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.¶
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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 <http://lists.w3.org/Archives/Public/ietf-http-wg/>.¶
The current issues list is at <http://tools.ietf.org/wg/httpbis/trac/report/3> and related documents (including fancy diffs) can be found at <http://tools.ietf.org/wg/httpbis/>.¶
The changes in this draft are summarized in Appendix D.1.¶
Table of ContentsHTTP 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.¶
An HTTP cache is a local store of response messages and the subsystem that controls storage, retrieval, and deletion of messages in it. A cache stores cacheable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests. Any client or server MAY employ a cache, though a cache cannot be used by a server that is acting as a tunnel.¶
The goal of caching in HTTP/1.1 is to significantly improve performance by reusing a prior response message to satisfy a current request. A stored response is considered "fresh", as defined in Section 4.2, if the response can be reused without "validation" (checking with the origin server to see if the cached response remains valid for this request). A fresh response can therefore reduce both latency and network overhead each time it is reused. When a cached response is not fresh, it might still be reusable if it can be freshened by validation (Section 4.3) or if the origin is unavailable (Section 4.2.4).¶
1.1. Conformance and Error HandlingThe key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶
Conformance criteria and considerations regarding error handling are defined in Section 2.5 of [Part1].¶
1.2. Syntax NotationThis specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] with the list rule extension defined in Section 7 of [Part1]. Appendix B describes rules imported from other documents. Appendix C shows the collected ABNF with the list rule expanded.¶
1.2.1. Delta SecondsThe delta-seconds rule specifies a non-negative integer, representing time in seconds.¶
A recipient parsing a delta-seconds value and converting it to binary form ought to use an arithmetic type of at least 31 bits of non-negative integer range. If a cache receives a delta-seconds value greater than the greatest integer it can represent, or if any of its subsequent calculations overflows, the cache MUST consider the value to be either 2147483648 (231) or the greatest positive integer it can conveniently represent.¶
Note: The value 2147483648 is here for historical reasons, effectively represents infinity (over 68 years), and does not need to be stored in binary form; an implementation could produce it as a canned string if any overflow occurs, even if the calculations are performed with an arithmetic type incapable of directly representing that number. What matters here is that an overflow be detected and not treated as a negative value in later calculations.¶
2. Overview of Cache OperationProper cache operation preserves the semantics of HTTP transfers ([Part2]) while eliminating the transfer of information already held in the cache. Although caching is an entirely OPTIONAL feature of HTTP, we assume that reusing the cached response is desirable and that such reuse is the default behavior when no requirement or local 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, rather than mandating that caches always store and reuse particular responses.¶
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 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 primary 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 caches simply decline other methods and use only the URI as the primary cache 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 4.1).¶
3. Storing Responses in CachesA cache MUST NOT store a response to any request, unless: ¶
Note that any of the requirements listed above can be overridden by a cache-control extension; see Section 5.2.3.¶
In this context, a cache has "understood" a request method or a response status code if it recognizes it and implements all specified caching-related behavior.¶
Note that, in normal operation, some 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.¶
3.1. Storing Incomplete ResponsesA response message is considered complete when all of the octets indicated by the message framing ([Part1]) are received prior to the connection being closed. If the request method is GET, the response status code is 200 (OK), and the entire response header section has been received, a cache MAY store an incomplete response message body if the cache entry is recorded as incomplete. Likewise, a 206 (Partial Content) response MAY be stored as if it were an incomplete 200 (OK) cache entry. However, a cache MUST NOT store incomplete or partial content responses if it does not support the Range and Content-Range header fields or if it does not understand the range units used in those fields.¶
A cache MAY complete a stored incomplete response by making a subsequent range request ([Part5]) and combining the successful response with the stored entry, as defined in Section 3.3. A cache MUST NOT use an incomplete response to answer requests unless the response has been made complete or the request is partial and specifies a range that is wholly within the incomplete response. A cache MUST NOT send a partial response to a client without explicitly marking it as such using the 206 (Partial Content) status code.¶
3.2. Storing Responses to Authenticated RequestsA shared cache MUST NOT use a cached response to a request with an Authorization header field (Section 4.1 of [Part7]) to satisfy any subsequent request unless a cache directive that allows such responses to be stored is present in the response.¶
In this specification, the following Cache-Control response directives (Section 5.2.2) have such an effect: must-revalidate, public, s-maxage.¶
Note that cached responses that contain the "must-revalidate" and/or "s-maxage" response directives are not allowed to be served stale (Section 4.2.4) 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.¶
3.3. Combining Partial ContentA 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 4.3 of [Part5].¶
When combining the new response with one or more stored responses, a cache MUST: ¶
When presented with a request, a cache MUST NOT reuse a stored response, unless: ¶
Note that any of the requirements listed above can be overridden by a cache-control extension; see Section 5.2.3.¶
When a stored response is used to satisfy a request without validation, a cache MUST generate an Age header field (Section 5.1), replacing any present in the response with a value equal to the stored response's current_age; see Section 4.2.3.¶
A cache MUST write through requests with methods that are unsafe (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 4.4.¶
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 the 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 upon every use.¶
4.1. Calculating Secondary Keys with VaryWhen a cache receives a request that can be satisfied by a stored 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: ¶
If (after any normalization that might take place) a header field is absent from a request, it can only match another request if it is also absent there.¶
A Vary header field-value of "*" always fails to match.¶
The stored response with matching selecting header fields is known as the selected response.¶
If multiple selected responses are available (potentially including responses without a Vary header field), the cache will need to choose one to use. When a selecting header field has a known mechanism for doing so (e.g., qvalues on Accept and similar request header fields), that mechanism MAY be used to select preferred responses; of the remainder, the most recent response (as determined by the Date header field) is used, as per Section 4.¶
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 4.3) request.¶
4.2. FreshnessA fresh response is one whose age has not yet exceeded its freshness lifetime. Conversely, a stale response is one where it has.¶
A response's freshness lifetime is the length of time between its generation by the origin server and its expiration time. An explicit expiration time is the time at which the origin server intends that a stored response can no longer be used by a cache without further validation, whereas a heuristic expiration time is assigned by a cache when no explicit expiration time is available.¶
A response's age is the time that has passed since it was generated by, or successfully validated with, the origin server.¶
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 5.3) or the max-age response cache directive (Section 5.2.2.8). 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 a stale cached response before reusing it for subsequent requests (see Section 4.2.4).¶
Since origin servers do not always provide explicit expiration times, caches are also allowed to use a heuristic to determine an expiration time under certain circumstances (see Section 4.2.2).¶
The calculation to determine if a response is fresh is:
response_is_fresh = (freshness_lifetime > current_age)
freshness_lifetime is defined in Section 4.2.1; current_age is defined in Section 4.2.3.¶
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 5.2.1).¶
When calculating freshness, to avoid common problems in date parsing:¶
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 6 for an explanation of the difference between caches and history mechanisms.¶
4.2.1. Calculating Freshness LifetimeA cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a response by using the first match of: ¶
Note that this calculation is not vulnerable to clock skew, since all of the information comes from the origin server.¶
When there is more than one value present for a given directive (e.g., two Expires header fields, multiple Cache-Control: max-age directives), the directive's value is considered invalid. Caches are encouraged to consider responses that have invalid freshness information to be stale.¶
4.2.2. Calculating Heuristic FreshnessSince 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 by default (see Section 6.1 of [Part2]), and those 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 generate a Warning header field with a 113 warn-code (see Section 5.5.4) in the response if its current_age is more than 24 hours and such a warning is not already present.¶
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, origin servers are encouraged to send explicit directives (e.g., Cache-Control: no-cache) if they wish to preclude caching.¶
4.2.3. Calculating AgeThe 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 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 ¶
date_value ¶
now ¶
request_time ¶
response_time ¶
A response's age can be calculated in two entirely independent ways: ¶
apparent_age = max(0, response_time - date_value); response_delay = response_time - request_time; corrected_age_value = age_value + response_delay;
These are combined as
corrected_initial_age = max(apparent_age, corrected_age_value);
unless the cache is confident in the value of the Age header field (e.g., because there are no HTTP/1.0 hops in the Via header field), in which case the corrected_age_value MAY be used as the corrected_initial_age.¶
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;4.2.4. 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 4.2.¶
A cache MUST NOT generate 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 5.2.2).¶
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 5.2.1).¶
A cache SHOULD generate a Warning header field with the 110 warn-code (see Section 5.5.1) in stale responses. Likewise, a cache SHOULD generate a 112 warn-code (see Section 5.5.3) in stale responses if the cache is disconnected.¶
A cache SHOULD NOT generate a new Warning header field when forwarding a response that does not have an Age header field, even if the response is already stale. A cache need not validate a response that merely became stale in transit.¶
4.3. ValidationWhen a cache has one or more stored responses for a requested URI, but cannot serve any of them (e.g., because they are not fresh, or one cannot be selected; see Section 4.1), it can use the conditional request mechanism [Part4] in the forwarded request to give the next inbound server an opportunity to select a valid stored response to use, updating the stored metadata in the process, or to replace the stored response(s) with a new response. This process is known as "validating" or "revalidating" the stored response.¶
4.3.1. Sending a Validation RequestWhen sending a conditional request for cache validation, a cache sends one or more precondition header fields containing validator metadata from its stored response(s), which is then compared by recipients to determine whether a stored response is equivalent to a current representation of the resource.¶
One such validator is the timestamp given in a Last-Modified header field (Section 2.2 of [Part4]), which can be used in an If-Modified-Since header field for response validation, or in an If-Unmodified-Since or If-Range header field for representation selection (i.e., the client is referring specifically to a previously obtained representation with that timestamp).¶
Another validator is the entity-tag given in an ETag header field (Section 2.3 of [Part4]). One or more entity-tags, indicating one or more stored responses, can be used in an If-None-Match header field for response validation, or in an If-Match or If-Range header field for representation selection (i.e., the client is referring specifically to one or more previously obtained representations with the listed entity-tags).¶
4.3.2. Handling a Received Validation RequestEach client in the request chain may have its own cache, so it is common for a cache at an intermediary to receive conditional requests from other (outbound) caches. Likewise, some user agents make use of conditional requests to limit data transfers to recently modified representations or to complete the transfer of a partially retrieved representation.¶
If a cache receives a request that can be satisfied by reusing one of its stored 200 (OK) or 206 (Partial Content) responses, the cache SHOULD evaluate any applicable conditional header field preconditions received in that request with respect to the corresponding validators contained within the selected response. A cache MUST NOT evaluate conditional header fields that are only applicable to an origin server, found in a request with semantics that cannot be satisfied with a cached response, or applied to a target resource for which it has no stored responses; such preconditions are likely intended for some other (inbound) server.¶
The proper evaluation of conditional requests by a cache depends on the received precondition header fields and their precedence, as defined in Section 6 of [Part4]. The If-Match and If-Unmodified-Since conditional header fields are not applicable to a cache.¶
A request containing an If-None-Match header field (Section 3.2 of [Part4]) indicates that the client wants to validate one or more of its own stored responses in comparison to whichever stored response is selected by the cache. If the field-value is "*", or if the field-value is a list of entity-tags and at least one of them match the entity-tag of the selected stored response, a cache recipient SHOULD generate a 304 (Not Modified) response (using the metadata of the selected stored response) instead of sending that stored response.¶
When a cache decides to revalidate its own stored responses for a request that contains an If-None-Match list of entity-tags, the cache MAY combine the received list with a list of entity-tags from its own stored set of responses (fresh or stale) and send the union of the two lists as a replacement If-None-Match header field value in the forwarded request. If a stored response contains only partial content, the cache MUST NOT include its entity-tag in the union unless the request is for a range that would be fully satisfied by that partial stored response. If the response to the forwarded request is 304 (Not Modified) and has an ETag header field value with an entity-tag that is not in the client's list, the cache MUST generate a 200 (OK) response for the client by reusing its corresponding stored response, as updated by the 304 response metadata (Section 4.3.4).¶
If an If-None-Match header field is not present, a request containing an If-Modified-Since header field (Section 3.3 of [Part4]) indicates that the client wants to validate one or more of its own stored responses by modification date. A cache recipient SHOULD generate a 304 (Not Modified) response (using the metadata of the selected stored response) if one of the following cases is true: 1) the selected stored response has a Last-Modified field-value that is earlier than or equal to the conditional timestamp; 2) no Last-Modified field is present in the selected stored response, but it has a Date field-value that is earlier than or equal to the conditional timestamp; or, 3) neither Last-Modified nor Date is present in the selected stored response, but the cache recorded it as having been received at a time earlier than or equal to the conditional timestamp.¶
A cache that implements partial responses to range requests, as defined in [Part5], also needs to evaluate a received If-Range header field (Section 3.2 of [Part5]) with respect to its selected stored response.¶
4.3.3. Handling a Validation ResponseCache handling of a response to a conditional request is dependent upon its status code:¶
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: ¶
If a stored response is selected for update, the cache MUST: ¶
A response to the HEAD method is identical to what an equivalent request made with a GET would have been, except it lacks a body. This property of HEAD responses can be used to invalidate or update a cached GET response if the more efficient conditional GET request mechanism is not available (due to no validators being present in the stored response) or if transmission of the representation body is not desired even if it has changed.¶
When a cache makes an inbound HEAD request for a given request target and receives a 200 (OK) response, the cache SHOULD update or invalidate each of its stored GET responses that could have been selected for that request (see Section 4.1).¶
For each of the stored responses that could have been selected, if the stored response and HEAD response have matching values for any received validator fields (ETag and Last-Modified) and, if the HEAD response has a Content-Length header field, the value of Content-Length matches that of the stored response, the cache SHOULD update the stored response a described below; otherwise, the cache SHOULD consider the stored response to be stale.¶
If a cache updates a stored response with the metadata provided in a HEAD response, the cache MUST: ¶
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 status code is received in response to an unsafe request method.¶
However, a cache MUST NOT invalidate a URI from a Location or Content-Location response header field if the host part of that URI differs from the host part in the effective request URI (Section 5.5 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 sent in response to a subsequent request.¶
Note that this does not guarantee that all appropriate responses are invalidated. For example, a state-changing request might invalidate responses in the caches it travels through, but relevant responses still might be stored in other caches that it has not.¶
6. History ListsUser agents often have history mechanisms, such as "Back" buttons and history lists, that can be used to redisplay a representation retrieved earlier in a session.¶
The freshness model (Section 4.2) does not necessarily apply to history mechanisms. I.e., a history mechanism can display a previous representation even if it has expired.¶
This does not prohibit the history mechanism from telling the user that a view might be stale, or from honoring cache directives (e.g., Cache-Control: no-store).¶
7. IANA Considerations 7.1. Cache Directive RegistryThe HTTP Cache Directive Registry defines the name space for the cache directives. It will be created and maintained at (the suggested URI) <http://www.iana.org/assignments/http-cache-directives>.¶
7.1.1. ProcedureA registration MUST include the following fields: ¶
Values to be added to this name space require IETF Review (see [RFC5226], Section 4.1).¶
7.1.2. Considerations for New Cache Control DirectivesNew extension directives ought to consider defining:¶
See also Section 5.2.3.¶
7.1.3. RegistrationsThe HTTP Cache Directive Registry shall be populated with the registrations below:¶
7.2. Warn Code RegistryThe HTTP Warn Code Registry defines the name space for warn codes. It will be created and maintained at (the suggested URI) <http://www.iana.org/assignments/http-warn-codes>.¶
7.2.1. ProcedureA registration MUST include the following fields: ¶
Values to be added to this name space require IETF Review (see [RFC5226], Section 4.1).¶
7.2.2. RegistrationsThe HTTP Warn Code Registry shall be populated with the registrations below:¶
8. Security ConsiderationsThis 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 unauthorized parties.¶
Note that the Set-Cookie response header field [RFC6265] does not inhibit caching; a cacheable response with a Set-Cookie header field 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 header fields.¶
9. AcknowledgmentsSee Section 10 of [Part1].¶
10. References 10.1. Normative ReferencesThe specification has been substantially rewritten for clarity.¶
The conditions under which an authenticated response can be cached have been clarified. (Section 3.2)¶
New status codes can now define that caches are allowed to use heuristic freshness with them. Caches are now allowed to calculate heuristic freshness for URIs with query components. (Section 4.2.2)¶
The algorithm for calculating age is now less conservative. Caches are now required to handle dates with timezones as if they're invalid, because it's not possible to accurately guess. (Section 4.2.3)¶
The Content-Location response header field is no longer used to determine the appropriate response to use when validating. (Section 4.3)¶
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 4.1)¶
Requirements regarding denial of service attack avoidance when performing invalidation have been clarified. (Section 4.4)¶
Cache invalidation only occurs when a successful response is received. (Section 4.4)¶
Cache directives are explicitly defined to be case-insensitive. Handling of multiple instances of cache directives when only one is expected is now defined. (Section 5.2)¶
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 5.2.1.5)¶
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 5.2.2)¶
The "no-cache" response cache directive's meaning has been clarified. (Section 5.2.2.2)¶
The one-year limit on Expires header field values has been removed; instead, the reasoning for using a sensible value is given. (Section 5.3)¶
The Pragma header field is now only defined for backwards compatibility; future pragmas are deprecated. (Section 5.4)¶
Some requirements regarding production and processing of the Warning header fields have been relaxed, as it is not widely implemented. Furthermore, the Warning header field no longer uses RFC 2047 encoding, nor allows multiple languages, as these aspects were not implemented. (Section 5.5)¶
This specification introduces the Cache Directive and Warn Code Registries, and defines considerations for new cache directives. (Section 7.1 and Section 7.2)¶
Appendix B. Imported ABNFThe 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, defined in [Part1], Section 3.2.3> field-name = <field-name, defined in [Part1], Section 3.2> quoted-string = <quoted-string, defined in [Part1], Section 3.2.6> token = <token, defined in [Part1], Section 3.2.6> port = <port, defined in [Part1], Section 2.7> pseudonym = <pseudonym, defined in [Part1], Section 5.7.1> uri-host = <uri-host, defined in [Part1], Section 2.7>
The rules below are defined in other parts:¶
Appendix C. Collected ABNFIn the collected ABNF below, list rules are expanded as per Section 1.2 of [Part1].¶
Age = delta-seconds Cache-Control = *( "," OWS ) cache-directive *( OWS "," [ OWS cache-directive ] ) Expires = HTTP-date HTTP-date = <HTTP-date, defined in [Part2], Section 7.1.1.1> OWS = <OWS, defined in [Part1], Section 3.2.3> 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 = <field-name, defined in [Part1], Section 3.2> port = <port, defined in [Part1], Section 2.7> pragma-directive = "no-cache" / extension-pragma pseudonym = <pseudonym, defined in [Part1], Section 5.7.1> quoted-string = <quoted-string, defined in [Part1], Section 3.2.6> token = <token, defined in [Part1], Section 3.2.6> uri-host = <uri-host, defined in [Part1], Section 2.7> warn-agent = ( uri-host [ ":" port ] ) / pseudonym warn-code = 3DIGIT warn-date = DQUOTE HTTP-date DQUOTE warn-text = quoted-string warning-value = warn-code SP warn-agent SP warn-text [ SP warn-date ]Appendix D. Change Log (to be removed by RFC Editor before publication)
Changes up to the IETF Last Call draft are summarized in <http://trac.tools.ietf.org/html/draft-ietf-httpbis-p6-cache-24#appendix-D>.¶
D.1. Since draft-ietf-httpbis-p6-cache-24 Index1 2 A B C D E F G H M N O P R S T V W
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