2004/id/draft-ietf-http-v10-spec-01.txt

HTTP Working Group                               T. Berners-Lee, MIT/W3C
INTERNET-DRAFT                                    R. Fielding, UC Irvine
<draft-ietf-http-v10-spec-01.txt>                    H. Nielsen, MIT/W3C
Expires February 3, 1996                                  August 3, 1995


                Hypertext Transfer Protocol -- HTTP/1.0


Status of this Memo

   This document is an Internet-Draft. Internet-Drafts are working 
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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".

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.  Protocol Parameters
       3.1  HTTP Version
       3.2  Uniform Resource Identifiers
            3.2.1  General Syntax
            3.2.2  http URL
       3.3  Date/Time Formats
            3.3.1  Full Date
            3.3.2  Delta Seconds
       3.4  Media Types
            3.4.1  Canonicalization and Text Defaults
            3.4.2  Multipart Types
       3.5  Character Set Encodings
       3.6  Encoding Mechanisms
       3.7  Transfer Encodings
       3.8  Language Tags
       3.9  Quality Values
       3.10 Product Tokens

   4.  HTTP Message
       4.1  Message Types
       4.2  Message Headers
       4.3  General Message Header Fields

   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  Request-URI
       5.4  Request Header Fields

   6.  Response
       6.1  Status-Line
       6.2  Status Codes and Reason Phrases
            6.2.1  Informational 1xx
            6.2.2  Successful 2xx
            6.2.3  Redirection 3xx
            6.2.4  Client Error 4xx
            6.2.5  Server Errors 5xx
       6.3  Response Header Fields

   7.  Entity
       7.1  Entity Header Fields
       7.2  Entity Body
            7.2.1  Type
            7.2.2  Length

   8.  Header Field Definitions
       8.1  Accept
       8.2  Accept-Charset
       8.3  Accept-Encoding
       8.4  Accept-Language
       8.5  Allow
       8.6  Authorization
       8.7  Content-Encoding
       8.8  Content-Language
       8.9  Content-Length
       8.10 Content-Transfer-Encoding
       8.11 Content-Type
       8.12 Date
       8.13 Expires
       8.14 Forwarded
       8.15 From
       8.16 If-Modified-Since
       8.17 Last-Modified
       8.18 Link
       8.19 Location
       8.20 MIME-Version
       8.21 Orig-URI
       8.22 Pragma
       8.23 Public
       8.24 Referer
       8.25 Retry-After
       8.26 Server
       8.27 Title
       8.28 URI
       8.29 User-Agent
       8.30 WWW-Authenticate

   9.  Content Negotiation

   10. Access Authentication
       10.1 Basic Authentication Scheme

   11. Security Considerations
       11.1 Authentication of Clients
       11.2 Idempotent Methods
       11.3 Abuse of Server Log Information
       11.4 Transfer of Sensitive Information

   12. Acknowledgments

   13. References

   14. Authors' Addresses

   Appendix A.   Internet Media Type message/http

   Appendix B.   Tolerant Applications

   Appendix C.   Relationship to MIME
       C.1  Conversion to Canonical Form
            C.1.1  Representation of Line Breaks
            C.1.2  Default Character Set Encoding
       C.2  Default Content-Transfer-Encoding
       C.3  Introduction of Content-Encoding


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 is 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 Uniform Resource Identifier (URI) [3], 
   as a location (URL) [5] or name (URN) [18], 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 [8] and the 
   Multipurpose Internet Mail Extensions (MIME) [6].

   HTTP/1.0 is also used for communication between user agents and 
   various gateways, allowing hypermedia access to existing Internet 
   protocols like SMTP [14], NNTP [12], FTP [16], Gopher [2], and 
   WAIS [9]. 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, entity 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 [17], 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 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 established between two parties for the 
       purpose of communication.

   message

       A structured sequence of octets transmitted via the connection 
       as the basic component 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.

   entity

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

   client

       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 [8]. 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, LWS, HT, CRLF, DIGIT, ALPHA, etc. 
       Angle brackets are used within definitions whenever their 
       presence will facilitate discerning the use of rule names.

   "literal"

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

   rule1 | rule2

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

   (rule1 rule2)

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

   *rule

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

   [rule]

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

   N rule

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

   #rule

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

   ; comment

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

   implied *LWS

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

2.2  Basic Rules

   The following rules are used throughout this specification to 
   describe basic parsing constructs. The US-ASCII character set 
   encoding is defined by [19].

       OCTET          = <any 8-bit sequence of data>
       CHAR           = <any US-ASCII character (octets 0 - 127)>
       UPALPHA        = <any US-ASCII uppercase letter "A".."Z">
       LOALPHA        = <any US-ASCII lowercase letter "a".."z">
       ALPHA          = UPALPHA | LOALPHA
       DIGIT          = <any US-ASCII digit "0".."9">
       CTL            = <any US-ASCII control character
                        (octets 0 - 31) and DEL (127)>
       CR             = <US-ASCII CR, carriage return (13)>
       LF             = <US-ASCII LF, linefeed (10)>
       SP             = <US-ASCII SP, space (32)>
       HT             = <US-ASCII HT, horizontal-tab (9)>
       <">            = <US-ASCII double-quote mark (34)>

   HTTP/1.0 defines the octet sequence CR LF as the end-of-line marker 
   for all protocol elements except the Entity-Body (see Appendix B 
   for tolerant applications). The end-of-line marker within an Entity-
   Body is defined by its associated media type, as described in 
   Section 3.4.

       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] 1*( SP | HT )

   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 | HT

   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.

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

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

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

   The backslash character ("\") may be used as a single-character 
   quoting mechanism only within quoted-string and comment constructs.

       quoted-pair    = "\" CHAR

   When left unquoted and not within a comment, HTTP uses angle 
   brackets to delimit machine-processable addresses; any LWS inside 
   the angle brackets should be ignored.

       addr-string    = ( "<" *(qatext) ">" )

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

   The text rule is only used for descriptive field contents and 
   values that are not intended to be interpreted by the message 
   parser. Words of *text may contain octets from character set 
   encodings other than US-ASCII only when encoded according to the 
   rules of RFC 1522 [13].

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

   Recipients of header field text containing octets outside the
   US-ASCII character set encoding may assume that they are ISO-8859-1 
   characters if there is no other encoding indicated by an RFC 1522 
   mechanism.

3.  Protocol Parameters

3.1  HTTP Version

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

   The version of an HTTP message is indicated by an HTTP-Version 
   field in the first line of the message. If the protocol version is 
   not specified, the recipient must assume that the message is in the 
   simple HTTP/0.9 format.

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

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

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

   HTTP servers must be able to recognize the format of the
   Request-Line for all lower-version requests, to understand any 
   valid request in the format of the immediately-prior major version 
   (<major-1>), to understand any valid request in the format of their 
   own native major version (<major>) with the same or lower minor 
   version, and to respond appropriately with a message within the 
   same <major> protocol version used by the client, even when the 
   response is simply an error message.

   HTTP clients must be able to recognize the format of the
   Status-Line for all lower-version responses, to understand any 
   valid response in the format of the immediately-prior major version 
   (<major-1>), and to understand any valid response in the format of 
   their own native major version (<major>) with the same or lower 
   minor version. The following hypothetical example illustrates the 
   required behavior.

      o A valid HTTP/3.5 request is received and the server's native 
        protocol version is

          o Less than 3.0: it should attempt to understand the request 
            and respond (possibly with an error) in its native format;

          o Major number of 3: It should understand the request and 
            respond in its native format;

          o Major number of 4: It should understand the request and 
            respond with a version 3 message;

          o Major number higher than 4: It should attempt to understand 
            the request and respond (possibly with an error) with a 
            version 3 message;

      o User agent receives a response to an HTTP/3.5 request, and the 
        response version is

          o Less than 2.0: It should attempt to understand the response 
            and unobtrusively warn the user of the version mismatch;

          o 2.0--3.4: It should understand the response and be aware 
            that its request may not have been fully understood by the 
            server;

          o 3.5 or higher 3: It should understand the response and can 
            assume that the server understood all aspects of the request 
            if the response does not indicate an error;

          o 4.0 or higher: It should attempt to understand the response 
            and unobtrusively warn the user of the version mismatch.

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

3.2  Uniform Resource Identifiers

   URIs have been known by many names: WWW addresses, Universal 
   Document Identifiers, Universal Resource Identifiers [3], and 
   finally the combination of Uniform Resource Locators (URL) [5] and 
   Names (URN) [18]. As far as HTTP is concerned, Uniform Resource 
   Identifiers are simply formatted strings which identify--via name, 
   location, or any other characteristic--a network resource.

3.2.1 General Syntax

   URIs in HTTP/1.0 can be represented in absolute form or relative to 
   some known base URI [10], depending upon the context of their use. 
   The two forms are differentiated by the fact that absolute URIs 
   always begin with a scheme name followed by a colon.

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

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

       relativeURI    = net_path | abs_path | rel_path

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

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

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

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

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

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

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

   For more information on URL syntax and semantics, see RFC 1738 [5] 
   and RFC 1808 [10]. The BNF above includes characters--all those 
   marked as national--not allowed in valid URLs as specified by RFC 
   1738, since HTTP servers are not restricted in the set of 
   unreserved characters allowed to represent the rel_path part of 
   addresses. In fact, the only real requirement for HTTP is that the 
   URI not contain any LWS; any other invalid URI can be identified 
   and rejected by the server.

3.2.2 http URL

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

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

       host           = <FQDN or IP address, as defined in RFC 1738>
       port           = *DIGIT

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

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

3.3  Date/Time Formats

3.3.1 Full Date

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

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

   The first format is preferred as an Internet standard and 
   represents a fixed-length subset of that defined by RFC 1123 [7] 
   (an update to RFC 822 [8]). The second format is in common use, but 
   is based on the obsolete RFC 850 [11] date format and lacks a four-
   digit year. HTTP/1.0 clients and servers must accept all three 
   formats, though they must never generate the third (asctime) 
   format. Future clients and servers must only generate the RFC 1123 
   format for representing date/time stamps in HTTP/1.0 requests and 
   responses.

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

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

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

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

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

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

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

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

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

   Comments and/or extra LWS are not permitted inside an HTTP-date 
   value generated by a conformant application.

       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.

3.3.2 Delta Seconds

   Some HTTP header fields allow a time value to be specified as an 
   integer number of seconds, represented in decimal, after the time 
   that the message was received. This format should only be used to 
   represent short time periods or periods that cannot start until 
   receipt of the message.

       delta-seconds  = 1*DIGIT

3.4  Media Types

   HTTP uses Internet Media Types [15] (formerly referred to as MIME 
   Content-Types [6]) 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 because it does not restrict itself to 
   the official IANA and x-token types.

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

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

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

   The type, subtype, and parameter attribute names are not case-
   sensitive. Parameter values may or may not be case-sensitive, 
   depending on the semantics of the parameter name. LWS should not be 
   generated between the type and subtype, nor between an attribute 
   and its value.

   If a given media-type value has been registered by the IANA, any 
   use of that value must be indicative of the registered data format. 
   Although HTTP allows the use of non-registered media types, such 
   usage must not conflict with the IANA registry. Data providers are 
   strongly encouraged to register their media types with IANA via the 
   procedures outlined in RFC 1590 [15].

   All media-type's registered by IANA must 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.

3.4.1 Canonicalization and Text Defaults

   Media types are registered in a canonical form. In general, entity 
   bodies transferred via HTTP must be represented in the appropriate 
   canonical form prior to transmission. If the body has been encoded 
   via a Content-Encoding and/or Content-Transfer-Encoding, the data 
   must be in canonical form prior to that encoding. However, HTTP 
   modifies the canonical form requirements for media of primary type 
   "text" and for "application" types consisting of text-like records.

   HTTP redefines the canonical form of text media to allow multiple 
   octet sequences to indicate a text line break. In addition to the 
   preferred form of CRLF, HTTP applications must accept a bare CR or 
   LF alone as representing a single line break in text media. 
   Furthermore, if the text media is represented in a character set 
   encoding which does not use octets 13 and 10 for CR and LF 
   respectively, as is the case for some multi-byte character set 
   encodings, HTTP allows the use of whatever octet sequence(s) is 
   defined by that character set encoding to represent the equivalent 
   of CRLF, bare CR, and bare LF. It is assumed that any recipient 
   capable of using such a character set encoding will know the 
   appropriate octet sequence for representing line breaks within that 
   character set encoding.

       Note: This interpretation of line breaks applies only to the 
       contents of an Entity-Body and only after any Content-
       Transfer-Encoding and/or Content-Encoding has been removed. 
       All other HTTP constructs use CRLF exclusively to indicate a 
       line break. Encoding mechanisms define their own line break 
       requirements.

   A recipient of an HTTP text entity should translate the received 
   entity line breaks to the local line break conventions before 
   saving the entity external to the application and its cache; 
   whether this translation takes place immediately upon receipt of 
   the entity, or only when prompted by the user, is entirely up to 
   the individual application.

   HTTP also redefines the default character set encoding for text 
   media in an entity body. If a textual media type defines a charset 
   parameter with a registered default value of "US-ASCII", HTTP 
   changes the default to be "ISO-8859-1". Since the ISO-8859-1 [20] 
   character set encoding is a superset of US-ASCII [19], this has no 
   effect upon the interpretation of entity bodies which only contain 
   octets within the US-ASCII set (0 - 127). The presence of a charset 
   parameter value in a Content-Type header field overrides the 
   default.

   It is recommended but not required that the character set encoding 
   of an entity body be labelled as the lowest common denominator of 
   the character codes used within a document, with the exception that 
   no label is preferred over the labels US-ASCII or ISO-8859-1.

3.4.2 Multipart Types

   MIME provides for a number of "multipart" types -- encapsulations of 
   several entities within a single message's Entity-Body. The 
   multipart types registered by IANA [17] 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. Ideally, an HTTP user agent should follow the same or similar 
   behavior as a MIME user agent does upon receipt of a multipart type.

   As in MIME [6], all multipart types share a common syntax and must 
   include a boundary parameter as part of the media type value. The 
   message body is itself a protocol element and must therefore use 
   only CRLF to represent line breaks between body-parts. Unlike in 
   MIME, multipart body-parts may contain HTTP header fields which are 
   significant to the meaning of that part.

   A URI-header field (Section 8.28) should be included in the body-
   part for each enclosed entity that can be identified by a URI.

3.5  Character Set Encodings

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

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

   However, since this is more commonly referred to as a character 
   encoding, this document will refer to them as character set 
   encodings. Character set encodings are identified by case-
   insensitive tokens. The complete set of tokens are defined by the 
   IANA Character Set registry [17]. However, because that registry 
   does not define a single, consistent token for each character set 
   encoding, we define here the preferred names for those character 
   set encodings most likely to be used with HTTP entities. This set 
   of charset values includes those registered by RFC 1521 [6] -- the 
   US-ASCII [19] and ISO-8859 [20] character set encodings -- and other 
   names specifically recommended for use within MIME charset 
   parameters.

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

   Although HTTP allows an arbitrary token to be used as a charset 
   value, any token that has a predefined value within the IANA 
   Character Set registry [17] must represent the character set 
   encoding defined by that registry. Applications are encouraged, but 
   not required, to limit their use of character set encodings to 
   those defined by the IANA registry. 

3.6  Encoding Mechanisms

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

       encoding-mechanism      = "gzip" | "compress" | token

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

   All encoding-mechanism values are case-insensitive. HTTP/1.0 uses 
   encoding-mechanism values in the Accept-Encoding (Section 8.3) and 
   Content-Encoding (Section 8.7) header fields. Although the value 
   describes the encoding-mechanism, what is more important is that it 
   indicates what decoding mechanism will be required to remove the 
   encoding. Note that a single program may be capable of decoding 
   multiple encoding-mechanism formats. Two values are defined by this 
   specification:

   gzip       An encoding format produced by the file compression
              program "gzip" (GNU zip) developed by Jean-loup Gailly.
              This format is typically a Lempel-Ziv coding (LZ77) with
              a 32 bit CRC. Gzip is available from the GNU project at 
              <URL:ftp://prep.ai.mit.edu/pub/gnu/>.

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

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

3.7  Transfer Encodings

   Transfer encoding values are used to indicate an encoding 
   transformation that has been, can be, or may need to be applied to 
   an Entity-Body in order to ensure safe transport through the 
   network. Current transfer encodings are only used with entities 
   destined for or retrieved from MIME-conformant systems, and thus 
   will rarely occur in an HTTP/1.0 message. This differs from an 
   encoding-mechanism in that the transfer encoding is a property of 
   the message, not of the original resource.

       transfer-encoding       = "binary" | "8bit" | "7bit"
                               | "quoted-printable" | "base64"
                               | token

   All transfer-encoding values are case-insensitive. HTTP/1.0 may use 
   transfer-encoding values in the Content-Transfer-Encoding 
   (Section 8.10) header field.

       Note: Transfer encodings were designed for MIME with the 
       assumption of their being used only within the context of 
       Internet mail and SMTP. "Safe transport" has a different 
       focus for an 8bit-clean transfer protocol. In HTTP, the only 
       unsafe characteristic of message bodies is the difficulty in 
       determining the exact body length (Section 7.2.2).

   The values "7bit", "8bit", and "binary" are used to indicate that 
   no transfer encoding has been performed. Instead, they describe the 
   sort of encoding that might be needed for transmission through an 
   unsafe transport system. Binary indicates that the body may contain 
   any set of octets. 8bit adds the restrictions that CR and LF 
   characters only occur as part of CRLF line separators, all lines 
   are short (less than 1000 octets), and no NULs (octet 0) are 
   present. 7bit adds a further restriction that all octets are 7-bit 
   US-ASCII characters.

   The "quoted-printable" and "base64" values indicate that the 
   associated encoding (as defined in MIME [6]) has been applied to 
   the body. These encodings consist entirely of 7-bit US-ASCII 
   characters.

3.8  Language Tags

   A language tag identifies a natural language spoken, written, or 
   otherwise conveyed by human beings for communication of information 
   to other human beings. Computer languages are explicitly excluded. 
   The HTTP/1.0 protocol uses language tags within the
   Accept-Language, Content-Language, and URI-header fields.

   The syntax and registry of HTTP language tags is the same as that 
   defined by RFC 1766 [1]. In summary, a language tag is composed of 
   1 or more parts: A primary language tag and a possibly empty series 
   of subtags:

        language-tag  = primary-tag *( "-" subtag )

        primary-tag   = 1*8ALPHA
        subtag        = 1*8ALPHA

   Whitespace is not allowed within the tag and all tags are not case-
   sensitive. The namespace of language tags is administered by the 
   IANA. Example tags include:

       en, en-US, en-cockney, i-cherokee, x-pig-latin

   where any two-letter primary-tag is an ISO 639 language 
   abbreviation and any two-letter initial subtag is an ISO 3166 
   country code.

   In the context of the Accept-Language header (Section 8.4), a 
   language tag is not to be interpreted as a single token, as per 
   RFC 1766, but as a hierarchy. A server should consider that it has a 
   match when a language tag received in an Accept-Language header 
   matches the initial portion of the language tag of a document. An 
   exact match should be preferred. This interpretation allows a 
   browser to send, for example:

       Accept-Language: en-US, en; ql=0.95

   when the intent is to access, in order of preference, documents in 
   US-English ("en-US"), 'plain' or 'international' English ("en"), 
   and any other variant of English (initial "en-").

       Note: Using the language tag as a hierarchy does not imply 
       that all languages with a common prefix will be understood 
       by those fluent in one or more of those languages; it simply 
       allows the user to request this commonality when it is true 
       for that user.

3.9  Quality Values

   HTTP content negotiation (Section 9) uses short "floating point" 
   numbers to indicate the relative importance ("weight") of various 
   negotiable parameters. The calculated weights are normalized to a 
   real number in the range 0 through 1, where 0 is the minimum and 1 
   the maximum value. In order to discourage misuse of this feature, 
   HTTP/1.0 applications must not generate more than three digits 
   after the decimal point. User configuration of these values should 
   also be limited in this fashion.

       qvalue         = ( "0" [ "." 0*3DIGIT ] )
                      | ( "." 0*3DIGIT )
                      | ( "1" [ "." 0*3("0") ] )

   "Quality values" is a slight misnomer, since these values actually 
   measure relative degradation in perceived quality. Thus, a value of 
   "0.8" represents a 20% degradation from the optimum rather than a 
   statement of 80% quality.

3.10  Product Tokens

   Product tokens are used to allow communicating applications to 
   identify themselves via a simple product token, with an optional 
   slash and version designator. Most fields using product tokens also 
   allow subproducts which form a significant part of the application 
   to be listed, separated by whitespace. By convention, the products 
   are listed in order of their significance for identifying the 
   application.

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

   Examples:

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

       Server: Apache/0.8.4

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

4.  HTTP Message

4.1  Message Types

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

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

   Full-Request and Full-Response use the generic message format of 
   RFC 822 [8] for transferring entities. Both messages may include 
   optional header fields (a.k.a. "headers") and an entity body. The 
   entity body is separated from the headers by a null line (i.e., a 
   line with nothing preceding the CRLF).

       Full-Request   = Request-Line             ; Section 5.1
                        *( General-Header        ; Section 4.3
                        |  Request-Header        ; Section 5.4
                        |  Entity-Header )       ; Section 7.1
                        CRLF
                        [ Entity-Body ]          ; Section 7.2

       Full-Response  = Status-Line              ; Section 6.1
                        *( General-Header        ; Section 4.3
                        |  Response-Header       ; Section 6.3
                        |  Entity-Header )       ; Section 7.1
                        CRLF
                        [ Entity-Body ]          ; Section 7.2

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

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

       Simple-Response = [ Entity-Body ]

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

4.2  Message Headers

   HTTP header fields, which include General-Header (Section 4.3), 
   Request-Header (Section 5.4), Response-Header (Section 6.3), and 
   Entity-Header (Section 7.1) fields, follow the same generic format 
   as that given in Section 3.1 of RFC 822 [8]. Each header field 
   consists of a name followed by a colon (":") and the field value. 
   Field names are never case-sensitive. The field value may be 
   preceded by any amount of LWS, though a single SP is preferred. 
   Header fields can be extended over multiple lines by preceding each 
   extra line with at least one LWS.

       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 "good practice" to send General-Header fields first, 
   followed by Request-Header or Response-Header fields prior to the 
   Entity-Header fields.

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

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 content 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 = Date                     ; Section 8.12
                      | Forwarded                ; Section 8.14
                      | MIME-Version             ; Section 8.20
                      | Pragma                   ; Section 8.22

   General header field names can be extended only via a change in the 
   protocol version. Unknown header fields are treated as
   Entity-Header fields.

5. Request

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

       Request        = Simple-Request | Full-Request

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

       Full-Request   = Request-Line             ; Section 5.1
                        *( General-Header        ; Section 4.3
                        |  Request-Header        ; Section 5.4
                        |  Entity-Header )       ; Section 7.1
                        CRLF
                        [ Entity-Body ]          ; Section 7.2

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

5.1  Request-Line

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

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

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

5.2  Method

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

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

       extension-method = token

   The list of methods acceptable by a specific resource can be 
   specified in an "Allow" Entity-Header (Section 8.5). 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. Servers should return the status code 
   "405 Method Not Allowed" if the method is known by the server but 
   not allowed for the requested resource, and "501 Not Implemented" 
   if the method is unknown or not implemented by the server.

   The methods GET and HEAD must be supported by all general-purpose 
   servers. Servers which provide Last-Modified dates for resources 
   must also support the conditional GET method.

   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 
   IANA [17].

5.2.1 GET

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

   The semantics of the GET method changes to a "conditional GET" if 
   the request message includes an If-Modified-Since header field. A 
   conditional GET method requests that the identified resource be 
   transferred only if it has been modified since the date given by 
   the If-Modified-Since header, as described in Section 8.16. The 
   conditional GET method is intended to reduce network usage by 
   allowing cached entities to be refreshed without requiring multiple 
   requests or transferring unnecessary data.

5.2.2 HEAD

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

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

5.2.3 POST

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

      o Annotation of existing resources; 

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

      o Providing a block of data, such as the result of submitting a 
        form [4], to a data-handling process;

      o Extending a database through an append operation.

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

   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 and may store the entity under a 
   different URI or without any URI.

   The client may apply relationships between the new resource and 
   other existing resources by including Link header fields, as 
   described in Section 8.18. The server may use the Link information 
   to perform other operations as a result of the new resource 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.

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

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

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

5.2.4 PUT

   The PUT method requests that the enclosed entity be stored under 
   the supplied Request-URI. If the Request-URI refers to an already 
   existing resource, the enclosed entity should be considered as a 
   modified version of the one residing on the origin server. If the 
   Request-URI does not point to an existing resource, and that URI is 
   capable of being defined as a new resource by the requesting user 
   agent, the origin server can create the resource with that URI. If 
   a new resource is created, the origin server must inform the user 
   agent via the 201 (created) response. If an existing resource is 
   modified, either the 200 (ok) or 204 (no content) response codes 
   should be sent to indicate successful completion of the request. If 
   the resource could not be created or modified with the Request-URI, 
   an appropriate error response should be given that reflects the 
   nature of the problem.

   The fundamental difference between the POST and PUT requests is 
   reflected in the different meaning of the Request-URI. The URI in a 
   POST request identifies the resource that will handle the enclosed 
   entity as an appendage. That resource may be a data-accepting 
   process, a gateway to some other protocol, or a separate entity 
   that accepts annotations. In contrast, the URI in a PUT request 
   identifies the entity enclosed with the request -- the user agent 
   knows what URI is intended and the server must not attempt to apply 
   the request to some other resource. If the server desires that the 
   request be applied to a different URI, it must send a 301 (moved 
   permanently) response; the user agent may then make its own 
   decision regarding whether or not to redirect the request.

   A single resource may be identified by many different URIs. For 
   example, an article may have a URI for identifying "the current 
   version" which is separate from the URI identifying each particular 
   version. In this case, a PUT request on a general URI may result in 
   several other URIs being defined by the origin server. The user 
   agent should be informed of these URIs via one or more URI header 
   fields in the response. The Location header field should be used to 
   identify the exact location URI if it is different than the
   Request-URI.

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

   The client can create or modify relationships between the enclosed 
   entity and other existing resources by including Link header 
   fields, as described in Section 8.18. 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 may 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, 
   then Link relationships should be defined by the server to help 
   identify and control revisions to a resource; suggested 
   relationship names include "Derived-From", "Obsoletes", and 
   "Updates".

       Note: The model of sending an entire PUT request within a 
       single message, without first checking if the server is 
       willing to accept that data, will break if the server is 
       unwilling to accept the request or desires some form of 
       authentication beforehand. Worse, the client won't be 
       notified of the reason for error if a TCP reset is received 
       prior to reading the response buffer (see note in 
       Section 6.2.4). It should therefore be recognized that 
       HTTP/1.0 PUT and large POST requests will only work reliably 
       if the client's intentions and server's desires are 
       negotiated prior to the request.

5.2.5 DELETE

   The DELETE method requests that the origin server delete the 
   resource identified by the Request-URI. This method may be 
   overridden by human intervention (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 or move it 
   to an inaccessible location.

   A successful response should be 200 (ok) if the response includes 
   an entity describing the status, 202 (accepted) if the action has 
   not yet been enacted, or 204 (no content) if the response is OK but 
   does not include an entity.

5.2.6 LINK

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

5.2.7 UNLINK

   The UNLINK method removes one or more Link relationships from the 
   existing resource identified by the Request-URI. These 
   relationships may have been established using the LINK method 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  Request-URI

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

       Request-URI    = "*" | absoluteURI | abs_path

   The three options for Request-URI are dependent on the nature of 
   the request. The asterisk "*" means that the request does not apply 
   to a particular resource, but to the server itself, and is only 
   allowed when the Method used does not necessarily apply to a 
   resource. Note that this is not the case for any of the methods 
   defined by this document; however, it may be true of extension 
   methods. One example would be

       OPTIONS * HTTP/1.0

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

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

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

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

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

5.4  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 = Accept                   ; Section 8.1
                      | Accept-Charset           ; Section 8.2
                      | Accept-Encoding          ; Section 8.3
                      | Accept-Language          ; Section 8.4
                      | Authorization            ; Section 8.6
                      | From                     ; Section 8.15
                      | If-Modified-Since        ; Section 8.16
                      | Orig-URI                 ; Section 8.21
                      | Referer                  ; Section 8.24
                      | User-Agent               ; Section 8.29

   Request-Header field names can be extended only via a change in the 
   protocol version. Unknown header fields are treated as
   Entity-Header fields.

6.  Response

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

       Response        = Simple-Response | Full-Response

       Simple-Response = [ Entity-Body ]

       Full-Response   = Status-Line             ; Section 6.1
                        *( General-Header        ; Section 4.3
                        |  Response-Header       ; Section 6.3
                        |  Entity-Header )       ; Section 7.1
                        CRLF
                        [ Entity-Body ]          ; Section 7.2

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

6.1  Status-Line

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

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

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

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

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

6.2  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 or display the Reason-Phrase.

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

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

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

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

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

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

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

       Status-Code    = "200"   ; OK
                      | "201"   ; Created
                      | "202"   ; Accepted
                      | "203"   ; Non-Authoritative Information
                      | "204"   ; No Content
                      | "300"   ; Multiple Choices
                      | "301"   ; Moved Permanently
                      | "302"   ; Moved Temporarily
                      | "303"   ; See Other
                      | "304"   ; Not Modified
                      | "400"   ; Bad Request
                      | "401"   ; Unauthorized
                      | "402"   ; Payment Required
                      | "403"   ; Forbidden
                      | "404"   ; Not Found
                      | "405"   ; Method Not Allowed
                      | "406"   ; None Acceptable
                      | "407"   ; Proxy Authentication Required
                      | "408"   ; Request Timeout
                      | "409"   ; Conflict
                      | "410"   ; Gone
                      | "411"   ; Authorization Refused
                      | "500"   ; Internal Server Error
                      | "501"   ; Not Implemented
                      | "502"   ; Bad Gateway
                      | "503"   ; Service Unavailable
                      | "504"   ; Gateway Timeout
                      | extension-code

       extension-code = 3DIGIT

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

   HTTP status codes are extensible and should be registered with the 
   IANA. HTTP applications are not required to understand the meaning 
   of all registered status codes, though such understanding is 
   obviously desirable. However, applications must understand the 
   class of any status code, as indicated by the first digit, and 
   treat any unknown response as being equivalent to the x00 status 
   code of that class. For example, if an unknown status code of 421 
   is received by the client, it can safely assume that there was 
   something wrong with its request and treat the response as if it 
   had received a 400 status code. In such cases, user agents are 
   encouraged to present the entity returned with the response to the 
   user, since that entity is likely to include human-readable 
   information which will explain the unusual status.

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

6.2.1 Informational 1xx

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

6.2.2 Successful 2xx

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

   200 OK

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

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

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

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

   PUT, DELETE, LINK, UNLINK
              an entity describing the result of the action;

   If the entity corresponds to a resource, the response may include a 
   Location header field giving the actual location of that specific 
   resource for later reference.

   201 Created

   The request has been fulfilled and resulted in a new resource being 
   created. The newly created resource can be referenced by the URI(s) 
   returned in the URI-header field of the response, with the most 
   specific URL for the resource given by a Location header field. The 
   origin server is encouraged, but not obliged, to actually create 
   the resource before using this Status-Code. If the action cannot be 
   carried out immediately, or within a clearly defined timeframe, the 
   server should respond with 202 (accepted) instead.

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

   202 Accepted

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

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

   203 Non-Authoritative Information

   The returned metainformation in the Entity-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 may be a 
   subset or superset of the original version. For example, including 
   local annotation information about the resource may result in a 
   superset of the metainformation known by the origin server. Use of 
   this response code is not required and is only appropriate when the 
   response would otherwise be 200 (ok).

   204 No Content

   The server has fulfilled the request but there is no new 
   information to send back. If the client is a user agent, it should 
   not change its document view from that which caused the request to 
   be generated. This response is primarily intended to allow input 
   for scripts or other actions to take place without causing a change 
   to the user agent's active document view. The response may include 
   new metainformation in the form of entity headers, which should 
   apply to the document currently in the user agent's active view.

6.2.3 Redirection 3xx

   This class of status code indicates that further action needs to be 
   taken by the user agent in order to fulfill the request. The action 
   required can sometimes be carried out by the user agent without 
   interaction with the user, but it is strongly recommended that this 
   only take place if the method used in the request is idempotent 
   (GET or HEAD). A user agent should never automatically redirect a 
   request more than 5 times, since such redirections usually indicate 
   an infinite loop.

   300 Multiple Choices

   The requested resource is available at one or more locations and a 
   preferred location could not be determined via content negotiation. 
   Unless it was a HEAD request, the response should include an entity 
   containing a list of resource characteristics and locations from 
   which the user or user agent can choose the one most appropriate. 
   The entity format is specified by the media type given in the 
   Content-Type header field. Depending upon the format and the 
   capabilities of the user agent, selection of the most appropriate 
   choice may be performed automatically. If the server has a 
   preferred choice, it should include its URL in a Location field; 
   user agents not capable of complex selection may use the Location 
   value for automatic redirection.

   301 Moved Permanently

   The requested resource has been assigned a new permanent URI and 
   any future references to this resource should be done using one of 
   the returned URIs. Clients with link editing capabilities are 
   encouraged to automatically relink references to the Request-URI to 
   one or more of the new references returned by the server, where 
   possible.

   If the new URI is a single location, its URL must be given by the 
   Location field in the response. If more than one URI exists for the 
   resource, the primary URL should be given in the Location field and 
   the other URIs given in one or more URI-header fields.  The Entity-
   Body of the response should contain a short hypertext note with a 
   hyperlink to the new URI(s).

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

   302 Moved Temporarily

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

   If the new URI is a single location, its URL must be given by the 
   Location field in the response. If more than one URI exists for the 
   resource, the primary URL should be given in the Location field and 
   the other URIs given in one or more URI-header fields. The Entity-
   Body of the response should contain a short hypertext note with a 
   hyperlink to the new URI(s).

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

   303 See Other

   The requested resource resides under a different URI and should be 
   accessed using a GET method on that resource. This method exists 
   primarily to allow the output of a POST-activated script to 
   redirect the user agent to a selected resource. The new resource is 
   not a replacement reference for the original Request-URI.

   If the new URI is a single location, its URL must be given by the 
   Location field in the response. If more than one URI exists for the 
   resource, the primary URL should be given in the Location field and 
   the other URIs given in one or more URI-header fields.  The Entity-
   Body of the response should contain a short hypertext note with a 
   hyperlink to the new URI(s).

   304 Not Modified

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

6.2.4 Client Error 4xx

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

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

   400 Bad Request

   The request could not be understood by the server due to it having 
   a malformed syntax. The client is discouraged from repeating the 
   request without modifications.

   401 Unauthorized

   The request requires user authentication. The response must include 
   a WWW-Authenticate header field (Section 8.30) containing a 
   challenge applicable to the requested resource. The client may 
   repeat the request with a suitable Authorization header field. HTTP 
   access authentication is explained in Section 10.

   402 Payment Required

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

   403 Forbidden

   The server understood the request, but is refusing to perform the 
   request because of an unspecified reason. Authorization will not 
   help and the request should not be repeated. This status code can 
   be used if the server does not want to make public why the request 
   has not been fulfilled.

   404 Not Found

   The server has not found anything matching the Request-URI. No 
   indication is given of whether the condition is temporary or 
   permanent. If the server does not wish to make this information 
   available to the client, the status code 403 (forbidden) can be 
   used instead. The 410 (gone) status code should be used if the 
   server knows, through some internally configurable mechanism, that 
   an old resource is permanently unavailable and has no forwarding 
   address.

   405 Method Not Allowed

   The method specified in the Request-Line is not allowed for the 
   resource identified by the Request-URI. The response must include 
   an Allow header containing a list of valid method's for the 
   requested resource.

   406 None Acceptable

   The server has found a resource matching the Request-URI, but not 
   one that satisfies the conditions identified by the Accept and 
   Accept-Encoding request headers. Unless it was a HEAD request, the 
   response should include an entity containing a list of resource 
   characteristics and locations from which the user or user agent can 
   choose the one most appropriate. The entity format is specified by 
   the media type given in the Content-Type header field. Depending 
   upon the format and the capabilities of the user agent, selection 
   of the most appropriate choice may be performed automatically.

   407 Proxy Authentication Required

   This code is reserved for future use. It is similar to 401 
   (unauthorized), but indicates that the client must first 
   authenticate itself with the proxy. HTTP/1.0 does not provide a 
   means for proxy authentication.

   408 Request Timeout

   The client did not produce a request within the time that the 
   server was prepared to wait. The client may repeat the request 
   without modifications at any later time.

   409 Conflict

   The request could not be completed due to a conflict with the 
   current state of the resource. This code is only allowed in 
   situations where it is expected that the user may be able to 
   resolve the conflict and resubmit the request. The response body 
   should include enough information for the user to recognize the 
   source of the conflict. Ideally, the response entity would include 
   enough information for the user or user-agent to fix the problem; 
   however, that may not be possible and is not required.

   Conflicts are most likely to occur in response to a PUT request. If 
   versioning is being used and the entity being PUT includes changes 
   to a resource which conflict with those made by an earlier (third-
   party) request, the server may use the 409 response to indicate 
   that it can't complete the PUT. In this case, the response entity 
   may contain a list of the differences between the two versions.

   410 Gone

   The requested resource is no longer available at the server and no 
   forwarding address is known. This condition should be considered 
   permanent. Clients with link editing capabilities are encouraged to 
   delete references to the Request-URI after user approval. If the 
   server does not know, or has no facility to determine, whether or 
   not the condition is permanent, the status code 404 (not found) 
   should be used instead.

   The 410 response is primarily intended to assist the task of web 
   maintenance by notifying the recipient that the resource is 
   intentionally unavailable and that the server owners desire that 
   remote links to that resource be removed. Such an event is common 
   for limited-time, promotional services and for resources belonging 
   to individuals no longer working at the server's site. It is not 
   necessary to mark all permanently unavailable resources as "gone" 
   or to keep the mark for any length of time -- that is left to the 
   discretion of the server owner.

   411 Authorization Refused

   The request credentials provided by the client were rejected by the 
   server or insufficient to grant authorization to access the 
   resource. This is similar to the 403 (forbidden) response, but 
   allows more information to be provided to the user. The content of 
   the response should contain a description of the problem and may 
   suggest corrective action. HTTP access authentication is explained 
   in Section 10.

   The response must include a WWW-Authenticate header field 
   (Section 8.30) containing a challenge applicable to the requested 
   resource. If the challenge is different from that assumed by the 
   last request, the client may repeat the request with a suitable 
   Authorization header field after obtaining the user's approval.

6.2.5 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. If the client has not completed the request 
   when a 5xx code is received, it should immediately cease sending 
   data to the server. Except when responding to a HEAD request, the 
   server is encouraged to include an entity containing an explanation 
   of the error situation, and whether it is a temporary or permanent 
   condition. These response codes are applicable to any request 
   method and there are no required header fields.

   500 Internal Server Error

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

   501 Not Implemented

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

   502 Bad Gateway

   The server received an invalid response from the gateway or 
   upstream server it accessed in attempting to fulfill the request.

   503 Service Unavailable

   The server is currently unable to handle the request due to a 
   temporary overloading or maintenance of the server. The implication 
   is that this is a temporary condition which will be alleviated 
   after some delay. If known, the length of the delay may be 
   indicated in a Retry-After header. If no Retry-After is given, the 
   client should handle the response as it would for a 500 response.

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

   504 Gateway Timeout

   The server did not receive a timely response from the gateway or 
   upstream server it accessed in attempting to complete the request.

6.3  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 Entity-Body returned in the response, but 
   about the server itself.

       Response-Header= Location                 ; Section 8.19
                      | Public                   ; Section 8.23
                      | Retry-After              ; Section 8.25
                      | Server                   ; Section 8.26
                      | WWW-Authenticate         ; Section 8.30

   Response-Header field names can be extended only via a change in 
   the protocol version. Unknown header fields are treated as
   Entity-Header fields.

7.  Entity

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

7.1  Entity Header Fields

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

       Entity-Header  = Allow                     ; Section 8.5
                      | Content-Encoding          ; Section 8.7
                      | Content-Language          ; Section 8.8
                      | Content-Length            ; Section 8.9
                      | Content-Transfer-Encoding ; Section 8.10
                      | Content-Type              ; Section 8.11
                      | Expires                   ; Section 8.13
                      | Last-Modified             ; Section 8.17
                      | Link                      ; Section 8.18
                      | Title                     ; Section 8.27
                      | URI-header                ; Section 8.28
                      | extension-header

       extension-header=HTTP-header

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

7.2  Entity Body

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

       Entity-Body    = *OCTET

   An entity-body is included with a request message only when the 
   request method calls for one. This specification defines two 
   request methods, "POST" and "PUT", that allow an entity-body. In 
   general, the presence of an entity-body in a request is signaled by 
   the inclusion of a Content-Length and/or Content-Transfer-Encoding 
   header field in the request message headers. HTTP/1.0 requests 
   containing content must include a valid Content-Length header field.

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

7.2.1 Type

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

       entity-body <-
          Content-Transfer-Encoding( Content-Encoding( Content-Type ) )

   The default for both encodings is none (i.e., the identity 
   function). A Content-Type specifies the media type of the 
   underlying data. A Content-Encoding may be used to indicate any 
   additional encoding mechanisms applied to the type, usually for the 
   purpose of data compression, that is a property of the resource 
   requested. A Content-Transfer-Encoding may be applied by a 
   transport agent to ensure safe and proper transfer of the message. 
   Note that the Content-Transfer-Encoding is a property of the 
   message, not of the resource.

   The Content-Type header field has no default value. If and only if 
   the media type is not given by a Content-Type header, as is always 
   the case for Simple-Response messages, the receiver may attempt to 
   guess the media type via inspection of its content and/or the name 
   extension(s) of the URL used to specify the resource. If the media 
   type remains unknown, the receiver should treat it as type 
   "application/octet-stream".

7.2.2 Length

   When an Entity-Body is included with a message, the length of that 
   body may be determined in one of several ways. If a Content-Length 
   header field is present, its value in bytes represents the length 
   of the Entity-Body. Otherwise, the body length is determined by the 
   Content-Type (for types with an explicit end-of-body delimiter), 
   the Content-Transfer-Encoding (for packetized encodings), or the 
   closing of the connection by the server.

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

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

8.  Header Field Definitions

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

8.1  Accept

   The Accept header field can be used to indicate a list of media 
   ranges which are acceptable as a response to the request. The 
   asterisk "*" character is used to group media types into ranges, 
   with "*/*" indicating all media types and "type/*" indicating all 
   subtypes of that type. The set of ranges given by the client should 
   represent what types are acceptable given the context of the 
   request. The Accept field should only be used when the request is 
   specifically limited to a set of desired types, as in the case of a 
   request for an in-line image, or to indicate qualitative 
   preferences for specific media types.

   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" ":" #(
                             media-range
                             [ ";" "q" "=" qvalue ]
                             [ ";" "mxb" "=" 1*DIGIT ] )

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

   The parameter q is used to indicate the quality factor, which 
   represents the user's preference for that range of media types. The 
   parameter mxb gives the maximum acceptable size of the Entity-Body, 
   in decimal number of octets, for that range of media types. 
   Section 9 describes the content negotiation algorithm which makes 
   use of these values. The default values are: q=1 and mxb=undefined 
   (i.e., infinity).

   The example

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

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

   If no Accept header is present, then it is assumed that the client 
   accepts all media types 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 would be interpreted as "text/html and text/x-c are 
   the preferred media types, but if they do not exist then send the 
   Entity-Body in text/x-dvi if the entity is less than 100000 bytes, 
   otherwise 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.

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

       Accept: text/*, text/html, text/html;version=2.0, */*

   have the following precedence:

       1) text/html;version=2.0
       2) text/html
       3) text/*
       4) */*

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

       Accept: text/*;q=0.3, text/html;q=0.7, text/html;version=2.0,
               */*;q=0.5

   would cause the following values to be associated:

       text/html;version=2.0                      = 1
       text/html                                  = 0.7
       text/plain                                 = 0.3
       image/jpeg                                 = 0.5
       text/html;level=3                          = 0.7

   It must be emphasized that the Accept field should only be used 
   when it is necessary to restrict the response media types to a 
   subset of those possible or when the user has been permitted to 
   specify qualitative values for ranges of media types. If no quality 
   factors have been set by the user, and the context of the request 
   is such that the user agent is capable of saving the entity to a 
   file if the received media type is unknown, then the only 
   appropriate value for Accept is "*/*".

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

8.2  Accept-Charset

   The Accept-Charset request header field can be used to indicate a 
   list of preferred character set encodings other than the default
   US-ASCII and ISO-8859-1. This field allows clients capable of 
   understanding more comprehensive or special-purpose character set 
   encodings to signal that capability to a server which is capable of 
   representing documents in those character set encodings.

       Accept-Charset = "Accept-Charset" ":" #charset

   Character set encoding values are described in Section 3.5. An 
   example is

       Accept-Charset: iso-8859-5, unicode-1-1

   The value of this field should not include "US-ASCII" or
   "ISO-8859-1", since those values are always assumed by default. If 
   a resource is only available in a character set encoding other than 
   the defaults, and that character set encoding is not listed in the 
   Accept-Charset field, it is only acceptable for the server to send 
   the entity if the character set encoding can be identified by an 
   appropriate charset parameter on the media type or within the 
   format of the media type itself.

       Note: User agents are not required to be able to render the 
       characters associated with the ISO-8859-1 character set 
       encoding. However, they must be able to interpret their 
       meaning to whatever extent is required to properly handle 
       messages in that character set encoding.

8.3  Accept-Encoding

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

       Accept-Encoding         = "Accept-Encoding" ":" 
                                 #( encoding-mechanism )

   An example of its use is

       Accept-Encoding: compress, gzip

   If no Accept-Encoding field is present in a request, the server 
   should assume that the client will accept any encoding-mechanism.

8.4  Accept-Language

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

       Accept-Language         = "Accept-Language" ":"
                                 #( language-tag [ ";" "ql" "=" qvalue ] )

   The language-tag is described in Section 3.8. Each language may be 
   given an associated quality value which represents an estimate of 
   the user's comprehension of that language. The quality value 
   defaults to "ql=1" (100% comprehension) for listed languages. This 
   value may be used in the server's content negotiation algorithm 
   (Section 9). For example,

       Accept-Language: da, en-gb;ql=0.8, de;ql=0.55

   would mean: "I prefer Danish, but will accept British English (with 
   80% comprehension) or German (with a 55% comprehension)."

   If the server cannot fulfill the request with one or more of the 
   languages given, or if the languages only represent a subset of a 
   multi-linguistic Entity-Body, it is acceptable to serve the request 
   in an unspecified language. This is equivalent to asssigning a 
   quality value of "ql=0.001" to any unlisted language.

   If no Accept-Language header is present in the request, the server 
   should assume that all languages are equally acceptable.

       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. If the choice is not made available, then the Accept-
       Language header field must not be given in the request.

8.5  Allow

   The Allow header field lists the set of methods supported by the 
   resource identified by the Request-URI. The purpose of this field 
   is strictly to inform the recipient of valid methods associated 
   with the resource. An Allow header field must be present in a 405 
   (method not allowed) response. The Allow header field is not 
   permitted in a request using the POST method, and thus should be 
   ignored if it is received as part of a POST entity.

       Allow          = "Allow" ":" #method

    Example of use:

       Allow: GET, HEAD, PUT

   This field cannot prevent a client from trying other methods. 
   However, the indications given by the Allow field value should 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.

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

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

   The Allow header field does not indicate what methods are 
   implemented at the server level. Servers must use the Public 
   response header field (Section 8.23) if they wish to describe what 
   methods are implemented on the server as a whole.

8.6  Authorization

   A user agent that wishes to authenticate itself with a server--
   usually, but not necessarily, after receiving a 401 or 411 response--
   may do so by including an Authorization header field with the 
   request. The Authorization field value consists of credentials 
   containing the authentication information of the user agent for the 
   realm of the resource being requested.

       Authorization  = "Authorization" ":" 1#credentials

   HTTP access authentication is described in Section 10. If a request 
   is authenticated and a realm specified, the same credentials should 
   be valid for all other requests within this realm, until the server 
   indicates otherwise with a 411 (authorization refused) response.

8.7  Content-Encoding

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

       Content-Encoding = "Content-Encoding" ":" 1#encoding-mechanism

   Encoding mechanisms are defined in Section 3.6. An example of its 
   use is

       Content-Encoding: gzip

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

   If multiple encodings have been applied to a resource, the
   encoding-mechanisms must be listed in the order in which they were 
   applied. Additional information about the encoding parameters may 
   be provided by other Entity-Header fields not defined by this 
   specification.

8.8  Content-Language

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

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

   Language tags are defined in Section 3.8. The primary purpose of 
   Content-Language is to allow a selective consumer to identify and 
   differentiate resources according to the consumer's own preferred 
   language. Thus, if the body content is intended only for a Danish-
   literate audience, the appropriate field is

       Content-Language: dk

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

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

       Content-Language: mi, en

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

   Content-Language may be applied to any media type -- it should not 
   be limited to textual documents.

8.9  Content-Length

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

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

   An example is

       Content-Length: 3495

   Although it is not required, applications are strongly encouraged 
   to use this field to indicate the size of the Entity-Body to be 
   transferred, regardless of the media type of the entity.

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

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

8.10  Content-Transfer-Encoding

   The Content-Transfer-Encoding (CTE) header indicates what (if any) 
   type of transformation has been applied to the entity in order to 
   safely transfer it between the sender and the recipient. This 
   differs from the Content-Encoding in that the CTE is a property of 
   the message, not of the original resource.

       Content-Transfer-Encoding = "Content-Transfer-Encoding" ":" 
                                   transfer-encoding

   Transfer encodings are defined in Section 3.7. Because all HTTP 
   transactions take place on an 8-bit clean connection, the default 
   Content-Transfer-Encoding for all messages is binary. However, HTTP 
   may be used to transfer MIME messages which already have a defined 
   CTE. An example is:

       Content-Transfer-Encoding: quoted-printable

   Many older HTTP/1.0 applications do not understand the
   Content-Transfer-Encoding header. However, since it may appear in 
   any MIME message (i.e., entities retrieved via a gateway to a MIME-
   conformant protocol), future HTTP/1.0 applications must understand 
   it upon receipt. Gateways are the only HTTP applications that would 
   generate a CTE.

8.11  Content-Type

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

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

   Media types are defined in Section 3.4. An example of the field is

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

   The Content-Type header field has no default value. Further 
   discussion of methods for identifying the media type of an entity 
   is provided in Section 7.2.1.

8.12  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.The field value is an HTTP-date, as described in Section 3.3.

       Date           = "Date" ":" HTTP-date

   An example is

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

   If a message is received via direct connection with the user agent 
   (in the case of requests) or the origin server (in the case of 
   responses), then the 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. 
   Clients should only send a Date header field in messages that 
   include an entity body, as in the case of the PUT and POST 
   requests, and even then it is optional. A 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.

   Only one Date header field is allowed per message. In theory, the 
   date should represent the moment just before the entity is 
   generated. In practice, the date can be generated at any time 
   during the message origination without affecting its semantic value.

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

8.13  Expires

   The Expires field gives the date/time after which the entity should 
   be considered stale. This allows information providers to suggest 
   the volatility of the resource. Caching clients, including proxies, 
   must not cache this copy of the resource beyond the date given, 
   unless its status has been updated by a later check of the origin 
   server. The presence of an Expires field does not imply that the 
   original resource will change or cease to exist at, before, or 
   after that time. However, information providers that know or even 
   suspect that a resource will change by a certain date are strongly 
   encouraged to include an Expires header with that date. The format 
   is an absolute date and time as defined by HTTP-date in Section 3.3.

       Expires        = "Expires" ":" HTTP-date

   An example of its use is

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

   The Expires field has no default value. If the date given is equal 
   to or earlier than the value of the Date header, the recipient must 
   not cache the enclosed entity. If a resource is dynamic by nature, 
   as is the case with many data-producing processes, copies of that 
   resource should be given an appropriate Expires value which 
   reflects that dynamism.

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

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

       Note: Applications are encouraged to be tolerant of bad or 
       misinformed implementations of the Expires header. In 
       particular, recipients may wish to recognize a delta-seconds 
       value (any decimal integer) as representing the number of 
       seconds after receipt of the message that its contents 
       should be considered expired. Likewise, a value of zero (0) 
       or an invalid date format should be considered equivalent to 
       an "expires immediately." Although these values are not 
       legitimate for HTTP/1.0, a robust implementation is always 
       desirable.

8.14  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 [8] 
   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, and any 
   Forwarded headers already present in the message (those added 
   behind the firewall) should be removed.

8.15  From

   The From header field, if given, should contain an Internet e-mail 
   address for the human user who controls the requesting user agent. 
   The address should be machine-usable, as defined by mailbox in RFC 
   822 [8] (as updated by RFC 1123 [7]):

       From           = "From" ":" mailbox

   An example is:

       From: webmaster@w3.org

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

   The Internet e-mail address in this field does not have to 
   correspond to the Internet host which issued the request. For 
   example, when a request is passed through a proxy the original 
   issuer's address should be used.

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

8.16  If-Modified-Since

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

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

   An example of the field is:

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

   A conditional GET method requests that the identified resource be 
   transferred only if it has been modified since the date given by 
   the If-Modified-Since header. The algorithm for determining this 
   includes the following cases:

      a)      If the request would normally result in anything other than 
              a "200 OK" status, or if the passed If-Modified-Since date 
              is invalid, the response is exactly the same as for a 
              normal GET.

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

      c)      If the resource has not been modified since the If-Modified-
              Since date, the server shall return a "304 Not Modified" 
              response.

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

       Note: 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 entity has been modified.

8.17  Last-Modified

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

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

   An example of its use is

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

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

8.18  Link

   The Link header provides a means for describing a relationship 
   between the entity and some other resource. An entity may include 
   multiple Link values. Links at the metainformation level typically 
   indicate relationships like hierarchical structure and navigation 
   paths. The Link field is semantically equivalent to the <LINK> 
   element in HTML [4].

       Link           = "Link" ":" #("<" URI ">"
                        [ ";" "rel" "=" relationship ]
                        [ ";" "rev" "=" relationship ]
                        [ ";" "title" "=" quoted-string ] )

       relationship   = sgml-name
                      | ( <"> sgml-name *( SP sgml-name) <"> )

       sgml-name      = ALPHA *( ALPHA | DIGIT | "." | "-" )

   Relation values are not case-sensitive and may be extended within 
   the constraints of the sgml-name syntax. There are no predefined 
   link relationship values for HTTP/1.0. The title parameter may be 
   used to label the destination of a link such that it can be used as 
   identification within a human-readable menu. Examples of usage 
   include:

       Link: <http://www.cern.ch/TheBook/chapter2>; rel="Previous"

       Link: <mailto:timbl@w3.org>; rev="Made"; title="Tim Berners-Lee"

   The first example indicates that the entity is previous to chapter2 
   in a logical navigation path. The second indicates that the person 
   responsible for making the resource available is identified by the 
   given e-mail address.

8.19  Location

   The Location response header field defines the exact location of 
   the resource that was identified by the Request-URI. For 2xx 
   responses, the location should be the URL needed to retrieve that 
   same resource again (i.e., if variants of that resource are 
   available, the value of the Location field should locate the 
   variant chosen by the server if it has its own specific URL). For 
   3xx responses, the location should indicate the server's preferred 
   URL for automatic redirection to the resource. Only one absolute 
   URL is allowed.

       Location       = "Location" ":" absoluteURI

   An example is

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

   If no base URL is provided by or within the entity, the value of 
   the Location field should be used as the base for resolving 
   relative URLs [10].

8.20  MIME-Version

   HTTP is not a MIME-conformant protocol (see Appendix C). 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 [6]). Unfortunately, current versions of HTTP/1.0 
   clients and servers use this field indiscriminately, 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.

8.21  Orig-URI

   The Orig-URI request header field allows the client to specify, for 
   the server's benefit, the original Uniform Resource Identifier 
   (Section 3.2) of the resource being requested, as it was obtained 
   from the user or the referring resource. This allows a server to 
   differentiate between internally-ambiguous URLs (such as the root 
   "/" URL of a server harboring multiple virtual hostnames), to learn 
   about new URNs used to reference resources on the server, and to 
   provide some additional assistance in identifying and redirecting 
   moved resources and resource fragments.

       Orig-URI       = "Orig-URI" ":" absoluteURI [ "#" fragment ]

   Example:

       Orig-URI: http://www.w3.org/

   The URI must be in absolute form and should include the fragment if 
   one is given to the client. It should include exactly what was 
   referenced by the Referer resource, with the exception that a 
   relative reference must first be resolved to its absolute form.

8.22  Pragma

   The Pragma message header field is used to specify directives that 
   should be applied to all intermediaries along the request/response 
   chain. The directives typically specify behavior intended to 
   prevent intermediate proxies or caches from adversely interfering 
   with the request or response. All pragma directives specify 
   optional behavior from the viewpoint of the protocol; however, some 
   systems may require that behavior be consistent with the 
   directives. HTTP/1.0 defines semantics for the "no-cache" and
   "max-age" directives.

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

       pragma-directive = "no-cache"
                        | "max-age" "=" delta-seconds
                        | extension-pragma
       extension-pragma = token [ "=" word ]

   When the "no-cache" directive is present in a request message, a 
   caching intermediary should forward the request toward the origin 
   server even if it has a cached copy of what is being requested. 
   This allows a client to insist upon receiving an authoritative 
   response to its request. It also allows a client to refresh a 
   cached copy which is known to be corrupted or stale.

   When the "no-cache" directive is present in a response message, 
   caching intermediaries are requested to not cache this response. 
   This allows an origin server to state that the message is intended 
   for only one recipient and may not be a valid response for other 
   requests.

   When the "max-age" directive is present in a request message, a 
   caching intermediary should forward the request toward the origin 
   server if it has no cached copy, or refresh its cached copy if it 
   is older than the age value given (in seconds) prior to returning a 
   response. A cached copy's "age" is determined by the cached 
   message's Date header field, or the equivalent as stored by the 
   cache manager. In most cases, a cached copy can be refreshed by 
   forwarding a conditional GET request toward the origin server with 
   the stored message's Date value in the If-Modified-Since field. If 
   a 304 (not modified) response is received, the cache should replace 
   the cached message's Date with that of the 304 response and send 
   this refreshed message as the response. Any other response should 
   be forwarded directly to the requestor and, depending on the 
   response code and the discretion of the cache manager, may replace 
   the message in the cache.

   When the "max-age" directive is present in a cached response 
   message, a caching intermediary should refresh the message if it is 
   older than the age value given (in seconds) at the time of a new 
   request for that resource. The behavior should be equivalent to 
   what would occur if the request had included that pragma directive. 
   If both the new request and the cached message have max-age 
   specified, then the lesser of the two values should be used.

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

   Pragma directives do not apply to the end-points of a 
   request/response chain. For example, a user agent's internal (non-
   shared) cache and/or history mechanism should ignore all pragma 
   directives in received messages. Similarly, pragma directives are 
   not applicable to the origin of a resource, though they may be 
   applicable to a server's internal response cache.

8.23  Public

   The Public response header field lists the set of non-standard 
   methods supported by the server. The purpose of this field is 
   strictly to inform the recipient of the capabilities of the server 
   regarding unusual methods. The methods listed may or may not be 
   applicable to the Request-URI; the Allow header field (Section 8.5) 
   should be used to indicate methods allowed for a particular URI. 
   This does not prevent a client from trying other methods. The field 
   value should not include the methods predefined for HTTP/1.0 in 
   Section 5.2.

       Public         = "Public" ":" #method

   Example of use:

       Public: OPTIONS, MGET, MHEAD

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

8.24  Referer

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

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

   Example:

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

   If a partial URI is given, it should be interpreted relative to the 
   Request-URI. The URI must not include a fragment.

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

8.25  Retry-After

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

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

   Two examples of its use are

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

       Retry-After: 120

   In the latter example, the delay is 2 minutes.

8.26  Server

   The Server response header field contains information about the 
   software used by the origin server to handle the request. The field 
   can contain multiple product tokens (Section 3.10) identifying the 
   server and any significant subproducts. By convention, the product 
   tokens are listed in order of their significance for identifying 
   the application.

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

   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 8.14).

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

8.27  Title

   The Title header field indicates the title of the entity 

       Title          = "Title" ":" *text

   An example of the field is

       Title: Hypertext Transfer Protocol -- HTTP/1.0

   This field is isomorphic with the <TITLE> element in HTML [4].

8.28  URI

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

       URI-header     = "URI" ":" #( "<" ( absoluteURI | relativeURI ) ">"
                        [ ";" vary ] *( ";" characteristic) )

       vary           = "vary" "="
                        ( vary-dimension | ( <"> 1#vary-dimension <"> ) )

       vary-dimension = "type" | "charset" | "language" | "encoding"
                      | "user-agent" | "version" | token

       characteristic = ( "type={" media-type "}" )
                      | ( "language={" 1#language-tag "}" )
                      | ( "encoding={" 1#encoding-mechanism "}" )
                      | ( "length=" 1*DIGIT )
                      | ( "qs=" qvalue )

   Any URI specified in this field can be either absolute or relative 
   to the Request-URI.

   If the Location header field is present in a 2xx response, its 
   value defines an implicit URI header with the characteristic 
   parameters defined by the associated Content-* header fields.

   The URI-header may be used by a client performing a POST request to 
   suggest a URI for the new entity. Whether or not the suggested URI 
   is used is entirely up to the server to decide. In any case, the 
   server's response must include the actual URI(s) of the new 
   resource if one is successfully created (status 201).

   If a URI refers to a set of variants, then the dimensions of that 
   variance must be given with a vary parameter. One example is:

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

   which indicates that the URI covers a group of entities that vary 
   in media type and natural language. A request for that URI will 
   result in a response that depends upon the client's request headers 
   for Accept and Accept-Language. Similar dimensions exist for the 
   Accept-Encoding, Accept-Charset, and User-Agent header fields, as 
   demonstrated in the following example.

       URI: <TheProject.ps>; vary="encoding,version";
            type={application/postscript},
            <TheProject.html>; vary="user-agent,charset,version";
            type={text/html},
            <TheProject.html3;v=25>; type={text/html; level=3}; qs=0.9

   User agents may use this information to notify the user of 
   additional formats.

   The vary parameter has an important effect on cache management, 
   particularly for caching intermediaries which service a diverse set 
   of user agents. Since the response to one user agent may differ 
   from the response to a second user agent if the two agents have 
   differing request profiles, a caching intermediary must keep track 
   of the content metainformation for resources with varying 
   dimensions. Thus, the vary parameter tells the intermediary what 
   entity headers must be part of the key for caching that URI. When 
   the caching proxy gets a request for that URI, it must forward the 
   request toward the origin server if the request profile includes a 
   variant dimension that has not already been cached.

   If the origin server provides the characteristics of each 
   identified resource as part of the URI header, then the recipient 
   may improve its cached response behavior by attempting to duplicate 
   the content negotiation that would be provided by the server. This 
   is not required by the protocol, but may improve the accuracy or 
   timeliness of responses to the end-user.

8.29  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. Although it is not required, user agents should 
   always include this field with requests. The field can contain 
   multiple product tokens (Section 3.10) identifying the agent and 
   any subproducts which form a significant part of the user agent.
   By convention, the product tokens are listed in order of their 
   significance for identifying the application.

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

   Example:

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

   The User-Agent field may include additional information within 
   comments.

       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 8.14.

8.30  WWW-Authenticate

   The WWW-Authenticate header field must be included in 401 
   (unauthorized) and 411 (authorization refused) response messages. 
   The field value consists of a challenge that indicates the 
   authentication scheme and parameters applicable to the Request-URI.

       WWW-Authenticate        = "WWW-Authenticate" ":" challenge

   The HTTP access authentication process is described in Section 10.

9.  Content Negotiation

   Content negotiation is an optional feature of the HTTP protocol. It 
   is designed to allow for selection of a preferred content 
   representation, within a single request-response round-trip, and 
   without intervention from the user. However, this may not always be 
   desirable for the user and is sometimes unnecessary for the content 
   provider. Implementors are encouraged to provide mechanisms whereby 
   the amount of preemptive content negotiation, and the parameters of 
   that negotiation, are configurable by the user and server 
   maintainer.

   The first step in the negotiation algorithm is for the server to 
   determine whether or not there are any content variants for the 
   requested resource. Content variants may be in the form of multiple 
   preexisting entities or a set of dynamic conversion filters. These 
   variants make up the set of entities which may be sent in response 
   to a request for the given Request-URI. In most cases, there will 
   only be one available form of the resource, and thus a single 
   "variant".

   For each variant form of the resource, the server identifies a set 
   of quality values (Section 3.9) which act as weights for measuring 
   the desirability of that resource as a response to the current 
   request. The calculated weights are all real numbers in the range
   0 through 1, where 0 is the minimum and 1 the maximum value. The 
   maximum acceptable bytes for each media range and the size of the 
   resource variant are also factors in the equation.

   The following parameters are included in the calculation:

      qs      Source quality is measured by the content provider as 
              representing the amount of degradation from the original 
              source. For example, a picture originally in JPEG form 
              would have a lower qs when translated to the XBM format, 
              and much lower qs when translated to an ASCII-art 
              representation. Note, however, that this is a function of 
              the source -- an original piece of ASCII-art may degrade in 
              quality if it is captured in JPEG form. The qs value should 
              be assigned to each variant by the content provider; if no 
              qs value has been assigned, the default is generally 
              "qs=1". A server may define its own default qs value based 
              on the resource characteristics, but only if individual 
              resources can override those defaults.

      qe      Encoding quality is measured by comparing the variant's 
              applied encoding-mechanisms (Section 3.6) to those listed 
              in the request message's Accept-Encoding field. If the 
              variant has no assigned Content-Encoding, or if no Accept-
              Encoding field is present, the value assigned is "qe=1". If 
              all of the variant's content encodings are listed in the 
              Accept-Encoding field, then the value assigned is "qe=1". 
              If any of the variant's content encodings are not listed in 
              the provided Accept-Encoding field, then the value assigned 
              is "qe=0.001".

      qc      Charset quality is measured by comparing the variant media-
              type's charset parameter value (if any) to those character 
              set encodings (Section 3.5) listed in the request message's 
              Accept-Charset field. If the variant's media-type has no 
              charset parameter, or the variant's charset is US-ASCII or 
              ISO-8859-1, or if no Accept-Charset field is present, then 
              the value assigned is "qc=1". If the variant's charset is 
              listed in the Accept-Charset field, then the value assigned 
              is "qc=1". Otherwise, if the variant's charset is not 
              listed in the provided Accept-Encoding field, then the 
              value assigned is "qc=0.001".

      ql      Language quality is measured by comparing the variant's 
              assigned language tag(s) (Section 3.8) to those listed in 
              the request message's Accept-Language field. If no variant 
              has an assigned Content-Language, or if no Accept-Language 
              field is present, the value assigned is "ql=1". If at least 
              one variant has an assigned content language, but the one 
              currently under consideration does not, then it should be 
              assigned the value "ql=0.5". If any of the variant's 
              content languages are listed in the Accept-Language field, 
              then the value assigned is the maximum of the "ql" 
              parameter values for those language tags (Section 8.4); if 
              there was no exact match and at least one of the Accept-
              Language field values is a complete subtag prefix of the 
              content language tag(s), then the "ql" parameter value of 
              the largest matching prefix is used. If none of the 
              variant's content language tags or tag prefixes are listed 
              in the provided Accept-Language field, then the value 
              assigned is "ql=0.001".

      q       Media type quality is measured by comparing the variant's 
              assigned media type (Section 3.4) to those listed in the 
              request message's Accept field. If no Accept field is 
              given, then the value assigned is "q=1". If at least one 
              listed media range (Section 8.1) matches the variant's 
              media type, then the "q" parameter value assigned to the 
              most specific of those matched is used (e.g., 
              "text/html;version=3.0" is more specific than "text/html", 
              which is more specific than "text/*", which in turn is more 
              specific than "*/*"). If no media range in the provided 
              Accept field matches the variant's media type, then the 
              value assigned is "q=0".

      mxb     The maximum number of bytes in an Entity-Body that the 
              client will accept is also obtained from the matching of 
              the variant's assigned media type to those listed in the 
              request message's Accept field. If no Accept field is 
              given, or if no media range in the provided Accept field 
              matches the variant's media type, then the value assigned 
              is "mxb=undefined"  (i.e., infinity). Otherwise, the value 
              used is that given to the "mxb" parameter in the media 
              range chosen above for the q value.

      bs      The actual number of bytes in the Entity-Body for the 
              variant when it is included in a response message. This 
              should equal the value of Content-Length.

   The mapping function is defined as:

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

   The variants with a maximal value for the Q function represent the 
   preferred representation(s) of the entity; those with a Q values 
   less than the maximal value are therefore excluded from further 
   consideration. If multiple representations exist that only vary by 
   Content-Encoding, then the smallest representation (lowest bs) is 
   preferred.

   If no variants remain with a value of Q greater than zero (0), the 
   server should respond with a 406 (none acceptable) response 
   message. If multiple variants remain with an equally high Q value, 
   the server may either choose one from those available and respond 
   with 200 (ok) or respond with 300 (multiple choices) and include an 
   entity describing the choices. In the latter case, the entity 
   should either be of type "text/html', such that the user can choose 
   from among the choices by following an exact link, or of some type 
   that would allow the user agent to perform the selection 
   automatically.

   The 300 (multiple choices) response can be given even if the server 
   does not perform any winnowing of the representation choices via 
   the content negotiation algorithm described above. Furthermore, it 
   may include choices that were not considered as part of the 
   negotiation algorithm and resources that may be located at other 
   servers.

   Servers that make use of content negotiated resources are strongly 
   encouraged to include URI response headers which accurately 
   describe the available variants and include the relevant parameters 
   necessary for the client (user agent or proxy) to evaluate those 
   variants.

   The algorithm presented above assumes that the user agent has 
   correctly implemented the protocol and is accurately communicating 
   its intentions in the form of Accept-related header fields. The 
   server may alter its response if it knows that the particular 
   version of user agent software making the request has incorrectly 
   or inadequately implemented these fields.

10.  Access Authentication

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

       auth-scheme    = "basic" | token

       auth-param     = token "=" quoted-string

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

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

       realm          = "realm" "=" quoted-string

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

   A user agent that wishes to authenticate itself with a server--
   usually, but not necessarily, after receiving a 401 or 411 response--
   may do so by including an Authorization header field with the 
   request. The Authorization field value consists of credentials 
   containing the authentication information of the user agent for the 
   realm of the resource being requested.

       credentials    = auth-scheme [ 1*LWS encoded-cookie ]
                        *(";" auth-param )

       encoded-cookie = <any valid base64 [6] encoded string,
                         except not limited to 76 char/line>

   The domain over which credentials can be automatically applied by a 
   user agent is determined by the authorization space. If a request 
   is authenticated, the credentials can be reused for all other 
   requests within that authorization space for a period of time 
   determined by the authentication scheme, parameters, and/or user 
   preference.

   If the server does not wish to accept the credentials sent with a 
   request, it should return either a 403 (forbidden) or 411 
   (authorization refused) response. In the latter case, the response 
   must include a WWW-Authenticate header field containing the 
   (possibly new) challenge applicable to the requested resource and 
   an entity explaining the refusal.

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

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

       Note: The names Proxy-Authenticate and Proxy-Authorization 
       have been suggested as headers, analogous to
       WWW-Authenticate and Authorization, but applying only to the 
       immediate connection with a proxy.

10.1  Basic Authentication Scheme

   The basic authentication scheme is based on the model that the 
   client must authenticate itself with a user-ID and a password for 
   each realm. The realm value should be considered an opaque string 
   which can only be compared for equality with other realms. The 
   server will service the request only if it can validate the user-ID 
   and password for the domain of the Request-URI.

       basic-challenge= "Basic" SP realm

   The client sends the user-ID and password, separated by a single 
   colon ":" character, within a base64 [6] encoded-cookie in the 
   credentials.

       basic-credentials="Basic" SP basic-cookie
       basic-cookie   = <base64 encoding of userid-password>
       userid-password= [ token ] ":" *text

   There are no optional authentication parameters for the basic 
   scheme. For example, if the user agent wishes to send the user-ID 
   "Aladdin" and password "open sesame", it would use the following 
   header field:

       Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

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

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

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 
   other than retrieval. 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 non-idempotent 
   action is being requested.

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

11.3  Abuse of Server Log Information

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

11.4  Transfer of Sensitive Information

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

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

   Proxies which serve as a gateway through a network firewall should 
   take special precautions regarding the transfer of header 
   information that identifies the hosts behind the firewall. In 
   particular, they should remove, or replace with sanitized versions, 
   any Forwarded fields generated behind the firewall.

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

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

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

12.  Acknowledgments

   This specification makes heavy use of the augmented BNF and generic 
   constructs defined by David H. Crocker for RFC 822 [8]. Similarly, 
   it reuses many of the definitions provided by Nathaniel Borenstein 
   and Ned Freed for MIME [6]. We hope that their inclusion in this 
   specification will help reduce past confusion over the relationship 
   between HTTP/1.0 and Internet mail message formats.

   The HTTP protocol has evolved considerably over the past 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, Robert Cailliau, Daniel W. Connolly, Bob Denny, 
   Jean Francois-Groff, Phillip M. Hallam-Baker, Haringkon W. Lie,
   Ari Luotonen, Rob McCool, Dave Raggett, Tony Sanders, and
   Marc VanHeyningen deserve special recognition for their efforts in 
   defining aspects of the protocol for early versions of this 
   specification.

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

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

13. References

   [1]  H. Alvestrand. "Tags for the identification of languages." 
        RFC 1766, UNINETT, March 1995.

   [2]  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, March 1993.

   [3]  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, June 1994.

   [4]  T. Berners-Lee and D. Connolly. "HyperText Markup Language 
        Specification - 2.0." Work in Progress
        (draft-ietf-html-spec-04.txt), MIT/W3C, June 1995.

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

   [6]  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, September 1993.

   [7]  R. Braden. "Requirements for Internet hosts - application and 
        support." STD 3, RFC 1123, IETF, October 1989.

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

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

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

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

   [12] 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, February 1986.

   [13] K. Moore. "MIME (Multipurpose Internet Mail Extensions) Part 
        Two: Message Header Extensions for Non-ASCII Text." RFC 1522, 
        University of Tennessee, September 1993.

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

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

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

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

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

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

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

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 5702
   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
   W3 Consortium
   MIT Laboratory for Computer Science
   545 Technology Square
   Cambridge, MA 02139, U.S.A.
   Tel: +1 (617) 258 8143
   Fax: +1 (617) 258 8682
   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.  Internet Media Type message/http

   In addition to defining the HTTP/1.0 protocol, this document serves 
   as the specification for the Internet media type "message/http". 
   The following is to be registered with IANA [15].

       Media Type name:         message

       Media subtype name:      http

       Required parameters:     none

       Optional parameters:     version, msgtype

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

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

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

       Security considerations: none

B.  Tolerant Applications

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

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

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

C.  Relationship to MIME

   HTTP/1.0 reuses many of the constructs defined for Internet Mail 
   (RFC 822 [8]) and the Multipurpose Internet Mail Extensions 
   (MIME [6]) to allow entities to be transmitted in an open variety 
   of representations and with extensible mechanisms. However, HTTP is 
   not a MIME-conforming application. HTTP's performance requirements 
   differ substantially from those of Internet mail. Since 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 appendix describes specific areas where HTTP differs from 
   MIME. Gateways to MIME-compliant protocols must be aware of these 
   differences and provide the appropriate conversions where 
   necessary. No conversion should be necessary for a MIME-conforming 
   entity to be transferred using HTTP.

C.1  Conversion to Canonical Form

   MIME requires that an entity be converted to canonical form prior 
   to being transferred, as described in Appendix G of RFC 1521 [6]. 
   Although HTTP does require media types to be transferred in 
   canonical form, it changes the definition of "canonical form" for 
   text-based media types as described in Section 3.4.1.

C.1.1 Representation of Line Breaks

   MIME requires that the canonical form of any text type represent 
   line breaks as CRLF and forbids the use of CR or LF outside of line 
   break sequences. Since HTTP allows CRLF, bare CR, and bare LF
   (or the octet sequence(s) to which they would be translated for the 
   given character set encoding) to indicate a line break within text 
   content, recipients of an HTTP message cannot rely upon receiving 
   MIME-canonical line breaks in text.

   Where it is possible, a gateway from HTTP to a MIME-conformant 
   protocol should translate all line breaks within text/* media types 
   to the MIME canonical form of CRLF. However, this may be 
   complicated by the presence of a Content-Encoding and by the fact 
   that HTTP allows the use of some character set encodings which do 
   not use octets 13 and 10 to represent CR and LF, as is the case for 
   some multi-byte character set encodings.

C.1.2 Default Character Set Encoding

   MIME requires that all subtypes of the top-level Content-Type 
   "text" have a default character set encoding of US-ASCII [19].
   In contrast, HTTP defines the default character set encoding for 
   "text" to be ISO-8859-1 [20] (a superset of US-ASCII). Therefore, 
   if a text/* media type given in the Content-Type header field does 
   not already include an explicit charset parameter, the parameter

       ;charset="iso-8859-1"

   should be added by the gateway if the entity contains any octets 
   greater than 127.

C.2  Default Content-Transfer-Encoding

   The default Content-Transfer-Encoding (CTE) for all MIME messages 
   is "7bit". In contrast, HTTP defines the default CTE to be 
   "binary". Therefore, if an entity does not include an explicit CTE 
   header field, the gateway should apply either the
   "quoted-printable" or "base64" transfer encodings and add the 
   appropriate Content-Transfer-Encoding field. At a minimum, the 
   explicit CTE field of

       Content-Transfer-Encoding: binary

   should be added by the gateway if it is unwilling to apply a
   mail-safe transfer encoding.

C.3  Introduction of Content-Encoding

   MIME does not include any concept equivalent to HTTP's
   Content-Encoding header field. Since this acts as a modifier on the 
   media type, gateways to MIME-conformant protocols should either 
   change the value of the Content-Type header field or decode the 
   Entity-Body before forwarding the message.

       Note: Some experimental applications of Content-Type for 
       Internet mail have used a media-type parameter of 
       ";conversions=<encoding-mechanisms>" to perform an 
       equivalent function as Content-Encoding. However, this 
       parameter is not part of the MIME specification at the time 
       of this writing.
