HTTPbis Working GroupR. Fielding, Editor
Internet-DraftAdobe
Obsoletes: 2616 (if approved)J. Gettys
Intended status: Standards TrackAlcatel-Lucent
Expires: May 3, 2012J. Mogul
HP
H. Frystyk
Microsoft
L. Masinter
Adobe
P. Leach
Microsoft
T. Berners-Lee
W3C/MIT
Y. Lafon, Editor
W3C
M. Nottingham, Editor
Rackspace
J. Reschke, Editor
greenbytes
October 31, 2011

HTTP/1.1, part 6: Caching
draft-ietf-httpbis-p6-cache-17

Abstract

The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. 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 (ietf-http-wg@w3.org), which is archived at <http://lists.w3.org/Archives/Public/ietf-http-wg/>.

The current issues list is at <http://tools.ietf.org/wg/httpbis/trac/report/3> and related documents (including fancy diffs) can be found at <http://tools.ietf.org/wg/httpbis/>.

The changes in this draft are summarized in Appendix C.18.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress”.

This Internet-Draft will expire on May 3, 2012.

Copyright Notice

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

The goal of caching in HTTP/1.1 is to significantly improve performance by reusing a prior response message to satisfy a current request. A stored response is considered "fresh", as defined in Section 2.3, if the response can be reused without "validation" (checking with the origin server to see if the cached response remains valid for this request). A fresh cache response can therefore reduce both latency and network transfers each time it is reused. When a cached response is not fresh, it might still be reusable if it can be freshened by validation (Section 2.4) or if the origin is unavailable.

1.2 Terminology

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

cache

shared cache

  • A cache that stores responses to be reused by more than one user; usually (but not always) deployed as part of an intermediary.

private cache

cacheable

explicit expiration time

heuristic expiration time

age

first-hand

freshness lifetime

fresh

stale

validator

strong validator

1.3 Conformance and Error Handling

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

This document defines conformance criteria for several roles in HTTP communication, including Senders, Recipients, Clients, Servers, User-Agents, Origin Servers, Intermediaries, Proxies and Gateways. See Section 2 of [Part1] for definitions of these terms.

An implementation is considered conformant if it complies with all of the requirements associated with its role(s). Note that SHOULD-level requirements are relevant here, unless one of the documented exceptions is applicable.

This document also uses ABNF to define valid protocol elements (Section 1.4). In addition to the prose requirements placed upon them, Senders MUST NOT generate protocol elements that are invalid.

Unless noted otherwise, Recipients MAY take steps to recover a usable protocol element from an invalid construct. However, HTTP does not define specific error handling mechanisms, except in cases where it has direct impact on security. This is because different uses of the protocol require different error handling strategies; for example, a Web browser may wish to transparently recover from a response where the Location header field doesn't parse according to the ABNF, whereby in a systems control protocol using HTTP, this type of error recovery could lead to dangerous consequences.

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), and VCHAR (any visible US-ASCII character).

1.4.1 Core Rules

The core rules below are defined in [Part1]:

  OWS           = <OWS, defined in [Part1], Section 1.2.2>
  quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
  token         = <token, defined in [Part1], Section 3.2.3>

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 3.2>
  HTTP-date     = <HTTP-date, defined in [Part2], Section 8>
  port          = <port, defined in [Part1], Section 2.7>
  pseudonym     = <pseudonym, defined in [Part1], Section 8.8> 
  uri-host      = <uri-host, defined in [Part1], Section 2.7>

1.5 Delta Seconds

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

  delta-seconds  = 1*DIGIT

If an implementation receives a delta-seconds value larger than the largest positive integer it can represent, or if any of its subsequent calculations overflows, it MUST consider the value to be 2147483648 (231). Recipients parsing a delta-seconds value MUST use an arithmetic type of at least 31 bits of range, and senders MUST NOT send delta-seconds with a value greater than 2147483648.

2. Cache Operation

Proper cache operation preserves the semantics of HTTP transfers ([Part2]) while eliminating the transfer of information already held in the cache. Although caching is an entirely OPTIONAL feature of HTTP, we assume that reusing the cached response is desirable and that such reuse is the default behavior when no requirement or locally-desired configuration prevents it. Therefore, HTTP cache requirements are focused on preventing a cache from either storing a non-reusable response or reusing a stored response inappropriately.

Each cache entry consists of a cache key and one or more HTTP responses corresponding to prior requests that used the same key. The most common form of cache entry is a successful result of a retrieval request: i.e., a 200 (OK) response containing a representation of the resource identified by the request target. However, it is also possible to cache negative results (e.g., 404 not found), incomplete results (e.g., 206 partial content), and responses to safe methods other than GET if the method's definition allows such caching and defines something suitable for use as a cache key.

The default cache key consists of the request method and target URI. However, since HTTP caches in common use today are typically limited to caching responses to GET, most implementations simply decline other methods and use only the URI as the key.

If a request target is subject to content negotiation, its cache entry might consist of multiple stored responses, each differentiated by a secondary key for the values of the original request's selecting header fields (Section 2.7).

2.1 Response Cacheability

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

Note that any of the requirements listed above can be overridden by a cache-control extension; see Section 3.2.3.

In this context, a cache has "understood" a request method or a response status code if it recognizes it and implements any cache-specific behavior.

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.

A response message is considered complete when all of the octets indicated by the message framing ([Part1]) are received prior to the connection being closed. If the request is GET, the response status is 200 (OK), and the entire response header block has been received, a cache MAY store an incomplete response message-body if the cache entry is recorded as incomplete. Likewise, a 206 (Partial Content) response MAY be stored as if it were an incomplete 200 (OK) cache entry. However, a cache MUST NOT store incomplete or partial content responses if it does not support the Range and Content-Range header fields or if it does not understand the range units used in those fields.

A cache MAY complete a stored incomplete response by making a subsequent range request ([Part5]) and combining the successful response with the stored entry, as defined in Section 2.8. A cache MUST NOT use an incomplete response to answer requests unless the response has been made complete or the request is partial and specifies a range that is wholly within the incomplete response. A cache MUST NOT send a partial response to a client without explicitly marking it as such using the 206 (Partial Content) status code.

2.2 Constructing Responses from Caches

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

Note that any of the requirements listed above can be overridden by a cache-control extension; see Section 3.2.3.

When a stored response is used to satisfy a request without validation, a cache 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.

A cache MUST write through requests with methods that are unsafe (Section 6.1.1 of [Part2]) to the origin server; i.e., a cache must not 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 2.5.

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

A cache that does not have a clock available MUST NOT use stored responses without revalidating them on every use. A cache, especially a shared cache, SHOULD use a mechanism, such as NTP [RFC1305], to synchronize its clock with a reliable external standard.

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 field (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 representation is not likely to change in a semantically significant way before the expiration time is reached.

If an origin server wishes to force a cache to validate every request, it can assign an explicit expiration time in the past to indicate that the response is already stale. Compliant caches will normally validate the cached response before reusing it for subsequent requests (see Section 2.3.3).

Since origin servers do not always provide explicit expiration times, a cache MAY assign a heuristic expiration time when an explicit time is not specified, employing algorithms that use other header field values (such as the Last-Modified time) to estimate a plausible expiration time. This specification does not provide specific algorithms, but does impose worst-case constraints on their results.

The calculation to determine if a response is fresh is:

   response_is_fresh = (freshness_lifetime > current_age)

The freshness_lifetime is defined in Section 2.3.1; the current_age is defined in Section 2.3.2.

Additionally, clients might 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.

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 field (Section 3.3) is present, use its value minus the value of the Date response header field, or
  • Otherwise, no explicit expiration time is present in the response. A heuristic freshness lifetime might be applicable; see Section 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 whose definition allows heuristic freshness to be used (including the following in Section 7 of [Part2]: 200, 203, 206, 300, 301 and 410), a cache MAY calculate a heuristic expiration time. A cache MUST NOT use heuristics to determine freshness for responses with status codes that do not explicitly allow it.

When a heuristic is used to calculate freshness lifetime, a cache SHOULD attach a Warning header field with a 113 warn-code to the response if its current_age is more than 24 hours and such a warning is not already present.

Also, if the response has a Last-Modified header field (Section 2.2 of [Part4]), caches are encouraged to use a heuristic expiration value that is no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.

Note: RFC 2616 ([RFC2616], Section 13.9) required that caches do not calculate heuristic freshness for URIs with query components (i.e., those containing '?'). In practice, this has not been widely implemented. Therefore, servers are encouraged to send explicit directives (e.g., Cache-Control: no-cache) if they wish to preclude caching.

2.3.2 Calculating Age

HTTP/1.1 uses the Age header field to convey the estimated age of the response message when obtained from a cache. The Age field value is the cache's estimate of the amount of time since the response was generated or validated by the origin server. In essence, the Age value is the sum of the time that the response has been resident in each of the caches along the path from the origin server, plus the amount of time it has been in transit along network paths.

The following data is used for the age calculation:

age_value

  • The term "age_value" denotes the value of the Age header field (Section 3.1), in a form appropriate for arithmetic operation; or 0, if not available.

date_value

  • HTTP/1.1 requires origin servers to send a Date header field, if possible, with every response, giving the time at which the response was generated. The term "date_value" denotes the value of the Date header field, in a form appropriate for arithmetic operations. See Section 9.2 of [Part2] for the definition of the Date header field, and for requirements regarding responses without it.

now

  • The term "now" means "the current value of the clock at the host performing the calculation". A cache SHOULD use NTP ([RFC1305]) or some similar protocol to synchronize its clocks to a globally accurate time standard.

request_time

  • The current value of the clock at the host at the time the request resulting in the stored response was made.

response_time

  • The current value of the clock at the host at the time the response was received.

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

  1. the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the origin server's clock. If the result is negative, the result is replaced by zero.
  2. the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1. A cache MUST interpret this value relative to the time the request was initiated, not the time that the response was received.
  apparent_age = max(0, response_time - date_value);

  response_delay = response_time - request_time;
  corrected_age_value = age_value + response_delay;  

These are combined as

  corrected_initial_age = max(apparent_age, corrected_age_value);

The current_age of a stored response can then be calculated by adding the amount of time (in seconds) since the stored response was last validated by the origin server to the corrected_initial_age.

  resident_time = now - response_time;
  current_age = corrected_initial_age + resident_time;

Additionally, to avoid common problems in date parsing:

  • HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date which appears to be more than 50 years in the future is in fact in the past (this helps solve the "year 2000" problem).
  • Although all date formats are specified to be case-sensitive, recipients SHOULD match day, week and timezone names case-insensitively.
  • An HTTP/1.1 implementation MAY internally represent a parsed Expires date as earlier than the proper value, but MUST NOT internally represent a parsed Expires date as later than the proper value.
  • All expiration-related calculations MUST be done in GMT. The local time zone MUST NOT influence the calculation or comparison of an age or expiration time.
  • If an HTTP header field incorrectly carries a date value with a time zone other than GMT, it MUST be converted into GMT using the most conservative possible conversion.

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.

A cache MUST NOT return a stale response if it is prohibited by an explicit in-protocol directive (e.g., by a "no-store" or "no-cache" cache directive, a "must-revalidate" cache-response-directive, or an applicable "s-maxage" or "proxy-revalidate" cache-response-directive; see Section 3.2.2).

A cache MUST NOT return stale responses unless it is disconnected (i.e., it cannot contact the origin server or otherwise find a forward path) or doing so is explicitly allowed (e.g., by the max-stale request directive; see Section 3.2.1).

A cache SHOULD append a Warning header field with the 110 warn-code (see Section 3.6) to stale responses. Likewise, a cache SHOULD add the 112 warn-code to stale responses if the cache is disconnected.

If a cache receives a first-hand response (either an entire response, or a 304 (Not Modified) response) that it would normally forward to the requesting client, and the received response is no longer fresh, the cache can forward it to the requesting client without adding a new Warning (but without removing any existing Warning header fields). A cache shouldn't attempt to validate a response simply because that response became stale in transit.

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 2.7), it can use the conditional request mechanism [Part4] in the forwarded request to give the origin server an opportunity to both select a valid stored response to be used, and to update it. This process is known as "validating" or "revalidating" the stored response.

When sending such a conditional request, a cache adds an If-Modified-Since header field whose value is that of the Last-Modified header field from the selected (see Section 2.7) stored response, if available.

Additionally, a cache can add an If-None-Match header field whose value is that of the ETag header field(s) from all responses stored for the requested URI, if present. However, if any of the stored responses contains only partial content, the cache shouldn't include its entity-tag in the If-None-Match header field unless the request is for a range that would be fully satisfied by that stored response.

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

2.4.1 Freshening Responses

When a cache receives a 304 (Not Modified) response and already has one or more stored 200 (OK) responses for the same cache key, the cache needs to identify which of the stored responses are updated by this new response and then update the stored response(s) with the new information provided in the 304 response.

  • If the new response contains a strong validator, then that strong validator identifies the selected representation. All of the stored responses with the same strong validator are selected. If none of the stored responses contain the same strong validator, then this new response corresponds to a new selected representation and MUST NOT update the existing stored responses.
  • If the new response contains a weak validator and that validator corresponds to one of the cache's stored responses, then the most recent of those matching stored responses is selected.
  • If the new response does not include any form of validator, there is only one stored response, and that stored response also lacks a validator, then that stored response is selected.

If a stored response is selected for update, the cache MUST:

  • delete any Warning header fields in the stored response with warn-code 1xx (see Section 3.6);
  • retain any Warning header fields in the stored response with warn-code 2xx; and,
  • use other header fields provided in the 304 response to replace all instances of the corresponding header fields in the stored response.

2.5 Request Methods that Invalidate

Because unsafe request methods (Section 6.1.1 of [Part2]) such as PUT, POST or DELETE have the potential for changing state on the origin server, intervening caches can use them to keep their contents up-to-date.

A cache MUST invalidate the effective Request URI (Section 4.3 of [Part1]) as well as the URI(s) in the Location and Content-Location header fields (if present) when a non-error response to a request with an unsafe method is received.

However, a cache MUST NOT invalidate a URI from a Location or Content-Location header field if the host part of that URI differs from the host part in the effective request URI (Section 4.3 of [Part1]). This helps prevent denial of service attacks.

A cache MUST invalidate the effective request URI (Section 4.3 of [Part1]) when it receives a non-error response to a request with a method whose safety is unknown.

Here, a "non-error response" is one with a 2xx or 3xx status code. "Invalidate" means that the cache will either remove all stored responses related to the effective 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.

2.6 Shared Caching of Authenticated Responses

A shared cache MUST NOT use a cached response to a request with an Authorization header field (Section 4.1 of [Part7]) to satisfy any subsequent request unless a cache directive that allows such responses to be stored is present in the response.

In this specification, the following Cache-Control response directives (Section 3.2.2) have such an effect: must-revalidate, public, s-maxage.

Note that cached responses that contain the "must-revalidate" and/or "s-maxage" response directives are not allowed to be served stale (Section 2.3.3) by shared caches. In particular, a response with either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to satisfy a subsequent request without revalidating it on the origin server.

2.7 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 header fields nominated by the Vary header field match in both the original request (i.e., that associated with the stored response), and the presented request.

The selecting header fields from two requests are defined to match if and only if those in the first request can be transformed to those in the second request by applying any of the following:

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

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

The stored response with matching selecting header fields is known as the selected response.

If multiple selected responses are available, the most recent response (as determined by the Date header field) is used; see Section 2.2.

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

2.8 Combining Partial Content

A response might transfer only a partial representation if the connection closed prematurely or if the request used one or more Range specifiers ([Part5]). After several such transfers, a cache might have received several ranges of the same representation. A cache MAY combine these ranges into a single stored response, and reuse that response to satisfy later requests, if they all share the same strong validator and the cache complies with the client requirements in Section 4 of [Part5].

When combining the new response with one or more stored responses, a cache MUST:

3. Header Field Definitions

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

3.1 Age

The "Age" header field conveys the sender's estimate of the amount of time since the response was generated or successfully validated at the origin server. Age values are calculated as specified in Section 2.3.2.

  Age = delta-seconds

Age field-values are non-negative integers, representing time in seconds (see Section 1.5).

The presence of an Age header field in a response implies that a response is not first-hand. However, the converse is not true, since HTTP/1.0 caches might not implement the Age header field.

3.2 Cache-Control

The "Cache-Control" header field is used to specify directives for caches along the request/response chain. Such cache directives are unidirectional in that the presence of a directive in a request does not imply that the same directive is to be given in the response.

A cache MUST obey the requirements of the Cache-Control directives defined in this section. See Section 3.2.3 for information about how Cache-Control directives defined elsewhere are handled.

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

A proxy, whether or not it implements a cache, MUST pass cache directives through in forwarded messages, regardless of their significance to that application, since the directives might be applicable to all recipients along the request/response chain. It is not possible to target a directive to a specific cache.

Cache directives are identified by a token, to be compared case-insensitively, and have an optional argument.

  Cache-Control   = 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 cache MUST NOT use a stored response 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 private and shared caches. "MUST NOT store" in this context means that the cache MUST NOT intentionally store the information in non-volatile storage, and MUST make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it.
  • This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.
  • Note that if a request containing this directive is satisfied from a cache, the no-store request directive does not apply to the already stored response.

max-age

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

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.

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 intermediary (whether or not it implements a cache) MUST NOT change the Content-Encoding, Content-Range or Content-Type request header fields, nor the request representation.

only-if-cached

  • The only-if-cached request directive indicates that the client only wishes to obtain a stored response. If it receives this directive, a cache SHOULD either respond using a stored response that is consistent with the other constraints of the request, or respond with a 504 (Gateway Timeout) status code. If a group of caches is being operated as a unified system with good internal connectivity, a member cache MAY forward such a request within that group of caches.

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 a response whose associated request contains an 'Authentication' header MAY be stored (see Section 2.6).

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 cache MAY store the response.
  • If the private response directive specifies one or more field-names, this requirement is limited to the field-values associated with the listed response header fields. That is, a shared cache MUST NOT store the specified field-names(s), whereas it MAY store the remainder of the response message.
  • Note: This usage of the word private only controls where the response can be stored; it cannot ensure the privacy of the message content. Also, private response directives with field-names are often handled by implementations as if an unqualified private directive was received; i.e., the special handling for the qualified form is not widely implemented.

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 a cache from using it to satisfy a request without contacting it, even by caches that have been configured to return stale responses.
  • If the no-cache response directive specifies one or more field-names, this requirement is limited to the field-values associated with the listed response header fields. That is, a cache MUST NOT send the specified field-name(s) 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. Also, no-cache response directives with field-names are often handled by implementations as if an unqualified no-cache directive was received; i.e., the special handling for the qualified form is not widely implemented.

no-store

  • The no-store response directive indicates that a cache MUST NOT store any part of either the immediate request or response. This directive applies to both private and shared caches. "MUST NOT store" in this context means that the cache MUST NOT intentionally store the information in non-volatile storage, and MUST make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it.
  • This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches might not recognize or obey this directive, and communications networks might be vulnerable to eavesdropping.

must-revalidate

  • The must-revalidate response directive indicates that once it has become stale, a cache MUST NOT use the response to satisfy subsequent requests without successful validation on the origin server.
  • The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances a cache MUST obey the must-revalidate directive; in particular, if a cache cannot reach the origin server for any reason, it MUST generate a 504 (Gateway Timeout) response.
  • The must-revalidate directive ought to be used by servers if and only if failure to validate a request on the representation could result in incorrect operation, such as a silently unexecuted financial transaction.

proxy-revalidate

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

max-age

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

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 field. The s-maxage directive also implies the semantics of the proxy-revalidate response directive.

no-transform

  • The no-transform response directive indicates that an intermediary (regardless of whether it implements a cache) MUST NOT change the Content-Encoding, Content-Range or Content-Type response header fields, nor the response representation.

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 private 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.

A cache MUST ignore unrecognized cache directives; it is assumed that any cache directive likely to be unrecognized by an HTTP/1.1 cache will be combined with standard directives (or the response's default cacheability) such that the cache behavior will remain minimally correct even if the cache does not understand the extension(s).

The HTTP Cache Directive Registry defines the name space for the cache directives.

A registration MUST include the following fields:

  • Cache Directive Name
  • Pointer to specification text

Values to be added to this name space are subject to IETF review ([RFC5226], Section 4.1).

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

3.3 Expires

The "Expires" header field 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 8 of [Part2]; a sender MUST use the rfc1123-date format.

  Expires = HTTP-date

For example

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

A cache MUST treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired").

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.

Historically, HTTP required the Expires field-value to be no more than a year in the future. While longer freshness lifetimes are no longer prohibited, extremely large values have been demonstrated to cause problems (e.g., clock overflows due to use of 32-bit integers for time values), and most caches will evict a response far sooner than that. Therefore, senders ought not produce them.

An origin server without a clock MUST NOT assign Expires values to a response unless these values were associated with the resource by a system or user with a reliable clock. It MAY assign an Expires value that is known, at or before server configuration time, to be in the past (this allows "pre-expiration" of responses without storing separate Expires values for each resource).

3.4 Pragma

The "Pragma" header field allows backwards compatibility with HTTP/1.0 caches, so that clients can specify a "no-cache" request that they will understand (as Cache-Control was not defined until HTTP/1.1). When the Cache-Control header is also present and understood in a request, Pragma is ignored.

In HTTP/1.0, Pragma was defined as an extensible field for implementation-specified directives for recipients. This specification deprecates such extensions to improve interoperability.

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

When the Cache-Control header is not present in a request, the no-cache request pragma-directive MUST have the same effect on caches as if "Cache-Control: no-cache" were present (see Section 3.2.1).

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

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

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

Note: Because the meaning of "Pragma: no-cache" in responses is not specified, it does not provide a reliable replacement for "Cache-Control: no-cache" in them.

3.5 Vary

The "Vary" header field conveys the set of header fields that were used to select the representation.

Caches use this information, in part, to determine whether a stored response can be used to satisfy a given request; see Section 2.7. 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 2.7.

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

  Vary = "*" / 1#field-name

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

A server SHOULD include a Vary header field with any cacheable response that is subject to server-driven negotiation. Doing so allows a cache to properly interpret future requests on that resource and informs the user agent about the presence of negotiation on that resource. A server MAY include a Vary header field with a non-cacheable response that is subject to server-driven negotiation, since this might provide the user agent with useful information about the dimensions over which the response varies at the time of the response.

A Vary field value of "*" signals that unspecified parameters not limited to the header fields (e.g., the network address of the client), play a role in the selection of the response representation; therefore, a cache cannot determine whether this response is appropriate. A proxy MUST NOT generate the "*" value.

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

3.6 Warning

The "Warning" header field is used to carry additional information about the status or transformation of a message that might not be reflected in the message. This information is typically used to warn about possible incorrectness introduced by caching operations or transformations applied to the payload of the message.

Warnings can be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status code, distinguishes these responses from true failures.

Warning header fields can in general be applied to any message, however some warn-codes are specific to caches and can only be applied to response messages.

  Warning       = 1#warning-value
  
  warning-value = warn-code SP warn-agent SP warn-text
                                        [SP warn-date]
  
  warn-code  = 3DIGIT
  warn-agent = ( uri-host [ ":" port ] ) / pseudonym
                  ; the name or pseudonym of the server adding
                  ; the Warning header field, for use in debugging
  warn-text  = quoted-string
  warn-date  = DQUOTE HTTP-date DQUOTE

Multiple warnings can be attached to a response (either by the origin server or by a cache), including multiple warnings with the same code number, only differing in warn-text.

When this occurs, the user agent SHOULD inform the user of as many of them as possible, in the order that they appear in the response.

Systems that generate multiple Warning header fields are encouraged to order them with this user agent behavior in mind. New Warning header fields are added after any existing Warning headers fields.

Warnings are assigned three digit warn-codes. The first digit indicates whether the Warning is required to be deleted from a stored response after validation:

If an implementation sends a message with one or more Warning header fields to a receiver whose version is HTTP/1.0 or lower, then the sender MUST include in each warning-value a warn-date that matches the Date header field in the message.

If a system receives a message with a warning-value that includes a warn-date, and that warn-date is different from the Date value in the response, then that warning-value MUST be deleted from the message before storing, forwarding, or using it. (preventing the consequences of naive caching of Warning header fields.) If all of the warning-values are deleted for this reason, the Warning header field MUST be deleted as well.

The following warn-codes are defined by this specification, each with a recommended warn-text in English, and a description of its meaning.

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 a representation retrieved earlier in a session.

The freshness model (Section 2.3) does not necessarily apply to history mechanisms. I.e., a history mechanism can display a previous representation even if it has expired.

This does not prohibit the history mechanism from telling the user that a view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).

5. IANA Considerations

5.1 Cache Directive Registry

The registration procedure for HTTP Cache Directives is defined by Section 3.2.3 of this document.

The HTTP Cache Directive Registry shall be created at <http://www.iana.org/assignments/http-cache-directives> and be populated with the registrations below:

Cache DirectiveReference
max-ageSection 3.2.1, Section 3.2.2
max-staleSection 3.2.1
min-freshSection 3.2.1
must-revalidateSection 3.2.2
no-cacheSection 3.2.1, Section 3.2.2
no-storeSection 3.2.1, Section 3.2.2
no-transformSection 3.2.1, Section 3.2.2
only-if-cachedSection 3.2.1
privateSection 3.2.2
proxy-revalidateSection 3.2.2
publicSection 3.2.2
s-maxageSection 3.2.2
stale-if-error[RFC5861], Section 4
stale-while-revalidate[RFC5861], Section 3

5.2 Header Field Registration

The Message Header Field Registry located at <http://www.iana.org/assignments/message-headers/message-header-index.html> shall be updated with the permanent registrations below (see [RFC3864]):

Header Field NameProtocolStatusReference
AgehttpstandardSection 3.1
Cache-ControlhttpstandardSection 3.2
ExpireshttpstandardSection 3.3
PragmahttpstandardSection 3.4
VaryhttpstandardSection 3.5
WarninghttpstandardSection 3.6

The change controller is: "IETF (iesg@ietf.org) - 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 need to be protected as sensitive information.

7. Acknowledgments

See Section 11 of [Part1].

8. References

8.1 Normative References

[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-17 (work in progress), October 2011.
[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-17 (work in progress), October 2011.
[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-17 (work in progress), October 2011.
[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-17 (work in progress), October 2011.
[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-17 (work in progress), October 2011.
[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.
[RFC5226]Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, May 2008.
[RFC5861]Nottingham, M., “HTTP Cache-Control Extensions for Stale Content”, RFC 5861, April 2010.

Authors' Addresses

Roy T. Fielding (editor)
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
Email: fielding@gbiv.com
URI: http://roy.gbiv.com/

Jim Gettys
Alcatel-Lucent Bell Labs
21 Oak Knoll Road
Carlisle, MA 01741
USA
Email: jg@freedesktop.org
URI: http://gettys.wordpress.com/

Jeffrey C. Mogul
Hewlett-Packard Company
HP Labs, Large Scale Systems Group
1501 Page Mill Road, MS 1177
Palo Alto, CA 94304
USA
Email: JeffMogul@acm.org

Henrik Frystyk Nielsen
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052
USA
Email: henrikn@microsoft.com

Larry Masinter
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
Email: LMM@acm.org
URI: http://larry.masinter.net/

Paul J. Leach
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052
Email: paulle@microsoft.com

Tim Berners-Lee
World Wide Web Consortium
MIT Computer Science and Artificial Intelligence Laboratory
The Stata Center, Building 32
32 Vassar Street
Cambridge, MA 02139
USA
Email: timbl@w3.org
URI: http://www.w3.org/People/Berners-Lee/

Yves Lafon (editor)
World Wide Web Consortium
W3C / ERCIM
2004, rte des Lucioles
Sophia-Antipolis, AM 06902
France
Email: ylafon@w3.org
URI: http://www.raubacapeu.net/people/yves/

Mark Nottingham (editor)
Rackspace
Email: mnot@mnot.net
URI: http://www.mnot.net/

Julian F. Reschke (editor)
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
Phone: +49 251 2807760
Fax: +49 251 2807761
Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/

A. Changes from RFC 2616

Make the specified age calculation algorithm less conservative. (Section 2.3.2)

Remove requirement to consider Content-Location in successful responses in order to determine the appropriate response to use. (Section 2.4)

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

Change ABNF productions for header fields to only define the field value. (Section 3)

Do not mention RFC 2047 encoding and multiple languages in Warning header fields anymore, as these aspects never were implemented. (Section 3.6)

B. Collected ABNF

Age = delta-seconds

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

Expires = HTTP-date

HTTP-date = <HTTP-date, defined in [Part2], Section 8>

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

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

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

Warning = *( "," 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-extension
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 3.2>

port = <port, defined in [Part1], Section 2.7>
pragma-directive = "no-cache" / extension-pragma
pseudonym = <pseudonym, defined in [Part1], Section 8.8>

quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>

token = <token, defined in [Part1], Section 3.2.3>

uri-host = <uri-host, defined in [Part1], Section 2.7>

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 RFC 2616

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 Field Registration (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):

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 (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

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 (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

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

Closed issues:

Affected issues:

C.9 Since draft-ietf-httpbis-p6-cache-07

Closed issues:

C.10 Since draft-ietf-httpbis-p6-cache-08

Closed issues:

Affected issues:

Partly resolved issues:

C.11 Since draft-ietf-httpbis-p6-cache-09

Closed issues:

Partly resolved issues:

C.12 Since draft-ietf-httpbis-p6-cache-10

Closed issues:

C.13 Since draft-ietf-httpbis-p6-cache-11

Closed issues:

C.14 Since draft-ietf-httpbis-p6-cache-12

Closed issues:

C.15 Since draft-ietf-httpbis-p6-cache-13

Closed issues:

C.16 Since draft-ietf-httpbis-p6-cache-14

Closed issues:

C.17 Since draft-ietf-httpbis-p6-cache-15

Closed issues:

C.18 Since draft-ietf-httpbis-p6-cache-16

Closed issues:

Index

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