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http1.1---9

作者:未知 来源:月光软件站 加入时间:2005-2-28 月光软件站

 

Fielding, et. al.           Standards Track                   [Page 116]

RFC 2068                        HTTP/1.1                    January 1997


   In theory, the date SHOULD 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.

   The format of the Date is an absolute date and time as defined by
   HTTP-date in section 3.3; it MUST be sent in RFC1123 [8]-date format.

14.20 ETag

   The ETag entity-header field defines the entity tag for the
   associated entity. The headers used with entity tags are described in
   sections 14.20, 14.25, 14.26 and 14.43. The entity tag may be used
   for comparison with other entities from the same resource (see
   section 13.3.2).

         ETag = "ETag" ":" entity-tag

   Examples:

         ETag: "xyzzy"
         ETag: W/"xyzzy"
         ETag: ""

14.21 Expires

   The Expires entity-header field gives the date/time after which the
   response should be considered stale. A stale cache entry may not
   normally be returned by a cache (either a proxy cache or an user
   agent cache) unless it is first validated with the origin server (or
   with an intermediate cache that has a fresh copy of the entity). See
   section 13.2 for further discussion of the expiration model.

   The presence of an Expires field does not imply that the original
   resource will change or cease to exist at, before, or after that
   time.

   The format is an absolute date and time as defined by HTTP-date in
   section 3.3; it MUST be in RFC1123-date format:

         Expires = "Expires" ":" HTTP-date

 

 

 

 


Fielding, et. al.           Standards Track                   [Page 117]

RFC 2068                        HTTP/1.1                    January 1997


   An example of its use is

         Expires: Thu, 01 Dec 1994 16:00:00 GMT

     Note: if a response includes a Cache-Control field with the max-age
     directive, that directive overrides the Expires field.

   HTTP/1.1 clients and caches MUST treat other invalid date formats,
   especially including the value "0", as in the past (i.e., "already
   expired").

   To mark a response as "already expired," an origin server should use
   an Expires date that is equal to the Date header value. (See the
   rules for expiration calculations in section 13.2.4.)

   To mark a response as "never expires," an origin server should use an
   Expires date approximately one year from the time the response is
   sent.  HTTP/1.1 servers should not send Expires dates more than one
   year in the future.

   The presence of an Expires header field with a date value of some
   time in the future on an response that otherwise would by default be
   non-cacheable indicates that the response is cachable, unless
   indicated otherwise by a Cache-Control header field (section 14.9).

14.22 From

   The From request-header field, if given, SHOULD contain an Internet
   e-mail address for the human user who controls the requesting user
   agent.  The address SHOULD be machine-usable, as defined by mailbox
   in RFC 822 (as updated by RFC 1123 ):

          From   = "From" ":" mailbox

   An example is:

          From: [email protected]

   This header field MAY be used for logging purposes and as a means for
   identifying the source of invalid or unwanted requests. It SHOULD NOT
   be used as an insecure form of access protection. The interpretation
   of this field is that the request is being performed on behalf of the
   person given, who accepts responsibility for the method performed. In
   particular, robot agents SHOULD include this header so that the
   person responsible for running the robot can be contacted if problems
   occur on the receiving end.

 

 

Fielding, et. al.           Standards Track                   [Page 118]

RFC 2068                        HTTP/1.1                    January 1997


   The Internet e-mail address in this field MAY be separate from the
   Internet host which issued the request. For example, when a request
   is passed through a proxy the original issuer's address SHOULD be
   used.

     Note: The client SHOULD not send the From header field without the
     user's approval, as it may conflict with the user's privacy
     interests or their site's security policy. It is strongly
     recommended that the user be able to disable, enable, and modify
     the value of this field at any time prior to a request.

14.23 Host

   The Host request-header field specifies the Internet host and port
   number of the resource being requested, as obtained from the original
   URL given by the user or referring resource (generally an HTTP URL,
   as described in section 3.2.2). The Host field value MUST represent
   the network location of the origin server or gateway given by the
   original URL. This allows the origin server or gateway to
   differentiate between internally-ambiguous URLs, such as the root "/"
   URL of a server for multiple host names on a single IP address.

          Host = "Host" ":" host [ ":" port ]    ; Section 3.2.2

   A "host" without any trailing port information implies the default
   port for the service requested (e.g., "80" for an HTTP URL). For
   example, a request on the origin server for
   <http://www.w3.org/pub/WWW/> MUST include:

          GET /pub/WWW/ HTTP/1.1
          Host: www.w3.org

   A client MUST include a Host header field in all HTTP/1.1 request
   messages on the Internet (i.e., on any message corresponding to a
   request for a URL which includes an Internet host address for the
   service being requested). If the Host field is not already present,
   an HTTP/1.1 proxy MUST add a Host field to the request message prior
   to forwarding it on the Internet. All Internet-based HTTP/1.1 servers
   MUST respond with a 400 status code to any HTTP/1.1 request message
   which lacks a Host header field.

   See sections 5.2 and 19.5.1 for other requirements relating to Host.

14.24 If-Modified-Since

   The If-Modified-Since request-header field is used with the GET
   method to make it conditional: if the requested variant has not been
   modified since the time specified in this field, an entity will not

 

Fielding, et. al.           Standards Track                   [Page 119]

RFC 2068                        HTTP/1.1                    January 1997


   be returned from the server; instead, a 304 (not modified) response
   will be returned without any message-body.

          If-Modified-Since = "If-Modified-Since" ":" HTTP-date

   An example of the field is:

          If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

   A GET method with an If-Modified-Since header and no Range header
   requests that the identified entity be transferred only if it has
   been modified since the date given by the If-Modified-Since header.
   The algorithm for determining this includes the following cases:

   a)If the request would normally result in anything other than a 200
     (OK) status, or if the passed If-Modified-Since date is invalid, the
     response is exactly the same as for a normal GET. A date which is
     later than the server's current time is invalid.

   b)If the variant has been modified since the If-Modified-Since date,
     the response is exactly the same as for a normal GET.

   c)If the variant has not been modified since a valid If-Modified-Since
     date, the server MUST return a 304 (Not Modified) response.

   The purpose of this feature is to allow efficient updates of cached
   information with a minimum amount of transaction overhead.

     Note that the Range request-header field modifies the meaning of
     If-Modified-Since; see section 14.36 for full details.

     Note that If-Modified-Since times are interpreted by the server,
     whose clock may not be synchronized with the client.

   Note that if a client uses an arbitrary date in the If-Modified-Since
   header instead of a date taken from the Last-Modified header for the
   same request, the client should be aware of the fact that this date
   is interpreted in the server's understanding of time. The client
   should consider unsynchronized clocks and rounding problems due to
   the different encodings of time between the client and server. This
   includes the possibility of race conditions if the document has
   changed between the time it was first requested and the If-Modified-
   Since date of a subsequent request, and the possibility of clock-
   skew-related problems if the If-Modified-Since date is derived from
   the client's clock without correction to the server's clock.
   Corrections for different time bases between client and server are at
   best approximate due to network latency.

 


Fielding, et. al.           Standards Track                   [Page 120]

RFC 2068                        HTTP/1.1                    January 1997


14.25 If-Match

   The If-Match request-header field is used with a method to make it
   conditional. A client that has one or more entities previously
   obtained from the resource can verify that one of those entities is
   current by including a list of their associated entity tags in the
   If-Match header field. The purpose of this feature is to allow
   efficient updates of cached information with a minimum amount of
   transaction overhead. It is also used, on updating requests, to
   prevent inadvertent modification of the wrong version of a resource.
   As a special case, the value "*" matches any current entity of the
   resource.

          If-Match = "If-Match" ":" ( "*" | 1#entity-tag )

   If any of the entity tags match the entity tag of the entity that
   would have been returned in the response to a similar GET request
   (without the If-Match header) on that resource, or if "*" is given
   and any current entity exists for that resource, then the server MAY
   perform the requested method as if the If-Match header field did not
   exist.

   A server MUST use the strong comparison function (see section 3.11)
   to compare the entity tags in If-Match.

   If none of the entity tags match, or if "*" is given and no current
   entity exists, the server MUST NOT perform the requested method, and
   MUST return a 412 (Precondition Failed) response. This behavior is
   most useful when the client wants to prevent an updating method, such
   as PUT, from modifying a resource that has changed since the client
   last retrieved it.

   If the request would, without the If-Match header field, result in
   anything other than a 2xx status, then the If-Match header MUST be
   ignored.

   The meaning of "If-Match: *" is that the method SHOULD be performed
   if the representation selected by the origin server (or by a cache,
   possibly using the Vary mechanism, see section 14.43) exists, and
   MUST NOT be performed if the representation does not exist.

 

 

 

 

 

Fielding, et. al.           Standards Track                   [Page 121]

RFC 2068                        HTTP/1.1                    January 1997


   A request intended to update a resource (e.g., a PUT) MAY include an
   If-Match header field to signal that the request method MUST NOT be
   applied if the entity corresponding to the If-Match value (a single
   entity tag) is no longer a representation of that resource.  This
   allows the user to indicate that they do not wish the request to be
   successful if the resource has been changed without their knowledge.
   Examples:

          If-Match: "xyzzy"
          If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
          If-Match: *

14.26 If-None-Match

   The If-None-Match request-header field is used with a method to make
   it conditional. A client that has one or more entities previously
   obtained from the resource can verify that none of those entities is
   current by including a list of their associated entity tags in the
   If-None-Match header field. The purpose of this feature is to allow
   efficient updates of cached information with a minimum amount of
   transaction overhead. It is also used, on updating requests, to
   prevent inadvertent modification of a resource which was not known to
   exist.

   As a special case, the value "*" matches any current entity of the
   resource.

          If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )

   If any of the entity tags match the entity tag of the entity that
   would have been returned in the response to a similar GET request
   (without the If-None-Match header) on that resource, or if "*" is
   given and any current entity exists for that resource, then the
   server MUST NOT perform the requested method. Instead, if the request
   method was GET or HEAD, the server SHOULD respond with a 304 (Not
   Modified) response, including the cache-related entity-header fields
   (particularly ETag) of one of the entities that matched. For all
   other request methods, the server MUST respond with a status of 412
   (Precondition Failed).

   See section 13.3.3 for rules on how to determine if two entity tags
   match. The weak comparison function can only be used with GET or HEAD
   requests.

   If none of the entity tags match, or if "*" is given and no current
   entity exists, then the server MAY perform the requested method as if
   the If-None-Match header field did not exist.

 


Fielding, et. al.           Standards Track                   [Page 122]

RFC 2068                        HTTP/1.1                    January 1997


   If the request would, without the If-None-Match header field, result
   in anything other than a 2xx status, then the If-None-Match header
   MUST be ignored.

   The meaning of "If-None-Match: *" is that the method MUST NOT be
   performed if the representation selected by the origin server (or by
   a cache, possibly using the Vary mechanism, see section 14.43)
   exists, and SHOULD be performed if the representation does not exist.
   This feature may be useful in preventing races between PUT
   operations.

   Examples:

          If-None-Match: "xyzzy"
          If-None-Match: W/"xyzzy"
          If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
          If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
          If-None-Match: *

14.27 If-Range

   If a client has a partial copy of an entity in its cache, and wishes
   to have an up-to-date copy of the entire entity in its cache, it
   could use the Range request-header with a conditional GET (using
   either or both of If-Unmodified-Since and If-Match.) However, if the
   condition fails because the entity has been modified, the client
   would then have to make a second request to obtain the entire current
   entity-body.

   The If-Range header allows a client to "short-circuit" the second
   request. Informally, its meaning is `if the entity is unchanged, send
   me the part(s) that I am missing; otherwise, send me the entire new
   entity.'

           If-Range = "If-Range" ":" ( entity-tag | HTTP-date )

   If the client has no entity tag for an entity, but does have a Last-
   Modified date, it may use that date in a If-Range header. (The server
   can distinguish between a valid HTTP-date and any form of entity-tag
   by examining no more than two characters.) The If-Range header should
   only be used together with a Range header, and must be ignored if the
   request does not include a Range header, or if the server does not
   support the sub-range operation.

 

 

 


Fielding, et. al.           Standards Track                   [Page 123]

RFC 2068                        HTTP/1.1                    January 1997


   If the entity tag given in the If-Range header matches the current
   entity tag for the entity, then the server should provide the
   specified sub-range of the entity using a 206 (Partial content)
   response. If the entity tag does not match, then the server should
   return the entire entity using a 200 (OK) response.

14.28 If-Unmodified-Since

   The If-Unmodified-Since request-header field is used with a method to
   make it conditional. If the requested resource has not been modified
   since the time specified in this field, the server should perform the
   requested operation as if the If-Unmodified-Since header were not
   present.

   If the requested variant has been modified since the specified time,
   the server MUST NOT perform the requested operation, and MUST return
   a 412 (Precondition Failed).

         If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date

   An example of the field is:

          If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT

   If the request normally (i.e., without the If-Unmodified-Since
   header) would result in anything other than a 2xx status, the If-
   Unmodified-Since header should be ignored.

   If the specified date is invalid, the header is ignored.

14.29 Last-Modified

   The Last-Modified entity-header field indicates the date and time at
   which the origin server believes the variant was last modified.

          Last-Modified  = "Last-Modified" ":" HTTP-date

   An example of its use is

          Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT

   The exact meaning of this header field depends on the implementation
   of the origin server and the nature of the original resource. For
   files, it may be just the file system last-modified time. For
   entities with dynamically included parts, it may be the most recent
   of the set of last-modify times for its component parts. For database
   gateways, it may be the last-update time stamp of the record. For
   virtual objects, it may be the last time the internal state changed.

 

Fielding, et. al.           Standards Track                   [Page 124]

RFC 2068                        HTTP/1.1                    January 1997


   An origin server MUST NOT send a Last-Modified date which is later
   than the server's time of message origination. In such cases, where
   the resource's last modification would indicate some time in the
   future, the server MUST replace that date with the message
   origination date.

   An origin server should obtain the Last-Modified value of the entity
   as close as possible to the time that it generates the Date value of
   its response. This allows a recipient to make an accurate assessment
   of the entity's modification time, especially if the entity changes
   near the time that the response is generated.

   HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.

14.30 Location

   The Location response-header field is used to redirect the recipient
   to a location other than the Request-URI for completion of the
   request or identification of a new resource. For 201 (Created)
   responses, the Location is that of the new resource which was created
   by the request.  For 3xx responses, the location SHOULD indicate the
   server's preferred URL for automatic redirection to the resource. The
   field value consists of a single absolute URL.

          Location       = "Location" ":" absoluteURI

   An example is

          Location: http://www.w3.org/pub/WWW/People.html

     Note: The Content-Location header field (section 14.15) differs
     from Location in that the Content-Location identifies the original
     location of the entity enclosed in the request. It is therefore
     possible for a response to contain header fields for both Location
     and Content-Location. Also see section 13.10 for cache requirements
     of some methods.

14.31 Max-Forwards

   The Max-Forwards request-header field may be used with the TRACE
   method (section 14.31) to limit the number of proxies or gateways
   that can forward the request to the next inbound server. This can be
   useful when the client is attempting to trace a request chain which
   appears to be failing or looping in mid-chain.

          Max-Forwards   = "Max-Forwards" ":" 1*DIGIT

 

 

Fielding, et. al.           Standards Track                   [Page 125]

RFC 2068                        HTTP/1.1                    January 1997


   The Max-Forwards value is a decimal integer indicating the remaining
   number of times this request message may be forwarded.

   Each proxy or gateway recipient of a TRACE request containing a Max-
   Forwards header field SHOULD check and update its value prior to
   forwarding the request. If the received value is zero (0), the
   recipient SHOULD NOT forward the request; instead, it SHOULD respond
   as the final recipient with a 200 (OK) response containing the
   received request message as the response entity-body (as described in
   section 9.8). If the received Max-Forwards value is greater than
   zero, then the forwarded message SHOULD contain an updated Max-
   Forwards field with a value decremented by one (1).

   The Max-Forwards header field SHOULD be ignored for all other methods
   defined by this specification and for any extension methods for which
   it is not explicitly referred to as part of that method definition.

14.32 Pragma

   The Pragma general-header field is used to include implementation-
   specific directives that may apply to any recipient along the
   request/response chain. All pragma directives specify optional
   behavior from the viewpoint of the protocol; however, some systems
   MAY require that behavior be consistent with the directives.

          Pragma            = "Pragma" ":" 1#pragma-directive

          pragma-directive  = "no-cache" | extension-pragma
          extension-pragma  = token [ "=" ( token | quoted-string ) ]

   When the no-cache directive is present in a request message, an
   application SHOULD forward the request toward the origin server even
   if it has a cached copy of what is being requested. This pragma
   directive has the same semantics as the no-cache cache-directive (see
   section 14.9) and is defined here for backwards compatibility with
   HTTP/1.0.  Clients SHOULD include both header fields when a no-cache
   request is sent to a server not known to be HTTP/1.1 compliant.

   Pragma directives MUST be passed through by a proxy or gateway
   application, regardless of their significance to that application,
   since the directives may be applicable to all recipients along the
   request/response chain. It is not possible to specify a pragma for a
   specific recipient; however, any pragma directive not relevant to a
   recipient SHOULD be ignored by that recipient.

 

 

 

Fielding, et. al.           Standards Track                   [Page 126]

RFC 2068                        HTTP/1.1                    January 1997


   HTTP/1.1 clients SHOULD NOT send the Pragma request-header. HTTP/1.1
   caches SHOULD treat "Pragma: no-cache" as if the client had sent
   "Cache-Control: no-cache". No new Pragma directives will be defined
   in HTTP.

14.33 Proxy-Authenticate

   The Proxy-Authenticate response-header field MUST be included as part
   of a 407 (Proxy Authentication Required) response. The field value
   consists of a challenge that indicates the authentication scheme and
   parameters applicable to the proxy for this Request-URI.

          Proxy-Authenticate  = "Proxy-Authenticate" ":" challenge

   The HTTP access authentication process is described in section 11.
   Unlike WWW-Authenticate, the Proxy-Authenticate header field applies
   only to the current connection and SHOULD NOT be passed on to
   downstream clients. However, an intermediate proxy may need to obtain
   its own credentials by requesting them from the downstream client,
   which in some circumstances will appear as if the proxy is forwarding
   the Proxy-Authenticate header field.

14.34 Proxy-Authorization

   The Proxy-Authorization request-header field allows the client to
   identify itself (or its user) to a proxy which requires
   authentication.  The Proxy-Authorization field value consists of
   credentials containing the authentication information of the user
   agent for the proxy and/or realm of the resource being requested.

       Proxy-Authorization     = "Proxy-Authorization" ":" credentials

   The HTTP access authentication process is described in section 11.
   Unlike Authorization, the Proxy-Authorization header field applies
   only to the next outbound proxy that demanded authentication using
   the Proxy-Authenticate field. When multiple proxies are used in a
   chain, the Proxy-Authorization header field is consumed by the first
   outbound proxy that was expecting to receive credentials. A proxy MAY
   relay the credentials from the client request to the next proxy if
   that is the mechanism by which the proxies cooperatively authenticate
   a given request.

14.35 Public

   The Public response-header field lists the set of methods supported
   by the server. The purpose of this field is strictly to inform the
   recipient of the capabilities of the server regarding unusual
   methods.  The methods listed may or may not be applicable to the

 

Fielding, et. al.           Standards Track                   [Page 127]

RFC 2068                        HTTP/1.1                    January 1997


   Request-URI; the Allow header field (section 14.7) MAY be used to
   indicate methods allowed for a particular URI.

          Public         = "Public" ":" 1#method

   Example of use:

          Public: OPTIONS, MGET, MHEAD, GET, HEAD

   This header field applies only to the server directly connected to
   the client (i.e., the nearest neighbor in a chain of connections). If
   the response passes through a proxy, the proxy MUST either remove the
   Public header field or replace it with one applicable to its own
   capabilities.

14.36 Range

14.36.1 Byte Ranges

   Since all HTTP entities are represented in HTTP messages as sequences
   of bytes, the concept of a byte range is meaningful for any HTTP
   entity.  (However, not all clients and servers need to support byte-
   range operations.)

   Byte range specifications in HTTP apply to the sequence of bytes in
   the entity-body (not necessarily the same as the message-body).

   A byte range operation may specify a single range of bytes, or a set
   of ranges within a single entity.

       ranges-specifier = byte-ranges-specifier

       byte-ranges-specifier = bytes-unit "=" byte-range-set

       byte-range-set  = 1#( byte-range-spec | suffix-byte-range-spec )

       byte-range-spec = first-byte-pos "-" [last-byte-pos]

       first-byte-pos  = 1*DIGIT

       last-byte-pos   = 1*DIGIT

   The first-byte-pos value in a byte-range-spec gives the byte-offset
   of the first byte in a range. The last-byte-pos value gives the
   byte-offset of the last byte in the range; that is, the byte
   positions specified are inclusive. Byte offsets start at zero.

 

 

Fielding, et. al.           Standards Track                   [Page 128]

RFC 2068                        HTTP/1.1                    January 1997


   If the last-byte-pos value is present, it must be greater than or
   equal to the first-byte-pos in that byte-range-spec, or the byte-
   range-spec is invalid. The recipient of an invalid byte-range-spec
   must ignore it.

   If the last-byte-pos value is absent, or if the value is greater than
   or equal to the current length of the entity-body, last-byte-pos is
   taken to be equal to one less than the current length of the entity-
   body in bytes.

   By its choice of last-byte-pos, a client can limit the number of
   bytes retrieved without knowing the size of the entity.

          suffix-byte-range-spec = "-" suffix-length

          suffix-length = 1*DIGIT

   A suffix-byte-range-spec is used to specify the suffix of the
   entity-body, of a length given by the suffix-length value. (That is,
   this form specifies the last N bytes of an entity-body.) If the
   entity is shorter than the specified suffix-length, the entire
   entity-body is used.

   Examples of byte-ranges-specifier values (assuming an entity-body of
   length 10000):

     o  The first 500 bytes (byte offsets 0-499, inclusive):

          bytes=0-499

     o  The second 500 bytes (byte offsets 500-999, inclusive):

          bytes=500-999

     o  The final 500 bytes (byte offsets 9500-9999, inclusive):

          bytes=-500

     o  Or

          bytes=9500-

     o  The first and last bytes only (bytes 0 and 9999):

          bytes=0-0,-1

 

 


Fielding, et. al.           Standards Track                   [Page 129]

RFC 2068                        HTTP/1.1                    January 1997


     o  Several legal but not canonical specifications of the second
        500 bytes (byte offsets 500-999, inclusive):

          bytes=500-600,601-999

          bytes=500-700,601-999

14.36.2 Range Retrieval Requests

   HTTP retrieval requests using conditional or unconditional GET
   methods may request one or more sub-ranges of the entity, instead of
   the entire entity, using the Range request header, which applies to
   the entity returned as the result of the request:

         Range = "Range" ":" ranges-specifier

   A server MAY ignore the Range header. However, HTTP/1.1 origin
   servers and intermediate caches SHOULD support byte ranges when
   possible, since Range supports efficient recovery from partially
   failed transfers, and supports efficient partial retrieval of large
   entities.

   If the server supports the Range header and the specified range or
   ranges are appropriate for the entity:

     o  The presence of a Range header in an unconditional GET modifies
        what is returned if the GET is otherwise successful. In other
        words, the response carries a status code of 206 (Partial
        Content) instead of 200 (OK).

     o  The presence of a Range header in a conditional GET (a request
        using one or both of If-Modified-Since and If-None-Match, or
        one or both of If-Unmodified-Since and If-Match) modifies what
        is returned if the GET is otherwise successful and the condition
        is true. It does not affect the 304 (Not Modified) response
        returned if the conditional is false.

   In some cases, it may be more appropriate to use the If-Range header
   (see section 14.27) in addition to the Range header.

   If a proxy that supports ranges receives a Range request, forwards
   the request to an inbound server, and receives an entire entity in
   reply, it SHOULD only return the requested range to its client. It
   SHOULD store the entire received response in its cache, if that is
   consistent with its cache allocation policies.

 

 


Fielding, et. al.           Standards Track                   [Page 130]

RFC 2068                        HTTP/1.1                    January 1997


14.37 Referer

   The Referer[sic] request-header field allows the client to specify,
   for the server's benefit, the address (URI) of the resource from
   which the Request-URI was obtained (the "referrer", although the
   header field is misspelled.) The Referer request-header allows a
   server to generate lists of back-links to resources for interest,
   logging, optimized caching, etc. It also allows obsolete or mistyped
   links to be traced for maintenance. The Referer field MUST NOT be
   sent if the Request-URI was obtained from a source that does not have
   its own URI, such as input from the user keyboard.

        Referer        = "Referer" ":" ( absoluteURI | relativeURI )

   Example:

        Referer: http://www.w3.org/hypertext/DataSources/Overview.html

   If the field value is a partial URI, it SHOULD be interpreted
   relative to the Request-URI. The URI MUST NOT include a fragment.

     Note: Because the source of a link may be private information or
     may reveal an otherwise private information source, it is strongly
     recommended that the user be able to select whether or not the
     Referer field is sent. For example, a browser client could have a
     toggle switch for browsing openly/anonymously, which would
     respectively enable/disable the sending of Referer and From
     information.

14.38 Retry-After

   The Retry-After response-header field can be used with a 503 (Service
   Unavailable) response to indicate how long the service is expected to
   be unavailable to the requesting client. The value of this field can
   be either an HTTP-date or an integer number of seconds (in decimal)
   after the time of the response.

          Retry-After  = "Retry-After" ":" ( HTTP-date | delta-seconds )

   Two examples of its use are

          Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
          Retry-After: 120

   In the latter example, the delay is 2 minutes.

 

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