HTTP/1.0

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                Hypertext Transfer Protocol -- HTTP/1.0

Status of this Memo

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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 and negotiation 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 preferred usage 
   of the protocol referred to as "HTTP/1.0", and is compatible with 
   the most commonly used HTTP server and client programs implemented
   prior to November 1994.

Table of Contents

   1.  Introduction
       1.1  Purpose
       1.2  Overall Operation
       1.3  Terminology
   2.  Notational Conventions and Generic Grammar
       2.1  Augmented BNF
       2.2  Basic Rules
   3.  HTTP Message
       3.1  Header Fields
       3.2  Object Body
   4.  Usage of RFC 822 and MIME Constructs
       4.1  Date/Time Format
       4.2  Media Types
            4.2.1  Multipart Types
                   4.2.1.1 Multipart/mixed
                   4.2.1.2 Multipart/parallel
                   4.2.1.3 Other Multipart Types
            4.2.2  Conversion to Canonical Form
       4.3  General Message Header Fields
            4.3.1  Connection
            4.3.2  Date    
            4.3.3  Forwarded
            4.3.4  Mandatory 
            4.3.5  Message-ID 
            4.3.6  MIME-Version
   5.  Request                
       5.1  Request-Line       
       5.2  Method              
            5.2.1  GET
            5.2.2  HEAD
            5.2.3  POST
            5.2.4  PUT
            5.2.5  DELETE
            5.2.6  LINK
            5.2.7  UNLINK
       5.3  HTTP-Version
       5.4  Universal Resource Identifier
       5.5  Request Header Fields
            5.5.1  User-Agent
            5.5.2  If-Modified-Since
            5.5.3  Pragma
            5.5.4  Authorization
            5.5.5  Proxy-Authorization
            5.5.6  Referer
            5.5.7  From
            5.5.8  Accept
            5.5.9  Accept-Encoding
            5.5.10 Accept-Language
   6.  Response
       6.1  Status-Line
       6.2  HTTP Version
       6.3  Status Codes and Reason Phrases
            6.3.1  Successful 2xx
            6.3.2  Redirection 3xx
            6.3.3  Client Error 4xx
            6.3.4  Server Errors 5xx
       6.4  Response Header Fields
            6.4.1  Server       
            6.4.2  WWW-Authenticate
            6.4.3  Proxy-Authenticate
            6.4.4  Retry-After
   7.  Object Header Fields
       7.1  Allow
       7.2  Content-Length
       7.3  Content-Type
       7.4  Content-Encoding
       7.5  Content-Transfer-Encoding
       7.6  Content-Language
       7.7  Expires
       7.8  Last-Modified
       7.9  URI Header
       7.10 Location  
       7.11 Version    
       7.12 Derived-From
       7.13 Title
       7.14 Link
   8.  HTTP Negotiation Algorithm
   9.  Basic Access Authentication Scheme
   10. Registration Authority
   11. Security Considerations
       11.1 Authentication of Clients
       11.2 Idempotent Methods
       11.3 Abuse of Server Log Information
   12. Acknowledgments
   13. References
   14. Authors Addresses 
   Appendix A. Tolerant Applications
       A.1  Request-Line, Status-Line, and Header Fields
       A.2  Object Body
       A.3  Backward Compatibility


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 preferred usage of the protocol 
   referred to as "HTTP/1.0". This specification does not necessarily 
   reflect the "current practice" of any single HTTP server or client 
   implementation. It does, however, seek to remain compatible with 
   existing implementations wherever possible, and should be 
   considered the reference for future implementations of HTTP/1.0.

   Practical information systems require more functionality than 
   simple retrieval, including search, front-end update, and 
   annotation. HTTP/1.0 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 Universal Resource Identifier 
   (URI) [2], as a location (URL) [3] or name (URN), 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) [4].

   HTTP/1.0 is also used for communication between user agents and 
   various gateways, allowing hypermedia access to existing Internet 
   protocols like SMTP [12], NNTP [11], FTP [14], Gopher [1], and 
   WAIS [8]. HTTP/1.0 is designed to allow such gateways, via proxy 
   servers, without any loss of the data conveyed by those earlier 
   protocols.

1.2  Overall Operation

   The HTTP protocol is based on a request/response paradigm. A 
   requesting program (termed a client) establishes a connection with 
   a receiving program (termed 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 status line (including its protocol version and a success or 
   error code), followed by a MIME-like message containing server 
   information, object metainformation, and possible body content. It 
   should be noted that a given program may be capable of being both a 
   client and a server; our use of those terms refers only to the role 
   being performed by the program during a particular connection, 
   rather than to the program's purpose in general.

   On the Internet, the communication generally takes place over a 
   TCP/IP connection. The default port is TCP 80 [15], but other ports
   can be used. This does not preclude the HTTP/1.0 protocol from 
   being implemented on top of any other protocol on the Internet, or 
   on other networks. 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.

   For most current implementations, the connection is established by 
   the client prior to each request and closed by the server after 
   sending the response. However, this is not a feature of the 
   protocol and is not required by this specification. Both clients 
   and servers must be capable of handling cases where either party 
   closes the connection prematurely, due to user action, automated 
   time-out, or program failure. In any case, the closing of the 
   connection by either or both parties always terminates the current 
   request, regardless of its status.

1.3  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 virtual circuit connecting two parties for the purpose of 
       communication.

   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.

   client
       A program that establishes connections for the purpose of 
       sending requests.

   user agent
       The client program which is closest to the user and which 
       initiates requests at their behest.

   server
       A 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 forwarding requests. Proxies are often used 
       to act as a portal through a network firewall. A proxy server 
       accepts requests from other clients and services them either 
       internally or by passing them (with possible translation) on to 
       other servers. A caching proxy is a proxy server with a local 
       cache of server responses -- some requested resources can be 
       serviced from the cache rather than from the origin server. Some 
       proxy servers also act as origin servers.

   gateway
       A proxy which services HTTP requests by translation into 
       protocols other than HTTP. The reply sent from the remote server 
       to the gateway is likewise translated into HTTP before being 
       forwarded to the user agent.

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:

   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, TAB, 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 ("|") 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.

   #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.

2.2  Basic Rules

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

       OCTET          = <any 8-bit character>
       CHAR           = <any US-ASCII character (octets 0 - 127)>
       UPALPHA        = <any US-ASCII uppercase letter "A".."Z">
       LOALPHA        = <any US-ASCII lowercase letter "a".."z">
       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)>
       HTAB           = <US-ASCII HT, horizontal-tab (9)>
       <">            = <US-ASCII double-quote mark>

   HTTP/1.0 defines the character sequence CR LF as the end-of-line 
   marker for all protocol elements except the Object-Body (see 
   Appendix A for tolerant applications). The end-of-line marker for 
   an Object-Body is defined by its associated media type, as 
   described in Section 4.2.2.

       CRLF           = CR LF

   HTTP/1.0 headers can be folded onto multiple lines if the 
   continuation lines begin with linear whitespace characters. All 
   linear whitespace (including folding) has the same semantics as SP.

       LWS            = [CRLF] ( SP | HTAB )

   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

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

       tspecials      = "(" | ")" | "<" | ">" | "@"
                      | "," | ";" | ":" | "¥" | <">
                      | "/" | "[" | "]" | "?" | "="
                      | SP | HTAB

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

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

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

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

   The text rule is only used for descriptive field contents. Words of
   *text may contain characters from character sets other than US-ASCII
   only when encoded according to the rules of RFC 1522 [5].

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

3.  HTTP Message

   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 objects. Both messages may include 
   optional header fields (a.k.a. "headers") and an object body. The 
   object body is separated from the headers by a null line (i.e., a 
   line with nothing preceding the CRLF).

       Full-Request   = Request-Line             ; see Section 5.1
                        *General-Header          ; see Section 4.3
                        *Request-Header          ; see Section 5.5
                        *Object-Header           ; see Section 7
                        CRLF
                        [ Object-Body ]

       Full-Response  = Status-Line              ; see Section 6.1
                        *General-Header          ; see Section 4.3
                        *Response-Header         ; see Section 6.4
                        *Object-Header           ; see Section 7
                        CRLF
                        [ Object-Body ]

   Simple-Request and Simple-Response, as defined in Section 5 and 
   Section 6 respectively, do not allow the use of any header 
   information and are limited to a single request method (GET). This 
   prevents the client from using content negotiation and the server 
   from identifying the media type of the returned object. Therefore, 
   clients are discouraged from using the Simple-Request format except 
   for the simplest of applications.

3.1  Header Fields

   HTTP header fields, which include Request-Header, Response-Header, 
   General-Header, Object-Header, and extension 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 by a colon (":") and 
   the field value. The field value may be preceded by any amount of 
   linear-white-space, though a single SP is preferred. Header fields 
   can be extended over multiple lines by preceding each extra line 
   with one or more linear white-space characters.

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

       field-name     = 1*<any CHAR, excluding CTLs, SP, and ":">
       field-value    = *( field-content | comment | 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 considered "good practice" to send General-Header 
   fields first, followed by Request-Header or Response-Header fields 
   prior to the Object-Header fields. Comments can be included in HTTP 
   header fields by surrounding the comment text with parentheses.

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

       Note: Use of comments within HTTP headers is generally 
       discouraged, since they are rarely seen by human eyes and 
       hence only increase network traffic. However, they may be 
       useful for messages posted or retrieved via NNTP and SMTP 
       gateways.

3.2  Object Body

   The object body (if any) sent with an HTTP/1.0 request or response 
   is in a format and encoding defined by the Object-Header fields.

       Object-Body    = *OCTET

   The actual length, encoding, and data type of the Object-Body is 
   determined via the header fields Content-Length, Content-Encoding, 
   Content-Transfer-Encoding, and Content-Type, similar to those 
   defined by MIME [4]. If the Content-Length header field is present, 
   its value in bytes (number of octets) represents the length of the 
   Object-Body. Otherwise, the body length is determined either by a 
   heuristic function of the Content-Type and Content-Encoding, or by 
   the closing of the connection by the server.

       Note: Closing the connection cannot be used to indicate the 
       value of the Content-Length header when the Object-Body is 
       part of a request message, as it leaves no possibility for 
       the server to send back a response.

4.  Usage of RFC 822 and MIME Constructs

   HTTP/1.0 reuses many of the constructs defined for Internet Mail 
   (RFC 822, [7]) and the Multipurpose Internet Mail Extensions 
   (MIME, [4]) to allow Object's to be transmitted in an open variety 
   of representations. However, because it is not limited by the 
   restrictions of existing mail protocols and gateways, HTTP does not 
   obey some of the constraints imposed by RFC 822 and MIME for mail 
   transport. This section describes how these common constructs are 
   defined within HTTP.

4.1  Date/Time Format

   For historical reasons, HTTP/1.0 allows 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

   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 
   today, but is based on the obsolete RFC 850 [10] date format and 
   lacks a four-digit year. HTTP/1.0 clients and servers must accept 
   all three formats, though they should never generate the third 
   (asctime) format. It is strongly recommended that future clients 
   and servers only generate the RFC 1123 format for representing 
   date/time stamps in HTTP/1.0 requests and responses.

   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/1.0 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 presentation, request logging, etc.

4.2  Media Types

   HTTP uses Internet Media Types [13], formerly referred to as MIME 
   Content-Types [4], in order to provide open and extensible data 
   typing and type negotiation. For mail applications, where there is 
   no type negotiation between sender and receiver, it is reasonable 
   to put strict limits on the set of allowed media types. With HTTP, 
   however, user agents can identify acceptable media types as part of 
   the connection, and thus are allowed more freedom in the use of non-
   registered types. The following grammar for media types is a 
   superset of that for MIME.

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

       parameter      = attribute "=" value
       attribute      = token                    ; case-insensitive
       value          = token | quoted-string    ; sometimes
                                                 ; case-sensitive

4.2.1 Multipart Types

   HTTP provides for a number of "multipart" types -- encapsulations of
   several object body parts within a single message's Object-Body. 
   Multipart responses should only be used in responses when the user 
   agent has indicated acceptability of the multipart type in addition 
   to the media types of each constituent body part. User agents may 
   use multipart types when submitting objects as part of a POST or 
   PUT request.

   As in MIME [4], all multipart types share a common syntax and must 
   include a boundary parameter as part of the media-type. Unlike in 
   MIME, multipart body parts may contain HTTP header fields which are 
   significant to the meaning of that part.

       boundary       = 0*69( bchar | SP ) bchar
       bchar          = DIGIT | ALPHA | "'" | "(" | ")" | "+"
                      | "_" | "," | "-" | "." | "/" | ":" | "=" | "?"

   The Object-Body of a multipart message is specified as follows:

       multipart-body = discard-text 1*encapsulation
                        close-delimiter discard-text

       encapsulation  = delimiter body-part CRLF

       delimiter      = "--" boundary CRLF       ; boundary is obtained
                                                 ; from Content-Type.

       close-delimiter= "--" boundary "--" CRLF

       discard-text   = *(*text CRLF)            ; ignored

       body-part      = *Object-Header
                        CRLF
                        [ Object-Body ]          ; May be recursive
                                                 ; if boundary differs

   A URI-header field (Section 7.9) should be included in the body-
   part for each enclosed object that can be identified by a URI.

4.2.1.1  Multipart/mixed

   The "multipart/mixed" media type is used when the first body-part 
   contains references to other parts which the sender wishes to send 
   at the same time. For example, the first body-part could be an HTML 
   document and the following body-parts could be annotations upon 
   that document. However, the use of "multipart/mixed" is strongly 
   discouraged in cases where the related objects are likely to have 
   already been retrieved and cached by a user agent or caching proxy.

4.2.1.2  Multipart/parallel

   The "multipart/parallel" media type is identical to 
   "multipart/mixed", but with the additional semantics that the parts 
   should be presented simultaneously by the user agent. This media 
   type would be appropriate for situations where simultaneous 
   presentation is an important aspect of the information, such as for 
   audio-annotated slides and movies.

       Note: This document does not define what is meant by 
       "simultaneous presentation." That is, HTTP/1.0 does not 
       provide any means of synchronization between the parts in 
       messages of type "multipart/parallel".

4.2.1.3  Other Multipart Types

   The other 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.

4.2.2 Conversion to Canonical Form

   Regardless of the media type, HTTP does not require any conversion 
   to canonical form of line terminators or other constructs in the 
   Object-Body while it is being transferred between an HTTP client 
   and server. However, gateway applications must understand that, 
   before an HTTP Object-Body can passed to a MIME-conforming 
   protocol, conversion to canonical form may be necessary [4]. 
   Furthermore, additional processing may be required in order to 
   convert an HTTP message to something that can be transferred via 
   other protocols, particularly when a Content-Encoding has been 
   applied to the enclosed object.

   In contrast, no conversion should be necessary for a MIME-
   conforming message to be tranferred using HTTP.

4.3  General Message 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 
   communicating parties or the object being transferred. Although 
   none of the General-Header fields are required, they are all 
   strongly recommended where their use is appropriate, and should be 
   understood by all future HTTP/1.0 clients and servers. These 
   headers apply only to the message being transmitted.

       General-Header = Connection
                      | Date
                      | Forwarded
                      | Mandatory
                      | Message-ID
                      | MIME-Version

4.3.1 Connection

   The Connection header is used to specify the parameters (desired or 
   actual) of the current connection. Clients can use this header to 
   indicate their desire to use a set of connection options. Servers 
   can use this header to indicate what options are actually being 
   applied. This field applies only to the current connection -- 
   receivers should not cache or otherwise save the connection 
   information after the connection is closed. Proxies must not 
   forward this header, though they may generate a separate Connection 
   header for their own connections.

       Connection     = "Connection" ":" 1#connect-option

       connect-option = token [ "=" word ]

   Although current HTTP/1.0 clients and servers do not make use of 
   the Connection header outside of experiments, this field will be 
   necessary to enable future extensibility of connection-specific 
   behavior. Most importantly, HTTP/1.0 proxies need to know that they 
   must not forward this header even when they do not understand or 
   make use of its contents. For example, an experimental client may 
   send:

       Connection: keep-alive

   to indicate that it desires to keep the connection open for 
   multiple requests. The server may then respond with a message 
   containing:

       Connection: keep-alive, timeout=10, maxreq=5

   to indicate that the connection will be kept open for a maximum of 
   5 requests, but will timeout if the next request is not received 
   within 10 seconds. Note that the semantics of these options are not 
   defined for HTTP/1.0, though similar options may be defined by 
   future versions of HTTP.

4.3.2 Date

   The Date header represents the date and time at which the message 
   was originated, having the same semantics as orig-date in RFC 822. 
   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 default 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. A 
   received message which does not have a Date header field should be 
   assigned one by the receiver if and only if the message will be 
   cached by that receiver or gatewayed via a protocol which requires 
   a Date. The field value is an HTTP-date, as described in Section 
   4.1.

       Date           = "Date" ":" HTTP-date

   An example is

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

   Only one Date header field is allowed per message.

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

4.3.3 Forwarded

   The Forwarded header is to be used by proxies to indicate the 
   intermediate steps between the user agent and the server (on 
   requests) and between the origin server and the client (on 
   responses). It is analogous to the "Received" field of RFC 822 and 
   is intended to be used for tracing transport problems and avoiding 
   request loops.

       Forwarded      = "Forwarded" ":" "by" URI [ "(" product ")" ]
                        [ "for" FQDN ]

       FQDN           = <Fully-Qualified Domain Name>

   For example, a message could be sent from a client on 
   ptsun00.cern.ch to a server at www.ics.uci.edu port 80, via an 
   intermediate HTTP proxy at info.cern.ch port 8000. The request 
   received by the server at www.ics.uci.edu would then have the 
   following Forwarded header field:

       Forwarded: by http://info.cern.ch:8000/ for ptsun00.cern.ch

   Multiple Forwarded header fields are allowed and should represent 
   each proxy that has forwarded the message. It is strongly 
   recommended that proxies used as a portal through a network 
   firewall do not, by default, send out information about the 
   internal hosts within the firewall region. This information should 
   only be propagated if explicitly enabled. If not enabled, the for 
   token and FQDN should not be included in the field value.

4.3.4 Mandatory

   The Mandatory header is used to indicate a list of other header 
   field names which must be understood by the receiver before the 
   contents of the message can be stored, cached, or presented to a 
   user. This header is used to alert the receiver that, unlike the 
   default behavior, it cannot safely ignore the semantics of the 
   listed field-names if they are not understood. It is anticipated 
   that the field names correspond to conditions that must be met by 
   the receiver in order to abide by the sender's licensing, 
   copyright, payment, or other fair-usage constraints.

       Mandatory      = "Mandatory" ":" 1#field-name

   Recommended usage of the Mandatory header is not defined for 
   HTTP/1.0. However, caching proxies must not cache responses which 
   contain mandatory header information which is not understood by the 
   proxy.

4.3.5 Message-ID

   The Message-ID field in HTTP is identical to that used by Internet 
   Mail and USENET messages, as defined in [10]. That is, it gives the 
   message a single, unique identifier which can be used for 
   identifying the message (not its contents) for "much longer" than 
   the expected lifetime of that message. 

       Message-ID     = "Message-ID" ":" "<" addr-spec ">"
       addr-spec      = unique-string "@" FQDN
       unique-string  = <1*CHAR, not including whitespace, "@", or ">">

   where unique-string must be unique within the host specified by 
   FQDN. An example is

       Message-ID: <9411151630.4256@info.cern.ch>

   which is composed using the time, date and process-ID on the host 
   info.cern.ch.

4.3.6 MIME-Version

   HTTP is not a MIME-conformant protocol. However, HTTP/1.0 messages 
   may include a single MIME-Version header field to indicate what 
   version of the MIME protocol was used to construct the message. Use 
   of the MIME-Version header field should indicate that the message 
   is in full compliance with the MIME protocol (as defined in [4]). 
   Unfortunately, current versions of HTTP/1.0 clients and servers use 
   this field indiscriminantly, and thus receivers must not take it 
   for granted that the message is indeed in full compliance with 
   MIME. Gateways are responsible for ensuring this compliance (where 
   possible) when exporting HTTP messages to strict MIME environments. 
   Future HTTP/1.0 applications must only use MIME-Version when the 
   message is intended to be MIME-conformant.

       MIME-Version   = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT

   MIME version "1.0" is the default for use in HTTP/1.0. However, 
   HTTP/1.0 message parsing and semantics are defined by this document 
   and not the MIME specification.

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 object 
   requested, the identifier of the object, 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 URI CRLF        ; HTTP/0.9 request

       Full-Request   = Request-Line             ; see Section 5.1
                        *General-Header          ; see Section 4.3
                        *Request-Header          ; see Section 5.5
                        *Object-Header           ; see Section 7
                        CRLF
                        [ Object-Body ]          ; see Section 3.2

   If an HTTP/1.0 server receives a Simple-Request, it must respond 
   with an HTTP/0.9 Simple-Response. Similarly, if a client 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.

5.1  Request-Line

   The Request-Line begins with a method token, followed by the 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 URI SP HTTP-Version CRLF

5.2  Method

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

       Method         = "GET" | "HEAD" | "PUT" | "POST"
                      | "DELETE" | "LINK" | "UNLINK"
                      | extension-method

       extension-method=token

   The methods GET and HEAD must be supported by all conforming 
   HTTP/1.0 servers. The list of methods acceptable by a specific 
   resource can be specified in an "Allow" Object-Header (Section 
   7.1). However, the client is always notified through the return 
   code of the response whether a method is currently allowed on a 
   specific resource, as this can change dynamically. The set of 
   common methods for HTTP/1.0 is described below. Although this set 
   can be easily expanded, additional methods cannot be assumed to 
   share the same semantics for separately extended clients and 
   servers. In order to maintain compatibility, the semantic 
   definition for extension methods should be registered with the HTTP 
   registration authority (Section 10). Servers should return the 
   Status-Code "501 Not Implemented" if the method is unknown.

5.2.1 GET

   The GET method means retrieve whatever object is identified by the 
   URI. In the case where the URI refers to a data-producing process, 
   or a script which can be run by such a process, it is the produced 
   data which shall be returned as the Object-Body in the response and 
   not the source text of the script or process (unless that happens 
   to be the output of the data-producing process).

   The semantics of the GET method is changed to a "conditional GET" 
   if the request message includes an If-Modified-Since header field. 
   A conditional GET method requests that the identified Object-Body 
   be transferred only if it has been modified since the date given by 
   the If-Modified-Since header. If it has been modified (or the 
   passed If-Modified-Since date is invalid), the response is exactly 
   the same as for a normal GET. If the object has not been modified 
   since the If-Modified-Since date, the server shall return a "304 
   Not Modified" response. The "conditional GET" method is intended to 
   reduce network usage by allowing cached objects to be refreshed 
   without requiring multiple requests and transferring unnecessary 
   data.

   Data originating from HTML forms [16] can be passed to the server 
   using the GET method by appending a "?" and a set of 
   attribute/value pairs. Section 11.2 describes when to use GET and 
   POST respectively when passing form data in a request.

5.2.2 HEAD

   The HEAD method is identical to GET except that the server must not 
   return any Object-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 
   object identified by the URI without transferring the Object-Body 
   itself. This method is often used for testing hypertext links for 
   validity, accessability, and recent modification.

5.2.3 POST

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

      o  Annotation of existing documents; 

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

      o  Providing a block of data (usually a form) to a data-handling 
         process, or a script which can be run by such a process;

      o  Extending a document during authorship.

   The posted object is considered to be subordinate to the specified 
   URI, in the way that a file is subordinate to a directory 
   containing it, or a news article is subordinate to a newsgroup to 
   which it is posted.

   The client can suggest a URI for identifying the new resource by 
   including a URI-header field in the request. However, the server 
   should treat that URI as advisory only and may store the object 
   under a different URI. The origin server must inform the user agent 
   of the allocated URI via a URI-header in the response.

   The client apply relationships between the new resource and other 
   existing resources by including Link header fields, as described in 
   Section 7.14. The server may use the Link information to perform 
   other operations as a result of the new object being added. For 
   example, lists and indexes might be updated. However, no mandatory 
   operation is imposed on the origin server. The origin server may 
   also generate its own or additional links to other resources.

   If a resource has been created on the origin server, the response 
   should contain the allocated URI and all applicable Link header 
   fields, along with an Object-Body (preferably of type "text/html") 
   which describes the status of the request and refers to the new 
   resource.

   A successful POST does not require that the object 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, a "200 OK" is the appropriate Status-Code returned in the 
   response. If a resource has been created, "201 Created" should be 
   the response.

       Note: The user agent may not assume any postconditions of 
       the method in terms of web topology. For example, if a POST 
       is accepted, the effect may be delayed or overruled by human 
       moderation, batch processing, etc. The user agent should not 
       rely on the resource being immediately (or ever) created.

5.2.4 PUT

   The PUT method specifies that the enclosed Object in the request is 
   to be stored under the supplied URI. If the URI points to an 
   already existing resource, the enclosed Object should be considered 
   a modified version of the one residing on the origin server. If the 
   URI does not point to an existing resource, and that URI is capable 
   of being defined as a new resource from the requesting user agent, 
   the origin server can create the resource at that URI.

   The client can create or modify relationships between the enclosed 
   Object and other existing resources by including Link header 
   fields, as described in Section 7.14. As with POST, the server may 
   use the Link information to perform other operations as a result of 
   the request. However, no mandatory operation is imposed on the 
   origin server. The origin server can generate its own or additional 
   links to other resources.

   The actual method for determining how the resource is placed, and 
   what happens to its predecessor, is defined entirely by the origin 
   server. If version control is implemented by the origin server, the 
   Version and Derived-From header fields should be used to help 
   identify and control revisions to a resource.

5.2.5 DELETE

   The DELETE method requests that the origin server delete the 
   resource identified by the given URI. This method may be overridden 
   by human interaction (or other means) on the origin server. The 
   client cannot be guaranteed that the operation has been carried 
   out, even if the status code returned from the origin server 
   indicates that the action has been completed successfully. However, 
   the server should not indicate success unless, at the time the 
   response is given, it intends to delete the resource.

5.2.6 LINK

   The LINK method establishes one or more Link relationships between 
   the existing resource identified by the URI and other existing 
   resources. The difference between LINK and other methods allowing 
   links to be established between the resources is that the LINK 
   method does not allow any Object-Body to be sent in the request and 
   does not result in the creation of new resources. The method is 
   uniquely defined to add metainformation to an existing resource.

5.2.7 UNLINK

   The UNLINK method removes one or more Link relationships from the 
   existing resource identified by the URI. These links may have been 
   established using LINK, or by any other method supporting the Link 
   header. The removal of a link to a resource does not imply that the 
   resource ceases to exist or becomes inaccessible for future 
   references.

5.3  HTTP-Version

   The HTTP-Version element defines the version of the HTTP protocol 
   being used for the request. If the protocol version is not 
   specified, the server shall assume that the client uses HTTP 
   version 0.9 and the response should be formatted as a Simple-
   Response.

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

   Full-Request messages which apply the protocol defined by this 
   document should use an HTTP-Version of "HTTP/1.0".

5.4  Universal Resource Identifier

   The URI is a Universal Resource Identifier, as defined in RFC 
   1630 [2], and identifies the resource upon which to apply the 
   request.

       URI            = <As defined in RFC 1630>

   Unless the server is being used as a proxy, a partial URI shall be 
   given with the assumptions of the protocol (http) and host name 
   (the server's address) being obvious. That is, if the full URI 
   looks like

       http://info.cern.ch/hypertext/WWW/TheProject.html

   then the corresponding partial URI in the Simple-Request or Full-
   Request is

       /hypertext/WWW/TheProject.html

   If the client is sending the request through a proxy, the protocol 
   and host name must be explicitly declared.

   The URI must be encoded using the escaping scheme described in [2].

5.5  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. All header fields are optional and conform to the generic 
   HTTP-header syntax.

       Request-Header = User-Agent
                      | If-Modified-Since
                      | Pragma
                      | Authorization
                      | Proxy-Authorization
                      | Referer
                      | From
                      | Accept
                      | Accept-Encoding
                      | Accept-Language

   Unknown header fields should be considered Object-Header fields.

5.5.1 User-Agent

   The User-Agent 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 or features. Although it is not required, user 
   agents should always include this field with requests. The field 
   can contain multiple tokens specifying the product name, with an 
   optional slash and version designator, and other products which 
   form a significant part of the user agent. By convention, the 
   products are listed in order of their significance for identifying 
   the application.

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

       product         = token ["/" product-version]
       product-version = 1*DIGIT "." 1*DIGIT

   Example:

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

   Product tokens should be short and to the point -- use of this field 
   for advertizing or other non-essential information is explicitly 
   deprecated and will be considered as non-conformance to the 
   protocol.

       Note: Some current proxy applications append their product 
       information to the list in the User-Agent field. This is no 
       longer recommended, since it makes machine interpretation of 
       these fields ambiguous. Instead, proxies should use the 
       Forwarded header described in Section 4.3.3.

5.5.2 If-Modified-Since

   The If-Modified-Since header field is used with the GET method to 
   make it conditional: if the requested document has not been 
   modified since the time specified in this field, the document will 
   not be returned from the server; instead, a "304 Not Modified" 
   response will be returned without any Object-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

   The purpose of this feature is to allow efficient updates of local 
   cache information with a minimum amount of transaction overhead. 
   The same functionality can be obtained, though with much greater 
   overhead, by issuing a HEAD request and following it with a GET 
   request if the server indicates that the object has been modified.

5.5.3 Pragma

   The Pragma header field is used to specify directives are to be be 
   applied by servers to which they are relevant (e.g. proxies). They 
   allow the client to request a certain behavior by all servers along 
   the length of a request chain. Although multiple pragma directives 
   can be listed as part of the request, HTTP/1.0 currently only 
   defines the semantics for the "no-cache" directive.

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

       pragma-directive = "no-cache" | extension-pragma
       extension-pragma = token

   When the "no-cache" directive is present, a caching proxy should 
   not return a document from the cache even though it has not 
   expired, but it should always request the document from the actual 
   server. 

   Pragmas must be passed through by proxies even though they might 
   have significance to the proxy itself. This is necessary in cases 
   when the request has to go through many proxies, and the pragma may 
   affect all of them. It is not possible to specify a pragma for a 
   specific proxy; however, any pragma-directive not relevant to a 
   gateway or proxy should be ignored.

5.5.4 Authorization

   This version of the HTTP protocol contains a simple access 
   authentication scheme which is explained in Section 9. The 
   Authorization header field identifies the user that originated the 
   request.

       Authorization = "Authorization" ":" (
                       ( "Basic" encoded-cookie )
                       | ( extension-scheme [ extension-encrypted ] ) )

       encoded-cookie      = <base64 encoding of userid-password>
       userid-password     = [ token ] ":" *text

       extension-scheme    = token
       extension-encrypted = word

   This field is extensible so that it can be used to identify more 
   advanced encryption schemes. The first word is a specification of 
   the authorization system in use, followed by an encoded version of 
   the User-ID and the password (separated by a ":" in decoded form). 
   The encoding used for the Basic scheme is identical to the base64 
   encoding of MIME [4] without line breaks. The Basic scheme provides 
   only a low level of authentication similar to the methods used by 
   unmodified FTP and Telnet. It cannot be considered a mechanism for 
   secure transfer.

       Note: Since both the User-ID and the password are optional, 
       the userid-password could be just the single colon character 
       (":"). However, that is not recommended practice.

5.5.5 Proxy-Authorization

   The Proxy-Authorization header field allows the client to identify 
   itself (or its user) to a proxy which requires authentication. The 
   format is the same as for Authorization.

       Proxy-Authorization
                      = "Proxy-Authorization" ":" (
                          ( "Basic" encoded-cookie )
                        | ( extension-scheme [ extension-encrypted ] ) )

   Unlike Authorization, the Proxy-Authorization applies only to the 
   current connection and must not be passed on to higher-level 
   servers or proxies.

5.5.6 Referer

   The Referer field allows the client to specify, for the server's 
   benefit, the address (URI) of the document (or element within the 
   document) from which the URI in the request was obtained. This 
   allows a server to generate lists of back-links to documents, for 
   interest, logging, optimized caching etc. It also allows obsolete 
   or mistyped links to be traced for maintenance. The format of the 
   field is:

       Referer        = "Referer" ":" URI

   Example:

       Referer: http://info.cern.ch/hypertext/DataSources/Overview.html

   If a partial URI is given, then it should be interpreted relative 
   to the URI of the object of the request.

       Note: Because the source of a link may be considered private 
       information or may reveal an otherwise secure 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.

5.5.7 From

   The From header field, if given, should contain an Internet e-mail 
   address for the human user who controls the requesting user agent. 
   It should contain a machine-usable address as defined by addr-spec 
   in RFC 822:

       From           = "From" ":" addr-spec

   An example is:

       From: webmaster@w3.org

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

   The Internet e-mail address in this field does not have to 
   correspond to the Internet host which issued the request. (For 
   example, when a request is passed through a proxy, then the 
   original issuer's address should be used). The address should, if 
   possible, be a valid Internet e-mail address, whether or not it is 
   in fact an Internet e-mail address or the Internet e-mail 
   representation of an address on some other mail system.

       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.

5.5.8 Accept

   The Accept header field can contain a list of media types which are 
   acceptable as a response to the request. The set given by the 
   client should represent what is acceptable given the context of the 
   request.

   The field may be folded onto several lines and more than one 
   occurrence of the field is allowed (with the semantics being the 
   same as if all the entries had been in one field value).

       Accept         = "Accept" ":" 1#(
                             ("*" | type) "/" ("*" | subtype)
                            *(";" parameter)
                             [ ";" "q" "=" ( "0" | "1" | float ) ]
                             [ ";" "mxb" "=" 1*DIGIT ] )

       float          = <ANSI-C floating point text representation,
                        where (0.0 < float < 1.0)>

   q is the quality factor of how well the client can handle the media 
   type and mxb is the maximum accepted size of the Object-Body in 
   number of octets (decimal). The definition does not prohibit 
   duplicate accept-param's, but leaves the interpretation undefined. 
   See Section 8 for a description of the negotiation algorithm. If at 
   least one Accept header is present, a quality factor of 0 is 
   equivalent to not sending an accept header field containing that 
   media-type or set of media-types. The default values are: q=1 and 
   mxb=undefined (i.e. infinity).

   In order to save time, and also allow clients to receive media 
   types of which they may not be aware, an asterisk "*" may be used 
   in place of either the type token and/or the subtype token. The 
   example

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

   should verbally be interpreted as "if you have audio/basic, send 
   it; otherwise send me some other audio". 

   If no Accept header is present, then it is assumed that the client 
   accepts all formats with quality factor 1. This is equivalent to 
   the client sending the following accept header field:

       Accept: */*; q=1

   or

       Accept: */*

   A more elaborate example is

       Accept: text/plain; q=0.5, text/html,
               text/x-dvi; q=0.8; mxb=100000, text/x-c

   Verbally, this should be interpreted as "text/html and text/x-c are 
   the preferred media types, but if they do not exist then send the 
   Object-Body in text/x-dvi if the object is less than 100000 bytes. 
   If this is not the case then send text/plain".

       Note: In earlier versions of this document, the mxs 
       parameter defined the maximum acceptable delay in seconds 
       before the response would arrive. This has been removed as 
       the server has no means of obtaining a useful reference 
       value. However, this does not prevent the client from 
       internally measuring the response time and optimizing the 
       accept header field accordingly.

5.5.9 Accept-Encoding

   The Accept-Encoding header field is similar to Accept, but lists 
   the Content-Encoding types which are acceptable in the response. 
   Content-Encoding is described in Section 7.4.

       Accept-Encoding = "Accept-Encoding" ":" 
                         1#(encoding-mechanism [ ";" encoding-param ] )

       encoding-mechanism = extension-encoding
       encoding-param     = "q" "=" ( "0" | "1" | float )
       extension-encoding = token

   q is the quality factor of how well the client can handle the 
   encoding type. The default value is q=1. As no encoding-mechanism's 
   have been registered by IANA [15], all values are of type extension-
   encoding. However, two well-known extension-encoding's are:

       "x-compress", "x-gzip"

   If no Accept-Encoding is present then it is assumed that the client 
   accepts no encoding at all. An example of use is

       Accept-Encoding: x-compress

5.5.10 Accept-Language

   The Accept-Language header field is similar to Accept, but lists 
   the set of natural languages acceptable for the response to the 
   request.

       Accept-Language = "Accept-Language" ":"
                         1#(language-dialect *1(";" language-param) )

       language-dialect = ("*" | language) ["/" dialect ]
       language-param   = "q" "=" ( "0" | "1" | float)
       language         = <As defined in ISO 639  but case-insensitive>
       dialect          = <As defined in ISO 3166 but case-insensitive>

   As with the Accept field, a quality factor q can be specified which 
   in this case describe the level of intelligibility to the user. The 
   default value is q=1. The definition does not prohibit duplicate 
   language-param's, but leaves the interpretation undefined. An 
   example of it's use is

       Accept-Language: dk, en/gb; q=0.5

   meaning: "Send me a Danish version if you have it; else a British 
   English version."

   If no Accept-Language is present, it is assumed that the client 
   accepts all natural languages with quality factor 1.

   If the server cannot service the request with the language 
   specified, or if the languages specified only represent a subset in 
   case of a multi-linguistic Object-Body, it is not illegal to serve 
   the request in an unspecified language. The character "*" can be 
   used to indicate "any language" and/or "any dialect".

       Note: As intelligibility is highly dependent on the 
       individual user, it is recommended that client applications 
       make the choice of linguistic preference available to the 
       user.

6.  Response

   If the client has issued an HTTP request, the response from the 
   server shall consist of the following: 

       Response       = Simple-Response | Full-Response

       Simple-Response= [Object-Body]

       Full-Response  = Status-Line              ; see Section 6.1
                        *General-Header          ; see Section 4.3
                        *Response-Header         ; see Section 6.4
                        *Object-Header           ; see Section 7
                        CRLF
                        [ Object-Body ]          ; see Section 3.2

   A Simple-Response should only be sent in response to an HTTP/0.9 
   Simple-Request. Note that the Simple-Response consists only of the 
   object that was requested and is terminated by the server closing 
   the connection.

6.1  Status-Line

   The Status-Line consists 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

6.2  HTTP Version

   The HTTP-Version element identifies the protocol version being used 
   by the server. The format of this field is identical to the 
   corresponding HTTP-Version field in the Request-Line described in 
   Section 5.3.

6.3  Status Codes and Reason Phrases

   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 the Reason-Phrase, nor to pass it on to the human user.

       Status-Code    = 3DIGIT

       Reason-Phrase  = token *( SP token )

   All responses, regardless of the Status-Code, may contain an Object-
   Header and/or an Object-Body. This can either be the object pointed 
   to by the requested URI or an object containing further explanation 
   of the Status-Code. In the latter case, the preferred media type is 
   "text/html", but "text/plain" is also acceptable.

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

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

      o  2xx: Success - The requested action was successfully received 
              and understood

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

      o  4xx: Client Error - The request contains bad syntax or is 
              inherently impossible to fulfill

      o  5xx: Server Error - The server could not fulfill the request

   The values of the numeric status codes and an example set of 
   corresponding Reason-Phrase's are presented below. Every Status-
   Code has a description of which method it can follow and any 
   metainformation required in the HTTP-header.

6.3.1 Successful 2xx

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

   200 OK

      o  Following:                 GET, HEAD, POST
      o  Required metainformation:  none

   The request could be fulfilled and an Object-Header should be 
   returned to the client in the response. In the case of GET, the 
   response should also contain an Object-Body.

   201 Created

      o  Following:                 POST, PUT
      o  Required metainformation:  URI-header

   This indicates that the POST has been successful or that the PUT 
   resulted in a new object. The newly created object can be 
   referenced by the URI returned in the URI-header field in the 
   response. This action can, at any time, be overridden at the origin 
   server (possibly by human intervention), so this status code is no 
   guarantee that the object continue to be available at the given URI.

   202 Accepted

      o  Following:                 GET, HEAD, PUT, POST, DELETE
      o  Required metainformation:  none

   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 operations such as this.

   203 Provisional Information

      o  Following:                 GET, HEAD, POST
      o  Required metainformation:  none

   When received in the response, this indicates that the returned 
   metainformation in the HTTP-header is not the definitive set as 
   available from the origin server, but is gathered from a local or a 
   third party copy. The set presented can either be a subset or a 
   superset of the original version, for example including annotation 
   information about the resource. 

   204 No Response

      o  Following:                 GET, HEAD, POST
      o  Required metainformation:  none

   The server has received the request but there is no information to 
   send back, and the client should stay in the same document view. 
   This is mainly to allow input for scripts without changing the 
   document at the same time.

   205 Deleted

      o  Following:                 DELETE
      o  Required metainformation:  none

   The DELETE method was successful and the object has been removed by 
   the requested server. This action can at any time be overridden by 
   the origin server, for example by human interaction, so this status 
   code is no guarantee that the operation has in fact been carried 
   out.

   206 Modified

      o  Following:                 PUT
      o  Required metainformation:  none

   The PUT method was successful and the object has been modified on 
   the requested server. This action can at any time be overridden at 
   the origin server, for example by human interaction so this status 
   code is no guarantee that the operation has in fact been carried 
   out.

6.3.2 Redirection 3xx

   This class of status codes indicates that further action needs to 
   be taken by the client in order to fulfill the request. The action 
   required can normally be carried out by the client without 
   interaction with the user, but it is strongly recommended that this 
   only takes place if the method used in the request is either GET or 
   HEAD.

   301 Moved Permanently

      o  Following:                 GET, HEAD, POST, PUT
      o  Required metainformation:  URI-header, Location

   The object requested has been assigned a new permanent URI, and any 
   future references to this object must be done using the returned 
   URI. Clients with link editing capabilities are encouraged to 
   automatically relink references to the URI requested to the new 
   reference returned by the server, where possible.

       Note: It is possible for the server to send back this status 
       code in response to a request using the PUT and POST 
       methods. However, as this might change the conditions under 
       which the request was issued, the user agent should not 
       automatically redirect the request unless it can be 
       confirmed by the user.

   302 Moved Temporarily

      o  Following:                 GET, HEAD, POST, PUT
      o  Required metainformation:  URI-header, Location

   The data requested resides temporarily under a different URI. As 
   the redirection may be altered on occasion, the client should on 
   future requests from the user continue to use the original URI used 
   for this request and not the URI returned in the URI-header field.

       Note: It is possible for the server to send back this status 
       code in response to a request using the PUT and POST 
       methods. However, as this might change the conditions under 
       which the request was issued, the user agent should not 
       automatically redirect the request unless it can be 
       confirmed by the user.

   303 Method

      o  Required metainformation:  none

   This code is obsolete.

   304 Not Modified

      o  Following:                 conditional GET
      o  Required metainformation:  none

   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 shall 
   respond with this status code and must not send the Object-Body to 
   the client. Metainformation contained in the response should only 
   contain information relevant to cache managers and which may have 
   changed independently of the object's Last-Modified date. Examples 
   of relevant header fields are: Date, Server, and Expires. However, 
   none of them are mandatory.

6.3.3 Client Error 4xx

   The 4xx class of status codes is intended for cases in which the 
   client seems to have erred. The codes can follow any method 
   described in Section 5.2, and the set consists of:

   400 Bad Request

      o  Required metainformation:  none

   The request had bad syntax or was inherently impossible to be 
   satisfied. The client is discouraged from repeating the request 
   without modifications.

   401 Unauthorized

      o  Required metainformation:  WWW-Authenticate

   The server must return a WWW-Authenticate header field as described 
   in Section 6.4.2 containing a list of authorization schemes in 
   which at least one must be fulfilled in order for the client to 
   obtain the Object-Body. The client should then retry the request 
   with a suitable Authorization header field. The HTTP access 
   authentication scheme is explained in Section 9.

   402 Payment Required

      o  Required metainformation:  none

   This code is not currently supported, but is reserved for future 
   use.

   403 Forbidden

      o  Required metainformation:  none

   The request is, for some reason unknown to the client, forbidden. 
   Authorization will not help and the request should not be repeated. 
   This status code can also be used if the server does not want to 
   make public whether the request can not be fulfilled due to 
   insufficient authorization from the client or because the object 
   does not exist.

   404 Not Found

      o  Required metainformation:  none

   The server has not found anything matching the URI given. No 
   indication is given whether the condition is temporary or permanent.

   405 Method Not Allowed

      o  Required metainformation:  Allow

   The method specified in the Request-Line is not allowed for the 
   object identified by the URI. The server should send back an Allow 
   header containing a list of valid method's as explained in Section 
   7.1.

   406 None Acceptable

      o  Required metainformation:  Content-Type, Content-Encoding, 
                                    Content-Language

   The server has found an object matching the URI given, but not one 
   that matches all of the conditions identified in the Accept, Accept-
   Encoding, and Accept-Language request headers. The response should 
   include at least the Content-Type, the Content-Encoding, and the 
   Content-Language, but is encouraged to include the object's 
   complete metainformation. No Object-Body can be included in the 
   response.

   407 Proxy Authentication Required

      o  Required metainformation:  Proxy-Authenticate

   This code is similar to "401 Unauthorized" but it indicates that 
   the user agent must first authenticate itself with the proxy. The 
   proxy must return a Proxy-Authenticate header field as described in 
   Section 6.4.3 containing a list of authorization schemes in which 
   at least one must be fulfilled in order for the client to use the 
   proxy. The client should then create a new request with the proxy 
   as the server destination and with a suitable Proxy-Authorization 
   header field. The HTTP access authentication scheme is explained in 
   Section 9.

   408 Request Timeout

      o  Required metainformation:  none

   This code indicates that the client did not produce a request 
   within a time that the server was prepared to wait. If the client 
   is still actively generating the request, it should immediately 
   stop sending further information to the server.

6.3.4 Server Errors 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. These codes can follow any method at any 
   time.

       Note: For all of the 5xx codes, the server is encouraged to 
       send back an HTTP-header and an Object-Body containing an 
       explanation of the error situation, and whether it is a 
       temporary or permanent condition.

   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 fulfil 
   the request.

   502 Bad Gateway

   This is equivalent to "500 Internal Server Error", but for the case 
   of a gateway or proxy accessing some other service, this indicates 
   that the response from the other service was invalid. As from the 
   point of view of the client and the HTTP transaction, the other 
   service is hidden within the gateway or proxy, this may be treated 
   identically to "500 Internal Server Error", but has more diagnostic 
   value.

   503 Service Unavailable

      o  Required metainformation:  Retry-After

   The server is currently unable to handle the request. This can 
   either be due to overload of the server or servicing of the server. 
   The implication is that this is a temporary condition which will be 
   alleviated after a delay indicated in the Retry-After header. If no 
   Retry-After is present in the response then the client should 
   handle the response as identical to a "500 Internal Server Error".

   504 Gateway Timeout

   This is equivalent to "500 Internal Server Error", but for the case 
   of a gateway or proxy accessing some other service, this indicates 
   that the response from the other service did not return within a 
   time that the gateway was prepared to wait. As from the point of 
   view of the client and the HTTP transaction, the other service is 
   hidden within the gateway or proxy, this may be treated identically 
   to "500 Internal Server Error", but has more diagnostic value.

6.4  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 are not intended to give information 
   about an Object-Body returned in the response, but about the server 
   itself.

       Response-Header= Server
                      | WWW-Authenticate
                      | Proxy-Authenticate
                      | Retry-After

   Unknown header fields should be considered Object-Header fields.

6.4.1 Server

   The Server header field contains information about the software 
   being used by the origin server program handling the request. The 
   field is analogous to the User-Agent field and has the following 
   format:

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

   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. Instead, it 
   should include a Forwarded field, as described in Section 4.3.3.

6.4.2 WWW-Authenticate

   The WWW-Authenticate header field must be included as part of the 
   response if the server sends back a "401 Unauthorized" Status-Code 
   on a request from the client as part of the Basic Authentication 
   Scheme described in Section 9. This header field indicates the 
   authentication scheme in use and the realm in which the requested 
   URI belongs.

       WWW-Authenticate        = "WWW-Authenticate" ":" (
                                   ( "Basic" realm )
                                 | ( extension-scheme realm ) )

       realm                   = "Realm" "=" 1#( "<" URI ">" )

   The first word of the field value identifies the authorization 
   scheme in use and is followed by the realm of the protected URI. 
   The realm is a comma separated list of URIs, where relative URLs 
   should be interpreted relative to the URI of the requested resource 
   in the RequestLine. If a request is authenticated and a realm 
   specified, the User-ID and password should be valid for all other 
   requests within this realm.

       Note: The realm may span more than one origin server.

6.4.3 Proxy-Authenticate

   The Proxy-Authenticate header field must be included as part of the 
   response if the proxy sends back a "407 Proxy Authentication 
   Required" Status-Code on a request from the client. This header 
   field indicates the authentication scheme in use and the realm for 
   which the proxy is protected.

       Proxy-Authenticate      = "Proxy-Authenticate" ":" (
                                   ( "Basic" realm )
                                 | ( extension-scheme realm )

   Unlike WWW-Authenticate, the Proxy-Authenticate applies only to the 
   current connection and must not be passed on to lower-level user 
   agents or proxies.

6.4.4 Retry-After

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

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

       delta-seconds  = 1*DIGIT

   Two examples of use are

       Retry-After: Wed, 14 Dec 1994 18:22:54 GMT

       Retry-After: 120

   In the latter example, the delay is 2 minutes.

7.  Object Header Fields

   Full-Request and Full-Response messages can contain Object-Header 
   fields and an Object-Body (as defined in Section 3). This section 
   specifies the format and contents of the Object-Header fields.

   Object-Header fields define metainformation about the Object-Body. 
   All are optional.

       Object-Header  = Allow
                      | Content-Length
                      | Content-Type
                      | Content-Encoding
                      | Content-Transfer-Encoding
                      | Content-Language
                      | Expires
                      | Last-Modified
                      | URI-header
                      | Location
                      | Version
                      | Derived-From
                      | Title
                      | Link
                      | extension-header

       extension-header=HTTP-header

   In this section, recipient refers to either the client or the 
   server, depending on who receives the object. Each object header 
   field is explained in the subsections below. Other header fields 
   are allowed but cannot be assumed to be recognizable by the 
   recipient. Unknown header fields should be ignored by the 
   recipient, but passed on to downstream recipients (if any).

7.1  Allow

   The "Allow" header field lists the set of methods supported by the 
   object identified by the requested URI. The purpose of this field 
   is strictly to inform the recipient of valid methods associated 
   with the object. This does not prevent a client from trying other 
   methods. However, it is recommended that the indications given by 
   this field be followed. This field has no default value. If left 
   undefined, the set of allowed methods is defined by the origin 
   server at the time of each request.

       Allow          = "Allow" ":" 1#method

    Example of use:

       Allow: GET, HEAD, PUT

       Note: If a response passes through a proxy which does not 
       understand one or more of the methods indicated in the Allow 
       header, the proxy should not try to modify the Allow header, 
       since the user agent may have other means of communicating 
       with the origin server.

7.2  Content-Length

   The Content-Length header field indicates the size of the Object-
   Body (in decimal number of octets) sent to the recipient or, in the 
   case of the HEAD method, the size of the Object-Body that would 
   have been sent had the request been a GET.

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

   An example is

       Content-Length: 3495

   Even though it is not mandatory, applications are strongly 
   encouraged to use this field to indicate the size of the Object-
   Body to be transferred regardless of the media type of the object.

   Any Content-Length of size greater than or equal to zero is a valid 
   value. If undefined, the length of the Object-Body may be 
   determined by the media type (if a multipart type is used), by the 
   content encoding (if a delimited encoding is used), or by the 
   closing of the connection by the server. However, since many 
   applications do not accept multipart types or delimited encodings, 
   a valid Content-Length is generally required for objects contained 
   in PUT and POST requests.

       Note: The meaning of this field is significantly different 
       from the corresponding specification in MIME, where it is an 
       optional field used within the "message/external-body" 
       Content-Type. In HTTP, it should be used whenever the 
       object's length can be determined prior to being transferred.

7.3  Content-Type

   The Content-Type header field indicates the Internet media type of 
   the Object-Body (as described in Section 4.2) 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

   An example of the field is

       Content-Type: text/html; charset=ISO-8859-1

   All media-type's registered by IANA should be preferred over 
   extension tokens. However, HTTP does not limit conforming 
   applications to the use of officially registered media types, nor 
   does it encourage the use of an "x-" prefix for unofficial types 
   outside of explicitly short experimental use between consenting 
   applications. Data providers are strongly encouraged to register 
   their media types with IANA via the procedures outlined in RFC 
   1590 [13].

   The Content-Type header field has no default value. If and only if 
   the media type is unknown, the receiver may attempt to guess the 
   media type via inspection of its content and/or the filename 
   extension(s) of the URL used to access the object. If the media 
   type remains unknown, the receiver should treat it as type 
   "application/octet-stream".

7.4  Content-Encoding

   The Content-Encoding header field, unique to HTTP, is used as a 
   modifier to the media-type. When present, its value indicates the 
   encoding mechanism applied to the associated Object-Body prior to 
   it being enclosed in the message, and thus what decoding mechanism 
   must be applied in order to obtain the media-type referenced by the 
   Content-Type header field. This is primarily used to allow object 
   compression without losing the identity of the underlying media 
   type.

       Content-Encoding = "Content-Encoding" ":" encoding-mechanism

   An example of its use is

       Content-Encoding: x-gzip

   This field should not be confused with the Content-Transfer-
   Encoding field defined below. The purpose of the Content-Transfer-
   Encoding is "to indicate the type of transformation that has been 
   used in order to represent the object in an acceptable manner for 
   transport." In contrast, Content-Encoding is used to indicate any 
   form of compression, encryption, or packetization mechanism used to 
   transform the data prior to being transported, such that it is in a 
   more convenient form for storage and/or transport. The result after 
   encoding is usually binary, but could be any type defined by the 
   Content-Transfer-Encoding field.

7.5  Content-Transfer-Encoding

   Because all HTTP communication takes place on an 8-bit clean 
   connection, the default Content-Transfer-Encoding for all messages 
   is "binary". Note that this differs from the required default in 
   MIME [4], so gateways between HTTP and MIME-compliant protocols 
   must add an explicit "Content-Transfer-Encoding: binary" to the 
   message header if a Content-Transfer-Encoding is not already 
   present.

       Content-Transfer-Encoding = "Content-Transfer-Encoding" ":"
                                   cte-mechanism

       cte-mechanism             = token

7.6  Content-Language

   The Content-Language field describes the natural language of the 
   Object-Body. It is defined as:

       Content-Language = "Content-Language" ":" 1#lang-dia

       lang-dia         = language ["/" dialect ]

   An example of its use is

       Content-Language: dk

   means that the content of the message is in Danish with no dialect 
   specified. The example

       Content-Language: en/gb, dk

   means that the language is Danish and British English.

   Multilinguistic Object-Body's can be described using a list of lang-
   dia codes. This document does not specify any means to indicate the 
   extent to which different natural languages are represented in the 
   Object-Body.

       Note: This field can be defined not only for textual 
       documents, but also for audio and possibly other media as 
       well. It should not be considered limited to Object-Body's 
       of type "text/*".

   In the absence of an explicit Content-Language header, the client 
   should make no assumptions about the language of the returned 
   Object-Body, as it might be a language-neutral or a multilingual 
   document.

7.7  Expires

   The Expires field gives the date and time after which the 
   information given ceases to be valid and should be retrieved again 
   if it has been kept as a local copy. This allows control of caching 
   mechanisms, but the date and time indicated does not necessarily 
   imply that the original object will cease to exist. This is 
   completely controlled by the server. The format is an absolute date 
   and time as defined by HTTP-date in Section 4.1. The formal 
   description is

       Expires        = "Expires" ":" HTTP-date

   and an example of the use is

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

       Note: This field can also be used for automatic refreshing 
       of dynamic or volatile data. However, this is completely 
       dependent on the implementation of the client application to 
       automatically issue a new request when the object has 
       expired.

   Object's generated by data-producing processes, or scripts which 
   can be run by such processes, are often dynamic by nature. 
   Therefore, it is strongly recommended that responses containing 
   such Objects's contain an Expires header field.

7.8  Last-Modified

   The Last-Modified header field indicates the date and time at which 
   the sender believes the object was last modified. The exact 
   semantics of this field are defined in terms of how the receiver 
   should interpret it: If the receiver has a copy of this object 
   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. For files, it may be just the file 
   system last-mod date. For entities with dynamically included parts, 
   it may be the most recent of the set of last-mod dates of 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 its internal state changed. In any case, the recipient should 
   only know (and care) about the result -- whatever gets stuck in the 
   Last-modified: header -- and not worry about how it was obtained.

7.9  URI Header

   The URI-header field contains a URI by which the object may be 
   found. It should not be confused with the token in the Request-Line 
   described in Section 5.4. As for a normal request, there is no 
   guarantee that the resource can be accessed using the URI 
   specified. The field is normally a part of a response having Status-
   Code "301 Moved Permanently" or "302 Moved Temporarily".

       URI-header     = "URI" ":" 1#( "<" URI ">" [ ";" vary ] )

       vary           = "vary" "=" <"> 1#vary-param <">
       vary-param     = "type" | "language" | "version" | "encoding"
                      | "user-agent" | extension-vary

       extension-vary = token

   All URI's specified in this field can be either absolute or 
   relative to the URI given in the RequestLine as specified by [9].

   If a URI refers to a set of variants, then the dimensions in which 
   the variants differ must be given with the vary parameters. 
   Multiple occurrences of vary-param in the vary field give 
   alternative access names or addresses for the object. An example of 
   the field is:

       URI: <http://info.cern.ch/hypertext/WWW/TheProject.multi>;
            vary="type,language"

   This indicates that the URI indicated covers a group of possible 
   resources which varies in media type and in natural language. The 
   client can specify which of these objects to be returned in the 
   response to a request using the request header fields: Accept, 
   Accept-Encoding, Accept-Language, and Version. Another example is:

       URI: <http://info.cern.ch/hypertext/WWW/TheProject.ps>;
            vary="encoding"

   This indicates that the resource pointed to by the URI exists in 
   different encodings as defined by the Content-Encoding field.

7.10  Location

   The Location header field is an earlier form of the URI-header and 
   is considered obsolete. However, HTTP/1.0 clients and servers 
   should continue to support the Location header in order to properly 
   interface with older applications. The purpose of Location is 
   identical to that of the URI-header, except that no variants can be 
   specified and only one absolute location URI is allowed.

       Location       = "Location" ":" URI

   An example is

       Location: http://info.cern.ch/hypertext/WWW/NewLocation.html

7.11  Version

   The Version field defines a version number referring to the current 
   contents of an evolving object resident on the origin server. 
   Together with the Derived-From field described in Section 7.12, it 
   allows groups of people to work simultaneously on the creation of a 
   work as an iterative process. The field should be used to indicate 
   evolution along a single path of a particular work. It should not 
   be used to indicate derived works or renditions in different 
   representations.

       Version        = "Version" ":" *text

       Note: The field should be present in the response if PUT is 
       an allowed method to perform on the object pointed to by the 
       requested URI. However the presence of the field can not be 
       taken as an indication whether PUT is allowed or not.

7.12  Derived-From

   The Derived-From field contains the most recent value of the 
   Version field before any modifications local to the transmitting 
   application have been carried out on an evolving resource. The 
   definition of the field value must be identical to the Version 
   field value.

       Derived-From   = "Derived-From" ":" *text

   The definition of this field allows both the server and the client 
   to employ a code management system to merge different versions of 
   an evolving resource. As for the Version field, the Derived-From 
   may only be used to indicate evolution along a single path of a 
   particular work. It should not be used to indicate derived works or 
   renditions in different representations.

       Note: The definition allows different code management 
       systems to be employed by the involved parties. The only 
       requirement is a conforming mapping between any internal 
       versioning system and the one defined by Derived-From and 
       Version.

7.13  Title

   This header field indicates the title of the document, which is not 
   to be considered as part of the object. The definition of the field 
   is:

       Title          = "Title" ":" *text

   The field differs from the "Subject" field described in RFC 822 in 
   that title is defined by the creator/author of a resource, but the 
   "Subject" field is defined by the originator. The field is to be 
   considered isomorphic with the <TITLE> element in HTML [16].

7.14  Link

   The Link header provides a means for describing the relationship 
   between HTTP-Object's. An object can have multiple Link elements 
   and can typically indicate relationships like hierarchical 
   structure. The field is semantically equivalent to the <LINK> 
   element in an HTML document.

       Link = "Link" ":" 1#("<" URI ">" [ ";" "REL" "=" relation ] )

       relation       = "UseIndex" | "UseGlossary" | "Contents"
                      | "Next" | "Previous" | "Parent"
                      | "BookMark" | "Made" | "Help"

   The reader is referred to the HTML specification [16] for a full 
   explanation of the semantics for LINK relationships. Examples of 
   usage include:

       Link: <http://info.cern.ch/previous>; REL="Previous"

       Link: <mailto:timbl@info.cern.ch>; REL="Made"

   The first example indicates that this object is logically a 
   continuation of the previous object identified by the URI. The 
   second indicates that the author of the object is identified by the 
   given e-mail address.

       Note: It has been proposed that any HTML metainformation 
       element (allowed within the <HEAD> as opposed to <BODY> 
       element of the document) be a valid candidate for an HTTP 
       object header. This document defines the two header fields 
       Link and Title which both are examples of this.

8.  HTTP Negotiation Algorithm

   The goal of the negotiation algorithm is to map a set of parameters 
   into a one-dimensional space where the calculated weights represent 
   the "degradation" figure of the resource. The maximum value of this 
   set represents the media-type in which the Object-Body optimally 
   should be returned to the client.

   It is assumed that it is possible to assign an absolute value 
   representing the amount of loss of value when the Object-Body is 
   rendered into a specific media type. Whilst this is a very 
   subjective measurement, and in fact largely a function of the 
   document in question, the approximation is made that one can define 
   this degradation figure as a function of merely the representation 
   involved.

   It is furthermore assumed that the cost to the user of viewing an 
   Object also is a function of the time taken for the presentation. 
   We first assume that the cost is linear in time, and then assume 
   that the time is linear in the size of the Object-Body.

   The calculated weights are normalized to a real number between 0 
   and 1 where 0 is the minimum value and 1 is the maximum value. This 
   document defines the following parameters to be included in the 
   algorithm:

   q    The quality factor representing the level of degradation when 
        rendering the Object-Body in a specific media-type in the 
        client application. The value is normalized so that q OE[0,1], 
        where the default value is q=1.

   qs   Equivalent to the q factor but for the server application in 
        case it can perform media-type conversions. The default value 
        is qs=1.

   mxb  The maximum number of bytes in the Object-Body accepted by the 
        client. The default value is mxb=undefined (i.e. infinity).

   bs   The actual number of bytes of the Object-Body as a function of 
        media-type and encoding-mechanism. This value equals the value 
        send in the Content-Length field. The default value is bs=0.

   The discrete mapping function is defined as:

                        { if mxb=undefined, then (qs * q) }
       Q(q,qs,mxb,bs) = { if mxb >= bs,     then (qs * q) }
                        { if mxb <  bs,     then 0        }

   The maximum of the Q function represents the preferred media-type 
   to be used for transmitting the Object-Body to the client.

   Use of content variants and/or this particular negotiation algorthm 
   is not mandatory. However, it is strongly recommended as it can 
   save a significant amount of bandwidth. The hope is that fine 
   decisions will not have to be made, since in most cases the results 
   for different formats will be differ widely, and there will be a 
   clear winner.

       Note: The algorithm described does not take into account the 
       cost of any conversion performed by a gateway or proxy. 
       Although this is an important topic, it is considered more 
       important to maintain any gateways or proxy as transparent 
       parties in the transmission between the client and the 
       server.

9. Basic Access Authentication Scheme

   The basic authentication scheme described here is to be considered 
   a non-secure way 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 access authentication scheme should be used accordingly. In 
   spite of this, clients are strongly encouraged to implement the 
   scheme in order to communicate with servers that use it.

   The basic authentication scheme is based on the model that the 
   client must authenticate itself using a user-ID and a password as a 
   function of the origin server, and the realm on that server. The 
   server will service the request only if it can validate the user-ID 
   and password in the realm where the resource identified by the URI 
   is located. The protected resources on the server can be divided 
   into a set of realms which can use different password files, so 
   that different realms can be accessible by different users.

   A typical example of an authenticated request has one of the 
   following two profiles:

      o  The client requests a document without sending an Authorization 
         header field

      o  The server responds with a "401 Unauthorized" status code as 
         described in Section 6.3.3 and an WWW-Authenticate header field

      o  The client knows already a valid user-ID and a password for the 
         realm indicated by the WWW-Authenticate header field or it 
         prompts the user.

      o  The clients generates a new request with an Authorization 
         header field

      o  The server replies by sending the requested Object to the 
         client.

   In this example, the client has no authorization information prior 
   to the initial request. In the next example, the client does have 
   such information due to a previous request within the realm 
   indicated by the server. The example would then look like:

      o  The client requests a document with an Authorization header 
         field

      o  The server replies by sending the requested Object to the 
         client.

   This specification of the HTTP protocol allows other authentication 
   schemes to be implemented using either the same frame as the basic 
   scheme or additional header fields. However, these can not be 
   assumed to be generally accepted by applications conforming to this 
   specification.

   The client is encouraged to only use the second approach after it 
   has verified that the requested URI is pointing to a resource 
   within the realm where the content of the authorization header 
   field is valid for accessing the URI. If no realm has been 
   indicated then the second approach should be used only when the 
   realm is known from previous requests.

   As described in Section 1.2, this specification does not require 
   that the connection be closed after each request. However, when 
   HTTP is used on top of TCP, it is recommended that the connection 
   be closed between the first and second client request in an 
   authenticated request.

   Proxies must be completely transparent in the basic access 
   authentication scheme. That is, they must forward the
   WWW-Authenticate and Authorization headers untouched. If a proxy
   wants to authenticate a client before a request is forwarded to
   the server, it can be done using the Proxy-Authenticate and
   Proxy-Authorization headers.

10. Registration Authority

   The HTTP Registration Authority is responsible for maintaining 
   lists of: 

      o  Authorization schemes (as described in Section 9)

      o  Common method semantics for a HTTP request and response

      o  Data format names (as MIME Content-Types or Internet Media 
         Types)

      o  Data encoding names (as MIME Content-Encoding)

   It is proposed that the Internet Assigned Numbers Authority [15] or 
   their successors take this role. 

11. 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.

11.1  Authentication of Clients

   As mentioned in Section 9, the Basic Authentication scheme used in 
   HTTP/1.0 is not considered to be a secure method of user 
   authentication, nor does it prevent the Object-Body from being 
   transmitted in clear text across the physical network used as the 
   carrier. The protocol does allow for additional authentication 
   schemes and encryption mechanisms to be employed to increase the 
   security level.

11.2  Idempotent Methods

   The writers of client software should be aware that the software 
   represents the user in their interactions over the net, 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. The link "click here to subscribe"--causing the reading of a 
   special "magic" document--is open to abuse by others making a link 
   "click here to see a pretty picture". These methods should be 
   considered "safe" and should not have side effects. This allows the 
   client software to represent other methods (such as POST, PUT and 
   DELETE) in a special way, so that the user is aware of the fact 
   that an action is being requested.

11.3  Abuse of Server Log Information

   A server is in the position to save personal data about information 
   requested by readers. This information is clearly confidential in 
   nature and its handling may be constrained by law in certain 
   countries. Server providers shall ensure that such material is not 
   distributed without the permission of any individuals that are 
   identifiable by the published results.

   Two header fields are worth special mention in this context: 
   Referer and From. 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 have indicated 
   a secure document's URI, whose revelation would itself be a breach 
   of security.

   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 prior to a 
   request.

12.  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 the Content-Type definitions provided by Nathaniel 
   Borenstein and Ned Freed for MIME [4]. We hope that their inclusion 
   in this specification will help reduce past confusion over the 
   relationship between HTTP/1.0 and Internet mail.

   The HTTP protocol has evolved considerably over the past three 
   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, Dan Connolly, Ari Luotonen, Rob McCool, Dave Raggett, 
   Tony Sanders, and Marc VanHeningen deserve special recognition for 
   their efforts in defining aspects of the protocol for early 
   versions of this specification. Bob Denny assisted in proof-reading 
   the specification and performing sanity-checks as it was being 
   rewritten.

13. References

   [1]  F. Anklesaria, M. McCahill, P. Lindner, D. Johnson, D. Torrey, 
        and B. Alberti. "The Internet Gopher Protocol: A distributed 
        document search and retrieval protocol." RFC 1436, University 
        of Minnesota,  <URL:http://ds.internic.net/rfc/rfc1436.txt>, 
        March 1993.

   [2]  T. Berners-Lee.  "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, <URL:http://ds.internic.net/rfc/rfc1630.txt>,
        June 1994.

   [3]  T. Berners-Lee, L. Masinter, and M. McCahill.  "Uniform Resource 
        Locators (URL)." Internet-Draft (work in progress), CERN, Xerox 
        PARC, University of Minnesota, <URI:http://ds.internic.net/
        internet-drafts/draft-ietf-uri-url-08.txt>, October 1994.

   [4]  N. Borenstein 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, <URL:http://ds.internic.net/rfc/rfc1521.ps>, 
        September 1993.

   [5]  K. Moore.  "MIME (Multipurpose Internet Mail Extensions) Part 
        Two: Message Header Extensions for Non-ASCII Text." RFC 1522, 
        University of Tennessee, 
        <URL:http://ds.internic.net/rfc/rfc1522.txt>, September 1993.

   [6]  R. Braden.  "Requirements for Internet hosts - application and 
        support." STD 3, RFC 1123, IETF, 
        <URL:http://ds.internic.net/rfc/rfc1123.txt>, October 1989.

   [7]  D. H. Crocker.  "Standard for the Format of ARPA Internet Text 
        Messages." STD 11, RFC 822, UDEL, 
        <URL:http://ds.internic.net/rfc/rfc822.txt>, 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 Corp.,
        <URL:ftp://quake.think.com/pub/wais/doc/protspec.txt>,
        April 1990.

   [9]  R. T. Fielding.  "Relative Uniform Resource Locators."
        Internet-Draft (work in progress), UC Irvine, 
        <URL:http://ds.internic.net/internet-drafts/
        draft-ietf-uri-relative-url-02.txt>, November 1994.

   [10] M. Horton and R. Adams.  "Standard for interchange of USENET 
        messages." RFC 1036 (Obsoletes RFC 850), AT&T Bell 
        Laboratories, Center for Seismic Studies, 
        <URL:http://ds.internic.net/rfc/rfc1036.txt>, December 1987.

   [11] B. Kantor and P. Lapsley.  "Network News Transfer Protocol: A 
        Proposed Standard for the Stream-Based Transmission of News." 
        RFC 977, UC San Diego, UC Berkeley, 
        <URL:http://ds.internic.net/rfc/rfc977.txt>, February 1986.

   [12] J. Postel.  "Simple Mail Transfer Protocol." STD 10, RFC 821, 
        USC/ISI, <URL:http://ds.internic.net/rfc/rfc821.txt>, August 
        1982.

   [13] J. Postel.  "Media Type Registration Procedure." RFC 1590, 
        USC/ISI, <URL:http://ds.internic.net/rfc/rfc1590.txt>, March 
        1994.

   [14] J. Postel and J. K. Reynolds.  "File Transfer Protocol (FTP)." 
        STD 9, RFC 959, USC/ISI, 
        <URL:http://ds.internic.net/rfc/rfc959.txt>, October 1985.

   [15] J. Reynolds and J. Postel.  "Assigned Numbers." STD 2, RFC 1700, 
        USC/ISI, <URL:http://ds.internic.net/rfc/rfc1700.txt>, October 
        1994.

   [16] T. Berners-Lee, D. Connolly, et al.  "HyperText Markup Language 
        Specification - 2.0." Internet-Draft (work in progress), CERN, 
        HaL Computer Systems, 
        <URL:http://www.ics.uci.edu/pub/ietf/html/>, November 1994.

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

14. Authors Addresses

   Tim Berners-Lee
   Director, W3 Consortium
   MIT Laboratory for Computer Science
   545 Technology Square
   Cambridge, MA 02139, U.S.A.
   Tel: +1 (617) 253 9670
   Fax: +1 (617) 258 8682
   Email: timbl@w3.org

   Roy T. Fielding
   Department of Information and Computer Science
   University of California
   Irvine, CA 92717-3425, U.S.A.
   Tel: +1 (714) 824-4049
   Fax: +1 (714) 824-4056
   Email: fielding@ics.uci.edu

   Henrik Frystyk Nielsen
   World-Wide Web Project
   CERN,
   1211 Geneva 23, Switzerland
   Tel: +41 (22) 767 8265
   Fax: +41 (22) 767 8730
   Email: frystyk@w3.org

Appendices

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

A.  Tolerant Applications

   While it may be appropriate for testing applications to verify full 
   conformance to this specification, it is recommended that 
   operational applications be tolerant of deviations. This appendix 
   mentions the most important topics where tolerance is recommended.

A.1  Request-Line, Status-Line, and Header Fields

   Clients should be tolerant in parsing the StatusLine and servers 
   tolerant when parsing the RequestLine. In particular, they should 
   accept any amount of SP and HTAB characters between fields, even 
   though only a single SP is specified.

   Line terminators for HTTP-header fields should be the sequence 
   CRLF. However it is recommended that applications, when parsing 
   such headers, also recognize a single LF as a line terminator and 
   ignore any leading CR.

   Any HTTP-Header which is not recognized should be ignored by 
   operational applications except when listed in the Mandatory field, 
   as described in Section 4.3.4. 

   It is recommended that servers use URIs free of "variant" 
   characters (whose representation differs in some of the national 
   variant character sets), punctuation characters, and spaces. This 
   will make URIs easier to handle by humans when the need arises 
   (such as for debugging or transmission through non hypertext 
   systems).

   If a Content-Type header is not present in a Object-Header, the 
   preferred action is to try and guess the media-type by looking at 
   the first part of the data in the Object-Body. This document does 
   not provide any algorithm for divining the media-type. If the media-
   type cannot be found by guessing, the application can either look 
   at the file suffix (if a URL is available) or can default to 
   "application/octet-stream".

A.2  Object Body

   As it is not required to convert any Object-Body to a canonical 
   form regardless of the media-type when transmitted using HTTP, the 
   client must be very tolerant when parsing Object-Body's of type 
   "text/*". It is recommended that the following terminators be 
   considered as line separators:

       CR, LF, CRLF

   If the line terminator is CRLF, the CR should be ignored.

A.3  Backward Compatibility

   Servers should be capable of both accepting Simple-Request's and 
   generating Simple-Response's. Many dedicated clients will find the 
   Simple-Request sufficient for their purposes, even though it is 
   generally recommended to use a Full-Request instead. For more 
   general purpose client applications, it is recommend the client be 
   able to generate a Simple-Request, and understand a Simple-Response,
   in order to communicate with native HTTP/0.9 servers.

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