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RFC 1945

 
 
 

Hypertext Transfer Protocol -- HTTP/1.0

Part 2 of 3, p. 21 to 46
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4.  HTTP Message

4.1  Message Types

   HTTP messages consist of requests from client to server and responses
   from server to client.

       HTTP-message   = Simple-Request           ; HTTP/0.9 messages
                      | Simple-Response
                      | Full-Request             ; HTTP/1.0 messages
                      | Full-Response

   Full-Request and Full-Response use the generic message format of RFC
   822 [7] for transferring entities. Both messages may include optional
   header fields (also known as "headers") and an entity body. The
   entity body is separated from the headers by a null line (i.e., a
   line with nothing preceding the CRLF).

       Full-Request   = Request-Line             ; Section 5.1
                        *( General-Header        ; Section 4.3
                         | Request-Header        ; Section 5.2
                         | Entity-Header )       ; Section 7.1
                        CRLF
                        [ Entity-Body ]          ; Section 7.2

       Full-Response  = Status-Line              ; Section 6.1
                        *( General-Header        ; Section 4.3
                         | Response-Header       ; Section 6.2

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                         | Entity-Header )       ; Section 7.1
                        CRLF
                        [ Entity-Body ]          ; Section 7.2

   Simple-Request and Simple-Response do not allow the use of any header
   information and are limited to a single request method (GET).

       Simple-Request  = "GET" SP Request-URI CRLF

       Simple-Response = [ Entity-Body ]

   Use of the Simple-Request format is discouraged because it prevents
   the server from identifying the media type of the returned entity.

4.2  Message Headers

   HTTP header fields, which include General-Header (Section 4.3),
   Request-Header (Section 5.2), Response-Header (Section 6.2), and
   Entity-Header (Section 7.1) fields, follow the same generic format as
   that given in Section 3.1 of RFC 822 [7]. Each header field consists
   of a name followed immediately by a colon (":"), a single space (SP)
   character, and the field value. Field names are case-insensitive.
   Header fields can be extended over multiple lines by preceding each
   extra line with at least one SP or HT, though this is not
   recommended.

       HTTP-header    = field-name ":" [ field-value ] CRLF

       field-name     = token
       field-value    = *( field-content | LWS )

       field-content  = <the OCTETs making up the field-value
                        and consisting of either *TEXT or combinations
                        of token, tspecials, and quoted-string>

   The order in which header fields are received is not significant.
   However, it is "good practice" to send General-Header fields first,
   followed by Request-Header or Response-Header fields prior to the
   Entity-Header fields.

   Multiple HTTP-header fields with the same field-name may be present
   in a message if and only if the entire field-value for that header
   field is defined as a comma-separated list [i.e., #(values)]. It must
   be possible to combine the multiple header fields into one "field-
   name: field-value" pair, without changing the semantics of the
   message, by appending each subsequent field-value to the first, each
   separated by a comma.

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4.3  General Header Fields

   There are a few header fields which have general applicability for
   both request and response messages, but which do not apply to the
   entity being transferred. These headers apply only to the message
   being transmitted.

       General-Header = Date                     ; Section 10.6
                      | Pragma                   ; Section 10.12

   General header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields may be given the semantics of general
   header fields if all parties in the communication recognize them to
   be general header fields. Unrecognized header fields are treated as
   Entity-Header fields.

5. Request

   A request message from a client to a server includes, within the
   first line of that message, the method to be applied to the resource,
   the identifier of the resource, and the protocol version in use. For
   backwards compatibility with the more limited HTTP/0.9 protocol,
   there are two valid formats for an HTTP request:

       Request        = Simple-Request | Full-Request

       Simple-Request = "GET" SP Request-URI CRLF

       Full-Request   = Request-Line             ; Section 5.1
                        *( General-Header        ; Section 4.3
                         | Request-Header        ; Section 5.2
                         | Entity-Header )       ; Section 7.1
                        CRLF
                        [ Entity-Body ]          ; Section 7.2

   If an HTTP/1.0 server receives a Simple-Request, it must respond with
   an HTTP/0.9 Simple-Response. An HTTP/1.0 client capable of receiving
   a Full-Response should never generate a Simple-Request.

5.1  Request-Line

   The Request-Line begins with a method token, followed by the
   Request-URI and the protocol version, and ending with CRLF. The
   elements are separated by SP characters. No CR or LF are allowed
   except in the final CRLF sequence.

       Request-Line = Method SP Request-URI SP HTTP-Version CRLF

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   Note that the difference between a Simple-Request and the Request-
   Line of a Full-Request is the presence of the HTTP-Version field and
   the availability of methods other than GET.

5.1.1 Method

   The Method token indicates the method to be performed on the resource
   identified by the Request-URI. The method is case-sensitive.

       Method         = "GET"                    ; Section 8.1
                      | "HEAD"                   ; Section 8.2
                      | "POST"                   ; Section 8.3
                      | extension-method

       extension-method = token

   The list of methods acceptable by a specific resource can change
   dynamically; the client is notified through the return code of the
   response if a method is not allowed on a resource. Servers should
   return the status code 501 (not implemented) if the method is
   unrecognized or not implemented.

   The methods commonly used by HTTP/1.0 applications are fully defined
   in Section 8.

5.1.2 Request-URI

   The Request-URI is a Uniform Resource Identifier (Section 3.2) and
   identifies the resource upon which to apply the request.

       Request-URI    = absoluteURI | abs_path

   The two options for Request-URI are dependent on the nature of the
   request.

   The absoluteURI form is only allowed when the request is being made
   to a proxy. The proxy is requested to forward the request and return
   the response. If the request is GET or HEAD and a prior response is
   cached, the proxy may use the cached message if it passes any
   restrictions in the Expires header field. Note that the proxy may
   forward the request on to another proxy or directly to the server
   specified by the absoluteURI. In order to avoid request loops, a
   proxy must be able to recognize all of its server names, including
   any aliases, local variations, and the numeric IP address. An example
   Request-Line would be:

       GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.0

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   The most common form of Request-URI is that used to identify a
   resource on an origin server or gateway. In this case, only the
   absolute path of the URI is transmitted (see Section 3.2.1,
   abs_path). For example, a client wishing to retrieve the resource
   above directly from the origin server would create a TCP connection
   to port 80 of the host "www.w3.org" and send the line:

       GET /pub/WWW/TheProject.html HTTP/1.0

   followed by the remainder of the Full-Request. Note that the absolute
   path cannot be empty; if none is present in the original URI, it must
   be given as "/" (the server root).

   The Request-URI is transmitted as an encoded string, where some
   characters may be escaped using the "% HEX HEX" encoding defined by
   RFC 1738 [4]. The origin server must decode the Request-URI in order
   to properly interpret the request.

5.2  Request Header Fields

   The request header fields allow the client to pass additional
   information about the request, and about the client itself, to the
   server. These fields act as request modifiers, with semantics
   equivalent to the parameters on a programming language method
   (procedure) invocation.

       Request-Header = Authorization            ; Section 10.2
                      | From                     ; Section 10.8
                      | If-Modified-Since        ; Section 10.9
                      | Referer                  ; Section 10.13
                      | User-Agent               ; Section 10.15

   Request-Header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields may be given the semantics of request
   header fields if all parties in the communication recognize them to
   be request header fields. Unrecognized header fields are treated as
   Entity-Header fields.

6.  Response

   After receiving and interpreting a request message, a server responds
   in the form of an HTTP response message.

       Response        = Simple-Response | Full-Response

       Simple-Response = [ Entity-Body ]

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       Full-Response   = Status-Line             ; Section 6.1
                         *( General-Header       ; Section 4.3
                          | Response-Header      ; Section 6.2
                          | Entity-Header )      ; Section 7.1
                         CRLF
                         [ Entity-Body ]         ; Section 7.2

   A Simple-Response should only be sent in response to an HTTP/0.9
   Simple-Request or if the server only supports the more limited
   HTTP/0.9 protocol. If a client sends an HTTP/1.0 Full-Request and
   receives a response that does not begin with a Status-Line, it should
   assume that the response is a Simple-Response and parse it
   accordingly. Note that the Simple-Response consists only of the
   entity body and is terminated by the server closing the connection.

6.1  Status-Line

   The first line of a Full-Response message is the Status-Line,
   consisting of the protocol version followed by a numeric status code
   and its associated textual phrase, with each element separated by SP
   characters. No CR or LF is allowed except in the final CRLF sequence.

       Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF

   Since a status line always begins with the protocol version and
   status code

       "HTTP/" 1*DIGIT "." 1*DIGIT SP 3DIGIT SP

   (e.g., "HTTP/1.0 200 "), the presence of that expression is
   sufficient to differentiate a Full-Response from a Simple-Response.
   Although the Simple-Response format may allow such an expression to
   occur at the beginning of an entity body, and thus cause a
   misinterpretation of the message if it was given in response to a
   Full-Request, most HTTP/0.9 servers are limited to responses of type
   "text/html" and therefore would never generate such a response.

6.1.1 Status Code and Reason Phrase

   The Status-Code element is a 3-digit integer result code of the
   attempt to understand and satisfy the request. The Reason-Phrase is
   intended to give a short textual description of the Status-Code. The
   Status-Code is intended for use by automata and the Reason-Phrase is
   intended for the human user. The client is not required to examine or
   display the Reason-Phrase.

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   The first digit of the Status-Code defines the class of response. The
   last two digits do not have any categorization role. There are 5
   values for the first digit:

      o 1xx: Informational - Not used, but reserved for future use

      o 2xx: Success - The action was successfully received,
             understood, and accepted.

      o 3xx: Redirection - Further action must be taken in order to
             complete the request

      o 4xx: Client Error - The request contains bad syntax or cannot
             be fulfilled

      o 5xx: Server Error - The server failed to fulfill an apparently
             valid request

   The individual values of the numeric status codes defined for
   HTTP/1.0, and an example set of corresponding Reason-Phrase's, are
   presented below. The reason phrases listed here are only recommended
   -- they may be replaced by local equivalents without affecting the
   protocol. These codes are fully defined in Section 9.

       Status-Code    = "200"   ; OK
                      | "201"   ; Created
                      | "202"   ; Accepted
                      | "204"   ; No Content
                      | "301"   ; Moved Permanently
                      | "302"   ; Moved Temporarily
                      | "304"   ; Not Modified
                      | "400"   ; Bad Request
                      | "401"   ; Unauthorized
                      | "403"   ; Forbidden
                      | "404"   ; Not Found
                      | "500"   ; Internal Server Error
                      | "501"   ; Not Implemented
                      | "502"   ; Bad Gateway
                      | "503"   ; Service Unavailable
                      | extension-code

       extension-code = 3DIGIT

       Reason-Phrase  = *<TEXT, excluding CR, LF>

   HTTP status codes are extensible, but the above codes are the only
   ones generally recognized in current practice. HTTP applications are
   not required to understand the meaning of all registered status

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   codes, though such understanding is obviously desirable. However,
   applications must understand the class of any status code, as
   indicated by the first digit, and treat any unrecognized response as
   being equivalent to the x00 status code of that class, with the
   exception that an unrecognized response must not be cached. For
   example, if an unrecognized status code of 431 is received by the
   client, it can safely assume that there was something wrong with its
   request and treat the response as if it had received a 400 status
   code. In such cases, user agents should present to the user the
   entity returned with the response, since that entity is likely to
   include human-readable information which will explain the unusual
   status.

6.2  Response Header Fields

   The response header fields allow the server to pass additional
   information about the response which cannot be placed in the Status-
   Line. These header fields give information about the server and about
   further access to the resource identified by the Request-URI.

       Response-Header = Location                ; Section 10.11
                       | Server                  ; Section 10.14
                       | WWW-Authenticate        ; Section 10.16

   Response-Header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields may be given the semantics of response
   header fields if all parties in the communication recognize them to
    be response header fields. Unrecognized header fields are treated as
   Entity-Header fields.

7.  Entity

   Full-Request and Full-Response messages may transfer an entity within
   some requests and responses. An entity consists of Entity-Header
   fields and (usually) an Entity-Body. In this section, both sender and
   recipient refer to either the client or the server, depending on who
   sends and who receives the entity.

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7.1  Entity Header Fields

   Entity-Header fields define optional metainformation about the
   Entity-Body or, if no body is present, about the resource identified
   by the request.

       Entity-Header  = Allow                    ; Section 10.1
                      | Content-Encoding         ; Section 10.3
                      | Content-Length           ; Section 10.4
                      | Content-Type             ; Section 10.5
                      | Expires                  ; Section 10.7
                      | Last-Modified            ; Section 10.10
                      | extension-header

       extension-header = HTTP-header

   The extension-header mechanism allows additional Entity-Header fields
   to be defined without changing the protocol, but these fields cannot
   be assumed to be recognizable by the recipient. Unrecognized header
   fields should be ignored by the recipient and forwarded by proxies.

7.2  Entity Body

   The entity body (if any) sent with an HTTP request or response is in
   a format and encoding defined by the Entity-Header fields.

       Entity-Body    = *OCTET

   An entity body is included with a request message only when the
   request method calls for one. The presence of an entity body in a
   request is signaled by the inclusion of a Content-Length header field
   in the request message headers. HTTP/1.0 requests containing an
   entity body must include a valid Content-Length header field.

   For response messages, whether or not an entity body is included with
   a message is dependent on both the request method and the response
   code. All responses to the HEAD request method must not include a
   body, even though the presence of entity header fields may lead one
   to believe they do. All 1xx (informational), 204 (no content), and
   304 (not modified) responses must not include a body. All other
   responses must include an entity body or a Content-Length header
   field defined with a value of zero (0).

7.2.1 Type

   When an Entity-Body is included with a message, the data type of that
   body is determined via the header fields Content-Type and Content-
   Encoding. These define a two-layer, ordered encoding model:

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       entity-body := Content-Encoding( Content-Type( data ) )

   A Content-Type specifies the media type of the underlying data. A
   Content-Encoding may be used to indicate any additional content
   coding applied to the type, usually for the purpose of data
   compression, that is a property of the resource requested. The
   default for the content encoding is none (i.e., the identity
   function).

   Any HTTP/1.0 message containing an entity body should include a
   Content-Type header field defining the media type of that body. If
   and only if the media type is not given by a Content-Type header, as
   is the case for Simple-Response messages, the recipient may attempt
   to guess the media type via inspection of its content and/or the name
   extension(s) of the URL used to identify the resource. If the media
   type remains unknown, the recipient should treat it as type
   "application/octet-stream".

7.2.2 Length

   When an Entity-Body is included with a message, the length of that
   body may be determined in one of two ways. If a Content-Length header
   field is present, its value in bytes represents the length of the
   Entity-Body. Otherwise, the body length is determined by the closing
   of the connection by the server.

   Closing the connection cannot be used to indicate the end of a
   request body, since it leaves no possibility for the server to send
   back a response. Therefore, HTTP/1.0 requests containing an entity
   body must include a valid Content-Length header field. If a request
   contains an entity body and Content-Length is not specified, and the
   server does not recognize or cannot calculate the length from other
   fields, then the server should send a 400 (bad request) response.

      Note: Some older servers supply an invalid Content-Length when
      sending a document that contains server-side includes dynamically
      inserted into the data stream. It must be emphasized that this
      will not be tolerated by future versions of HTTP. Unless the
      client knows that it is receiving a response from a compliant
      server, it should not depend on the Content-Length value being
      correct.

8.  Method Definitions

   The set of common methods for HTTP/1.0 is defined below. Although
   this set can be expanded, additional methods cannot be assumed to
   share the same semantics for separately extended clients and servers.

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8.1  GET

   The GET method means retrieve whatever information (in the form of an
   entity) is identified by the Request-URI. If the Request-URI refers
   to a data-producing process, it is the produced data which shall be
   returned as the entity in the response and not the source text of the
   process, unless that text happens to be the output of the process.

   The semantics of the GET method changes to a "conditional GET" if the
   request message includes an If-Modified-Since header field. A
   conditional GET method requests that the identified resource be
   transferred only if it has been modified since the date given by the
   If-Modified-Since header, as described in Section 10.9. The
   conditional GET method is intended to reduce network usage by
   allowing cached entities to be refreshed without requiring multiple
   requests or transferring unnecessary data.

8.2  HEAD

   The HEAD method is identical to GET except that the server must not
   return any Entity-Body in the response. The metainformation contained
   in the HTTP headers in response to a HEAD request should be identical
   to the information sent in response to a GET request. This method can
   be used for obtaining metainformation about the resource identified
   by the Request-URI without transferring the Entity-Body itself. This
   method is often used for testing hypertext links for validity,
   accessibility, and recent modification.

   There is no "conditional HEAD" request analogous to the conditional
   GET. If an If-Modified-Since header field is included with a HEAD
   request, it should be ignored.

8.3  POST

   The POST method is used to request that the destination server accept
   the entity enclosed in the request as a new subordinate of the
   resource identified by the Request-URI in the Request-Line. POST is
   designed to allow a uniform method to cover the following functions:

      o Annotation of existing resources;

      o Posting a message to a bulletin board, newsgroup, mailing list,
        or similar group of articles;

      o Providing a block of data, such as the result of submitting a
        form [3], to a data-handling process;

      o Extending a database through an append operation.

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   The actual function performed by the POST method is determined by the
   server and is usually dependent on the Request-URI. The posted entity
   is subordinate to that URI in the same way that a file is subordinate
   to a directory containing it, a news article is subordinate to a
   newsgroup to which it is posted, or a record is subordinate to a
   database.

   A successful POST does not require that the entity be created as a
   resource on the origin server or made accessible for future
   reference. That is, the action performed by the POST method might not
   result in a resource that can be identified by a URI. In this case,
   either 200 (ok) or 204 (no content) is the appropriate response
   status, depending on whether or not the response includes an entity
   that describes the result.

   If a resource has been created on the origin server, the response
   should be 201 (created) and contain an entity (preferably of type
   "text/html") which describes the status of the request and refers to
   the new resource.

   A valid Content-Length is required on all HTTP/1.0 POST requests. An
   HTTP/1.0 server should respond with a 400 (bad request) message if it
   cannot determine the length of the request message's content.

   Applications must not cache responses to a POST request because the
   application has no way of knowing that the server would return an
   equivalent response on some future request.

9.  Status Code Definitions

   Each Status-Code is described below, including a description of which
   method(s) it can follow and any metainformation required in the
   response.

9.1  Informational 1xx

   This class of status code indicates a provisional response,
   consisting only of the Status-Line and optional headers, and is
   terminated by an empty line. HTTP/1.0 does not define any 1xx status
   codes and they are not a valid response to a HTTP/1.0 request.
   However, they may be useful for experimental applications which are
   outside the scope of this specification.

9.2  Successful 2xx

   This class of status code indicates that the client's request was
   successfully received, understood, and accepted.

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   200 OK

   The request has succeeded. The information returned with the
   response is dependent on the method used in the request, as follows:

   GET    an entity corresponding to the requested resource is sent
          in the response;

   HEAD   the response must only contain the header information and
          no Entity-Body;

   POST   an entity describing or containing the result of the action.

   201 Created

   The request has been fulfilled and resulted in a new resource being
   created. The newly created resource can be referenced by the URI(s)
   returned in the entity of the response. The origin server should
   create the resource before using this Status-Code. If the action
   cannot be carried out immediately, the server must include in the
   response body a description of when the resource will be available;
   otherwise, the server should respond with 202 (accepted).

   Of the methods defined by this specification, only POST can create a
   resource.

   202 Accepted

   The request has been accepted for processing, but the processing
   has not been completed. The request may or may not eventually be
   acted upon, as it may be disallowed when processing actually takes
   place. There is no facility for re-sending a status code from an
   asynchronous operation such as this.

   The 202 response is intentionally non-committal. Its purpose is to
   allow a server to accept a request for some other process (perhaps
   a batch-oriented process that is only run once per day) without
   requiring that the user agent's connection to the server persist
   until the process is completed. The entity returned with this
   response should include an indication of the request's current
   status and either a pointer to a status monitor or some estimate of
   when the user can expect the request to be fulfilled.

   204 No Content

   The server has fulfilled the request but there is no new
   information to send back. If the client is a user agent, it should
   not change its document view from that which caused the request to

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   be generated. This response is primarily intended to allow input
   for scripts or other actions to take place without causing a change
   to the user agent's active document view. The response may include
   new metainformation in the form of entity headers, which should
   apply to the document currently in the user agent's active view.

9.3  Redirection 3xx

   This class of status code indicates that further action needs to be
   taken by the user agent in order to fulfill the request. The action
   required may be carried out by the user agent without interaction
   with the user if and only if the method used in the subsequent
   request is GET or HEAD. A user agent should never automatically
   redirect a request more than 5 times, since such redirections usually
   indicate an infinite loop.

   300 Multiple Choices

   This response code is not directly used by HTTP/1.0 applications,
   but serves as the default for interpreting the 3xx class of
   responses.

   The requested resource is available at one or more locations.
   Unless it was a HEAD request, the response should include an entity
   containing a list of resource characteristics and locations from
   which the user or user agent can choose the one most appropriate.
   If the server has a preferred choice, it should include the URL in
   a Location field; user agents may use this field value for
   automatic redirection.

   301 Moved Permanently

   The requested resource has been assigned a new permanent URL and
   any future references to this resource should be done using that
   URL. Clients with link editing capabilities should automatically
   relink references to the Request-URI to the new reference returned
   by the server, where possible.

   The new URL must be given by the Location field in the response.
   Unless it was a HEAD request, the Entity-Body of the response
   should contain a short note with a hyperlink to the new URL.

   If the 301 status code is received in response to a request using
   the POST method, the user agent must not automatically redirect the
   request unless it can be confirmed by the user, since this might
   change the conditions under which the request was issued.

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       Note: When automatically redirecting a POST request after
       receiving a 301 status code, some existing user agents will
       erroneously change it into a GET request.

   302 Moved Temporarily

   The requested resource resides temporarily under a different URL.
   Since the redirection may be altered on occasion, the client should
   continue to use the Request-URI for future requests.

   The URL must be given by the Location field in the response. Unless
   it was a HEAD request, the Entity-Body of the response should
   contain a short note with a hyperlink to the new URI(s).

   If the 302 status code is received in response to a request using
   the POST method, the user agent must not automatically redirect the
   request unless it can be confirmed by the user, since this might
   change the conditions under which the request was issued.

       Note: When automatically redirecting a POST request after
       receiving a 302 status code, some existing user agents will
       erroneously change it into a GET request.

   304 Not Modified

   If the client has performed a conditional GET request and access is
   allowed, but the document has not been modified since the date and
   time specified in the If-Modified-Since field, the server must
   respond with this status code and not send an Entity-Body to the
   client. Header fields contained in the response should only include
   information which is relevant to cache managers or which may have
   changed independently of the entity's Last-Modified date. Examples
   of relevant header fields include: Date, Server, and Expires. A
   cache should update its cached entity to reflect any new field
   values given in the 304 response.

9.4  Client Error 4xx

   The 4xx class of status code is intended for cases in which the
   client seems to have erred. If the client has not completed the
   request when a 4xx code is received, it should immediately cease
   sending data to the server. Except when responding to a HEAD request,
   the server should include an entity containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition. These status codes are applicable to any request method.

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      Note: If the client is sending data, server implementations on TCP
      should be careful to ensure that the client acknowledges receipt
      of the packet(s) containing the response prior to closing the
      input connection. If the client continues sending data to the
      server after the close, the server's controller will send a reset
      packet to the client, which may erase the client's unacknowledged
      input buffers before they can be read and interpreted by the HTTP
      application.

   400 Bad Request

   The request could not be understood by the server due to malformed
   syntax. The client should not repeat the request without
   modifications.

   401 Unauthorized

   The request requires user authentication. The response must include
   a WWW-Authenticate header field (Section 10.16) containing a
   challenge applicable to the requested resource. The client may
   repeat the request with a suitable Authorization header field
   (Section 10.2). If the request already included Authorization
   credentials, then the 401 response indicates that authorization has
   been refused for those credentials. If the 401 response contains
   the same challenge as the prior response, and the user agent has
   already attempted authentication at least once, then the user
   should be presented the entity that was given in the response,
   since that entity may include relevant diagnostic information. HTTP
   access authentication is explained in Section 11.

   403 Forbidden

   The server understood the request, but is refusing to fulfill it.
   Authorization will not help and the request should not be repeated.
   If the request method was not HEAD and the server wishes to make
   public why the request has not been fulfilled, it should describe
   the reason for the refusal in the entity body. This status code is
   commonly used when the server does not wish to reveal exactly why
   the request has been refused, or when no other response is
   applicable.

   404 Not Found

   The server has not found anything matching the Request-URI. No
   indication is given of whether the condition is temporary or
   permanent. If the server does not wish to make this information
   available to the client, the status code 403 (forbidden) can be
   used instead.

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9.5  Server Error 5xx

   Response status codes beginning with the digit "5" indicate cases in
   which the server is aware that it has erred or is incapable of
   performing the request. If the client has not completed the request
   when a 5xx code is received, it should immediately cease sending data
   to the server. Except when responding to a HEAD request, the server
   should include an entity containing an explanation of the error
   situation, and whether it is a temporary or permanent condition.
   These response codes are applicable to any request method and there
   are no required header fields.

   500 Internal Server Error

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

   501 Not Implemented

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

   502 Bad Gateway

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

   503 Service Unavailable

   The server is currently unable to handle the request due to a
   temporary overloading or maintenance of the server. The implication
   is that this is a temporary condition which will be alleviated
   after some delay.

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

10.  Header Field Definitions

   This section defines the syntax and semantics of all commonly used
   HTTP/1.0 header fields. For general and entity header fields, both
   sender and recipient refer to either the client or the server,
   depending on who sends and who receives the message.

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10.1  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. The Allow header field is not permitted in a request
   using the POST method, and thus should be ignored if it is received
   as part of a POST entity.

       Allow          = "Allow" ":" 1#method

    Example of use:

       Allow: GET, HEAD

   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.

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

   The Allow header field does not indicate what methods are implemented
   by the server.

10.2  Authorization

   A user agent that wishes to authenticate itself with a server--
   usually, but not necessarily, after receiving a 401 response--may do
   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 Section 11. If a request
   is authenticated and a realm specified, the same credentials should
   be valid for all other requests within this realm.

   Responses to requests containing an Authorization field are not
   cachable.

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10.3  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
   coding has been applied to the resource, and thus what decoding
   mechanism must be applied in order to obtain the media-type
   referenced by the Content-Type header field. The 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" ":" content-coding

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

       Content-Encoding: x-gzip

   The Content-Encoding is a characteristic of the resource identified
   by the Request-URI. Typically, the resource is stored with this
   encoding and is only decoded before rendering or analogous usage.

10.4  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 size of the
   Entity-Body to be transferred, regardless of the media type of the
   entity. A valid Content-Length field value is required on all
   HTTP/1.0 request messages containing an entity body.

   Any Content-Length greater than or equal to zero is a valid value.
   Section 7.2.2 describes how to determine the length of a response
   entity body if a Content-Length is not given.

      Note: 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 used whenever the entity's length can be
      determined prior to being transferred.

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10.5  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 3.6. An example of the field is

       Content-Type: text/html

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

10.6  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.

       Date           = "Date" ":" HTTP-date

   An example is

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

   If a message is received via direct connection with the user agent
   (in the case of requests) or the origin server (in the case of
   responses), then the date can be assumed to be the current date at
   the receiving end. However, since the date--as it is believed by the
   origin--is important for evaluating cached responses, origin servers
   should always include a Date header. Clients should only send a Date
   header field in messages that include an entity body, as in the case
   of the POST request, and even then it is optional. A received message
   which does not have a Date header field should be assigned one by the
   recipient if the message will be cached by that recipient or
   gatewayed via a protocol which requires a Date.

   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.

      Note: An earlier version of this document incorrectly specified
      that this field should contain the creation date of the enclosed
      Entity-Body. This has been changed to reflect actual (and proper)

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      usage.

10.7  Expires

   The Expires entity-header field gives the date/time after which the
   entity should be considered stale. This allows information providers
   to suggest the volatility of the resource, or a date after which the
   information may no longer be valid. Applications must not cache this
   entity beyond the date given. 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. However, information providers that
   know or even suspect that a resource will change by a certain date
   should include an Expires header with that date. The format is an
   absolute date and time as defined by HTTP-date in Section 3.3.

       Expires        = "Expires" ":" HTTP-date

   An example of its use is

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

   If the date given is equal to or earlier than the value of the Date
   header, the recipient must not cache the enclosed entity. If a
   resource is dynamic by nature, as is the case with many data-
   producing processes, entities from that resource should be given an
   appropriate Expires value which reflects that dynamism.

   The Expires field cannot be used to force a user agent to refresh its
   display or reload a resource; its semantics apply only to caching
   mechanisms, and such mechanisms need only check a resource's
   expiration status when a new request for that resource is initiated.

   User agents often have history mechanisms, such as "Back" buttons and
   history lists, which can be used to redisplay an entity retrieved
   earlier in a session. By default, the Expires field does not apply to
   history mechanisms. If the entity is still in storage, a history
   mechanism should display it even if the entity has expired, unless
   the user has specifically configured the agent to refresh expired
   history documents.

      Note: Applications are encouraged to be tolerant of bad or
      misinformed implementations of the Expires header. A value of zero
      (0) or an invalid date format should be considered equivalent to
      an "expires immediately." Although these values are not legitimate
      for HTTP/1.0, a robust implementation is always desirable.

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10.8  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 [7] (as updated by RFC 1123 [6]):

       From           = "From" ":" mailbox

   An example is:

       From: webmaster@w3.org

   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.

   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.

10.9  If-Modified-Since

   The If-Modified-Since request-header field is used with the GET
   method to make it conditional: if the requested resource has not been
   modified since the time specified in this field, a copy of the
   resource will not be returned from the server; instead, a 304 (not
   modified) response will be returned without any Entity-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

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   A conditional GET method requests that the identified resource 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 resource has been modified since the
         If-Modified-Since date, the response is exactly the same as
         for a normal GET.

      c) If the resource has not been modified since a valid
         If-Modified-Since date, the server shall 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.

10.10  Last-Modified

   The Last-Modified entity-header field indicates the date and time at
   which the sender believes the resource was last modified. The exact
   semantics of this field are defined in terms of how the recipient
   should interpret it:  if the recipient has a copy of this resource
   which is older than the date given by the Last-Modified field, that
   copy should be considered stale.

       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 sender 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 timestamp of the record. For virtual objects,
   it may be the last time the internal state changed.

   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

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   future, the server must replace that date with the message
   origination date.

10.11  Location

   The Location response-header field defines the exact location of the
   resource that was identified by the Request-URI. For 3xx responses,
   the location must indicate the server's preferred URL for automatic
   redirection to the resource. Only one absolute URL is allowed.

       Location       = "Location" ":" absoluteURI

   An example is

       Location: http://www.w3.org/hypertext/WWW/NewLocation.html

10.12  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 [ "=" word ]

   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 allows a
   client to insist upon receiving an authoritative response to its
   request. It also allows a client to refresh a cached copy which is
   known to be corrupted or stale.

   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.

10.13  Referer

   The Referer 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. This allows a server to generate lists

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   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 a partial URI is given, 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.

10.14  Server

   The Server response-header field contains information about the
   software used by the origin server to handle the request. The field
   can contain multiple product tokens (Section 3.7) and comments
   identifying the server and any significant subproducts. By
   convention, the product tokens are listed in order of their
   significance for identifying the application.

       Server         = "Server" ":" 1*( product | comment )

   Example:

       Server: CERN/3.0 libwww/2.17

   If the response is being forwarded through a proxy, the proxy
   application must not add its data to the product list.

      Note: Revealing the specific software version of the server may
      allow the server machine to become more vulnerable to attacks
      against software that is known to contain security holes. Server
      implementors are encouraged to make this field a configurable
      option.

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      Note: Some existing servers fail to restrict themselves to the
      product token syntax within the Server field.

10.15  User-Agent

   The User-Agent request-header field contains information about the
   user agent originating the request. This is for statistical purposes,
   the tracing of protocol violations, and automated recognition of user
   agents for the sake of tailoring responses to avoid particular user
   agent limitations. Although it is not required, user agents should
   include this field with requests. The field can contain multiple
   product tokens (Section 3.7) and comments identifying the agent and
   any subproducts which form a significant part of the user agent. By
   convention, the product tokens are listed in order of their
   significance for identifying the application.

       User-Agent     = "User-Agent" ":" 1*( product | comment )

   Example:

       User-Agent: CERN-LineMode/2.15 libwww/2.17b3

       Note: Some current proxy applications append their product
       information to the list in the User-Agent field. This is not
       recommended, since it makes machine interpretation of these
       fields ambiguous.

       Note: Some existing clients fail to restrict themselves to
       the product token syntax within the User-Agent field.

10.16  WWW-Authenticate

   The WWW-Authenticate response-header field must be included in 401
   (unauthorized) response messages. The field value consists of at
   least one challenge that indicates the authentication scheme(s) and
   parameters applicable to the Request-URI.

       WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge

   The HTTP access authentication process is described in Section 11.
   User agents must take special care in parsing the WWW-Authenticate
   field value if it contains more than one challenge, or if more than
   one WWW-Authenticate header field is provided, since the contents of
   a challenge may itself contain a comma-separated list of
   authentication parameters.


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