RFC 2616 (rfc2616) - Hypertext Transfer Protocol -- HTTP/1.1
Description: RFC 2616 - Hypertext Transfer Protocol -- HTTP/1.1

# RFC 2616 (RFC2616)

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Comment on RFC 2616

### RFC 2616 - Hypertext Transfer Protocol -- HTTP/1.1

    Network Working Group                                      R. Fielding
    Request for Comments: 2616                                   UC Irvine
    Obsoletes: 2068                                              J. Gettys
    Category: Standards Track                                   Compaq/W3C
                                                                  J. Mogul
                                                                    Compaq
                                                                H. Frystyk
                                                                   W3C/MIT
                                                               L. Masinter
                                                                     Xerox
                                                                  P. Leach
                                                                 Microsoft
                                                            T. Berners-Lee
                                                                   W3C/MIT
                                                                 June 1999

                    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.

    Copyright Notice

       Copyright (C) The Internet Society (1999).  All Rights Reserved.

    Abstract

       The Hypertext Transfer Protocol (HTTP) is an application-level
       protocol for distributed, collaborative, hypermedia information
       systems. It is a generic, stateless, protocol which can be used for
       many tasks beyond its use for hypertext, such as name servers and
       distributed object management systems, through extension of its
       request methods, error codes and headers [47]. 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", and is an update to RFC 2068 [33].

    Table of Contents

       1   Introduction ...................................................7
       1.1    Purpose......................................................7
       1.2   Requirements .................................................8
       1.3   Terminology ..................................................8
       1.4   Overall Operation ...........................................12
       2   Notational Conventions and Generic Grammar ....................14
       2.1   Augmented BNF ...............................................14
       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 ...........................................19
       3.2.2    http URL .................................................19
       3.2.3    URI Comparison ...........................................20
       3.3   Date/Time Formats ...........................................20
       3.3.1    Full Date ................................................20
       3.3.2    Delta Seconds ............................................21
       3.4   Character Sets ..............................................21
       3.4.1    Missing Charset ..........................................22
       3.5   Content Codings .............................................23
       3.6   Transfer Codings ............................................24
       3.6.1    Chunked Transfer Coding ..................................25
       3.7   Media Types .................................................26
       3.7.1    Canonicalization and Text Defaults .......................27
       3.7.2    Multipart Types ..........................................27
       3.8   Product Tokens ..............................................28
       3.9   Quality Values ..............................................29
       3.10  Language Tags ...............................................29
       3.11  Entity Tags .................................................30
       3.12  Range Units .................................................30
       4   HTTP Message ..................................................31
       4.1   Message Types ...............................................31
       4.2   Message Headers .............................................31
       4.3   Message Body ................................................32
       4.4   Message Length ..............................................33
       4.5   General Header Fields .......................................34
       5   Request .......................................................35
       5.1   Request-Line ................................................35
       5.1.1    Method ...................................................36
       5.1.2    Request-URI ..............................................36
       5.2   The Resource Identified by a Request ........................38
       5.3   Request Header Fields .......................................38
       6   Response ......................................................39
       6.1   Status-Line .................................................39
       6.1.1    Status Code and Reason Phrase ............................39
       6.2   Response Header Fields ......................................41

       7   Entity ........................................................42
       7.1   Entity Header Fields ........................................42
       7.2   Entity Body .................................................43
       7.2.1    Type .....................................................43
       7.2.2    Entity Length ............................................43
       8   Connections ...................................................44
       8.1   Persistent Connections ......................................44
       8.1.1    Purpose ..................................................44
       8.1.2    Overall Operation ........................................45
       8.1.3    Proxy Servers ............................................46
       8.1.4    Practical Considerations .................................46
       8.2   Message Transmission Requirements ...........................47
       8.2.1    Persistent Connections and Flow Control ..................47
       8.2.2    Monitoring Connections for Error Status Messages .........48
       8.2.3    Use of the 100 (Continue) Status .........................48
       8.2.4    Client Behavior if Server Prematurely Closes Connection ..50
       9   Method Definitions ............................................51
       9.1   Safe and Idempotent Methods .................................51
       9.1.1    Safe Methods .............................................51
       9.1.2    Idempotent Methods .......................................51
       9.2   OPTIONS .....................................................52
       9.3   GET .........................................................53
       9.4   HEAD ........................................................54
       9.5   POST ........................................................54
       9.6   PUT .........................................................55
       9.7   DELETE ......................................................56
       9.8   TRACE .......................................................56
       9.9   CONNECT .....................................................57
       10   Status Code Definitions ......................................57
       10.1  Informational 1xx ...........................................57
       10.1.1   100 Continue .............................................58
       10.1.2   101 Switching Protocols ..................................58
       10.2  Successful 2xx ..............................................58
       10.2.1   200 OK ...................................................58
       10.2.2   201 Created ..............................................59
       10.2.3   202 Accepted .............................................59
       10.2.4   203 Non-Authoritative Information ........................59
       10.2.5   204 No Content ...........................................60
       10.2.6   205 Reset Content ........................................60
       10.2.7   206 Partial Content ......................................60
       10.3  Redirection 3xx .............................................61
       10.3.1   300 Multiple Choices .....................................61
       10.3.2   301 Moved Permanently ....................................62
       10.3.3   302 Found ................................................62
       10.3.4   303 See Other ............................................63
       10.3.5   304 Not Modified .........................................63
       10.3.6   305 Use Proxy ............................................64
       10.3.7   306 (Unused) .............................................64

       10.3.8   307 Temporary Redirect ...................................65
       10.4  Client Error 4xx ............................................65
       10.4.1    400 Bad Request .........................................65
       10.4.2    401 Unauthorized ........................................66
       10.4.3    402 Payment Required ....................................66
       10.4.4    403 Forbidden ...........................................66
       10.4.5    404 Not Found ...........................................66
       10.4.6    405 Method Not Allowed ..................................66
       10.4.7    406 Not Acceptable ......................................67
       10.4.8    407 Proxy Authentication Required .......................67
       10.4.9    408 Request Timeout .....................................67
       10.4.10   409 Conflict ............................................67
       10.4.11   410 Gone ................................................68
       10.4.12   411 Length Required .....................................68
       10.4.13   412 Precondition Failed .................................68
       10.4.14   413 Request Entity Too Large ............................69
       10.4.15   414 Request-URI Too Long ................................69
       10.4.16   415 Unsupported Media Type ..............................69
       10.4.17   416 Requested Range Not Satisfiable .....................69
       10.4.18   417 Expectation Failed ..................................70
       10.5  Server Error 5xx ............................................70
       10.5.1   500 Internal Server Error ................................70
       10.5.2   501 Not Implemented ......................................70
       10.5.3   502 Bad Gateway ..........................................70
       10.5.4   503 Service Unavailable ..................................70
       10.5.5   504 Gateway Timeout ......................................71
       10.5.6   505 HTTP Version Not Supported ...........................71
       11   Access Authentication ........................................71
       12   Content Negotiation ..........................................71
       12.1  Server-driven Negotiation ...................................72
       12.2  Agent-driven Negotiation ....................................73
       12.3  Transparent Negotiation .....................................74
       13   Caching in HTTP ..............................................74
       13.1.1   Cache Correctness ........................................75
       13.1.2   Warnings .................................................76
       13.1.3   Cache-control Mechanisms .................................77
       13.1.4   Explicit User Agent Warnings .............................78
       13.1.5   Exceptions to the Rules and Warnings .....................78
       13.1.6   Client-controlled Behavior ...............................79
       13.2  Expiration Model ............................................79
       13.2.1   Server-Specified Expiration ..............................79
       13.2.2   Heuristic Expiration .....................................80
       13.2.3   Age Calculations .........................................80
       13.2.4   Expiration Calculations ..................................83
       13.2.5   Disambiguating Expiration Values .........................84
       13.2.6   Disambiguating Multiple Responses ........................84
       13.3  Validation Model ............................................85
       13.3.1   Last-Modified Dates ......................................86

       13.3.2   Entity Tag Cache Validators ..............................86
       13.3.3   Weak and Strong Validators ...............................86
       13.3.4   Rules for When to Use Entity Tags and Last-Modified Dates.89
       13.3.5   Non-validating Conditionals ..............................90
       13.4  Response Cacheability .......................................91
       13.5  Constructing Responses From Caches ..........................92
       13.5.1   End-to-end and Hop-by-hop Headers ........................92
       13.5.2   Non-modifiable Headers ...................................92
       13.5.3   Combining Headers ........................................94
       13.5.4   Combining Byte Ranges ....................................95
       13.6  Caching Negotiated Responses ................................95
       13.7  Shared and Non-Shared Caches ................................96
       13.8  Errors or Incomplete Response Cache Behavior ................97
       13.9  Side Effects of GET and HEAD ................................97
       13.10   Invalidation After Updates or Deletions ...................97
       13.11   Write-Through Mandatory ...................................98
       13.12   Cache Replacement .........................................99
       13.13   History Lists .............................................99
       14   Header Field Definitions ....................................100
       14.1  Accept .....................................................100
       14.2  Accept-Charset .............................................102
       14.3  Accept-Encoding ............................................102
       14.4  Accept-Language ............................................104
       14.5  Accept-Ranges ..............................................105
       14.6  Age ........................................................106
       14.7  Allow ......................................................106
       14.8  Authorization ..............................................107
       14.9  Cache-Control ..............................................108
       14.9.1   What is Cacheable .......................................109
       14.9.2   What May be Stored by Caches ............................110
       14.9.3   Modifications of the Basic Expiration Mechanism .........111
       14.9.4   Cache Revalidation and Reload Controls ..................113
       14.9.5   No-Transform Directive ..................................115
       14.9.6   Cache Control Extensions ................................116
       14.10   Connection ...............................................117
       14.11   Content-Encoding .........................................118
       14.12   Content-Language .........................................118
       14.13   Content-Length ...........................................119
       14.14   Content-Location .........................................120
       14.15   Content-MD5 ..............................................121
       14.16   Content-Range ............................................122
       14.17   Content-Type .............................................124
       14.18   Date .....................................................124
       14.18.1   Clockless Origin Server Operation ......................125
       14.19   ETag .....................................................126
       14.20   Expect ...................................................126
       14.21   Expires ..................................................127
       14.22   From .....................................................128

       14.23   Host .....................................................128
       14.24   If-Match .................................................129
       14.25   If-Modified-Since ........................................130
       14.26   If-None-Match ............................................132
       14.27   If-Range .................................................133
       14.28   If-Unmodified-Since ......................................134
       14.29   Last-Modified ............................................134
       14.30   Location .................................................135
       14.31   Max-Forwards .............................................136
       14.32   Pragma ...................................................136
       14.33   Proxy-Authenticate .......................................137
       14.34   Proxy-Authorization ......................................137
       14.35   Range ....................................................138
       14.35.1    Byte Ranges ...........................................138
       14.35.2    Range Retrieval Requests ..............................139
       14.36   Referer ..................................................140
       14.37   Retry-After ..............................................141
       14.38   Server ...................................................141
       14.39   TE .......................................................142
       14.40   Trailer ..................................................143
       14.41  Transfer-Encoding..........................................143
       14.42   Upgrade ..................................................144
       14.43   User-Agent ...............................................145
       14.44   Vary .....................................................145
       14.45   Via ......................................................146
       14.46   Warning ..................................................148
       14.47   WWW-Authenticate .........................................150
       15 Security Considerations .......................................150
       15.1      Personal Information....................................151
       15.1.1   Abuse of Server Log Information .........................151
       15.1.2   Transfer of Sensitive Information .......................151
       15.1.3   Encoding Sensitive Information in URI's .................152
       15.1.4   Privacy Issues Connected to Accept Headers ..............152
       15.2  Attacks Based On File and Path Names .......................153
       15.3  DNS Spoofing ...............................................154
       15.4  Location Headers and Spoofing ..............................154
       15.5  Content-Disposition Issues .................................154
       15.6  Authentication Credentials and Idle Clients ................155
       15.7  Proxies and Caching ........................................155
       15.7.1    Denial of Service Attacks on Proxies....................156
       16   Acknowledgments .............................................156
       17   References ..................................................158
       18   Authors' Addresses ..........................................162
       19   Appendices ..................................................164
       19.1  Internet Media Type message/http and application/http ......164
       19.2  Internet Media Type multipart/byteranges ...................165
       19.3  Tolerant Applications ......................................166
       19.4  Differences Between HTTP Entities and RFC 2045 Entities ....167

       19.4.1   MIME-Version ............................................167
       19.4.2   Conversion to Canonical Form ............................167
       19.4.3   Conversion of Date Formats ..............................168
       19.4.4   Introduction of Content-Encoding ........................168
       19.4.5   No Content-Transfer-Encoding ............................168
       19.4.6   Introduction of Transfer-Encoding .......................169
       19.4.7   MHTML and Line Length Limitations .......................169
       19.5  Additional Features ........................................169
       19.5.1   Content-Disposition .....................................170
       19.6  Compatibility with Previous Versions .......................170
       19.6.1   Changes from HTTP/1.0 ...................................171
       19.6.2   Compatibility with HTTP/1.0 Persistent Connections ......172
       19.6.3   Changes from RFC 2068 ...................................172
       20   Index .......................................................175
       21   Full Copyright Statement ....................................176

    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, or 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 and headers that indicate the
       purpose of a request [47]. It builds on the discipline of reference
       provided by the Uniform Resource Identifier (URI) [3], as a location
       (URL) [4] or name (URN) [20], for indicating the resource to which a

       method is to be applied. Messages are passed in a format similar to
       that used by Internet mail [9] as defined by the Multipurpose
       Internet Mail Extensions (MIME) [7].

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

    1.2 Requirements

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in RFC 2119 [34].

       An implementation is not compliant if it fails to satisfy one or more
       of the MUST or REQUIRED level requirements for the protocols it
       implements. An implementation that satisfies all the MUST or REQUIRED
       level and all the SHOULD level requirements for its protocols is said
       to be "unconditionally compliant"; one that satisfies all the MUST
       level requirements but not all the SHOULD level 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, and
          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 `varriant'.  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. A
          "transparent proxy" is a proxy that does not modify the request or
          response beyond what is required for proxy authentication and
          identification. A "non-transparent proxy" is a proxy that modifies
          the request or response in order to provide some added service to
          the user agent, such as group annotation services, media type
          transformation, protocol reduction, or anonymity filtering. Except
          where either transparent or non-transparent behavior is explicitly
          stated, the HTTP proxy requirements apply to both types of
          proxies.

       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 cacheable 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.

       cacheable
          A response is cacheable if a cache is allowed to store a copy of
          the response message for use in answering subsequent requests. The
          rules for determining the cacheability of HTTP responses are
          defined in section 13. Even if a resource is cacheable, 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.

       upstream/downstream
          Upstream and downstream describe the flow of a message: all
          messages flow from upstream to downstream.

       inbound/outbound
          Inbound and outbound refer to the request and response paths for
          messages: "inbound" means "traveling toward the origin server",
          and "outbound" means "traveling toward the user agent"

    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 cacheable, and some requests may
       contain modifiers which place special requirements on cache behavior.
       HTTP requirements for cache behavior and cacheable responses are
       defined in section 13.

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

       HTTP communication usually takes place over TCP/IP connections. The
       default port is TCP 80 [19], but other ports can be used. This does
       not preclude HTTP from being implemented on top of any other protocol
       on the Internet, or on other networks. HTTP only presumes a reliable
       transport; any protocol that provides such guarantees can be used;
       the mapping of the HTTP/1.1 request and response structures onto the
       transport data units of the protocol in question is outside the scope
       of this specification.

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

    2 Notational Conventions and Generic Grammar

    2.1 Augmented BNF

       All of the mechanisms specified in this document are described in
       both prose and an augmented Backus-Naur Form (BNF) similar to that
       used by RFC 822 [9]. Implementors 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. White space 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 white space (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 white space (LWS) can be included
          between any two adjacent words (token or quoted-string), and
          between adjacent words and separators, without changing the
          interpretation of a field. At least one delimiter (LWS and/or

          separators) MUST exist between any two tokens (for the definition
          of "token" below), 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 header field values 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. A
       recipient MAY replace any linear white space with a single SP before
       interpreting the field value or forwarding the message downstream.

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

       The TEXT rule is only used for descriptive field contents and values
       that are not intended to be interpreted by the message parser. Words
       of *TEXT MAY contain characters from character sets other than ISO-
       8859-1 [22] only when encoded according to the rules of RFC 2047
       [14].

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

       A CRLF is allowed in the definition of TEXT only as part of a header
       field continuation. It is expected that the folding LWS will be
       replaced with a single SP before interpretation of the TEXT value.

       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 (as defined in section
       3.6).

           token          = 1*<any CHAR except CTLs or separators>
           separators     = "(" | ")" | "<" | ">" | "@"
                          | "," | ";" | ":" | "\" | <">
                          | "/" | "[" | "]" | "?" | "="
                          | "{" | "}" | SP | HT

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

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

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

           quoted-string  = ( <"> *(qdtext | quoted-pair ) <"> )
           qdtext         = <any TEXT except <">>

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

           quoted-pair    = "\" CHAR

    3 Protocol Parameters

    3.1 HTTP Version

       HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
       of the protocol. The protocol versioning policy is intended to allow
       the sender to indicate the format of a message and its capacity for
       understanding further HTTP communication, rather than the features
       obtained via that communication. No change is made to the version
       number for the addition of message components which do not affect
       communication behavior or which only add to extensible field values.
       The <minor> number is incremented when the changes made to the
       protocol add features which do not change the general message parsing
       algorithm, but which may add to the message semantics and imply
       additional capabilities of the sender. The <major> number is
       incremented when the format of a message within the protocol is
       changed. See RFC 2145 [36] for a fuller explanation.

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

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

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

       An application that sends a request or response message that includes
       HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant
       with this specification. Applications that are at least conditionally
       compliant with this specification SHOULD use an HTTP-Version of
       "HTTP/1.1" in their messages, and MUST do so for any message that is
       not compatible with HTTP/1.0. For more details on when to send
       specific HTTP-Version values, see RFC 2145 [36].

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

       Proxy and gateway applications need to 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 NOT send a message with a version
       indicator which is greater than its actual version. If a higher
       version request is received, the proxy/gateway MUST either downgrade
       the request version, or respond with an error, or switch to tunnel
       behavior.

       Due to interoperability problems with HTTP/1.0 proxies discovered
       since the publication of RFC 2068[33], caching proxies MUST, gateways
       MAY, and tunnels MUST NOT upgrade the request to the highest version
       they support. The proxy/gateway's response to that request MUST be in
       the same major version as the request.

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

    3.2 Uniform Resource Identifiers

       URIs have been known by many names: WWW addresses, Universal Document
       Identifiers, Universal Resource Identifiers [3], and finally the
       combination of Uniform Resource Locators (URL) [4] and Names (URN)
       [20]. As far as HTTP is concerned, Uniform Resource Identifiers are
       simply formatted strings which identify--via name, location, or any
       other characteristic--a resource.

    3.2.1 General Syntax

       URIs in HTTP can be represented in absolute form or relative to some
       known base URI [11], depending upon the context of their use. The two
       forms are differentiated by the fact that absolute URIs always begin
       with a scheme name followed by a colon. For definitive information on
       URL syntax and semantics, see "Uniform Resource Identifiers (URI):
       Generic Syntax and Semantics," RFC 2396 [42] (which replaces RFCs
       1738 [4] and RFC 1808 [11]). This specification adopts the
       definitions of "URI-reference", "absoluteURI", "relativeURI", "port",
       "host","abs_path", "rel_path", and "authority" from that
       specification.

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

          Note: Servers ought to be cautious about depending on URI lengths
          above 255 bytes, because some older client or proxy
          implementations might 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 [ "?" query ]]

       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 (section 5.1.2). The use of IP addresses
       in URLs 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). If a proxy
       receives a host name which is not a fully qualified domain name, it
       MAY add its domain to the host name it received. If a proxy receives
       a fully qualified domain name, the proxy MUST NOT change the host
       name.

    3.2.3 URI Comparison

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

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

            - Comparisons of host names MUST be case-insensitive;

            - Comparisons of scheme names MUST be case-insensitive;

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

       Characters other than those in the "reserved" and "unsafe" sets (see
       RFC 2396 [42]) are equivalent to their ""%" HEX HEX" encoding.

       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 [8] (an update to
       RFC 822 [9]). The second format is in common use, but is based on the
       obsolete RFC 850 [12] date format and lacks a four-digit year.
       HTTP/1.1 clients and servers that parse the date value MUST accept
       all three formats (for compatibility with HTTP/1.0), though they MUST
       only generate the RFC 1123 format for representing HTTP-date values
       in header fields. See section 19.3 for further information.

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

       All HTTP date/time stamps MUST be represented in Greenwich Mean Time
       (GMT), without exception. For the purposes of HTTP, GMT is exactly
       equal to UTC (Coordinated Universal Time). This is indicated in the
       first two formats by the inclusion of "GMT" as the three-letter
       abbreviation for time zone, and MUST be assumed when reading the
       asctime format. HTTP-date is case sensitive and MUST NOT include
       additional LWS beyond that specifically included as SP in the
       grammar.

           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
       encoding, from simple single-table mappings such as US-ASCII to
       complex table switching methods such as those that use ISO-2022's
       techniques. However, the definition associated with a MIME character
       set name MUST fully specify the mapping to be performed from octets
       to characters. In particular, use of external profiling information
       to determine the exact mapping is not permitted.

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

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

           charset = token

       Although HTTP allows an arbitrary token to be used as a charset
       value, any token that has a predefined value within the IANA
       Character Set registry [19] MUST represent the character set defined
       by that registry. Applications SHOULD limit their use of character
       sets to those defined by the IANA registry.

       Implementors should be aware of IETF character set requirements [38]
       [41].

    3.4.1 Missing Charset

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

       Unfortunately, some older HTTP/1.0 clients did not deal properly with
       an explicit charset parameter. HTTP/1.1 recipients MUST respect the
       charset label provided by the sender; and those user agents that have
       a provision to "guess" a charset MUST use the charset from the

       content-type field if they support that charset, rather than the
       recipient's preference, when initially displaying a document. See
       section 3.7.1.

    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 14.11) 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).

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

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

       identity
            The default (identity) encoding; the use of no transformation
            whatsoever. This content-coding is used only in the Accept-
            Encoding header, and SHOULD NOT be used in the Content-Encoding
            header.

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

    3.6 Transfer Codings

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

           transfer-coding         = "chunked" | transfer-extension
           transfer-extension      = token *( ";" parameter )

       Parameters are in  the form of attribute/value pairs.

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

       All transfer-coding values are case-insensitive. HTTP/1.1 uses
       transfer-coding values in the TE header field (section 14.39) and in
       the Transfer-Encoding header field (section 14.41).

       Whenever a transfer-coding is applied to a message-body, the set of
       transfer-codings MUST include "chunked", unless the message is
       terminated by closing the connection. When the "chunked" transfer-
       coding is used, it MUST be the last transfer-coding applied to the
       message-body. The "chunked" transfer-coding MUST NOT be applied more
       than once to a message-body. These rules allow the recipient to
       determine the transfer-length of the message (section 4.4).

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

       The Internet Assigned Numbers Authority (IANA) acts as a registry for
       transfer-coding value tokens. Initially, the registry contains the
       following tokens: "chunked" (section 3.6.1), "identity" (section
       3.6.2), "gzip" (section 3.5), "compress" (section 3.5), and "deflate"
       (section 3.5).

       New transfer-coding value tokens SHOULD be registered in the same way
       as new content-coding value tokens (section 3.5).

       A server which receives an entity-body with a transfer-coding it does
       not understand SHOULD return 501 (Unimplemented), and close the
       connection. A server MUST NOT send transfer-codings to an HTTP/1.0
       client.

    3.6.1 Chunked Transfer Coding

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

           Chunked-Body   = *chunk
                            last-chunk
                            trailer
                            CRLF

           chunk          = chunk-size [ chunk-extension ] CRLF
                            chunk-data CRLF
           chunk-size     = 1*HEX
           last-chunk     = 1*("0") [ chunk-extension ] CRLF

           chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
           chunk-ext-name = token
           chunk-ext-val  = token | quoted-string
           chunk-data     = chunk-size(OCTET)
           trailer        = *(entity-header CRLF)

       The chunk-size field is a string of hex digits indicating the size of
       the chunk. The chunked encoding is ended by any chunk whose size is
       zero, followed by the trailer, which is terminated by an empty line.

       The trailer allows the sender to include additional HTTP header
       fields at the end of the message. The Trailer header field can be
       used to indicate which header fields are included in a trailer (see
       section 14.40).

       A server using chunked transfer-coding in a response MUST NOT use the
       trailer for any header fields unless at least one of the following is
       true:

       a)the request included a TE header field that indicates "trailers" is
         acceptable in the transfer-coding of the  response, as described in
         section 14.39; or,

       b)the server is the origin server for the response, the trailer
         fields consist entirely of optional metadata, and the recipient
         could use the message (in a manner acceptable to the origin server)
         without receiving this metadata.  In other words, the origin server
         is willing to accept the possibility that the trailer fields might
         be silently discarded along the path to the client.

       This requirement prevents an interoperability failure when the
       message is being received by an HTTP/1.1 (or later) proxy and
       forwarded to an HTTP/1.0 recipient. It avoids a situation where
       compliance with the protocol would have necessitated a possibly
       infinite buffer on the proxy.

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

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

    3.7 Media Types

       HTTP uses Internet Media Types [17] in the Content-Type (section
       14.17) 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 (as defined in section 3.6).

       The type, subtype, and parameter attribute names are case-
       insensitive. Parameter values might or might 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. The presence or absence of a parameter might
       be significant to the processing of a media-type, depending on its
       definition within the media type registry.

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

       Media-type values are registered with the Internet Assigned Number
       Authority (IANA [19]). The media type registration process is
       outlined in RFC 1590 [17]. Use of non-registered media types is
       discouraged.

    3.7.1 Canonicalization and Text Defaults

       Internet media types are registered with a canonical form. An
       entity-body transferred via HTTP messages MUST be represented in the
       appropriate canonical form prior to its transmission except for
       "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-coding, the underlying
       data MUST be in a form defined above prior to being encoded.

       The "charset" parameter is used with some media types to define the
       character set (section 3.4) of the data. When no explicit charset
       parameter is provided by the sender, media subtypes of the "text"
       type are defined to have a default charset value of "ISO-8859-1" when
       received via HTTP. Data in character sets other than "ISO-8859-1" or
       its subsets MUST be labeled with an appropriate charset value. See
       section 3.4.1 for compatibility problems.

    3.7.2 Multipart Types

       MIME provides for a number of "multipart" types -- encapsulations of
       one or more entities within a single message-body. All multipart
       types share a common syntax, as defined in section 5.1.1 of RFC 2046

       [40], and MUST include a boundary parameter as part of the media type
       value. The message body is itself a protocol element and MUST
       therefore use only CRLF to represent line breaks between body-parts.
       Unlike in RFC 2046, the epilogue of any multipart message MUST be
       empty; HTTP applications MUST NOT transmit the epilogue (even if the
       original multipart contains an epilogue). These restrictions exist in
       order to preserve the self-delimiting nature of a multipart message-
       body, wherein the "end" of the message-body is indicated by the
       ending multipart boundary.

       In general, HTTP treats a multipart message-body no differently than
       any other media type: strictly as payload. The one exception is the
       "multipart/byteranges" type (appendix 19.2) when it appears in a 206
       (Partial Content) response, which will be interpreted by some HTTP
       caching mechanisms as described in sections 13.5.4 and 14.16. In all
       other cases, an HTTP user agent SHOULD follow the same or similar
       behavior as a MIME user agent would upon receipt of a multipart type.
       The MIME header fields within each body-part of a multipart message-
       body do not have any significance to HTTP beyond that defined by
       their MIME semantics.

       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 white space. 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. They MUST NOT be
       used for advertising or other non-essential information. Although any
       token character MAY appear in a product-version, this token SHOULD
       only be used for a version identifier (i.e., successive versions of
       the same product SHOULD only differ in the product-version portion of
       the product value).

    3.9 Quality Values

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

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

       "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

       White space 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
       14.19), If-Match (section 14.24), 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. The use
       of the same entity tag value in conjunction with entities obtained by
       requests on different URIs does not imply 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 14.35) and Content-Range (section 14.16)
       header fields. An entity can be broken down into subranges according
       to various structural units.

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

       The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1
       implementations MAY ignore ranges specified using other units.

       HTTP/1.1 has been designed to allow implementations of applications
       that do not depend on knowledge of ranges.

    4 HTTP Message

    4.1 Message Types

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

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

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

            generic-message = start-line
                              *(message-header CRLF)
                              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.

       Certain buggy HTTP/1.0 client implementations generate 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 ought to follow "common form", where
       one is known or indicated, 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 ]
           field-name     = token
           field-value    = *( field-content | LWS )
           field-content  = <the OCTETs making up the field-value
                            and consisting of either *TEXT or combinations
                            of token, separators, and quoted-string>

       The field-content does not include any leading or trailing LWS:
       linear white space occurring before the first non-whitespace
       character of the field-value or after the last non-whitespace
       character of the field-value. Such leading or trailing LWS MAY be
       removed without changing the semantics of the field value. Any LWS
       that occurs between field-content MAY be replaced with a single SP
       before interpreting the field value or forwarding the message
       downstream.

       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
       14.41).

           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 MAY be added or removed by any application along the
       request/response chain. (However, section 3.6 places restrictions on
       when certain transfer-codings may be used.)

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

       The presence of a message-body in a request is signaled by the
       inclusion of a Content-Length or Transfer-Encoding header field in
       the request's message-headers. A message-body MUST NOT be included in
       a request if the specification of the request method (section 5.1.1)
       does not allow sending an entity-body in requests. A server SHOULD
       read and forward a message-body on any request; if the request method
       does not include defined semantics for an entity-body, then the
       message-body SHOULD be ignored when handling the request.

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

    4.4 Message Length

       The transfer-length of a message is the length of the message-body as
       it appears in the message; that is, after any transfer-codings have
       been applied. When a message-body is included with a message, the
       transfer-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 14.41) is present and
         has any value other than "identity", then the transfer-length is
         defined by use of the "chunked" transfer-coding (section 3.6),
         unless the message is terminated by closing the connection.

       3.If a Content-Length header field (section 14.13) is present, its
         decimal value in OCTETs represents both the entity-length and the
         transfer-length. The Content-Length header field MUST NOT be sent
         if these two lengths are different (i.e., if a Transfer-Encoding

         header field is present). If a message is received with both a
         Transfer-Encoding header field and a Content-Length header field,
         the latter MUST be ignored.

       4.If the message uses the media type "multipart/byteranges", and the
         ransfer-length is not otherwise specified, then this self-
         elimiting media type defines the transfer-length. This media type
         UST NOT be used unless the sender knows that the recipient can arse
         it; the presence in a request of a Range header with ultiple byte-
         range specifiers from a 1.1 client implies that the lient can parse
         multipart/byteranges responses.

           A range header might be forwarded by a 1.0 proxy that does not
           understand multipart/byteranges; in this case the server MUST
           delimit the message using methods defined in items 1,3 or 5 of
           this section.

       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 a
       non-identity transfer-coding. If the message does include a non-
       identity transfer-coding, 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 14.18
                          | Pragma                   ; Section 14.32
                          | Trailer                  ; Section 14.40
                          | Transfer-Encoding        ; Section 14.41
                          | Upgrade                  ; Section 14.42
                          | Via                      ; Section 14.45
                          | Warning                  ; Section 14.46

       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 ) CRLF)  ; Section 7.1
                            CRLF
                            [ message-body ]          ; Section 4.3

    5.1 Request-Line

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

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

    5.1.1 Method

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

           Method         = "OPTIONS"                ; Section 9.2
                          | "GET"                    ; Section 9.3
                          | "HEAD"                   ; Section 9.4
                          | "POST"                   ; Section 9.5
                          | "PUT"                    ; Section 9.6
                          | "DELETE"                 ; Section 9.7
                          | "TRACE"                  ; Section 9.8
                          | "CONNECT"                ; Section 9.9
                          | extension-method
           extension-method = token

       The list of methods allowed by a resource can be specified in an
       Allow header field (section 14.7). The return code of the response
       always notifies the client whether a method is currently allowed on a
       resource, since the set of allowed methods can change dynamically. An
       origin server SHOULD return the status code 405 (Method Not Allowed)
       if the method is known by the origin server but not allowed for the
       requested resource, and 501 (Not Implemented) if the method is
       unrecognized or not implemented by the origin server. The methods GET
       and HEAD MUST be supported by all general-purpose servers. All other
       methods are OPTIONAL; however, if the above methods are implemented,
       they MUST be implemented with the same semantics as those specified
       in section 9.

    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 | authority

       The four 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 authority form is only used by the CONNECT method (section 9.9).

       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 (authority) MUST
       be transmitted in a Host header field. For example, a client wishing
       to retrieve the resource above directly from the origin server would
       create a TCP connection to port 80 of the host "www.w3.org" and send
       the lines:

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

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

       The Request-URI is transmitted in the format specified in section
       3.2.1. If the Request-URI is encoded using the "% HEX HEX" encoding
       [42], 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.

       A transparent proxy MUST NOT rewrite the "abs_path" part of the
       received Request-URI when forwarding it to the next inbound server,
       except as noted above to replace a null abs_path with "/".

          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 URI character for a reserved purpose.  Implementors
          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

       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 when
       determining the resource identified by an HTTP/1.1 request. (But see
       section 19.6.1.1 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 host
       names) 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
                          | Expect                   ; Section 14.20
                          | From                     ; Section 14.22
                          | Host                     ; Section 14.23
                          | If-Match                 ; Section 14.24

                          | If-Modified-Since        ; 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 14.35
                          | Referer                  ; Section 14.36
                          | TE                       ; Section 14.39
                          | User-Agent               ; Section 14.43

       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 ) CRLF)  ; 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:

          - 1xx: Informational - Request received, continuing process

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

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

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

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

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

          Status-Code    =
                "100"  ; Section 10.1.1: Continue
              | "101"  ; Section 10.1.2: Switching Protocols
              | "200"  ; Section 10.2.1: OK
              | "201"  ; Section 10.2.2: Created
              | "202"  ; Section 10.2.3: Accepted
              | "203"  ; Section 10.2.4: Non-Authoritative Information
              | "204"  ; Section 10.2.5: No Content
              | "205"  ; Section 10.2.6: Reset Content
              | "206"  ; Section 10.2.7: Partial Content
              | "300"  ; Section 10.3.1: Multiple Choices
              | "301"  ; Section 10.3.2: Moved Permanently
              | "302"  ; Section 10.3.3: Found
              | "303"  ; Section 10.3.4: See Other
              | "304"  ; Section 10.3.5: Not Modified
              | "305"  ; Section 10.3.6: Use Proxy
              | "307"  ; Section 10.3.8: Temporary Redirect
              | "400"  ; Section 10.4.1: Bad Request
              | "401"  ; Section 10.4.2: Unauthorized
              | "402"  ; Section 10.4.3: Payment Required
              | "403"  ; Section 10.4.4: Forbidden
              | "404"  ; Section 10.4.5: Not Found
              | "405"  ; Section 10.4.6: Method Not Allowed
              | "406"  ; Section 10.4.7: Not Acceptable

              | "407"  ; Section 10.4.8: Proxy Authentication Required
              | "408"  ; Section 10.4.9: Request Time-out
              | "409"  ; Section 10.4.10: Conflict
              | "410"  ; Section 10.4.11: Gone
              | "411"  ; Section 10.4.12: Length Required
              | "412"  ; Section 10.4.13: Precondition Failed
              | "413"  ; Section 10.4.14: Request Entity Too Large
              | "414"  ; Section 10.4.15: Request-URI Too Large
              | "415"  ; Section 10.4.16: Unsupported Media Type
              | "416"  ; Section 10.4.17: Requested range not satisfiable
              | "417"  ; Section 10.4.18: Expectation Failed
              | "500"  ; Section 10.5.1: Internal Server Error
              | "501"  ; Section 10.5.2: Not Implemented
              | "502"  ; Section 10.5.3: Bad Gateway
              | "503"  ; Section 10.5.4: Service Unavailable
              | "504"  ; Section 10.5.5: Gateway Time-out
              | "505"  ; Section 10.5.6: HTTP Version not supported
              | extension-code

          extension-code = 3DIGIT
          Reason-Phrase  = *<TEXT, excluding CR, LF>

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

    6.2 Response Header Fields

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

           response-header = Accept-Ranges           ; Section 14.5
                           | Age                     ; Section 14.6
                           | ETag                    ; Section 14.19
                           | Location                ; Section 14.30
                           | Proxy-Authenticate      ; Section 14.33

                           | Retry-After             ; Section 14.37
                           | Server                  ; Section 14.38
                           | Vary                    ; Section 14.44
                           | WWW-Authenticate        ; Section 14.47

       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 metainformation about the entity-body or,
       if no body is present, about the resource identified by the request.
       Some of this metainformation is OPTIONAL; some might be REQUIRED by
       portions of this specification.

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

           extension-header = message-header

       The extension-header mechanism allows additional entity-header fields
       to be defined without changing the protocol, but these fields cannot
       be assumed to be recognizable by the recipient. Unrecognized header
       fields SHOULD be ignored by the recipient and MUST be forwarded by
       transparent 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 might
       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 URI used to identify the
       resource. If the media type remains unknown, the recipient SHOULD
       treat it as type "application/octet-stream".

    7.2.2 Entity Length

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

    8 Connections

    8.1 Persistent Connections

    8.1.1 Purpose

       Prior to persistent connections, a separate TCP connection was
       established to fetch each URL, increasing the load on HTTP servers
       and causing congestion on the Internet. The use of inline images and
       other associated data often require a client to make multiple
       requests of the same server in a short amount of time. Analysis of
       these performance problems and results from a prototype
       implementation are available [26] [30]. Implementation experience and
       measurements of actual HTTP/1.1 (RFC 2068) implementations show good
       results [39]. Alternatives have also been explored, for example,
       T/TCP [27].

       Persistent HTTP connections have a number of advantages:

          - By opening and closing fewer TCP connections, CPU time is saved
            in routers and hosts (clients, servers, proxies, gateways,
            tunnels, or caches), and memory used for TCP protocol control
            blocks can be saved in hosts.

          - HTTP requests and responses can be pipelined on a connection.
            Pipelining allows a client to make multiple requests without
            waiting for each response, allowing a single TCP connection to
            be used much more efficiently, with much lower elapsed time.

          - Network congestion is reduced by reducing the number of packets
            caused by TCP opens, and by allowing TCP sufficient time to
            determine the congestion state of the network.

          - Latency on subsequent requests is reduced since there is no time
            spent in TCP's connection opening handshake.

          - 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
       SHOULD assume that the server will maintain a persistent connection,
       even after error responses from the server.

       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 (section 14.10). 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 19.6.2 for more information on backward
       compatibility with HTTP/1.0 clients.

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

    8.1.2.2 Pipelining

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

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

       Clients SHOULD NOT pipeline requests using non-idempotent methods or
       non-idempotent sequences of methods (see section 9.1.2). Otherwise, a
       premature termination of the transport connection could lead to
       indeterminate results. A client wishing to send a non-idempotent
       request SHOULD wait to send that request until it has received the
       response status for the previous request.

    8.1.3 Proxy Servers

       It is especially important that proxies correctly implement the
       properties of the Connection header field as specified in section
       14.10.

       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 HTTP/1.1 persistent connection
       with an HTTP/1.0 client (but see RFC 2068 [33] for information and
       discussion of the problems with the Keep-Alive header implemented by
       many HTTP/1.0 clients).

    8.1.4 Practical Considerations

       Servers will usually have some time-out value beyond which they will
       no longer maintain an inactive connection. Proxy servers might make
       this a higher value since it is likely that the client will be making
       more connections through the same server. The use of persistent
       connections places no requirements on the length (or existence) 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 might have started to send a new request
       at the same time that the server has decided to close the "idle"
       connection. From the server's point of view, the connection is being
       closed while it was idle, but from the client's point of view, a
       request is in progress.

       This means that clients, servers, and proxies MUST be able to recover
       from asynchronous close events. Client software SHOULD reopen the
       transport connection and retransmit the aborted sequence of requests
       without user interaction so long as the request sequence is
       idempotent (see section 9.1.2). Non-idempotent methods or sequences
       MUST NOT be automatically retried, although user agents MAY offer a
       human operator the choice of retrying the request(s). Confirmation by
       user-agent software with semantic understanding of the application
       MAY substitute for user confirmation. The automatic retry SHOULD NOT
       be repeated if the second sequence of requests 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 NOT maintain more than 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.

    8.2 Message Transmission Requirements

    8.2.1 Persistent Connections and Flow Control

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

    8.2.2 Monitoring Connections for Error Status Messages

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

    8.2.3 Use of the 100 (Continue) Status

       The purpose of the 100 (Continue) status (see section 10.1.1) is to
       allow a client that is sending a request message with a request body
       to determine if the origin server is willing to accept the request
       (based on the request headers) before the client sends the request
       body. In some cases, it might either be inappropriate or highly
       inefficient for the client to send the body if the server will reject
       the message without looking at the body.

       Requirements for HTTP/1.1 clients:

          - If a client will wait for a 100 (Continue) response before
            sending the request body, it MUST send an Expect request-header
            field (section 14.20) with the "100-continue" expectation.

          - A client MUST NOT send an Expect request-header field (section
            14.20) with the "100-continue" expectation if it does not intend
            to send a request body.

       Because of the presence of older implementations, the protocol allows
       ambiguous situations in which a client may send "Expect: 100-
       continue" without receiving either a 417 (Expectation Failed) status
       or a 100 (Continue) status. Therefore, when a client sends this
       header field to an origin server (possibly via a proxy) from which it
       has never seen a 100 (Continue) status, the client SHOULD NOT wait
       for an indefinite period before sending the request body.

       Requirements for HTTP/1.1 origin servers:

          - Upon receiving a request which includes an Expect request-header
            field with the "100-continue" expectation, an origin server MUST
            either respond with 100 (Continue) status and continue to read
            from the input stream, or respond with a final status code. The
            origin server MUST NOT wait for the request body before sending
            the 100 (Continue) response. If it responds with a final status
            code, it MAY close the transport connection or it MAY continue

            to read and discard the rest of the request.  It MUST NOT
            perform the requested method if it returns a final status code.

          - An origin server SHOULD NOT send a 100 (Continue) response if
            the request message does not include an Expect request-header
            field with the "100-continue" expectation, and MUST NOT send a
            100 (Continue) response if such a request comes from an HTTP/1.0
            (or earlier) client. There is an exception to this rule: for
            compatibility with RFC 2068, a server MAY send a 100 (Continue)
            status in response to an HTTP/1.1 PUT or POST request that does
            not include an Expect request-header field with the "100-
            continue" expectation. This exception, the purpose of which is
            to minimize any client processing delays associated with an
            undeclared wait for 100 (Continue) status, applies only to
            HTTP/1.1 requests, and not to requests with any other HTTP-
            version value.

          - An origin server MAY omit a 100 (Continue) response if it has
            already received some or all of the request body for the
            corresponding request.

          - An origin server that sends a 100 (Continue) response MUST
            ultimately send a final status code, once the request body is
            received and processed, unless it terminates the transport
            connection prematurely.

          - If an origin server receives a request that does not include an
            Expect request-header field with the "100-continue" expectation,
            the request includes a request body, and the server responds
            with a final status code before reading the entire request body
            from the transport connection, then the server SHOULD NOT close
            the transport connection until it has read the entire request,
            or until the client closes the connection. Otherwise, the client
            might not reliably receive the response message. However, this
            requirement is not be construed as preventing a server from
            defending itself against denial-of-service attacks, or from
            badly broken client implementations.

       Requirements for HTTP/1.1 proxies:

          - If a proxy receives a request that includes an Expect request-
            header field with the "100-continue" expectation, and the proxy
            either knows that the next-hop server complies with HTTP/1.1 or
            higher, or does not know the HTTP version of the next-hop
            server, it MUST forward the request, including the Expect header
            field.

          - If the proxy knows that the version of the next-hop server is
            HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
            respond with a 417 (Expectation Failed) status.

          - Proxies SHOULD maintain a cache recording the HTTP version
            numbers received from recently-referenced next-hop servers.

          - A proxy MUST NOT forward a 100 (Continue) response if the
            request message was received from an HTTP/1.0 (or earlier)
            client and did not include an Expect request-header field with
            the "100-continue" expectation. This requirement overrides the
            general rule for forwarding of 1xx responses (see section 10.1).

    8.2.4 Client Behavior if Server Prematurely Closes Connection

       If an HTTP/1.1 client sends a request which includes a request body,
       but which does not include an Expect request-header field with the
       "100-continue" expectation, and if the client is not directly
       connected to an HTTP/1.1 origin server, and if the client sees the
       connection close before receiving any status from the server, the
       client SHOULD retry the request.  If the client does retry this
       request, it MAY use the following "binary exponential backoff"
       algorithm to be assured of obtaining a reliable response:

          1. Initiate a new connection to the server

          2. Transmit the request-headers

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

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

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

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

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

       If at any point an error status is received, the client

          - SHOULD NOT continue and

          - SHOULD close the connection if it has not completed sending the
            request message.

    9 Method Definitions

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

       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

       Implementors 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 might 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 NOT have the significance of taking an action
       other than retrieval. These methods ought to 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 can also have the property of "idempotence" in that (aside
       from error or expiration issues) the side-effects of N > 0 identical
       requests is the same as for a single request. The methods GET, HEAD,
       PUT and DELETE share this property. Also, the methods OPTIONS and
       TRACE SHOULD NOT have side effects, and so are inherently idempotent.

       However, it is possible that a sequence of several requests is non-
       idempotent, even if all of the methods executed in that sequence are
       idempotent. (A sequence is idempotent if a single execution of the
       entire sequence always yields a result that is not changed by a
       reexecution of all, or part, of that sequence.) For example, a
       sequence is non-idempotent if its result depends on a value that is
       later modified in the same sequence.

       A sequence that never has side effects is idempotent, by definition
       (provided that no concurrent operations are being executed on the
       same set of resources).

    9.2 OPTIONS

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

       Responses to this method are not cacheable.

       If the OPTIONS request includes an entity-body (as indicated by the
       presence of Content-Length or Transfer-Encoding), then the media type
       MUST be indicated by a Content-Type field. Although this
       specification does not define any use for such a body, future
       extensions to HTTP might use the OPTIONS body to make more detailed
       queries on the server. A server that does not support such an
       extension MAY discard the request body.

       If the Request-URI is an asterisk ("*"), the OPTIONS request is
       intended to apply to the server in general rather than to a specific
       resource. Since a server's communication options typically depend on
       the resource, the "*" request is only useful as a "ping" or "no-op"
       type of method; it does nothing beyond allowing the client to test
       the capabilities of the server. For example, this can be used to test
       a proxy for HTTP/1.1 compliance (or lack thereof).

       If the Request-URI is not an asterisk, the OPTIONS request applies
       only to the options that are available when communicating with that
       resource.

       A 200 response SHOULD include any header fields that indicate
       optional features implemented by the server and applicable to that
       resource (e.g., Allow), possibly including extensions not defined by
       this specification. The response body, if any, SHOULD also include
       information about the communication options. The format for such a

       body is not defined by this specification, but might be defined by
       future extensions to HTTP. Content negotiation MAY be used to select
       the appropriate response format. If no response body is included, the
       response MUST include a Content-Length field with a field-value of
       "0".

       The Max-Forwards request-header field MAY be used to target a
       specific proxy in the request chain. When a proxy receives an OPTIONS
       request on an absoluteURI for which request forwarding is permitted,
       the proxy MUST check for a Max-Forwards field. If the Max-Forwards
       field-value is zero ("0"), the proxy MUST NOT forward the message;
       instead, the proxy SHOULD respond with its own communication options.
       If the Max-Forwards field-value is an integer greater than zero, the
       proxy MUST decrement the field-value when it forwards the request. If
       no Max-Forwards field is present in the request, then the forwarded
       request MUST NOT include a Max-Forwards field.

    9.3 GET

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

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

       The semantics of the GET method change to a "partial GET" if the
       request message includes a Range header field. A partial GET requests
       that only part of the entity be transferred, as described in section
       14.35. 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 cacheable if and only if it meets
       the requirements for HTTP caching described in section 13.

       See section 15.1.3 for security considerations when used for forms.

    9.4 HEAD

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

       The response to a HEAD request MAY be cacheable 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 origin server accept the
       entity enclosed in the request as a new subordinate of the resource
       identified by the Request-URI in the Request-Line. POST is designed
       to allow a uniform method to cover the following functions:

          - Annotation of existing resources;

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

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

          - Extending a database through an append operation.

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

       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 cacheable, 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 cacheable resource.

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

       See section 15.1.3 for security considerations.

    9.6 PUT

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

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

       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 might 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 might 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 might 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.

       Unless otherwise specified for a particular entity-header, the
       entity-headers in the PUT request SHOULD be applied to the resource
       created or modified by the PUT.

    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 action has been enacted
       but the response 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 cacheable.

    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 14.45) 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 the request is valid, 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.

    9.9 CONNECT

       This specification reserves the method name CONNECT for use with a
       proxy that can dynamically switch to being a tunnel (e.g. SSL
       tunneling [44]).

    10 Status Code Definitions

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

    10.1 Informational 1xx

       This class of status code indicates a provisional response,
       consisting only of the Status-Line and optional headers, and is
       terminated by an empty line. There are no required headers for this
       class of status code. Since HTTP/1.0 did not define any 1xx status
       codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client
       except under experimental conditions.

       A client MUST be prepared to accept one or more 1xx status responses
       prior to a regular response, even if the client does not expect a 100
       (Continue) status message. Unexpected 1xx status responses MAY be
       ignored by a user agent.

       Proxies MUST forward 1xx responses, unless the connection between the
       proxy and its client has been closed, or unless the proxy itself
       requested the generation of the 1xx response. (For example, if a

       proxy adds a "Expect: 100-continue" field when it forwards a request,
       then it need not forward the corresponding 100 (Continue)
       response(s).)

    10.1.1 100 Continue

       The client SHOULD continue with its request. This interim response is
       used to inform the client that the initial part of the request has
       been received and has not yet been rejected by the server. The client
       SHOULD continue by sending the remainder of the request or, if the
       request has already been completed, ignore this response. The server
       MUST send a final response after the request has been completed. See
       section 8.2.3 for detailed discussion of the use and handling of this
       status code.

    10.1.2 101 Switching Protocols

       The server understands and is willing to comply with the client's
       request, via the Upgrade message header field (section 14.42), 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 be switched only 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 might 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 URI
       for the resource given by a Location header field. 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. 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.

       A 201 response MAY contain an ETag response header field indicating
       the current value of the entity tag for the requested variant just
       created, see section 14.19.

    10.2.3 202 Accepted

       The request has been accepted for processing, but the processing has
       not been completed.  The request might or might not eventually be
       acted upon, as it might 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 might 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 does not need to return an
       entity-body, and might want to return updated metainformation. The
       response MAY include new or updated metainformation in the form of
       entity-headers, which if present SHOULD be associated with the
       requested variant.

       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, although
       any new or updated metainformation SHOULD be applied 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 14.35)
       indicating the desired range, and MAY have included an If-Range
       header field (section 14.27) to make the request conditional.

       The response MUST include the following header fields:

          - Either a Content-Range header field (section 14.16) indicating
            the range included with this response, or a multipart/byteranges
            Content-Type including Content-Range fields for each part. If a
            Content-Length header field is present in the response, its
            value MUST match the actual number of OCTETs transmitted in the
            message-body.

          - Date

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

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

       If the 206 response is the result of an If-Range request that used a
       strong cache validator (see section 13.3.3), the response SHOULD NOT
       include other entity-headers. If the response is the result of an
       If-Range request that used a weak validator, the response MUST NOT
       include other entity-headers; this prevents inconsistencies between
       cached entity-bodies and updated headers. Otherwise, the response
       MUST include all of the entity-headers that would have been returned
       with a 200 (OK) response to the same request.

       A cache MUST NOT combine a 206 response with other previously cached
       content if the ETag or Last-Modified headers do not match exactly,
       see 13.5.4.

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

    10.3 Redirection 3xx

       This class of status code indicates that further action needs to be
       taken by the user agent in order to fulfill the request.  The action
       required MAY be carried out by the user agent without interaction
       with the user if and only if the method used in the second request is
       GET or HEAD. A client SHOULD detect infinite redirection loops, since
       such loops generate network traffic for each redirection.

          Note: previous versions of this specification recommended a
          maximum of five redirections. Content developers should be aware
          that there might be clients that implement such a fixed
          limitation.

    10.3.1 300 Multiple Choices

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

       Unless it was a HEAD request, the response SHOULD include an entity
       containing a list of resource characteristics and location(s) from
       which the user or user agent can choose the one most appropriate. The
       entity format is specified by the media type given in the Content-
       Type header field. Depending upon the format and the capabilities of

       the user agent, selection of the most appropriate choice MAY be
       performed automatically. However, this specification does not define
       any standard for such automatic selection.

       If the server has a preferred choice of representation, it SHOULD
       include the specific URI for that representation in the Location
       field; user agents MAY use the Location field value for automatic
       redirection. This response is cacheable 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 use one of the returned
       URIs.  Clients with link editing capabilities ought to 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
       cacheable unless indicated otherwise.

       The new permanent URI 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 Found

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

       The temporary URI 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: RFC 1945 and RFC 2068 specify that the client is not allowed
          to change the method on the redirected request.  However, most
          existing user agent implementations treat 302 as if it were a 303
          response, performing a GET on the Location field-value regardless
          of the original request method. The status codes 303 and 307 have
          been added for servers that wish to make unambiguously clear which
          kind of reaction is expected of the client.

    10.3.4 303 See Other

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

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

          Note: Many pre-HTTP/1.1 user agents do not understand the 303
          status. When interoperability with such clients is a concern, the
          302 status code may be used instead, since most user agents react
          to a 302 response as described here for 303.

    10.3.5 304 Not Modified

       If the client has performed a conditional GET request and access is
       allowed, but the document has not been modified, the server SHOULD
       respond with this status code. The 304 response MUST NOT contain a
       message-body, and thus is always terminated by the first empty line
       after the header fields.

       The response MUST include the following header fields:

          - Date, unless its omission is required by section 14.18.1

       If a clockless origin server obeys these rules, and proxies and
       clients add their own Date to any response received without one (as
       already specified by [RFC 2068], section 14.19), caches will operate
       correctly.

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

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

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

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

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

    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 URI of the proxy.
       The recipient is expected to repeat this single request via the
       proxy. 305 responses MUST only be generated by origin servers.

          Note: RFC 2068 was not clear that 305 was intended to redirect a
          single request, and to be generated by origin servers only.  Not
          observing these limitations has significant security consequences.

    10.3.7 306 (Unused)

       The 306 status code was used in a previous version of the
       specification, is no longer used, and the code is reserved.

    10.3.8 307 Temporary Redirect

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

       The temporary URI 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) , since many pre-HTTP/1.1 user agents do not
       understand the 307 status. Therefore, the note SHOULD contain the
       information necessary for a user to repeat the original request on
       the new URI.

       If the 307 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.

    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.

       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 14.47) 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 might
       include relevant diagnostic information. HTTP access authentication
       is explained in "HTTP Authentication: Basic and Digest Access
       Authentication" [43].

    10.4.3 402 Payment Required

       This code is reserved for future use.

    10.4.4 403 Forbidden

       The server understood the request, but is refusing to fulfill it.
       Authorization will not help and the request SHOULD NOT be repeated.
       If the request method was not HEAD and the server wishes to make
       public why the request has not been fulfilled, it SHOULD describe the
       reason for the refusal in the entity.  If the server does not wish to
       make this information available to the client, the status code 404
       (Not Found) can be used instead.

    10.4.5 404 Not Found

       The server has not found anything matching the Request-URI. No
       indication is given of whether the condition is temporary or
       permanent. The 410 (Gone) status code SHOULD be used if the server
       knows, through some internally configurable mechanism, that an old
       resource is permanently unavailable and has no forwarding address.
       This status code is commonly used when the server does not wish to
       reveal exactly why the request has been refused, or when no other
       response is applicable.

    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 "HTTP Authentication: Basic and Digest Access Authentication"
       [43].

    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 might not be
       possible and is not required.

       Conflicts are most likely to occur in response to a PUT request. For
       example, if versioning were being used and the entity being PUT
       included changes to a resource which conflict with those made by an
       earlier (third-party) request, the server might use the 409 response
       to indicate that it can't complete the request. In this case, the
       response entity would likely 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 is expected to 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 cacheable 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 URI "black hole" of
       redirection (e.g., a redirected URI prefix that points to a suffix of
       itself), or when the server is under attack by a client attempting to
       exploit security holes present in some servers using fixed-length
       buffers for reading or manipulating the Request-URI.

    10.4.16 415 Unsupported Media Type

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

    10.4.17 416 Requested Range Not Satisfiable

       A server SHOULD return a response with this status code if a request
       included a Range request-header field (section 14.35), and none of
       the range-specifier values in this field overlap the current extent
       of the selected resource, and the request did not include an If-Range
       request-header field. (For byte-ranges, this means that the first-
       byte-pos of all of the byte-range-spec values were greater than the
       current length of the selected resource.)

       When this status code is returned for a byte-range request, the
       response SHOULD include a Content-Range entity-header field
       specifying the current length of the selected resource (see section
       14.16). This response MUST NOT use the multipart/byteranges content-
       type.

    10.4.18 417 Expectation Failed

       The expectation given in an Expect request-header field (see section
       14.20) could not be met by this server, or, if the server is a proxy,
       the server has unambiguous evidence that the request could not be met
       by the next-hop server.

    10.5 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 specified by the URI (e.g.
       HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
       to access in attempting to complete the request.

          Note: Note to implementors: some deployed proxies are known to
          return 400 or 500 when DNS lookups time out.

    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 several OPTIONAL challenge-response authentication
       mechanisms which can be used by a server to challenge a client
       request and by a client to provide authentication information. The
       general framework for access authentication, and the specification of
       "basic" and "digest" authentication, are specified in "HTTP
       Authentication: Basic and Digest Access Authentication" [43]. This
       specification adopts the definitions of "challenge" and "credentials"
       from that specification.

    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 14.43). 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.

       The Vary  header field can be used to express the parameters the
       server uses to select a representation that is subject to server-
       driven negotiation. See section 13.6 for use of the Vary header field
       by caches and section 14.44 for use of the Vary header field by
       servers.

    12.2 Agent-driven Negotiation

       With agent-driven negotiation, selection of the best representation
       for a response is performed by the user agent after receiving an
       initial response from the origin server. Selection is based on a list
       of the available representations of the response included within the
       header fields 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 that could be used within HTTP/1.1.

    13 Caching in HTTP

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

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

       Requirements for performance, availability, and disconnected
       operation require us to be able to relax the goal of semantic
       transparency. The HTTP/1.1 protocol allows origin servers, caches,

       and clients to explicitly reduce transparency when necessary.
       However, because non-transparent operation may confuse non-expert
       users, and might be incompatible with certain server applications
       (such as those for ordering merchandise), the protocol requires that
       transparency be relaxed

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

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

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

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

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

          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 implementor might be faced with
          design decisions not explicitly discussed in this specification.
          If a decision might affect semantic transparency, the implementor
          ought to err on the side of maintaining transparency unless a
          careful and complete analysis shows significant benefits in
          breaking transparency.

    13.1.1 Cache Correctness

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

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

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

             If a stored response is not "fresh enough" by the most
             restrictive freshness requirement of both the client and the
             origin server, in carefully considered circumstances the cache
             MAY still return the response with the appropriate Warning
             header (see section 13.1.5 and 14.46), unless such a response
             is prohibited (e.g., by a "no-store" cache-directive, or by a
             "no-cache" cache-request-directive; see section 14.9).

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

       If the cache can not communicate with the origin server, then a
       correct cache SHOULD respond as above if the response can be
       correctly served from the cache; if not it MUST return an error or
       warning indicating that there was a communication failure.

       If a cache receives a response (either an entire response, or a 304
       (Not Modified) response) that it would normally forward to the
       requesting client, and the received response is no longer fresh, the
       cache SHOULD 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. A 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 general-header.
       The Warning header and the currently defined warnings are described
       in section 14.46. The 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 assigned three digit warn-codes. The first digit
       indicates whether the Warning MUST or MUST NOT be deleted from a
       stored cache entry after a successful revalidation:

       1xx  Warnings that describe the freshness or revalidation status of
         the response, and so MUST be deleted after a successful
         revalidation. 1XX warn-codes MAY be generated by a cache only when
         validating a cached entry. It MUST NOT be generated by clients.

       2xx  Warnings that describe some aspect of the entity body or entity
         headers that is not rectified by a revalidation (for example, a
         lossy compression of the entity bodies) and which MUST NOT be
         deleted after a successful revalidation.

       See section 14.46 for the definitions of the codes themselves.

       HTTP/1.0 caches will cache all Warnings in responses, without
       deleting the ones in the first category. Warnings in responses that
       are passed to HTTP/1.0 caches carry an extra warning-date field,
       which prevents a future HTTP/1.1 recipient from believing an
       erroneously cached Warning.

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

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

       When multiple warnings are attached to a response, it might 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.

    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 might 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 is 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 might 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 agent SHOULD NOT
       default to either non-transparent behavior, or behavior that results
       in abnormally ineffective caching, but MAY be explicitly configured
       to do so by an explicit action of the user.

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

       If the user has overridden the caching mechanisms in a way that would
       abnormally reduce the effectiveness of caches, the user agent SHOULD
       continually indicate this state to the user (for example, by a
       display of 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 ought 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) enabling
       the client software to alert the user that there might 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 might 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 might violate the origin server's specified
       constraints on semantic transparency, but might 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 an 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 might compromise semantic transparency, they ought to 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.

       HTTP/1.1 requires origin servers to send a Date header, if possible,
       with every response, giving the time at which the response was
       generated (see section 14.18). 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 convey the estimated age of
       the response message when obtained from a cache. The Age field value
       is the cache's estimate of the amount of time since the response was
       generated or revalidated by the origin server.

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

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

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

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

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

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

           corrected_received_age = max(now - date_value, age_value)

       and as long as we have either nearly synchronized clocks or all-
       HTTP/1.1 paths, one gets a reliable (conservative) result.

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

       Because the request that resulted in the returned Age value must have
       been initiated prior to that Age value's generation, we can correct
       for delays imposed by the network by recording the time at which the
       request was initiated. Then, when an Age value is received, it MUST
       be interpreted relative to the time the request was initiated, not

       the time that the response was received. This algorithm results in
       conservative behavior no matter how much delay is experienced. So, we
       compute:

          corrected_initial_age = corrected_received_age
                                + (now - request_time)

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

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

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

          apparent_age = max(0, response_time - date_value);
          corrected_received_age = max(apparent_age, age_value);
          response_delay = response_time - request_time;
          corrected_initial_age = corrected_received_age + response_delay;
          resident_time = now - response_time;
          current_age   = corrected_initial_age + resident_time;

       The current_age of a cache entry is calculated by adding the amount
       of time (in seconds) since the cache entry was last validated by the
       origin server to the corrected_initial_age. When a response is
       generated from a cache entry, the cache MUST include a single Age
       header field in the response with a value equal to the cache entry's
       current_age.

       The presence of an Age header field in a response implies that a
       response is not first-hand. However, the converse is not true, since
       the lack of an Age header field in a response does not imply that the

       response is first-hand unless all caches along the request path are
       compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
       the Age header field).

    13.2.4 Expiration Calculations

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

       We use the term "expires_value" to denote the value of the Expires
       header. We use the term "max_age_value" to denote an appropriate
       value of the number of seconds carried by the "max-age" directive of
       the Cache-Control header in a response (see section 14.9.3).

       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 none of Expires, Cache-Control: max-age, or Cache-Control: s-
       maxage (see section 14.9.3) appears in the response, and the response
       does not include other restrictions on caching, the cache MAY compute
       a freshness lifetime using a heuristic. The cache MUST attach Warning
       113 to any response whose age is more than 24 hours if such warning
       has not already been added.

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

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

          response_is_fresh = (freshness_lifetime > current_age)

    13.2.5 Disambiguating Expiration Values

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

       If a client performing a retrieval receives a non-first-hand response
       for a request that was already fresh in its own cache, and the Date
       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 might 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 might 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 might
       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. 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 (see section 13.3.3), 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.

       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 response-header field value, an entity tag, provides for an
       "opaque" cache validator. This might 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 might 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 14.19, 14.24, 14.26 and 14.44.

    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 might 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 might 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 might end up with an internally
       inconsistent entity.

       Clients MAY issue simple (non-subrange) GET requests with either weak
       validators or strong validators. Clients MUST NOT use weak validators
       in other forms of request.

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

          - The strong comparison function: in order to be considered equal,
            both validators MUST be identical in every way, and both MUST
            NOT be weak.

          - The weak comparison function: in order to be considered equal,
            both validators MUST be identical in every way, but either or
            both of them MAY be tagged as "weak" without affecting the
            result.

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

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

          - The validator is being compared by an origin server to the
            actual current validator for the entity and,

          - That origin server reliably knows that the associated entity did
            not change twice during the second covered by the presented
            validator.

       or

          - The validator is about to be used by a client in an If-
            Modified-Since or If-Unmodified-Since header, because the client
            has a cache entry for the associated entity, and

          - That cache entry includes a Date value, which gives the time
            when the origin server sent the original response, and

          - The presented Last-Modified time is at least 60 seconds before
            the Date value.

       or

          - The validator is being compared by an intermediate cache to the
            validator stored in its cache entry for the entity, and

          - That cache entry includes a Date value, which gives the time
            when the origin server sent the original response, and

          - The presented Last-Modified time is at least 60 seconds before
            the Date value.

       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 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 ought to
       be used, and for what purposes.

       HTTP/1.1 origin servers:

          - SHOULD send an entity tag validator unless it is not feasible to
            generate one.

          - MAY send a weak entity tag instead of a strong entity tag, if
            performance considerations support the use of weak entity tags,
            or if it is unfeasible to send a strong entity tag.

          - SHOULD send a Last-Modified value if it is feasible to send one,
            unless the risk of a breakdown in semantic transparency that
            could result from using this date in an If-Modified-Since header
            would lead to serious problems.

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

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

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

       HTTP/1.1 clients:

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

          - If only a Last-Modified value has been provided by the origin
            server, SHOULD use that value in non-subrange cache-conditional
            requests (using If-Modified-Since).

          - If only a Last-Modified value has been provided by an HTTP/1.0
            origin server, MAY use that value in subrange cache-conditional
            requests (using If-Unmodified-Since:). The user agent SHOULD
            provide a way to disable this, in case of difficulty.

          - If both an entity tag and a Last-Modified value have been
            provided by the origin server, SHOULD use both validators in
            cache-conditional requests. This allows both HTTP/1.0 and
            HTTP/1.1 caches to respond appropriately.

       An HTTP/1.1 origin server, upon receiving a conditional request that
       includes both a Last-Modified date (e.g., in an If-Modified-Since or
       If-Unmodified-Since header field) and one or more entity tags (e.g.,
       in an If-Match, If-None-Match, or If-Range header field) as cache
       validators, MUST NOT return a response status of 304 (Not Modified)
       unless doing so is consistent with all of the conditional header
       fields in the request.

       An HTTP/1.1 caching proxy, upon receiving a conditional request that
       includes both a Last-Modified date and one or more entity tags as
       cache validators, MUST NOT return a locally cached response to the
       client unless that cached response is consistent with all of the
       conditional header fields in the request.

          Note: 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 Cacheability

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

       However, in some cases it might be inappropriate for a cache to
       retain an entity, or to return it in response to a subsequent
       request. This might 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, are not to 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 (e.g. status codes 302
       and 307) MUST NOT be returned in a reply to a subsequent request
       unless there are cache-control directives or another header(s) that
       explicitly allow it. For example, these include the following: an
       Expires header (section 14.21); a "max-age", "s-maxage",  "must-
       revalidate", "proxy-revalidate", "public" or "private" cache-control
       directive (section 14.9).

    13.5 Constructing Responses From Caches

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

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

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

          - End-to-end headers, which are  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 MUST be
            transmitted in any response formed from a cache entry.

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

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

          - Connection
          - Keep-Alive
          - Proxy-Authenticate
          - Proxy-Authorization
          - TE
          - Trailers
          - Transfer-Encoding
          - Upgrade

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

       Other hop-by-hop headers MUST be listed in a Connection header,
       (section 14.10) to be introduced into HTTP/1.1 (or later).

    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
       transparent proxy SHOULD NOT modify an end-to-end header unless the
       definition of that header requires or specifically allows that.

       A transparent proxy MUST NOT modify any of the following fields in a
       request or response, and it MUST NOT add any of these fields if not
       already present:

          - Content-Location

          - Content-MD5

          - ETag

          - Last-Modified

       A transparent proxy MUST NOT modify any of the following fields in a
       response:

          - Expires

       but it MAY add any of these fields if not already present. If an
       Expires header is added, it MUST be given a field-value identical to
       that of the Date header in that response.

       A  proxy MUST NOT modify or add any of the following fields in a
       message that contains the no-transform cache-control directive, or in
       any request:

          - Content-Encoding

          - Content-Range

          - Content-Type

       A non-transparent proxy MAY modify or add these fields to a message
       that does not include no-transform, but if it does so, it MUST add a
       Warning 214 (Transformation applied) if one does not already appear
       in the message (see section 14.46).

          Warning: unnecessary modification of end-to-end headers might
          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.

       The Content-Length field of a request or response is added or deleted
       according to the rules in section 4.4. A transparent proxy MUST
       preserve the entity-length (section 7.2.2) of the entity-body,
       although it MAY change the transfer-length (section 4.4).

    13.5.3 Combining Headers

       When a cache makes a validating request to a server, and the server
       provides a 304 (Not Modified) response or a 206 (Partial Content)
       response, the cache then constructs a response to send to the
       requesting client.

       If the status code is 304 (Not Modified), the cache uses the entity-
       body stored in the cache entry as the entity-body of this outgoing
       response. If the status code is 206 (Partial Content) and the ETag or
       Last-Modified headers match exactly, the cache MAY combine the
       contents stored in the cache entry with the new contents received in
       the response and use the result as the entity-body of this outgoing
       response, (see 13.5.4).

       The end-to-end headers stored in the cache entry are used for the
       constructed response, except that

          - any stored Warning headers with warn-code 1xx (see section
            14.46) MUST be deleted from the cache entry and the forwarded
            response.

          - any stored Warning headers with warn-code 2xx MUST be retained
            in the cache entry and the forwarded response.

          - any end-to-end headers provided in the 304 or 206 response MUST
            replace the corresponding headers from the cache entry.

       Unless the cache decides to remove the cache entry, it MUST also
       replace the end-to-end headers stored with the cache entry with
       corresponding headers received in the incoming response, except for
       Warning headers as described immediately above. If a header field-
       name in the incoming response matches more than one header in the
       cache entry, all such old headers MUST be replaced.

       In other words, the set of end-to-end headers received in the
       incoming response overrides all corresponding end-to-end headers
       stored with the cache entry (except for stored Warning headers with
       warn-code 1xx, which are deleted even if not overridden).

          Note: this rule allows an origin server to use a 304 (Not
          Modified) or a 206 (Partial Content) response to update any header
          associated with a previous response for the same entity or sub-
          ranges thereof, although it might not always be meaningful or
          correct to do so. This rule does not allow an origin server to use
          a 304 (Not Modified) or a 206 (Partial Content) response to
          entirely delete a header that it had provided with a previous
          response.

    13.5.4 Combining Byte Ranges

       A response might 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 might have received several ranges of
       the same entity-body.

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

          - Both the incoming response and the cache entry have a cache
            validator.

          - The two cache validators match using the strong comparison
            function (see section 13.3.3).

       If either requirement is not met, 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 12.1), as indicated
       by the presence of a Vary header field in a response, alters the
       conditions and procedure by which a cache can use the response for
       subsequent requests. See section 14.44 for use of the Vary header
       field by servers.

       A server SHOULD use the Vary header field to inform a cache of what
       request-header fields were used to select among multiple
       representations of a cacheable response subject to server-driven
       negotiation. The set of header fields named by the Vary field value
       is known as the "selecting" request-headers.

       When the cache receives a subsequent request whose Request-URI
       specifies one or more cache entries including a Vary header field,
       the cache MUST NOT use such a cache entry to construct a response to
       the new request unless all of the selecting request-headers present
       in the new request match the corresponding stored request-headers in
       the original request.

       The selecting request-headers from two requests are defined to match
       if and only if the selecting request-headers in the first request can
       be transformed to the selecting request-headers in the second request

       by adding or removing linear white space (LWS) at places where this
       is allowed by the corresponding BNF, and/or combining multiple
       message-header fields with the same field name following the rules
       about message headers in section 4.2.

       A Vary header field-value of "*" always fails to match and subsequent
       requests on that resource can only be properly interpreted by the
       origin server.

       If the selecting request header fields for the cached entry do not
       match the selecting request header fields of the new request, then
       the cache MUST NOT use a cached entry to satisfy the request unless
       it first relays the new request to the origin server in a conditional
       request and the server responds with 304 (Not Modified), including an
       entity tag or Content-Location that indicates the entity to be used.

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

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

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

    13.7 Shared and Non-Shared Caches

       For reasons of security and privacy, it is necessary to make a
       distinction between "shared" and "non-shared" caches. A non-shared
       cache is one that is accessible only to a single user. Accessibility
       in this case SHOULD be enforced by appropriate security mechanisms.
       All other caches are considered to be "shared." Other sections of

       this specification place certain constraints on the operation of
       shared caches in order to prevent loss of privacy or failure of
       access controls.

    13.8 Errors or Incomplete Response Cache Behavior

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

       If a cache receives a 5xx response while attempting to revalidate an
       entry, it MAY either forward this response to the requesting client,
       or act as if the server failed to respond. In the latter case, it MAY
       return a previously received response unless the cached entry
       includes the "must-revalidate" cache-control directive (see section
       14.9).

    13.9 Side Effects of GET and HEAD

       Unless the origin server explicitly prohibits the caching of their
       responses, the application of GET and HEAD methods to any resources
       SHOULD NOT have side effects that would lead to erroneous behavior if
       these responses are taken from a cache. They MAY still have side
       effects, but a cache is not required to consider such side effects in
       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 URIs as fresh unless
       the server provides an explicit expiration time. This specifically
       means that responses from HTTP/1.0 servers for such URIs SHOULD NOT
       be taken from a cache. See section 9.1.1 for related information.

    13.10 Invalidation After Updates or Deletions

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

       There is no way for the HTTP protocol to guarantee that all such
       cache entries are marked invalid. For example, the request that
       caused the change at the origin server might 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 will either remove all instances of that entity from its
       storage, or will mark these as "invalid" and in need of a mandatory
       revalidation before they can be returned in response to a subsequent
       request.

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

          - PUT

          - DELETE

          - POST

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

       A cache that passes through requests for methods it does not
       understand SHOULD invalidate any entities referred to by the
       Request-URI.

    13.11 Write-Through Mandatory

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

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

    13.12 Cache Replacement

       If a new cacheable (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  the
       rules in section 13.5.3 apply.

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

    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 is not to be construed to prohibit the history mechanism from
       telling the user that a view might 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 are 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 might 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 ought to 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.

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

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

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

       The special value "*", if present in the Accept-Charset field,
       matches every character set (including ISO-8859-1) which is not
       mentioned elsewhere in the Accept-Charset field. If no "*" is present
       in an Accept-Charset field, then all character sets not explicitly
       mentioned get a quality value of 0, except for ISO-8859-1, which gets
       a quality value of 1 if not explicitly mentioned.

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

    14.3 Accept-Encoding

       The Accept-Encoding request-header field is similar to Accept, but
       restricts the content-codings (section 3.5) that are acceptable in
       the response.

           Accept-Encoding  = "Accept-Encoding" ":"

                              1#( codings [ ";" "q" "=" qvalue ] )
           codings          = ( content-coding | "*" )

       Examples of its use are:

           Accept-Encoding: compress, gzip
           Accept-Encoding:
           Accept-Encoding: *
           Accept-Encoding: compress;q=0.5, gzip;q=1.0
           Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0

       A server tests whether a content-coding is acceptable, according to
       an Accept-Encoding field, using these rules:

          1. If the content-coding is one of the content-codings listed in
             the Accept-Encoding field, then it is acceptable, unless it is
             accompanied by a qvalue of 0. (As defined in section 3.9, a
             qvalue of 0 means "not acceptable.")

          2. The special "*" symbol in an Accept-Encoding field matches any
             available content-coding not explicitly listed in the header
             field.

          3. If multiple content-codings are acceptable, then the acceptable
             content-coding with the highest non-zero qvalue is preferred.

          4. The "identity" content-coding is always acceptable, unless
             specifically refused because the Accept-Encoding field includes
             "identity;q=0", or because the field includes "*;q=0" and does
             not explicitly include the "identity" content-coding. If the
             Accept-Encoding field-value is empty, then only the "identity"
             encoding is acceptable.

       If an Accept-Encoding field is present in a request, and if the
       server cannot send a response which is acceptable according to the
       Accept-Encoding header, then the server SHOULD send an error response
       with the 406 (Not Acceptable) status code.

       If no Accept-Encoding field is present in a request, the server MAY
       assume that the client will accept any content coding. In this case,
       if "identity" is one of the available content-codings, then the
       server SHOULD use the "identity" content-coding, unless it has
       additional information that a different content-coding is meaningful
       to the client.

          Note: If the request does not include an Accept-Encoding field,
          and if the "identity" content-coding is unavailable, then
          content-codings commonly understood by HTTP/1.0 clients (i.e.,

          "gzip" and "compress") are preferred; some older clients
          improperly display messages sent with other content-codings.  The
          server might also make this decision based on information about
          the particular user-agent or client.

          Note: Most HTTP/1.0 applications do not recognize or obey qvalues
          associated with content-codings. This means that qvalues will not
          work and are not permitted with x-gzip or x-compress.

    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. Language tags are defined in section 3.10.

           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 might 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 15.1.4.

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

          Note: When making the choice of linguistic preference available to
          the user, we remind implementors of  the fact that users are not
          familiar with the details of language matching as described above,
          and should provide appropriate guidance. As an example, users
          might assume that on selecting "en-gb", they will be served any
          kind of English document if British English is not available. A
          user agent might suggest in such a case to add "en" to get the
          best matching behavior.

    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. Range units are defined in section 3.12.

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

              Accept-Ranges: none

          to advise the client not to attempt a range request.

    14.6 Age

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

               Age = "Age" ":" age-value
               age-value = delta-seconds

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

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

    14.7 Allow

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

              Allow   = "Allow" ":" #Method

          Example of use:

              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 might
          have other means of communicating with the origin server.

    14.8 Authorization

          A user agent that wishes to authenticate itself with a server--
          usually, but not necessarily, after receiving a 401 response--does
          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 "HTTP Authentication:
          Basic and Digest Access Authentication" [43]. If a request is
          authenticated and a realm specified, the same credentials SHOULD
          be valid for all other requests within this realm (assuming that
          the authentication scheme itself does not require otherwise, such
          as credentials that vary according to a challenge value or using
          synchronized clocks).

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

          1. If the response includes the "s-maxage" cache-control
             directive, the cache MAY use that response in replying to a
             subsequent request. But (if the specified maximum age has
             passed) a proxy cache MUST first revalidate it with the origin
             server, using the request-headers from the new request to allow
             the origin server to authenticate the new request. (This is the
             defined behavior for s-maxage.) If the response includes "s-
             maxage=0", the proxy MUST always revalidate it before re-using
             it.

          2. If the response includes the "must-revalidate" cache-control
             directive, the cache MAY use that response in replying to a
             subsequent request. But if the response is stale, all caches
             MUST first revalidate it with the origin server, using the
             request-headers from the new request to allow the origin server
             to authenticate the new request.

          3. If the response includes the "public" cache-control directive,
             it MAY be returned in reply to any subsequent request.

    14.9 Cache-Control

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

          Note that HTTP/1.0 caches might not implement Cache-Control and
          might 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 might 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"                          ; Section 14.9.1
             | "no-store"                          ; Section 14.9.2
             | "max-age" "=" delta-seconds         ; Section 14.9.3, 14.9.4
             | "max-stale" [ "=" delta-seconds ]   ; Section 14.9.3
             | "min-fresh" "=" delta-seconds       ; Section 14.9.3
             | "no-transform"                      ; Section 14.9.5
             | "only-if-cached"                    ; Section 14.9.4
             | cache-extension                     ; Section 14.9.6

         cache-response-directive =
               "public"                               ; Section 14.9.1
             | "private" [ "=" <"> 1#field-name <"> ] ; Section 14.9.1
             | "no-cache" [ "=" <"> 1#field-name <"> ]; Section 14.9.1
             | "no-store"                             ; Section 14.9.2
             | "no-transform"                         ; Section 14.9.5
             | "must-revalidate"                      ; Section 14.9.4
             | "proxy-revalidate"                     ; Section 14.9.4
             | "max-age" "=" delta-seconds            ; Section 14.9.3
             | "s-maxage" "=" delta-seconds           ; Section 14.9.3
             | cache-extension                        ; Section 14.9.6

        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 might apply these directives to
       header fields not defined in HTTP/1.1.

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

          - Restrictions on what are cacheable; these may only be imposed by
            the origin server.

          - Restrictions on what may be stored by a cache; these may be
            imposed by either the origin server or the user agent.

          - Modifications of the basic expiration mechanism; these may be
            imposed by either the origin server or the user agent.

          - Controls over cache revalidation and reload; these may only be
            imposed by a user agent.

          - Control over transformation of entities.

          - Extensions to the caching system.

    14.9.1 What is Cacheable

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

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

       private
          Indicates that all or part of the response message is intended for
          a single user and MUST NOT be cached by a shared cache. This
          allows an origin server to state that the specified parts of the

          response are intended for only one user and are not a valid
          response for requests by other users. A private (non-shared) cache
          MAY cache the response.

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

       no-cache
           If the no-cache directive does not specify a field-name, then a
          cache MUST NOT use the response to satisfy a subsequent request
          without successful revalidation with the origin server. This
          allows an origin server to prevent caching even by caches that
          have been configured to return stale responses to client requests.

          If the no-cache directive does specify one or more field-names,
          then a cache MAY use the response to satisfy a subsequent request,
          subject to any other restrictions on caching. However, the
          specified field-name(s) MUST NOT be sent in the response to a
          subsequent request without successful revalidation with the origin
          server. This allows an origin server to prevent the re-use of
          certain header fields in a response, while still allowing caching
          of the rest of the response.

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

    14.9.2 What May be Stored by Caches

       no-store
          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
          might 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 might
          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 might not recognize or
          obey this directive, and communications networks might be
          vulnerable to eavesdropping.

    14.9.3 Modifications of the Basic Expiration Mechanism

       The expiration time of an entity MAY be specified by the origin
       server using the Expires header (see section 14.21). Alternatively,
       it MAY be specified using the max-age directive in a response. When
       the max-age cache-control directive is present in a cached response,
       the response is stale if its current age is greater than the age
       value given (in seconds) at the time of a new request for that
       resource. The max-age directive on a response implies that the
       response is cacheable (i.e., "public") unless some other, more
       restrictive cache directive is also present.

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

       Many HTTP/1.0 cache implementations will treat an Expires value that
       is less than or equal to the response Date value as being equivalent
       to the Cache-Control response directive "no-cache". If an HTTP/1.1
       cache receives such a response, and the response does not include a
       Cache-Control header field, it SHOULD consider the response to be
       non-cacheable in order to retain compatibility with HTTP/1.0 servers.

           Note: An origin server might wish to use a relatively new HTTP
           cache control feature, such as the "private" directive, on a
           network including older caches that do not understand that
           feature. The origin server will need to combine the new feature
           with an Expires field whose value is less than or equal to the
           Date value. This will prevent older caches from improperly
           caching the response.

       s-maxage
           If a response includes an s-maxage directive, then for a shared
           cache (but not for a private cache), the maximum age specified by
           this directive overrides the maximum age specified by either the
           max-age directive or the Expires header. The s-maxage directive
           also implies the semantics of the proxy-revalidate directive (see
           section 14.9.4), i.e., that the shared cache must not use the
           entry after it becomes stale to respond to a subsequent request
           without first revalidating it with the origin server. The s-
           maxage directive is always ignored by a private cache.

       Note that 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 pre-HTTP/1.1-compliant caches do not observe the
       max-age directive.

       Other directives allow a 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 110 (Response is
       stale).

       A cache MAY be configured to return stale responses without
       validation, but only if this does not conflict with any "MUST"-level
       requirements concerning cache validation (e.g., a "must-revalidate"
       cache-control directive).

       If both the new request and the cached entry include "max-age"
       directives, then the lesser of the two values is used for determining
       the freshness of the cached entry for that request.

    14.9.4 Cache Revalidation and Reload Controls

       Sometimes a user agent might 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 might 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". Field
          names MUST NOT 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.

       max-age
          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
          might 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 might 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 can 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
          can 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.

       only-if-cached
          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.

       must-revalidate
          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.

       proxy-revalidate
          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

       no-transform
          Implementors of intermediate caches (proxies) have found it useful
          to convert the media type of certain entity bodies. A non-
          transparent 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.

          Serious operational problems occur, however, when these
          transformations are applied to entity bodies intended for certain
          kinds of applications. For example, applications for medical

          imaging, scientific data analysis and those using end-to-end
          authentication, all depend on receiving an entity body that is bit
          for bit identical to the original entity-body.

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

    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 an 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) could 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
       cacheability) such that the cache behavior will remain minimally
       correct even if the cache does not understand the extension(s).

    14.10 Connection

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

       The Connection header has the following grammar:

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

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

       Message headers listed in the Connection header MUST NOT include
       end-to-end headers, such as Cache-Control.

       HTTP/1.1 defines the "close" connection option for the sender to
       signal that the connection will be closed after completion of the
       response. For example,

           Connection: close

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

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

       A system receiving an HTTP/1.0 (or lower-version) message that
       includes a Connection header MUST, for each connection-token in this
       field, remove and ignore any header field(s) from the message with
       the same name as the connection-token. This protects against mistaken
       forwarding of such header fields by pre-HTTP/1.1 proxies. See section
       19.6.2.

    14.11 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-coding is a characteristic of the entity identified by
       the Request-URI. Typically, the entity-body is stored with this
       encoding and is only decoded before rendering or analogous usage.
       However, a non-transparent proxy MAY modify the content-coding if the
       new coding is known to be acceptable to the recipient, unless the
       "no-transform" cache-control directive is present in the message.

       If the content-coding of an entity is not "identity", then the
       response MUST include a Content-Encoding entity-header (section
       14.11) that lists the non-identity content-coding(s) used.

       If the content-coding of an entity in a request message is not
       acceptable to the origin server, the server SHOULD respond with a
       status code of 415 (Unsupported Media Type).

       If multiple encodings have been applied to an entity, the content
       codings MUST be listed in the order in which they were applied.
       Additional information about the encoding parameters MAY be provided
       by other entity-header fields not defined by this specification.

    14.12 Content-Language

       The Content-Language entity-header field describes the natural
       language(s) of the intended audience for the enclosed entity. Note
       that this might 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 might 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 would
       properly only include "en".

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

    14.13 Content-Length

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

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

       An example is

           Content-Length: 3495

       Applications SHOULD use this field to indicate the transfer-length of
       the message-body, unless this is prohibited by the rules in section
       4.4.

       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 that 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, unless this is prohibited by the rules in
       section 4.4.

    14.14 Content-Location

       The Content-Location entity-header field MAY be used to supply the
       resource location for the entity enclosed in the message when that
       entity is accessible from a location separate from the requested
       resource's URI. A server SHOULD provide a Content-Location for the
       variant corresponding to the response entity; especially 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.

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

       The value of Content-Location also defines the base URI for the
       entity.

       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 specify the Content-Location URI as the request-
       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 relative URI is
       interpreted relative to the Request-URI.

       The meaning of the Content-Location header in PUT or POST requests is
       undefined; servers are free to ignore it in those cases.

    14.15 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 or
       client to function as an integrity check of the entity-body. Only
       origin servers or clients 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-coding that has been applied, but not including
       any transfer-encoding 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.

       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.

       Conversion of all line breaks to CRLF MUST NOT be done before
       computing or checking the digest: the line break convention used in
       the text actually transmitted MUST be left unaltered when computing
       the digest.

          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.

    14.16 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
       applied. Range units are defined in section 3.12.

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

           content-range-spec      = byte-content-range-spec
           byte-content-range-spec = bytes-unit SP
                                     byte-range-resp-spec "/"
                                     ( instance-length | "*" )

           byte-range-resp-spec = (first-byte-pos "-" last-byte-pos)
                                          | "*"
           instance-length           = 1*DIGIT

       The header SHOULD indicate the total length of the full entity-body,
       unless this length is unknown or difficult to determine. The asterisk
       "*" character means that the instance-length is unknown at the time
       when the response was generated.

       Unlike byte-ranges-specifier values (see section 14.35.1), a byte-
       range-resp-spec MUST only specify one range, and MUST contain
       absolute byte positions for both the first and last byte of the
       range.

       A byte-content-range-spec with a byte-range-resp-spec whose last-
       byte-pos value is less than its first-byte-pos value, or whose
       instance-length value is less than or equal to its last-byte-pos
       value, is invalid. The recipient of an invalid byte-content-range-
       spec MUST ignore it and any content transferred along with it.

       A server sending a response with status code 416 (Requested range not
       satisfiable) SHOULD include a Content-Range field with a byte-range-
       resp-spec of "*". The instance-length specifies the current length of

       the selected resource. A response with status code 206 (Partial
       Content) MUST NOT include a Content-Range field with a byte-range-
       resp-spec of "*".

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

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

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

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

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

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

           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 message. The multipart
       media type used for this purpose is "multipart/byteranges" as defined
       in appendix 19.2. See appendix 19.6.3 for a compatibility issue.

       A response to a request for a single range MUST NOT be sent using the
       multipart/byteranges media type.  A response to a request for
       multiple ranges, whose result is a single range, MAY be sent as a
       multipart/byteranges media type with one part. A client that cannot
       decode a multipart/byteranges message MUST NOT ask for multiple
       byte-ranges in a single request.

       When a client requests multiple byte-ranges in one request, the
       server SHOULD return them in the order that they appeared in the
       request.

       If the server ignores a byte-range-spec because it is syntactically
       invalid, the server SHOULD treat the request as if the invalid Range
       header field did not exist. (Normally, this means return a 200
       response containing the full entity).

       If the server receives a request (other than one including an If-
       Range request-header field) with an unsatisfiable Range request-
       header field (that is, all of whose byte-range-spec values have a
       first-byte-pos value greater than the current length of the selected
       resource), it SHOULD return a response code of 416 (Requested range
       not satisfiable) (section 10.4.17).

          Note: clients cannot depend on servers to send a 416 (Requested
          range not satisfiable) response instead of a 200 (OK) response for
          an unsatisfiable Range request-header, since not all servers
          implement this request-header.

    14.17 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.

    14.18 Date

       The Date general-header field represents the date and time at which
       the message was originated, having the same semantics as orig-date in
       RFC 822. The field value is an HTTP-date, as described in section
       3.3.1; it MUST be sent in RFC 1123 [8]-date format.

           Date  = "Date" ":" HTTP-date

       An example is

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

       Origin servers MUST include a Date header field in all responses,
       except in these cases:

          1. If the response status code is 100 (Continue) or 101 (Switching
             Protocols), the response MAY include a Date header field, at
             the server's option.

          2. If the response status code conveys a server error, e.g. 500
             (Internal Server Error) or 503 (Service Unavailable), and it is
             inconvenient or impossible to generate a valid Date.

          3. If the server does not have a clock that can provide a
             reasonable approximation of the current time, its responses
             MUST NOT include a Date header field. In this case, the rules
             in section 14.18.1 MUST be followed.

       A received message that does not have a Date header field MUST be
       assigned one by the recipient if the message will be cached by that
       recipient or gatewayed via a protocol which requires a Date. An HTTP
       implementation without a clock MUST NOT cache responses without
       revalidating them on every use. An HTTP cache, especially a shared
       cache, SHOULD use a mechanism, such as NTP [28], to synchronize its
       clock with a reliable external standard.

       Clients SHOULD only send a Date header field in messages that include
       an entity-body, as in the case of the PUT and POST requests, and even
       then it is optional. A client without a clock MUST NOT send a Date
       header field in a request.

       The HTTP-date sent in a Date header SHOULD NOT represent a date and
       time subsequent to the generation of the message. It SHOULD represent
       the best available approximation of the date and time of message
       generation, unless the implementation has no means of generating a
       reasonably accurate date and time. In theory, the date ought to
       represent the moment just before the entity is generated. In
       practice, the date can be generated at any time during the message
       origination without affecting its semantic value.

    14.18.1 Clockless Origin Server Operation

       Some origin server implementations might not have a clock available.
       An origin server without a clock MUST NOT assign Expires or Last-
       Modified values to a response, unless these values were associated
       with the resource by a system or user with a reliable clock. It MAY
       assign an Expires value that is known, at or before server
       configuration time, to be in the past (this allows "pre-expiration"
       of responses without storing separate Expires values for each
       resource).

    14.19 ETag

       The ETag response-header field provides the current value of the
       entity tag for the requested variant. The headers used with entity
       tags are described in sections 14.24, 14.26 and 14.44. The entity tag
       MAY be used for comparison with other entities from the same resource
       (see section 13.3.3).

          ETag = "ETag" ":" entity-tag

       Examples:

          ETag: "xyzzy"
          ETag: W/"xyzzy"
          ETag: ""

    14.20 Expect

       The Expect request-header field is used to indicate that particular
       server behaviors are required by the client.

          Expect       =  "Expect" ":" 1#expectation

          expectation  =  "100-continue" | expectation-extension
          expectation-extension =  token [ "=" ( token | quoted-string )
                                   *expect-params ]
          expect-params =  ";" token [ "=" ( token | quoted-string ) ]

       A server that does not understand or is unable to comply with any of
       the expectation values in the Expect field of a request MUST respond
       with appropriate error status. The server MUST respond with a 417
       (Expectation Failed) status if any of the expectations cannot be met
       or, if there are other problems with the request, some other 4xx
       status.

       This header field is defined with extensible syntax to allow for
       future extensions. If a server receives a request containing an
       Expect field that includes an expectation-extension that it does not
       support, it MUST respond with a 417 (Expectation Failed) status.

       Comparison of expectation values is case-insensitive for unquoted
       tokens (including the 100-continue token), and is case-sensitive for
       quoted-string expectation-extensions.

       The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST
       return a 417 (Expectation Failed) status if it receives a request
       with an expectation that it cannot meet. However, the Expect
       request-header itself is end-to-end; it MUST be forwarded if the
       request is forwarded.

       Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
       Expect header.

       See section 8.2.3 for the use of the 100 (continue) status.

    14.21 Expires

       The Expires entity-header field gives the date/time after which the
       response is considered stale. A stale cache entry may not normally be
       returned by a cache (either a proxy cache or a 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 3.3.1; it MUST be in RFC 1123 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 (see section 14.9.3), 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 sends 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 sends 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 a response that otherwise would by default be
       non-cacheable indicates that the response is cacheable, unless
       indicated otherwise by a Cache-Control header field (section 14.9).

    14.22 From

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

           From   = "From" ":" mailbox

       An example is:

           From: webmaster@w3.org

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

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

       The client SHOULD NOT send the From header field without the user's
       approval, as it might 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
       URI 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 naming authority 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/> would properly 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 . If the requested URI does not include an Internet host
       name for the service being requested, then the Host header field MUST
       be given with an empty value. An HTTP/1.1 proxy MUST ensure that any
       request message it forwards does contain an appropriate Host header
       field that identifies the service being requested by the proxy. All
       Internet-based HTTP/1.1 servers MUST respond with a 400 (Bad Request)
       status code to any HTTP/1.1 request message which lacks a Host header
       field.

       See sections 5.2 and 19.6.1.1 for other requirements relating to
       Host.

    14.24 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. Entity tags are defined in section 3.11. 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 13.3.3)
       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 or 412 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 14.44) 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: *

       The result of a request having both an If-Match header field and
       either an If-None-Match or an If-Modified-Since header fields is
       undefined by this specification.

    14.25 If-Modified-Since

       The If-Modified-Since request-header field is used with a 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 SHOULD 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: The Range request-header field modifies the meaning of If-
          Modified-Since; see section 14.35 for full details.

          Note: If-Modified-Since times are interpreted by the server, whose
          clock might not be synchronized with the client.

          Note: When handling an If-Modified-Since header field, some
          servers will use an exact date comparison function, rather than a
          less-than function, for deciding whether to send a 304 (Not
          Modified) response. To get best results when sending an If-
          Modified-Since header field for cache validation, clients are
          advised to use the exact date string received in a previous Last-
          Modified header field whenever possible.

          Note: 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.

       The result of a request having both an If-Modified-Since header field
       and either an If-Match or an If-Unmodified-Since header fields is
       undefined by this specification.

    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 to prevent a method (e.g. PUT)
       from inadvertently modifying an existing resource when the client
       believes that the resource does not exist.

       As a special case, the value "*" matches any current entity of the
       resource.

           If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )

       If any of the entity tags match the entity tag of the entity that
       would have been returned in the response to a similar GET request
       (without the If-None-Match header) on that resource, or if "*" is
       given and any current entity exists for that resource, then the
       server MUST NOT perform the requested method, unless required to do
       so because the resource's modification date fails to match that
       supplied in an If-Modified-Since header field in the request.
       Instead, if the request method was GET or HEAD, the server SHOULD
       respond with a 304 (Not Modified) response, including the cache-
       related 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 entities tags
       match. The weak comparison function can only be used with GET or HEAD
       requests.

       If none of the entity tags match, then the server MAY perform the
       requested method as if the If-None-Match header field did not exist,
       but MUST also ignore any If-Modified-Since header field(s) in the
       request. That is, if no entity tags match, then the server MUST NOT
       return a 304 (Not Modified) response.

       If the request would, without the If-None-Match header field, result
       in anything other than a 2xx or 304 status, then the If-None-Match
       header MUST be ignored. (See section 13.3.4 for a discussion of
       server behavior when both If-Modified-Since and If-None-Match appear
       in the same request.)

       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 14.44)
       exists, and SHOULD be performed if the representation does not exist.
       This feature is intended to 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: *

       The result of a request having both an If-None-Match header field and
       either an If-Match or an If-Unmodified-Since header fields is
       undefined by this specification.

    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 an 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 or 412 status, the
       If-Unmodified-Since header SHOULD be ignored.

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

       The result of a request having both an If-Unmodified-Since header
       field and either an If-None-Match or an If-Modified-Since header
       fields is undefined by this specification.

    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 URI for automatic redirection to the resource. The
       field value consists of a single absolute URI.

           Location       = "Location" ":" absoluteURI

       An example is:

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

          Note: The Content-Location header field (section 14.14) 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 provides a mechanism with the
       TRACE (section 9.8) and OPTIONS (section 9.2) methods to limit the
       number of proxies or gateways that can forward the request to the
       next inbound server. This can be useful when the client is attempting
       to trace a request chain which appears to be failing or looping in
       mid-chain.

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

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

       Each proxy or gateway recipient of a TRACE or OPTIONS request
       containing a Max-Forwards header field MUST check and update its
       value prior to forwarding the request. If the received value is zero
       (0), the recipient MUST NOT forward the request; instead, it MUST
       respond as the final recipient. If the received Max-Forwards value is
       greater than zero, then the forwarded message MUST contain an updated
       Max-Forwards field with a value decremented by one (1).

       The Max-Forwards header field MAY be ignored for all other methods
       defined by this specification and for any extension methods for which
       it is not explicitly referred to as part of that method definition.

    14.32 Pragma

       The Pragma general-header field is used to include implementation-
       specific directives that might 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 backward 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 might 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 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.

          Note: because the meaning of "Pragma: no-cache as a response
          header field is not actually specified, it does not provide a
          reliable replacement for "Cache-Control: no-cache" in a response

    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" ":" 1#challenge

       The HTTP access authentication process is described in "HTTP
       Authentication: Basic and Digest Access Authentication" [43]. 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 might 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 "HTTP
       Authentication: Basic and Digest Access Authentication" [43] . 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 Range

    14.35.1 Byte Ranges

       Since all HTTP entities are represented in HTTP messages as sequences
       of bytes, the concept of a byte range is meaningful for any HTTP
       entity. (However, not all clients and servers need to support byte-
       range operations.)

       Byte range specifications in HTTP apply to the sequence of bytes in
       the entity-body (not necessarily the same as the message-body).

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

           ranges-specifier = byte-ranges-specifier
           byte-ranges-specifier = bytes-unit "=" byte-range-set
           byte-range-set  = 1#( byte-range-spec | suffix-byte-range-spec )
           byte-range-spec = first-byte-pos "-" [last-byte-pos]
           first-byte-pos  = 1*DIGIT
           last-byte-pos   = 1*DIGIT

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

       If the last-byte-pos value is present, it MUST be greater than or
       equal to the first-byte-pos in that byte-range-spec, or the byte-
       range-spec is syntactically invalid. The recipient of a byte-range-
       set that includes one or more syntactically invalid byte-range-spec
       values MUST ignore the header field that includes that byte-range-
       set.

       If the last-byte-pos value is absent, or if the value is greater than
       or equal to the current length of the entity-body, last-byte-pos is
       taken to be equal to one less than the current length of the entity-
       body in bytes.

       By its choice of last-byte-pos, a client can limit the number of
       bytes retrieved without knowing the size of the entity.

           suffix-byte-range-spec = "-" suffix-length
           suffix-length = 1*DIGIT

       A suffix-byte-range-spec is used to specify the suffix of the
       entity-body, of a length given by the suffix-length value. (That is,
       this form specifies the last N bytes of an entity-body.) If the
       entity is shorter than the specified suffix-length, the entire
       entity-body is used.

       If a syntactically valid byte-range-set includes at least one byte-
       range-spec whose first-byte-pos is less than the current length of
       the entity-body, or at least one suffix-byte-range-spec with a non-
       zero suffix-length, then the byte-range-set is satisfiable.
       Otherwise, the byte-range-set is unsatisfiable. If the byte-range-set
       is unsatisfiable, the server SHOULD return a response with a status
       of 416 (Requested range not satisfiable). Otherwise, the server
       SHOULD return a response with a status of 206 (Partial Content)
       containing the satisfiable ranges of the entity-body.

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

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

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

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

          - Or bytes=9500-

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

          - Several legal but not canonical specifications of the second 500
            bytes (byte offsets 500-999, inclusive):
             bytes=500-600,601-999
             bytes=500-700,601-999

    14.35.2 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 ought to support byte ranges when
       possible, since Range supports efficient recovery from partially
       failed transfers, and supports efficient partial retrieval of large
       entities.

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

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

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

       In some cases, it might 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.

    14.36 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 relative URI, it SHOULD be interpreted
       relative to the Request-URI. The URI MUST NOT include a fragment. See
       section 15.1.3 for security considerations.

    14.37 Retry-After

       The Retry-After response-header field can be used with a 503 (Service
       Unavailable) response to indicate how long the service is expected to
       be unavailable to the requesting client. This field MAY also be used
       with any 3xx (Redirection) response to indicate the minimum time the
       user-agent is asked wait before issuing the redirected request. The
       value of this field can be either an HTTP-date or an integer number
       of seconds (in decimal) after the time of the response.

           Retry-After  = "Retry-After" ":" ( HTTP-date | delta-seconds )

       Two examples of its use are

           Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
           Retry-After: 120

       In the latter example, the delay is 2 minutes.

    14.38 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 14.45).

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

    14.39 TE

       The TE request-header field indicates what extension transfer-codings
       it is willing to accept in the response and whether or not it is
       willing to accept trailer fields in a chunked transfer-coding. Its
       value may consist of the keyword "trailers" and/or a comma-separated
       list of extension transfer-coding names with optional accept
       parameters (as described in section 3.6).

           TE        = "TE" ":" #( t-codings )
           t-codings = "trailers" | ( transfer-extension [ accept-params ] )

       The presence of the keyword "trailers" indicates that the client is
       willing to accept trailer fields in a chunked transfer-coding, as
       defined in section 3.6.1. This keyword is reserved for use with
       transfer-coding values even though it does not itself represent a
       transfer-coding.

       Examples of its use are:

           TE: deflate
           TE:
           TE: trailers, deflate;q=0.5

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

       A server tests whether a transfer-coding is acceptable, according to
       a TE field, using these rules:

          1. The "chunked" transfer-coding is always acceptable. If the
             keyword "trailers" is listed, the client indicates that it is
             willing to accept trailer fields in the chunked response on
             behalf of itself and any downstream clients. The implication is
             that, if given, the client is stating that either all
             downstream clients are willing to accept trailer fields in the
             forwarded response, or that it will attempt to buffer the
             response on behalf of downstream recipients.

             Note: HTTP/1.1 does not define any means to limit the size of a
             chunked response such that a client can be assured of buffering
             the entire response.

          2. If the transfer-coding being tested is one of the transfer-
             codings listed in the TE field, then it is acceptable unless it
             is accompanied by a qvalue of 0. (As defined in section 3.9, a
             qvalue of 0 means "not acceptable.")

          3. If multiple transfer-codings are acceptable, then the
             acceptable transfer-coding with the highest non-zero qvalue is
             preferred.  The "chunked" transfer-coding always has a qvalue
             of 1.

       If the TE field-value is empty or if no TE field is present, the only
       transfer-coding  is "chunked". A message with no transfer-coding is
       always acceptable.

    14.40 Trailer

       The Trailer general field value indicates that the given set of
       header fields is present in the trailer of a message encoded with
       chunked transfer-coding.

           Trailer  = "Trailer" ":" 1#field-name

       An HTTP/1.1 message SHOULD include a Trailer header field in a
       message using chunked transfer-coding with a non-empty trailer. Doing
       so allows the recipient to know which header fields to expect in the
       trailer.

       If no Trailer header field is present, the trailer SHOULD NOT include
       any header fields. See section 3.6.1 for restrictions on the use of
       trailer fields in a "chunked" transfer-coding.

       Message header fields listed in the Trailer header field MUST NOT
       include the following header fields:

          . Transfer-Encoding

          . Content-Length

          . Trailer

    14.41 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-coding 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

       If multiple encodings have been applied to an entity, the transfer-
       codings MUST be listed in the order in which they were applied.
       Additional information about the encoding parameters MAY be provided
       by other entity-header fields not defined by this specification.

       Many older HTTP/1.0 applications do not understand the Transfer-
       Encoding header.

    14.42 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.

    14.43 User-Agent

       The User-Agent request-header field contains information about the
       user agent originating the request. This is for statistical purposes,
       the tracing of protocol violations, and automated recognition of user
       agents for the sake of tailoring responses to avoid particular user
       agent limitations. 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

    14.44 Vary

       The Vary field value indicates the set of request-header fields that
       fully determines, while the response is fresh, whether a cache is
       permitted to use the response to reply to a subsequent request
       without revalidation. For uncacheable or stale responses, the Vary
       field value advises the user agent about the criteria that were used
       to select the representation. A Vary field value of "*" implies that
       a cache cannot determine from the request headers of a subsequent
       request whether this response is the appropriate representation. See
       section 13.6 for use of the Vary header field by caches.

           Vary  = "Vary" ":" ( "*" | 1#field-name )

       An HTTP/1.1 server SHOULD include a Vary header field with any
       cacheable response that is subject to server-driven negotiation.
       Doing so allows a cache to properly interpret future requests on that
       resource and informs the user agent about the presence of negotiation

       on that resource. A server MAY include a Vary header field with a
       non-cacheable response that is subject to server-driven negotiation,
       since this might provide the user agent with useful information about
       the dimensions over which the response varies at the time of the
       response.

       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 for
       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.

       A Vary field value of "*" signals that unspecified parameters not
       limited to the request-headers (e.g., the network address of the
       client), play a role in the selection of the response representation.
       The "*" value MUST NOT be generated by a proxy server; it may only be
       generated by an origin server.

    14.45  Via

       The Via general-header field MUST be used by gateways and proxies to
       indicate the intermediate protocols and recipients between the user
       agent and the server on requests, and between the origin server and
       the client on responses. It is analogous to the "Received" field of
       RFC 822 [9] and is intended to be used for tracking message forwards,
       avoiding request loops, and identifying the protocol capabilities of
       all senders along the request/response chain.

          Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
          received-protocol = [ protocol-name "/" ] protocol-version
          protocol-name     = token
          protocol-version  = token
          received-by       = ( host [ ":" port ] ) | pseudonym
          pseudonym         = token

       The received-protocol indicates the protocol version of the message
       received by the server or client along each segment of the
       request/response chain. The received-protocol version is appended to
       the Via field value when the message is forwarded so that information
       about the protocol capabilities of upstream applications remains
       visible to all recipients.

       The protocol-name is optional if and only if it would be "HTTP". The
       received-by field is normally the host and optional port number of a
       recipient server or client that subsequently forwarded the message.
       However, if the real host is considered to be sensitive information,
       it MAY be replaced by a pseudonym. If the port is not given, it MAY
       be assumed to be the default port of the received-protocol.

       Multiple Via field values represents 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.

    14.46 Warning

       The Warning general-header field is used to carry additional
       information about the status or transformation of a message which
       might not be reflected in the message. This information is typically
       used to warn about a possible lack of semantic transparency from
       caching operations or transformations applied to the entity body of
       the message.

       Warning headers are sent with responses using:

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

           warning-value = warn-code SP warn-agent SP warn-text
                                                 [SP warn-date]

           warn-code  = 3DIGIT
           warn-agent = ( host [ ":" port ] ) | pseudonym
                           ; the name or pseudonym of the server adding
                           ; the Warning header, for use in debugging
           warn-text  = quoted-string
           warn-date  = <"> HTTP-date <">

       A response MAY carry more than one Warning header.

       The warn-text SHOULD be in a natural language and character set that
       is most likely to be intelligible to the human user receiving the
       response. This decision MAY be based on any available knowledge, such
       as the location of the cache or user, the Accept-Language field in a
       request, the Content-Language field in a response, etc. The default
       language is English and the default character set is ISO-8859-1.

       If a character set other than ISO-8859-1 is used, it MUST be encoded
       in the warn-text using the method described in RFC 2047 [14].

       Warning headers can in general be applied to any message, however
       some specific warn-codes are specific to caches and can only be
       applied to response messages. New Warning headers SHOULD be added
       after any existing Warning headers. A cache MUST NOT delete any
       Warning header that it received with a message. 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 ought to inform the user of as many of them as possible, in the
       order that they appear in the response. If it is not possible to
       inform the user of all of the warnings, the user agent SHOULD follow
       these heuristics:

          - Warnings that appear early in the response take priority over
            those appearing later in the response.

          - Warnings in the user's preferred character set take priority
            over warnings in other character sets but with identical warn-
            codes and warn-agents.

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

       Requirements for the behavior of caches with respect to Warnings are
       stated in section 13.1.2.

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

       110 Response is stale
         MUST be included whenever the returned response is stale.

       111 Revalidation failed
         MUST be included if a cache returns a stale response because an
         attempt to revalidate the response failed, due to an inability to
         reach the server.

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

       113 Heuristic expiration
         MUST be included if the cache heuristically chose a freshness
         lifetime greater than 24 hours and the response's age is greater
         than 24 hours.

       199 Miscellaneous warning
         The warning text MAY include arbitrary information to be presented
         to a human user, or logged. A system receiving this warning MUST
         NOT take any automated action, besides presenting the warning to
         the user.

       214 Transformation applied
         MUST be added by an intermediate cache or proxy if it applies any
         transformation changing the content-coding (as specified in the
         Content-Encoding header) or media-type (as specified in the
         Content-Type header) of the response, or the entity-body of the
         response, unless this Warning code already appears in the response.

       299 Miscellaneous persistent warning
         The warning text MAY include arbitrary information to be presented
         to a human user, or logged. A system receiving this warning MUST
         NOT take any automated action.

       If an implementation sends a message with one or more Warning headers
       whose version is HTTP/1.0 or lower, then the sender MUST include in
       each warning-value a warn-date that matches the date in the response.

       If an implementation receives a message with a warning-value that
       includes a warn-date, and that warn-date is different from the Date
       value in the response, then that warning-value MUST be deleted from
       the message before storing, forwarding, or using it. (This prevents
       bad consequences of naive caching of Warning header fields.) If all
       of the warning-values are deleted for this reason, the Warning header
       MUST be deleted as well.

    14.47 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 "HTTP
       Authentication: Basic and Digest Access Authentication" [43]. User
       agents are advised to take special care in parsing the WWW-
       Authenticate field value as it might contain more than one challenge,
       or if more than one WWW-Authenticate header field is provided, the
       contents of a challenge itself can 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 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 implementors be particularly careful in this area.
       History shows that errors in this area often create serious security
       and/or privacy problems and generate highly adverse publicity for the
       implementor's company.

    15.1.1 Abuse of Server Log Information

       A server is in the position to save personal data about a user's
       requests which might identify their reading patterns or subjects of
       interest. This information is clearly confidential in nature and its
       handling can 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.

    15.1.2 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 might allow the
       server machine to become more vulnerable to attacks against software
       that is known to contain security holes. Implementors 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 header 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 header might 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.

       The User-Agent (section 14.43) or Server (section 14.38) header
       fields can sometimes be used to determine that a specific client or
       server have a particular security hole which might be exploited.
       Unfortunately, this same information is often used for other valuable
       purposes for which HTTP currently has no better mechanism.

    15.1.3 Encoding Sensitive Information in URI's

       Because the source of a link might be private information or might
       reveal an otherwise private information source, it is strongly
       recommended that the user be able to select whether or not the
       Referer field is sent. For example, a browser client could have a
       toggle switch for browsing openly/anonymously, which would
       respectively enable/disable the sending of Referer and From
       information.

       Clients SHOULD NOT include a Referer header field in a (non-secure)
       HTTP request if the referring page was transferred with a secure
       protocol.

       Authors of services which use the HTTP protocol SHOULD NOT use GET
       based forms for the submission of sensitive data, because this will
       cause this data to be encoded in the Request-URI. Many existing
       servers, proxies, and user agents will log the request URI in some
       place where it might be visible to third parties. Servers can use
       POST-based form submission instead

    15.1.4 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 or not to 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 ought to 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.

    15.2 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.

    15.3 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. It is proper for these lookups to
       be cached, however, only when the TTL (Time To Live) information
       reported by the name server makes it likely that the cached
       information will remain useful.

       If HTTP clients cache the results of host name lookups in order to
       achieve a performance improvement, they MUST observe the TTL
       information reported by DNS.

       If HTTP clients do not observe this rule, they could be spoofed when
       a previously-accessed server's IP address changes. As network
       renumbering is expected to become increasingly common [24], the
       possibility of this form of attack will grow. Observing this
       requirement thus reduces this potential security vulnerability.

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

    15.4 Location Headers and Spoofing

       If a single server supports multiple organizations that do not trust
       one another, then it MUST check the values of Location and Content-
       Location headers in responses that are generated under control of
       said organizations to make sure that they do not attempt to
       invalidate resources over which they have no authority.

    15.5 Content-Disposition Issues

       RFC 1806 [35], from which the often implemented Content-Disposition
       (see section 19.5.1) header in HTTP is derived, has a number of very
       serious security considerations. Content-Disposition is not part of
       the HTTP standard, but since it is widely implemented, we are
       documenting its use and risks for implementors. See RFC 2183 [49]
       (which updates RFC 1806) for details.

    15.6 Authentication Credentials and Idle Clients

       Existing HTTP clients and user agents typically retain authentication
       information indefinitely. HTTP/1.1. does not provide a method for a
       server to direct clients to discard these cached credentials. This is
       a significant defect that requires further extensions to HTTP.
       Circumstances under which credential caching can interfere with the
       application's security model include but are not limited to:

          - Clients which have been idle for an extended period following
            which the server might wish to cause the client to reprompt the
            user for credentials.

          - Applications which include a session termination indication
            (such as a `logout' or `commit' button on a page) after which
            the server side of the application `knows' that there is no
            further reason for the client to retain the credentials.

       This is currently under separate study. There are a number of work-
       arounds to parts of this problem, and we encourage the use of
       password protection in screen savers, idle time-outs, and other
       methods which mitigate the security problems inherent in this
       problem. In particular, user agents which cache credentials are
       encouraged to provide a readily accessible mechanism for discarding
       cached credentials under user control.

    15.7 Proxies and Caching

       By their very nature, HTTP proxies are men-in-the-middle, and
       represent an opportunity for man-in-the-middle attacks. Compromise of
       the systems on which the proxies run can result in serious security
       and privacy problems. Proxies have access to security-related
       information, personal information about individual users and
       organizations, and proprietary information belonging to users and
       content providers. A compromised proxy, or a proxy implemented or
       configured without regard to security and privacy considerations,
       might be used in the commission of a wide range of potential attacks.

       Proxy operators should protect the systems on which proxies run as
       they would protect any system that contains or transports sensitive
       information. In particular, log information gathered at proxies often
       contains highly sensitive personal information, and/or information
       about organizations. Log information should be carefully guarded, and
       appropriate guidelines for use developed and followed. (Section
       15.1.1).

       Caching proxies provide additional potential vulnerabilities, since
       the contents of the cache represent an attractive target for
       malicious exploitation. Because cache contents persist after an HTTP
       request is complete, an attack on the cache can reveal information
       long after a user believes that the information has been removed from
       the network. Therefore, cache contents should be protected as
       sensitive information.

       Proxy implementors should consider the privacy and security
       implications of their design and coding decisions, and of the
       configuration options they provide to proxy operators (especially the
       default configuration).

       Users of a proxy need to be aware that they are no trustworthier than
       the people who run the proxy; HTTP itself cannot solve this problem.

       The judicious use of cryptography, when appropriate, may suffice to
       protect against a broad range of security and privacy attacks. Such
       cryptography is beyond the scope of the HTTP/1.1 specification.

    15.7.1 Denial of Service Attacks on Proxies

       They exist. They are hard to defend against. Research continues.
       Beware.

    16 Acknowledgments

       This specification makes heavy use of the augmented BNF and generic
       constructs defined by David H. Crocker for RFC 822 [9]. Similarly, it
       reuses many of the definitions provided by Nathaniel Borenstein and
       Ned Freed for MIME [7]. We hope that their inclusion in this
       specification will help reduce past confusion over the relationship
       between HTTP and Internet mail message formats.

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

       Much of the content and presentation of the caching design is due to
       suggestions and comments from individuals including: Shel Kaphan,
       Paul Leach, Koen Holtman, David Morris, and Larry Masinter.

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

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

       Jim Gettys (the current editor of this document) wishes particularly
       to thank Roy Fielding, the previous editor of this document, along
       with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
       Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
       Larry Masinter for their help. And thanks go particularly to Jeff
       Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.

       The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
       Frystyk implemented RFC 2068 early, and we wish to thank them for the
       discovery of many of the problems that this document attempts to
       rectify.

    17 References

       [1] Alvestrand, H., "Tags for the Identification of Languages", RFC
           1766, 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, March 1993.

       [3] Berners-Lee, T., "Universal Resource Identifiers in WWW", RFC
           1630, June 1994.

       [4] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform Resource
           Locators (URL)", RFC 1738, December 1994.

       [5] Berners-Lee, T. and D. Connolly, "Hypertext Markup Language -
           2.0", RFC 1866, November 1995.

       [6] Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext Transfer
           Protocol -- HTTP/1.0", RFC 1945, May 1996.

       [7] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
           Extensions (MIME) Part One: Format of Internet Message Bodies",
           RFC 2045, November 1996.

       [8] Braden, R., "Requirements for Internet Hosts -- Communication
           Layers", STD 3, RFC 1123, October 1989.

       [9] Crocker, D., "Standard for The Format of ARPA Internet Text
           Messages", STD 11, RFC 822, 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,
            June 1995.

       [12] Horton, M. and R. Adams, "Standard for Interchange of USENET
            Messages", RFC 1036, December 1987.

       [13] Kantor, B. and P. Lapsley, "Network News Transfer Protocol", RFC
            977, February 1986.

       [14] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
            Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
            November 1996.

       [15] Nebel, E. and L. Masinter, "Form-based File Upload in HTML", RFC
            1867, November 1995.

       [16] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
            August 1982.

       [17] Postel, J., "Media Type Registration Procedure", RFC 1590,
            November 1996.

       [18] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC
            959, October 1985.

       [19] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
            October 1994.

       [20] Sollins, K. and L. Masinter, "Functional Requirements for
            Uniform Resource Names", RFC 1737, 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, October 1995.

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

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

       [26] Venkata N. Padmanabhan, and Jeffrey C. Mogul. "Improving HTTP
            Latency", Computer Networks and ISDN Systems, v. 28, pp. 25-35,
            Dec. 1995. Slightly revised version of paper in Proc. 2nd
            International WWW Conference '94: Mosaic and the Web, Oct. 1994,
            which is available at
            http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLat
            ency.html.

       [27] Joe Touch, John Heidemann, and Katia Obraczka. "Analysis of HTTP
            Performance", <URL: http://www.isi.edu/touch/pubs/http-perf96/>,
            ISI Research Report ISI/RR-98-463, (original report dated Aug.
            1996), USC/Information Sciences Institute, August 1998.

       [28] Mills, D., "Network Time Protocol (Version 3) Specification,
            Implementation and Analysis", RFC 1305, March 1992.

       [29] Deutsch, P., "DEFLATE Compressed Data Format Specification
            version 1.3", RFC 1951, May 1996.

       [30] S. Spero, "Analysis of HTTP Performance Problems,"
            http://sunsite.unc.edu/mdma-release/http-prob.html.

       [31] Deutsch, P. and J. Gailly, "ZLIB Compressed Data Format
            Specification version 3.3", RFC 1950, May 1996.

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

       [33] Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.
            Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
            2068, January 1997.

       [34] Bradner, S., "Key words for use in RFCs to Indicate Requirement
            Levels", BCP 14, RFC 2119, March 1997.

       [35] Troost, R. and Dorner, S., "Communicating Presentation
            Information in Internet Messages: The Content-Disposition
            Header", RFC 1806, June 1995.

       [36] Mogul, J., Fielding, R., Gettys, J. and H. Frystyk, "Use and
            Interpretation of HTTP Version Numbers", RFC 2145, May 1997.
            [jg639]

       [37] Palme, J., "Common Internet Message Headers", RFC 2076, February
            1997. [jg640]

       [38] Yergeau, F., "UTF-8, a transformation format of Unicode and
            ISO-10646", RFC 2279, January 1998. [jg641]

       [39] Nielsen, H.F., Gettys, J., Baird-Smith, A., Prud'hommeaux, E.,
            Lie, H., and C. Lilley. "Network Performance Effects of
            HTTP/1.1, CSS1, and PNG," Proceedings of ACM SIGCOMM '97, Cannes
            France, September 1997.[jg642]

       [40] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part Two: Media Types", RFC 2046, November
            1996. [jg643]

       [41] Alvestrand, H., "IETF Policy on Character Sets and Languages",
            BCP 18, RFC 2277, January 1998. [jg644]

       [42] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
            Identifiers (URI): Generic Syntax and Semantics", RFC 2396,
            August 1998. [jg645]

       [43] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
            Leach, P., Luotonen, A., Sink, E. and L. Stewart, "HTTP
            Authentication: Basic and Digest Access Authentication", RFC
            2617, June 1999. [jg646]

       [44] Luotonen, A., "Tunneling TCP based protocols through Web proxy
            servers," Work in Progress. [jg647]

       [45] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
            Aggregate Documents, such as HTML (MHTML)", RFC 2110, March
            1997.

       [46] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
            9, RFC 2026, October 1996.

       [47] Masinter, L., "Hyper Text Coffee Pot Control Protocol
            (HTCPCP/1.0)", RFC 2324, 1 April 1998.

       [48] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
            Extensions (MIME) Part Five: Conformance Criteria and Examples",
            RFC 2049, November 1996.

       [49] Troost, R., Dorner, S. and K. Moore, "Communicating Presentation
            Information in Internet Messages: The Content-Disposition Header
            Field", RFC 2183, August 1997.

    18 Authors' Addresses

       Roy T. Fielding
       Information and Computer Science
       University of California, Irvine
       Irvine, CA 92697-3425, USA

       Fax: +1 (949) 824-1715
       EMail: fielding@ics.uci.edu

       James Gettys
       World Wide Web Consortium
       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
       Compaq Computer Corporation
       250 University Avenue
       Palo Alto, California, 94305, USA

       EMail: mogul@wrl.dec.com

       Henrik Frystyk Nielsen
       World Wide Web Consortium
       MIT Laboratory for Computer Science
       545 Technology Square
       Cambridge, MA 02139, USA

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

       Larry Masinter
       Xerox Corporation
       3333 Coyote Hill Road
       Palo Alto, CA 94034, USA

       EMail: masinter@parc.xerox.com

       Paul J. Leach
       Microsoft Corporation
       1 Microsoft Way
       Redmond, WA 98052, USA

       EMail: paulle@microsoft.com

       Tim Berners-Lee
       Director, World Wide Web 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 and application/http

       In addition to defining the HTTP/1.1 protocol, this document serves
       as the specification for the Internet media type "message/http" and
       "application/http". The message/http type can be used to enclose a
       single HTTP request or response message, provided that it obeys the
       MIME restrictions for all "message" types regarding line length and
       encodings. The application/http type can be used to enclose a
       pipeline of one or more HTTP request or response messages (not
       intermixed). The following is to be registered with IANA [17].

           Media Type name:         message
           Media subtype name:      http
           Required parameters:     none
           Optional parameters:     version, msgtype
            version: The HTTP-Version number of the enclosed message
                     (e.g., "1.1"). If not present, the version can be
                     determined from the first line of the body.
            msgtype: The message type -- "request" or "response". If not
                     present, the type can be determined from the first
                     line of the body.
           Encoding considerations: only "7bit", "8bit", or "binary" are
                                    permitted
           Security considerations: none

           Media Type name:         application
           Media subtype name:      http
           Required parameters:     none
           Optional parameters:     version, msgtype
            version: The HTTP-Version number of the enclosed messages
                     (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: HTTP messages enclosed by this type
                     are in "binary" format; use of an appropriate
                     Content-Transfer-Encoding is required when
                     transmitted via E-mail.
           Security considerations: none

    19.2 Internet Media Type multipart/byteranges

       When an HTTP 206 (Partial Content) response message includes the
       content of multiple ranges (a response to a request for multiple
       non-overlapping ranges), these are transmitted as a multipart
       message-body. The 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 required
       boundary parameter specifies the boundary string used to separate
       each body-part.

           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--

          Notes:

          1) Additional CRLFs may precede the first boundary string in the
             entity.

          2) Although RFC 2046 [40] permits the boundary string to be
             quoted, some existing implementations handle a quoted boundary
             string incorrectly.

          3) A number of browsers and servers were coded to an early draft
             of the byteranges specification to use a media type of
             multipart/x-byteranges, which is almost, but not quite
             compatible with the version documented in HTTP/1.1.

    19.3 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 not labeling the entity is preferred over labeling
       the entity with the labels US-ASCII or ISO-8859-1. See section 3.7.1
       and 3.4.1.

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

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

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

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

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

    19.4 Differences Between HTTP Entities and RFC 2045 Entities

       HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC
       822 [9]) and the Multipurpose Internet Mail Extensions (MIME [7]) to
       allow entities to be transmitted in an open variety of
       representations and with extensible mechanisms. However, RFC 2045
       discusses mail, and HTTP has a few features that are different from
       those described in RFC 2045. These differences were carefully chosen
       to optimize performance over binary connections, to allow greater
       freedom in the use of new media types, to make date comparisons
       easier, and to acknowledge the practice of some early HTTP servers
       and clients.

       This appendix describes specific areas where HTTP differs from RFC
       2045. Proxies and gateways to strict MIME environments SHOULD be
       aware of these differences and provide the appropriate conversions
       where necessary. Proxies and gateways from MIME environments to HTTP
       also need to be aware of the differences because some conversions
       might be required.

    19.4.1 MIME-Version

       HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages MAY
       include a single MIME-Version general-header field to indicate what
       version of the MIME protocol was used to construct the message. Use
       of the MIME-Version header field indicates that the message is in
       full compliance with the MIME protocol (as defined in RFC 2045[7]).
       Proxies/gateways are responsible for ensuring full compliance (where
       possible) when exporting HTTP messages to strict MIME environments.

           MIME-Version   = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT

       MIME version "1.0" is the default for use in HTTP/1.1. However,
       HTTP/1.1 message parsing and semantics are defined by this document
       and not the MIME specification.

    19.4.2 Conversion to Canonical Form

       RFC 2045 [7] requires that an Internet mail entity be converted to
       canonical form prior to being transferred, as described in section 4
       of RFC 2049 [48]. Section 3.7.1 of this document describes the forms
       allowed for subtypes of the "text" media type when transmitted over
       HTTP. RFC 2046 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 RFC 2049
       canonical form of CRLF. Note, however, that this might 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.

       Implementors should note that conversion will break any cryptographic
       checksums applied to the original content unless the original content
       is already in canonical form. Therefore, the canonical form is
       recommended for any content that uses such checksums in HTTP.

    19.4.3 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.

    19.4.4 Introduction of Content-Encoding

       RFC 2045 does not include any concept equivalent to HTTP/1.1's
       Content-Encoding header field. Since this acts as a modifier on the
       media type, proxies and gateways from HTTP to MIME-compliant
       protocols MUST either change the value of the Content-Type header
       field or decode the entity-body before forwarding the message. (Some
       experimental applications of Content-Type for Internet mail have used
       a media-type parameter of ";conversions=<content-coding>" to perform
       a function equivalent to Content-Encoding. However, this parameter is
       not part of RFC 2045.)

    19.4.5 No Content-Transfer-Encoding

       HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC
       2045. Proxies and gateways from MIME-compliant protocols to HTTP MUST
       remove any non-identity CTE ("quoted-printable" or "base64") encoding
       prior to delivering the response message to an HTTP client.

       Proxies and gateways from HTTP to MIME-compliant protocols are
       responsible for ensuring that the message is in the correct format
       and encoding for safe transport on that protocol, where "safe

       transport" is defined by the limitations of the protocol being used.
       Such a proxy or gateway SHOULD label the data with an appropriate
       Content-Transfer-Encoding if doing so will improve the likelihood of
       safe transport over the destination protocol.

    19.4.6 Introduction of Transfer-Encoding

       HTTP/1.1 introduces the Transfer-Encoding header field (section
       14.41). Proxies/gateways MUST remove any transfer-coding prior to
       forwarding a message via a MIME-compliant protocol.

       A process for decoding the "chunked" transfer-coding (section 3.6)
       can be represented in pseudo-code as:

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

    19.4.7 MHTML and Line Length Limitations

       HTTP implementations which share code with MHTML [45] implementations
       need to be aware of MIME line length limitations. Since HTTP does not
       have this limitation, HTTP does not fold long lines. MHTML messages
       being transported by HTTP follow all conventions of MHTML, including
       line length limitations and folding, canonicalization, etc., since
       HTTP transports all message-bodies as payload (see section 3.7.2) and
       does not interpret the content or any MIME header lines that might be
       contained therein.

    19.5 Additional Features

       RFC 1945 and RFC 2068 document protocol elements used by some
       existing HTTP implementations, but not consistently and correctly
       across most HTTP/1.1 applications. Implementors are advised to be
       aware of these features, but cannot rely upon their presence in, or
       interoperability with, other HTTP/1.1 applications. Some of these

       describe proposed experimental features, and some describe features
       that experimental deployment found lacking that are now addressed in
       the base HTTP/1.1 specification.

       A number of other headers, such as Content-Disposition and Title,
       from SMTP and MIME are also often implemented (see RFC 2076 [37]).

    19.5.1 Content-Disposition

       The Content-Disposition response-header field has been proposed as a
       means for the origin server to suggest a default filename if the user
       requests that the content is saved to a file. This usage is derived
       from the definition of Content-Disposition in RFC 1806 [35].

            content-disposition = "Content-Disposition" ":"
                                  disposition-type *( ";" disposition-parm )
            disposition-type = "attachment" | disp-extension-token
            disposition-parm = filename-parm | disp-extension-parm
            filename-parm = "filename" "=" quoted-string
            disp-extension-token = token
            disp-extension-parm = token "=" ( token | quoted-string )

       An example is

            Content-Disposition: attachment; filename="fname.ext"

       The receiving user agent SHOULD NOT respect any directory path
       information present in the filename-parm parameter, which is the only
       parameter believed to apply to HTTP implementations at this time. The
       filename SHOULD be treated as a terminal component only.

       If this header is used in a response with the application/octet-
       stream content-type, the implied suggestion is that the user agent
       should not display the response, but directly enter a `save response
       as...' dialog.

       See section 15.5 for Content-Disposition security issues.

    19.6 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
       (1996), we would expect commercial HTTP/1.1 servers to:

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

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

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

       And we would expect HTTP/1.1 clients to:

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

          - 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. Some implementations implement the Keep-Alive
       version of persistent connections described in section 19.7.1 of RFC
       2068 [33].

    19.6.1 Changes from HTTP/1.0

       This section summarizes major differences between versions HTTP/1.0
       and HTTP/1.1.

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

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

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

       important that all implementations of HTTP (including updates to
       existing HTTP/1.0 applications) correctly implement these
       requirements:

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

          - A client that sends an HTTP/1.1 request MUST send a Host header.

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

          - Servers MUST accept absolute URIs.

    19.6.2 Compatibility with HTTP/1.0 Persistent Connections

       Some clients and servers might wish to be compatible with some
       previous implementations of persistent connections in HTTP/1.0
       clients and servers. Persistent connections in HTTP/1.0 are
       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. See section 14.10.

       The original HTTP/1.0 form of persistent connections (the Connection:
       Keep-Alive and Keep-Alive header) is documented in RFC 2068. [33]

    19.6.3 Changes from RFC 2068

       This specification has been carefully audited to correct and
       disambiguate key word usage; RFC 2068 had many problems in respect to
       the conventions laid out in RFC 2119 [34].

       Clarified which error code should be used for inbound server failures
       (e.g. DNS failures). (Section 10.5.5).

       CREATE had a race that required an Etag be sent when a resource is
       first created. (Section 10.2.2).

       Content-Base was deleted from the specification: it was not
       implemented widely, and there is no simple, safe way to introduce it
       without a robust extension mechanism. In addition, it is used in a
       similar, but not identical fashion in MHTML [45].

       Transfer-coding and message lengths all interact in ways that
       required fixing exactly when chunked encoding is used (to allow for
       transfer encoding that may not be self delimiting); it was important
       to straighten out exactly how message lengths are computed. (Sections
       3.6, 4.4, 7.2.2, 13.5.2, 14.13, 14.16)

       A content-coding of "identity" was introduced, to solve problems
       discovered in caching. (section 3.5)

       Quality Values of zero should indicate that "I don't want something"
       to allow clients to refuse a representation. (Section 3.9)

       The use and interpretation of HTTP version numbers has been clarified
       by RFC 2145. Require proxies to upgrade requests to highest protocol
       version they support to deal with problems discovered in HTTP/1.0
       implementations (Section 3.1)

       Charset wildcarding is introduced to avoid explosion of character set
       names in accept headers. (Section 14.2)

       A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
       was introduced to add this missing case. (Sections 13.4, 14.8, 14.9,
       14.9.3)

       The Cache-Control: max-age directive was not properly defined for
       responses. (Section 14.9.3)

       There are situations where a server (especially a proxy) does not
       know the full length of a response but is capable of serving a
       byterange request. We therefore need a mechanism to allow byteranges
       with a content-range not indicating the full length of the message.
       (Section 14.16)

       Range request responses would become very verbose if all meta-data
       were always returned; by allowing the server to only send needed
       headers in a 206 response, this problem can be avoided. (Section
       10.2.7, 13.5.3, and 14.27)

       Fix problem with unsatisfiable range requests; there are two cases:
       syntactic problems, and range doesn't exist in the document. The 416
       status code was needed to resolve this ambiguity needed to indicate
       an error for a byte range request that falls outside of the actual
       contents of a document. (Section 10.4.17, 14.16)

       Rewrite of message transmission requirements to make it much harder
       for implementors to get it wrong, as the consequences of errors here
       can have significant impact on the Internet, and to deal with the
       following problems:

          1. Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where
             this was incorrectly placing a requirement on the behavior of
             an implementation of a future version of HTTP/1.x

          2. Made it clear that user-agents should retry requests, not
             "clients" in general.

          3. Converted requirements for clients to ignore unexpected 100
             (Continue) responses, and for proxies to forward 100 responses,
             into a general requirement for 1xx responses.

          4. Modified some TCP-specific language, to make it clearer that
             non-TCP transports are possible for HTTP.

          5. Require that the origin server MUST NOT wait for the request
             body before it sends a required 100 (Continue) response.

          6. Allow, rather than require, a server to omit 100 (Continue) if
             it has already seen some of the request body.

          7. Allow servers to defend against denial-of-service attacks and
             broken clients.

       This change adds the Expect header and 417 status code. The message
       transmission requirements fixes are in sections 8.2, 10.4.18,
       8.1.2.2, 13.11, and 14.20.

       Proxies should be able to add Content-Length when appropriate.
       (Section 13.5.2)

       Clean up confusion between 403 and 404 responses. (Section 10.4.4,
       10.4.5, and 10.4.11)

       Warnings could be cached incorrectly, or not updated appropriately.
       (Section 13.1.2, 13.2.4, 13.5.2, 13.5.3, 14.9.3, and 14.46) Warning
       also needed to be a general header, as PUT or other methods may have
       need for it in requests.

       Transfer-coding had significant problems, particularly with
       interactions with chunked encoding. The solution is that transfer-
       codings become as full fledged as content-codings. This involves
       adding an IANA registry for transfer-codings (separate from content
       codings), a new header field (TE) and enabling trailer headers in the
       future. Transfer encoding is a major performance benefit, so it was
       worth fixing [39]. TE also solves another, obscure, downward
       interoperability problem that could have occurred due to interactions
       between authentication trailers, chunked encoding and HTTP/1.0
       clients.(Section 3.6, 3.6.1, and 14.39)

       The PATCH, LINK, UNLINK methods were defined but not commonly
       implemented in previous versions of this specification. See RFC 2068
       [33].

       The Alternates, Content-Version, Derived-From, Link, URI, Public and
       Content-Base header fields were defined in previous versions of this
       specification, but not commonly implemented. See RFC 2068 [33].

    20 Index

       Please see the PostScript version of this RFC for the INDEX.

    21.  Full Copyright Statement

       Copyright (C) The Internet Society (1999).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

Comment on RFC 2616

Comments about this RFC: RFC 2616: Very basic HTTP Server written in Emacs Lisp: http://www.chez.com/emarsden/downl... by Alex Schröder (5/12/2004) RFC 2616: If someone has got sample code for a basic HTTP/1.1 Server please let me know...... by Mike (2/4/2004) RFC 2616: I am seeking ladies for nothing but sex that is all i want from the lady sorry... by treblav (1/16/2004) RFC 2616: Hello. I have realized that point 8.1.4 of the RFC 2616 is a big threat to... by Doolyo (8/20/2005) RFC 2616: êêêöïïäììóì::::::@::@"""@:>c :::@"$$%^^*^^*((((99( <<>:"? by óäöóöóöóö (12/16/2004) RFC 2616: Man Such a big and boring doc.. thats why ppl shy away from RFC stuff. Make it... by AnurgaM (6/15/2005) RFC 2616: igeve uo 100 dolars iwant password farh51@hotmail.com by farh (7/24/2005) RFC 2616: I have realized that point 8.1.4 of this RFC is a big threat to the load speed... by Doolyo (8/15/2005) RFC 2616: Would it possible to extend the protocol to allow another GET, HEAD, POST,... by CCaldwell (9/15/2004) RFC 2616: Wonder if he got his lady? by SinJax (3/26/2004) RFC 2616: it's for shareaza that the program go fasther by sylle (5/24/2005) RFC 2616: Only 2 simultaneous connections is a big threat to the load speed of HTML... by Doolyo (9/30/2005) RFC 2616: yo to the southampton uni coursework massive by anon (4/6/2004) RFC 2616: If you know everything about HTTP this rfc will be helpful to you (as a recap... by just a developer (1/5/2005) RFC 2616: Please note that section 3.6.2, referenced in section 3.5 (page 25) may most... by Patrick Powell (8/12/2004) RFC 2616: gtu87t 86t8o;koip]0-n df bhvfjkhfgfter6ecvgjlhhjuhjioy uftyei vn bk.jljpoi =- ... by jkl (10/20/2004) RFC 2616: I HOPE KNOW MANY OF SOME RFC by MARRY (5/5/2005)

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