HTTP/1.1, part 6: Caching

The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. This document is Part 6 of the seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 6 defines requirements on HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages.¶

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 include a cache, though a cache cannot be used by a server that is acting as a tunnel.¶

Caching would be useless if it did not significantly improve performance. The goal of caching in HTTP/1.1 is to reuse a prior response message to satisfy a current request. In some cases, a stored response can be reused without the need for a network request, reducing latency and network round-trips; a "freshness" mechanism is used for this purpose (see Section 2.3). Even when a new request is required, it is often possible to reuse all or parts of the payload of a prior response to satisfy the request, thereby reducing network bandwidth usage; a "validation" mechanism is used for this purpose (see Section 2.4).¶

1.2. Terminology

This specification uses a number of terms to refer to the roles played by participants in, and objects of, HTTP caching.¶

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 may be additional constraints on whether a cache can use the cached copy to satisfy a particular request.

explicit expiration time ¶

The time at which the origin server intends that an entity should 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 an entity.

shared cache ¶

A cache that is accessible to more than one user. A non-shared cache is dedicated to a single user.

1.3. Requirements

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].¶

An implementation is not compliant if it fails to satisfy one or more of the MUST or REQUIRED level requirements for the protocols it implements. An implementation that satisfies all the MUST or REQUIRED level and all the SHOULD level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the MUST level requirements but not all the SHOULD level requirements for its protocols is said to be "conditionally compliant."¶

1.4. Syntax Notation

This specification uses the ABNF syntax defined in Section 1.2 of [Part1] (which extends the syntax defined in [RFC5234] with a list rule). Appendix B shows the collected ABNF, with the list rule expanded.¶

The following core rules are included by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), VCHAR (any visible USASCII character), and WSP (whitespace).¶

1.4.1. Core Rules

The core rules below are defined in Section 1.2.2 of [Part1]:¶

  quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
  token         = <token, defined in [Part1], Section 1.2.2>
  OWS           = <OWS, defined in [Part1], Section 1.2.2>

1.4.2. ABNF Rules defined in other Parts of the Specification

The ABNF rules below are defined in other parts:¶

  field-name    = <field-name, defined in [Part1], Section 4.2>
  HTTP-date     = <HTTP-date, defined in [Part1], Section 3.2.1>
  port          = <port, defined in [Part1], Section 2.1>
  pseudonym     = <pseudonym, defined in [Part1], Section 8.9> 
  uri-host      = <uri-host, defined in [Part1], Section 2.1>

2. Cache Operation

2.1. Response Cacheability

A cache MUST NOT store a response to any request, unless: ¶

The request method is defined as being cacheable, and
the "no-store" cache directive (see Section 3.2) does not appear in request or response headers, and
the "private" cache response directive (see Section 3.2 does not appear in the response, if the cache is shared, and
the "Authorization" header (see Section 3.1 of [Part7]) does not appear in the request, if the cache is shared (unless the "public" directive is present; see Section 3.2), and
the cache understands partial responses, if the response is partial or incomplete (see Section 2.1.1).

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

2.1.1. Storing Partial and Incomplete Responses

A cache that receives an incomplete response (for example, with fewer bytes of data than specified in a Content-Length header) can store the response, but MUST treat it as a partial response [Part5]. Partial responses can be combined as described in Section 4 of [Part5]; the result might be a full response or might still be partial. A cache MUST NOT return a partial response to a client without explicitly marking it as such using the 206 (Partial Content) status code.¶

A cache that does not support the Range and Content-Range headers MUST NOT store incomplete or partial responses.¶

2.2. Constructing Responses from Caches

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

The presented Request-URI and that of the stored response match (see [rfc.comment.1: TBD]), and
the request method associated with the stored response allows it to be used for the presented request, and
selecting request-headers nominated by the stored response (if any) match those presented (see Section 2.6), and
the presented request and stored response are free from directives that would prevent its use (see Section 3.2 and Section 3.4), and
the stored response is either:
- fresh (see Section 2.3), or
- allowed to be served stale (see Section 2.3.3), or
- successfully validated (see Section 2.4).

[rfc.comment.2: TODO: define method cacheability for GET, HEAD and POST in p2-semantics.] ¶

When a stored response is used to satisfy a request, caches MUST include a single Age header field Section 3.1 in the response with a value equal to the stored response's current_age; see Section 2.3.2. [rfc.comment.3: DISCUSS: this currently includes successfully validated responses.] ¶

Requests with methods that are unsafe (Section 7.1.1 of [Part2]) MUST be written through the cache to the origin server; i.e., A cache must not 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 2.5.¶

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

[rfc.comment.4: TODO: end-to-end and hop-by-hop headers, non-modifiable headers removed; re-spec in p1] ¶

2.3. 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 (Section 3.3) or the max-age response cache directive (Section 3.2.2). Generally, origin servers will assign future explicit expiration times to responses in the belief that the entity 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. This means that the response is always stale, so that caches should validate it before using it for subsequent requests. [rfc.comment.5: This wording may cause confusion, because the response may still be served stale.] ¶

Since origin servers do not always provide explicit expiration times, HTTP caches may also assign heuristic expiration times when they are not specified, employing algorithms that use other header values (such as the Last-Modified time) to estimate a plausible expiration time. The HTTP/1.1 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 2.3.1; the current_age is defined in Section 2.3.2.¶

Additionally, clients may need to influence freshness calculation. They can do this using several request cache directives, with the effect of either increasing or loosening constraints on freshness. See Section 3.2.1.¶

[rfc.comment.6: ISSUE: there are not requirements directly applying to cache-request-directives and freshness.] ¶

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

2.3.1. 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 3.2.2) is present, use its value, or
If the max-age response cache directive (Section 3.2.2) is present, use its value, or
If the Expires response header (Section 3.3) is present, use its value minus the value of the Date response header, or
Otherwise, no explicit expiration time is present in the response, but a heuristic may be used; see Section 2.3.1.1.

Note that this calculation is not vulnerable to clock skew, since all of the information comes from the origin server.¶

2.3.1.1. Calculating Heuristic Freshness

If no explicit expiration time is present in a stored response that has a status code of 200, 203, 206, 300, 301 or 410, a heuristic expiration time can be calculated. Heuristics MUST NOT be used for other response status codes.¶

When a heuristic is used to calculate freshness lifetime, the cache SHOULD attach a Warning header 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 (Section 6.6 of [Part4]), the heuristic expiration value SHOULD be no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.¶

[rfc.comment.7: REVIEW: took away HTTP/1.0 query string heuristic uncacheability.] ¶

2.3.2. Calculating Age

HTTP/1.1 uses the Age response-header 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 term "age_value" denotes the value of the Age header, in a form appropriate for arithmetic operations.¶

HTTP/1.1 requires origin servers to send a Date header, if possible, with every response, giving the time at which the response was generated (see Section 8.3 of [Part1]). The term "date_value" denotes the value of the Date header, in a form appropriate for arithmetic operations.¶

The term "now" means "the current value of the clock at the host performing the calculation." Hosts that use HTTP, but especially hosts running origin servers and caches, SHOULD use NTP [RFC1305] or some similar protocol to synchronize their clocks to a globally accurate time standard.¶

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

now 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.
age_value, if all of the caches along the response path implement HTTP/1.1.

These are combined as

    corrected_received_age = max(now - date_value, age_value)

When an Age value is received, it MUST be interpreted relative to the time the request was initiated, not the time that the response was received.¶

   corrected_initial_age = corrected_received_age
                         + (now - request_time)

where "request_time" is the time (according to the local clock) when the request that elicited this response was sent.¶

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.¶

In summary:¶

  age_value     - Age header field-value received with the response
  date_value    - Date header field-value received with the response
  request_time  - local time when the cache made the request 
                 resulting in the stored response
  response_time - local time when the cache received the response
  now           - current local time
  
  apparent_age = max(0, response_time - date_value);
  corrected_received_age = max(apparent_age, age_value);
  response_delay = response_time - request_time;
  corrected_initial_age = corrected_received_age + response_delay;
  resident_time = now - response_time;
  current_age   = corrected_initial_age + resident_time;

2.3.3. 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 2.3.¶

Caches 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 3.2.2).¶

Caches SHOULD NOT return stale responses unless they are disconnected (i.e., it cannot contact the origin server or otherwise find a forward path) or otherwise explicitly allowed (e.g., the max-stale request directive; see Section 3.2.1).¶

Stale responses SHOULD have a Warning header with the 110 warn-code (see Section 3.6). Likewise, the 112 warn-code SHOULD be sent on 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 SHOULD forward it to the requesting client without adding a new Warning (but without removing any existing Warning headers). A cache SHOULD NOT attempt to validate a response simply because that response became stale in transit.¶

2.4. Validation Model

Checking with the origin server to see if a stale or otherwise unusable cached response can be reused is called "validating" or "revalidating." Doing so potentially avoids the overhead of retransmitting the response body when the stored response is valid.¶

HTTP's conditional request mechanism [Part4] is used for this purpose. When a stored response includes one or more validators, such as the field values of an ETag or Last-Modified header field, then a validating request SHOULD be made conditional to those field values.¶

A 304 (Not Modified) response status code indicates that the stored response can be updated and reused; see Section 2.7.¶

If instead the cache receives a full response (i.e., one with a response body), it is used to satisfy the request and replace the stored response. [rfc.comment.8: Should there be a requirement here?] ¶

If a cache receives a 5xx response while attempting to validate a response, it MAY either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, it MAY return a previously stored response (which SHOULD include the 111 warn-code; see Section 3.6) unless the stored response includes the "must-revalidate" cache directive (see Section 2.3.3).¶

2.5. Request Methods that Invalidate

Because unsafe methods (Section 7.1.1 of [Part2]) have the potential for changing state on the origin server, intervening caches can use them to keep their contents up-to-date.¶

The following HTTP methods MUST cause a cache to invalidate the Request-URI as well as the Location and Content-Location headers (if present): ¶

PUT
DELETE
POST

An invalidation based on the URI in a Location or Content-Location header MUST NOT be performed if the host part of that URI differs from the host part in the Request-URI. This helps prevent denial of service attacks.¶

[rfc.comment.9: TODO: "host part" needs to be specified better.] ¶

A cache that passes through requests for methods it does not understand SHOULD invalidate the Request-URI.¶

Here, "invalidate" means that the cache will either remove all stored responses related to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation before they can be returned in response to a subsequent request.¶

Note that this does not guarantee that all appropriate responses are invalidated. For example, the request that caused the change at the origin server might not have gone through the cache where a response is stored.¶

[rfc.comment.10: TODO: specify that only successful (2xx, 3xx?) responses invalidate.] ¶

2.6. Caching Negotiated Responses

Use of server-driven content negotiation (Section 4.1 of [Part3]) alters the conditions under which a cache can use the response for subsequent requests.¶

When a cache receives a request that can be satisfied by a stored response that includes a Vary header field (Section 3.5), it MUST NOT use that response unless all of the selecting request-headers in the presented request match the corresponding stored request-headers from the original request.¶

The selecting request-headers from two requests are defined to match if and only if the selecting request-headers in the first request can be transformed to the selecting request-headers in the second request by adding or removing linear white space [rfc.comment.11: [ref]] at places where this is allowed by the corresponding ABNF, and/or combining multiple message-header fields with the same field name following the rules about message headers in Section 4.2 of [Part1]. [rfc.comment.12: DISCUSS: header-specific canonicalisation] ¶

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.¶

If no stored response matches, the cache MAY forward the presented request to the origin server in a conditional request, and SHOULD include all ETags stored with potentially suitable responses in an If-None-Match request header. If the server responds with 304 (Not Modified) and includes an entity tag or Content-Location that indicates the entity to be used, that cached response MUST be used to satisfy the presented request, and SHOULD be used to update the corresponding stored response; see Section 2.7.¶

If any of the stored responses contains only partial content, its entity-tag SHOULD NOT be included in the If-None-Match header field unless the request is for a range that would be fully satisfied by that stored response.¶

If a cache receives a successful response whose Content-Location field matches that of an existing stored response for the same Request-URI, whose entity-tag differs from that of the existing stored response, and whose Date is more recent than that of the existing response, the existing response SHOULD NOT be returned in response to future requests and SHOULD be deleted from the cache.[rfc.comment.13: DISCUSS: Not sure if this is necessary.] ¶

2.7. Combining Responses

When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response, it needs to update the stored response with the new one, so that the updated response can be sent to the client.¶

If the status code is 304 (Not Modified), the cache SHOULD use the stored entity-body as the updated entity-body. If the status code is 206 (Partial Content) and the ETag or Last-Modified headers match exactly, the cache MAY combine the stored entity-body in the stored response with the updated entity-body received in the response and use the result as the updated entity-body (see Section 4 of [Part5]).¶

The stored response headers are used for the updated response, except that ¶

any stored Warning headers with warn-code 1xx (see Section 3.6) MUST be deleted from the stored response and the forwarded response.
any stored Warning headers with warn-code 2xx MUST be retained in the stored response and the forwarded response.
any headers provided in the 304 or 206 response MUST replace the corresponding headers from the stored response.

A cache MUST also replace any stored headers with corresponding headers received in the incoming response, except for Warning headers as described immediately above. If a header field-name in the incoming response matches more than one header in the stored response, all such old headers MUST be replaced. It MAY store the combined entity-body.¶

[rfc.comment.14: ISSUE: discuss how to handle HEAD updates] ¶

3. Header Field Definitions

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

For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who receives the entity.¶

3.1. Age

The response-header field "Age" conveys the sender's estimate of the amount of time since the response (or its validation) was generated at the origin server. Age values are calculated as specified in Section 2.3.2.¶

  Age   = "Age" ":" OWS Age-v
  Age-v = delta-seconds

Age field-values are non-negative decimal integers, representing time in seconds.¶

  delta-seconds  = 1*DIGIT

If a cache receives a value larger than the largest positive integer it can represent, or if any of its age calculations overflows, it MUST transmit an Age header with a field-value of 2147483648 (2³¹). Caches SHOULD use an arithmetic type of at least 31 bits of range.¶

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 may not implement the Age header field.¶

3.2. Cache-Control

The general-header field "Cache-Control" is used to specify directives that MUST be obeyed by all caches along the request/response chain. The directives specify behavior intended to prevent caches from adversely interfering with the request or response. 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.¶

Cache directives MUST be passed through by a proxy or gateway application, 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-Control   = "Cache-Control" ":" OWS Cache-Control-v
  Cache-Control-v = 1#cache-directive

  cache-directive = cache-request-directive
     / cache-response-directive

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

3.2.1. Request Cache-Control Directives

  cache-request-directive =
       "no-cache"
     / "no-store"
     / "max-age" "=" delta-seconds
     / "max-stale" [ "=" delta-seconds ]
     / "min-fresh" "=" delta-seconds
     / "no-transform"
     / "only-if-cached"
     / cache-extension

no-cache ¶

The no-cache request directive indicates that a stored response MUST NOT be used to satisfy the request without successful validation on the origin server.

no-store ¶

The no-store request directive indicates that a cache MUST NOT store any part of either this request or any response to it. This directive applies to both non-shared 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 may be vulnerable to eavesdropping.

max-age ¶

The max-age request directive indicates that the client is willing to accept a response whose age is no greater than the specified time in seconds. Unless max-stale directive is also included, the client is not willing to accept a stale response.

max-stale ¶

The max-stale request directive indicates that the client is willing to accept a response that has exceeded its expiration time. If max-stale is assigned a value, then the client is willing to accept a response that has exceeded its expiration time by no more than the specified number of seconds. If no value is assigned to max-stale, then the client is willing to accept a stale response of any age. [rfc.comment.15: of any staleness? --mnot]

min-fresh ¶

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

no-transform ¶

The no-transform request directive indicates that an intermediate cache or proxy MUST NOT change the Content-Encoding, Content-Range or Content-Type request headers, nor the request entity-body.

only-if-cached ¶

The only-if-cached request directive indicates that the client only wishes to return 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. If a group of caches is being operated as a unified system with good internal connectivity, such a request MAY be forwarded within that group of caches.

3.2.2. Response Cache-Control Directives

  cache-response-directive =
       "public"
     / "private" [ "=" DQUOTE 1#field-name DQUOTE ]
     / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
     / "no-store"
     / "no-transform"
     / "must-revalidate"
     / "proxy-revalidate"
     / "max-age" "=" delta-seconds
     / "s-maxage" "=" delta-seconds
     / cache-extension

public ¶

The public response directive indicates that the response MAY be cached, even if it would normally be non-cacheable or cacheable only within a non-shared cache. (See also Authorization, Section 3.1 of [Part7], for additional details.)

private ¶

The private response directive indicates that the response message is intended for a single user and MUST NOT be stored by a shared cache. A private (non-shared) 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 headers. That is, the specified field-names(s) MUST NOT be stored by a shared cache, whereas the remainder of the response message MAY be.
Note: This usage of the word private only controls where the response may be stored, and cannot ensure the privacy of the message content.

no-cache ¶

The no-cache response directive indicates that the response MUST NOT be used to satisfy a subsequent request without successful validation on the origin server. This allows an origin server to prevent caching even by caches that have been configured to return stale responses.
If the no-cache response directive specifies one or more field-names, this requirement is limited to the field-values assosicated with the listed response headers. That is, the specified field-name(s) MUST NOT be sent in the response to a subsequent request without successful validation on the origin server. This allows an origin server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response.
Note: Most HTTP/1.0 caches will not recognize or obey this directive.

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 non-shared 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 may be vulnerable to eavesdropping.

must-revalidate ¶

The must-revalidate response directive indicates that once it has become stale, the response MUST NOT be used 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 an HTTP/1.1 cache MUST obey the must-revalidate directive; in particular, if the cache cannot reach the origin server for any reason, it MUST generate a 504 (Gateway Timeout) response.
Servers SHOULD send the must-revalidate directive if and only if failure to validate a request on the entity could result in incorrect operation, such as a silently unexecuted financial transaction.

proxy-revalidate ¶

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

max-age ¶

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

s-maxage ¶

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

no-transform ¶

The no-transform response directive indicates that an intermediate cache or proxy MUST NOT change the Content-Encoding, Content-Range or Content-Type response headers, nor the response entity-body.

3.2.3. 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 non-shared cache, any cache that is shared only by members of the community named within its value may cache the response. An origin server wishing to allow the UCI community to use an otherwise private response in their shared cache(s) could do so by including¶

  Cache-Control: private, community="UCI"

A cache seeing this header field will act correctly even if the cache does not understand the community cache-extension, since it will also see and understand the private directive and thus default to the safe behavior.¶

Unrecognized cache directives MUST be ignored; 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).¶

3.3. Expires

The entity-header field "Expires" gives the date/time after which the response is considered stale. See Section 2.3 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 3.2.1 of [Part1]; it MUST be sent in rfc1123-date format.¶

  Expires   = "Expires" ":" OWS Expires-v
  Expires-v = HTTP-date

For example

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

HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in the future.¶

HTTP/1.1 clients and caches MUST treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").¶

3.4. Pragma

The general-header field "Pragma" is used to include implementation-specific directives that might apply to any recipient along the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however, some systems MAY require that behavior be consistent with the directives.¶

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

When the no-cache directive is present in a request message, an application SHOULD forward the request toward the origin server even if it has a cached copy of what is being requested. This pragma directive has the same semantics as the no-cache response directive (see Section 3.2.2) and is defined here for backward compatibility with HTTP/1.0. Clients SHOULD include both header fields when a no-cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache".¶

This mechanism is deprecated; no new Pragma directives will be defined in HTTP.¶

3.5. Vary

The "Vary" response-header field's value indicates the set of request-header fields that determines, while the response is fresh, whether a cache is permitted to use the response to reply to a subsequent request without validation; see Section 2.6.¶

In uncacheable or stale responses, the Vary field value advises the user agent about the criteria that were used to select the representation.¶

  Vary   = "Vary" ":" OWS Vary-v
  Vary-v = "*" / 1#field-name

The set of header fields named by the Vary field value is known as the selecting request-headers.¶

Servers 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 request-headers (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. The "*" value MUST NOT be generated by a proxy server; it may only be generated by an origin server.¶

The field-names given are not limited to the set of standard request-header fields defined by this specification. Field names are case-insensitive.¶

3.6. Warning

The general-header field "Warning" 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 entity body 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, distinguish these responses from true failures.¶

Warning headers 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    = "Warning" ":" OWS Warning-v
  Warning-v  = 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, 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. For example, a server might provide the same warning with texts in both English and Basque.¶

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. If it is not possible to inform the user of all of the warnings, the user agent SHOULD follow these heuristics: ¶

Warnings that appear early in the response take priority over those appearing later in the response.
Warnings in the user's preferred character set take priority over warnings in other character sets but with identical warn-codes and warn-agents.

Systems that generate multiple Warning headers SHOULD order them with this user agent behavior in mind. New Warning headers SHOULD be added after any existing Warning headers.¶

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 that describe the freshness or validation status of the response, and so MUST be deleted by caches after validation. They MUST NOT be generated by a cache except when validating a cached entry, and MUST NOT be generated by clients.
2xx Warnings that describe some aspect of the entity body or entity headers that is not rectified by a validation (for example, a lossy compression of the entity bodies) and MUST NOT be deleted by caches after validation, unless a full response is returned, in which case they MUST be.

The warn-text SHOULD be in a natural language and character set that is most likely to be intelligible to the human user receiving the response. This decision can be based on any available knowledge, such as the location of the cache or user, the Accept-Language field in a request, the Content-Language field in a response, etc. The default language is English and the default character set is ISO-8859-1 ([ISO-8859-1]).¶

If a character set other than ISO-8859-1 is used, it MUST be encoded in the warn-text using the method described in [RFC2047].¶

If an implementation sends a message with one or more Warning headers 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 in the message.¶

If an implementation 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 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.¶

110 Response is stale ¶

SHOULD be included whenever the returned response is stale.

111 Revalidation failed ¶

SHOULD be included if a cache returns a stale response because an attempt to validate the response failed, due to an inability to reach the server.

112 Disconnected operation ¶

SHOULD be included if the cache is intentionally disconnected from the rest of the network for a period of time.

113 Heuristic expiration ¶

SHOULD be included if the cache heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than 24 hours.

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.

214 Transformation applied ¶

MUST be added by an intermediate cache or proxy if it applies any transformation changing the content-coding (as specified in the Content-Encoding header) or media-type (as specified in the Content-Type header) of the response, or the entity-body of the response, unless this Warning code already appears in the response.

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.

8. References

8.1. Normative References

8.2. Informative References