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authorKlaus Weidenbach <Klaus.Weidenbach@gmx.net>2014-06-28 22:28:08 +0200
committerKlaus Weidenbach <Klaus.Weidenbach@gmx.net>2014-06-29 01:17:07 +0200
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-
-
-
-
-
-
-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]
-
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
-3.2.1 General Syntax
-
- URIs in HTTP can be represented in absolute form or relative to some
- known base URI [11], depending upon the context of their use. The two
- forms are differentiated by the fact that absolute URIs always begin
- with a scheme name followed by a colon. For definitive information on
- URL syntax and semantics, see "Uniform Resource Identifiers (URI):
- Generic Syntax and Semantics," RFC 2396 [42] (which replaces RFCs
- 1738 [4] and RFC 1808 [11]). This specification adopts the
- definitions of "URI-reference", "absoluteURI", "relativeURI", "port",
- "host","abs_path", "rel_path", and "authority" from that
- specification.
-
- The HTTP protocol does not place any a priori limit on the length of
- a URI. Servers MUST be able to handle the URI of any resource they
- serve, and SHOULD be able to handle URIs of unbounded length if they
- provide GET-based forms that could generate such URIs. A server
- SHOULD return 414 (Request-URI Too Long) status if a URI is longer
- than the server can handle (see section 10.4.15).
-
- Note: Servers ought to be cautious about depending on URI lengths
- above 255 bytes, because some older client or proxy
- implementations might not properly support these lengths.
-
-3.2.2 http URL
-
- The "http" scheme is used to locate network resources via the HTTP
- protocol. This section defines the scheme-specific syntax and
- semantics for http URLs.
-
- http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
-
- If the port is empty or not given, port 80 is assumed. The semantics
- are that the identified resource is located at the server listening
- for TCP connections on that port of that host, and the Request-URI
- for the resource is abs_path (section 5.1.2). The use of IP addresses
- in URLs SHOULD be avoided whenever possible (see RFC 1900 [24]). If
- the abs_path is not present in the URL, it MUST be given as "/" when
- used as a Request-URI for a resource (section 5.1.2). If a proxy
- receives a host name which is not a fully qualified domain name, it
- MAY add its domain to the host name it received. If a proxy receives
- a fully qualified domain name, the proxy MUST NOT change the host
- name.
-
-
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 19]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
-3.2.3 URI Comparison
-
- When comparing two URIs to decide if they match or not, a client
- SHOULD use a case-sensitive octet-by-octet comparison of the entire
- URIs, with these exceptions:
-
- - A port that is empty or not given is equivalent to the default
- port for that URI-reference;
-
- - Comparisons of host names MUST be case-insensitive;
-
- - Comparisons of scheme names MUST be case-insensitive;
-
- - An empty abs_path is equivalent to an abs_path of "/".
-
- Characters other than those in the "reserved" and "unsafe" sets (see
- RFC 2396 [42]) are equivalent to their ""%" HEX HEX" encoding.
-
- For example, the following three URIs are equivalent:
-
- http://abc.com:80/~smith/home.html
- http://ABC.com/%7Esmith/home.html
- http://ABC.com:/%7esmith/home.html
-
-3.3 Date/Time Formats
-
-3.3.1 Full Date
-
- HTTP applications have historically allowed three different formats
- for the representation of date/time stamps:
-
- Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123
- Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
- Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
-
- The first format is preferred as an Internet standard and represents
- a fixed-length subset of that defined by RFC 1123 [8] (an update to
- RFC 822 [9]). The second format is in common use, but is based on the
- obsolete RFC 850 [12] date format and lacks a four-digit year.
- HTTP/1.1 clients and servers that parse the date value MUST accept
- all three formats (for compatibility with HTTP/1.0), though they MUST
- only generate the RFC 1123 format for representing HTTP-date values
- in header fields. See section 19.3 for further information.
-
- Note: Recipients of date values are encouraged to be robust in
- accepting date values that may have been sent by non-HTTP
- applications, as is sometimes the case when retrieving or posting
- messages via proxies/gateways to SMTP or NNTP.
-
-
-
-Fielding, et al. Standards Track [Page 20]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- All HTTP date/time stamps MUST be represented in Greenwich Mean Time
- (GMT), without exception. For the purposes of HTTP, GMT is exactly
- equal to UTC (Coordinated Universal Time). This is indicated in the
- first two formats by the inclusion of "GMT" as the three-letter
- abbreviation for time zone, and MUST be assumed when reading the
- asctime format. HTTP-date is case sensitive and MUST NOT include
- additional LWS beyond that specifically included as SP in the
- grammar.
-
- HTTP-date = rfc1123-date | rfc850-date | asctime-date
- rfc1123-date = wkday "," SP date1 SP time SP "GMT"
- rfc850-date = weekday "," SP date2 SP time SP "GMT"
- asctime-date = wkday SP date3 SP time SP 4DIGIT
- date1 = 2DIGIT SP month SP 4DIGIT
- ; day month year (e.g., 02 Jun 1982)
- date2 = 2DIGIT "-" month "-" 2DIGIT
- ; day-month-year (e.g., 02-Jun-82)
- date3 = month SP ( 2DIGIT | ( SP 1DIGIT ))
- ; month day (e.g., Jun 2)
- time = 2DIGIT ":" 2DIGIT ":" 2DIGIT
- ; 00:00:00 - 23:59:59
- wkday = "Mon" | "Tue" | "Wed"
- | "Thu" | "Fri" | "Sat" | "Sun"
- weekday = "Monday" | "Tuesday" | "Wednesday"
- | "Thursday" | "Friday" | "Saturday" | "Sunday"
- month = "Jan" | "Feb" | "Mar" | "Apr"
- | "May" | "Jun" | "Jul" | "Aug"
- | "Sep" | "Oct" | "Nov" | "Dec"
-
- Note: HTTP requirements for the date/time stamp format apply only
- to their usage within the protocol stream. Clients and servers are
- not required to use these formats for user presentation, request
- logging, etc.
-
-3.3.2 Delta Seconds
-
- Some HTTP header fields allow a time value to be specified as an
- integer number of seconds, represented in decimal, after the time
- that the message was received.
-
- delta-seconds = 1*DIGIT
-
-3.4 Character Sets
-
- HTTP uses the same definition of the term "character set" as that
- described for MIME:
-
-
-
-
-
-Fielding, et al. Standards Track [Page 21]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- The term "character set" is used in this document to refer to a
- method used with one or more tables to convert a sequence of octets
- into a sequence of characters. Note that unconditional conversion in
- the other direction is not required, in that not all characters may
- be available in a given character set and a character set may provide
- more than one sequence of octets to represent a particular character.
- This definition is intended to allow various kinds of character
- encoding, from simple single-table mappings such as US-ASCII to
- complex table switching methods such as those that use ISO-2022's
- techniques. However, the definition associated with a MIME character
- set name MUST fully specify the mapping to be performed from octets
- to characters. In particular, use of external profiling information
- to determine the exact mapping is not permitted.
-
- Note: This use of the term "character set" is more commonly
- referred to as a "character encoding." However, since HTTP and
- MIME share the same registry, it is important that the terminology
- also be shared.
-
- HTTP character sets are identified by case-insensitive tokens. The
- complete set of tokens is defined by the IANA Character Set registry
- [19].
-
- charset = token
-
- Although HTTP allows an arbitrary token to be used as a charset
- value, any token that has a predefined value within the IANA
- Character Set registry [19] MUST represent the character set defined
- by that registry. Applications SHOULD limit their use of character
- sets to those defined by the IANA registry.
-
- Implementors should be aware of IETF character set requirements [38]
- [41].
-
-3.4.1 Missing Charset
-
- Some HTTP/1.0 software has interpreted a Content-Type header without
- charset parameter incorrectly to mean "recipient should guess."
- Senders wishing to defeat this behavior MAY include a charset
- parameter even when the charset is ISO-8859-1 and SHOULD do so when
- it is known that it will not confuse the recipient.
-
- Unfortunately, some older HTTP/1.0 clients did not deal properly with
- an explicit charset parameter. HTTP/1.1 recipients MUST respect the
- charset label provided by the sender; and those user agents that have
- a provision to "guess" a charset MUST use the charset from the
-
-
-
-
-
-Fielding, et al. Standards Track [Page 22]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- content-type field if they support that charset, rather than the
- recipient's preference, when initially displaying a document. See
- section 3.7.1.
-
-3.5 Content Codings
-
- Content coding values indicate an encoding transformation that has
- been or can be applied to an entity. Content codings are primarily
- used to allow a document to be compressed or otherwise usefully
- transformed without losing the identity of its underlying media type
- and without loss of information. Frequently, the entity is stored in
- coded form, transmitted directly, and only decoded by the recipient.
-
- content-coding = token
-
- All content-coding values are case-insensitive. HTTP/1.1 uses
- content-coding values in the Accept-Encoding (section 14.3) and
- Content-Encoding (section 14.11) header fields. Although the value
- describes the content-coding, what is more important is that it
- indicates what decoding mechanism will be required to remove the
- encoding.
-
- The Internet Assigned Numbers Authority (IANA) acts as a registry for
- content-coding value tokens. Initially, the registry contains the
- following tokens:
-
- gzip An encoding format produced by the file compression program
- "gzip" (GNU zip) as described in RFC 1952 [25]. This format is a
- Lempel-Ziv coding (LZ77) with a 32 bit CRC.
-
- compress
- The encoding format produced by the common UNIX file compression
- program "compress". This format is an adaptive Lempel-Ziv-Welch
- coding (LZW).
-
- Use of program names for the identification of encoding formats
- is not desirable and is discouraged for future encodings. Their
- use here is representative of historical practice, not good
- design. For compatibility with previous implementations of HTTP,
- applications SHOULD consider "x-gzip" and "x-compress" to be
- equivalent to "gzip" and "compress" respectively.
-
- deflate
- The "zlib" format defined in RFC 1950 [31] in combination with
- the "deflate" compression mechanism described in RFC 1951 [29].
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 23]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- identity
- The default (identity) encoding; the use of no transformation
- whatsoever. This content-coding is used only in the Accept-
- Encoding header, and SHOULD NOT be used in the Content-Encoding
- header.
-
- New content-coding value tokens SHOULD be registered; to allow
- interoperability between clients and servers, specifications of the
- content coding algorithms needed to implement a new value SHOULD be
- publicly available and adequate for independent implementation, and
- conform to the purpose of content coding defined in this section.
-
-3.6 Transfer Codings
-
- Transfer-coding values are used to indicate an encoding
- transformation that has been, can be, or may need to be applied to an
- entity-body in order to ensure "safe transport" through the network.
- This differs from a content coding in that the transfer-coding is a
- property of the message, not of the original entity.
-
- transfer-coding = "chunked" | transfer-extension
- transfer-extension = token *( ";" parameter )
-
- Parameters are in the form of attribute/value pairs.
-
- parameter = attribute "=" value
- attribute = token
- value = token | quoted-string
-
- All transfer-coding values are case-insensitive. HTTP/1.1 uses
- transfer-coding values in the TE header field (section 14.39) and in
- the Transfer-Encoding header field (section 14.41).
-
- Whenever a transfer-coding is applied to a message-body, the set of
- transfer-codings MUST include "chunked", unless the message is
- terminated by closing the connection. When the "chunked" transfer-
- coding is used, it MUST be the last transfer-coding applied to the
- message-body. The "chunked" transfer-coding MUST NOT be applied more
- than once to a message-body. These rules allow the recipient to
- determine the transfer-length of the message (section 4.4).
-
- Transfer-codings are analogous to the Content-Transfer-Encoding
- values of MIME [7], which were designed to enable safe transport of
- binary data over a 7-bit transport service. However, safe transport
- has a different focus for an 8bit-clean transfer protocol. In HTTP,
- the only unsafe characteristic of message-bodies is the difficulty in
- determining the exact body length (section 7.2.2), or the desire to
- encrypt data over a shared transport.
-
-
-
-Fielding, et al. Standards Track [Page 24]
-
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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]
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
- [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]
-
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
- | If-Modified-Since ; Section 14.25
- | If-None-Match ; Section 14.26
- | If-Range ; Section 14.27
- | If-Unmodified-Since ; Section 14.28
- | Max-Forwards ; Section 14.31
- | Proxy-Authorization ; Section 14.34
- | Range ; Section 14.35
- | Referer ; Section 14.36
- | TE ; Section 14.39
- | User-Agent ; Section 14.43
-
- Request-header field names can be extended reliably only in
- combination with a change in the protocol version. However, new or
- experimental header fields MAY be given the semantics of request-
- header fields if all parties in the communication recognize them to
- be request-header fields. Unrecognized header fields are treated as
- entity-header fields.
-
-6 Response
-
- After receiving and interpreting a request message, a server responds
- with an HTTP response message.
-
- Response = Status-Line ; Section 6.1
- *(( general-header ; Section 4.5
- | response-header ; Section 6.2
- | entity-header ) CRLF) ; Section 7.1
- CRLF
- [ message-body ] ; Section 7.2
-
-6.1 Status-Line
-
- The first line of a Response message is the Status-Line, consisting
- of the protocol version followed by a numeric status code and its
- associated textual phrase, with each element separated by SP
- characters. No CR or LF is allowed except in the final CRLF sequence.
-
- Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
-
-6.1.1 Status Code and Reason Phrase
-
- The Status-Code element is a 3-digit integer result code of the
- attempt to understand and satisfy the request. These codes are fully
- defined in section 10. The Reason-Phrase is intended to give a short
- textual description of the Status-Code. The Status-Code is intended
- for use by automata and the Reason-Phrase is intended for the human
- user. The client is not required to examine or display the Reason-
- Phrase.
-
-
-
-Fielding, et al. Standards Track [Page 39]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- The first digit of the Status-Code defines the class of response. The
- last two digits do not have any categorization role. There are 5
- values for the first digit:
-
- - 1xx: Informational - Request received, continuing process
-
- - 2xx: Success - The action was successfully received,
- understood, and accepted
-
- - 3xx: Redirection - Further action must be taken in order to
- complete the request
-
- - 4xx: Client Error - The request contains bad syntax or cannot
- be fulfilled
-
- - 5xx: Server Error - The server failed to fulfill an apparently
- valid request
-
- The individual values of the numeric status codes defined for
- HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
- presented below. The reason phrases listed here are only
- recommendations -- they MAY be replaced by local equivalents without
- affecting the protocol.
-
- Status-Code =
- "100" ; Section 10.1.1: Continue
- | "101" ; Section 10.1.2: Switching Protocols
- | "200" ; Section 10.2.1: OK
- | "201" ; Section 10.2.2: Created
- | "202" ; Section 10.2.3: Accepted
- | "203" ; Section 10.2.4: Non-Authoritative Information
- | "204" ; Section 10.2.5: No Content
- | "205" ; Section 10.2.6: Reset Content
- | "206" ; Section 10.2.7: Partial Content
- | "300" ; Section 10.3.1: Multiple Choices
- | "301" ; Section 10.3.2: Moved Permanently
- | "302" ; Section 10.3.3: Found
- | "303" ; Section 10.3.4: See Other
- | "304" ; Section 10.3.5: Not Modified
- | "305" ; Section 10.3.6: Use Proxy
- | "307" ; Section 10.3.8: Temporary Redirect
- | "400" ; Section 10.4.1: Bad Request
- | "401" ; Section 10.4.2: Unauthorized
- | "402" ; Section 10.4.3: Payment Required
- | "403" ; Section 10.4.4: Forbidden
- | "404" ; Section 10.4.5: Not Found
- | "405" ; Section 10.4.6: Method Not Allowed
- | "406" ; Section 10.4.7: Not Acceptable
-
-
-
-Fielding, et al. Standards Track [Page 40]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- | "407" ; Section 10.4.8: Proxy Authentication Required
- | "408" ; Section 10.4.9: Request Time-out
- | "409" ; Section 10.4.10: Conflict
- | "410" ; Section 10.4.11: Gone
- | "411" ; Section 10.4.12: Length Required
- | "412" ; Section 10.4.13: Precondition Failed
- | "413" ; Section 10.4.14: Request Entity Too Large
- | "414" ; Section 10.4.15: Request-URI Too Large
- | "415" ; Section 10.4.16: Unsupported Media Type
- | "416" ; Section 10.4.17: Requested range not satisfiable
- | "417" ; Section 10.4.18: Expectation Failed
- | "500" ; Section 10.5.1: Internal Server Error
- | "501" ; Section 10.5.2: Not Implemented
- | "502" ; Section 10.5.3: Bad Gateway
- | "503" ; Section 10.5.4: Service Unavailable
- | "504" ; Section 10.5.5: Gateway Time-out
- | "505" ; Section 10.5.6: HTTP Version not supported
- | extension-code
-
- extension-code = 3DIGIT
- Reason-Phrase = *<TEXT, excluding CR, LF>
-
- HTTP status codes are extensible. HTTP applications are not required
- to understand the meaning of all registered status codes, though such
- understanding is obviously desirable. However, applications MUST
- understand the class of any status code, as indicated by the first
- digit, and treat any unrecognized response as being equivalent to the
- x00 status code of that class, with the exception that an
- unrecognized response MUST NOT be cached. For example, if an
- unrecognized status code of 431 is received by the client, it can
- safely assume that there was something wrong with its request and
- treat the response as if it had received a 400 status code. In such
- cases, user agents SHOULD present to the user the entity returned
- with the response, since that entity is likely to include human-
- readable information which will explain the unusual status.
-
-6.2 Response Header Fields
-
- The response-header fields allow the server to pass additional
- information about the response which cannot be placed in the Status-
- Line. These header fields give information about the server and about
- further access to the resource identified by the Request-URI.
-
- response-header = Accept-Ranges ; Section 14.5
- | Age ; Section 14.6
- | ETag ; Section 14.19
- | Location ; Section 14.30
- | Proxy-Authenticate ; Section 14.33
-
-
-
-Fielding, et al. Standards Track [Page 41]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- | Retry-After ; Section 14.37
- | Server ; Section 14.38
- | Vary ; Section 14.44
- | WWW-Authenticate ; Section 14.47
-
- Response-header field names can be extended reliably only in
- combination with a change in the protocol version. However, new or
- experimental header fields MAY be given the semantics of response-
- header fields if all parties in the communication recognize them to
- be response-header fields. Unrecognized header fields are treated as
- entity-header fields.
-
-7 Entity
-
- Request and Response messages MAY transfer an entity if not otherwise
- restricted by the request method or response status code. An entity
- consists of entity-header fields and an entity-body, although some
- responses will only include the entity-headers.
-
- In this section, both sender and recipient refer to either the client
- or the server, depending on who sends and who receives the entity.
-
-7.1 Entity Header Fields
-
- Entity-header fields define metainformation about the entity-body or,
- if no body is present, about the resource identified by the request.
- Some of this metainformation is OPTIONAL; some might be REQUIRED by
- portions of this specification.
-
- entity-header = Allow ; Section 14.7
- | Content-Encoding ; Section 14.11
- | Content-Language ; Section 14.12
- | Content-Length ; Section 14.13
- | Content-Location ; Section 14.14
- | Content-MD5 ; Section 14.15
- | Content-Range ; Section 14.16
- | Content-Type ; Section 14.17
- | Expires ; Section 14.21
- | Last-Modified ; Section 14.29
- | extension-header
-
- extension-header = message-header
-
- The extension-header mechanism allows additional entity-header fields
- to be defined without changing the protocol, but these fields cannot
- be assumed to be recognizable by the recipient. Unrecognized header
- fields SHOULD be ignored by the recipient and MUST be forwarded by
- transparent proxies.
-
-
-
-Fielding, et al. Standards Track [Page 42]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
-7.2 Entity Body
-
- The entity-body (if any) sent with an HTTP request or response is in
- a format and encoding defined by the entity-header fields.
-
- entity-body = *OCTET
-
- An entity-body is only present in a message when a message-body is
- present, as described in section 4.3. The entity-body is obtained
- from the message-body by decoding any Transfer-Encoding that might
- have been applied to ensure safe and proper transfer of the message.
-
-7.2.1 Type
-
- When an entity-body is included with a message, the data type of that
- body is determined via the header fields Content-Type and Content-
- Encoding. These define a two-layer, ordered encoding model:
-
- entity-body := Content-Encoding( Content-Type( data ) )
-
- Content-Type specifies the media type of the underlying data.
- Content-Encoding may be used to indicate any additional content
- codings applied to the data, usually for the purpose of data
- compression, that are a property of the requested resource. There is
- no default encoding.
-
- Any HTTP/1.1 message containing an entity-body SHOULD include a
- Content-Type header field defining the media type of that body. If
- and only if the media type is not given by a Content-Type field, the
- recipient MAY attempt to guess the media type via inspection of its
- content and/or the name extension(s) of the URI used to identify the
- resource. If the media type remains unknown, the recipient SHOULD
- treat it as type "application/octet-stream".
-
-7.2.2 Entity Length
-
- The entity-length of a message is the length of the message-body
- before any transfer-codings have been applied. Section 4.4 defines
- how the transfer-length of a message-body is determined.
-
-
-
-
-
-
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 43]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
-8 Connections
-
-8.1 Persistent Connections
-
-8.1.1 Purpose
-
- Prior to persistent connections, a separate TCP connection was
- established to fetch each URL, increasing the load on HTTP servers
- and causing congestion on the Internet. The use of inline images and
- other associated data often require a client to make multiple
- requests of the same server in a short amount of time. Analysis of
- these performance problems and results from a prototype
- implementation are available [26] [30]. Implementation experience and
- measurements of actual HTTP/1.1 (RFC 2068) implementations show good
- results [39]. Alternatives have also been explored, for example,
- T/TCP [27].
-
- Persistent HTTP connections have a number of advantages:
-
- - By opening and closing fewer TCP connections, CPU time is saved
- in routers and hosts (clients, servers, proxies, gateways,
- tunnels, or caches), and memory used for TCP protocol control
- blocks can be saved in hosts.
-
- - HTTP requests and responses can be pipelined on a connection.
- Pipelining allows a client to make multiple requests without
- waiting for each response, allowing a single TCP connection to
- be used much more efficiently, with much lower elapsed time.
-
- - Network congestion is reduced by reducing the number of packets
- caused by TCP opens, and by allowing TCP sufficient time to
- determine the congestion state of the network.
-
- - Latency on subsequent requests is reduced since there is no time
- spent in TCP's connection opening handshake.
-
- - HTTP can evolve more gracefully, since errors can be reported
- without the penalty of closing the TCP connection. Clients using
- future versions of HTTP might optimistically try a new feature,
- but if communicating with an older server, retry with old
- semantics after an error is reported.
-
- HTTP implementations SHOULD implement persistent connections.
-
-
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 44]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
-8.1.2 Overall Operation
-
- A significant difference between HTTP/1.1 and earlier versions of
- HTTP is that persistent connections are the default behavior of any
- HTTP connection. That is, unless otherwise indicated, the client
- SHOULD assume that the server will maintain a persistent connection,
- even after error responses from the server.
-
- Persistent connections provide a mechanism by which a client and a
- server can signal the close of a TCP connection. This signaling takes
- place using the Connection header field (section 14.10). Once a close
- has been signaled, the client MUST NOT send any more requests on that
- connection.
-
-8.1.2.1 Negotiation
-
- An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to
- maintain a persistent connection unless a Connection header including
- the connection-token "close" was sent in the request. If the server
- chooses to close the connection immediately after sending the
- response, it SHOULD send a Connection header including the
- connection-token close.
-
- An HTTP/1.1 client MAY expect a connection to remain open, but would
- decide to keep it open based on whether the response from a server
- contains a Connection header with the connection-token close. In case
- the client does not want to maintain a connection for more than that
- request, it SHOULD send a Connection header including the
- connection-token close.
-
- If either the client or the server sends the close token in the
- Connection header, that request becomes the last one for the
- connection.
-
- Clients and servers SHOULD NOT assume that a persistent connection is
- maintained for HTTP versions less than 1.1 unless it is explicitly
- signaled. See section 19.6.2 for more information on backward
- compatibility with HTTP/1.0 clients.
-
- In order to remain persistent, all messages on the connection MUST
- have a self-defined message length (i.e., one not defined by closure
- of the connection), as described in section 4.4.
-
-
-
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 45]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
-8.1.2.2 Pipelining
-
- A client that supports persistent connections MAY "pipeline" its
- requests (i.e., send multiple requests without waiting for each
- response). A server MUST send its responses to those requests in the
- same order that the requests were received.
-
- Clients which assume persistent connections and pipeline immediately
- after connection establishment SHOULD be prepared to retry their
- connection if the first pipelined attempt fails. If a client does
- such a retry, it MUST NOT pipeline before it knows the connection is
- persistent. Clients MUST also be prepared to resend their requests if
- the server closes the connection before sending all of the
- corresponding responses.
-
- Clients SHOULD NOT pipeline requests using non-idempotent methods or
- non-idempotent sequences of methods (see section 9.1.2). Otherwise, a
- premature termination of the transport connection could lead to
- indeterminate results. A client wishing to send a non-idempotent
- request SHOULD wait to send that request until it has received the
- response status for the previous request.
-
-8.1.3 Proxy Servers
-
- It is especially important that proxies correctly implement the
- properties of the Connection header field as specified in section
- 14.10.
-
- The proxy server MUST signal persistent connections separately with
- its clients and the origin servers (or other proxy servers) that it
- connects to. Each persistent connection applies to only one transport
- link.
-
- A proxy server MUST NOT establish a HTTP/1.1 persistent connection
- with an HTTP/1.0 client (but see RFC 2068 [33] for information and
- discussion of the problems with the Keep-Alive header implemented by
- many HTTP/1.0 clients).
-
-8.1.4 Practical Considerations
-
- Servers will usually have some time-out value beyond which they will
- no longer maintain an inactive connection. Proxy servers might make
- this a higher value since it is likely that the client will be making
- more connections through the same server. The use of persistent
- connections places no requirements on the length (or existence) of
- this time-out for either the client or the server.
-
-
-
-
-
-Fielding, et al. Standards Track [Page 46]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- When a client or server wishes to time-out it SHOULD issue a graceful
- close on the transport connection. Clients and servers SHOULD both
- constantly watch for the other side of the transport close, and
- respond to it as appropriate. If a client or server does not detect
- the other side's close promptly it could cause unnecessary resource
- drain on the network.
-
- A client, server, or proxy MAY close the transport connection at any
- time. For example, a client might have started to send a new request
- at the same time that the server has decided to close the "idle"
- connection. From the server's point of view, the connection is being
- closed while it was idle, but from the client's point of view, a
- request is in progress.
-
- This means that clients, servers, and proxies MUST be able to recover
- from asynchronous close events. Client software SHOULD reopen the
- transport connection and retransmit the aborted sequence of requests
- without user interaction so long as the request sequence is
- idempotent (see section 9.1.2). Non-idempotent methods or sequences
- MUST NOT be automatically retried, although user agents MAY offer a
- human operator the choice of retrying the request(s). Confirmation by
- user-agent software with semantic understanding of the application
- MAY substitute for user confirmation. The automatic retry SHOULD NOT
- be repeated if the second sequence of requests fails.
-
- Servers SHOULD always respond to at least one request per connection,
- if at all possible. Servers SHOULD NOT close a connection in the
- middle of transmitting a response, unless a network or client failure
- is suspected.
-
- Clients that use persistent connections SHOULD limit the number of
- simultaneous connections that they maintain to a given server. A
- single-user client SHOULD NOT maintain more than 2 connections with
- any server or proxy. A proxy SHOULD use up to 2*N connections to
- another server or proxy, where N is the number of simultaneously
- active users. These guidelines are intended to improve HTTP response
- times and avoid congestion.
-
-8.2 Message Transmission Requirements
-
-8.2.1 Persistent Connections and Flow Control
-
- HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's
- flow control mechanisms to resolve temporary overloads, rather than
- terminating connections with the expectation that clients will retry.
- The latter technique can exacerbate network congestion.
-
-
-
-
-
-Fielding, et al. Standards Track [Page 47]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
-8.2.2 Monitoring Connections for Error Status Messages
-
- An HTTP/1.1 (or later) client sending a message-body SHOULD monitor
- the network connection for an error status while it is transmitting
- the request. If the client sees an error status, it SHOULD
- immediately cease transmitting the body. If the body is being sent
- using a "chunked" encoding (section 3.6), a zero length chunk and
- empty trailer MAY be used to prematurely mark the end of the message.
- If the body was preceded by a Content-Length header, the client MUST
- close the connection.
-
-8.2.3 Use of the 100 (Continue) Status
-
- The purpose of the 100 (Continue) status (see section 10.1.1) is to
- allow a client that is sending a request message with a request body
- to determine if the origin server is willing to accept the request
- (based on the request headers) before the client sends the request
- body. In some cases, it might either be inappropriate or highly
- inefficient for the client to send the body if the server will reject
- the message without looking at the body.
-
- Requirements for HTTP/1.1 clients:
-
- - If a client will wait for a 100 (Continue) response before
- sending the request body, it MUST send an Expect request-header
- field (section 14.20) with the "100-continue" expectation.
-
- - A client MUST NOT send an Expect request-header field (section
- 14.20) with the "100-continue" expectation if it does not intend
- to send a request body.
-
- Because of the presence of older implementations, the protocol allows
- ambiguous situations in which a client may send "Expect: 100-
- continue" without receiving either a 417 (Expectation Failed) status
- or a 100 (Continue) status. Therefore, when a client sends this
- header field to an origin server (possibly via a proxy) from which it
- has never seen a 100 (Continue) status, the client SHOULD NOT wait
- for an indefinite period before sending the request body.
-
- Requirements for HTTP/1.1 origin servers:
-
- - Upon receiving a request which includes an Expect request-header
- field with the "100-continue" expectation, an origin server MUST
- either respond with 100 (Continue) status and continue to read
- from the input stream, or respond with a final status code. The
- origin server MUST NOT wait for the request body before sending
- the 100 (Continue) response. If it responds with a final status
- code, it MAY close the transport connection or it MAY continue
-
-
-
-Fielding, et al. Standards Track [Page 48]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- to read and discard the rest of the request. It MUST NOT
- perform the requested method if it returns a final status code.
-
- - An origin server SHOULD NOT send a 100 (Continue) response if
- the request message does not include an Expect request-header
- field with the "100-continue" expectation, and MUST NOT send a
- 100 (Continue) response if such a request comes from an HTTP/1.0
- (or earlier) client. There is an exception to this rule: for
- compatibility with RFC 2068, a server MAY send a 100 (Continue)
- status in response to an HTTP/1.1 PUT or POST request that does
- not include an Expect request-header field with the "100-
- continue" expectation. This exception, the purpose of which is
- to minimize any client processing delays associated with an
- undeclared wait for 100 (Continue) status, applies only to
- HTTP/1.1 requests, and not to requests with any other HTTP-
- version value.
-
- - An origin server MAY omit a 100 (Continue) response if it has
- already received some or all of the request body for the
- corresponding request.
-
- - An origin server that sends a 100 (Continue) response MUST
- ultimately send a final status code, once the request body is
- received and processed, unless it terminates the transport
- connection prematurely.
-
- - If an origin server receives a request that does not include an
- Expect request-header field with the "100-continue" expectation,
- the request includes a request body, and the server responds
- with a final status code before reading the entire request body
- from the transport connection, then the server SHOULD NOT close
- the transport connection until it has read the entire request,
- or until the client closes the connection. Otherwise, the client
- might not reliably receive the response message. However, this
- requirement is not be construed as preventing a server from
- defending itself against denial-of-service attacks, or from
- badly broken client implementations.
-
- Requirements for HTTP/1.1 proxies:
-
- - If a proxy receives a request that includes an Expect request-
- header field with the "100-continue" expectation, and the proxy
- either knows that the next-hop server complies with HTTP/1.1 or
- higher, or does not know the HTTP version of the next-hop
- server, it MUST forward the request, including the Expect header
- field.
-
-
-
-
-
-Fielding, et al. Standards Track [Page 49]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- - If the proxy knows that the version of the next-hop server is
- HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
- respond with a 417 (Expectation Failed) status.
-
- - Proxies SHOULD maintain a cache recording the HTTP version
- numbers received from recently-referenced next-hop servers.
-
- - A proxy MUST NOT forward a 100 (Continue) response if the
- request message was received from an HTTP/1.0 (or earlier)
- client and did not include an Expect request-header field with
- the "100-continue" expectation. This requirement overrides the
- general rule for forwarding of 1xx responses (see section 10.1).
-
-8.2.4 Client Behavior if Server Prematurely Closes Connection
-
- If an HTTP/1.1 client sends a request which includes a request body,
- but which does not include an Expect request-header field with the
- "100-continue" expectation, and if the client is not directly
- connected to an HTTP/1.1 origin server, and if the client sees the
- connection close before receiving any status from the server, the
- client SHOULD retry the request. If the client does retry this
- request, it MAY use the following "binary exponential backoff"
- algorithm to be assured of obtaining a reliable response:
-
- 1. Initiate a new connection to the server
-
- 2. Transmit the request-headers
-
- 3. Initialize a variable R to the estimated round-trip time to the
- server (e.g., based on the time it took to establish the
- connection), or to a constant value of 5 seconds if the round-
- trip time is not available.
-
- 4. Compute T = R * (2**N), where N is the number of previous
- retries of this request.
-
- 5. Wait either for an error response from the server, or for T
- seconds (whichever comes first)
-
- 6. If no error response is received, after T seconds transmit the
- body of the request.
-
- 7. If client sees that the connection is closed prematurely,
- repeat from step 1 until the request is accepted, an error
- response is received, or the user becomes impatient and
- terminates the retry process.
-
-
-
-
-
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-
-
- If at any point an error status is received, the client
-
- - SHOULD NOT continue and
-
- - SHOULD close the connection if it has not completed sending the
- request message.
-
-9 Method Definitions
-
- The set of common methods for HTTP/1.1 is defined below. Although
- this set can be expanded, additional methods cannot be assumed to
- share the same semantics for separately extended clients and servers.
-
- The Host request-header field (section 14.23) MUST accompany all
- HTTP/1.1 requests.
-
-9.1 Safe and Idempotent Methods
-
-9.1.1 Safe Methods
-
- Implementors should be aware that the software represents the user in
- their interactions over the Internet, and should be careful to allow
- the user to be aware of any actions they might take which may have an
- unexpected significance to themselves or others.
-
- In particular, the convention has been established that the GET and
- HEAD methods SHOULD NOT have the significance of taking an action
- other than retrieval. These methods ought to be considered "safe".
- This allows user agents to represent other methods, such as POST, PUT
- and DELETE, in a special way, so that the user is made aware of the
- fact that a possibly unsafe action is being requested.
-
- Naturally, it is not possible to ensure that the server does not
- generate side-effects as a result of performing a GET request; in
- fact, some dynamic resources consider that a feature. The important
- distinction here is that the user did not request the side-effects,
- so therefore cannot be held accountable for them.
-
-9.1.2 Idempotent Methods
-
- Methods can also have the property of "idempotence" in that (aside
- from error or expiration issues) the side-effects of N > 0 identical
- requests is the same as for a single request. The methods GET, HEAD,
- PUT and DELETE share this property. Also, the methods OPTIONS and
- TRACE SHOULD NOT have side effects, and so are inherently idempotent.
-
-
-
-
-
-
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-
-
- 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
-
-
-
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-
-
- 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.
-
-
-
-
-
-
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-
-
-9.4 HEAD
-
- The HEAD method is identical to GET except that the server MUST NOT
- return a message-body in the response. The metainformation contained
- in the HTTP headers in response to a HEAD request SHOULD be identical
- to the information sent in response to a GET request. This method can
- be used for obtaining metainformation about the entity implied by the
- request without transferring the entity-body itself. This method is
- often used for testing hypertext links for validity, accessibility,
- and recent modification.
-
- The response to a HEAD request MAY be cacheable in the sense that the
- information contained in the response MAY be used to update a
- previously cached entity from that resource. If the new field values
- indicate that the cached entity differs from the current entity (as
- would be indicated by a change in Content-Length, Content-MD5, ETag
- or Last-Modified), then the cache MUST treat the cache entry as
- stale.
-
-9.5 POST
-
- The POST method is used to request that the origin server accept the
- entity enclosed in the request as a new subordinate of the resource
- identified by the Request-URI in the Request-Line. POST is designed
- to allow a uniform method to cover the following functions:
-
- - Annotation of existing resources;
-
- - Posting a message to a bulletin board, newsgroup, mailing list,
- or similar group of articles;
-
- - Providing a block of data, such as the result of submitting a
- form, to a data-handling process;
-
- - Extending a database through an append operation.
-
- The actual function performed by the POST method is determined by the
- server and is usually dependent on the Request-URI. The posted entity
- is subordinate to that URI in the same way that a file is subordinate
- to a directory containing it, a news article is subordinate to a
- newsgroup to which it is posted, or a record is subordinate to a
- database.
-
- The action performed by the POST method might not result in a
- resource that can be identified by a URI. In this case, either 200
- (OK) or 204 (No Content) is the appropriate response status,
- depending on whether or not the response includes an entity that
- describes the result.
-
-
-
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-
-
- 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,
-
-
-
-
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-
-
- 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
-
-
-
-
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-
-
- 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
-
-
-
-
-
-
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-
-
- 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;
-
-
-
-
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-
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-
-
- 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:
-
- - Either a Content-Range header field (section 14.16) indicating
- the range included with this response, or a multipart/byteranges
- Content-Type including Content-Range fields for each part. If a
- Content-Length header field is present in the response, its
- value MUST match the actual number of OCTETs transmitted in the
- message-body.
-
- - Date
-
- - ETag and/or Content-Location, if the header would have been sent
- in a 200 response to the same request
-
-
-
-
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-
-
- - 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:
-
- - 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.
-
- - ETag and/or Content-Location, if the header would have been sent
- in a 200 response to the same request
-
- - Expires, Cache-Control, and/or Vary, if the field-value might
- differ from that sent in any previous response for the same
- variant
-
- If the conditional GET used a strong cache validator (see section
- 13.3.3), the response SHOULD NOT include other entity-headers.
- Otherwise (i.e., the conditional GET used a weak validator), the
- response MUST NOT include other entity-headers; this prevents
- inconsistencies between cached entity-bodies and updated headers.
-
- If a 304 response indicates an entity not currently cached, then the
- cache MUST disregard the response and repeat the request without the
- conditional.
-
- If a cache uses a received 304 response to update a cache entry, the
- cache MUST update the entry to reflect any new field values given in
- the response.
-
-10.3.6 305 Use Proxy
-
- The requested resource MUST be accessed through the proxy given by
- the Location field. The Location field gives the URI of the proxy.
- The recipient is expected to repeat this single request via the
- proxy. 305 responses MUST only be generated by origin servers.
-
- Note: RFC 2068 was not clear that 305 was intended to redirect a
- single request, and to be generated by origin servers only. Not
- observing these limitations has significant security consequences.
-
-10.3.7 306 (Unused)
-
- The 306 status code was used in a previous version of the
- specification, is no longer used, and the code is reserved.
-
-
-
-
-
-
-
-
-
-
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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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-
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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.
-
-
-
-
-
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-
-
- 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.
-
-
-
-
-
-
-
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-
-
- 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,
-
-
-
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-
-
- and clients to explicitly reduce transparency when necessary.
- However, because non-transparent operation may confuse non-expert
- users, and might be incompatible with certain server applications
- (such as those for ordering merchandise), the protocol requires that
- transparency be relaxed
-
- - only by an explicit protocol-level request when relaxed by
- client or origin server
-
- - only with an explicit warning to the end user when relaxed by
- cache or client
-
- Therefore, the HTTP/1.1 protocol provides these important elements:
-
- 1. Protocol features that provide full semantic transparency when
- this is required by all parties.
-
- 2. Protocol features that allow an origin server or user agent to
- explicitly request and control non-transparent operation.
-
- 3. Protocol features that allow a cache to attach warnings to
- responses that do not preserve the requested approximation of
- semantic transparency.
-
- A basic principle is that it must be possible for the clients to
- detect any potential relaxation of semantic transparency.
-
- Note: The server, cache, or client implementor might be faced with
- design decisions not explicitly discussed in this specification.
- If a decision might affect semantic transparency, the implementor
- ought to err on the side of maintaining transparency unless a
- careful and complete analysis shows significant benefits in
- breaking transparency.
-
-13.1.1 Cache Correctness
-
- A correct cache MUST respond to a request with the most up-to-date
- response held by the cache that is appropriate to the request (see
- sections 13.2.5, 13.2.6, and 13.12) which meets one of the following
- conditions:
-
- 1. It has been checked for equivalence with what the origin server
- would have returned by revalidating the response with the
- origin server (section 13.3);
-
-
-
-
-
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-
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-
-
- 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]
-
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-
-
- 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,
-
-
-
-
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-
-
- 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.
-
-
-
-
-
-
-
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-
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-
-
- 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.
-
-
-
-
-
-
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-
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-
-
- 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.
-
-
-
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-
-
- 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
-
-
-
-
-
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-
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-
-
- 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
-
-
-
-
-
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-
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-
-
- 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)
-
-
-
-
-
-
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-
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-
-
-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]
-
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-
-
- 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.
-
-
-
-
-
-
-
-
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-
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-
-
- 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]
-
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-
-
- An entity's modification time, if represented with one-second
- resolution, could be a weak validator, since it is possible that
- the resource might be modified twice during a single second.
-
- Support for weak validators is optional. However, weak validators
- allow for more efficient caching of equivalent objects; for
- example, a hit counter on a site is probably good enough if it is
- updated every few days or weeks, and any value during that period
- is likely "good enough" to be equivalent.
-
- A "use" of a validator is either when a client generates a request
- and includes the validator in a validating header field, or when a
- server compares two validators.
-
- Strong validators are usable in any context. Weak validators are only
- usable in contexts that do not depend on exact equality of an entity.
- For example, either kind is usable for a conditional GET of a full
- entity. However, only a strong validator is usable for a sub-range
- retrieval, since otherwise the client might end up with an internally
- inconsistent entity.
-
- Clients MAY issue simple (non-subrange) GET requests with either weak
- validators or strong validators. Clients MUST NOT use weak validators
- in other forms of request.
-
- The only function that the HTTP/1.1 protocol defines on validators is
- comparison. There are two validator comparison functions, depending
- on whether the comparison context allows the use of weak validators
- or not:
-
- - The strong comparison function: in order to be considered equal,
- both validators MUST be identical in every way, and both MUST
- NOT be weak.
-
- - The weak comparison function: in order to be considered equal,
- both validators MUST be identical in every way, but either or
- both of them MAY be tagged as "weak" without affecting the
- result.
-
- An entity tag is strong unless it is explicitly tagged as weak.
- Section 3.11 gives the syntax for entity tags.
-
- A Last-Modified time, when used as a validator in a request, is
- implicitly weak unless it is possible to deduce that it is strong,
- using the following rules:
-
- - The validator is being compared by an origin server to the
- actual current validator for the entity and,
-
-
-
-Fielding, et al. Standards Track [Page 87]
-
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-
-
- - That origin server reliably knows that the associated entity did
- not change twice during the second covered by the presented
- validator.
-
- or
-
- - The validator is about to be used by a client in an If-
- Modified-Since or If-Unmodified-Since header, because the client
- has a cache entry for the associated entity, and
-
- - That cache entry includes a Date value, which gives the time
- when the origin server sent the original response, and
-
- - The presented Last-Modified time is at least 60 seconds before
- the Date value.
-
- or
-
- - The validator is being compared by an intermediate cache to the
- validator stored in its cache entry for the entity, and
-
- - That cache entry includes a Date value, which gives the time
- when the origin server sent the original response, and
-
- - The presented Last-Modified time is at least 60 seconds before
- the Date value.
-
- This method relies on the fact that if two different responses were
- sent by the origin server during the same second, but both had the
- same Last-Modified time, then at least one of those responses would
- have a Date value equal to its Last-Modified time. The arbitrary 60-
- second limit guards against the possibility that the Date and Last-
- Modified values are generated from different clocks, or at somewhat
- different times during the preparation of the response. An
- implementation MAY use a value larger than 60 seconds, if it is
- believed that 60 seconds is too short.
-
- If a client wishes to perform a sub-range retrieval on a value for
- which it has only a Last-Modified time and no opaque validator, it
- MAY do this only if the Last-Modified time is strong in the sense
- described here.
-
- A cache or origin server receiving a conditional request, other than
- a full-body GET request, MUST use the strong comparison function to
- evaluate the condition.
-
- These rules allow HTTP/1.1 caches and clients to safely perform sub-
- range retrievals on values that have been obtained from HTTP/1.0
-
-
-
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-
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-
-
- servers.
-
-13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates
-
- We adopt a set of rules and recommendations for origin servers,
- clients, and caches regarding when various validator types ought to
- be used, and for what purposes.
-
- HTTP/1.1 origin servers:
-
- - SHOULD send an entity tag validator unless it is not feasible to
- generate one.
-
- - MAY send a weak entity tag instead of a strong entity tag, if
- performance considerations support the use of weak entity tags,
- or if it is unfeasible to send a strong entity tag.
-
- - SHOULD send a Last-Modified value if it is feasible to send one,
- unless the risk of a breakdown in semantic transparency that
- could result from using this date in an If-Modified-Since header
- would lead to serious problems.
-
- In other words, the preferred behavior for an HTTP/1.1 origin server
- is to send both a strong entity tag and a Last-Modified value.
-
- In order to be legal, a strong entity tag MUST change whenever the
- associated entity value changes in any way. A weak entity tag SHOULD
- change whenever the associated entity changes in a semantically
- significant way.
-
- Note: in order to provide semantically transparent caching, an
- origin server must avoid reusing a specific strong entity tag
- value for two different entities, or reusing a specific weak
- entity tag value for two semantically different entities. Cache
- entries might persist for arbitrarily long periods, regardless of
- expiration times, so it might be inappropriate to expect that a
- cache will never again attempt to validate an entry using a
- validator that it obtained at some point in the past.
-
- HTTP/1.1 clients:
-
- - If an entity tag has been provided by the origin server, MUST
- use that entity tag in any cache-conditional request (using If-
- Match or If-None-Match).
-
- - If only a Last-Modified value has been provided by the origin
- server, SHOULD use that value in non-subrange cache-conditional
- requests (using If-Modified-Since).
-
-
-
-Fielding, et al. Standards Track [Page 89]
-
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-
-
- - If only a Last-Modified value has been provided by an HTTP/1.0
- origin server, MAY use that value in subrange cache-conditional
- requests (using If-Unmodified-Since:). The user agent SHOULD
- provide a way to disable this, in case of difficulty.
-
- - If both an entity tag and a Last-Modified value have been
- provided by the origin server, SHOULD use both validators in
- cache-conditional requests. This allows both HTTP/1.0 and
- HTTP/1.1 caches to respond appropriately.
-
- An HTTP/1.1 origin server, upon receiving a conditional request that
- includes both a Last-Modified date (e.g., in an If-Modified-Since or
- If-Unmodified-Since header field) and one or more entity tags (e.g.,
- in an If-Match, If-None-Match, or If-Range header field) as cache
- validators, MUST NOT return a response status of 304 (Not Modified)
- unless doing so is consistent with all of the conditional header
- fields in the request.
-
- An HTTP/1.1 caching proxy, upon receiving a conditional request that
- includes both a Last-Modified date and one or more entity tags as
- cache validators, MUST NOT return a locally cached response to the
- client unless that cached response is consistent with all of the
- conditional header fields in the request.
-
- Note: The general principle behind these rules is that HTTP/1.1
- servers and clients should transmit as much non-redundant
- information as is available in their responses and requests.
- HTTP/1.1 systems receiving this information will make the most
- conservative assumptions about the validators they receive.
-
- HTTP/1.0 clients and caches will ignore entity tags. Generally,
- last-modified values received or used by these systems will
- support transparent and efficient caching, and so HTTP/1.1 origin
- servers should provide Last-Modified values. In those rare cases
- where the use of a Last-Modified value as a validator by an
- HTTP/1.0 system could result in a serious problem, then HTTP/1.1
- origin servers should not provide one.
-
-13.3.5 Non-validating Conditionals
-
- The principle behind entity tags is that only the service author
- knows the semantics of a resource well enough to select an
- appropriate cache validation mechanism, and the specification of any
- validator comparison function more complex than byte-equality would
- open up a can of worms. Thus, comparisons of any other headers
- (except Last-Modified, for compatibility with HTTP/1.0) are never
- used for purposes of validating a cache entry.
-
-
-
-
-Fielding, et al. Standards Track [Page 90]
-
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-
-
-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]
-
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-
-
-13.5 Constructing Responses From Caches
-
- The purpose of an HTTP cache is to store information received in
- response to requests for use in responding to future requests. In
- many cases, a cache simply returns the appropriate parts of a
- response to the requester. However, if the cache holds a cache entry
- based on a previous response, it might have to combine parts of a new
- response with what is held in the cache entry.
-
-13.5.1 End-to-end and Hop-by-hop Headers
-
- For the purpose of defining the behavior of caches and non-caching
- proxies, we divide HTTP headers into two categories:
-
- - End-to-end headers, which are transmitted to the ultimate
- recipient of a request or response. End-to-end headers in
- responses MUST be stored as part of a cache entry and MUST be
- transmitted in any response formed from a cache entry.
-
- - Hop-by-hop headers, which are meaningful only for a single
- transport-level connection, and are not stored by caches or
- forwarded by proxies.
-
- The following HTTP/1.1 headers are hop-by-hop headers:
-
- - Connection
- - Keep-Alive
- - Proxy-Authenticate
- - Proxy-Authorization
- - TE
- - Trailers
- - Transfer-Encoding
- - Upgrade
-
- All other headers defined by HTTP/1.1 are end-to-end headers.
-
- Other hop-by-hop headers MUST be listed in a Connection header,
- (section 14.10) to be introduced into HTTP/1.1 (or later).
-
-13.5.2 Non-modifiable Headers
-
- Some features of the HTTP/1.1 protocol, such as Digest
- Authentication, depend on the value of certain end-to-end headers. A
- transparent proxy SHOULD NOT modify an end-to-end header unless the
- definition of that header requires or specifically allows that.
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 92]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- A transparent proxy MUST NOT modify any of the following fields in a
- request or response, and it MUST NOT add any of these fields if not
- already present:
-
- - Content-Location
-
- - Content-MD5
-
- - ETag
-
- - Last-Modified
-
- A transparent proxy MUST NOT modify any of the following fields in a
- response:
-
- - Expires
-
- but it MAY add any of these fields if not already present. If an
- Expires header is added, it MUST be given a field-value identical to
- that of the Date header in that response.
-
- A proxy MUST NOT modify or add any of the following fields in a
- message that contains the no-transform cache-control directive, or in
- any request:
-
- - Content-Encoding
-
- - Content-Range
-
- - Content-Type
-
- A non-transparent proxy MAY modify or add these fields to a message
- that does not include no-transform, but if it does so, it MUST add a
- Warning 214 (Transformation applied) if one does not already appear
- in the message (see section 14.46).
-
- Warning: unnecessary modification of end-to-end headers might
- cause authentication failures if stronger authentication
- mechanisms are introduced in later versions of HTTP. Such
- authentication mechanisms MAY rely on the values of header fields
- not listed here.
-
- The Content-Length field of a request or response is added or deleted
- according to the rules in section 4.4. A transparent proxy MUST
- preserve the entity-length (section 7.2.2) of the entity-body,
- although it MAY change the transfer-length (section 4.4).
-
-
-
-
-
-Fielding, et al. Standards Track [Page 93]
-
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-
-
-13.5.3 Combining Headers
-
- When a cache makes a validating request to a server, and the server
- provides a 304 (Not Modified) response or a 206 (Partial Content)
- response, the cache then constructs a response to send to the
- requesting client.
-
- If the status code is 304 (Not Modified), the cache uses the entity-
- body stored in the cache entry as the entity-body of this outgoing
- response. If the status code is 206 (Partial Content) and the ETag or
- Last-Modified headers match exactly, the cache MAY combine the
- contents stored in the cache entry with the new contents received in
- the response and use the result as the entity-body of this outgoing
- response, (see 13.5.4).
-
- The end-to-end headers stored in the cache entry are used for the
- constructed response, except that
-
- - any stored Warning headers with warn-code 1xx (see section
- 14.46) MUST be deleted from the cache entry and the forwarded
- response.
-
- - any stored Warning headers with warn-code 2xx MUST be retained
- in the cache entry and the forwarded response.
-
- - any end-to-end headers provided in the 304 or 206 response MUST
- replace the corresponding headers from the cache entry.
-
- Unless the cache decides to remove the cache entry, it MUST also
- replace the end-to-end headers stored with the cache entry with
- corresponding headers received in the incoming response, except for
- Warning headers as described immediately above. If a header field-
- name in the incoming response matches more than one header in the
- cache entry, all such old headers MUST be replaced.
-
- In other words, the set of end-to-end headers received in the
- incoming response overrides all corresponding end-to-end headers
- stored with the cache entry (except for stored Warning headers with
- warn-code 1xx, which are deleted even if not overridden).
-
- Note: this rule allows an origin server to use a 304 (Not
- Modified) or a 206 (Partial Content) response to update any header
- associated with a previous response for the same entity or sub-
- ranges thereof, although it might not always be meaningful or
- correct to do so. This rule does not allow an origin server to use
- a 304 (Not Modified) or a 206 (Partial Content) response to
- entirely delete a header that it had provided with a previous
- response.
-
-
-
-Fielding, et al. Standards Track [Page 94]
-
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-
-
-13.5.4 Combining Byte Ranges
-
- A response might transfer only a subrange of the bytes of an entity-
- body, either because the request included one or more Range
- specifications, or because a connection was broken prematurely. After
- several such transfers, a cache might have received several ranges of
- the same entity-body.
-
- If a cache has a stored non-empty set of subranges for an entity, and
- an incoming response transfers another subrange, the cache MAY
- combine the new subrange with the existing set if both the following
- conditions are met:
-
- - Both the incoming response and the cache entry have a cache
- validator.
-
- - The two cache validators match using the strong comparison
- function (see section 13.3.3).
-
- If either requirement is not met, the cache MUST use only the most
- recent partial response (based on the Date values transmitted with
- every response, and using the incoming response if these values are
- equal or missing), and MUST discard the other partial information.
-
-13.6 Caching Negotiated Responses
-
- Use of server-driven content negotiation (section 12.1), as indicated
- by the presence of a Vary header field in a response, alters the
- conditions and procedure by which a cache can use the response for
- subsequent requests. See section 14.44 for use of the Vary header
- field by servers.
-
- A server SHOULD use the Vary header field to inform a cache of what
- request-header fields were used to select among multiple
- representations of a cacheable response subject to server-driven
- negotiation. The set of header fields named by the Vary field value
- is known as the "selecting" request-headers.
-
- When the cache receives a subsequent request whose Request-URI
- specifies one or more cache entries including a Vary header field,
- the cache MUST NOT use such a cache entry to construct a response to
- the new request unless all of the selecting request-headers present
- in the new request match the corresponding stored request-headers in
- the original request.
-
- The selecting request-headers from two requests are defined to match
- if and only if the selecting request-headers in the first request can
- be transformed to the selecting request-headers in the second request
-
-
-
-Fielding, et al. Standards Track [Page 95]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- by adding or removing linear white space (LWS) at places where this
- is allowed by the corresponding BNF, and/or combining multiple
- message-header fields with the same field name following the rules
- about message headers in section 4.2.
-
- A Vary header field-value of "*" always fails to match and subsequent
- requests on that resource can only be properly interpreted by the
- origin server.
-
- If the selecting request header fields for the cached entry do not
- match the selecting request header fields of the new request, then
- the cache MUST NOT use a cached entry to satisfy the request unless
- it first relays the new request to the origin server in a conditional
- request and the server responds with 304 (Not Modified), including an
- entity tag or Content-Location that indicates the entity to be used.
-
- If an entity tag was assigned to a cached representation, the
- forwarded request SHOULD be conditional and include the entity tags
- in an If-None-Match header field from all its cache entries for the
- resource. This conveys to the server the set of entities currently
- held by the cache, so that if any one of these entities matches the
- requested entity, the server can use the ETag header field in its 304
- (Not Modified) response to tell the cache which entry is appropriate.
- If the entity-tag of the new response matches that of an existing
- entry, the new response SHOULD be used to update the header fields of
- the existing entry, and the result MUST be returned to the client.
-
- If any of the existing cache entries contains only partial content
- for the associated entity, its entity-tag SHOULD NOT be included in
- the If-None-Match header field unless the request is for a range that
- would be fully satisfied by that entry.
-
- If a cache receives a successful response whose Content-Location
- field matches that of an existing cache entry for the same Request-
- ]URI, whose entity-tag differs from that of the existing entry, and
- whose Date is more recent than that of the existing entry, the
- existing entry SHOULD NOT be returned in response to future requests
- and SHOULD be deleted from the cache.
-
-13.7 Shared and Non-Shared Caches
-
- For reasons of security and privacy, it is necessary to make a
- distinction between "shared" and "non-shared" caches. A non-shared
- cache is one that is accessible only to a single user. Accessibility
- in this case SHOULD be enforced by appropriate security mechanisms.
- All other caches are considered to be "shared." Other sections of
-
-
-
-
-
-Fielding, et al. Standards Track [Page 96]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- this specification place certain constraints on the operation of
- shared caches in order to prevent loss of privacy or failure of
- access controls.
-
-13.8 Errors or Incomplete Response Cache Behavior
-
- A cache that receives an incomplete response (for example, with fewer
- bytes of data than specified in a Content-Length header) MAY store
- the response. However, the cache MUST treat this as a partial
- response. Partial responses MAY be combined as described in section
- 13.5.4; the result might be a full response or might still be
- partial. A cache MUST NOT return a partial response to a client
- without explicitly marking it as such, using the 206 (Partial
- Content) status code. A cache MUST NOT return a partial response
- using a status code of 200 (OK).
-
- If a cache receives a 5xx response while attempting to revalidate an
- entry, it MAY either forward this response to the requesting client,
- or act as if the server failed to respond. In the latter case, it MAY
- return a previously received response unless the cached entry
- includes the "must-revalidate" cache-control directive (see section
- 14.9).
-
-13.9 Side Effects of GET and HEAD
-
- Unless the origin server explicitly prohibits the caching of their
- responses, the application of GET and HEAD methods to any resources
- SHOULD NOT have side effects that would lead to erroneous behavior if
- these responses are taken from a cache. They MAY still have side
- effects, but a cache is not required to consider such side effects in
- its caching decisions. Caches are always expected to observe an
- origin server's explicit restrictions on caching.
-
- We note one exception to this rule: since some applications have
- traditionally used GETs and HEADs with query URLs (those containing a
- "?" in the rel_path part) to perform operations with significant side
- effects, caches MUST NOT treat responses to such URIs as fresh unless
- the server provides an explicit expiration time. This specifically
- means that responses from HTTP/1.0 servers for such URIs SHOULD NOT
- be taken from a cache. See section 9.1.1 for related information.
-
-13.10 Invalidation After Updates or Deletions
-
- The effect of certain methods performed on a resource at the origin
- server might cause one or more existing cache entries to become non-
- transparently invalid. That is, although they might continue to be
- "fresh," they do not accurately reflect what the origin server would
- return for a new request on that resource.
-
-
-
-Fielding, et al. Standards Track [Page 97]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- There is no way for the HTTP protocol to guarantee that all such
- cache entries are marked invalid. For example, the request that
- caused the change at the origin server might not have gone through
- the proxy where a cache entry is stored. However, several rules help
- reduce the likelihood of erroneous behavior.
-
- In this section, the phrase "invalidate an entity" means that the
- cache will either remove all instances of that entity from its
- storage, or will mark these as "invalid" and in need of a mandatory
- revalidation before they can be returned in response to a subsequent
- request.
-
- Some HTTP methods MUST cause a cache to invalidate an entity. This is
- either the entity referred to by the Request-URI, or by the Location
- or Content-Location headers (if present). These methods are:
-
- - PUT
-
- - DELETE
-
- - POST
-
- In order to prevent denial of service attacks, an invalidation based
- on the URI in a Location or Content-Location header MUST only be
- performed if the host part is the same as in the Request-URI.
-
- A cache that passes through requests for methods it does not
- understand SHOULD invalidate any entities referred to by the
- Request-URI.
-
-13.11 Write-Through Mandatory
-
- All methods that might be expected to cause modifications to the
- origin server's resources MUST be written through to the origin
- server. This currently includes all methods except for GET and HEAD.
- A cache MUST NOT reply to such a request from a client before having
- transmitted the request to the inbound server, and having received a
- corresponding response from the inbound server. This does not prevent
- a proxy cache from sending a 100 (Continue) response before the
- inbound server has sent its final reply.
-
- The alternative (known as "write-back" or "copy-back" caching) is not
- allowed in HTTP/1.1, due to the difficulty of providing consistent
- updates and the problems arising from server, cache, or network
- failure prior to write-back.
-
-
-
-
-
-
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-
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-
-
-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]
-
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
-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]
-
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-
-
-14.9 Cache-Control
-
- The Cache-Control general-header field is used to specify directives
- that MUST be obeyed by all caching mechanisms along the
- request/response chain. The directives specify behavior intended to
- prevent caches from adversely interfering with the request or
- response. These directives typically override the default caching
- algorithms. Cache directives are unidirectional in that the presence
- of a directive in a request does not imply that the same directive is
- to be given in the response.
-
- Note that HTTP/1.0 caches might not implement Cache-Control and
- might only implement Pragma: no-cache (see section 14.32).
-
- Cache directives MUST be passed through by a proxy or gateway
- application, regardless of their significance to that application,
- since the directives might be applicable to all recipients along the
- request/response chain. It is not possible to specify a cache-
- directive for a specific cache.
-
- Cache-Control = "Cache-Control" ":" 1#cache-directive
-
- cache-directive = cache-request-directive
- | cache-response-directive
-
- cache-request-directive =
- "no-cache" ; Section 14.9.1
- | "no-store" ; Section 14.9.2
- | "max-age" "=" delta-seconds ; Section 14.9.3, 14.9.4
- | "max-stale" [ "=" delta-seconds ] ; Section 14.9.3
- | "min-fresh" "=" delta-seconds ; Section 14.9.3
- | "no-transform" ; Section 14.9.5
- | "only-if-cached" ; Section 14.9.4
- | cache-extension ; Section 14.9.6
-
- cache-response-directive =
- "public" ; Section 14.9.1
- | "private" [ "=" <"> 1#field-name <"> ] ; Section 14.9.1
- | "no-cache" [ "=" <"> 1#field-name <"> ]; Section 14.9.1
- | "no-store" ; Section 14.9.2
- | "no-transform" ; Section 14.9.5
- | "must-revalidate" ; Section 14.9.4
- | "proxy-revalidate" ; Section 14.9.4
- | "max-age" "=" delta-seconds ; Section 14.9.3
- | "s-maxage" "=" delta-seconds ; Section 14.9.3
- | cache-extension ; Section 14.9.6
-
- cache-extension = token [ "=" ( token | quoted-string ) ]
-
-
-
-Fielding, et al. Standards Track [Page 108]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- When a directive appears without any 1#field-name parameter, the
- directive applies to the entire request or response. When such a
- directive appears with a 1#field-name parameter, it applies only to
- the named field or fields, and not to the rest of the request or
- response. This mechanism supports extensibility; implementations of
- future versions of the HTTP protocol might apply these directives to
- header fields not defined in HTTP/1.1.
-
- The cache-control directives can be broken down into these general
- categories:
-
- - Restrictions on what are cacheable; these may only be imposed by
- the origin server.
-
- - Restrictions on what may be stored by a cache; these may be
- imposed by either the origin server or the user agent.
-
- - Modifications of the basic expiration mechanism; these may be
- imposed by either the origin server or the user agent.
-
- - Controls over cache revalidation and reload; these may only be
- imposed by a user agent.
-
- - Control over transformation of entities.
-
- - Extensions to the caching system.
-
-14.9.1 What is Cacheable
-
- By default, a response is cacheable if the requirements of the
- request method, request header fields, and the response status
- indicate that it is cacheable. Section 13.4 summarizes these defaults
- for cacheability. The following Cache-Control response directives
- allow an origin server to override the default cacheability of a
- response:
-
- public
- Indicates that the response MAY be cached by any cache, even if it
- would normally be non-cacheable or cacheable only within a non-
- shared cache. (See also Authorization, section 14.8, for
- additional details.)
-
- private
- Indicates that all or part of the response message is intended for
- a single user and MUST NOT be cached by a shared cache. This
- allows an origin server to state that the specified parts of the
-
-
-
-
-
-Fielding, et al. Standards Track [Page 109]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- response are intended for only one user and are not a valid
- response for requests by other users. A private (non-shared) cache
- MAY cache the response.
-
- Note: This usage of the word private only controls where the
- response may be cached, and cannot ensure the privacy of the
- message content.
-
- no-cache
- If the no-cache directive does not specify a field-name, then a
- cache MUST NOT use the response to satisfy a subsequent request
- without successful revalidation with the origin server. This
- allows an origin server to prevent caching even by caches that
- have been configured to return stale responses to client requests.
-
- If the no-cache directive does specify one or more field-names,
- then a cache MAY use the response to satisfy a subsequent request,
- subject to any other restrictions on caching. However, the
- specified field-name(s) MUST NOT be sent in the response to a
- subsequent request without successful revalidation with the origin
- server. This allows an origin server to prevent the re-use of
- certain header fields in a response, while still allowing caching
- of the rest of the response.
-
- Note: Most HTTP/1.0 caches will not recognize or obey this
- directive.
-
-14.9.2 What May be Stored by Caches
-
- no-store
- The purpose of the no-store directive is to prevent the
- inadvertent release or retention of sensitive information (for
- example, on backup tapes). The no-store directive applies to the
- entire message, and MAY be sent either in a response or in a
- request. If sent in a request, a cache MUST NOT store any part of
- either this request or any response to it. If sent in a response,
- a cache MUST NOT store any part of either this response or the
- request that elicited it. This directive applies to both non-
- shared and shared caches. "MUST NOT store" in this context means
- that the cache MUST NOT intentionally store the information in
- non-volatile storage, and MUST make a best-effort attempt to
- remove the information from volatile storage as promptly as
- possible after forwarding it.
-
- Even when this directive is associated with a response, users
- might explicitly store such a response outside of the caching
- system (e.g., with a "Save As" dialog). History buffers MAY store
- such responses as part of their normal operation.
-
-
-
-Fielding, et al. Standards Track [Page 110]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- The purpose of this directive is to meet the stated requirements
- of certain users and service authors who are concerned about
- accidental releases of information via unanticipated accesses to
- cache data structures. While the use of this directive might
- improve privacy in some cases, we caution that it is NOT in any
- way a reliable or sufficient mechanism for ensuring privacy. In
- particular, malicious or compromised caches might not recognize or
- obey this directive, and communications networks might be
- vulnerable to eavesdropping.
-
-14.9.3 Modifications of the Basic Expiration Mechanism
-
- The expiration time of an entity MAY be specified by the origin
- server using the Expires header (see section 14.21). Alternatively,
- it MAY be specified using the max-age directive in a response. When
- the max-age cache-control directive is present in a cached response,
- the response is stale if its current age is greater than the age
- value given (in seconds) at the time of a new request for that
- resource. The max-age directive on a response implies that the
- response is cacheable (i.e., "public") unless some other, more
- restrictive cache directive is also present.
-
- If a response includes both an Expires header and a max-age
- directive, the max-age directive overrides the Expires header, even
- if the Expires header is more restrictive. This rule allows an origin
- server to provide, for a given response, a longer expiration time to
- an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
- useful if certain HTTP/1.0 caches improperly calculate ages or
- expiration times, perhaps due to desynchronized clocks.
-
- Many HTTP/1.0 cache implementations will treat an Expires value that
- is less than or equal to the response Date value as being equivalent
- to the Cache-Control response directive "no-cache". If an HTTP/1.1
- cache receives such a response, and the response does not include a
- Cache-Control header field, it SHOULD consider the response to be
- non-cacheable in order to retain compatibility with HTTP/1.0 servers.
-
- Note: An origin server might wish to use a relatively new HTTP
- cache control feature, such as the "private" directive, on a
- network including older caches that do not understand that
- feature. The origin server will need to combine the new feature
- with an Expires field whose value is less than or equal to the
- Date value. This will prevent older caches from improperly
- caching the response.
-
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 111]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- s-maxage
- If a response includes an s-maxage directive, then for a shared
- cache (but not for a private cache), the maximum age specified by
- this directive overrides the maximum age specified by either the
- max-age directive or the Expires header. The s-maxage directive
- also implies the semantics of the proxy-revalidate directive (see
- section 14.9.4), i.e., that the shared cache must not use the
- entry after it becomes stale to respond to a subsequent request
- without first revalidating it with the origin server. The s-
- maxage directive is always ignored by a private cache.
-
- Note that most older caches, not compliant with this specification,
- do not implement any cache-control directives. An origin server
- wishing to use a cache-control directive that restricts, but does not
- prevent, caching by an HTTP/1.1-compliant cache MAY exploit the
- requirement that the max-age directive overrides the Expires header,
- and the fact that pre-HTTP/1.1-compliant caches do not observe the
- max-age directive.
-
- Other directives allow a user agent to modify the basic expiration
- mechanism. These directives MAY be specified on a request:
-
- max-age
- Indicates that the client is willing to accept a response whose
- age is no greater than the specified time in seconds. Unless max-
- stale directive is also included, the client is not willing to
- accept a stale response.
-
- min-fresh
- Indicates that the client is willing to accept a response whose
- freshness lifetime is no less than its current age plus the
- specified time in seconds. That is, the client wants a response
- that will still be fresh for at least the specified number of
- seconds.
-
- max-stale
- Indicates that the client is willing to accept a response that has
- exceeded its expiration time. If max-stale is assigned a value,
- then the client is willing to accept a response that has exceeded
- its expiration time by no more than the specified number of
- seconds. If no value is assigned to max-stale, then the client is
- willing to accept a stale response of any age.
-
- If a cache returns a stale response, either because of a max-stale
- directive on a request, or because the cache is configured to
- override the expiration time of a response, the cache MUST attach a
- Warning header to the stale response, using Warning 110 (Response is
- stale).
-
-
-
-Fielding, et al. Standards Track [Page 112]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- A cache MAY be configured to return stale responses without
- validation, but only if this does not conflict with any "MUST"-level
- requirements concerning cache validation (e.g., a "must-revalidate"
- cache-control directive).
-
- If both the new request and the cached entry include "max-age"
- directives, then the lesser of the two values is used for determining
- the freshness of the cached entry for that request.
-
-14.9.4 Cache Revalidation and Reload Controls
-
- Sometimes a user agent might want or need to insist that a cache
- revalidate its cache entry with the origin server (and not just with
- the next cache along the path to the origin server), or to reload its
- cache entry from the origin server. End-to-end revalidation might be
- necessary if either the cache or the origin server has overestimated
- the expiration time of the cached response. End-to-end reload may be
- necessary if the cache entry has become corrupted for some reason.
-
- End-to-end revalidation may be requested either when the client does
- not have its own local cached copy, in which case we call it
- "unspecified end-to-end revalidation", or when the client does have a
- local cached copy, in which case we call it "specific end-to-end
- revalidation."
-
- The client can specify these three kinds of action using Cache-
- Control request directives:
-
- End-to-end reload
- The request includes a "no-cache" cache-control directive or, for
- compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
- names MUST NOT be included with the no-cache directive in a
- request. The server MUST NOT use a cached copy when responding to
- such a request.
-
- Specific end-to-end revalidation
- The request includes a "max-age=0" cache-control directive, which
- forces each cache along the path to the origin server to
- revalidate its own entry, if any, with the next cache or server.
- The initial request includes a cache-validating conditional with
- the client's current validator.
-
- Unspecified end-to-end revalidation
- The request includes "max-age=0" cache-control directive, which
- forces each cache along the path to the origin server to
- revalidate its own entry, if any, with the next cache or server.
- The initial request does not include a cache-validating
-
-
-
-
-Fielding, et al. Standards Track [Page 113]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- conditional; the first cache along the path (if any) that holds a
- cache entry for this resource includes a cache-validating
- conditional with its current validator.
-
- max-age
- When an intermediate cache is forced, by means of a max-age=0
- directive, to revalidate its own cache entry, and the client has
- supplied its own validator in the request, the supplied validator
- might differ from the validator currently stored with the cache
- entry. In this case, the cache MAY use either validator in making
- its own request without affecting semantic transparency.
-
- However, the choice of validator might affect performance. The
- best approach is for the intermediate cache to use its own
- validator when making its request. If the server replies with 304
- (Not Modified), then the cache can return its now validated copy
- to the client with a 200 (OK) response. If the server replies with
- a new entity and cache validator, however, the intermediate cache
- can compare the returned validator with the one provided in the
- client's request, using the strong comparison function. If the
- client's validator is equal to the origin server's, then the
- intermediate cache simply returns 304 (Not Modified). Otherwise,
- it returns the new entity with a 200 (OK) response.
-
- If a request includes the no-cache directive, it SHOULD NOT
- include min-fresh, max-stale, or max-age.
-
- only-if-cached
- In some cases, such as times of extremely poor network
- connectivity, a client may want a cache to return only those
- responses that it currently has stored, and not to reload or
- revalidate with the origin server. To do this, the client may
- include the only-if-cached directive in a request. If it receives
- this directive, a cache SHOULD either respond using a cached entry
- that is consistent with the other constraints of the request, or
- respond with a 504 (Gateway Timeout) status. However, if a group
- of caches is being operated as a unified system with good internal
- connectivity, such a request MAY be forwarded within that group of
- caches.
-
- must-revalidate
- Because a cache MAY be configured to ignore a server's specified
- expiration time, and because a client request MAY include a max-
- stale directive (which has a similar effect), the protocol also
- includes a mechanism for the origin server to require revalidation
- of a cache entry on any subsequent use. When the must-revalidate
- directive is present in a response received by a cache, that cache
- MUST NOT use the entry after it becomes stale to respond to a
-
-
-
-Fielding, et al. Standards Track [Page 114]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- subsequent request without first revalidating it with the origin
- server. (I.e., the cache MUST do an end-to-end revalidation every
- time, if, based solely on the origin server's Expires or max-age
- value, the cached response is stale.)
-
- The must-revalidate directive is necessary to support reliable
- operation for certain protocol features. In all circumstances an
- HTTP/1.1 cache MUST obey the must-revalidate directive; in
- particular, if the cache cannot reach the origin server for any
- reason, it MUST generate a 504 (Gateway Timeout) response.
-
- Servers SHOULD send the must-revalidate directive if and only if
- failure to revalidate a request on the entity could result in
- incorrect operation, such as a silently unexecuted financial
- transaction. Recipients MUST NOT take any automated action that
- violates this directive, and MUST NOT automatically provide an
- unvalidated copy of the entity if revalidation fails.
-
- Although this is not recommended, user agents operating under
- severe connectivity constraints MAY violate this directive but, if
- so, MUST explicitly warn the user that an unvalidated response has
- been provided. The warning MUST be provided on each unvalidated
- access, and SHOULD require explicit user confirmation.
-
- proxy-revalidate
- The proxy-revalidate directive has the same meaning as the must-
- revalidate directive, except that it does not apply to non-shared
- user agent caches. It can be used on a response to an
- authenticated request to permit the user's cache to store and
- later return the response without needing to revalidate it (since
- it has already been authenticated once by that user), while still
- requiring proxies that service many users to revalidate each time
- (in order to make sure that each user has been authenticated).
- Note that such authenticated responses also need the public cache
- control directive in order to allow them to be cached at all.
-
-14.9.5 No-Transform Directive
-
- no-transform
- Implementors of intermediate caches (proxies) have found it useful
- to convert the media type of certain entity bodies. A non-
- transparent proxy might, for example, convert between image
- formats in order to save cache space or to reduce the amount of
- traffic on a slow link.
-
- Serious operational problems occur, however, when these
- transformations are applied to entity bodies intended for certain
- kinds of applications. For example, applications for medical
-
-
-
-Fielding, et al. Standards Track [Page 115]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- imaging, scientific data analysis and those using end-to-end
- authentication, all depend on receiving an entity body that is bit
- for bit identical to the original entity-body.
-
- Therefore, if a message includes the no-transform directive, an
- intermediate cache or proxy MUST NOT change those headers that are
- listed in section 13.5.2 as being subject to the no-transform
- directive. This implies that the cache or proxy MUST NOT change
- any aspect of the entity-body that is specified by these headers,
- including the value of the entity-body itself.
-
-14.9.6 Cache Control Extensions
-
- The Cache-Control header field can be extended through the use of one
- or more cache-extension tokens, each with an optional assigned value.
- Informational extensions (those which do not require a change in
- cache behavior) MAY be added without changing the semantics of other
- directives. Behavioral extensions are designed to work by acting as
- modifiers to the existing base of cache directives. Both the new
- directive and the standard directive are supplied, such that
- applications which do not understand the new directive will default
- to the behavior specified by the standard directive, and those that
- understand the new directive will recognize it as modifying the
- requirements associated with the standard directive. In this way,
- extensions to the cache-control directives can be made without
- requiring changes to the base protocol.
-
- This extension mechanism depends on an HTTP cache obeying all of the
- cache-control directives defined for its native HTTP-version, obeying
- certain extensions, and ignoring all directives that it does not
- understand.
-
- For example, consider a hypothetical new response directive called
- community which acts as a modifier to the private directive. We
- define this new directive to mean that, in addition to any non-shared
- cache, any cache which is shared only by members of the community
- named within its value may cache the response. An origin server
- wishing to allow the UCI community to use an otherwise private
- response in their shared cache(s) could do so by including
-
- Cache-Control: private, community="UCI"
-
- A cache seeing this header field will act correctly even if the cache
- does not understand the community cache-extension, since it will also
- see and understand the private directive and thus default to the safe
- behavior.
-
-
-
-
-
-Fielding, et al. Standards Track [Page 116]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- Unrecognized cache-directives MUST be ignored; it is assumed that any
- cache-directive likely to be unrecognized by an HTTP/1.1 cache will
- be combined with standard directives (or the response's default
- cacheability) such that the cache behavior will remain minimally
- correct even if the cache does not understand the extension(s).
-
-14.10 Connection
-
- The Connection general-header field allows the sender to specify
- options that are desired for that particular connection and MUST NOT
- be communicated by proxies over further connections.
-
- The Connection header has the following grammar:
-
- Connection = "Connection" ":" 1#(connection-token)
- connection-token = token
-
- HTTP/1.1 proxies MUST parse the Connection header field before a
- message is forwarded and, for each connection-token in this field,
- remove any header field(s) from the message with the same name as the
- connection-token. Connection options are signaled by the presence of
- a connection-token in the Connection header field, not by any
- corresponding additional header field(s), since the additional header
- field may not be sent if there are no parameters associated with that
- connection option.
-
- Message headers listed in the Connection header MUST NOT include
- end-to-end headers, such as Cache-Control.
-
- HTTP/1.1 defines the "close" connection option for the sender to
- signal that the connection will be closed after completion of the
- response. For example,
-
- Connection: close
-
- in either the request or the response header fields indicates that
- the connection SHOULD NOT be considered `persistent' (section 8.1)
- after the current request/response is complete.
-
- HTTP/1.1 applications that do not support persistent connections MUST
- include the "close" connection option in every message.
-
- A system receiving an HTTP/1.0 (or lower-version) message that
- includes a Connection header MUST, for each connection-token in this
- field, remove and ignore any header field(s) from the message with
- the same name as the connection-token. This protects against mistaken
- forwarding of such header fields by pre-HTTP/1.1 proxies. See section
- 19.6.2.
-
-
-
-Fielding, et al. Standards Track [Page 117]
-
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-
-
-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]
-
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-
-
- and any current entity exists for that resource, then the server MAY
- perform the requested method as if the If-Match header field did not
- exist.
-
- A server MUST use the strong comparison function (see section 13.3.3)
- to compare the entity tags in If-Match.
-
- If none of the entity tags match, or if "*" is given and no current
- entity exists, the server MUST NOT perform the requested method, and
- MUST return a 412 (Precondition Failed) response. This behavior is
- most useful when the client wants to prevent an updating method, such
- as PUT, from modifying a resource that has changed since the client
- last retrieved it.
-
- If the request would, without the If-Match header field, result in
- anything other than a 2xx or 412 status, then the If-Match header
- MUST be ignored.
-
- The meaning of "If-Match: *" is that the method SHOULD be performed
- if the representation selected by the origin server (or by a cache,
- possibly using the Vary mechanism, see section 14.44) exists, and
- MUST NOT be performed if the representation does not exist.
-
- A request intended to update a resource (e.g., a PUT) MAY include an
- If-Match header field to signal that the request method MUST NOT be
- applied if the entity corresponding to the If-Match value (a single
- entity tag) is no longer a representation of that resource. This
- allows the user to indicate that they do not wish the request to be
- successful if the resource has been changed without their knowledge.
- Examples:
-
- If-Match: "xyzzy"
- If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
- If-Match: *
-
- The result of a request having both an If-Match header field and
- either an If-None-Match or an If-Modified-Since header fields is
- undefined by this specification.
-
-14.25 If-Modified-Since
-
- The If-Modified-Since request-header field is used with a method to
- make it conditional: if the requested variant has not been modified
- since the time specified in this field, an entity will not be
- returned from the server; instead, a 304 (not modified) response will
- be returned without any message-body.
-
- If-Modified-Since = "If-Modified-Since" ":" HTTP-date
-
-
-
-Fielding, et al. Standards Track [Page 130]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- An example of the field is:
-
- If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
-
- A GET method with an If-Modified-Since header and no Range header
- requests that the identified entity be transferred only if it has
- been modified since the date given by the If-Modified-Since header.
- The algorithm for determining this includes the following cases:
-
- a) If the request would normally result in anything other than a
- 200 (OK) status, or if the passed If-Modified-Since date is
- invalid, the response is exactly the same as for a normal GET.
- A date which is later than the server's current time is
- invalid.
-
- b) If the variant has been modified since the If-Modified-Since
- date, the response is exactly the same as for a normal GET.
-
- c) If the variant has not been modified since a valid If-
- Modified-Since date, the server SHOULD return a 304 (Not
- Modified) response.
-
- The purpose of this feature is to allow efficient updates of cached
- information with a minimum amount of transaction overhead.
-
- Note: The Range request-header field modifies the meaning of If-
- Modified-Since; see section 14.35 for full details.
-
- Note: If-Modified-Since times are interpreted by the server, whose
- clock might not be synchronized with the client.
-
- Note: When handling an If-Modified-Since header field, some
- servers will use an exact date comparison function, rather than a
- less-than function, for deciding whether to send a 304 (Not
- Modified) response. To get best results when sending an If-
- Modified-Since header field for cache validation, clients are
- advised to use the exact date string received in a previous Last-
- Modified header field whenever possible.
-
- Note: If a client uses an arbitrary date in the If-Modified-Since
- header instead of a date taken from the Last-Modified header for
- the same request, the client should be aware of the fact that this
- date is interpreted in the server's understanding of time. The
- client should consider unsynchronized clocks and rounding problems
- due to the different encodings of time between the client and
- server. This includes the possibility of race conditions if the
- document has changed between the time it was first requested and
- the If-Modified-Since date of a subsequent request, and the
-
-
-
-Fielding, et al. Standards Track [Page 131]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- possibility of clock-skew-related problems if the If-Modified-
- Since date is derived from the client's clock without correction
- to the server's clock. Corrections for different time bases
- between client and server are at best approximate due to network
- latency.
-
- The result of a request having both an If-Modified-Since header field
- and either an If-Match or an If-Unmodified-Since header fields is
- undefined by this specification.
-
-14.26 If-None-Match
-
- The If-None-Match request-header field is used with a method to make
- it conditional. A client that has one or more entities previously
- obtained from the resource can verify that none of those entities is
- current by including a list of their associated entity tags in the
- If-None-Match header field. The purpose of this feature is to allow
- efficient updates of cached information with a minimum amount of
- transaction overhead. It is also used to prevent a method (e.g. PUT)
- from inadvertently modifying an existing resource when the client
- believes that the resource does not exist.
-
- As a special case, the value "*" matches any current entity of the
- resource.
-
- If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )
-
- If any of the entity tags match the entity tag of the entity that
- would have been returned in the response to a similar GET request
- (without the If-None-Match header) on that resource, or if "*" is
- given and any current entity exists for that resource, then the
- server MUST NOT perform the requested method, unless required to do
- so because the resource's modification date fails to match that
- supplied in an If-Modified-Since header field in the request.
- Instead, if the request method was GET or HEAD, the server SHOULD
- respond with a 304 (Not Modified) response, including the cache-
- related header fields (particularly ETag) of one of the entities that
- matched. For all other request methods, the server MUST respond with
- a status of 412 (Precondition Failed).
-
- See section 13.3.3 for rules on how to determine if two entities tags
- match. The weak comparison function can only be used with GET or HEAD
- requests.
-
-
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 132]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- If none of the entity tags match, then the server MAY perform the
- requested method as if the If-None-Match header field did not exist,
- but MUST also ignore any If-Modified-Since header field(s) in the
- request. That is, if no entity tags match, then the server MUST NOT
- return a 304 (Not Modified) response.
-
- If the request would, without the If-None-Match header field, result
- in anything other than a 2xx or 304 status, then the If-None-Match
- header MUST be ignored. (See section 13.3.4 for a discussion of
- server behavior when both If-Modified-Since and If-None-Match appear
- in the same request.)
-
- The meaning of "If-None-Match: *" is that the method MUST NOT be
- performed if the representation selected by the origin server (or by
- a cache, possibly using the Vary mechanism, see section 14.44)
- exists, and SHOULD be performed if the representation does not exist.
- This feature is intended to be useful in preventing races between PUT
- operations.
-
- Examples:
-
- If-None-Match: "xyzzy"
- If-None-Match: W/"xyzzy"
- If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
- If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
- If-None-Match: *
-
- The result of a request having both an If-None-Match header field and
- either an If-Match or an If-Unmodified-Since header fields is
- undefined by this specification.
-
-14.27 If-Range
-
- If a client has a partial copy of an entity in its cache, and wishes
- to have an up-to-date copy of the entire entity in its cache, it
- could use the Range request-header with a conditional GET (using
- either or both of If-Unmodified-Since and If-Match.) However, if the
- condition fails because the entity has been modified, the client
- would then have to make a second request to obtain the entire current
- entity-body.
-
- The If-Range header allows a client to "short-circuit" the second
- request. Informally, its meaning is `if the entity is unchanged, send
- me the part(s) that I am missing; otherwise, send me the entire new
- entity'.
-
- If-Range = "If-Range" ":" ( entity-tag | HTTP-date )
-
-
-
-
-Fielding, et al. Standards Track [Page 133]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- If the client has no entity tag for an entity, but does have a Last-
- Modified date, it MAY use that date in an If-Range header. (The
- server can distinguish between a valid HTTP-date and any form of
- entity-tag by examining no more than two characters.) The If-Range
- header SHOULD only be used together with a Range header, and MUST be
- ignored if the request does not include a Range header, or if the
- server does not support the sub-range operation.
-
- If the entity tag given in the If-Range header matches the current
- entity tag for the entity, then the server SHOULD provide the
- specified sub-range of the entity using a 206 (Partial content)
- response. If the entity tag does not match, then the server SHOULD
- return the entire entity using a 200 (OK) response.
-
-14.28 If-Unmodified-Since
-
- The If-Unmodified-Since request-header field is used with a method to
- make it conditional. If the requested resource has not been modified
- since the time specified in this field, the server SHOULD perform the
- requested operation as if the If-Unmodified-Since header were not
- present.
-
- If the requested variant has been modified since the specified time,
- the server MUST NOT perform the requested operation, and MUST return
- a 412 (Precondition Failed).
-
- If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date
-
- An example of the field is:
-
- If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
-
- If the request normally (i.e., without the If-Unmodified-Since
- header) would result in anything other than a 2xx or 412 status, the
- If-Unmodified-Since header SHOULD be ignored.
-
- If the specified date is invalid, the header is ignored.
-
- The result of a request having both an If-Unmodified-Since header
- field and either an If-None-Match or an If-Modified-Since header
- fields is undefined by this specification.
-
-14.29 Last-Modified
-
- The Last-Modified entity-header field indicates the date and time at
- which the origin server believes the variant was last modified.
-
- Last-Modified = "Last-Modified" ":" HTTP-date
-
-
-
-Fielding, et al. Standards Track [Page 134]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- An example of its use is
-
- Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
-
- The exact meaning of this header field depends on the implementation
- of the origin server and the nature of the original resource. For
- files, it may be just the file system last-modified time. For
- entities with dynamically included parts, it may be the most recent
- of the set of last-modify times for its component parts. For database
- gateways, it may be the last-update time stamp of the record. For
- virtual objects, it may be the last time the internal state changed.
-
- An origin server MUST NOT send a Last-Modified date which is later
- than the server's time of message origination. In such cases, where
- the resource's last modification would indicate some time in the
- future, the server MUST replace that date with the message
- origination date.
-
- An origin server SHOULD obtain the Last-Modified value of the entity
- as close as possible to the time that it generates the Date value of
- its response. This allows a recipient to make an accurate assessment
- of the entity's modification time, especially if the entity changes
- near the time that the response is generated.
-
- HTTP/1.1 servers SHOULD send Last-Modified whenever feasible.
-
-14.30 Location
-
- The Location response-header field is used to redirect the recipient
- to a location other than the Request-URI for completion of the
- request or identification of a new resource. For 201 (Created)
- responses, the Location is that of the new resource which was created
- by the request. For 3xx responses, the location SHOULD indicate the
- server's preferred URI for automatic redirection to the resource. The
- field value consists of a single absolute URI.
-
- Location = "Location" ":" absoluteURI
-
- An example is:
-
- Location: http://www.w3.org/pub/WWW/People.html
-
- Note: The Content-Location header field (section 14.14) differs
- from Location in that the Content-Location identifies the original
- location of the entity enclosed in the request. It is therefore
- possible for a response to contain header fields for both Location
- and Content-Location. Also see section 13.10 for cache
- requirements of some methods.
-
-
-
-Fielding, et al. Standards Track [Page 135]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
-14.31 Max-Forwards
-
- The Max-Forwards request-header field provides a mechanism with the
- TRACE (section 9.8) and OPTIONS (section 9.2) methods to limit the
- number of proxies or gateways that can forward the request to the
- next inbound server. This can be useful when the client is attempting
- to trace a request chain which appears to be failing or looping in
- mid-chain.
-
- Max-Forwards = "Max-Forwards" ":" 1*DIGIT
-
- The Max-Forwards value is a decimal integer indicating the remaining
- number of times this request message may be forwarded.
-
- Each proxy or gateway recipient of a TRACE or OPTIONS request
- containing a Max-Forwards header field MUST check and update its
- value prior to forwarding the request. If the received value is zero
- (0), the recipient MUST NOT forward the request; instead, it MUST
- respond as the final recipient. If the received Max-Forwards value is
- greater than zero, then the forwarded message MUST contain an updated
- Max-Forwards field with a value decremented by one (1).
-
- The Max-Forwards header field MAY be ignored for all other methods
- defined by this specification and for any extension methods for which
- it is not explicitly referred to as part of that method definition.
-
-14.32 Pragma
-
- The Pragma general-header field is used to include implementation-
- specific directives that might apply to any recipient along the
- request/response chain. All pragma directives specify optional
- behavior from the viewpoint of the protocol; however, some systems
- MAY require that behavior be consistent with the directives.
-
- Pragma = "Pragma" ":" 1#pragma-directive
- pragma-directive = "no-cache" | extension-pragma
- extension-pragma = token [ "=" ( token | quoted-string ) ]
-
- When the no-cache directive is present in a request message, an
- application SHOULD forward the request toward the origin server even
- if it has a cached copy of what is being requested. This pragma
- directive has the same semantics as the no-cache cache-directive (see
- section 14.9) and is defined here for backward compatibility with
- HTTP/1.0. Clients SHOULD include both header fields when a no-cache
- request is sent to a server not known to be HTTP/1.1 compliant.
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 136]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- Pragma directives MUST be passed through by a proxy or gateway
- application, regardless of their significance to that application,
- since the directives might be applicable to all recipients along the
- request/response chain. It is not possible to specify a pragma for a
- specific recipient; however, any pragma directive not relevant to a
- recipient SHOULD be ignored by that recipient.
-
- HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
- sent "Cache-Control: no-cache". No new Pragma directives will be
- defined in HTTP.
-
- Note: because the meaning of "Pragma: no-cache as a response
- header field is not actually specified, it does not provide a
- reliable replacement for "Cache-Control: no-cache" in a response
-
-14.33 Proxy-Authenticate
-
- The Proxy-Authenticate response-header field MUST be included as part
- of a 407 (Proxy Authentication Required) response. The field value
- consists of a challenge that indicates the authentication scheme and
- parameters applicable to the proxy for this Request-URI.
-
- Proxy-Authenticate = "Proxy-Authenticate" ":" 1#challenge
-
- The HTTP access authentication process is described in "HTTP
- Authentication: Basic and Digest Access Authentication" [43]. Unlike
- WWW-Authenticate, the Proxy-Authenticate header field applies only to
- the current connection and SHOULD NOT be passed on to downstream
- clients. However, an intermediate proxy might need to obtain its own
- credentials by requesting them from the downstream client, which in
- some circumstances will appear as if the proxy is forwarding the
- Proxy-Authenticate header field.
-
-14.34 Proxy-Authorization
-
- The Proxy-Authorization request-header field allows the client to
- identify itself (or its user) to a proxy which requires
- authentication. The Proxy-Authorization field value consists of
- credentials containing the authentication information of the user
- agent for the proxy and/or realm of the resource being requested.
-
- Proxy-Authorization = "Proxy-Authorization" ":" credentials
-
- The HTTP access authentication process is described in "HTTP
- Authentication: Basic and Digest Access Authentication" [43] . Unlike
- Authorization, the Proxy-Authorization header field applies only to
- the next outbound proxy that demanded authentication using the Proxy-
- Authenticate field. When multiple proxies are used in a chain, the
-
-
-
-Fielding, et al. Standards Track [Page 137]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- Proxy-Authorization header field is consumed by the first outbound
- proxy that was expecting to receive credentials. A proxy MAY relay
- the credentials from the client request to the next proxy if that is
- the mechanism by which the proxies cooperatively authenticate a given
- request.
-
-14.35 Range
-
-14.35.1 Byte Ranges
-
- Since all HTTP entities are represented in HTTP messages as sequences
- of bytes, the concept of a byte range is meaningful for any HTTP
- entity. (However, not all clients and servers need to support byte-
- range operations.)
-
- Byte range specifications in HTTP apply to the sequence of bytes in
- the entity-body (not necessarily the same as the message-body).
-
- A byte range operation MAY specify a single range of bytes, or a set
- of ranges within a single entity.
-
- ranges-specifier = byte-ranges-specifier
- byte-ranges-specifier = bytes-unit "=" byte-range-set
- byte-range-set = 1#( byte-range-spec | suffix-byte-range-spec )
- byte-range-spec = first-byte-pos "-" [last-byte-pos]
- first-byte-pos = 1*DIGIT
- last-byte-pos = 1*DIGIT
-
- The first-byte-pos value in a byte-range-spec gives the byte-offset
- of the first byte in a range. The last-byte-pos value gives the
- byte-offset of the last byte in the range; that is, the byte
- positions specified are inclusive. Byte offsets start at zero.
-
- If the last-byte-pos value is present, it MUST be greater than or
- equal to the first-byte-pos in that byte-range-spec, or the byte-
- range-spec is syntactically invalid. The recipient of a byte-range-
- set that includes one or more syntactically invalid byte-range-spec
- values MUST ignore the header field that includes that byte-range-
- set.
-
- If the last-byte-pos value is absent, or if the value is greater than
- or equal to the current length of the entity-body, last-byte-pos is
- taken to be equal to one less than the current length of the entity-
- body in bytes.
-
- By its choice of last-byte-pos, a client can limit the number of
- bytes retrieved without knowing the size of the entity.
-
-
-
-
-Fielding, et al. Standards Track [Page 138]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- suffix-byte-range-spec = "-" suffix-length
- suffix-length = 1*DIGIT
-
- A suffix-byte-range-spec is used to specify the suffix of the
- entity-body, of a length given by the suffix-length value. (That is,
- this form specifies the last N bytes of an entity-body.) If the
- entity is shorter than the specified suffix-length, the entire
- entity-body is used.
-
- If a syntactically valid byte-range-set includes at least one byte-
- range-spec whose first-byte-pos is less than the current length of
- the entity-body, or at least one suffix-byte-range-spec with a non-
- zero suffix-length, then the byte-range-set is satisfiable.
- Otherwise, the byte-range-set is unsatisfiable. If the byte-range-set
- is unsatisfiable, the server SHOULD return a response with a status
- of 416 (Requested range not satisfiable). Otherwise, the server
- SHOULD return a response with a status of 206 (Partial Content)
- containing the satisfiable ranges of the entity-body.
-
- Examples of byte-ranges-specifier values (assuming an entity-body of
- length 10000):
-
- - The first 500 bytes (byte offsets 0-499, inclusive): bytes=0-
- 499
-
- - The second 500 bytes (byte offsets 500-999, inclusive):
- bytes=500-999
-
- - The final 500 bytes (byte offsets 9500-9999, inclusive):
- bytes=-500
-
- - Or bytes=9500-
-
- - The first and last bytes only (bytes 0 and 9999): bytes=0-0,-1
-
- - Several legal but not canonical specifications of the second 500
- bytes (byte offsets 500-999, inclusive):
- bytes=500-600,601-999
- bytes=500-700,601-999
-
-14.35.2 Range Retrieval Requests
-
- HTTP retrieval requests using conditional or unconditional GET
- methods MAY request one or more sub-ranges of the entity, instead of
- the entire entity, using the Range request header, which applies to
- the entity returned as the result of the request:
-
- Range = "Range" ":" ranges-specifier
-
-
-
-Fielding, et al. Standards Track [Page 139]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- A server MAY ignore the Range header. However, HTTP/1.1 origin
- servers and intermediate caches ought to support byte ranges when
- possible, since Range supports efficient recovery from partially
- failed transfers, and supports efficient partial retrieval of large
- entities.
-
- If the server supports the Range header and the specified range or
- ranges are appropriate for the entity:
-
- - The presence of a Range header in an unconditional GET modifies
- what is returned if the GET is otherwise successful. In other
- words, the response carries a status code of 206 (Partial
- Content) instead of 200 (OK).
-
- - The presence of a Range header in a conditional GET (a request
- using one or both of If-Modified-Since and If-None-Match, or
- one or both of If-Unmodified-Since and If-Match) modifies what
- is returned if the GET is otherwise successful and the
- condition is true. It does not affect the 304 (Not Modified)
- response returned if the conditional is false.
-
- In some cases, it might be more appropriate to use the If-Range
- header (see section 14.27) in addition to the Range header.
-
- If a proxy that supports ranges receives a Range request, forwards
- the request to an inbound server, and receives an entire entity in
- reply, it SHOULD only return the requested range to its client. It
- SHOULD store the entire received response in its cache if that is
- consistent with its cache allocation policies.
-
-14.36 Referer
-
- The Referer[sic] request-header field allows the client to specify,
- for the server's benefit, the address (URI) of the resource from
- which the Request-URI was obtained (the "referrer", although the
- header field is misspelled.) The Referer request-header allows a
- server to generate lists of back-links to resources for interest,
- logging, optimized caching, etc. It also allows obsolete or mistyped
- links to be traced for maintenance. The Referer field MUST NOT be
- sent if the Request-URI was obtained from a source that does not have
- its own URI, such as input from the user keyboard.
-
- Referer = "Referer" ":" ( absoluteURI | relativeURI )
-
- Example:
-
- Referer: http://www.w3.org/hypertext/DataSources/Overview.html
-
-
-
-
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-
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-
-
- 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.
-
-
-
-
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-
-
-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.")
-
-
-
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-
-
- 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
-
-
-
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-
-
- 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.
-
-
-
-
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-
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
- 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]
-
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-
-
- specific Warning codes. It MUST then add any Warning headers received
- in the validating response. In other words, Warning headers are those
- that would be attached to the most recent relevant response.
-
- When multiple Warning headers are attached to a response, the user
- agent ought to inform the user of as many of them as possible, in the
- order that they appear in the response. If it is not possible to
- inform the user of all of the warnings, the user agent SHOULD follow
- these heuristics:
-
- - Warnings that appear early in the response take priority over
- those appearing later in the response.
-
- - Warnings in the user's preferred character set take priority
- over warnings in other character sets but with identical warn-
- codes and warn-agents.
-
- Systems that generate multiple Warning headers SHOULD order them with
- this user agent behavior in mind.
-
- Requirements for the behavior of caches with respect to Warnings are
- stated in section 13.1.2.
-
- This is a list of the currently-defined warn-codes, each with a
- recommended warn-text in English, and a description of its meaning.
-
- 110 Response is stale
- MUST be included whenever the returned response is stale.
-
- 111 Revalidation failed
- MUST be included if a cache returns a stale response because an
- attempt to revalidate the response failed, due to an inability to
- reach the server.
-
- 112 Disconnected operation
- SHOULD be included if the cache is intentionally disconnected from
- the rest of the network for a period of time.
-
- 113 Heuristic expiration
- MUST be included if the cache heuristically chose a freshness
- lifetime greater than 24 hours and the response's age is greater
- than 24 hours.
-
- 199 Miscellaneous warning
- The warning text MAY include arbitrary information to be presented
- to a human user, or logged. A system receiving this warning MUST
- NOT take any automated action, besides presenting the warning to
- the user.
-
-
-
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-
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-
-
- 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.
-
-
-
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-
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-
-
-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
-
-
-
-
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-
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-
-
- 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
-
-
-
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-
-
- strongly correlated to the membership of a particular ethnic group.
- User agents which offer the option to configure the contents of an
- Accept-Language header to be sent in every request are strongly
- encouraged to let the configuration process include a message which
- makes the user aware of the loss of privacy involved.
-
- An approach that limits the loss of privacy would be for a user agent
- to omit the sending of Accept-Language headers by default, and to ask
- the user whether or not to start sending Accept-Language headers to a
- server if it detects, by looking for any Vary response-header fields
- generated by the server, that such sending could improve the quality
- of service.
-
- Elaborate user-customized accept header fields sent in every request,
- in particular if these include quality values, can be used by servers
- as relatively reliable and long-lived user identifiers. Such user
- identifiers would allow content providers to do click-trail tracking,
- and would allow collaborating content providers to match cross-server
- click-trails or form submissions of individual users. Note that for
- many users not behind a proxy, the network address of the host
- running the user agent will also serve as a long-lived user
- identifier. In environments where proxies are used to enhance
- privacy, user agents ought to be conservative in offering accept
- header configuration options to end users. As an extreme privacy
- measure, proxies could filter the accept headers in relayed requests.
- General purpose user agents which provide a high degree of header
- configurability SHOULD warn users about the loss of privacy which can
- be involved.
-
-15.2 Attacks Based On File and Path Names
-
- Implementations of HTTP origin servers SHOULD be careful to restrict
- the documents returned by HTTP requests to be only those that were
- intended by the server administrators. If an HTTP server translates
- HTTP URIs directly into file system calls, the server MUST take
- special care not to serve files that were not intended to be
- delivered to HTTP clients. For example, UNIX, Microsoft Windows, and
- other operating systems use ".." as a path component to indicate a
- directory level above the current one. On such a system, an HTTP
- server MUST disallow any such construct in the Request-URI if it
- would otherwise allow access to a resource outside those intended to
- be accessible via the HTTP server. Similarly, files intended for
- reference only internally to the server (such as access control
- files, configuration files, and script code) MUST be protected from
- inappropriate retrieval, since they might contain sensitive
- information. Experience has shown that minor bugs in such HTTP server
- implementations have turned into security risks.
-
-
-
-
-Fielding, et al. Standards Track [Page 153]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
-15.3 DNS Spoofing
-
- Clients using HTTP rely heavily on the Domain Name Service, and are
- thus generally prone to security attacks based on the deliberate
- mis-association of IP addresses and DNS names. Clients need to be
- cautious in assuming the continuing validity of an IP number/DNS name
- association.
-
- In particular, HTTP clients SHOULD rely on their name resolver for
- confirmation of an IP number/DNS name association, rather than
- caching the result of previous host name lookups. Many platforms
- already can cache host name lookups locally when appropriate, and
- they SHOULD be configured to do so. It is proper for these lookups to
- be cached, however, only when the TTL (Time To Live) information
- reported by the name server makes it likely that the cached
- information will remain useful.
-
- If HTTP clients cache the results of host name lookups in order to
- achieve a performance improvement, they MUST observe the TTL
- information reported by DNS.
-
- If HTTP clients do not observe this rule, they could be spoofed when
- a previously-accessed server's IP address changes. As network
- renumbering is expected to become increasingly common [24], the
- possibility of this form of attack will grow. Observing this
- requirement thus reduces this potential security vulnerability.
-
- This requirement also improves the load-balancing behavior of clients
- for replicated servers using the same DNS name and reduces the
- likelihood of a user's experiencing failure in accessing sites which
- use that strategy.
-
-15.4 Location Headers and Spoofing
-
- If a single server supports multiple organizations that do not trust
- one another, then it MUST check the values of Location and Content-
- Location headers in responses that are generated under control of
- said organizations to make sure that they do not attempt to
- invalidate resources over which they have no authority.
-
-15.5 Content-Disposition Issues
-
- RFC 1806 [35], from which the often implemented Content-Disposition
- (see section 19.5.1) header in HTTP is derived, has a number of very
- serious security considerations. Content-Disposition is not part of
- the HTTP standard, but since it is widely implemented, we are
- documenting its use and risks for implementors. See RFC 2183 [49]
- (which updates RFC 1806) for details.
-
-
-
-Fielding, et al. Standards Track [Page 154]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
-15.6 Authentication Credentials and Idle Clients
-
- Existing HTTP clients and user agents typically retain authentication
- information indefinitely. HTTP/1.1. does not provide a method for a
- server to direct clients to discard these cached credentials. This is
- a significant defect that requires further extensions to HTTP.
- Circumstances under which credential caching can interfere with the
- application's security model include but are not limited to:
-
- - Clients which have been idle for an extended period following
- which the server might wish to cause the client to reprompt the
- user for credentials.
-
- - Applications which include a session termination indication
- (such as a `logout' or `commit' button on a page) after which
- the server side of the application `knows' that there is no
- further reason for the client to retain the credentials.
-
- This is currently under separate study. There are a number of work-
- arounds to parts of this problem, and we encourage the use of
- password protection in screen savers, idle time-outs, and other
- methods which mitigate the security problems inherent in this
- problem. In particular, user agents which cache credentials are
- encouraged to provide a readily accessible mechanism for discarding
- cached credentials under user control.
-
-15.7 Proxies and Caching
-
- By their very nature, HTTP proxies are men-in-the-middle, and
- represent an opportunity for man-in-the-middle attacks. Compromise of
- the systems on which the proxies run can result in serious security
- and privacy problems. Proxies have access to security-related
- information, personal information about individual users and
- organizations, and proprietary information belonging to users and
- content providers. A compromised proxy, or a proxy implemented or
- configured without regard to security and privacy considerations,
- might be used in the commission of a wide range of potential attacks.
-
- Proxy operators should protect the systems on which proxies run as
- they would protect any system that contains or transports sensitive
- information. In particular, log information gathered at proxies often
- contains highly sensitive personal information, and/or information
- about organizations. Log information should be carefully guarded, and
- appropriate guidelines for use developed and followed. (Section
- 15.1.1).
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 155]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- Caching proxies provide additional potential vulnerabilities, since
- the contents of the cache represent an attractive target for
- malicious exploitation. Because cache contents persist after an HTTP
- request is complete, an attack on the cache can reveal information
- long after a user believes that the information has been removed from
- the network. Therefore, cache contents should be protected as
- sensitive information.
-
- Proxy implementors should consider the privacy and security
- implications of their design and coding decisions, and of the
- configuration options they provide to proxy operators (especially the
- default configuration).
-
- Users of a proxy need to be aware that they are no trustworthier than
- the people who run the proxy; HTTP itself cannot solve this problem.
-
- The judicious use of cryptography, when appropriate, may suffice to
- protect against a broad range of security and privacy attacks. Such
- cryptography is beyond the scope of the HTTP/1.1 specification.
-
-15.7.1 Denial of Service Attacks on Proxies
-
- They exist. They are hard to defend against. Research continues.
- Beware.
-
-16 Acknowledgments
-
- This specification makes heavy use of the augmented BNF and generic
- constructs defined by David H. Crocker for RFC 822 [9]. Similarly, it
- reuses many of the definitions provided by Nathaniel Borenstein and
- Ned Freed for MIME [7]. We hope that their inclusion in this
- specification will help reduce past confusion over the relationship
- between HTTP and Internet mail message formats.
-
- The HTTP protocol has evolved considerably over the years. It has
- benefited from a large and active developer community--the many
- people who have participated on the www-talk mailing list--and it is
- that community which has been most responsible for the success of
- HTTP and of the World-Wide Web in general. Marc Andreessen, Robert
- Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
- Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
- McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
- VanHeyningen deserve special recognition for their efforts in
- defining early aspects of the protocol.
-
- This document has benefited greatly from the comments of all those
- participating in the HTTP-WG. In addition to those already mentioned,
- the following individuals have contributed to this specification:
-
-
-
-Fielding, et al. Standards Track [Page 156]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- Gary Adams Ross Patterson
- Harald Tveit Alvestrand Albert Lunde
- Keith Ball John C. Mallery
- Brian Behlendorf Jean-Philippe Martin-Flatin
- Paul Burchard Mitra
- Maurizio Codogno David Morris
- Mike Cowlishaw Gavin Nicol
- Roman Czyborra Bill Perry
- Michael A. Dolan Jeffrey Perry
- David J. Fiander Scott Powers
- Alan Freier Owen Rees
- Marc Hedlund Luigi Rizzo
- Greg Herlihy David Robinson
- Koen Holtman Marc Salomon
- Alex Hopmann Rich Salz
- Bob Jernigan Allan M. Schiffman
- Shel Kaphan Jim Seidman
- Rohit Khare Chuck Shotton
- John Klensin Eric W. Sink
- Martijn Koster Simon E. Spero
- Alexei Kosut Richard N. Taylor
- David M. Kristol Robert S. Thau
- Daniel LaLiberte Bill (BearHeart) Weinman
- Ben Laurie Francois Yergeau
- Paul J. Leach Mary Ellen Zurko
- Daniel DuBois Josh Cohen
-
-
- Much of the content and presentation of the caching design is due to
- suggestions and comments from individuals including: Shel Kaphan,
- Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
-
- Most of the specification of ranges is based on work originally done
- by Ari Luotonen and John Franks, with additional input from Steve
- Zilles.
-
- Thanks to the "cave men" of Palo Alto. You know who you are.
-
- Jim Gettys (the current editor of this document) wishes particularly
- to thank Roy Fielding, the previous editor of this document, along
- with John Klensin, Jeff Mogul, Paul Leach, Dave Kristol, Koen
- Holtman, John Franks, Josh Cohen, Alex Hopmann, Scott Lawrence, and
- Larry Masinter for their help. And thanks go particularly to Jeff
- Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
-
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 157]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
- Frystyk implemented RFC 2068 early, and we wish to thank them for the
- discovery of many of the problems that this document attempts to
- rectify.
-
-17 References
-
- [1] Alvestrand, H., "Tags for the Identification of Languages", RFC
- 1766, March 1995.
-
- [2] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey,
- D. and B. Alberti, "The Internet Gopher Protocol (a distributed
- document search and retrieval protocol)", RFC 1436, March 1993.
-
- [3] Berners-Lee, T., "Universal Resource Identifiers in WWW", RFC
- 1630, June 1994.
-
- [4] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform Resource
- Locators (URL)", RFC 1738, December 1994.
-
- [5] Berners-Lee, T. and D. Connolly, "Hypertext Markup Language -
- 2.0", RFC 1866, November 1995.
-
- [6] Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext Transfer
- Protocol -- HTTP/1.0", RFC 1945, May 1996.
-
- [7] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
- Extensions (MIME) Part One: Format of Internet Message Bodies",
- RFC 2045, November 1996.
-
- [8] Braden, R., "Requirements for Internet Hosts -- Communication
- Layers", STD 3, RFC 1123, October 1989.
-
- [9] Crocker, D., "Standard for The Format of ARPA Internet Text
- Messages", STD 11, RFC 822, August 1982.
-
- [10] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R.,
- Sui, J., and M. Grinbaum, "WAIS Interface Protocol Prototype
- Functional Specification," (v1.5), Thinking Machines
- Corporation, April 1990.
-
- [11] Fielding, R., "Relative Uniform Resource Locators", RFC 1808,
- June 1995.
-
- [12] Horton, M. and R. Adams, "Standard for Interchange of USENET
- Messages", RFC 1036, December 1987.
-
-
-
-
-
-Fielding, et al. Standards Track [Page 158]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- [13] Kantor, B. and P. Lapsley, "Network News Transfer Protocol", RFC
- 977, February 1986.
-
- [14] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
- Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
- November 1996.
-
- [15] Nebel, E. and L. Masinter, "Form-based File Upload in HTML", RFC
- 1867, November 1995.
-
- [16] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
- August 1982.
-
- [17] Postel, J., "Media Type Registration Procedure", RFC 1590,
- November 1996.
-
- [18] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC
- 959, October 1985.
-
- [19] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
- October 1994.
-
- [20] Sollins, K. and L. Masinter, "Functional Requirements for
- Uniform Resource Names", RFC 1737, December 1994.
-
- [21] US-ASCII. Coded Character Set - 7-Bit American Standard Code for
- Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.
-
- [22] ISO-8859. International Standard -- Information Processing --
- 8-bit Single-Byte Coded Graphic Character Sets --
- Part 1: Latin alphabet No. 1, ISO-8859-1:1987.
- Part 2: Latin alphabet No. 2, ISO-8859-2, 1987.
- Part 3: Latin alphabet No. 3, ISO-8859-3, 1988.
- Part 4: Latin alphabet No. 4, ISO-8859-4, 1988.
- Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988.
- Part 6: Latin/Arabic alphabet, ISO-8859-6, 1987.
- Part 7: Latin/Greek alphabet, ISO-8859-7, 1987.
- Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988.
- Part 9: Latin alphabet No. 5, ISO-8859-9, 1990.
-
- [23] Meyers, J. and M. Rose, "The Content-MD5 Header Field", RFC
- 1864, October 1995.
-
- [24] Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", RFC
- 1900, February 1996.
-
- [25] Deutsch, P., "GZIP file format specification version 4.3", RFC
- 1952, May 1996.
-
-
-
-Fielding, et al. Standards Track [Page 159]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- [26] Venkata N. Padmanabhan, and Jeffrey C. Mogul. "Improving HTTP
- Latency", Computer Networks and ISDN Systems, v. 28, pp. 25-35,
- Dec. 1995. Slightly revised version of paper in Proc. 2nd
- International WWW Conference '94: Mosaic and the Web, Oct. 1994,
- which is available at
- http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLat
- ency.html.
-
- [27] Joe Touch, John Heidemann, and Katia Obraczka. "Analysis of HTTP
- Performance", <URL: http://www.isi.edu/touch/pubs/http-perf96/>,
- ISI Research Report ISI/RR-98-463, (original report dated Aug.
- 1996), USC/Information Sciences Institute, August 1998.
-
- [28] Mills, D., "Network Time Protocol (Version 3) Specification,
- Implementation and Analysis", RFC 1305, March 1992.
-
- [29] Deutsch, P., "DEFLATE Compressed Data Format Specification
- version 1.3", RFC 1951, May 1996.
-
- [30] S. Spero, "Analysis of HTTP Performance Problems,"
- http://sunsite.unc.edu/mdma-release/http-prob.html.
-
- [31] Deutsch, P. and J. Gailly, "ZLIB Compressed Data Format
- Specification version 3.3", RFC 1950, May 1996.
-
- [32] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
- Luotonen, A., Sink, E. and L. Stewart, "An Extension to HTTP:
- Digest Access Authentication", RFC 2069, January 1997.
-
- [33] Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.
- Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
- 2068, January 1997.
-
- [34] Bradner, S., "Key words for use in RFCs to Indicate Requirement
- Levels", BCP 14, RFC 2119, March 1997.
-
- [35] Troost, R. and Dorner, S., "Communicating Presentation
- Information in Internet Messages: The Content-Disposition
- Header", RFC 1806, June 1995.
-
- [36] Mogul, J., Fielding, R., Gettys, J. and H. Frystyk, "Use and
- Interpretation of HTTP Version Numbers", RFC 2145, May 1997.
- [jg639]
-
- [37] Palme, J., "Common Internet Message Headers", RFC 2076, February
- 1997. [jg640]
-
-
-
-
-
-Fielding, et al. Standards Track [Page 160]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- [38] Yergeau, F., "UTF-8, a transformation format of Unicode and
- ISO-10646", RFC 2279, January 1998. [jg641]
-
- [39] Nielsen, H.F., Gettys, J., Baird-Smith, A., Prud'hommeaux, E.,
- Lie, H., and C. Lilley. "Network Performance Effects of
- HTTP/1.1, CSS1, and PNG," Proceedings of ACM SIGCOMM '97, Cannes
- France, September 1997.[jg642]
-
- [40] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
- Extensions (MIME) Part Two: Media Types", RFC 2046, November
- 1996. [jg643]
-
- [41] Alvestrand, H., "IETF Policy on Character Sets and Languages",
- BCP 18, RFC 2277, January 1998. [jg644]
-
- [42] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
- Identifiers (URI): Generic Syntax and Semantics", RFC 2396,
- August 1998. [jg645]
-
- [43] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
- Leach, P., Luotonen, A., Sink, E. and L. Stewart, "HTTP
- Authentication: Basic and Digest Access Authentication", RFC
- 2617, June 1999. [jg646]
-
- [44] Luotonen, A., "Tunneling TCP based protocols through Web proxy
- servers," Work in Progress. [jg647]
-
- [45] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
- Aggregate Documents, such as HTML (MHTML)", RFC 2110, March
- 1997.
-
- [46] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
- 9, RFC 2026, October 1996.
-
- [47] Masinter, L., "Hyper Text Coffee Pot Control Protocol
- (HTCPCP/1.0)", RFC 2324, 1 April 1998.
-
- [48] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
- Extensions (MIME) Part Five: Conformance Criteria and Examples",
- RFC 2049, November 1996.
-
- [49] Troost, R., Dorner, S. and K. Moore, "Communicating Presentation
- Information in Internet Messages: The Content-Disposition Header
- Field", RFC 2183, August 1997.
-
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 161]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
-18 Authors' Addresses
-
- Roy T. Fielding
- Information and Computer Science
- University of California, Irvine
- Irvine, CA 92697-3425, USA
-
- Fax: +1 (949) 824-1715
- EMail: fielding@ics.uci.edu
-
-
- James Gettys
- World Wide Web Consortium
- MIT Laboratory for Computer Science
- 545 Technology Square
- Cambridge, MA 02139, USA
-
- Fax: +1 (617) 258 8682
- EMail: jg@w3.org
-
-
- Jeffrey C. Mogul
- Western Research Laboratory
- Compaq Computer Corporation
- 250 University Avenue
- Palo Alto, California, 94305, USA
-
- EMail: mogul@wrl.dec.com
-
-
- Henrik Frystyk Nielsen
- World Wide Web Consortium
- MIT Laboratory for Computer Science
- 545 Technology Square
- Cambridge, MA 02139, USA
-
- Fax: +1 (617) 258 8682
- EMail: frystyk@w3.org
-
-
- Larry Masinter
- Xerox Corporation
- 3333 Coyote Hill Road
- Palo Alto, CA 94034, USA
-
- EMail: masinter@parc.xerox.com
-
-
-
-
-
-Fielding, et al. Standards Track [Page 162]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- Paul J. Leach
- Microsoft Corporation
- 1 Microsoft Way
- Redmond, WA 98052, USA
-
- EMail: paulle@microsoft.com
-
-
- Tim Berners-Lee
- Director, World Wide Web Consortium
- MIT Laboratory for Computer Science
- 545 Technology Square
- Cambridge, MA 02139, USA
-
- Fax: +1 (617) 258 8682
- EMail: timbl@w3.org
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 163]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
-19 Appendices
-
-19.1 Internet Media Type message/http and application/http
-
- In addition to defining the HTTP/1.1 protocol, this document serves
- as the specification for the Internet media type "message/http" and
- "application/http". The message/http type can be used to enclose a
- single HTTP request or response message, provided that it obeys the
- MIME restrictions for all "message" types regarding line length and
- encodings. The application/http type can be used to enclose a
- pipeline of one or more HTTP request or response messages (not
- intermixed). The following is to be registered with IANA [17].
-
- Media Type name: message
- Media subtype name: http
- Required parameters: none
- Optional parameters: version, msgtype
- version: The HTTP-Version number of the enclosed message
- (e.g., "1.1"). If not present, the version can be
- determined from the first line of the body.
- msgtype: The message type -- "request" or "response". If not
- present, the type can be determined from the first
- line of the body.
- Encoding considerations: only "7bit", "8bit", or "binary" are
- permitted
- Security considerations: none
-
- Media Type name: application
- Media subtype name: http
- Required parameters: none
- Optional parameters: version, msgtype
- version: The HTTP-Version number of the enclosed messages
- (e.g., "1.1"). If not present, the version can be
- determined from the first line of the body.
- msgtype: The message type -- "request" or "response". If not
- present, the type can be determined from the first
- line of the body.
- Encoding considerations: HTTP messages enclosed by this type
- are in "binary" format; use of an appropriate
- Content-Transfer-Encoding is required when
- transmitted via E-mail.
- Security considerations: none
-
-
-
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 164]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
-19.2 Internet Media Type multipart/byteranges
-
- When an HTTP 206 (Partial Content) response message includes the
- content of multiple ranges (a response to a request for multiple
- non-overlapping ranges), these are transmitted as a multipart
- message-body. The media type for this purpose is called
- "multipart/byteranges".
-
- The multipart/byteranges media type includes two or more parts, each
- with its own Content-Type and Content-Range fields. The required
- boundary parameter specifies the boundary string used to separate
- each body-part.
-
- Media Type name: multipart
- Media subtype name: byteranges
- Required parameters: boundary
- Optional parameters: none
- Encoding considerations: only "7bit", "8bit", or "binary" are
- permitted
- Security considerations: none
-
-
- For example:
-
- HTTP/1.1 206 Partial Content
- Date: Wed, 15 Nov 1995 06:25:24 GMT
- Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
- Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES
-
- --THIS_STRING_SEPARATES
- Content-type: application/pdf
- Content-range: bytes 500-999/8000
-
- ...the first range...
- --THIS_STRING_SEPARATES
- Content-type: application/pdf
- Content-range: bytes 7000-7999/8000
-
- ...the second range
- --THIS_STRING_SEPARATES--
-
- Notes:
-
- 1) Additional CRLFs may precede the first boundary string in the
- entity.
-
-
-
-
-
-
-Fielding, et al. Standards Track [Page 165]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- 2) Although RFC 2046 [40] permits the boundary string to be
- quoted, some existing implementations handle a quoted boundary
- string incorrectly.
-
- 3) A number of browsers and servers were coded to an early draft
- of the byteranges specification to use a media type of
- multipart/x-byteranges, which is almost, but not quite
- compatible with the version documented in HTTP/1.1.
-
-19.3 Tolerant Applications
-
- Although this document specifies the requirements for the generation
- of HTTP/1.1 messages, not all applications will be correct in their
- implementation. We therefore recommend that operational applications
- be tolerant of deviations whenever those deviations can be
- interpreted unambiguously.
-
- Clients SHOULD be tolerant in parsing the Status-Line and servers
- tolerant when parsing the Request-Line. In particular, they SHOULD
- accept any amount of SP or HT characters between fields, even though
- only a single SP is required.
-
- The line terminator for message-header fields is the sequence CRLF.
- However, we recommend that applications, when parsing such headers,
- recognize a single LF as a line terminator and ignore the leading CR.
-
- The character set of an entity-body SHOULD be labeled as the lowest
- common denominator of the character codes used within that body, with
- the exception that not labeling the entity is preferred over labeling
- the entity with the labels US-ASCII or ISO-8859-1. See section 3.7.1
- and 3.4.1.
-
- Additional rules for requirements on parsing and encoding of dates
- and other potential problems with date encodings include:
-
- - HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date
- which appears to be more than 50 years in the future is in fact
- in the past (this helps solve the "year 2000" problem).
-
- - An HTTP/1.1 implementation MAY internally represent a parsed
- Expires date as earlier than the proper value, but MUST NOT
- internally represent a parsed Expires date as later than the
- proper value.
-
- - All expiration-related calculations MUST be done in GMT. The
- local time zone MUST NOT influence the calculation or comparison
- of an age or expiration time.
-
-
-
-
-Fielding, et al. Standards Track [Page 166]
-
-RFC 2616 HTTP/1.1 June 1999
-
-
- - If an HTTP header incorrectly carries a date value with a time
- zone other than GMT, it MUST be converted into GMT using the
- most conservative possible conversion.
-
-19.4 Differences Between HTTP Entities and RFC 2045 Entities
-
- HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC
- 822 [9]) and the Multipurpose Internet Mail Extensions (MIME [7]) to
- allow entities to be transmitted in an open variety of
- representations and with extensible mechanisms. However, RFC 2045
- discusses mail, and HTTP has a few features that are different from
- those described in RFC 2045. These differences were carefully chosen
- to optimize performance over binary connections, to allow greater
- freedom in the use of new media types, to make date comparisons
- easier, and to acknowledge the practice of some early HTTP servers
- and clients.
-
- This appendix describes specific areas where HTTP differs from RFC
- 2045. Proxies and gateways to strict MIME environments SHOULD be
- aware of these differences and provide the appropriate conversions
- where necessary. Proxies and gateways from MIME environments to HTTP
- also need to be aware of the differences because some conversions
- might be required.
-
-19.4.1 MIME-Version
-
- HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages MAY
- include a single MIME-Version general-header field to indicate what
- version of the MIME protocol was used to construct the message. Use
- of the MIME-Version header field indicates that the message is in
- full compliance with the MIME protocol (as defined in RFC 2045[7]).
- Proxies/gateways are responsible for ensuring full compliance (where
- possible) when exporting HTTP messages to strict MIME environments.
-
- MIME-Version = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
-
- MIME version "1.0" is the default for use in HTTP/1.1. However,
- HTTP/1.1 message parsing and semantics are defined by this document
- and not the MIME specification.
-
-19.4.2 Conversion to Canonical Form
-
- RFC 2045 [7] requires that an Internet mail entity be converted to
- canonical form prior to being transferred, as described in section 4
- of RFC 2049 [48]. Section 3.7.1 of this document describes the forms
- allowed for subtypes of the "text" media type when transmitted over
- HTTP. RFC 2046 requires that content with a type of "text" represent
- line breaks as CRLF and forbids the use of CR or LF outside of line
-
-
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-
- 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
-
-
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-
- 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
-
-
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-
- describe proposed experimental features, and some describe features
- that experimental deployment found lacking that are now addressed in
- the base HTTP/1.1 specification.
-
- A number of other headers, such as Content-Disposition and Title,
- from SMTP and MIME are also often implemented (see RFC 2076 [37]).
-
-19.5.1 Content-Disposition
-
- The Content-Disposition response-header field has been proposed as a
- means for the origin server to suggest a default filename if the user
- requests that the content is saved to a file. This usage is derived
- from the definition of Content-Disposition in RFC 1806 [35].
-
- content-disposition = "Content-Disposition" ":"
- disposition-type *( ";" disposition-parm )
- disposition-type = "attachment" | disp-extension-token
- disposition-parm = filename-parm | disp-extension-parm
- filename-parm = "filename" "=" quoted-string
- disp-extension-token = token
- disp-extension-parm = token "=" ( token | quoted-string )
-
- An example is
-
- Content-Disposition: attachment; filename="fname.ext"
-
- The receiving user agent SHOULD NOT respect any directory path
- information present in the filename-parm parameter, which is the only
- parameter believed to apply to HTTP implementations at this time. The
- filename SHOULD be treated as a terminal component only.
-
- If this header is used in a response with the application/octet-
- stream content-type, the implied suggestion is that the user agent
- should not display the response, but directly enter a `save response
- as...' dialog.
-
- See section 15.5 for Content-Disposition security issues.
-
-19.6 Compatibility with Previous Versions
-
- It is beyond the scope of a protocol specification to mandate
- compliance with previous versions. HTTP/1.1 was deliberately
- designed, however, to make supporting previous versions easy. It is
- worth noting that, at the time of composing this specification
- (1996), we would expect commercial HTTP/1.1 servers to:
-
- - recognize the format of the Request-Line for HTTP/0.9, 1.0, and
- 1.1 requests;
-
-
-
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-
-
- - understand any valid request in the format of HTTP/0.9, 1.0, or
- 1.1;
-
- - respond appropriately with a message in the same major version
- used by the client.
-
- And we would expect HTTP/1.1 clients to:
-
- - recognize the format of the Status-Line for HTTP/1.0 and 1.1
- responses;
-
- - understand any valid response in the format of HTTP/0.9, 1.0, or
- 1.1.
-
- For most implementations of HTTP/1.0, each connection is established
- by the client prior to the request and closed by the server after
- sending the response. Some implementations implement the Keep-Alive
- version of persistent connections described in section 19.7.1 of RFC
- 2068 [33].
-
-19.6.1 Changes from HTTP/1.0
-
- This section summarizes major differences between versions HTTP/1.0
- and HTTP/1.1.
-
-19.6.1.1 Changes to Simplify Multi-homed Web Servers and Conserve IP
- Addresses
-
- The requirements that clients and servers support the Host request-
- header, report an error if the Host request-header (section 14.23) is
- missing from an HTTP/1.1 request, and accept absolute URIs (section
- 5.1.2) are among the most important changes defined by this
- specification.
-
- Older HTTP/1.0 clients assumed a one-to-one relationship of IP
- addresses and servers; there was no other established mechanism for
- distinguishing the intended server of a request than the IP address
- to which that request was directed. The changes outlined above will
- allow the Internet, once older HTTP clients are no longer common, to
- support multiple Web sites from a single IP address, greatly
- simplifying large operational Web servers, where allocation of many
- IP addresses to a single host has created serious problems. The
- Internet will also be able to recover the IP addresses that have been
- allocated for the sole purpose of allowing special-purpose domain
- names to be used in root-level HTTP URLs. Given the rate of growth of
- the Web, and the number of servers already deployed, it is extremely
-
-
-
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-
- important that all implementations of HTTP (including updates to
- existing HTTP/1.0 applications) correctly implement these
- requirements:
-
- - Both clients and servers MUST support the Host request-header.
-
- - A client that sends an HTTP/1.1 request MUST send a Host header.
-
- - Servers MUST report a 400 (Bad Request) error if an HTTP/1.1
- request does not include a Host request-header.
-
- - Servers MUST accept absolute URIs.
-
-19.6.2 Compatibility with HTTP/1.0 Persistent Connections
-
- Some clients and servers might wish to be compatible with some
- previous implementations of persistent connections in HTTP/1.0
- clients and servers. Persistent connections in HTTP/1.0 are
- explicitly negotiated as they are not the default behavior. HTTP/1.0
- experimental implementations of persistent connections are faulty,
- and the new facilities in HTTP/1.1 are designed to rectify these
- problems. The problem was that some existing 1.0 clients may be
- sending Keep-Alive to a proxy server that doesn't understand
- Connection, which would then erroneously forward it to the next
- inbound server, which would establish the Keep-Alive connection and
- result in a hung HTTP/1.0 proxy waiting for the close on the
- response. The result is that HTTP/1.0 clients must be prevented from
- using Keep-Alive when talking to proxies.
-
- However, talking to proxies is the most important use of persistent
- connections, so that prohibition is clearly unacceptable. Therefore,
- we need some other mechanism for indicating a persistent connection
- is desired, which is safe to use even when talking to an old proxy
- that ignores Connection. Persistent connections are the default for
- HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
- declaring non-persistence. See section 14.10.
-
- The original HTTP/1.0 form of persistent connections (the Connection:
- Keep-Alive and Keep-Alive header) is documented in RFC 2068. [33]
-
-19.6.3 Changes from RFC 2068
-
- This specification has been carefully audited to correct and
- disambiguate key word usage; RFC 2068 had many problems in respect to
- the conventions laid out in RFC 2119 [34].
-
- Clarified which error code should be used for inbound server failures
- (e.g. DNS failures). (Section 10.5.5).
-
-
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-
-
- 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)
-
-
-
-
-
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-
-
- 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.
-
-
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-
-
- 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.
-
-
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-
-
-21. Full Copyright Statement
-
- Copyright (C) The Internet Society (1999). All Rights Reserved.
-
- This document and translations of it may be copied and furnished to
- others, and derivative works that comment on or otherwise explain it
- or assist in its implementation may be prepared, copied, published
- and distributed, in whole or in part, without restriction of any
- kind, provided that the above copyright notice and this paragraph are
- included on all such copies and derivative works. However, this
- document itself may not be modified in any way, such as by removing
- the copyright notice or references to the Internet Society or other
- Internet organizations, except as needed for the purpose of
- developing Internet standards in which case the procedures for
- copyrights defined in the Internet Standards process must be
- followed, or as required to translate it into languages other than
- English.
-
- The limited permissions granted above are perpetual and will not be
- revoked by the Internet Society or its successors or assigns.
-
- This document and the information contained herein is provided on an
- "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
- TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
- BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
- HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
- MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
-
-Acknowledgement
-
- Funding for the RFC Editor function is currently provided by the
- Internet Society.
-
-
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