1 files changed, 9859 insertions, 0 deletions

diff --git a/misc/rfc2616-http.txt b/misc/rfc2616-http.txt
new file mode 100644
index 0000000..45d7d08
--- /dev/null
+++ b/misc/rfc2616-http.txt
@@ -0,0 +1,9859 @@
+
+
+
+
+
+
+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].
+
+
+
+
+
+
+Fielding, et al.            Standards Track                     [Page 1]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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
+
+
+
+Fielding, et al.            Standards Track                     [Page 2]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                     [Page 3]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                     [Page 4]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                     [Page 5]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                     [Page 6]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+
+Fielding, et al.            Standards Track                     [Page 7]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                     [Page 8]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+Fielding, et al.            Standards Track                     [Page 9]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 10]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 11]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                    [Page 12]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 13]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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)".
+
+
+
+Fielding, et al.            Standards Track                    [Page 14]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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].
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 15]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+       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
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 16]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 17]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 18]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 19]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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
+      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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 20]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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:
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 21]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 22]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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].
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 23]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 24]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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).
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 25]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 26]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                    [Page 27]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   [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
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 28]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 29]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 30]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                    [Page 31]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                    [Page 32]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                    [Page 33]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+     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
+
+
+
+Fielding, et al.            Standards Track                    [Page 34]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 35]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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
+
+
+
+Fielding, et al.            Standards Track                    [Page 36]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 37]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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
+
+
+
+Fielding, et al.            Standards Track                    [Page 38]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+                      | 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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 39]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                    [Page 40]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+          | "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
+
+
+
+Fielding, et al.            Standards Track                    [Page 41]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+                       | 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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 42]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 43]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 44]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 45]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 46]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 47]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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
+
+
+
+Fielding, et al.            Standards Track                    [Page 48]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+        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.
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 49]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      - 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.
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 50]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 51]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                    [Page 52]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 53]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 54]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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,
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 55]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 56]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 57]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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;
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 58]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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).
+
+
+
+Fielding, et al.            Standards Track                    [Page 59]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 60]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      - 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
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 61]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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).
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 62]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 63]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 64]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 65]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 66]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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
+
+
+
+Fielding, et al.            Standards Track                    [Page 67]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 68]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 69]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 70]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 71]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 72]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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.
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 73]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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,
+
+
+
+Fielding, et al.            Standards Track                    [Page 74]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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);
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 75]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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:
+
+
+
+Fielding, et al.            Standards Track                    [Page 76]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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,
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 77]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 78]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 79]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 80]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 81]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 82]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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)
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 83]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 84]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 85]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 86]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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,
+
+
+
+Fielding, et al.            Standards Track                    [Page 87]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      - 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
+
+
+
+Fielding, et al.            Standards Track                    [Page 88]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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).
+
+
+
+Fielding, et al.            Standards Track                    [Page 89]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      - 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.
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 90]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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).
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 91]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 92]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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).
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 93]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 94]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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
+
+
+
+Fielding, et al.            Standards Track                    [Page 95]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 96]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                    [Page 97]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 98]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+Fielding, et al.            Standards Track                    [Page 99]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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".
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 100]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                   [Page 101]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+       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" ":"
+
+
+
+Fielding, et al.            Standards Track                   [Page 102]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+                          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.,
+
+
+
+Fielding, et al.            Standards Track                   [Page 103]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      "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
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 104]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 105]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 106]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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.
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 107]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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 ) ]
+
+
+
+Fielding, et al.            Standards Track                   [Page 108]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 109]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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.
+
+
+
+Fielding, et al.            Standards Track                   [Page 110]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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.
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 111]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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).
+
+
+
+Fielding, et al.            Standards Track                   [Page 112]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 113]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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
+
+
+
+Fielding, et al.            Standards Track                   [Page 114]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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
+
+
+
+Fielding, et al.            Standards Track                   [Page 115]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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.
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 116]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                   [Page 117]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 118]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 119]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                   [Page 120]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+Fielding, et al.            Standards Track                   [Page 121]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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
+
+
+
+Fielding, et al.            Standards Track                   [Page 122]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                   [Page 123]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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:
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 124]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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).
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 125]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 126]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.)
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 127]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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,
+
+
+
+Fielding, et al.            Standards Track                   [Page 128]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 129]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                   [Page 130]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                   [Page 131]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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.
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 132]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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 )
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 133]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                   [Page 134]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                   [Page 135]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 136]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                   [Page 137]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 138]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+       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
+
+
+
+Fielding, et al.            Standards Track                   [Page 139]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 140]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 141]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.")
+
+
+
+Fielding, et al.            Standards Track                   [Page 142]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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
+
+
+
+Fielding, et al.            Standards Track                   [Page 143]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 144]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                   [Page 145]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 146]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 147]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 148]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                   [Page 149]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                   [Page 150]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 151]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                   [Page 152]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 153]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+Fielding, et al.            Standards Track                   [Page 154]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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).
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 155]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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:
+
+
+
+Fielding, et al.            Standards Track                   [Page 156]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+       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.
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 157]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 158]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   [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.
+
+
+
+Fielding, et al.            Standards Track                   [Page 159]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   [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]
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 160]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   [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.
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 161]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 162]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 163]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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
+
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 164]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 165]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      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.
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 166]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      - 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
+
+
+
+Fielding, et al.            Standards Track                   [Page 167]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                   [Page 168]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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
+
+
+
+Fielding, et al.            Standards Track                   [Page 169]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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;
+
+
+
+Fielding, et al.            Standards Track                   [Page 170]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+      - 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
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 171]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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).
+
+
+
+Fielding, et al.            Standards Track                   [Page 172]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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)
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 173]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+Fielding, et al.            Standards Track                   [Page 174]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+   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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 175]
+
+RFC 2616                        HTTP/1.1                       June 1999
+
+
+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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Fielding, et al.            Standards Track                   [Page 176]
+