--- 1/draft-ietf-httpbis-p6-cache-21.txt 2013-02-23 15:29:13.460290659 +0100 +++ 2/draft-ietf-httpbis-p6-cache-22.txt 2013-02-23 15:29:13.520291012 +0100 @@ -1,62 +1,62 @@ HTTPbis Working Group R. Fielding, Ed. Internet-Draft Adobe Obsoletes: 2616 (if approved) M. Nottingham, Ed. Intended status: Standards Track Akamai -Expires: April 7, 2013 J. Reschke, Ed. +Expires: August 27, 2013 J. Reschke, Ed. greenbytes - October 4, 2012 + February 23, 2013 Hypertext Transfer Protocol (HTTP/1.1): Caching - draft-ietf-httpbis-p6-cache-21 + draft-ietf-httpbis-p6-cache-22 Abstract The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. This document 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 takes place on the HTTPBIS working group mailing list (ietf-http-wg@w3.org), which is archived at . 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 D.2. + The changes in this draft are summarized in Appendix D.3. 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 April 7, 2013. + This Internet-Draft will expire on August 27, 2013. Copyright Notice - Copyright (c) 2012 IETF Trust and the persons identified as the + Copyright (c) 2013 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 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 @@ -78,68 +78,69 @@ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 1.3. Conformance and Error Handling . . . . . . . . . . . . . . 6 1.4. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6 1.4.1. Delta Seconds . . . . . . . . . . . . . . . . . . . . 6 2. Overview of Cache Operation . . . . . . . . . . . . . . . . . 6 3. Storing Responses in Caches . . . . . . . . . . . . . . . . . 7 3.1. Storing Incomplete Responses . . . . . . . . . . . . . . . 8 - 3.2. Storing Responses to Authenticated Requests . . . . . . . 8 + 3.2. Storing Responses to Authenticated Requests . . . . . . . 9 4. Constructing Responses from Caches . . . . . . . . . . . . . . 9 4.1. Freshness Model . . . . . . . . . . . . . . . . . . . . . 10 4.1.1. Calculating Freshness Lifetime . . . . . . . . . . . . 11 4.1.2. Calculating Heuristic Freshness . . . . . . . . . . . 12 4.1.3. Calculating Age . . . . . . . . . . . . . . . . . . . 12 4.1.4. Serving Stale Responses . . . . . . . . . . . . . . . 14 4.2. Validation Model . . . . . . . . . . . . . . . . . . . . . 15 4.2.1. Freshening Responses with 304 Not Modified . . . . . . 16 - 4.3. Using Negotiated Responses . . . . . . . . . . . . . . . . 16 + 4.3. Using Negotiated Responses . . . . . . . . . . . . . . . . 17 4.4. Combining Partial Content . . . . . . . . . . . . . . . . 17 5. Updating Caches with HEAD Responses . . . . . . . . . . . . . 18 - 6. Request Methods that Invalidate . . . . . . . . . . . . . . . 18 + 6. Request Methods that Invalidate . . . . . . . . . . . . . . . 19 7. Header Field Definitions . . . . . . . . . . . . . . . . . . . 19 7.1. Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 7.2. Cache-Control . . . . . . . . . . . . . . . . . . . . . . 20 7.2.1. Request Cache-Control Directives . . . . . . . . . . . 20 7.2.2. Response Cache-Control Directives . . . . . . . . . . 22 - 7.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 25 + 7.2.3. Cache Control Extensions . . . . . . . . . . . . . . . 26 7.3. Expires . . . . . . . . . . . . . . . . . . . . . . . . . 27 7.4. Pragma . . . . . . . . . . . . . . . . . . . . . . . . . . 28 - 7.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 28 + 7.5. Warning . . . . . . . . . . . . . . . . . . . . . . . . . 29 7.5.1. 110 Response is Stale . . . . . . . . . . . . . . . . 30 7.5.2. 111 Revalidation Failed . . . . . . . . . . . . . . . 30 7.5.3. 112 Disconnected Operation . . . . . . . . . . . . . . 30 7.5.4. 113 Heuristic Expiration . . . . . . . . . . . . . . . 30 - 7.5.5. 199 Miscellaneous Warning . . . . . . . . . . . . . . 30 - 7.5.6. 214 Transformation Applied . . . . . . . . . . . . . . 30 + 7.5.5. 199 Miscellaneous Warning . . . . . . . . . . . . . . 31 + 7.5.6. 214 Transformation Applied . . . . . . . . . . . . . . 31 7.5.7. 299 Miscellaneous Persistent Warning . . . . . . . . . 31 7.5.8. Warn Code Extensions . . . . . . . . . . . . . . . . . 31 8. History Lists . . . . . . . . . . . . . . . . . . . . . . . . 31 - 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31 - 9.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 31 + 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32 + 9.1. Cache Directive Registry . . . . . . . . . . . . . . . . . 32 9.2. Warn Code Registry . . . . . . . . . . . . . . . . . . . . 32 9.3. Header Field Registration . . . . . . . . . . . . . . . . 33 10. Security Considerations . . . . . . . . . . . . . . . . . . . 33 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34 12.1. Normative References . . . . . . . . . . . . . . . . . . . 34 - 12.2. Informative References . . . . . . . . . . . . . . . . . . 34 + 12.2. Informative References . . . . . . . . . . . . . . . . . . 35 Appendix A. Changes from RFC 2616 . . . . . . . . . . . . . . . . 35 - Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 35 - Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 37 + Appendix B. Imported ABNF . . . . . . . . . . . . . . . . . . . . 37 + Appendix C. Collected ABNF . . . . . . . . . . . . . . . . . . . 38 Appendix D. Change Log (to be removed by RFC Editor before - publication) . . . . . . . . . . . . . . . . . . . . 38 - D.1. Since draft-ietf-httpbis-p6-cache-19 . . . . . . . . . . . 38 - D.2. Since draft-ietf-httpbis-p6-cache-20 . . . . . . . . . . . 38 - Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 + publication) . . . . . . . . . . . . . . . . . . . . 39 + D.1. Since draft-ietf-httpbis-p6-cache-19 . . . . . . . . . . . 39 + D.2. Since draft-ietf-httpbis-p6-cache-20 . . . . . . . . . . . 39 + D.3. Since draft-ietf-httpbis-p6-cache-21 . . . . . . . . . . . 40 + Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 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 @@ -161,21 +162,21 @@ reusable if it can be freshened by validation (Section 4.2) 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 - A conformant implementation of a HTTP cache. Note that this + A conformant implementation of an HTTP cache. Note that this implies an HTTP/1.1 cache; this specification does not define conformance for HTTP/1.0 caches. 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 @@ -184,22 +185,22 @@ 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 might be additional constraints on whether a cache can use the stored copy to satisfy a particular request. explicit expiration time - The time at which the origin server intends that a representation - no longer be returned by a cache without further validation. + The time at which the origin server intends that a stored response + no longer be used by a cache without further validation. heuristic expiration time An expiration time assigned by a cache when no explicit expiration time is available. age The age of a response is the time since it was sent by, or successfully validated with, the origin server. @@ -276,26 +277,27 @@ 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 methods other than GET if the method's definition allows - such caching and defines something suitable for use as a cache key. + retrieval request: i.e., a 200 (OK) response to a GET request, which + contains a representation of the resource identified by the request + target (Section 4.3.1 of [Part2]). However, it is also possible to + cache permanent redirects, negative results (e.g., 404 (Not Found)), + incomplete results (e.g., 206 (Partial Content)), and responses to + 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, many 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 4.3). @@ -325,22 +327,25 @@ * contains a max-age response cache directive (see Section 7.2.2.7), or * contains a s-maxage response cache directive and the cache is shared, or * contains a Cache Control Extension (see Section 7.2.3) that allows it to be cached, or - * has a status code that can be served with heuristic freshness - (see Section 4.1.2). + * has a status code that is defined as cacheable (see + Section 4.1.2), or + + * contains a public response cache directive (see + Section 7.2.2.1). Note that any of the requirements listed above can be overridden by a cache-control extension; see Section 7.2.3. 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, many caches will not store a response that has neither a cache validator nor an explicit expiration time, @@ -383,21 +388,21 @@ Note that cached responses that contain the "must-revalidate" and/or "s-maxage" response directives are not allowed to be served stale (Section 4.1.4) 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. 4. Constructing Responses from Caches - For a presented request, a cache MUST NOT return a stored response, + For a presented request, a cache MUST NOT send a stored response, unless: o The presented effective request URI (Section 5.5 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 header fields nominated by the stored response (if any) match those presented (see Section 4.3), and @@ -409,87 +414,80 @@ o the stored response does not contain the no-cache cache directive (Section 7.2.2.3), unless it is successfully validated (Section 4.2), and o the stored response is either: * fresh (see Section 4.1), or * allowed to be served stale (see Section 4.1.4), or - * successfully validated (see Section 4.2). Note that any of the requirements listed above can be overridden by a cache-control extension; see Section 7.2.3. When a stored response is used to satisfy a request without - validation, a cache MUST include a single Age header field - (Section 7.1) in the response with a value equal to the stored - response's current_age; see Section 4.1.3. + validation, a cache MUST send a single Age header field (Section 7.1) + in the response with a value equal to the stored response's + current_age; see Section 4.1.3. A cache MUST write through requests with methods that are unsafe - (Section 5.2.1 of [Part2]) to the origin server; i.e., a cache is not + (Section 4.2.1 of [Part2]) to the origin server; i.e., a cache is not allowed to 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 6. 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. + responses without revalidating them on every use. 4.1. 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 7.3) or the max-age response cache directive (Section 7.2.2.7). 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 + normally validate a stale cached response before reusing it for subsequent requests (see Section 4.1.4). 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. + caches are also allowed to use a heuristic to determine an expiration + time under certain circumstances (see Section 4.1.2). The calculation to determine if a response is fresh is: response_is_fresh = (freshness_lifetime > current_age) The freshness_lifetime is defined in Section 4.1.1; the current_age is defined in Section 4.1.3. - Additionally, clients can influence freshness calculation -- either - constraining it relaxing it -- by using the max-age and min-fresh - request cache directives. See Section 7.2.1 for details. + Clients can send the max-age or min-fresh cache directives in a + request to constrain or relax freshness calculations for the + corresponding response (Section 7.2.1). 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 8 for an explanation of the difference between caches and history mechanisms. 4.1.1. Calculating Freshness Lifetime A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a response by using the first match of: @@ -511,80 +509,83 @@ of the information comes from the origin server. When there is more than one value present for a given directive (e.g., two Expires header fields, multiple Cache-Control: max-age directives), it is considered invalid. Caches are encouraged to consider responses that have invalid freshness information to be stale. 4.1.2. 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 (OK), 203 - (Non-Authoritative Information), 206 (Partial Content), 300 (Multiple - Choices), 301 (Moved Permanently) and 410 (Gone)), 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. + 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. + + A cache MUST NOT use heuristics to determine freshness when an + explicit expiration time is present in the stored response. Because + of the requirements in Section 3, this means that, effectively, + heuristics can only be used on responses without explicit freshness + whose status codes are defined as cacheable, and responses without + explicit freshness that have been marked as explicitly cacheable + (e.g., with a "public" response cache directive). + + 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%. 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: Section 13.9 of [RFC2616] prohibited caches from calculating 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. 4.1.3. Calculating Age - HTTP/1.1 uses the Age header field to convey the estimated age of the + The Age header field is used to convey an 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 + the cache's estimate of the number of seconds 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 7.1), 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 8.1.1.2 of [Part2] for the definition of - the Date header field, and for requirements regarding responses - without it. + The term "date_value" denotes the value of the Date header field, + in a form appropriate for arithmetic operations. See Section + 7.1.1.2 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. + performing the calculation". A host ought to use NTP ([RFC1305]) + or some similar protocol to synchronize its clocks to Coordinated + Universal Time. 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. @@ -617,52 +618,48 @@ 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: - o Recipients 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). - o Although all date formats are specified to be case-sensitive, - recipients SHOULD match day, week and timezone names case- + cache recipients SHOULD match day, week and timezone names case- insensitively. - o An 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. + o If a cache recipient's internal implementation of time has less + resolution than the value of an HTTP-date, the recipient MUST + internally represent a parsed Expires date as the nearest time + equal to or earlier than the received value. - o Recipients MUST perform all expiration-related calculations in - GMT. The local time zone MUST NOT influence the calculation or - comparison of an age or expiration time. + o Cache recipients MUST NOT allow local time zones to influence the + calculation or comparison of an age or expiration time. - o Caches SHOULD consider dates with time zones other than "GMT" - invalid. + o Cache recipients SHOULD consider a date with a zone abbreviation + other than "GMT" to be invalid for calculating expiration. 4.1.4. 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 4.1. - A cache MUST NOT return a stale response if it is prohibited by an + A cache MUST NOT send 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 7.2.2). - A cache MUST NOT return stale responses unless it is disconnected + A cache MUST NOT send 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 7.2.1). A cache SHOULD append a Warning header field with the 110 warn-code (see Section 7.5) 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 @@ -701,62 +698,68 @@ stored response can be updated and reused; see Section 4.2.1. o A full response (i.e., one with a payload body) indicates that none of the stored responses nominated in the conditional request is suitable. Instead, the cache can use the full response to satisfy the request and MAY replace the stored response(s). o However, if a cache receives a 5xx (Server Error) response while attempting to validate a response, it can either forward this response to the requesting client, or act as if the server failed - to respond. In the latter case, it can return a previously stored + to respond. In the latter case, it can send a previously stored response (see Section 4.1.4). 4.2.1. Freshening Responses with 304 Not Modified 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. + The stored response to update is identified by using the first match + (if any) of: + o 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. + then the new response MUST NOT be used to update any stored + responses. o 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. - o 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. + o If the new response does not include any form of validator (such + as in the case where a client generates an If-Modified-Since + request from a source other than the Last-Modified response header + field), and 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: o delete any Warning header fields in the stored response with warn- code 1xx (see Section 7.5); o retain any Warning header fields in the stored response with warn- code 2xx; and, o use other header fields provided in the 304 (Not Modified) response to replace all instances of the corresponding header fields in the stored response. 4.3. Using Negotiated Responses When a cache receives a request that can be satisfied by a stored - response that has a Vary header field (Section 8.2.1 of [Part2]), it + response that has a Vary header field (Section 7.1.4 of [Part2]), 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: o adding or removing whitespace, where allowed in the header field's @@ -779,34 +782,34 @@ 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 4. - If no selected response is available, the cache can forward the - presented request to the origin server in a conditional request; see - Section 4.2. + If no selected response is available, the cache cannot satisfy the + presented request. Typically, it is forwarded to the origin server + in a (possibly conditional; see Section 4.2) request. 4.4. 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 4.2 of [Part5]. + requirements in Section 4.3 of [Part5]. When combining the new response with one or more stored responses, a cache MUST: o delete any Warning header fields in the stored response with warn- code 1xx (see Section 7.5); o retain any Warning header fields in the stored response with warn- code 2xx; and, @@ -837,21 +840,21 @@ o retain any Warning header fields in the stored response with warn- code 2xx; and, o use other header fields provided in the response to replace all instances of the corresponding header fields in the stored response. 6. Request Methods that Invalidate - Because unsafe request methods (Section 5.2.1 of [Part2]) such as + Because unsafe request methods (Section 4.2.1 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 5.5 of [Part1]) as well as the URI(s) in the Location and Content-Location response 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 @@ -860,21 +863,21 @@ of [Part1]). This helps prevent denial of service attacks. A cache MUST invalidate the effective request URI (Section 5.5 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 (Successful) or 3xx (Redirection) 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 + validation before they can be sent 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. 7. Header Field Definitions This section defines the syntax and semantics of HTTP/1.1 header @@ -1003,51 +1006,55 @@ client wants a response that will still be fresh for at least the specified number of seconds. Note: This directive uses the token form of the argument syntax; e.g., 'min-fresh=20', not 'min-fresh="20"'. Senders SHOULD NOT use the quoted-string form. 7.2.1.6. 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. + (whether or not it implements a cache) MUST NOT transform the + payload, as defined in Section 5.7.2 of [Part1]. 7.2.1.7. 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. 7.2.2. Response Cache-Control Directives 7.2.2.1. public - The "public" response directive indicates that a response whose - associated request contains an 'Authentication' header MAY be stored - (see Section 3.2). + The "public" response directive indicates that any cache MAY store + the response, even if the response would normally be non-cacheable or + cacheable only within a non-shared cache. (See Section 3.2 for + additional details related to the use of public in response to a + request containing Authorization, and Section 3 for details of how + public affects responses that would normally not be stored, due to + their status codes not being defined as cacheable.) 7.2.2.2. private Argument syntax: #field-name 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. + cache. A private cache MAY store the response and reuse it for later + requests, even if the response would normally be non-cacheable. 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. The field-names given are not limited to the set of standard header fields defined by this specification. Field names are case- insensitive. @@ -1066,22 +1073,21 @@ 7.2.2.3. no-cache Argument syntax: #field-name 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. + it, even by caches that have been configured to send stale responses. If the no-cache response directive specifies one or more field-names, then a cache MAY use the response to satisfy a subsequent request, subject to any other restrictions on caching. However, any header fields in the response that have the field-name(s) listed MUST NOT be sent in the response to a subsequent request without successful revalidation with 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. @@ -1163,23 +1169,22 @@ field. The s-maxage directive also implies the semantics of the proxy-revalidate response directive. Note: This directive uses the token form of the argument syntax; e.g., 's-maxage=10', not 's-maxage="10"'. Senders SHOULD NOT use the quoted-string form. 7.2.2.9. 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. + (regardless of whether it implements a cache) MUST NOT transform the + payload, as defined in Section 5.7.2 of [Part1]. 7.2.3. 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 @@ -1244,51 +1249,51 @@ 7.3. Expires The "Expires" header field gives the date/time after which the response is considered stale. See Section 4.1 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 8.1.1.1 of [Part2]; a sender MUST use the rfc1123-date - format. + The Expires value is an HTTP-date timestamp, as defined in Section + 7.1.1.1 of [Part2]. 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"). + A cache recipient MUST interpret invalid date formats, especially the + value "0", as representing a time in the past (i.e., "already + expired"). - Note: If a response includes a Cache-Control field with the max- - age directive (see Section 7.2.2.7), that directive overrides the - Expires field. Likewise, the s-maxage directive (Section 7.2.2.8) - overrides the Expires header fieldin shared caches. + If a response includes a Cache-Control field with the max-age + directive (Section 7.2.2.7), a recipient MUST ignore the Expires + field. Likewise, if a response includes the s-maxage directive + (Section 7.2.2.8), a shared cache recipient MUST ignore the Expires + field. In both these cases, the value in Expires is only intended + for recipients that have not yet implemented the Cache-Control field. + + An origin server without a clock MUST NOT generate an Expires field + unless its value represents a fixed time in the past (always expired) + or its value has been associated with the resource by a system or + user with a reliable clock. 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 many 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). + than that. 7.4. 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 field is also present and understood in a request, Pragma is ignored. In HTTP/1.0, Pragma was defined as an extensible field for @@ -1365,57 +1370,57 @@ response after validation: o 1xx Warnings describe the freshness or validation status of the response, and so MUST be deleted by a cache 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 representation that is not rectified by a validation (for example, a lossy compression of the representation) and MUST NOT be deleted by a cache after - validation, unless a full response is returned, in which case they + validation, unless a full response is sent, in which case they MUST be. 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. 7.5.1. 110 Response is Stale - A cache SHOULD include this whenever the returned response is stale. + A cache SHOULD generate this whenever the sent response is stale. 7.5.2. 111 Revalidation Failed - A cache SHOULD include this when returning a stale response because - an attempt to validate the response failed, due to an inability to - reach the server. + A cache SHOULD generate this when sending a stale response because an + attempt to validate the response failed, due to an inability to reach + the server. 7.5.3. 112 Disconnected Operation - A cache SHOULD include this if it is intentionally disconnected from + A cache SHOULD generate this if it is intentionally disconnected from the rest of the network for a period of time. 7.5.4. 113 Heuristic Expiration - A cache SHOULD include this if it heuristically chose a freshness + A cache SHOULD generate this if it heuristically chose a freshness lifetime greater than 24 hours and the response's age is greater than 24 hours. 7.5.5. 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. @@ -1512,199 +1517,262 @@ | 113 | Heuristic Expiration | Section 7.5.4 | | 199 | Miscellaneous Warning | Section 7.5.5 | | 214 | Transformation Applied | Section 7.5.6 | | 299 | Miscellaneous Persistent Warning | Section 7.5.7 | +-----------+----------------------------------+---------------+ 9.3. Header Field Registration The Message Header Field Registry located at shall be - updated with the permanent registrations below (see [RFC3864]): + updated with the permanent registrations below (see [BCP90]): +-------------------+----------+----------+-------------+ | Header Field Name | Protocol | Status | Reference | +-------------------+----------+----------+-------------+ | Age | http | standard | Section 7.1 | | Cache-Control | http | standard | Section 7.2 | | Expires | http | standard | Section 7.3 | | Pragma | http | standard | Section 7.4 | | Warning | http | standard | Section 7.5 | +-------------------+----------+----------+-------------+ The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force". 10. Security Considerations + This section is meant to inform developers, information providers, + and users of known security concerns specific to HTTP/1.1 caching. + More general security considerations are addressed in HTTP messaging + [Part1] and semantics [Part2]. + 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. + Furthermore, the very use of a cache can bring about privacy + concerns. For example, if two users share a cache, and the first one + browses to a site, the second may be able to detect that the other + has been to that site, because the resources from it load more + quickly, thanks to the cache. + Implementation flaws might allow attackers to insert content into a cache ("cache poisoning"), leading to compromise of clients that trust that content. Because of their nature, these attacks are difficult to mitigate. Likewise, implementation flaws (as well as misunderstanding of cache operation) might lead to caching of sensitive information (e.g., authentication credentials) that is thought to be private, exposing - it to unauthorised parties. + it to unauthorized parties. Note that the Set-Cookie response header [RFC6265] does not inhibit caching; a cacheable response with a Set-Cookie header can be (and often is) used to satisfy subsequent requests to caches. Servers who wish to control caching of these responses are encouraged to emit appropriate Cache-Control response headers. 11. Acknowledgments See Section 9 of [Part1]. 12. References 12.1. Normative References [Part1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", - draft-ietf-httpbis-p1-messaging-21 (work in progress), - October 2012. + draft-ietf-httpbis-p1-messaging-22 (work in progress), + February 2013. [Part2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", - draft-ietf-httpbis-p2-semantics-21 (work in progress), - October 2012. + draft-ietf-httpbis-p2-semantics-22 (work in progress), + February 2013. [Part4] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests", - draft-ietf-httpbis-p4-conditional-21 (work in progress), - October 2012. + draft-ietf-httpbis-p4-conditional-22 (work in progress), + February 2013. [Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Range Requests", - draft-ietf-httpbis-p5-range-21 (work in progress), - October 2012. + draft-ietf-httpbis-p5-range-22 (work in progress), + February 2013. [Part7] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Authentication", - draft-ietf-httpbis-p7-auth-21 (work in progress), - October 2012. + draft-ietf-httpbis-p7-auth-22 (work in progress), + February 2013. [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. 12.2. Informative References + [BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration + Procedures for Message Header Fields", BCP 90, RFC 3864, + September 2004. + [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. [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, April 2011. Appendix A. Changes from RFC 2616 - Make the specified age calculation algorithm less conservative. + Caching-related text has been substantially rewritten for clarity. + + The algorithm for calculating age is now less conservative. (Section 4.1.3) - Remove requirement to consider "Content-Location" in successful - responses in order to determine the appropriate response to use. + Caches are now required to handle dates with timezones as if they're + invalid, because it's not possible to accurately guess. + (Section 4.1.3) + + The Content-Location response header field is no longer used to + determine the appropriate response to use when validating. (Section 4.2) + The algorithm for selecting a cached negotiated response to use has + been clarified in several ways. In particular, it now explicitly + allows header-specific canonicalization when processing selecting + header fields. (Section 4.3) - Clarify denial of service attack avoidance requirement. (Section 6) + Requirements regarding denial of service attack avoidance when + performing invalidation have been clarified. (Section 6) - Do not mention RFC 2047 encoding and multiple languages in "Warning" - header fields anymore, as these aspects never were implemented. + Cache invalidation only occurs when a successful response is + received. (Section 6) + + The conditions under which an authenticated response can be cached + have been clarified. (Section 3.2) + + The one-year limit on Expires header field values has been removed; + instead, the reasoning for using a sensible value is given. + (Section 7.3) + + The Pragma header field is now only defined for backwards + compatibility; future pragmas are deprecated. (Section 7.4) + + Cache directives are explicitly defined to be case-insensitive. + (Section 7.2) + + Handling of multiple instances of cache directives when only one is + expected is now defined. (Section 7.2) + + The qualified forms of the private and no-cache cache directives are + noted to not be widely implemented; e.g., "private=foo" is + interpreted by many caches as simply "private". Additionally, the + meaning of the qualified form of no-cache has been clarified. + (Section 7.2.2) + + The "no-store" cache request directive doesn't apply to responses; + i.e., a cache can satisfy a request with no-store on it, and does not + invalidate it. (Section 7.2.1.2) + + The "no-cache" response cache directive's meaning has been clarified. + (Section 7.2.2.3) + + New status codes can now define that caches are allowed to use + heuristic freshness with them. (Section 4.1.2) + + Caches are now allow to calculate heuristic freshness for URLs with + query components. (Section 4.1.2) + + Some requirements regarding production of the Warning header have + been relaxed, as it is not widely implemented. (Section 7.5) + The Warning header field no longer uses RFC 2047 encoding, nor allows + multiple languages, as these aspects were not implemented. (Section 7.5) - Introduce Cache Directive and Warn Code Registries. (Section 7.2.3 - and Section 7.5.8) + This specification introduces the Cache Directive and Warn Code + Registries, and defines considerations for new cache directives. + (Section 7.2.3 and Section 7.5.8) Appendix B. Imported ABNF The following core rules are included by reference, as defined in Appendix B.1 of [RFC5234]: 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). The rules below are defined in [Part1]: - OWS = + OWS = field-name = - quoted-string = - token = + quoted-string = + token = port = - pseudonym = + pseudonym = uri-host = The rules below are defined in other parts: - HTTP-date = + HTTP-date = Appendix C. Collected ABNF Age = delta-seconds Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS cache-directive ] ) Expires = HTTP-date - HTTP-date = + HTTP-date = - OWS = + OWS = Pragma = *( "," OWS ) pragma-directive *( OWS "," [ OWS pragma-directive ] ) Warning = *( "," OWS ) warning-value *( OWS "," [ OWS warning-value ] ) cache-directive = token [ "=" ( token / quoted-string ) ] delta-seconds = 1*DIGIT extension-pragma = token [ "=" ( token / quoted-string ) ] field-name = port = pragma-directive = "no-cache" / extension-pragma - pseudonym = + pseudonym = - quoted-string = + quoted-string = - token = + token = uri-host = 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 ] @@ -1752,31 +1820,46 @@ Other changes: o Conformance criteria and considerations regarding error handling are now defined in Part 1. o Move definition of "Vary" header field into Part 2. o Add security considerations with respect to cache poisoning and the "Set-Cookie" header field. +D.3. Since draft-ietf-httpbis-p6-cache-21 + + Closed issues: + + o : "Allowing + heuristic caching for new status codes" + + o : "304 without + validator" + + o : "No-Transform" + + o : "Revert prior + change to the meaning of the public cache response directive. + Index 1 110 Response is Stale (warn code) 30 111 Revalidation Failed (warn code) 30 112 Disconnected Operation (warn code) 30 113 Heuristic Expiration (warn code) 30 - 199 Miscellaneous Warning (warn code) 30 + 199 Miscellaneous Warning (warn code) 31 2 - 214 Transformation Applied (warn code) 30 + 214 Transformation Applied (warn code) 31 299 Miscellaneous Persistent Warning (warn code) 31 A age 5 Age header field 19 C cache 4 cache entry 6 cache key 6 @@ -1813,45 +1896,45 @@ heuristic expiration time 5 M max-age (cache directive) 21, 25 max-stale (cache directive) 21 min-fresh (cache directive) 22 must-revalidate (cache directive) 24 N no-cache (cache directive) 20, 23 - no-store (cache directive) 20, 24 + no-store (cache directive) 21, 24 no-transform (cache directive) 22, 25 O only-if-cached (cache directive) 22 P Pragma header field 28 - private (cache directive) 22 + private (cache directive) 23 private cache 4 - proxy-revalidate (cache directive) 24 + proxy-revalidate (cache directive) 25 public (cache directive) 22 S s-maxage (cache directive) 25 shared cache 4 stale 5 strong validator 6 V validator 5 strong 6 W - Warning header field 28 + Warning header field 29 Authors' Addresses Roy T. Fielding (editor) Adobe Systems Incorporated 345 Park Ave San Jose, CA 95110 USA EMail: fielding@gbiv.com