Internet Engineering Task Force (IETF) R. Fielding, Ed. Request for Comments: 7234 Adobe Obsoletes: 2616 M. Nottingham, Ed. Category: Standards Track Akamai ISSN: 2070-1721 J. Reschke, Ed. greenbytes June 2014 Hypertext Transfer Protocol (HTTP/1.1): Caching Abstract The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7234.
Copyright Notice Copyright (c) 2014 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 described in the Simplified BSD License. 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 ....................................................4 1.1. Conformance and Error Handling .............................4 1.2. Syntax Notation ............................................4 1.2.1. Delta Seconds .......................................5 2. Overview of Cache Operation .....................................5 3. Storing Responses in Caches .....................................6 3.1. Storing Incomplete Responses ...............................7 3.2. Storing Responses to Authenticated Requests ................7 3.3. Combining Partial Content ..................................8 4. Constructing Responses from Caches ..............................8 4.1. Calculating Secondary Keys with Vary .......................9 4.2. Freshness .................................................11 4.2.1. Calculating Freshness Lifetime .....................12 4.2.2. Calculating Heuristic Freshness ....................13 4.2.3. Calculating Age ....................................13 4.2.4. Serving Stale Responses ............................15 4.3. Validation ................................................16 4.3.1. Sending a Validation Request .......................16 4.3.2. Handling a Received Validation Request .............16
4.3.3. Handling a Validation Response .....................18 4.3.4. Freshening Stored Responses upon Validation ........18 4.3.5. Freshening Responses via HEAD ......................19 4.4. Invalidation ..............................................20 5. Header Field Definitions .......................................21 5.1. Age .......................................................21 5.2. Cache-Control .............................................21 5.2.1. Request Cache-Control Directives ...................22 5.2.2. Response Cache-Control Directives ..................24 5.2.3. Cache Control Extensions ...........................27 5.3. Expires ...................................................28 5.4. Pragma ....................................................29 5.5. Warning ...................................................29 5.5.1. Warning: 110 - "Response is Stale" .................31 5.5.2. Warning: 111 - "Revalidation Failed" ...............31 5.5.3. Warning: 112 - "Disconnected Operation" ............31 5.5.4. Warning: 113 - "Heuristic Expiration" ..............31 5.5.5. Warning: 199 - "Miscellaneous Warning" .............32 5.5.6. Warning: 214 - "Transformation Applied" ............32 5.5.7. Warning: 299 - "Miscellaneous Persistent Warning" ..32 6. History Lists ..................................................32 7. IANA Considerations ............................................32 7.1. Cache Directive Registry ..................................32 7.1.1. Procedure ..........................................32 7.1.2. Considerations for New Cache Control Directives ....33 7.1.3. Registrations ......................................33 7.2. Warn Code Registry ........................................34 7.2.1. Procedure ..........................................34 7.2.2. Registrations ......................................34 7.3. Header Field Registration .................................34 8. Security Considerations ........................................35 9. Acknowledgments ................................................36 10. References ....................................................36 10.1. Normative References .....................................36 10.2. Informative References ...................................37 Appendix A. Changes from RFC 2616 .................................38 Appendix B. Imported ABNF .........................................39 Appendix C. Collected ABNF ........................................39 Index .............................................................41
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. 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. A shared cache is a cache that stores responses to be reused by more than one user; shared caches are usually (but not always) deployed as a part of an intermediary. A private cache, in contrast, is dedicated to a single user; often, they are deployed as a component of a user agent. 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 Handling The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Conformance criteria and considerations regarding error handling are defined in Section 2.5 of [RFC7230]. 1.2. Syntax Notation This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] with a list extension, defined in Section 7 of [RFC7230], that allows for compact definition of comma-separated lists using a '#' operator (similar to how the '*' operator indicates
repetition). Appendix B describes rules imported from other documents. Appendix C shows the collected grammar with all list operators expanded to standard ABNF notation. 1.2.1. Delta Seconds The delta-seconds rule specifies a non-negative integer, representing time in seconds. delta-seconds = 1*DIGIT 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 (2^31) 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 Operation Proper cache operation preserves the semantics of HTTP transfers ([RFC7231]) while eliminating the transfer of information already held in the cache. Although caching is an entirely OPTIONAL feature of HTTP, it can be assumed that reusing a 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 [RFC7231]). 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 Caches A cache MUST NOT store a response to any request, unless: 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 5.2) does not appear in request or response header fields, and o the "private" response directive (see Section 18.104.22.168) does not appear in the response, if the cache is shared, and o the Authorization header field (see Section 4.2 of [RFC7235]) does not appear in the request, if the cache is shared, unless the response explicitly allows it (see Section 3.2), and o the response either: * contains an Expires header field (see Section 5.3), or * contains a max-age response directive (see Section 22.214.171.124), or * contains a s-maxage response directive (see Section 126.96.36.199) and the cache is shared, or * contains a Cache Control Extension (see Section 5.2.3) that allows it to be cached, or * has a status code that is defined as cacheable by default (see Section 4.2.2), or
* contains a public response directive (see Section 188.8.131.52). 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 Responses A response message is considered complete when all of the octets indicated by the message framing ([RFC7230]) 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 ([RFC7233]) 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 Requests A shared cache MUST NOT use a cached response to a request with an Authorization header field (Section 4.2 of [RFC7235]) 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, and 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 Content A response might transfer only a partial representation if the connection closed prematurely or if the request used one or more Range specifiers ([RFC7233]). 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 [RFC7233]. 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 5.5); o retain any Warning header fields in the stored response with warn-code 2xx; and, o use other header fields provided in the new response, aside from Content-Range, to replace all instances of the corresponding header fields in the stored response. 4. Constructing Responses from Caches When presented with a request, a cache MUST NOT reuse a stored response, unless: o The presented effective request URI (Section 5.5 of [RFC7230]) 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 4.1), and
o the presented request does not contain the no-cache pragma (Section 5.4), nor the no-cache cache directive (Section 5.2.1), unless the stored response is successfully validated (Section 4.3), and o the stored response does not contain the no-cache cache directive (Section 184.108.40.206), unless it is successfully validated (Section 4.3), and o the stored response is either: * fresh (see Section 4.2), or * allowed to be served stale (see Section 4.2.4), or * successfully validated (see Section 4.3). 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 [RFC7231]) 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 Vary When a cache receives a request that can be satisfied by a stored response that has a Vary header field (Section 7.1.4 of [RFC7231]), 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 syntax o combining multiple header fields with the same field name (see Section 3.2 of [RFC7230]) o normalizing both header field values in a way that is known to have identical semantics, according to the header field's specification (e.g., reordering field values when order is not significant; case-normalization, where values are defined to be case-insensitive) 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. Freshness A 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 directive (Section 220.127.116.11). 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: o Although all date formats are specified to be case-sensitive, a cache recipient SHOULD match day, week, and time-zone names case-insensitively. 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 A cache recipient MUST NOT allow local time zones to influence the calculation or comparison of an age or expiration time. o A cache recipient SHOULD consider a date with a zone abbreviation other than GMT or UTC to be invalid for calculating expiration. 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 Lifetime A cache can calculate the freshness lifetime (denoted as freshness_lifetime) of a response by using the first match of the following: o If the cache is shared and the s-maxage response directive (Section 18.104.22.168) is present, use its value, or o If the max-age response directive (Section 22.214.171.124) is present, use its value, or o If the Expires response header field (Section 5.3) is present, use its value minus the value of the Date response header field, or o Otherwise, no explicit expiration time is present in the response. A heuristic freshness lifetime might be applicable; see Section 4.2.2. 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 Freshness 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 by default (see Section 6.1 of [RFC7231]), and those responses without explicit freshness that have been marked as explicitly cacheable (e.g., with a "public" response directive). If the response has a Last-Modified header field (Section 2.2 of [RFC7232]), 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 Age 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 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 5.1), in a form appropriate for arithmetic operation; or 0, if not available. date_value The term "date_value" denotes the value of the Date header field, in a form appropriate for arithmetic operations. See Section 126.96.36.199 of [RFC7231] 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 host ought to use NTP ([RFC5905]) 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. A response's age can be calculated in two entirely independent ways: 1. the "apparent_age": response_time minus date_value, if the local clock is reasonably well synchronized to the origin server's clock. If the result is negative, the result is replaced by zero. 2. the "corrected_age_value", if all of the caches along the response path implement HTTP/1.1. A cache MUST interpret this value relative to the time the request was initiated, not the time that the response was received.
apparent_age = max(0, response_time - date_value); response_delay = response_time - request_time; corrected_age_value = age_value + response_delay; These are combined as corrected_initial_age = max(apparent_age, corrected_age_value); 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. Validation When a cache has one or more stored responses for a requested URI, but cannot serve any of them (e.g., because they are not fresh, or one cannot be selected; see Section 4.1), it can use the conditional request mechanism [RFC7232] 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 Request When 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 [RFC7232]), 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 [RFC7232]). 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 Request Each 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 [RFC7232]. 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 [RFC7232]) 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 matches 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 [RFC7232]) 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 [RFC7233], also needs to evaluate a received If-Range header field (Section 3.2 of [RFC7233]) with respect to its selected stored response. 4.3.3. Handling a Validation Response Cache handling of a response to a conditional request is dependent upon its status code: o A 304 (Not Modified) response status code indicates that the stored response can be updated and reused; see Section 4.3.4. 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 MUST 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, the cache MAY send a previously stored response (see Section 4.2.4). 4.3.4. Freshening Stored Responses upon Validation 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 the following: o If the new response contains a strong validator (see Section 2.1 of [RFC7232]), then that strong validator identifies the selected representation for update. All of the stored responses with the same strong validator are selected. If none of the stored responses contain the same strong validator, then the cache MUST NOT use the new response 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 for update. 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 for update. 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 5.5); 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. 4.3.5. Freshening Responses via HEAD 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 as 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: o delete any Warning header fields in the stored response with warn-code 1xx (see Section 5.5); o retain any Warning header fields in the stored response with warn-code 2xx; and, o use other header fields provided in the HEAD response to replace all instances of the corresponding header fields in the stored response and append new header fields to the stored response's header section unless otherwise restricted by the Cache-Control header field. 4.4. Invalidation Because unsafe request methods (Section 4.2.1 of [RFC7231]) 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 [RFC7230]) 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 [RFC7230]). This helps prevent denial-of-service attacks. A cache MUST invalidate the effective request URI (Section 5.5 of [RFC7230]) 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.