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Hypertext Transfer Protocol -- HTTP/1.1
RFC 2616

Document type: RFC - Draft Standard (June 1999; Errata)
Obsoletes RFC 2068
Document stream: IETF
Last updated: 2013-03-02
Other versions: plain text, pdf, ps, html

IETF State: (None)
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IESG State: RFC 2616 (Draft Standard)
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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].

Fielding, et al.            Standards Track                     [Page 1]
RFC 2616                        HTTP/1.1                       June 1999

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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RFC 2616                        HTTP/1.1                       June 1999

   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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RFC 2616                        HTTP/1.1                       June 1999

   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

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 [36] for a fuller explanation.

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   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 [36].

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

   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[33], 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 [3], and finally the
   combination of Uniform Resource Locators (URL) [4] and Names (URN)
   [20]. As far as HTTP is concerned, Uniform Resource Identifiers are
   simply formatted strings which identify--via name, location, or any
   other characteristic--a resource.

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3.2.1 General Syntax

   URIs in HTTP can be represented in absolute form or relative to some
   known base URI [11], depending upon the context of their use. The two
   forms are differentiated by the fact that absolute URIs always begin
   with a scheme name followed by a colon. For definitive information on
   URL syntax and semantics, see "Uniform Resource Identifiers (URI):
   Generic Syntax and Semantics," RFC 2396 [42] (which replaces RFCs
   1738 [4] and RFC 1808 [11]). 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 10.4.15).

      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 [24]). 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.

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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:

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

        - Comparisons of host names MUST be case-insensitive;

        - Comparisons of scheme names MUST be case-insensitive;

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

   Characters other than those in the "reserved" and "unsafe" sets (see
   RFC 2396 [42]) 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

3.3 Date/Time Formats

3.3.1 Full Date

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

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

   The first format is preferred as an Internet standard and represents
   a fixed-length subset of that defined by RFC 1123 [8] (an update to
   RFC 822 [9]). The second format is in common use, but is based on the
   obsolete RFC 850 [12] date format and lacks a four-digit year.
   HTTP/1.1 clients and servers that parse the date value MUST accept
   all three formats (for compatibility with HTTP/1.0), though they MUST
   only generate the RFC 1123 format for representing HTTP-date values
   in header fields. See section 19.3 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.

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   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.

       HTTP-date    = rfc1123-date | rfc850-date | asctime-date
       rfc1123-date = wkday "," SP date1 SP time SP "GMT"
       rfc850-date  = weekday "," SP date2 SP time SP "GMT"
       asctime-date = wkday SP date3 SP time SP 4DIGIT
       date1        = 2DIGIT SP month SP 4DIGIT
                      ; day month year (e.g., 02 Jun 1982)
       date2        = 2DIGIT "-" month "-" 2DIGIT
                      ; day-month-year (e.g., 02-Jun-82)
       date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
                      ; month day (e.g., Jun  2)
       time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                      ; 00:00:00 - 23:59:59
       wkday        = "Mon" | "Tue" | "Wed"
                    | "Thu" | "Fri" | "Sat" | "Sun"
       weekday      = "Monday" | "Tuesday" | "Wednesday"
                    | "Thursday" | "Friday" | "Saturday" | "Sunday"
       month        = "Jan" | "Feb" | "Mar" | "Apr"
                    | "May" | "Jun" | "Jul" | "Aug"
                    | "Sep" | "Oct" | "Nov" | "Dec"

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

3.3.2 Delta Seconds

   Some HTTP header fields allow a time value to be specified as an
   integer number of seconds, represented in decimal, after the time
   that the message was received.

       delta-seconds  = 1*DIGIT

3.4 Character Sets

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

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

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

   HTTP character sets are identified by case-insensitive tokens. The
   complete set of tokens is defined by the IANA Character Set registry
   [19].

       charset = token

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

   Implementors should be aware of IETF character set requirements [38]
   [41].

3.4.1 Missing Charset

   Some HTTP/1.0 software has interpreted a Content-Type header without
   charset parameter incorrectly to mean "recipient should guess."
   Senders wishing to defeat this behavior MAY include a charset
   parameter even when the charset is ISO-8859-1 and SHOULD do so when
   it is known that it will not confuse the recipient.

   Unfortunately, some older HTTP/1.0 clients did not deal properly with
   an explicit charset parameter. HTTP/1.1 recipients MUST respect the
   charset label provided by the sender; and those user agents that have
   a provision to "guess" a charset MUST use the charset from the

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   content-type field if they support that charset, rather than the
   recipient's preference, when initially displaying a document. See
   section 3.7.1.

3.5 Content Codings

   Content coding values indicate an encoding transformation that has
   been or can be applied to an entity. Content codings are primarily
   used to allow a document to be compressed or otherwise usefully
   transformed without losing the identity of its underlying media type
   and without loss of information. Frequently, the entity is stored in
   coded form, transmitted directly, and only decoded by the recipient.

       content-coding   = token

   All content-coding values are case-insensitive. HTTP/1.1 uses
   content-coding values in the Accept-Encoding (section 14.3) and
   Content-Encoding (section 14.11) header fields. Although the value
   describes the content-coding, what is more important is that it
   indicates what decoding mechanism will be required to remove the
   encoding.

   The Internet Assigned Numbers Authority (IANA) acts as a registry for
   content-coding value tokens. Initially, the registry contains the
   following tokens:

   gzip An encoding format produced by the file compression program
        "gzip" (GNU zip) as described in RFC 1952 [25]. This format is a
        Lempel-Ziv coding (LZ77) with a 32 bit CRC.

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

        Use of program names for the identification of encoding formats
        is not desirable and is discouraged for future encodings. Their
        use here is representative of historical practice, not good
        design. For compatibility with previous implementations of HTTP,
        applications SHOULD consider "x-gzip" and "x-compress" to be
        equivalent to "gzip" and "compress" respectively.

   deflate
        The "zlib" format defined in RFC 1950 [31] in combination with
        the "deflate" compression mechanism described in RFC 1951 [29].

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   identity
        The default (identity) encoding; the use of no transformation
        whatsoever. This content-coding is used only in the Accept-
        Encoding header, and SHOULD NOT be used in the Content-Encoding
        header.

   New content-coding value tokens SHOULD be registered; to allow
   interoperability between clients and servers, specifications of the
   content coding algorithms needed to implement a new value SHOULD be
   publicly available and adequate for independent implementation, and
   conform to the purpose of content coding defined in this section.

3.6 Transfer Codings

   Transfer-coding values are used to indicate an encoding
   transformation that has been, can be, or may need to be applied to an
   entity-body in order to ensure "safe transport" through the network.
   This differs from a content coding in that the transfer-coding is a
   property of the message, not of the original 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 14.39) and in
   the Transfer-Encoding header field (section 14.41).

   Whenever a transfer-coding is applied to a message-body, the set of
   transfer-codings MUST include "chunked", unless the message is
   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 [7], which were designed to enable safe transport of
   binary data over a 7-bit transport service. However, safe transport
   has a different focus for an 8bit-clean transfer protocol. In HTTP,
   the only unsafe characteristic of message-bodies is the difficulty in
   determining the exact body length (section 7.2.2), or the desire to
   encrypt data over a shared transport.

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   The Internet Assigned Numbers Authority (IANA) acts as a registry for
   transfer-coding value tokens. Initially, the registry contains the
   following tokens: "chunked" (section 3.6.1), "identity" (section
   3.6.2), "gzip" (section 3.5), "compress" (section 3.5), and "deflate"
   (section 3.5).

   New transfer-coding value tokens SHOULD be registered in the same way
   as new content-coding value tokens (section 3.5).

   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.

3.6.1 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.

       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 14.40).

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   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:

   a)the request included a TE header field that indicates "trailers" is
     acceptable in the transfer-coding of the  response, as described in
     section 14.39; or,

   b)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.

   An example process for decoding a Chunked-Body is presented in
   appendix 19.4.6.

   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.

3.7 Media Types

   HTTP uses Internet Media Types [17] in the Content-Type (section
   14.17) and Accept (section 14.1) header fields in order to provide
   open and extensible data typing and type negotiation.

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

   Parameters MAY follow the type/subtype in the form of attribute/value
   pairs (as defined in section 3.6).

   The type, subtype, and parameter attribute names are case-
   insensitive. Parameter values might or might not be case-sensitive,
   depending on the semantics of the parameter name. Linear white space
   (LWS) MUST NOT be used between the type and subtype, nor between an
   attribute and its value. The presence or absence of a parameter might
   be significant to the processing of a media-type, depending on its
   definition within the media type registry.

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   Note that some older HTTP applications do not recognize media type
   parameters. When sending data to older HTTP applications,
   implementations SHOULD only use media type parameters when they are
   required by that type/subtype definition.

   Media-type values are registered with the Internet Assigned Number
   Authority (IANA [19]). The media type registration process is
   outlined in RFC 1590 [17]. Use of non-registered media types is
   discouraged.

3.7.1 Canonicalization and Text Defaults

   Internet media types are registered with a canonical form. An
   entity-body transferred via HTTP messages MUST be represented in the
   appropriate canonical form prior to its transmission except for
   "text" types, as defined in the next paragraph.

   When in canonical form, media subtypes of the "text" type use CRLF as
   the text line break. HTTP relaxes this requirement and allows the
   transport of text media with plain CR or LF alone representing a line
   break when it is done consistently for an entire entity-body. HTTP
   applications MUST accept CRLF, bare CR, and bare LF as being
   representative of a line break in text media received via HTTP. In
   addition, if the text is represented in a character set that does not
   use octets 13 and 10 for CR and LF respectively, as is the case for
   some multi-byte character sets, HTTP allows the use of whatever octet
   sequences are defined by that character set to represent the
   equivalent of CR and LF for line breaks. This flexibility regarding
   line breaks applies only to text media in the entity-body; a bare CR
   or LF MUST NOT be substituted for CRLF within any of the HTTP control
   structures (such as header fields and multipart boundaries).

   If an entity-body is encoded with a content-coding, the underlying
   data MUST be in a form defined above prior to being encoded.

   The "charset" parameter is used with some media types to define the
   character set (section 3.4) of the data. When no explicit charset
   parameter is provided by the sender, media subtypes of the "text"
   type are defined to have a default charset value of "ISO-8859-1" when
   received via HTTP. Data in character sets other than "ISO-8859-1" or
   its subsets MUST be labeled with an appropriate charset value. See
   section 3.4.1 for compatibility problems.

3.7.2 Multipart Types

   MIME provides for a number of "multipart" types -- encapsulations of
   one or more entities within a single message-body. All multipart
   types share a common syntax, as defined in section 5.1.1 of RFC 2046

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   [40], and MUST include a boundary parameter as part of the media type
   value. The message body is itself a protocol element and MUST
   therefore use only CRLF to represent line breaks between body-parts.
   Unlike in RFC 2046, the epilogue of any multipart message MUST be
   empty; HTTP applications MUST NOT transmit the epilogue (even if the
   original multipart contains an epilogue). These restrictions exist in
   order to preserve the self-delimiting nature of a multipart message-
   body, wherein the "end" of the message-body is indicated by the
   ending multipart boundary.

   In general, HTTP treats a multipart message-body no differently than
   any other media type: strictly as payload. The one exception is the
   "multipart/byteranges" type (appendix 19.2) when it appears in a 206
   (Partial Content) response, which will be interpreted by some HTTP
   caching mechanisms as described in sections 13.5.4 and 14.16. In all
   other cases, an HTTP user agent SHOULD follow the same or similar
   behavior as a MIME user agent would upon receipt of a multipart type.
   The MIME header fields within each body-part of a multipart message-
   body do not have any significance to HTTP beyond that defined by
   their MIME semantics.

   In general, an HTTP user agent SHOULD follow the same or similar
   behavior as a MIME user agent would upon receipt of a multipart type.
   If an application receives an unrecognized multipart subtype, the
   application MUST treat it as being equivalent to "multipart/mixed".

      Note: The "multipart/form-data" type has been specifically defined
      for carrying form data suitable for processing via the POST
      request method, as described in RFC 1867 [15].

3.8 Product Tokens

   Product tokens are used to allow communicating applications to
   identify themselves by software name and version. Most fields using
   product tokens also allow sub-products which form a significant part
   of the application to be listed, separated by white space. By
   convention, the products are listed in order of their significance
   for identifying the application.

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

   Examples:

       User-Agent: CERN-LineMode/2.15 libwww/2.17b3
       Server: Apache/0.8.4

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   Product tokens SHOULD be short and to the point. They MUST NOT be
   used for advertising or other non-essential information. Although any
   token character MAY appear in a product-version, this token SHOULD
   only be used for a version identifier (i.e., successive versions of
   the same product SHOULD only differ in the product-version portion of
   the product value).

3.9 Quality Values

   HTTP content negotiation (section 12) uses short "floating point"
   numbers to indicate the relative importance ("weight") of various
   negotiable parameters.  A weight is normalized to a real number in
   the range 0 through 1, where 0 is the minimum and 1 the maximum
   value. If a parameter has a quality value of 0, then content with
   this parameter is `not acceptable' for the client. HTTP/1.1
   applications MUST NOT generate more than three digits after the
   decimal point. User configuration of these values SHOULD also be
   limited in this fashion.

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

   "Quality values" is a misnomer, since these values merely represent
   relative degradation in desired quality.

3.10 Language Tags

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

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

        language-tag  = primary-tag *( "-" subtag )
        primary-tag   = 1*8ALPHA
        subtag        = 1*8ALPHA

   White space is not allowed within the tag and all tags are case-
   insensitive. The name space of language tags is administered by the
   IANA. Example tags include:

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

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   where any two-letter primary-tag is an ISO-639 language abbreviation
   and any two-letter initial subtag is an ISO-3166 country code. (The
   last three tags above are not registered tags; all but the last are
   examples of tags which could be registered in future.)

3.11 Entity Tags

   Entity tags are used for comparing two or more entities from the same
   requested resource. HTTP/1.1 uses entity tags in the ETag (section
   14.19), If-Match (section 14.24), If-None-Match (section 14.26), and
   If-Range (section 14.27) header fields. The definition of how they
   are used and compared as cache validators is in section 13.3.3. An
   entity tag consists of an opaque quoted string, possibly prefixed by
   a weakness indicator.

      entity-tag = [ weak ] opaque-tag
      weak       = "W/"
      opaque-tag = quoted-string

   A "strong entity tag" MAY be shared by two entities of a resource
   only if they are equivalent by octet equality.

   A "weak entity tag," indicated by the "W/" prefix, MAY be shared by
   two entities of a resource only if the entities are equivalent and
   could be substituted for each other with no significant change in
   semantics. A weak entity tag can only be used for weak comparison.

   An entity tag MUST be unique across all versions of all entities
   associated with a particular resource. A given entity tag value MAY
   be used for entities obtained by requests on different URIs. The use
   of the same entity tag value in conjunction with entities obtained by
   requests on different URIs does not imply the equivalence of those
   entities.

3.12 Range Units

   HTTP/1.1 allows a client to request that only part (a range of) the
   response entity be included within the response. HTTP/1.1 uses range
   units in the Range (section 14.35) and Content-Range (section 14.16)
   header fields. An entity can be broken down into subranges according
   to various structural units.

      range-unit       = bytes-unit | other-range-unit
      bytes-unit       = "bytes"
      other-range-unit = token

   The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1
   implementations MAY ignore ranges specified using other units.

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   HTTP/1.1 has been designed to allow implementations of applications
   that do not depend on knowledge of ranges.

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 [9] 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
   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 5.3), response-header (section 6.2), and
   entity-header (section 7.1) fields, follow the same generic format as
   that given in Section 3.1 of RFC 822 [9]. Each header field consists
   of a name followed by a colon (":") and the field value. Field names
   are case-insensitive. The field value MAY be preceded by any amount
   of LWS, though a single SP is preferred. Header fields can be
   extended over multiple lines by preceding each extra line with at
   least one SP or HT. 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

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   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
   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
   14.41).

       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

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   entity, and thus MAY be added or removed by any application along the
   request/response chain. (However, section 3.6 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 5.1.1)
   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.

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 14.41) is present and
     has any value other than "identity", then the transfer-length is
     defined by use of the "chunked" transfer-coding (section 3.6),
     unless the message is terminated by closing the connection.

   3.If a Content-Length header field (section 14.13) 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

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     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
   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 3.6), 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

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   message being transmitted.

       general-header = Cache-Control            ; Section 14.9
                      | Connection               ; Section 14.10
                      | Date                     ; Section 14.18
                      | Pragma                   ; Section 14.32
                      | Trailer                  ; Section 14.40
                      | Transfer-Encoding        ; Section 14.41
                      | Upgrade                  ; Section 14.42
                      | Via                      ; Section 14.45
                      | Warning                  ; Section 14.46

   General-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields may be given the semantics of general
   header fields if all parties in the communication recognize them to
   be general-header fields. Unrecognized header fields are treated as
   entity-header fields.

5 Request

   A request message from a client to a server includes, within the
   first line of that message, the method to be applied to the resource,
   the identifier of the resource, and the protocol version in use.

        Request       = Request-Line              ; Section 5.1
                        *(( general-header        ; Section 4.5
                         | request-header         ; Section 5.3
                         | entity-header ) CRLF)  ; Section 7.1
                        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

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5.1.1 Method

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

       Method         = "OPTIONS"                ; Section 9.2
                      | "GET"                    ; Section 9.3
                      | "HEAD"                   ; Section 9.4
                      | "POST"                   ; Section 9.5
                      | "PUT"                    ; Section 9.6
                      | "DELETE"                 ; Section 9.7
                      | "TRACE"                  ; Section 9.8
                      | "CONNECT"                ; Section 9.9
                      | extension-method
       extension-method = token

   The list of methods allowed by a resource can be specified in an
   Allow header field (section 14.7). The return code of the response
   always notifies the client whether a method is currently allowed on a
   resource, since the set of allowed methods can change dynamically. An
   origin server SHOULD return the status code 405 (Method Not Allowed)
   if the method is known by the origin server but not allowed for the
   requested resource, and 501 (Not Implemented) if the method is
   unrecognized or not implemented by the origin server. The methods GET
   and HEAD MUST be supported by all general-purpose servers. All other
   methods are OPTIONAL; however, if the above methods are implemented,
   they MUST be implemented with the same semantics as those specified
   in section 9.

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

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   specified by the absoluteURI. In order to avoid request loops, a
   proxy MUST be able to recognize all of its server names, including
   any aliases, local variations, and the numeric IP address. An example
   Request-Line would be:

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

   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 9.9).

   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
   [42], 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.

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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
   requested host MAY ignore the Host header field value when
   determining the resource identified by an HTTP/1.1 request. (But see
   section 19.6.1.1 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.

5.3 Request Header Fields

   The request-header fields allow the client to pass additional
   information about the request, and about the client itself, to the
   server. These fields act as request modifiers, with semantics
   equivalent to the parameters on a programming language method
   invocation.

       request-header = Accept                   ; Section 14.1
                      | Accept-Charset           ; Section 14.2
                      | Accept-Encoding          ; Section 14.3
                      | Accept-Language          ; Section 14.4
                      | Authorization            ; Section 14.8
                      | Expect                   ; Section 14.20
                      | From                     ; Section 14.22
                      | Host                     ; Section 14.23
                      | If-Match                 ; Section 14.24

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                      | If-Modified-Since        ; Section 14.25
                      | If-None-Match            ; Section 14.26
                      | If-Range                 ; Section 14.27
                      | If-Unmodified-Since      ; Section 14.28
                      | Max-Forwards             ; Section 14.31
                      | Proxy-Authorization      ; Section 14.34
                      | Range                    ; Section 14.35
                      | Referer                  ; Section 14.36
                      | TE                       ; Section 14.39
                      | User-Agent               ; Section 14.43

   Request-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields MAY be given the semantics of request-
   header fields if all parties in the communication recognize them to
   be request-header fields. Unrecognized header fields are treated as
   entity-header fields.

6 Response

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

       Response      = Status-Line               ; Section 6.1
                       *(( general-header        ; Section 4.5
                        | response-header        ; Section 6.2
                        | entity-header ) CRLF)  ; Section 7.1
                       CRLF
                       [ message-body ]          ; Section 7.2

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

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 10. 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.

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   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:

      - 1xx: Informational - Request received, continuing process

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

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

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

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

   The individual values of the numeric status codes defined for
   HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
   presented below. The reason phrases listed here are only
   recommendations -- they MAY be replaced by local equivalents without
   affecting the protocol.

      Status-Code    =
            "100"  ; Section 10.1.1: Continue
          | "101"  ; Section 10.1.2: Switching Protocols
          | "200"  ; Section 10.2.1: OK
          | "201"  ; Section 10.2.2: Created
          | "202"  ; Section 10.2.3: Accepted
          | "203"  ; Section 10.2.4: Non-Authoritative Information
          | "204"  ; Section 10.2.5: No Content
          | "205"  ; Section 10.2.6: Reset Content
          | "206"  ; Section 10.2.7: Partial Content
          | "300"  ; Section 10.3.1: Multiple Choices
          | "301"  ; Section 10.3.2: Moved Permanently
          | "302"  ; Section 10.3.3: Found
          | "303"  ; Section 10.3.4: See Other
          | "304"  ; Section 10.3.5: Not Modified
          | "305"  ; Section 10.3.6: Use Proxy
          | "307"  ; Section 10.3.8: Temporary Redirect
          | "400"  ; Section 10.4.1: Bad Request
          | "401"  ; Section 10.4.2: Unauthorized
          | "402"  ; Section 10.4.3: Payment Required
          | "403"  ; Section 10.4.4: Forbidden
          | "404"  ; Section 10.4.5: Not Found
          | "405"  ; Section 10.4.6: Method Not Allowed
          | "406"  ; Section 10.4.7: Not Acceptable

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          | "407"  ; Section 10.4.8: Proxy Authentication Required
          | "408"  ; Section 10.4.9: Request Time-out
          | "409"  ; Section 10.4.10: Conflict
          | "410"  ; Section 10.4.11: Gone
          | "411"  ; Section 10.4.12: Length Required
          | "412"  ; Section 10.4.13: Precondition Failed
          | "413"  ; Section 10.4.14: Request Entity Too Large
          | "414"  ; Section 10.4.15: Request-URI Too Large
          | "415"  ; Section 10.4.16: Unsupported Media Type
          | "416"  ; Section 10.4.17: Requested range not satisfiable
          | "417"  ; Section 10.4.18: Expectation Failed
          | "500"  ; Section 10.5.1: Internal Server Error
          | "501"  ; Section 10.5.2: Not Implemented
          | "502"  ; Section 10.5.3: Bad Gateway
          | "503"  ; Section 10.5.4: Service Unavailable
          | "504"  ; Section 10.5.5: Gateway Time-out
          | "505"  ; Section 10.5.6: HTTP Version not supported
          | extension-code

      extension-code = 3DIGIT
      Reason-Phrase  = *<TEXT, excluding CR, LF>

   HTTP status codes are extensible. HTTP applications are not required
   to understand the meaning of all registered status codes, though such
   understanding is obviously desirable. However, applications MUST
   understand the class of any status code, as indicated by the first
   digit, and treat any unrecognized response as being equivalent to the
   x00 status code of that class, with the exception that an
   unrecognized response MUST NOT be cached. For example, if an
   unrecognized status code of 431 is received by the client, it can
   safely assume that there was something wrong with its request and
   treat the response as if it had received a 400 status code. In such
   cases, user agents SHOULD present to the user the entity returned
   with the response, since that entity is likely to include human-
   readable information which will explain the unusual status.

6.2 Response Header Fields

   The response-header fields allow the server to pass additional
   information about the response which cannot be placed in the Status-
   Line. These header fields give information about the server and about
   further access to the resource identified by the Request-URI.

       response-header = Accept-Ranges           ; Section 14.5
                       | Age                     ; Section 14.6
                       | ETag                    ; Section 14.19
                       | Location                ; Section 14.30
                       | Proxy-Authenticate      ; Section 14.33

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                       | Retry-After             ; Section 14.37
                       | Server                  ; Section 14.38
                       | Vary                    ; Section 14.44
                       | WWW-Authenticate        ; Section 14.47

   Response-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields MAY be given the semantics of response-
   header fields if all parties in the communication recognize them to
   be response-header fields. Unrecognized header fields are treated as
   entity-header fields.

7 Entity

   Request and Response messages MAY transfer an entity if not otherwise
   restricted by the request method or response status code. An entity
   consists of entity-header fields and an entity-body, although some
   responses will only include the entity-headers.

   In this section, both sender and recipient refer to either the client
   or the server, depending on who sends and who receives the entity.

7.1 Entity Header Fields

   Entity-header fields define metainformation about the entity-body or,
   if no body is present, about the resource identified by the request.
   Some of this metainformation is OPTIONAL; some might be REQUIRED by
   portions of this specification.

       entity-header  = Allow                    ; Section 14.7
                      | Content-Encoding         ; Section 14.11
                      | Content-Language         ; Section 14.12
                      | Content-Length           ; Section 14.13
                      | Content-Location         ; Section 14.14
                      | Content-MD5              ; Section 14.15
                      | Content-Range            ; Section 14.16
                      | Content-Type             ; Section 14.17
                      | Expires                  ; Section 14.21
                      | Last-Modified            ; Section 14.29
                      | extension-header

       extension-header = message-header

   The extension-header mechanism allows additional entity-header fields
   to be defined without changing the protocol, but these fields cannot
   be assumed to be recognizable by the recipient. Unrecognized header
   fields SHOULD be ignored by the recipient and MUST be forwarded by
   transparent proxies.

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7.2 Entity Body

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

       entity-body    = *OCTET

   An entity-body is only present in a message when a message-body is
   present, as described in section 4.3. The entity-body is obtained
   from the message-body by decoding any Transfer-Encoding that might
   have been applied to ensure safe and proper transfer of the message.

7.2.1 Type

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

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

   Content-Type specifies the media type of the underlying data.
   Content-Encoding may be used to indicate any additional content
   codings applied to the data, usually for the purpose of data
   compression, that are a property of the requested resource. There is
   no default encoding.

   Any HTTP/1.1 message containing an entity-body SHOULD include a
   Content-Type header field defining the media type of that body. If
   and only if the media type is not given by a Content-Type field, the
   recipient MAY attempt to guess the media type via inspection of its
   content and/or the name extension(s) of the URI used to identify the
   resource. If the media type remains unknown, the recipient SHOULD
   treat it as type "application/octet-stream".

7.2.2 Entity Length

   The entity-length of a message is the length of the message-body
   before any transfer-codings have been applied. Section 4.4 defines
   how the transfer-length of a message-body is determined.

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8 Connections

8.1 Persistent Connections

8.1.1 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 [26] [30]. Implementation experience and
   measurements of actual HTTP/1.1 (RFC 2068) implementations show good
   results [39]. Alternatives have also been explored, for example,
   T/TCP [27].

   Persistent HTTP connections have a number of advantages:

      - 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.

      - 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.

      - 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.

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

      - 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.

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8.1.2 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 14.10). Once a close
   has been signaled, the client MUST NOT send any more requests on that
   connection.

8.1.2.1 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
   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 section 19.6.2 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.

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8.1.2.2 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 9.1.2). 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.

8.1.3 Proxy Servers

   It is especially important that proxies correctly implement the
   properties of the Connection header field as specified in section
   14.10.

   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 [33] for information and
   discussion of the problems with the Keep-Alive header implemented by
   many HTTP/1.0 clients).

8.1.4 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.

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   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 9.1.2). 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 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.

8.2 Message Transmission Requirements

8.2.1 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.

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8.2.2 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 3.6), 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.

8.2.3 Use of the 100 (Continue) Status

   The purpose of the 100 (Continue) status (see section 10.1.1) 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:

      - If a client will wait for a 100 (Continue) response before
        sending the request body, it MUST send an Expect request-header
        field (section 14.20) with the "100-continue" expectation.

      - A client MUST NOT send an Expect request-header field (section
        14.20) 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:

      - 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

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        to read and discard the rest of the request.  It MUST NOT
        perform the requested method if it returns a final status code.

      - 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.

      - 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.

      - 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
        connection prematurely.

      - 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:

      - 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.

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      - 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.

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

      - 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 10.1).

8.2.4 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

      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.

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

      - SHOULD NOT continue and

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

9 Method Definitions

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

   The Host request-header field (section 14.23) MUST accompany all
   HTTP/1.1 requests.

9.1 Safe and Idempotent Methods

9.1.1 Safe Methods

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

   In particular, the convention has been established that the GET and
   HEAD methods SHOULD NOT have the significance of taking an action
   other than retrieval. These methods ought to be considered "safe".
   This allows user agents to represent other methods, such as POST, PUT
   and DELETE, in a special way, so that the user is made aware of the
   fact that a possibly unsafe action is being requested.

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

9.1.2 Idempotent Methods

   Methods can also have the property of "idempotence" in that (aside
   from error or expiration issues) the side-effects of N > 0 identical
   requests is the same as for a single request. The methods GET, HEAD,
   PUT and DELETE share this property. Also, the methods OPTIONS and
   TRACE SHOULD NOT have side effects, and so are inherently idempotent.

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   However, it is possible that a sequence of several requests is non-
   idempotent, even if all of the methods executed in that sequence are
   idempotent. (A sequence is idempotent if a single execution of the
   entire sequence always yields a result that is not changed by a
   reexecution of all, or part, of that sequence.) For example, a
   sequence is non-idempotent if its result depends on a value that is
   later modified in the same sequence.

   A sequence that never has side effects is idempotent, by definition
   (provided that no concurrent operations are being executed on the
   same set of resources).

9.2 OPTIONS

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

   Responses to this method are not cacheable.

   If the OPTIONS request includes an entity-body (as indicated by the
   presence of Content-Length or Transfer-Encoding), then the media type
   MUST be indicated by a Content-Type field. Although this
   specification does not define any use for such a body, future
   extensions to HTTP might use the OPTIONS body to make more detailed
   queries on the server. A server that does not support such an
   extension MAY discard the request body.

   If the Request-URI is an asterisk ("*"), the OPTIONS request is
   intended to apply to the server in general rather than to a specific
   resource. Since a server's communication options typically depend on
   the resource, the "*" request is only useful as a "ping" or "no-op"
   type of method; it does nothing beyond allowing the client to test
   the capabilities of the server. For example, this can be used to test
   a proxy for HTTP/1.1 compliance (or lack thereof).

   If the Request-URI is not an asterisk, the OPTIONS request applies
   only to the options that are available when communicating with that
   resource.

   A 200 response SHOULD include any header fields that indicate
   optional features implemented by the server and applicable to that
   resource (e.g., Allow), possibly including extensions not defined by
   this specification. The response body, if any, SHOULD also include
   information about the communication options. The format for such a

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   body is not defined by this specification, but might be defined by
   future extensions to HTTP. Content negotiation MAY be used to select
   the appropriate response format. If no response body is included, the
   response MUST include a Content-Length field with a field-value of
   "0".

   The Max-Forwards request-header field MAY be used to target a
   specific proxy in the request chain. When a proxy receives an OPTIONS
   request on an absoluteURI for which request forwarding is permitted,
   the proxy MUST check for a Max-Forwards field. If the Max-Forwards
   field-value is zero ("0"), the proxy MUST NOT forward the message;
   instead, the proxy SHOULD respond with its own communication options.
   If the Max-Forwards field-value is an integer greater than zero, the
   proxy MUST decrement the field-value when it forwards the request. If
   no Max-Forwards field is present in the request, then the forwarded
   request MUST NOT include a Max-Forwards field.

9.3 GET

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

   The semantics of the GET method change to a "conditional GET" if the
   request message includes an If-Modified-Since, If-Unmodified-Since,
   If-Match, If-None-Match, or If-Range header field. A conditional GET
   method requests that the entity be transferred only under the
   circumstances described by the conditional header field(s). The
   conditional GET method is intended to reduce unnecessary network
   usage by allowing cached entities to be refreshed without requiring
   multiple requests or transferring data already held by the client.

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

   The response to a GET request is cacheable if and only if it meets
   the requirements for HTTP caching described in section 13.

   See section 15.1.3 for security considerations when used for forms.

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9.4 HEAD

   The HEAD method is identical to GET except that the server MUST NOT
   return a message-body in the response. The metainformation contained
   in the HTTP headers in response to a HEAD request SHOULD be identical
   to the information sent in response to a GET request. This method can
   be used for obtaining metainformation about the entity implied by the
   request without transferring the entity-body itself. This method is
   often used for testing hypertext links for validity, accessibility,
   and recent modification.

   The response to a HEAD request MAY be cacheable in the sense that the
   information contained in the response MAY be used to update a
   previously cached entity from that resource. If the new field values
   indicate that the cached entity differs from the current entity (as
   would be indicated by a change in Content-Length, Content-MD5, ETag
   or Last-Modified), then the cache MUST treat the cache entry as
   stale.

9.5 POST

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

      - Annotation of existing resources;

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

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

      - Extending a database through an append operation.

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

   The action performed by the POST method might not result in a
   resource that can be identified by a URI. In this case, either 200
   (OK) or 204 (No Content) is the appropriate response status,
   depending on whether or not the response includes an entity that
   describes the result.

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   If a resource has been created on the origin server, the response
   SHOULD be 201 (Created) and contain an entity which describes the
   status of the request and refers to the new resource, and a Location
   header (see section 14.30).

   Responses to this method are not cacheable, unless the response
   includes appropriate Cache-Control or Expires header fields. However,
   the 303 (See Other) response can be used to direct the user agent to
   retrieve a cacheable resource.

   POST requests MUST obey the message transmission requirements set out
   in section 8.2.

   See section 15.1.3 for security considerations.

9.6 PUT

   The PUT method requests that the enclosed entity be stored under the
   supplied Request-URI. If the Request-URI refers to an already
   existing resource, the enclosed entity SHOULD be considered as a
   modified version of the one residing on the origin server. If the
   Request-URI does not point to an existing resource, and that URI is
   capable of being defined as a new resource by the requesting user
   agent, the origin server can create the resource with that URI. If a
   new resource is created, the origin server MUST inform the user agent
   via the 201 (Created) response. If an existing resource is modified,
   either the 200 (OK) or 204 (No Content) response codes SHOULD be sent
   to indicate successful completion of the request. If the resource
   could not be created or modified with the Request-URI, an appropriate
   error response SHOULD be given that reflects the nature of the
   problem. The recipient of the entity MUST NOT ignore any Content-*
   (e.g. Content-Range) headers that it does not understand or implement
   and MUST return a 501 (Not Implemented) response in such cases.

   If the request passes through a cache and the Request-URI identifies
   one or more currently cached entities, those entries SHOULD be
   treated as stale. Responses to this method are not cacheable.

   The fundamental difference between the POST and PUT requests is
   reflected in the different meaning of the Request-URI. The URI in a
   POST request identifies the resource that will handle the enclosed
   entity. That resource might be a data-accepting process, a gateway to
   some other protocol, or a separate entity that accepts annotations.
   In contrast, the URI in a PUT request identifies the entity enclosed
   with the request -- the user agent knows what URI is intended and the
   server MUST NOT attempt to apply the request to some other resource.
   If the server desires that the request be applied to a different URI,

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   it MUST send a 301 (Moved Permanently) response; the user agent MAY
   then make its own decision regarding whether or not to redirect the
   request.

   A single resource MAY be identified by many different URIs. For
   example, an article might have a URI for identifying "the current
   version" which is separate from the URI identifying each particular
   version. In this case, a PUT request on a general URI might result in
   several other URIs being defined by the origin server.

   HTTP/1.1 does not define how a PUT method affects the state of an
   origin server.

   PUT requests MUST obey the message transmission requirements set out
   in section 8.2.

   Unless otherwise specified for a particular entity-header, the
   entity-headers in the PUT request SHOULD be applied to the resource
   created or modified by the PUT.

9.7 DELETE

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

   A successful response SHOULD be 200 (OK) if the response includes an
   entity describing the status, 202 (Accepted) if the action has not
   yet been enacted, or 204 (No Content) if the action has been enacted
   but the response does not include an entity.

   If the request passes through a cache and the Request-URI identifies
   one or more currently cached entities, those entries SHOULD be
   treated as stale. Responses to this method are not cacheable.

9.8 TRACE

   The TRACE method is used to invoke a remote, application-layer loop-
   back of the request message. The final recipient of the request
   SHOULD reflect the message received back to the client as the
   entity-body of a 200 (OK) response. The final recipient is either the

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   origin server or the first proxy or gateway to receive a Max-Forwards
   value of zero (0) in the request (see section 14.31). A TRACE request
   MUST NOT include an entity.

   TRACE allows the client to see what is being received at the other
   end of the request chain and use that data for testing or diagnostic
   information. The value of the Via header field (section 14.45) is of
   particular interest, since it acts as a trace of the request chain.
   Use of the Max-Forwards header field allows the client to limit the
   length of the request chain, which is useful for testing a chain of
   proxies forwarding messages in an infinite loop.

   If the request is valid, the response SHOULD contain the entire
   request message in the entity-body, with a Content-Type of
   "message/http". Responses to this method MUST NOT be cached.

9.9 CONNECT

   This specification reserves the method name CONNECT for use with a
   proxy that can dynamically switch to being a tunnel (e.g. SSL
   tunneling [44]).

10 Status Code Definitions

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

10.1 Informational 1xx

   This class of status code indicates a provisional response,
   consisting only of the Status-Line and optional headers, and is
   terminated by an empty line. There are no required headers for this
   class of status code. Since HTTP/1.0 did not define any 1xx status
   codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client
   except under experimental conditions.

   A client MUST be prepared to accept one or more 1xx status responses
   prior to a regular response, even if the client does not expect a 100
   (Continue) status message. Unexpected 1xx status responses MAY be
   ignored by a user agent.

   Proxies MUST forward 1xx responses, unless the connection between the
   proxy and its client has been closed, or unless the proxy itself
   requested the generation of the 1xx response. (For example, if a

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   proxy adds a "Expect: 100-continue" field when it forwards a request,
   then it need not forward the corresponding 100 (Continue)
   response(s).)

10.1.1 100 Continue

   The client SHOULD continue with its request. This interim response is
   used to inform the client that the initial part of the request has
   been received and has not yet been rejected by the server. The client
   SHOULD continue by sending the remainder of the request or, if the
   request has already been completed, ignore this response. The server
   MUST send a final response after the request has been completed. See
   section 8.2.3 for detailed discussion of the use and handling of this
   status code.

10.1.2 101 Switching Protocols

   The server understands and is willing to comply with the client's
   request, via the Upgrade message header field (section 14.42), for a
   change in the application protocol being used on this connection. The
   server will switch protocols to those defined by the response's
   Upgrade header field immediately after the empty line which
   terminates the 101 response.

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

10.2 Successful 2xx

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

10.2.1 200 OK

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

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

   HEAD   the entity-header fields corresponding to the requested
          resource are sent in the response without any message-body;

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

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

10.2.2 201 Created

   The request has been fulfilled and resulted in a new resource being
   created. The newly created resource can be referenced by the URI(s)
   returned in the entity of the response, with the most specific URI
   for the resource given by a Location header field. The response
   SHOULD include an entity containing a list of resource
   characteristics and location(s) from which the user or user agent can
   choose the one most appropriate. The entity format is specified by
   the media type given in the Content-Type header field. The origin
   server MUST create the resource before returning the 201 status code.
   If the action cannot be carried out immediately, the server SHOULD
   respond with 202 (Accepted) response instead.

   A 201 response MAY contain an ETag response header field indicating
   the current value of the entity tag for the requested variant just
   created, see section 14.19.

10.2.3 202 Accepted

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

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

10.2.4 203 Non-Authoritative Information

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

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10.2.5 204 No Content

   The server has fulfilled the request but does not need to return an
   entity-body, and might want to return updated metainformation. The
   response MAY include new or updated metainformation in the form of
   entity-headers, which if present SHOULD be associated with the
   requested variant.

   If the client is a user agent, it SHOULD NOT change its document view
   from that which caused the request to be sent. This response is
   primarily intended to allow input for actions to take place without
   causing a change to the user agent's active document view, although
   any new or updated metainformation SHOULD be applied to the document
   currently in the user agent's active view.

   The 204 response MUST NOT include a message-body, and thus is always
   terminated by the first empty line after the header fields.

10.2.6 205 Reset Content

   The server has fulfilled the request and the user agent SHOULD reset
   the document view which caused the request to be sent. This response
   is primarily intended to allow input for actions to take place via
   user input, followed by a clearing of the form in which the input is
   given so that the user can easily initiate another input action. The
   response MUST NOT include an entity.

10.2.7 206 Partial Content

   The server has fulfilled the partial GET request for the resource.
   The request MUST have included a Range header field (section 14.35)
   indicating the desired range, and MAY have included an If-Range
   header field (section 14.27) to make the request conditional.

   The response MUST include the following header fields:

      - Either a Content-Range header field (section 14.16) indicating
        the range included with this response, or a multipart/byteranges
        Content-Type including Content-Range fields for each part. If a
        Content-Length header field is present in the response, its
        value MUST match the actual number of OCTETs transmitted in the
        message-body.

      - Date

      - ETag and/or Content-Location, if the header would have been sent
        in a 200 response to the same request

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      - Expires, Cache-Control, and/or Vary, if the field-value might
        differ from that sent in any previous response for the same
        variant

   If the 206 response is the result of an If-Range request that used a
   strong cache validator (see section 13.3.3), the response SHOULD NOT
   include other entity-headers. If the response is the result of an
   If-Range request that used a weak validator, the response MUST NOT
   include other entity-headers; this prevents inconsistencies between
   cached entity-bodies and updated headers. Otherwise, the response
   MUST include all of the entity-headers that would have been returned
   with a 200 (OK) response to the same request.

   A cache MUST NOT combine a 206 response with other previously cached
   content if the ETag or Last-Modified headers do not match exactly,
   see 13.5.4.

   A cache that does not support the Range and Content-Range headers
   MUST NOT cache 206 (Partial) responses.

10.3 Redirection 3xx

   This class of status code indicates that further action needs to be
   taken by the user agent in order to fulfill the request.  The action
   required MAY be carried out by the user agent without interaction
   with the user if and only if the method used in the second request is
   GET or HEAD. A client SHOULD detect infinite redirection loops, since
   such loops generate network traffic for each redirection.

      Note: previous versions of this specification recommended a
      maximum of five redirections. Content developers should be aware
      that there might be clients that implement such a fixed
      limitation.

10.3.1 300 Multiple Choices

   The requested resource corresponds to any one of a set of
   representations, each with its own specific location, and agent-
   driven negotiation information (section 12) is being provided so that
   the user (or user agent) can select a preferred representation and
   redirect its request to that location.

   Unless it was a HEAD request, the response SHOULD include an entity
   containing a list of resource characteristics and location(s) from
   which the user or user agent can choose the one most appropriate. The
   entity format is specified by the media type given in the Content-
   Type header field. Depending upon the format and the capabilities of

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   the user agent, selection of the most appropriate choice MAY be
   performed automatically. However, this specification does not define
   any standard for such automatic selection.

   If the server has a preferred choice of representation, it SHOULD
   include the specific URI for that representation in the Location
   field; user agents MAY use the Location field value for automatic
   redirection. This response is cacheable unless indicated otherwise.

10.3.2 301 Moved Permanently

   The requested resource has been assigned a new permanent URI and any
   future references to this resource SHOULD use one of the returned
   URIs.  Clients with link editing capabilities ought to automatically
   re-link references to the Request-URI to one or more of the new
   references returned by the server, where possible. This response is
   cacheable unless indicated otherwise.

   The new permanent URI SHOULD be given by the Location field in the
   response. Unless the request method was HEAD, the entity of the
   response SHOULD contain a short hypertext note with a hyperlink to
   the new URI(s).

   If the 301 status code is received in response to a request other
   than GET or HEAD, the user agent MUST NOT automatically redirect the
   request unless it can be confirmed by the user, since this might
   change the conditions under which the request was issued.

      Note: When automatically redirecting a POST request after
      receiving a 301 status code, some existing HTTP/1.0 user agents
      will erroneously change it into a GET request.

10.3.3 302 Found

   The requested resource resides temporarily under a different URI.
   Since the redirection might be altered on occasion, the client SHOULD
   continue to use the Request-URI for future requests.  This response
   is only cacheable if indicated by a Cache-Control or Expires header
   field.

   The temporary URI SHOULD be given by the Location field in the
   response. Unless the request method was HEAD, the entity of the
   response SHOULD contain a short hypertext note with a hyperlink to
   the new URI(s).

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   If the 302 status code is received in response to a request other
   than GET or HEAD, the user agent MUST NOT automatically redirect the
   request unless it can be confirmed by the user, since this might
   change the conditions under which the request was issued.

      Note: RFC 1945 and RFC 2068 specify that the client is not allowed
      to change the method on the redirected request.  However, most
      existing user agent implementations treat 302 as if it were a 303
      response, performing a GET on the Location field-value regardless
      of the original request method. The status codes 303 and 307 have
      been added for servers that wish to make unambiguously clear which
      kind of reaction is expected of the client.

10.3.4 303 See Other

   The response to the request can be found under a different URI and
   SHOULD be retrieved using a GET method on that resource. This method
   exists primarily to allow the output of a POST-activated script to
   redirect the user agent to a selected resource. The new URI is not a
   substitute reference for the originally requested resource. The 303
   response MUST NOT be cached, but the response to the second
   (redirected) request might be cacheable.

   The different URI SHOULD be given by the Location field in the
   response. Unless the request method was HEAD, the entity of the
   response SHOULD contain a short hypertext note with a hyperlink to
   the new URI(s).

      Note: Many pre-HTTP/1.1 user agents do not understand the 303
      status. When interoperability with such clients is a concern, the
      302 status code may be used instead, since most user agents react
      to a 302 response as described here for 303.

10.3.5 304 Not Modified

   If the client has performed a conditional GET request and access is
   allowed, but the document has not been modified, the server SHOULD
   respond with this status code. The 304 response MUST NOT contain a
   message-body, and thus is always terminated by the first empty line
   after the header fields.

   The response MUST include the following header fields:

      - Date, unless its omission is required by section 14.18.1

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   If a clockless origin server obeys these rules, and proxies and
   clients add their own Date to any response received without one (as
   already specified by [RFC 2068], section 14.19), caches will operate
   correctly.

      - ETag and/or Content-Location, if the header would have been sent
        in a 200 response to the same request

      - Expires, Cache-Control, and/or Vary, if the field-value might
        differ from that sent in any previous response for the same
        variant

   If the conditional GET used a strong cache validator (see section
   13.3.3), the response SHOULD NOT include other entity-headers.
   Otherwise (i.e., the conditional GET used a weak validator), the
   response MUST NOT include other entity-headers; this prevents
   inconsistencies between cached entity-bodies and updated headers.

   If a 304 response indicates an entity not currently cached, then the
   cache MUST disregard the response and repeat the request without the
   conditional.

   If a cache uses a received 304 response to update a cache entry, the
   cache MUST update the entry to reflect any new field values given in
   the response.

10.3.6 305 Use Proxy

   The requested resource MUST be accessed through the proxy given by
   the Location field. The Location field gives the URI of the proxy.
   The recipient is expected to repeat this single request via the
   proxy. 305 responses MUST only be generated by origin servers.

      Note: RFC 2068 was not clear that 305 was intended to redirect a
      single request, and to be generated by origin servers only.  Not
      observing these limitations has significant security consequences.

10.3.7 306 (Unused)

   The 306 status code was used in a previous version of the
   specification, is no longer used, and the code is reserved.

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10.3.8 307 Temporary Redirect

   The requested resource resides temporarily under a different URI.
   Since the redirection MAY be altered on occasion, the client SHOULD
   continue to use the Request-URI for future requests.  This response
   is only cacheable if indicated by a Cache-Control or Expires header
   field.

   The temporary URI SHOULD be given by the Location field in the
   response. Unless the request method was HEAD, the entity of the
   response SHOULD contain a short hypertext note with a hyperlink to
   the new URI(s) , since many pre-HTTP/1.1 user agents do not
   understand the 307 status. Therefore, the note SHOULD contain the
   information necessary for a user to repeat the original request on
   the new URI.

   If the 307 status code is received in response to a request other
   than GET or HEAD, the user agent MUST NOT automatically redirect the
   request unless it can be confirmed by the user, since this might
   change the conditions under which the request was issued.

10.4 Client Error 4xx

   The 4xx class of status code is intended for cases in which the
   client seems to have erred. Except when responding to a HEAD request,
   the server SHOULD include an entity containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition. These status codes are applicable to any request method.
   User agents SHOULD display any included entity to the user.

   If the client is sending data, a server implementation using TCP
   SHOULD be careful to ensure that the client acknowledges receipt of
   the packet(s) containing the response, before the server closes the
   input connection. If the client continues sending data to the server
   after the close, the server's TCP stack will send a reset packet to
   the client, which may erase the client's unacknowledged input buffers
   before they can be read and interpreted by the HTTP application.

10.4.1 400 Bad Request

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

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10.4.2 401 Unauthorized

   The request requires user authentication. The response MUST include a
   WWW-Authenticate header field (section 14.47) containing a challenge
   applicable to the requested resource. The client MAY repeat the
   request with a suitable Authorization header field (section 14.8). If
   the request already included Authorization credentials, then the 401
   response indicates that authorization has been refused for those
   credentials. If the 401 response contains the same challenge as the
   prior response, and the user agent has already attempted
   authentication at least once, then the user SHOULD be presented the
   entity that was given in the response, since that entity might
   include relevant diagnostic information. HTTP access authentication
   is explained in "HTTP Authentication: Basic and Digest Access
   Authentication" [43].

10.4.3 402 Payment Required

   This code is reserved for future use.

10.4.4 403 Forbidden

   The server understood the request, but is refusing to fulfill it.
   Authorization will not help and the request SHOULD NOT be repeated.
   If the request method was not HEAD and the server wishes to make
   public why the request has not been fulfilled, it SHOULD describe the
   reason for the refusal in the entity.  If the server does not wish to
   make this information available to the client, the status code 404
   (Not Found) can be used instead.

10.4.5 404 Not Found

   The server has not found anything matching the Request-URI. No
   indication is given of whether the condition is temporary or
   permanent. The 410 (Gone) status code SHOULD be used if the server
   knows, through some internally configurable mechanism, that an old
   resource is permanently unavailable and has no forwarding address.
   This status code is commonly used when the server does not wish to
   reveal exactly why the request has been refused, or when no other
   response is applicable.

10.4.6 405 Method Not Allowed

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

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10.4.7 406 Not Acceptable

   The resource identified by the request is only capable of generating
   response entities which have content characteristics not acceptable
   according to the accept headers sent in the request.

   Unless it was a HEAD request, the response SHOULD include an entity
   containing a list of available entity characteristics and location(s)
   from which the user or user agent can choose the one most
   appropriate. The entity format is specified by the media type given
   in the Content-Type header field. Depending upon the format and the
   capabilities of the user agent, selection of the most appropriate
   choice MAY be performed automatically. However, this specification
   does not define any standard for such automatic selection.

      Note: HTTP/1.1 servers are allowed to return responses which are
      not acceptable according to the accept headers sent in the
      request. In some cases, this may even be preferable to sending a
      406 response. User agents are encouraged to inspect the headers of
      an incoming response to determine if it is acceptable.

   If the response could be unacceptable, a user agent SHOULD
   temporarily stop receipt of more data and query the user for a
   decision on further actions.

10.4.8 407 Proxy Authentication Required

   This code is similar to 401 (Unauthorized), but indicates that the
   client must first authenticate itself with the proxy. The proxy MUST
   return a Proxy-Authenticate header field (section 14.33) containing a
   challenge applicable to the proxy for the requested resource. The
   client MAY repeat the request with a suitable Proxy-Authorization
   header field (section 14.34). HTTP access authentication is explained
   in "HTTP Authentication: Basic and Digest Access Authentication"
   [43].

10.4.9 408 Request Timeout

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

10.4.10 409 Conflict

   The request could not be completed due to a conflict with the current
   state of the resource. This code is only allowed in situations where
   it is expected that the user might be able to resolve the conflict
   and resubmit the request. The response body SHOULD include enough

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   information for the user to recognize the source of the conflict.
   Ideally, the response entity would include enough information for the
   user or user agent to fix the problem; however, that might not be
   possible and is not required.

   Conflicts are most likely to occur in response to a PUT request. For
   example, if versioning were being used and the entity being PUT
   included changes to a resource which conflict with those made by an
   earlier (third-party) request, the server might use the 409 response
   to indicate that it can't complete the request. In this case, the
   response entity would likely contain a list of the differences
   between the two versions in a format defined by the response
   Content-Type.

10.4.11 410 Gone

   The requested resource is no longer available at the server and no
   forwarding address is known. This condition is expected to be
   considered permanent. Clients with link editing capabilities SHOULD
   delete references to the Request-URI after user approval. If the
   server does not know, or has no facility to determine, whether or not
   the condition is permanent, the status code 404 (Not Found) SHOULD be
   used instead. This response is cacheable unless indicated otherwise.

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

10.4.12 411 Length Required

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

10.4.13 412 Precondition Failed

   The precondition given in one or more of the request-header fields
   evaluated to false when it was tested on the server. This response
   code allows the client to place preconditions on the current resource
   metainformation (header field data) and thus prevent the requested
   method from being applied to a resource other than the one intended.

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10.4.14 413 Request Entity Too Large

   The server is refusing to process a request because the request
   entity is larger than the server is willing or able to process. The
   server MAY close the connection to prevent the client from continuing
   the request.

   If the condition is temporary, the server SHOULD include a Retry-
   After header field to indicate that it is temporary and after what
   time the client MAY try again.

10.4.15 414 Request-URI Too Long

   The server is refusing to service the request because the Request-URI
   is longer than the server is willing to interpret. This rare
   condition is only likely to occur when a client has improperly
   converted a POST request to a GET request with long query
   information, when the client has descended into a URI "black hole" of
   redirection (e.g., a redirected URI prefix that points to a suffix of
   itself), or when the server is under attack by a client attempting to
   exploit security holes present in some servers using fixed-length
   buffers for reading or manipulating the Request-URI.

10.4.16 415 Unsupported Media Type

   The server is refusing to service the request because the entity of
   the request is in a format not supported by the requested resource
   for the requested method.

10.4.17 416 Requested Range Not Satisfiable

   A server SHOULD return a response with this status code if a request
   included a Range request-header field (section 14.35), and none of
   the range-specifier values in this field overlap the current extent
   of the selected resource, and the request did not include an If-Range
   request-header field. (For byte-ranges, this means that the first-
   byte-pos of all of the byte-range-spec values were greater than the
   current length of the selected resource.)

   When this status code is returned for a byte-range request, the
   response SHOULD include a Content-Range entity-header field
   specifying the current length of the selected resource (see section
   14.16). This response MUST NOT use the multipart/byteranges content-
   type.

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10.4.18 417 Expectation Failed

   The expectation given in an Expect request-header field (see section
   14.20) could not be met by this server, or, if the server is a proxy,
   the server has unambiguous evidence that the request could not be met
   by the next-hop server.

10.5 Server Error 5xx

   Response status codes beginning with the digit "5" indicate cases in
   which the server is aware that it has erred or is incapable of
   performing the request. Except when responding to a HEAD request, the
   server SHOULD include an entity containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition. User agents SHOULD display any included entity to the
   user. These response codes are applicable to any request method.

10.5.1 500 Internal Server Error

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

10.5.2 501 Not Implemented

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

10.5.3 502 Bad Gateway

   The server, while acting as a gateway or proxy, received an invalid
   response from the upstream server it accessed in attempting to
   fulfill the request.

10.5.4 503 Service Unavailable

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

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

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10.5.5 504 Gateway Timeout

   The server, while acting as a gateway or proxy, did not receive a
   timely response from the upstream server specified by the URI (e.g.
   HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
   to access in attempting to complete the request.

      Note: Note to implementors: some deployed proxies are known to
      return 400 or 500 when DNS lookups time out.

10.5.6 505 HTTP Version Not Supported

   The server does not support, or refuses to support, the HTTP protocol
   version that was used in the request message. The server is
   indicating that it is unable or unwilling to complete the request
   using the same major version as the client, as described in section
   3.1, other than with this error message. The response SHOULD contain
   an entity describing why that version is not supported and what other
   protocols are supported by that server.

11 Access Authentication

   HTTP provides several OPTIONAL challenge-response authentication
   mechanisms which can be used by a server to challenge a client
   request and by a client to provide authentication information. The
   general framework for access authentication, and the specification of
   "basic" and "digest" authentication, are specified in "HTTP
   Authentication: Basic and Digest Access Authentication" [43]. This
   specification adopts the definitions of "challenge" and "credentials"
   from that specification.

12 Content Negotiation

   Most HTTP responses include an entity which contains information for
   interpretation by a human user. Naturally, it is desirable to supply
   the user with the "best available" entity corresponding to the
   request. Unfortunately for servers and caches, not all users have the
   same preferences for what is "best," and not all user agents are
   equally capable of rendering all entity types. For that reason, HTTP
   has provisions for several mechanisms for "content negotiation" --
   the process of selecting the best representation for a given response
   when there are multiple representations available.

      Note: This is not called "format negotiation" because the
      alternate representations may be of the same media type, but use
      different capabilities of that type, be in different languages,
      etc.

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   Any response containing an entity-body MAY be subject to negotiation,
   including error responses.

   There are two kinds of content negotiation which are possible in
   HTTP: server-driven and agent-driven negotiation. These two kinds of
   negotiation are orthogonal and thus may be used separately or in
   combination. One method of combination, referred to as transparent
   negotiation, occurs when a cache uses the agent-driven negotiation
   information provided by the origin server in order to provide
   server-driven negotiation for subsequent requests.

12.1 Server-driven Negotiation

   If the selection of the best representation for a response is made by
   an algorithm located at the server, it is called server-driven
   negotiation. Selection is based on the available representations of
   the response (the dimensions over which it can vary; e.g. language,
   content-coding, etc.) and the contents of particular header fields in
   the request message or on other information pertaining to the request
   (such as the network address of the client).

   Server-driven negotiation is advantageous when the algorithm for
   selecting from among the available representations is difficult to
   describe to the user agent, or when the server desires to send its
   "best guess" to the client along with the first response (hoping to
   avoid the round-trip delay of a subsequent request if the "best
   guess" is good enough for the user). In order to improve the server's
   guess, the user agent MAY include request header fields (Accept,
   Accept-Language, Accept-Encoding, etc.) which describe its
   preferences for such a response.

   Server-driven negotiation has disadvantages:

      1. It is impossible for the server to accurately determine what
         might be "best" for any given user, since that would require
         complete knowledge of both the capabilities of the user agent
         and the intended use for the response (e.g., does the user want
         to view it on screen or print it on paper?).

      2. Having the user agent describe its capabilities in every
         request can be both very inefficient (given that only a small
         percentage of responses have multiple representations) and a
         potential violation of the user's privacy.

      3. It complicates the implementation of an origin server and the
         algorithms for generating responses to a request.

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      4. It may limit a public cache's ability to use the same response
         for multiple user's requests.

   HTTP/1.1 includes the following request-header fields for enabling
   server-driven negotiation through description of user agent
   capabilities and user preferences: Accept (section 14.1), Accept-
   Charset (section 14.2), Accept-Encoding (section 14.3), Accept-
   Language (section 14.4), and User-Agent (section 14.43). However, an
   origin server is not limited to these dimensions and MAY vary the
   response based on any aspect of the request, including information
   outside the request-header fields or within extension header fields
   not defined by this specification.

   The Vary  header field can be used to express the parameters the
   server uses to select a representation that is subject to server-
   driven negotiation. See section 13.6 for use of the Vary header field
   by caches and section 14.44 for use of the Vary header field by
   servers.

12.2 Agent-driven Negotiation

   With agent-driven negotiation, selection of the best representation
   for a response is performed by the user agent after receiving an
   initial response from the origin server. Selection is based on a list
   of the available representations of the response included within the
   header fields or entity-body of the initial response, with each
   representation identified by its own URI. Selection from among the
   representations may be performed automatically (if the user agent is
   capable of doing so) or manually by the user selecting from a
   generated (possibly hypertext) menu.

   Agent-driven negotiation is advantageous when the response would vary
   over commonly-used dimensions (such as type, language, or encoding),
   when the origin server is unable to determine a user agent's
   capabilities from examining the request, and generally when public
   caches are used to distribute server load and reduce network usage.

   Agent-driven negotiation suffers from the disadvantage of needing a
   second request to obtain the best alternate representation. This
   second request is only efficient when caching is used. In addition,
   this specification does not define any mechanism for supporting
   automatic selection, though it also does not prevent any such
   mechanism from being developed as an extension and used within
   HTTP/1.1.

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   HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)
   status codes for enabling agent-driven negotiation when the server is
   unwilling or unable to provide a varying response using server-driven
   negotiation.

12.3 Transparent Negotiation

   Transparent negotiation is a combination of both server-driven and
   agent-driven negotiation. When a cache is supplied with a form of the
   list of available representations of the response (as in agent-driven
   negotiation) and the dimensions of variance are completely understood
   by the cache, then the cache becomes capable of performing server-
   driven negotiation on behalf of the origin server for subsequent
   requests on that resource.

   Transparent negotiation has the advantage of distributing the
   negotiation work that would otherwise be required of the origin
   server and also removing the second request delay of agent-driven
   negotiation when the cache is able to correctly guess the right
   response.

   This specification does not define any mechanism for transparent
   negotiation, though it also does not prevent any such mechanism from
   being developed as an extension that could be used within HTTP/1.1.

13 Caching in HTTP

   HTTP is typically used for distributed information systems, where
   performance can be improved by the use of response caches. The
   HTTP/1.1 protocol includes a number of elements intended to make
   caching work as well as possible. Because these elements are
   inextricable from other aspects of the protocol, and because they
   interact with each other, it is useful to describe the basic caching
   design of HTTP separately from the detailed descriptions of methods,
   headers, response codes, etc.

   Caching would be useless if it did not significantly improve
   performance. The goal of caching in HTTP/1.1 is to eliminate the need
   to send requests in many cases, and to eliminate the need to send
   full responses in many other cases. The former reduces the number of
   network round-trips required for many operations; we use an
   "expiration" mechanism for this purpose (see section 13.2). The
   latter reduces network bandwidth requirements; we use a "validation"
   mechanism for this purpose (see section 13.3).

   Requirements for performance, availability, and disconnected
   operation require us to be able to relax the goal of semantic
   transparency. The HTTP/1.1 protocol allows origin servers, caches,

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   and clients to explicitly reduce transparency when necessary.
   However, because non-transparent operation may confuse non-expert
   users, and might be incompatible with certain server applications
   (such as those for ordering merchandise), the protocol requires that
   transparency be relaxed

      - only by an explicit protocol-level request when relaxed by
        client or origin server

      - only with an explicit warning to the end user when relaxed by
        cache or client

   Therefore, the HTTP/1.1 protocol provides these important elements:

      1. Protocol features that provide full semantic transparency when
         this is required by all parties.

      2. Protocol features that allow an origin server or user agent to
         explicitly request and control non-transparent operation.

      3. Protocol features that allow a cache to attach warnings to
         responses that do not preserve the requested approximation of
         semantic transparency.

   A basic principle is that it must be possible for the clients to
   detect any potential relaxation of semantic transparency.

      Note: The server, cache, or client implementor might be faced with
      design decisions not explicitly discussed in this specification.
      If a decision might affect semantic transparency, the implementor
      ought to err on the side of maintaining transparency unless a
      careful and complete analysis shows significant benefits in
      breaking transparency.

13.1.1 Cache Correctness

   A correct cache MUST respond to a request with the most up-to-date
   response held by the cache that is appropriate to the request (see
   sections 13.2.5, 13.2.6, and 13.12) which meets one of the following
   conditions:

      1. It has been checked for equivalence with what the origin server
         would have returned by revalidating the response with the
         origin server (section 13.3);

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      2. It is "fresh enough" (see section 13.2). In the default case,
         this means it meets the least restrictive freshness requirement
         of the client, origin server, and cache (see section 14.9); if
         the origin server so specifies, it is the freshness requirement
         of the origin server alone.

         If a stored response is not "fresh enough" by the most
         restrictive freshness requirement of both the client and the
         origin server, in carefully considered circumstances the cache
         MAY still return the response with the appropriate Warning
         header (see section 13.1.5 and 14.46), unless such a response
         is prohibited (e.g., by a "no-store" cache-directive, or by a
         "no-cache" cache-request-directive; see section 14.9).

      3. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect),
         or error (4xx or 5xx) response message.

   If the cache can not communicate with the origin server, then a
   correct cache SHOULD respond as above if the response can be
   correctly served from the cache; if not it MUST return an error or
   warning indicating that there was a communication failure.

   If a cache receives a response (either an entire response, or a 304
   (Not Modified) response) that it would normally forward to the
   requesting client, and the received response is no longer fresh, the
   cache SHOULD forward it to the requesting client without adding a new
   Warning (but without removing any existing Warning headers). A cache
   SHOULD NOT attempt to revalidate a response simply because that
   response became stale in transit; this might lead to an infinite
   loop. A user agent that receives a stale response without a Warning
   MAY display a warning indication to the user.

13.1.2 Warnings

   Whenever a cache returns a response that is neither first-hand nor
   "fresh enough" (in the sense of condition 2 in section 13.1.1), it
   MUST attach a warning to that effect, using a Warning general-header.
   The Warning header and the currently defined warnings are described
   in section 14.46. The warning allows clients to take appropriate
   action.

   Warnings MAY be used for other purposes, both cache-related and
   otherwise. The use of a warning, rather than an error status code,
   distinguish these responses from true failures.

   Warnings are assigned three digit warn-codes. The first digit
   indicates whether the Warning MUST or MUST NOT be deleted from a
   stored cache entry after a successful revalidation:

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   1xx  Warnings that describe the freshness or revalidation status of
     the response, and so MUST be deleted after a successful
     revalidation. 1XX warn-codes MAY be generated by a cache only when
     validating a cached entry. It MUST NOT be generated by clients.

   2xx  Warnings that describe some aspect of the entity body or entity
     headers that is not rectified by a revalidation (for example, a
     lossy compression of the entity bodies) and which MUST NOT be
     deleted after a successful revalidation.

   See section 14.46 for the definitions of the codes themselves.

   HTTP/1.0 caches will cache all Warnings in responses, without
   deleting the ones in the first category. Warnings in responses that
   are passed to HTTP/1.0 caches carry an extra warning-date field,
   which prevents a future HTTP/1.1 recipient from believing an
   erroneously cached Warning.

   Warnings also carry a warning text. The text MAY be in any
   appropriate natural language (perhaps based on the client's Accept
   headers), and include an OPTIONAL indication of what character set is
   used.

   Multiple warnings MAY be attached to a response (either by the origin
   server or by a cache), including multiple warnings with the same code
   number. For example, a server might provide the same warning with
   texts in both English and Basque.

   When multiple warnings are attached to a response, it might not be
   practical or reasonable to display all of them to the user. This
   version of HTTP does not specify strict priority rules for deciding
   which warnings to display and in what order, but does suggest some
   heuristics.

13.1.3 Cache-control Mechanisms

   The basic cache mechanisms in HTTP/1.1 (server-specified expiration
   times and validators) are implicit directives to caches. In some
   cases, a server or client might need to provide explicit directives
   to the HTTP caches. We use the Cache-Control header for this purpose.

   The Cache-Control header allows a client or server to transmit a
   variety of directives in either requests or responses. These
   directives typically override the default caching algorithms. As a
   general rule, if there is any apparent conflict between header
   values, the most restrictive interpretation is applied (that is, the
   one that is most likely to preserve semantic transparency). However,

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   in some cases, cache-control directives are explicitly specified as
   weakening the approximation of semantic transparency (for example,
   "max-stale" or "public").

   The cache-control directives are described in detail in section 14.9.

13.1.4 Explicit User Agent Warnings

   Many user agents make it possible for users to override the basic
   caching mechanisms. For example, the user agent might allow the user
   to specify that cached entities (even explicitly stale ones) are
   never validated. Or the user agent might habitually add "Cache-
   Control: max-stale=3600" to every request. The user agent SHOULD NOT
   default to either non-transparent behavior, or behavior that results
   in abnormally ineffective caching, but MAY be explicitly configured
   to do so by an explicit action of the user.

   If the user has overridden the basic caching mechanisms, the user
   agent SHOULD explicitly indicate to the user whenever this results in
   the display of information that might not meet the server's
   transparency requirements (in particular, if the displayed entity is
   known to be stale). Since the protocol normally allows the user agent
   to determine if responses are stale or not, this indication need only
   be displayed when this actually happens. The indication need not be a
   dialog box; it could be an icon (for example, a picture of a rotting
   fish) or some other indicator.

   If the user has overridden the caching mechanisms in a way that would
   abnormally reduce the effectiveness of caches, the user agent SHOULD
   continually indicate this state to the user (for example, by a
   display of a picture of currency in flames) so that the user does not
   inadvertently consume excess resources or suffer from excessive
   latency.

13.1.5 Exceptions to the Rules and Warnings

   In some cases, the operator of a cache MAY choose to configure it to
   return stale responses even when not requested by clients. This
   decision ought not be made lightly, but may be necessary for reasons
   of availability or performance, especially when the cache is poorly
   connected to the origin server. Whenever a cache returns a stale
   response, it MUST mark it as such (using a Warning header) enabling
   the client software to alert the user that there might be a potential
   problem.

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   It also allows the user agent to take steps to obtain a first-hand or
   fresh response. For this reason, a cache SHOULD NOT return a stale
   response if the client explicitly requests a first-hand or fresh one,
   unless it is impossible to comply for technical or policy reasons.

13.1.6 Client-controlled Behavior

   While the origin server (and to a lesser extent, intermediate caches,
   by their contribution to the age of a response) are the primary
   source of expiration information, in some cases the client might need
   to control a cache's decision about whether to return a cached
   response without validating it. Clients do this using several
   directives of the Cache-Control header.

   A client's request MAY specify the maximum age it is willing to
   accept of an unvalidated response; specifying a value of zero forces
   the cache(s) to revalidate all responses. A client MAY also specify
   the minimum time remaining before a response expires. Both of these
   options increase constraints on the behavior of caches, and so cannot
   further relax the cache's approximation of semantic transparency.

   A client MAY also specify that it will accept stale responses, up to
   some maximum amount of staleness. This loosens the constraints on the
   caches, and so might violate the origin server's specified
   constraints on semantic transparency, but might be necessary to
   support disconnected operation, or high availability in the face of
   poor connectivity.

13.2 Expiration Model

13.2.1 Server-Specified Expiration

   HTTP caching works best when caches can entirely avoid making
   requests to the origin server. The primary mechanism for avoiding
   requests is for an origin server to provide an explicit expiration
   time in the future, indicating that a response MAY be used to satisfy
   subsequent requests. In other words, a cache can return a fresh
   response without first contacting the server.

   Our expectation is that servers will assign future explicit
   expiration times to responses in the belief that the entity is not
   likely to change, in a semantically significant way, before the
   expiration time is reached. This normally preserves semantic
   transparency, as long as the server's expiration times are carefully
   chosen.

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   The expiration mechanism applies only to responses taken from a cache
   and not to first-hand responses forwarded immediately to the
   requesting client.

   If an origin server wishes to force a semantically transparent cache
   to validate every request, it MAY assign an explicit expiration time
   in the past. This means that the response is always stale, and so the
   cache SHOULD validate it before using it for subsequent requests. See
   section 14.9.4 for a more restrictive way to force revalidation.

   If an origin server wishes to force any HTTP/1.1 cache, no matter how
   it is configured, to validate every request, it SHOULD use the "must-
   revalidate" cache-control directive (see section 14.9).

   Servers specify explicit expiration times using either the Expires
   header, or the max-age directive of the Cache-Control header.

   An expiration time cannot be used to force a user agent to refresh
   its display or reload a resource; its semantics apply only to caching
   mechanisms, and such mechanisms need only check a resource's
   expiration status when a new request for that resource is initiated.
   See section 13.13 for an explanation of the difference between caches
   and history mechanisms.

13.2.2 Heuristic Expiration

   Since origin servers do not always provide explicit expiration times,
   HTTP caches typically assign heuristic expiration times, employing
   algorithms that use other header values (such as the Last-Modified
   time) to estimate a plausible expiration time. The HTTP/1.1
   specification does not provide specific algorithms, but does impose
   worst-case constraints on their results. Since heuristic expiration
   times might compromise semantic transparency, they ought to used
   cautiously, and we encourage origin servers to provide explicit
   expiration times as much as possible.

13.2.3 Age Calculations

   In order to know if a cached entry is fresh, a cache needs to know if
   its age exceeds its freshness lifetime. We discuss how to calculate
   the latter in section 13.2.4; this section describes how to calculate
   the age of a response or cache entry.

   In this discussion, we use the term "now" to mean "the current value
   of the clock at the host performing the calculation." Hosts that use
   HTTP, but especially hosts running origin servers and caches, SHOULD
   use NTP [28] or some similar protocol to synchronize their clocks to
   a globally accurate time standard.

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   HTTP/1.1 requires origin servers to send a Date header, if possible,
   with every response, giving the time at which the response was
   generated (see section 14.18). We use the term "date_value" to denote
   the value of the Date header, in a form appropriate for arithmetic
   operations.

   HTTP/1.1 uses the Age response-header to convey the estimated age of
   the response message when obtained from a cache. The Age field value
   is the cache's estimate of the amount of time since the response was
   generated or revalidated by the origin server.

   In essence, the Age value is the sum of the time that the response
   has been resident in each of the caches along the path from the
   origin server, plus the amount of time it has been in transit along
   network paths.

   We use the term "age_value" to denote the value of the Age header, in
   a form appropriate for arithmetic operations.

   A response's age can be calculated in two entirely independent ways:

      1. now minus date_value, if the local clock is reasonably well
         synchronized to the origin server's clock. If the result is
         negative, the result is replaced by zero.

      2. age_value, if all of the caches along the response path
         implement HTTP/1.1.

   Given that we have two independent ways to compute the age of a
   response when it is received, we can combine these as

       corrected_received_age = max(now - date_value, age_value)

   and as long as we have either nearly synchronized clocks or all-
   HTTP/1.1 paths, one gets a reliable (conservative) result.

   Because of network-imposed delays, some significant interval might
   pass between the time that a server generates a response and the time
   it is received at the next outbound cache or client. If uncorrected,
   this delay could result in improperly low ages.

   Because the request that resulted in the returned Age value must have
   been initiated prior to that Age value's generation, we can correct
   for delays imposed by the network by recording the time at which the
   request was initiated. Then, when an Age value is received, it MUST
   be interpreted relative to the time the request was initiated, not

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   the time that the response was received. This algorithm results in
   conservative behavior no matter how much delay is experienced. So, we
   compute:

      corrected_initial_age = corrected_received_age
                            + (now - request_time)

   where "request_time" is the time (according to the local clock) when
   the request that elicited this response was sent.

   Summary of age calculation algorithm, when a cache receives a
   response:

      /*
       * age_value
       *      is the value of Age: header received by the cache with
       *              this response.
       * date_value
       *      is the value of the origin server's Date: header
       * request_time
       *      is the (local) time when the cache made the request
       *              that resulted in this cached response
       * response_time
       *      is the (local) time when the cache received the
       *              response
       * now
       *      is the current (local) time
       */

      apparent_age = max(0, response_time - date_value);
      corrected_received_age = max(apparent_age, age_value);
      response_delay = response_time - request_time;
      corrected_initial_age = corrected_received_age + response_delay;
      resident_time = now - response_time;
      current_age   = corrected_initial_age + resident_time;

   The current_age of a cache entry is calculated by adding the amount
   of time (in seconds) since the cache entry was last validated by the
   origin server to the corrected_initial_age. When a response is
   generated from a cache entry, the cache MUST include a single Age
   header field in the response with a value equal to the cache entry's
   current_age.

   The presence of an Age header field in a response implies that a
   response is not first-hand. However, the converse is not true, since
   the lack of an Age header field in a response does not imply that the

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   response is first-hand unless all caches along the request path are
   compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
   the Age header field).

13.2.4 Expiration Calculations

   In order to decide whether a response is fresh or stale, we need to
   compare its freshness lifetime to its age. The age is calculated as
   described in section 13.2.3; this section describes how to calculate
   the freshness lifetime, and to determine if a response has expired.
   In the discussion below, the values can be represented in any form
   appropriate for arithmetic operations.

   We use the term "expires_value" to denote the value of the Expires
   header. We use the term "max_age_value" to denote an appropriate
   value of the number of seconds carried by the "max-age" directive of
   the Cache-Control header in a response (see section 14.9.3).

   The max-age directive takes priority over Expires, so if max-age is
   present in a response, the calculation is simply:

      freshness_lifetime = max_age_value

   Otherwise, if Expires is present in the response, the calculation is:

      freshness_lifetime = expires_value - date_value

   Note that neither of these calculations is vulnerable to clock skew,
   since all of the information comes from the origin server.

   If none of Expires, Cache-Control: max-age, or Cache-Control: s-
   maxage (see section 14.9.3) appears in the response, and the response
   does not include other restrictions on caching, the cache MAY compute
   a freshness lifetime using a heuristic. The cache MUST attach Warning
   113 to any response whose age is more than 24 hours if such warning
   has not already been added.

   Also, if the response does have a Last-Modified time, the heuristic
   expiration value SHOULD be no more than some fraction of the interval
   since that time. A typical setting of this fraction might be 10%.

   The calculation to determine if a response has expired is quite
   simple:

      response_is_fresh = (freshness_lifetime > current_age)

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13.2.5 Disambiguating Expiration Values

   Because expiration values are assigned optimistically, it is possible
   for two caches to contain fresh values for the same resource that are
   different.

   If a client performing a retrieval receives a non-first-hand response
   for a request that was already fresh in its own cache, and the Date
   header in its existing cache entry is newer than the Date on the new
   response, then the client MAY ignore the response. If so, it MAY
   retry the request with a "Cache-Control: max-age=0" directive (see
   section 14.9), to force a check with the origin server.

   If a cache has two fresh responses for the same representation with
   different validators, it MUST use the one with the more recent Date
   header. This situation might arise because the cache is pooling
   responses from other caches, or because a client has asked for a
   reload or a revalidation of an apparently fresh cache entry.

13.2.6 Disambiguating Multiple Responses

   Because a client might be receiving responses via multiple paths, so
   that some responses flow through one set of caches and other
   responses flow through a different set of caches, a client might
   receive responses in an order different from that in which the origin
   server sent them. We would like the client to use the most recently
   generated response, even if older responses are still apparently
   fresh.

   Neither the entity tag nor the expiration value can impose an
   ordering on responses, since it is possible that a later response
   intentionally carries an earlier expiration time. The Date values are
   ordered to a granularity of one second.

   When a client tries to revalidate a cache entry, and the response it
   receives contains a Date header that appears to be older than the one
   for the existing entry, then the client SHOULD repeat the request
   unconditionally, and include

       Cache-Control: max-age=0

   to force any intermediate caches to validate their copies directly
   with the origin server, or

       Cache-Control: no-cache

   to force any intermediate caches to obtain a new copy from the origin
   server.

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   If the Date values are equal, then the client MAY use either response
   (or MAY, if it is being extremely prudent, request a new response).
   Servers MUST NOT depend on clients being able to choose
   deterministically between responses generated during the same second,
   if their expiration times overlap.

13.3 Validation Model

   When a cache has a stale entry that it would like to use as a
   response to a client's request, it first has to check with the origin
   server (or possibly an intermediate cache with a fresh response) to
   see if its cached entry is still usable. We call this "validating"
   the cache entry. Since we do not want to have to pay the overhead of
   retransmitting the full response if the cached entry is good, and we
   do not want to pay the overhead of an extra round trip if the cached
   entry is invalid, the HTTP/1.1 protocol supports the use of
   conditional methods.

   The key protocol features for supporting conditional methods are
   those concerned with "cache validators." When an origin server
   generates a full response, it attaches some sort of validator to it,
   which is kept with the cache entry. When a client (user agent or
   proxy cache) makes a conditional request for a resource for which it
   has a cache entry, it includes the associated validator in the
   request.

   The server then checks that validator against the current validator
   for the entity, and, if they match (see section 13.3.3), it responds
   with a special status code (usually, 304 (Not Modified)) and no
   entity-body. Otherwise, it returns a full response (including
   entity-body). Thus, we avoid transmitting the full response if the
   validator matches, and we avoid an extra round trip if it does not
   match.

   In HTTP/1.1, a conditional request looks exactly the same as a normal
   request for the same resource, except that it carries a special
   header (which includes the validator) that implicitly turns the
   method (usually, GET) into a conditional.

   The protocol includes both positive and negative senses of cache-
   validating conditions. That is, it is possible to request either that
   a method be performed if and only if a validator matches or if and
   only if no validators match.

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      Note: a response that lacks a validator may still be cached, and
      served from cache until it expires, unless this is explicitly
      prohibited by a cache-control directive. However, a cache cannot
      do a conditional retrieval if it does not have a validator for the
      entity, which means it will not be refreshable after it expires.

13.3.1 Last-Modified Dates

   The Last-Modified entity-header field value is often used as a cache
   validator. In simple terms, a cache entry is considered to be valid
   if the entity has not been modified since the Last-Modified value.

13.3.2 Entity Tag Cache Validators

   The ETag response-header field value, an entity tag, provides for an
   "opaque" cache validator. This might allow more reliable validation
   in situations where it is inconvenient to store modification dates,
   where the one-second resolution of HTTP date values is not
   sufficient, or where the origin server wishes to avoid certain
   paradoxes that might arise from the use of modification dates.

   Entity Tags are described in section 3.11. The headers used with
   entity tags are described in sections 14.19, 14.24, 14.26 and 14.44.

13.3.3 Weak and Strong Validators

   Since both origin servers and caches will compare two validators to
   decide if they represent the same or different entities, one normally
   would expect that if the entity (the entity-body or any entity-
   headers) changes in any way, then the associated validator would
   change as well. If this is true, then we call this validator a
   "strong validator."

   However, there might be cases when a server prefers to change the
   validator only on semantically significant changes, and not when
   insignificant aspects of the entity change. A validator that does not
   always change when the resource changes is a "weak validator."

   Entity tags are normally "strong validators," but the protocol
   provides a mechanism to tag an entity tag as "weak." One can think of
   a strong validator as one that changes whenever the bits of an entity
   changes, while a weak value changes whenever the meaning of an entity
   changes. Alternatively, one can think of a strong validator as part
   of an identifier for a specific entity, while a weak validator is
   part of an identifier for a set of semantically equivalent entities.

      Note: One example of a strong validator is an integer that is
      incremented in stable storage every time an entity is changed.

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      An entity's modification time, if represented with one-second
      resolution, could be a weak validator, since it is possible that
      the resource might be modified twice during a single second.

      Support for weak validators is optional. However, weak validators
      allow for more efficient caching of equivalent objects; for
      example, a hit counter on a site is probably good enough if it is
      updated every few days or weeks, and any value during that period
      is likely "good enough" to be equivalent.

   A "use" of a validator is either when a client generates a request
   and includes the validator in a validating header field, or when a
   server compares two validators.

   Strong validators are usable in any context. Weak validators are only
   usable in contexts that do not depend on exact equality of an entity.
   For example, either kind is usable for a conditional GET of a full
   entity. However, only a strong validator is usable for a sub-range
   retrieval, since otherwise the client might end up with an internally
   inconsistent entity.

   Clients MAY issue simple (non-subrange) GET requests with either weak
   validators or strong validators. Clients MUST NOT use weak validators
   in other forms of request.

   The only function that the HTTP/1.1 protocol defines on validators is
   comparison. There are two validator comparison functions, depending
   on whether the comparison context allows the use of weak validators
   or not:

      - The strong comparison function: in order to be considered equal,
        both validators MUST be identical in every way, and both MUST
        NOT be weak.

      - The weak comparison function: in order to be considered equal,
        both validators MUST be identical in every way, but either or
        both of them MAY be tagged as "weak" without affecting the
        result.

   An entity tag is strong unless it is explicitly tagged as weak.
   Section 3.11 gives the syntax for entity tags.

   A Last-Modified time, when used as a validator in a request, is
   implicitly weak unless it is possible to deduce that it is strong,
   using the following rules:

      - The validator is being compared by an origin server to the
        actual current validator for the entity and,

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      - That origin server reliably knows that the associated entity did
        not change twice during the second covered by the presented
        validator.

   or

      - The validator is about to be used by a client in an If-
        Modified-Since or If-Unmodified-Since header, because the client
        has a cache entry for the associated entity, and

      - That cache entry includes a Date value, which gives the time
        when the origin server sent the original response, and

      - The presented Last-Modified time is at least 60 seconds before
        the Date value.

   or

      - The validator is being compared by an intermediate cache to the
        validator stored in its cache entry for the entity, and

      - That cache entry includes a Date value, which gives the time
        when the origin server sent the original response, and

      - The presented Last-Modified time is at least 60 seconds before
        the Date value.

   This method relies on the fact that if two different responses were
   sent by the origin server during the same second, but both had the
   same Last-Modified time, then at least one of those responses would
   have a Date value equal to its Last-Modified time. The arbitrary 60-
   second limit guards against the possibility that the Date and Last-
   Modified values are generated from different clocks, or at somewhat
   different times during the preparation of the response. An
   implementation MAY use a value larger than 60 seconds, if it is
   believed that 60 seconds is too short.

   If a client wishes to perform a sub-range retrieval on a value for
   which it has only a Last-Modified time and no opaque validator, it
   MAY do this only if the Last-Modified time is strong in the sense
   described here.

   A cache or origin server receiving a conditional request, other than
   a full-body GET request, MUST use the strong comparison function to
   evaluate the condition.

   These rules allow HTTP/1.1 caches and clients to safely perform sub-
   range retrievals on values that have been obtained from HTTP/1.0

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   servers.

13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates

   We adopt a set of rules and recommendations for origin servers,
   clients, and caches regarding when various validator types ought to
   be used, and for what purposes.

   HTTP/1.1 origin servers:

      - SHOULD send an entity tag validator unless it is not feasible to
        generate one.

      - MAY send a weak entity tag instead of a strong entity tag, if
        performance considerations support the use of weak entity tags,
        or if it is unfeasible to send a strong entity tag.

      - SHOULD send a Last-Modified value if it is feasible to send one,
        unless the risk of a breakdown in semantic transparency that
        could result from using this date in an If-Modified-Since header
        would lead to serious problems.

   In other words, the preferred behavior for an HTTP/1.1 origin server
   is to send both a strong entity tag and a Last-Modified value.

   In order to be legal, a strong entity tag MUST change whenever the
   associated entity value changes in any way. A weak entity tag SHOULD
   change whenever the associated entity changes in a semantically
   significant way.

      Note: in order to provide semantically transparent caching, an
      origin server must avoid reusing a specific strong entity tag
      value for two different entities, or reusing a specific weak
      entity tag value for two semantically different entities. Cache
      entries might persist for arbitrarily long periods, regardless of
      expiration times, so it might be inappropriate to expect that a
      cache will never again attempt to validate an entry using a
      validator that it obtained at some point in the past.

   HTTP/1.1 clients:

      - If an entity tag has been provided by the origin server, MUST
        use that entity tag in any cache-conditional request (using If-
        Match or If-None-Match).

      - If only a Last-Modified value has been provided by the origin
        server, SHOULD use that value in non-subrange cache-conditional
        requests (using If-Modified-Since).

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      - If only a Last-Modified value has been provided by an HTTP/1.0
        origin server, MAY use that value in subrange cache-conditional
        requests (using If-Unmodified-Since:). The user agent SHOULD
        provide a way to disable this, in case of difficulty.

      - If both an entity tag and a Last-Modified value have been
        provided by the origin server, SHOULD use both validators in
        cache-conditional requests. This allows both HTTP/1.0 and
        HTTP/1.1 caches to respond appropriately.

   An HTTP/1.1 origin server, upon receiving a conditional request that
   includes both a Last-Modified date (e.g., in an If-Modified-Since or
   If-Unmodified-Since header field) and one or more entity tags (e.g.,
   in an If-Match, If-None-Match, or If-Range header field) as cache
   validators, MUST NOT return a response status of 304 (Not Modified)
   unless doing so is consistent with all of the conditional header
   fields in the request.

   An HTTP/1.1 caching proxy, upon receiving a conditional request that
   includes both a Last-Modified date and one or more entity tags as
   cache validators, MUST NOT return a locally cached response to the
   client unless that cached response is consistent with all of the
   conditional header fields in the request.

      Note: The general principle behind these rules is that HTTP/1.1
      servers and clients should transmit as much non-redundant
      information as is available in their responses and requests.
      HTTP/1.1 systems receiving this information will make the most
      conservative assumptions about the validators they receive.

      HTTP/1.0 clients and caches will ignore entity tags. Generally,
      last-modified values received or used by these systems will
      support transparent and efficient caching, and so HTTP/1.1 origin
      servers should provide Last-Modified values. In those rare cases
      where the use of a Last-Modified value as a validator by an
      HTTP/1.0 system could result in a serious problem, then HTTP/1.1
      origin servers should not provide one.

13.3.5 Non-validating Conditionals

   The principle behind entity tags is that only the service author
   knows the semantics of a resource well enough to select an
   appropriate cache validation mechanism, and the specification of any
   validator comparison function more complex than byte-equality would
   open up a can of worms. Thus, comparisons of any other headers
   (except Last-Modified, for compatibility with HTTP/1.0) are never
   used for purposes of validating a cache entry.

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13.4 Response Cacheability

   Unless specifically constrained by a cache-control (section 14.9)
   directive, a caching system MAY always store a successful response
   (see section 13.8) as a cache entry, MAY return it without validation
   if it is fresh, and MAY return it after successful validation. If
   there is neither a cache validator nor an explicit expiration time
   associated with a response, we do not expect it to be cached, but
   certain caches MAY violate this expectation (for example, when little
   or no network connectivity is available). A client can usually detect
   that such a response was taken from a cache by comparing the Date
   header to the current time.

      Note: some HTTP/1.0 caches are known to violate this expectation
      without providing any Warning.

   However, in some cases it might be inappropriate for a cache to
   retain an entity, or to return it in response to a subsequent
   request. This might be because absolute semantic transparency is
   deemed necessary by the service author, or because of security or
   privacy considerations. Certain cache-control directives are
   therefore provided so that the server can indicate that certain
   resource entities, or portions thereof, are not to be cached
   regardless of other considerations.

   Note that section 14.8 normally prevents a shared cache from saving
   and returning a response to a previous request if that request
   included an Authorization header.

   A response received with a status code of 200, 203, 206, 300, 301 or
   410 MAY be stored by a cache and used in reply to a subsequent
   request, subject to the expiration mechanism, unless a cache-control
   directive prohibits caching. However, a cache that does not support
   the Range and Content-Range headers MUST NOT cache 206 (Partial
   Content) responses.

   A response received with any other status code (e.g. status codes 302
   and 307) MUST NOT be returned in a reply to a subsequent request
   unless there are cache-control directives or another header(s) that
   explicitly allow it. For example, these include the following: an
   Expires header (section 14.21); a "max-age", "s-maxage",  "must-
   revalidate", "proxy-revalidate", "public" or "private" cache-control
   directive (section 14.9).

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13.5 Constructing Responses From Caches

   The purpose of an HTTP cache is to store information received in
   response to requests for use in responding to future requests. In
   many cases, a cache simply returns the appropriate parts of a
   response to the requester. However, if the cache holds a cache entry
   based on a previous response, it might have to combine parts of a new
   response with what is held in the cache entry.

13.5.1 End-to-end and Hop-by-hop Headers

   For the purpose of defining the behavior of caches and non-caching
   proxies, we divide HTTP headers into two categories:

      - End-to-end headers, which are  transmitted to the ultimate
        recipient of a request or response. End-to-end headers in
        responses MUST be stored as part of a cache entry and MUST be
        transmitted in any response formed from a cache entry.

      - Hop-by-hop headers, which are meaningful only for a single
        transport-level connection, and are not stored by caches or
        forwarded by proxies.

   The following HTTP/1.1 headers are hop-by-hop headers:

      - Connection
      - Keep-Alive
      - Proxy-Authenticate
      - Proxy-Authorization
      - TE
      - Trailers
      - Transfer-Encoding
      - Upgrade

   All other headers defined by HTTP/1.1 are end-to-end headers.

   Other hop-by-hop headers MUST be listed in a Connection header,
   (section 14.10) to be introduced into HTTP/1.1 (or later).

13.5.2 Non-modifiable Headers

   Some features of the HTTP/1.1 protocol, such as Digest
   Authentication, depend on the value of certain end-to-end headers. A
   transparent proxy SHOULD NOT modify an end-to-end header unless the
   definition of that header requires or specifically allows that.

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   A transparent proxy MUST NOT modify any of the following fields in a
   request or response, and it MUST NOT add any of these fields if not
   already present:

      - Content-Location

      - Content-MD5

      - ETag

      - Last-Modified

   A transparent proxy MUST NOT modify any of the following fields in a
   response:

      - Expires

   but it MAY add any of these fields if not already present. If an
   Expires header is added, it MUST be given a field-value identical to
   that of the Date header in that response.

   A  proxy MUST NOT modify or add any of the following fields in a
   message that contains the no-transform cache-control directive, or in
   any request:

      - Content-Encoding

      - Content-Range

      - Content-Type

   A non-transparent proxy MAY modify or add these fields to a message
   that does not include no-transform, but if it does so, it MUST add a
   Warning 214 (Transformation applied) if one does not already appear
   in the message (see section 14.46).

      Warning: unnecessary modification of end-to-end headers might
      cause authentication failures if stronger authentication
      mechanisms are introduced in later versions of HTTP. Such
      authentication mechanisms MAY rely on the values of header fields
      not listed here.

   The Content-Length field of a request or response is added or deleted
   according to the rules in section 4.4. A transparent proxy MUST
   preserve the entity-length (section 7.2.2) of the entity-body,
   although it MAY change the transfer-length (section 4.4).

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13.5.3 Combining Headers

   When a cache makes a validating request to a server, and the server
   provides a 304 (Not Modified) response or a 206 (Partial Content)
   response, the cache then constructs a response to send to the
   requesting client.

   If the status code is 304 (Not Modified), the cache uses the entity-
   body stored in the cache entry as the entity-body of this outgoing
   response. If the status code is 206 (Partial Content) and the ETag or
   Last-Modified headers match exactly, the cache MAY combine the
   contents stored in the cache entry with the new contents received in
   the response and use the result as the entity-body of this outgoing
   response, (see 13.5.4).

   The end-to-end headers stored in the cache entry are used for the
   constructed response, except that

      - any stored Warning headers with warn-code 1xx (see section
        14.46) MUST be deleted from the cache entry and the forwarded
        response.

      - any stored Warning headers with warn-code 2xx MUST be retained
        in the cache entry and the forwarded response.

      - any end-to-end headers provided in the 304 or 206 response MUST
        replace the corresponding headers from the cache entry.

   Unless the cache decides to remove the cache entry, it MUST also
   replace the end-to-end headers stored with the cache entry with
   corresponding headers received in the incoming response, except for
   Warning headers as described immediately above. If a header field-
   name in the incoming response matches more than one header in the
   cache entry, all such old headers MUST be replaced.

   In other words, the set of end-to-end headers received in the
   incoming response overrides all corresponding end-to-end headers
   stored with the cache entry (except for stored Warning headers with
   warn-code 1xx, which are deleted even if not overridden).

      Note: this rule allows an origin server to use a 304 (Not
      Modified) or a 206 (Partial Content) response to update any header
      associated with a previous response for the same entity or sub-
      ranges thereof, although it might not always be meaningful or
      correct to do so. This rule does not allow an origin server to use
      a 304 (Not Modified) or a 206 (Partial Content) response to
      entirely delete a header that it had provided with a previous
      response.

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13.5.4 Combining Byte Ranges

   A response might transfer only a subrange of the bytes of an entity-
   body, either because the request included one or more Range
   specifications, or because a connection was broken prematurely. After
   several such transfers, a cache might have received several ranges of
   the same entity-body.

   If a cache has a stored non-empty set of subranges for an entity, and
   an incoming response transfers another subrange, the cache MAY
   combine the new subrange with the existing set if both the following
   conditions are met:

      - Both the incoming response and the cache entry have a cache
        validator.

      - The two cache validators match using the strong comparison
        function (see section 13.3.3).

   If either requirement is not met, the cache MUST use only the most
   recent partial response (based on the Date values transmitted with
   every response, and using the incoming response if these values are
   equal or missing), and MUST discard the other partial information.

13.6 Caching Negotiated Responses

   Use of server-driven content negotiation (section 12.1), as indicated
   by the presence of a Vary header field in a response, alters the
   conditions and procedure by which a cache can use the response for
   subsequent requests. See section 14.44 for use of the Vary header
   field by servers.

   A server SHOULD use the Vary header field to inform a cache of what
   request-header fields were used to select among multiple
   representations of a cacheable response subject to server-driven
   negotiation. The set of header fields named by the Vary field value
   is known as the "selecting" request-headers.

   When the cache receives a subsequent request whose Request-URI
   specifies one or more cache entries including a Vary header field,
   the cache MUST NOT use such a cache entry to construct a response to
   the new request unless all of the selecting request-headers present
   in the new request match the corresponding stored request-headers in
   the original request.

   The selecting request-headers from two requests are defined to match
   if and only if the selecting request-headers in the first request can
   be transformed to the selecting request-headers in the second request

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   by adding or removing linear white space (LWS) at places where this
   is allowed by the corresponding BNF, and/or combining multiple
   message-header fields with the same field name following the rules
   about message headers in section 4.2.

   A Vary header field-value of "*" always fails to match and subsequent
   requests on that resource can only be properly interpreted by the
   origin server.

   If the selecting request header fields for the cached entry do not
   match the selecting request header fields of the new request, then
   the cache MUST NOT use a cached entry to satisfy the request unless
   it first relays the new request to the origin server in a conditional
   request and the server responds with 304 (Not Modified), including an
   entity tag or Content-Location that indicates the entity to be used.

   If an entity tag was assigned to a cached representation, the
   forwarded request SHOULD be conditional and include the entity tags
   in an If-None-Match header field from all its cache entries for the
   resource. This conveys to the server the set of entities currently
   held by the cache, so that if any one of these entities matches the
   requested entity, the server can use the ETag header field in its 304
   (Not Modified) response to tell the cache which entry is appropriate.
   If the entity-tag of the new response matches that of an existing
   entry, the new response SHOULD be used to update the header fields of
   the existing entry, and the result MUST be returned to the client.

   If any of the existing cache entries contains only partial content
   for the associated entity, its entity-tag SHOULD NOT be included in
   the If-None-Match header field unless the request is for a range that
   would be fully satisfied by that entry.

   If a cache receives a successful response whose Content-Location
   field matches that of an existing cache entry for the same Request-
   ]URI, whose entity-tag differs from that of the existing entry, and
   whose Date is more recent than that of the existing entry, the
   existing entry SHOULD NOT be returned in response to future requests
   and SHOULD be deleted from the cache.

13.7 Shared and Non-Shared Caches

   For reasons of security and privacy, it is necessary to make a
   distinction between "shared" and "non-shared" caches. A non-shared
   cache is one that is accessible only to a single user. Accessibility
   in this case SHOULD be enforced by appropriate security mechanisms.
   All other caches are considered to be "shared." Other sections of

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   this specification place certain constraints on the operation of
   shared caches in order to prevent loss of privacy or failure of
   access controls.

13.8 Errors or Incomplete Response Cache Behavior

   A cache that receives an incomplete response (for example, with fewer
   bytes of data than specified in a Content-Length header) MAY store
   the response. However, the cache MUST treat this as a partial
   response. Partial responses MAY be combined as described in section
   13.5.4; the result might be a full response or might still be
   partial. A cache MUST NOT return a partial response to a client
   without explicitly marking it as such, using the 206 (Partial
   Content) status code. A cache MUST NOT return a partial response
   using a status code of 200 (OK).

   If a cache receives a 5xx response while attempting to revalidate an
   entry, it MAY either forward this response to the requesting client,
   or act as if the server failed to respond. In the latter case, it MAY
   return a previously received response unless the cached entry
   includes the "must-revalidate" cache-control directive (see section
   14.9).

13.9 Side Effects of GET and HEAD

   Unless the origin server explicitly prohibits the caching of their
   responses, the application of GET and HEAD methods to any resources
   SHOULD NOT have side effects that would lead to erroneous behavior if
   these responses are taken from a cache. They MAY still have side
   effects, but a cache is not required to consider such side effects in
   its caching decisions. Caches are always expected to observe an
   origin server's explicit restrictions on caching.

   We note one exception to this rule: since some applications have
   traditionally used GETs and HEADs with query URLs (those containing a
   "?" in the rel_path part) to perform operations with significant side
   effects, caches MUST NOT treat responses to such URIs as fresh unless
   the server provides an explicit expiration time. This specifically
   means that responses from HTTP/1.0 servers for such URIs SHOULD NOT
   be taken from a cache. See section 9.1.1 for related information.

13.10 Invalidation After Updates or Deletions

   The effect of certain methods performed on a resource at the origin
   server might cause one or more existing cache entries to become non-
   transparently invalid. That is, although they might continue to be
   "fresh," they do not accurately reflect what the origin server would
   return for a new request on that resource.

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   There is no way for the HTTP protocol to guarantee that all such
   cache entries are marked invalid. For example, the request that
   caused the change at the origin server might not have gone through
   the proxy where a cache entry is stored. However, several rules help
   reduce the likelihood of erroneous behavior.

   In this section, the phrase "invalidate an entity" means that the
   cache will either remove all instances of that entity from its
   storage, or will mark these as "invalid" and in need of a mandatory
   revalidation before they can be returned in response to a subsequent
   request.

   Some HTTP methods MUST cause a cache to invalidate an entity. This is
   either the entity referred to by the Request-URI, or by the Location
   or Content-Location headers (if present). These methods are:

      - PUT

      - DELETE

      - POST

   In order to prevent denial of service attacks, an invalidation based
   on the URI in a Location or Content-Location header MUST only be
   performed if the host part is the same as in the Request-URI.

   A cache that passes through requests for methods it does not
   understand SHOULD invalidate any entities referred to by the
   Request-URI.

13.11 Write-Through Mandatory

   All methods that might be expected to cause modifications to the
   origin server's resources MUST be written through to the origin
   server. This currently includes all methods except for GET and HEAD.
   A cache MUST NOT reply to such a request from a client before having
   transmitted the request to the inbound server, and having received a
   corresponding response from the inbound server. This does not prevent
   a proxy cache from sending a 100 (Continue) response before the
   inbound server has sent its final reply.

   The alternative (known as "write-back" or "copy-back" caching) is not
   allowed in HTTP/1.1, due to the difficulty of providing consistent
   updates and the problems arising from server, cache, or network
   failure prior to write-back.

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13.12 Cache Replacement

   If a new cacheable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8)
   response is received from a resource while any existing responses for
   the same resource are cached, the cache SHOULD use the new response
   to reply to the current request. It MAY insert it into cache storage
   and MAY, if it meets all other requirements, use it to respond to any
   future requests that would previously have caused the old response to
   be returned. If it inserts the new response into cache storage  the
   rules in section 13.5.3 apply.

      Note: a new response that has an older Date header value than
      existing cached responses is not cacheable.

13.13 History Lists

   User agents often have history mechanisms, such as "Back" buttons and
   history lists, which can be used to redisplay an entity retrieved
   earlier in a session.

   History mechanisms and caches are different. In particular history
   mechanisms SHOULD NOT try to show a semantically transparent view of
   the current state of a resource. Rather, a history mechanism is meant
   to show exactly what the user saw at the time when the resource was
   retrieved.

   By default, an expiration time does not apply to history mechanisms.
   If the entity is still in storage, a history mechanism SHOULD display
   it even if the entity has expired, unless the user has specifically
   configured the agent to refresh expired history documents.

   This is not to be construed to prohibit the history mechanism from
   telling the user that a view might be stale.

      Note: if history list mechanisms unnecessarily prevent users from
      viewing stale resources, this will tend to force service authors
      to avoid using HTTP expiration controls and cache controls when
      they would otherwise like to. Service authors may consider it
      important that users not be presented with error messages or
      warning messages when they use navigation controls (such as BACK)
      to view previously fetched resources. Even though sometimes such
      resources ought not to cached, or ought to expire quickly, user
      interface considerations may force service authors to resort to
      other means of preventing caching (e.g. "once-only" URLs) in order
      not to suffer the effects of improperly functioning history
      mechanisms.

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14 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.

14.1 Accept

   The Accept request-header field can be used to specify certain media
   types which are acceptable for the response. Accept headers can be
   used to indicate that the request is specifically limited to a small
   set of desired types, as in the case of a request for an in-line
   image.

       Accept         = "Accept" ":"
                        #( media-range [ accept-params ] )

       media-range    = ( "*/*"
                        | ( type "/" "*" )
                        | ( type "/" subtype )
                        ) *( ";" parameter )
       accept-params  = ";" "q" "=" qvalue *( accept-extension )
       accept-extension = ";" token [ "=" ( token | quoted-string ) ]

   The asterisk "*" character is used to group media types into ranges,
   with "*/*" indicating all media types and "type/*" indicating all
   subtypes of that type. The media-range MAY include media type
   parameters that are applicable to that range.

   Each media-range MAY be followed by one or more accept-params,
   beginning with the "q" parameter for indicating a relative quality
   factor. The first "q" parameter (if any) separates the media-range
   parameter(s) from the accept-params. Quality factors allow the user
   or user agent to indicate the relative degree of preference for that
   media-range, using the qvalue scale from 0 to 1 (section 3.9). The
   default value is q=1.

      Note: Use of the "q" parameter name to separate media type
      parameters from Accept extension parameters is due to historical
      practice. Although this prevents any media type parameter named
      "q" from being used with a media range, such an event is believed
      to be unlikely given the lack of any "q" parameters in the IANA
      media type registry and the rare usage of any media type
      parameters in Accept. Future media types are discouraged from
      registering any parameter named "q".

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   The example

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

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

   If no Accept header field is present, then it is assumed that the
   client accepts all media types. If an Accept header field is present,
   and if the server cannot send a response which is acceptable
   according to the combined Accept field value, then the server SHOULD
   send a 406 (not acceptable) response.

   A more elaborate example is

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

   Verbally, this would be interpreted as "text/html and text/x-c are
   the preferred media types, but if they do not exist, then send the
   text/x-dvi entity, and if that does not exist, send the text/plain
   entity."

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

       Accept: text/*, text/html, text/html;level=1, */*

   have the following precedence:

       1) text/html;level=1
       2) text/html
       3) text/*
       4) */*

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

       Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
               text/html;level=2;q=0.4, */*;q=0.5

   would cause the following values to be associated:

       text/html;level=1         = 1
       text/html                 = 0.7
       text/plain                = 0.3

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       image/jpeg                = 0.5
       text/html;level=2         = 0.4
       text/html;level=3         = 0.7

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

14.2 Accept-Charset

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

      Accept-Charset = "Accept-Charset" ":"
              1#( ( charset | "*" )[ ";" "q" "=" qvalue ] )

   Character set values are described in section 3.4. Each charset MAY
   be given an associated quality value which represents the user's
   preference for that charset. The default value is q=1. An example is

      Accept-Charset: iso-8859-5, unicode-1-1;q=0.8

   The special value "*", if present in the Accept-Charset field,
   matches every character set (including ISO-8859-1) which is not
   mentioned elsewhere in the Accept-Charset field. If no "*" is present
   in an Accept-Charset field, then all character sets not explicitly
   mentioned get a quality value of 0, except for ISO-8859-1, which gets
   a quality value of 1 if not explicitly mentioned.

   If no Accept-Charset header is present, the default is that any
   character set is acceptable. If an Accept-Charset header is present,
   and if the server cannot send a response which is acceptable
   according to the Accept-Charset header, then the server SHOULD send
   an error response with the 406 (not acceptable) status code, though
   the sending of an unacceptable response is also allowed.

14.3 Accept-Encoding

   The Accept-Encoding request-header field is similar to Accept, but
   restricts the content-codings (section 3.5) that are acceptable in
   the response.

       Accept-Encoding  = "Accept-Encoding" ":"

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                          1#( codings [ ";" "q" "=" qvalue ] )
       codings          = ( content-coding | "*" )

   Examples of its use are:

       Accept-Encoding: compress, gzip
       Accept-Encoding:
       Accept-Encoding: *
       Accept-Encoding: compress;q=0.5, gzip;q=1.0
       Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0

   A server tests whether a content-coding is acceptable, according to
   an Accept-Encoding field, using these rules:

      1. If the content-coding is one of the content-codings listed in
         the Accept-Encoding field, then it is acceptable, unless it is
         accompanied by a qvalue of 0. (As defined in section 3.9, a
         qvalue of 0 means "not acceptable.")

      2. The special "*" symbol in an Accept-Encoding field matches any
         available content-coding not explicitly listed in the header
         field.

      3. If multiple content-codings are acceptable, then the acceptable
         content-coding with the highest non-zero qvalue is preferred.

      4. The "identity" content-coding is always acceptable, unless
         specifically refused because the Accept-Encoding field includes
         "identity;q=0", or because the field includes "*;q=0" and does
         not explicitly include the "identity" content-coding. If the
         Accept-Encoding field-value is empty, then only the "identity"
         encoding is acceptable.

   If an Accept-Encoding field is present in a request, and if the
   server cannot send a response which is acceptable according to the
   Accept-Encoding header, then the server SHOULD send an error response
   with the 406 (Not Acceptable) status code.

   If no Accept-Encoding field is present in a request, the server MAY
   assume that the client will accept any content coding. In this case,
   if "identity" is one of the available content-codings, then the
   server SHOULD use the "identity" content-coding, unless it has
   additional information that a different content-coding is meaningful
   to the client.

      Note: If the request does not include an Accept-Encoding field,
      and if the "identity" content-coding is unavailable, then
      content-codings commonly understood by HTTP/1.0 clients (i.e.,

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      "gzip" and "compress") are preferred; some older clients
      improperly display messages sent with other content-codings.  The
      server might also make this decision based on information about
      the particular user-agent or client.

      Note: Most HTTP/1.0 applications do not recognize or obey qvalues
      associated with content-codings. This means that qvalues will not
      work and are not permitted with x-gzip or x-compress.

14.4 Accept-Language

   The Accept-Language request-header field is similar to Accept, but
   restricts the set of natural languages that are preferred as a
   response to the request. Language tags are defined in section 3.10.

       Accept-Language = "Accept-Language" ":"
                         1#( language-range [ ";" "q" "=" qvalue ] )
       language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )

   Each language-range MAY be given an associated quality value which
   represents an estimate of the user's preference for the languages
   specified by that range. The quality value defaults to "q=1". For
   example,

       Accept-Language: da, en-gb;q=0.8, en;q=0.7

   would mean: "I prefer Danish, but will accept British English and
   other types of English." A language-range matches a language-tag if
   it exactly equals the tag, or if it exactly equals a prefix of the
   tag such that the first tag character following the prefix is "-".
   The special range "*", if present in the Accept-Language field,
   matches every tag not matched by any other range present in the
   Accept-Language field.

      Note: This use of a prefix matching rule does not imply that
      language tags are assigned to languages in such a way that it is
      always true that if a user understands a language with a certain
      tag, then this user will also understand all languages with tags
      for which this tag is a prefix. The prefix rule simply allows the
      use of prefix tags if this is the case.

   The language quality factor assigned to a language-tag by the
   Accept-Language field is the quality value of the longest language-
   range in the field that matches the language-tag. If no language-
   range in the field matches the tag, the language quality factor
   assigned is 0. If no Accept-Language header is present in the
   request, the server

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   SHOULD assume that all languages are equally acceptable. If an
   Accept-Language header is present, then all languages which are
   assigned a quality factor greater than 0 are acceptable.

   It might be contrary to the privacy expectations of the user to send
   an Accept-Language header with the complete linguistic preferences of
   the user in every request. For a discussion of this issue, see
   section 15.1.4.

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

      Note: When making the choice of linguistic preference available to
      the user, we remind implementors of  the fact that users are not
      familiar with the details of language matching as described above,
      and should provide appropriate guidance. As an example, users
      might assume that on selecting "en-gb", they will be served any
      kind of English document if British English is not available. A
      user agent might suggest in such a case to add "en" to get the
      best matching behavior.

14.5 Accept-Ranges

      The Accept-Ranges response-header field allows the server to
      indicate its acceptance of range requests for a resource:

          Accept-Ranges     = "Accept-Ranges" ":" acceptable-ranges
          acceptable-ranges = 1#range-unit | "none"

      Origin servers that accept byte-range requests MAY send

          Accept-Ranges: bytes

      but are not required to do so. Clients MAY generate byte-range
      requests without having received this header for the resource
      involved. Range units are defined in section 3.12.

      Servers that do not accept any kind of range request for a
      resource MAY send

          Accept-Ranges: none

      to advise the client not to attempt a range request.

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14.6 Age

      The Age response-header field conveys the sender's estimate of the
      amount of time since the response (or its revalidation) was
      generated at the origin server. A cached response is "fresh" if
      its age does not exceed its freshness lifetime. Age values are
      calculated as specified in section 13.2.3.

           Age = "Age" ":" age-value
           age-value = delta-seconds

      Age values are non-negative decimal integers, representing time in
      seconds.

      If a cache receives a value larger than the largest positive
      integer it can represent, or if any of its age calculations
      overflows, it MUST transmit an Age header with a value of
      2147483648 (2^31). An HTTP/1.1 server that includes a cache MUST
      include an Age header field in every response generated from its
      own cache. Caches SHOULD use an arithmetic type of at least 31
      bits of range.

14.7 Allow

      The Allow entity-header field lists the set of methods supported
      by the resource identified by the Request-URI. The purpose of this
      field is strictly to inform the recipient of valid methods
      associated with the resource. An Allow header field MUST be
      present in a 405 (Method Not Allowed) response.

          Allow   = "Allow" ":" #Method

      Example of use:

          Allow: GET, HEAD, PUT

      This field cannot prevent a client from trying other methods.
      However, the indications given by the Allow header field value
      SHOULD be followed. The actual set of allowed methods is defined
      by the origin server at the time of each request.

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

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      A proxy MUST NOT modify the Allow header field even if it does not
      understand all the methods specified, since the user agent might
      have other means of communicating with the origin server.

14.8 Authorization

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

          Authorization  = "Authorization" ":" credentials

      HTTP access authentication is described in "HTTP Authentication:
      Basic and Digest Access Authentication" [43]. If a request is
      authenticated and a realm specified, the same credentials SHOULD
      be valid for all other requests within this realm (assuming that
      the authentication scheme itself does not require otherwise, such
      as credentials that vary according to a challenge value or using
      synchronized clocks).

      When a shared cache (see section 13.7) receives a request
      containing an Authorization field, it MUST NOT return the
      corresponding response as a reply to any other request, unless one
      of the following specific exceptions holds:

      1. If the response includes the "s-maxage" cache-control
         directive, the cache MAY use that response in replying to a
         subsequent request. But (if the specified maximum age has
         passed) a proxy cache MUST first revalidate it with the origin
         server, using the request-headers from the new request to allow
         the origin server to authenticate the new request. (This is the
         defined behavior for s-maxage.) If the response includes "s-
         maxage=0", the proxy MUST always revalidate it before re-using
         it.

      2. If the response includes the "must-revalidate" cache-control
         directive, the cache MAY use that response in replying to a
         subsequent request. But if the response is stale, all caches
         MUST first revalidate it with the origin server, using the
         request-headers from the new request to allow the origin server
         to authenticate the new request.

      3. If the response includes the "public" cache-control directive,
         it MAY be returned in reply to any subsequent request.

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14.9 Cache-Control

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

      Note that HTTP/1.0 caches might not implement Cache-Control and
      might only implement Pragma: no-cache (see section 14.32).

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

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

    cache-directive = cache-request-directive
         | cache-response-directive

    cache-request-directive =
           "no-cache"                          ; Section 14.9.1
         | "no-store"                          ; Section 14.9.2
         | "max-age" "=" delta-seconds         ; Section 14.9.3, 14.9.4
         | "max-stale" [ "=" delta-seconds ]   ; Section 14.9.3
         | "min-fresh" "=" delta-seconds       ; Section 14.9.3
         | "no-transform"                      ; Section 14.9.5
         | "only-if-cached"                    ; Section 14.9.4
         | cache-extension                     ; Section 14.9.6

     cache-response-directive =
           "public"                               ; Section 14.9.1
         | "private" [ "=" <"> 1#field-name <"> ] ; Section 14.9.1
         | "no-cache" [ "=" <"> 1#field-name <"> ]; Section 14.9.1
         | "no-store"                             ; Section 14.9.2
         | "no-transform"                         ; Section 14.9.5
         | "must-revalidate"                      ; Section 14.9.4
         | "proxy-revalidate"                     ; Section 14.9.4
         | "max-age" "=" delta-seconds            ; Section 14.9.3
         | "s-maxage" "=" delta-seconds           ; Section 14.9.3
         | cache-extension                        ; Section 14.9.6

    cache-extension = token [ "=" ( token | quoted-string ) ]

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   When a directive appears without any 1#field-name parameter, the
   directive applies to the entire request or response. When such a
   directive appears with a 1#field-name parameter, it applies only to
   the named field or fields, and not to the rest of the request or
   response. This mechanism supports extensibility; implementations of
   future versions of the HTTP protocol might apply these directives to
   header fields not defined in HTTP/1.1.

   The cache-control directives can be broken down into these general
   categories:

      - Restrictions on what are cacheable; these may only be imposed by
        the origin server.

      - Restrictions on what may be stored by a cache; these may be
        imposed by either the origin server or the user agent.

      - Modifications of the basic expiration mechanism; these may be
        imposed by either the origin server or the user agent.

      - Controls over cache revalidation and reload; these may only be
        imposed by a user agent.

      - Control over transformation of entities.

      - Extensions to the caching system.

14.9.1 What is Cacheable

   By default, a response is cacheable if the requirements of the
   request method, request header fields, and the response status
   indicate that it is cacheable. Section 13.4 summarizes these defaults
   for cacheability. The following Cache-Control response directives
   allow an origin server to override the default cacheability of a
   response:

   public
      Indicates that the response MAY be cached by any cache, even if it
      would normally be non-cacheable or cacheable only within a non-
      shared cache. (See also Authorization, section 14.8, for
      additional details.)

   private
      Indicates that all or part of the response message is intended for
      a single user and MUST NOT be cached by a shared cache. This
      allows an origin server to state that the specified parts of the

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      response are intended for only one user and are not a valid
      response for requests by other users. A private (non-shared) cache
      MAY cache the response.

       Note: This usage of the word private only controls where the
       response may be cached, and cannot ensure the privacy of the
       message content.

   no-cache
       If the no-cache directive does not specify a field-name, then a
      cache MUST NOT use the response to satisfy a subsequent request
      without successful revalidation with the origin server. This
      allows an origin server to prevent caching even by caches that
      have been configured to return stale responses to client requests.

      If the no-cache directive does specify one or more field-names,
      then a cache MAY use the response to satisfy a subsequent request,
      subject to any other restrictions on caching. However, the
      specified field-name(s) MUST NOT be sent in the response to a
      subsequent request without successful revalidation with the origin
      server. This allows an origin server to prevent the re-use of
      certain header fields in a response, while still allowing caching
      of the rest of the response.

       Note: Most HTTP/1.0 caches will not recognize or obey this
       directive.

14.9.2 What May be Stored by Caches

   no-store
      The purpose of the no-store directive is to prevent the
      inadvertent release or retention of sensitive information (for
      example, on backup tapes). The no-store directive applies to the
      entire message, and MAY be sent either in a response or in a
      request. If sent in a request, a cache MUST NOT store any part of
      either this request or any response to it. If sent in a response,
      a cache MUST NOT store any part of either this response or the
      request that elicited it. This directive applies to both non-
      shared and shared caches. "MUST NOT store" in this context means
      that the cache MUST NOT intentionally store the information in
      non-volatile storage, and MUST make a best-effort attempt to
      remove the information from volatile storage as promptly as
      possible after forwarding it.

      Even when this directive is associated with a response, users
      might explicitly store such a response outside of the caching
      system (e.g., with a "Save As" dialog). History buffers MAY store
      such responses as part of their normal operation.

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      The purpose of this directive is to meet the stated requirements
      of certain users and service authors who are concerned about
      accidental releases of information via unanticipated accesses to
      cache data structures. While the use of this directive might
      improve privacy in some cases, we caution that it is NOT in any
      way a reliable or sufficient mechanism for ensuring privacy. In
      particular, malicious or compromised caches might not recognize or
      obey this directive, and communications networks might be
      vulnerable to eavesdropping.

14.9.3 Modifications of the Basic Expiration Mechanism

   The expiration time of an entity MAY be specified by the origin
   server using the Expires header (see section 14.21). Alternatively,
   it MAY be specified using the max-age directive in a response. When
   the max-age cache-control directive is present in a cached response,
   the response is stale if its current age is greater than the age
   value given (in seconds) at the time of a new request for that
   resource. The max-age directive on a response implies that the
   response is cacheable (i.e., "public") unless some other, more
   restrictive cache directive is also present.

   If a response includes both an Expires header and a max-age
   directive, the max-age directive overrides the Expires header, even
   if the Expires header is more restrictive. This rule allows an origin
   server to provide, for a given response, a longer expiration time to
   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
   useful if certain HTTP/1.0 caches improperly calculate ages or
   expiration times, perhaps due to desynchronized clocks.

   Many HTTP/1.0 cache implementations will treat an Expires value that
   is less than or equal to the response Date value as being equivalent
   to the Cache-Control response directive "no-cache". If an HTTP/1.1
   cache receives such a response, and the response does not include a
   Cache-Control header field, it SHOULD consider the response to be
   non-cacheable in order to retain compatibility with HTTP/1.0 servers.

       Note: An origin server might wish to use a relatively new HTTP
       cache control feature, such as the "private" directive, on a
       network including older caches that do not understand that
       feature. The origin server will need to combine the new feature
       with an Expires field whose value is less than or equal to the
       Date value. This will prevent older caches from improperly
       caching the response.

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   s-maxage
       If a response includes an s-maxage directive, then for a shared
       cache (but not for a private cache), the maximum age specified by
       this directive overrides the maximum age specified by either the
       max-age directive or the Expires header. The s-maxage directive
       also implies the semantics of the proxy-revalidate directive (see
       section 14.9.4), i.e., that the shared cache must not use the
       entry after it becomes stale to respond to a subsequent request
       without first revalidating it with the origin server. The s-
       maxage directive is always ignored by a private cache.

   Note that most older caches, not compliant with this specification,
   do not implement any cache-control directives. An origin server
   wishing to use a cache-control directive that restricts, but does not
   prevent, caching by an HTTP/1.1-compliant cache MAY exploit the
   requirement that the max-age directive overrides the Expires header,
   and the fact that pre-HTTP/1.1-compliant caches do not observe the
   max-age directive.

   Other directives allow a user agent to modify the basic expiration
   mechanism. These directives MAY be specified on a request:

   max-age
      Indicates that the client is willing to accept a response whose
      age is no greater than the specified time in seconds. Unless max-
      stale directive is also included, the client is not willing to
      accept a stale response.

   min-fresh
      Indicates that the client is willing to accept a response whose
      freshness lifetime is no less than its current age plus the
      specified time in seconds. That is, the client wants a response
      that will still be fresh for at least the specified number of
      seconds.

   max-stale
      Indicates that the client is willing to accept a response that has
      exceeded its expiration time. If max-stale is assigned a value,
      then the client is willing to accept a response that has exceeded
      its expiration time by no more than the specified number of
      seconds. If no value is assigned to max-stale, then the client is
      willing to accept a stale response of any age.

   If a cache returns a stale response, either because of a max-stale
   directive on a request, or because the cache is configured to
   override the expiration time of a response, the cache MUST attach a
   Warning header to the stale response, using Warning 110 (Response is
   stale).

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   A cache MAY be configured to return stale responses without
   validation, but only if this does not conflict with any "MUST"-level
   requirements concerning cache validation (e.g., a "must-revalidate"
   cache-control directive).

   If both the new request and the cached entry include "max-age"
   directives, then the lesser of the two values is used for determining
   the freshness of the cached entry for that request.

14.9.4 Cache Revalidation and Reload Controls

   Sometimes a user agent might want or need to insist that a cache
   revalidate its cache entry with the origin server (and not just with
   the next cache along the path to the origin server), or to reload its
   cache entry from the origin server. End-to-end revalidation might be
   necessary if either the cache or the origin server has overestimated
   the expiration time of the cached response. End-to-end reload may be
   necessary if the cache entry has become corrupted for some reason.

   End-to-end revalidation may be requested either when the client does
   not have its own local cached copy, in which case we call it
   "unspecified end-to-end revalidation", or when the client does have a
   local cached copy, in which case we call it "specific end-to-end
   revalidation."

   The client can specify these three kinds of action using Cache-
   Control request directives:

   End-to-end reload
      The request includes a "no-cache" cache-control directive or, for
      compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
      names MUST NOT be included with the no-cache directive in a
      request. The server MUST NOT use a cached copy when responding to
      such a request.

   Specific end-to-end revalidation
      The request includes a "max-age=0" cache-control directive, which
      forces each cache along the path to the origin server to
      revalidate its own entry, if any, with the next cache or server.
      The initial request includes a cache-validating conditional with
      the client's current validator.

   Unspecified end-to-end revalidation
      The request includes "max-age=0" cache-control directive, which
      forces each cache along the path to the origin server to
      revalidate its own entry, if any, with the next cache or server.
      The initial request does not include a cache-validating

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      conditional; the first cache along the path (if any) that holds a
      cache entry for this resource includes a cache-validating
      conditional with its current validator.

   max-age
      When an intermediate cache is forced, by means of a max-age=0
      directive, to revalidate its own cache entry, and the client has
      supplied its own validator in the request, the supplied validator
      might differ from the validator currently stored with the cache
      entry. In this case, the cache MAY use either validator in making
      its own request without affecting semantic transparency.

      However, the choice of validator might affect performance. The
      best approach is for the intermediate cache to use its own
      validator when making its request. If the server replies with 304
      (Not Modified), then the cache can return its now validated copy
      to the client with a 200 (OK) response. If the server replies with
      a new entity and cache validator, however, the intermediate cache
      can compare the returned validator with the one provided in the
      client's request, using the strong comparison function. If the
      client's validator is equal to the origin server's, then the
      intermediate cache simply returns 304 (Not Modified). Otherwise,
      it returns the new entity with a 200 (OK) response.

      If a request includes the no-cache directive, it SHOULD NOT
      include min-fresh, max-stale, or max-age.

   only-if-cached
      In some cases, such as times of extremely poor network
      connectivity, a client may want a cache to return only those
      responses that it currently has stored, and not to reload or
      revalidate with the origin server. To do this, the client may
      include the only-if-cached directive in a request. If it receives
      this directive, a cache SHOULD either respond using a cached entry
      that is consistent with the other constraints of the request, or
      respond with a 504 (Gateway Timeout) status. However, if a group
      of caches is being operated as a unified system with good internal
      connectivity, such a request MAY be forwarded within that group of
      caches.

   must-revalidate
      Because a cache MAY be configured to ignore a server's specified
      expiration time, and because a client request MAY include a max-
      stale directive (which has a similar effect), the protocol also
      includes a mechanism for the origin server to require revalidation
      of a cache entry on any subsequent use. When the must-revalidate
      directive is present in a response received by a cache, that cache
      MUST NOT use the entry after it becomes stale to respond to a

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      subsequent request without first revalidating it with the origin
      server. (I.e., the cache MUST do an end-to-end revalidation every
      time, if, based solely on the origin server's Expires or max-age
      value, the cached response is stale.)

      The must-revalidate directive is necessary to support reliable
      operation for certain protocol features. In all circumstances an
      HTTP/1.1 cache MUST obey the must-revalidate directive; in
      particular, if the cache cannot reach the origin server for any
      reason, it MUST generate a 504 (Gateway Timeout) response.

      Servers SHOULD send the must-revalidate directive if and only if
      failure to revalidate a request on the entity could result in
      incorrect operation, such as a silently unexecuted financial
      transaction. Recipients MUST NOT take any automated action that
      violates this directive, and MUST NOT automatically provide an
      unvalidated copy of the entity if revalidation fails.

      Although this is not recommended, user agents operating under
      severe connectivity constraints MAY violate this directive but, if
      so, MUST explicitly warn the user that an unvalidated response has
      been provided. The warning MUST be provided on each unvalidated
      access, and SHOULD require explicit user confirmation.

   proxy-revalidate
      The proxy-revalidate directive has the same meaning as the must-
      revalidate directive, except that it does not apply to non-shared
      user agent caches. It can be used on a response to an
      authenticated request to permit the user's cache to store and
      later return the response without needing to revalidate it (since
      it has already been authenticated once by that user), while still
      requiring proxies that service many users to revalidate each time
      (in order to make sure that each user has been authenticated).
      Note that such authenticated responses also need the public cache
      control directive in order to allow them to be cached at all.

14.9.5 No-Transform Directive

   no-transform
      Implementors of intermediate caches (proxies) have found it useful
      to convert the media type of certain entity bodies. A non-
      transparent proxy might, for example, convert between image
      formats in order to save cache space or to reduce the amount of
      traffic on a slow link.

      Serious operational problems occur, however, when these
      transformations are applied to entity bodies intended for certain
      kinds of applications. For example, applications for medical

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      imaging, scientific data analysis and those using end-to-end
      authentication, all depend on receiving an entity body that is bit
      for bit identical to the original entity-body.

      Therefore, if a message includes the no-transform directive, an
      intermediate cache or proxy MUST NOT change those headers that are
      listed in section 13.5.2 as being subject to the no-transform
      directive. This implies that the cache or proxy MUST NOT change
      any aspect of the entity-body that is specified by these headers,
      including the value of the entity-body itself.

14.9.6 Cache Control Extensions

   The Cache-Control header field can be extended through the use of one
   or more cache-extension tokens, each with an optional assigned value.
   Informational extensions (those which do not require a change in
   cache behavior) MAY be added without changing the semantics of other
   directives. Behavioral extensions are designed to work by acting as
   modifiers to the existing base of cache directives. Both the new
   directive and the standard directive are supplied, such that
   applications which do not understand the new directive will default
   to the behavior specified by the standard directive, and those that
   understand the new directive will recognize it as modifying the
   requirements associated with the standard directive. In this way,
   extensions to the cache-control directives can be made without
   requiring changes to the base protocol.

   This extension mechanism depends on an HTTP cache obeying all of the
   cache-control directives defined for its native HTTP-version, obeying
   certain extensions, and ignoring all directives that it does not
   understand.

   For example, consider a hypothetical new response directive called
   community which acts as a modifier to the private directive. We
   define this new directive to mean that, in addition to any non-shared
   cache, any cache which is shared only by members of the community
   named within its value may cache the response. An origin server
   wishing to allow the UCI community to use an otherwise private
   response in their shared cache(s) could do so by including

       Cache-Control: private, community="UCI"

   A cache seeing this header field will act correctly even if the cache
   does not understand the community cache-extension, since it will also
   see and understand the private directive and thus default to the safe
   behavior.

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   Unrecognized cache-directives MUST be ignored; it is assumed that any
   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
   be combined with standard directives (or the response's default
   cacheability) such that the cache behavior will remain minimally
   correct even if the cache does not understand the extension(s).

14.10 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
   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 8.1)
   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 section
   19.6.2.

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14.11 Content-Encoding

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

       Content-Encoding  = "Content-Encoding" ":" 1#content-coding

   Content codings are defined in section 3.5. An example of its use is

       Content-Encoding: gzip

   The content-coding is a characteristic of the entity identified by
   the Request-URI. Typically, the entity-body is stored with this
   encoding and is only decoded before rendering or analogous usage.
   However, a non-transparent proxy MAY modify the content-coding if the
   new coding is known to be acceptable to the recipient, unless the
   "no-transform" cache-control directive is present in the message.

   If the content-coding of an entity is not "identity", then the
   response MUST include a Content-Encoding entity-header (section
   14.11) that lists the non-identity content-coding(s) used.

   If the content-coding of an entity in a request message is not
   acceptable to the origin server, the server SHOULD respond with a
   status code of 415 (Unsupported Media Type).

   If multiple encodings have been applied to an entity, the content
   codings MUST be listed in the order in which they were applied.
   Additional information about the encoding parameters MAY be provided
   by other entity-header fields not defined by this specification.

14.12 Content-Language

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

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

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   Language tags are defined in section 3.10. The primary purpose of
   Content-Language is to allow a user to identify and differentiate
   entities according to the user's own preferred language. Thus, if the
   body content is intended only for a Danish-literate audience, the
   appropriate field is

       Content-Language: da

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

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

       Content-Language: mi, en

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

   Content-Language MAY be applied to any media type -- it is not
   limited to textual documents.

14.13 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.

14.14 Content-Location

   The Content-Location entity-header field MAY be used to supply the
   resource location for the entity enclosed in the message when that
   entity is accessible from a location separate from the requested
   resource's URI. A server SHOULD provide a Content-Location for the
   variant corresponding to the response entity; especially in the case
   where a resource has multiple entities associated with it, and those
   entities actually have separate locations by which they might be
   individually accessed, the server SHOULD provide a Content-Location
   for the particular variant which is returned.

       Content-Location = "Content-Location" ":"
                         ( absoluteURI | relativeURI )

   The value of Content-Location also defines the base URI for the
   entity.

   The Content-Location value is not a replacement for the original
   requested URI; it is only a statement of the location of the resource
   corresponding to this particular entity at the time of the request.
   Future requests MAY specify the Content-Location URI as the request-
   URI if the desire is to identify the source of that particular
   entity.

   A cache cannot assume that an entity with a Content-Location
   different from the URI used to retrieve it can be used to respond to
   later requests on that Content-Location URI. However, the Content-
   Location can be used to differentiate between multiple entities
   retrieved from a single requested resource, as described in section
   13.6.

   If the Content-Location is a relative URI, the relative URI is
   interpreted relative to the Request-URI.

   The meaning of the Content-Location header in PUT or POST requests is
   undefined; servers are free to ignore it in those cases.

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14.15 Content-MD5

   The Content-MD5 entity-header field, as defined in RFC 1864 [23], is
   an MD5 digest of the entity-body for the purpose of providing an
   end-to-end message integrity check (MIC) of the entity-body. (Note: a
   MIC is good for detecting accidental modification of the entity-body
   in transit, but is not proof against malicious attacks.)

        Content-MD5   = "Content-MD5" ":" md5-digest
        md5-digest   = <base64 of 128 bit MD5 digest as per RFC 1864>

   The Content-MD5 header field MAY be generated by an origin server or
   client to function as an integrity check of the entity-body. Only
   origin servers or clients MAY generate the Content-MD5 header field;
   proxies and gateways MUST NOT generate it, as this would defeat its
   value as an end-to-end integrity check. Any recipient of the entity-
   body, including gateways and proxies, MAY check that the digest value
   in this header field matches that of the entity-body as received.

   The MD5 digest is computed based on the content of the entity-body,
   including any content-coding that has been applied, but not including
   any transfer-encoding applied to the message-body. If the message is
   received with a transfer-encoding, that encoding MUST be removed
   prior to checking the Content-MD5 value against the received entity.

   This has the result that the digest is computed on the octets of the
   entity-body exactly as, and in the order that, they would be sent if
   no transfer-encoding were being applied.

   HTTP extends RFC 1864 to permit the digest to be computed for MIME
   composite media-types (e.g., multipart/* and message/rfc822), but
   this does not change how the digest is computed as defined in the
   preceding paragraph.

   There are several consequences of this. The entity-body for composite
   types MAY contain many body-parts, each with its own MIME and HTTP
   headers (including Content-MD5, Content-Transfer-Encoding, and
   Content-Encoding headers). If a body-part has a Content-Transfer-
   Encoding or Content-Encoding header, it is assumed that the content
   of the body-part has had the encoding applied, and the body-part is
   included in the Content-MD5 digest as is -- i.e., after the
   application. The Transfer-Encoding header field is not allowed within
   body-parts.

   Conversion of all line breaks to CRLF MUST NOT be done before
   computing or checking the digest: the line break convention used in
   the text actually transmitted MUST be left unaltered when computing
   the digest.

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      Note: while the definition of Content-MD5 is exactly the same for
      HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
      in which the application of Content-MD5 to HTTP entity-bodies
      differs from its application to MIME entity-bodies. One is that
      HTTP, unlike MIME, does not use Content-Transfer-Encoding, and
      does use Transfer-Encoding and Content-Encoding. Another is that
      HTTP more frequently uses binary content types than MIME, so it is
      worth noting that, in such cases, the byte order used to compute
      the digest is the transmission byte order defined for the type.
      Lastly, HTTP allows transmission of text types with any of several
      line break conventions and not just the canonical form using CRLF.

14.16 Content-Range

   The Content-Range entity-header is sent with a partial entity-body to
   specify where in the full entity-body the partial body should be
   applied. Range units are defined in section 3.12.

       Content-Range = "Content-Range" ":" content-range-spec

       content-range-spec      = byte-content-range-spec
       byte-content-range-spec = bytes-unit SP
                                 byte-range-resp-spec "/"
                                 ( instance-length | "*" )

       byte-range-resp-spec = (first-byte-pos "-" last-byte-pos)
                                      | "*"
       instance-length           = 1*DIGIT

   The header SHOULD indicate the total length of the full entity-body,
   unless this length is unknown or difficult to determine. The asterisk
   "*" character means that the instance-length is unknown at the time
   when the response was generated.

   Unlike byte-ranges-specifier values (see section 14.35.1), a byte-
   range-resp-spec MUST only specify one range, and MUST contain
   absolute byte positions for both the first and last byte of the
   range.

   A byte-content-range-spec with a byte-range-resp-spec whose last-
   byte-pos value is less than its first-byte-pos value, or whose
   instance-length value is less than or equal to its last-byte-pos
   value, is invalid. The recipient of an invalid byte-content-range-
   spec MUST ignore it and any content transferred along with it.

   A server sending a response with status code 416 (Requested range not
   satisfiable) SHOULD include a Content-Range field with a byte-range-
   resp-spec of "*". The instance-length specifies the current length of

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   the selected resource. A response with status code 206 (Partial
   Content) MUST NOT include a Content-Range field with a byte-range-
   resp-spec of "*".

   Examples of byte-content-range-spec values, assuming that the entity
   contains a total of 1234 bytes:

      . The first 500 bytes:
       bytes 0-499/1234

      . The second 500 bytes:
       bytes 500-999/1234

      . All except for the first 500 bytes:
       bytes 500-1233/1234

      . The last 500 bytes:
       bytes 734-1233/1234

   When an HTTP message includes the content of a single range (for
   example, a response to a request for a single range, or to a request
   for a set of ranges that overlap without any holes), this content is
   transmitted with a Content-Range header, and a Content-Length header
   showing the number of bytes actually transferred. For example,

       HTTP/1.1 206 Partial content
       Date: Wed, 15 Nov 1995 06:25:24 GMT
       Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
       Content-Range: bytes 21010-47021/47022
       Content-Length: 26012
       Content-Type: image/gif

   When an HTTP message includes the content of multiple ranges (for
   example, a response to a request for multiple non-overlapping
   ranges), these are transmitted as a multipart message. The multipart
   media type used for this purpose is "multipart/byteranges" as defined
   in appendix 19.2. See appendix 19.6.3 for a compatibility issue.

   A response to a request for a single range MUST NOT be sent using the
   multipart/byteranges media type.  A response to a request for
   multiple ranges, whose result is a single range, MAY be sent as a
   multipart/byteranges media type with one part. A client that cannot
   decode a multipart/byteranges message MUST NOT ask for multiple
   byte-ranges in a single request.

   When a client requests multiple byte-ranges in one request, the
   server SHOULD return them in the order that they appeared in the
   request.

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   If the server ignores a byte-range-spec because it is syntactically
   invalid, the server SHOULD treat the request as if the invalid Range
   header field did not exist. (Normally, this means return a 200
   response containing the full entity).

   If the server receives a request (other than one including an If-
   Range request-header field) with an unsatisfiable Range request-
   header field (that is, all of whose byte-range-spec values have a
   first-byte-pos value greater than the current length of the selected
   resource), it SHOULD return a response code of 416 (Requested range
   not satisfiable) (section 10.4.17).

      Note: clients cannot depend on servers to send a 416 (Requested
      range not satisfiable) response instead of a 200 (OK) response for
      an unsatisfiable Range request-header, since not all servers
      implement this request-header.

14.17 Content-Type

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

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

   Media types are defined in section 3.7. An example of the field is

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

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

14.18 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 [8]-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:

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      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 14.18.1 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 [28], to synchronize its
   clock with a reliable external standard.

   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.

14.18.1 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).

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14.19 ETag

   The ETag response-header field provides the current value of the
   entity tag for the requested variant. The headers used with entity
   tags are described in sections 14.24, 14.26 and 14.44. The entity tag
   MAY be used for comparison with other entities from the same resource
   (see section 13.3.3).

      ETag = "ETag" ":" entity-tag

   Examples:

      ETag: "xyzzy"
      ETag: W/"xyzzy"
      ETag: ""

14.20 Expect

   The Expect request-header field is used to indicate that particular
   server behaviors are required by the client.

      Expect       =  "Expect" ":" 1#expectation

      expectation  =  "100-continue" | expectation-extension
      expectation-extension =  token [ "=" ( token | quoted-string )
                               *expect-params ]
      expect-params =  ";" token [ "=" ( token | quoted-string ) ]

   A server that does not understand or is unable to comply with any of
   the expectation values in the Expect field of a request MUST respond
   with appropriate error status. The server MUST respond with a 417
   (Expectation Failed) status if any of the expectations cannot be met
   or, if there are other problems with the request, some other 4xx
   status.

   This header field is defined with extensible syntax to allow for
   future extensions. If a server receives a request containing an
   Expect field that includes an expectation-extension that it does not
   support, it MUST respond with a 417 (Expectation Failed) status.

   Comparison of expectation values is case-insensitive for unquoted
   tokens (including the 100-continue token), and is case-sensitive for
   quoted-string expectation-extensions.

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   The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST
   return a 417 (Expectation Failed) status if it receives a request
   with an expectation that it cannot meet. However, the Expect
   request-header itself is end-to-end; it MUST be forwarded if the
   request is forwarded.

   Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
   Expect header.

   See section 8.2.3 for the use of the 100 (continue) status.

14.21 Expires

   The Expires entity-header field gives the date/time after which the
   response is considered stale. A stale cache entry may not normally be
   returned by a cache (either a proxy cache or a user agent cache)
   unless it is first validated with the origin server (or with an
   intermediate cache that has a fresh copy of the entity). See section
   13.2 for further discussion of the expiration model.

   The presence of an Expires field does not imply that the original
   resource will change or cease to exist at, before, or after that
   time.

   The format is an absolute date and time as defined by HTTP-date in
   section 3.3.1; it MUST be in RFC 1123 date format:

      Expires = "Expires" ":" HTTP-date

   An example of its use is

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

      Note: if a response includes a Cache-Control field with the max-
      age directive (see section 14.9.3), that directive overrides the
      Expires field.

   HTTP/1.1 clients and caches MUST treat other invalid date formats,
   especially including the value "0", as in the past (i.e., "already
   expired").

   To mark a response as "already expired," an origin server sends an
   Expires date that is equal to the Date header value. (See the rules
   for expiration calculations in section 13.2.4.)

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   To mark a response as "never expires," an origin server sends an
   Expires date approximately one year from the time the response is
   sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one
   year in the future.

   The presence of an Expires header field with a date value of some
   time in the future on a response that otherwise would by default be
   non-cacheable indicates that the response is cacheable, unless
   indicated otherwise by a Cache-Control header field (section 14.9).

14.22 From

   The From request-header field, if given, SHOULD contain an Internet
   e-mail address for the human user who controls the requesting user
   agent. The address SHOULD be machine-usable, as defined by "mailbox"
   in RFC 822 [9] as updated by RFC 1123 [8]:

       From   = "From" ":" mailbox

   An example is:

       From: webmaster@w3.org

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

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

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

14.23 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,

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   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
   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 19.6.1.1 for other requirements relating to
   Host.

14.24 If-Match

   The If-Match request-header field is used with a method to make it
   conditional. A client that has one or more entities previously
   obtained from the resource can verify that one of those entities is
   current by including a list of their associated entity tags in the
   If-Match header field. Entity tags are defined in section 3.11. The
   purpose of this feature is to allow efficient updates of cached
   information with a minimum amount of transaction overhead. It is also
   used, on updating requests, to prevent inadvertent modification of
   the wrong version of a resource. As a special case, the value "*"
   matches any current entity of the resource.

       If-Match = "If-Match" ":" ( "*" | 1#entity-tag )

   If any of the entity tags match the entity tag of the entity that
   would have been returned in the response to a similar GET request
   (without the If-Match header) on that resource, or if "*" is given

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   and any current entity exists for that resource, then the server MAY
   perform the requested method as if the If-Match header field did not
   exist.

   A server MUST use the strong comparison function (see section 13.3.3)
   to compare the entity tags in If-Match.

   If none of the entity tags match, or if "*" is given and no current
   entity exists, the server MUST NOT perform the requested method, and
   MUST return a 412 (Precondition Failed) response. This behavior is
   most useful when the client wants to prevent an updating method, such
   as PUT, from modifying a resource that has changed since the client
   last retrieved it.

   If the request would, without the If-Match header field, result in
   anything other than a 2xx or 412 status, then the If-Match header
   MUST be ignored.

   The meaning of "If-Match: *" is that the method SHOULD be performed
   if the representation selected by the origin server (or by a cache,
   possibly using the Vary mechanism, see section 14.44) exists, and
   MUST NOT be performed if the representation does not exist.

   A request intended to update a resource (e.g., a PUT) MAY include an
   If-Match header field to signal that the request method MUST NOT be
   applied if the entity corresponding to the If-Match value (a single
   entity tag) is no longer a representation of that resource. This
   allows the user to indicate that they do not wish the request to be
   successful if the resource has been changed without their knowledge.
   Examples:

       If-Match: "xyzzy"
       If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
       If-Match: *

   The result of a request having both an If-Match header field and
   either an If-None-Match or an If-Modified-Since header fields is
   undefined by this specification.

14.25 If-Modified-Since

   The If-Modified-Since request-header field is used with a method to
   make it conditional: if the requested variant has not been modified
   since the time specified in this field, an entity will not be
   returned from the server; instead, a 304 (not modified) response will
   be returned without any message-body.

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

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   An example of the field is:

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

   A GET method with an If-Modified-Since header and no Range header
   requests that the identified entity be transferred only if it has
   been modified since the date given by the If-Modified-Since header.
   The algorithm for determining this includes the following cases:

      a) If the request would normally result in anything other than a
         200 (OK) status, or if the passed If-Modified-Since date is
         invalid, the response is exactly the same as for a normal GET.
         A date which is later than the server's current time is
         invalid.

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

      c) If the variant has not been modified since a valid If-
         Modified-Since date, the server SHOULD return a 304 (Not
         Modified) response.

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

      Note: The Range request-header field modifies the meaning of If-
      Modified-Since; see section 14.35 for full details.

      Note: If-Modified-Since times are interpreted by the server, whose
      clock might not be synchronized with the client.

      Note: When handling an If-Modified-Since header field, some
      servers will use an exact date comparison function, rather than a
      less-than function, for deciding whether to send a 304 (Not
      Modified) response. To get best results when sending an If-
      Modified-Since header field for cache validation, clients are
      advised to use the exact date string received in a previous Last-
      Modified header field whenever possible.

      Note: If a client uses an arbitrary date in the If-Modified-Since
      header instead of a date taken from the Last-Modified header for
      the same request, the client should be aware of the fact that this
      date is interpreted in the server's understanding of time. The
      client should consider unsynchronized clocks and rounding problems
      due to the different encodings of time between the client and
      server. This includes the possibility of race conditions if the
      document has changed between the time it was first requested and
      the If-Modified-Since date of a subsequent request, and the

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      possibility of clock-skew-related problems if the If-Modified-
      Since date is derived from the client's clock without correction
      to the server's clock. Corrections for different time bases
      between client and server are at best approximate due to network
      latency.

   The result of a request having both an If-Modified-Since header field
   and either an If-Match or an If-Unmodified-Since header fields is
   undefined by this specification.

14.26 If-None-Match

   The If-None-Match request-header field is used with a method to make
   it conditional. A client that has one or more entities previously
   obtained from the resource can verify that none of those entities is
   current by including a list of their associated entity tags in the
   If-None-Match header field. The purpose of this feature is to allow
   efficient updates of cached information with a minimum amount of
   transaction overhead. It is also used to prevent a method (e.g. PUT)
   from inadvertently modifying an existing resource when the client
   believes that the resource does not exist.

   As a special case, the value "*" matches any current entity of the
   resource.

       If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )

   If any of the entity tags match the entity tag of the entity that
   would have been returned in the response to a similar GET request
   (without the If-None-Match header) on that resource, or if "*" is
   given and any current entity exists for that resource, then the
   server MUST NOT perform the requested method, unless required to do
   so because the resource's modification date fails to match that
   supplied in an If-Modified-Since header field in the request.
   Instead, if the request method was GET or HEAD, the server SHOULD
   respond with a 304 (Not Modified) response, including the cache-
   related header fields (particularly ETag) of one of the entities that
   matched. For all other request methods, the server MUST respond with
   a status of 412 (Precondition Failed).

   See section 13.3.3 for rules on how to determine if two entities tags
   match. The weak comparison function can only be used with GET or HEAD
   requests.

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   If none of the entity tags match, then the server MAY perform the
   requested method as if the If-None-Match header field did not exist,
   but MUST also ignore any If-Modified-Since header field(s) in the
   request. That is, if no entity tags match, then the server MUST NOT
   return a 304 (Not Modified) response.

   If the request would, without the If-None-Match header field, result
   in anything other than a 2xx or 304 status, then the If-None-Match
   header MUST be ignored. (See section 13.3.4 for a discussion of
   server behavior when both If-Modified-Since and If-None-Match appear
   in the same request.)

   The meaning of "If-None-Match: *" is that the method MUST NOT be
   performed if the representation selected by the origin server (or by
   a cache, possibly using the Vary mechanism, see section 14.44)
   exists, and SHOULD be performed if the representation does not exist.
   This feature is intended to be useful in preventing races between PUT
   operations.

   Examples:

       If-None-Match: "xyzzy"
       If-None-Match: W/"xyzzy"
       If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
       If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
       If-None-Match: *

   The result of a request having both an If-None-Match header field and
   either an If-Match or an If-Unmodified-Since header fields is
   undefined by this specification.

14.27 If-Range

   If a client has a partial copy of an entity in its cache, and wishes
   to have an up-to-date copy of the entire entity in its cache, it
   could use the Range request-header with a conditional GET (using
   either or both of If-Unmodified-Since and If-Match.) However, if the
   condition fails because the entity has been modified, the client
   would then have to make a second request to obtain the entire current
   entity-body.

   The If-Range header allows a client to "short-circuit" the second
   request. Informally, its meaning is `if the entity is unchanged, send
   me the part(s) that I am missing; otherwise, send me the entire new
   entity'.

        If-Range = "If-Range" ":" ( entity-tag | HTTP-date )

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   If the client has no entity tag for an entity, but does have a Last-
   Modified date, it MAY use that date in an If-Range header. (The
   server can distinguish between a valid HTTP-date and any form of
   entity-tag by examining no more than two characters.) The If-Range
   header SHOULD only be used together with a Range header, and MUST be
   ignored if the request does not include a Range header, or if the
   server does not support the sub-range operation.

   If the entity tag given in the If-Range header matches the current
   entity tag for the entity, then the server SHOULD provide the
   specified sub-range of the entity using a 206 (Partial content)
   response. If the entity tag does not match, then the server SHOULD
   return the entire entity using a 200 (OK) response.

14.28 If-Unmodified-Since

   The If-Unmodified-Since request-header field is used with a method to
   make it conditional. If the requested resource has not been modified
   since the time specified in this field, the server SHOULD perform the
   requested operation as if the If-Unmodified-Since header were not
   present.

   If the requested variant has been modified since the specified time,
   the server MUST NOT perform the requested operation, and MUST return
   a 412 (Precondition Failed).

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

   An example of the field is:

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

   If the request normally (i.e., without the If-Unmodified-Since
   header) would result in anything other than a 2xx or 412 status, the
   If-Unmodified-Since header SHOULD be ignored.

   If the specified date is invalid, the header is ignored.

   The result of a request having both an If-Unmodified-Since header
   field and either an If-None-Match or an If-Modified-Since header
   fields is undefined by this specification.

14.29 Last-Modified

   The Last-Modified entity-header field indicates the date and time at
   which the origin server believes the variant was last modified.

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

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   An example of its use is

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

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

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

   An origin server SHOULD obtain the Last-Modified value of the entity
   as close as possible to the time that it generates the Date value of
   its response. This allows a recipient to make an accurate assessment
   of the entity's modification time, especially if the entity changes
   near the time that the response is generated.

   HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.

14.30 Location

   The Location response-header field is used to redirect the recipient
   to a location other than the Request-URI for completion of the
   request or identification of a new resource. For 201 (Created)
   responses, the Location is that of the new resource which was created
   by the request. For 3xx responses, the location SHOULD indicate the
   server's preferred URI for automatic redirection to the resource. The
   field value consists of a single absolute URI.

       Location       = "Location" ":" absoluteURI

   An example is:

       Location: http://www.w3.org/pub/WWW/People.html

      Note: The Content-Location header field (section 14.14) differs
      from Location in that the Content-Location identifies the original
      location of the entity enclosed in the request. It is therefore
      possible for a response to contain header fields for both Location
      and Content-Location. Also see section 13.10 for cache
      requirements of some methods.

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14.31 Max-Forwards

   The Max-Forwards request-header field provides a mechanism with the
   TRACE (section 9.8) and OPTIONS (section 9.2) methods to limit the
   number of proxies or gateways that can forward the request to the
   next inbound server. This can be useful when the client is attempting
   to trace a request chain which appears to be failing or looping in
   mid-chain.

       Max-Forwards   = "Max-Forwards" ":" 1*DIGIT

   The Max-Forwards value is a decimal integer indicating the remaining
   number of times this request message may be forwarded.

   Each proxy or gateway recipient of a TRACE or OPTIONS request
   containing a Max-Forwards header field MUST check and update its
   value prior to forwarding the request. If the received value is zero
   (0), the recipient MUST NOT forward the request; instead, it MUST
   respond as the final recipient. If the received Max-Forwards value is
   greater than zero, then the forwarded message MUST contain an updated
   Max-Forwards field with a value decremented by one (1).

   The Max-Forwards header field MAY be ignored for all other methods
   defined by this specification and for any extension methods for which
   it is not explicitly referred to as part of that method definition.

14.32 Pragma

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

       Pragma            = "Pragma" ":" 1#pragma-directive
       pragma-directive  = "no-cache" | extension-pragma
       extension-pragma  = token [ "=" ( token | quoted-string ) ]

   When the no-cache directive is present in a request message, an
   application SHOULD forward the request toward the origin server even
   if it has a cached copy of what is being requested. This pragma
   directive has the same semantics as the no-cache cache-directive (see
   section 14.9) and is defined here for backward compatibility with
   HTTP/1.0. Clients SHOULD include both header fields when a no-cache
   request is sent to a server not known to be HTTP/1.1 compliant.

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

   HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
   sent "Cache-Control: no-cache". No new Pragma directives will be
   defined in HTTP.

      Note: because the meaning of "Pragma: no-cache as a response
      header field is not actually specified, it does not provide a
      reliable replacement for "Cache-Control: no-cache" in a response

14.33 Proxy-Authenticate

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

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

   The HTTP access authentication process is described in "HTTP
   Authentication: Basic and Digest Access Authentication" [43]. Unlike
   WWW-Authenticate, the Proxy-Authenticate header field applies only to
   the current connection and SHOULD NOT be passed on to downstream
   clients. However, an intermediate proxy might need to obtain its own
   credentials by requesting them from the downstream client, which in
   some circumstances will appear as if the proxy is forwarding the
   Proxy-Authenticate header field.

14.34 Proxy-Authorization

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

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

   The HTTP access authentication process is described in "HTTP
   Authentication: Basic and Digest Access Authentication" [43] . Unlike
   Authorization, the Proxy-Authorization header field applies only to
   the next outbound proxy that demanded authentication using the Proxy-
   Authenticate field. When multiple proxies are used in a chain, the

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   Proxy-Authorization header field is consumed by the first outbound
   proxy that was expecting to receive credentials. A proxy MAY relay
   the credentials from the client request to the next proxy if that is
   the mechanism by which the proxies cooperatively authenticate a given
   request.

14.35 Range

14.35.1 Byte Ranges

   Since all HTTP entities are represented in HTTP messages as sequences
   of bytes, the concept of a byte range is meaningful for any HTTP
   entity. (However, not all clients and servers need to support byte-
   range operations.)

   Byte range specifications in HTTP apply to the sequence of bytes in
   the entity-body (not necessarily the same as the message-body).

   A byte range operation MAY specify a single range of bytes, or a set
   of ranges within a single entity.

       ranges-specifier = byte-ranges-specifier
       byte-ranges-specifier = bytes-unit "=" byte-range-set
       byte-range-set  = 1#( byte-range-spec | suffix-byte-range-spec )
       byte-range-spec = first-byte-pos "-" [last-byte-pos]
       first-byte-pos  = 1*DIGIT
       last-byte-pos   = 1*DIGIT

   The first-byte-pos value in a byte-range-spec gives the byte-offset
   of the first byte in a range. The last-byte-pos value gives the
   byte-offset of the last byte in the range; that is, the byte
   positions specified are inclusive. Byte offsets start at zero.

   If the last-byte-pos value is present, it MUST be greater than or
   equal to the first-byte-pos in that byte-range-spec, or the byte-
   range-spec is syntactically invalid. The recipient of a byte-range-
   set that includes one or more syntactically invalid byte-range-spec
   values MUST ignore the header field that includes that byte-range-
   set.

   If the last-byte-pos value is absent, or if the value is greater than
   or equal to the current length of the entity-body, last-byte-pos is
   taken to be equal to one less than the current length of the entity-
   body in bytes.

   By its choice of last-byte-pos, a client can limit the number of
   bytes retrieved without knowing the size of the entity.

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       suffix-byte-range-spec = "-" suffix-length
       suffix-length = 1*DIGIT

   A suffix-byte-range-spec is used to specify the suffix of the
   entity-body, of a length given by the suffix-length value. (That is,
   this form specifies the last N bytes of an entity-body.) If the
   entity is shorter than the specified suffix-length, the entire
   entity-body is used.

   If a syntactically valid byte-range-set includes at least one byte-
   range-spec whose first-byte-pos is less than the current length of
   the entity-body, or at least one suffix-byte-range-spec with a non-
   zero suffix-length, then the byte-range-set is satisfiable.
   Otherwise, the byte-range-set is unsatisfiable. If the byte-range-set
   is unsatisfiable, the server SHOULD return a response with a status
   of 416 (Requested range not satisfiable). Otherwise, the server
   SHOULD return a response with a status of 206 (Partial Content)
   containing the satisfiable ranges of the entity-body.

   Examples of byte-ranges-specifier values (assuming an entity-body of
   length 10000):

      - The first 500 bytes (byte offsets 0-499, inclusive):  bytes=0-
        499

      - The second 500 bytes (byte offsets 500-999, inclusive):
        bytes=500-999

      - The final 500 bytes (byte offsets 9500-9999, inclusive):
        bytes=-500

      - Or bytes=9500-

      - The first and last bytes only (bytes 0 and 9999):  bytes=0-0,-1

      - Several legal but not canonical specifications of the second 500
        bytes (byte offsets 500-999, inclusive):
         bytes=500-600,601-999
         bytes=500-700,601-999

14.35.2 Range Retrieval Requests

   HTTP retrieval requests using conditional or unconditional GET
   methods MAY request one or more sub-ranges of the entity, instead of
   the entire entity, using the Range request header, which applies to
   the entity returned as the result of the request:

      Range = "Range" ":" ranges-specifier

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   A server MAY ignore the Range header. However, HTTP/1.1 origin
   servers and intermediate caches ought to support byte ranges when
   possible, since Range supports efficient recovery from partially
   failed transfers, and supports efficient partial retrieval of large
   entities.

   If the server supports the Range header and the specified range or
   ranges are appropriate for the entity:

      - The presence of a Range header in an unconditional GET modifies
        what is returned if the GET is otherwise successful. In other
        words, the response carries a status code of 206 (Partial
        Content) instead of 200 (OK).

      - The presence of a Range header in a conditional GET (a request
        using one or both of If-Modified-Since and If-None-Match, or
        one or both of If-Unmodified-Since and If-Match) modifies what
        is returned if the GET is otherwise successful and the
        condition is true. It does not affect the 304 (Not Modified)
        response returned if the conditional is false.

   In some cases, it might be more appropriate to use the If-Range
   header (see section 14.27) in addition to the Range header.

   If a proxy that supports ranges receives a Range request, forwards
   the request to an inbound server, and receives an entire entity in
   reply, it SHOULD only return the requested range to its client. It
   SHOULD store the entire received response in its cache if that is
   consistent with its cache allocation policies.

14.36 Referer

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

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

   Example:

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

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   If the field value is a relative URI, it SHOULD be interpreted
   relative to the Request-URI. The URI MUST NOT include a fragment. See
   section 15.1.3 for security considerations.

14.37 Retry-After

   The Retry-After response-header field can be used with a 503 (Service
   Unavailable) response to indicate how long the service is expected to
   be unavailable to the requesting client. This field MAY also be used
   with any 3xx (Redirection) response to indicate the minimum time the
   user-agent is asked wait before issuing the redirected request. The
   value of this field can be either an HTTP-date or an integer number
   of seconds (in decimal) after the time of the response.

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

   Two examples of its use are

       Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
       Retry-After: 120

   In the latter example, the delay is 2 minutes.

14.38 Server

   The Server response-header field contains information about the
   software used by the origin server to handle the request. The field
   can contain multiple product tokens (section 3.8) and comments
   identifying the server and any significant subproducts. The product
   tokens are listed in order of their significance for identifying the
   application.

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

   Example:

       Server: CERN/3.0 libwww/2.17

   If the response is being forwarded through a proxy, the proxy
   application MUST NOT modify the Server response-header. Instead, it
   SHOULD include a Via field (as described in section 14.45).

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

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14.39 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 3.6).

       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 3.6.1. 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 14.10) 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 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 3.9, a
         qvalue of 0 means "not acceptable.")

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      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.

14.40 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 3.6.1 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:

      . Transfer-Encoding

      . Content-Length

      . Trailer

14.41 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 3.6. An example is:

     Transfer-Encoding: chunked

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   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.

14.42 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).

   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 14.10) whenever Upgrade is present in an
   HTTP/1.1 message.

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   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.

14.43 User-Agent

   The User-Agent request-header field contains information about the
   user agent originating the request. This is for statistical purposes,
   the tracing of protocol violations, and automated recognition of user
   agents for the sake of tailoring responses to avoid particular user
   agent limitations. User agents SHOULD include this field with
   requests. The field can contain multiple product tokens (section 3.8)
   and comments identifying the agent and any subproducts which form a
   significant part of the user agent. By convention, the product tokens
   are listed in order of their significance for identifying the
   application.

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

   Example:

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

14.44 Vary

   The Vary field value indicates the set of request-header fields that
   fully determines, while the response is fresh, whether a cache is
   permitted to use the response to reply to a subsequent request
   without revalidation. For uncacheable or stale responses, the Vary
   field value advises the user agent about the criteria that were used
   to select the representation. A Vary field value of "*" implies that
   a cache cannot determine from the request headers of a subsequent
   request whether this response is the appropriate representation. See
   section 13.6 for use of the Vary header field by caches.

       Vary  = "Vary" ":" ( "*" | 1#field-name )

   An HTTP/1.1 server SHOULD include a Vary header field with any
   cacheable response that is subject to server-driven negotiation.
   Doing so allows a cache to properly interpret future requests on that
   resource and informs the user agent about the presence of negotiation

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   on that resource. A server MAY include a Vary header field with a
   non-cacheable response that is subject to server-driven negotiation,
   since this might provide the user agent with useful information about
   the dimensions over which the response varies at the time of the
   response.

   A Vary field value consisting of a list of field-names signals that
   the representation selected for the response is based on a selection
   algorithm which considers ONLY the listed request-header field values
   in selecting the most appropriate representation. A cache MAY assume
   that the same selection will be made for future requests with the
   same values for the listed field names, for the duration of time for
   which the response is fresh.

   The field-names given are not limited to the set of standard
   request-header fields defined by this specification. Field names are
   case-insensitive.

   A Vary field value of "*" signals that unspecified parameters not
   limited to the request-headers (e.g., the network address of the
   client), play a role in the selection of the response representation.
   The "*" value MUST NOT be generated by a proxy server; it may only be
   generated by an origin server.

14.45  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 [9] 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

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   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
   have different received-protocol values.

14.46 Warning

   The Warning general-header field is used to carry additional
   information about the status or transformation of a message which
   might not be reflected in the message. This information is typically
   used to warn about a possible lack of semantic transparency from
   caching operations or transformations applied to the entity body of
   the message.

   Warning headers are sent with responses using:

       Warning    = "Warning" ":" 1#warning-value

       warning-value = warn-code SP warn-agent SP warn-text
                                             [SP warn-date]

       warn-code  = 3DIGIT
       warn-agent = ( host [ ":" port ] ) | pseudonym
                       ; the name or pseudonym of the server adding
                       ; the Warning header, for use in debugging
       warn-text  = quoted-string
       warn-date  = <"> HTTP-date <">

   A response MAY carry more than one Warning header.

   The warn-text SHOULD be in a natural language and character set that
   is most likely to be intelligible to the human user receiving the
   response. This decision MAY be based on any available knowledge, such
   as the location of the cache or user, the Accept-Language field in a
   request, the Content-Language field in a response, etc. The default
   language is English and the default character set is ISO-8859-1.

   If a character set other than ISO-8859-1 is used, it MUST be encoded
   in the warn-text using the method described in RFC 2047 [14].

   Warning headers can in general be applied to any message, however
   some specific warn-codes are specific to caches and can only be
   applied to response messages. New Warning headers SHOULD be added
   after any existing Warning headers. A cache MUST NOT delete any
   Warning header that it received with a message. However, if a cache
   successfully validates a cache entry, it SHOULD remove any Warning
   headers previously attached to that entry except as specified for

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   specific Warning codes. It MUST then add any Warning headers received
   in the validating response. In other words, Warning headers are those
   that would be attached to the most recent relevant response.

   When multiple Warning headers are attached to a response, the user
   agent ought to inform the user of as many of them as possible, in the
   order that they appear in the response. If it is not possible to
   inform the user of all of the warnings, the user agent SHOULD follow
   these heuristics:

      - Warnings that appear early in the response take priority over
        those appearing later in the response.

      - Warnings in the user's preferred character set take priority
        over warnings in other character sets but with identical warn-
        codes and warn-agents.

   Systems that generate multiple Warning headers SHOULD order them with
   this user agent behavior in mind.

   Requirements for the behavior of caches with respect to Warnings are
   stated in section 13.1.2.

   This is a list of the currently-defined warn-codes, each with a
   recommended warn-text in English, and a description of its meaning.

   110 Response is stale
     MUST be included whenever the returned response is stale.

   111 Revalidation failed
     MUST be included if a cache returns a stale response because an
     attempt to revalidate the response failed, due to an inability to
     reach the server.

   112 Disconnected operation
     SHOULD be included if the cache is intentionally disconnected from
     the rest of the network for a period of time.

   113 Heuristic expiration
     MUST be included if the cache heuristically chose a freshness
     lifetime greater than 24 hours and the response's age is greater
     than 24 hours.

   199 Miscellaneous warning
     The warning text MAY include arbitrary information to be presented
     to a human user, or logged. A system receiving this warning MUST
     NOT take any automated action, besides presenting the warning to
     the user.

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   214 Transformation applied
     MUST be added by an intermediate cache or proxy if it applies any
     transformation changing the content-coding (as specified in the
     Content-Encoding header) or media-type (as specified in the
     Content-Type header) of the response, or the entity-body of the
     response, unless this Warning code already appears in the response.

   299 Miscellaneous persistent warning
     The warning text MAY include arbitrary information to be presented
     to a human user, or logged. A system receiving this warning MUST
     NOT take any automated action.

   If an implementation sends a message with one or more Warning headers
   whose version is HTTP/1.0 or lower, then the sender MUST include in
   each warning-value a warn-date that matches the date in the response.

   If an implementation receives a message with a warning-value that
   includes a warn-date, and that warn-date is different from the Date
   value in the response, then that warning-value MUST be deleted from
   the message before storing, forwarding, or using it. (This prevents
   bad consequences of naive caching of Warning header fields.) If all
   of the warning-values are deleted for this reason, the Warning header
   MUST be deleted as well.

14.47 WWW-Authenticate

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

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

   The HTTP access authentication process is described in "HTTP
   Authentication: Basic and Digest Access Authentication" [43]. User
   agents are advised to take special care in parsing the WWW-
   Authenticate field value as it might contain more than one challenge,
   or if more than one WWW-Authenticate header field is provided, the
   contents of a challenge itself can contain a comma-separated list of
   authentication parameters.

15 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.

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15.1 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.

15.1.1 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.

15.1.2 Transfer of Sensitive Information

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

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

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

   The Referer header allows reading patterns to be studied and reverse
   links drawn. Although it can be very useful, its power can be abused
   if user details are not separated from the information contained in

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   the Referer. Even when the personal information has been removed, the
   Referer header might indicate a private document's URI whose
   publication would be inappropriate.

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

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

   The User-Agent (section 14.43) or Server (section 14.38) header
   fields can sometimes be used to determine that a specific client or
   server have a particular security hole which might be exploited.
   Unfortunately, this same information is often used for other valuable
   purposes for which HTTP currently has no better mechanism.

15.1.3 Encoding Sensitive Information in URI's

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

   Clients SHOULD NOT include a Referer header field in a (non-secure)
   HTTP request if the referring page was transferred with a secure
   protocol.

   Authors of services which use the HTTP protocol SHOULD NOT use GET
   based forms for the submission of sensitive data, because this will
   cause this data to be encoded in the Request-URI. Many existing
   servers, proxies, and user agents will log the request URI in some
   place where it might be visible to third parties. Servers can use
   POST-based form submission instead

15.1.4 Privacy Issues Connected to Accept Headers

   Accept request-headers can reveal information about the user to all
   servers which are accessed. The Accept-Language header in particular
   can reveal information the user would consider to be of a private
   nature, because the understanding of particular languages is often

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   strongly correlated to the membership of a particular ethnic group.
   User agents which offer the option to configure the contents of an
   Accept-Language header to be sent in every request are strongly
   encouraged to let the configuration process include a message which
   makes the user aware of the loss of privacy involved.

   An approach that limits the loss of privacy would be for a user agent
   to omit the sending of Accept-Language headers by default, and to ask
   the user whether or not to start sending Accept-Language headers to a
   server if it detects, by looking for any Vary response-header fields
   generated by the server, that such sending could improve the quality
   of service.

   Elaborate user-customized accept header fields sent in every request,
   in particular if these include quality values, can be used by servers
   as relatively reliable and long-lived user identifiers. Such user
   identifiers would allow content providers to do click-trail tracking,
   and would allow collaborating content providers to match cross-server
   click-trails or form submissions of individual users. Note that for
   many users not behind a proxy, the network address of the host
   running the user agent will also serve as a long-lived user
   identifier. In environments where proxies are used to enhance
   privacy, user agents ought to be conservative in offering accept
   header configuration options to end users. As an extreme privacy
   measure, proxies could filter the accept headers in relayed requests.
   General purpose user agents which provide a high degree of header
   configurability SHOULD warn users about the loss of privacy which can
   be involved.

15.2 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
   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.

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15.3 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 [24], 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.

15.4 Location Headers and Spoofing

   If a single server supports multiple organizations that do not trust
   one another, then it MUST check the values of Location and Content-
   Location headers in responses that are generated under control of
   said organizations to make sure that they do not attempt to
   invalidate resources over which they have no authority.

15.5 Content-Disposition Issues

   RFC 1806 [35], from which the often implemented Content-Disposition
   (see section 19.5.1) header in HTTP is derived, has a number of very
   serious security considerations. Content-Disposition is not part of
   the HTTP standard, but since it is widely implemented, we are
   documenting its use and risks for implementors. See RFC 2183 [49]
   (which updates RFC 1806) for details.

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15.6 Authentication Credentials and Idle Clients

   Existing HTTP clients and user agents typically retain authentication
   information indefinitely. HTTP/1.1. does not provide a method for a
   server to direct clients to discard these cached credentials. This is
   a significant defect that requires further extensions to HTTP.
   Circumstances under which credential caching can interfere with the
   application's security model include but are not limited to:

      - Clients which have been idle for an extended period following
        which the server might wish to cause the client to reprompt the
        user for credentials.

      - Applications which include a session termination indication
        (such as a `logout' or `commit' button on a page) after which
        the server side of the application `knows' that there is no
        further reason for the client to retain the credentials.

   This is currently under separate study. There are a number of work-
   arounds to parts of this problem, and we encourage the use of
   password protection in screen savers, idle time-outs, and other
   methods which mitigate the security problems inherent in this
   problem. In particular, user agents which cache credentials are
   encouraged to provide a readily accessible mechanism for discarding
   cached credentials under user control.

15.7 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-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
   15.1.1).

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   Caching proxies provide additional potential vulnerabilities, since
   the contents of the cache represent an attractive target for
   malicious exploitation. Because cache contents persist after an HTTP
   request is complete, an attack on the cache can reveal information
   long after a user believes that the information has been removed from
   the network. Therefore, cache contents should be protected as
   sensitive information.

   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.

15.7.1 Denial of Service Attacks on Proxies

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

16 Acknowledgments

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

   The HTTP protocol has evolved considerably over the 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
   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:

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       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

   Much of the content and presentation of the caching design is due to
   suggestions and comments from individuals including: Shel Kaphan,
   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.

   Most of the specification of ranges is based on work originally done
   by Ari Luotonen and John Franks, with additional input from Steve
   Zilles.

   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.

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   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
   rectify.

17 References

   [1] Alvestrand, H., "Tags for the Identification of Languages", RFC
       1766, March 1995.

   [2] 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.

   [3] Berners-Lee, T., "Universal Resource Identifiers in WWW", RFC
       1630, June 1994.

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

   [5] Berners-Lee, T. and D. Connolly, "Hypertext Markup Language -
       2.0", RFC 1866, November 1995.

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

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

   [8] Braden, R., "Requirements for Internet Hosts -- Communication
       Layers", STD 3, RFC 1123, October 1989.

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

   [10] 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.

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

   [12] Horton, M. and R. Adams, "Standard for Interchange of USENET
        Messages", RFC 1036, December 1987.

Fielding, et al.            Standards Track                   [Page 158]
RFC 2616                        HTTP/1.1                       June 1999

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

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

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

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

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

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

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

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

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

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

   [23] Meyers, J. and M. Rose, "The Content-MD5 Header Field", RFC
        1864, October 1995.

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

   [25] Deutsch, P., "GZIP file format specification version 4.3", RFC
        1952, May 1996.

Fielding, et al.            Standards Track                   [Page 159]
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   [26] Venkata N. Padmanabhan, and Jeffrey C. 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/HTTPLat
        ency.html.

   [27] Joe Touch, John Heidemann, and Katia Obraczka. "Analysis of HTTP
        Performance", <URL: http://www.isi.edu/touch/pubs/http-perf96/>,
        ISI Research Report ISI/RR-98-463, (original report dated Aug.
        1996), USC/Information Sciences Institute, August 1998.

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

   [29] Deutsch, P., "DEFLATE Compressed Data Format Specification
        version 1.3", RFC 1951, May 1996.

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

   [31] Deutsch, P. and J. Gailly, "ZLIB Compressed Data Format
        Specification version 3.3", RFC 1950, May 1996.

   [32] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
        Luotonen, A., Sink, E. and L. Stewart, "An Extension to HTTP:
        Digest Access Authentication", RFC 2069, January 1997.

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

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

   [35] Troost, R. and Dorner, S., "Communicating Presentation
        Information in Internet Messages: The Content-Disposition
        Header", RFC 1806, June 1995.

   [36] Mogul, J., Fielding, R., Gettys, J. and H. Frystyk, "Use and
        Interpretation of HTTP Version Numbers", RFC 2145, May 1997.
        [jg639]

   [37] Palme, J., "Common Internet Message Headers", RFC 2076, February
        1997. [jg640]

Fielding, et al.            Standards Track                   [Page 160]
RFC 2616                        HTTP/1.1                       June 1999

   [38] Yergeau, F., "UTF-8, a transformation format of Unicode and
        ISO-10646", RFC 2279, January 1998. [jg641]

   [39] Nielsen, H.F., Gettys, J., Baird-Smith, A., 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, September 1997.[jg642]

   [40] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part Two: Media Types", RFC 2046, November
        1996. [jg643]

   [41] Alvestrand, H., "IETF Policy on Character Sets and Languages",
        BCP 18, RFC 2277, January 1998. [jg644]

   [42] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
        Identifiers (URI): Generic Syntax and Semantics", RFC 2396,
        August 1998. [jg645]

   [43] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
        Leach, P., Luotonen, A., Sink, E. and L. Stewart, "HTTP
        Authentication: Basic and Digest Access Authentication", RFC
        2617, June 1999. [jg646]

   [44] Luotonen, A., "Tunneling TCP based protocols through Web proxy
        servers," Work in Progress. [jg647]

   [45] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
        Aggregate Documents, such as HTML (MHTML)", RFC 2110, March
        1997.

   [46] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
        9, RFC 2026, October 1996.

   [47] Masinter, L., "Hyper Text Coffee Pot Control Protocol
        (HTCPCP/1.0)", RFC 2324, 1 April 1998.

   [48] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part Five: Conformance Criteria and Examples",
        RFC 2049, November 1996.

   [49] Troost, R., Dorner, S. and K. Moore, "Communicating Presentation
        Information in Internet Messages: The Content-Disposition Header
        Field", RFC 2183, August 1997.

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18 Authors' Addresses

   Roy T. Fielding
   Information and Computer Science
   University of California, Irvine
   Irvine, CA 92697-3425, USA

   Fax: +1 (949) 824-1715
   EMail: fielding@ics.uci.edu

   James Gettys
   World Wide Web Consortium
   MIT Laboratory for Computer Science
   545 Technology Square
   Cambridge, MA 02139, USA

   Fax: +1 (617) 258 8682
   EMail: jg@w3.org

   Jeffrey C. Mogul
   Western Research Laboratory
   Compaq Computer Corporation
   250 University Avenue
   Palo Alto, California, 94305, USA

   EMail: mogul@wrl.dec.com

   Henrik Frystyk Nielsen
   World Wide Web Consortium
   MIT Laboratory for Computer Science
   545 Technology Square
   Cambridge, MA 02139, USA

   Fax: +1 (617) 258 8682
   EMail: frystyk@w3.org

   Larry Masinter
   Xerox Corporation
   3333 Coyote Hill Road
   Palo Alto, CA 94034, USA

   EMail: masinter@parc.xerox.com

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

   EMail: paulle@microsoft.com

   Tim Berners-Lee
   Director, World Wide Web Consortium
   MIT Laboratory for Computer Science
   545 Technology Square
   Cambridge, MA 02139, USA

   Fax: +1 (617) 258 8682
   EMail: timbl@w3.org

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19 Appendices

19.1 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 [17].

       Media Type name:         message
       Media subtype name:      http
       Required parameters:     none
       Optional parameters:     version, msgtype
        version: The HTTP-Version number of the enclosed message
                 (e.g., "1.1"). If not present, the version can be
                 determined from the first line of the body.
        msgtype: The message type -- "request" or "response". If not
                 present, the type can be determined from the first
                 line of the body.
       Encoding considerations: only "7bit", "8bit", or "binary" are
                                permitted
       Security considerations: none

       Media Type name:         application
       Media subtype name:      http
       Required parameters:     none
       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

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19.2 Internet Media Type multipart/byteranges

   When an HTTP 206 (Partial Content) response message includes the
   content of multiple ranges (a response to a request for multiple
   non-overlapping ranges), these are transmitted as a multipart
   message-body. The media type for this purpose is called
   "multipart/byteranges".

   The multipart/byteranges media type includes two or more parts, each
   with its own Content-Type and Content-Range fields. The required
   boundary parameter specifies the boundary string used to separate
   each body-part.

       Media Type name:         multipart
       Media subtype name:      byteranges
       Required parameters:     boundary
       Optional parameters:     none
       Encoding considerations: only "7bit", "8bit", or "binary" are
                                permitted
       Security considerations: none

   For example:

   HTTP/1.1 206 Partial Content
   Date: Wed, 15 Nov 1995 06:25:24 GMT
   Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
   Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES

   --THIS_STRING_SEPARATES
   Content-type: application/pdf
   Content-range: bytes 500-999/8000

   ...the first range...
   --THIS_STRING_SEPARATES
   Content-type: application/pdf
   Content-range: bytes 7000-7999/8000

   ...the second range
   --THIS_STRING_SEPARATES--

      Notes:

      1) Additional CRLFs may precede the first boundary string in the
         entity.

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      2) Although RFC 2046 [40] permits the boundary string to be
         quoted, some existing implementations handle a quoted boundary
         string incorrectly.

      3) A number of browsers and servers were coded to an early draft
         of the byteranges specification to use a media type of
         multipart/x-byteranges, which is almost, but not quite
         compatible with the version documented in HTTP/1.1.

19.3 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 section 3.7.1
   and 3.4.1.

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

      - 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).

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

      - 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.

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      - 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.

19.4 Differences Between HTTP Entities and RFC 2045 Entities

   HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC
   822 [9]) and the Multipurpose Internet Mail Extensions (MIME [7]) to
   allow entities to be transmitted in an open variety of
   representations and with extensible mechanisms. However, RFC 2045
   discusses mail, and HTTP has a few features that are different from
   those described in RFC 2045. These differences were carefully chosen
   to optimize performance over binary connections, to allow greater
   freedom in the use of new media types, to make date comparisons
   easier, and to acknowledge the practice of some early HTTP servers
   and clients.

   This appendix describes specific areas where HTTP differs from RFC
   2045. Proxies and gateways to strict MIME environments SHOULD be
   aware of these differences and provide the appropriate conversions
   where necessary. Proxies and gateways from MIME environments to HTTP
   also need to be aware of the differences because some conversions
   might be required.

19.4.1 MIME-Version

   HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages MAY
   include a single MIME-Version general-header field to indicate what
   version of the MIME protocol was used to construct the message. Use
   of the MIME-Version header field indicates that the message is in
   full compliance with the MIME protocol (as defined in RFC 2045[7]).
   Proxies/gateways are responsible for ensuring full compliance (where
   possible) when exporting HTTP messages to strict MIME environments.

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

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

19.4.2 Conversion to Canonical Form

   RFC 2045 [7] requires that an Internet mail entity be converted to
   canonical form prior to being transferred, as described in section 4
   of RFC 2049 [48]. Section 3.7.1 of this document describes the forms
   allowed for subtypes of the "text" media type when transmitted over
   HTTP. RFC 2046 requires that content with a type of "text" represent
   line breaks as CRLF and forbids the use of CR or LF outside of line

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   break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a
   line break within text content when a message is transmitted over
   HTTP.

   Where it is possible, a proxy or gateway from HTTP to a strict MIME
   environment SHOULD translate all line breaks within the text media
   types described in section 3.7.1 of this document to the RFC 2049
   canonical form of CRLF. Note, however, that this might be complicated
   by the presence of a Content-Encoding and by the fact that HTTP
   allows the use of some character sets which do not use octets 13 and
   10 to represent CR and LF, as is the case for some multi-byte
   character sets.

   Implementors should note that conversion will break any cryptographic
   checksums applied to the original content unless the original content
   is already in canonical form. Therefore, the canonical form is
   recommended for any content that uses such checksums in HTTP.

19.4.3 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.

19.4.4 Introduction of Content-Encoding

   RFC 2045 does not include any concept equivalent to HTTP/1.1's
   Content-Encoding header field. Since this acts as a modifier on the
   media type, proxies and gateways from HTTP to MIME-compliant
   protocols MUST either change the value of the Content-Type header
   field or decode the entity-body before forwarding the message. (Some
   experimental applications of Content-Type for Internet mail have used
   a media-type parameter of ";conversions=<content-coding>" to perform
   a function equivalent to Content-Encoding. However, this parameter is
   not part of RFC 2045.)

19.4.5 No Content-Transfer-Encoding

   HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC
   2045. Proxies and gateways from MIME-compliant protocols to HTTP MUST
   remove any non-identity CTE ("quoted-printable" or "base64") encoding
   prior to delivering the response message to an HTTP client.

   Proxies and gateways from HTTP to MIME-compliant protocols are
   responsible for ensuring that the message is in the correct format
   and encoding for safe transport on that protocol, where "safe

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   transport" is defined by the limitations of the protocol being used.
   Such a proxy or gateway SHOULD label the data with an appropriate
   Content-Transfer-Encoding if doing so will improve the likelihood of
   safe transport over the destination protocol.

19.4.6 Introduction of Transfer-Encoding

   HTTP/1.1 introduces the Transfer-Encoding header field (section
   14.41). Proxies/gateways MUST remove any transfer-coding prior to
   forwarding a message via a MIME-compliant protocol.

   A process for decoding the "chunked" transfer-coding (section 3.6)
   can be 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

19.4.7 MHTML and Line Length Limitations

   HTTP implementations which share code with MHTML [45] implementations
   need to be aware of MIME line length limitations. Since HTTP does not
   have this limitation, HTTP does not fold long lines. MHTML messages
   being transported by HTTP follow all conventions of MHTML, including
   line length limitations and folding, canonicalization, etc., since
   HTTP transports all message-bodies as payload (see section 3.7.2) and
   does not interpret the content or any MIME header lines that might be
   contained therein.

19.5 Additional Features

   RFC 1945 and RFC 2068 document protocol elements used by some
   existing HTTP implementations, but not consistently and correctly
   across most HTTP/1.1 applications. Implementors are advised to be
   aware of these features, but cannot rely upon their presence in, or
   interoperability with, other HTTP/1.1 applications. Some of these

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   describe proposed experimental features, and some describe features
   that experimental deployment found lacking that are now addressed in
   the base HTTP/1.1 specification.

   A number of other headers, such as Content-Disposition and Title,
   from SMTP and MIME are also often implemented (see RFC 2076 [37]).

19.5.1 Content-Disposition

   The Content-Disposition response-header field has been proposed as a
   means for the origin server to suggest a default filename if the user
   requests that the content is saved to a file. This usage is derived
   from the definition of Content-Disposition in RFC 1806 [35].

        content-disposition = "Content-Disposition" ":"
                              disposition-type *( ";" disposition-parm )
        disposition-type = "attachment" | disp-extension-token
        disposition-parm = filename-parm | disp-extension-parm
        filename-parm = "filename" "=" quoted-string
        disp-extension-token = token
        disp-extension-parm = token "=" ( token | quoted-string )

   An example is

        Content-Disposition: attachment; filename="fname.ext"

   The receiving user agent SHOULD NOT respect any directory path
   information present in the filename-parm parameter, which is the only
   parameter believed to apply to HTTP implementations at this time. The
   filename SHOULD be treated as a terminal component only.

   If this header is used in a response with the application/octet-
   stream content-type, the implied suggestion is that the user agent
   should not display the response, but directly enter a `save response
   as...' dialog.

   See section 15.5 for Content-Disposition security issues.

19.6 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:

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

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

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

   And we would expect HTTP/1.1 clients to:

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

      - 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
   sending the response. Some implementations implement the Keep-Alive
   version of persistent connections described in section 19.7.1 of RFC
   2068 [33].

19.6.1 Changes from HTTP/1.0

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

19.6.1.1 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 14.23) 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

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   important that all implementations of HTTP (including updates to
   existing HTTP/1.0 applications) correctly implement these
   requirements:

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

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

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

      - Servers MUST accept absolute URIs.

19.6.2 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
   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 14.10.

   The original HTTP/1.0 form of persistent connections (the Connection:
   Keep-Alive and Keep-Alive header) is documented in RFC 2068. [33]

19.6.3 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 [34].

   Clarified which error code should be used for inbound server failures
   (e.g. DNS failures). (Section 10.5.5).

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   CREATE had a race that required an Etag be sent when a resource is
   first created. (Section 10.2.2).

   Content-Base was deleted from the specification: it was not
   implemented widely, and there is no simple, safe way to introduce it
   without a robust extension mechanism. In addition, it is used in a
   similar, but not identical fashion in MHTML [45].

   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. (Sections
   3.6, 4.4, 7.2.2, 13.5.2, 14.13, 14.16)

   A content-coding of "identity" was introduced, to solve problems
   discovered in caching. (section 3.5)

   Quality Values of zero should indicate that "I don't want something"
   to allow clients to refuse a representation. (Section 3.9)

   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)

   Charset wildcarding is introduced to avoid explosion of character set
   names in accept headers. (Section 14.2)

   A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
   was introduced to add this missing case. (Sections 13.4, 14.8, 14.9,
   14.9.3)

   The Cache-Control: max-age directive was not properly defined for
   responses. (Section 14.9.3)

   There are situations where a server (especially a proxy) does not
   know the full length of a response but is capable of serving a
   byterange request. We therefore need a mechanism to allow byteranges
   with a content-range not indicating the full length of the message.
   (Section 14.16)

   Range request responses would become very verbose if all meta-data
   were always returned; by allowing the server to only send needed
   headers in a 206 response, this problem can be avoided. (Section
   10.2.7, 13.5.3, and 14.27)

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   Fix problem with unsatisfiable range requests; there are two cases:
   syntactic problems, and range doesn't exist in the document. The 416
   status code was needed to resolve this ambiguity needed to indicate
   an error for a byte range request that falls outside of the actual
   contents of a document. (Section 10.4.17, 14.16)

   Rewrite of message transmission requirements to make it much harder
   for implementors to get it wrong, as the consequences of errors here
   can have significant impact on the Internet, and to deal with the
   following problems:

      1. Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where
         this was incorrectly placing a requirement on the behavior of
         an implementation of a future version of HTTP/1.x

      2. Made it clear that user-agents should retry requests, not
         "clients" in general.

      3. Converted requirements for clients to ignore unexpected 100
         (Continue) responses, and for proxies to forward 100 responses,
         into a general requirement for 1xx responses.

      4. Modified some TCP-specific language, to make it clearer that
         non-TCP transports are possible for HTTP.

      5. Require that the origin server MUST NOT wait for the request
         body before it sends a required 100 (Continue) response.

      6. Allow, rather than require, a server to omit 100 (Continue) if
         it has already seen some of the request body.

      7. Allow servers to defend against denial-of-service attacks and
         broken clients.

   This change adds the Expect header and 417 status code. The message
   transmission requirements fixes are in sections 8.2, 10.4.18,
   8.1.2.2, 13.11, and 14.20.

   Proxies should be able to add Content-Length when appropriate.
   (Section 13.5.2)

   Clean up confusion between 403 and 404 responses. (Section 10.4.4,
   10.4.5, and 10.4.11)

   Warnings could be cached incorrectly, or not updated appropriately.
   (Section 13.1.2, 13.2.4, 13.5.2, 13.5.3, 14.9.3, and 14.46) Warning
   also needed to be a general header, as PUT or other methods may have
   need for it in requests.

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   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 [39]. TE also solves another, obscure, downward
   interoperability problem that could have occurred due to interactions
   between authentication trailers, chunked encoding and HTTP/1.0
   clients.(Section 3.6, 3.6.1, and 14.39)

   The PATCH, LINK, UNLINK methods were defined but not commonly
   implemented in previous versions of this specification. See RFC 2068
   [33].

   The Alternates, Content-Version, Derived-From, Link, URI, Public and
   Content-Base header fields were defined in previous versions of this
   specification, but not commonly implemented. See RFC 2068 [33].

20 Index

   Please see the PostScript version of this RFC for the INDEX.

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21.  Full Copyright Statement

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

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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