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RFC 2068 - Hypertext Transfer Protocol -- HTTP/1.1


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

                Hypertext Transfer Protocol -- HTTP/1.1

Status of this Memo

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

Abstract

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

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

Table of Contents

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

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

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

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

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

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

1 Introduction

1.1 Purpose

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypermedia information
   systems. HTTP has been in use by the World-Wide Web global
   information initiative since 1990. The first version of HTTP,
   referred to as HTTP/0.9, was a simple protocol for raw data transfer
   across the Internet. HTTP/1.0, as defined by RFC 1945 [6], improved
   the protocol by allowing messages to be in the format of MIME-like
   messages, containing metainformation about the data transferred and
   modifiers on the request/response semantics. However, HTTP/1.0 does
   not sufficiently take into consideration the effects of hierarchical
   proxies, caching, the need for persistent connections, and virtual
   hosts. In addition, the proliferation of incompletely-implemented
   applications calling themselves "HTTP/1.0" has necessitated a
   protocol version change in order for two communicating applications
   to determine each other's true capabilities.

   This specification defines the protocol referred to as "HTTP/1.1".
   This protocol includes more stringent requirements than HTTP/1.0 in
   order to ensure reliable implementation of its features.

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

   HTTP is also used as a generic protocol for communication between
   user agents and proxies/gateways to other Internet systems, including
   those supported by the SMTP [16], NNTP [13], FTP [18], Gopher [2],
   and WAIS [10] protocols. In this way, HTTP allows basic hypermedia
   access to resources available from diverse applications.

1.2 Requirements

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

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

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

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

   An implementation is not compliant if it fails to satisfy one or more
   of the MUST requirements for the protocols it implements. An
   implementation that satisfies all the MUST and all the SHOULD
   requirements for its protocols is said to be "unconditionally
   compliant"; one that satisfies all the MUST requirements but not all
   the SHOULD requirements for its protocols is said to be
   "conditionally compliant."

1.3 Terminology

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

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

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

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

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

   resource
      A network data object or service that can be identified by a URI,
      as defined in section 3.2. Resources may be available in multiple
      representations (e.g. multiple languages, data formats, size,
      resolutions) or vary in other ways.

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

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

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

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

   client
      A program that establishes connections for the purpose of sending
      requests.

   user agent
      The client which initiates a request. These are often browsers,
      editors, spiders (web-traversing robots), or other end user tools.

   server
      An application program that accepts connections in order to
      service requests by sending back responses. Any given program may
      be capable of being both a client and a server; our use of these
      terms refers only to the role being performed by the program for a
      particular connection, rather than to the program's capabilities
      in general.  Likewise, any server may act as an origin server,
      proxy, gateway, or tunnel, switching behavior based on the nature
      of each request.

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

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

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

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

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

   cachable
      A response is cachable if a cache is allowed to store a copy of
      the response message for use in answering subsequent requests. The
      rules for determining the cachability of HTTP responses are
      defined in section 13. Even if a resource is cachable, there may
      be additional constraints on whether a cache can use the cached
      copy for a particular request.

   first-hand
      A response is first-hand if it comes directly and without
      unnecessary delay from the origin server, perhaps via one or more
      proxies. A response is also first-hand if its validity has just
      been checked directly with the origin server.

   explicit expiration time
      The time at which the origin server intends that an entity should
      no longer be returned by a cache without further validation.

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

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

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

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

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

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

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

1.4 Overall Operation

   The HTTP protocol is a request/response protocol. A client sends a
   request to the server in the form of a request method, URI, and
   protocol version, followed by a MIME-like message containing request
   modifiers, client information, and possible body content over a
   connection with a server. The server responds with a status line,
   including the message's protocol version and a success or error code,
   followed by a MIME-like message containing server information, entity
   metainformation, and possible entity-body content. The relationship
   between HTTP and MIME is described in appendix 19.4.

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

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

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

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

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

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

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

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

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

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

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

2 Notational Conventions and Generic Grammar

2.1 Augmented BNF

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

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

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

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

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

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

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

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

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

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

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

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

2.2 Basic Rules

   The following rules are used throughout this specification to
   describe basic parsing constructs. The US-ASCII coded character set
   is defined by ANSI X3.4-1986 [21].

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

   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
   protocol elements except the entity-body (see appendix 19.3 for
   tolerant applications). The end-of-line marker within an entity-body
   is defined by its associated media type, as described in section 3.7.

          CRLF           = CR LF

   HTTP/1.1 headers can be folded onto multiple lines if the
   continuation line begins with a space or horizontal tab. All linear
   white space, including folding, has the same semantics as SP.

          LWS            = [CRLF] 1*( SP | HT )

   The TEXT rule is only used for descriptive field contents and values
   that are not intended to be interpreted by the message parser. Words
   of *TEXT may contain characters from character sets other than ISO
   8859-1 [22] only when encoded according to the rules of RFC 1522
   [14].

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

   Hexadecimal numeric characters are used in several protocol elements.

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

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

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

          tspecials      = "(" | ")" | "<" | ">" | "@"
                         | "," | ";" | ":" | "\" | <">
                         | "/" | "[" | "]" | "?" | "="
                         | "{" | "}" | SP | HT

   Comments can be included in some HTTP header fields by surrounding
   the comment text with parentheses. Comments are only allowed in
   fields containing "comment" as part of their field value definition.
   In all other fields, parentheses are considered part of the field
   value.

          comment        = "(" *( ctext | comment ) ")"
          ctext          = <any TEXT excluding "(" and ")">

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

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

          qdtext         = <any TEXT except <">>

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

          quoted-pair    = "\" CHAR

3 Protocol Parameters

3.1 HTTP Version

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

   The version of an HTTP message is indicated by an HTTP-Version field
   in the first line of the message.

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

   Note that the major and minor numbers MUST be treated as separate
   integers and that each may be incremented higher than a single digit.
   Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
   lower than HTTP/12.3. Leading zeros MUST be ignored by recipients and
   MUST NOT be sent.

   Applications sending Request or Response messages, as defined by this
   specification, MUST include an HTTP-Version of "HTTP/1.1". Use of
   this version number indicates that the sending application is at
   least conditionally compliant with this specification.

   The HTTP version of an application is the highest HTTP version for
   which the application is at least conditionally compliant.

   Proxy and gateway applications must be careful when forwarding
   messages in protocol versions different from that of the application.
   Since the protocol version indicates the protocol capability of the
   sender, a proxy/gateway MUST never send a message with a version
   indicator which is greater than its actual version; if a higher
   version request is received, the proxy/gateway MUST either downgrade
   the request version, respond with an error, or switch to tunnel
   behavior. Requests with a version lower than that of the
   proxy/gateway's version MAY be upgraded before being forwarded; the
   proxy/gateway's response to that request MUST be in the same major
   version as the request.

     Note: Converting between versions of HTTP may involve modification
     of header fields required or forbidden by the versions involved.

3.2 Uniform Resource Identifiers

   URIs have been known by many names: WWW addresses, Universal Document
   Identifiers, Universal Resource Identifiers , and finally the
   combination of Uniform Resource Locators (URL)  and Names (URN). As
   far as HTTP is concerned, Uniform Resource Identifiers are simply
   formatted strings which identify--via name, location, or any other
   characteristic--a resource.

3.2.1 General Syntax

   URIs in HTTP can be represented in absolute form or relative to some
   known base URI, depending upon the context of their use. The two
   forms are differentiated by the fact that absolute URIs always begin
   with a scheme name followed by a colon.

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

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

          relativeURI    = net_path | abs_path | rel_path

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

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

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

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

          query          = *( uchar | reserved )
          fragment       = *( uchar | reserved )

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

          escape         = "%" HEX HEX
          reserved       = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
          extra          = "!" | "*" | "'" | "(" | ")" | ","
          safe           = "$" | "-" | "_" | "."
          unsafe         = CTL | SP | <"> | "#" | "%" | "<" | ">"
          national       = <any OCTET excluding ALPHA, DIGIT,
                           reserved, extra, safe, and unsafe>

   For definitive information on URL syntax and semantics, see RFC 1738
   [4] and RFC 1808 [11]. The BNF above includes national characters not
   allowed in valid URLs as specified by RFC 1738, since HTTP servers
   are not restricted in the set of unreserved characters allowed to
   represent the rel_path part of addresses, and HTTP proxies may
   receive requests for URIs not defined by RFC 1738.

   The HTTP protocol does not place any a priori limit on the length of
   a URI. Servers MUST be able to handle the URI of any resource they
   serve, and SHOULD be able to handle URIs of unbounded length if they
   provide GET-based forms that could generate such URIs. A server
   SHOULD return 414 (Request-URI Too Long) status if a URI is longer
   than the server can handle (see section 10.4.15).

     Note: Servers should be cautious about depending on URI lengths
     above 255 bytes, because some older client or proxy implementations
     may not properly support these lengths.

3.2.2 http URL

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

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

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

          port           = *DIGIT

   If the port is empty or not given, port 80 is assumed. The semantics
   are that the identified resource is located at the server listening
   for TCP connections on that port of that host, and the Request-URI
   for the resource is abs_path. The use of IP addresses in URL's SHOULD
   be avoided whenever possible (see RFC 1900 [24]). If the abs_path is
   not present in the URL, it MUST be given as "/" when used as a
   Request-URI for a resource (section 5.1.2).

3.2.3 URI Comparison

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

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

     o  Comparisons of host names MUST be case-insensitive;

     o  Comparisons of scheme names MUST be case-insensitive;

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

   Characters other than those in the "reserved" and "unsafe" sets (see
   section 3.2) are equivalent to their ""%" HEX HEX" encodings.

   For example, the following three URIs are equivalent:

         http://abc.com:80/~smith/home.html
         http://ABC.com/%7Esmith/home.html
         /ABC.com:/%7esmith/home.html">http://ABC.com:/%7esmith/home.html

3.3 Date/Time Formats

3.3.1 Full Date

   HTTP applications have historically allowed three different formats
   for the representation of date/time stamps:

          Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
          Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
          Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format

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

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

   All HTTP date/time stamps MUST be represented in Greenwich Mean Time
   (GMT), without exception. This is indicated in the first two formats
   by the inclusion of "GMT" as the three-letter abbreviation for time
   zone, and MUST be assumed when reading the asctime format.

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

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

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

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

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

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

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

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

3.3.2 Delta Seconds

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

          delta-seconds  = 1*DIGIT

3.4 Character Sets

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

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

     Note: This use of the term "character set" is more commonly
     referred to as a "character encoding." However, since HTTP and MIME
     share the same registry, it is important that the terminology also
     be shared.

   HTTP character sets are identified by case-insensitive tokens. The
   complete set of tokens is defined by the IANA Character Set registry
   [19].

          charset = token

   Although HTTP allows an arbitrary token to be used as a charset
   value, any token that has a predefined value within the IANA
   Character Set registry MUST represent the character set defined by
   that registry.  Applications SHOULD limit their use of character sets
   to those defined by the IANA registry.

3.5 Content Codings

   Content coding values indicate an encoding transformation that has
   been or can be applied to an entity. Content codings are primarily
   used to allow a document to be compressed or otherwise usefully
   transformed without losing the identity of its underlying media type
   and without loss of information. Frequently, the entity is stored in
   coded form, transmitted directly, and only decoded by the recipient.

          content-coding   = token

   All content-coding values are case-insensitive. HTTP/1.1 uses
   content-coding values in the Accept-Encoding (section 14.3) and
   Content-Encoding (section 14.12) header fields. Although the value
   describes the content-coding, what is more important is that it
   indicates what decoding mechanism will be required to remove the
   encoding.

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

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

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

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

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

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

3.6 Transfer Codings

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

          transfer-coding         = "chunked" | transfer-extension

          transfer-extension      = token

   All transfer-coding values are case-insensitive. HTTP/1.1 uses
   transfer coding values in the Transfer-Encoding header field (section
   14.40).

   Transfer codings are analogous to the Content-Transfer-Encoding
   values of MIME , which were designed to enable safe transport of
   binary data over a 7-bit transport service. However, safe transport
   has a different focus for an 8bit-clean transfer protocol. In HTTP,
   the only unsafe characteristic of message-bodies is the difficulty in
   determining the exact body length (section 7.2.2), or the desire to
   encrypt data over a shared transport.

   The chunked encoding modifies the body of a message in order to
   transfer it as a series of chunks, each with its own size indicator,
   followed by an optional footer containing entity-header fields. This
   allows dynamically-produced content to be transferred along with the
   information necessary for the recipient to verify that it has
   received the full message.

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

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

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

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

       footer         = *entity-header

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

   An example process for decoding a Chunked-Body is presented in
   appendix 19.4.6.

   All HTTP/1.1 applications MUST be able to receive and decode the
   "chunked" transfer coding, and MUST ignore transfer coding extensions
   they do not understand. A server which receives an entity-body with a
   transfer-coding it does not understand SHOULD return 501
   (Unimplemented), and close the connection. A server MUST NOT send
   transfer-codings to an HTTP/1.0 client.

3.7 Media Types

   HTTP uses Internet Media Types  in the Content-Type (section 14.18)
   and Accept (section 14.1) header fields in order to provide open and
   extensible data typing and type negotiation.

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

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

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

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

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

   Media-type values are registered with the Internet Assigned Number
   Authority (IANA). The media type registration process is outlined in
   RFC 2048 [17]. Use of non-registered media types is discouraged.

3.7.1 Canonicalization and Text Defaults

   Internet media types are registered with a canonical form. In
   general, an entity-body transferred via HTTP messages MUST be
   represented in the appropriate canonical form prior to its
   transmission; the exception is "text" types, as defined in the next
   paragraph.

   When in canonical form, media subtypes of the "text" type use CRLF as
   the text line break. HTTP relaxes this requirement and allows the
   transport of text media with plain CR or LF alone representing a line
   break when it is done consistently for an entire entity-body. HTTP
   applications MUST accept CRLF, bare CR, and bare LF as being
   representative of a line break in text media received via HTTP. In
   addition, if the text is represented in a character set that does not
   use octets 13 and 10 for CR and LF respectively, as is the case for
   some multi-byte character sets, HTTP allows the use of whatever octet
   sequences are defined by that character set to represent the
   equivalent of CR and LF for line breaks. This flexibility regarding
   line breaks applies only to text media in the entity-body; a bare CR
   or LF MUST NOT be substituted for CRLF within any of the HTTP control
   structures (such as header fields and multipart boundaries).

   If an entity-body is encoded with a Content-Encoding, the underlying
   data MUST be in a form defined above prior to being encoded.

   The "charset" parameter is used with some media types to define the
   character set (section 3.4) of the data. When no explicit charset
   parameter is provided by the sender, media subtypes of the "text"
   type are defined to have a default charset value of "ISO-8859-1" when
   received via HTTP. Data in character sets other than "ISO-8859-1" or
   its subsets MUST be labeled with an appropriate charset value.

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

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

3.7.2 Multipart Types

   MIME provides for a number of "multipart" types -- encapsulations of
   one or more entities within a single message-body. All multipart
   types share a common syntax, as defined in  MIME [7], and MUST
   include a boundary parameter as part of the media type value. The
   message body is itself a protocol element and MUST therefore use only
   CRLF to represent line breaks between body-parts. Unlike in MIME, the
   epilogue of any multipart message MUST be empty; HTTP applications
   MUST NOT transmit the epilogue (even if the original multipart
   contains an epilogue).

   In HTTP, multipart body-parts MAY contain header fields which are
   significant to the meaning of that part. A Content-Location header
   field (section 14.15) SHOULD be included in the body-part of each
   enclosed entity that can be identified by a URL.

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

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

3.8 Product Tokens

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

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

   Examples:

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

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

3.9 Quality Values

   HTTP content negotiation (section 12) uses short "floating point"
   numbers to indicate the relative importance ("weight") of various
   negotiable parameters. A weight is normalized to a real number in the
   range 0 through 1, where 0 is the minimum and 1 the maximum value.
   HTTP/1.1 applications MUST NOT generate more than three digits after
   the decimal point. User configuration of these values SHOULD also be
   limited in this fashion.

          qvalue         = ( "0" [ "." 0*3DIGIT ] )
                         | ( "1" [ "." 0*3("0") ] )

   "Quality values" is a misnomer, since these values merely represent
   relative degradation in desired quality.

3.10 Language Tags

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

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

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

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

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

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

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

3.11 Entity Tags

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

         entity-tag = [ weak ] opaque-tag

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

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

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

   An entity tag MUST be unique across all versions of all entities
   associated with a particular resource. A given entity tag value may
   be used for entities obtained by requests on different URIs without
   implying anything about the equivalence of those entities.

3.12 Range Units

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

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

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

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

4 HTTP Message

4.1 Message Types

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

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

   Request (section 5) and Response (section 6) messages use the generic
   message format of RFC 822 [9] for transferring entities (the payload
   of the message). Both types of message consist of a start-line, one
   or more header fields (also known as "headers"), an empty line (i.e.,
   a line with nothing preceding the CRLF) indicating the end of the
   header fields, and an optional message-body.

           generic-message = start-line
                             *message-header
                             CRLF
                             [ message-body ]

           start-line      = Request-Line | Status-Line

   In the interest of robustness, servers SHOULD ignore any empty
   line(s) received where a Request-Line is expected. In other words, if
   the server is reading the protocol stream at the beginning of a
   message and receives a CRLF first, it should ignore the CRLF.

     Note: certain buggy HTTP/1.0 client implementations generate an
     extra CRLF's after a POST request. To restate what is explicitly
     forbidden by the BNF, an HTTP/1.1 client must not preface or follow
     a request with an extra CRLF.

4.2 Message Headers

   HTTP header fields, which include general-header (section 4.5),
   request-header (section 5.3), response-header (section 6.2), and
   entity-header (section 7.1) fields, follow the same generic format as
   that given in Section 3.1 of RFC 822 [9]. Each header field consists
   of a name followed by a colon (":") and the field value. Field names
   are case-insensitive. The field value may be preceded by any amount
   of LWS, though a single SP is preferred. Header fields can be
   extended over multiple lines by preceding each extra line with at
   least one SP or HT.  Applications SHOULD follow "common form" when
   generating HTTP constructs, since there might exist some
   implementations that fail to accept anything beyond the common forms.

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

          field-name     = token
          field-value    = *( field-content | LWS )

          field-content  = <the OCTETs making up the field-value
                           and consisting of either *TEXT or combinations
                           of token, tspecials, and quoted-string>

   The order in which header fields with differing field names are
   received is not significant. However, it is "good practice" to send
   general-header fields first, followed by request-header or response-
   header fields, and ending with the entity-header fields.

   Multiple message-header fields with the same field-name may be
   present in a message if and only if the entire field-value for that
   header field is defined as a comma-separated list [i.e., #(values)].
   It MUST be possible to combine the multiple header fields into one
   "field-name: field-value" pair, without changing the semantics of the
   message, by appending each subsequent field-value to the first, each
   separated by a comma. The order in which header fields with the same
   field-name are received is therefore significant to the
   interpretation of the combined field value, and thus a proxy MUST NOT
   change the order of these field values when a message is forwarded.

4.3 Message Body

   The message-body (if any) of an HTTP message is used to carry the
   entity-body associated with the request or response. The message-body
   differs from the entity-body only when a transfer coding has been
   applied, as indicated by the Transfer-Encoding header field (section
   14.40).

          message-body = entity-body
                       | <entity-body encoded as per Transfer-Encoding>

   Transfer-Encoding MUST be used to indicate any transfer codings
   applied by an application to ensure safe and proper transfer of the
   message.  Transfer-Encoding is a property of the message, not of the
   entity, and thus can be added or removed by any application along the
   request/response chain.

   The rules for when a message-body is allowed in a message differ for
   requests and responses.

   The presence of a message-body in a request is signaled by the
   inclusion of a Content-Length or Transfer-Encoding header field in
   the request's message-headers. A message-body MAY be included in a
   request only when the request method (section 5.1.1) allows an
   entity-body.

   For response messages, whether or not a message-body is included with
   a message is dependent on both the request method and the response
   status code (section 6.1.1). All responses to the HEAD request method
   MUST NOT include a message-body, even though the presence of entity-
   header fields might lead one to believe they do. All 1xx
   (informational), 204 (no content), and 304 (not modified) responses
   MUST NOT include a message-body. All other responses do include a
   message-body, although it may be of zero length.

4.4 Message Length

   When a message-body is included with a message, the length of that
   body is determined by one of the following (in order of precedence):

   1. Any response message which MUST NOT include a message-body
     (such as the 1xx, 204, and 304 responses and any response to a HEAD
     request) is always terminated by the first empty line after the
     header fields, regardless of the entity-header fields present in the
     message.

   2. If a Transfer-Encoding header field (section 14.40) is present and
     indicates that the "chunked" transfer coding has been applied, then

     the length is defined by the chunked encoding (section 3.6).

   3. If a Content-Length header field (section 14.14) is present, its
     value in bytes represents the length of the message-body.

   4. If the message uses the media type "multipart/byteranges", which is
     self-delimiting, then that defines the length. This media type MUST
     NOT be used unless the sender knows that the recipient can parse it;
     the presence in a request of a Range header with multiple byte-range
     specifiers implies that the client can parse multipart/byteranges
     responses.

   5. By the server closing the connection. (Closing the connection
     cannot be used to indicate the end of a request body, since that
     would leave no possibility for the server to send back a response.)

   For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
   containing a message-body MUST include a valid Content-Length header
   field unless the server is known to be HTTP/1.1 compliant. If a
   request contains a message-body and a Content-Length is not given,
   the server SHOULD respond with 400 (bad request) if it cannot
   determine the length of the message, or with 411 (length required) if
   it wishes to insist on receiving a valid Content-Length.

   All HTTP/1.1 applications that receive entities MUST accept the
   "chunked" transfer coding (section 3.6), thus allowing this mechanism
   to be used for messages when the message length cannot be determined
   in advance.

   Messages MUST NOT include both a Content-Length header field and the
   "chunked" transfer coding. If both are received, the Content-Length
   MUST be ignored.

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

4.5 General Header Fields

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

          general-header = Cache-Control            ; Section 14.9
                         | Connection               ; Section 14.10
                         | Date                     ; Section 14.19
                         | Pragma                   ; Section 14.32
                         | Transfer-Encoding        ; Section 14.40
                         | Upgrade                  ; Section 14.41
                         | Via                      ; Section 14.44

   General-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields may be given the semantics of general
   header fields if all parties in the communication recognize them to
   be general-header fields.  Unrecognized header fields are treated as
   entity-header fields.

5 Request

   A request message from a client to a server includes, within the
   first line of that message, the method to be applied to the resource,
   the identifier of the resource, and the protocol version in use.

           Request       = Request-Line              ; Section 5.1
                           *( general-header         ; Section 4.5
                            | request-header         ; Section 5.3
                            | entity-header )        ; Section 7.1
                           CRLF
                           [ message-body ]          ; Section 7.2

5.1 Request-Line

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

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

5.1.1 Method

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

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

          extension-method = token

   The list of methods allowed by a resource can be specified in an
   Allow header field (section 14.7). The return code of the response
   always notifies the client whether a method is currently allowed on a
   resource, since the set of allowed methods can change dynamically.
   Servers SHOULD return the status code 405 (Method Not Allowed) if the
   method is known by the server but not allowed for the requested
   resource, and 501 (Not Implemented) if the method is unrecognized or
   not implemented by the server. The list of methods known by a server
   can be listed in a Public response-header field (section 14.35).

   The methods GET and HEAD MUST be supported by all general-purpose
   servers. All other methods are optional; however, if the above
   methods are implemented, they MUST be implemented with the same
   semantics as those specified in section 9.

5.1.2 Request-URI

   The Request-URI is a Uniform Resource Identifier (section 3.2) and
   identifies the resource upon which to apply the request.

          Request-URI    = "*" | absoluteURI | abs_path

   The three options for Request-URI are dependent on the nature of the
   request. The asterisk "*" means that the request does not apply to a
   particular resource, but to the server itself, and is only allowed
   when the method used does not necessarily apply to a resource. One
   example would be

          OPTIONS * HTTP/1.1

   The absoluteURI form is required when the request is being made to a
   proxy. The proxy is requested to forward the request or service it

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

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

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

   The most common form of Request-URI is that used to identify a
   resource on an origin server or gateway. In this case the absolute
   path of the URI MUST be transmitted (see section 3.2.1, abs_path) as
   the Request-URI, and the network location of the URI (net_loc) MUST
   be transmitted in a Host header field. For example, a client wishing
   to retrieve the resource above directly from the origin server would
   create a TCP connection to port 80 of the host "www.w3.org" and send
   the lines:

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

   followed by the remainder of the Request. Note that the absolute path
   cannot be empty; if none is present in the original URI, it MUST be
   given as "/" (the server root).

   If a proxy receives a request without any path in the Request-URI and
   the method specified is capable of supporting the asterisk form of
   request, then the last proxy on the request chain MUST forward the
   request with "*" as the final Request-URI. For example, the request

          OPTIONS http://www.ics.uci.edu:8001 HTTP/1.1

   would be forwarded by the proxy as

          OPTIONS * HTTP/1.1
          Host: www.ics.uci.edu:8001

   after connecting to port 8001 of host "www.ics.uci.edu".

   The Request-URI is transmitted in the format specified in section
   3.2.1.  The origin server MUST decode the Request-URI in order to
   properly interpret the request. Servers SHOULD respond to invalid
   Request-URIs with an appropriate status code.

   In requests that they forward, proxies MUST NOT rewrite the
   "abs_path" part of a Request-URI in any way except as noted above to
   replace a null abs_path with "*", no matter what the proxy does in
   its internal implementation.

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

5.2 The Resource Identified by a Request

   HTTP/1.1 origin servers SHOULD be aware that the exact resource
   identified by an Internet request is determined by examining both the
   Request-URI and the Host header field.

   An origin server that does not allow resources to differ by the
   requested host MAY ignore the Host header field value. (But see
   section 19.5.1 for other requirements on Host support in HTTP/1.1.)

   An origin server that does differentiate resources based on the host
   requested (sometimes referred to as virtual hosts or vanity
   hostnames) MUST use the following rules for determining the requested
   resource on an HTTP/1.1 request:

     1. If Request-URI is an absoluteURI, the host is part of the
        Request-URI. Any Host header field value in the request MUST be
        ignored.

     2. If the Request-URI is not an absoluteURI, and the request
        includes a Host header field, the host is determined by the Host
        header field value.

     3. If the host as determined by rule 1 or 2 is not a valid host on
        the server, the response MUST be a 400 (Bad Request) error
        message.

   Recipients of an HTTP/1.0 request that lacks a Host header field MAY
   attempt to use heuristics (e.g., examination of the URI path for
   something unique to a particular host) in order to determine what
   exact resource is being requested.

5.3 Request Header Fields

   The request-header fields allow the client to pass additional
   information about the request, and about the client itself, to the
   server. These fields act as request modifiers, with semantics

   equivalent to the parameters on a programming language method
   invocation.

          request-header = Accept                   ; Section 14.1
                         | Accept-Charset           ; Section 14.2
                         | Accept-Encoding          ; Section 14.3
                         | Accept-Language          ; Section 14.4
                         | Authorization            ; Section 14.8
                         | From                     ; Section 14.22
                         | Host                     ; Section 14.23
                         | If-Modified-Since        ; Section 14.24
                         | If-Match                 ; Section 14.25
                         | If-None-Match            ; Section 14.26
                         | If-Range                 ; Section 14.27
                         | If-Unmodified-Since      ; Section 14.28
                         | Max-Forwards             ; Section 14.31
                         | Proxy-Authorization      ; Section 14.34
                         | Range                    ; Section 14.36
                         | Referer                  ; Section 14.37
                         | User-Agent               ; Section 14.42

   Request-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields MAY be given the semantics of request-
   header fields if all parties in the communication recognize them to
   be request-header fields.  Unrecognized header fields are treated as
   entity-header fields.

6 Response

   After receiving and interpreting a request message, a server responds
   with an HTTP response message.

       Response      = Status-Line               ; Section 6.1
                       *( general-header         ; Section 4.5
                        | response-header        ; Section 6.2
                        | entity-header )        ; Section 7.1
                       CRLF
                       [ message-body ]          ; Section 7.2

6.1 Status-Line

   The first line of a Response message is the Status-Line, consisting
   of the protocol version followed by a numeric status code and its
   associated textual phrase, with each element separated by SP
   characters.  No CR or LF is allowed except in the final CRLF
   sequence.

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

6.1.1 Status Code and Reason Phrase

   The Status-Code element is a 3-digit integer result code of the
   attempt to understand and satisfy the request. These codes are fully
   defined in section 10. The Reason-Phrase is intended to give a short
   textual description of the Status-Code. The Status-Code is intended
   for use by automata and the Reason-Phrase is intended for the human
   user. The client is not required to examine or display the Reason-
   Phrase.

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

     o  1xx: Informational - Request received, continuing process

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

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

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

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

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

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

                         | "303"   ; See Other
                         | "304"   ; Not Modified
                         | "305"   ; Use Proxy
                         | "400"   ; Bad Request
                         | "401"   ; Unauthorized
                         | "402"   ; Payment Required
                         | "403"   ; Forbidden
                         | "404"   ; Not Found
                         | "405"   ; Method Not Allowed
                         | "406"   ; Not Acceptable
                         | "407"   ; Proxy Authentication Required
                         | "408"   ; Request Time-out
                         | "409"   ; Conflict
                         | "410"   ; Gone
                         | "411"   ; Length Required
                         | "412"   ; Precondition Failed
                         | "413"   ; Request Entity Too Large
                         | "414"   ; Request-URI Too Large
                         | "415"   ; Unsupported Media Type
                         | "500"   ; Internal Server Error
                         | "501"   ; Not Implemented
                         | "502"   ; Bad Gateway
                         | "503"   ; Service Unavailable
                         | "504"   ; Gateway Time-out
                         | "505"   ; HTTP Version not supported
                         | extension-code

          extension-code = 3DIGIT

          Reason-Phrase  = *<TEXT, excluding CR, LF>

   HTTP status codes are extensible. HTTP applications are not required
   to understand the meaning of all registered status codes, though such
   understanding is obviously desirable. However, applications MUST
   understand the class of any status code, as indicated by the first
   digit, and treat any unrecognized response as being equivalent to the
   x00 status code of that class, with the exception that an
   unrecognized response MUST NOT be cached. For example, if an
   unrecognized status code of 431 is received by the client, it can
   safely assume that there was something wrong with its request and
   treat the response as if it had received a 400 status code. In such
   cases, user agents SHOULD present to the user the entity returned
   with the response, since that entity is likely to include human-
   readable information which will explain the unusual status.

6.2 Response Header Fields

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

          response-header = Age                     ; Section 14.6
                          | Location                ; Section 14.30
                          | Proxy-Authenticate      ; Section 14.33
                          | Public                  ; Section 14.35
                          | Retry-After             ; Section 14.38
                          | Server                  ; Section 14.39
                          | Vary                    ; Section 14.43
                          | Warning                 ; Section 14.45
                          | WWW-Authenticate        ; Section 14.46

   Response-header field names can be extended reliably only in
   combination with a change in the protocol version. However, new or
   experimental header fields MAY be given the semantics of response-
   header fields if all parties in the communication recognize them to
   be response-header fields. Unrecognized header fields are treated as
   entity-header fields.

7 Entity

   Request and Response messages MAY transfer an entity if not otherwise
   restricted by the request method or response status code. An entity
   consists of entity-header fields and an entity-body, although some
   responses will only include the entity-headers.

   In this section, both sender and recipient refer to either the client
   or the server, depending on who sends and who receives the entity.

7.1 Entity Header Fields

   Entity-header fields define optional metainformation about the
   entity-body or, if no body is present, about the resource identified
   by the request.

          entity-header  = Allow                    ; Section 14.7
                         | Content-Base             ; Section 14.11
                         | Content-Encoding         ; Section 14.12
                         | Content-Language         ; Section 14.13
                         | Content-Length           ; Section 14.14
                         | Content-Location         ; Section 14.15
                         | Content-MD5              ; Section 14.16
                         | Content-Range            ; Section 14.17
                         | Content-Type             ; Section 14.18
                         | ETag                     ; Section 14.20
                         | Expires                  ; Section 14.21
                         | Last-Modified            ; Section 14.29
                         | extension-header

          extension-header = message-header

   The extension-header mechanism allows additional entity-header fields
   to be defined without changing the protocol, but these fields cannot
   be assumed to be recognizable by the recipient. Unrecognized header
   fields SHOULD be ignored by the recipient and forwarded by proxies.

7.2 Entity Body

   The entity-body (if any) sent with an HTTP request or response is in
   a format and encoding defined by the entity-header fields.

          entity-body    = *OCTET

   An entity-body is only present in a message when a message-body is
   present, as described in section 4.3. The entity-body is obtained
   from the message-body by decoding any Transfer-Encoding that may have
   been applied to ensure safe and proper transfer of the message.

7.2.1 Type

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

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

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

   Any HTTP/1.1 message containing an entity-body SHOULD include a
   Content-Type header field defining the media type of that body. If
   and only if the media type is not given by a Content-Type field, the
   recipient MAY attempt to guess the media type via inspection of its
   content and/or the name extension(s) of the URL used to identify the
   resource. If the media type remains unknown, the recipient SHOULD
   treat it as type "application/octet-stream".

7.2.2 Length

   The length of an entity-body is the length of the message-body after
   any transfer codings have been removed. Section 4.4 defines how the
   length of a message-body is determined.

8 Connections

8.1 Persistent Connections

8.1.1 Purpose

   Prior to persistent connections, a separate TCP connection was
   established to fetch each URL, increasing the load on HTTP servers
   and causing congestion on the Internet. The use of inline images and
   other associated data often requires a client to make multiple
   requests of the same server in a short amount of time. Analyses of
   these performance problems are available [30][27]; analysis and
   results from a prototype implementation are in [26].

   Persistent HTTP connections have a number of advantages:

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

   HTTP implementations SHOULD implement persistent connections.

8.1.2 Overall Operation

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

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

8.1.2.1 Negotiation

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

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

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

   Clients and servers SHOULD NOT assume that a persistent connection is
   maintained for HTTP versions less than 1.1 unless it is explicitly
   signaled. See section 19.7.1 for more information on backwards
   compatibility with HTTP/1.0 clients.

   In order to remain persistent, all messages on the connection must
   have a self-defined message length (i.e., one not defined by closure
   of the connection), as described in section 4.4.

8.1.2.2 Pipelining

   A client that supports persistent connections MAY "pipeline" its
   requests (i.e., send multiple requests without waiting for each
   response). A server MUST send its responses to those requests in the
   same order that the requests were received.

   Clients which assume persistent connections and pipeline immediately
   after connection establishment SHOULD be prepared to retry their
   connection if the first pipelined attempt fails. If a client does
   such a retry, it MUST NOT pipeline before it knows the connection is
   persistent. Clients MUST also be prepared to resend their requests if
   the server closes the connection before sending all of the
   corresponding responses.

8.1.3 Proxy Servers

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

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

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

8.1.4 Practical Considerations

   Servers will usually have some time-out value beyond which they will
   no longer maintain an inactive connection. Proxy servers might make
   this a higher value since it is likely that the client will be making
   more connections through the same server. The use of persistent
   connections places no requirements on the length of this time-out for
   either the client or the server.

   When a client or server wishes to time-out it SHOULD issue a graceful
   close on the transport connection. Clients and servers SHOULD both
   constantly watch for the other side of the transport close, and
   respond to it as appropriate. If a client or server does not detect
   the other side's close promptly it could cause unnecessary resource
   drain on the network.

   A client, server, or proxy MAY close the transport connection at any
   time. For example, a client MAY have started to send a new request at
   the same time that the server has decided to close the "idle"
   connection. From the server's point of view, the connection is being
   closed while it was idle, but from the client's point of view, a
   request is in progress.

   This means that clients, servers, and proxies MUST be able to recover
   from asynchronous close events. Client software SHOULD reopen the
   transport connection and retransmit the aborted request without user
   interaction so long as the request method is idempotent (see section

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

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

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

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

8.2 Message Transmission Requirements

General requirements:

o  HTTP/1.1 servers SHOULD maintain persistent connections and use
   TCP's flow control mechanisms to resolve temporary overloads,
   rather than terminating connections with the expectation that
   clients will retry. The latter technique can exacerbate network
   congestion.

o  An HTTP/1.1 (or later) client sending a message-body SHOULD monitor
   the network connection for an error status while it is transmitting
   the request. If the client sees an error status, it SHOULD
   immediately cease transmitting the body. If the body is being sent
   using a "chunked" encoding (section 3.6), a zero length chunk and
   empty footer MAY be used to prematurely mark the end of the
   message. If the body was preceded by a Content-Length header, the
   client MUST close the connection.

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

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

   Upon receiving a method subject to these requirements from an
   HTTP/1.1 (or later) client, an HTTP/1.1 (or later) server MUST either
   respond with 100 (Continue) status and continue to read from the
   input stream, or respond with an error status. If it responds with an
   error status, it MAY close the transport (TCP) connection or it MAY
   continue to read and discard the rest of the request. It MUST NOT
   perform the requested method if it returns an error status.

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

     o  MUST first send the request header fields, and then

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

   If an HTTP/1.1 client has not seen an HTTP/1.1 or later response from
   the server, it should assume that the server implements HTTP/1.0 or
   older and will not use the 100 (Continue) response. If in this case
   the client sees the connection close before receiving any status from
   the server, the client SHOULD retry the request. If the client does
   retry the request to this HTTP/1.0 server, it should use the
   following "binary exponential backoff" algorithm to be assured of
   obtaining a reliable response:

  1. Initiate a new connection to the server

  2. Transmit the request-headers

  3. Initialize a variable R to the estimated round-trip time to the
     server (e.g., based on the time it took to establish the
     connection), or to a constant value of 5 seconds if the round-trip
     time is not available.

  4. Compute T = R * (2**N), where N is the number of previous retries
     of this request.

  5. Wait either for an error response from the server, or for T seconds
     (whichever comes first)

  6. If no error response is received, after T seconds transmit the body
     of the request.

  7. If client sees that the connection is closed prematurely, repeat
     from step 1 until the request is accepted, an error response is
     received, or the user becomes impatient and terminates the retry
     process.

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

  o  MUST NOT continue and

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

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

9 Method Definitions

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

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

9.1 Safe and Idempotent Methods

9.1.1 Safe Methods

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

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

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

   fact, some dynamic resources consider that a feature. The important
   distinction here is that the user did not request the side-effects,
   so therefore cannot be held accountable for them.

9.1.2 Idempotent Methods

   Methods may also have the property of "idempotence" in that (aside
   from error or expiration issues) the side-effects of  N > 0 identical
   requests is the same as for a single request. The methods GET, HEAD,
   PUT and DELETE share this property.

9.2 OPTIONS

   The OPTIONS method represents a request for information about the
   communication options available on the request/response chain
   identified by the Request-URI. This method allows the client to
   determine the options and/or requirements associated with a resource,
   or the capabilities of a server, without implying a resource action
   or initiating a resource retrieval.

   Unless the server's response is an error, the response MUST NOT
   include entity information other than what can be considered as
   communication options (e.g., Allow is appropriate, but Content-Type
   is not). Responses to this method are not cachable.

   If the Request-URI is an asterisk ("*"), the OPTIONS request is
   intended to apply to the server as a whole. A 200 response SHOULD
   include any header fields which indicate optional features
   implemented by the server (e.g., Public), including any extensions
   not defined by this specification, in addition to any applicable
   general or response-header fields. As described in section 5.1.2, an
   "OPTIONS *" request can be applied through a proxy by specifying the
   destination server in the Request-URI without any path information.

   If the Request-URI is not an asterisk, the OPTIONS request applies
   only to the options that are available when communicating with that
   resource.  A 200 response SHOULD include any header fields which
   indicate optional features implemented by the server and applicable
   to that resource (e.g., Allow), including any extensions not defined
   by this specification, in addition to any applicable general or
   response-header fields. If the OPTIONS request passes through a
   proxy, the proxy MUST edit the response to exclude those options
   which apply to a proxy's capabilities and which are known to be
   unavailable through that proxy.

9.3 GET

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

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

   The semantics of the GET method change to a "partial GET" if the
   request message includes a Range header field. A partial GET requests
   that only part of the entity be transferred, as described in section
   14.36. The partial GET method is intended to reduce unnecessary
   network usage by allowing partially-retrieved entities to be
   completed without transferring data already held by the client.

   The response to a GET request is cachable if and only if it meets the
   requirements for HTTP caching described in section 13.

9.4 HEAD

   The HEAD method is identical to GET except that the server MUST NOT
   return a message-body in the response. The metainformation contained
   in the HTTP headers in response to a HEAD request SHOULD be identical
   to the information sent in response to a GET request. This method can
   be used for obtaining metainformation about the entity implied by the
   request without transferring the entity-body itself. This method is
   often used for testing hypertext links for validity, accessibility,
   and recent modification.

   The response to a HEAD request may be cachable in the sense that the
   information contained in the response may be used to update a
   previously cached entity from that resource. If the new field values
   indicate that the cached entity differs from the current entity (as
   would be indicated by a change in Content-Length, Content-MD5, ETag
   or Last-Modified), then the cache MUST treat the cache entry as
   stale.

9.5 POST

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

     o  Annotation of existing resources;

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

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

     o  Extending a database through an append operation.

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

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

   If a resource has been created on the origin server, the response
   SHOULD be 201 (Created) and contain an entity which describes the
   status of the request and refers to the new resource, and a Location
   header (see section 14.30).

   Responses to this method are not cachable, unless the response
   includes appropriate Cache-Control or Expires header fields. However,
   the 303 (See Other) response can be used to direct the user agent to
   retrieve a cachable resource.

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

9.6 PUT

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

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

   The fundamental difference between the POST and PUT requests is
   reflected in the different meaning of the Request-URI. The URI in a
   POST request identifies the resource that will handle the enclosed
   entity.  That resource may be a data-accepting process, a gateway to
   some other protocol, or a separate entity that accepts annotations.
   In contrast, the URI in a PUT request identifies the entity enclosed
   with the request -- the user agent knows what URI is intended and the
   server MUST NOT attempt to apply the request to some other resource.
   If the server desires that the request be applied to a different URI,
   it MUST send a 301 (Moved Permanently) response; the user agent MAY
   then make its own decision regarding whether or not to redirect the
   request.

   A single resource MAY be identified by many different URIs. For
   example, an article may have a URI for identifying "the current
   version" which is separate from the URI identifying each particular
   version. In this case, a PUT request on a general URI may result in
   several other URIs being defined by the origin server.

   HTTP/1.1 does not define how a PUT method affects the state of an
   origin server.

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

9.7 DELETE

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

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

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

9.8 TRACE

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

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

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

10 Status Code Definitions

   Each Status-Code is described below, including a description of which
   method(s) it can follow and any metainformation required in the
   response.

10.1 Informational 1xx

   This class of status code indicates a provisional response,
   consisting only of the Status-Line and optional headers, and is
   terminated by an empty line. Since HTTP/1.0 did not define any 1xx
   status codes, servers MUST NOT send a 1xx response to an HTTP/1.0
   client except under experimental conditions.

10.1.1 100 Continue

   The client may continue with its request. This interim response is
   used to inform the client that the initial part of the request has
   been received and has not yet been rejected by the server. The client
   SHOULD continue by sending the remainder of the request or, if the
   request has already been completed, ignore this response. The server
   MUST send a final response after the request has been completed.

10.1.2 101 Switching Protocols

   The server understands and is willing to comply with the client's
   request, via the Upgrade message header field (section 14.41), for a
   change in the application protocol being used on this connection. The
   server will switch protocols to those defined by the response's
   Upgrade header field immediately after the empty line which
   terminates the 101 response.

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

10.2 Successful 2xx

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

10.2.1 200 OK

   The request has succeeded. The information returned with the response
   is dependent on the method used in the request, for example:

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

   HEAD the entity-header fields corresponding to the requested resource
        are sent in the response without any message-body;

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

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

10.2.2 201 Created

   The request has been fulfilled and resulted in a new resource being
   created. The newly created resource can be referenced by the URI(s)
   returned in the entity of the response, with the most specific URL
   for the resource given by a Location header field. The origin server
   MUST create the resource before returning the 201 status code. If the
   action cannot be carried out immediately, the server should respond
   with 202 (Accepted) response instead.

10.2.3 202 Accepted

   The request has been accepted for processing, but the processing has
   not been completed. The request MAY or MAY NOT eventually be acted
   upon, as it MAY be disallowed when processing actually takes place.
   There is no facility for re-sending a status code from an
   asynchronous operation such as this.

   The 202 response is intentionally non-committal. Its purpose is to
   allow a server to accept a request for some other process (perhaps a
   batch-oriented process that is only run once per day) without
   requiring that the user agent's connection to the server persist
   until the process is completed. The entity returned with this
   response SHOULD include an indication of the request's current status
   and either a pointer to a status monitor or some estimate of when the
   user can expect the request to be fulfilled.

10.2.4 203 Non-Authoritative Information

   The returned metainformation in the entity-header is not the
   definitive set as available from the origin server, but is gathered
   from a local or a third-party copy. The set presented MAY be a subset
   or superset of the original version. For example, including local
   annotation information about the resource MAY result in a superset of
   the metainformation known by the origin server. Use of this response
   code is not required and is only appropriate when the response would
   otherwise be 200 (OK).

10.2.5 204 No Content

   The server has fulfilled the request but there is no new information
   to send back. If the client is a user agent, it SHOULD NOT change its
   document view from that which caused the request to be sent. This

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

   The 204 response MUST NOT include a message-body, and thus is always
   terminated by the first empty line after the header fields.

10.2.6 205 Reset Content

   The server has fulfilled the request and the user agent SHOULD reset
   the document view which caused the request to be sent. This response
   is primarily intended to allow input for actions to take place via
   user input, followed by a clearing of the form in which the input is
   given so that the user can easily initiate another input action. The
   response MUST NOT include an entity.

10.2.7 206 Partial Content

   The server has fulfilled the partial GET request for the resource.
   The request must have included a Range header field (section 14.36)
   indicating the desired range. The response MUST include either a
   Content-Range header field (section 14.17) indicating the range
   included with this response, or a multipart/byteranges Content-Type
   including Content-Range fields for each part. If multipart/byteranges
   is not used, the Content-Length header field in the response MUST
   match the actual number of OCTETs transmitted in the message-body.

   A cache that does not support the Range and Content-Range headers
   MUST NOT cache 206 (Partial) responses.

10.3 Redirection 3xx

   This class of status code indicates that further action needs to be
   taken by the user agent in order to fulfill the request. The action
   required MAY be carried out by the user agent without interaction
   with the user if and only if the method used in the second request is
   GET or HEAD. A user agent SHOULD NOT automatically redirect a request
   more than 5 times, since such redirections usually indicate an
   infinite loop.

10.3.1 300 Multiple Choices

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

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

   If the server has a preferred choice of representation, it SHOULD
   include the specific URL for that representation in the Location
   field; user agents MAY use the Location field value for automatic
   redirection.  This response is cachable unless indicated otherwise.

10.3.2 301 Moved Permanently

   The requested resource has been assigned a new permanent URI and any
   future references to this resource SHOULD be done using one of the
   returned URIs. Clients with link editing capabilities SHOULD
   automatically re-link references to the Request-URI to one or more of
   the new references returned by the server, where possible. This
   response is cachable unless indicated otherwise.

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

   If the 301 status code is received in response to a request other
   than GET or HEAD, the user agent MUST NOT automatically redirect the
   request unless it can be confirmed by the user, since this might
   change the conditions under which the request was issued.

     Note: When automatically redirecting a POST request after receiving
     a 301 status code, some existing HTTP/1.0 user agents will
     erroneously change it into a GET request.

10.3.3 302 Moved Temporarily

   The requested resource resides temporarily under a different URI.
   Since the redirection may be altered on occasion, the client SHOULD
   continue to use the Request-URI for future requests. This response is
   only cachable if indicated by a Cache-Control or Expires header
   field.

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

   If the 302 status code is received in response to a request other
   than GET or HEAD, the user agent MUST NOT automatically redirect the
   request unless it can be confirmed by the user, since this might
   change the conditions under which the request was issued.

     Note: When automatically redirecting a POST request after receiving
     a 302 status code, some existing HTTP/1.0 user agents will
     erroneously change it into a GET request.

10.3.4 303 See Other

   The response to the request can be found under a different URI and
   SHOULD be retrieved using a GET method on that resource. This method
   exists primarily to allow the output of a POST-activated script to
   redirect the user agent to a selected resource. The new URI is not a
   substitute reference for the originally requested resource. The 303
   response is not cachable, but the response to the second (redirected)
   request MAY be cachable.

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

10.3.5 304 Not Modified

   If the client has performed a conditional GET request and access is
   allowed, but the document has not been modified, the server SHOULD
   respond with this status code. The response MUST NOT contain a
   message-body.

   The response MUST include the following header fields:

  o  Date

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

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

   If the conditional GET used a strong cache validator (see section
   13.3.3), the response SHOULD NOT include other entity-headers.
   Otherwise (i.e., the conditional GET used a weak validator), the
   response MUST NOT include other entity-headers; this prevents
   inconsistencies between cached entity-bodies and updated headers.

   If a 304 response indicates an entity not currently cached, then the
   cache MUST disregard the response and repeat the request without the
   conditional.

   If a cache uses a received 304 response to update a cache entry, the
   cache MUST update the entry to reflect any new field values given in
   the response.

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

10.3.6 305 Use Proxy

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

10.4 Client Error 4xx

   The 4xx class of status code is intended for cases in which the
   client seems to have erred. Except when responding to a HEAD request,
   the server SHOULD include an entity containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition. These status codes are applicable to any request method.
   User agents SHOULD display any included entity to the user.

     Note: If the client is sending data, a server implementation using
     TCP should be careful to ensure that the client acknowledges
     receipt of the packet(s) containing the response, before the server
     closes the input connection. If the client continues sending data
     to the server after the close, the server's TCP stack will send a
     reset packet to the client, which may erase the client's

     unacknowledged input buffers before they can be read and
     interpreted by the HTTP application.

10.4.1 400 Bad Request

   The request could not be understood by the server due to malformed
   syntax. The client SHOULD NOT repeat the request without
   modifications.

10.4.2 401 Unauthorized

   The request requires user authentication. The response MUST include a
   WWW-Authenticate header field (section 14.46) containing a challenge
   applicable to the requested resource. The client MAY repeat the
   request with a suitable Authorization header field (section 14.8). If
   the request already included Authorization credentials, then the 401
   response indicates that authorization has been refused for those
   credentials. If the 401 response contains the same challenge as the
   prior response, and the user agent has already attempted
   authentication at least once, then the user SHOULD be presented the
   entity that was given in the response, since that entity MAY include
   relevant diagnostic information. HTTP access authentication is
   explained in section 11.

10.4.3 402 Payment Required

   This code is reserved for future use.

10.4.4 403 Forbidden

   The server understood the request, but is refusing to fulfill it.
   Authorization will not help and the request SHOULD NOT be repeated.
   If the request method was not HEAD and the server wishes to make
   public why the request has not been fulfilled, it SHOULD describe the
   reason for the refusal in the entity. This status code is commonly
   used when the server does not wish to reveal exactly why the request
   has been refused, or when no other response is applicable.

10.4.5 404 Not Found

   The server has not found anything matching the Request-URI. No
   indication is given of whether the condition is temporary or
   permanent.

   If the server does not wish to make this information available to the
   client, the status code 403 (Forbidden) can be used instead. The 410
   (Gone) status code SHOULD be used if the server knows, through some
   internally configurable mechanism, that an old resource is
   permanently unavailable and has no forwarding address.

10.4.6 405 Method Not Allowed

   The method specified in the Request-Line is not allowed for the
   resource identified by the Request-URI. The response MUST include an
   Allow header containing a list of valid methods for the requested
   resource.

10.4.7 406 Not Acceptable

   The resource identified by the request is only capable of generating
   response entities which have content characteristics not acceptable
   according to the accept headers sent in the request.

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

     Note: HTTP/1.1 servers are allowed to return responses which are
     not acceptable according to the accept headers sent in the request.
     In some cases, this may even be preferable to sending a 406
     response. User agents are encouraged to inspect the headers of an
     incoming response to determine if it is acceptable. If the response
     could be unacceptable, a user agent SHOULD temporarily stop receipt
     of more data and query the user for a decision on further actions.

10.4.8 407 Proxy Authentication Required

   This code is similar to 401 (Unauthorized), but indicates that the
   client MUST first authenticate itself with the proxy. The proxy MUST
   return a Proxy-Authenticate header field (section 14.33) containing a
   challenge applicable to the proxy for the requested resource. The
   client MAY repeat the request with a suitable Proxy-Authorization
   header field (section 14.34). HTTP access authentication is explained
   in section 11.

10.4.9 408 Request Timeout

   The client did not produce a request within the time that the server
   was prepared to wait. The client MAY repeat the request without
   modifications at any later time.

10.4.10 409 Conflict

   The request could not be completed due to a conflict with the current
   state of the resource. This code is only allowed in situations where
   it is expected that the user might be able to resolve the conflict
   and resubmit the request. The response body SHOULD include enough
   information for the user to recognize the source of the conflict.
   Ideally, the response entity would include enough information for the
   user or user agent to fix the problem; however, that may not be
   possible and is not required.

   Conflicts are most likely to occur in response to a PUT request. If
   versioning is being used and the entity being PUT includes changes to
   a resource which conflict with those made by an earlier (third-party)
   request, the server MAY use the 409 response to indicate that it
   can't complete the request. In this case, the response entity SHOULD
   contain a list of the differences between the two versions in a
   format defined by the response Content-Type.

10.4.11 410 Gone

   The requested resource is no longer available at the server and no
   forwarding address is known. This condition SHOULD be considered
   permanent. Clients with link editing capabilities SHOULD delete
   references to the Request-URI after user approval. If the server does
   not know, or has no facility to determine, whether or not the
   condition is permanent, the status code 404 (Not Found) SHOULD be
   used instead.  This response is cachable unless indicated otherwise.

   The 410 response is primarily intended to assist the task of web
   maintenance by notifying the recipient that the resource is
   intentionally unavailable and that the server owners desire that
   remote links to that resource be removed. Such an event is common for
   limited-time, promotional services and for resources belonging to
   individuals no longer working at the server's site. It is not
   necessary to mark all permanently unavailable resources as "gone" or
   to keep the mark for any length of time -- that is left to the
   discretion of the server owner.

10.4.12 411 Length Required

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

10.4.13 412 Precondition Failed

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

10.4.14 413 Request Entity Too Large

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

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

10.4.15 414 Request-URI Too Long

   The server is refusing to service the request because the Request-URI
   is longer than the server is willing to interpret. This rare
   condition is only likely to occur when a client has improperly
   converted a POST request to a GET request with long query
   information, when the client has descended into a URL "black hole" of
   redirection (e.g., a redirected URL prefix that points to a suffix of
   itself), or when the server is under attack by a client attempting to
   exploit security holes present in some servers using fixed-length
   buffers for reading or manipulating the Request-URI.

10.4.16 415 Unsupported Media Type

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

10.5 Server Error 5xx

   Response status codes beginning with the digit "5" indicate cases in
   which the server is aware that it has erred or is incapable of
   performing the request. Except when responding to a HEAD request, the
   server SHOULD include an entity containing an explanation of the
   error situation, and whether it is a temporary or permanent
   condition. User agents SHOULD display any included entity to the
   user. These response codes are applicable to any request method.

10.5.1 500 Internal Server Error

   The server encountered an unexpected condition which prevented it
   from fulfilling the request.

10.5.2 501 Not Implemented

   The server does not support the functionality required to fulfill the
   request. This is the appropriate response when the server does not
   recognize the request method and is not capable of supporting it for
   any resource.

10.5.3 502 Bad Gateway

   The server, while acting as a gateway or proxy, received an invalid
   response from the upstream server it accessed in attempting to
   fulfill the request.

10.5.4 503 Service Unavailable

   The server is currently unable to handle the request due to a
   temporary overloading or maintenance of the server. The implication
   is that this is a temporary condition which will be alleviated after
   some delay. If known, the length of the delay may be indicated in a
   Retry-After header.  If no Retry-After is given, the client SHOULD
   handle the response as it would for a 500 response.

     Note: The existence of the 503 status code does not imply that a
     server must use it when becoming overloaded. Some servers may wish
     to simply refuse the connection.

10.5.5 504 Gateway Timeout

   The server, while acting as a gateway or proxy, did not receive a
   timely response from the upstream server it accessed in attempting to
   complete the request.

10.5.6 505 HTTP Version Not Supported

   The server does not support, or refuses to support, the HTTP protocol
   version that was used in the request message. The server is
   indicating that it is unable or unwilling to complete the request
   using the same major version as the client, as described in section
   3.1, other than with this error message. The response SHOULD contain
   an entity describing why that version is not supported and what other
   protocols are supported by that server.

11 Access Authentication

   HTTP provides a simple challenge-response authentication mechanism
   which MAY be used by a server to challenge a client request and by a
   client to provide authentication information. It uses an extensible,
   case-insensitive token to identify the authentication scheme,
   followed by a comma-separated list of attribute-value pairs which
   carry the parameters necessary for achieving authentication via that
   scheme.

          auth-scheme    = token

          auth-param     = token "=" quoted-string

   The 401 (Unauthorized) response message is used by an origin server
   to challenge the authorization of a user agent. This response MUST
   include a WWW-Authenticate header field containing at least one
   challenge applicable to the requested resource.

          challenge      = auth-scheme 1*SP realm *( "," auth-param )

          realm          = "realm" "=" realm-value
          realm-value    = quoted-string

   The realm attribute (case-insensitive) is required for all
   authentication schemes which issue a challenge. The realm value
   (case-sensitive), in combination with the canonical root URL (see
   section 5.1.2) of the server being accessed, defines the protection
   space. These realms allow the protected resources on a server to be
   partitioned into a set of protection spaces, each with its own
   authentication scheme and/or authorization database. The realm value
   is a string, generally assigned by the origin server, which may have
   additional semantics specific to the authentication scheme.

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

   containing the authentication information of the user agent for the
   realm of the resource being requested.

          credentials    = basic-credentials
                         | auth-scheme #auth-param

   The domain over which credentials can be automatically applied by a
   user agent is determined by the protection space. If a prior request
   has been authorized, the same credentials MAY be reused for all other
   requests within that protection space for a period of time determined
   by the authentication scheme, parameters, and/or user preference.
   Unless otherwise defined by the authentication scheme, a single
   protection space cannot extend outside the scope of its server.

   If the server does not wish to accept the credentials sent with a
   request, it SHOULD return a 401 (Unauthorized) response. The response
   MUST include a WWW-Authenticate header field containing the (possibly
   new) challenge applicable to the requested resource and an entity
   explaining the refusal.

   The HTTP protocol does not restrict applications to this simple
   challenge-response mechanism for access authentication. Additional
   mechanisms MAY be used, such as encryption at the transport level or
   via message encapsulation, and with additional header fields
   specifying authentication information. However, these additional
   mechanisms are not defined by this specification.

   Proxies MUST be completely transparent regarding user agent
   authentication. That is, they MUST forward the WWW-Authenticate and
   Authorization headers untouched, and follow the rules found in
   section 14.8.

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

11.1 Basic Authentication Scheme

   The "basic" authentication scheme is based on the model that the user
   agent must authenticate itself with a user-ID and a password for each
   realm. The realm value should be considered an opaque string which
   can only be compared for equality with other realms on that server.
   The server will service the request only if it can validate the
   user-ID and password for the protection space of the Request-URI.
   There are no optional authentication parameters.

   Upon receipt of an unauthorized request for a URI within the
   protection space, the server MAY respond with a challenge like the
   following:

          WWW-Authenticate: Basic realm="WallyWorld"

   where "WallyWorld" is the string assigned by the server to identify
   the protection space of the Request-URI.

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

          basic-credentials = "Basic" SP basic-cookie

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

          user-pass   = userid ":" password

          userid      = *<TEXT excluding ":">

          password    = *TEXT

   Userids might be case sensitive.

   If the user agent wishes to send the userid "Aladdin" and password
   "open sesame", it would use the following header field:

          Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

   See section 15 for security considerations associated with Basic
   authentication.

11.2 Digest Authentication Scheme

   A digest authentication for HTTP is specified in RFC 2069 [32].

12 Content Negotiation

   Most HTTP responses include an entity which contains information for
   interpretation by a human user. Naturally, it is desirable to supply
   the user with the "best available" entity corresponding to the
   request.  Unfortunately for servers and caches, not all users have
   the same preferences for what is "best," and not all user agents are
   equally capable of rendering all entity types. For that reason, HTTP
   has provisions for several mechanisms for "content negotiation" --
   the process of selecting the best representation for a given response

   when there are multiple representations available.

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

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

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

12.1 Server-driven Negotiation

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

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

   Server-driven negotiation has disadvantages:

1. It is impossible for the server to accurately determine what might be
  "best" for any given user, since that would require complete
  knowledge of both the capabilities of the user agent and the intended
  use for the response (e.g., does the user want to view it on screen
  or print it on paper?).

2. Having the user agent describe its capabilities in every request can
  be both very inefficient (given that only a small percentage of
  responses have multiple representations) and a potential violation of

  the user's privacy.

3. It complicates the implementation of an origin server and the
  algorithms for generating responses to a request.

4. It may limit a public cache's ability to use the same response for
  multiple user's requests.

   HTTP/1.1 includes the following request-header fields for enabling
   server-driven negotiation through description of user agent
   capabilities and user preferences: Accept (section 14.1), Accept-
   Charset (section 14.2), Accept-Encoding (section 14.3), Accept-
   Language (section 14.4), and User-Agent (section 14.42). However, an
   origin server is not limited to these dimensions and MAY vary the
   response based on any aspect of the request, including information
   outside the request-header fields or within extension header fields
   not defined by this specification.

   HTTP/1.1 origin servers MUST include an appropriate Vary header field
   (section 14.43) in any cachable response based on server-driven
   negotiation. The Vary header field describes the dimensions over
   which the response might vary (i.e. the dimensions over which the
   origin server picks its "best guess" response from multiple
   representations).

   HTTP/1.1 public caches MUST recognize the Vary header field when it
   is included in a response and obey the requirements described in
   section 13.6 that describes the interactions between caching and
   content negotiation.

12.2 Agent-driven Negotiation

   With agent-driven negotiation, selection of the best representation
   for a response is performed by the user agent after receiving an
   initial response from the origin server. Selection is based on a list
   of the available representations of the response included within the
   header fields (this specification reserves the field-name Alternates,
   as described in appendix 19.6.2.1) or entity-body of the initial
   response, with each representation identified by its own URI.
   Selection from among the representations may be performed
   automatically (if the user agent is capable of doing so) or manually
   by the user selecting from a generated (possibly hypertext) menu.

   Agent-driven negotiation is advantageous when the response would vary
   over commonly-used dimensions (such as type, language, or encoding),
   when the origin server is unable to determine a user agent's
   capabilities from examining the request, and generally when public
   caches are used to distribute server load and reduce network usage.

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

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

12.3 Transparent Negotiation

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

   Transparent negotiation has the advantage of distributing the
   negotiation work that would otherwise be required of the origin
   server and also removing the second request delay of agent-driven
   negotiation when the cache is able to correctly guess the right
   response.

   This specification does not define any mechanism for transparent
   negotiation, though it also does not prevent any such mechanism from
   being developed as an extension and used within HTTP/1.1. An HTTP/1.1
   cache performing transparent negotiation MUST include a Vary header
   field in the response (defining the dimensions of its variance) if it
   is cachable to ensure correct interoperation with all HTTP/1.1
   clients. The agent-driven negotiation information supplied by the
   origin server SHOULD be included with the transparently negotiated
   response.

13 Caching in HTTP

   HTTP is typically used for distributed information systems, where
   performance can be improved by the use of response caches. The
   HTTP/1.1 protocol includes a number of elements intended to make
   caching work as well as possible. Because these elements are
   inextricable from other aspects of the protocol, and because they
   interact with each other, it is useful to describe the basic caching
   design of HTTP separately from the detailed descriptions of methods,

   headers, response codes, etc.

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

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

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

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

   Therefore, the HTTP/1.1 protocol provides these important elements:

  1. Protocol features that provide full semantic transparency when this
     is required by all parties.

  2. Protocol features that allow an origin server or user agent to
     explicitly request and control non-transparent operation.

  3. Protocol features that allow a cache to attach warnings to
     responses that do not preserve the requested approximation of
     semantic transparency.

   A basic principle is that it must be possible for the clients to
   detect any potential relaxation of semantic transparency.

     Note: The server, cache, or client implementer may be faced with
     design decisions not explicitly discussed in this specification. If
     a decision may affect semantic transparency, the implementer ought
     to err on the side of maintaining transparency unless a careful and
     complete analysis shows significant benefits in breaking
     transparency.

13.1.1 Cache Correctness

   A correct cache MUST respond to a request with the most up-to-date
   response held by the cache that is appropriate to the request (see
   sections 13.2.5, 13.2.6, and 13.12) which meets one of the following
   conditions:

  1. It has been checked for equivalence with what the origin server
     would have returned by revalidating the response with the origin
     server (section 13.3);

  2. It is "fresh enough" (see section 13.2). In the default case, this
     means it meets the least restrictive freshness requirement of the
     client, server, and cache (see section 14.9); if the origin server
     so specifies, it is the freshness requirement of the origin server
     alone.

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

  4. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect), or
     error (4xx or 5xx) response message.

   If the cache can not communicate with the origin server, then a
   correct cache SHOULD respond as above if the response can be
   correctly served from the cache; if not it MUST return an error or

   warning indicating that there was a communication failure.

   If a cache receives a response (either an entire response, or a 304
   (Not Modified) response) that it would normally forward to the
   requesting client, and the received response is no longer fresh, the
   cache SHOULD forward it to the requesting client without adding a new
   Warning (but without removing any existing Warning headers). A cache
   SHOULD NOT attempt to revalidate a response simply because that
   response became stale in transit; this might lead to an infinite
   loop. An user agent that receives a stale response without a Warning
   MAY display a warning indication to the user.

13.1.2 Warnings

   Whenever a cache returns a response that is neither first-hand nor
   "fresh enough" (in the sense of condition 2 in section 13.1.1), it
   must attach a warning to that effect, using a Warning response-
   header. This warning allows clients to take appropriate action.

   Warnings may be used for other purposes, both cache-related and
   otherwise. The use of a warning, rather than an error status code,
   distinguish these responses from true failures.

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

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

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

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

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

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

13.1.3 Cache-control Mechanisms

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

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

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

13.1.4 Explicit User Agent Warnings

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

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

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

13.1.5 Exceptions to the Rules and Warnings

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

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

13.1.6 Client-controlled Behavior

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

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

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

13.2 Expiration Model

13.2.1 Server-Specified Expiration

   HTTP caching works best when caches can entirely avoid making
   requests to the origin server. The primary mechanism for avoiding
   requests is for an origin server to provide an explicit expiration
   time in the future, indicating that a response may be used to satisfy
   subsequent requests.  In other words, a cache can return a fresh

   response without first contacting the server.

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

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

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

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

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

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

13.2.2 Heuristic Expiration

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

13.2.3 Age Calculations

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

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

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

   HTTP/1.1 uses the Age response-header to help convey age information
   between caches. The Age header value is the sender's estimate of the
   amount of time since the response was generated at the origin server.
   In the case of a cached response that has been revalidated with the
   origin server, the Age value is based on the time of revalidation,
   not of the original response.

   In essence, the Age value is the sum of the time that the response
   has been resident in each of the caches along the path from the
   origin server, plus the amount of time it has been in transit along
   network paths.

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

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

     1. now minus date_value, if the local clock is reasonably well
        synchronized to the origin server's clock. If the result is
        negative, the result is replaced by zero.

     2. age_value, if all of the caches along the response path
        implement HTTP/1.1.

   Given that we have two independent ways to compute the age of a
   response when it is received, we can combine these as

          corrected_received_age = max(now - date_value, age_value)

   and as long as we have either nearly synchronized clocks or all-

   HTTP/1.1 paths, one gets a reliable (conservative) result.

   Note that this correction is applied at each HTTP/1.1 cache along the
   path, so that if there is an HTTP/1.0 cache in the path, the correct
   received age is computed as long as the receiving cache's clock is
   nearly in sync. We don't need end-to-end clock synchronization
   (although it is good to have), and there is no explicit clock
   synchronization step.

   Because of network-imposed delays, some significant interval may pass
   from the time that a server generates a response and the time it is
   received at the next outbound cache or client. If uncorrected, this
   delay could result in improperly low ages.

   Because the request that resulted in the returned Age value must have
   been initiated prior to that Age value's generation, we can correct
   for delays imposed by the network by recording the time at which the
   request was initiated. Then, when an Age value is received, it MUST
   be interpreted relative to the time the request was initiated, not
   the time that the response was received. This algorithm results in
   conservative behavior no matter how much delay is experienced. So, we
   compute:

         corrected_initial_age = corrected_received_age
                               + (now - request_time)

   where "request_time" is the time (according to the local clock) when
   the request that elicited this response was sent.

   Summary of age calculation algorithm, when a cache receives a
   response:

      /*
       * age_value
       *      is the value of Age: header received by the cache with
       *              this response.
       * date_value
       *      is the value of the origin server's Date: header
       * request_time
       *      is the (local) time when the cache made the request
       *              that resulted in this cached response
       * response_time
       *      is the (local) time when the cache received the
       *              response
       * now
       *      is the current (local) time
       */
      apparent_age = max(0, response_time - date_value);

      corrected_received_age = max(apparent_age, age_value);
      response_delay = response_time - request_time;
      corrected_initial_age = corrected_received_age + response_delay;
      resident_time = now - response_time;
      current_age   = corrected_initial_age + resident_time;

   When a cache sends a response, it must add to the
   corrected_initial_age the amount of time that the response was
   resident locally. It must then transmit this total age, using the Age
   header, to the next recipient cache.

     Note that a client cannot reliably tell that a response is first-
     hand, but the presence of an Age header indicates that a response
     is definitely not first-hand. Also, if the Date in a response is
     earlier than the client's local request time, the response is
     probably not first-hand (in the absence of serious clock skew).

13.2.4 Expiration Calculations

   In order to decide whether a response is fresh or stale, we need to
   compare its freshness lifetime to its age. The age is calculated as
   described in section 13.2.3; this section describes how to calculate
   the freshness lifetime, and to determine if a response has expired.
   In the discussion below, the values can be represented in any form
   appropriate for arithmetic operations.

   We use the term "expires_value" to denote the value of the Expires
   header. We use the term "max_age_value" to denote an appropriate
   value of the number of seconds carried by the max-age directive of
   the Cache-Control header in a response (see section 14.10.

   The max-age directive takes priority over Expires, so if max-age is
   present in a response, the calculation is simply:

         freshness_lifetime = max_age_value

   Otherwise, if Expires is present in the response, the calculation is:

         freshness_lifetime = expires_value - date_value

   Note that neither of these calculations is vulnerable to clock skew,
   since all of the information comes from the origin server.

   If neither Expires nor Cache-Control: max-age appears in the
   response, and the response does not include other restrictions on
   caching, the cache MAY compute a freshness lifetime using a
   heuristic. If the value is greater than 24 hours, the cache must
   attach Warning 13 to any response whose age is more than 24 hours if

   such warning has not already been added.

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

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

         response_is_fresh = (freshness_lifetime > current_age)

13.2.5 Disambiguating Expiration Values

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

   If a client performing a retrieval receives a non-first-hand response
   for a request that was already fresh in its own cache, and the Date
   header in its existing cache entry is newer than the Date on the new
   response, then the client MAY ignore the response. If so, it MAY
   retry the request with a "Cache-Control: max-age=0" directive (see
   section 14.9), to force a check with the origin server.

   If a cache has two fresh responses for the same representation with
   different validators, it MUST use the one with the more recent Date
   header. This situation may arise because the cache is pooling
   responses from other caches, or because a client has asked for a
   reload or a revalidation of an apparently fresh cache entry.

13.2.6 Disambiguating Multiple Responses

   Because a client may be receiving responses via multiple paths, so
   that some responses flow through one set of caches and other
   responses flow through a different set of caches, a client may
   receive responses in an order different from that in which the origin
   server sent them. We would like the client to use the most recently
   generated response, even if older responses are still apparently
   fresh.

   Neither the entity tag nor the expiration value can impose an
   ordering on responses, since it is possible that a later response
   intentionally carries an earlier expiration time. However, the
   HTTP/1.1 specification requires the transmission of Date headers on
   every response, and the Date values are ordered to a granularity of
   one second.

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

          Cache-Control: max-age=0

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

          Cache-Control: no-cache

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

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

13.3 Validation Model

   When a cache has a stale entry that it would like to use as a
   response to a client's request, it first has to check with the origin
   server (or possibly an intermediate cache with a fresh response) to
   see if its cached entry is still usable. We call this "validating"
   the cache entry.  Since we do not want to have to pay the overhead of
   retransmitting the full response if the cached entry is good, and we
   do not want to pay the overhead of an extra round trip if the cached
   entry is invalid, the HTTP/1.1 protocol supports the use of
   conditional methods.

   The key protocol features for supporting conditional methods are
   those concerned with "cache validators." When an origin server
   generates a full response, it attaches some sort of validator to it,
   which is kept with the cache entry. When a client (user agent or
   proxy cache) makes a conditional request for a resource for which it
   has a cache entry, it includes the associated validator in the
   request.

   The server then checks that validator against the current validator
   for the entity, and, if they match, it responds with a special status
   code (usually, 304 (Not Modified)) and no entity-body. Otherwise, it
   returns a full response (including entity-body). Thus, we avoid
   transmitting the full response if the validator matches, and we avoid
   an extra round trip if it does not match.

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

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

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

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

13.3.1 Last-modified Dates

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

13.3.2 Entity Tag Cache Validators

   The ETag entity-header field value, an entity tag, provides for an
   "opaque" cache validator. This may allow more reliable validation in
   situations where it is inconvenient to store modification dates,
   where the one-second resolution of HTTP date values is not
   sufficient, or where the origin server wishes to avoid certain
   paradoxes that may arise from the use of modification dates.

   Entity Tags are described in section 3.11. The headers used with
   entity tags are described in sections 14.20, 14.25, 14.26 and 14.43.

13.3.3 Weak and Strong Validators

   Since both origin servers and caches will compare two validators to
   decide if they represent the same or different entities, one normally
   would expect that if the entity (the entity-body or any entity-
   headers) changes in any way, then the associated validator would
   change as well.  If this is true, then we call this validator a
   "strong validator."

   However, there may be cases when a server prefers to change the
   validator only on semantically significant changes, and not when

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

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

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

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

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

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

   Strong validators are usable in any context. Weak validators are only
   usable in contexts that do not depend on exact equality of an entity.
   For example, either kind is usable for a conditional GET of a full
   entity. However, only a strong validator is usable for a sub-range
   retrieval, since otherwise the client may end up with an internally
   inconsistent entity.

   The only function that the HTTP/1.1 protocol defines on validators is
   comparison. There are two validator comparison functions, depending
   on whether the comparison context allows the use of weak validators
   or not:

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

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

   GET requests. The strong comparison function MUST be used in all
   other cases.

   An entity tag is strong unless it is explicitly tagged as weak.
   Section 3.11 gives the syntax for entity tags.

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

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

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

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

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

   If a client wishes to perform a sub-range retrieval on a value for
   which it has only a Last-Modified time and no opaque validator, it
   may do this only if the Last-Modified time is strong in the sense
   described here.

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

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

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

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

   HTTP/1.1 origin servers:

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

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

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

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

   HTTP/1.1 clients:

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

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

   An HTTP/1.1 cache, upon receiving a request, MUST use the most
   restrictive validator when deciding whether the client's cache entry
   matches the cache's own cache entry. This is only an issue when the
   request contains both an entity tag and a last-modified-date
   validator (If-Modified-Since or If-Unmodified-Since).

     A note on rationale: The general principle behind these rules is
     that HTTP/1.1 servers and clients should transmit as much non-
     redundant information as is available in their responses and
     requests. HTTP/1.1 systems receiving this information will make the
     most conservative assumptions about the validators they receive.

     HTTP/1.0 clients and caches will ignore entity tags. Generally,
     last-modified values received or used by these systems will support
     transparent and efficient caching, and so HTTP/1.1 origin servers
     should provide Last-Modified values. In those rare cases where the
     use of a Last-Modified value as a validator by an HTTP/1.0 system
     could result in a serious problem, then HTTP/1.1 origin servers
     should not provide one.

13.3.5 Non-validating Conditionals

   The principle behind entity tags is that only the service author
   knows the semantics of a resource well enough to select an
   appropriate cache validation mechanism, and the specification of any
   validator comparison function more complex than byte-equality would
   open up a can of worms.  Thus, comparisons of any other headers
   (except Last-Modified, for compatibility with HTTP/1.0) are never
   used for purposes of validating a cache entry.

13.4 Response Cachability

   Unless specifically constrained by a Cache-Control (section 14.9)
   directive, a caching system may always store a successful response
   (see section 13.8) as a cache entry, may return it without validation
   if it is fresh, and may return it after successful validation. If

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

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

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

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

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

   A response received with any other status code MUST NOT be returned
   in a reply to a subsequent request unless there are Cache-Control
   directives or another header(s) that explicitly allow it. For
   example, these include the following: an Expires header (section
   14.21); a "max-age", "must-revalidate", "proxy-revalidate", "public"
   or "private" Cache-Control directive (section 14.9).

13.5 Constructing Responses From Caches

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

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

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

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

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

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

   All other headers defined by HTTP/1.1 are end-to-end headers.

   Hop-by-hop headers introduced in future versions of HTTP MUST be
   listed in a Connection header, as described in section 14.10.

13.5.2 Non-modifiable Headers

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

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

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

   A cache or non-caching proxy MUST NOT modify or add any of the
   following fields in a response that contains the no-transform Cache-
   Control directive, or in any request:

     o  Content-Encoding
     o  Content-Length
     o  Content-Range
     o  Content-Type

   A cache or non-caching proxy MAY modify or add these fields in a
   response that does not include no-transform, but if it does so, it
   MUST add a Warning 14 (Transformation applied) if one does not
   already appear in the response.

     Warning: unnecessary modification of end-to-end headers may cause
     authentication failures if stronger authentication mechanisms are
     introduced in later versions of HTTP. Such authentication
     mechanisms may rely on the values of header fields not listed here.

13.5.3 Combining Headers

   When a cache makes a validating request to a server, and the server
   provides a 304 (Not Modified) response, the cache must construct a
   response to send to the requesting client. The cache uses the
   entity-body stored in the cache entry as the entity-body of this
   outgoing response. The end-to-end headers stored in the cache entry
   are used for the constructed response, except that any end-to-end
   headers provided in the 304 response MUST replace the corresponding
   headers from the cache entry. Unless the cache decides to remove the
   cache entry, it MUST also replace the end-to-end headers stored with
   the cache entry with corresponding headers received in the incoming
   response.

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

   If a header field-name in the incoming response matches more than one
   header in the cache entry, all such old headers are replaced.

     Note: this rule allows an origin server to use a 304 (Not Modified)
     response to update any header associated with a previous response
     for the same entity, although it might not always be meaningful or
     correct to do so. This rule does not allow an origin server to use
     a 304 (not Modified) response to entirely delete a header that it
     had provided with a previous response.

13.5.4 Combining Byte Ranges

   A response may transfer only a subrange of the bytes of an entity-
   body, either because the request included one or more Range
   specifications, or because a connection was broken prematurely. After
   several such transfers, a cache may have received several ranges of
   the same entity-body.

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

     o  Both the incoming response and the cache entry must have a cache
        validator.
     o  The two cache validators must match using the strong comparison
        function (see section 13.3.3).

   If either requirement is not meant, the cache must use only the most
   recent partial response (based on the Date values transmitted with
   every response, and using the incoming response if these values are
   equal or missing), and must discard the other partial information.

13.6 Caching Negotiated Responses

   Use of server-driven content negotiation (section 12), as indicated
   by the presence of a Vary header field in a response, alters the
   conditions and procedure by which a cache can use the response for
   subsequent requests.

   A server MUST use the Vary header field (section 14.43) to inform a
   cache of what header field dimensions are used to select among
   multiple representations of a cachable response. A cache may use the
   selected representation (the entity included with that particular
   response) for replying to subsequent requests on that resource only
   when the subsequent requests have the same or equivalent values for
   all header fields specified in the Vary response-header. Requests
   with a different value for one or more of those header fields would
   be forwarded toward the origin server.

   If an entity tag was assigned to the representation, the forwarded
   request SHOULD be conditional and include the entity tags in an If-
   None-Match header field from all its cache entries for the Request-
   URI. This conveys to the server the set of entities currently held by
   the cache, so that if any one of these entities matches the requested
   entity, the server can use the ETag header in its 304 (Not Modified)
   response to tell the cache which entry is appropriate. If the
   entity-tag of the new response matches that of an existing entry, the

   new response SHOULD be used to update the header fields of the
   existing entry, and the result MUST be returned to the client.

   The Vary header field may also inform the cache that the
   representation was selected using criteria not limited to the
   request-headers; in this case, a cache MUST NOT use the response in a
   reply to a subsequent request unless the cache relays the new request
   to the origin server in a conditional request and the server responds
   with 304 (Not Modified), including an entity tag or Content-Location
   that indicates which entity should be used.

   If any of the existing cache entries contains only partial content
   for the associated entity, its entity-tag SHOULD NOT be included in
   the If-None-Match header unless the request is for a range that would
   be fully satisfied by that entry.

   If a cache receives a successful response whose Content-Location
   field matches that of an existing cache entry for the same Request-
   URI, whose entity-tag differs from that of the existing entry, and
   whose Date is more recent than that of the existing entry, the
   existing entry SHOULD NOT be returned in response to future requests,
   and should be deleted from the cache.

13.7 Shared and Non-Shared Caches

   For reasons of security and privacy, it is necessary to make a
   distinction between "shared" and "non-shared" caches. A non-shared
   cache is one that is accessible only to a single user. Accessibility
   in this case SHOULD be enforced by appropriate security mechanisms.
   All other caches are considered to be "shared." Other sections of
   this specification place certain constraints on the operation of
   shared caches in order to prevent loss of privacy or failure of
   access controls.

13.8 Errors or Incomplete Response Cache Behavior

   A cache that receives an incomplete response (for example, with fewer
   bytes of data than specified in a Content-Length header) may store
   the response. However, the cache MUST treat this as a partial
   response.  Partial responses may be combined as described in section
   13.5.4; the result might be a full response or might still be
   partial. A cache MUST NOT return a partial response to a client
   without explicitly marking it as such, using the 206 (Partial
   Content) status code. A cache MUST NOT return a partial response
   using a status code of 200 (OK).

   If a cache receives a 5xx response while attempting to revalidate an
   entry, it may either forward this response to the requesting client,

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

13.9 Side Effects of GET and HEAD

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

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

13.10 Invalidation After Updates or Deletions

   The effect of certain methods at the origin server may cause one or
   more existing cache entries to become non-transparently invalid. That
   is, although they may continue to be "fresh," they do not accurately
   reflect what the origin server would return for a new request.

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

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

   Some HTTP methods may invalidate an entity. This is either the entity
   referred to by the Request-URI, or by the Location or Content-
   Location response-headers (if present). These methods are:

     o  PUT
     o  DELETE
     o  POST

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

13.11 Write-Through Mandatory

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

   The alternative (known as "write-back" or "copy-back" caching) is not
   allowed in HTTP/1.1, due to the difficulty of providing consistent
   updates and the problems arising from server, cache, or network
   failure prior to write-back.

13.12 Cache Replacement

   If a new cachable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8)
   response is received from a resource while any existing responses for
   the same resource are cached, the cache SHOULD use the new response
   to reply to the current request. It may insert it into cache storage
   and may, if it meets all other requirements, use it to respond to any
   future requests that would previously have caused the old response to
   be returned. If it inserts the new response into cache storage it
   should follow the rules in section 13.5.3.

     Note: a new response that has an older Date header value than
     existing cached responses is not cachable.

13.13 History Lists

   User agents often have history mechanisms, such as "Back" buttons and
   history lists, which can be used to redisplay an entity retrieved
   earlier in a session.

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

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

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

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

14 Header Field Definitions

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

14.1 Accept

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

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

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

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

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

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

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

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

   The example

          Accept: audio/*; q=0.2, audio/basic

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

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

   A more elaborate example is

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

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

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

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

   have the following precedence:

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

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

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

   would cause the following values to be associated:

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

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

     this default set should be configurable by the user.

14.2 Accept-Charset

   The Accept-Charset request-header field can be used to indicate what
   character sets are acceptable for the response. This field allows
   clients capable of understanding more comprehensive or special-
   purpose character sets to signal that capability to a server which is
   capable of representing documents in those character sets. The ISO-
   8859-1 character set can be assumed to be acceptable to all user
   agents.

          Accept-Charset = "Accept-Charset" ":"
                    1#( charset [ ";" "q" "=" qvalue ] )

   Character set values are described in section 3.4. Each charset may
   be given an associated quality value which represents the user's
   preference for that charset. The default value is q=1. An example is

          Accept-Charset: iso-8859-5, unicode-1-1;q=0.8

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

14.3 Accept-Encoding

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

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

   An example of its use is

          Accept-Encoding: compress, gzip

   If no Accept-Encoding header is present in a request, the server MAY
   assume that the client will accept any content coding. If an Accept-
   Encoding header is present, and if the server cannot send a response
   which is acceptable according to the Accept-Encoding header, then the
   server SHOULD send an error response with the 406 (Not Acceptable)
   status code.

   An empty Accept-Encoding value indicates none are acceptable.

14.4 Accept-Language

   The Accept-Language request-header field is similar to Accept, but
   restricts the set of natural languages that are preferred as a
   response to the request.

          Accept-Language = "Accept-Language" ":"
                            1#( language-range [ ";" "q" "=" qvalue ] )

          language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )

   Each language-range MAY be given an associated quality value which
   represents an estimate of the user's preference for the languages
   specified by that range. The quality value defaults to "q=1". For
   example,

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

   would mean: "I prefer Danish, but will accept British English and
   other types of English." A language-range matches a language-tag if
   it exactly equals the tag, or if it exactly equals a prefix of the
   tag such that the first tag character following the prefix is "-".
   The special range "*", if present in the Accept-Language field,
   matches every tag not matched by any other range present in the
   Accept-Language field.

     Note: This use of a prefix matching rule does not imply that
     language tags are assigned to languages in such a way that it is
     always true that if a user understands a language with a certain
     tag, then this user will also understand all languages with tags
     for which this tag is a prefix. The prefix rule simply allows the
     use of prefix tags if this is the case.

   The language quality factor assigned to a language-tag by the
   Accept-Language field is the quality value of the longest language-
   range in the field that matches the language-tag. If no language-
   range in the field matches the tag, the language quality factor
   assigned is 0. If no Accept-Language header is present in the
   request, the server SHOULD assume that all languages are equally
   acceptable. If an Accept-Language header is present, then all
   languages which are assigned a quality factor greater than 0 are
   acceptable.

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

   section 15.7.

     Note: As intelligibility is highly dependent on the individual
     user, it is recommended that client applications make the choice of
     linguistic preference available to the user. If the choice is not
     made available, then the Accept-Language header field must not be
     given in the request.

14.5 Accept-Ranges

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

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

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

   Origin servers that accept byte-range requests MAY send

          Accept-Ranges: bytes

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

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

          Accept-Ranges: none

   to advise the client not to attempt a range request.

14.6 Age

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

           Age = "Age" ":" age-value

           age-value = delta-seconds

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

   If a cache receives a value larger than the largest positive integer
   it can represent, or if any of its age calculations overflows, it
   MUST transmit an Age header with a value of 2147483648 (2^31).
   HTTP/1.1 caches MUST send an Age header in every response. Caches
   SHOULD use an arithmetic type of at least 31 bits of range.

14.7 Allow

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

          Allow          = "Allow" ":" 1#method

   Example of use:

          Allow: GET, HEAD, PUT

   This field cannot prevent a client from trying other methods.
   However, the indications given by the Allow header field value SHOULD
   be followed. The actual set of allowed methods is defined by the
   origin server at the time of each request.

   The Allow header field MAY be provided with a PUT request to
   recommend the methods to be supported by the new or modified
   resource. The server is not required to support these methods and
   SHOULD include an Allow header in the response giving the actual
   supported methods.

   A proxy MUST NOT modify the Allow header field even if it does not
   understand all the methods specified, since the user agent MAY have
   other means of communicating with the origin server.

   The Allow header field does not indicate what methods are implemented
   at the server level. Servers MAY use the Public response-header field
   (section 14.35) to describe what methods are implemented on the
   server as a whole.

14.8 Authorization

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

          Authorization  = "Authorization" ":" credentials

   HTTP access authentication is described in section 11. If a request
   is authenticated and a realm specified, the same credentials SHOULD
   be valid for all other requests within this realm.

   When a shared cache (see section 13.7) receives a request containing
   an Authorization field, it MUST NOT return the corresponding response
   as a reply to any other request, unless one of the following specific
   exceptions holds:

     1. If the response includes the "proxy-revalidate" Cache-Control
        directive, the cache MAY use that response in replying to a
        subsequent request, but a proxy cache MUST first revalidate it with
        the origin server, using the request-headers from the new request
        to allow the origin server to authenticate the new request.
     2. If the response includes the "must-revalidate" Cache-Control
        directive, the cache MAY use that response in replying to a
        subsequent request, but all caches MUST first revalidate it with
        the origin server, using the request-headers from the new request
        to allow the origin server to authenticate the new request.
     3. If the response includes the "public" Cache-Control directive, it
        may be returned in reply to any subsequent request.

14.9 Cache-Control

   The Cache-Control general-header field is used to specify directives
   that MUST be obeyed by all caching mechanisms along the
   request/response chain. The directives specify behavior intended to
   prevent caches from adversely interfering with the request or
   response. These directives typically override the default caching
   algorithms. Cache directives are unidirectional in that the presence
   of a directive in a request does not imply that the same directive
   should be given in the response.

     Note that HTTP/1.0 caches may not implement Cache-Control and may
     only implement Pragma: no-cache (see section 14.32).

   Cache directives must be passed through by a proxy or gateway
   application, regardless of their significance to that application,
   since the directives may be applicable to all recipients along the
   request/response chain. It is not possible to specify a cache-
   directive for a specific cache.

          Cache-Control   = "Cache-Control" ":" 1#cache-directive

          cache-directive = cache-request-directive
                          | cache-response-directive

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

          cache-response-directive =
                            "public"
                          | "private" [ "=" <"> 1#field-name <"> ]
                          | "no-cache" [ "=" <"> 1#field-name <"> ]
                          | "no-store"
                          | "no-transform"
                          | "must-revalidate"
                          | "proxy-revalidate"
                          | "max-age" "=" delta-seconds
                          | cache-extension

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

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

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

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

14.9.1 What is Cachable

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

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

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

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

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

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

14.9.2 What May be Stored by Caches

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

   Even when this directive is associated with a response, users may
   explicitly store such a response outside of the caching system (e.g.,
   with a "Save As" dialog). History buffers may store such responses as
   part of their normal operation.

   The purpose of this directive is to meet the stated requirements of
   certain users and service authors who are concerned about accidental
   releases of information via unanticipated accesses to cache data
   structures. While the use of this directive may improve privacy in
   some cases, we caution that it is NOT in any way a reliable or
   sufficient mechanism for ensuring privacy. In particular, malicious
   or compromised caches may not recognize or obey this directive; and
   communications networks may be vulnerable to eavesdropping.

14.9.3 Modifications of the Basic Expiration Mechanism

   The expiration time of an entity may be specified by the origin
   server using the Expires header (see section 14.21). Alternatively,
   it may be specified using the max-age directive in a response.

   If a response includes both an Expires header and a max-age
   directive, the max-age directive overrides the Expires header, even
   if the Expires header is more restrictive. This rule allows an origin
   server to provide, for a given response, a longer expiration time to
   an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This may be
   useful if certain HTTP/1.0 caches improperly calculate ages or
   expiration times, perhaps due to desynchronized clocks.

     Note: most older caches, not compliant with this specification, do
     not implement any Cache-Control directives.  An origin server
     wishing to use a Cache-Control directive that restricts, but does
     not prevent, caching by an HTTP/1.1-compliant cache may exploit the
     requirement that the max-age directive overrides the Expires
     header, and the fact that non-HTTP/1.1-compliant caches do not
     observe the max-age directive.

   Other directives allow an user agent to modify the basic expiration
   mechanism. These directives may be specified on a request:

   max-age
     Indicates that the client is willing to accept a response whose age
     is no greater than the specified time in seconds. Unless max-stale
     directive is also included, the client is not willing to accept a
     stale response.

   min-fresh
     Indicates that the client is willing to accept a response whose
     freshness lifetime is no less than its current age plus the

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

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

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

14.9.4 Cache Revalidation and Reload Controls

   Sometimes an user agent may want or need to insist that a cache
   revalidate its cache entry with the origin server (and not just with
   the next cache along the path to the origin server), or to reload its
   cache entry from the origin server. End-to-end revalidation may be
   necessary if either the cache or the origin server has overestimated
   the expiration time of the cached response. End-to-end reload may be
   necessary if the cache entry has become corrupted for some reason.

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

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

   End-to-end reload
     The request includes a "no-cache" Cache-Control directive or, for
     compatibility with HTTP/1.0 clients, "Pragma: no-cache". No field
     names may be included with the no-cache directive in a request. The
     server MUST NOT use a cached copy when responding to such a
     request.

   Specific end-to-end revalidation
     The request includes a "max-age=0" Cache-Control directive, which
     forces each cache along the path to the origin server to revalidate
     its own entry, if any, with the next cache or server. The initial

     request includes a cache-validating conditional with the client's
     current validator.

   Unspecified end-to-end revalidation
     The request includes "max-age=0" Cache-Control directive, which
     forces each cache along the path to the origin server to revalidate
     its own entry, if any, with the next cache or server. The initial
     request does not include a cache-validating conditional; the first
     cache along the path (if any) that holds a cache entry for this
     resource includes a cache-validating conditional with its current
     validator.

   When an intermediate cache is forced, by means of a max-age=0
   directive, to revalidate its own cache entry, and the client has
   supplied its own validator in the request, the supplied validator may
   differ from the validator currently stored with the cache entry. In
   this case, the cache may use either validator in making its own
   request without affecting semantic transparency.

   However, the choice of validator may affect performance. The best
   approach is for the intermediate cache to use its own validator when
   making its request. If the server replies with 304 (Not Modified),
   then the cache should return its now validated copy to the client
   with a 200 (OK) response. If the server replies with a new entity and
   cache validator, however, the intermediate cache should compare the
   returned validator with the one provided in the client's request,
   using the strong comparison function. If the client's validator is
   equal to the origin server's, then the intermediate cache simply
   returns 304 (Not Modified). Otherwise, it returns the new entity with
   a 200 (OK) response.

   If a request includes the no-cache directive, it should not include
   min-fresh, max-stale, or max-age.

   In some cases, such as times of extremely poor network connectivity,
   a client may want a cache to return only those responses that it
   currently has stored, and not to reload or revalidate with the origin
   server. To do this, the client may include the only-if-cached
   directive in a request. If it receives this directive, a cache SHOULD
   either respond using a cached entry that is consistent with the other
   constraints of the request, or respond with a 504 (Gateway Timeout)
   status. However, if a group of caches is being operated as a unified
   system with good internal connectivity, such a request MAY be
   forwarded within that group of caches.

   Because a cache may be configured to ignore a server's specified
   expiration time, and because a client request may include a max-stale
   directive (which has a similar effect), the protocol also includes a

   mechanism for the origin server to require revalidation of a cache
   entry on any subsequent use. When the must-revalidate directive is
   present in a response received by a cache, that cache MUST NOT use
   the entry after it becomes stale to respond to a subsequent request
   without first revalidating it with the origin server. (I.e., the
   cache must do an end-to-end revalidation every time, if, based solely
   on the origin server's Expires or max-age value, the cached response
   is stale.)

   The must-revalidate directive is necessary to support reliable
   operation for certain protocol features. In all circumstances an
   HTTP/1.1 cache MUST obey the must-revalidate directive; in
   particular, if the cache cannot reach the origin server for any
   reason, it MUST generate a 504 (Gateway Timeout) response.

   Servers should send the must-revalidate directive if and only if
   failure to revalidate a request on the entity could result in
   incorrect operation, such as a silently unexecuted financial
   transaction.  Recipients MUST NOT take any automated action that
   violates this directive, and MUST NOT automatically provide an
   unvalidated copy of the entity if revalidation fails.

   Although this is not recommended, user agents operating under severe
   connectivity constraints may violate this directive but, if so, MUST
   explicitly warn the user that an unvalidated response has been
   provided.  The warning MUST be provided on each unvalidated access,
   and SHOULD require explicit user confirmation.

   The proxy-revalidate directive has the same meaning as the must-
   revalidate directive, except that it does not apply to non-shared
   user agent caches. It can be used on a response to an authenticated
   request to permit the user's cache to store and later return the
   response without needing to revalidate it (since it has already been
   authenticated once by that user), while still requiring proxies that
   service many users to revalidate each time (in order to make sure
   that each user has been authenticated). Note that such authenticated
   responses also need the public cache control directive in order to
   allow them to be cached at all.

14.9.5 No-Transform Directive

   Implementers of intermediate caches (proxies) have found it useful to
   convert the media type of certain entity bodies. A proxy might, for
   example, convert between image formats in order to save cache space
   or to reduce the amount of traffic on a slow link. HTTP has to date
   been silent on these transformations.

   Serious operational problems have already occurred, however, when
   these transformations have been applied to entity bodies intended for
   certain kinds of applications. For example, applications for medical
   imaging, scientific data analysis and those using end-to-end
   authentication, all depend on receiving an entity body that is bit
   for bit identical to the original entity-body.

   Therefore, if a response includes the no-transform directive, an
   intermediate cache or proxy MUST NOT change those headers that are
   listed in section 13.5.2 as being subject to the no-transform
   directive.  This implies that the cache or proxy must not change any
   aspect of the entity-body that is specified by these headers.

14.9.6 Cache Control Extensions

   The Cache-Control header field can be extended through the use of one
   or more cache-extension tokens, each with an optional assigned value.
   Informational extensions (those which do not require a change in
   cache behavior) may be added without changing the semantics of other
   directives. Behavioral extensions are designed to work by acting as
   modifiers to the existing base of cache directives. Both the new
   directive and the standard directive are supplied, such that
   applications which do not understand the new directive will default
   to the behavior specified by the standard directive, and those that
   understand the new directive will recognize it as modifying the
   requirements associated with the standard directive.  In this way,
   extensions to the Cache-Control directives can be made without
   requiring changes to the base protocol.

   This extension mechanism depends on a HTTP cache obeying all of the
   cache-control directives defined for its native HTTP-version, obeying
   certain extensions, and ignoring all directives that it does not
   understand.

   For example, consider a hypothetical new response directive called
   "community" which acts as a modifier to the "private" directive. We
   define this new directive to mean that, in addition to any non-shared
   cache, any cache which is shared only by members of the community
   named within its value may cache the response. An origin server
   wishing to allow the "UCI" community to use an otherwise private
   response in their shared cache(s) may do so by including

          Cache-Control: private, community="UCI"

   A cache seeing this header field will act correctly even if the cache
   does not understand the "community" cache-extension, since it will
   also see and understand the "private" directive and thus default to
   the safe behavior.

   Unrecognized cache-directives MUST be ignored; it is assumed that any
   cache-directive likely to be unrecognized by an HTTP/1.1 cache will
   be combined with standard directives (or the response's default
   cachability) such that the cache behavior will remain minimally
   correct even if the cache does not understand the extension(s).

14.10 Connection

   The Connection general-header field allows the sender to specify
   options that are desired for that particular connection and MUST NOT
   be communicated by proxies over further connections.

   The Connection header has the following grammar:

          Connection-header = "Connection" ":" 1#(connection-token)
          connection-token  = token

   HTTP/1.1 proxies MUST parse the Connection header field before a
   message is forwarded and, for each connection-token in this field,
   remove any header field(s) from the message with the same name as the
   connection-token. Connection options are signaled by the presence of
   a connection-token in the Connection header field, not by any
   corresponding additional header field(s), since the additional header
   field may not be sent if there are no parameters associated with that
   connection option.  HTTP/1.1 defines the "close" connection option
   for the sender to signal that the connection will be closed after
   completion of the response. For example,

          Connection: close

   in either the request or the response header fields indicates that
   the connection should not be considered `persistent' (section 8.1)
   after the current request/response is complete.

   HTTP/1.1 applications that do not support persistent connections MUST
   include the "close" connection option in every message.

14.11 Content-Base

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

          Content-Base      = "Content-Base" ":" absoluteURI

   If no Content-Base field is present, the base URI of an entity is
   defined either by its Content-Location (if that Content-Location URI
   is an absolute URI) or the URI used to initiate the request, in that

   order of precedence. Note, however, that the base URI of the contents
   within the entity-body may be redefined within that entity-body.

14.12 Content-Encoding

   The Content-Encoding entity-header field is used as a modifier to the
   media-type. When present, its value indicates what additional content
   codings have been applied to the entity-body, and thus what decoding
   mechanisms MUST be applied in order to obtain the media-type
   referenced by the Content-Type header field. Content-Encoding is
   primarily used to allow a document to be compressed without losing
   the identity of its underlying media type.

          Content-Encoding  = "Content-Encoding" ":" 1#content-coding

   Content codings are defined in section 3.5. An example of its use is

          Content-Encoding: gzip

   The Content-Encoding is a characteristic of the entity identified by
   the Request-URI. Typically, the entity-body is stored with this
   encoding and is only decoded before rendering or analogous usage.

   If multiple encodings have been applied to an entity, the content
   codings MUST be listed in the order in which they were applied.

   Additional information about the encoding parameters MAY be provided
   by other entity-header fields not defined by this specification.

14.13 Content-Language

   The Content-Language entity-header field describes the natural
   language(s) of the intended audience for the enclosed entity. Note
   that this may not be equivalent to all the languages used within the
   entity-body.

          Content-Language  = "Content-Language" ":" 1#language-tag

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

          Content-Language: da

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

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

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

          Content-Language: mi, en

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

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

14.14 Content-Length

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

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

   An example is

          Content-Length: 3495

   Applications SHOULD use this field to indicate the size of the
   message-body to be transferred, regardless of the media type of the
   entity. It must be possible for the recipient to reliably determine
   the end of HTTP/1.1 requests containing an entity-body, e.g., because
   the request has a valid Content-Length field, uses Transfer-Encoding:
   chunked or a multipart body.

   Any Content-Length greater than or equal to zero is a valid value.
   Section 4.4 describes how to determine the length of a message-body
   if a Content-Length is not given.

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

14.15 Content-Location

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

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

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

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

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

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

14.16 Content-MD5

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

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

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

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

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

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

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

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

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

     in which the application of Content-MD5 to HTTP entity-bodies
     differs from its application to MIME entity-bodies. One is that
     HTTP, unlike MIME, does not use Content-Transfer-Encoding, and does
     use Transfer-Encoding and Content-Encoding. Another is that HTTP
     more frequently uses binary content types than MIME, so it is worth
     noting that, in such cases, the byte order used to compute the
     digest is the transmission byte order defined for the type. Lastly,
     HTTP allows transmission of text types with any of several line
     break conventions and not just the canonical form using CRLF.
     Conversion of all line breaks to CRLF should not be done before
     computing or checking the digest: the line break convention used in
     the text actually transmitted should be left unaltered when
     computing the digest.

14.17 Content-Range

   The Content-Range entity-header is sent with a partial entity-body to
   specify where in the full entity-body the partial body should be
   inserted. It also indicates the total size of the full entity-body.
   When a server returns a partial response to a client, it must
   describe both the extent of the range covered by the response, and
   the length of the entire entity-body.

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

          content-range-spec      = byte-content-range-spec

          byte-content-range-spec = bytes-unit SP first-byte-pos "-"
                                    last-byte-pos "/" entity-length

          entity-length           = 1*DIGIT

   Unlike byte-ranges-specifier values, a byte-content-range-spec may
   only specify one range, and must contain absolute byte positions for
   both the first and last byte of the range.

   A byte-content-range-spec whose last-byte-pos value is less than its
   first-byte-pos value, or whose entity-length value is less than or
   equal to its last-byte-pos value, is invalid. The recipient of an
   invalid byte-content-range-spec MUST ignore it and any content
   transferred along with it.

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

     o  The first 500 bytes:

          bytes 0-499/1234

     o  The second 500 bytes:

          bytes 500-999/1234

     o  All except for the first 500 bytes:

          bytes 500-1233/1234

     o  The last 500 bytes:

          bytes 734-1233/1234

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

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

   When an HTTP message includes the content of multiple ranges (for
   example, a response to a request for multiple non-overlapping
   ranges), these are transmitted as a multipart MIME message. The
   multipart MIME content-type used for this purpose is defined in this
   specification to be "multipart/byteranges". See appendix 19.2 for its
   definition.

   A client that cannot decode a MIME multipart/byteranges message
   should not ask for multiple byte-ranges in a single request.

   When a client requests multiple byte-ranges in one request, the
   server SHOULD return them in the order that they appeared in the
   request.

   If the server ignores a byte-range-spec because it is invalid, the
   server should treat the request as if the invalid Range header field

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

14.18 Content-Type

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

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

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

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

14.19 Date

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

          Date  = "Date" ":" HTTP-date

   An example is

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

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

   In theory, the date SHOULD represent the moment just before the
   entity is generated. In practice, the date can be generated at any
   time during the message origination without affecting its semantic
   value.

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

14.20 ETag

   The ETag entity-header field defines the entity tag for the
   associated entity. The headers used with entity tags are described in
   sections 14.20, 14.25, 14.26 and 14.43. The entity tag may be used
   for comparison with other entities from the same resource (see
   section 13.3.2).

         ETag = "ETag" ":" entity-tag

   Examples:

         ETag: "xyzzy"
         ETag: W/"xyzzy"
         ETag: ""

14.21 Expires

   The Expires entity-header field gives the date/time after which the
   response should be considered stale. A stale cache entry may not
   normally be returned by a cache (either a proxy cache or an user
   agent cache) unless it is first validated with the origin server (or
   with an intermediate cache that has a fresh copy of the entity). See
   section 13.2 for further discussion of the expiration model.

   The presence of an Expires field does not imply that the original
   resource will change or cease to exist at, before, or after that
   time.

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

         Expires = "Expires" ":" HTTP-date

   An example of its use is

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

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

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

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

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

   The presence of an Expires header field with a date value of some
   time in the future on an response that otherwise would by default be
   non-cacheable indicates that the response is cachable, unless
   indicated otherwise by a Cache-Control header field (section 14.9).

14.22 From

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

          From   = "From" ":" mailbox

   An example is:

          From: webmaster@w3.org

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

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

     Note: The client SHOULD not send the From header field without the
     user's approval, as it may conflict with the user's privacy
     interests or their site's security policy. It is strongly
     recommended that the user be able to disable, enable, and modify
     the value of this field at any time prior to a request.

14.23 Host

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

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

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

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

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

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

14.24 If-Modified-Since

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

   be returned from the server; instead, a 304 (not modified) response
   will be returned without any message-body.

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

   An example of the field is:

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

   A GET method with an If-Modified-Since header and no Range header
   requests that the identified entity be transferred only if it has
   been modified since the date given by the If-Modified-Since header.
   The algorithm for determining this includes the following cases:

   a)If the request would normally result in anything other than a 200
     (OK) status, or if the passed If-Modified-Since date is invalid, the
     response is exactly the same as for a normal GET. A date which is
     later than the server's current time is invalid.

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

   c)If the variant has not been modified since a valid If-Modified-Since
     date, the server MUST return a 304 (Not Modified) response.

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

     Note that the Range request-header field modifies the meaning of
     If-Modified-Since; see section 14.36 for full details.

     Note that If-Modified-Since times are interpreted by the server,
     whose clock may not be synchronized with the client.

   Note that if a client uses an arbitrary date in the If-Modified-Since
   header instead of a date taken from the Last-Modified header for the
   same request, the client should be aware of the fact that this date
   is interpreted in the server's understanding of time. The client
   should consider unsynchronized clocks and rounding problems due to
   the different encodings of time between the client and server. This
   includes the possibility of race conditions if the document has
   changed between the time it was first requested and the If-Modified-
   Since date of a subsequent request, and the possibility of clock-
   skew-related problems if the If-Modified-Since date is derived from
   the client's clock without correction to the server's clock.
   Corrections for different time bases between client and server are at
   best approximate due to network latency.

14.25 If-Match

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

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

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

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

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

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

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

   A request intended to update a resource (e.g., a PUT) MAY include an
   If-Match header field to signal that the request method MUST NOT be
   applied if the entity corresponding to the If-Match value (a single
   entity tag) is no longer a representation of that resource.  This
   allows the user to indicate that they do not wish the request to be
   successful if the resource has been changed without their knowledge.
   Examples:

          If-Match: "xyzzy"
          If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
          If-Match: *

14.26 If-None-Match

   The If-None-Match request-header field is used with a method to make
   it conditional. A client that has one or more entities previously
   obtained from the resource can verify that none of those entities is
   current by including a list of their associated entity tags in the
   If-None-Match header field. The purpose of this feature is to allow
   efficient updates of cached information with a minimum amount of
   transaction overhead. It is also used, on updating requests, to
   prevent inadvertent modification of a resource which was not known to
   exist.

   As a special case, the value "*" matches any current entity of the
   resource.

          If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )

   If any of the entity tags match the entity tag of the entity that
   would have been returned in the response to a similar GET request
   (without the If-None-Match header) on that resource, or if "*" is
   given and any current entity exists for that resource, then the
   server MUST NOT perform the requested method. Instead, if the request
   method was GET or HEAD, the server SHOULD respond with a 304 (Not
   Modified) response, including the cache-related entity-header fields
   (particularly ETag) of one of the entities that matched. For all
   other request methods, the server MUST respond with a status of 412
   (Precondition Failed).

   See section 13.3.3 for rules on how to determine if two entity tags
   match. The weak comparison function can only be used with GET or HEAD
   requests.

   If none of the entity tags match, or if "*" is given and no current
   entity exists, then the server MAY perform the requested method as if
   the If-None-Match header field did not exist.

   If the request would, without the If-None-Match header field, result
   in anything other than a 2xx status, then the If-None-Match header
   MUST be ignored.

   The meaning of "If-None-Match: *" is that the method MUST NOT be
   performed if the representation selected by the origin server (or by
   a cache, possibly using the Vary mechanism, see section 14.43)
   exists, and SHOULD be performed if the representation does not exist.
   This feature may be useful in preventing races between PUT
   operations.

   Examples:

          If-None-Match: "xyzzy"
          If-None-Match: W/"xyzzy"
          If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
          If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
          If-None-Match: *

14.27 If-Range

   If a client has a partial copy of an entity in its cache, and wishes
   to have an up-to-date copy of the entire entity in its cache, it
   could use the Range request-header with a conditional GET (using
   either or both of If-Unmodified-Since and If-Match.) However, if the
   condition fails because the entity has been modified, the client
   would then have to make a second request to obtain the entire current
   entity-body.

   The If-Range header allows a client to "short-circuit" the second
   request. Informally, its meaning is `if the entity is unchanged, send
   me the part(s) that I am missing; otherwise, send me the entire new
   entity.'

           If-Range = "If-Range" ":" ( entity-tag | HTTP-date )

   If the client has no entity tag for an entity, but does have a Last-
   Modified date, it may use that date in a If-Range header. (The server
   can distinguish between a valid HTTP-date and any form of entity-tag
   by examining no more than two characters.) The If-Range header should
   only be used together with a Range header, and must be ignored if the
   request does not include a Range header, or if the server does not
   support the sub-range operation.

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

14.28 If-Unmodified-Since

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

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

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

   An example of the field is:

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

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

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

14.29 Last-Modified

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

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

   An example of its use is

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

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

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

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

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

14.30 Location

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

          Location       = "Location" ":" absoluteURI

   An example is

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

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

14.31 Max-Forwards

   The Max-Forwards request-header field may be used with the TRACE
   method (section 14.31) to limit the number of proxies or gateways
   that can forward the request to the next inbound server. This can be
   useful when the client is attempting to trace a request chain which
   appears to be failing or looping in mid-chain.

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

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

   Each proxy or gateway recipient of a TRACE request containing a Max-
   Forwards header field SHOULD check and update its value prior to
   forwarding the request. If the received value is zero (0), the
   recipient SHOULD NOT forward the request; instead, it SHOULD respond
   as the final recipient with a 200 (OK) response containing the
   received request message as the response entity-body (as described in
   section 9.8). If the received Max-Forwards value is greater than
   zero, then the forwarded message SHOULD contain an updated Max-
   Forwards field with a value decremented by one (1).

   The Max-Forwards header field SHOULD be ignored for all other methods
   defined by this specification and for any extension methods for which
   it is not explicitly referred to as part of that method definition.

14.32 Pragma

   The Pragma general-header field is used to include implementation-
   specific directives that may apply to any recipient along the
   request/response chain. All pragma directives specify optional
   behavior from the viewpoint of the protocol; however, some systems
   MAY require that behavior be consistent with the directives.

          Pragma            = "Pragma" ":" 1#pragma-directive

          pragma-directive  = "no-cache" | extension-pragma
          extension-pragma  = token [ "=" ( token | quoted-string ) ]

   When the no-cache directive is present in a request message, an
   application SHOULD forward the request toward the origin server even
   if it has a cached copy of what is being requested. This pragma
   directive has the same semantics as the no-cache cache-directive (see
   section 14.9) and is defined here for backwards compatibility with
   HTTP/1.0.  Clients SHOULD include both header fields when a no-cache
   request is sent to a server not known to be HTTP/1.1 compliant.

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

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

14.33 Proxy-Authenticate

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

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

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

14.34 Proxy-Authorization

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

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

   The HTTP access authentication process is described in section 11.
   Unlike Authorization, the Proxy-Authorization header field applies
   only to the next outbound proxy that demanded authentication using
   the Proxy-Authenticate field. When multiple proxies are used in a
   chain, the Proxy-Authorization header field is consumed by the first
   outbound proxy that was expecting to receive credentials. A proxy MAY
   relay the credentials from the client request to the next proxy if
   that is the mechanism by which the proxies cooperatively authenticate
   a given request.

14.35 Public

   The Public response-header field lists the set of methods supported
   by the server. The purpose of this field is strictly to inform the
   recipient of the capabilities of the server regarding unusual
   methods.  The methods listed may or may not be applicable to the

   Request-URI; the Allow header field (section 14.7) MAY be used to
   indicate methods allowed for a particular URI.

          Public         = "Public" ":" 1#method

   Example of use:

          Public: OPTIONS, MGET, MHEAD, GET, HEAD

   This header field applies only to the server directly connected to
   the client (i.e., the nearest neighbor in a chain of connections). If
   the response passes through a proxy, the proxy MUST either remove the
   Public header field or replace it with one applicable to its own
   capabilities.

14.36 Range

14.36.1 Byte Ranges

   Since all HTTP entities are represented in HTTP messages as sequences
   of bytes, the concept of a byte range is meaningful for any HTTP
   entity.  (However, not all clients and servers need to support byte-
   range operations.)

   Byte range specifications in HTTP apply to the sequence of bytes in
   the entity-body (not necessarily the same as the message-body).

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

       ranges-specifier = byte-ranges-specifier

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

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

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

       first-byte-pos  = 1*DIGIT

       last-byte-pos   = 1*DIGIT

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

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

   If the last-byte-pos value is absent, or if the value is greater than
   or equal to the current length of the entity-body, last-byte-pos is
   taken to be equal to one less than the current length of the entity-
   body in bytes.

   By its choice of last-byte-pos, a client can limit the number of
   bytes retrieved without knowing the size of the entity.

          suffix-byte-range-spec = "-" suffix-length

          suffix-length = 1*DIGIT

   A suffix-byte-range-spec is used to specify the suffix of the
   entity-body, of a length given by the suffix-length value. (That is,
   this form specifies the last N bytes of an entity-body.) If the
   entity is shorter than the specified suffix-length, the entire
   entity-body is used.

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

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

          bytes=0-499

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

          bytes=500-999

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

          bytes=-500

     o  Or

          bytes=9500-

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

          bytes=0-0,-1

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

          bytes=500-600,601-999

          bytes=500-700,601-999

14.36.2 Range Retrieval Requests

   HTTP retrieval requests using conditional or unconditional GET
   methods may request one or more sub-ranges of the entity, instead of
   the entire entity, using the Range request header, which applies to
   the entity returned as the result of the request:

         Range = "Range" ":" ranges-specifier

   A server MAY ignore the Range header. However, HTTP/1.1 origin
   servers and intermediate caches SHOULD support byte ranges when
   possible, since Range supports efficient recovery from partially
   failed transfers, and supports efficient partial retrieval of large
   entities.

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

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

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

   In some cases, it may be more appropriate to use the If-Range header
   (see section 14.27) in addition to the Range header.

   If a proxy that supports ranges receives a Range request, forwards
   the request to an inbound server, and receives an entire entity in
   reply, it SHOULD only return the requested range to its client. It
   SHOULD store the entire received response in its cache, if that is
   consistent with its cache allocation policies.

14.37 Referer

   The Referer[sic] request-header field allows the client to specify,
   for the server's benefit, the address (URI) of the resource from
   which the Request-URI was obtained (the "referrer", although the
   header field is misspelled.) The Referer request-header allows a
   server to generate lists of back-links to resources for interest,
   logging, optimized caching, etc. It also allows obsolete or mistyped
   links to be traced for maintenance. The Referer field MUST NOT be
   sent if the Request-URI was obtained from a source that does not have
   its own URI, such as input from the user keyboard.

        Referer        = "Referer" ":" ( absoluteURI | relativeURI )

   Example:

        Referer: http://www.w3.org/hypertext/DataSources/Overview.html

   If the field value is a partial URI, it SHOULD be interpreted
   relative to the Request-URI. The URI MUST NOT include a fragment.

     Note: Because the source of a link may be private information or
     may reveal an otherwise private information source, it is strongly
     recommended that the user be able to select whether or not the
     Referer field is sent. For example, a browser client could have a
     toggle switch for browsing openly/anonymously, which would
     respectively enable/disable the sending of Referer and From
     information.

14.38 Retry-After

   The Retry-After response-header field can be used with a 503 (Service
   Unavailable) response to indicate how long the service is expected to
   be unavailable to the requesting client. The value of this field can
   be either an HTTP-date or an integer number of seconds (in decimal)
   after the time of the response.

          Retry-After  = "Retry-After" ":" ( HTTP-date | delta-seconds )

   Two examples of its use are

          Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
          Retry-After: 120

   In the latter example, the delay is 2 minutes.

14.39 Server

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

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

   Example:

          Server: CERN/3.0 libwww/2.17

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

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

14.40 Transfer-Encoding

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

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

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

          Transfer-Encoding: chunked

   Many older HTTP/1.0 applications do not understand the Transfer-
   Encoding header.

14.41 Upgrade

   The Upgrade general-header allows the client to specify what
   additional communication protocols it supports and would like to use
   if the server finds it appropriate to switch protocols. The server

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

          Upgrade        = "Upgrade" ":" 1#product

   For example,

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

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

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

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

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

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

14.42 User-Agent

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

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

   Example:

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

14.43 Vary

   The Vary response-header field is used by a server to signal that the
   response entity was selected from the available representations of
   the response using server-driven negotiation (section 12). Field-
   names listed in Vary headers are those of request-headers. The Vary
   field value indicates either that the given set of header fields
   encompass the dimensions over which the representation might vary, or
   that the dimensions of variance are unspecified ("*") and thus may
   vary over any aspect of future requests.

          Vary  = "Vary" ":" ( "*" | 1#field-name )

   An HTTP/1.1 server MUST include an appropriate Vary header field with
   any cachable response that is subject to server-driven negotiation.
   Doing so allows a cache to properly interpret future requests on that
   resource and informs the user agent about the presence of negotiation
   on that resource. A server SHOULD include an appropriate Vary header
   field with a non-cachable response that is subject to server-driven
   negotiation, since this might provide the user agent with useful
   information about the dimensions over which the response might vary.

   The set of header fields named by the Vary field value is known as
   the "selecting" request-headers.

   When the cache receives a subsequent request whose Request-URI
   specifies one or more cache entries including a Vary header, the
   cache MUST NOT use such a cache entry to construct a response to the
   new request unless all of the headers named in the cached Vary header

   are present in the new request, and all of the stored selecting
   request-headers from the previous request match the corresponding
   headers in the new request.

   The selecting request-headers from two requests are defined to match
   if and only if the selecting request-headers in the first request can
   be transformed to the selecting request-headers in the second request
   by adding or removing linear whitespace (LWS) at places where this is
   allowed by the corresponding BNF, and/or combining multiple message-
   header fields with the same field name following the rules about
   message headers in section 4.2.

   A Vary field value of "*" signals that unspecified parameters,
   possibly other than the contents of request-header fields (e.g., the
   network address of the client), play a role in the selection of the
   response representation. Subsequent requests on that resource can
   only be properly interpreted by the origin server, and thus a cache
   MUST forward a (possibly conditional) request even when it has a
   fresh response cached for the resource. See section 13.6 for use of
   the Vary header by caches.

   A Vary field value consisting of a list of field-names signals that
   the representation selected for the response is based on a selection
   algorithm which considers ONLY the listed request-header field values
   in selecting the most appropriate representation. A cache MAY assume
   that the same selection will be made for future requests with the
   same values for the listed field names, for the duration of time in
   which the response is fresh.

   The field-names given are not limited to the set of standard
   request-header fields defined by this specification. Field names are
   case-insensitive.

14.44 Via

   The Via general-header field MUST be used by gateways and proxies to
   indicate the intermediate protocols and recipients between the user
   agent and the server on requests, and between the origin server and
   the client on responses. It is analogous to the "Received" field of
   RFC 822 and is intended to be used for tracking message forwards,
   avoiding request loops, and identifying the protocol capabilities of
   all senders along the request/response chain.

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

      received-protocol = [ protocol-name "/" ] protocol-version
      protocol-name     = token
      protocol-version  = token
      received-by       = ( host [ ":" port ] ) | pseudonym
      pseudonym         = token

   The received-protocol indicates the protocol version of the message
   received by the server or client along each segment of the
   request/response chain. The received-protocol version is appended to
   the Via field value when the message is forwarded so that information
   about the protocol capabilities of upstream applications remains
   visible to all recipients.

   The protocol-name is optional if and only if it would be "HTTP". The
   received-by field is normally the host and optional port number of a
   recipient server or client that subsequently forwarded the message.
   However, if the real host is considered to be sensitive information,
   it MAY be replaced by a pseudonym. If the port is not given, it MAY
   be assumed to be the default port of the received-protocol.

   Multiple Via field values represent each proxy or gateway that has
   forwarded the message. Each recipient MUST append its information
   such that the end result is ordered according to the sequence of
   forwarding applications.

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

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

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

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

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

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

           could be collapsed to

          Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

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

14.45 Warning

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

   Warning headers are sent with responses using:

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

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

   A response may carry more than one Warning header.

   The warn-text should be in a natural language and character set that
   is most likely to be intelligible to the human user receiving the
   response.  This decision may be based on any available knowledge,
   such as the location of the cache or user, the Accept-Language field
   in a request, the Content-Language field in a response, etc. The
   default language is English and the default character set is ISO-
   8859-1.

   If a character set other than ISO-8859-1 is used, it MUST be encoded
   in the warn-text using the method described in RFC 1522 [14].

   Any server or cache may add Warning headers to a response. New
   Warning headers should be added after any existing Warning headers. A
   cache MUST NOT delete any Warning header that it received with a
   response. However, if a cache successfully validates a cache entry,
   it SHOULD remove any Warning headers previously attached to that
   entry except as specified for specific Warning codes. It MUST then
   add any Warning headers received in the validating response. In other
   words, Warning headers are those that would be attached to the most
   recent relevant response.

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

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

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

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

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

11 Revalidation failed
  MUST be included if a cache returns a stale response because an
  attempt to revalidate the response failed, due to an inability to
  reach the server. A cache may add this warning to any response, but
  may never remove it until the response is successfully revalidated.

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

13 Heuristic expiration
  MUST be included if the cache heuristically chose a freshness
  lifetime greater than 24 hours and the response's age is greater than
  24 hours.

14 Transformation applied
  MUST be added by an intermediate cache or proxy if it applies any
  transformation changing the content-coding (as specified in the
  Content-Encoding header) or media-type (as specified in the
  Content-Type header) of the response, unless this Warning code
  already appears in the response. MUST NOT be deleted from a response
  even after revalidation.

99 Miscellaneous warning
  The warning text may include arbitrary information to be presented to
  a human user, or logged. A system receiving this warning MUST NOT
  take any automated action.

14.46 WWW-Authenticate

   The WWW-Authenticate response-header field MUST be included in 401
   (Unauthorized) response messages. The field value consists of at
   least one challenge that indicates the authentication scheme(s) and
   parameters applicable to the Request-URI.

          WWW-Authenticate  = "WWW-Authenticate" ":" 1#challenge

   The HTTP access authentication process is described in section 11.
   User agents MUST take special care in parsing the WWW-Authenticate
   field value if it contains more than one challenge, or if more than
   one WWW-Authenticate header field is provided, since the contents of
   a challenge may itself contain a comma-separated list of
   authentication parameters.

15 Security Considerations

   This section is meant to inform application developers, information
   providers, and users of the security limitations in HTTP/1.1 as
   described by this document. The discussion does not include
   definitive solutions to the problems revealed, though it does make
   some suggestions for reducing security risks.

15.1 Authentication of Clients

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

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

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

   A common use of Basic authentication is for identification purposes
   -- requiring the user to provide a user name and password as a means
   of identification, for example, for purposes of gathering accurate
   usage statistics on a server. When used in this way it is tempting to
   think that there is no danger in its use if illicit access to the
   protected documents is not a major concern. This is only correct if
   the server issues both user name and password to the users and in
   particular does not allow the user to choose his or her own password.
   The danger arises because naive users frequently reuse a single
   password to avoid the task of maintaining multiple passwords.

   If a server permits users to select their own passwords, then the
   threat is not only illicit access to documents on the server but also
   illicit access to the accounts of all users who have chosen to use
   their account password. If users are allowed to choose their own
   password that also means the server must maintain files containing
   the (presumably encrypted) passwords. Many of these may be the
   account passwords of users perhaps at distant sites. The owner or
   administrator of such a system could conceivably incur liability if
   this information is not maintained in a secure fashion.

   Basic Authentication is also vulnerable to spoofing by counterfeit
   servers. If a user can be led to believe that he is connecting to a
   host containing information protected by basic authentication when in
   fact he is connecting to a hostile server or gateway then the
   attacker can request a password, store it for later use, and feign an
   error. This type of attack is not possible with Digest Authentication
   [32]. Server implementers SHOULD guard against the possibility of
   this sort of counterfeiting by gateways or CGI scripts. In particular
   it is very dangerous for a server to simply turn over a connection to
   a gateway since that gateway can then use the persistent connection
   mechanism to engage in multiple transactions with the client while
   impersonating the original server in a way that is not detectable by
   the client.

15.2 Offering a Choice of Authentication Schemes

   An HTTP/1.1 server may return multiple challenges with a 401
   (Authenticate) response, and each challenge may use a different

   scheme.  The order of the challenges returned to the user agent is in
   the order that the server would prefer they be chosen. The server
   should order its challenges with the "most secure" authentication
   scheme first. A user agent should choose as the challenge to be made
   to the user the first one that the user agent understands.

   When the server offers choices of authentication schemes using the
   WWW-Authenticate header, the "security" of the authentication is only
   as malicious user could capture the set of challenges and try to
   authenticate him/herself using the weakest of the authentication
   schemes. Thus, the ordering serves more to protect the user's
   credentials than the server's information.

   A possible man-in-the-middle (MITM) attack would be to add a weak
   authentication scheme to the set of choices, hoping that the client
   will use one that exposes the user's credentials (e.g. password). For
   this reason, the client should always use the strongest scheme that
   it understands from the choices accepted.

   An even better MITM attack would be to remove all offered choices,
   and to insert a challenge that requests Basic authentication. For
   this reason, user agents that are concerned about this kind of attack
   could remember the strongest authentication scheme ever requested by
   a server and produce a warning message that requires user
   confirmation before using a weaker one. A particularly insidious way
   to mount such a MITM attack would be to offer a "free" proxy caching
   service to gullible users.

15.3 Abuse of Server Log Information

   A server is in the position to save personal data about a user's
   requests which may identify their reading patterns or subjects of
   interest. This information is clearly confidential in nature and its
   handling may be constrained by law in certain countries. People using
   the HTTP protocol to provide data are responsible for ensuring that
   such material is not distributed without the permission of any
   individuals that are identifiable by the published results.

15.4 Transfer of Sensitive Information

   Like any generic data transfer protocol, HTTP cannot regulate the
   content of the data that is transferred, nor is there any a priori
   method of determining the sensitivity of any particular piece of
   information within the context of any given request. Therefore,
   applications SHOULD supply as much control over this information as
   possible to the provider of that information. Four header fields are
   worth special mention in this context: Server, Via, Referer and From.

   Revealing the specific software version of the server may allow the
   server machine to become more vulnerable to attacks against software
   that is known to contain security holes. Implementers SHOULD make the
   Server header field a configurable option.

   Proxies which serve as a portal through a network firewall SHOULD
   take special precautions regarding the transfer of header information
   that identifies the hosts behind the firewall. In particular, they
   SHOULD remove, or replace with sanitized versions, any Via fields
   generated behind the firewall.

   The Referer field allows reading patterns to be studied and reverse
   links drawn. Although it can be very useful, its power can be abused
   if user details are not separated from the information contained in
   the Referer. Even when the personal information has been removed, the
   Referer field may indicate a private document's URI whose publication
   would be inappropriate.

   The information sent in the From field might conflict with the user's
   privacy interests or their site's security policy, and hence it
   SHOULD NOT be transmitted without the user being able to disable,
   enable, and modify the contents of the field. The user MUST be able
   to set the contents of this field within a user preference or
   application defaults configuration.

   We suggest, though do not require, that a convenient toggle interface
   be provided for the user to enable or disable the sending of From and
   Referer information.

15.5 Attacks Based On File and Path Names

   Implementations of HTTP origin servers SHOULD be careful to restrict
   the documents returned by HTTP requests to be only those that were
   intended by the server administrators. If an HTTP server translates
   HTTP URIs directly into file system calls, the server MUST take
   special care not to serve files that were not intended to be
   delivered to HTTP clients.  For example, UNIX, Microsoft Windows, and
   other operating systems use ".." as a path component to indicate a
   directory level above the current one. On such a system, an HTTP
   server MUST disallow any such construct in the Request-URI if it
   would otherwise allow access to a resource outside those intended to
   be accessible via the HTTP server. Similarly, files intended for
   reference only internally to the server (such as access control
   files, configuration files, and script code) MUST be protected from
   inappropriate retrieval, since they might contain sensitive
   information. Experience has shown that minor bugs in such HTTP server
   implementations have turned into security risks.

15.6 Personal Information

   HTTP clients are often privy to large amounts of personal information
   (e.g. the user's name, location, mail address, passwords, encryption
   keys, etc.), and SHOULD be very careful to prevent unintentional
   leakage of this information via the HTTP protocol to other sources.
   We very strongly recommend that a convenient interface be provided
   for the user to control dissemination of such information, and that
   designers and implementers be particularly careful in this area.
   History shows that errors in this area are often both serious
   security and/or privacy problems, and often generate highly adverse
   publicity for the implementer's company.

15.7 Privacy Issues Connected to Accept Headers

   Accept request-headers can reveal information about the user to all
   servers which are accessed. The Accept-Language header in particular
   can reveal information the user would consider to be of a private
   nature, because the understanding of particular languages is often
   strongly correlated to the membership of a particular ethnic group.
   User agents which offer the option to configure the contents of an
   Accept-Language header to be sent in every request are strongly
   encouraged to let the configuration process include a message which
   makes the user aware of the loss of privacy involved.

   An approach that limits the loss of privacy would be for a user agent
   to omit the sending of Accept-Language headers by default, and to ask
   the user whether it should start sending Accept-Language headers to a
   server if it detects, by looking for any Vary response-header fields
   generated by the server, that such sending could improve the quality
   of service.

   Elaborate user-customized accept header fields sent in every request,
   in particular if these include quality values, can be used by servers
   as relatively reliable and long-lived user identifiers. Such user
   identifiers would allow content providers to do click-trail tracking,
   and would allow collaborating content providers to match cross-server
   click-trails or form submissions of individual users. Note that for
   many users not behind a proxy, the network address of the host
   running the user agent will also serve as a long-lived user
   identifier. In environments where proxies are used to enhance
   privacy, user agents should be conservative in offering accept header
   configuration options to end users. As an extreme privacy measure,
   proxies could filter the accept headers in relayed requests. General
   purpose user agents which provide a high degree of header
   configurability should warn users about the loss of privacy which can
   be involved.

15.8 DNS Spoofing

   Clients using HTTP rely heavily on the Domain Name Service, and are
   thus generally prone to security attacks based on the deliberate
   mis-association of IP addresses and DNS names. Clients need to be
   cautious in assuming the continuing validity of an IP number/DNS name
   association.

   In particular, HTTP clients SHOULD rely on their name resolver for
   confirmation of an IP number/DNS name association, rather than
   caching the result of previous host name lookups. Many platforms
   already can cache host name lookups locally when appropriate, and
   they SHOULD be configured to do so. These lookups should be cached,
   however, only when the TTL (Time To Live) information reported by the
   name server makes it likely that the cached information will remain
   useful.

   If HTTP clients cache the results of host name lookups in order to
   achieve a performance improvement, they MUST observe the TTL
   information reported by DNS.

   If HTTP clients do not observe this rule, they could be spoofed when
   a previously-accessed server's IP address changes. As network
   renumbering is expected to become increasingly common, the
   possibility of this form of attack will grow. Observing this
   requirement thus reduces this potential security vulnerability.

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

15.9 Location Headers and Spoofing

   If a single server supports multiple organizations that do not trust
   one another, then it must check the values of Location and Content-
   Location headers in responses that are generated under control of
   said organizations to make sure that they do not attempt to
   invalidate resources over which they have no authority.

16 Acknowledgments

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

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

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

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

   Much of the content and presentation of the caching design is due to
   suggestions and comments from individuals including: Shel Kaphan,
   Paul Leach, Koen Holtman, David Morris, and Larry Masinter.

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

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

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

17 References

   [1] Alvestrand, H., "Tags for the identification of languages", RFC
   1766, UNINETT, March 1995.

   [2] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey,
   D., and B. Alberti. "The Internet Gopher Protocol: (a distributed
   document search and retrieval protocol)", RFC 1436, University of
   Minnesota, March 1993.

   [3] Berners-Lee, T., "Universal Resource Identifiers in WWW", A
   Unifying Syntax for the Expression of Names and Addresses of Objects
   on the Network as used in the World-Wide Web", RFC 1630, CERN, June
   1994.

   [4] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform Resource
   Locators (URL)", RFC 1738, CERN, Xerox PARC, University of Minnesota,
   December 1994.

   [5] Berners-Lee, T., and D. Connolly, "HyperText Markup Language
   Specification - 2.0", RFC 1866, MIT/LCS, November 1995.

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

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

   [8] Braden, R., "Requirements for Internet hosts - application and
   support", STD 3,  RFC 1123, IETF, October 1989.

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

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

   [11] Fielding, R., "Relative Uniform Resource Locators", RFC 1808, UC
   Irvine, June 1995.

   [12] Horton, M., and R. Adams. "Standard for interchange of USENET
   messages", RFC 1036, AT&T Bell Laboratories, Center for Seismic
   Studies, December 1987.

   [13] Kantor, B., and P. Lapsley. "Network News Transfer Protocol." A
   Proposed Standard for the Stream-Based Transmission of News", RFC
   977, UC San Diego, UC Berkeley, February 1986.

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

   [15] Nebel, E., and L. Masinter. "Form-based File Upload in HTML",
   RFC 1867, Xerox Corporation, November 1995.

   [16] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
   USC/ISI, August 1982.

   [17] Postel, J., "Media Type Registration Procedure", RFC 2048,
   USC/ISI, November 1996.

   [18] Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)", STD
   9, RFC 959, USC/ISI, October 1985.

   [19] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
   1700, USC/ISI, October 1994.

   [20] Sollins, K., and L. Masinter, "Functional Requirements for
   Uniform Resource Names", RFC 1737, MIT/LCS, Xerox Corporation,
   December 1994.

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

   [22] ISO-8859. International Standard -- Information Processing --
     8-bit Single-Byte Coded Graphic Character Sets --
     Part 1: Latin alphabet No. 1, ISO 8859-1:1987.
     Part 2: Latin alphabet No. 2, ISO 8859-2, 1987.
     Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.
     Part 4: Latin alphabet No. 4, ISO 8859-4, 1988.

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

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

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

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

   [26] Venkata N. Padmanabhan and Jeffrey C. Mogul. Improving HTTP
   Latency. Computer Networks and ISDN Systems, v. 28, pp. 25-35, Dec.
   1995.  Slightly revised version of paper in Proc. 2nd International
   WWW Conf. '94: Mosaic and the Web, Oct. 1994, which is available at
   http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/
   HTTPLatency.html.

   [27] Joe Touch, John Heidemann, and Katia Obraczka, "Analysis of HTTP
   Performance", <URL: http://www.isi.edu/lsam/ib/http-perf/>,
   USC/Information Sciences Institute, June 1996

   [28] Mills, D., "Network Time Protocol, Version 3, Specification,
   Implementation and Analysis", RFC 1305, University of Delaware, March
   1992.

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

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

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

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

18 Authors' Addresses

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

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

   Jim Gettys
   MIT Laboratory for Computer Science
   545 Technology Square
   Cambridge, MA 02139, USA

   Fax: +1 (617) 258 8682
   EMail: jg@w3.org

   Jeffrey C. Mogul
   Western Research Laboratory
   Digital Equipment Corporation
   250 University Avenue
   Palo Alto, California, 94305, USA

   EMail: mogul@wrl.dec.com

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

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

   Tim Berners-Lee
   Director, W3 Consortium
   MIT Laboratory for Computer Science
   545 Technology Square
   Cambridge, MA 02139, USA

   Fax: +1 (617) 258 8682
   EMail: timbl@w3.org

19 Appendices

19.1 Internet Media Type message/http

   In addition to defining the HTTP/1.1 protocol, this document serves
   as the specification for the Internet media type "message/http". The
   following is to be registered with IANA.

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

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

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

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

       Security considerations: none

19.2 Internet Media Type multipart/byteranges

   When an HTTP message includes the content of multiple ranges (for
   example, a response to a request for multiple non-overlapping
   ranges), these are transmitted as a multipart MIME message. The
   multipart media type for this purpose is called
   "multipart/byteranges".

   The multipart/byteranges media type includes two or more parts, each
   with its own Content-Type and Content-Range fields. The parts are
   separated using a MIME boundary parameter.

          Media Type name:         multipart
          Media subtype name:      byteranges
          Required parameters:     boundary
          Optional parameters:     none

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

          Security considerations: none

For example:

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

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

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

   ...the second range
   --THIS_STRING_SEPARATES--

19.3 Tolerant Applications

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

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

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

   The character set of an entity-body should be labeled as the lowest
   common denominator of the character codes used within that body, with
   the exception that no label is preferred over the labels US-ASCII or
   ISO-8859-1.

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

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

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

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

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

19.4 Differences Between HTTP Entities and MIME Entities

   HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC
   822) and the Multipurpose Internet Mail Extensions (MIME ) to allow
   entities to be transmitted in an open variety of representations and
   with extensible mechanisms. However, MIME [7] discusses mail, and
   HTTP has a few features that are different from those described in
   MIME.  These differences were carefully chosen to optimize
   performance over binary connections, to allow greater freedom in the
   use of new media types, to make date comparisons easier, and to
   acknowledge the practice of some early HTTP servers and clients.

   This appendix describes specific areas where HTTP differs from MIME.
   Proxies and gateways to strict MIME environments SHOULD be aware of
   these differences and provide the appropriate conversions where
   necessary. Proxies and gateways from MIME environments to HTTP also
   need to be aware of the differences because some conversions may be
   required.

19.4.1 Conversion to Canonical Form

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

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

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

19.4.2 Conversion of Date Formats

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

19.4.3 Introduction of Content-Encoding

   MIME does not include any concept equivalent to HTTP/1.1's Content-
   Encoding header field. Since this acts as a modifier on the media
   type, proxies and gateways from HTTP to MIME-compliant protocols MUST
   either change the value of the Content-Type header field or decode
   the entity-body before forwarding the message. (Some experimental
   applications of Content-Type for Internet mail have used a media-type
   parameter of ";conversions=<content-coding>" to perform an equivalent
   function as Content-Encoding. However, this parameter is not part of
   MIME.)

19.4.4 No Content-Transfer-Encoding

   HTTP does not use the Content-Transfer-Encoding (CTE) field of MIME.
   Proxies and gateways from MIME-compliant protocols to HTTP MUST
   remove any non-identity CTE ("quoted-printable" or "base64") encoding
   prior to delivering the response message to an HTTP client.

   Proxies and gateways from HTTP to MIME-compliant protocols are
   responsible for ensuring that the message is in the correct format
   and encoding for safe transport on that protocol, where "safe
   transport" is defined by the limitations of the protocol being used.
   Such a proxy or gateway SHOULD label the data with an appropriate
   Content-Transfer-Encoding if doing so will improve the likelihood of
   safe transport over the destination protocol.

19.4.5 HTTP Header Fields in Multipart Body-Parts

   In MIME, most header fields in multipart body-parts are generally
   ignored unless the field name begins with "Content-". In HTTP/1.1,
   multipart body-parts may contain any HTTP header fields which are
   significant to the meaning of that part.

19.4.6 Introduction of Transfer-Encoding

   HTTP/1.1 introduces the Transfer-Encoding header field (section
   14.40).  Proxies/gateways MUST remove any transfer coding prior to
   forwarding a message via a MIME-compliant protocol.

   A process for decoding the "chunked" transfer coding (section 3.6)
   can be represented in pseudo-code as:

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

19.4.7 MIME-Version

   HTTP is not a MIME-compliant protocol (see appendix 19.4). However,
   HTTP/1.1 messages may include a single MIME-Version general-header
   field to indicate what version of the MIME protocol was used to
   construct the message. Use of the MIME-Version header field indicates
   that the message is in full compliance with the MIME protocol.
   Proxies/gateways are responsible for ensuring full compliance (where
   possible) when exporting HTTP messages to strict MIME environments.

          MIME-Version   = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT

   MIME version "1.0" is the default for use in HTTP/1.1. However,
   HTTP/1.1 message parsing and semantics are defined by this document
   and not the MIME specification.

19.5 Changes from HTTP/1.0

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

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

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

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

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

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

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

     o  Servers MUST accept absolute URIs.

19.6 Additional Features

   This appendix documents protocol elements used by some existing HTTP
   implementations, but not consistently and correctly across most
   HTTP/1.1 applications. Implementers should be aware of these
   features, but cannot rely upon their presence in, or interoperability
   with, other HTTP/1.1 applications. Some of these describe proposed
   experimental features, and some describe features that experimental
   deployment found lacking that are now addressed in the base HTTP/1.1
   specification.

19.6.1 Additional Request Methods

19.6.1.1 PATCH

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

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

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

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

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

19.6.1.2 LINK

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

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

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

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

19.6.1.3 UNLINK

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

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

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

19.6.2 Additional Header Field Definitions

19.6.2.1 Alternates

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

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

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

19.6.2.2 Content-Version

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

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

   Examples of the Content-Version field include:

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

19.6.2.3 Derived-From

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

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

   An example use of the field is:

          Derived-From: "2.1.1"

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

19.6.2.4 Link

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

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

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

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

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

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

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

   Examples of usage include:

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

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

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

19.6.2.5 URI

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

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

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

19.7 Compatibility with Previous Versions

   It is beyond the scope of a protocol specification to mandate
   compliance with previous versions. HTTP/1.1 was deliberately
   designed, however, to make supporting previous versions easy. It is
   worth noting that at the time of composing this specification, we
   would expect commercial HTTP/1.1 servers to:

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

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

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

   And we would expect HTTP/1.1 clients to:

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

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

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

19.7.1 Compatibility with HTTP/1.0 Persistent Connections

   Some clients and servers may wish to be compatible with some previous
   implementations of persistent connections in HTTP/1.0 clients and
   servers. Persistent connections in HTTP/1.0 must be explicitly
   negotiated as they are not the default behavior. HTTP/1.0
   experimental implementations of persistent connections are faulty,
   and the new facilities in HTTP/1.1 are designed to rectify these
   problems. The problem was that some existing 1.0 clients may be
   sending Keep-Alive to a proxy server that doesn't understand
   Connection, which would then erroneously forward it to the next
   inbound server, which would establish the Keep-Alive connection and
   result in a hung HTTP/1.0 proxy waiting for the close on the
   response. The result is that HTTP/1.0 clients must be prevented from
   using Keep-Alive when talking to proxies.

   However, talking to proxies is the most important use of persistent
   connections, so that prohibition is clearly unacceptable. Therefore,
   we need some other mechanism for indicating a persistent connection
   is desired, which is safe to use even when talking to an old proxy
   that ignores Connection. Persistent connections are the default for
   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
   declaring non-persistence.

   The following describes the original HTTP/1.0 form of persistent
   connections.

   When it connects to an origin server, an HTTP client MAY send the
   Keep-Alive connection-token in addition to the Persist connection-
   token:

          Connection: Keep-Alive

   An HTTP/1.0 server would then respond with the Keep-Alive connection
   token and the client may proceed with an HTTP/1.0 (or Keep-Alive)
   persistent connection.

   An HTTP/1.1 server may also establish persistent connections with
   HTTP/1.0 clients upon receipt of a Keep-Alive connection token.
   However, a persistent connection with an HTTP/1.0 client cannot make
   use of the chunked transfer-coding, and therefore MUST use a
   Content-Length for marking the ending boundary of each message.

   A client MUST NOT send the Keep-Alive connection token to a proxy
   server as HTTP/1.0 proxy servers do not obey the rules of HTTP/1.1
   for parsing the Connection header field.

19.7.1.1 The Keep-Alive Header

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

          Keep-Alive-header = "Keep-Alive" ":" 0# keepalive-param

          keepalive-param = param-name "=" value

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

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

 

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