    Network Working Group                                      R. Fielding
    Request for Comments: 2068                                   UC Irvine
    Category: Standards Track                                    J. Gettys
                                                                  J. Mogul
                                                                       DEC
                                                                H. Frystyk
                                                            T. Berners-Lee
                                                                   MIT/LCS
                                                              January 1997

                    Hypertext Transfer Protocol -- HTTP/1.1

    Status of this Memo

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

    Abstract

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

       HTTP has been in use by the World-Wide Web global information
       initiative since 1990. This specification defines the protocol
       referred to as "HTTP/1.1".

    Table of Contents

       1 Introduction.............................................7
        1.1 Purpose ..............................................7
        1.2 Requirements .........................................7
        1.3 Terminology ..........................................8
        1.4 Overall Operation ...................................11
       2 Notational Conventions and Generic Grammar..............13
        2.1 Augmented BNF .......................................13
        2.2 Basic Rules .........................................15
       3 Protocol Parameters.....................................17
        3.1 HTTP Version ........................................17

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        3.2 Uniform Resource Identifiers ........................18
         3.2.1 General Syntax ...................................18
         3.2.2 http URL .........................................19
         3.2.3 URI Comparison ...................................20
        3.3 Date/Time Formats ...................................21
         3.3.1 Full Date ........................................21
         3.3.2 Delta Seconds ....................................22
        3.4 Character Sets ......................................22
        3.5 Content Codings .....................................23
        3.6 Transfer Codings ....................................24
        3.7 Media Types .........................................25
         3.7.1 Canonicalization and Text Defaults ...............26
         3.7.2 Multipart Types ..................................27
        3.8 Product Tokens ......................................28
        3.9 Quality Values ......................................28
        3.10 Language Tags ......................................28
        3.11 Entity Tags ........................................29
        3.12 Range Units ........................................30
       4 HTTP Message............................................30
        4.1 Message Types .......................................30
        4.2 Message Headers .....................................31
        4.3 Message Body ........................................32
        4.4 Message Length ......................................32
        4.5 General Header Fields ...............................34
       5 Request.................................................34
        5.1 Request-Line ........................................34
         5.1.1 Method ...........................................35
         5.1.2 Request-URI ......................................35
        5.2 The Resource Identified by a Request ................37
        5.3 Request Header Fields ...............................37
       6 Response................................................38
        6.1 Status-Line .........................................38
         6.1.1 Status Code and Reason Phrase ....................39
        6.2 Response Header Fields ..............................41
       7 Entity..................................................41
        7.1 Entity Header Fields ................................41
        7.2 Entity Body .........................................42
         7.2.1 Type .............................................42
         7.2.2 Length ...........................................43
       8 Connections.............................................43
        8.1 Persistent Connections ..............................43
         8.1.1 Purpose ..........................................43
         8.1.2 Overall Operation ................................44
         8.1.3 Proxy Servers ....................................45
         8.1.4 Practical Considerations .........................45
        8.2 Message Transmission Requirements ...................46
       9 Method Definitions......................................48
        9.1 Safe and Idempotent Methods .........................48

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         9.1.1 Safe Methods .....................................48
         9.1.2 Idempotent Methods ...............................49
        9.2 OPTIONS .............................................49
        9.3 GET .................................................50
        9.4 HEAD ................................................50
        9.5 POST ................................................51
        9.6 PUT .................................................52
        9.7 DELETE ..............................................53
        9.8 TRACE ...............................................53
       10 Status Code Definitions................................53
        10.1 Informational 1xx ..................................54
         10.1.1 100 Continue ....................................54
         10.1.2 101 Switching Protocols .........................54
        10.2 Successful 2xx .....................................54
         10.2.1 200 OK ..........................................54
         10.2.2 201 Created .....................................55
         10.2.3 202 Accepted ....................................55
         10.2.4 203 Non-Authoritative Information ...............55
         10.2.5 204 No Content ..................................55
         10.2.6 205 Reset Content ...............................56
         10.2.7 206 Partial Content .............................56
        10.3 Redirection 3xx ....................................56
         10.3.1 300 Multiple Choices ............................57
         10.3.2 301 Moved Permanently ...........................57
         10.3.3 302 Moved Temporarily ...........................58
         10.3.4 303 See Other ...................................58
         10.3.5 304 Not Modified ................................58
         10.3.6 305 Use Proxy ...................................59
        10.4 Client Error 4xx ...................................59
         10.4.1 400 Bad Request .................................60
         10.4.2 401 Unauthorized ................................60
         10.4.3 402 Payment Required ............................60
         10.4.4 403 Forbidden ...................................60
         10.4.5 404 Not Found ...................................60
         10.4.6 405 Method Not Allowed ..........................61
         10.4.7 406 Not Acceptable ..............................61
         10.4.8 407 Proxy Authentication Required ...............61
         10.4.9 408 Request Timeout .............................62
         10.4.10 409 Conflict ...................................62
         10.4.11 410 Gone .......................................62
         10.4.12 411 Length Required ............................63
         10.4.13 412 Precondition Failed ........................63
         10.4.14 413 Request Entity Too Large ...................63
         10.4.15 414 Request-URI Too Long .......................63
         10.4.16 415 Unsupported Media Type .....................63
        10.5 Server Error 5xx ...................................64
         10.5.1 500 Internal Server Error .......................64
         10.5.2 501 Not Implemented .............................64

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         10.5.3 502 Bad Gateway .................................64
         10.5.4 503 Service Unavailable .........................64
         10.5.5 504 Gateway Timeout .............................64
         10.5.6 505 HTTP Version Not Supported ..................65
       11 Access Authentication..................................65
        11.1 Basic Authentication Scheme ........................66
        11.2 Digest Authentication Scheme .......................67
       12 Content Negotiation....................................67
        12.1 Server-driven Negotiation ..........................68
        12.2 Agent-driven Negotiation ...........................69
        12.3 Transparent Negotiation ............................70
       13 Caching in HTTP........................................70
         13.1.1 Cache Correctness ...............................72
         13.1.2 Warnings ........................................73
         13.1.3 Cache-control Mechanisms ........................74
         13.1.4 Explicit User Agent Warnings ....................74
         13.1.5 Exceptions to the Rules and Warnings ............75
         13.1.6 Client-controlled Behavior ......................75
        13.2 Expiration Model ...................................75
         13.2.1 Server-Specified Expiration .....................75
         13.2.2 Heuristic Expiration ............................76
         13.2.3 Age Calculations ................................77
         13.2.4 Expiration Calculations .........................79
         13.2.5 Disambiguating Expiration Values ................80
         13.2.6 Disambiguating Multiple Responses ...............80
        13.3 Validation Model ...................................81
         13.3.1 Last-modified Dates .............................82
         13.3.2 Entity Tag Cache Validators .....................82
         13.3.3 Weak and Strong Validators ......................82
         13.3.4 Rules for When to Use Entity Tags and Last-
         modified Dates..........................................85
         13.3.5 Non-validating Conditionals .....................86
        13.4 Response Cachability ...............................86
        13.5 Constructing Responses From Caches .................87
         13.5.1 End-to-end and Hop-by-hop Headers ...............88
         13.5.2 Non-modifiable Headers ..........................88
         13.5.3 Combining Headers ...............................89
         13.5.4 Combining Byte Ranges ...........................90
        13.6 Caching Negotiated Responses .......................90
        13.7 Shared and Non-Shared Caches .......................91
        13.8 Errors or Incomplete Response Cache Behavior .......91
        13.9 Side Effects of GET and HEAD .......................92
        13.10 Invalidation After Updates or Deletions ...........92
        13.11 Write-Through Mandatory ...........................93
        13.12 Cache Replacement .................................93
        13.13 History Lists .....................................93
       14 Header Field Definitions...............................94
        14.1 Accept .............................................95

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        14.2 Accept-Charset .....................................97
        14.3 Accept-Encoding ....................................97
        14.4 Accept-Language ....................................98
        14.5 Accept-Ranges ......................................99
        14.6 Age ................................................99
        14.7 Allow .............................................100
        14.8 Authorization .....................................100
        14.9 Cache-Control .....................................101
         14.9.1 What is Cachable ...............................103
         14.9.2 What May be Stored by Caches ...................103
         14.9.3 Modifications of the Basic Expiration Mechanism 104
         14.9.4 Cache Revalidation and Reload Controls .........105
         14.9.5 No-Transform Directive .........................107
         14.9.6 Cache Control Extensions .......................108
        14.10 Connection .......................................109
        14.11 Content-Base .....................................109
        14.12 Content-Encoding .................................110
        14.13 Content-Language .................................110
        14.14 Content-Length ...................................111
        14.15 Content-Location .................................112
        14.16 Content-MD5 ......................................113
        14.17 Content-Range ....................................114
        14.18 Content-Type .....................................116
        14.19 Date .............................................116
        14.20 ETag .............................................117
        14.21 Expires ..........................................117
        14.22 From .............................................118
        14.23 Host .............................................119
        14.24 If-Modified-Since ................................119
        14.25 If-Match .........................................121
        14.26 If-None-Match ....................................122
        14.27 If-Range .........................................123
        14.28 If-Unmodified-Since ..............................124
        14.29 Last-Modified ....................................124
        14.30 Location .........................................125
        14.31 Max-Forwards .....................................125
        14.32 Pragma ...........................................126
        14.33 Proxy-Authenticate ...............................127
        14.34 Proxy-Authorization ..............................127
        14.35 Public ...........................................127
        14.36 Range ............................................128
         14.36.1 Byte Ranges ...................................128
         14.36.2 Range Retrieval Requests ......................130
        14.37 Referer ..........................................131
        14.38 Retry-After ......................................131
        14.39 Server ...........................................132
        14.40 Transfer-Encoding ................................132
        14.41 Upgrade ..........................................132

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        14.42 User-Agent .......................................134
        14.43 Vary .............................................134
        14.44 Via ..............................................135
        14.45 Warning ..........................................137
        14.46 WWW-Authenticate .................................139
       15 Security Considerations...............................139
        15.1 Authentication of Clients .........................139
        15.2 Offering a Choice of Authentication Schemes .......140
        15.3 Abuse of Server Log Information ...................141
        15.4 Transfer of Sensitive Information .................141
        15.5 Attacks Based On File and Path Names ..............142
        15.6 Personal Information ..............................143
        15.7 Privacy Issues Connected to Accept Headers ........143
        15.8 DNS Spoofing ......................................144
        15.9 Location Headers and Spoofing .....................144
       16 Acknowledgments.......................................144
       17 References............................................146
       18 Authors' Addresses....................................149
       19 Appendices............................................150
        19.1 Internet Media Type message/http ..................150
        19.2 Internet Media Type multipart/byteranges ..........150
        19.3 Tolerant Applications .............................151
        19.4 Differences Between HTTP Entities and
        MIME Entities...........................................152
         19.4.1 Conversion to Canonical Form ...................152
         19.4.2 Conversion of Date Formats .....................153
         19.4.3 Introduction of Content-Encoding ...............153
         19.4.4 No Content-Transfer-Encoding ...................153
         19.4.5 HTTP Header Fields in Multipart Body-Parts .....153
         19.4.6 Introduction of Transfer-Encoding ..............154
         19.4.7 MIME-Version ...................................154
        19.5 Changes from HTTP/1.0 .............................154
         19.5.1 Changes to Simplify Multi-homed Web Servers and
         Conserve IP Addresses .................................155
        19.6 Additional Features ...............................156
         19.6.1 Additional Request Methods .....................156
         19.6.2 Additional Header Field Definitions ............156
        19.7 Compatibility with Previous Versions ..............160
         19.7.1 Compatibility with HTTP/1.0 Persistent
         Connections............................................161

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    RFC 2068                        HTTP/1.1                    January 1997

    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][20], as a location (URL) [4] or
       name (URN) , 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 as defined by the Multipurpose Internet Mail Extensions
       (MIME).

       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

       This specification uses the same words as RFC 1123 [8] for defining
       the significance of each particular requirement. These words are:

       MUST
          This word or the adjective "required" means that the item is an
          absolute requirement of the specification.

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       SHOULD
          This word or the adjective "recommended" means that there may
          exist valid reasons in particular circumstances to ignore this
          item, but the full implications should be understood and the case
          carefully weighed before choosing a different course.

       MAY
          This word or the adjective "optional" means that this item is
          truly optional. One vendor may choose to include the item because
          a particular marketplace requires it or because it enhances the
          product, for example; another vendor may omit the same item.

       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.

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       entity
          The information transferred as the payload of a request or
          response. An entity consists of metainformation in the form of
          entity-header fields and content in the form of an entity-body, as
          described in section 7.

       representation
          An entity included with a response that is subject to content
          negotiation, as described in section 12. There may exist multiple
          representations associated with a particular response status.

       content negotiation
          The mechanism for selecting the appropriate representation when
          servicing a request, as described in section 12. The
          representation of entities in any response can be negotiated
          (including error responses).

       variant
          A resource may have one, or more than one, representation(s)
          associated with it at any given instant. Each of these
          representations is termed a `variant.' Use of the term `variant'
          does not necessarily imply that the resource is subject to content
          negotiation.

       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.

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       proxy
          An intermediary program which acts as both a server and a client
          for the purpose of making requests on behalf of other clients.
          Requests are serviced internally or by passing them on, with
          possible translation, to other servers. A proxy must implement
          both the client and server requirements of this specification.

       gateway
          A server which acts as an intermediary for some other server.
          Unlike a proxy, a gateway receives requests as if it were the
          origin server for the requested resource; the requesting client
          may not be aware that it is communicating with a gateway.

       tunnel
          An intermediary program which is acting as a blind relay between
          two connections. Once active, a tunnel is not considered a party
          to the HTTP communication, though the tunnel may have been
          initiated by an HTTP request. The tunnel ceases to exist when both
          ends of the relayed connections are closed.

       cache
          A program's local store of response messages and the subsystem
          that controls its message storage, retrieval, and deletion. A
          cache stores cachable responses in order to reduce the response
          time and network bandwidth consumption on future, equivalent
          requests. Any client or server may include a cache, though a cache
          cannot be used by a server that is acting as a tunnel.

       cachable
          A response is cachable if a cache is allowed to store a copy of
          the response message for use in answering subsequent requests. The
          rules 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.

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

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

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

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

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

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

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

    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.

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       Most HTTP communication is initiated by a user agent and consists of
       a request to be applied to a resource on some origin server. In the
       simplest case, this may be accomplished via a single connection (v)
       between the user agent (UA) and the origin server (O).

                 request chain ------------------------>
              UA -------------------v------------------- O
                 <----------------------- response chain

       A more complicated situation occurs when one or more intermediaries
       are present in the request/response chain. There are three common
       forms of intermediary: proxy, gateway, and tunnel. A proxy is a
       forwarding agent, receiving requests for a URI in its absolute form,
       rewriting all or part of the message, and forwarding the reformatted
       request toward the server identified by the URI. A gateway is a
       receiving agent, acting as a layer above some other server(s) and, if
       necessary, translating the requests to the underlying server's
       protocol. A tunnel acts as a relay point between two connections
       without changing the messages; tunnels are used when the
       communication needs to pass through an intermediary (such as a
       firewall) even when the intermediary cannot understand the contents
       of the messages.

                 request chain -------------------------------------->
              UA -----v----- A -----v----- B -----v----- C -----v----- O
                 <------------------------------------- response chain

       The figure above shows three intermediaries (A, B, and C) between the
       user agent and origin server. A request or response message that
       travels the whole chain will pass through four separate connections.
       This distinction is important because some HTTP communication options
       may apply only to the connection with the nearest, non-tunnel
       neighbor, only to the end-points of the chain, or to all connections
       along the chain.  Although the diagram is linear, each participant
       may be engaged in multiple, simultaneous communications. For example,
       B may be receiving requests from many clients other than A, and/or
       forwarding requests to servers other than C, at the same time that it
       is handling A's request.

       Any party to the communication which is not acting as a tunnel may
       employ an internal cache for handling requests. The effect of a cache
       is that the request/response chain is shortened if one of the
       participants along the chain has a cached response applicable to that
       request. The 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.

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                 request chain ---------->
              UA -----v----- A -----v----- B - - - - - - C - - - - - - O
                 <--------- response chain

       Not all responses are usefully cachable, and some requests may
       contain modifiers which place special requirements on cache behavior.
       HTTP requirements for cache behavior and cachable responses are
       defined in section 13.

       In fact, there are a wide variety of architectures and configurations
       of caches and proxies currently being experimented with or deployed
       across the World Wide Web; these systems include national hierarchies
       of proxy caches to save transoceanic bandwidth, systems that
       broadcast or multicast cache entries, organizations that distribute
       subsets of cached data via CD-ROM, and so on. HTTP systems are used
       in corporate intranets over high-bandwidth links, and for access via
       PDAs with low-power radio links and intermittent connectivity. The
       goal of HTTP/1.1 is to support the wide diversity of configurations
       already deployed while introducing protocol constructs that meet the
       needs of those who build web applications that require high
       reliability and, failing that, at least reliable indications of
       failure.

       HTTP communication usually takes place over TCP/IP connections. The
       default port is TCP 80, but other ports can be used. This does not
       preclude HTTP from being implemented on top of any other protocol on
       the Internet, or on other networks. HTTP only presumes a reliable
       transport; any protocol that provides such guarantees can be used;
       the mapping of the HTTP/1.1 request and response structures onto the
       transport data units of the protocol in question is outside the scope
       of this specification.

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

    2 Notational Conventions and Generic Grammar

    2.1 Augmented BNF

       All of the mechanisms specified in this document are described in
       both prose and an augmented Backus-Naur Form (BNF) similar to that
       used by RFC 822 [9]. Implementers will need to be familiar with the
       notation in order to understand this specification. The augmented BNF
       includes the following constructs:

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    name = definition
         The name of a rule is simply the name itself (without any enclosing
         "<" and ">") and is separated from its definition by the equal "="
         character. Whitespace is only significant in that indentation of
         continuation lines is used to indicate a rule definition that spans
         more than one line. Certain basic rules are in uppercase, such as
         SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle brackets are used
         within definitions whenever their presence will facilitate
         discerning the use of rule names.

    "literal"
         Quotation marks surround literal text. Unless stated otherwise, the
              text is case-insensitive.

    rule1 | rule2
         Elements separated by a bar ("|") are alternatives, e.g., "yes |
         no" will accept yes or no.

    (rule1 rule2)
         Elements enclosed in parentheses are treated as a single element.
         Thus, "(elem (foo | bar) elem)" allows the token sequences "elem
         foo elem" and "elem bar elem".

    *rule
         The character "*" preceding an element indicates repetition. The
         full form is "<n>*<m>element" indicating at least <n> and at most
         <m> occurrences of element. Default values are 0 and infinity so
         that "*(element)" allows any number, including zero; "1*element"
         requires at least one; and "1*2element" allows one or two.

    [rule]
         Square brackets enclose optional elements; "[foo bar]" is
         equivalent to "*1(foo bar)".

    N rule
         Specific repetition: "<n>(element)" is equivalent to
         "<n>*<n>(element)"; that is, exactly <n> occurrences of (element).
         Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
         alphabetic characters.

    #rule
         A construct "#" is defined, similar to "*", for defining lists of
         elements. The full form is "<n>#<m>element " indicating at least
         <n> and at most <m> elements, each separated by one or more commas
         (",") and optional linear whitespace (LWS). This makes the usual
         form of lists very easy; a rule such as "( *LWS element *( *LWS ","
         *LWS element )) " can be shown as "1#element". Wherever this
         construct is used, null elements are allowed, but do not contribute

    Fielding, et. al.           Standards Track                    [Page 14]

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         to the count of elements present.  That is, "(element), , (element)
         " is permitted, but counts as only two elements. Therefore, where
         at least one element is required, at least one non-null element
         must be present. Default values are 0 and infinity so that
         "#element" allows any number, including zero; "1#element" requires
         at least one; and "1#2element" allows one or two.

    ; comment
         A semi-colon, set off some distance to the right of rule text,
         starts a comment that continues to the end of line. This is a
         simple way of including useful notes in parallel with the
         specifications.

    implied *LWS
         The grammar described by this specification is word-based. Except
         where noted otherwise, linear whitespace (LWS) can be included
         between any two adjacent words (token or quoted-string), and
         between adjacent tokens and delimiters (tspecials), without
         changing the interpretation of a field. At least one delimiter
         (tspecials) 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)>

    Fielding, et. al.           Standards Track                    [Page 15]

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       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 1522
       [14].

              TEXT           = <any OCTET except CTLs,
                               but including LWS>

       Hexadecimal numeric characters are used in several protocol elements.

              HEX            = "A" | "B" | "C" | "D" | "E" | "F"
                             | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT

       Many HTTP/1.1 header field values consist of words separated by LWS
       or special characters. These special characters MUST be in a quoted
       string to be used within a parameter value.

              token          = 1*<any CHAR except CTLs or tspecials>

              tspecials      = "(" | ")" | "<" | ">" | "@"
                             | "," | ";" | ":" | "\" | <">
                             | "/" | "[" | "]" | "?" | "="
                             | "{" | "}" | 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 | comment ) ")"
              ctext          = <any TEXT excluding "(" and ")">

    Fielding, et. al.           Standards Track                    [Page 16]

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       A string of text is parsed as a single word if it is quoted using
       double-quote marks.

              quoted-string  = ( <"> *(qdtext) <"> )

              qdtext         = <any TEXT except <">>

       The backslash character ("\") may be used as a single-character quoting
       mechanism only within quoted-string and comment constructs.

              quoted-pair    = "\" CHAR

    3 Protocol Parameters

    3.1 HTTP Version

       HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
       of the protocol. The protocol versioning policy is intended to allow
       the sender to indicate the format of a message and its capacity for
       understanding further HTTP communication, rather than the features
       obtained via that communication. No change is made to the version
       number for the addition of message components which do not affect
       communication behavior or which only add to extensible field values.
       The <minor> number is incremented when the changes made to the
       protocol add features which do not change the general message parsing
       algorithm, but which may add to the message semantics and imply
       additional capabilities of the sender. The <major> number is
       incremented when the format of a message within the protocol is
       changed.

       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.

    Fielding, et. al.           Standards Track                    [Page 17]

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       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.

    3.2 Uniform Resource Identifiers

       URIs have been known by many names: WWW addresses, Universal Document
       Identifiers, Universal Resource Identifiers , and finally the
       combination of Uniform Resource Locators (URL)  and Names (URN). 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, 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 | "/" )

    Fielding, et. al.           Standards Track                    [Page 18]

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              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           = "$" | "-" | "_" | "."
              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.

    Fielding, et. al.           Standards Track                    [Page 19]

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              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).

    3.2.3 URI Comparison

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

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

         o  Comparisons of host names MUST be case-insensitive;

         o  Comparisons of scheme names MUST be case-insensitive;

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

       Characters other than those in the "reserved" 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

    Fielding, et. al.           Standards Track                    [Page 20]

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    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  (an update to RFC
       822).  The second format is in common use, but is based on the
       obsolete RFC 850 [12] date format and lacks a four-digit year.
       HTTP/1.1 clients and servers that parse the date value MUST accept
       all three formats (for compatibility with HTTP/1.0), though they MUST
       only generate the RFC 1123 format for representing HTTP-date values
       in header fields.

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

       All HTTP date/time stamps MUST be represented in Greenwich Mean Time
       (GMT), without exception. 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"

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              weekday      = "Monday" | "Tuesday" | "Wednesday"
                           | "Thursday" | "Friday" | "Saturday" | "Sunday"

              month        = "Jan" | "Feb" | "Mar" | "Apr"
                           | "May" | "Jun" | "Jul" | "Aug"
                           | "Sep" | "Oct" | "Nov" | "Dec"

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

    3.3.2 Delta Seconds

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

              delta-seconds  = 1*DIGIT

    3.4 Character Sets

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

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

         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.

    Fielding, et. al.           Standards Track                    [Page 22]

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       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 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 more important is that it
       indicates what decoding mechanism will be required to remove the
       encoding.

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

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

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

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

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

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

    3.6 Transfer Codings

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

              transfer-coding         = "chunked" | transfer-extension

              transfer-extension      = token

       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 , 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 footer 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.

    Fielding, et. al.           Standards Track                    [Page 24]

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           Chunked-Body   = *chunk
                            "0" CRLF
                            footer
                            CRLF

           chunk          = chunk-size [ chunk-ext ] CRLF
                            chunk-data CRLF

           hex-no-zero    = <HEX excluding "0">

           chunk-size     = hex-no-zero *HEX
           chunk-ext      = *( ";" chunk-ext-name [ "=" chunk-ext-value ] )
           chunk-ext-name = token
           chunk-ext-val  = token | quoted-string
           chunk-data     = chunk-size(OCTET)

           footer         = *entity-header

       The chunked encoding is ended by a zero-sized chunk followed by the
       footer, which is terminated by an empty line. The purpose of the
       footer is to provide an efficient way to supply information about an
       entity that is generated dynamically; applications MUST NOT send
       header fields in the footer which are not explicitly defined as being
       appropriate for the footer, such as Content-MD5 or future extensions
       to HTTP for digital signatures or other facilities.

       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 transfer coding 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  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.

    Fielding, et. al.           Standards Track                    [Page 25]

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              parameter      = attribute "=" value
              attribute      = token
              value          = token | quoted-string

       The type, subtype, and parameter attribute names are case-
       insensitive.  Parameter values may or may not be case-sensitive,
       depending on the semantics of the parameter name. Linear white space
       (LWS) MUST NOT be used between the type and subtype, nor between an
       attribute and its value. User agents that recognize the media-type
       MUST process (or arrange to be processed by any external applications
       used to process that type/subtype by the user agent) the parameters
       for that MIME type as described by that type/subtype definition to
       the and inform the user of any problems discovered.

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

       Media-type values are registered with the Internet Assigned Number
       Authority (IANA). The media type registration process is outlined in
       RFC 2048 [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.

    Fielding, et. al.           Standards Track                    [Page 26]

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       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.

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

       Unfortunately, some older HTTP/1.0 clients did not deal properly with
       an explicit charset parameter. HTTP/1.1 recipients MUST respect the
       charset label provided by the sender; and those user agents that have
       a provision to "guess" a charset MUST use the charset from the
       content-type field if they support that charset, rather than the
       recipient's preference, when initially displaying a document.

    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  MIME [7], 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 MIME, 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].

    Fielding, et. al.           Standards Track                    [Page 27]

    RFC 2068                        HTTP/1.1                    January 1997

    3.8 Product Tokens

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

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

       Examples:

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

       Product tokens should be short and to the point -- use of them for
       advertising or other non-essential information is explicitly
       forbidden.  Although any token character may appear in a product-
       version, this token SHOULD only be used for a version identifier
       (i.e., successive versions of the same product SHOULD only differ in
       the product-version portion of the product value).

    3.9 Quality Values

       HTTP content negotiation (section 12) uses short "floating point"
       numbers to indicate the relative importance ("weight") of various
       negotiable parameters. A weight is normalized to a real number in the
       range 0 through 1, where 0 is the minimum and 1 the maximum value.
       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.

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

               language-tag  = primary-tag *( "-" subtag )

               primary-tag   = 1*8ALPHA
               subtag        = 1*8ALPHA

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

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

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

    3.11 Entity Tags

       Entity tags are used for comparing two or more entities from the same
       requested resource. HTTP/1.1 uses entity tags in the ETag (section
       14.20), If-Match (section 14.25), If-None-Match (section 14.26), and
       If-Range (section 14.27) header fields. The definition of how they
       are used and compared as cache validators is in section 13.3.3. An
       entity tag consists of an opaque quoted string, possibly prefixed by
       a weakness indicator.

             entity-tag = [ weak ] opaque-tag

             weak       = "W/"
             opaque-tag = quoted-string

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

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

       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.

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    3.12 Range Units

       HTTP/1.1 allows a client to request that only part (a range of) the
       response entity be included within the response. HTTP/1.1 uses range
       units in the Range (section 14.36) and Content-Range (section 14.17)
       header fields. An entity may be broken down into subranges according
       to various structural units.

             range-unit       = bytes-unit | other-range-unit

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

    The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1
       implementations may ignore ranges specified using other units.
       HTTP/1.1 has been designed to allow implementations of applications
       that do not depend on knowledge of ranges.

    4 HTTP Message

    4.1 Message Types

       HTTP messages consist of requests from client to server and responses
       from server to client.

              HTTP-message   = Request | Response     ; HTTP/1.1 messages

       Request (section 5) and Response (section 6) messages use the generic
       message format of RFC 822 [9] for transferring entities (the payload
       of the message). Both types of message consist of a start-line, 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.

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         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.

              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, tspecials, 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.

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    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.

       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 MAY be included in a
       request only when the request method (section 5.1.1) allows an
       entity-body.

       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

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         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.)

       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.

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    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.

    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 7.2

    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

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    5.1.1 Method

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

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

              extension-method = token

       The list of methods allowed by a resource can be specified in an
       Allow header field (section 14.7). The return code of the response
       always notifies the client whether a method is currently allowed on a
       resource, since the set of allowed methods can change dynamically.
       Servers SHOULD return the status code 405 (Method Not Allowed) if the
       method is known by the server but not allowed for the requested
       resource, and 501 (Not Implemented) if the method is unrecognized or
       not implemented by the server. The 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

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       from a valid cache, and return the response. Note that the proxy MAY
       forward the request on to another proxy or directly to the server
       specified by the absoluteURI. In order to avoid request loops, a
       proxy MUST be able to recognize all of its server names, including
       any aliases, local variations, and the numeric IP address. An example
       Request-Line would be:

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

       To allow for transition to absoluteURIs in all requests in future
       versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI
       form in requests, even though HTTP/1.1 clients will only generate
       them in requests to proxies.

       The most common form of Request-URI is that used to identify a
       resource on an origin server or gateway. In this case the absolute
       path of the URI MUST be transmitted (see section 3.2.1, abs_path) as
       the Request-URI, and the network location of the URI (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).

       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".

       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.

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       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.

    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

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       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
                             | 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

    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.

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           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:

         o  1xx: Informational - Request received, continuing process

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

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

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

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

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

              Status-Code    = "100"   ; Continue
                             | "101"   ; Switching Protocols
                             | "200"   ; OK
                             | "201"   ; Created
                             | "202"   ; Accepted
                             | "203"   ; Non-Authoritative Information
                             | "204"   ; No Content
                             | "205"   ; Reset Content
                             | "206"   ; Partial Content
                             | "300"   ; Multiple Choices
                             | "301"   ; Moved Permanently
                             | "302"   ; Moved Temporarily

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                             | "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
                             | "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 = 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
                              | 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.

    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.

    Fielding, et. al.           Standards Track                    [Page 41]

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              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 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.

    7.2.1 Type

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

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

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

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       Any HTTP/1.1 message containing an entity-body SHOULD include a
       Content-Type header field defining the media type of that body. If
       and only if the media type is not given by a Content-Type field, the
       recipient MAY attempt to guess the media type via inspection of its
       content and/or the name extension(s) of the 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 requires a client to make multiple
       requests of the same server in a short amount of time. Analyses of
       these performance problems are available [30][27]; analysis and
       results from a prototype implementation are in [26].

       Persistent HTTP connections have a number of advantages:

         o  By opening and closing fewer TCP connections, CPU time is saved,
            and memory used for TCP protocol control blocks is also saved.
         o  HTTP requests and responses can be pipelined on a connection.
            Pipelining allows a client to make multiple requests without
            waiting for each response, allowing a single TCP connection to be
            used much more efficiently, with much lower elapsed time.
         o  Network congestion is reduced by reducing the number of packets
            caused by TCP opens, and by allowing TCP sufficient time to
            determine the congestion state of the network.
         o  HTTP can evolve more gracefully; since errors can be reported
            without the penalty of closing the TCP connection. Clients using
            future versions of HTTP might optimistically try a new feature, but
            if communicating with an older server, retry with old semantics
            after an error is reported.

       HTTP implementations SHOULD implement persistent connections.

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    8.1.2 Overall Operation

       A significant difference between HTTP/1.1 and earlier versions of
       HTTP is that persistent connections are the default behavior of any
       HTTP connection. That is, unless otherwise indicated, the client may
       assume that the server will maintain a persistent connection.

       Persistent connections provide a mechanism by which a client and a
       server can signal the close of a TCP connection. This signaling takes
       place using the Connection header field. Once a close has been
       signaled, the client MUST not send any more requests on that
       connection.

    8.1.2.1 Negotiation

       An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to
       maintain a persistent connection unless a Connection header including
       the connection-token "close" was sent in the request. If the server
       chooses to close the connection immediately after sending the
       response, it SHOULD send a Connection header including the
       connection-token close.

       An HTTP/1.1 client MAY expect a connection to remain open, but would
       decide to keep it open based on whether the response from a server
       contains a Connection header with the connection-token close. In case
       the client does not want to maintain a connection for more than that
       request, it SHOULD send a Connection header including the
       connection-token close.

       If either the client or the server sends the close token in the
       Connection header, that request becomes the last one for the
       connection.

       Clients and servers SHOULD NOT assume that a persistent connection is
       maintained for HTTP versions less than 1.1 unless it is explicitly
       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.

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       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.

    8.1.3 Proxy Servers

       It is especially important that proxies correctly implement the
       properties of the Connection header field as specified in 14.2.1.

       The proxy server MUST signal persistent connections separately with
       its clients and the origin servers (or other proxy servers) that it
       connects to. Each persistent connection applies to only one transport
       link.

       A proxy server MUST NOT establish a persistent connection with an
       HTTP/1.0 client.

    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 request without user
       interaction so long as the request method is idempotent (see section

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       9.1.2); other methods MUST NOT be automatically retried, although
       user agents MAY offer a human operator the choice of retrying the
       request.

       However, this automatic retry SHOULD NOT be repeated if the second
       request fails.

       Servers SHOULD always respond to at least one request per connection,
       if at all possible. Servers SHOULD NOT close a connection in the
       middle of transmitting a response, unless a network or client failure
       is suspected.

       Clients that use persistent connections SHOULD limit the number of
       simultaneous connections that they maintain to a given server. A
       single-user client SHOULD maintain AT MOST 2 connections with any
       server or proxy. A proxy SHOULD use up to 2*N connections to another
       server or proxy, where N is the number of simultaneously active
       users. These guidelines are intended to improve HTTP response times
       and avoid congestion of the Internet or other networks.

    8.2 Message Transmission Requirements

    General requirements:

    o  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.

    o  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 footer 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.

    o  An HTTP/1.1 (or later) client MUST be prepared to accept a 100
       (Continue) status followed by a regular response.

    o  An HTTP/1.1 (or later) server that receives a request from a
       HTTP/1.0 (or earlier) client MUST NOT transmit the 100 (continue)
       response; it SHOULD either wait for the request to be completed
       normally (thus avoiding an interrupted request) or close the
       connection prematurely.

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       Upon receiving a method subject to these requirements from an
       HTTP/1.1 (or later) client, an HTTP/1.1 (or later) server MUST either
       respond with 100 (Continue) status and continue to read from the
       input stream, or respond with an error status. If it responds with an
       error status, it MAY close the transport (TCP) 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.

       Clients SHOULD remember the version number of at least the most
       recently used server; if an HTTP/1.1 client has seen an HTTP/1.1 or
       later response from the server, and it sees the connection close
       before receiving any status from the server, the client SHOULD retry
       the request without user interaction so long as the request method is
       idempotent (see section 9.1.2); other methods MUST NOT be
       automatically retried, although user agents MAY offer a human
       operator the choice of retrying the request.. If the client does
       retry the request, the client

         o  MUST first send the request header fields, and then

         o  MUST wait for the server to respond with either a 100 (Continue)
            response, in which case the client should continue, or with an
            error status.

       If an HTTP/1.1 client has not seen an HTTP/1.1 or later response from
       the server, it should assume that the server implements HTTP/1.0 or
       older and will not use the 100 (Continue) response. If in this case
       the client sees the connection close before receiving any status from
       the server, the client SHOULD retry the request. If the client does
       retry the request to this HTTP/1.0 server, it should 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)

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      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.

       No matter what the server version, if an error status is received,
       the client

      o  MUST NOT continue and

      o  MUST close the connection if it has not completed sending the
         message.

       An HTTP/1.1 (or later) client that sees the connection close after
       receiving a 100 (Continue) but before receiving any other status
       SHOULD retry the request, and need not wait for 100 (Continue)
       response (but MAY do so if this simplifies the implementation).

    9 Method Definitions

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

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

    9.1 Safe and Idempotent Methods

    9.1.1 Safe Methods

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

       In particular, the convention has been established that the GET and
       HEAD methods should never have the significance of taking an action
       other than retrieval. These methods should be considered "safe." This
       allows user agents to represent other methods, such as POST, PUT and
       DELETE, in a special way, so that the user is made aware of the fact
       that a possibly unsafe action is being requested.

       Naturally, it is not possible to ensure that the server does not
       generate side-effects as a result of performing a GET request; in

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       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.

    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.

       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.

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    9.3 GET

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

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

       The semantics of the GET method change to a "partial GET" if the
       request message includes a Range header field. A partial GET requests
       that only part of the entity be transferred, as described in section
       14.36. The partial GET method is intended to reduce unnecessary
       network usage by 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.

    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.

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    9.5 POST

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

         o  Annotation of existing resources;

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

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

         o  Extending a database through an append operation.

       The actual function performed by the POST method is determined by the
       server and is usually dependent on the Request-URI. The 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.

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    9.6 PUT

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

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

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

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    9.7 DELETE

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

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

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

    9.8 TRACE

       The TRACE method is used to invoke a remote, application-layer loop-
       back of the request message. The final recipient of the request
       SHOULD reflect the message received back to the client as the
       entity-body of a 200 (OK) response. The final recipient is either the
       origin server or the first proxy or gateway to receive a Max-Forwards
       value of zero (0) in the request (see section 14.31). A TRACE request
       MUST NOT include an entity.

       TRACE allows the client to see what is being received at the other
       end of the request chain and use that data for testing or diagnostic
       information. The value of the Via header field (section 14.44) is of
       particular interest, since it acts as a trace of the request chain.
       Use of the Max-Forwards header field allows the client to limit the
       length of the request chain, which is useful for testing a chain of
       proxies forwarding messages in an infinite loop.

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

    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. 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.

    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.

    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.

    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;

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       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 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

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       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. The response MUST include 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.

       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.

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    10.3.1 300 Multiple Choices

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

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

       If the server has a preferred choice of representation, it SHOULD
       include the specific 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.

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    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 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.

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       The response MUST include the following header fields:

      o  Date

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

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

       If the 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 requested resource MUST be accessed through the proxy given by
       the Location field. The Location field gives the URL of the proxy.
       The recipient is expected to repeat the request via the proxy.

    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

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         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 information. HTTP access authentication is
       explained in section 11.

    10.4.3 402 Payment Required

       This code is reserved for future use.

    10.4.4 403 Forbidden

       The server understood the request, but is refusing to fulfill it.
       Authorization will not help and the request SHOULD NOT be repeated.
       If the request method was not HEAD and the server wishes to make
       public why the request has not been fulfilled, it SHOULD describe the
       reason for the refusal in the entity. 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.

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       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.

       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.

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    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 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.

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    10.4.12 411 Length Required

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

    10.4.13 412 Precondition Failed

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

    10.4.14 413 Request Entity Too Large

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

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

    10.4.15 414 Request-URI Too Long

       The server is refusing to service the request because the Request-URI
       is longer than the server is willing to interpret. This rare
       condition is only likely to occur when a client has improperly
       converted a POST request to a GET request with long query
       information, when the client has descended into a 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.

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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.

    11 Access Authentication

       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.

              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 a server--
       usually, but not necessarily, after receiving a 401 or 411 response-
       -MAY do so by including an Authorization header field with the
       request. The Authorization field value consists of credentials

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       containing the authentication information of the user agent 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
       user agent 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 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 the (possibly
       new) challenge applicable to the requested resource and an entity
       explaining the refusal.

       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. That is, they MUST forward the WWW-Authenticate and
       Authorization headers untouched, and follow the rules found in
       section 14.8.

       HTTP/1.1 allows a client to pass authentication information to and
       from a proxy via the Proxy-Authenticate and Proxy-Authorization
       headers.

    11.1 Basic Authentication Scheme

       The "basic" authentication scheme is based on the model that the user
       agent 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.

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       Upon receipt of an unauthorized request for a URI within the
       protection space, the 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.

       To receive authorization, the client sends the userid and password,
       separated by a single colon (":") character, within a base64  encoded
       string in the credentials.

              basic-credentials = "Basic" SP basic-cookie

              basic-cookie   = <base64 [7] encoding of user-pass,
                               except not limited to 76 char/line>

              user-pass   = userid ":" password

              userid      = *<TEXT excluding ":">

              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==

       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

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       when there are multiple representations available.

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

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

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

    12.1 Server-driven Negotiation

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

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

       Server-driven negotiation has disadvantages:

    1. It is impossible for the server to accurately determine what might be
      "best" for any given user, since that would require complete
      knowledge of both the capabilities of the user agent and the 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

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      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.

       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 19.6.2.1) 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.

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

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

    12.3 Transparent Negotiation

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

       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,

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       headers, response codes, etc.

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

       Requirements for performance, availability, and disconnected
       operation require us to be able to relax the goal of semantic
       transparency. The HTTP/1.1 protocol allows origin servers, caches,
       and clients to explicitly reduce transparency when necessary.
       However, because non-transparent operation may confuse non-expert
       users, and may be incompatible with certain server applications (such
       as those for ordering merchandise), the protocol requires that
       transparency be relaxed

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

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

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       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, server, and cache (see section 14.9); if the origin server
         so specifies, it is the freshness requirement of the origin server
         alone.

      3. It includes a warning if the freshness demand of the client or the
         origin server is violated (see section 13.1.5 and 14.45).

      4. 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

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       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 are always cachable, because they never weaken the
       transparency of a response. This means that warnings can be passed to
       HTTP/1.0 caches without danger; such caches will simply pass the
       warning along as an entity-header in the response.

       Warnings are assigned numbers between 0 and 99. This specification
       defines the code numbers and meanings of each currently assigned
       warnings, allowing a client or cache to take automated action in some
       (but not all) cases.

       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.

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       The Warning header and the currently defined warnings are described
       in section 14.45.

    13.1.3 Cache-control Mechanisms

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

       The Cache-Control header allows a client or server to transmit a
       variety of directives in either requests or responses. These
       directives typically override the default caching algorithms. As a
       general rule, if there is any apparent conflict between header
       values, the most restrictive interpretation should be applied (that
       is, the one that is most likely to preserve semantic transparency).
       However, in some cases, Cache-Control directives are explicitly
       specified as weakening the approximation of semantic transparency
       (for example, "max-stale" or "public").

       The Cache-Control directives are described in detail in section 14.9.

    13.1.4 Explicit User Agent Warnings

       Many user agents make it possible for users to override the basic
       caching mechanisms. For example, the user agent may allow the user to
       specify that cached entities (even explicitly stale ones) are never
       validated. Or the user agent might habitually add "Cache-Control:
       max-stale=3600" to every request. The user should have to explicitly
       request either non-transparent behavior, or behavior that results in
       abnormally ineffective caching.

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

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    13.1.5 Exceptions to the Rules and Warnings

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

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

    13.1.6 Client-controlled Behavior

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

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

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

    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

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       response without first contacting the server.

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

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

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

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

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

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

    13.2.2 Heuristic Expiration

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

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    13.2.3 Age Calculations

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

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

       Also note that HTTP/1.1 requires origin servers to send a Date header
       with every response, giving the time at which the response was
       generated. We use the term "date_value" to denote the value of the
       Date header, in a form appropriate for arithmetic operations.

       HTTP/1.1 uses the Age response-header to 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-

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       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 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);

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          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 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 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

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       such warning has not already been added.

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

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

             response_is_fresh = (freshness_lifetime > current_age)

    13.2.5 Disambiguating Expiration Values

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

       If a client performing a retrieval receives a non-first-hand response
       for a request that was already fresh in its own cache, and the Date
       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.

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

              Cache-Control: max-age=0

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

              Cache-Control: no-cache

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

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

    13.3 Validation Model

       When a cache has a stale entry that it would like to use as a
       response to a client's request, it first has to check with the origin
       server (or 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.

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         Note: the comparison functions used to decide if validators match
         are defined in section 13.3.3.

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

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

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

    13.3.1 Last-modified Dates

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

    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

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       insignificant aspects of the entity change. A validator that does not
       always change when the resource changes is a "weak validator."

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

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

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

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

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

       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:

      o  The strong comparison function: in order to be considered equal,
         both validators must be identical in every way, and neither may be
         weak.
      o  The weak comparison function: in order to be considered equal, both
         validators must be identical in every way, but either or both of
         them may be tagged as "weak" without affecting the result.

       The weak comparison function MAY be used for simple (non-subrange)

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       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:

      o  The validator is being compared by an origin server to the actual
         current validator for the entity and,
      o  That origin server reliably knows that the associated entity did
         not change twice during the second covered by the presented
         validator.
    or

      o  The validator is about to be used by a client in an If-Modified-
         Since or If-Unmodified-Since header, because the client has a cache
         entry for the associated entity, and
      o  That cache entry includes a Date value, which gives the time when
         the origin server sent the original response, and
      o  The presented Last-Modified time is at least 60 seconds before the
         Date value.
    or

      o  The validator is being compared by an intermediate cache to the
         validator stored in its cache entry for the entity, and
      o  That cache entry includes a Date value, which gives the time when
         the origin server sent the original response, and
      o  The presented Last-Modified time is at least 60 seconds before the
         Date value.

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

       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.

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

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

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

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

       HTTP/1.1 origin servers:

      o  SHOULD send an entity tag validator unless it is not feasible to
         generate one.
      o  MAY send a weak entity tag instead of a strong entity tag, if
         performance considerations support the use of weak entity tags, or
         if it is unfeasible to send a strong entity tag.
      o  SHOULD send a Last-Modified value if it is feasible to send one,
         unless the risk of a breakdown in semantic transparency that could
         result from using this date in an If-Modified-Since header would
         lead to serious problems.

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

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

         Note: in order to provide semantically transparent caching, an
         origin server must avoid reusing a specific strong entity tag value
         for two different entities, or reusing a specific weak entity tag
         value for two semantically different entities. Cache entries 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:

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

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         o  If only a Last-Modified value has been provided by the origin
            server, SHOULD use that value in non-subrange cache-conditional
            requests (using If-Modified-Since).
         o  If only a Last-Modified value has been provided by an HTTP/1.0
            origin server, MAY use that value in subrange cache-conditional
            requests (using If-Unmodified-Since:). The user agent should
            provide a way to disable this, in case of difficulty.
         o  If both an entity tag and a Last-Modified value have been
            provided by the origin server, SHOULD use both validators in
            cache-conditional requests. This allows both HTTP/1.0 and
            HTTP/1.1 caches to respond appropriately.

       An HTTP/1.1 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

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       there is neither a cache validator nor an explicit expiration time
       associated with a response, we do not expect it to be cached, but
       certain caches may violate this expectation (for example, when little
       or no network connectivity is available). A client can usually detect
       that such a response was taken from a cache by comparing the Date
       header to the current time.

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

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

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

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

       A response received with any other status code MUST NOT be returned
       in a reply to a subsequent request unless there are Cache-Control
       directives or another header(s) that explicitly allow it. For
       example, these include the following: an Expires header (section
       14.21); a "max-age", "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.

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

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

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

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

         o  Connection
         o  Keep-Alive
         o  Public
         o  Proxy-Authenticate
         o  Transfer-Encoding
         o  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:

         o  Content-Location
         o  ETag
         o  Expires
         o  Last-Modified

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       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:

         o  Content-Encoding
         o  Content-Length
         o  Content-Range
         o  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, the cache must construct a
       response to send to the requesting client. The cache uses the
       entity-body stored in the cache entry as the entity-body of this
       outgoing response. The end-to-end headers stored in the cache entry
       are used for the constructed response, except that any end-to-end
       headers provided in the 304 response MUST replace the corresponding
       headers from the cache entry. Unless the cache decides to remove the
       cache entry, it MUST also replace the end-to-end headers stored with
       the cache entry with corresponding headers received in the incoming
       response.

       In other words, the set of end-to-end headers received in the
       incoming response overrides all corresponding end-to-end headers
       stored with the cache entry. The cache may add Warning headers (see
       section 14.45) to this set.

       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)
         response to update any header associated with a previous response
         for the same entity, 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) response to entirely delete a header that it
         had provided with a previous response.

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    13.5.4 Combining Byte Ranges

       A response 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:

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

    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

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       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.

    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,

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       or act as if the server failed to respond. In the latter case, it MAY
       return a previously received response unless the cached entry
       includes the "must-revalidate" Cache-Control directive (see section
       14.9).

    13.9 Side Effects of GET and HEAD

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

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

    13.10 Invalidation After Updates or Deletions

       The effect of certain methods 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.

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       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 response-headers (if present). These methods are:

         o  PUT
         o  DELETE
         o  POST

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

    13.11 Write-Through Mandatory

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

       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.

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

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

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

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

    14 Header Field Definitions

       This section defines the syntax and semantics of all standard
       HTTP/1.1 header fields. For entity-header fields, both sender and
       recipient refer to either the client or the server, depending on who
       sends and who receives the entity.

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    14.1 Accept

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

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

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

              accept-params  = ";" "q" "=" qvalue *( accept-extension )

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

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

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

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

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

       A more elaborate example is

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

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

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

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

       have the following precedence:

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

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

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

       would cause the following values to be associated:

              text/html;level=1         = 1
              text/html                 = 0.7
              text/plain                = 0.3
              image/jpeg                = 0.5
              text/html;level=2         = 0.4
              text/html;level=3         = 0.7

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

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         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

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

    14.3 Accept-Encoding

       The Accept-Encoding request-header field is similar to Accept, but
       restricts the content-coding values (section 14.12) which are
       acceptable in the response.

              Accept-Encoding  = "Accept-Encoding" ":"
                                        #( content-coding )

       An example of its use is

              Accept-Encoding: compress, gzip

       If no Accept-Encoding header is present in a request, the server MAY
       assume that the client will accept any content coding. If an Accept-
       Encoding header is present, 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.

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       An empty Accept-Encoding value indicates none are acceptable.

    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 "*", 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

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       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.

    14.5 Accept-Ranges

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

              Accept-Ranges     = "Accept-Ranges" ":" acceptable-ranges

              acceptable-ranges = 1#range-unit | "none"

       Origin servers that accept byte-range requests MAY send

              Accept-Ranges: bytes

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

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

              Accept-Ranges: none

       to advise the client not to attempt a range request.

    14.6 Age

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

               Age = "Age" ":" age-value

               age-value = delta-seconds

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

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       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

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

              Allow          = "Allow" ":" 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.

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              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 "proxy-revalidate" Cache-Control
            directive, the cache MAY use that response in replying to a
            subsequent request, but 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.
         2. If the response includes the "must-revalidate" Cache-Control
            directive, the cache MAY use that response in replying to a
            subsequent request, but 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

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              cache-request-directive =
                                "no-cache" [ "=" <"> 1#field-name <"> ]
                              | "no-store"
                              | "max-age" "=" delta-seconds
                              | "max-stale" [ "=" delta-seconds ]
                              | "min-fresh" "=" delta-seconds
                              | "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
                              | 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:

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

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    14.9.1 What is Cachable

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

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

    private
      Indicates that all or part of the response message is intended for a
      single user and MUST NOT be cached by a shared cache. This allows an
      origin server to state that the specified parts of the response are
      intended for only one user and are not a valid response for requests
      by other users. A private (non-shared) cache may cache the response.

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

    no-cache
      Indicates that all or part of the response message MUST NOT be cached
      anywhere. This allows an origin server to prevent caching even by
      caches that have been configured to return stale responses to client
      requests.

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

    14.9.2 What May be Stored by Caches

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

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       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.

       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.

         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

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         specified time in seconds. That is, the client wants a response
         that will still be fresh for at least the specified number of
         seconds.

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

       If a cache returns a stale response, either because of a max-stale
       directive on a request, or because the cache is configured to
       override the expiration time of a response, the cache MUST attach a
       Warning header to the stale response, using Warning 10 (Response is
       stale).

    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 the next cache or server. The initial

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         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), the protocol also includes a

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       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 response 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.  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.

    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, 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

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       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.

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

              Content-Language: da

       If no Content-Language is specified, the default is that the content
       is intended for all language audiences. This may mean that the sender

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       does not consider it to be specific to any natural language, or that
       the sender does not know for which language it is intended.

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

              Content-Language: mi, en

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

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

    14.14 Content-Length

       The Content-Length entity-header field indicates the size of the
       message-body, in decimal number of octets, sent to the recipient or,
       in the case of the HEAD method, the size of the entity-body that
       would have been sent had the request been a GET.

              Content-Length    = "Content-Length" ":" 1*DIGIT

       An example is

              Content-Length: 3495

       Applications SHOULD use this field to indicate the size of the
       message-body to be transferred, regardless of the media type of the
       entity. It must be possible for the recipient to reliably determine
       the end of HTTP/1.1 requests containing an entity-body, e.g., because
       the request 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.

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         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. In the case
       where a resource has multiple entities associated with it, and those
       entities actually have separate locations by which they might be
       individually accessed, the server should provide a Content-Location
       for the particular variant which is returned. In addition, a server
       SHOULD provide a Content-Location for the resource corresponding to
       the response entity.

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

       If no Content-Base header field is present, the value of Content-
       Location also defines the base URL for the entity (see section
       14.11).

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

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

       If the Content-Location is a relative URI, the URI is interpreted
       relative to any Content-Base URI provided in the response. If no
       Content-Base is provided, the relative URI is interpreted relative to
       the Request-URI.

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    14.16 Content-MD5

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

               Content-MD5   = "Content-MD5" ":" md5-digest

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

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

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

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

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

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

         Note: while the definition of Content-MD5 is exactly the same for
         HTTP as in RFC 1864 for MIME entity-bodies, there are several ways

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         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

       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 also indicates the total size of the full entity-body.
       When a server returns a partial response to a client, it must
       describe both the extent of the range covered by the response, and
       the length of the entire entity-body.

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

              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

       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.

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

         o  The first 500 bytes:

              bytes 0-499/1234

         o  The second 500 bytes:

              bytes 500-999/1234

         o  All except for the first 500 bytes:

              bytes 500-1233/1234

         o  The last 500 bytes:

              bytes 734-1233/1234

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

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

       When an HTTP message includes the content of multiple ranges (for
       example, a response to a request for multiple non-overlapping
       ranges), these are transmitted as a multipart 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.

       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 invalid, the
       server should treat the request as if the invalid Range header field

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       did not exist. (Normally, this means return a 200 response containing
       the full entity). The reason is that the only time a client will make
       such an invalid request is when the entity is smaller than the entity
       retrieved by a prior request.

    14.18 Content-Type

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

              Content-Type   = "Content-Type" ":" media-type
       Media types are defined in section 3.7. An example of the field is

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

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

    14.19 Date

       The Date general-header field represents the date and time at which
       the message was originated, having the same semantics as orig-date in
       RFC 822. The field value is an HTTP-date, as described in section
       3.3.1.

              Date  = "Date" ":" HTTP-date

       An example is

              Date: Tue, 15 Nov 1994 08:12:31 GMT

       If a message is received via direct connection with the user agent
       (in the case of requests) or the origin server (in the case of
       responses), then the date can be assumed to be the current date at
       the receiving end. However, since the date--as it is believed by the
       origin--is important for evaluating cached responses, origin servers
       MUST include a Date header field in all responses. 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 received message which does not have a Date header
       field SHOULD be assigned one by the recipient if the message will be
       cached by that recipient or gatewayed via a protocol which requires a
       Date.

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       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.

       The format of the Date is an absolute date and time as defined by
       HTTP-date in section 3.3; it MUST be sent in RFC1123 [8]-date format.

    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.

       The format is an absolute date and time as defined by HTTP-date in
       section 3.3; it MUST be in RFC1123-date format:

             Expires = "Expires" ":" HTTP-date

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       An example of its use is

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

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

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

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

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

       The presence of an Expires header field with a date value of some
       time in the future on an response that otherwise would by default be
       non-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 (as updated by RFC 1123 ):

              From   = "From" ":" mailbox

       An example is:

              From: webmaster@w3.org

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

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       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

       The Host request-header field specifies the Internet host and port
       number of the resource being requested, as obtained from the original
       URL given by the user or referring resource (generally an HTTP URL,
       as described in section 3.2.2). The Host field value MUST represent
       the network location of the origin server or gateway given by the
       original URL. This allows the origin server or gateway to
       differentiate between internally-ambiguous URLs, such as the root "/"
       URL of a server for multiple host names on a single IP address.

              Host = "Host" ":" host [ ":" port ]    ; Section 3.2.2

       A "host" without any trailing port information implies the default
       port for the service requested (e.g., "80" for an HTTP URL). For
       example, a request on the origin server for
       <http://www.w3.org/pub/WWW/> MUST include:

              GET /pub/WWW/ HTTP/1.1
              Host: www.w3.org

       A client MUST include a Host header field in all HTTP/1.1 request
       messages on the Internet (i.e., on any message corresponding to a
       request for a URL which includes an Internet host address for the
       service being requested). If the Host field is not already present,
       an HTTP/1.1 proxy MUST add a Host field to the request message prior
       to forwarding it on the Internet. All Internet-based HTTP/1.1 servers
       MUST respond with a 400 status code to any HTTP/1.1 request message
       which lacks a Host header field.

       See sections 5.2 and 19.5.1 for other requirements relating to Host.

    14.24 If-Modified-Since

       The If-Modified-Since request-header field is used with the GET
       method to make it conditional: if the requested variant has not been
       modified since the time specified in this field, an entity will not

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       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.

         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.

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    14.25 If-Match

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

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

       If any of the entity tags match the entity tag of the entity that
       would have been returned in the response to a similar GET request
       (without the If-Match header) on that resource, or if "*" is given
       and any current entity exists for that resource, then the server MAY
       perform the requested method as if the If-Match header field did not
       exist.

       A server MUST use the strong comparison function (see section 3.11)
       to compare the entity tags in If-Match.

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

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

       The meaning of "If-Match: *" is that the method SHOULD be performed
       if the representation selected by the origin server (or by a cache,
       possibly using the Vary mechanism, see section 14.43) exists, and
       MUST NOT be performed if the representation does not exist.

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       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 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.

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       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 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.

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

    14.28 If-Unmodified-Since

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

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

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

       An example of the field is:

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

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

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

    14.29 Last-Modified

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

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

       An example of its use is

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

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

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

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

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

    14.30 Location

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

              Location       = "Location" ":" absoluteURI

       An example is

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

         Note: The Content-Location header field (section 14.15) differs
         from Location in that the Content-Location identifies the original
         location of the entity enclosed in the request. It is therefore
         possible for a response to contain header fields for both Location
         and Content-Location. Also see section 13.10 for cache requirements
         of some methods.

    14.31 Max-Forwards

       The Max-Forwards request-header field may be used with the TRACE
       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

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       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.

              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.

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       HTTP/1.1 clients SHOULD NOT send the Pragma request-header. HTTP/1.1
       caches SHOULD treat "Pragma: no-cache" as if the client had sent
       "Cache-Control: no-cache". No new Pragma directives will be defined
       in HTTP.

    14.33 Proxy-Authenticate

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

              Proxy-Authenticate  = "Proxy-Authenticate" ":" challenge

       The HTTP access authentication process is described in section 11.
       Unlike WWW-Authenticate, the Proxy-Authenticate header field applies
       only to the current connection and SHOULD NOT be passed on to
       downstream clients. However, an intermediate proxy may need to obtain
       its own credentials by requesting them from the downstream client,
       which in some circumstances will appear as if the proxy is forwarding
       the Proxy-Authenticate header field.

    14.34 Proxy-Authorization

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

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

       The HTTP access authentication process is described in section 11.
       Unlike Authorization, the Proxy-Authorization header field applies
       only 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

       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

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       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.

    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.

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       If the last-byte-pos value is present, it must be greater than or
       equal to the first-byte-pos in that byte-range-spec, or the byte-
       range-spec is 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

       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):

         o  The first 500 bytes (byte offsets 0-499, inclusive):

              bytes=0-499

         o  The second 500 bytes (byte offsets 500-999, inclusive):

              bytes=500-999

         o  The final 500 bytes (byte offsets 9500-9999, inclusive):

              bytes=-500

         o  Or

              bytes=9500-

         o  The first and last bytes only (bytes 0 and 9999):

              bytes=0-0,-1

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         o  Several legal but not canonical specifications of the second
            500 bytes (byte offsets 500-999, inclusive):

              bytes=500-600,601-999

              bytes=500-700,601-999

    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:

         o  The presence of a Range header in an unconditional GET modifies
            what is returned if the GET is otherwise successful. In other
            words, the response carries a status code of 206 (Partial
            Content) instead of 200 (OK).

         o  The presence of a Range header in a conditional GET (a request
            using one or both of If-Modified-Since and If-None-Match, or
            one or both of If-Unmodified-Since and If-Match) modifies what
            is returned if the GET is otherwise successful and the condition
            is true. It does not affect the 304 (Not Modified) response
            returned if the conditional is false.

       In some cases, it 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.

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    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.

         Note: 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.

    14.38 Retry-After

       The Retry-After response-header field can be used with a 503 (Service
       Unavailable) response to indicate how long the service is expected to
       be unavailable to the requesting client. 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.

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    14.39 Server

       The Server response-header field contains information about the
       software used by the origin server to handle the request. The field
       can contain multiple product tokens (section 3.8) and comments
       identifying the server and any significant subproducts. The product
       tokens are listed in order of their significance for identifying the
       application.

              Server         = "Server" ":" 1*( product | comment )

       Example:

              Server: CERN/3.0 libwww/2.17

       If the response is being forwarded through a proxy, the proxy
       application MUST NOT modify the Server response-header. Instead, it
       SHOULD include a Via field (as described in section 14.44).

         Note: Revealing the specific software version of the server may
         allow the server machine to become more vulnerable to attacks
         against software that is known to contain security holes. Server
         implementers are encouraged to make this field a configurable
         option.

    14.40 Transfer-Encoding

       The Transfer-Encoding general-header field indicates what (if any)
       type of transformation has been applied to the message body in order
       to safely transfer it between the sender and the recipient. This
       differs from the Content-Encoding in that the transfer coding is a
       property of the message, not of the entity.

              Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-
       coding

       Transfer codings are defined in section 3.6. An example is:

              Transfer-Encoding: chunked

       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

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       MUST use the Upgrade header field within a 101 (Switching Protocols)
       response to indicate which protocol(s) are being switched.

              Upgrade        = "Upgrade" ":" 1#product

       For example,

              Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11

       The Upgrade header field is intended to provide a simple mechanism
       for transition from HTTP/1.1 to some other, incompatible protocol. It
       does so by allowing the client to advertise its desire to use another
       protocol, such as a later version of HTTP with a higher major version
       number, even though the current request has been made using HTTP/1.1.
       This eases the difficult transition between incompatible protocols by
       allowing the client to initiate a request in the more commonly
       supported protocol while indicating to the server that it would like
       to use a "better" protocol if available (where "better" is determined
       by the server, possibly according to the nature of the method and/or
       resource being requested).

       The Upgrade header field only applies to switching application-layer
       protocols upon the existing transport-layer connection. Upgrade
       cannot be used to insist on a protocol change; its acceptance and use
       by the server is optional. The capabilities and nature of the
       application-layer communication after the protocol change is entirely
       dependent upon the new protocol chosen, although the first action
       after changing the protocol MUST be a response to the initial HTTP
       request containing the Upgrade header field.

       The Upgrade header field only applies to the immediate connection.
       Therefore, the upgrade keyword MUST be supplied within a Connection
       header field (section 14.10) whenever Upgrade is present in an
       HTTP/1.1 message.

       The Upgrade header field cannot be used to indicate a switch to a
       protocol on a different connection. For that purpose, it is more
       appropriate to use a 301, 302, 303, or 305 redirection response.

       This specification only defines the protocol name "HTTP" for use by
       the family of Hypertext Transfer Protocols, as defined by the HTTP
       version rules of section 3.1 and future updates to this
       specification. Any token can be used as a protocol name; however, it
       will only be useful if both the client and server associate the name
       with the same protocol.

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    14.42 User-Agent

       The User-Agent request-header field contains information about the
       user agent originating the request. This is for statistical purposes,
       the tracing of protocol violations, and automated recognition of user
       agents for the sake of tailoring responses to avoid particular user
       agent limitations. User agents SHOULD include this field with
       requests. The field can contain multiple product tokens (section 3.8)
       and comments identifying the agent and any subproducts which form a
       significant part of the user agent. By convention, the product tokens
       are listed in order of their significance for identifying the
       application.

              User-Agent     = "User-Agent" ":" 1*( product | comment )

       Example:

              User-Agent: CERN-LineMode/2.15 libwww/2.17b3

    14.43 Vary

       The Vary 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 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

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       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 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 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.

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          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.

       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.

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

              Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

               could be collapsed to

              Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

       Applications SHOULD NOT combine multiple entries unless they are all
       under the same organizational control and the hosts have already been
       replaced by pseudonyms. Applications MUST NOT combine entries which
       have different received-protocol values.

    14.45 Warning

       The Warning response-header field is used to carry additional
       information about the status of a response which may not be reflected
       by the response status code. This information is typically, though
       not exclusively, used to warn about a possible lack of semantic
       transparency from caching operations.

       Warning headers are sent with responses using:

              Warning    = "Warning" ":" 1#warning-value

              warning-value = warn-code SP warn-agent SP warn-text
              warn-code  = 2DIGIT
              warn-agent = ( host [ ":" port ] ) | pseudonym
                              ; the name or pseudonym of the server adding
                              ; the Warning header, for use in debugging
              warn-text  = quoted-string

       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 1522 [14].

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       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:

         o  Warnings that appear early in the response take priority over those
            appearing later in the response.
         o  Warnings in the user's preferred character set take priority over
            warnings in other character sets but with identical warn-codes and
            warn-agents.

       Systems that generate multiple Warning headers should order them with
       this user agent behavior in mind.

       This is a list of the currently-defined warn-codes, each with a
       recommended warn-text in English, and a description of its meaning.

    10 Response is stale
      MUST be included whenever the returned response is stale. A cache may
      add this warning to any response, but may never remove it until the
      response is known to be fresh.

    11 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. A cache may add this warning to any response, but
      may never remove it until the response is successfully revalidated.

    12 Disconnected operation
       SHOULD be included if the cache is intentionally disconnected from
      the rest of the network for a period of time.

    13 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.

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    14 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. MUST NOT be deleted from a response
      even after revalidation.

    99 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.

    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.

    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 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.

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       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
       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

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       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 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.

    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.

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       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 example, UNIX, Microsoft Windows, and
       other operating systems use ".." as a path component to indicate a
       directory level above the current one. On such a system, an HTTP
       server MUST disallow any such construct in the Request-URI if it
       would otherwise allow access to a resource outside those intended to
       be accessible via the HTTP server. Similarly, files intended for
       reference only internally to the server (such as access control
       files, configuration files, and script code) MUST be protected from
       inappropriate retrieval, since they might contain sensitive
       information. Experience has shown that minor bugs in such HTTP server
       implementations have turned into security risks.

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    15.6 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 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.

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    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, the
       possibility of this form of attack will grow. Observing this
       requirement thus reduces this potential security vulnerability.

       This requirement also improves the load-balancing behavior of clients
       for replicated servers using the same DNS name and reduces the
       likelihood of a user's experiencing failure in accessing sites which
       use that strategy.

    15.9 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.

    16 Acknowledgments

       This specification makes heavy use of the augmented BNF and generic
       constructs defined by David H. Crocker for RFC 822. Similarly, it
       reuses many of the definitions provided by Nathaniel Borenstein and
       Ned Freed for MIME. We hope that their inclusion in this
       specification will help reduce past confusion over the relationship
       between HTTP and Internet mail message formats.

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       The HTTP protocol has evolved considerably over the past four years.
       It has benefited from a large and active developer community--the
       many people who have participated on the www-talk mailing list--and
       it is that community which has been most responsible for the success
       of HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
       Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
       Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
       McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
       VanHeyningen deserve special recognition for their efforts in
       defining early aspects of the protocol.

       This document has benefited greatly from the comments of all those
       participating in the HTTP-WG. In addition to those already mentioned,
       the following individuals have contributed to this specification:

              Gary Adams                  Albert Lunde
              Harald Tveit Alvestrand     John C. Mallery
              Keith Ball                  Jean-Philippe Martin-Flatin
              Brian Behlendorf            Larry Masinter
              Paul Burchard               Mitra
              Maurizio Codogno            David Morris
              Mike Cowlishaw              Gavin Nicol
              Roman Czyborra              Bill Perry
              Michael A. Dolan            Jeffrey Perry
              David J. Fiander            Scott Powers
              Alan Freier                 Owen Rees
              Marc Hedlund                Luigi Rizzo
              Greg Herlihy                David Robinson
              Koen Holtman                Marc Salomon
              Alex Hopmann                Rich Salz
              Bob Jernigan                Allan M. Schiffman
              Shel Kaphan                 Jim Seidman
              Rohit Khare                 Chuck Shotton
              John Klensin                Eric W. Sink
              Martijn Koster              Simon E. Spero
              Alexei Kosut                Richard N. Taylor
              David M. Kristol            Robert S. Thau
              Daniel LaLiberte            Bill (BearHeart) Weinman
              Ben Laurie                  Francois Yergeau
              Paul J. Leach               Mary Ellen Zurko
              Daniel DuBois

       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.

    Fielding, et. al.           Standards Track                   [Page 145]

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       Most of the specification of ranges is based on work originally done
       by Ari Luotonen and John Franks, with additional input from Steve
       Zilles.

       Thanks to the "cave men" of Palo Alto. You know who you are.

       Jim Gettys (the current editor of this document) wishes particularly
       to thank Roy Fielding, the previous editor of this document, along
       with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
       Holtman, John Franks, 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.

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

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    RFC 2068                        HTTP/1.1                    January 1997

       [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, AT&T Bell Laboratories, Center for Seismic
       Studies, December 1987.

       [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.

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         Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988.
         Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987.
         Part 7: Latin/Greek alphabet, ISO 8859-7, 1987.
         Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988.
         Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.

       [23] Meyers, J., and M. Rose "The Content-MD5 Header Field", RFC
       1864, Carnegie Mellon, Dover Beach Consulting, October, 1995.

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

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

       [26] Venkata N. Padmanabhan and Jeffrey C. Mogul. Improving HTTP
       Latency. Computer Networks and ISDN Systems, v. 28, pp. 25-35, Dec.
       1995.  Slightly revised version of paper in Proc. 2nd International
       WWW Conf. '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/ib/http-perf/>,
       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] Spero, S., "Analysis of HTTP Performance Problems"
       <URL:http://sunsite.unc.edu/mdma-release/http-prob.html>.

       [31] Deutsch, P., and J-L. Gailly, "ZLIB Compressed Data Format
       Specification version 3.3", RFC 1950, Aladdin Enterprises, Info-ZIP,
       May 1996.

       [32] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
       Luotonen, A., Sink, E., and L. Stewart, "An Extension to HTTP :
       Digest Access Authentication", RFC 2069, January 1997.

    Fielding, et. al.           Standards Track                   [Page 148]

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    18 Authors' Addresses

       Roy T. Fielding
       Department of Information and Computer Science
       University of California
       Irvine, CA 92717-3425, USA

       Fax: +1 (714) 824-4056
       EMail: fielding@ics.uci.edu

       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

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    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.

           Media Type name:         message
           Media subtype name:      http
           Required parameters:     none
           Optional parameters:     version, msgtype

            version: The HTTP-Version number of the enclosed message
                     (e.g., "1.1"). If not present, the version can be
                     determined from the first line of the body.

            msgtype: The message type -- "request" or "response". If not
                     present, the type can be determined from the first
                     line of the body.

           Encoding considerations: only "7bit", "8bit", or "binary" are
                                    permitted

           Security considerations: none

    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

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    For example:

       HTTP/1.1 206 Partial content
       Date: Wed, 15 Nov 1995 06:25:24 GMT
       Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
       Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES

       --THIS_STRING_SEPARATES
       Content-type: application/pdf
       Content-range: bytes 500-999/8000

       ...the first range...
       --THIS_STRING_SEPARATES
       Content-type: application/pdf
       Content-range: bytes 7000-7999/8000

       ...the second range
       --THIS_STRING_SEPARATES--

    19.3 Tolerant Applications

       Although this document specifies the requirements for the generation
       of HTTP/1.1 messages, not all applications will be correct in their
       implementation. We therefore recommend that operational applications
       be tolerant of deviations whenever those deviations can be
       interpreted unambiguously.

       Clients SHOULD be tolerant in parsing the Status-Line and servers
       tolerant when parsing the Request-Line. In particular, they SHOULD
       accept any amount of SP or HT characters between fields, even though
       only a single SP is required.

       The line terminator for message-header fields is the sequence CRLF.
       However, we recommend that applications, when parsing such headers,
       recognize a single LF as a line terminator and ignore the leading CR.

       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:

      o  HTTP/1.1 clients and caches should assume that an RFC-850 date
         which appears to be more than 50 years in the future is in fact
         in the past (this helps solve the "year 2000" problem).

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      o  An HTTP/1.1 implementation may internally represent a parsed
         Expires date as earlier than the proper value, but MUST NOT
         internally represent a parsed Expires date as later than the
         proper value.

      o  All expiration-related calculations must be done in GMT. The
         local time zone MUST NOT influence the calculation or comparison
         of an age or expiration time.

      o  If an HTTP header incorrectly carries a date value with a time
         zone other than GMT, it must be converted into GMT using the most
         conservative possible conversion.

    19.4 Differences Between HTTP Entities and MIME Entities

       HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC
       822) and the Multipurpose Internet Mail Extensions (MIME ) to allow
       entities to be transmitted in an open variety of representations and
       with extensible mechanisms. However, MIME [7] discusses mail, and
       HTTP has a few features that are different from those described in
       MIME.  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 MIME.
       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

       MIME requires that an Internet mail entity be converted to canonical
       form prior to being transferred.  Section 3.7.1 of this document
       describes the forms allowed for subtypes of the "text" media type
       when transmitted over HTTP. MIME requires that content with a type of
       "text" represent line breaks as CRLF and forbids the use of CR or LF
       outside of line break sequences.  HTTP allows CRLF, bare CR, and bare
       LF to indicate a line break within text content when a message is
       transmitted over HTTP.

       Where it is possible, a proxy or gateway from HTTP to a strict MIME
       environment SHOULD translate all line breaks within the text media
       types described in section 3.7.1 of this document to the MIME
       canonical form of CRLF. Note, however, that this may be complicated
       by the presence of a Content-Encoding and by the fact that HTTP

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       allows the use of some character sets which do not use octets 13 and
       10 to represent CR and LF, as is the case for some multi-byte
       character sets.

    19.4.2 Conversion of Date Formats

       HTTP/1.1 uses a restricted set of date formats (section 3.3.1) to
       simplify the process of date comparison. Proxies and gateways from
       other protocols SHOULD ensure that any Date header field present in a
       message conforms to one of the HTTP/1.1 formats and rewrite the date
       if necessary.

    19.4.3 Introduction of Content-Encoding

       MIME 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
       MIME.)

    19.4.4 No Content-Transfer-Encoding

       HTTP does not use the Content-Transfer-Encoding (CTE) field of MIME.
       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 MIME, 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.

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    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-ext (if any) and CRLF
              while (chunk-size > 0) {
                 read chunk-data and CRLF
                 append chunk-data to entity-body
                 length := length + chunk-size
                 read chunk-size and CRLF
              }
              read entity-header
              while (entity-header not empty) {
                 append entity-header to existing header fields
                 read entity-header
              }
              Content-Length := length
              Remove "chunked" from Transfer-Encoding

    19.4.7 MIME-Version

       HTTP is not a MIME-compliant protocol (see appendix 19.4). However,
       HTTP/1.1 messages may include a single MIME-Version general-header
       field to indicate what version of the MIME protocol was used to
       construct the message. Use of the MIME-Version header field indicates
       that the message is in full compliance with the MIME protocol.
       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.

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    19.5.1 Changes to Simplify Multi-homed Web Servers and Conserve IP
           Addresses

       The requirements that clients and servers support the Host request-
       header, report an error if the Host request-header (section 14.23) is
       missing from an HTTP/1.1 request, and accept absolute URIs (section
       5.1.2) are among the most important changes defined by this
       specification.

       Older HTTP/1.0 clients assumed a one-to-one relationship of IP
       addresses and servers; there was no other established mechanism for
       distinguishing the intended server of a request than the IP address
       to which that request was directed. The changes outlined above will
       allow the Internet, once older HTTP clients are no longer common, to
       support multiple Web sites from a single IP address, greatly
       simplifying large operational Web servers, where allocation of many
       IP addresses to a single host has created serious problems. The
       Internet will also be able to recover the IP addresses that have been
       allocated for the sole purpose of allowing special-purpose domain
       names to be used in root-level HTTP URLs. Given the rate of growth of
       the Web, and the number of servers already deployed, it is extremely
       important that all implementations of HTTP (including updates to
       existing HTTP/1.0 applications) correctly implement these
       requirements:

         o  Both clients and servers MUST support the Host request-header.

         o  Host request-headers are required in HTTP/1.1 requests.

         o  Servers MUST report a 400 (Bad Request) error if an HTTP/1.1
            request does not include a Host request-header.

         o  Servers MUST accept absolute URIs.

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    19.6 Additional Features

       This appendix documents 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.

    19.6.1 Additional Request Methods

    19.6.1.1 PATCH

       The PATCH method is similar to PUT except that the entity contains a
       list of differences between the original version of the resource
       identified by the Request-URI and the desired content of the resource
       after the PATCH action has been applied. The list of differences is
       in a format defined by the media type of the entity (e.g.,
       "application/diff") and MUST include sufficient information to allow
       the server to recreate the changes necessary to convert the original
       version of the resource to the desired version.

       If the request passes through a cache and the Request-URI identifies
       a currently cached entity, that entity MUST be removed from the
       cache.  Responses to this method are not cachable.

       The actual method for determining how the patched resource is placed,
       and what happens to its predecessor, is defined entirely by the
       origin server. If the original version of the resource being patched
       included a Content-Version header field, the request entity MUST
       include a Derived-From header field corresponding to the value of the
       original Content-Version header field. Applications are encouraged to
       use these fields for constructing versioning relationships and
       resolving version conflicts.

       PATCH requests must obey the message transmission requirements set
       out in section 8.2.

       Caches that implement PATCH should invalidate cached responses as
       defined in section 13.10 for PUT.

    19.6.1.2 LINK

       The LINK method establishes one or more Link relationships between
       the existing resource identified by the Request-URI and other
       existing resources. The difference between LINK and other methods

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       allowing links to be established between resources is that the LINK
       method does not allow any message-body to be sent in the request and
       does not directly result in the creation of new resources.

       If the request passes through a cache and the Request-URI identifies
       a currently cached entity, that entity MUST be removed from the
       cache.  Responses to this method are not cachable.

       Caches that implement LINK should invalidate cached responses as
       defined in section 13.10 for PUT.

    19.6.1.3 UNLINK

       The UNLINK method removes one or more Link relationships from the
       existing resource identified by the Request-URI. These relationships
       may have been established using the LINK method or by any other
       method supporting the Link header. The removal of a link to a
       resource does not imply that the resource ceases to exist or becomes
       inaccessible for future references.

       If the request passes through a cache and the Request-URI identifies
       a currently cached entity, that entity MUST be removed from the
       cache.  Responses to this method are not cachable.

       Caches that implement UNLINK should invalidate cached responses as
       defined in section 13.10 for PUT.

    19.6.2 Additional Header Field Definitions

    19.6.2.1 Alternates

       The Alternates response-header field has been proposed as a means for
       the origin server to inform the client about other available
       representations of the requested resource, along with their
       distinguishing attributes, and thus providing a more reliable means
       for a user agent to perform subsequent selection of another
       representation which better fits the desires of its user (described
       as agent-driven negotiation in section 12).

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       The Alternates header field is orthogonal to the Vary header field in
       that both may coexist in a message without affecting the
       interpretation of the response or the available representations. It
       is expected that Alternates will provide a significant improvement
       over the server-driven negotiation provided by the Vary field for
       those resources that vary over common dimensions like type and
       language.

       The Alternates header field will be defined in a future
       specification.

    19.6.2.2 Content-Version

       The Content-Version entity-header field defines the version tag
       associated with a rendition of an evolving entity. Together with the
       Derived-From field described in section 19.6.2.3, it allows a group
       of people to work simultaneously on the creation of a work as an
       iterative process. The field should be used to allow evolution of a
       particular work along a single path rather than derived works or
       renditions in different representations.

              Content-Version = "Content-Version" ":" quoted-string

       Examples of the Content-Version field include:

              Content-Version: "2.1.2"
              Content-Version: "Fred 19950116-12:26:48"
              Content-Version: "2.5a4-omega7"

    19.6.2.3 Derived-From

       The Derived-From entity-header field can be used to indicate the
       version tag of the resource from which the enclosed entity was
       derived before modifications were made by the sender. This field is
       used to help manage the process of merging successive changes to a
       resource, particularly when such changes are being made in parallel
       and from multiple sources.

              Derived-From   = "Derived-From" ":" quoted-string

       An example use of the field is:

              Derived-From: "2.1.1"

       The Derived-From field is required for PUT and PATCH requests if the
       entity being sent was previously retrieved from the same URI and a
       Content-Version header was included with the entity when it was last
       retrieved.

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    19.6.2.4 Link

       The Link entity-header field provides a means for describing a
       relationship between two resources, generally between the requested
       resource and some other resource. An entity MAY include multiple Link
       values. Links at the metainformation level typically indicate
       relationships like hierarchical structure and navigation paths. The
       Link field is semantically equivalent to the <LINK> element in
       HTML.[5]

              Link           = "Link" ":" #("<" URI ">" *( ";" link-param )

              link-param     = ( ( "rel" "=" relationship )
                                 | ( "rev" "=" relationship )
                                 | ( "title" "=" quoted-string )
                                 | ( "anchor" "=" <"> URI <"> )
                                 | ( link-extension ) )

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

              relationship   = sgml-name
                             | ( <"> sgml-name *( SP sgml-name) <"> )

              sgml-name      = ALPHA *( ALPHA | DIGIT | "." | "-" )

       Relationship values are case-insensitive and MAY be extended within
       the constraints of the sgml-name syntax. The title parameter MAY be
       used to label the destination of a link such that it can be used as
       identification within a human-readable menu. The anchor parameter MAY
       be used to indicate a source anchor other than the entire current
       resource, such as a fragment of this resource or a third resource.

       Examples of usage include:

           Link: <http://www.cern.ch/TheBook/chapter2>; rel="Previous"

           Link: <mailto:timbl@w3.org>; rev="Made"; title="Tim Berners-Lee"

       The first example indicates that chapter2 is previous to this
       resource in a logical navigation path. The second indicates that the
       person responsible for making the resource available is identified by
       the given e-mail address.

    19.6.2.5 URI

       The URI header field has, in past versions of this specification,
       been used as a combination of the existing Location, Content-
       Location, and Vary header fields as well as the future Alternates

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       field (above). Its primary purpose has been to include a list of
       additional URIs for the resource, including names and mirror
       locations. However, it has become clear that the combination of many
       different functions within this single field has been a barrier to
       consistently and correctly implementing any of those functions.
       Furthermore, we believe that the identification of names and mirror
       locations would be better performed via the Link header field. The
       URI header field is therefore deprecated in favor of those other
       fields.

              URI-header    = "URI" ":" 1#( "<" URI ">" )

    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:

      o  recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1
         requests;

      o  understand any valid request in the format of HTTP/0.9, 1.0, or
         1.1;

      o  respond appropriately with a message in the same major version used
         by the client.

       And we would expect HTTP/1.1 clients to:

      o  recognize the format of the Status-Line for HTTP/1.0 and 1.1
         responses;

      o  understand any valid response in the format of HTTP/0.9, 1.0, or
         1.1.

       For most implementations of HTTP/1.0, each connection is established
       by the client prior to the request and closed by the server after
       sending the response. A few implementations implement the Keep-Alive
       version of persistent connections described in section 19.7.1.1.

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    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 in addition to the Persist 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.

       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.

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    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.

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