2004/id/draft-ietf-http-v11-spec-02.txt


      HTTP Working Group                                R. Fielding, UC Irvine
      INTERNET-DRAFT                                       H. Frystyk, MIT/LCS
      <draft-ietf-http-v11-spec-02.txt>                T. Berners-Lee, MIT/LCS
                                                                J. Gettys, DEC
                                                         Jeffrey C. Mogul, DEC
      Expires September 23, 1996                                April 23, 1996


                      Hypertext Transfer Protocol -- HTTP/1.1

      Status of this Memo

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

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

      To learn the current status of any Internet-Draft, please check the
      _1id-abstracts.txt_ listing contained in the Internet-Drafts Shadow
      Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
      munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
      ftp.isi.edu (US West Coast).

      Distribution of this document is unlimited. Please send comments to the
      HTTP working group at <http-wg@cuckoo.hpl.hp.com>. Discussions of the
      working group are archived at
      <URL:http://www.ics.uci.edu/pub/ietf/http/>. General discussions about
      HTTP and the applications which use HTTP should take place on the <www-
      talk@w3.org> mailing list.

        NOTE: This specification is for discussion purposes only. It is
        not claimed to represent the consensus of the HTTP working
        group, and contains a number of proposals that either have not
        been discussed or are controversial. The working group is
        discussing significant changes in many areas, including -
        support for caching, persistent connections, range retrieval,
        content negotiation, MIME compatibility, authentication, timing
        of the PUT operation.


      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 (commands). A feature of HTTP
      is the typing and negotiation of data representation, allowing systems
      to be built independently of the data being transferred.

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

      Note to Readers of This Document
      This document is still organized to minimize changes from the previous
      draft, to ease reviewers work in finding new material (and because the
      editor has not had time to reorganize it)..  However, the current
      organization is now quite poor for new readers of this document.  We
      recommend that new readers of this document not read it in the current
      order of presentation, but may want to skip ahead after reading sections
      1-9 and read sections 11, 12 13 and 14 before reading section 10 which
      defines the header field definitions.  Section 10 itself is now also not
      in alphabetical order, again, to avoid renumbering sections to be able
      to easily compare between drafts.

      If you are reading the version of this document showing revision markup,
      note that we've tried to preserve significant changes from the previous
      version, though a few changes may have slipped through unmarked. We make
      no guarantees that all changes have revision marks, though we've tried
      to preserve them as an aid to those who wish to check a specific change
      has been reflected in this draft.

      Note that some sections are still marked as SLUSHY and a few are marked
      FLUID; these are still undergoing drafting.

      Note that text in bold in the text are as yet incompletely resolved
      issues.  Opinions are solicited_


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      Table of Contents


      HYPERTEXT TRANSFER PROTOCOL -- HTTP/1.1................................1

      Status of this Memo....................................................1

      Abstract...............................................................1

      Note to Readers of This Document.......................................2

      Table of Contents......................................................3

      1. Introduction........................................................9
       1.1 Purpose ..........................................................9
       1.2 Requirements .....................................................9
       1.3 Terminology .....................................................10
       1.4 Overall Operation ...............................................12
       1.4 HTTP and MIME ...................................................14

      2. Notational Conventions and Generic Grammar.........................14
       2.1 Augmented BNF ...................................................14
       2.2 Basic Rules .....................................................16

      3. Protocol Parameters................................................18
       3.1 HTTP Version ....................................................18
       3.2 Uniform Resource Identifiers ....................................19
        3.2.1 General Syntax ...............................................19
        3.2.2 http URL .....................................................21
       3.3 Date/Time Formats ...............................................22
        3.3.1 Full Date ....................................................22
        3.3.2 Delta Seconds ................................................24
       3.4 Character Sets ..................................................24
       3.5 Content Codings .................................................25
       3.6 Transfer Codings ................................................26
       3.7 Media Types .....................................................27
        3.7.1 Canonicalization and Text Defaults ...........................28
        3.7.2 Multipart Types ..............................................29
       3.8 Product Tokens ..................................................29
       3.9 Quality Values ..................................................30
       3.10 Language Tags ..................................................30
       3.12 Full Date Values ...............................................31
       3.13 Opaque Validators ..............................................31
       3.14 Variant IDs ....................................................32
       3.15 Validator Sets .................................................32
       3.16 Variant Sets ...................................................32
       3.17 HTTP Protocol Parameters Related to Ranges .....................32
        3.17.1SLUSHY Range Units ...........................................32
        3.17.2 SLUSHY Byte Ranges ..........................................33
        3.17.3 SLUSHY: Content Ranges ......................................34

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      4. HTTP Message.......................................................35
       4.1 Message Types ...................................................35
       4.2 Message Headers .................................................36
       4.3 General Header Fields ...........................................37

      5. Request............................................................38
       5.1 Request-Line ....................................................38
        5.1.1 Method .......................................................38
        5.1.2 Request-URI ..................................................39
       5.2 Request Header Fields ...........................................41

      6. Response...........................................................42
       6.1 Status-Line .....................................................43
        6.1.1 Status Code and Reason Phrase ................................43
       6.2 Response Header Fields ..........................................46

      7. Entity.............................................................46
       7.1 Entity Header Fields ............................................46
       7.2 Entity Body .....................................................47
        7.2.1 Type .........................................................48
        7.2.2 Length .......................................................48

      8. Method Definitions.................................................49
       8.1 OPTIONS .........................................................49
       8.2 GET .............................................................50
       8.3 HEAD ............................................................50
       8.4 POST ............................................................51
        8.4.1 SLUSHY: Entity Transmission Requirements .....................52
       8.5 PUT .............................................................53
       8.9 DELETE ..........................................................54
       8.12 TRACE ..........................................................54

      9. Status Code Definitions............................................55
       9.1 Informational 1xx ...............................................55
       9.2 Successful 2xx ..................................................56
       9.3 Redirection 3xx .................................................58
       9.4 Client Error 4xx ................................................60
       9.5 Server Error 5xx ................................................63

      10. Header Field Definitions..........................................65
       10.1 Accept .........................................................65
       10.2 Accept-Charset .................................................67
       10.3 Accept-Encoding ................................................67
       10.4 Accept-Language ................................................68
       10.5 Allow ..........................................................69
       10.6 Authorization ..................................................70
       10.7 Cache-Control ..................................................70
        Check: is this true? ...............................................72
        10.7.1 SLUSHY: Restrictions on What is Cachable ....................72
        10.7.2 Restrictions On What May be Stored by a Cache ...............73
        10.7.3 Modifications of the Basic Expiration Mechanism .............73
        10.7.4 SLUSHY: Controls over cache revalidation and reload .........74
        10.7.5 FLUID: Restrictions on use count and demographic reporting ..76
        10.7.6 Miscellaneous restrictions ..................................77

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       10.8 Connection .....................................................77
       10.8.1 Persist ......................................................78
       10.9 Content-Base ...................................................78
       10.10 Content-Encoding ..............................................78
       10.11 Content-Language ..............................................79
       10.12 Content-Length ................................................80
       10.13 Content-MD5 ...................................................80
       10.14 SLUSHY Content-Range ..........................................82
        10.14.1 MIME multipart/byteranges content-type .....................82
        10.14.2 Additional rules for Content-Range .........................83
       10.15 Content-Type ..................................................83
       10.16 Content-Location ..............................................84
       10.17 Date ..........................................................84
       10.19 SLUSHY Expires ................................................85
       10.20 Via ...........................................................86
       10.21 From ..........................................................88
       10.22 Host ..........................................................88
       10.23 If-Modified-Since .............................................89
       10.25 Last-Modified .................................................90
       10.27 Location ......................................................91
       10.29 Pragma ........................................................91
       10.30 Proxy-Authenticate ............................................92
       10.31 Proxy-Authorization ...........................................92
       10.32 Public ........................................................93
       10.33 Range .........................................................93
       10.34 Referer .......................................................94
       10.36 Retry-After ...................................................95
       10.37 Server ........................................................95
       10.38 Title .........................................................95
       10.39 Transfer Encoding .............................................96
       10.41 Upgrade .......................................................96
       10.43 User-Agent ....................................................97
       10.44 WWW-Authenticate ..............................................98
       10.45 Max-Forwards ..................................................98
       10.46 Age ...........................................................99
       10.47 CVal ..........................................................99
       10.48 If-Invalid ....................................................99
       10.49 If-Valid .....................................................100
       10.50 If-Unmodified-Since ..........................................101
       10.51 Warning ......................................................102
       10.52 Vary .........................................................103
       10.53 Alternates ...................................................106
       10.54 SLUSHY: Accept-Ranges ........................................107
       10.55 SLUSHY: Range-If .............................................107

      11. Access Authentication............................................108
       11.1 Basic Authentication Scheme ...................................109
       11.2 Digest Authentication Scheme ..................................110

      12. Content Negotiation..............................................111
       12.1  Negotiation facilities defined in this specification .........111

      13 Caching in HTTP...................................................112
       13.1 Semantic Transparency .........................................112

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       13.2 Expiration Model ..............................................113
        13.2.1 Server-Specified Expiration ................................113
        13.2.2 Limitations on the Effect of Expiration Times ..............114
        13.2.3 Heuristic Expiration .......................................114
        13.2.4 Client-controlled Behavior .................................114
        13.2.5 Exceptions to the Rules and Warnings .......................115
        13.2.6 Age Calculations ...........................................115
        13.2.7 Expiration Calculations ....................................117
       13.2.8 UT Mandatory ................................................118
       13.3 Validation Model ..............................................118
        13.3.1 Last-modified Dates ........................................119
        13.3.2 Opaque Validators ..........................................119
        13.3.3 Weak and Strong Validators .................................120
        13.3.4 Rules for When to Use Opaque Validators and Last-modified
        Dates .............................................................122
        13.3.5 SLUSHY: Non-validating conditionals ........................123
        13.3.6 FLUID: Other Issues ........................................123
       13.4 Cache-control Mechanisms ......................................123
       13.5 Warnings ......................................................124
       13.6 Explicit Indications Regarding User-specified Overrides .......124
       13.7 Constructing Responses From Caches ............................125
        13.7.1 End-to-end and Hop-by-hop Headers ..........................125
        13.7.2 Non-modifiable Headers .....................................126
        13.7.3 Combining Headers ..........................................126
        13.7.4 Combining Byte Ranges ......................................126
        13.7.5 SLUSHY: Scope of Expiration ................................127
       13.8 Caching and Content Negotiation ...............................127
        13.8.1 Use of the Vary header .....................................127
        13.8.2 SLUSHY: Use of the Alternates header .......................128
        13.8.3 Use of Variant-IDs .........................................128
        13.8.4 Use of Selecting Opaque Validators .........................129
       13.10 Shared and Non-Shared Caches .................................130
       13.11 SLUSHY: Miscellaneous Considerations .........................130
        13.11.1 Detecting Firsthand Responses .............................130
        13.11.2 Disambiguating Expiration values ..........................130
        13.11.3 Disambiguating Multiple Responses .........................131
       13.12 SLUSHY: Cache Keys ...........................................131
        13.12.1 Non-varying Resources .....................................132
        13.12.2 SLUSHY: Varying Resources .................................132
        13.12.3 SLUSHY: Key-Matching Procedure ............................133
        13.12.4 Canonicalization of URIs ..................................134
       13.13 FLUID: Cache-Related Problems Not Addressed in HTTP/1.1 ......134
       13.14 Cache Operation When Receiving Errors or Incomplete Responses 134
        13.14.1 Caching and Status Codes ..................................135
        13.14.2 Handling of Retry-After ...................................135
       13.15 FLUID: Compatibility With Earlier Versions of HTTP ...........135
       13.16 SLUSHY: Side Effects of GET and HEAD .........................135
       13.17 SLUSHY: Invalidation After Updates or Deletions ..............136
       13.18 Write-Through Mandatory ......................................136
       13.19  Interoperability of Varying Resources with HTTP/1.0 Proxy
       Caches .............................................................136
       13.20 Cache Replacement for Varying Resources ......................137
       13.22 FLUID: Network Partitions ....................................138
       13.23 FLUID: Caching of Negative Responses .........................138

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       13.24 History Lists ................................................138

      14 Persistent Connections............................................138
       14.1 Purpose .......................................................138
       14.2 Overall Operation .............................................139
        14.2.3 Negotiation ................................................139
        14.2.4 Pipe-lining ................................................139
        14.2.5 Delimiting Entity-Bodies ...................................139
       14.3 Proxy Servers .................................................140
       14.4 Interaction with Security Protocols ...........................140
       14.5 Practical Considerations ......................................140

      15. Security Considerations..........................................141
       15.1 Authentication of Clients .....................................141
       15.2 Safe Methods ..................................................142
       15.3 Abuse of Server Log Information ...............................143
       15.4 Transfer of Sensitive Information .............................143
       15.5 Attacks Based On File and Path Names ..........................143
       15.6 Personal Information ..........................................144
       15.7 Privacy issues connected to Accept headers ....................144
       15.8 DNS Spoofing ..................................................145
       15.9 SLUSHY: Location Headers and Spoofing .........................145

      16. Acknowledgments..................................................145

      17. References.......................................................147

      18. Authors' Addresses...............................................150

      Appendices...........................................................151

      A. Internet Media Type message/http..................................151

      B. Tolerant Applications.............................................152

      C. Differences Between  HTTP Bodies and RFC 1521 Internet Message Bodies
      .....................................................................152
       C.1 Conversion to Canonical Form ...................................153
       C.2 Conversion of Date Formats .....................................153
       C.3 Introduction of Content-Encoding ...............................153
       C.4 No Content-Transfer-Encoding ...................................154
       C.5 HTTP Header Fields in Multipart Body-Parts .....................154
       C.6 Introduction of Transfer-Encoding ..............................154
       C.7 MIME-Version ...................................................155

      D. Changes from HTTP/1.0.............................................155
       D.1 Changes to Simplify Multi-homed Web Servers and Conserve IP
       Addresses ..........................................................155

      E. Additional Features...............................................156
       E.1 Additional Request Methods .....................................156
        E.1.1 PATCH .......................................................156
        E.1.2 LINK ........................................................157
        E.1.3 UNLINK ......................................................157

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       E.2 Additional Header Field Definitions ............................157
        E.2.1 Content-Version .............................................157
        E.2.2 Derived-From ................................................158
        E.2.3 Link ........................................................158
        E.2.4 URI .........................................................159
        E.2.5 Compatibility with HTTP/1.0 Persistent Connections ..........160
       F.1 Compatibility with Previous Versions ...........................160
       G. Proxy Cache Implementation Guidelines ...........................161
       G.1 Support for Content Negotiation by Proxy Caches ................161
       G.2  Propagation of Changes in Opaque Selection ....................163
       G.3 SLUSHY: State ..................................................163
       G.4 FLUID: Cache Replacement Algorithms ............................163
       G.5 FLUID: Bypassing in Caching Hierarchies ........................164


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      1. Introduction
      1.1 Purpose
      The Hypertext Transfer Protocol (HTTP) is an application-level protocol
      for distributed, collaborative, hypermedia information systems. HTTP has
      been in use by the World-Wide Web global information initiative since
      1990. The first version of HTTP, referred to as HTTP/0.9, was a simple
      protocol for raw data transfer across the Internet. HTTP/1.0, as defined
      by RFC xxxx [6], improved the protocol by allowing messages to be in the
      format of MIME-like entities, containing metainformation about the data
      transferred and modifiers on the request/response semantics. However,
      HTTP/1.0 does not sufficiently take into consideration the effect of
      hierarchical proxies and caching, the desire for persistent connections
      and virtual hosts, and a number of other details that slipped through
      the cracks of existing implementations. 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 is backwards-compatible with HTTP/1.0, but includes more
      stringent requirements in order to ensure reliable implementation of its
      features.

      Practical information systems require more functionality than simple
      retrieval, including search, front-end update, and annotation. HTTP
      allows an open-ended set of methods that indicate the purpose of a
      request. It builds on the discipline of reference provided by the
      Uniform Resource Identifier (URI) [3], as a location (URL) [4] or name
      (URN) [20], for indicating the resource on which a method is to be
      applied. Messages are passed in a format similar to that used by
      Internet Mail [9] and the Multipurpose Internet Mail Extensions (MIME)
      [7].

      HTTP is also used as a generic protocol for communication between user
      agents and proxies/gateways to other Internet protocols, such as SMTP
      [16], NNTP [13], FTP [18], Gopher [2], and WAIS [10], allowing basic

      hypermedia access to resources available from diverse applications and
      simplifying the implementation of user agents.


      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.

      SHOULD
           This word or the adjective _recommended_ means that there may exist
           valid reasons in particular circumstances to ignore this item, but
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           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
           application 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
           (Section 3.2).


      entity
           A particular representation, rendition, encoding, or presentation
           of a resource.  Resources not supporting content negotiation are
           bound to a single entity.  Resources supporting content negotiation

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           are bound to a set of one or more entities, whose membership may
           vary over time.

      entity instance
           The definite value of an entity at a given
           point in time.  The HTTP protocol transfers
           entity instances in request or response
           messages.  An entity instance is transferred as
           metainformation in the form of entity headers
           and content in the form of an entity body.


      client
           An application 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.


      origin server
           The server on which a given resource resides or is to be created.


      proxy
           An intermediary program which acts as both a server and a client
           for the purpose of making requests on behalf of other clients.
           Requests are serviced internally or by passing them, with possible
           translation, on to other servers. A proxy MUST interpret and, if
           necessary, rewrite a request message before forwarding it. Proxies
           are often used as client-side portals through network firewalls and
           as helper applications for handling requests via protocols not
           implemented by the user agent.


      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. Gateways are
           often used as server-side portals through network firewalls and as
           protocol translators for access to resources stored on non-HTTP
           systems.


      tunnel
           A tunnel is an intermediary program which is acting as a blind
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           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. Tunnels are
           used when a portal is necessary and the intermediary cannot, or
           should not, interpret the relayed communication.


      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 while it is acting as a tunnel.
      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.


      1.4 Overall Operation
      The HTTP protocol is based on a request/response paradigm. 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 body content.

      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
      parts 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
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      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 MUST pass through four separate connections. This
      distinction is important because some HTTP communication options may
      apply only to the connection with the nearest, non-tunnel neighbor, only
      to the end-points of the chain, or to all connections along the chain.
      Although the diagram is linear, each participant may be engaged in
      multiple, simultaneous communications. For example, B may be receiving
      requests from many clients other than A, and/or forwarding requests to
      servers other than C, at the same time that it is handling A's request.

      Any party to the communication which is not acting as a tunnel may
      employ an internal cache for handling requests. The effect of a cache is
      that the request/response chain is shortened if one of the participants
      along the chain has a cached response applicable to that request. The
      following illustrates the resulting chain if B has a cached copy of an
      earlier response from O (via C) for a request which has not been cached
      by UA or A.

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


      Not all responses are 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.


      On the Internet, HTTP communication generally takes place over TCP/IP
      connections. The default port is TCP 80 [19], but other ports can be

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

      However, HTTP/1.1 implementations SHOULD implement persistent
      connections (See section 14). Both clients and servers MUST be capable
      of handling cases where either party closes the connection prematurely,
      due to user action, automated time-out, or program failure. In any case,

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      the closing of the connection by either or both parties always
      terminates the current request, regardless of its status.


      1.4 HTTP and MIME
      HTTP/1.1 uses many of the constructs defined for MIME, as defined in RFC
      1521 [7]. Appendix C describes the ways in which the context of HTTP

      allows for different use of Internet Media Types than is typically found
      in Internet mail, and gives the rationale for those differences.


      2. Notational Conventions and Generic Grammar

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

      order to understand this specification. The augmented BNF includes the
      following constructs:


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


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


      rule1 | rule2
           Elements separated by a bar ("I") 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
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           that _*(element)_ allows any number, including zero; _1*element_
           requires at least one; and _1*2element_ allows one or two.


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


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


      #rule
           A construct "#" is defined, similar to "*", for defining lists of
           elements. The full form is "<n>#<m>element" indicating at least <n>
           and at most <m> elements, each separated by one or more commas
           (",") and optional linear whitespace (LWS). This makes the usual
           form of lists very easy; a rule such as "( *LWS element *( *LWS ","
           *LWS element ))" can be shown as "1#element". Wherever this
           construct is used, null elements are allowed, but do not contribute
           to the count of elements present. That is, "(element), , (element)"
           is permitted, but counts as only two elements. Therefore, where at
           least one element is required, at least one non-null element MUST
           be present. Default values are 0 and infinity so that "#(element)"
           allows any number, including zero; "1#element" requires at least
           one; and _1#2element_ allows one or two.


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


      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. However, applications
           SHOULD attempt to follow _common form_ when generating HTTP
           constructs, since there exist some implementations that fail to
           accept anything beyond the common forms.


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      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
      [21].


             OCTET          = <any 8-bit sequence of data>
             CHAR           = <any US-ASCII character (octets 0 - 127)>
             UPALPHA        = <any US-ASCII uppercase letter "A".."Z">
             LOALPHA        = <any US-ASCII lowercase letter "a".."z">
             ALPHA          = UPALPHA | LOALPHA
             DIGIT          = <any US-ASCII digit "0".."9">
             CTL            = <any US-ASCII control character
                              (octets 0 - 31) and DEL (127)>
             CR             = <US-ASCII CR, carriage return (13)>
             LF             = <US-ASCII LF, linefeed (10)>
             SP             = <US-ASCII SP, space (32)>
             HT             = <US-ASCII HT, horizontal-tab (9)>
             <">            = <US-ASCII double-quote mark (34)>


      HTTP/1.1 defines the octet sequence CR LF as the end-of-line marker for
      all protocol elements except the Entity-Body (see Appendix B 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 whitespace,
      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 octets from character sets other than US-ASCII only
      when encoded according to the rules of RFC 1522 [14].


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


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      Recipients of header field TEXT containing octets outside the US-ASCII
      character set range MAY assume that they represent ISO-8859-1 characters
      if there is no other encoding indicated by an RFC 1522 mechanism.

      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.

             word           = token | quoted-string


             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 ")">


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

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


             qdtext         = <any CHAR except <"> and CTLs,
                              but including LWS>


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


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             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. If the protocol version is not specified,
      the recipient MUST assume that the message is in the simple HTTP/0.9
      format [6].


             HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT


      Note that the major and minor numbers SHOULD 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 SHOULD be ignored by recipients and never
      generated by senders.

      Applications sending Full-Request or Full-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.

      Proxy and gateway applications MUST be careful in forwarding requests
      that are received in a format different than that of the application's
      native HTTP version. 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 native 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
      application's native format MAY be upgraded before being forwarded; the
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      proxy/gateway's response to that request MUST follow the server
      requirements listed above.

        Note: Converting between versions of HTTP may involve addition
        or deletion of headers required or forbidden by the version
        involved.  It is likely more involved than just changing the
        version indicator.


      3.2 Uniform Resource Identifiers
      URIs have been known by many names: WWW addresses, Universal Document
      Identifiers, Universal Resource Identifiers [3], and finally the

      combination of Uniform Resource Locators (URL) [4] and Names (URN) [20].

      As far as HTTP is concerned, Uniform Resource Identifiers are simply
      formatted strings which identify--via name, location, or any other
      characteristic--a network resource.


      3.2.1 General Syntax
      URIs in HTTP can be represented in absolute form or relative to some
      known base URI [11], depending upon the context of their use. The two

      forms are differentiated by the fact that absolute URIs always begin
      with a scheme name followed by a colon.


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             URI            = ( absoluteURI | relativeURI ) [ "#" fragment ]


             absoluteURI    = scheme ":" *( uchar | reserved )


             relativeURI    = net_path | abs_path | rel_path


             net_path       = "//" net_loc [ abs_path ]
             abs_path       = "/" rel_path
             rel_path       = [ path ] [ ";" params ] [ "?" query ]


             path           = fsegment *( "/" segment )
             fsegment       = 1*pchar
             segment        = *pchar


             params         = param *( ";" param )
             param          = *( pchar | "/" )


             scheme         = 1*( ALPHA | DIGIT | "+" | "-" | "." )
             net_loc        = *( pchar | ";" | "?" )
             query          = *( uchar | reserved )
             fragment       = *( uchar | reserved )


             pchar          = uchar | ":" | "@" | "&" | "=" | "+"
             uchar          = unreserved | escape
             unreserved     = ALPHA | DIGIT | safe | extra | national


             escape         = "%" HEX HEX
             reserved       = ";" | "/" | "?" | ":" | "@" | "&" | "="
             extra          = "!" | "*" | "'" | "(" | ")" | ","
             safe           = "$" | "-" | "_" | "." | "+"
             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.


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      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 a status code of


       if a URI is longer than the server can handle.  See section 9.4.


        Note: Servers SHOULD be cautious about depending on URI lengths       
        above 255 bytes, because some older client or proxy            414 Request-URI Too Large
        implementations may not properly support these.

       All client and proxy implementations MUST be able to handle a URI of
      any finite length.


      3.2.2 http URL
      The _http_ scheme is used to locate network resources via the HTTP
      protocol. This section defines the scheme-specific syntax and semantics
      for http URLs.

             http_URL       = "http:" "//" host [ ":" port ] [ abs_path ]


             host           = <A legal Internet host domain name
                               or IP address (in dotted-decimal form),
                               as defined by Section 2.1 of RFC 1123>


             port           = *DIGIT


      If the port is empty or not given, port 80 is assumed. The semantics are
      that the identified resource is located at the server listening for TCP
      connections on that port of that host, and the Request-URI for the
      resource is abs_path.  The use of IP addresses in URL's SHOULD be
      avoided whenever possible.  See RFC 1900[24].  If the abs_path is not

      present in the URL, it MUST be given as _/_ when used as a Request-URI
      for a resource (Section 5.1.2).


        Note: Although the HTTP protocol is independent of the transport
        layer protocol, the http URL only identifies resources by their
        TCP location, and thus non-TCP resources MUST be identified by
        some other URI scheme.

      The canonical form for _http_ URLs is obtained by converting any UPALPHA
      characters in host to their LOALPHA equivalent (hostnames are case-

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      insensitive), eliding the [ ":" port ] if the port is 80, and replacing
      an empty abs_path with _/_.


      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, made obsolete by RFC 1036
             Sun Nov  6 08:49:37 1994         ; ANSI C's asctime() format


      The first format is preferred as an Internet standard and represents a
      fixed-length subset of that defined by RFC 1123 [8] (an update to RFC

      822 [9]). The second format is in common use, but is based on the

      obsolete RFC 850 [12] date format and lacks a four-digit year. HTTP/1.1

      clients and servers that parse the date value MUST accept all three
      formats, though they MUST only generate the RFC 1123 format for
      representing date/time stamps in HTTP message fields.

        Note: Recipients of date values are encouraged to be robust in
        accepting date values that may have been generated 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 Universal Time (UT),
      also known as 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 SHOULD be assumed when
      reading the asctime format.


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             HTTP-date      = rfc1123-date | rfc850-date | asctime-date


             rfc1123-date   = wkday "," SP date1 SP time SP "GMT"
             rfc850-date    = weekday "," SP date2 SP time SP "GMT"
             asctime-date   = wkday SP date3 SP time SP 4DIGIT


             date1          = 2DIGIT SP month SP 4DIGIT
                              ; day month year (e.g., 02 Jun 1982)
             date2          = 2DIGIT "-" month "-" 2DIGIT
                              ; day-month-year (e.g., 02-Jun-82)
             date3          = month SP ( 2DIGIT | ( SP 1DIGIT ))
                              ; month day (e.g., Jun  2)


             time           = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                              ; 00:00:00 - 23:59:59


             wkday          = "Mon" | "Tue" | "Wed"
                            | "Thu" | "Fri" | "Sat" | "Sun"


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


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


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

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

        .  HTTP/1.1 clients and caches should assume that an RFC-850 date
           which appears to be more than 50 years in the future is in fact in
           the past (this helps solve the _year 2000_ problem).
        .  An HTTP/1.1 implementation may internally represent a parsed
           Expires date as earlier than the proper value, but MUST NOT
           internally represent a parsed Expires date as later than the proper
           value.


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      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. This format SHOULD only be used to represent short
      time periods or periods that cannot start until receipt of the message.

             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.

      HTTP character sets are identified by case-insensitive tokens. The
      complete set of tokens is defined by the IANA Character Set registry
      [19]. However, because that registry does not define a single,

      consistent token for each character set, we define here the preferred
      names for those character sets most likely to be used with HTTP
      entities. These character sets include those registered by RFC 1521 [7]

      -- the US-ASCII [21] and ISO-8859 [22] character sets -- and other names

      specifically recommended for use within MIME charset parameters.


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             charset = "US-ASCII"
                     | "ISO-8859-1" | "ISO-8859-2" | "ISO-8859-3"
                     | "ISO-8859-4" | "ISO-8859-5" | "ISO-8859-6"

                     | "ISO-2022-JP" | "ISO-2022-JP-2" | "ISO-2022-KR"
                     | "UNICODE-1-1" | "UNICODE-1-1-UTF-7" | "UNICODE-1-1-UTF-8"
                     | token


      registry [19] MUST represent the character set defined by that registry.

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

                                   _             _ is more commonly      Although HTTP allows an arbitrary token to be used as a charset value,                     | "ISO-8859-7" | "ISO-8859-8" | "ISO-8859-9"      any token that has a predefined value within the IANA Character Set        Note: This use of the term  character set
        referred to as a _character encoding._ However, since HTTP and
        MIME share the same registry, it is important that the
        terminology also be shared.

      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.


      3.5 Content Codings
      Content coding values indicate an encoding transformation that has been
      or can be applied to a resource. Content codings are primarily used to
      allow a document to be compressed or encrypted without losing the
      identity of its underlying media type. Typically, the resource is stored
      in this encoding and only decoded before rendering or analogous usage.

             content-coding   = "gzip" | "x-gzip" | "compress" | "x-compress" | token


        Note: For historical reasons, HTTP applications SHOULD consider
        _x-gzip_ and
        _x-compress_ to be equivalent to _gzip_ and _compress_,
        respectively.

      All content-coding values are case-insensitive. HTTP/1.1 uses content-
      coding values in the Accept-Encoding (Section 10.3) and Content-Encoding

      (Section 10.10) 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. Note that a single
      program MAY be capable of decoding multiple content-coding formats. Two
      values are defined by this specification:

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      gzip
           An encoding format produced by the file compression program _gzip_
           (GNU zip) developed by Jean-loup Gailly[25]. This format is

           typically a Lempel-Ziv coding (LZ77) with a 32 bit CRC.

      compress
           The encoding format produced by the file compression program
           _compress_. This format is an adaptive Lempel-Ziv-Welch coding
           (LZW).
        Note: Use of program names for the identification of encoding
        formats is not desirable and should be discouraged for future
        encodings. Their use here is representative of historical
        practice, not good design.

      HTTP defines a registration process which uses the Internet Assigned
      Numbers Authority (IANA) as a central registry for content-coding value
      tokens.  Additional content-coding value tokens beyond the four defined
      in this document (gzip x-gzip compress x-compress) SHOULD be registered
      with the IANA. To allow interoperability between clients and servers,
      specifications of the content coding algorithms used to implement a new
      value SHOULD be publicly available and adequate for independent
      implementation, and MUST 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 resource.

             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 10.39).


      Transfer codings are analogous to the Content-Transfer-Encoding values
      of MIME [7], which were designed to enable safe transport of binary data

      over a 7-bit transport service. However, _safe transport_ has a
      different focus for an 8bit-clean transfer protocol. In HTTP, the only
      unsafe characteristic of message bodies is the difficulty in determining
      the exact body length (Section 7.2.2), or the desire to encrypt data

      over a shared transport.


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      All HTTP/1.1 applications MUST be able to receive and decode the
      _chunked_ transfer coding , and MUST ignore chunked extensions they do
      not understand. 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.

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


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


             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         = *< Content-MD5 and future headers that specify
                               they are allowed in footer>>


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


      Note that the chunks are ended by a zero-sized chunk, followed by the
      footer and terminated by an empty line. An example process for decoding
      a Chunked-Body is presented in Appendix C.5.


      3.7 Media Types
      HTTP uses Internet Media Types [17] in the Content-Type (Section 10.15)

      and Accept (Section 10.1) header fields in order to provide open and

      extensible data typing and type negotiation.

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


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      Parameters may follow the type/subtype in the form of attribute/value
      pairs.

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


      The type, subtype, and parameter attribute names are case-insensitive.
      Parameter values may or may not be case-sensitive, depending on the
      semantics of the parameter name. LWS MUST NOT be generated between the
      type and subtype, nor between an attribute and its value. Upon receipt
      of a media type with an unrecognized parameter, a user agent SHOULD
      treat the media type as if the unrecognized parameter and its value were
      not present.

      Some older HTTP applications do not recognize media type parameters.
      HTTP/1.1 applications SHOULD only use media type parameters when they
      are necessary to define the content of a message.

      Media-type values are registered with the Internet Assigned Number
      Authority (IANA [19]). The media type registration process is outlined

      in RFC 1590 [17]. Use of non-registered media types is discouraged.


      3.7.1 Canonicalization and Text Defaults
      Internet media types are registered with a canonical form. In general,
      an Entity-Body transferred via HTTP MUST be represented in the
      appropriate canonical form prior to its transmission. If the body has
      been encoded with a Content-Encoding, the underlying data SHOULD be in
      canonical form prior to being encoded.

      Media subtypes of the _text_ type use CRLF as the text line break when
      in canonical form. However, HTTP allows the transport of text media with
      plain CR or LF alone representing a line break when used consistently
      within the 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 media 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
      SHOULD NOT be substituted for CRLF within any of the HTTP control
      structures (such as header fields and multipart boundaries).

      The _charset_ parameter is used with some media types to define the
      character set (Section 3.4) of the data. When no explicit charset

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      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 in order to be
      consistently interpreted by the recipient.

        Note: Many current HTTP servers provide data using charsets
        other than _ISO-8859-1_ without proper labeling. This situation
        reduces interoperability and is not recommended. To compensate
        for this, some HTTP user agents provide a configuration option
        to allow the user to change the default interpretation of the
        media type character set when no charset parameter is given.


      3.7.2 Multipart Types
      MIME provides for a number of _multipart_ types -- encapsulations of one
      or more entities within a single message's Entity-Body. All multipart
      types share a common syntax, as defined in Section 7.2.1 of RFC 1521 [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 RFC
      1521, the epilogue of any multipart message MUST be empty; HTTP
      applications MUST NOT transmit the epilogue even if the original
      resource contains an epilogue.

      In HTTP, multipart body-parts MAY contain header fields which are
      significant to the meaning of that part.

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

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


      3.8 Product Tokens
      Product tokens are used to allow communicating applications to identify
      themselves via a simple product token, with an optional slash and
      version designator. 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.

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             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. The weights are normalized to a real number in
      the range 0 through 1, where 0 is the minimum and 1 the maximum value.
      In order to discourage misuse of this feature, 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 ] )
                            | ( "." 0*3DIGIT )
                            | ( "1" [ "." 0*3("0") ] )


      _Quality values_ is a slight misnomer, since these values actually
      measure relative degradation in perceived quality. Thus, a value of
      _0.8_ represents a 20% degradation from the optimum rather than a
      statement of 80% quality.


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

      The syntax and registry of HTTP language tags is the same as that
      defined by RFC 1766 [1]. In summary, a language tag is composed of 1 or


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      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 namespace 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, but examples of tags which
      could be registered in future.


      3.12 Full Date Values
      Contents moved to section 3.3.


      3.13 Opaque Validators
      Opaque validators are quoted strings whose internal structure is not
      visible to clients or caches.

            opaque-validator = strong-opaque-validator | weak-opaque-validator
                                    | null-validator
            strong-opaque-validator = quoted-string
            weak-opaque-validator = quoted-string "/W"
            null-validator = <"> <">


        Note that the _/W_ tag is considered part of a weak opaque
        validator; it MUST NOT be removed by any cache or client.

      There are two comparison functions on opaque validators:

        .  The strong comparison function: in order to be considered equal,
           both validators must be identical in every way, and neither may be
           weak.
        .  The weak comparison function: in order to be considered equal, both
           validators must be identical in every way, except for the presence
           or absence of a _weak_ tag.
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      The weak comparison function MAY be used for simple (non-subrange) GET
      requests. The strong comparison function MUST be used in all other
      cases.

      The null validator is a special value, defined as never matching the
      current validator of an existing resource, and always matching the
      _current_ validator of a resource that does not exist.


      3.14 Variant IDs
      Variant-IDs are used to identify specific entities (variants) of a
      varying resource; see section 13.8.3 for how they are used.

            variant-id = quoted-string


      Variant-IDs are compared using string octet-equality; case is
      significant.


      3.15 Validator Sets
      Validator sets are used for doing conditional retrievals on varying
      resources; see section 13.8.4.

            validator-set = 1#validator-set-item
            validator-set-item = opaque-validator


      3.16 Variant Sets
      Validator sets are used for doing conditional retrievals on varying
      resources; see section 13.8.3.

            variant-set = 1#variant-set-item
            variant-set-item = opaque-validator ";" variant-id


      3.17 HTTP Protocol Parameters Related to Ranges
      This section defines certain HTTP protocol parameters used in range
      requests and related responses.


      3.17.1SLUSHY Range Units
      A resource may be broken down into subranges according to various
      structural units.


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            bytes-unit = "bytes"

      The only range unit defined by HTTP/1.1 is        .  HTTP/1.1            range-unit = bytes-unit            other-range-unit                     _bytes_
      implementations may ignore ranges specified using other units. other-
      range-unit = token


      3.17.2 SLUSHY 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 that
      would be transferred by the protocol if no transfer-encoding were being
      applied.

        This means that if Content-encoding is applied to the data, the
        byte range specification applies to the resulting content-
        encoded byte stream, not to the unencoded byte stream.  It also
        means that if the entity-body's media-type is a composite type
        (e.g., multipart/* and message/rfc822), then the composite's
        body-parts may have their own content-encoding and content-
        transfer-encoding, and the byte range applies to the result of
        the those encodings.

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

             ranges-specifier = byte-ranges-specifier

             byte-ranges-specifier = bytes-unit "=" byte-range-set

             byte-range-set = 1#( byte-range-spec | suffix-byte-range-spec )

             byte-range-spec = first-byte-pos "-" [last-byte-pos]

             first-byte-pos = 1*DIGIT

             last-byte-pos = 1*DIGIT

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

      If the last-byte-pos value is present, it must be greater than or equal
      to the first-byte-pos in that byte-range-spec, or the byte-range-spec is
      invalid.  The recipient of an invalid byte-range-spec must ignore it.

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      If the last-byte-pos value is absent, it is assumed to be equal to the
      current length of the entity in bytes.

      If the last-byte-pos value is larger than the current length of the
      entity, it is assumed to be equal to the current length 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, of
      a length given by the suffix-length value.  (That is, this form
      specifies the last N bytes of an entity.)  If the entity is shorter than
      the specified suffix-length, the entire entity is used.

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

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

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

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

        .  Or
             bytes=9500-

        .  The first and last bytes only (bytes 0 and 9999):
             bytes=0-0,-1

        .  Several legal but not canonical specifications of the second 500
           bytes (byte offsets 500-999, inclusive):
             bytes=500-600,601-999

             bytes=500-700,601-999


      3.17.3 SLUSHY: Content Ranges
      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.

             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


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      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 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.

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

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

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

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

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


      4. HTTP Message

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

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

      A NULL-Request (an empty line where a request would normally be
      expected) MUST be ignored. Clients SHOULD NOT send a NULL-Request, but
      there are some error and testing circumstances in which a NULL-Request
      might be sent by mistake and MUST NOT cause failure on the server.

             NULL-Request   = CRLF

      Full-Request and Full-Response use the generic message format of RFC 822
      [9] for transferring entities. Both messages may include optional header

      fields (also known as _headers_) and an entity body. The entity body is
      separated from the headers by a null line (i.e., a line with nothing
      preceding the CRLF).


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             Full-Request   = Request-Line              ; Section 5.1

                              *( General-Header         ; Section 4.3

                               | Request-Header         ; Section 5.2

                               | Entity-Header )        ; Section 7.1

                              CRLF
                              [ Entity-Body ]           ; Section 7.2


             Full-Response  = Status-Line               ; Section 6.1

                              *( General-Header         ; Section 4.3

                               | Response-Header        ; Section 6.2

                               | Entity-Header )        ; Section 7.1

                              CRLF
                              [ Entity-Body ]           ; Section 7.2


      4.2 Message Headers
      HTTP header fields, which include                (Section 4.3), Request-

      Header                                (                                        General-Header             (Section 5.2), 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.


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             HTTP-header    = field-name ":" [ field-value ] CRLF


                              and consisting of either *TEXT or combinations


      The order in which header fields with differing field names are received
                                         _             field-name     = token             field-value    = *( field-content | LWS )             field-content  = <the OCTETs making up the field-value                              of token, tspecials, and quoted-string>      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 HTTP-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.  Thus,
      the order in which multiple header fields with the same field-name are
      received may be significant to the interpretation of the combined field-
      value.


      4.3 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 headers apply only to the message being
      transmitted.

             General-Header = Cache-Control            ; Section 10.8

                            | Connection               ; Section 10.9

                            | Date                     ; Section 10.17

                            | Via                      ; Section 10.20

                            | Keep-Alive               ; Section 10.24

                            | Pragma                   ; Section 10.29

                            | Upgrade                  ; Section 10.41


      General header field names can be extended reliably only in combination
      with a change in the protocol version. However, new or experimental
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      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. For
      backwards compatibility with the more limited HTTP/0.9 protocol, there
      are two valid formats for an HTTP request:


             Full-Request   = Request-Line              ; Section 5.1

                              *( General-Header         ; Section 4.3

                               | Request-Header         ; Section 5.2

                               | Entity-Header )        ; Section 7.1

                              CRLF
                              [ Entity-Body ]           ; Section 7.2


             NULL-Request   = CRLF

      A NULL-Request MUST be ignored.


      5.1 Request-Line             Request        = Full-Request | NULL-Request
      The Request-Line begins with a method token, followed by the Request-URI
      and the protocol version, and ending with CRLF. The elements are
      separated by SP characters. No CR or LF are allowed except in the final
      CRLF sequence.

             Request-Line   = Method SP Request-URI SP HTTP-Version CRLF


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


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             Method         = "OPTIONS"                ; 

                            | "GET"                    ; 

                            | "HEAD"                   ; Section 8.3

                            | "POST"                   ; Section 8.4

                            | "PUT"                    ; Section 8.5

                            | "DELETE"                 ;                      |
      "TRACE"                  ; Section 8.12

                            | extension-method


             extension-method = token


      The list of methods acceptable by a specific resource can be specified
            Allow                           ). However, the client is always                                                         Section 8.1                                                         Section 8.2      in an       header field (Section 10.5

      notified through the return code of the response whether a method is
      currently allowed on a specific resource, as this 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 10.32).


      The methods GET and HEAD MUST be supported by all general-purpose
      servers. Servers which provide Last-Modified dates for resources MUST
      also support the conditional GET method. 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 8.


      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


      To allow for transition to absoluteURIs in all requests in future
      versions of HTTP, HTTP/1.1 servers MUST accept the absoluteURI form in
      requests, even though HTTP/1.1 clients will not normally generate them.
      Versions of HTTP after HTTP/1.1 may require absoluteURIs everywhere,
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      after HTTP/1.1 or later have become the dominant implementations. 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 only allowed to an origin server if the client
      knows the server supports HTTP/1.1 or later.  If the absoluteURI form is
      used, any Host request-header included with the request MUST be ignored.
      The absoluteURI form is required when the request is being made to a
      proxy. The proxy is requested to forward the request and return the
      response. If the request is GET or HEAD and a prior response is cached,
      the proxy may use the cached message if it passes any restrictions in
      the Cache-Control and Expires header fields. 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


      The most common form of Request-URI is that used to identify a resource
      on an origin server or gateway. In this case, only the absolute path of
      the URI is transmitted (see Section 3.2.1, abs_path). 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 Full-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 used 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


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      would be forwarded by the proxy as

             OPTIONS * HTTP/1.1


                                            _www.ics.uci.edu_.

                      is transmitted as an encoded string, where some      after connecting to port 8001 of host       The Request-URI
      characters may be escaped using the _% HEX HEX_ encoding defined by RFC
      1738 [4]. The origin server MUST decode the Request-URI in order to

      properly interpret the request.  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 _*_. Illegal Request-URIs
      SHOULD be responded to with an appropriate status code.  (Proxies MAY
      transform the Request-URI for internal processing purposes, but SHOULD
      NOT send such a transformed Request-URI  in forwarded requests.
      Transformations for use in cache updates and lookups are subject to
      additional requirements; see section 13 on caching.  The main reason for
      this rule is to make sure that the form of Request-URIs is well
      specified, to enable future extensions without fear that they will break
      in the face of some rewritings. Another is that one consequence of
      rewriting the Request-URI is that integrity or authentication checks by
      the server may fail; since rewriting MUST be avoided in this case, it
      may as well be proscribed in general.

        Note: servers writers SHOULD be aware that some existing proxies
        do some rewriting.


      5.2 Request Header Fields
      The request header fields allow the client to pass additional
      information about the request, and about the client itself, to the
      server. These fields act as request modifiers, with semantics equivalent
      to the parameters on a programming language method (procedure)
      invocation.


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             Request-Header = Accept                   ; Section 10.1

                            | Accept-Charset           ; Section 10.2

                            | Accept-Encoding          ; Section 10.3

                            | Accept-Language          ; Section 10.4

                            | Authorization            ; Section 10.6

                            | From                     ; Section 10.21

                            | Host                     ; Section 10.22

                            | If-Modified-Since        ; Section 10.23

                            | Proxy-Authorization      ; Section 10.31

                            | Range                    ; Section 10.33

                            | Referer                  ; Section 10.34

                            | User-Agent               ; Section 10.43

                            | Max-Forwards             ; Section 10.45


      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 in
      the form of an HTTP response message.


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             Response        = Full-Response


             Full-Response   = Status-Line               ; Section 6.1

                               *( General-Header         ; Section 4.3

                                | Response-Header        ; Section 6.2

                                | Entity-Header )        ; Section 7.1

                               CRLF
                               [ Entity-Body ]           ; Section 7.2


      6.1 Status-Line
      The first line of a Full-Response message is the Status-Line, consisting

      associated textual phrase, with each element separated by SP characters.
      No CR or LF is allowed except in the final CRLF sequence.

             Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF


      6.1.1 Status Code and Reason Phrase
                      element is a 3-digit integer result code of the attempt      of the protocol version followed by a numeric status code and its      The Status-Code
      to understand and satisfy the request. The Reason-Phrase is intended to
      give a short textual description of the Status-Code. The Status-Code is
      intended for use by automata and the Reason-Phrase is intended for the
      human user. The client is not required to examine or display the Reason-
      Phrase.

      The first digit of the Status-Code defines the class of response. The
      last two digits do not have any categorization role. There are 5 values
      for the first digit:


        .  1xx: Informational - Request received, continuing process

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

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

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


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        .  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. These
      codes are fully defined in Section 9.


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             Status-Code    = "100"   ; Continue
                            | "101"   ; Switching Protocols
                            | "200"   ; OK
                            | "201"   ; Created
                            | "202"   ; Accepted
                            | "203"   ; Non-Authoritative Information
                            | "204"   ; No Content
                            | "205"   ; Reset Content
                            | "206"   ; Partial Content
                            | "300"   ; Multiple Choices
                            | "301"   ; Moved Permanently
                            | "302"   ; Moved Temporarily
                            | "303"   ; See Other
                            | "304"   ; Not Modified
                            | "305"   ; Use Proxy
                            | "400"   ; Bad Request
                            | "401"   ; Unauthorized
                            | "402"   ; Payment Required
                            | "403"   ; Forbidden
                            | "404"   ; Not Found
                            | "405"   ; Method Not Allowed
                            | "406"   ; Not Acceptable
                            | "407"   ; Proxy Authentication Required
                            | "408"   ; Request Time-out
                            | "409"   ; Conflict
                            | "410"   ; Gone
                            | "411"   ; Length Required
                            | "412"   ; Precondition Failed
                            | "413"   ; Request Entity Too Large
                            | "414"   ; Request URI Too Large
                            | "415"   ; Unsupported Media Type
                            | "416"   ; None Acceptable
                            | "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
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      response MUST not be cached. For example, if an unrecognized status code
      of 431 is received by the client, it can safely assume that there was
      something wrong with its request and treat the response as if it had
      received a 400 status code. In such cases, user agents SHOULD present to
      the user the entity returned with the response, since that entity is
      likely to include human-readable information which will explain the
      unusual status.


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

             Response-Header = Location                ; Section 10.27

                             | Proxy-Authenticate      ; Section 10.30

                             | Public                  ; Section 10.32

                             | Retry-After             ; Section 10.36

                             | Server                  ; Section 10.37

                             | WWW-Authenticate        ; Section 10.44


      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
      Full-Request and Full-Response messages MAY transfer an entity within
      some requests and responses. An entity consists of Entity-Header fields
      and (usually) an Entity-Body. 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.


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             Entity-Header  = Allow                    ; Section 10.5

                            | Content-Base             ; Section 10.9

                            | Content-Encoding         ; Section 10.10

                            | Content-Language         ; Section 10.11

                            | Content-Length           ; Section 10.12

                            | Content-Location         ; Section 10.16

                            | Content-MD5              ; Section 10.13

                            | Content-Range            ; Section 10.14

                            | Content-Type             ; Section 10.15

                            | Expires                  ; Section 10.19

                            | Last-Modified            ; Section 10.25

                            | Title                    ; Section 10.38

                            | Transfer-Encoding        ; Section 10.39

                            | extension-header


             extension-header = HTTP-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 included with a request message only when the request
      method calls for one. The presence of an entity body in a request is
      signaled by the inclusion of a Content-Length and/or Content-Type header
      field in the request message headers.

      For response messages, whether or not an entity body is included with a
      message is dependent on both the request method and the response code.
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      All responses to the HEAD request method MUST not include a body, even
      though the presence of entity header fields may lead one to believe they
      do. All 1xx (informational), 204 (no content), and 304 (not modified)
      responses MUST not include a body. All other responses MUST include an
      entity body or a Content-Length header field defined with a value of
      zero (0).


      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, Content-Encoding,
      and Transfer-Encoding. These define a three-layer, ordered encoding
      model:

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


      The default for both encodings is none (i.e., the identity function).
      Content-Type specifies the media type of the underlying data. Content-
      Encoding may be used to indicate any additional content codings applied
      to the type, usually for the purpose of data compression, that are a
      property of the resource requested. Transfer-Encoding may be used to
      indicate any additional transfer codings applied by an application to
      ensure safe and proper transfer of the message. Note that Transfer-
      Encoding is a property of the message, not of the resource.

      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 header, 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
      When an entity body is included with a message, the length of that body
      may be determined in one of several ways. If a Content-Length header
      field is present, its value in bytes represents the length of the entity
      body. Otherwise, the body length is determined by the Transfer-Encoding
      (if the _chunked_ transfer coding has been applied) or by the server
      closing the connection.

        Note: Any response message which MUST NOT include an entity 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.

      Closing the connection cannot be used to indicate the end of a request
      body, since it leaves no possibility for the server to send back a
      response. For compatibility with HTTP/1.0 applications, HTTP/1.1
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      requests containing an entity body MUST include a valid Content-Length
      header field unless the server is known to be HTTP/1.1 compliant.
      HTTP/1.1 servers MUST accept the _chunked_ transfer coding (Section 3.6

      ), thus allowing this  mechanism to be used for a request when Content-
      Length is unknown.

      If a request contains an entity body and Content-Length is not
      specified, the server SHOULD respond with 400 (bad request) if it cannot
      determine the length of the request message's content, or with 411
      (length required) if it wishes to insist on receiving a valid Content-
      Length.

      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 an entity body is
      allowed, its field value MUST exactly match the number of OCTETs in the
      entity body. HTTP/1.1 user agents MUST notify the user when an invalid
      length is received and detected.


      8. 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 10.22) MUST accompany all

      HTTP/1.1 requests.


      8.1 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) and MUST
      include a Content-Length with a value of zero (0). 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

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      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 known to be unavailable
      through that proxy.


      8.2 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 header field. A
      conditional GET method requests that the identified resource be
      transferred only if it has been modified since the date given by the If-
      Modified-Since header, as described in Section 10.23. 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 identified resource be transferred, as described in
      Section 10.33. 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 may be cachable if and only if it meets
      the requirements for HTTP caching described in Section 13.


      8.3 HEAD
      The HEAD method is identical to GET except that the server MUST not
      return any Entity-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 resource identified by the
      Request-URI without transferring the Entity-Body itself. This method is
      often used for testing hypertext links for validity, accessibility, and
      recent modification.

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      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 resource (as would be
      indicated by a change in Content-Length, Content-MD5, or Content-
      Version), then the cache MUST discard the cached entity.

      There is no _conditional HEAD_ or _partial HEAD_ request analogous to
      those associated with the GET method. If an If-Modified-Since and/or
      Range header field is included with a HEAD request, they SHOULD be
      ignored.


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


        .  Annotation of existing resources;

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

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


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

      For compatibility with HTTP/1.0 applications, all POST requests MUST
      include a valid Content-Length header field unless the server is known
      to be HTTP/1.1 compliant. When sending a POST request to an HTTP/1.1
      server, a client MUST use a valid Content-Length or the _chunked_
      Transfer-Encoding. The server SHOULD respond with a 400 (bad request)
      message if it cannot determine the length of the request message's
      content, or with 411 (length required) if it wishes to insist on
      receiving a valid Content-Length.

      A successful POST does not require that the entity be created as a
      resource on the origin server or made accessible for future reference.
      That is, the action performed by the POST method might not result in a
      resource that can be identified by a URI. In this case, either 200 (ok)
      or 204 (no content) is the appropriate response status, depending on
      whether or not the response includes an entity that describes the
      result.


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      If a resource has been created on the origin server, the response SHOULD
      be 201 (created) and contain an entity (preferably of type _text/html_)
      which describes the status of the request and refers to the new
      resource.

      Responses to this method are not cachable. However, the 303 (see other)
      response can be used to direct the user agent to retrieve a cachable
      resource.

      POST requests must obey the entity transmission requirements set out in
      section 8.4.1.


      8.4.1 SLUSHY: Entity Transmission Requirements
      The following rules apply to any method that is subject to the two-phase
      mechanism.

      Upon receiving such a method from an HTTP/1.1 (or later) client, an
      HTTP/1.1 (or later) server immediately either respond with _100
      Continue_ 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 action if
      returns an error status.

      HTTP/1.1 servers are encouraged to 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.

      An HTTP/1.1 (or later) client doing a PUT-like method SHOULD monitor the
      network connection for an error status while it is transmitting the body
      of the request including any encoding mechanism used to transmit the
      body.  If  the client sees an error status, it SHOULD immediately  cease
      transmitting the body.  If the body was proceeded by a Content-length
      header, the client MUST either close the connection  or if the body is
      being sent using a Chunked encoding, use a 0 length chunk, to mark the
      end of the message.

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

      An HTTP/1.1 (or later) client that sees the connection close before
      receiving any status from the server SHOULD retry the request, but if it
      does so, it MUST use the two-phase mechanism.  In the two-phase
      mechanism, the client first sends the request headers, then waits for
      the server to respond with either a 100 Continue, in which case the
      client SHOULD continue, or an error status, in which case the client
      MUST NOT continue and MUST close the connection if it has not already
      completed sending the full request body including any encoding mechanism
      used to transmit the body.

      If the client knows that the server is an HTTP/1.1 (or later) server,
      because of the server protocol version returned with a previous request
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      on the same persistent connection [alternatively:  within the past <N>
      hours], it MUST wait for a response.  If the client believes that the
      server is a 1.0 or earlier server, it    SHOULD continue transmitting
      its request after waiting at least [5] seconds for a status response.

      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 use the two-phase method (but MAY do so
      if this simplifies the implementation).

      An HTTP/1.1 (or later) server that receives a request from a 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.


      8.5 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.

      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 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 as
      an appendage. 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.


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      For compatibility with HTTP/1.0 applications, all PUT requests MUST
      include a valid Content-Length header field unless the server is known
      to be HTTP/1.1 compliant. When sending a PUT request to an HTTP/1.1
      server, a client MUST use a valid Content-Length or the _chunked_
      Transfer-Encoding. The server SHOULD respond with a 400 (bad request)
      message if it cannot determine the length of the request message's
      content, or with 411 (length required) if it wishes to insist on
      receiving a valid Content-Length.

      The actual method for determining how the resource is placed, and what
      happens to its predecessor, is defined entirely by the origin server. If
      the entity being PUT was derived from an existing resource which
      included a Content-Version header field, the new entity MUST include a
      Derived-From header field corresponding to the value of the original
      Content-Version header field. Multiple Derived-From values may be
      included if the entity was derived from multiple resources with Content-
      Version information. Applications are encouraged to use these fields for
      constructing versioning relationships and resolving version conflicts.

      PUT requests must obey the entity transmission requirements set out in
      section 8.4.1.


      8.9 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 a
      currently cached entity, that entity MUST be removed from the cache.
      Responses to this method are not cachable.


      8.12 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 10.45).  A TRACE request MUST NOT include an

      entity body and MUST include a Content-Length header field with a value
      of zero (0).

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      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 10.20) 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_,
      _application/http_, or _text/plain_.  Responses to this method MUST NOT
      be cached.


      9. 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.


      9.1 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.


      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.


      101 Switching Protocols
      The server understands and is willing to comply with the client's
      request, via the Upgrade message header field (Section 10.41), for a

      change in the application protocol being used on this connection. The
      server will switch protocols to those defined by the response's Upgrade
      header field immediately after the empty line which terminates the 101
      response.

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


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      9.2 Successful 2xx
      This class of status code indicates that the client's request was
      successfully received, understood, and accepted.


      200 OK
      The request has succeeded. The information returned with the response is
      dependent on the method used in the request, as follows:


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

      HEAD
        the response MUST only contain the header information and no Entity-
        Body;

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

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

      otherwise,
        an entity describing the result of the action;
      If the entity corresponds to a resource, the response MAY include a
      Content-Location header field giving the actual location of that
      specific resource for later reference.


      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 SHOULD
      create the resource before using this Status-Code. If the action cannot
      be carried out immediately, the server MUST include in the response body
      a description of when the resource will be available; otherwise, the
      server SHOULD respond with 202 (accepted).


      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
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      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.


      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).


      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 generated. This
      response is primarily intended to allow input for actions to take place
      without causing a change to the user agent's active document view. The
      response MAY include new metainformation in the form of entity headers,
      which SHOULD apply to the document currently in the user agent's active
      view.

      The 204 response MUST not include an entity body, and thus is always
      terminated by the first empty line after the header fields.


      205 Reset Content
      The server has fulfilled the request and the user agent SHOULD reset the
      document view which caused the request to be generated. 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 include a Content-Length with a value of zero (0) and no entity
      body.


      206 Partial Content
      The server has fulfilled the partial GET request for the resource. The
      request MUST have included a Range header field (Section 10.33)

      indicating the desired range. The response MUST include a Content-Range
      header field (Section 10.14) indicating the range included with this

      response. All entity header fields in the response MUST describe the
      partial entity transmitted rather than what would have been transmitted
      in a full response. In particular, the Content-Length header field in
      the response MUST match the actual number of OCTETs transmitted in the
      entity body. It is assumed that the client already has the complete
      entity's header field data.

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      207 Range Out Of Bounds
      The server has determined that the requested range(s) are not present in
      the requested resource, and so there is no content to return. This
      status code should be handled by the client the same as 204 No Content.

        This could be a compatibility problem if there is an installed
        base. If treating this status code as the generic 2xx code by
        such implementations would lead to an error, it will have to be
        replace by 204.


      9.3 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.


      300 Multiple Choices
      This status code is reserved for future use by a planned content
      negotiation mechanism.  HTTP/1.1 user agents receiving a 300 response
      which includes a Location header field can treat this response as they
      would treat a 303 (See Other) response.  If no Location header field is
      included, the appropriate action is to display the entity enclosed in
      the response to the user.


      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 MUST be given by the Location
      field in the response. Unless it was a HEAD request, the Entity-Body 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.


      302 Moved Temporarily

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      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 MUST be given by the Location
      field in the response. Unless it was a HEAD request, the Entity-Body 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.


      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 resource is not
      a update reference for the original Request-URI. The 303 response is not
      cachable, but the response to the second request MAY be cachable.

      If the new URI is a location, its URL MUST be given by the Location
      field in the response. Unless it was a HEAD request, the Entity-Body of
      the response SHOULD contain a short hypertext note with a hyperlink to
      the new URI(s).

        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.


      304 Not Modified
      If the client has performed a conditional GET request and access is
      allowed, but the document has not been modified since the date and time
      specified in the If-Modified-Since field, the server MUST respond with
      this status code and not send an Entity-Body to the client. Header
      fields contained in the response SHOULD only include information which
      is relevant to cache managers or which MAY have changed independently of
      the entity's Last-Modified date. Examples of relevant header fields
      include: Date, Server, Content-Length, Content-MD5, Content-Version,
      Cache-Control and Expires.

      A cache SHOULD update its cached entity to reflect any new field values
      given in the 304 response. If the new field values indicate that the
      cached entity differs from the current resource (as would be indicated
      by a change in Content-Length, Content-MD5, or Content-Version), then
      the cache MUST disregard the 304 response and repeat the request without
      an If-Modified-Since field.

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      The 304 response MUST NOT include an entity body, and thus is always
      terminated by the first empty line after the header fields.


      305 Use Proxy
      The requested resource MUST be accessed through the proxy given by the
      Location field in the response. In other words, this is a proxy
      redirect.


      9.4 Client Error 4xx
      The 4xx class of status code is intended for cases in which the client
      seems to have erred. If the client has not completed the request when a
      4xx code is received, it SHOULD immediately cease sending data to the
      server. 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.

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


      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.


      401 Unauthorized
      The request requires user authentication. The response MUST include a
      WWW-Authenticate header field (Section 10.44) containing a challenge

      applicable to the requested resource. The client MAY repeat the request
      with a suitable Authorization header field (Section 10.6). 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.


      402 Payment Required
      This code is reserved for future use.

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      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 body. 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.


      404 Not Found
      The server has not found anything matching the Request-URI. No
      indication is given of whether the condition is temporary or permanent.
      If the server does not wish to make this information available to the
      client, the status code 403 (forbidden) can be used instead. The 410
      (gone) status code SHOULD be used if the server knows, through some
      internally configurable mechanism, that an old resource is permanently
      unavailable and has no forwarding address.


      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.


      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.

      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 over 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 is not acceptable, user
      agents SHOULD interrupt the receipt of the response if doing so would
      save network resources.  If it is unknown whether an incoming response
      would be acceptable, a user agent SHOULD temporarily stop receipt of
      more data and query the user for a decision on further

      actions.


      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 10.30) 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 10.31). HTTP access authentication is explained in Section 11.


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      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.


      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 MAY 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.


      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.


      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 entity body in the
      request message.


      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
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      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.


       413 Request Entity Too Large
      The server is refusing to process a request because it considers the
      request entity to be larger than it is willing or able to process. The
      server SHOULD close the connection if that is necessary to prevent the
      client from continuing the request.

      If the client manages to read the 413 response, it MUST honor it and
      SHOULD reflect it to the user.

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


      414 Request-URI Too Large
      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.


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


      416 None Acceptable
      This status code is reserved for future use by a planned content
      negotiation mechanism.  HTTP/1.1 user agents receiving a 416 response
      which includes a Location header can treat this response as they would
      treat a 303 (See Other) response. If no Location header is included, the
      appropriate action is to display the entity enclosed in the response to
      the user.


      9.5 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. If the client has not completed the request when
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      a 5xx code is received, it SHOULD immediately cease sending data to the
      server. 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 response codes
      are applicable to any request method and there are no required header
      fields.


      500 Internal Server Error
      The server encountered an unexpected condition which prevented it from
      fulfilling the request.


      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.


      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.


      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.


      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.


      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.
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      10. 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.


      10.1 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.

      The field MAY be folded onto several lines and more than one occurrence
      of the field is allowed, with the semantics being the same as if all the
      entries had been in one field value.

             Accept         = "Accept" ":" #(
                                   media-range
                                   [ ( ":" | ";" )

                                     range-parameter

                                     *( ";" range-parameter ) ]

                                  | extension-token )


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


             range-parameter = ( "q" "=" qvalue )
                             | extension-range-parameter

             extension-range-parameter = ( token "=" token )

             extension-token = token


      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 range-parameter q is used to indicate the
      media type quality factor for the range, which represents the user's
      preference for that range of media types.  The default value is q=1.  In
      Accept headers generated by HTTP/1.1 clients, the character separating
      media-ranges from range-parameters SHOULD be a _:_.  HTTP/1.1 servers
      SHOULD be tolerant of use of the _;_ separator by HTTP/1.0 clients.

      The example
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             Accept: audio/*: q=0.2, audio/basic


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

      If no Accept header is present, then it is assumed that the client
      accepts all media types.  If Accept headers are present, and if the
      server cannot send a response which is acceptable according to the
      Accept headers, 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.

      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,
                     */*:q=0.5


      would cause the following values to be associated:


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             text/html;level=1                  = 1


             image/jpeg                                 = 0.5
             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             text/html                                  = 0.7             text/plain                                 = 0.3
        agents, this default set SHOULD be configurable by the user.


      10.2 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.


      10.3 Accept-Encoding
      The Accept-Encoding request-header field is similar to Accept, but
      restricts the content-coding values (Section 3.5) which are acceptable

      in the response.


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             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.


      10.4 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 comprehension of the languages
      specified by that range.  The quality value defaults to _q=1_ (100%
      comprehension).For example,

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


      would mean: _I prefer Danish, but will accept British English (with 80%
      comprehension) and other types of English(with 70% comprehension)._  A
      language-range matches a language-tag if it exactly equals the tag, or
      if it exactly equals a prefix (a sub-sequence starting at the first
      character) 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 ranges 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
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        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-range.  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.  If the server cannot generate a
      response for an audience capable of understanding at least one
      acceptable language, it can send a response that uses one or more un-
      accepted languages.

      It may be contrary to the privacy expectations of the user to send an
      Accept-Language header with the complete linguistic preferences of the
      user in every request.  For a discussion of this issue, see Section 14.7

      .

        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.


      10.5 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. The Allow header field is not permitted in a request
      using the POST method, and thus SHOULD be ignored if it is received as
      part of a POST entity.

             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.

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      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 10.32) to describe what methods are implemented on the server

      as a whole.


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

             Authorization  = "Authorization" ":" credentials


      HTTP access authentication is described in Section 11. If a request is

      authenticated and a realm specified, the same credentials SHOULD be
      valid for all other requests within this realm.

      When a shared cache (see section 13.10) 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.

      10.7 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
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      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.

      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 = "public"
                             | "private" [ "=" <"> 1#field-name <"> ]
                             | "no-cache" [ "=" <"> 1#field-name <"> ]
                             | "no-store"
                             | "no-transform"
                             | "must-revalidate"
                             | "proxy-revalidate"
                             | "only-if-cached"
                             | "max-age" "=" delta-seconds
                             | "max-stale" "=" delta-seconds
                             | "min-fresh" "=" delta-seconds
                             | "min-vers" "=" HTTP-Version

            and perhaps
                             | "max-uses" "=" 1*DIGIT
                             | "use-count" "=" 1*DIGIT


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

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

        .  Restrictions on what is cachable; these may only be imposed by the
           origin server.
        .  Restrictions on what may be stored by a cache; these may be imposed
           by either the origin server or the end-user client.
        .  Modifications of the basic expiration mechanism; these may be
           imposed by either the origin server or the end-user client.
        .  Controls over cache revalidation and reload; these may only be
           imposed by an end-user client.
        .  Restrictions on the number of times a cache entry may be used, and
           related demographic reporting mechanisms.
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        .  Miscellaneous restrictions
      Caches never add or remove Cache-Control directives to requests or
      responses.


      Check: is this true?

      10.7.1 SLUSHY: Restrictions on What is Cachable
      Unless specifically constrained by a Cache-Control directive, a caching
      system may always store a successful response as a cache entry, may
      return it without validation if it is fresh, and may return it after
      successful validation. If there is neither a cache validator nor an
      explicit expiration time associated with a response, we do not expect it
      to be cached, but certain caches may violate this expectation (for
      example, when little or no network connectivity is available) as long as
      they explicit mark their responses using the Warning mechanism describe
      in section 10.51.

        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 a
      resource value, 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 resources, or portions thereof, may not
      be cached regardless of other considerations.

      Note that section 10.6 normally prevents a shared cache from saving and
      returning a response to a previous request if that request included an
      Authorization header.

      The following Cache-Control response directives add or remove
      restrictions on what is cachable:

      public 
        Overrides the restriction in section 10.6 that prevents a shared
        cache from saving and returning a response to a previous request if
        that request included an Authorization header. However, any other
        constraints on caching still apply.
      private
        Indicates that all or parts of the response message are 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. applicable to responses and must not be
        generated by clients. A private (non-shared) cache may ignore this
        directive.
        Note: This usage of the word _private_ only controls where the
        response may cached, and cannot ensure the privacy of the
        message content. Note in particular that HTTP/1.0 caches will
        not recognize or obey this directive.

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      no-cache
        indicates that all or parts of the response message MUST NOT be
        cached. This allows an origin server to prevent caching even by
        caches that have been configured to return stale responses to client
        requests.
        Note: HTTP/1.0 caches will not recognize or obey this directive.

      TBS: precedence relations between public, private, and no-cache.


      10.7.2 Restrictions On What May be Stored by a Cache
      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.

      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, HTTP/1.0 caches will not
      recognize or obey this directive, malicious or compromised caches may
      not recognize or obey this directive, and all communications networks
      may be vulnerable to eavesdropping.

      The _min-vers_ 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
      whose HTTP version number is less than the specified version MUST NOT
      store any part of either this request or any response to it. If sent in
      a response, a cache whose HTTP version number is less than the specified
      version MUST NOT store any part of either this response or the request
      that elicited it, nor may any cache transmit a stored (non-firsthand)
      copy of the response to any client with a lower HTTP version number.
      This directive applies to both non-shared and shared caches, and is made
      mandatory to allow for future protocol extensions that may affect
      caching.

        Note that the lowest version that can be sensibly included in a
        _min-vers_ directive is HTTP/1.1, since HTTP/1.0 caches do not
        obey it.


      10.7.3 Modifications of the Basic Expiration Mechanism
      The expiration time of a resource may be specified by the origin server
      using the Expires header (see section TBS). Alternatively, it may be
      specified using the _max-age_ directive in a response.
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      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 badly unsynchronized clocks.

      Other directives allow an end-user client to modify the basic expiration
      mechanism, making it either stricter or looser. 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_
      is also included, the client is not willing to accept a stale response.
      This directive overrides any policy of the cache.

      min-fresh Indicates that the client is willing to accept a response
      whose freshness lifetime is no less than its current age plus the
      specified time in seconds. That is, the client wants a that response
      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 by no more than the specified number of
      seconds. If a cache returns a stale response in response to such a
      request, it MUST mark it as stale using the Warning header.

        Note that HTTP/1.0 caches will ignore these directives.

      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).


      10.7.4 SLUSHY: Controls over cache revalidation and reload
      Sometimes an end-user client 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 response value has become corrupted for some reason,
      and the fact that its validator is up-to-date is irrelevant.

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

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


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      End-to-end reload The request includes _Cache-Control: no-cache_ 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 _Cache-Control:
      max-age=0_, which forces each cache along the path to the origin server
      to revalidate its own entry, if any, with the next cache or server. The
      initial request includes a cache-validating conditional with the
      client's current validator.

      Unspecified end-to-end revalidation The request includes _Cache-Control:
      max-age=0_, 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.

        Note that HTTP/1.0 caches will ignore these directives, except
        perhaps for _Pragma: no-cache_.

      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-body 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-body with a 200 (OK)
      response.

      If a request includes the _no-cache_ directive, it should not include
      _fresh-min_, _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 value 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.
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      Because a cache may be configured to ignore a server's specified
      expiration time, and because a client request may include a max-stale
      directive, which has a similar effect, the protocol also includes a
      mechanism for the origin server to require revalidation of a cache entry
      on any subsequent use. When the _must-revalidate_ directive is present
      in a response received by a cache, that cache MUST NOT use the value
      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.)

      The _must-revalidate_ directive is necessary to support reliable
      operation for cookies and certain other 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. Note
      that HTTP/1.0 caches will ignore this directive.

      Servers should send the _must-revalidate_ directive if and only if
      failure to revalidate a request on the entity could result in
      significantly 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 user-agent
      caches. This directive is meant to support digest authentication.


      10.7.5 FLUID: Restrictions on use count and demographic reporting
      This section is highly debatable and is likely to be removed to a
      separate I.D.

      The _max-uses_ response directive allows a cache to use a response at
      most a certain limited number of times.  For example, _max-uses=10_
      means that the response should be returned in reply to the current
      request, and may be returned in reply to no more than nine subsequent
      requests (subject to other caching constraints), unless revalidated.

      A cache may subdivide its remaining use-count among several of its own
      clients.  For example, if the incoming response includes _max-uses=10_,
      the recipient may forward this as two responses, each with _max-uses=5_.
      The idea is that the total number of uses allowed in a cache hierarchy
      should not exceed the specified limit. (The heuristics a cache uses to
      sub-allocate its max-uses value are beyond the scope of the HTTP spec.)

      The _use-count_ request directive allows a cache to tell a server how
      many times it has actually used the cache entry specified in the
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      associated request.  If a cache receives a use-count value from one of
      its clients, and it has a corresponding cache entry, it should add the
      incoming use-count to its local count.

      When a cache removes an entry, it MAY first send a HEAD request on the
      associated URI, including its use-count value, to inform the server of
      the actual use-count.  If the server has sent a max-uses limit, the
      cache SHOULD perform this notification.

      A cache that is willing to perform such notifications and that is
      willing to obey the max-uses limit SHOULD send a ``use-count=0''
      directive on its first (non-conditional) request on a resource.  This
      informs the server that the cache intends to use these two directives in
      the manner described here.


      10.7.6 Miscellaneous restrictions
      In certain circumstances, an intermediate cache (proxy) may find it
      useful to convert the encoding of an entity body. For example, a proxy
      might use a compressed content-coding to transfer the body to a client
      on a slow link.

      Because end-to-end authentication of entity bodies and/or entity headers
      relies on the specific encoding of these values, such transformations
      may cause authentication failures. Therefore, an intermediate cache MUST
      NOT change the encoding of an entity body if the response includes the
      _no-transform_ directive.


      10.8 Connection
      HTTP version 1.1 provides a new request and response header field called
      _Connection_. This header field allows the client and server to specify
      options which should only exist over that particular connection and MUST
      NOT be communicated by proxies over further connections. The connection
      header field MAY have multiple tokens separated by commas (referred to
      as connection-tokens).

      HTTP version 1.1 proxies MUST parse the Connection header field and for
      every connection-token in this field, remove a corresponding header
      field from the request before the request is forwarded. The use of a
      connection option is specified by the presence of a connection token in
      the Connection header field, not by the corresponding additional header
      field (which may not be present).

      When a client wishes to establish a persistent connection it MUST send a
      _Persist_ connection-token:

             Connection: persist

      The Connection header has the following grammar:


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             Connection-header = "Connection" ":"

                                  connection-token 0#( "," connection-token )


      When the Persist connection-token has been transmitted with a request or
      a response a Persist header field MAY also be included. The       10.8.1 Persist                                              Persist
      header field takes the following form:

             Persist-header = "Persist" ":" 0#pers-param

             pers-param = param-name "=" value

      The Persist header itself is optional, and is used only if a parameter
      is being sent. HTTP/1.1 does not define any parameters.

      If the Persist header is sent, the corresponding connection token MUST
      be transmitted. The Persist header MUST be ignored if received without
      the connection token.


      10.9 Content-Base
      The Content-Base entity-header field may be used to specify the base URI
      for resolving relative URLs within the entity. This header field is
      described as "Base" in RFC 1808 [11], which is expected to be revised

      soon.

             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 or the URI used to initiate the
      request, in that order of precedence. Note, however, that the base URI
      of the contents within the entity body may be redefined within that
      entity body.


      10.10 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 resource, 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


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      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 resource identified by
      the Request-URI. Typically, the resource is stored with this encoding
      and is only decoded before rendering or analogous usage.

      If multiple encodings have been applied to a resource, 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.


      10.11 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.

             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 selective consumer to identify and
      differentiate resources according to the consumer's own preferred
      language. Thus, if the body content is intended only for a Danish-
      literate audience, the appropriate field is

             Content-Language: dk


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

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

             Content-Language: mi, en


      However, just because multiple languages are present within an entity
      does not mean that it is intended for multiple linguistic audiences. An
      example would be a beginner's language primer, such as _A First Lesson
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      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 SHOULD not be
      limited to textual documents.


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

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


      An example is

             Content-Length: 3495


      Applications SHOULD use this field to indicate the size of the Entity-
      Body to be transferred, regardless of the media type of the entity. A
      valid Content-Length field value is required on all HTTP/1.1 request
      messages containing an entity body.

      Any Content-Length greater than or equal to zero is a valid value.
      Section 7.2.2 describes how to determine the length of an Entity-Body if

      a Content-Length is not given.

        Note: The meaning of this field is significantly different from
        the corresponding definition in MIME, where it is an optional
        field used within the _message/external-body_ content-type. In
        HTTP, it SHOULD be used whenever the entity's length can be
        determined prior to being transferred.


      10.13 Content-MD5
      The Content-MD5 entity-header field is an MD5 digest of the entity-body,
      as defined in RFC 1864 [23], for the purpose of providing an end-to-end

      message integrity check (MIC) of the entity-body. (Note: an MIC is good
      for detecting accidental modification of the entity-body in transit, but
      is not proof against malicious attacks.)

              ContentMD5      = "Content-MD5" ":" md5-digest

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


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      The Content-MD5 header 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.  If the entity 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 many 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. Also, the HTTP
        Transfer-Encoding header makes no sense within body-parts; if it
        is present, it is ignored -- i.e. treated as ordinary text.

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


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      10.14 SLUSHY Content-Range
      The Content-Range 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 entity.

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

      When an HTTP message includes the content of a single range (for
      example, a response to a request for a single range, or to 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/1.0 206 Partial content

             Date: Wed, 15 Nov 1995 06:25:24 GMT

             Last-modified: Wed, 15 Nov 1995 04:58:08 GMT

             Content-range: 21010-47021/47022

             Content-length: 26012

             Content-type: image/gif


      10.14.1 MIME multipart/byteranges content-type
      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".

      The MIME multipart/byteranges content-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.


      For example:


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             HTTP/1.0 206 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_


      10.14.2 Additional rules for Content-Range
      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, or
      because it specifies a range that starts beyond the end of the entity,
      it may omit the corresponding Content-Range field and partial entity
      body.

      If none of the byte-range-spec values in a request specify part of the
      current entity (i.e., start before the last byte of the entity), then
      the server should return a status of 207 (Range Out Of Bounds).


      10.15 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.
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             Content-Type   = "Content-Type" ":" media-type


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


      Further discussion of methods for identifying the media type of an
      entity is provided in Section 7.2.1.             Content-Type: text/html; charset=ISO-8859-4


      10.16 Content-Location
      The Content-Location entity-header field is used to define the location
      of the specific resource associated with the entity enclosed in the
      message. A server SHOULD provide a Content-Location if, when including
      an entity in response to a GET request on a negotiated resource, the
      entity corresponds to a specific, non-negotiated location which can be
      accessed via the Content-Location URI. A server SHOULD provide a
      Content-Location with any 200 (OK) response which was internally (not
      visible to the client) redirected to a resource other than the one
      identified by the request and for which correct interpretation of that
      resource MAY require knowledge of its actual location. The recipient MAY
      make future requests on this location instead of on the Request-URI.

              Content-Location = "Content-Location" ":" absoluteURI

      If no Content-Base header field is present, the value of Content-
      Location also defines the base URL for the entity (see Section 10.9).

        Note: Since the Content-Location field re-interprets the source
        of an entity, recipients must take care in not allowing a
        _spoofed_ location to deny access to the real resource. This is
        described in Section 15.9.


      10.17 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.


             Date           = "Date" ":" HTTP-date


      An example is


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             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 SHOULD always
      include a Date header. 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.

      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.

        Note: An earlier version of this document incorrectly specified
        that this field SHOULD contain the creation date of the enclosed
        Entity-Body. This has been changed to reflect actual (and
        proper) usage.

      Origin servers MUST send a Date field in every response.  However, if a
      cache receives a response without a Date field, it SHOULD attach one
      with the cache's best estimate of the time at which the response was
      originally generated.

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


      10.19 SLUSHY Expires
      The Expires entity-header field gives the date/time after which the
      entity should be considered stale. A stale cache entry may not normally
      be returned by a cache (either a proxy cache or an end-user cache)
      unless it is first validated with the origin server (or with an
      intermediate cache that has a fresh copy of the resource). 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 13.2.7.)

      To mark a response as _never expires,_ an origin server should use
      Expires date approximately one year from the time the response is
      generated. HTTP/1.1 servers should not send Expires dates more than one
      year in the future.


      10.20 Via
      The Via general-header field MUST be used by gateways and proxies to
      indicate the intermediate protocols and recipients between the user
      agent and the server on requests, and between the origin server and the
      client on responses. It is analogous to the _Received_ field of RFC 822
      [9]and is intended to be used for tracking message forwards, avoiding

      request loops, and identifying the protocol capabilities of all senders
      along the request/response chain.

            Via   =   "Via" ":" 1#( received-protocol received-by [ comment ] )


            received-protocol = [ protocol-name "/" ] protocol-version

            protocol-name     = token

            protocol-version  = token


            received-by       = ( host [ ":" port ] ) | pseudonym

            pseudonym         = token


      The received-protocol indicates the protocol version of the message
      received by the server or client along each segment of the
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      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 their information
      such that the end result is ordered according to the sequence of
      forwarding applications.

      Comments MAY be used in the Via header field to identify the software of
      the recipient proxy or gateway, analogous to the User-Agent and  Server
      header fields.  However, all comments in the Via field are optional and
      MAY be removed by any recipient prior to forwarding the message.

      For example, a request message could be sent from an HTTP/1.0 user agent
      to an internal proxy code-named _fred_, which uses HTTP/1.1 to forward
      the request to a public proxy at nowhere.com, which completes the
      request by forwarding it to the origin server at www.ics.uci.edu.  The
      request received by www.ics.uci.edu would then have the following Via
      header field:

            Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)


      Proxies and gateways used as a portal through a network firewall SHOULD
      NOT, by default, forward the names and ports of hosts within the
      firewall region. This information SHOULD only be propagated if
      explicitly enabled. If not enabled, the received-by host of any host
      behind the firewall SHOULD be replaced by an appropriate pseudonym for
      that host.

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

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


         could be collapsed to

              Via: 1.0 ricky, 1.1 mertz, 1.0 lucy


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

        Note: The Via header field replaces the Forwarded header field
        which was present in earlier drafts of this specification.


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


             From           = "From" ":" mailbox


      An example is:

             From: webmaster@w3.org


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

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

        Note: The client SHOULD not send the From header field without
        the user's approval, as it may conflict with the user's privacy
        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.


      10.22 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

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      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 ]    ; see Section 3.2.2


        _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


       The      header field MUST be included in all HTTP/1.1 request messages      A    Host
      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.


      10.23 If-Modified-Since
      The If-Modified-Since request-header field is used with the GET method
      to make it conditional: if the requested resource has not been modified
      since the time specified in this field, a copy of the resource will not
      be returned from the server; instead, a 304 (not modified) response will
      be returned without any Entity-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 resource 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:


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

      c)
        If the resource 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 13.9 for full details.

        Note that If-Modified-Since is ignored for varying resources.

        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 representations 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 request and the If-Modified-Since date of a subsequent
        request, and the possibility of clock-skew-related problems if
        the If-Modified-Date 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.


      10.25 Last-Modified
      The Last-Modified entity-header field indicates the date and time at
      which the sender believes the resource was last modified. The exact
      semantics of this field are defined in terms of how the recipient SHOULD
      interpret it: if the recipient has a copy of this resource which is
      older than the date given by the Last-Modified field, that copy SHOULD
      be considered stale.

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


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      An example of its use is

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


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

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

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


      10.27 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 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 10.16) differs
        from Location in that the former 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.


      10.29 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
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      from the viewpoint of the protocol; however, some systems MAY require
      that behavior be consistent with the directives.


      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             Pragma                  = "Pragma" ":" 1#pragma-directive             extension-pragma        = token [ "=" word ]
      has the same semantics as the _no-cache_ cache-directive (see             pragma-directive        = "no-cache" | extension-pragma
      Section 10.8) and is defined here for backwards compatibility with

      HTTP/1.0. Clients SHOULD include both header fields when a _no-cache_
      request is sent to a server not known to be HTTP/1.1 compliant.

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

      HTTP/1.1 clients SHOULD NOT send the Pragma request header. HTTP/1.1
      caches SHOULD treat _Pragma: no-cache_ as if the client had sent _Cache-
      control: no-cache_.  No new Pragma directives will be defined in HTTP.


      10.30 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-Authentication    = "Proxy-Authentication" ":" 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 MUST not be passed on to downstream
      clients.


      10.31 Proxy-Authorization
      The Proxy-Authorization request-header field allows the client to
      identify itself (or its user) to a proxy which requires authentication.
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      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 applies only to the
      current connection and MUST not be passed on to upstream servers. If a
      request is authenticated and a realm specified, the same credentials
      SHOULD be valid for all other requests within this realm.


      10.32 Public
      The Public response-header field lists the set of non-standard methods
      supported by the server. The purpose of this field is strictly to inform
      the recipient of the capabilities of the server regarding unusual
      methods. The methods listed may or may not be applicable to the Request-
      URI; the Allow header field (Section 10.5) SHOULD be used to indicate

      methods allowed for a particular URI. This does not prevent a client
      from trying other methods. The field value SHOULD not include the
      methods predefined for HTTP/1.1 in Section 5.1.1.


             Public         = "Public" ":" 1#method


      Example of use:

             Public: OPTIONS, MGET, MHEAD


      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.


      10.33 Range
      HTTP retrieval requests using conditional or unconditional GET methods
      may request one or more sub-ranges of the entity, instead of the entire
      entity.  This is done using the Range request header:

            Range = "Range" ":" ranges-specifier

      A server MAY ignore the Range header.  However, HTTP/1.1 origin servers
      and intermediate caches SHOULD support byte ranges whenever possible,

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      since this supports efficient recovery from partially failed transfers,
      and it supports efficient partial retrieval of large entities.

      I the server supports the Range header and the specified range or ranges
      are appropriate for the entity:

        .  The presence of a Range header in an unconditional GET modifies
           what is returned if the GET is otherwise successful.  In other
           words, the response carries a status code of 206 (Partial content)
           instead of 200 (OK).
        .  The presence of a Range header in a conditional GET (a request
           using one or both of If-Modified-Since and If-Invalid, or one or
           both of If-Unmodified-Since and If-Valid) 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 Range-If header
      (see section 10.104) instead of the Range header.


      10.34 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. This 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 a partial URI is given, 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.


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      10.36 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: Wed, 14 Dec 1994 18:22:54 GMT
             Retry-After: 120


      In the latter example, the delay is 2 minutes.


      10.37 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. By convention, 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 add its data to the product list. Instead, it
      SHOULD include a Via field (as described in Section 10.20).


        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.


      10.38 Title
      The Title entity-header field indicates the title of the entity
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             Title          = "Title" ":" *TEXT


      An example of the field is

             Title: Hypertext Transfer Protocol -- HTTP/1.1


      This field is isomorphic with the <TITLE> element in HTML [5].


      10.39 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 original resource.

             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.


      10.41 Upgrade
      The Upgrade general-header allows the client to specify what additional
      communication protocols it supports and would like to use if the server
      finds it appropriate to switch protocols. The server MUST use the
      Upgrade header field within a 101 (switching protocols) response to
      indicate which protocol(s) are being switched.

             Upgrade        = "Upgrade" ":" 1#product


      For example,


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             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 10.8) 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.


      10.43 User-Agent
      The User-Agent request-header field contains information about the user
      agent originating the request. This is for statistical purposes, the
      tracing of protocol violations, and automated recognition of user agents
      for the sake of tailoring responses to avoid particular user agent
      limitations. Although it is not required, 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.

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


      Example:

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


      10.44 WWW-Authenticate
      The WWW-Authenticate response-header field MUST be included in 401
      (unauthorized) response messages. The field value consists of at least
      one           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          challenge                   header field is provided, since the contents of a challenge
      may itself contain a comma-separated list of authentication parameters.


      10.45 Max-Forwards
      [JG17]The Max-Forwards general-header field may be used with the TRACE
      method (Section 8.12) to limit the number of times that a proxy or

      gateway can forward the request to the next inbound server.  This can be
      useful when the client is attempting to trace a request chain which
      appears to be failing or looping in mid-chain.

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


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

      Each proxy or gateway recipient of a TRACE request containing a Max-
      Forwards header field SHOULD check and update its value prior to
      forwarding the request.  If the received value is zero (0), the
      recipient SHOULD NOT forward the request; instead, it SHOULD respond as
      the final recipient with a 200 response containing the received request
      message as the response entity body (as described in Section 8.12).  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).

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       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.


      10.46 Age
      Caches transmit age values using:

              Age = "Age" ":" age-value

              age-value = delta-seconds


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

      If a cache receives a value larger than the largest positive integer it
      can represent, or if any of its age calculations overflows, it MUST NOT
      transmit an Age header.  Otherwise, HTTP/1.1 caches MUST send an Age
      header in every response.  Caches SHOULD use a representation with at
      least 31 bits of range.


      10.47 CVal
      The CVal header is used to transmit opaque cache validators in HTTP/1.1
      responses.

            CVal = "CVal" ":" cval-info
            cval-info = opaque-validator [ ";" variant-id ]


      Examples:

            CVal: "xyzzy"
            CVal: "xyzzy"/W
            CVal: "xyzzy";3
            CVal: "xyzzy"/W;3
            CVal: ""


        Note that the variant-id is not part of the opaque validator.
        The CVal field is used to transmit a variant-id simply as a
        matter of compact representation of responses.

      TBS: does the protocol allow the combination of a null validator and a
      variant-ID?


      10.48 If-Invalid
      The If-Invalid request-header field is used with a method to make it
      conditional. A client that has a cache entry for the relevant entity
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      supplies the associated validator using the If-Invalid header; if this
      validator matches the server's current validator for the entity, the
      server SHOULD return a 304 (Not modified) response without any Entity-
      Body.

      If the validators do not match, the server should treat the request as
      if the If-Invalid header was not present.

      See section 13.3.3 for rules on how to determine if two validators
      match.

      If the If-Invalid header is used to make a conditional request on
      varying resource, it may be used to pass a set of validators. This is
      done using the variant-set mechanism if the client has variant IDs for
      the corresponding cache entries (see sections 13.8.3 and 3.16), or the
      validator-set mechanism if the client has no variant IDs (see sections
      13.8.4 and 3.15).

            If-Invalid = "If-Invalid" ":" if-invalid-rhs
            if-invalid-rhs = variant-set | validator-set


      Examples of single-entity form:

             If-Invalid: "xyzzy"
             If-Invalid: "xyzzy"/W


      Examples of multiple-entity form:

             If-Invalid: "xyzzy";4
             If-Invalid: "xyzzy";3, "r2d2xxxx";5, "c3piozzzz";7
             If-Invalid: "xyzzy"/W;3, "r2d2xxxx"/W;5, "c3piozzzz"/W;7
             If-Invalid: "xyzzy", "r2d2xxxx", "c3piozzzz"


      If the request would, without the If-Invalid header, result in anything
      other than a 2xx status, then the If-Invalid header is ignored.

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


      10.49 If-Valid
      The If-Valid request-header field is used with a method to make it
      conditional. A client that has a cache entry for the relevant entity
      supplies the associated validator using the If-Valid header; if this
      validator matches the server's current validator for the entity, the
      server SHOULD perform the requested operation as if the If-Valid header
      were not present.

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      If the validators do not match, the server MUST NOT perform the
      requested operation, and MUST return a 412 (Precondition failed)
      response with no Entity-Body. This behavior is most useful when the
      client wants to prevent an updating method, such as PUT or POST, from
      modifying a resource whose value has changed since the client last
      checked it.

      When the If-Valid header is used, the server should use the strong
      comparison function (see section 3.13) to compare validators.

      If the If-Valid header is used to make a conditional request on varying
      resource, it may be used to pass a set of validators. This is done using
      the variant-set mechanism if the client has variant IDs for the
      corresponding cache entries (see sections 13.8.3 and 3.16), or the
      validator-set mechanism if the client has no variant IDs (see sections
      13.8.4 and 3.15).

            If-Valid = "If-Valid" ":" if-valid-rhs
            if-valid-rhs = validator-set | variant-set


      An updating request (e.g., a PUT or POST) on a multi-entity resource
      should include only one variant-set-item, the one associated with the
      particular variant whose value is being conditionally updated.

      Examples of single-entity form:

            - If-Valid: "xyzzy"
            - If-Valid: "xyzzy"/W


      Examples of multiple-entity form:

            - If-Valid: "xyzzy";4
            - If-Valid: "xyzzy";3, "r2d2xxxx";5, "c3piozzzz";7
            - If-Valid: "xyzzy", "r2d2xxxx", "c3piozzzz"
            - If-Valid: "xyzzy"/W;3, "r2d2xxxx"/W;5, "c3piozzzz"/W;7


      If the request would, without the If-Valid header, result in anything
      other than a 2xx status, then the If-Valid header is ignored.

      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 instance of a resource.


      10.50 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
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      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 resource has been modified since the specified time,
      the server MUST NOT perform the requested operation, and MUST return a
      412 (Precondition failed) response with no Entity-Body.


      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.


      10.51 Warning            If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
      Warning headers are sent with responses using:

             Warning = "Warning" ":" warn-code SP warn-agent SP warn-text
                     [SP language-tag [SP charset]]
             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-8599-1.

      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. 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.

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      This needs clarification. Someplace else in the specification, we need
      to make a clear distinction between headers that are stored with a cache
      entry and those that aren't, and we have to define carefully what
      headers are simply deleted when a cache entry is updated. Section 13.7.3
      already talks about combining headers, but doesn't provide a way to
      remove, say, a _Response is stale_ Warning after a fresh response is
      received.

      When multiple Warning headers are attached to a response, the user agent
      SHOULD display as many of them as possible, in the order that they
      appear in the response. If it is not possible to display all of the
      warnings, the user agent should follow these heuristics:

        .  Warnings that appear early in the response take priority over those
           appearing later in the response.
        .  Warnings in the user's preferred language and character set take
           priority over warnings in other languages or character sets but
           with identical warn-codes and warn-agents.
        .  TBS
      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.
      13 _Disconnected operation_
         SHOULD be included if the cache is intentionally disconnected from
        the rest of the network for a period of time.
      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.
      TBS XXX anything else?


      10.52 Vary
      The Vary response-header field is used by an origin server to signal
      that the resource identified by the current request is a varying
      resource.  A varying resource has multiple entities associated with it,
      all of which are representations of the content of the resource.  If a
      GET or HEAD request on a varying resource is received, the origin server
      will select one of the associated entities as the entity best matching
      the request.  Selection of this entity is based on the contents of
      particular header fields in the request message, or on other information
      pertaining to the request, like the network address of the sending
      client.


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      If a resource is varying, this has an important effect on cache
      management, particularly for caching proxies which service a diverse set
      of user agents.  All 200 (OK) responses from varying resources MUST
      contain at least one Vary header or Alternates header (Section 10.53) to
      signal variance.

      If no Vary headers and no Alternates headers are present in a 200 (OK)
      response, then caches may assume, as long as the response is fresh, that
      the resource in question is not varying, and has only one associated
      entity.  Note however that this entity can still change through time, as
      possibly indicated by a Cache-Control response header (section 10.cc).

      After selection of the entity best matching the current request, the
      origin server will usually generate a 200 (OK) response, but it can also
      generate other responses like 206 (Partial Content) or 304 (Not
      modified) if headers which modify the semantics of the request, like
      Range (Section 10.ran) or If-Valid (Section 10.ifva), are present.  An
      origin server need not be capable of selecting an entity for every
      possible incoming request on a varying resource; it can choose to
      generate a 3xx (redirection) or 4xx (client error) type response for
      some requests.

      In a request message on a varying resource, the selecting request
      headers are those request headers whose contents were used by the origin
      server to select the entity best matching the request. The Vary header
      field specifies the selecting request headers and any other selection
      parameters that were used by the origin server.

            Vary                 = "Vary" ":" 1#selection-parameter

            selection-parameter  = request-header-name
                                  | "{accept-headers}"
                                  | "{other}"
                                  | "{" extension-parameter "}"

             request-header-name  = field-name

             extension-parameter  = token


      The presence of a request-header-name signals that the request-header
      field with this name is selecting.  Note that the name need not belong
      to a request-header field defined in this specification, and that header
      names are case-insensitive.  The presence of the _{accept-headers}_
      parameter signals that all request headers whose names start with
      _accept_ are selecting.

      The inclusion of the _{other}_ parameter in a Vary field signals that
      parameters other than the contents of request headers, for example the
      network address of the sending party, play a role in the selection of
      the response.


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        Note: This specification allows the origin server to express
        that other parameters were used, but does not allow the origin
        server to specify the exact nature of these parameters.  This is
        left to future extensions.

      If an extension-parameter unknown to the cache is present in a Vary
      header, the cache MUST treat it as the _{other}_ parameter. If multiple
      Vary and Alternates header fields are present in a response, these MUST
      be combined to give all selecting parameters.

      The field name _Host_ MUST never be included into a Vary header; clients
      MUST ignore it if it is present.  The names of fields which change the
      semantics of a GET request, like _Range_ and _If-Valid_ MUST also never
      be included, and MUST be ignored when present.

      Servers which use access authentication are not obliged to send _Vary:
      Authorization_ headers in responses.  It MUST be assumed that requests
      on authenticated resources can always produce different responses for
      different users.  Note that servers can signal the absence of
      authentication by including a _Cache-Control: public_ header in the
      response.

      A cache MAY store and refresh 200 (OK) responses from a varying resource
      according to the rules in Section 13.7.2.  The partial entities in 206
      (Partial Content) responses from varying resources MAY also be used by
      the cache.

      When getting a request on a varying resource, a cache can only return a
      cached 200 (OK) response to one of its clients in two particular cases.

      First, if a cache gets a request on a varying resource for which it has
      cached one or more responses with Vary or Alternates headers, it can
      relay that request towards the origin server, adding an If-Invalid
      header listing the cval-info values in the CVal headers (Section 10.47)
      of the cached responses.  If it then gets back a 304 (Not Modified)
      response with the cval-info of a cached 200 (OK) response in its CVal
      header, it can return this cached 200 (OK) response to its client, after
      merging in any of the 304 response headers as specified in Section
      13.7.2.

      Second, if a cache gets a request on a varying resource, it can return
      to its client a cached, fresh 200 (OK) response which has Vary or
      Alternates headers, provided that


        .  the Vary and Alternates headers of this fresh response specify that
           only request header fields are selecting parameters,

        .  the specified selecting request header fields of the current
           request match the specified selecting request header fields of a
           previous request on the resource relayed towards the origin server,


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        .  this previous request got a 200 (OK) or 304 (Not Modified) response
           which had the same cval-info value in its CVal header as the
           cached, fresh 200 (OK) response.
      Two sequences of selecting request header fields match if and only if
      the first sequence can be transformed into the second sequence by only
      adding or removing whitespace at places in fields where this is allowed
      according to the syntax rules in this specification.

      If a cached 200 (OK) response MAY be returned to a request on a varying
      resource which includes a Range request header, then a cache MAY also
      use this 200 (OK) response to construct and return a 206 (Partial
      Content) response with the requested range.

        Note: Implementation of support for the second case above is
        mainly interesting in user agent caches, as a user agent cache
        will generally have an easy way of determining whether the
        sequence of request header fields of the current request equals
        the sequence sent in an earlier request on the same resource.
        Proxy caches supporting the second case would have to record
        diverse sequences of request header fields previously relayed;
        the implementation effort associated with this may not be
        balanced by a sufficient payoff in traffic savings.  A planned
        specification of a content negotiation mechanism will define
        additional cases in which proxy caches can return a cached 200
        (OK) response without contacting the origin server.  The
        implementation effort associated with support for these
        additional cases is expected to have a much better cost/benefit
        ratio.


      10.53 Alternates
      The Alternates response-header field is used by origin servers to signal
      that the resource identified by the current request has the capability
      to send different responses depending on the accept headers in the
      request message.  This has an important effect on cache management,
      particularly for caching proxies which service a diverse set of user
      agents.  This effect is covered in Section 10.v.

             Alternates           = "Alternates" ":" opaque-field

             opaque-field         = field-value


      The Alternates header is included into HTTP/1.1 to make HTTP/1.1 caches
      compatible with a planned content negotiation mechanism.  HTTP/1.1
      allows a future content negotiation standard to define the format of the
      Alternates header field-value, as long as the defined format satisfies
      the general rules in Section 4.2.

      To ensure compatibility with future experimental or standardized
      software, caching HTTP/1.1 clients MUST treat all Alternates headers in
      a response as synonymous to the following Vary header:

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               Vary: {accept-headers}

      and follow the caching rules associated with the presence of this Vary
      header, as covered in Section 10.v.  HTTP/1.1 allows origin servers to
      send Alternates headers under experimental conditions.


      10.54 SLUSHY: Accept-Ranges
      In some cases, a client may want to know if the server accepts range
      requests using a certain range unit. The server may indicate its
      acceptance of range requests for a resource by providing this header in
      a response for that 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 specific resource involved,
      but the server MAY ignore such requests.

        Should this say that the server SHOULD send "Accept-Ranges:
        bytes", or is MAY good enough

      Origin servers that do not accept any kind of range request for a
      specific resource MAY send

             Accept-Ranges: none

      to advise the client not to attempt a range request.

        We're still not quite sure why this header is in the protocol.
        We gather that Netscape uses it for something, but nobody from
        Netscape has even tried to explain to me whether it is necessary
        for anything.  The only thing we can think of is that a client
        would have to know in advance if a server accepted partial-
        content PUTs (i.e., PUT+Content-Range), but we don't see any
        indication that this is what Netscape wants.


      10.55 SLUSHY: Range-If
      If a client has a partial copy of an entity in its cache, and wished to
      have an up-to-date copy of the entire entity in its cache, it could use
      Range request header with a conditional GET (using either of both of If-
      Unmodified-Since and If-Valid.)  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.

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      The Range-If 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.''

             Range-If: if-valid-rhs

      The Range-If header should only be used together with a Range header,and
      must be ignored if the request does not include a Range header, or if
      the server does not support the sub-range operation.

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

        This description may need slight modification to deal with(1)
        the use of a last-modified date as a validator (but this |can
        perhaps be hidden in the definition of if-valid-rhs), and|(2)
        its application to multi-entity resources.


      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 of the server
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      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 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 MUST not cache the response to a
      request containing Authorization.

      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
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      server will service the request only if it can validate the user-ID and
      password for the protection space of the Request-URI. There are no
      optional authentication parameters.

      Upon receipt of an unauthorized request for a URI within the protection
      space, the server SHOULD 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 user-ID and password,
      separated by a single colon (_:_) character, within a base64 [7] encoded

      string in the credentials.

             basic-credentials = "Basic" SP basic-cookie


             basic-cookie   = <base64 [7] encoding of userid-password,

                               except not limited to 76 char/line>


             userid-password = [ token ] ":" *TEXT


      If the user agent wishes to send the user-ID _Aladdin_ and password
      _open sesame_, it would use the following header field:

             Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==


      The basic authentication scheme is a non-secure method of filtering
      unauthorized access to resources on an HTTP server. It is based on the
      assumption that the connection between the client and the server can be
      regarded as a trusted carrier. As this is not generally true on an open
      network, the basic authentication scheme should be used accordingly. In
      spite of this, clients SHOULD implement the scheme in order to
      communicate with servers that use it.


      11.2 Digest Authentication Scheme
      The _digest_ authentication scheme is [currently described in an expired
      Internet-Draft, and this description will have to be improved to
      reference a new draft or include the old one].


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      12. Content Negotiation
      A varying resource has multiple entities associated with it, all of
      which are representations of the content of the resource.  Content
      negotiation is the process of selecting the best representation when a
      GET or HEAD request is made on the varying resource.  HTTP/1.1 has
      provisions for two kinds of content negotiation: opaque negotiation and
      transparent negotiation.

      With opaque negotiation, the selection of the best representation is
      done by an algorithm located at the origin server, and unknown to the
      proxies and user agents involved.  Selection is based on the contents of
      particular header fields in the request message, or on other information
      pertaining to the request, like the network address of the sending
      client.  A typical example of opaque negotiation would be the selection
      of a text/html response in a particular language based on the contents
      of the Accept-Language request header field.  A disadvantage of opaque
      negotiation is that the request headers may not always contain enough
      information to allow for selection.  If the Accept header

              Accept: text/*: q=0.3, text/html, */*: q=0.5

      is sent in a request on a varying resource which has a video/mpeg and a
      video/quicktime representation, the selection algorithm in the origin
      server will either have to make a default choice, or return an error
      response which allows the user to decide on further actions.

      With transparent negotiation, the selection of the best representation
      is done by a distributed algorithm which can perform computation steps
      in the origin server, in proxies, or in the user agent.  Transparent
      negotiation guarantees that, if the user agent supports the transparent
      negotiation algorithm and is correctly configured, the request will
      always correctly yield either the video/mpeg representation, the
      video/quicktime representation, or an error message indicating that the
      resource cannot be displayed by the user agent.


      12.1  Negotiation facilities defined in this specification
      This specification defines all protocol facilities for opaque
      negotiation, but does not define the distributed algorithm for
      transparent negotiation.  This specification only defines the basic
      facilities (Vary, Alternates, Accept) in the core protocol allowing
      requests on transparently negotiated resources to be correctly handled
      by HTTP/1.1 caches.  All other information about transparent content
      negotiation is found in a separate document[29].

      If a varying resource is opaquely negotiated, successful responses to
      requests on the resource will always include a Vary header.  If a
      varying resource is transparently negotiated, successful responses to
      requests on the resource will always include an Alternates header.  If a
      successful response contains an Alternates header, it will also always
      contain a Content-Location header.  A future specification may allow a
      combination of opaque and transparent negotiation that would lead to the
      inclusion of both a Vary header and an Alternates header in a response.

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      .


      13 Caching in HTTP
      The World Wide Web is a distributed system, and so its performance can
      be improved by the use of caches. These caches are typically placed at
      proxies and in the clients themselves. The HTTP/1.1 protocol includes a
      number of elements intended to make caching work as well as possible.
      Because these elements are inextricable from other aspects of the
      protocol, and because they interact with each other, it is useful to
      describe the basic caching design of HTTP separately from the detailed
      descriptions of methods, headers, response codes, etc.


      13.1 Semantic Transparency
      Ideally, an HTTP/1.1 cache would be _semantically transparent._ That is,
      use of the cache would not affect either the clients or the servers in
      any way except to improve performance. When a client makes a request via
      a semantically transparent cache, it receives exactly the same entity
      headers and entity body it would have received if it had made the same
      request to the origin server, at the same time.

      In the real world, requirements for performance, availability, and
      disconnected operation require us to relax the goal of semantic
      transparency in many cases. 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 may
      not be relaxed

        .  without an explicit protocol-level request (when relaxed by client
           or origin server)
        .  without a means for warning the end user (when relaxed by cache or
           client)
      Therefore, the HTTP/1.1 protocol provides these important elements:

        1.            Protocol features that provide full semantic transparency when this
           is desired by all parties.
        2.            Protocol features that allow an origin server or end-user client to
           explicitly request and control non-transparent operation.
        3.            Protocol features that allow a cache to attach warnings to
           responses that do not preserve semantic transparency.
      A basic principle is that it must be possible for the clients to detect
      any potential breakdown of semantic transparency.

      Caching would be useless if it did not significantly improve performance
      in many cases. 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).
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      The server, cache, or client implementer may be faced with design
      decisions not explicitly discussed in this specification. If 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.

      A note on terminology: we say that a resource is _cachable_ if a cache
      is allowed to store a copy of this resource, when it arrives in a
      response message, and then later use that copy to respond to a
      subsequent request. Even if a resource is cachable, there may be
      additional constraints on when and whether a cache can use a cached copy
      of it.


      13.2 Expiration Model
      In order to describe the associated mechanisms, we introduce several
      terms for describing responses returned by a cache in response to a
      client's request:

      firsthand
        A response is firsthand if it comes directly and without unnecessary
        delay from the origin server, perhaps via one or more proxies.  A
        response is also firsthand if its validity has just been checked
        directly with the origin server.
      explicit expiration time
        The time at which the origin server intends that an entity should no
        longer be returned by a cache without further validation.
      heuristic expiration time
        An expiration time assigned by a cache when no explicit expiration
        time is available.
      age
        The age of a response is the time since it was generated 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 reached its freshness
        lifetime.
      stale
        A response is stale if its age has passed its freshness lifetime.

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

      Our expectation is that servers will assign future explicit expiration
      times to responses in the belief that the entity is not likely to
      change, in a semantically significant way, before the expiration time is

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      reached. This normally preserves semantic transparency, as long as the
      server's expiration times are carefully chosen.

      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. (Note that a
      firsthand response MUST always be returned to the requesting client,
      independent of its expiration time, unless the connection to the client
      is lost.)

      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 10.8).

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


      13.2.2 Limitations on the Effect of Expiration Times
      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.

      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. 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.


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


      13.2.4 Client-controlled Behavior
      While the origin server (and to a lesser extent, intermediate caches)
      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
      for an unvalidated response; specifying a value of zero forces the
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      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 decrease
      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 semantic transparency, but may be necessary
      to support disconnected operation, or high availability in the face of
      poor connectivity.


      13.2.5 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
      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 to take steps to obtain a firsthand or fresh
      response, if the user so desires. For this reason, a cache MUST NOT
      return a stale response if the client explicitly requests a first-hand
      or fresh one, unless it is impossible to comply.


      13.2.6 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 13.2.7; 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._ All HTTP
      implementations, but especially origin servers and caches, should use
      NTP [RFC1305] 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 a representation of
      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
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      server, plus the amount of time it has been in transit along network
      paths.

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

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

        1.            now - date_value, if the local clock is reasonably well
           synchronized to the origin server's clock. If the result is
           negative, this 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


      and as long as we have either nearly synchronized clocks or all-HTTP/1.1             corrected_received_age = max(now - date_value, age_value)
      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:


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            /*
             * 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, now - date_value);
            corrected_received_age = max(apparent_age, age_value);
            response_delay = now - 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
                                         Age header, to the next recipient
      cache.


      13.2.7 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 13.2.6; this section describes how to calculate the
      freshness lifetime, and to determine if a response has expired.

                      _      transmit this total age, using the       We use the term  expires_value_ to denote a representation of the value
      of the Expires header, in a form appropriate for arithmetic operations.
      We use the term _max_age_value_ to denote an appropriate representation
      of the number of seconds carried by the max-age directive of the Cache-
      Control header in a response (see section 10.8).

      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:


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            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. This heuristic
      is subject to certain limitations; the minimum value may be zero, and
      the maximum value MUST be no more than 24 hours.

      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)


      13.2.8 UT Mandatory
      All expiration-related calculations must be done in Universal Time
      (GMT).  The local time zone MUST not influence the calculation or
      comparison of an age or expiration time.

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


      13.3 Validation Model
      When a cache has a stale value 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 value 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 value is good, and we do not want to pay the
      overhead of an extra round trip if the cached value 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 (end-user or 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 resource, and if they match, it responds with a special status code
      (usually, _304 Not Modified_) and no entity body. Otherwise, it returns
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      a full response (including entity body). Thus, we avoid transmitting the
      full response if the validator matches, and we avoid an extra round trip
      if it does not match.

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

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

      The protocol includes both positive and negative senses of cache-
      validating conditions. That is, it is possible to request either that a
      method be performed if and only if the validators match, or if and only
      if the validators do not match.

        Note: a response that lacks a cache 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
        cache validator for the entity, which means it will not be
        refreshable after it expires.


      13.3.1 Last-modified Dates
      In HTTP/1.0, the only cache validator is the Last-Modified time carried
      by a response. Clients validate entities using the If-Modified-Since
      header. In simple terms, a cache entry is considered to be valid if the
      actual resource has not been modified since the original response was
      generated.


      13.3.2 Opaque Validators
      HTTP/1.1 introduces the possibility of using an _opaque_ validator, for
      situations where the Last-Modified date is not appropriate. This may
      include server implementations where it is not convenient to store
      modification dates, or where the one-second resolution of HTTP date
      values is insufficient, or where the origin server wishes to avoid
      certain paradoxes that may arise from the use of modification dates.

      An opaque validator is simply a string of octets whose internal
      structure is not known to clients or caches. Caches store opaque
      validators and return them when making conditional requests. Also, when
      a cache receives a conditional request for a resource for which it has a
      fresh cache entry, it may compare opaque validators using strict octet-
      equality. Otherwise, opaque validators have no semantic value to clients
      or caches.

      To preserve compatibility with HTTP/1.0 clients and caches, and because
      the Last-Modified date may be useful for purposes other than cache

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

      The headers used to convey opaque validators are described in sections
      10.47, 10.48, 10.49, and 10.55.


      13.3.3 Weak and Strong Validators
      Since both origin servers and caches will compare two validator values
      to decide if they represent the same or different values for the entire
      resource, one normally would expect that if the resource value (the
      entity body or any entity headers) changes in any way, then the
      associated validator would change as well. If this is true, then we call
      this validator a _strong validator._

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

      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 instance of an entity,
      while a weak validator is part of an identifier for a set of
      semantically equivalent instances of an entity.

        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.

      Opaque validators are normally _strong,_ but the protocol provides a
      mechanism to tag an opaque validator as _weak._

      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 body.

      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:


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        .  The strong comparison function: in order to be considered equal,
           both validators must be identical in every way, and neither may be
           weak.
        .  The weak comparison function: in order to be considered equal, both
           validators must be identical in every way, but either or both of
           them may be tagged as _weak_ without affecting the result.
      The weak comparison function should be used for simple (non-subrange)
      GET requests. The strong comparison function must be used in all other
      cases.

      An opaque validator is strong unless it is explicitly tagged as weak.
      Section 3.13 gives the syntax for opaque validators.

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

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

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

        .  The validator is being compared by an intermediate cache to the
           validator stored in its cache entry for the entity, and
        .  That cache entry include a Date value, which gives the time when
           the origin server generated the original response, and
        .  The presented Last-Modified time is at least 60 seconds before the
           Date value.
      This method relies on the fact that if two different responses were
      generated 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.

      This allows 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 Opaque Validators and Last-modified Dates
      We adopt a set of rules and recommendations for origin servers, clients,
      and caches regarding when various validator types should be used, and
      for what purposes.

      HTTP/1.1 origin servers:

        .  SHOULD send a strong opaque validator unless performance
           considerations support the use of weak opaque validators, or unless
           it is unfeasible to send a strong opaque validator.
        .  MAY send a weak opaque validator instead of a strong one.
        .  MAY send no opaque validator if it is infeasible to generate one.
        .  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 opaque validator and a Last-Modified value.

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

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

      HTTP/1.1 clients:

        .  If an opaque validator has been provided by the origin server, MUST
           use that validator in any cache-conditional request (using If-Valid
           or If-Invalid).
        .  If only a Last-Modified value has been provided by the origin
           server, SHOULD use that value in non-subrange cache-conditional
           requests (using If-Modified-Since).
        .  If only a Last-Modified value has been provided by an HTTP/1.0
           origin server, MAY use that value in subrange cache-conditional
           requests (using If-Unmodified-Since:). The user agent should
           provide a way to disable this, in case of difficulty.
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        .  If both an opaque validator 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 opaque validator 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 opaque validators.
        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 SLUSHY: Non-validating conditionals
      TBS

      The principle behind opaque validators 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.


      13.3.6 FLUID: Other Issues
      TBS: what if no validator present in response?


       13.4 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,
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      Cache-Control directives are explicitly specified as weakening semantic
      transparency (for example, _max-stale_ or _public_).

      The Cache-Control directives are described in detail in section 10.7.


      13.5 Warnings
      Whenever a cache returns a response that is not semantically
      transparent, it must attach a warning to that effect, using a Warning
      response header. This warning allows clients and user agents 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 a
      entity header in the response.

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

      Warnings also carry a warning message text in any appropriate natural
      language (perhaps based on the client's Accept headers), and an optional
      indication of what language and character set are used.

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

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

      The Warning header and the currently defined warnings are described in
      section 10.106.


      13.6 Explicit Indications Regarding User-specified Overrides
      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_ or _Cache-Control: reload_ to every request. We recognize
      that there may be situations which require such overrides, although user
      agents SHOULD NOT default to any behavior contrary to the HTTP/1.1
      specification. That is, the user should have to explicitly request
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      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 resource is known to be
      stale). Since the protocol normally allows the user agent to determine
      if responses are stale or not, this indication need only be displayed
      when this actually happens. The indication need not be a dialog box; it
      could be an icon (for example, a picture of a rotting fish) or some
      other visual indicator.

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


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


      13.7.1 End-to-end and Hop-by-hop Headers
      For the purpose of defining the behavior of caches and non-caching
      proxies, we divide HTTP headers into two categories:

        .  End-to-end headers, which must be transmitted to the ultimate
           recipient of a request or response. End-to-end headers in responses
           must be stored as part of a cache entry and transmitted in any
           response formed from a cache entry.
        .  Hop-by-hop headers, which are meaningful only for a single
           transport-level connection, and are not stored by caches or
           forwarded by proxies.
      The following HTTP/1.1 headers are hop-by-hop headers:

        .  Connection
        .  Keep-Alive
        .  Upgrade
        .  Public
        .  Proxy-Authenticate
        .  Transfer-Encoding
      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 10.9.


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      13.7.2 Non-modifiable Headers
      Some features of the HTTP/1.1 protocol, such as Digest Authentication
      (see TBS), depend on the value of certain end-to-end headers. A cache or
      non-caching proxy SHOULD NOT modify an end-to-end header unless the
      definition of that header requires or specifically allows that.

      A cache or non-caching proxy MUST NOT modify any of the following fields
      in a request or response, nor may it add any of these fields if not
      already present:

        .  Content-Type
        .  Content-Encoding
        .  Content-Length
        .  Expires
        .  Last-Modified
        .  Content-Range
        .  Content-Location
        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.


      13.7.3 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. It uses the end-to-end headers from the incoming response, not
      from the cache entry.  Unless it decides to remove the cache entry, it
      must also replace the end-to-end headers stored with the cache entry
      with those received in the incoming response.

      In other words, the complete set of end-to-end headers received in the
      incoming response overrides all end-to-end headers stored with the cache
      entry. The cache may add Warning headers (see section 10.106) to this
      set.

      A cache MUST preserve the order of all headers as received in an
      incoming response.

      These rule allows an origin server to completely control the response
      seen by the client of a cache when the cache revalidates an entry, and
      may be necessary for preserving semantic transparency or for certain
      kinds of security mechanisms or future extensions.


      13.7.4 Combining Byte Ranges
      A response may transfer only a subrange of the bytes of an entity,
      either because the request included one or more Range specifications, or

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      because a connection was broken prematurely. After several such
      transfers, a cache may have received several ranges of the same entity.

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

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


      13.7.5 SLUSHY: Scope of Expiration
      HTTP/1.1's expiration model is that as soon as any variant of a URI
      becomes stale, all variants becomes stale as well.  Thus, _freshness_
      applies to all the variants of URI, rather than any particular variant.
      Dates and expires etc. apply to any cached variant that a proxy might
      have with a URI and not just the one particular entity.


      13.8 Caching and Content Negotiation
      The HTTP content negotiation mechanism interacts with caching in several
      ways:

        .  A varying resource (one subject to content negotiation) may be
           bound to more than one entity. Each of these entities is called a
           _variant_ of the resource.
        .  The request-URI may be only one part of the cache key.

      13.8.1 Use of the Vary header
      Origin servers may respond to requests for varying resources use the
      Vary header (see section 10.vary for a full description) to inform the
      cache which header fields of the request were used to select the variant
      returned in the response. A cache can use that response to reply to a
      subsequent request only if the two requests not only specify the same
      URI, but also have the same value for all headers specified in the Vary
      response-header.

      The Vary header may also inform the cache that the variant was selected
      using criteria not limited to the request headers; in this case, the
      response MUST NOT be used in a reply to a subsequent request except if
      the cache relays the new request to the origin server in a conditional
      request, and the origin server responds with 304 (Not Modified) and
      includes the same variant-ID (see 13.8.3).


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      13.8.2 SLUSHY: Use of the Alternates header
      Origin servers may respond to requests for varying resources with a
      status of 300 (Multiple choice), using the Alternates header (see
      section 10.alternates) to inform the requesting client that describes
      the set of possible choices, including specific URIs for each variant.

      Roy says this response also includes a Content-Location header.

      In this case, the client may choose one of the available variants and
      make a subsequent request using the specific URI for that variant. Since
      such an URI is bound to just one entity, the origin server's response to
      this request includes neither a Vary header nor an Alternates header,
      and a cache may treat it as it would any non-varying resource.

      If a cache receives an Alternates header in a response from the origin
      server, it should act as if the response carried a "Vary:{accept-
      headers}" header.  This means that the response may be returned in reply
      to a subsequent request with Accept-* headers identical to those in the
      current request.

        Note that section 13.14.1 prevents caching of 300 (Multiple
        choices) responses unless this is explicitly allowed by an
        Expires or Cache-control header.


      13.8.3 Use of Variant-IDs
      A cache stores copies of specific entity instances, not copies of
      varying resources per se. Therefore, the URI of a varying resource is
      not sufficient for use as a cache key. In certain interactions between a
      cache and an origin server, it is convenient to encode the cache key
      using a more compact representation than the full set of selecting
      request headers. Or, if the selection criteria are not known to the
      cache, it may be impossible to express the actual cache key to the
      cache. For these reasons, the HTTP protocol provides two different
      optional mechanisms to encode a cache key:

        .  Variant-IDs: an opaque identifier for a specific variant of a
           varying resource.
        .  Selecting opaque validators: a special kind of opaque validator
           that is defined to be unique across all variants of a varying
           resource.
      Variant-IDs are the preferred mechanism, since they generally allow more
      efficient management of caches.

      If an origin server chooses to use the variant-ID mechanism, it assigns
      a variant-ID (see section 3.14) to each distinct variant. This
      assignment can only be done by the origin server. It then returns the
      appropriate variant-ID with each response that applies to a specific
      variant, using the CVal header (see 10.47).

      If an origin server provides a variant-ID for any variant of a resource,
      it SHOULD provide a variant-ID for all variants of that resource.


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      When a cache receives a successful response with a variant-ID, it SHOULD
      use this information to replace any existing cache entries for the same
      variant of the corresponding URI. That is, it forms a cache key using
      the URI of the request and the variant-ID of the response. If this key
      matches the key of an existing cache entry, it SHOULD replace the
      existing entry with the new response (subject to all of the other rules
      on caching). See section 13.12 for more details on update.

      When a cache performs a conditional request on a varying resource, and
      it has one or more cache entries for the resource that include variant-
      IDs, the cache MUST transmit the (cache-validator, variant-ID) tuples in
      the conditional request, using the variant-set mechanism (see section
      3.16). This tells the server which variants are currently in the
      requester's cache.

        The client MAY choose to transmit only a subset of the (cache-
        validator, variant-ID) tuples corresponding to its cache entries
        for this resource.

      When a server receives a conditional request that includes a variant-
      set, and the server is able to reply with an appropriate variant (either
      because it is the origin server, or because it is an intermediate cache
      that can properly implement the variant selection algorithm), once it
      has selected the variant it should examine the elements of the supplied
      variant-set. If one of these matches the variant-ID of the selected
      variant, and if the cache validators match, the server SHOULD reply with
      a 304 (Not Modified) response, including the variant-ID of the selected
      variant. Otherwise, the server should reply as if the request were
      unconditional.

      The server may optionally use the variant-set information in its
      selection algorithm. For example, if the selection algorithm yields
      several variants with equal preference, and one of these is already in
      the requester's cache, the server could select that variant and avoid an
      extra data transfer. This is a performance optimization; otherwise, the
      variant-selection mechanism is orthogonal to the variant-ID mechanism.


      13.8.4 Use of Selecting Opaque Validators
      If the origin server prefers not to provide variant-IDs, it MAY at its
      option use the _selecting opaque validator_ mechanism. A selecting
      opaque validator is an opaque validator whose value is unique across all
      variants of a resource.

      If the origin server cannot generate opaque validators that are
      guaranteed to be unique across all variants of a varying resource, it
      MUST NOT send any opaque validators for that resource.

      When a cache receives a successful response with an opaque validator and
      no variant-ID, it MAY either replace any cache entries for the resource
      with the new response, or it may keep multiple such entries.  See
      section 13.12 for more details on update.


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      When a cache performs a conditional request on a varying resource, and
      it has one or more cache entries for the resource that include opaque
      validators, the cache SHOULD transmit the set of opaque validators in
      the conditional request, using the validator-set mechanism (see section
      3.15). This tells the server which variants are currently in the
      requester's cache.

      The client MAY chose to transmit only a subset of the opaque validators
      from its cache entries for this resource.

      When a server receives a conditional request that includes a validator-
      set, and the server is able to reply with an appropriate variant (either
      because it is the origin server, or because it is an intermediate cache
      that can properly implement the variant selection algorithm), once it
      has selected the variant it should examine the elements of the supplied
      validator-set. If one of these matches the cache validator of the
      selected variant, the server SHOULD reply with a 304 (Not Modified)
      response, including that cache validator. Otherwise, the server should
      reply as if the request were unconditional.


      13.10 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.


      13.11 SLUSHY: Miscellaneous Considerations
      This section is somewhat miscellaneous, and its contents might be
      shifted to other locations in the document.


      13.11.1 Detecting Firsthand Responses
      Note that a client can usually tell if a response is firsthand by
      comparing the Date to its local request-time, and hoping that the clocks
      are not badly skewed.


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

      If a client performing a retrieval receives a non-firsthand response for
      a resource 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 10.8), to force
      a check with the origin server.
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      If a cache that is pooling cached responses from other caches sees two
      fresh responses for the same resource with different validators, it
      SHOULD use the one with the newer Date header.


      13.11.3 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 generated them. We
      would like the client to use the most recently generated response, even
      if older responses are still apparently fresh.

      Neither the opaque validator 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.

      If a client performs a request for a resource that it already has in its
      cache, and the response it receives contains a Date header that appears
      to be older than the one it already has in its cache, 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. This prevents certain paradoxes arising from the use of multiple
      caches.

      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.


      13.12 SLUSHY: Cache Keys
      A _cache key_ is a value used to identify a cache entry.  HTTP caches
      three different kinds of cache keys, for use in different contexts:

        .  Some subset of the fields stored with a cache entry constitute the
           _entry key_ for that entry.  These may include the Request-URI,
           some request-header fields, and some response-header fields.
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        .  Some subset of the fields of a response, together with perhaps the
           Request-URI, constitute the _update key_ of a response.
        .  Some subset of the fields of a request, together with the Request-
           URI, constitute the _lookup key_ of a request.
      When a cache receives a request, it builds a lookup key from that
      request, then tries to find (lookup) a cache entry with a matching entry
      key according to the key matching procedure in section 13.12.3.  If such
      a match exists, then the cache can decide (based on the other caching
      rules) whether to return that entry in reply to the request.

      When a cache receives a response, it builds a update key from that
      response, and from the request that elicited it.  It uses this key to
      find any previously stored entry with a matching entry key. If such an
      entry exists, the cache replaces the old entry with the new one.

        The term _update_ means to remove the old entry from the cache,
        and then to insert the new entry.  It does not imply a
        modification of an existing entry.

      This section describes specifically how the three kinds of keys are
      constructed, and how a cache determines if keys match.


      13.12.1 Non-varying Resources
      When a response is received for a non-varying resource (that is, the
      response includes no Vary, Alternates, or Content-Location headers), the
      update key for the response is simply the Request-URI of the request
      that elicited it: (Request-URI, null).  The entry key for the response
      is (Request-URI, null, null).


      13.12.2 SLUSHY: Varying Resources
      If a response includes a Vary header, then we use the notation _sel-hdr-
      values_ to denote the canonical form of the headers in the corresponding
      request whose field-names are given in the Vary header. If the response
      does not include a Vary header, then sel-hdr-values is assigned the null
      value.  Section 10.52 on Vary defines the canonical form for selecting
      headers.

      The canonical form of the headers is defined to be a set whose elements
      are sequences of request headers with identical field-names.  For a
      given field-name, the corresponding element is the concatenation of the
      request headers with that field-name, in exactly the order that these
      fields appear in the request

      If the response contains "Vary: {other}", then sel-hdr-values is
      assigned a non-null value that is defined as never matching a set of
      request headers.

      When a response is received that includes a variant-ID in a CVal header
      (see section 10.102), but no Content-Location header, then the update
      key is (Request-URI, variant-ID), and the entry key for the response is
      (Request-URI, variant-ID, sel-hdr-values).

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      When a response is received that includes a Vary header and an opaque
      validator, but no variant-ID or Content-Location header, then the update
      key is (Request-URI, opaque-validator), and the entry key for the
      response is (Request-URI, opaque-validator, sel-hdr-values).

      This rule supports the _selecting opaque validators_ mechanism described
      in section 13.8.4.  The cache should distinguish between actual variant-
      IDs and opaque-validators in the variant-ID element of the entry key; a
      non-null opaque-validator in an entry key DOES match a null variant-ID
      in a lookup key.

      When a response is received that includes both a variant-ID in a CVal
      header, and a Content-Location header, then the update key is (content-
      location-URI, variant-ID), and the entry key for the response is
      (content-location-URI, variant-ID, sel-hdr-values).

       When a response is received that includes a Content-Location header but
      no variant-ID, then the update key is (content-location-URI, null), and
      the entry key for the response is (content-location-URI, null, sel-hdr-
      values).


      13.12.3 SLUSHY: Key-Matching Procedure
      We express entry keys as the tuple (URI, variant-ID, sel-hdr-values), in
      which the variant-ID may be null, and the sel-hdr-values may either be
      null, or may be a set of request headers.

      We express update keys as a tuple (URI, variant-ID), in which the
      variant-ID may be null. A update key matches an entry key if both their
      URI elements match and their variant-ID elements match.  (A null
      variant-ID does not match a non-null variant-ID.)

      We express lookup keys as a tuple (URI, variant-ID, all-request-
      headers), in which the variant-ID may be null.  The all-request-headers
      element of the tuple is not always used, but is included here as a
      notational convenience.  A lookup key matches an entry key if both their
      URI elements match and their variant-ID elements match, and either

        .  the sel-hdr-values element of the entry key is null
       or

        .  the sel-hdr-values element of the entry key matches the appropriate
           headers in the all-request-headers element of the lookup key,
           according to the matching rules in section on Vary, section 10.52.
      This description matching algorithm is clearly not the most efficient
      implementation of an equivalent algorithm.  A cache may use any
      algorithm that yields equivalent results.  For example, it may use a
      hierarchical approach where cache entries are grouped into sets by the
      URI and variant-ID, and only if a set includes non-null sel-hdr-values
      elements does the cache need to consider the other request headers.

      If on a cache lookup there are two or more fresh entries that appear to
      match the request, then the one with the most recent Date value MUST be
      used.
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      13.12.4 Canonicalization of URIs
      A cache, when comparing two URIs to decide if they match or not, a cache
      MUST use a case-sensitive octet-by-octet comparison of the entire URIs,
      with these exceptions:

      Following the rules from section 3.2.2:

        .  A port that is empty or not given is equivalent to port 80.
        .  Comparisons of host names MUST be case-insensitive.
        .  Comparisons of scheme names MUST be case-insensitive.
        .  An empty abs_path is equivalent to an abs_path of _/_
      Characters except those in the reserved set and the unsafe set (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


      13.13 FLUID: Cache-Related Problems Not Addressed in HTTP/1.1
      TBS

      This section will list a few problems that are NOT addressed in
      HTTP/1.1, with the intention of encouraging implementers not to adopt
      proprietary solutions inconsistent with possible future protocol
      revisions..

        .  Server-driven invalidation
        .  Demographics

      13.14 Cache Operation When Receiving Errors or Incomplete Responses
      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 13.7.4; 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).

      A cache that receives a response with a zero-length Entity-body and no
      explicit indication that the correct length is zero (such as _Content-
      Length: 0_) MUST NOT not store the response. The same rule applies to a
      response of any length received without an explicit length indication if
      the transport connection was terminated in any unusual way.

      If a cache receives a response carrying Retry-After header (see section
      10.36), it may either forward this response to the requesting client, or
      act as if the server failed to respond. In the latter case, it MAY
      return a previously received response, although it MUST follow all of
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      the rules applying to stale responses. In particular, it MUST NOT
      override the _must-revalidate_ Cache-Control directive (see section
      10.7).


      13.14.1 Caching and Status Codes
      A response received with a status code of 200 or 206 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.

      A response received with any other status code MUST not be returned in a
      reply to a subsequent request unless it carries at least one of the
      following:

        .  an Expires header
        .  a max-age Cache-control directive
        .  a must-revalidate Cache-control directive
        .  a public Cache-control directive

      13.14.2 Handling of Retry-After
      If a cache receives a response carrying a Retry-After header (see
      section 10.36), it may either forward this response to the requesting
      client, or act as if the server failed to respond.  In the latter case,
      it MAY return a previously received response, although it MUST follow
      all of the rules applying to stale responses.  In particular, it MUST
      not override the _must-revalidate_ Cache-control directive (see section
      10.7).


      13.15 FLUID: Compatibility With Earlier Versions of HTTP
      TBS

      If anything should be here, it should be a collection of warnings about
      what HTTP/1.1 systems should not assume about HTTP/1.0 systems.


      13.16 SLUSHY: 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.
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      See section 15.2 for related information.


      13.17 SLUSHY: Invalidation After Updates or Deletions
      The effect of certain methods at the origin server may cause one or more
      existing cache entries to become non-transparently invalid. That is,
      although they may continue to be _fresh,_ they do not accurately reflect
      what the origin server would return for a new request.

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

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

      Some HTTP methods invalidate a single 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:

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


      13.18 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.

      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.


      13.19  Interoperability of Varying Resources with HTTP/1.0 Proxy Caches
      If the correct handling of responses from a varying resource (Section
      10.xxx) by HTTP/1.0 proxy caches in the response chain is important,
      HTTP/1.1 origin servers can include the following Expires (Section
      10.exp) response header in all responses from the varying resource:

           Expires: Thu, 01 Jan 1980 00:00:00 GMT

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      If this Expires header is included, the server should usually also
      include a Cache-Control header for the benefit of HTTP/1.1 caches, for
      example


      which overrides the freshness lifetime of zero seconds specified by the
      included            Cache-Control: max-age=604800               Expires header.


      13.20 Cache Replacement for Varying Resources
      If a new 200 (OK) response is received from a non-varying resource while
      an old 200 (OK) response is cached, caches can delete this old response
      from cache memory and insert the new response.  For 200 (OK) responses
      from varying resources (Section 13.12.3), cache replacement is more
      complex.

      HTTP/1.1 allows the authors of varying resources to guide cache update
      by the inclusion of elements of so-called update keys in the responses
      of these resources.  The update key of a varying response consists of
      two elements, both of which may be empty strings, separated by a
      semicolon:

             update-key  =  variant-id ";" absoluteURI

      The variant-id element of the update key is the variant-id value in the
      CVal header of the response, if a CVal header which such a value is
      present, and an empty string otherwise.  The absoluteURI element of the
      update key is the absolute URI given in, or derived from, the Content-
      Location header of the response if present, and an empty string if no
      Content-Location header is present.

      If a cache has stored in memory a 200 (OK) response with a certain
      update key, and receives, from the same resource, a new 200 (OK)
      response which has the same update key, this should be interpreted as a
      signal from the resource author that the old response can be deleted
      from cache memory and replaced by the new response.

      The update key mechanism cannot cause deletion from cache memory of old
      responses with update keys that will no longer be used.  It is expected
      that the normal  _least recently used_ update heuristics employed by
      caches will eventually cause such old responses to be deleted.

      All 200 (OK) responses from varying resources should include update key
      elements.  Resource authors may not assume that caches will be able to
      cache responses not including update key elements.  If a Vary header is
      used to signal variance, the response should include a variant-id value
      as the update key element. The Content-Location header should only be
      used to supply a update key element if an Alternates header is present
      in the response.


      13.22 FLUID: Network Partitions
      TBS
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      There may be enough said elsewhere already, but we haven't checked.


      13.23 FLUID: Caching of Negative Responses
      TBS


      13.24 History Lists
      History lists as implemented in many user agents and caches are
      different.  In particular history lists SHOULD NOT try to show a
      semantically transparent view of the current state of a resource.
      Rather, a history list is meant to show exactly what the user saw at the
      time when the resource was retrieved .

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


      14 Persistent Connections

      14.1 Purpose
      HTTP's greatest strength and its greatest weakness has been its
      simplicity.  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.   An excellent analysis of
      these performance problems is available [2]; analysis and results from a
      prototype implementation are in [32, 33].

       Persistent HTTP connections have a number of advantages, including:

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


      14.2 Overall Operation
      Persistent connections provides a mechanism by which a client and a
      server can negotiate the use of a TCP connection for an extended
      conversation.. This negotiation takes place using the Connection and

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      Persist header fields. Once this option has been negotiated the client
      can make multiple HTTP requests over a single transport connection.


      14.2.3 Negotiation
      To request the use of persistent connections, a client sends a
      Connection header with a connection-token _Persist_. If the server
      wishes to accept persistent connections it will respond with the same
      connection-token. Both the client and server MUST send this connection-
      token with every request and response for the duration of the persistent
      connection. If either the client or the server omits the Persist token
      from the Connection header, that request becomes the last one for the
      connection.

      A server MUST NOT establish a persistent connection with an HTTP/1.0
      client that uses the above form of the Persist header due to problems
      with the interactions between 1.1 clients and 1.0 proxy servers (See
      section E.2.5 for more information on backwards compatibility with HTTP
      1.0 clients).


      14.2.4 Pipe-lining
      Clients and servers which support persistent connections MAY _pipe-line_
      their requests and responses. When pipe-lining, a client will send
      multiple requests without waiting for the responses. The server MUST
      then send all of the responses in the same order that the requests were
      made.

      A client MAY pipeline multiple requests immediately if it has previous
      knowledge that the server it is connecting to supports persistent
      connections. A client MAY assume that a server supports persistent
      connections if the same server has accepted persistent connections
      within the past 24 hours. Clients which assume persistent connections
      and pipeline immediately SHOULD be prepared to retry their connection if
      the first pipe-lined attempt fails. If a client does such a retry, it
      MUST NOT pipeline without first receiving an explicit Persist token from
      the server. Clients MUST also be prepared to resend their requests if
      the server closes the connection before sending all of the corresponding
      responses.


      14.2.5 Delimiting Entity-Bodies
      When using persistent connections both the client and the server MUST
      mark the exact endings of transmitted entity-bodies using one of the
      following three techniques:

        1.            Send a Content-length field in the header with the exact number of
           bytes in the entity-body.
        2.            Send the message using Chunked transfer encoding as described in
           section 3.6. Chunked transfer encoding allows the server to
           transmit the data to the client a piece at a time while still
           communicating an exact ending of the entity-body.
        3.            Close the transport connection after the entity body.

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      Sending the Content-length is the preferred technique. Chunked encoding
      SHOULD be used when the size of the entity-body is not known before
      beginning to transmit the entity-body.  Finally, the connection MAY be
      closed and fall back to non-persistent connections, if neither 1 or 2
      are possible.

      Clients and servers that support persistent connections MUST correctly
      support receiving via all three techniques.


      14.3 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 negotiate 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.


      14.4 Interaction with Security Protocols
      It is expected that the Session extension will operate with both SHTTP
      [31] and SSL [32]. When used in conjunction with SHTTP, the SHTTP
      request is prepared normally and the persist connection-token is placed
      in the outermost request block (the one containing the _Secure_ method).
      When used in conjunction with SSL, a SSL session is started as normal
      and the first HTTP request made using SSL contains the persistent
      connection header.


      14.5 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
      sides 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.
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      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. 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.

      It is suggested that clients which 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 of 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.


      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
      As mentioned in Section 11.1, the Basic authentication scheme is not a

      secure method of user authentication, nor does it in any way protect the
      Entity-Body, which is transmitted in clear text across the physical
      network used as the carrier. HTTP does not prevent additional
      authentication schemes and encryption mechanisms from being employed to
      increase security or the addition of enhancements (such as schemes to
      use one-time passwords) to Basic authentication.

      The most serious flaw in Basic authentication is that it results in the
      essentially clear text transmission of the user's password over the
      physical network.  It is this problem which Digest Authentication
      attempts to address.

      Because Basic authentication involves the clear text transmission of
      passwords it SHOULD never be used (without enhancements) to protect
      sensitive or valuable information.

      A common use of Basic authentication is for identification purposes --
      requiring the user to provide a user name and password as a means of
      identification, for example, for purposes of gathering accurate usage
      statistics on a server.  When used in this way it is tempting to think
      that there is no danger in its use if illicit access to the protected
      documents is not a major concern.  This is only correct if the server
      issues both user name and password to the users and in particular does
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      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[26].
      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 Safe Methods
      The writers of client software should be aware that the software
      represents the user in their interactions over the Internet, and should
      be careful to allow the user to be aware of any actions they may take
      which may have an unexpected significance to themselves or others.

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

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


      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
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      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.

      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
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      in the Request-URI if it would otherwise allow access to a resource
      outside those intended to be accessible via the HTTP server. Similarly,
      files intended for reference only internally to the server (such as
      access control files, configuration files, and script code) MUST be
      protected from inappropriate retrieval, since they might contain
      sensitive information. Experience has shown that minor bugs in such HTTP
      server implementations have turned into security risks.


      15.6 Personal Information
      HTTP clients are often privy to large amounts of personal information
      (e.g. the user's name, location, mail address, passwords, encryption
      keys, etc.), and SHOULD be very careful to prevent unintentional leakage
      of this information via the HTTP protocol to other sources.  We very
      strongly recommend that a convenient interface be provided for the user
      to control dissemination of such information, and that designers and
      implementers be particularly careful in this area. History shows that
      errors in this area are often both serious security and/or privacy
      problems, and often generate very 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 or Alternates response headers
      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 miss-
      association of IP addresses and DNS names.  The deployment of DNSSEC[27]

      should help this situation.  In advance of this deployment, however,
      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 a 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 renumbering is
      expected to become increasingly common[24], the possibility of this form

      of attack will grow.  Observing this requirement thus reduces this
      potential security vulnerability.

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


      15.9 SLUSHY: 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 [9]. Similarly, it
      reuses many of the definitions provided by Nathaniel Borenstein and Ned
      Freed for MIME [7]. We hope that their inclusion in this specification
      will help reduce past confusion over the relationship between HTTP and
      Internet mail message formats.

      The HTTP protocol has evolved considerably over the past four years. It
      has benefited from a large and active developer community--the many
      people who have participated on the www-talk mailing list--and it is
      that community which has been most responsible for the success of HTTP
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      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                         Harald Tveit Alvestrand
             Keith Ball                         Brian Behlendorf
             Paul Burchard                      Maurizio Codogno
             Mike Cowlishaw                     Roman Czyborra
             Michael A. Dolan                   Jim Gettys
             Marc Hedlund                       Koen Holtman
             Alex Hopmann                       Bob Jernigan
             Shel Kaphan                        Rohit Khare
             Martijn Koster                     Alexei Kosut
             David M. Kristol                    Daniel LaLiberte
             Paul J. Leach                      Albert Lunde
             John C. Mallery                    Jean-Philippe Martin-Flatin
             Larry Masinter                     Mitra
             Jeffrey Mogul                      Gavin Nicol
             Bill Perry                         Jeffrey Perry
             Owen Rees                          Luigi Rizzo
             David Robinson                     Marc Salomon
             Rich Salz                          Jim Seidman
             Chuck Shotton                      Eric W. Sink
             Simon E. Spero                     Richard N. Taylor
             Robert S. Thau                     Francois Yergeau
             Mary Ellen Zurko                   David Morris
             Greg Herlihy                       Scott Powers
             Allan M. Schiffman                 Alan Freier
             Bill (BearHeart) Weinman


      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, Larry Masinter, and Roy Fielding.

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

      XXX need acks for subgroup work.


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      17. References

      [1]
        H. Alvestrand. _Tags for the identification of languages._ RFC 1766,

        UNINETT, March 1995.

      [2]
        F. Anklesaria, M. McCahill, P. Lindner, D. Johnson, D. Torrey, B.
        Alberti. _The Internet Gopher Protocol: (a distributed document

        search and retrieval protocol)_, RFC 1436, University of Minnesota,
        March 1993.

      [3]
        T. Berners-Lee. _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]
        T. Berners-Lee, L. Masinter, M. McCahill.
        _Uniform Resource Locators (URL)._ RFC 1738, CERN, Xerox PARC,

        University of Minnesota, December 1994.

      [5]
        T. Berners-Lee, D. Connolly.
        _HyperText Markup Language Specification - 2.0._ RFC 1866, MIT/LCS,

        November 1995.

      [6]
        T. Berners-Lee, R. Fielding, H. Frystyk.
        "Hypertext Transfer Protocol - HTTP/1.0." Work in Progress (draft-

        ietf-http-v10-spec-04.txt), MIT/LCS, UC Irvine, September 1995.

      [7]
        N. Borenstein, N. Freed.
        _MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms

        for Specifying and Describing the Format of Internet Message Bodies."
        RFC 1521, Bellcore, Innosoft, September 1993.

      [8]
        R. Braden.
        _Requirements for Internet hosts - application and support._ STD 3,

        RFC 1123, IETF, October 1989.

      [9]
        D. H. Crocker.

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      INTERNET-DRAFT            HTTP/1.1               Monday, April 22, 1996


        _Standard for the Format of ARPA Internet Text Messages._ STD 11, RFC

        822, UDEL, August 1982.

      [10]
        F. Davis, B. Kahle, H. Morris, J. Salem, T. Shen, R. Wang, J. Sui, M.
        Grinbaum. _WAIS Interface Protocol Prototype Functional
        Specification._ (v1.5), Thinking Machines Corporation, April 1990.

      [11]
        R. Fielding. _Relative Uniform Resource Locators._ RFC 1808, UC

        Irvine, June 1995.

      [12]
        M. Horton, R. Adams. _Standard for interchange of USENET messages._

        RFC 1036 (Obsoletes RFC 850), AT&T Bell Laboratories, Center for
        Seismic Studies, December 1987.

      [13]
        B. Kantor, 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]
        K. Moore. _MIME (Multipurpose Internet Mail Extensions) Part Two :

        Message Header Extensions for Non-ASCII Text._ RFC 1522, University
        of Tennessee, September 1993.

      [15]
        E. Nebel, L. Masinter. _Form-based File Upload in HTML._ RFC 1867,

        Xerox Corporation, November 1995.

      [16]
        J. Postel. _Simple Mail Transfer Protocol._ STD 10, RFC 821, USC/ISI,

        August 1982.

      [17]
        J. Postel. _Media Type Registration Procedure._ RFC 1590, USC/ISI,

        March 1994.

      [18]
        J. Postel, J. K. Reynolds. _File Transfer Protocol (FTP)_ STD 9, RFC

        959, USC/ISI, October 1985.


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      [19]
        J. Reynolds, J. Postel. _Assigned Numbers._ STD 2, RFC 1700, USC/ISI,

        October 1994.

      [20]
        K. Sollins, L. Masinter.
        _Functional Requirements for Uniform Resource Names._ RFC 1737,

        MIT/LCS, Xerox Corporation, December 1994.

      [21]
        US-ASCII. Coded Character Set - 7-Bit American Standard Code for
        Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.

      [22]
        ISO-8859. International Standard -- Information Processing --
        8-bit Single-Byte Coded Graphic Character Sets --
        Part 1: Latin alphabet No. 1, ISO 8859-1:1987.
        Part 2: Latin alphabet No. 2, ISO 8859-2, 1987.
        Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.
        Part 4: Latin alphabet No. 4, ISO 8859-4, 1988.
        Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988.
        Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987.
        Part 7: Latin/Greek alphabet, ISO 8859-7, 1987.
        Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988.
        Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.

      [23]
        Meyers, M. Rose _The Content-MD5 Header Field._ RFC 1864, Carnegie

        Mellon, Dover Beach Consulting, October, 1995.

      [24]
        B. Carpenter, Y. Rekhter, _Renumbering Needs Work_. RFC 1900, IAB,

        February 1996.

      [25]
        Gzip is available from the GNU project at
        <URL:ftp://prep.ai.mit.edu/pub/gnu/>.  A more formal specification is

        currently a work in progress.

      [26]
        Work In Progress for Digest authentication of the IETF HTTP working
        group.

      [27]
        TBS, Work in progress (XXX should put RFC in here_ )

      [28]
        Mills, D, _Network Time Protocol, Version 3_, Specification,

      Fielding, Frystyk, Berners-Lee, Gettys, and Mogul        [Page   149]


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        Implementation and Analysis RFC 1305, University of Delaware, March,
        1992.

      [29]
        Work in progress of the HTTP working group (XXX is this correct
        reference for incomplete work?).

      [30]
        S. Spero. _Analysis of HTTP Performance Problems_
        <URL:http://sunsite.unc.edu/mdma-release/http-prob.html>

      [31]
        E. Rescorla, A. Schiffman _The Secure HyperText Transfer Protocol_
        Internet-Draft (work in progress).

      [32]
        A. Freier, P Karlton, P. Kocher. _SSL Version 3.0" Internet-Draft_
        (work in progress).

      [33]
        Jeffrey C. Mogul. _The Case for Persistent-Connection HTTP_.  In
        Proc.SIGCOMM '95 Symposium on Communications Architectures and
        Protocols, pages 299-313. Cambridge, MA, August, 1995.

      [34]
        Jeffrey C. Mogul. _The Case for Persistent-Connection HTTP_.
        Research, Report 95/4, Digital Equipment Corporation Western Research
        Laboratory, May, 1995.,
        <URL
        :http://www.research.digital.com/wrl/techreports/abstracts/95.4.html>

      [35]
        Work in progress of the HTTP working group on state management.


      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

      Henrik Frystyk Nielsen

      W3 Consortium
      MIT Laboratory for Computer Science
      545 Technology Square
      Cambridge, MA 02139, USA

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      Fax: +1 (617) 258 8682
      Email: frystyk@w3.org

      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

      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, U.S.A.
      Email: mogul@wrl.dec.com


      Appendices
      These appendices are provided for informational reasons only -- they do
      not form a part of the HTTP/1.1 specification.


      A. Internet Media Type message/http
      In addition to defining the HTTP/1.1 protocol, this document serves as
      the specification for the Internet media type _message/http_. The
      following is to be registered with IANA [17].


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             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


      B. 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


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

      The line terminator for HTTP-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.


      C. Differences Between  HTTP Bodies and RFC 1521 Internet Message Bodies
      HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC 822
      [9]) and the Multipurpose Internet Mail Extensions (MIME [7]) to allow

      entities to be transmitted in an open variety of representations and
      with extensible mechanisms. However, RFC 1521 discusses mail, and HTTP
      has a few features that are different than those described in RFC 1521.
      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.


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      At the time of this writing, it is expected that RFC 1521 will be
      revised. The revisions may include some of the practices found in
      HTTP/1.1 but not in RFC 1521.

      This appendix describes specific areas where HTTP differs from RFC 1521.
      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.


      C.1 Conversion to Canonical Form
      RFC 1521 requires that an Internet mail entity be converted to canonical
      form prior to being transferred, as described in Appendix G of RFC 1521
      [7]. Section 3.6.1 of this document describes the forms allowed for

      subtypes of the _text_ media type when transmitted over HTTP.  RFC 1521
      requires that content with a  typeof _text_ represent line breaks as
      CRLF and forbids the use of CR or LF outside of line break sequences.
      HTTP allows CRLF, bare CR, and bare LF to indicate a line break within
      text content when a message is transmitted over HTTP.

      Where it is possible, a proxy or gateway from HTTP to a strict RFC 1521
      environment SHOULD translate all line breaks within the text media types
      described in Section 3.6.1 of this document to the RFC 1521 canonical

      form of CRLF. Note, however, that this may be complicated by the
      presence of a Content-Encoding and by the fact that HTTP allows the use
      of some character sets which do not use octets 13 and 10 to represent CR
      and LF, as is the case for some multi-byte character sets.


      C.2 Conversion of Date Formats
      HTTP/1.1 uses a restricted set of date formats (Section 3.3) 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.


      C.3 Introduction of Content-Encoding
      RFC 1521 does not include any concept equivalent to HTTP/1.1's Content-
      Encoding header field. Since this acts as a modifier on the media type,
      proxies and gateways from HTTP to MIME-compliant protocols MUST either
      change the value of the Content-Type header field or decode the Entity-
      Body before forwarding the message. (Some experimental applications of
      Content-Type for Internet mail have used a media-type parameter of
      _;conversions=<content-coding>_ to perform an equivalent function as
      Content-Encoding. However, this parameter is not part of RFC 1521.)


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      C.4 No Content-Transfer-Encoding
      HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC 1521.
      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.


      C.5 HTTP Header Fields in Multipart Body-Parts
      In RFC 1521, 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.


      C.6 Introduction of Transfer-Encoding
      HTTP/1.1 introduces the Transfer-Encoding header field (Section 10.39).

      Proxies/gateways MUST remove any transfer coding prior to forwarding a
      message via a MIME-compliant protocol. The process for decoding the
      _chunked_ transfer coding (Section 3.6) can be represented in pseudo-

      code as:

             length := 0
             read chunk-size 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


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      C.7 MIME-Version
      HTTP is not a MIME-compliant protocol (see Appendix C). However,

      HTTP/1.1 messages may include a single MIME-Version general-header field
      to indicate what version of the MIME protocol was used to construct the
      message. Use of the MIME-Version header field indicates that the message
      is in full compliance with the MIME protocol (as defined in [7]).

      Proxies/gateways are responsible for ensuring full compliance (where
      possible) when exporting HTTP messages to strict MIME environments.


      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.


      D. Changes from HTTP/1.0
      This section will summarize major differences between versions 1.0 and             MIME-Version   = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
      1.1 of the Hypertext Transfer Protocol.


      D.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 is  missing from an
      HTTP/1.1 request (Section 10.22),  and accept absolute URIs (Section
      5.1.2) are among the most important changes from HTTP/1.0.

      In HTTP/1.0 there is a one-to-one relationship of IP addresses and
      servers. There is no other way to distinguish the intended server of a
      request than the IP address to which that request is directed. The
      HTTP/1.1 change will allow the Internet, once HTTP/1.0 clients and
      servers 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 used for the sole purpose of allowing root-
      level domain names to be used in HTTP URLs. Given the rate of growth of
      the Web, and the number of servers already deployed, it is extremely
      important that implementations of HTTP/1.1 correctly implement these new
      requirements:


        .  both clients and servers MUST support the Host request-header

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

        .  servers MUST report an error if an HTTP/1.1 request does not
           include a Host request-header

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        .  servers MUST accept absolute URIs

      E. 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.


      E.1 Additional Request Methods

      E.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.

      For compatibility with HTTP/1.0 applications, all PATCH requests MUST
      include a valid Content-Length header field unless the server is known
      to be HTTP/1.1 compliant. When sending a PATCH request to an HTTP/1.1
      server, a client MUST use a valid Content-Length or the _chunked_
      Transfer-Encoding. The server SHOULD respond with a 400 (bad request)
      message if it cannot determine the length of the request message's
      content, or with 411 (length required) if it wishes to insist on
      receiving a valid Content-Length.

      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 entity transmission requirements set out in
      section 8.4.1.

      Caches that implement PATCH should invalidate cached responses as
      defined in section 13.17 for PUT.


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      E.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 allowing links
      to be established between resources is that the LINK method does not
      allow any Entity-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 13.17 for PUT.


      E.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 13.17 for PUT.


      E.2 Additional Header Field Definitions

      E.2.1 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 10.18, 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. It SHOULD NOT be used to indicate derived
      works or renditions in different representations. It MAY also me used as
      an opaque value for comparing a cached entity's version with that of the
      current resource.

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


      Examples of the Content-Version field include:


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             Content-Version: "2.1.2"

             Content-Version: "Fred 19950116-12:26:48"

             Content-Version: "2.5a4-omega7"


      The value of the Content-Version field SHOULD be considered opaque to
      all parties but the origin server. A user agent MAY suggest a value for
      the version of an entity transferred via a PUT request; however, only
      the origin server can reliably assign that value.


      E.2.2 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.


      E.2.3 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].


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             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.


      E.2.4 URI
      The URI header field has, in past versions of this specification, been
      used as a combination of the existing Location, Content-Location, and
      Alternates header fields. 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.


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


      E.2.5 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 version 1.0 clients
      and servers.

      When connecting to an origin server an HTTP client MAY send the Keep-
      Alive connection-token in addition to the Persist connection-token:

             Connection: Keep-Alive,Persist

      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.

      A persistent connection based on the Keep-Alive connection token MUST
      only use the _Content-Length_ technique for marking the ending
      boundaries of entity-bodies. It MAY use pipe-lining.

      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.


      E.2.5.1 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.


      F.1 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.  While we are contemplating a
      separate document containing advice to implementers, we feel it worth
      noting that at the time of composing this specification, we would expect
      commercial HTTP/1.1 servers to::

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

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

        .  respond appropriately with a message in the same major version used
           by the client.
      And  we would expect HTTP/1.1 clients to:


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

        .  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 E.2.5.1.


      G. Proxy Cache Implementation Guidelines

      G.1 Support for Content Negotiation by Proxy Caches
      The material in appendix G should go into a separate implementation
      guide as an informational RFC, rather than in this specification. (since
      it mostly describes 3 possible cache implementation strategies possible
      within the protocol, rather than just the two protocol facilities
      (transparent and opaque)). For the purposes of this (02) draft, we will
      leave it in as an appendix as it clarifies some points of how caching
      might work in the context of the HTTP/1.1 protocol.

      If a resource is varying, this has an important effect on cache
      management, particularly for caching proxies which service a diverse set
      of user agents.  Such proxy caches must correctly handle requests on
      varying resources in order not to disturb the negotiation process.

      This specification distinguishes six levels of correct support for
      content negotiation by proxy caches.  The text below describes these
      levels, but does not exhaustively list all mechanisms associated with
      support on these levels.  In particular, mechanisms for handling partial
      requests on varying resources are not discussed.

        1.            A proxy cache providing level 1 support will never store in cache
           memory responses from varying resources (such responses always
           include at least one Vary header or Alternates header).  When
           receiving a request on a varying resource, the proxy will thus
           always forward the request towards an upstream server.  A level 1
           proxy cache never makes selection decisions itself.
        2.            A proxy cache providing level 2 support is able to maintain in
           cache memory a mapping from the varying resource URI to a set of
           200 (OK) response messages.  When receiving a request on the
           varying resource, the proxy will forward this request to an
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           upstream server after including an If-Invalid request header field
           listing the CVal header values of the associated cached 200
           responses, as described in Section 10.52.  If a 304 (Not Modified)
           response is received from the upstream server, the proxy updates,
           with the 304 response headers, the stored 200 response which has
           the same CVal header field as the 304 response. It then passes
           either the updated 200 response or the 304 response on to its
           client, the choice depending on the presence and contents of an If-
           Invalid header in the original request.  If a 200 response is
           received from the upstream server, the proxy will update the set of
           responses it has for the varying resource by using the cache update
           algorithm described in Section 13.20, and pass on the 200 response
           to its client.
        3.            A proxy cache providing level 3 support is able to maintain in
           cache memory a mapping from the varying resource URI to a set of
           200 (OK) response messages.  In addition, it is able to maintain,
           for each cached 200 response belonging to the varying resource, a
           list of selecting request header sequences.  This list of selecting
           request header sequences starts with the sequence taken from the
           request which initially caused an upstream server to return the
           cached 200 response, and continues with any additional sequences
           taken from subsequent requests which caused an upstream server to
           return a 304 response with a CVal header identical to the CVal
           header of that cached 200 response.  When receiving a request on
           the varying resource, the proxy will iterate over all cached, fresh
           200 responses associated with the resource.  For each fresh 200
           response, it will search the associated list of selecting request
           header sequences to see if a match to the headers of the current
           request can be found.  If a match is found, the proxy will return
           the fresh 200 response in question.  If no match is found, the
           proxy will switch to level 1 behavior and pass on the request to an
           upstream server.  The response received from the upstream server
           may refresh a stale 200 response that was cached for the varying
           resource a side effect. XXX previous sentence doesn't make sense_
        4.            A proxy cache providing level 4 support provides transparent
           negotiation services for transparently negotiated resources, and
           provides level 1 support for opaquely negotiated resources.
        5.            A proxy cache providing level 5 support provides transparent
           negotiation services for transparently negotiated resources, and
           provides level 2 support for opaquely negotiated resources.
        6.            A proxy cache providing level 6 support provides transparent
           negotiation services for transparently negotiated resources, and
           provides level 3 support for opaquely negotiated resources.
        Note: Implementation of support levels 4 to 6 is only possible
        when the planned content negotiation specification [29] is
        completed.  The level numbers above were assigned to reflect
        expected caching efficiency in an environment where the proxy
        cache is serving a diverse set of clients.  It is expected that
        level 4 proxies will be easier to implement than level 3
        proxies.


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      G.2  Propagation of Changes in Opaque Selection
      Level 3 and level 6 proxy caches not only cache the responses from an
      opaquely varying resource, they also cache the mappings from request
      headers to particular entities computed by the opaque selection
      algorithm located at the origin server.  If this selection algorithm is
      changed by the resource author, for example because a Spanish text
      entity is added to a resource which previously only had English and
      French entities available, it is important to make the level 3 and 6
      caches refresh their cached mappings.  This can be done by changing the
      CVal header fields sent along with the original English and French
      responses.  This change will eventually cause the proxies to replace the
      old English and French responses in cache memory, along with their
      associated lists of selecting request header sequences, by `new' English
      and French responses with fresh lists of selecting request header
      sequences. In order to guarantee an upper time bound for this update
      process, the resource author can include an appropriate Cache-control:
      max-age=... directive in the responses from the varying resource.


      G.3 SLUSHY: State
      This should probably be in the cookie ID, and not in this document at
      all.

      HTTP implementations often support facilities for state management,
      often called _cookies_[35].  Cookies can not be cached by public
      (shared) caches, but since public documents may make up part of a
      _stateful dialog,_ and in particular the first document in a stateful
      dialog may be (for example) a public and cachable home page, servers
      that wish to receive the client's cookie on each request, or to issue a
      new cookie on requests for a document, must set the document up to
      require validation on each request (Cache-Control: must-revalidate)

      In general, the cache control headers for responses control what a proxy
      has to do.  If a document is fresh in a cache, a request containing a
      cookie does not have to be forwarded to the origin server, since (by
      definition) if the document can be served from a cache the origin server
      must have said there are no important side effects at the origin
      relating to requests for that document, and so, no changes to the
      cookie.

      One important state issue bearing on caching is that for conditional
      requests that go through to the origin server, for which the origin
      server responds with 304 and also with a set-cookie header, caches must
      splice the set-cookie sent by the origin server into their own response.
      For example, this allows a home page to be cached, but stale, so that
      the only traffic to the origin server is to validate the home page,
      receiving a 304 and potentially a new cookie.


      G.4 FLUID: Cache Replacement Algorithms
      TBS

      Should go into an implementers Informational RFC.

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      G.5 FLUID: Bypassing in Caching Hierarchies
      This should also go into an implementers Informational RFC, and become
      grist for HTML's mill.

      Many HTTP caching configurations involve hierarchies of caches, often
      designed to reduce bandwidth requirements rather than improving latency.
      However, if a cache at a low level in the hierarchy is sure that the
      cache(s) above it do not contain a cache entry to match a given request,
      that low-level cache can transmit the request directly to the origin
      server.  This improves retrieval latency without increasing total
      bandwidth requirements (it even eliminates some packet transmissions)
      and is entirely appropriate for resources whose values are explicitly
      not cached.

      We call this technique _request bypassing._ Note that although the
      bypassing decision might be done by the ultimate client, in many cases
      the use of firewalls or unsophisticated clients means that the decision
      must be made by an intermediate-level cache.

      In order for request bypassing to work in the most efficient possible
      way, the caches must be able to determine from the request whether the
      response is likely to be cachable.  (It is important to err on the side
      of assuming cachability,  since the assuming converse could seriously
      reduce the effectiveness of the higher-level caches.)

      The current HTTP/1.1 draft specification does not include a foolproof
      mechanism to mark requests in this way.  While we generally do not allow
      caching of responses to GET requests for URLs with a _?_ in the rel_path
      part (see section 13.16), we also allow the origin server to mark
      responses to  such queries as cachable.  Therefore, any bypassing done
      using this heuristic runs the risk of giving up perfectly good
      opportunities to cache some resources.

      XXX we have discussed various approaches for marking requests, all of
      which apparently require some kind of change to HTML to allow the origin
      server to pass the marks to the ultimate client.  Some people suggest
      using  special methods that are explicitly always cachable
      (_POST_WITH_NO_SIDE_EFFECTS_, or more concisely _POSTC_) or never
      cachable (_GET_QUERY_, or more concisely _GETQ_).  Others have suggested
      adding tags to HTML that would cause subsequent requests to carry some
      special sort of header.  Neither solution has resulted in a consensus.

      An origin server would be able to use POSTC only withHTTP/1.1 clients
      and proxies, and so would have to return different HTML forms depending
      on the protocol version in the request header.  This would also imply
      using the proposed Vary:  header with some token that indicates _varies
      based on request HTTP version,_  since we don't want a cache returning
      one of these HTML responses to an HTTP/1.0 client


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