Network Working Group                                     T. Berners-Lee
Request for Comments: 1945                                       MIT/LCS
Category: Informational                                      R. Fielding
                                                              UC Irvine
                                                             H. Frystyk
                                                                MIT/LCS
                                                               May 1996


               Hypertext Transfer Protocol -- HTTP/1.0

Status of This Memo

  This memo provides information for the Internet community.  This memo
  does not specify an Internet standard of any kind.  Distribution of
  this memo is unlimited.

IESG Note:

  The IESG has concerns about this protocol, and expects this document
  to be replaced relatively soon by a standards track document.

Abstract

  The Hypertext Transfer Protocol (HTTP) is an application-level
  protocol with the lightness and speed necessary for distributed,
  collaborative, hypermedia information systems. It is a generic,
  stateless, object-oriented protocol which can be used for many tasks,
  such as name servers and distributed object management systems,
  through extension of its request methods (commands). A feature of
  HTTP is the typing of data representation, allowing systems to be
  built independently of the data being transferred.

  HTTP has been in use by the World-Wide Web global information
  initiative since 1990. This specification reflects common usage of
  the protocol referred to as "HTTP/1.0".

Table of Contents

  1.  Introduction ..............................................  4
      1.1  Purpose ..............................................  4
      1.2  Terminology ..........................................  4
      1.3  Overall Operation ....................................  6
      1.4  HTTP and MIME ........................................  8
  2.  Notational Conventions and Generic Grammar ................  8
      2.1  Augmented BNF ........................................  8
      2.2  Basic Rules .......................................... 10
  3.  Protocol Parameters ....................................... 12



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      3.1  HTTP Version ......................................... 12
      3.2  Uniform Resource Identifiers ......................... 14
           3.2.1  General Syntax ................................ 14
           3.2.2  http URL ...................................... 15
      3.3  Date/Time Formats .................................... 15
      3.4  Character Sets ....................................... 17
      3.5  Content Codings ...................................... 18
      3.6  Media Types .......................................... 19
           3.6.1  Canonicalization and Text Defaults ............ 19
           3.6.2  Multipart Types ............................... 20
      3.7  Product Tokens ....................................... 20
  4.  HTTP Message .............................................. 21
      4.1  Message Types ........................................ 21
      4.2  Message Headers ...................................... 22
      4.3  General Header Fields ................................ 23
  5.  Request ................................................... 23
      5.1  Request-Line ......................................... 23
           5.1.1  Method ........................................ 24
           5.1.2  Request-URI ................................... 24
      5.2  Request Header Fields ................................ 25
  6.  Response .................................................. 25
      6.1  Status-Line .......................................... 26
           6.1.1  Status Code and Reason Phrase ................. 26
      6.2  Response Header Fields ............................... 28
  7.  Entity .................................................... 28
      7.1  Entity Header Fields ................................. 29
      7.2  Entity Body .......................................... 29
           7.2.1  Type .......................................... 29
           7.2.2  Length ........................................ 30
  8.  Method Definitions ........................................ 30
      8.1  GET .................................................. 31
      8.2  HEAD ................................................. 31
      8.3  POST ................................................. 31
  9.  Status Code Definitions ................................... 32
      9.1  Informational 1xx .................................... 32
      9.2  Successful 2xx ....................................... 32
      9.3  Redirection 3xx ...................................... 34
      9.4  Client Error 4xx ..................................... 35
      9.5  Server Error 5xx ..................................... 37
  10. Header Field Definitions .................................. 37
      10.1  Allow ............................................... 38
      10.2  Authorization ....................................... 38
      10.3  Content-Encoding .................................... 39
      10.4  Content-Length ...................................... 39
      10.5  Content-Type ........................................ 40
      10.6  Date ................................................ 40
      10.7  Expires ............................................. 41
      10.8  From ................................................ 42



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      10.9  If-Modified-Since ................................... 42
      10.10 Last-Modified ....................................... 43
      10.11 Location ............................................ 44
      10.12 Pragma .............................................. 44
      10.13 Referer ............................................. 44
      10.14 Server .............................................. 45
      10.15 User-Agent .......................................... 46
      10.16 WWW-Authenticate .................................... 46
  11. Access Authentication ..................................... 47
      11.1  Basic Authentication Scheme ......................... 48
  12. Security Considerations ................................... 49
      12.1  Authentication of Clients ........................... 49
      12.2  Safe Methods ........................................ 49
      12.3  Abuse of Server Log Information ..................... 50
      12.4  Transfer of Sensitive Information ................... 50
      12.5  Attacks Based On File and Path Names ................ 51
  13. Acknowledgments ........................................... 51
  14. References ................................................ 52
  15. Authors' Addresses ........................................ 54
  Appendix A.   Internet Media Type message/http ................ 55
  Appendix B.   Tolerant Applications ........................... 55
  Appendix C.   Relationship to MIME ............................ 56
      C.1  Conversion to Canonical Form ......................... 56
      C.2  Conversion of Date Formats ........................... 57
      C.3  Introduction of Content-Encoding ..................... 57
      C.4  No Content-Transfer-Encoding ......................... 57
      C.5  HTTP Header Fields in Multipart Body-Parts ........... 57
  Appendix D.   Additional Features ............................. 57
      D.1  Additional Request Methods ........................... 58
           D.1.1  PUT ........................................... 58
           D.1.2  DELETE ........................................ 58
           D.1.3  LINK .......................................... 58
           D.1.4  UNLINK ........................................ 58
      D.2  Additional Header Field Definitions .................. 58
           D.2.1  Accept ........................................ 58
           D.2.2  Accept-Charset ................................ 59
           D.2.3  Accept-Encoding ............................... 59
           D.2.4  Accept-Language ............................... 59
           D.2.5  Content-Language .............................. 59
           D.2.6  Link .......................................... 59
           D.2.7  MIME-Version .................................. 59
           D.2.8  Retry-After ................................... 60
           D.2.9  Title ......................................... 60
           D.2.10 URI ........................................... 60







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1.  Introduction

1.1  Purpose

  The Hypertext Transfer Protocol (HTTP) is an application-level
  protocol with the lightness and speed necessary for distributed,
  collaborative, hypermedia information systems. HTTP has been in use
  by the World-Wide Web global information initiative since 1990. This
  specification reflects common usage of the protocol referred too as
  "HTTP/1.0". This specification describes the features that seem to be
  consistently implemented in most HTTP/1.0 clients and servers. The
  specification is split into two sections. Those features of HTTP for
  which implementations are usually consistent are described in the
  main body of this document. Those features which have few or
  inconsistent implementations are listed in Appendix D.

  Practical information systems require more functionality than simple
  retrieval, including search, front-end update, and annotation. HTTP
  allows an open-ended set of methods to be used to indicate the
  purpose of a request. It builds on the discipline of reference
  provided by the Uniform Resource Identifier (URI) [2], as a location
  (URL) [4] or name (URN) [16], for indicating the resource on which a
  method is to be applied. Messages are passed in a format similar to
  that used by Internet Mail [7] and the Multipurpose Internet Mail
  Extensions (MIME) [5].

  HTTP is also used as a generic protocol for communication between
  user agents and proxies/gateways to other Internet protocols, such as
  SMTP [12], NNTP [11], FTP [14], Gopher [1], and WAIS [8], allowing
  basic hypermedia access to resources available from diverse
  applications and simplifying the implementation of user agents.

1.2  Terminology

  This specification uses a number of terms to refer to the roles
  played by participants in, and objects of, the HTTP communication.

  connection

      A transport layer virtual circuit established between two
      application programs for the purpose of communication.

  message

      The basic unit of HTTP communication, consisting of a structured
      sequence of octets matching the syntax defined in Section 4 and
      transmitted via the connection.




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  request

      An HTTP request message (as defined in Section 5).

  response

      An HTTP response message (as defined in Section 6).

  resource

      A network data object or service which can be identified by a
      URI (Section 3.2).

  entity

      A particular representation or rendition of a data resource, or
      reply from a service resource, that may be enclosed within a
      request or response message. An entity consists of
      metainformation in the form of entity headers and content in the
      form of an entity body.

  client

      An application program that establishes connections for the
      purpose of sending requests.

  user agent

      The client which initiates a request. These are often browsers,
      editors, spiders (web-traversing robots), or other end user
      tools.

  server

      An application program that accepts connections in order to
      service requests by sending back responses.

  origin server

      The server on which a given resource resides or is to be created.

  proxy

      An intermediary program which acts as both a server and a client
      for the purpose of making requests on behalf of other clients.
      Requests are serviced internally or by passing them, with
      possible translation, on to other servers. A proxy must
      interpret and, if necessary, rewrite a request message before



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      forwarding it. Proxies are often used as client-side portals
      through network firewalls and as helper applications for
      handling requests via protocols not implemented by the user
      agent.

  gateway

      A server which acts as an intermediary for some other server.
      Unlike a proxy, a gateway receives requests as if it were the
      origin server for the requested resource; the requesting client
      may not be aware that it is communicating with a gateway.
      Gateways are often used as server-side portals through network
      firewalls and as protocol translators for access to resources
      stored on non-HTTP systems.

  tunnel

      A tunnel is an intermediary program which is acting as a blind
      relay between two connections. Once active, a tunnel is not
      considered a party to the HTTP communication, though the tunnel
      may have been initiated by an HTTP request. The tunnel ceases to
      exist when both ends of the relayed connections are closed.
      Tunnels are used when a portal is necessary and the intermediary
      cannot, or should not, interpret the relayed communication.

  cache

      A program's local store of response messages and the subsystem
      that controls its message storage, retrieval, and deletion. A
      cache stores cachable responses in order to reduce the response
      time and network bandwidth consumption on future, equivalent
      requests. Any client or server may include a cache, though a
      cache cannot be used by a server while it is acting as a tunnel.

  Any given program may be capable of being both a client and a server;
  our use of these terms refers only to the role being performed by the
  program for a particular connection, rather than to the program's
  capabilities in general. Likewise, any server may act as an origin
  server, proxy, gateway, or tunnel, switching behavior based on the
  nature of each request.

1.3  Overall Operation

  The HTTP protocol is based on a request/response paradigm. A client
  establishes a connection with a server and sends a request to the
  server in the form of a request method, URI, and protocol version,
  followed by a MIME-like message containing request modifiers, client
  information, and possible body content. The server responds with a



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  status line, including the message's protocol version and a success
  or error code, followed by a MIME-like message containing server
  information, entity metainformation, and possible body content.

  Most HTTP communication is initiated by a user agent and consists of
  a request to be applied to a resource on some origin server. In the
  simplest case, this may be accomplished via a single connection (v)
  between the user agent (UA) and the origin server (O).

         request chain ------------------------>
      UA -------------------v------------------- O
         <----------------------- response chain

  A more complicated situation occurs when one or more intermediaries
  are present in the request/response chain. There are three common
  forms of intermediary: proxy, gateway, and tunnel. A proxy is a
  forwarding agent, receiving requests for a URI in its absolute form,
  rewriting all or parts of the message, and forwarding the reformatted
  request toward the server identified by the URI. A gateway is a
  receiving agent, acting as a layer above some other server(s) and, if
  necessary, translating the requests to the underlying server's
  protocol. A tunnel acts as a relay point between two connections
  without changing the messages; tunnels are used when the
  communication needs to pass through an intermediary (such as a
  firewall) even when the intermediary cannot understand the contents
  of the messages.

         request chain -------------------------------------->
      UA -----v----- A -----v----- B -----v----- C -----v----- O
         <------------------------------------- response chain

  The figure above shows three intermediaries (A, B, and C) between the
  user agent and origin server. A request or response message that
  travels the whole chain must pass through four separate connections.
  This distinction is important because some HTTP communication options
  may apply only to the connection with the nearest, non-tunnel
  neighbor, only to the end-points of the chain, or to all connections
  along the chain. Although the diagram is linear, each participant may
  be engaged in multiple, simultaneous communications. For example, B
  may be receiving requests from many clients other than A, and/or
  forwarding requests to servers other than C, at the same time that it
  is handling A's request.

  Any party to the communication which is not acting as a tunnel may
  employ an internal cache for handling requests. The effect of a cache
  is that the request/response chain is shortened if one of the
  participants along the chain has a cached response applicable to that
  request. The following illustrates the resulting chain if B has a



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  cached copy of an earlier response from O (via C) for a request which
  has not been cached by UA or A.

         request chain ---------->
      UA -----v----- A -----v----- B - - - - - - C - - - - - - O
         <--------- response chain

  Not all responses are cachable, and some requests may contain
  modifiers which place special requirements on cache behavior. Some
  HTTP/1.0 applications use heuristics to describe what is or is not a
  "cachable" response, but these rules are not standardized.

  On the Internet, HTTP communication generally takes place over TCP/IP
  connections. The default port is TCP 80 [15], but other ports can be
  used. This does not preclude HTTP from being implemented on top of
  any other protocol on the Internet, or on other networks. HTTP only
  presumes a reliable transport; any protocol that provides such
  guarantees can be used, and the mapping of the HTTP/1.0 request and
  response structures onto the transport data units of the protocol in
  question is outside the scope of this specification.

  Except for experimental applications, current practice requires that
  the connection be established by the client prior to each request and
  closed by the server after sending the response. Both clients and
  servers should be aware that either party may close the connection
  prematurely, due to user action, automated time-out, or program
  failure, and should handle such closing in a predictable fashion. In
  any case, the closing of the connection by either or both parties
  always terminates the current request, regardless of its status.

1.4  HTTP and MIME

  HTTP/1.0 uses many of the constructs defined for MIME, as defined in
  RFC 1521 [5]. Appendix C describes the ways in which the context of
  HTTP allows for different use of Internet Media Types than is
  typically found in Internet mail, and gives the rationale for those
  differences.

2.  Notational Conventions and Generic Grammar

2.1  Augmented BNF

  All of the mechanisms specified in this document are described in
  both prose and an augmented Backus-Naur Form (BNF) similar to that
  used by RFC 822 [7]. Implementors will need to be familiar with the
  notation in order to understand this specification. The augmented BNF
  includes the following constructs:




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  name = definition

      The name of a rule is simply the name itself (without any
      enclosing "<" and ">") and is separated from its definition by
      the equal character "=". Whitespace is only significant in that
      indentation of continuation lines is used to indicate a rule
      definition that spans more than one line. Certain basic rules
      are in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc.
      Angle brackets are used within definitions whenever their
      presence will facilitate discerning the use of rule names.

  "literal"

      Quotation marks surround literal text. Unless stated otherwise,
      the text is case-insensitive.

  rule1 | rule2

      Elements separated by a bar ("I") are alternatives,
      e.g., "yes | no" will accept yes or no.

  (rule1 rule2)

      Elements enclosed in parentheses are treated as a single
      element. Thus, "(elem (foo | bar) elem)" allows the token
      sequences "elem foo elem" and "elem bar elem".

  *rule

      The character "*" preceding an element indicates repetition. The
      full form is "<n>*<m>element" indicating at least <n> and at
      most <m> occurrences of element. Default values are 0 and
      infinity so that "*(element)" allows any number, including zero;
      "1*element" requires at least one; and "1*2element" allows one
      or two.

  [rule]

      Square brackets enclose optional elements; "[foo bar]" is
      equivalent to "*1(foo bar)".

  N rule

      Specific repetition: "<n>(element)" is equivalent to
      "<n>*<n>(element)"; that is, exactly <n> occurrences of
      (element). Thus 2DIGIT is a 2-digit number, and 3ALPHA is a
      string of three alphabetic characters.




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  #rule

      A construct "#" is defined, similar to "*", for defining lists
      of elements. The full form is "<n>#<m>element" indicating at
      least <n> and at most <m> elements, each separated by one or
      more commas (",") and optional linear whitespace (LWS). This
      makes the usual form of lists very easy; a rule such as
      "( *LWS element *( *LWS "," *LWS element ))" can be shown as
      "1#element". Wherever this construct is used, null elements are
      allowed, but do not contribute to the count of elements present.
      That is, "(element), , (element)" is permitted, but counts as
      only two elements. Therefore, where at least one element is
      required, at least one non-null element must be present. Default
      values are 0 and infinity so that "#(element)" allows any
      number, including zero; "1#element" requires at least one; and
      "1#2element" allows one or two.

  ; comment

      A semi-colon, set off some distance to the right of rule text,
      starts a comment that continues to the end of line. This is a
      simple way of including useful notes in parallel with the
      specifications.

  implied *LWS

      The grammar described by this specification is word-based.
      Except where noted otherwise, linear whitespace (LWS) can be
      included between any two adjacent words (token or
      quoted-string), and between adjacent tokens and delimiters
      (tspecials), without changing the interpretation of a field. At
      least one delimiter (tspecials) must exist between any two
      tokens, since they would otherwise be interpreted as a single
      token. However, applications should attempt to follow "common
      form" when generating HTTP constructs, since there exist some
      implementations that fail to accept anything beyond the common
      forms.

2.2  Basic Rules

  The following rules are used throughout this specification to
  describe basic parsing constructs. The US-ASCII coded character set
  is defined by [17].

      OCTET          = <any 8-bit sequence of data>
      CHAR           = <any US-ASCII character (octets 0 - 127)>
      UPALPHA        = <any US-ASCII uppercase letter "A".."Z">
      LOALPHA        = <any US-ASCII lowercase letter "a".."z">



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      ALPHA          = UPALPHA | LOALPHA
      DIGIT          = <any US-ASCII digit "0".."9">
      CTL            = <any US-ASCII control character
                       (octets 0 - 31) and DEL (127)>
      CR             = <US-ASCII CR, carriage return (13)>
      LF             = <US-ASCII LF, linefeed (10)>
      SP             = <US-ASCII SP, space (32)>
      HT             = <US-ASCII HT, horizontal-tab (9)>
      <">            = <US-ASCII double-quote mark (34)>

  HTTP/1.0 defines the octet sequence CR LF as the end-of-line marker
  for all protocol elements except the Entity-Body (see Appendix B for
  tolerant applications). The end-of-line marker within an Entity-Body
  is defined by its associated media type, as described in Section 3.6.

      CRLF           = CR LF

  HTTP/1.0 headers may be folded onto multiple lines if each
  continuation line begins with a space or horizontal tab. All linear
  whitespace, including folding, has the same semantics as SP.

      LWS            = [CRLF] 1*( SP | HT )

  However, folding of header lines is not expected by some
  applications, and should not be generated by HTTP/1.0 applications.

  The TEXT rule is only used for descriptive field contents and values
  that are not intended to be interpreted by the message parser. Words
  of *TEXT may contain octets from character sets other than US-ASCII.

      TEXT           = <any OCTET except CTLs,
                       but including LWS>

  Recipients of header field TEXT containing octets outside the US-
  ASCII character set may assume that they represent ISO-8859-1
  characters.

  Hexadecimal numeric characters are used in several protocol elements.

      HEX            = "A" | "B" | "C" | "D" | "E" | "F"
                     | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT

  Many HTTP/1.0 header field values consist of words separated by LWS
  or special characters. These special characters must be in a quoted
  string to be used within a parameter value.

      word           = token | quoted-string




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      token          = 1*<any CHAR except CTLs or tspecials>

      tspecials      = "(" | ")" | "<" | ">" | "@"
                     | "," | ";" | ":" | "\" | <">
                     | "/" | "[" | "]" | "?" | "="
                     | "{" | "}" | SP | HT

  Comments may be included in some HTTP header fields by surrounding
  the comment text with parentheses. Comments are only allowed in
  fields containing "comment" as part of their field value definition.
  In all other fields, parentheses are considered part of the field
  value.

      comment        = "(" *( ctext | comment ) ")"
      ctext          = <any TEXT excluding "(" and ")">

  A string of text is parsed as a single word if it is quoted using
  double-quote marks.

      quoted-string  = ( <"> *(qdtext) <"> )

      qdtext         = <any CHAR except <"> and CTLs,
                       but including LWS>

  Single-character quoting using the backslash ("\") character is not
  permitted in HTTP/1.0.

3.  Protocol Parameters

3.1  HTTP Version

  HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
  of the protocol. The protocol versioning policy is intended to allow
  the sender to indicate the format of a message and its capacity for
  understanding further HTTP communication, rather than the features
  obtained via that communication. No change is made to the version
  number for the addition of message components which do not affect
  communication behavior or which only add to extensible field values.
  The <minor> number is incremented when the changes made to the
  protocol add features which do not change the general message parsing
  algorithm, but which may add to the message semantics and imply
  additional capabilities of the sender. The <major> number is
  incremented when the format of a message within the protocol is
  changed.

  The version of an HTTP message is indicated by an HTTP-Version field
  in the first line of the message. If the protocol version is not
  specified, the recipient must assume that the message is in the



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  simple HTTP/0.9 format.

      HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT

  Note that the major and minor numbers should be treated as separate
  integers and that each may be incremented higher than a single digit.
  Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
  lower than HTTP/12.3. Leading zeros should be ignored by recipients
  and never generated by senders.

  This document defines both the 0.9 and 1.0 versions of the HTTP
  protocol. Applications sending Full-Request or Full-Response
  messages, as defined by this specification, must include an HTTP-
  Version of "HTTP/1.0".

  HTTP/1.0 servers must:

     o recognize the format of the Request-Line for HTTP/0.9 and
       HTTP/1.0 requests;

     o understand any valid request in the format of HTTP/0.9 or
       HTTP/1.0;

     o respond appropriately with a message in the same protocol
       version used by the client.

  HTTP/1.0 clients must:

     o recognize the format of the Status-Line for HTTP/1.0 responses;

     o understand any valid response in the format of HTTP/0.9 or
       HTTP/1.0.

  Proxy and gateway applications must be careful in forwarding requests
  that are received in a format different than that of the
  application's native HTTP version. Since the protocol version
  indicates the protocol capability of the sender, a proxy/gateway must
  never send a message with a version indicator which is greater than
  its native version; if a higher version request is received, the
  proxy/gateway must either downgrade the request version or respond
  with an error. Requests with a version lower than that of the
  application's native format may be upgraded before being forwarded;
  the proxy/gateway's response to that request must follow the server
  requirements listed above.







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3.2  Uniform Resource Identifiers

  URIs have been known by many names: WWW addresses, Universal Document
  Identifiers, Universal Resource Identifiers [2], and finally the
  combination of Uniform Resource Locators (URL) [4] and Names (URN)
  [16]. As far as HTTP is concerned, Uniform Resource Identifiers are
  simply formatted strings which identify--via name, location, or any
  other characteristic--a network resource.

3.2.1 General Syntax

  URIs in HTTP can be represented in absolute form or relative to some
  known base URI [9], depending upon the context of their use. The two
  forms are differentiated by the fact that absolute URIs always begin
  with a scheme name followed by a colon.

      URI            = ( absoluteURI | relativeURI ) [ "#" fragment ]

      absoluteURI    = scheme ":" *( uchar | reserved )

      relativeURI    = net_path | abs_path | rel_path

      net_path       = "//" net_loc [ abs_path ]
      abs_path       = "/" rel_path
      rel_path       = [ path ] [ ";" params ] [ "?" query ]

      path           = fsegment *( "/" segment )
      fsegment       = 1*pchar
      segment        = *pchar

      params         = param *( ";" param )
      param          = *( pchar | "/" )

      scheme         = 1*( ALPHA | DIGIT | "+" | "-" | "." )
      net_loc        = *( pchar | ";" | "?" )
      query          = *( uchar | reserved )
      fragment       = *( uchar | reserved )

      pchar          = uchar | ":" | "@" | "&" | "=" | "+"
      uchar          = unreserved | escape
      unreserved     = ALPHA | DIGIT | safe | extra | national

      escape         = "%" HEX HEX
      reserved       = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
      extra          = "!" | "*" | "'" | "(" | ")" | ","
      safe           = "$" | "-" | "_" | "."
      unsafe         = CTL | SP | <"> | "#" | "%" | "<" | ">"
      national       = <any OCTET excluding ALPHA, DIGIT,



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                       reserved, extra, safe, and unsafe>

  For definitive information on URL syntax and semantics, see RFC 1738
  [4] and RFC 1808 [9]. The BNF above includes national characters not
  allowed in valid URLs as specified by RFC 1738, since HTTP servers
  are not restricted in the set of unreserved characters allowed to
  represent the rel_path part of addresses, and HTTP proxies may
  receive requests for URIs not defined by RFC 1738.

3.2.2 http URL

  The "http" scheme is used to locate network resources via the HTTP
  protocol. This section defines the scheme-specific syntax and
  semantics for http URLs.

      http_URL       = "http:" "//" host [ ":" port ] [ abs_path ]

      host           = <A legal Internet host domain name
                        or IP address (in dotted-decimal form),
                        as defined by Section 2.1 of RFC 1123>

      port           = *DIGIT

  If the port is empty or not given, port 80 is assumed. The semantics
  are that the identified resource is located at the server listening
  for TCP connections on that port of that host, and the Request-URI
  for the resource is abs_path. If the abs_path is not present in the
  URL, it must be given as "/" when used as a Request-URI (Section
  5.1.2).

     Note: Although the HTTP protocol is independent of the transport
     layer protocol, the http URL only identifies resources by their
     TCP location, and thus non-TCP resources must be identified by
     some other URI scheme.

  The canonical form for "http" URLs is obtained by converting any
  UPALPHA characters in host to their LOALPHA equivalent (hostnames are
  case-insensitive), eliding the [ ":" port ] if the port is 80, and
  replacing an empty abs_path with "/".

3.3  Date/Time Formats

  HTTP/1.0 applications have historically allowed three different
  formats for the representation of date/time stamps:

      Sun, 06 Nov 1994 08:49:37 GMT    ; RFC 822, updated by RFC 1123
      Sunday, 06-Nov-94 08:49:37 GMT   ; RFC 850, obsoleted by RFC 1036
      Sun Nov  6 08:49:37 1994         ; ANSI C's asctime() format



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  The first format is preferred as an Internet standard and represents
  a fixed-length subset of that defined by RFC 1123 [6] (an update to
  RFC 822 [7]). The second format is in common use, but is based on the
  obsolete RFC 850 [10] date format and lacks a four-digit year.
  HTTP/1.0 clients and servers that parse the date value should accept
  all three formats, though they must never generate the third
  (asctime) format.

     Note: Recipients of date values are encouraged to be robust in
     accepting date values that may have been generated by non-HTTP
     applications, as is sometimes the case when retrieving or posting
     messages via proxies/gateways to SMTP or NNTP.

  All HTTP/1.0 date/time stamps must be represented in Universal Time
  (UT), also known as Greenwich Mean Time (GMT), without exception.
  This is indicated in the first two formats by the inclusion of "GMT"
  as the three-letter abbreviation for time zone, and should be assumed
  when reading the asctime format.

      HTTP-date      = rfc1123-date | rfc850-date | asctime-date

      rfc1123-date   = wkday "," SP date1 SP time SP "GMT"
      rfc850-date    = weekday "," SP date2 SP time SP "GMT"
      asctime-date   = wkday SP date3 SP time SP 4DIGIT

      date1          = 2DIGIT SP month SP 4DIGIT
                       ; day month year (e.g., 02 Jun 1982)
      date2          = 2DIGIT "-" month "-" 2DIGIT
                       ; day-month-year (e.g., 02-Jun-82)
      date3          = month SP ( 2DIGIT | ( SP 1DIGIT ))
                       ; month day (e.g., Jun  2)

      time           = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                       ; 00:00:00 - 23:59:59

      wkday          = "Mon" | "Tue" | "Wed"
                     | "Thu" | "Fri" | "Sat" | "Sun"

      weekday        = "Monday" | "Tuesday" | "Wednesday"
                     | "Thursday" | "Friday" | "Saturday" | "Sunday"

      month          = "Jan" | "Feb" | "Mar" | "Apr"
                     | "May" | "Jun" | "Jul" | "Aug"
                     | "Sep" | "Oct" | "Nov" | "Dec"

      Note: HTTP requirements for the date/time stamp format apply
      only to their usage within the protocol stream. Clients and
      servers are not required to use these formats for user



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      presentation, request logging, etc.

3.4  Character Sets

  HTTP uses the same definition of the term "character set" as that
  described for MIME:

     The term "character set" is used in this document to refer to a
     method used with one or more tables to convert a sequence of
     octets into a sequence of characters. Note that unconditional
     conversion in the other direction is not required, in that not all
     characters may be available in a given character set and a
     character set may provide more than one sequence of octets to
     represent a particular character. This definition is intended to
     allow various kinds of character encodings, from simple single-
     table mappings such as US-ASCII to complex table switching methods
     such as those that use ISO 2022's techniques. However, the
     definition associated with a MIME character set name must fully
     specify the mapping to be performed from octets to characters. In
     particular, use of external profiling information to determine the
     exact mapping is not permitted.

     Note: This use of the term "character set" is more commonly
     referred to as a "character encoding." However, since HTTP and
     MIME share the same registry, it is important that the terminology
     also be shared.

  HTTP character sets are identified by case-insensitive tokens. The
  complete set of tokens are defined by the IANA Character Set registry
  [15]. However, because that registry does not define a single,
  consistent token for each character set, we define here the preferred
  names for those character sets most likely to be used with HTTP
  entities. These character sets include those registered by RFC 1521
  [5] -- the US-ASCII [17] and ISO-8859 [18] character sets -- and
  other names specifically recommended for use within MIME charset
  parameters.

    charset = "US-ASCII"
            | "ISO-8859-1" | "ISO-8859-2" | "ISO-8859-3"
            | "ISO-8859-4" | "ISO-8859-5" | "ISO-8859-6"
            | "ISO-8859-7" | "ISO-8859-8" | "ISO-8859-9"
            | "ISO-2022-JP" | "ISO-2022-JP-2" | "ISO-2022-KR"
            | "UNICODE-1-1" | "UNICODE-1-1-UTF-7" | "UNICODE-1-1-UTF-8"
            | token

  Although HTTP allows an arbitrary token to be used as a charset
  value, any token that has a predefined value within the IANA
  Character Set registry [15] must represent the character set defined



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  by that registry. Applications should limit their use of character
  sets to those defined by the IANA registry.

  The character set of an entity body should be labelled as the lowest
  common denominator of the character codes used within that body, with
  the exception that no label is preferred over the labels US-ASCII or
  ISO-8859-1.

3.5  Content Codings

  Content coding values are used to indicate an encoding transformation
  that has been applied to a resource. Content codings are primarily
  used to allow a document to be compressed or encrypted without losing
  the identity of its underlying media type. Typically, the resource is
  stored in this encoding and only decoded before rendering or
  analogous usage.

      content-coding = "x-gzip" | "x-compress" | token

      Note: For future compatibility, HTTP/1.0 applications should
      consider "gzip" and "compress" to be equivalent to "x-gzip"
      and "x-compress", respectively.

  All content-coding values are case-insensitive. HTTP/1.0 uses
  content-coding values in the Content-Encoding (Section 10.3) header
  field. Although the value describes the content-coding, what is more
  important is that it indicates what decoding mechanism will be
  required to remove the encoding. Note that a single program may be
  capable of decoding multiple content-coding formats. Two values are
  defined by this specification:

  x-gzip
      An encoding format produced by the file compression program
      "gzip" (GNU zip) developed by Jean-loup Gailly. This format is
      typically a Lempel-Ziv coding (LZ77) with a 32 bit CRC.

  x-compress
      The encoding format produced by the file compression program
      "compress". This format is an adaptive Lempel-Ziv-Welch coding
      (LZW).

      Note: Use of program names for the identification of
      encoding formats is not desirable and should be discouraged
      for future encodings. Their use here is representative of
      historical practice, not good design.






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3.6  Media Types

  HTTP uses Internet Media Types [13] in the Content-Type header field
  (Section 10.5) in order to provide open and extensible data typing.

      media-type     = type "/" subtype *( ";" parameter )
      type           = token
      subtype        = token

  Parameters may follow the type/subtype in the form of attribute/value
  pairs.

      parameter      = attribute "=" value
      attribute      = token
      value          = token | quoted-string

  The type, subtype, and parameter attribute names are case-
  insensitive. Parameter values may or may not be case-sensitive,
  depending on the semantics of the parameter name. LWS must not be
  generated between the type and subtype, nor between an attribute and
  its value. Upon receipt of a media type with an unrecognized
  parameter, a user agent should treat the media type as if the
  unrecognized parameter and its value were not present.

  Some older HTTP applications do not recognize media type parameters.
  HTTP/1.0 applications should only use media type parameters when they
  are necessary to define the content of a message.

  Media-type values are registered with the Internet Assigned Number
  Authority (IANA [15]). The media type registration process is
  outlined in RFC 1590 [13]. Use of non-registered media types is
  discouraged.

3.6.1 Canonicalization and Text Defaults

  Internet media types are registered with a canonical form. In
  general, an Entity-Body transferred via HTTP must be represented in
  the appropriate canonical form prior to its transmission. If the body
  has been encoded with a Content-Encoding, the underlying data should
  be in canonical form prior to being encoded.

  Media subtypes of the "text" type use CRLF as the text line break
  when in canonical form. However, HTTP allows the transport of text
  media with plain CR or LF alone representing a line break when used
  consistently within the Entity-Body. HTTP applications must accept
  CRLF, bare CR, and bare LF as being representative of a line break in
  text media received via HTTP.




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  In addition, if the text media is represented in a character set that
  does not use octets 13 and 10 for CR and LF respectively, as is the
  case for some multi-byte character sets, HTTP allows the use of
  whatever octet sequences are defined by that character set to
  represent the equivalent of CR and LF for line breaks. This
  flexibility regarding line breaks applies only to text media in the
  Entity-Body; a bare CR or LF should not be substituted for CRLF
  within any of the HTTP control structures (such as header fields and
  multipart boundaries).

  The "charset" parameter is used with some media types to define the
  character set (Section 3.4) of the data. When no explicit charset
  parameter is provided by the sender, media subtypes of the "text"
  type are defined to have a default charset value of "ISO-8859-1" when
  received via HTTP. Data in character sets other than "ISO-8859-1" or
  its subsets must be labelled with an appropriate charset value in
  order to be consistently interpreted by the recipient.

     Note: Many current HTTP servers provide data using charsets other
     than "ISO-8859-1" without proper labelling. This situation reduces
     interoperability and is not recommended. To compensate for this,
     some HTTP user agents provide a configuration option to allow the
     user to change the default interpretation of the media type
     character set when no charset parameter is given.

3.6.2 Multipart Types

  MIME provides for a number of "multipart" types -- encapsulations of
  several entities within a single message's Entity-Body. The multipart
  types registered by IANA [15] do not have any special meaning for
  HTTP/1.0, though user agents may need to understand each type in
  order to correctly interpret the purpose of each body-part. An HTTP
  user agent should follow the same or similar behavior as a MIME user
  agent does upon receipt of a multipart type. HTTP servers should not
  assume that all HTTP clients are prepared to handle multipart types.

  All multipart types share a common syntax and must include a boundary
  parameter as part of the media type value. The message body is itself
  a protocol element and must therefore use only CRLF to represent line
  breaks between body-parts. Multipart body-parts may contain HTTP
  header fields which are significant to the meaning of that part.

3.7  Product Tokens

  Product tokens are used to allow communicating applications to
  identify themselves via a simple product token, with an optional
  slash and version designator. Most fields using product tokens also
  allow subproducts which form a significant part of the application to



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  be listed, separated by whitespace. By convention, the products are
  listed in order of their significance for identifying the
  application.

      product         = token ["/" product-version]
      product-version = token

  Examples:

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

      Server: Apache/0.8.4

  Product tokens should be short and to the point -- use of them for
  advertizing or other non-essential information is explicitly
  forbidden. Although any token character may appear in a product-
  version, this token should only be used for a version identifier
  (i.e., successive versions of the same product should only differ in
  the product-version portion of the product value).

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: [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.

  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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11.  Access Authentication

  HTTP provides a simple challenge-response authentication mechanism
  which may be used by a server to challenge a client request and by a
  client to provide authentication information. It uses an extensible,
  case-insensitive token to identify the authentication scheme,
  followed by a comma-separated list of attribute-value pairs which
  carry the parameters necessary for achieving authentication via that
  scheme.

      auth-scheme    = token

      auth-param     = token "=" quoted-string

  The 401 (unauthorized) response message is used by an origin server
  to challenge the authorization of a user agent. This response must
  include a WWW-Authenticate header field containing at least one
  challenge applicable to the requested resource.

      challenge      = auth-scheme 1*SP realm *( "," auth-param )

      realm          = "realm" "=" realm-value
      realm-value    = quoted-string

  The realm attribute (case-insensitive) is required for all
  authentication schemes which issue a challenge. The realm value
  (case-sensitive), in combination with the canonical root URL of the
  server being accessed, defines the protection space. These realms
  allow the protected resources on a server to be partitioned into a
  set of protection spaces, each with its own authentication scheme
  and/or authorization database. The realm value is a string, generally
  assigned by the origin server, which may have additional semantics
  specific to the authentication scheme.

  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 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.

      credentials    = basic-credentials
                     | ( auth-scheme #auth-param )

  The domain over which credentials can be automatically applied by a
  user agent is determined by the protection space. If a prior request
  has been authorized, the same credentials may be reused for all other
  requests within that protection space for a period of time determined



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  by the authentication scheme, parameters, and/or user preference.
  Unless otherwise defined by the authentication scheme, a single
  protection space cannot extend outside the scope of its server.

  If the server does not wish to accept the credentials sent with a
  request, it should return a 403 (forbidden) response.

  The HTTP protocol does not restrict applications to this simple
  challenge-response mechanism for access authentication. Additional
  mechanisms may be used, such as encryption at the transport level or
  via message encapsulation, and with additional header fields
  specifying authentication information. However, these additional
  mechanisms are not defined by this specification.

  Proxies must be completely transparent regarding user agent
  authentication. That is, they must forward the WWW-Authenticate and
  Authorization headers untouched, and must not cache the response to a
  request containing Authorization. HTTP/1.0 does not provide a means
  for a client to be authenticated with a proxy.

11.1  Basic Authentication Scheme

  The "basic" authentication scheme is based on the model that the user
  agent must authenticate itself with a user-ID and a password for each
  realm. The realm value should be considered an opaque string which
  can only be compared for equality with other realms on that server.
  The server will authorize the request only if it can validate the
  user-ID and password for the protection space of the Request-URI.
  There are no optional authentication parameters.

  Upon receipt of an unauthorized request for a URI within the
  protection space, the server should respond with a challenge like the
  following:

      WWW-Authenticate: Basic realm="WallyWorld"

  where "WallyWorld" is the string assigned by the server to identify
  the protection space of the Request-URI.

  To receive authorization, the client sends the user-ID and password,
  separated by a single colon (":") character, within a base64 [5]
  encoded string in the credentials.

      basic-credentials = "Basic" SP basic-cookie

      basic-cookie      = <base64 [5] encoding of userid-password,
                           except not limited to 76 char/line>




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      userid-password   = [ token ] ":" *TEXT

  If the user agent wishes to send the user-ID "Aladdin" and password
  "open sesame", it would use the following header field:

      Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

  The basic authentication scheme is a non-secure method of filtering
  unauthorized access to resources on an HTTP server. It is based on
  the assumption that the connection between the client and the server
  can be regarded as a trusted carrier. As this is not generally true
  on an open network, the basic authentication scheme should be used
  accordingly. In spite of this, clients should implement the scheme in
  order to communicate with servers that use it.

12.  Security Considerations

  This section is meant to inform application developers, information
  providers, and users of the security limitations in HTTP/1.0 as
  described by this document. The discussion does not include
  definitive solutions to the problems revealed, though it does make
  some suggestions for reducing security risks.

12.1  Authentication of Clients

  As mentioned in Section 11.1, the Basic authentication scheme is not
  a secure method of user authentication, nor does it prevent the
  Entity-Body from being transmitted in clear text across the physical
  network used as the carrier. HTTP/1.0 does not prevent additional
  authentication schemes and encryption mechanisms from being employed
  to increase security.

12.2  Safe Methods

  The writers of client software should be aware that the software
  represents the user in their interactions over the Internet, and
  should be careful to allow the user to be aware of any actions they
  may take which may have an unexpected significance to themselves or
  others.

  In particular, the convention has been established that the GET and
  HEAD methods should never have the significance of taking an action
  other than retrieval. These methods should be considered "safe." This
  allows user agents to represent other methods, such as POST, in a
  special way, so that the user is made aware of the fact that a
  possibly unsafe action is being requested.





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  Naturally, it is not possible to ensure that the server does not
  generate side-effects as a result of performing a GET request; in
  fact, some dynamic resources consider that a feature. The important
  distinction here is that the user did not request the side-effects,
  so therefore cannot be held accountable for them.

12.3  Abuse of Server Log Information

  A server is in the position to save personal data about a user's
  requests which may identify their reading patterns or subjects of
  interest. This information is clearly confidential in nature and its
  handling may be constrained by law in certain countries. People using
  the HTTP protocol to provide data are responsible for ensuring that
  such material is not distributed without the permission of any
  individuals that are identifiable by the published results.

12.4  Transfer of Sensitive Information

  Like any generic data transfer protocol, HTTP cannot regulate the
  content of the data that is transferred, nor is there any a priori
  method of determining the sensitivity of any particular piece of
  information within the context of any given request. Therefore,
  applications should supply as much control over this information as
  possible to the provider of that information. Three header fields are
  worth special mention in this context: Server, Referer and From.

  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. Implementors should make the
  Server header field a configurable option.

  The Referer field allows reading patterns to be studied and reverse
  links drawn. Although it can be very useful, its power can be abused
  if user details are not separated from the information contained in
  the Referer. Even when the personal information has been removed, the
  Referer field may indicate a private document's URI whose publication
  would be inappropriate.

  The information sent in the From field might conflict with the user's
  privacy interests or their site's security policy, and hence it
  should not be transmitted without the user being able to disable,
  enable, and modify the contents of the field. The user must be able
  to set the contents of this field within a user preference or
  application defaults configuration.

  We suggest, though do not require, that a convenient toggle interface
  be provided for the user to enable or disable the sending of From and
  Referer information.



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12.5  Attacks Based On File and Path Names

  Implementations of HTTP origin servers should be careful to restrict
  the documents returned by HTTP requests to be only those that were
  intended by the server administrators. If an HTTP server translates
  HTTP URIs directly into file system calls, the server must take
  special care not to serve files that were not intended to be
  delivered to HTTP clients. For example, Unix, Microsoft Windows, and
  other operating systems use ".." as a path component to indicate a
  directory level above the current one. On such a system, an HTTP
  server must disallow any such construct in the Request-URI if it
  would otherwise allow access to a resource outside those intended to
  be accessible via the HTTP server. Similarly, files intended for
  reference only internally to the server (such as access control
  files, configuration files, and script code) must be protected from
  inappropriate retrieval, since they might contain sensitive
  information. Experience has shown that minor bugs in such HTTP server
  implementations have turned into security risks.

13.  Acknowledgments

  This specification makes heavy use of the augmented BNF and generic
  constructs defined by David H. Crocker for RFC 822 [7]. Similarly, it
  reuses many of the definitions provided by Nathaniel Borenstein and
  Ned Freed for MIME [5]. We hope that their inclusion in this
  specification will help reduce past confusion over the relationship
  between HTTP/1.0 and Internet mail message formats.

  The HTTP protocol has evolved considerably over the past four years.
  It has benefited from a large and active developer community--the
  many people who have participated on the www-talk mailing list--and
  it is that community which has been most responsible for the success
  of HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
  Cailliau, Daniel W. Connolly, Bob Denny, Jean-Francois Groff, Phillip
  M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob McCool, Lou
  Montulli, Dave Raggett, Tony Sanders, and Marc VanHeyningen deserve
  special recognition for their efforts in defining aspects of the
  protocol for early versions of this specification.

  Paul Hoffman contributed sections regarding the informational status
  of this document and Appendices C and D.










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  This document has benefited greatly from the comments of all those
  participating in the HTTP-WG. In addition to those already mentioned,
  the following individuals have contributed to this specification:

      Gary Adams                         Harald Tveit Alvestrand
      Keith Ball                         Brian Behlendorf
      Paul Burchard                      Maurizio Codogno
      Mike Cowlishaw                     Roman Czyborra
      Michael A. Dolan                   John Franks
      Jim Gettys                         Marc Hedlund
      Koen Holtman                       Alex Hopmann
      Bob Jernigan                       Shel Kaphan
      Martijn Koster                     Dave Kristol
      Daniel LaLiberte                   Paul Leach
      Albert Lunde                       John C. Mallery
      Larry Masinter                     Mitra
      Jeffrey Mogul                      Gavin Nicol
      Bill Perry                         Jeffrey Perry
      Owen Rees                          Luigi Rizzo
      David Robinson                     Marc Salomon
      Rich Salz                          Jim Seidman
      Chuck Shotton                      Eric W. Sink
      Simon E. Spero                     Robert S. Thau
      Francois Yergeau                   Mary Ellen Zurko
      Jean-Philippe Martin-Flatin

14. References

  [1]  Anklesaria, F., McCahill, M., Lindner, P., Johnson, D.,
       Torrey, D., and B. Alberti, "The Internet Gopher Protocol: A
       Distributed Document Search and Retrieval Protocol", RFC 1436,
       University of Minnesota, March 1993.

  [2]  Berners-Lee, T., "Universal Resource Identifiers in WWW: A
       Unifying Syntax for the Expression of Names and Addresses of
       Objects on the Network as used in the World-Wide Web",
       RFC 1630, CERN, June 1994.

  [3]  Berners-Lee, T., and D. Connolly, "Hypertext Markup Language -
       2.0", RFC 1866, MIT/W3C, November 1995.

  [4]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
       Resource Locators (URL)", RFC 1738, CERN, Xerox PARC,
       University of Minnesota, December 1994.







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  [5]  Borenstein, N., and N. Freed, "MIME (Multipurpose Internet Mail
       Extensions) Part One: Mechanisms for Specifying and Describing
       the Format of Internet Message Bodies", RFC 1521, Bellcore,
       Innosoft, September 1993.

  [6]  Braden, R., "Requirements for Internet hosts - Application and
       Support", STD 3, RFC 1123, IETF, October 1989.

  [7]  Crocker, D., "Standard for the Format of ARPA Internet Text
       Messages", STD 11, RFC 822, UDEL, August 1982.

  [8]  F. Davis, B. Kahle, H. Morris, J. Salem, T. Shen, R. Wang,
       J. Sui, and M. Grinbaum. "WAIS Interface Protocol Prototype
       Functional Specification." (v1.5), Thinking Machines
       Corporation, April 1990.

  [9]  Fielding, R., "Relative Uniform Resource Locators", RFC 1808,
       UC Irvine, June 1995.

  [10] Horton, M., and R. Adams, "Standard for interchange of USENET
       Messages", RFC 1036 (Obsoletes RFC 850), AT&T Bell
       Laboratories, Center for Seismic Studies, December 1987.

  [11] Kantor, B., and P. Lapsley, "Network News Transfer Protocol:
       A Proposed Standard for the Stream-Based Transmission of News",
       RFC 977, UC San Diego, UC Berkeley, February 1986.

  [12] Postel, J., "Simple Mail Transfer Protocol." STD 10, RFC 821,
       USC/ISI, August 1982.

  [13] Postel, J., "Media Type Registration Procedure." RFC 1590,
       USC/ISI, March 1994.

  [14] Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)",
       STD 9, RFC 959, USC/ISI, October 1985.

  [15] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
       1700, USC/ISI, October 1994.

  [16] Sollins, K., and L. Masinter, "Functional Requirements for
       Uniform Resource Names", RFC 1737, MIT/LCS, Xerox Corporation,
       December 1994.

  [17] US-ASCII. Coded Character Set - 7-Bit American Standard Code
       for Information Interchange. Standard ANSI X3.4-1986, ANSI,
       1986.





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  [18] ISO-8859. International Standard -- Information Processing --
       8-bit Single-Byte Coded Graphic Character Sets --
       Part 1: Latin alphabet No. 1, ISO 8859-1:1987.
       Part 2: Latin alphabet No. 2, ISO 8859-2, 1987.
       Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.
       Part 4: Latin alphabet No. 4, ISO 8859-4, 1988.
       Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988.
       Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987.
       Part 7: Latin/Greek alphabet, ISO 8859-7, 1987.
       Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988.
       Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.

15.  Authors' Addresses

  Tim Berners-Lee
  Director, W3 Consortium
  MIT Laboratory for Computer Science
  545 Technology Square
  Cambridge, MA 02139, U.S.A.

  Fax: +1 (617) 258 8682
  EMail: [email protected]


  Roy T. Fielding
  Department of Information and Computer Science
  University of California
  Irvine, CA 92717-3425, U.S.A.

  Fax: +1 (714) 824-4056
  EMail: [email protected]


  Henrik Frystyk Nielsen
  W3 Consortium
  MIT Laboratory for Computer Science
  545 Technology Square
  Cambridge, MA 02139, U.S.A.

  Fax: +1 (617) 258 8682
  EMail: [email protected]










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Appendices

  These appendices are provided for informational reasons only -- they
  do not form a part of the HTTP/1.0 specification.

A.  Internet Media Type message/http

  In addition to defining the HTTP/1.0 protocol, this document serves
  as the specification for the Internet media type "message/http". The
  following is to be registered with IANA [13].

      Media Type name:         message

      Media subtype name:      http

      Required parameters:     none

      Optional parameters:     version, msgtype

             version: The HTTP-Version number of the enclosed message
                      (e.g., "1.0"). If not present, the version can be
                      determined from the first line of the body.

             msgtype: The message type -- "request" or "response". If
                      not present, the type can be determined from the
                      first line of the body.

      Encoding considerations: only "7bit", "8bit", or "binary" are
                               permitted

      Security considerations: none

B.  Tolerant Applications

  Although this document specifies the requirements for the generation
  of HTTP/1.0 messages, not all applications will be correct in their
  implementation. We therefore recommend that operational applications
  be tolerant of deviations whenever those deviations can be
  interpreted unambiguously.

  Clients should be tolerant in parsing the Status-Line and servers
  tolerant when parsing the Request-Line. In particular, they should
  accept any amount of SP or HT characters between fields, even though
  only a single SP is required.

  The line terminator for HTTP-header fields is the sequence CRLF.
  However, we recommend that applications, when parsing such headers,
  recognize a single LF as a line terminator and ignore the leading CR.



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C.  Relationship to MIME

  HTTP/1.0 uses many of the constructs defined for Internet Mail (RFC
  822 [7]) and the Multipurpose Internet Mail Extensions (MIME [5]) to
  allow entities to be transmitted in an open variety of
  representations and with extensible mechanisms. However, RFC 1521
  discusses mail, and HTTP has a few features that are different than
  those described in RFC 1521. These differences were carefully chosen
  to optimize performance over binary connections, to allow greater
  freedom in the use of new media types, to make date comparisons
  easier, and to acknowledge the practice of some early HTTP servers
  and clients.

  At the time of this writing, it is expected that RFC 1521 will be
  revised. The revisions may include some of the practices found in
  HTTP/1.0 but not in RFC 1521.

  This appendix describes specific areas where HTTP differs from RFC
  1521. Proxies and gateways to strict MIME environments should be
  aware of these differences and provide the appropriate conversions
  where necessary. Proxies and gateways from MIME environments to HTTP
  also need to be aware of the differences because some conversions may
  be required.

C.1  Conversion to Canonical Form

  RFC 1521 requires that an Internet mail entity be converted to
  canonical form prior to being transferred, as described in Appendix G
  of RFC 1521 [5]. Section 3.6.1 of this document describes the forms
  allowed for subtypes of the "text" media type when transmitted over
  HTTP.

  RFC 1521 requires that content with a Content-Type of "text"
  represent line breaks as CRLF and forbids the use of CR or LF outside
  of line break sequences. HTTP allows CRLF, bare CR, and bare LF to
  indicate a line break within text content when a message is
  transmitted over HTTP.

  Where it is possible, a proxy or gateway from HTTP to a strict RFC
  1521 environment should translate all line breaks within the text
  media types described in Section 3.6.1 of this document to the RFC
  1521 canonical form of CRLF. Note, however, that this may be
  complicated by the presence of a Content-Encoding and by the fact
  that HTTP allows the use of some character sets which do not use
  octets 13 and 10 to represent CR and LF, as is the case for some
  multi-byte character sets.





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C.2  Conversion of Date Formats

  HTTP/1.0 uses a restricted set of date formats (Section 3.3) to
  simplify the process of date comparison. Proxies and gateways from
  other protocols should ensure that any Date header field present in a
  message conforms to one of the HTTP/1.0 formats and rewrite the date
  if necessary.

C.3  Introduction of Content-Encoding

  RFC 1521 does not include any concept equivalent to HTTP/1.0's
  Content-Encoding header field. Since this acts as a modifier on the
  media type, proxies and gateways from HTTP to MIME-compliant
  protocols must either change the value of the Content-Type header
  field or decode the Entity-Body before forwarding the message. (Some
  experimental applications of Content-Type for Internet mail have used
  a media-type parameter of ";conversions=<content-coding>" to perform
  an equivalent function as Content-Encoding. However, this parameter
  is not part of RFC 1521.)

C.4  No Content-Transfer-Encoding

  HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC
  1521. Proxies and gateways from MIME-compliant protocols to HTTP must
  remove any non-identity CTE ("quoted-printable" or "base64") encoding
  prior to delivering the response message to an HTTP client.

  Proxies and gateways from HTTP to MIME-compliant protocols are
  responsible for ensuring that the message is in the correct format
  and encoding for safe transport on that protocol, where "safe
  transport" is defined by the limitations of the protocol being used.
  Such a proxy or gateway should label the data with an appropriate
  Content-Transfer-Encoding if doing so will improve the likelihood of
  safe transport over the destination protocol.

C.5  HTTP Header Fields in Multipart Body-Parts

  In RFC 1521, most header fields in multipart body-parts are generally
  ignored unless the field name begins with "Content-". In HTTP/1.0,
  multipart body-parts may contain any HTTP header fields which are
  significant to the meaning of that part.

D.  Additional Features

  This appendix documents protocol elements used by some existing HTTP
  implementations, but not consistently and correctly across most
  HTTP/1.0 applications. Implementors should be aware of these
  features, but cannot rely upon their presence in, or interoperability



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  with, other HTTP/1.0 applications.

D.1  Additional Request Methods

D.1.1 PUT

  The PUT method requests that the enclosed entity be stored under the
  supplied Request-URI. If the Request-URI refers to an already
  existing resource, the enclosed entity should be considered as a
  modified version of the one residing on the origin server. If the
  Request-URI does not point to an existing resource, and that URI is
  capable of being defined as a new resource by the requesting user
  agent, the origin server can create the resource with that URI.

  The fundamental difference between the POST and PUT requests is
  reflected in the different meaning of the Request-URI. The URI in a
  POST request identifies the resource that will handle the enclosed
  entity as data to be processed. That resource may be a data-accepting
  process, a gateway to some other protocol, or a separate entity that
  accepts annotations. In contrast, the URI in a PUT request identifies
  the entity enclosed with the request -- the user agent knows what URI
  is intended and the server should not apply the request to some other
  resource.

D.1.2 DELETE

  The DELETE method requests that the origin server delete the resource
  identified by the Request-URI.

D.1.3 LINK

  The LINK method establishes one or more Link relationships between
  the existing resource identified by the Request-URI and other
  existing resources.

D.1.4 UNLINK

  The UNLINK method removes one or more Link relationships from the
  existing resource identified by the Request-URI.

D.2  Additional Header Field Definitions

D.2.1 Accept

  The Accept request-header field can be used to indicate a list of
  media ranges which are acceptable as a response to the request. The
  asterisk "*" character is used to group media types into ranges, with
  "*/*" indicating all media types and "type/*" indicating all subtypes



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  of that type. The set of ranges given by the client should represent
  what types are acceptable given the context of the request.

D.2.2 Accept-Charset

  The Accept-Charset request-header field can be used to indicate a
  list of preferred character sets other than the default US-ASCII and
  ISO-8859-1. 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.

D.2.3 Accept-Encoding

  The Accept-Encoding request-header field is similar to Accept, but
  restricts the content-coding values which are acceptable in the
  response.

D.2.4 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.

D.2.5 Content-Language

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

D.2.6 Link

  The Link entity-header field provides a means for describing a
  relationship between the entity and some other resource. An entity
  may include multiple Link values. Links at the metainformation level
  typically indicate relationships like hierarchical structure and
  navigation paths.

D.2.7 MIME-Version

  HTTP messages may include a single MIME-Version general-header field
  to indicate what version of the MIME protocol was used to construct
  the message. Use of the MIME-Version header field, as defined by RFC
  1521 [5], should indicate that the message is MIME-conformant.
  Unfortunately, some older HTTP/1.0 servers send it indiscriminately,
  and thus this field should be ignored.




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D.2.8 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.

D.2.9 Title

  The Title entity-header field indicates the title of the entity.

D.2.10 URI

  The URI entity-header field may contain some or all of the Uniform
  Resource Identifiers (Section 3.2) by which the Request-URI resource
  can be identified. There is no guarantee that the resource can be
  accessed using the URI(s) specified.

































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