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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 RFC 7230

Network Working Group                                   R. Fielding, Ed.
Internet-Draft                                              Day Software
Obsoletes: 2068, 2616                                          J. Gettys
(if approved)                                       One Laptop per Child
Intended status: Standards Track                                J. Mogul
Expires: June 22, 2008                                                HP
                                                              H. Frystyk
                                                               Microsoft
                                                             L. Masinter
                                                           Adobe Systems
                                                                P. Leach
                                                               Microsoft
                                                          T. Berners-Lee
                                                                 W3C/MIT
                                                       December 20, 2007


        HTTP/1.1, part 1: URIs, Connections, and Message Parsing
                   draft-ietf-httpbis-p1-messaging-00

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   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on June 22, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).



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Abstract

   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.  This document is Part 1 of the
   seven-part specification that defines the protocol referred to as
   "HTTP/1.1" and, taken together, obsoletes RFC 2616.  Part 1 provides
   an overview of HTTP and its associated terminology, defines the
   "http" and "https" Uniform Resource Identifier (URI) schemes, defines
   the generic message syntax and parsing requirements for HTTP message
   frames, and describes general security concerns for implementations.

Editorial Note (To be removed by RFC Editor)

   This version of the HTTP specification contains only minimal
   editorial changes from [RFC2616] (abstract, introductory paragraph,
   and authors' addresses).  All other changes are due to partitioning
   the original into seven mostly independent parts.  The intent is for
   readers of future drafts to able to use draft 00 as the basis for
   comparison when the WG makes later changes to the specification text.
   This draft will shortly be followed by draft 01 (containing the first
   round of changes that have already been agreed to on the mailing
   list).  There is no point in reviewing this draft other than to
   verify that the partitioning has been done correctly.  Roy T.
   Fielding, Yves Lafon, and Julian Reschke will be the editors after
   draft 00 is submitted.

   Discussion of this draft should take place on the HTTPBIS working
   group mailing list (ietf-http-wg@w3.org).  The current issues list is
   at <http://www3.tools.ietf.org/wg/httpbis/trac/report/11> and related
   documents (including fancy diffs) can be found at
   <http://www3.tools.ietf.org/wg/httpbis/>.


















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.2.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  6
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
     1.4.  Overall Operation  . . . . . . . . . . . . . . . . . . . .  9
   2.  Notational Conventions and Generic Grammar . . . . . . . . . . 11
     2.1.  Augmented BNF  . . . . . . . . . . . . . . . . . . . . . . 11
     2.2.  Basic Rules  . . . . . . . . . . . . . . . . . . . . . . . 13
   3.  Protocol Parameters  . . . . . . . . . . . . . . . . . . . . . 15
     3.1.  HTTP Version . . . . . . . . . . . . . . . . . . . . . . . 15
     3.2.  Uniform Resource Identifiers . . . . . . . . . . . . . . . 16
       3.2.1.  General Syntax . . . . . . . . . . . . . . . . . . . . 16
       3.2.2.  http URL . . . . . . . . . . . . . . . . . . . . . . . 17
       3.2.3.  URI Comparison . . . . . . . . . . . . . . . . . . . . 17
     3.3.  Date/Time Formats  . . . . . . . . . . . . . . . . . . . . 18
       3.3.1.  Full Date  . . . . . . . . . . . . . . . . . . . . . . 18
     3.4.  Transfer Codings . . . . . . . . . . . . . . . . . . . . . 19
       3.4.1.  Chunked Transfer Coding  . . . . . . . . . . . . . . . 20
   4.  HTTP Message . . . . . . . . . . . . . . . . . . . . . . . . . 22
     4.1.  Message Types  . . . . . . . . . . . . . . . . . . . . . . 22
     4.2.  Message Headers  . . . . . . . . . . . . . . . . . . . . . 23
     4.3.  Message Body . . . . . . . . . . . . . . . . . . . . . . . 24
     4.4.  Message Length . . . . . . . . . . . . . . . . . . . . . . 25
     4.5.  General Header Fields  . . . . . . . . . . . . . . . . . . 26
   5.  Request  . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     5.1.  Request-Line . . . . . . . . . . . . . . . . . . . . . . . 27
       5.1.1.  Method . . . . . . . . . . . . . . . . . . . . . . . . 27
       5.1.2.  Request-URI  . . . . . . . . . . . . . . . . . . . . . 27
     5.2.  The Resource Identified by a Request . . . . . . . . . . . 28
   6.  Response . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     6.1.  Status-Line  . . . . . . . . . . . . . . . . . . . . . . . 29
       6.1.1.  Status Code and Reason Phrase  . . . . . . . . . . . . 30
   7.  Connections  . . . . . . . . . . . . . . . . . . . . . . . . . 30
     7.1.  Persistent Connections . . . . . . . . . . . . . . . . . . 30
       7.1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . 30
       7.1.2.  Overall Operation  . . . . . . . . . . . . . . . . . . 31
       7.1.3.  Proxy Servers  . . . . . . . . . . . . . . . . . . . . 33
       7.1.4.  Practical Considerations . . . . . . . . . . . . . . . 33
     7.2.  Message Transmission Requirements  . . . . . . . . . . . . 34
       7.2.1.  Persistent Connections and Flow Control  . . . . . . . 34
       7.2.2.  Monitoring Connections for Error Status Messages . . . 34
       7.2.3.  Use of the 100 (Continue) Status . . . . . . . . . . . 34
       7.2.4.  Client Behavior if Server Prematurely Closes
               Connection . . . . . . . . . . . . . . . . . . . . . . 36
   8.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 37
     8.1.  Connection . . . . . . . . . . . . . . . . . . . . . . . . 37



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     8.2.  Content-Length . . . . . . . . . . . . . . . . . . . . . . 38
     8.3.  Date . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
       8.3.1.  Clockless Origin Server Operation  . . . . . . . . . . 40
     8.4.  Host . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     8.5.  TE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     8.6.  Trailer  . . . . . . . . . . . . . . . . . . . . . . . . . 42
     8.7.  Transfer-Encoding  . . . . . . . . . . . . . . . . . . . . 43
     8.8.  Upgrade  . . . . . . . . . . . . . . . . . . . . . . . . . 43
     8.9.  Via  . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 46
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 46
     10.1. Personal Information . . . . . . . . . . . . . . . . . . . 46
     10.2. Abuse of Server Log Information  . . . . . . . . . . . . . 46
     10.3. Attacks Based On File and Path Names . . . . . . . . . . . 46
     10.4. DNS Spoofing . . . . . . . . . . . . . . . . . . . . . . . 47
     10.5. Proxies and Caching  . . . . . . . . . . . . . . . . . . . 47
     10.6. Denial of Service Attacks on Proxies . . . . . . . . . . . 48
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 48
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 50
   Appendix A.  Internet Media Type message/http and
                application/http  . . . . . . . . . . . . . . . . . . 53
   Appendix B.  Tolerant Applications . . . . . . . . . . . . . . . . 54
   Appendix C.  Conversion of Date Formats  . . . . . . . . . . . . . 55
   Appendix D.  Compatibility with Previous Versions  . . . . . . . . 55
     D.1.  Changes from HTTP/1.0  . . . . . . . . . . . . . . . . . . 56
       D.1.1.  Changes to Simplify Multi-homed Web Servers and
               Conserve IP Addresses  . . . . . . . . . . . . . . . . 56
     D.2.  Compatibility with HTTP/1.0 Persistent Connections . . . . 56
     D.3.  Changes from RFC 2068  . . . . . . . . . . . . . . . . . . 57
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 61
   Intellectual Property and Copyright Statements . . . . . . . . . . 63



















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1.  Introduction

   This document will define aspects of HTTP related to overall network
   operation, message framing, interaction with transport protocols, and
   URI schemes.  Right now it only includes the extracted relevant
   sections of [RFC2616].

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 1945 [RFC1945],
   improved the protocol by allowing messages to be in the format of
   MIME-like messages, containing metainformation about the data
   transferred and modifiers on the request/response semantics.
   However, HTTP/1.0 does not sufficiently take into consideration the
   effects of hierarchical proxies, caching, the need for persistent
   connections, or virtual hosts.  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 includes more stringent requirements than HTTP/1.0 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 and headers that indicate the
   purpose of a request [RFC2324].  It builds on the discipline of
   reference provided by the Uniform Resource Identifier (URI)
   [RFC1630], as a location (URL) [RFC1738] or name (URN) [RFC1737], for
   indicating the resource to which a method is to be applied.  Messages
   are passed in a format similar to that used by Internet mail [RFC822]
   as defined by the Multipurpose Internet Mail Extensions (MIME)
   [RFC2045].

   HTTP is also used as a generic protocol for communication between
   user agents and proxies/gateways to other Internet systems, including
   those supported by the SMTP [RFC821], NNTP [RFC977], FTP [RFC959],
   Gopher [RFC1436], and WAIS [WAIS] protocols.  In this way, HTTP
   allows basic hypermedia access to resources available from diverse
   applications.




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1.2.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   An implementation is not compliant if it fails to satisfy one or more
   of the MUST or REQUIRED level requirements for the protocols it
   implements.  An implementation that satisfies all the MUST or
   REQUIRED level and all the SHOULD level requirements for its
   protocols is said to be "unconditionally compliant"; one that
   satisfies all the MUST level requirements but not all the SHOULD
   level 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 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 that can be identified by a URI,
      as defined in Section 3.2.  Resources may be available in multiple
      representations (e.g. multiple languages, data formats, size, and
      resolutions) or vary in other ways.

   entity




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      The information transferred as the payload of a request or
      response.  An entity consists of metainformation in the form of
      entity-header fields and content in the form of an entity-body, as
      described in Section 3 of [Part3].

   representation

      An entity included with a response that is subject to content
      negotiation, as described in Section 4 of [Part3].  There may
      exist multiple representations associated with a particular
      response status.

   content negotiation

      The mechanism for selecting the appropriate representation when
      servicing a request, as described in Section 4 of [Part3].  The
      representation of entities in any response can be negotiated
      (including error responses).

   variant

      A resource may have one, or more than one, representation(s)
      associated with it at any given instant.  Each of these
      representations is termed a `varriant'.  Use of the term `variant'
      does not necessarily imply that the resource is subject to content
      negotiation.

   client

      A 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.  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.




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   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 on, with
      possible translation, to other servers.  A proxy MUST implement
      both the client and server requirements of this specification.  A
      "transparent proxy" is a proxy that does not modify the request or
      response beyond what is required for proxy authentication and
      identification.  A "non-transparent proxy" is a proxy that
      modifies the request or response in order to provide some added
      service to the user agent, such as group annotation services,
      media type transformation, protocol reduction, or anonymity
      filtering.  Except where either transparent or non-transparent
      behavior is explicitly stated, the HTTP proxy requirements apply
      to both types of proxies.

   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.

   tunnel

      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.

   cache

      A program's local store of response messages and the subsystem
      that controls its message storage, retrieval, and deletion.  A
      cache stores cacheable 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 that is acting as a tunnel.

   cacheable





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      A response is cacheable if a cache is allowed to store a copy of
      the response message for use in answering subsequent requests.
      The rules for determining the cacheability of HTTP responses are
      defined in Section 2 of [Part6].  Even if a resource is cacheable,
      there may be additional constraints on whether a cache can use the
      cached copy for a particular request.

   upstream/downstream

      Upstream and downstream describe the flow of a message: all
      messages flow from upstream to downstream.

   inbound/outbound

      Inbound and outbound refer to the request and response paths for
      messages: "inbound" means "traveling toward the origin server",
      and "outbound" means "traveling toward the user agent"

1.4.  Overall Operation

   The HTTP protocol is a request/response protocol.  A client 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 over a
   connection with a server.  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 entity-body content.  The relationship
   between HTTP and MIME is described in Appendix A of [Part3].

   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 part 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



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   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 will 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 usefully cacheable, and some requests may
   contain modifiers which place special requirements on cache behavior.
   HTTP requirements for cache behavior and cacheable responses are
   defined in Section 2 of [Part6].

   In fact, there are a wide variety of architectures and configurations
   of caches and proxies currently being experimented with or deployed
   across the World Wide Web. These systems include national hierarchies
   of proxy caches to save transoceanic bandwidth, systems that
   broadcast or multicast cache entries, organizations that distribute
   subsets of cached data via CD-ROM, and so on.  HTTP systems are used
   in corporate intranets over high-bandwidth links, and for access via
   PDAs with low-power radio links and intermittent connectivity.  The
   goal of HTTP/1.1 is to support the wide diversity of configurations
   already deployed while introducing protocol constructs that meet the



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   needs of those who build web applications that require high
   reliability and, failing that, at least reliable indications of
   failure.

   HTTP communication usually takes place over TCP/IP connections.  The
   default port is TCP 80 [RFC1700], 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; 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.

   In HTTP/1.0, most implementations used a new connection for each
   request/response exchange.  In HTTP/1.1, a connection may be used for
   one or more request/response exchanges, although connections may be
   closed for a variety of reasons (see Section 7.1).


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 [RFC822].  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.  White space 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





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      Elements separated by a bar ("|") 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 white space (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



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      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 white space (LWS) can be included
      between any two adjacent words (token or quoted-string), and
      between adjacent words and separators, without changing the
      interpretation of a field.  At least one delimiter (LWS and/or
      separators) MUST exist between any two tokens (for the definition
      of "token" below), since they would otherwise be interpreted as a
      single token.

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 ANSI X3.4-1986 [USASCII].

       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 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 [Part3].

       CRLF           = CR LF



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   HTTP/1.1 header field values can be folded onto multiple lines if the
   continuation line begins with a space or horizontal tab.  All linear
   white space, including folding, has the same semantics as SP.  A
   recipient MAY replace any linear white space with a single SP before
   interpreting the field value or forwarding the message downstream.

       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 characters from character sets other than ISO-
   8859-1 [ISO-8859] only when encoded according to the rules of RFC
   2047 [RFC2047].

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

   A CRLF is allowed in the definition of TEXT only as part of a header
   field continuation.  It is expected that the folding LWS will be
   replaced with a single SP before interpretation of the TEXT value.

   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 (as defined in
   Section 3.4).

       token          = 1*<any CHAR except CTLs or separators>
       separators     = "(" | ")" | "<" | ">" | "@"
                      | "," | ";" | ":" | "\" | <">
                      | "/" | "[" | "]" | "?" | "="
                      | "{" | "}" | 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 | quoted-pair | comment ) ")"
       ctext          = <any TEXT excluding "(" and ")">

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



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       quoted-string  = ( <"> *(qdtext | quoted-pair ) <"> )
       qdtext         = <any TEXT except <">>

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

       quoted-pair    = "\" CHAR


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.  See RFC 2145 [RFC2145] for a fuller explanation.

   The version of an HTTP message is indicated by an HTTP-Version field
   in the first line of the message.

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

   Note that the major and minor numbers MUST 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 MUST be ignored by recipients
   and MUST NOT be sent.

   An application that sends a request or response message that includes
   HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant
   with this specification.  Applications that are at least
   conditionally compliant with this specification SHOULD use an HTTP-
   Version of "HTTP/1.1" in their messages, and MUST do so for any
   message that is not compatible with HTTP/1.0.  For more details on
   when to send specific HTTP-Version values, see RFC 2145 [RFC2145].

   The HTTP version of an application is the highest HTTP version for
   which the application is at least conditionally compliant.



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   Proxy and gateway applications need to be careful when forwarding
   messages in protocol versions different from that of the application.
   Since the protocol version indicates the protocol capability of the
   sender, a proxy/gateway MUST NOT send a message with a version
   indicator which is greater than its actual version.  If a higher
   version request is received, the proxy/gateway MUST either downgrade
   the request version, or respond with an error, or switch to tunnel
   behavior.

   Due to interoperability problems with HTTP/1.0 proxies discovered
   since the publication of RFC 2068 [RFC2068], caching proxies MUST,
   gateways MAY, and tunnels MUST NOT upgrade the request to the highest
   version they support.  The proxy/gateway's response to that request
   MUST be in the same major version as the request.

      Note: Converting between versions of HTTP may involve modification
      of header fields required or forbidden by the versions involved.

3.2.  Uniform Resource Identifiers

   URIs have been known by many names: WWW addresses, Universal Document
   Identifiers, Universal Resource Identifiers [RFC1630], and finally
   the combination of Uniform Resource Locators (URL) [RFC1738] and
   Names (URN) [RFC1737].  As far as HTTP is concerned, Uniform Resource
   Identifiers are simply formatted strings which identify--via name,
   location, or any other characteristic--a resource.

3.2.1.  General Syntax

   URIs in HTTP can be represented in absolute form or relative to some
   known base URI [RFC1808], 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.  For definitive
   information on URL syntax and semantics, see "Uniform Resource
   Identifiers (URI): Generic Syntax and Semantics," RFC 2396 [RFC2396]
   (which replaces RFCs 1738 [RFC1738] and RFC 1808 [RFC1808]).  This
   specification adopts the definitions of "URI-reference",
   "absoluteURI", "relativeURI", "port", "host","abs_path", "rel_path",
   and "authority" from that specification.

   The HTTP protocol does not place any a priori limit on the length of
   a URI.  Servers MUST be able to handle the URI of any resource they
   serve, and SHOULD be able to handle URIs of unbounded length if they
   provide GET-based forms that could generate such URIs.  A server
   SHOULD return 414 (Request-URI Too Long) status if a URI is longer
   than the server can handle (see Section 9.4.15 of [Part2]).





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      Note: Servers ought to be cautious about depending on URI lengths
      above 255 bytes, because some older client or proxy
      implementations might not properly support these lengths.

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 [ "?" query ]]

   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 (Section 5.1.2).  The use of IP
   addresses in URLs SHOULD be avoided whenever possible (see RFC 1900
   [RFC1900]).  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).  If a proxy receives a host name which is not a
   fully qualified domain name, it MAY add its domain to the host name
   it received.  If a proxy receives a fully qualified domain name, the
   proxy MUST NOT change the host name.

3.2.3.  URI Comparison

   When comparing two URIs to decide if they match or not, a client
   SHOULD use a case-sensitive octet-by-octet comparison of the entire
   URIs, with these exceptions:

   o  A port that is empty or not given is equivalent to the default
      port for that URI-reference;

   o  Comparisons of host names MUST be case-insensitive;

   o  Comparisons of scheme names MUST be case-insensitive;

   o  An empty abs_path is equivalent to an abs_path of "/".

   Characters other than those in the "reserved" and "unsafe" sets (see
   RFC 2396 [RFC2396]) are equivalent to their ""%" HEX HEX" encoding.

   For example, the following three URIs are equivalent:

      http://abc.com:80/~smith/home.html
      http://ABC.com/%7Esmith/home.html
      http://ABC.com:/%7esmith/home.html




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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 [RFC1123] (an
   update to RFC 822 [RFC822]).  The second format is in common use, but
   is based on the obsolete RFC 850 [RFC1036] date format and lacks a
   four-digit year.  HTTP/1.1 clients and servers that parse the date
   value MUST accept all three formats (for compatibility with
   HTTP/1.0), though they MUST only generate the RFC 1123 format for
   representing HTTP-date values in header fields.  See Appendix B for
   further information.

      Note: Recipients of date values are encouraged to be robust in
      accepting date values that may have been sent 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 Greenwich Mean Time
   (GMT), without exception.  For the purposes of HTTP, GMT is exactly
   equal to UTC (Coordinated Universal Time).  This is indicated in the
   first two formats by the inclusion of "GMT" as the three-letter
   abbreviation for time zone, and MUST be assumed when reading the
   asctime format.  HTTP-date is case sensitive and MUST NOT include
   additional LWS beyond that specifically included as SP in the
   grammar.

















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       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.

3.4.  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 entity.

       transfer-coding         = "chunked" | transfer-extension
       transfer-extension      = token *( ";" parameter )

   Parameters are in the form of attribute/value pairs.

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

   All transfer-coding values are case-insensitive.  HTTP/1.1 uses
   transfer-coding values in the TE header field (Section 8.5) and in
   the Transfer-Encoding header field (Section 8.7).

   Whenever a transfer-coding is applied to a message-body, the set of
   transfer-codings MUST include "chunked", unless the message is



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   terminated by closing the connection.  When the "chunked" transfer-
   coding is used, it MUST be the last transfer-coding applied to the
   message-body.  The "chunked" transfer-coding MUST NOT be applied more
   than once to a message-body.  These rules allow the recipient to
   determine the transfer-length of the message (Section 4.4).

   Transfer-codings are analogous to the Content-Transfer-Encoding
   values of MIME [RFC2045], 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 4.4),
   or the desire to encrypt data over a shared transport.

   The Internet Assigned Numbers Authority (IANA) acts as a registry for
   transfer-coding value tokens.  Initially, the registry contains the
   following tokens: "chunked" (Section 3.4.1), "identity" (section
   3.6.2), "gzip" ([Part3]), "compress" ([Part3]), and "deflate"
   ([Part3]).

   New transfer-coding value tokens SHOULD be registered in the same way
   as new content-coding value tokens ([Part3]).

   A server which receives an entity-body with a transfer-coding it does
   not understand SHOULD return 501 (Unimplemented), and close the
   connection.  A server MUST NOT send transfer-codings to an HTTP/1.0
   client.

3.4.1.  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 trailer 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.















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       Chunked-Body   = *chunk
                        last-chunk
                        trailer
                        CRLF

       chunk          = chunk-size [ chunk-extension ] CRLF
                        chunk-data CRLF
       chunk-size     = 1*HEX
       last-chunk     = 1*("0") [ chunk-extension ] CRLF

       chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
       chunk-ext-name = token
       chunk-ext-val  = token | quoted-string
       chunk-data     = chunk-size(OCTET)
       trailer        = *(entity-header CRLF)

   The chunk-size field is a string of hex digits indicating the size of
   the chunk.  The chunked encoding is ended by any chunk whose size is
   zero, followed by the trailer, which is terminated by an empty line.

   The trailer allows the sender to include additional HTTP header
   fields at the end of the message.  The Trailer header field can be
   used to indicate which header fields are included in a trailer (see
   Section 8.6).

   A server using chunked transfer-coding in a response MUST NOT use the
   trailer for any header fields unless at least one of the following is
   true:

   1.  the request included a TE header field that indicates "trailers"
       is acceptable in the transfer-coding of the response, as
       described in Section 8.5; or,

   2.  the server is the origin server for the response, the trailer
       fields consist entirely of optional metadata, and the recipient
       could use the message (in a manner acceptable to the origin
       server) without receiving this metadata.  In other words, the
       origin server is willing to accept the possibility that the
       trailer fields might be silently discarded along the path to the
       client.

   This requirement prevents an interoperability failure when the
   message is being received by an HTTP/1.1 (or later) proxy and
   forwarded to an HTTP/1.0 recipient.  It avoids a situation where
   compliance with the protocol would have necessitated a possibly
   infinite buffer on the proxy.

   A process for decoding the "chunked" transfer-coding can be



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   represented in pseudo-code as:

       length := 0
       read chunk-size, chunk-extension (if any) 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

   All HTTP/1.1 applications MUST be able to receive and decode the
   "chunked" transfer-coding, and MUST ignore chunk-extension extensions
   they do not understand.


4.  HTTP Message

4.1.  Message Types

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

       HTTP-message   = Request | Response     ; HTTP/1.1 messages

   Request (Section 5) and Response (Section 6) messages use the generic
   message format of RFC 822 [RFC822] for transferring entities (the
   payload of the message).  Both types of message consist of a start-
   line, zero or more header fields (also known as "headers"), an empty
   line (i.e., a line with nothing preceding the CRLF) indicating the
   end of the header fields, and possibly a message-body.

        generic-message = start-line
                          *(message-header CRLF)
                          CRLF
                          [ message-body ]
        start-line      = Request-Line | Status-Line

   In the interest of robustness, servers SHOULD ignore any empty
   line(s) received where a Request-Line is expected.  In other words,
   if the server is reading the protocol stream at the beginning of a



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   message and receives a CRLF first, it should ignore the CRLF.

   Certain buggy HTTP/1.0 client implementations generate extra CRLF's
   after a POST request.  To restate what is explicitly forbidden by the
   BNF, an HTTP/1.1 client MUST NOT preface or follow a request with an
   extra CRLF.

4.2.  Message Headers

   HTTP header fields, which include general-header (Section 4.5),
   request-header (Section 4 of [Part2]), response-header (Section 6 of
   [Part2]), and entity-header (Section 3.1 of [Part3]) fields, follow
   the same generic format as that given in Section 3.1 of RFC 822
   [RFC822].  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.  Applications
   ought to follow "common form", where one is known or indicated, when
   generating HTTP constructs, since there might exist some
   implementations that fail to accept anything beyond the common forms.

       message-header = field-name ":" [ field-value ]
       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, separators, and quoted-string>

   The field-content does not include any leading or trailing LWS:
   linear white space occurring before the first non-whitespace
   character of the field-value or after the last non-whitespace
   character of the field-value.  Such leading or trailing LWS MAY be
   removed without changing the semantics of the field value.  Any LWS
   that occurs between field-content MAY be replaced with a single SP
   before interpreting the field value or forwarding the message
   downstream.

   The order in which header fields with differing field names are
   received is not significant.  However, it is "good practice" to send
   general-header fields first, followed by request-header or response-
   header fields, and ending with the entity-header fields.

   Multiple message-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



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   message, by appending each subsequent field-value to the first, each
   separated by a comma.  The order in which header fields with the same
   field-name are received is therefore significant to the
   interpretation of the combined field value, and thus a proxy MUST NOT
   change the order of these field values when a message is forwarded.

4.3.  Message Body

   The message-body (if any) of an HTTP message is used to carry the
   entity-body associated with the request or response.  The message-
   body differs from the entity-body only when a transfer-coding has
   been applied, as indicated by the Transfer-Encoding header field
   (Section 8.7).

       message-body = entity-body
                    | <entity-body encoded as per Transfer-Encoding>

   Transfer-Encoding MUST be used to indicate any transfer-codings
   applied by an application to ensure safe and proper transfer of the
   message.  Transfer-Encoding is a property of the message, not of the
   entity, and thus MAY be added or removed by any application along the
   request/response chain.  (However, Section 3.4 places restrictions on
   when certain transfer-codings may be used.)

   The rules for when a message-body is allowed in a message differ for
   requests and responses.

   The presence of a message-body in a request is signaled by the
   inclusion of a Content-Length or Transfer-Encoding header field in
   the request's message-headers.  A message-body MUST NOT be included
   in a request if the specification of the request method (Section 3 of
   [Part2]) does not allow sending an entity-body in requests.  A server
   SHOULD read and forward a message-body on any request; if the request
   method does not include defined semantics for an entity-body, then
   the message-body SHOULD be ignored when handling the request.

   For response messages, whether or not a message-body is included with
   a message is dependent on both the request method and the response
   status code (Section 6.1.1).  All responses to the HEAD request
   method MUST NOT include a message-body, even though the presence of
   entity-header fields might lead one to believe they do.  All 1xx
   (informational), 204 (no content), and 304 (not modified) responses
   MUST NOT include a message-body.  All other responses do include a
   message-body, although it MAY be of zero length.







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4.4.  Message Length

   The transfer-length of a message is the length of the message-body as
   it appears in the message; that is, after any transfer-codings have
   been applied.  When a message-body is included with a message, the
   transfer-length of that body is determined by one of the following
   (in order of precedence):

   1.  Any response message which "MUST NOT" include a message-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.

   2.  If a Transfer-Encoding header field (Section 8.7) is present and
       has any value other than "identity", then the transfer-length is
       defined by use of the "chunked" transfer-coding (Section 3.4),
       unless the message is terminated by closing the connection.

   3.  If a Content-Length header field (Section 8.2) is present, its
       decimal value in OCTETs represents both the entity-length and the
       transfer-length.  The Content-Length header field MUST NOT be
       sent if these two lengths are different (i.e., if a Transfer-
       Encoding header field is present).  If a message is received with
       both a Transfer-Encoding header field and a Content-Length header
       field, the latter MUST be ignored.

   4.  If the message uses the media type "multipart/byteranges", and
       the ransfer-length is not otherwise specified, then this self-
       elimiting media type defines the transfer-length.  This media
       type UST NOT be used unless the sender knows that the recipient
       can arse it; the presence in a request of a Range header with
       ultiple byte-range specifiers from a 1.1 client implies that the
       lient can parse multipart/byteranges responses.

          A range header might be forwarded by a 1.0 proxy that does not
          understand multipart/byteranges; in this case the server MUST
          delimit the message using methods defined in items 1, 3 or 5
          of this section.

   5.  By the server closing the connection.  (Closing the connection
       cannot be used to indicate the end of a request body, since that
       would leave no possibility for the server to send back a
       response.)

   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
   containing a message-body MUST include a valid Content-Length header
   field unless the server is known to be HTTP/1.1 compliant.  If a



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   request contains a message-body and a Content-Length is not given,
   the server SHOULD respond with 400 (bad request) if it cannot
   determine the length of the message, or with 411 (length required) if
   it wishes to insist on receiving a valid Content-Length.

   All HTTP/1.1 applications that receive entities MUST accept the
   "chunked" transfer-coding (Section 3.4), thus allowing this mechanism
   to be used for messages when the message length cannot be determined
   in advance.

   Messages MUST NOT include both a Content-Length header field and a
   non-identity transfer-coding.  If the message does include a non-
   identity transfer-coding, the Content-Length MUST be ignored.

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

4.5.  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 header fields apply only to the
   message being transmitted.

       general-header = Cache-Control            ; [Part6], Section 3.2
                      | Connection               ; Section 8.1
                      | Date                     ; Section 8.3
                      | Pragma                   ; [Part6], Section 3.4
                      | Trailer                  ; Section 8.6
                      | Transfer-Encoding        ; Section 8.7
                      | Upgrade                  ; Section 8.8
                      | Via                      ; Section 8.9
                      | Warning                  ; [Part6], Section 3.6

   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,



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   the identifier of the resource, and the protocol version in use.

        Request       = Request-Line              ; Section 5.1
                        *(( general-header        ; Section 4.5
                         | request-header         ; [Part2], Section 4
                         | entity-header ) CRLF)  ; [Part3], Section 3.1
                        CRLF
                        [ message-body ]          ; Section 4.3

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 is allowed except in the
   final CRLF sequence.

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

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         = token

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 | authority

   The four 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 REQUIRED when the request is being made to a
   proxy.  The proxy is requested to forward the request or service it
   from a valid cache, and return the response.  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:



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       GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1

   To allow for transition to absoluteURIs in all requests in future
   versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI
   form in requests, even though HTTP/1.1 clients will only generate
   them in requests to proxies.

   The authority form is only used by the CONNECT method (Section 8.9 of
   [Part2]).

   The most common form of Request-URI is that used to identify a
   resource on an origin server or gateway.  In this case the absolute
   path of the URI MUST be transmitted (see Section 3.2.1, abs_path) as
   the Request-URI, and the network location of the URI (authority) MUST
   be transmitted in a Host header field.  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 lines:

       GET /pub/WWW/TheProject.html HTTP/1.1
       Host: www.w3.org

   followed by the remainder of the 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).

   The Request-URI is transmitted in the format specified in
   Section 3.2.1.  If the Request-URI is encoded using the "% HEX HEX"
   encoding [RFC2396], the origin server MUST decode the Request-URI in
   order to properly interpret the request.  Servers SHOULD respond to
   invalid Request-URIs with an appropriate status code.

   A transparent proxy MUST NOT rewrite the "abs_path" part of the
   received Request-URI when forwarding it to the next inbound server,
   except as noted above to replace a null abs_path with "/".

      Note: The "no rewrite" rule prevents the proxy from changing the
      meaning of the request when the origin server is improperly using
      a non-reserved URI character for a reserved purpose.  Implementors
      should be aware that some pre-HTTP/1.1 proxies have been known to
      rewrite the Request-URI.

5.2.  The Resource Identified by a Request

   The exact resource identified by an Internet request is determined by
   examining both the Request-URI and the Host header field.

   An origin server that does not allow resources to differ by the



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   requested host MAY ignore the Host header field value when
   determining the resource identified by an HTTP/1.1 request.  (But see
   Appendix D.1.1 for other requirements on Host support in HTTP/1.1.)

   An origin server that does differentiate resources based on the host
   requested (sometimes referred to as virtual hosts or vanity host
   names) MUST use the following rules for determining the requested
   resource on an HTTP/1.1 request:

   1.  If Request-URI is an absoluteURI, the host is part of the
       Request-URI.  Any Host header field value in the request MUST be
       ignored.

   2.  If the Request-URI is not an absoluteURI, and the request
       includes a Host header field, the host is determined by the Host
       header field value.

   3.  If the host as determined by rule 1 or 2 is not a valid host on
       the server, the response MUST be a 400 (Bad Request) error
       message.

   Recipients of an HTTP/1.0 request that lacks a Host header field MAY
   attempt to use heuristics (e.g., examination of the URI path for
   something unique to a particular host) in order to determine what
   exact resource is being requested.


6.  Response

   After receiving and interpreting a request message, a server responds
   with an HTTP response message.

       Response      = Status-Line               ; Section 6.1
                       *(( general-header        ; Section 4.5
                        | response-header        ; [Part2], Section 6
                        | entity-header ) CRLF)  ; [Part3], Section 3.1
                       CRLF
                       [ message-body ]          ; Section 4.3

6.1.  Status-Line

   The first line of a 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



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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.  These codes are fully
   defined in Section 9 of [Part2].  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


      Status-Code    = 3DIGIT
      Reason-Phrase  = *<TEXT, excluding CR, LF>


7.  Connections

7.1.  Persistent Connections

7.1.1.  Purpose

   Prior to persistent connections, a separate TCP connection was
   established to fetch each URL, increasing the load on HTTP servers
   and causing congestion on the Internet.  The use of inline images and
   other associated data often require a client to make multiple
   requests of the same server in a short amount of time.  Analysis of
   these performance problems and results from a prototype
   implementation are available [Pad1995] [Spe].  Implementation
   experience and measurements of actual HTTP/1.1 (RFC 2068)
   implementations show good results [Nie1997].  Alternatives have also



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   been explored, for example, T/TCP [Tou1998].

   Persistent HTTP connections have a number of advantages:

   o  By opening and closing fewer TCP connections, CPU time is saved in
      routers and hosts (clients, servers, proxies, gateways, tunnels,
      or caches), and memory used for TCP protocol control blocks can be
      saved in hosts.

   o  HTTP requests and responses can be pipelined on a connection.
      Pipelining allows a client to make multiple requests without
      waiting for each response, allowing a single TCP connection to be
      used much more efficiently, with much lower elapsed time.

   o  Network congestion is reduced by reducing the number of packets
      caused by TCP opens, and by allowing TCP sufficient time to
      determine the congestion state of the network.

   o  Latency on subsequent requests is reduced since there is no time
      spent in TCP's connection opening handshake.

   o  HTTP can evolve more gracefully, since errors can be reported
      without the penalty of closing the TCP connection.  Clients using
      future versions of HTTP might optimistically try a new feature,
      but if communicating with an older server, retry with old
      semantics after an error is reported.

   HTTP implementations SHOULD implement persistent connections.

7.1.2.  Overall Operation

   A significant difference between HTTP/1.1 and earlier versions of
   HTTP is that persistent connections are the default behavior of any
   HTTP connection.  That is, unless otherwise indicated, the client
   SHOULD assume that the server will maintain a persistent connection,
   even after error responses from the server.

   Persistent connections provide a mechanism by which a client and a
   server can signal the close of a TCP connection.  This signaling
   takes place using the Connection header field (Section 8.1).  Once a
   close has been signaled, the client MUST NOT send any more requests
   on that connection.

7.1.2.1.  Negotiation

   An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to
   maintain a persistent connection unless a Connection header including
   the connection-token "close" was sent in the request.  If the server



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   chooses to close the connection immediately after sending the
   response, it SHOULD send a Connection header including the
   connection-token close.

   An HTTP/1.1 client MAY expect a connection to remain open, but would
   decide to keep it open based on whether the response from a server
   contains a Connection header with the connection-token close.  In
   case the client does not want to maintain a connection for more than
   that request, it SHOULD send a Connection header including the
   connection-token close.

   If either the client or the server sends the close token in the
   Connection header, that request becomes the last one for the
   connection.

   Clients and servers SHOULD NOT assume that a persistent connection is
   maintained for HTTP versions less than 1.1 unless it is explicitly
   signaled.  See Appendix D.2 for more information on backward
   compatibility with HTTP/1.0 clients.

   In order to remain persistent, all messages on the connection MUST
   have a self-defined message length (i.e., one not defined by closure
   of the connection), as described in Section 4.4.

7.1.2.2.  Pipelining

   A client that supports persistent connections MAY "pipeline" its
   requests (i.e., send multiple requests without waiting for each
   response).  A server MUST send its responses to those requests in the
   same order that the requests were received.

   Clients which assume persistent connections and pipeline immediately
   after connection establishment SHOULD be prepared to retry their
   connection if the first pipelined attempt fails.  If a client does
   such a retry, it MUST NOT pipeline before it knows the connection is
   persistent.  Clients MUST also be prepared to resend their requests
   if the server closes the connection before sending all of the
   corresponding responses.

   Clients SHOULD NOT pipeline requests using non-idempotent methods or
   non-idempotent sequences of methods (see Section 8.1.2 of [Part2]).
   Otherwise, a premature termination of the transport connection could
   lead to indeterminate results.  A client wishing to send a non-
   idempotent request SHOULD wait to send that request until it has
   received the response status for the previous request.






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7.1.3.  Proxy Servers

   It is especially important that proxies correctly implement the
   properties of the Connection header field as specified in
   Section 8.1.

   The proxy server MUST signal persistent connections separately with
   its clients and the origin servers (or other proxy servers) that it
   connects to.  Each persistent connection applies to only one
   transport link.

   A proxy server MUST NOT establish a HTTP/1.1 persistent connection
   with an HTTP/1.0 client (but see RFC 2068 [RFC2068] for information
   and discussion of the problems with the Keep-Alive header implemented
   by many HTTP/1.0 clients).

7.1.4.  Practical Considerations

   Servers will usually have some time-out value beyond which they will
   no longer maintain an inactive connection.  Proxy servers might make
   this a higher value since it is likely that the client will be making
   more connections through the same server.  The use of persistent
   connections places no requirements on the length (or existence) of
   this time-out for either the client or the server.

   When a client or server wishes to time-out it SHOULD issue a graceful
   close on the transport connection.  Clients and servers SHOULD both
   constantly watch for the other side of the transport close, and
   respond to it as appropriate.  If a client or server does not detect
   the other side's close promptly it could cause unnecessary resource
   drain on the network.

   A client, server, or proxy MAY close the transport connection at any
   time.  For example, a client might have started to send a new request
   at the same time that the server has decided to close the "idle"
   connection.  From the server's point of view, the connection is being
   closed while it was idle, but from the client's point of view, a
   request is in progress.

   This means that clients, servers, and proxies MUST be able to recover
   from asynchronous close events.  Client software SHOULD reopen the
   transport connection and retransmit the aborted sequence of requests
   without user interaction so long as the request sequence is
   idempotent (see Section 8.1.2 of [Part2]).  Non-idempotent methods or
   sequences MUST NOT be automatically retried, although user agents MAY
   offer a human operator the choice of retrying the request(s).
   Confirmation by user-agent software with semantic understanding of
   the application MAY substitute for user confirmation.  The automatic



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   retry SHOULD NOT be repeated if the second sequence of requests
   fails.

   Servers SHOULD always respond to at least one request per connection,
   if at all possible.  Servers SHOULD NOT close a connection in the
   middle of transmitting a response, unless a network or client failure
   is suspected.

   Clients that use persistent connections SHOULD limit the number of
   simultaneous connections that they maintain to a given server.  A
   single-user client SHOULD NOT maintain more than 2 connections with
   any server or proxy.  A proxy SHOULD use up to 2*N connections to
   another server or proxy, where N is the number of simultaneously
   active users.  These guidelines are intended to improve HTTP response
   times and avoid congestion.

7.2.  Message Transmission Requirements

7.2.1.  Persistent Connections and Flow Control

   HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's
   flow control mechanisms to resolve temporary overloads, rather than
   terminating connections with the expectation that clients will retry.
   The latter technique can exacerbate network congestion.

7.2.2.  Monitoring Connections for Error Status Messages

   An HTTP/1.1 (or later) client sending a message-body SHOULD monitor
   the network connection for an error status while it is transmitting
   the request.  If the client sees an error status, it SHOULD
   immediately cease transmitting the body.  If the body is being sent
   using a "chunked" encoding (Section 3.4), a zero length chunk and
   empty trailer MAY be used to prematurely mark the end of the message.
   If the body was preceded by a Content-Length header, the client MUST
   close the connection.

7.2.3.  Use of the 100 (Continue) Status

   The purpose of the 100 (Continue) status (see Section 9.1.1 of
   [Part2]) is to allow a client that is sending a request message with
   a request body to determine if the origin server is willing to accept
   the request (based on the request headers) before the client sends
   the request body.  In some cases, it might either be inappropriate or
   highly inefficient for the client to send the body if the server will
   reject the message without looking at the body.

   Requirements for HTTP/1.1 clients:




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   o  If a client will wait for a 100 (Continue) response before sending
      the request body, it MUST send an Expect request-header field
      (Section 10.2 of [Part2]) with the "100-continue" expectation.

   o  A client MUST NOT send an Expect request-header field (Section
      10.2 of [Part2]) with the "100-continue" expectation if it does
      not intend to send a request body.

   Because of the presence of older implementations, the protocol allows
   ambiguous situations in which a client may send "Expect: 100-
   continue" without receiving either a 417 (Expectation Failed) status
   or a 100 (Continue) status.  Therefore, when a client sends this
   header field to an origin server (possibly via a proxy) from which it
   has never seen a 100 (Continue) status, the client SHOULD NOT wait
   for an indefinite period before sending the request body.

   Requirements for HTTP/1.1 origin servers:

   o  Upon receiving a request which includes an Expect request-header
      field with the "100-continue" expectation, an origin server MUST
      either respond with 100 (Continue) status and continue to read
      from the input stream, or respond with a final status code.  The
      origin server MUST NOT wait for the request body before sending
      the 100 (Continue) response.  If it responds with a final status
      code, it MAY close the transport connection or it MAY continue to
      read and discard the rest of the request.  It MUST NOT perform the
      requested method if it returns a final status code.

   o  An origin server SHOULD NOT send a 100 (Continue) response if the
      request message does not include an Expect request-header field
      with the "100-continue" expectation, and MUST NOT send a 100
      (Continue) response if such a request comes from an HTTP/1.0 (or
      earlier) client.  There is an exception to this rule: for
      compatibility with RFC 2068, a server MAY send a 100 (Continue)
      status in response to an HTTP/1.1 PUT or POST request that does
      not include an Expect request-header field with the "100-continue"
      expectation.  This exception, the purpose of which is to minimize
      any client processing delays associated with an undeclared wait
      for 100 (Continue) status, applies only to HTTP/1.1 requests, and
      not to requests with any other HTTP-version value.

   o  An origin server MAY omit a 100 (Continue) response if it has
      already received some or all of the request body for the
      corresponding request.

   o  An origin server that sends a 100 (Continue) response MUST
      ultimately send a final status code, once the request body is
      received and processed, unless it terminates the transport



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      connection prematurely.

   o  If an origin server receives a request that does not include an
      Expect request-header field with the "100-continue" expectation,
      the request includes a request body, and the server responds with
      a final status code before reading the entire request body from
      the transport connection, then the server SHOULD NOT close the
      transport connection until it has read the entire request, or
      until the client closes the connection.  Otherwise, the client
      might not reliably receive the response message.  However, this
      requirement is not be construed as preventing a server from
      defending itself against denial-of-service attacks, or from badly
      broken client implementations.

   Requirements for HTTP/1.1 proxies:

   o  If a proxy receives a request that includes an Expect request-
      header field with the "100-continue" expectation, and the proxy
      either knows that the next-hop server complies with HTTP/1.1 or
      higher, or does not know the HTTP version of the next-hop server,
      it MUST forward the request, including the Expect header field.

   o  If the proxy knows that the version of the next-hop server is
      HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
      respond with a 417 (Expectation Failed) status.

   o  Proxies SHOULD maintain a cache recording the HTTP version numbers
      received from recently-referenced next-hop servers.

   o  A proxy MUST NOT forward a 100 (Continue) response if the request
      message was received from an HTTP/1.0 (or earlier) client and did
      not include an Expect request-header field with the "100-continue"
      expectation.  This requirement overrides the general rule for
      forwarding of 1xx responses (see Section 9.1 of [Part2]).

7.2.4.  Client Behavior if Server Prematurely Closes Connection

   If an HTTP/1.1 client sends a request which includes a request body,
   but which does not include an Expect request-header field with the
   "100-continue" expectation, and if the client is not directly
   connected to an HTTP/1.1 origin server, and if the client sees the
   connection close before receiving any status from the server, the
   client SHOULD retry the request.  If the client does retry this
   request, it MAY use the following "binary exponential backoff"
   algorithm to be assured of obtaining a reliable response:

   1.  Initiate a new connection to the server




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   2.  Transmit the request-headers

   3.  Initialize a variable R to the estimated round-trip time to the
       server (e.g., based on the time it took to establish the
       connection), or to a constant value of 5 seconds if the round-
       trip time is not available.

   4.  Compute T = R * (2**N), where N is the number of previous retries
       of this request.

   5.  Wait either for an error response from the server, or for T
       seconds (whichever comes first)

   6.  If no error response is received, after T seconds transmit the
       body of the request.

   7.  If client sees that the connection is closed prematurely, repeat
       from step 1 until the request is accepted, an error response is
       received, or the user becomes impatient and terminates the retry
       process.

   If at any point an error status is received, the client

   o  SHOULD NOT continue and

   o  SHOULD close the connection if it has not completed sending the
      request message.


8.  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.

8.1.  Connection

   The Connection general-header field allows the sender to specify
   options that are desired for that particular connection and MUST NOT
   be communicated by proxies over further connections.

   The Connection header has the following grammar:

       Connection = "Connection" ":" 1#(connection-token)
       connection-token  = token

   HTTP/1.1 proxies MUST parse the Connection header field before a



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   message is forwarded and, for each connection-token in this field,
   remove any header field(s) from the message with the same name as the
   connection-token.  Connection options are signaled by the presence of
   a connection-token in the Connection header field, not by any
   corresponding additional header field(s), since the additional header
   field may not be sent if there are no parameters associated with that
   connection option.

   Message headers listed in the Connection header MUST NOT include end-
   to-end headers, such as Cache-Control.

   HTTP/1.1 defines the "close" connection option for the sender to
   signal that the connection will be closed after completion of the
   response.  For example,

       Connection: close

   in either the request or the response header fields indicates that
   the connection SHOULD NOT be considered `persistent' (Section 7.1)
   after the current request/response is complete.

   HTTP/1.1 applications that do not support persistent connections MUST
   include the "close" connection option in every message.

   A system receiving an HTTP/1.0 (or lower-version) message that
   includes a Connection header MUST, for each connection-token in this
   field, remove and ignore any header field(s) from the message with
   the same name as the connection-token.  This protects against
   mistaken forwarding of such header fields by pre-HTTP/1.1 proxies.
   See Appendix D.2.

8.2.  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 transfer-length of
   the message-body, unless this is prohibited by the rules in
   Section 4.4.




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   Any Content-Length greater than or equal to zero is a valid value.
   Section 4.4 describes how to determine the length of a message-body
   if a Content-Length is not given.

   Note that 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 sent whenever the message's length can be determined prior
   to being transferred, unless this is prohibited by the rules in
   Section 4.4.

8.3.  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 3.3.1; it MUST be sent in RFC 1123 [RFC1123]-date format.

       Date  = "Date" ":" HTTP-date

   An example is

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

   Origin servers MUST include a Date header field in all responses,
   except in these cases:

   1.  If the response status code is 100 (Continue) or 101 (Switching
       Protocols), the response MAY include a Date header field, at the
       server's option.

   2.  If the response status code conveys a server error, e.g. 500
       (Internal Server Error) or 503 (Service Unavailable), and it is
       inconvenient or impossible to generate a valid Date.

   3.  If the server does not have a clock that can provide a reasonable
       approximation of the current time, its responses MUST NOT include
       a Date header field.  In this case, the rules in Section 8.3.1
       MUST be followed.

   A received message that does not have a Date header field MUST be
   assigned one by the recipient if the message will be cached by that
   recipient or gatewayed via a protocol which requires a Date.  An HTTP
   implementation without a clock MUST NOT cache responses without
   revalidating them on every use.  An HTTP cache, especially a shared
   cache, SHOULD use a mechanism, such as NTP [RFC1305], to synchronize
   its clock with a reliable external standard.




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   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 client without a clock MUST NOT send a Date
   header field in a request.

   The HTTP-date sent in a Date header SHOULD NOT represent a date and
   time subsequent to the generation of the message.  It SHOULD
   represent the best available approximation of the date and time of
   message generation, unless the implementation has no means of
   generating a reasonably accurate date and time.  In theory, the date
   ought to 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.

8.3.1.  Clockless Origin Server Operation

   Some origin server implementations might not have a clock available.
   An origin server without a clock MUST NOT assign Expires or Last-
   Modified values to a response, unless these values were associated
   with the resource by a system or user with a reliable clock.  It MAY
   assign an Expires value that is known, at or before server
   configuration time, to be in the past (this allows "pre-expiration"
   of responses without storing separate Expires values for each
   resource).

8.4.  Host

   The Host request-header field specifies the Internet host and port
   number of the resource being requested, as obtained from the original
   URI given by the user or referring resource (generally an HTTP URL,
   as described in Section 3.2.2).  The Host field value MUST represent
   the naming authority of the origin server or gateway given by the
   original URL.  This allows the origin server or gateway to
   differentiate between internally-ambiguous URLs, such as the root "/"
   URL of a server for multiple host names on a single IP address.

       Host = "Host" ":" host [ ":" port ] ; Section 3.2.2

   A "host" without any trailing port information implies the default
   port for the service requested (e.g., "80" for an HTTP URL).  For
   example, a request on the origin server for
   <http://www.w3.org/pub/WWW/> would properly include:

       GET /pub/WWW/ HTTP/1.1
       Host: www.w3.org

   A client MUST include a Host header field in all HTTP/1.1 request
   messages .  If the requested URI does not include an Internet host



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   name for the service being requested, then the Host header field MUST
   be given with an empty value.  An HTTP/1.1 proxy MUST ensure that any
   request message it forwards does contain an appropriate Host header
   field that identifies the service being requested by the proxy.  All
   Internet-based HTTP/1.1 servers MUST respond with a 400 (Bad Request)
   status code to any HTTP/1.1 request message which lacks a Host header
   field.

   See sections 5.2 and D.1.1 for other requirements relating to Host.

8.5.  TE

   The TE request-header field indicates what extension transfer-codings
   it is willing to accept in the response and whether or not it is
   willing to accept trailer fields in a chunked transfer-coding.  Its
   value may consist of the keyword "trailers" and/or a comma-separated
   list of extension transfer-coding names with optional accept
   parameters (as described in Section 3.4).

       TE        = "TE" ":" #( t-codings )
       t-codings = "trailers" | ( transfer-extension [ accept-params ] )

   The presence of the keyword "trailers" indicates that the client is
   willing to accept trailer fields in a chunked transfer-coding, as
   defined in Section 3.4.1.  This keyword is reserved for use with
   transfer-coding values even though it does not itself represent a
   transfer-coding.

   Examples of its use are:

       TE: deflate
       TE:
       TE: trailers, deflate;q=0.5

   The TE header field only applies to the immediate connection.
   Therefore, the keyword MUST be supplied within a Connection header
   field (Section 8.1) whenever TE is present in an HTTP/1.1 message.

   A server tests whether a transfer-coding is acceptable, according to
   a TE field, using these rules:

   1.  The "chunked" transfer-coding is always acceptable.  If the
       keyword "trailers" is listed, the client indicates that it is
       willing to accept trailer fields in the chunked response on
       behalf of itself and any downstream clients.  The implication is
       that, if given, the client is stating that either all downstream
       clients are willing to accept trailer fields in the forwarded
       response, or that it will attempt to buffer the response on



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       behalf of downstream recipients.

       Note: HTTP/1.1 does not define any means to limit the size of a
       chunked response such that a client can be assured of buffering
       the entire response.

   2.  If the transfer-coding being tested is one of the transfer-
       codings listed in the TE field, then it is acceptable unless it
       is accompanied by a qvalue of 0.  (As defined in Section 2.4 of
       [Part3], a qvalue of 0 means "not acceptable.")

   3.  If multiple transfer-codings are acceptable, then the acceptable
       transfer-coding with the highest non-zero qvalue is preferred.
       The "chunked" transfer-coding always has a qvalue of 1.

   If the TE field-value is empty or if no TE field is present, the only
   transfer-coding is "chunked".  A message with no transfer-coding is
   always acceptable.

8.6.  Trailer

   The Trailer general field value indicates that the given set of
   header fields is present in the trailer of a message encoded with
   chunked transfer-coding.

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

   An HTTP/1.1 message SHOULD include a Trailer header field in a
   message using chunked transfer-coding with a non-empty trailer.
   Doing so allows the recipient to know which header fields to expect
   in the trailer.

   If no Trailer header field is present, the trailer SHOULD NOT include
   any header fields.  See Section 3.4.1 for restrictions on the use of
   trailer fields in a "chunked" transfer-coding.

   Message header fields listed in the Trailer header field MUST NOT
   include the following header fields:

   o  Transfer-Encoding

   o  Content-Length

   o  Trailer







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8.7.  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-coding in that the transfer-coding is a
   property of the message, not of the entity.

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

   Transfer-codings are defined in Section 3.4.  An example is:

     Transfer-Encoding: chunked

   If multiple encodings have been applied to an entity, the transfer-
   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.

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

8.8.  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 Upgrade header field is intended to provide a simple mechanism
   for transition from HTTP/1.1 to some other, incompatible protocol.
   It does so by allowing the client to advertise its desire to use
   another protocol, such as a later version of HTTP with a higher major
   version number, even though the current request has been made using
   HTTP/1.1.  This eases the difficult transition between incompatible
   protocols by allowing the client to initiate a request in the more
   commonly supported protocol while indicating to the server that it
   would like to use a "better" protocol if available (where "better" is
   determined by the server, possibly according to the nature of the
   method and/or resource being requested).




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   The Upgrade header field only applies to switching application-layer
   protocols upon the existing transport-layer connection.  Upgrade
   cannot be used to insist on a protocol change; its acceptance and use
   by the server is optional.  The capabilities and nature of the
   application-layer communication after the protocol change is entirely
   dependent upon the new protocol chosen, although the first action
   after changing the protocol MUST be a response to the initial HTTP
   request containing the Upgrade header field.

   The Upgrade header field only applies to the immediate connection.
   Therefore, the upgrade keyword MUST be supplied within a Connection
   header field (Section 8.1) whenever Upgrade is present in an HTTP/1.1
   message.

   The Upgrade header field cannot be used to indicate a switch to a
   protocol on a different connection.  For that purpose, it is more
   appropriate to use a 301, 302, 303, or 305 redirection response.

   This specification only defines the protocol name "HTTP" for use by
   the family of Hypertext Transfer Protocols, as defined by the HTTP
   version rules of Section 3.1 and future updates to this
   specification.  Any token can be used as a protocol name; however, it
   will only be useful if both the client and server associate the name
   with the same protocol.

8.9.  Via

   The Via general-header field MUST be used by gateways and proxies to
   indicate the intermediate protocols and recipients 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 [RFC822] and is intended to be used for tracking message
   forwards, avoiding request loops, and identifying the protocol
   capabilities of all senders along the request/response chain.

      Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
      received-protocol = [ protocol-name "/" ] protocol-version
      protocol-name     = token
      protocol-version  = token
      received-by       = ( host [ ":" port ] ) | pseudonym
      pseudonym         = token

   The received-protocol indicates the protocol version of the message
   received by the server or client along each segment of the request/
   response chain.  The received-protocol version is appended to the Via
   field value when the message is forwarded so that information about
   the protocol capabilities of upstream applications remains visible to
   all recipients.



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   The protocol-name is optional if and only if it would be "HTTP".  The
   received-by field is normally the host and optional port number of a
   recipient server or client that subsequently forwarded the message.
   However, if the real host is considered to be sensitive information,
   it MAY be replaced by a pseudonym.  If the port is not given, it MAY
   be assumed to be the default port of the received-protocol.

   Multiple Via field values represents each proxy or gateway that has
   forwarded the message.  Each recipient MUST append its information
   such that the end result is ordered according to the sequence of
   forwarding applications.

   Comments MAY be used in the Via header field to identify the software
   of the recipient proxy or gateway, analogous to the User-Agent and
   Server header fields.  However, all comments in the Via field are
   optional and MAY be removed by any recipient prior to forwarding the
   message.

   For example, a request message could be sent from an HTTP/1.0 user
   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
   forward the request to a public proxy at nowhere.com, which completes
   the request by forwarding it to the origin server at www.ics.uci.edu.
   The request received by www.ics.uci.edu would then have the following
   Via header field:

       Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)

   Proxies and gateways used as a portal through a network firewall
   SHOULD NOT, by default, forward the names and ports of hosts within
   the firewall region.  This information SHOULD only be propagated if
   explicitly enabled.  If not enabled, the received-by host of any host
   behind the firewall SHOULD be replaced by an appropriate pseudonym
   for that host.

   For organizations that have strong privacy requirements for hiding
   internal structures, a proxy MAY combine an ordered subsequence of
   Via header field entries with identical received-protocol values into
   a single such entry.  For example,

       Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

   could be collapsed to

       Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

   Applications SHOULD NOT combine multiple entries unless they are all
   under the same organizational control and the hosts have already been
   replaced by pseudonyms.  Applications MUST NOT combine entries which



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   have different received-protocol values.


9.  IANA Considerations

   TBD.


10.  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.

10.1.  Personal Information

   HTTP clients are often privy to large amounts of personal information
   (e.g. the user's name, location, mail address, passwords, encryption
   keys, etc.), and SHOULD be very careful to prevent unintentional
   leakage of this information via the HTTP protocol to other sources.
   We very strongly recommend that a convenient interface be provided
   for the user to control dissemination of such information, and that
   designers and implementors be particularly careful in this area.
   History shows that errors in this area often create serious security
   and/or privacy problems and generate highly adverse publicity for the
   implementor's company.

10.2.  Abuse of Server Log Information

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

10.3.  Attacks Based On File and Path Names

   Implementations of HTTP origin servers SHOULD be careful to restrict
   the documents returned by HTTP requests to be only those that were
   intended by the server administrators.  If an HTTP server translates
   HTTP URIs directly into file system calls, the server MUST take
   special care not to serve files that were not intended to be
   delivered to HTTP clients.  For example, UNIX, Microsoft Windows, and
   other operating systems use ".." as a path component to indicate a



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   directory level above the current one.  On such a system, an HTTP
   server MUST disallow any such construct in the Request-URI if it
   would otherwise allow access to a resource outside those intended to
   be accessible via the HTTP server.  Similarly, files intended for
   reference only internally to the server (such as access control
   files, configuration files, and script code) MUST be protected from
   inappropriate retrieval, since they might contain sensitive
   information.  Experience has shown that minor bugs in such HTTP
   server implementations have turned into security risks.

10.4.  DNS Spoofing

   Clients using HTTP rely heavily on the Domain Name Service, and are
   thus generally prone to security attacks based on the deliberate mis-
   association of IP addresses and DNS names.  Clients need to be
   cautious in assuming the continuing validity of an IP number/DNS name
   association.

   In particular, HTTP clients SHOULD rely on their name resolver for
   confirmation of an IP number/DNS name association, rather than
   caching the result of previous host name lookups.  Many platforms
   already can cache host name lookups locally when appropriate, and
   they SHOULD be configured to do so.  It is proper for these lookups
   to be cached, however, only when the TTL (Time To Live) information
   reported by the name server makes it likely that the cached
   information will remain useful.

   If HTTP clients cache the results of host name lookups in order to
   achieve a performance improvement, they MUST observe the TTL
   information reported by DNS.

   If HTTP clients do not observe this rule, they could be spoofed when
   a previously-accessed server's IP address changes.  As network
   renumbering is expected to become increasingly common [RFC1900], the
   possibility of this form of attack will grow.  Observing this
   requirement thus reduces this potential security vulnerability.

   This requirement also improves the load-balancing behavior of clients
   for replicated servers using the same DNS name and reduces the
   likelihood of a user's experiencing failure in accessing sites which
   use that strategy.

10.5.  Proxies and Caching

   By their very nature, HTTP proxies are men-in-the-middle, and
   represent an opportunity for man-in-the-middle attacks.  Compromise
   of the systems on which the proxies run can result in serious
   security and privacy problems.  Proxies have access to security-



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   related information, personal information about individual users and
   organizations, and proprietary information belonging to users and
   content providers.  A compromised proxy, or a proxy implemented or
   configured without regard to security and privacy considerations,
   might be used in the commission of a wide range of potential attacks.

   Proxy operators should protect the systems on which proxies run as
   they would protect any system that contains or transports sensitive
   information.  In particular, log information gathered at proxies
   often contains highly sensitive personal information, and/or
   information about organizations.  Log information should be carefully
   guarded, and appropriate guidelines for use developed and followed.
   (Section 10.2).

   Proxy implementors should consider the privacy and security
   implications of their design and coding decisions, and of the
   configuration options they provide to proxy operators (especially the
   default configuration).

   Users of a proxy need to be aware that they are no trustworthier than
   the people who run the proxy; HTTP itself cannot solve this problem.

   The judicious use of cryptography, when appropriate, may suffice to
   protect against a broad range of security and privacy attacks.  Such
   cryptography is beyond the scope of the HTTP/1.1 specification.

10.6.  Denial of Service Attacks on Proxies

   They exist.  They are hard to defend against.  Research continues.
   Beware.


11.  Acknowledgments

   This specification makes heavy use of the augmented BNF and generic
   constructs defined by David H. Crocker for RFC 822 [RFC822].
   Similarly, it reuses many of the definitions provided by Nathaniel
   Borenstein and Ned Freed for MIME [RFC2045].  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 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, Hakon W. Lie, Ari Luotonen, Rob



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   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                  Ross Patterson
       Harald Tveit Alvestrand     Albert Lunde
       Keith Ball                  John C. Mallery
       Brian Behlendorf            Jean-Philippe Martin-Flatin
       Paul Burchard               Mitra
       Maurizio Codogno            David Morris
       Mike Cowlishaw              Gavin Nicol
       Roman Czyborra              Bill Perry
       Michael A. Dolan            Jeffrey Perry
       David J. Fiander            Scott Powers
       Alan Freier                 Owen Rees
       Marc Hedlund                Luigi Rizzo
       Greg Herlihy                David Robinson
       Koen Holtman                Marc Salomon
       Alex Hopmann                Rich Salz
       Bob Jernigan                Allan M. Schiffman
       Shel Kaphan                 Jim Seidman
       Rohit Khare                 Chuck Shotton
       John Klensin                Eric W. Sink
       Martijn Koster              Simon E. Spero
       Alexei Kosut                Richard N. Taylor
       David M. Kristol            Robert S. Thau
       Daniel LaLiberte            Bill (BearHeart) Weinman
       Ben Laurie                  Francois Yergeau
       Paul J. Leach               Mary Ellen Zurko
       Daniel DuBois               Josh Cohen

   Thanks to the "cave men" of Palo Alto.  You know who you are.

   Jim Gettys (the current editor of this document) wishes particularly
   to thank Roy Fielding, the previous editor of this document, along
   with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
   Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
   Larry Masinter for their help.  And thanks go particularly to Jeff
   Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.

   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
   Frystyk implemented RFC 2068 early, and we wish to thank them for the
   discovery of many of the problems that this document attempts to



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   rectify.

   Based on an XML translation of RFC 2616 by Julian Reschke.


12.  References

   [ISO-8859]
              International Organization for Standardization,
              "Information technology - 8-bit single byte coded graphic
              - character sets", 1987-1990.

              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.

   [Nie1997]  Nielsen, H., Gettys, J., Prud'hommeaux, E., Lie, H., and
              C. Lilley, "Network Performance Effects of HTTP/1.1, CSS1,
              and PNG", Proceedings of ACM SIGCOMM '97, Cannes France ,
              Sep 1997.

   [Pad1995]  Padmanabhan, V. and J. Mogul, "Improving HTTP Latency",
              Computer Networks and ISDN Systems v. 28, pp. 25-35,
              Dec 1995.

              Slightly revised version of paper in Proc. 2nd
              International WWW Conference '94: Mosaic and the Web, Oct.
              1994, which is available at <http://www.ncsa.uiuc.edu/SDG/
              IT94/Proceedings/DDay/mogul/HTTPLatency.html>.

   [Part2]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
              part 2: Message Semantics",
              draft-ietf-httpbis-p2-semantics-00 (work in progress),
              December 2007.

   [Part3]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
              part 3: Message Payload and Content Negotiation",
              draft-ietf-httpbis-p3-payload-00 (work in progress),
              December 2007.

   [Part6]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,



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              Masinter, L., Leach, P., and T. Berners-Lee, "HTTP/1.1,
              part 6: Caching", draft-ietf-httpbis-p6-cache-00 (work in
              progress), December 2007.

   [RFC1036]  Horton, M. and R. Adams, "Standard for interchange of
              USENET messages", RFC 1036, December 1987.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
              Specification, Implementation", RFC 1305, March 1992.

   [RFC1436]  Anklesaria, F., McCahill, M., Lindner, P., Johnson, D.,
              Torrey, D., and B. Alberti, "The Internet Gopher Protocol
              (a distributed document search and retrieval protocol)",
              RFC 1436, March 1993.

   [RFC1590]  Postel, J., "Media Type Registration Procedure", RFC 1590,
              November 1996.

   [RFC1630]  Berners-Lee, T., "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, June 1994.

   [RFC1700]  Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,
              RFC 1700, October 1994.

   [RFC1737]  Masinter, L. and K. Sollins, "Functional Requirements for
              Uniform Resource Names", RFC 1737, December 1994.

   [RFC1738]  Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
              Resource Locators (URL)", RFC 1738, December 1994.

   [RFC1808]  Fielding, R., "Relative Uniform Resource Locators",
              RFC 1808, June 1995.

   [RFC1900]  Carpenter, B. and Y. Rekhter, "Renumbering Needs Work",
              RFC 1900, February 1996.

   [RFC1945]  Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.




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   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, November 1996.

   [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
              Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
              RFC 2068, January 1997.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2145]  Mogul, J., Fielding, R., Gettys, J., and H. Nielsen, "Use
              and Interpretation of HTTP Version Numbers", RFC 2145,
              May 1997.

   [RFC2324]  Masinter, L., "Hyper Text Coffee Pot Control Protocol
              (HTCPCP/1.0)", RFC 2324, April 1998.

   [RFC2396]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC821]   Postel, J., "Simple Mail Transfer Protocol", STD 10,
              RFC 821, August 1982.

   [RFC822]   Crocker, D., "Standard for the format of ARPA Internet
              text messages", STD 11, RFC 822, August 1982.

   [RFC959]   Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 959, October 1985.

   [RFC977]   Kantor, B. and P. Lapsley, "Network News Transfer
              Protocol", RFC 977, February 1986.

   [Spe]      Spero, S., "Analysis of HTTP Performance Problems",
              <http://sunsite.unc.edu/mdma-release/http-prob.html>.

   [Tou1998]  Touch, J., Heidemann, J., and K. Obraczka, "Analysis of
              HTTP Performance", ISI Research Report ISI/RR-98-463
              (original report dated Aug.1996), Aug 1998,
              <http://www.isi.edu/touch/pubs/http-perf96/>.

   [USASCII]  American National Standards Institute, "Coded Character
              Set -- 7-bit American Standard Code for Information



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              Interchange", ANSI X3.4, 1986.

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


Appendix A.  Internet Media Type message/http and application/http

   In addition to defining the HTTP/1.1 protocol, this document serves
   as the specification for the Internet media type "message/http" and
   "application/http".  The message/http type can be used to enclose a
   single HTTP request or response message, provided that it obeys the
   MIME restrictions for all "message" types regarding line length and
   encodings.  The application/http type can be used to enclose a
   pipeline of one or more HTTP request or response messages (not
   intermixed).  The following is to be registered with IANA [RFC1590].

   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

   Media Type name:  application

   Media subtype name:  http

   Required parameters:  none






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   Optional parameters:  version, msgtype

      version:  The HTTP-Version number of the enclosed messages (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:  HTTP messages enclosed by this type are in
      "binary" format; use of an appropriate Content-Transfer-Encoding
      is required when transmitted via E-mail.

   Security considerations:  none


Appendix 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 message-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.

   The character set of an entity-body SHOULD be labeled as the lowest
   common denominator of the character codes used within that body, with
   the exception that not labeling the entity is preferred over labeling
   the entity with the labels US-ASCII or ISO-8859-1.  See [Part3].

   Additional rules for requirements on parsing and encoding of dates
   and other potential problems with date encodings include:

   o  HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date
      which appears to be more than 50 years in the future is in fact in
      the past (this helps solve the "year 2000" problem).

   o  An HTTP/1.1 implementation MAY internally represent a parsed
      Expires date as earlier than the proper value, but MUST NOT



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      internally represent a parsed Expires date as later than the
      proper value.

   o  All expiration-related calculations MUST be done in GMT.  The
      local time zone MUST NOT influence the calculation or comparison
      of an age or expiration time.

   o  If an HTTP header incorrectly carries a date value with a time
      zone other than GMT, it MUST be converted into GMT using the most
      conservative possible conversion.


Appendix C.  Conversion of Date Formats

   HTTP/1.1 uses a restricted set of date formats (Section 3.3.1) to
   simplify the process of date comparison.  Proxies and 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.


Appendix D.  Compatibility with Previous Versions

   It is beyond the scope of a protocol specification to mandate
   compliance with previous versions.  HTTP/1.1 was deliberately
   designed, however, to make supporting previous versions easy.  It is
   worth noting that, at the time of composing this specification
   (1996), we would expect commercial HTTP/1.1 servers to:

   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.

   And we would expect HTTP/1.1 clients to:

   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.

   For most implementations of HTTP/1.0, each connection is established
   by the client prior to the request and closed by the server after



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   sending the response.  Some implementations implement the Keep-Alive
   version of persistent connections described in Section 19.7.1 of RFC
   2068 [RFC2068].

D.1.  Changes from HTTP/1.0

   This section summarizes major differences between versions HTTP/1.0
   and HTTP/1.1.

D.1.1.  Changes to Simplify Multi-homed Web Servers and Conserve IP
        Addresses

   The requirements that clients and servers support the Host request-
   header, report an error if the Host request-header (Section 8.4) is
   missing from an HTTP/1.1 request, and accept absolute URIs
   (Section 5.1.2) are among the most important changes defined by this
   specification.

   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
   addresses and servers; there was no other established mechanism for
   distinguishing the intended server of a request than the IP address
   to which that request was directed.  The changes outlined above will
   allow the Internet, once older HTTP clients are no longer common, to
   support multiple Web sites from a single IP address, greatly
   simplifying large operational Web servers, where allocation of many
   IP addresses to a single host has created serious problems.  The
   Internet will also be able to recover the IP addresses that have been
   allocated for the sole purpose of allowing special-purpose domain
   names to be used in root-level HTTP URLs.  Given the rate of growth
   of the Web, and the number of servers already deployed, it is
   extremely important that all implementations of HTTP (including
   updates to existing HTTP/1.0 applications) correctly implement these
   requirements:

   o  Both clients and servers MUST support the Host request-header.

   o  A client that sends an HTTP/1.1 request MUST send a Host header.

   o  Servers MUST report a 400 (Bad Request) error if an HTTP/1.1
      request does not include a Host request-header.

   o  Servers MUST accept absolute URIs.

D.2.  Compatibility with HTTP/1.0 Persistent Connections

   Some clients and servers might wish to be compatible with some
   previous implementations of persistent connections in HTTP/1.0
   clients and servers.  Persistent connections in HTTP/1.0 are



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   explicitly negotiated as they are not the default behavior.  HTTP/1.0
   experimental implementations of persistent connections are faulty,
   and the new facilities in HTTP/1.1 are designed to rectify these
   problems.  The problem was that some existing 1.0 clients may be
   sending Keep-Alive to a proxy server that doesn't understand
   Connection, which would then erroneously forward it to the next
   inbound server, which would establish the Keep-Alive connection and
   result in a hung HTTP/1.0 proxy waiting for the close on the
   response.  The result is that HTTP/1.0 clients must be prevented from
   using Keep-Alive when talking to proxies.

   However, talking to proxies is the most important use of persistent
   connections, so that prohibition is clearly unacceptable.  Therefore,
   we need some other mechanism for indicating a persistent connection
   is desired, which is safe to use even when talking to an old proxy
   that ignores Connection.  Persistent connections are the default for
   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
   declaring non-persistence.  See Section 8.1.

   The original HTTP/1.0 form of persistent connections (the Connection:
   Keep-Alive and Keep-Alive header) is documented in RFC 2068.
   [RFC2068]

D.3.  Changes from RFC 2068

   This specification has been carefully audited to correct and
   disambiguate key word usage; RFC 2068 had many problems in respect to
   the conventions laid out in RFC 2119 [RFC2119].

   Transfer-coding and message lengths all interact in ways that
   required fixing exactly when chunked encoding is used (to allow for
   transfer encoding that may not be self delimiting); it was important
   to straighten out exactly how message lengths are computed.

   The use and interpretation of HTTP version numbers has been clarified
   by RFC 2145.  Require proxies to upgrade requests to highest protocol
   version they support to deal with problems discovered in HTTP/1.0
   implementations (Section 3.1)

   Proxies should be able to add Content-Length when appropriate.

   Transfer-coding had significant problems, particularly with
   interactions with chunked encoding.  The solution is that transfer-
   codings become as full fledged as content-codings.  This involves
   adding an IANA registry for transfer-codings (separate from content
   codings), a new header field (TE) and enabling trailer headers in the
   future.  Transfer encoding is a major performance benefit, so it was
   worth fixing [Nie1997].  TE also solves another, obscure, downward



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   interoperability problem that could have occurred due to interactions
   between authentication trailers, chunked encoding and HTTP/1.0
   clients.(Section 3.4, 3.4.1, and 8.5)


Index

   A
      application/http Media Type  53

   C
      cache  8
      cacheable  8
      client  7
      connection  6
      Connection header  37
      content negotiation  7
      Content-Length header  38

   D
      Date header  39
      downstream  9

   E
      entity  6

   G
      gateway  8
      Grammar
         ALPHA  13
         asctime-date  19
         attribute  19
         CHAR  13
         chunk  21
         chunk-data  21
         chunk-ext-name  21
         chunk-ext-val  21
         chunk-extension  21
         chunk-size  21
         Chunked-Body  21
         comment  14
         Connection  37
         connection-token  37
         Content-Length  38
         CR  13
         CRLF  13
         ctext  14
         CTL  13



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         Date  39
         date1  19
         date2  19
         date3  19
         DIGIT  13
         extension-code  30
         extension-method  27
         field-content  23
         field-name  23
         field-value  23
         general-header  26
         generic-message  22
         HEX  14
         Host  40
         HT  13
         HTTP-date  19
         HTTP-message  22
         HTTP-Version  15
         http_URL  17
         last-chunk  21
         LF  13
         LOALPHA  13
         LWS  14
         message-body  24
         message-header  23
         Method  27
         month  19
         OCTET  13
         parameter  19
         protocol-name  44
         protocol-version  44
         pseudonym  44
         qdtext  15
         quoted-pair  15
         quoted-string  15
         Reason-Phrase  30
         received-by  44
         received-protocol  44
         Request  27
         Request-Line  27
         Request-URI  27
         Response  29
         rfc850-date  19
         rfc1123-date  19
         separators  14
         SP  13
         start-line  22
         Status-Code  30



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         Status-Line  29
         t-codings  41
         TE  41
         TEXT  14
         time  19
         token  14
         Trailer  42
         trailer  21
         transfer-coding  19
         Transfer-Encoding  43
         transfer-extension  19
         UPALPHA  13
         Upgrade  43
         value  19
         Via  44
         weekday  19
         wkday  19

   H
      Headers
         Connection  37
         Content-Length  38
         Date  39
         Host  40
         TE  41
         Trailer  42
         Transfer-Encoding  43
         Upgrade  43
         Via  44
      Host header  40

   I
      inbound  9

   M
      Media Type
         application/http  53
         message/http  53
      message  6
      message/http Media Type  53

   O
      origin server  8
      outbound  9

   P
      proxy  8




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   R
      representation  7
      request  6
      resource  6
      response  6

   S
      server  7

   T
      TE header  41
      Trailer header  42
      Transfer-Encoding header  43
      tunnel  8

   U
      Upgrade header  43
      upstream  9
      user agent  7

   V
      variant  7
      Via header  44


Authors' Addresses

   Roy T. Fielding (editor)
   Day Software
   23 Corporate Plaza DR, Suite 280
   Newport Beach, CA  92660
   USA

   Phone: +1-949-706-5300
   Fax:   +1-949-706-5305
   Email: fielding@gbiv.com
   URI:   http://roy.gbiv.com/


   Jim Gettys
   One Laptop per Child
   21 Oak Knoll Road
   Carlisle, MA  01741
   USA

   Email: jg@laptop.org
   URI:   http://www.laptop.org/




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   Jeffrey C. Mogul
   Hewlett-Packard Company
   HP Labs, Large Scale Systems Group
   1501 Page Mill Road, MS 1177
   Palo Alto, CA  94304
   USA

   Email: JeffMogul@acm.org


   Henrik Frystyk Nielsen
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052
   USA

   Email: henrikn@microsoft.com


   Larry Masinter
   Adobe Systems, Incorporated
   345 Park Ave
   San Jose, CA  95110
   USA

   Email: LMM@acm.org
   URI:   http://larry.masinter.net/


   Paul J. Leach
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052

   Email: paulle@microsoft.com


   Tim Berners-Lee
   World Wide Web Consortium
   MIT Computer Science and Artificial Intelligence Laboratory
   The Stata Center, Building 32
   32 Vassar Street
   Cambridge, MA  02139
   USA

   Email: timbl@w3.org
   URI:   http://www.w3.org/People/Berners-Lee/




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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
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