HTTP/1.1, part 6: Caching

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

1. Introduction

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.¶

1.1. Purpose

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 Section 4.1, 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 (Section 4.2) or if the origin is unavailable.¶

1.2. Terminology

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 Section 2.1 of [Part4].

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 (Section 2.3 of [Part4]) or, if no entity-tag is provided, a Last-Modified value that is strong in the sense defined by Section 2.2.2 of [Part4].

1.3. Conformance and Error Handling

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 [RFC2119].¶

This specification targets conformance criteria according to the role of a participant in HTTP communication. Hence, HTTP requirements are placed on senders, recipients, clients, servers, user agents, intermediaries, origin servers, proxies, gateways, or caches, depending on what behavior is being constrained by the requirement. See Section 2 of [Part1] for definitions of these terms.¶

The verb "generate" is used instead of "send" where a requirement differentiates between creating a protocol element and merely forwarding a received element downstream.¶

An implementation is considered conformant if it complies with all of the requirements associated with the roles it partakes in HTTP. 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 (Section 1.4). In addition to the prose requirements placed upon them, senders MUST NOT generate protocol elements that do not match the grammar defined by the ABNF rules for those protocol elements that are applicable to the sender's role. If a received protocol element is processed, the recipient MUST be able to parse any value that would match the ABNF rules for that protocol element, excluding only those rules not applicable to the recipient's role.¶

Unless noted otherwise, a recipient MAY attempt to recover a usable protocol element from an invalid construct. HTTP does not define specific error handling mechanisms except when they have a direct impact on security, since different applications of the protocol require different error handling strategies. For example, a Web browser might wish to transparently recover from a response where the Location header field doesn't parse according to the ABNF, whereas a systems control client might consider any form of error recovery to be dangerous.¶

1.4. Syntax Notation

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] with the list rule extension defined in Section 1.2 of [Part1]. Appendix B describes rules imported from other documents. Appendix C shows the collected ABNF with the list rule expanded.¶

1.4.1. Delta Seconds

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.¶

4. Constructing Responses from Caches

For a presented request, a cache MUST NOT return a stored response, unless: ¶

The presented effective request URI (Section 5.5 of [Part1]) 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 Section 4.3), and
the presented request does not contain the no-cache pragma (Section 7.4), nor the no-cache cache directive (Section 7.2.1), unless the stored response is successfully validated (Section 4.2), and
the stored response does not contain the no-cache cache directive (Section 7.2.2.3), unless it is successfully validated (Section 4.2), and
the stored response is either:
- fresh (see Section 4.1), or
- allowed to be served stale (see Section 4.1.4), or
- successfully validated (see Section 4.2).

Note that any of the requirements listed above can be overridden by a cache-control extension; see Section 7.2.3.¶

When a stored response is used to satisfy a request without validation, a cache MUST include a single Age header field (Section 7.1) in the response with a value equal to the stored response's current_age; see Section 4.1.3.¶

A cache MUST write through requests with methods that are unsafe (Section 2.1.1 of [Part2]) 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 Section 6.¶

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 [RFC1305], to synchronize its clock with a reliable external standard.¶

4.1. Freshness Model

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 (Section 7.3) or the max-age response cache directive (Section 7.2.2.7). 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 Section 4.1.4).¶

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 Section 4.1.1; the current_age is defined in Section 4.1.3.¶

Additionally, clients can influence freshness calculation — either constraining it relaxing it — by using the max-age and min-fresh request cache directives. See Section 7.2.1 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 Section 8 for an explanation of the difference between caches and history mechanisms.¶

4.1.1. Calculating Freshness Lifetime

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 (Section 7.2.2.8) is present, use its value, or
If the max-age response cache directive (Section 7.2.2.7) is present, use its value, or
If the Expires response header field (Section 7.3) 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 Section 4.1.2.

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), it is considered invalid. Caches are encouraged to consider responses that have invalid freshness information to be stale.¶

4.1.2. Calculating Heuristic Freshness

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 Section 4 of [Part2]: 200 (OK), 203 (Non-Authoritative Information), 206 (Partial Content), 300 (Multiple Choices), 301 (Moved Permanently) and 410 (Gone)), 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 (Section 2.2 of [Part4]), 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%.¶

4.1.3. Calculating Age

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 (Section 7.1), 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 Section 9.10 of [Part2] 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 ([RFC1305]) 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 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;

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.

4.1.4. Serving Stale Responses

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 Section 4.1.¶

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 Section 7.2.2).¶

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 Section 7.2.1).¶

A cache SHOULD append a Warning header field with the 110 warn-code (see Section 7.6) 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.¶

4.2. Validation Model

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 Section 4.3), it can use the conditional request mechanism [Part4] 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 Section 4.3) 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 Section 4.2.1.
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 (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, it can return a previously stored response (see Section 4.1.4).

4.2.1. Freshening Responses with 304 Not Modified

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 Section 7.6);
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.

4.3. Using Negotiated Responses

When a cache receives a request that can be satisfied by a stored response that has a Vary header field (Section 7.5), 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 Section 3.2 of [Part1])
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 Section 4.¶

If no selected response is available, the cache can forward the presented request to the origin server in a conditional request; see Section 4.2.¶

4.4. Combining Partial Content

A response might transfer only a partial representation if the connection closed prematurely or if the request used one or more Range specifiers ([Part5]). 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 Section 4.2 of [Part5].¶

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 Section 7.6);
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.

7. Header Field Definitions

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

7.1. Age

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 Section 4.1.3.¶

  Age = delta-seconds

Age field-values are non-negative integers, representing time in seconds (see Section 1.4.1).¶

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.¶

7.2. Cache-Control

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 Section 7.2.3 for information about how Cache-Control directives defined elsewhere are handled.¶

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, recipients 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) otherwise stated otherwise.¶

7.2.1. Request Cache-Control Directives

7.2.1.1. 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.¶

7.2.1.2. 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.¶

7.2.1.3. max-age

Argument syntax: ¶

delta-seconds (see Section 1.4.1)

The "max-age" request directive indicates that the client is unwilling to accept a response whose age is greater than the specified number of seconds. Unless the max-stale request directive is also present, the client is not willing to accept a stale response.¶

Note: This directive uses the token form of the argument syntax; e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the quoted-string form.¶

7.2.1.4. max-stale

Argument syntax: ¶

delta-seconds (see Section 1.4.1)

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.¶

Note: This directive uses the token form of the argument syntax; e.g., 'max-stale=10', not 'max-stale="10"'. Senders SHOULD NOT use the quoted-string form.¶

7.2.1.5. min-fresh

Argument syntax: ¶

delta-seconds (see Section 1.4.1)

The "min-fresh" request directive indicates that the client is willing to accept a response whose freshness lifetime is no less than its current age plus the specified time in seconds. That is, the client wants a response that will still be fresh for at least the specified number of seconds.¶

Note: This directive uses the token form of the argument syntax; e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders SHOULD NOT use the quoted-string form.¶

7.2.1.6. 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.¶

7.2.1.7. 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.¶

7.2.2. Response Cache-Control Directives

7.2.2.1. public

The "public" response directive indicates that a response whose associated request contains an 'Authentication' header MAY be stored (see Section 3.2).¶

7.2.2.2. private

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.¶

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 standard header fields defined by this specification. Field names are case-insensitive.¶

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.¶

Note: This directive uses the quoted-string form of the argument syntax. Senders SHOULD NOT use the token form (even if quoting appears not to be needed for single-entry lists).¶

7.2.2.3. no-cache

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 return 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 standard header fields defined by this specification. Field names are case-insensitive.¶

Note: Many 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.¶

Note: This directive uses the quoted-string form of the argument syntax. Senders SHOULD NOT use the token form (even if quoting appears not to be needed for single-entry lists).¶

7.2.2.4. 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.¶

7.2.2.5. 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.¶

7.2.2.6. 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.¶

7.2.2.7. max-age

Argument syntax: ¶

delta-seconds (see Section 1.4.1)

The "max-age" response directive indicates that the response is to be considered stale after its age is greater than the specified number of seconds.¶

Note: This directive uses the token form of the argument syntax; e.g., 'max-age=5', not 'max-age="5"'. Senders SHOULD NOT use the quoted-string form.¶

7.2.2.8. s-maxage

Argument syntax: ¶

delta-seconds (see Section 1.4.1)

The "s-maxage" response directive indicates that, in shared caches, the maximum age specified by this directive overrides the maximum age specified by either the max-age directive or the Expires header field. The s-maxage directive also implies the semantics of the proxy-revalidate response directive.¶

Note: This directive uses the token form of the argument syntax; e.g., 's-maxage=10', not 's-maxage="10"'. Senders SHOULD NOT use the quoted-string form.¶

7.2.2.9. 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.¶

7.2.3. Cache Control Extensions

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 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 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).¶

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.

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 require IETF Review (see [RFC5226], Section 4.1).¶

The registry itself is maintained at <http://www.iana.org/assignments/http-cache-directives>.¶

7.3. Expires

The "Expires" header field gives the date/time after which the response is considered stale. See Section 4.1 for further discussion of the freshness model.¶

The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after that time.¶

The field-value is an absolute date and time as defined by HTTP-date in Section 5.1 of [Part2]; 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").¶

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. 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).¶

7.4. Pragma

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, the no-cache request pragma-directive MUST have the same effect on caches as if "Cache-Control: no-cache" were present (see Section 7.2.1).¶

When sending a no-cache request, a client ought to include both the pragma and cache-control directives, unless Cache-Control: no-cache is purposefully omitted to target other Cache-Control response directives at HTTP/1.1 caches. For example:¶

GET / HTTP/1.1
Host: www.example.com
Cache-Control: max-age=30
Pragma: no-cache

will constrain HTTP/1.1 caches to serve a response no older than 30 seconds, while precluding implementations that do not understand Cache-Control from serving a cached response.¶

7.5. Vary

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 Section 4.3.¶

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.¶

7.6. Warning

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 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 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.¶

7.6.1. 110 Response is Stale

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

7.6.2. 111 Revalidation Failed

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.¶

7.6.3. 112 Disconnected Operation

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

7.6.4. 113 Heuristic Expiration

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.¶

7.6.5. 199 Miscellaneous Warning

The warning text can include arbitrary information to be presented to a human user, or logged. A system receiving this warning MUST NOT take any automated action, besides presenting the warning to the user.¶

7.6.6. 214 Transformation Applied

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.¶

7.6.7. 299 Miscellaneous Persistent Warning

The warning text can include arbitrary information to be presented to a human user, or logged. A system receiving this warning MUST NOT take any automated action.¶

7.6.8. Warn Code Extensions

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 require IETF Review (see [RFC5226], Section 4.1).¶

The registry itself is maintained at <http://www.iana.org/assignments/http-warn-codes>.¶

3.1. Storing Incomplete Responses

A response message is considered complete when all of the octets indicated by the message framing ([Part1]) 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 ([Part5]) and combining the successful response with the stored entry, as defined in Section 4.4. 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.¶

3.2. Storing Responses to Authenticated Requests

A shared cache MUST NOT use a cached response to a request with an Authorization header field (Section 4.1 of [Part7]) 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 (Section 7.2.2) 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 (Section 4.1.4) 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.¶

Cache Directive	Reference
max-age	Section 7.2.1.3, Section 7.2.2.7
max-stale	Section 7.2.1.4
min-fresh	Section 7.2.1.5
must-revalidate	Section 7.2.2.5
no-cache	Section 7.2.1.1, Section 7.2.2.3
no-store	Section 7.2.1.2, Section 7.2.2.4
no-transform	Section 7.2.1.6, Section 7.2.2.9
only-if-cached	Section 7.2.1.7
private	Section 7.2.2.2
proxy-revalidate	Section 7.2.2.6
public	Section 7.2.2.1
s-maxage	Section 7.2.2.8
stale-if-error	[RFC5861], Section 4
stale-while-revalidate	[RFC5861], Section 3

Warn Code	Short Description	Reference
110	Response is Stale	Section 7.6.1
111	Revalidation Failed	Section 7.6.2
112	Disconnected Operation	Section 7.6.3
113	Heuristic Expiration	Section 7.6.4
199	Miscellaneous Warning	Section 7.6.5
214	Transformation Applied	Section 7.6.6
299	Miscellaneous Persistent Warning	Section 7.6.7

12. References

12.1. Normative References

12.2. Informative References