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Versions: 00 01 02 03 04 05 RFC 1945

HTTP Working Group                               T. Berners-Lee, MIT/W3C
INTERNET-DRAFT                                    R. Fielding, UC Irvine
<draft-ietf-http-v10-spec-02.txt>                    H. Frystyk, MIT/W3C
Expires February 13, 1996                                August 13, 1995


                Hypertext Transfer Protocol -- HTTP/1.0


Status of this Memo

   This document is an Internet-Draft. Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups. Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other
   documents at any time. It is inappropriate to use Internet-Drafts
   as reference material or to cite them other than as "work in
   progress".

   To learn the current status of any Internet-Draft, please check the
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   ftp.isi.edu (US West Coast).

   Distribution of this document is unlimited. Please send comments to
   the HTTP working group at <http-wg@cuckoo.hpl.hp.com>. Discussions
   of the working group are archived at
   <URL:http://www.ics.uci.edu/pub/ietf/http/>. General discussions
   about HTTP and the applications which use HTTP should take place on
   the <www-talk@w3.org> mailing list.

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 common usage of
   the protocol referred to as "HTTP/1.0".

Table of Contents

   1.  Introduction
       1.1  Purpose
       1.2  Overall Operation
       1.3  Terminology

   2.  Notational Conventions and Generic Grammar
       2.1  Augmented BNF
       2.2  Basic Rules

   3.  Protocol Parameters
       3.1  HTTP Version
       3.2  Uniform Resource Identifiers
            3.2.1  General Syntax
            3.2.2  http URL
       3.3  Date/Time Formats
       3.4  Coded Character Sets
       3.5  Encoding Mechanisms
       3.6  Media Types
            3.6.1  Canonicalization and Text Defaults
            3.6.2  Multipart Types
       3.7  Product Tokens

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

   5.  Request
       5.1  Request-Line
       5.2  Method
            5.2.1  GET
            5.2.2  HEAD
            5.2.3  POST
       5.3  Request-URI
       5.4  Request Header Fields

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

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

   8.  Header Field Definitions
       8.1  Allow
       8.2  Authorization
       8.3  Content-Encoding
       8.4  Content-Length
       8.5  Content-Type
       8.6  Date
       8.7  Expires
       8.8  From
       8.9  If-Modified-Since
       8.10 Last-Modified
       8.11 Location
       8.12 MIME-Version
       8.13 Pragma
       8.14 Referer
       8.15 Server
       8.16 User-Agent
       8.17 WWW-Authenticate

   9.  Access Authentication
       9.1  Basic Authentication Scheme

   10. Security Considerations
       10.1 Authentication of Clients
       10.2 Idempotent Methods
       10.3 Abuse of Server Log Information
       10.4 Transfer of Sensitive Information

   11. Acknowledgments

   12. References

   13. Authors' Addresses

   Appendix A.   Internet Media Type message/http

   Appendix B.   Tolerant Applications

   Appendix C.   Relationship to MIME
       C.1  Conversion to Canonical Form
            C.1.1  Representation of Line Breaks
            C.1.2  Default Coded Character Set
       C.2  Conversion of Date Formats
       C.3  Introduction of Content-Encoding
       C.4  No Content-Transfer-Encoding



1.  Introduction

1.1  Purpose

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol with the lightness and speed necessary for distributed,
   collaborative, hypermedia information systems. HTTP has been in use
   by the World-Wide Web global information initiative since 1990.
   This specification reflects common usage of the protocol referred
   to as "HTTP/1.0". This specification is not intended to become an
   Internet standard; rather, it defines those features of the HTTP
   protocol that can reasonably be expected of any implementation
   which claims to be using HTTP/1.0.

   Practical information systems require more functionality than
   simple retrieval, including search, front-end update, and
   annotation. HTTP/1.0 allows an open-ended set of methods to be used
   to indicate the purpose of a request. It builds on the discipline
   of reference provided by the Uniform Resource Identifier (URI) [2],
   as a location (URL) [4] or name (URN) [16], for indicating the
   resource on which a method is to be applied. Messages are passed in
   a format similar to that used by Internet Mail [7] and the
   Multipurpose Internet Mail Extensions (MIME) [5].

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

1.2  Overall Operation

   The HTTP protocol is based on a request/response paradigm. A
   requesting program (termed a client) establishes a connection with
   a receiving program (termed a server) and sends a request to the
   server in the form of a request method, URI, and protocol version,
   followed by a MIME-like message containing request modifiers,
   client information, and possible body content. The server responds
   with a status line, including its protocol version and a success or
   error code, followed by a MIME-like message containing server
   information, 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 [15], but other ports
   can be used. This does not preclude the HTTP/1.0 protocol from
   being implemented on top of any other protocol on the Internet, or
   on other networks. HTTP only presumes a reliable transport; any
   protocol that provides such guarantees can be used, and the mapping
   of the HTTP/1.0 request and response structures onto the transport
   data units of the protocol in question is outside the scope of this
   specification.

   Current practice requires that the connection be established by the
   client prior to each request and closed by the server after sending
   the response. Both clients and servers 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 (Section 3.2).

   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. These are often browsers,
       editors, spiders (web-traversing robots), or other end user
       tools.

   server

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

   origin server

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

   proxy

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

   gateway

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

2.  Notational Conventions and Generic Grammar

2.1  Augmented BNF

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

   name = definition

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

   "literal"

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

   rule1 | rule2

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

   (rule1 rule2)

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

   *rule

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

   [rule]

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

   N rule

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

   #rule

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

   ; comment

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

   implied *LWS

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

2.2  Basic Rules

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

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

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

       CRLF           = CR LF

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

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

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

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

       word           = token | quoted-string

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

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

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

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

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

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

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

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

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

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

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

3.  Protocol Parameters

3.1  HTTP Version

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

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

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

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

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

   HTTP/1.0 servers must:

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

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

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

   HTTP/1.0 clients must:

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

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

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

3.2  Uniform Resource Identifiers

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

3.2.1 General Syntax

   URIs in HTTP/1.0 can be represented in absolute form or relative to
   some known base URI [9], depending upon the context of their use.
   The two forms are differentiated by the fact that absolute URIs
   always begin with a scheme name followed by a colon.

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

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

       relativeURI    = net_path | abs_path | rel_path

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

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

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

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

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

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

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

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

3.2.2 http URL

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

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

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

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

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

3.3  Date/Time Formats

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

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

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

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

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

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

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

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

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

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

       Note: HTTP/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.

3.4  Coded 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.

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

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

   Although HTTP allows an arbitrary token to be used as a charset
   value, any token that has a predefined value within the IANA
   Character Set registry [15] must represent the coded character set
   defined by that registry. Applications are encouraged, but not
   required, to limit their use of coded character sets to those
   defined by the IANA registry.

3.5  Encoding Mechanisms

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

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

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

   All encoding-mechanism values are case-insensitive. HTTP/1.0 uses
   encoding-mechanism values in the Content-Encoding (Section 8.3)
   header field. 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:

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

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

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

3.6  Media Types

   HTTP uses Internet Media Types [13] (formerly referred to as MIME
   Content-Types [5]) 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,
   where the sender and recipient can communicate directly,
   applications 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 case-
   insensitive. Parameter values may or may not be case-sensitive,
   depending on the semantics of the parameter name. LWS must not be
   generated between the type and subtype, nor between an attribute
   and its value.

   Many current applications do not recognize media type parameters.
   Since parameters are a fundamental aspect of media types, this must
   be considered an error in those applications. Nevertheless,
   HTTP/1.0 applications should only use media type parameters when
   they are necessary to define the content of a message.

   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 [13].

   All media-type's registered by IANA must be preferred over
   extension tokens. However, HTTP does not limit conforming
   applications to the use of officially registered media types, nor
   does it encourage the use of an "x-" prefix for unofficial types
   outside of explicitly short experimental use between consenting
   applications.

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

       Note: This interpretation of line breaks applies only to the
       contents of an Entity-Body and only after any 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 coded 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 ISO-8859-1 [18]
   coded character set is a superset of US-ASCII [17], this has no
   effect upon the interpretation of entity bodies which only contain
   octets within the US-ASCII set (0 - 127). The presence of a charset
   parameter value in a Content-Type header field overrides the
   default.

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

3.6.2 Multipart Types

   MIME provides for a number of "multipart" types -- encapsulations of
   several entities within a single message's Entity-Body. The
   multipart types registered by IANA [15] do not have any special
   meaning for HTTP/1.0, though user agents may need to understand
   each type in order to correctly interpret the purpose of each body-
   part. 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 [5], 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.

3.7  Product Tokens

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

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

   Examples:

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

       Server: Apache/0.8.4

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

4.  HTTP Message

4.1  Message Types

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

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

   Full-Request and Full-Response use the generic message format of
   RFC 822 [7] for transferring entities. Both messages may include
   optional header fields (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 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 [7]. Each header field
   consists of a name followed immediately by a colon (":"), a single
   space (SP) character, and the field value. Field names are case-
   insensitive. Header fields can be extended over multiple lines by
   preceding each extra line with at least one LWS, though this is not
   recommended.

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

       field-name     = 1*<any CHAR, excluding CTLs, SP, and ":">
       field-value    = *( field-content | LWS )

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

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

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

4.3  General Message Header Fields

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

       General-Header = Date                     ; Section 8.6
                      | MIME-Version             ; Section 8.12
                      | Pragma                   ; Section 8.13

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

5. Request

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

       Request        = Simple-Request | Full-Request

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

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

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

5.1  Request-Line

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

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

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

5.2  Method

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

       Method         = "GET" | "HEAD" | "POST"
                      | extension-method

       extension-method=token

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

   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.

5.2.1 GET

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

   The semantics of the GET method changes to a "conditional GET" if
   the request message includes an If-Modified-Since header field. A
   conditional GET method requests that the identified resource be
   transferred only if it has been modified since the date given by
   the If-Modified-Since header, as described in Section 8.9. The
   conditional GET method is intended to reduce network usage by
   allowing cached entities to be refreshed without requiring multiple
   requests or transferring unnecessary data.

5.2.2 HEAD

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

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

5.2.3 POST

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

      o Annotation of existing resources;

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

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

      o Extending a database through an append operation.

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

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

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

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

5.3  Request-URI

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

       Request-URI    = absoluteURI | abs_path

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

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

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

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

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

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

5.4  Request Header Fields

   The request header fields allow the client to pass additional
   information about the request, and about the client itself, to the
   server. All header fields are optional and conform to the generic
   HTTP-header syntax.

       Request-Header = Authorization            ; Section 8.2
                      | From                     ; Section 8.8
                      | If-Modified-Since        ; Section 8.9
                      | Referer                  ; Section 8.14
                      | User-Agent               ; Section 8.16

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

6.  Response

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

       Response       = Simple-Response | Full-Response

       Simple-Response= [ Entity-Body ]

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

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

6.1  Status-Line

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

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

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

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

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

6.2  Status Codes and Reason Phrases

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

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

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

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

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

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

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

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

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

       extension-code = 3DIGIT

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

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

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

6.2.1 Informational 1xx

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

6.2.2 Successful 2xx

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

   200 OK

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

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

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

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

   201 Created

   The request has been fulfilled and resulted in a new resource being
   created. The newly created resource can be referenced by the URI(s)
   returned in the entity of the response. The origin server 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 POST can create
   a resource.

   202 Accepted

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

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

   204 No Content

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

6.2.3 Redirection 3xx

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

   300 Multiple Choices

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

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

   301 Moved Permanently

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

   The new URL must be given by the Location field in the response.
   The Entity-Body of the response should contain a short hypertext
   note with a hyperlink to the new URL.

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

   302 Moved Temporarily

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

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

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

   304 Not Modified

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

6.2.4 Client Error 4xx

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

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

   400 Bad Request

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

   401 Unauthorized

   The request requires user authentication. The response must include
   a WWW-Authenticate header field (Section 8.17) 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 9.

   403 Forbidden

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

   404 Not Found

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

6.2.5 Server Errors 5xx

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

   500 Internal Server Error

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

   501 Not Implemented

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

   502 Bad Gateway

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

   503 Service Unavailable

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

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

6.3  Response Header Fields

   The response header fields allow the server to pass additional
   information about the response which cannot be placed in the
   Status-Line. These header fields are not intended to give
   information about an Entity-Body returned in the response, but
   about the server itself.

       Response-Header= Location                 ; Section 8.11
                      | Server                   ; Section 8.15
                      | WWW-Authenticate         ; Section 8.17

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

7.  Entity

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

7.1  Entity Header Fields

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

       Entity-Header  = Allow                    ; Section 8.1
                      | Content-Encoding         ; Section 8.3
                      | Content-Length           ; Section 8.4
                      | Content-Type             ; Section 8.5
                      | Expires                  ; Section 8.7
                      | Last-Modified            ; Section 8.10
                      | extension-header

       extension-header = HTTP-header

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

7.2  Entity Body

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

       Entity-Body    = *OCTET

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

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

7.2.1 Type

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

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

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

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

7.2.2 Length

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

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

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

8.  Header Field Definitions

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

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

       Allow          = "Allow" ":" 1#method

    Example of use:

       Allow: GET, HEAD

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

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

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

8.2  Authorization

   A user agent that wishes to authenticate itself with a server--
   usually, but not necessarily, after receiving a 401 response--may do
   so by including an Authorization 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 9. If a request
   is authenticated and a realm specified, the same credentials should
   be valid for all other requests within this realm.

   Proxies must not cache the response to a request containing an
   Authorization field.

8.3  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 3.5. An example of its
   use is

       Content-Encoding: x-gzip

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

8.4  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.

8.5  Content-Type

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

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

   Media types are defined in Section 3.6. An example of the field is

       Content-Type: text/html

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

8.6  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 POST request, and
   even then it is optional. A received message which does not have a
   Date header field should be assigned one by the receiver if and
   only if the message will be cached by that receiver or gatewayed
   via a protocol which requires a Date.

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

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

8.7  Expires

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

       Expires        = "Expires" ":" HTTP-date

   An example of its use is

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

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

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

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

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

8.8  From

   The From header field, if given, should contain an Internet e-mail
   address for the human user who controls the requesting user agent.
   The address should be machine-usable, as defined by mailbox in
   RFC 822 [7] (as updated by RFC 1123 [6]):

       From           = "From" ":" mailbox

   An example is:

       From: webmaster@w3.org

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

   The Internet e-mail address in this field may be separate from the
   Internet host which issued the request. For example, when a request
   is passed through a proxy, the original issuer's address should be
   used.

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

8.9  If-Modified-Since

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

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

   An example of the field is:

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

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

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

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

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

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

8.10  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-modified
   time. For entities with dynamically included parts, it may be the
   most recent of the set of last-modify times for its component
   parts. For database gateways, it may be the last-update timestamp
   of the record. For virtual objects, it may be the last time the
   internal state changed.

8.11  Location

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

       Location       = "Location" ":" absoluteURI

   An example is

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

8.12  MIME-Version

   HTTP is not a MIME-conformant protocol (see Appendix C). However,
   HTTP/1.0 messages may include a single MIME-Version header field to
   indicate what version of the MIME protocol was used to construct
   the message. Use of the MIME-Version header field should indicate
   that the message is in full compliance with the MIME protocol (as
   defined in [5]). Unfortunately, current versions of HTTP/1.0
   clients and servers use this field indiscriminately, and thus
   receivers must not take it for granted that the message is indeed
   in full compliance with MIME. Gateways are responsible for ensuring
   this compliance (where possible) when exporting HTTP messages to
   strict MIME environments. Future HTTP/1.0 applications must only
   use MIME-Version when the message is intended to be MIME-conformant.

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

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

8.13  Pragma

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

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

       pragma-directive = "no-cache"
                        | extension-pragma
       extension-pragma = token [ "=" word ]

   When the "no-cache" directive is present in a request message, a
   caching intermediary 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 is known to be corrupted or stale.

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

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

8.14  Referer

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

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

   Example:

       Referer: http://www.w3.org/hypertext/DataSources/Overview.html

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

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

8.15  Server

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

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

   Example:

       Server: CERN/3.0 libwww/2.17

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

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

8.16  User-Agent

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

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

   Example:

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

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

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

8.17  WWW-Authenticate

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

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

   The HTTP access authentication process is described in Section 9.
   User agents must take special care in parsing the WWW-Authenticate
   field value if it contains more than one challenge, or if more than
   one WWW-Authenticate header field is provided, since the contents
   of a challenge may itself contain a comma-separated list of
   authentication parameters.

9.  Access Authentication

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

       auth-scheme    = token

       auth-param     = token "=" quoted-string

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

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

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

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

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

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

   The domain over which credentials can be automatically applied by a
   user agent is determined by the protection space. If a prior
   request has been authorized, the same credentials may be reused for
   all other requests within that protection space for a period of
   time determined by the authentication scheme, parameters, and/or
   user preference. Unless otherwise defined by the authentication
   scheme, a single protection space cannot extend outside the scope
   of its server.

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

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

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

9.1  Basic Authentication Scheme

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

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

       WWW-Authenticate: Basic realm="WallyWorld"

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

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

       basic-credentials = "Basic" SP basic-cookie

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

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

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

       Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

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

10.  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.

10.1  Authentication of Clients

   As mentioned in Section 9.1, the Basic authentication scheme is not
   a secure method of user authentication, nor does it prevent the
   Entity-Body from being transmitted in clear text across the
   physical network used as the carrier. HTTP/1.0 does not prevent
   additional authentication schemes and encryption mechanisms from
   being employed to increase security.

10.2  Idempotent Methods

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

   In particular, the convention has been established that the GET and
   HEAD methods should never have the significance of taking an action
   other than retrieval. These methods should be considered "safe" and
   should not have side effects. This allows the client software to
   represent other methods, such as POST, in a special way so that the
   user is made aware of the fact that an non-idempotent action is
   being requested.

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

10.3  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.

10.4  Transfer of Sensitive Information

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

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

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

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

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

11.  Acknowledgments

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

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

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

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

12. References

   [1]  F. Anklesaria, M. McCahill, P. Lindner, D. Johnson, D. Torrey,
        and B. Alberti. "The Internet Gopher Protocol: A distributed
        document search and retrieval protocol." RFC 1436, University
        of Minnesota, March 1993.

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

   [3]  T. Berners-Lee and D. Connolly. "HyperText Markup Language
        Specification - 2.0." Work in Progress
        (draft-ietf-html-spec-05.txt), MIT/W3C, August 1995.

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

   [5]  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.

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

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

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

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

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

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

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

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

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

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

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

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

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

13.  Authors' Addresses

   Tim Berners-Lee
   Director, W3 Consortium
   MIT Laboratory for Computer Science
   545 Technology Square
   Cambridge, MA 02139, U.S.A.
   Tel: +1 (617) 253 5702
   Fax: +1 (617) 258 8682
   Email: timbl@w3.org

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

   Henrik Frystyk Nielsen
   W3 Consortium
   MIT Laboratory for Computer Science
   545 Technology Square
   Cambridge, MA 02139, U.S.A.
   Tel: +1 (617) 258 8143
   Fax: +1 (617) 258 8682
   Email: frystyk@w3.org

Appendices

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

A.  Internet Media Type message/http

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

       Media Type name:         message

       Media subtype name:      http

       Required parameters:     none

       Optional parameters:     version, msgtype

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

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

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

       Security considerations: none

B.  Tolerant Applications

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

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

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

C.  Relationship to MIME

   HTTP/1.0 reuses many of the constructs defined for Internet Mail
   (RFC 822 [7]) and the Multipurpose Internet Mail Extensions
   (MIME [5]) to allow entities to be transmitted in an open variety
   of representations and with extensible mechanisms. However, 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.

C.1  Conversion to Canonical Form

   MIME requires that an entity be converted to canonical form prior
   to being transferred, as described in Appendix G of RFC 1521 [5].
   Although HTTP does require media types to be transferred in
   canonical form, it changes the definition of "canonical form" for
   text-based media types as described in Section 3.6.1.

C.1.1 Representation of Line Breaks

   MIME requires that the canonical form of any text type represent
   line breaks as CRLF and forbids the use of CR or LF outside of line
   break sequences. Since HTTP allows CRLF, bare CR, and bare LF (or
   the octet sequence(s) to which they would be translated for the
   given coded 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 coded character sets which do not
   use octets 13 and 10 to represent CR and LF, as is the case for
   some multi-byte coded character sets. If canonicalization is
   performed, the Content-Length header field value must be updated to
   reflect the new body length.

C.1.2 Default Coded Character Set

   MIME requires that all subtypes of the top-level Content-Type
   "text" have a default coded character set of US-ASCII [17]. In
   contrast, HTTP defines the default coded character set for "text"
   to be ISO-8859-1 [18] (a superset of US-ASCII). Therefore, if a
   text/* media type given in the Content-Type header field does not
   already include an explicit charset parameter, the parameter

       ;charset="iso-8859-1"

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

C.2  Conversion of Date Formats

   HTTP/1.0 uses a restricted subset of date formats to simplify the
   process of date comparison. Gateways from other protocols should
   ensure that any Date header field present in a message conforms to
   one of the HTTP/1.0 formats and rewrite the date if necessary.

C.3  Introduction of Content-Encoding

   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 must 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.

C.4  No Content-Transfer-Encoding

   HTTP does not use the Content-Transfer-Encoding (CTE) field of
   MIME. Gateways from MIME-compliant protocols must remove any non-
   identity CTE ("quoted-printable" or "base64") encoding prior to
   delivering the response message to an HTTP client. Gateways to MIME-
   compliant protocols are responsible for ensuring that the message
   is in the correct format and encoding for safe transport on that
   protocol, where "safe transport" is defined by the limitations of
   the protocol being used. At a minimum, the CTE field of

       Content-Transfer-Encoding: binary

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

   An HTTP client may include a Content-Transfer-Encoding as an
   extension Entity-Header in a POST request when it knows the
   destination of that request is a gateway to a MIME-compliant
   protocol.


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