2004/id/draft-ietf-http-v11-spec-00.txt

HTTP Working Group                                R. Fielding, UC Irvine
INTERNET-DRAFT                                       H. Frystyk, MIT/LCS
<draft-ietf-http-v11-spec-00.txt>                T. Berners-Lee, MIT/LCS
Expires May 22, 1996                                   November 22, 1995


               Hypertext Transfer Protocol -- HTTP/1.1


Status of this Memo

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

   The Hypertext Transfer Protocol (HTTP) is an application-level 
   protocol 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.
