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Versions: 00 01 02 03 04 05 06 RFC 2616

       HTTP Working Group                              R. Fielding, UC Irvine
       INTERNET-DRAFT                                      J. Gettys, Digital
       <draft-ietf-http-v11-spec-rev-01>                 J. C. Mogul, Digital
                                                           L. Masinter, Xerox
                                                          P. Leach, Microsoft
                                                          H. Frystyk, MIT/LCS
                                                      T. Berners-Lee, MIT/LCS
       Expires May 21, 1998                                 November 21, 1997
       
                       Hypertext Transfer Protocol -- HTTP/1.1
       
       Status of this Memo
       
       This document is an Internet-Draft. 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".
       
       To learn the current status of any Internet-Draft, please check the
       "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
       Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
       munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
       ftp.isi.edu (US West Coast).
       
       Distribution of this document is unlimited. Please send comments to the
       HTTP working group at <http-wg@cuckoo.hpl.hp.com>. Discussions of the
       working group are archived at
       <URL:http://www.ics.uci.edu/pub/ietf/http/>. General discussions about
       HTTP and the applications which use HTTP should take place on the <www-
       talk@w3.org> mailing list.
       
       Abstract
       
       The Hypertext Transfer Protocol (HTTP) is an application-level protocol
       for distributed, collaborative, hypermedia information systems. It is a
       generic, stateless, object-oriented protocol which can be used for many
       tasks, such as name servers and distributed object management systems,
       through extension of its request methods. A feature of HTTP is the
       typing and negotiation of data representation, allowing systems to be
       built independently of the data being transferred.
       
       HTTP has been in use by the World-Wide Web global information initiative
       since 1990. This specification defines the protocol referred to as
       "HTTP/1.1".
       
       The issues list for HTTP/1.1 can be found at:
       http://www.w3.org/Protocols/HTTP/Issues/.
       
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       This draft does not resolve all open issues in the HTTP/1.1
       specification requiring closure before HTTP/1.1 goes to draft standard.
       It does, however, close most of them, and note where in the document
       there are still significant issues under discussion.  The best way to
       view this document is to get a copy of the Word 97 document found at:
       http://www.w3.org/Protocols/HTTP/1.1/draft-ietf-http-v11-spec-rev-
       01.doc; all issues are noted as comments in the source document, with
       hyperlinks to the Issues list.
       
       This document has had the basic ambiguity problems with OPTIONS
       resolved; but does not solve the underlying issue of identifying
       extensions to HTTP. The issue OPTIONS is therefore still open; at
       Munich, there was strong sentiment to some sort of OPTIONS facility, but
       we've not been able to converge; it is on the agenda for Washington.
       See draft-ietf-http-options-02.txt for details.
       
       Summary Of Issues since Rev-01 (Rev-01 and Auth-00 under preparation)
       
       4 Open Technical issues: RE-AUTHENTICATION-REQUESTED, PROXY-REDIRECT,
       CONTENT-ENCODING, OPTIONS.
       
       Minimal OPTIONS is in Rev-01, PROXY-REDIRECT proposal that was in Rev-00
       draft (which interacts with OPTIONS) was removed from rev-01.
       
       8 Open Editorial issues: XREFS,  REQUEST_TARGET, UPDATE_ACKS,
       DISPOSITION, REQUIREMENTS, CLEAN_INDEXES, CODE_REGISTRY,
       IANA_DIRECTIONS.
       
       4 Ready for Last Call: PUT-RANGE, RE-AUTHENTICATION-REQUESTED,
       TRAILER_FIELDS, RANGE_WITH_CONTENTCODING.
       
       12 Last call issues (all edited into Rev-01): DIGEST_SYNTAX,
       PROTECTION_SPACE, CONTRADICTION, IMS_INM_MISMATCH, CONNECTION_METHOD,
       BYTERANGE_SYNTAX,  RE-VERSION, HOST, AUTH-CHUNKED, REQUIRE-DIGEST,
       DOCKDIGEST, GENERIC_MESSAGE.
       
       9 Technical Issues closed since Rev-00 (all edited into Rev-01): 301-
       302, REDIRECTS, CACHING-CGI, WARNINGS, DATE-IF-MODIFIED, 403VS404, AGE-
       CALCULATION (Yeah!), VARY, RANGE-ERROR.
       
       12 Editorial issues closed since Rev-00 (all edited into Rev-01):
       REMOVE_19.6, GENERIC_MESSAGE, RANGE_OPTIONAL, EBNF, CODES, UTF-8,
       DOCKDIGEST, URL-SYNTAX, 1310_CACHE, MESSAGE-BODY, LENGTH_WORDING,
       CLEANUP.
       
       
       
       
       
       
       
       
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       Table of Contents
       
       
       
       HYPERTEXT TRANSFER PROTOCOL -- HTTP/1.1....................1
       
       Status of this Memo........................................1
       
       Abstract...................................................1
       
       Table of Contents..........................................3
       
       1   Introduction ..........................................8
        1.1  Purpose .............................................8
        1.2  Requirements ........................................9
        1.3  Terminology ........................................10
        1.4  Overall Operation ..................................13
       
       2   Notational Conventions and Generic Grammar ...........14
        2.1  Augmented BNF ......................................14
        2.2  Basic Rules ........................................16
       
       3   Protocol Parameters ..................................17
        3.1  HTTP Version .......................................17
        3.2  Uniform Resource Identifiers .......................19
         3.2.1  General Syntax ..................................19
         3.2.2  http URL ........................................19
         3.2.3  URI Comparison ..................................20
        1.3  Date/Time Formats ..................................20
         1.3.1  Full Date .......................................20
         1.3.2  Delta Seconds ...................................21
        1.4  Character Sets .....................................21
        1.5  Content Codings ....................................22
        1.6  Transfer Codings ...................................23
         1.6.1  Chunked Transfer Coding .........................24
         1.6.2  Identity Transfer Coding ........................25
        1.7  Media Types ........................................25
         1.7.1  Canonicalization and Text Defaults ..............26
         1.7.2  Multipart Types .................................26
        1.8  Product Tokens .....................................27
        1.9  Quality Values .....................................27
        1.10 Language Tags ......................................28
        1.11 Entity Tags ........................................28
        1.12 Range Units ........................................29
       
       4   HTTP Message .........................................29
        4.1  Message Types ......................................29
        4.2  Message Headers ....................................30
        4.3  Message Body .......................................30
        4.4  Message Length .....................................31
        4.5  General Header Fields ..............................32
       
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       5   Request ..............................................33
        5.1  Request-Line .......................................33
         5.1.1  Method ..........................................33
         5.1.2  Request-URI .....................................34
        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
        1.2  Response Header Fields .............................38
       
       7   Entity ...............................................39
        7.1  Entity Header Fields ...............................39
        7.2  Entity Body ........................................39
         7.2.1  Type ............................................40
         7.2.2  Length ..........................................40
       
       8   Connections ..........................................40
        8.1  Persistent Connections .............................40
         8.1.1  Purpose .........................................40
         1.1.2  Overall Operation ...............................41
         1.1.3  Proxy Servers ...................................42
         1.1.4  Practical Considerations ........................43
        1.2  Message Transmission Requirements ..................44
         1.2.1  Persistent connections and flow control .........44
         1.2.2  Monitoring connections for error status messages 44
         1.2.3  Automatic retrying of requests ..................44
         1.2.4  Use of the 100 (Continue) status ................44
         1.2.5  Client behavior if server prematurely closes connection
                .................................................46
       
       9   Method Definitions ...................................47
        9.1  Safe and Idempotent Methods ........................47
         9.1.1  Safe Methods ....................................47
         9.1.2  Idempotent Methods ..............................48
        9.2  OPTIONS ............................................48
        9.3  GET ................................................49
        9.4  HEAD ...............................................50
        9.5  POST ...............................................50
        1.6  PUT ................................................51
         1.6.1  Partial PUT (PUT with Content-Range) ............52
        1.7  DELETE .............................................53
        1.8  TRACE ..............................................53
        1.9  CONNECT ............................................54
       
       10   Status Code Definitions .............................54
        10.1 Informational 1xx ..................................54
         10.1.1 100 Continue ....................................54
         10.1.2 101 Switching Protocols .........................54
        10.2 Successful 2xx .....................................55
       
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         10.2.1 200 OK ..........................................55
         10.2.2 201 Created .....................................55
         10.2.3 202 Accepted ....................................55
         10.2.4 203 Non-Authoritative Information ...............56
         10.2.5 204 No Content ..................................56
         10.2.6 205 Reset Content ...............................56
         10.2.7 206 Partial Content .............................57
         10.2.8 207 Partial Update OK ...........................57
        10.3 Redirection 3xx ....................................58
         10.3.1 300 Multiple Choices ............................58
         10.3.2 301 Moved Permanently ...........................58
         10.3.3 302 Found .......................................59
         10.3.4 303 See Other ...................................59
         10.3.5 304 Not Modified ................................60
         10.3.6 305 Use Proxy ...................................61
         10.3.7 307 Temporary Redirect ..........................61
        10.4 Client Error 4xx ...................................61
         10.4.1 400 Bad Request .................................62
         10.4.2 401 Unauthorized ................................62
         10.4.3 402 Payment Required ............................62
         10.4.4 403 Forbidden ...................................62
         10.4.5 404 Not Found ...................................62
         10.4.6 405 Method Not Allowed ..........................63
         10.4.7 406 Not Acceptable ..............................63
         10.4.8 407 Proxy Authentication Required ...............63
         10.4.9 408 Request Timeout .............................63
         10.4.10 ......................................409 Conflict    64
         10.4.11 ..........................................410 Gone    64
         10.4.12 ...............................411 Length Required    64
         10.4.13 ...........................412 Precondition Failed    65
         10.4.14 ......................413 Request Entity Too Large    65
         10.4.15 ..........................414 Request-URI Too Long    65
         10.4.16 ........................415 Unsupported Media Type    65
         10.4.17 ...............416 Requested range not satisfiable    65
         10.4.18 ............................417 Expectation Failed    66
         10.4.19 .....................418 Reauthentication Required    66
         10.4.20 ...............419 Proxy Reauthentication Required    66
        10.5 Server Error 5xx ...................................67
         10.5.1 500 Internal Server Error .......................67
         10.5.2 501 Not Implemented .............................67
         10.5.3 502 Bad Gateway .................................67
         10.5.4 503 Service Unavailable .........................67
         10.5.5 504 Gateway Timeout .............................67
         10.5.6 505 HTTP Version Not Supported ..................68
         10.5.7 506 Partial Update Not Implemented ..............68
       
       11   Access Authentication ...............................68
        11.1 Digest Authentication ..............................68
       
       12   Content Negotiation .................................68
        12.1 Server-driven Negotiation ..........................69
       
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        12.2 Agent-driven Negotiation ...........................70
        12.3 Transparent Negotiation ............................70
       
       13   Caching in HTTP .....................................71
         1.1.1  Cache Correctness ...............................72
         1.1.2  Warnings ........................................73
         1.1.3  Cache-control Mechanisms ........................74
         1.1.4  Explicit User Agent Warnings ....................74
         1.1.5  Exceptions to the Rules and Warnings ............75
         1.1.6  Client-controlled Behavior ......................75
        1.2  Expiration Model ...................................76
         1.2.1  Server-Specified Expiration .....................76
         1.2.2  Heuristic Expiration ............................76
         1.2.3  Age Calculations ................................77
         1.2.4  Expiration Calculations .........................79
         1.2.5  Disambiguating Expiration Values ................79
         1.2.6  Disambiguating Multiple Responses ...............80
        1.3  Validation Model ...................................80
         1.3.1  Last-modified Dates .............................81
         1.3.2  Entity Tag Cache Validators .....................82
         1.3.3  Weak and Strong Validators ......................82
         1.1.4  Rules for When to Use Entity Tags and Last-modified Dates
                .................................................84
         1.1.5  Non-validating Conditionals .....................86
        1.4  Response Cachability ...............................86
        1.5  Constructing Responses From Caches .................87
         1.5.1  End-to-end and Hop-by-hop Headers ...............87
         1.1.2  Non-modifiable Headers ..........................87
         1.1.3  Combining Headers ...............................88
         1.1.4  Combining Byte Ranges ...........................89
        1.6  Caching Negotiated Responses .......................90
        1.7  Shared and Non-Shared Caches .......................91
        1.8  Errors or Incomplete Response Cache Behavior .......91
        1.9  Side Effects of GET and HEAD .......................91
        1.10 Invalidation After Updates or Deletions ............92
        1.11 Write-Through Mandatory ............................92
        1.12 Cache Replacement ..................................93
        1.13 History Lists ......................................93
       
       14   Header Field Definitions ............................94
        14.1 Accept .............................................94
        14.2 Accept-Charset .....................................96
        14.3 Accept-Encoding ....................................96
        14.4 Accept-Language ....................................97
        14.5 Accept-Ranges ......................................99
        14.6 Age ................................................99
        14.7 Allow ..............................................99
        14.8 Authorization .....................................100
        14.9 Cache-Control .....................................101
         1.1.1  What is Cachable ...............................102
         1.1.2  What May be Stored by Caches ...................103
       
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         1.1.3  Modifications of the Basic Expiration Mechanism 103
         1.1.4  Cache Revalidation and Reload Controls .........105
         1.1.5  No-Transform Directive .........................107
         1.1.6  Cache Control Extensions .......................108
        1.10 Connection ........................................109
        1.11 Content-Base ......................................110
        1.12 Content-Encoding ..................................110
        1.13 Content-Language ..................................111
        1.14 Content-Length ....................................111
        1.15 Content-Location ..................................112
        1.16 Content-MD5 .......................................113
        1.17 Content-Range .....................................114
        1.18 Content-Type ......................................116
        1.19 Date ..............................................116
         1.19.1 Clockless Origin Server Operation ..............117
        1.20 ETag ..............................................117
        1.21 Expires ...........................................117
        1.22 From ..............................................118
        1.23 Host ..............................................119
        1.24 If-Modified-Since .................................119
        1.25 If-Match ..........................................121
        1.26 If-None-Match .....................................122
        1.27 If-Range ..........................................123
        1.28 If-Unmodified-Since ...............................123
        1.29 Last-Modified .....................................124
        1.30 Location ..........................................124
        1.31 Max-Forwards ......................................125
        1.32 Pragma ............................................125
        1.33 Proxy-Authenticate ................................126
        1.34 Proxy-Authorization ...............................126
        1.35 Public ............................................127
        1.36 Range .............................................127
         1.36.1 Byte Ranges ....................................127
         1.1.2  Range Retrieval Requests .......................128
        1.37 Referer ...........................................129
        1.38 Retry-After .......................................130
        1.39 Server ............................................130
        1.40 Transfer-Encoding .................................130
        1.41 Upgrade ...........................................131
        1.42 User-Agent ........................................132
        1.43 Vary ..............................................132
        1.44 Via ...............................................133
        1.45 Warning ...........................................134
        1.46 WWW-Authenticate ..................................137
        1.47 Expect ............................................137
         1.47.1 Expect 100-continue ............................137
        1.48 TE ................................................138
        1.49 Trailer ...........................................139
       
       15   Security Considerations ............................139
        15.1 Authentication of Clients .........................140
       
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        15.2 Abuse of Server Log Information ...................141
        15.3 Transfer of Sensitive Information .................141
        15.4 Attacks Based On File and Path Names ..............142
        15.5 Personal Information ..............................142
        15.6 Privacy Issues Connected to Accept Headers ........142
        15.7 DNS Spoofing ......................................143
        15.8 Location Headers and Spoofing .....................144
        15.9 Content-Disposition Issues ........................144
        15.10  Encoding Sensitive Information in URL's .........144
        15.11  Authetication Credentials and Idle Clients ......144
       
       16   Acknowledgments ....................................145
       
       17   References .........................................146
       
       18   Authors' Addresses .................................150
       
       19   Appendices .........................................151
        19.1 Internet Media Type message/http ..................151
        19.2 Internet Media Type multipart/byteranges ..........152
        19.3 Tolerant Applications .............................152
        1.4  Differences Between HTTP Entities and RFC 2045 Entities   153
         1.4.1  Conversion to Canonical Form ...................153
         1.4.2  Conversion of Date Formats .....................154
         1.4.3  Introduction of Content-Encoding ...............154
         1.4.4  No Content-Transfer-Encoding ...................154
         1.4.5  HTTP Header Fields in Multipart Body-Parts .....155
         1.4.6  Introduction of Transfer-Encoding ..............155
         1.4.7  MIME-Version ...................................155
        1.5  Changes from HTTP/1.0 .............................156
         1.5.1  Changes to Simplify Multi-homed Web Servers and Conserve IP
         Addresses .............................................156
        1.6  Additional Features ...............................156
         1.1.1  Content-Disposition ............................157
        1.7  Compatibility with Previous Versions ..............157
         1.1.1  Compatibility with HTTP/1.0 Persistent Connections158
        1.8  Backward Compatibility ............................158
         1.8.1  CRLF's in Quoted Strings .......................159
         1.8.2  Missing Content Type ...........................159
         1.8.3  Multipart/x-byteranges .........................159
       
       20   Index ..............................................160
       
       
       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
       
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       1990. The first version of HTTP, referred to as HTTP/0.9, was a simple
       protocol for raw data transfer across the Internet. HTTP/1.0, as defined
       by RFC 1945 [6], improved the protocol by allowing messages to be in the
       format of MIME-like messages, containing metainformation about the data
       transferred and modifiers on the request/response semantics. However,
       HTTP/1.0 does not sufficiently take into consideration the effects of
       hierarchical proxies, caching, the need for persistent connections, and
       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 that indicate the purpose of a
       request. It builds on the discipline of reference provided by the
       Uniform Resource Identifier (URI) [3], as a location (URL) [4] or name
       (URN) [20], for indicating the resource to which a method is to be
       applied. Messages are passed in a format similar to that used by
       Internet mail [9] as defined by the Multipurpose Internet Mail
       Extensions (MIME) [7].
       
       HTTP is also used as a generic protocol for communication between user
       agents and proxies/gateways to other Internet systems, including those
       supported by the SMTP [16], NNTP [13], FTP [18], Gopher [2], and WAIS
       [10] protocols. In this way, HTTP allows basic hypermedia access to
       resources available from diverse applications.
       
       
       1.2 Requirements
       
       
       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
            "SHOULD", SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
            this document are to be interpreted as described in RFC 2119 [34].
       
       An implementation is not compliant if it fails to satisfy one or more of
       the MUST requirements for the protocols it implements. An implementation
       that satisfies all the MUST and all the SHOULD requirements for its
       protocols is said to be "unconditionally compliant"; one that satisfies
       all the MUST requirements but not all the SHOULD requirements for its
       protocols is said to be "conditionally compliant."
       
       
       
       
       
       
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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,
         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 program's capabilities in general.
         Likewise, any server may act as an origin server, proxy, gateway, or
         tunnel, switching behavior based on the nature of each request.
       
       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.
       
       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
         cachable responses in order to reduce the response time and network
         bandwidth consumption on future, equivalent requests. Any client or
         server may include a cache, though a cache cannot be used by a server
         that is acting as a tunnel.
       
       cachable
         A response is cachable if a cache is allowed to store a copy of the
         response message for use in answering subsequent requests. The rules
       
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         for determining the cachability of HTTP responses are defined in
         section 13. Even if a resource is cachable, 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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       1.4 Overall Operation
       
       The HTTP protocol is a request/response protocol. A client sends a
       request to the server in the form of a request method, URI, and protocol
       version, followed by a MIME-like message containing request modifiers,
       client information, and possible body content over a connection with a
       server. The server responds with a status line, including the message's
       protocol version and a success or error code, followed by a MIME-like
       message containing server information, entity metainformation, and
       possible entity-body content. The relationship between HTTP and MIME is
       described in appendix 19.4.
       
       Most HTTP communication is initiated by a user agent and consists of a
       request to be applied to a resource on some origin server. In the
       simplest case, this may be accomplished via a single connection (v)
       between the user agent (UA) and the origin server (O).
       
                 request chain ------------------------>
              UA -------------------v------------------- O
                 <----------------------- response chain
       A more complicated situation occurs when one or more intermediaries are
       present in the request/response chain. There are three common forms of
       intermediary: proxy, gateway, and tunnel. A proxy is a forwarding agent,
       receiving requests for a URI in its absolute form, rewriting all or part
       of the message, and forwarding the reformatted request toward the server
       identified by the URI. A gateway is a receiving agent, acting as a layer
       above some other server(s) and, if necessary, translating the requests
       to the underlying server's protocol. A tunnel acts as a relay point
       between two connections without changing the messages; tunnels are used
       when the communication needs to pass through an intermediary (such as a
       firewall) even when the intermediary cannot understand the contents of
       the messages.
       
                 request chain -------------------------------------->
              UA -----v----- A -----v----- B -----v----- C -----v----- O
                 <------------------------------------- response chain
       The figure above shows three intermediaries (A, B, and C) between the
       user agent and origin server. A request or response message that travels
       the whole chain will pass through four separate connections. This
       distinction is important because some HTTP communication options may
       apply only to the connection with the nearest, non-tunnel neighbor, only
       to the end-points of the chain, or to all connections along the chain.
       Although the diagram is linear, each participant may be engaged in
       multiple, simultaneous communications. For example, B may be receiving
       requests from many clients other than A, and/or forwarding requests to
       servers other than C, at the same time that it is handling A's request.
       
       Any party to the communication which is not acting as a tunnel may
       employ an internal cache for handling requests. The effect of a cache is
       that the request/response chain is shortened if one of the participants
       along the chain has a cached response applicable to that request. The
       
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       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 cachable, and some requests may contain
       modifiers which place special requirements on cache behavior. HTTP
       requirements for cache behavior and cachable responses are defined in
       section 13.
       
       In fact, there are a wide variety of architectures and configurations of
       caches and proxies currently being experimented with or deployed across
       the World Wide Web. These systems include national hierarchies of proxy
       caches to save transoceanic bandwidth, systems that broadcast or
       multicast cache entries, organizations that distribute subsets of cached
       data via CD-ROM, and so on. HTTP systems are used in corporate intranets
       over high-bandwidth links, and for access via PDAs with low-power radio
       links and intermittent connectivity. The goal of HTTP/1.1 is to support
       the wide diversity of configurations already deployed while introducing
       protocol constructs that meet the needs of those who build web
       applications that require high reliability and, failing that, at least
       reliable indications of failure.
       
       HTTP communication usually takes place over TCP/IP connections. The
       default port is TCP 80 [19], but other ports can be used. This does not
       preclude HTTP from being implemented on top of any other protocol on the
       Internet, or on other networks. HTTP only presumes a reliable transport;
       any protocol that provides such guarantees can be used; the mapping of
       the HTTP/1.1 request and response structures onto the transport data
       units of the protocol in question is outside the scope of this
       specification.
       
       In HTTP/1.0, most implementations used a new connection for each
       request/response exchange. In HTTP/1.1, a connection may be used for one
       or more request/response exchanges, although connections may be closed
       for a variety of reasons (see section 8.1).
       
       
       2 Notational Conventions and Generic Grammar
       
       
       2.1 Augmented BNF
       
       All of the mechanisms specified in this document are described in both
       prose and an augmented Backus-Naur Form (BNF) similar to that used by
       RFC 822 [9]. Implementers will need to be familiar with the notation in
       order to understand this specification. The augmented BNF includes the
       following constructs:
       
       
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       name = definition
         The name of a rule is simply the name itself (without any enclosing
         "<" and ">") and is separated from its definition by the equal "="
         character. Whitespace is only significant in that indentation of
         continuation lines is used to indicate a rule definition that spans
         more than one line. Certain basic rules are in uppercase, such as SP,
         LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle brackets are used within
         definitions whenever their presence will facilitate discerning the
         use of rule names.
       
       "literal"
         Quotation marks surround literal text. Unless stated otherwise, the
         text is case-insensitive.
       
       rule1 | rule2
         Elements separated by a bar ("|") are alternatives, e.g., "yes | no"
         will accept yes or no.
       
       (rule1 rule2)
         Elements enclosed in parentheses are treated as a single element.
         Thus, "(elem (foo | bar) elem)" allows the token sequences
         "elem foo elem" and "elem bar elem".
       
       *rule
         The character "*" preceding an element indicates repetition. The full
         form is "<n>*<m>element" indicating at least <n> and at most <m>
         occurrences of element. Default values are 0 and infinity so that
         "*(element)" allows any number, including zero; "1*element" requires
         at least one; and "1*2element" allows one or two.
       
       [rule]
         Square brackets enclose optional elements; "[foo bar]" is equivalent
         to "*1(foo bar)".
       
       N rule
         Specific repetition: "<n>(element)" is equivalent to
         "<n>*<n>(element)"; that is, exactly <n> occurrences of (element).
         Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
         alphabetic characters.
       
       #rule
         A construct "#" is defined, similar to "*", for defining lists of
         elements. The full form is "<n>#<m>element " indicating at least <n>
         and at most <m> elements, each separated by one or more commas (",")
         and optional linear whitespace (LWS). This makes the usual form of
         lists very easy; a rule such as
         "( *LWS element *( *LWS "," *LWS element )) " can be shown as
         "1#element". Wherever this construct is used, null elements are
         allowed, but do not contribute to the count of elements present. That
         is, "(element), , (element) " is permitted, but counts as only two
         elements. Therefore, where at least one element is required, at least
       
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         one non-null element must be present. Default values are 0 and
         infinity so that "#element" allows any number, including zero;
         "1#element" requires at least one; and "1#2element" allows one or
         two.
       
       ; comment
         A semi-colon, set off some distance to the right of rule text, starts
         a comment that continues to the end of line. This is a simple way of
         including useful notes in parallel with the specifications.
       
       implied *LWS
         The grammar described by this specification is word-based. Except
         where noted otherwise, linear whitespace (LWS) can be included
         between any two adjacent words (token or quoted-string), and between
         adjacent tokens and separators, without changing the interpretation
         of a field. At least one delimiter (LWS and/or separators[jg3]) must
         exist between any two tokens, since they would otherwise be
         interpreted as a single token.
       
       
       2.2 Basic Rules
       
       The following rules are used throughout this specification to describe
       basic parsing constructs. The US-ASCII coded character set is defined by
       ANSI X3.4-1986 [21].
       
              OCTET          = <any 8-bit sequence of data>
              CHAR           = <any US-ASCII character (octets 0 - 127)>
              UPALPHA        = <any US-ASCII uppercase letter "A".."Z">
              LOALPHA        = <any US-ASCII lowercase letter "a".."z">
              ALPHA          = UPALPHA | LOALPHA
              DIGIT          = <any US-ASCII digit "0".."9">
              CTL            = <any US-ASCII control character
                               (octets 0 - 31) and DEL (127)>
              CR             = <US-ASCII CR, carriage return (13)>
              LF             = <US-ASCII LF, linefeed (10)>
              SP             = <US-ASCII SP, space (32)>
              HT             = <US-ASCII HT, horizontal-tab (9)>
              <">            = <US-ASCII double-quote mark (34)>
       
       HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
       protocol elements except the entity-body (see appendix 19.3 for tolerant
       applications). The end-of-line marker within an entity-body is defined
       by its associated media type, as described in section 3.7.
       
              CRLF           = CR LF
       
       HTTP/1.1 headers 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.
       
       
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              LWS            = [CRLF] 1*( SP | HT )
       
       The TEXT rule is only used for descriptive field contents and values
       that are not intended to be interpreted by the message parser. Words of
       *TEXT may contain characters from character sets other than ISO 8859-1
       [22] only when encoded according to the rules of RFC 2047 [14].
       
              TEXT           = <any OCTET except CTLs,
                               but including LWS>
       
       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.
       
              token          = 1*<any CHAR except CTLs or separators>
              separators     = "(" | ")" | "<" | ">" | "@"
                             | "," | ";" | ":" | "\" | <">
                             | "/" | "[" | "]" | "?" | "="
                             | "{" | "}" | SP | HT
       
       Comments can be included in some HTTP header fields by surrounding the
       comment text with parentheses. Comments are only allowed in fields
       containing "comment" as part of their field value definition. In all
       other fields, parentheses are considered part of the field value.
       
              comment        = "(" *( ctext | quoted-pair | comment ) ")"
              ctext          = <any TEXT excluding "(" and ")">
       
       A string of text is parsed as a single word if it is quoted using
       double-quote marks.
       
              quoted-string  = ( <"> *(qdtext | quoted-pair ) <"> )
              qdtext         = <any TEXT except <">>
       
       The backslash character ("\") may be used as a single-character quoting
       mechanism only within quoted-string and comment constructs.
       
              quoted-pair    = "\" CHAR
       
       3 Protocol Parameters
       
       
       3.1 HTTP Version
       
       HTTP uses a "<major>.<minor>" numbering scheme to indicate versions of
       the protocol. The protocol versioning policy is intended to allow the
       
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       sender to indicate the format of a message and its capacity for
       understanding further HTTP communication, rather than the features
       obtained via that communication. No change is made to the version number
       for the addition of message components which do not affect communication
       behavior or which only add to extensible field values. The <minor>
       number is incremented when the changes made to the protocol add features
       which do not change the general message parsing algorithm, but which may
       add to the message semantics and imply additional capabilities of the
       sender. The <major> number is incremented when the format of a message
       within the protocol is changed. See RFC 2145 [36] for a fuller
       explanation.
       
       The version of an HTTP message is indicated by an HTTP-Version field in
       the first line of the message.
       
              HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT
       
       Note that the major and minor numbers MUST be treated as separate
       integers and that each may be incremented higher than a single digit.
       Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is lower
       than HTTP/12.3. Leading zeros MUST be ignored by recipients and MUST NOT
       be sent.
       
       Applications sending Request or Response messages, as defined by this
       specification, MUST include an HTTP-Version of "HTTP/1.1". Use of this
       version number indicates that the sending application is at least
       conditionally compliant with this specification.
       
       The HTTP version of an application is the highest HTTP version for which
       the application is at least conditionally compliant.
       
       Proxy and gateway applications must 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 never send a message with a version indicator which
       is greater than its actual version. If a higher version request is
       received, the proxy/gateway MUST either downgrade the request version,
       or respond with an error, or switch to tunnel behavior.
       
       Due to interoperability problems with HTTP/1.0 proxies discovered since
       the publication of RFC 2068, 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.
       
       
       
       
       
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       3.2 Uniform Resource Identifiers
       
       URIs have been known by many names: WWW addresses, Universal Document
       Identifiers, Universal Resource Identifiers [3], and finally the
       combination of Uniform Resource Locators (URL) [4] and Names (URN) [20].
       As far as HTTP is concerned, Uniform Resource Identifiers are simply
       formatted strings which identify--via name, location, or any other
       characteristic--a resource.
       
       
       3.2.1 General Syntax
       
       URIs in HTTP can be represented in absolute form or relative to some
       known base URI [11], depending upon the context of their use. The two
       forms are differentiated by the fact that absolute URIs always begin
       with a scheme name followed by a colon. For definitive information on
       URL syntax and semantics, see RFC XURI [42] (which replaces RFCs 1738
       [4] and RFC 1808 [11]). This specification adopts the definitions of
       "URI-reference", "absoluteURI", "relativeURI", "port",
       "host","abs_path", "rel_path", and "site" 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 should be cautious about depending on URI lengths
         above 255 bytes, because some older client or proxy implementations
         may 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 ]
       
       If the port is empty or not given, port 80 is assumed. The semantics are
       that the identified resource is located at the server listening for TCP
       connections on that port of that host, and the Request-URI for the
       resource is abs_path. The use of IP addresses in URL's SHOULD be avoided
       whenever possible (see RFC 1900 [24]). If the abs_path is not present in
       the URL, it MUST be given as "/" when used as a Request-URI for a
       resource (section 5.1.2).
       
       
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       3.2.3 URI Comparison
       
       When comparing two URIs to decide if they match or not, a client SHOULD
       use a case-sensitive octet-by-octet comparison of the entire URIs, with
       these exceptions:
       
         .  A port that is empty or not given is equivalent to the default port
            for that URI-reference;
         .  Comparisons of host names MUST be case-insensitive;
         .  Comparisons of scheme names MUST be case-insensitive;
         .  An empty abs_path is equivalent to an abs_path of "/".
       Characters other than those in the "reserved" and "unsafe" sets (see
       section 3.2) are equivalent to their ""%" HEX HEX" encodings.
       
       For example, the following three URIs are equivalent:
       
             http://abc.com:80/~smith/home.html
             http://ABC.com/%7Esmith/home.html
             http://ABC.com:/%7esmith/home.html
       
       3.3 Date/Time Formats
       
       
       3.3.1 Full Date
       
       HTTP applications have historically allowed three different formats for
       the representation of date/time stamps:
       
              Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
              Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
              Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
       The first format is preferred as an Internet standard and represents a
       fixed-length subset of that defined by RFC 1123 [8] (an update to RFC
       822 [9]). The second format is in common use, but is based on the
       obsolete RFC 850 [12] date format and lacks a four-digit year. HTTP/1.1
       clients and servers that parse the date value MUST accept all three
       formats (for compatibility with HTTP/1.0), though they MUST only
       generate the RFC 1123 format for representing HTTP-date values in header
       fields.
       
         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.
       
       
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              HTTP-date    = rfc1123-date | rfc850-date | asctime-date
              rfc1123-date = wkday "," SP date1 SP time SP "GMT"
              rfc850-date  = weekday "," SP date2 SP time SP "GMT"
              asctime-date = wkday SP date3 SP time SP 4DIGIT
              date1        = 2DIGIT SP month SP 4DIGIT
                             ; day month year (e.g., 02 Jun 1982)
              date2        = 2DIGIT "-" month "-" 2DIGIT
                             ; day-month-year (e.g., 02-Jun-82)
              date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
                             ; month day (e.g., Jun  2)
              time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                             ; 00:00:00 - 23:59:59
              wkday        = "Mon" | "Tue" | "Wed"
                           | "Thu" | "Fri" | "Sat" | "Sun"
              weekday      = "Monday" | "Tuesday" | "Wednesday"
                           | "Thursday" | "Friday" | "Saturday" | "Sunday"
              month        = "Jan" | "Feb" | "Mar" | "Apr"
                           | "May" | "Jun" | "Jul" | "Aug"
                           | "Sep" | "Oct" | "Nov" | "Dec"
         Note: HTTP requirements for the date/time stamp format apply only
         to their usage within the protocol stream. Clients and servers are
         not required to use these formats for user presentation, request
         logging, etc.
       
       
       3.3.2 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
         in the other direction is not required, in that not all characters
         may be available in a given character set and a character set may
         provide more than one sequence of octets to represent a particular
         character. This definition is intended to allow various kinds of
         character encodings, from simple single-table mappings such as US-
         ASCII to complex table switching methods such as those that use ISO
         2022's techniques. However, the definition associated with a MIME
         character set name MUST fully specify the mapping to be performed
         from octets to characters. In particular, use of external profiling
         information to determine the exact mapping is not permitted.
       
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         Note: This use of the term "character set" is more commonly
         referred to as a "character encoding." However, since HTTP and MIME
         share the same registry, it is important that the terminology also
         be shared.
       
       HTTP character sets are identified by case-insensitive tokens. The
       complete set of tokens is defined by the IANA Character Set registry
       [19].
       
              charset = token
       Although HTTP allows an arbitrary token to be used as a charset value,
       any token that has a predefined value within the IANA Character Set
       registry [19] MUST represent the character set defined by that registry.
       Applications SHOULD limit their use of character sets to those defined
       by the IANA registry.
       
       Implementers should be aware of IETF character set requirements [38]
       [41].
       
       
       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 14.12) header fields. Although the value describes the content-
       coding, what is more important is that it indicates what decoding
       mechanism will be required to remove the encoding.
       
       The Internet Assigned Numbers Authority (IANA) acts as a registry for
       content-coding value tokens. Initially, the registry contains the
       following tokens:
       
       
       gzip An encoding format produced by the file compression program "gzip"
            (GNU zip) as described in RFC 1952 [25]. This format is a Lempel-
            Ziv coding (LZ77) with a 32 bit CRC.
       
       
       compress
            The encoding format produced by the common UNIX file compression
            program "compress". This format is an adaptive Lempel-Ziv-Welch
            coding (LZW).
       
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         Note: Use of program names for the identification of encoding
         formats is not desirable and should be discouraged for future
         encodings. Their use here is representative of historical practice,
         not good design. For compatibility with previous implementations of
         HTTP, applications should consider "x-gzip" and "x-compress" to be
         equivalent to "gzip" and "compress" respectively.
       
       deflate The "zlib" format defined in RFC 1950 [31] in combination with
            the "deflate" compression mechanism described in RFC 1951 [29].
       
       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 Content-Encoding header.
       
       New content-coding value tokens should be registered; to allow
       interoperability between clients and servers, specifications of the
       content coding algorithms needed to implement a new value should be
       publicly available and adequate for independent implementation, and
       conform to the purpose of content coding defined in this section.
       
       
       3.6 Transfer Codings
       
       Transfer coding values are used to indicate an encoding transformation
       that has been, can be, or may need to be applied to an entity-body in
       order to ensure "safe transport" through the network. This differs from
       a content coding in that the transfer coding is a property of the
       message, not of the original entity.
       
              transfer-coding         = "chunked" | transfer-extension
              transfer-extension      = token *( ";" parameter )
       Parameters may be 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.Y) and in the Transfer-
       Encoding header field (section 14.40).
       
       Transfer codings are analogous to the Content-Transfer-Encoding values
       of MIME [7], which were designed to enable safe transport of binary data
       over a 7-bit transport service. However, safe transport has a different
       focus for an 8bit-clean transfer protocol. In HTTP, the only unsafe
       characteristic of message-bodies is the difficulty in determining the
       exact body length (section 7.2.2), or the desire to encrypt data over a
       shared transport.
       
       The Internet Assigned Numbers Authority (IANA) acts as a registry for
       transfer-coding value tokens. Initially, the registry contains the
       
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       following tokens: "chunked" (section 3.6.1), "identity" (section 3.6.2),
       "gzip" (section 3.5), "compress" (section 3.5), and "deflate" (section
       3.5).
       
       New transfer-coding value tokens should be registered in the same way as
       new content-coding value tokens (section 3.5).
       
       A server which receives an entity-body with a transfer-coding it does
       not understand SHOULD return 501 (Unimplemented), and close the
       connection. A server MUST NOT send transfer-codings to an HTTP/1.0
       client.
       
       
       3.6.1 Chunked Transfer Coding
       
       The chunked encoding modifies the body of a message in order to transfer
       it as a series of chunks, each with its own size indicator, followed by
       an optional trailer containing entity-header fields. This allows
       dynamically-produced content to be transferred along with the
       information necessary for the recipient to verify that it has received
       the full message.
       
              Chunked-Body   = *chunk
                               last-chunk
                               trailer
                               CRLF
              chunk          = chunk-size [ chunk-extension ] CRLF
                               chunk-data CRLF
              chunk-size     = 1*HEX
              last-chunk     = 1*("0") [ chunk-extension ] CRLF
       
              chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
              chunk-ext-name = token
              chunk-ext-val  = token | quoted-string
              chunk-data     = chunk-size(OCTET)
              trailer        = *entity-header
       The chunk-size field is a string of hex digits indicating the size of
       the chunk. The chunked encoding is ended by any chunk whose size is
       zero, followed by the trailer, which is terminated by an empty line.
       
       The trailer allows the sender to include additional HTTP header fields
       at the end of the message. The Trailer header field can be used to
       indicate which header fields are included in a trailer (see section
       14.49).
       
       
       
       A server using chunked transfer-coding in a response MUST NOT use the
       trailer for other header fields than Content-MD5 and Authentication-Info
       unless the "chunked" transfer-coding is present in the request as an
       accepted transfer-coding in the TE field (section 14.48). The
       
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       Authentication-Info header is defined by RFC 2069 [32] or its successor
       [43].
       
       
       
       An example process for decoding a Chunked-Body is presented in appendix
       19.4.6.
       
       All HTTP/1.1 applications MUST be able to receive and decode the
       "chunked" transfer coding, and MUST ignore chunk-extension extensions
       they do not understand.
       
       
       3.6.2 Identity Transfer Coding
       
       The identity transfer-encoding is the default (identity) encoding; the
       use of no transformation whatsoever. This transfer-coding is used only
       in the TE header field, and SHOULD NOT be used in any Transfer-Encoding
       header field.
       
       
       3.7 Media Types
       
       HTTP uses Internet Media Types [17] in the Content-Type (section 14.18)
       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.
       
              parameter      = attribute "=" value
              attribute      = token
              value          = token | quoted-string
       The type, subtype, and parameter attribute names are case-insensitive.
       Parameter values may or may not be case-sensitive, depending on the
       semantics of the parameter name. Linear white space (LWS) MUST NOT be
       used between the type and subtype, nor between an attribute and its
       value. User agents that recognize the media-type MUST process (or
       arrange to be processed by any external applications used to process
       that type/subtype by the user agent) the parameters for that MIME type
       as described by that type/subtype definition to the and inform the user
       of any problems discovered.
       
         Note: 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.
       
       
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       Media-type values are registered with the Internet Assigned Number
       Authority (IANA [19]). The media type registration process is outlined
       in RFC 1590 [17]. Use of non-registered media types is discouraged.
       
       
       3.7.1 Canonicalization and Text Defaults
       
       Internet media types are registered with a canonical form. In general,
       an entity-body transferred via HTTP messages MUST be represented in the
       appropriate canonical form prior to its transmission; the exception is
       "text" types, as defined in the next paragraph.
       
       When in canonical form, media subtypes of the "text" type use CRLF as
       the text line break. HTTP relaxes this requirement and allows the
       transport of text media with plain CR or LF alone representing a line
       break when it is done consistently for an entire entity-body. HTTP
       applications MUST accept CRLF, bare CR, and bare LF as being
       representative of a line break in text media received via HTTP. In
       addition, if the text is represented in a character set that does not
       use octets 13 and 10 for CR and LF respectively, as is the case for some
       multi-byte character sets, HTTP allows the use of whatever octet
       sequences are defined by that character set to represent the equivalent
       of CR and LF for line breaks. This flexibility regarding line breaks
       applies only to text media in the entity-body; a bare CR or LF MUST NOT
       be substituted for CRLF within any of the HTTP control structures (such
       as header fields and multipart boundaries).
       
       If an entity-body is encoded with a Content-Encoding, 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
       19.8.2 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 [40], and MUST
       include a boundary parameter as part of the media type value. The
       message body is itself a protocol element and MUST therefore use only
       CRLF to represent line breaks between body-parts. Unlike in RFC 2046,
       the epilogue of any multipart message MUST be empty; HTTP applications
       MUST NOT transmit the epilogue (even if the original multipart contains
       an epilogue).
       
       
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       In HTTP, multipart body-parts MAY contain header fields which are
       significant to the meaning of that part. A Content-Location header field
       (section 14.15) SHOULD be included in the body-part of each enclosed
       entity that can be identified by a URL.
       
       In general, an HTTP user agent SHOULD follow the same or similar
       behavior as a MIME user agent would upon receipt of a multipart type. If
       an application receives an unrecognized multipart subtype, the
       application MUST treat it as being equivalent to "multipart/mixed".
       
         Note: The "multipart/form-data" type has been specifically defined
         for carrying form data suitable for processing via the POST request
         method, as described in RFC 1867 [15].
       
       
       3.8 Product Tokens
       
       Product tokens are used to allow communicating applications to identify
       themselves by software name and version. Most fields using product
       tokens also allow sub-products which form a significant part of the
       application to be listed, separated by whitespace. By convention, the
       products are listed in order of their significance for identifying the
       application.
       
              product         = token ["/" product-version]
              product-version = token
       Examples:
       
              User-Agent: CERN-LineMode/2.15 libwww/2.17b3
              Server: Apache/0.8.4
       Product tokens should be short and to the point -- use of them for
       advertising or other non-essential information is explicitly forbidden.
       Although any token character may appear in a product-version, this token
       SHOULD only be used for a version identifier (i.e., successive versions
       of the same product SHOULD only differ in the product-version portion of
       the product value).
       
       
       3.9 Quality Values
       
       HTTP content negotiation (section 12) uses short "floating point"
       numbers to indicate the relative importance ("weight") of various
       negotiable parameters. 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") ] )
       
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       "Quality values" is a misnomer, since these values merely represent
       relative degradation in desired quality.
       
       
       3.10 Language Tags
       
       A language tag identifies a natural language spoken, written, or
       otherwise conveyed by human beings for communication of information to
       other human beings. Computer languages are explicitly excluded. HTTP
       uses language tags within the Accept-Language and Content-Language
       fields.
       
       The syntax and registry of HTTP language tags is the same as that
       defined by RFC 1766 [1]. In summary, a language tag is composed of 1 or
       more parts: A primary language tag and a possibly empty series of
       subtags:
       
               language-tag  = primary-tag *( "-" subtag )
               primary-tag   = 1*8ALPHA
               subtag        = 1*8ALPHA
       Whitespace 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
       14.20), If-Match (section 14.25), 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.
       
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       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 without
       implying anything about 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 14.36) and Content-Range (section 14.17)
       header fields. An entity may 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. HTTP/1.1
       has been designed to allow implementations of applications that do not
       depend on knowledge of ranges.
       
       
       4 HTTP Message
       
       
       4.1 Message Types
       
       HTTP messages consist of requests from client to server and responses
       from server to client.
       
              HTTP-message   = Request | Response     ; HTTP/1.1 messages
       Request (section 5) and Response (section 6) messages use the generic
       message format of RFC 822 [9] for transferring entities (the payload of
       the message). Both types of message consist of a start-line, zero or
       more header fields (also known as "headers"), an empty line (i.e., a
       line with nothing preceding the CRLF) indicating the end of the header
       fields, and an optional message-body.
       
               generic-message = start-line
                                 *message-header
                                 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.
       
         Note: certain buggy HTTP/1.0 client implementations generate an
         extra CRLF's after a POST request. To restate what is explicitly
       
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         forbidden by the BNF, an HTTP/1.1 client must not preface or follow
         a request with an extra CRLF.
       
       
       4.2 Message Headers
       
       HTTP header fields, which include general-header (section 4.5), request-
       header (section 5.3), response-header (section 6.2), and entity-header
       (section 7.1) fields, follow the same generic format as that given in
       Section 3.1 of RFC 822 [9]. Each header field consists of a name
       followed by a colon (":") and the field value. Field names are case-
       insensitive. The field value may be preceded by any amount of LWS,
       though a single SP is preferred. Header fields can be extended over
       multiple lines by preceding each extra line with at least one SP or HT.
       Applications SHOULD follow "common form" 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 ] CRLF
              field-name     = token
              field-value    = *( field-content | LWS )
              field-content  = <the OCTETs making up the field-value
                               and consisting of either *TEXT or combinations
                               of token, separators, and quoted-string>
       The order in which header fields with differing field names are received
       is not significant. However, it is "good practice" to send general-
       header fields first, followed by request-header or response-header
       fields, and ending with the entity-header fields.
       
       Multiple message-header fields with the same field-name may be present
       in a message if and only if the entire field-value for that header field
       is defined as a comma-separated list [i.e., #(values)]. It MUST be
       possible to combine the multiple header fields into one "field-name:
       field-value" pair, without changing the semantics of the message, by
       appending each subsequent field-value to the first, each separated by a
       comma. The order in which header fields with the same field-name are
       received is therefore significant to the interpretation of the combined
       field value, and thus a proxy MUST NOT change the order of these field
       values when a message is forwarded.
       
       
       4.3 Message Body
       
       The message-body (if any) of an HTTP message is used to carry the
       entity-body associated with the request or response. The message-body
       differs from the entity-body only when a transfer coding has been
       applied, as indicated by the Transfer-Encoding header field (section
       14.40).
       
              message-body = entity-body
                           | <entity-body encoded as per Transfer-Encoding>
       
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       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 can be added or removed by any application along the
       request/response chain.
       
       The rules for when a message-body is allowed in a message differ for
       requests and responses.
       
       The presence of a message-body in a request is signaled by the inclusion
       of a Content-Length or Transfer-Encoding header field in the request's
       message-headers. A message-body MUST NOT be included in a request if the
       specification of the request method (section 5.1.1) does not allow
       sending an entity-body in requests. A server SHOULD read and forward a
       message-body on any request; if the request method does not include
       defined semantics for an entity-body, then the message-body SHOULD be
       ignored when handling the request.
       
       For response messages, whether or not a message-body is included with a
       message is dependent on both the request method and the response status
       code (section 6.1.1). All responses to the HEAD request method MUST NOT
       include a message-body, even though the presence of entity-header fields
       might lead one to believe they do. All 1xx (informational), 204 (no
       content), and 304 (not modified) responses MUST NOT include a message-
       body. All other responses do include a message-body, although it may be
       of zero length.
       
       
       4.4 Message Length
       
       When a message-body is included with a message, the 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 14.40) is present and
            indicates that the "chunked" transfer coding has been applied, then
            the length is defined by the chunked encoding (section 3.6).
       
         3. If a Content-Length header field (section 14.14) is present, its
            decimal value in OCTETs represents the length of the message-body.
       
         4.    If the message uses the media type "multipart/byteranges", which
            is self-delimiting, then that defines the 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
       
       
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            byte-range specifiers implies that the client can parse
            multipart/byteranges responses.
       
         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 the
       "chunked" transfer coding. If both are received, 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 14.19
                             | Pragma                   ; Section 14.32
                             | Transfer-Encoding        ; Section 14.40
                             | Upgrade                  ; Section 14.41
                             | Trailer                  ; Section 14.49
                             | Via                      ; Section 14.44
       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 )        ; 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. Servers SHOULD
       return the status code 405 (Method Not Allowed) if the method is known
       by the server but not allowed for the requested resource, and 501 (Not
       Implemented) if the method is unrecognized or not implemented by the
       server. The 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.
       
       
       
       
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       5.1.2 Request-URI
       
       The Request-URI is a Uniform Resource Identifier (section 3.2) and
       identifies the resource upon which to apply the request.
       
              Request-URI    = "*" | absoluteURI | abs_path
       The three options for Request-URI are dependent on the nature of the
       request. The asterisk "*" means that the request does not apply to a
       particular resource, but to the server itself, and is only allowed when
       the method used does not necessarily apply to a resource. One example
       would be
       
              OPTIONS * HTTP/1.1
       The absoluteURI form is 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 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 (site) 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.
       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.
       
       In requests that they forward, proxies MUST NOT rewrite the "abs_path"
       part of a Request-URI in any way except as noted above to replace a null
       abs_path with "*", no matter what the proxy does in its internal
       implementation.
       
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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 URL character for a reserved purpose. Implementers
         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
       
       HTTP/1.1 origin servers SHOULD be aware that 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. (But see section
       19.5.1 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 hostnames)
       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
       
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                             | Expect                   ; Section 14.47
                             | From                     ; Section 14.22
                             | Host                     ; Section 14.23
                             | If-Modified-Since        ; Section 14.24
                             | If-Match                 ; 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 14.36
                             | Referer                  ; Section 14.37
                             | TE                       ; Section 14.48
                             | User-Agent               ; Section 14.42
       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 )        ; Section 7.1
                              CRLF
                              [ message-body ]          ; Section 7.2
       
       6.1 Status-Line
       
       The first line of a Response message is the Status-Line, consisting of
       the protocol version followed by a numeric status code and its
       associated textual phrase, with each element separated by SP characters.
       No CR or LF is allowed except in the final CRLF sequence.
       
              Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
       
       6.1.1 Status Code and Reason Phrase
       
       The Status-Code element is a 3-digit integer result code of the attempt
       to understand and satisfy the request. These codes are fully defined in
       section 10. The Reason-Phrase is intended to give a short textual
       description of the Status-Code. The Status-Code is intended for use by
       automata and the Reason-Phrase is intended for the human user. The
       client is not required to examine or display the Reason-Phrase.
       
       
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       The first digit of the Status-Code defines the class of response. The
       last two digits do not have any categorization role. There are 5 values
       for the first digit:
       
       
         .  1xx: Informational - Request received, continuing process
       
         .  2xx: Success - The action was successfully received, understood,
            and accepted
       
         .  3xx: Redirection - Further action must be taken in order to
            complete the request
       
         .  4xx: Client Error - The request contains bad syntax or cannot be
            fulfilled
       
         .  5xx: Server Error - The server failed to fulfill an apparently
            valid request
       The individual values of the numeric status codes defined for HTTP/1.1,
       and an example set of corresponding Reason-Phrase's, are presented
       below. The reason phrases listed here are only recommended -- they may
       be replaced by local equivalents without affecting the protocol.
       
          Status-Code    = "100"  ; Continue
                         | "101"  ; Switching Protocols
                         | "200"  ; OK
                         | "201"  ; Created
                         | "202"  ; Accepted
                         | "203"  ; Non-Authoritative Information
                         | "204"  ; No Content
                         | "205"  ; Reset Content
                         | "206"  ; Partial Content
                         | "300"  ; Multiple Choices
                         | "301"  ; Moved Permanently
                         | "302"  ; Moved Temporarily
                         | "303"  ; See Other
                         | "304"  ; Not Modified
                         | "305"  ; Use Proxy
                         | "400"  ; Bad Request
                         | "401"  ; Unauthorized
                         | "402"  ; Payment Required
                         | "403"  ; Forbidden
                         | "404"  ; Not Found
                         | "405"  ; Method Not Allowed
                         | "406"  ; Not Acceptable
                         | "407"  ; Proxy Authentication Required
                         | "408"  ; Request Time-out
                         | "409"  ; Conflict
                         | "410"  ; Gone
                         | "411"  ; Length Required
                         | "412"  ; Precondition Failed
       
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                         | "413"  ; Request Entity Too Large
                         | "414"  ; Request-URI Too Large
                         | "415"  ; Unsupported Media Type
                         | "416"  ; Requested range not satisfiable
                         | "500"  ; Internal Server Error
                         | "501"  ; Not Implemented
                         | "502"  ; Bad Gateway
                         | "503"  ; Service Unavailable
                         | "504"  ; Gateway Time-out
                         | "505"  ; 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
                              | Location                ; Section 14.30
                              | Proxy-Authenticate      ; Section 14.33
       
                              | Retry-After             ; Section 14.38
                              | Server                  ; Section 14.39
       
                              | Vary                    ; Section 14.43
                              | Warning                 ; Section 14.45
                              | WWW-Authenticate        ; Section 14.46
       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
       
       
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       all parties in the communication recognize them to be response-header
       fields. Unrecognized header fields are treated as entity-header fields.
       
       
       7 Entity
       
       Request and Response messages MAY transfer an entity if not otherwise
       restricted by the request method or response status code. An entity
       consists of entity-header fields and an entity-body, although some
       responses will only include the entity-headers.
       
       In this section, both sender and recipient refer to either the client or
       the server, depending on who sends and who receives the entity.
       
       
       7.1 Entity Header Fields
       
       Entity-header fields define optional metainformation about the entity-
       body or, if no body is present, about the resource identified by the
       request.
       
              entity-header  = Allow                    ; Section 14.7
                             | Content-Base             ; Section 14.11
                             | Content-Encoding         ; Section 14.12
                             | Content-Language         ; Section 14.13
                             | Content-Length           ; Section 14.14
                             | Content-Location         ; Section 14.15
                             | Content-MD5              ; Section 14.16
                             | Content-Range            ; Section 14.17
                             | Content-Type             ; Section 14.18
                             | ETag                     ; Section 14.20
                             | 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 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 may have been
       applied to ensure safe and proper transfer of the message.
       
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       7.2.1 Type
       
       When an entity-body is included with a message, the data type of that
       body is determined via the header fields Content-Type and Content-
       Encoding. These define a two-layer, ordered encoding model:
       
              entity-body := Content-Encoding( Content-Type( data ) )
       Content-Type specifies the media type of the underlying data. Content-
       Encoding may be used to indicate any additional content codings applied
       to the data, usually for the purpose of data compression, that are a
       property of the requested resource. There is no default encoding.
       
       Any HTTP/1.1 message containing an entity-body SHOULD include a Content-
       Type header field defining the media type of that body. If and only if
       the media type is not given by a Content-Type field, the recipient MAY
       attempt to guess the media type via inspection of its content and/or the
       name extension(s) of the URL used to identify the resource. If the media
       type remains unknown, the recipient SHOULD treat it as type
       "application/octet-stream".
       
       
       7.2.2 Length
       
       The length of an entity-body is the length of the message-body after any
       transfer codings have been removed. Section 4.4 defines how the 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. Analyses of these performance
       problems are available [30]; analysis and results from a prototype
       implementation are in [26]. Implementation experience and measurements
       of actual HTTP/1.1 (RFC 2068) implementations show good results [39].
       
       Alternatives have also been explored, for example, T/TCP  [27].
       
       
       
       Persistent HTTP connections have a number of advantages:
       
       
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         .  By opening and closing fewer TCP connections, CPU time is saved,
            and memory used for TCP protocol control blocks is also saved.
         .  HTTP requests and responses can be pipelined on a connection.
            Pipelining allows a client to make multiple requests without
            waiting for each response, allowing a single TCP connection to be
            used much more efficiently, with much lower elapsed time.
         .  Network congestion is reduced by reducing the number of packets
            caused by TCP opens, and by allowing TCP sufficient time to
            determine the congestion state of the network.
         .  HTTP can evolve more gracefully; since errors can be reported
            without the penalty of closing the TCP connection. Clients using
            future versions of HTTP might optimistically try a new feature, but
            if communicating with an older server, retry with old semantics
            after an error is reported.
       HTTP implementations SHOULD implement persistent connections.
       
       
       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 may assume
       that the server will maintain a persistent connection.
       
       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. 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
       
       
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       signaled. See section 19.7.1 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
       
       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 may lead
       toindeterminate 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 14.2.1.
       
       The proxy server MUST signal persistent connections separately with its
       clients and the origin servers (or other proxy servers) that it connects
       to. Each persistent connection applies to only one transport link.
       
       A proxy server MUST NOT establish a persistent connection with an
       HTTP/1.0 client (but see section Error! Reference source not found. for
       information about the Keep-Alive header implemented by many HTTP/1.0
       clients).
       
       
       
       
       
       
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       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 of this time-out for either the
       client or the server.
       
       When a client or server wishes to time-out it SHOULD issue a graceful
       close on the transport connection. Clients and servers SHOULD both
       constantly watch for the other side of the transport close, and respond
       to it as appropriate. If a client or server does not detect the other
       side's close promptly it could cause unnecessary resource drain on the
       network.
       
       A client, server, or proxy MAY close the transport connection at any
       time. For example, a client MAY have started to send a new request at
       the same time that the server has decided to close the "idle"
       connection. From the server's point of view, the connection is being
       closed while it was idle, but from the client's point of view, a request
       is in progress.
       
       This means that clients, servers, and proxies MUST be able to recover
       from asynchronous close events. Client software SHOULD reopen the
       transport connection and retransmit the aborted sequence of requests
       without user interaction so long as the request sequence is idempotent
       (see section 9.1.2);.Non-idempotent methods or sequences MUST NOT be
       automatically retried, although user agents MAY offer a human operator
       the choice of retrying the request(s). However, this automatic retry
       SHOULD NOT be repeated if the second request 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 maintain AT MOST 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 of the Internet or other networks.
       
       
       
       
       
       
       
       
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       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.
       
       
       8.2.3 Automatic retrying of requests
       
       If a user agent sees the transport connection close before it receives a
       final response to its request, if the request method is idempotent (see
       section 9.1.2), the user agent SHOULD retry the request without user
       interaction.  If the request method is not idempotent, the user agent
       SHOULD NOT retry the request without user confirmation.  (Confirmation
       by user-agent software with semantic understanding of the application
       MAY substitute for user confirmation.)
       
       
       8.2.4 Use of the 100 (Continue) status
       
       The purpose of the 100 (Continue) status (see section 10.1.1) is to
       allow an end-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 may either be inappropriate or highly inefficient for
       the client to send the body if the server will reject the message
       without looking at the body.
       
       Requirements for HTTP/1.1 clients:
       
         .  If a client will wait for a 100 (Continue) response before sending
            the request body, it MUST send an Expect request-header field
            (section 14.47) with the "100-continue" expectation.
       
       
       
       
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         .  A client MUST NOT send an Expect request-header field (section
            14.47) with the "100-continue" expectation if it does not intend to
            send a request body.
       
         Note: 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 or lengthy period before sending
         the request body.
       
       Requirements for HTTP/1.1 origin servers:
       
         .  Upon receiving a request which includes an Expect request-header
            field with the "100-continue" expectation, an origin server MUST
            either respond with 100 (Continue) status and continue to read from
            the input stream, or respond with an error status.  The origin
            server MUST NOT wait for the request body before sending the 100
            (Continue) response.  If it responds with an error status, 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 an error status.
       
         .  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.
       
         .  An origin server MAY omit a 100 (Continue) response if has already
            received some or all of the request body for the corresponding
            request.
       
         .  An origin server that sends a 100 (Continue) response MUST
            ultimately send a final status code, once the request body is
            received and processed, unless it terminates the transport
            connection prematurely.
       
         .  If an origin server receives a request that does not include an
            Expect request-header field with the "100-continue" expectation,
            the request includes a request body, and the server responds with
            an error status 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 may not
            reliably receive the response message.  However, this requirement
            should not be construed as preventing a server from defending
            itself against denial-of-service attacks, or from badly broken
            client implementations.
       
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       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.
       
       Requirements for HTTP/1.1 proxies:
       
         .  If a proxy receives a request that includes an Expect request-
            header field with the "100-continue" expectation, and the proxy
            either knows that the next-hop server complies with HTTP/1.1 or
            higher, or does not know the HTTP version of the next-hop server,
            it MUST forward the request, including the Expect header field.
       
         .  If the proxy knows that the version of the next-hop server is
            HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
            respond with a 417 (Expectation Failed) status.
       
         .  Proxies SHOULD maintain a cache recording the HTTP version numbers
            received from recently-referenced next-hop servers.
       
         .  A proxy MUST NOT forward a 100 (Continue) response if the request
            message was received from an HTTP/1.0 (or earlier) client and did
            not include an Expect request-header field with the "100-continue"
            expectation.  This requirement overrides the general rule for
            forwarding of 1xx responses (see section 10.1).
       
       
       8.2.5 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 the client sees the connection close
       before receiving any status from the server, the client SHOULD retry the
       request, subject to the restrictions in section 8.2.3. If the client
       does retry this request, it MAY use the following "binary exponential
       backoff" algorithm to be assured of obtaining a reliable response:
       
         1. Initiate a new connection to the server
       
         2. Transmit the request-headers
       
         3. Initialize a variable R to the estimated round-trip time to the
            server (e.g., based on the time it took to establish the
            connection), or to a constant value of 5 seconds if the round-trip
            time is not available.
       
       
       
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         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
       
         .  SHOULD NOT continue and
       
         .  SHOULD close the connection if it has not completed sending the
            request message.
       
       
       
       
       9 Method Definitions
       
       The set of common methods for HTTP/1.1 is defined below. Although this
       set can be expanded, additional methods cannot be assumed to share the
       same semantics for separately extended clients and servers.
       
       The Host request-header field (section 14.23) MUST accompany all
       HTTP/1.1 requests.
       
       
       9.1 Safe and Idempotent Methods
       
       
       9.1.1 Safe Methods
       
       Implementers should be aware that the software represents the user in
       their interactions over the Internet, and should be careful to allow the
       user to be aware of any actions they may take which may have an
       unexpected significance to themselves or others.
       
       In particular, the convention has been established that the GET and HEAD
       methods should never have the significance of taking an action other
       than retrieval. These methods should be considered "safe." This allows
       user agents to represent other methods, such as POST, PUT and DELETE, in
       a special way, so that the user is made aware of the fact that a
       possibly unsafe action is being requested.
       
       
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       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 may 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 have no 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 is
       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 cachable.
       
       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 may
       use the OPTIONS body to make more detailed queries on the server. A
       server that does not support such an extension MAY discard the request
       body.
       
       If the Request-URI is an asterisk ("*"), the OPTIONS request is intended
       to apply to the server in general rather than to a specific resource.
       Since a server's communication options typically depend on the resource,
       the "*" request is only useful as a "ping" or "no-op" type of method; it
       
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       does nothing beyond allowing the client to test the capabilities of the
       server. For example, this can be used to test a proxy for HTTP/1.1
       compliance (or lack thereof).
       
       If the Request-URI is not an asterisk, the OPTIONS request applies only
       to the options that are available when communicating with that resource.
       
       A 200 response SHOULD include any header fields that indicate optional
       features implemented by the server and applicable to that resource
       (e.g., Allow), possibly including extensions not defined by this
       specification. The response body, if any, SHOULD also include
       information about the communication options. The format for such a body
       is not defined by this specification, but may 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 14.36. The
       partial GET method is intended to reduce unnecessary network usage by
       
       
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       allowing partially-retrieved entities to be completed without
       transferring data already held by the client.
       
       The response to a GET request is cachable if and only if it meets the
       requirements for HTTP caching described in section 13.
       
       See section 15.10 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 cachable in the sense that the
       information contained in the response may be used to update a previously
       cached entity from that resource. If the new field values indicate that
       the cached entity differs from the current 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 destination server accept
       the entity enclosed in the request as a new subordinate of the resource
       identified by the Request-URI in the Request-Line. POST is designed to
       allow a uniform method to cover the following functions:
       
       
         .  Annotation of existing resources;
       
         .  Posting a message to a bulletin board, newsgroup, mailing list, or
            similar group of articles;
       
         .  Providing a block of data, such as the result of submitting a form,
            to a data-handling process;
       
         .  Extending a database through an append operation.
       The actual function performed by the POST method is determined by the
       server and is usually dependent on the Request-URI. The posted entity is
       subordinate to that URI in the same way that a file is subordinate to a
       directory containing it, a news article is subordinate to a newsgroup to
       which it is posted, or a record is subordinate to a database.
       
       
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       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 status of the
       request and refers to the new resource, and a Location header (see
       section 14.30).
       
       Responses to this method are not cachable, 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
       cachable resource.
       
       POST requests must obey the message transmission requirements set out in
       section 8.2.
       
       See section 15.10 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 cachable.
       
       The fundamental difference between the POST and PUT requests is
       reflected in the different meaning of the Request-URI. The URI in a POST
       request identifies the resource that will handle the enclosed entity.
       That resource may be a data-accepting process, a gateway to some other
       protocol, or a separate entity that accepts annotations. In contrast,
       the URI in a PUT request identifies the entity enclosed with the request
       -- the user agent knows what URI is intended and the server MUST NOT
       
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       attempt to apply the request to some other resource. If the server
       desires that the request be applied to a different URI, it MUST send a
       301 (Moved Permanently) response; the user agent MAY then make its own
       decision regarding whether or not to redirect the request.
       
       A single resource MAY be identified by many different URIs. For example,
       an article may have a URI for identifying "the current version" which is
       separate from the URI identifying each particular version. In this case,
       a PUT request on a general URI may result in several other URIs being
       defined by the origin server.
       
       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.
       
       
       9.6.1 Partial PUT (PUT with Content-Range)
       
       A PUT method MAY supply a partial entity body specified by a Content-
       Range header. The PUT requests that the resource be updated or created
       with the partial entity-body, in a manner entirely up to the origin
       server. However, the origin server may indicate to caches that they may
       perform the same update to a cached copy.
       
         Note: Even if partial PUT is not exceedingly interesting, it should
         establish a paradigm for possible future methods that do
         incremental updates to resources and want to allow incremental
         updates to cached copies.
       
         1. An HTTP/1.1 server that does not support PUT with Content-Range
            MUST reply with 506 (Partial Update Not Implemented) and MUST NOT
            perform any action on the requested resource.
       
         2. An HTTP/1.1 client MUST know that that the origin server for the
            request supports PUT with Content-Range header (but not
            neccessarily on the specified resource) before attempting such a
            request. This is  necessary since a pre-1.1 server that does not
            understand PUT with Content-Range header will overwrite the whole
            resource instead of just the Content-Range the client requested.
       
         3. An HTTP/1.1 origin server MAY reply to a PUT with Content-Range
            with a 207 (Partial Update OK) if the change in the entity body of
            a cached response can be duplicated by a cache by merely replacing
            the bytes at the byte positions specified in the Content-Range
            with the corresponding bytes in the entity-body in the request.
            Client and proxies that do not understand 207 (Range Update) will
            be correct if they treat it as "200 OK". (Recall that responses to
            PUT are not cachable)
       
       
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         4. An HTTP/1.1 cache receiving a 207 (Partial Update OK) status-code
            in the response to a PUT with Content-Range MAY update the entity
            in the cache as described above; if it does not, or if the status-
            code is anything else, it MUST invalidate the entity, as per
            section 9.6)
       
       
       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 response is OK but 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 cachable.
       
       
       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 14.44) 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 successful, 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 by SSL
       tunnelling. [44]
       
       
       10 Status Code Definitions
       
       Each Status-Code is described below, including a description of which
       method(s) it can follow and any metainformation required in the
       response.
       
       Status codes are assigned per IANA registry [45].
       
       
       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
       codes. 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 may continue with its request. This interim response is used
       to inform the client that the initial part of the request has been
       received and has not yet been rejected by the server. The client SHOULD
       continue by sending the remainder of the request or, if the request has
       already been completed, ignore this response. The server MUST send a
       final response after the request has been completed. See section 8.2.4
       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 14.41), for a
       
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       change in the application protocol being used on this connection. The
       server will switch protocols to those defined by the response's Upgrade
       header field immediately after the empty line which terminates the 101
       response.
       
       The protocol should only be switched when it is advantageous to do so.
       For example, switching to a newer version of HTTP is advantageous over
       older versions, and switching to a real-time, synchronous protocol may
       be advantageous when delivering resources that use such features.
       
       
       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 URL for
       the resource given by a Location 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.
       
       
       10.2.3 202 Accepted
       
       The request has been accepted for processing, but the processing has not
       been completed.  The request MAY or MAY NOT eventually be acted upon, as
       it MAY be disallowed when processing actually takes place. There is no
       
       
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       facility for re-sending a status code from an asynchronous operation
       such as this.
       
       The 202 response is intentionally non-committal. Its purpose is to allow
       a server to accept a request for some other process (perhaps a batch-
       oriented process that is only run once per day) without requiring that
       the user agent's connection to the server persist until the process is
       completed. The entity returned with this response SHOULD include an
       indication of the request's current status and either a pointer to a
       status monitor or some estimate of when the user can expect the request
       to be fulfilled.
       
       
       10.2.4 203 Non-Authoritative Information
       
       The returned metainformation in the entity-header is not the definitive
       set as available from the origin server, but is gathered from a local or
       a third-party copy. The set presented MAY be a subset or superset of the
       original version. For example, including local annotation information
       about the resource MAY result in a superset of the metainformation known
       by the origin server. Use of this response code is not required and is
       only appropriate when the response would otherwise be 200 (OK).
       
       
       10.2.5 204 No Content
       
       The server has fulfilled the request but there is no new information to
       send back.  If the client is a user agent, it SHOULD NOT change its
       document view from that which caused the request to be sent. This
       response is primarily intended to allow input for actions to take place
       without causing a change to the user agent's active document view. The
       response MAY include new metainformation in the form of entity-headers,
       which SHOULD apply to the document currently in the user agent's active
       view.
       
       The 204 response MUST NOT include 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 14.36)
       indicating the desired range , and may have included an If-Range header
       field (section 14.27) to make the request conditional.
       
       The response MUST include the following header fields:
       
         .  Either a Content-Range header field (section 14.17) indicating the
            range included with this response, or a multipart/byteranges
            Content-Type including Content-Range fields for each part. If
            multipart/byteranges is not used, the Content-Length header field
            in the response MUST match the actual number of OCTETs transmitted
            in the message-body.
       
         .  Date
       
         .  ETag and/or Content-Location, if the header would have been sent in
            a 200 response to the same request
       
         .  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.
       
       
       10.2.8 207 Partial Update OK
       
       The server has fulfilled a method that specified a partial update, and
       the change in the resource may be mimiced by a cache that understands
       the method. In the case of partial PUT the change in the resource can be
       duplicated by a cache merely by replacing the bytes at the byte
       positions specified with the corresponding bytes in the entity-body in
       the request.
       
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       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 implement 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
         may be clients that implement such a fixed limitation.
       
       
       10.3.1 300 Multiple Choices
       
       The requested resource corresponds to any one of a set of
       representations, each with its own specific location, and agent-driven
       negotiation information (section 12) is being provided so that the user
       (or user agent) can select a preferred representation and redirect its
       request to that location.
       
       Unless it was a HEAD request, the response SHOULD include an entity
       containing a list of resource characteristics and location(s) from which
       the user or user agent can choose the one most appropriate. The entity
       format is specified by the media type given in the Content-Type header
       field. Depending upon the format and the capabilities of 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 URL for that representation in the Location field;
       user agents MAY use the Location field value for automatic redirection.
       This response is cachable 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 be done using one of the
       returned URIs.  Clients with link editing capabilities SHOULD
       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 cachable unless indicated otherwise.
       
       If the new URI is a location, its URL SHOULD be given by the Location
       field in the response. Unless the request method was HEAD, the entity of
       the response SHOULD contain a short hypertext note with a hyperlink to
       the new URI(s).
       
       
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       If the 301 status code is received in response to a request other than
       GET or HEAD, the user agent MUST NOT automatically redirect the request
       unless it can be confirmed by the user, since this might change the
       conditions under which the request was issued.
       
         Note: When automatically redirecting a POST request after receiving
         a 301 status code, some existing HTTP/1.0 user agents will
         erroneously change it into a GET request.
       
       
       10.3.3 302 Found
       
       The requested resource resides temporarily under a different URI. Since
       the redirection may be altered on occasion, the client SHOULD continue
       to use the Request-URI for future requests.  This response is only
       cachable if indicated by a Cache-Control or Expires header field.
       
       If the new URI is a location, its URL 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 other than
       GET or HEAD, the user agent MUST NOT automatically redirect the request
       unless it can be confirmed by the user, since this might change the
       conditions under which the request was issued.
       
         Note: When automatically redirecting a POST request after receiving
         a 302 status code, some existing HTTP/1.0 user agents will
         erroneously change it into a GET request.
       
         Note: RFC 1945 and RFC 2068 specify that the client should not
         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 is not cachable, but the response to the second (redirected)
       request MAY be cachable.
       
       
       
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       If the new URI is a location, its URL 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 response MUST NOT contain a message-
       body.
       
       The response MUST include the following header fields:
       
         .  Date, unless its omission is required by section 14.19.1
         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.
       
         .  ETag and/or Content-Location, if the header would have been sent in
            a 200 response to the same request
         .  Expires, Cache-Control, and/or Vary, if the field-value might
            differ from that sent in any previous response for the same variant
       If the conditional GET used a strong cache validator (see section
       13.3.3), the response SHOULD NOT include other entity-headers. Otherwise
       (i.e., the conditional GET used a weak validator), the response MUST NOT
       include other entity-headers; this prevents inconsistencies between
       cached entity-bodies and updated headers.
       
       If a 304 response indicates an entity not currently cached, then the
       cache MUST disregard the response and repeat the request without the
       conditional.
       
       If a cache uses a received 304 response to update a cache entry, the
       cache MUST update the entry to reflect any new field values given in the
       response.
       
       The 304 response MUST NOT include a message-body, and thus is always
       terminated by the first empty line after the header fields.
       
       
       
       
       
       
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       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 URL of the proxy. The
       recipient is expected to repeat this single request via the proxy. 305
       responses MUST only be generated by origin servers.
       
       
       10.3.7 307 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
       cachable if indicated by a Cache-Control or Expires header field.
       
       If the new URI is a location, its URL 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 other than
       GET or HEAD, the user agent MUST NOT automatically redirect the request
       unless it can be confirmed by the user, since this might change the
       conditions under which the request was issued.
       
         Note: Many pre-HTTP/1.1 user agents do not understand the 307
         status.  An appropriate response entity should contain the
         information necessary for a user to repeat the original request on
         the new URL.
       
       
       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.
       
         Note: 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.
       
       
       
       
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       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 14.46) 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 MAY include relevant diagnostic
       information. HTTP access authentication is explained in section 11.
       
       
       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.
       
       
       
       
       
       
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       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
         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 section 11.
       
       
       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.
       
       
       
       
       
       
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       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 may not be possible
       and is not required.
       
       Conflicts are most likely to occur in response to a PUT request. If
       versioning is being used and the entity being PUT includes changes to a
       resource which conflict with those made by an earlier (third-party)
       request, the server MAY use the 409 response to indicate that it can't
       complete the request. In this case, the response entity SHOULD contain a
       list of the differences between the two versions in a format defined by
       the response Content-Type.
       
       
       10.4.11 410 Gone
       
       The requested resource is no longer available at the server and no
       forwarding address is known. This condition SHOULD be considered
       permanent. Clients with link editing capabilities SHOULD delete
       references to the Request-URI after user approval. If the server does
       not know, or has no facility to determine, whether or not the condition
       is permanent, the status code 404 (Not Found) SHOULD be used instead.
       This response is cachable unless indicated otherwise.
       
       The 410 response is primarily intended to assist the task of web
       maintenance by notifying the recipient that the resource is
       intentionally unavailable and that the server owners desire that remote
       links to that resource be removed. Such an event is common for limited-
       time, promotional services and for resources belonging to individuals no
       longer working at the server's site. It is not necessary to mark all
       permanently unavailable resources as "gone" or to keep the mark for any
       length of time -- that is left to the discretion of the server owner.
       
       
       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.
       
       
       
       
       
       
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       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 URL "black hole" of redirection (e.g., a redirected
       URL prefix that points to a suffix of itself), or when the server is
       under attack by a client attempting to exploit security holes present in
       some servers using fixed-length buffers for reading or manipulating the
       Request-URI.
       
       
       10.4.16 415 Unsupported Media Type
       
       The server is refusing to service the request because the entity of the
       request is in a format not supported by the requested resource for the
       requested method.
       
       
       10.4.17 416 Requested range not satisfiable
       
       A server SHOULD return a response with this status code if a request
       included a Range request-header field (section 14.36) , 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.)
       
       
       
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       When this status code is returned for a byte-range request, the response
       MUST include a Content-Range entity-header field specifying the current
       length of the selected resource (see section 14.17).  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
       14.47) 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
       
       
       10.4.19 418 Reauthentication Required
       
       Editor's note:  I wonder if the user agent should not actually be client
       in this and the following sections
       
       This header is similar to 401 (Unauthorized), except that the user agent
       MUST request credentials from the user before resubmitting the request,
       even if the challenge is the same as on a prior response or if the user
       agent has already obtained credentials from the user. The user agent
       should not assume that the current credentials are invalid if the
       request contained an Authorization header. The server can use this
       status code to cause the browser to verify that the current user is the
       same as the one who supplied the original credentials (say, after a
       period of inactivity). The server SHOULD send an entity-body explaining
       the reason for requiring reauthentication, because user agents that do
       not understand the status code will treat it as a generic 400 error and
       display the message. See section 15.11 for security considerations
       involving idle clients.
       
       
       10.4.20 419 Proxy Reauthentication Required
       
       This header is similar to 407 (Proxy Authentication Required), except
       that the user agent MUST request credentials from the user before
       resubmitting the request, even if the challenge is the same as on a
       prior response or if the user agent has already obtained credentials
       from the user. The user agent should not assume that the current
       credentials are invalid if the request contained an Proxy-Authorization
       header. The server can use this status code to cause the browser to
       verify that the current user is the same as the one who supplied the
       original credentials (say, after a period of inactivity). The server
       SHOULD send an entity-body explaining the reason for requiring
       reauthentication, because user agents that do not understand the status
       code will treat it as a generic 400 error and display the message. See
       section 15.11 for security considerations involving idle clients.
       
       
       
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       10.5 Server Error 5xx
       
       Response status codes beginning with the digit "5" indicate cases in
       which the server is aware that it has erred or is incapable of
       performing the request. Except when responding to a HEAD request, the
       server SHOULD include an entity containing an explanation of the error
       situation, and whether it is a temporary or permanent condition. User
       agents SHOULD display any included entity to the user. These response
       codes are applicable to any request method.
       
       
       10.5.1 500 Internal Server Error
       
       The server encountered an unexpected condition which prevented it from
       fulfilling the request.
       
       
       10.5.2 501 Not Implemented
       
       The server does not support the functionality required to fulfill the
       request. This is the appropriate response when the server does not
       recognize the request method and is not capable of supporting it for any
       resource.
       
       
       10.5.3 502 Bad Gateway
       
       The server, while acting as a gateway or proxy, received an invalid
       response from the upstream server it accessed in attempting to fulfill
       the request.
       
       
       10.5.4 503 Service Unavailable
       
       The server is currently unable to handle the request due to a temporary
       overloading or maintenance of the server. The implication is that this
       is a temporary condition which will be alleviated after some delay. If
       known, the length of the delay may be indicated in a Retry-After header.
       If no Retry-After is given, the client SHOULD handle the response as it
       would for a 500 response.
       
         Note: The existence of the 503 status code does not imply that a
         server must use it when becoming overloaded. Some servers may wish
         to simply refuse the connection.
       
       
       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 it accessed in attempting to complete
       the request.
       
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       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.
       
       
       10.5.7 506 Partial Update Not Implemented
       
       The server does not support paritial update on the specified resource.
       
       
       11 Access Authentication
       
       HTTP provides a several optional challenge-response authentication
       mechanism which MAY 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 specification of "basic" and
       "digest" authentication are specified in RFC XAAA .[
       
       
       11.1 Digest Authentication
       
       A digest authentication for HTTP is specified in RFC 2069 [32] Scheme
       
       
       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
       
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       negotiation are orthogonal and thus may be used separately or in
       combination. One method of combination, referred to as transparent
       negotiation, occurs when a cache uses the agent-driven negotiation
       information provided by the origin server in order to provide server-
       driven negotiation for subsequent requests.
       
       
       12.1 Server-driven Negotiation
       
       If the selection of the best representation for a response is made by an
       algorithm located at the server, it is called server-driven negotiation.
       Selection is based on the available representations of the response (the
       dimensions over which it can vary; e.g. language, content-coding, etc.)
       and the contents of particular header fields in the request message or
       on other information pertaining to the request (such as the network
       address of the client).
       
       Server-driven negotiation is advantageous when the algorithm for
       selecting from among the available representations is difficult to
       describe to the user agent, or when the server desires to send its "best
       guess" to the client along with the first response (hoping to avoid the
       round-trip delay of a subsequent request if the "best guess" is good
       enough for the user). In order to improve the server's guess, the user
       agent MAY include request header fields (Accept, Accept-Language,
       Accept-Encoding, etc.) which describe its preferences for such a
       response.
       
       Server-driven negotiation has disadvantages:
       
         1. It is impossible for the server to accurately determine what might
            be "best" for any given user, since that would require complete
            knowledge of both the capabilities of the user agent and the
            intended use for the response (e.g., does the user want to view it
            on screen or print it on paper?).
       
         2. Having the user agent describe its capabilities in every request
            can be both very inefficient (given that only a small percentage of
            responses have multiple representations) and a potential violation
            of the user's privacy.
       
         3. It complicates the implementation of an origin server and the
            algorithms for generating responses to a request.
       
         4. It may limit a public cache's ability to use the same response for
            multiple user's requests.
       
       HTTP/1.1 includes the following request-header fields for enabling
       server-driven negotiation through description of user agent capabilities
       and user preferences: Accept (section 14.1), Accept-Charset (section
       14.2), Accept-Encoding (section 14.3), Accept-Language (section 14.4),
       and User-Agent (section 14.42). However, an origin server is not limited
       
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       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 used to
       select a representation subject to server-driven negotiation.  See
       section 13.6 for use of the Vary header field by caches and section
       14.43 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 (this specification reserves the field-name Alternates, as
       described in appendix Error! Reference source not found.) or entity-body
       of the initial response, with each representation identified by its own
       URI. Selection from among the representations may be performed
       automatically (if the user agent is capable of doing so) or manually by
       the user selecting from a generated (possibly hypertext) menu.
       
       Agent-driven negotiation is advantageous when the response would vary
       over commonly-used dimensions (such as type, language, or encoding),
       when the origin server is unable to determine a user agent's
       capabilities from examining the request, and generally when public
       caches are used to distribute server load and reduce network usage.
       
       Agent-driven negotiation suffers from the disadvantage of needing a
       second request to obtain the best alternate representation. This second
       request is only efficient when caching is used. In addition, this
       specification does not define any mechanism for supporting automatic
       selection, though it also does not prevent any such mechanism from being
       developed as an extension and used within HTTP/1.1.
       
       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.
       
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       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 and 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 users, and may be
       incompatible with certain server applications (such as those for
       ordering merchandise), the protocol requires that transparency be
       relaxed
       
         .  only by an explicit protocol-level request when relaxed by client
            or origin server
         .  only with an explicit warning to the end user when relaxed by cache
            or client
       Therefore, the HTTP/1.1 protocol provides these important elements:
       
         1. Protocol features that provide full semantic transparency when this
            is required by all parties.
       
         2. Protocol features that allow an origin server or user agent to
            explicitly request and control non-transparent operation.
       
       
       
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         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 implementer may be faced with
         design decisions not explicitly discussed in this specification. If
         a decision may affect semantic transparency, the implementer 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
            response with the appropriate Warning header (see section 13.1.5
            and 14.45), 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
       
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       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. An 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 response-header. This
       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 come in two categories:
       
         1. Those that describe the freshness or revalidation status of the
            response, and so MUST be deleted after a successful revalidation
            (see section 13.3 for a definition of revalidation).
       
         2. Those that describe some aspect of the entity body or entity
            headers that are not rectified by a revalidation; for example, a
            lossy compression of the entity bodys.  These warnings MUST NOT be
            deleted after a successful revalidation.
       
       Warnings are assigned 3-digit code numbers.  The first digit indicates
       whether the Warning must or must not be deleted from a cached response
       after it is successfully revalidated. This specification defines the
       code numbers and meanings of each currently assigned warning, allowing a
       client or cache to take automated action in some (but not all) cases.
       
       HTTP/1.0 caches will cache all Warnings, without deleting the ones in
       the first category.  Warnings 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 may provide the same warning with texts in
       both English and Basque.
       
       
       
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       When multiple warnings are attached to a response, it may 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.
       
       The Warning header and the currently defined warnings are described in
       section 14.45.
       
       
       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 may 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 should be 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 may 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 should have to explicitly request
       either non-transparent behavior, or behavior that results in abnormally
       ineffective caching.
       
       If the user has overridden the basic caching mechanisms, the user agent
       should explicitly indicate to the user whenever this results in the
       display of information that might not meet the server's transparency
       requirements (in particular, if the displayed entity is known to be
       stale). Since the protocol normally allows the user agent to determine
       if responses are stale or not, this indication need only be displayed
       when this actually happens. The indication need not be a dialog box; it
       could be an icon (for example, a picture of a rotting fish) or some
       other indicator.
       
       If the user has overridden the caching mechanisms in a way that would
       abnormally reduce the effectiveness of caches, the user agent should
       
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       continually display an indication (for example, 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
       should 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). This allows
       the client software to alert the user that there may 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 may 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 may violate the origin server's specified constraints on
       semantic transparency, but may be necessary to support disconnected
       operation, or high availability in the face of poor connectivity.
       
       
       
       
       
       
       
       
       
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       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 explanation of the difference between caches and history
       mechanisms.
       
       
       13.2.2 Heuristic Expiration
       
       Since origin servers do not always provide explicit expiration times,
       HTTP caches typically assign heuristic expiration times, employing
       algorithms that use other header values (such as the Last-Modified time)
       to estimate a plausible expiration time. The HTTP/1.1 specification does
       not provide specific algorithms, but does impose worst-case constraints
       on their results. Since heuristic expiration times may compromise
       
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       semantic transparency, they should be used cautiously, and we encourage
       origin servers to provide explicit expiration times as much as possible.
       
       
       13.2.3 Age Calculations
       
       In order to know if a cached entry is fresh, a cache needs to know if
       its age exceeds its freshness lifetime. We discuss how to calculate the
       latter in section 13.2.4; this section describes how to calculate the
       age of a response or cache entry.
       
       In this discussion, we use the term "now" to mean "the current value of
       the clock at the host performing the calculation." Hosts that use HTTP,
       but especially hosts running origin servers and caches, should use NTP
       [28] or some similar protocol to synchronize their clocks to a globally
       accurate time standard.
       
       Also note that HTTP/1.1 requires origin servers to send a Date header
       with every response, giving the time at which the response was
       generated. 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 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 may pass
       from the time that a server generates a response and the time it is
       received at the next outbound cache or client. If uncorrected, this
       delay could result in improperly low ages.
       
       Because the request that resulted in the returned Age value must have
       been initiated prior to that Age value's generation, we can correct for
       delays imposed by the network by recording the time at which the request
       was initiated. Then, when an Age value is received, it MUST be
       interpreted relative to the time the request was initiated, not 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.  hen a response is generated from a
       cache entry, the server must include a single Age header field in the
       response with a value equal to the cache entry's current_age.
       
       The presence of an Age header field in a response implies that a
       response is not first-hand. However, the converse is not true, since the
       lack of an Age header field in a response does not imply that the
       response is first-hand unless all caches along the request path are
       
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       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 14.10.
       
       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 neither Expires nor Cache-Control: max-age or 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. If the value is greater than 24 hours, 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.
       
       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
       
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       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 may 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 may 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 may 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. However, the HTTP/1.1 specification
       requires the transmission of Date headers on every response, and 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 client's request, it first has to check with the origin server (or
       
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       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, 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.
       
         Note: the comparison functions used to decide if validators match
         are defined in section 13.3.3.
       
       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.
       
       
       
       
       
       
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       13.3.2 Entity Tag Cache Validators
       
       The ETag entity-header field value, an entity tag, provides for an
       "opaque" cache validator. This may 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 may arise
       from the use of modification dates.
       
       Entity Tags are described in section 3.11. The headers used with entity
       tags are described in sections 14.20, 14.25, 14.26 and 14.43.
       
       
       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 may 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 may 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.
       
       
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       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 may end up with an internally
       inconsistent entity.
       
       The only function that the HTTP/1.1 protocol defines on validators is
       comparison. There are two validator comparison functions, depending on
       whether the comparison context allows the use of weak validators or not:
       
         .  The strong comparison function: in order to be considered equal,
            both validators must be identical in every way, and neither may be
            weak.
         .  The weak comparison function: in order to be considered equal, both
            validators must be identical in every way, but either or both of
            them may be tagged as "weak" without affecting the result.
       The weak comparison function MAY be used for simple (non-subrange) GET
       requests. The strong comparison function MUST be used in all other
       cases.
       
       An entity tag is strong unless it is explicitly tagged as weak. Section
       3.11 gives the syntax for entity tags.
       
       A Last-Modified time, when used as a validator in a request, is
       implicitly weak unless it is possible to deduce that it is strong, using
       the following rules:
       
         .  The validator is being compared by an origin server to the actual
            current validator for the entity and,
         .  That origin server reliably knows that the associated entity did
            not change twice during the second covered by the presented
            validator.
       or
       
         .  The validator is about to be used by a client in an If-Modified-
            Since or If-Unmodified-Since header, because the client has a cache
            entry for the associated entity, and
         .  That cache entry includes a Date value, which gives the time when
            the origin server sent the original response, and
         .  The presented Last-Modified time is at least 60 seconds before the
            Date value.
       or
       
         .  The validator is being compared by an intermediate cache to the
            validator stored in its cache entry for the entity, and
         .  That cache entry includes a Date value, which gives the time when
            the origin server sent the original response, and
         .  The presented Last-Modified time is at least 60 seconds before the
            Date value.
       
       
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       This method relies on the fact that if two different responses were sent
       by the origin server during the same second, but both had the same Last-
       Modified time, then at least one of those responses would have a Date
       value equal to its Last-Modified time. The arbitrary 60-second limit
       guards against the possibility that the Date and Last-Modified values
       are generated from different clocks, or at somewhat different times
       during the preparation of the response. An implementation may use a
       value larger than 60 seconds, if it is believed that 60 seconds is too
       short.
       
       If a client wishes to perform a sub-range retrieval on a value for which
       it has only a Last-Modified time and no opaque validator, it may do this
       only if the Last-Modified time is strong in the sense described here.
       
       A cache or origin server receiving a cache-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 should be used, and
       for what purposes.
       
       HTTP/1.1 origin servers:
       
         .  SHOULD send an entity tag validator unless it is not feasible to
            generate one.
         .  MAY send a weak entity tag instead of a strong entity tag, if
            performance considerations support the use of weak entity tags, or
            if it is unfeasible to send a strong entity tag.
         .  SHOULD send a Last-Modified value if it is feasible to send one,
            unless the risk of a breakdown in semantic transparency that could
            result from using this date in an If-Modified-Since header would
            lead to serious problems.
       In other words, the preferred behavior for an HTTP/1.1 origin server is
       to send both a strong entity tag and a Last-Modified value.
       
       In order to be legal, a strong entity tag MUST change whenever the
       associated entity value changes in any way. A weak entity tag SHOULD
       change whenever the associated entity changes in a semantically
       significant way.
       
         Note: in order to provide semantically transparent caching, an
         origin server must avoid reusing a specific strong entity tag value
         for two different entities, or reusing a specific weak entity tag
       
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         value for two semantically different entities. Cache entries may
         persist for arbitrarily long periods, regardless of expiration
         times, so it may be inappropriate to expect that a cache will never
         again attempt to validate an entry using a validator that it
         obtained at some point in the past.
       
       HTTP/1.1 clients:
       
         .  If an entity tag has been provided by the origin server, MUST use
            that entity tag in any cache-conditional request (using If-Match or
            If-None-Match).
         .  If only a Last-Modified value has been provided by the origin
            server, SHOULD use that value in non-subrange cache-conditional
            requests (using If-Modified-Since).
         .  If only a Last-Modified value has been provided by an HTTP/1.0
            origin server, MAY use that value in subrange cache-conditional
            requests (using If-Unmodified-Since:). The user agent should
            provide a way to disable this, in case of difficulty.
         .  If both an entity tag and a Last-Modified value have been provided
            by the origin server, SHOULD use both validators in cache-
            conditional requests. This allows both HTTP/1.0 and HTTP/1.1 caches
            to respond appropriately.
       An HTTP/1.1 origin server, upon receiving a conditional request that
       includes both a Last-modified date (e.g., in an If-Modified-Since or If-
       Unmodified-Since header field) and one or more entity tags (e.g., in an
       If-Match, If-None-Match, or If-Range header field) as cache validators,
       MUST NOT return a response status of 304 (Not Modified) unless doing so
       is consistent with all of the conditional header fields in the request.
       
       An HTTP/1.1 caching proxy, upon receiving a conditional request that
       includes both a Last-modified date and one or more entity tags as cache
       validators, MUST NOT return a locally cached response to the client
       unless that cached response is consistent with all of the conditional
       header fields in the request.
       
         A note on rationale: 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.
       
       
       
       
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       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.
       
       
       13.4 Response Cachability
       
       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 that some HTTP/1.0 caches are known to violate this
         expectation without providing any Warning.
       
       However, in some cases it may be inappropriate for a cache to retain an
       entity, or to return it in response to a subsequent request. This may 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,
       may not 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 MUST NOT be returned in a
       reply to a subsequent request unless there are Cache-Control directives
       or another header(s) that explicitly allow it. For example, these
       include the following: an Expires header (section 14.21); a "max-age",
       "s-maxage" "must-revalidate", "proxy-revalidate", "public" or "private"
       Cache-Control directive (section 14.9).
       
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       13.5 Constructing Responses From Caches
       
       The purpose of an HTTP cache is to store information received in
       response to requests, for use in responding to future requests. In many
       cases, a cache simply returns the appropriate parts of a response to the
       requester. However, if the cache holds a cache entry based on a previous
       response, it may have to combine parts of a new response with what is
       held in the cache entry.
       
       
       13.5.1 End-to-end and Hop-by-hop Headers
       
       For the purpose of defining the behavior of caches and non-caching
       proxies, we divide HTTP headers into two categories:
       
         .  End-to-end headers, which must be 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 transmitted in any
            response formed from a cache entry.
         .  Hop-by-hop headers, which are meaningful only for a single
            transport-level connection, and are not stored by caches or
            forwarded by proxies.
       The following HTTP/1.1 headers are hop-by-hop headers:
       
         .  Connection
         .  Keep-Alive
         .  .  Proxy-Authenticate
         .  Transfer-Encoding
         .  Upgrade
       All other headers defined by HTTP/1.1 are end-to-end headers.
       
       Hop-by-hop headers introduced in future versions of HTTP MUST be listed
       in a Connection header, as described in section 14.10.
       
       
       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 cache or non-
       caching proxy SHOULD NOT modify an end-to-end header unless the
       definition of that header requires or specifically allows that.
       
       A cache or non-caching proxy MUST NOT modify any of the following fields
       in a request or response, nor may it add any of these fields if not
       already present:
       
         .  Content-Location
         .  Content-MD5
         .  ETag
         .  Last-Modified
       
       
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       A cache or non-caching proxy MUST NOT modify any of the following fields
       in a response:
       
         .  Expires
         .  Content-Length
       
       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. If a Content-Length header is added, it
       MUST correctly reflect the length of the entity-body.
       
         Note: a typical reason for adding the Content-Length header is that
         the origin server sent the content chunked encoded.
       
       A cache or non-caching proxy MUST NOT modify or add any of the following
       fields in a response that contains the no-transform Cache-Control
       directive, or in any request:
       
         .  Content-Encoding
         .
         .  Content-Range
         .  Content-Type
       A cache or non-caching proxy MAY modify or add these fields in a
       response that does not include no-transform, but if it does so, it MUST
       add a Warning 114 (Transformation applied) if one does not already
       appear in the response.
       
         Warning: unnecessary modification of end-to-end headers may 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.
       
       
       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 must construct a response to send to the requesting
       client.
       
       In 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, see 13.5.4, 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
       
       
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         .  any stored Warning headers with warn-code 1XX (see section 14.45)
            are deleted from the cache entry and the forwarded response.
         .  any stored Warning headers with warn-code 2XX are retained in the
            cache entry and the forwarded response.
         .  any end-to-end headers provided in the 304 or 206 response MUST
            replace the corresponding headers from the cache entry.
       Unless the cache decides to remove the cache entry, it MUST also replace
       the end-to-end headers stored with the cache entry with corresponding
       headers received in the incoming response.
       
       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).
       
       If a header field-name in the incoming response matches more than one
       header in the cache entry, all such old headers are replaced.
       
         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 may 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 may have received several ranges of the same entity-
       body.
       
       If a cache has a stored non-empty set of subranges for an entity, and an
       incoming response transfers another subrange, the cache MAY combine the
       new subrange with the existing set if both the following conditions are
       met:
       
         .  Both the incoming response and the cache entry must have a cache
            validator.
         .  The two cache validators must match using the strong comparison
            function (see section 13.3.3).
       If either requirement is not meant, 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.
       
       
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       13.6 Caching Negotiated Responses
       
       Use of server-driven content negotiation (section 12), as indicated by
       the presence of a Vary header field in a response, alters the conditions
       and procedure by which a cache can use the response for subsequent
       requests. See section 14.43 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 cachable 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 whitespace (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 fail 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 which entity should 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-
       
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       None-Match header field unless the request is for a range that would be
       fully satisfied by that entry.
       
       If a cache receives a successful response whose Content-Location field
       matches that of an existing cache entry for the same Request-URI, whose
       entity-tag differs from that of the existing entry, and whose Date is
       more recent than that of the existing entry, the existing entry SHOULD
       NOT be returned in response to future requests and should be deleted
       from the cache.
       
       
       
       
       13.7 Shared and Non-Shared Caches
       
       For reasons of security and privacy, it is necessary to make a
       distinction between "shared" and "non-shared" caches. A non-shared cache
       is one that is accessible only to a single user. Accessibility in this
       case SHOULD be enforced by appropriate security mechanisms. All other
       caches are considered to be "shared." Other sections of 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
       
       
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       its caching decisions. Caches are always expected to observe an origin
       server's explicit restrictions on caching.
       
       We note one exception to this rule: since some applications have
       traditionally used GETs and HEADs with query URLs (those containing a
       "?" in the rel_path part) to perform operations with significant side
       effects, caches MUST NOT treat responses to such URLs as fresh unless
       the server provides an explicit expiration time. This specifically means
       that responses from HTTP/1.0 servers for such URIs should not be taken
       from a cache. See section 9.1.1 for related information.
       
       
       13.10 Invalidation After Updates or Deletions
       
       The effect of certain methods performed on a resource at the origin
       server may cause one or more existing cache entries to become non-
       transparently invalid. That is, although they may 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 may 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
       should either remove all instances of that entity from its storage, or
       should mark these as "invalid" and in need of a mandatory revalidation
       before they can be returned in response to a subsequent request.
       
       Some HTTP methods MUST cause a cache to invalidate an entity. This is
       either the entity referred to by the Request-URI, or by the Location or
       Content-Location headers (if present). These methods are:
       
         .  PUT
         .  DELETE
         .  POST
       In order to prevent denial of service attacks, an invalidation based on
       the URI in a Location or Content-Location header MUST only be performed
       if the host part is the same as in the Request-URI.
       
       
       13.11 Write-Through Mandatory
       
       All methods that may 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
       
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       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.
       
       
       13.12 Cache Replacement
       
       If a new cachable (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 it should
       follow the rules in section 13.5.3.
       
         Note: a new response that has an older Date header value than
         existing cached responses is not cachable.
       
       
       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 should not be construed to prohibit the history mechanism from
       telling the user that a view may 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
       
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         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.
       
       
       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 should be discouraged from
         registering any parameter named "q".
       
       
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       The example
       
              Accept: audio/*; q=0.2, audio/basic
       SHOULD be interpreted as "I prefer audio/basic, but send me any audio
       type if it is the best available after an 80% mark-down in quality."
       
       If no Accept header field is present, then it is assumed that the client
       accepts all media types. If an Accept header field is present, and if
       the server cannot send a response which is acceptable according to the
       combined Accept field value, then the server SHOULD send a 406 (not
       acceptable) response.
       
       A more elaborate example is
       
              Accept: text/plain; q=0.5, text/html,
                      text/x-dvi; q=0.8, text/x-c
       Verbally, this would be interpreted as "text/html and text/x-c are the
       preferred media types, but if they do not exist, then send the text/x-
       dvi entity, and if that does not exist, send the text/plain entity."
       
       Media ranges can be overridden by more specific media ranges or specific
       media types. If more than one media range applies to a given type, the
       most specific reference has precedence. For example,
       
              Accept: text/*, text/html, text/html;level=1, */*
       have the following precedence:
       
              1) text/html;level=1
              2) text/html
              3) text/*
              4) */*
       The media type quality factor associated with a given type is determined
       by finding the media range with the highest precedence which matches
       that type. For example,
       
              Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
                      text/html;level=2;q=0.4, */*;q=0.5
       would cause the following values to be associated:
       
              text/html;level=1         = 1
              text/html                 = 0.7
              text/plain                = 0.3
              image/jpeg                = 0.5
              text/html;level=2         = 0.4
              text/html;level=3         = 0.7
         Note: A user agent may be provided with a default set of quality
         values for certain media ranges. However, unless the user agent is
         a closed system which cannot interact with other rendering agents,
         this default set should be configurable by the user.
       
       
       
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       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. The ISO-8859-1 character
       set can be assumed to be acceptable to all user agents.
       
          Accept-Charset = "Accept-Charset" ":"
                     1#( ( charset | "*" )[ ";" "q" "=" qvalue ] )
       
       Character set values are described in section 3.4. Each charset may be
       given an associated quality value which represents the user's preference
       for that charset. The default value is q=1. An example is
       
          Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
       
       The special value "*", if present in the Accept-Charset field, matches
       every character set (including ISO-8859-1) which is not mentioned
       elsewhere in the Accept-Charset field.  If no "*" is present in an
       Accept-Charset field, then all character sets not explicitly mentioned
       get a quality value of 0, except for ISO-8859-1, which gets a quality
       value of 1 if not explicitly mentioned.
       
       If no Accept-Charset header is present, the default is that any
       character set is acceptable. If an Accept-Charset header is present, and
       if the server cannot send a response which is acceptable according to
       the Accept-Charset header, then the server SHOULD send an error response
       with the 406 (not acceptable) status code, though the sending of an
       unacceptable response is also allowed.
       
       
       14.3 Accept-Encoding
       
       The Accept-Encoding request-header field is similar to Accept, but
       restricts the content-coding (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=0.5; *;q=0
       A server tests whether a content-coding is acceptable, according to an
       Accept-Encoding field, using these rules:
       
       
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            1. If the content-coding is one of the content-codings listed in
            the Accept-Encoding field, then it is acceptable, unless it is
            accompanied by a qvalue of 0.  (As defined in section 3.9, a qvalue
            of 0 means "not acceptable.")
       
            2. The special "*" symbol in an Accept-Encoding field matches any
            available content-coding not explicitly listed in the header field.
       
            3. If multiple content-codings are acceptable, then the acceptable
            content-coding with the highest non-zero qvalue is preferred.
       
            4. The "identity" content-coding is always acceptable, unless
            specifically refused because the Accept-Encoding field includes
            "identity;q=0", or because the field includes "*;q=0" and does not
            explictly 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 preference
         should be given to content-codings commonly understood by HTTP/1.0
         clients (i.e., "gzip" and "compress"); some older clients
         improperly display messages sent with other content-encodings.  The
         server may 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.
       
       
       
       
       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.
       
              Accept-Language = "Accept-Language" ":"
                                1#( language-range [ ";" "q" "=" qvalue ] )
       
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              language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )
       Each language-range MAY be given an associated quality value which
       represents an estimate of the user's preference for the languages
       specified by that range. The quality value defaults to "q=1". For
       example,
       
              Accept-Language: da, en-gb;q=0.8, en;q=0.7
       would mean: "I prefer Danish, but will accept British English and other
       types of English." A language-range matches a language-tag if it exactly
       equals the tag, or if it exactly equals a prefix of the tag such that
       the first tag character following the prefix is "-". The special range
       "*", if present in the Accept-Language field, matches every tag not
       matched by any other range present in the Accept-Language field.
       
         Note: This use of a prefix matching rule does not imply that
         language tags are assigned to languages in such a way that it is
         always true that if a user understands a language with a certain
         tag, then this user will also understand all languages with tags
         for which this tag is a prefix. The prefix rule simply allows the
         use of prefix tags if this is the case.
       
       The language quality factor assigned to a language-tag by the Accept-
       Language field is the quality value of the longest language-range in the
       field that matches the language-tag. If no language-range in the field
       matches the tag, the language quality factor assigned is 0. If no
       Accept-Language header is present in the request, the server 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 may 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 15.6.
       
         Note: As intelligibility is highly dependent on the individual
         user, it is recommended that client applications make the choice of
         linguistic preference available to the user. If the choice is not
         made available, then the Accept-Language header field must not be
         given in the request.
       
         Note: When making the choice of linguistic preference available to
         the user, implementors should take into account 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 may assume that on selecting "en-gb", they will be served any
         kind of English document if British English is not available. A
         user agent may suggest in such a case to add "en" to get the best
         matching behaviour.
       
       
       
       
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       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.
       
       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:
       
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              Allow: GET, HEAD, PUT
       This field cannot prevent a client from trying other methods. However,
       the indications given by the Allow header field value SHOULD be
       followed. The actual set of allowed methods is defined by the origin
       server at the time of each request.
       
       The Allow header field MAY be provided with a PUT request to recommend
       the methods to be supported by the new or modified resource. The server
       is not required to support these methods and SHOULD include an Allow
       header in the response giving the actual supported methods.
       
       A proxy MUST NOT modify the Allow header field even if it does not
       understand all the methods specified, since the user agent MAY have
       other means of communicating with the origin server.
       
       
       14.8 Authorization
       
       A user agent that wishes to authenticate itself with a server--usually,
       but not necessarily, after receiving a 401 response--MAY do so by
       including an Authorization request-header field with the request. The
       Authorization field value consists of credentials containing the
       authentication information of the user agent for the realm of the
       resource being requested.
       
              Authorization  = "Authorization" ":" credentials
       HTTP access authentication is described in section 11. If a request is
       authenticated and a realm specified, the same credentials SHOULD be
       valid for all other requests within this realm.
       
       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
            proxy-maxage.) If the response includes "proxy-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.
       
       
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         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 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 should be given in the response.
       
         Note that HTTP/1.0 caches may not implement Cache-Control and may
         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 may 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"
                           | "no-store"
                           | "max-age" "=" delta-seconds
                           | "max-stale" [ "=" delta-seconds ]
                           | "min-fresh" "=" delta-seconds
                           | "no-transform"
                           | "only-if-cached"
                           | cache-extension
            cache-response-directive =
                             "public"
                           | "private" [ "=" <"> 1#field-name <"> ]
                           | "no-cache" [ "=" <"> 1#field-name <"> ]
                           | "no-store"
                           | "no-transform"
                           | "must-revalidate"
                           | "proxy-revalidate"
                           | "max-age" "=" delta-seconds
                           | "s-maxage" "=" delta-seconds
                           | cache-extension
           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
       
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       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 may apply these directives to header
       fields not defined in HTTP/1.1.
       
       The cache-control directives can be broken down into these general
       categories:
       
         .  Restrictions on what is cachable; these may only be imposed by the
            origin server.
         .  Restrictions on what may be stored by a cache; these may be imposed
            by either the origin server or the user agent.
         .  Modifications of the basic expiration mechanism; these may be
            imposed by either the origin server or the user agent.
         .  Controls over cache revalidation and reload; these may only be
            imposed by a user agent.
         .  Control over transformation of entities.
         .  Extensions to the caching system.
       
       14.9.1 What is Cachable
       
       By default, a response is cachable if the requirements of the request
       method, request header fields, and the response status indicate that it
       is cachable. Section 13.4 summarizes these defaults for cachability. The
       following Cache-Control response directives allow an origin server to
       override the default cachability of a response:
       
       public
         Indicates that the response is cachable by any cache, even if it
         would normally be non-cachable or cachable 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.
       
       
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         If the no-cache directive does specify one or more field-names, then
         a cache MAY use the response to satisfy a subsequent request, subject
         to any other restrictions on caching. However, the specified field-
         name(s) MUST NOT be sent in the response to a  subsequent request
         without successful revalidation with the origin server.  This allows
         an origin server to prevent the re-use of certain header fields in a
         response, while still allowing caching of the rest of the response.
       
         Note: Most HTTP/1.0 caches will not recognize or obey this
         directive.
       
       
       14.9.2 What May be Stored by Caches
       
       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 may
       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 may 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 may not recognize or obey this directive; and communications
       networks may 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"
       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
       
       
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       response implies that the response is cachable (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 may be useful if
       certain HTTP/1.0 caches improperly calculate ages or expiration times,
       perhaps due to desynchronized clocks.
       
       Many HTTP/1.0 cache implementations will treat an Expires value that is
       less than or equal to the response Date value as being equivalent to the
       Cache-Control response directive "no-cache".  If an HTTP/1.1 cache
       receives such a response, and the response does not include a Cache-
       Control header field, it SHOULD consider the response to be non-cachable
       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.
       
       If a response includes a 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: 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 non-HTTP/1.1-compliant caches do not
         observe the max-age directive.
       
       Other directives allow an 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
       
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         is no greater than the specified time in seconds. Unless max-stale
         directive is also included, the client is not willing to accept a
         stale response.
       
       min-fresh
         Indicates that the client is willing to accept a response whose
         freshness lifetime is no less than its current age plus the specified
         time in seconds. That is, the client wants a response that will still
         be fresh for at least the specified number of seconds.
       
       max-stale
         Indicates that the client is willing to accept a response that has
         exceeded its expiration time. If max-stale is assigned a value, then
         the client is willing to accept a response that has exceeded its
         expiration time by no more than the specified number of seconds. If
         no value is assigned to max-stale, then the client is willing to
         accept a stale response of any age.
       
       If a cache returns a stale response, either because of a max-stale
       directive on a request, or because the cache is configured to override
       the expiration time of a response, the cache MUST attach a Warning
       header to the stale response, using Warning 110 (Response is stale).
       
         Note: 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 an user agent may 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 may 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:
       
       
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       End-to-end reload
         The request includes a "no-cache" Cache-Control directive or, for
         compatibility with HTTP/1.0 clients, "Pragma: no-cache".
         No field names may be included with the no-cache directive in a
         request. The server MUST NOT use a cached copy when responding to
         such a request.[jg127]
       
       Specific end-to-end revalidation
         The request includes a "max-age=0" Cache-Control directive, which
         forces each cache along the path to the origin server to revalidate
         its own entry, if any, with the next cache or server. The initial
         request includes a cache-validating conditional with the client's
         current validator.
       
       Unspecified end-to-end revalidation
         The request includes "max-age=0" Cache-Control directive, which
         forces each cache along the path to the origin server to revalidate
         its own entry, if any, with the next cache or server. The initial
         request does not include a cache-validating conditional; the first
         cache along the path (if any) that holds a cache entry for this
         resource includes a cache-validating conditional with its current
         validator.
       
       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 may 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 may 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 should 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 should compare the returned
       validator with the one provided in the client's request, using the
       strong comparison function. If the client's validator is equal to the
       origin server's, then the intermediate cache simply returns 304 (Not
       Modified). Otherwise, it returns the new entity with a 200 (OK)
       response.
       
       If a request includes the no-cache directive, it should not include min-
       fresh, max-stale, or max-age.
       
       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
       
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       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.
       
       Because a cache may be configured to ignore a server's specified
       expiration time, and because a client request may include a max-stale
       directive (which has a similar effect), the protocol also includes a
       mechanism for the origin server to require revalidation of a cache entry
       on any subsequent use. When the must-revalidate directive is present in
       a response received by a cache, that cache MUST NOT use the entry after
       it becomes stale to respond to a subsequent request without first
       revalidating it with the origin server. (I.e., the cache must do an end-
       to-end revalidation every time, if, based solely on the origin server's
       Expires or max-age value, the cached response is stale.)
       
       The must-revalidate directive is necessary to support reliable operation
       for certain protocol features. In all circumstances an HTTP/1.1 cache
       MUST obey the must-revalidate directive; in particular, if the cache
       cannot reach the origin server for any reason, it MUST generate a 504
       (Gateway Timeout) response.
       
       Servers should send the must-revalidate directive if and only if failure
       to revalidate a request on the entity could result in incorrect
       operation, such as a silently unexecuted financial transaction.
       Recipients MUST NOT take any automated action that violates this
       directive, and MUST NOT automatically provide an unvalidated copy of the
       entity if revalidation fails.
       
       Although this is not recommended, user agents operating under severe
       connectivity constraints may violate this directive but, if so, MUST
       explicitly warn the user that an unvalidated response has been provided.
       The warning MUST be provided on each unvalidated access, and SHOULD
       require explicit user confirmation.
       
       The proxy-revalidate directive has the same meaning as the must-
       revalidate directive, except that it does not apply to non-shared user
       agent caches. It can be used on a response to an authenticated request
       to permit the user's cache to store and later return the response
       without needing to revalidate it (since it has already been
       authenticated once by that user), while still requiring proxies that
       service many users to revalidate each time (in order to make sure that
       each user has been authenticated). Note that such authenticated
       responses also need the public cache control directive in order to allow
       them to be cached at all.
       
       
       14.9.5 No-Transform Directive
       
       Implementers of intermediate caches (proxies) have found it useful to
       convert the media type of certain entity bodies. A proxy might, for
       
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       example, convert between image formats in order to save cache space or
       to reduce the amount of traffic on a slow link. HTTP has to date been
       silent on these transformations.
       
       Serious operational problems have already occurred, however, when these
       transformations have been 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.
       
       
       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 a 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) may 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
       
       
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       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 response's default
       cachability) such that the cache behavior will remain minimally correct
       even if the cache does not understand the extension(s).
       
       
       14.10 Connection
       
       The Connection general-header field allows the sender to specify options
       that are desired for that particular connection and MUST NOT be
       communicated by proxies over further connections.
       
       The Connection header has the following grammar:
       
              Connection = "Connection" ":" 1#(connection-token)
              connection-token  = token
       
       HTTP/1.1 proxies MUST parse the Connection header field before a message
       is forwarded and, for each connection-token in this field, remove any
       header field(s) from the message with the same name as the connection-
       token. Connection options are signaled by the presence of a connection-
       token in the Connection header field, not by any corresponding
       additional header field(s), since the additional header field may not be
       sent if there are no parameters associated with that connection option.
       
       Message headers listed in the Connection header MUST NOT include end-to-
       end headers, such as Cache-Control.
       
       HTTP/1.1 defines the "close" connection option for the sender to signal
       that the connection will be closed after completion of the response. For
       example,
       
              Connection: close
       
       in either the request or the response header fields indicates that the
       connection should not be considered `persistent' (section 8.1) after the
       current request/response is complete.
       
       HTTP/1.1 applications that do not support persistent connections MUST
       include the "close" connection option in every message.
       
       A system receiving an HTTP/1.0 (or lower-version) message that includes
       a Connection header MUST, for each connection-token in this field,
       remove and ignore any header field(s) from the message with the same
       name as the connection-token.  This protects against mistaken forwarding
       of such header fields by pre-HTTP/1.1 proxies.
       
       
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       14.11 Content-Base
       
       The Content-Base entity-header field may be used to specify the base URI
       for resolving relative URLs within the entity.
       
              Content-Base      = "Content-Base" ":" absoluteURI
       If no Content-Base field is present, the base URI of an entity is
       defined either by its Content-Location (if that Content-Location URI is
       an absolute URI) or the URI used to initiate the request, in that order
       of precedence. Note, however, that the base URI of the contents within
       the entity-body may be redefined within that entity-body.
       
       
       14.12 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-Encoding is a characteristic of the entity identified by the
       Request-URI. Typically, the entity-body is stored with this encoding and
       is only decoded before rendering or analogous usage. However, a 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 including a Content-Encoding entity-header (section 14.12) 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.
       
       
       
       
       
       
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       14.13 Content-Language
       
       The Content-Language entity-header field describes the natural
       language(s) of the intended audience for the enclosed entity. Note that
       this may not be equivalent to all the languages used within the entity-
       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 may mean that the sender does
       not consider it to be specific to any natural language, or that the
       sender does not know for which language it is intended.
       
       Multiple languages MAY be listed for content that is intended for
       multiple audiences. For example, a rendition of the "Treaty of
       Waitangi," presented simultaneously in the original Maori and English
       versions, would call for
       
              Content-Language: mi, en
       However, just because multiple languages are present within an entity
       does not mean that it is intended for multiple linguistic audiences. An
       example would be a beginner's language primer, such as "A First Lesson
       in Latin," which is clearly intended to be used by an English-literate
       audience. In this case, the Content-Language should only include "en".
       
       Content-Language may be applied to any media type -- it is not limited
       to textual documents.
       
       
       14.14 Content-Length
       
       The Content-Length entity-header field indicates the size of the
       message-body, in decimal number of OCTETs, sent to the recipient or, in
       the case of the HEAD method, the size of the entity-body that would have
       been sent had the request been a GET.
       
              Content-Length    = "Content-Length" ":" 1*DIGIT
       An example is
       
              Content-Length: 3495
       Applications SHOULD use this field to indicate the size of the message-
       body to be transferred, regardless of the media type of the entity. It
       must be possible for the recipient to reliably determine the end of
       HTTP/1.1 requests containing an entity-body, e.g., because the request
       
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       has a valid Content-Length field, uses Transfer-Encoding: chunked or a
       multipart body.
       
       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: 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.
       
       
       14.15 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.. 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. In
       addition, a server SHOULD provide a Content-Location for the resource
       corresponding to the response entity.
       
              Content-Location = "Content-Location" ":"
                                ( absoluteURI | relativeURI )
       If no Content-Base header field is present, the value of Content-
       Location also defines the base URL for the entity (see section 14.11).
       
       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 use the Content-Location 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 URI is interpreted
       relative to any Content-Base URI provided in the response. If no
       Content-Base is provided, the relative URI is interpreted relative to
       the Request-URI.
       
       
       
       
       
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       14.16 Content-MD5
       
       The Content-MD5 entity-header field, as defined in RFC 1864 [23], is an
       MD5 digest of the entity-body for the purpose of providing an end-to-end
       message integrity check (MIC) of the entity-body. (Note: a MIC is good
       for detecting accidental modification of the entity-body in transit, but
       is not proof against malicious attacks.)
       
               Content-MD5   = "Content-MD5" ":" md5-digest
               md5-digest   = <base64 of 128 bit MD5 digest as per RFC 1864>
       The Content-MD5 header field may be generated by an origin server to
       function as an integrity check of the entity-body. Only origin servers
       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-Encoding that has been applied, but not including
       any Transfer-Encoding that may have been 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.
       
         Note: There are several consequences of this. The entity-body for
         composite types may contain many body-parts, each with its own MIME
         and HTTP headers (including Content-MD5, Content-Transfer-Encoding,
         and Content-Encoding headers). If a body-part has a Content-
         Transfer-Encoding or Content-Encoding header, it is assumed that
         the content of the body-part has had the encoding applied, and the
         body-part is included in the Content-MD5 digest as is -- i.e.,
         after the application. The Transfer-Encoding header field is not
         allowed within body-parts.
       
         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
       
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         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.
         Conversion of all line breaks to CRLF should not be done before
         computing or checking the digest: the line break convention used in
         the text actually transmitted should be left unaltered when
         computing the digest.
       
       
       14.17 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
       inserted. It SHOULD indicate the total length of the full entity-body,
       unless length this is unknown or difficult to determine.
       
              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 "/"
                                        ( entity-length | "*" )
       
              byte-range-resp-spec = (first-byte-pos "-" last-byte-pos)
                                             | "*"
              entity-length           = 1*DIGIT
       The asterisk "*" character means that the entity-length is unknown at
       the time when the response was generated.
       
       Unlike byte-ranges-specifier values, a byte-range-resp-spec may 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 entity-length
       value is less than or equal to its last-byte-pos value, is invalid. The
       recipient of an invalid byte-content-range-spec MUST ignore it and any
       content transferred along with it.
       
       A server sending a response with status code 416 (Requested range not
       satisfiable) SHOULD include a Content-range field with a byte-range-
       resp-spec of "*". The entity-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 content-range-spec of "*".
       
       Examples of byte-content-range-spec values, assuming that the entity
       contains a total of 1234 bytes:
       
         .  The first 500 bytes:
              bytes 0-499/1234
         .  The second 500 bytes:
       
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              bytes 500-999/1234
         .  All except for the first 500 bytes:
              bytes 500-1233/1234
         .  The last 500 bytes:
              bytes 734-1233/1234
       When an HTTP message includes the content of a single range (for
       example, a response to a request for a single range, or to a request for
       a set of ranges that overlap without any holes), this content is
       transmitted with a Content-Range header, and a Content-Length header
       showing the number of bytes actually transferred. For example,
       
              HTTP/1.1 206 Partial content
              Date: Wed, 15 Nov 1995 06:25:24 GMT
              Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
              Content-Range: bytes 21010-47021/47022
              Content-Length: 26012
              Content-Type: image/gif
       When an HTTP message includes the content of multiple ranges (for
       example, a response to a request for multiple non-overlapping ranges),
       these are transmitted as a multipart MIME message. The multipart MIME
       content-type used for this purpose is defined in this specification to
       be "multipart/byteranges". See appendix 19.2 for its definition. See
       appendix 19.8.3 for a compatibility issue.
       
       A client that cannot decode a MIME multipart/byteranges message should
       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).
       
       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.
       
       
       
       
       
       
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       14.18 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.
       
       
       14.19 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 RFC1123 [8]-date format.
       
              Date  = "Date" ":" HTTP-date
       An example is
       
              Date: Tue, 15 Nov 1994 08:12:31 GMT
       Origin servers MUST include a Date header field in all responses, except
       in these cases:
       
         1. If the response status code is 100 (Continue) or 101 (Switching
            Protocols), the response MAY include a Date header field, at the
            server's option.
       
         2. If the response status code conveys a server error, e.g. 500
            (Internal Server Error) or 503 (Service Unavailable), and it is
            inconvenient or impossible to generate a valid Date.
       
         3. If the server does not have a clock that can provide a reasonable
            approximation of the current time, its responses MUST NOT include a
            Date header field.  In this case, the rules in section 14.19.1 MUST
            be followed.
       
       A received message that does not have a Date header field MUST be
       assigned one by the recipient if the message will be cached by that
       recipient or gatewayed via a protocol which requires a Date. An HTTP
       implementation without a clock MUST NOT cache responses without
       revalidating them on every use. An HTTP cache, especially a shared
       cache, SHOULD use a mechanism, such as NTP [28], to synchronize its
       clock with a reliable external standard.
       
       Clients SHOULD only send a Date header field in messages that include an
       entity-body, as in the case of the PUT and POST requests, and even then
       
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       it is optional.  A client without a clock MUST NOT send a Date header
       field in a request.
       
       In theory, the date SHOULD represent the moment just before the entity
       is generated. In practice, the date can be generated at any time during
       the message origination without affecting its semantic value.
       
       
       
       
       14.19.1 Clockless Origin Server Operation
       
       Some origin server implementations may 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).
       
       
       14.20 ETag
       
       The ETag entity-header field defines the entity tag for the associated
       entity. The headers used with entity tags are described in sections
       14.20, 14.25, 14.26 and 14.43. The entity tag may be used for comparison
       with other entities from the same resource (see section 13.2.3).
       
             ETag = "ETag" ":" entity-tag
       Examples:
       
             ETag: "xyzzy"
             ETag: W/"xyzzy"
             ETag: ""
       
       14.21 Expires
       
       The Expires entity-header field gives the date/time after which the
       response should be considered stale. A stale cache entry may not
       normally be returned by a cache (either a proxy cache or an 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; it MUST be in RFC1123-date format:
       
             Expires = "Expires" ":" HTTP-date
       
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       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, 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 should use 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 should use 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 an response that otherwise would by default be non-
       cachable indicates that the response is cachable, unless indicated
       otherwise by a Cache-Control header field (section 14.9).
       
       
       14.22 From
       
       The From request-header field, if given, SHOULD contain an Internet e-
       mail address for the human user who controls the requesting user agent.
       The address SHOULD be machine-usable, as defined by mailbox in RFC 822
       [9] (as updated by RFC 1123 [8]):
       
              From   = "From" ":" mailbox
       An example is:
       
              From: webmaster@w3.org
       This header field MAY be used for logging purposes and as a means for
       identifying the source of invalid or unwanted requests. It SHOULD NOT be
       used as an insecure form of access protection. The interpretation of
       this field is that the request is being performed on behalf of the
       person given, who accepts responsibility for the method performed. In
       particular, robot agents SHOULD include this header so that the person
       responsible for running the robot can be contacted if problems occur on
       the receiving end.
       
       The Internet e-mail address in this field MAY be separate from the
       Internet host which issued the request. For example, when a request is
       passed through a proxy the original issuer's address SHOULD be used.
       
         Note: The client SHOULD not send the From header field without the
         user's approval, as it may conflict with the user's privacy
       
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         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
       URL 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
       network location 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/> MUST
       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 on the Internet (i.e., on any message corresponding to a
       request for a URL which includes an Internet host address for the
       service being requested). If the Host field is not already present, an
       HTTP/1.1 proxy MUST add a Host field to the request message prior to
       forwarding it on the Internet. All Internet-based HTTP/1.1 servers MUST
       respond with a 400 status code to any HTTP/1.1 request message which
       lacks a Host header field.
       
       See sections 5.2 and 19.5.1 for other requirements relating to Host.
       
       
       14.24 If-Modified-Since
       
       The If-Modified-Since request-header field is used with the GET 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:
       
              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
       
       
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       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 MUST return a 304 (Not Modified) response.
       
       The purpose of this feature is to allow efficient updates of cached
       information with a minimum amount of transaction overhead.
       
         Note that the Range request-header field modifies the meaning of
         If-Modified-Since; see section 14.36 for full details.
       
         Note that If-Modified-Since times are interpreted by the server,
         whose clock may 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 should use the exact
         date string received in a previous Last-Modified header field
         whenever possible.
       
       Note that 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.
       
       
       
       
       
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       14.25 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. The purpose of this feature is to allow efficient updates of
       cached information with a minimum amount of transaction overhead. It is
       also used, on updating requests, to prevent inadvertent modification of
       the wrong version of a resource. As a special case, the value "*"
       matches any current entity of the resource.
       
              If-Match = "If-Match" ":" ( "*" | 1#entity-tag )
       If any of the entity tags match the entity tag of the entity that would
       have been returned in the response to a similar GET request (without the
       If-Match header) on that resource, or if "*" is given 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 3.11) 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 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 14.43) 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: *
       
       
       
       
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       14.26 If-None-Match
       
       The If-None-Match request-header field is used with a method to make the
       method 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 entity-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 entity 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 "*" is given and no current entity exists, then the server MAY
       perform the requested method as if the If-None-Match header field did
       not exist.
       
       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.)
       
       
       
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       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 14.43) exists, and
       SHOULD be performed if the representation does not exist. This feature
       may be useful in preventing races between PUT operations.
       
       Examples:
       
              If-None-Match: "xyzzy"
              If-None-Match: W/"xyzzy"
              If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
              If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
              If-None-Match: *
       
       14.27 If-Range
       
       If a client has a partial copy of an entity in its cache, and wishes to
       have an up-to-date copy of the entire entity in its cache, it could use
       the Range request-header with a conditional GET (using either or both of
       If-Unmodified-Since and If-Match.) However, if the condition fails
       because the entity has been modified, the client would then have to make
       a second request to obtain the entire current entity-body.
       
       The If-Range header allows a client to "short-circuit" the second
       request. Informally, its meaning is `if the entity is unchanged, send me
       the part(s) that I am missing; otherwise, send me the entire new
       entity.'
       
               If-Range = "If-Range" ":" ( entity-tag | HTTP-date )
       If the client has no entity tag for an entity, but does have a Last-
       Modified date, it may use that date in a 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.
       
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       If the requested variant has been modified since the specified time, the
       server MUST NOT perform the requested operation, and MUST return a 412
       (Precondition Failed).
       
             If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
       An example of the field is:
       
              If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
       
       If the request normally (i.e., without the If-Unmodified-Since header)
       would result in anything other than a 2xx status, the If-Unmodified-
       Since header should be ignored.
       
       If the specified date is invalid, the header is ignored.
       
       
       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
       
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       identification of a new resource. For 201 (Created) responses, the
       Location is that of the new resource which was created by the request.
       For 3xx responses, the location SHOULD indicate the server's preferred
       URL for automatic redirection to the resource. The field value consists
       of a single absolute URL.
       
              Location       = "Location" ":" absoluteURI
       An example is:
       
              Location: http://www.w3.org/pub/WWW/People.html
         Note: The Content-Location header field (section 14.15) 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.
       
       
       14.31 Max-Forwards
       
       The Max-Forwards request-header field may be used with the TRACE
       (section 14.31) 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 SHOULD check and update its value prior to
       forwarding the request. If the received value is zero (0), the recipient
       MUST NOT forward the request; instead, it MUST respond as the final
       recipient. If the received Max-Forwards value is greater than zero, then
       the forwarded message MUST contain an updated Max-Forwards field with a
       value decremented by one (1).
       
       The Max-Forwards header field MAY be ignored for all other methods
       defined by this specification and for any extension methods for which it
       is not explicitly referred to as part of that method definition.
       
       
       14.32 Pragma
       
       The Pragma general-header field is used to include implementation-
       specific directives that may apply to any recipient along the
       request/response chain. All pragma directives specify optional behavior
       from the viewpoint of the protocol; however, some systems MAY require
       that behavior be consistent with the directives.
       
       
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              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 backwards compatibility with HTTP/1.0.
       Clients SHOULD include both header fields when a no-cache request is
       sent to a server not known to be HTTP/1.1 compliant.
       
       Pragma directives MUST be passed through by a proxy or gateway
       application, regardless of their significance to that application, since
       the directives may be applicable to all recipients along the
       request/response chain. It is not possible to specify a pragma for a
       specific recipient; however, any pragma directive not relevant to a
       recipient SHOULD be ignored by that recipient.
       
       HTTP/1.1 clients SHOULD NOT send the Pragma request-header. 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.
       
       
       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" ":" challenge
       The HTTP access authentication process is described in section 11.
       Unlike WWW-Authenticate, the Proxy-Authenticate header field applies
       only to the current connection and SHOULD NOT be passed on to downstream
       clients. However, an intermediate proxy may need to obtain its own
       credentials by requesting them from the downstream client, which in some
       circumstances will appear as if the proxy is forwarding the Proxy-
       Authenticate header field.
       
       
       14.34 Proxy-Authorization
       
       The Proxy-Authorization request-header field allows the client to
       identify itself (or its user) to a proxy which requires authentication.
       The Proxy-Authorization field value consists of credentials containing
       the authentication information of the user agent for the proxy and/or
       realm of the resource being requested.
       
              Proxy-Authorization     = "Proxy-Authorization" ":" credentials
       The HTTP access authentication process is described in section 11.
       Unlike Authorization, the Proxy-Authorization header field applies only
       
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       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 Public
       
       Editor's Note: The Public header here is left so as to preserve the
       numbering of the sections in section 14; the next draft will have delete
       this placeholder and will reorganize some material.
       
       
       14.36 Range
       
       
       14.36.1 Byte Ranges
       
       Since all HTTP entities are represented in HTTP messages as sequences of
       bytes, the concept of a byte range is meaningful for any HTTP entity.
       (However, not all clients and servers need to support byte-range
       operations.)
       
       Byte range specifications in HTTP apply to the sequence of bytes in the
       entity-body (not necessarily the same as the message-body).
       
       A byte range operation may specify a single range of bytes, or a set of
       ranges within a single entity.
       
              ranges-specifier = byte-ranges-specifier
              byte-ranges-specifier = bytes-unit "=" byte-range-set
              byte-range-set  = 1#( byte-range-spec | suffix-byte-range-spec )
              byte-range-spec = first-byte-pos "-" [last-byte-pos]
              first-byte-pos  = 1*DIGIT
              last-byte-pos   = 1*DIGIT
       The first-byte-pos value in a byte-range-spec gives the byte-offset of
       the first byte in a range. The last-byte-pos value gives the byte-offset
       of the last byte in the range; that is, the byte positions specified are
       inclusive. Byte offsets start at zero.
       
       If the last-byte-pos value is present, it must be greater than or equal
       to the first-byte-pos in that byte-range-spec, or the byte-range-spec is
       syntactically invalid. The recipient of a byte-range-set that includes
       one or more syntactically invalid byte-range-spec values  MUST ignore
       the header field that includes that byte-range-set.
       
       If the last-byte-pos value is absent, or if the value is greater than or
       equal to the current length of the entity-body, last-byte-pos is taken
       
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       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):
       
         .  The first 500 bytes (byte offsets 0-499, inclusive):
              bytes=0-499
         .  The second 500 bytes (byte offsets 500-999, inclusive):
              bytes=500-999
         .  The final 500 bytes (byte offsets 9500-9999, inclusive):
              bytes=-500
         .  Or
              bytes=9500-
         .  The first and last bytes only (bytes 0 and 9999):
              bytes=0-0,-1
         .  Several legal but not canonical specifications of the second 500
            bytes (byte offsets 500-999, inclusive):
              bytes=500-600,601-999
              bytes=500-700,601-999
       
       14.36.2 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 should support byte ranges when possible, since
       
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       Range supports efficient recovery from partially failed transfers, and
       supports efficient partial retrieval of large entities.
       
       If the server supports the Range header and the specified range or
       ranges are appropriate for the entity:
       
         .  The presence of a Range header in an unconditional GET modifies
            what is returned if the GET is otherwise successful. In other
            words, the response carries a status code of 206 (Partial Content)
            instead of 200 (OK).
         .  The presence of a Range header in a conditional GET (a request
            using one or both of If-Modified-Since and If-None-Match, or one or
            both of If-Unmodified-Since and If-Match) modifies what is returned
            if the GET is otherwise successful and the condition is true. It
            does not affect the 304 (Not Modified) response returned if the
            conditional is false.
       In some cases, it may 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.
       
       
       14.37 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
       If the field value is a partial URI, it SHOULD be interpreted relative
       to the Request-URI. The URI MUST NOT include a fragment.See section
       15.10 for security considerations.
       
       
       
       
       
       
       
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       14.38 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 should 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.
       
       
       14.39 Server
       
       The Server response-header field contains information about the software
       used by the origin server to handle the request. The field can contain
       multiple product tokens (section 3.8) and comments identifying the
       server and any significant subproducts. The product tokens are listed in
       order of their significance for identifying the application.
       
              Server         = "Server" ":" 1*( product | comment )
       Example:
       
              Server: CERN/3.0 libwww/2.17
       If the response is being forwarded through a proxy, the proxy
       application MUST NOT modify the Server response-header. Instead, it
       SHOULD include a Via field (as described in section 14.44).
       
         Note: Revealing the specific software version of the server may
         allow the server machine to become more vulnerable to attacks
         against software that is known to contain security holes. Server
         implementers are encouraged to make this field a configurable
         option.
       
       
       14.40 Transfer-Encoding
       
       The Transfer-Encoding general-header field indicates what (if any) type
       of transformation has been applied to the message body in order to
       safely transfer it between the sender and the recipient. This differs
       from the Content-Encoding in that the transfer coding is a property of
       the message, not of the entity.
       
              Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-
       coding
       
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       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.
       
       
       14.41 Upgrade
       
       The Upgrade general-header allows the client to specify what additional
       communication protocols it supports and would like to use if the server
       finds it appropriate to switch protocols. The server MUST use the
       Upgrade header field within a 101 (Switching Protocols) response to
       indicate which protocol(s) are being switched.
       
              Upgrade        = "Upgrade" ":" 1#product
       For example,
       
              Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
       The Upgrade header field is intended to provide a simple mechanism for
       transition from HTTP/1.1 to some other, incompatible protocol. It does
       so by allowing the client to advertise its desire to use another
       protocol, such as a later version of HTTP with a higher major version
       number, even though the current request has been made using HTTP/1.1.
       This eases the difficult transition between incompatible protocols by
       allowing the client to initiate a request in the more commonly supported
       protocol while indicating to the server that it would like to use a
       "better" protocol if available (where "better" is determined by the
       server, possibly according to the nature of the method and/or resource
       being requested).
       
       The Upgrade header field only applies to switching application-layer
       protocols upon the existing transport-layer connection. Upgrade cannot
       be used to insist on a protocol change; its acceptance and use by the
       server is optional. The capabilities and nature of the application-layer
       communication after the protocol change is entirely dependent upon the
       new protocol chosen, although the first action after changing the
       protocol MUST be a response to the initial HTTP request containing the
       Upgrade header field.
       
       The Upgrade header field only applies to the immediate connection.
       Therefore, the upgrade keyword MUST be supplied within a Connection
       header field (section 14.10) whenever Upgrade is present in an HTTP/1.1
       message.
       
       
       
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       The Upgrade header field cannot be used to indicate a switch to a
       protocol on a different connection. For that purpose, it is more
       appropriate to use a 301, 302, 303, or 305 redirection response.
       
       This specification only defines the protocol name "HTTP" for use by the
       family of Hypertext Transfer Protocols, as defined by the HTTP version
       rules of section 3.1 and future updates to this specification. Any token
       can be used as a protocol name; however, it will only be useful if both
       the client and server associate the name with the same protocol.
       
       
       14.42 User-Agent
       
       The User-Agent request-header field contains information about the user
       agent originating the request. This is for statistical purposes, the
       tracing of protocol violations, and automated recognition of user agents
       for the sake of tailoring responses to avoid particular user agent
       limitations. User agents SHOULD include this field with requests. The
       field can contain multiple product tokens (section 3.8) and comments
       identifying the agent and any subproducts which form a significant part
       of the user agent. By convention, the product tokens are listed in order
       of their significance for identifying the application.
       
              User-Agent     = "User-Agent" ":" 1*( product | comment )
       Example:
       
              User-Agent: CERN-LineMode/2.15 libwww/2.17b3
       
       14.43 Vary
       
       The Vary field value indicates the set of request-header fields that
       fully determines, while the response is fresh, whether a cache may use
       the response to reply to a subsequent request without revalidation. For
       uncachable 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 cachable
       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-cachable 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.
       
       
       
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       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 in which the
       response is fresh.
       
       The field-names given are not limited to the set of standard request-
       headerfields defined by this specification. Field names are case-
       insensitive.
       
       A Vary field value of "*" signals that unspecified parameters not
       limited to the request-headers (e.g., the network address of the
       client), play a role in the selection of the response representation.
       The "*" value MUST NOT be generated by a proxy server; it may only be
       generated by an origin server.
       
       
       14.44  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 represent each proxy or gateway that has
       forwarded the message. Each recipient MUST append its information such
       
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       that the end result is ordered according to the sequence of forwarding
       applications.
       
       Comments MAY be used in the Via header field to identify the software of
       the recipient proxy or gateway, analogous to the User-Agent and Server
       header fields. However, all comments in the Via field are optional and
       MAY be removed by any recipient prior to forwarding the message.
       
       For example, a request message could be sent from an HTTP/1.0 user agent
       to an internal proxy code-named "fred", which uses HTTP/1.1 to forward
       the request to a public proxy at nowhere.com, which completes the
       request by forwarding it to the origin server at www.ics.uci.edu. The
       request received by www.ics.uci.edu would then have the following Via
       header field:
       
              Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
       Proxies and gateways used as a portal through a network firewall SHOULD
       NOT, by default, forward the names and ports of hosts within the
       firewall region. This information SHOULD only be propagated if
       explicitly enabled. If not enabled, the received-by host of any host
       behind the firewall SHOULD be replaced by an appropriate pseudonym for
       that host.
       
       For organizations that have strong privacy requirements for hiding
       internal structures, a proxy MAY combine an ordered subsequence of Via
       header field entries with identical received-protocol values into a
       single such entry. For example,
       
              Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
               could be collapsed to
       
              Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
       Applications SHOULD NOT combine multiple entries unless they are all
       under the same organizational control and the hosts have already been
       replaced by pseudonyms. Applications MUST NOT combine entries which have
       different received-protocol values.
       
       
       14.45 Warning
       
       The Warning response-header field is used to carry additional
       information about the status of a response which may not be reflected by
       the response status code. This information is typically, though not
       exclusively, used to warn about a possible lack of semantic transparency
       from caching operations.
       
       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]
       
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              warn-code  = 3DIGIT
              warn-agent = ( host [ ":" port ] ) | pseudonym
                              ; the name or pseudonym of the server adding
                              ; the Warning header, for use in debugging
              warn-text  = quoted-string
              warn-date  = <"> HTTP-date <">
       A response may carry more than one Warning header.
       
       The warn-text should be in a natural language and character set that is
       most likely to be intelligible to the human user receiving the response.
       This decision may be based on any available knowledge, such as the
       location of the cache or user, the Accept-Language field in a request,
       the Content-Language field in a response, etc. The default language is
       English and the default character set is ISO-8859-1.
       
       If a character set other than ISO-8859-1 is used, it MUST be encoded in
       the warn-text using the method described in RFC 2047 [14].
       
       Any server or cache may add Warning headers to a response. New Warning
       headers should be added after any existing Warning headers. A cache MUST
       NOT delete any Warning header that it received with a response. 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
       SHOULD display as many of them as possible, in the order that they
       appear in the response. If it is not possible to display all of the
       warnings, the user agent should follow these heuristics:
       
         .  Warnings that appear early in the response take priority over those
            appearing later in the response.
         .  Warnings in the user's preferred character set take priority over
            warnings in other character sets but with identical warn-codes and
            warn-agents.
       Systems that generate multiple Warning headers should order them with
       this user agent behavior in mind.
       
       The warn-code consists of three digits.  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.
       
       2XX  Warnings that describe some aspect of the entity body or entity
         headers that is not rectified by a revalidation, and which MUST NOT
         be deleted after a successful revalidation.
       
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       This is a list of the currently-defined warn-codes, each with a
       recommended warn-text in English, and a description of its meaning.
       
       110 Response is stale
         MUST be included whenever the returned response is stale.
       
       111 Revalidation failed
         MUST be included if a cache returns a stale response because an
         attempt to revalidate the response failed, due to an inability to
         reach the server.
       
       112 Disconnected operation
          SHOULD be included if the cache is intentionally disconnected from
         the rest of the network for a period of time.
       
       113 Heuristic expiration
         MUST be included if the cache heuristically chose a freshness
         lifetime greater than 24 hours and the response's age is greater than
         24 hours.
       
       199 Miscellaneous warning
         The warning text may include arbitrary information to be presented to
         a human user, or logged. A system receiving this warning MUST NOT
         take any automated action, besides presenting the warning to the
         user.
       
       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, 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 response with one or more Warning headers
       to a client whose version is HTTP/1.0 or lower, then the sender MUST
       include a warn-date in each warning-value.
       
       If an implementation receives a response 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.
       
       
       
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       14.46 WWW-Authenticate
       
       The WWW-Authenticate response-header field MUST be included in 401
       (Unauthorized) response messages. The field value consists of at least
       one challenge that indicates the authentication scheme(s) and parameters
       applicable to the Request-URI.
       
              WWW-Authenticate  = "WWW-Authenticate" ":" 1#challenge
       The HTTP access authentication process is described in section 11. User
       agents MUST take special care in parsing the WWW-Authenticate field
       value if it contains more than one challenge, or if more than one WWW-
       Authenticate header field is provided, since the contents of a challenge
       may itself contain a comma-separated list of authentication parameters.
       
       
       14.47 Expect
       
       The Expect request-header field is used to indicate that particular
       server behaviors are required by the client.  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.
       
             Expect       =  "Expect" ":" 1#expectation
             expectation  =  "100-continue" | expectation-extension
             expectation-extension =  token [ "=" ( token | quoted-string )
                                      *expect-params ]
             expect-params =  ";" token [ "=" ( token | quoted-string ) ]
       The server SHOULD respond with a 417 (Expectation Failed) status if any
       of the expectations cannot be met.
       
       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.
       
       
       14.47.1 Expect 100-continue
       
       When the "100-continue" expectation is present on a request that
       includes a body, the requesting client will wait after sending the
       request headers before sending the content-body.  In this case, the
       server MUST conform to the requirements of section 8.2.4: it MUST either
       send a 100 (Continue) status, or an error status, after receiving the
       "Expect: 100-continue" request header.
       
       If a proxy receives a request 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.  If the
       proxy knows that the version of the next-hop server is HTTP/1.0 or
       
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       lower, it MUST NOT forward the request, and it MUST respond with a 417
       (Expectation Failed) status.  Proxies SHOULD maintain a cache recording
       the HTTP version numbers received from recently-referenced next-hop
       servers.
       
         Note: 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 or lengthy period before sending
         the request body.
       
       
       14.48 TE
       
       The TE request-header field is similar to Accept-Encoding, but restricts
       the transfer-codings (section 3.6) that are acceptable in the response.
       
              TE           = "TE" ":" #( t-codings )
       
               t-codings   = "chunked" | ( transfer-extension [ accept-params ]
       )
       Examples of its use are:
       
              TE: deflate
              TE:
              TE: chunked; deflate;q=0.5
       The TE header field only applies to the immediate connection. Therefore,
       the 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 an
       TE field, using these rules:
       
         1. If the transfer-coding 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.")
       
         2. If multiple transfer-codings are acceptable, then the acceptable
            transfer-coding with the highest non-zero qvalue is preferred.
       
         3. The "identity"  transfer-coding is always acceptable, unless
            specifically refused because the TE field includes "identity;q=0".
            The "chunked" transfer-coding is always acceptable. The Trailer
            header field (section 14.49) can be used to indicate the set of
            header fields included in the trailer.
       
       
       
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         4.    If the TE field-value is empty, only the "identity" and the
            "chunked" transfer-codings are acceptable.
       
       If an TE field is present in a request, and if a server cannot send a
       response which is acceptable according to the TE header field, then the
       server SHOULD send an error response with the 406 (Not Acceptable)
       status code.
       
       If no TE field is present, the sender MAY assume that the recipient will
       accept the "identity" and "chunked" transfer-codings.
       
       A server using chunked transfer-coding in a response MUST NOT use the
       trailer for other header fields than Content-MD5 and Authentication-Info
       unless the "chunked" transfer-coding is present in the request as an
       accepted transfer-coding in the TE field.
       
       
       14.49 Trailer
       
       The Trailer general field value indicates that the given set of header
       fields are present in the trailer of a message encoded with chunked
       transfer-coding.
       
              Trailer  = "Trailer" ":" 1#field-name
       An HTTP/1.1 sender 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 other header fields than Content-MD5 and Authentication-Info.
       
       A server MUST NOT include any other header fields unless the "chunked"
       transfer-coding is present in the request as an accepted transfer-coding
       in the TE field.
       
       Message header fields listed in the Trailer header field MUST NOT
       include the following header fields:
       
         .  Transfer-Encoding
       
         .  Content-Length
       
         .  Trailer
       
       
       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
       
       
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       solutions to the problems revealed, though it does make some suggestions
       for reducing security risks.
       
       
       15.1 Authentication of Clients
       
       The Basic authentication scheme is not a secure method of user
       authentication, nor does it in any way protect the entity, which is
       transmitted in clear text across the physical network used as the
       carrier. HTTP does not prevent additional authentication schemes and
       encryption mechanisms from being employed to increase security or the
       addition of enhancements (such as schemes to use one-time passwords) to
       Basic authentication.
       
       The most serious flaw in Basic authentication is that it results in the
       essentially clear text transmission of the user's password over the
       physical network. It is this problem which Digest Authentication
       attempts to address.
       
       Because Basic authentication involves the clear text transmission of
       passwords it SHOULD never be used (without enhancements) to protect
       sensitive or valuable information.
       
       A common use of Basic authentication is for identification purposes --
       requiring the user to provide a user name and password as a means of
       identification, for example, for purposes of gathering accurate usage
       statistics on a server. When used in this way it is tempting to think
       that there is no danger in its use if illicit access to the protected
       documents is not a major concern. This is only correct if the server
       issues both user name and password to the users and in particular does
       not allow the user to choose his or her own password. The danger arises
       because naive users frequently reuse a single password to avoid the task
       of maintaining multiple passwords.
       
       If a server permits users to select their own passwords, then the threat
       is not only illicit access to documents on the server but also illicit
       access to the accounts of all users who have chosen to use their account
       password. If users are allowed to choose their own password that also
       means the server must maintain files containing the (presumably
       encrypted) passwords. Many of these may be the account passwords of
       users perhaps at distant sites. The owner or administrator of such a
       system could conceivably incur liability if this information is not
       maintained in a secure fashion.
       
       Basic Authentication is also vulnerable to spoofing by counterfeit
       servers. If a user can be led to believe that he is connecting to a host
       containing information protected by basic authentication when in fact he
       is connecting to a hostile server or gateway then the attacker can
       request a password, store it for later use, and feign an error. This
       type of attack is not possible with Digest Authentication [32]. Server
       implementers SHOULD guard against the possibility of this sort of
       
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       counterfeiting by gateways or CGI scripts. In particular it is very
       dangerous for a server to simply turn over a connection to a gateway
       since that gateway can then use the persistent connection mechanism to
       engage in multiple transactions with the client while impersonating the
       original server in a way that is not detectable by the client.
       
       
       15.2 Abuse of Server Log Information
       
       A server is in the position to save personal data about a user's
       requests which may identify their reading patterns or subjects of
       interest. This information is clearly confidential in nature and its
       handling may be constrained by law in certain countries. People using
       the HTTP protocol to provide data are responsible for ensuring that such
       material is not distributed without the permission of any individuals
       that are identifiable by the published results.
       
       
       15.3 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 may allow the
       server machine to become more vulnerable to attacks against software
       that is known to contain security holes. Implementers 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 field allows reading patterns to be studied and reverse
       links drawn. Although it can be very useful, its power can be abused if
       user details are not separated from the information contained in the
       Referer. Even when the personal information has been removed, the
       Referer field may indicate a private document's URI whose publication
       would be inappropriate.
       
       The information sent in the From field might conflict with the user's
       privacy interests or their site's security policy, and hence it SHOULD
       NOT be transmitted without the user being able to disable, enable, and
       modify the contents of the field. The user MUST be able to set the
       
       
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       contents of this field within a user preference or application defaults
       configuration.
       
       We suggest, though do not require, that a convenient toggle interface be
       provided for the user to enable or disable the sending of From and
       Referer information.
       
       
       15.4 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.
       
       
       15.5 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
       implementers be particularly careful in this area. History shows that
       errors in this area are often both serious security and/or privacy
       problems, and often generate highly adverse publicity for the
       implementer's company.
       
       
       15.6 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
       
       
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       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 it should start sending Accept-Language headers to a server
       if it detects, by looking for any Vary response-header fields generated
       by the server, that such sending could improve the quality of service.
       
       Elaborate user-customized accept header fields sent in every request, in
       particular if these include quality values, can be used by servers as
       relatively reliable and long-lived user identifiers. Such user
       identifiers would allow content providers to do click-trail tracking,
       and would allow collaborating content providers to match cross-server
       click-trails or form submissions of individual users. Note that for many
       users not behind a proxy, the network address of the host running the
       user agent will also serve as a long-lived user identifier. In
       environments where proxies are used to enhance privacy, user agents
       should 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.
       
       
       15.7 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. These lookups should be cached, however, only when
       the TTL (Time To Live) information reported by the name server makes it
       likely that the cached information will remain useful.
       
       If HTTP clients cache the results of host name lookups in order to
       achieve a performance improvement, they MUST observe the TTL information
       reported by DNS.
       
       If HTTP clients do not observe this rule, they could be spoofed when a
       previously-accessed server's IP address changes. As network renumbering
       is expected to become increasingly common [24], the possibility of this
       form of attack will grow. Observing this requirement thus reduces this
       potential security vulnerability.
       
       
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       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.
       
       
       15.8 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.
       
       
       15.9 Content-Disposition Issues
       
       RFC 1806, from which the often implemented Content-Disposition (see
       section 19.6.1) 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 implementers.  See RFC 1806 [35] for details.
       
       
       15.10 Encoding Sensitive Information in URL's
       
       Because the source of a link may be private information or may reveal an
       otherwise private information source, it is strongly recommended that
       the user be able to select whether or not the Referer field is sent. For
       example, a browser client could have a toggle switch for browsing
       openly/anonymously, which would respectively enable/disable the sending
       of Referer and From information.
       
       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
       may be visible to third parties.  Servers can use POST based form
       submission instead
       
       
       15.11 Authetication Credentials and Idle Clients
       
       Editor's note: The RE-AUTHENTICATION-REQUESTED issue has made it clear
       more needs to be said here. See also code 418 and 418 in sections
       10.4.19 and 10.4.20
       
       
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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 past four years. It
       has benefited from a large and active developer community--the many
       people who have participated on the www-talk mailing list--and it is
       that community which has been most responsible for the success of HTTP
       and of the World-Wide Web in general. Marc Andreessen, Robert Cailliau,
       Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois Groff, Phillip
       M. Hallam-Baker, 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                  Albert Lunde
              Harald Tveit Alvestrand     John C. Mallery
              Keith Ball                  Jean-Philippe Martin-Flatin
              Brian Behlendorf            Larry Masinter
              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
       
       
       
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       Much of the content and presentation of the caching design is due to
       suggestions and comments from individuals including: Shel Kaphan, Paul
       Leach, Koen Holtman, David Morris, and Larry Masinter.
       
       Most of the specification of ranges is based on work originally done by
       Ari Luotonen and John Franks, with additional input from Steve Zilles.
       
       Thanks to the "cave men" of Palo Alto. You know who you are.
       
       Jim Gettys (the current editor of this document) wishes particularly to
       thank Roy Fielding, the previous editor of this document, along with
       John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen Holtman, John
       Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and Larry Masinter for
       their help.
       
       The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
       Frystyk implemented RFC2068 early on, and we wish to thank them for the
       discovery of many of the problems that this document attempts to
       rectify.
       
       
       17 References
       
       
       [1]     Alvestrand, H., "Tags for the Identification of Languages" RFC
         1766, UNINETT, March 1995.
       
       
       [2]     Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey,
         D., and B. Alberti. "The Internet Gopher Protocol (a distributed
         document search and retrieval protocol)", RFC 1436, University of
         Minnesota, March 1993.
       
       
       [3]     Berners-Lee, T., "Universal Resource Identifiers in WWW,"  RFC
         1630, CERN, June 1994.
       
       
       [4]     Berners-Lee, T., Masinter, L., and M. McCahill. "Uniform
         Resource Locators (URL)," RFC 1738, CERN, Xerox PARC, University of
         Minnesota, December 1994.
       
       
       [5]     Berners-Lee, T. and D. Connolly . "Hypertext Markup Language -
         2.0," RFC 1866, MIT/LCS, November 1995.
       
       
       [6]     Berners-Lee, T., Fielding, R. and H. Frystyk. "Hypertext
         Transfer Protocol -- HTTP/1.0," RFC 1945, MIT/LCS, UC Irvine, May
         1996.
       
       
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       [7]     Freed, N., and N. Borenstein. "Multipurpose Internet Mail
         Extensions (MIME) Part One: Format of Internet Message Bodies." RFC
         2045, Innosoft, First Virtual, November 1996.
       
       
       [8]     Braden, R., "Requirements for Internet Hosts -- Communication
         Layers," STD 3, RFC 1123, IETF, October 1989.
       
       
       [9]     D. H. Crocker, "Standard for The Format of ARPA Internet Text
         Messages," STD 11, RFC 822, UDEL, August 1982.
       
       
       [10] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R.,
            Sui, J., and M. Grinbaum, "WAIS Interface Protocol Prototype
            Functional Specification." (v1.5), Thinking Machines Corporation,
            April 1990.
       
       
       [11]    Fielding, R., "Relative Uniform Resource Locators," RFC 1808, UC
         Irvine, June 1995.
       
       
       [12]    Horton, M., and R. Adams. "Standard for Interchange of USENET
         Messages," RFC 1036 (Obsoletes RFC 850), AT&T Bell Laboratories,
         Center for Seismic Studies, December 1987.
       
       
       [13]    Kantor, B. and P. Lapsley. "Network News Transfer Protocol," RFC
         977, UC San Diego, UC Berkeley, February 1986.
       
       [14] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
         Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
         University of Tennessee, November 1996.
       
       
       [15]    Nebel, E., and L. Masinter. "Form-based File Upload in HTML,"
         RFC 1867, Xerox Corporation, November 1995.
       
       
       [16]    Postel, J., "Simple Mail Transfer Protocol," STD 10, RFC 821,
         USC/ISI, August 1982.
       
       
       [17]    Postel, J., "Media Type Registration Procedure," RFC  1590,
         USC/ISI, November 1996.
       
       
       [18]    Postel, J. and J. Reynolds. "File Transfer Protocol," STD 9, RFC
         959, USC/ISI, October 1985.
       
       
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       [19]    Reynolds, J. and J. Postel. "Assigned Numbers," STD 2, RFC 1700,
         USC/ISI, October 1994.
       
       
       [20]    Sollins, K. and L. Masinter. "Functional Requirements for
         Uniform Resource Names," RFC 1737, MIT/LCS, Xerox Corporation,
         December 1994.
       
       
       [21]    US-ASCII. Coded Character Set - 7-Bit American Standard Code for
         Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.
       
       
       [22]    ISO-8859. International Standard -- Information Processing --
         8-bit Single-Byte Coded Graphic Character Sets --
         Part 1: Latin alphabet No. 1, ISO 8859-1:1987.
         Part 2: Latin alphabet No. 2, ISO 8859-2, 1987.
         Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.
         Part 4: Latin alphabet No. 4, ISO 8859-4, 1988.
         Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988.
         Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987.
         Part 7: Latin/Greek alphabet, ISO 8859-7, 1987.
         Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988.
         Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.
       
       
       [23]    Meyers, J., and M. Rose. "The Content-MD5 Header Field," RFC
         1864, Carnegie Mellon, Dover Beach Consulting, October, 1995.
       
       
       [24]    Carpenter, B. and Y. Rekhter. "Renumbering Needs Work," RFC
         1900, IAB, February 1996.
       
       
       [25]    Deutsch, P., "GZIP file format specification version 4.3,." RFC
         1952, Aladdin Enterprises, May, 1996.
       
       
       [26]    Venkata N. Padmanabhan,  and Jeffrey C. Mogul. "Improving HTTP
         Latency", Computer Networks and ISDN Systems, v. 28, pp. 25-35, Dec.
         1995. Slightly revised version of paper in Proc. 2nd International
         WWW Conference '94: Mosaic and the Web, Oct. 1994, which is available
         at
         http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLatency.
         html.
       
       
       [27]    Joe Touch, John Heidemann, and Katia Obraczka. "Analysis of HTTP
         Performance", <URL: http://www.isi.edu/lsam/publications/http-
         perf/index.html>, USC/Information Sciences Institute, June 1996.
       
       
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       [28]    Mills, D., "Network Time Protocol (Version 3) Specification,
         Implementation and Analysis." RFC 1305, University of Delaware,
         March, 1992.
       
       
       [29]    Deutsch, P., "DEFLATE Compressed Data Format Specification
         version 1.3." RFC 1951, Aladdin Enterprises, May 1996.
       
       [30]    S. Spero, "Analysis of HTTP Performance Problems"
         <URL:http://sunsite.unc.edu/mdma-release/http-prob.html>.
       
       
       [31]    Deutsch, P. and J-L. Gailly. "ZLIB Compressed Data Format
         Specification version 3.3," RFC 1950, Aladdin Enterprises, Info-ZIP,
         May 1996.
       
       
       [32] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P., Luotonen,
         A., Sink, E., and L. Stewart. "An Extension to HTTP: Digest Access
         Authentication," RFC 2069, January 1997.
       
       
       [33] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Berners-Lee, T.,
         "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2068, UC Irvine,
         Digital Equipment Corporation, M.I.T., January, 1997.
       
       
       [34] Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels," RFC 2119, Harvard University, March 1997.
       
       [35]    Troost, R., and Dorner, S., "Communicating Presentation
         Information in Internet Messages: The Content-Disposition Header,"
         RFC 1806, New Century Systems, QUALCOMM, Inc., June 1995.
       
       
       [36] Mogul, J.C., Fielding, R., Gettys, J, Frystyk, H., "Use and
         Interpretation of HTTP Version Numbers", RFC 2145, Digital Equipment
         Corporation, U.C. Irvine, M.I.T., May 1997.
       
       [37]    Palme, J,  "Common Internet Message Headers," RFC 2076,
         Stockholm University, KTH, February, 1997.
       
       [38] Yergeau, F.,  "UTF-8, a transformation format of Unicode and ISO
         10646," Work In Progress (draft-yergeau-utf8-rev-00.txt, obsoletes
         RFC 2044), Alis Technologies,.
       
       [39]  Nielsen, H.F., Gettys, J., Baird-Smith, A., Prud'hommeaux, E.,
         Lie, H., and C. Lilley. "Network Performance Effects of HTTP/1.1,
         CSS1, and PNG," Proceedings of ACM SIGCOMM '97, Cannes France,
         September 1997.
       
       
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       INTERNET-DRAFT            HTTP/1.1 Friday, November 21, 1997
       
       
       [40]    Freed, N., and N. Borenstein. "Multipurpose Internet Mail
         Extensions (MIME) Part Two: Media Types." RFC 2046, Innosoft, First
         Virtual, November 1996.
       
       [41] Alvestrand, H. T.,  "IETF Policy on Character Sets and Languages,"
         Work in Progress, UNINETT,  October, 1997.
       
       [42] Berners Lee, T, Fielding, R., Masinter, L., "Uniform Resource
         Identifiers (URI): Generic Syntax and Semantics ," Work in Progress,
         November, 1997.
       
       [43] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P., Luotonen,
         A., Sink, E., Stewart, L., "HTTP Authentication: Basic and Digest
         Access Authentication ," Work in Progress, November, 1997.
       
       [44] Luotonen, A., "Tunnelling SSL Through a WWW Proxy," Work in
         Progress, November, 1997.
       
       [45] Schulzrinne, H., "Assignment of Status Codes for HTTP and HTTP-
         Derived Protocols," Work in Progress, July, 1997.
       
       
       18 Authors' Addresses
       
       Roy T. Fielding
       Department of Information and Computer Science
       University of California
       Irvine, CA 92717-3425, USA
       Fax: +1 (714) 824-4056
       Email: fielding@ics.uci.edu
       
       James Gettys
       MIT Laboratory for Computer Science
       545 Technology Square
       Cambridge, MA 02139, USA
       Fax: +1 (617) 258 8682
       Email: jg@w3.org
       
       Jeffrey C. Mogul
       Western Research Laboratory
       Digital Equipment Corporation
       250 University Avenue
       Palo Alto, California, 94305, USA
       Email: mogul@wrl.dec.com
       
       Henrik Frystyk Nielsen
       W3 Consortium
       MIT Laboratory for Computer Science
       545 Technology Square
       Cambridge, MA 02139, USA
       
       
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       Fax: +1 (617) 258 8682
       Email: frystyk@w3.org
       
       Larry Masinter
       Xerox PARC
       3333 Coyote Hill Road
       Palo Alto, CA 94034, USA
       Fax:+1 (415) 812-4333
       Email: masinter@parc.xerox.com
       
       Paul J. Leach
       Microsoft Corporation
       1 Microsoft Way
       Redmond, WA 98052, USA
       Email: paulle@microsoft.com
       
       Tim Berners-Lee
       Director, W3 Consortium
       MIT Laboratory for Computer Science
       545 Technology Square
       Cambridge, MA 02139, USA
       Fax: +1 (617) 258 8682
       Email: timbl@w3.org
       
       
       19 Appendices
       
       
       19.1 Internet Media Type message/http
       
       In addition to defining the HTTP/1.1 protocol, this document serves as
       the specification for the Internet media type "message/http". The
       following is to be registered with IANA [17].
       
              Media Type name:         message
              Media subtype name:      http
              Required parameters:     none
              Optional parameters:     version, msgtype
               version: The HTTP-Version number of the enclosed message
                        (e.g., "1.1"). If not present, the version can be
                        determined from the first line of the body.
               msgtype: The message type -- "request" or "response". If not
                        present, the type can be determined from the first
                        line of the body.
              Encoding considerations: only "7bit", "8bit", or "binary" are
                                       permitted
              Security considerations: none
       
       
       
       
       
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       19.2 Internet Media Type multipart/byteranges
       
       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 MIME message. The multipart 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 parts are
       separated using a MIME boundary parameter.
       
              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--
       
       19.3 Tolerant Applications
       
       Although this document specifies the requirements for the generation of
       HTTP/1.1 messages, not all applications will be correct in their
       implementation. We therefore recommend that operational applications be
       tolerant of deviations whenever those deviations can be interpreted
       unambiguously.
       
       Clients SHOULD be tolerant in parsing the Status-Line and servers
       tolerant when parsing the Request-Line. In particular, they SHOULD
       accept any amount of SP or HT characters between fields, even though
       only a single SP is required.
       
       The line terminator for message-header fields is the sequence CRLF.
       However, we recommend that applications, when parsing such headers,
       recognize a single LF as a line terminator and ignore the leading CR.
       
       
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       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 no label is preferred over the labels US-ASCII or
       ISO-8859-1.
       
       Additional rules for requirements on parsing and encoding of dates and
       other potential problems with date encodings include:
       
         .  HTTP/1.1 clients and caches should assume that an RFC-850 date
            which appears to be more than 50 years in the future is in fact in
            the past (this helps solve the "year 2000" problem).
         .  An HTTP/1.1 implementation may internally represent a parsed
            Expires date as earlier than the proper value, but MUST NOT
            internally represent a parsed Expires date as later than the proper
            value.
         .  All expiration-related calculations must be done in GMT. The local
            time zone MUST NOT influence the calculation or comparison of an
            age or expiration time.
         .  If an HTTP header incorrectly carries a date value with a time zone
            other than GMT, it must be converted into GMT using the most
            conservative possible conversion.
       
       19.4 Differences Between HTTP Entities and RFC 2045 Entities
       
       HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC 822
       [9]) and the Multipurpose Internet Mail Extensions (MIME [7]) to allow
       entities to be transmitted in an open variety of representations and
       with extensible mechanisms. However, RFC 2045 discusses mail, and HTTP
       has a few features that are different from those described in RFC 2045.
       These differences were carefully chosen to optimize performance over
       binary connections, to allow greater freedom in the use of new media
       types, to make date comparisons easier, and to acknowledge the practice
       of some early HTTP servers and clients.
       
       
       
       This appendix describes specific areas where HTTP differs from RFC 2045.
       Proxies and gateways to strict MIME environments SHOULD be aware of
       these differences and provide the appropriate conversions where
       necessary. Proxies and gateways from MIME environments to HTTP also need
       to be aware of the differences because some conversions may be required.
       
       
       19.4.1 Conversion to Canonical Form
       
       RFC 2045 requires that an Internet mail entity be converted to canonical
       form prior to being transferred, as described in Appendix G of RFC 2045
       [7]. Section 3.7.1 of this document describes the forms allowed for
       subtypes of the "text" media type when transmitted over HTTP. RFC 2045
       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.
       
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       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 RFC 2045
       environment SHOULD translate all line breaks within the text media types
       described in section 3.7.1 of this document to the RFC 2045 canonical
       form of CRLF. Note, however, that this may be complicated by the
       presence of a Content-Encoding and by the fact that HTTP allows the use
       of some character sets which do not use octets 13 and 10 to represent CR
       and LF, as is the case for some multi-byte character sets.
       
       
       19.4.2 Conversion of Date Formats
       
       HTTP/1.1 uses a restricted set of date formats (section 3.3.1) to
       simplify the process of date comparison. Proxies and gateways from other
       protocols SHOULD ensure that any Date header field present in a message
       conforms to one of the HTTP/1.1 formats and rewrite the date if
       necessary.
       
       
       19.4.3 Introduction of Content-Encoding
       
       RFC 2045 does not include any concept equivalent to HTTP/1.1's Content-
       Encoding header field. Since this acts as a modifier on the media type,
       proxies and gateways from HTTP to MIME-compliant protocols MUST either
       change the value of the Content-Type header field or decode the entity-
       body before forwarding the message. (Some experimental applications of
       Content-Type for Internet mail have used a media-type parameter of
       ";conversions=<content-coding>" to perform an equivalent function as
       Content-Encoding. However, this parameter is not part of RFC 2045.)
       
       
       19.4.4 No Content-Transfer-Encoding
       
       HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC 2045.
       Proxies and gateways from MIME-compliant protocols to HTTP MUST remove
       any non-identity CTE ("quoted-printable" or "base64") encoding prior to
       delivering the response message to an HTTP client.
       
       Proxies and gateways from HTTP to MIME-compliant protocols are
       responsible for ensuring that the message is in the correct format and
       encoding for safe transport on that protocol, where "safe 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.
       
       
       
       
       
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       19.4.5 HTTP Header Fields in Multipart Body-Parts
       
       In RFC 2045, most header fields in multipart body-parts are generally
       ignored unless the field name begins with "Content-". In HTTP/1.1,
       multipart body-parts may contain any HTTP header fields which are
       significant to the meaning of that part.
       
       
       19.4.6 Introduction of Transfer-Encoding
       
       HTTP/1.1 introduces the Transfer-Encoding header field (section 14.40).
       Proxies/gateways MUST remove any transfer coding prior to forwarding a
       message via a MIME-compliant protocol.
       
       A process for decoding the "chunked" transfer coding (section 3.6) can
       be represented in pseudo-code as:
       
              length := 0
              read chunk-size, chunk-extension (if any) and CRLF
              while (chunk-size > 0) {
                 read chunk-data and CRLF
                 append chunk-data to entity-body
                 length := length + chunk-size
                 read chunk-size and CRLF
              }
              read entity-header
              while (entity-header not empty) {
                 append entity-header to existing header fields
                 read entity-header
              }
              Content-Length := length
              Remove "chunked" from Transfer-Encoding
       
       19.4.7 MIME-Version
       
       HTTP is not a MIME-compliant protocol (see appendix 19.4). However,
       HTTP/1.1 messages may include a single MIME-Version general-header field
       to indicate what version of the MIME protocol was used to construct the
       message. Use of the MIME-Version header field indicates that the message
       is in full compliance with the MIME protocol (as defined in RFC
       2045[7]). Proxies/gateways are responsible for ensuring full compliance
       (where possible) when exporting HTTP messages to strict MIME
       environments.
       
              MIME-Version   = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
       MIME version "1.0" is the default for use in HTTP/1.1. However, HTTP/1.1
       message parsing and semantics are defined by this document and not the
       MIME specification.
       
       
       
       
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       19.5 Changes from HTTP/1.0
       
       This section summarizes major differences between versions HTTP/1.0 and
       HTTP/1.1.
       
       
       19.5.1 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:
       
       
         .  Both clients and servers MUST support the Host request-header.
       
         .  Host request-headers are required in HTTP/1.1 requests.
       
         .  Servers MUST report a 400 (Bad Request) error if an HTTP/1.1
            request does not include a Host request-header.
       
         .  Servers MUST accept absolute URIs.
       
       19.6 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. Implementers should 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.
       
       
       
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       The Public header of RFC 2068 [33] has been removed, as it was not
       widely implemented.
       
       A number of other headers, such as Content-Disposition and Title, from
       SMTP and MIME are also often implemented (see RFC 2076 [37]).
       
       
       19.6.1 Content-Disposition
       
       The Content-Disposition response-header field has been proposed as a
       means for the origin server to suggest a default filename if the user
       requests that the content is saved to a file.  This usage is derived
       from the definition of Content-Disposition in RFC 1806 [35].
       
               content-disposition = "Content-Disposition" ":"
                                     disposition-type *( ";" disposition-parm )
               disposition-type = "attachment" | disp-extension-token
               disposition-parm = filename-parm | disp-extension-parm
               filename-parm = "filename" "=" quoted-string
               disp-extension-token = token
               disp-extension-parm = token "=" ( token | quoted-string )
       An example is
       
               Content-Disposition: attachment; filename="fname.ext"
       The receiving user agent should not respect any directory path
       information that may seem to be present in the filename parameter. 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 15.9 for Content-Disposition  security issues.
       
       
       
       
       19.7 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, we would expect commercial
       HTTP/1.1 servers to:
       
       
         .  recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1
            requests;
       
         .  understand any valid request in the format of HTTP/0.9, 1.0, or
            1.1;
       
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         .  respond appropriately with a message in the same major version used
            by the client.
       And we would expect HTTP/1.1 clients to:
       
       
         .  recognize the format of the Status-Line for HTTP/1.0 and 1.1
            responses;
       
         .  understand any valid response in the format of HTTP/0.9, 1.0, or
            1.1.
       For most implementations of HTTP/1.0, each connection is established by
       the client prior to the request and closed by the server after sending
       the response. A few implementations implement the Keep-Alive version of
       persistent connections described in section Error! Reference source not
       found..
       
       
       19.7.1 Compatibility with HTTP/1.0 Persistent Connections
       
       Some clients and servers may wish to be compatible with some previous
       implementations of persistent connections in HTTP/1.0 clients and
       servers. Persistent connections in HTTP/1.0 must be explicitly
       negotiated as they are not the default behavior. HTTP/1.0 experimental
       implementations of persistent connections are faulty, and the new
       facilities in HTTP/1.1 are designed to rectify these problems. The
       problem was that some existing 1.0 clients may be sending Keep-Alive to
       a proxy server that doesn't understand Connection, which would then
       erroneously forward it to the next inbound server, which would establish
       the Keep-Alive connection and result in a hung HTTP/1.0 proxy waiting
       for the close on the response. The result is that HTTP/1.0 clients must
       be prevented from using Keep-Alive when talking to proxies.
       
       However, talking to proxies is the most important use of persistent
       connections, so that prohibition is clearly unacceptable. Therefore, we
       need some other mechanism for indicating a persistent connection is
       desired, which is safe to use even when talking to an old proxy that
       ignores Connection. Persistent connections are the default for HTTP/1.1
       messages; we introduce a new keyword (Connection: close) for declaring
       non-persistence.
       
       The original HTTP/10 form of persistent connections (the Connection:
       Keep-Alive and Keep-Alive header) is documented in RFC 2068. [33]
       
       
       19.8 Backward Compatibility
       
       Editor's Note: We (the editorial group) have discussed moving many of
       the implementation notes having to do with backward compatibility (often
       bug work-arounds) out of the mainline specification into an appendix.
       This is mostly a placeholder in case this work gets done. _ JG.
       
       
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       19.8.1 CRLF's in Quoted Strings
       
       CRLF in a quoted string is legal, but only in a strange way: as part of
       a header continuation, as in "part of
       a
       quoted-string".  This is strange, and CRLF's should be allowed in
       general, but backward compatibility constraints mean that they are not
       allowed in general. .
       
       
       19.8.2 Missing Content Type
       
       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.
       
       
       19.8.3 Multipart/x-byteranges
       
       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.
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
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       20 Index
       
       While some care was taken producing this index, there is no guarantee
       that all occurences of an index term have been entered into the index.
       Italics indicate the definition of a term; bold face is used for the
       definition of a header.
       
       
       "literal", 15
       #rule, 15                             410, 37, 62,   , 86
       (rule1 rule2), 15                     411, 32, 37, 64
            ,                                412, 37, 65, 121, 122, 124                                             409, 37, 64                                                          64       *rule  15
       ; comment, 16                         413, 38, 65
       [rule], 15                            414, 19, 38, 65
       100, 37, 44, 45, 46, 47, 54, 93,      415, 38, 65, 110
        116, 137, 138                        416, 38, 65, 114, 115, 128
       101, 37, 54, 55, 116, 131             417, 66
       1xx Informational Status Codes,       418, 66
        54                                   419, 66
       200, 37, 49, 51, 53, 55, 56, 57,      4xx Client Error Status Codes, 61
        60, 86, 91, 106, 115, 120, 123,      500, 38, 67
        129                                  501, 24, 33, 38, 51, 67
       201, 37, 51, 55, 125                  502, 38, 67
       202, 37, 53, 55, 56                   503, 38, 67, 116, 130
       203, 37, 56, 86                       504, 38, 67, 107
       204, 31, 37, 51, 53, 56               505, 38, 68
       205, 37, 56                           506, 52, 68
       206, 37, 57, 86, 88, 89, 91, 114,     5xx Server Error Status Codes, 67
        115, 123, 128, 129, 152              abs_path, 19, 20, 34
       207, 52, 53, 57                       absoluteURI, 19, 34, 35, 110,
       2xx, 122                               112, 125, 129
       2xx Successful Status Codes, 55       Accept, 25, 35, 69, 73, 94, 95,
       300, 37, 58, 70, 86                    96, 97, 142
       301, 37, 52, 58, 59, 86, 132          Accept-Charset, 35, 69, 96
       302, 37, 59, 61, 132                  Accept-Encoding, 22, 23, 35, 69,
       303, 37, 51, 59, 132                   96, 97, 138
       304, 31, 37, 60, 72, 81, 85, 88,      accept-extension, 94
        89, 90, 106, 119, 120, 122, 129      Accept-Language, 28, 35, 69, 97,
       305, 37, 61, 72, 132                   98, 135, 142, 143
       3xx Redirection Status Codes, 58      accept-params, 94
       400, 32, 35, 37, 38, 62, 66, 119,     Accept-Ranges, 38, 99
        156                                  Access Authentication, 68
       401, 37, 62, 63, 66, 100, 137          Basic and Digest. See [43]
       402, 37, 62                           Acknowlegements, 145
       403, 37, 62                           age, 12
       404, 37, 62, 64                       Age, 38, 77, 78, 99
       405, 33, 37, 63, 99                   age-value, 99
       406, 37, 63, 70, 95, 96, 97           Allow, 33, 39, 49, 63, 99, 100
       407, 37, 63, 66, 126                  ALPHA, 16
       408, 37, 63                           Alternates. See RFC 2068
       
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       asctime-date, 21                       Warnings, 73
       attribute, 25                          weak and strong cache
       augmented Backus-Naur Form, 14           validators, 82
       Authentication-Info, 139. See          write-through mandantory, 92
        [43]                                 Cache-Control, 32, 51, 57, 59,
       Authorization, 35, 36, 62, 63,         60, 61, 74, 75, 76, 79, 80, 81,
        86, 100, 102, 126                     86, 88, 91, 100, 101, 102, 104,
                                              105, 106, 108, 109, 110, 118,
       Basic Authentication. See [43]         126
       byte-content-range-spec, 114           max-age, 76, 79, 80, 86, 101,
       byte-range, 127                          103, 104, 105, 106, 107, 118
       byte-range-resp-spec, 114              max-stale, 74, 101, 105, 106,
       byte-range-set, 127, 128                 107
       byte-range-spec, 65, 115, 127          min-fresh, 105
       byte-ranges-specifier, 127             no-cache, 101, 102, 103, 104,
       bytes-unit, 29                           106, 126
       cachable, 11                           public, 62, 69, 70, 74, 86, 101,
       cache, 11                                102, 104, 107, 134
       Cache                                  s-maxage, 79, 86, 100, 101, 104       Basic authentication, 140
        cachability of responses, 86         cache-directive, 72, 101, 109,
        calculating the age of a              126
         response, 77                        cache-request-directive, 72, 101
        combining byte ranges, 89            Changes from HTTP/1.0. See RFC
        combining headers, 88                 1945 and RFC 2068
        combining negotiated responses,       Host requirement, 156
         90                                  CHAR, 16
        constructing responses, 87           charset, 22
        Correctness, 72                      chunk, 24
        disambiguating expiration            chunk-data, 24
         values, 79                          chunked, 24
        disambiguating multiple              Chunked-Body, 24
         responses, 80                       chunk-extension, 24
        entity tags used as cache            chunk-ext-name, 24
         validators, 82                      chunk-ext-val, 24
        entry validation, 81                 chunk-size, 24
        errors or incomplete responses,      client, 11
         91                                  codings, 96
        expiration calculation, 79           comment, 17
        explicit expiration time, 76         Compatibility
        GET and HEAD cannot affect            CRLF in a quoted string, 159
         caching, 92                          missing Content-Type, 159
        heuristic expiration, 76              multipart/x-byteranges, 159
        history list behavior, 93            Compatibility with previous HTTP
        invalidation cannot be complete,      versions, 157
         92                                  compress, 24
        Last-Modified values used as         CONNECT, 33. See [44].
         validators, 81                      connection, 10
        mechanisms, 74                       Connection, 32, 41, 42, 87, 109,
        replacement of cached responses,      131, 138, 158
         93                                   Keep-Alive, 158. See RFC 2068
        shared and non-shared, 91            Content Codings
       
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        compress, 22                          MIME-Version, 155
        deflate, 23
        gzip, 22
        identity, 23                                              Transfer-Encoding, 155                                             Digest Authentication, 87, 140.                                              See [43]
       content negotiation,                  Digest Authentication Scheme, 68
       Content Negotiation, 68               DIGIT, 16
       Content-Base, 39, 110, 112            disp-extension-token, 157
       content-coding, 22, 24                disposition-parm, 157
       content-disposition, 157              disposition-type, 157
       Content-Disposition, 144, 149,        DNS, 143, 144
        157                                   must obey TTL information, 143                            10
       Content-Encoding, 22, 23, 26, 39,     End-to-end headers, 87
        40, 88, 110, 113, 130, 136, 154      entity, 10
       Content-Language, 28, 39, 111,        Entity, 39
        135                                  Entity body, 39
       Content-Length, 31, 32, 39, 44,       Entity Tags, 28, 82
        48, 50, 57, 64, 91, 111, 112,        entity-body, 39
        115, 139, 155                        entity-header, 39
       Content-Location, 90, 91, 92,         Entity-header fields, 39
        110, 112, 125, 144                   entity-length, 114
       Content-MD5, 24, 39, 50, 87, 113,     entity-tag, 28
        139, 148                             ETag, 28, 39, 50, 57, 60, 82, 87,
       Content-Range, 52, 57, 114             88, 90, 117, 122
       Content-Transfer-Encoding, 23,        Expect, 36, 44, 45, 46, 54, 66,
        154                                   137, 138
       Content-Type, 25, 39, 40, 48, 53,     expectation, 137
        57, 58, 63, 64, 88, 110, 115,        expectation-extension, 137
        116, 136, 152, 154, 159              expect-params, 137
       Content-Version. See RFC 2068         Expires, 39, 51, 57, 59, 60, 61,
       CR, 16                                 76, 79, 86, 87, 88, 103, 104,
       CRLF, 16                               107, 117, 118, 153
       ctext, 17                             explicit expiration time, 12
       CTL, 16                               extension-code, 38
       Date, 32, 57, 60, 77, 79, 80, 83,     extension-header, 39
        84, 86, 89, 91, 93, 104, 116,        field-content, 30
        118, 124, 136, 154                   field-name, 30
       date1, 21                             field-value, 30
       date2, 21                             filename-parm, 157
       date3, 21                             first-byte-pos, 65, 114, 115,
       deflate, 24                            127, 128
       DELETE, 33, 47, 48, 53, 92            first-hand, 12
       delta-seconds, 21                     fresh, 12
       Derived-From. See RFC 2068            freshness lifetime, 12
       Differences between MIME and          From, 36, 43, 118, 141, 142, 144
        HTTP, 153                            gateway, 11
        canonical form, 153                  General Header Fields, 32
        Content-Encoding, 154                general-header, 32
        Content-Transfer-Encoding, 154       generic-message, 29
        date formats, 154                    GET, 19, 33, 34, 47, 48, 49, 50,
        header fileds in multipart body-      55, 57, 58, 59, 60, 61, 65, 81,
         parts, 155                           83, 84, 91, 92, 100, 111, 116,
       
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        119, 120, 121, 122, 123, 128,        LF,
        129, 144
           , 24                                                 16                                             lifetime, 12, 77, 79, 99, 105,       gzip                                   136
       HEAD, 31, 33, 47, 48,   , 55, 58,     Link. See RFC 2068
        59, 60, 61, 62, 63, 67, 91, 92,      LINK. See RFC 2068
        100, 111, 116, 122                   LOALPHA, 16
       Headers                               Location, 27, 38, 39, 51, 55, 57,
        end-to-end, 87, 105, 106              58, 59, 60, 61, 87, 92, 110,
        Hop-by-hop, 87                        112, 124, 125, 144
        non-modifiable headers, 87           LWS, 17
       Henrik Frystyk Nielsen, 150           Max-Forwards, 36, 49, 53, 125
       heuristic expiration time,            media type, 16, 22, 25, 26, 31,                             50   12
       HEX, 17                                40, 58, 63, 68, 94, 95, 107,
       Hop-by-hop headers, 87                 110, 111, 116, 151, 152, 153,
       host, 19                               154, 159
       Host, 34, 35, 36, 47, 119, 156        Media Types, 25
       HT, 16                                media-range, 94
       http_URL, 19                          media-type, 25
       HTTP-date, 20, 21, 116, 117, 119,     message, 10
        123, 124, 130, 135                   Message Body, 30
       HTTP-message, 29                      Message Headers, 30
       HTTP-Version, 18                      Message Length, 31
       IANA, 22, 23, 26, 28, 94, 151         Message Transmission
       identity, 24                           Requirements, 44
       If-Match, 28, 36, 49, 85, 121,        Message Types, 29
        123, 129                             message-body, 30
       If-Modified-Since, 36, 49, 83,        message-header, 30
        84, 85, 119, 120, 122, 129           Method, 33
       If-None-Match, 28, 36, 49, 85,        Method Definitions, 47
        90, 91, 122, 123, 129                Methods
       If-Range, 28, 36, 49, 57, 65, 85,      Idempotent, 48
        115, 123, 129                         Safe, 47
       If-Unmodified-Since, 36, 49, 83,      MIME, 9, 13, 21, 22, 23, 25, 26,
        85, 123, 124, 129                     27, 112, 113, 115, 145, 147,
       implied *LWS, 16                       150, 152, 153, 154, 155, 157
       James Gettys, 150                      multipart, 26
       Jeffrey C. Mogul, 150                 MIME-Version, 155
       Keep-Alive, 42, 87, 158. See RFC      month, 21
        2068                                 multipart/byteranges, 31, 57, 66,
       Language Tags, 28                      115, 152
       language-range, 97, 98                multipart/x-byteranges, 159
       language-tag, 28                      N rule, 15
       Larry Masinter, 151                   name, 15
       last-byte-pos, 114, 127, 128          non-shared cache, 91, 102, 108
       last-chunk, 24                        OCTET, 16
       Last-Modified, 12, 39, 50, 57,        opaque-tag, 28
        76, 79, 81, 83, 84, 85, 86, 87,      OPTIONS, 33, 34, 48, 49, 125
        88, 117, 120, 123, 124               origin server, 11
       Length                                other-range-unit, 29
        computed after transfer codings      parameter, 25
         have been removed, 40               Partial PUT, 52
       
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       PATCH. See RFC 2068                   Request header fields, 35
       Paul J. Leach, 151                    request-header, 35
       Persistent Connections,               Request-Line, 29, 33, 34, 50, 63,
                                              152, 157
        Purpose, 40                          Request-URI, 19, 33, 34, 35, 38,
        Use of Connection Header, 41          48, 49, 50, 51, 53, 58, 59, 61,
       Pipelining, 42                         62, 63, 64, 65, 90, 91, 92, 99,
       port, 19                               110, 112, 124, 126, 129, 137,
       POST, 27, 29, 33, 46, 47, 50, 51,      142
        55, 59, 65, 92, 116, 144             Requirements        Overall Operation, 41                               40
       Pragma, 32, 101, 106, 125, 126         compliance, 9
        no-cache, 72, 80, 101, 126            key words, 9
       primary-tag, 28                       resource, 10
       product, 27                           response, 10
       Product tokens, 27                    Response, 36
       product-version, 27                   Response Header Fields, 38
       protocol-name, 133                    response-header, 38
       protocol-version, 133                 Retry-After, 38, 65, 67, 130
       proxy, 11                             rfc1123-date, 21
       Proxy-Authenticate, 38, 63, 87,       rfc850-date, 21
        126, 127                             Roy T. Fielding, 150
       Proxy-Authorization, 66, 126          rule1 | rule2, 15
       pseudonym, 133, 134, 135              Safe and Idempotent Methods, 47
       Public, 157. See RFC 2068             Security Considerations, 139
       PUT, 33, 46, 47, 48, 51, 52, 57,       abuse of server logs, 141
        64, 92, 100, 116, 121, 123            Accept headers reveal ethnic
       qdtext, 17                               information, 142
       Quality Values, 27                     attacks based on path names, 142
       quoted-pair, 17                        basic scheme is insecure, 140
       quoted-string, 16, 17, 24, 25,         be careful about personal
        28, 30, 94, 101, 126, 135, 137,         information, 142
        157, 159                              Content-Disposition, 144
       qvalue, 27, 138                        encoding information in URL's,
       Range, 29, 31, 36, 39, 49, 51,           144
        57, 65, 66, 86, 88, 89, 114,          Location headers and spoofing,
        115, 119, 120, 123, 127, 128,           144
        129, 152                              sensitive headers, 141
        Used with PUT, 52                    selecting request-headers, 90
       Range Units, 29                       semantically transparent, 12
       ranges-specifier, 114, 127, 128       separators, 17
       range-unit, 29                        server, 11
       Reason-Phrase, 36, 37, 38             Server, 27, 38, 130, 134, 141
       received-by, 133, 134                 shared caches, 91, 103
       received-protocol, 133, 134           site, 19, 34
       References, 146                       SP, 16
       Referer, 36, 129, 141, 142, 144       stale, 12
       rel_path, 19, 92                      start-line, 29
       relativeURI, 19, 112, 129             Status Code Definitions, 54
       representation, 10                    Status-Code, 36, 37, 54
       request, 10                           Status-Line, 29, 36, 38, 54, 152,
       Request, 33                            158
       
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       strong entity tag, 28                 tunnel, 11
       strong validators, 83                 type, 25
       subtag, 28                            UNLINK. See RFC 2068
       subtype, 25                           UPALPHA, 16
       suffix-byte-range-spec, 127, 128      Upgrade, 32, 54, 55, 87, 131, 132
       suffix-length, 128                    URI. See RFC 2068
       TE, 25, 36, 138, 139                  URI-reference, 19
       TEXT, 17                              user agent, 11
       Tim Berners.Lee, 151                  User-Agent, 27, 36, 69, 132, 134
       time, 21                              validator, 12
       token, 15, 16, 17, 22, 23, 24,        validators, 28, 74, 80, 81, 82,
        25, 27, 29, 30, 33, 41, 94, 101,      83, 85, 89
        109, 126, 132, 133, 137, 157          rules on use of, 84
       Tolerant Applications, 152            value, 25
        bad dates, 153                       variant, 10
        should tolerate whitespace in        Vary, 38, 57, 60, 70, 90, 121,
         request and status lines, 152        123, 132, 143
        tolerate LF and ignore CR in         Via, 32, 53, 130, 133, 134, 141
         line terminators, 152               warn-agent, 134, 135
        use lowest common denominator of     warn-code, 89, 134, 135
         character set, 152                  warn-date, 134, 135, 136
       TRACE, 33, 48, 53, 55, 125            Warning, 38, 72, 73, 74, 75, 79,
       trailer, 24                            86, 88, 89, 105, 134, 135, 136
       Trailer, 24, 32, 138, 139             warning-value, 134, 136
       Transfer coding, 23                   warn-text, 134, 135, 136
       Transfer Codings                      weak, 28
        chunked, 23                          weak entity tag, 28
       transfer-coding, 23, 24, 138          weak validators, 82, 83
       transfer-codings, 138                 weekday, 21
       Transfer-Encoding, 23, 25, 30,        wkday, 21
        31, 32, 39, 48, 87, 112, 113,        WWW-Authenticate, 38, 62, 126,
        130, 131, 139, 154, 155               137
       transfer-extension, 23
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       
       Fielding, et al                                   [Page 165]
       

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