2004/id/draft-ietf-http-v10-spec-00.txt

HTTP Working Group                                        T. Berners-Lee
INTERNET-DRAFT                                            R. T. Fielding
<draft-ietf-http-v10-spec-00.txt>                     H. Frystyk Nielsen
Expires September 8, 1995                                  March 8, 1995


                Hypertext Transfer Protocol -- HTTP/1.0


Status of this Memo

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Abstract

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

   HTTP has been in use by the World-Wide Web global information 
   initiative since 1990. This specification reflects preferred usage 
   of the protocol referred to as "HTTP/1.0", and is compatible with 
   the most commonly used HTTP server and client programs implemented 
   prior to November 1994.

Table of Contents

   1.  Introduction
       1.1  Purpose
       1.2  Overall Operation
       1.3  Terminology
   2.  Notational Conventions and Generic Grammar
       2.1  Augmented BNF
       2.2  Basic Rules
   3.  Protocol Parameters
       3.1  HTTP Version
       3.2  Universal Resource Identifiers
       3.3  Date/Time Formats
            3.3.1  Full Date
            3.3.2  Delta Seconds
   4.  HTTP Message
       4.1  Message Types
       4.2  Message Headers
       4.3  General Message Header Fields
            4.3.1  Date
            4.3.2  Forwarded
            4.3.3  Message-ID
            4.3.4  MIME-Version
   5.  Request
       5.1  Request-Line
       5.2  Method
            5.2.1  GET
            5.2.2  HEAD
            5.2.3  POST
            5.2.4  PUT
            5.2.5  DELETE
            5.2.6  LINK
            5.2.7  UNLINK
       5.3  Request-URI
       5.4  Request Header Fields
            5.4.1  Accept
            5.4.2  Accept-Charset
            5.4.3  Accept-Encoding
            5.4.4  Accept-Language
            5.4.5  Authorization
            5.4.6  From
            5.4.7  If-Modified-Since
            5.4.8  Pragma
            5.4.9  Referer
            5.4.10 User-Agent
   6.  Response
       6.1  Status-Line
       6.2  Status Codes and Reason Phrases
            6.2.1  Successful 2xx
            6.2.2  Redirection 3xx
            6.2.3  Client Error 4xx
            6.2.4  Server Errors 5xx
       6.3  Response Header Fields
            6.3.1  Public
            6.3.2  Retry-After
            6.3.3  Server
            6.3.4  WWW-Authenticate
   7.  Entity
       7.1  Entity Header Fields
            7.1.1  Allow
            7.1.2  Content-Encoding
            7.1.3  Content-Language
            7.1.4  Content-Length
            7.1.5  Content-Transfer-Encoding
            7.1.6  Content-Type
            7.1.7  Derived-From
            7.1.8  Expires
            7.1.9  Last-Modified
            7.1.10 Link
            7.1.11 Location
            7.1.12 Title
            7.1.13 URI
            7.1.14 Version
       7.2  Entity Body
            7.2.1  Type
            7.2.2  Length
   8.  Content Parameters
       8.1  Media Types
            8.1.1  Canonicalization and Text Defaults
            8.1.2  Multipart Types
       8.2  Language Tags
       8.3  Character Sets
       8.4  Encoding Mechanisms
       8.5  Transfer Encodings
   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
   12. Acknowledgments
   13. References
   14. Authors' Addresses
   Appendix A.   Internet Media Type message/http
   Appendix B.   Minimum Compliance
       B.1  Servers
   Appendix C.   Tolerant Applications
       C.1  Request-Line, Status-Line, and Header Fields
   Appendix D.   Relationship to MIME
       D.1  Conversion to Canonical Form
            D.1.1  Representation of Line Breaks
            D.1.2  Default Character Set
       D.2  Default Content-Transfer-Encoding
       D.3  Introduction of Content-Encoding
   Appendix E.   Example of Version Control


1.  Introduction

1.1  Purpose

   The Hypertext Transfer Protocol (HTTP) is an application-level 
   protocol with the lightness and speed necessary for distributed, 
   collaborative, hypermedia information systems. HTTP has been in use 
   by the World-Wide Web global information initiative since 1990. 
   This specification reflects preferred usage of the protocol 
   referred to as "HTTP/1.0". This specification does not necessarily 
   reflect the "current practice" of any single HTTP server or client 
   implementation. It does, however, seek to remain compatible with 
   existing implementations wherever possible, and should be 
   considered the reference for future implementations of HTTP/1.0.

   Practical information systems require more functionality than 
   simple retrieval, including search, front-end update, and 
   annotation. HTTP/1.0 allows an open-ended set of methods to be used 
   to indicate the purpose of a request. It builds on the discipline 
   of reference provided by the Universal Resource Identifier 
   (URI) [3], as a location (URL) [5] or name (URN), for indicating 
   the resource on which a method is to be applied. Messages are 
   passed in a format similar to that used by Internet Mail [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, HTAB, 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 words (token or 
       quoted-string) 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 is 
   defined by [18].

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

   HTTP/1.0 defines the octet sequence CR LF as the end-of-line marker 
   for all protocol elements except the Entity-Body (see Appendix C 
   for tolerant applications). The end-of-line marker for an 
   Entity-Body is defined by its associated media type, as described 
   in Section 8.1.

       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 | HTAB )

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

       word           = token | quoted-string

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

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

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

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

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

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

   The text rule is only used for descriptive field contents. Words of 
   *text may contain characters from character sets other than 
   US-ASCII only when encoded according to the rules of RFC 1522 [13].

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

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 zeroes 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 are required to be able to recognize the format of the 
   Request-Line for all lower-version requests, understand requests 
   with a format within one major number of their native version (i.e. 
   <major1> and <major>), and respond appropriately with a message 
   within the same <major> protocol number (even if the response is 
   simply an error message). HTTP clients are required to be able to 
   recognize the format of the Status-Line for all lower-version 
   responses and understand responses with a format within one major 
   number of their request 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  Universal Resource Identifiers

   This section is To-Be-Specified. It will contain a brief 
   description of URIs as defined by RFC 1630, including mention of 
   the allowed characters and the hex encoding, and provide a full 
   definition of the http URL scheme.

       URI            = <As defined in RFC 1630>

   All URI protocol elements must be encoded using the escaping scheme 
   described in RFC 1630 [3].

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 should 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.

   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. However, recipients of 
   date values should 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).

       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

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. 
   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 considered "good practice" to send General-Header 
   fields first, followed by Request-Header or Response-Header fields 
   prior to the Entity-Header fields. Comments can be included in HTTP 
   header fields by surrounding the comment text with parentheses.

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

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

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 entity 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
                      | Forwarded
                      | Message-ID
                      | MIME-Version
                      | extension-header

   Although additional general header fields may be implemented via 
   the extension mechanism, applications which do not recognize those 
   fields should treat them as Entity-Header fields. 

4.3.1 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 status/request-
   line is generated (i.e. the time at which the originator made the 
   determination of what the request/response should be). 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.

4.3.2 Forwarded

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

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

       FQDN           = <Fully-Qualified Domain Name>

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

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

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

4.3.3 Message-ID

   The Message-ID field in HTTP is identical to that used by Internet 
   Mail and USENET messages, as defined in [11]. That is, it gives the 
   message a single, unique identifier which can be used for 
   identifying the message (not its contents) for "much longer" than 
   the expected lifetime of that message. 

       Message-ID     = "Message-ID" ":" "<" addr-spec ">"
       addr-spec      = <as defined in RFC 822 [8]>

   Although it is not required, the addr-spec format typically used 
   within a Message-ID consists of a string that is unique at the 
   originator's machine, followed by the required "@" character and 
   the fully-qualified domain name of that machine. An example is

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

   which is composed using the time, date and process-ID on the host 
   info.cern.ch. However, this is only one of many possible methods 
   for generating a unique Message-ID and recipients of a message 
   should consider the entire value opaque.

   The Message-ID field is normally not generated by HTTP applications 
   and is never required. It should only be generated by a gateway 
   application when the message is being posted to some other protocol 
   that desires a Message-ID. HTTP responses should only include a 
   Message-ID header field when the entity being transferred already 
   has one assigned to it (as in the case of resources that were 
   originally posted via Internet Mail or USENET).

4.3.4 MIME-Version

   HTTP is not a MIME-conformant protocol. However, HTTP/1.0 messages 
   may include a single MIME-Version header field to indicate what 
   version of the MIME protocol was used to construct the message. Use 
   of the MIME-Version header field should indicate that the message 
   is in full compliance with the MIME protocol (as defined in [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.

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.

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 and extensible.

       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 7.1.1). However, the 
   client is always notified through the return code of the response 
   whether a method is currently allowed on a specific resource, as 
   this can change dynamically. 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 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. 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 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" requests 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 (usually a form) 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 considered to be 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 only 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 7.1.10. 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.

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 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, the "200 OK" response 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 requestor of a 
   PUT knows what URI is intended and the receiver must not attempt to 
   apply the request to some other resource. If the receiver desires 
   that the request be applied to a different URI, it must send a
   "301 Moved Permanently" response; the requestor may then make its
   own decision regarding whether or not to redirect the request.

   The client can create or modify relationships between the enclosed 
   entity and other existing resources by including Link header 
   fields, as described in Section 7.1.10. 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, the 
   Version and Derived-From header fields should be used to help 
   identify and control revisions to a resource.

5.2.5 DELETE

   The DELETE method requests that the origin server delete the 
   resource identified by the 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 Universal Resource Identifier (Section 3.2) 
   and identifies the resource upon which to apply the request.

       Request-URI    = URI

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

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

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

       /hypertext/WWW/TheProject.html

   If the client is sending the request through a proxy, the absolute 
   form of the URI (including scheme and server address) must be used. 
   A proxy must be able to recognize all of its server names, 
   including any aliases, local variations, and the numeric IP address.

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
                      | Accept-Charset
                      | Accept-Encoding
                      | Accept-Language
                      | Authorization
                      | From
                      | If-Modified-Since
                      | Pragma
                      | Referer
                      | User-Agent
                      | extension-header

   Although additional request header fields may be implemented via 
   the extension mechanism, applications which do not recognize those 
   fields should treat them as Entity-Header fields.

5.4.1 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), to indicate qualitative preferences 
   for specific media types, or to indicate acceptance of unusual 
   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" ":" 1#(
                             media-range
                             [ ";" "q" "=" ( "0" | "1" | float ) ]
                             [ ";" "mxb" "=" 1*DIGIT ] )

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

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

   The parameter q is used to indicate the quality factor, which 
   represents the user's preference for that range of media types, and 
   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. If at least one Accept header is present, a 
   quality factor of 0 is equivalent to not sending an Accept header 
   field containing that media-type or set of media-types. The default 
   values are: q=1 and mxb=undefined (i.e. infinity).

   The example

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

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

   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.

   It must be emphasized that this field should only be used when it 
   is necessary to restrict the response media types to a subset of 
   those possible, to indicate the acceptance of unusual media types, 
   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 "*/*" and the list of unusual types. Whether or not a particular 
   media type is deemed "unusual" should be a configurable aspect of 
   the user agent.

5.4.2 Accept-Charset

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

       Accept-Charset = "Accept-Charset" ":" 1#charset

   Character set values are described in Section 8.3. 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 other than the 
   defaults, and that character set is not listed in the Accept-
   Charset field, it is only acceptable for the server to send the 
   entity if the character set 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. 
       However, they must be able to interpret their meaning to 
       whatever extent is required to properly handle messages in 
       that character set..

5.4.3 Accept-Encoding

   The Accept-Encoding header field is similar to Accept, but lists 
   the encoding-mechanisms and transfer-encoding values which are 
   acceptable in the response.

       Accept-Encoding = "Accept-Encoding" ":" 
                         1#( encoding-mechanism | transfer-encoding )

   An example of its use is

       Accept-Encoding: compress, base64, gzip, quoted-printable

   The field value should never include the identity transfer-encoding 
   values ("7bit", "8bit", and "binary") since they actually represent 
   "no encoding." If no Accept-Encoding field is present in a request, 
   it must be assumed that the client does not accept any encoding-
   mechanism and only the identity transfer-encodings.

5.4.4 Accept-Language

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

       Accept-Language = "Accept-Language" ":" 1#language-tag

   The language-tag is described in Section 8.2. Languages are listed 
   in the order of their preference to the user. For example,

       Accept-Language: dk, en-gb

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

   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.

       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.

5.4.5 Authorization

   A user agent that wishes to authenticate itself with a server 
   (usually, but not necessarily, after receiving a "401 Unauthorized" 
   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" ":" 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.

5.4.6 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 addr-spec in 
   RFC 822:

       From           = "From" ":" addr-spec

   An example is:

       From: webmaster@w3.org

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

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

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

5.4.7 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

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

5.4.8 Pragma

   The Pragma header field is used to specify directives that must be 
   applied to all servers along the request chain (where relevant). 
   The directives typically specify behavior that prevents 
   intermediate proxies from changing the nature of the request. 
   Although multiple pragma directives can be listed as part of the 
   request, HTTP/1.0 only defines semantics for the
   "no-cache" directive.

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

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

   When the "no-cache" directive is present, a caching proxy must 
   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 has become 
   corrupted or is known to be stale.

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

5.4.9 Referer

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

       Referer        = "Referer" ":" URI

   Example:

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

   If a partial URI is given, it should be interpreted relative to the 
   Request-URI.

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

5.4.10 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 tokens specifying the product name, with an optional slash 
   and version designator, and other products which form a significant 
   part of the user agent. By convention, the products are listed in 
   order of their significance for identifying the application.

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

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

   Example:

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

   Product tokens should be short and to the point -- use of this field 
   for advertizing or other non-essential information is explicitly 
   deprecated and will be considered as non-conformance to the 
   protocol. 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). 
   The User-Agent field may include additional information within 
   comments that are not part of the value of the field.

       Note: Some current proxy applications append their product 
       information to the list in the User-Agent field. This is no 
       longer recommended, since it makes machine interpretation of 
       these fields ambiguous. Instead, proxies should use the 
       Forwarded header described in Section 4.3.2.

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's 

       "HTTP/" 1*DIGIT "." 1*DIGIT

   (e.g. "HTTP/1.0"), the presence of that expression is considered 
   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), the likelihood of such an occurrence is negligible.

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"   ; Provisional Information
                      | "204"   ; No Content
                      | "300"   ; Multiple Choices
                      | "301"   ; Moved Permanently
                      | "302"   ; Moved Temporarily
                      | "303"   ; Method
                      | "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
                      | "500"   ; Internal Server Error
                      | "501"   ; Not Implemented
                      | "502"   ; Bad Gateway
                      | "503"   ; Service Unavailable
                      | "504"   ; Gateway Timeout
                      | extension-code

       extension-code = 3DIGIT

       Reason-Phrase  = token *( SP token )

   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 are required to 
   understand the class of any status code (as indicated by the first 
   digit) and to treat the 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 Successful 2xx

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

   200 OK

      o Following:        any method
      o Required headers: none

   The request has been fulfilled and an entity corresponding to the 
   requested resource is being sent in the response. If the HEAD 
   method was used, the response should only contain the Entity-Header 
   information and no Entity-Body.

   201 Created

      o Following:        any method that may request a new resource
                          be created
      o Required headers: URI-header

   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. 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

      o Following:        any method
      o Required headers: none

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

   The "202 Accepted" 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 enacted.

   203 Provisional Information

      o Following:        GET, HEAD
      o Required headers: none

   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. 

   204 No Content

      o Following:        any method
      o Required headers: none

   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. 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 current document view.

6.2.2 Redirection 3xx

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

   300 Multiple Choices

      o Following:        any method
      o Required headers: none

   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 must include an entity 
   containing a formatted list of resource characteristics and 
   locations from which the 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.

   301 Moved Permanently

      o Following:        any method
      o Required headers: URI-header, Location

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

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

   302 Moved Temporarily

      o Following:        any method
      o Required headers: URI-header, Location

   The requested resource resides temporarily under a different URI. 
   Since the redirection may be altered on occasion, the client should 
   on future requests from the user continue to use the original 
   Request-URI and not the URI returned in the URI-header field and 
   Location fields.

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

   303 Method

   This code is obsolete.

   304 Not Modified

      o Following:        conditional GET
      o Required headers: none

   If the client has performed a conditional GET request and access is 
   allowed, but the document has not been modified since the date and 
   time specified in the If-Modified-Since field, the server shall 
   respond with this status code and 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.

6.2.3 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.

   400 Bad Request

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

   401 Unauthorized

      o Required headers: WWW-Authenticate

   The request requires user authentication. The response must include 
   a WWW-Authenticate header field (Section 6.3.4) 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 request is forbidden because of some reason that remains 
   unknown to the client. 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 cannot be 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 method) that an 
   old resource is permanently unavailable and has no forwarding 
   address.

   405 Method Not Allowed

      o Required headers: Allow

   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

      o Required headers: Content-*, where applicable to the Request-URI

   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. The response must include (when 
   applicable) the Content-Type, Content-Encoding, and Content-
   Language of the resource, and is encouraged to include the 
   resource's complete metainformation. No Entity-Body can be included 
   in the response.

   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 -- this feature will be available in 
   future versions.

   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 Conflict" 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" can 
   be used instead.

   The "410 Gone" 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.

6.2.4 Server Errors 5xx

   Response status codes beginning with the digit "5" indicate cases 
   in which the server is aware that it has erred or is incapable of 
   performing the request. 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 complete 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 a "500 Internal 
   Server Error".

       Note: The presence 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= Public
                      | Retry-After
                      | Server
                      | WWW-Authenticate
                      | extension-header

   Although additional response header fields may be implemented via 
   the extension mechanism, applications which do not recognize those 
   fields should treat them as Entity-Header fields.

6.3.1 Public

   The Public 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 7.1.1) 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" ":" 1#method

   Example of use:

       Public: OPTIONS, MGET, MHEAD

   This header field applies only to the current connection. 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.

6.3.2 Retry-After

   The Retry-After header field can be used with "503 Service 
   Unavailable" to indicate how long the service is expected to be 
   unavailable to the requesting client. The value of this field can 
   be either an full 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.

6.3.3 Server

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

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

   Example:

       Server: CERN/3.0 libwww/2.17

   If the response is being forwarded through a proxy, the proxy 
   application must not add its data to the product list. Instead, it 
   should include a Forwarded field, as described in Section 4.3.2.

6.3.4 WWW-Authenticate

   The WWW-Authenticate header field must be included as part of a 
   "401 Unauthorized" response. 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.

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

   This section specifies the format and semantics of the 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
                      | Content-Encoding
                      | Content-Language
                      | Content-Length
                      | Content-Transfer-Encoding
                      | Content-Type
                      | Derived-From
                      | Expires
                      | Last-Modified
                      | Link
                      | Location
                      | Title
                      | URI-header
                      | Version
                      | extension-header

       extension-header = HTTP-header

   Each entity header field is explained in the subsections below. 
   Other header fields are allowed but cannot be assumed to be 
   recognizable by the recipient. Unknown header fields should be 
   ignored by the recipient and forwarded by proxies.

       Note: It has been proposed that any HTML metainformation 
       element (allowed within the <HEAD> element of a document) be 
       a valid candidate for an HTTP entity header. This document 
       defines two header fields, Link and Title, which are both 
       examples of this. Base will be used in future versions of 
       HTTP.

7.1.1 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; it must be present in a response with status 
   code "405 Method Not Allowed".

       Allow          = "Allow" ":" 1#method

    Example of use:

       Allow: GET, HEAD, PUT

   This does not prevent a client from trying other methods. However, 
   it is recommended that the indications given by this field be 
   followed. This field has no default value; if left undefined, the 
   set of allowed methods is defined by the origin server at the time 
   of each request.

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

7.1.2 Content-Encoding

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

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

   Encoding mechanisms are defined in Section 8.4. 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.

7.1.3 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" ":" 1#language-tag

   Language tags are defined in Section 8.2. 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 
   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 audience. In this case, the Content-Language should 
   only include "en".

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

7.1.4 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.

7.1.5 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 8.5. 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 are required to understand 
   it upon receipt. Gateways are the only HTTP applications that would 
   generate a CTE.

7.1.6 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 8.1. 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.

7.1.7 Derived-From

   The Derived-From header field can be used to indicate the version 
   tag of the resource from which the enclosed entity was derived 
   before modifications were made by the sender. This field is used to 
   help manage the process of merging successive changes to a 
   resource, particularly when such changes are being made in parallel 
   and from multiple sources.

       Derived-From   = "Derived-From" ":" version-tag

   An example use of the field is:

       Derived-From: 2.1.1

   A longer example of version control is included in Appendix C. The 
   Derived-From field is required for PUT requests if the entity being 
   put was previously retrieved from the same URI and a Version header 
   was included with the entity when it was last retrieved. 

7.1.8 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.

       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 may be considered equivalent to an 
       "expires immediately." Although these values are not 
       legitimate for HTTP/1.0, a robust implementation is always 
       desirable.

7.1.9 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-mod dates of its component parts. For 
   database gateways, it may be the last-update timestamp of the 
   record. For virtual objects, it may be the last time the internal 
   state changed. In any case, the recipient should only know (and 
   care) about the result -- whatever gets stuck in the Last-Modified 
   field -- and not worry about how that result was obtained.

7.1.10 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" ":" 1#("<" URI ">"
                        [ ";" "rel" "=" relation ]
                        [ ";" "rev" "=" relation ]
                        [ ";" "title" "=" quoted-string ] )

       relation       = sgml-name

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

   Relation values are not case-sensitive and may be extended within 
   the constraints of the sgmlname 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 
   publisher of the resource is identified by the given e-mail address.

7.1.11 Location

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

       Location       = "Location" ":" URI

   An example is

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

7.1.12 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 to be considered isomorphic with the <TITLE> element 
   in HTML [4].

7.1.13 URI

   The URI-header field may contain one or more Universal Resource 
   Identifiers (URIs) by which the resource origin of the entity can 
   be identified. There is no guarantee that the resource can be 
   accessed using the URI(s) specified. This field is required for the 
   201, 301, and 302 response messages, and may be included in any 
   message that contains resource metainformation.

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

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

       extension-vary = token

   Any URI specified in this field can be either absolute or relative 
   to the URI given in the Request-Line. The URI-header improves upon 
   the Location header field described in Section 7.1.11. For 
   backwards compatibility with older clients, servers are encouraged 
   to include both header fields in 301 and 302 responses.

   The URI-header may also 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, Version, and User-Agent header 
   fields, as demonstrated in the following example.

       URI: <TheProject.ps>;vary="encoding,version",
            <TheProject.html>;vary="user-agent,charset,version"

   The vary parameter has an important effect on cache management, 
   particularly for caching proxies 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 proxy must keep track of the 
   content metainformation for resources with varying dimensions. 
   Thus, the vary parameter tells the caching proxy 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.

7.1.14 Version

   The Version field defines the version tag associated with a 
   rendition of an evolving entity. Together with the Derived-From 
   field described in Section 7.1.7, it allows a group of people to 
   work simultaneously on the creation of a work as an iterative 
   process. The field should be used to allow evolution of a 
   particular work along a single path. It should not be used to 
   indicate derived works or renditions in different representations.

       Version        = "Version" ":" version-tag

       version-tag    = token | quoted-string

   Examples of the Version field include:

       Version: 2.1.2
       Version: "Fred 19950116-12:26:48"
       Version: 2.5a4-omega7

   The version tag should be considered opaque to all parties but the 
   origin server. A user agent can request a particular version of an 
   entity by including its tag in a Version header as part of the 
   request. Similarly, a user agent may suggest a value for the 
   version of an entity transferred via a PUT or POST request. 
   However, only the origin server can reliably assign or increment 
   the version tag of an entity.

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.

       Note: Most current implementations of the POST and PUT 
       methods require a valid Content-Length header field. This 
       can cause problems for some systems that do not know the 
       size of the entity-body before transmission. Experimental 
       implementations (and future versions of HTTP) may use a 
       packetized Content-Transfer-Encoding to obviate the need for 
       a Content-Length.

   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 should. Similarly, the responses "204 
   No Content", "304 Not Modified", and "406 None Acceptable" must not 
   include a 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 an 
   additional encoding mechanism 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/or 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 access 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 (number of octets) 
   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. Note that the latter cannot be used to indicate the end of 
   a request body, since it leaves no possibility for the server to 
   send back a 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.  Content Parameters

8.1  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. No LWS is allowed 
   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.

8.1.1 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 
   which does not use octets 13 and 10 for CR and LF respectively (as 
   is the case for some multi-byte character sets), HTTP allows the 
   use of whatever octet sequence(s) is defined by that character set 
   to represent the equivalent of CRLF, bare CR, and bare LF. It is 
   assumed that any recipient capable of using such a character set 
   will know the appropriate octet sequence for representing line 
   breaks within that character set.

       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 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 character set ISO-8859-1 [19] 
   is a superset of USASCII [18], 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.

   HTTP does not require that the character set of an entity body be 
   labelled as the lowest common denominator of the character codes 
   used within a document.

8.1.2 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 7.1.13) should be included in the body-
   part for each enclosed entity that can be identified by a URI.

8.2  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 
   and Content-Language 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 to be 
   treated as case insensitive. 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.

       Note: Earlier versions of this document specified an 
       incomplete language tag, where values were limited to ISO 
       639 language abbreviations with an optional ISO 3166 country 
       code appended after an underscore ("_") or slash ("/") 
       character. This format was abandoned in favor of the 
       recently proposed standard for Internet protocols.

8.3  Character Sets

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

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

   Character sets are identified by case-insensitive tokens. The 
   complete set of allowed charset values are defined by the IANA 
   Character Set registry [17]. However, because that registry does 
   not define a single, consistent token for each character set, we 
   define here the preferred names for those character sets most 
   likely to be used with HTTP entities. This set of charset values 
   includes those registered by RFC 1521 [6] -- the US-ASCII [18] and 
   ISO8859 [19] character sets -- and other character set 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 character 
   set value, any token that has a predefined value within the IANA 
   Character Set registry [17] must represent the character set 
   defined by that registry. Applications are encouraged, but not 
   required, to limit their use of character sets to those defined by 
   the IANA registry. 

8.4  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 with this 
   encoding and is 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 5.4.3) 
   and Content-Encoding (Section 7.1.2) 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).

8.5  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. 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 uses 
   transfer-encoding values in the Accept-Encoding (Section 5.4.3) and 
   Content-Transfer-Encoding (Section 7.1.5) header fields.

   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.

9.  Content Negotiation

   Content negotiation is an optional feature of the HTTP protocol. It 
   is designed to allow for preemptive 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. If 
   there are no variant forms of the resource, the "negotiation" is 
   limited to whether or not that single media type is acceptable 
   under the constraints given by the Accept request header field
   (if any).

   If variants are available, those variants that are completely 
   unacceptable should be removed from consideration first. 
   Unacceptable variants include those with a Content-Encoding not 
   listed in an Accept-Encoding field, those with a character subset 
   (other than the default ISO-8859-1) not listed in an Accept-Charset 
   field, and those with a media type not within any of the media 
   ranges of an explicitly constrained Accept field (or listed with a 
   zero quality parameter). 

   If no acceptable variants remain at this point, the server should 
   respond with a "406 None Acceptable" response message. If more than 
   one variant remains, and at least one has a Content-Language within 
   those listed by an Accept-Language field, any variants which do not 
   match the language constraint are removed from further 
   consideration.

   If multiple choices still remain, the selection is further winnowed 
   by calculating and comparing the relative quality of the available 
   media types. 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. The following parameters are included in the calculation:

      q    The quality factor chosen by the user agent (and 
           configurable by the user) that represents the desirability 
           of that media type. In this case, desirability is usually a 
           measure of the clients ability to faithfully represent the 
           contents of that media type to the user. The value is in 
           the range [0,1], where the default value is 1.

      qs   The quality factor, as chosen by the server (via some 
           unspecified mechanism), to represent the relative quality 
           of a particular variant representation of the 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 has the same range and 
           default as q.

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

      bs   The actual number of bytes in the Entity-Body for a 
           particular source representation. This should equal the 
           value sent as Content-Length. The default value is 0.

   The discrete mapping function is defined as:

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

   The variants with a maximal value for the Q function represent the 
   preferred representation(s) of the entity. If multiple 
   representations exist for a single media type (as would be the case 
   if they only varied by Content-Encoding), then the smallest 
   representation (lowest bs) is preferred.

   Finally, there may still be multiple choices available to the user. 
   If so, the server may either choose one (possibly at random) from 
   those available and respond with "200 OK", or it may 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 (no such type is 
   available at the time of this writing).

   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.

10.  Access Authentication

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

       auth-scheme    = "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 the challenge 
   applicable to the requested resource.

       challenge      = auth-scheme 1*LWS realm [ "," 1#auth-param ]

       realm          = "Realm" "=" quoted-string

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

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

       credentials    = auth-scheme [ 1*LWS encoded-cookie ] #auth-param

       encoded-cookie = 1*<any CHAR except CTLs or tspecials,
                           but including "=" and "/">

   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.

   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 access 
   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 -- this feature will 
   be available in future versions of HTTP.

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

11.3  Abuse of Server Log Information

   A server is in the position to save personal data about 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.

   Two header fields are worth special mention in this context: 
   Referer and From. The Referer field allows reading patterns to be 
   studied and reverse links drawn. Although it can be very useful, 
   its power can be abused if user details are not separated from the 
   information contained in the Referer. Even when the personal 
   information has been removed, the Referer field may have indicated 
   a secure document's URI, whose revelation itself would be a breach 
   of security.

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

   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,
   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 HTTPWG. 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                     Michael A. Dolan
       John Franks                        Alex Hopmann
       Bob Jernigan                       Martijn Koster
       Dave Kristol                       Daniel LaLiberte
       Albert Lunde                       John C. Mallery
       Larry Masinter                     Mitra
       Gavin Nicol                        Marc Salomon
       Chuck Shotton                      Eric W. Sink
       Simon E. Spero

13. References

   [1]  H. T. 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-
        01.txt), CERN, HaL Computer Systems, February 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. T. Fielding. "Relative Uniform Resource Locators." Work in 
        Progress (draft-ietf-uri-relative-url-05.txt), UC Irvine, 
        January 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] US-ASCII. Coded Character Set - 7-Bit American Standard Code for
        Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.

   [19] 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 9670
   Fax: +1 (617) 258 8682
   Email: timbl@w3.org

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

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

Appendices

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

A.  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, type

             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.

             type:    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.  Minimum Compliance

   Early reviews of this specification have indicated the need for a 
   statement of the minimum requirements for an implementation to be 
   considered in compliance with HTTP/1.0. This section will be 
   written soon.

       Note: The primary difficulty in determining a standard for 
       minimum compliance rests in the fact that HTTP is a flexible 
       protocol which can be used for many purposes. The 
       requirements for special purpose applications often differ 
       from those of general purpose applications.

B.1  Servers

   Servers have a special responsibility for being honest when 
   generating their responses to requesting clients. The Status-Code 
   sent in the Status-Line must correspond to the actual action taken 
   by the server. This is especially the case when the method used in 
   the request is one of PUT, POST, DELETE, LINK, and UNLINK. If a 
   Status-Code of 200 is returned, the client must be able to assume 
   that the action has been carried out. If the server is not able to 
   fulfill the requested action immediately, the correct status code 
   to use is "202 Accepted".

   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.

C.  Tolerant Applications

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

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

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

   The line terminator for HTTP-header fields should be 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.

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

D.  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.

D.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 8.1.1.

D.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) 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 sets which do not use 
   octets 13 and 10 to represent CR and LF (as is the case for some 
   multi-byte character sets).

D.1.2 Default Character Set

   MIME requires that all subtypes of the top-level Content-Type 
   "text" have a default character set of US-ASCII [18]. In contrast, 
   HTTP defines the default character set for "text" to be 
   ISO88591 [19] (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.

D.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 encoding.

D.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.

E.  Example of Version Control

   This appendix gives an example on how the Entity-Header fields 
   Version and Derived-From can be used to apply version control to 
   the creation and parallel development of a work. In order to 
   simplify the example, only two user agents (A and B) are considered 
   together with an origin server S.

      o A sends a POST request to S, including the header
        "Version: 1.0" and an entity

      o S replies "201 Created" to A, including the header
        "Version: 1.0" and a URI-header which should be used for
        future references

      o A starts editing the entity

      o B sends a GET request to S

      o S replies "200 OK" to B, including the entity with a header 
        "Version: 1.0"

      o B starts editing the entity

      o B sends a PUT request to S, including the entity and a header 
        "Derived-From: 1.0"

      o S replies "204 No Content" to B, including a header
        "Version: 1.1" but no entity

      o A sends a PUT request to S, including the entity and a header 
        "Derived-From: 1.0"

      o S replies "409 Conflict" to A, including "Version: 1.1" and the 
        list of problems with merging A's changes to 1.0 with those 
        already applied for B and version 1.1

      o A merges B's changes with its own, possibly with help from the 
        user of A

      o A sends a PUT request to S including the entity and the header 
        "Derived-From: 1.1"

      o S replies "204 No Content" to A, including the header 
        "Version: 1.2" but no entity

   The example can be expanded to any number of involved user agents, 
   though the likelihood of conflicts and the difficulty of resolving 
   them may increase.
