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Versions: 00 01 02 03 04 05 06 07 08 RFC 2068

            HTTP Working Group                            R. Fielding
            INTERNET-DRAFT                                  UC Irvine
            <draft-ietf-http-v11-spec-08>                   J. Gettys
                                                          J. C. Mogul
                                                                  DEC
                                                           H. Frystyk
                                                       T. Berners-Lee
                                                              MIT/LCS
            Expires January 30, 1998                    July 30, 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.
            
            Fielding, et al                                     [Page 1]
            

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            HTTP has been in use by the World-Wide Web global
            information initiative since 1990. This specification
            defines the protocol referred to as "HTTP/1.1".
            
            The issues list for HTTP/1.1 can be found at:
            http://www.w3.org/Protocols/HTTP/Issues/.
            
            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/diff-v11-
            RFC2068to08.doc; all issues are noted as comments in the
            source document, with hyperlinks to the Issues list.
            
            The most significant outstanding issue is OPTIONS; there is
            a separate internet draft on the topic that you should
            review NOT incorporated into this draft (though editorial
            notes identify where changes may occur).  This draft is
            draft-ietf-http-options-00.txt.
            
            Also an issue: AGE-CALCULATION; Roy Fielding has issued an
            ID on the topic; Jeff Mogul intends to issue a draft as
            well.
            
            The editorial group is very interested in feedback on the
            sample table of requirements in this draft (issue
            REQUIREMENTS, section 1.9). Is it useful?  How could it be
            improved?
            
            Open or drafting  issues not incorporated into this draft
            include: REDIRECTS, ENCODING-NOT-CONNEG, DATE_IF_MODIFIED,
            403VS404, PUT-RANGE, HOST, AGE-CALCULATION, RE-
            AUTHENTICATION-REQUESTED, VARY
            
            Issues incorporated into this draft where there is still
            controversy are noted in bold italic with an editor's note.
            These are issues: CONTENT-ENCODING, CACHING-CGI.
            
            Issues incorporated into this draft being working group last
            called are: AUTH-CHUNKED, RETRY-AFTER, PROXY-REDIRECT
            
            Closed issues incorporated into this draft include: PROXY-
            AUTHORIZATION, PROXY-LENGTH, LANGUAGE-TAG, TSPECIALS,
            STATUS100, QZERO, RANGE-ERROR, CLARIFY-NO-CACHE, COMMENT,
            CONTENT-LOCATION, QUOTED-BACK, CACHE-CONTRA, CACHE-
            DIRECTIVE, BYTE-RANGE, LWS-DELIMITER, CRLF, MAX-AGE,
            100DATE, DISPOSITION, CHUNKED, CACHING, WARNINGS, VERSION,
            PROXY-MAXAGE, CHARSET-WILDCARD, PADDING, CONNECTION, RANGES,
            WARNING-8859, SHOULD-8859, X-BYTERANGES, MULTIPLE-TRANSFER-
            CODINGS, LINK_HEADER.
            
            
            Fielding, et al                                     [Page 2]
            

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            Editorial issues still open include: CLEANUP, UTF-8, URL-
            SYNTAX, ENTITY, DOCKDIGEST, 1310_CACHE.
            
            Editorial issues closed include: ACCEPT-RANGES, KEEP-ALIVE,
            BNFNAME, KEYWORDS, RESPONSE-VERSION, XREF, COMMON-HEADERS,
            NO-CACHE, FIX-REF, PERSIST-CONFUSED, CONNECTION2, GMT-UTC,
            PROXY-FORWARD, REFERER-SEC, CHUNK-EXT, REMOVE_19.6,
            IDEMPOTENT, REF-SIGCOMM, 1521-OBSOLETE, MESSAGE-BODY
            
            Apologies for the extreme length; Microsoft Word exhibited a
            fatal bug whenever trying to adjust margins when converting to
            ascii text; therefore, the margins are extreme and the document
            very long in ascii.  Get the Postscript version off the Issues
            list!
            
            
            Fielding, et al                                     [Page 3]
            

            Table of Contents
            
            
            
            HYPERTEXT TRANSFER PROTOCOL -- HTTP/1.1 ...................1
            
            Status of this Memo .......................................1
            
            Abstract ..................................................1
            
            Table of Contents .........................................5
            
            1   Introduction .........................................11
             1.1   Purpose ...........................................11
             1.2   Requirements ......................................11
             1.3   Terminology .......................................12
             1.4   Overall Operation .................................15
            
            2   Notational Conventions and Generic Grammar ...........17
             2.1   Augmented BNF .....................................17
             2.2   Basic Rules .......................................19
            
            3   Protocol Parameters ..................................20
             3.1   HTTP Version ......................................20
             3.2   Uniform Resource Identifiers ......................22
               3.2.1  General Syntax .................................22
               3.2.2  http URL .......................................23
               3.2.3  URI Comparison .................................24
             3.3   Date/Time Formats .................................24
               3.3.1  Full Date ......................................24
               3.3.2  Delta Seconds ..................................25
             3.4   Character Sets ....................................25
             3.5   Content Codings ...................................26
             3.6   Transfer Codings ..................................27
             3.7   Media Types .......................................29
               3.7.1  Canonicalization and Text Defaults .............30
               3.7.2  Multipart Types ................................31
             3.8   Product Tokens ....................................31
             3.9   Quality Values ....................................32
             3.10  Language Tags .....................................32
             3.11  Entity Tags .......................................33
             3.12  Range Units .......................................33
            
            4   HTTP Message .........................................34
             4.1   Message Types .....................................34
             4.2   Message Headers ...................................34
             4.3   Message Body ......................................35
             4.4   Message Length ....................................36
             4.5   General Header Fields .............................37
            
            5   Request ..............................................38
             5.1   Request-Line ......................................38
               5.1.1  Method .........................................38
               5.1.2  Request-URI ....................................39
            
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             5.2   The Resource Identified by a Request ..............41
             5.3   Request Header Fields .............................41
            
            6   Response .............................................42
             6.1   Status-Line .......................................43
               6.1.1  Status Code and Reason Phrase ..................43
             6.2   Response Header Fields ............................45
            
            7   Entity ...............................................45
             7.1   Entity Header Fields ..............................46
             7.2   Entity Body .......................................46
               7.2.1  Type ...........................................47
               7.2.2  Length .........................................47
            
            8   Connections ..........................................47
             8.1   Persistent Connections ............................47
               8.1.1  Purpose ........................................47
               8.1.2  Overall Operation ..............................48
               8.1.3  Proxy Servers ..................................49
               8.1.4  Practical Considerations .......................50
             8.2   Message Transmission Requirements .................51
            
            9   Method Definitions ...................................54
             9.1   Safe and Idempotent Methods .......................55
               9.1.1  Safe Methods ...................................55
               9.1.2  Idempotent Methods .............................55
             9.2   OPTIONS ...........................................56
             9.3   GET ...............................................56
             9.4   HEAD ..............................................57
             9.5   POST ..............................................57
             9.6   PUT ...............................................58
             9.7   DELETE ............................................59
             9.8   TRACE .............................................60
            
            10    Status Code Definitions ............................60
             10.1  Informational 1xx .................................61
               10.1.1 100 Continue ...................................61
               10.1.2 101 Switching Protocols ........................61
             10.2  Successful 2xx ....................................62
               10.2.1 200 OK .........................................62
               10.2.2 201 Created ....................................62
               10.2.3 202 Accepted ...................................62
               10.2.4 203 Non-Authoritative Information ..............63
               10.2.5 204 No Content .................................63
               10.2.6 205 Reset Content ..............................63
               10.2.7 206 Partial Content ............................63
             10.3  Redirection 3xx ...................................64
               10.3.1 300 Multiple Choices ...........................64
               10.3.2 301 Moved Permanently ..........................65
               10.3.3 302 Moved Temporarily ..........................65
               10.3.4 303 See Other ..................................66
               10.3.5 304 Not Modified ...............................66
               10.3.6 305 Use Proxy ..................................67
             10.4  Client Error 4xx ..................................68
            
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               10.4.1 400 Bad Request ................................68
               10.4.2 401 Unauthorized ...............................68
               10.4.3 402 Payment Required ...........................69
               10.4.4 403 Forbidden ..................................69
               10.4.5 404 Not Found ..................................69
               10.4.6 405 Method Not Allowed .........................69
               10.4.7 406 Not Acceptable .............................69
               10.4.8 407 Proxy Authentication Required ..............70
               10.4.9 408 Request Timeout ............................70
               10.4.10 ...............................409 Conflict    70
               10.4.11 ...................................410 Gone    71
               10.4.12 ........................411 Length Required    71
               10.4.13 ....................412 Precondition Failed    71
               10.4.14 ...............413 Request Entity Too Large    71
               10.4.15 ...................414 Request-URI Too Long    72
               10.4.16 .................415 Unsupported Media Type    72
             10.5  Server Error 5xx ..................................73
               10.5.1 500 Internal Server Error ......................73
               10.5.2 501 Not Implemented ............................73
               10.5.3 502 Bad Gateway ................................73
               10.5.4 503 Service Unavailable ........................73
               10.5.5 504 Gateway Timeout ............................73
               10.5.6 505 HTTP Version Not Supported .................74
            
            11    Access Authentication ..............................74
             11.1  Basic Authentication Scheme .......................76
             11.2  Digest Authentication Scheme ......................77
            
            12    Content Negotiation ................................77
             12.1  Server-driven Negotiation .........................78
             12.2  Agent-driven Negotiation ..........................79
             12.3  Transparent Negotiation ...........................79
            
            13    Caching in HTTP ....................................80
               13.1.1 Cache Correctness ..............................81
               13.1.2 Warnings .......................................82
               13.1.3 Cache-control Mechanisms .......................83
               13.1.4 Explicit User Agent Warnings ...................84
               13.1.5 Exceptions to the Rules and Warnings ...........84
               13.1.6 Client-controlled Behavior .....................85
             13.2  Expiration Model ..................................85
               13.2.1 Server-Specified Expiration ....................85
               13.2.2 Heuristic Expiration ...........................86
               13.2.3 Age Calculations ...............................86
               13.2.4 Expiration Calculations ........................89
               13.2.5 Disambiguating Expiration Values ...............90
               13.2.6 Disambiguating Multiple Responses ..............90
             13.3  Validation Model ..................................91
               13.3.1 Last-modified Dates ............................92
               13.3.2 Entity Tag Cache Validators ....................92
               13.3.3 Weak and Strong Validators .....................92
               13.3.4 Rules for When to Use Entity Tags and Last-
               modified Dates ........................................95
               13.3.5 Non-validating Conditionals ....................96
            
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             13.4  Response Cachability ..............................96
             13.5  Constructing Responses From Caches ................97
               13.5.1 End-to-end and Hop-by-hop Headers ..............97
               13.5.2 Non-modifiable Headers .........................98
               13.5.3 Combining Headers ..............................99
               13.5.4 Combining Byte Ranges .........................100
             13.6  Caching Negotiated Responses .....................101
             13.7  Shared and Non-Shared Caches .....................102
             13.8  Errors or Incomplete Response Cache Behavior .....102
             13.9  Side Effects of GET and HEAD .....................102
             13.10  Invalidation After Updates or Deletions .........103
             13.11  Write-Through Mandatory .........................103
             13.12  Cache Replacement ...............................104
             13.13  History Lists ...................................104
            
            14    Header Field Definitions ..........................105
             14.1  Accept ...........................................105
             14.2  Accept-Charset ...................................107
             14.3  Accept-Encoding ..................................108
             14.4  Accept-Language ..................................109
             14.5  Accept-Ranges ....................................110
             14.6  Age ..............................................111
             14.7  Allow ............................................111
             14.8  Authorization ....................................112
             14.9  Cache-Control ....................................113
               14.9.1 What is Cachable ..............................114
               14.9.2 What May be Stored by Caches ..................115
               14.9.3 Modifications of the Basic Expiration Mechanism116
               14.9.4 Cache Revalidation and Reload Controls ........118
               14.9.5 No-Transform Directive ........................120
               14.9.6 Cache Control Extensions ......................121
             14.10  Connection ......................................122
             14.11  Content-Base ....................................123
             14.12  Content-Encoding ................................123
             14.13  Content-Language ................................124
             14.14  Content-Length ..................................125
             14.15  Content-Location ................................125
             14.16  Content-MD5 .....................................126
             14.17  Content-Range ...................................127
             14.18  Content-Type ....................................129
             14.19  Date ............................................130
             14.20  ETag ............................................131
             14.21  Expires .........................................131
             14.22  From ............................................132
             14.23  Host ............................................133
             14.24  If-Modified-Since ...............................134
             14.25  If-Match ........................................135
             14.26  If-None-Match ...................................136
             14.27  If-Range ........................................137
             14.28  If-Unmodified-Since .............................138
             14.29  Last-Modified ...................................138
             14.30  Location ........................................139
             14.31  Max-Forwards ....................................140
             14.32  Pragma ..........................................140
            
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             14.33  Proxy-Authenticate ..............................141
             14.34  Proxy-Authorization .............................142
             14.35  Public ..........................................142
             14.36  Range ...........................................143
               14.36.1 ...............................Byte Ranges    143
               14.36.2 ..................Range Retrieval Requests    144
             14.37  Referer .........................................145
             14.38  Retry-After .....................................145
             14.39  Server ..........................................146
             14.40  Transfer-Encoding ...............................146
             14.41  Upgrade .........................................147
             14.42  User-Agent ......................................148
             14.43  Vary ............................................148
             14.44  Via .............................................150
             14.45  Warning .........................................151
             14.46  WWW-Authenticate ................................154
            
            15    Security Considerations ...........................157
             15.1  Authentication of Clients ........................157
             15.2  Offering a Choice of Authentication Schemes ......159
             15.3  Abuse of Server Log Information ..................159
             15.4  Transfer of Sensitive Information ................160
             15.5  Attacks Based On File and Path Names .............160
             15.6  Personal Information .............................161
             15.7  Privacy Issues Connected to Accept Headers .......161
             15.8  DNS Spoofing .....................................162
             15.9  Location Headers and Spoofing ....................163
            
            16    Acknowledgments ...................................165
            
            17    References ........................................166
            
            18    Authors' Addresses ................................170
            
            19    Appendices ........................................171
             19.1  Internet Media Type message/http .................171
             19.2  Internet Media Type multipart/byteranges .........172
             19.3  Tolerant Applications ............................173
             19.4  Differences Between HTTP Entities and RFC 2045
             Entities ...............................................173
               19.4.1 Conversion to Canonical Form ..................174
               19.4.2 Conversion of Date Formats ....................174
               19.4.3 Introduction of Content-Encoding ..............175
               19.4.4 No Content-Transfer-Encoding ..................175
               19.4.5 HTTP Header Fields in Multipart Body-Parts ....175
               19.4.6 Introduction of Transfer-Encoding .............175
               19.4.7 MIME-Version ..................................176
             19.5  Changes from HTTP/1.0 ............................176
               19.5.1 Changes to Simplify Multi-homed Web Servers and
               Conserve IP Addresses ................................176
             19.6  Additional Features ..............................177
               19.6.1 Additional Request Methods ....................178
               19.6.2 Additional Header Field Definitions ...........178
             19.7  Compatibility with Previous Versions .............178
            
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               19.7.1 Compatibility with HTTP/1.0 Persistent Connections179
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            Fielding, et al                                    [Page 10]
            

            1 Introduction
            
            1.1 Purpose
            
            The Hypertext Transfer Protocol (HTTP) is an application-
            level protocol for distributed, collaborative, hypermedia
            information systems. HTTP has been in use by the World-Wide
            Web global information initiative since 1990. The first
            version of HTTP, referred to as HTTP/0.9, was a simple
            protocol for raw data transfer across the Internet.
            HTTP/1.0, as defined by RFC 1945 [6], improved the protocol
            by allowing messages to be in the format of MIME-like
            messages, containing metainformation about the data
            transferred and modifiers on the request/response semantics.
            However, HTTP/1.0 does not sufficiently take into
            consideration the effects of hierarchical proxies, caching,
            the need for persistent connections, 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].
            
            
            
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            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."
            
            
            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).
            
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            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.
            
            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.
            
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            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 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
            
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               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.
            
            
            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
            
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                <------------------------------------- response chain
            
            
            The figure above shows three intermediaries (A, B, and C)
            between the user agent and origin server. A request or
            response message that travels the whole chain will pass
            through four separate connections. This distinction is
            important because some HTTP communication options may apply
            only to the connection with the nearest, non-tunnel
            neighbor, only to the end-points of the chain, or to all
            connections along the chain. Although the diagram is linear,
            each participant may be engaged in multiple, simultaneous
            communications. For example, B may be receiving requests
            from many clients other than A, and/or forwarding requests
            to servers other than C, at the same time that it is
            handling A's request.
            
            Any party to the communication which is not acting as a
            tunnel may employ an internal cache for handling requests.
            The effect of a cache is that the request/response chain is
            shortened if one of the participants along the chain has a
            cached response applicable to that request. The following
            illustrates the resulting chain if B has a cached copy of an
            earlier response from O (via C) for a request which has not
            been cached by UA or A.
            
                request chain ---------->
              UA -----v----- A -----v----- B - - - - - - C - - - - - - O
                <--------- response chain
            
            
            Not all responses are usefully 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.
            
            
            
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            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:
            
            
            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.
            
            
            
            
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            (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 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.
            
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            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)
                 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.
            
                   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].
            
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                   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 sender to indicate the format of a
            
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            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, respond with an error, or switch to tunnel
            behavior. Requests with a version lower than that of the
            proxy/gateway's version MAY be upgraded before being
            forwarded; 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.
            
                   URI      = ( absoluteURI | relativeURI ) [ "#" fragment ]
            
                   absoluteURI    = scheme ":" *( uchar | reserved )
            
                   relativeURI    = net_path | abs_path | rel_path
            
                   net_path       = "//" net_loc [ abs_path ]
                   abs_path       = "/" rel_path
                   rel_path       = [ path ] [ ";" params ] [ "?" query ]
            
                   path           = fsegment *( "/" segment )
                   fsegment       = 1*pchar
                   segment        = *pchar
            
                   params         = param *( ";" param )
                   param          = *( pchar | "/" )
            
                   scheme         = 1*( ALPHA | DIGIT | "+" | "-" | "." )
                   net_loc        = *( pchar | ";" | "?" )
                   query          = *( uchar | reserved )
                   fragment       = *( uchar | reserved )
            
                   pchar          = uchar | ":" | "@" | "&" | "=" | "+"
                   uchar          = unreserved | escape
                   unreserved     = ALPHA | DIGIT | safe | extra | national
            
                   escape         = "%" HEX HEX
                   reserved       = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
                   extra          = "!" | "*" | "'" | "(" | ")" | ","
                   safe           = "$" | "-" | "_" | "."
            
            
            
            
            
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                   unsafe         = CTL | SP | <"> | "#" | "%" | "<" | ">"
                   national       = <any OCTET excluding ALPHA, DIGIT,
                                    reserved, extra, safe, and unsafe>
            
            For definitive information on URL syntax and semantics, see
            RFC 1738 [4] and RFC 1808 [11]. The BNF above includes
            national characters not allowed in valid URLs as specified
            by RFC 1738, since HTTP servers are not restricted in the
            set of unreserved characters allowed to represent the
            rel_path part of addresses, and HTTP proxies may receive
            requests for URIs not defined by RFC 1738.
            
            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 ]
            
                   host    = <A legal Internet host domain name or IP
                              address (in dotted-decimal form),
                              as defined by Section 2.1 of RFC 1123>
            
                   port    = *DIGIT
            
            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;
               . 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.
            
            
            
            
            
            
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            All HTTP date/time stamps MUST be represented in Greenwich
            Mean Time (GMT), without exception. For the purposes of
            HTTP, GMT is exactly equal to UTC (Coordinated Universal
            Time). This is indicated in the first two formats by the
            inclusion of "GMT" as the three-letter abbreviation for time
            zone, and MUST be assumed when reading the asctime format.
            
                   HTTP-date = rfc1123-date | rfc850-date | asctime-date
            
                   rfc1123-date = wkday "," SP date1 SP time SP "GMT"
                   rfc850-date  = weekday "," SP date2 SP time SP "GMT"
                   asctime-date = wkday SP date3 SP time SP 4DIGIT
            
                   date1        = 2DIGIT SP month SP 4DIGIT
                                  ; day month year (e.g., 02 Jun 1982)
                   date2        = 2DIGIT "-" month "-" 2DIGIT
                                  ; day-month-year (e.g., 02-Jun-82)
                   date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
                                  ; month day (e.g., Jun  2)
            
                   time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                                  ; 00:00:00 - 23:59:59
            
                   wkday        = "Mon" | "Tue" | "Wed"
                                | "Thu" | "Fri" | "Sat" | "Sun"
            
                   weekday      = "Monday" | "Tuesday" | "Wednesday"
                                | "Thursday" | "Friday" | "Saturday"
                                | "Sunday"
            
                   month        = "Jan" | "Feb" | "Mar" | "Apr"
                                | "May" | "Jun" | "Jul" | "Aug"
                                | "Sep" | "Oct" | "Nov" | "Dec"
            
               Note: HTTP requirements for the date/time stamp format
               apply only to their usage within the protocol stream.
               Clients and servers are not required to use these
               formats for user presentation, request logging, etc.
            
            
            3.3.2 Delta Seconds
            
            Some HTTP header fields allow a time value to be specified
            as an integer number of seconds, represented in decimal,
            after the time that the message was received.
            
                   delta-seconds  = 1*DIGIT
            
            
            3.4 Character Sets
            
            HTTP uses the same definition of the term "character set" as
            that described for MIME:
            
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               The term "character set" is used in this document to
               refer to a method used with one or more tables to
               convert a sequence of octets into a sequence of
               characters. Note that unconditional conversion in the
               other direction is not required, in that not all
               characters may be available in a given character set
               and a character set may provide more than one sequence
               of octets to represent a particular character. This
               definition is intended to allow various kinds of
               character 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.
            
               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.
            
            
            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
            
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            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).
            
               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.
            
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                   transfer-coding   = "chunked" | transfer-extension
            
                   transfer-extension      = token
            
            All transfer-coding values are case-insensitive. HTTP/1.1
            uses transfer coding values 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 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-value ] )
                   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 purpose of the trailer  is
            to provide an efficient way to supply information about an
            entity that is generated dynamically. Applications MUST NOT
            
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            send header fields in the trailer which are not explicitly
            defined as being appropriate for the trailer.
            
            The Content-MD5 header (section 14.16) is appropriate for
            the trailer.
            
            The Authentication-Info header defined by RFC 2069 [32] (An
            Extension to HTTP: Digest Access Authentication), or its
            successor is appropriate for the trailer.
            
            
            
            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. 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.
            
            
            3.7 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.
            
            
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               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.
            
            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.
            
            
            
            
            
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            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).
            
            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
            
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            for a version identifier (i.e., successive versions of the
            same product SHOULD only differ in the product-version
            portion of the product value).
            
            
            3.9 Quality Values
            
            HTTP content negotiation (section 12) uses short "floating
            point" numbers to indicate the relative importance
            ("weight") of various negotiable parameters. A weight is
            normalized to a real number in the range 0 through 1, where
            0 is the minimum and 1 the maximum value. If a parameter has
            a quality value of 0, then content with this  parameter is
            `not acceptable' for the client. HTTP/1.1 applications MUST
            NOT generate more than three digits after the decimal point.
            User configuration of these values SHOULD also be limited in
            this fashion.
            
                   qvalue         = ( "0" [ "." 0*3DIGIT ] )
                                  | ( "1" [ "." 0*3("0") ] )
            
            "Quality values" is a misnomer, since these values merely
            represent relative degradation in desired quality.
            
            
            3.10 Language Tags
            
            A language tag identifies a natural language spoken,
            written, or otherwise conveyed by human beings for
            communication of information to other human beings. Computer
            languages are explicitly excluded. HTTP uses language tags
            within the Accept-Language and Content-Language fields.
            
            The syntax and registry of HTTP language tags is the same as
            that defined by RFC 1766 [1]. In summary, a language tag is
            composed of 1 or more parts: A primary language tag and a
            possibly empty series of subtags:
            
                    language-tag  = primary-tag *( "-" subtag )
            
                    primary-tag   = 1*8ALPHA
                    subtag        = 1*8ALPHA
            
            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.)
            
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            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.
            
            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.
            
            
            
            
            
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            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, one 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 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.
            
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                   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>
            
            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.
            
            
            
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            The presence of a message-body in a request is signaled by
            the inclusion of a Content-Length or Transfer-Encoding
            header field in the 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.
            
            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 value in bytes 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 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.)
            
            
            
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            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
                                  | 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 are
            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
                                  | extension-method
            
                   extension-method = token
            
            
            
            
            
            
            
            
            
            
            
            
            
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            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 list of methods known by
            a server can be listed in a Public response-header field
            (section 14.35).
            
            The methods GET and HEAD MUST be supported by all general-
            purpose servers. All other methods are optional; however, if
            the above methods are implemented, they MUST be implemented
            with the same semantics as those specified in section 9.
            
            
            5.1.2 Request-URI
            
            The Request-URI is a Uniform Resource Identifier (section
            3.2) and identifies the resource upon which to apply the
            request.
            
                   Request-URI    = "*" | absoluteURI | abs_path
            
            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
            
            
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            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 (net_loc) 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).
            
            Editorial note: The proposed changes to OPTIONS  will remove
            the following down to ***END***. See draft-ietf-http-options-00.txt
            
            If a proxy receives a request without any path in the
            Request-URI and the method specified is capable of
            supporting the asterisk form of request, then the last proxy
            on the request chain MUST forward the request with "*" as
            the final Request-URI. For example, the request
            
            
                   OPTIONS http://www.ics.uci.edu:8001 HTTP/1.1
            
            would be forwarded by the proxy as
            
                   OPTIONS * HTTP/1.1
                   Host: www.ics.uci.edu:8001
            
            after connecting to port 8001 of host "www.ics.uci.edu".
            
            ***END***
            
            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.
            
               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.
            
            
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            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
                                  | 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
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
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            6.1 Status-Line
            
            The first line of a Response message is the Status-Line,
            consisting of the protocol version followed by a numeric
            status code and its associated textual phrase, with each
            element separated by SP characters. No CR or LF is allowed
            except in the final CRLF sequence.
            
                   Status-Line = HTTP-Version SP Status-Code SP
                                 Reason-Phrase CRLF
            
            
            6.1.1 Status Code and Reason Phrase
            
            The Status-Code element is a 3-digit integer result code of
            the attempt to understand and satisfy the request. These
            codes are fully defined in section 10. The Reason-Phrase is
            intended to give a short textual description of the Status-
            Code. The Status-Code is intended for use by automata and
            the Reason-Phrase is intended for the human user. The client
            is not required to examine or display the Reason-Phrase.
            
            The first digit of the Status-Code defines the class of
            response. The last two digits do not have any categorization
            role. There are 5 values for the first digit:
            
            
               . 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
            
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                                  | "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
                                  | "413"   ; Request Entity Too Large
                                  | "414"   ; Request-URI Too Large
                                  | "415"   ; Unsupported Media Type
                                  | "416"   ; Requested range not valid
                                  | "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.
            
            
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            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
                                   | Public              ; Section 14.35
                                   | Retry-After         ; Section 14.38
                                   | Server              ; Section 14.39
                                   | Set-Proxy           ; Section 14.48
                                   | 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 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.
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            
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            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].
            
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            Persistent HTTP connections have a number of advantages:
            
               . 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
            
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            maintain a connection for more than that request, it SHOULD
            send a Connection header including the connection-token
            close.
            
            If either the client or the server sends the close token in
            the Connection header, that request becomes the last one for
            the connection.
            
            Clients and servers SHOULD NOT assume that a persistent
            connection is maintained for HTTP versions less than 1.1
            unless it is explicitly signaled. See section 19.7.1 for
            more information on backwards 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.
            
            
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            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 19.7.1.1 for
            information about the Keep-Alive header implemented by many
            HTTP/1.0 clients).
            
            
            8.1.4 Practical Considerations
            
            Servers will usually have some time-out value beyond which
            they will no longer maintain an inactive connection. Proxy
            servers might make this a higher value since it is likely
            that the client will be making more connections through the
            same server. The use of persistent connections places no
            requirements on the length 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.
            
            
            
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            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.
            
            
            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
            
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            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.
            
               . 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 419  (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.
            
            
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               . 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.
            
            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 419 (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).
            
            
            
            
            
            
            
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            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.
            
               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.
            
            
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            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.
            
            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).
            
            
            
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            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.
            
            Editorial note: The proposed changes to OPTIONS  will change
            the following down to ***END***. See draft-ietf-http-
            options-00.txt.
            
            Unless the server's response is an error, the response MUST
            NOT include entity information other than what can be
            considered as communication options (e.g., Allow is
            appropriate, but Content-Type is not). Responses to this
            method are not cachable.
            
            If the Request-URI is an asterisk ("*"), the OPTIONS request
            is intended to apply to the server as a whole. A 200
            response SHOULD include any header fields which indicate
            optional features implemented by the server (e.g., Public),
            including any extensions not defined by this specification,
            in addition to any applicable general or response-header
            fields. As described in section 5.1.2, an "OPTIONS *"
            request can be applied through a proxy by specifying the
            destination server in the Request-URI without any path
            information.
            
            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 which indicate optional features
            implemented by the server and applicable to that resource
            (e.g., Allow), including any extensions not defined by this
            specification, in addition to any applicable general or
            response-header fields. If the OPTIONS request passes
            through a proxy, the proxy MUST edit the response to exclude
            those options which apply to a proxy's capabilities and
            which are known to be unavailable through that proxy.
            
            ***END***
            
            
            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.
            
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            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 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 15.11 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
            
            
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            the Request-Line. POST is designed to allow a uniform method
            to cover the following functions:
            
            
               . Annotation of existing resources;
            
               . Posting a message to a bulletin board, newsgroup,
                 mailing list, or similar group of articles;
            
               . Providing a block of data, such as the result of
                 submitting a form, to a data-handling process;
            
               . Extending a database through an append operation.
            The actual function performed by the POST method is
            determined by the server and is usually dependent on the
            Request-URI. The posted entity is subordinate to that URI in
            the same way that a file is subordinate to a directory
            containing it, a news article is subordinate to a newsgroup
            to which it is posted, or a record is subordinate to a
            database.
            
            The action performed by the POST method might not result in
            a resource that can be identified by a URI. In this case,
            either 200 (OK) or 204 (No Content) is the appropriate
            response status, depending on whether or not the response
            includes an entity that describes the result.
            
            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 15.11 for security considerations.
            
            
            9.6 PUT
            
            Editor's note: Paul Leach is circulating changes that would
            define how to do PUTs with byte ranges; the spec is
            currently silent on the topic.
            
            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
            
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            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 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.
            
            
            9.7 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
            
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            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.
            
            
            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.
            
            
            
            
            
            
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            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 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.
            
            
            
            
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            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 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
            
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            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.
            
            
            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:
            
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               . 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.
            
            
            10.3 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 user
            agent SHOULD NOT automatically redirect a request more than
            5 times, since such redirections usually indicate an
            infinite loop.
            
            
            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
            
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            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).
            
            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 Moved Temporarily
            
            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
            
            
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            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.
            
            
            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.
            
            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).
            
            
            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.
            
            
            
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               . 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.
            
            
            10.3.6 305 Use Proxy
            
            The 305 is generated by an origin server to indicate that
            the client, or proxy, should use a proxy to access the
            requested resource.
            
            The request SHOULD be accompanied by a Set-Proxy response
            header indicating what proxy is to be used. The client will
            parse the Set-Proxy header as defined below to decide how
            long and for what URLs it should use the specified proxy.
            
            If the 305 response is not accompanied by a Set-Proxy
            header, it MUST be accompanied by a Location header.  The
            Location header will specify a URL to the proxy.
            
            If both headers are present in the response, the client
            SHOULD only use the Set-Proxy header only.
            
            
            10.3.7 306 Switch Proxy
            
            The 306 response is generated by a proxy server to indicate
            that the client or proxy should use the information in the
            accompanying Set-Proxy header to choose a proxy for
            subsequent requests.
            
            The 306 response code MUST be accompanied by the Set-Proxy
            response header.  The client or proxy will parse the Set-
            
            
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            Proxy header to determine which proxy to use, how long to
            use it, and for which URLs to use it.
            
            The scope in the Set-Proxy header is considered an optional
            advisory. The client or proxy may choose to ignore it, and
            use it for just this request, for all requests, or for a
            scope previously or implicitly defined by another
            configuration method or autoconfiguration system.
            
            
            
            
            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.
            
            
            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
            
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            information. HTTP access authentication is explained in
            section 11.
            
            
            10.4.3 402 Payment Required
            
            This code is reserved for future use.
            
            Editor's Note: Henrik Frystyk will be drafting language to
            deal with 403 vs. 404. Issue. Current wording says:
            Description for "404 Not found" says "403 Forbidden" can be
            used instead. As Ari Luotonen points out - this should be
            the other way round .
            
            
            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. This status code is commonly used
            when the server does not wish to reveal exactly why the
            request has been refused, or when no other response is
            applicable.
            
            
            10.4.5 404 Not Found
            
            The server has not found anything matching the Request-URI.
            No indication is given of whether the condition is temporary
            or permanent. If the server does not wish to make this
            information available to the client, the status code 403
            (Forbidden) can be used instead. 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.
            
            
            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.
            
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            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.
            
            
            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
            
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            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.
            
            
            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
            
            
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            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 valid
            
            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.)
            
            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.
            
            
            10.4.18 419 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
            
            
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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 Redirection Failed
            
            The 506 response is returned when a redirection fails or is
            refused by a proxy or client.  If the redirection response
            included a body, then it SHOULD be included in the 506
            response.
            
            This response is returned by a proxy, to a downstream proxy
            or client, when it cannot or chooses not to honor a
            redirection.
            
            
            11 Access Authentication
            
            Editor's note: This section (11) will be removed from future
            drafts of this document, and combined with Digest
            authentication, which will then become a more general
            document "Authentication in HTTP".
            
            HTTP provides a simple challenge-response authentication
            mechanism which MAY be used by a server to challenge a
            client request and by a client to provide authentication
            information. It uses an extensible, case-insensitive token
            to identify the authentication scheme, followed by a comma-
            separated list of attribute-value pairs which carry the
            parameters necessary for achieving authentication via that
            scheme.
            
                   auth-scheme    = token
            
                   auth-param     = token "=" quoted-string
            
            The 401 (Unauthorized) response message is used by an origin
            server to challenge the authorization of a user agent. This
            response MUST include a WWW-Authenticate header field
            containing at least one challenge applicable to the
            requested resource. The 407 (Proxy Authentication Required)
            response message is used by a proxy to challenge the
            authorization of a client and MUST include a Proxy-
            Authenticate header field containing a challenge applicable
            to the proxy for the requested resource.
            
            
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                   challenge      = auth-scheme 1*SP realm *( "," auth-
            param )
            
                   realm          = "realm" "=" realm-value
                   realm-value    = quoted-string
            
            The realm attribute (case-insensitive) is required for all
            authentication schemes which issue a challenge. The realm
            value (case-sensitive), in combination with the canonical
            root URL (see section 5.1.2) of the server being accessed,
            defines the protection space. These realms allow the
            protected resources on a server to be partitioned into a set
            of protection spaces, each with its own authentication
            scheme and/or authorization database. The realm value is a
            string, generally assigned by the origin server, which may
            have additional semantics specific to the authentication
            scheme.
            
            A user agent that wishes to authenticate itself with an
            origin server--usually, but not necessarily, after receiving
            a 401 (Unauthorized)--MAY do so by including an
            Authorization header field with the request. A client that
            wishes to authenticate itself with a proxy--usually, but not
            necessarily, after receiving a 407 (Proxy Authentication
            Required)--MAY do so by including a Proxy-Authoraiztion
            header field with the request. Both the Authorization field
            value and the Proxy-Authorization field value consists of
            credentials containing the authentication information of the
            client for the realm of the resource being requested.
            
                   credentials    = basic-credentials
                                  | auth-scheme #auth-param
            
            The domain over which credentials can be automatically
            applied by a client is determined by the protection space.
            If a prior request has been authorized, the same credentials
            MAY be reused for all other requests within that protection
            space for a period of time determined by the authentication
            scheme, parameters, and/or user preference. Unless otherwise
            defined by the authentication scheme, a single protection
            space cannot extend outside the scope of its server.
            
            If the origin server does not wish to accept the credentials
            sent with a request, it SHOULD return a 401 (Unauthorized)
            response. The response MUST include a WWW-Authenticate
            header field containing at least one (possibly new)
            challenge applicable to the requested resource. If a proxy
            does not accept the credentials sent with a request, it
            SHOULD return a 407 (Proxy Authentication Required). The
            response MUST include a Proxy-Authenticate header field
            containing a (possibly new) challenge applicable to the
            proxy for the requested resource.
            
            
            
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            The HTTP protocol does not restrict applications to this
            simple challenge-response mechanism for access
            authentication. Additional mechanisms MAY be used, such as
            encryption at the transport level or via message
            encapsulation, and with additional header fields specifying
            authentication information. However, these additional
            mechanisms are not defined by this specification.
            
            Proxies MUST be completely transparent regarding user agent
            authentication by origin servers. That is, they MUST forward
            the WWW-Authenticate and Authorization headers untouched,
            and follow the rules found in section 14.8. Both the Proxy-
            Authenticate and the Proxy-Authorization header fields are
            hop-by-hop headers (see section 13.5.1).
            
            
            
            
            11.1 Basic Authentication Scheme
            
            The "basic" authentication scheme is based on the model that
            the client must authenticate itself with a user-ID and a
            password for each realm. The realm value should be
            considered an opaque string which can only be compared for
            equality with other realms on that server. The server will
            service the request only if it can validate the user-ID and
            password for the protection space of the Request-URI. There
            are no optional authentication parameters.
            
            Upon receipt of an unauthorized request for a URI within the
            protection space, the origin server MAY respond with a
            challenge like the following:
            
                   WWW-Authenticate: Basic realm="WallyWorld"
            
            where "WallyWorld" is the string assigned by the server to
            identify the protection space of the Request-URI. A proxy
            may respond with the same challenge using the Proxy-
            Authenticate header field.
            
            To receive authorization, the client sends the userid and
            password, separated by a single colon (":") character,
            within a base64 [7] encoded string in the credentials.
            
                   basic-credentials = "Basic" SP base64-user-pass
            
                   base64-user-pass  = <base64 [7] encoding of user-pass,
                                    except not limited to 76 char/line>
            
                   user-pass   = userid ":" password
            
                   userid      = *<TEXT excluding ":">
            
            
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                   password    = *TEXT
            
            Userids might be case sensitive.
            
            If the user agent wishes to send the userid "Aladdin" and
            password "open sesame", it would use the following header
            field:
            
                   Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
            
            If a client wishes to send the same userid and password to a
            proxy, it would use the Proxy-Authorization header field.
            See section 15 for security considerations associated with
            Basic authentication.
            
            
            11.2 Digest Authentication Scheme
            
            A digest authentication for HTTP is specified in RFC 2069
            [32].
            
            
            12 Content Negotiation
            
            Most HTTP responses include an entity which contains
            information for interpretation by a human user. Naturally,
            it is desirable to supply the user with the "best available"
            entity corresponding to the request. Unfortunately for
            servers and caches, not all users have the same preferences
            for what is "best," and not all user agents are equally
            capable of rendering all entity types. For that reason, HTTP
            has provisions for several mechanisms for "content
            negotiation" -- the process of selecting the best
            representation for a given response when there are multiple
            representations available.
            
               Note: This is not called "format negotiation" because
               the alternate representations may be of the same media
               type, but use different capabilities of that type, be
               in different languages, etc.
            
            Any response containing an entity-body MAY be subject to
            negotiation, including error responses.
            
            There are two kinds of content negotiation which are
            possible in HTTP: server-driven and agent-driven
            negotiation. These two kinds of negotiation are orthogonal
            and thus may be used separately or in combination. One
            method of combination, referred to as transparent
            negotiation, occurs when a cache uses the agent-driven
            negotiation information provided by the origin server in
            order to provide server-driven negotiation for subsequent
            requests.
            
            
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            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 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.
            
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            HTTP/1.1 origin servers MUST include an appropriate Vary
            header field (section 14.43) in any cachable response based
            on server-driven negotiation. The Vary header field
            describes the dimensions over which the response might vary
            (i.e. the dimensions over which the origin server picks its
            "best guess" response from multiple representations).
            
            HTTP/1.1 public caches MUST recognize the Vary header field
            when it is included in a response and obey the requirements
            described in section 13.6 that describes the interactions
            between caching and content negotiation.
            
            
            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 19.6.1.0) 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
            
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            representations of the response (as in agent-driven
            negotiation) and the dimensions of variance are completely
            understood by the cache, then the cache becomes capable of
            performing server-driven negotiation on behalf of the origin
            server for subsequent requests on that resource.
            
            Transparent negotiation has the advantage of distributing
            the negotiation work that would otherwise be required of the
            origin server and also removing the second request delay of
            agent-driven negotiation when the cache is able to correctly
            guess the right response.
            
            This specification does not define any mechanism for
            transparent negotiation, though it also does not prevent any
            such mechanism from being developed as an extension and used
            within HTTP/1.1. An HTTP/1.1 cache performing transparent
            negotiation MUST include a Vary header field in the response
            (defining the dimensions of its variance) if it is cachable
            to ensure correct interoperation with all HTTP/1.1 clients.
            The agent-driven negotiation information supplied by the
            origin server SHOULD be included with the transparently
            negotiated response.
            
            
            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
            
            
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            those for ordering merchandise), the protocol requires that
            transparency be relaxed
            
               . only by an explicit protocol-level request when relaxed
                 by client or origin server
               . only with an explicit warning to the end user when
                 relaxed by cache or client
            Therefore, the HTTP/1.1 protocol provides these important
            elements:
            
               1.Protocol features that provide full semantic
                 transparency when this is required by all parties.
            
               2.Protocol features that allow an origin server or user
                 agent to explicitly request and control non-transparent
                 operation.
            
               3.Protocol features that allow a cache to attach warnings
                 to responses that do not preserve the requested
                 approximation of semantic transparency.
            
            A basic principle is that it must be possible for the
            clients to detect any potential relaxation of semantic
            transparency.
            
               Note: The server, cache, or client 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
            
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                 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). \*
                 MERGEFORMAT  \* MERGEFORMAT
            
               3.It is an appropriate 304 (Not Modified), 305 (Proxy
                 Redirect), or error (4xx or 5xx) response message.
            
            If the cache can not communicate with the origin server,
            then a correct cache SHOULD respond as above if the response
            can be correctly served from the cache; if not it MUST
            return an error or warning indicating that there was a
            communication failure.
            
            If a cache receives a response (either an entire response,
            or a 304 (Not Modified) response) that it would normally
            forward to the requesting client, and the received response
            is no longer fresh, the cache SHOULD forward it to the
            requesting client without adding a new Warning (but without
            removing any existing Warning headers). A cache SHOULD NOT
            attempt to revalidate a response simply because that
            response became stale in transit; this might lead to an
            infinite loop. 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.
            
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            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.
            
            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").
            
            
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            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 visual indicator.
            
            If the user has overridden the caching mechanisms in a way
            that would abnormally reduce the effectiveness of caches,
            the user agent should continually 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.
            
            
            
            
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            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.
            
            
            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
            
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            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 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
            
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            denote the value of the Date header, in a form appropriate
            for arithmetic operations.
            
            HTTP/1.1 uses the Age response-header to help convey age
            information between caches. The Age header value is the
            sender's estimate of the amount of time since the response
            was generated at the origin server. In the case of a cached
            response that has been revalidated with the origin server,
            the Age value is based on the time of revalidation, not of
            the original response.
            
            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.
            
            Note that this correction is applied at each HTTP/1.1 cache
            along the path, so that if there is an HTTP/1.0 cache in the
            path, the correct received age is computed as long as the
            receiving cache's clock is nearly in sync. We don't need
            end-to-end clock synchronization (although it is good to
            have), and there is no explicit clock synchronization step.
            
            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
            
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            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;
            
            When a cache sends a response, it must add to the
            corrected_initial_age the amount of time that the response
            was resident locally. It must then transmit this total age,
            using the Age header, to the next recipient cache.
            
               Note that a client cannot reliably tell that a response
               is first-hand, but the presence of an Age header
               indicates that a response is definitely not first-hand.
               Also, if the Date in a response is earlier than the
            
            
            
            
            
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               client's local request time, the response is probably
               not first-hand (in the absence of serious clock skew).
            
            
            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 13 to any response whose age is more than 24 hours
            if such warning has not already been added.
            
            Also, if the response does have a Last-Modified time, the
            heuristic expiration value SHOULD be no more than some
            fraction of the interval since that time. A typical setting
            of this fraction might be 10%.
            
            The calculation to determine if a response has expired is
            quite simple:
            
                  response_is_fresh = (freshness_lifetime > current_age)
            
            
            
            
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            13.2.5 Disambiguating Expiration Values
            
            Because expiration values are assigned optimistically, it is
            possible for two caches to contain fresh values for the same
            resource that are different.
            
            If a client performing a retrieval receives a non-first-hand
            response for a request that was already fresh in its own
            cache, and the Date header in its existing cache entry is
            newer than the Date on the new response, then the client MAY
            ignore the response. If so, it MAY retry the request with a
            "Cache-Control: max-age=0" directive (see section 14.9), to
            force a check with the origin server.
            
            If a cache has two fresh responses for the same
            representation with different validators, it MUST use the
            one with the more recent Date header. This situation 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.
            
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            If the Date values are equal, then the client may use either
            response (or may, if it is being extremely prudent, request
            a new response). Servers MUST NOT depend on clients being
            able to choose deterministically between responses generated
            during the same second, if their expiration times overlap.
            
            
            13.3 Validation Model
            
            When a cache has a stale entry that it would like to use as
            a response to a client's request, it first has to check with
            the origin server (or possibly an intermediate cache with a
            fresh response) to see if its cached entry is still usable.
            We call this "validating" the cache entry. Since we do not
            want to have to pay the overhead of retransmitting the full
            response if the cached entry is good, and we do not want to
            pay the overhead of an extra round trip if the cached entry
            is invalid, the HTTP/1.1 protocol supports the use of
            conditional methods.
            
            The key protocol features for supporting conditional methods
            are those concerned with "cache validators." When an origin
            server generates a full response, it attaches some sort of
            validator to it, which is kept with the cache entry. When a
            client (user agent or proxy cache) makes a conditional
            request for a resource for which it has a cache entry, it
            includes the associated validator in the request.
            
            The server then checks that validator against the current
            validator for the entity, and, if they match, 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
            
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               retrieval if it does not have a validator for the
               entity, which means it will not be refreshable after it
               expires.
            
            
            13.3.1 Last-modified Dates
            
            The Last-Modified entity-header field value is often used as
            a cache validator. In simple terms, a cache entry is
            considered to be valid if the entity has not been modified
            since the Last-Modified value.
            
            
            13.3.2 Entity Tag Cache Validators
            
            The ETag 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.
            
            
            
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               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.
            
            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,
            
            
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               . That origin server reliably knows that the associated
                 entity did not change twice during the second covered
                 by the presented validator.
            or
            
               . The validator is about to be used by a client in an If-
                 Modified-Since or If-Unmodified-Since header, because
                 the client has a cache entry for the associated entity,
                 and
               . That cache entry includes a Date value, which gives the
                 time when the origin server sent the original response,
                 and
               . The presented Last-Modified time is at least 60 seconds
                 before the Date value.
            or
            
               . The validator is being compared by an intermediate
                 cache to the validator stored in its cache entry for
                 the entity, and
               . That cache entry includes a Date value, which gives the
                 time when the origin server sent the original response,
                 and
               . The presented Last-Modified time is at least 60 seconds
                 before the Date value.
            This method relies on the fact that if two different
            responses were sent by the origin server during the same
            second, but both had the same Last-Modified time, then at
            least one of those responses would have a Date value equal
            to its Last-Modified time. The arbitrary 60-second limit
            guards against the possibility that the Date and Last-
            Modified values are generated from different clocks, or at
            somewhat different times during the preparation of the
            response. An implementation may use a value larger than 60
            seconds, if it is believed that 60 seconds is too short.
            
            If a client wishes to perform a sub-range retrieval on a
            value for which it has only a Last-Modified time and no
            opaque validator, it may do this only if the Last-Modified
            time is strong in the sense described here.
            
            A cache or origin server receiving a 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.
            
            
            
            
            
            
            
            
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            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 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.
            
            
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               . 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 cache, upon receiving a request, MUST use the
            most restrictive validator when deciding whether the
            client's cache entry matches the cache's own cache entry.
            This is only an issue when the request contains both an
            entity tag and a last-modified-date validator (If-Modified-
            Since or If-Unmodified-Since).
            
               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.
            
            
            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
            
            
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            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).
            
            
            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:
            
            
            
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               . 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
               . Public
               . 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
            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.
            
            
            
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            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-Length
               . 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 14 (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
            
               . 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.
            
            
            
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            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.
            
            A server MUST use the Vary header field (section 14.43) to
            inform a cache of what header field dimensions are used to
            select among multiple representations of a cachable
            response. A cache may use the selected representation (the
            entity included with that particular response) for replying
            to subsequent requests on that resource only when the
            subsequent requests have the same or equivalent values for
            all header fields specified in the Vary response-header.
            Requests with a different value for one or more of those
            header fields would be forwarded toward the origin server.
            
            If an entity tag was assigned to the 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 Request-URI. 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 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.
            
            The Vary header field may also inform the cache that the
            representation was selected using criteria not limited to
            the request-headers; in this case, a cache MUST NOT use the
            response in a reply to a subsequent request unless the cache
            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 any of the existing cache entries contains only partial
            content for the associated entity, its entity-tag SHOULD NOT
            be included in the If-None-Match header 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.
            
            
            
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            13.7 Shared and Non-Shared Caches
            
            For reasons of security and privacy, it is necessary to make
            a distinction between "shared" and "non-shared" caches. A
            non-shared cache is one that is accessible only to a single
            user. Accessibility in this case SHOULD be enforced by
            appropriate security mechanisms. All other caches are
            considered to be "shared." Other sections of this
            specification place certain constraints on the operation of
            shared caches in order to prevent loss of privacy or failure
            of access controls.
            
            
            13.8 Errors or Incomplete Response Cache Behavior
            
            A cache that receives an incomplete response (for example,
            with fewer bytes of data than specified in a Content-Length
            header) may store the response. However, the cache MUST
            treat this as a partial response. Partial responses may be
            combined as described in section 13.5.4; the result might be
            a full response or might still be partial. A cache MUST NOT
            return a partial response to a client without explicitly
            marking it as such, using the 206 (Partial Content) status
            code. A cache MUST NOT return a partial response using a
            status code of 200 (OK).
            
            If a cache receives a 5xx response while attempting to
            revalidate an entry, it may either forward this response to
            the requesting client, or act as if the server failed to
            respond. In the latter case, it MAY return a previously
            received response unless the cached entry includes the
            "must-revalidate" Cache-Control directive (see section
            14.9).
            
            
            13.9 Side Effects of GET and HEAD
            
            Unless the origin server explicitly prohibits the caching of
            their responses, the application of GET and HEAD methods to
            any resources SHOULD NOT have side effects that would lead
            to erroneous behavior if these responses are taken from a
            cache. They may still have side effects, but a cache is not
            required to consider such side effects in its caching
            decisions. Caches are always expected to observe an origin
            server's explicit restrictions on caching.
            
            EDITOR's Note: Roy Fielding is opposed to the change
            represented  by the following two paragraphs:
            
            Some HTTP/1.0 cache operators have found that it is
            dangerous to cache and reuse without revalidation responses
            to requests for URLs that include any of the strings "cgi-
            bin", "htbin", or "?".  Applications have traditionally used
            these URLs in conjunction with operations with significant
            
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            side effects for GET or HEAD methods.  However, if such a
            response includes an explicit, future, expiration time, then
            this implies that the response may be cached and reused
            without revalidation until it expires.  If such a response
            includes a Last-Modified or Etag header, this implies that
            the response may be reused after revalidation (or without
            revalidation if explicitly fresh).
            
            A cache MUST NOT assign a heuristic expiration time to a
            response for a URL that includes the strings "htbin", "cgi-
            bin", or "?" in its rel_path part.  If such a response does
            not  carry an explicit expiration time, it must be treated
            as if it expires immediately.
            
            
            13.10 Invalidation After Updates or Deletions
            
            The effect of certain methods 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.
            
            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 may 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
            
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            origin server. This currently includes all methods except
            for GET and HEAD. A cache MUST NOT reply to such a request
            from a client before having transmitted the request to the
            inbound server, and having received a corresponding response
            from the inbound server. This does not prevent a proxy cache
            from sending a 100 (Continue) response before the inbound
            server has sent its final reply.
            
            The alternative (known as "write-back" or "copy-back"
            caching) is not allowed in HTTP/1.1, due to the difficulty
            of providing consistent updates and the problems arising
            from server, cache, or network failure prior to write-back.
            
            
            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
            
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               controls and cache controls when they would otherwise
               like to. Service authors may consider it important that
               users not be presented with error messages or warning
               messages when they use navigation controls (such as
               BACK) to view previously fetched resources. Even though
               sometimes such resources ought not to cached, or ought
               to expire quickly, user interface considerations may
               force service authors to resort to other means of
               preventing caching (e.g. "once-only" URLs) in order not
               to suffer the effects of improperly functioning history
               mechanisms.
            
            
            14 Header Field Definitions
            
            This section defines the syntax and semantics of all
            standard HTTP/1.1 header fields. For entity-header fields,
            both sender and recipient refer to either the client or the
            server, depending on who sends and who receives the entity.
            
            
            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-
            
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            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".
            
            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) */*
            
            
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            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.
            
            
            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.
            
            
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            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
            
            Editor's note: there is still some nervousness about
            introducing q values into accept encoding; concrete examples
            of implementations which will break are needed to make the
            case against them, as q values are common across other
            Accept-* headers. This does seem the cleanest solution to
            the problem right now.
            
            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-codings | "*" )
            
            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=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:
            
            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
            
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            "*;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.
            
            
            
            
            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 ] )
            
                   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
            
            
            
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            "*", 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 15.7.
            
               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.
            
            
            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
            
            
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                   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). HTTP/1.1 caches MUST send an Age header
            in every response. Caches SHOULD use an arithmetic type of
            at least 31 bits of range.
            
            
            14.7 Allow
            
            Editors Note: The OPTIONS changes would cause possible
            changes to Allow and/or Public for consistency with each
            other and with section 9.2 (OPTIONS).
            
            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.
            
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                   Allow          = "Allow" ":" 1#method
            
            Example of use:
            
                   Allow: GET, HEAD, PUT
            
            This field cannot prevent a client from trying other
            methods. However, the indications given by the Allow header
            field value SHOULD be followed. The actual set of allowed
            methods is defined by the origin server at the time of each
            request.
            
            The Allow header field MAY be provided with a PUT request to
            recommend the methods to be supported by the new or modified
            resource. The server is not required to support these
            methods and SHOULD include an Allow header in the response
            giving the actual supported methods.
            
            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.
            
            The Allow header field does not indicate what methods are
            implemented at the server level. Servers MAY use the Public
            response-header field (section 14.35) to describe what
            methods are implemented on the server as a whole.
            
            
            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
            
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                 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.
            
               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 ]
            
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                                   | "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 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
            
            
            
            
            
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            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.
            
               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.
            
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            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 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.
            
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               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 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
            
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               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 10 (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:
            
            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.
            
            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
            
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               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 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),
            
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            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 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.
            
            
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            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 see and understand the
            "private" directive and thus default to the safe behavior.
            
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            Unrecognized cache-directives MUST be ignored; it is assumed
            that any cache-directive likely to be unrecognized by an
            HTTP/1.1 cache will be combined with standard directives (or
            the response's default 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-header = "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.
            
            
            
            
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            This protects against mistaken forwarding of such header
            fields by pre-HTTP/1.1 proxies.
            
            
            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.
            This header field is described as Base in RFC 1808[11],
            which is expected to be revised.
            
                   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.
            
            
            
            
            
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            If the content-coding of an entity in a request message is
            not acceptable to the origin server, the server SHOULD
            respond with a status code of 415 (Unsupported Media Type).
            
            If multiple encodings have been applied to an entity, the
            content codings MUST be listed in the order in which they
            were applied. Additional information about the encoding
            parameters MAY be provided by other entity-header fields not
            defined by this specification.
            
            
            14.13 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 Error! Reference source
            not found.. 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.
            
            
            
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            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 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).
            
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            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.
            
            
            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,
            
            
            
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            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 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
            
            Editor's note: Paul Leach is circulating changes that would
            define how to do PUTs with byte ranges; the spec is
            currently silent on the topic
            
            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
            
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                content-range-spec      = byte-content-range-spec | "*"
            
                byte-content-range-spec = bytes-unit SP first-byte-pos "-"
                                          last-byte-pos "/"
                                          ( entity-length | "*" )
            
                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-content-range-
            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 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 valid) SHOULD include a Content-range field with
            a content-range-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:
                   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,
            
            
            
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                   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 valid)
            (section 10.4.17).
            
               Note: clients cannot depend on servers to send a 416
               (Requested range not valid) response instead of a 200
               (OK) response for an unsatisfiable Range request-
               header, since not all servers implement this request-
               header.
            
            
            
            
            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
            
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            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 it is optional.  A client
            
            
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            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 13.3.2).
            
                  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.
            
            
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            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
            
            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-cacheable 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.
            
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            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 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
            
            Editor's note: not yet drafted is a change that would allow
            a proxy to add a host header if not present, but not change
            it if it is already present.
            
            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.
            
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            See sections 5.2 and 19.5.1 for other requirements relating
            to Host.
            
            
            14.24 If-Modified-Since
            
            Editor's Note: issue DATE-IF-MODIFIED this still needs some
            advice to implementers  that would suggest that an HTTP
            client ought to act as if the If-Modified-Since headers that
            it sends *for cache validation* are going to be interpreted
            as "If-Modification-date-does-not-match-exactly".
            
            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 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.
            
            
            
            
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               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 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.
            
            
            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
            
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            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: *
            
            
            14.26 If-None-Match
            
            The If-None-Match request-header field is used with a method
            to make it conditional. A client that has one or more
            entities previously obtained from the resource can verify
            that none of those entities is current by including a list
            of their associated entity tags in the If-None-Match header
            field. The purpose of this feature is to allow efficient
            updates of cached information with a minimum amount of
            transaction overhead. It is also used, on updating requests,
            to prevent inadvertent modification of a resource which was
            not known to 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. Instead, if the request method was GET or
            
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            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, or 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 status, then the
            If-None-Match header MUST be ignored.
            
            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
            
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            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
            SHOULDprovide the specified sub-range of the entity using a
            206 (Partial content) response. If the entity tag does not
            match, then the server SHOULDreturn the entire entity using
            a 200 (OK) response.
            
            
            14.28 If-Unmodified-Since
            
            The If-Unmodified-Since request-header field is used with a
            method to make it conditional. If the requested resource has
            not been modified since the time specified in this field,
            the server should perform the requested operation as if the
            If-Unmodified-Since header were not present.
            
            If the requested variant has been modified since the
            specified time, the server MUST NOT perform the requested
            operation, and MUST return a 412 (Precondition Failed).
            
                If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
            
            An example of the field is:
            
                If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
            
            If the request normally (i.e., without the If-Unmodified-
            Since header) would result in anything other than a 2xx
            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
            
            
            
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            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
            
            In RFC 2068, the Location header was used to indicate the
            proxy setting.  Its use is DEPRECATED by the Set-Proxy
            header in the context of a 305 response. All new
            implementations MUST send the Set-proxy header.
            Implementations MAY send the Location header so as to allow
            backward  compatibility.
            
            If the Location header is specified, it should contain a URI
            of the proxy.  If the Set-Proxy header is not specified, the
            client should use this proxy for just one request, and only
            for the originally requested exact URL.
            
            The Location response-header field is used to redirect the
            recipient to a location other than the Request-URI for
            completion of the request or identification of a new
            resource. For 201 (Created) responses, the Location is that
            of the new resource which was created by the request. For
            3xx responses, the location SHOULD indicate the server's
            preferred 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
            
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               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
            
            Editor's note: The OPTIONS changes would allow Max-Forward
            with OPTIONS, not just with TRACE.
            
            The Max-Forwards request-header field may be used with the
            TRACE method (section 14.31) 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 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 SHOULD NOT forward
            the request; instead, it SHOULD respond as the final
            recipient with a 200 (OK) response containing the received
            request message as the response entity-body (as described in
            section 9.8). If the received Max-Forwards value is greater
            than zero, then the forwarded message SHOULD contain an
            updated Max-Forwards field with a value decremented by one
            (1).
            
            The Max-Forwards header field SHOULD 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.
            
            
            
            
            
            
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            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 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
            
            Editors Note: The OPTIONS changes would cause possible
            changes to Allow and/or Public for consistency with each
            other and with section 9.2 (OPTIONS )
            
            The Public response-header field lists the set of methods
            supported by the server. The purpose of this field is
            strictly to inform the recipient of the capabilities of the
            server regarding unusual methods. The methods listed may or
            may not be applicable to the Request-URI; the Allow header
            field (section 14.7) MAY be used to indicate methods allowed
            for a particular URI.
            
                   Public         = "Public" ":" 1#method
            
            Example of use:
            
                   Public: OPTIONS, MGET, MHEAD, GET, HEAD
            
            This header field applies only to the server directly
            connected to the client (i.e., the nearest neighbor in a
            chain of connections). If the response passes through a
            proxy, the proxy MUST either remove the Public header field
            or replace it with one applicable to its own capabilities.
            
            
            
            
            
            
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            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 invalid. The recipient of an
            invalid byte-range-spec must ignore it.
            
            If the last-byte-pos value is absent, or if the value is
            greater than or equal to the current length of the entity-
            body, last-byte-pos is taken to be equal to one less than
            the current length of the entity-body in bytes.
            
            By its choice of last-byte-pos, a client can limit the
            number of bytes retrieved without knowing the size of the
            entity.
            
                   suffix-byte-range-spec = "-" suffix-length
            
                   suffix-length = 1*DIGIT
            
            
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            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.
            
            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 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
            
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                 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 15.11 for security
            considerations.
            
            
            
            
            14.38 Retry-After
            
            The Retry-After response-header field can be used with a 503
            (Service Unavailable) response to indicate how long the
            
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            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.
            
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                   Transfer-Encoding = "Transfer-Encoding" ":"
                                       1#transfer-coding
            
            Transfer codings are defined in section 3.6. An example is:
            
                   Transfer-Encoding: chunked
            
            If multiple encodings have been applied to an entity, the
            transfer codings MUST be listed in the order in which they
            were applied. Additional information about the encoding
            parameters MAY be provided by other entity-header fields not
            defined by this specification.
            
            Many older HTTP/1.0 applications do not understand the
            Transfer-Encoding header.
            
            
            14.41 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
            
            
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            action after changing the protocol MUST be a response to the
            initial HTTP request containing the Upgrade header field.
            
            The Upgrade header field only applies to the immediate
            connection. Therefore, the upgrade keyword MUST be supplied
            within a Connection header field (section 14.10) whenever
            Upgrade is present in an HTTP/1.1 message.
            
            The Upgrade header field cannot be used to indicate a switch
            to a protocol on a different connection. For that purpose,
            it is more appropriate to use a 301, 302, 303, or 305
            redirection response.
            
            This specification only defines the protocol name "HTTP" for
            use by the family of Hypertext Transfer Protocols, as
            defined by the HTTP version rules of section 3.1 and future
            updates to this specification. Any token can be used as a
            protocol name; however, it will only be useful if both the
            client and server associate the name with the same protocol.
            
            
            14.42 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
            
            Editors Note: Henrik Frystyk has drafted language to fix an
            editorial problem (VARY) around Vary. Vary is really cache
            advice. The description varies throughout the spec.  Time
            did not permit me to incorporate a final version of these
            changes.
            
            The Vary response-header field is used by a server to signal
            that the response entity was selected from the available
            representations of the response using server-driven
            
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            negotiation (section 12). Field-names listed in Vary headers
            are those of request-headers. The Vary field value indicates
            either that the given set of header fields encompass the
            dimensions over which the representation might vary, or that
            the dimensions of variance are unspecified ("*") and thus
            may vary over any aspect of future requests.
            
                   Vary  = "Vary" ":" ( "*" | 1#field-name )
            
            An HTTP/1.1 server MUST include an appropriate 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 SHOULD include an appropriate 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 might vary.
            
            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, the cache MUST NOT use such a cache entry to
            construct a response to the new request unless all of the
            headers named in the cached Vary header are present in the
            new request, and all of the stored selecting request-headers
            from the previous request match the corresponding headers in
            the new 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 field value of "*" signals that unspecified
            parameters, possibly other than the contents of request-
            header fields (e.g., the network address of the client),
            play a role in the selection of the response representation.
            Subsequent requests on that resource can only be properly
            interpreted by the origin server, and thus a cache MUST
            forward a (possibly conditional) request even when it has a
            fresh response cached for the resource. See section 13.6 for
            use of the Vary header by caches.
            
            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
            
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            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-header fields defined by this specification. Field
            names are case-insensitive.
            
            
            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 that the end result is ordered
            according to the sequence of forwarding applications.
            
            
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            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
            
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                   warning-value = warn-code SP warn-agent SP warn-text
                                            [SP warn-date]
            
            
                   warn-code  = 3DIGIT
                   warn-agent = ( host [ ":" port ] ) | pseudonym
                             ; the name or pseudonym of the server adding
                             ; the Warning header, for use in debugging
                   warn-text  = quoted-string
            
                   warn-date  = <"> HTTP-date <">
            
            A response may carry more than one Warning header.
            
            The warn-text should be in a natural language and character
            set that is most likely to be intelligible to the human user
            receiving the response. This decision may be based on any
            available knowledge, such as the location of the cache or
            user, the Accept-Language field in a request, the Content-
            Language field in a response, etc. The default language is
            English and the default character set is ISO-8859-1.
            
            If a character set other than ISO-8859-1 is used, it MUST be
            encoded in the warn-text using the method described in RFC
            2047 [14].
            
            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.
            
            
            
            
            
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            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.
            
            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.
            
            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.
            
            
            
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            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.  If all of the warning-
            values are deleted for this reason, the Warning header MUST
            be deleted as well.
            
            
            
            
            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 ) ]
            
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            The server SHOULD respond with a 419 (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
            419 (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
            lower, it MUST NOT forward the request, and it MUST respond
            with a 419 (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 419 (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 Set-Proxy
            
            The Set-Proxy response-header  is used to carry information
            to redirect a client to use a different proxy.
            
                  Set-Proxy: "Set-Proxy" ":" action [ ";" 1#parameters ]
            
                  parameters   = ( "scope" "=" scopePattern ) |
                                 ( proxyURI "=" URI ) | lifetime
            
                  lifetime     = ( "seconds"  "=" integer )
                                 | ( "hits"      "=" integer )
            
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                  action       = ( "direct" | "ipl" | "set" )
            
                  scopePattern = "*" | "-" | URIpattern
            
                  URIpattern   = character | "*"
            
                  character    = Any character legal in the definition
                                 of a URL/URI in the context of RFC2068
            
            An example header:
            
               Set-proxy: set ; proxyURI = "http://proxy.me.com:8080/",
                       scope="http://", seconds=5
            
            Scope Meaning: all URLS beginning with "http://"
            
            Another example header:
            
               Set-Proxy: set ; proxyURI = "http://proxy.me.com:8080/",
                       scope="http://*.ups.com/", seconds=5
            
            Scope meaning: all URLS beginning which are for hosts in the
            ups.com domain.
            
            The action response directive specifies the type or mode of
            the change.
            
            direct
               Attempt to connect directly, with no proxy
            
            ipl
               Initial Program Load, the client or proxy should attempt
               to revert back to its default or initial proxy setting.
               This is meant to instruct a client to re-fetch its proxy
               configuration, or PAC file. When set, the accompanying
               scope field MUST be "*" A client receiving this response
               SHOULD prompt the user for confirmation.
            
               If accompanied by a proxyURI parameter, a proxy or client
               MAY use the value as a URL containing a configuration to
               retrieve.  If a client  does so, it MUST prompt the user
               for confirmation.
            
            set
               Set to parameter proxyURI.  The client should use the URL
               specified for proxyURI as the proxy.  If the SET mode is
               specified, the parameter,  proxyURI, MUST be present.
            
            Scope refers to an expression pattern that specifies which
            URIs that are subject to this header setting.  URIs should
            be matched against the scope with this rule :
            
                 The scope "*" means all requests.
            
            
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                 The scope "-" means this EXACT URL ONLY
            
            Otherwise, the URL is compared with the scope in the
            following manner.
            
            The scope is a prefix of matching URLs.
            
            The character "*" is allowed in the DNS name portion of a
            URL, or in the path portion of the URL, but ONLY when used
            with a 306, not a 305.
            
            It matches any sequence of characters except '/'.
            
            This is intended to be a simple matching scheme to allow a
            prefix match to take place.
            
            See the examples section in section 15.12.2.
            
            The lifetime parameter specifies how long the specified
            proxy should be used.  If lifetime is specified as "seconds"
            then the proxy setting remains in effect for `integer'
            seconds.  If lifetime is specified in `hits' then the proxy
            setting remains in effect for `integer' transactions.
            
            
            14.49 Compliance
            
            Editor's note: The OPTIONS changes would introduce a new
            "Compliance" header.
            
            
            14.50 Non-Compliance
            
            Editor's note: The OPTIONS changes would introduce a new
            "Compliance" header.
            
            
            
            
            15 Security Considerations
            
            This section is meant to inform application developers,
            information providers, and users of the security limitations
            in HTTP/1.1 as described by this document. The discussion
            does not include definitive solutions to the problems
            revealed, though it does make some suggestions for reducing
            security risks.
            
            
            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
            
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            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 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
            
            
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            impersonating the original server in a way that is not
            detectable by the client.
            
            
            15.2 Offering a Choice of Authentication Schemes
            
            An HTTP/1.1 server may return multiple challenges with a 401
            (Authenticate) response, and each challenge may use a
            different scheme. The order of the challenges returned to
            the user agent is in the order that the server would prefer
            they be chosen. The server should order its challenges with
            the "most secure" authentication scheme first. A user agent
            should choose as the challenge to be made to the user the
            first one that the user agent understands.
            
            When the server offers choices of authentication schemes
            using the WWW-Authenticate header, the "security" of the
            authentication is only as good as the security of the
            weakest of the authentication schemes. A malicious user
            could capture the set of challenges and try to authenticate
            him/herself using the weakest of the authentication schemes.
            Thus, the ordering serves more to protect the user's
            credentials than the server's information.
            
            A possible man-in-the-middle (MITM) attack would be to add a
            weak authentication scheme to the set of choices, hoping
            that the client will use one that exposes the user's
            credentials (e.g. password). For this reason, the client
            should always use the strongest scheme that it understands
            from the choices accepted.
            
            An even better MITM attack would be to remove all offered
            choices, and to insert a challenge that requests Basic
            authentication. For this reason, user agents that are
            concerned about this kind of attack could remember the
            strongest authentication scheme ever requested by a server
            and produce a warning message that requires user
            confirmation before using a weaker one. A particularly
            insidious way to mount such a MITM attack would be to offer
            a "free" proxy caching service to gullible users.
            
            
            15.3 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.
            
            
            
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            15.4 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 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.
            
            
            15.5 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
            
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            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.
            
            
            15.6 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.
            
            
            15.7 Privacy Issues Connected to Accept Headers
            
            Accept request-headers can reveal information about the user
            to all servers which are accessed. The Accept-Language
            header in particular can reveal information the user would
            consider to be of a private nature, because the
            understanding of particular languages is often strongly
            correlated to the membership of a particular ethnic group.
            User agents which offer the option to configure the contents
            of an Accept-Language header to be sent in every request are
            strongly encouraged to let the configuration process include
            a message which makes the user aware of the loss of privacy
            involved.
            
            An approach that limits the loss of privacy would be for a
            user agent to omit the sending of Accept-Language headers by
            default, and to ask the user whether 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
            
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            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.
            
            
            15.8 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.
            
            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.
            
            
            
            
            
            
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            15.9 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.
            
            
            15.10 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.
            
            
            15.11 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.
            
            
            15.12 Using 305/306 response codes and 'Set-Proxy' header
            
            Editor's note:  This presumes the OPTIONS issue gets closed
            quickly and incorporated in the next draft of this document.
            
            
            15.12.1 Methods
            
            A client or proxy receiving a 305 or 306, should use the
            OPTIONS method to determine if the server or proxy it is
            
            
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            talking to actually is an HTTP/1.1 server supporting set-
            proxy header 305 and 306 responses.
            
            
            15.12.2  Operational Contraints
            
            Both the 305 and 306 response codes are HOP by HOP.  A proxy
            server MUST not forward a 305 or 306 respose code (unless it
            generated the 306). A webserver MUST NOT send a 306 response
            under any circumstances. A proxy server MUST NOT generate a
            305 response. A client or proxy SHOULD NOT accept a 306 from
            a proxy that it learned of via a 305 response code. A client
            or proxy MAY maintain state and allow a lifetime to extend
            beyond a session or restart. A "Set-Proxy: ipl" SHOULD
            override any previous Set-Proxy header. A 305 or 306
            response MAY contain a body containing an explanation of the
            redirect for clients which do not understand the redirect.
            In the absence of any parameter, the following defaults
            should be used:
            
                 lifetime = this transaction only
            
                 scope = this exact URL only
            
            When receiving a 305 response, the client or proxy will
            enforce the following rule with respect to the scope.The
            scope specified must be more restrictive than the
            transformed URL in question based on the rightmost slash in
            the URI.
            
            Example: (in order of restrictiveness)
            
                 for URI = http://www.ups.com/services/index.html
            
                 http://www.ups.com/services/  (allowed)
                 http://www.ups.com/services/express/ ( allowed )
                 http://www.ups.com/ (NOT allowed)
            
            Using "*" in a 306 response Set-Proxy header:
            
            The scope may be set to:
            
                 http://*.foo.com/
            
                 which would apply to all URLs in to domain foo.com
            
            If the scope returned with a 305 response is less
            restrictive than the requested URL, the client may reject
            the redirection and return 506 Redirection Failed.  If the
            client wished to honor the redirect, it client MUST prompt
            the user for confirmation before accepting the new proxy
            setting.
            
            
            
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            Since HTTP/1.0 proxies may unknowingly forward a 305 or 306
            response code that was generated maliciously or in good
            faith, the client must attempt to ascertain if the proxy
            with which it is directly communicating is HTTP/1.1 and if
            it supports the Set-Proxy header.  To determine this, the
            client or proxy should use the OPTIONS method to make a
            request check for this feature.  The extension string should
            be HDR='set-proxy', or, should this be defined in the
            Standard RFC for HTTP/1.1, then the string should be
            RFC='rfcXXXX'  in the OPTIONS request.
            
            Great care should be taken when implementing client side
            actions based on the 305 or 306.  Since older proxies may
            unknowingly forward either of these reponses, clients should
            be prepared to check the validity. A client or proxy MUST
            NOT accept a 305 response from a proxy. A client or proxy
            MUST NOT accept a 306 response from an origin server. When
            receiving a 306 response from a proxy, the client MUST
            verify that the proxy supports the 306 response with an
            OPTIONS request.
            
            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
            
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                   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
            
            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, Alex
            Hopmann, and Larry Masinter for their help.
            
            
            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.
            
            
            
            
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            [3]     Berners-Lee, T., "Universal Resource Identifiers in WWW:
               A Unifying Syntax for the Expression of Names and Addresses of
               Objects on the Network as used in the World-Wide Web."
               RFC 1630, 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 Specification - 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.
            
            
            [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 - application
               and support." 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.
            
            
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            [13]    Kantor, B. and P. Lapsley. "Network News Transfer Protocol
                A Proposed Standard for the Stream-Based Transmission of News."
               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
               2048, USC/ISI, November 1996.
            
            
            [18]    Postel, J. and J. Reynolds.
               "File Transfer Protocol (FTP)." STD 9, RFC 959, USC/ISI,
               October 1985.
            
            
            [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.
            
            
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               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.
            
            
            [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
            
            
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               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," RFC 2044, Alis Technologies,
               October, 1996.
            
            [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.
            
            
            [40]    Freed, N., and N. Borenstein. "Multipurpose Internet
               Mail Extensions (MIME) Part Two: Media Types." RFC 2046,
               Innosoft, First Virtual, November 1996.
            
            
            
            
            
            
            18 Authors' Addresses
            
            Roy T. Fielding
            Department of Information and Computer Science
            University of California
            Irvine, CA 92717-3425, USA
            
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            Fax: +1 (714) 824-4056
            Email: fielding@ics.uci.edu
            
            Jim 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
            Fax: +1 (617) 258 8682
            Email: frystyk@w3.org
            
            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.
            
            
            
            
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                    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
            
            
            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
            
            
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               ...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.
            
            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
            
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            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. 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.
            
            
            
            
            
            
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            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.
            
            
            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) {
            
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                      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.
            
            
            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
            
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            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.
            
            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 )
            
            
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                    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 15.10 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;
            
               . 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;
            
            
            
            
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               . 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 19.7.1.1.
            
            
            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 following describes the original HTTP/1.0 form of
            persistent connections.
            
            When it connects to an origin server, an HTTP client MAY
            send the Keep-Alive connection-token:
            
                   Connection: Keep-Alive
            
            An HTTP/1.0 server would then respond with the Keep-Alive
            connection token and the client may proceed with an HTTP/1.0
            (or Keep-Alive) persistent connection.
            
            An HTTP/1.1 server may also establish persistent connections
            with HTTP/1.0 clients upon receipt of a Keep-Alive
            connection token. However, a persistent connection with an
            HTTP/1.0 client cannot make use of the chunked transfer-
            coding, and therefore MUST use a Content-Length for marking
            the ending boundary of each message.
            
            
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            A client MUST NOT send the Keep-Alive connection token to a
            proxy server as HTTP/1.0 proxy servers do not obey the rules
            of HTTP/1.1 for parsing the Connection header field.
            
            
            19.7.1.1 The Keep-Alive Header
            
            When the Keep-Alive connection-token has been transmitted
            with a request or a response, a Keep-Alive header field MAY
            also be included. The Keep-Alive header field takes the
            following form:
            
                   Keep-Alive-header = "Keep-Alive" ":" 0# keepalive-param
            
                   keepalive-param = param-name "=" value
            
            The Keep-Alive header itself is optional, and is used only
            if a parameter is being sent. HTTP/1.1 does not define any
            parameters.
            
            If the Keep-Alive header is sent, the corresponding
            connection token MUST be transmitted. The Keep-Alive header
            MUST be ignored if received without the connection token.
            
            
            19.8 Backward Compatibility
            
            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.
            
            
            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.
            
            
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            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.
            
            
            19.9 Requirements Summary
            
            This section summarizes the requirements of the HTTP/1.1
            specification.  (Requirements are those aspects of the
            protocol defined with the words "    "
                                             MUST , "SHOULD", or "MAY.")
            
            This list is not a normative part of the HTTP/1.1
            specification, and if there is any conflict between this
            listing and another part of the specification, the
            statements elsewhere in the specification take absolute
            priority.
            
            Requirements are listed in the order that they appear in the
            the specification.  For each requirement, the list includes
            
               . a very brief summary of the feature; this is meant for
                 identification purposes only, and must not be used as a
                 specification of the feature.
            
               . the section of the document in which the feature is
                 specified
            
               . A column for each of three categories of implementation
                 (Server, Proxy, and Client), showing whether the listed
                 feature is a MUST, SHOULD, or MAY requirement.  ("MUST
                 NOT" is abbreviated as "MST NT"; "SHOULD NOT" is
                 abbreviated as "SH NOT".)
            
               . A column for additional footnotes Note that some
                 aspects of the protocol may be specified in multiple
                 sections in separated part of the document.
            
            Editor's Note: this draft of the HTTP/1.1 specification does
            not include a requirements summary.  A summary will be
            provided in a subsequent draft.  The format of the list may
            change, based on experience with the creation of the list.
            
            Fielding, et al                                   [Page 181]
            

            INTERNET-DRAFT            HTTP/1.1  Wednesday, July 30, 1997
            
            
            What follows is meant only as an example of the final
            listing.
            
            
            Feature summary            Section Server Proxy  Client  Note
            
            
            Send From header in          14.22 Na     Na     MAY     1
            requests
            
            
            From header contains user's  14.22 Na     Na     SHOULD
            email address
            
            
            From not meant for           14.22 SH     SH     Na
            authentication                     NOT    NOT
            
            
            User approves sending of     14.22 Na     Na     SHOULD
            From header
            
            
            Send Host header in          14.23 Na     Na     MUST    2
            requests
            
            
            Add Host hdr to forwarded    14.23 Na     MUST   na
            HTTP/1.1 req if missing
            
            
            Require Host header in       14.23 MUST   MUST   na
            HTTP/1.1 requests
            
            
               Footnotes:
            
               (1) From header SHOULD be sent by robots
            
               (2) Not required on non-Internet networks
            
            
            
            
            
            
            
            
            
            
            
            
            
            
            Fielding, et al                                   [Page 182]
            

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