10.5 Server Error 5xx

   Response status codes beginning with the digit "5" indicate cases in
   which the server is aware that it has erred or is incapable of
   performing the request. Except when responding to a HEAD request, the
   server SHOULD include an entity containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition. User agents SHOULD display any included entity to the
   user. These response codes are applicable to any request method.

10.5.1 500 Internal Server Error

   The server encountered an unexpected condition which prevented it
   from fulfilling the request.

10.5.2 501 Not Implemented

   The server does not support the functionality required to fulfill the
   request. This is the appropriate response when the server does not
   recognize the request method and is not capable of supporting it for
   any resource.

10.5.3 502 Bad Gateway

   The server, while acting as a gateway or proxy, received an invalid
   response from the upstream server it accessed in attempting to
   fulfill the request.

10.5.4 503 Service Unavailable

   The server is currently unable to handle the request due to a
   temporary overloading or maintenance of the server. The implication
   is that this is a temporary condition which will be alleviated after
   some delay. If known, the length of the delay MAY be indicated in a
   Retry-After header. If no Retry-After is given, the client SHOULD
   handle the response as it would for a 500 response.

      Note: The existence of the 503 status code does not imply that a
      server must use it when becoming overloaded. Some servers may wish
      to simply refuse the connection.

10.5.5 504 Gateway Timeout

   The server, while acting as a gateway or proxy, did not receive a
   timely response from the upstream server specified by the URI (e.g.
   HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
   to access in attempting to complete the request.

      Note: Note to implementors: some deployed proxies are known to
      return 400 or 500 when DNS lookups time out.

10.5.6 505 HTTP Version Not Supported

   The server does not support, or refuses to support, the HTTP protocol
   version that was used in the request message. The server is
   indicating that it is unable or unwilling to complete the request
   using the same major version as the client, as described in section
   3.1, other than with this error message. The response SHOULD contain
   an entity describing why that version is not supported and what other
   protocols are supported by that server.

11 Access Authentication

   HTTP provides several OPTIONAL challenge-response authentication
   mechanisms which can be used by a server to challenge a client
   request and by a client to provide authentication information. The
   general framework for access authentication, and the specification of
   "basic" and "digest" authentication, are specified in "HTTP
   Authentication: Basic and Digest Access Authentication" [43]. This
   specification adopts the definitions of "challenge" and "credentials"
   from that specification.

12 Content Negotiation

   Most HTTP responses include an entity which contains information for
   interpretation by a human user. Naturally, it is desirable to supply
   the user with the "best available" entity corresponding to the
   request. Unfortunately for servers and caches, not all users have the
   same preferences for what is "best," and not all user agents are
   equally capable of rendering all entity types. For that reason, HTTP
   has provisions for several mechanisms for "content negotiation" --
   the process of selecting the best representation for a given response
   when there are multiple representations available.

      Note: This is not called "format negotiation" because the
      alternate representations may be of the same media type, but use
      different capabilities of that type, be in different languages,
      etc.

   Any response containing an entity-body MAY be subject to negotiation,
   including error responses.

   There are two kinds of content negotiation which are possible in
   HTTP: server-driven and agent-driven negotiation. These two kinds of
   negotiation are orthogonal and thus may be used separately or in
   combination. One method of combination, referred to as transparent
   negotiation, occurs when a cache uses the agent-driven negotiation
   information provided by the origin server in order to provide
   server-driven negotiation for subsequent requests.

12.1 Server-driven Negotiation

   If the selection of the best representation for a response is made by
   an algorithm located at the server, it is called server-driven
   negotiation. Selection is based on the available representations of
   the response (the dimensions over which it can vary; e.g. language,
   content-coding, etc.) and the contents of particular header fields in
   the request message or on other information pertaining to the request
   (such as the network address of the client).

   Server-driven negotiation is advantageous when the algorithm for
   selecting from among the available representations is difficult to
   describe to the user agent, or when the server desires to send its
   "best guess" to the client along with the first response (hoping to
   avoid the round-trip delay of a subsequent request if the "best
   guess" is good enough for the user). In order to improve the server's
   guess, the user agent MAY include request header fields (Accept,
   Accept-Language, Accept-Encoding, etc.) which describe its
   preferences for such a response.

   Server-driven negotiation has disadvantages:

      1. It is impossible for the server to accurately determine what
         might be "best" for any given user, since that would require
         complete knowledge of both the capabilities of the user agent
         and the intended use for the response (e.g., does the user want
         to view it on screen or print it on paper?).

      2. Having the user agent describe its capabilities in every
         request can be both very inefficient (given that only a small
         percentage of responses have multiple representations) and a
         potential violation of the user's privacy.

      3. It complicates the implementation of an origin server and the
         algorithms for generating responses to a request.

      4. It may limit a public cache's ability to use the same response
         for multiple user's requests.

   HTTP/1.1 includes the following request-header fields for enabling
   server-driven negotiation through description of user agent
   capabilities and user preferences: Accept (section 14.1), Accept-
   Charset (section 14.2), Accept-Encoding (section 14.3), Accept-
   Language (section 14.4), and User-Agent (section 14.43). However, an
   origin server is not limited to these dimensions and MAY vary the
   response based on any aspect of the request, including information
   outside the request-header fields or within extension header fields
   not defined by this specification.

   The Vary  header field can be used to express the parameters the
   server uses to select a representation that is subject to server-
   driven negotiation. See section 13.6 for use of the Vary header field
   by caches and section 14.44 for use of the Vary header field by
   servers.

12.2 Agent-driven Negotiation

   With agent-driven negotiation, selection of the best representation
   for a response is performed by the user agent after receiving an
   initial response from the origin server. Selection is based on a list
   of the available representations of the response included within the
   header fields or entity-body of the initial response, with each
   representation identified by its own URI. Selection from among the
   representations may be performed automatically (if the user agent is
   capable of doing so) or manually by the user selecting from a
   generated (possibly hypertext) menu.

   Agent-driven negotiation is advantageous when the response would vary
   over commonly-used dimensions (such as type, language, or encoding),
   when the origin server is unable to determine a user agent's
   capabilities from examining the request, and generally when public
   caches are used to distribute server load and reduce network usage.

   Agent-driven negotiation suffers from the disadvantage of needing a
   second request to obtain the best alternate representation. This
   second request is only efficient when caching is used. In addition,
   this specification does not define any mechanism for supporting
   automatic selection, though it also does not prevent any such
   mechanism from being developed as an extension and used within
   HTTP/1.1.

   HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)
   status codes for enabling agent-driven negotiation when the server is
   unwilling or unable to provide a varying response using server-driven
   negotiation.

12.3 Transparent Negotiation

   Transparent negotiation is a combination of both server-driven and
   agent-driven negotiation. When a cache is supplied with a form of the
   list of available representations of the response (as in agent-driven
   negotiation) and the dimensions of variance are completely understood
   by the cache, then the cache becomes capable of performing server-
   driven negotiation on behalf of the origin server for subsequent
   requests on that resource.

   Transparent negotiation has the advantage of distributing the
   negotiation work that would otherwise be required of the origin
   server and also removing the second request delay of agent-driven
   negotiation when the cache is able to correctly guess the right
   response.

   This specification does not define any mechanism for transparent
   negotiation, though it also does not prevent any such mechanism from
   being developed as an extension that could be used within HTTP/1.1.

13 Caching in HTTP

   HTTP is typically used for distributed information systems, where
   performance can be improved by the use of response caches. The
   HTTP/1.1 protocol includes a number of elements intended to make
   caching work as well as possible. Because these elements are
   inextricable from other aspects of the protocol, and because they
   interact with each other, it is useful to describe the basic caching
   design of HTTP separately from the detailed descriptions of methods,
   headers, response codes, etc.

   Caching would be useless if it did not significantly improve
   performance. The goal of caching in HTTP/1.1 is to eliminate the need
   to send requests in many cases, and to eliminate the need to send
   full responses in many other cases. The former reduces the number of
   network round-trips required for many operations; we use an
   "expiration" mechanism for this purpose (see section 13.2). The
   latter reduces network bandwidth requirements; we use a "validation"
   mechanism for this purpose (see section 13.3).

   Requirements for performance, availability, and disconnected
   operation require us to be able to relax the goal of semantic
   transparency. The HTTP/1.1 protocol allows origin servers, caches,

   and clients to explicitly reduce transparency when necessary.
   However, because non-transparent operation may confuse non-expert
   users, and might be incompatible with certain server applications
   (such as those for ordering merchandise), the protocol requires that
   transparency be relaxed

      - only by an explicit protocol-level request when relaxed by
        client or origin server

      - only with an explicit warning to the end user when relaxed by
        cache or client

   Therefore, the HTTP/1.1 protocol provides these important elements:

      1. Protocol features that provide full semantic transparency when
         this is required by all parties.

      2. Protocol features that allow an origin server or user agent to
         explicitly request and control non-transparent operation.

      3. Protocol features that allow a cache to attach warnings to
         responses that do not preserve the requested approximation of
         semantic transparency.

   A basic principle is that it must be possible for the clients to
   detect any potential relaxation of semantic transparency.

      Note: The server, cache, or client implementor might be faced with
      design decisions not explicitly discussed in this specification.
      If a decision might affect semantic transparency, the implementor
      ought to err on the side of maintaining transparency unless a
      careful and complete analysis shows significant benefits in
      breaking transparency.

13.1.1 Cache Correctness

   A correct cache MUST respond to a request with the most up-to-date
   response held by the cache that is appropriate to the request (see
   sections 13.2.5, 13.2.6, and 13.12) which meets one of the following
   conditions:

      1. It has been checked for equivalence with what the origin server
         would have returned by revalidating the response with the
         origin server (section 13.3);

      2. It is "fresh enough" (see section 13.2). In the default case,
         this means it meets the least restrictive freshness requirement
         of the client, origin server, and cache (see section 14.9); if
         the origin server so specifies, it is the freshness requirement
         of the origin server alone.

         If a stored response is not "fresh enough" by the most
         restrictive freshness requirement of both the client and the
         origin server, in carefully considered circumstances the cache
         MAY still return the response with the appropriate Warning
         header (see section 13.1.5 and 14.46), unless such a response
         is prohibited (e.g., by a "no-store" cache-directive, or by a
         "no-cache" cache-request-directive; see section 14.9).

      3. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect),
         or error (4xx or 5xx) response message.

   If the cache can not communicate with the origin server, then a
   correct cache SHOULD respond as above if the response can be
   correctly served from the cache; if not it MUST return an error or
   warning indicating that there was a communication failure.

   If a cache receives a response (either an entire response, or a 304
   (Not Modified) response) that it would normally forward to the
   requesting client, and the received response is no longer fresh, the
   cache SHOULD forward it to the requesting client without adding a new
   Warning (but without removing any existing Warning headers). A cache
   SHOULD NOT attempt to revalidate a response simply because that
   response became stale in transit; this might lead to an infinite
   loop. A user agent that receives a stale response without a Warning
   MAY display a warning indication to the user.

13.1.2 Warnings

   Whenever a cache returns a response that is neither first-hand nor
   "fresh enough" (in the sense of condition 2 in section 13.1.1), it
   MUST attach a warning to that effect, using a Warning general-header.
   The Warning header and the currently defined warnings are described
   in section 14.46. The warning allows clients to take appropriate
   action.

   Warnings MAY be used for other purposes, both cache-related and
   otherwise. The use of a warning, rather than an error status code,
   distinguish these responses from true failures.

   Warnings are assigned three digit warn-codes. The first digit
   indicates whether the Warning MUST or MUST NOT be deleted from a
   stored cache entry after a successful revalidation:

   1xx  Warnings that describe the freshness or revalidation status of
     the response, and so MUST be deleted after a successful
     revalidation. 1XX warn-codes MAY be generated by a cache only when
     validating a cached entry. It MUST NOT be generated by clients.

   2xx  Warnings that describe some aspect of the entity body or entity
     headers that is not rectified by a revalidation (for example, a
     lossy compression of the entity bodies) and which MUST NOT be
     deleted after a successful revalidation.

   See section 14.46 for the definitions of the codes themselves.

   HTTP/1.0 caches will cache all Warnings in responses, without
   deleting the ones in the first category. Warnings in responses that
   are passed to HTTP/1.0 caches carry an extra warning-date field,
   which prevents a future HTTP/1.1 recipient from believing an
   erroneously cached Warning.

   Warnings also carry a warning text. The text MAY be in any
   appropriate natural language (perhaps based on the client's Accept
   headers), and include an OPTIONAL indication of what character set is
   used.

   Multiple warnings MAY be attached to a response (either by the origin
   server or by a cache), including multiple warnings with the same code
   number. For example, a server might provide the same warning with
   texts in both English and Basque.

   When multiple warnings are attached to a response, it might not be
   practical or reasonable to display all of them to the user. This
   version of HTTP does not specify strict priority rules for deciding
   which warnings to display and in what order, but does suggest some
   heuristics.

13.1.3 Cache-control Mechanisms

   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
   times and validators) are implicit directives to caches. In some
   cases, a server or client might need to provide explicit directives
   to the HTTP caches. We use the Cache-Control header for this purpose.

   The Cache-Control header allows a client or server to transmit a
   variety of directives in either requests or responses. These
   directives typically override the default caching algorithms. As a
   general rule, if there is any apparent conflict between header
   values, the most restrictive interpretation is applied (that is, the
   one that is most likely to preserve semantic transparency). However,

   in some cases, cache-control directives are explicitly specified as
   weakening the approximation of semantic transparency (for example,
   "max-stale" or "public").

   The cache-control directives are described in detail in section 14.9.

13.1.4 Explicit User Agent Warnings

   Many user agents make it possible for users to override the basic
   caching mechanisms. For example, the user agent might allow the user
   to specify that cached entities (even explicitly stale ones) are
   never validated. Or the user agent might habitually add "Cache-
   Control: max-stale=3600" to every request. The user agent SHOULD NOT
   default to either non-transparent behavior, or behavior that results
   in abnormally ineffective caching, but MAY be explicitly configured
   to do so by an explicit action of the user.

   If the user has overridden the basic caching mechanisms, the user
   agent SHOULD explicitly indicate to the user whenever this results in
   the display of information that might not meet the server's
   transparency requirements (in particular, if the displayed entity is
   known to be stale). Since the protocol normally allows the user agent
   to determine if responses are stale or not, this indication need only
   be displayed when this actually happens. The indication need not be a
   dialog box; it could be an icon (for example, a picture of a rotting
   fish) or some other indicator.

   If the user has overridden the caching mechanisms in a way that would
   abnormally reduce the effectiveness of caches, the user agent SHOULD
   continually indicate this state to the user (for example, by a
   display of a picture of currency in flames) so that the user does not
   inadvertently consume excess resources or suffer from excessive
   latency.

13.1.5 Exceptions to the Rules and Warnings

   In some cases, the operator of a cache MAY choose to configure it to
   return stale responses even when not requested by clients. This
   decision ought not be made lightly, but may be necessary for reasons
   of availability or performance, especially when the cache is poorly
   connected to the origin server. Whenever a cache returns a stale
   response, it MUST mark it as such (using a Warning header) enabling
   the client software to alert the user that there might be a potential
   problem.

   It also allows the user agent to take steps to obtain a first-hand or
   fresh response. For this reason, a cache SHOULD NOT return a stale
   response if the client explicitly requests a first-hand or fresh one,
   unless it is impossible to comply for technical or policy reasons.

13.1.6 Client-controlled Behavior

   While the origin server (and to a lesser extent, intermediate caches,
   by their contribution to the age of a response) are the primary
   source of expiration information, in some cases the client might need
   to control a cache's decision about whether to return a cached
   response without validating it. Clients do this using several
   directives of the Cache-Control header.

   A client's request MAY specify the maximum age it is willing to
   accept of an unvalidated response; specifying a value of zero forces
   the cache(s) to revalidate all responses. A client MAY also specify
   the minimum time remaining before a response expires. Both of these
   options increase constraints on the behavior of caches, and so cannot
   further relax the cache's approximation of semantic transparency.

   A client MAY also specify that it will accept stale responses, up to
   some maximum amount of staleness. This loosens the constraints on the
   caches, and so might violate the origin server's specified
   constraints on semantic transparency, but might be necessary to
   support disconnected operation, or high availability in the face of
   poor connectivity.

13.2 Expiration Model

13.2.1 Server-Specified Expiration

   HTTP caching works best when caches can entirely avoid making
   requests to the origin server. The primary mechanism for avoiding
   requests is for an origin server to provide an explicit expiration
   time in the future, indicating that a response MAY be used to satisfy
   subsequent requests. In other words, a cache can return a fresh
   response without first contacting the server.

   Our expectation is that servers will assign future explicit
   expiration times to responses in the belief that the entity is not
   likely to change, in a semantically significant way, before the
   expiration time is reached. This normally preserves semantic
   transparency, as long as the server's expiration times are carefully
   chosen.

   The expiration mechanism applies only to responses taken from a cache
   and not to first-hand responses forwarded immediately to the
   requesting client.

   If an origin server wishes to force a semantically transparent cache
   to validate every request, it MAY assign an explicit expiration time
   in the past. This means that the response is always stale, and so the
   cache SHOULD validate it before using it for subsequent requests. See
   section 14.9.4 for a more restrictive way to force revalidation.

   If an origin server wishes to force any HTTP/1.1 cache, no matter how
   it is configured, to validate every request, it SHOULD use the "must-
   revalidate" cache-control directive (see section 14.9).

   Servers specify explicit expiration times using either the Expires
   header, or the max-age directive of the Cache-Control header.

   An expiration time cannot be used to force a user agent to refresh
   its display or reload a resource; its semantics apply only to caching
   mechanisms, and such mechanisms need only check a resource's
   expiration status when a new request for that resource is initiated.
   See section 13.13 for an explanation of the difference between caches
   and history mechanisms.

13.2.2 Heuristic Expiration

   Since origin servers do not always provide explicit expiration times,
   HTTP caches typically assign heuristic expiration times, employing
   algorithms that use other header values (such as the Last-Modified
   time) to estimate a plausible expiration time. The HTTP/1.1
   specification does not provide specific algorithms, but does impose
   worst-case constraints on their results. Since heuristic expiration
   times might compromise semantic transparency, they ought to used
   cautiously, and we encourage origin servers to provide explicit
   expiration times as much as possible.

13.2.3 Age Calculations

   In order to know if a cached entry is fresh, a cache needs to know if
   its age exceeds its freshness lifetime. We discuss how to calculate
   the latter in section 13.2.4; this section describes how to calculate
   the age of a response or cache entry.

   In this discussion, we use the term "now" to mean "the current value
   of the clock at the host performing the calculation." Hosts that use
   HTTP, but especially hosts running origin servers and caches, SHOULD
   use NTP [28] or some similar protocol to synchronize their clocks to
   a globally accurate time standard.

   HTTP/1.1 requires origin servers to send a Date header, if possible,
   with every response, giving the time at which the response was
   generated (see section 14.18). We use the term "date_value" to denote
   the value of the Date header, in a form appropriate for arithmetic
   operations.

   HTTP/1.1 uses the Age response-header to convey the estimated age of
   the response message when obtained from a cache. The Age field value
   is the cache's estimate of the amount of time since the response was
   generated or revalidated 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.

   We use the term "age_value" to denote the value of the Age header, in
   a form appropriate for arithmetic operations.

   A response's age can be calculated in two entirely independent ways:

      1. now 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. age_value, if all of the caches along the response path
         implement HTTP/1.1.

   Given that we have two independent ways to compute the age of a
   response when it is received, we can combine these as

       corrected_received_age = max(now - date_value, age_value)

   and as long as we have either nearly synchronized clocks or all-
   HTTP/1.1 paths, one gets a reliable (conservative) result.

   Because of network-imposed delays, some significant interval might
   pass between the time that a server generates a response and the time
   it is received at the next outbound cache or client. If uncorrected,
   this delay could result in improperly low ages.

   Because the request that resulted in the returned Age value must have
   been initiated prior to that Age value's generation, we can correct
   for delays imposed by the network by recording the time at which the
   request was initiated. Then, when an Age value is received, it MUST
   be interpreted relative to the time the request was initiated, not

   the time that the response was received. This algorithm results in
   conservative behavior no matter how much delay is experienced. So, we
   compute:

      corrected_initial_age = corrected_received_age
                            + (now - request_time)

   where "request_time" is the time (according to the local clock) when
   the request that elicited this response was sent.

   Summary of age calculation algorithm, when a cache receives a
   response:

      /*
       * age_value
       *      is the value of Age: header received by the cache with
       *              this response.
       * date_value
       *      is the value of the origin server's Date: header
       * request_time
       *      is the (local) time when the cache made the request
       *              that resulted in this cached response
       * response_time
       *      is the (local) time when the cache received the
       *              response
       * now
       *      is the current (local) time
       */

      apparent_age = max(0, response_time - date_value);
      corrected_received_age = max(apparent_age, age_value);
      response_delay = response_time - request_time;
      corrected_initial_age = corrected_received_age + response_delay;
      resident_time = now - response_time;
      current_age   = corrected_initial_age + resident_time;

   The current_age of a cache entry is calculated by adding the amount
   of time (in seconds) since the cache entry was last validated by the
   origin server to the corrected_initial_age. When a response is
   generated from a cache entry, the cache MUST include a single Age
   header field in the response with a value equal to the cache entry's
   current_age.

   The presence of an Age header field in a response implies that a
   response is not first-hand. However, the converse is not true, since
   the lack of an Age header field in a response does not imply that the

   response is first-hand unless all caches along the request path are
   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
   the Age header field).

13.2.4 Expiration Calculations

   In order to decide whether a response is fresh or stale, we need to
   compare its freshness lifetime to its age. The age is calculated as
   described in section 13.2.3; this section describes how to calculate
   the freshness lifetime, and to determine if a response has expired.
   In the discussion below, the values can be represented in any form
   appropriate for arithmetic operations.

   We use the term "expires_value" to denote the value of the Expires
   header. We use the term "max_age_value" to denote an appropriate
   value of the number of seconds carried by the "max-age" directive of
   the Cache-Control header in a response (see section 14.9.3).

   The max-age directive takes priority over Expires, so if max-age is
   present in a response, the calculation is simply:

      freshness_lifetime = max_age_value

   Otherwise, if Expires is present in the response, the calculation is:

      freshness_lifetime = expires_value - date_value

   Note that neither of these calculations is vulnerable to clock skew,
   since all of the information comes from the origin server.

   If none of Expires, Cache-Control: max-age, or Cache-Control: s-
   maxage (see section 14.9.3) appears in the response, and the response
   does not include other restrictions on caching, the cache MAY compute
   a freshness lifetime using a heuristic. The cache MUST attach Warning
   113 to any response whose age is more than 24 hours if such warning
   has not already been added.

   Also, if the response does have a Last-Modified time, the heuristic
   expiration value SHOULD be no more than some fraction of the interval
   since that time. A typical setting of this fraction might be 10%.

   The calculation to determine if a response has expired is quite
   simple:

      response_is_fresh = (freshness_lifetime > current_age)

13.2.5 Disambiguating Expiration Values

   Because expiration values are assigned optimistically, it is possible
   for two caches to contain fresh values for the same resource that are
   different.

   If a client performing a retrieval receives a non-first-hand response
   for a request that was already fresh in its own cache, and the Date
   header in its existing cache entry is newer than the Date on the new
   response, then the client MAY ignore the response. If so, it MAY
   retry the request with a "Cache-Control: max-age=0" directive (see
   section 14.9), to force a check with the origin server.

   If a cache has two fresh responses for the same representation with
   different validators, it MUST use the one with the more recent Date
   header. This situation might arise because the cache is pooling
   responses from other caches, or because a client has asked for a
   reload or a revalidation of an apparently fresh cache entry.

13.2.6 Disambiguating Multiple Responses

   Because a client might be receiving responses via multiple paths, so
   that some responses flow through one set of caches and other
   responses flow through a different set of caches, a client might
   receive responses in an order different from that in which the origin
   server sent them. We would like the client to use the most recently
   generated response, even if older responses are still apparently
   fresh.

   Neither the entity tag nor the expiration value can impose an
   ordering on responses, since it is possible that a later response
   intentionally carries an earlier expiration time. The Date values are
   ordered to a granularity of one second.

   When a client tries to revalidate a cache entry, and the response it
   receives contains a Date header that appears to be older than the one
   for the existing entry, then the client SHOULD repeat the request
   unconditionally, and include

       Cache-Control: max-age=0

   to force any intermediate caches to validate their copies directly
   with the origin server, or

       Cache-Control: no-cache

   to force any intermediate caches to obtain a new copy from the origin
   server.

   If the Date values are equal, then the client MAY use either response
   (or MAY, if it is being extremely prudent, request a new response).
   Servers MUST NOT depend on clients being able to choose
   deterministically between responses generated during the same second,
   if their expiration times overlap.

13.3 Validation Model

   When a cache has a stale entry that it would like to use as a
   response to a client's request, it first has to check with the origin
   server (or possibly an intermediate cache with a fresh response) to
   see if its cached entry is still usable. We call this "validating"
   the cache entry. Since we do not want to have to pay the overhead of
   retransmitting the full response if the cached entry is good, and we
   do not want to pay the overhead of an extra round trip if the cached
   entry is invalid, the HTTP/1.1 protocol supports the use of
   conditional methods.

   The key protocol features for supporting conditional methods are
   those concerned with "cache validators." When an origin server
   generates a full response, it attaches some sort of validator to it,
   which is kept with the cache entry. When a client (user agent or
   proxy cache) makes a conditional request for a resource for which it
   has a cache entry, it includes the associated validator in the
   request.

   The server then checks that validator against the current validator
   for the entity, and, if they match (see section 13.3.3), it responds
   with a special status code (usually, 304 (Not Modified)) and no
   entity-body. Otherwise, it returns a full response (including
   entity-body). Thus, we avoid transmitting the full response if the
   validator matches, and we avoid an extra round trip if it does not
   match.

   In HTTP/1.1, a conditional request looks exactly the same as a normal
   request for the same resource, except that it carries a special
   header (which includes the validator) that implicitly turns the
   method (usually, GET) into a conditional.

   The protocol includes both positive and negative senses of cache-
   validating conditions. That is, it is possible to request either that
   a method be performed if and only if a validator matches or if and
   only if no validators match.

      Note: a response that lacks a validator may still be cached, and
      served from cache until it expires, unless this is explicitly
      prohibited by a cache-control directive. However, a cache cannot
      do a conditional retrieval if it does not have a validator for the
      entity, which means it will not be refreshable after it expires.

13.3.1 Last-Modified Dates

   The Last-Modified entity-header field value is often used as a cache
   validator. In simple terms, a cache entry is considered to be valid
   if the entity has not been modified since the Last-Modified value.

13.3.2 Entity Tag Cache Validators

   The ETag response-header field value, an entity tag, provides for an
   "opaque" cache validator. This might allow more reliable validation
   in situations where it is inconvenient to store modification dates,
   where the one-second resolution of HTTP date values is not
   sufficient, or where the origin server wishes to avoid certain
   paradoxes that might arise from the use of modification dates.

   Entity Tags are described in section 3.11. The headers used with
   entity tags are described in sections 14.19, 14.24, 14.26 and 14.44.

13.3.3 Weak and Strong Validators

   Since both origin servers and caches will compare two validators to
   decide if they represent the same or different entities, one normally
   would expect that if the entity (the entity-body or any entity-
   headers) changes in any way, then the associated validator would
   change as well. If this is true, then we call this validator a
   "strong validator."

   However, there might be cases when a server prefers to change the
   validator only on semantically significant changes, and not when
   insignificant aspects of the entity change. A validator that does not
   always change when the resource changes is a "weak validator."

   Entity tags are normally "strong validators," but the protocol
   provides a mechanism to tag an entity tag as "weak." One can think of
   a strong validator as one that changes whenever the bits of an entity
   changes, while a weak value changes whenever the meaning of an entity
   changes. Alternatively, one can think of a strong validator as part
   of an identifier for a specific entity, while a weak validator is
   part of an identifier for a set of semantically equivalent entities.

      Note: One example of a strong validator is an integer that is
      incremented in stable storage every time an entity is changed.

      An entity's modification time, if represented with one-second
      resolution, could be a weak validator, since it is possible that
      the resource might be modified twice during a single second.

      Support for weak validators is optional. However, weak validators
      allow for more efficient caching of equivalent objects; for
      example, a hit counter on a site is probably good enough if it is
      updated every few days or weeks, and any value during that period
      is likely "good enough" to be equivalent.

   A "use" of a validator is either when a client generates a request
   and includes the validator in a validating header field, or when a
   server compares two validators.

   Strong validators are usable in any context. Weak validators are only
   usable in contexts that do not depend on exact equality of an entity.
   For example, either kind is usable for a conditional GET of a full
   entity. However, only a strong validator is usable for a sub-range
   retrieval, since otherwise the client might end up with an internally
   inconsistent entity.

   Clients MAY issue simple (non-subrange) GET requests with either weak
   validators or strong validators. Clients MUST NOT use weak validators
   in other forms of request.

   The only function that the HTTP/1.1 protocol defines on validators is
   comparison. There are two validator comparison functions, depending
   on whether the comparison context allows the use of weak validators
   or not:

      - The strong comparison function: in order to be considered equal,
        both validators MUST be identical in every way, and both MUST
        NOT be weak.

      - The weak comparison function: in order to be considered equal,
        both validators MUST be identical in every way, but either or
        both of them MAY be tagged as "weak" without affecting the
        result.

   An entity tag is strong unless it is explicitly tagged as weak.
   Section 3.11 gives the syntax for entity tags.

   A Last-Modified time, when used as a validator in a request, is
   implicitly weak unless it is possible to deduce that it is strong,
   using the following rules:

      - The validator is being compared by an origin server to the
        actual current validator for the entity and,

      - That origin server reliably knows that the associated entity did
        not change twice during the second covered by the presented
        validator.

   or

      - The validator is about to be used by a client in an If-
        Modified-Since or If-Unmodified-Since header, because the client
        has a cache entry for the associated entity, and

      - That cache entry includes a Date value, which gives the time
        when the origin server sent the original response, and

      - The presented Last-Modified time is at least 60 seconds before
        the Date value.

   or

      - The validator is being compared by an intermediate cache to the
        validator stored in its cache entry for the entity, and

      - That cache entry includes a Date value, which gives the time
        when the origin server sent the original response, and

      - The presented Last-Modified time is at least 60 seconds before
        the Date value.

   This method relies on the fact that if two different responses were
   sent by the origin server during the same second, but both had the
   same Last-Modified time, then at least one of those responses would
   have a Date value equal to its Last-Modified time. The arbitrary 60-
   second limit guards against the possibility that the Date and Last-
   Modified values are generated from different clocks, or at somewhat
   different times during the preparation of the response. An
   implementation MAY use a value larger than 60 seconds, if it is
   believed that 60 seconds is too short.

   If a client wishes to perform a sub-range retrieval on a value for
   which it has only a Last-Modified time and no opaque validator, it
   MAY do this only if the Last-Modified time is strong in the sense
   described here.

   A cache or origin server receiving a conditional request, other than
   a full-body GET request, MUST use the strong comparison function to
   evaluate the condition.

   These rules allow HTTP/1.1 caches and clients to safely perform sub-
   range retrievals on values that have been obtained from HTTP/1.0

   servers.

13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates

   We adopt a set of rules and recommendations for origin servers,
   clients, and caches regarding when various validator types ought to
   be used, and for what purposes.

   HTTP/1.1 origin servers:

      - SHOULD send an entity tag validator unless it is not feasible to
        generate one.

      - MAY send a weak entity tag instead of a strong entity tag, if
        performance considerations support the use of weak entity tags,
        or if it is unfeasible to send a strong entity tag.

      - SHOULD send a Last-Modified value if it is feasible to send one,
        unless the risk of a breakdown in semantic transparency that
        could result from using this date in an If-Modified-Since header
        would lead to serious problems.

   In other words, the preferred behavior for an HTTP/1.1 origin server
   is to send both a strong entity tag and a Last-Modified value.

   In order to be legal, a strong entity tag MUST change whenever the
   associated entity value changes in any way. A weak entity tag SHOULD
   change whenever the associated entity changes in a semantically
   significant way.

      Note: in order to provide semantically transparent caching, an
      origin server must avoid reusing a specific strong entity tag
      value for two different entities, or reusing a specific weak
      entity tag value for two semantically different entities. Cache
      entries might persist for arbitrarily long periods, regardless of
      expiration times, so it might be inappropriate to expect that a
      cache will never again attempt to validate an entry using a
      validator that it obtained at some point in the past.

   HTTP/1.1 clients:

      - If an entity tag has been provided by the origin server, MUST
        use that entity tag in any cache-conditional request (using If-
        Match or If-None-Match).

      - If only a Last-Modified value has been provided by the origin
        server, SHOULD use that value in non-subrange cache-conditional
        requests (using If-Modified-Since).

      - If only a Last-Modified value has been provided by an HTTP/1.0
        origin server, MAY use that value in subrange cache-conditional
        requests (using If-Unmodified-Since:). The user agent SHOULD
        provide a way to disable this, in case of difficulty.

      - If both an entity tag and a Last-Modified value have been
        provided by the origin server, SHOULD use both validators in
        cache-conditional requests. This allows both HTTP/1.0 and
        HTTP/1.1 caches to respond appropriately.

   An HTTP/1.1 origin server, upon receiving a conditional request that
   includes both a Last-Modified date (e.g., in an If-Modified-Since or
   If-Unmodified-Since header field) and one or more entity tags (e.g.,
   in an If-Match, If-None-Match, or If-Range header field) as cache
   validators, MUST NOT return a response status of 304 (Not Modified)
   unless doing so is consistent with all of the conditional header
   fields in the request.

   An HTTP/1.1 caching proxy, upon receiving a conditional request that
   includes both a Last-Modified date and one or more entity tags as
   cache validators, MUST NOT return a locally cached response to the
   client unless that cached response is consistent with all of the
   conditional header fields in the request.

      Note: The general principle behind these rules is that HTTP/1.1
      servers and clients should transmit as much non-redundant
      information as is available in their responses and requests.
      HTTP/1.1 systems receiving this information will make the most
      conservative assumptions about the validators they receive.

      HTTP/1.0 clients and caches will ignore entity tags. Generally,
      last-modified values received or used by these systems will
      support transparent and efficient caching, and so HTTP/1.1 origin
      servers should provide Last-Modified values. In those rare cases
      where the use of a Last-Modified value as a validator by an
      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
      origin servers should not provide one.

13.3.5 Non-validating Conditionals

   The principle behind entity tags is that only the service author
   knows the semantics of a resource well enough to select an
   appropriate cache validation mechanism, and the specification of any
   validator comparison function more complex than byte-equality would
   open up a can of worms. Thus, comparisons of any other headers
   (except Last-Modified, for compatibility with HTTP/1.0) are never
   used for purposes of validating a cache entry.

13.4 Response Cacheability

   Unless specifically constrained by a cache-control (section 14.9)
   directive, a caching system MAY always store a successful response
   (see section 13.8) as a cache entry, MAY return it without validation
   if it is fresh, and MAY return it after successful validation. If
   there is neither a cache validator nor an explicit expiration time
   associated with a response, we do not expect it to be cached, but
   certain caches MAY violate this expectation (for example, when little
   or no network connectivity is available). A client can usually detect
   that such a response was taken from a cache by comparing the Date
   header to the current time.

      Note: some HTTP/1.0 caches are known to violate this expectation
      without providing any Warning.

   However, in some cases it might be inappropriate for a cache to
   retain an entity, or to return it in response to a subsequent
   request. This might be because absolute semantic transparency is
   deemed necessary by the service author, or because of security or
   privacy considerations. Certain cache-control directives are
   therefore provided so that the server can indicate that certain
   resource entities, or portions thereof, are not to be cached
   regardless of other considerations.

   Note that section 14.8 normally prevents a shared cache from saving
   and returning a response to a previous request if that request
   included an Authorization header.

   A response received with a status code of 200, 203, 206, 300, 301 or
   410 MAY be stored by a cache and used in reply to a subsequent
   request, subject to the expiration mechanism, unless a cache-control
   directive prohibits caching. However, a cache that does not support
   the Range and Content-Range headers MUST NOT cache 206 (Partial
   Content) responses.

   A response received with any other status code (e.g. status codes 302
   and 307) MUST NOT be returned in a reply to a subsequent request
   unless there are cache-control directives or another header(s) that
   explicitly allow it. For example, these include the following: an
   Expires header (section 14.21); a "max-age", "s-maxage",  "must-
   revalidate", "proxy-revalidate", "public" or "private" cache-control
   directive (section 14.9).

13.5 Constructing Responses From Caches

   The purpose of an HTTP cache is to store information received in
   response to requests for use in responding to future requests. In
   many cases, a cache simply returns the appropriate parts of a
   response to the requester. However, if the cache holds a cache entry
   based on a previous response, it might have to combine parts of a new
   response with what is held in the cache entry.

13.5.1 End-to-end and Hop-by-hop Headers

   For the purpose of defining the behavior of caches and non-caching
   proxies, we divide HTTP headers into two categories:

      - End-to-end headers, which are  transmitted to the ultimate
        recipient of a request or response. End-to-end headers in
        responses MUST be stored as part of a cache entry and MUST be
        transmitted in any response formed from a cache entry.

      - Hop-by-hop headers, which are meaningful only for a single
        transport-level connection, and are not stored by caches or
        forwarded by proxies.

   The following HTTP/1.1 headers are hop-by-hop headers:

      - Connection
      - Keep-Alive
      - Proxy-Authenticate
      - Proxy-Authorization
      - TE
      - Trailers
      - Transfer-Encoding
      - Upgrade

   All other headers defined by HTTP/1.1 are end-to-end headers.

   Other hop-by-hop headers MUST be listed in a Connection header,
   (section 14.10) to be introduced into HTTP/1.1 (or later).

13.5.2 Non-modifiable Headers

   Some features of the HTTP/1.1 protocol, such as Digest
   Authentication, depend on the value of certain end-to-end headers. A
   transparent proxy SHOULD NOT modify an end-to-end header unless the
   definition of that header requires or specifically allows that.

   A transparent proxy MUST NOT modify any of the following fields in a
   request or response, and it MUST NOT add any of these fields if not
   already present:

      - Content-Location

      - Content-MD5

      - ETag

      - Last-Modified

   A transparent proxy MUST NOT modify any of the following fields in a
   response:

      - Expires

   but it MAY add any of these fields if not already present. If an
   Expires header is added, it MUST be given a field-value identical to
   that of the Date header in that response.

   A  proxy MUST NOT modify or add any of the following fields in a
   message that contains the no-transform cache-control directive, or in
   any request:

      - Content-Encoding

      - Content-Range

      - Content-Type

   A non-transparent proxy MAY modify or add these fields to a message
   that does not include no-transform, but if it does so, it MUST add a
   Warning 214 (Transformation applied) if one does not already appear
   in the message (see section 14.46).

      Warning: unnecessary modification of end-to-end headers might
      cause authentication failures if stronger authentication
      mechanisms are introduced in later versions of HTTP. Such
      authentication mechanisms MAY rely on the values of header fields
      not listed here.

   The Content-Length field of a request or response is added or deleted
   according to the rules in section 4.4. A transparent proxy MUST
   preserve the entity-length (section 7.2.2) of the entity-body,
   although it MAY change the transfer-length (section 4.4).

13.5.3 Combining Headers

   When a cache makes a validating request to a server, and the server
   provides a 304 (Not Modified) response or a 206 (Partial Content)
   response, the cache then constructs a response to send to the
   requesting client.

   If the status code is 304 (Not Modified), the cache uses the entity-
   body stored in the cache entry as the entity-body of this outgoing
   response. If the status code is 206 (Partial Content) and the ETag or
   Last-Modified headers match exactly, the cache MAY combine the
   contents stored in the cache entry with the new contents received in
   the response and use the result as the entity-body of this outgoing
   response, (see 13.5.4).

   The end-to-end headers stored in the cache entry are used for the
   constructed response, except that

      - any stored Warning headers with warn-code 1xx (see section
        14.46) MUST be deleted from the cache entry and the forwarded
        response.

      - any stored Warning headers with warn-code 2xx MUST be retained
        in the cache entry and the forwarded response.

      - any end-to-end headers provided in the 304 or 206 response MUST
        replace the corresponding headers from the cache entry.

   Unless the cache decides to remove the cache entry, it MUST also
   replace the end-to-end headers stored with the cache entry with
   corresponding headers received in the incoming response, except for
   Warning headers as described immediately above. If a header field-
   name in the incoming response matches more than one header in the
   cache entry, all such old headers MUST be replaced.

   In other words, the set of end-to-end headers received in the
   incoming response overrides all corresponding end-to-end headers
   stored with the cache entry (except for stored Warning headers with
   warn-code 1xx, which are deleted even if not overridden).

      Note: this rule allows an origin server to use a 304 (Not
      Modified) or a 206 (Partial Content) response to update any header
      associated with a previous response for the same entity or sub-
      ranges thereof, although it might not always be meaningful or
      correct to do so. This rule does not allow an origin server to use
      a 304 (Not Modified) or a 206 (Partial Content) response to
      entirely delete a header that it had provided with a previous
      response.

13.5.4 Combining Byte Ranges

   A response might transfer only a subrange of the bytes of an entity-
   body, either because the request included one or more Range
   specifications, or because a connection was broken prematurely. After
   several such transfers, a cache might have received several ranges of
   the same entity-body.

   If a cache has a stored non-empty set of subranges for an entity, and
   an incoming response transfers another subrange, the cache MAY
   combine the new subrange with the existing set if both the following
   conditions are met:

      - Both the incoming response and the cache entry have a cache
        validator.

      - The two cache validators match using the strong comparison
        function (see section 13.3.3).

   If either requirement is not met, the cache MUST use only the most
   recent partial response (based on the Date values transmitted with
   every response, and using the incoming response if these values are
   equal or missing), and MUST discard the other partial information.

13.6 Caching Negotiated Responses

   Use of server-driven content negotiation (section 12.1), as indicated
   by the presence of a Vary header field in a response, alters the
   conditions and procedure by which a cache can use the response for
   subsequent requests. See section 14.44 for use of the Vary header
   field by servers.

   A server SHOULD use the Vary header field to inform a cache of what
   request-header fields were used to select among multiple
   representations of a cacheable response subject to server-driven
   negotiation. The set of header fields named by the Vary field value
   is known as the "selecting" request-headers.

   When the cache receives a subsequent request whose Request-URI
   specifies one or more cache entries including a Vary header field,
   the cache MUST NOT use such a cache entry to construct a response to
   the new request unless all of the selecting request-headers present
   in the new request match the corresponding stored request-headers in
   the original request.

   The selecting request-headers from two requests are defined to match
   if and only if the selecting request-headers in the first request can
   be transformed to the selecting request-headers in the second request

   by adding or removing linear white space (LWS) at places where this
   is allowed by the corresponding BNF, and/or combining multiple
   message-header fields with the same field name following the rules
   about message headers in section 4.2.

   A Vary header field-value of "*" always fails to match and subsequent
   requests on that resource can only be properly interpreted by the
   origin server.

   If the selecting request header fields for the cached entry do not
   match the selecting request header fields of the new request, then
   the cache MUST NOT use a cached entry to satisfy the request unless
   it first relays the new request to the origin server in a conditional
   request and the server responds with 304 (Not Modified), including an
   entity tag or Content-Location that indicates the entity to be used.

   If an entity tag was assigned to a cached representation, the
   forwarded request SHOULD be conditional and include the entity tags
   in an If-None-Match header field from all its cache entries for the
   resource. This conveys to the server the set of entities currently
   held by the cache, so that if any one of these entities matches the
   requested entity, the server can use the ETag header field in its 304
   (Not Modified) response to tell the cache which entry is appropriate.
   If the entity-tag of the new response matches that of an existing
   entry, the new response SHOULD be used to update the header fields of
   the existing entry, and the result MUST be returned to the client.

   If any of the existing cache entries contains only partial content
   for the associated entity, its entity-tag SHOULD NOT be included in
   the If-None-Match header field unless the request is for a range that
   would be fully satisfied by that entry.

   If a cache receives a successful response whose Content-Location
   field matches that of an existing cache entry for the same Request-
   ]URI, whose entity-tag differs from that of the existing entry, and
   whose Date is more recent than that of the existing entry, the
   existing entry SHOULD NOT be returned in response to future requests
   and SHOULD be deleted from the cache.

13.7 Shared and Non-Shared Caches

   For reasons of security and privacy, it is necessary to make a
   distinction between "shared" and "non-shared" caches. A non-shared
   cache is one that is accessible only to a single user. Accessibility
   in this case SHOULD be enforced by appropriate security mechanisms.
   All other caches are considered to be "shared." Other sections of

   this specification place certain constraints on the operation of
   shared caches in order to prevent loss of privacy or failure of
   access controls.

13.8 Errors or Incomplete Response Cache Behavior

   A cache that receives an incomplete response (for example, with fewer
   bytes of data than specified in a Content-Length header) MAY store
   the response. However, the cache MUST treat this as a partial
   response. Partial responses MAY be combined as described in section
   13.5.4; the result might be a full response or might still be
   partial. A cache MUST NOT return a partial response to a client
   without explicitly marking it as such, using the 206 (Partial
   Content) status code. A cache MUST NOT return a partial response
   using a status code of 200 (OK).

   If a cache receives a 5xx response while attempting to revalidate an
   entry, it MAY either forward this response to the requesting client,
   or act as if the server failed to respond. In the latter case, it MAY
   return a previously received response unless the cached entry
   includes the "must-revalidate" cache-control directive (see section
   14.9).

13.9 Side Effects of GET and HEAD

   Unless the origin server explicitly prohibits the caching of their
   responses, the application of GET and HEAD methods to any resources
   SHOULD NOT have side effects that would lead to erroneous behavior if
   these responses are taken from a cache. They MAY still have side
   effects, but a cache is not required to consider such side effects in
   its caching decisions. Caches are always expected to observe an
   origin server's explicit restrictions on caching.

   We note one exception to this rule: since some applications have
   traditionally used GETs and HEADs with query URLs (those containing a
   "?" in the rel_path part) to perform operations with significant side
   effects, caches MUST NOT treat responses to such URIs as fresh unless
   the server provides an explicit expiration time. This specifically
   means that responses from HTTP/1.0 servers for such URIs SHOULD NOT
   be taken from a cache. See section 9.1.1 for related information.

13.10 Invalidation After Updates or Deletions

   The effect of certain methods performed on a resource at the origin
   server might cause one or more existing cache entries to become non-
   transparently invalid. That is, although they might continue to be
   "fresh," they do not accurately reflect what the origin server would
   return for a new request on that resource.

   There is no way for the HTTP protocol to guarantee that all such
   cache entries are marked invalid. For example, the request that
   caused the change at the origin server might not have gone through
   the proxy where a cache entry is stored. However, several rules help
   reduce the likelihood of erroneous behavior.

   In this section, the phrase "invalidate an entity" means that the
   cache will either remove all instances of that entity from its
   storage, or will mark these as "invalid" and in need of a mandatory
   revalidation before they can be returned in response to a subsequent
   request.

   Some HTTP methods MUST cause a cache to invalidate an entity. This is
   either the entity referred to by the Request-URI, or by the Location
   or Content-Location headers (if present). These methods are:

      - PUT

      - DELETE

      - POST

   In order to prevent denial of service attacks, an invalidation based
   on the URI in a Location or Content-Location header MUST only be
   performed if the host part is the same as in the Request-URI.

   A cache that passes through requests for methods it does not
   understand SHOULD invalidate any entities referred to by the
   Request-URI.

13.11 Write-Through Mandatory

   All methods that might be expected to cause modifications to the
   origin server's resources MUST be written through to the origin
   server. This currently includes all methods except for GET and HEAD.
   A cache MUST NOT reply to such a request from a client before having
   transmitted the request to the inbound server, and having received a
   corresponding response from the inbound server. This does not prevent
   a proxy cache from sending a 100 (Continue) response before the
   inbound server has sent its final reply.

   The alternative (known as "write-back" or "copy-back" caching) is not
   allowed in HTTP/1.1, due to the difficulty of providing consistent
   updates and the problems arising from server, cache, or network
   failure prior to write-back.

13.12 Cache Replacement

   If a new cacheable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8)
   response is received from a resource while any existing responses for
   the same resource are cached, the cache SHOULD use the new response
   to reply to the current request. It MAY insert it into cache storage
   and MAY, if it meets all other requirements, use it to respond to any
   future requests that would previously have caused the old response to
   be returned. If it inserts the new response into cache storage  the
   rules in section 13.5.3 apply.

      Note: a new response that has an older Date header value than
      existing cached responses is not cacheable.

13.13 History Lists

   User agents often have history mechanisms, such as "Back" buttons and
   history lists, which can be used to redisplay an entity retrieved
   earlier in a session.

   History mechanisms and caches are different. In particular history
   mechanisms SHOULD NOT try to show a semantically transparent view of
   the current state of a resource. Rather, a history mechanism is meant
   to show exactly what the user saw at the time when the resource was
   retrieved.

   By default, an expiration time does not apply to history mechanisms.
   If the entity is still in storage, a history mechanism SHOULD display
   it even if the entity has expired, unless the user has specifically
   configured the agent to refresh expired history documents.

   This is not to be construed to prohibit the history mechanism from
   telling the user that a view might be stale.

      Note: if history list mechanisms unnecessarily prevent users from
      viewing stale resources, this will tend to force service authors
      to avoid using HTTP expiration controls and cache controls when
      they would otherwise like to. Service authors may consider it
      important that users not be presented with error messages or
      warning messages when they use navigation controls (such as BACK)
      to view previously fetched resources. Even though sometimes such
      resources ought not to cached, or ought to expire quickly, user
      interface considerations may force service authors to resort to
      other means of preventing caching (e.g. "once-only" URLs) in order
      not to suffer the effects of improperly functioning history
      mechanisms.