Internet DRAFT - draft-gettys-http-v11-spec-rev

draft-gettys-http-v11-spec-rev




   INTERNET-DRAFT                                           R. Fielding 
   <draft-gettys-http-v11-spec-rev-00>                     Day Software 
   Obsoletes: 2616                                            J. Gettys 
   Category: Standards Track                                J. C. Mogul 
   Expires: June 2004                                                HP 
                                                             H. Frystyk               
                                                              Microsoft 
                                                            L. Masinter 
                                                                  Adobe 
                                                               P. Leach 
                                                              Microsoft 
                                                         T. Berners-Lee 
                                                                W3C/MIT 
                                                         December, 2003 

    
                  Hypertext Transfer Protocol -- HTTP/1.1 

Status of this Memo 

   This document is an Internet-Draft and is in full conformance with 
   all provisions of Section 10 of RFC2026. 

   Internet-Drafts are working documents of the Internet Engineering 
   Task Force (IETF), its areas, and its working groups. Note that other 
   groups may also distribute working documents as Internet-Drafts. 

   Internet-Drafts are draft documents valid for a maximum of six months 
   and may be updated, replaced, or made obsolete by other documents at 
   any time. It is inappropriate to use Internet-Drafts as reference 
   material or to cite them other than as "work in progress." 

   Comments are welcome should be submitted to the mailing list ietf-
   http-wg@w3.org 

   The list of current Internet-Drafts can be accessed at 
   http://www.ietf.org/ietf/1id-abstracts.txt 

   The list of Internet-Draft Shadow Directories can be accessed at 
   http://www.ietf.org/shadow.html. 

Copyright Notice 

   Copyright (C) The Internet Society (2003). All Rights Reserved. See 
   section 20 for the full copyright notice. 

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 [I36]. A feature of HTTP is 
   the typing and negotiation of data representation, allowing systems 
   to be built independently of the data being transferred. 




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   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 obsoletes RFC 2616 [I39], which 
   obsoleted RFC 2068 [I25]. 




















































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Table of Contents 

   HYPERTEXT TRANSFER PROTOCOL -- HTTP/1.1                          1 
   Status of this Memo..............................................1 
   Copyright Notice.................................................1 
   Abstract.........................................................1 
   Table of Contents................................................3 
   1  Introduction..................................................8 
      1.1 Purpose...................................................8 
      1.2 Requirements..............................................8 
      1.3 Terminology...............................................9 
      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......................................18 
         3.2.2 http URL............................................18 
         3.2.3 URI Comparison......................................19 
      3.3 Date/Time Formats........................................19 
         3.3.1 Full Date...........................................19 
         3.3.2 Delta Seconds.......................................20 
      3.4 Character Sets...........................................20 
      3.5 Content Codings..........................................22 
      3.6 Transfer Codings.........................................23 
         3.6.1 Chunked Transfer Coding.............................23 
      3.7 Media Types..............................................25 
         3.7.1 Canonicalization and Text Defaults..................25 
         3.7.2 Multipart Types.....................................26 
      3.8 Product Tokens...........................................26 
      3.9 Quality Values...........................................27 
      3.10 Language Tags...........................................27 
      3.11 Entity Tags.............................................28 
      3.12 Range Units.............................................28 
   4  HTTP Message.................................................29 
      4.1 Message Types............................................29 
      4.2 Message Headers..........................................29 
      4.3 Message Body.............................................30 
      4.4 Message Length...........................................31 
      4.5 General Header Fields....................................32 
   5  Request......................................................33 
      5.1 Request-Line.............................................33 
         5.1.1 Method..............................................33 
         5.1.2 Request-URI.........................................33 
      5.2 The Resource Identified by a Request.....................35 
      5.3 Request Header Fields....................................35 
   6  Response.....................................................36 
      6.1 Status-Line..............................................36 
         6.1.1 Status Code and Reason Phrase.......................36 
      6.2 Response Header Fields...................................38 



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   7  Entity.......................................................38 
      7.1 Entity Header Fields.....................................39 
      7.2 Entity Body..............................................39 
         7.2.1 Type................................................39 
         7.2.2 Entity Length.......................................40 
   8  Connections..................................................40 
      8.1 Persistent Connections...................................40 
         8.1.1 Purpose.............................................40 
         8.1.2 Overall Operation...................................41 
         8.1.3 Proxy Servers.......................................42 
         8.1.4 Practical Considerations............................42 
      8.2 Message Transmission Requirements........................43 
         8.2.1 Persistent Connections and Flow Control.............43 
         8.2.2 Monitoring Connections for Error Status Messages....43 
         8.2.3 Use of the 100 (Continue) Status....................44 
         8.2.4 Client Behavior if Server Prematurely Closes Connection
               45 
   9  Method Definitions...........................................46 
      9.1 Safe and Idempotent Methods..............................46 
         9.1.1 Safe Methods........................................46 
         9.1.2 Idempotent Methods..................................47 
      9.2 OPTIONS..................................................47 
      9.3 GET......................................................48 
      9.4 HEAD.....................................................49 
      9.5 POST.....................................................49 
      9.6 PUT......................................................50 
      9.7 DELETE...................................................51 
      9.8 TRACE....................................................51 
      9.9 CONNECT..................................................52 
   10    Status Code Definitions...................................52 
      10.1 Informational 1xx.......................................52 
         10.1.1 100 Continue.......................................52 
         10.1.2 101 Switching Protocols............................52 
      10.2 Successful 2xx..........................................53 
         10.2.1 200 OK.............................................53 
         10.2.2 201 Created........................................53 
         10.2.3 202 Accepted.......................................53 
         10.2.4 203 Non-Authoritative Information..................54 
         10.2.5 204 No Content.....................................54 
         10.2.6 205 Reset Content..................................54 
         10.2.7 206 Partial Content................................55 
      10.3 Redirection 3xx.........................................55 
         10.3.1 300 Multiple Choices...............................56 
         10.3.2 301 Moved Permanently..............................56 
         10.3.3 302 Found..........................................56 
         10.3.4 303 See Other......................................57 
         10.3.5 304 Not Modified...................................57 
         10.3.6 305 Use Proxy......................................58 
         10.3.7 306 (Unused).......................................58 
         10.3.8 307 Temporary Redirect.............................58 
      10.4 Client Error 4xx........................................59 
         10.4.1 400 Bad Request....................................59 
         10.4.2 401 Unauthorized...................................59 
         10.4.3 402 Payment Required...............................60 
         10.4.4 403 Forbidden......................................60 


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         10.4.5 404 Not Found......................................60 
         10.4.6 405 Method Not Allowed.............................60 
         10.4.7 406 Not Acceptable.................................60 
         10.4.8 407 Proxy Authentication Required..................61 
         10.4.9 408 Request Timeout................................61 
         10.4.10 409 Conflict......................................61 
         10.4.11 410 Gone..........................................61 
         10.4.12 411 Length Required...............................62 
         10.4.13 412 Precondition Failed...........................62 
         10.4.14 413 Request Entity Too Large......................62 
         10.4.15 414 Request-URI Too Long..........................62 
         10.4.16 415 Unsupported Media Type........................63 
         10.4.17 416 Requested Range Not Satisfiable...............63 
         10.4.18 417 Expectation Failed............................63 
      10.5 Server Error 5xx........................................63 
         10.5.1 500 Internal Server Error..........................63 
         10.5.2 501 Not Implemented................................63 
         10.5.3 502 Bad Gateway....................................64 
         10.5.4 503 Service Unavailable............................64 
         10.5.5 504 Gateway Timeout................................64 
         10.5.6 505 HTTP Version Not Supported.....................64 
   11    Access Authentication.....................................64 
   12    Content Negotiation.......................................65 
      12.1 Server-driven Negotiation...............................65 
      12.2 Agent-driven Negotiation................................66 
      12.3 Transparent Negotiation.................................67 
   13    Caching in HTTP...........................................67 
         13.1.1 Cache Correctness..................................68 
         13.1.2 Warnings...........................................69 
         13.1.3 Cache-control Mechanisms...........................70 
         13.1.4 Explicit User Agent Warnings.......................70 
         13.1.5 Exceptions to the Rules and Warnings...............71 
         13.1.6 Client-controlled Behavior.........................71 
      13.2 Expiration Model........................................72 
         13.2.1 Server-Specified Expiration........................72 
         13.2.2 Heuristic Expiration...............................72 
         13.2.3 Age Calculations...................................73 
         13.2.4 Expiration Calculations............................75 
         13.2.5 Disambiguating Expiration Values...................75 
         13.2.6 Disambiguating Multiple Responses..................76 
      13.3 Validation Model........................................76 
         13.3.1 Last-Modified Dates................................77 
         13.3.2 Entity Tag Cache Validators........................77 
         13.3.3 Weak and Strong Validators.........................78 
         13.3.4 Rules for When to Use Entity Tags and Last-Modified 
         Dates  80 
         13.3.5 Non-validating Conditionals........................81 
      13.4 Response Cacheability...................................82 
      13.5 Constructing Responses From Caches......................82 
         13.5.1 End-to-end and Hop-by-hop Headers..................83 
         13.5.2 Non-modifiable Headers.............................83 
         13.5.3 Combining Headers..................................84 
         13.5.4 Combining Byte Ranges..............................85 
      13.6 Caching Negotiated Responses............................85 
      13.7 Shared and Non-Shared Caches............................87 


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      13.8 Errors or Incomplete Response Cache Behavior............87 
      13.9 Side Effects of GET and HEAD............................87 
      13.10 Invalidation After Updates or Deletions................88 
      13.11 Write-Through Mandatory................................88 
      13.12 Cache Replacement......................................89 
      13.13 History Lists..........................................89 
   14    Header Field Definitions..................................89 
      14.1 Accept..................................................90 
      14.2 Accept-Charset..........................................91 
      14.3 Accept-Encoding.........................................92 
      14.4 Accept-Language.........................................93 
      14.5 Accept-Ranges...........................................94 
      14.6 Age.....................................................95 
      14.7 Allow...................................................95 
      14.8 Authorization...........................................96 
      14.9 Cache-Control...........................................96 
         14.9.1 What is Cacheable..................................98 
         14.9.2 What May be Stored by Caches.......................99 
         14.9.3 Modifications of the Basic Expiration Mechanism....99 
         14.9.4 Cache Revalidation and Reload Controls............101 
         14.9.5 No-Transform Directive............................103 
         14.9.6 Cache Control Extensions..........................104 
      14.10 Connection............................................104 
      14.11 Content-Encoding......................................105 
      14.12 Content-Language......................................106 
      14.13 Content-Length........................................107 
      14.14 Content-Location......................................107 
      14.15 Content-MD5...........................................108 
      14.16 Content-Range.........................................109 
      14.17 Content-Type..........................................111 
      14.18 Date..................................................111 
         14.18.1 Clockless Origin Server Operation................112 
      14.19 ETag..................................................112 
      14.20 Expect................................................113 
      14.21 Expires...............................................113 
      14.22 From..................................................114 
      14.23 Host..................................................115 
      14.24 If-Match..............................................115 
      14.25 If-Modified-Since.....................................116 
      14.26 If-None-Match.........................................118 
      14.27 If-Range..............................................119 
      14.28 If-Unmodified-Since...................................119 
      14.29 Last-Modified.........................................120 
      14.30 Location..............................................120 
      14.31 Max-Forwards..........................................121 
      14.32 Pragma................................................122 
      14.33 Proxy-Authenticate....................................122 
      14.34 Proxy-Authorization...................................123 
      14.35 Range.................................................123 
         14.35.1 Byte Ranges......................................123 
         14.35.2 Range Retrieval Requests.........................125 
      14.36 Referer...............................................125 
      14.37 Retry-After...........................................126 
      14.38 Server................................................126 
      14.39 TE....................................................126 


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      14.40 Trailer...............................................128 
      14.41 Transfer-Encoding.....................................128 
      14.42 Upgrade...............................................128 
      14.43 User-Agent............................................129 
      14.44 Vary..................................................130 
      14.45 Via...................................................131 
      14.46 Warning...............................................132 
      14.47 WWW-Authenticate......................................134 
   15    Security Considerations..................................134 
      15.1 Personal Information...................................135 
         15.1.1 Abuse of Server Log Information...................135 
         15.1.2 Transfer of Sensitive Information.................135 
         15.1.3 Encoding Sensitive Information in URI's...........136 
         15.1.4 Privacy Issues Connected to Accept Headers........136 
      15.2 Attacks Based On File and Path Names...................137 
      15.3 DNS Spoofing...........................................137 
      15.4 Location Headers and Spoofing..........................138 
      15.5 Content-Disposition Issues.............................138 
      15.6 Authentication Credentials and Idle Clients............138 
      15.7 Proxies and Caching....................................139 
         15.7.1 Denial of Service Attacks on Proxies..............139 
   16    Acknowledgments..........................................140 
   17    Appendices...............................................141 
      17.1 IANA Considerations - Internet Media Type message/http and 
      application/http............................................141 
      17.2 IANA Considerations - Internet Media Type 
      multipart/byteranges........................................142 
      17.3 Tolerant Applications..................................143 
      17.4 Differences Between HTTP Entities and RFC 2045 Entities143 
         17.4.1 MIME-Version......................................144 
         17.4.2 Conversion to Canonical Form......................144 
         17.4.3 Conversion of Date Formats........................145 
         17.4.4 Introduction of Content-Encoding..................145 
         17.4.5 No Content-Transfer-Encoding......................145 
         17.4.6 Introduction of Transfer-Encoding.................145 
         17.4.7 MHTML and Line Length Limitations.................146 
      17.5 Additional Features....................................146 
         17.5.1 Content-Disposition...............................146 
      17.6 Compatibility with Previous Versions...................147 
         17.6.1 Changes from HTTP/1.0.............................147 
         17.6.2 Compatibility with HTTP/1.0 Persistent Connections148 
         17.6.3 Changes from RFC 2616.............................149 
   18    References...............................................150 
      18.1 Normative References...................................150 
      18.2 Informative References.................................151 
   19    Authors' Addresses.......................................154 
   20    Full Copyright Statement.................................156 
      20.1 Acknowledgement........................................156 
   21    Index....................................................157 

    






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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 [I6], 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 [I36]. It builds on the discipline of reference 
   provided by the Uniform Resource Identifier (URI) [I3], [N9], as a 
   location (URL) [I4] or name (URN) [I15], for indicating the resource 
   to which a method is to be applied. Messages are passed in a format 
   similar to that used by Internet mail [I9] as defined by the 
   Multipurpose Internet Mail Extensions (MIME) [N1]. 

   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 [I12], NNTP [I10], FTP [I13], Gopher 
   [I2], and WAIS [I7] 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 [N34].  
   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." 




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1.3 Terminology  

   This specification uses a number of terms to refer to the roles 
   played by participants in, and objects of, the HTTP communication. 


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

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

request 
  An HTTP request message, as defined in section 5. 

response 
  An HTTP response message, as defined in section 6. 

resource 
  A network data object or service that can be identified by a URI, as 

  defined in section 3.2. Resources may be available in multiple 
  representations (e.g. multiple languages, data formats, size, and 
  resolutions) or vary in other ways. 

entity 
  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 "variant." Use of the term "variant" does 
  not necessarily imply that the resource is subject to content 
  negotiation. 





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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 programÆs 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. 

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 


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  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.  

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.  

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

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

1.4 Overall Operation  

   The HTTP protocol is a request/response protocol. A client sends a 
   request to the server in the form of a request method, URI, and 
   protocol version, followed by a MIME-like message containing request 
   modifiers, client information, and possible body content over a 
   connection with a server. The server responds with a status line, 
   including the message's protocol version and a success or error code, 
   followed by a MIME-like message containing server information, entity 
   metainformation, and possible entity-body content. The relationship 
   between HTTP and MIME is described in appendix 17.4. 

   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 
   may apply only to the connection with the nearest, non-tunnel 
   neighbor, only to the end-points of the chain, or to all connections 


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   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 [I14], but other ports can be used. This does 
   not preclude HTTP from being implemented on top of any other protocol 
   on the Internet, or on other networks. HTTP only presumes a reliable 
   transport; any protocol that provides such guarantees can be used; 
   the mapping of the HTTP/1.1 request and response structures onto the 
   transport data units of the protocol in question is outside the scope 
   of this specification. 

   In HTTP/1.0, most implementations used a new connection for each 
   request/response exchange. In HTTP/1.1, a connection may be used for 
   one or more request/response exchanges, although connections may be 
   closed for a variety of reasons (see section 8.1). 






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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 [I9]. 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)". 

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 

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  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 [N6]. 

          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 17.3 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. 


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          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 [N7] only when encoded according to the rules of RFC 2047 
   [N14]. 

          TEXT           = <any OCTET except CTLs, 
                           but including LWS> 
    
   A CRLF is allowed in the definition of TEXT only as part of a header 
   field continuation. It is expected that the folding LWS will be 
   replaced with a single SP before interpretation of the TEXT value. 


   Hexadecimal numeric characters are used in several protocol elements. 

          HEX            = "A" | "B" | "C" | "D" | "E" | "F" 
                         | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT 
    
   Many HTTP/1.1 header field values consist of words separated by LWS 
   or special characters. These special characters MUST be in a quoted 
   string to be used within a parameter value (as defined in section 
   3.6). 

          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

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   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 [I28] for a fuller explanation. 
   The version of an HTTP message is indicated by an HTTP-Version field 
   in the first line of the message. HTTP-Version is case-sensitive. 

          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 [I28]. 

   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.  


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   Due to interoperability problems with HTTP/1.0 proxies discovered 
   since the publication of RFC 2068[I25], 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 [I3], [N9], and finally 
   the combination of Uniform Resource Locators (URL) [I4] and Names 
   (URN) [I15]. As far as HTTP is concerned, Uniform Resource 
   Identifiers are simply formatted strings which identify--via name, 
   location, or any other characteristic--a resource. 

3.2.1 General Syntax  

   URIs in HTTP can be represented in absolute form or relative to some 
   known base URI [I8], 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 [N9] (which replaces RFCs 
   1738 [I4] and RFC 1808 [I8]). 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 



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   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 [I17]). If 
   the abs_path is not present in the URL, it MUST be given as "/" when 
   used as a Request-URI for a resource (section 5.1.2). If a proxy 
   receives a host name which is not a fully qualified domain name, it 
   MAY add its domain to the host name it received. If a proxy receives 
   a fully qualified domain name, the proxy MUST NOT change the host 
   name. 


3.2.3 URI Comparison 

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

     o A port that is empty or not given is equivalent to the default port 
     for that URI-reference; 
     o Comparisons of host names MUST be case-insensitive;  
     o Comparisons of scheme names MUST be case-insensitive; 
     o An empty abs_path is equivalent to an abs_path of "/". 
   
   Characters other than those in the "reserved" set (see RFC 2396 [N9]) 
   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 [N2] (an update to 
   RFC 822 [I9]). The second format is in common use, but is based on 
   the obsolete RFC 850 [I9] 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 17.3 for further information. 



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  Note: Recipients of date values are encouraged to be robust in 
  accepting date values that may have been sent by non-HTTP 
  applications, as is sometimes the case when retrieving or posting 
  messages via proxies/gateways to SMTP or NNTP.  

   All HTTP date/time stamps MUST be represented in Greenwich Mean Time 
   (GMT), without exception. For the purposes of HTTP, GMT is exactly 
   equal to UTC (Coordinated Universal Time). This is indicated in the 
   first two formats by the inclusion of "GMT" as the three-letter 
   abbreviation for time zone, and MUST be assumed when reading the 
   asctime format. HTTP-date is case sensitive and MUST NOT include 
   additional LWS beyond that specifically included as SP in the 
   grammar. 

          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: 

  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 

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  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-2022Æs 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 
   [I14].  

          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 MUST represent the character set defined by 
   that registry. Applications SHOULD limit their use of character sets 
   to those defined by the IANA registry. 

   HTTP uses charset in two contexts: within an Accept-Charset request 
   header (in which the charset value is an unquoted token) and as the 
   value of a parameter in a Content-Type header (within a request or 
   response), in which case the parameter value of the charset parameter 
   may be quoted. 

   Implementors should be aware of IETF character set requirements [I30] 
   [I32]. 


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



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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 [I18]. 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 [I24] in combination with the 
     "deflate" compression mechanism described in RFC 1951 [I22]. 

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. New 
   registrations are reviewed and approved by the IESG according to 
   these criteria. 

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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 [I7], 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. 

   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), "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, 


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   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-data in octets. 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). 

   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 17.4.6. 


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   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 [N5] 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. 

   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 [I14]). The media type registration process is 
   outlined in RFC 2048 [N5]. 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 


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   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 0 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 
   [N8], 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 17.2) 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 2388 [I11]. 

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 


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   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 
    
   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 3066 [I1]. 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*8(ALPHA / DIGIT) 
    

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   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 

   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 [I9] 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 [I9]. Each header field consists 
   of a name followed by a colon (":") and the field value. Field names 
   are case-insensitive. The field value MAY be preceded by any amount 
   of LWS, though a single SP is preferred. Header fields can be 
   extended over multiple lines by preceding each extra line with at 
   least one SP or HT. Applications ought to follow "common form", where 
   one is known or indicated, when generating HTTP constructs, since 
   there might exist some implementations that fail to accept anything 

   beyond the common forms. 

          message-header = field-name ":" [ field-value ] 
          field-name     = token 
          field-value    = *( field-content | LWS ) 

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          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. 

   All HTTP header field-names are registered according to the procedure 
   in [I40]. 


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 
   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. 


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   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 requestÆs 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 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  
     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 
     transfer-length is not otherwise specified, then this self-
     delimiting media type defines the transfer-length. This media type 
     MUST NOT be used unless the sender knows that the recipient can 
     parse it; the presence in a request of a Range header with multiple 
     byte-range specifiers from a 1.1 client implies that the client can 
     parse multipart/byteranges responses. 


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  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 
   transfer-coding. If the message does include a non-identity transfer-
   coding, the Content-Length MUST be ignored.  

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

4.5 General Header Fields  

   There are a few header fields which have general applicability for 
   both request and response messages, but which do not apply to the 
   entity being transferred. These header fields apply only to the 
   message being transmitted. 

          general-header = Cache-Control            ; 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. 

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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 

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. 

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          Request-URI    = "*" | absoluteURI  
                        | abs_path ["?" query ]| authority 
    
   The four options for Request-URI are dependent on the nature of the 
   request. The asterisk "*" means that the request does not apply to a 
   particular resource, but to the server itself, and is only allowed 
   when the method used does not necessarily apply to a resource. One 
   example would be 

          OPTIONS * HTTP/1.1 
    
   The absoluteURI form is REQUIRED when the request is being made to a 
   proxy. The proxy is requested to forward the request or service it 
   from a valid cache, and return the response. Note that the proxy MAY 
   forward the request on to another proxy or directly to the server 
   specified by the absoluteURI. In order to avoid request loops, a 
   proxy MUST be able to recognize all of its server names, including 
   any aliases, local variations, and the numeric IP address. An example 
   Request-Line would be: 

          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 
   [N9], 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 "/". 


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


5.2 The Resource Identified by a Request 

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

   An origin server that does not allow resources to differ by the 
   requested host MAY ignore the Host header field value when 
   determining the resource identified by an HTTP/1.1 request. (But see 
   section 17.6.1.1 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 


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                         | If-Match                 ; Section 14.24 
                         | 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. 

   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: 


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      o 1xx: Informational - Request received, continuing process 
      o 2xx: Success - The action was successfully received, understood, 
        and accepted 
      o 3xx: Redirection - Further action must be taken in order to 
        complete the request 
      o 4xx: Client Error - The request contains bad syntax or cannot be 
        fulfilled 
      o 5xx: Server Error - The server failed to fulfill an apparently 
        valid request  
   The individual values of the numeric status codes defined for 
   HTTP/1.1, and an example set of corresponding Reason-Phrase's, are 
   presented below. The reason phrases listed here are only 
   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 
             | "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 


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             | "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 
                          | 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.  


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   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. 

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.  

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   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. 

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 [I19] [I23]. Implementation experience 
   and measurements of actual HTTP/1.1 (RFC 2068) implementations show 
   good results [I31]. Alternatives have also been explored, for 
   example, T/TCP [I20]. 

   Persistent HTTP connections have a number of advantages: 

     o By opening and closing fewer TCP connections, CPU time is saved in 
       routers and hosts (clients, servers, proxies, gateways, tunnels, or 
       caches), and memory used for TCP protocol control blocks can be 
       saved in hosts. 
     o HTTP requests and responses can be pipelined on a connection. 
       Pipelining allows a client to make multiple requests without 
       waiting for each response, allowing a single TCP connection to be 
       used much more efficiently, with much lower elapsed time. 
     o Network congestion is reduced by reducing the number of packets 
       caused by TCP opens, and by allowing TCP sufficient time to 
       determine the congestion state of the network. 
     o Latency on subsequent requests is reduced since there is no time 
       spent in TCPÆs connection opening handshake. 
     o HTTP can evolve more gracefully, since errors can be reported 
       without the penalty of closing the TCP connection. Clients using 
       future versions of HTTP might optimistically try a new feature, but 


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     if communicating with an older server, retry with old semantics 
     after an error is reported.  
   
   HTTP implementations SHOULD implement persistent connections. 


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 17.6.2 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. 


8.1.2.2 Pipelining 



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   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 [I25] 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. 

   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. 



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   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 serverÆs 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. 

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. 




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

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

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




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

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

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

   Requirements for HTTP/1.1 proxies: 

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

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

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

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



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  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. 

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

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

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". 


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   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. 

   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" 


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   type of method. For example, this can be used to test a proxy for 
   OPTIONS method support (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 
   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. 



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   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. 


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 data to be processed by the 
   resource identified by the Request-URI in the Request-Line. POST is 
   designed to allow a uniform method to cover the following functions: 

     o Annotation of existing resources;  
     o Posting a message to a bulletin board, newsgroup, mailing list, or 
       similar group of articles; 
     o Providing a block of data, such as the result of submitting a form, 
       to a data-handling process; 
     o Extending a database through an append operation.  
   
   The actual function performed by the POST method is determined by the 
   server and is usually dependent on the Request-URI.  

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

   If a resource has been created on the origin server, the response 
   SHOULD be 201 (Created) and contain an entity which describes the 


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   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, 
   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. 

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   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 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.  



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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 [I33]). 

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 
   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 responseÆs 
   Upgrade header field immediately after the empty line which 
   terminates the 101 response. 

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   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;  

   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 


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   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 requestÆs 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). 


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 agentÆs active document view, although 
   any new or updated metainformation SHOULD be applied to the document 
   currently in the user agentÆs 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. 




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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: 

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



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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 
   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 method 
   that is known to be "safe", as defined in section 9.1.1, then the 
   request MAY be automatically redirected by the user agent without 
   confirmation.  Otherwise, 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 



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   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). 

   If the 302 status code is received in response to a request  method 
   that is known to be "safe", as defined in section 9.1.1, then the 
   request MAY be automatically redirected by the user agent without 
   confirmation.  Otherwise, 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: 


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     o Date, unless its omission is required by section 14.18.1 
       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. 
     o ETag and/or Content-Location, if the header would have been sent in 
     a 200 response to the same request 
     o 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. 


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 


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   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 method 
   that is known to be "safe", as defined in section 9.1.1, then the 
   request MAY be automatically redirected by the user agent without 
   confirmation.  Otherwise, 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. 


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" [N10]. 



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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. 


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 


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  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" 
   [N10]. 


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 
   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 canÆt 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 

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   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 serverÆs 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. 



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. 


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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. 


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. 

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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.  


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" [N10]. This 
   specification adopts the definitions of "challenge" and "credentials" 
   from that specification. 



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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. 

   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 



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     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.  

  4. It may limit a public cacheÆs ability to use the same response for 
     multiple userÆs 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 

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   mechanism from being developed as an extension and used within 
   HTTP/1.1. 

   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, 
   and clients to explicitly reduce transparency when necessary. 
   However, because non-transparent operation may confuse non-expert 

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   users, and might be incompatible with certain server applications 
   (such as those for ordering merchandise), the protocol requires that 
   transparency be relaxed 

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

  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 


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     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:  

   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. 



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   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, 
   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. 



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   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 serverÆs 
   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. 

   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 

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   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. 

   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 


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   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 [I21] or some similar protocol to synchronize their clocks to 
   a globally accurate time standard. 

   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. 

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   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 valueÆs 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 
   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 


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   the lack of an Age header field in a response does not imply that the 
   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) 

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. 



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   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.  

   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 clientÆs request, it first has to check with the origin 

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   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. 

  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. 

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   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.  
  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. 



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   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: 

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

     o The validator is being compared by an origin server to the actual 
       current validator for the entity and, 
     o That origin server reliably knows that the associated entity did 
       not change twice during the second covered by the presented 
       validator.  
   or 
     o 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 
     o That cache entry includes a Date value, which gives the time when 
       the origin server sent the original response, and 
     o The presented Last-Modified time is at least 60 seconds before the 
       Date value.  
   or 
     o The validator is being compared by an intermediate cache to the 
       validator stored in its cache entry for the entity, and 
     o That cache entry includes a Date value, which gives the time when 
       the origin server sent the original response, and 
     o 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. 



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   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 
   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: 

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

     o 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). 

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     o 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). 
     o 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. 
     o 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). 


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. 



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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: 

     o 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. 
     o 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: 

     o Connection 
     o Keep-Alive 
     o Proxy-Authenticate  
     o Proxy-Authorization 
     o TE 
     o Trailer 
     o Transfer-Encoding 
     o 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. 
   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: 

     o Content-Location 
     o Content-MD5 
     o ETag 
     o Last-Modified  
    

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

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     o 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: 

     o Content-Encoding 
     o Content-Range 
     o 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). 


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 
     o any stored Warning headers with warn-code 1xx (see section 14.46) 
       MUST be deleted from the cache entry and the forwarded response. 
     o any stored Warning headers with warn-code 2xx MUST be retained in 
       the cache entry and the forwarded response.  



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     o 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. 


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: 

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


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   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 
   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 

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   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 
   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 serverÆs 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. 



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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. 

   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: 

     o PUT 
     o DELETE 
     o POST 
   
   An invalidation based on the URI in a Location or Content-Location 
   header MUST NOT be performed if the host part of that URI differs 
   from the host part in the Request-URI. This helps prevent denial of 
   service attacks. 

   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. 

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. 



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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". 

   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, 

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                  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 
          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 ] ) 
    



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   Character set values are described in section 3.4. Each charset MAY 
   be given an associated quality value which represents the userÆs 
   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" ":"  
                             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 

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     "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., "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, 


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   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 
   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. 

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   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. 

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" [N10]. 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.  

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 



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   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, 
						  ; Section 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 ) ] 
    
   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: 

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


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     o Restrictions on what may be stored by a cache; these may be imposed 
       by either the origin server or the user agent. 
     o Modifications of the basic expiration mechanism; these may be 
       imposed by either the origin server or the user agent. 
     o Controls over cache revalidation and reload; these may only be 
       imposed by a user agent. 
     o Control over transformation of entities. 
     o 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 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.  

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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. 

  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 


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   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. 

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. 


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   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). 

   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 clientÆs 
  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 conditional; the first 
  cache along the path (if any) that holds a cache entry for this 

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  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 clientÆs request, using the 
  strong comparison function. If the clientÆs validator is equal to the 
  origin serverÆs, 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 serverÆs 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 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 serverÆs 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 

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  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 userÆs 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 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. 



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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. 

   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 responseÆs 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 
    


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   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 æpersistentÆ (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 request message. 

   An HTTP/1.1 server that does not support persistent connections MUST 
   include the "close" connection option in every response message that 
   does not have a 1xx (informational) status code. 

   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 
   17.6.2. 

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 

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   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 
    
   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". 


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   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.  

   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-



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   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. 

14.15 Content-MD5 

   The Content-MD5 entity-header field, as defined in RFC 1864 [I16], 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-

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   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. 

  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. 

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   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 
   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: 

     o The first 500 bytes: 
          bytes 0-499/1234 
     o The second 500 bytes: 
          bytes 500-999/1234 
     o All except for the first 500 bytes: 
          bytes 500-1233/1234 
     o 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 17.2. See appendix 17.6.3 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. 

   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).  


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   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 [N2]-date format. 

          Date  = "Date" ":" HTTP-date 
    
   An example is 

          Date: Tue, 15 Nov 1994 08:12:31 GMT 
    
   Origin servers MUST include a Date header field in all responses, 
   except in these cases: 

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

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

   3. If the server does not have a clock that can provide a reasonable 
      approximation of the current time, its responses MUST NOT include a 

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     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 [I21], 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). 

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: "" 



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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. 

   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: 


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         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.) 

   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 [N3] as updated by RFC 1123 [N2]: 

          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 issuerÆs address SHOULD be 
   used. 

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   The client SHOULD NOT send the From header field without the userÆs 
   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, 
   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 17.6.1.1 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 ) 

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   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 
   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 
    
   An example of the field is: 

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          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 
  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. 


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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. 

   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: 


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          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 æif 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 ) 
    
   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). 


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         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 
    
   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) 

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   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 [ "#" fragment ]  
    
   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. 

There are circumstances in which a fragment identifier in a Location 
URL would not be appropriate: 

     o With a 201 Created response, because in this usage the Location 
     header specifies the URL for the entire created resource. 

     o With a 300 Multiple Choices, since the choice decision is 
     intended to be made on resource characteristics and not fragment 

     characteristics. 

     o With 305 Use Proxy.  

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). 




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   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. 

   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" [N10]. 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 


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   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" [N10] . 
   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 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. 


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   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. 

          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): 

     o The first 500 bytes (byte offsets 0-499, inclusive): 
          bytes=0-499 
     o The second 500 bytes (byte offsets 500-999, inclusive): 
          bytes=500-999 
     o The final 500 bytes (byte offsets 9500-9999, inclusive): 
          bytes=-500 
     o Or 
          bytes=9500- 
     o The first and last bytes only (bytes 0 and 9999): 
          bytes=0-0,-1 
     o 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 




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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 
    
   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: 

     o 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). 
     o 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 
   MUST 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.  


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 

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   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.") 

   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. 

    


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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: 

     o Transfer-Encoding 
     o Content-Length 
     o 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 
    
   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 



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   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. 

   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) 

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   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 
   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. 




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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. 

   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 


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   explicitly enabled. If not enabled, the received-by host of any host 
   behind the firewall SHOULD be replaced by an appropriate pseudonym 
   for that host. 

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

          Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy 
        could be collapsed to 

          Via: 1.0 ricky, 1.1 mertz, 1.0 lucy 
    
   Applications SHOULD NOT combine multiple entries unless they are all 
   under the same organizational control and the hosts have already been 
   replaced by pseudonyms. Applications MUST NOT combine entries which 
   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 [N4]. 



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   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 
   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: 

     o Warnings that appear early in the response take priority over those 
       appearing later in the response. 
     o 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'ss 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 

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  take any automated action, besides presenting the warning to the 
  user. 

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" [N10]. 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 
   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 

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   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 URIÆs 

   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 
   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 



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   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. 


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 


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   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 [I27], from which the often implemented Content-Disposition 
   (see section 17.5.1) 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 [I38] 
   (which updates RFC 1806) for details. 


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. 

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   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). 

   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. 


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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: 

          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. 


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   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. 

   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 Appendices  


17.1 IANA Considerations - 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 [N5]. 

          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 


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                    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 

17.2 IANA Considerations - 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 [N8] 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. 


17.3 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 0. 

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

     o HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date 
       which appears to be more than 50 years in the future is in fact in 
       the past (this helps solve the "year 2000" problem). 
     o An HTTP/1.1 implementation MAY internally represent a parsed 
       Expires date as earlier than the proper value, but MUST NOT 
       internally represent a parsed Expires date as later than the proper 
       value. 
     o All expiration-related calculations MUST be done in GMT. The local 
       time zone MUST NOT influence the calculation or comparison of an 
       age or expiration time. 
     o If an HTTP header incorrectly carries a date value with a time zone 
       other than GMT, it MUST be converted into GMT using the most 
       conservative possible conversion. 

17.4 Differences Between HTTP Entities and RFC 2045 Entities 

   HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC 
   822 [N3]) and the Multipurpose Internet Mail Extensions (MIME [7]) to 

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   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. 


17.4.1 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[N1]). 

   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. 


17.4.2 Conversion to Canonical Form  

   RFC 2045 [N1] requires that an Internet mail entity be converted to 
   canonical form prior to being transferred, as described in section 4 
   of RFC 2049 [I37]. 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 
   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. 


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   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. 


17.4.3 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. 


17.4.4 Introduction of Content-Encoding  

   RFC 2045 does not include any concept equivalent to HTTP/1.1Æs 
   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.) 


17.4.5 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 CTE 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 
   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. 


17.4.6 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 

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          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 

17.4.7 MHTML and Line Length Limitations 

   HTTP implementations which share code with MHTML [I34] 
   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.  


17.5 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 
   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 [I29]). 



17.5.1 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 [I27]. 

           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 

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           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. 


17.6 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: 
     o recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1 
     requests; 
     o understand any valid request in the format of HTTP/0.9, 1.0, or 
     1.1; 
     o respond appropriately with a message in the same major version used 
     by the client.  
   And we would expect HTTP/1.1 clients to: 
     o recognize the format of the Status-Line for HTTP/1.0 and 1.1 
     responses; 
     o understand any valid response in the format of HTTP/0.9, 1.0, or 
     1.1.  
   For most implementations of HTTP/1.0, each connection is established 
   by the client prior to the request and closed by the server after 
   sending the response. Some implementations implement the Keep-Alive 
   version of persistent connections described in section 19.7.1 of RFC 
   2068 [I25]. 


17.6.1 Changes from HTTP/1.0  

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


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17.6.1.1 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 
   important that all implementations of HTTP (including updates to 
   existing HTTP/1.0 applications) correctly implement these 
   requirements: 

     o Both clients and servers MUST support the Host request-header. 
     o A client that sends an HTTP/1.1 request MUST send a Host header. 
     o Servers MUST report a 400 (Bad Request) error if an HTTP/1.1 
       request does not include a Host request-header. 
     o Servers MUST accept absolute URIs. 

17.6.2 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 doesnÆt 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 



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   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. [I25] 



17.6.3 Changes from RFC 2616 

   Section 3.1: Clarify that HTTP-Version is case sensitive. 

   Section 3.2.3: Eliminate overlooked reference to "unsafe" characters. 

   Section 3.4: Clarify contexts that charset is used in. 

   Section 3.6: Remove reference to non-existant identity transfer-
   coding value tokens. 

   Section 3.6.1: Clarification that the chunk length does not include 

   the count of the octets in the chunk header and trailer. 

   Section 3.7: Fix reference to media type registration procedure. 
   Update reference for multipart/form-data (RFC 2388) 

   Section 3.10: Update reference from RFC 1766 to RFC 3066.  Fix BNF 
   inconsistency with that document. 

   Section 4.4: Remove reference to non-existant identity transfer-
   coding value tokens. 

   Section 5.1.2: Fix BNF to add query, as RFC 2396 doesn't define it. 


   Section 9.4: Clarify definition of POST. 

   Section 10.3.2, 10.3.3, 10.3.8: failed to consider that there are 
   many other request methods that are safe to automatically redirect, 
   and further that the user agent is able to make that determination 
   based on the request method semantics. In particular, the OPTIONS 
   method is always safe to automatically redirect. Unfortunately, the 
   paragraph was written long before there was OPTIONS, and was never 
   updated to reflect the extensibility of methods. The same problem 
   paragraph is found in sections 10.3.3 and 10.3.8. 

   Section 13.5.1: Fix misspelled header. 

   Section 13.10: Clarify denial of service attack avoidance 
   requirement. 

   Section 14.10: Clarify exactly when close connection options must be 
   sent. 

   Section 14.38: In the description of the Server header, the Via field 
   was described as a SHOULD. The requirement was and is stated 
   correctly in the description of the Via header, [section 14.45]. 


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   Section 14.31: Correct syntax of Location header to allow fragment, 
   as referred symbol wasnÆt what was expected, and add some 
   clarifications as to when it would not be appropriate. 

   Section 17: Update to RFC3066, BCP 47;  Update to RFC2388;  Update to 
   RFC3232;  add ISO-8859 û10 through û16, -12 doesnÆt exist, 
   apparently; Update to RFC 2617; Add RFC 2616 draft standard 
   reference. 

   Section 19.4.5: Remove reference to non-existant identity transfer-
   coding value tokens. 

18 References  


18.1 Normative References 

[N1] Freed, N., and N. Borenstein. "Multipurpose Internet Mail 
  Extensions (MIME) Part One: Format of Internet Message Bodies." RFC 
  2045, November 1996.  

[N2] Braden, R., "Requirements for Internet Hosts -- Communication 
  Layers," STD 3, RFC 1123, October 1989.  

[N3] D. H. Crocker, "Standard for The Format of ARPA Internet Text 
  Messages," STD 11, RFC 822, August 1982.  

[N4] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part 
  Three: Message Header Extensions for Non-ASCII Text", RFC 2047, 
  November 1996.  

[N5] Freed, N., Klensin, J., and Postel, J., "Mulitpurpose Internet Mail 
  Extensions (MIME) Part Four: Registration Procedure", RFC 2048, 
  November 1996.  

[N6] US-ASCII. Coded Character Set - 7-Bit American Standard Code for 

  Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.  

[N7] 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.  
  Part 10: Latin alphabet No. 6, ISO-8859-10, 1998. 
  Part 11: Latin/Thai alphabet, ISO-8859-11, 2001. 

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  Part 13: Latin alphabet No. 7, ISO-8859-13, 1998. 
  Part 14: Latin alphabet No. 8 (Celtic), ISO-8859-14, 1998. 
  Part 15: Latin alphabet No. 9, ISO-8859-15, 1999. 
  Part 16: Latin alphabet No. 10, ISO-8859-16, 2001. 

[N8] Freed, N., and N. Borenstein. "Multipurpose Internet Mail 
  Extensions (MIME) Part Two: Media Types." RFC 2046, November 1996.  


[N9] Berners-Lee, T., Fielding, R., and L. Masinter,"Uniform Resource 
  Identifiers (URI): Generic Syntax and Semantics," RFC 2396, August 
  1998. 

[N10] 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. 


18.2 Informative References 

[I1] Alvestrand, H., "Tags for the Identification of Languages", RFC 
  3066, BCP 47, January 2001.  

[I2] 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.  

[I3] Berners-Lee, T., "Universal Resource Identifiers in WWW," RFC 1630, 
  June 1994.  

[I4] Berners-Lee, T., Masinter, L., and M. McCahill. "Uniform Resource 
  Locators (URL)," RFC 1738, December 1994.  

[I5] Berners-Lee, T. and D. Connolly. "Hypertext Markup Language - 2.0," 
  RFC 1866, November 1995.  

[I6] Berners-Lee, T., Fielding, R. and H. Frystyk. "Hypertext Transfer 
  Protocol -- HTTP/1.0," RFC 1945, May 1996.  

[I7] 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.  

[I8] Fielding, R., "Relative Uniform Resource Locators," RFC 1808, June 
  1995.  

[I9] Horton, M., and R. Adams. "Standard for Interchange of USENET 
  Messages," RFC 1036, December 1987.  



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[I10] Kantor, B. and P. Lapsley. "Network News Transfer Protocol," RFC 
  977, February 1986.  

[I11] L. Masinter. " Returning Values from Forms:  multipart/form-data," 
  RFC 2388, August 1998.  

[I12] Postel, J., "Simple Mail Transfer Protocol," STD 10, RFC 821, 
  August 1982.  

[I13] Postel, J. and J. Reynolds. "File Transfer Protocol," STD 9, RFC 
  959, October 1985.  

[I14] Reynolds, J. "Assigned Numbers : RFC 1700 is Replaced by an On-
   line Database,"  RFC 3232, January 2002.  

[I15] Sollins, K. and L. Masinter. "Functional Requirements for Uniform 
  Resource Names," RFC 1737, December 1994.  

[I16] Meyers, J., and M. Rose. "The Content-MD5 Header Field," RFC 1864, 
  October 1995. 

[I17] Carpenter, B. and Y. Rekhter. "Renumbering Needs Work," RFC 1900, 
  February 1996. 

[I18] Deutsch, P., "GZIP file format specification version 4.3,." RFC 
  1952, May 1996. 

[I19] 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/HTTPLatency.
  html.  

[I20] 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. 

[I21] Mills, D., "Network Time Protocol (Version 3) Specification, 
  Implementation and Analysis." RFC 1305, March 1992. 

[I22] Deutsch, P., "DEFLATE Compressed Data Format Specification version 
  1.3." RFC 1951, May 1996. 

[I23] S. Spero, "Analysis of HTTP Performance Problems," 
  http://sunsite.unc.edu/mdma-release/http-prob.html. 


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[I24] Deutsch, P. and J. Gailly. "ZLIB Compressed Data Format 
  Specification version 3.3," RFC 1950, May 1996. 

[I25] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T. Berners-
  Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2068, January 
  1997. 

[I26] Bradner, S., "Key words for use in RFCs to Indicate Requirement 

  Levels," RFC 2119, March 1997. 

[I27] Troost, R., and Dorner, S., "Communicating Presentation 
  Information in Internet Messages: The Content-Disposition Header," 
  RFC 1806, June 1995. 

[I28] Mogul, J.C., Fielding, R., Gettys, J., and H. Frystyk,., "Use and 
  Interpretation of HTTP Version Numbers", RFC 2145, May 1997.  

[I29] Palme, J, "Common Internet Message Headers," RFC 2076, February 
  1997. 

[I30] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO-
  10646," RFC 2279, January 1998.  

[I31] Nielsen, H.F., Gettys, J., Baird-Smith, A., PrudÆhommeaux, 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. 

[I32] Alvestrand, H. T., "IETF Policy on Character Sets and Languages," 
  RFC 2277, BCP 18, January 1998.  

[I33] Luotonen, A., "Tunneling TCP based protocols through Web proxy 
  servers," Work in Progress.  

[I34] Palme, J., and A. Hopmann, "MIME E-mail Encapsulation of Aggregate 
  Documents, such as HTML (MHTML)," RFC 2110, March 1997 

[I35] Bradner, S., "The Internet Standards Process -- Revision 3," BCP 
  9, RFC 2026, Harvard University, October 1996. 

[I36] Masinter, L., "Hyper Text Coffee Pot Control Protocol 
  (HTCPCP/1.0)," RFC 2324, April 1998. 

[I37] Freed, N., and N. Borenstein, "Multipurpose Internet Mail 
  Extensions (MIME) Part Five: Conformance Criteria and Examples," RFC 
  2049, November 1996. 

[I38] 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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[I39] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T. Berners-
  Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June, 1999. 


[I40] Klyne, G., Nottingham, M. and Mogul, J., "Registration procedures 
  for message header fields", Work-in-progress. 

19 AuthorsÆ Addresses 

   Roy T. Fielding 
   Chief Scientist 
   Day Software 
   5251 California Avenue 
   Suite 110, 
   Irvine, CA 92612-3074, USA 
     
   EMail: roy.fielding@day.com 

   James Gettys 
   HP Labs, Cambridge Research Laboratory 
   Hewlett-Packard Company 
   One Cambridge Center 
   Cambride, MA 02138 
    
    EMail: Jim.Gettys@hp.com 

   Jeffrey C. Mogul 
   HP Labs, Large Scale Systems Group 
   Hewlett-Packard Company 
   1501 Page Mill Road, MS 1177 
   Palo Alto, California, 94304, USA 
    
   EMail: JeffMogul@acm.org 

   Henrik Frystyk Nielsen 
   Microsoft Corporation 
   1 Microsoft Way 
   Redmond, WA 98052, USA  
    
    EMail: henrikn@microsoft.com 

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

   EMail: LMM2acm.org 
   URI: http://larry.masinter.net 

   Paul J. Leach 
   Microsoft Corporation 
   1 Microsoft Way 
   Redmond, WA 98052, USA 
    
   EMail: paulle@microsoft.com 

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   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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20 Full Copyright Statement 

   Copyright (C) The Internet Society (2003). 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. 


20.1  Acknowledgement 

    Funding for the RFC Editor function is currently provided by the 
   Internet Society. 























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21 Index 

   While some care was taken producing this index, there is no guarantee 
   that all occurrences of an index term have been entered into the 
   index. Bold face italic is used for the definition of a term. 

"literal", 14                         414, 18, 36, 60 
#rule, 14                             415, 36, 61, 102 
(rule1 rule2), 14                     416, 36, 61, 106, 107, 120 
*rule, 14                             417, 36, 61, 109 
; comment, 14                         4xx Client Error Status Codes, 57 
[rule], 14                            500, 36, 61, 62, 108 
<">, 15                               501, 23, 32, 36, 48, 61 
100, 36, 42, 43, 44, 50, 51, 85,      502, 36, 62 
 108, 109                             503, 36, 62, 108, 122 
101, 36, 51, 108, 124                 504, 36, 62, 99 
1xx Informational Status Codes,       505, 36, 62 
 50                                   5xx Server Error Status Codes, 61 
200, 36, 46, 48, 49, 50, 51, 52,      abs_path, 18, 33 
 53, 56, 79, 84, 98, 107, 113,        absoluteURI, 18, 32, 33, 34, 104, 
 115, 121                              117, 121 
201, 36, 48, 52, 117                  Accept, 24, 34, 63, 64, 67, 87, 
202, 36, 49, 52                        89, 90, 132 
203, 36, 52, 79                       acceptable-ranges, 91 
204, 30, 36, 48, 49, 52, 53           Accept-Charset, 20, 34, 64, 88, 
205, 36, 53                            89 
206, 36, 53, 79, 81, 82, 84, 106,     Accept-Encoding, 21, 22, 34, 63, 
 115, 120, 121, 137                    64, 89, 90 
2xx, 114                              accept-extension, 87 
2xx Successful Status Codes, 51       Accept-Language, 26, 34, 63, 64, 
300, 36, 54, 64, 79                    90, 91, 128, 132 
301, 36, 49, 54, 55, 79, 125          accept-params, 87, 123 
302, 36, 55, 57, 79, 125              Accept-Ranges, 37, 91 
303, 36, 48, 55, 125                  Access Authentication, 62 
304, 30, 36, 56, 66, 67, 74, 78,       Basic and Digest. See [43] 
 81, 82, 83, 98, 113, 114, 121        Acknowledgements, 135 
305, 36, 56, 66, 125                  age, 11 
306, 56                               Age, 37, 70, 71, 72, 92 
307, 36, 55, 57, 79                   age-value, 92 
3xx Redirection Status Codes, 54      Allow, 32, 38, 46, 58, 92 
400, 31, 34, 36, 37, 57, 111, 143     ALPHA, 13, 15 
401, 36, 57, 59, 92, 130              ANSI X3.4-1986, 15, 145 
402, 36, 58                           asctime-date, 19 
403, 36, 58                           attribute, 22 
404, 36, 58, 60                       authority, 18, 33 
405, 32, 36, 58, 92                   Authorization, 34, 57, 58, 79, 
406, 36, 58, 59, 64, 87, 89, 90        92, 93, 95, 119 
407, 36, 59, 118                      Backus-Naur Form, 13 
408, 36, 59                           Basic Authentication. See [43] 
409, 36, 59                           BCP 18, 148 
410, 36, 59, 60, 79                   BCP 9, 148 
411, 31, 36, 60                       byte-content-range-spec, 105, 106 
412, 36, 60, 112, 114, 116            byte-range, 119 
413, 36, 60                           byte-range-resp-spec, 106 

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byte-range-set, 119, 120               must-revalidate, 93, 94, 97, 99 
byte-range-spec, 61, 107, 119,         no-cache, 66, 74, 94, 95, 96, 
 120                                    98, 99, 118 
byte-ranges-specifier, 119             no-store, 66, 94, 95 
bytes, 91                              no-transform, 94, 100, 102 
bytes-unit, 28                         only-if-cached, 94, 99 
cachable, 10                           private, 79, 94, 95, 96, 101 
cache, 10                              proxy-revalidate, 79, 94, 100 
Cache                                  public, 68, 79, 93, 94, 95, 96, 
 cachability of responses, 79           100 
 calculating the age of a              s-maxage, 73, 79, 93, 94, 97 
  response, 70                       cache-directive, 94, 101, 118 
 combining byte ranges, 82            cache-request-directive, 66, 94 
 combining headers, 81                Changes from HTTP/1.0. See RFC 
 combining negotiated responses,       1945 and RFC 2068 
  83                                   Host requirement, 143 
 constructing responses, 80           CHAR, 15 
 correctness, 66                      charset, 20, 88 
 disambiguating expiration            chunk, 23 
  values, 73                         chunk-data, 23 
 disambiguating multiple              chunked, 123, 124 
  responses, 73                      Chunked-Body, 23 
 entity tags used as cache            chunk-extension, 23 
  validators, 75                     chunk-ext-name, 23 
 entry validation, 74                 chunk-ext-val, 23 
 errors or incomplete responses,      chunk-size, 23 
  84                                 client, 9 
 expiration calculation, 72           codings, 89 
 explicit expiration time, 69         comment, 16, 125, 127 
 GET and HEAD cannot affect           Compatibility 
  caching, 84                          missing charset, 21 
 heuristic expiration, 70              multipart/x-byteranges, 138 
 history list behavior, 86            Compatibility with previous HTTP 
 invalidation cannot be complete,      versions, 142 
  85                                 CONNECT, 32, 33. See [44]. 
 Last-Modified values used as         connection, 8 
  validators, 75                     Connection, 31, 40, 41, 80, 101, 
 mechanisms, 68                        102, 123, 125, 143, 144 
 replacement of cached responses,      close, 40, 101, 102, 144 
  86                                   Keep-Alive, 144. See RFC 2068 
 shared and non-shared, 84            connection-token, 101, 102 
 Warnings, 67                         Content Codings 
 weak and strong cache                 compress, 21 
  validators, 75                       deflate, 21 
 write-through mandatory, 85           gzip, 21 
Cache-Control, 31, 48, 53, 55,         identity, 22 
 56, 57, 68, 69, 70, 72, 73, 74,      content negotiation, 9 
 79, 81, 84, 93, 94, 95, 96, 97,      Content Negotiation, 63 
 98, 101, 110, 118                    content-cncoding, 102 
 cache-extension, 94                  content-coding, 21, 22, 23, 25, 
 extensions, 100                       63, 89, 90, 102, 124, 129 
 max-age, 70, 72, 73, 74, 79, 94,      new tokens SHOULD be registered 
  96, 97, 98, 99, 110                   with IANA, 22 
 max-stale, 68, 94, 97, 99             qvalues used with, 90 
 min-fresh, 94, 97                    content-disposition, 142 

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Content-Disposition, 134, 141,        Entity body, 38 
 142, 148                             Entity Tags, 27, 75 
Content-Encoding, 21, 22, 38, 81,     entity-body, 38 
 102, 105, 129, 140                   entity-header, 32, 35, 38 
Content-Language, 26, 38, 102,        Entity-header fields, 37 
 103, 128                             entity-length, 39, 81 
Content-Length, 30, 31, 38, 42,       entity-tag, 27, 114, 115 
 46, 47, 53, 60, 81, 84, 103,         ETag, 27, 37, 47, 52, 53, 56, 75, 
 106, 124, 141                         81, 83, 109, 114 
Content-Location, 38, 53, 56, 81,     Expect, 34, 42, 43, 44, 50, 61, 
 83, 85, 104, 117, 133                 109 
Content-MD5, 38, 47, 81, 104,         expectation, 109 
 105, 147                             expectation-extension, 109 
Content-Range, 53, 79, 105            expect-params, 109 
content-range-spec, 105               Expires, 38, 48, 53, 55, 56, 57, 
Content-Transfer-Encoding, 22,         70, 72, 73, 79, 81, 96, 97, 99, 
 105, 140                              108, 110, 139 
Content-Type, 20, 21, 24, 38, 46,     explicit expiration time, 11 
 50, 52, 53, 54, 58, 59, 81, 102,     extension-code, 36 
 106, 107, 129, 137, 140              extension-header, 38 
CR, 15, 25, 32, 35, 36, 138, 140      extension-pragma, 118 
CRLF, 13, 15, 23, 24, 25, 28, 32,     field-content, 29 
 35, 105, 138, 140                    field-name, 29 
ctext, 16                             field-value, 29 
CTL, 15                               filename-parm, 142 
Date, 31, 53, 56, 70, 73, 74, 76,     first-byte-pos, 61, 106, 107, 
 77, 79, 82, 84, 86, 96, 107,          119, 120 
 108, 110, 116, 130, 140              first-hand, 10 
date1, 19                             fresh, 11 
date2, 19                             freshness lifetime, 11 
date3, 19                             freshness_lifetime, 72 
DELETE, 32, 45, 49, 85                From, 34, 41, 110, 111, 131, 132 
delta-seconds, 20, 122                gateway, 10 
Differences between MIME and          General Header Fields, 31 
 HTTP, 139                            general-header, 31, 32, 35 
 canonical form, 139                  generic-message, 28 
 Content-Encoding, 140                GET, 18, 32, 33, 45, 47, 51, 53, 
 Content-Transfer-Encoding, 140        54, 55, 56, 57, 60, 74, 76, 77, 
 date formats, 140                     84, 85, 92, 103, 107, 112, 113, 
 MIME-Version, 139                     114, 115, 121, 132 
 Transfer-Encoding, 141               HEAD, 30, 32, 45, 47, 51, 54, 55, 
Digest Authentication, 80. See         57, 58, 61, 84, 85, 92, 103, 
 [43]                                  107, 114 
DIGIT, 13, 14, 15, 17, 19, 26,        Headers 
 117, 139                              end-to-end, 80, 81, 82, 101, 109 
disp-extension-token, 142              hop-by-hop, 11, 80 
disposition-parm, 142                  non-modifiable headers, 80 
disposition-type, 142                 Henrik Frystyk Nielsen, 149 
DNS, 133                              heuristic expiration time, 11 
 HTTP applications MUST obey TTL      HEX, 16, 18, 23 
  information, 133                   Hop-by-hop headers, 80 
downstream, 11                        host, 18, 127, 128 
End-to-end headers, 80                Host, 33, 34, 45, 111, 143 
entity, 9                             HT, 13, 15, 16, 28, 138 
Entity, 37                            http_URL, 18 

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INTERNET-DRAFT                HTTP/1.1                  December, 2003 


HTTP-date, 19, 107, 108, 110,         Media Types, 24 
 113, 115, 116, 122, 128              media-range, 87 
HTTP-message, 28                      media-type, 24, 102, 105, 129 
HTTP-Version, 17, 32, 35              message, 9 
IANA, 20, 21, 22, 24, 27, 87, 136     Message Body, 29 
identity, 22, 89, 90, 102             Message Headers, 28 
If-Match, 27, 34, 47, 78, 112,        Message Length, 30 
 114, 115, 121                        Message Transmission 
If-Modified-Since, 34, 47, 76,         Requirements, 42 
 77, 78, 112, 113, 114, 115, 116,     Message Types, 28 
 121                                  message-body, 28, 29, 32, 35, 39 
If-None-Match, 27, 34, 47, 78,        message-header, 28, 29, 38 
 83, 112, 114, 115, 116, 121          Method, 32, 92 
If-Range, 27, 34, 47, 53, 61, 78,     Method Definitions, 45 
 107, 115, 121                        Methods 
If-Unmodified-Since, 34, 47, 76,       Idempotent, 45 
 78, 114, 115, 116, 121                Safe and Idempotent, 45 
If-Unmodified-Since, 116              MIME, 8, 12, 20, 22, 25, 103, 
implied *LWS, 14                       105, 135, 139, 140, 141, 145, 
inbound, 11                            146 
instance-length, 106                   multipart, 25 
ISO-10646, 148                        MIME-Version, 139 
ISO-2022, 20                          month, 19 
ISO-3166, 27                          multipart/byteranges, 25, 30, 53, 
ISO-639, 27                            61, 107, 137 
ISO-8859, 145                         multipart/x-byteranges, 138 
ISO-8859-1, 15, 21, 25, 89, 128,      MUST, 8 
 138                                  MUST NOT, 8 
James Gettys, 149                     N rule, 14 
Jeffrey C. Mogul, 149                 name, 13 
Keep-Alive, 41, 80, 142, 143,         non-shared cache, 84, 95, 101 
 144. See RFC 2068                    non-transparent proxy. See proxy: 
Language Tags, 26                      non-transparent 
language-range, 90, 91                OCTET, 15, 38 
language-tag, 27, 90                  opaque-tag, 27 
Larry Masinter, 149                   OPTIONAL, 8 
last-byte-pos, 106, 119, 120          OPTIONS, 32, 33, 45, 46, 117, 118 
last-chunk, 23                        origin server, 10 
Last-Modified, 11, 38, 47, 53,        other-range-unit, 28 
 70, 73, 75, 76, 77, 78, 79, 81,      outbound, 11 
 108, 113, 115, 116, 117              parameter, 22 
LF, 15, 25, 32, 35, 36, 138, 140      Paul J. Leach, 149 
lifetime, 11, 70, 72, 73, 92, 97,     Persistent Connections, 39 
 129                                   Overall Operation, 39 
LOALPHA, 15                            Purpose, 39 
Location, 37, 48, 51, 54, 55, 56,      Use of Connection Header, 40 
 57, 85, 117, 133                     Pipelining, 40 
LWS, 13, 15, 29                       port, 18, 127, 128 
Max-Forwards, 35, 46, 50, 117,        POST, 26, 28, 32, 43, 45, 47, 48, 
 118                                   49, 51, 55, 60, 85, 108, 132 
MAY, 8                                Pragma, 31, 93, 98, 118 
media type, 15, 21, 24, 25, 30,        no-cache, 66, 74, 93, 118 
 38, 52, 54, 58, 63, 87, 88, 100,     pragma-directive, 118 
 102, 103, 107, 137, 138, 139,        primary-tag, 27 
 140                                  product, 26, 125 

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Product tokens, 26                    response, 9 
product-version, 26                   Response, 35 
protocol-name, 127                    Response Header Fields, 37 
protocol-version, 127                 response-header, 35, 37 
proxy, 10                             Retry-After, 37, 60, 62, 122 
 non-transparent, 10, 81, 100,        Revalidation 
  102                                  end-to-end, 98 
 transparent, 10, 38, 80               end-to-end reload, 98 
Proxy-Authenticate, 37, 59, 80,        end-to-end specific 
 118, 119                               revalidation, 98 
Proxy-Authorization, 35, 59, 80,       end-to-end unspecific 
 119                                    revalidation, 98 
pseudonym, 127, 128                   RFC 1036, 19, 146 
public cache, 64                      RFC 1123, 19, 107, 110, 145 
PUT, 32, 43, 45, 48, 49, 59, 85,      RFC 1305, 147 
 92, 108, 112, 115                    RFC 1436, 146 
qdtext, 16                            , 24 
Quality Values, 26                    RFC 1630, 146 
query, 18                             RFC 1737, 147 
quoted-pair, 16                       RFC 1738, 17, 146 
quoted-string, 15, 16, 23, 27,        , 27, 146 
 29, 87, 94, 109, 118, 128, 142       RFC 1806, 134, 142, 148 
qvalue, 26, 87, 88, 89                RFC 1808, 17, 146 
Range, 27, 35, 38, 47, 48, 53,        RFC 1864, 104, 105, 147 
 61, 79, 81, 82, 106, 107, 113,       RFC 1866, 146 
 115, 119, 121, 137                   RFC 1867, 147 
Range Units, 27                       RFC 1900, 18, 147 
ranges-specifier, 106, 119, 120,      RFC 1945, 8, 55, 141, 146 
 121                                  RFC 1950, 21, 147 
range-unit, 28, 91                    RFC 1951, 21, 147 
Reason-Phrase, 35, 36                 RFC 1952, 147 
received-by, 127                      RFC 2026, 148 
received-protocol, 127, 128           RFC 2045, 139, 140, 145 
RECOMMENDED, 8                        RFC 2046, 25, 138, 139, 146 
References, 146                       RFC 2047, 15, 128, 145 
Referer, 35, 121, 131, 132            RFC 2048, 145 
rel_path, 18, 84                      RFC 2049, 139, 140, 148 
relativeURI, 18, 104, 121             RFC 2068, 2, 17, 39, 41, 43, 55, 
representation, 9                      56, 136, 141, 142, 144, 148 
request, 9                            RFC 2076, 141, 148 
Request, 32                           RFC 2110, 148 
Request header fields, 34             RFC 2119, 8, 148 
request-header, 32, 34                RFC 2145, 16, 148 
Request-Line, 28, 32, 33, 48, 58,     RFC 2277, 148 
 138, 142                             RFC 2279, 148 
Request-URI, 18, 32, 33, 34, 36,      RFC 2324, 148 
 37, 46, 47, 48, 49, 54, 55, 57,      RFC 2388, 26 
 58, 59, 60, 83, 85, 92, 102,         RFC 2396, 17, 146 
 104, 117, 118, 121, 122, 130,        RFC 2616, 148 
 132, 133                             RFC 3066, 27 
REQUIRED, 8                           RFC 3232, 147 
Requirements                          RFC 821, 147 
 compliance, 8                        RFC 822, 13, 19, 28, 107, 110, 
 key words, 8                          126, 135, 139, 145 
resource, 9                           RFC 850, 19 

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RFC 959, 147                          subtype, 24 
RFC 977, 146                          suffix-byte-range-spec, 119, 120 
rfc1123-date, 19                      suffix-length, 120 
RFC-850, 138                          T/TCP, 39 
rfc850-date, 19                       t-codings, 123 
Roy T. Fielding, 149                  TE, 23, 35, 80, 122, 123 
rule1 | rule2, 14                     TEXT, 15 
Safe and Idempotent Methods, 45       Tim Berners-Lee, 149 
Security Considerations, 130          time, 19 
 abuse of server logs, 130            token, 14, 15, 16, 20, 21, 22, 
 Accept header, 132                    23, 24, 26, 28, 29, 32, 87, 94, 
 Accept headers can reveal ethnic      101, 109, 118, 125, 127, 142 
  information, 132                   Tolerant Applications, 138 
 attacks based on path names, 132      bad dates, 138 
 Authentication Credentials and        should tolerate whitespace in 
  Idle Clients, 134                     request and status lines, 138 
 be careful about personal             tolerate LF and ignore CR in 
  information, 130                      line terminators, 138 
 Content-Disposition Header, 134       use lowest common denominator of 
 Content-Location header, 133           character set, 138 
 encoding information in URI's,       TRACE, 32, 45, 50, 51, 117, 118 
  131                                trailer, 23 
 From header, 131, 132                Trailer, 23, 31, 124 
 GET method, 132                      trailers, 122, 123 
 Location header, 133                 Trailers, 80 
 Location headers and spoofing,       Transfer Encoding 
  133                                  chunked, 22 
 Proxies and Caching, 134             transfer-coding 
 Referer header, 131, 132              chunked, 22 
 sensitive headers, 130                deflate, 22 
 Server header, 131                    gzip, 22 
 Transfer of Sensitive                 , 22 
  Information, 131                   transfer-coding, 22, 23, 29, 30, 
 Via header, 131                       31, 39, 105, 122, 123, 124, 141 
selecting request-headers, 83          chunked, 22, 23, 30, 42, 123, 
semantically transparent, 11             124, 141 
separators, 16                         chunked REQUIRED, 31 
server, 9                              compress, 22 
Server, 26, 37, 122, 127, 131          , 30 
SHALL, 8                               trailers, 123 
SHALL NOT, 8                          Transfer-Encoding, 22, 29, 30, 
shared caches, 84, 96                  31, 38, 46, 80, 124, 141 
SHOULD, 8                             transfer-extension, 22, 123 
SHOULD NOT, 8                         transfer-length, 39, 81 
SP, 13, 15, 16, 19, 28, 29, 32,       transparent 
 35, 105, 128, 138                     proxy, 81 
stale, 11                             transparent proxy. See proxy: 
start-line, 28                         transparent 
Status Code Definitions, 50           tunnel, 10 
Status-Code, 35, 36, 50               type, 24 
Status-Line, 28, 35, 37, 50, 138,     UPALPHA, 15 
 142                                  Upgrade, 31, 51, 80, 124, 125 
strong entity tag, 27                 upstream, 11 
strong validators, 76                 URI-reference, 18 
subtag, 27                            US-ASCII, 15, 20, 138 

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INTERNET-DRAFT                HTTP/1.1                  December, 2003 


user agent, 9                          111 Revalidation failed, 129 
User-Agent, 26, 35, 64, 125, 126,      112 Disconnected operation, 129 
 127, 131                              113 Heuristic expiration, 129 
validators, 11, 27, 68, 73, 74,        199 Miscellaneous warning, 129 
 75, 76, 78, 82                        214 Transformation applied, 129 
 rules on use of, 77                   299 Miscellaneous persistent 
value, 22                                warning, 129 
variant, 9                            warning-value, 128, 129, 130 
Vary, 37, 53, 56, 64, 83, 112,        warn-text, 128, 129 
 115, 126, 132                        weak, 27 
Via, 31, 50, 122, 126, 127, 131       weak entity tag, 27 
warn-agent, 128                       weak validators, 75, 76 
warn-code, 82, 128                    weekday, 19 
warn-codes, 67                        wkday, 19 
warn-date, 128, 129, 130              WWW-Authenticate, 37, 57, 119, 
Warning, 31, 66, 67, 69, 73, 79,       130 
 81, 82, 97, 128, 129, 130            x-compress, 90 
Warnings                              x-gzip, 90 
 110 Response is stale, 129 
    


































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