD; assume that an RFC-850 date which appears to be more than 50 years in the future is in fact in the past (this helps solve the "year 2000" problem). Although all date formats are specified to be case-sensitive, recipients &SHOULD; match day, week and timezone names case-insensitively. An HTTP/1.1 implementation &MAY; internally represent a parsed Expires date as earlier than the proper value, but &MUST-NOT; internally represent a parsed Expires date as later than the proper value. All expiration-related calculations &MUST; be done in GMT. The local time zone &MUST-NOT; influence the calculation or comparison of an age or expiration time. If an HTTP header field incorrectly carries a date value with a time zone other than GMT, it &MUST; be converted into GMT using the most conservative possible conversion.

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; 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 ). A cache &MUST-NOT; return 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; append a Warning header field with the 110 warn-code (see ) to stale responses. Likewise, a cache &SHOULD; add the 112 warn-code to 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 can forward it to the requesting client without adding a new Warning (but without removing any existing Warning header fields). A cache shouldn't 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, a cache adds an If-Modified-Since header field whose value is that of the Last-Modified header field from the selected (see ) stored response, if available. Additionally, a cache can add an If-None-Match header field whose value is that of the ETag header field(s) from all responses stored for the requested URI, if present. However, if any of the stored responses contains only partial content, the cache shouldn't include its entity-tag in the If-None-Match header field unless the request is for a range that would be fully satisfied by that 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 response body) indicates that none of the stored responses nominated in the conditional request is suitable. Instead, the cache can use the full response to satisfy the request and &MAY; replace the stored response(s). However, if a cache receives a 5xx 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, it can return 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. If the new response contains a strong validator, then that strong validator identifies the selected representation. All of the stored responses with the same strong validator are selected. If none of the stored responses contain the same strong validator, then this new response corresponds to a new selected representation and &MUST-NOT; update the existing 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. If the new response does not include any form of validator, there is only one stored response, and that stored response also lacks a validator, then that stored response is selected. 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 response to replace all instances of the corresponding header fields in the stored response.

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 header fields (if present) when a non-error response to a request with an unsafe method is received. However, a cache &MUST-NOT; invalidate a URI from a Location or Content-Location 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 or 3xx 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 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.

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, 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 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., 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 header fields is known as the selected response. If multiple selected responses are available, the most recent response (as determined by the Date header field) is used; see . If no selected response is available, the cache can forward the presented request to the origin server in a conditional request; see .

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.

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

Age field-values are non-negative integers, representing time in seconds (see ). 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" 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: HTTP/1.0 caches might not implement Cache-Control and might only implement Pragma: no-cache (see ). 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.

Cache-Control = 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 cache &MUST-NOT; use a stored response 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 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. max-age 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. 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 intermediary (whether or not it implements a cache) &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type request header fields, nor the request representation. only-if-cached 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.

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 a response whose associated request contains an 'Authentication' header &MAY; be stored (see ). 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 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 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. 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 header fields. That is, a cache &MUST-NOT; send the specified field-name(s) 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 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. must-revalidate 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. proxy-revalidate The proxy-revalidate response directive has the same meaning as the must-revalidate response directive, except that it does not apply to private caches. max-age 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. 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 field. The s-maxage directive also implies the semantics of the proxy-revalidate response directive. no-transform The no-transform response directive indicates that an intermediary (regardless of whether it implements a cache) &MUST-NOT; change the Content-Encoding, Content-Range or Content-Type response header fields, 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 private 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. A cache &MUST; ignore unrecognized cache directives; 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. A registration &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 &http-date;; a sender &MUST; use the rfc1123-date format.

Expires = HTTP-date

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

A cache &MUST; treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired"). 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. 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 most caches will evict a response far sooner than that. Therefore, senders ought not produce them. An origin server without a clock &MUST-NOT; assign Expires values to a response unless these values were associated with the resource by a system or user with a reliable clock. It &MAY; assign an Expires value that is known, at or before server configuration time, to be in the past (this allows "pre-expiration" of responses without storing separate Expires values for each resource).

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 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 is not present in a request, the no-cache request pragma-directive &MUST; have the same effect on caches 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 "Vary" header field conveys the set of 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 = "*" / 1#field-name

The set of header fields named by the Vary field value is known as the selecting header fields. A server &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 header fields (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. A proxy &MUST-NOT; generate the "*" value. The field-names given are not limited to the set of standard header fields defined by this specification. Field names are case-insensitive.

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 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 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 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 header fields are encouraged to order them with this user agent behavior in mind. New Warning header fields are added after any existing Warning headers 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 Warnings describe the freshness or validation status of the response, and so &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 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 a cache 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 header fields 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 field in the message. If a system 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 field &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.

A cache &SHOULD; include this whenever the returned response is stale.

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

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

A cache &SHOULD; include 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.

&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.

The HTTP Warn Code Registry defines the name space for warn codes. A registration &MUST; include the following fields: Warn Code (3 digits) Short Description Pointer to specification text Values to be added to this name space are subject to IETF review (). The registry itself is maintained at .

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 registration procedure for HTTP Warn Codes is defined by of this document. The HTTP Warn Code Registry shall be created at and be 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

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.

See &acks;.

Adobe Systems Incorporated

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

Adobe Systems Incorporated

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

Adobe Systems Incorporated

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

Adobe Systems Incorporated

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

Adobe Systems Incorporated

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. () Change ABNF productions for header fields to only define the field value. () Do not mention RFC 2047 encoding and multiple languages in Warning header fields anymore, as these aspects never were implemented. ()

Age = delta-seconds Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS cache-directive ] ) Expires = HTTP-date HTTP-date = <HTTP-date, defined in [Part2], Section 8> OWS = <OWS, defined in [Part1], Section 1.2.2> Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS pragma-directive ] ) Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ] ) ) Warning = *( "," 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 2.7> pragma-directive = "no-cache" / extension-pragma pseudonym = <pseudonym, defined in [Part1], Section 8.8> quoted-string = <quoted-string, defined in [Part1], Section 3.2.3> token = <token, defined in [Part1], Section 3.2.3> uri-host = <uri-host, defined in [Part1], 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 ]

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 Field Registration (): Reference RFC 3984, and update header field registrations for header fields 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 field 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" : "Cache Extensions can override no-store, etc." 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" Clean up TODOs and prose in "Combining Responses."

Closed issues: : "Text about clock requirement for caches belongs in p6"

Closed issues: : "Header Classification" : "Clarify 'public'"

Closed issues: : "untangle ABNFs for header fields"

Closed issues: : "Mismatch Vary" : "Cache Invalidation only happens upon successful responses" : "Recommend minimum sizes for protocol elements" : "Proxies don't 'understand' methods" : "Cache Extensions can override no-store, etc." : "Pragma"

Closed issues: : "Motivate one-year limit for Expires"

Closed issues: : "Document HTTP's error-handling philosophy" : "Cache-Control directive case sensitivity"

Closed issues: : "Interaction of request and response Cache-Control" : "Refining age for 1.1 proxy chains" : "warn-code registry"