[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

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-01.txt>                    H. Nielsen, MIT/W3C
Expires February 3, 1996                                  August 3, 1995


                Hypertext Transfer Protocol -- HTTP/1.0


Status of this Memo

   This document is an Internet-Draft. Internet-Drafts are working
   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
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   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 preferred usage
   of the protocol referred to as "HTTP/1.0".

Table of Contents

   1.  Introduction
       1.1  Purpose
       1.2  Overall Operation
       1.3  Terminology

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

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

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

   5.  Request
       5.1  Request-Line
       5.2  Method
            5.2.1  GET
            5.2.2  HEAD
            5.2.3  POST
            5.2.4  PUT
            5.2.5  DELETE
            5.2.6  LINK
            5.2.7  UNLINK
       5.3  Request-URI
       5.4  Request Header Fields

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

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

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

   9.  Content Negotiation

   10. Access Authentication
       10.1 Basic Authentication Scheme

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

   12. Acknowledgments

   13. References

   14. Authors' Addresses

   Appendix A.   Internet Media Type message/http

   Appendix B.   Tolerant Applications

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



1.  Introduction

1.1  Purpose

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

   Practical information systems require more functionality than
   simple retrieval, including search, front-end update, and
   annotation. HTTP/1.0 allows an open-ended set of methods to be used
   to indicate the purpose of a request. It builds on the discipline
   of reference provided by the Uniform Resource Identifier (URI) [3],
   as a location (URL) [5] or name (URN) [18], for indicating the
   resource on which a method is to be applied. Messages are passed in
   a format similar to that used by Internet Mail [8] and the
   Multipurpose Internet Mail Extensions (MIME) [6].

   HTTP/1.0 is also used for communication between user agents and
   various gateways, allowing hypermedia access to existing Internet
   protocols like SMTP [14], NNTP [12], FTP [16], Gopher [2], and
   WAIS [9]. HTTP/1.0 is designed to allow such gateways, via proxy
   servers, without any loss of the data conveyed by those earlier
   protocols.

1.2  Overall Operation

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

   On the Internet, the communication generally takes place over a
   TCP/IP connection. The default port is TCP 80 [17], but other ports
   can be used. This does not preclude the HTTP/1.0 protocol from
   being implemented on top of any other protocol on the Internet, or
   on other networks. The mapping of the HTTP/1.0 request and response
   structures onto the transport data units of the protocol in
   question is outside the scope of this specification.

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

1.3  Terminology

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

   connection

       A virtual circuit established between two parties for the
       purpose of communication.

   message

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

   request

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

   response

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

   resource

       A network data object or service which can be identified by a
       URI.

   entity

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

   client

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

   user agent

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

   server

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

   origin server

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

   proxy

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

   gateway

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

2.  Notational Conventions and Generic Grammar

2.1  Augmented BNF

   All of the mechanisms specified in this document are described in
   both prose and an augmented Backus-Naur Form (BNF) similar to that
   used by RFC 822 [8]. Implementors will need to be familiar with the
   notation in order to understand this specification. The augmented
   BNF includes the following constructs:

   name = definition

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

   "literal"

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

   rule1 | rule2

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

   (rule1 rule2)

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

   *rule

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

   [rule]

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

   N rule

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

   #rule

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

   ; comment

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

   implied *LWS

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

2.2  Basic Rules

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

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

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

       CRLF           = CR LF

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

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

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

       word           = token | quoted-string

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

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

   Comments can be included in HTTP header fields by surrounding the
   comment text with parentheses.

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

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

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

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

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

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

       quoted-pair    = "\" CHAR

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

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

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

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

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

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

3.  Protocol Parameters

3.1  HTTP Version

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

3.2  Uniform Resource Identifiers

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

3.2.1 General Syntax

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

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

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

       relativeURI    = net_path | abs_path | rel_path

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

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

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

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

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

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

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

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

3.2.2 http URL

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

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

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

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

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

3.3  Date/Time Formats

3.3.1 Full Date

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

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

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

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

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

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

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

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

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

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

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

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

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

       Note: HTTP/1.0 requirements for the date/time stamp format
       apply only to their usage within the protocol stream.
       Clients and servers are not required to use these formats
       for user presentation, request logging, etc.

3.3.2 Delta Seconds

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

       delta-seconds  = 1*DIGIT

3.4  Media Types

   HTTP uses Internet Media Types [15] (formerly referred to as MIME
   Content-Types [6]) in order to provide open and extensible data
   typing and type negotiation. For mail applications, where there is
   no type negotiation between sender and receiver, it is reasonable
   to put strict limits on the set of allowed media types. With HTTP,
   however, user agents can identify acceptable media types as part of
   the connection, and thus are allowed more freedom in the use of non-
   registered types. The following grammar for media types is a
   superset of that for MIME because it does not restrict itself to
   the official IANA and x-token types.

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

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

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

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

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

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

3.4.1 Canonicalization and Text Defaults

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

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

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

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

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

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

3.4.2 Multipart Types

   MIME provides for a number of "multipart" types -- encapsulations of
   several entities within a single message's Entity-Body. The
   multipart types registered by IANA [17] do not have any special
   meaning for HTTP/1.0, though user agents may need to understand
   each type in order to correctly interpret the purpose of each body-
   part. Ideally, an HTTP user agent should follow the same or similar
   behavior as a MIME user agent does upon receipt of a multipart type.

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

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

3.5  Character Set Encodings

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

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

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

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

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

3.6  Encoding Mechanisms

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

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

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

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

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

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

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

3.7  Transfer Encodings

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

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

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

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

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

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

3.8  Language Tags

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

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

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

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

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

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

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

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

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

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

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

3.9  Quality Values

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

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

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

3.10  Product Tokens

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

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

   Examples:

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

       Server: Apache/0.8.4

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

4.  HTTP Message

4.1  Message Types

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

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

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

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

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

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

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

       Simple-Response = [ Entity-Body ]

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

4.2  Message Headers

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

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

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

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

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

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

4.3  General Message Header Fields

   There are a few header fields which have general applicability for
   both request and response messages, but which do not apply to the
   communicating parties or the content being transferred. Although
   none of the General-Header fields are required, they are all
   strongly recommended where their use is appropriate, and should be
   understood by all future HTTP/1.0 clients and servers. These
   headers apply only to the message being transmitted.

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

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

5. Request

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

       Request        = Simple-Request | Full-Request

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

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

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

5.1  Request-Line

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

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

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

5.2  Method

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

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

       extension-method = token

   The list of methods acceptable by a specific resource can be
   specified in an "Allow" Entity-Header (Section 8.5). However, the
   client is always notified through the return code of the response
   whether a method is currently allowed on a specific resource, as
   this can change dynamically. Servers should return the status code
   "405 Method Not Allowed" if the method is known by the server but
   not allowed for the requested resource, and "501 Not Implemented"
   if the method is unknown or not implemented by the server.

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

   The set of common methods for HTTP/1.0 is described below. Although
   this set can be easily expanded, additional methods cannot be
   assumed to share the same semantics for separately extended clients
   and servers. In order to maintain compatibility, the semantic
   definition for extension methods should be registered with the
   IANA [17].

5.2.1 GET

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

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

5.2.2 HEAD

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

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

5.2.3 POST

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

      o Annotation of existing resources;

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

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

      o Extending a database through an append operation.

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

   The client can suggest a URI for identifying the new resource by
   including a URI-header field in the request. However, the server
   should treat that URI as advisory and may store the entity under a
   different URI or without any URI.

   The client may apply relationships between the new resource and
   other existing resources by including Link header fields, as
   described in Section 8.18. The server may use the Link information
   to perform other operations as a result of the new resource being
   added. For example, lists and indexes might be updated. However, no
   mandatory operation is imposed on the origin server. The origin
   server may also generate its own or additional links to other
   resources.

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

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

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

5.2.4 PUT

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

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

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

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

   The client can create or modify relationships between the enclosed
   entity and other existing resources by including Link header
   fields, as described in Section 8.18. As with POST, the server may
   use the Link information to perform other operations as a result of
   the request. However, no mandatory operation is imposed on the
   origin server. The origin server may generate its own or additional
   links to other resources.

   The actual method for determining how the resource is placed, and
   what happens to its predecessor, is defined entirely by the origin
   server. If version control is implemented by the origin server,
   then Link relationships should be defined by the server to help
   identify and control revisions to a resource; suggested
   relationship names include "Derived-From", "Obsoletes", and
   "Updates".

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

5.2.5 DELETE

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

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

5.2.6 LINK

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

5.2.7 UNLINK

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

5.3  Request-URI

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

       Request-URI    = "*" | absoluteURI | abs_path

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

       OPTIONS * HTTP/1.0

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

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

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

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

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

5.4  Request Header Fields

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

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

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

6.  Response

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

       Response        = Simple-Response | Full-Response

       Simple-Response = [ Entity-Body ]

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

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

6.1  Status-Line

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

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

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

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

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

6.2  Status Codes and Reason Phrases

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

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

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

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

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

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

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

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

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

       extension-code = 3DIGIT

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

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

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

6.2.1 Informational 1xx

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

6.2.2 Successful 2xx

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

   200 OK

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

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

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

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

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

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

   201 Created

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

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

   202 Accepted

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

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

   203 Non-Authoritative Information

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

   204 No Content

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

6.2.3 Redirection 3xx

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

   300 Multiple Choices

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

   301 Moved Permanently

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

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

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

   302 Moved Temporarily

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

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

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

   303 See Other

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

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

   304 Not Modified

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

6.2.4 Client Error 4xx

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

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

   400 Bad Request

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

   401 Unauthorized

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

   402 Payment Required

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

   403 Forbidden

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

   404 Not Found

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

   405 Method Not Allowed

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

   406 None Acceptable

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

   407 Proxy Authentication Required

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

   408 Request Timeout

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

   409 Conflict

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

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

   410 Gone

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

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

   411 Authorization Refused

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

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

6.2.5 Server Errors 5xx

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

   500 Internal Server Error

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

   501 Not Implemented

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

   502 Bad Gateway

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

   503 Service Unavailable

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

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

   504 Gateway Timeout

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

6.3  Response Header Fields

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

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

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

7.  Entity

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

7.1  Entity Header Fields

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

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

       extension-header=HTTP-header

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

7.2  Entity Body

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

       Entity-Body    = *OCTET

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

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

7.2.1 Type

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

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

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

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

7.2.2 Length

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

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

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

8.  Header Field Definitions

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

8.1  Accept

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

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

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

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

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

   The example

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

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

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

       Accept: */*; q=1

   or

       Accept: */*

   A more elaborate example is

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

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

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

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

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

   have the following precedence:

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

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

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

   would cause the following values to be associated:

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

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

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

8.2  Accept-Charset

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

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

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

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

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

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

8.3  Accept-Encoding

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

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

   An example of its use is

       Accept-Encoding: compress, gzip

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

8.4  Accept-Language

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

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

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

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

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

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

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

       Note: As intelligibility is highly dependent on the
       individual user, it is recommended that client applications
       make the choice of linguistic preference available to the
       user. If the choice is not made available, then the Accept-
       Language header field must not be given in the request.

8.5  Allow

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

       Allow          = "Allow" ":" #method

    Example of use:

       Allow: GET, HEAD, PUT

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

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

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

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

8.6  Authorization

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

       Authorization  = "Authorization" ":" 1#credentials

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

8.7  Content-Encoding

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

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

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

       Content-Encoding: gzip

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

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

8.8  Content-Language

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

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

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

       Content-Language: dk

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

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

       Content-Language: mi, en

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

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

8.9  Content-Length

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

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

   An example is

       Content-Length: 3495

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

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

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

8.10  Content-Transfer-Encoding

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

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

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

       Content-Transfer-Encoding: quoted-printable

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

8.11  Content-Type

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

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

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

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

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

8.12  Date

   The Date header represents the date and time at which the message
   was originated, having the same semantics as orig-date in RFC
   822.The field value is an HTTP-date, as described in Section 3.3.

       Date           = "Date" ":" HTTP-date

   An example is

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

   If a message is received via direct connection with the user agent
   (in the case of requests) or the origin server (in the case of
   responses), then the default date can be assumed to be the current
   date at the receiving end. However, since the date--as it is
   believed by the origin--is important for evaluating cached
   responses, origin servers should always include a Date header.
   Clients should only send a Date header field in messages that
   include an entity body, as in the case of the PUT and POST
   requests, and even then it is optional. A received message which
   does not have a Date header field should be assigned one by the
   receiver if and only if the message will be cached by that receiver
   or gatewayed via a protocol which requires a Date.

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

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

8.13  Expires

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

       Expires        = "Expires" ":" HTTP-date

   An example of its use is

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

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

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

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

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

8.14  Forwarded

   The Forwarded header is to be used by proxies to indicate the
   intermediate steps between the user agent and the server on
   requests, and between the origin server and the client on
   responses. It is analogous to the "Received" field of RFC 822 [8]
   and is intended to be used for tracing transport problems and
   avoiding request loops.

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

       FQDN           = <Fully-Qualified Domain Name>

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

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

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

8.15  From

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

       From           = "From" ":" mailbox

   An example is:

       From: webmaster@w3.org

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

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

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

8.16  If-Modified-Since

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

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

   An example of the field is:

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

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

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

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

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

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

       Note: The same functionality can be obtained, though with
       much greater overhead, by issuing a HEAD request and
       following it with a GET request if the server indicates that
       the entity has been modified.

8.17  Last-Modified

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

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

   An example of its use is

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

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

8.18  Link

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

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

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

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

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

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

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

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

8.19  Location

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

       Location       = "Location" ":" absoluteURI

   An example is

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

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

8.20  MIME-Version

   HTTP is not a MIME-conformant protocol (see Appendix C). However,
   HTTP/1.0 messages may include a single MIME-Version header field to
   indicate what version of the MIME protocol was used to construct
   the message. Use of the MIME-Version header field should indicate
   that the message is in full compliance with the MIME protocol (as
   defined in [6]). Unfortunately, current versions of HTTP/1.0
   clients and servers use this field indiscriminately, and thus
   receivers must not take it for granted that the message is indeed
   in full compliance with MIME. Gateways are responsible for ensuring
   this compliance (where possible) when exporting HTTP messages to
   strict MIME environments. Future HTTP/1.0 applications must only
   use MIME-Version when the message is intended to be MIME-conformant.

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

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

8.21  Orig-URI

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

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

   Example:

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

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

8.22  Pragma

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

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

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

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

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

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

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

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

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

8.23  Public

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

       Public         = "Public" ":" #method

   Example of use:

       Public: OPTIONS, MGET, MHEAD

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

8.24  Referer

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

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

   Example:

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

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

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

8.25  Retry-After

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

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

   Two examples of its use are

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

       Retry-After: 120

   In the latter example, the delay is 2 minutes.

8.26  Server

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

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

   Example:

       Server: CERN/3.0 libwww/2.17

   If the response is being forwarded through a proxy, the proxy
   application must not add its data to the product list. Instead, it
   should include a Forwarded field (as described in Section 8.14).

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

8.27  Title

   The Title header field indicates the title of the entity

       Title          = "Title" ":" *text

   An example of the field is

       Title: Hypertext Transfer Protocol -- HTTP/1.0

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

8.28  URI

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

8.29  User-Agent

   The User-Agent field contains information about the user agent
   originating the request. This is for statistical purposes, the
   tracing of protocol violations, and automated recognition of user
   agents for the sake of tailoring responses to avoid particular user
   agent limitations. Although it is not required, user agents should
   always include this field with requests. The field can contain
   multiple product tokens (Section 3.10) identifying the agent and
   any subproducts which form a significant part of the user agent.
   By convention, the product tokens are listed in order of their
   significance for identifying the application.

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

   Example:

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

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

       Note: Some current proxy applications append their product
       information to the list in the User-Agent field. This is no
       longer recommended, since it makes machine interpretation of
       these fields ambiguous. Instead, proxies should use the
       Forwarded header described in Section 8.14.

8.30  WWW-Authenticate

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

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

   The HTTP access authentication process is described in Section 10.

9.  Content Negotiation

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

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

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

   The following parameters are included in the calculation:

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

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

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

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

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

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

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

   The mapping function is defined as:

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

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

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

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

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

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

10.  Access Authentication

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

       auth-scheme    = "basic" | token

       auth-param     = token "=" quoted-string

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

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

       realm          = "realm" "=" quoted-string

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

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

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

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

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

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

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

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

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

10.1  Basic Authentication Scheme

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

       basic-challenge= "Basic" SP realm

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

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

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

       Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

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

11.  Security Considerations

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

11.1  Authentication of Clients

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

11.2  Idempotent Methods

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

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

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

11.3  Abuse of Server Log Information

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

11.4  Transfer of Sensitive Information

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

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

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

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

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

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

12.  Acknowledgments

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

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

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

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

13. References

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

14.  Authors' Addresses

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

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

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

Appendices

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

A.  Internet Media Type message/http

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

       Media Type name:         message

       Media subtype name:      http

       Required parameters:     none

       Optional parameters:     version, msgtype

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

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

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

       Security considerations: none

B.  Tolerant Applications

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

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

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

C.  Relationship to MIME

   HTTP/1.0 reuses many of the constructs defined for Internet Mail
   (RFC 822 [8]) and the Multipurpose Internet Mail Extensions
   (MIME [6]) to allow entities to be transmitted in an open variety
   of representations and with extensible mechanisms. However, HTTP is
   not a MIME-conforming application. HTTP's performance requirements
   differ substantially from those of Internet mail. Since it is not
   limited by the restrictions of existing mail protocols and
   gateways, HTTP does not obey some of the constraints imposed by
   RFC 822 and MIME for mail transport.

   This appendix describes specific areas where HTTP differs from
   MIME. Gateways to MIME-compliant protocols must be aware of these
   differences and provide the appropriate conversions where
   necessary. No conversion should be necessary for a MIME-conforming
   entity to be transferred using HTTP.

C.1  Conversion to Canonical Form

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

C.1.1 Representation of Line Breaks

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

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

C.1.2 Default Character Set Encoding

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

       ;charset="iso-8859-1"

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

C.2  Default Content-Transfer-Encoding

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

       Content-Transfer-Encoding: binary

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

C.3  Introduction of Content-Encoding

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

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


Html markup produced by rfcmarkup 1.129b, available from https://tools.ietf.org/tools/rfcmarkup/