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Versions: 00 01 02 03 04 05 06 07 08 RFC 2068

HTTP Working Group                                R. Fielding, UC Irvine
INTERNET-DRAFT                                       H. Frystyk, MIT/LCS
<draft-ietf-http-v11-spec-01.txt>                T. Berners-Lee, MIT/LCS
Expires in six months                                   January 19, 1996


               Hypertext Transfer Protocol -- HTTP/1.1


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
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   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.

   ================================================================
   NOTE: This specification is for discussion purposes only. It is not
   claimed to represent the consensus of the HTTP working group, and
   contains a number of proposals that either have not been discussed or
   are controversial.  The working group is discussing significant
   changes in many areas, including logic bags, support for caching,
   range retrieval, content negotiation, MIME compatibility,
   authentication, timing of the PUT operation.
   ================================================================

Abstract

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol 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 defines the protocol
   referred to as "HTTP/1.1".

Table of Contents

   1.  Introduction
       1.1  Purpose
       1.2  Requirements
       1.3  Terminology
       1.4  Overall Operation

   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  Character Sets
       3.5  Content Codings
       3.6  Transfer Codings
       3.7  Media Types
            3.7.1  Canonicalization and Text Defaults
            3.7.2  Multipart Types
       3.8  Product Tokens
       3.9  Quality Values
       3.10 Language Tags
       3.11 Logic Bags

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

   5.  Request
       5.1  Request-Line
            5.1.1  Method
            5.1.2  Request-URI
       5.2  Request Header Fields

   6.  Response
       6.1  Status-Line
            6.1.1  Status Code and Reason Phrase
       6.2  Response Header Fields

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

   8.  Method Definitions
       8.1  OPTIONS
       8.2  GET
       8.3  HEAD
       8.4  POST
       8.5  PUT
       8.6  PATCH
       8.7  COPY
       8.8  MOVE
       8.9  DELETE
       8.10 LINK
       8.11 UNLINK
       8.12 TRACE
       8.13 WRAPPED

   9.  Status Code Definitions
       9.1  Informational 1xx
       9.2  Successful 2xx
       9.3  Redirection 3xx
       9.4  Client Error 4xx
       9.5  Server Error 5xx

   10. Header Field Definitions
       10.1  Accept
       10.2  Accept-Charset
       10.3  Accept-Encoding
       10.4  Accept-Language
       10.5  Allow
       10.6  Authorization
       10.7  Base
       10.8  Cache-Control
       10.9  Connection
             10.9.1 Persistent Connections
       10.10 Content-Encoding
       10.11 Content-Language
       10.12 Content-Length
       10.13 Content-MD5
       10.14 Content-Range
       10.15 Content-Type
       10.16 Content-Version
       10.17 Date
       10.18 Derived-From
       10.19 Expires
       10.20 Forwarded
       10.21 From
       10.22 Host
       10.23 If-Modified-Since
       10.24 Keep-Alive
       10.25 Last-Modified
       10.26 Link
       10.27 Location
       10.28 MIME-Version
       10.29 Pragma
       10.30 Proxy-Authenticate
       10.31 Proxy-Authorization
       10.32 Public
       10.33 Range
       10.34 Referer
       10.35 Refresh
       10.36 Retry-After
       10.37 Server
       10.38 Title
       10.39 Transfer Encoding
       10.40 Unless
       10.41 Upgrade
       10.42 URI
       10.43 User-Agent
       10.44 WWW-Authenticate

   11. Access Authentication
       11.1 Basic Authentication Scheme
       11.2 Digest Authentication Scheme

   12. Content Negotiation
       12.1 Preemptive Negotiation

   13. Caching

   14. Security Considerations
       14.1 Authentication of Clients
       14.2 Safe Methods
       14.3 Abuse of Server Log Information
       14.4 Transfer of Sensitive Information

   15. Acknowledgments

   16. References

   17. 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
       C.2  Conversion of Date Formats
       C.3  Introduction of Content-Encoding
       C.4  No Content-Transfer-Encoding
       C.5  Introduction of Transfer-Encoding
   Appendix D.   Changes from HTTP/1.0



1.  Introduction

1.1  Purpose

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypermedia information
   systems. HTTP has been in use by the World-Wide Web global
   information initiative since 1990. The first version of HTTP,
   referred to as HTTP/0.9, was a simple protocol for raw data
   transfer across the Internet. HTTP/1.0, as defined by RFC xxxx [6],
   improved the protocol by allowing messages to be in the format of
   MIME-like entities, containing metainformation about the data
   transferred and modifiers on the request/response semantics.
   However, HTTP/1.0 does not sufficiently take into consideration the
   effect of hierarchical proxies and caching, the desire for
   persistent connections and virtual hosts, and a number of other
   details that slipped through the cracks of existing
   implementations. In addition, the proliferation of incompletely-
   implemented applications calling themselves "HTTP/1.0" has
   necessitated a protocol version change in order for two
   communicating applications to determine each other's true
   capabilities.

   This specification defines the protocol referred to as "HTTP/1.1".
   This protocol is backwards-compatible with HTTP/1.0, but includes
   more stringent requirements in order to ensure reliable
   implementation of its features.

   Practical information systems require more functionality than
   simple retrieval, including search, front-end update, and
   annotation. HTTP 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) [4] or name (URN) [20], 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 [9] and the
   Multipurpose Internet Mail Extensions (MIME) [7].

   HTTP is also used as a generic protocol for communication between
   user agents and proxies/gateways to other Internet protocols, such
   as SMTP [16], NNTP [13], FTP [18], Gopher [2], and WAIS [10],
   allowing basic hypermedia access to resources available from
   diverse applications and simplifying the implementation of user
   agents.

1.2  Requirements

   This specification uses the same words as RFC 1123 [8] for defining
   the significance of each particular requirement. These words are:

   must

       This word or the adjective "required" means that the item is an
       absolute requirement of the specification.

   should

       This word or the adjective "recommended" means that there may
       exist valid reasons in particular circumstances to ignore this
       item, but the full implications should be understood and the
       case carefully weighed before choosing a different course.

   may

       This word or the adjective "optional" means that this item is
       truly optional. One vendor may choose to include the item
       because a particular marketplace requires it or because it
       enhances the product, for example; another vendor may omit the
       same item.

   An implementation is not compliant if it fails to satisfy one or
   more of the must requirements for the protocols it implements. An
   implementation that satisfies all the must and all the should
   requirements for its protocols is said to be "unconditionally
   compliant"; one that satisfies all the must requirements but not
   all the should requirements for its protocols is said to be
   "conditionally compliant".

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 transport layer virtual circuit established between two
       application programs for the purpose of communication.

   message

       The basic unit of HTTP communication, consisting of a structured
       sequence of octets matching the syntax defined in Section 4 and
       transmitted via the connection.

   request

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

   response

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

   resource

       A network data object or service which can be identified by a
       URI (Section 3.2).

   entity

       A particular representation or rendition of a data resource, or
       reply from a service 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

       An application program that establishes connections for the
       purpose of sending requests.

   user agent

       The client which initiates a request. These are often browsers,
       editors, spiders (web-traversing robots), or other end user
       tools.

   server

       An application 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 making requests on behalf of other clients.
       Requests are serviced internally or by passing them, with
       possible translation, on to other servers. A proxy must
       interpret and, if necessary, rewrite a request message before
       forwarding it. Proxies are often used as client-side portals
       through network firewalls and as helper applications for
       handling requests via protocols not implemented by the user
       agent.

   gateway

       A server which acts as an intermediary for some other server.
       Unlike a proxy, a gateway receives requests as if it were the
       origin server for the requested resource; the requesting client
       may not be aware that it is communicating with a gateway.
       Gateways are often used as server-side portals through network
       firewalls and as protocol translators for access to resources
       stored on non-HTTP systems.

   tunnel

       A tunnel is an intermediary program which is acting as a blind
       relay between two connections. Once active, a tunnel is not
       considered a party to the HTTP communication, though the tunnel
       may have been initiated by an HTTP request. The tunnel ceases to
       exist when both ends of the relayed connections are closed.
       Tunnels are used when a portal is necessary and the intermediary
       cannot, or should not, interpret the relayed communication.

   cache

       A program's local store of response messages and the subsystem
       that controls its message storage, retrieval, and deletion. A
       cache stores cachable responses in order to reduce the response
       time and network bandwidth consumption on future, equivalent
       requests. Any client or server may include a cache, though a
       cache cannot be used by a server while it is acting as a tunnel.

   Any given program may be capable of being both a client and a
   server; our use of these terms refers only to the role being
   performed by the program for a particular connection, rather than
   to the program's capabilities in general. Likewise, any server may
   act as an origin server, proxy, gateway, or tunnel, switching
   behavior based on the nature of each request.

1.4  Overall Operation

   The HTTP protocol is based on a request/response paradigm. A client
   establishes a connection with 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 the message's protocol version and a
   success or error code, followed by a MIME-like message containing
   server information, entity metainformation, and possible body
   content.

   Most HTTP communication is initiated by a user agent and consists
   of a request to be applied to a resource on some origin server. In
   the simplest case, this may be accomplished via a single connection
   (v) between the user agent (UA) and the origin server (O).

          request chain ------------------------>
       UA -------------------v------------------- O
          <----------------------- response chain

   A more complicated situation occurs when one or more intermediaries
   are present in the request/response chain. There are three common
   forms of intermediary: proxy, gateway, and tunnel. A proxy is a
   forwarding agent, receiving requests for a URI in its absolute
   form, rewriting all or parts of the message, and forwarding the
   reformatted request toward the server identified by the URI. A
   gateway is a receiving agent, acting as a layer above some other
   server(s) and, if necessary, translating the requests to the
   underlying server's protocol. A tunnel acts as a relay point
   between two connections without changing the messages; tunnels are
   used when the communication needs to pass through an intermediary
   (such as a firewall) even when the intermediary cannot understand
   the contents of the messages.

          request chain -------------------------------------->
       UA -----v----- A -----v----- B -----v----- C -----v----- O
          <------------------------------------- response chain

   The figure above shows three intermediaries (A, B, and C) between
   the user agent and origin server. A request or response message
   that travels the whole chain must pass through four separate
   connections. This distinction is important because some HTTP
   communication options may apply only to the connection with the
   nearest, non-tunnel neighbor, only to the end-points of the chain,
   or to all connections along the chain. Although the diagram is
   linear, each participant may be engaged in multiple, simultaneous
   communications. For example, B may be receiving requests from many
   clients other than A, and/or forwarding requests to servers other
   than C, at the same time that it is handling A's request.

   Any party to the communication which is not acting as a tunnel may
   employ an internal cache for handling requests. The effect of a
   cache is that the request/response chain is shortened if one of the
   participants along the chain has a cached response applicable to
   that request. The following illustrates the resulting chain if B
   has a cached copy of an earlier response from O (via C) for a
   request which has not been cached by UA or A.

          request chain ---------->
       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
          <--------- response chain

   Not all responses are cachable, and some requests may contain
   modifiers which place special requirements on cache behavior. HTTP
   requirements for cache behavior and cachable responses are defined
   in Section 13.

   On the Internet, HTTP communication generally takes place over
   TCP/IP connections. The default port is TCP 80 [19], but other
   ports can be used. This does not preclude HTTP from being
   implemented on top of any other protocol on the Internet, or on
   other networks. HTTP only presumes a reliable transport; any
   protocol that provides such guarantees can be used, and the mapping
   of the HTTP/1.1 request and response structures onto the transport
   data units of the protocol in question is outside the scope of this
   specification.

   For most implementations, each connection is established by the
   client prior to the 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.

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 [9]. 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, 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. At
       least one delimiter (tspecials) must exist between any two
       tokens, since they would otherwise be interpreted as a single
       token. However, applications should attempt to follow "common
       form" when generating HTTP constructs, since there exist some
       implementations that fail to accept anything beyond the common
       forms.

2.2  Basic Rules

   The following rules are used throughout this specification to
   describe basic parsing constructs. The US-ASCII coded character set
   is defined by [21].

       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/1.1 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 3.7.

       CRLF           = CR LF

   HTTP/1.1 headers can be folded onto multiple lines if the
   continuation line begins with a space or horizontal tab. All linear
   whitespace, including folding, has the same semantics as SP.

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

   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 sets other
   than US-ASCII only when encoded according to the rules of
   RFC 1522 [14].

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

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

   Hexadecimal numeric characters are used in several protocol
   elements.

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

   Many HTTP/1.1 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 some HTTP header fields by surrounding
   the comment text with parentheses. Comments are only allowed in
   fields containing "comment" as part of their field value
   definition. In all other fields, parentheses are considered part of
   the field value.

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

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

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

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

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

       quoted-pair    = "\" CHAR

   Braces are used to delimit an attribute-value bag, which may
   consist of a set, list, or recursively defined tokens and quoted
   strings. The bag semantics are defined by its context and the bag
   name, which may be a Uniform Resource Identifier (Section 3.2) in
   some fields.

       bag            = "{" bagname 1*LWS *bagitem "}"
       bagname        = token | URI
       bagitem        = bag | token | quoted-string

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 [6].

       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.

   Applications sending Full-Request or Full-Response messages, as
   defined by this specification, must include an HTTP-Version of
   "HTTP/1.1". Use of this version number indicates that the sending
   application is at least conditionally compliant with this
   specification.

   HTTP/1.1 servers must:

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

      o understand any valid request in the format of HTTP/0.9, 1.0, or
        1.1;

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

   HTTP/1.1 clients must:

      o recognize the format of the Status-Line for HTTP/1.0 and 1.1
        responses;

      o understand any valid response in the format of HTTP/0.9, 1.0,
        or 1.1.

   Proxy and gateway applications must be careful in forwarding
   requests that are received in a format different than that of the
   application's native HTTP version. Since the protocol version
   indicates the protocol capability of the sender, a proxy/gateway
   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/gateway must either downgrade the request version,
   respond with an error, or switch to tunnel behavior. Requests with
   a version lower than that of the application's native format may be
   upgraded before being forwarded; the proxy/gateway's response to
   that request must follow the server requirements listed above.

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) [4] and
   Names (URN) [20]. 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 can be represented in absolute form or relative to
   some known base URI [11], 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
       reserved       = ";" | "/" | "?" | ":" | "@" | "&" | "="
       extra          = "!" | "*" | "'" | "(" | ")" | ","
       safe           = "$" | "-" | "_" | "." | "+"
       unsafe         = CTL | SP | <"> | "#" | "%" | "<" | ">"
       national       = <any OCTET excluding ALPHA, DIGIT,
                        reserved, extra, safe, and unsafe>

   For definitive information on URL syntax and semantics, see RFC
   1738 [4] and RFC 1808 [11]. The BNF above includes national
   characters not allowed in valid URLs as specified by RFC 1738,
   since HTTP servers are not restricted in the set of unreserved
   characters allowed to represent the rel_path part of addresses, and
   HTTP proxies may receive requests for URIs not defined by RFC 1738.

3.2.2 http URL

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

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

       host           = <A legal Internet host domain name
                         or IP address (in dotted-decimal form),
                         as defined by Section 2.1 of RFC 1123>

       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 for a resource (Section 5.1.2).

       Note: Although the HTTP protocol is independent of the
       transport layer protocol, the http URL only identifies
       resources by their TCP location, and thus non-TCP resources
       must be identified by some other URI scheme.

   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 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 [8]
   (an update to RFC 822 [9]). The second format is in common use, but
   is based on the obsolete RFC 850 [12] date format and lacks a
   four-digit year. HTTP/1.1 clients and servers that parse the date
   value must accept all three formats, though they must only generate
   the RFC 1123 format for representing date/time stamps in HTTP
   message fields.

       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 proxies/gateways to SMTP
       or NNTP.

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

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

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

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

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

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

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

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

       Note: HTTP 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  Character Sets

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

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

   HTTP character sets are identified by case-insensitive tokens. The
   complete set of tokens are defined by the IANA Character Set
   registry [19]. However, because that registry does not define a
   single, consistent token for each character set, we define here the
   preferred names for those character sets most likely to be used
   with HTTP entities. These character sets include those registered
   by RFC 1521 [7] -- the US-ASCII [21] and ISO-8859 [22] character
   sets -- and other names specifically recommended for use within MIME
   charset parameters.

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

   Although HTTP allows an arbitrary token to be used as a charset
   value, any token that has a predefined value within the IANA
   Character Set registry [19] must represent the character set
   defined by that registry. Applications should limit their use of
   character sets to those defined by the IANA registry.

       Note: This use of the term "character set" is more commonly
       referred to as a "character encoding." However, since HTTP
       and MIME share the same registry, it is important that the
       terminology also be shared.

3.5  Content Codings

   Content coding values are used to indicate an encoding
   transformation that has been or can be applied to a resource.
   Content codings 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.

       content-coding          = "gzip" | "compress" | token

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

   All content-coding values are case-insensitive. HTTP/1.1 uses
   content-coding values in the Accept-Encoding (Section 10.3) and
   Content-Encoding (Section 10.10) header fields. Although the value
   describes the content-coding, 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 content-coding 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.

3.6  Transfer Codings

   Transfer coding 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. This differs from a content coding in that the transfer
   coding is a property of the message, not of the original resource.

       transfer-coding         = "chunked" | token

   All transfer-coding values are case-insensitive. HTTP/1.1 uses
   transfer coding values in the Transfer-Encoding header field
   (Section 10.39).

   Transfer codings are analogous to the Content-Transfer-Encoding
   values of MIME [7], which were designed to enable safe transport of
   binary data over a 7-bit transport service. However, "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), or the desire to encrypt data over a shared
   transport.

   All HTTP/1.1 applications must be able to receive and decode the
   "chunked" transfer coding. The chunked encoding modifies the body
   of a message in order to transfer it as a series of chunks, each
   with its own size indicator, followed by an optional footer
   containing entity-header fields. This allows dynamically-produced
   content to be transferred along with the information necessary for
   the recipient to verify that it has received the full message.

       Chunked-Body   = *chunk
                        "0" CRLF
                        footer
                        CRLF

       chunk          = chunk-size CRLF
                        chunk-data CRLF

       chunk-size     = hex-no-zero *HEX
       chunk-data     = chunk-size(OCTET)

       footer         = *<Entity-Header, excluding Content-Length
                          and Transfer-Encoding>

       hex-no-zero    = <HEX excluding "0">

   Note that the chunks are ended by a zero-sized chunk, followed by
   the footer and terminated by an empty line. An example process for
   decoding a Chunked-Body is presented in Appendix C.5.

3.7  Media Types

   HTTP uses Internet Media Types [17] in the Content-Type
   (Section 10.15) and Accept (Section 10.1) header fields in order to
   provide open and extensible data typing and type negotiation. For
   mail applications, where there is no type negotiation between
   sender and recipient, it is reasonable to put strict limits on the
   set of allowed media types. With HTTP, where the sender and
   recipient can communicate directly, applications are allowed more
   freedom in the use of non-registered types. The following grammar
   for media types is a superset of that for MIME because it does not
   restrict itself to the official IANA and x-token types.

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

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

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

   The type, subtype, and parameter attribute names are
   case-insensitive. Parameter values may or may not be
   case-sensitive, depending on the semantics of the parameter name.
   LWS 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 [17].

   All media-type's registered by IANA must be preferred over
   extension tokens. However, HTTP does not limit 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.

3.7.1 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 Transfer-Encoding, the data must be
   in canonical form prior to that encoding. However, HTTP modifies
   the canonical form requirements for media of primary type "text"
   and for "application" types consisting of text-like records.

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

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

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

   HTTP also redefines the default character set for text media in an
   entity body. If a textual media type defines a charset parameter
   with a registered default value of "US-ASCII", HTTP changes the
   default to be "ISO-8859-1". Since the ISO-8859-1 [22] character set
   is a superset of US-ASCII [21], this does not affect the
   interpretation of entity bodies which only contain octets within
   the US-ASCII character set (0 - 127). The presence of a charset
   parameter value in a Content-Type header field overrides the
   default.

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

3.7.2 Multipart Types

   MIME provides for a number of "multipart" types -- encapsulations of
   one or more entities within a single message's Entity-Body. All
   multipart types share a common syntax, as defined in Section 7.2.1
   of RFC 1521 [7], 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, the epilogue of any multipart message
   must be empty; HTTP applications must not transmit the epilogue
   even if the original resource contains an epilogue.

   In HTTP, multipart body-parts may contain header fields which are
   significant to the meaning of that part. A URI entity-header field
   (Section 10.42) should be included in the body-part for each
   enclosed entity that can be identified by a URI.

   In general, an HTTP user agent should follow the same or similar
   behavior as a MIME user agent would upon receipt of a multipart
   type. The following subtypes have been defined:

   multipart/mixed

       The mixed subtype is used when there are no additional semantics
       implied beyond the fact that one or more entities are
       encaspsulated. HTTP servers should not use this type to send
       groups of entities if it is possible for those entities to be
       individually retrieved and cached.

   multipart/alternative

       The alternative subtype implies that each of the parts is an
       alternative format for the same information; the user agent
       should present only the part most preferred by the user. HTTP
       servers should use some form of content negotiation (Section 12)
       instead of this type.

   multipart/digest

       The digest subtype implies that each of the parts is a message
       (normally of type "message/rfc822") and thus the whole entity is
       a collected sequence of message traffic. This type does not have
       any special significance for HTTP.

   multipart/form-data

       The form-data subtype is defined by RFC 1867 [15] for use in
       submitting the data that comes about from filling-in a form.

   multipart/parallel

       The parallel subtype implies that the parts should be presented
       simultaneously by the user agent. This media type would be
       appropriate for situations where simultaneous presentation is an
       important aspect of the information, such as for audio-annotated
       slides.

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

   Other multipart subtypes may be registered by IANA [19] according
   to the procedures defined in RFC 1590 [17]. If an application
   receives an unrecognized multipart subtype, the application must
   treat it as being equivalent to "multipart/mixed".

3.8  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).

3.9  Quality Values

   HTTP content negotiation (Section 12) uses short "floating point"
   numbers to indicate the relative importance ("weight") of various
   negotiable parameters. The 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/1.1
   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  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.
   HTTP 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
   case-insensitive. The namespace of language tags is administered by
   the IANA. Example tags include:

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

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

   In the context of the Accept-Language header (Section 10.4), 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.

3.11  Logic Bags

   A logic bag is a binary logic expression tree represented in prefix
   notation using the generic bag syntax. Logic bags are used by HTTP
   in the Unless (Section 10.40) header field as expressions to be
   tested against the requested resource's header field
   metainformation.

       logic-bag   = "{" expression "}"

       expression  = ( log-op 1*logic-bag )
                   | ( rel-op 1*field-tuple )
                   | ( "def" 1*field-name )

       log-op      = "and" | "or" | "xor" | "not"
       rel-op      = "eq" | "ne" | "lt" | "le" | "ge" | "gt" | "in"

       field-tuple = "{" field-name ( bag | token | quoted-string ) "}"

   The recursive structure of a logic bag allows a complex expression
   tree to be formed by joining together subexpressions with logical
   operators. Expressions with relational operators are used to
   compare the requested resource's corresponding metainformation
   (header field values) to those inside the expression field-tuples.
   For example,

       {or {ne {Content-MD5 "Q2hlY2sgSW50ZWdyaXR5IQ=="}}
           {ne {Content-Length 10036}}
           {ne {Content-Version "12.4.8"}}
           {gt {Last-Modified "Mon, 04 Dec 1995 01:23:45 GMT"}}}

   The expression is evaluated recursively by depth-first traversal
   and bottom-up evaluation of the subexpressions until a true or
   false value can be determined. Multiple operands to an operator
   imply a conjunctive ("and") expression; e.g.,

       {eq {A "a"} {B "b"} {C "c"}}

   is equivalent to

       {and {eq {A "a"}} {eq {B "b"}} {eq {C "c"}}}

   Each expression is evaluated as defined by the operator:

   and   True if all of the operands evaluate true.

   or    True if any of the operands evaluate true.

   xor   True if one and only one operand evaluates true.

   not   True if all of the operands evaluate false.

   eq    True if all field-tuple values exactly match the resource's
         corresponding field values.

   ne    True if all field-tuple values do not match the resource's
         corresponding field values.

   lt    True if, for each field-tuple, the resource's corresponding
         field value is less than the one given in the expression.

   le    True if, for each field-tuple, the resource's corresponding
         field value is less than or equal to the one given in the
         expression.

   ge    True if, for each field-tuple, the resource's corresponding
         field value is greater than or equal to the one given in the
         expression.

   gt    True if, for each field-tuple, the resource's corresponding
         field value is greater than the one given in the expression.

   in    True if, for each field-tuple, the resource's corresponding
         field value contains the component value given in the
         expression.

   def   True if, for each field-name operand, the resource defines a
         value for that field.

   A field-tuple consists of a field-name (assumed to be an HTTP
   header field name, though not constrained to those defined by this
   specification) and the field-value component which is to be
   compared against the resource's field value. The actual method of
   comparison (e.g., byte equivalence, substring matching, numeric
   order, substructure containment, etc.) is defined by the logical
   definition of the operator and the type of field-value allowed for
   that field-name. Server implementors must use an appropriate
   comparison function for each type of field-value given in this
   specification. The default functions for unrecognized fields are
   numeric comparison (for values consisting of 1*DIGIT) and lexical
   comparison (for all others).

   Except for "ne", any comparison to a field not defined by the
   resource evaluates to false.

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/1.1 messages
                      | Full-Response

   Full-Request and Full-Response use the generic message format of
   RFC 822 [9] for transferring entities. Both messages may include
   optional header fields (also known as "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 5.2
                        |  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 6.2
                        |  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 5.2), Response-Header (Section 6.2), and
   Entity-Header (Section 7.1) fields, follow the same generic format
   as that given in Section 3.1 of RFC 822 [9]. Each header field
   consists of a name followed by a colon (":") and the field value.
   Field names are case-insensitive. 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 SP or HT.

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

       field-name     = token
       field-value    = *( field-content | LWS )

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

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

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

4.3  General 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
   entity being transferred. These headers apply only to the message
   being transmitted.

       General-Header = Cache-Control            ; Section 10.8
                      | Connection               ; Section 10.9
                      | Date                     ; Section 10.17
                      | Forwarded                ; Section 10.20
                      | Keep-Alive               ; Section 10.24
                      | MIME-Version             ; Section 10.28
                      | Pragma                   ; Section 10.29
                      | Upgrade                  ; Section 10.41

   General header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields may be given the semantics of general
   header fields if all parties in the communication recognize them to
   be general header fields. Unrecognized 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, 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 5.2
                        |  Entity-Header )        ; Section 7.1
                        CRLF
                        [ Entity-Body ]           ; Section 7.2

   If an HTTP/1.1 server receives a Simple-Request, it must respond
   with an HTTP/0.9 Simple-Response. An HTTP/1.1 client must 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.

5.1.1 Method

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

       Method         = "OPTIONS"                ; Section 8.1
                      | "GET"                    ; Section 8.2
                      | "HEAD"                   ; Section 8.3
                      | "POST"                   ; Section 8.4
                      | "PUT"                    ; Section 8.5
                      | "PATCH"                  ; Section 8.6
                      | "COPY"                   ; Section 8.7
                      | "MOVE"                   ; Section 8.8
                      | "DELETE"                 ; Section 8.9
                      | "LINK"                   ; Section 8.10
                      | "UNLINK"                 ; Section 8.11
                      | "TRACE"                  ; Section 8.12
                      | "WRAPPED"                ; Section 8.13
                      | extension-method

       extension-method = token

   The list of methods acceptable by a specific resource can be
   specified in an Allow header field (Section 10.5). 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 unrecognized or not implemented by the server. The
   list of methods known by a server can be listed in a Public
   response header field (Section 10.32).

   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. All other methods are
   optional; however, if the above methods are implemented, they must
   be implemented with the same semantics as those specified in
   Section 8.

5.1.2 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. One example would be

       OPTIONS * HTTP/1.1

   The absoluteURI form is only allowed when the request is being made
   to a proxy. The proxy is requested to forward the request and
   return the response. If the request is GET or HEAD and a prior
   response is cached, the proxy may use the cached message if it
   passes any restrictions in the Cache-Control and Expires header
   fields. Note that the proxy may forward the request on to another
   proxy or directly to the 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/pub/WWW/TheProject.html HTTP/1.1

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

       GET /pub/WWW/TheProject.html HTTP/1.1

   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).

   If a proxy receives a request without any path in the Request-URI
   and the method used is capable of supporting the asterisk form of
   request, then the last proxy on the request chain must forward the
   request with "*" as the final Request-URI. For example, the request

       OPTIONS http://www.ics.uci.edu:8001 HTTP/1.1

   would be forwarded by the proxy as

       OPTIONS * HTTP/1.1

   after connecting to port 8001 of host "www.ics.uci.edu".

   The Request-URI is transmitted as an encoded string, where some
   characters may be escaped using the "% hex hex" encoding defined by
   RFC 1738 [4]. The origin server must decode the Request-URI in
   order to properly interpret the request.

5.2  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. These fields act as request modifiers, with semantics
   equivalent to the parameters on a programming language method
   (procedure) invocation.

       Request-Header = Accept                   ; Section 10.1
                      | Accept-Charset           ; Section 10.2
                      | Accept-Encoding          ; Section 10.3
                      | Accept-Language          ; Section 10.4
                      | Authorization            ; Section 10.6
                      | From                     ; Section 10.21
                      | Host                     ; Section 10.22
                      | If-Modified-Since        ; Section 10.23
                      | Proxy-Authorization      ; Section 10.31
                      | Range                    ; Section 10.33
                      | Referer                  ; Section 10.34
                      | Unless                   ; Section 10.40
                      | User-Agent               ; Section 10.43

   Request-Header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields may be given the semantics of request
   header fields if all parties in the communication recognize them to
   be request header fields. Unrecognized 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 6.2
                         |  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/1.1 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/1.1 200 "), the presence of that expression is
   sufficient to differentiate a Full-Response from a Simple-Response.
   Although the Simple-Response format may allow such an expression to
   occur at the beginning of an entity body, and thus cause a
   misinterpretation of the message if it was given in response to a
   Full-Request, most HTTP/0.9 servers are limited to responses of
   type "text/html" and therefore would never generate such a response.

6.1.1 Status Code and Reason Phrase

   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 - Request received, continuing process

      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/1.1, 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. These codes are fully defined in Section 9.

       Status-Code    = "100"   ; Continue
                      | "101"   ; Switching Protocols
                      | "200"   ; OK
                      | "201"   ; Created
                      | "202"   ; Accepted
                      | "203"   ; Non-Authoritative Information
                      | "204"   ; No Content
                      | "205"   ; Reset Content
                      | "206"   ; Partial Content
                      | "300"   ; Multiple Choices
                      | "301"   ; Moved Permanently
                      | "302"   ; Moved Temporarily
                      | "303"   ; See Other
                      | "304"   ; Not Modified
                      | "305"   ; Use Proxy
                      | "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"   ; Length Required
                      | "412"   ; Unless True
                      | "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. 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 unrecognized response
   as being equivalent to the x00 status code of that class, with the
   exception that an unrecognized response must not be cached. For
   example, if an unrecognized status code of 431 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 should present to the user
   the entity returned with the response, since that entity is likely
   to include human-readable information which will explain the
   unusual status.

6.2  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 access to the resource or the server itself.

       Response-Header= Location                 ; Section 10.27
                      | Proxy-Authenticate       ; Section 10.30
                      | Public                   ; Section 10.32
                      | Retry-After              ; Section 10.36
                      | Server                   ; Section 10.37
                      | WWW-Authenticate         ; Section 10.44

   Response-Header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields may be given the semantics of response
   header fields if all parties in the communication recognize them to
   be response header fields. Unrecognized 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 10.5
                      | Content-Encoding         ; Section 10.10
                      | Content-Language         ; Section 10.11
                      | Content-Length           ; Section 10.12
                      | Content-MD5              ; Section 10.13
                      | Content-Range            ; Section 10.14
                      | Content-Type             ; Section 10.15
                      | Content-Version          ; Section 10.16
                      | Derived-From             ; Section 10.18
                      | Expires                  ; Section 10.19
                      | Last-Modified            ; Section 10.25
                      | Link                     ; Section 10.26
                      | Title                    ; Section 10.38
                      | Transfer-Encoding        ; Section 10.39
                      | URI-header               ; Section 10.42
                      | extension-header

       extension-header=HTTP-header

   The extension-header mechanism allows additional Entity-Header
   fields to be defined without changing the protocol, but these
   fields cannot be assumed to be recognizable by the recipient.
   Unrecognized 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 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. The presence of an entity body in a
   request is signaled by the inclusion of a Content-Length and/or
   Content-Type header field in the request message headers.

   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 entity header fields
   may lead one to believe they do. All 1xx (informational), 204 (no
   content), and 304 (not modified) responses must not include a body.
   All other responses must include an entity body or a Content-Length
   header field defined with a value of zero (0).

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 Transfer-Encoding. These define a
   three-layer, ordered encoding model:

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

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

   Any HTTP/1.1 message containing an entity body should include a
   Content-Type header field defining the media type of that body. If
   and only if the media type is not given by a Content-Type header,
   as is the case for Simple-Response messages, the recipient may
   attempt to guess the media type via inspection of its content
   and/or the name extension(s) of the URL used to identify the
   resource. If the media type remains unknown, the recipient 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
   Transfer-Encoding (if the "chunked" transfer coding has been
   applied), by the Content-Type (for multipart types with an explicit
   end-of-body delimiter), or by the server closing the connection.

       Note: Any response message which must not include an entity
       body (such as the 1xx, 204, and 304 responses and any
       response to a HEAD request) is always terminated by the
       first empty line after the header fields, regardless of the
       entity header fields present in the message.

   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. For compatibility with HTTP/1.0 applications,
   HTTP/1.1 requests containing an entity body must include a valid
   Content-Length header field unless the server is known to be
   HTTP/1.1 compliant. HTTP/1.1 servers must accept the "chunked"
   transfer coding (Section 3.6) and multipart media types
   (Section 3.7.2), thus allowing either mechanism to be used for a
   request when Content-Length is unknown.

   If a request contains an entity body and Content-Length is not
   specified, the server should respond with 400 (bad request) if it
   cannot determine the length of the request message's content, or
   with 411 (length required) if it wishes to insist on receiving a
   valid Content-Length.

   Messages must not include both a Content-Length header field and
   the "chunked" transfer coding. If both are received, the
   Content-Length must be ignored.

   When a Content-Length is given in a message where an entity body is
   allowed, its field value must exactly match the number of OCTETs in
   the entity body. HTTP/1.1 user agents must notify the user when an
   invalid length is received and detected.

8.  Method Definitions

   The set of common methods for HTTP/1.1 is defined below. Although
   this set can be expanded, additional methods cannot be assumed to
   share the same semantics for separately extended clients and
   servers.

   The semantics of all methods may be affected by the presence of an
   Unless request header field, as described in Section 10.40.

8.1  OPTIONS

   The OPTIONS method represents a request for information about the
   communication options available on the request/response chain
   identified by the Request-URI. This method allows the client to
   determine the options and/or requirements associated with a
   resource, or the capabilities of a server, without implying a
   resource action or initiating a resource retrieval.

   Unless the server's response is an error, the response must not
   include entity information other than what can be considered as
   communication options (e.g., Allow is appropriate, but Content-Type
   is not) and must include a Content-Length with a value of zero (0).
   Responses to this method are not cachable.

   If the Request-URI is an asterisk ("*"), the OPTIONS request is
   intended to apply to the server as a whole. A 200 response should
   include any header fields which indicate optional features
   implemented by the server (e.g., Public), including any extensions
   not defined by this specification, in addition to any applicable
   general or response header fields. As described in Section 5.1.2,
   an "OPTIONS *" request can be applied through a proxy by specifying
   the destination server in the Request-URI without any path
   information.

   If the Request-URI is not an asterisk, the OPTIONS request applies
   only to the options that are available when communicating with that
   resource. A 200 response should include any header fields which
   indicate optional features implemented by the server and applicable
   to that resource (e.g., Allow), including any extensions not
   defined by this specification, in addition to any applicable
   general or response header fields. If the OPTIONS request passes
   through a proxy, the proxy must edit the response to exclude those
   options known to be unavailable through that proxy.

8.2  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 change 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 10.23. The
   conditional GET method is intended to reduce unnecessary network
   usage by allowing cached entities to be refreshed without requiring
   multiple requests or transferring data already held by the client.

   The semantics of the GET method change to a "partial GET" if the
   request message includes a Range header field. A partial GET
   requests that only part of the identified resource be transferred,
   as described in Section 10.33. The partial GET method is intended
   to reduce unnecessary network usage by allowing partially-retrieved
   entities to be completed without transferring data already held by
   the client.

   The response to a GET request may be cachable if and only if it
   meets the requirements for HTTP caching described in Section 13.

8.3  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.

   The response to a HEAD request may be cachable in the sense that
   the information contained in the response may be used to update a
   previously cached entity from that resource. If the new field
   values indicate that the cached entity differs from the current
   resource (as would be indicated by a change in Content-Length,
   Content-MD5, or Content-Version), then the cache must discard the
   cached entity.

   There is no "conditional HEAD" or "partial HEAD" request analogous
   to those associated with the GET method. If an If-Modified-Since
   and/or Range header field is included with a HEAD request, they
   should be ignored.

8.4  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 [5], 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.

   HTTP/1.1 allows for a two-phase process to occur in accepting and
   processing a POST request. If the media type of the posted entity
   is not "application/x-www-form-urlencoded" [5], an HTTP/1.1 client
   must pause between sending the message header fields (including the
   empty line signifying the end of the headers) and sending the
   message body; the duration of the pause is five (5) seconds or
   until a response is received from the server, whichever is shorter.
   If no response is received during the pause period, or if the
   initial response is 100 (continue), the client may continue sending
   the POST request. If the response indicates an error, the client
   must discontinue the request and close the connection with the
   server after reading the response.

   Upon receipt of a POST request, the server must examine the header
   fields and determine whether or not the client should continue its
   request. If any of the header fields indicate the request is
   insufficient or unacceptable to the server (i.e., will result in a
   4xx or 5xx response), or if the server can determine the response
   without reading the entity body (e.g., a 301 or 302 response due to
   an old Request-URI), the server must send that response immediately
   upon its determination. If, on the other hand, the request appears
   (at least initially) to be acceptable and the client has indicated
   HTTP/1.1 compliance, the server must transmit an interim 100
   response message after receiving the empty line terminating the
   request headers and continue processing the request. After
   processing has finished, a final response message must be sent to
   indicate the actual result of the request. A 100 response should
   not be sent in response to an HTTP/1.0 request except under
   experimental conditions, since an HTTP/1.0 client may mistake the
   100 response for the final response.

   For compatibility with HTTP/1.0 applications, all POST requests
   must include a valid Content-Length header field unless the server
   is known to be HTTP/1.1 compliant. When sending a POST request to
   an HTTP/1.1 server, a client must use at least one of: a valid
   Content-Length, a multipart Content-Type, or the "chunked"
   Transfer-Encoding. The server should respond with a 400 (bad
   request) message if it cannot determine the length of the request
   message's content, or with 411 (length required) if it wishes to
   insist on receiving a valid Content-Length.

   The client can suggest one or more URIs for the new resource by
   including a URI header field in the request. However, the server
   should treat those URIs as advisory and may store the entity under
   a different URI, additional URIs, 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 10.26. 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 an entity (preferably of type
   "text/html") which describes the status of the request and refers
   to the new resource.

   Responses to this method are not cachable. However, the 303 (see
   other) response can be used to direct the user agent to retrieve a
   cachable resource.

8.5  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.

   If the request passes through a cache and the Request-URI
   identifies a currently cached entity, that entity must be removed
   from the cache. Responses to this method are not cachable.

   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.

   HTTP/1.1 allows for a two-phase process to occur in accepting and
   processing a PUT request. An HTTP/1.1 client must pause between
   sending the message header fields (including the empty line
   signifying the end of the headers) and sending the message body;
   the duration of the pause is five (5) seconds or until a response
   is received from the server, whichever is shorter. If no response
   is received during the pause period, or if the initial response is
   100 (continue), the client may continue sending the PUT request. If
   the response indicates an error, the client must discontinue the
   request and close the connection with the server after reading the
   response.

   Upon receipt of a PUT request, the server must examine the header
   fields and determine whether or not the client should continue its
   request. If any of the header fields indicate the request is
   insufficient or unacceptable to the server (i.e., will result in a
   4xx or 5xx response), or if the server can determine the response
   without reading the entity body (e.g., a 301 or 302 response due to
   an old Request-URI), the server must send that response immediately
   upon its determination. If, on the other hand, the request appears
   (at least initially) to be acceptable and the client has indicated
   HTTP/1.1 compliance, the server must transmit an interim 100
   response message after receiving the empty line terminating the
   request headers and continue processing the request. After
   processing has finished, a final response message must be sent to
   indicate the actual result of the request. A 100 response should
   not be sent in response to an HTTP/1.0 request except under
   experimental conditions, since an HTTP/1.0 client may mistake the
   100 response for the final response.

   For compatibility with HTTP/1.0 applications, all PUT requests must
   include a valid Content-Length header field unless the server is
   known to be HTTP/1.1 compliant. When sending a PUT request to an
   HTTP/1.1 server, a client must use at least one of: a valid
   Content-Length, a multipart Content-Type, or the "chunked"
   Transfer-Encoding. The server should respond with a 400 (bad
   request) message if it cannot determine the length of the request
   message's content, or with 411 (length required) if it wishes to
   insist on receiving a valid Content-Length.

   The client can create or modify relationships between the enclosed
   entity and other existing resources by including Link header
   fields, as described in Section 10.26. 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. If the entity being
   PUT was derived from an existing resource which included a
   Content-Version header field, the new entity must include a
   Derived-From header field corresponding to the value of the
   original Content-Version header field. Multiple Derived-From values
   may be included if the entity was derived from multiple resources
   with Content-Version information. Applications are encouraged to
   use these fields for constructing versioning relationships and
   resolving version conflicts.

8.6  PATCH

   The PATCH method is similar to PUT except that the entity contains
   a list of differences between the original version of the resource
   identified by the Request-URI and the desired content of the
   resource after the PATCH action has been applied. The list of
   differences is in a format defined by the media type of the entity
   (e.g., "application/diff") and must include sufficient information
   to allow the server to recreate the changes necessary to convert
   the original version of the resource to the desired version.

   If the request passes through a cache and the Request-URI
   identifies a currently cached entity, that entity must be removed
   from the cache. Responses to this method are not cachable.

   HTTP/1.1 allows for a two-phase process to occur in accepting and
   processing a PATCH request. An HTTP/1.1 client must pause between
   sending the message header fields (including the empty line
   signifying the end of the headers) and sending the message body;
   the duration of the pause is five (5) seconds or until a response
   is received from the server, whichever is shorter. If no response
   is received during the pause period, or if the initial response is
   100 (continue), the client may continue sending the PATCH request.
   If the response indicates an error, the client must discontinue the
   request and close the connection with the server after reading the
   response.

   Upon receipt of a PATCH request, the server must examine the header
   fields and determine whether or not the client should continue its
   request. If any of the header fields indicate the request is
   insufficient or unacceptable to the server (i.e., will result in a
   4xx or 5xx response), or if the server can determine the response
   without reading the entity body (e.g., a 301 or 302 response due to
   an old Request-URI), the server must send that response immediately
   upon its determination. If, on the other hand, the request appears
   (at least initially) to be acceptable and the client has indicated
   HTTP/1.1 compliance, the server must transmit an interim 100
   response message after receiving the empty line terminating the
   request headers and continue processing the request. After
   processing has finished, a final response message must be sent to
   indicate the actual result of the request. A 100 response should
   not be sent in response to an HTTP/1.0 request except under
   experimental conditions, since an HTTP/1.0 client may mistake the
   100 response for the final response.

   For compatibility with HTTP/1.0 applications, all PATCH requests
   must include a valid Content-Length header field unless the server
   is known to be HTTP/1.1 compliant. When sending a PATCH request to
   an HTTP/1.1 server, a client must use at least one of: a valid
   Content-Length, a multipart Content-Type, or the "chunked"
   Transfer-Encoding. The server should respond with a 400 (bad
   request) message if it cannot determine the length of the request
   message's content, or with 411 (length required) if it wishes to
   insist on receiving a valid Content-Length.

   The client can create or modify relationships between the new
   resource and other existing resources by including Link header
   fields, as described in Section 10.26. 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 patched 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. If the original
   version of the resource being patched included a Content-Version
   header field, the request entity must include a Derived-From header
   field corresponding to the value of the original Content-Version
   header field. Applications are encouraged to use these fields for
   constructing versioning relationships and resolving version
   conflicts.

8.7  COPY

   The COPY method requests that the resource identified by the
   Request-URI be copied to the location(s) given in the URI header
   field of the request. Responses to this method are not cachable.

8.8  MOVE

   The MOVE method requests that the resource identified by the
   Request-URI be moved to the location(s) given in the URI header
   field of the request. This method is equivalent to a COPY
   immediately followed by a DELETE, but enables both to occur within
   a single transaction.

   If the request passes through a cache and the Request-URI
   identifies a currently cached entity, that entity must be removed
   from the cache. Responses to this method are not cachable.

8.9  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.

   If the request passes through a cache and the Request-URI
   identifies a currently cached entity, that entity must be removed
   from the cache. Responses to this method are not cachable.

8.10  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 directly result in the creation of new resources.

   If the request passes through a cache and the Request-URI
   identifies a currently cached entity, that entity must be removed
   from the cache. Responses to this method are not cachable.

8.11  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.

   If the request passes through a cache and the Request-URI
   identifies a currently cached entity, that entity must be removed
   from the cache. Responses to this method are not cachable.

8.12  TRACE

   The TRACE method requests that the server identified by the
   Request-URI reflect whatever is received back to the client as the
   entity body of the response. In this way, the client can see what
   is being received at the other end of the request chain, and may
   use this data for testing or diagnostic information.

   If successful, the response should contain the entire, unedited
   request message in the entity body, with a Content-Type of
   "message/http", "application/http", or "text/plain". Responses to
   this method are not cachable.

8.13  WRAPPED

   The WRAPPED method allows a client to send one or more encapsulated
   requests to the server identified by the Request-URI. This method
   is intended to allow the request(s) to be wrapped together,
   possibly encrypted in order to improve the security and/or privacy
   of the request, and delivered for unwrapping by the destination
   server. Upon receipt of the WRAPPED request, the destination server
   must unwrap the message and feed it to the appropriate protocol
   handler as if it were an incoming request stream.

   Responses to this method are not cachable. Applications should not
   use this method for making requests that would normally be public
   and cachable.

   The request entity must include at least one encapsulated message,
   with the media type identifying the protocol of that message. For
   example, if the wrapped request is another HTTP request message,
   then the media type must be either "message/http" (for a single
   message) or "application/http" (for a request stream containing one
   or more requests), with any codings identied by the
   Content-Encoding and Transfer-Encoding header fields.

   HTTP/1.1 allows for a two-phase process to occur in accepting and
   processing a WRAPPED request. An HTTP/1.1 client must pause between
   sending the message header fields (including the empty line
   signifying the end of the headers) and sending the message body;
   the duration of the pause is five (5) seconds or until a response
   is received from the server, whichever is shorter. If no response
   is received during the pause period, or if the initial response is
   100 (continue), the client may continue sending the WRAPPED
   request. If the response indicates an error, the client must
   discontinue the request and close the connection with the server
   after reading the response.

   Upon receipt of a WRAPPED request, the server must examine the
   header fields and determine whether or not the client should
   continue its request. If any of the header fields indicate the
   request is insufficient or unacceptable to the server (i.e., will
   result in a 4xx or 5xx response), or if the server can determine
   the response without reading the entity body (e.g., a 301 or 302
   response due to an old Request-URI), the server must send that
   response immediately upon its determination. If, on the other hand,
   the request appears (at least initially) to be acceptable and the
   client has indicated HTTP/1.1 compliance, the server must transmit
   an interim 100 response message after receiving the empty line
   terminating the request headers and continue processing the
   request. After processing has finished, a final response message
   must be sent to indicate the actual result of the request. A 100
   response should not be sent in response to an HTTP/1.0 request
   except under experimental conditions, since an HTTP/1.0 client may
   mistake the 100 response for the final response.

   For compatibility with HTTP/1.0 applications, all WRAPPED requests
   must include a valid Content-Length header field unless the server
   is known to be HTTP/1.1 compliant. When sending a WRAPPED request
   to an HTTP/1.1 server, a client must use at least one of: a valid
   Content-Length, a multipart Content-Type, or the "chunked"
   Transfer-Encoding. The server should respond with a 400 (bad
   request) message if it cannot determine the length of the request
   message's content, or with 411 (length required) if it wishes to
   insist on receiving a valid Content-Length.

9.  Status Code Definitions

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

9.1  Informational 1xx

   This class of status code indicates a provisional response,
   consisting only of the Status-Line and optional headers, and is
   terminated by an empty line. Since HTTP/1.0 did not define any 1xx
   status codes, servers should not send a 1xx response to an HTTP/1.0
   client except under experimental conditions.

   100 Continue

   The client may continue with its request. This interim response is
   used to inform the client that the initial part of the request has
   been received and has not yet been rejected by the server. The
   client should continue by sending the remainder of the request or,
   if the request has already been completed, ignore this response.
   The server must send a final response after the request has been
   completed.

   101 Switching Protocols

   The server understands and is willing to comply with the client's
   request, via the Upgrade message header field (Section 10.41), for
   a change in the application protocol being used on this connection.
   The server will switch protocols to those defined by the response's
   Upgrade header field immediately after the empty line which
   terminates the 101 response.

   The protocol should only be switched when it is advantageous to do
   so. For example, switching to a newer version of HTTP is
   advantageous over older versions, and switching to a real-time,
   synchronous protocol may be advantageous when delivering resources
   that use such features.

9.2  Successful 2xx

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

   200 OK

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

   GET    an entity corresponding to the requested resource is 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;

   TRACE  an entity containing the request message as received by the
          end server;

   otherwise, 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 and/or the entity of the response,
   with the most specific URL for the resource given by a Location
   header field. The origin server should create the resource before
   using this Status-Code. If the action cannot be carried out
   immediately, the server must include in the response body a
   description of when the resource will be available; otherwise, the
   server should respond with 202 (accepted).

   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 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.

   The 204 response must not include an entity body, and thus is
   always ternminated by the first empty line after the header fields.

   205 Reset Content

   The server has fulfilled the request and the user agent should
   reset the document view which caused the request to be generated.
   This response is primarily intended to allow input for actions to
   take place via user input, followed by a clearing of the form in
   which the input is given so that the user can easily initiate
   another input action. The response must include a Content-Length
   with a value of zero (0) and no entity body.

   206 Partial Content

   The server has fulfilled the partial GET request for the resource.
   The request must have included a Range header field (Section 10.33)
   indicating the desired range. The response must include a
   Content-Range header field (Section 10.14) indicating the range
   included with this response. All entity header fields in the
   response must describe the actual entity transmitted rather than
   what would have been transmitted in a full response. In particular,
   the Content-Length header field in the response must match the
   actual number of OCTETs transmitted in the entity body. It is
   assumed that the client already has the complete entity's header
   field data.

9.3  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 may be carried out by the user agent without interaction
   with the user if and only if the method used in the second request
   is 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 preemptive content
   negotiation (Section 12). 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 the URL in a
   Location field; user agents not capable of complex selection may
   use this field value for automatic redirection. This response is
   cachable unless indicated otherwise.

   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 should
   automatically relink references to the Request-URI to one or more
   of the new references returned by the server, where possible. This
   response is cachable unless indicated otherwise.

   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. Unless it
   was a HEAD request, 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 other
   than GET or HEAD, 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. This response
   is only cachable if indicated by a Cache-Control or Expires header
   field.

   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. Unless it
   was a HEAD request, 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 other
   than GET or HEAD, 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 response to the request can be found under a different URI and
   should be retrieved 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. The 303 response is not cachable, but the response to
   the second request may be cachable.

   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. Unless it
   was a HEAD request, 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 must
   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 or which may have
   changed independently of the entity's Last-Modified date. Examples
   of relevant header fields include: Date, Server, Content-Length,
   Content-MD5, Content-Version, Cache-Control and Expires.

   A cache should update its cached entity to reflect any new field
   values given in the 304 response. If the new field values indicate
   that the cached entity differs from the current resource (as would
   be indicated by a change in Content-Length, Content-MD5, or
   Content-Version), then the cache must disregard the 304 response
   and repeat the request without an If-Modified-Since field.

   The 304 response must not include an entity body, and thus is
   always ternminated by the first empty line after the header fields.

   305 Use Proxy

   The requested resource must be accessed through the proxy given by
   the Location field in the response. In other words, this is a proxy
   redirect.

9.4  Client Error 4xx

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

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

   400 Bad Request

   The request could not be understood by the server due to malformed
   syntax. The client should not repeat the request without
   modifications.

   401 Unauthorized

   The request requires user authentication. The response must include
   a WWW-Authenticate header field (Section 10.44) containing a
   challenge applicable to the requested resource. The client may
   repeat the request with a suitable Authorization header field
   (Section 10.6). If the request already included Authorization
   credentials, then the 401 response indicates that authorization has
   been refused for those credentials. If the 401 response contains
   the same challenge as the prior response, and the user agent has
   already attempted authentication at least once, then the user
   should be presented the entity that was given in the response,
   since that entity may include relevant diagnostic information. HTTP
   access authentication is explained in Section 11.

   402 Payment Required

   This code is reserved for future use.

   403 Forbidden

   The server understood the request, but is refusing to fulfill it.
   Authorization will not help and the request should not be repeated.
   If the request method was not HEAD and the server wishes to make
   public why the request has not been fulfilled, it should describe
   the reason for the refusal in the entity body. This status code is
   commonly used when the server does not wish to reveal exactly why
   the request has been refused, or when no other response is
   applicable.

   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 methods 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 similar to 401 (unauthorized), but indicates that the
   client must first authenticate itself with the proxy. The proxy
   must return a Proxy-Authenticate header field (Section 10.30)
   containing a challenge applicable to the proxy for the requested
   resource. The client may repeat the request with a suitable
   Proxy-Authorization header field (Section 10.31). HTTP access
   authentication is explained in Section 11.

   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 or PATCH
   request. If versioning is being used and the entity being PUT or
   PATCHed 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 request. In
   this case, the response entity should contain a list of the
   differences between the two versions in a format defined by the
   response Content-Type.

   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 should 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. This response is cachable unless indicated otherwise.

   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 Length Required

   The server refuses to accept the request without a defined
   Content-Length. The client may repeat the request if it adds a
   valid Content-Length header field containing the length of the
   entity body in the request message.

   412 Unless True

   The condition given in the Unless request-header field
   (Section 10.40) evaluated to true when it was tested on the server
   and the request did not include a Range header field (which would
   indicate a partial GET) or an If-Modified-Since header field (which
   would indicate a conditional GET). This response code allows the
   client to place arbitrary preconditions on the current resource
   metainformation (header field data) and thus prevent the requested
   method from being applied to a resource other than the one intended.

9.5  Server Error 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 should 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, while acting as a gateway or proxy, received an invalid
   response from the 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, while acting as a gateway or proxy, did not receive a
   timely response from the upstream server it accessed in attempting
   to complete the request.

10.  Header Field Definitions

   This section defines the syntax and semantics of all standard
   HTTP/1.1 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.

10.1  Accept

   The Accept response-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 12 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: */*

   If a single Accept header is provided and it contains no field
   value, then the server must interpret it as a request to not
   perform any preemptive content negotiation (Section 12) and instead
   return a 406 (none acceptable) response if there are variants
   available for the Request-URI.

   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
   text/x-dvi entity if it is less than 100000 bytes, otherwise send
   the text/plain entity."

   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 "*/*", or an empty value if the
   user desires reactive negotiation.

       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 closed system which cannot interact with
       other rendering agents, this default set should be
       configurable by the user.

10.2  Accept-Charset

   The Accept-Charset request-header field can be used to indicate
   what character sets are acceptable for the response. This field
   allows clients capable of understanding more comprehensive or
   special-purpose character sets to signal that capability to a
   server which is capable of representing documents in those
   character sets. The US-ASCII character set can be assumed to be
   acceptable to all user agents.

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

   Character set values are described in Section 3.4. An example is

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

   If no Accept-Charset field is given, the default is that any
   character set is acceptable. If the Accept-Charset field is given
   and the requested resource is not available in one of the listed
   character sets, then the server should respond with the 406 (none
   acceptable) status code.

10.3  Accept-Encoding

   The Accept-Encoding request-header field is similar to Accept, but
   restricts the content-coding values (Section 3.5) which are
   acceptable in the response.

       Accept-Encoding         = "Accept-Encoding" ":"
                                 #( content-coding )

   An example of its use is

       Accept-Encoding: compress, gzip

   If no Accept-Encoding field is present in a request, the server may
   assume that the client will accept any content coding. If an
   Accept-Encoding field is present, but contains an empty field
   value, then the user agent is refusing to accept any content coding.

10.4  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" ":"
                         1#( language-tag [ ";" "q" "=" qvalue ] )

   The language-tag is described in Section 3.10. 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 "q=1" (100% comprehension) for listed languages. This
   value may be used in the server's content negotiation algorithm
   (Section 12). For example,

       Accept-Language: da, en-gb;q=0.8, de;q=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 assigning a
   quality value of "q=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.

10.5  Allow

   The Allow entity-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" ":" 1#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 header field value
   should be followed. The actual 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 field 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 may use the Public
   response header field (Section 10.32) to describe what methods are
   implemented on the server as a whole.

10.6  Authorization

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

       Authorization  = "Authorization" ":" credentials

   HTTP access authentication is described in Section 11. If a request
   is authenticated and a realm specified, the same credentials should
   be valid for all other requests within this realm.

   Responses to requests containing an Authorization field are not
   cachable.

10.7  Base

   The Base entity-header field may be used to specify the base URI
   for resolving relative URLs, as described in RFC 1808 [11].

10.8  Cache-Control

   The Cache-Control general-header field is used to specify
   directives that must be obeyed by all caching mechanisms along the
   request/response chain. The directives specify behavior intended to
   prevent caches from adversely interfering with the request or
   response. Cache directives are unidirectional in that the presence
   of a directive in a request does not imply that the same directive
   should be given in the response.

       Cache-Control   = "Cache-Control" ":" 1#cache-directive

       cache-directive = "cachable"
                       | "max-age" "=" delta-seconds
                       | "private" [ "=" <"> 1#field-name <"> ]
                       | "no-cache" [ "=" <"> 1#field-name <"> ]

   The Cache-Control header field may be used to modify the optional
   behavior of caching mechanisms, and the default cachability of a
   response message; it cannot be used to modify the required behavior
   of caching mechanisms. HTTP requirements for caching and cachable
   messages are described in Section 13.

   The "cachable" directive indicates that the entire response message
   is cachable unless required otherwise by HTTP restrictions on the
   request method and response code. In other words, this directive
   indicates that the server believes the response to be cachable.
   This directive applies only to responses and must not be used with
   any other cache directive.

   When the "max-age" directive is present in a request message, an
   application must 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 Last-Modified value in the If-Modified-Since field. The
   Unless header field may be used to add further restrictions to the
   modification test on the server. 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, an application must 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 the max-age directive. If both
   the new request and the cached message have max-age specified, then
   the lesser of the two values must be used. A max-age value of zero
   (0) forces a cache to perform a refresh (If-Modified-Since) on
   every request. The max-age directive on a response implies that the
   server believes it to be cachable.

   The "private" directive indicates that parts of the response
   message are intended for a single user and must not be cached
   except within a private (non-shared) cache controlled by the user
   agent. If no list of field names is given, then the entire message
   is private; otherwise, only the information within the header
   fields identified by the list of names is private and the remainder
   of the message is believed to be cachable by any application. This
   allows an origin server to state that the specified parts of the
   message are intended for only one user and are not a valid response
   for requests by other agents. The "private" directive is only
   applicable to responses and must not be generated by clients.

       Note: This usage of the word "private" implies only that the
       message must not be cached publically; it does not ensure
       the privacy of the message content.

   The "no-cache" directive on a request message requires any cache to
   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. This is equivalent to the "no-cache"
   pragma-directive in Section 10.29. The list of field names is not
   used with requests and must not be generated by clients. The
   no-cache directive overrides any max-age directive.

   The "no-cache" directive on a response message indicates that parts
   of the message must never be cached. If no list of field names is
   given, then the entire message must not be cached; otherwise, only
   the information within the header fields identified by the list of
   names must not be cached and the remainder of the message is
   believed to be cachable. This allows an origin server to state that
   the specified parts of the message are intended for only one
   recipient and must not be stored unless the user explicitly
   requests it through a separate action.

   The max-age, private, and no-cache directives may be used in
   combination to define the cachability of each part of the message.
   In all cases, no-cache takes precedence over private, which in turn
   takes precedence over max-age.

   Cache directives must be passed through by a proxy or gateway
   application, regardless of their significance to that application,
   since the directives may be applicable to all recipients along the
   request/response chain. It is not possible to specify a
   cache-directive for a specific cache.

10.9  Connection

   The Connection general-header field is used to indicate a list of
   keywords and header field names containing information which is
   only applicable to the current connection between the sender and
   the nearest non-tunnel recipient on the request/response chain.
   This information must not be forwarded or cached. Unlike the
   default behavior, the recipient cannot safely ignore the semantics
   of the listed field-names if they are not understood, since
   forwarding them may imply that understanding.

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

   Proxies and gateways must discard the named header fields, and the
   Connection header itself, before forwarding the message. Proxies
   and gateways may add their own Connection information to forwarded
   messages if such options are desired for the forwarding connection.
   These restrictions do not apply to a tunnel, since the tunnel is
   acting as a relay between two connections and does not affect the
   connection options.

   Whether or not the listed field-name(s) occur as header fields in
   the message is optional. If no corresponding header field is
   present, then the field name is treated as a keyword. Keywords are
   useful for indicating a desired option without assigning parameters
   to that option. This allows for a minimal syntax to provide
   connection-based options without pre-restricting the syntax or
   number of those options. HTTP/1.1 only defines the "keep-alive"
   keyword.

   The semantics of Connection are defined by HTTP/1.1 in order to
   provide a safe transition to connection-based features. Connection
   header fields received in an HTTP/1.0 message, as would be the case
   if an older proxy mistakenly forwards the field, cannot be trusted
   and must be discarded except under experimental conditions.

10.9.1 Persistent Connections

   The "keep-alive" keyword in a Connection header field allows the
   sender to indicate its desire for a persistent connection (i.e., a
   connection that lasts beyond the current request/response
   transaction). Persistent connections allow the client to perform
   multiple requests without the overhead of connection tear-down and
   set-up between each request.

   As an example, a client would send

       Connection: Keep-Alive

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

       Connection: Keep-Alive

   to indicate that the connection will be kept open for the next
   request. The Connection header field with a keep-alive keyword must
   be sent on all requests and responses that wish to continue the
   persistence. The client sends requests as normal and the server
   responds as normal, except that all messages containing an entity
   body must have a length that can be determined without closing the
   connection (i.e., each message containg an entity body must have a
   valid Content-Length, be a multipart media type, or be encoded
   using the "chunked" transfer coding, as described in Section 7.2.2).

   The Keep-Alive header field (Section 10.24) may be used to include
   diagnostic information and other optional parameters. For example,
   the server may responds with

       Connection: Keep-Alive
       Keep-Alive: timeout=10, max=5

   to indicate that the server has selected (perhaps dynamically) a
   maximum of 5 requests, but will timeout if the next request is not
   received within 10 seconds. Note, however, that this additional
   information is optional and the Keep-Alive header field does not
   need to be present. If it is present, the semantics of the
   Connection header field prevents it from being accidentally
   forwarded to downstream connections.

   The persistent connection ends when either side closes the
   connection or after the receipt of a response which lacks the
   "keep-alive" keyword. The server may close the connection
   immediately after responding to a request without a "keep-alive"
   keyword. A client can tell if the connection will be closed by
   looking for a "keep-alive" in the response.

10.10  Content-Encoding

   The Content-Encoding entity-header field is used as a modifier to
   the media-type. When present, its value indicates what additional
   content codings 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.
   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#content-coding

   Content codings are defined in Section 3.5. 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 content
   codings 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.

10.11  Content-Language

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

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

   Language tags are defined in Section 3.10. 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.

10.12  Content-Length

   The Content-Length entity-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

   Applications should use this field to indicate the size of the
   Entity-Body to be transferred, regardless of the media type of the
   entity. A valid Content-Length field value is required on all
   HTTP/1.1 request messages containing an entity body.

   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.

10.13  Content-MD5

   TBS

10.14  Content-Range

   TBS

10.15  Content-Type

   The Content-Type entity-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 3.7. An example of the field is

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

   Further discussion of methods for identifying the media type of an
   entity is provided in Section 7.2.1.

10.16  Content-Version

   The Content-Version entity-header field defines the version tag
   associated with a rendition of an evolving entity. Together with
   the Derived-From field described in Section 10.18, it allows a
   group of people to work simultaneously on the creation of a work as
   an iterative process. The field should be used to allow evolution
   of a particular work along a single path. It should not be used to
   indicate derived works or renditions in different representations.
   It may also me used as an opaque value for comparing a cached
   entity's version with that of the current resource.

       Content-Version= "Content-Version" ":" quoted-string

   Examples of the Content-Version field include:

       Content-Version: "2.1.2"

       Content-Version: "Fred 19950116-12:26:48"

       Content-Version: "2.5a4-omega7"

   The value of the Content-Version field should be considered opaque
   to all parties but the origin server. A user agent may suggest a
   value for the version of an entity transferred via a PUT request;
   however, only the origin server can reliably assign that value.

10.17  Date

   The Date general-header field 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 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 recipient if the message will
   be cached by that recipient or gatewayed via a protocol which
   requires a Date.

   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.

10.18  Derived-From

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

       Derived-From   = "Derived-From" ":" quoted-string

   An example use of the field is:

       Derived-From: "2.1.1"

   The Derived-From field is required for PUT and PATCH requests if
   the entity being sent was previously retrieved from the same URI
   and a Content-Version header was included with the entity when it
   was last retrieved.

10.19  Expires

   The Expires entity-header field gives the date/time after which the
   entity should be considered stale. This allows information
   providers to suggest the volatility of the resource, or a date
   after which the information may no longer be valid. Applications
   must not cache this entity beyond the date given. 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 should 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

   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, entities from 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. By default, 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, unless the user has specifically configured the agent to
   refresh expired history documents.

       Note: Applications are encouraged to be tolerant of bad or
       misinformed implementations of the Expires header. A value
       of zero (0) or an invalid date format should be considered
       equivalent to an "expires immediately." Although these
       values are not legitimate for HTTP/1.1, a robust
       implementation is always desirable.

10.20  Forwarded

   The Forwarded general-header field is to be used by gateways and
   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 [9] 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/gateway that has forwarded the message. It is strongly
   recommended that proxies/gateways 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.

10.21  From

   The From request-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 [9] (as updated by RFC 1123 [8]):

       From           = "From" ":" mailbox

   An example is:

       From: webmaster@w3.org

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

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

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

10.22  Host

   The Host request-header field allows the client to specify, for the
   server's benefit, the Internet host given by 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). This field is required on all HTTP/1.1 requests which
   do not already include the host in the Request-URI.

       Host           = "Host" ":" host          ; Section 3.2.2

   Example:

       Host: www.w3.org

   The contents of the Host header field should exactly match the host
   information used to contact the origin server or gateway in
   question. It must not include the trailing ":port" information
   which may also be found in the net_loc portion of a URL
   (Section 3.2).

10.23  If-Modified-Since

   The If-Modified-Since request-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. A date which is later than the server's current
         time is invalid.

      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 a valid
         If-Modified-Since date, the server must 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.

10.24  Keep-Alive

   The Keep-Alive general-header field may be used to include
   diagnostic information and other optional parameters associated
   with the "keep-alive" keyword of the Connection header field
   (Section 10.9). This Keep-Alive field must only be used when the
   "keep-alive" keyword is present (Section 10.9.1).

       Keep-Alive     = "Keep-Alive" ":" 1#kaparam

       kaparam        = ( "timeout" "=" delta-seconds )
                      | ( "max" "=" 1*DIGIT )
                      | ( attribute [ "=" value ] )

   The Keep-Alive header field and the additional information it
   provides are optional and do not need to be present to indicate a
   persistent connection has been established. The semantics of the
   Connection header field prevent the Keep-Alive field from being
   accidentally forwarded to downstream connections.

   HTTP/1.1 defines semantics for the optional "timeout" and "max"
   parameters on responses; other parameters may be added and the
   field may also be used on request messages. The "timeout" parameter
   allows the server to indicate, for diagnostic purposes only, the
   amount of time in seconds it is currently allowing between when the
   response was generated and when the next request is received from
   the client (i.e., the request timeout limit). Similarly, the "max"
   parameter allows the server to indicate the maximum additional
   requests that it will allow on the current persistent connection.

   For example, the server may respond to a request for a persistent
   connection with

       Connection: Keep-Alive
       Keep-Alive: timeout=10, max=5

   to indicate that the server has selected (perhaps dynamically) a
   maximum of 5 requests, but will timeout the connection if the next
   request is not received within 10 seconds. Although these
   parameters have no affect on the operational requirements of the
   connection, they are sometimes useful for testing functionality and
   monitoring server behavior.

10.25  Last-Modified

   The Last-Modified entity-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
   recipient should interpret it:  if the recipient has a copy of this
   resource which is older than the date given by the Last-Modified
   field, that copy should be considered stale.

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

   An example of its use is

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

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

   An origin server must not send a Last-Modified date which is later
   than the server's time of message origination. In such cases, where
   the resource's last modification would indicate some time in the
   future, the server must replace that date with the message
   origination date.

10.26  Link

   The Link entity-header field 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 [5].

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

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

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

   Relationship values are case-insensitive and may be extended within
   the constraints of the sgml-name syntax. 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 chapter2  is previous to the
   entity 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.

10.27  Location

   The Location response-header field defines the exact location of
   the resource that was identified by the Request-URI. For 2xx
   responses, if the Request-URI corresponds to a negotiable set of
   variants and the response includes one of those variants, then the
   response must also include a Location header field containing the
   exact location of the chosen variant. For 3xx responses, the
   location should indicate the server's preferred URL for automatic
   redirection to the resource. The field value consists of a single
   absolute URL.

       Location       = "Location" ":" absoluteURI

   An example is

       Location: http://www.w3.org/pub/WWW/People.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 [11].

10.28  MIME-Version

   HTTP is not a MIME-compliant protocol (see Appendix C). However,
   HTTP/1.1 messages may include a single MIME-Version general-header
   field to indicate what version of the MIME protocol was used to
   construct the message. Use of the MIME-Version header field
   indicates that the message is in full compliance with the MIME
   protocol (as defined in [7]). Proxies/gateways are responsible for
   ensuring full compliance (where possible) when exporting HTTP
   messages to strict MIME environments.

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

   MIME version "1.0" is the default for use in HTTP/1.1. However,
   HTTP/1.1 message parsing and semantics are defined by this document
   and not the MIME specification.

10.29  Pragma

   The Pragma general-header field is used to include
   implementation-specific directives that may apply to any recipient
   along the request/response chain. All pragma directives specify
   optional behavior from the viewpoint of the protocol; however, some
   systems may require that behavior be consistent with the directives.

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

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

   When the "no-cache" directive is present in a request message, an
   application should forward the request toward the origin server
   even if it has a cached copy of what is being requested. This
   pragma directive has the same semantics as the "no-cache"
   cache-directive (see Section 10.8) and is defined here for
   backwards compatibility with HTTP/1.0. Clients should include both
   header fields when a "no-cache" request is sent to a server not
   known to be HTTP/1.1 compliant.

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

10.30  Proxy-Authenticate

   The Proxy-Authenticate response-header field must be included as
   part of a 407 (proxy authentication required) response. The field
   value consists of a challenge that indicates the authentication
   scheme and parameters applicable to the proxy for this Request-URI.

       Proxy-Authentication    = "Proxy-Authentication" ":" challenge

   The HTTP access authentication process is described in Section 11.
   Unlike WWW-Authenticate, the Proxy-Authenticate header field
   applies only to the current connection and must not be passed on to
   downstream clients.

10.31  Proxy-Authorization

   The Proxy-Authorization request-header field allows the client to
   identify itself (or its user) to a proxy which requires
   authentication. The Proxy-Authorization field value consists of
   credentials containing the authentication information of the user
   agent for the proxy and/or realm of the resource being requested.

       Proxy-Authorization     = "Proxy-Authorization" ":" credentials

   The HTTP access authentication process is described in Section 11.
   Unlike Authorization, the Proxy-Authorization applies only to the
   current connection and must not be passed on to upstream servers.
   If a request is authenticated and a realm specified, the same
   credentials should be valid for all other requests within this
   realm.

10.32  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 10.5) 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/1.1 in Section 5.1.1.

       Public         = "Public" ":" 1#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.

10.33  Range

   TBS

10.34  Referer

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

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

   Example:

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

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

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

10.35  Refresh

   TBS

10.36  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.

10.37  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 3.8) and comments
   identifying the server and any significant subproducts. By
   convention, the product tokens are listed in order of their
   significance for identifying the application.

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

   Example:

       Server: CERN/3.0 libwww/2.17

   If the response is being forwarded through a proxy, the proxy
   application must not add its data to the product list. Instead, it
   should include a Forwarded field (as described in Section 10.20).

       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.

10.38  Title

   The Title entity-header field indicates the title of the entity

       Title          = "Title" ":" *TEXT

   An example of the field is

       Title: Hypertext Transfer Protocol -- HTTP/1.1

   This field is isomorphic with the <TITLE> element in HTML [5].

10.39  Transfer Encoding

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

       Transfer-Encoding = "Transfer-Encoding" ":" 1#transfer-coding

   Transfer codings are defined in Section 3.6. An example is:

       Transfer-Encoding: chunked

   Many older HTTP/1.0 applications do not understand the
   Transfer-Encoding header.

10.40  Unless

   The Unless request-header field performs a similar function as
   If-Modified-Since, but the comparison is based on any Entity-Header
   field value of the resource and is not restricted to the GET method.

       Unless         = "Unless" ":" 1#logic-bag

   For example,

       Unless: {or {ne {Content-MD5 "Q2hlY2sgSW50ZWdyaXR5IQ=="}}
                   {ne {Content-Length 10036}}
                   {ne {Content-Version "12.4.8"}}
                   {gt {Last-Modified "Mon, 04 Dec 1995 01:23:45 GMT"}}}

   Multiple Unless headers, or multiple bags separated by commas, can
   be combined by OR'ing them together:

       Unless: {eq {A "a"}}
       Unless: {eq {B "b"}}

   is equivalent to

       Unless: {eq {A "a"}},{eq {B "b"}}

   which in turn is equivalent to

       Unless: {or {eq {A "a"}} {eq {B "b"}}}

   When a request containing an Unless header field is received, the
   server must evaluate the expression defined by the listed
   logic-bags (Section 3.11). If the expression evaluates to false,
   then no change is made to the semantics of the request. If it
   evaluates true and the request is not a conditional GET
   (If-Modified-Since, Section 10.23) or a partial GET (Range,
   Section 10.33), then the server must abort the request and respond
   with the 412 (unless true) status code. If the request is a
   conditional GET, then the server must disregard the
   If-Modified-Since value and respond as it would for a normal GET.
   Similarly, if the request is a partial GET, then the server must
   disregard the Range value and respond as it would for a normal GET.

10.41  Upgrade

   The Upgrade general-header allows the client to specify what
   additional communication protocols it supports and would like to
   use if the server finds it appropriate to switch protocols. The
   server must use the Upgrade header field within a 101 (switching
   protocols) response to indicate which protocol(s) are being
   switched.

       Upgrade        = "Upgrade" ":" 1#product

   For example,

       Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11

   The purpose of the Upgrade header is to allow easier migration
   across protocols in order to better match the application needs
   with protocol capabilities.

10.42  URI

   The URI entity-header field is used to inform the recipient of
   other Uniform Resource Identifiers (Section 3.2) by which the
   resource can be identified, and of all negotiable variants
   corresponding to the Request-URI.

       URI-header  = "URI" ":" 1#( uri-mirror | uri-name | uri-variant )

       uri-mirror  = "{" "mirror" <"> URI <"> "}"
       uri-name    = "{" "name" <"> URI <"> "}"
       uri-variant = "{" "variant" <"> URI <"> qvalue
                         [ "{" "type" <"> media-type <"> "}" ]
                         [ "{" "language" <"> 1#language-tag <"> "}" ]
                         [ "{" "encoding" <"> 1#content-coding <"> "}" ]
                         [ "{" "length" 1*DIGIT "}" ]
                         [ "{" "user-agent" "}" ]
                     "}"

   Any URI specified in this field can be absolute or relative to the
   Request-URI. The "mirror" form of URI refers to a location which is
   a mirror copy of the Request-URI. The "name" form refers to a
   location-independent name corresponding to the Request-URI. The
   "variant" form refers to one of the set of negotiable variants that
   may be retrieved via a request on the Request-URI.

   If the Request-URI maps to a set of variants, then the dimensions
   of that variance must be given in any response containing one of
   those variants. If the Location header field is present in a 2xx
   response, its value identifies which one of the variants is
   included with the response. An example is:

       Location: http://www.w3.org/pub/WWW/TheProject.en.html

       URI: {variant "TheProject.fr.html" 1.0
                    {type "text/html"} {language "fr"}},
            {variant "TheProject.en.html" 1.0
                    {type "text/html"} {language "en"}},
            {variant "TheProject.fr.txt" 0.7
                    {type "text/plain"} {language "fr"}},
            {variant "TheProject.en.txt" 0.8
                    {type "text/plain"} {language "en"}}

   which indicates that the negotiable Request-URI covers a group of
   four individual resources that vary in media type and natural
   language. The type, language, encoding, and length attributes refer
   to their Content-* counterparts for each resource. The user-agent
   attribute indicates that the associated URI is negotiable based on
   the User-Agent header field.

   User agents may use this information to notify the user of
   additional formats and to guide the process of reactive content
   negotiation (Section 12).

10.43  User-Agent

   The User-Agent request-header 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 include this field with requests. The
   field can contain multiple product tokens (Section 3.8) and
   comments identifying the agent and any subproducts which form a
   significant part of the user agent. By convention, the product
   tokens are listed in order of their significance for identifying
   the application.

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

   Example:

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

10.44  WWW-Authenticate

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

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

   The HTTP access authentication process is described in Section 11.
   User agents must take special care in parsing the WWW-Authenticate
   field value if it contains more than one challenge, or if more than
   one WWW-Authenticate header field is provided, since the contents
   of a challenge may itself contain a comma-separated list of
   authentication parameters.

11.  Access Authentication

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

       auth-scheme    = token

       auth-param     = token "=" quoted-string

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

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

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

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

   A user agent that wishes to authenticate itself with a
   server--usually, but not necessarily, after receiving a 401 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    = basic-credentials
                      | auth-scheme *("," auth-param )

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

   If the server does not wish to accept the credentials sent with a
   request, it should return a 401 (unauthorized) response. 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, such as encryption at the transport level
   or via message encapsulation, and with additional header fields
   specifying authentication information. However, these additional
   mechanisms are not defined by this specification.

   Proxies must be completely transparent regarding user agent
   authentication. That is, they must forward the WWW-Authenticate and
   Authorization headers untouched, and must not cache the response to
   a request containing Authorization.

   HTTP/1.1 allows a client pass authentication information to and
   from a proxy via the Proxy-Authenticate and Proxy-Authorization
   headers.

11.1  Basic Authentication Scheme

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

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

       WWW-Authenticate: Basic realm="WallyWorld"

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

   To receive authorization, the client sends the user-ID and
   password, separated by a single colon (":") character, within a
   base64 [7] encoded string in the credentials.

       basic-credentials = " Basic" SP basic-cookie

       basic-cookie      = <base64 [7] encoding of userid-password,
                           except not limited to 76 char/line>

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

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

       Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

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

11.2  Digest Authentication Scheme

   The "digest" authentication scheme is [currently described in an
   expired Internet-Draft, and this description will have to be
   improved to reference a new draft or include the old one].

12.  Content Negotiation

   Content negotiation is an optional feature of the HTTP protocol. It
   is designed to allow for selection of a preferred content
   representation based upon the attributes of the negotiable variants
   corresponding to the requested resource. HTTP/1.1 provides for two
   types of negotiation: preemptive and reactive.

   Servers that make use of content negotiated resources must 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.

12.1  Preemptive Negotiation

   Preemptive negotiation attempts to "negotiate" the variant
   parameters by including the user agent preferences within each
   request. In this way, the preferred representation of the resource
   may be negotiated and obtained within a single request-response
   round-trip, and without intervention from the user. However, this
   also means that the user agent preferences are all the time, even
   though relatively few resources are ever negotiable. Preemptive
   negotiation may not always be desirable for the user and is
   sometimes unnecessary for the content provider. Implementors should
   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 content-codings (Section 3.5) 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".

      qc   Charset quality is measured by comparing the variant
           media-type's charset parameter value (if any) to those
           character sets (Section 3.4) 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 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".

      ql   Language quality is measured by comparing the variant's
           assigned language tag(s) (Section 3.10) 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 "q" parameter
           values for those language tags (Section 10.4); 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 "q" 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 3.7) 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 10.1) 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.

   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.

13.  Caching

   [This will be a summary of what is already defined in the Methods,
   Status Codes, Cache-Control, Unless, and If-Modified-Since
   sections.]

14.  Security Considerations

   This section is meant to inform application developers, information
   providers, and users of the security limitations in HTTP/1.1 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.

14.1  Authentication of Clients

   As mentioned in Section 11.1, 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 does not prevent
   additional authentication schemes and encryption mechanisms from
   being employed to increase security.

14.2  Safe Methods

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

   In particular, the convention has been established that the GET and
   HEAD methods should never have the significance of taking an action
   other than retrieval. These methods should be considered "safe."
   This allows user agents to represent other methods, such as POST,
   PUT and DELETE, in a special way, so that the user is made aware of
   the fact that a possibly unsafe 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.

14.3  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.

14.4  Transfer of Sensitive Information

   Like any generic data transfer protocol, HTTP cannot regulate the
   content of the data that is transferred, nor is there any a priori
   method of determining the sensitivity of any particular piece of
   information within the context of any given request. Therefore,
   applications should 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
   should make the Server header field a configurable option.

   Proxies which serve as a portal 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.

15.  Acknowledgments

   This specification makes heavy use of the augmented BNF and generic
   constructs defined by David H. Crocker for RFC 822 [9]. Similarly,
   it reuses many of the definitions provided by Nathaniel Borenstein
   and Ned Freed for MIME [7]. We hope that their inclusion in this
   specification will help reduce past confusion over the relationship
   between HTTP and Internet mail message formats.

   The HTTP protocol has evolved considerably over the past four
   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, John Franks, Jean-Francois Groff, Phillip M. Hallam-Baker,
   H&kon W. Lie, Ari Luotonen, Rob McCool, Lou Montulli, Dave Raggett,
   Tony Sanders, and Marc VanHeyningen deserve special recognition for
   their efforts in defining early aspects of the protocol.

   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                   Jim Gettys
       Marc Hedlund                       Koen Holtman
       Alex Hopmann                       Bob Jernigan
       Shel Kaphan                        Rohit Khare
       Martijn Koster                     Alexei Kosut
       Dave Kristol                       Daniel LaLiberte
       Paul Leach                         Albert Lunde
       John C. Mallery                    Jean-Philippe Martin-Flatin
       Larry Masinter                     Mitra
       Jeffrey Mogul                      Gavin Nicol
       Bill Perry                         Jeffrey Perry
       Owen Rees                          Luigi Rizzo
       David Robinson                     Marc Salomon
       Rich Salz                          Jim Seidman
       Chuck Shotton                      Eric W. Sink
       Simon E. Spero                     Richard N. Taylor
       Robert S. Thau                     Franccedillaois Yergeau
       Mary Ellen Zurko

16. 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,
        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, L. Masinter, M. McCahill. "Uniform Resource
        Locators (URL)." RFC 1738, CERN, Xerox PARC, University of
        Minnesota, December 1994.

   [5]  T. Berners-Lee, D. Connolly. "HyperText Markup Language
        Specification - 2.0." RFC 1866, MIT/LCS, November 1995.

   [6]  T. Berners-Lee, R. Fielding, H. Frystyk. "Hypertext Transfer
        Protocol - HTTP/1.0." Work in Progress
        (draft-ietf-http-v10-spec-04.txt), MIT/LCS, UC Irvine,
        September 1995.

   [7]  N. Borenstein, 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.

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

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

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

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

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

   [13] B. Kantor, 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.

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

   [15] E. Nebel, L. Masinter. "Form-based File Upload in HTML."
        RFC 1867, Xerox Corporation, November 1995.

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

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

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

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

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

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

   [22] 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.

17.  Authors' Addresses

   Roy T. Fielding
   Department of Information and Computer Science
   University of California
   Irvine, CA 92717-3425, U.S.A.
   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.
   Fax: +1 (617) 258 8682
   Email: frystyk@w3.org

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

Appendices

   These appendices are provided for informational reasons only -- they
   do not form a part of the HTTP/1.1 specification.

A.  Internet Media Type message/http

   In addition to defining the HTTP/1.1 protocol, this document serves
   as the specification for the Internet media type "message/http".
   The following is to be registered with IANA [17].

       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., "1.1"). 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/1.1 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 Status-Line and servers
   tolerant when parsing the Request-Line. 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/1.1 reuses many of the constructs defined for Internet Mail
   (RFC 822 [9]) and the Multipurpose Internet Mail Extensions
   (MIME [7]) to allow entities to be transmitted in an open variety
   of representations and with extensible mechanisms. However, HTTP is
   not a MIME-compliant 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 SMTP
   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. Proxies/gateways to MIME-compliant protocols must be aware of
   these differences and provide the appropriate conversions where
   necessary.

C.1  Conversion to Canonical Form

   MIME requires that an entity be converted to canonical form prior
   to being transferred, as described in Appendix G of RFC 1521 [7].
   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 3.7.1.

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) 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 proxy/gateway from HTTP to a MIME-compliant
   protocol should translate all line breaks within text/* media types
   to the MIME canonical form of CRLF. However, this may be
   complicated by the presence of a Content-Encoding and by the fact
   that HTTP allows the use of some character sets which do not use
   octets 13 and 10 to represent CR and LF, as is the case for some
   multi-byte character sets. If canonicalization is performed, the
   Content-Length header field value must be updated to reflect the
   new body length.

C.1.2 Default Character Set

   MIME requires that all subtypes of the top-level Content-Type
   "text" have a default character set of US-ASCII [21]. In contrast,
   HTTP defines the default character set for "text" to be
   ISO-8859-1 [22] (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 proxy/gateway if the entity contains any
   octets greater than 127.

C.2  Conversion of Date Formats

   HTTP/1.1 uses a restricted subset of date formats to simplify the
   process of date comparison. Proxies/gateways from other protocols
   should ensure that any Date header field present in a message
   conforms to one of the HTTP/1.1 formats and rewrite the date if
   necessary.

C.3  Introduction of Content-Encoding

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

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

C.4  No Content-Transfer-Encoding

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

       Content-Transfer-Encoding: binary

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

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

C.5  Introduction of Transfer-Encoding

   HTTP/1.1 introduces the Transfer-Encoding header field
   (Section 10.39). Proxies/gateways must remove any transfer coding
   prior to forwarding a message via a MIME-compliant protocol. The
   process for decoding the "chunked" transfer coding (Section 3.6)
   can be represented in pseudo-code as:

       length := 0
       read chunk-size and CRLF
       while (chunk-size > 0) {
          read chunk-data and CRLF
          append chunk-data to Entity-Body
          length := length + chunk-size
          read chunk-size and CRLF
       }
       read entity-header
       while (entity-header not empty) {
          append entity-header to existing header fields
          read entity-header
       }
       Content-Length := length
       Remove "chunked" from Transfer-Encoding

D.  Changes from HTTP/1.0

   This section will summarize the differences between versions 1.0
   and 1.1 of the Hypertext Transfer Protocol.


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