--- 1/draft-ietf-httpbis-p6-cache-10.txt 2010-08-04 15:12:50.000000000 +0200 +++ 2/draft-ietf-httpbis-p6-cache-11.txt 2010-08-04 15:12:50.000000000 +0200 @@ -1,71 +1,71 @@ HTTPbis Working Group R. Fielding, Ed. Internet-Draft Day Software Obsoletes: 2616 (if approved) J. Gettys Intended status: Standards Track Alcatel-Lucent -Expires: January 13, 2011 J. Mogul +Expires: February 5, 2011 J. Mogul HP H. Frystyk Microsoft L. Masinter Adobe Systems P. Leach Microsoft T. Berners-Lee W3C/MIT Y. Lafon, Ed. W3C M. Nottingham, Ed. J. Reschke, Ed. greenbytes - July 12, 2010 + August 4, 2010 HTTP/1.1, part 6: Caching - draft-ietf-httpbis-p6-cache-10 + draft-ietf-httpbis-p6-cache-11 Abstract 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 (ietf-http-wg@w3.org). 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.11. + The changes in this draft are summarized in Appendix C.12. 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 January 13, 2011. + This Internet-Draft will expire on February 5, 2011. Copyright Notice Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents @@ -106,70 +106,69 @@ 2.3.2. Calculating Age . . . . . . . . . . . . . . . . . . . 12 2.3.3. Serving Stale Responses . . . . . . . . . . . . . . . 13 2.4. Validation Model . . . . . . . . . . . . . . . . . . . . . 14 2.5. Request Methods that Invalidate . . . . . . . . . . . . . 14 2.6. Shared Caching of Authenticated Responses . . . . . . . . 15 2.7. Caching Negotiated Responses . . . . . . . . . . . . . . . 16 2.8. Combining Responses . . . . . . . . . . . . . . . . . . . 16 3. Header Field Definitions . . . . . . . . . . . . . . . . . . . 17 3.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 18 - 3.2.1. Request Cache-Control Directives . . . . . . . . . . . 19 + 3.2.1. Request Cache-Control Directives . . . . . . . . . . . 18 3.2.2. Response Cache-Control Directives . . . . . . . . . . 20 - 3.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 23 + 3.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 22 3.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 24 - 3.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 25 + 3.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3.6. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 26 - 4. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 29 - 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29 - 5.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 29 - 5.2. Message Header Registration . . . . . . . . . . . . . . . 30 - 6. Security Considerations . . . . . . . . . . . . . . . . . . . 30 + 4. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 28 + 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 + 5.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 28 + 5.2. Header Field Registration . . . . . . . . . . . . . . . . 29 + 6. Security Considerations . . . . . . . . . . . . . . . . . . . 29 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30 8.1. Normative References . . . . . . . . . . . . . . . . . . . 30 8.2. Informative References . . . . . . . . . . . . . . . . . . 31 - Appendix A. Compatibility with Previous Versions . . . . . . . . 32 - A.1. Changes from RFC 2068 . . . . . . . . . . . . . . . . . . 32 - A.2. Changes from RFC 2616 . . . . . . . . . . . . . . . . . . 32 - Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 32 + Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 31 + Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 31 Appendix C. Change Log (to be removed by RFC Editor before - publication) . . . . . . . . . . . . . . . . . . . . 34 - C.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 34 - C.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 34 - C.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 35 - C.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 35 - C.5. Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 35 - C.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 35 - C.7. Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 36 - C.8. Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 36 - C.9. Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 36 - C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 37 - C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 37 - Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 + publication) . . . . . . . . . . . . . . . . . . . . 33 + C.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 33 + C.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 33 + C.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 34 + C.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 34 + C.5. Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 34 + C.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 34 + C.7. Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 35 + C.8. Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 35 + C.9. Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 35 + C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 36 + C.11. Since draft-ietf-httpbis-p6-cache-09 . . . . . . . . . . . 36 + C.12. Since draft-ietf-httpbis-p6-cache-10 . . . . . . . . . . . 37 + Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 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. + client or server MAY employ 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 @@ -177,27 +176,27 @@ 1.2. Terminology This specification uses a number of terms to refer to the roles played by participants in, and objects of, HTTP caching. cacheable A response is cacheable if a cache is allowed to store a copy of the response message for use in answering subsequent requests. - Even when a response is cacheable, there may be additional + Even when a response is cacheable, there might be additional constraints on whether a cache can use the cached copy to satisfy a particular request. explicit expiration time - The time at which the origin server intends that an entity should + The time at which the origin server intends that a representation no longer be returned by a cache without further validation. heuristic expiration time An expiration time assigned by a cache when no explicit expiration time is available. age The age of a response is the time since it was sent by, or @@ -218,23 +217,23 @@ A response is fresh if its age has not yet exceeded its freshness lifetime. stale A response is stale if its age has passed its freshness lifetime (either explicit or heuristic). validator - A protocol element (e.g., an entity tag or a Last-Modified time) - that is used to find out whether a stored response is an - equivalent copy of an entity. + A protocol element (e.g., an entity-tag or a Last-Modified time) + that is used to find out whether a stored response has an + equivalent copy of a representation. shared cache A cache that is accessible to more than one user. A non-shared cache is dedicated to a single user. 1.3. Requirements The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this @@ -341,48 +340,44 @@ 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: - o The presented Effective Request URI (Section 4.3 of [Part1]) and + o The presented effective request URI (Section 4.3 of [Part1]) and that of the stored response match, and o the request method associated with the stored response allows it to be used for the presented request, and o selecting request-headers nominated by the stored response (if any) match those presented (see Section 2.7), and o the presented request and stored response are free from directives that would prevent its use (see Section 3.2 and Section 3.4), and o the stored response is either: * fresh (see Section 2.3), or * allowed to be served stale (see Section 2.3.3), or * successfully validated (see Section 2.4). - [[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. [[DISCUSS-includes-validated: this currently includes - successfully validated responses.]] + When a stored response is used to satisfy a request without + validation, 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. 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 @@ -394,45 +389,44 @@ 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. + 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. This - means that the response is always stale, so that caches should - validate it before using it for subsequent requests. [[TODO- - response-stale: This wording may cause confusion, because the - response may still be served stale.]] + request, it can assign an explicit expiration time in the past to + indicate that the response is already stale. Compliant caches will + validate the cached response before reusing it for subsequent + requests. 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 + HTTP caches MAY assign heuristic expiration times when explicit times + 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. + 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. [[ISSUE-no-req-for-directives: 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 @@ -455,23 +449,25 @@ o 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 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. + has a status code whose definition allows heuristic freshness to be + used (including the following in Section 8 of [Part2]: 200, 203, 206, + 300, 301 and 410), a heuristic expiration time MAY be calculated. + Heuristics MUST NOT be used for response status codes that do not + explicitly allow it. 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%. @@ -605,69 +601,63 @@ 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 2.8. 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. [[TODO-req-missing: Should - there be a requirement here?]] + suitable. Instead, the full response SHOULD be used to satisfy the + request and MAY replace the stored response. 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). 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 - Effective Request URI (Section 4.3 of [Part1]) as well as the URI(s) + effective Request URI (Section 4.3 of [Part1]) as well as the URI(s) in the Location and Content-Location headers (if present): o PUT o DELETE o POST 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 Effective Request URI (Section 4.3 of + from the host part in the effective request URI (Section 4.3 of [Part1]). This helps prevent denial of service attacks. - [[TODO-def-host-part: "host part" needs to be specified better.]] - A cache that passes through requests for methods it does not - understand SHOULD invalidate the Effective Request URI (Section 4.3 + understand SHOULD invalidate the effective request URI (Section 4.3 of [Part1]). Here, "invalidate" means that the cache will either remove all stored - responses related to the Effective Request URI, or will mark these as + 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. - [[TODO-spec-success-invalidate: specify that only successful (2xx, - 3xx?) responses invalidate.]] - 2.6. Shared Caching of Authenticated Responses Shared caches MUST NOT use a cached response to a request with an Authorization header (Section 3.1 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. @@ -696,84 +686,74 @@ o combining multiple message-header fields with the same field name (see Section 3.2 of [Part1]) o normalizing both header values in a way that is known to have identical semantics, according to the header's specification (e.g., re-ordering field values when order is not significant; case-normalization, where values are defined to be case- insensitive) - If (after any normalisation that may take place) a header field is + 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 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. 2.8. 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. + request, and potentially update the cached response. If the new response contains an ETag, it identifies the stored - response to use. [[TODO-mention-CL: may need language about Content- - Location here]][[TODO-inm-mult-etags: cover case where INM with - multiple etags was sent]] + response to use. [[TODO-mention-CL: might need language about + Content-Location here]][[TODO-select-for-combine: Shouldn't this be + the selected response?]] - If the status code is 206 (partial content), both the stored and new - responses MUST have validators, and those validators MUST match using - the strong comparison function (see Section 4 of [Part4]). - Otherwise, the responses MUST NOT be combined. + If the new response's status code is 206 (partial content), both the + stored and new responses MUST have validators, and those validators + 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 o any stored Warning headers with warn-code 1xx (see Section 3.6) - MUST be deleted from the stored response and the updated response. - - o any stored Warning headers with warn-code 2xx MUST be retained in - the stored response and the updated response. - - o any headers provided in the new response MUST replace the - corresponding headers from the stored response. + MUST be deleted. - 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. + o any stored Warning headers with warn-code 2xx MUST be retained. - The updated response can [[TODO-is-req: requirement?]] be used to - replace the stored response in cache. In the case of a 206 response, - the combined entity-body MAY be stored. + o any other headers provided in the new response MUST replace all + instances of the corresponding headers from the stored response. - [[ISSUE-how-head: discuss how to handle HEAD updates]] + The updated response headers MUST be used to replace those of the + stored response in cache (unless the stored response is removed from + cache). In the case of a 206 response, the combined representation + MAY be stored. 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 "Age" response-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 = "Age" ":" OWS Age-v Age-v = delta-seconds @@ -778,25 +758,26 @@ 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 (2^31). + 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. + HTTP/1.0 caches might not implement the Age header field. 3.2. Cache-Control The "Cache-Control" general-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. HTTP/1.1 caches MUST obey the requirements of the Cache-Control @@ -845,62 +826,61 @@ store any part of either this request or any response to it. This directive applies to both non-shared and shared caches. "MUST NOT store" in this context means that the cache MUST NOT intentionally store the information in non-volatile storage, and MUST make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it. This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches might not recognize or obey this directive, and communications - networks may be vulnerable to eavesdropping. + networks might be vulnerable to eavesdropping. max-age The max-age request directive indicates that the client is willing to accept a response whose age is no greater than the specified time in seconds. Unless 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. [[TODO-staleness: of any staleness? - --mnot]] + 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 intermediate cache or proxy MUST NOT change the Content-Encoding, Content-Range - or Content-Type request headers, nor the request entity-body. + or Content-Type request headers, nor the request representation. only-if-cached The only-if-cached request directive indicates that the client only wishes to return a stored response. If it receives this directive, a cache SHOULD either respond using a stored response that is consistent with the other constraints of the request, or - respond with a 504 (Gateway Timeout) status. If a group of caches - is being operated as a unified system with good internal + respond with a 504 (Gateway Timeout) status code. If a group of + caches is being operated as a unified system with good internal connectivity, such a request MAY be forwarded within that group of caches. 3.2.2. Response Cache-Control Directives cache-response-directive = "public" / "private" [ "=" DQUOTE 1#field-name DQUOTE ] / "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ] / "no-store" @@ -924,32 +904,33 @@ is intended for a single user and MUST NOT be stored by a shared cache. A private (non-shared) cache MAY store the response. If the private response directive specifies one or more field- names, this requirement is limited to the field-values associated with the listed response headers. That is, the specified field- names(s) MUST NOT be stored by a shared cache, whereas the remainder of the response message MAY be. Note: This usage of the word private only controls where the - response may be stored, and cannot ensure the privacy of the + 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 caching even by caches that have been configured to return + 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 headers. That is, the specified field- name(s) MUST NOT be sent in the response to a subsequent request without successful validation on the origin server. This allows an origin server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response. @@ -966,37 +947,37 @@ store any part of either the immediate request or response. This directive applies to both non-shared and shared caches. "MUST NOT store" in this context means that the cache MUST NOT intentionally store the information in non-volatile storage, and MUST make a best-effort attempt to remove the information from volatile storage as promptly as possible after forwarding it. This directive is NOT a reliable or sufficient mechanism for ensuring privacy. In particular, malicious or compromised caches might not recognize or obey this directive, and communications - networks may be vulnerable to eavesdropping. + networks might be vulnerable to eavesdropping. must-revalidate The must-revalidate response directive indicates that once it has become stale, the response MUST NOT be used to satisfy subsequent requests without successful validation on the origin server. The must-revalidate directive is necessary to support reliable operation for certain protocol features. In all circumstances an HTTP/1.1 cache MUST obey the must-revalidate directive; in particular, if the cache cannot reach the origin server for any reason, it MUST generate a 504 (Gateway Timeout) response. Servers SHOULD send the must-revalidate directive if and only if - failure to validate a request on the entity could result in - incorrect operation, such as a silently unexecuted financial + 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 non-shared caches. max-age @@ -1009,21 +990,21 @@ The s-maxage response directive indicates that, in shared caches, the maximum age specified by this directive overrides the maximum age specified by either the max-age directive or the Expires header. The s-maxage directive also implies the semantics of the proxy-revalidate response directive. no-transform The no-transform response directive indicates that an intermediate cache or proxy MUST NOT change the Content-Encoding, Content-Range - or Content-Type response headers, nor the response entity-body. + or Content-Type response headers, 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 @@ -1070,21 +1051,21 @@ o 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 . 3.3. Expires - The "Expires" entity-header field gives the date/time after which the + 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 6.1 of [Part1]; it MUST be sent in rfc1123-date format. @@ -1165,35 +1146,33 @@ 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. + appropriate. The "*" value MUST NOT be generated by a proxy 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 "Warning" general-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 entity body of the - message. + 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 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 @@ -1222,25 +1201,25 @@ 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: o 1xx Warnings describe the freshness or validation status of the response, and so MUST be deleted by caches after validation. They can only be generated by a cache when validating a cached entry, and MUST NOT be generated in any other situation. - o 2xx Warnings describe some aspect of the entity body or entity - headers that is not rectified by a validation (for example, a - lossy compression of the entity bodies) and MUST NOT be deleted by - caches after validation, unless a full response is returned, in - which case they MUST be. + o 2xx Warnings describe some aspect of the representation that is + not rectified by a validation (for example, a lossy compression of + the representation) and MUST NOT be deleted by caches after + validation, unless a full response is returned, in which case they + MUST be. 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 @@ -1274,55 +1253,53 @@ 199 Miscellaneous warning The warning text can include arbitrary information to be presented to a human user, or logged. A system receiving this warning MUST NOT take any automated action, besides presenting the warning to the user. 214 Transformation applied - MUST be added by an intermediate cache or proxy if it applies any - transformation changing the content-coding (as specified in the - Content-Encoding header) or media-type (as specified in the - Content-Type header) of the response, or the entity-body of the - response, unless this Warning code already appears in the - response. + MUST be added by an intermediate proxy if it applies any + transformation to the representation, such as changing the + content-coding, media-type, or modifying the representation data, + unless this Warning code already appears in the response. 299 Miscellaneous persistent warning The warning text can include arbitrary information to be presented to a human user, or logged. A system receiving this warning MUST NOT take any automated action. 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 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 should be created at + The HTTP Cache Directive Registry shall be created at and be populated with the registrations below: +------------------------+------------------------------+ | Cache Directive | Reference | +------------------------+------------------------------+ | max-age | Section 3.2.1, Section 3.2.2 | | max-stale | Section 3.2.1 | | min-fresh | Section 3.2.1 | | must-revalidate | Section 3.2.2 | @@ -1331,24 +1308,24 @@ | no-transform | Section 3.2.1, Section 3.2.2 | | only-if-cached | Section 3.2.1 | | private | Section 3.2.2 | | proxy-revalidate | Section 3.2.2 | | public | Section 3.2.2 | | s-maxage | Section 3.2.2 | | stale-if-error | [RFC5861], Section 4 | | stale-while-revalidate | [RFC5861], Section 3 | +------------------------+------------------------------+ -5.2. Message Header Registration +5.2. Header Field Registration - The Message Header Registry located at should be + The Message Header Field Registry located at shall 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 | @@ -1358,62 +1335,62 @@ 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 should be protected as sensitive + network. Therefore, cache contents need to 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 [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", draft-ietf-httpbis-p1-messaging-10 - (work in progress), July 2010. + and Message Parsing", draft-ietf-httpbis-p1-messaging-11 + (work in progress), August 2010. [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", draft-ietf-httpbis-p2-semantics-10 (work in - progress), July 2010. + Semantics", draft-ietf-httpbis-p2-semantics-11 (work in + progress), August 2010. [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", draft-ietf-httpbis-p4-conditional-10 (work in - progress), July 2010. + Requests", draft-ietf-httpbis-p4-conditional-11 (work in + progress), August 2010. [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", draft-ietf-httpbis-p5-range-10 (work - in progress), July 2010. + Partial Responses", draft-ietf-httpbis-p5-range-11 (work + in progress), August 2010. [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", - draft-ietf-httpbis-p7-auth-10 (work in progress), - July 2010. + draft-ietf-httpbis-p7-auth-11 (work in progress), + August 2010. [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) @@ -1427,40 +1404,21 @@ 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. -Appendix 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). - - 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 2.8) - - 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, 2.8, 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 +Appendix 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) @@ -1717,115 +1676,133 @@ for cache-control directives" o : "Heuristic caching of URLs with query components" Partly resolved issues: o : "Term for the requested resource's URI" +C.12. Since draft-ietf-httpbis-p6-cache-10 + + Closed issues: + + o : "Clarify + entity / representation / variant terminology" + + o : "consider + removing the 'changes from 2068' sections" + + o : "Allowing + heuristic caching for new status codes" + + o : "Allowing + heuristic caching for new status codes" + + o Clean up TODOs and prose in "Combining Responses." + Index A age 6 Age header 17 C cache 5 Cache Directives max-age 19, 22 max-stale 19 - min-fresh 20 + min-fresh 19 must-revalidate 22 no-cache 19, 21 - no-store 19, 22 - no-transform 20, 23 + no-store 19, 21 + no-transform 20, 22 only-if-cached 20 - private 21 + private 20 proxy-revalidate 22 public 20 s-maxage 22 Cache-Control header 18 cacheable 5 E Expires header 24 explicit expiration time 5 F first-hand 6 fresh 6 freshness lifetime 6 G Grammar - Age 18 - Age-v 18 + Age 17 + Age-v 17 Cache-Control 18 Cache-Control-v 18 cache-extension 18 - cache-request-directive 19 + cache-request-directive 18 cache-response-directive 20 - delta-seconds 18 + delta-seconds 17 Expires 24 Expires-v 24 - extension-pragma 25 - Pragma 25 - pragma-directive 25 - Pragma-v 25 - Vary 26 - Vary-v 26 - warn-agent 27 - warn-code 27 - warn-date 27 - warn-text 27 - Warning 27 - Warning-v 27 - warning-value 27 + extension-pragma 24 + Pragma 24 + pragma-directive 24 + Pragma-v 24 + Vary 25 + Vary-v 25 + warn-agent 26 + warn-code 26 + warn-date 26 + warn-text 26 + Warning 26 + Warning-v 26 + warning-value 26 H Headers Age 17 Cache-Control 18 Expires 24 - Pragma 25 + Pragma 24 Vary 25 Warning 26 heuristic expiration time 5 M max-age Cache Directive 19, 22 max-stale Cache Directive 19 min-fresh - Cache Directive 20 + Cache Directive 19 must-revalidate Cache Directive 22 N no-cache Cache Directive 19, 21 no-store - Cache Directive 19, 22 + Cache Directive 19, 21 no-transform - Cache Directive 20, 23 + Cache Directive 20, 22 O only-if-cached Cache Directive 20 P - Pragma header 25 + Pragma header 24 private - Cache Directive 21 + Cache Directive 20 proxy-revalidate Cache Directive 22 public Cache Directive 20 S s-maxage Cache Directive 22 stale 6