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rfc:rfc2068

Network Working Group R. Fielding Request for Comments: 2068 UC Irvine Category: Standards Track J. Gettys

                                                            J. Mogul
                                                                 DEC
                                                          H. Frystyk
                                                      T. Berners-Lee
                                                             MIT/LCS
                                                        January 1997
              Hypertext Transfer Protocol -- HTTP/1.1

Status of this Memo

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

Abstract

 The Hypertext Transfer Protocol (HTTP) is an application-level
 protocol for distributed, collaborative, hypermedia information
 systems. It is a generic, stateless, object-oriented protocol which
 can be used for many tasks, such as name servers and distributed
 object management systems, through extension of its request methods.
 A feature of HTTP is the typing and negotiation of data
 representation, allowing systems to be built independently of the
 data being transferred.
 HTTP has been in use by the World-Wide Web global information
 initiative since 1990. This specification defines the protocol
 referred to as "HTTP/1.1".

Table of Contents

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

Fielding, et. al. Standards Track [Page 1] RFC 2068 HTTP/1.1 January 1997

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

Fielding, et. al. Standards Track [Page 2] RFC 2068 HTTP/1.1 January 1997

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

Fielding, et. al. Standards Track [Page 3] RFC 2068 HTTP/1.1 January 1997

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

Fielding, et. al. Standards Track [Page 4] RFC 2068 HTTP/1.1 January 1997

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

Fielding, et. al. Standards Track [Page 5] RFC 2068 HTTP/1.1 January 1997

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

Fielding, et. al. Standards Track [Page 6] RFC 2068 HTTP/1.1 January 1997

1 Introduction

1.1 Purpose

 The Hypertext Transfer Protocol (HTTP) is an application-level
 protocol for distributed, collaborative, hypermedia information
 systems. HTTP has been in use by the World-Wide Web global
 information initiative since 1990. The first version of HTTP,
 referred to as HTTP/0.9, was a simple protocol for raw data transfer
 across the Internet. HTTP/1.0, as defined by RFC 1945 [6], improved
 the protocol by allowing messages to be in the format of MIME-like
 messages, containing metainformation about the data transferred and
 modifiers on the request/response semantics. However, HTTP/1.0 does
 not sufficiently take into consideration the effects of hierarchical
 proxies, caching, the need for persistent connections, and virtual
 hosts. In addition, the proliferation of incompletely-implemented
 applications calling themselves "HTTP/1.0" has necessitated a
 protocol version change in order for two communicating applications
 to determine each other's true capabilities.
 This specification defines the protocol referred to as "HTTP/1.1".
 This protocol includes more stringent requirements than HTTP/1.0 in
 order to ensure reliable implementation of its features.
 Practical information systems require more functionality than simple
 retrieval, including search, front-end update, and annotation. HTTP
 allows an open-ended set of methods that indicate the purpose of a
 request. It builds on the discipline of reference provided by the
 Uniform Resource Identifier (URI) [3][20], as a location (URL) [4] or
 name (URN) , for indicating the resource to which a method is to be
 applied. Messages are passed in a format similar to that used by
 Internet mail as defined by the Multipurpose Internet Mail Extensions
 (MIME).
 HTTP is also used as a generic protocol for communication between
 user agents and proxies/gateways to other Internet systems, including
 those supported by the SMTP [16], NNTP [13], FTP [18], Gopher [2],
 and WAIS [10] protocols. In this way, HTTP allows basic hypermedia
 access to resources available from diverse applications.

1.2 Requirements

 This specification uses the same words as RFC 1123 [8] for defining
 the significance of each particular requirement. These words are:
 MUST
    This word or the adjective "required" means that the item is an
    absolute requirement of the specification.

Fielding, et. al. Standards Track [Page 7] RFC 2068 HTTP/1.1 January 1997

 SHOULD
    This word or the adjective "recommended" means that there may
    exist valid reasons in particular circumstances to ignore this
    item, but the full implications should be understood and the case
    carefully weighed before choosing a different course.
 MAY
    This word or the adjective "optional" means that this item is
    truly optional. One vendor may choose to include the item because
    a particular marketplace requires it or because it enhances the
    product, for example; another vendor may omit the same item.
 An implementation is not compliant if it fails to satisfy one or more
 of the MUST requirements for the protocols it implements. An
 implementation that satisfies all the MUST and all the SHOULD
 requirements for its protocols is said to be "unconditionally
 compliant"; one that satisfies all the MUST requirements but not all
 the SHOULD requirements for its protocols is said to be
 "conditionally compliant."

1.3 Terminology

 This specification uses a number of terms to refer to the roles
 played by participants in, and objects of, the HTTP communication.
 connection
    A transport layer virtual circuit established between two programs
    for the purpose of communication.
 message
    The basic unit of HTTP communication, consisting of a structured
    sequence of octets matching the syntax defined in section 4 and
    transmitted via the connection.
 request
    An HTTP request message, as defined in section 5.
 response
    An HTTP response message, as defined in section 6.
 resource
    A network data object or service that can be identified by a URI,
    as defined in section 3.2. Resources may be available in multiple
    representations (e.g. multiple languages, data formats, size,
    resolutions) or vary in other ways.

Fielding, et. al. Standards Track [Page 8] RFC 2068 HTTP/1.1 January 1997

 entity
    The information transferred as the payload of a request or
    response. An entity consists of metainformation in the form of
    entity-header fields and content in the form of an entity-body, as
    described in section 7.
 representation
    An entity included with a response that is subject to content
    negotiation, as described in section 12. There may exist multiple
    representations associated with a particular response status.
 content negotiation
    The mechanism for selecting the appropriate representation when
    servicing a request, as described in section 12. The
    representation of entities in any response can be negotiated
    (including error responses).
 variant
    A resource may have one, or more than one, representation(s)
    associated with it at any given instant. Each of these
    representations is termed a `variant.' Use of the term `variant'
    does not necessarily imply that the resource is subject to content
    negotiation.
 client
    A program that establishes connections for the purpose of sending
    requests.
 user agent
    The client which initiates a request. These are often browsers,
    editors, spiders (web-traversing robots), or other end user tools.
 server
    An application program that accepts connections in order to
    service requests by sending back responses. Any given program may
    be capable of being both a client and a server; our use of these
    terms refers only to the role being performed by the program for a
    particular connection, rather than to the program's capabilities
    in general.  Likewise, any server may act as an origin server,
    proxy, gateway, or tunnel, switching behavior based on the nature
    of each request.
 origin server
    The server on which a given resource resides or is to be created.

Fielding, et. al. Standards Track [Page 9] RFC 2068 HTTP/1.1 January 1997

 proxy
    An intermediary program which acts as both a server and a client
    for the purpose of making requests on behalf of other clients.
    Requests are serviced internally or by passing them on, with
    possible translation, to other servers. A proxy must implement
    both the client and server requirements of this specification.
 gateway
    A server which acts as an intermediary for some other server.
    Unlike a proxy, a gateway receives requests as if it were the
    origin server for the requested resource; the requesting client
    may not be aware that it is communicating with a gateway.
 tunnel
    An intermediary program which is acting as a blind relay between
    two connections. Once active, a tunnel is not considered a party
    to the HTTP communication, though the tunnel may have been
    initiated by an HTTP request. The tunnel ceases to exist when both
    ends of the relayed connections are closed.
 cache
    A program's local store of response messages and the subsystem
    that controls its message storage, retrieval, and deletion. A
    cache stores cachable responses in order to reduce the response
    time and network bandwidth consumption on future, equivalent
    requests. Any client or server may include a cache, though a cache
    cannot be used by a server that is acting as a tunnel.
 cachable
    A response is cachable if a cache is allowed to store a copy of
    the response message for use in answering subsequent requests. The
    rules for determining the cachability of HTTP responses are
    defined in section 13. Even if a resource is cachable, there may
    be additional constraints on whether a cache can use the cached
    copy for a particular request.
 first-hand
    A response is first-hand if it comes directly and without
    unnecessary delay from the origin server, perhaps via one or more
    proxies. A response is also first-hand if its validity has just
    been checked directly with the origin server.
 explicit expiration time
    The time at which the origin server intends that an entity should
    no longer be returned by a cache without further validation.

Fielding, et. al. Standards Track [Page 10] RFC 2068 HTTP/1.1 January 1997

 heuristic expiration time
    An expiration time assigned by a cache when no explicit expiration
    time is available.
 age
    The age of a response is the time since it was sent by, or
    successfully validated with, the origin server.
 freshness lifetime
    The length of time between the generation of a response and its
    expiration time.
 fresh
    A response is fresh if its age has not yet exceeded its freshness
    lifetime.
 stale
    A response is stale if its age has passed its freshness lifetime.
 semantically transparent
    A cache behaves in a "semantically transparent" manner, with
    respect to a particular response, when its use affects neither the
    requesting client nor the origin server, except to improve
    performance. When a cache is semantically transparent, the client
    receives exactly the same response (except for hop-by-hop headers)
    that it would have received had its request been handled directly
    by the origin server.
 validator
    A protocol element (e.g., an entity tag or a Last-Modified time)
    that is used to find out whether a cache entry is an equivalent
    copy of an entity.

1.4 Overall Operation

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

Fielding, et. al. Standards Track [Page 11] RFC 2068 HTTP/1.1 January 1997

 Most HTTP communication is initiated by a user agent and consists of
 a request to be applied to a resource on some origin server. In the
 simplest case, this may be accomplished via a single connection (v)
 between the user agent (UA) and the origin server (O).
           request chain ------------------------>
        UA -------------------v------------------- O
           <----------------------- response chain
 A more complicated situation occurs when one or more intermediaries
 are present in the request/response chain. There are three common
 forms of intermediary: proxy, gateway, and tunnel. A proxy is a
 forwarding agent, receiving requests for a URI in its absolute form,
 rewriting all or part of the message, and forwarding the reformatted
 request toward the server identified by the URI. A gateway is a
 receiving agent, acting as a layer above some other server(s) and, if
 necessary, translating the requests to the underlying server's
 protocol. A tunnel acts as a relay point between two connections
 without changing the messages; tunnels are used when the
 communication needs to pass through an intermediary (such as a
 firewall) even when the intermediary cannot understand the contents
 of the messages.
           request chain -------------------------------------->
        UA -----v----- A -----v----- B -----v----- C -----v----- O
           <------------------------------------- response chain
 The figure above shows three intermediaries (A, B, and C) between the
 user agent and origin server. A request or response message that
 travels the whole chain will pass through four separate connections.
 This distinction is important because some HTTP communication options
 may apply only to the connection with the nearest, non-tunnel
 neighbor, only to the end-points of the chain, or to all connections
 along the chain.  Although the diagram is linear, each participant
 may be engaged in multiple, simultaneous communications. For example,
 B may be receiving requests from many clients other than A, and/or
 forwarding requests to servers other than C, at the same time that it
 is handling A's request.
 Any party to the communication which is not acting as a tunnel may
 employ an internal cache for handling requests. The effect of a cache
 is that the request/response chain is shortened if one of the
 participants along the chain has a cached response applicable to that
 request. The following illustrates the resulting chain if B has a
 cached copy of an earlier response from O (via C) for a request which
 has not been cached by UA or A.

Fielding, et. al. Standards Track [Page 12] RFC 2068 HTTP/1.1 January 1997

           request chain ---------->
        UA -----v----- A -----v----- B - - - - - - C - - - - - - O
           <--------- response chain
 Not all responses are usefully cachable, and some requests may
 contain modifiers which place special requirements on cache behavior.
 HTTP requirements for cache behavior and cachable responses are
 defined in section 13.
 In fact, there are a wide variety of architectures and configurations
 of caches and proxies currently being experimented with or deployed
 across the World Wide Web; these systems include national hierarchies
 of proxy caches to save transoceanic bandwidth, systems that
 broadcast or multicast cache entries, organizations that distribute
 subsets of cached data via CD-ROM, and so on. HTTP systems are used
 in corporate intranets over high-bandwidth links, and for access via
 PDAs with low-power radio links and intermittent connectivity. The
 goal of HTTP/1.1 is to support the wide diversity of configurations
 already deployed while introducing protocol constructs that meet the
 needs of those who build web applications that require high
 reliability and, failing that, at least reliable indications of
 failure.
 HTTP communication usually takes place over TCP/IP connections. The
 default port is TCP 80, but other ports can be used. This does not
 preclude HTTP from being implemented on top of any other protocol on
 the Internet, or on other networks. HTTP only presumes a reliable
 transport; any protocol that provides such guarantees can be used;
 the mapping of the HTTP/1.1 request and response structures onto the
 transport data units of the protocol in question is outside the scope
 of this specification.
 In HTTP/1.0, most implementations used a new connection for each
 request/response exchange. In HTTP/1.1, a connection may be used for
 one or more request/response exchanges, although connections may be
 closed for a variety of reasons (see section 8.1).

2 Notational Conventions and Generic Grammar

2.1 Augmented BNF

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

Fielding, et. al. Standards Track [Page 13] RFC 2068 HTTP/1.1 January 1997

name = definition

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

"literal"

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

rule1 | rule2

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

(rule1 rule2)

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

*rule

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

[rule]

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

N rule

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

#rule

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

Fielding, et. al. Standards Track [Page 14] RFC 2068 HTTP/1.1 January 1997

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

; comment

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

implied *LWS

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

2.2 Basic Rules

 The following rules are used throughout this specification to
 describe basic parsing constructs. The US-ASCII coded character set
 is defined by ANSI X3.4-1986 [21].
        OCTET          = <any 8-bit sequence of data>
        CHAR           = <any US-ASCII character (octets 0 - 127)>
        UPALPHA        = <any US-ASCII uppercase letter "A".."Z">
        LOALPHA        = <any US-ASCII lowercase letter "a".."z">
        ALPHA          = UPALPHA | LOALPHA
        DIGIT          = <any US-ASCII digit "0".."9">
        CTL            = <any US-ASCII control character
                         (octets 0 - 31) and DEL (127)>
        CR             = <US-ASCII CR, carriage return (13)>
        LF             = <US-ASCII LF, linefeed (10)>
        SP             = <US-ASCII SP, space (32)>
        HT             = <US-ASCII HT, horizontal-tab (9)>
        <">            = <US-ASCII double-quote mark (34)>

Fielding, et. al. Standards Track [Page 15] RFC 2068 HTTP/1.1 January 1997

 HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
 protocol elements except the entity-body (see appendix 19.3 for
 tolerant applications). The end-of-line marker within an entity-body
 is defined by its associated media type, as described in section 3.7.
        CRLF           = CR LF
 HTTP/1.1 headers can be folded onto multiple lines if the
 continuation line begins with a space or horizontal tab. All linear
 white space, including folding, has the same semantics as SP.
        LWS            = [CRLF] 1*( SP | HT )
 The TEXT rule is only used for descriptive field contents and values
 that are not intended to be interpreted by the message parser. Words
 of *TEXT may contain characters from character sets other than ISO
 8859-1 [22] only when encoded according to the rules of RFC 1522
 [14].
        TEXT           = <any OCTET except CTLs,
                         but including LWS>
 Hexadecimal numeric characters are used in several protocol elements.
        HEX            = "A" | "B" | "C" | "D" | "E" | "F"
                       | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
 Many HTTP/1.1 header field values consist of words separated by LWS
 or special characters. These special characters MUST be in a quoted
 string to be used within a parameter value.
        token          = 1*<any CHAR except CTLs or tspecials>
        tspecials      = "(" | ")" | "<" | ">" | "@"
                       | "," | ";" | ":" | "\" | <">
                       | "/" | "[" | "]" | "?" | "="
                       | "{" | "}" | SP | HT
 Comments can be included in some HTTP header fields by surrounding
 the comment text with parentheses. Comments are only allowed in
 fields containing "comment" as part of their field value definition.
 In all other fields, parentheses are considered part of the field
 value.
        comment        = "(" *( ctext | comment ) ")"
        ctext          = <any TEXT excluding "(" and ")">

Fielding, et. al. Standards Track [Page 16] RFC 2068 HTTP/1.1 January 1997

 A string of text is parsed as a single word if it is quoted using
 double-quote marks.
        quoted-string  = ( <"> *(qdtext) <"> )
        qdtext         = <any TEXT except <">>
 The backslash character ("\") may be used as a single-character quoting
 mechanism only within quoted-string and comment constructs.
        quoted-pair    = "\" CHAR

3 Protocol Parameters

3.1 HTTP Version

 HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
 of the protocol. The protocol versioning policy is intended to allow
 the sender to indicate the format of a message and its capacity for
 understanding further HTTP communication, rather than the features
 obtained via that communication. No change is made to the version
 number for the addition of message components which do not affect
 communication behavior or which only add to extensible field values.
 The <minor> number is incremented when the changes made to the
 protocol add features which do not change the general message parsing
 algorithm, but which may add to the message semantics and imply
 additional capabilities of the sender. The <major> number is
 incremented when the format of a message within the protocol is
 changed.
 The version of an HTTP message is indicated by an HTTP-Version field
 in the first line of the message.
        HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT
 Note that the major and minor numbers MUST be treated as separate
 integers and that each may be incremented higher than a single digit.
 Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
 lower than HTTP/12.3. Leading zeros MUST be ignored by recipients and
 MUST NOT be sent.
 Applications sending Request or Response messages, as defined by this
 specification, MUST include an HTTP-Version of "HTTP/1.1". Use of
 this version number indicates that the sending application is at
 least conditionally compliant with this specification.
 The HTTP version of an application is the highest HTTP version for
 which the application is at least conditionally compliant.

Fielding, et. al. Standards Track [Page 17] RFC 2068 HTTP/1.1 January 1997

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

3.2 Uniform Resource Identifiers

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

3.2.1 General Syntax

 URIs in HTTP can be represented in absolute form or relative to some
 known base URI, depending upon the context of their use. The two
 forms are differentiated by the fact that absolute URIs always begin
 with a scheme name followed by a colon.
        URI            = ( absoluteURI | relativeURI ) [ "#" fragment ]
        absoluteURI    = scheme ":" *( uchar | reserved )
        relativeURI    = net_path | abs_path | rel_path
        net_path       = "//" net_loc [ abs_path ]
        abs_path       = "/" rel_path
        rel_path       = [ path ] [ ";" params ] [ "?" query ]
        path           = fsegment *( "/" segment )
        fsegment       = 1*pchar
        segment        = *pchar
        params         = param *( ";" param )
        param          = *( pchar | "/" )

Fielding, et. al. Standards Track [Page 18] RFC 2068 HTTP/1.1 January 1997

        scheme         = 1*( ALPHA | DIGIT | "+" | "-" | "." )
        net_loc        = *( pchar | ";" | "?" )
        query          = *( uchar | reserved )
        fragment       = *( uchar | reserved )
        pchar          = uchar | ":" | "@" | "&" | "=" | "+"
        uchar          = unreserved | escape
        unreserved     = ALPHA | DIGIT | safe | extra | national
        escape         = "%" HEX HEX
        reserved       = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
        extra          = "!" | "*" | "'" | "(" | ")" | ","
        safe           = "$" | "-" | "_" | "."
        unsafe         = CTL | SP | <"> | "#" | "%" | "<" | ">"
        national       = <any OCTET excluding ALPHA, DIGIT,
                         reserved, extra, safe, and unsafe>
 For definitive information on URL syntax and semantics, see RFC 1738
 [4] and RFC 1808 [11]. The BNF above includes national characters not
 allowed in valid URLs as specified by RFC 1738, since HTTP servers
 are not restricted in the set of unreserved characters allowed to
 represent the rel_path part of addresses, and HTTP proxies may
 receive requests for URIs not defined by RFC 1738.
 The HTTP protocol does not place any a priori limit on the length of
 a URI. Servers MUST be able to handle the URI of any resource they
 serve, and SHOULD be able to handle URIs of unbounded length if they
 provide GET-based forms that could generate such URIs. A server
 SHOULD return 414 (Request-URI Too Long) status if a URI is longer
 than the server can handle (see section 10.4.15).
   Note: Servers should be cautious about depending on URI lengths
   above 255 bytes, because some older client or proxy implementations
   may not properly support these lengths.

3.2.2 http URL

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

Fielding, et. al. Standards Track [Page 19] RFC 2068 HTTP/1.1 January 1997

        http_URL       = "http:" "//" host [ ":" port ] [ abs_path ]
        host           = <A legal Internet host domain name
                          or IP address (in dotted-decimal form),
                          as defined by Section 2.1 of RFC 1123>
        port           = *DIGIT
 If the port is empty or not given, port 80 is assumed. The semantics
 are that the identified resource is located at the server listening
 for TCP connections on that port of that host, and the Request-URI
 for the resource is abs_path. The use of IP addresses in URL's SHOULD
 be avoided whenever possible (see RFC 1900 [24]). If the abs_path is
 not present in the URL, it MUST be given as "/" when used as a
 Request-URI for a resource (section 5.1.2).

3.2.3 URI Comparison

 When comparing two URIs to decide if they match or not, a client
 SHOULD use a case-sensitive octet-by-octet comparison of the entire
 URIs, with these exceptions:
   o  A port that is empty or not given is equivalent to the default
      port for that URI;
   o  Comparisons of host names MUST be case-insensitive;
   o  Comparisons of scheme names MUST be case-insensitive;
   o  An empty abs_path is equivalent to an abs_path of "/".
 Characters other than those in the "reserved" and "unsafe" sets (see
 section 3.2) are equivalent to their ""%" HEX HEX" encodings.
 For example, the following three URIs are equivalent:
       http://abc.com:80/~smith/home.html
       http://ABC.com/%7Esmith/home.html
       http://ABC.com:/%7esmith/home.html

Fielding, et. al. Standards Track [Page 20] RFC 2068 HTTP/1.1 January 1997

3.3 Date/Time Formats

3.3.1 Full Date

 HTTP applications have historically allowed three different formats
 for the representation of date/time stamps:
        Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
        Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
        Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
 The first format is preferred as an Internet standard and represents
 a fixed-length subset of that defined by RFC 1123  (an update to RFC
 822).  The second format is in common use, but is based on the
 obsolete RFC 850 [12] date format and lacks a four-digit year.
 HTTP/1.1 clients and servers that parse the date value MUST accept
 all three formats (for compatibility with HTTP/1.0), though they MUST
 only generate the RFC 1123 format for representing HTTP-date values
 in header fields.
   Note: Recipients of date values are encouraged to be robust in
   accepting date values that may have been sent by non-HTTP
   applications, as is sometimes the case when retrieving or posting
   messages via proxies/gateways to SMTP or NNTP.
 All HTTP date/time stamps MUST be represented in Greenwich Mean Time
 (GMT), without exception. This is indicated in the first two formats
 by the inclusion of "GMT" as the three-letter abbreviation for time
 zone, and MUST be assumed when reading the asctime format.
        HTTP-date    = rfc1123-date | rfc850-date | asctime-date
        rfc1123-date = wkday "," SP date1 SP time SP "GMT"
        rfc850-date  = weekday "," SP date2 SP time SP "GMT"
        asctime-date = wkday SP date3 SP time SP 4DIGIT
        date1        = 2DIGIT SP month SP 4DIGIT
                       ; day month year (e.g., 02 Jun 1982)
        date2        = 2DIGIT "-" month "-" 2DIGIT
                       ; day-month-year (e.g., 02-Jun-82)
        date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
                       ; month day (e.g., Jun  2)
        time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                       ; 00:00:00 - 23:59:59
        wkday        = "Mon" | "Tue" | "Wed"
                     | "Thu" | "Fri" | "Sat" | "Sun"

Fielding, et. al. Standards Track [Page 21] RFC 2068 HTTP/1.1 January 1997

        weekday      = "Monday" | "Tuesday" | "Wednesday"
                     | "Thursday" | "Friday" | "Saturday" | "Sunday"
        month        = "Jan" | "Feb" | "Mar" | "Apr"
                     | "May" | "Jun" | "Jul" | "Aug"
                     | "Sep" | "Oct" | "Nov" | "Dec"
   Note: HTTP requirements for the date/time stamp format apply only
   to their usage within the protocol stream. Clients and servers are
   not required to use these formats for user presentation, request
   logging, etc.

3.3.2 Delta Seconds

 Some HTTP header fields allow a time value to be specified as an
 integer number of seconds, represented in decimal, after the time
 that the message was received.
        delta-seconds  = 1*DIGIT

3.4 Character Sets

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

Fielding, et. al. Standards Track [Page 22] RFC 2068 HTTP/1.1 January 1997

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

3.5 Content Codings

 Content coding values indicate an encoding transformation that has
 been or can be applied to an entity. Content codings are primarily
 used to allow a document to be compressed or otherwise usefully
 transformed without losing the identity of its underlying media type
 and without loss of information. Frequently, the entity is stored in
 coded form, transmitted directly, and only decoded by the recipient.
        content-coding   = token
 All content-coding values are case-insensitive. HTTP/1.1 uses
 content-coding values in the Accept-Encoding (section 14.3) and
 Content-Encoding (section 14.12) header fields. Although the value
 describes the content-coding, what is more important is that it
 indicates what decoding mechanism will be required to remove the
 encoding.
 The Internet Assigned Numbers Authority (IANA) acts as a registry for
 content-coding value tokens. Initially, the registry contains the
 following tokens:
 gzip An encoding format produced by the file compression program "gzip"
      (GNU zip) as described in RFC 1952 [25]. This format is a Lempel-
      Ziv coding (LZ77) with a 32 bit CRC.
 compress
      The encoding format produced by the common UNIX file compression
      program "compress". This format is an adaptive Lempel-Ziv-Welch
      coding (LZW).

Fielding, et. al. Standards Track [Page 23] RFC 2068 HTTP/1.1 January 1997

   Note: Use of program names for the identification of encoding
   formats is not desirable and should be discouraged for future
   encodings. Their use here is representative of historical practice,
   not good design. For compatibility with previous implementations of
   HTTP, applications should consider "x-gzip" and "x-compress" to be
   equivalent to "gzip" and "compress" respectively.
 deflate The "zlib" format defined in RFC 1950[31] in combination with
      the "deflate" compression mechanism described in RFC 1951[29].
 New content-coding value tokens should be registered; to allow
 interoperability between clients and servers, specifications of the
 content coding algorithms needed to implement a new value should be
 publicly available and adequate for independent implementation, and
 conform to the purpose of content coding defined in this section.

3.6 Transfer Codings

 Transfer coding values are used to indicate an encoding
 transformation that has been, can be, or may need to be applied to an
 entity-body in order to ensure "safe transport" through the network.
 This differs from a content coding in that the transfer coding is a
 property of the message, not of the original entity.
        transfer-coding         = "chunked" | transfer-extension
        transfer-extension      = token
 All transfer-coding values are case-insensitive. HTTP/1.1 uses
 transfer coding values in the Transfer-Encoding header field (section
 14.40).
 Transfer codings are analogous to the Content-Transfer-Encoding
 values of MIME , which were designed to enable safe transport of
 binary data over a 7-bit transport service. However, safe transport
 has a different focus for an 8bit-clean transfer protocol. In HTTP,
 the only unsafe characteristic of message-bodies is the difficulty in
 determining the exact body length (section 7.2.2), or the desire to
 encrypt data over a shared transport.
 The chunked encoding modifies the body of a message in order to
 transfer it as a series of chunks, each with its own size indicator,
 followed by an optional footer containing entity-header fields. This
 allows dynamically-produced content to be transferred along with the
 information necessary for the recipient to verify that it has
 received the full message.

Fielding, et. al. Standards Track [Page 24] RFC 2068 HTTP/1.1 January 1997

     Chunked-Body   = *chunk
                      "0" CRLF
                      footer
                      CRLF
     chunk          = chunk-size [ chunk-ext ] CRLF
                      chunk-data CRLF
     hex-no-zero    = <HEX excluding "0">
     chunk-size     = hex-no-zero *HEX
     chunk-ext      = *( ";" chunk-ext-name [ "=" chunk-ext-value ] )
     chunk-ext-name = token
     chunk-ext-val  = token | quoted-string
     chunk-data     = chunk-size(OCTET)
     footer         = *entity-header
 The chunked encoding is ended by a zero-sized chunk followed by the
 footer, which is terminated by an empty line. The purpose of the
 footer is to provide an efficient way to supply information about an
 entity that is generated dynamically; applications MUST NOT send
 header fields in the footer which are not explicitly defined as being
 appropriate for the footer, such as Content-MD5 or future extensions
 to HTTP for digital signatures or other facilities.
 An example process for decoding a Chunked-Body is presented in
 appendix 19.4.6.
 All HTTP/1.1 applications MUST be able to receive and decode the
 "chunked" transfer coding, and MUST ignore transfer coding extensions
 they do not understand. A server which receives an entity-body with a
 transfer-coding it does not understand SHOULD return 501
 (Unimplemented), and close the connection. A server MUST NOT send
 transfer-codings to an HTTP/1.0 client.

3.7 Media Types

 HTTP uses Internet Media Types  in the Content-Type (section 14.18)
 and Accept (section 14.1) header fields in order to provide open and
 extensible data typing and type negotiation.
        media-type     = type "/" subtype *( ";" parameter )
        type           = token
        subtype        = token
 Parameters may follow the type/subtype in the form of attribute/value
 pairs.

Fielding, et. al. Standards Track [Page 25] RFC 2068 HTTP/1.1 January 1997

        parameter      = attribute "=" value
        attribute      = token
        value          = token | quoted-string
 The type, subtype, and parameter attribute names are case-
 insensitive.  Parameter values may or may not be case-sensitive,
 depending on the semantics of the parameter name. Linear white space
 (LWS) MUST NOT be used between the type and subtype, nor between an
 attribute and its value. User agents that recognize the media-type
 MUST process (or arrange to be processed by any external applications
 used to process that type/subtype by the user agent) the parameters
 for that MIME type as described by that type/subtype definition to
 the and inform the user of any problems discovered.
   Note: some older HTTP applications do not recognize media type
   parameters. When sending data to older HTTP applications,
   implementations should only use media type parameters when they are
   required by that type/subtype definition.
 Media-type values are registered with the Internet Assigned Number
 Authority (IANA). The media type registration process is outlined in
 RFC 2048 [17]. Use of non-registered media types is discouraged.

3.7.1 Canonicalization and Text Defaults

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

Fielding, et. al. Standards Track [Page 26] RFC 2068 HTTP/1.1 January 1997

 The "charset" parameter is used with some media types to define the
 character set (section 3.4) of the data. When no explicit charset
 parameter is provided by the sender, media subtypes of the "text"
 type are defined to have a default charset value of "ISO-8859-1" when
 received via HTTP. Data in character sets other than "ISO-8859-1" or
 its subsets MUST be labeled with an appropriate charset value.
 Some HTTP/1.0 software has interpreted a Content-Type header without
 charset parameter incorrectly to mean "recipient should guess."
 Senders wishing to defeat this behavior MAY include a charset
 parameter even when the charset is ISO-8859-1 and SHOULD do so when
 it is known that it will not confuse the recipient.
 Unfortunately, some older HTTP/1.0 clients did not deal properly with
 an explicit charset parameter. HTTP/1.1 recipients MUST respect the
 charset label provided by the sender; and those user agents that have
 a provision to "guess" a charset MUST use the charset from the
 content-type field if they support that charset, rather than the
 recipient's preference, when initially displaying a document.

3.7.2 Multipart Types

 MIME provides for a number of "multipart" types -- encapsulations of
 one or more entities within a single message-body. All multipart
 types share a common syntax, as defined in  MIME [7], and MUST
 include a boundary parameter as part of the media type value. The
 message body is itself a protocol element and MUST therefore use only
 CRLF to represent line breaks between body-parts. Unlike in MIME, the
 epilogue of any multipart message MUST be empty; HTTP applications
 MUST NOT transmit the epilogue (even if the original multipart
 contains an epilogue).
 In HTTP, multipart body-parts MAY contain header fields which are
 significant to the meaning of that part. A Content-Location header
 field (section 14.15) SHOULD be included in the body-part of each
 enclosed entity that can be identified by a URL.
 In general, an HTTP user agent SHOULD follow the same or similar
 behavior as a MIME user agent would upon receipt of a multipart type.
 If an application receives an unrecognized multipart subtype, the
 application MUST treat it as being equivalent to "multipart/mixed".
   Note: The "multipart/form-data" type has been specifically defined
   for carrying form data suitable for processing via the POST request
   method, as described in RFC 1867 [15].

Fielding, et. al. Standards Track [Page 27] RFC 2068 HTTP/1.1 January 1997

3.8 Product Tokens

 Product tokens are used to allow communicating applications to
 identify themselves by software name and version. Most fields using
 product tokens also allow sub-products which form a significant part
 of the application to be listed, separated by whitespace. By
 convention, the products are listed in order of their significance
 for identifying the application.
        product         = token ["/" product-version]
        product-version = token
 Examples:
        User-Agent: CERN-LineMode/2.15 libwww/2.17b3
        Server: Apache/0.8.4
 Product tokens should be short and to the point -- use of them for
 advertising or other non-essential information is explicitly
 forbidden.  Although any token character may appear in a product-
 version, this token SHOULD only be used for a version identifier
 (i.e., successive versions of the same product SHOULD only differ in
 the product-version portion of the product value).

3.9 Quality Values

 HTTP content negotiation (section 12) uses short "floating point"
 numbers to indicate the relative importance ("weight") of various
 negotiable parameters. A weight is normalized to a real number in the
 range 0 through 1, where 0 is the minimum and 1 the maximum value.
 HTTP/1.1 applications MUST NOT generate more than three digits after
 the decimal point. User configuration of these values SHOULD also be
 limited in this fashion.
        qvalue         = ( "0" [ "." 0*3DIGIT ] )
                       | ( "1" [ "." 0*3("0") ] )
 "Quality values" is a misnomer, since these values merely represent
 relative degradation in desired quality.

3.10 Language Tags

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

Fielding, et. al. Standards Track [Page 28] RFC 2068 HTTP/1.1 January 1997

 The syntax and registry of HTTP language tags is the same as that
 defined by RFC 1766 [1]. In summary, a language tag is composed of 1
 or more parts: A primary language tag and a possibly empty series of
 subtags:
         language-tag  = primary-tag *( "-" subtag )
         primary-tag   = 1*8ALPHA
         subtag        = 1*8ALPHA
 Whitespace is not allowed within the tag and all tags are case-
 insensitive. The name space of language tags is administered by the
 IANA. Example tags include:
        en, en-US, en-cockney, i-cherokee, x-pig-latin
 where any two-letter primary-tag is an ISO 639 language abbreviation
 and any two-letter initial subtag is an ISO 3166 country code. (The
 last three tags above are not registered tags; all but the last are
 examples of tags which could be registered in future.)

3.11 Entity Tags

 Entity tags are used for comparing two or more entities from the same
 requested resource. HTTP/1.1 uses entity tags in the ETag (section
 14.20), If-Match (section 14.25), If-None-Match (section 14.26), and
 If-Range (section 14.27) header fields. The definition of how they
 are used and compared as cache validators is in section 13.3.3. An
 entity tag consists of an opaque quoted string, possibly prefixed by
 a weakness indicator.
       entity-tag = [ weak ] opaque-tag
       weak       = "W/"
       opaque-tag = quoted-string
 A "strong entity tag" may be shared by two entities of a resource
 only if they are equivalent by octet equality.
 A "weak entity tag," indicated by the "W/" prefix, may be shared by
 two entities of a resource only if the entities are equivalent and
 could be substituted for each other with no significant change in
 semantics. A weak entity tag can only be used for weak comparison.
 An entity tag MUST be unique across all versions of all entities
 associated with a particular resource. A given entity tag value may
 be used for entities obtained by requests on different URIs without
 implying anything about the equivalence of those entities.

Fielding, et. al. Standards Track [Page 29] RFC 2068 HTTP/1.1 January 1997

3.12 Range Units

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

The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1

 implementations may ignore ranges specified using other units.
 HTTP/1.1 has been designed to allow implementations of applications
 that do not depend on knowledge of ranges.

4 HTTP Message

4.1 Message Types

 HTTP messages consist of requests from client to server and responses
 from server to client.
        HTTP-message   = Request | Response     ; HTTP/1.1 messages
 Request (section 5) and Response (section 6) messages use the generic
 message format of RFC 822 [9] for transferring entities (the payload
 of the message). Both types of message consist of a start-line, one
 or more header fields (also known as "headers"), an empty line (i.e.,
 a line with nothing preceding the CRLF) indicating the end of the
 header fields, and an optional message-body.
         generic-message = start-line
                           *message-header
                           CRLF
                           [ message-body ]
         start-line      = Request-Line | Status-Line
 In the interest of robustness, servers SHOULD ignore any empty
 line(s) received where a Request-Line is expected. In other words, if
 the server is reading the protocol stream at the beginning of a
 message and receives a CRLF first, it should ignore the CRLF.

Fielding, et. al. Standards Track [Page 30] RFC 2068 HTTP/1.1 January 1997

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

4.2 Message Headers

 HTTP header fields, which include general-header (section 4.5),
 request-header (section 5.3), response-header (section 6.2), and
 entity-header (section 7.1) fields, follow the same generic format as
 that given in Section 3.1 of RFC 822 [9]. Each header field consists
 of a name followed by a colon (":") and the field value. Field names
 are case-insensitive. The field value may be preceded by any amount
 of LWS, though a single SP is preferred. Header fields can be
 extended over multiple lines by preceding each extra line with at
 least one SP or HT.  Applications SHOULD follow "common form" when
 generating HTTP constructs, since there might exist some
 implementations that fail to accept anything beyond the common forms.
        message-header = field-name ":" [ field-value ] CRLF
        field-name     = token
        field-value    = *( field-content | LWS )
        field-content  = <the OCTETs making up the field-value
                         and consisting of either *TEXT or combinations
                         of token, tspecials, and quoted-string>
 The order in which header fields with differing field names are
 received is not significant. However, it is "good practice" to send
 general-header fields first, followed by request-header or response-
 header fields, and ending with the entity-header fields.
 Multiple message-header fields with the same field-name may be
 present in a message if and only if the entire field-value for that
 header field is defined as a comma-separated list [i.e., #(values)].
 It MUST be possible to combine the multiple header fields into one
 "field-name: field-value" pair, without changing the semantics of the
 message, by appending each subsequent field-value to the first, each
 separated by a comma. The order in which header fields with the same
 field-name are received is therefore significant to the
 interpretation of the combined field value, and thus a proxy MUST NOT
 change the order of these field values when a message is forwarded.

Fielding, et. al. Standards Track [Page 31] RFC 2068 HTTP/1.1 January 1997

4.3 Message Body

 The message-body (if any) of an HTTP message is used to carry the
 entity-body associated with the request or response. The message-body
 differs from the entity-body only when a transfer coding has been
 applied, as indicated by the Transfer-Encoding header field (section
 14.40).
        message-body = entity-body
                     | <entity-body encoded as per Transfer-Encoding>
 Transfer-Encoding MUST be used to indicate any transfer codings
 applied by an application to ensure safe and proper transfer of the
 message.  Transfer-Encoding is a property of the message, not of the
 entity, and thus can be added or removed by any application along the
 request/response chain.
 The rules for when a message-body is allowed in a message differ for
 requests and responses.
 The presence of a message-body in a request is signaled by the
 inclusion of a Content-Length or Transfer-Encoding header field in
 the request's message-headers. A message-body MAY be included in a
 request only when the request method (section 5.1.1) allows an
 entity-body.
 For response messages, whether or not a message-body is included with
 a message is dependent on both the request method and the response
 status code (section 6.1.1). All responses to the HEAD request method
 MUST NOT include a message-body, even though the presence of entity-
 header fields might lead one to believe they do. All 1xx
 (informational), 204 (no content), and 304 (not modified) responses
 MUST NOT include a message-body. All other responses do include a
 message-body, although it may be of zero length.

4.4 Message Length

 When a message-body is included with a message, the length of that
 body is determined by one of the following (in order of precedence):
 1. Any response message which MUST NOT include a message-body
   (such as the 1xx, 204, and 304 responses and any response to a HEAD
   request) is always terminated by the first empty line after the
   header fields, regardless of the entity-header fields present in the
   message.
 2. If a Transfer-Encoding header field (section 14.40) is present and
   indicates that the "chunked" transfer coding has been applied, then

Fielding, et. al. Standards Track [Page 32] RFC 2068 HTTP/1.1 January 1997

   the length is defined by the chunked encoding (section 3.6).
 3. If a Content-Length header field (section 14.14) is present, its
   value in bytes represents the length of the message-body.
 4. If the message uses the media type "multipart/byteranges", which is
   self-delimiting, then that defines the length. This media type MUST
   NOT be used unless the sender knows that the recipient can parse it;
   the presence in a request of a Range header with multiple byte-range
   specifiers implies that the client can parse multipart/byteranges
   responses.
 5. By the server closing the connection. (Closing the connection
   cannot be used to indicate the end of a request body, since that
   would leave no possibility for the server to send back a response.)
 For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
 containing a message-body MUST include a valid Content-Length header
 field unless the server is known to be HTTP/1.1 compliant. If a
 request contains a message-body and a Content-Length is not given,
 the server SHOULD respond with 400 (bad request) if it cannot
 determine the length of the message, or with 411 (length required) if
 it wishes to insist on receiving a valid Content-Length.
 All HTTP/1.1 applications that receive entities MUST accept the
 "chunked" transfer coding (section 3.6), thus allowing this mechanism
 to be used for messages when the message length cannot be determined
 in advance.
 Messages MUST NOT include both a Content-Length header field and the
 "chunked" transfer coding. If both are received, the Content-Length
 MUST be ignored.
 When a Content-Length is given in a message where a message-body is
 allowed, its field value MUST exactly match the number of OCTETs in
 the message-body. HTTP/1.1 user agents MUST notify the user when an
 invalid length is received and detected.

Fielding, et. al. Standards Track [Page 33] RFC 2068 HTTP/1.1 January 1997

4.5 General Header Fields

 There are a few header fields which have general applicability for
 both request and response messages, but which do not apply to the
 entity being transferred. These header fields apply only to the
 message being transmitted.
        general-header = Cache-Control            ; Section 14.9
                       | Connection               ; Section 14.10
                       | Date                     ; Section 14.19
                       | Pragma                   ; Section 14.32
                       | Transfer-Encoding        ; Section 14.40
                       | Upgrade                  ; Section 14.41
                       | Via                      ; Section 14.44
 General-header field names can be extended reliably only in
 combination with a change in the protocol version. However, new or
 experimental header fields may be given the semantics of general
 header fields if all parties in the communication recognize them to
 be general-header fields.  Unrecognized header fields are treated as
 entity-header fields.

5 Request

 A request message from a client to a server includes, within the
 first line of that message, the method to be applied to the resource,
 the identifier of the resource, and the protocol version in use.
         Request       = Request-Line              ; Section 5.1
                         *( general-header         ; Section 4.5
                          | request-header         ; Section 5.3
                          | entity-header )        ; Section 7.1
                         CRLF
                         [ message-body ]          ; Section 7.2

5.1 Request-Line

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

Fielding, et. al. Standards Track [Page 34] RFC 2068 HTTP/1.1 January 1997

5.1.1 Method

 The Method token indicates the method to be performed on the resource
 identified by the Request-URI. The method is case-sensitive.
        Method         = "OPTIONS"                ; Section 9.2
                       | "GET"                    ; Section 9.3
                       | "HEAD"                   ; Section 9.4
                       | "POST"                   ; Section 9.5
                       | "PUT"                    ; Section 9.6
                       | "DELETE"                 ; Section 9.7
                       | "TRACE"                  ; Section 9.8
                       | extension-method
        extension-method = token
 The list of methods allowed by a resource can be specified in an
 Allow header field (section 14.7). The return code of the response
 always notifies the client whether a method is currently allowed on a
 resource, since the set of allowed methods can change dynamically.
 Servers SHOULD return the status code 405 (Method Not Allowed) if the
 method is known by the server but not allowed for the requested
 resource, and 501 (Not Implemented) if the method is unrecognized or
 not implemented by the server. The list of methods known by a server
 can be listed in a Public response-header field (section 14.35).
 The methods GET and HEAD MUST be supported by all general-purpose
 servers. All other methods are optional; however, if the above
 methods are implemented, they MUST be implemented with the same
 semantics as those specified in section 9.

5.1.2 Request-URI

 The Request-URI is a Uniform Resource Identifier (section 3.2) and
 identifies the resource upon which to apply the request.
        Request-URI    = "*" | absoluteURI | abs_path
 The three options for Request-URI are dependent on the nature of the
 request. The asterisk "*" means that the request does not apply to a
 particular resource, but to the server itself, and is only allowed
 when the method used does not necessarily apply to a resource. One
 example would be
        OPTIONS * HTTP/1.1
 The absoluteURI form is required when the request is being made to a
 proxy. The proxy is requested to forward the request or service it

Fielding, et. al. Standards Track [Page 35] RFC 2068 HTTP/1.1 January 1997

 from a valid cache, and return the response. Note that the proxy MAY
 forward the request on to another proxy or directly to the server
 specified by the absoluteURI. In order to avoid request loops, a
 proxy MUST be able to recognize all of its server names, including
 any aliases, local variations, and the numeric IP address. An example
 Request-Line would be:
        GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1
 To allow for transition to absoluteURIs in all requests in future
 versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI
 form in requests, even though HTTP/1.1 clients will only generate
 them in requests to proxies.
 The most common form of Request-URI is that used to identify a
 resource on an origin server or gateway. In this case the absolute
 path of the URI MUST be transmitted (see section 3.2.1, abs_path) as
 the Request-URI, and the network location of the URI (net_loc) MUST
 be transmitted in a Host header field. For example, a client wishing
 to retrieve the resource above directly from the origin server would
 create a TCP connection to port 80 of the host "www.w3.org" and send
 the lines:
        GET /pub/WWW/TheProject.html HTTP/1.1
        Host: www.w3.org
 followed by the remainder of the Request. Note that the absolute path
 cannot be empty; if none is present in the original URI, it MUST be
 given as "/" (the server root).
 If a proxy receives a request without any path in the Request-URI and
 the method specified is capable of supporting the asterisk form of
 request, then the last proxy on the request chain MUST forward the
 request with "*" as the final Request-URI. For example, the request
        OPTIONS http://www.ics.uci.edu:8001 HTTP/1.1
 would be forwarded by the proxy as
        OPTIONS * HTTP/1.1
        Host: www.ics.uci.edu:8001
 after connecting to port 8001 of host "www.ics.uci.edu".
 The Request-URI is transmitted in the format specified in section
 3.2.1.  The origin server MUST decode the Request-URI in order to
 properly interpret the request. Servers SHOULD respond to invalid
 Request-URIs with an appropriate status code.

Fielding, et. al. Standards Track [Page 36] RFC 2068 HTTP/1.1 January 1997

 In requests that they forward, proxies MUST NOT rewrite the
 "abs_path" part of a Request-URI in any way except as noted above to
 replace a null abs_path with "*", no matter what the proxy does in
 its internal implementation.
   Note: The "no rewrite" rule prevents the proxy from changing the
   meaning of the request when the origin server is improperly using a
   non-reserved URL character for a reserved purpose. Implementers
   should be aware that some pre-HTTP/1.1 proxies have been known to
   rewrite the Request-URI.

5.2 The Resource Identified by a Request

 HTTP/1.1 origin servers SHOULD be aware that the exact resource
 identified by an Internet request is determined by examining both the
 Request-URI and the Host header field.
 An origin server that does not allow resources to differ by the
 requested host MAY ignore the Host header field value. (But see
 section 19.5.1 for other requirements on Host support in HTTP/1.1.)
 An origin server that does differentiate resources based on the host
 requested (sometimes referred to as virtual hosts or vanity
 hostnames) MUST use the following rules for determining the requested
 resource on an HTTP/1.1 request:
   1. If Request-URI is an absoluteURI, the host is part of the
      Request-URI. Any Host header field value in the request MUST be
      ignored.
   2. If the Request-URI is not an absoluteURI, and the request
      includes a Host header field, the host is determined by the Host
      header field value.
   3. If the host as determined by rule 1 or 2 is not a valid host on
      the server, the response MUST be a 400 (Bad Request) error
      message.
 Recipients of an HTTP/1.0 request that lacks a Host header field MAY
 attempt to use heuristics (e.g., examination of the URI path for
 something unique to a particular host) in order to determine what
 exact resource is being requested.

5.3 Request Header Fields

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

Fielding, et. al. Standards Track [Page 37] RFC 2068 HTTP/1.1 January 1997

 equivalent to the parameters on a programming language method
 invocation.
        request-header = Accept                   ; Section 14.1
                       | Accept-Charset           ; Section 14.2
                       | Accept-Encoding          ; Section 14.3
                       | Accept-Language          ; Section 14.4
                       | Authorization            ; Section 14.8
                       | From                     ; Section 14.22
                       | Host                     ; Section 14.23
                       | If-Modified-Since        ; Section 14.24
                       | If-Match                 ; Section 14.25
                       | If-None-Match            ; Section 14.26
                       | If-Range                 ; Section 14.27
                       | If-Unmodified-Since      ; Section 14.28
                       | Max-Forwards             ; Section 14.31
                       | Proxy-Authorization      ; Section 14.34
                       | Range                    ; Section 14.36
                       | Referer                  ; Section 14.37
                       | User-Agent               ; Section 14.42
 Request-header field names can be extended reliably only in
 combination with a change in the protocol version. However, new or
 experimental header fields MAY be given the semantics of request-
 header fields if all parties in the communication recognize them to
 be request-header fields.  Unrecognized header fields are treated as
 entity-header fields.

6 Response

 After receiving and interpreting a request message, a server responds
 with an HTTP response message.
     Response      = Status-Line               ; Section 6.1
                     *( general-header         ; Section 4.5
                      | response-header        ; Section 6.2
                      | entity-header )        ; Section 7.1
                     CRLF
                     [ message-body ]          ; Section 7.2

6.1 Status-Line

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

Fielding, et. al. Standards Track [Page 38] RFC 2068 HTTP/1.1 January 1997

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

6.1.1 Status Code and Reason Phrase

 The Status-Code element is a 3-digit integer result code of the
 attempt to understand and satisfy the request. These codes are fully
 defined in section 10. The Reason-Phrase is intended to give a short
 textual description of the Status-Code. The Status-Code is intended
 for use by automata and the Reason-Phrase is intended for the human
 user. The client is not required to examine or display the Reason-
 Phrase.
 The first digit of the Status-Code defines the class of response. The
 last two digits do not have any categorization role. There are 5
 values for the first digit:
   o  1xx: Informational - Request received, continuing process
   o  2xx: Success - The action was successfully received, understood,
      and accepted
   o  3xx: Redirection - Further action must be taken in order to
      complete the request
   o  4xx: Client Error - The request contains bad syntax or cannot be
      fulfilled
   o  5xx: Server Error - The server failed to fulfill an apparently
      valid request
 The individual values of the numeric status codes defined for
 HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
 presented below. The reason phrases listed here are only recommended
 -- they may be replaced by local equivalents without affecting the
 protocol.
        Status-Code    = "100"   ; Continue
                       | "101"   ; Switching Protocols
                       | "200"   ; OK
                       | "201"   ; Created
                       | "202"   ; Accepted
                       | "203"   ; Non-Authoritative Information
                       | "204"   ; No Content
                       | "205"   ; Reset Content
                       | "206"   ; Partial Content
                       | "300"   ; Multiple Choices
                       | "301"   ; Moved Permanently
                       | "302"   ; Moved Temporarily

Fielding, et. al. Standards Track [Page 39] RFC 2068 HTTP/1.1 January 1997

                       | "303"   ; See Other
                       | "304"   ; Not Modified
                       | "305"   ; Use Proxy
                       | "400"   ; Bad Request
                       | "401"   ; Unauthorized
                       | "402"   ; Payment Required
                       | "403"   ; Forbidden
                       | "404"   ; Not Found
                       | "405"   ; Method Not Allowed
                       | "406"   ; Not Acceptable
                       | "407"   ; Proxy Authentication Required
                       | "408"   ; Request Time-out
                       | "409"   ; Conflict
                       | "410"   ; Gone
                       | "411"   ; Length Required
                       | "412"   ; Precondition Failed
                       | "413"   ; Request Entity Too Large
                       | "414"   ; Request-URI Too Large
                       | "415"   ; Unsupported Media Type
                       | "500"   ; Internal Server Error
                       | "501"   ; Not Implemented
                       | "502"   ; Bad Gateway
                       | "503"   ; Service Unavailable
                       | "504"   ; Gateway Time-out
                       | "505"   ; HTTP Version not supported
                       | extension-code
        extension-code = 3DIGIT
        Reason-Phrase  = *<TEXT, excluding CR, LF>
 HTTP status codes are extensible. HTTP applications are not required
 to understand the meaning of all registered status codes, though such
 understanding is obviously desirable. However, applications MUST
 understand the class of any status code, as indicated by the first
 digit, and treat any unrecognized response as being equivalent to the
 x00 status code of that class, with the exception that an
 unrecognized response MUST NOT be cached. For example, if an
 unrecognized status code of 431 is received by the client, it can
 safely assume that there was something wrong with its request and
 treat the response as if it had received a 400 status code. In such
 cases, user agents SHOULD present to the user the entity returned
 with the response, since that entity is likely to include human-
 readable information which will explain the unusual status.

Fielding, et. al. Standards Track [Page 40] RFC 2068 HTTP/1.1 January 1997

6.2 Response Header Fields

 The response-header fields allow the server to pass additional
 information about the response which cannot be placed in the Status-
 Line. These header fields give information about the server and about
 further access to the resource identified by the Request-URI.
        response-header = Age                     ; Section 14.6
                        | Location                ; Section 14.30
                        | Proxy-Authenticate      ; Section 14.33
                        | Public                  ; Section 14.35
                        | Retry-After             ; Section 14.38
                        | Server                  ; Section 14.39
                        | Vary                    ; Section 14.43
                        | Warning                 ; Section 14.45
                        | WWW-Authenticate        ; Section 14.46
 Response-header field names can be extended reliably only in
 combination with a change in the protocol version. However, new or
 experimental header fields MAY be given the semantics of response-
 header fields if all parties in the communication recognize them to
 be response-header fields. Unrecognized header fields are treated as
 entity-header fields.

7 Entity

 Request and Response messages MAY transfer an entity if not otherwise
 restricted by the request method or response status code. An entity
 consists of entity-header fields and an entity-body, although some
 responses will only include the entity-headers.
 In this section, both sender and recipient refer to either the client
 or the server, depending on who sends and who receives the entity.

7.1 Entity Header Fields

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

Fielding, et. al. Standards Track [Page 41] RFC 2068 HTTP/1.1 January 1997

        entity-header  = Allow                    ; Section 14.7
                       | Content-Base             ; Section 14.11
                       | Content-Encoding         ; Section 14.12
                       | Content-Language         ; Section 14.13
                       | Content-Length           ; Section 14.14
                       | Content-Location         ; Section 14.15
                       | Content-MD5              ; Section 14.16
                       | Content-Range            ; Section 14.17
                       | Content-Type             ; Section 14.18
                       | ETag                     ; Section 14.20
                       | Expires                  ; Section 14.21
                       | Last-Modified            ; Section 14.29
                       | extension-header
        extension-header = message-header
 The extension-header mechanism allows additional entity-header fields
 to be defined without changing the protocol, but these fields cannot
 be assumed to be recognizable by the recipient. Unrecognized header
 fields SHOULD be ignored by the recipient and forwarded by proxies.

7.2 Entity Body

 The entity-body (if any) sent with an HTTP request or response is in
 a format and encoding defined by the entity-header fields.
        entity-body    = *OCTET
 An entity-body is only present in a message when a message-body is
 present, as described in section 4.3. The entity-body is obtained
 from the message-body by decoding any Transfer-Encoding that may have
 been applied to ensure safe and proper transfer of the message.

7.2.1 Type

 When an entity-body is included with a message, the data type of that
 body is determined via the header fields Content-Type and Content-
 Encoding. These define a two-layer, ordered encoding model:
        entity-body := Content-Encoding( Content-Type( data ) )
 Content-Type specifies the media type of the underlying data.
 Content-Encoding may be used to indicate any additional content
 codings applied to the data, usually for the purpose of data
 compression, that are a property of the requested resource. There is
 no default encoding.

Fielding, et. al. Standards Track [Page 42] RFC 2068 HTTP/1.1 January 1997

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

7.2.2 Length

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

8 Connections

8.1 Persistent Connections

8.1.1 Purpose

 Prior to persistent connections, a separate TCP connection was
 established to fetch each URL, increasing the load on HTTP servers
 and causing congestion on the Internet. The use of inline images and
 other associated data often requires a client to make multiple
 requests of the same server in a short amount of time. Analyses of
 these performance problems are available [30][27]; analysis and
 results from a prototype implementation are in [26].
 Persistent HTTP connections have a number of advantages:
   o  By opening and closing fewer TCP connections, CPU time is saved,
      and memory used for TCP protocol control blocks is also saved.
   o  HTTP requests and responses can be pipelined on a connection.
      Pipelining allows a client to make multiple requests without
      waiting for each response, allowing a single TCP connection to be
      used much more efficiently, with much lower elapsed time.
   o  Network congestion is reduced by reducing the number of packets
      caused by TCP opens, and by allowing TCP sufficient time to
      determine the congestion state of the network.
   o  HTTP can evolve more gracefully; since errors can be reported
      without the penalty of closing the TCP connection. Clients using
      future versions of HTTP might optimistically try a new feature, but
      if communicating with an older server, retry with old semantics
      after an error is reported.
 HTTP implementations SHOULD implement persistent connections.

Fielding, et. al. Standards Track [Page 43] RFC 2068 HTTP/1.1 January 1997

8.1.2 Overall Operation

 A significant difference between HTTP/1.1 and earlier versions of
 HTTP is that persistent connections are the default behavior of any
 HTTP connection. That is, unless otherwise indicated, the client may
 assume that the server will maintain a persistent connection.
 Persistent connections provide a mechanism by which a client and a
 server can signal the close of a TCP connection. This signaling takes
 place using the Connection header field. Once a close has been
 signaled, the client MUST not send any more requests on that
 connection.

8.1.2.1 Negotiation

 An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to
 maintain a persistent connection unless a Connection header including
 the connection-token "close" was sent in the request. If the server
 chooses to close the connection immediately after sending the
 response, it SHOULD send a Connection header including the
 connection-token close.
 An HTTP/1.1 client MAY expect a connection to remain open, but would
 decide to keep it open based on whether the response from a server
 contains a Connection header with the connection-token close. In case
 the client does not want to maintain a connection for more than that
 request, it SHOULD send a Connection header including the
 connection-token close.
 If either the client or the server sends the close token in the
 Connection header, that request becomes the last one for the
 connection.
 Clients and servers SHOULD NOT assume that a persistent connection is
 maintained for HTTP versions less than 1.1 unless it is explicitly
 signaled. See section 19.7.1 for more information on backwards
 compatibility with HTTP/1.0 clients.
 In order to remain persistent, all messages on the connection must
 have a self-defined message length (i.e., one not defined by closure
 of the connection), as described in section 4.4.

8.1.2.2 Pipelining

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

Fielding, et. al. Standards Track [Page 44] RFC 2068 HTTP/1.1 January 1997

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

8.1.3 Proxy Servers

 It is especially important that proxies correctly implement the
 properties of the Connection header field as specified in 14.2.1.
 The proxy server MUST signal persistent connections separately with
 its clients and the origin servers (or other proxy servers) that it
 connects to. Each persistent connection applies to only one transport
 link.
 A proxy server MUST NOT establish a persistent connection with an
 HTTP/1.0 client.

8.1.4 Practical Considerations

 Servers will usually have some time-out value beyond which they will
 no longer maintain an inactive connection. Proxy servers might make
 this a higher value since it is likely that the client will be making
 more connections through the same server. The use of persistent
 connections places no requirements on the length of this time-out for
 either the client or the server.
 When a client or server wishes to time-out it SHOULD issue a graceful
 close on the transport connection. Clients and servers SHOULD both
 constantly watch for the other side of the transport close, and
 respond to it as appropriate. If a client or server does not detect
 the other side's close promptly it could cause unnecessary resource
 drain on the network.
 A client, server, or proxy MAY close the transport connection at any
 time. For example, a client MAY have started to send a new request at
 the same time that the server has decided to close the "idle"
 connection. From the server's point of view, the connection is being
 closed while it was idle, but from the client's point of view, a
 request is in progress.
 This means that clients, servers, and proxies MUST be able to recover
 from asynchronous close events. Client software SHOULD reopen the
 transport connection and retransmit the aborted request without user
 interaction so long as the request method is idempotent (see section

Fielding, et. al. Standards Track [Page 45] RFC 2068 HTTP/1.1 January 1997

 9.1.2); other methods MUST NOT be automatically retried, although
 user agents MAY offer a human operator the choice of retrying the
 request.
 However, this automatic retry SHOULD NOT be repeated if the second
 request fails.
 Servers SHOULD always respond to at least one request per connection,
 if at all possible. Servers SHOULD NOT close a connection in the
 middle of transmitting a response, unless a network or client failure
 is suspected.
 Clients that use persistent connections SHOULD limit the number of
 simultaneous connections that they maintain to a given server. A
 single-user client SHOULD maintain AT MOST 2 connections with any
 server or proxy. A proxy SHOULD use up to 2*N connections to another
 server or proxy, where N is the number of simultaneously active
 users. These guidelines are intended to improve HTTP response times
 and avoid congestion of the Internet or other networks.

8.2 Message Transmission Requirements

General requirements:

o HTTP/1.1 servers SHOULD maintain persistent connections and use

 TCP's flow control mechanisms to resolve temporary overloads,
 rather than terminating connections with the expectation that
 clients will retry. The latter technique can exacerbate network
 congestion.

o An HTTP/1.1 (or later) client sending a message-body SHOULD monitor

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

o An HTTP/1.1 (or later) client MUST be prepared to accept a 100

 (Continue) status followed by a regular response.

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

 HTTP/1.0 (or earlier) client MUST NOT transmit the 100 (continue)
 response; it SHOULD either wait for the request to be completed
 normally (thus avoiding an interrupted request) or close the
 connection prematurely.

Fielding, et. al. Standards Track [Page 46] RFC 2068 HTTP/1.1 January 1997

 Upon receiving a method subject to these requirements from an
 HTTP/1.1 (or later) client, an HTTP/1.1 (or later) server MUST either
 respond with 100 (Continue) status and continue to read from the
 input stream, or respond with an error status. If it responds with an
 error status, it MAY close the transport (TCP) connection or it MAY
 continue to read and discard the rest of the request. It MUST NOT
 perform the requested method if it returns an error status.
 Clients SHOULD remember the version number of at least the most
 recently used server; if an HTTP/1.1 client has seen an HTTP/1.1 or
 later response from the server, and it sees the connection close
 before receiving any status from the server, the client SHOULD retry
 the request without user interaction so long as the request method is
 idempotent (see section 9.1.2); other methods MUST NOT be
 automatically retried, although user agents MAY offer a human
 operator the choice of retrying the request.. If the client does
 retry the request, the client
   o  MUST first send the request header fields, and then
   o  MUST wait for the server to respond with either a 100 (Continue)
      response, in which case the client should continue, or with an
      error status.
 If an HTTP/1.1 client has not seen an HTTP/1.1 or later response from
 the server, it should assume that the server implements HTTP/1.0 or
 older and will not use the 100 (Continue) response. If in this case
 the client sees the connection close before receiving any status from
 the server, the client SHOULD retry the request. If the client does
 retry the request to this HTTP/1.0 server, it should use the
 following "binary exponential backoff" algorithm to be assured of
 obtaining a reliable response:
1. Initiate a new connection to the server
2. Transmit the request-headers
3. Initialize a variable R to the estimated round-trip time to the
   server (e.g., based on the time it took to establish the
   connection), or to a constant value of 5 seconds if the round-trip
   time is not available.
4. Compute T = R * (2**N), where N is the number of previous retries
   of this request.
5. Wait either for an error response from the server, or for T seconds
   (whichever comes first)

Fielding, et. al. Standards Track [Page 47] RFC 2068 HTTP/1.1 January 1997

6. If no error response is received, after T seconds transmit the body
   of the request.
7. If client sees that the connection is closed prematurely, repeat
   from step 1 until the request is accepted, an error response is
   received, or the user becomes impatient and terminates the retry
   process.
 No matter what the server version, if an error status is received,
 the client
o  MUST NOT continue and
o  MUST close the connection if it has not completed sending the
   message.
 An HTTP/1.1 (or later) client that sees the connection close after
 receiving a 100 (Continue) but before receiving any other status
 SHOULD retry the request, and need not wait for 100 (Continue)
 response (but MAY do so if this simplifies the implementation).

9 Method Definitions

 The set of common methods for HTTP/1.1 is defined below. Although
 this set can be expanded, additional methods cannot be assumed to
 share the same semantics for separately extended clients and servers.
 The Host request-header field (section 14.23) MUST accompany all
 HTTP/1.1 requests.

9.1 Safe and Idempotent Methods

9.1.1 Safe Methods

 Implementers should be aware that the software represents the user in
 their interactions over the Internet, and should be careful to allow
 the user to be aware of any actions they may take which may have an
 unexpected significance to themselves or others.
 In particular, the convention has been established that the GET and
 HEAD methods should never have the significance of taking an action
 other than retrieval. These methods should be considered "safe." This
 allows user agents to represent other methods, such as POST, PUT and
 DELETE, in a special way, so that the user is made aware of the fact
 that a possibly unsafe action is being requested.
 Naturally, it is not possible to ensure that the server does not
 generate side-effects as a result of performing a GET request; in

Fielding, et. al. Standards Track [Page 48] RFC 2068 HTTP/1.1 January 1997

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

9.1.2 Idempotent Methods

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

9.2 OPTIONS

 The OPTIONS method represents a request for information about the
 communication options available on the request/response chain
 identified by the Request-URI. This method allows the client to
 determine the options and/or requirements associated with a resource,
 or the capabilities of a server, without implying a resource action
 or initiating a resource retrieval.
 Unless the server's response is an error, the response MUST NOT
 include entity information other than what can be considered as
 communication options (e.g., Allow is appropriate, but Content-Type
 is not). Responses to this method are not cachable.
 If the Request-URI is an asterisk ("*"), the OPTIONS request is
 intended to apply to the server as a whole. A 200 response SHOULD
 include any header fields which indicate optional features
 implemented by the server (e.g., Public), including any extensions
 not defined by this specification, in addition to any applicable
 general or response-header fields. As described in section 5.1.2, an
 "OPTIONS *" request can be applied through a proxy by specifying the
 destination server in the Request-URI without any path information.
 If the Request-URI is not an asterisk, the OPTIONS request applies
 only to the options that are available when communicating with that
 resource.  A 200 response SHOULD include any header fields which
 indicate optional features implemented by the server and applicable
 to that resource (e.g., Allow), including any extensions not defined
 by this specification, in addition to any applicable general or
 response-header fields. If the OPTIONS request passes through a
 proxy, the proxy MUST edit the response to exclude those options
 which apply to a proxy's capabilities and which are known to be
 unavailable through that proxy.

Fielding, et. al. Standards Track [Page 49] RFC 2068 HTTP/1.1 January 1997

9.3 GET

 The GET method means retrieve whatever information (in the form of an
 entity) is identified by the Request-URI. If the Request-URI refers
 to a data-producing process, it is the produced data which shall be
 returned as the entity in the response and not the source text of the
 process, unless that text happens to be the output of the process.
 The semantics of the GET method change to a "conditional GET" if the
 request message includes an If-Modified-Since, If-Unmodified-Since,
 If-Match, If-None-Match, or If-Range header field. A conditional GET
 method requests that the entity be transferred only under the
 circumstances described by the conditional header field(s). The
 conditional GET method is intended to reduce unnecessary network
 usage by allowing cached entities to be refreshed without requiring
 multiple requests or transferring data already held by the client.
 The semantics of the GET method change to a "partial GET" if the
 request message includes a Range header field. A partial GET requests
 that only part of the entity be transferred, as described in section
 14.36. The partial GET method is intended to reduce unnecessary
 network usage by allowing partially-retrieved entities to be
 completed without transferring data already held by the client.
 The response to a GET request is cachable if and only if it meets the
 requirements for HTTP caching described in section 13.

9.4 HEAD

 The HEAD method is identical to GET except that the server MUST NOT
 return a message-body in the response. The metainformation contained
 in the HTTP headers in response to a HEAD request SHOULD be identical
 to the information sent in response to a GET request. This method can
 be used for obtaining metainformation about the entity implied by the
 request without transferring the entity-body itself. This method is
 often used for testing hypertext links for validity, accessibility,
 and recent modification.
 The response to a HEAD request may be cachable in the sense that the
 information contained in the response may be used to update a
 previously cached entity from that resource. If the new field values
 indicate that the cached entity differs from the current entity (as
 would be indicated by a change in Content-Length, Content-MD5, ETag
 or Last-Modified), then the cache MUST treat the cache entry as
 stale.

Fielding, et. al. Standards Track [Page 50] RFC 2068 HTTP/1.1 January 1997

9.5 POST

 The POST method is used to request that the destination server accept
 the entity enclosed in the request as a new subordinate of the
 resource identified by the Request-URI in the Request-Line. POST is
 designed to allow a uniform method to cover the following functions:
   o  Annotation of existing resources;
   o  Posting a message to a bulletin board, newsgroup, mailing list,
      or similar group of articles;
   o  Providing a block of data, such as the result of submitting a
      form, to a data-handling process;
   o  Extending a database through an append operation.
 The actual function performed by the POST method is determined by the
 server and is usually dependent on the Request-URI. The posted entity
 is subordinate to that URI in the same way that a file is subordinate
 to a directory containing it, a news article is subordinate to a
 newsgroup to which it is posted, or a record is subordinate to a
 database.
 The action performed by the POST method might not result in a
 resource that can be identified by a URI. In this case, either 200
 (OK) or 204 (No Content) is the appropriate response status,
 depending on whether or not the response includes an entity that
 describes the result.
 If a resource has been created on the origin server, the response
 SHOULD be 201 (Created) and contain an entity which describes the
 status of the request and refers to the new resource, and a Location
 header (see section 14.30).
 Responses to this method are not cachable, unless the response
 includes appropriate Cache-Control or Expires header fields. However,
 the 303 (See Other) response can be used to direct the user agent to
 retrieve a cachable resource.
 POST requests must obey the message transmission requirements set out
 in section 8.2.

Fielding, et. al. Standards Track [Page 51] RFC 2068 HTTP/1.1 January 1997

9.6 PUT

 The PUT method requests that the enclosed entity be stored under the
 supplied Request-URI. If the Request-URI refers to an already
 existing resource, the enclosed entity SHOULD be considered as a
 modified version of the one residing on the origin server. If the
 Request-URI does not point to an existing resource, and that URI is
 capable of being defined as a new resource by the requesting user
 agent, the origin server can create the resource with that URI. If a
 new resource is created, the origin server MUST inform the user agent
 via the 201 (Created) response.  If an existing resource is modified,
 either the 200 (OK) or 204 (No Content) response codes SHOULD be sent
 to indicate successful completion of the request. If the resource
 could not be created or modified with the Request-URI, an appropriate
 error response SHOULD be given that reflects the nature of the
 problem. The recipient of the entity MUST NOT ignore any Content-*
 (e.g. Content-Range) headers that it does not understand or implement
 and MUST return a 501 (Not Implemented) response in such cases.
 If the request passes through a cache and the Request-URI identifies
 one or more currently cached entities, those entries should be
 treated as stale. Responses to this method are not cachable.
 The fundamental difference between the POST and PUT requests is
 reflected in the different meaning of the Request-URI. The URI in a
 POST request identifies the resource that will handle the enclosed
 entity.  That resource may be a data-accepting process, a gateway to
 some other protocol, or a separate entity that accepts annotations.
 In contrast, the URI in a PUT request identifies the entity enclosed
 with the request -- the user agent knows what URI is intended and the
 server MUST NOT attempt to apply the request to some other resource.
 If the server desires that the request be applied to a different URI,
 it MUST send a 301 (Moved Permanently) response; the user agent MAY
 then make its own decision regarding whether or not to redirect the
 request.
 A single resource MAY be identified by many different URIs. For
 example, an article may have a URI for identifying "the current
 version" which is separate from the URI identifying each particular
 version. In this case, a PUT request on a general URI may result in
 several other URIs being defined by the origin server.
 HTTP/1.1 does not define how a PUT method affects the state of an
 origin server.
 PUT requests must obey the message transmission requirements set out
 in section 8.2.

Fielding, et. al. Standards Track [Page 52] RFC 2068 HTTP/1.1 January 1997

9.7 DELETE

 The DELETE method requests that the origin server delete the resource
 identified by the Request-URI. This method MAY be overridden by human
 intervention (or other means) on the origin server. The client cannot
 be guaranteed that the operation has been carried out, even if the
 status code returned from the origin server indicates that the action
 has been completed successfully. However, the server SHOULD not
 indicate success unless, at the time the response is given, it
 intends to delete the resource or move it to an inaccessible
 location.
 A successful response SHOULD be 200 (OK) if the response includes an
 entity describing the status, 202 (Accepted) if the action has not
 yet been enacted, or 204 (No Content) if the response is OK but does
 not include an entity.
 If the request passes through a cache and the Request-URI identifies
 one or more currently cached entities, those entries should be
 treated as stale. Responses to this method are not cachable.

9.8 TRACE

 The TRACE method is used to invoke a remote, application-layer loop-
 back of the request message. The final recipient of the request
 SHOULD reflect the message received back to the client as the
 entity-body of a 200 (OK) response. The final recipient is either the
 origin server or the first proxy or gateway to receive a Max-Forwards
 value of zero (0) in the request (see section 14.31). A TRACE request
 MUST NOT include an entity.
 TRACE allows the client to see what is being received at the other
 end of the request chain and use that data for testing or diagnostic
 information. The value of the Via header field (section 14.44) is of
 particular interest, since it acts as a trace of the request chain.
 Use of the Max-Forwards header field allows the client to limit the
 length of the request chain, which is useful for testing a chain of
 proxies forwarding messages in an infinite loop.
 If successful, the response SHOULD contain the entire request message
 in the entity-body, with a Content-Type of "message/http". Responses
 to this method MUST NOT be cached.

10 Status Code Definitions

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

Fielding, et. al. Standards Track [Page 53] RFC 2068 HTTP/1.1 January 1997

10.1 Informational 1xx

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

10.1.1 100 Continue

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

10.1.2 101 Switching Protocols

 The server understands and is willing to comply with the client's
 request, via the Upgrade message header field (section 14.41), for a
 change in the application protocol being used on this connection. The
 server will switch protocols to those defined by the response's
 Upgrade header field immediately after the empty line which
 terminates the 101 response.
 The protocol should only be switched when it is advantageous to do
 so.  For example, switching to a newer version of HTTP is
 advantageous over older versions, and switching to a real-time,
 synchronous protocol may be advantageous when delivering resources
 that use such features.

10.2 Successful 2xx

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

10.2.1 200 OK

 The request has succeeded. The information returned with the response
 is dependent on the method used in the request, for example:
 GET  an entity corresponding to the requested resource is sent in the
      response;
 HEAD the entity-header fields corresponding to the requested resource
      are sent in the response without any message-body;

Fielding, et. al. Standards Track [Page 54] RFC 2068 HTTP/1.1 January 1997

 POST an entity describing or containing the result of the action;
 TRACE an entity containing the request message as received by the end
      server.

10.2.2 201 Created

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

10.2.3 202 Accepted

 The request has been accepted for processing, but the processing has
 not been completed. The request MAY or MAY NOT eventually be acted
 upon, as it MAY be disallowed when processing actually takes place.
 There is no facility for re-sending a status code from an
 asynchronous operation such as this.
 The 202 response is intentionally non-committal. Its purpose is to
 allow a server to accept a request for some other process (perhaps a
 batch-oriented process that is only run once per day) without
 requiring that the user agent's connection to the server persist
 until the process is completed. The entity returned with this
 response SHOULD include an indication of the request's current status
 and either a pointer to a status monitor or some estimate of when the
 user can expect the request to be fulfilled.

10.2.4 203 Non-Authoritative Information

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

10.2.5 204 No Content

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

Fielding, et. al. Standards Track [Page 55] RFC 2068 HTTP/1.1 January 1997

 response is primarily intended to allow input for actions to take
 place without causing a change to the user agent's active document
 view. The response MAY include new metainformation in the form of
 entity-headers, which SHOULD apply to the document currently in the
 user agent's active view.
 The 204 response MUST NOT include a message-body, and thus is always
 terminated by the first empty line after the header fields.

10.2.6 205 Reset Content

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

10.2.7 206 Partial Content

 The server has fulfilled the partial GET request for the resource.
 The request must have included a Range header field (section 14.36)
 indicating the desired range. The response MUST include either a
 Content-Range header field (section 14.17) indicating the range
 included with this response, or a multipart/byteranges Content-Type
 including Content-Range fields for each part. If multipart/byteranges
 is not used, the Content-Length header field in the response MUST
 match the actual number of OCTETs transmitted in the message-body.
 A cache that does not support the Range and Content-Range headers
 MUST NOT cache 206 (Partial) responses.

10.3 Redirection 3xx

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

Fielding, et. al. Standards Track [Page 56] RFC 2068 HTTP/1.1 January 1997

10.3.1 300 Multiple Choices

 The requested resource corresponds to any one of a set of
 representations, each with its own specific location, and agent-
 driven negotiation information (section 12) is being provided so that
 the user (or user agent) can select a preferred representation and
 redirect its request to that location.
 Unless it was a HEAD request, the response SHOULD include an entity
 containing a list of resource characteristics and location(s) from
 which the user or user agent can choose the one most appropriate. The
 entity format is specified by the media type given in the Content-
 Type header field. Depending upon the format and the capabilities of
 the user agent, selection of the most appropriate choice may be
 performed automatically.  However, this specification does not define
 any standard for such automatic selection.
 If the server has a preferred choice of representation, it SHOULD
 include the specific URL for that representation in the Location
 field; user agents MAY use the Location field value for automatic
 redirection.  This response is cachable unless indicated otherwise.

10.3.2 301 Moved Permanently

 The requested resource has been assigned a new permanent URI and any
 future references to this resource SHOULD be done using one of the
 returned URIs. Clients with link editing capabilities SHOULD
 automatically re-link references to the Request-URI to one or more of
 the new references returned by the server, where possible. This
 response is cachable unless indicated otherwise.
 If the new URI is a location, its URL SHOULD be given by the Location
 field in the response. Unless the request method was HEAD, the entity
 of the response SHOULD contain a short hypertext note with a
 hyperlink to the new URI(s).
 If the 301 status code is received in response to a request other
 than GET or HEAD, the user agent MUST NOT automatically redirect the
 request unless it can be confirmed by the user, since this might
 change the conditions under which the request was issued.
   Note: When automatically redirecting a POST request after receiving
   a 301 status code, some existing HTTP/1.0 user agents will
   erroneously change it into a GET request.

Fielding, et. al. Standards Track [Page 57] RFC 2068 HTTP/1.1 January 1997

10.3.3 302 Moved Temporarily

 The requested resource resides temporarily under a different URI.
 Since the redirection may be altered on occasion, the client SHOULD
 continue to use the Request-URI for future requests. This response is
 only cachable if indicated by a Cache-Control or Expires header
 field.
 If the new URI is a location, its URL SHOULD be given by the Location
 field in the response. Unless the request method was HEAD, the entity
 of the response SHOULD contain a short hypertext note with a
 hyperlink to the new URI(s).
 If the 302 status code is received in response to a request other
 than GET or HEAD, the user agent MUST NOT automatically redirect the
 request unless it can be confirmed by the user, since this might
 change the conditions under which the request was issued.
   Note: When automatically redirecting a POST request after receiving
   a 302 status code, some existing HTTP/1.0 user agents will
   erroneously change it into a GET request.

10.3.4 303 See Other

 The response to the request can be found under a different URI and
 SHOULD be retrieved using a GET method on that resource. This method
 exists primarily to allow the output of a POST-activated script to
 redirect the user agent to a selected resource. The new URI is not a
 substitute reference for the originally requested resource. The 303
 response is not cachable, but the response to the second (redirected)
 request MAY be cachable.
 If the new URI is a location, its URL SHOULD be given by the Location
 field in the response. Unless the request method was HEAD, the entity
 of the response SHOULD contain a short hypertext note with a
 hyperlink to the new URI(s).

10.3.5 304 Not Modified

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

Fielding, et. al. Standards Track [Page 58] RFC 2068 HTTP/1.1 January 1997

 The response MUST include the following header fields:
o  Date
o  ETag and/or Content-Location, if the header would have been sent in
   a 200 response to the same request
o  Expires, Cache-Control, and/or Vary, if the field-value might
   differ from that sent in any previous response for the same variant
 If the conditional GET used a strong cache validator (see section
 13.3.3), the response SHOULD NOT include other entity-headers.
 Otherwise (i.e., the conditional GET used a weak validator), the
 response MUST NOT include other entity-headers; this prevents
 inconsistencies between cached entity-bodies and updated headers.
 If a 304 response indicates an entity not currently cached, then the
 cache MUST disregard the response and repeat the request without the
 conditional.
 If a cache uses a received 304 response to update a cache entry, the
 cache MUST update the entry to reflect any new field values given in
 the response.
 The 304 response MUST NOT include a message-body, and thus is always
 terminated by the first empty line after the header fields.

10.3.6 305 Use Proxy

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

10.4 Client Error 4xx

 The 4xx class of status code is intended for cases in which the
 client seems to have erred. Except when responding to a HEAD request,
 the server SHOULD include an entity containing an explanation of the
 error situation, and whether it is a temporary or permanent
 condition. These status codes are applicable to any request method.
 User agents SHOULD display any included entity to the user.
   Note: If the client is sending data, a server implementation using
   TCP should be careful to ensure that the client acknowledges
   receipt of the packet(s) containing the response, before the server
   closes the input connection. If the client continues sending data
   to the server after the close, the server's TCP stack will send a
   reset packet to the client, which may erase the client's

Fielding, et. al. Standards Track [Page 59] RFC 2068 HTTP/1.1 January 1997

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

10.4.1 400 Bad Request

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

10.4.2 401 Unauthorized

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

10.4.3 402 Payment Required

 This code is reserved for future use.

10.4.4 403 Forbidden

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

10.4.5 404 Not Found

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

Fielding, et. al. Standards Track [Page 60] RFC 2068 HTTP/1.1 January 1997

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

10.4.6 405 Method Not Allowed

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

10.4.7 406 Not Acceptable

 The resource identified by the request is only capable of generating
 response entities which have content characteristics not acceptable
 according to the accept headers sent in the request.
 Unless it was a HEAD request, the response SHOULD include an entity
 containing a list of available entity characteristics and location(s)
 from which the user or user agent can choose the one most
 appropriate.  The entity format is specified by the media type given
 in the Content-Type header field. Depending upon the format and the
 capabilities of the user agent, selection of the most appropriate
 choice may be performed automatically. However, this specification
 does not define any standard for such automatic selection.
   Note: HTTP/1.1 servers are allowed to return responses which are
   not acceptable according to the accept headers sent in the request.
   In some cases, this may even be preferable to sending a 406
   response. User agents are encouraged to inspect the headers of an
   incoming response to determine if it is acceptable. If the response
   could be unacceptable, a user agent SHOULD temporarily stop receipt
   of more data and query the user for a decision on further actions.

10.4.8 407 Proxy Authentication Required

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

Fielding, et. al. Standards Track [Page 61] RFC 2068 HTTP/1.1 January 1997

10.4.9 408 Request Timeout

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

10.4.10 409 Conflict

 The request could not be completed due to a conflict with the current
 state of the resource. This code is only allowed in situations where
 it is expected that the user might be able to resolve the conflict
 and resubmit the request. The response body SHOULD include enough
 information for the user to recognize the source of the conflict.
 Ideally, the response entity would include enough information for the
 user or user agent to fix the problem; however, that may not be
 possible and is not required.
 Conflicts are most likely to occur in response to a PUT request. If
 versioning is being used and the entity being PUT includes changes to
 a resource which conflict with those made by an earlier (third-party)
 request, the server MAY use the 409 response to indicate that it
 can't complete the request. In this case, the response entity SHOULD
 contain a list of the differences between the two versions in a
 format defined by the response Content-Type.

10.4.11 410 Gone

 The requested resource is no longer available at the server and no
 forwarding address is known. This condition SHOULD be considered
 permanent. Clients with link editing capabilities SHOULD delete
 references to the Request-URI after user approval. If the server does
 not know, or has no facility to determine, whether or not the
 condition is permanent, the status code 404 (Not Found) SHOULD be
 used instead.  This response is cachable unless indicated otherwise.
 The 410 response is primarily intended to assist the task of web
 maintenance by notifying the recipient that the resource is
 intentionally unavailable and that the server owners desire that
 remote links to that resource be removed. Such an event is common for
 limited-time, promotional services and for resources belonging to
 individuals no longer working at the server's site. It is not
 necessary to mark all permanently unavailable resources as "gone" or
 to keep the mark for any length of time -- that is left to the
 discretion of the server owner.

Fielding, et. al. Standards Track [Page 62] RFC 2068 HTTP/1.1 January 1997

10.4.12 411 Length Required

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

10.4.13 412 Precondition Failed

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

10.4.14 413 Request Entity Too Large

 The server is refusing to process a request because the request
 entity is larger than the server is willing or able to process. The
 server may close the connection to prevent the client from continuing
 the request.
 If the condition is temporary, the server SHOULD include a Retry-
 After header field to indicate that it is temporary and after what
 time the client may try again.

10.4.15 414 Request-URI Too Long

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

10.4.16 415 Unsupported Media Type

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

Fielding, et. al. Standards Track [Page 63] RFC 2068 HTTP/1.1 January 1997

10.5 Server Error 5xx

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

10.5.1 500 Internal Server Error

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

10.5.2 501 Not Implemented

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

10.5.3 502 Bad Gateway

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

10.5.4 503 Service Unavailable

 The server is currently unable to handle the request due to a
 temporary overloading or maintenance of the server. The implication
 is that this is a temporary condition which will be alleviated after
 some delay. If known, the length of the delay may be indicated in a
 Retry-After header.  If no Retry-After is given, the client SHOULD
 handle the response as it would for a 500 response.
   Note: The existence of the 503 status code does not imply that a
   server must use it when becoming overloaded. Some servers may wish
   to simply refuse the connection.

10.5.5 504 Gateway Timeout

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

Fielding, et. al. Standards Track [Page 64] RFC 2068 HTTP/1.1 January 1997

10.5.6 505 HTTP Version Not Supported

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

11 Access Authentication

 HTTP provides a simple challenge-response authentication mechanism
 which MAY be used by a server to challenge a client request and by a
 client to provide authentication information. It uses an extensible,
 case-insensitive token to identify the authentication scheme,
 followed by a comma-separated list of attribute-value pairs which
 carry the parameters necessary for achieving authentication via that
 scheme.
        auth-scheme    = token
        auth-param     = token "=" quoted-string
 The 401 (Unauthorized) response message is used by an origin server
 to challenge the authorization of a user agent. This response MUST
 include a WWW-Authenticate header field containing at least one
 challenge applicable to the requested resource.
        challenge      = auth-scheme 1*SP realm *( "," auth-param )
        realm          = "realm" "=" realm-value
        realm-value    = quoted-string
 The realm attribute (case-insensitive) is required for all
 authentication schemes which issue a challenge. The realm value
 (case-sensitive), in combination with the canonical root URL (see
 section 5.1.2) of the server being accessed, defines the protection
 space. These realms allow the protected resources on a server to be
 partitioned into a set of protection spaces, each with its own
 authentication scheme and/or authorization database. The realm value
 is a string, generally assigned by the origin server, which may have
 additional semantics specific to the authentication scheme.
 A user agent that wishes to authenticate itself with a server--
 usually, but not necessarily, after receiving a 401 or 411 response-
 -MAY do so by including an Authorization header field with the
 request. The Authorization field value consists of credentials

Fielding, et. al. Standards Track [Page 65] RFC 2068 HTTP/1.1 January 1997

 containing the authentication information of the user agent for the
 realm of the resource being requested.
        credentials    = basic-credentials
                       | auth-scheme #auth-param
 The domain over which credentials can be automatically applied by a
 user agent is determined by the protection space. If a prior request
 has been authorized, the same credentials MAY be reused for all other
 requests within that protection space for a period of time determined
 by the authentication scheme, parameters, and/or user preference.
 Unless otherwise defined by the authentication scheme, a single
 protection space cannot extend outside the scope of its server.
 If the server does not wish to accept the credentials sent with a
 request, it SHOULD return a 401 (Unauthorized) response. The response
 MUST include a WWW-Authenticate header field containing the (possibly
 new) challenge applicable to the requested resource and an entity
 explaining the refusal.
 The HTTP protocol does not restrict applications to this simple
 challenge-response mechanism for access authentication. Additional
 mechanisms MAY be used, such as encryption at the transport level or
 via message encapsulation, and with additional header fields
 specifying authentication information. However, these additional
 mechanisms are not defined by this specification.
 Proxies MUST be completely transparent regarding user agent
 authentication. That is, they MUST forward the WWW-Authenticate and
 Authorization headers untouched, and follow the rules found in
 section 14.8.
 HTTP/1.1 allows a client to pass authentication information to and
 from a proxy via the Proxy-Authenticate and Proxy-Authorization
 headers.

11.1 Basic Authentication Scheme

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

Fielding, et. al. Standards Track [Page 66] RFC 2068 HTTP/1.1 January 1997

 Upon receipt of an unauthorized request for a URI within the
 protection space, the server MAY respond with a challenge like the
 following:
        WWW-Authenticate: Basic realm="WallyWorld"
 where "WallyWorld" is the string assigned by the server to identify
 the protection space of the Request-URI.
 To receive authorization, the client sends the userid and password,
 separated by a single colon (":") character, within a base64  encoded
 string in the credentials.
        basic-credentials = "Basic" SP basic-cookie
        basic-cookie   = <base64 [7] encoding of user-pass,
                         except not limited to 76 char/line>
        user-pass   = userid ":" password
        userid      = *<TEXT excluding ":">
        password    = *TEXT
 Userids might be case sensitive.
 If the user agent wishes to send the userid "Aladdin" and password
 "open sesame", it would use the following header field:
        Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
 See section 15 for security considerations associated with Basic
 authentication.

11.2 Digest Authentication Scheme

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

12 Content Negotiation

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

Fielding, et. al. Standards Track [Page 67] RFC 2068 HTTP/1.1 January 1997

 when there are multiple representations available.
   Note: This is not called "format negotiation" because the alternate
   representations may be of the same media type, but use different
   capabilities of that type, be in different languages, etc.
 Any response containing an entity-body MAY be subject to negotiation,
 including error responses.
 There are two kinds of content negotiation which are possible in
 HTTP: server-driven and agent-driven negotiation. These two kinds of
 negotiation are orthogonal and thus may be used separately or in
 combination. One method of combination, referred to as transparent
 negotiation, occurs when a cache uses the agent-driven negotiation
 information provided by the origin server in order to provide
 server-driven negotiation for subsequent requests.

12.1 Server-driven Negotiation

 If the selection of the best representation for a response is made by
 an algorithm located at the server, it is called server-driven
 negotiation.  Selection is based on the available representations of
 the response (the dimensions over which it can vary; e.g. language,
 content-coding, etc.) and the contents of particular header fields in
 the request message or on other information pertaining to the request
 (such as the network address of the client).
 Server-driven negotiation is advantageous when the algorithm for
 selecting from among the available representations is difficult to
 describe to the user agent, or when the server desires to send its
 "best guess" to the client along with the first response (hoping to
 avoid the round-trip delay of a subsequent request if the "best
 guess" is good enough for the user). In order to improve the server's
 guess, the user agent MAY include request header fields (Accept,
 Accept-Language, Accept-Encoding, etc.) which describe its
 preferences for such a response.
 Server-driven negotiation has disadvantages:

1. It is impossible for the server to accurately determine what might be

"best" for any given user, since that would require complete
knowledge of both the capabilities of the user agent and the intended
use for the response (e.g., does the user want to view it on screen
or print it on paper?).

2. Having the user agent describe its capabilities in every request can

be both very inefficient (given that only a small percentage of
responses have multiple representations) and a potential violation of

Fielding, et. al. Standards Track [Page 68] RFC 2068 HTTP/1.1 January 1997

the user's privacy.

3. It complicates the implementation of an origin server and the

algorithms for generating responses to a request.

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

multiple user's requests.
 HTTP/1.1 includes the following request-header fields for enabling
 server-driven negotiation through description of user agent
 capabilities and user preferences: Accept (section 14.1), Accept-
 Charset (section 14.2), Accept-Encoding (section 14.3), Accept-
 Language (section 14.4), and User-Agent (section 14.42). However, an
 origin server is not limited to these dimensions and MAY vary the
 response based on any aspect of the request, including information
 outside the request-header fields or within extension header fields
 not defined by this specification.
 HTTP/1.1 origin servers MUST include an appropriate Vary header field
 (section 14.43) in any cachable response based on server-driven
 negotiation. The Vary header field describes the dimensions over
 which the response might vary (i.e. the dimensions over which the
 origin server picks its "best guess" response from multiple
 representations).
 HTTP/1.1 public caches MUST recognize the Vary header field when it
 is included in a response and obey the requirements described in
 section 13.6 that describes the interactions between caching and
 content negotiation.

12.2 Agent-driven Negotiation

 With agent-driven negotiation, selection of the best representation
 for a response is performed by the user agent after receiving an
 initial response from the origin server. Selection is based on a list
 of the available representations of the response included within the
 header fields (this specification reserves the field-name Alternates,
 as described in appendix 19.6.2.1) or entity-body of the initial
 response, with each representation identified by its own URI.
 Selection from among the representations may be performed
 automatically (if the user agent is capable of doing so) or manually
 by the user selecting from a generated (possibly hypertext) menu.
 Agent-driven negotiation is advantageous when the response would vary
 over commonly-used dimensions (such as type, language, or encoding),
 when the origin server is unable to determine a user agent's
 capabilities from examining the request, and generally when public
 caches are used to distribute server load and reduce network usage.

Fielding, et. al. Standards Track [Page 69] RFC 2068 HTTP/1.1 January 1997

 Agent-driven negotiation suffers from the disadvantage of needing a
 second request to obtain the best alternate representation. This
 second request is only efficient when caching is used. In addition,
 this specification does not define any mechanism for supporting
 automatic selection, though it also does not prevent any such
 mechanism from being developed as an extension and used within
 HTTP/1.1.
 HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)
 status codes for enabling agent-driven negotiation when the server is
 unwilling or unable to provide a varying response using server-driven
 negotiation.

12.3 Transparent Negotiation

 Transparent negotiation is a combination of both server-driven and
 agent-driven negotiation. When a cache is supplied with a form of the
 list of available representations of the response (as in agent-driven
 negotiation) and the dimensions of variance are completely understood
 by the cache, then the cache becomes capable of performing server-
 driven negotiation on behalf of the origin server for subsequent
 requests on that resource.
 Transparent negotiation has the advantage of distributing the
 negotiation work that would otherwise be required of the origin
 server and also removing the second request delay of agent-driven
 negotiation when the cache is able to correctly guess the right
 response.
 This specification does not define any mechanism for transparent
 negotiation, though it also does not prevent any such mechanism from
 being developed as an extension and used within HTTP/1.1. An HTTP/1.1
 cache performing transparent negotiation MUST include a Vary header
 field in the response (defining the dimensions of its variance) if it
 is cachable to ensure correct interoperation with all HTTP/1.1
 clients. The agent-driven negotiation information supplied by the
 origin server SHOULD be included with the transparently negotiated
 response.

13 Caching in HTTP

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

Fielding, et. al. Standards Track [Page 70] RFC 2068 HTTP/1.1 January 1997

 headers, response codes, etc.
 Caching would be useless if it did not significantly improve
 performance. The goal of caching in HTTP/1.1 is to eliminate the need
 to send requests in many cases, and to eliminate the need to send
 full responses in many other cases. The former reduces the number of
 network round-trips required for many operations; we use an
 "expiration" mechanism for this purpose (see section 13.2). The
 latter reduces network bandwidth requirements; we use a "validation"
 mechanism for this purpose (see section 13.3).
 Requirements for performance, availability, and disconnected
 operation require us to be able to relax the goal of semantic
 transparency. The HTTP/1.1 protocol allows origin servers, caches,
 and clients to explicitly reduce transparency when necessary.
 However, because non-transparent operation may confuse non-expert
 users, and may be incompatible with certain server applications (such
 as those for ordering merchandise), the protocol requires that
 transparency be relaxed
o  only by an explicit protocol-level request when relaxed by client
   or origin server
o  only with an explicit warning to the end user when relaxed by cache
   or client

Fielding, et. al. Standards Track [Page 71] RFC 2068 HTTP/1.1 January 1997

 Therefore, the HTTP/1.1 protocol provides these important elements:
1. Protocol features that provide full semantic transparency when this
   is required by all parties.
2. Protocol features that allow an origin server or user agent to
   explicitly request and control non-transparent operation.
3. Protocol features that allow a cache to attach warnings to
   responses that do not preserve the requested approximation of
   semantic transparency.
 A basic principle is that it must be possible for the clients to
 detect any potential relaxation of semantic transparency.
   Note: The server, cache, or client implementer may be faced with
   design decisions not explicitly discussed in this specification. If
   a decision may affect semantic transparency, the implementer ought
   to err on the side of maintaining transparency unless a careful and
   complete analysis shows significant benefits in breaking
   transparency.

13.1.1 Cache Correctness

 A correct cache MUST respond to a request with the most up-to-date
 response held by the cache that is appropriate to the request (see
 sections 13.2.5, 13.2.6, and 13.12) which meets one of the following
 conditions:
1. It has been checked for equivalence with what the origin server
   would have returned by revalidating the response with the origin
   server (section 13.3);
2. It is "fresh enough" (see section 13.2). In the default case, this
   means it meets the least restrictive freshness requirement of the
   client, server, and cache (see section 14.9); if the origin server
   so specifies, it is the freshness requirement of the origin server
   alone.
3. It includes a warning if the freshness demand of the client or the
   origin server is violated (see section 13.1.5 and 14.45).
4. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect), or
   error (4xx or 5xx) response message.
 If the cache can not communicate with the origin server, then a
 correct cache SHOULD respond as above if the response can be
 correctly served from the cache; if not it MUST return an error or

Fielding, et. al. Standards Track [Page 72] RFC 2068 HTTP/1.1 January 1997

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

13.1.2 Warnings

 Whenever a cache returns a response that is neither first-hand nor
 "fresh enough" (in the sense of condition 2 in section 13.1.1), it
 must attach a warning to that effect, using a Warning response-
 header. This warning allows clients to take appropriate action.
 Warnings may be used for other purposes, both cache-related and
 otherwise. The use of a warning, rather than an error status code,
 distinguish these responses from true failures.
 Warnings are always cachable, because they never weaken the
 transparency of a response. This means that warnings can be passed to
 HTTP/1.0 caches without danger; such caches will simply pass the
 warning along as an entity-header in the response.
 Warnings are assigned numbers between 0 and 99. This specification
 defines the code numbers and meanings of each currently assigned
 warnings, allowing a client or cache to take automated action in some
 (but not all) cases.
 Warnings also carry a warning text. The text may be in any
 appropriate natural language (perhaps based on the client's Accept
 headers), and include an optional indication of what character set is
 used.
 Multiple warnings may be attached to a response (either by the origin
 server or by a cache), including multiple warnings with the same code
 number. For example, a server may provide the same warning with texts
 in both English and Basque.
 When multiple warnings are attached to a response, it may not be
 practical or reasonable to display all of them to the user. This
 version of HTTP does not specify strict priority rules for deciding
 which warnings to display and in what order, but does suggest some
 heuristics.

Fielding, et. al. Standards Track [Page 73] RFC 2068 HTTP/1.1 January 1997

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

13.1.3 Cache-control Mechanisms

 The basic cache mechanisms in HTTP/1.1 (server-specified expiration
 times and validators) are implicit directives to caches. In some
 cases, a server or client may need to provide explicit directives to
 the HTTP caches. We use the Cache-Control header for this purpose.
 The Cache-Control header allows a client or server to transmit a
 variety of directives in either requests or responses. These
 directives typically override the default caching algorithms. As a
 general rule, if there is any apparent conflict between header
 values, the most restrictive interpretation should be applied (that
 is, the one that is most likely to preserve semantic transparency).
 However, in some cases, Cache-Control directives are explicitly
 specified as weakening the approximation of semantic transparency
 (for example, "max-stale" or "public").
 The Cache-Control directives are described in detail in section 14.9.

13.1.4 Explicit User Agent Warnings

 Many user agents make it possible for users to override the basic
 caching mechanisms. For example, the user agent may allow the user to
 specify that cached entities (even explicitly stale ones) are never
 validated. Or the user agent might habitually add "Cache-Control:
 max-stale=3600" to every request. The user should have to explicitly
 request either non-transparent behavior, or behavior that results in
 abnormally ineffective caching.
 If the user has overridden the basic caching mechanisms, the user
 agent should explicitly indicate to the user whenever this results in
 the display of information that might not meet the server's
 transparency requirements (in particular, if the displayed entity is
 known to be stale). Since the protocol normally allows the user agent
 to determine if responses are stale or not, this indication need only
 be displayed when this actually happens. The indication need not be a
 dialog box; it could be an icon (for example, a picture of a rotting
 fish) or some other visual indicator.
 If the user has overridden the caching mechanisms in a way that would
 abnormally reduce the effectiveness of caches, the user agent should
 continually display an indication (for example, a picture of currency
 in flames) so that the user does not inadvertently consume excess
 resources or suffer from excessive latency.

Fielding, et. al. Standards Track [Page 74] RFC 2068 HTTP/1.1 January 1997

13.1.5 Exceptions to the Rules and Warnings

 In some cases, the operator of a cache may choose to configure it to
 return stale responses even when not requested by clients. This
 decision should not be made lightly, but may be necessary for reasons
 of availability or performance, especially when the cache is poorly
 connected to the origin server. Whenever a cache returns a stale
 response, it MUST mark it as such (using a Warning header). This
 allows the client software to alert the user that there may be a
 potential problem.
 It also allows the user agent to take steps to obtain a first-hand or
 fresh response. For this reason, a cache SHOULD NOT return a stale
 response if the client explicitly requests a first-hand or fresh one,
 unless it is impossible to comply for technical or policy reasons.

13.1.6 Client-controlled Behavior

 While the origin server (and to a lesser extent, intermediate caches,
 by their contribution to the age of a response) are the primary
 source of expiration information, in some cases the client may need
 to control a cache's decision about whether to return a cached
 response without validating it. Clients do this using several
 directives of the Cache-Control header.
 A client's request may specify the maximum age it is willing to
 accept of an unvalidated response; specifying a value of zero forces
 the cache(s) to revalidate all responses. A client may also specify
 the minimum time remaining before a response expires. Both of these
 options increase constraints on the behavior of caches, and so cannot
 further relax the cache's approximation of semantic transparency.
 A client may also specify that it will accept stale responses, up to
 some maximum amount of staleness. This loosens the constraints on the
 caches, and so may violate the origin server's specified constraints
 on semantic transparency, but may be necessary to support
 disconnected operation, or high availability in the face of poor
 connectivity.

13.2 Expiration Model

13.2.1 Server-Specified Expiration

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

Fielding, et. al. Standards Track [Page 75] RFC 2068 HTTP/1.1 January 1997

 response without first contacting the server.
 Our expectation is that servers will assign future explicit
 expiration times to responses in the belief that the entity is not
 likely to change, in a semantically significant way, before the
 expiration time is reached. This normally preserves semantic
 transparency, as long as the server's expiration times are carefully
 chosen.
 The expiration mechanism applies only to responses taken from a cache
 and not to first-hand responses forwarded immediately to the
 requesting client.
 If an origin server wishes to force a semantically transparent cache
 to validate every request, it may assign an explicit expiration time
 in the past. This means that the response is always stale, and so the
 cache SHOULD validate it before using it for subsequent requests. See
 section 14.9.4 for a more restrictive way to force revalidation.
 If an origin server wishes to force any HTTP/1.1 cache, no matter how
 it is configured, to validate every request, it should use the
 "must-revalidate" Cache-Control directive (see section 14.9).
 Servers specify explicit expiration times using either the Expires
 header, or the max-age directive of the Cache-Control header.
 An expiration time cannot be used to force a user agent to refresh
 its display or reload a resource; its semantics apply only to caching
 mechanisms, and such mechanisms need only check a resource's
 expiration status when a new request for that resource is initiated.
 See section 13.13 for explanation of the difference between caches
 and history mechanisms.

13.2.2 Heuristic Expiration

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

Fielding, et. al. Standards Track [Page 76] RFC 2068 HTTP/1.1 January 1997

13.2.3 Age Calculations

 In order to know if a cached entry is fresh, a cache needs to know if
 its age exceeds its freshness lifetime. We discuss how to calculate
 the latter in section 13.2.4; this section describes how to calculate
 the age of a response or cache entry.
 In this discussion, we use the term "now" to mean "the current value
 of the clock at the host performing the calculation." Hosts that use
 HTTP, but especially hosts running origin servers and caches, should
 use NTP [28] or some similar protocol to synchronize their clocks to
 a globally accurate time standard.
 Also note that HTTP/1.1 requires origin servers to send a Date header
 with every response, giving the time at which the response was
 generated. We use the term "date_value" to denote the value of the
 Date header, in a form appropriate for arithmetic operations.
 HTTP/1.1 uses the Age response-header to help convey age information
 between caches. The Age header value is the sender's estimate of the
 amount of time since the response was generated at the origin server.
 In the case of a cached response that has been revalidated with the
 origin server, the Age value is based on the time of revalidation,
 not of the original response.
 In essence, the Age value is the sum of the time that the response
 has been resident in each of the caches along the path from the
 origin server, plus the amount of time it has been in transit along
 network paths.
 We use the term "age_value" to denote the value of the Age header, in
 a form appropriate for arithmetic operations.
 A response's age can be calculated in two entirely independent ways:
   1. now minus date_value, if the local clock is reasonably well
      synchronized to the origin server's clock. If the result is
      negative, the result is replaced by zero.
   2. age_value, if all of the caches along the response path
      implement HTTP/1.1.
 Given that we have two independent ways to compute the age of a
 response when it is received, we can combine these as
        corrected_received_age = max(now - date_value, age_value)
 and as long as we have either nearly synchronized clocks or all-

Fielding, et. al. Standards Track [Page 77] RFC 2068 HTTP/1.1 January 1997

 HTTP/1.1 paths, one gets a reliable (conservative) result.
 Note that this correction is applied at each HTTP/1.1 cache along the
 path, so that if there is an HTTP/1.0 cache in the path, the correct
 received age is computed as long as the receiving cache's clock is
 nearly in sync. We don't need end-to-end clock synchronization
 (although it is good to have), and there is no explicit clock
 synchronization step.
 Because of network-imposed delays, some significant interval may pass
 from the time that a server generates a response and the time it is
 received at the next outbound cache or client. If uncorrected, this
 delay could result in improperly low ages.
 Because the request that resulted in the returned Age value must have
 been initiated prior to that Age value's generation, we can correct
 for delays imposed by the network by recording the time at which the
 request was initiated. Then, when an Age value is received, it MUST
 be interpreted relative to the time the request was initiated, not
 the time that the response was received. This algorithm results in
 conservative behavior no matter how much delay is experienced. So, we
 compute:
       corrected_initial_age = corrected_received_age
                             + (now - request_time)
 where "request_time" is the time (according to the local clock) when
 the request that elicited this response was sent.
 Summary of age calculation algorithm, when a cache receives a
 response:
    /*
     * age_value
     *      is the value of Age: header received by the cache with
     *              this response.
     * date_value
     *      is the value of the origin server's Date: header
     * request_time
     *      is the (local) time when the cache made the request
     *              that resulted in this cached response
     * response_time
     *      is the (local) time when the cache received the
     *              response
     * now
     *      is the current (local) time
     */
    apparent_age = max(0, response_time - date_value);

Fielding, et. al. Standards Track [Page 78] RFC 2068 HTTP/1.1 January 1997

    corrected_received_age = max(apparent_age, age_value);
    response_delay = response_time - request_time;
    corrected_initial_age = corrected_received_age + response_delay;
    resident_time = now - response_time;
    current_age   = corrected_initial_age + resident_time;
 When a cache sends a response, it must add to the
 corrected_initial_age the amount of time that the response was
 resident locally. It must then transmit this total age, using the Age
 header, to the next recipient cache.
   Note that a client cannot reliably tell that a response is first-
   hand, but the presence of an Age header indicates that a response
   is definitely not first-hand. Also, if the Date in a response is
   earlier than the client's local request time, the response is
   probably not first-hand (in the absence of serious clock skew).

13.2.4 Expiration Calculations

 In order to decide whether a response is fresh or stale, we need to
 compare its freshness lifetime to its age. The age is calculated as
 described in section 13.2.3; this section describes how to calculate
 the freshness lifetime, and to determine if a response has expired.
 In the discussion below, the values can be represented in any form
 appropriate for arithmetic operations.
 We use the term "expires_value" to denote the value of the Expires
 header. We use the term "max_age_value" to denote an appropriate
 value of the number of seconds carried by the max-age directive of
 the Cache-Control header in a response (see section 14.10.
 The max-age directive takes priority over Expires, so if max-age is
 present in a response, the calculation is simply:
       freshness_lifetime = max_age_value
 Otherwise, if Expires is present in the response, the calculation is:
       freshness_lifetime = expires_value - date_value
 Note that neither of these calculations is vulnerable to clock skew,
 since all of the information comes from the origin server.
 If neither Expires nor Cache-Control: max-age appears in the
 response, and the response does not include other restrictions on
 caching, the cache MAY compute a freshness lifetime using a
 heuristic. If the value is greater than 24 hours, the cache must
 attach Warning 13 to any response whose age is more than 24 hours if

Fielding, et. al. Standards Track [Page 79] RFC 2068 HTTP/1.1 January 1997

 such warning has not already been added.
 Also, if the response does have a Last-Modified time, the heuristic
 expiration value SHOULD be no more than some fraction of the interval
 since that time. A typical setting of this fraction might be 10%.
 The calculation to determine if a response has expired is quite
 simple:
       response_is_fresh = (freshness_lifetime > current_age)

13.2.5 Disambiguating Expiration Values

 Because expiration values are assigned optimistically, it is possible
 for two caches to contain fresh values for the same resource that are
 different.
 If a client performing a retrieval receives a non-first-hand response
 for a request that was already fresh in its own cache, and the Date
 header in its existing cache entry is newer than the Date on the new
 response, then the client MAY ignore the response. If so, it MAY
 retry the request with a "Cache-Control: max-age=0" directive (see
 section 14.9), to force a check with the origin server.
 If a cache has two fresh responses for the same representation with
 different validators, it MUST use the one with the more recent Date
 header. This situation may arise because the cache is pooling
 responses from other caches, or because a client has asked for a
 reload or a revalidation of an apparently fresh cache entry.

13.2.6 Disambiguating Multiple Responses

 Because a client may be receiving responses via multiple paths, so
 that some responses flow through one set of caches and other
 responses flow through a different set of caches, a client may
 receive responses in an order different from that in which the origin
 server sent them. We would like the client to use the most recently
 generated response, even if older responses are still apparently
 fresh.
 Neither the entity tag nor the expiration value can impose an
 ordering on responses, since it is possible that a later response
 intentionally carries an earlier expiration time. However, the
 HTTP/1.1 specification requires the transmission of Date headers on
 every response, and the Date values are ordered to a granularity of
 one second.

Fielding, et. al. Standards Track [Page 80] RFC 2068 HTTP/1.1 January 1997

 When a client tries to revalidate a cache entry, and the response it
 receives contains a Date header that appears to be older than the one
 for the existing entry, then the client SHOULD repeat the request
 unconditionally, and include
        Cache-Control: max-age=0
 to force any intermediate caches to validate their copies directly
 with the origin server, or
        Cache-Control: no-cache
 to force any intermediate caches to obtain a new copy from the origin
 server.
 If the Date values are equal, then the client may use either response
 (or may, if it is being extremely prudent, request a new response).
 Servers MUST NOT depend on clients being able to choose
 deterministically between responses generated during the same second,
 if their expiration times overlap.

13.3 Validation Model

 When a cache has a stale entry that it would like to use as a
 response to a client's request, it first has to check with the origin
 server (or possibly an intermediate cache with a fresh response) to
 see if its cached entry is still usable. We call this "validating"
 the cache entry.  Since we do not want to have to pay the overhead of
 retransmitting the full response if the cached entry is good, and we
 do not want to pay the overhead of an extra round trip if the cached
 entry is invalid, the HTTP/1.1 protocol supports the use of
 conditional methods.
 The key protocol features for supporting conditional methods are
 those concerned with "cache validators." When an origin server
 generates a full response, it attaches some sort of validator to it,
 which is kept with the cache entry. When a client (user agent or
 proxy cache) makes a conditional request for a resource for which it
 has a cache entry, it includes the associated validator in the
 request.
 The server then checks that validator against the current validator
 for the entity, and, if they match, it responds with a special status
 code (usually, 304 (Not Modified)) and no entity-body. Otherwise, it
 returns a full response (including entity-body). Thus, we avoid
 transmitting the full response if the validator matches, and we avoid
 an extra round trip if it does not match.

Fielding, et. al. Standards Track [Page 81] RFC 2068 HTTP/1.1 January 1997

   Note: the comparison functions used to decide if validators match
   are defined in section 13.3.3.
 In HTTP/1.1, a conditional request looks exactly the same as a normal
 request for the same resource, except that it carries a special
 header (which includes the validator) that implicitly turns the
 method (usually, GET) into a conditional.
 The protocol includes both positive and negative senses of cache-
 validating conditions. That is, it is possible to request either that
 a method be performed if and only if a validator matches or if and
 only if no validators match.
   Note: a response that lacks a validator may still be cached, and
   served from cache until it expires, unless this is explicitly
   prohibited by a Cache-Control directive. However, a cache cannot do
   a conditional retrieval if it does not have a validator for the
   entity, which means it will not be refreshable after it expires.

13.3.1 Last-modified Dates

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

13.3.2 Entity Tag Cache Validators

 The ETag entity-header field value, an entity tag, provides for an
 "opaque" cache validator. This may allow more reliable validation in
 situations where it is inconvenient to store modification dates,
 where the one-second resolution of HTTP date values is not
 sufficient, or where the origin server wishes to avoid certain
 paradoxes that may arise from the use of modification dates.
 Entity Tags are described in section 3.11. The headers used with
 entity tags are described in sections 14.20, 14.25, 14.26 and 14.43.

13.3.3 Weak and Strong Validators

 Since both origin servers and caches will compare two validators to
 decide if they represent the same or different entities, one normally
 would expect that if the entity (the entity-body or any entity-
 headers) changes in any way, then the associated validator would
 change as well.  If this is true, then we call this validator a
 "strong validator."
 However, there may be cases when a server prefers to change the
 validator only on semantically significant changes, and not when

Fielding, et. al. Standards Track [Page 82] RFC 2068 HTTP/1.1 January 1997

 insignificant aspects of the entity change. A validator that does not
 always change when the resource changes is a "weak validator."
 Entity tags are normally "strong validators," but the protocol
 provides a mechanism to tag an entity tag as "weak." One can think of
 a strong validator as one that changes whenever the bits of an entity
 changes, while a weak value changes whenever the meaning of an entity
 changes.  Alternatively, one can think of a strong validator as part
 of an identifier for a specific entity, while a weak validator is
 part of an identifier for a set of semantically equivalent entities.
   Note: One example of a strong validator is an integer that is
   incremented in stable storage every time an entity is changed.
   An entity's modification time, if represented with one-second
   resolution, could be a weak validator, since it is possible that
   the resource may be modified twice during a single second.
   Support for weak validators is optional; however, weak validators
   allow for more efficient caching of equivalent objects; for
   example, a hit counter on a site is probably good enough if it is
   updated every few days or weeks, and any value during that period
   is likely "good enough" to be equivalent.
   A "use" of a validator is either when a client generates a request
   and includes the validator in a validating header field, or when a
   server compares two validators.
 Strong validators are usable in any context. Weak validators are only
 usable in contexts that do not depend on exact equality of an entity.
 For example, either kind is usable for a conditional GET of a full
 entity. However, only a strong validator is usable for a sub-range
 retrieval, since otherwise the client may end up with an internally
 inconsistent entity.
 The only function that the HTTP/1.1 protocol defines on validators is
 comparison. There are two validator comparison functions, depending
 on whether the comparison context allows the use of weak validators
 or not:
o  The strong comparison function: in order to be considered equal,
   both validators must be identical in every way, and neither may be
   weak.
o  The weak comparison function: in order to be considered equal, both
   validators must be identical in every way, but either or both of
   them may be tagged as "weak" without affecting the result.
 The weak comparison function MAY be used for simple (non-subrange)

Fielding, et. al. Standards Track [Page 83] RFC 2068 HTTP/1.1 January 1997

 GET requests. The strong comparison function MUST be used in all
 other cases.
 An entity tag is strong unless it is explicitly tagged as weak.
 Section 3.11 gives the syntax for entity tags.
 A Last-Modified time, when used as a validator in a request, is
 implicitly weak unless it is possible to deduce that it is strong,
 using the following rules:
o  The validator is being compared by an origin server to the actual
   current validator for the entity and,
o  That origin server reliably knows that the associated entity did
   not change twice during the second covered by the presented
   validator.

or

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

or

o  The validator is being compared by an intermediate cache to the
   validator stored in its cache entry for the entity, and
o  That cache entry includes a Date value, which gives the time when
   the origin server sent the original response, and
o  The presented Last-Modified time is at least 60 seconds before the
   Date value.
 This method relies on the fact that if two different responses were
 sent by the origin server during the same second, but both had the
 same Last-Modified time, then at least one of those responses would
 have a Date value equal to its Last-Modified time. The arbitrary 60-
 second limit guards against the possibility that the Date and Last-
 Modified values are generated from different clocks, or at somewhat
 different times during the preparation of the response. An
 implementation may use a value larger than 60 seconds, if it is
 believed that 60 seconds is too short.
 If a client wishes to perform a sub-range retrieval on a value for
 which it has only a Last-Modified time and no opaque validator, it
 may do this only if the Last-Modified time is strong in the sense
 described here.

Fielding, et. al. Standards Track [Page 84] RFC 2068 HTTP/1.1 January 1997

 A cache or origin server receiving a cache-conditional request, other
 than a full-body GET request, MUST use the strong comparison function
 to evaluate the condition.
 These rules allow HTTP/1.1 caches and clients to safely perform sub-
 range retrievals on values that have been obtained from HTTP/1.0
 servers.

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

 We adopt a set of rules and recommendations for origin servers,
 clients, and caches regarding when various validator types should be
 used, and for what purposes.
 HTTP/1.1 origin servers:
o  SHOULD send an entity tag validator unless it is not feasible to
   generate one.
o  MAY send a weak entity tag instead of a strong entity tag, if
   performance considerations support the use of weak entity tags, or
   if it is unfeasible to send a strong entity tag.
o  SHOULD send a Last-Modified value if it is feasible to send one,
   unless the risk of a breakdown in semantic transparency that could
   result from using this date in an If-Modified-Since header would
   lead to serious problems.
 In other words, the preferred behavior for an HTTP/1.1 origin server
 is to send both a strong entity tag and a Last-Modified value.
 In order to be legal, a strong entity tag MUST change whenever the
 associated entity value changes in any way. A weak entity tag SHOULD
 change whenever the associated entity changes in a semantically
 significant way.
   Note: in order to provide semantically transparent caching, an
   origin server must avoid reusing a specific strong entity tag value
   for two different entities, or reusing a specific weak entity tag
   value for two semantically different entities. Cache entries may
   persist for arbitrarily long periods, regardless of expiration
   times, so it may be inappropriate to expect that a cache will never
   again attempt to validate an entry using a validator that it
   obtained at some point in the past.
 HTTP/1.1 clients:
   o  If an entity tag has been provided by the origin server, MUST
      use that entity tag in any cache-conditional request (using
      If-Match or If-None-Match).

Fielding, et. al. Standards Track [Page 85] RFC 2068 HTTP/1.1 January 1997

   o  If only a Last-Modified value has been provided by the origin
      server, SHOULD use that value in non-subrange cache-conditional
      requests (using If-Modified-Since).
   o  If only a Last-Modified value has been provided by an HTTP/1.0
      origin server, MAY use that value in subrange cache-conditional
      requests (using If-Unmodified-Since:). The user agent should
      provide a way to disable this, in case of difficulty.
   o  If both an entity tag and a Last-Modified value have been
      provided by the origin server, SHOULD use both validators in
      cache-conditional requests. This allows both HTTP/1.0 and
      HTTP/1.1 caches to respond appropriately.
 An HTTP/1.1 cache, upon receiving a request, MUST use the most
 restrictive validator when deciding whether the client's cache entry
 matches the cache's own cache entry. This is only an issue when the
 request contains both an entity tag and a last-modified-date
 validator (If-Modified-Since or If-Unmodified-Since).
   A note on rationale: The general principle behind these rules is
   that HTTP/1.1 servers and clients should transmit as much non-
   redundant information as is available in their responses and
   requests. HTTP/1.1 systems receiving this information will make the
   most conservative assumptions about the validators they receive.
   HTTP/1.0 clients and caches will ignore entity tags. Generally,
   last-modified values received or used by these systems will support
   transparent and efficient caching, and so HTTP/1.1 origin servers
   should provide Last-Modified values. In those rare cases where the
   use of a Last-Modified value as a validator by an HTTP/1.0 system
   could result in a serious problem, then HTTP/1.1 origin servers
   should not provide one.

13.3.5 Non-validating Conditionals

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

13.4 Response Cachability

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

Fielding, et. al. Standards Track [Page 86] RFC 2068 HTTP/1.1 January 1997

 there is neither a cache validator nor an explicit expiration time
 associated with a response, we do not expect it to be cached, but
 certain caches may violate this expectation (for example, when little
 or no network connectivity is available). A client can usually detect
 that such a response was taken from a cache by comparing the Date
 header to the current time.
   Note that some HTTP/1.0 caches are known to violate this
   expectation without providing any Warning.
 However, in some cases it may be inappropriate for a cache to retain
 an entity, or to return it in response to a subsequent request. This
 may be because absolute semantic transparency is deemed necessary by
 the service author, or because of security or privacy considerations.
 Certain Cache-Control directives are therefore provided so that the
 server can indicate that certain resource entities, or portions
 thereof, may not be cached regardless of other considerations.
 Note that section 14.8 normally prevents a shared cache from saving
 and returning a response to a previous request if that request
 included an Authorization header.
 A response received with a status code of 200, 203, 206, 300, 301 or
 410 may be stored by a cache and used in reply to a subsequent
 request, subject to the expiration mechanism, unless a Cache-Control
 directive prohibits caching. However, a cache that does not support
 the Range and Content-Range headers MUST NOT cache 206 (Partial
 Content) responses.
 A response received with any other status code MUST NOT be returned
 in a reply to a subsequent request unless there are Cache-Control
 directives or another header(s) that explicitly allow it. For
 example, these include the following: an Expires header (section
 14.21); a "max-age", "must-revalidate", "proxy-revalidate", "public"
 or "private" Cache-Control directive (section 14.9).

13.5 Constructing Responses From Caches

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

Fielding, et. al. Standards Track [Page 87] RFC 2068 HTTP/1.1 January 1997

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

 For the purpose of defining the behavior of caches and non-caching
 proxies, we divide HTTP headers into two categories:
o  End-to-end headers, which must be transmitted to the
   ultimate recipient of a request or response. End-to-end
   headers in responses must be stored as part of a cache entry
   and transmitted in any response formed from a cache entry.
o  Hop-by-hop headers, which are meaningful only for a single
   transport-level connection, and are not stored by caches or
   forwarded by proxies.
 The following HTTP/1.1 headers are hop-by-hop headers:
   o  Connection
   o  Keep-Alive
   o  Public
   o  Proxy-Authenticate
   o  Transfer-Encoding
   o  Upgrade
 All other headers defined by HTTP/1.1 are end-to-end headers.
 Hop-by-hop headers introduced in future versions of HTTP MUST be
 listed in a Connection header, as described in section 14.10.

13.5.2 Non-modifiable Headers

 Some features of the HTTP/1.1 protocol, such as Digest
 Authentication, depend on the value of certain end-to-end headers. A
 cache or non-caching proxy SHOULD NOT modify an end-to-end header
 unless the definition of that header requires or specifically allows
 that.
 A cache or non-caching proxy MUST NOT modify any of the following
 fields in a request or response, nor may it add any of these fields
 if not already present:
   o  Content-Location
   o  ETag
   o  Expires
   o  Last-Modified

Fielding, et. al. Standards Track [Page 88] RFC 2068 HTTP/1.1 January 1997

 A cache or non-caching proxy MUST NOT modify or add any of the
 following fields in a response that contains the no-transform Cache-
 Control directive, or in any request:
   o  Content-Encoding
   o  Content-Length
   o  Content-Range
   o  Content-Type
 A cache or non-caching proxy MAY modify or add these fields in a
 response that does not include no-transform, but if it does so, it
 MUST add a Warning 14 (Transformation applied) if one does not
 already appear in the response.
   Warning: unnecessary modification of end-to-end headers may cause
   authentication failures if stronger authentication mechanisms are
   introduced in later versions of HTTP. Such authentication
   mechanisms may rely on the values of header fields not listed here.

13.5.3 Combining Headers

 When a cache makes a validating request to a server, and the server
 provides a 304 (Not Modified) response, the cache must construct a
 response to send to the requesting client. The cache uses the
 entity-body stored in the cache entry as the entity-body of this
 outgoing response. The end-to-end headers stored in the cache entry
 are used for the constructed response, except that any end-to-end
 headers provided in the 304 response MUST replace the corresponding
 headers from the cache entry. Unless the cache decides to remove the
 cache entry, it MUST also replace the end-to-end headers stored with
 the cache entry with corresponding headers received in the incoming
 response.
 In other words, the set of end-to-end headers received in the
 incoming response overrides all corresponding end-to-end headers
 stored with the cache entry. The cache may add Warning headers (see
 section 14.45) to this set.
 If a header field-name in the incoming response matches more than one
 header in the cache entry, all such old headers are replaced.
   Note: this rule allows an origin server to use a 304 (Not Modified)
   response to update any header associated with a previous response
   for the same entity, although it might not always be meaningful or
   correct to do so. This rule does not allow an origin server to use
   a 304 (not Modified) response to entirely delete a header that it
   had provided with a previous response.

Fielding, et. al. Standards Track [Page 89] RFC 2068 HTTP/1.1 January 1997

13.5.4 Combining Byte Ranges

 A response may transfer only a subrange of the bytes of an entity-
 body, either because the request included one or more Range
 specifications, or because a connection was broken prematurely. After
 several such transfers, a cache may have received several ranges of
 the same entity-body.
 If a cache has a stored non-empty set of subranges for an entity, and
 an incoming response transfers another subrange, the cache MAY
 combine the new subrange with the existing set if both the following
 conditions are met:
   o  Both the incoming response and the cache entry must have a cache
      validator.
   o  The two cache validators must match using the strong comparison
      function (see section 13.3.3).
 If either requirement is not meant, the cache must use only the most
 recent partial response (based on the Date values transmitted with
 every response, and using the incoming response if these values are
 equal or missing), and must discard the other partial information.

13.6 Caching Negotiated Responses

 Use of server-driven content negotiation (section 12), as indicated
 by the presence of a Vary header field in a response, alters the
 conditions and procedure by which a cache can use the response for
 subsequent requests.
 A server MUST use the Vary header field (section 14.43) to inform a
 cache of what header field dimensions are used to select among
 multiple representations of a cachable response. A cache may use the
 selected representation (the entity included with that particular
 response) for replying to subsequent requests on that resource only
 when the subsequent requests have the same or equivalent values for
 all header fields specified in the Vary response-header. Requests
 with a different value for one or more of those header fields would
 be forwarded toward the origin server.
 If an entity tag was assigned to the representation, the forwarded
 request SHOULD be conditional and include the entity tags in an If-
 None-Match header field from all its cache entries for the Request-
 URI. This conveys to the server the set of entities currently held by
 the cache, so that if any one of these entities matches the requested
 entity, the server can use the ETag header in its 304 (Not Modified)
 response to tell the cache which entry is appropriate. If the
 entity-tag of the new response matches that of an existing entry, the

Fielding, et. al. Standards Track [Page 90] RFC 2068 HTTP/1.1 January 1997

 new response SHOULD be used to update the header fields of the
 existing entry, and the result MUST be returned to the client.
 The Vary header field may also inform the cache that the
 representation was selected using criteria not limited to the
 request-headers; in this case, a cache MUST NOT use the response in a
 reply to a subsequent request unless the cache relays the new request
 to the origin server in a conditional request and the server responds
 with 304 (Not Modified), including an entity tag or Content-Location
 that indicates which entity should be used.
 If any of the existing cache entries contains only partial content
 for the associated entity, its entity-tag SHOULD NOT be included in
 the If-None-Match header unless the request is for a range that would
 be fully satisfied by that entry.
 If a cache receives a successful response whose Content-Location
 field matches that of an existing cache entry for the same Request-
 URI, whose entity-tag differs from that of the existing entry, and
 whose Date is more recent than that of the existing entry, the
 existing entry SHOULD NOT be returned in response to future requests,
 and should be deleted from the cache.

13.7 Shared and Non-Shared Caches

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

13.8 Errors or Incomplete Response Cache Behavior

 A cache that receives an incomplete response (for example, with fewer
 bytes of data than specified in a Content-Length header) may store
 the response. However, the cache MUST treat this as a partial
 response.  Partial responses may be combined as described in section
 13.5.4; the result might be a full response or might still be
 partial. A cache MUST NOT return a partial response to a client
 without explicitly marking it as such, using the 206 (Partial
 Content) status code. A cache MUST NOT return a partial response
 using a status code of 200 (OK).
 If a cache receives a 5xx response while attempting to revalidate an
 entry, it may either forward this response to the requesting client,

Fielding, et. al. Standards Track [Page 91] RFC 2068 HTTP/1.1 January 1997

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

13.9 Side Effects of GET and HEAD

 Unless the origin server explicitly prohibits the caching of their
 responses, the application of GET and HEAD methods to any resources
 SHOULD NOT have side effects that would lead to erroneous behavior if
 these responses are taken from a cache. They may still have side
 effects, but a cache is not required to consider such side effects in
 its caching decisions. Caches are always expected to observe an
 origin server's explicit restrictions on caching.
 We note one exception to this rule: since some applications have
 traditionally used GETs and HEADs with query URLs (those containing a
 "?" in the rel_path part) to perform operations with significant side
 effects, caches MUST NOT treat responses to such URLs as fresh unless
 the server provides an explicit expiration time. This specifically
 means that responses from HTTP/1.0 servers for such URIs should not
 be taken from a cache. See section 9.1.1 for related information.

13.10 Invalidation After Updates or Deletions

 The effect of certain methods at the origin server may cause one or
 more existing cache entries to become non-transparently invalid. That
 is, although they may continue to be "fresh," they do not accurately
 reflect what the origin server would return for a new request.
 There is no way for the HTTP protocol to guarantee that all such
 cache entries are marked invalid. For example, the request that
 caused the change at the origin server may not have gone through the
 proxy where a cache entry is stored. However, several rules help
 reduce the likelihood of erroneous behavior.
 In this section, the phrase "invalidate an entity" means that the
 cache should either remove all instances of that entity from its
 storage, or should mark these as "invalid" and in need of a mandatory
 revalidation before they can be returned in response to a subsequent
 request.

Fielding, et. al. Standards Track [Page 92] RFC 2068 HTTP/1.1 January 1997

 Some HTTP methods may invalidate an entity. This is either the entity
 referred to by the Request-URI, or by the Location or Content-
 Location response-headers (if present). These methods are:
   o  PUT
   o  DELETE
   o  POST
 In order to prevent denial of service attacks, an invalidation based
 on the URI in a Location or Content-Location header MUST only be
 performed if the host part is the same as in the Request-URI.

13.11 Write-Through Mandatory

 All methods that may be expected to cause modifications to the origin
 server's resources MUST be written through to the origin server. This
 currently includes all methods except for GET and HEAD. A cache MUST
 NOT reply to such a request from a client before having transmitted
 the request to the inbound server, and having received a
 corresponding response from the inbound server. This does not prevent
 a cache from sending a 100 (Continue) response before the inbound
 server has replied.
 The alternative (known as "write-back" or "copy-back" caching) is not
 allowed in HTTP/1.1, due to the difficulty of providing consistent
 updates and the problems arising from server, cache, or network
 failure prior to write-back.

13.12 Cache Replacement

 If a new cachable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8)
 response is received from a resource while any existing responses for
 the same resource are cached, the cache SHOULD use the new response
 to reply to the current request. It may insert it into cache storage
 and may, if it meets all other requirements, use it to respond to any
 future requests that would previously have caused the old response to
 be returned. If it inserts the new response into cache storage it
 should follow the rules in section 13.5.3.
   Note: a new response that has an older Date header value than
   existing cached responses is not cachable.

13.13 History Lists

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

Fielding, et. al. Standards Track [Page 93] RFC 2068 HTTP/1.1 January 1997

 History mechanisms and caches are different. In particular history
 mechanisms SHOULD NOT try to show a semantically transparent view of
 the current state of a resource. Rather, a history mechanism is meant
 to show exactly what the user saw at the time when the resource was
 retrieved.
 By default, an expiration time does not apply to history mechanisms.
 If the entity is still in storage, a history mechanism should display
 it even if the entity has expired, unless the user has specifically
 configured the agent to refresh expired history documents.
 This should not be construed to prohibit the history mechanism from
 telling the user that a view may be stale.
   Note: if history list mechanisms unnecessarily prevent users from
   viewing stale resources, this will tend to force service authors to
   avoid using HTTP expiration controls and cache controls when they
   would otherwise like to. Service authors may consider it important
   that users not be presented with error messages or warning messages
   when they use navigation controls (such as BACK) to view previously
   fetched resources. Even though sometimes such resources ought not
   to cached, or ought to expire quickly, user interface
   considerations may force service authors to resort to other means
   of preventing caching (e.g. "once-only" URLs) in order not to
   suffer the effects of improperly functioning history mechanisms.

14 Header Field Definitions

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

Fielding, et. al. Standards Track [Page 94] RFC 2068 HTTP/1.1 January 1997

14.1 Accept

 The Accept request-header field can be used to specify certain media
 types which are acceptable for the response. Accept headers can be
 used to indicate that the request is specifically limited to a small
 set of desired types, as in the case of a request for an in-line
 image.
        Accept         = "Accept" ":"
                         #( media-range [ accept-params ] )
        media-range    = ( "*/*"
                         | ( type "/" "*" )
                         | ( type "/" subtype )
                         ) *( ";" parameter )
        accept-params  = ";" "q" "=" qvalue *( accept-extension )
        accept-extension = ";" token [ "=" ( token | quoted-string ) ]
 The asterisk "*" character is used to group media types into ranges,
 with "*/*" indicating all media types and "type/*" indicating all
 subtypes of that type. The media-range MAY include media type
 parameters that are applicable to that range.
 Each media-range MAY be followed by one or more accept-params,
 beginning with the "q" parameter for indicating a relative quality
 factor. The first "q" parameter (if any) separates the media-range
 parameter(s) from the accept-params. Quality factors allow the user
 or user agent to indicate the relative degree of preference for that
 media-range, using the qvalue scale from 0 to 1 (section 3.9). The
 default value is q=1.
   Note: Use of the "q" parameter name to separate media type
   parameters from Accept extension parameters is due to historical
   practice.  Although this prevents any media type parameter named
   "q" from being used with a media range, such an event is believed
   to be unlikely given the lack of any "q" parameters in the IANA
   media type registry and the rare usage of any media type parameters
   in Accept. Future media types should be discouraged from
   registering any parameter named "q".
 The example
        Accept: audio/*; q=0.2, audio/basic
 SHOULD be interpreted as "I prefer audio/basic, but send me any audio
 type if it is the best available after an 80% mark-down in quality."

Fielding, et. al. Standards Track [Page 95] RFC 2068 HTTP/1.1 January 1997

 If no Accept header field is present, then it is assumed that the
 client accepts all media types. If an Accept header field is present,
 and if the server cannot send a response which is acceptable
 according to the combined Accept field value, then the server SHOULD
 send a 406 (not acceptable) response.
 A more elaborate example is
        Accept: text/plain; q=0.5, text/html,
                text/x-dvi; q=0.8, text/x-c
 Verbally, this would be interpreted as "text/html and text/x-c are
 the preferred media types, but if they do not exist, then send the
 text/x-dvi entity, and if that does not exist, send the text/plain
 entity."
 Media ranges can be overridden by more specific media ranges or
 specific media types. If more than one media range applies to a given
 type, the most specific reference has precedence. For example,
        Accept: text/*, text/html, text/html;level=1, */*
 have the following precedence:
        1) text/html;level=1
        2) text/html
        3) text/*
        4) */*
 The media type quality factor associated with a given type is
 determined by finding the media range with the highest precedence
 which matches that type. For example,
        Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
                text/html;level=2;q=0.4, */*;q=0.5
 would cause the following values to be associated:
        text/html;level=1         = 1
        text/html                 = 0.7
        text/plain                = 0.3
        image/jpeg                = 0.5
        text/html;level=2         = 0.4
        text/html;level=3         = 0.7
   Note: A user agent may be provided with a default set of quality
   values for certain media ranges. However, unless the user agent is
   a closed system which cannot interact with other rendering agents,

Fielding, et. al. Standards Track [Page 96] RFC 2068 HTTP/1.1 January 1997

   this default set should be configurable by the user.

14.2 Accept-Charset

 The Accept-Charset request-header field can be used to indicate what
 character sets are acceptable for the response. This field allows
 clients capable of understanding more comprehensive or special-
 purpose character sets to signal that capability to a server which is
 capable of representing documents in those character sets. The ISO-
 8859-1 character set can be assumed to be acceptable to all user
 agents.
        Accept-Charset = "Accept-Charset" ":"
                  1#( charset [ ";" "q" "=" qvalue ] )
 Character set values are described in section 3.4. Each charset may
 be given an associated quality value which represents the user's
 preference for that charset. The default value is q=1. An example is
        Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
 If no Accept-Charset header is present, the default is that any
 character set is acceptable. If an Accept-Charset header is present,
 and if the server cannot send a response which is acceptable
 according to the Accept-Charset header, then the server SHOULD send
 an error response with the 406 (not acceptable) status code, though
 the sending of an unacceptable response is also allowed.

14.3 Accept-Encoding

 The Accept-Encoding request-header field is similar to Accept, but
 restricts the content-coding values (section 14.12) which are
 acceptable in the response.
        Accept-Encoding  = "Accept-Encoding" ":"
                                  #( content-coding )
 An example of its use is
        Accept-Encoding: compress, gzip
 If no Accept-Encoding header is present in a request, the server MAY
 assume that the client will accept any content coding. If an Accept-
 Encoding header is present, and if the server cannot send a response
 which is acceptable according to the Accept-Encoding header, then the
 server SHOULD send an error response with the 406 (Not Acceptable)
 status code.

Fielding, et. al. Standards Track [Page 97] RFC 2068 HTTP/1.1 January 1997

 An empty Accept-Encoding value indicates none are acceptable.

14.4 Accept-Language

 The Accept-Language request-header field is similar to Accept, but
 restricts the set of natural languages that are preferred as a
 response to the request.
        Accept-Language = "Accept-Language" ":"
                          1#( language-range [ ";" "q" "=" qvalue ] )
        language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )
 Each language-range MAY be given an associated quality value which
 represents an estimate of the user's preference for the languages
 specified by that range. The quality value defaults to "q=1". For
 example,
        Accept-Language: da, en-gb;q=0.8, en;q=0.7
 would mean: "I prefer Danish, but will accept British English and
 other types of English." A language-range matches a language-tag if
 it exactly equals the tag, or if it exactly equals a prefix of the
 tag such that the first tag character following the prefix is "-".
 The special range "*", if present in the Accept-Language field,
 matches every tag not matched by any other range present in the
 Accept-Language field.
   Note: This use of a prefix matching rule does not imply that
   language tags are assigned to languages in such a way that it is
   always true that if a user understands a language with a certain
   tag, then this user will also understand all languages with tags
   for which this tag is a prefix. The prefix rule simply allows the
   use of prefix tags if this is the case.
 The language quality factor assigned to a language-tag by the
 Accept-Language field is the quality value of the longest language-
 range in the field that matches the language-tag. If no language-
 range in the field matches the tag, the language quality factor
 assigned is 0. If no Accept-Language header is present in the
 request, the server SHOULD assume that all languages are equally
 acceptable. If an Accept-Language header is present, then all
 languages which are assigned a quality factor greater than 0 are
 acceptable.
 It may be contrary to the privacy expectations of the user to send an
 Accept-Language header with the complete linguistic preferences of
 the user in every request. For a discussion of this issue, see

Fielding, et. al. Standards Track [Page 98] RFC 2068 HTTP/1.1 January 1997

 section 15.7.
   Note: As intelligibility is highly dependent on the individual
   user, it is recommended that client applications make the choice of
   linguistic preference available to the user. If the choice is not
   made available, then the Accept-Language header field must not be
   given in the request.

14.5 Accept-Ranges

 The Accept-Ranges response-header field allows the server to indicate
 its acceptance of range requests for a resource:
        Accept-Ranges     = "Accept-Ranges" ":" acceptable-ranges
        acceptable-ranges = 1#range-unit | "none"
 Origin servers that accept byte-range requests MAY send
        Accept-Ranges: bytes
 but are not required to do so. Clients MAY generate byte-range
 requests without having received this header for the resource
 involved.
 Servers that do not accept any kind of range request for a  resource
 MAY send
        Accept-Ranges: none
 to advise the client not to attempt a range request.

14.6 Age

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

Fielding, et. al. Standards Track [Page 99] RFC 2068 HTTP/1.1 January 1997

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

14.7 Allow

 The Allow entity-header field lists the set of methods supported by
 the resource identified by the Request-URI. The purpose of this field
 is strictly to inform the recipient of valid methods associated with
 the resource. An Allow header field MUST be present in a 405 (Method
 Not Allowed) response.
        Allow          = "Allow" ":" 1#method
 Example of use:
        Allow: GET, HEAD, PUT
 This field cannot prevent a client from trying other methods.
 However, the indications given by the Allow header field value SHOULD
 be followed. The actual set of allowed methods is defined by the
 origin server at the time of each request.
 The Allow header field MAY be provided with a PUT request to
 recommend the methods to be supported by the new or modified
 resource. The server is not required to support these methods and
 SHOULD include an Allow header in the response giving the actual
 supported methods.
 A proxy MUST NOT modify the Allow header field even if it does not
 understand all the methods specified, since the user agent MAY have
 other means of communicating with the origin server.
 The Allow header field does not indicate what methods are implemented
 at the server level. Servers MAY use the Public response-header field
 (section 14.35) to describe what methods are implemented on the
 server as a whole.

14.8 Authorization

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

Fielding, et. al. Standards Track [Page 100] RFC 2068 HTTP/1.1 January 1997

        Authorization  = "Authorization" ":" credentials
 HTTP access authentication is described in section 11. If a request
 is authenticated and a realm specified, the same credentials SHOULD
 be valid for all other requests within this realm.
 When a shared cache (see section 13.7) receives a request containing
 an Authorization field, it MUST NOT return the corresponding response
 as a reply to any other request, unless one of the following specific
 exceptions holds:
   1. If the response includes the "proxy-revalidate" Cache-Control
      directive, the cache MAY use that response in replying to a
      subsequent request, but a proxy cache MUST first revalidate it with
      the origin server, using the request-headers from the new request
      to allow the origin server to authenticate the new request.
   2. If the response includes the "must-revalidate" Cache-Control
      directive, the cache MAY use that response in replying to a
      subsequent request, but all caches MUST first revalidate it with
      the origin server, using the request-headers from the new request
      to allow the origin server to authenticate the new request.
   3. If the response includes the "public" Cache-Control directive, it
      may be returned in reply to any subsequent request.

14.9 Cache-Control

 The Cache-Control general-header field is used to specify directives
 that MUST be obeyed by all caching mechanisms along the
 request/response chain. The directives specify behavior intended to
 prevent caches from adversely interfering with the request or
 response. These directives typically override the default caching
 algorithms. Cache directives are unidirectional in that the presence
 of a directive in a request does not imply that the same directive
 should be given in the response.
   Note that HTTP/1.0 caches may not implement Cache-Control and may
   only implement Pragma: no-cache (see section 14.32).
 Cache directives must be passed through by a proxy or gateway
 application, regardless of their significance to that application,
 since the directives may be applicable to all recipients along the
 request/response chain. It is not possible to specify a cache-
 directive for a specific cache.
        Cache-Control   = "Cache-Control" ":" 1#cache-directive
        cache-directive = cache-request-directive
                        | cache-response-directive

Fielding, et. al. Standards Track [Page 101] RFC 2068 HTTP/1.1 January 1997

        cache-request-directive =
                          "no-cache" [ "=" <"> 1#field-name <"> ]
                        | "no-store"
                        | "max-age" "=" delta-seconds
                        | "max-stale" [ "=" delta-seconds ]
                        | "min-fresh" "=" delta-seconds
                        | "only-if-cached"
                        | cache-extension
        cache-response-directive =
                          "public"
                        | "private" [ "=" <"> 1#field-name <"> ]
                        | "no-cache" [ "=" <"> 1#field-name <"> ]
                        | "no-store"
                        | "no-transform"
                        | "must-revalidate"
                        | "proxy-revalidate"
                        | "max-age" "=" delta-seconds
                        | cache-extension
        cache-extension = token [ "=" ( token | quoted-string ) ]
 When a directive appears without any 1#field-name parameter, the
 directive applies to the entire request or response. When such a
 directive appears with a 1#field-name parameter, it applies only to
 the named field or fields, and not to the rest of the request or
 response.  This mechanism supports extensibility; implementations of
 future versions of the HTTP protocol may apply these directives to
 header fields not defined in HTTP/1.1.
 The cache-control directives can be broken down into these general
 categories:
   o  Restrictions on what is cachable; these may only be imposed by the
      origin server.
   o  Restrictions on what may be stored by a cache; these may be imposed
      by either the origin server or the user agent.
   o  Modifications of the basic expiration mechanism; these may be
      imposed by either the origin server or the user agent.
   o  Controls over cache revalidation and reload; these may only be
      imposed by a user agent.
   o  Control over transformation of entities.
   o  Extensions to the caching system.

Fielding, et. al. Standards Track [Page 102] RFC 2068 HTTP/1.1 January 1997

14.9.1 What is Cachable

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

public

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

private

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

no-cache

Indicates that all or part of the response message MUST NOT be cached
anywhere. This allows an origin server to prevent caching even by
caches that have been configured to return stale responses to client
requests.
Note: Most HTTP/1.0 caches will not recognize or obey this
directive.

14.9.2 What May be Stored by Caches

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

Fielding, et. al. Standards Track [Page 103] RFC 2068 HTTP/1.1 January 1997

 Even when this directive is associated with a response, users may
 explicitly store such a response outside of the caching system (e.g.,
 with a "Save As" dialog). History buffers may store such responses as
 part of their normal operation.
 The purpose of this directive is to meet the stated requirements of
 certain users and service authors who are concerned about accidental
 releases of information via unanticipated accesses to cache data
 structures. While the use of this directive may improve privacy in
 some cases, we caution that it is NOT in any way a reliable or
 sufficient mechanism for ensuring privacy. In particular, malicious
 or compromised caches may not recognize or obey this directive; and
 communications networks may be vulnerable to eavesdropping.

14.9.3 Modifications of the Basic Expiration Mechanism

 The expiration time of an entity may be specified by the origin
 server using the Expires header (see section 14.21). Alternatively,
 it may be specified using the max-age directive in a response.
 If a response includes both an Expires header and a max-age
 directive, the max-age directive overrides the Expires header, even
 if the Expires header is more restrictive. This rule allows an origin
 server to provide, for a given response, a longer expiration time to
 an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This may be
 useful if certain HTTP/1.0 caches improperly calculate ages or
 expiration times, perhaps due to desynchronized clocks.
   Note: most older caches, not compliant with this specification, do
   not implement any Cache-Control directives.  An origin server
   wishing to use a Cache-Control directive that restricts, but does
   not prevent, caching by an HTTP/1.1-compliant cache may exploit the
   requirement that the max-age directive overrides the Expires
   header, and the fact that non-HTTP/1.1-compliant caches do not
   observe the max-age directive.
 Other directives allow an user agent to modify the basic expiration
 mechanism. These directives may be specified on a request:
 max-age
   Indicates that the client is willing to accept a response whose age
   is no greater than the specified time in seconds. Unless max-stale
   directive is also included, the client is not willing to accept a
   stale response.
 min-fresh
   Indicates that the client is willing to accept a response whose
   freshness lifetime is no less than its current age plus the

Fielding, et. al. Standards Track [Page 104] RFC 2068 HTTP/1.1 January 1997

   specified time in seconds. That is, the client wants a response
   that will still be fresh for at least the specified number of
   seconds.
 max-stale
   Indicates that the client is willing to accept a response that has
   exceeded its expiration time. If max-stale is assigned a value,
   then the client is willing to accept a response that has exceeded
   its expiration time by no more than the specified number of
   seconds. If no value is assigned to max-stale, then the client is
   willing to accept a stale response of any age.
 If a cache returns a stale response, either because of a max-stale
 directive on a request, or because the cache is configured to
 override the expiration time of a response, the cache MUST attach a
 Warning header to the stale response, using Warning 10 (Response is
 stale).

14.9.4 Cache Revalidation and Reload Controls

 Sometimes an user agent may want or need to insist that a cache
 revalidate its cache entry with the origin server (and not just with
 the next cache along the path to the origin server), or to reload its
 cache entry from the origin server. End-to-end revalidation may be
 necessary if either the cache or the origin server has overestimated
 the expiration time of the cached response. End-to-end reload may be
 necessary if the cache entry has become corrupted for some reason.
 End-to-end revalidation may be requested either when the client does
 not have its own local cached copy, in which case we call it
 "unspecified end-to-end revalidation", or when the client does have a
 local cached copy, in which case we call it "specific end-to-end
 revalidation."
 The client can specify these three kinds of action using Cache-
 Control request directives:
 End-to-end reload
   The request includes a "no-cache" Cache-Control directive or, for
   compatibility with HTTP/1.0 clients, "Pragma: no-cache". No field
   names may be included with the no-cache directive in a request. The
   server MUST NOT use a cached copy when responding to such a
   request.
 Specific end-to-end revalidation
   The request includes a "max-age=0" Cache-Control directive, which
   forces each cache along the path to the origin server to revalidate
   its own entry, if any, with the next cache or server. The initial

Fielding, et. al. Standards Track [Page 105] RFC 2068 HTTP/1.1 January 1997

   request includes a cache-validating conditional with the client's
   current validator.
 Unspecified end-to-end revalidation
   The request includes "max-age=0" Cache-Control directive, which
   forces each cache along the path to the origin server to revalidate
   its own entry, if any, with the next cache or server. The initial
   request does not include a cache-validating conditional; the first
   cache along the path (if any) that holds a cache entry for this
   resource includes a cache-validating conditional with its current
   validator.
 When an intermediate cache is forced, by means of a max-age=0
 directive, to revalidate its own cache entry, and the client has
 supplied its own validator in the request, the supplied validator may
 differ from the validator currently stored with the cache entry. In
 this case, the cache may use either validator in making its own
 request without affecting semantic transparency.
 However, the choice of validator may affect performance. The best
 approach is for the intermediate cache to use its own validator when
 making its request. If the server replies with 304 (Not Modified),
 then the cache should return its now validated copy to the client
 with a 200 (OK) response. If the server replies with a new entity and
 cache validator, however, the intermediate cache should compare the
 returned validator with the one provided in the client's request,
 using the strong comparison function. If the client's validator is
 equal to the origin server's, then the intermediate cache simply
 returns 304 (Not Modified). Otherwise, it returns the new entity with
 a 200 (OK) response.
 If a request includes the no-cache directive, it should not include
 min-fresh, max-stale, or max-age.
 In some cases, such as times of extremely poor network connectivity,
 a client may want a cache to return only those responses that it
 currently has stored, and not to reload or revalidate with the origin
 server. To do this, the client may include the only-if-cached
 directive in a request. If it receives this directive, a cache SHOULD
 either respond using a cached entry that is consistent with the other
 constraints of the request, or respond with a 504 (Gateway Timeout)
 status. However, if a group of caches is being operated as a unified
 system with good internal connectivity, such a request MAY be
 forwarded within that group of caches.
 Because a cache may be configured to ignore a server's specified
 expiration time, and because a client request may include a max-stale
 directive (which has a similar effect), the protocol also includes a

Fielding, et. al. Standards Track [Page 106] RFC 2068 HTTP/1.1 January 1997

 mechanism for the origin server to require revalidation of a cache
 entry on any subsequent use. When the must-revalidate directive is
 present in a response received by a cache, that cache MUST NOT use
 the entry after it becomes stale to respond to a subsequent request
 without first revalidating it with the origin server. (I.e., the
 cache must do an end-to-end revalidation every time, if, based solely
 on the origin server's Expires or max-age value, the cached response
 is stale.)
 The must-revalidate directive is necessary to support reliable
 operation for certain protocol features. In all circumstances an
 HTTP/1.1 cache MUST obey the must-revalidate directive; in
 particular, if the cache cannot reach the origin server for any
 reason, it MUST generate a 504 (Gateway Timeout) response.
 Servers should send the must-revalidate directive if and only if
 failure to revalidate a request on the entity could result in
 incorrect operation, such as a silently unexecuted financial
 transaction.  Recipients MUST NOT take any automated action that
 violates this directive, and MUST NOT automatically provide an
 unvalidated copy of the entity if revalidation fails.
 Although this is not recommended, user agents operating under severe
 connectivity constraints may violate this directive but, if so, MUST
 explicitly warn the user that an unvalidated response has been
 provided.  The warning MUST be provided on each unvalidated access,
 and SHOULD require explicit user confirmation.
 The proxy-revalidate directive has the same meaning as the must-
 revalidate directive, except that it does not apply to non-shared
 user agent caches. It can be used on a response to an authenticated
 request to permit the user's cache to store and later return the
 response without needing to revalidate it (since it has already been
 authenticated once by that user), while still requiring proxies that
 service many users to revalidate each time (in order to make sure
 that each user has been authenticated). Note that such authenticated
 responses also need the public cache control directive in order to
 allow them to be cached at all.

14.9.5 No-Transform Directive

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

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 Serious operational problems have already occurred, however, when
 these transformations have been applied to entity bodies intended for
 certain kinds of applications. For example, applications for medical
 imaging, scientific data analysis and those using end-to-end
 authentication, all depend on receiving an entity body that is bit
 for bit identical to the original entity-body.
 Therefore, if a response includes the no-transform directive, an
 intermediate cache or proxy MUST NOT change those headers that are
 listed in section 13.5.2 as being subject to the no-transform
 directive.  This implies that the cache or proxy must not change any
 aspect of the entity-body that is specified by these headers.

14.9.6 Cache Control Extensions

 The Cache-Control header field can be extended through the use of one
 or more cache-extension tokens, each with an optional assigned value.
 Informational extensions (those which do not require a change in
 cache behavior) may be added without changing the semantics of other
 directives. Behavioral extensions are designed to work by acting as
 modifiers to the existing base of cache directives. Both the new
 directive and the standard directive are supplied, such that
 applications which do not understand the new directive will default
 to the behavior specified by the standard directive, and those that
 understand the new directive will recognize it as modifying the
 requirements associated with the standard directive.  In this way,
 extensions to the Cache-Control directives can be made without
 requiring changes to the base protocol.
 This extension mechanism depends on a HTTP cache obeying all of the
 cache-control directives defined for its native HTTP-version, obeying
 certain extensions, and ignoring all directives that it does not
 understand.
 For example, consider a hypothetical new response directive called
 "community" which acts as a modifier to the "private" directive. We
 define this new directive to mean that, in addition to any non-shared
 cache, any cache which is shared only by members of the community
 named within its value may cache the response. An origin server
 wishing to allow the "UCI" community to use an otherwise private
 response in their shared cache(s) may do so by including
        Cache-Control: private, community="UCI"
 A cache seeing this header field will act correctly even if the cache
 does not understand the "community" cache-extension, since it will
 also see and understand the "private" directive and thus default to
 the safe behavior.

Fielding, et. al. Standards Track [Page 108] RFC 2068 HTTP/1.1 January 1997

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

14.10 Connection

 The Connection general-header field allows the sender to specify
 options that are desired for that particular connection and MUST NOT
 be communicated by proxies over further connections.
 The Connection header has the following grammar:
        Connection-header = "Connection" ":" 1#(connection-token)
        connection-token  = token
 HTTP/1.1 proxies MUST parse the Connection header field before a
 message is forwarded and, for each connection-token in this field,
 remove any header field(s) from the message with the same name as the
 connection-token. Connection options are signaled by the presence of
 a connection-token in the Connection header field, not by any
 corresponding additional header field(s), since the additional header
 field may not be sent if there are no parameters associated with that
 connection option.  HTTP/1.1 defines the "close" connection option
 for the sender to signal that the connection will be closed after
 completion of the response. For example,
        Connection: close
 in either the request or the response header fields indicates that
 the connection should not be considered `persistent' (section 8.1)
 after the current request/response is complete.
 HTTP/1.1 applications that do not support persistent connections MUST
 include the "close" connection option in every message.

14.11 Content-Base

 The Content-Base entity-header field may be used to specify the base
 URI for resolving relative URLs within the entity. This header field
 is described as Base in RFC 1808, which is expected to be revised.
        Content-Base      = "Content-Base" ":" absoluteURI
 If no Content-Base field is present, the base URI of an entity is
 defined either by its Content-Location (if that Content-Location URI
 is an absolute URI) or the URI used to initiate the request, in that

Fielding, et. al. Standards Track [Page 109] RFC 2068 HTTP/1.1 January 1997

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

14.12 Content-Encoding

 The Content-Encoding entity-header field is used as a modifier to the
 media-type. When present, its value indicates what additional content
 codings have been applied to the entity-body, and thus what decoding
 mechanisms MUST be applied in order to obtain the media-type
 referenced by the Content-Type header field. Content-Encoding is
 primarily used to allow a document to be compressed without losing
 the identity of its underlying media type.
        Content-Encoding  = "Content-Encoding" ":" 1#content-coding
 Content codings are defined in section 3.5. An example of its use is
        Content-Encoding: gzip
 The Content-Encoding is a characteristic of the entity identified by
 the Request-URI. Typically, the entity-body is stored with this
 encoding and is only decoded before rendering or analogous usage.
 If multiple encodings have been applied to an entity, the content
 codings MUST be listed in the order in which they were applied.
 Additional information about the encoding parameters MAY be provided
 by other entity-header fields not defined by this specification.

14.13 Content-Language

 The Content-Language entity-header field describes the natural
 language(s) of the intended audience for the enclosed entity. Note
 that this may not be equivalent to all the languages used within the
 entity-body.
        Content-Language  = "Content-Language" ":" 1#language-tag
 Language tags are defined in section 3.10. The primary purpose of
 Content-Language is to allow a user to identify and differentiate
 entities according to the user's own preferred language. Thus, if the
 body content is intended only for a Danish-literate audience, the
 appropriate field is
        Content-Language: da
 If no Content-Language is specified, the default is that the content
 is intended for all language audiences. This may mean that the sender

Fielding, et. al. Standards Track [Page 110] RFC 2068 HTTP/1.1 January 1997

 does not consider it to be specific to any natural language, or that
 the sender does not know for which language it is intended.
 Multiple languages MAY be listed for content that is intended for
 multiple audiences. For example, a rendition of the "Treaty of
 Waitangi," presented simultaneously in the original Maori and English
 versions, would call for
        Content-Language: mi, en
 However, just because multiple languages are present within an entity
 does not mean that it is intended for multiple linguistic audiences.
 An example would be a beginner's language primer, such as "A First
 Lesson in Latin," which is clearly intended to be used by an
 English-literate audience. In this case, the Content-Language should
 only include "en".
 Content-Language may be applied to any media type -- it is not
 limited to textual documents.

14.14 Content-Length

 The Content-Length entity-header field indicates the size of the
 message-body, in decimal number of octets, sent to the recipient or,
 in the case of the HEAD method, the size of the entity-body that
 would have been sent had the request been a GET.
        Content-Length    = "Content-Length" ":" 1*DIGIT
 An example is
        Content-Length: 3495
 Applications SHOULD use this field to indicate the size of the
 message-body to be transferred, regardless of the media type of the
 entity. It must be possible for the recipient to reliably determine
 the end of HTTP/1.1 requests containing an entity-body, e.g., because
 the request has a valid Content-Length field, uses Transfer-Encoding:
 chunked or a multipart body.
 Any Content-Length greater than or equal to zero is a valid value.
 Section 4.4 describes how to determine the length of a message-body
 if a Content-Length is not given.

Fielding, et. al. Standards Track [Page 111] RFC 2068 HTTP/1.1 January 1997

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

14.15 Content-Location

 The Content-Location entity-header field may be used to supply the
 resource location for the entity enclosed in the message. In the case
 where a resource has multiple entities associated with it, and those
 entities actually have separate locations by which they might be
 individually accessed, the server should provide a Content-Location
 for the particular variant which is returned. In addition, a server
 SHOULD provide a Content-Location for the resource corresponding to
 the response entity.
        Content-Location = "Content-Location" ":"
                          ( absoluteURI | relativeURI )
 If no Content-Base header field is present, the value of Content-
 Location also defines the base URL for the entity (see section
 14.11).
 The Content-Location value is not a replacement for the original
 requested URI; it is only a statement of the location of the resource
 corresponding to this particular entity at the time of the request.
 Future requests MAY use the Content-Location URI if the desire is to
 identify the source of that particular entity.
 A cache cannot assume that an entity with a Content-Location
 different from the URI used to retrieve it can be used to respond to
 later requests on that Content-Location URI. However, the Content-
 Location can be used to differentiate between multiple entities
 retrieved from a single requested resource, as described in section
 13.6.
 If the Content-Location is a relative URI, the URI is interpreted
 relative to any Content-Base URI provided in the response. If no
 Content-Base is provided, the relative URI is interpreted relative to
 the Request-URI.

Fielding, et. al. Standards Track [Page 112] RFC 2068 HTTP/1.1 January 1997

14.16 Content-MD5

 The Content-MD5 entity-header field, as defined in RFC 1864 [23], is
 an MD5 digest of the entity-body for the purpose of providing an
 end-to-end message integrity check (MIC) of the entity-body. (Note: a
 MIC is good for detecting accidental modification of the entity-body
 in transit, but is not proof against malicious attacks.)
         Content-MD5   = "Content-MD5" ":" md5-digest
         md5-digest   = <base64 of 128 bit MD5 digest as per RFC 1864>
 The Content-MD5 header field may be generated by an origin server to
 function as an integrity check of the entity-body. Only origin
 servers may generate the Content-MD5 header field; proxies and
 gateways MUST NOT generate it, as this would defeat its value as an
 end-to-end integrity check. Any recipient of the entity-body,
 including gateways and proxies, MAY check that the digest value in
 this header field matches that of the entity-body as received.
 The MD5 digest is computed based on the content of the entity-body,
 including any Content-Encoding that has been applied, but not
 including any Transfer-Encoding that may have been applied to the
 message-body. If the message is received with a Transfer-Encoding,
 that encoding must be removed prior to checking the Content-MD5 value
 against the received entity.
 This has the result that the digest is computed on the octets of the
 entity-body exactly as, and in the order that, they would be sent if
 no Transfer-Encoding were being applied.
 HTTP extends RFC 1864 to permit the digest to be computed for MIME
 composite media-types (e.g., multipart/* and message/rfc822), but
 this does not change how the digest is computed as defined in the
 preceding paragraph.
   Note: There are several consequences of this. The entity-body for
   composite types may contain many body-parts, each with its own MIME
   and HTTP headers (including Content-MD5, Content-Transfer-Encoding,
   and Content-Encoding headers). If a body-part has a Content-
   Transfer-Encoding or Content-Encoding header, it is assumed that
   the content of the body-part has had the encoding applied, and the
   body-part is included in the Content-MD5 digest as is -- i.e.,
   after the application. The Transfer-Encoding header field is not
   allowed within body-parts.
   Note: while the definition of Content-MD5 is exactly the same for
   HTTP as in RFC 1864 for MIME entity-bodies, there are several ways

Fielding, et. al. Standards Track [Page 113] RFC 2068 HTTP/1.1 January 1997

   in which the application of Content-MD5 to HTTP entity-bodies
   differs from its application to MIME entity-bodies. One is that
   HTTP, unlike MIME, does not use Content-Transfer-Encoding, and does
   use Transfer-Encoding and Content-Encoding. Another is that HTTP
   more frequently uses binary content types than MIME, so it is worth
   noting that, in such cases, the byte order used to compute the
   digest is the transmission byte order defined for the type. Lastly,
   HTTP allows transmission of text types with any of several line
   break conventions and not just the canonical form using CRLF.
   Conversion of all line breaks to CRLF should not be done before
   computing or checking the digest: the line break convention used in
   the text actually transmitted should be left unaltered when
   computing the digest.

14.17 Content-Range

 The Content-Range entity-header is sent with a partial entity-body to
 specify where in the full entity-body the partial body should be
 inserted. It also indicates the total size of the full entity-body.
 When a server returns a partial response to a client, it must
 describe both the extent of the range covered by the response, and
 the length of the entire entity-body.
        Content-Range = "Content-Range" ":" content-range-spec
        content-range-spec      = byte-content-range-spec
        byte-content-range-spec = bytes-unit SP first-byte-pos "-"
                                  last-byte-pos "/" entity-length
        entity-length           = 1*DIGIT
 Unlike byte-ranges-specifier values, a byte-content-range-spec may
 only specify one range, and must contain absolute byte positions for
 both the first and last byte of the range.
 A byte-content-range-spec whose last-byte-pos value is less than its
 first-byte-pos value, or whose entity-length value is less than or
 equal to its last-byte-pos value, is invalid. The recipient of an
 invalid byte-content-range-spec MUST ignore it and any content
 transferred along with it.

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 Examples of byte-content-range-spec values, assuming that the entity
 contains a total of 1234 bytes:
   o  The first 500 bytes:
        bytes 0-499/1234
   o  The second 500 bytes:
        bytes 500-999/1234
   o  All except for the first 500 bytes:
        bytes 500-1233/1234
   o  The last 500 bytes:
        bytes 734-1233/1234
 When an HTTP message includes the content of a single range (for
 example, a response to a request for a single range, or to a request
 for a set of ranges that overlap without any holes), this content is
 transmitted with a Content-Range header, and a Content-Length header
 showing the number of bytes actually transferred. For example,
        HTTP/1.1 206 Partial content
        Date: Wed, 15 Nov 1995 06:25:24 GMT
        Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
        Content-Range: bytes 21010-47021/47022
        Content-Length: 26012
        Content-Type: image/gif
 When an HTTP message includes the content of multiple ranges (for
 example, a response to a request for multiple non-overlapping
 ranges), these are transmitted as a multipart MIME message. The
 multipart MIME content-type used for this purpose is defined in this
 specification to be "multipart/byteranges". See appendix 19.2 for its
 definition.
 A client that cannot decode a MIME multipart/byteranges message
 should not ask for multiple byte-ranges in a single request.
 When a client requests multiple byte-ranges in one request, the
 server SHOULD return them in the order that they appeared in the
 request.
 If the server ignores a byte-range-spec because it is invalid, the
 server should treat the request as if the invalid Range header field

Fielding, et. al. Standards Track [Page 115] RFC 2068 HTTP/1.1 January 1997

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

14.18 Content-Type

 The Content-Type entity-header field indicates the media type of the
 entity-body sent to the recipient or, in the case of the HEAD method,
 the media type that would have been sent had the request been a GET.
        Content-Type   = "Content-Type" ":" media-type
 Media types are defined in section 3.7. An example of the field is
        Content-Type: text/html; charset=ISO-8859-4
 Further discussion of methods for identifying the media type of an
 entity is provided in section 7.2.1.

14.19 Date

 The Date general-header field represents the date and time at which
 the message was originated, having the same semantics as orig-date in
 RFC 822. The field value is an HTTP-date, as described in section
 3.3.1.
        Date  = "Date" ":" HTTP-date
 An example is
        Date: Tue, 15 Nov 1994 08:12:31 GMT
 If a message is received via direct connection with the user agent
 (in the case of requests) or the origin server (in the case of
 responses), then the date can be assumed to be the current date at
 the receiving end. However, since the date--as it is believed by the
 origin--is important for evaluating cached responses, origin servers
 MUST include a Date header field in all responses. Clients SHOULD
 only send a Date header field in messages that include an entity-
 body, as in the case of the PUT and POST requests, and even then it
 is optional. A received message which does not have a Date header
 field SHOULD be assigned one by the recipient if the message will be
 cached by that recipient or gatewayed via a protocol which requires a
 Date.

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 In theory, the date SHOULD represent the moment just before the
 entity is generated. In practice, the date can be generated at any
 time during the message origination without affecting its semantic
 value.
 The format of the Date is an absolute date and time as defined by
 HTTP-date in section 3.3; it MUST be sent in RFC1123 [8]-date format.

14.20 ETag

 The ETag entity-header field defines the entity tag for the
 associated entity. The headers used with entity tags are described in
 sections 14.20, 14.25, 14.26 and 14.43. The entity tag may be used
 for comparison with other entities from the same resource (see
 section 13.3.2).
       ETag = "ETag" ":" entity-tag
 Examples:
       ETag: "xyzzy"
       ETag: W/"xyzzy"
       ETag: ""

14.21 Expires

 The Expires entity-header field gives the date/time after which the
 response should be considered stale. A stale cache entry may not
 normally be returned by a cache (either a proxy cache or an user
 agent cache) unless it is first validated with the origin server (or
 with an intermediate cache that has a fresh copy of the entity). See
 section 13.2 for further discussion of the expiration model.
 The presence of an Expires field does not imply that the original
 resource will change or cease to exist at, before, or after that
 time.
 The format is an absolute date and time as defined by HTTP-date in
 section 3.3; it MUST be in RFC1123-date format:
       Expires = "Expires" ":" HTTP-date

Fielding, et. al. Standards Track [Page 117] RFC 2068 HTTP/1.1 January 1997

 An example of its use is
       Expires: Thu, 01 Dec 1994 16:00:00 GMT
   Note: if a response includes a Cache-Control field with the max-age
   directive, that directive overrides the Expires field.
 HTTP/1.1 clients and caches MUST treat other invalid date formats,
 especially including the value "0", as in the past (i.e., "already
 expired").
 To mark a response as "already expired," an origin server should use
 an Expires date that is equal to the Date header value. (See the
 rules for expiration calculations in section 13.2.4.)
 To mark a response as "never expires," an origin server should use an
 Expires date approximately one year from the time the response is
 sent.  HTTP/1.1 servers should not send Expires dates more than one
 year in the future.
 The presence of an Expires header field with a date value of some
 time in the future on an response that otherwise would by default be
 non-cacheable indicates that the response is cachable, unless
 indicated otherwise by a Cache-Control header field (section 14.9).

14.22 From

 The From request-header field, if given, SHOULD contain an Internet
 e-mail address for the human user who controls the requesting user
 agent.  The address SHOULD be machine-usable, as defined by mailbox
 in RFC 822 (as updated by RFC 1123 ):
        From   = "From" ":" mailbox
 An example is:
        From: webmaster@w3.org
 This header field MAY be used for logging purposes and as a means for
 identifying the source of invalid or unwanted requests. It SHOULD NOT
 be used as an insecure form of access protection. The interpretation
 of this field is that the request is being performed on behalf of the
 person given, who accepts responsibility for the method performed. In
 particular, robot agents SHOULD include this header so that the
 person responsible for running the robot can be contacted if problems
 occur on the receiving end.

Fielding, et. al. Standards Track [Page 118] RFC 2068 HTTP/1.1 January 1997

 The Internet e-mail address in this field MAY be separate from the
 Internet host which issued the request. For example, when a request
 is passed through a proxy the original issuer's address SHOULD be
 used.
   Note: The client SHOULD not send the From header field without the
   user's approval, as it may conflict with the user's privacy
   interests or their site's security policy. It is strongly
   recommended that the user be able to disable, enable, and modify
   the value of this field at any time prior to a request.

14.23 Host

 The Host request-header field specifies the Internet host and port
 number of the resource being requested, as obtained from the original
 URL given by the user or referring resource (generally an HTTP URL,
 as described in section 3.2.2). The Host field value MUST represent
 the network location of the origin server or gateway given by the
 original URL. This allows the origin server or gateway to
 differentiate between internally-ambiguous URLs, such as the root "/"
 URL of a server for multiple host names on a single IP address.
        Host = "Host" ":" host [ ":" port ]    ; Section 3.2.2
 A "host" without any trailing port information implies the default
 port for the service requested (e.g., "80" for an HTTP URL). For
 example, a request on the origin server for
 <http://www.w3.org/pub/WWW/> MUST include:
        GET /pub/WWW/ HTTP/1.1
        Host: www.w3.org
 A client MUST include a Host header field in all HTTP/1.1 request
 messages on the Internet (i.e., on any message corresponding to a
 request for a URL which includes an Internet host address for the
 service being requested). If the Host field is not already present,
 an HTTP/1.1 proxy MUST add a Host field to the request message prior
 to forwarding it on the Internet. All Internet-based HTTP/1.1 servers
 MUST respond with a 400 status code to any HTTP/1.1 request message
 which lacks a Host header field.
 See sections 5.2 and 19.5.1 for other requirements relating to Host.

14.24 If-Modified-Since

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

Fielding, et. al. Standards Track [Page 119] RFC 2068 HTTP/1.1 January 1997

 be returned from the server; instead, a 304 (not modified) response
 will be returned without any message-body.
        If-Modified-Since = "If-Modified-Since" ":" HTTP-date
 An example of the field is:
        If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
 A GET method with an If-Modified-Since header and no Range header
 requests that the identified entity be transferred only if it has
 been modified since the date given by the If-Modified-Since header.
 The algorithm for determining this includes the following cases:
 a)If the request would normally result in anything other than a 200
   (OK) status, or if the passed If-Modified-Since date is invalid, the
   response is exactly the same as for a normal GET. A date which is
   later than the server's current time is invalid.
 b)If the variant has been modified since the If-Modified-Since date,
   the response is exactly the same as for a normal GET.
 c)If the variant has not been modified since a valid If-Modified-Since
   date, the server MUST return a 304 (Not Modified) response.
 The purpose of this feature is to allow efficient updates of cached
 information with a minimum amount of transaction overhead.
   Note that the Range request-header field modifies the meaning of
   If-Modified-Since; see section 14.36 for full details.
   Note that If-Modified-Since times are interpreted by the server,
   whose clock may not be synchronized with the client.
 Note that if a client uses an arbitrary date in the If-Modified-Since
 header instead of a date taken from the Last-Modified header for the
 same request, the client should be aware of the fact that this date
 is interpreted in the server's understanding of time. The client
 should consider unsynchronized clocks and rounding problems due to
 the different encodings of time between the client and server. This
 includes the possibility of race conditions if the document has
 changed between the time it was first requested and the If-Modified-
 Since date of a subsequent request, and the possibility of clock-
 skew-related problems if the If-Modified-Since date is derived from
 the client's clock without correction to the server's clock.
 Corrections for different time bases between client and server are at
 best approximate due to network latency.

Fielding, et. al. Standards Track [Page 120] RFC 2068 HTTP/1.1 January 1997

14.25 If-Match

 The If-Match request-header field is used with a method to make it
 conditional. A client that has one or more entities previously
 obtained from the resource can verify that one of those entities is
 current by including a list of their associated entity tags in the
 If-Match header field. The purpose of this feature is to allow
 efficient updates of cached information with a minimum amount of
 transaction overhead. It is also used, on updating requests, to
 prevent inadvertent modification of the wrong version of a resource.
 As a special case, the value "*" matches any current entity of the
 resource.
        If-Match = "If-Match" ":" ( "*" | 1#entity-tag )
 If any of the entity tags match the entity tag of the entity that
 would have been returned in the response to a similar GET request
 (without the If-Match header) on that resource, or if "*" is given
 and any current entity exists for that resource, then the server MAY
 perform the requested method as if the If-Match header field did not
 exist.
 A server MUST use the strong comparison function (see section 3.11)
 to compare the entity tags in If-Match.
 If none of the entity tags match, or if "*" is given and no current
 entity exists, the server MUST NOT perform the requested method, and
 MUST return a 412 (Precondition Failed) response. This behavior is
 most useful when the client wants to prevent an updating method, such
 as PUT, from modifying a resource that has changed since the client
 last retrieved it.
 If the request would, without the If-Match header field, result in
 anything other than a 2xx status, then the If-Match header MUST be
 ignored.
 The meaning of "If-Match: *" is that the method SHOULD be performed
 if the representation selected by the origin server (or by a cache,
 possibly using the Vary mechanism, see section 14.43) exists, and
 MUST NOT be performed if the representation does not exist.

Fielding, et. al. Standards Track [Page 121] RFC 2068 HTTP/1.1 January 1997

 A request intended to update a resource (e.g., a PUT) MAY include an
 If-Match header field to signal that the request method MUST NOT be
 applied if the entity corresponding to the If-Match value (a single
 entity tag) is no longer a representation of that resource.  This
 allows the user to indicate that they do not wish the request to be
 successful if the resource has been changed without their knowledge.
 Examples:
        If-Match: "xyzzy"
        If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
        If-Match: *

14.26 If-None-Match

 The If-None-Match request-header field is used with a method to make
 it conditional. A client that has one or more entities previously
 obtained from the resource can verify that none of those entities is
 current by including a list of their associated entity tags in the
 If-None-Match header field. The purpose of this feature is to allow
 efficient updates of cached information with a minimum amount of
 transaction overhead. It is also used, on updating requests, to
 prevent inadvertent modification of a resource which was not known to
 exist.
 As a special case, the value "*" matches any current entity of the
 resource.
        If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
 If any of the entity tags match the entity tag of the entity that
 would have been returned in the response to a similar GET request
 (without the If-None-Match header) on that resource, or if "*" is
 given and any current entity exists for that resource, then the
 server MUST NOT perform the requested method. Instead, if the request
 method was GET or HEAD, the server SHOULD respond with a 304 (Not
 Modified) response, including the cache-related entity-header fields
 (particularly ETag) of one of the entities that matched. For all
 other request methods, the server MUST respond with a status of 412
 (Precondition Failed).
 See section 13.3.3 for rules on how to determine if two entity tags
 match. The weak comparison function can only be used with GET or HEAD
 requests.
 If none of the entity tags match, or if "*" is given and no current
 entity exists, then the server MAY perform the requested method as if
 the If-None-Match header field did not exist.

Fielding, et. al. Standards Track [Page 122] RFC 2068 HTTP/1.1 January 1997

 If the request would, without the If-None-Match header field, result
 in anything other than a 2xx status, then the If-None-Match header
 MUST be ignored.
 The meaning of "If-None-Match: *" is that the method MUST NOT be
 performed if the representation selected by the origin server (or by
 a cache, possibly using the Vary mechanism, see section 14.43)
 exists, and SHOULD be performed if the representation does not exist.
 This feature may be useful in preventing races between PUT
 operations.
 Examples:
        If-None-Match: "xyzzy"
        If-None-Match: W/"xyzzy"
        If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
        If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
        If-None-Match: *

14.27 If-Range

 If a client has a partial copy of an entity in its cache, and wishes
 to have an up-to-date copy of the entire entity in its cache, it
 could use the Range request-header with a conditional GET (using
 either or both of If-Unmodified-Since and If-Match.) However, if the
 condition fails because the entity has been modified, the client
 would then have to make a second request to obtain the entire current
 entity-body.
 The If-Range header allows a client to "short-circuit" the second
 request. Informally, its meaning is `if the entity is unchanged, send
 me the part(s) that I am missing; otherwise, send me the entire new
 entity.'
         If-Range = "If-Range" ":" ( entity-tag | HTTP-date )
 If the client has no entity tag for an entity, but does have a Last-
 Modified date, it may use that date in a If-Range header. (The server
 can distinguish between a valid HTTP-date and any form of entity-tag
 by examining no more than two characters.) The If-Range header should
 only be used together with a Range header, and must be ignored if the
 request does not include a Range header, or if the server does not
 support the sub-range operation.

Fielding, et. al. Standards Track [Page 123] RFC 2068 HTTP/1.1 January 1997

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

14.28 If-Unmodified-Since

 The If-Unmodified-Since request-header field is used with a method to
 make it conditional. If the requested resource has not been modified
 since the time specified in this field, the server should perform the
 requested operation as if the If-Unmodified-Since header were not
 present.
 If the requested variant has been modified since the specified time,
 the server MUST NOT perform the requested operation, and MUST return
 a 412 (Precondition Failed).
       If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
 An example of the field is:
        If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
 If the request normally (i.e., without the If-Unmodified-Since
 header) would result in anything other than a 2xx status, the If-
 Unmodified-Since header should be ignored.
 If the specified date is invalid, the header is ignored.

14.29 Last-Modified

 The Last-Modified entity-header field indicates the date and time at
 which the origin server believes the variant was last modified.
        Last-Modified  = "Last-Modified" ":" HTTP-date
 An example of its use is
        Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
 The exact meaning of this header field depends on the implementation
 of the origin server and the nature of the original resource. For
 files, it may be just the file system last-modified time. For
 entities with dynamically included parts, it may be the most recent
 of the set of last-modify times for its component parts. For database
 gateways, it may be the last-update time stamp of the record. For
 virtual objects, it may be the last time the internal state changed.

Fielding, et. al. Standards Track [Page 124] RFC 2068 HTTP/1.1 January 1997

 An origin server MUST NOT send a Last-Modified date which is later
 than the server's time of message origination. In such cases, where
 the resource's last modification would indicate some time in the
 future, the server MUST replace that date with the message
 origination date.
 An origin server should obtain the Last-Modified value of the entity
 as close as possible to the time that it generates the Date value of
 its response. This allows a recipient to make an accurate assessment
 of the entity's modification time, especially if the entity changes
 near the time that the response is generated.
 HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.

14.30 Location

 The Location response-header field is used to redirect the recipient
 to a location other than the Request-URI for completion of the
 request or identification of a new resource. For 201 (Created)
 responses, the Location is that of the new resource which was created
 by the request.  For 3xx responses, the location SHOULD indicate the
 server's preferred URL for automatic redirection to the resource. The
 field value consists of a single absolute URL.
        Location       = "Location" ":" absoluteURI
 An example is
        Location: http://www.w3.org/pub/WWW/People.html
   Note: The Content-Location header field (section 14.15) differs
   from Location in that the Content-Location identifies the original
   location of the entity enclosed in the request. It is therefore
   possible for a response to contain header fields for both Location
   and Content-Location. Also see section 13.10 for cache requirements
   of some methods.

14.31 Max-Forwards

 The Max-Forwards request-header field may be used with the TRACE
 method (section 14.31) to limit the number of proxies or gateways
 that can forward the request to the next inbound server. This can be
 useful when the client is attempting to trace a request chain which
 appears to be failing or looping in mid-chain.
        Max-Forwards   = "Max-Forwards" ":" 1*DIGIT

Fielding, et. al. Standards Track [Page 125] RFC 2068 HTTP/1.1 January 1997

 The Max-Forwards value is a decimal integer indicating the remaining
 number of times this request message may be forwarded.
 Each proxy or gateway recipient of a TRACE request containing a Max-
 Forwards header field SHOULD check and update its value prior to
 forwarding the request. If the received value is zero (0), the
 recipient SHOULD NOT forward the request; instead, it SHOULD respond
 as the final recipient with a 200 (OK) response containing the
 received request message as the response entity-body (as described in
 section 9.8). If the received Max-Forwards value is greater than
 zero, then the forwarded message SHOULD contain an updated Max-
 Forwards field with a value decremented by one (1).
 The Max-Forwards header field SHOULD be ignored for all other methods
 defined by this specification and for any extension methods for which
 it is not explicitly referred to as part of that method definition.

14.32 Pragma

 The Pragma general-header field is used to include implementation-
 specific directives that may apply to any recipient along the
 request/response chain. All pragma directives specify optional
 behavior from the viewpoint of the protocol; however, some systems
 MAY require that behavior be consistent with the directives.
        Pragma            = "Pragma" ":" 1#pragma-directive
        pragma-directive  = "no-cache" | extension-pragma
        extension-pragma  = token [ "=" ( token | quoted-string ) ]
 When the no-cache directive is present in a request message, an
 application SHOULD forward the request toward the origin server even
 if it has a cached copy of what is being requested. This pragma
 directive has the same semantics as the no-cache cache-directive (see
 section 14.9) and is defined here for backwards compatibility with
 HTTP/1.0.  Clients SHOULD include both header fields when a no-cache
 request is sent to a server not known to be HTTP/1.1 compliant.
 Pragma directives MUST be passed through by a proxy or gateway
 application, regardless of their significance to that application,
 since the directives may be applicable to all recipients along the
 request/response chain. It is not possible to specify a pragma for a
 specific recipient; however, any pragma directive not relevant to a
 recipient SHOULD be ignored by that recipient.

Fielding, et. al. Standards Track [Page 126] RFC 2068 HTTP/1.1 January 1997

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

14.33 Proxy-Authenticate

 The Proxy-Authenticate response-header field MUST be included as part
 of a 407 (Proxy Authentication Required) response. The field value
 consists of a challenge that indicates the authentication scheme and
 parameters applicable to the proxy for this Request-URI.
        Proxy-Authenticate  = "Proxy-Authenticate" ":" challenge
 The HTTP access authentication process is described in section 11.
 Unlike WWW-Authenticate, the Proxy-Authenticate header field applies
 only to the current connection and SHOULD NOT be passed on to
 downstream clients. However, an intermediate proxy may need to obtain
 its own credentials by requesting them from the downstream client,
 which in some circumstances will appear as if the proxy is forwarding
 the Proxy-Authenticate header field.

14.34 Proxy-Authorization

 The Proxy-Authorization request-header field allows the client to
 identify itself (or its user) to a proxy which requires
 authentication.  The Proxy-Authorization field value consists of
 credentials containing the authentication information of the user
 agent for the proxy and/or realm of the resource being requested.
     Proxy-Authorization     = "Proxy-Authorization" ":" credentials
 The HTTP access authentication process is described in section 11.
 Unlike Authorization, the Proxy-Authorization header field applies
 only to the next outbound proxy that demanded authentication using
 the Proxy-Authenticate field. When multiple proxies are used in a
 chain, the Proxy-Authorization header field is consumed by the first
 outbound proxy that was expecting to receive credentials. A proxy MAY
 relay the credentials from the client request to the next proxy if
 that is the mechanism by which the proxies cooperatively authenticate
 a given request.

14.35 Public

 The Public response-header field lists the set of methods supported
 by the server. The purpose of this field is strictly to inform the
 recipient of the capabilities of the server regarding unusual
 methods.  The methods listed may or may not be applicable to the

Fielding, et. al. Standards Track [Page 127] RFC 2068 HTTP/1.1 January 1997

 Request-URI; the Allow header field (section 14.7) MAY be used to
 indicate methods allowed for a particular URI.
        Public         = "Public" ":" 1#method
 Example of use:
        Public: OPTIONS, MGET, MHEAD, GET, HEAD
 This header field applies only to the server directly connected to
 the client (i.e., the nearest neighbor in a chain of connections). If
 the response passes through a proxy, the proxy MUST either remove the
 Public header field or replace it with one applicable to its own
 capabilities.

14.36 Range

14.36.1 Byte Ranges

 Since all HTTP entities are represented in HTTP messages as sequences
 of bytes, the concept of a byte range is meaningful for any HTTP
 entity.  (However, not all clients and servers need to support byte-
 range operations.)
 Byte range specifications in HTTP apply to the sequence of bytes in
 the entity-body (not necessarily the same as the message-body).
 A byte range operation may specify a single range of bytes, or a set
 of ranges within a single entity.
     ranges-specifier = byte-ranges-specifier
     byte-ranges-specifier = bytes-unit "=" byte-range-set
     byte-range-set  = 1#( byte-range-spec | suffix-byte-range-spec )
     byte-range-spec = first-byte-pos "-" [last-byte-pos]
     first-byte-pos  = 1*DIGIT
     last-byte-pos   = 1*DIGIT
 The first-byte-pos value in a byte-range-spec gives the byte-offset
 of the first byte in a range. The last-byte-pos value gives the
 byte-offset of the last byte in the range; that is, the byte
 positions specified are inclusive. Byte offsets start at zero.

Fielding, et. al. Standards Track [Page 128] RFC 2068 HTTP/1.1 January 1997

 If the last-byte-pos value is present, it must be greater than or
 equal to the first-byte-pos in that byte-range-spec, or the byte-
 range-spec is invalid. The recipient of an invalid byte-range-spec
 must ignore it.
 If the last-byte-pos value is absent, or if the value is greater than
 or equal to the current length of the entity-body, last-byte-pos is
 taken to be equal to one less than the current length of the entity-
 body in bytes.
 By its choice of last-byte-pos, a client can limit the number of
 bytes retrieved without knowing the size of the entity.
        suffix-byte-range-spec = "-" suffix-length
        suffix-length = 1*DIGIT
 A suffix-byte-range-spec is used to specify the suffix of the
 entity-body, of a length given by the suffix-length value. (That is,
 this form specifies the last N bytes of an entity-body.) If the
 entity is shorter than the specified suffix-length, the entire
 entity-body is used.
 Examples of byte-ranges-specifier values (assuming an entity-body of
 length 10000):
   o  The first 500 bytes (byte offsets 0-499, inclusive):
        bytes=0-499
   o  The second 500 bytes (byte offsets 500-999, inclusive):
        bytes=500-999
   o  The final 500 bytes (byte offsets 9500-9999, inclusive):
        bytes=-500
   o  Or
        bytes=9500-
   o  The first and last bytes only (bytes 0 and 9999):
        bytes=0-0,-1

Fielding, et. al. Standards Track [Page 129] RFC 2068 HTTP/1.1 January 1997

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

14.36.2 Range Retrieval Requests

 HTTP retrieval requests using conditional or unconditional GET
 methods may request one or more sub-ranges of the entity, instead of
 the entire entity, using the Range request header, which applies to
 the entity returned as the result of the request:
       Range = "Range" ":" ranges-specifier
 A server MAY ignore the Range header. However, HTTP/1.1 origin
 servers and intermediate caches SHOULD support byte ranges when
 possible, since Range supports efficient recovery from partially
 failed transfers, and supports efficient partial retrieval of large
 entities.
 If the server supports the Range header and the specified range or
 ranges are appropriate for the entity:
   o  The presence of a Range header in an unconditional GET modifies
      what is returned if the GET is otherwise successful. In other
      words, the response carries a status code of 206 (Partial
      Content) instead of 200 (OK).
   o  The presence of a Range header in a conditional GET (a request
      using one or both of If-Modified-Since and If-None-Match, or
      one or both of If-Unmodified-Since and If-Match) modifies what
      is returned if the GET is otherwise successful and the condition
      is true. It does not affect the 304 (Not Modified) response
      returned if the conditional is false.
 In some cases, it may be more appropriate to use the If-Range header
 (see section 14.27) in addition to the Range header.
 If a proxy that supports ranges receives a Range request, forwards
 the request to an inbound server, and receives an entire entity in
 reply, it SHOULD only return the requested range to its client. It
 SHOULD store the entire received response in its cache, if that is
 consistent with its cache allocation policies.

Fielding, et. al. Standards Track [Page 130] RFC 2068 HTTP/1.1 January 1997

14.37 Referer

 The Referer[sic] request-header field allows the client to specify,
 for the server's benefit, the address (URI) of the resource from
 which the Request-URI was obtained (the "referrer", although the
 header field is misspelled.) The Referer request-header allows a
 server to generate lists of back-links to resources for interest,
 logging, optimized caching, etc. It also allows obsolete or mistyped
 links to be traced for maintenance. The Referer field MUST NOT be
 sent if the Request-URI was obtained from a source that does not have
 its own URI, such as input from the user keyboard.
      Referer        = "Referer" ":" ( absoluteURI | relativeURI )
 Example:
      Referer: http://www.w3.org/hypertext/DataSources/Overview.html
 If the field value is a partial URI, it SHOULD be interpreted
 relative to the Request-URI. The URI MUST NOT include a fragment.
   Note: Because the source of a link may be private information or
   may reveal an otherwise private information source, it is strongly
   recommended that the user be able to select whether or not the
   Referer field is sent. For example, a browser client could have a
   toggle switch for browsing openly/anonymously, which would
   respectively enable/disable the sending of Referer and From
   information.

14.38 Retry-After

 The Retry-After response-header field can be used with a 503 (Service
 Unavailable) response to indicate how long the service is expected to
 be unavailable to the requesting client. The value of this field can
 be either an HTTP-date or an integer number of seconds (in decimal)
 after the time of the response.
        Retry-After  = "Retry-After" ":" ( HTTP-date | delta-seconds )
 Two examples of its use are
        Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
        Retry-After: 120
 In the latter example, the delay is 2 minutes.

Fielding, et. al. Standards Track [Page 131] RFC 2068 HTTP/1.1 January 1997

14.39 Server

 The Server response-header field contains information about the
 software used by the origin server to handle the request. The field
 can contain multiple product tokens (section 3.8) and comments
 identifying the server and any significant subproducts. The product
 tokens are listed in order of their significance for identifying the
 application.
        Server         = "Server" ":" 1*( product | comment )
 Example:
        Server: CERN/3.0 libwww/2.17
 If the response is being forwarded through a proxy, the proxy
 application MUST NOT modify the Server response-header. Instead, it
 SHOULD include a Via field (as described in section 14.44).
   Note: Revealing the specific software version of the server may
   allow the server machine to become more vulnerable to attacks
   against software that is known to contain security holes. Server
   implementers are encouraged to make this field a configurable
   option.

14.40 Transfer-Encoding

 The Transfer-Encoding general-header field indicates what (if any)
 type of transformation has been applied to the message body in order
 to safely transfer it between the sender and the recipient. This
 differs from the Content-Encoding in that the transfer coding is a
 property of the message, not of the entity.
        Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer-
 coding
 Transfer codings are defined in section 3.6. An example is:
        Transfer-Encoding: chunked
 Many older HTTP/1.0 applications do not understand the Transfer-
 Encoding header.

14.41 Upgrade

 The Upgrade general-header allows the client to specify what
 additional communication protocols it supports and would like to use
 if the server finds it appropriate to switch protocols. The server

Fielding, et. al. Standards Track [Page 132] RFC 2068 HTTP/1.1 January 1997

 MUST use the Upgrade header field within a 101 (Switching Protocols)
 response to indicate which protocol(s) are being switched.
        Upgrade        = "Upgrade" ":" 1#product
 For example,
        Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
 The Upgrade header field is intended to provide a simple mechanism
 for transition from HTTP/1.1 to some other, incompatible protocol. It
 does so by allowing the client to advertise its desire to use another
 protocol, such as a later version of HTTP with a higher major version
 number, even though the current request has been made using HTTP/1.1.
 This eases the difficult transition between incompatible protocols by
 allowing the client to initiate a request in the more commonly
 supported protocol while indicating to the server that it would like
 to use a "better" protocol if available (where "better" is determined
 by the server, possibly according to the nature of the method and/or
 resource being requested).
 The Upgrade header field only applies to switching application-layer
 protocols upon the existing transport-layer connection. Upgrade
 cannot be used to insist on a protocol change; its acceptance and use
 by the server is optional. The capabilities and nature of the
 application-layer communication after the protocol change is entirely
 dependent upon the new protocol chosen, although the first action
 after changing the protocol MUST be a response to the initial HTTP
 request containing the Upgrade header field.
 The Upgrade header field only applies to the immediate connection.
 Therefore, the upgrade keyword MUST be supplied within a Connection
 header field (section 14.10) whenever Upgrade is present in an
 HTTP/1.1 message.
 The Upgrade header field cannot be used to indicate a switch to a
 protocol on a different connection. For that purpose, it is more
 appropriate to use a 301, 302, 303, or 305 redirection response.
 This specification only defines the protocol name "HTTP" for use by
 the family of Hypertext Transfer Protocols, as defined by the HTTP
 version rules of section 3.1 and future updates to this
 specification. Any token can be used as a protocol name; however, it
 will only be useful if both the client and server associate the name
 with the same protocol.

Fielding, et. al. Standards Track [Page 133] RFC 2068 HTTP/1.1 January 1997

14.42 User-Agent

 The User-Agent request-header field contains information about the
 user agent originating the request. This is for statistical purposes,
 the tracing of protocol violations, and automated recognition of user
 agents for the sake of tailoring responses to avoid particular user
 agent limitations. User agents SHOULD include this field with
 requests. The field can contain multiple product tokens (section 3.8)
 and comments identifying the agent and any subproducts which form a
 significant part of the user agent. By convention, the product tokens
 are listed in order of their significance for identifying the
 application.
        User-Agent     = "User-Agent" ":" 1*( product | comment )
 Example:
        User-Agent: CERN-LineMode/2.15 libwww/2.17b3

14.43 Vary

 The Vary response-header field is used by a server to signal that the
 response entity was selected from the available representations of
 the response using server-driven negotiation (section 12). Field-
 names listed in Vary headers are those of request-headers. The Vary
 field value indicates either that the given set of header fields
 encompass the dimensions over which the representation might vary, or
 that the dimensions of variance are unspecified ("*") and thus may
 vary over any aspect of future requests.
        Vary  = "Vary" ":" ( "*" | 1#field-name )
 An HTTP/1.1 server MUST include an appropriate Vary header field with
 any cachable response that is subject to server-driven negotiation.
 Doing so allows a cache to properly interpret future requests on that
 resource and informs the user agent about the presence of negotiation
 on that resource. A server SHOULD include an appropriate Vary header
 field with a non-cachable response that is subject to server-driven
 negotiation, since this might provide the user agent with useful
 information about the dimensions over which the response might vary.
 The set of header fields named by the Vary field value is known as
 the "selecting" request-headers.
 When the cache receives a subsequent request whose Request-URI
 specifies one or more cache entries including a Vary header, the
 cache MUST NOT use such a cache entry to construct a response to the
 new request unless all of the headers named in the cached Vary header

Fielding, et. al. Standards Track [Page 134] RFC 2068 HTTP/1.1 January 1997

 are present in the new request, and all of the stored selecting
 request-headers from the previous request match the corresponding
 headers in the new request.
 The selecting request-headers from two requests are defined to match
 if and only if the selecting request-headers in the first request can
 be transformed to the selecting request-headers in the second request
 by adding or removing linear whitespace (LWS) at places where this is
 allowed by the corresponding BNF, and/or combining multiple message-
 header fields with the same field name following the rules about
 message headers in section 4.2.
 A Vary field value of "*" signals that unspecified parameters,
 possibly other than the contents of request-header fields (e.g., the
 network address of the client), play a role in the selection of the
 response representation. Subsequent requests on that resource can
 only be properly interpreted by the origin server, and thus a cache
 MUST forward a (possibly conditional) request even when it has a
 fresh response cached for the resource. See section 13.6 for use of
 the Vary header by caches.
 A Vary field value consisting of a list of field-names signals that
 the representation selected for the response is based on a selection
 algorithm which considers ONLY the listed request-header field values
 in selecting the most appropriate representation. A cache MAY assume
 that the same selection will be made for future requests with the
 same values for the listed field names, for the duration of time in
 which the response is fresh.
 The field-names given are not limited to the set of standard
 request-header fields defined by this specification. Field names are
 case-insensitive.

14.44 Via

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

Fielding, et. al. Standards Track [Page 135] RFC 2068 HTTP/1.1 January 1997

    Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
    received-protocol = [ protocol-name "/" ] protocol-version
    protocol-name     = token
    protocol-version  = token
    received-by       = ( host [ ":" port ] ) | pseudonym
    pseudonym         = token
 The received-protocol indicates the protocol version of the message
 received by the server or client along each segment of the
 request/response chain. The received-protocol version is appended to
 the Via field value when the message is forwarded so that information
 about the protocol capabilities of upstream applications remains
 visible to all recipients.
 The protocol-name is optional if and only if it would be "HTTP". The
 received-by field is normally the host and optional port number of a
 recipient server or client that subsequently forwarded the message.
 However, if the real host is considered to be sensitive information,
 it MAY be replaced by a pseudonym. If the port is not given, it MAY
 be assumed to be the default port of the received-protocol.
 Multiple Via field values represent each proxy or gateway that has
 forwarded the message. Each recipient MUST append its information
 such that the end result is ordered according to the sequence of
 forwarding applications.
 Comments MAY be used in the Via header field to identify the software
 of the recipient proxy or gateway, analogous to the User-Agent and
 Server header fields. However, all comments in the Via field are
 optional and MAY be removed by any recipient prior to forwarding the
 message.
 For example, a request message could be sent from an HTTP/1.0 user
 agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
 forward the request to a public proxy at nowhere.com, which completes
 the request by forwarding it to the origin server at www.ics.uci.edu.
 The request received by www.ics.uci.edu would then have the following
 Via header field:
        Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
 Proxies and gateways used as a portal through a network firewall
 SHOULD NOT, by default, forward the names and ports of hosts within
 the firewall region. This information SHOULD only be propagated if
 explicitly enabled. If not enabled, the received-by host of any host
 behind the firewall SHOULD be replaced by an appropriate pseudonym
 for that host.

Fielding, et. al. Standards Track [Page 136] RFC 2068 HTTP/1.1 January 1997

 For organizations that have strong privacy requirements for hiding
 internal structures, a proxy MAY combine an ordered subsequence of
 Via header field entries with identical received-protocol values into
 a single such entry. For example,
        Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
         could be collapsed to
        Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
 Applications SHOULD NOT combine multiple entries unless they are all
 under the same organizational control and the hosts have already been
 replaced by pseudonyms. Applications MUST NOT combine entries which
 have different received-protocol values.

14.45 Warning

 The Warning response-header field is used to carry additional
 information about the status of a response which may not be reflected
 by the response status code. This information is typically, though
 not exclusively, used to warn about a possible lack of semantic
 transparency from caching operations.
 Warning headers are sent with responses using:
        Warning    = "Warning" ":" 1#warning-value
        warning-value = warn-code SP warn-agent SP warn-text
        warn-code  = 2DIGIT
        warn-agent = ( host [ ":" port ] ) | pseudonym
                        ; the name or pseudonym of the server adding
                        ; the Warning header, for use in debugging
        warn-text  = quoted-string
 A response may carry more than one Warning header.
 The warn-text should be in a natural language and character set that
 is most likely to be intelligible to the human user receiving the
 response.  This decision may be based on any available knowledge,
 such as the location of the cache or user, the Accept-Language field
 in a request, the Content-Language field in a response, etc. The
 default language is English and the default character set is ISO-
 8859-1.
 If a character set other than ISO-8859-1 is used, it MUST be encoded
 in the warn-text using the method described in RFC 1522 [14].

Fielding, et. al. Standards Track [Page 137] RFC 2068 HTTP/1.1 January 1997

 Any server or cache may add Warning headers to a response. New
 Warning headers should be added after any existing Warning headers. A
 cache MUST NOT delete any Warning header that it received with a
 response. However, if a cache successfully validates a cache entry,
 it SHOULD remove any Warning headers previously attached to that
 entry except as specified for specific Warning codes. It MUST then
 add any Warning headers received in the validating response. In other
 words, Warning headers are those that would be attached to the most
 recent relevant response.
 When multiple Warning headers are attached to a response, the user
 agent SHOULD display as many of them as possible, in the order that
 they appear in the response. If it is not possible to display all of
 the warnings, the user agent should follow these heuristics:
   o  Warnings that appear early in the response take priority over those
      appearing later in the response.
   o  Warnings in the user's preferred character set take priority over
      warnings in other character sets but with identical warn-codes and
      warn-agents.
 Systems that generate multiple Warning headers should order them with
 this user agent behavior in mind.
 This is a list of the currently-defined warn-codes, each with a
 recommended warn-text in English, and a description of its meaning.

10 Response is stale

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

11 Revalidation failed

MUST be included if a cache returns a stale response because an
attempt to revalidate the response failed, due to an inability to
reach the server. A cache may add this warning to any response, but
may never remove it until the response is successfully revalidated.

12 Disconnected operation

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

13 Heuristic expiration

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

Fielding, et. al. Standards Track [Page 138] RFC 2068 HTTP/1.1 January 1997

14 Transformation applied

MUST be added by an intermediate cache or proxy if it applies any
transformation changing the content-coding (as specified in the
Content-Encoding header) or media-type (as specified in the
Content-Type header) of the response, unless this Warning code
already appears in the response. MUST NOT be deleted from a response
even after revalidation.

99 Miscellaneous warning

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

14.46 WWW-Authenticate

 The WWW-Authenticate response-header field MUST be included in 401
 (Unauthorized) response messages. The field value consists of at
 least one challenge that indicates the authentication scheme(s) and
 parameters applicable to the Request-URI.
        WWW-Authenticate  = "WWW-Authenticate" ":" 1#challenge
 The HTTP access authentication process is described in section 11.
 User agents MUST take special care in parsing the WWW-Authenticate
 field value if it contains more than one challenge, or if more than
 one WWW-Authenticate header field is provided, since the contents of
 a challenge may itself contain a comma-separated list of
 authentication parameters.

15 Security Considerations

 This section is meant to inform application developers, information
 providers, and users of the security limitations in HTTP/1.1 as
 described by this document. The discussion does not include
 definitive solutions to the problems revealed, though it does make
 some suggestions for reducing security risks.

15.1 Authentication of Clients

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

Fielding, et. al. Standards Track [Page 139] RFC 2068 HTTP/1.1 January 1997

 The most serious flaw in Basic authentication is that it results in
 the essentially clear text transmission of the user's password over
 the physical network. It is this problem which Digest Authentication
 attempts to address.
 Because Basic authentication involves the clear text transmission of
 passwords it SHOULD never be used (without enhancements) to protect
 sensitive or valuable information.
 A common use of Basic authentication is for identification purposes
 -- requiring the user to provide a user name and password as a means
 of identification, for example, for purposes of gathering accurate
 usage statistics on a server. When used in this way it is tempting to
 think that there is no danger in its use if illicit access to the
 protected documents is not a major concern. This is only correct if
 the server issues both user name and password to the users and in
 particular does not allow the user to choose his or her own password.
 The danger arises because naive users frequently reuse a single
 password to avoid the task of maintaining multiple passwords.
 If a server permits users to select their own passwords, then the
 threat is not only illicit access to documents on the server but also
 illicit access to the accounts of all users who have chosen to use
 their account password. If users are allowed to choose their own
 password that also means the server must maintain files containing
 the (presumably encrypted) passwords. Many of these may be the
 account passwords of users perhaps at distant sites. The owner or
 administrator of such a system could conceivably incur liability if
 this information is not maintained in a secure fashion.
 Basic Authentication is also vulnerable to spoofing by counterfeit
 servers. If a user can be led to believe that he is connecting to a
 host containing information protected by basic authentication when in
 fact he is connecting to a hostile server or gateway then the
 attacker can request a password, store it for later use, and feign an
 error. This type of attack is not possible with Digest Authentication
 [32]. Server implementers SHOULD guard against the possibility of
 this sort of counterfeiting by gateways or CGI scripts. In particular
 it is very dangerous for a server to simply turn over a connection to
 a gateway since that gateway can then use the persistent connection
 mechanism to engage in multiple transactions with the client while
 impersonating the original server in a way that is not detectable by
 the client.

15.2 Offering a Choice of Authentication Schemes

 An HTTP/1.1 server may return multiple challenges with a 401
 (Authenticate) response, and each challenge may use a different

Fielding, et. al. Standards Track [Page 140] RFC 2068 HTTP/1.1 January 1997

 scheme.  The order of the challenges returned to the user agent is in
 the order that the server would prefer they be chosen. The server
 should order its challenges with the "most secure" authentication
 scheme first. A user agent should choose as the challenge to be made
 to the user the first one that the user agent understands.
 When the server offers choices of authentication schemes using the
 WWW-Authenticate header, the "security" of the authentication is only
 as malicious user could capture the set of challenges and try to
 authenticate him/herself using the weakest of the authentication
 schemes. Thus, the ordering serves more to protect the user's
 credentials than the server's information.
 A possible man-in-the-middle (MITM) attack would be to add a weak
 authentication scheme to the set of choices, hoping that the client
 will use one that exposes the user's credentials (e.g. password). For
 this reason, the client should always use the strongest scheme that
 it understands from the choices accepted.
 An even better MITM attack would be to remove all offered choices,
 and to insert a challenge that requests Basic authentication. For
 this reason, user agents that are concerned about this kind of attack
 could remember the strongest authentication scheme ever requested by
 a server and produce a warning message that requires user
 confirmation before using a weaker one. A particularly insidious way
 to mount such a MITM attack would be to offer a "free" proxy caching
 service to gullible users.

15.3 Abuse of Server Log Information

 A server is in the position to save personal data about a user's
 requests which may identify their reading patterns or subjects of
 interest. This information is clearly confidential in nature and its
 handling may be constrained by law in certain countries. People using
 the HTTP protocol to provide data are responsible for ensuring that
 such material is not distributed without the permission of any
 individuals that are identifiable by the published results.

15.4 Transfer of Sensitive Information

 Like any generic data transfer protocol, HTTP cannot regulate the
 content of the data that is transferred, nor is there any a priori
 method of determining the sensitivity of any particular piece of
 information within the context of any given request. Therefore,
 applications SHOULD supply as much control over this information as
 possible to the provider of that information. Four header fields are
 worth special mention in this context: Server, Via, Referer and From.

Fielding, et. al. Standards Track [Page 141] RFC 2068 HTTP/1.1 January 1997

 Revealing the specific software version of the server may allow the
 server machine to become more vulnerable to attacks against software
 that is known to contain security holes. Implementers SHOULD make the
 Server header field a configurable option.
 Proxies which serve as a portal through a network firewall SHOULD
 take special precautions regarding the transfer of header information
 that identifies the hosts behind the firewall. In particular, they
 SHOULD remove, or replace with sanitized versions, any Via fields
 generated behind the firewall.
 The Referer field allows reading patterns to be studied and reverse
 links drawn. Although it can be very useful, its power can be abused
 if user details are not separated from the information contained in
 the Referer. Even when the personal information has been removed, the
 Referer field may indicate a private document's URI whose publication
 would be inappropriate.
 The information sent in the From field might conflict with the user's
 privacy interests or their site's security policy, and hence it
 SHOULD NOT be transmitted without the user being able to disable,
 enable, and modify the contents of the field. The user MUST be able
 to set the contents of this field within a user preference or
 application defaults configuration.
 We suggest, though do not require, that a convenient toggle interface
 be provided for the user to enable or disable the sending of From and
 Referer information.

15.5 Attacks Based On File and Path Names

 Implementations of HTTP origin servers SHOULD be careful to restrict
 the documents returned by HTTP requests to be only those that were
 intended by the server administrators. If an HTTP server translates
 HTTP URIs directly into file system calls, the server MUST take
 special care not to serve files that were not intended to be
 delivered to HTTP clients.  For example, UNIX, Microsoft Windows, and
 other operating systems use ".." as a path component to indicate a
 directory level above the current one. On such a system, an HTTP
 server MUST disallow any such construct in the Request-URI if it
 would otherwise allow access to a resource outside those intended to
 be accessible via the HTTP server. Similarly, files intended for
 reference only internally to the server (such as access control
 files, configuration files, and script code) MUST be protected from
 inappropriate retrieval, since they might contain sensitive
 information. Experience has shown that minor bugs in such HTTP server
 implementations have turned into security risks.

Fielding, et. al. Standards Track [Page 142] RFC 2068 HTTP/1.1 January 1997

15.6 Personal Information

 HTTP clients are often privy to large amounts of personal information
 (e.g. the user's name, location, mail address, passwords, encryption
 keys, etc.), and SHOULD be very careful to prevent unintentional
 leakage of this information via the HTTP protocol to other sources.
 We very strongly recommend that a convenient interface be provided
 for the user to control dissemination of such information, and that
 designers and implementers be particularly careful in this area.
 History shows that errors in this area are often both serious
 security and/or privacy problems, and often generate highly adverse
 publicity for the implementer's company.

15.7 Privacy Issues Connected to Accept Headers

 Accept request-headers can reveal information about the user to all
 servers which are accessed. The Accept-Language header in particular
 can reveal information the user would consider to be of a private
 nature, because the understanding of particular languages is often
 strongly correlated to the membership of a particular ethnic group.
 User agents which offer the option to configure the contents of an
 Accept-Language header to be sent in every request are strongly
 encouraged to let the configuration process include a message which
 makes the user aware of the loss of privacy involved.
 An approach that limits the loss of privacy would be for a user agent
 to omit the sending of Accept-Language headers by default, and to ask
 the user whether it should start sending Accept-Language headers to a
 server if it detects, by looking for any Vary response-header fields
 generated by the server, that such sending could improve the quality
 of service.
 Elaborate user-customized accept header fields sent in every request,
 in particular if these include quality values, can be used by servers
 as relatively reliable and long-lived user identifiers. Such user
 identifiers would allow content providers to do click-trail tracking,
 and would allow collaborating content providers to match cross-server
 click-trails or form submissions of individual users. Note that for
 many users not behind a proxy, the network address of the host
 running the user agent will also serve as a long-lived user
 identifier. In environments where proxies are used to enhance
 privacy, user agents should be conservative in offering accept header
 configuration options to end users. As an extreme privacy measure,
 proxies could filter the accept headers in relayed requests. General
 purpose user agents which provide a high degree of header
 configurability should warn users about the loss of privacy which can
 be involved.

Fielding, et. al. Standards Track [Page 143] RFC 2068 HTTP/1.1 January 1997

15.8 DNS Spoofing

 Clients using HTTP rely heavily on the Domain Name Service, and are
 thus generally prone to security attacks based on the deliberate
 mis-association of IP addresses and DNS names. Clients need to be
 cautious in assuming the continuing validity of an IP number/DNS name
 association.
 In particular, HTTP clients SHOULD rely on their name resolver for
 confirmation of an IP number/DNS name association, rather than
 caching the result of previous host name lookups. Many platforms
 already can cache host name lookups locally when appropriate, and
 they SHOULD be configured to do so. These lookups should be cached,
 however, only when the TTL (Time To Live) information reported by the
 name server makes it likely that the cached information will remain
 useful.
 If HTTP clients cache the results of host name lookups in order to
 achieve a performance improvement, they MUST observe the TTL
 information reported by DNS.
 If HTTP clients do not observe this rule, they could be spoofed when
 a previously-accessed server's IP address changes. As network
 renumbering is expected to become increasingly common, the
 possibility of this form of attack will grow. Observing this
 requirement thus reduces this potential security vulnerability.
 This requirement also improves the load-balancing behavior of clients
 for replicated servers using the same DNS name and reduces the
 likelihood of a user's experiencing failure in accessing sites which
 use that strategy.

15.9 Location Headers and Spoofing

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

16 Acknowledgments

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

Fielding, et. al. Standards Track [Page 144] RFC 2068 HTTP/1.1 January 1997

 The HTTP protocol has evolved considerably over the past four years.
 It has benefited from a large and active developer community--the
 many people who have participated on the www-talk mailing list--and
 it is that community which has been most responsible for the success
 of HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
 Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
 Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
 McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
 VanHeyningen deserve special recognition for their efforts in
 defining early aspects of the protocol.
 This document has benefited greatly from the comments of all those
 participating in the HTTP-WG. In addition to those already mentioned,
 the following individuals have contributed to this specification:
        Gary Adams                  Albert Lunde
        Harald Tveit Alvestrand     John C. Mallery
        Keith Ball                  Jean-Philippe Martin-Flatin
        Brian Behlendorf            Larry Masinter
        Paul Burchard               Mitra
        Maurizio Codogno            David Morris
        Mike Cowlishaw              Gavin Nicol
        Roman Czyborra              Bill Perry
        Michael A. Dolan            Jeffrey Perry
        David J. Fiander            Scott Powers
        Alan Freier                 Owen Rees
        Marc Hedlund                Luigi Rizzo
        Greg Herlihy                David Robinson
        Koen Holtman                Marc Salomon
        Alex Hopmann                Rich Salz
        Bob Jernigan                Allan M. Schiffman
        Shel Kaphan                 Jim Seidman
        Rohit Khare                 Chuck Shotton
        John Klensin                Eric W. Sink
        Martijn Koster              Simon E. Spero
        Alexei Kosut                Richard N. Taylor
        David M. Kristol            Robert S. Thau
        Daniel LaLiberte            Bill (BearHeart) Weinman
        Ben Laurie                  Francois Yergeau
        Paul J. Leach               Mary Ellen Zurko
        Daniel DuBois
 Much of the content and presentation of the caching design is due to
 suggestions and comments from individuals including: Shel Kaphan,
 Paul Leach, Koen Holtman, David Morris, and Larry Masinter.

Fielding, et. al. Standards Track [Page 145] RFC 2068 HTTP/1.1 January 1997

 Most of the specification of ranges is based on work originally done
 by Ari Luotonen and John Franks, with additional input from Steve
 Zilles.
 Thanks to the "cave men" of Palo Alto. You know who you are.
 Jim Gettys (the current editor of this document) wishes particularly
 to thank Roy Fielding, the previous editor of this document, along
 with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
 Holtman, John Franks, Alex Hopmann, and Larry Masinter for their
 help.

17 References

 [1] Alvestrand, H., "Tags for the identification of languages", RFC
 1766, UNINETT, March 1995.
 [2] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey,
 D., and B. Alberti. "The Internet Gopher Protocol: (a distributed
 document search and retrieval protocol)", RFC 1436, University of
 Minnesota, March 1993.
 [3] Berners-Lee, T., "Universal Resource Identifiers in WWW", A
 Unifying Syntax for the Expression of Names and Addresses of Objects
 on the Network as used in the World-Wide Web", RFC 1630, CERN, June
 1994.
 [4] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform Resource
 Locators (URL)", RFC 1738, CERN, Xerox PARC, University of Minnesota,
 December 1994.
 [5] Berners-Lee, T., and D. Connolly, "HyperText Markup Language
 Specification - 2.0", RFC 1866, MIT/LCS, November 1995.
 [6] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
 Transfer Protocol -- HTTP/1.0.", RFC 1945 MIT/LCS, UC Irvine, May
 1996.
 [7] Freed, N., and N. Borenstein, "Multipurpose Internet Mail
 Extensions (MIME) Part One: Format of Internet Message Bodies", RFC
 2045, Innosoft, First Virtual, November 1996.
 [8] Braden, R., "Requirements for Internet hosts - application and
 support", STD 3,  RFC 1123, IETF, October 1989.
 [9] Crocker, D., "Standard for the Format of ARPA Internet Text
 Messages", STD 11, RFC 822, UDEL, August 1982.

Fielding, et. al. Standards Track [Page 146] RFC 2068 HTTP/1.1 January 1997

 [10] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R.,
 Sui, J., and M. Grinbaum. "WAIS Interface Protocol Prototype
 Functional Specification", (v1.5), Thinking Machines Corporation,
 April 1990.
 [11] Fielding, R., "Relative Uniform Resource Locators", RFC 1808, UC
 Irvine, June 1995.
 [12] Horton, M., and R. Adams. "Standard for interchange of USENET
 messages", RFC 1036, AT&T Bell Laboratories, Center for Seismic
 Studies, December 1987.
 [13] Kantor, B., and P. Lapsley. "Network News Transfer Protocol." A
 Proposed Standard for the Stream-Based Transmission of News", RFC
 977, UC San Diego, UC Berkeley, February 1986.
 [14] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
 Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
 University of Tennessee, November 1996.
 [15] Nebel, E., and L. Masinter. "Form-based File Upload in HTML",
 RFC 1867, Xerox Corporation, November 1995.
 [16] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
 USC/ISI, August 1982.
 [17] Postel, J., "Media Type Registration Procedure", RFC 2048,
 USC/ISI, November 1996.
 [18] Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)", STD
 9, RFC 959, USC/ISI, October 1985.
 [19] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
 1700, USC/ISI, October 1994.
 [20] Sollins, K., and L. Masinter, "Functional Requirements for
 Uniform Resource Names", RFC 1737, MIT/LCS, Xerox Corporation,
 December 1994.
 [21] US-ASCII. Coded Character Set - 7-Bit American Standard Code for
 Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.
 [22] ISO-8859. International Standard -- Information Processing --
   8-bit Single-Byte Coded Graphic Character Sets --
   Part 1: Latin alphabet No. 1, ISO 8859-1:1987.
   Part 2: Latin alphabet No. 2, ISO 8859-2, 1987.
   Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.
   Part 4: Latin alphabet No. 4, ISO 8859-4, 1988.

Fielding, et. al. Standards Track [Page 147] RFC 2068 HTTP/1.1 January 1997

   Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988.
   Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987.
   Part 7: Latin/Greek alphabet, ISO 8859-7, 1987.
   Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988.
   Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.
 [23] Meyers, J., and M. Rose "The Content-MD5 Header Field", RFC
 1864, Carnegie Mellon, Dover Beach Consulting, October, 1995.
 [24] Carpenter, B., and Y. Rekhter, "Renumbering Needs Work", RFC
 1900, IAB, February 1996.
 [25] Deutsch, P., "GZIP file format specification version 4.3." RFC
 1952, Aladdin Enterprises, May 1996.
 [26] Venkata N. Padmanabhan and Jeffrey C. Mogul. Improving HTTP
 Latency. Computer Networks and ISDN Systems, v. 28, pp. 25-35, Dec.
 1995.  Slightly revised version of paper in Proc. 2nd International
 WWW Conf. '94: Mosaic and the Web, Oct. 1994, which is available at
 http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/
 HTTPLatency.html.
 [27] Joe Touch, John Heidemann, and Katia Obraczka, "Analysis of HTTP
 Performance", <URL: http://www.isi.edu/lsam/ib/http-perf/>,
 USC/Information Sciences Institute, June 1996
 [28] Mills, D., "Network Time Protocol, Version 3, Specification,
 Implementation and Analysis", RFC 1305, University of Delaware, March
 1992.
 [29] Deutsch, P., "DEFLATE Compressed Data Format Specification
 version 1.3." RFC 1951, Aladdin Enterprises, May 1996.
 [30] Spero, S., "Analysis of HTTP Performance Problems"
 <URL:http://sunsite.unc.edu/mdma-release/http-prob.html>.
 [31] Deutsch, P., and J-L. Gailly, "ZLIB Compressed Data Format
 Specification version 3.3", RFC 1950, Aladdin Enterprises, Info-ZIP,
 May 1996.
 [32] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
 Luotonen, A., Sink, E., and L. Stewart, "An Extension to HTTP :
 Digest Access Authentication", RFC 2069, January 1997.

Fielding, et. al. Standards Track [Page 148] RFC 2068 HTTP/1.1 January 1997

18 Authors' Addresses

 Roy T. Fielding
 Department of Information and Computer Science
 University of California
 Irvine, CA 92717-3425, USA
 Fax: +1 (714) 824-4056
 EMail: fielding@ics.uci.edu
 Jim Gettys
 MIT Laboratory for Computer Science
 545 Technology Square
 Cambridge, MA 02139, USA
 Fax: +1 (617) 258 8682
 EMail: jg@w3.org
 Jeffrey C. Mogul
 Western Research Laboratory
 Digital Equipment Corporation
 250 University Avenue
 Palo Alto, California, 94305, USA
 EMail: mogul@wrl.dec.com
 Henrik Frystyk Nielsen
 W3 Consortium
 MIT Laboratory for Computer Science
 545 Technology Square
 Cambridge, MA 02139, USA
 Fax: +1 (617) 258 8682
 EMail: frystyk@w3.org
 Tim Berners-Lee
 Director, W3 Consortium
 MIT Laboratory for Computer Science
 545 Technology Square
 Cambridge, MA 02139, USA
 Fax: +1 (617) 258 8682
 EMail: timbl@w3.org

Fielding, et. al. Standards Track [Page 149] RFC 2068 HTTP/1.1 January 1997

19 Appendices

19.1 Internet Media Type message/http

 In addition to defining the HTTP/1.1 protocol, this document serves
 as the specification for the Internet media type "message/http". The
 following is to be registered with IANA.
     Media Type name:         message
     Media subtype name:      http
     Required parameters:     none
     Optional parameters:     version, msgtype
      version: The HTTP-Version number of the enclosed message
               (e.g., "1.1"). If not present, the version can be
               determined from the first line of the body.
      msgtype: The message type -- "request" or "response". If not
               present, the type can be determined from the first
               line of the body.
     Encoding considerations: only "7bit", "8bit", or "binary" are
                              permitted
     Security considerations: none

19.2 Internet Media Type multipart/byteranges

 When an HTTP message includes the content of multiple ranges (for
 example, a response to a request for multiple non-overlapping
 ranges), these are transmitted as a multipart MIME message. The
 multipart media type for this purpose is called
 "multipart/byteranges".
 The multipart/byteranges media type includes two or more parts, each
 with its own Content-Type and Content-Range fields. The parts are
 separated using a MIME boundary parameter.
        Media Type name:         multipart
        Media subtype name:      byteranges
        Required parameters:     boundary
        Optional parameters:     none
        Encoding considerations: only "7bit", "8bit", or "binary" are
                                 permitted
        Security considerations: none

Fielding, et. al. Standards Track [Page 150] RFC 2068 HTTP/1.1 January 1997

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
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19.3 Tolerant Applications

 Although this document specifies the requirements for the generation
 of HTTP/1.1 messages, not all applications will be correct in their
 implementation. We therefore recommend that operational applications
 be tolerant of deviations whenever those deviations can be
 interpreted unambiguously.
 Clients SHOULD be tolerant in parsing the Status-Line and servers
 tolerant when parsing the Request-Line. In particular, they SHOULD
 accept any amount of SP or HT characters between fields, even though
 only a single SP is required.
 The line terminator for message-header fields is the sequence CRLF.
 However, we recommend that applications, when parsing such headers,
 recognize a single LF as a line terminator and ignore the leading CR.
 The character set of an entity-body should be labeled as the lowest
 common denominator of the character codes used within that body, with
 the exception that no label is preferred over the labels US-ASCII or
 ISO-8859-1.
 Additional rules for requirements on parsing and encoding of dates
 and other potential problems with date encodings include:
o  HTTP/1.1 clients and caches should assume that an RFC-850 date
   which appears to be more than 50 years in the future is in fact
   in the past (this helps solve the "year 2000" problem).

Fielding, et. al. Standards Track [Page 151] RFC 2068 HTTP/1.1 January 1997

o  An HTTP/1.1 implementation may internally represent a parsed
   Expires date as earlier than the proper value, but MUST NOT
   internally represent a parsed Expires date as later than the
   proper value.
o  All expiration-related calculations must be done in GMT. The
   local time zone MUST NOT influence the calculation or comparison
   of an age or expiration time.
o  If an HTTP header incorrectly carries a date value with a time
   zone other than GMT, it must be converted into GMT using the most
   conservative possible conversion.

19.4 Differences Between HTTP Entities and MIME Entities

 HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC
 822) and the Multipurpose Internet Mail Extensions (MIME ) to allow
 entities to be transmitted in an open variety of representations and
 with extensible mechanisms. However, MIME [7] discusses mail, and
 HTTP has a few features that are different from those described in
 MIME.  These differences were carefully chosen to optimize
 performance over binary connections, to allow greater freedom in the
 use of new media types, to make date comparisons easier, and to
 acknowledge the practice of some early HTTP servers and clients.
 This appendix describes specific areas where HTTP differs from MIME.
 Proxies and gateways to strict MIME environments SHOULD be aware of
 these differences and provide the appropriate conversions where
 necessary. Proxies and gateways from MIME environments to HTTP also
 need to be aware of the differences because some conversions may be
 required.

19.4.1 Conversion to Canonical Form

 MIME requires that an Internet mail entity be converted to canonical
 form prior to being transferred.  Section 3.7.1 of this document
 describes the forms allowed for subtypes of the "text" media type
 when transmitted over HTTP. MIME requires that content with a type of
 "text" represent line breaks as CRLF and forbids the use of CR or LF
 outside of line break sequences.  HTTP allows CRLF, bare CR, and bare
 LF to indicate a line break within text content when a message is
 transmitted over HTTP.
 Where it is possible, a proxy or gateway from HTTP to a strict MIME
 environment SHOULD translate all line breaks within the text media
 types described in section 3.7.1 of this document to the MIME
 canonical form of CRLF. Note, however, that this may be complicated
 by the presence of a Content-Encoding and by the fact that HTTP

Fielding, et. al. Standards Track [Page 152] RFC 2068 HTTP/1.1 January 1997

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

19.4.2 Conversion of Date Formats

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

19.4.3 Introduction of Content-Encoding

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

19.4.4 No Content-Transfer-Encoding

 HTTP does not use the Content-Transfer-Encoding (CTE) field of MIME.
 Proxies and gateways from MIME-compliant protocols to HTTP MUST
 remove any non-identity CTE ("quoted-printable" or "base64") encoding
 prior to delivering the response message to an HTTP client.
 Proxies and gateways from HTTP to MIME-compliant protocols are
 responsible for ensuring that the message is in the correct format
 and encoding for safe transport on that protocol, where "safe
 transport" is defined by the limitations of the protocol being used.
 Such a proxy or gateway SHOULD label the data with an appropriate
 Content-Transfer-Encoding if doing so will improve the likelihood of
 safe transport over the destination protocol.

19.4.5 HTTP Header Fields in Multipart Body-Parts

 In MIME, most header fields in multipart body-parts are generally
 ignored unless the field name begins with "Content-". In HTTP/1.1,
 multipart body-parts may contain any HTTP header fields which are
 significant to the meaning of that part.

Fielding, et. al. Standards Track [Page 153] RFC 2068 HTTP/1.1 January 1997

19.4.6 Introduction of Transfer-Encoding

 HTTP/1.1 introduces the Transfer-Encoding header field (section
 14.40).  Proxies/gateways MUST remove any transfer coding prior to
 forwarding a message via a MIME-compliant protocol.
 A process for decoding the "chunked" transfer coding (section 3.6)
 can be represented in pseudo-code as:
        length := 0
        read chunk-size, chunk-ext (if any) and CRLF
        while (chunk-size > 0) {
           read chunk-data and CRLF
           append chunk-data to entity-body
           length := length + chunk-size
           read chunk-size and CRLF
        }
        read entity-header
        while (entity-header not empty) {
           append entity-header to existing header fields
           read entity-header
        }
        Content-Length := length
        Remove "chunked" from Transfer-Encoding

19.4.7 MIME-Version

 HTTP is not a MIME-compliant protocol (see appendix 19.4). However,
 HTTP/1.1 messages may include a single MIME-Version general-header
 field to indicate what version of the MIME protocol was used to
 construct the message. Use of the MIME-Version header field indicates
 that the message is in full compliance with the MIME protocol.
 Proxies/gateways are responsible for ensuring full compliance (where
 possible) when exporting HTTP messages to strict MIME environments.
        MIME-Version   = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
 MIME version "1.0" is the default for use in HTTP/1.1. However,
 HTTP/1.1 message parsing and semantics are defined by this document
 and not the MIME specification.

19.5 Changes from HTTP/1.0

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

Fielding, et. al. Standards Track [Page 154] RFC 2068 HTTP/1.1 January 1997

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

     Addresses
 The requirements that clients and servers support the Host request-
 header, report an error if the Host request-header (section 14.23) is
 missing from an HTTP/1.1 request, and accept absolute URIs (section
 5.1.2) are among the most important changes defined by this
 specification.
 Older HTTP/1.0 clients assumed a one-to-one relationship of IP
 addresses and servers; there was no other established mechanism for
 distinguishing the intended server of a request than the IP address
 to which that request was directed. The changes outlined above will
 allow the Internet, once older HTTP clients are no longer common, to
 support multiple Web sites from a single IP address, greatly
 simplifying large operational Web servers, where allocation of many
 IP addresses to a single host has created serious problems. The
 Internet will also be able to recover the IP addresses that have been
 allocated for the sole purpose of allowing special-purpose domain
 names to be used in root-level HTTP URLs. Given the rate of growth of
 the Web, and the number of servers already deployed, it is extremely
 important that all implementations of HTTP (including updates to
 existing HTTP/1.0 applications) correctly implement these
 requirements:
   o  Both clients and servers MUST support the Host request-header.
   o  Host request-headers are required in HTTP/1.1 requests.
   o  Servers MUST report a 400 (Bad Request) error if an HTTP/1.1
      request does not include a Host request-header.
   o  Servers MUST accept absolute URIs.

Fielding, et. al. Standards Track [Page 155] RFC 2068 HTTP/1.1 January 1997

19.6 Additional Features

 This appendix documents protocol elements used by some existing HTTP
 implementations, but not consistently and correctly across most
 HTTP/1.1 applications. Implementers should be aware of these
 features, but cannot rely upon their presence in, or interoperability
 with, other HTTP/1.1 applications. Some of these describe proposed
 experimental features, and some describe features that experimental
 deployment found lacking that are now addressed in the base HTTP/1.1
 specification.

19.6.1 Additional Request Methods

19.6.1.1 PATCH

 The PATCH method is similar to PUT except that the entity contains a
 list of differences between the original version of the resource
 identified by the Request-URI and the desired content of the resource
 after the PATCH action has been applied. The list of differences is
 in a format defined by the media type of the entity (e.g.,
 "application/diff") and MUST include sufficient information to allow
 the server to recreate the changes necessary to convert the original
 version of the resource to the desired version.
 If the request passes through a cache and the Request-URI identifies
 a currently cached entity, that entity MUST be removed from the
 cache.  Responses to this method are not cachable.
 The actual method for determining how the patched resource is placed,
 and what happens to its predecessor, is defined entirely by the
 origin server. If the original version of the resource being patched
 included a Content-Version header field, the request entity MUST
 include a Derived-From header field corresponding to the value of the
 original Content-Version header field. Applications are encouraged to
 use these fields for constructing versioning relationships and
 resolving version conflicts.
 PATCH requests must obey the message transmission requirements set
 out in section 8.2.
 Caches that implement PATCH should invalidate cached responses as
 defined in section 13.10 for PUT.

19.6.1.2 LINK

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

Fielding, et. al. Standards Track [Page 156] RFC 2068 HTTP/1.1 January 1997

 allowing links to be established between resources is that the LINK
 method does not allow any message-body to be sent in the request and
 does not directly result in the creation of new resources.
 If the request passes through a cache and the Request-URI identifies
 a currently cached entity, that entity MUST be removed from the
 cache.  Responses to this method are not cachable.
 Caches that implement LINK should invalidate cached responses as
 defined in section 13.10 for PUT.

19.6.1.3 UNLINK

 The UNLINK method removes one or more Link relationships from the
 existing resource identified by the Request-URI. These relationships
 may have been established using the LINK method or by any other
 method supporting the Link header. The removal of a link to a
 resource does not imply that the resource ceases to exist or becomes
 inaccessible for future references.
 If the request passes through a cache and the Request-URI identifies
 a currently cached entity, that entity MUST be removed from the
 cache.  Responses to this method are not cachable.
 Caches that implement UNLINK should invalidate cached responses as
 defined in section 13.10 for PUT.

19.6.2 Additional Header Field Definitions

19.6.2.1 Alternates

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

Fielding, et. al. Standards Track [Page 157] RFC 2068 HTTP/1.1 January 1997

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

19.6.2.2 Content-Version

 The Content-Version entity-header field defines the version tag
 associated with a rendition of an evolving entity. Together with the
 Derived-From field described in section 19.6.2.3, it allows a group
 of people to work simultaneously on the creation of a work as an
 iterative process. The field should be used to allow evolution of a
 particular work along a single path rather than derived works or
 renditions in different representations.
        Content-Version = "Content-Version" ":" quoted-string
 Examples of the Content-Version field include:
        Content-Version: "2.1.2"
        Content-Version: "Fred 19950116-12:26:48"
        Content-Version: "2.5a4-omega7"

19.6.2.3 Derived-From

 The Derived-From entity-header field can be used to indicate the
 version tag of the resource from which the enclosed entity was
 derived before modifications were made by the sender. This field is
 used to help manage the process of merging successive changes to a
 resource, particularly when such changes are being made in parallel
 and from multiple sources.
        Derived-From   = "Derived-From" ":" quoted-string
 An example use of the field is:
        Derived-From: "2.1.1"
 The Derived-From field is required for PUT and PATCH requests if the
 entity being sent was previously retrieved from the same URI and a
 Content-Version header was included with the entity when it was last
 retrieved.

Fielding, et. al. Standards Track [Page 158] RFC 2068 HTTP/1.1 January 1997

19.6.2.4 Link

 The Link entity-header field provides a means for describing a
 relationship between two resources, generally between the requested
 resource and some other resource. An entity MAY include multiple Link
 values. Links at the metainformation level typically indicate
 relationships like hierarchical structure and navigation paths. The
 Link field is semantically equivalent to the <LINK> element in
 HTML.[5]
        Link           = "Link" ":" #("<" URI ">" *( ";" link-param )
        link-param     = ( ( "rel" "=" relationship )
                           | ( "rev" "=" relationship )
                           | ( "title" "=" quoted-string )
                           | ( "anchor" "=" <"> URI <"> )
                           | ( link-extension ) )
        link-extension = token [ "=" ( token | quoted-string ) ]
        relationship   = sgml-name
                       | ( <"> sgml-name *( SP sgml-name) <"> )
        sgml-name      = ALPHA *( ALPHA | DIGIT | "." | "-" )
 Relationship values are case-insensitive and MAY be extended within
 the constraints of the sgml-name syntax. The title parameter MAY be
 used to label the destination of a link such that it can be used as
 identification within a human-readable menu. The anchor parameter MAY
 be used to indicate a source anchor other than the entire current
 resource, such as a fragment of this resource or a third resource.
 Examples of usage include:
     Link: <http://www.cern.ch/TheBook/chapter2>; rel="Previous"
     Link: <mailto:timbl@w3.org>; rev="Made"; title="Tim Berners-Lee"
 The first example indicates that chapter2 is previous to this
 resource in a logical navigation path. The second indicates that the
 person responsible for making the resource available is identified by
 the given e-mail address.

19.6.2.5 URI

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

Fielding, et. al. Standards Track [Page 159] RFC 2068 HTTP/1.1 January 1997

 field (above). Its primary purpose has been to include a list of
 additional URIs for the resource, including names and mirror
 locations. However, it has become clear that the combination of many
 different functions within this single field has been a barrier to
 consistently and correctly implementing any of those functions.
 Furthermore, we believe that the identification of names and mirror
 locations would be better performed via the Link header field. The
 URI header field is therefore deprecated in favor of those other
 fields.
        URI-header    = "URI" ":" 1#( "<" URI ">" )

19.7 Compatibility with Previous Versions

 It is beyond the scope of a protocol specification to mandate
 compliance with previous versions. HTTP/1.1 was deliberately
 designed, however, to make supporting previous versions easy. It is
 worth noting that at the time of composing this specification, we
 would expect commercial HTTP/1.1 servers to:
o  recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1
   requests;
o  understand any valid request in the format of HTTP/0.9, 1.0, or
   1.1;
o  respond appropriately with a message in the same major version used
   by the client.
 And we would expect HTTP/1.1 clients to:
o  recognize the format of the Status-Line for HTTP/1.0 and 1.1
   responses;
o  understand any valid response in the format of HTTP/0.9, 1.0, or
   1.1.
 For most implementations of HTTP/1.0, each connection is established
 by the client prior to the request and closed by the server after
 sending the response. A few implementations implement the Keep-Alive
 version of persistent connections described in section 19.7.1.1.

Fielding, et. al. Standards Track [Page 160] RFC 2068 HTTP/1.1 January 1997

19.7.1 Compatibility with HTTP/1.0 Persistent Connections

 Some clients and servers may wish to be compatible with some previous
 implementations of persistent connections in HTTP/1.0 clients and
 servers. Persistent connections in HTTP/1.0 must be explicitly
 negotiated as they are not the default behavior. HTTP/1.0
 experimental implementations of persistent connections are faulty,
 and the new facilities in HTTP/1.1 are designed to rectify these
 problems. The problem was that some existing 1.0 clients may be
 sending Keep-Alive to a proxy server that doesn't understand
 Connection, which would then erroneously forward it to the next
 inbound server, which would establish the Keep-Alive connection and
 result in a hung HTTP/1.0 proxy waiting for the close on the
 response. The result is that HTTP/1.0 clients must be prevented from
 using Keep-Alive when talking to proxies.
 However, talking to proxies is the most important use of persistent
 connections, so that prohibition is clearly unacceptable. Therefore,
 we need some other mechanism for indicating a persistent connection
 is desired, which is safe to use even when talking to an old proxy
 that ignores Connection. Persistent connections are the default for
 HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
 declaring non-persistence.
 The following describes the original HTTP/1.0 form of persistent
 connections.
 When it connects to an origin server, an HTTP client MAY send the
 Keep-Alive connection-token in addition to the Persist connection-
 token:
        Connection: Keep-Alive
 An HTTP/1.0 server would then respond with the Keep-Alive connection
 token and the client may proceed with an HTTP/1.0 (or Keep-Alive)
 persistent connection.
 An HTTP/1.1 server may also establish persistent connections with
 HTTP/1.0 clients upon receipt of a Keep-Alive connection token.
 However, a persistent connection with an HTTP/1.0 client cannot make
 use of the chunked transfer-coding, and therefore MUST use a
 Content-Length for marking the ending boundary of each message.
 A client MUST NOT send the Keep-Alive connection token to a proxy
 server as HTTP/1.0 proxy servers do not obey the rules of HTTP/1.1
 for parsing the Connection header field.

Fielding, et. al. Standards Track [Page 161] RFC 2068 HTTP/1.1 January 1997

19.7.1.1 The Keep-Alive Header

 When the Keep-Alive connection-token has been transmitted with a
 request or a response, a Keep-Alive header field MAY also be
 included. The Keep-Alive header field takes the following form:
        Keep-Alive-header = "Keep-Alive" ":" 0# keepalive-param
        keepalive-param = param-name "=" value
 The Keep-Alive header itself is optional, and is used only if a
 parameter is being sent. HTTP/1.1 does not define any parameters.
 If the Keep-Alive header is sent, the corresponding connection token
 MUST be transmitted. The Keep-Alive header MUST be ignored if
 received without the connection token.

Fielding, et. al. Standards Track [Page 162]

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