RFC 9111 - HTTP Caching

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

4. Constructing Responses from Caches

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

the presented target URI (Section 7.1 of [HTTP]) 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
request header fields nominated by the stored response (if any) match those presented (see Section 4.1), and
the stored response does not contain the no-cache directive (Section 5.2.2.4), unless it is successfully validated (Section 4.3), and
the stored response is one of the following:
- fresh (see Section 4.2), or
- allowed to be served stale (see Section 4.2.4), or
- successfully validated (see Section 4.3).

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:¶

adding or removing whitespace, where allowed in the header field's syntax
combining multiple header field lines with the same field name (see Section 5.2 of [HTTP])
normalizing both header field values in a way that is known to have identical semantics, according to the header field's specification (e.g., reordering field values when order is not significant; case-normalization, where values are defined to be case-insensitive)

If (after any normalization that might take place) a header field is absent from a request, it can only match another request if it is also absent there.¶

A 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:¶

Although all date formats are specified to be case-sensitive, a cache recipient SHOULD match the field value case-insensitively.
If a cache recipient's internal implementation of time has less resolution than the value of an HTTP-date, the recipient MUST internally represent a parsed Expires date as the nearest time equal to or earlier than the received value.
A cache recipient MUST NOT allow local time zones to influence the calculation or comparison of an age or expiration time.
A cache recipient SHOULD consider a date with a zone abbreviation other than "GMT" to be invalid for calculating expiration.

Note that freshness applies only to cache operation; it cannot be used to force a user agent to refresh its display or reload a resource. See 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:¶

If the cache is shared and the s-maxage response directive (Section 5.2.2.10) is present, use its value, or
If the max-age response directive (Section 5.2.2.1) is present, use its value, or
If the Expires response header field (Section 5.3) is present, use its value minus the value of the Date response header field (using the time the message was received if it is not present, as per Section 6.6.1 of [HTTP]), or
Otherwise, no explicit expiration time is present in the response. A heuristic freshness lifetime might be applicable; see Section 4.2.2.

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%.¶

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:¶

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.
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:¶

MUST send the relevant entity tags (using If-Match, If-None-Match, or If-Range) if the entity tags were provided in the stored response(s) being validated.
SHOULD send the Last-Modified value (using If-Modified-Since) if the request is not for a subrange, a single stored response is being validated, and that response contains a Last-Modified value.
MAY send the Last-Modified value (using If-Unmodified-Since or If-Range) if the request is for a subrange, a single stored response is being validated, and that response contains only a Last-Modified value (not an entity tag).

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:¶

A 304 (Not Modified) response status code indicates that the stored response can be updated and reused; see Section 4.3.4.
A full response (i.e., one containing content) indicates that none of the stored responses nominated in the conditional request are suitable. Instead, the cache MUST use the full response to satisfy the request. The cache MAY store such a full response, subject to its constraints (see Section 3).
However, if a cache receives a 5xx (Server Error) response while attempting to validate a response, it can either forward this response to the requesting client or act as if the server failed to respond. In the latter case, the cache can send a previously stored response, subject to its constraints on doing so (see Section 4.2.4), or retry the validation request.

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:¶

If the new response contains one or more strong validators (see Section 8.8.1 of [HTTP]), then each of those strong validators identifies a selected representation for update. All the stored responses in the initial set with one of those same strong validators are identified for update. If none of the initial set contains at least one of the same strong validators, then the cache MUST NOT use the new response to update any stored responses.
If the new response contains no strong validators but does contain one or more weak validators, and those validators correspond to one of the initial set's stored responses, then the most recent of those matching stored responses is identified for update.
If the new response does not include any form of validator (such as where a client generates an If-Modified-Since request from a source other than the Last-Modified response header field), and there is only one stored response in the initial set, and that stored response also lacks a validator, then that stored response is identified for update.

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.¶

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.¶

3.5. Storing Responses to Authenticated Requests

A shared cache MUST NOT use a cached response to a request with an Authorization header field (Section 11.6.2 of [HTTP]) to satisfy any subsequent request unless the response contains a Cache-Control field with a response directive (Section 5.2.2) that allows it to be stored by a shared cache, and the cache conforms to the requirements of that directive for that response.¶

In this specification, the following response directives have such an effect: must-revalidate (Section 5.2.2.2), public (Section 5.2.2.9), and s-maxage (Section 5.2.2.10).¶

Field Name	Status	Section
Age	permanent	5.1
Cache-Control	permanent	5.2
Expires	permanent	5.3
Pragma	deprecated	5.4
Warning	obsoleted	5.5

Cache Directive	Section
max-age	5.2.1.1, 5.2.2.1
max-stale	5.2.1.2
min-fresh	5.2.1.3
must-revalidate	5.2.2.2
must-understand	5.2.2.3
no-cache	5.2.1.4, 5.2.2.4
no-store	5.2.1.5, 5.2.2.5
no-transform	5.2.1.6, 5.2.2.6
only-if-cached	5.2.1.7
private	5.2.2.7
proxy-revalidate	5.2.2.8
public	5.2.2.9
s-maxage	5.2.2.10

HTTP Caching

Abstract

1.2.1. Imported Rules

1.2.2. Delta Seconds