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RFC 2616


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Hypertext Transfer Protocol -- HTTP/1.1

Part 1 of 7, p. 1 to 17
None       Next RFC Part

Obsoleted by:    7230    7231    7232    7233    7234    7235
Obsoletes:    2068
Updated by:    2817    5785    6266    6585


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Network Working Group                                      R. Fielding
Request for Comments: 2616                                   UC Irvine
Obsoletes: 2068                                              J. Gettys
Category: Standards Track                                   Compaq/W3C
                                                              J. Mogul
                                                                Compaq
                                                            H. Frystyk
                                                               W3C/MIT
                                                           L. Masinter
                                                                 Xerox
                                                              P. Leach
                                                             Microsoft
                                                        T. Berners-Lee
                                                               W3C/MIT
                                                             June 1999


                Hypertext Transfer Protocol -- HTTP/1.1

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypermedia information
   systems. It is a generic, stateless, protocol which can be used for
   many tasks beyond its use for hypertext, such as name servers and
   distributed object management systems, through extension of its
   request methods, error codes and headers [47]. A feature of HTTP is
   the typing and negotiation of data representation, allowing systems
   to be built independently of the data being transferred.

   HTTP has been in use by the World-Wide Web global information
   initiative since 1990. This specification defines the protocol
   referred to as "HTTP/1.1", and is an update to RFC 2068 [33].

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Table of Contents

   1   Introduction ...................................................7
   1.1    Purpose......................................................7
   1.2   Requirements .................................................8
   1.3   Terminology ..................................................8
   1.4   Overall Operation ...........................................12
   2   Notational Conventions and Generic Grammar ....................14
   2.1   Augmented BNF ...............................................14
   2.2   Basic Rules .................................................15
   3   Protocol Parameters ...........................................17
   3.1   HTTP Version ................................................17
   3.2   Uniform Resource Identifiers ................................18
   3.2.1    General Syntax ...........................................19
   3.2.2    http URL .................................................19
   3.2.3    URI Comparison ...........................................20
   3.3   Date/Time Formats ...........................................20
   3.3.1    Full Date ................................................20
   3.3.2    Delta Seconds ............................................21
   3.4   Character Sets ..............................................21
   3.4.1    Missing Charset ..........................................22
   3.5   Content Codings .............................................23
   3.6   Transfer Codings ............................................24
   3.6.1    Chunked Transfer Coding ..................................25
   3.7   Media Types .................................................26
   3.7.1    Canonicalization and Text Defaults .......................27
   3.7.2    Multipart Types ..........................................27
   3.8   Product Tokens ..............................................28
   3.9   Quality Values ..............................................29
   3.10  Language Tags ...............................................29
   3.11  Entity Tags .................................................30
   3.12  Range Units .................................................30
   4   HTTP Message ..................................................31
   4.1   Message Types ...............................................31
   4.2   Message Headers .............................................31
   4.3   Message Body ................................................32
   4.4   Message Length ..............................................33
   4.5   General Header Fields .......................................34
   5   Request .......................................................35
   5.1   Request-Line ................................................35
   5.1.1    Method ...................................................36
   5.1.2    Request-URI ..............................................36
   5.2   The Resource Identified by a Request ........................38
   5.3   Request Header Fields .......................................38
   6   Response ......................................................39
   6.1   Status-Line .................................................39
   6.1.1    Status Code and Reason Phrase ............................39
   6.2   Response Header Fields ......................................41

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   7   Entity ........................................................42
   7.1   Entity Header Fields ........................................42
   7.2   Entity Body .................................................43
   7.2.1    Type .....................................................43
   7.2.2    Entity Length ............................................43
   8   Connections ...................................................44
   8.1   Persistent Connections ......................................44
   8.1.1    Purpose ..................................................44
   8.1.2    Overall Operation ........................................45
   8.1.3    Proxy Servers ............................................46
   8.1.4    Practical Considerations .................................46
   8.2   Message Transmission Requirements ...........................47
   8.2.1    Persistent Connections and Flow Control ..................47
   8.2.2    Monitoring Connections for Error Status Messages .........48
   8.2.3    Use of the 100 (Continue) Status .........................48
   8.2.4    Client Behavior if Server Prematurely Closes Connection ..50
   9   Method Definitions ............................................51
   9.1   Safe and Idempotent Methods .................................51
   9.1.1    Safe Methods .............................................51
   9.1.2    Idempotent Methods .......................................51
   9.2   OPTIONS .....................................................52
   9.3   GET .........................................................53
   9.4   HEAD ........................................................54
   9.5   POST ........................................................54
   9.6   PUT .........................................................55
   9.7   DELETE ......................................................56
   9.8   TRACE .......................................................56
   9.9   CONNECT .....................................................57
   10   Status Code Definitions ......................................57
   10.1  Informational 1xx ...........................................57
   10.1.1   100 Continue .............................................58
   10.1.2   101 Switching Protocols ..................................58
   10.2  Successful 2xx ..............................................58
   10.2.1   200 OK ...................................................58
   10.2.2   201 Created ..............................................59
   10.2.3   202 Accepted .............................................59
   10.2.4   203 Non-Authoritative Information ........................59
   10.2.5   204 No Content ...........................................60
   10.2.6   205 Reset Content ........................................60
   10.2.7   206 Partial Content ......................................60
   10.3  Redirection 3xx .............................................61
   10.3.1   300 Multiple Choices .....................................61
   10.3.2   301 Moved Permanently ....................................62
   10.3.3   302 Found ................................................62
   10.3.4   303 See Other ............................................63
   10.3.5   304 Not Modified .........................................63
   10.3.6   305 Use Proxy ............................................64
   10.3.7   306 (Unused) .............................................64

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   10.3.8   307 Temporary Redirect ...................................65
   10.4  Client Error 4xx ............................................65
   10.4.1    400 Bad Request .........................................65
   10.4.2    401 Unauthorized ........................................66
   10.4.3    402 Payment Required ....................................66
   10.4.4    403 Forbidden ...........................................66
   10.4.5    404 Not Found ...........................................66
   10.4.6    405 Method Not Allowed ..................................66
   10.4.7    406 Not Acceptable ......................................67
   10.4.8    407 Proxy Authentication Required .......................67
   10.4.9    408 Request Timeout .....................................67
   10.4.10   409 Conflict ............................................67
   10.4.11   410 Gone ................................................68
   10.4.12   411 Length Required .....................................68
   10.4.13   412 Precondition Failed .................................68
   10.4.14   413 Request Entity Too Large ............................69
   10.4.15   414 Request-URI Too Long ................................69
   10.4.16   415 Unsupported Media Type ..............................69
   10.4.17   416 Requested Range Not Satisfiable .....................69
   10.4.18   417 Expectation Failed ..................................70
   10.5  Server Error 5xx ............................................70
   10.5.1   500 Internal Server Error ................................70
   10.5.2   501 Not Implemented ......................................70
   10.5.3   502 Bad Gateway ..........................................70
   10.5.4   503 Service Unavailable ..................................70
   10.5.5   504 Gateway Timeout ......................................71
   10.5.6   505 HTTP Version Not Supported ...........................71
   11   Access Authentication ........................................71
   12   Content Negotiation ..........................................71
   12.1  Server-driven Negotiation ...................................72
   12.2  Agent-driven Negotiation ....................................73
   12.3  Transparent Negotiation .....................................74
   13   Caching in HTTP ..............................................74
   13.1.1   Cache Correctness ........................................75
   13.1.2   Warnings .................................................76
   13.1.3   Cache-control Mechanisms .................................77
   13.1.4   Explicit User Agent Warnings .............................78
   13.1.5   Exceptions to the Rules and Warnings .....................78
   13.1.6   Client-controlled Behavior ...............................79
   13.2  Expiration Model ............................................79
   13.2.1   Server-Specified Expiration ..............................79
   13.2.2   Heuristic Expiration .....................................80
   13.2.3   Age Calculations .........................................80
   13.2.4   Expiration Calculations ..................................83
   13.2.5   Disambiguating Expiration Values .........................84
   13.2.6   Disambiguating Multiple Responses ........................84
   13.3  Validation Model ............................................85
   13.3.1   Last-Modified Dates ......................................86

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   13.3.2   Entity Tag Cache Validators ..............................86
   13.3.3   Weak and Strong Validators ...............................86
   13.3.4   Rules for When to Use Entity Tags and Last-Modified Dates.89
   13.3.5   Non-validating Conditionals ..............................90
   13.4  Response Cacheability .......................................91
   13.5  Constructing Responses From Caches ..........................92
   13.5.1   End-to-end and Hop-by-hop Headers ........................92
   13.5.2   Non-modifiable Headers ...................................92
   13.5.3   Combining Headers ........................................94
   13.5.4   Combining Byte Ranges ....................................95
   13.6  Caching Negotiated Responses ................................95
   13.7  Shared and Non-Shared Caches ................................96
   13.8  Errors or Incomplete Response Cache Behavior ................97
   13.9  Side Effects of GET and HEAD ................................97
   13.10   Invalidation After Updates or Deletions ...................97
   13.11   Write-Through Mandatory ...................................98
   13.12   Cache Replacement .........................................99
   13.13   History Lists .............................................99
   14   Header Field Definitions ....................................100
   14.1  Accept .....................................................100
   14.2  Accept-Charset .............................................102
   14.3  Accept-Encoding ............................................102
   14.4  Accept-Language ............................................104
   14.5  Accept-Ranges ..............................................105
   14.6  Age ........................................................106
   14.7  Allow ......................................................106
   14.8  Authorization ..............................................107
   14.9  Cache-Control ..............................................108
   14.9.1   What is Cacheable .......................................109
   14.9.2   What May be Stored by Caches ............................110
   14.9.3   Modifications of the Basic Expiration Mechanism .........111
   14.9.4   Cache Revalidation and Reload Controls ..................113
   14.9.5   No-Transform Directive ..................................115
   14.9.6   Cache Control Extensions ................................116
   14.10   Connection ...............................................117
   14.11   Content-Encoding .........................................118
   14.12   Content-Language .........................................118
   14.13   Content-Length ...........................................119
   14.14   Content-Location .........................................120
   14.15   Content-MD5 ..............................................121
   14.16   Content-Range ............................................122
   14.17   Content-Type .............................................124
   14.18   Date .....................................................124
   14.18.1   Clockless Origin Server Operation ......................125
   14.19   ETag .....................................................126
   14.20   Expect ...................................................126
   14.21   Expires ..................................................127
   14.22   From .....................................................128

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   14.23   Host .....................................................128
   14.24   If-Match .................................................129
   14.25   If-Modified-Since ........................................130
   14.26   If-None-Match ............................................132
   14.27   If-Range .................................................133
   14.28   If-Unmodified-Since ......................................134
   14.29   Last-Modified ............................................134
   14.30   Location .................................................135
   14.31   Max-Forwards .............................................136
   14.32   Pragma ...................................................136
   14.33   Proxy-Authenticate .......................................137
   14.34   Proxy-Authorization ......................................137
   14.35   Range ....................................................138
   14.35.1    Byte Ranges ...........................................138
   14.35.2    Range Retrieval Requests ..............................139
   14.36   Referer ..................................................140
   14.37   Retry-After ..............................................141
   14.38   Server ...................................................141
   14.39   TE .......................................................142
   14.40   Trailer ..................................................143
   14.41  Transfer-Encoding..........................................143
   14.42   Upgrade ..................................................144
   14.43   User-Agent ...............................................145
   14.44   Vary .....................................................145
   14.45   Via ......................................................146
   14.46   Warning ..................................................148
   14.47   WWW-Authenticate .........................................150
   15 Security Considerations .......................................150
   15.1      Personal Information....................................151
   15.1.1   Abuse of Server Log Information .........................151
   15.1.2   Transfer of Sensitive Information .......................151
   15.1.3   Encoding Sensitive Information in URI's .................152
   15.1.4   Privacy Issues Connected to Accept Headers ..............152
   15.2  Attacks Based On File and Path Names .......................153
   15.3  DNS Spoofing ...............................................154
   15.4  Location Headers and Spoofing ..............................154
   15.5  Content-Disposition Issues .................................154
   15.6  Authentication Credentials and Idle Clients ................155
   15.7  Proxies and Caching ........................................155
   15.7.1    Denial of Service Attacks on Proxies....................156
   16   Acknowledgments .............................................156
   17   References ..................................................158
   18   Authors' Addresses ..........................................162
   19   Appendices ..................................................164
   19.1  Internet Media Type message/http and application/http ......164
   19.2  Internet Media Type multipart/byteranges ...................165
   19.3  Tolerant Applications ......................................166
   19.4  Differences Between HTTP Entities and RFC 2045 Entities ....167

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   19.4.1   MIME-Version ............................................167
   19.4.2   Conversion to Canonical Form ............................167
   19.4.3   Conversion of Date Formats ..............................168
   19.4.4   Introduction of Content-Encoding ........................168
   19.4.5   No Content-Transfer-Encoding ............................168
   19.4.6   Introduction of Transfer-Encoding .......................169
   19.4.7   MHTML and Line Length Limitations .......................169
   19.5  Additional Features ........................................169
   19.5.1   Content-Disposition .....................................170
   19.6  Compatibility with Previous Versions .......................170
   19.6.1   Changes from HTTP/1.0 ...................................171
   19.6.2   Compatibility with HTTP/1.0 Persistent Connections ......172
   19.6.3   Changes from RFC 2068 ...................................172
   20   Index .......................................................175
   21   Full Copyright Statement ....................................176

1 Introduction

1.1 Purpose

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypermedia information
   systems. HTTP has been in use by the World-Wide Web global
   information initiative since 1990. The first version of HTTP,
   referred to as HTTP/0.9, was a simple protocol for raw data transfer
   across the Internet. HTTP/1.0, as defined by RFC 1945 [6], 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 [47]. It builds on the discipline of reference
   provided by the Uniform Resource Identifier (URI) [3], as a location
   (URL) [4] or name (URN) [20], for indicating the resource to which a

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   method is to be applied. Messages are passed in a format similar to
   that used by Internet mail [9] as defined by the Multipurpose
   Internet Mail Extensions (MIME) [7].

   HTTP is also used as a generic protocol for communication between
   user agents and proxies/gateways to other Internet systems, including
   those supported by the SMTP [16], NNTP [13], FTP [18], Gopher [2],
   and WAIS [10] protocols. In this way, HTTP allows basic hypermedia
   access to resources available from diverse applications.

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 [34].

   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.

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   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
      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 7.

   representation
      An entity included with a response that is subject to content
      negotiation, as described in section 12. 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 12. 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
      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 13. Even if a resource is cacheable, there may
      be additional constraints on whether a cache can use the cached
      copy for a particular request.

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   first-hand
      A response is first-hand if it comes directly and without
      unnecessary delay from the origin server, perhaps via one or more
      proxies. A response is also first-hand if its validity has just
      been checked directly with the origin server.

   explicit expiration time
      The time at which the origin server intends that an entity should
      no longer be returned by a cache without further validation.

   heuristic expiration time
      An expiration time assigned by a cache when no explicit expiration
      time is available.

   age
      The age of a response is the time since it was sent by, or
      successfully validated with, the origin server.

   freshness lifetime
      The length of time between the generation of a response and its
      expiration time.

   fresh
      A response is fresh if its age has not yet exceeded its freshness
      lifetime.

   stale
      A response is stale if its age has passed its freshness lifetime.

   semantically transparent
      A cache behaves in a "semantically transparent" manner, with
      respect to a particular response, when its use affects neither the
      requesting client nor the origin server, except to improve
      performance. When a cache is semantically transparent, the client
      receives exactly the same response (except for hop-by-hop headers)
      that it would have received had its request been handled directly
      by the origin server.

   validator
      A protocol element (e.g., an entity tag or a Last-Modified time)
      that is used to find out whether a cache entry is an equivalent
      copy of an entity.

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

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   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 19.4.

   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
   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

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   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 13.

   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
   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 [19], but other ports can be used. This does
   not preclude HTTP from being implemented on top of any other protocol
   on the Internet, or on other networks. HTTP only presumes a reliable
   transport; any protocol that provides such guarantees can be used;
   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.

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   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 8.1).

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 [9]. Implementors will need to be familiar with the
   notation in order to understand this specification. The augmented BNF
   includes the following constructs:

   name = definition
      The name of a rule is simply the name itself (without any
      enclosing "<" and ">") and is separated from its definition by the
      equal "=" character. 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
      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)".

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   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
      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 [21].

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       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 19.3 for
   tolerant applications). The end-of-line marker within an entity-body
   is defined by its associated media type, as described in section 3.7.

       CRLF           = CR LF

   HTTP/1.1 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 [22] only when encoded according to the rules of RFC 2047
   [14].

       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

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   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
   3.6).

       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.

       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



(page 17 continued on part 2)

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