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rfc:rfc2616

Network Working Group R. Fielding Request for Comments: 2616 UC Irvine Obsoletes: 2068 J. Gettys Category: Standards Track Compaq/W3C

                                                            J. Mogul
                                                              Compaq
                                                          H. Frystyk
                                                             W3C/MIT
                                                         L. Masinter
                                                               Xerox
                                                            P. Leach
                                                           Microsoft
                                                      T. Berners-Lee
                                                             W3C/MIT
                                                           June 1999
              Hypertext Transfer Protocol -- HTTP/1.1

Status of this Memo

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

Copyright Notice

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

Abstract

 The Hypertext Transfer Protocol (HTTP) is an application-level
 protocol for distributed, collaborative, hypermedia information
 systems. It is a generic, stateless, protocol which can be used for
 many tasks beyond its use for hypertext, such as name servers and
 distributed object management systems, through extension of its
 request methods, error codes and headers [47]. A feature of HTTP is
 the typing and negotiation of data representation, allowing systems
 to be built independently of the data being transferred.
 HTTP has been in use by the World-Wide Web global information
 initiative since 1990. This specification defines the protocol
 referred to as "HTTP/1.1", and is an update to RFC 2068 [33].

Fielding, et al. Standards Track [Page 1] RFC 2616 HTTP/1.1 June 1999

Table of Contents

 1   Introduction ...................................................7
 1.1    Purpose......................................................7
 1.2   Requirements .................................................8
 1.3   Terminology ..................................................8
 1.4   Overall Operation ...........................................12
 2   Notational Conventions and Generic Grammar ....................14
 2.1   Augmented BNF ...............................................14
 2.2   Basic Rules .................................................15
 3   Protocol Parameters ...........................................17
 3.1   HTTP Version ................................................17
 3.2   Uniform Resource Identifiers ................................18
 3.2.1    General Syntax ...........................................19
 3.2.2    http URL .................................................19
 3.2.3    URI Comparison ...........................................20
 3.3   Date/Time Formats ...........................................20
 3.3.1    Full Date ................................................20
 3.3.2    Delta Seconds ............................................21
 3.4   Character Sets ..............................................21
 3.4.1    Missing Charset ..........................................22
 3.5   Content Codings .............................................23
 3.6   Transfer Codings ............................................24
 3.6.1    Chunked Transfer Coding ..................................25
 3.7   Media Types .................................................26
 3.7.1    Canonicalization and Text Defaults .......................27
 3.7.2    Multipart Types ..........................................27
 3.8   Product Tokens ..............................................28
 3.9   Quality Values ..............................................29
 3.10  Language Tags ...............................................29
 3.11  Entity Tags .................................................30
 3.12  Range Units .................................................30
 4   HTTP Message ..................................................31
 4.1   Message Types ...............................................31
 4.2   Message Headers .............................................31
 4.3   Message Body ................................................32
 4.4   Message Length ..............................................33
 4.5   General Header Fields .......................................34
 5   Request .......................................................35
 5.1   Request-Line ................................................35
 5.1.1    Method ...................................................36
 5.1.2    Request-URI ..............................................36
 5.2   The Resource Identified by a Request ........................38
 5.3   Request Header Fields .......................................38
 6   Response ......................................................39
 6.1   Status-Line .................................................39
 6.1.1    Status Code and Reason Phrase ............................39
 6.2   Response Header Fields ......................................41

Fielding, et al. Standards Track [Page 2] RFC 2616 HTTP/1.1 June 1999

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

Fielding, et al. Standards Track [Page 3] RFC 2616 HTTP/1.1 June 1999

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

Fielding, et al. Standards Track [Page 4] RFC 2616 HTTP/1.1 June 1999

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

Fielding, et al. Standards Track [Page 5] RFC 2616 HTTP/1.1 June 1999

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

Fielding, et al. Standards Track [Page 6] RFC 2616 HTTP/1.1 June 1999

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

1 Introduction

1.1 Purpose

 The Hypertext Transfer Protocol (HTTP) is an application-level
 protocol for distributed, collaborative, hypermedia information
 systems. HTTP has been in use by the World-Wide Web global
 information initiative since 1990. The first version of HTTP,
 referred to as HTTP/0.9, was a simple protocol for raw data transfer
 across the Internet. HTTP/1.0, as defined by RFC 1945 [6], improved
 the protocol by allowing messages to be in the format of MIME-like
 messages, containing metainformation about the data transferred and
 modifiers on the request/response semantics. However, HTTP/1.0 does
 not sufficiently take into consideration the effects of hierarchical
 proxies, caching, the need for persistent connections, or virtual
 hosts. In addition, the proliferation of incompletely-implemented
 applications calling themselves "HTTP/1.0" has necessitated a
 protocol version change in order for two communicating applications
 to determine each other's true capabilities.
 This specification defines the protocol referred to as "HTTP/1.1".
 This protocol includes more stringent requirements than HTTP/1.0 in
 order to ensure reliable implementation of its features.
 Practical information systems require more functionality than simple
 retrieval, including search, front-end update, and annotation. HTTP
 allows an open-ended set of methods and headers that indicate the
 purpose of a request [47]. It builds on the discipline of reference
 provided by the Uniform Resource Identifier (URI) [3], as a location
 (URL) [4] or name (URN) [20], for indicating the resource to which a

Fielding, et al. Standards Track [Page 7] RFC 2616 HTTP/1.1 June 1999

 method is to be applied. Messages are passed in a format similar to
 that used by Internet mail [9] as defined by the Multipurpose
 Internet Mail Extensions (MIME) [7].
 HTTP is also used as a generic protocol for communication between
 user agents and proxies/gateways to other Internet systems, including
 those supported by the SMTP [16], NNTP [13], FTP [18], Gopher [2],
 and WAIS [10] protocols. In this way, HTTP allows basic hypermedia
 access to resources available from diverse applications.

1.2 Requirements

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [34].
 An implementation is not compliant if it fails to satisfy one or more
 of the MUST or REQUIRED level requirements for the protocols it
 implements. An implementation that satisfies all the MUST or REQUIRED
 level and all the SHOULD level requirements for its protocols is said
 to be "unconditionally compliant"; one that satisfies all the MUST
 level requirements but not all the SHOULD level requirements for its
 protocols is said to be "conditionally compliant."

1.3 Terminology

 This specification uses a number of terms to refer to the roles
 played by participants in, and objects of, the HTTP communication.
 connection
    A transport layer virtual circuit established between two programs
    for the purpose of communication.
 message
    The basic unit of HTTP communication, consisting of a structured
    sequence of octets matching the syntax defined in section 4 and
    transmitted via the connection.
 request
    An HTTP request message, as defined in section 5.
 response
    An HTTP response message, as defined in section 6.

Fielding, et al. Standards Track [Page 8] RFC 2616 HTTP/1.1 June 1999

 resource
    A network data object or service that can be identified by a URI,
    as defined in section 3.2. Resources may be available in multiple
    representations (e.g. multiple languages, data formats, size, and
    resolutions) or vary in other ways.
 entity
    The information transferred as the payload of a request or
    response. An entity consists of metainformation in the form of
    entity-header fields and content in the form of an entity-body, as
    described in section 7.
 representation
    An entity included with a response that is subject to content
    negotiation, as described in section 12. There may exist multiple
    representations associated with a particular response status.
 content negotiation
    The mechanism for selecting the appropriate representation when
    servicing a request, as described in section 12. The
    representation of entities in any response can be negotiated
    (including error responses).
 variant
    A resource may have one, or more than one, representation(s)
    associated with it at any given instant. Each of these
    representations is termed a `varriant'.  Use of the term `variant'
    does not necessarily imply that the resource is subject to content
    negotiation.
 client
    A program that establishes connections for the purpose of sending
    requests.
 user agent
    The client which initiates a request. These are often browsers,
    editors, spiders (web-traversing robots), or other end user tools.
 server
    An application program that accepts connections in order to
    service requests by sending back responses. Any given program may
    be capable of being both a client and a server; our use of these
    terms refers only to the role being performed by the program for a
    particular connection, rather than to the program's capabilities
    in general. Likewise, any server may act as an origin server,
    proxy, gateway, or tunnel, switching behavior based on the nature
    of each request.

Fielding, et al. Standards Track [Page 9] RFC 2616 HTTP/1.1 June 1999

 origin server
    The server on which a given resource resides or is to be created.
 proxy
    An intermediary program which acts as both a server and a client
    for the purpose of making requests on behalf of other clients.
    Requests are serviced internally or by passing them on, with
    possible translation, to other servers. A proxy MUST implement
    both the client and server requirements of this specification. A
    "transparent proxy" is a proxy that does not modify the request or
    response beyond what is required for proxy authentication and
    identification. A "non-transparent proxy" is a proxy that modifies
    the request or response in order to provide some added service to
    the user agent, such as group annotation services, media type
    transformation, protocol reduction, or anonymity filtering. Except
    where either transparent or non-transparent behavior is explicitly
    stated, the HTTP proxy requirements apply to both types of
    proxies.
 gateway
    A server which acts as an intermediary for some other server.
    Unlike a proxy, a gateway receives requests as if it were the
    origin server for the requested resource; the requesting client
    may not be aware that it is communicating with a gateway.
 tunnel
    An intermediary program which is acting as a blind relay between
    two connections. Once active, a tunnel is not considered a party
    to the HTTP communication, though the tunnel may have been
    initiated by an HTTP request. The tunnel ceases to exist when both
    ends of the relayed connections are closed.
 cache
    A program's local store of response messages and the subsystem
    that controls its message storage, retrieval, and deletion. A
    cache stores cacheable responses in order to reduce the response
    time and network bandwidth consumption on future, equivalent
    requests. Any client or server may include a cache, though a cache
    cannot be used by a server that is acting as a tunnel.
 cacheable
    A response is cacheable if a cache is allowed to store a copy of
    the response message for use in answering subsequent requests. The
    rules for determining the cacheability of HTTP responses are
    defined in section 13. Even if a resource is cacheable, there may
    be additional constraints on whether a cache can use the cached
    copy for a particular request.

Fielding, et al. Standards Track [Page 10] RFC 2616 HTTP/1.1 June 1999

 first-hand
    A response is first-hand if it comes directly and without
    unnecessary delay from the origin server, perhaps via one or more
    proxies. A response is also first-hand if its validity has just
    been checked directly with the origin server.
 explicit expiration time
    The time at which the origin server intends that an entity should
    no longer be returned by a cache without further validation.
 heuristic expiration time
    An expiration time assigned by a cache when no explicit expiration
    time is available.
 age
    The age of a response is the time since it was sent by, or
    successfully validated with, the origin server.
 freshness lifetime
    The length of time between the generation of a response and its
    expiration time.
 fresh
    A response is fresh if its age has not yet exceeded its freshness
    lifetime.
 stale
    A response is stale if its age has passed its freshness lifetime.
 semantically transparent
    A cache behaves in a "semantically transparent" manner, with
    respect to a particular response, when its use affects neither the
    requesting client nor the origin server, except to improve
    performance. When a cache is semantically transparent, the client
    receives exactly the same response (except for hop-by-hop headers)
    that it would have received had its request been handled directly
    by the origin server.
 validator
    A protocol element (e.g., an entity tag or a Last-Modified time)
    that is used to find out whether a cache entry is an equivalent
    copy of an entity.
 upstream/downstream
    Upstream and downstream describe the flow of a message: all
    messages flow from upstream to downstream.

Fielding, et al. Standards Track [Page 11] RFC 2616 HTTP/1.1 June 1999

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

1.4 Overall Operation

 The HTTP protocol is a request/response protocol. A client sends a
 request to the server in the form of a request method, URI, and
 protocol version, followed by a MIME-like message containing request
 modifiers, client information, and possible body content over a
 connection with a server. The server responds with a status line,
 including the message's protocol version and a success or error code,
 followed by a MIME-like message containing server information, entity
 metainformation, and possible entity-body content. The relationship
 between HTTP and MIME is described in appendix 19.4.
 Most HTTP communication is initiated by a user agent and consists of
 a request to be applied to a resource on some origin server. In the
 simplest case, this may be accomplished via a single connection (v)
 between the user agent (UA) and the origin server (O).
        request chain ------------------------>
     UA -------------------v------------------- O
        <----------------------- response chain
 A more complicated situation occurs when one or more intermediaries
 are present in the request/response chain. There are three common
 forms of intermediary: proxy, gateway, and tunnel. A proxy is a
 forwarding agent, receiving requests for a URI in its absolute form,
 rewriting all or part of the message, and forwarding the reformatted
 request toward the server identified by the URI. A gateway is a
 receiving agent, acting as a layer above some other server(s) and, if
 necessary, translating the requests to the underlying server's
 protocol. A tunnel acts as a relay point between two connections
 without changing the messages; tunnels are used when the
 communication needs to pass through an intermediary (such as a
 firewall) even when the intermediary cannot understand the contents
 of the messages.
        request chain -------------------------------------->
     UA -----v----- A -----v----- B -----v----- C -----v----- O
        <------------------------------------- response chain
 The figure above shows three intermediaries (A, B, and C) between the
 user agent and origin server. A request or response message that
 travels the whole chain will pass through four separate connections.
 This distinction is important because some HTTP communication options

Fielding, et al. Standards Track [Page 12] RFC 2616 HTTP/1.1 June 1999

 may apply only to the connection with the nearest, non-tunnel
 neighbor, only to the end-points of the chain, or to all connections
 along the chain. Although the diagram is linear, each participant may
 be engaged in multiple, simultaneous communications. For example, B
 may be receiving requests from many clients other than A, and/or
 forwarding requests to servers other than C, at the same time that it
 is handling A's request.
 Any party to the communication which is not acting as a tunnel may
 employ an internal cache for handling requests. The effect of a cache
 is that the request/response chain is shortened if one of the
 participants along the chain has a cached response applicable to that
 request. The following illustrates the resulting chain if B has a
 cached copy of an earlier response from O (via C) for a request which
 has not been cached by UA or A.
        request chain ---------->
     UA -----v----- A -----v----- B - - - - - - C - - - - - - O
        <--------- response chain
 Not all responses are usefully cacheable, and some requests may
 contain modifiers which place special requirements on cache behavior.
 HTTP requirements for cache behavior and cacheable responses are
 defined in section 13.
 In fact, there are a wide variety of architectures and configurations
 of caches and proxies currently being experimented with or deployed
 across the World Wide Web. These systems include national hierarchies
 of proxy caches to save transoceanic bandwidth, systems that
 broadcast or multicast cache entries, organizations that distribute
 subsets of cached data via CD-ROM, and so on. HTTP systems are used
 in corporate intranets over high-bandwidth links, and for access via
 PDAs with low-power radio links and intermittent connectivity. The
 goal of HTTP/1.1 is to support the wide diversity of configurations
 already deployed while introducing protocol constructs that meet the
 needs of those who build web applications that require high
 reliability and, failing that, at least reliable indications of
 failure.
 HTTP communication usually takes place over TCP/IP connections. The
 default port is TCP 80 [19], but other ports can be used. This does
 not preclude HTTP from being implemented on top of any other protocol
 on the Internet, or on other networks. HTTP only presumes a reliable
 transport; any protocol that provides such guarantees can be used;
 the mapping of the HTTP/1.1 request and response structures onto the
 transport data units of the protocol in question is outside the scope
 of this specification.

Fielding, et al. Standards Track [Page 13] RFC 2616 HTTP/1.1 June 1999

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

2 Notational Conventions and Generic Grammar

2.1 Augmented BNF

 All of the mechanisms specified in this document are described in
 both prose and an augmented Backus-Naur Form (BNF) similar to that
 used by RFC 822 [9]. Implementors will need to be familiar with the
 notation in order to understand this specification. The augmented BNF
 includes the following constructs:
 name = definition
    The name of a rule is simply the name itself (without any
    enclosing "<" and ">") and is separated from its definition by the
    equal "=" character. White space is only significant in that
    indentation of continuation lines is used to indicate a rule
    definition that spans more than one line. Certain basic rules are
    in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle
    brackets are used within definitions whenever their presence will
    facilitate discerning the use of rule names.
 "literal"
    Quotation marks surround literal text. Unless stated otherwise,
    the text is case-insensitive.
 rule1 | rule2
    Elements separated by a bar ("|") are alternatives, e.g., "yes |
    no" will accept yes or no.
 (rule1 rule2)
    Elements enclosed in parentheses are treated as a single element.
    Thus, "(elem (foo | bar) elem)" allows the token sequences "elem
    foo elem" and "elem bar elem".
  • rule

The character "*" preceding an element indicates repetition. The

    full form is "<n>*<m>element" indicating at least <n> and at most
    <m> occurrences of element. Default values are 0 and infinity so
    that "*(element)" allows any number, including zero; "1*element"
    requires at least one; and "1*2element" allows one or two.
 [rule]
    Square brackets enclose optional elements; "[foo bar]" is
    equivalent to "*1(foo bar)".

Fielding, et al. Standards Track [Page 14] RFC 2616 HTTP/1.1 June 1999

 N rule
    Specific repetition: "<n>(element)" is equivalent to
    "<n>*<n>(element)"; that is, exactly <n> occurrences of (element).
    Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
    alphabetic characters.
 #rule
    A construct "#" is defined, similar to "*", for defining lists of
    elements. The full form is "<n>#<m>element" indicating at least
    <n> and at most <m> elements, each separated by one or more commas
    (",") and OPTIONAL linear white space (LWS). This makes the usual
    form of lists very easy; a rule such as
       ( *LWS element *( *LWS "," *LWS element ))
    can be shown as
       1#element
    Wherever this construct is used, null elements are allowed, but do
    not contribute to the count of elements present. That is,
    "(element), , (element) " is permitted, but counts as only two
    elements. Therefore, where at least one element is required, at
    least one non-null element MUST be present. Default values are 0
    and infinity so that "#element" allows any number, including zero;
    "1#element" requires at least one; and "1#2element" allows one or
    two.
 ; comment
    A semi-colon, set off some distance to the right of rule text,
    starts a comment that continues to the end of line. This is a
    simple way of including useful notes in parallel with the
    specifications.
 implied *LWS
    The grammar described by this specification is word-based. Except
    where noted otherwise, linear white space (LWS) can be included
    between any two adjacent words (token or quoted-string), and
    between adjacent words and separators, without changing the
    interpretation of a field. At least one delimiter (LWS and/or
    separators) MUST exist between any two tokens (for the definition
    of "token" below), since they would otherwise be interpreted as a
    single token.

2.2 Basic Rules

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

Fielding, et al. Standards Track [Page 15] RFC 2616 HTTP/1.1 June 1999

     OCTET          = <any 8-bit sequence of data>
     CHAR           = <any US-ASCII character (octets 0 - 127)>
     UPALPHA        = <any US-ASCII uppercase letter "A".."Z">
     LOALPHA        = <any US-ASCII lowercase letter "a".."z">
     ALPHA          = UPALPHA | LOALPHA
     DIGIT          = <any US-ASCII digit "0".."9">
     CTL            = <any US-ASCII control character
                      (octets 0 - 31) and DEL (127)>
     CR             = <US-ASCII CR, carriage return (13)>
     LF             = <US-ASCII LF, linefeed (10)>
     SP             = <US-ASCII SP, space (32)>
     HT             = <US-ASCII HT, horizontal-tab (9)>
     <">            = <US-ASCII double-quote mark (34)>
 HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
 protocol elements except the entity-body (see appendix 19.3 for
 tolerant applications). The end-of-line marker within an entity-body
 is defined by its associated media type, as described in section 3.7.
     CRLF           = CR LF
 HTTP/1.1 header field values can be folded onto multiple lines if the
 continuation line begins with a space or horizontal tab. All linear
 white space, including folding, has the same semantics as SP. A
 recipient MAY replace any linear white space with a single SP before
 interpreting the field value or forwarding the message downstream.
     LWS            = [CRLF] 1*( SP | HT )
 The TEXT rule is only used for descriptive field contents and values
 that are not intended to be interpreted by the message parser. Words
 of *TEXT MAY contain characters from character sets other than ISO-
 8859-1 [22] only when encoded according to the rules of RFC 2047
 [14].
     TEXT           = <any OCTET except CTLs,
                      but including LWS>
 A CRLF is allowed in the definition of TEXT only as part of a header
 field continuation. It is expected that the folding LWS will be
 replaced with a single SP before interpretation of the TEXT value.
 Hexadecimal numeric characters are used in several protocol elements.
     HEX            = "A" | "B" | "C" | "D" | "E" | "F"
                    | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT

Fielding, et al. Standards Track [Page 16] RFC 2616 HTTP/1.1 June 1999

 Many HTTP/1.1 header field values consist of words separated by LWS
 or special characters. These special characters MUST be in a quoted
 string to be used within a parameter value (as defined in section
 3.6).
     token          = 1*<any CHAR except CTLs or separators>
     separators     = "(" | ")" | "<" | ">" | "@"
                    | "," | ";" | ":" | "\" | <">
                    | "/" | "[" | "]" | "?" | "="
                    | "{" | "}" | SP | HT
 Comments can be included in some HTTP header fields by surrounding
 the comment text with parentheses. Comments are only allowed in
 fields containing "comment" as part of their field value definition.
 In all other fields, parentheses are considered part of the field
 value.
     comment        = "(" *( ctext | quoted-pair | comment ) ")"
     ctext          = <any TEXT excluding "(" and ")">
 A string of text is parsed as a single word if it is quoted using
 double-quote marks.
     quoted-string  = ( <"> *(qdtext | quoted-pair ) <"> )
     qdtext         = <any TEXT except <">>
 The backslash character ("\") MAY be used as a single-character
 quoting mechanism only within quoted-string and comment constructs.
     quoted-pair    = "\" CHAR

3 Protocol Parameters

3.1 HTTP Version

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

Fielding, et al. Standards Track [Page 17] RFC 2616 HTTP/1.1 June 1999

 The version of an HTTP message is indicated by an HTTP-Version field
 in the first line of the message.
     HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT
 Note that the major and minor numbers MUST be treated as separate
 integers and that each MAY be incremented higher than a single digit.
 Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
 lower than HTTP/12.3. Leading zeros MUST be ignored by recipients and
 MUST NOT be sent.
 An application that sends a request or response message that includes
 HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant
 with this specification. Applications that are at least conditionally
 compliant with this specification SHOULD use an HTTP-Version of
 "HTTP/1.1" in their messages, and MUST do so for any message that is
 not compatible with HTTP/1.0. For more details on when to send
 specific HTTP-Version values, see RFC 2145 [36].
 The HTTP version of an application is the highest HTTP version for
 which the application is at least conditionally compliant.
 Proxy and gateway applications need to be careful when forwarding
 messages in protocol versions different from that of the application.
 Since the protocol version indicates the protocol capability of the
 sender, a proxy/gateway MUST NOT send a message with a version
 indicator which is greater than its actual version. If a higher
 version request is received, the proxy/gateway MUST either downgrade
 the request version, or respond with an error, or switch to tunnel
 behavior.
 Due to interoperability problems with HTTP/1.0 proxies discovered
 since the publication of RFC 2068[33], caching proxies MUST, gateways
 MAY, and tunnels MUST NOT upgrade the request to the highest version
 they support. The proxy/gateway's response to that request MUST be in
 the same major version as the request.
    Note: Converting between versions of HTTP may involve modification
    of header fields required or forbidden by the versions involved.

3.2 Uniform Resource Identifiers

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

Fielding, et al. Standards Track [Page 18] RFC 2616 HTTP/1.1 June 1999

3.2.1 General Syntax

 URIs in HTTP can be represented in absolute form or relative to some
 known base URI [11], depending upon the context of their use. The two
 forms are differentiated by the fact that absolute URIs always begin
 with a scheme name followed by a colon. For definitive information on
 URL syntax and semantics, see "Uniform Resource Identifiers (URI):
 Generic Syntax and Semantics," RFC 2396 [42] (which replaces RFCs
 1738 [4] and RFC 1808 [11]). This specification adopts the
 definitions of "URI-reference", "absoluteURI", "relativeURI", "port",
 "host","abs_path", "rel_path", and "authority" from that
 specification.
 The HTTP protocol does not place any a priori limit on the length of
 a URI. Servers MUST be able to handle the URI of any resource they
 serve, and SHOULD be able to handle URIs of unbounded length if they
 provide GET-based forms that could generate such URIs. A server
 SHOULD return 414 (Request-URI Too Long) status if a URI is longer
 than the server can handle (see section 10.4.15).
    Note: Servers ought to be cautious about depending on URI lengths
    above 255 bytes, because some older client or proxy
    implementations might not properly support these lengths.

3.2.2 http URL

 The "http" scheme is used to locate network resources via the HTTP
 protocol. This section defines the scheme-specific syntax and
 semantics for http URLs.
 http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
 If the port is empty or not given, port 80 is assumed. The semantics
 are that the identified resource is located at the server listening
 for TCP connections on that port of that host, and the Request-URI
 for the resource is abs_path (section 5.1.2). The use of IP addresses
 in URLs SHOULD be avoided whenever possible (see RFC 1900 [24]). If
 the abs_path is not present in the URL, it MUST be given as "/" when
 used as a Request-URI for a resource (section 5.1.2). If a proxy
 receives a host name which is not a fully qualified domain name, it
 MAY add its domain to the host name it received. If a proxy receives
 a fully qualified domain name, the proxy MUST NOT change the host
 name.

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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:
  1. A port that is empty or not given is equivalent to the default

port for that URI-reference;

  1. Comparisons of host names MUST be case-insensitive;
  1. Comparisons of scheme names MUST be case-insensitive;
  1. An empty abs_path is equivalent to an abs_path of "/".
 Characters other than those in the "reserved" and "unsafe" sets (see
 RFC 2396 [42]) are equivalent to their ""%" HEX HEX" encoding.
 For example, the following three URIs are equivalent:
    http://abc.com:80/~smith/home.html
    http://ABC.com/%7Esmith/home.html
    http://ABC.com:/%7esmith/home.html

3.3 Date/Time Formats

3.3.1 Full Date

 HTTP applications have historically allowed three different formats
 for the representation of date/time stamps:
    Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
    Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
    Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format
 The first format is preferred as an Internet standard and represents
 a fixed-length subset of that defined by RFC 1123 [8] (an update to
 RFC 822 [9]). The second format is in common use, but is based on the
 obsolete RFC 850 [12] date format and lacks a four-digit year.
 HTTP/1.1 clients and servers that parse the date value MUST accept
 all three formats (for compatibility with HTTP/1.0), though they MUST
 only generate the RFC 1123 format for representing HTTP-date values
 in header fields. See section 19.3 for further information.
    Note: Recipients of date values are encouraged to be robust in
    accepting date values that may have been sent by non-HTTP
    applications, as is sometimes the case when retrieving or posting
    messages via proxies/gateways to SMTP or NNTP.

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 All HTTP date/time stamps MUST be represented in Greenwich Mean Time
 (GMT), without exception. For the purposes of HTTP, GMT is exactly
 equal to UTC (Coordinated Universal Time). This is indicated in the
 first two formats by the inclusion of "GMT" as the three-letter
 abbreviation for time zone, and MUST be assumed when reading the
 asctime format. HTTP-date is case sensitive and MUST NOT include
 additional LWS beyond that specifically included as SP in the
 grammar.
     HTTP-date    = rfc1123-date | rfc850-date | asctime-date
     rfc1123-date = wkday "," SP date1 SP time SP "GMT"
     rfc850-date  = weekday "," SP date2 SP time SP "GMT"
     asctime-date = wkday SP date3 SP time SP 4DIGIT
     date1        = 2DIGIT SP month SP 4DIGIT
                    ; day month year (e.g., 02 Jun 1982)
     date2        = 2DIGIT "-" month "-" 2DIGIT
                    ; day-month-year (e.g., 02-Jun-82)
     date3        = month SP ( 2DIGIT | ( SP 1DIGIT ))
                    ; month day (e.g., Jun  2)
     time         = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                    ; 00:00:00 - 23:59:59
     wkday        = "Mon" | "Tue" | "Wed"
                  | "Thu" | "Fri" | "Sat" | "Sun"
     weekday      = "Monday" | "Tuesday" | "Wednesday"
                  | "Thursday" | "Friday" | "Saturday" | "Sunday"
     month        = "Jan" | "Feb" | "Mar" | "Apr"
                  | "May" | "Jun" | "Jul" | "Aug"
                  | "Sep" | "Oct" | "Nov" | "Dec"
    Note: HTTP requirements for the date/time stamp format apply only
    to their usage within the protocol stream. Clients and servers are
    not required to use these formats for user presentation, request
    logging, etc.

3.3.2 Delta Seconds

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

3.4 Character Sets

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

Fielding, et al. Standards Track [Page 21] RFC 2616 HTTP/1.1 June 1999

 The term "character set" is used in this document to refer to a
 method used with one or more tables to convert a sequence of octets
 into a sequence of characters. Note that unconditional conversion in
 the other direction is not required, in that not all characters may
 be available in a given character set and a character set may provide
 more than one sequence of octets to represent a particular character.
 This definition is intended to allow various kinds of character
 encoding, from simple single-table mappings such as US-ASCII to
 complex table switching methods such as those that use ISO-2022's
 techniques. However, the definition associated with a MIME character
 set name MUST fully specify the mapping to be performed from octets
 to characters. In particular, use of external profiling information
 to determine the exact mapping is not permitted.
    Note: This use of the term "character set" is more commonly
    referred to as a "character encoding." However, since HTTP and
    MIME share the same registry, it is important that the terminology
    also be shared.
 HTTP character sets are identified by case-insensitive tokens. The
 complete set of tokens is defined by the IANA Character Set registry
 [19].
     charset = token
 Although HTTP allows an arbitrary token to be used as a charset
 value, any token that has a predefined value within the IANA
 Character Set registry [19] MUST represent the character set defined
 by that registry. Applications SHOULD limit their use of character
 sets to those defined by the IANA registry.
 Implementors should be aware of IETF character set requirements [38]
 [41].

3.4.1 Missing Charset

 Some HTTP/1.0 software has interpreted a Content-Type header without
 charset parameter incorrectly to mean "recipient should guess."
 Senders wishing to defeat this behavior MAY include a charset
 parameter even when the charset is ISO-8859-1 and SHOULD do so when
 it is known that it will not confuse the recipient.
 Unfortunately, some older HTTP/1.0 clients did not deal properly with
 an explicit charset parameter. HTTP/1.1 recipients MUST respect the
 charset label provided by the sender; and those user agents that have
 a provision to "guess" a charset MUST use the charset from the

Fielding, et al. Standards Track [Page 22] RFC 2616 HTTP/1.1 June 1999

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

3.5 Content Codings

 Content coding values indicate an encoding transformation that has
 been or can be applied to an entity. Content codings are primarily
 used to allow a document to be compressed or otherwise usefully
 transformed without losing the identity of its underlying media type
 and without loss of information. Frequently, the entity is stored in
 coded form, transmitted directly, and only decoded by the recipient.
     content-coding   = token
 All content-coding values are case-insensitive. HTTP/1.1 uses
 content-coding values in the Accept-Encoding (section 14.3) and
 Content-Encoding (section 14.11) header fields. Although the value
 describes the content-coding, what is more important is that it
 indicates what decoding mechanism will be required to remove the
 encoding.
 The Internet Assigned Numbers Authority (IANA) acts as a registry for
 content-coding value tokens. Initially, the registry contains the
 following tokens:
 gzip An encoding format produced by the file compression program
      "gzip" (GNU zip) as described in RFC 1952 [25]. This format is a
      Lempel-Ziv coding (LZ77) with a 32 bit CRC.
 compress
      The encoding format produced by the common UNIX file compression
      program "compress". This format is an adaptive Lempel-Ziv-Welch
      coding (LZW).
      Use of program names for the identification of encoding formats
      is not desirable and is discouraged for future encodings. Their
      use here is representative of historical practice, not good
      design. For compatibility with previous implementations of HTTP,
      applications SHOULD consider "x-gzip" and "x-compress" to be
      equivalent to "gzip" and "compress" respectively.
 deflate
      The "zlib" format defined in RFC 1950 [31] in combination with
      the "deflate" compression mechanism described in RFC 1951 [29].

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 identity
      The default (identity) encoding; the use of no transformation
      whatsoever. This content-coding is used only in the Accept-
      Encoding header, and SHOULD NOT be used in the Content-Encoding
      header.
 New content-coding value tokens SHOULD be registered; to allow
 interoperability between clients and servers, specifications of the
 content coding algorithms needed to implement a new value SHOULD be
 publicly available and adequate for independent implementation, and
 conform to the purpose of content coding defined in this section.

3.6 Transfer Codings

 Transfer-coding values are used to indicate an encoding
 transformation that has been, can be, or may need to be applied to an
 entity-body in order to ensure "safe transport" through the network.
 This differs from a content coding in that the transfer-coding is a
 property of the message, not of the original entity.
     transfer-coding         = "chunked" | transfer-extension
     transfer-extension      = token *( ";" parameter )
 Parameters are in  the form of attribute/value pairs.
     parameter               = attribute "=" value
     attribute               = token
     value                   = token | quoted-string
 All transfer-coding values are case-insensitive. HTTP/1.1 uses
 transfer-coding values in the TE header field (section 14.39) and in
 the Transfer-Encoding header field (section 14.41).
 Whenever a transfer-coding is applied to a message-body, the set of
 transfer-codings MUST include "chunked", unless the message is
 terminated by closing the connection. When the "chunked" transfer-
 coding is used, it MUST be the last transfer-coding applied to the
 message-body. The "chunked" transfer-coding MUST NOT be applied more
 than once to a message-body. These rules allow the recipient to
 determine the transfer-length of the message (section 4.4).
 Transfer-codings are analogous to the Content-Transfer-Encoding
 values of MIME [7], which were designed to enable safe transport of
 binary data over a 7-bit transport service. However, safe transport
 has a different focus for an 8bit-clean transfer protocol. In HTTP,
 the only unsafe characteristic of message-bodies is the difficulty in
 determining the exact body length (section 7.2.2), or the desire to
 encrypt data over a shared transport.

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 The Internet Assigned Numbers Authority (IANA) acts as a registry for
 transfer-coding value tokens. Initially, the registry contains the
 following tokens: "chunked" (section 3.6.1), "identity" (section
 3.6.2), "gzip" (section 3.5), "compress" (section 3.5), and "deflate"
 (section 3.5).
 New transfer-coding value tokens SHOULD be registered in the same way
 as new content-coding value tokens (section 3.5).
 A server which receives an entity-body with a transfer-coding it does
 not understand SHOULD return 501 (Unimplemented), and close the
 connection. A server MUST NOT send transfer-codings to an HTTP/1.0
 client.

3.6.1 Chunked Transfer Coding

 The chunked encoding modifies the body of a message in order to
 transfer it as a series of chunks, each with its own size indicator,
 followed by an OPTIONAL trailer containing entity-header fields. This
 allows dynamically produced content to be transferred along with the
 information necessary for the recipient to verify that it has
 received the full message.
     Chunked-Body   = *chunk
                      last-chunk
                      trailer
                      CRLF
     chunk          = chunk-size [ chunk-extension ] CRLF
                      chunk-data CRLF
     chunk-size     = 1*HEX
     last-chunk     = 1*("0") [ chunk-extension ] CRLF
     chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
     chunk-ext-name = token
     chunk-ext-val  = token | quoted-string
     chunk-data     = chunk-size(OCTET)
     trailer        = *(entity-header CRLF)
 The chunk-size field is a string of hex digits indicating the size of
 the chunk. The chunked encoding is ended by any chunk whose size is
 zero, followed by the trailer, which is terminated by an empty line.
 The trailer allows the sender to include additional HTTP header
 fields at the end of the message. The Trailer header field can be
 used to indicate which header fields are included in a trailer (see
 section 14.40).

Fielding, et al. Standards Track [Page 25] RFC 2616 HTTP/1.1 June 1999

 A server using chunked transfer-coding in a response MUST NOT use the
 trailer for any header fields unless at least one of the following is
 true:
 a)the request included a TE header field that indicates "trailers" is
   acceptable in the transfer-coding of the  response, as described in
   section 14.39; or,
 b)the server is the origin server for the response, the trailer
   fields consist entirely of optional metadata, and the recipient
   could use the message (in a manner acceptable to the origin server)
   without receiving this metadata.  In other words, the origin server
   is willing to accept the possibility that the trailer fields might
   be silently discarded along the path to the client.
 This requirement prevents an interoperability failure when the
 message is being received by an HTTP/1.1 (or later) proxy and
 forwarded to an HTTP/1.0 recipient. It avoids a situation where
 compliance with the protocol would have necessitated a possibly
 infinite buffer on the proxy.
 An example process for decoding a Chunked-Body is presented in
 appendix 19.4.6.
 All HTTP/1.1 applications MUST be able to receive and decode the
 "chunked" transfer-coding, and MUST ignore chunk-extension extensions
 they do not understand.

3.7 Media Types

 HTTP uses Internet Media Types [17] in the Content-Type (section
 14.17) and Accept (section 14.1) header fields in order to provide
 open and extensible data typing and type negotiation.
     media-type     = type "/" subtype *( ";" parameter )
     type           = token
     subtype        = token
 Parameters MAY follow the type/subtype in the form of attribute/value
 pairs (as defined in section 3.6).
 The type, subtype, and parameter attribute names are case-
 insensitive. Parameter values might or might not be case-sensitive,
 depending on the semantics of the parameter name. Linear white space
 (LWS) MUST NOT be used between the type and subtype, nor between an
 attribute and its value. The presence or absence of a parameter might
 be significant to the processing of a media-type, depending on its
 definition within the media type registry.

Fielding, et al. Standards Track [Page 26] RFC 2616 HTTP/1.1 June 1999

 Note that some older HTTP applications do not recognize media type
 parameters. When sending data to older HTTP applications,
 implementations SHOULD only use media type parameters when they are
 required by that type/subtype definition.
 Media-type values are registered with the Internet Assigned Number
 Authority (IANA [19]). The media type registration process is
 outlined in RFC 1590 [17]. Use of non-registered media types is
 discouraged.

3.7.1 Canonicalization and Text Defaults

 Internet media types are registered with a canonical form. An
 entity-body transferred via HTTP messages MUST be represented in the
 appropriate canonical form prior to its transmission except for
 "text" types, as defined in the next paragraph.
 When in canonical form, media subtypes of the "text" type use CRLF as
 the text line break. HTTP relaxes this requirement and allows the
 transport of text media with plain CR or LF alone representing a line
 break when it is done consistently for an entire entity-body. HTTP
 applications MUST accept CRLF, bare CR, and bare LF as being
 representative of a line break in text media received via HTTP. In
 addition, if the text is represented in a character set that does not
 use octets 13 and 10 for CR and LF respectively, as is the case for
 some multi-byte character sets, HTTP allows the use of whatever octet
 sequences are defined by that character set to represent the
 equivalent of CR and LF for line breaks. This flexibility regarding
 line breaks applies only to text media in the entity-body; a bare CR
 or LF MUST NOT be substituted for CRLF within any of the HTTP control
 structures (such as header fields and multipart boundaries).
 If an entity-body is encoded with a content-coding, the underlying
 data MUST be in a form defined above prior to being encoded.
 The "charset" parameter is used with some media types to define the
 character set (section 3.4) of the data. When no explicit charset
 parameter is provided by the sender, media subtypes of the "text"
 type are defined to have a default charset value of "ISO-8859-1" when
 received via HTTP. Data in character sets other than "ISO-8859-1" or
 its subsets MUST be labeled with an appropriate charset value. See
 section 3.4.1 for compatibility problems.

3.7.2 Multipart Types

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

Fielding, et al. Standards Track [Page 27] RFC 2616 HTTP/1.1 June 1999

 [40], and MUST include a boundary parameter as part of the media type
 value. The message body is itself a protocol element and MUST
 therefore use only CRLF to represent line breaks between body-parts.
 Unlike in RFC 2046, the epilogue of any multipart message MUST be
 empty; HTTP applications MUST NOT transmit the epilogue (even if the
 original multipart contains an epilogue). These restrictions exist in
 order to preserve the self-delimiting nature of a multipart message-
 body, wherein the "end" of the message-body is indicated by the
 ending multipart boundary.
 In general, HTTP treats a multipart message-body no differently than
 any other media type: strictly as payload. The one exception is the
 "multipart/byteranges" type (appendix 19.2) when it appears in a 206
 (Partial Content) response, which will be interpreted by some HTTP
 caching mechanisms as described in sections 13.5.4 and 14.16. In all
 other cases, an HTTP user agent SHOULD follow the same or similar
 behavior as a MIME user agent would upon receipt of a multipart type.
 The MIME header fields within each body-part of a multipart message-
 body do not have any significance to HTTP beyond that defined by
 their MIME semantics.
 In general, an HTTP user agent SHOULD follow the same or similar
 behavior as a MIME user agent would upon receipt of a multipart type.
 If an application receives an unrecognized multipart subtype, the
 application MUST treat it as being equivalent to "multipart/mixed".
    Note: The "multipart/form-data" type has been specifically defined
    for carrying form data suitable for processing via the POST
    request method, as described in RFC 1867 [15].

3.8 Product Tokens

 Product tokens are used to allow communicating applications to
 identify themselves by software name and version. Most fields using
 product tokens also allow sub-products which form a significant part
 of the application to be listed, separated by white space. By
 convention, the products are listed in order of their significance
 for identifying the application.
     product         = token ["/" product-version]
     product-version = token
 Examples:
     User-Agent: CERN-LineMode/2.15 libwww/2.17b3
     Server: Apache/0.8.4

Fielding, et al. Standards Track [Page 28] RFC 2616 HTTP/1.1 June 1999

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

3.9 Quality Values

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

3.10 Language Tags

 A language tag identifies a natural language spoken, written, or
 otherwise conveyed by human beings for communication of information
 to other human beings. Computer languages are explicitly excluded.
 HTTP uses language tags within the Accept-Language and Content-
 Language fields.
 The syntax and registry of HTTP language tags is the same as that
 defined by RFC 1766 [1]. In summary, a language tag is composed of 1
 or more parts: A primary language tag and a possibly empty series of
 subtags:
      language-tag  = primary-tag *( "-" subtag )
      primary-tag   = 1*8ALPHA
      subtag        = 1*8ALPHA
 White space is not allowed within the tag and all tags are case-
 insensitive. The name space of language tags is administered by the
 IANA. Example tags include:
     en, en-US, en-cockney, i-cherokee, x-pig-latin

Fielding, et al. Standards Track [Page 29] RFC 2616 HTTP/1.1 June 1999

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

3.11 Entity Tags

 Entity tags are used for comparing two or more entities from the same
 requested resource. HTTP/1.1 uses entity tags in the ETag (section
 14.19), If-Match (section 14.24), If-None-Match (section 14.26), and
 If-Range (section 14.27) header fields. The definition of how they
 are used and compared as cache validators is in section 13.3.3. An
 entity tag consists of an opaque quoted string, possibly prefixed by
 a weakness indicator.
    entity-tag = [ weak ] opaque-tag
    weak       = "W/"
    opaque-tag = quoted-string
 A "strong entity tag" MAY be shared by two entities of a resource
 only if they are equivalent by octet equality.
 A "weak entity tag," indicated by the "W/" prefix, MAY be shared by
 two entities of a resource only if the entities are equivalent and
 could be substituted for each other with no significant change in
 semantics. A weak entity tag can only be used for weak comparison.
 An entity tag MUST be unique across all versions of all entities
 associated with a particular resource. A given entity tag value MAY
 be used for entities obtained by requests on different URIs. The use
 of the same entity tag value in conjunction with entities obtained by
 requests on different URIs does not imply the equivalence of those
 entities.

3.12 Range Units

 HTTP/1.1 allows a client to request that only part (a range of) the
 response entity be included within the response. HTTP/1.1 uses range
 units in the Range (section 14.35) and Content-Range (section 14.16)
 header fields. An entity can be broken down into subranges according
 to various structural units.
    range-unit       = bytes-unit | other-range-unit
    bytes-unit       = "bytes"
    other-range-unit = token
 The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1
 implementations MAY ignore ranges specified using other units.

Fielding, et al. Standards Track [Page 30] RFC 2616 HTTP/1.1 June 1999

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

4 HTTP Message

4.1 Message Types

 HTTP messages consist of requests from client to server and responses
 from server to client.
     HTTP-message   = Request | Response     ; HTTP/1.1 messages
 Request (section 5) and Response (section 6) messages use the generic
 message format of RFC 822 [9] for transferring entities (the payload
 of the message). Both types of message consist of a start-line, zero
 or more header fields (also known as "headers"), an empty line (i.e.,
 a line with nothing preceding the CRLF) indicating the end of the
 header fields, and possibly a message-body.
      generic-message = start-line
                        *(message-header CRLF)
                        CRLF
                        [ message-body ]
      start-line      = Request-Line | Status-Line
 In the interest of robustness, servers SHOULD ignore any empty
 line(s) received where a Request-Line is expected. In other words, if
 the server is reading the protocol stream at the beginning of a
 message and receives a CRLF first, it should ignore the CRLF.
 Certain buggy HTTP/1.0 client implementations generate extra CRLF's
 after a POST request. To restate what is explicitly forbidden by the
 BNF, an HTTP/1.1 client MUST NOT preface or follow a request with an
 extra CRLF.

4.2 Message Headers

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

Fielding, et al. Standards Track [Page 31] RFC 2616 HTTP/1.1 June 1999

 beyond the common forms.
     message-header = field-name ":" [ field-value ]
     field-name     = token
     field-value    = *( field-content | LWS )
     field-content  = <the OCTETs making up the field-value
                      and consisting of either *TEXT or combinations
                      of token, separators, and quoted-string>
 The field-content does not include any leading or trailing LWS:
 linear white space occurring before the first non-whitespace
 character of the field-value or after the last non-whitespace
 character of the field-value. Such leading or trailing LWS MAY be
 removed without changing the semantics of the field value. Any LWS
 that occurs between field-content MAY be replaced with a single SP
 before interpreting the field value or forwarding the message
 downstream.
 The order in which header fields with differing field names are
 received is not significant. However, it is "good practice" to send
 general-header fields first, followed by request-header or response-
 header fields, and ending with the entity-header fields.
 Multiple message-header fields with the same field-name MAY be
 present in a message if and only if the entire field-value for that
 header field is defined as a comma-separated list [i.e., #(values)].
 It MUST be possible to combine the multiple header fields into one
 "field-name: field-value" pair, without changing the semantics of the
 message, by appending each subsequent field-value to the first, each
 separated by a comma. The order in which header fields with the same
 field-name are received is therefore significant to the
 interpretation of the combined field value, and thus a proxy MUST NOT
 change the order of these field values when a message is forwarded.

4.3 Message Body

 The message-body (if any) of an HTTP message is used to carry the
 entity-body associated with the request or response. The message-body
 differs from the entity-body only when a transfer-coding has been
 applied, as indicated by the Transfer-Encoding header field (section
 14.41).
     message-body = entity-body
                  | <entity-body encoded as per Transfer-Encoding>
 Transfer-Encoding MUST be used to indicate any transfer-codings
 applied by an application to ensure safe and proper transfer of the
 message. Transfer-Encoding is a property of the message, not of the

Fielding, et al. Standards Track [Page 32] RFC 2616 HTTP/1.1 June 1999

 entity, and thus MAY be added or removed by any application along the
 request/response chain. (However, section 3.6 places restrictions on
 when certain transfer-codings may be used.)
 The rules for when a message-body is allowed in a message differ for
 requests and responses.
 The presence of a message-body in a request is signaled by the
 inclusion of a Content-Length or Transfer-Encoding header field in
 the request's message-headers. A message-body MUST NOT be included in
 a request if the specification of the request method (section 5.1.1)
 does not allow sending an entity-body in requests. A server SHOULD
 read and forward a message-body on any request; if the request method
 does not include defined semantics for an entity-body, then the
 message-body SHOULD be ignored when handling the request.
 For response messages, whether or not a message-body is included with
 a message is dependent on both the request method and the response
 status code (section 6.1.1). All responses to the HEAD request method
 MUST NOT include a message-body, even though the presence of entity-
 header fields might lead one to believe they do. All 1xx
 (informational), 204 (no content), and 304 (not modified) responses
 MUST NOT include a message-body. All other responses do include a
 message-body, although it MAY be of zero length.

4.4 Message Length

 The transfer-length of a message is the length of the message-body as
 it appears in the message; that is, after any transfer-codings have
 been applied. When a message-body is included with a message, the
 transfer-length of that body is determined by one of the following
 (in order of precedence):
 1.Any response message which "MUST NOT" include a message-body (such
   as the 1xx, 204, and 304 responses and any response to a HEAD
   request) is always terminated by the first empty line after the
   header fields, regardless of the entity-header fields present in
   the message.
 2.If a Transfer-Encoding header field (section 14.41) is present and
   has any value other than "identity", then the transfer-length is
   defined by use of the "chunked" transfer-coding (section 3.6),
   unless the message is terminated by closing the connection.
 3.If a Content-Length header field (section 14.13) is present, its
   decimal value in OCTETs represents both the entity-length and the
   transfer-length. The Content-Length header field MUST NOT be sent
   if these two lengths are different (i.e., if a Transfer-Encoding

Fielding, et al. Standards Track [Page 33] RFC 2616 HTTP/1.1 June 1999

   header field is present). If a message is received with both a
   Transfer-Encoding header field and a Content-Length header field,
   the latter MUST be ignored.
 4.If the message uses the media type "multipart/byteranges", and the
   ransfer-length is not otherwise specified, then this self-
   elimiting media type defines the transfer-length. This media type
   UST NOT be used unless the sender knows that the recipient can arse
   it; the presence in a request of a Range header with ultiple byte-
   range specifiers from a 1.1 client implies that the lient can parse
   multipart/byteranges responses.
     A range header might be forwarded by a 1.0 proxy that does not
     understand multipart/byteranges; in this case the server MUST
     delimit the message using methods defined in items 1,3 or 5 of
     this section.
 5.By the server closing the connection. (Closing the connection
   cannot be used to indicate the end of a request body, since that
   would leave no possibility for the server to send back a response.)
 For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
 containing a message-body MUST include a valid Content-Length header
 field unless the server is known to be HTTP/1.1 compliant. If a
 request contains a message-body and a Content-Length is not given,
 the server SHOULD respond with 400 (bad request) if it cannot
 determine the length of the message, or with 411 (length required) if
 it wishes to insist on receiving a valid Content-Length.
 All HTTP/1.1 applications that receive entities MUST accept the
 "chunked" transfer-coding (section 3.6), thus allowing this mechanism
 to be used for messages when the message length cannot be determined
 in advance.
 Messages MUST NOT include both a Content-Length header field and a
 non-identity transfer-coding. If the message does include a non-
 identity transfer-coding, the Content-Length MUST be ignored.
 When a Content-Length is given in a message where a message-body is
 allowed, its field value MUST exactly match the number of OCTETs in
 the message-body. HTTP/1.1 user agents MUST notify the user when an
 invalid length is received and detected.

4.5 General Header Fields

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

Fielding, et al. Standards Track [Page 34] RFC 2616 HTTP/1.1 June 1999

 message being transmitted.
     general-header = Cache-Control            ; Section 14.9
                    | Connection               ; Section 14.10
                    | Date                     ; Section 14.18
                    | Pragma                   ; Section 14.32
                    | Trailer                  ; Section 14.40
                    | Transfer-Encoding        ; Section 14.41
                    | Upgrade                  ; Section 14.42
                    | Via                      ; Section 14.45
                    | Warning                  ; Section 14.46
 General-header field names can be extended reliably only in
 combination with a change in the protocol version. However, new or
 experimental header fields may be given the semantics of general
 header fields if all parties in the communication recognize them to
 be general-header fields. Unrecognized header fields are treated as
 entity-header fields.

5 Request

 A request message from a client to a server includes, within the
 first line of that message, the method to be applied to the resource,
 the identifier of the resource, and the protocol version in use.
      Request       = Request-Line              ; Section 5.1
                      *(( general-header        ; Section 4.5
                       | request-header         ; Section 5.3
                       | entity-header ) CRLF)  ; Section 7.1
                      CRLF
                      [ message-body ]          ; Section 4.3

5.1 Request-Line

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

Fielding, et al. Standards Track [Page 35] RFC 2616 HTTP/1.1 June 1999

5.1.1 Method

 The Method  token indicates the method to be performed on the
 resource identified by the Request-URI. The method is case-sensitive.
     Method         = "OPTIONS"                ; Section 9.2
                    | "GET"                    ; Section 9.3
                    | "HEAD"                   ; Section 9.4
                    | "POST"                   ; Section 9.5
                    | "PUT"                    ; Section 9.6
                    | "DELETE"                 ; Section 9.7
                    | "TRACE"                  ; Section 9.8
                    | "CONNECT"                ; Section 9.9
                    | extension-method
     extension-method = token
 The list of methods allowed by a resource can be specified in an
 Allow header field (section 14.7). The return code of the response
 always notifies the client whether a method is currently allowed on a
 resource, since the set of allowed methods can change dynamically. An
 origin server SHOULD return the status code 405 (Method Not Allowed)
 if the method is known by the origin server but not allowed for the
 requested resource, and 501 (Not Implemented) if the method is
 unrecognized or not implemented by the origin server. The methods GET
 and HEAD MUST be supported by all general-purpose servers. All other
 methods are OPTIONAL; however, if the above methods are implemented,
 they MUST be implemented with the same semantics as those specified
 in section 9.

5.1.2 Request-URI

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

Fielding, et al. Standards Track [Page 36] RFC 2616 HTTP/1.1 June 1999

 specified by the absoluteURI. In order to avoid request loops, a
 proxy MUST be able to recognize all of its server names, including
 any aliases, local variations, and the numeric IP address. An example
 Request-Line would be:
     GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1
 To allow for transition to absoluteURIs in all requests in future
 versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI
 form in requests, even though HTTP/1.1 clients will only generate
 them in requests to proxies.
 The authority form is only used by the CONNECT method (section 9.9).
 The most common form of Request-URI is that used to identify a
 resource on an origin server or gateway. In this case the absolute
 path of the URI MUST be transmitted (see section 3.2.1, abs_path) as
 the Request-URI, and the network location of the URI (authority) MUST
 be transmitted in a Host header field. For example, a client wishing
 to retrieve the resource above directly from the origin server would
 create a TCP connection to port 80 of the host "www.w3.org" and send
 the lines:
     GET /pub/WWW/TheProject.html HTTP/1.1
     Host: www.w3.org
 followed by the remainder of the Request. Note that the absolute path
 cannot be empty; if none is present in the original URI, it MUST be
 given as "/" (the server root).
 The Request-URI is transmitted in the format specified in section
 3.2.1. If the Request-URI is encoded using the "% HEX HEX" encoding
 [42], the origin server MUST decode the Request-URI in order to
 properly interpret the request. Servers SHOULD respond to invalid
 Request-URIs with an appropriate status code.
 A transparent proxy MUST NOT rewrite the "abs_path" part of the
 received Request-URI when forwarding it to the next inbound server,
 except as noted above to replace a null abs_path with "/".
    Note: The "no rewrite" rule prevents the proxy from changing the
    meaning of the request when the origin server is improperly using
    a non-reserved URI character for a reserved purpose.  Implementors
    should be aware that some pre-HTTP/1.1 proxies have been known to
    rewrite the Request-URI.

Fielding, et al. Standards Track [Page 37] RFC 2616 HTTP/1.1 June 1999

5.2 The Resource Identified by a Request

 The exact resource identified by an Internet request is determined by
 examining both the Request-URI and the Host header field.
 An origin server that does not allow resources to differ by the
 requested host MAY ignore the Host header field value when
 determining the resource identified by an HTTP/1.1 request. (But see
 section 19.6.1.1 for other requirements on Host support in HTTP/1.1.)
 An origin server that does differentiate resources based on the host
 requested (sometimes referred to as virtual hosts or vanity host
 names) MUST use the following rules for determining the requested
 resource on an HTTP/1.1 request:
 1. If Request-URI is an absoluteURI, the host is part of the
   Request-URI. Any Host header field value in the request MUST be
   ignored.
 2. If the Request-URI is not an absoluteURI, and the request includes
   a Host header field, the host is determined by the Host header
   field value.
 3. If the host as determined by rule 1 or 2 is not a valid host on
   the server, the response MUST be a 400 (Bad Request) error message.
 Recipients of an HTTP/1.0 request that lacks a Host header field MAY
 attempt to use heuristics (e.g., examination of the URI path for
 something unique to a particular host) in order to determine what
 exact resource is being requested.

5.3 Request Header Fields

 The request-header fields allow the client to pass additional
 information about the request, and about the client itself, to the
 server. These fields act as request modifiers, with semantics
 equivalent to the parameters on a programming language method
 invocation.
     request-header = Accept                   ; Section 14.1
                    | Accept-Charset           ; Section 14.2
                    | Accept-Encoding          ; Section 14.3
                    | Accept-Language          ; Section 14.4
                    | Authorization            ; Section 14.8
                    | Expect                   ; Section 14.20
                    | From                     ; Section 14.22
                    | Host                     ; Section 14.23
                    | If-Match                 ; Section 14.24

Fielding, et al. Standards Track [Page 38] RFC 2616 HTTP/1.1 June 1999

                    | If-Modified-Since        ; Section 14.25
                    | If-None-Match            ; Section 14.26
                    | If-Range                 ; Section 14.27
                    | If-Unmodified-Since      ; Section 14.28
                    | Max-Forwards             ; Section 14.31
                    | Proxy-Authorization      ; Section 14.34
                    | Range                    ; Section 14.35
                    | Referer                  ; Section 14.36
                    | TE                       ; Section 14.39
                    | User-Agent               ; Section 14.43
 Request-header field names can be extended reliably only in
 combination with a change in the protocol version. However, new or
 experimental header fields MAY be given the semantics of request-
 header fields if all parties in the communication recognize them to
 be request-header fields. Unrecognized header fields are treated as
 entity-header fields.

6 Response

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

6.1 Status-Line

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

6.1.1 Status Code and Reason Phrase

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

Fielding, et al. Standards Track [Page 39] RFC 2616 HTTP/1.1 June 1999

 The first digit of the Status-Code defines the class of response. The
 last two digits do not have any categorization role. There are 5
 values for the first digit:
  1. 1xx: Informational - Request received, continuing process
  1. 2xx: Success - The action was successfully received,

understood, and accepted

  1. 3xx: Redirection - Further action must be taken in order to

complete the request

  1. 4xx: Client Error - The request contains bad syntax or cannot

be fulfilled

  1. 5xx: Server Error - The server failed to fulfill an apparently

valid request

 The individual values of the numeric status codes defined for
 HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
 presented below. The reason phrases listed here are only
 recommendations -- they MAY be replaced by local equivalents without
 affecting the protocol.
    Status-Code    =
          "100"  ; Section 10.1.1: Continue
        | "101"  ; Section 10.1.2: Switching Protocols
        | "200"  ; Section 10.2.1: OK
        | "201"  ; Section 10.2.2: Created
        | "202"  ; Section 10.2.3: Accepted
        | "203"  ; Section 10.2.4: Non-Authoritative Information
        | "204"  ; Section 10.2.5: No Content
        | "205"  ; Section 10.2.6: Reset Content
        | "206"  ; Section 10.2.7: Partial Content
        | "300"  ; Section 10.3.1: Multiple Choices
        | "301"  ; Section 10.3.2: Moved Permanently
        | "302"  ; Section 10.3.3: Found
        | "303"  ; Section 10.3.4: See Other
        | "304"  ; Section 10.3.5: Not Modified
        | "305"  ; Section 10.3.6: Use Proxy
        | "307"  ; Section 10.3.8: Temporary Redirect
        | "400"  ; Section 10.4.1: Bad Request
        | "401"  ; Section 10.4.2: Unauthorized
        | "402"  ; Section 10.4.3: Payment Required
        | "403"  ; Section 10.4.4: Forbidden
        | "404"  ; Section 10.4.5: Not Found
        | "405"  ; Section 10.4.6: Method Not Allowed
        | "406"  ; Section 10.4.7: Not Acceptable

Fielding, et al. Standards Track [Page 40] RFC 2616 HTTP/1.1 June 1999

        | "407"  ; Section 10.4.8: Proxy Authentication Required
        | "408"  ; Section 10.4.9: Request Time-out
        | "409"  ; Section 10.4.10: Conflict
        | "410"  ; Section 10.4.11: Gone
        | "411"  ; Section 10.4.12: Length Required
        | "412"  ; Section 10.4.13: Precondition Failed
        | "413"  ; Section 10.4.14: Request Entity Too Large
        | "414"  ; Section 10.4.15: Request-URI Too Large
        | "415"  ; Section 10.4.16: Unsupported Media Type
        | "416"  ; Section 10.4.17: Requested range not satisfiable
        | "417"  ; Section 10.4.18: Expectation Failed
        | "500"  ; Section 10.5.1: Internal Server Error
        | "501"  ; Section 10.5.2: Not Implemented
        | "502"  ; Section 10.5.3: Bad Gateway
        | "503"  ; Section 10.5.4: Service Unavailable
        | "504"  ; Section 10.5.5: Gateway Time-out
        | "505"  ; Section 10.5.6: HTTP Version not supported
        | extension-code
    extension-code = 3DIGIT
    Reason-Phrase  = *<TEXT, excluding CR, LF>
 HTTP status codes are extensible. HTTP applications are not required
 to understand the meaning of all registered status codes, though such
 understanding is obviously desirable. However, applications MUST
 understand the class of any status code, as indicated by the first
 digit, and treat any unrecognized response as being equivalent to the
 x00 status code of that class, with the exception that an
 unrecognized response MUST NOT be cached. For example, if an
 unrecognized status code of 431 is received by the client, it can
 safely assume that there was something wrong with its request and
 treat the response as if it had received a 400 status code. In such
 cases, user agents SHOULD present to the user the entity returned
 with the response, since that entity is likely to include human-
 readable information which will explain the unusual status.

6.2 Response Header Fields

 The response-header fields allow the server to pass additional
 information about the response which cannot be placed in the Status-
 Line. These header fields give information about the server and about
 further access to the resource identified by the Request-URI.
     response-header = Accept-Ranges           ; Section 14.5
                     | Age                     ; Section 14.6
                     | ETag                    ; Section 14.19
                     | Location                ; Section 14.30
                     | Proxy-Authenticate      ; Section 14.33

Fielding, et al. Standards Track [Page 41] RFC 2616 HTTP/1.1 June 1999

                     | Retry-After             ; Section 14.37
                     | Server                  ; Section 14.38
                     | Vary                    ; Section 14.44
                     | WWW-Authenticate        ; Section 14.47
 Response-header field names can be extended reliably only in
 combination with a change in the protocol version. However, new or
 experimental header fields MAY be given the semantics of response-
 header fields if all parties in the communication recognize them to
 be response-header fields. Unrecognized header fields are treated as
 entity-header fields.

7 Entity

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

7.1 Entity Header Fields

 Entity-header fields define metainformation about the entity-body or,
 if no body is present, about the resource identified by the request.
 Some of this metainformation is OPTIONAL; some might be REQUIRED by
 portions of this specification.
     entity-header  = Allow                    ; Section 14.7
                    | Content-Encoding         ; Section 14.11
                    | Content-Language         ; Section 14.12
                    | Content-Length           ; Section 14.13
                    | Content-Location         ; Section 14.14
                    | Content-MD5              ; Section 14.15
                    | Content-Range            ; Section 14.16
                    | Content-Type             ; Section 14.17
                    | Expires                  ; Section 14.21
                    | Last-Modified            ; Section 14.29
                    | extension-header
     extension-header = message-header
 The extension-header mechanism allows additional entity-header fields
 to be defined without changing the protocol, but these fields cannot
 be assumed to be recognizable by the recipient. Unrecognized header
 fields SHOULD be ignored by the recipient and MUST be forwarded by
 transparent proxies.

Fielding, et al. Standards Track [Page 42] RFC 2616 HTTP/1.1 June 1999

7.2 Entity Body

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

7.2.1 Type

 When an entity-body is included with a message, the data type of that
 body is determined via the header fields Content-Type and Content-
 Encoding. These define a two-layer, ordered encoding model:
     entity-body := Content-Encoding( Content-Type( data ) )
 Content-Type specifies the media type of the underlying data.
 Content-Encoding may be used to indicate any additional content
 codings applied to the data, usually for the purpose of data
 compression, that are a property of the requested resource. There is
 no default encoding.
 Any HTTP/1.1 message containing an entity-body SHOULD include a
 Content-Type header field defining the media type of that body. If
 and only if the media type is not given by a Content-Type field, the
 recipient MAY attempt to guess the media type via inspection of its
 content and/or the name extension(s) of the URI used to identify the
 resource. If the media type remains unknown, the recipient SHOULD
 treat it as type "application/octet-stream".

7.2.2 Entity Length

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

Fielding, et al. Standards Track [Page 43] RFC 2616 HTTP/1.1 June 1999

8 Connections

8.1 Persistent Connections

8.1.1 Purpose

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

in routers and hosts (clients, servers, proxies, gateways,

      tunnels, or caches), and memory used for TCP protocol control
      blocks can be saved in hosts.
  1. 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.
  1. 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.
  1. Latency on subsequent requests is reduced since there is no time

spent in TCP's connection opening handshake.

  1. HTTP can evolve more gracefully, since errors can be reported

without the penalty of closing the TCP connection. Clients using

      future versions of HTTP might optimistically try a new feature,
      but if communicating with an older server, retry with old
      semantics after an error is reported.
 HTTP implementations SHOULD implement persistent connections.

Fielding, et al. Standards Track [Page 44] RFC 2616 HTTP/1.1 June 1999

8.1.2 Overall Operation

 A significant difference between HTTP/1.1 and earlier versions of
 HTTP is that persistent connections are the default behavior of any
 HTTP connection. That is, unless otherwise indicated, the client
 SHOULD assume that the server will maintain a persistent connection,
 even after error responses from the server.
 Persistent connections provide a mechanism by which a client and a
 server can signal the close of a TCP connection. This signaling takes
 place using the Connection header field (section 14.10). Once a close
 has been signaled, the client MUST NOT send any more requests on that
 connection.

8.1.2.1 Negotiation

 An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to
 maintain a persistent connection unless a Connection header including
 the connection-token "close" was sent in the request. If the server
 chooses to close the connection immediately after sending the
 response, it SHOULD send a Connection header including the
 connection-token close.
 An HTTP/1.1 client MAY expect a connection to remain open, but would
 decide to keep it open based on whether the response from a server
 contains a Connection header with the connection-token close. In case
 the client does not want to maintain a connection for more than that
 request, it SHOULD send a Connection header including the
 connection-token close.
 If either the client or the server sends the close token in the
 Connection header, that request becomes the last one for the
 connection.
 Clients and servers SHOULD NOT assume that a persistent connection is
 maintained for HTTP versions less than 1.1 unless it is explicitly
 signaled. See section 19.6.2 for more information on backward
 compatibility with HTTP/1.0 clients.
 In order to remain persistent, all messages on the connection MUST
 have a self-defined message length (i.e., one not defined by closure
 of the connection), as described in section 4.4.

Fielding, et al. Standards Track [Page 45] RFC 2616 HTTP/1.1 June 1999

8.1.2.2 Pipelining

 A client that supports persistent connections MAY "pipeline" its
 requests (i.e., send multiple requests without waiting for each
 response). A server MUST send its responses to those requests in the
 same order that the requests were received.
 Clients which assume persistent connections and pipeline immediately
 after connection establishment SHOULD be prepared to retry their
 connection if the first pipelined attempt fails. If a client does
 such a retry, it MUST NOT pipeline before it knows the connection is
 persistent. Clients MUST also be prepared to resend their requests if
 the server closes the connection before sending all of the
 corresponding responses.
 Clients SHOULD NOT pipeline requests using non-idempotent methods or
 non-idempotent sequences of methods (see section 9.1.2). Otherwise, a
 premature termination of the transport connection could lead to
 indeterminate results. A client wishing to send a non-idempotent
 request SHOULD wait to send that request until it has received the
 response status for the previous request.

8.1.3 Proxy Servers

 It is especially important that proxies correctly implement the
 properties of the Connection header field as specified in section
 14.10.
 The proxy server MUST signal persistent connections separately with
 its clients and the origin servers (or other proxy servers) that it
 connects to. Each persistent connection applies to only one transport
 link.
 A proxy server MUST NOT establish a HTTP/1.1 persistent connection
 with an HTTP/1.0 client (but see RFC 2068 [33] for information and
 discussion of the problems with the Keep-Alive header implemented by
 many HTTP/1.0 clients).

8.1.4 Practical Considerations

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

Fielding, et al. Standards Track [Page 46] RFC 2616 HTTP/1.1 June 1999

 When a client or server wishes to time-out it SHOULD issue a graceful
 close on the transport connection. Clients and servers SHOULD both
 constantly watch for the other side of the transport close, and
 respond to it as appropriate. If a client or server does not detect
 the other side's close promptly it could cause unnecessary resource
 drain on the network.
 A client, server, or proxy MAY close the transport connection at any
 time. For example, a client might have started to send a new request
 at the same time that the server has decided to close the "idle"
 connection. From the server's point of view, the connection is being
 closed while it was idle, but from the client's point of view, a
 request is in progress.
 This means that clients, servers, and proxies MUST be able to recover
 from asynchronous close events. Client software SHOULD reopen the
 transport connection and retransmit the aborted sequence of requests
 without user interaction so long as the request sequence is
 idempotent (see section 9.1.2). Non-idempotent methods or sequences
 MUST NOT be automatically retried, although user agents MAY offer a
 human operator the choice of retrying the request(s). Confirmation by
 user-agent software with semantic understanding of the application
 MAY substitute for user confirmation. The automatic retry SHOULD NOT
 be repeated if the second sequence of requests fails.
 Servers SHOULD always respond to at least one request per connection,
 if at all possible. Servers SHOULD NOT close a connection in the
 middle of transmitting a response, unless a network or client failure
 is suspected.
 Clients that use persistent connections SHOULD limit the number of
 simultaneous connections that they maintain to a given server. A
 single-user client SHOULD NOT maintain more than 2 connections with
 any server or proxy. A proxy SHOULD use up to 2*N connections to
 another server or proxy, where N is the number of simultaneously
 active users. These guidelines are intended to improve HTTP response
 times and avoid congestion.

8.2 Message Transmission Requirements

8.2.1 Persistent Connections and Flow Control

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

Fielding, et al. Standards Track [Page 47] RFC 2616 HTTP/1.1 June 1999

8.2.2 Monitoring Connections for Error Status Messages

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

8.2.3 Use of the 100 (Continue) Status

 The purpose of the 100 (Continue) status (see section 10.1.1) is to
 allow a client that is sending a request message with a request body
 to determine if the origin server is willing to accept the request
 (based on the request headers) before the client sends the request
 body. In some cases, it might either be inappropriate or highly
 inefficient for the client to send the body if the server will reject
 the message without looking at the body.
 Requirements for HTTP/1.1 clients:
  1. If a client will wait for a 100 (Continue) response before

sending the request body, it MUST send an Expect request-header

      field (section 14.20) with the "100-continue" expectation.
  1. A client MUST NOT send an Expect request-header field (section

14.20) with the "100-continue" expectation if it does not intend

      to send a request body.
 Because of the presence of older implementations, the protocol allows
 ambiguous situations in which a client may send "Expect: 100-
 continue" without receiving either a 417 (Expectation Failed) status
 or a 100 (Continue) status. Therefore, when a client sends this
 header field to an origin server (possibly via a proxy) from which it
 has never seen a 100 (Continue) status, the client SHOULD NOT wait
 for an indefinite period before sending the request body.
 Requirements for HTTP/1.1 origin servers:
  1. Upon receiving a request which includes an Expect request-header

field with the "100-continue" expectation, an origin server MUST

      either respond with 100 (Continue) status and continue to read
      from the input stream, or respond with a final status code. The
      origin server MUST NOT wait for the request body before sending
      the 100 (Continue) response. If it responds with a final status
      code, it MAY close the transport connection or it MAY continue

Fielding, et al. Standards Track [Page 48] RFC 2616 HTTP/1.1 June 1999

      to read and discard the rest of the request.  It MUST NOT
      perform the requested method if it returns a final status code.
  1. An origin server SHOULD NOT send a 100 (Continue) response if

the request message does not include an Expect request-header

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

already received some or all of the request body for the

      corresponding request.
  1. An origin server that sends a 100 (Continue) response MUST

ultimately send a final status code, once the request body is

      received and processed, unless it terminates the transport
      connection prematurely.
  1. If an origin server receives a request that does not include an

Expect request-header field with the "100-continue" expectation,

      the request includes a request body, and the server responds
      with a final status code before reading the entire request body
      from the transport connection, then the server SHOULD NOT close
      the transport connection until it has read the entire request,
      or until the client closes the connection. Otherwise, the client
      might not reliably receive the response message. However, this
      requirement is not be construed as preventing a server from
      defending itself against denial-of-service attacks, or from
      badly broken client implementations.
 Requirements for HTTP/1.1 proxies:
  1. If a proxy receives a request that includes an Expect request-

header field with the "100-continue" expectation, and the proxy

      either knows that the next-hop server complies with HTTP/1.1 or
      higher, or does not know the HTTP version of the next-hop
      server, it MUST forward the request, including the Expect header
      field.

Fielding, et al. Standards Track [Page 49] RFC 2616 HTTP/1.1 June 1999

  1. If the proxy knows that the version of the next-hop server is

HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST

      respond with a 417 (Expectation Failed) status.
  1. Proxies SHOULD maintain a cache recording the HTTP version

numbers received from recently-referenced next-hop servers.

  1. A proxy MUST NOT forward a 100 (Continue) response if the

request message was received from an HTTP/1.0 (or earlier)

      client and did not include an Expect request-header field with
      the "100-continue" expectation. This requirement overrides the
      general rule for forwarding of 1xx responses (see section 10.1).

8.2.4 Client Behavior if Server Prematurely Closes Connection

 If an HTTP/1.1 client sends a request which includes a request body,
 but which does not include an Expect request-header field with the
 "100-continue" expectation, and if the client is not directly
 connected to an HTTP/1.1 origin server, and if the client sees the
 connection close before receiving any status from the server, the
 client SHOULD retry the request.  If the client does retry this
 request, it MAY use the following "binary exponential backoff"
 algorithm to be assured of obtaining a reliable response:
    1. Initiate a new connection to the server
    2. Transmit the request-headers
    3. Initialize a variable R to the estimated round-trip time to the
       server (e.g., based on the time it took to establish the
       connection), or to a constant value of 5 seconds if the round-
       trip time is not available.
    4. Compute T = R * (2**N), where N is the number of previous
       retries of this request.
    5. Wait either for an error response from the server, or for T
       seconds (whichever comes first)
    6. If no error response is received, after T seconds transmit the
       body of the request.
    7. If client sees that the connection is closed prematurely,
       repeat from step 1 until the request is accepted, an error
       response is received, or the user becomes impatient and
       terminates the retry process.

Fielding, et al. Standards Track [Page 50] RFC 2616 HTTP/1.1 June 1999

 If at any point an error status is received, the client
  1. SHOULD NOT continue and
  1. SHOULD close the connection if it has not completed sending the

request message.

9 Method Definitions

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

9.1 Safe and Idempotent Methods

9.1.1 Safe Methods

 Implementors should be aware that the software represents the user in
 their interactions over the Internet, and should be careful to allow
 the user to be aware of any actions they might take which may have an
 unexpected significance to themselves or others.
 In particular, the convention has been established that the GET and
 HEAD methods SHOULD NOT have the significance of taking an action
 other than retrieval. These methods ought to be considered "safe".
 This allows user agents to represent other methods, such as POST, PUT
 and DELETE, in a special way, so that the user is made aware of the
 fact that a possibly unsafe action is being requested.
 Naturally, it is not possible to ensure that the server does not
 generate side-effects as a result of performing a GET request; in
 fact, some dynamic resources consider that a feature. The important
 distinction here is that the user did not request the side-effects,
 so therefore cannot be held accountable for them.

9.1.2 Idempotent Methods

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

Fielding, et al. Standards Track [Page 51] RFC 2616 HTTP/1.1 June 1999

 However, it is possible that a sequence of several requests is non-
 idempotent, even if all of the methods executed in that sequence are
 idempotent. (A sequence is idempotent if a single execution of the
 entire sequence always yields a result that is not changed by a
 reexecution of all, or part, of that sequence.) For example, a
 sequence is non-idempotent if its result depends on a value that is
 later modified in the same sequence.
 A sequence that never has side effects is idempotent, by definition
 (provided that no concurrent operations are being executed on the
 same set of resources).

9.2 OPTIONS

 The OPTIONS method represents a request for information about the
 communication options available on the request/response chain
 identified by the Request-URI. This method allows the client to
 determine the options and/or requirements associated with a resource,
 or the capabilities of a server, without implying a resource action
 or initiating a resource retrieval.
 Responses to this method are not cacheable.
 If the OPTIONS request includes an entity-body (as indicated by the
 presence of Content-Length or Transfer-Encoding), then the media type
 MUST be indicated by a Content-Type field. Although this
 specification does not define any use for such a body, future
 extensions to HTTP might use the OPTIONS body to make more detailed
 queries on the server. A server that does not support such an
 extension MAY discard the request body.
 If the Request-URI is an asterisk ("*"), the OPTIONS request is
 intended to apply to the server in general rather than to a specific
 resource. Since a server's communication options typically depend on
 the resource, the "*" request is only useful as a "ping" or "no-op"
 type of method; it does nothing beyond allowing the client to test
 the capabilities of the server. For example, this can be used to test
 a proxy for HTTP/1.1 compliance (or lack thereof).
 If the Request-URI is not an asterisk, the OPTIONS request applies
 only to the options that are available when communicating with that
 resource.
 A 200 response SHOULD include any header fields that indicate
 optional features implemented by the server and applicable to that
 resource (e.g., Allow), possibly including extensions not defined by
 this specification. The response body, if any, SHOULD also include
 information about the communication options. The format for such a

Fielding, et al. Standards Track [Page 52] RFC 2616 HTTP/1.1 June 1999

 body is not defined by this specification, but might be defined by
 future extensions to HTTP. Content negotiation MAY be used to select
 the appropriate response format. If no response body is included, the
 response MUST include a Content-Length field with a field-value of
 "0".
 The Max-Forwards request-header field MAY be used to target a
 specific proxy in the request chain. When a proxy receives an OPTIONS
 request on an absoluteURI for which request forwarding is permitted,
 the proxy MUST check for a Max-Forwards field. If the Max-Forwards
 field-value is zero ("0"), the proxy MUST NOT forward the message;
 instead, the proxy SHOULD respond with its own communication options.
 If the Max-Forwards field-value is an integer greater than zero, the
 proxy MUST decrement the field-value when it forwards the request. If
 no Max-Forwards field is present in the request, then the forwarded
 request MUST NOT include a Max-Forwards field.

9.3 GET

 The GET method means retrieve whatever information (in the form of an
 entity) is identified by the Request-URI. If the Request-URI refers
 to a data-producing process, it is the produced data which shall be
 returned as the entity in the response and not the source text of the
 process, unless that text happens to be the output of the process.
 The semantics of the GET method change to a "conditional GET" if the
 request message includes an If-Modified-Since, If-Unmodified-Since,
 If-Match, If-None-Match, or If-Range header field. A conditional GET
 method requests that the entity be transferred only under the
 circumstances described by the conditional header field(s). The
 conditional GET method is intended to reduce unnecessary network
 usage by allowing cached entities to be refreshed without requiring
 multiple requests or transferring data already held by the client.
 The semantics of the GET method change to a "partial GET" if the
 request message includes a Range header field. A partial GET requests
 that only part of the entity be transferred, as described in section
 14.35. The partial GET method is intended to reduce unnecessary
 network usage by allowing partially-retrieved entities to be
 completed without transferring data already held by the client.
 The response to a GET request is cacheable if and only if it meets
 the requirements for HTTP caching described in section 13.
 See section 15.1.3 for security considerations when used for forms.

Fielding, et al. Standards Track [Page 53] RFC 2616 HTTP/1.1 June 1999

9.4 HEAD

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

9.5 POST

 The POST method is used to request that the origin server accept the
 entity enclosed in the request as a new subordinate of the resource
 identified by the Request-URI in the Request-Line. POST is designed
 to allow a uniform method to cover the following functions:
  1. Annotation of existing resources;
  1. Posting a message to a bulletin board, newsgroup, mailing list,

or similar group of articles;

  1. Providing a block of data, such as the result of submitting a

form, to a data-handling process;

  1. Extending a database through an append operation.
 The actual function performed by the POST method is determined by the
 server and is usually dependent on the Request-URI. The posted entity
 is subordinate to that URI in the same way that a file is subordinate
 to a directory containing it, a news article is subordinate to a
 newsgroup to which it is posted, or a record is subordinate to a
 database.
 The action performed by the POST method might not result in a
 resource that can be identified by a URI. In this case, either 200
 (OK) or 204 (No Content) is the appropriate response status,
 depending on whether or not the response includes an entity that
 describes the result.

Fielding, et al. Standards Track [Page 54] RFC 2616 HTTP/1.1 June 1999

 If a resource has been created on the origin server, the response
 SHOULD be 201 (Created) and contain an entity which describes the
 status of the request and refers to the new resource, and a Location
 header (see section 14.30).
 Responses to this method are not cacheable, unless the response
 includes appropriate Cache-Control or Expires header fields. However,
 the 303 (See Other) response can be used to direct the user agent to
 retrieve a cacheable resource.
 POST requests MUST obey the message transmission requirements set out
 in section 8.2.
 See section 15.1.3 for security considerations.

9.6 PUT

 The PUT method requests that the enclosed entity be stored under the
 supplied Request-URI. If the Request-URI refers to an already
 existing resource, the enclosed entity SHOULD be considered as a
 modified version of the one residing on the origin server. If the
 Request-URI does not point to an existing resource, and that URI is
 capable of being defined as a new resource by the requesting user
 agent, the origin server can create the resource with that URI. If a
 new resource is created, the origin server MUST inform the user agent
 via the 201 (Created) response. If an existing resource is modified,
 either the 200 (OK) or 204 (No Content) response codes SHOULD be sent
 to indicate successful completion of the request. If the resource
 could not be created or modified with the Request-URI, an appropriate
 error response SHOULD be given that reflects the nature of the
 problem. The recipient of the entity MUST NOT ignore any Content-*
 (e.g. Content-Range) headers that it does not understand or implement
 and MUST return a 501 (Not Implemented) response in such cases.
 If the request passes through a cache and the Request-URI identifies
 one or more currently cached entities, those entries SHOULD be
 treated as stale. Responses to this method are not cacheable.
 The fundamental difference between the POST and PUT requests is
 reflected in the different meaning of the Request-URI. The URI in a
 POST request identifies the resource that will handle the enclosed
 entity. That resource might be a data-accepting process, a gateway to
 some other protocol, or a separate entity that accepts annotations.
 In contrast, the URI in a PUT request identifies the entity enclosed
 with the request -- the user agent knows what URI is intended and the
 server MUST NOT attempt to apply the request to some other resource.
 If the server desires that the request be applied to a different URI,

Fielding, et al. Standards Track [Page 55] RFC 2616 HTTP/1.1 June 1999

 it MUST send a 301 (Moved Permanently) response; the user agent MAY
 then make its own decision regarding whether or not to redirect the
 request.
 A single resource MAY be identified by many different URIs. For
 example, an article might have a URI for identifying "the current
 version" which is separate from the URI identifying each particular
 version. In this case, a PUT request on a general URI might result in
 several other URIs being defined by the origin server.
 HTTP/1.1 does not define how a PUT method affects the state of an
 origin server.
 PUT requests MUST obey the message transmission requirements set out
 in section 8.2.
 Unless otherwise specified for a particular entity-header, the
 entity-headers in the PUT request SHOULD be applied to the resource
 created or modified by the PUT.

9.7 DELETE

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

9.8 TRACE

 The TRACE method is used to invoke a remote, application-layer loop-
 back of the request message. The final recipient of the request
 SHOULD reflect the message received back to the client as the
 entity-body of a 200 (OK) response. The final recipient is either the

Fielding, et al. Standards Track [Page 56] RFC 2616 HTTP/1.1 June 1999

 origin server or the first proxy or gateway to receive a Max-Forwards
 value of zero (0) in the request (see section 14.31). A TRACE request
 MUST NOT include an entity.
 TRACE allows the client to see what is being received at the other
 end of the request chain and use that data for testing or diagnostic
 information. The value of the Via header field (section 14.45) is of
 particular interest, since it acts as a trace of the request chain.
 Use of the Max-Forwards header field allows the client to limit the
 length of the request chain, which is useful for testing a chain of
 proxies forwarding messages in an infinite loop.
 If the request is valid, the response SHOULD contain the entire
 request message in the entity-body, with a Content-Type of
 "message/http". Responses to this method MUST NOT be cached.

9.9 CONNECT

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

10 Status Code Definitions

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

10.1 Informational 1xx

 This class of status code indicates a provisional response,
 consisting only of the Status-Line and optional headers, and is
 terminated by an empty line. There are no required headers for this
 class of status code. Since HTTP/1.0 did not define any 1xx status
 codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client
 except under experimental conditions.
 A client MUST be prepared to accept one or more 1xx status responses
 prior to a regular response, even if the client does not expect a 100
 (Continue) status message. Unexpected 1xx status responses MAY be
 ignored by a user agent.
 Proxies MUST forward 1xx responses, unless the connection between the
 proxy and its client has been closed, or unless the proxy itself
 requested the generation of the 1xx response. (For example, if a

Fielding, et al. Standards Track [Page 57] RFC 2616 HTTP/1.1 June 1999

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

10.1.1 100 Continue

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

10.1.2 101 Switching Protocols

 The server understands and is willing to comply with the client's
 request, via the Upgrade message header field (section 14.42), for a
 change in the application protocol being used on this connection. The
 server will switch protocols to those defined by the response's
 Upgrade header field immediately after the empty line which
 terminates the 101 response.
 The protocol SHOULD be switched only when it is advantageous to do
 so. For example, switching to a newer version of HTTP is advantageous
 over older versions, and switching to a real-time, synchronous
 protocol might be advantageous when delivering resources that use
 such features.

10.2 Successful 2xx

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

10.2.1 200 OK

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

Fielding, et al. Standards Track [Page 58] RFC 2616 HTTP/1.1 June 1999

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

10.2.2 201 Created

 The request has been fulfilled and resulted in a new resource being
 created. The newly created resource can be referenced by the URI(s)
 returned in the entity of the response, with the most specific URI
 for the resource given by a Location header field. The response
 SHOULD include an entity containing a list of resource
 characteristics and location(s) from which the user or user agent can
 choose the one most appropriate. The entity format is specified by
 the media type given in the Content-Type header field. The origin
 server MUST create the resource before returning the 201 status code.
 If the action cannot be carried out immediately, the server SHOULD
 respond with 202 (Accepted) response instead.
 A 201 response MAY contain an ETag response header field indicating
 the current value of the entity tag for the requested variant just
 created, see section 14.19.

10.2.3 202 Accepted

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

10.2.4 203 Non-Authoritative Information

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

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10.2.5 204 No Content

 The server has fulfilled the request but does not need to return an
 entity-body, and might want to return updated metainformation. The
 response MAY include new or updated metainformation in the form of
 entity-headers, which if present SHOULD be associated with the
 requested variant.
 If the client is a user agent, it SHOULD NOT change its document view
 from that which caused the request to be sent. This response is
 primarily intended to allow input for actions to take place without
 causing a change to the user agent's active document view, although
 any new or updated metainformation SHOULD be applied to the document
 currently in the user agent's active view.
 The 204 response MUST NOT include a message-body, and thus is always
 terminated by the first empty line after the header fields.

10.2.6 205 Reset Content

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

10.2.7 206 Partial Content

 The server has fulfilled the partial GET request for the resource.
 The request MUST have included a Range header field (section 14.35)
 indicating the desired range, and MAY have included an If-Range
 header field (section 14.27) to make the request conditional.
 The response MUST include the following header fields:
  1. Either a Content-Range header field (section 14.16) indicating

the range included with this response, or a multipart/byteranges

      Content-Type including Content-Range fields for each part. If a
      Content-Length header field is present in the response, its
      value MUST match the actual number of OCTETs transmitted in the
      message-body.
  1. Date
  1. ETag and/or Content-Location, if the header would have been sent

in a 200 response to the same request

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  1. Expires, Cache-Control, and/or Vary, if the field-value might

differ from that sent in any previous response for the same

      variant
 If the 206 response is the result of an If-Range request that used a
 strong cache validator (see section 13.3.3), the response SHOULD NOT
 include other entity-headers. If the response is the result of an
 If-Range request that used a weak validator, the response MUST NOT
 include other entity-headers; this prevents inconsistencies between
 cached entity-bodies and updated headers. Otherwise, the response
 MUST include all of the entity-headers that would have been returned
 with a 200 (OK) response to the same request.
 A cache MUST NOT combine a 206 response with other previously cached
 content if the ETag or Last-Modified headers do not match exactly,
 see 13.5.4.
 A cache that does not support the Range and Content-Range headers
 MUST NOT cache 206 (Partial) responses.

10.3 Redirection 3xx

 This class of status code indicates that further action needs to be
 taken by the user agent in order to fulfill the request.  The action
 required MAY be carried out by the user agent without interaction
 with the user if and only if the method used in the second request is
 GET or HEAD. A client SHOULD detect infinite redirection loops, since
 such loops generate network traffic for each redirection.
    Note: previous versions of this specification recommended a
    maximum of five redirections. Content developers should be aware
    that there might be clients that implement such a fixed
    limitation.

10.3.1 300 Multiple Choices

 The requested resource corresponds to any one of a set of
 representations, each with its own specific location, and agent-
 driven negotiation information (section 12) is being provided so that
 the user (or user agent) can select a preferred representation and
 redirect its request to that location.
 Unless it was a HEAD request, the response SHOULD include an entity
 containing a list of resource characteristics and location(s) from
 which the user or user agent can choose the one most appropriate. The
 entity format is specified by the media type given in the Content-
 Type header field. Depending upon the format and the capabilities of

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 the user agent, selection of the most appropriate choice MAY be
 performed automatically. However, this specification does not define
 any standard for such automatic selection.
 If the server has a preferred choice of representation, it SHOULD
 include the specific URI for that representation in the Location
 field; user agents MAY use the Location field value for automatic
 redirection. This response is cacheable unless indicated otherwise.

10.3.2 301 Moved Permanently

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

10.3.3 302 Found

 The requested resource resides temporarily under a different URI.
 Since the redirection might be altered on occasion, the client SHOULD
 continue to use the Request-URI for future requests.  This response
 is only cacheable if indicated by a Cache-Control or Expires header
 field.
 The temporary URI SHOULD be given by the Location field in the
 response. Unless the request method was HEAD, the entity of the
 response SHOULD contain a short hypertext note with a hyperlink to
 the new URI(s).

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 If the 302 status code is received in response to a request other
 than GET or HEAD, the user agent MUST NOT automatically redirect the
 request unless it can be confirmed by the user, since this might
 change the conditions under which the request was issued.
    Note: RFC 1945 and RFC 2068 specify that the client is not allowed
    to change the method on the redirected request.  However, most
    existing user agent implementations treat 302 as if it were a 303
    response, performing a GET on the Location field-value regardless
    of the original request method. The status codes 303 and 307 have
    been added for servers that wish to make unambiguously clear which
    kind of reaction is expected of the client.

10.3.4 303 See Other

 The response to the request can be found under a different URI and
 SHOULD be retrieved using a GET method on that resource. This method
 exists primarily to allow the output of a POST-activated script to
 redirect the user agent to a selected resource. The new URI is not a
 substitute reference for the originally requested resource. The 303
 response MUST NOT be cached, but the response to the second
 (redirected) request might be cacheable.
 The different URI SHOULD be given by the Location field in the
 response. Unless the request method was HEAD, the entity of the
 response SHOULD contain a short hypertext note with a hyperlink to
 the new URI(s).
    Note: Many pre-HTTP/1.1 user agents do not understand the 303
    status. When interoperability with such clients is a concern, the
    302 status code may be used instead, since most user agents react
    to a 302 response as described here for 303.

10.3.5 304 Not Modified

 If the client has performed a conditional GET request and access is
 allowed, but the document has not been modified, the server SHOULD
 respond with this status code. The 304 response MUST NOT contain a
 message-body, and thus is always terminated by the first empty line
 after the header fields.
 The response MUST include the following header fields:
  1. Date, unless its omission is required by section 14.18.1

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 If a clockless origin server obeys these rules, and proxies and
 clients add their own Date to any response received without one (as
 already specified by [RFC 2068], section 14.19), caches will operate
 correctly.
  1. ETag and/or Content-Location, if the header would have been sent

in a 200 response to the same request

  1. Expires, Cache-Control, and/or Vary, if the field-value might

differ from that sent in any previous response for the same

      variant
 If the conditional GET used a strong cache validator (see section
 13.3.3), the response SHOULD NOT include other entity-headers.
 Otherwise (i.e., the conditional GET used a weak validator), the
 response MUST NOT include other entity-headers; this prevents
 inconsistencies between cached entity-bodies and updated headers.
 If a 304 response indicates an entity not currently cached, then the
 cache MUST disregard the response and repeat the request without the
 conditional.
 If a cache uses a received 304 response to update a cache entry, the
 cache MUST update the entry to reflect any new field values given in
 the response.

10.3.6 305 Use Proxy

 The requested resource MUST be accessed through the proxy given by
 the Location field. The Location field gives the URI of the proxy.
 The recipient is expected to repeat this single request via the
 proxy. 305 responses MUST only be generated by origin servers.
    Note: RFC 2068 was not clear that 305 was intended to redirect a
    single request, and to be generated by origin servers only.  Not
    observing these limitations has significant security consequences.

10.3.7 306 (Unused)

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

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10.3.8 307 Temporary Redirect

 The requested resource resides temporarily under a different URI.
 Since the redirection MAY be altered on occasion, the client SHOULD
 continue to use the Request-URI for future requests.  This response
 is only cacheable if indicated by a Cache-Control or Expires header
 field.
 The temporary URI SHOULD be given by the Location field in the
 response. Unless the request method was HEAD, the entity of the
 response SHOULD contain a short hypertext note with a hyperlink to
 the new URI(s) , since many pre-HTTP/1.1 user agents do not
 understand the 307 status. Therefore, the note SHOULD contain the
 information necessary for a user to repeat the original request on
 the new URI.
 If the 307 status code is received in response to a request other
 than GET or HEAD, the user agent MUST NOT automatically redirect the
 request unless it can be confirmed by the user, since this might
 change the conditions under which the request was issued.

10.4 Client Error 4xx

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

10.4.1 400 Bad Request

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

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10.4.2 401 Unauthorized

 The request requires user authentication. The response MUST include a
 WWW-Authenticate header field (section 14.47) containing a challenge
 applicable to the requested resource. The client MAY repeat the
 request with a suitable Authorization header field (section 14.8). If
 the request already included Authorization credentials, then the 401
 response indicates that authorization has been refused for those
 credentials. If the 401 response contains the same challenge as the
 prior response, and the user agent has already attempted
 authentication at least once, then the user SHOULD be presented the
 entity that was given in the response, since that entity might
 include relevant diagnostic information. HTTP access authentication
 is explained in "HTTP Authentication: Basic and Digest Access
 Authentication" [43].

10.4.3 402 Payment Required

 This code is reserved for future use.

10.4.4 403 Forbidden

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

10.4.5 404 Not Found

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

10.4.6 405 Method Not Allowed

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

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10.4.7 406 Not Acceptable

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

10.4.8 407 Proxy Authentication Required

 This code is similar to 401 (Unauthorized), but indicates that the
 client must first authenticate itself with the proxy. The proxy MUST
 return a Proxy-Authenticate header field (section 14.33) containing a
 challenge applicable to the proxy for the requested resource. The
 client MAY repeat the request with a suitable Proxy-Authorization
 header field (section 14.34). HTTP access authentication is explained
 in "HTTP Authentication: Basic and Digest Access Authentication"
 [43].

10.4.9 408 Request Timeout

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

10.4.10 409 Conflict

 The request could not be completed due to a conflict with the current
 state of the resource. This code is only allowed in situations where
 it is expected that the user might be able to resolve the conflict
 and resubmit the request. The response body SHOULD include enough

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 information for the user to recognize the source of the conflict.
 Ideally, the response entity would include enough information for the
 user or user agent to fix the problem; however, that might not be
 possible and is not required.
 Conflicts are most likely to occur in response to a PUT request. For
 example, if versioning were being used and the entity being PUT
 included changes to a resource which conflict with those made by an
 earlier (third-party) request, the server might use the 409 response
 to indicate that it can't complete the request. In this case, the
 response entity would likely contain a list of the differences
 between the two versions in a format defined by the response
 Content-Type.

10.4.11 410 Gone

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

10.4.12 411 Length Required

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

10.4.13 412 Precondition Failed

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

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10.4.14 413 Request Entity Too Large

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

10.4.15 414 Request-URI Too Long

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

10.4.16 415 Unsupported Media Type

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

10.4.17 416 Requested Range Not Satisfiable

 A server SHOULD return a response with this status code if a request
 included a Range request-header field (section 14.35), and none of
 the range-specifier values in this field overlap the current extent
 of the selected resource, and the request did not include an If-Range
 request-header field. (For byte-ranges, this means that the first-
 byte-pos of all of the byte-range-spec values were greater than the
 current length of the selected resource.)
 When this status code is returned for a byte-range request, the
 response SHOULD include a Content-Range entity-header field
 specifying the current length of the selected resource (see section
 14.16). This response MUST NOT use the multipart/byteranges content-
 type.

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10.4.18 417 Expectation Failed

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

10.5 Server Error 5xx

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

10.5.1 500 Internal Server Error

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

10.5.2 501 Not Implemented

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

10.5.3 502 Bad Gateway

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

10.5.4 503 Service Unavailable

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

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10.5.5 504 Gateway Timeout

 The server, while acting as a gateway or proxy, did not receive a
 timely response from the upstream server specified by the URI (e.g.
 HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
 to access in attempting to complete the request.
    Note: Note to implementors: some deployed proxies are known to
    return 400 or 500 when DNS lookups time out.

10.5.6 505 HTTP Version Not Supported

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

11 Access Authentication

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

12 Content Negotiation

 Most HTTP responses include an entity which contains information for
 interpretation by a human user. Naturally, it is desirable to supply
 the user with the "best available" entity corresponding to the
 request. Unfortunately for servers and caches, not all users have the
 same preferences for what is "best," and not all user agents are
 equally capable of rendering all entity types. For that reason, HTTP
 has provisions for several mechanisms for "content negotiation" --
 the process of selecting the best representation for a given response
 when there are multiple representations available.
    Note: This is not called "format negotiation" because the
    alternate representations may be of the same media type, but use
    different capabilities of that type, be in different languages,
    etc.

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

12.1 Server-driven Negotiation

 If the selection of the best representation for a response is made by
 an algorithm located at the server, it is called server-driven
 negotiation. Selection is based on the available representations of
 the response (the dimensions over which it can vary; e.g. language,
 content-coding, etc.) and the contents of particular header fields in
 the request message or on other information pertaining to the request
 (such as the network address of the client).
 Server-driven negotiation is advantageous when the algorithm for
 selecting from among the available representations is difficult to
 describe to the user agent, or when the server desires to send its
 "best guess" to the client along with the first response (hoping to
 avoid the round-trip delay of a subsequent request if the "best
 guess" is good enough for the user). In order to improve the server's
 guess, the user agent MAY include request header fields (Accept,
 Accept-Language, Accept-Encoding, etc.) which describe its
 preferences for such a response.
 Server-driven negotiation has disadvantages:
    1. It is impossible for the server to accurately determine what
       might be "best" for any given user, since that would require
       complete knowledge of both the capabilities of the user agent
       and the intended use for the response (e.g., does the user want
       to view it on screen or print it on paper?).
    2. Having the user agent describe its capabilities in every
       request can be both very inefficient (given that only a small
       percentage of responses have multiple representations) and a
       potential violation of the user's privacy.
    3. It complicates the implementation of an origin server and the
       algorithms for generating responses to a request.

Fielding, et al. Standards Track [Page 72] RFC 2616 HTTP/1.1 June 1999

    4. It may limit a public cache's ability to use the same response
       for multiple user's requests.
 HTTP/1.1 includes the following request-header fields for enabling
 server-driven negotiation through description of user agent
 capabilities and user preferences: Accept (section 14.1), Accept-
 Charset (section 14.2), Accept-Encoding (section 14.3), Accept-
 Language (section 14.4), and User-Agent (section 14.43). However, an
 origin server is not limited to these dimensions and MAY vary the
 response based on any aspect of the request, including information
 outside the request-header fields or within extension header fields
 not defined by this specification.
 The Vary  header field can be used to express the parameters the
 server uses to select a representation that is subject to server-
 driven negotiation. See section 13.6 for use of the Vary header field
 by caches and section 14.44 for use of the Vary header field by
 servers.

12.2 Agent-driven Negotiation

 With agent-driven negotiation, selection of the best representation
 for a response is performed by the user agent after receiving an
 initial response from the origin server. Selection is based on a list
 of the available representations of the response included within the
 header fields or entity-body of the initial response, with each
 representation identified by its own URI. Selection from among the
 representations may be performed automatically (if the user agent is
 capable of doing so) or manually by the user selecting from a
 generated (possibly hypertext) menu.
 Agent-driven negotiation is advantageous when the response would vary
 over commonly-used dimensions (such as type, language, or encoding),
 when the origin server is unable to determine a user agent's
 capabilities from examining the request, and generally when public
 caches are used to distribute server load and reduce network usage.
 Agent-driven negotiation suffers from the disadvantage of needing a
 second request to obtain the best alternate representation. This
 second request is only efficient when caching is used. In addition,
 this specification does not define any mechanism for supporting
 automatic selection, though it also does not prevent any such
 mechanism from being developed as an extension and used within
 HTTP/1.1.

Fielding, et al. Standards Track [Page 73] RFC 2616 HTTP/1.1 June 1999

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

12.3 Transparent Negotiation

 Transparent negotiation is a combination of both server-driven and
 agent-driven negotiation. When a cache is supplied with a form of the
 list of available representations of the response (as in agent-driven
 negotiation) and the dimensions of variance are completely understood
 by the cache, then the cache becomes capable of performing server-
 driven negotiation on behalf of the origin server for subsequent
 requests on that resource.
 Transparent negotiation has the advantage of distributing the
 negotiation work that would otherwise be required of the origin
 server and also removing the second request delay of agent-driven
 negotiation when the cache is able to correctly guess the right
 response.
 This specification does not define any mechanism for transparent
 negotiation, though it also does not prevent any such mechanism from
 being developed as an extension that could be used within HTTP/1.1.

13 Caching in HTTP

 HTTP is typically used for distributed information systems, where
 performance can be improved by the use of response caches. The
 HTTP/1.1 protocol includes a number of elements intended to make
 caching work as well as possible. Because these elements are
 inextricable from other aspects of the protocol, and because they
 interact with each other, it is useful to describe the basic caching
 design of HTTP separately from the detailed descriptions of methods,
 headers, response codes, etc.
 Caching would be useless if it did not significantly improve
 performance. The goal of caching in HTTP/1.1 is to eliminate the need
 to send requests in many cases, and to eliminate the need to send
 full responses in many other cases. The former reduces the number of
 network round-trips required for many operations; we use an
 "expiration" mechanism for this purpose (see section 13.2). The
 latter reduces network bandwidth requirements; we use a "validation"
 mechanism for this purpose (see section 13.3).
 Requirements for performance, availability, and disconnected
 operation require us to be able to relax the goal of semantic
 transparency. The HTTP/1.1 protocol allows origin servers, caches,

Fielding, et al. Standards Track [Page 74] RFC 2616 HTTP/1.1 June 1999

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

client or origin server

  1. only with an explicit warning to the end user when relaxed by

cache or client

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

13.1.1 Cache Correctness

 A correct cache MUST respond to a request with the most up-to-date
 response held by the cache that is appropriate to the request (see
 sections 13.2.5, 13.2.6, and 13.12) which meets one of the following
 conditions:
    1. It has been checked for equivalence with what the origin server
       would have returned by revalidating the response with the
       origin server (section 13.3);

Fielding, et al. Standards Track [Page 75] RFC 2616 HTTP/1.1 June 1999

    2. It is "fresh enough" (see section 13.2). In the default case,
       this means it meets the least restrictive freshness requirement
       of the client, origin server, and cache (see section 14.9); if
       the origin server so specifies, it is the freshness requirement
       of the origin server alone.
       If a stored response is not "fresh enough" by the most
       restrictive freshness requirement of both the client and the
       origin server, in carefully considered circumstances the cache
       MAY still return the response with the appropriate Warning
       header (see section 13.1.5 and 14.46), unless such a response
       is prohibited (e.g., by a "no-store" cache-directive, or by a
       "no-cache" cache-request-directive; see section 14.9).
    3. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect),
       or error (4xx or 5xx) response message.
 If the cache can not communicate with the origin server, then a
 correct cache SHOULD respond as above if the response can be
 correctly served from the cache; if not it MUST return an error or
 warning indicating that there was a communication failure.
 If a cache receives a response (either an entire response, or a 304
 (Not Modified) response) that it would normally forward to the
 requesting client, and the received response is no longer fresh, the
 cache SHOULD forward it to the requesting client without adding a new
 Warning (but without removing any existing Warning headers). A cache
 SHOULD NOT attempt to revalidate a response simply because that
 response became stale in transit; this might lead to an infinite
 loop. A user agent that receives a stale response without a Warning
 MAY display a warning indication to the user.

13.1.2 Warnings

 Whenever a cache returns a response that is neither first-hand nor
 "fresh enough" (in the sense of condition 2 in section 13.1.1), it
 MUST attach a warning to that effect, using a Warning general-header.
 The Warning header and the currently defined warnings are described
 in section 14.46. The warning allows clients to take appropriate
 action.
 Warnings MAY be used for other purposes, both cache-related and
 otherwise. The use of a warning, rather than an error status code,
 distinguish these responses from true failures.
 Warnings are assigned three digit warn-codes. The first digit
 indicates whether the Warning MUST or MUST NOT be deleted from a
 stored cache entry after a successful revalidation:

Fielding, et al. Standards Track [Page 76] RFC 2616 HTTP/1.1 June 1999

 1xx  Warnings that describe the freshness or revalidation status of
   the response, and so MUST be deleted after a successful
   revalidation. 1XX warn-codes MAY be generated by a cache only when
   validating a cached entry. It MUST NOT be generated by clients.
 2xx  Warnings that describe some aspect of the entity body or entity
   headers that is not rectified by a revalidation (for example, a
   lossy compression of the entity bodies) and which MUST NOT be
   deleted after a successful revalidation.
 See section 14.46 for the definitions of the codes themselves.
 HTTP/1.0 caches will cache all Warnings in responses, without
 deleting the ones in the first category. Warnings in responses that
 are passed to HTTP/1.0 caches carry an extra warning-date field,
 which prevents a future HTTP/1.1 recipient from believing an
 erroneously cached Warning.
 Warnings also carry a warning text. The text MAY be in any
 appropriate natural language (perhaps based on the client's Accept
 headers), and include an OPTIONAL indication of what character set is
 used.
 Multiple warnings MAY be attached to a response (either by the origin
 server or by a cache), including multiple warnings with the same code
 number. For example, a server might provide the same warning with
 texts in both English and Basque.
 When multiple warnings are attached to a response, it might not be
 practical or reasonable to display all of them to the user. This
 version of HTTP does not specify strict priority rules for deciding
 which warnings to display and in what order, but does suggest some
 heuristics.

13.1.3 Cache-control Mechanisms

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

Fielding, et al. Standards Track [Page 77] RFC 2616 HTTP/1.1 June 1999

 in some cases, cache-control directives are explicitly specified as
 weakening the approximation of semantic transparency (for example,
 "max-stale" or "public").
 The cache-control directives are described in detail in section 14.9.

13.1.4 Explicit User Agent Warnings

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

13.1.5 Exceptions to the Rules and Warnings

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

Fielding, et al. Standards Track [Page 78] RFC 2616 HTTP/1.1 June 1999

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

13.1.6 Client-controlled Behavior

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

13.2 Expiration Model

13.2.1 Server-Specified Expiration

 HTTP caching works best when caches can entirely avoid making
 requests to the origin server. The primary mechanism for avoiding
 requests is for an origin server to provide an explicit expiration
 time in the future, indicating that a response MAY be used to satisfy
 subsequent requests. In other words, a cache can return a fresh
 response without first contacting the server.
 Our expectation is that servers will assign future explicit
 expiration times to responses in the belief that the entity is not
 likely to change, in a semantically significant way, before the
 expiration time is reached. This normally preserves semantic
 transparency, as long as the server's expiration times are carefully
 chosen.

Fielding, et al. Standards Track [Page 79] RFC 2616 HTTP/1.1 June 1999

 The expiration mechanism applies only to responses taken from a cache
 and not to first-hand responses forwarded immediately to the
 requesting client.
 If an origin server wishes to force a semantically transparent cache
 to validate every request, it MAY assign an explicit expiration time
 in the past. This means that the response is always stale, and so the
 cache SHOULD validate it before using it for subsequent requests. See
 section 14.9.4 for a more restrictive way to force revalidation.
 If an origin server wishes to force any HTTP/1.1 cache, no matter how
 it is configured, to validate every request, it SHOULD use the "must-
 revalidate" cache-control directive (see section 14.9).
 Servers specify explicit expiration times using either the Expires
 header, or the max-age directive of the Cache-Control header.
 An expiration time cannot be used to force a user agent to refresh
 its display or reload a resource; its semantics apply only to caching
 mechanisms, and such mechanisms need only check a resource's
 expiration status when a new request for that resource is initiated.
 See section 13.13 for an explanation of the difference between caches
 and history mechanisms.

13.2.2 Heuristic Expiration

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

13.2.3 Age Calculations

 In order to know if a cached entry is fresh, a cache needs to know if
 its age exceeds its freshness lifetime. We discuss how to calculate
 the latter in section 13.2.4; this section describes how to calculate
 the age of a response or cache entry.
 In this discussion, we use the term "now" to mean "the current value
 of the clock at the host performing the calculation." Hosts that use
 HTTP, but especially hosts running origin servers and caches, SHOULD
 use NTP [28] or some similar protocol to synchronize their clocks to
 a globally accurate time standard.

Fielding, et al. Standards Track [Page 80] RFC 2616 HTTP/1.1 June 1999

 HTTP/1.1 requires origin servers to send a Date header, if possible,
 with every response, giving the time at which the response was
 generated (see section 14.18). We use the term "date_value" to denote
 the value of the Date header, in a form appropriate for arithmetic
 operations.
 HTTP/1.1 uses the Age response-header to convey the estimated age of
 the response message when obtained from a cache. The Age field value
 is the cache's estimate of the amount of time since the response was
 generated or revalidated by the origin server.
 In essence, the Age value is the sum of the time that the response
 has been resident in each of the caches along the path from the
 origin server, plus the amount of time it has been in transit along
 network paths.
 We use the term "age_value" to denote the value of the Age header, in
 a form appropriate for arithmetic operations.
 A response's age can be calculated in two entirely independent ways:
    1. now minus date_value, if the local clock is reasonably well
       synchronized to the origin server's clock. If the result is
       negative, the result is replaced by zero.
    2. age_value, if all of the caches along the response path
       implement HTTP/1.1.
 Given that we have two independent ways to compute the age of a
 response when it is received, we can combine these as
     corrected_received_age = max(now - date_value, age_value)
 and as long as we have either nearly synchronized clocks or all-
 HTTP/1.1 paths, one gets a reliable (conservative) result.
 Because of network-imposed delays, some significant interval might
 pass between the time that a server generates a response and the time
 it is received at the next outbound cache or client. If uncorrected,
 this delay could result in improperly low ages.
 Because the request that resulted in the returned Age value must have
 been initiated prior to that Age value's generation, we can correct
 for delays imposed by the network by recording the time at which the
 request was initiated. Then, when an Age value is received, it MUST
 be interpreted relative to the time the request was initiated, not

Fielding, et al. Standards Track [Page 81] RFC 2616 HTTP/1.1 June 1999

 the time that the response was received. This algorithm results in
 conservative behavior no matter how much delay is experienced. So, we
 compute:
    corrected_initial_age = corrected_received_age
                          + (now - request_time)
 where "request_time" is the time (according to the local clock) when
 the request that elicited this response was sent.
 Summary of age calculation algorithm, when a cache receives a
 response:
    /*
     * age_value
     *      is the value of Age: header received by the cache with
     *              this response.
     * date_value
     *      is the value of the origin server's Date: header
     * request_time
     *      is the (local) time when the cache made the request
     *              that resulted in this cached response
     * response_time
     *      is the (local) time when the cache received the
     *              response
     * now
     *      is the current (local) time
     */
    apparent_age = max(0, response_time - date_value);
    corrected_received_age = max(apparent_age, age_value);
    response_delay = response_time - request_time;
    corrected_initial_age = corrected_received_age + response_delay;
    resident_time = now - response_time;
    current_age   = corrected_initial_age + resident_time;
 The current_age of a cache entry is calculated by adding the amount
 of time (in seconds) since the cache entry was last validated by the
 origin server to the corrected_initial_age. When a response is
 generated from a cache entry, the cache MUST include a single Age
 header field in the response with a value equal to the cache entry's
 current_age.
 The presence of an Age header field in a response implies that a
 response is not first-hand. However, the converse is not true, since
 the lack of an Age header field in a response does not imply that the

Fielding, et al. Standards Track [Page 82] RFC 2616 HTTP/1.1 June 1999

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

13.2.4 Expiration Calculations

 In order to decide whether a response is fresh or stale, we need to
 compare its freshness lifetime to its age. The age is calculated as
 described in section 13.2.3; this section describes how to calculate
 the freshness lifetime, and to determine if a response has expired.
 In the discussion below, the values can be represented in any form
 appropriate for arithmetic operations.
 We use the term "expires_value" to denote the value of the Expires
 header. We use the term "max_age_value" to denote an appropriate
 value of the number of seconds carried by the "max-age" directive of
 the Cache-Control header in a response (see section 14.9.3).
 The max-age directive takes priority over Expires, so if max-age is
 present in a response, the calculation is simply:
    freshness_lifetime = max_age_value
 Otherwise, if Expires is present in the response, the calculation is:
    freshness_lifetime = expires_value - date_value
 Note that neither of these calculations is vulnerable to clock skew,
 since all of the information comes from the origin server.
 If none of Expires, Cache-Control: max-age, or Cache-Control: s-
 maxage (see section 14.9.3) appears in the response, and the response
 does not include other restrictions on caching, the cache MAY compute
 a freshness lifetime using a heuristic. The cache MUST attach Warning
 113 to any response whose age is more than 24 hours if such warning
 has not already been added.
 Also, if the response does have a Last-Modified time, the heuristic
 expiration value SHOULD be no more than some fraction of the interval
 since that time. A typical setting of this fraction might be 10%.
 The calculation to determine if a response has expired is quite
 simple:
    response_is_fresh = (freshness_lifetime > current_age)

Fielding, et al. Standards Track [Page 83] RFC 2616 HTTP/1.1 June 1999

13.2.5 Disambiguating Expiration Values

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

13.2.6 Disambiguating Multiple Responses

 Because a client might be receiving responses via multiple paths, so
 that some responses flow through one set of caches and other
 responses flow through a different set of caches, a client might
 receive responses in an order different from that in which the origin
 server sent them. We would like the client to use the most recently
 generated response, even if older responses are still apparently
 fresh.
 Neither the entity tag nor the expiration value can impose an
 ordering on responses, since it is possible that a later response
 intentionally carries an earlier expiration time. The Date values are
 ordered to a granularity of one second.
 When a client tries to revalidate a cache entry, and the response it
 receives contains a Date header that appears to be older than the one
 for the existing entry, then the client SHOULD repeat the request
 unconditionally, and include
     Cache-Control: max-age=0
 to force any intermediate caches to validate their copies directly
 with the origin server, or
     Cache-Control: no-cache
 to force any intermediate caches to obtain a new copy from the origin
 server.

Fielding, et al. Standards Track [Page 84] RFC 2616 HTTP/1.1 June 1999

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

13.3 Validation Model

 When a cache has a stale entry that it would like to use as a
 response to a client's request, it first has to check with the origin
 server (or possibly an intermediate cache with a fresh response) to
 see if its cached entry is still usable. We call this "validating"
 the cache entry. Since we do not want to have to pay the overhead of
 retransmitting the full response if the cached entry is good, and we
 do not want to pay the overhead of an extra round trip if the cached
 entry is invalid, the HTTP/1.1 protocol supports the use of
 conditional methods.
 The key protocol features for supporting conditional methods are
 those concerned with "cache validators." When an origin server
 generates a full response, it attaches some sort of validator to it,
 which is kept with the cache entry. When a client (user agent or
 proxy cache) makes a conditional request for a resource for which it
 has a cache entry, it includes the associated validator in the
 request.
 The server then checks that validator against the current validator
 for the entity, and, if they match (see section 13.3.3), it responds
 with a special status code (usually, 304 (Not Modified)) and no
 entity-body. Otherwise, it returns a full response (including
 entity-body). Thus, we avoid transmitting the full response if the
 validator matches, and we avoid an extra round trip if it does not
 match.
 In HTTP/1.1, a conditional request looks exactly the same as a normal
 request for the same resource, except that it carries a special
 header (which includes the validator) that implicitly turns the
 method (usually, GET) into a conditional.
 The protocol includes both positive and negative senses of cache-
 validating conditions. That is, it is possible to request either that
 a method be performed if and only if a validator matches or if and
 only if no validators match.

Fielding, et al. Standards Track [Page 85] RFC 2616 HTTP/1.1 June 1999

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

13.3.1 Last-Modified Dates

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

13.3.2 Entity Tag Cache Validators

 The ETag response-header field value, an entity tag, provides for an
 "opaque" cache validator. This might allow more reliable validation
 in situations where it is inconvenient to store modification dates,
 where the one-second resolution of HTTP date values is not
 sufficient, or where the origin server wishes to avoid certain
 paradoxes that might arise from the use of modification dates.
 Entity Tags are described in section 3.11. The headers used with
 entity tags are described in sections 14.19, 14.24, 14.26 and 14.44.

13.3.3 Weak and Strong Validators

 Since both origin servers and caches will compare two validators to
 decide if they represent the same or different entities, one normally
 would expect that if the entity (the entity-body or any entity-
 headers) changes in any way, then the associated validator would
 change as well. If this is true, then we call this validator a
 "strong validator."
 However, there might be cases when a server prefers to change the
 validator only on semantically significant changes, and not when
 insignificant aspects of the entity change. A validator that does not
 always change when the resource changes is a "weak validator."
 Entity tags are normally "strong validators," but the protocol
 provides a mechanism to tag an entity tag as "weak." One can think of
 a strong validator as one that changes whenever the bits of an entity
 changes, while a weak value changes whenever the meaning of an entity
 changes. Alternatively, one can think of a strong validator as part
 of an identifier for a specific entity, while a weak validator is
 part of an identifier for a set of semantically equivalent entities.
    Note: One example of a strong validator is an integer that is
    incremented in stable storage every time an entity is changed.

Fielding, et al. Standards Track [Page 86] RFC 2616 HTTP/1.1 June 1999

    An entity's modification time, if represented with one-second
    resolution, could be a weak validator, since it is possible that
    the resource might be modified twice during a single second.
    Support for weak validators is optional. However, weak validators
    allow for more efficient caching of equivalent objects; for
    example, a hit counter on a site is probably good enough if it is
    updated every few days or weeks, and any value during that period
    is likely "good enough" to be equivalent.
 A "use" of a validator is either when a client generates a request
 and includes the validator in a validating header field, or when a
 server compares two validators.
 Strong validators are usable in any context. Weak validators are only
 usable in contexts that do not depend on exact equality of an entity.
 For example, either kind is usable for a conditional GET of a full
 entity. However, only a strong validator is usable for a sub-range
 retrieval, since otherwise the client might end up with an internally
 inconsistent entity.
 Clients MAY issue simple (non-subrange) GET requests with either weak
 validators or strong validators. Clients MUST NOT use weak validators
 in other forms of request.
 The only function that the HTTP/1.1 protocol defines on validators is
 comparison. There are two validator comparison functions, depending
 on whether the comparison context allows the use of weak validators
 or not:
  1. The strong comparison function: in order to be considered equal,

both validators MUST be identical in every way, and both MUST

      NOT be weak.
  1. The weak comparison function: in order to be considered equal,

both validators MUST be identical in every way, but either or

      both of them MAY be tagged as "weak" without affecting the
      result.
 An entity tag is strong unless it is explicitly tagged as weak.
 Section 3.11 gives the syntax for entity tags.
 A Last-Modified time, when used as a validator in a request, is
 implicitly weak unless it is possible to deduce that it is strong,
 using the following rules:
  1. The validator is being compared by an origin server to the

actual current validator for the entity and,

Fielding, et al. Standards Track [Page 87] RFC 2616 HTTP/1.1 June 1999

  1. That origin server reliably knows that the associated entity did

not change twice during the second covered by the presented

      validator.
 or
  1. 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
  1. That cache entry includes a Date value, which gives the time

when the origin server sent the original response, and

  1. The presented Last-Modified time is at least 60 seconds before

the Date value.

 or
  1. The validator is being compared by an intermediate cache to the

validator stored in its cache entry for the entity, and

  1. That cache entry includes a Date value, which gives the time

when the origin server sent the original response, and

  1. The presented Last-Modified time is at least 60 seconds before

the Date value.

 This method relies on the fact that if two different responses were
 sent by the origin server during the same second, but both had the
 same Last-Modified time, then at least one of those responses would
 have a Date value equal to its Last-Modified time. The arbitrary 60-
 second limit guards against the possibility that the Date and Last-
 Modified values are generated from different clocks, or at somewhat
 different times during the preparation of the response. An
 implementation MAY use a value larger than 60 seconds, if it is
 believed that 60 seconds is too short.
 If a client wishes to perform a sub-range retrieval on a value for
 which it has only a Last-Modified time and no opaque validator, it
 MAY do this only if the Last-Modified time is strong in the sense
 described here.
 A cache or origin server receiving a conditional request, other than
 a full-body GET request, MUST use the strong comparison function to
 evaluate the condition.
 These rules allow HTTP/1.1 caches and clients to safely perform sub-
 range retrievals on values that have been obtained from HTTP/1.0

Fielding, et al. Standards Track [Page 88] RFC 2616 HTTP/1.1 June 1999

 servers.

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

 We adopt a set of rules and recommendations for origin servers,
 clients, and caches regarding when various validator types ought to
 be used, and for what purposes.
 HTTP/1.1 origin servers:
  1. SHOULD send an entity tag validator unless it is not feasible to

generate one.

  1. 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.
  1. SHOULD send a Last-Modified value if it is feasible to send one,

unless the risk of a breakdown in semantic transparency that

      could result from using this date in an If-Modified-Since header
      would lead to serious problems.
 In other words, the preferred behavior for an HTTP/1.1 origin server
 is to send both a strong entity tag and a Last-Modified value.
 In order to be legal, a strong entity tag MUST change whenever the
 associated entity value changes in any way. A weak entity tag SHOULD
 change whenever the associated entity changes in a semantically
 significant way.
    Note: in order to provide semantically transparent caching, an
    origin server must avoid reusing a specific strong entity tag
    value for two different entities, or reusing a specific weak
    entity tag value for two semantically different entities. Cache
    entries might persist for arbitrarily long periods, regardless of
    expiration times, so it might be inappropriate to expect that a
    cache will never again attempt to validate an entry using a
    validator that it obtained at some point in the past.
 HTTP/1.1 clients:
  1. 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).
  1. If only a Last-Modified value has been provided by the origin

server, SHOULD use that value in non-subrange cache-conditional

      requests (using If-Modified-Since).

Fielding, et al. Standards Track [Page 89] RFC 2616 HTTP/1.1 June 1999

  1. 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.
  1. If both an entity tag and a Last-Modified value have been

provided by the origin server, SHOULD use both validators in

      cache-conditional requests. This allows both HTTP/1.0 and
      HTTP/1.1 caches to respond appropriately.
 An HTTP/1.1 origin server, upon receiving a conditional request that
 includes both a Last-Modified date (e.g., in an If-Modified-Since or
 If-Unmodified-Since header field) and one or more entity tags (e.g.,
 in an If-Match, If-None-Match, or If-Range header field) as cache
 validators, MUST NOT return a response status of 304 (Not Modified)
 unless doing so is consistent with all of the conditional header
 fields in the request.
 An HTTP/1.1 caching proxy, upon receiving a conditional request that
 includes both a Last-Modified date and one or more entity tags as
 cache validators, MUST NOT return a locally cached response to the
 client unless that cached response is consistent with all of the
 conditional header fields in the request.
    Note: The general principle behind these rules is that HTTP/1.1
    servers and clients should transmit as much non-redundant
    information as is available in their responses and requests.
    HTTP/1.1 systems receiving this information will make the most
    conservative assumptions about the validators they receive.
    HTTP/1.0 clients and caches will ignore entity tags. Generally,
    last-modified values received or used by these systems will
    support transparent and efficient caching, and so HTTP/1.1 origin
    servers should provide Last-Modified values. In those rare cases
    where the use of a Last-Modified value as a validator by an
    HTTP/1.0 system could result in a serious problem, then HTTP/1.1
    origin servers should not provide one.

13.3.5 Non-validating Conditionals

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

Fielding, et al. Standards Track [Page 90] RFC 2616 HTTP/1.1 June 1999

13.4 Response Cacheability

 Unless specifically constrained by a cache-control (section 14.9)
 directive, a caching system MAY always store a successful response
 (see section 13.8) as a cache entry, MAY return it without validation
 if it is fresh, and MAY return it after successful validation. If
 there is neither a cache validator nor an explicit expiration time
 associated with a response, we do not expect it to be cached, but
 certain caches MAY violate this expectation (for example, when little
 or no network connectivity is available). A client can usually detect
 that such a response was taken from a cache by comparing the Date
 header to the current time.
    Note: some HTTP/1.0 caches are known to violate this expectation
    without providing any Warning.
 However, in some cases it might be inappropriate for a cache to
 retain an entity, or to return it in response to a subsequent
 request. This might be because absolute semantic transparency is
 deemed necessary by the service author, or because of security or
 privacy considerations. Certain cache-control directives are
 therefore provided so that the server can indicate that certain
 resource entities, or portions thereof, are not to be cached
 regardless of other considerations.
 Note that section 14.8 normally prevents a shared cache from saving
 and returning a response to a previous request if that request
 included an Authorization header.
 A response received with a status code of 200, 203, 206, 300, 301 or
 410 MAY be stored by a cache and used in reply to a subsequent
 request, subject to the expiration mechanism, unless a cache-control
 directive prohibits caching. However, a cache that does not support
 the Range and Content-Range headers MUST NOT cache 206 (Partial
 Content) responses.
 A response received with any other status code (e.g. status codes 302
 and 307) MUST NOT be returned in a reply to a subsequent request
 unless there are cache-control directives or another header(s) that
 explicitly allow it. For example, these include the following: an
 Expires header (section 14.21); a "max-age", "s-maxage",  "must-
 revalidate", "proxy-revalidate", "public" or "private" cache-control
 directive (section 14.9).

Fielding, et al. Standards Track [Page 91] RFC 2616 HTTP/1.1 June 1999

13.5 Constructing Responses From Caches

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

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

 For the purpose of defining the behavior of caches and non-caching
 proxies, we divide HTTP headers into two categories:
  1. End-to-end headers, which are transmitted to the ultimate

recipient of a request or response. End-to-end headers in

      responses MUST be stored as part of a cache entry and MUST be
      transmitted in any response formed from a cache entry.
  1. 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:
  1. Connection
  2. Keep-Alive
  3. Proxy-Authenticate
  4. Proxy-Authorization
  5. TE
  6. Trailers
  7. Transfer-Encoding
  8. Upgrade
 All other headers defined by HTTP/1.1 are end-to-end headers.
 Other hop-by-hop headers MUST be listed in a Connection header,
 (section 14.10) to be introduced into HTTP/1.1 (or later).

13.5.2 Non-modifiable Headers

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

Fielding, et al. Standards Track [Page 92] RFC 2616 HTTP/1.1 June 1999

 A transparent proxy MUST NOT modify any of the following fields in a
 request or response, and it MUST NOT add any of these fields if not
 already present:
  1. Content-Location
  1. Content-MD5
  1. ETag
  1. Last-Modified
 A transparent proxy MUST NOT modify any of the following fields in a
 response:
  1. Expires
 but it MAY add any of these fields if not already present. If an
 Expires header is added, it MUST be given a field-value identical to
 that of the Date header in that response.
 A  proxy MUST NOT modify or add any of the following fields in a
 message that contains the no-transform cache-control directive, or in
 any request:
  1. Content-Encoding
  1. Content-Range
  1. Content-Type
 A non-transparent proxy MAY modify or add these fields to a message
 that does not include no-transform, but if it does so, it MUST add a
 Warning 214 (Transformation applied) if one does not already appear
 in the message (see section 14.46).
    Warning: unnecessary modification of end-to-end headers might
    cause authentication failures if stronger authentication
    mechanisms are introduced in later versions of HTTP. Such
    authentication mechanisms MAY rely on the values of header fields
    not listed here.
 The Content-Length field of a request or response is added or deleted
 according to the rules in section 4.4. A transparent proxy MUST
 preserve the entity-length (section 7.2.2) of the entity-body,
 although it MAY change the transfer-length (section 4.4).

Fielding, et al. Standards Track [Page 93] RFC 2616 HTTP/1.1 June 1999

13.5.3 Combining Headers

 When a cache makes a validating request to a server, and the server
 provides a 304 (Not Modified) response or a 206 (Partial Content)
 response, the cache then constructs a response to send to the
 requesting client.
 If the status code is 304 (Not Modified), the cache uses the entity-
 body stored in the cache entry as the entity-body of this outgoing
 response. If the status code is 206 (Partial Content) and the ETag or
 Last-Modified headers match exactly, the cache MAY combine the
 contents stored in the cache entry with the new contents received in
 the response and use the result as the entity-body of this outgoing
 response, (see 13.5.4).
 The end-to-end headers stored in the cache entry are used for the
 constructed response, except that
  1. any stored Warning headers with warn-code 1xx (see section

14.46) MUST be deleted from the cache entry and the forwarded

      response.
  1. any stored Warning headers with warn-code 2xx MUST be retained

in the cache entry and the forwarded response.

  1. any end-to-end headers provided in the 304 or 206 response MUST

replace the corresponding headers from the cache entry.

 Unless the cache decides to remove the cache entry, it MUST also
 replace the end-to-end headers stored with the cache entry with
 corresponding headers received in the incoming response, except for
 Warning headers as described immediately above. If a header field-
 name in the incoming response matches more than one header in the
 cache entry, all such old headers MUST be replaced.
 In other words, the set of end-to-end headers received in the
 incoming response overrides all corresponding end-to-end headers
 stored with the cache entry (except for stored Warning headers with
 warn-code 1xx, which are deleted even if not overridden).
    Note: this rule allows an origin server to use a 304 (Not
    Modified) or a 206 (Partial Content) response to update any header
    associated with a previous response for the same entity or sub-
    ranges thereof, although it might not always be meaningful or
    correct to do so. This rule does not allow an origin server to use
    a 304 (Not Modified) or a 206 (Partial Content) response to
    entirely delete a header that it had provided with a previous
    response.

Fielding, et al. Standards Track [Page 94] RFC 2616 HTTP/1.1 June 1999

13.5.4 Combining Byte Ranges

 A response might transfer only a subrange of the bytes of an entity-
 body, either because the request included one or more Range
 specifications, or because a connection was broken prematurely. After
 several such transfers, a cache might have received several ranges of
 the same entity-body.
 If a cache has a stored non-empty set of subranges for an entity, and
 an incoming response transfers another subrange, the cache MAY
 combine the new subrange with the existing set if both the following
 conditions are met:
  1. Both the incoming response and the cache entry have a cache

validator.

  1. The two cache validators match using the strong comparison

function (see section 13.3.3).

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

13.6 Caching Negotiated Responses

 Use of server-driven content negotiation (section 12.1), as indicated
 by the presence of a Vary header field in a response, alters the
 conditions and procedure by which a cache can use the response for
 subsequent requests. See section 14.44 for use of the Vary header
 field by servers.
 A server SHOULD use the Vary header field to inform a cache of what
 request-header fields were used to select among multiple
 representations of a cacheable response subject to server-driven
 negotiation. The set of header fields named by the Vary field value
 is known as the "selecting" request-headers.
 When the cache receives a subsequent request whose Request-URI
 specifies one or more cache entries including a Vary header field,
 the cache MUST NOT use such a cache entry to construct a response to
 the new request unless all of the selecting request-headers present
 in the new request match the corresponding stored request-headers in
 the original request.
 The selecting request-headers from two requests are defined to match
 if and only if the selecting request-headers in the first request can
 be transformed to the selecting request-headers in the second request

Fielding, et al. Standards Track [Page 95] RFC 2616 HTTP/1.1 June 1999

 by adding or removing linear white space (LWS) at places where this
 is allowed by the corresponding BNF, and/or combining multiple
 message-header fields with the same field name following the rules
 about message headers in section 4.2.
 A Vary header field-value of "*" always fails to match and subsequent
 requests on that resource can only be properly interpreted by the
 origin server.
 If the selecting request header fields for the cached entry do not
 match the selecting request header fields of the new request, then
 the cache MUST NOT use a cached entry to satisfy the request unless
 it first relays the new request to the origin server in a conditional
 request and the server responds with 304 (Not Modified), including an
 entity tag or Content-Location that indicates the entity to be used.
 If an entity tag was assigned to a cached representation, the
 forwarded request SHOULD be conditional and include the entity tags
 in an If-None-Match header field from all its cache entries for the
 resource. This conveys to the server the set of entities currently
 held by the cache, so that if any one of these entities matches the
 requested entity, the server can use the ETag header field in its 304
 (Not Modified) response to tell the cache which entry is appropriate.
 If the entity-tag of the new response matches that of an existing
 entry, the new response SHOULD be used to update the header fields of
 the existing entry, and the result MUST be returned to the client.
 If any of the existing cache entries contains only partial content
 for the associated entity, its entity-tag SHOULD NOT be included in
 the If-None-Match header field unless the request is for a range that
 would be fully satisfied by that entry.
 If a cache receives a successful response whose Content-Location
 field matches that of an existing cache entry for the same Request-
 ]URI, whose entity-tag differs from that of the existing entry, and
 whose Date is more recent than that of the existing entry, the
 existing entry SHOULD NOT be returned in response to future requests
 and SHOULD be deleted from the cache.

13.7 Shared and Non-Shared Caches

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

Fielding, et al. Standards Track [Page 96] RFC 2616 HTTP/1.1 June 1999

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

13.8 Errors or Incomplete Response Cache Behavior

 A cache that receives an incomplete response (for example, with fewer
 bytes of data than specified in a Content-Length header) MAY store
 the response. However, the cache MUST treat this as a partial
 response. Partial responses MAY be combined as described in section
 13.5.4; the result might be a full response or might still be
 partial. A cache MUST NOT return a partial response to a client
 without explicitly marking it as such, using the 206 (Partial
 Content) status code. A cache MUST NOT return a partial response
 using a status code of 200 (OK).
 If a cache receives a 5xx response while attempting to revalidate an
 entry, it MAY either forward this response to the requesting client,
 or act as if the server failed to respond. In the latter case, it MAY
 return a previously received response unless the cached entry
 includes the "must-revalidate" cache-control directive (see section
 14.9).

13.9 Side Effects of GET and HEAD

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

13.10 Invalidation After Updates or Deletions

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

Fielding, et al. Standards Track [Page 97] RFC 2616 HTTP/1.1 June 1999

 There is no way for the HTTP protocol to guarantee that all such
 cache entries are marked invalid. For example, the request that
 caused the change at the origin server might not have gone through
 the proxy where a cache entry is stored. However, several rules help
 reduce the likelihood of erroneous behavior.
 In this section, the phrase "invalidate an entity" means that the
 cache will either remove all instances of that entity from its
 storage, or will mark these as "invalid" and in need of a mandatory
 revalidation before they can be returned in response to a subsequent
 request.
 Some HTTP methods MUST cause a cache to invalidate an entity. This is
 either the entity referred to by the Request-URI, or by the Location
 or Content-Location headers (if present). These methods are:
  1. PUT
  1. DELETE
  1. POST
 In order to prevent denial of service attacks, an invalidation based
 on the URI in a Location or Content-Location header MUST only be
 performed if the host part is the same as in the Request-URI.
 A cache that passes through requests for methods it does not
 understand SHOULD invalidate any entities referred to by the
 Request-URI.

13.11 Write-Through Mandatory

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

Fielding, et al. Standards Track [Page 98] RFC 2616 HTTP/1.1 June 1999

13.12 Cache Replacement

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

13.13 History Lists

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

Fielding, et al. Standards Track [Page 99] RFC 2616 HTTP/1.1 June 1999

14 Header Field Definitions

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

14.1 Accept

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

Fielding, et al. Standards Track [Page 100] RFC 2616 HTTP/1.1 June 1999

 The example
     Accept: audio/*; q=0.2, audio/basic
 SHOULD be interpreted as "I prefer audio/basic, but send me any audio
 type if it is the best available after an 80% mark-down in quality."
 If no Accept header field is present, then it is assumed that the
 client accepts all media types. If an Accept header field is present,
 and if the server cannot send a response which is acceptable
 according to the combined Accept field value, then the server SHOULD
 send a 406 (not acceptable) response.
 A more elaborate example is
     Accept: text/plain; q=0.5, text/html,
             text/x-dvi; q=0.8, text/x-c
 Verbally, this would be interpreted as "text/html and text/x-c are
 the preferred media types, but if they do not exist, then send the
 text/x-dvi entity, and if that does not exist, send the text/plain
 entity."
 Media ranges can be overridden by more specific media ranges or
 specific media types. If more than one media range applies to a given
 type, the most specific reference has precedence. For example,
     Accept: text/*, text/html, text/html;level=1, */*
 have the following precedence:
     1) text/html;level=1
     2) text/html
     3) text/*
     4) */*
 The media type quality factor associated with a given type is
 determined by finding the media range with the highest precedence
 which matches that type. For example,
     Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
             text/html;level=2;q=0.4, */*;q=0.5
 would cause the following values to be associated:
     text/html;level=1         = 1
     text/html                 = 0.7
     text/plain                = 0.3

Fielding, et al. Standards Track [Page 101] RFC 2616 HTTP/1.1 June 1999

     image/jpeg                = 0.5
     text/html;level=2         = 0.4
     text/html;level=3         = 0.7
    Note: A user agent might be provided with a default set of quality
    values for certain media ranges. However, unless the user agent is
    a closed system which cannot interact with other rendering agents,
    this default set ought to be configurable by the user.

14.2 Accept-Charset

 The Accept-Charset request-header field can be used to indicate what
 character sets are acceptable for the response. This field allows
 clients capable of understanding more comprehensive or special-
 purpose character sets to signal that capability to a server which is
 capable of representing documents in those character sets.
    Accept-Charset = "Accept-Charset" ":"
            1#( ( charset | "*" )[ ";" "q" "=" qvalue ] )
 Character set values are described in section 3.4. Each charset MAY
 be given an associated quality value which represents the user's
 preference for that charset. The default value is q=1. An example is
    Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
 The special value "*", if present in the Accept-Charset field,
 matches every character set (including ISO-8859-1) which is not
 mentioned elsewhere in the Accept-Charset field. If no "*" is present
 in an Accept-Charset field, then all character sets not explicitly
 mentioned get a quality value of 0, except for ISO-8859-1, which gets
 a quality value of 1 if not explicitly mentioned.
 If no Accept-Charset header is present, the default is that any
 character set is acceptable. If an Accept-Charset header is present,
 and if the server cannot send a response which is acceptable
 according to the Accept-Charset header, then the server SHOULD send
 an error response with the 406 (not acceptable) status code, though
 the sending of an unacceptable response is also allowed.

14.3 Accept-Encoding

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

Fielding, et al. Standards Track [Page 102] RFC 2616 HTTP/1.1 June 1999

                        1#( codings [ ";" "q" "=" qvalue ] )
     codings          = ( content-coding | "*" )
 Examples of its use are:
     Accept-Encoding: compress, gzip
     Accept-Encoding:
     Accept-Encoding: *
     Accept-Encoding: compress;q=0.5, gzip;q=1.0
     Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
 A server tests whether a content-coding is acceptable, according to
 an Accept-Encoding field, using these rules:
    1. If the content-coding is one of the content-codings listed in
       the Accept-Encoding field, then it is acceptable, unless it is
       accompanied by a qvalue of 0. (As defined in section 3.9, a
       qvalue of 0 means "not acceptable.")
    2. The special "*" symbol in an Accept-Encoding field matches any
       available content-coding not explicitly listed in the header
       field.
    3. If multiple content-codings are acceptable, then the acceptable
       content-coding with the highest non-zero qvalue is preferred.
    4. The "identity" content-coding is always acceptable, unless
       specifically refused because the Accept-Encoding field includes
       "identity;q=0", or because the field includes "*;q=0" and does
       not explicitly include the "identity" content-coding. If the
       Accept-Encoding field-value is empty, then only the "identity"
       encoding is acceptable.
 If an Accept-Encoding field is present in a request, and if the
 server cannot send a response which is acceptable according to the
 Accept-Encoding header, then the server SHOULD send an error response
 with the 406 (Not Acceptable) status code.
 If no Accept-Encoding field is present in a request, the server MAY
 assume that the client will accept any content coding. In this case,
 if "identity" is one of the available content-codings, then the
 server SHOULD use the "identity" content-coding, unless it has
 additional information that a different content-coding is meaningful
 to the client.
    Note: If the request does not include an Accept-Encoding field,
    and if the "identity" content-coding is unavailable, then
    content-codings commonly understood by HTTP/1.0 clients (i.e.,

Fielding, et al. Standards Track [Page 103] RFC 2616 HTTP/1.1 June 1999

    "gzip" and "compress") are preferred; some older clients
    improperly display messages sent with other content-codings.  The
    server might also make this decision based on information about
    the particular user-agent or client.
    Note: Most HTTP/1.0 applications do not recognize or obey qvalues
    associated with content-codings. This means that qvalues will not
    work and are not permitted with x-gzip or x-compress.

14.4 Accept-Language

 The Accept-Language request-header field is similar to Accept, but
 restricts the set of natural languages that are preferred as a
 response to the request. Language tags are defined in section 3.10.
     Accept-Language = "Accept-Language" ":"
                       1#( language-range [ ";" "q" "=" qvalue ] )
     language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )
 Each language-range MAY be given an associated quality value which
 represents an estimate of the user's preference for the languages
 specified by that range. The quality value defaults to "q=1". For
 example,
     Accept-Language: da, en-gb;q=0.8, en;q=0.7
 would mean: "I prefer Danish, but will accept British English and
 other types of English." A language-range matches a language-tag if
 it exactly equals the tag, or if it exactly equals a prefix of the
 tag such that the first tag character following the prefix is "-".
 The special range "*", if present in the Accept-Language field,
 matches every tag not matched by any other range present in the
 Accept-Language field.
    Note: This use of a prefix matching rule does not imply that
    language tags are assigned to languages in such a way that it is
    always true that if a user understands a language with a certain
    tag, then this user will also understand all languages with tags
    for which this tag is a prefix. The prefix rule simply allows the
    use of prefix tags if this is the case.
 The language quality factor assigned to a language-tag by the
 Accept-Language field is the quality value of the longest language-
 range in the field that matches the language-tag. If no language-
 range in the field matches the tag, the language quality factor
 assigned is 0. If no Accept-Language header is present in the
 request, the server

Fielding, et al. Standards Track [Page 104] RFC 2616 HTTP/1.1 June 1999

 SHOULD assume that all languages are equally acceptable. If an
 Accept-Language header is present, then all languages which are
 assigned a quality factor greater than 0 are acceptable.
 It might be contrary to the privacy expectations of the user to send
 an Accept-Language header with the complete linguistic preferences of
 the user in every request. For a discussion of this issue, see
 section 15.1.4.
 As intelligibility is highly dependent on the individual user, it is
 recommended that client applications make the choice of linguistic
 preference available to the user. If the choice is not made
 available, then the Accept-Language header field MUST NOT be given in
 the request.
    Note: When making the choice of linguistic preference available to
    the user, we remind implementors of  the fact that users are not
    familiar with the details of language matching as described above,
    and should provide appropriate guidance. As an example, users
    might assume that on selecting "en-gb", they will be served any
    kind of English document if British English is not available. A
    user agent might suggest in such a case to add "en" to get the
    best matching behavior.

14.5 Accept-Ranges

    The Accept-Ranges response-header field allows the server to
    indicate its acceptance of range requests for a resource:
        Accept-Ranges     = "Accept-Ranges" ":" acceptable-ranges
        acceptable-ranges = 1#range-unit | "none"
    Origin servers that accept byte-range requests MAY send
        Accept-Ranges: bytes
    but are not required to do so. Clients MAY generate byte-range
    requests without having received this header for the resource
    involved. Range units are defined in section 3.12.
    Servers that do not accept any kind of range request for a
    resource MAY send
        Accept-Ranges: none
    to advise the client not to attempt a range request.

Fielding, et al. Standards Track [Page 105] RFC 2616 HTTP/1.1 June 1999

14.6 Age

    The Age response-header field conveys the sender's estimate of the
    amount of time since the response (or its revalidation) was
    generated at the origin server. A cached response is "fresh" if
    its age does not exceed its freshness lifetime. Age values are
    calculated as specified in section 13.2.3.
         Age = "Age" ":" age-value
         age-value = delta-seconds
    Age values are non-negative decimal integers, representing time in
    seconds.
    If a cache receives a value larger than the largest positive
    integer it can represent, or if any of its age calculations
    overflows, it MUST transmit an Age header with a value of
    2147483648 (2^31). An HTTP/1.1 server that includes a cache MUST
    include an Age header field in every response generated from its
    own cache. Caches SHOULD use an arithmetic type of at least 31
    bits of range.

14.7 Allow

    The Allow entity-header field lists the set of methods supported
    by the resource identified by the Request-URI. The purpose of this
    field is strictly to inform the recipient of valid methods
    associated with the resource. An Allow header field MUST be
    present in a 405 (Method Not Allowed) response.
        Allow   = "Allow" ":" #Method
    Example of use:
        Allow: GET, HEAD, PUT
    This field cannot prevent a client from trying other methods.
    However, the indications given by the Allow header field value
    SHOULD be followed. The actual set of allowed methods is defined
    by the origin server at the time of each request.
    The Allow header field MAY be provided with a PUT request to
    recommend the methods to be supported by the new or modified
    resource. The server is not required to support these methods and
    SHOULD include an Allow header in the response giving the actual
    supported methods.

Fielding, et al. Standards Track [Page 106] RFC 2616 HTTP/1.1 June 1999

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

14.8 Authorization

    A user agent that wishes to authenticate itself with a server--
    usually, but not necessarily, after receiving a 401 response--does
    so by including an Authorization request-header field with the
    request.  The Authorization field value consists of credentials
    containing the authentication information of the user agent for
    the realm of the resource being requested.
        Authorization  = "Authorization" ":" credentials
    HTTP access authentication is described in "HTTP Authentication:
    Basic and Digest Access Authentication" [43]. If a request is
    authenticated and a realm specified, the same credentials SHOULD
    be valid for all other requests within this realm (assuming that
    the authentication scheme itself does not require otherwise, such
    as credentials that vary according to a challenge value or using
    synchronized clocks).
    When a shared cache (see section 13.7) receives a request
    containing an Authorization field, it MUST NOT return the
    corresponding response as a reply to any other request, unless one
    of the following specific exceptions holds:
    1. If the response includes the "s-maxage" cache-control
       directive, the cache MAY use that response in replying to a
       subsequent request. But (if the specified maximum age has
       passed) a proxy cache MUST first revalidate it with the origin
       server, using the request-headers from the new request to allow
       the origin server to authenticate the new request. (This is the
       defined behavior for s-maxage.) If the response includes "s-
       maxage=0", the proxy MUST always revalidate it before re-using
       it.
    2. If the response includes the "must-revalidate" cache-control
       directive, the cache MAY use that response in replying to a
       subsequent request. But if the response is stale, all caches
       MUST first revalidate it with the origin server, using the
       request-headers from the new request to allow the origin server
       to authenticate the new request.
    3. If the response includes the "public" cache-control directive,
       it MAY be returned in reply to any subsequent request.

Fielding, et al. Standards Track [Page 107] RFC 2616 HTTP/1.1 June 1999

14.9 Cache-Control

 The Cache-Control general-header field is used to specify directives
 that MUST be obeyed by all caching mechanisms along the
 request/response chain. The directives specify behavior intended to
 prevent caches from adversely interfering with the request or
 response. These directives typically override the default caching
 algorithms. Cache directives are unidirectional in that the presence
 of a directive in a request does not imply that the same directive is
 to be given in the response.
    Note that HTTP/1.0 caches might not implement Cache-Control and
    might only implement Pragma: no-cache (see section 14.32).
 Cache directives MUST be passed through by a proxy or gateway
 application, regardless of their significance to that application,
 since the directives might be applicable to all recipients along the
 request/response chain. It is not possible to specify a cache-
 directive for a specific cache.
  Cache-Control   = "Cache-Control" ":" 1#cache-directive
  cache-directive = cache-request-directive
       | cache-response-directive
  cache-request-directive =
         "no-cache"                          ; Section 14.9.1
       | "no-store"                          ; Section 14.9.2
       | "max-age" "=" delta-seconds         ; Section 14.9.3, 14.9.4
       | "max-stale" [ "=" delta-seconds ]   ; Section 14.9.3
       | "min-fresh" "=" delta-seconds       ; Section 14.9.3
       | "no-transform"                      ; Section 14.9.5
       | "only-if-cached"                    ; Section 14.9.4
       | cache-extension                     ; Section 14.9.6
   cache-response-directive =
         "public"                               ; Section 14.9.1
       | "private" [ "=" <"> 1#field-name <"> ] ; Section 14.9.1
       | "no-cache" [ "=" <"> 1#field-name <"> ]; Section 14.9.1
       | "no-store"                             ; Section 14.9.2
       | "no-transform"                         ; Section 14.9.5
       | "must-revalidate"                      ; Section 14.9.4
       | "proxy-revalidate"                     ; Section 14.9.4
       | "max-age" "=" delta-seconds            ; Section 14.9.3
       | "s-maxage" "=" delta-seconds           ; Section 14.9.3
       | cache-extension                        ; Section 14.9.6
  cache-extension = token [ "=" ( token | quoted-string ) ]

Fielding, et al. Standards Track [Page 108] RFC 2616 HTTP/1.1 June 1999

 When a directive appears without any 1#field-name parameter, the
 directive applies to the entire request or response. When such a
 directive appears with a 1#field-name parameter, it applies only to
 the named field or fields, and not to the rest of the request or
 response. This mechanism supports extensibility; implementations of
 future versions of the HTTP protocol might apply these directives to
 header fields not defined in HTTP/1.1.
 The cache-control directives can be broken down into these general
 categories:
  1. Restrictions on what are cacheable; these may only be imposed by

the origin server.

  1. Restrictions on what may be stored by a cache; these may be

imposed by either the origin server or the user agent.

  1. Modifications of the basic expiration mechanism; these may be

imposed by either the origin server or the user agent.

  1. Controls over cache revalidation and reload; these may only be

imposed by a user agent.

  1. Control over transformation of entities.
  1. Extensions to the caching system.

14.9.1 What is Cacheable

 By default, a response is cacheable if the requirements of the
 request method, request header fields, and the response status
 indicate that it is cacheable. Section 13.4 summarizes these defaults
 for cacheability. The following Cache-Control response directives
 allow an origin server to override the default cacheability of a
 response:
 public
    Indicates that the response MAY be cached by any cache, even if it
    would normally be non-cacheable or cacheable only within a non-
    shared cache. (See also Authorization, section 14.8, for
    additional details.)
 private
    Indicates that all or part of the response message is intended for
    a single user and MUST NOT be cached by a shared cache. This
    allows an origin server to state that the specified parts of the

Fielding, et al. Standards Track [Page 109] RFC 2616 HTTP/1.1 June 1999

    response are intended for only one user and are not a valid
    response for requests by other users. A private (non-shared) cache
    MAY cache the response.
     Note: This usage of the word private only controls where the
     response may be cached, and cannot ensure the privacy of the
     message content.
 no-cache
     If the no-cache directive does not specify a field-name, then a
    cache MUST NOT use the response to satisfy a subsequent request
    without successful revalidation with the origin server. This
    allows an origin server to prevent caching even by caches that
    have been configured to return stale responses to client requests.
    If the no-cache directive does specify one or more field-names,
    then a cache MAY use the response to satisfy a subsequent request,
    subject to any other restrictions on caching. However, the
    specified field-name(s) MUST NOT be sent in the response to a
    subsequent request without successful revalidation with the origin
    server. This allows an origin server to prevent the re-use of
    certain header fields in a response, while still allowing caching
    of the rest of the response.
     Note: Most HTTP/1.0 caches will not recognize or obey this
     directive.

14.9.2 What May be Stored by Caches

 no-store
    The purpose of the no-store directive is to prevent the
    inadvertent release or retention of sensitive information (for
    example, on backup tapes). The no-store directive applies to the
    entire message, and MAY be sent either in a response or in a
    request. If sent in a request, a cache MUST NOT store any part of
    either this request or any response to it. If sent in a response,
    a cache MUST NOT store any part of either this response or the
    request that elicited it. This directive applies to both non-
    shared and shared caches. "MUST NOT store" in this context means
    that the cache MUST NOT intentionally store the information in
    non-volatile storage, and MUST make a best-effort attempt to
    remove the information from volatile storage as promptly as
    possible after forwarding it.
    Even when this directive is associated with a response, users
    might explicitly store such a response outside of the caching
    system (e.g., with a "Save As" dialog). History buffers MAY store
    such responses as part of their normal operation.

Fielding, et al. Standards Track [Page 110] RFC 2616 HTTP/1.1 June 1999

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

14.9.3 Modifications of the Basic Expiration Mechanism

 The expiration time of an entity MAY be specified by the origin
 server using the Expires header (see section 14.21). Alternatively,
 it MAY be specified using the max-age directive in a response. When
 the max-age cache-control directive is present in a cached response,
 the response is stale if its current age is greater than the age
 value given (in seconds) at the time of a new request for that
 resource. The max-age directive on a response implies that the
 response is cacheable (i.e., "public") unless some other, more
 restrictive cache directive is also present.
 If a response includes both an Expires header and a max-age
 directive, the max-age directive overrides the Expires header, even
 if the Expires header is more restrictive. This rule allows an origin
 server to provide, for a given response, a longer expiration time to
 an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
 useful if certain HTTP/1.0 caches improperly calculate ages or
 expiration times, perhaps due to desynchronized clocks.
 Many HTTP/1.0 cache implementations will treat an Expires value that
 is less than or equal to the response Date value as being equivalent
 to the Cache-Control response directive "no-cache". If an HTTP/1.1
 cache receives such a response, and the response does not include a
 Cache-Control header field, it SHOULD consider the response to be
 non-cacheable in order to retain compatibility with HTTP/1.0 servers.
     Note: An origin server might wish to use a relatively new HTTP
     cache control feature, such as the "private" directive, on a
     network including older caches that do not understand that
     feature. The origin server will need to combine the new feature
     with an Expires field whose value is less than or equal to the
     Date value. This will prevent older caches from improperly
     caching the response.

Fielding, et al. Standards Track [Page 111] RFC 2616 HTTP/1.1 June 1999

 s-maxage
     If a response includes an s-maxage directive, then for a shared
     cache (but not for a private cache), the maximum age specified by
     this directive overrides the maximum age specified by either the
     max-age directive or the Expires header. The s-maxage directive
     also implies the semantics of the proxy-revalidate directive (see
     section 14.9.4), i.e., that the shared cache must not use the
     entry after it becomes stale to respond to a subsequent request
     without first revalidating it with the origin server. The s-
     maxage directive is always ignored by a private cache.
 Note that most older caches, not compliant with this specification,
 do not implement any cache-control directives. An origin server
 wishing to use a cache-control directive that restricts, but does not
 prevent, caching by an HTTP/1.1-compliant cache MAY exploit the
 requirement that the max-age directive overrides the Expires header,
 and the fact that pre-HTTP/1.1-compliant caches do not observe the
 max-age directive.
 Other directives allow a user agent to modify the basic expiration
 mechanism. These directives MAY be specified on a request:
 max-age
    Indicates that the client is willing to accept a response whose
    age is no greater than the specified time in seconds. Unless max-
    stale directive is also included, the client is not willing to
    accept a stale response.
 min-fresh
    Indicates that the client is willing to accept a response whose
    freshness lifetime is no less than its current age plus the
    specified time in seconds. That is, the client wants a response
    that will still be fresh for at least the specified number of
    seconds.
 max-stale
    Indicates that the client is willing to accept a response that has
    exceeded its expiration time. If max-stale is assigned a value,
    then the client is willing to accept a response that has exceeded
    its expiration time by no more than the specified number of
    seconds. If no value is assigned to max-stale, then the client is
    willing to accept a stale response of any age.
 If a cache returns a stale response, either because of a max-stale
 directive on a request, or because the cache is configured to
 override the expiration time of a response, the cache MUST attach a
 Warning header to the stale response, using Warning 110 (Response is
 stale).

Fielding, et al. Standards Track [Page 112] RFC 2616 HTTP/1.1 June 1999

 A cache MAY be configured to return stale responses without
 validation, but only if this does not conflict with any "MUST"-level
 requirements concerning cache validation (e.g., a "must-revalidate"
 cache-control directive).
 If both the new request and the cached entry include "max-age"
 directives, then the lesser of the two values is used for determining
 the freshness of the cached entry for that request.

14.9.4 Cache Revalidation and Reload Controls

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

Fielding, et al. Standards Track [Page 113] RFC 2616 HTTP/1.1 June 1999

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

Fielding, et al. Standards Track [Page 114] RFC 2616 HTTP/1.1 June 1999

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

14.9.5 No-Transform Directive

 no-transform
    Implementors of intermediate caches (proxies) have found it useful
    to convert the media type of certain entity bodies. A non-
    transparent proxy might, for example, convert between image
    formats in order to save cache space or to reduce the amount of
    traffic on a slow link.
    Serious operational problems occur, however, when these
    transformations are applied to entity bodies intended for certain
    kinds of applications. For example, applications for medical

Fielding, et al. Standards Track [Page 115] RFC 2616 HTTP/1.1 June 1999

    imaging, scientific data analysis and those using end-to-end
    authentication, all depend on receiving an entity body that is bit
    for bit identical to the original entity-body.
    Therefore, if a message includes the no-transform directive, an
    intermediate cache or proxy MUST NOT change those headers that are
    listed in section 13.5.2 as being subject to the no-transform
    directive. This implies that the cache or proxy MUST NOT change
    any aspect of the entity-body that is specified by these headers,
    including the value of the entity-body itself.

14.9.6 Cache Control Extensions

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

Fielding, et al. Standards Track [Page 116] RFC 2616 HTTP/1.1 June 1999

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

14.10 Connection

 The Connection general-header field allows the sender to specify
 options that are desired for that particular connection and MUST NOT
 be communicated by proxies over further connections.
 The Connection header has the following grammar:
     Connection = "Connection" ":" 1#(connection-token)
     connection-token  = token
 HTTP/1.1 proxies MUST parse the Connection header field before a
 message is forwarded and, for each connection-token in this field,
 remove any header field(s) from the message with the same name as the
 connection-token. Connection options are signaled by the presence of
 a connection-token in the Connection header field, not by any
 corresponding additional header field(s), since the additional header
 field may not be sent if there are no parameters associated with that
 connection option.
 Message headers listed in the Connection header MUST NOT include
 end-to-end headers, such as Cache-Control.
 HTTP/1.1 defines the "close" connection option for the sender to
 signal that the connection will be closed after completion of the
 response. For example,
     Connection: close
 in either the request or the response header fields indicates that
 the connection SHOULD NOT be considered `persistent' (section 8.1)
 after the current request/response is complete.
 HTTP/1.1 applications that do not support persistent connections MUST
 include the "close" connection option in every message.
 A system receiving an HTTP/1.0 (or lower-version) message that
 includes a Connection header MUST, for each connection-token in this
 field, remove and ignore any header field(s) from the message with
 the same name as the connection-token. This protects against mistaken
 forwarding of such header fields by pre-HTTP/1.1 proxies. See section
 19.6.2.

Fielding, et al. Standards Track [Page 117] RFC 2616 HTTP/1.1 June 1999

14.11 Content-Encoding

 The Content-Encoding entity-header field is used as a modifier to the
 media-type. When present, its value indicates what additional content
 codings have been applied to the entity-body, and thus what decoding
 mechanisms must be applied in order to obtain the media-type
 referenced by the Content-Type header field. Content-Encoding is
 primarily used to allow a document to be compressed without losing
 the identity of its underlying media type.
     Content-Encoding  = "Content-Encoding" ":" 1#content-coding
 Content codings are defined in section 3.5. An example of its use is
     Content-Encoding: gzip
 The content-coding is a characteristic of the entity identified by
 the Request-URI. Typically, the entity-body is stored with this
 encoding and is only decoded before rendering or analogous usage.
 However, a non-transparent proxy MAY modify the content-coding if the
 new coding is known to be acceptable to the recipient, unless the
 "no-transform" cache-control directive is present in the message.
 If the content-coding of an entity is not "identity", then the
 response MUST include a Content-Encoding entity-header (section
 14.11) that lists the non-identity content-coding(s) used.
 If the content-coding of an entity in a request message is not
 acceptable to the origin server, the server SHOULD respond with a
 status code of 415 (Unsupported Media Type).
 If multiple encodings have been applied to an entity, the content
 codings MUST be listed in the order in which they were applied.
 Additional information about the encoding parameters MAY be provided
 by other entity-header fields not defined by this specification.

14.12 Content-Language

 The Content-Language entity-header field describes the natural
 language(s) of the intended audience for the enclosed entity. Note
 that this might not be equivalent to all the languages used within
 the entity-body.
     Content-Language  = "Content-Language" ":" 1#language-tag

Fielding, et al. Standards Track [Page 118] RFC 2616 HTTP/1.1 June 1999

 Language tags are defined in section 3.10. The primary purpose of
 Content-Language is to allow a user to identify and differentiate
 entities according to the user's own preferred language. Thus, if the
 body content is intended only for a Danish-literate audience, the
 appropriate field is
     Content-Language: da
 If no Content-Language is specified, the default is that the content
 is intended for all language audiences. This might mean that the
 sender does not consider it to be specific to any natural language,
 or that the sender does not know for which language it is intended.
 Multiple languages MAY be listed for content that is intended for
 multiple audiences. For example, a rendition of the "Treaty of
 Waitangi," presented simultaneously in the original Maori and English
 versions, would call for
     Content-Language: mi, en
 However, just because multiple languages are present within an entity
 does not mean that it is intended for multiple linguistic audiences.
 An example would be a beginner's language primer, such as "A First
 Lesson in Latin," which is clearly intended to be used by an
 English-literate audience. In this case, the Content-Language would
 properly only include "en".
 Content-Language MAY be applied to any media type -- it is not
 limited to textual documents.

14.13 Content-Length

 The Content-Length entity-header field indicates the size of the
 entity-body, in decimal number of OCTETs, sent to the recipient or,
 in the case of the HEAD method, the size of the entity-body that
 would have been sent had the request been a GET.
     Content-Length    = "Content-Length" ":" 1*DIGIT
 An example is
     Content-Length: 3495
 Applications SHOULD use this field to indicate the transfer-length of
 the message-body, unless this is prohibited by the rules in section
 4.4.

Fielding, et al. Standards Track [Page 119] RFC 2616 HTTP/1.1 June 1999

 Any Content-Length greater than or equal to zero is a valid value.
 Section 4.4 describes how to determine the length of a message-body
 if a Content-Length is not given.
 Note that the meaning of this field is significantly different from
 the corresponding definition in MIME, where it is an optional field
 used within the "message/external-body" content-type. In HTTP, it
 SHOULD be sent whenever the message's length can be determined prior
 to being transferred, unless this is prohibited by the rules in
 section 4.4.

14.14 Content-Location

 The Content-Location entity-header field MAY be used to supply the
 resource location for the entity enclosed in the message when that
 entity is accessible from a location separate from the requested
 resource's URI. A server SHOULD provide a Content-Location for the
 variant corresponding to the response entity; especially in the case
 where a resource has multiple entities associated with it, and those
 entities actually have separate locations by which they might be
 individually accessed, the server SHOULD provide a Content-Location
 for the particular variant which is returned.
     Content-Location = "Content-Location" ":"
                       ( absoluteURI | relativeURI )
 The value of Content-Location also defines the base URI for the
 entity.
 The Content-Location value is not a replacement for the original
 requested URI; it is only a statement of the location of the resource
 corresponding to this particular entity at the time of the request.
 Future requests MAY specify the Content-Location URI as the request-
 URI if the desire is to identify the source of that particular
 entity.
 A cache cannot assume that an entity with a Content-Location
 different from the URI used to retrieve it can be used to respond to
 later requests on that Content-Location URI. However, the Content-
 Location can be used to differentiate between multiple entities
 retrieved from a single requested resource, as described in section
 13.6.
 If the Content-Location is a relative URI, the relative URI is
 interpreted relative to the Request-URI.
 The meaning of the Content-Location header in PUT or POST requests is
 undefined; servers are free to ignore it in those cases.

Fielding, et al. Standards Track [Page 120] RFC 2616 HTTP/1.1 June 1999

14.15 Content-MD5

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

Fielding, et al. Standards Track [Page 121] RFC 2616 HTTP/1.1 June 1999

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

14.16 Content-Range

 The Content-Range entity-header is sent with a partial entity-body to
 specify where in the full entity-body the partial body should be
 applied. Range units are defined in section 3.12.
     Content-Range = "Content-Range" ":" content-range-spec
     content-range-spec      = byte-content-range-spec
     byte-content-range-spec = bytes-unit SP
                               byte-range-resp-spec "/"
                               ( instance-length | "*" )
     byte-range-resp-spec = (first-byte-pos "-" last-byte-pos)
                                    | "*"
     instance-length           = 1*DIGIT
 The header SHOULD indicate the total length of the full entity-body,
 unless this length is unknown or difficult to determine. The asterisk
 "*" character means that the instance-length is unknown at the time
 when the response was generated.
 Unlike byte-ranges-specifier values (see section 14.35.1), a byte-
 range-resp-spec MUST only specify one range, and MUST contain
 absolute byte positions for both the first and last byte of the
 range.
 A byte-content-range-spec with a byte-range-resp-spec whose last-
 byte-pos value is less than its first-byte-pos value, or whose
 instance-length value is less than or equal to its last-byte-pos
 value, is invalid. The recipient of an invalid byte-content-range-
 spec MUST ignore it and any content transferred along with it.
 A server sending a response with status code 416 (Requested range not
 satisfiable) SHOULD include a Content-Range field with a byte-range-
 resp-spec of "*". The instance-length specifies the current length of

Fielding, et al. Standards Track [Page 122] RFC 2616 HTTP/1.1 June 1999

 the selected resource. A response with status code 206 (Partial
 Content) MUST NOT include a Content-Range field with a byte-range-
 resp-spec of "*".
 Examples of byte-content-range-spec values, assuming that the entity
 contains a total of 1234 bytes:
    . The first 500 bytes:
     bytes 0-499/1234
    . The second 500 bytes:
     bytes 500-999/1234
    . All except for the first 500 bytes:
     bytes 500-1233/1234
    . The last 500 bytes:
     bytes 734-1233/1234
 When an HTTP message includes the content of a single range (for
 example, a response to a request for a single range, or to a request
 for a set of ranges that overlap without any holes), this content is
 transmitted with a Content-Range header, and a Content-Length header
 showing the number of bytes actually transferred. For example,
     HTTP/1.1 206 Partial content
     Date: Wed, 15 Nov 1995 06:25:24 GMT
     Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
     Content-Range: bytes 21010-47021/47022
     Content-Length: 26012
     Content-Type: image/gif
 When an HTTP message includes the content of multiple ranges (for
 example, a response to a request for multiple non-overlapping
 ranges), these are transmitted as a multipart message. The multipart
 media type used for this purpose is "multipart/byteranges" as defined
 in appendix 19.2. See appendix 19.6.3 for a compatibility issue.
 A response to a request for a single range MUST NOT be sent using the
 multipart/byteranges media type.  A response to a request for
 multiple ranges, whose result is a single range, MAY be sent as a
 multipart/byteranges media type with one part. A client that cannot
 decode a multipart/byteranges message MUST NOT ask for multiple
 byte-ranges in a single request.
 When a client requests multiple byte-ranges in one request, the
 server SHOULD return them in the order that they appeared in the
 request.

Fielding, et al. Standards Track [Page 123] RFC 2616 HTTP/1.1 June 1999

 If the server ignores a byte-range-spec because it is syntactically
 invalid, the server SHOULD treat the request as if the invalid Range
 header field did not exist. (Normally, this means return a 200
 response containing the full entity).
 If the server receives a request (other than one including an If-
 Range request-header field) with an unsatisfiable Range request-
 header field (that is, all of whose byte-range-spec values have a
 first-byte-pos value greater than the current length of the selected
 resource), it SHOULD return a response code of 416 (Requested range
 not satisfiable) (section 10.4.17).
    Note: clients cannot depend on servers to send a 416 (Requested
    range not satisfiable) response instead of a 200 (OK) response for
    an unsatisfiable Range request-header, since not all servers
    implement this request-header.

14.17 Content-Type

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

14.18 Date

 The Date general-header field represents the date and time at which
 the message was originated, having the same semantics as orig-date in
 RFC 822. The field value is an HTTP-date, as described in section
 3.3.1; it MUST be sent in RFC 1123 [8]-date format.
     Date  = "Date" ":" HTTP-date
 An example is
     Date: Tue, 15 Nov 1994 08:12:31 GMT
 Origin servers MUST include a Date header field in all responses,
 except in these cases:

Fielding, et al. Standards Track [Page 124] RFC 2616 HTTP/1.1 June 1999

    1. If the response status code is 100 (Continue) or 101 (Switching
       Protocols), the response MAY include a Date header field, at
       the server's option.
    2. If the response status code conveys a server error, e.g. 500
       (Internal Server Error) or 503 (Service Unavailable), and it is
       inconvenient or impossible to generate a valid Date.
    3. If the server does not have a clock that can provide a
       reasonable approximation of the current time, its responses
       MUST NOT include a Date header field. In this case, the rules
       in section 14.18.1 MUST be followed.
 A received message that does not have a Date header field MUST be
 assigned one by the recipient if the message will be cached by that
 recipient or gatewayed via a protocol which requires a Date. An HTTP
 implementation without a clock MUST NOT cache responses without
 revalidating them on every use. An HTTP cache, especially a shared
 cache, SHOULD use a mechanism, such as NTP [28], to synchronize its
 clock with a reliable external standard.
 Clients SHOULD only send a Date header field in messages that include
 an entity-body, as in the case of the PUT and POST requests, and even
 then it is optional. A client without a clock MUST NOT send a Date
 header field in a request.
 The HTTP-date sent in a Date header SHOULD NOT represent a date and
 time subsequent to the generation of the message. It SHOULD represent
 the best available approximation of the date and time of message
 generation, unless the implementation has no means of generating a
 reasonably accurate date and time. In theory, the date ought to
 represent the moment just before the entity is generated. In
 practice, the date can be generated at any time during the message
 origination without affecting its semantic value.

14.18.1 Clockless Origin Server Operation

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

Fielding, et al. Standards Track [Page 125] RFC 2616 HTTP/1.1 June 1999

14.19 ETag

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

14.20 Expect

 The Expect request-header field is used to indicate that particular
 server behaviors are required by the client.
    Expect       =  "Expect" ":" 1#expectation
    expectation  =  "100-continue" | expectation-extension
    expectation-extension =  token [ "=" ( token | quoted-string )
                             *expect-params ]
    expect-params =  ";" token [ "=" ( token | quoted-string ) ]
 A server that does not understand or is unable to comply with any of
 the expectation values in the Expect field of a request MUST respond
 with appropriate error status. The server MUST respond with a 417
 (Expectation Failed) status if any of the expectations cannot be met
 or, if there are other problems with the request, some other 4xx
 status.
 This header field is defined with extensible syntax to allow for
 future extensions. If a server receives a request containing an
 Expect field that includes an expectation-extension that it does not
 support, it MUST respond with a 417 (Expectation Failed) status.
 Comparison of expectation values is case-insensitive for unquoted
 tokens (including the 100-continue token), and is case-sensitive for
 quoted-string expectation-extensions.

Fielding, et al. Standards Track [Page 126] RFC 2616 HTTP/1.1 June 1999

 The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST
 return a 417 (Expectation Failed) status if it receives a request
 with an expectation that it cannot meet. However, the Expect
 request-header itself is end-to-end; it MUST be forwarded if the
 request is forwarded.
 Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
 Expect header.
 See section 8.2.3 for the use of the 100 (continue) status.

14.21 Expires

 The Expires entity-header field gives the date/time after which the
 response is considered stale. A stale cache entry may not normally be
 returned by a cache (either a proxy cache or a user agent cache)
 unless it is first validated with the origin server (or with an
 intermediate cache that has a fresh copy of the entity). See section
 13.2 for further discussion of the expiration model.
 The presence of an Expires field does not imply that the original
 resource will change or cease to exist at, before, or after that
 time.
 The format is an absolute date and time as defined by HTTP-date in
 section 3.3.1; it MUST be in RFC 1123 date format:
    Expires = "Expires" ":" HTTP-date
 An example of its use is
    Expires: Thu, 01 Dec 1994 16:00:00 GMT
    Note: if a response includes a Cache-Control field with the max-
    age directive (see section 14.9.3), that directive overrides the
    Expires field.
 HTTP/1.1 clients and caches MUST treat other invalid date formats,
 especially including the value "0", as in the past (i.e., "already
 expired").
 To mark a response as "already expired," an origin server sends an
 Expires date that is equal to the Date header value. (See the rules
 for expiration calculations in section 13.2.4.)

Fielding, et al. Standards Track [Page 127] RFC 2616 HTTP/1.1 June 1999

 To mark a response as "never expires," an origin server sends an
 Expires date approximately one year from the time the response is
 sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one
 year in the future.
 The presence of an Expires header field with a date value of some
 time in the future on a response that otherwise would by default be
 non-cacheable indicates that the response is cacheable, unless
 indicated otherwise by a Cache-Control header field (section 14.9).

14.22 From

 The From request-header field, if given, SHOULD contain an Internet
 e-mail address for the human user who controls the requesting user
 agent. The address SHOULD be machine-usable, as defined by "mailbox"
 in RFC 822 [9] as updated by RFC 1123 [8]:
     From   = "From" ":" mailbox
 An example is:
     From: webmaster@w3.org
 This header field MAY be used for logging purposes and as a means for
 identifying the source of invalid or unwanted requests. It SHOULD NOT
 be used as an insecure form of access protection. The interpretation
 of this field is that the request is being performed on behalf of the
 person given, who accepts responsibility for the method performed. In
 particular, robot agents SHOULD include this header so that the
 person responsible for running the robot can be contacted if problems
 occur on the receiving end.
 The Internet e-mail address in this field MAY be separate from the
 Internet host which issued the request. For example, when a request
 is passed through a proxy the original issuer's address SHOULD be
 used.
 The client SHOULD NOT send the From header field without the user's
 approval, as it might conflict with the user's privacy interests or
 their site's security policy. It is strongly recommended that the
 user be able to disable, enable, and modify the value of this field
 at any time prior to a request.

14.23 Host

 The Host request-header field specifies the Internet host and port
 number of the resource being requested, as obtained from the original
 URI given by the user or referring resource (generally an HTTP URL,

Fielding, et al. Standards Track [Page 128] RFC 2616 HTTP/1.1 June 1999

 as described in section 3.2.2). The Host field value MUST represent
 the naming authority of the origin server or gateway given by the
 original URL. This allows the origin server or gateway to
 differentiate between internally-ambiguous URLs, such as the root "/"
 URL of a server for multiple host names on a single IP address.
     Host = "Host" ":" host [ ":" port ] ; Section 3.2.2
 A "host" without any trailing port information implies the default
 port for the service requested (e.g., "80" for an HTTP URL). For
 example, a request on the origin server for
 <http://www.w3.org/pub/WWW/> would properly include:
     GET /pub/WWW/ HTTP/1.1
     Host: www.w3.org
 A client MUST include a Host header field in all HTTP/1.1 request
 messages . If the requested URI does not include an Internet host
 name for the service being requested, then the Host header field MUST
 be given with an empty value. An HTTP/1.1 proxy MUST ensure that any
 request message it forwards does contain an appropriate Host header
 field that identifies the service being requested by the proxy. All
 Internet-based HTTP/1.1 servers MUST respond with a 400 (Bad Request)
 status code to any HTTP/1.1 request message which lacks a Host header
 field.
 See sections 5.2 and 19.6.1.1 for other requirements relating to
 Host.

14.24 If-Match

 The If-Match request-header field is used with a method to make it
 conditional. A client that has one or more entities previously
 obtained from the resource can verify that one of those entities is
 current by including a list of their associated entity tags in the
 If-Match header field. Entity tags are defined in section 3.11. The
 purpose of this feature is to allow efficient updates of cached
 information with a minimum amount of transaction overhead. It is also
 used, on updating requests, to prevent inadvertent modification of
 the wrong version of a resource. As a special case, the value "*"
 matches any current entity of the resource.
     If-Match = "If-Match" ":" ( "*" | 1#entity-tag )
 If any of the entity tags match the entity tag of the entity that
 would have been returned in the response to a similar GET request
 (without the If-Match header) on that resource, or if "*" is given

Fielding, et al. Standards Track [Page 129] RFC 2616 HTTP/1.1 June 1999

 and any current entity exists for that resource, then the server MAY
 perform the requested method as if the If-Match header field did not
 exist.
 A server MUST use the strong comparison function (see section 13.3.3)
 to compare the entity tags in If-Match.
 If none of the entity tags match, or if "*" is given and no current
 entity exists, the server MUST NOT perform the requested method, and
 MUST return a 412 (Precondition Failed) response. This behavior is
 most useful when the client wants to prevent an updating method, such
 as PUT, from modifying a resource that has changed since the client
 last retrieved it.
 If the request would, without the If-Match header field, result in
 anything other than a 2xx or 412 status, then the If-Match header
 MUST be ignored.
 The meaning of "If-Match: *" is that the method SHOULD be performed
 if the representation selected by the origin server (or by a cache,
 possibly using the Vary mechanism, see section 14.44) exists, and
 MUST NOT be performed if the representation does not exist.
 A request intended to update a resource (e.g., a PUT) MAY include an
 If-Match header field to signal that the request method MUST NOT be
 applied if the entity corresponding to the If-Match value (a single
 entity tag) is no longer a representation of that resource. This
 allows the user to indicate that they do not wish the request to be
 successful if the resource has been changed without their knowledge.
 Examples:
     If-Match: "xyzzy"
     If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
     If-Match: *
 The result of a request having both an If-Match header field and
 either an If-None-Match or an If-Modified-Since header fields is
 undefined by this specification.

14.25 If-Modified-Since

 The If-Modified-Since request-header field is used with a method to
 make it conditional: if the requested variant has not been modified
 since the time specified in this field, an entity will not be
 returned from the server; instead, a 304 (not modified) response will
 be returned without any message-body.
     If-Modified-Since = "If-Modified-Since" ":" HTTP-date

Fielding, et al. Standards Track [Page 130] RFC 2616 HTTP/1.1 June 1999

 An example of the field is:
     If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
 A GET method with an If-Modified-Since header and no Range header
 requests that the identified entity be transferred only if it has
 been modified since the date given by the If-Modified-Since header.
 The algorithm for determining this includes the following cases:
    a) If the request would normally result in anything other than a
       200 (OK) status, or if the passed If-Modified-Since date is
       invalid, the response is exactly the same as for a normal GET.
       A date which is later than the server's current time is
       invalid.
    b) If the variant has been modified since the If-Modified-Since
       date, the response is exactly the same as for a normal GET.
    c) If the variant has not been modified since a valid If-
       Modified-Since date, the server SHOULD return a 304 (Not
       Modified) response.
 The purpose of this feature is to allow efficient updates of cached
 information with a minimum amount of transaction overhead.
    Note: The Range request-header field modifies the meaning of If-
    Modified-Since; see section 14.35 for full details.
    Note: If-Modified-Since times are interpreted by the server, whose
    clock might not be synchronized with the client.
    Note: When handling an If-Modified-Since header field, some
    servers will use an exact date comparison function, rather than a
    less-than function, for deciding whether to send a 304 (Not
    Modified) response. To get best results when sending an If-
    Modified-Since header field for cache validation, clients are
    advised to use the exact date string received in a previous Last-
    Modified header field whenever possible.
    Note: If a client uses an arbitrary date in the If-Modified-Since
    header instead of a date taken from the Last-Modified header for
    the same request, the client should be aware of the fact that this
    date is interpreted in the server's understanding of time. The
    client should consider unsynchronized clocks and rounding problems
    due to the different encodings of time between the client and
    server. This includes the possibility of race conditions if the
    document has changed between the time it was first requested and
    the If-Modified-Since date of a subsequent request, and the

Fielding, et al. Standards Track [Page 131] RFC 2616 HTTP/1.1 June 1999

    possibility of clock-skew-related problems if the If-Modified-
    Since date is derived from the client's clock without correction
    to the server's clock. Corrections for different time bases
    between client and server are at best approximate due to network
    latency.
 The result of a request having both an If-Modified-Since header field
 and either an If-Match or an If-Unmodified-Since header fields is
 undefined by this specification.

14.26 If-None-Match

 The If-None-Match request-header field is used with a method to make
 it conditional. A client that has one or more entities previously
 obtained from the resource can verify that none of those entities is
 current by including a list of their associated entity tags in the
 If-None-Match header field. The purpose of this feature is to allow
 efficient updates of cached information with a minimum amount of
 transaction overhead. It is also used to prevent a method (e.g. PUT)
 from inadvertently modifying an existing resource when the client
 believes that the resource does not exist.
 As a special case, the value "*" matches any current entity of the
 resource.
     If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
 If any of the entity tags match the entity tag of the entity that
 would have been returned in the response to a similar GET request
 (without the If-None-Match header) on that resource, or if "*" is
 given and any current entity exists for that resource, then the
 server MUST NOT perform the requested method, unless required to do
 so because the resource's modification date fails to match that
 supplied in an If-Modified-Since header field in the request.
 Instead, if the request method was GET or HEAD, the server SHOULD
 respond with a 304 (Not Modified) response, including the cache-
 related header fields (particularly ETag) of one of the entities that
 matched. For all other request methods, the server MUST respond with
 a status of 412 (Precondition Failed).
 See section 13.3.3 for rules on how to determine if two entities tags
 match. The weak comparison function can only be used with GET or HEAD
 requests.

Fielding, et al. Standards Track [Page 132] RFC 2616 HTTP/1.1 June 1999

 If none of the entity tags match, then the server MAY perform the
 requested method as if the If-None-Match header field did not exist,
 but MUST also ignore any If-Modified-Since header field(s) in the
 request. That is, if no entity tags match, then the server MUST NOT
 return a 304 (Not Modified) response.
 If the request would, without the If-None-Match header field, result
 in anything other than a 2xx or 304 status, then the If-None-Match
 header MUST be ignored. (See section 13.3.4 for a discussion of
 server behavior when both If-Modified-Since and If-None-Match appear
 in the same request.)
 The meaning of "If-None-Match: *" is that the method MUST NOT be
 performed if the representation selected by the origin server (or by
 a cache, possibly using the Vary mechanism, see section 14.44)
 exists, and SHOULD be performed if the representation does not exist.
 This feature is intended to be useful in preventing races between PUT
 operations.
 Examples:
     If-None-Match: "xyzzy"
     If-None-Match: W/"xyzzy"
     If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
     If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
     If-None-Match: *
 The result of a request having both an If-None-Match header field and
 either an If-Match or an If-Unmodified-Since header fields is
 undefined by this specification.

14.27 If-Range

 If a client has a partial copy of an entity in its cache, and wishes
 to have an up-to-date copy of the entire entity in its cache, it
 could use the Range request-header with a conditional GET (using
 either or both of If-Unmodified-Since and If-Match.) However, if the
 condition fails because the entity has been modified, the client
 would then have to make a second request to obtain the entire current
 entity-body.
 The If-Range header allows a client to "short-circuit" the second
 request. Informally, its meaning is `if the entity is unchanged, send
 me the part(s) that I am missing; otherwise, send me the entire new
 entity'.
      If-Range = "If-Range" ":" ( entity-tag | HTTP-date )

Fielding, et al. Standards Track [Page 133] RFC 2616 HTTP/1.1 June 1999

 If the client has no entity tag for an entity, but does have a Last-
 Modified date, it MAY use that date in an If-Range header. (The
 server can distinguish between a valid HTTP-date and any form of
 entity-tag by examining no more than two characters.) The If-Range
 header SHOULD only be used together with a Range header, and MUST be
 ignored if the request does not include a Range header, or if the
 server does not support the sub-range operation.
 If the entity tag given in the If-Range header matches the current
 entity tag for the entity, then the server SHOULD provide the
 specified sub-range of the entity using a 206 (Partial content)
 response. If the entity tag does not match, then the server SHOULD
 return the entire entity using a 200 (OK) response.

14.28 If-Unmodified-Since

 The If-Unmodified-Since request-header field is used with a method to
 make it conditional. If the requested resource has not been modified
 since the time specified in this field, the server SHOULD perform the
 requested operation as if the If-Unmodified-Since header were not
 present.
 If the requested variant has been modified since the specified time,
 the server MUST NOT perform the requested operation, and MUST return
 a 412 (Precondition Failed).
    If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
 An example of the field is:
     If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
 If the request normally (i.e., without the If-Unmodified-Since
 header) would result in anything other than a 2xx or 412 status, the
 If-Unmodified-Since header SHOULD be ignored.
 If the specified date is invalid, the header is ignored.
 The result of a request having both an If-Unmodified-Since header
 field and either an If-None-Match or an If-Modified-Since header
 fields is undefined by this specification.

14.29 Last-Modified

 The Last-Modified entity-header field indicates the date and time at
 which the origin server believes the variant was last modified.
     Last-Modified  = "Last-Modified" ":" HTTP-date

Fielding, et al. Standards Track [Page 134] RFC 2616 HTTP/1.1 June 1999

 An example of its use is
     Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
 The exact meaning of this header field depends on the implementation
 of the origin server and the nature of the original resource. For
 files, it may be just the file system last-modified time. For
 entities with dynamically included parts, it may be the most recent
 of the set of last-modify times for its component parts. For database
 gateways, it may be the last-update time stamp of the record. For
 virtual objects, it may be the last time the internal state changed.
 An origin server MUST NOT send a Last-Modified date which is later
 than the server's time of message origination. In such cases, where
 the resource's last modification would indicate some time in the
 future, the server MUST replace that date with the message
 origination date.
 An origin server SHOULD obtain the Last-Modified value of the entity
 as close as possible to the time that it generates the Date value of
 its response. This allows a recipient to make an accurate assessment
 of the entity's modification time, especially if the entity changes
 near the time that the response is generated.
 HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.

14.30 Location

 The Location response-header field is used to redirect the recipient
 to a location other than the Request-URI for completion of the
 request or identification of a new resource. For 201 (Created)
 responses, the Location is that of the new resource which was created
 by the request. For 3xx responses, the location SHOULD indicate the
 server's preferred URI for automatic redirection to the resource. The
 field value consists of a single absolute URI.
     Location       = "Location" ":" absoluteURI
 An example is:
     Location: http://www.w3.org/pub/WWW/People.html
    Note: The Content-Location header field (section 14.14) differs
    from Location in that the Content-Location identifies the original
    location of the entity enclosed in the request. It is therefore
    possible for a response to contain header fields for both Location
    and Content-Location. Also see section 13.10 for cache
    requirements of some methods.

Fielding, et al. Standards Track [Page 135] RFC 2616 HTTP/1.1 June 1999

14.31 Max-Forwards

 The Max-Forwards request-header field provides a mechanism with the
 TRACE (section 9.8) and OPTIONS (section 9.2) methods to limit the
 number of proxies or gateways that can forward the request to the
 next inbound server. This can be useful when the client is attempting
 to trace a request chain which appears to be failing or looping in
 mid-chain.
     Max-Forwards   = "Max-Forwards" ":" 1*DIGIT
 The Max-Forwards value is a decimal integer indicating the remaining
 number of times this request message may be forwarded.
 Each proxy or gateway recipient of a TRACE or OPTIONS request
 containing a Max-Forwards header field MUST check and update its
 value prior to forwarding the request. If the received value is zero
 (0), the recipient MUST NOT forward the request; instead, it MUST
 respond as the final recipient. If the received Max-Forwards value is
 greater than zero, then the forwarded message MUST contain an updated
 Max-Forwards field with a value decremented by one (1).
 The Max-Forwards header field MAY be ignored for all other methods
 defined by this specification and for any extension methods for which
 it is not explicitly referred to as part of that method definition.

14.32 Pragma

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

Fielding, et al. Standards Track [Page 136] RFC 2616 HTTP/1.1 June 1999

 Pragma directives MUST be passed through by a proxy or gateway
 application, regardless of their significance to that application,
 since the directives might be applicable to all recipients along the
 request/response chain. It is not possible to specify a pragma for a
 specific recipient; however, any pragma directive not relevant to a
 recipient SHOULD be ignored by that recipient.
 HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
 sent "Cache-Control: no-cache". No new Pragma directives will be
 defined in HTTP.
    Note: because the meaning of "Pragma: no-cache as a response
    header field is not actually specified, it does not provide a
    reliable replacement for "Cache-Control: no-cache" in a response

14.33 Proxy-Authenticate

 The Proxy-Authenticate response-header field MUST be included as part
 of a 407 (Proxy Authentication Required) response. The field value
 consists of a challenge that indicates the authentication scheme and
 parameters applicable to the proxy for this Request-URI.
     Proxy-Authenticate  = "Proxy-Authenticate" ":" 1#challenge
 The HTTP access authentication process is described in "HTTP
 Authentication: Basic and Digest Access Authentication" [43]. Unlike
 WWW-Authenticate, the Proxy-Authenticate header field applies only to
 the current connection and SHOULD NOT be passed on to downstream
 clients. However, an intermediate proxy might need to obtain its own
 credentials by requesting them from the downstream client, which in
 some circumstances will appear as if the proxy is forwarding the
 Proxy-Authenticate header field.

14.34 Proxy-Authorization

 The Proxy-Authorization request-header field allows the client to
 identify itself (or its user) to a proxy which requires
 authentication. The Proxy-Authorization field value consists of
 credentials containing the authentication information of the user
 agent for the proxy and/or realm of the resource being requested.
     Proxy-Authorization     = "Proxy-Authorization" ":" credentials
 The HTTP access authentication process is described in "HTTP
 Authentication: Basic and Digest Access Authentication" [43] . Unlike
 Authorization, the Proxy-Authorization header field applies only to
 the next outbound proxy that demanded authentication using the Proxy-
 Authenticate field. When multiple proxies are used in a chain, the

Fielding, et al. Standards Track [Page 137] RFC 2616 HTTP/1.1 June 1999

 Proxy-Authorization header field is consumed by the first outbound
 proxy that was expecting to receive credentials. A proxy MAY relay
 the credentials from the client request to the next proxy if that is
 the mechanism by which the proxies cooperatively authenticate a given
 request.

14.35 Range

14.35.1 Byte Ranges

 Since all HTTP entities are represented in HTTP messages as sequences
 of bytes, the concept of a byte range is meaningful for any HTTP
 entity. (However, not all clients and servers need to support byte-
 range operations.)
 Byte range specifications in HTTP apply to the sequence of bytes in
 the entity-body (not necessarily the same as the message-body).
 A byte range operation MAY specify a single range of bytes, or a set
 of ranges within a single entity.
     ranges-specifier = byte-ranges-specifier
     byte-ranges-specifier = bytes-unit "=" byte-range-set
     byte-range-set  = 1#( byte-range-spec | suffix-byte-range-spec )
     byte-range-spec = first-byte-pos "-" [last-byte-pos]
     first-byte-pos  = 1*DIGIT
     last-byte-pos   = 1*DIGIT
 The first-byte-pos value in a byte-range-spec gives the byte-offset
 of the first byte in a range. The last-byte-pos value gives the
 byte-offset of the last byte in the range; that is, the byte
 positions specified are inclusive. Byte offsets start at zero.
 If the last-byte-pos value is present, it MUST be greater than or
 equal to the first-byte-pos in that byte-range-spec, or the byte-
 range-spec is syntactically invalid. The recipient of a byte-range-
 set that includes one or more syntactically invalid byte-range-spec
 values MUST ignore the header field that includes that byte-range-
 set.
 If the last-byte-pos value is absent, or if the value is greater than
 or equal to the current length of the entity-body, last-byte-pos is
 taken to be equal to one less than the current length of the entity-
 body in bytes.
 By its choice of last-byte-pos, a client can limit the number of
 bytes retrieved without knowing the size of the entity.

Fielding, et al. Standards Track [Page 138] RFC 2616 HTTP/1.1 June 1999

     suffix-byte-range-spec = "-" suffix-length
     suffix-length = 1*DIGIT
 A suffix-byte-range-spec is used to specify the suffix of the
 entity-body, of a length given by the suffix-length value. (That is,
 this form specifies the last N bytes of an entity-body.) If the
 entity is shorter than the specified suffix-length, the entire
 entity-body is used.
 If a syntactically valid byte-range-set includes at least one byte-
 range-spec whose first-byte-pos is less than the current length of
 the entity-body, or at least one suffix-byte-range-spec with a non-
 zero suffix-length, then the byte-range-set is satisfiable.
 Otherwise, the byte-range-set is unsatisfiable. If the byte-range-set
 is unsatisfiable, the server SHOULD return a response with a status
 of 416 (Requested range not satisfiable). Otherwise, the server
 SHOULD return a response with a status of 206 (Partial Content)
 containing the satisfiable ranges of the entity-body.
 Examples of byte-ranges-specifier values (assuming an entity-body of
 length 10000):
  1. The first 500 bytes (byte offsets 0-499, inclusive): bytes=0-

499

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

bytes=500-999

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

bytes=-500

  1. Or bytes=9500-
  1. The first and last bytes only (bytes 0 and 9999): bytes=0-0,-1
  1. Several legal but not canonical specifications of the second 500

bytes (byte offsets 500-999, inclusive):

       bytes=500-600,601-999
       bytes=500-700,601-999

14.35.2 Range Retrieval Requests

 HTTP retrieval requests using conditional or unconditional GET
 methods MAY request one or more sub-ranges of the entity, instead of
 the entire entity, using the Range request header, which applies to
 the entity returned as the result of the request:
    Range = "Range" ":" ranges-specifier

Fielding, et al. Standards Track [Page 139] RFC 2616 HTTP/1.1 June 1999

 A server MAY ignore the Range header. However, HTTP/1.1 origin
 servers and intermediate caches ought to support byte ranges when
 possible, since Range supports efficient recovery from partially
 failed transfers, and supports efficient partial retrieval of large
 entities.
 If the server supports the Range header and the specified range or
 ranges are appropriate for the entity:
  1. 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).
  1. The presence of a Range header in a conditional GET (a request

using one or both of If-Modified-Since and If-None-Match, or

      one or both of If-Unmodified-Since and If-Match) modifies what
      is returned if the GET is otherwise successful and the
      condition is true. It does not affect the 304 (Not Modified)
      response returned if the conditional is false.
 In some cases, it might be more appropriate to use the If-Range
 header (see section 14.27) in addition to the Range header.
 If a proxy that supports ranges receives a Range request, forwards
 the request to an inbound server, and receives an entire entity in
 reply, it SHOULD only return the requested range to its client. It
 SHOULD store the entire received response in its cache if that is
 consistent with its cache allocation policies.

14.36 Referer

 The Referer[sic] request-header field allows the client to specify,
 for the server's benefit, the address (URI) of the resource from
 which the Request-URI was obtained (the "referrer", although the
 header field is misspelled.) The Referer request-header allows a
 server to generate lists of back-links to resources for interest,
 logging, optimized caching, etc. It also allows obsolete or mistyped
 links to be traced for maintenance. The Referer field MUST NOT be
 sent if the Request-URI was obtained from a source that does not have
 its own URI, such as input from the user keyboard.
     Referer        = "Referer" ":" ( absoluteURI | relativeURI )
 Example:
     Referer: http://www.w3.org/hypertext/DataSources/Overview.html

Fielding, et al. Standards Track [Page 140] RFC 2616 HTTP/1.1 June 1999

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

14.37 Retry-After

 The Retry-After response-header field can be used with a 503 (Service
 Unavailable) response to indicate how long the service is expected to
 be unavailable to the requesting client. This field MAY also be used
 with any 3xx (Redirection) response to indicate the minimum time the
 user-agent is asked wait before issuing the redirected request. The
 value of this field can be either an HTTP-date or an integer number
 of seconds (in decimal) after the time of the response.
     Retry-After  = "Retry-After" ":" ( HTTP-date | delta-seconds )
 Two examples of its use are
     Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
     Retry-After: 120
 In the latter example, the delay is 2 minutes.

14.38 Server

 The Server response-header field contains information about the
 software used by the origin server to handle the request. The field
 can contain multiple product tokens (section 3.8) and comments
 identifying the server and any significant subproducts. The product
 tokens are listed in order of their significance for identifying the
 application.
     Server         = "Server" ":" 1*( product | comment )
 Example:
     Server: CERN/3.0 libwww/2.17
 If the response is being forwarded through a proxy, the proxy
 application MUST NOT modify the Server response-header. Instead, it
 SHOULD include a Via field (as described in section 14.45).
    Note: Revealing the specific software version of the server might
    allow the server machine to become more vulnerable to attacks
    against software that is known to contain security holes. Server
    implementors are encouraged to make this field a configurable
    option.

Fielding, et al. Standards Track [Page 141] RFC 2616 HTTP/1.1 June 1999

14.39 TE

 The TE request-header field indicates what extension transfer-codings
 it is willing to accept in the response and whether or not it is
 willing to accept trailer fields in a chunked transfer-coding. Its
 value may consist of the keyword "trailers" and/or a comma-separated
 list of extension transfer-coding names with optional accept
 parameters (as described in section 3.6).
     TE        = "TE" ":" #( t-codings )
     t-codings = "trailers" | ( transfer-extension [ accept-params ] )
 The presence of the keyword "trailers" indicates that the client is
 willing to accept trailer fields in a chunked transfer-coding, as
 defined in section 3.6.1. This keyword is reserved for use with
 transfer-coding values even though it does not itself represent a
 transfer-coding.
 Examples of its use are:
     TE: deflate
     TE:
     TE: trailers, deflate;q=0.5
 The TE header field only applies to the immediate connection.
 Therefore, the keyword MUST be supplied within a Connection header
 field (section 14.10) whenever TE is present in an HTTP/1.1 message.
 A server tests whether a transfer-coding is acceptable, according to
 a TE field, using these rules:
    1. The "chunked" transfer-coding is always acceptable. If the
       keyword "trailers" is listed, the client indicates that it is
       willing to accept trailer fields in the chunked response on
       behalf of itself and any downstream clients. The implication is
       that, if given, the client is stating that either all
       downstream clients are willing to accept trailer fields in the
       forwarded response, or that it will attempt to buffer the
       response on behalf of downstream recipients.
       Note: HTTP/1.1 does not define any means to limit the size of a
       chunked response such that a client can be assured of buffering
       the entire response.
    2. If the transfer-coding being tested is one of the transfer-
       codings listed in the TE field, then it is acceptable unless it
       is accompanied by a qvalue of 0. (As defined in section 3.9, a
       qvalue of 0 means "not acceptable.")

Fielding, et al. Standards Track [Page 142] RFC 2616 HTTP/1.1 June 1999

    3. If multiple transfer-codings are acceptable, then the
       acceptable transfer-coding with the highest non-zero qvalue is
       preferred.  The "chunked" transfer-coding always has a qvalue
       of 1.
 If the TE field-value is empty or if no TE field is present, the only
 transfer-coding  is "chunked". A message with no transfer-coding is
 always acceptable.

14.40 Trailer

 The Trailer general field value indicates that the given set of
 header fields is present in the trailer of a message encoded with
 chunked transfer-coding.
     Trailer  = "Trailer" ":" 1#field-name
 An HTTP/1.1 message SHOULD include a Trailer header field in a
 message using chunked transfer-coding with a non-empty trailer. Doing
 so allows the recipient to know which header fields to expect in the
 trailer.
 If no Trailer header field is present, the trailer SHOULD NOT include
 any header fields. See section 3.6.1 for restrictions on the use of
 trailer fields in a "chunked" transfer-coding.
 Message header fields listed in the Trailer header field MUST NOT
 include the following header fields:
    . Transfer-Encoding
    . Content-Length
    . Trailer

14.41 Transfer-Encoding

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

Fielding, et al. Standards Track [Page 143] RFC 2616 HTTP/1.1 June 1999

 If multiple encodings have been applied to an entity, the transfer-
 codings MUST be listed in the order in which they were applied.
 Additional information about the encoding parameters MAY be provided
 by other entity-header fields not defined by this specification.
 Many older HTTP/1.0 applications do not understand the Transfer-
 Encoding header.

14.42 Upgrade

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

Fielding, et al. Standards Track [Page 144] RFC 2616 HTTP/1.1 June 1999

 The Upgrade header field cannot be used to indicate a switch to a
 protocol on a different connection. For that purpose, it is more
 appropriate to use a 301, 302, 303, or 305 redirection response.
 This specification only defines the protocol name "HTTP" for use by
 the family of Hypertext Transfer Protocols, as defined by the HTTP
 version rules of section 3.1 and future updates to this
 specification. Any token can be used as a protocol name; however, it
 will only be useful if both the client and server associate the name
 with the same protocol.

14.43 User-Agent

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

14.44 Vary

 The Vary field value indicates the set of request-header fields that
 fully determines, while the response is fresh, whether a cache is
 permitted to use the response to reply to a subsequent request
 without revalidation. For uncacheable or stale responses, the Vary
 field value advises the user agent about the criteria that were used
 to select the representation. A Vary field value of "*" implies that
 a cache cannot determine from the request headers of a subsequent
 request whether this response is the appropriate representation. See
 section 13.6 for use of the Vary header field by caches.
     Vary  = "Vary" ":" ( "*" | 1#field-name )
 An HTTP/1.1 server SHOULD include a Vary header field with any
 cacheable response that is subject to server-driven negotiation.
 Doing so allows a cache to properly interpret future requests on that
 resource and informs the user agent about the presence of negotiation

Fielding, et al. Standards Track [Page 145] RFC 2616 HTTP/1.1 June 1999

 on that resource. A server MAY include a Vary header field with a
 non-cacheable response that is subject to server-driven negotiation,
 since this might provide the user agent with useful information about
 the dimensions over which the response varies at the time of the
 response.
 A Vary field value consisting of a list of field-names signals that
 the representation selected for the response is based on a selection
 algorithm which considers ONLY the listed request-header field values
 in selecting the most appropriate representation. A cache MAY assume
 that the same selection will be made for future requests with the
 same values for the listed field names, for the duration of time for
 which the response is fresh.
 The field-names given are not limited to the set of standard
 request-header fields defined by this specification. Field names are
 case-insensitive.
 A Vary field value of "*" signals that unspecified parameters not
 limited to the request-headers (e.g., the network address of the
 client), play a role in the selection of the response representation.
 The "*" value MUST NOT be generated by a proxy server; it may only be
 generated by an origin server.

14.45 Via

 The Via general-header field MUST be used by gateways and proxies to
 indicate the intermediate protocols and recipients between the user
 agent and the server on requests, and between the origin server and
 the client on responses. It is analogous to the "Received" field of
 RFC 822 [9] and is intended to be used for tracking message forwards,
 avoiding request loops, and identifying the protocol capabilities of
 all senders along the request/response chain.
    Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
    received-protocol = [ protocol-name "/" ] protocol-version
    protocol-name     = token
    protocol-version  = token
    received-by       = ( host [ ":" port ] ) | pseudonym
    pseudonym         = token
 The received-protocol indicates the protocol version of the message
 received by the server or client along each segment of the
 request/response chain. The received-protocol version is appended to
 the Via field value when the message is forwarded so that information
 about the protocol capabilities of upstream applications remains
 visible to all recipients.

Fielding, et al. Standards Track [Page 146] RFC 2616 HTTP/1.1 June 1999

 The protocol-name is optional if and only if it would be "HTTP". The
 received-by field is normally the host and optional port number of a
 recipient server or client that subsequently forwarded the message.
 However, if the real host is considered to be sensitive information,
 it MAY be replaced by a pseudonym. If the port is not given, it MAY
 be assumed to be the default port of the received-protocol.
 Multiple Via field values represents each proxy or gateway that has
 forwarded the message. Each recipient MUST append its information
 such that the end result is ordered according to the sequence of
 forwarding applications.
 Comments MAY be used in the Via header field to identify the software
 of the recipient proxy or gateway, analogous to the User-Agent and
 Server header fields. However, all comments in the Via field are
 optional and MAY be removed by any recipient prior to forwarding the
 message.
 For example, a request message could be sent from an HTTP/1.0 user
 agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
 forward the request to a public proxy at nowhere.com, which completes
 the request by forwarding it to the origin server at www.ics.uci.edu.
 The request received by www.ics.uci.edu would then have the following
 Via header field:
     Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
 Proxies and gateways used as a portal through a network firewall
 SHOULD NOT, by default, forward the names and ports of hosts within
 the firewall region. This information SHOULD only be propagated if
 explicitly enabled. If not enabled, the received-by host of any host
 behind the firewall SHOULD be replaced by an appropriate pseudonym
 for that host.
 For organizations that have strong privacy requirements for hiding
 internal structures, a proxy MAY combine an ordered subsequence of
 Via header field entries with identical received-protocol values into
 a single such entry. For example,
     Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
      could be collapsed to
     Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

Fielding, et al. Standards Track [Page 147] RFC 2616 HTTP/1.1 June 1999

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

14.46 Warning

 The Warning general-header field is used to carry additional
 information about the status or transformation of a message which
 might not be reflected in the message. This information is typically
 used to warn about a possible lack of semantic transparency from
 caching operations or transformations applied to the entity body of
 the message.
 Warning headers are sent with responses using:
     Warning    = "Warning" ":" 1#warning-value
     warning-value = warn-code SP warn-agent SP warn-text
                                           [SP warn-date]
     warn-code  = 3DIGIT
     warn-agent = ( host [ ":" port ] ) | pseudonym
                     ; the name or pseudonym of the server adding
                     ; the Warning header, for use in debugging
     warn-text  = quoted-string
     warn-date  = <"> HTTP-date <">
 A response MAY carry more than one Warning header.
 The warn-text SHOULD be in a natural language and character set that
 is most likely to be intelligible to the human user receiving the
 response. This decision MAY be based on any available knowledge, such
 as the location of the cache or user, the Accept-Language field in a
 request, the Content-Language field in a response, etc. The default
 language is English and the default character set is ISO-8859-1.
 If a character set other than ISO-8859-1 is used, it MUST be encoded
 in the warn-text using the method described in RFC 2047 [14].
 Warning headers can in general be applied to any message, however
 some specific warn-codes are specific to caches and can only be
 applied to response messages. New Warning headers SHOULD be added
 after any existing Warning headers. A cache MUST NOT delete any
 Warning header that it received with a message. However, if a cache
 successfully validates a cache entry, it SHOULD remove any Warning
 headers previously attached to that entry except as specified for

Fielding, et al. Standards Track [Page 148] RFC 2616 HTTP/1.1 June 1999

 specific Warning codes. It MUST then add any Warning headers received
 in the validating response. In other words, Warning headers are those
 that would be attached to the most recent relevant response.
 When multiple Warning headers are attached to a response, the user
 agent ought to inform the user of as many of them as possible, in the
 order that they appear in the response. If it is not possible to
 inform the user of all of the warnings, the user agent SHOULD follow
 these heuristics:
  1. Warnings that appear early in the response take priority over

those appearing later in the response.

  1. Warnings in the user's preferred character set take priority

over warnings in other character sets but with identical warn-

      codes and warn-agents.
 Systems that generate multiple Warning headers SHOULD order them with
 this user agent behavior in mind.
 Requirements for the behavior of caches with respect to Warnings are
 stated in section 13.1.2.
 This is a list of the currently-defined warn-codes, each with a
 recommended warn-text in English, and a description of its meaning.
 110 Response is stale
   MUST be included whenever the returned response is stale.
 111 Revalidation failed
   MUST be included if a cache returns a stale response because an
   attempt to revalidate the response failed, due to an inability to
   reach the server.
 112 Disconnected operation
   SHOULD be included if the cache is intentionally disconnected from
   the rest of the network for a period of time.
 113 Heuristic expiration
   MUST be included if the cache heuristically chose a freshness
   lifetime greater than 24 hours and the response's age is greater
   than 24 hours.
 199 Miscellaneous warning
   The warning text MAY include arbitrary information to be presented
   to a human user, or logged. A system receiving this warning MUST
   NOT take any automated action, besides presenting the warning to
   the user.

Fielding, et al. Standards Track [Page 149] RFC 2616 HTTP/1.1 June 1999

 214 Transformation applied
   MUST be added by an intermediate cache or proxy if it applies any
   transformation changing the content-coding (as specified in the
   Content-Encoding header) or media-type (as specified in the
   Content-Type header) of the response, or the entity-body of the
   response, unless this Warning code already appears in the response.
 299 Miscellaneous persistent warning
   The warning text MAY include arbitrary information to be presented
   to a human user, or logged. A system receiving this warning MUST
   NOT take any automated action.
 If an implementation sends a message with one or more Warning headers
 whose version is HTTP/1.0 or lower, then the sender MUST include in
 each warning-value a warn-date that matches the date in the response.
 If an implementation receives a message with a warning-value that
 includes a warn-date, and that warn-date is different from the Date
 value in the response, then that warning-value MUST be deleted from
 the message before storing, forwarding, or using it. (This prevents
 bad consequences of naive caching of Warning header fields.) If all
 of the warning-values are deleted for this reason, the Warning header
 MUST be deleted as well.

14.47 WWW-Authenticate

 The WWW-Authenticate response-header field MUST be included in 401
 (Unauthorized) response messages. The field value consists of at
 least one challenge that indicates the authentication scheme(s) and
 parameters applicable to the Request-URI.
     WWW-Authenticate  = "WWW-Authenticate" ":" 1#challenge
 The HTTP access authentication process is described in "HTTP
 Authentication: Basic and Digest Access Authentication" [43]. User
 agents are advised to take special care in parsing the WWW-
 Authenticate field value as it might contain more than one challenge,
 or if more than one WWW-Authenticate header field is provided, the
 contents of a challenge itself can contain a comma-separated list of
 authentication parameters.

15 Security Considerations

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

Fielding, et al. Standards Track [Page 150] RFC 2616 HTTP/1.1 June 1999

15.1 Personal Information

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

15.1.1 Abuse of Server Log Information

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

15.1.2 Transfer of Sensitive Information

 Like any generic data transfer protocol, HTTP cannot regulate the
 content of the data that is transferred, nor is there any a priori
 method of determining the sensitivity of any particular piece of
 information within the context of any given request. Therefore,
 applications SHOULD supply as much control over this information as
 possible to the provider of that information. Four header fields are
 worth special mention in this context: Server, Via, Referer and From.
 Revealing the specific software version of the server might allow the
 server machine to become more vulnerable to attacks against software
 that is known to contain security holes. Implementors SHOULD make the
 Server header field a configurable option.
 Proxies which serve as a portal through a network firewall SHOULD
 take special precautions regarding the transfer of header information
 that identifies the hosts behind the firewall. In particular, they
 SHOULD remove, or replace with sanitized versions, any Via fields
 generated behind the firewall.
 The Referer header allows reading patterns to be studied and reverse
 links drawn. Although it can be very useful, its power can be abused
 if user details are not separated from the information contained in

Fielding, et al. Standards Track [Page 151] RFC 2616 HTTP/1.1 June 1999

 the Referer. Even when the personal information has been removed, the
 Referer header might indicate a private document's URI whose
 publication would be inappropriate.
 The information sent in the From field might conflict with the user's
 privacy interests or their site's security policy, and hence it
 SHOULD NOT be transmitted without the user being able to disable,
 enable, and modify the contents of the field. The user MUST be able
 to set the contents of this field within a user preference or
 application defaults configuration.
 We suggest, though do not require, that a convenient toggle interface
 be provided for the user to enable or disable the sending of From and
 Referer information.
 The User-Agent (section 14.43) or Server (section 14.38) header
 fields can sometimes be used to determine that a specific client or
 server have a particular security hole which might be exploited.
 Unfortunately, this same information is often used for other valuable
 purposes for which HTTP currently has no better mechanism.

15.1.3 Encoding Sensitive Information in URI's

 Because the source of a link might be private information or might
 reveal an otherwise private information source, it is strongly
 recommended that the user be able to select whether or not the
 Referer field is sent. For example, a browser client could have a
 toggle switch for browsing openly/anonymously, which would
 respectively enable/disable the sending of Referer and From
 information.
 Clients SHOULD NOT include a Referer header field in a (non-secure)
 HTTP request if the referring page was transferred with a secure
 protocol.
 Authors of services which use the HTTP protocol SHOULD NOT use GET
 based forms for the submission of sensitive data, because this will
 cause this data to be encoded in the Request-URI. Many existing
 servers, proxies, and user agents will log the request URI in some
 place where it might be visible to third parties. Servers can use
 POST-based form submission instead

15.1.4 Privacy Issues Connected to Accept Headers

 Accept request-headers can reveal information about the user to all
 servers which are accessed. The Accept-Language header in particular
 can reveal information the user would consider to be of a private
 nature, because the understanding of particular languages is often

Fielding, et al. Standards Track [Page 152] RFC 2616 HTTP/1.1 June 1999

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

15.2 Attacks Based On File and Path Names

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

Fielding, et al. Standards Track [Page 153] RFC 2616 HTTP/1.1 June 1999

15.3 DNS Spoofing

 Clients using HTTP rely heavily on the Domain Name Service, and are
 thus generally prone to security attacks based on the deliberate
 mis-association of IP addresses and DNS names. Clients need to be
 cautious in assuming the continuing validity of an IP number/DNS name
 association.
 In particular, HTTP clients SHOULD rely on their name resolver for
 confirmation of an IP number/DNS name association, rather than
 caching the result of previous host name lookups. Many platforms
 already can cache host name lookups locally when appropriate, and
 they SHOULD be configured to do so. It is proper for these lookups to
 be cached, however, only when the TTL (Time To Live) information
 reported by the name server makes it likely that the cached
 information will remain useful.
 If HTTP clients cache the results of host name lookups in order to
 achieve a performance improvement, they MUST observe the TTL
 information reported by DNS.
 If HTTP clients do not observe this rule, they could be spoofed when
 a previously-accessed server's IP address changes. As network
 renumbering is expected to become increasingly common [24], the
 possibility of this form of attack will grow. Observing this
 requirement thus reduces this potential security vulnerability.
 This requirement also improves the load-balancing behavior of clients
 for replicated servers using the same DNS name and reduces the
 likelihood of a user's experiencing failure in accessing sites which
 use that strategy.

15.4 Location Headers and Spoofing

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

15.5 Content-Disposition Issues

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

Fielding, et al. Standards Track [Page 154] RFC 2616 HTTP/1.1 June 1999

15.6 Authentication Credentials and Idle Clients

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

which the server might wish to cause the client to reprompt the

      user for credentials.
  1. Applications which include a session termination indication

(such as a `logout' or `commit' button on a page) after which

      the server side of the application `knows' that there is no
      further reason for the client to retain the credentials.
 This is currently under separate study. There are a number of work-
 arounds to parts of this problem, and we encourage the use of
 password protection in screen savers, idle time-outs, and other
 methods which mitigate the security problems inherent in this
 problem. In particular, user agents which cache credentials are
 encouraged to provide a readily accessible mechanism for discarding
 cached credentials under user control.

15.7 Proxies and Caching

 By their very nature, HTTP proxies are men-in-the-middle, and
 represent an opportunity for man-in-the-middle attacks. Compromise of
 the systems on which the proxies run can result in serious security
 and privacy problems. Proxies have access to security-related
 information, personal information about individual users and
 organizations, and proprietary information belonging to users and
 content providers. A compromised proxy, or a proxy implemented or
 configured without regard to security and privacy considerations,
 might be used in the commission of a wide range of potential attacks.
 Proxy operators should protect the systems on which proxies run as
 they would protect any system that contains or transports sensitive
 information. In particular, log information gathered at proxies often
 contains highly sensitive personal information, and/or information
 about organizations. Log information should be carefully guarded, and
 appropriate guidelines for use developed and followed. (Section
 15.1.1).

Fielding, et al. Standards Track [Page 155] RFC 2616 HTTP/1.1 June 1999

 Caching proxies provide additional potential vulnerabilities, since
 the contents of the cache represent an attractive target for
 malicious exploitation. Because cache contents persist after an HTTP
 request is complete, an attack on the cache can reveal information
 long after a user believes that the information has been removed from
 the network. Therefore, cache contents should be protected as
 sensitive information.
 Proxy implementors should consider the privacy and security
 implications of their design and coding decisions, and of the
 configuration options they provide to proxy operators (especially the
 default configuration).
 Users of a proxy need to be aware that they are no trustworthier than
 the people who run the proxy; HTTP itself cannot solve this problem.
 The judicious use of cryptography, when appropriate, may suffice to
 protect against a broad range of security and privacy attacks. Such
 cryptography is beyond the scope of the HTTP/1.1 specification.

15.7.1 Denial of Service Attacks on Proxies

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

16 Acknowledgments

 This specification makes heavy use of the augmented BNF and generic
 constructs defined by David H. Crocker for RFC 822 [9]. Similarly, it
 reuses many of the definitions provided by Nathaniel Borenstein and
 Ned Freed for MIME [7]. We hope that their inclusion in this
 specification will help reduce past confusion over the relationship
 between HTTP and Internet mail message formats.
 The HTTP protocol has evolved considerably over the years. It has
 benefited from a large and active developer community--the many
 people who have participated on the www-talk mailing list--and it is
 that community which has been most responsible for the success of
 HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
 Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
 Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
 McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
 VanHeyningen deserve special recognition for their efforts in
 defining early aspects of the protocol.
 This document has benefited greatly from the comments of all those
 participating in the HTTP-WG. In addition to those already mentioned,
 the following individuals have contributed to this specification:

Fielding, et al. Standards Track [Page 156] RFC 2616 HTTP/1.1 June 1999

     Gary Adams                  Ross Patterson
     Harald Tveit Alvestrand     Albert Lunde
     Keith Ball                  John C. Mallery
     Brian Behlendorf            Jean-Philippe Martin-Flatin
     Paul Burchard               Mitra
     Maurizio Codogno            David Morris
     Mike Cowlishaw              Gavin Nicol
     Roman Czyborra              Bill Perry
     Michael A. Dolan            Jeffrey Perry
     David J. Fiander            Scott Powers
     Alan Freier                 Owen Rees
     Marc Hedlund                Luigi Rizzo
     Greg Herlihy                David Robinson
     Koen Holtman                Marc Salomon
     Alex Hopmann                Rich Salz
     Bob Jernigan                Allan M. Schiffman
     Shel Kaphan                 Jim Seidman
     Rohit Khare                 Chuck Shotton
     John Klensin                Eric W. Sink
     Martijn Koster              Simon E. Spero
     Alexei Kosut                Richard N. Taylor
     David M. Kristol            Robert S. Thau
     Daniel LaLiberte            Bill (BearHeart) Weinman
     Ben Laurie                  Francois Yergeau
     Paul J. Leach               Mary Ellen Zurko
     Daniel DuBois               Josh Cohen
 Much of the content and presentation of the caching design is due to
 suggestions and comments from individuals including: Shel Kaphan,
 Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
 Most of the specification of ranges is based on work originally done
 by Ari Luotonen and John Franks, with additional input from Steve
 Zilles.
 Thanks to the "cave men" of Palo Alto. You know who you are.
 Jim Gettys (the current editor of this document) wishes particularly
 to thank Roy Fielding, the previous editor of this document, along
 with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
 Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
 Larry Masinter for their help. And thanks go particularly to Jeff
 Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.

Fielding, et al. Standards Track [Page 157] RFC 2616 HTTP/1.1 June 1999

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

17 References

 [1] Alvestrand, H., "Tags for the Identification of Languages", RFC
     1766, March 1995.
 [2] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey,
     D. and B. Alberti, "The Internet Gopher Protocol (a distributed
     document search and retrieval protocol)", RFC 1436, March 1993.
 [3] Berners-Lee, T., "Universal Resource Identifiers in WWW", RFC
     1630, June 1994.
 [4] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform Resource
     Locators (URL)", RFC 1738, December 1994.
 [5] Berners-Lee, T. and D. Connolly, "Hypertext Markup Language -
     2.0", RFC 1866, November 1995.
 [6] Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext Transfer
     Protocol -- HTTP/1.0", RFC 1945, May 1996.
 [7] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
     Extensions (MIME) Part One: Format of Internet Message Bodies",
     RFC 2045, November 1996.
 [8] Braden, R., "Requirements for Internet Hosts -- Communication
     Layers", STD 3, RFC 1123, October 1989.
 [9] Crocker, D., "Standard for The Format of ARPA Internet Text
     Messages", STD 11, RFC 822, August 1982.
 [10] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R.,
      Sui, J., and M. Grinbaum, "WAIS Interface Protocol Prototype
      Functional Specification," (v1.5), Thinking Machines
      Corporation, April 1990.
 [11] Fielding, R., "Relative Uniform Resource Locators", RFC 1808,
      June 1995.
 [12] Horton, M. and R. Adams, "Standard for Interchange of USENET
      Messages", RFC 1036, December 1987.

Fielding, et al. Standards Track [Page 158] RFC 2616 HTTP/1.1 June 1999

 [13] Kantor, B. and P. Lapsley, "Network News Transfer Protocol", RFC
      977, February 1986.
 [14] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
      Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
      November 1996.
 [15] Nebel, E. and L. Masinter, "Form-based File Upload in HTML", RFC
      1867, November 1995.
 [16] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
      August 1982.
 [17] Postel, J., "Media Type Registration Procedure", RFC 1590,
      November 1996.
 [18] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC
      959, October 1985.
 [19] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
      October 1994.
 [20] Sollins, K. and L. Masinter, "Functional Requirements for
      Uniform Resource Names", RFC 1737, December 1994.
 [21] US-ASCII. Coded Character Set - 7-Bit American Standard Code for
      Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.
 [22] ISO-8859. International Standard -- Information Processing --
      8-bit Single-Byte Coded Graphic Character Sets --
      Part 1: Latin alphabet No. 1, ISO-8859-1:1987.
      Part 2: Latin alphabet No. 2, ISO-8859-2, 1987.
      Part 3: Latin alphabet No. 3, ISO-8859-3, 1988.
      Part 4: Latin alphabet No. 4, ISO-8859-4, 1988.
      Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988.
      Part 6: Latin/Arabic alphabet, ISO-8859-6, 1987.
      Part 7: Latin/Greek alphabet, ISO-8859-7, 1987.
      Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988.
      Part 9: Latin alphabet No. 5, ISO-8859-9, 1990.
 [23] Meyers, J. and M. Rose, "The Content-MD5 Header Field", RFC
      1864, October 1995.
 [24] Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", RFC
      1900, February 1996.
 [25] Deutsch, P., "GZIP file format specification version 4.3", RFC
      1952, May 1996.

Fielding, et al. Standards Track [Page 159] RFC 2616 HTTP/1.1 June 1999

 [26] Venkata N. Padmanabhan, and Jeffrey C. Mogul. "Improving HTTP
      Latency", Computer Networks and ISDN Systems, v. 28, pp. 25-35,
      Dec. 1995. Slightly revised version of paper in Proc. 2nd
      International WWW Conference '94: Mosaic and the Web, Oct. 1994,
      which is available at
      http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLat
      ency.html.
 [27] Joe Touch, John Heidemann, and Katia Obraczka. "Analysis of HTTP
      Performance", <URL: http://www.isi.edu/touch/pubs/http-perf96/>,
      ISI Research Report ISI/RR-98-463, (original report dated Aug.
      1996), USC/Information Sciences Institute, August 1998.
 [28] Mills, D., "Network Time Protocol (Version 3) Specification,
      Implementation and Analysis", RFC 1305, March 1992.
 [29] Deutsch, P., "DEFLATE Compressed Data Format Specification
      version 1.3", RFC 1951, May 1996.
 [30] S. Spero, "Analysis of HTTP Performance Problems,"
      http://sunsite.unc.edu/mdma-release/http-prob.html.
 [31] Deutsch, P. and J. Gailly, "ZLIB Compressed Data Format
      Specification version 3.3", RFC 1950, May 1996.
 [32] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
      Luotonen, A., Sink, E. and L. Stewart, "An Extension to HTTP:
      Digest Access Authentication", RFC 2069, January 1997.
 [33] Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.
      Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
      2068, January 1997.
 [34] Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [35] Troost, R. and Dorner, S., "Communicating Presentation
      Information in Internet Messages: The Content-Disposition
      Header", RFC 1806, June 1995.
 [36] Mogul, J., Fielding, R., Gettys, J. and H. Frystyk, "Use and
      Interpretation of HTTP Version Numbers", RFC 2145, May 1997.
      [jg639]
 [37] Palme, J., "Common Internet Message Headers", RFC 2076, February
      1997. [jg640]

Fielding, et al. Standards Track [Page 160] RFC 2616 HTTP/1.1 June 1999

 [38] Yergeau, F., "UTF-8, a transformation format of Unicode and
      ISO-10646", RFC 2279, January 1998. [jg641]
 [39] Nielsen, H.F., Gettys, J., Baird-Smith, A., Prud'hommeaux, E.,
      Lie, H., and C. Lilley. "Network Performance Effects of
      HTTP/1.1, CSS1, and PNG," Proceedings of ACM SIGCOMM '97, Cannes
      France, September 1997.[jg642]
 [40] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
      Extensions (MIME) Part Two: Media Types", RFC 2046, November
      1996. [jg643]
 [41] Alvestrand, H., "IETF Policy on Character Sets and Languages",
      BCP 18, RFC 2277, January 1998. [jg644]
 [42] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
      Identifiers (URI): Generic Syntax and Semantics", RFC 2396,
      August 1998. [jg645]
 [43] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
      Leach, P., Luotonen, A., Sink, E. and L. Stewart, "HTTP
      Authentication: Basic and Digest Access Authentication", RFC
      2617, June 1999. [jg646]
 [44] Luotonen, A., "Tunneling TCP based protocols through Web proxy
      servers," Work in Progress. [jg647]
 [45] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
      Aggregate Documents, such as HTML (MHTML)", RFC 2110, March
      1997.
 [46] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.
 [47] Masinter, L., "Hyper Text Coffee Pot Control Protocol
      (HTCPCP/1.0)", RFC 2324, 1 April 1998.
 [48] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
      Extensions (MIME) Part Five: Conformance Criteria and Examples",
      RFC 2049, November 1996.
 [49] Troost, R., Dorner, S. and K. Moore, "Communicating Presentation
      Information in Internet Messages: The Content-Disposition Header
      Field", RFC 2183, August 1997.

Fielding, et al. Standards Track [Page 161] RFC 2616 HTTP/1.1 June 1999

18 Authors' Addresses

 Roy T. Fielding
 Information and Computer Science
 University of California, Irvine
 Irvine, CA 92697-3425, USA
 Fax: +1 (949) 824-1715
 EMail: fielding@ics.uci.edu
 James Gettys
 World Wide Web Consortium
 MIT Laboratory for Computer Science
 545 Technology Square
 Cambridge, MA 02139, USA
 Fax: +1 (617) 258 8682
 EMail: jg@w3.org
 Jeffrey C. Mogul
 Western Research Laboratory
 Compaq Computer Corporation
 250 University Avenue
 Palo Alto, California, 94305, USA
 EMail: mogul@wrl.dec.com
 Henrik Frystyk Nielsen
 World Wide Web Consortium
 MIT Laboratory for Computer Science
 545 Technology Square
 Cambridge, MA 02139, USA
 Fax: +1 (617) 258 8682
 EMail: frystyk@w3.org
 Larry Masinter
 Xerox Corporation
 3333 Coyote Hill Road
 Palo Alto, CA 94034, USA
 EMail: masinter@parc.xerox.com

Fielding, et al. Standards Track [Page 162] RFC 2616 HTTP/1.1 June 1999

 Paul J. Leach
 Microsoft Corporation
 1 Microsoft Way
 Redmond, WA 98052, USA
 EMail: paulle@microsoft.com
 Tim Berners-Lee
 Director, World Wide Web Consortium
 MIT Laboratory for Computer Science
 545 Technology Square
 Cambridge, MA 02139, USA
 Fax: +1 (617) 258 8682
 EMail: timbl@w3.org

Fielding, et al. Standards Track [Page 163] RFC 2616 HTTP/1.1 June 1999

19 Appendices

19.1 Internet Media Type message/http and application/http

 In addition to defining the HTTP/1.1 protocol, this document serves
 as the specification for the Internet media type "message/http" and
 "application/http". The message/http type can be used to enclose a
 single HTTP request or response message, provided that it obeys the
 MIME restrictions for all "message" types regarding line length and
 encodings. The application/http type can be used to enclose a
 pipeline of one or more HTTP request or response messages (not
 intermixed). The following is to be registered with IANA [17].
     Media Type name:         message
     Media subtype name:      http
     Required parameters:     none
     Optional parameters:     version, msgtype
      version: The HTTP-Version number of the enclosed message
               (e.g., "1.1"). If not present, the version can be
               determined from the first line of the body.
      msgtype: The message type -- "request" or "response". If not
               present, the type can be determined from the first
               line of the body.
     Encoding considerations: only "7bit", "8bit", or "binary" are
                              permitted
     Security considerations: none
     Media Type name:         application
     Media subtype name:      http
     Required parameters:     none
     Optional parameters:     version, msgtype
      version: The HTTP-Version number of the enclosed messages
               (e.g., "1.1"). If not present, the version can be
               determined from the first line of the body.
      msgtype: The message type -- "request" or "response". If not
               present, the type can be determined from the first
               line of the body.
     Encoding considerations: HTTP messages enclosed by this type
               are in "binary" format; use of an appropriate
               Content-Transfer-Encoding is required when
               transmitted via E-mail.
     Security considerations: none

Fielding, et al. Standards Track [Page 164] RFC 2616 HTTP/1.1 June 1999

19.2 Internet Media Type multipart/byteranges

 When an HTTP 206 (Partial Content) response message includes the
 content of multiple ranges (a response to a request for multiple
 non-overlapping ranges), these are transmitted as a multipart
 message-body. The media type for this purpose is called
 "multipart/byteranges".
 The multipart/byteranges media type includes two or more parts, each
 with its own Content-Type and Content-Range fields. The required
 boundary parameter specifies the boundary string used to separate
 each body-part.
     Media Type name:         multipart
     Media subtype name:      byteranges
     Required parameters:     boundary
     Optional parameters:     none
     Encoding considerations: only "7bit", "8bit", or "binary" are
                              permitted
     Security considerations: none
 For example:
 HTTP/1.1 206 Partial Content
 Date: Wed, 15 Nov 1995 06:25:24 GMT
 Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
 Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES
  1. -THIS_STRING_SEPARATES

Content-type: application/pdf

 Content-range: bytes 500-999/8000
 ...the first range...
 --THIS_STRING_SEPARATES
 Content-type: application/pdf
 Content-range: bytes 7000-7999/8000
 ...the second range
 --THIS_STRING_SEPARATES--
    Notes:
    1) Additional CRLFs may precede the first boundary string in the
       entity.

Fielding, et al. Standards Track [Page 165] RFC 2616 HTTP/1.1 June 1999

    2) Although RFC 2046 [40] permits the boundary string to be
       quoted, some existing implementations handle a quoted boundary
       string incorrectly.
    3) A number of browsers and servers were coded to an early draft
       of the byteranges specification to use a media type of
       multipart/x-byteranges, which is almost, but not quite
       compatible with the version documented in HTTP/1.1.

19.3 Tolerant Applications

 Although this document specifies the requirements for the generation
 of HTTP/1.1 messages, not all applications will be correct in their
 implementation. We therefore recommend that operational applications
 be tolerant of deviations whenever those deviations can be
 interpreted unambiguously.
 Clients SHOULD be tolerant in parsing the Status-Line and servers
 tolerant when parsing the Request-Line. In particular, they SHOULD
 accept any amount of SP or HT characters between fields, even though
 only a single SP is required.
 The line terminator for message-header fields is the sequence CRLF.
 However, we recommend that applications, when parsing such headers,
 recognize a single LF as a line terminator and ignore the leading CR.
 The character set of an entity-body SHOULD be labeled as the lowest
 common denominator of the character codes used within that body, with
 the exception that not labeling the entity is preferred over labeling
 the entity with the labels US-ASCII or ISO-8859-1. See section 3.7.1
 and 3.4.1.
 Additional rules for requirements on parsing and encoding of dates
 and other potential problems with date encodings include:
  1. 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).
  1. 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.
  1. All expiration-related calculations MUST be done in GMT. The

local time zone MUST NOT influence the calculation or comparison

      of an age or expiration time.

Fielding, et al. Standards Track [Page 166] RFC 2616 HTTP/1.1 June 1999

  1. If an HTTP header incorrectly carries a date value with a time

zone other than GMT, it MUST be converted into GMT using the

      most conservative possible conversion.

19.4 Differences Between HTTP Entities and RFC 2045 Entities

 HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC
 822 [9]) and the Multipurpose Internet Mail Extensions (MIME [7]) to
 allow entities to be transmitted in an open variety of
 representations and with extensible mechanisms. However, RFC 2045
 discusses mail, and HTTP has a few features that are different from
 those described in RFC 2045. These differences were carefully chosen
 to optimize performance over binary connections, to allow greater
 freedom in the use of new media types, to make date comparisons
 easier, and to acknowledge the practice of some early HTTP servers
 and clients.
 This appendix describes specific areas where HTTP differs from RFC
 2045. Proxies and gateways to strict MIME environments SHOULD be
 aware of these differences and provide the appropriate conversions
 where necessary. Proxies and gateways from MIME environments to HTTP
 also need to be aware of the differences because some conversions
 might be required.

19.4.1 MIME-Version

 HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages MAY
 include a single MIME-Version general-header field to indicate what
 version of the MIME protocol was used to construct the message. Use
 of the MIME-Version header field indicates that the message is in
 full compliance with the MIME protocol (as defined in RFC 2045[7]).
 Proxies/gateways are responsible for ensuring full compliance (where
 possible) when exporting HTTP messages to strict MIME environments.
     MIME-Version   = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
 MIME version "1.0" is the default for use in HTTP/1.1. However,
 HTTP/1.1 message parsing and semantics are defined by this document
 and not the MIME specification.

19.4.2 Conversion to Canonical Form

 RFC 2045 [7] requires that an Internet mail entity be converted to
 canonical form prior to being transferred, as described in section 4
 of RFC 2049 [48]. Section 3.7.1 of this document describes the forms
 allowed for subtypes of the "text" media type when transmitted over
 HTTP. RFC 2046 requires that content with a type of "text" represent
 line breaks as CRLF and forbids the use of CR or LF outside of line

Fielding, et al. Standards Track [Page 167] RFC 2616 HTTP/1.1 June 1999

 break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a
 line break within text content when a message is transmitted over
 HTTP.
 Where it is possible, a proxy or gateway from HTTP to a strict MIME
 environment SHOULD translate all line breaks within the text media
 types described in section 3.7.1 of this document to the RFC 2049
 canonical form of CRLF. Note, however, that this might be complicated
 by the presence of a Content-Encoding and by the fact that HTTP
 allows the use of some character sets which do not use octets 13 and
 10 to represent CR and LF, as is the case for some multi-byte
 character sets.
 Implementors should note that conversion will break any cryptographic
 checksums applied to the original content unless the original content
 is already in canonical form. Therefore, the canonical form is
 recommended for any content that uses such checksums in HTTP.

19.4.3 Conversion of Date Formats

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

19.4.4 Introduction of Content-Encoding

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

19.4.5 No Content-Transfer-Encoding

 HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC
 2045. Proxies and gateways from MIME-compliant protocols to HTTP MUST
 remove any non-identity CTE ("quoted-printable" or "base64") encoding
 prior to delivering the response message to an HTTP client.
 Proxies and gateways from HTTP to MIME-compliant protocols are
 responsible for ensuring that the message is in the correct format
 and encoding for safe transport on that protocol, where "safe

Fielding, et al. Standards Track [Page 168] RFC 2616 HTTP/1.1 June 1999

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

19.4.6 Introduction of Transfer-Encoding

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

19.4.7 MHTML and Line Length Limitations

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

19.5 Additional Features

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

Fielding, et al. Standards Track [Page 169] RFC 2616 HTTP/1.1 June 1999

 describe proposed experimental features, and some describe features
 that experimental deployment found lacking that are now addressed in
 the base HTTP/1.1 specification.
 A number of other headers, such as Content-Disposition and Title,
 from SMTP and MIME are also often implemented (see RFC 2076 [37]).

19.5.1 Content-Disposition

 The Content-Disposition response-header field has been proposed as a
 means for the origin server to suggest a default filename if the user
 requests that the content is saved to a file. This usage is derived
 from the definition of Content-Disposition in RFC 1806 [35].
      content-disposition = "Content-Disposition" ":"
                            disposition-type *( ";" disposition-parm )
      disposition-type = "attachment" | disp-extension-token
      disposition-parm = filename-parm | disp-extension-parm
      filename-parm = "filename" "=" quoted-string
      disp-extension-token = token
      disp-extension-parm = token "=" ( token | quoted-string )
 An example is
      Content-Disposition: attachment; filename="fname.ext"
 The receiving user agent SHOULD NOT respect any directory path
 information present in the filename-parm parameter, which is the only
 parameter believed to apply to HTTP implementations at this time. The
 filename SHOULD be treated as a terminal component only.
 If this header is used in a response with the application/octet-
 stream content-type, the implied suggestion is that the user agent
 should not display the response, but directly enter a `save response
 as...' dialog.
 See section 15.5 for Content-Disposition security issues.

19.6 Compatibility with Previous Versions

 It is beyond the scope of a protocol specification to mandate
 compliance with previous versions. HTTP/1.1 was deliberately
 designed, however, to make supporting previous versions easy. It is
 worth noting that, at the time of composing this specification
 (1996), we would expect commercial HTTP/1.1 servers to:
  1. recognize the format of the Request-Line for HTTP/0.9, 1.0, and

1.1 requests;

Fielding, et al. Standards Track [Page 170] RFC 2616 HTTP/1.1 June 1999

  1. understand any valid request in the format of HTTP/0.9, 1.0, or

1.1;

  1. respond appropriately with a message in the same major version

used by the client.

 And we would expect HTTP/1.1 clients to:
  1. recognize the format of the Status-Line for HTTP/1.0 and 1.1

responses;

  1. understand any valid response in the format of HTTP/0.9, 1.0, or

1.1.

 For most implementations of HTTP/1.0, each connection is established
 by the client prior to the request and closed by the server after
 sending the response. Some implementations implement the Keep-Alive
 version of persistent connections described in section 19.7.1 of RFC
 2068 [33].

19.6.1 Changes from HTTP/1.0

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

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

       Addresses
 The requirements that clients and servers support the Host request-
 header, report an error if the Host request-header (section 14.23) is
 missing from an HTTP/1.1 request, and accept absolute URIs (section
 5.1.2) are among the most important changes defined by this
 specification.
 Older HTTP/1.0 clients assumed a one-to-one relationship of IP
 addresses and servers; there was no other established mechanism for
 distinguishing the intended server of a request than the IP address
 to which that request was directed. The changes outlined above will
 allow the Internet, once older HTTP clients are no longer common, to
 support multiple Web sites from a single IP address, greatly
 simplifying large operational Web servers, where allocation of many
 IP addresses to a single host has created serious problems. The
 Internet will also be able to recover the IP addresses that have been
 allocated for the sole purpose of allowing special-purpose domain
 names to be used in root-level HTTP URLs. Given the rate of growth of
 the Web, and the number of servers already deployed, it is extremely

Fielding, et al. Standards Track [Page 171] RFC 2616 HTTP/1.1 June 1999

 important that all implementations of HTTP (including updates to
 existing HTTP/1.0 applications) correctly implement these
 requirements:
  1. Both clients and servers MUST support the Host request-header.
  1. A client that sends an HTTP/1.1 request MUST send a Host header.
  1. Servers MUST report a 400 (Bad Request) error if an HTTP/1.1

request does not include a Host request-header.

  1. Servers MUST accept absolute URIs.

19.6.2 Compatibility with HTTP/1.0 Persistent Connections

 Some clients and servers might wish to be compatible with some
 previous implementations of persistent connections in HTTP/1.0
 clients and servers. Persistent connections in HTTP/1.0 are
 explicitly negotiated as they are not the default behavior. HTTP/1.0
 experimental implementations of persistent connections are faulty,
 and the new facilities in HTTP/1.1 are designed to rectify these
 problems. The problem was that some existing 1.0 clients may be
 sending Keep-Alive to a proxy server that doesn't understand
 Connection, which would then erroneously forward it to the next
 inbound server, which would establish the Keep-Alive connection and
 result in a hung HTTP/1.0 proxy waiting for the close on the
 response. The result is that HTTP/1.0 clients must be prevented from
 using Keep-Alive when talking to proxies.
 However, talking to proxies is the most important use of persistent
 connections, so that prohibition is clearly unacceptable. Therefore,
 we need some other mechanism for indicating a persistent connection
 is desired, which is safe to use even when talking to an old proxy
 that ignores Connection. Persistent connections are the default for
 HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
 declaring non-persistence. See section 14.10.
 The original HTTP/1.0 form of persistent connections (the Connection:
 Keep-Alive and Keep-Alive header) is documented in RFC 2068. [33]

19.6.3 Changes from RFC 2068

 This specification has been carefully audited to correct and
 disambiguate key word usage; RFC 2068 had many problems in respect to
 the conventions laid out in RFC 2119 [34].
 Clarified which error code should be used for inbound server failures
 (e.g. DNS failures). (Section 10.5.5).

Fielding, et al. Standards Track [Page 172] RFC 2616 HTTP/1.1 June 1999

 CREATE had a race that required an Etag be sent when a resource is
 first created. (Section 10.2.2).
 Content-Base was deleted from the specification: it was not
 implemented widely, and there is no simple, safe way to introduce it
 without a robust extension mechanism. In addition, it is used in a
 similar, but not identical fashion in MHTML [45].
 Transfer-coding and message lengths all interact in ways that
 required fixing exactly when chunked encoding is used (to allow for
 transfer encoding that may not be self delimiting); it was important
 to straighten out exactly how message lengths are computed. (Sections
 3.6, 4.4, 7.2.2, 13.5.2, 14.13, 14.16)
 A content-coding of "identity" was introduced, to solve problems
 discovered in caching. (section 3.5)
 Quality Values of zero should indicate that "I don't want something"
 to allow clients to refuse a representation. (Section 3.9)
 The use and interpretation of HTTP version numbers has been clarified
 by RFC 2145. Require proxies to upgrade requests to highest protocol
 version they support to deal with problems discovered in HTTP/1.0
 implementations (Section 3.1)
 Charset wildcarding is introduced to avoid explosion of character set
 names in accept headers. (Section 14.2)
 A case was missed in the Cache-Control model of HTTP/1.1; s-maxage
 was introduced to add this missing case. (Sections 13.4, 14.8, 14.9,
 14.9.3)
 The Cache-Control: max-age directive was not properly defined for
 responses. (Section 14.9.3)
 There are situations where a server (especially a proxy) does not
 know the full length of a response but is capable of serving a
 byterange request. We therefore need a mechanism to allow byteranges
 with a content-range not indicating the full length of the message.
 (Section 14.16)
 Range request responses would become very verbose if all meta-data
 were always returned; by allowing the server to only send needed
 headers in a 206 response, this problem can be avoided. (Section
 10.2.7, 13.5.3, and 14.27)

Fielding, et al. Standards Track [Page 173] RFC 2616 HTTP/1.1 June 1999

 Fix problem with unsatisfiable range requests; there are two cases:
 syntactic problems, and range doesn't exist in the document. The 416
 status code was needed to resolve this ambiguity needed to indicate
 an error for a byte range request that falls outside of the actual
 contents of a document. (Section 10.4.17, 14.16)
 Rewrite of message transmission requirements to make it much harder
 for implementors to get it wrong, as the consequences of errors here
 can have significant impact on the Internet, and to deal with the
 following problems:
    1. Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where
       this was incorrectly placing a requirement on the behavior of
       an implementation of a future version of HTTP/1.x
    2. Made it clear that user-agents should retry requests, not
       "clients" in general.
    3. Converted requirements for clients to ignore unexpected 100
       (Continue) responses, and for proxies to forward 100 responses,
       into a general requirement for 1xx responses.
    4. Modified some TCP-specific language, to make it clearer that
       non-TCP transports are possible for HTTP.
    5. Require that the origin server MUST NOT wait for the request
       body before it sends a required 100 (Continue) response.
    6. Allow, rather than require, a server to omit 100 (Continue) if
       it has already seen some of the request body.
    7. Allow servers to defend against denial-of-service attacks and
       broken clients.
 This change adds the Expect header and 417 status code. The message
 transmission requirements fixes are in sections 8.2, 10.4.18,
 8.1.2.2, 13.11, and 14.20.
 Proxies should be able to add Content-Length when appropriate.
 (Section 13.5.2)
 Clean up confusion between 403 and 404 responses. (Section 10.4.4,
 10.4.5, and 10.4.11)
 Warnings could be cached incorrectly, or not updated appropriately.
 (Section 13.1.2, 13.2.4, 13.5.2, 13.5.3, 14.9.3, and 14.46) Warning
 also needed to be a general header, as PUT or other methods may have
 need for it in requests.

Fielding, et al. Standards Track [Page 174] RFC 2616 HTTP/1.1 June 1999

 Transfer-coding had significant problems, particularly with
 interactions with chunked encoding. The solution is that transfer-
 codings become as full fledged as content-codings. This involves
 adding an IANA registry for transfer-codings (separate from content
 codings), a new header field (TE) and enabling trailer headers in the
 future. Transfer encoding is a major performance benefit, so it was
 worth fixing [39]. TE also solves another, obscure, downward
 interoperability problem that could have occurred due to interactions
 between authentication trailers, chunked encoding and HTTP/1.0
 clients.(Section 3.6, 3.6.1, and 14.39)
 The PATCH, LINK, UNLINK methods were defined but not commonly
 implemented in previous versions of this specification. See RFC 2068
 [33].
 The Alternates, Content-Version, Derived-From, Link, URI, Public and
 Content-Base header fields were defined in previous versions of this
 specification, but not commonly implemented. See RFC 2068 [33].

20 Index

 Please see the PostScript version of this RFC for the INDEX.

Fielding, et al. Standards Track [Page 175] RFC 2616 HTTP/1.1 June 1999

21. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
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Fielding, et al. Standards Track [Page 176]

/data/webs/external/dokuwiki/data/pages/rfc/rfc2616.txt · Last modified: 1999/06/11 18:46 (external edit)