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RFC 2616


Hypertext Transfer Protocol -- HTTP/1.1

Part 5 of 7, p. 100 to 125
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14 Header Field Definitions

   This section defines the syntax and semantics of all standard
   HTTP/1.1 header fields. For entity-header fields, both sender and
   recipient refer to either the client or the server, depending on who
   sends and who receives the entity.

14.1 Accept

   The Accept request-header field can be used to specify certain media
   types which are acceptable for the response. Accept headers can be
   used to indicate that the request is specifically limited to a small
   set of desired types, as in the case of a request for an in-line

       Accept         = "Accept" ":"
                        #( media-range [ accept-params ] )

       media-range    = ( "*/*"
                        | ( type "/" "*" )
                        | ( type "/" subtype )
                        ) *( ";" parameter )
       accept-params  = ";" "q" "=" qvalue *( accept-extension )
       accept-extension = ";" token [ "=" ( token | quoted-string ) ]

   The asterisk "*" character is used to group media types into ranges,
   with "*/*" indicating all media types and "type/*" indicating all
   subtypes of that type. The media-range MAY include media type
   parameters that are applicable to that range.

   Each media-range MAY be followed by one or more accept-params,
   beginning with the "q" parameter for indicating a relative quality
   factor. The first "q" parameter (if any) separates the media-range
   parameter(s) from the accept-params. Quality factors allow the user
   or user agent to indicate the relative degree of preference for that
   media-range, using the qvalue scale from 0 to 1 (section 3.9). The
   default value is q=1.

      Note: Use of the "q" parameter name to separate media type
      parameters from Accept extension parameters is due to historical
      practice. Although this prevents any media type parameter named
      "q" from being used with a media range, such an event is believed
      to be unlikely given the lack of any "q" parameters in the IANA
      media type registry and the rare usage of any media type
      parameters in Accept. Future media types are discouraged from
      registering any parameter named "q".

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   The example

       Accept: audio/*; q=0.2, audio/basic

   SHOULD be interpreted as "I prefer audio/basic, but send me any audio
   type if it is the best available after an 80% mark-down in quality."

   If no Accept header field is present, then it is assumed that the
   client accepts all media types. If an Accept header field is present,
   and if the server cannot send a response which is acceptable
   according to the combined Accept field value, then the server SHOULD
   send a 406 (not acceptable) response.

   A more elaborate example is

       Accept: text/plain; q=0.5, text/html,
               text/x-dvi; q=0.8, text/x-c

   Verbally, this would be interpreted as "text/html and text/x-c are
   the preferred media types, but if they do not exist, then send the
   text/x-dvi entity, and if that does not exist, send the text/plain

   Media ranges can be overridden by more specific media ranges or
   specific media types. If more than one media range applies to a given
   type, the most specific reference has precedence. For example,

       Accept: text/*, text/html, text/html;level=1, */*

   have the following precedence:

       1) text/html;level=1
       2) text/html
       3) text/*
       4) */*

   The media type quality factor associated with a given type is
   determined by finding the media range with the highest precedence
   which matches that type. For example,

       Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
               text/html;level=2;q=0.4, */*;q=0.5

   would cause the following values to be associated:

       text/html;level=1         = 1
       text/html                 = 0.7
       text/plain                = 0.3

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       image/jpeg                = 0.5
       text/html;level=2         = 0.4
       text/html;level=3         = 0.7

      Note: A user agent might be provided with a default set of quality
      values for certain media ranges. However, unless the user agent is
      a closed system which cannot interact with other rendering agents,
      this default set ought to be configurable by the user.

14.2 Accept-Charset

   The Accept-Charset request-header field can be used to indicate what
   character sets are acceptable for the response. This field allows
   clients capable of understanding more comprehensive or special-
   purpose character sets to signal that capability to a server which is
   capable of representing documents in those character sets.

      Accept-Charset = "Accept-Charset" ":"
              1#( ( charset | "*" )[ ";" "q" "=" qvalue ] )

   Character set values are described in section 3.4. Each charset MAY
   be given an associated quality value which represents the user's
   preference for that charset. The default value is q=1. An example is

      Accept-Charset: iso-8859-5, unicode-1-1;q=0.8

   The special value "*", if present in the Accept-Charset field,
   matches every character set (including ISO-8859-1) which is not
   mentioned elsewhere in the Accept-Charset field. If no "*" is present
   in an Accept-Charset field, then all character sets not explicitly
   mentioned get a quality value of 0, except for ISO-8859-1, which gets
   a quality value of 1 if not explicitly mentioned.

   If no Accept-Charset header is present, the default is that any
   character set is acceptable. If an Accept-Charset header is present,
   and if the server cannot send a response which is acceptable
   according to the Accept-Charset header, then the server SHOULD send
   an error response with the 406 (not acceptable) status code, though
   the sending of an unacceptable response is also allowed.

14.3 Accept-Encoding

   The Accept-Encoding request-header field is similar to Accept, but
   restricts the content-codings (section 3.5) that are acceptable in
   the response.

       Accept-Encoding  = "Accept-Encoding" ":"

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                          1#( codings [ ";" "q" "=" qvalue ] )
       codings          = ( content-coding | "*" )

   Examples of its use are:

       Accept-Encoding: compress, gzip
       Accept-Encoding: *
       Accept-Encoding: compress;q=0.5, gzip;q=1.0
       Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0

   A server tests whether a content-coding is acceptable, according to
   an Accept-Encoding field, using these rules:

      1. If the content-coding is one of the content-codings listed in
         the Accept-Encoding field, then it is acceptable, unless it is
         accompanied by a qvalue of 0. (As defined in section 3.9, a
         qvalue of 0 means "not acceptable.")

      2. The special "*" symbol in an Accept-Encoding field matches any
         available content-coding not explicitly listed in the header

      3. If multiple content-codings are acceptable, then the acceptable
         content-coding with the highest non-zero qvalue is preferred.

      4. The "identity" content-coding is always acceptable, unless
         specifically refused because the Accept-Encoding field includes
         "identity;q=0", or because the field includes "*;q=0" and does
         not explicitly include the "identity" content-coding. If the
         Accept-Encoding field-value is empty, then only the "identity"
         encoding is acceptable.

   If an Accept-Encoding field is present in a request, and if the
   server cannot send a response which is acceptable according to the
   Accept-Encoding header, then the server SHOULD send an error response
   with the 406 (Not Acceptable) status code.

   If no Accept-Encoding field is present in a request, the server MAY
   assume that the client will accept any content coding. In this case,
   if "identity" is one of the available content-codings, then the
   server SHOULD use the "identity" content-coding, unless it has
   additional information that a different content-coding is meaningful
   to the client.

      Note: If the request does not include an Accept-Encoding field,
      and if the "identity" content-coding is unavailable, then
      content-codings commonly understood by HTTP/1.0 clients (i.e.,

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      "gzip" and "compress") are preferred; some older clients
      improperly display messages sent with other content-codings.  The
      server might also make this decision based on information about
      the particular user-agent or client.

      Note: Most HTTP/1.0 applications do not recognize or obey qvalues
      associated with content-codings. This means that qvalues will not
      work and are not permitted with x-gzip or x-compress.

14.4 Accept-Language

   The Accept-Language request-header field is similar to Accept, but
   restricts the set of natural languages that are preferred as a
   response to the request. Language tags are defined in section 3.10.

       Accept-Language = "Accept-Language" ":"
                         1#( language-range [ ";" "q" "=" qvalue ] )
       language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )

   Each language-range MAY be given an associated quality value which
   represents an estimate of the user's preference for the languages
   specified by that range. The quality value defaults to "q=1". For

       Accept-Language: da, en-gb;q=0.8, en;q=0.7

   would mean: "I prefer Danish, but will accept British English and
   other types of English." A language-range matches a language-tag if
   it exactly equals the tag, or if it exactly equals a prefix of the
   tag such that the first tag character following the prefix is "-".
   The special range "*", if present in the Accept-Language field,
   matches every tag not matched by any other range present in the
   Accept-Language field.

      Note: This use of a prefix matching rule does not imply that
      language tags are assigned to languages in such a way that it is
      always true that if a user understands a language with a certain
      tag, then this user will also understand all languages with tags
      for which this tag is a prefix. The prefix rule simply allows the
      use of prefix tags if this is the case.

   The language quality factor assigned to a language-tag by the
   Accept-Language field is the quality value of the longest language-
   range in the field that matches the language-tag. If no language-
   range in the field matches the tag, the language quality factor
   assigned is 0. If no Accept-Language header is present in the
   request, the server

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   SHOULD assume that all languages are equally acceptable. If an
   Accept-Language header is present, then all languages which are
   assigned a quality factor greater than 0 are acceptable.

   It might be contrary to the privacy expectations of the user to send
   an Accept-Language header with the complete linguistic preferences of
   the user in every request. For a discussion of this issue, see
   section 15.1.4.

   As intelligibility is highly dependent on the individual user, it is
   recommended that client applications make the choice of linguistic
   preference available to the user. If the choice is not made
   available, then the Accept-Language header field MUST NOT be given in
   the request.

      Note: When making the choice of linguistic preference available to
      the user, we remind implementors of  the fact that users are not
      familiar with the details of language matching as described above,
      and should provide appropriate guidance. As an example, users
      might assume that on selecting "en-gb", they will be served any
      kind of English document if British English is not available. A
      user agent might suggest in such a case to add "en" to get the
      best matching behavior.

14.5 Accept-Ranges

      The Accept-Ranges response-header field allows the server to
      indicate its acceptance of range requests for a resource:

          Accept-Ranges     = "Accept-Ranges" ":" acceptable-ranges
          acceptable-ranges = 1#range-unit | "none"

      Origin servers that accept byte-range requests MAY send

          Accept-Ranges: bytes

      but are not required to do so. Clients MAY generate byte-range
      requests without having received this header for the resource
      involved. Range units are defined in section 3.12.

      Servers that do not accept any kind of range request for a
      resource MAY send

          Accept-Ranges: none

      to advise the client not to attempt a range request.

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14.6 Age

      The Age response-header field conveys the sender's estimate of the
      amount of time since the response (or its revalidation) was
      generated at the origin server. A cached response is "fresh" if
      its age does not exceed its freshness lifetime. Age values are
      calculated as specified in section 13.2.3.

           Age = "Age" ":" age-value
           age-value = delta-seconds

      Age values are non-negative decimal integers, representing time in

      If a cache receives a value larger than the largest positive
      integer it can represent, or if any of its age calculations
      overflows, it MUST transmit an Age header with a value of
      2147483648 (2^31). An HTTP/1.1 server that includes a cache MUST
      include an Age header field in every response generated from its
      own cache. Caches SHOULD use an arithmetic type of at least 31
      bits of range.

14.7 Allow

      The Allow entity-header field lists the set of methods supported
      by the resource identified by the Request-URI. The purpose of this
      field is strictly to inform the recipient of valid methods
      associated with the resource. An Allow header field MUST be
      present in a 405 (Method Not Allowed) response.

          Allow   = "Allow" ":" #Method

      Example of use:

          Allow: GET, HEAD, PUT

      This field cannot prevent a client from trying other methods.
      However, the indications given by the Allow header field value
      SHOULD be followed. The actual set of allowed methods is defined
      by the origin server at the time of each request.

      The Allow header field MAY be provided with a PUT request to
      recommend the methods to be supported by the new or modified
      resource. The server is not required to support these methods and
      SHOULD include an Allow header in the response giving the actual
      supported methods.

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      A proxy MUST NOT modify the Allow header field even if it does not
      understand all the methods specified, since the user agent might
      have other means of communicating with the origin server.

14.8 Authorization

      A user agent that wishes to authenticate itself with a server--
      usually, but not necessarily, after receiving a 401 response--does
      so by including an Authorization request-header field with the
      request.  The Authorization field value consists of credentials
      containing the authentication information of the user agent for
      the realm of the resource being requested.

          Authorization  = "Authorization" ":" credentials

      HTTP access authentication is described in "HTTP Authentication:
      Basic and Digest Access Authentication" [43]. If a request is
      authenticated and a realm specified, the same credentials SHOULD
      be valid for all other requests within this realm (assuming that
      the authentication scheme itself does not require otherwise, such
      as credentials that vary according to a challenge value or using
      synchronized clocks).

      When a shared cache (see section 13.7) receives a request
      containing an Authorization field, it MUST NOT return the
      corresponding response as a reply to any other request, unless one
      of the following specific exceptions holds:

      1. If the response includes the "s-maxage" cache-control
         directive, the cache MAY use that response in replying to a
         subsequent request. But (if the specified maximum age has
         passed) a proxy cache MUST first revalidate it with the origin
         server, using the request-headers from the new request to allow
         the origin server to authenticate the new request. (This is the
         defined behavior for s-maxage.) If the response includes "s-
         maxage=0", the proxy MUST always revalidate it before re-using

      2. If the response includes the "must-revalidate" cache-control
         directive, the cache MAY use that response in replying to a
         subsequent request. But if the response is stale, all caches
         MUST first revalidate it with the origin server, using the
         request-headers from the new request to allow the origin server
         to authenticate the new request.

      3. If the response includes the "public" cache-control directive,
         it MAY be returned in reply to any subsequent request.

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14.9 Cache-Control

   The Cache-Control general-header field is used to specify directives
   that MUST be obeyed by all caching mechanisms along the
   request/response chain. The directives specify behavior intended to
   prevent caches from adversely interfering with the request or
   response. These directives typically override the default caching
   algorithms. 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.

      Note that HTTP/1.0 caches might not implement Cache-Control and
      might only implement Pragma: no-cache (see section 14.32).

   Cache directives MUST be passed through by a proxy or gateway
   application, 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 specify a cache-
   directive for a specific cache.

    Cache-Control   = "Cache-Control" ":" 1#cache-directive

    cache-directive = cache-request-directive
         | cache-response-directive

    cache-request-directive =
           "no-cache"                          ; Section 14.9.1
         | "no-store"                          ; Section 14.9.2
         | "max-age" "=" delta-seconds         ; Section 14.9.3, 14.9.4
         | "max-stale" [ "=" delta-seconds ]   ; Section 14.9.3
         | "min-fresh" "=" delta-seconds       ; Section 14.9.3
         | "no-transform"                      ; Section 14.9.5
         | "only-if-cached"                    ; Section 14.9.4
         | cache-extension                     ; Section 14.9.6

     cache-response-directive =
           "public"                               ; Section 14.9.1
         | "private" [ "=" <"> 1#field-name <"> ] ; Section 14.9.1
         | "no-cache" [ "=" <"> 1#field-name <"> ]; Section 14.9.1
         | "no-store"                             ; Section 14.9.2
         | "no-transform"                         ; Section 14.9.5
         | "must-revalidate"                      ; Section 14.9.4
         | "proxy-revalidate"                     ; Section 14.9.4
         | "max-age" "=" delta-seconds            ; Section 14.9.3
         | "s-maxage" "=" delta-seconds           ; Section 14.9.3
         | cache-extension                        ; Section 14.9.6

    cache-extension = token [ "=" ( token | quoted-string ) ]

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   When a directive appears without any 1#field-name parameter, the
   directive applies to the entire request or response. When such a
   directive appears with a 1#field-name parameter, it applies only to
   the named field or fields, and not to the rest of the request or
   response. This mechanism supports extensibility; implementations of
   future versions of the HTTP protocol might apply these directives to
   header fields not defined in HTTP/1.1.

   The cache-control directives can be broken down into these general

      - Restrictions on what are cacheable; these may only be imposed by
        the origin server.

      - Restrictions on what may be stored by a cache; these may be
        imposed by either the origin server or the user agent.

      - Modifications of the basic expiration mechanism; these may be
        imposed by either the origin server or the user agent.

      - Controls over cache revalidation and reload; these may only be
        imposed by a user agent.

      - Control over transformation of entities.

      - Extensions to the caching system.

14.9.1 What is Cacheable

   By default, a response is cacheable if the requirements of the
   request method, request header fields, and the response status
   indicate that it is cacheable. Section 13.4 summarizes these defaults
   for cacheability. The following Cache-Control response directives
   allow an origin server to override the default cacheability of a

      Indicates that the response MAY be cached by any cache, even if it
      would normally be non-cacheable or cacheable only within a non-
      shared cache. (See also Authorization, section 14.8, for
      additional details.)

      Indicates that all or part of the response message is intended for
      a single user and MUST NOT be cached by a shared cache. This
      allows an origin server to state that the specified parts of the

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      response are intended for only one user and are not a valid
      response for requests by other users. A private (non-shared) cache
      MAY cache the response.

       Note: This usage of the word private only controls where the
       response may be cached, and cannot ensure the privacy of the
       message content.

       If the no-cache directive does not specify a field-name, then a
      cache MUST NOT use the response to satisfy a subsequent request
      without successful revalidation with the origin server. This
      allows an origin server to prevent caching even by caches that
      have been configured to return stale responses to client requests.

      If the no-cache directive does specify 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, the
      specified field-name(s) 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.

       Note: Most HTTP/1.0 caches will not recognize or obey this

14.9.2 What May be Stored by Caches

      The purpose of the no-store directive is to prevent the
      inadvertent release or retention of sensitive information (for
      example, on backup tapes). The no-store directive applies to the
      entire message, and MAY be sent either in a response or in a
      request. If sent in a request, a cache MUST NOT store any part of
      either this request or any response to it. If sent in a response,
      a cache MUST NOT store any part of either this response or the
      request that elicited it. This directive applies to both non-
      shared 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.

      Even when this directive is associated with a response, users
      might explicitly store such a response outside of the caching
      system (e.g., with a "Save As" dialog). History buffers MAY store
      such responses as part of their normal operation.

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      The purpose of this directive is to meet the stated requirements
      of certain users and service authors who are concerned about
      accidental releases of information via unanticipated accesses to
      cache data structures. While the use of this directive might
      improve privacy in some cases, we caution that it is NOT in any
      way 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.

14.9.3 Modifications of the Basic Expiration Mechanism

   The expiration time of an entity MAY be specified by the origin
   server using the Expires header (see section 14.21). Alternatively,
   it MAY be specified using the max-age directive in a response. When
   the max-age cache-control directive is present in a cached response,
   the response is stale if its current age is greater than the age
   value given (in seconds) at the time of a new request for that
   resource. The max-age directive on a response implies that the
   response is cacheable (i.e., "public") unless some other, more
   restrictive cache directive is also present.

   If a response includes both an Expires header and a max-age
   directive, the max-age directive overrides the Expires header, even
   if the Expires header is more restrictive. This rule allows an origin
   server to provide, for a given response, a longer expiration time to
   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
   useful if certain HTTP/1.0 caches improperly calculate ages or
   expiration times, perhaps due to desynchronized clocks.

   Many HTTP/1.0 cache implementations will treat an Expires value that
   is less than or equal to the response Date value as being equivalent
   to the Cache-Control response directive "no-cache". If an HTTP/1.1
   cache receives such a response, and the response does not include a
   Cache-Control header field, it SHOULD consider the response to be
   non-cacheable in order to retain compatibility with HTTP/1.0 servers.

       Note: An origin server might wish to use a relatively new HTTP
       cache control feature, such as the "private" directive, on a
       network including older caches that do not understand that
       feature. The origin server will need to combine the new feature
       with an Expires field whose value is less than or equal to the
       Date value. This will prevent older caches from improperly
       caching the response.

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       If a response includes an s-maxage directive, then for a shared
       cache (but not for a private cache), the maximum age specified by
       this directive overrides the maximum age specified by either the
       max-age directive or the Expires header. The s-maxage directive
       also implies the semantics of the proxy-revalidate directive (see
       section 14.9.4), i.e., that the shared cache must not use the
       entry after it becomes stale to respond to a subsequent request
       without first revalidating it with the origin server. The s-
       maxage directive is always ignored by a private cache.

   Note that most older caches, not compliant with this specification,
   do not implement any cache-control directives. An origin server
   wishing to use a cache-control directive that restricts, but does not
   prevent, caching by an HTTP/1.1-compliant cache MAY exploit the
   requirement that the max-age directive overrides the Expires header,
   and the fact that pre-HTTP/1.1-compliant caches do not observe the
   max-age directive.

   Other directives allow a user agent to modify the basic expiration
   mechanism. These directives MAY be specified on a request:

      Indicates that the client is willing to accept a response whose
      age is no greater than the specified time in seconds. Unless max-
      stale directive is also included, the client is not willing to
      accept a stale response.

      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

      Indicates that the client is willing to accept a response that has
      exceeded its expiration time. If max-stale is assigned a value,
      then the client is willing to accept a response that has exceeded
      its expiration time 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.

   If a cache returns a stale response, either because of a max-stale
   directive on a request, or because the cache is configured to
   override the expiration time of a response, the cache MUST attach a
   Warning header to the stale response, using Warning 110 (Response is

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   A cache MAY be configured to return stale responses without
   validation, but only if this does not conflict with any "MUST"-level
   requirements concerning cache validation (e.g., a "must-revalidate"
   cache-control directive).

   If both the new request and the cached entry include "max-age"
   directives, then the lesser of the two values is used for determining
   the freshness of the cached entry for that request.

14.9.4 Cache Revalidation and Reload Controls

   Sometimes a user agent might want or need to insist that a cache
   revalidate its cache entry with the origin server (and not just with
   the next cache along the path to the origin server), or to reload its
   cache entry from the origin server. End-to-end revalidation might be
   necessary if either the cache or the origin server has overestimated
   the expiration time of the cached response. End-to-end reload may be
   necessary if the cache entry has become corrupted for some reason.

   End-to-end revalidation may be requested either when the client does
   not have its own local cached copy, in which case we call it
   "unspecified end-to-end revalidation", or when the client does have a
   local cached copy, in which case we call it "specific end-to-end

   The client can specify these three kinds of action using Cache-
   Control request directives:

   End-to-end reload
      The request includes a "no-cache" cache-control directive or, for
      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
      names MUST NOT be included with the no-cache directive in a
      request. The server MUST NOT use a cached copy when responding to
      such a request.

   Specific end-to-end revalidation
      The request includes a "max-age=0" cache-control directive, which
      forces each cache along the path to the origin server to
      revalidate its own entry, if any, with the next cache or server.
      The initial request includes a cache-validating conditional with
      the client's current validator.

   Unspecified end-to-end revalidation
      The request includes "max-age=0" cache-control directive, which
      forces each cache along the path to the origin server to
      revalidate its own entry, if any, with the next cache or server.
      The initial request does not include a cache-validating

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      conditional; the first cache along the path (if any) that holds a
      cache entry for this resource includes a cache-validating
      conditional with its current validator.

      When an intermediate cache is forced, by means of a max-age=0
      directive, to revalidate its own cache entry, and the client has
      supplied its own validator in the request, the supplied validator
      might differ from the validator currently stored with the cache
      entry. In this case, the cache MAY use either validator in making
      its own request without affecting semantic transparency.

      However, the choice of validator might affect performance. The
      best approach is for the intermediate cache to use its own
      validator when making its request. If the server replies with 304
      (Not Modified), then the cache can return its now validated copy
      to the client with a 200 (OK) response. If the server replies with
      a new entity and cache validator, however, the intermediate cache
      can compare the returned validator with the one provided in the
      client's request, using the strong comparison function. If the
      client's validator is equal to the origin server's, then the
      intermediate cache simply returns 304 (Not Modified). Otherwise,
      it returns the new entity with a 200 (OK) response.

      If a request includes the no-cache directive, it SHOULD NOT
      include min-fresh, max-stale, or max-age.

      In some cases, such as times of extremely poor network
      connectivity, a client may want a cache to return only those
      responses that it currently has stored, and not to reload or
      revalidate with the origin server. To do this, the client may
      include the only-if-cached directive in a request. If it receives
      this directive, a cache SHOULD either respond using a cached entry
      that is consistent with the other constraints of the request, or
      respond with a 504 (Gateway Timeout) status. However, if a group
      of caches is being operated as a unified system with good internal
      connectivity, such a request MAY be forwarded within that group of

      Because a cache MAY be configured to ignore a server's specified
      expiration time, and because a client request MAY include a max-
      stale directive (which has a similar effect), the protocol also
      includes a mechanism for the origin server to require revalidation
      of a cache entry on any subsequent use. When the must-revalidate
      directive is present in a response received by a cache, that cache
      MUST NOT use the entry after it becomes stale to respond to a

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      subsequent request without first revalidating it with the origin
      server. (I.e., the cache MUST do an end-to-end revalidation every
      time, if, based solely on the origin server's Expires or max-age
      value, the cached response is stale.)

      The must-revalidate directive is necessary to support reliable
      operation for certain protocol features. In all circumstances an
      HTTP/1.1 cache MUST obey the must-revalidate directive; in
      particular, if the cache cannot reach the origin server for any
      reason, it MUST generate a 504 (Gateway Timeout) response.

      Servers SHOULD send the must-revalidate directive if and only if
      failure to revalidate a request on the entity could result in
      incorrect operation, such as a silently unexecuted financial
      transaction. Recipients MUST NOT take any automated action that
      violates this directive, and MUST NOT automatically provide an
      unvalidated copy of the entity if revalidation fails.

      Although this is not recommended, user agents operating under
      severe connectivity constraints MAY violate this directive but, if
      so, MUST explicitly warn the user that an unvalidated response has
      been provided. The warning MUST be provided on each unvalidated
      access, and SHOULD require explicit user confirmation.

      The proxy-revalidate directive has the same meaning as the must-
      revalidate directive, except that it does not apply to non-shared
      user agent caches. It can be used on a response to an
      authenticated request to permit the user's cache to store and
      later return the response without needing to revalidate it (since
      it has already been authenticated once by that user), while still
      requiring proxies that service many users to revalidate each time
      (in order to make sure that each user has been authenticated).
      Note that such authenticated responses also need the public cache
      control directive in order to allow them to be cached at all.

14.9.5 No-Transform Directive

      Implementors of intermediate caches (proxies) have found it useful
      to convert the media type of certain entity bodies. A non-
      transparent proxy might, for example, convert between image
      formats in order to save cache space or to reduce the amount of
      traffic on a slow link.

      Serious operational problems occur, however, when these
      transformations are applied to entity bodies intended for certain
      kinds of applications. For example, applications for medical

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      imaging, scientific data analysis and those using end-to-end
      authentication, all depend on receiving an entity body that is bit
      for bit identical to the original entity-body.

      Therefore, if a message includes the no-transform directive, an
      intermediate cache or proxy MUST NOT change those headers that are
      listed in section 13.5.2 as being subject to the no-transform
      directive. This implies that the cache or proxy MUST NOT change
      any aspect of the entity-body that is specified by these headers,
      including the value of the entity-body itself.

14.9.6 Cache Control Extensions

   The Cache-Control header field can be extended through the use of one
   or more cache-extension tokens, each with an optional assigned value.
   Informational extensions (those which do not require a change in
   cache behavior) MAY be added without changing the semantics of other
   directives. Behavioral extensions are designed to work by acting as
   modifiers to the existing base of cache directives. Both the new
   directive and the standard directive are supplied, such that
   applications which do not understand the new directive will default
   to the behavior specified by the standard directive, and those that
   understand the new directive will recognize it as modifying the
   requirements associated with the standard directive. In this way,
   extensions to the cache-control directives can be made without
   requiring changes to the base protocol.

   This extension mechanism depends on an HTTP cache obeying all of the
   cache-control directives defined for its native HTTP-version, obeying
   certain extensions, and ignoring all directives that it does not

   For example, consider a hypothetical new response directive called
   community which acts as a modifier to the private directive. We
   define this new directive to mean that, in addition to any non-shared
   cache, any cache which is shared only by members of the community
   named within its value may 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 seeing this header field will act correctly even if the cache
   does not understand the community cache-extension, since it will also
   see and understand the private directive and thus default to the safe

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   Unrecognized cache-directives MUST be ignored; it is assumed that any
   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
   be combined with standard directives (or the response's default
   cacheability) such that the cache behavior will remain minimally
   correct even if the cache does not understand the extension(s).

14.10 Connection

   The Connection general-header field allows the sender to specify
   options that are desired for that particular connection and MUST NOT
   be communicated by proxies over further connections.

   The Connection header has the following grammar:

       Connection = "Connection" ":" 1#(connection-token)
       connection-token  = token

   HTTP/1.1 proxies MUST parse the Connection header field before a
   message is forwarded and, for each connection-token in this field,
   remove any header field(s) from the message with the same name as the
   connection-token. Connection options are signaled by the presence of
   a connection-token in the Connection header field, not by any
   corresponding additional header field(s), since the additional header
   field may not be sent if there are no parameters associated with that
   connection option.

   Message headers listed in the Connection header MUST NOT include
   end-to-end headers, such as Cache-Control.

   HTTP/1.1 defines the "close" connection option for the sender to
   signal that the connection will be closed after completion of the
   response. For example,

       Connection: close

   in either the request or the response header fields indicates that
   the connection SHOULD NOT be considered `persistent' (section 8.1)
   after the current request/response is complete.

   HTTP/1.1 applications that do not support persistent connections MUST
   include the "close" connection option in every message.

   A system receiving an HTTP/1.0 (or lower-version) message that
   includes a Connection header MUST, for each connection-token in this
   field, remove and ignore any header field(s) from the message with
   the same name as the connection-token. This protects against mistaken
   forwarding of such header fields by pre-HTTP/1.1 proxies. See section

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14.11 Content-Encoding

   The Content-Encoding entity-header field is used as a modifier to the
   media-type. When present, its value indicates what additional content
   codings have been applied to the entity-body, and thus what decoding
   mechanisms must be applied in order to obtain the media-type
   referenced by the Content-Type header field. Content-Encoding is
   primarily used to allow a document to be compressed without losing
   the identity of its underlying media type.

       Content-Encoding  = "Content-Encoding" ":" 1#content-coding

   Content codings are defined in section 3.5. An example of its use is

       Content-Encoding: gzip

   The content-coding is a characteristic of the entity identified by
   the Request-URI. Typically, the entity-body is stored with this
   encoding and is only decoded before rendering or analogous usage.
   However, a non-transparent proxy MAY modify the content-coding if the
   new coding is known to be acceptable to the recipient, unless the
   "no-transform" cache-control directive is present in the message.

   If the content-coding of an entity is not "identity", then the
   response MUST include a Content-Encoding entity-header (section
   14.11) that lists the non-identity content-coding(s) used.

   If the content-coding of an entity in a request message is not
   acceptable to the origin server, the server SHOULD respond with a
   status code of 415 (Unsupported Media Type).

   If multiple encodings have been applied to an entity, the content
   codings MUST be listed in the order in which they were applied.
   Additional information about the encoding parameters MAY be provided
   by other entity-header fields not defined by this specification.

14.12 Content-Language

   The Content-Language entity-header field describes the natural
   language(s) of the intended audience for the enclosed entity. Note
   that this might not be equivalent to all the languages used within
   the entity-body.

       Content-Language  = "Content-Language" ":" 1#language-tag

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   Language tags are defined in section 3.10. The primary purpose of
   Content-Language is to allow a user to identify and differentiate
   entities according to the user's own preferred language. Thus, if the
   body content is intended only for a Danish-literate audience, the
   appropriate field is

       Content-Language: da

   If no Content-Language is specified, the default is that the content
   is intended for all language audiences. This might mean that the
   sender does not consider it to be specific to any natural language,
   or that the sender does not know for which language it is intended.

   Multiple languages MAY be listed for content that is intended for
   multiple audiences. For example, a rendition of the "Treaty of
   Waitangi," presented simultaneously in the original Maori and English
   versions, would call for

       Content-Language: mi, en

   However, just because multiple languages are present within an entity
   does not mean that it is intended for multiple linguistic audiences.
   An example would be a beginner's language primer, such as "A First
   Lesson in Latin," which is clearly intended to be used by an
   English-literate audience. In this case, the Content-Language would
   properly only include "en".

   Content-Language MAY be applied to any media type -- it is not
   limited to textual documents.

14.13 Content-Length

   The Content-Length entity-header field indicates the size of the
   entity-body, in decimal number of OCTETs, sent to the recipient or,
   in the case of the HEAD method, the size of the entity-body that
   would have been sent had the request been a GET.

       Content-Length    = "Content-Length" ":" 1*DIGIT

   An example is

       Content-Length: 3495

   Applications SHOULD use this field to indicate the transfer-length of
   the message-body, unless this is prohibited by the rules in section

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   Any Content-Length greater than or equal to zero is a valid value.
   Section 4.4 describes how to determine the length of a message-body
   if a Content-Length is not given.

   Note that the meaning of this field is significantly different from
   the corresponding definition in MIME, where it is an optional field
   used within the "message/external-body" content-type. In HTTP, it
   SHOULD be sent whenever the message's length can be determined prior
   to being transferred, unless this is prohibited by the rules in
   section 4.4.

14.14 Content-Location

   The Content-Location entity-header field MAY be used to supply the
   resource location for the entity enclosed in the message when that
   entity is accessible from a location separate from the requested
   resource's URI. A server SHOULD provide a Content-Location for the
   variant corresponding to the response entity; especially in the case
   where a resource has multiple entities associated with it, and those
   entities actually have separate locations by which they might be
   individually accessed, the server SHOULD provide a Content-Location
   for the particular variant which is returned.

       Content-Location = "Content-Location" ":"
                         ( absoluteURI | relativeURI )

   The value of Content-Location also defines the base URI for the

   The Content-Location value is not a replacement for the original
   requested URI; it is only a statement of the location of the resource
   corresponding to this particular entity at the time of the request.
   Future requests MAY specify the Content-Location URI as the request-
   URI if the desire is to identify the source of that particular

   A cache cannot assume that an entity with a Content-Location
   different from the URI used to retrieve it can be used to respond to
   later requests on that Content-Location URI. However, the Content-
   Location can be used to differentiate between multiple entities
   retrieved from a single requested resource, as described in section

   If the Content-Location is a relative URI, the relative URI is
   interpreted relative to the Request-URI.

   The meaning of the Content-Location header in PUT or POST requests is
   undefined; servers are free to ignore it in those cases.

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14.15 Content-MD5

   The Content-MD5 entity-header field, as defined in RFC 1864 [23], is
   an MD5 digest of the entity-body for the purpose of providing an
   end-to-end message integrity check (MIC) of the entity-body. (Note: a
   MIC is good for detecting accidental modification of the entity-body
   in transit, but is not proof against malicious attacks.)

        Content-MD5   = "Content-MD5" ":" md5-digest
        md5-digest   = <base64 of 128 bit MD5 digest as per RFC 1864>

   The Content-MD5 header field MAY be generated by an origin server or
   client to function as an integrity check of the entity-body. Only
   origin servers or clients MAY generate the Content-MD5 header field;
   proxies and gateways MUST NOT generate it, as this would defeat its
   value as an end-to-end integrity check. Any recipient of the entity-
   body, including gateways and proxies, MAY check that the digest value
   in this header field matches that of the entity-body as received.

   The MD5 digest is computed based on the content of the entity-body,
   including any content-coding that has been applied, but not including
   any transfer-encoding applied to the message-body. If the message is
   received with a transfer-encoding, that encoding MUST be removed
   prior to checking the Content-MD5 value against the received entity.

   This has the result that the digest is computed on the octets of the
   entity-body exactly as, and in the order that, they would be sent if
   no transfer-encoding were being applied.

   HTTP extends RFC 1864 to permit the digest to be computed for MIME
   composite media-types (e.g., multipart/* and message/rfc822), but
   this does not change how the digest is computed as defined in the
   preceding paragraph.

   There are several consequences of this. The entity-body for composite
   types MAY contain many body-parts, each with its own MIME and HTTP
   headers (including Content-MD5, Content-Transfer-Encoding, and
   Content-Encoding headers). If a body-part has a Content-Transfer-
   Encoding or Content-Encoding header, it is assumed that the content
   of the body-part has had the encoding applied, and the body-part is
   included in the Content-MD5 digest as is -- i.e., after the
   application. The Transfer-Encoding header field is not allowed within

   Conversion of all line breaks to CRLF MUST NOT be done before
   computing or checking the digest: the line break convention used in
   the text actually transmitted MUST be left unaltered when computing
   the digest.

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      Note: while the definition of Content-MD5 is exactly the same for
      HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
      in which the application of Content-MD5 to HTTP entity-bodies
      differs from its application to MIME entity-bodies. One is that
      HTTP, unlike MIME, does not use Content-Transfer-Encoding, and
      does use Transfer-Encoding and Content-Encoding. Another is that
      HTTP more frequently uses binary content types than MIME, so it is
      worth noting that, in such cases, the byte order used to compute
      the digest is the transmission byte order defined for the type.
      Lastly, HTTP allows transmission of text types with any of several
      line break conventions and not just the canonical form using CRLF.

14.16 Content-Range

   The Content-Range entity-header is sent with a partial entity-body to
   specify where in the full entity-body the partial body should be
   applied. Range units are defined in section 3.12.

       Content-Range = "Content-Range" ":" content-range-spec

       content-range-spec      = byte-content-range-spec
       byte-content-range-spec = bytes-unit SP
                                 byte-range-resp-spec "/"
                                 ( instance-length | "*" )

       byte-range-resp-spec = (first-byte-pos "-" last-byte-pos)
                                      | "*"
       instance-length           = 1*DIGIT

   The header SHOULD indicate the total length of the full entity-body,
   unless this length is unknown or difficult to determine. The asterisk
   "*" character means that the instance-length is unknown at the time
   when the response was generated.

   Unlike byte-ranges-specifier values (see section 14.35.1), a byte-
   range-resp-spec MUST only specify one range, and MUST contain
   absolute byte positions for both the first and last byte of the

   A byte-content-range-spec with a byte-range-resp-spec whose last-
   byte-pos value is less than its first-byte-pos value, or whose
   instance-length value is less than or equal to its last-byte-pos
   value, is invalid. The recipient of an invalid byte-content-range-
   spec MUST ignore it and any content transferred along with it.

   A server sending a response with status code 416 (Requested range not
   satisfiable) SHOULD include a Content-Range field with a byte-range-
   resp-spec of "*". The instance-length specifies the current length of

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   the selected resource. A response with status code 206 (Partial
   Content) MUST NOT include a Content-Range field with a byte-range-
   resp-spec of "*".

   Examples of byte-content-range-spec values, assuming that the entity
   contains a total of 1234 bytes:

      . The first 500 bytes:
       bytes 0-499/1234

      . The second 500 bytes:
       bytes 500-999/1234

      . All except for the first 500 bytes:
       bytes 500-1233/1234

      . The last 500 bytes:
       bytes 734-1233/1234

   When an HTTP message includes the content of a single range (for
   example, a response to a request for a single range, or to a request
   for a set of ranges that overlap without any holes), this content is
   transmitted with a Content-Range header, and a Content-Length header
   showing the number of bytes actually transferred. For example,

       HTTP/1.1 206 Partial content
       Date: Wed, 15 Nov 1995 06:25:24 GMT
       Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
       Content-Range: bytes 21010-47021/47022
       Content-Length: 26012
       Content-Type: image/gif

   When an HTTP message includes the content of multiple ranges (for
   example, a response to a request for multiple non-overlapping
   ranges), these are transmitted as a multipart message. The multipart
   media type used for this purpose is "multipart/byteranges" as defined
   in appendix 19.2. See appendix 19.6.3 for a compatibility issue.

   A response to a request for a single range MUST NOT be sent using the
   multipart/byteranges media type.  A response to a request for
   multiple ranges, whose result is a single range, MAY be sent as a
   multipart/byteranges media type with one part. A client that cannot
   decode a multipart/byteranges message MUST NOT ask for multiple
   byte-ranges in a single request.

   When a client requests multiple byte-ranges in one request, the
   server SHOULD return them in the order that they appeared in the

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   If the server ignores a byte-range-spec because it is syntactically
   invalid, the server SHOULD treat the request as if the invalid Range
   header field did not exist. (Normally, this means return a 200
   response containing the full entity).

   If the server receives a request (other than one including an If-
   Range request-header field) with an unsatisfiable Range request-
   header field (that is, all of whose byte-range-spec values have a
   first-byte-pos value greater than the current length of the selected
   resource), it SHOULD return a response code of 416 (Requested range
   not satisfiable) (section 10.4.17).

      Note: clients cannot depend on servers to send a 416 (Requested
      range not satisfiable) response instead of a 200 (OK) response for
      an unsatisfiable Range request-header, since not all servers
      implement this request-header.

14.17 Content-Type

   The Content-Type entity-header field indicates the media type of the
   entity-body sent to the recipient or, in the case of the HEAD method,
   the media type that would have been sent had the request been a GET.

       Content-Type   = "Content-Type" ":" media-type

   Media types are defined in section 3.7. An example of the field is

       Content-Type: text/html; charset=ISO-8859-4

   Further discussion of methods for identifying the media type of an
   entity is provided in section 7.2.1.

14.18 Date

   The Date general-header field represents the date and time at which
   the message was originated, having the same semantics as orig-date in
   RFC 822. The field value is an HTTP-date, as described in section
   3.3.1; it MUST be sent in RFC 1123 [8]-date format.

       Date  = "Date" ":" HTTP-date

   An example is

       Date: Tue, 15 Nov 1994 08:12:31 GMT

   Origin servers MUST include a Date header field in all responses,
   except in these cases:

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      1. If the response status code is 100 (Continue) or 101 (Switching
         Protocols), the response MAY include a Date header field, at
         the server's option.

      2. If the response status code conveys a server error, e.g. 500
         (Internal Server Error) or 503 (Service Unavailable), and it is
         inconvenient or impossible to generate a valid Date.

      3. If the server does not have a clock that can provide a
         reasonable approximation of the current time, its responses
         MUST NOT include a Date header field. In this case, the rules
         in section 14.18.1 MUST be followed.

   A received message that does not have a Date header field MUST be
   assigned one by the recipient if the message will be cached by that
   recipient or gatewayed via a protocol which requires a Date. An HTTP
   implementation without a clock MUST NOT cache responses without
   revalidating them on every use. An HTTP cache, especially a shared
   cache, SHOULD use a mechanism, such as NTP [28], to synchronize its
   clock with a reliable external standard.

   Clients SHOULD only send a Date header field in messages that include
   an entity-body, as in the case of the PUT and POST requests, and even
   then it is optional. A client without a clock MUST NOT send a Date
   header field in a request.

   The HTTP-date sent in a Date header SHOULD NOT represent a date and
   time subsequent to the generation of the message. It SHOULD represent
   the best available approximation of the date and time of message
   generation, unless the implementation has no means of generating a
   reasonably accurate date and time. In theory, the date ought to
   represent the moment just before the entity is generated. In
   practice, the date can be generated at any time during the message
   origination without affecting its semantic value.

14.18.1 Clockless Origin Server Operation

   Some origin server implementations might not have a clock available.
   An origin server without a clock MUST NOT assign Expires or Last-
   Modified values to a response, unless these values were associated
   with the resource by a system or user with a reliable clock. It MAY
   assign an Expires value that is known, at or before server
   configuration time, to be in the past (this allows "pre-expiration"
   of responses without storing separate Expires values for each

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