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RFC 9111: HTTP Caching

RFC 9111 HTTP Caching June 2022 Fielding, et al. Standards Track [Page] RFC 9111 HTTP Caching Abstract

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

This document obsoletes RFC 7234.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9111.

Copyright Notice

Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

1. Introduction

The Hypertext Transfer Protocol (HTTP) is a stateless application-level request/response protocol that uses extensible semantics and self-descriptive messages for flexible interaction with network-based hypertext information systems. It is typically used for distributed information systems, where the use of response caches can improve performance. This document defines aspects of HTTP related to caching and reusing response messages.

An HTTP "cache" is a local store of response messages and the subsystem that controls storage, retrieval, and deletion of messages in it. A cache stores cacheable responses to reduce the response time and network bandwidth consumption on future equivalent requests. Any client or server MAY use a cache, though not when acting as a tunnel (Section 3.7 of [HTTP]).

A "shared cache" is a cache that stores responses for reuse by more than one user; shared caches are usually (but not always) deployed as a part of an intermediary. A "private cache", in contrast, is dedicated to a single user; often, they are deployed as a component of a user agent.

The goal of HTTP caching is significantly improving performance by reusing a prior response message to satisfy a current request. A cache considers a stored response "fresh", as defined in Section 4.2, if it can be reused without "validation" (checking with the origin server to see if the cached response remains valid for this request). A fresh response can therefore reduce both latency and network overhead each time the cache reuses it. When a cached response is not fresh, it might still be reusable if validation can freshen it (Section 4.3) or if the origin is unavailable (Section 4.2.4).

This document obsoletes RFC 7234, with the changes being summarized in Appendix B.

1.1. Requirements Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

Section 2 of [HTTP] defines conformance criteria and contains considerations regarding error handling.

1.2. Syntax Notation

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234], extended with the notation for case-sensitivity in strings defined in [RFC7405].

It also uses a list extension, defined in Section 5.6.1 of [HTTP], that allows for compact definition of comma-separated lists using a "#" operator (similar to how the "*" operator indicates repetition). Appendix A shows the collected grammar with all list operators expanded to standard ABNF notation.

1.2.1. Imported Rules

The following core rule is included by reference, as defined in [RFC5234], Appendix B.1: DIGIT (decimal 0-9).

[HTTP] defines the following rules:

  HTTP-date     = <HTTP-date, see [HTTP], Section 5.6.7>
  OWS           = <OWS, see [HTTP], Section 5.6.3>
  field-name    = <field-name, see [HTTP], Section 5.1>
  quoted-string = <quoted-string, see [HTTP], Section 5.6.4>
  token         = <token, see [HTTP], Section 5.6.2>
1.2.2. Delta Seconds

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

  delta-seconds  = 1*DIGIT

A recipient parsing a delta-seconds value and converting it to binary form ought to use an arithmetic type of at least 31 bits of non-negative integer range. If a cache receives a delta-seconds value greater than the greatest integer it can represent, or if any of its subsequent calculations overflows, the cache MUST consider the value to be 2147483648 (231) or the greatest positive integer it can conveniently represent.

Note: The value 2147483648 is here for historical reasons, represents infinity (over 68 years), and does not need to be stored in binary form; an implementation could produce it as a string if any overflow occurs, even if the calculations are performed with an arithmetic type incapable of directly representing that number. What matters here is that an overflow be detected and not treated as a negative value in later calculations.

2. Overview of Cache Operation

Proper cache operation preserves the semantics of HTTP transfers while reducing the transmission of information already held in the cache. See Section 3 of [HTTP] for the general terminology and core concepts of HTTP.

Although caching is an entirely OPTIONAL feature of HTTP, it can be assumed that reusing a cached response is desirable and that such reuse is the default behavior when no requirement or local configuration prevents it. Therefore, HTTP cache requirements are focused on preventing a cache from either storing a non-reusable response or reusing a stored response inappropriately, rather than mandating that caches always store and reuse particular responses.

The "cache key" is the information a cache uses to choose a response and is composed from, at a minimum, the request method and target URI used to retrieve the stored response; the method determines under which circumstances that response can be used to satisfy a subsequent request. However, many HTTP caches in common use today only cache GET responses and therefore only use the URI as the cache key.

A cache might store multiple responses for a request target that is subject to content negotiation. Caches differentiate these responses by incorporating some of the original request's header fields into the cache key as well, using information in the Vary response header field, as per Section 4.1.

Caches might incorporate additional material into the cache key. For example, user agent caches might include the referring site's identity, thereby "double keying" the cache to avoid some privacy risks (see Section 7.2).

Most commonly, caches store the successful result of a retrieval request: i.e., a 200 (OK) response to a GET request, which contains a representation of the target resource (Section 9.3.1 of [HTTP]). However, it is also possible to store redirects, negative results (e.g., 404 (Not Found)), incomplete results (e.g., 206 (Partial Content)), and responses to methods other than GET if the method's definition allows such caching and defines something suitable for use as a cache key.

A cache is "disconnected" when it cannot contact the origin server or otherwise find a forward path for a request. A disconnected cache can serve stale responses in some circumstances (Section 4.2.4).

3. Storing Responses in Caches

A cache MUST NOT store a response to a request unless:

Note that a cache extension can override any of the requirements listed; see Section 5.2.3.

In this context, a cache has "understood" a request method or a response status code if it recognizes it and implements all specified caching-related behavior.

Note that, in normal operation, some caches will not store a response that has neither a cache validator nor an explicit expiration time, as such responses are not usually useful to store. However, caches are not prohibited from storing such responses.

3.1. Storing Header and Trailer Fields

Caches MUST include all received response header fields -- including unrecognized ones -- when storing a response; this assures that new HTTP header fields can be successfully deployed. However, the following exceptions are made:

Caches MAY either store trailer fields separate from header fields or discard them. Caches MUST NOT combine trailer fields with header fields.

Caches are required to update a stored response's header fields from another (typically newer) response in several situations; for example, see Sections 3.4, 4.3.4, and 4.3.5.

When doing so, the cache MUST add each header field in the provided response to the stored response, replacing field values that are already present, with the following exceptions:

In some cases, caches (especially in user agents) store the results of processing the received response, rather than the response itself, and updating header fields that affect that processing can result in inconsistent behavior and security issues. Caches in this situation MAY omit these header fields from updating stored responses on an exceptional basis but SHOULD limit such omission to those fields necessary to assure integrity of the stored response.

For example, a browser might decode the content coding of a response while it is being received, creating a disconnect between the data it has stored and the response's original metadata. Updating that stored metadata with a different Content-Encoding header field would be problematic. Likewise, a browser might store a post-parse HTML tree rather than the content received in the response; updating the Content-Type header field would not be workable in this case because any assumptions about the format made in parsing would now be invalid.

Furthermore, some fields are automatically processed and removed by the HTTP implementation, such as the Content-Range header field. Implementations MAY automatically omit such header fields from updates, even when the processing does not actually occur.

Note that the Content-* prefix is not a signal that a header field is omitted from update; it is a convention for MIME header fields, not HTTP.

3.3. Storing Incomplete Responses

If the request method is GET, the response status code is 200 (OK), and the entire response header section has been received, a cache MAY store a response that is not complete (Section 6.1 of [HTTP]) provided that the stored response is recorded as being incomplete. Likewise, a 206 (Partial Content) response MAY be stored as if it were an incomplete 200 (OK) response. 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 (Section 14.2 of [HTTP]) and combining the successful response with the stored response, as defined in Section 3.4. 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 wholly within the incomplete response. A cache MUST NOT send a partial response to a client without explicitly marking it using the 206 (Partial Content) status code.

3.4. 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 (Section 14.2 of [HTTP]). 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 15.3.7.3 of [HTTP].

When combining the new response with one or more stored responses, a cache MUST update the stored response header fields using the header fields provided in the new response, as per Section 3.2.

4. Constructing Responses from Caches

When presented with a request, a cache MUST NOT reuse a stored response unless:

Note that a cache extension can override any of the requirements listed; see Section 5.2.3.

When a stored response is used to satisfy a request without validation, a cache MUST generate an Age header field (Section 5.1), replacing any present in the response with a value equal to the stored response's current_age; see Section 4.2.3.

A cache MUST write through requests with methods that are unsafe (Section 9.2.1 of [HTTP]) 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 4.4.

A cache can use a response that is stored or storable to satisfy multiple requests, provided that it is allowed to reuse that response for the requests in question. This enables a cache to "collapse requests" -- or combine multiple incoming requests into a single forward request upon a cache miss -- thereby reducing load on the origin server and network. Note, however, that if the cache cannot use the returned response for some or all of the collapsed requests, it will need to forward the requests in order to satisfy them, potentially introducing additional latency.

When more than one suitable response is stored, a cache MUST use the most recent one (as determined by the Date header field). It can also forward the request with "Cache-Control: max-age=0" or "Cache-Control: no-cache" to disambiguate which response to use.

A cache without a clock (Section 5.6.7 of [HTTP]) MUST revalidate stored responses upon every use.

4.1. Calculating Cache Keys with the Vary Header Field

When a cache receives a request that can be satisfied by a stored response and that stored response contains a Vary header field (Section 12.5.5 of [HTTP]), the cache MUST NOT use that stored response without revalidation unless all the presented request header fields nominated by that Vary field value match those fields in the original request (i.e., the request that caused the cached response to be stored).

The 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:

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 stored response with a Vary header field value containing a member "*" always fails to match.

If multiple stored responses match, the cache will need to choose one to use. When a nominated request header field has a known mechanism for ranking preference (e.g., qvalues on Accept and similar request header fields), that mechanism MAY be used to choose a preferred response. If such a mechanism is not available, or leads to equally preferred responses, the most recent response (as determined by the Date header field) is chosen, as per Section 4.

Some resources mistakenly omit the Vary header field from their default response (i.e., the one sent when the request does not express any preferences), with the effect of choosing it for subsequent requests to that resource even when more preferable responses are available. When a cache has multiple stored responses for a target URI and one or more omits the Vary header field, the cache SHOULD choose the most recent (see Section 4.2.3) stored response with a valid Vary field value.

If no stored response matches, the cache cannot satisfy the presented request. Typically, the request is forwarded to the origin server, potentially with preconditions added to describe what responses the cache has already stored (Section 4.3).

4.2. Freshness

A "fresh" response is one whose age has not yet exceeded its freshness lifetime. Conversely, a "stale" response is one where it has.

A response's "freshness lifetime" is the length of time between its generation by the origin server and its expiration time. An "explicit expiration time" is the time at which the origin server intends that a stored response can no longer be used by a cache without further validation, whereas a "heuristic expiration time" is assigned by a cache when no explicit expiration time is available.

A response's "age" is the time that has passed since it was generated by, or successfully validated with, the origin server.

When a response is fresh, 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 5.3) or the max-age response directive (Section 5.2.2.1). Generally, origin servers will assign future explicit expiration times to responses in the belief that the representation is not likely to change in a semantically significant way before the expiration time is reached.

If an origin server wishes to force a cache to validate every request, it can assign an explicit expiration time in the past to indicate that the response is already stale. Compliant caches will normally validate a stale cached response before reusing it for subsequent requests (see Section 4.2.4).

Since origin servers do not always provide explicit expiration times, caches are also allowed to use a heuristic to determine an expiration time under certain circumstances (see Section 4.2.2).

The calculation to determine if a response is fresh is:

   response_is_fresh = (freshness_lifetime > current_age)

freshness_lifetime is defined in Section 4.2.1; current_age is defined in Section 4.2.3.

Clients can send the max-age or min-fresh request directives (Section 5.2.1) to suggest limits on the freshness calculations for the corresponding response. However, caches are not required to honor them.

When calculating freshness, to avoid common problems in date parsing:

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 6 for an explanation of the difference between caches and history mechanisms.

4.2.1. Calculating Freshness Lifetime

A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a response by evaluating the following rules and using the first match:

Note that this calculation is intended to reduce clock skew by using the clock information provided by the origin server whenever possible.

When there is more than one value present for a given directive (e.g., two Expires header field lines or multiple Cache-Control: max-age directives), either the first occurrence should be used or the response should be considered stale. If directives conflict (e.g., both max-age and no-cache are present), the most restrictive directive should be honored. Caches are encouraged to consider responses that have invalid freshness information (e.g., a max-age directive with non-integer content) to be stale.

4.2.2. Calculating Heuristic Freshness

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 field values (such as the Last-Modified time) to estimate a plausible expiration time. This specification does not provide specific algorithms, but it does impose worst-case constraints on their results.

A cache MUST NOT use heuristics to determine freshness when an explicit expiration time is present in the stored response. Because of the requirements in Section 3, heuristics can only be used on responses without explicit freshness whose status codes are defined as "heuristically cacheable" (e.g., see Section 15.1 of [HTTP]) and on responses without explicit freshness that have been marked as explicitly cacheable (e.g., with a public response directive).

Note that in previous specifications, heuristically cacheable response status codes were called "cacheable by default".

If the response has a Last-Modified header field (Section 8.8.2 of [HTTP]), 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: A previous version of the HTTP specification (Section 13.9 of [RFC2616]) prohibited caches from calculating heuristic freshness for URIs with query components (i.e., those containing "?"). In practice, this has not been widely implemented. Therefore, origin servers are encouraged to send explicit directives (e.g., Cache-Control: no-cache) if they wish to prevent caching.

4.2.3. Calculating Age

The Age header field is used to convey an estimated age of the response message when obtained from a cache. The Age field value is the cache's estimate of the number of seconds since the origin server generated or validated the response. The Age value is therefore 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 time it has been in transit along network paths.

Age calculation uses the following data:

"age_value"
The term "age_value" denotes the value of the Age header field (Section 5.1), in a form appropriate for arithmetic operation; or 0, if not available.
"date_value"
The term "date_value" denotes the value of the Date header field, in a form appropriate for arithmetic operations. See Section 6.6.1 of [HTTP] for the definition of the Date header field and for requirements regarding responses without it.
"now"
The term "now" means the current value of this implementation's clock (Section 5.6.7 of [HTTP]).
"request_time"
The value of the clock at the time of the request that resulted in the stored response.
"response_time"
The value of the clock at the time the response was received.

A response's age can be calculated in two entirely independent ways:

  1. the "apparent_age": response_time minus date_value, if the implementation's clock is reasonably well synchronized to the origin server's clock. If the result is negative, the result is replaced by zero.
  2. the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1 or greater. 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;

The corrected_age_value MAY be used as the corrected_initial_age. In circumstances where very old cache implementations that might not correctly insert Age are present, corrected_initial_age can be calculated more conservatively as

  corrected_initial_age = max(apparent_age, corrected_age_value);

The current_age of a stored response can then be calculated by adding the 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;
4.2.4. 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 4.2.

A cache MUST NOT generate a stale response if it is prohibited by an explicit in-protocol directive (e.g., by a no-cache response directive, a must-revalidate response directive, or an applicable s-maxage or proxy-revalidate response directive; see Section 5.2.2).

A cache MUST NOT generate a stale response unless it is disconnected or doing so is explicitly permitted by the client or origin server (e.g., by the max-stale request directive in Section 5.2.1, extension directives such as those defined in [RFC5861], or configuration in accordance with an out-of-band contract).

4.3. Validation

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 chosen; see Section 4.1), it can use the conditional request mechanism (Section 13 of [HTTP]) in the forwarded request to give the next inbound server an opportunity to choose a valid stored response to use, updating the stored metadata in the process, or to replace the stored response(s) with a new response. This process is known as "validating" or "revalidating" the stored response.

4.3.1. Sending a Validation Request

When generating a conditional request for validation, a cache either starts with a request it is attempting to satisfy or -- if it is initiating the request independently -- synthesizes a request using a stored response by copying the method, target URI, and request header fields identified by the Vary header field (Section 4.1).

It then updates that request with one or more precondition header fields. These contain validator metadata sourced from a stored response(s) that has the same URI. Typically, this will include only the stored response(s) that has the same cache key, although a cache is allowed to validate a response that it cannot choose with the request header fields it is sending (see Section 4.1).

The precondition header fields are then compared by recipients to determine whether any stored response is equivalent to a current representation of the resource.

One such validator is the timestamp given in a Last-Modified header field (Section 8.8.2 of [HTTP]), which can be used in an If-Modified-Since header field for response validation, or in an If-Unmodified-Since or If-Range header field for representation selection (i.e., the client is referring specifically to a previously obtained representation with that timestamp).

Another validator is the entity tag given in an ETag field (Section 8.8.3 of [HTTP]). One or more entity tags, indicating one or more stored responses, can be used in an If-None-Match header field for response validation, or in an If-Match or If-Range header field for representation selection (i.e., the client is referring specifically to one or more previously obtained representations with the listed entity tags).

When generating a conditional request for validation, a cache:

In most cases, both validators are generated in cache validation requests, even when entity tags are clearly superior, to allow old intermediaries that do not understand entity tag preconditions to respond appropriately.

4.3.2. Handling a Received Validation Request

Each client in the request chain may have its own cache, so it is common for a cache at an intermediary to receive conditional requests from other (outbound) caches. Likewise, some user agents make use of conditional requests to limit data transfers to recently modified representations or to complete the transfer of a partially retrieved representation.

If a cache receives a request that can be satisfied by reusing a stored 200 (OK) or 206 (Partial Content) response, as per Section 4, the cache SHOULD evaluate any applicable conditional header field preconditions received in that request with respect to the corresponding validators contained within the stored response.

A cache MUST NOT evaluate conditional header fields that only apply to an origin server, occur in a request with semantics that cannot be satisfied with a cached response, or occur in a request with a target resource for which it has no stored responses; such preconditions are likely intended for some other (inbound) server.

The proper evaluation of conditional requests by a cache depends on the received precondition header fields and their precedence. In summary, the If-Match and If-Unmodified-Since conditional header fields are not applicable to a cache, and If-None-Match takes precedence over If-Modified-Since. See Section 13.2.2 of [HTTP] for a complete specification of precondition precedence.

A request containing an If-None-Match header field (Section 13.1.2 of [HTTP]) indicates that the client wants to validate one or more of its own stored responses in comparison to the stored response chosen by the cache (as per Section 4).

If an If-None-Match header field is not present, a request containing an If-Modified-Since header field (Section 13.1.3 of [HTTP]) indicates that the client wants to validate one or more of its own stored responses by modification date.

If a request contains an If-Modified-Since header field and the Last-Modified header field is not present in a stored response, a cache SHOULD use the stored response's Date field value (or, if no Date field is present, the time that the stored response was received) to evaluate the conditional.

A cache that implements partial responses to range requests, as defined in Section 14.2 of [HTTP], also needs to evaluate a received If-Range header field (Section 13.1.5 of [HTTP]) with respect to the cache's chosen response.

When a cache decides to forward a request to revalidate its own stored responses for a request that contains an If-None-Match list of entity tags, the cache MAY combine the received list with a list of entity tags from its own stored set of responses (fresh or stale) and send the union of the two lists as a replacement If-None-Match header field value in the forwarded request. If a stored response contains only partial content, the cache MUST NOT include its entity tag in the union unless the request is for a range that would be fully satisfied by that partial stored response. If the response to the forwarded request is 304 (Not Modified) and has an ETag field value with an entity tag that is not in the client's list, the cache MUST generate a 200 (OK) response for the client by reusing its corresponding stored response, as updated by the 304 response metadata (Section 4.3.4).

4.3.3. Handling a Validation Response

Cache handling of a response to a conditional request depends upon its status code:

4.3.4. Freshening Stored Responses upon Validation

When a cache receives a 304 (Not Modified) response, it needs to identify stored responses that are suitable for updating with the new information provided, and then do so.

The initial set of stored responses to update are those that could have been chosen for that request -- i.e., those that meet the requirements in Section 4, except the last requirement to be fresh, able to be served stale, or just validated.

Then, that initial set of stored responses is further filtered by the first match of:

For each stored response identified, the cache MUST update its header fields with the header fields provided in the 304 (Not Modified) response, as per Section 3.2.

4.3.5. Freshening Responses with HEAD

A response to the HEAD method is identical to what an equivalent request made with a GET would have been, without sending the content. This property of HEAD responses can be used to invalidate or update a cached GET response if the more efficient conditional GET request mechanism is not available (due to no validators being present in the stored response) or if transmission of the content is not desired even if it has changed.

When a cache makes an inbound HEAD request for a target URI and receives a 200 (OK) response, the cache SHOULD update or invalidate each of its stored GET responses that could have been chosen for that request (see Section 4.1).

For each of the stored responses that could have been chosen, if the stored response and HEAD response have matching values for any received validator fields (ETag and Last-Modified) and, if the HEAD response has a Content-Length header field, the value of Content-Length matches that of the stored response, the cache SHOULD update the stored response as described below; otherwise, the cache SHOULD consider the stored response to be stale.

If a cache updates a stored response with the metadata provided in a HEAD response, the cache MUST use the header fields provided in the HEAD response to update the stored response (see Section 3.2).

4.4. Invalidating Stored Responses

Because unsafe request methods (Section 9.2.1 of [HTTP]) such as PUT, POST, or DELETE have the potential for changing state on the origin server, intervening caches are required to invalidate stored responses to keep their contents up to date.

A cache MUST invalidate the target URI (Section 7.1 of [HTTP]) when it receives a non-error status code in response to an unsafe request method (including methods whose safety is unknown).

A cache MAY invalidate other URIs when it receives a non-error status code in response to an unsafe request method (including methods whose safety is unknown). In particular, the URI(s) in the Location and Content-Location response header fields (if present) are candidates for invalidation; other URIs might be discovered through mechanisms not specified in this document. However, a cache MUST NOT trigger an invalidation under these conditions if the origin (Section 4.3.1 of [HTTP]) of the URI to be invalidated differs from that of the target URI (Section 7.1 of [HTTP]). This helps prevent denial-of-service attacks.

"Invalidate" means that the cache will either remove all stored responses whose target URI matches the given URI or mark them as "invalid" and in need of a mandatory validation before they can be sent in response to a subsequent request.

A "non-error response" is one with a 2xx (Successful) or 3xx (Redirection) status code.

Note that this does not guarantee that all appropriate responses are invalidated globally; a state-changing request would only invalidate responses in the caches it travels through.

6. Relationship to Applications and Other Caches

Applications using HTTP often specify additional forms of caching. For example, Web browsers often have history mechanisms such as "Back" buttons that can be used to redisplay a representation retrieved earlier in a session.

Likewise, some Web browsers implement caching of images and other assets within a page view; they may or may not honor HTTP caching semantics.

The requirements in this specification do not necessarily apply to how applications use data after it is retrieved from an HTTP cache. For example, a history mechanism can display a previous representation even if it has expired, and an application can use cached data in other ways beyond its freshness lifetime.

This specification does not prohibit the application from taking HTTP caching into account; for example, a history mechanism might tell the user that a view is stale, or it might honor cache directives (e.g., Cache-Control: no-store).

However, when an application caches data and does not make this apparent to or easily controllable by the user, it is strongly encouraged to define its operation with respect to HTTP cache directives so as not to surprise authors who expect caching semantics to be honored. For example, while it might be reasonable to define an application cache "above" HTTP that allows a response containing Cache-Control: no-store to be reused for requests that are directly related to the request that fetched it (such as those created during the same page load), it would likely be surprising and confusing to users and authors if it were allowed to be reused for requests unrelated in any way to the one from which it was obtained.

7. Security Considerations

This section is meant to inform developers, information providers, and users of known security concerns specific to HTTP caching. More general security considerations are addressed in "HTTP/1.1" (Section 11 of [HTTP/1.1]) and "HTTP Semantics" (Section 17 of [HTTP]).

Caches expose an additional attack surface because the contents of the cache represent an attractive target for malicious exploitation. Since cache contents persist after an HTTP request is complete, an attack on the cache can reveal information long after a user believes that the information has been removed from the network. Therefore, cache contents need to be protected as sensitive information.

In particular, because private caches are restricted to a single user, they can be used to reconstruct a user's activity. As a result, it is important for user agents to allow end users to control them, for example, by allowing stored responses to be removed for some or all origin servers.

7.1. Cache Poisoning

Storing malicious content in a cache can extend the reach of an attacker to affect multiple users. Such "cache poisoning" attacks happen when an attacker uses implementation flaws, elevated privileges, or other techniques to insert a response into a cache. This is especially effective when shared caches are used to distribute malicious content to many clients.

One common attack vector for cache poisoning is to exploit differences in message parsing on proxies and in user agents; see Section 6.3 of [HTTP/1.1] for the relevant requirements regarding HTTP/1.1.

7.2. Timing Attacks

Because one of the primary uses of a cache is to optimize performance, its use can "leak" information about which resources have been previously requested.

For example, if a user visits a site and their browser caches some of its responses and then navigates to a second site, that site can attempt to load responses it knows exist on the first site. If they load quickly, it can be assumed that the user has visited that site, or even a specific page on it.

Such "timing attacks" can be mitigated by adding more information to the cache key, such as the identity of the referring site (to prevent the attack described above). This is sometimes called "double keying".

7.3. Caching of Sensitive Information

Implementation and deployment flaws (often led to by the misunderstanding of cache operation) might lead to the caching of sensitive information (e.g., authentication credentials) that is thought to be private, exposing it to unauthorized parties.

Note that the Set-Cookie response header field [COOKIE] does not inhibit caching; a cacheable response with a Set-Cookie header field can be (and often is) used to satisfy subsequent requests to caches. Servers that wish to control caching of these responses are encouraged to emit appropriate Cache-Control response header fields.

8. IANA Considerations

The change controller for the following registrations is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".

9. References 9.1. Normative References
[HTTP]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, June 2022, <https://www.rfc-editor.org/info/rfc9110>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC5234]
Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <https://www.rfc-editor.org/info/rfc5234>.
[RFC7405]
Kyzivat, P., "Case-Sensitive String Support in ABNF", RFC 7405, DOI 10.17487/RFC7405, December 2014, <https://www.rfc-editor.org/info/rfc7405>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[COOKIE]
Barth, A., "HTTP State Management Mechanism", RFC 6265, DOI 10.17487/RFC6265, April 2011, <https://www.rfc-editor.org/info/rfc6265>.
[HTTP/1.1]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112, June 2022, <https://www.rfc-editor.org/info/rfc9112>.
[RFC2616]
Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, DOI 10.17487/RFC2616, June 1999, <https://www.rfc-editor.org/info/rfc2616>.
[RFC5861]
Nottingham, M., "HTTP Cache-Control Extensions for Stale Content", RFC 5861, DOI 10.17487/RFC5861, May 2010, <https://www.rfc-editor.org/info/rfc5861>.
[RFC7234]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234, DOI 10.17487/RFC7234, June 2014, <https://www.rfc-editor.org/info/rfc7234>.
[RFC8126]
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/info/rfc8126>.
Appendix A. Collected ABNF

In the collected ABNF below, list rules are expanded per Section 5.6.1 of [HTTP].

Age = delta-seconds

Cache-Control = [ cache-directive *( OWS "," OWS cache-directive ) ]

Expires = HTTP-date

HTTP-date = <HTTP-date, see [HTTP], Section 5.6.7>

OWS = <OWS, see [HTTP], Section 5.6.3>

cache-directive = token [ "=" ( token / quoted-string ) ]

delta-seconds = 1*DIGIT

field-name = <field-name, see [HTTP], Section 5.1>

quoted-string = <quoted-string, see [HTTP], Section 5.6.4>

token = <token, see [HTTP], Section 5.6.2>
Appendix B. Changes from RFC 7234

Handling of duplicate and conflicting cache directives has been clarified. (Section 4.2.1)

Cache invalidation of the URIs in the Location and Content-Location header fields is no longer required but is still allowed. (Section 4.4)

Cache invalidation of the URIs in the Location and Content-Location header fields is disallowed when the origin is different; previously, it was the host. (Section 4.4)

Handling invalid and multiple Age header field values has been clarified. (Section 5.1)

Some cache directives defined by this specification now have stronger prohibitions against generating the quoted form of their values, since this has been found to create interoperability problems. Consumers of extension cache directives are no longer required to accept both token and quoted-string forms, but they still need to parse them properly for unknown extensions. (Section 5.2)

The public and private cache directives were clarified, so that they do not make responses reusable under any condition. (Section 5.2.2)

The must-understand cache directive was introduced; caches are no longer required to understand the semantics of new response status codes unless it is present. (Section 5.2.2.3)

The Warning response header was obsoleted. Much of the information supported by Warning could be gleaned by examining the response, and the remaining information -- although potentially useful -- was entirely advisory. In practice, Warning was not added by caches or intermediaries. (Section 5.5)

Index

A C E F G H M N O P S V W

Authors' Addresses

Roy T. Fielding (editor)

Adobe

345 Park Ave
San Jose, CA 95110

United States of America

Mark Nottingham (editor)

Fastly

Prahran

Australia

Julian Reschke (editor)

greenbytes GmbH

Hafenweg 16
48155 Münster

Germany

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