RFC 7234: Caching

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](about:blank#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](about:blank#section-4.3)) or if the origin is    unavailable ([Section 4.2.4](about:blank#section-4.2.4)).  

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](https://tools.ietf.org/html/rfc2119)].     Conformance criteria and considerations regarding error handling are    defined in [Section 2.5 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-2.5).  

This specification uses the Augmented Backus-Naur Form (ABNF)    notation of [[RFC5234](https://tools.ietf.org/html/rfc5234)] with a list extension, defined in [Section 7 of    [RFC7230]](https://tools.ietf.org/html/rfc7230#section-7), that allows for compact definition of comma-separated    lists using a '#' operator (similar to how the '\*' operator indicates      repetition).  [Appendix B](about:blank#appendix-B) describes rules imported from other    documents.  [Appendix C](about:blank#appendix-C) shows the collected grammar with all list    operators expanded to standard ABNF notation.  

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.  

Proper cache operation preserves the semantics of HTTP transfers    ([[RFC7231](https://tools.ietf.org/html/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]](https://tools.ietf.org/html/rfc7231#section-4.3.1)).  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](about:blank#section-4.1)).  

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](about:blank#section-5.2)) does not appear       in request or response header fields, and     o  the "private" response directive (see [Section 5.2.2.6](about:blank#section-5.2.2.6)) does not       appear in the response, if the cache is shared, and     o  the Authorization header field (see [Section 4.2 of [RFC7235]](https://tools.ietf.org/html/rfc7235#section-4.2)) does       not appear in the request, if the cache is shared, unless the       response explicitly allows it (see [Section 3.2](about:blank#section-3.2)), and     o  the response either:        \*  contains an Expires header field (see [Section 5.3](about:blank#section-5.3)), or        \*  contains a max-age response directive (see [Section 5.2.2.8](about:blank#section-5.2.2.8)), or        \*  contains a s-maxage response directive (see [Section 5.2.2.9](about:blank#section-5.2.2.9))          and the cache is shared, or        \*  contains a Cache Control Extension (see [Section 5.2.3](about:blank#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](about:blank#section-4.2.2)), or         \*  contains a public response directive (see [Section 5.2.2.5](about:blank#section-5.2.2.5)).     Note that any of the requirements listed above can be overridden by a    cache-control extension; see [Section 5.2.3](about:blank#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.  

A response message is considered complete when all of the octets    indicated by the message framing ([[RFC7230](https://tools.ietf.org/html/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](https://tools.ietf.org/html/rfc7233)]) and combining the successful    response with the stored entry, as defined in [Section 3.3](about:blank#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.  

A shared cache MUST NOT use a cached response to a request with an    Authorization header field ([Section 4.2 of [RFC7235]](https://tools.ietf.org/html/rfc7235#section-4.2)) 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](about:blank#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](about:blank#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.  

A response might transfer only a partial representation if the    connection closed prematurely or if the request used one or more    Range specifiers ([[RFC7233](https://tools.ietf.org/html/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]](https://tools.ietf.org/html/rfc7233#section-4.3).     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](about:blank#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.  

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]](https://tools.ietf.org/html/rfc7230#section-5.5)) 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](about:blank#section-4.1)), and      o  the presented request does not contain the no-cache pragma       ([Section 5.4](about:blank#section-5.4)), nor the no-cache cache directive ([Section 5.2.1](about:blank#section-5.2.1)),       unless the stored response is successfully validated       ([Section 4.3](about:blank#section-4.3)), and     o  the stored response does not contain the no-cache cache directive       ([Section 5.2.2.2](about:blank#section-5.2.2.2)), unless it is successfully validated       ([Section 4.3](about:blank#section-4.3)), and     o  the stored response is either:        \*  fresh (see [Section 4.2](about:blank#section-4.2)), or        \*  allowed to be served stale (see [Section 4.2.4](about:blank#section-4.2.4)), or        \*  successfully validated (see [Section 4.3](about:blank#section-4.3)).     Note that any of the requirements listed above can be overridden by a    cache-control extension; see [Section 5.2.3](about:blank#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](about:blank#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](about:blank#section-4.2.3).     A cache MUST write through requests with methods that are unsafe    ([Section 4.2.1 of [RFC7231]](https://tools.ietf.org/html/rfc7231#section-4.2.1)) 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](about:blank#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.  

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]](https://tools.ietf.org/html/rfc7231#section-7.1.4)),    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]](https://tools.ietf.org/html/rfc7230#section-3.2))     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](about:blank#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](about:blank#section-4.3)) request.   

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](about:blank#section-5.3)) or the max-age response    directive ([Section 5.2.2.8](about:blank#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](about:blank#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](about:blank#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](about:blank#section-4.2.1); current\_age is    defined in [Section 4.2.3](about:blank#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](about:blank#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](about:blank#section-6) for an explanation of the difference between    caches and history mechanisms.  

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 5.2.2.9](about:blank#section-5.2.2.9)) is present, use its value, or     o  If the max-age response directive ([Section 5.2.2.8](about:blank#section-5.2.2.8)) is present,       use its value, or     o  If the Expires response header field ([Section 5.3](about:blank#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](about:blank#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.  

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](about:blank#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](https://tools.ietf.org/html/rfc7231#section-6.1) [6.1 of [RFC7231]](https://tools.ietf.org/html/rfc7231#section-6.1)), 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]](https://tools.ietf.org/html/rfc7232#section-2.2)), 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](about:blank#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]](https://tools.ietf.org/html/rfc2616#section-13.9) 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.  

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](about:blank#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](https://tools.ietf.org/html/rfc7231#section-7.1.1.2) [7.1.1.2 of [RFC7231]](https://tools.ietf.org/html/rfc7231#section-7.1.1.2) 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](https://tools.ietf.org/html/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;  

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](about:blank#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](about:blank#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](about:blank#section-5.2.1)).     A cache SHOULD generate a Warning header field with the 110 warn-code    (see [Section 5.5.1](about:blank#section-5.5.1)) in stale responses.  Likewise, a cache SHOULD    generate a 112 warn-code (see [Section 5.5.3](about:blank#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.   

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](about:blank#section-4.1)), it can use the conditional    request mechanism [[RFC7232](https://tools.ietf.org/html/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.  

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]](https://tools.ietf.org/html/rfc7232#section-2.2)), 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]](https://tools.ietf.org/html/rfc7232#section-2.3)).  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).  

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]](https://tools.ietf.org/html/rfc7232#section-6).  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]](https://tools.ietf.org/html/rfc7232#section-3.2)) 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](about:blank#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]](https://tools.ietf.org/html/rfc7232#section-3.3))    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](https://tools.ietf.org/html/rfc7233)], also needs to evaluate a received If-Range    header field ([Section 3.2 of [RFC7233]](https://tools.ietf.org/html/rfc7233#section-3.2)) with respect to its selected    stored 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](about:blank#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](about:blank#section-4.2.4)).  

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]](https://tools.ietf.org/html/rfc7232#section-2.1)), 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](about:blank#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.  

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](about:blank#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](about:blank#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.  

Because unsafe request methods ([Section 4.2.1 of [RFC7231]](https://tools.ietf.org/html/rfc7231#section-4.2.1)) 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]](https://tools.ietf.org/html/rfc7230#section-5.5)) 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]](https://tools.ietf.org/html/rfc7230#section-5.5)).  This helps prevent denial-of-service attacks.     A cache MUST invalidate the effective request URI ([Section 5.5 of    [RFC7230]](https://tools.ietf.org/html/rfc7230#section-5.5)) 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.   

This section defines the syntax and semantics of HTTP/1.1 header    fields related to caching.  

The "Age" header field conveys the sender's estimate of the amount of    time since the response was generated or successfully validated at    the origin server.  Age values are calculated as specified in    [Section 4.2.3](about:blank#section-4.2.3).       Age = delta-seconds     The Age field-value is a non-negative integer, representing time in    seconds (see [Section 1.2.1](about:blank#section-1.2.1)).     The presence of an Age header field implies that the response was not    generated or validated by the origin server for this request.    However, lack of an Age header field does not imply the origin was    contacted, since the response might have been received from an    HTTP/1.0 cache that does not implement Age.  

The "Cache-Control" header field is used to specify directives for    caches along the request/response chain.  Such cache directives are    unidirectional in that the presence of a directive in a request does    not imply that the same directive is to be given in the response.     A cache MUST obey the requirements of the Cache-Control directives    defined in this section.  See [Section 5.2.3](about:blank#section-5.2.3) for information about how    Cache-Control directives defined elsewhere are handled.        Note: Some HTTP/1.0 caches might not implement Cache-Control.     A proxy, whether or not it implements a cache, MUST pass cache    directives through in forwarded messages, regardless of their    significance to that application, since the directives might be    applicable to all recipients along the request/response chain.  It is    not possible to target a directive to a specific cache.     Cache directives are identified by a token, to be compared    case-insensitively, and have an optional argument, that can use both    token and quoted-string syntax.  For the directives defined below    that define arguments, recipients ought to accept both forms, even if    one is documented to be preferred.  For any directive not defined by    this specification, a recipient MUST accept both forms.        Cache-Control   = 1#cache-directive       cache-directive = token [ "=" ( token / quoted-string ) ]     For the cache directives defined below, no argument is defined (nor    allowed) unless stated otherwise.  

Argument syntax:        delta-seconds (see [Section 1.2.1](about:blank#section-1.2.1))     The "max-age" request directive indicates that the client is    unwilling to accept a response whose age is greater than the    specified number of seconds.  Unless the max-stale request directive    is also present, the client is not willing to accept a stale    response.     This directive uses the token form of the argument syntax: e.g.,    'max-age=5' not 'max-age="5"'.  A sender SHOULD NOT generate the    quoted-string form.  

Argument syntax:        delta-seconds (see [Section 1.2.1](about:blank#section-1.2.1))     The "max-stale" request directive indicates that the client is    willing to accept a response that has exceeded its freshness    lifetime.  If max-stale is assigned a value, then the client is    willing to accept a response that has exceeded its freshness lifetime    by no more than the specified number of seconds.  If no value is    assigned to max-stale, then the client is willing to accept a stale    response of any age.     This directive uses the token form of the argument syntax: e.g.,    'max-stale=10' not 'max-stale="10"'.  A sender SHOULD NOT generate    the quoted-string form.  

Argument syntax:        delta-seconds (see [Section 1.2.1](about:blank#section-1.2.1))      The "min-fresh" request directive indicates that the client is    willing to accept a response whose freshness lifetime is no less than    its current age plus the specified time in seconds.  That is, the    client wants a response that will still be fresh for at least the    specified number of seconds.     This directive uses the token form of the argument syntax: e.g.,    'min-fresh=20' not 'min-fresh="20"'.  A sender SHOULD NOT generate    the quoted-string form.  

The "no-cache" request directive indicates that a cache MUST NOT use    a stored response to satisfy the request without successful    validation on the origin server.  

The "no-store" request directive indicates that a cache MUST NOT    store any part of either this request or any response to it.  This    directive applies to both private and shared caches.  "MUST NOT    store" in this context means that the cache MUST NOT intentionally    store the information in non-volatile storage, and MUST make a    best-effort attempt to remove the information from volatile storage    as promptly as possible after forwarding it.     This directive is NOT a reliable or sufficient mechanism for ensuring    privacy.  In particular, malicious or compromised caches might not    recognize or obey this directive, and communications networks might    be vulnerable to eavesdropping.     Note that if a request containing this directive is satisfied from a    cache, the no-store request directive does not apply to the already    stored response.  

The "no-transform" request directive indicates that an intermediary    (whether or not it implements a cache) MUST NOT transform the    payload, as defined in [Section 5.7.2 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-5.7.2).  

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.  

The "must-revalidate" response directive indicates that once it has    become stale, a cache MUST NOT use the response to satisfy subsequent    requests without successful validation on the origin server.     The must-revalidate directive is necessary to support reliable    operation for certain protocol features.  In all circumstances a    cache MUST obey the must-revalidate directive; in particular, if a    cache cannot reach the origin server for any reason, it MUST generate    a 504 (Gateway Timeout) response.     The must-revalidate directive ought to be used by servers if and only    if failure to validate a request on the representation could result    in incorrect operation, such as a silently unexecuted financial    transaction.  

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 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.     The field-names given are not limited to the set of header fields    defined by this specification.  Field names are case-insensitive.      This directive uses the quoted-string form of the argument syntax.  A    sender SHOULD NOT generate the token form (even if quoting appears    not to be needed for single-entry lists).     Note: Although it has been back-ported to many implementations, some    HTTP/1.0 caches will not recognize or obey this directive.  Also,    no-cache response directives with field-names are often handled by    caches as if an unqualified no-cache directive was received; i.e.,    the special handling for the qualified form is not widely    implemented.  

The "no-store" response directive indicates that a cache MUST NOT    store any part of either the immediate request or response.  This    directive applies to both private and shared caches.  "MUST NOT    store" in this context means that the cache MUST NOT intentionally    store the information in non-volatile storage, and MUST make a    best-effort attempt to remove the information from volatile storage    as promptly as possible after forwarding it.     This directive is NOT a reliable or sufficient mechanism for ensuring    privacy.  In particular, malicious or compromised caches might not    recognize or obey this directive, and communications networks might    be vulnerable to eavesdropping.  

The "no-transform" response directive indicates that an intermediary    (regardless of whether it implements a cache) MUST NOT transform the    payload, as defined in [Section 5.7.2 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-5.7.2).  

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 private cache.  (See [Section 3.2](about:blank#section-3.2) for    additional details related to the use of public in response to a    request containing Authorization, and [Section 3](about:blank#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 by default; see    [Section 4.2.2](about:blank#section-4.2.2).)  

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 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 header fields    defined by this specification.  Field names are case-insensitive.     This directive uses the quoted-string form of the argument syntax.  A    sender SHOULD NOT generate the token form (even if quoting appears    not to be needed for single-entry lists).     Note: This usage of the word "private" only controls where the    response can be stored; it cannot ensure the privacy of the message    content.  Also, private response directives with field-names are    often handled by caches as if an unqualified private directive was    received; i.e., the special handling for the qualified form is not    widely implemented.  

The "proxy-revalidate" response directive has the same meaning as the    must-revalidate response directive, except that it does not apply to    private caches.  

Argument syntax:        delta-seconds (see [Section 1.2.1](about:blank#section-1.2.1))     The "max-age" response directive indicates that the response is to be    considered stale after its age is greater than the specified number    of seconds.     This directive uses the token form of the argument syntax: e.g.,    'max-age=5' not 'max-age="5"'.  A sender SHOULD NOT generate the    quoted-string form.   

Argument syntax:        delta-seconds (see [Section 1.2.1](about:blank#section-1.2.1))     The "s-maxage" response directive indicates that, in shared caches,    the maximum age specified by this directive overrides the maximum age    specified by either the max-age directive or the Expires header    field.  The s-maxage directive also implies the semantics of the    proxy-revalidate response directive.     This directive uses the token form of the argument syntax: e.g.,    's-maxage=10' not 's-maxage="10"'.  A sender SHOULD NOT generate the    quoted-string form.  

The Cache-Control header field can be extended through the use of one    or more cache-extension tokens, each with an optional value.  A cache    MUST ignore unrecognized cache directives.     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 old directive are supplied, such that applications that do not    understand the new directive will default to the behavior specified    by the old directive, and those that understand the new directive    will recognize it as modifying the requirements associated with the    old directive.  In this way, extensions to the existing cache-control    directives can be made without breaking deployed caches.     For example, consider a hypothetical new response directive called    "community" that acts as a modifier to the private directive: in    addition to private caches, any cache that is shared only by members    of the named community is allowed to cache the response.  An origin    server wishing to allow the UCI community to use an otherwise private    response in their shared cache(s) could do so by including       Cache-Control: private, community="UCI"     A cache that recognizes such a community cache-extension could    broaden its behavior in accordance with that extension.  A cache that    does not recognize the community cache-extension would ignore it and    adhere to the private directive.   

The "Expires" header field gives the date/time after which the    response is considered stale.  See [Section 4.2](about:blank#section-4.2) 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 Expires value is an HTTP-date timestamp, as defined in [Section](https://tools.ietf.org/html/rfc7231#section-7.1.1.1) [7.1.1.1 of [RFC7231]](https://tools.ietf.org/html/rfc7231#section-7.1.1.1).       Expires = HTTP-date     For example       Expires: Thu, 01 Dec 1994 16:00:00 GMT     A cache recipient MUST interpret invalid date formats, especially the    value "0", as representing a time in the past (i.e., "already    expired").     If a response includes a Cache-Control field with the max-age    directive ([Section 5.2.2.8](about:blank#section-5.2.2.8)), a recipient MUST ignore the Expires    field.  Likewise, if a response includes the s-maxage directive    ([Section 5.2.2.9](about:blank#section-5.2.2.9)), 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.   

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    implementation-specified directives for recipients.  This    specification deprecates such extensions to improve interoperability.       Pragma           = 1#pragma-directive      pragma-directive = "no-cache" / extension-pragma      extension-pragma = token [ "=" ( token / quoted-string ) ]     When the Cache-Control header field is not present in a request,    caches MUST consider the no-cache request pragma-directive as having    the same effect as if "Cache-Control: no-cache" were present (see    [Section 5.2.1](about:blank#section-5.2.1)).     When sending a no-cache request, a client ought to include both the    pragma and cache-control directives, unless Cache-Control: no-cache    is purposefully omitted to target other Cache-Control response    directives at HTTP/1.1 caches.  For example:       GET / HTTP/1.1      Host: www.example.com      Cache-Control: max-age=30      Pragma: no-cache     will constrain HTTP/1.1 caches to serve a response no older than 30    seconds, while precluding implementations that do not understand    Cache-Control from serving a cached response.        Note: Because the meaning of "Pragma: no-cache" in responses is       not specified, it does not provide a reliable replacement for       "Cache-Control: no-cache" in them.  

The "Warning" header field is used to carry additional information    about the status or transformation of a message that might not be    reflected in the status code.  This information is typically used to    warn about possible incorrectness introduced by caching operations or    transformations applied to the payload of the message.      Warnings can be used for other purposes, both cache-related and    otherwise.  The use of a warning, rather than an error status code,    distinguishes these responses from true failures.     Warning header fields can in general be applied to any message,    however some warn-codes are specific to caches and can only be    applied to response messages.       Warning       = 1#warning-value       warning-value = warn-code SP warn-agent SP warn-text                                            [ SP warn-date ]       warn-code  = 3DIGIT      warn-agent = ( uri-host [ ":" port ] ) / pseudonym                      ; the name or pseudonym of the server adding                      ; the Warning header field, for use in debugging                      ; a single "-" is recommended when agent unknown      warn-text  = quoted-string      warn-date  = DQUOTE HTTP-date DQUOTE     Multiple warnings can be generated in a response (either by the    origin server or by a cache), including multiple warnings with the    same warn-code number that only differ in warn-text.     A user agent that receives one or more Warning header fields SHOULD    inform the user of as many of them as possible, in the order that    they appear in the response.  Senders that generate multiple Warning    header fields are encouraged to order them with this user agent    behavior in mind.  A sender that generates new Warning header fields    MUST append them after any existing Warning header fields.     Warnings are assigned three digit warn-codes.  The first digit    indicates whether the Warning is required to be deleted from a stored    response after validation:     o  1xx warn-codes describe the freshness or validation status of the       response, and so they 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 warn-codes describe some aspect of the representation that is       not rectified by a validation (for example, a lossy compression of       the representation) and they MUST NOT be deleted by a cache after       validation, unless a full response is sent, in which case they       MUST be.      If a sender generates one or more 1xx warn-codes in a message to be    sent to a recipient known to implement only HTTP/1.0, the sender MUST    include in each corresponding warning-value a warn-date that matches    the Date header field in the message.  For example:       HTTP/1.1 200 OK      Date: Sat, 25 Aug 2012 23:34:45 GMT      Warning: 112 - "network down" "Sat, 25 Aug 2012 23:34:45 GMT"      Warnings have accompanying warn-text that describes the error, e.g.,    for logging.  It is advisory only, and its content does not affect    interpretation of the warn-code.     If a recipient that uses, evaluates, or displays Warning header    fields receives a warn-date that is different from the Date value in    the same message, the recipient MUST exclude the warning-value    containing that warn-date before storing, forwarding, or using the    message.  This allows recipients to exclude warning-values that were    improperly retained after a cache validation.  If all of the    warning-values are excluded, the recipient MUST exclude the Warning    header field 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.    The procedure for defining additional warn codes is described in    [Section 7.2.1](about:blank#section-7.2.1).  

A cache SHOULD generate this whenever the sent response is stale.  

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.  

A cache SHOULD generate this if it is intentionally disconnected from    the rest of the network for a period of time.  

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.   

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.  

This Warning code MUST be added by a proxy if it applies any    transformation to the representation, such as changing the    content-coding, media-type, or modifying the representation data,    unless this Warning code already appears in the response.  

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.  

User 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](about:blank#section-4.2)) does not necessarily apply to    history mechanisms.  That is, 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).  

The "Hypertext Transfer Protocol (HTTP) Cache Directive Registry"    defines the namespace for the cache directives.  It has been created    and is now maintained at    <[http://www.iana.org/assignments/http-cache-directives](http://www.iana.org/assignments/http-cache-directives)>.  

A registration MUST include the following fields:     o  Cache Directive Name      o  Pointer to specification text     Values to be added to this namespace require IETF Review (see    [[RFC5226], Section 4.1](https://tools.ietf.org/html/rfc5226#section-4.1)).  

New extension directives ought to consider defining:     o  What it means for a directive to be specified multiple times,     o  When the directive does not take an argument, what it means when       an argument is present,     o  When the directive requires an argument, what it means when it is       missing,     o  Whether the directive is specific to requests, responses, or able       to be used in either.     See also [Section 5.2.3](about:blank#section-5.2.3).  

The registry has been populated with the registrations below:     +------------------------+----------------------------------+    | Cache Directive        | Reference                        |    +------------------------+----------------------------------+    | max-age                | [Section 5.2.1.1](about:blank#section-5.2.1.1), [Section 5.2.2.8](about:blank#section-5.2.2.8) |    | max-stale              | [Section 5.2.1.2](about:blank#section-5.2.1.2)                  |    | min-fresh              | [Section 5.2.1.3](about:blank#section-5.2.1.3)                  |    | must-revalidate        | [Section 5.2.2.1](about:blank#section-5.2.2.1)                  |    | no-cache               | [Section 5.2.1.4](about:blank#section-5.2.1.4), [Section 5.2.2.2](about:blank#section-5.2.2.2) |    | no-store               | [Section 5.2.1.5](about:blank#section-5.2.1.5), [Section 5.2.2.3](about:blank#section-5.2.2.3) |    | no-transform           | [Section 5.2.1.6](about:blank#section-5.2.1.6), [Section 5.2.2.4](about:blank#section-5.2.2.4) |    | only-if-cached         | [Section 5.2.1.7](about:blank#section-5.2.1.7)                  |    | private                | [Section 5.2.2.6](about:blank#section-5.2.2.6)                  |    | proxy-revalidate       | [Section 5.2.2.7](about:blank#section-5.2.2.7)                  |    | public                 | [Section 5.2.2.5](about:blank#section-5.2.2.5)                  |    | s-maxage               | [Section 5.2.2.9](about:blank#section-5.2.2.9)                  |    | stale-if-error         | [[RFC5861], Section 4](https://tools.ietf.org/html/rfc5861#section-4)             |    | stale-while-revalidate | [[RFC5861], Section 3](https://tools.ietf.org/html/rfc5861#section-3)             |    +------------------------+----------------------------------+   

The "Hypertext Transfer Protocol (HTTP) Warn Codes" registry defines    the namespace for warn codes.  It has been created and is now    maintained at <[http://www.iana.org/assignments/http-warn-codes](http://www.iana.org/assignments/http-warn-codes)>.  

A registration MUST include the following fields:     o  Warn Code (3 digits)     o  Short Description     o  Pointer to specification text     Values to be added to this namespace require IETF Review (see    [[RFC5226], Section 4.1](https://tools.ietf.org/html/rfc5226#section-4.1)).  

The registry has been populated with the registrations below:     +-----------+----------------------------------+---------------+    | Warn Code | Short Description                | Reference     |    +-----------+----------------------------------+---------------+    | 110       | Response is Stale                | [Section 5.5.1](about:blank#section-5.5.1) |    | 111       | Revalidation Failed              | [Section 5.5.2](about:blank#section-5.5.2) |    | 112       | Disconnected Operation           | [Section 5.5.3](about:blank#section-5.5.3) |    | 113       | Heuristic Expiration             | [Section 5.5.4](about:blank#section-5.5.4) |    | 199       | Miscellaneous Warning            | [Section 5.5.5](about:blank#section-5.5.5) |    | 214       | Transformation Applied           | [Section 5.5.6](about:blank#section-5.5.6) |    | 299       | Miscellaneous Persistent Warning | [Section 5.5.7](about:blank#section-5.5.7) |    +-----------+----------------------------------+---------------+  

HTTP header fields are registered within the "Message Headers"    registry maintained at    <[http://www.iana.org/assignments/message-headers/](http://www.iana.org/assignments/message-headers/)>.      This document defines the following HTTP header fields, so the    "Permanent Message Header Field Names" registry has been updated    accordingly (see [[BCP90](about:blank#ref-BCP90)]).     +-------------------+----------+----------+-------------+    | Header Field Name | Protocol | Status   | Reference   |    +-------------------+----------+----------+-------------+    | Age               | http     | standard | [Section 5.1](about:blank#section-5.1) |    | Cache-Control     | http     | standard | [Section 5.2](about:blank#section-5.2) |    | Expires           | http     | standard | [Section 5.3](about:blank#section-5.3) |    | Pragma            | http     | standard | [Section 5.4](about:blank#section-5.4) |    | Warning           | http     | standard | [Section 5.5](about:blank#section-5.5) |    +-------------------+----------+----------+-------------+     The change controller is: "IETF (iesg@ietf.org) - Internet    Engineering Task Force".  

This section is meant to inform developers, information providers,    and users of known security concerns specific to HTTP caching.  More    general security considerations are addressed in HTTP messaging    [[RFC7230](https://tools.ietf.org/html/rfc7230)] and semantics [[RFC7231](https://tools.ietf.org/html/rfc7231)].     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.     In particular, various attacks might be amplified by being stored in    a shared cache; such "cache poisoning" attacks use the cache to    distribute a malicious payload to many clients, and are especially    effective when an attacker can use implementation flaws, elevated    privileges, or other techniques to insert such a response into a    cache.  One common attack vector for cache poisoning is to exploit    differences in message parsing on proxies and in user agents; see    [Section 3.3.3 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-3.3.3) for the relevant requirements.     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.      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.     Note that the Set-Cookie response header field [[RFC6265](https://tools.ietf.org/html/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.  

See [Section 10 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-10).  

[[RFC2119]()]  Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels", [BCP 14](https://tools.ietf.org/html/bcp14), [RFC 2119](https://tools.ietf.org/html/rfc2119), March 1997.     [[RFC5234]()]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax               Specifications: ABNF", STD 68, [RFC 5234](https://tools.ietf.org/html/rfc5234), January 2008.     [[RFC7230]()]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer               Protocol (HTTP/1.1): Message Syntax and Routing",               [RFC 7230](https://tools.ietf.org/html/rfc7230), June 2014.     [[RFC7231]()]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer               Protocol (HTTP/1.1): Semantics and Content", [RFC 7231](https://tools.ietf.org/html/rfc7231),               June 2014.     [[RFC7232]()]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer               Protocol (HTTP/1.1): Conditional Requests", [RFC 7232](https://tools.ietf.org/html/rfc7232),               June 2014.     [[RFC7233]()]  Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,               "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",               [RFC 7233](https://tools.ietf.org/html/rfc7233), June 2014.     [[RFC7235]()]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer               Protocol (HTTP/1.1): Authentication", [RFC 7235](https://tools.ietf.org/html/rfc7235), June 2014.   

[[BCP90]()]    Klyne, G., Nottingham, M., and J. Mogul, "Registration               Procedures for Message Header Fields", [BCP 90](https://tools.ietf.org/html/bcp90), [RFC 3864](https://tools.ietf.org/html/rfc3864),               September 2004.     [[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](https://tools.ietf.org/html/rfc2616), June 1999.     [[RFC5226]()]  Narten, T. and H. Alvestrand, "Guidelines for Writing an               IANA Considerations Section in RFCs", [BCP 26](https://tools.ietf.org/html/bcp26), [RFC 5226](https://tools.ietf.org/html/rfc5226),               May 2008.     [[RFC5861]()]  Nottingham, M., "HTTP Cache-Control Extensions for Stale               Content", [RFC 5861](https://tools.ietf.org/html/rfc5861), April 2010.     [[RFC5905]()]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,               "Network Time Protocol Version 4: Protocol and Algorithms               Specification", [RFC 5905](https://tools.ietf.org/html/rfc5905), June 2010.     [[RFC6265]()]  Barth, A., "HTTP State Management Mechanism", [RFC 6265](https://tools.ietf.org/html/rfc6265),               April 2011.   

The specification has been substantially rewritten for clarity.     The conditions under which an authenticated response can be cached    have been clarified.  ([Section 3.2](about:blank#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](about:blank#section-4.2.2))     The algorithm for calculating age is now less conservative.  Caches    are now required to handle dates with time zones as if they're    invalid, because it's not possible to accurately guess.    ([Section 4.2.3](about:blank#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](about:blank#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](about:blank#section-4.1))     Requirements regarding denial-of-service attack avoidance when    performing invalidation have been clarified.  ([Section 4.4](about:blank#section-4.4))     Cache invalidation only occurs when a successful response is    received.  ([Section 4.4](about:blank#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](about:blank#section-5.2))     The "no-store" 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](about:blank#section-5.2.1.5))     The qualified forms of the private and no-cache cache directives are    noted to not be widely implemented; for example, "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](about:blank#section-5.2.2))     The "no-cache" response directive's meaning has been clarified.    ([Section 5.2.2.2](about:blank#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](about:blank#section-5.3))     The Pragma header field is now only defined for backwards    compatibility; future pragmas are deprecated.  ([Section 5.4](about:blank#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](https://tools.ietf.org/html/rfc2047)    encoding, nor does it allow multiple languages, as these aspects were    not implemented.  ([Section 5.5](about:blank#section-5.5))     This specification introduces the Cache Directive and Warn Code    Registries, and defines considerations for new cache directives.    ([Section 7.1](about:blank#section-7.1) and [Section 7.2](about:blank#section-7.2))  

The following core rules are included by reference, as defined in    [Appendix B.1 of [RFC5234]](https://tools.ietf.org/html/rfc5234#appendix-B.1): 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 [[RFC7230](https://tools.ietf.org/html/rfc7230)]:       OWS           = <OWS, see [[RFC7230], Section 3.2.3](https://tools.ietf.org/html/rfc7230#section-3.2.3)>      field-name    = <field-name, see [[RFC7230], Section 3.2](https://tools.ietf.org/html/rfc7230#section-3.2)>      quoted-string = <quoted-string, see [[RFC7230], Section 3.2.6](https://tools.ietf.org/html/rfc7230#section-3.2.6)>      token         = <token, see [[RFC7230], Section 3.2.6](https://tools.ietf.org/html/rfc7230#section-3.2.6)>       port          = <port, see [[RFC7230], Section 2.7](https://tools.ietf.org/html/rfc7230#section-2.7)>      pseudonym     = <pseudonym, see [[RFC7230], Section 5.7.1](https://tools.ietf.org/html/rfc7230#section-5.7.1)>      uri-host      = <uri-host, see [[RFC7230], Section 2.7](https://tools.ietf.org/html/rfc7230#section-2.7)>     The rules below are defined in other parts:       HTTP-date     = <HTTP-date, see [[RFC7231], Section 7.1.1.1](https://tools.ietf.org/html/rfc7231#section-7.1.1.1)>   

In the collected ABNF below, list rules are expanded as per [Section](https://tools.ietf.org/html/rfc7230#section-1.2) [1.2 of [RFC7230]](https://tools.ietf.org/html/rfc7230#section-1.2).     Age = delta-seconds     Cache-Control = \*( "," OWS ) cache-directive \*( OWS "," [ OWS     cache-directive ] )     Expires = HTTP-date     HTTP-date = <HTTP-date, see [[RFC7231], Section 7.1.1.1](https://tools.ietf.org/html/rfc7231#section-7.1.1.1)>     OWS = <OWS, see [[RFC7230], Section 3.2.3](https://tools.ietf.org/html/rfc7230#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, see [[RFC7230], Section 3.2](https://tools.ietf.org/html/rfc7230#section-3.2)>     port = <port, see [[RFC7230], Section 2.7](https://tools.ietf.org/html/rfc7230#section-2.7)>    pragma-directive = "no-cache" / extension-pragma    pseudonym = <pseudonym, see [[RFC7230], Section 5.7.1](https://tools.ietf.org/html/rfc7230#section-5.7.1)>     quoted-string = <quoted-string, see [[RFC7230], Section 3.2.6](https://tools.ietf.org/html/rfc7230#section-3.2.6)>     token = <token, see [[RFC7230], Section 3.2.6](https://tools.ietf.org/html/rfc7230#section-3.2.6)>     uri-host = <uri-host, see [[RFC7230], Section 2.7](https://tools.ietf.org/html/rfc7230#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     ]   Index     1       110 (warn-code)  31       111 (warn-code)  31       112 (warn-code)  31       113 (warn-code)  31       199 (warn-code)  32     2       214 (warn-code)  32       299 (warn-code)  32     A       age  11       Age header field  21     C       cache  4       cache entry  5       cache key  5-6       Cache-Control header field  21     D       Disconnected Operation (warn-text)  31     E       Expires header field  28       explicit expiration time  11     F       fresh  11       freshness lifetime  11     G       Grammar          Age  21          Cache-Control  22          cache-directive  22          delta-seconds  5          Expires  28          extension-pragma  29          Pragma  29          pragma-directive  29          warn-agent  29          warn-code  29          warn-date  29          warn-text  29            Warning  29          warning-value  29     H       Heuristic Expiration (warn-text)  31       heuristic expiration time  11    M       max-age (cache directive)  22, 26       max-stale (cache directive)  22       min-fresh (cache directive)  22       Miscellaneous Persistent Warning (warn-text)  32       Miscellaneous Warning (warn-text)  32       must-revalidate (cache directive)  24     N       no-cache (cache directive)  23, 25       no-store (cache directive)  23, 24       no-transform (cache directive)  23, 25     O       only-if-cached (cache directive)  23     P       Pragma header field  29       private (cache directive)  25       private cache  4       proxy-revalidate (cache directive)  26       public (cache directive)  25     R       Response is Stale (warn-text)  30       Revalidation Failed (warn-text)  31     S       s-maxage (cache directive)  27       shared cache  4       stale  11       strong validator  18     T       Transformation Applied (warn-text)  32     V       validator  16     W       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    URI:   [http://roy.gbiv.com/](http://roy.gbiv.com/)      Mark Nottingham (editor)    Akamai     EMail: mnot@mnot.net    URI:   [http://www.mnot.net/](http://www.mnot.net/)      Julian F. Reschke (editor)    greenbytes GmbH    Hafenweg 16    Muenster, NW  48155    Germany     EMail: julian.reschke@greenbytes.de    URI:   [http://greenbytes.de/tech/webdav/](http://greenbytes.de/tech/webdav/)   Fielding, et al.             Standards Track                   [Page 43]

 © document authors. All rights reserved.