--- 1/draft-ietf-httpbis-p6-cache-08.txt 2010-03-08 11:11:05.000000000 +0100 +++ 2/draft-ietf-httpbis-p6-cache-09.txt 2010-03-08 11:11:05.000000000 +0100 @@ -1,155 +1,161 @@ HTTPbis Working Group R. Fielding, Ed. Internet-Draft Day Software Obsoletes: 2616 (if approved) J. Gettys Intended status: Standards Track One Laptop per Child -Expires: April 29, 2010 J. Mogul +Expires: September 9, 2010 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 - October 26, 2009 + March 8, 2010 HTTP/1.1, part 6: Caching - draft-ietf-httpbis-p6-cache-08 + draft-ietf-httpbis-p6-cache-09 -Status of this Memo +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.10. +Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the - provisions of BCP 78 and BCP 79. This document may contain material - from IETF Documents or IETF Contributions published or made publicly - available before November 10, 2008. The person(s) controlling the - copyright in some of this material may not have granted the IETF - Trust the right to allow modifications of such material outside the - IETF Standards Process. Without obtaining an adequate license from - the person(s) controlling the copyright in such materials, this - document may not be modified outside the IETF Standards Process, and - derivative works of it may not be created outside the IETF Standards - Process, except to format it for publication as an RFC or to - translate it into languages other than English. + provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. - This Internet-Draft will expire on April 29, 2010. + This Internet-Draft will expire on September 9, 2010. Copyright Notice - Copyright (c) 2009 IETF Trust and the persons identified as the + 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 in effect on the date of - publication of this document (http://trustee.ietf.org/license-info). - Please review these documents carefully, as they describe your rights - and restrictions with respect to this document. - -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 - . + 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 + carefully, as they describe your rights and restrictions with respect + to this document. Code Components extracted from this document must + include Simplified BSD License text as described in Section 4.e of + the Trust Legal Provisions and are provided without warranty as + described in the BSD License. - The changes in this draft are summarized in Appendix C.9. + This document may contain material from IETF Documents or IETF + Contributions published or made publicly available before November + 10, 2008. The person(s) controlling the copyright in some of this + material may not have granted the IETF Trust the right to allow + modifications of such material outside the IETF Standards Process. + Without obtaining an adequate license from the person(s) controlling + the copyright in such materials, this document may not be modified + outside the IETF Standards Process, and derivative works of it may + not be created outside the IETF Standards Process, except to format + it for publication as an RFC or to translate it into languages other + than English. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 1.3. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6 1.4. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 7 1.4.1. Core Rules . . . . . . . . . . . . . . . . . . . . . . 7 1.4.2. ABNF Rules defined in other Parts of the Specification . . . . . . . . . . . . . . . . . . . . 7 2. Cache Operation . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Response Cacheability . . . . . . . . . . . . . . . . . . 7 2.1.1. Storing Partial and Incomplete Responses . . . . . . . 8 - 2.2. Constructing Responses from Caches . . . . . . . . . . . . 8 - 2.3. Freshness Model . . . . . . . . . . . . . . . . . . . . . 9 - 2.3.1. Calculating Freshness Lifetime . . . . . . . . . . . . 10 - 2.3.2. Calculating Age . . . . . . . . . . . . . . . . . . . 11 + 2.2. Constructing Responses from Caches . . . . . . . . . . . . 9 + 2.3. Freshness Model . . . . . . . . . . . . . . . . . . . . . 10 + 2.3.1. Calculating Freshness Lifetime . . . . . . . . . . . . 11 + 2.3.2. Calculating Age . . . . . . . . . . . . . . . . . . . 12 2.3.3. Serving Stale Responses . . . . . . . . . . . . . . . 13 - 2.4. Validation Model . . . . . . . . . . . . . . . . . . . . . 13 + 2.4. Validation Model . . . . . . . . . . . . . . . . . . . . . 14 2.5. Request Methods that Invalidate . . . . . . . . . . . . . 14 2.6. Caching Negotiated Responses . . . . . . . . . . . . . . . 15 2.7. Combining Responses . . . . . . . . . . . . . . . . . . . 16 - 3. Header Field Definitions . . . . . . . . . . . . . . . . . . . 16 + 3. Header Field Definitions . . . . . . . . . . . . . . . . . . . 17 3.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 - 3.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 17 + 3.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 18 3.2.1. Request Cache-Control Directives . . . . . . . . . . . 18 3.2.2. Response Cache-Control Directives . . . . . . . . . . 20 3.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 22 3.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 23 3.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.5. Vary . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.6. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 25 4. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 28 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28 5.1. Message Header Registration . . . . . . . . . . . . . . . 28 - 6. Security Considerations . . . . . . . . . . . . . . . . . . . 29 + 6. Security Considerations . . . . . . . . . . . . . . . . . . . 28 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 29 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29 8.1. Normative References . . . . . . . . . . . . . . . . . . . 29 8.2. Informative References . . . . . . . . . . . . . . . . . . 30 Appendix A. Compatibility with Previous Versions . . . . . . . . 30 - A.1. Changes from RFC 2068 . . . . . . . . . . . . . . . . . . 31 - A.2. Changes from RFC 2616 . . . . . . . . . . . . . . . . . . 31 + A.1. Changes from RFC 2068 . . . . . . . . . . . . . . . . . . 30 + A.2. Changes from RFC 2616 . . . . . . . . . . . . . . . . . . 30 Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 31 Appendix C. Change Log (to be removed by RFC Editor before - 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 + publication) . . . . . . . . . . . . . . . . . . . . 32 + C.1. Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 32 + C.2. Since draft-ietf-httpbis-p6-cache-00 . . . . . . . . . . . 32 + C.3. Since draft-ietf-httpbis-p6-cache-01 . . . . . . . . . . . 33 + C.4. Since draft-ietf-httpbis-p6-cache-02 . . . . . . . . . . . 33 C.5. Since draft-ietf-httpbis-p6-cache-03 . . . . . . . . . . . 34 - C.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 35 - C.7. Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 35 + C.6. Since draft-ietf-httpbis-p6-cache-04 . . . . . . . . . . . 34 + C.7. Since draft-ietf-httpbis-p6-cache-05 . . . . . . . . . . . 34 C.8. Since draft-ietf-httpbis-p6-cache-06 . . . . . . . . . . . 35 - C.9. Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 36 + C.9. Since draft-ietf-httpbis-p6-cache-07 . . . . . . . . . . . 35 + C.10. Since draft-ietf-httpbis-p6-cache-08 . . . . . . . . . . . 35 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38 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. @@ -278,34 +284,56 @@ port = pseudonym = uri-host = 2. Cache Operation 2.1. Response Cacheability A cache MUST NOT store a response to any request, unless: - o The request method is defined as being cacheable, and + o The request method is understood by the cache and defined as being + cacheable, and + + o the response status code is understood by the cache, and o the "no-store" cache directive (see Section 3.2) does not appear in request or response headers, and - o the "private" cache response directive (see Section 3.2 does not + o the "private" cache response directive (see Section 3.2.2 does not appear in the response, if the cache is shared, and o the "Authorization" header (see Section 3.1 of [Part7]) does not appear in the request, if the cache is shared (unless the "public" directive is present; see Section 3.2), and - o the cache understands partial responses, if the response is - partial or incomplete (see Section 2.1.1). + o the response either: + + * contains an Expires header (see Section 3.3), or + + * contains a max-age response cache directive (see + Section 3.2.2), or + + * contains a s-maxage response cache directive and the cache is + shared, or + + * contains a Cache Control Extension (see Section 3.2.3) that + allows it to be cached, or + + * has a status code that can be served with heuristic freshness + (see Section 2.3.1.1). + + In this context, a cache has "understood" a request method or a + response status code if it recognises it and implements any cache- + specific behaviour. In particular, 206 Partial Content responses + cannot be cached by an implementation that does not handle partial + content (see Section 2.1.1). Note that in normal operation, most caches will not store a response that has neither a cache validator nor an explicit expiration time, as such responses are not usually useful to store. However, caches are not prohibited from storing such responses. 2.1.1. Storing Partial and Incomplete Responses A cache that receives an incomplete response (for example, with fewer bytes of data than specified in a Content-Length header) can store @@ -346,25 +375,25 @@ * 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. [[anchor1: DISCUSS: this currently includes + Section 2.3.2. [[DISCUSS-includes-validated: this currently includes successfully validated responses.]] Requests with methods that are unsafe (Section 7.1.1 of [Part2]) MUST - be written through the cache to the origin server; i.e., A cache must + be written through the cache to the origin server; i.e., a cache must not reply to such a request before having forwarded the request and having received a corresponding response. Also, note that unsafe requests might invalidate already stored responses; see Section 2.5. Caches MUST use the most recent response (as determined by the Date header) when more than one suitable response is stored. They can also forward a request with "Cache-Control: max-age=0" or "Cache- Control: no-cache" to disambiguate which response to use. @@ -382,45 +411,45 @@ server to provide an explicit expiration time in the future, using either the Expires header (Section 3.3) or the max-age response cache directive (Section 3.2.2). Generally, origin servers will assign future explicit expiration times to responses in the belief that the entity is not likely to change in a semantically significant way before the expiration time is reached. If an origin server wishes to force a cache to validate every request, it can assign an explicit expiration time in the past. This means that the response is always stale, so that caches should - validate it before using it for subsequent requests. [[anchor2: This - wording may cause confusion, because the response may still be served - stale.]] + validate it before using it for subsequent requests. [[TODO- + response-stale: This wording may cause confusion, because the + response may still be served stale.]] Since origin servers do not always provide explicit expiration times, HTTP caches may also assign heuristic expiration times when they are not specified, employing algorithms that use other header values (such as the Last-Modified time) to estimate a plausible expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does impose worst-case constraints on their results. The calculation to determine if a response is fresh is: response_is_fresh = (freshness_lifetime > current_age) The freshness_lifetime is defined in Section 2.3.1; the current_age is defined in Section 2.3.2. Additionally, clients may need to influence freshness calculation. They can do this using several request cache directives, with the effect of either increasing or loosening constraints on freshness. See Section 3.2.1. - [[anchor3: ISSUE: there are not requirements directly applying to - cache-request-directives and freshness.]] + [[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 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: @@ -450,22 +479,22 @@ 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%. - [[anchor4: REVIEW: took away HTTP/1.0 query string heuristic - uncacheability.]] + [[REVIEW-query-string-heuristics: took away HTTP/1.0 query string + heuristic uncacheability.]] 2.3.2. Calculating Age HTTP/1.1 uses the Age response-header to convey the estimated age of the response message when obtained from a cache. The Age field value is the cache's estimate of the amount of time since the response was generated or validated by the origin server. In essence, the Age value is the sum of the time that the response has been resident in each of the caches along the path from the origin server, plus the amount of time it has been in transit along network paths. @@ -577,22 +606,22 @@ contains only partial content, its entity-tag SHOULD NOT be included in the If-None-Match header field unless the request is for a range that would be fully satisfied by that stored response. A 304 (Not Modified) response status code indicates that the stored response can be updated and reused; see Section 2.7. A full response (i.e., one with a response body) indicates that none of the stored responses nominated in the conditional request is suitable. Instead, the full response is used both to satisfy the - request and replace the stored response. [[anchor5: Should there be a - requirement here?]] + request and replace the stored response. [[TODO-req-missing: Should + there be a requirement here?]] If a cache receives a 5xx response while attempting to validate a response, it MAY either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, it MAY return a previously stored response (see Section 2.3.3). 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 @@ -606,56 +635,65 @@ 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 Request-URI. This helps prevent denial of service attacks. - [[anchor6: TODO: "host part" needs to be specified better.]] + [[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 Request-URI. Here, "invalidate" means that the cache will either remove all stored responses related to the Request-URI, or will mark these as "invalid" and in need of a mandatory validation before they can be returned in response to a subsequent request. Note that this does not guarantee that all appropriate responses are invalidated. For example, the request that caused the change at the origin server might not have gone through the cache where a response is stored. - [[anchor7: TODO: specify that only successful (2xx, 3xx?) responses - invalidate.]] + [[TODO-spec-success-invalidate: specify that only successful (2xx, + 3xx?) responses invalidate.]] 2.6. Caching Negotiated Responses When a cache receives a request that can be satisfied by a stored response that has a Vary header field (Section 3.5), it MUST NOT use that response unless all of the selecting request-headers nominated by the Vary header match in both the original request (i.e., that associated with the stored response), and the presented request. The selecting request-headers from two requests are defined to match - if and only if the selecting request-headers in the first request can - be transformed to the selecting request-headers in the second request - by adding or removing linear white space [[anchor8: [ref]]] at places - where this is allowed by the corresponding ABNF, and/or combining - multiple message-header fields with the same field name following the - rules about header fields in Section 3.2 of [Part1]. + 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 a header field is absent from a request, it can only match another - request if it is also absent there. + o adding or removing whitespace, where allowed in the header's + syntax + + 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 + 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 @@ -663,22 +701,23 @@ 2.7. Combining Responses When a cache receives a 304 (Not Modified) response or a 206 (Partial Content) response (in this section, the "new" response"), it needs to created an updated response by combining the stored response with the new one, so that the updated response can be used to satisfy the request. If the new response contains an ETag, it identifies the stored - response to use. [[anchor9: may need language about Content-Location - here]][[anchor10: cover case where INM with multiple etags was sent]] + 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]] 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. 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) @@ -687,25 +726,25 @@ 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. If a header field-name in the new response matches more than one header in the stored response, all such stored headers MUST be replaced. - The updated response can [[[[anchor11: requirement?]]]] be used to + 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. - [[anchor12: ISSUE: discuss how to handle HEAD updates]] + [[ISSUE-how-head: discuss how to handle HEAD updates]] 3. Header Field Definitions This section defines the syntax and semantics of HTTP/1.1 header fields related to caching. For entity-header fields, both sender and recipient refer to either the client or the server, depending on who sends and who receives the entity. @@ -799,21 +837,22 @@ 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. [[anchor13: of any staleness? --mnot]] + stale response of any age. [[TODO-staleness: of any staleness? + --mnot]] min-fresh The min-fresh request directive indicates that the client is willing to accept a response whose freshness lifetime is no less than its current age plus the specified time in seconds. That is, the client wants a response that will still be fresh for at least the specified number of seconds. no-transform @@ -863,21 +901,21 @@ If the private response directive specifies one or more field- names, this requirement is limited to the field-values associated with the listed response headers. That is, the specified field- names(s) MUST NOT be stored by a shared cache, whereas the remainder of the response message MAY be. Note: This usage of the word private only controls where the response may be stored, and cannot ensure the privacy of the message content. Also, private response directives with field- names are often handled by implementations as if an unqualified - private directive was recieved; i.e., the special handling for the + 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 stale responses. @@ -886,21 +924,21 @@ with the listed response headers. That is, the specified field- name(s) MUST NOT be sent in the response to a subsequent request without successful validation on the origin server. This allows an origin server to prevent the re-use of certain header fields in a response, while still allowing caching of the rest of the response. Note: Most HTTP/1.0 caches will not recognize or obey this directive. Also, no-cache response directives with field-names are often handled by implementations as if an unqualified no-cache - directive was recieved; i.e., the special handling for the + 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 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 @@ -1007,22 +1046,21 @@ 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. Expires = "Expires" ":" OWS Expires-v Expires-v = HTTP-date For example Expires: Thu, 01 Dec 1994 16:00:00 GMT - - Note: if a response includes a Cache-Control field with the max- + Note: If a response includes a Cache-Control field with the max- age directive (see Section 3.2.2), that directive overrides the Expires field. Likewise, the s-maxage directive overrides Expires in shared caches. HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in the future. HTTP/1.1 clients and caches MUST treat other invalid date formats, especially including the value "0", as in the past (i.e., "already expired"). @@ -1043,34 +1081,34 @@ When the no-cache directive is present in a request message, an application SHOULD forward the request toward the origin server even if it has a cached copy of what is being requested. This pragma directive has the same semantics as the no-cache response directive (see Section 3.2.2) and is defined here for backward compatibility with HTTP/1.0. Clients SHOULD include both header fields when a no- cache request is sent to a server not known to be HTTP/1.1 compliant. HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had sent "Cache-Control: no-cache". - Note: because the meaning of "Pragma: no-cache" as a response- + Note: Because the meaning of "Pragma: no-cache" as a response- header field is not actually specified, it does not provide a reliable replacement for "Cache-Control: no-cache" in a response. This mechanism is deprecated; no new Pragma directives will be defined in HTTP. 3.5. Vary The "Vary" response-header field conveys the set of request-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 satisdy a given request; see Section 2.6. + response can be used to satisfy a given request; see Section 2.6. determines, while the response is fresh, whether a cache is permitted to use the response to reply to a subsequent request without validation; see Section 2.6. In uncacheable or stale responses, the Vary field value advises the user agent about the criteria that were used to select the representation. Vary = "Vary" ":" OWS Vary-v Vary-v = "*" / 1#field-name @@ -1103,21 +1141,21 @@ 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. Warnings can be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status code, - distinguish these responses from true failures. + 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 Warning-v = 1#warning-value warning-value = warn-code SP warn-agent SP warn-text [SP warn-date] @@ -1213,45 +1251,27 @@ 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 mechanisms and caches are different. In particular history - mechanisms SHOULD NOT try to show a correct view of the current state - of a resource. Rather, a history mechanism is meant to show exactly - what the user saw at the time when the resource was retrieved. - - By default, an expiration time does not apply to history mechanisms. - If the entity is still in storage, a history mechanism SHOULD display - it even if the entity has expired, unless the user has specifically - configured the agent to refresh expired history documents. - - This is not to be construed to prohibit the history mechanism from - telling the user that a view might be stale. + 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. - Note: if history list mechanisms unnecessarily prevent users from - viewing stale resources, this will tend to force service authors - to avoid using HTTP expiration controls and cache controls when - they would otherwise like to. Service authors may consider it - important that users not be presented with error messages or - warning messages when they use navigation controls (such as BACK) - to view previously fetched resources. Even though sometimes such - resources ought not be cached, or ought to expire quickly, user - interface considerations may force service authors to resort to - other means of preventing caching (e.g. "once-only" URLs) in order - not to suffer the effects of improperly functioning history - mechanisms. + 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. Message Header Registration The Message Header Registry located at should be updated with the permanent registrations below (see [RFC3864]): +-------------------+----------+----------+-------------+ @@ -1284,52 +1304,46 @@ 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-08 - (work in progress), October 2009. + and Message Parsing", draft-ietf-httpbis-p1-messaging-09 + (work in progress), March 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-08 (work in - progress), October 2009. - - [Part3] Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., - Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., - and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload - and Content Negotiation", draft-ietf-httpbis-p3-payload-08 - (work in progress), October 2009. + Semantics", draft-ietf-httpbis-p2-semantics-09 (work in + progress), March 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-08 (work in - progress), October 2009. + Requests", draft-ietf-httpbis-p4-conditional-09 (work in + progress), March 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-08 (work - in progress), October 2009. + Partial Responses", draft-ietf-httpbis-p5-range-09 (work + in progress), March 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-08 (work in progress), - October 2009. + draft-ietf-httpbis-p7-auth-09 (work in progress), + March 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) @@ -1337,37 +1351,26 @@ [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. 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). - Transfer-coding and message lengths all interact in ways that - required fixing exactly when chunked encoding is used (to allow for - transfer encoding that may not be self delimiting); it was important - to straighten out exactly how message lengths are computed. (see also - [Part1], [Part3] and [Part5]) [[anchor16: This used to refer to the - text about non-modifiable headers, and will have to be updated later - on. --jre]] - - Proxies should be able to add Content-Length when appropriate. - [[anchor17: This used to refer to the text about non-modifiable - headers, and will have to be updated later on. --jre]] - Range request responses would become very verbose if all meta-data were always returned; by allowing the server to only send needed headers in a 206 response, this problem can be avoided. (Section 2.7) The Cache-Control: max-age directive was not properly defined for responses. (Section 3.2.2) Warnings could be cached incorrectly, or not updated appropriately. (Section 2.3, 2.7, 3.2, and 3.6) Warning also needed to be a general @@ -1588,42 +1591,62 @@ o : "Content- Location on 304 responses" o : "private and no-cache CC directives with headers" o : "RFC2047 and warn-text" +C.10. Since draft-ietf-httpbis-p6-cache-08 + + Closed issues: + + o : "serving + negotiated responses from cache: header-specific canonicalization" + + o : "Effect of CC + directives on history lists" + + Affected issues: + + o : Status codes + and caching + + Partly resolved issues: + + o : "Placement of + 13.5.1 and 13.5.2" + Index A age 6 Age header 17 C cache 5 Cache Directives - max-age 18, 22 + max-age 19, 22 max-stale 19 min-fresh 19 must-revalidate 21 - no-cache 18, 20 + no-cache 18, 21 no-store 18, 21 no-transform 19, 22 only-if-cached 19 private 20 - proxy-revalidate 21 + proxy-revalidate 22 public 20 s-maxage 22 - Cache-Control header 17 + Cache-Control header 18 cacheable 5 E Expires header 23 explicit expiration time 5 F first-hand 6 fresh 6 freshness lifetime 6 @@ -1637,68 +1660,68 @@ cache-extension 18 cache-request-directive 18 cache-response-directive 20 delta-seconds 17 Expires 23 Expires-v 23 extension-pragma 24 Pragma 24 pragma-directive 24 Pragma-v 24 - Vary 24 - Vary-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 17 + Cache-Control 18 Expires 23 Pragma 24 Vary 24 Warning 25 heuristic expiration time 5 M max-age - Cache Directive 18, 22 + Cache Directive 19, 22 max-stale Cache Directive 19 min-fresh Cache Directive 19 must-revalidate Cache Directive 21 N no-cache - Cache Directive 18, 20 + Cache Directive 18, 21 no-store Cache Directive 18, 21 no-transform Cache Directive 19, 22 O only-if-cached Cache Directive 19 P Pragma header 24 private Cache Directive 20 proxy-revalidate - Cache Directive 21 + Cache Directive 22 public Cache Directive 20 S s-maxage Cache Directive 22 stale 6 V validator 6