HTTPbis Working Group R. Fielding, Editor
Internet-Draft Day Software
Obsoletes: 2616 (if approved) J. Gettys
Intended status: Standards Track One Laptop per Child
Expires: January 14, 2010 J. Mogul
H. Frystyk
L. Masinter
Adobe Systems
P. Leach
T. Berners-Lee
Y. Lafon, Editor
M. Nottingham, Editor
J. Reschke, Editor
July 13, 2009

HTTP/1.1, part 6: Caching

Status of this Memo

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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.

Editorial Note (To be removed by RFC Editor)

Discussion of this draft should take place on the HTTPBIS working group mailing list ( The current issues list is at <> and related documents (including fancy diffs) can be found at <>.

The changes in this draft are summarized in Appendix C.8.

Table of Contents

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.


explicit expiration time

heuristic expiration time



freshness lifetime




shared cache

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>
  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:

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:

[TODO-method-cacheability: 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.1: 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.

[TODO-header-properties: 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.2: 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.3: 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:

Note that this calculation is not vulnerable to clock skew, since all of the information comes from the origin server. 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.4: 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:

  1. 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.
  2. 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

When a cache has one or more stored responses for a requested URI, but cannot serve any of them (e.g., because they are not fresh, or one cannot be selected; see Section 2.6), it can use the conditional request mechanism [Part4] in the forwarded request to give the origin server an opportunity to both select a valid stored response to be used, and to update it. This process is known as "validating" or "revalidating" the stored response.

When sending such a conditional request, the cache SHOULD add an If-Modified-Since header whose value is that of the Last-Modified header from the selected (see Section 2.6) stored response, if available.

Additionally, the cache SHOULD add an If-None-Match header whose value is that of the ETag header(s) from all responses stored for the requested URI, if present. However, 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.

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

A full response (i.e., one with a response body) indicates that none of the stored responses nominated in the conditional request is suitable. Instead, the full response is used both to satisfy the request and replace the stored response. [rfc.comment.5: 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 (see Section 2.3.3).

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.6: DISCUSS: Not sure if this is necessary.]

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 URI(s) in the Location and Content-Location headers (if present):

An invalidation based on a URI from 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.7: 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.8: TODO: specify that only successful (2xx, 3xx?) responses invalidate.]

2.6 Caching Negotiated Responses

When a cache receives a request that can be satisfied by a stored response that has a Vary header field (Section 3.5), it MUST NOT use that response unless all of the selecting request-headers nominated by the Vary header match in both the original request (i.e., that associated with the stored response), and the presented 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.9: [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].

If a header field is absent from a request, it can only match another request if it is also absent there.

A Vary header field-value of "*" always fails to match, and subsequent requests to that resource can only be properly interpreted by the origin server.

The stored response with matching selecting request-headers is known as the selected response.

If no selected response is available, the cache MAY forward the presented request to the origin server in a conditional request; see Section 2.4.

2.7 Combining Responses

When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response (in this section, the "new" response"), it needs to created an updated response by combining the stored response with the new one, so that the updated response can be used to satisfy the request.

If the new response contains an ETag, it identifies the stored response to use. [rfc.comment.10: may need language about Content-Location here][rfc.comment.11: cover case where INM with multiple etags was sent]

If the status code is 206 (partial content), both the stored and new responses MUST have ETags, and those ETags MUST match using the strong comparison function (see Section 4 of [Part4]). Otherwise, the responses MUST NOT be combined.

The stored response headers are used as those of the updated response, except that

If a header field-name in the new response matches more than one header in the stored response, all such stored headers MUST be replaced.

The updated response can [[[rfc.comment.12: requirement?]]] be used to replace the stored response in cache. In the case of a 206 response, the combined entity-body MAY be stored.

[rfc.comment.13: 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 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.

Note that HTTP/1.0 caches might not implement Cache-Control and might only implement Pragma: no-cache (see Section 3.4).

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








3.2.2 Response Cache-Control Directives

  cache-response-directive =
     / "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










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 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

Note: if a response includes a Cache-Control field with the max-age directive (see Section 3.2.2), that directive overrides the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches.

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".

Note: because the meaning of "Pragma: no-cache" as a response-header field is not actually specified, it does not provide a reliable replacement for "Cache-Control: no-cache" in a response.

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:

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:

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

111 Revalidation failed

112 Disconnected operation

113 Heuristic expiration

199 Miscellaneous warning

214 Transformation applied

299 Miscellaneous persistent warning

4. History Lists

User agents often have history mechanisms, such as "Back" buttons and history lists, that can be used to redisplay an entity retrieved earlier in a session.

History mechanisms and caches are different. In particular history mechanisms SHOULD NOT try to show a correct view of the current state of a resource. Rather, a history mechanism is meant to show exactly what the user saw at the time when the resource was retrieved.

By default, an expiration time does not apply to history mechanisms. If the entity is still in storage, a history mechanism SHOULD display it even if the entity has expired, unless the user has specifically configured the agent to refresh expired history documents.

This is not to be construed to prohibit the history mechanism from telling the user that a view might be stale.

Note: if history list mechanisms unnecessarily prevent users from viewing stale resources, this will tend to force service authors to avoid using HTTP expiration controls and cache controls when they would otherwise like to. Service authors may consider it important that users not be presented with error messages or warning messages when they use navigation controls (such as BACK) to view previously fetched resources. Even though sometimes such resources ought not be cached, or ought to expire quickly, user interface considerations may force service authors to resort to other means of preventing caching (e.g. "once-only" URLs) in order not to suffer the effects of improperly functioning history mechanisms.

5. IANA Considerations

5.1 Message Header Registration

The Message Header Registry located at <> should be updated with the permanent registrations below (see [RFC3864]):

Header Field Name Protocol Status Reference
Age http standard Section 3.1
Cache-Control http standard Section 3.2
Expires http standard Section 3.3
Pragma http standard Section 3.4
Vary http standard Section 3.5
Warning http standard Section 3.6

The change controller is: "IETF ( - Internet Engineering Task Force".

6. Security Considerations

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

7. Acknowledgments

Much of the content and presentation of the caching design is due to suggestions and comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris, and Larry Masinter.

8. References

8.1 Normative References

[ISO-8859-1] International Organization for Standardization, “Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1”, ISO/IEC 8859-1:1998, 1998.
[Part1] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 1: URIs, Connections, and Message Parsing”, Internet-Draft draft-ietf-httpbis-p1-messaging-07 (work in progress), July 2009.
[Part2] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 2: Message Semantics”, Internet-Draft draft-ietf-httpbis-p2-semantics-07 (work in progress), July 2009.
[Part3] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 3: Message Payload and Content Negotiation”, Internet-Draft draft-ietf-httpbis-p3-payload-07 (work in progress), July 2009.
[Part4] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 4: Conditional Requests”, Internet-Draft draft-ietf-httpbis-p4-conditional-07 (work in progress), July 2009.
[Part5] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 5: Range Requests and Partial Responses”, Internet-Draft draft-ietf-httpbis-p5-range-07 (work in progress), July 2009.
[Part7] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 7: Authentication”, Internet-Draft draft-ietf-httpbis-p7-auth-07 (work in progress), July 2009.
[RFC2047] Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text”, RFC 2047, November 1996.
[RFC2119] Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, January 2008.

8.2 Informative References

[RFC1305] Mills, D., “Network Time Protocol (Version 3) Specification, Implementation”, RFC 1305, March 1992.
[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, June 1999.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.

Authors' Addresses

Roy T. Fielding (editor) Day Software23 Corporate Plaza DR, Suite 280Newport Beach, CA 92660USAPhone: +1-949-706-5300Fax: +1-949-706-5305EMail: URI:
Jim GettysOne Laptop per Child21 Oak Knoll RoadCarlisle, MA 01741USAEMail: URI:
Jeffrey C. MogulHewlett-Packard CompanyHP Labs, Large Scale Systems Group1501 Page Mill Road, MS 1177Palo Alto, CA 94304USAEMail:
Henrik Frystyk NielsenMicrosoft Corporation1 Microsoft WayRedmond, WA 98052USAEMail:
Larry MasinterAdobe Systems, Incorporated345 Park AveSan Jose, CA 95110USAEMail: URI:
Paul J. LeachMicrosoft Corporation1 Microsoft WayRedmond, WA 98052EMail:
Tim Berners-LeeWorld Wide Web ConsortiumMIT Computer Science and Artificial Intelligence LaboratoryThe Stata Center, Building 3232 Vassar StreetCambridge, MA 02139USAEMail: URI:
Yves Lafon (editor) World Wide Web ConsortiumW3C / ERCIM2004, rte des LuciolesSophia-Antipolis, AM 06902FranceEMail: URI:
Mark Nottingham (editor) EMail: URI:
Julian F. Reschke (editor) greenbytes GmbHHafenweg 16Muenster, NW 48155GermanyPhone: +49 251 2807760Fax: +49 251 2807761EMail: URI:

A. Compatibility with Previous Versions

A.1 Changes from RFC 2068

A case was missed in the Cache-Control model of HTTP/1.1; s-maxage was introduced to add this missing case. (Sections 2.1, 3.2).

Transfer-coding and message lengths all interact in ways that required fixing exactly when chunked encoding is used (to allow for transfer encoding that may not be self delimiting); it was important to straighten out exactly how message lengths are computed. (see also [Part1], [Part3] and [Part5]) [rfc.comment.15: This used to refer to the text about non-modifiable headers, and will have to be updated later on. --jre]

Proxies should be able to add Content-Length when appropriate. [rfc.comment.16: This used to refer to the text about non-modifiable headers, and will have to be updated later on. --jre]

Range request responses would become very verbose if all meta-data were always returned; by allowing the server to only send needed headers in a 206 response, this problem can be avoided. (Section 2.7)

The Cache-Control: max-age directive was not properly defined for responses. (Section 3.2.2)

Warnings could be cached incorrectly, or not updated appropriately. (Section 2.3, 2.7, 3.2, and 3.6) Warning also needed to be a general header, as PUT or other methods may have need for it in requests.

A.2 Changes from RFC 2616

Clarify denial of service attack avoidance requirement. (Section 2.5)

B. Collected ABNF

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

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

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

HTTP-date = <HTTP-date, defined in [Part1], Section 3.2>

OWS = <OWS, defined in [Part1], Section 1.2.2>

Pragma = "Pragma:" OWS Pragma-v
Pragma-v = *( "," OWS ) pragma-directive *( OWS "," [ OWS
 pragma-directive ] )

Vary = "Vary:" OWS Vary-v
Vary-v = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name
 ] ) )

Warning = "Warning:" OWS Warning-v
Warning-v = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value
 ] )

cache-directive = cache-request-directive / cache-response-directive
cache-extension = token [ "=" ( token / quoted-string ) ]
cache-request-directive = "no-cache" / "no-store" / ( "max-age="
 delta-seconds ) / ( "max-stale" [ "=" delta-seconds ] ) / (
 "min-fresh=" delta-seconds ) / "no-transform" / "only-if-cached" /
cache-response-directive = "public" / ( "private" [ "=" DQUOTE *( ","
 OWS ) field-name *( OWS "," [ OWS field-name ] ) DQUOTE ] ) / (
 "no-cache" [ "=" DQUOTE *( "," OWS ) field-name *( OWS "," [ OWS
 field-name ] ) DQUOTE ] ) / "no-store" / "no-transform" /
 "must-revalidate" / "proxy-revalidate" / ( "max-age=" delta-seconds
 ) / ( "s-maxage=" delta-seconds ) / cache-extension

delta-seconds = 1*DIGIT

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

field-name = <field-name, defined in [Part1], Section 4.2>

port = <port, defined in [Part1], Section 2.1>
pragma-directive = "no-cache" / extension-pragma
pseudonym = <pseudonym, defined in [Part1], Section 8.9>

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

token = <token, defined in [Part1], Section 1.2.2>

uri-host = <uri-host, defined in [Part1], Section 2.1>

warn-agent = ( uri-host [ ":" port ] ) / pseudonym
warn-code = 3DIGIT
warn-date = DQUOTE HTTP-date DQUOTE
warn-text = quoted-string
warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date

ABNF diagnostics:

; Age defined but not used
; Cache-Control defined but not used
; Expires defined but not used
; Pragma defined but not used
; Vary defined but not used
; Warning defined but not used

C. Change Log (to be removed by RFC Editor before publication)

C.1 Since RFC2616

Extracted relevant partitions from [RFC2616].

C.2 Since draft-ietf-httpbis-p6-cache-00

Closed issues:

Other changes:

C.3 Since draft-ietf-httpbis-p6-cache-01

Closed issues:

Other changes:

C.4 Since draft-ietf-httpbis-p6-cache-02

Ongoing work on IANA Message Header Registration (<>):

C.5 Since draft-ietf-httpbis-p6-cache-03

Closed issues:

C.6 Since draft-ietf-httpbis-p6-cache-04

Ongoing work on ABNF conversion (<>):

C.7 Since draft-ietf-httpbis-p6-cache-05

This is a total rewrite of this part of the specification.

Affected issues:

In addition: Final work on ABNF conversion (<>):

C.8 Since draft-ietf-httpbis-p6-cache-06

Closed issues:

Affected issues: