rfc2616.txt   draft-gettys-http-v11-spec-rev-00.txt 
Network Working Group R. Fielding INTERNET-DRAFT R. Fielding
Request for Comments: 2616 UC Irvine <draft-gettys-http-v11-spec-rev-00> Day Software
Obsoletes: 2068 J. Gettys Obsoletes: 2616 J. Gettys
Category: Standards Track Compaq/W3C Category: Standards Track J. C. Mogul
J. Mogul Expires: June 2004 HP
Compaq
H. Frystyk H. Frystyk
W3C/MIT Microsoft
L. Masinter L. Masinter
Xerox Adobe
P. Leach P. Leach
Microsoft Microsoft
T. Berners-Lee T. Berners-Lee
W3C/MIT W3C/MIT
June 1999 December, 2003
Hypertext Transfer Protocol -- HTTP/1.1 Hypertext Transfer Protocol -- HTTP/1.1
Status of this Memo Status of this Memo
This document specifies an Internet standards track protocol for the This document is an Internet-Draft and is in full conformance with
Internet community, and requests discussion and suggestions for all provisions of Section 10 of RFC2026.
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state Internet-Drafts are working documents of the Internet Engineering
and status of this protocol. Distribution of this memo is unlimited. Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or made obsolete by other documents at
any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
Comments are welcome should be submitted to the mailing list ietf-
http-wg@w3.org
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved. See
section 20 for the full copyright notice.
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypermedia information protocol for distributed, collaborative, hypermedia information
systems. It is a generic, stateless, protocol which can be used for systems. It is a generic, stateless, protocol which can be used for
many tasks beyond its use for hypertext, such as name servers and many tasks beyond its use for hypertext, such as name servers and
distributed object management systems, through extension of its distributed object management systems, through extension of its
request methods, error codes and headers [47]. A feature of HTTP is request methods, error codes and headers [I36]. A feature of HTTP is
the typing and negotiation of data representation, allowing systems the typing and negotiation of data representation, allowing systems
to be built independently of the data being transferred. to be built independently of the data being transferred.
Fielding, et al Standards Track [Page 1]
HTTP has been in use by the World-Wide Web global information HTTP has been in use by the World-Wide Web global information
initiative since 1990. This specification defines the protocol initiative since 1990. This specification defines the protocol
referred to as "HTTP/1.1", and is an update to RFC 2068 [33]. referred to as "HTTP/1.1", and obsoletes RFC 2616 [I39], which
obsoleted RFC 2068 [I25].
Fielding, et al [Page 2]
Table of Contents Table of Contents
1 Introduction ...................................................7 HYPERTEXT TRANSFER PROTOCOL -- HTTP/1.1 1
1.1 Purpose......................................................7 Status of this Memo..............................................1
1.2 Requirements .................................................8 Copyright Notice.................................................1
1.3 Terminology ..................................................8 Abstract.........................................................1
1.4 Overall Operation ...........................................12 Table of Contents................................................3
2 Notational Conventions and Generic Grammar ....................14 1 Introduction..................................................8
2.1 Augmented BNF ...............................................14 1.1 Purpose...................................................8
2.2 Basic Rules .................................................15 1.2 Requirements..............................................8
3 Protocol Parameters ...........................................17 1.3 Terminology...............................................9
3.1 HTTP Version ................................................17 1.4 Overall Operation........................................12
3.2 Uniform Resource Identifiers ................................18 2 Notational Conventions and Generic Grammar...................14
3.2.1 General Syntax ...........................................19 2.1 Augmented BNF............................................14
3.2.2 http URL .................................................19 2.2 Basic Rules..............................................15
3.2.3 URI Comparison ...........................................20 3 Protocol Parameters..........................................17
3.3 Date/Time Formats ...........................................20 3.1 HTTP Version.............................................17
3.3.1 Full Date ................................................20 3.2 Uniform Resource Identifiers.............................18
3.3.2 Delta Seconds ............................................21 3.2.1 General Syntax......................................18
3.4 Character Sets ..............................................21 3.2.2 http URL............................................18
3.4.1 Missing Charset ..........................................22 3.2.3 URI Comparison......................................19
3.5 Content Codings .............................................23 3.3 Date/Time Formats........................................19
3.6 Transfer Codings ............................................24 3.3.1 Full Date...........................................19
3.6.1 Chunked Transfer Coding ..................................25 3.3.2 Delta Seconds.......................................20
3.7 Media Types .................................................26 3.4 Character Sets...........................................20
3.7.1 Canonicalization and Text Defaults .......................27 3.5 Content Codings..........................................22
3.7.2 Multipart Types ..........................................27 3.6 Transfer Codings.........................................23
3.8 Product Tokens ..............................................28 3.6.1 Chunked Transfer Coding.............................23
3.9 Quality Values ..............................................29 3.7 Media Types..............................................25
3.10 Language Tags ...............................................29 3.7.1 Canonicalization and Text Defaults..................25
3.11 Entity Tags .................................................30 3.7.2 Multipart Types.....................................26
3.12 Range Units .................................................30 3.8 Product Tokens...........................................26
4 HTTP Message ..................................................31 3.9 Quality Values...........................................27
4.1 Message Types ...............................................31 3.10 Language Tags...........................................27
4.2 Message Headers .............................................31 3.11 Entity Tags.............................................28
4.3 Message Body ................................................32 3.12 Range Units.............................................28
4.4 Message Length ..............................................33 4 HTTP Message.................................................29
4.5 General Header Fields .......................................34 4.1 Message Types............................................29
5 Request .......................................................35 4.2 Message Headers..........................................29
5.1 Request-Line ................................................35 4.3 Message Body.............................................30
5.1.1 Method ...................................................36 4.4 Message Length...........................................31
5.1.2 Request-URI ..............................................36 4.5 General Header Fields....................................32
5.2 The Resource Identified by a Request ........................38 5 Request......................................................33
5.3 Request Header Fields .......................................38 5.1 Request-Line.............................................33
6 Response ......................................................39 5.1.1 Method..............................................33
6.1 Status-Line .................................................39 5.1.2 Request-URI.........................................33
6.1.1 Status Code and Reason Phrase ............................39 5.2 The Resource Identified by a Request.....................35
6.2 Response Header Fields ......................................41 5.3 Request Header Fields....................................35
7 Entity ........................................................42 6 Response.....................................................36
7.1 Entity Header Fields ........................................42 6.1 Status-Line..............................................36
7.2 Entity Body .................................................43 6.1.1 Status Code and Reason Phrase.......................36
7.2.1 Type .....................................................43 6.2 Response Header Fields...................................38
7.2.2 Entity Length ............................................43
8 Connections ...................................................44 Fielding, et al [Page 3]
8.1 Persistent Connections ......................................44 7 Entity.......................................................38
8.1.1 Purpose ..................................................44 7.1 Entity Header Fields.....................................39
8.1.2 Overall Operation ........................................45 7.2 Entity Body..............................................39
8.1.3 Proxy Servers ............................................46 7.2.1 Type................................................39
8.1.4 Practical Considerations .................................46 7.2.2 Entity Length.......................................40
8.2 Message Transmission Requirements ...........................47 8 Connections..................................................40
8.2.1 Persistent Connections and Flow Control ..................47 8.1 Persistent Connections...................................40
8.2.2 Monitoring Connections for Error Status Messages .........48 8.1.1 Purpose.............................................40
8.2.3 Use of the 100 (Continue) Status .........................48 8.1.2 Overall Operation...................................41
8.2.4 Client Behavior if Server Prematurely Closes Connection ..50 8.1.3 Proxy Servers.......................................42
9 Method Definitions ............................................51 8.1.4 Practical Considerations............................42
9.1 Safe and Idempotent Methods .................................51 8.2 Message Transmission Requirements........................43
9.1.1 Safe Methods .............................................51 8.2.1 Persistent Connections and Flow Control.............43
9.1.2 Idempotent Methods .......................................51 8.2.2 Monitoring Connections for Error Status Messages....43
9.2 OPTIONS .....................................................52 8.2.3 Use of the 100 (Continue) Status....................44
9.3 GET .........................................................53 8.2.4 Client Behavior if Server Prematurely Closes Connection
9.4 HEAD ........................................................54 45
9.5 POST ........................................................54 9 Method Definitions...........................................46
9.6 PUT .........................................................55 9.1 Safe and Idempotent Methods..............................46
9.7 DELETE ......................................................56 9.1.1 Safe Methods........................................46
9.8 TRACE .......................................................56 9.1.2 Idempotent Methods..................................47
9.9 CONNECT .....................................................57 9.2 OPTIONS..................................................47
10 Status Code Definitions ......................................57 9.3 GET......................................................48
10.1 Informational 1xx ...........................................57 9.4 HEAD.....................................................49
10.1.1 100 Continue .............................................58 9.5 POST.....................................................49
10.1.2 101 Switching Protocols ..................................58 9.6 PUT......................................................50
10.2 Successful 2xx ..............................................58 9.7 DELETE...................................................51
10.2.1 200 OK ...................................................58 9.8 TRACE....................................................51
10.2.2 201 Created ..............................................59 9.9 CONNECT..................................................52
10.2.3 202 Accepted .............................................59 10 Status Code Definitions...................................52
10.2.4 203 Non-Authoritative Information ........................59 10.1 Informational 1xx.......................................52
10.2.5 204 No Content ...........................................60 10.1.1 100 Continue.......................................52
10.2.6 205 Reset Content ........................................60 10.1.2 101 Switching Protocols............................52
10.2.7 206 Partial Content ......................................60 10.2 Successful 2xx..........................................53
10.3 Redirection 3xx .............................................61 10.2.1 200 OK.............................................53
10.3.1 300 Multiple Choices .....................................61 10.2.2 201 Created........................................53
10.3.2 301 Moved Permanently ....................................62 10.2.3 202 Accepted.......................................53
10.3.3 302 Found ................................................62 10.2.4 203 Non-Authoritative Information..................54
10.3.4 303 See Other ............................................63 10.2.5 204 No Content.....................................54
10.3.5 304 Not Modified .........................................63 10.2.6 205 Reset Content..................................54
10.3.6 305 Use Proxy ............................................64 10.2.7 206 Partial Content................................55
10.3.7 306 (Unused) .............................................64 10.3 Redirection 3xx.........................................55
10.3.8 307 Temporary Redirect ...................................65 10.3.1 300 Multiple Choices...............................56
10.4 Client Error 4xx ............................................65 10.3.2 301 Moved Permanently..............................56
10.4.1 400 Bad Request .........................................65 10.3.3 302 Found..........................................56
10.4.2 401 Unauthorized ........................................66 10.3.4 303 See Other......................................57
10.4.3 402 Payment Required ....................................66 10.3.5 304 Not Modified...................................57
10.4.4 403 Forbidden ...........................................66 10.3.6 305 Use Proxy......................................58
10.4.5 404 Not Found ...........................................66 10.3.7 306 (Unused).......................................58
10.4.6 405 Method Not Allowed ..................................66 10.3.8 307 Temporary Redirect.............................58
10.4.7 406 Not Acceptable ......................................67 10.4 Client Error 4xx........................................59
10.4.8 407 Proxy Authentication Required .......................67 10.4.1 400 Bad Request....................................59
10.4.9 408 Request Timeout .....................................67 10.4.2 401 Unauthorized...................................59
10.4.10 409 Conflict ............................................67 10.4.3 402 Payment Required...............................60
10.4.11 410 Gone ................................................68 10.4.4 403 Forbidden......................................60
10.4.12 411 Length Required .....................................68
10.4.13 412 Precondition Failed .................................68 Fielding, et al [Page 4]
10.4.14 413 Request Entity Too Large ............................69 10.4.5 404 Not Found......................................60
10.4.15 414 Request-URI Too Long ................................69 10.4.6 405 Method Not Allowed.............................60
10.4.16 415 Unsupported Media Type ..............................69 10.4.7 406 Not Acceptable.................................60
10.4.17 416 Requested Range Not Satisfiable .....................69 10.4.8 407 Proxy Authentication Required..................61
10.4.18 417 Expectation Failed ..................................70 10.4.9 408 Request Timeout................................61
10.5 Server Error 5xx ............................................70 10.4.10 409 Conflict......................................61
10.5.1 500 Internal Server Error ................................70 10.4.11 410 Gone..........................................61
10.5.2 501 Not Implemented ......................................70 10.4.12 411 Length Required...............................62
10.5.3 502 Bad Gateway ..........................................70 10.4.13 412 Precondition Failed...........................62
10.5.4 503 Service Unavailable ..................................70 10.4.14 413 Request Entity Too Large......................62
10.5.5 504 Gateway Timeout ......................................71 10.4.15 414 Request-URI Too Long..........................62
10.5.6 505 HTTP Version Not Supported ...........................71 10.4.16 415 Unsupported Media Type........................63
11 Access Authentication ........................................71 10.4.17 416 Requested Range Not Satisfiable...............63
12 Content Negotiation ..........................................71 10.4.18 417 Expectation Failed............................63
12.1 Server-driven Negotiation ...................................72 10.5 Server Error 5xx........................................63
12.2 Agent-driven Negotiation ....................................73 10.5.1 500 Internal Server Error..........................63
12.3 Transparent Negotiation .....................................74 10.5.2 501 Not Implemented................................63
13 Caching in HTTP ..............................................74 10.5.3 502 Bad Gateway....................................64
13.1.1 Cache Correctness ........................................75 10.5.4 503 Service Unavailable............................64
13.1.2 Warnings .................................................76 10.5.5 504 Gateway Timeout................................64
13.1.3 Cache-control Mechanisms .................................77 10.5.6 505 HTTP Version Not Supported.....................64
13.1.4 Explicit User Agent Warnings .............................78 11 Access Authentication.....................................64
13.1.5 Exceptions to the Rules and Warnings .....................78 12 Content Negotiation.......................................65
13.1.6 Client-controlled Behavior ...............................79 12.1 Server-driven Negotiation...............................65
13.2 Expiration Model ............................................79 12.2 Agent-driven Negotiation................................66
13.2.1 Server-Specified Expiration ..............................79 12.3 Transparent Negotiation.................................67
13.2.2 Heuristic Expiration .....................................80 13 Caching in HTTP...........................................67
13.2.3 Age Calculations .........................................80 13.1.1 Cache Correctness..................................68
13.2.4 Expiration Calculations ..................................83 13.1.2 Warnings...........................................69
13.2.5 Disambiguating Expiration Values .........................84 13.1.3 Cache-control Mechanisms...........................70
13.2.6 Disambiguating Multiple Responses ........................84 13.1.4 Explicit User Agent Warnings.......................70
13.3 Validation Model ............................................85 13.1.5 Exceptions to the Rules and Warnings...............71
13.3.1 Last-Modified Dates ......................................86 13.1.6 Client-controlled Behavior.........................71
13.3.2 Entity Tag Cache Validators ..............................86 13.2 Expiration Model........................................72
13.3.3 Weak and Strong Validators ...............................86 13.2.1 Server-Specified Expiration........................72
13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates.89 13.2.2 Heuristic Expiration...............................72
13.3.5 Non-validating Conditionals ..............................90 13.2.3 Age Calculations...................................73
13.4 Response Cacheability .......................................91 13.2.4 Expiration Calculations............................75
13.5 Constructing Responses From Caches ..........................92 13.2.5 Disambiguating Expiration Values...................75
13.5.1 End-to-end and Hop-by-hop Headers ........................92 13.2.6 Disambiguating Multiple Responses..................76
13.5.2 Non-modifiable Headers ...................................92 13.3 Validation Model........................................76
13.5.3 Combining Headers ........................................94 13.3.1 Last-Modified Dates................................77
13.5.4 Combining Byte Ranges ....................................95 13.3.2 Entity Tag Cache Validators........................77
13.6 Caching Negotiated Responses ................................95 13.3.3 Weak and Strong Validators.........................78
13.7 Shared and Non-Shared Caches ................................96 13.3.4 Rules for When to Use Entity Tags and Last-Modified
13.8 Errors or Incomplete Response Cache Behavior ................97 Dates 80
13.9 Side Effects of GET and HEAD ................................97 13.3.5 Non-validating Conditionals........................81
13.10 Invalidation After Updates or Deletions ...................97 13.4 Response Cacheability...................................82
13.11 Write-Through Mandatory ...................................98 13.5 Constructing Responses From Caches......................82
13.12 Cache Replacement .........................................99 13.5.1 End-to-end and Hop-by-hop Headers..................83
13.13 History Lists .............................................99 13.5.2 Non-modifiable Headers.............................83
14 Header Field Definitions ....................................100 13.5.3 Combining Headers..................................84
14.1 Accept .....................................................100 13.5.4 Combining Byte Ranges..............................85
14.2 Accept-Charset .............................................102 13.6 Caching Negotiated Responses............................85
14.3 Accept-Encoding ............................................102 13.7 Shared and Non-Shared Caches............................87
14.4 Accept-Language ............................................104
14.5 Accept-Ranges ..............................................105 Fielding, et al [Page 5]
14.6 Age ........................................................106 13.8 Errors or Incomplete Response Cache Behavior............87
14.7 Allow ......................................................106 13.9 Side Effects of GET and HEAD............................87
14.8 Authorization ..............................................107 13.10 Invalidation After Updates or Deletions................88
14.9 Cache-Control ..............................................108 13.11 Write-Through Mandatory................................88
14.9.1 What is Cacheable .......................................109 13.12 Cache Replacement......................................89
14.9.2 What May be Stored by Caches ............................110 13.13 History Lists..........................................89
14.9.3 Modifications of the Basic Expiration Mechanism .........111 14 Header Field Definitions..................................89
14.9.4 Cache Revalidation and Reload Controls ..................113 14.1 Accept..................................................90
14.9.5 No-Transform Directive ..................................115 14.2 Accept-Charset..........................................91
14.9.6 Cache Control Extensions ................................116 14.3 Accept-Encoding.........................................92
14.10 Connection ...............................................117 14.4 Accept-Language.........................................93
14.11 Content-Encoding .........................................118 14.5 Accept-Ranges...........................................94
14.12 Content-Language .........................................118 14.6 Age.....................................................95
14.13 Content-Length ...........................................119 14.7 Allow...................................................95
14.14 Content-Location .........................................120 14.8 Authorization...........................................96
14.15 Content-MD5 ..............................................121 14.9 Cache-Control...........................................96
14.16 Content-Range ............................................122 14.9.1 What is Cacheable..................................98
14.17 Content-Type .............................................124 14.9.2 What May be Stored by Caches.......................99
14.18 Date .....................................................124 14.9.3 Modifications of the Basic Expiration Mechanism....99
14.18.1 Clockless Origin Server Operation ......................125 14.9.4 Cache Revalidation and Reload Controls............101
14.19 ETag .....................................................126 14.9.5 No-Transform Directive............................103
14.20 Expect ...................................................126 14.9.6 Cache Control Extensions..........................104
14.21 Expires ..................................................127 14.10 Connection............................................104
14.22 From .....................................................128 14.11 Content-Encoding......................................105
14.23 Host .....................................................128 14.12 Content-Language......................................106
14.24 If-Match .................................................129 14.13 Content-Length........................................107
14.25 If-Modified-Since ........................................130 14.14 Content-Location......................................107
14.26 If-None-Match ............................................132 14.15 Content-MD5...........................................108
14.27 If-Range .................................................133 14.16 Content-Range.........................................109
14.28 If-Unmodified-Since ......................................134 14.17 Content-Type..........................................111
14.29 Last-Modified ............................................134 14.18 Date..................................................111
14.30 Location .................................................135 14.18.1 Clockless Origin Server Operation................112
14.31 Max-Forwards .............................................136 14.19 ETag..................................................112
14.32 Pragma ...................................................136 14.20 Expect................................................113
14.33 Proxy-Authenticate .......................................137 14.21 Expires...............................................113
14.34 Proxy-Authorization ......................................137 14.22 From..................................................114
14.35 Range ....................................................138 14.23 Host..................................................115
14.35.1 Byte Ranges ...........................................138 14.24 If-Match..............................................115
14.35.2 Range Retrieval Requests ..............................139 14.25 If-Modified-Since.....................................116
14.36 Referer ..................................................140 14.26 If-None-Match.........................................118
14.37 Retry-After ..............................................141 14.27 If-Range..............................................119
14.38 Server ...................................................141 14.28 If-Unmodified-Since...................................119
14.39 TE .......................................................142 14.29 Last-Modified.........................................120
14.40 Trailer ..................................................143 14.30 Location..............................................120
14.41 Transfer-Encoding..........................................143 14.31 Max-Forwards..........................................121
14.42 Upgrade ..................................................144 14.32 Pragma................................................122
14.43 User-Agent ...............................................145 14.33 Proxy-Authenticate....................................122
14.44 Vary .....................................................145 14.34 Proxy-Authorization...................................123
14.45 Via ......................................................146 14.35 Range.................................................123
14.46 Warning ..................................................148 14.35.1 Byte Ranges......................................123
14.47 WWW-Authenticate .........................................150 14.35.2 Range Retrieval Requests.........................125
15 Security Considerations .......................................150 14.36 Referer...............................................125
15.1 Personal Information....................................151 14.37 Retry-After...........................................126
15.1.1 Abuse of Server Log Information .........................151 14.38 Server................................................126
15.1.2 Transfer of Sensitive Information .......................151 14.39 TE....................................................126
15.1.3 Encoding Sensitive Information in URI's .................152
15.1.4 Privacy Issues Connected to Accept Headers ..............152 Fielding, et al [Page 6]
15.2 Attacks Based On File and Path Names .......................153 14.40 Trailer...............................................128
15.3 DNS Spoofing ...............................................154 14.41 Transfer-Encoding.....................................128
15.4 Location Headers and Spoofing ..............................154 14.42 Upgrade...............................................128
15.5 Content-Disposition Issues .................................154 14.43 User-Agent............................................129
15.6 Authentication Credentials and Idle Clients ................155 14.44 Vary..................................................130
15.7 Proxies and Caching ........................................155 14.45 Via...................................................131
15.7.1 Denial of Service Attacks on Proxies....................156 14.46 Warning...............................................132
16 Acknowledgments .............................................156 14.47 WWW-Authenticate......................................134
17 References ..................................................158 15 Security Considerations..................................134
18 Authors' Addresses ..........................................162 15.1 Personal Information...................................135
19 Appendices ..................................................164 15.1.1 Abuse of Server Log Information...................135
19.1 Internet Media Type message/http and application/http ......164 15.1.2 Transfer of Sensitive Information.................135
19.2 Internet Media Type multipart/byteranges ...................165 15.1.3 Encoding Sensitive Information in URI's...........136
19.3 Tolerant Applications ......................................166 15.1.4 Privacy Issues Connected to Accept Headers........136
19.4 Differences Between HTTP Entities and RFC 2045 Entities ....167 15.2 Attacks Based On File and Path Names...................137
19.4.1 MIME-Version ............................................167 15.3 DNS Spoofing...........................................137
19.4.2 Conversion to Canonical Form ............................167 15.4 Location Headers and Spoofing..........................138
19.4.3 Conversion of Date Formats ..............................168 15.5 Content-Disposition Issues.............................138
19.4.4 Introduction of Content-Encoding ........................168 15.6 Authentication Credentials and Idle Clients............138
19.4.5 No Content-Transfer-Encoding ............................168 15.7 Proxies and Caching....................................139
19.4.6 Introduction of Transfer-Encoding .......................169 15.7.1 Denial of Service Attacks on Proxies..............139
19.4.7 MHTML and Line Length Limitations .......................169 16 Acknowledgments..........................................140
19.5 Additional Features ........................................169 17 Appendices...............................................141
19.5.1 Content-Disposition .....................................170 17.1 IANA Considerations - Internet Media Type message/http and
19.6 Compatibility with Previous Versions .......................170 application/http............................................141
19.6.1 Changes from HTTP/1.0 ...................................171 17.2 IANA Considerations - Internet Media Type
19.6.2 Compatibility with HTTP/1.0 Persistent Connections ......172 multipart/byteranges........................................142
19.6.3 Changes from RFC 2068 ...................................172 17.3 Tolerant Applications..................................143
20 Index .......................................................175 17.4 Differences Between HTTP Entities and RFC 2045 Entities143
21 Full Copyright Statement ....................................176 17.4.1 MIME-Version......................................144
17.4.2 Conversion to Canonical Form......................144
17.4.3 Conversion of Date Formats........................145
17.4.4 Introduction of Content-Encoding..................145
17.4.5 No Content-Transfer-Encoding......................145
17.4.6 Introduction of Transfer-Encoding.................145
17.4.7 MHTML and Line Length Limitations.................146
17.5 Additional Features....................................146
17.5.1 Content-Disposition...............................146
17.6 Compatibility with Previous Versions...................147
17.6.1 Changes from HTTP/1.0.............................147
17.6.2 Compatibility with HTTP/1.0 Persistent Connections148
17.6.3 Changes from RFC 2616.............................149
18 References...............................................150
18.1 Normative References...................................150
18.2 Informative References.................................151
19 Authors' Addresses.......................................154
20 Full Copyright Statement.................................156
20.1 Acknowledgement........................................156
21 Index....................................................157
Fielding, et al [Page 7]
1 Introduction 1 Introduction
1.1 Purpose 1.1 Purpose
The Hypertext Transfer Protocol (HTTP) is an application-level The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypermedia information protocol for distributed, collaborative, hypermedia information
systems. HTTP has been in use by the World-Wide Web global systems. HTTP has been in use by the World-Wide Web global
information initiative since 1990. The first version of HTTP, information initiative since 1990. The first version of HTTP,
referred to as HTTP/0.9, was a simple protocol for raw data transfer 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 across the Internet. HTTP/1.0, as defined by RFC 1945 [I6], improved
the protocol by allowing messages to be in the format of MIME-like the protocol by allowing messages to be in the format of MIME-like
messages, containing metainformation about the data transferred and messages, containing metainformation about the data transferred and
modifiers on the request/response semantics. However, HTTP/1.0 does modifiers on the request/response semantics. However, HTTP/1.0 does
not sufficiently take into consideration the effects of hierarchical not sufficiently take into consideration the effects of hierarchical
proxies, caching, the need for persistent connections, or virtual proxies, caching, the need for persistent connections, or virtual
hosts. In addition, the proliferation of incompletely-implemented hosts. In addition, the proliferation of incompletely-implemented
applications calling themselves "HTTP/1.0" has necessitated a applications calling themselves "HTTP/1.0" has necessitated a
protocol version change in order for two communicating applications protocol version change in order for two communicating applications
to determine each other's true capabilities. to determine each other's true capabilities.
This specification defines the protocol referred to as "HTTP/1.1". This specification defines the protocol referred to as "HTTP/1.1".
This protocol includes more stringent requirements than HTTP/1.0 in This protocol includes more stringent requirements than HTTP/1.0 in
order to ensure reliable implementation of its features. order to ensure reliable implementation of its features.
Practical information systems require more functionality than simple Practical information systems require more functionality than simple
retrieval, including search, front-end update, and annotation. HTTP retrieval, including search, front-end update, and annotation. HTTP
allows an open-ended set of methods and headers that indicate the allows an open-ended set of methods and headers that indicate the
purpose of a request [47]. It builds on the discipline of reference purpose of a request [I36]. It builds on the discipline of reference
provided by the Uniform Resource Identifier (URI) [3], as a location provided by the Uniform Resource Identifier (URI) [I3], [N9], as a
(URL) [4] or name (URN) [20], for indicating the resource to which a location (URL) [I4] or name (URN) [I15], for indicating the resource
method is to be applied. Messages are passed in a format similar to to which a method is to be applied. Messages are passed in a format
that used by Internet mail [9] as defined by the Multipurpose similar to that used by Internet mail [I9] as defined by the
Internet Mail Extensions (MIME) [7]. Multipurpose Internet Mail Extensions (MIME) [N1].
HTTP is also used as a generic protocol for communication between HTTP is also used as a generic protocol for communication between
user agents and proxies/gateways to other Internet systems, including user agents and proxies/gateways to other Internet systems, including
those supported by the SMTP [16], NNTP [13], FTP [18], Gopher [2], those supported by the SMTP [I12], NNTP [I10], FTP [I13], Gopher
and WAIS [10] protocols. In this way, HTTP allows basic hypermedia [I2], and WAIS [I7] protocols. In this way, HTTP allows basic
access to resources available from diverse applications. hypermedia access to resources available from diverse applications.
1.2 Requirements 1.2 Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [34]. document are to be interpreted as described in RFC 2119 [N34].
An implementation is not compliant if it fails to satisfy one or more An implementation is not compliant if it fails to satisfy one or more
of the MUST or REQUIRED level requirements for the protocols it of the MUST or REQUIRED level requirements for the protocols it
implements. An implementation that satisfies all the MUST or REQUIRED implements. An implementation that satisfies all the MUST or REQUIRED
level and all the SHOULD level requirements for its protocols is said level and all the SHOULD level requirements for its protocols is said
to be "unconditionally compliant"; one that satisfies all the MUST to be "unconditionally compliant"; one that satisfies all the MUST
level requirements but not all the SHOULD level requirements for its level requirements but not all the SHOULD level requirements for its
protocols is said to be "conditionally compliant." protocols is said to be "conditionally compliant."
Fielding, et al [Page 8]
1.3 Terminology 1.3 Terminology
This specification uses a number of terms to refer to the roles This specification uses a number of terms to refer to the roles
played by participants in, and objects of, the HTTP communication. played by participants in, and objects of, the HTTP communication.
connection connection
A transport layer virtual circuit established between two programs A transport layer virtual circuit established between two programs
for the purpose of communication. for the purpose of communication.
message message
skipping to change at page 9, line 6 skipping to change at line 419
sequence of octets matching the syntax defined in section 4 and sequence of octets matching the syntax defined in section 4 and
transmitted via the connection. transmitted via the connection.
request request
An HTTP request message, as defined in section 5. An HTTP request message, as defined in section 5.
response response
An HTTP response message, as defined in section 6. An HTTP response message, as defined in section 6.
resource resource
A network data object or service that can be identified by a URI, A network data object or service that can be identified by a URI, as
as defined in section 3.2. Resources may be available in multiple
defined in section 3.2. Resources may be available in multiple
representations (e.g. multiple languages, data formats, size, and representations (e.g. multiple languages, data formats, size, and
resolutions) or vary in other ways. resolutions) or vary in other ways.
entity entity
The information transferred as the payload of a request or The information transferred as the payload of a request or response.
response. An entity consists of metainformation in the form of An entity consists of metainformation in the form of entity-header
entity-header fields and content in the form of an entity-body, as fields and content in the form of an entity-body, as described in
described in section 7. section 7.
representation representation
An entity included with a response that is subject to content An entity included with a response that is subject to content
negotiation, as described in section 12. There may exist multiple negotiation, as described in section 12. There may exist multiple
representations associated with a particular response status. representations associated with a particular response status.
content negotiation content negotiation
The mechanism for selecting the appropriate representation when The mechanism for selecting the appropriate representation when
servicing a request, as described in section 12. The servicing a request, as described in section 12. The representation
representation of entities in any response can be negotiated of entities in any response can be negotiated (including error
(including error responses). responses).
variant variant
A resource may have one, or more than one, representation(s) A resource may have one, or more than one, representation(s)
associated with it at any given instant. Each of these associated with it at any given instant. Each of these
representations is termed a `varriant'. Use of the term `variant' representations is termed a "variant." Use of the term "variant" does
does not necessarily imply that the resource is subject to content not necessarily imply that the resource is subject to content
negotiation. negotiation.
Fielding, et al Expires May, 2004 [Page 9]
client client
A program that establishes connections for the purpose of sending A program that establishes connections for the purpose of sending
requests. requests.
user agent user agent
The client which initiates a request. These are often browsers, The client which initiates a request. These are often browsers,
editors, spiders (web-traversing robots), or other end user tools. editors, spiders (web-traversing robots), or other end user tools.
server server
An application program that accepts connections in order to An application program that accepts connections in order to service
service requests by sending back responses. Any given program may requests by sending back responses. Any given program may be capable
be capable of being both a client and a server; our use of these of being both a client and a server; our use of these terms refers
terms refers only to the role being performed by the program for a only to the role being performed by the program for a particular
particular connection, rather than to the program's capabilities connection, rather than to the program∆s capabilities in general.
in general. Likewise, any server may act as an origin server, Likewise, any server may act as an origin server, proxy, gateway, or
proxy, gateway, or tunnel, switching behavior based on the nature tunnel, switching behavior based on the nature of each request.
of each request.
origin server origin server
The server on which a given resource resides or is to be created. The server on which a given resource resides or is to be created.
proxy proxy
An intermediary program which acts as both a server and a client An intermediary program which acts as both a server and a client for
for the purpose of making requests on behalf of other clients. the purpose of making requests on behalf of other clients. Requests
Requests are serviced internally or by passing them on, with are serviced internally or by passing them on, with possible
possible translation, to other servers. A proxy MUST implement translation, to other servers. A proxy MUST implement both the client
both the client and server requirements of this specification. A and server requirements of this specification. A "transparent proxy"
"transparent proxy" is a proxy that does not modify the request or is a proxy that does not modify the request or response beyond what
response beyond what is required for proxy authentication and is required for proxy authentication and identification. A "non-
identification. A "non-transparent proxy" is a proxy that modifies transparent proxy" is a proxy that modifies the request or response
the request or response in order to provide some added service to in order to provide some added service to the user agent, such as
the user agent, such as group annotation services, media type group annotation services, media type transformation, protocol
transformation, protocol reduction, or anonymity filtering. Except reduction, or anonymity filtering. Except where either transparent or
where either transparent or non-transparent behavior is explicitly non-transparent behavior is explicitly stated, the HTTP proxy
stated, the HTTP proxy requirements apply to both types of requirements apply to both types of proxies.
proxies.
gateway gateway
A server which acts as an intermediary for some other server. A server which acts as an intermediary for some other server. Unlike
Unlike a proxy, a gateway receives requests as if it were the a proxy, a gateway receives requests as if it were the origin server
origin server for the requested resource; the requesting client for the requested resource; the requesting client may not be aware
may not be aware that it is communicating with a gateway. that it is communicating with a gateway.
tunnel tunnel
An intermediary program which is acting as a blind relay between An intermediary program which is acting as a blind relay between two
two connections. Once active, a tunnel is not considered a party connections. Once active, a tunnel is not considered a party to the
to the HTTP communication, though the tunnel may have been HTTP communication, though the tunnel may have been initiated by an
initiated by an HTTP request. The tunnel ceases to exist when both HTTP request. The tunnel ceases to exist when both ends of the
ends of the relayed connections are closed. relayed connections are closed.
cache cache
A program's local store of response messages and the subsystem A program's local store of response messages and the subsystem that
that controls its message storage, retrieval, and deletion. A controls its message storage, retrieval, and deletion. A cache stores
cache stores cacheable responses in order to reduce the response cacheable responses in order to reduce the response time and network
time and network bandwidth consumption on future, equivalent bandwidth consumption on future, equivalent requests. Any client or
requests. Any client or server may include a cache, though a cache
cannot be used by a server that is acting as a tunnel. Fielding, et al Expires May, 2004 [Page 10]
server may include a cache, though a cache cannot be used by a server
that is acting as a tunnel.
cacheable cacheable
A response is cacheable if a cache is allowed to store a copy of A response is cacheable if a cache is allowed to store a copy of the
the response message for use in answering subsequent requests. The response message for use in answering subsequent requests. The rules
rules for determining the cacheability of HTTP responses are for determining the cacheability of HTTP responses are defined in
defined in section 13. Even if a resource is cacheable, there may section 13. Even if a resource is cacheable, there may be additional
be additional constraints on whether a cache can use the cached constraints on whether a cache can use the cached copy for a
copy for a particular request. particular request.
first-hand first-hand
A response is first-hand if it comes directly and without A response is first-hand if it comes directly and without unnecessary
unnecessary delay from the origin server, perhaps via one or more delay from the origin server, perhaps via one or more proxies. A
proxies. A response is also first-hand if its validity has just response is also first-hand if its validity has just been checked
been checked directly with the origin server. directly with the origin server.
explicit expiration time explicit expiration time
The time at which the origin server intends that an entity should The time at which the origin server intends that an entity should no
no longer be returned by a cache without further validation. longer be returned by a cache without further validation.
heuristic expiration time heuristic expiration time
An expiration time assigned by a cache when no explicit expiration An expiration time assigned by a cache when no explicit expiration
time is available. time is available.
age age
The age of a response is the time since it was sent by, or The age of a response is the time since it was sent by, or
successfully validated with, the origin server. successfully validated with, the origin server.
freshness lifetime freshness lifetime
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expiration time. expiration time.
fresh fresh
A response is fresh if its age has not yet exceeded its freshness A response is fresh if its age has not yet exceeded its freshness
lifetime. lifetime.
stale stale
A response is stale if its age has passed its freshness lifetime. A response is stale if its age has passed its freshness lifetime.
semantically transparent semantically transparent
A cache behaves in a "semantically transparent" manner, with A cache behaves in a "semantically transparent" manner, with respect
respect to a particular response, when its use affects neither the to a particular response, when its use affects neither the requesting
requesting client nor the origin server, except to improve client nor the origin server, except to improve performance. When a
performance. When a cache is semantically transparent, the client cache is semantically transparent, the client receives exactly the
receives exactly the same response (except for hop-by-hop headers) same response (except for hop-by-hop headers) that it would have
that it would have received had its request been handled directly received had its request been handled directly by the origin server.
by the origin server.
validator validator
A protocol element (e.g., an entity tag or a Last-Modified time) A protocol element (e.g., an entity tag or a Last-Modified time) that
that is used to find out whether a cache entry is an equivalent is used to find out whether a cache entry is an equivalent copy of an
copy of an entity. entity.
Fielding, et al Expires May, 2004 [Page 11]
upstream/downstream upstream/downstream
Upstream and downstream describe the flow of a message: all Upstream and downstream describe the flow of a message: all messages
messages flow from upstream to downstream. flow from upstream to downstream.
inbound/outbound inbound/outbound
Inbound and outbound refer to the request and response paths for Inbound and outbound refer to the request and response paths for
messages: "inbound" means "traveling toward the origin server", messages: "inbound" means "traveling toward the origin server", and
and "outbound" means "traveling toward the user agent" "outbound" means "traveling toward the user agent"
1.4 Overall Operation 1.4 Overall Operation
The HTTP protocol is a request/response protocol. A client sends a 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 request to the server in the form of a request method, URI, and
protocol version, followed by a MIME-like message containing request protocol version, followed by a MIME-like message containing request
modifiers, client information, and possible body content over a modifiers, client information, and possible body content over a
connection with a server. The server responds with a status line, connection with a server. The server responds with a status line,
including the message's protocol version and a success or error code, including the message's protocol version and a success or error code,
followed by a MIME-like message containing server information, entity followed by a MIME-like message containing server information, entity
metainformation, and possible entity-body content. The relationship metainformation, and possible entity-body content. The relationship
between HTTP and MIME is described in appendix 19.4. between HTTP and MIME is described in appendix 17.4.
Most HTTP communication is initiated by a user agent and consists of 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 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) simplest case, this may be accomplished via a single connection (v)
between the user agent (UA) and the origin server (O). between the user agent (UA) and the origin server (O).
request chain ------------------------> request chain ------------------------>
UA -------------------v------------------- O UA -------------------v------------------- O
<----------------------- response chain <----------------------- response chain
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firewall) even when the intermediary cannot understand the contents firewall) even when the intermediary cannot understand the contents
of the messages. of the messages.
request chain --------------------------------------> request chain -------------------------------------->
UA -----v----- A -----v----- B -----v----- C -----v----- O UA -----v----- A -----v----- B -----v----- C -----v----- O
<------------------------------------- response chain <------------------------------------- response chain
The figure above shows three intermediaries (A, B, and C) between the The figure above shows three intermediaries (A, B, and C) between the
user agent and origin server. A request or response message that user agent and origin server. A request or response message that
travels the whole chain will pass through four separate connections. travels the whole chain will pass through four separate connections.
This distinction is important because some HTTP communication options This distinction is important because some HTTP communication options
may apply only to the connection with the nearest, non-tunnel may apply only to the connection with the nearest, non-tunnel
neighbor, only to the end-points of the chain, or to all connections neighbor, only to the end-points of the chain, or to all connections
Fielding, et al Expires May, 2004 [Page 12]
along the chain. Although the diagram is linear, each participant may along the chain. Although the diagram is linear, each participant may
be engaged in multiple, simultaneous communications. For example, B be engaged in multiple, simultaneous communications. For example, B
may be receiving requests from many clients other than A, and/or 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 forwarding requests to servers other than C, at the same time that it
is handling A's request. is handling A's request.
Any party to the communication which is not acting as a tunnel may 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 employ an internal cache for handling requests. The effect of a cache
is that the request/response chain is shortened if one of the is that the request/response chain is shortened if one of the
participants along the chain has a cached response applicable to that participants along the chain has a cached response applicable to that
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subsets of cached data via CD-ROM, and so on. HTTP systems are used 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 in corporate intranets over high-bandwidth links, and for access via
PDAs with low-power radio links and intermittent connectivity. The PDAs with low-power radio links and intermittent connectivity. The
goal of HTTP/1.1 is to support the wide diversity of configurations goal of HTTP/1.1 is to support the wide diversity of configurations
already deployed while introducing protocol constructs that meet the already deployed while introducing protocol constructs that meet the
needs of those who build web applications that require high needs of those who build web applications that require high
reliability and, failing that, at least reliable indications of reliability and, failing that, at least reliable indications of
failure. failure.
HTTP communication usually takes place over TCP/IP connections. The HTTP communication usually takes place over TCP/IP connections. The
default port is TCP 80 [19], but other ports can be used. This does default port is TCP 80 [I14], but other ports can be used. This does
not preclude HTTP from being implemented on top of any other protocol not preclude HTTP from being implemented on top of any other protocol
on the Internet, or on other networks. HTTP only presumes a reliable on the Internet, or on other networks. HTTP only presumes a reliable
transport; any protocol that provides such guarantees can be used; transport; any protocol that provides such guarantees can be used;
the mapping of the HTTP/1.1 request and response structures onto the 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 transport data units of the protocol in question is outside the scope
of this specification. of this specification.
In HTTP/1.0, most implementations used a new connection for each 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 request/response exchange. In HTTP/1.1, a connection may be used for
one or more request/response exchanges, although connections may be one or more request/response exchanges, although connections may be
closed for a variety of reasons (see section 8.1). closed for a variety of reasons (see section 8.1).
Fielding, et al Expires May, 2004 [Page 13]
2 Notational Conventions and Generic Grammar 2 Notational Conventions and Generic Grammar
2.1 Augmented BNF 2.1 Augmented BNF
All of the mechanisms specified in this document are described in All of the mechanisms specified in this document are described in
both prose and an augmented Backus-Naur Form (BNF) similar to that 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 used by RFC 822 [I9]. Implementors will need to be familiar with the
notation in order to understand this specification. The augmented BNF notation in order to understand this specification. The augmented BNF
includes the following constructs: includes the following constructs:
name = definition name = definition
The name of a rule is simply the name itself (without any The name of a rule is simply the name itself (without any enclosing
enclosing "<" and ">") and is separated from its definition by the "<" and ">") and is separated from its definition by the equal "="
equal "=" character. White space is only significant in that character. White space is only significant in that indentation of
indentation of continuation lines is used to indicate a rule continuation lines is used to indicate a rule definition that spans
definition that spans more than one line. Certain basic rules are more than one line. Certain basic rules are in uppercase, such as SP,
in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle brackets are used within
brackets are used within definitions whenever their presence will definitions whenever their presence will facilitate discerning the
facilitate discerning the use of rule names. use of rule names.
"literal" "literal"
Quotation marks surround literal text. Unless stated otherwise, Quotation marks surround literal text. Unless stated otherwise, the
the text is case-insensitive. text is case-insensitive.
rule1 | rule2 rule1 | rule2
Elements separated by a bar ("|") are alternatives, e.g., "yes | Elements separated by a bar ("|") are alternatives, e.g., "yes | no"
no" will accept yes or no. will accept yes or no.
(rule1 rule2) (rule1 rule2)
Elements enclosed in parentheses are treated as a single element. Elements enclosed in parentheses are treated as a single element.
Thus, "(elem (foo | bar) elem)" allows the token sequences "elem Thus, "(elem (foo | bar) elem)" allows the token sequences
foo elem" and "elem bar elem". "elem foo elem" and "elem bar elem".
*rule *rule
The character "*" preceding an element indicates repetition. The The character "*" preceding an element indicates repetition. The full
full form is "<n>*<m>element" indicating at least <n> and at most form is "<n>*<m>element" indicating at least <n> and at most <m>
<m> occurrences of element. Default values are 0 and infinity so occurrences of element. Default values are 0 and infinity so that
that "*(element)" allows any number, including zero; "1*element" "*(element)" allows any number, including zero; "1*element" requires
requires at least one; and "1*2element" allows one or two. at least one; and "1*2element" allows one or two.
[rule] [rule]
Square brackets enclose optional elements; "[foo bar]" is Square brackets enclose optional elements; "[foo bar]" is equivalent
equivalent to "*1(foo bar)". to "*1(foo bar)".
N rule N rule
Specific repetition: "<n>(element)" is equivalent to Specific repetition: "<n>(element)" is equivalent to
"<n>*<n>(element)"; that is, exactly <n> occurrences of (element). "<n>*<n>(element)"; that is, exactly <n> occurrences of (element).
Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
alphabetic characters. alphabetic characters.
#rule #rule
A construct "#" is defined, similar to "*", for defining lists of 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 Fielding, et al Expires May, 2004 [Page 14]
(",") and OPTIONAL linear white space (LWS). This makes the usual elements. The full form is "<n>#<m>element" indicating at least <n>
form of lists very easy; a rule such as 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 )) ( *LWS element *( *LWS "," *LWS element ))
can be shown as can be shown as
1#element 1#element
Wherever this construct is used, null elements are allowed, but do Wherever this construct is used, null elements are allowed, but do
not contribute to the count of elements present. That is, not contribute to the count of elements present. That is, "(element),
"(element), , (element) " is permitted, but counts as only two , (element) " is permitted, but counts as only two elements.
elements. Therefore, where at least one element is required, at Therefore, where at least one element is required, at least one non-
least one non-null element MUST be present. Default values are 0 null element MUST be present. Default values are 0 and infinity so
and infinity so that "#element" allows any number, including zero; that "#element" allows any number, including zero; "1#element"
"1#element" requires at least one; and "1#2element" allows one or requires at least one; and "1#2element" allows one or two.
two.
; comment ; comment
A semi-colon, set off some distance to the right of rule text, A semi-colon, set off some distance to the right of rule text, starts
starts a comment that continues to the end of line. This is a a comment that continues to the end of line. This is a simple way of
simple way of including useful notes in parallel with the including useful notes in parallel with the specifications.
specifications.
implied *LWS implied *LWS
The grammar described by this specification is word-based. Except The grammar described by this specification is word-based. Except
where noted otherwise, linear white space (LWS) can be included where noted otherwise, linear white space (LWS) can be included
between any two adjacent words (token or quoted-string), and between any two adjacent words (token or quoted-string), and between
between adjacent words and separators, without changing the adjacent words and separators, without changing the interpretation of
interpretation of a field. At least one delimiter (LWS and/or a field. At least one delimiter (LWS and/or separators) MUST exist
between any two tokens (for the definition of "token" below), since
separators) MUST exist between any two tokens (for the definition they would otherwise be interpreted as a single token.
of "token" below), since they would otherwise be interpreted as a
single token.
2.2 Basic Rules 2.2 Basic Rules
The following rules are used throughout this specification to The following rules are used throughout this specification to
describe basic parsing constructs. The US-ASCII coded character set describe basic parsing constructs. The US-ASCII coded character set
is defined by ANSI X3.4-1986 [21]. is defined by ANSI X3.4-1986 [N6].
OCTET = <any 8-bit sequence of data> OCTET = <any 8-bit sequence of data>
CHAR = <any US-ASCII character (octets 0 - 127)> CHAR = <any US-ASCII character (octets 0 - 127)>
UPALPHA = <any US-ASCII uppercase letter "A".."Z"> UPALPHA = <any US-ASCII uppercase letter "A".."Z">
LOALPHA = <any US-ASCII lowercase letter "a".."z"> LOALPHA = <any US-ASCII lowercase letter "a".."z">
ALPHA = UPALPHA | LOALPHA ALPHA = UPALPHA | LOALPHA
DIGIT = <any US-ASCII digit "0".."9"> DIGIT = <any US-ASCII digit "0".."9">
CTL = <any US-ASCII control character CTL = <any US-ASCII control character
(octets 0 - 31) and DEL (127)> (octets 0 - 31) and DEL (127)>
CR = <US-ASCII CR, carriage return (13)> CR = <US-ASCII CR, carriage return (13)>
LF = <US-ASCII LF, linefeed (10)> LF = <US-ASCII LF, linefeed (10)>
SP = <US-ASCII SP, space (32)> SP = <US-ASCII SP, space (32)>
HT = <US-ASCII HT, horizontal-tab (9)> HT = <US-ASCII HT, horizontal-tab (9)>
<"> = <US-ASCII double-quote mark (34)> <"> = <US-ASCII double-quote mark (34)>
HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all 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 protocol elements except the entity-body (see appendix 17.3 for
tolerant applications). The end-of-line marker within an entity-body tolerant applications). The end-of-line marker within an entity-body
is defined by its associated media type, as described in section 3.7. is defined by its associated media type, as described in section 3.7.
Fielding, et al Expires May, 2004 [Page 15]
CRLF = CR LF CRLF = CR LF
HTTP/1.1 header field values can be folded onto multiple lines if the 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 continuation line begins with a space or horizontal tab. All linear
white space, including folding, has the same semantics as SP. A white space, including folding, has the same semantics as SP. A
recipient MAY replace any linear white space with a single SP before recipient MAY replace any linear white space with a single SP before
interpreting the field value or forwarding the message downstream. interpreting the field value or forwarding the message downstream.
LWS = [CRLF] 1*( SP | HT ) LWS = [CRLF] 1*( SP | HT )
The TEXT rule is only used for descriptive field contents and values The TEXT rule is only used for descriptive field contents and values
that are not intended to be interpreted by the message parser. Words that are not intended to be interpreted by the message parser. Words
of *TEXT MAY contain characters from character sets other than ISO- of *TEXT MAY contain characters from character sets other than ISO-
8859-1 [22] only when encoded according to the rules of RFC 2047 8859-1 [N7] only when encoded according to the rules of RFC 2047
[14]. [N14].
TEXT = <any OCTET except CTLs, TEXT = <any OCTET except CTLs,
but including LWS> but including LWS>
A CRLF is allowed in the definition of TEXT only as part of a header 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 field continuation. It is expected that the folding LWS will be
replaced with a single SP before interpretation of the TEXT value. replaced with a single SP before interpretation of the TEXT value.
Hexadecimal numeric characters are used in several protocol elements. Hexadecimal numeric characters are used in several protocol elements.
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comment = "(" *( ctext | quoted-pair | comment ) ")" comment = "(" *( ctext | quoted-pair | comment ) ")"
ctext = <any TEXT excluding "(" and ")"> ctext = <any TEXT excluding "(" and ")">
A string of text is parsed as a single word if it is quoted using A string of text is parsed as a single word if it is quoted using
double-quote marks. double-quote marks.
quoted-string = ( <"> *(qdtext | quoted-pair ) <"> ) quoted-string = ( <"> *(qdtext | quoted-pair ) <"> )
qdtext = <any TEXT except <">> qdtext = <any TEXT except <">>
The backslash character ("\") MAY be used as a single-character The backslash character ("\") MAY be used as a single-character
Fielding, et al Expires May, 2004 [Page 16]
quoting mechanism only within quoted-string and comment constructs. quoting mechanism only within quoted-string and comment constructs.
quoted-pair = "\" CHAR quoted-pair = "\" CHAR
3 Protocol Parameters 3 Protocol Parameters
3.1 HTTP Version 3.1 HTTP Version
HTTP uses a "<major>.<minor>" numbering scheme to indicate versions HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
of the protocol. The protocol versioning policy is intended to allow of the protocol. The protocol versioning policy is intended to allow
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3.1 HTTP Version 3.1 HTTP Version
HTTP uses a "<major>.<minor>" numbering scheme to indicate versions HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
of the protocol. The protocol versioning policy is intended to allow of the protocol. The protocol versioning policy is intended to allow
the sender to indicate the format of a message and its capacity for the sender to indicate the format of a message and its capacity for
understanding further HTTP communication, rather than the features understanding further HTTP communication, rather than the features
obtained via that communication. No change is made to the version obtained via that communication. No change is made to the version
number for the addition of message components which do not affect number for the addition of message components which do not affect
communication behavior or which only add to extensible field values. communication behavior or which only add to extensible field values.
The <minor> number is incremented when the changes made to the The <minor> number is incremented when the changes made to the
protocol add features which do not change the general message parsing protocol add features which do not change the general message parsing
algorithm, but which may add to the message semantics and imply algorithm, but which may add to the message semantics and imply
additional capabilities of the sender. The <major> number is additional capabilities of the sender. The <major> number is
incremented when the format of a message within the protocol is incremented when the format of a message within the protocol is
changed. See RFC 2145 [36] for a fuller explanation. changed. See RFC 2145 [I28] for a fuller explanation.
The version of an HTTP message is indicated by an HTTP-Version field The version of an HTTP message is indicated by an HTTP-Version field
in the first line of the message. in the first line of the message. HTTP-Version is case-sensitive.
HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT
Note that the major and minor numbers MUST be treated as separate Note that the major and minor numbers MUST be treated as separate
integers and that each MAY be incremented higher than a single digit. 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 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 lower than HTTP/12.3. Leading zeros MUST be ignored by recipients and
MUST NOT be sent. MUST NOT be sent.
An application that sends a request or response message that includes An application that sends a request or response message that includes
HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant
with this specification. Applications that are at least conditionally with this specification. Applications that are at least conditionally
compliant with this specification SHOULD use an HTTP-Version of 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 "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 not compatible with HTTP/1.0. For more details on when to send
specific HTTP-Version values, see RFC 2145 [36]. specific HTTP-Version values, see RFC 2145 [I28].
The HTTP version of an application is the highest HTTP version for The HTTP version of an application is the highest HTTP version for
which the application is at least conditionally compliant. which the application is at least conditionally compliant.
Proxy and gateway applications need to be careful when forwarding Proxy and gateway applications need to be careful when forwarding
messages in protocol versions different from that of the application. messages in protocol versions different from that of the application.
Since the protocol version indicates the protocol capability of the Since the protocol version indicates the protocol capability of the
sender, a proxy/gateway MUST NOT send a message with a version sender, a proxy/gateway MUST NOT send a message with a version
indicator which is greater than its actual version. If a higher indicator which is greater than its actual version. If a higher
version request is received, the proxy/gateway MUST either downgrade version request is received, the proxy/gateway MUST either downgrade
the request version, or respond with an error, or switch to tunnel the request version, or respond with an error, or switch to tunnel
behavior. behavior.
Fielding, et al Expires May, 2004 [Page 17]
Due to interoperability problems with HTTP/1.0 proxies discovered Due to interoperability problems with HTTP/1.0 proxies discovered
since the publication of RFC 2068[33], caching proxies MUST, gateways since the publication of RFC 2068[I25], caching proxies MUST,
MAY, and tunnels MUST NOT upgrade the request to the highest version gateways MAY, and tunnels MUST NOT upgrade the request to the highest
they support. The proxy/gateway's response to that request MUST be in version they support. The proxy/gateway's response to that request
the same major version as the request. MUST be in the same major version as the request.
Note: Converting between versions of HTTP may involve modification Note: Converting between versions of HTTP may involve modification
of header fields required or forbidden by the versions involved. of header fields required or forbidden by the versions involved.
3.2 Uniform Resource Identifiers 3.2 Uniform Resource Identifiers
URIs have been known by many names: WWW addresses, Universal Document URIs have been known by many names: WWW addresses, Universal Document
Identifiers, Universal Resource Identifiers [3], and finally the Identifiers, Universal Resource Identifiers [I3], [N9], and finally
combination of Uniform Resource Locators (URL) [4] and Names (URN) the combination of Uniform Resource Locators (URL) [I4] and Names
[20]. As far as HTTP is concerned, Uniform Resource Identifiers are (URN) [I15]. As far as HTTP is concerned, Uniform Resource
simply formatted strings which identify--via name, location, or any Identifiers are simply formatted strings which identify--via name,
other characteristic--a resource. location, or any other characteristic--a resource.
3.2.1 General Syntax 3.2.1 General Syntax
URIs in HTTP can be represented in absolute form or relative to some 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 known base URI [I8], depending upon the context of their use. The two
forms are differentiated by the fact that absolute URIs always begin forms are differentiated by the fact that absolute URIs always begin
with a scheme name followed by a colon. For definitive information on with a scheme name followed by a colon. For definitive information on
URL syntax and semantics, see "Uniform Resource Identifiers (URI): URL syntax and semantics, see "Uniform Resource Identifiers (URI):
Generic Syntax and Semantics," RFC 2396 [42] (which replaces RFCs Generic Syntax and Semantics," RFC 2396 [N9] (which replaces RFCs
1738 [4] and RFC 1808 [11]). This specification adopts the 1738 [I4] and RFC 1808 [I8]). This specification adopts the
definitions of "URI-reference", "absoluteURI", "relativeURI", "port", definitions of "URI-reference", "absoluteURI", "relativeURI", "port",
"host","abs_path", "rel_path", and "authority" from that "host","abs_path", "rel_path", and "authority" from that
specification. specification.
The HTTP protocol does not place any a priori limit on the length of 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 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 serve, and SHOULD be able to handle URIs of unbounded length if they
provide GET-based forms that could generate such URIs. A server provide GET-based forms that could generate such URIs. A server
SHOULD return 414 (Request-URI Too Long) status if a URI is longer SHOULD return 414 (Request-URI Too Long) status if a URI is longer
than the server can handle (see section 10.4.15). than the server can handle (see section 10.4.15).
Note: Servers ought to be cautious about depending on URI lengths Note: Servers ought to be cautious about depending on URI lengths
above 255 bytes, because some older client or proxy above 255 bytes, because some older client or proxy implementations
implementations might not properly support these lengths. might not properly support these lengths.
3.2.2 http URL 3.2.2 http URL
The "http" scheme is used to locate network resources via the HTTP The "http" scheme is used to locate network resources via the HTTP
protocol. This section defines the scheme-specific syntax and protocol. This section defines the scheme-specific syntax and
semantics for http URLs. semantics for http URLs.
http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]] http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
If the port is empty or not given, port 80 is assumed. The semantics 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 are that the identified resource is located at the server listening
Fielding, et al Expires May, 2004 [Page 18]
for TCP connections on that port of that host, and the Request-URI 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 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 in URLs SHOULD be avoided whenever possible (see RFC 1900 [I17]). If
the abs_path is not present in the URL, it MUST be given as "/" when 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 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 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 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 a fully qualified domain name, the proxy MUST NOT change the host
name. name.
3.2.3 URI Comparison 3.2.3 URI Comparison
When comparing two URIs to decide if they match or not, a client 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 SHOULD use a case-sensitive octet-by-octet comparison of the entire
URIs, with these exceptions: URIs, with these exceptions:
- A port that is empty or not given is equivalent to the default o A port that is empty or not given is equivalent to the default port
port for that URI-reference; for that URI-reference;
o Comparisons of host names MUST be case-insensitive;
- Comparisons of host names MUST be case-insensitive; o Comparisons of scheme names MUST be case-insensitive;
o An empty abs_path is equivalent to an abs_path of "/".
- 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 Characters other than those in the "reserved" set (see RFC 2396 [N9])
RFC 2396 [42]) are equivalent to their ""%" HEX HEX" encoding. are equivalent to their ""%" HEX HEX" encoding.
For example, the following three URIs are equivalent: For example, the following three URIs are equivalent:
http://abc.com:80/~smith/home.html http://abc.com:80/~smith/home.html
http://ABC.com/%7Esmith/home.html http://ABC.com/%7Esmith/home.html
http://ABC.com:/%7esmith/home.html http://ABC.com:/%7esmith/home.html
3.3 Date/Time Formats 3.3 Date/Time Formats
3.3.1 Full Date 3.3.1 Full Date
HTTP applications have historically allowed three different formats HTTP applications have historically allowed three different formats
for the representation of date/time stamps: for the representation of date/time stamps:
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123 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 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 Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
The first format is preferred as an Internet standard and represents 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 a fixed-length subset of that defined by RFC 1123 [N2] (an update to
RFC 822 [9]). The second format is in common use, but is based on the RFC 822 [I9]). The second format is in common use, but is based on
obsolete RFC 850 [12] date format and lacks a four-digit year. the obsolete RFC 850 [I9] date format and lacks a four-digit year.
HTTP/1.1 clients and servers that parse the date value MUST accept 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 all three formats (for compatibility with HTTP/1.0), though they MUST
only generate the RFC 1123 format for representing HTTP-date values only generate the RFC 1123 format for representing HTTP-date values
in header fields. See section 19.3 for further information. in header fields. See section 17.3 for further information.
Fielding, et al Expires May, 2004 [Page 19]
Note: Recipients of date values are encouraged to be robust in Note: Recipients of date values are encouraged to be robust in
accepting date values that may have been sent by non-HTTP accepting date values that may have been sent by non-HTTP
applications, as is sometimes the case when retrieving or posting applications, as is sometimes the case when retrieving or posting
messages via proxies/gateways to SMTP or NNTP. messages via proxies/gateways to SMTP or NNTP.
All HTTP date/time stamps MUST be represented in Greenwich Mean Time All HTTP date/time stamps MUST be represented in Greenwich Mean Time
(GMT), without exception. For the purposes of HTTP, GMT is exactly (GMT), without exception. For the purposes of HTTP, GMT is exactly
equal to UTC (Coordinated Universal Time). This is indicated in the equal to UTC (Coordinated Universal Time). This is indicated in the
first two formats by the inclusion of "GMT" as the three-letter first two formats by the inclusion of "GMT" as the three-letter
abbreviation for time zone, and MUST be assumed when reading the abbreviation for time zone, and MUST be assumed when reading the
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delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
3.4 Character Sets 3.4 Character Sets
HTTP uses the same definition of the term "character set" as that HTTP uses the same definition of the term "character set" as that
described for MIME: described for MIME:
The term "character set" is used in this document to refer to a 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 method used with one or more tables to convert a sequence of octets
into a sequence of characters. Note that unconditional conversion in into a sequence of characters. Note that unconditional conversion
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 Fielding, et al Expires May, 2004 [Page 20]
more than one sequence of octets to represent a particular character. in the other direction is not required, in that not all characters
This definition is intended to allow various kinds of character may be available in a given character set and a character set may
encoding, from simple single-table mappings such as US-ASCII to provide more than one sequence of octets to represent a particular
complex table switching methods such as those that use ISO-2022's character. This definition is intended to allow various kinds of
techniques. However, the definition associated with a MIME character character encoding, from simple single-table mappings such as US-
set name MUST fully specify the mapping to be performed from octets ASCII to complex table switching methods such as those that use
to characters. In particular, use of external profiling information ISO-2022∆s techniques. However, the definition associated with a
to determine the exact mapping is not permitted. 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 Note: This use of the term "character set" is more commonly
referred to as a "character encoding." However, since HTTP and referred to as a "character encoding." However, since HTTP and MIME
MIME share the same registry, it is important that the terminology share the same registry, it is important that the terminology also
also be shared. be shared.
HTTP character sets are identified by case-insensitive tokens. The HTTP character sets are identified by case-insensitive tokens. The
complete set of tokens is defined by the IANA Character Set registry complete set of tokens is defined by the IANA Character Set registry
[19]. [I14].
charset = token charset = token
Although HTTP allows an arbitrary token to be used as a charset Although HTTP allows an arbitrary token to be used as a charset
value, any token that has a predefined value within the IANA value, any token that has a predefined value within the IANA
Character Set registry [19] MUST represent the character set defined Character Set registry MUST represent the character set defined by
by that registry. Applications SHOULD limit their use of character that registry. Applications SHOULD limit their use of character sets
sets to those defined by the IANA registry. to those defined by the IANA registry.
Implementors should be aware of IETF character set requirements [38] HTTP uses charset in two contexts: within an Accept-Charset request
[41]. header (in which the charset value is an unquoted token) and as the
value of a parameter in a Content-Type header (within a request or
response), in which case the parameter value of the charset parameter
may be quoted.
3.4.1 Missing Charset Implementors should be aware of IETF character set requirements [I30]
[I32].
Missing Charset
Some HTTP/1.0 software has interpreted a Content-Type header without Some HTTP/1.0 software has interpreted a Content-Type header without
charset parameter incorrectly to mean "recipient should guess." charset parameter incorrectly to mean "recipient should guess."
Senders wishing to defeat this behavior MAY include a charset Senders wishing to defeat this behavior MAY include a charset
parameter even when the charset is ISO-8859-1 and SHOULD do so when parameter even when the charset is ISO-8859-1 and SHOULD do so when
it is known that it will not confuse the recipient. it is known that it will not confuse the recipient.
Unfortunately, some older HTTP/1.0 clients did not deal properly with Unfortunately, some older HTTP/1.0 clients did not deal properly with
an explicit charset parameter. HTTP/1.1 recipients MUST respect the an explicit charset parameter. HTTP/1.1 recipients MUST respect the
charset label provided by the sender; and those user agents that have charset label provided by the sender; and those user agents that have
a provision to "guess" a charset MUST use the charset from the a provision to "guess" a charset MUST use the charset from the
content-type field if they support that charset, rather than the content-type field if they support that charset, rather than the
recipient's preference, when initially displaying a document. See recipient's preference, when initially displaying a document. See
section 3.7.1. section 3.7.1.
Fielding, et al Expires May, 2004 [Page 21]
3.5 Content Codings 3.5 Content Codings
Content coding values indicate an encoding transformation that has Content coding values indicate an encoding transformation that has
been or can be applied to an entity. Content codings are primarily been or can be applied to an entity. Content codings are primarily
used to allow a document to be compressed or otherwise usefully used to allow a document to be compressed or otherwise usefully
transformed without losing the identity of its underlying media type transformed without losing the identity of its underlying media type
and without loss of information. Frequently, the entity is stored in and without loss of information. Frequently, the entity is stored in
coded form, transmitted directly, and only decoded by the recipient. coded form, transmitted directly, and only decoded by the recipient.
content-coding = token content-coding = token
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coded form, transmitted directly, and only decoded by the recipient. coded form, transmitted directly, and only decoded by the recipient.
content-coding = token content-coding = token
All content-coding values are case-insensitive. HTTP/1.1 uses All content-coding values are case-insensitive. HTTP/1.1 uses
content-coding values in the Accept-Encoding (section 14.3) and content-coding values in the Accept-Encoding (section 14.3) and
Content-Encoding (section 14.11) header fields. Although the value Content-Encoding (section 14.11) header fields. Although the value
describes the content-coding, what is more important is that it describes the content-coding, what is more important is that it
indicates what decoding mechanism will be required to remove the indicates what decoding mechanism will be required to remove the
encoding. encoding.
The Internet Assigned Numbers Authority (IANA) acts as a registry for The Internet Assigned Numbers Authority (IANA) acts as a registry for
content-coding value tokens. Initially, the registry contains the content-coding value tokens. Initially, the registry contains the
following tokens: following tokens:
gzip An encoding format produced by the file compression program gzip An encoding format produced by the file compression program "gzip"
"gzip" (GNU zip) as described in RFC 1952 [25]. This format is a (GNU zip) as described in RFC 1952 [I18]. This format is a Lempel-
Lempel-Ziv coding (LZ77) with a 32 bit CRC. Ziv coding (LZ77) with a 32 bit CRC.
compress compress
The encoding format produced by the common UNIX file compression The encoding format produced by the common UNIX file compression
program "compress". This format is an adaptive Lempel-Ziv-Welch program "compress". This format is an adaptive Lempel-Ziv-Welch
coding (LZW). coding (LZW).
Use of program names for the identification of encoding formats Use of program names for the identification of encoding formats is
is not desirable and is discouraged for future encodings. Their not desirable and is discouraged for future encodings. Their use
use here is representative of historical practice, not good here is representative of historical practice, not good design. For
design. For compatibility with previous implementations of HTTP, compatibility with previous implementations of HTTP, applications
applications SHOULD consider "x-gzip" and "x-compress" to be SHOULD consider "x-gzip" and "x-compress" to be equivalent to
equivalent to "gzip" and "compress" respectively. "gzip" and "compress" respectively.
deflate deflate
The "zlib" format defined in RFC 1950 [31] in combination with The "zlib" format defined in RFC 1950 [I24] in combination with the
the "deflate" compression mechanism described in RFC 1951 [29]. "deflate" compression mechanism described in RFC 1951 [I22].
identity identity
The default (identity) encoding; the use of no transformation The default (identity) encoding; the use of no transformation
whatsoever. This content-coding is used only in the Accept- whatsoever. This content-coding is used only in the Accept-Encoding
Encoding header, and SHOULD NOT be used in the Content-Encoding header, and SHOULD NOT be used in the Content-Encoding header.
header.
New content-coding value tokens SHOULD be registered; to allow New content-coding value tokens SHOULD be registered; to allow
interoperability between clients and servers, specifications of the interoperability between clients and servers, specifications of the
content coding algorithms needed to implement a new value SHOULD be content coding algorithms needed to implement a new value SHOULD be
publicly available and adequate for independent implementation, and publicly available and adequate for independent implementation, and
conform to the purpose of content coding defined in this section. conform to the purpose of content coding defined in this section. New
registrations are reviewed and approved by the IESG according to
these criteria.
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3.6 Transfer Codings 3.6 Transfer Codings
Transfer-coding values are used to indicate an encoding Transfer-coding values are used to indicate an encoding
transformation that has been, can be, or may need to be applied to an 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. entity-body in order to ensure "safe transport" through the network.
This differs from a content coding in that the transfer-coding is a This differs from a content coding in that the transfer-coding is a
property of the message, not of the original entity. property of the message, not of the original entity.
transfer-coding = "chunked" | transfer-extension transfer-coding = "chunked" | transfer-extension
transfer-extension = token *( ";" parameter ) transfer-extension = token *( ";" parameter )
Parameters are in the form of attribute/value pairs. Parameters are in the form of attribute/value pairs.
parameter = attribute "=" value parameter = attribute "=" value
attribute = token attribute = token
skipping to change at page 24, line 47 skipping to change at line 1205
Whenever a transfer-coding is applied to a message-body, the set of Whenever a transfer-coding is applied to a message-body, the set of
transfer-codings MUST include "chunked", unless the message is transfer-codings MUST include "chunked", unless the message is
terminated by closing the connection. When the "chunked" transfer- terminated by closing the connection. When the "chunked" transfer-
coding is used, it MUST be the last transfer-coding applied to the coding is used, it MUST be the last transfer-coding applied to the
message-body. The "chunked" transfer-coding MUST NOT be applied more message-body. The "chunked" transfer-coding MUST NOT be applied more
than once to a message-body. These rules allow the recipient to than once to a message-body. These rules allow the recipient to
determine the transfer-length of the message (section 4.4). determine the transfer-length of the message (section 4.4).
Transfer-codings are analogous to the Content-Transfer-Encoding Transfer-codings are analogous to the Content-Transfer-Encoding
values of MIME [7], which were designed to enable safe transport of values of MIME [I7], which were designed to enable safe transport of
binary data over a 7-bit transport service. However, safe transport binary data over a 7-bit transport service. However, safe transport
has a different focus for an 8bit-clean transfer protocol. In HTTP, has a different focus for an 8bit-clean transfer protocol. In HTTP,
the only unsafe characteristic of message-bodies is the difficulty in the only unsafe characteristic of message-bodies is the difficulty in
determining the exact body length (section 7.2.2), or the desire to determining the exact body length (section 7.2.2), or the desire to
encrypt data over a shared transport. encrypt data over a shared transport.
The Internet Assigned Numbers Authority (IANA) acts as a registry for The Internet Assigned Numbers Authority (IANA) acts as a registry for
transfer-coding value tokens. Initially, the registry contains the transfer-coding value tokens. Initially, the registry contains the
following tokens: "chunked" (section 3.6.1), "identity" (section following tokens: "chunked" (section 3.6.1), "gzip" (section 3.5),
3.6.2), "gzip" (section 3.5), "compress" (section 3.5), and "deflate" "compress" (section 3.5), and "deflate" (section 3.5).
(section 3.5).
New transfer-coding value tokens SHOULD be registered in the same way New transfer-coding value tokens SHOULD be registered in the same way
as new content-coding value tokens (section 3.5). as new content-coding value tokens (section 3.5).
A server which receives an entity-body with a transfer-coding it does A server which receives an entity-body with a transfer-coding it does
not understand SHOULD return 501 (Unimplemented), and close the not understand SHOULD return 501 (Unimplemented), and close the
connection. A server MUST NOT send transfer-codings to an HTTP/1.0 connection. A server MUST NOT send transfer-codings to an HTTP/1.0
client. client.
3.6.1 Chunked Transfer Coding 3.6.1 Chunked Transfer Coding
The chunked encoding modifies the body of a message in order to 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, transfer it as a series of chunks, each with its own size indicator,
Fielding, et al Expires May, 2004 [Page 23]
followed by an OPTIONAL trailer containing entity-header fields. This followed by an OPTIONAL trailer containing entity-header fields. This
allows dynamically produced content to be transferred along with the allows dynamically produced content to be transferred along with the
information necessary for the recipient to verify that it has information necessary for the recipient to verify that it has
received the full message. received the full message.
Chunked-Body = *chunk Chunked-Body = *chunk
last-chunk last-chunk
trailer trailer
CRLF CRLF
chunk = chunk-size [ chunk-extension ] CRLF chunk = chunk-size [ chunk-extension ] CRLF
chunk-data CRLF chunk-data CRLF
chunk-size = 1*HEX chunk-size = 1*HEX
last-chunk = 1*("0") [ chunk-extension ] CRLF last-chunk = 1*("0") [ chunk-extension ] CRLF
chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] ) chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
chunk-ext-name = token chunk-ext-name = token
chunk-ext-val = token | quoted-string chunk-ext-val = token | quoted-string
chunk-data = chunk-size(OCTET) chunk-data = chunk-size(OCTET)
trailer = *(entity-header CRLF) trailer = *(entity-header CRLF)
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chunk-size = 1*HEX chunk-size = 1*HEX
last-chunk = 1*("0") [ chunk-extension ] CRLF last-chunk = 1*("0") [ chunk-extension ] CRLF
chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] ) chunk-extension= *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
chunk-ext-name = token chunk-ext-name = token
chunk-ext-val = token | quoted-string chunk-ext-val = token | quoted-string
chunk-data = chunk-size(OCTET) chunk-data = chunk-size(OCTET)
trailer = *(entity-header CRLF) trailer = *(entity-header CRLF)
The chunk-size field is a string of hex digits indicating the size of 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 the chunk-data in octets. The chunked encoding is ended by any chunk
zero, followed by the trailer, which is terminated by an empty line. 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 The trailer allows the sender to include additional HTTP header
fields at the end of the message. The Trailer header field can be 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 used to indicate which header fields are included in a trailer (see
section 14.40). section 14.40).
A server using chunked transfer-coding in a response MUST NOT use the 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 trailer for any header fields unless at least one of the following is
true: true:
a)the request included a TE header field that indicates "trailers" is a) the request included a TE header field that indicates "trailers"
acceptable in the transfer-coding of the response, as described in is acceptable in the transfer-coding of the response, as described
section 14.39; or, in section 14.39; or,
b)the server is the origin server for the response, the trailer b)the server is the origin server for the response, the trailer
fields consist entirely of optional metadata, and the recipient fields consist entirely of optional metadata, and the recipient
could use the message (in a manner acceptable to the origin server) could use the message (in a manner acceptable to the origin server)
without receiving this metadata. In other words, the origin server without receiving this metadata. In other words, the origin server
is willing to accept the possibility that the trailer fields might is willing to accept the possibility that the trailer fields might
be silently discarded along the path to the client. be silently discarded along the path to the client.
This requirement prevents an interoperability failure when the This requirement prevents an interoperability failure when the
message is being received by an HTTP/1.1 (or later) proxy and 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 forwarded to an HTTP/1.0 recipient. It avoids a situation where
compliance with the protocol would have necessitated a possibly compliance with the protocol would have necessitated a possibly
infinite buffer on the proxy. infinite buffer on the proxy.
An example process for decoding a Chunked-Body is presented in An example process for decoding a Chunked-Body is presented in
appendix 19.4.6. appendix 17.4.6.
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All HTTP/1.1 applications MUST be able to receive and decode the All HTTP/1.1 applications MUST be able to receive and decode the
"chunked" transfer-coding, and MUST ignore chunk-extension extensions "chunked" transfer-coding, and MUST ignore chunk-extension extensions
they do not understand. they do not understand.
3.7 Media Types 3.7 Media Types
HTTP uses Internet Media Types [17] in the Content-Type (section HTTP uses Internet Media Types [N5] in the Content-Type (section
14.17) and Accept (section 14.1) header fields in order to provide 14.17) and Accept (section 14.1) header fields in order to provide
open and extensible data typing and type negotiation. open and extensible data typing and type negotiation.
media-type = type "/" subtype *( ";" parameter ) media-type = type "/" subtype *( ";" parameter )
type = token type = token
subtype = token subtype = token
Parameters MAY follow the type/subtype in the form of attribute/value Parameters MAY follow the type/subtype in the form of attribute/value
pairs (as defined in section 3.6). pairs (as defined in section 3.6).
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attribute and its value. The presence or absence of a parameter might attribute and its value. The presence or absence of a parameter might
be significant to the processing of a media-type, depending on its be significant to the processing of a media-type, depending on its
definition within the media type registry. definition within the media type registry.
Note that some older HTTP applications do not recognize media type Note that some older HTTP applications do not recognize media type
parameters. When sending data to older HTTP applications, parameters. When sending data to older HTTP applications,
implementations SHOULD only use media type parameters when they are implementations SHOULD only use media type parameters when they are
required by that type/subtype definition. required by that type/subtype definition.
Media-type values are registered with the Internet Assigned Number Media-type values are registered with the Internet Assigned Number
Authority (IANA [19]). The media type registration process is Authority (IANA [I14]). The media type registration process is
outlined in RFC 1590 [17]. Use of non-registered media types is outlined in RFC 2048 [N5]. Use of non-registered media types is
discouraged. discouraged.
3.7.1 Canonicalization and Text Defaults 3.7.1 Canonicalization and Text Defaults
Internet media types are registered with a canonical form. An Internet media types are registered with a canonical form. An entity-
entity-body transferred via HTTP messages MUST be represented in the body transferred via HTTP messages MUST be represented in the
appropriate canonical form prior to its transmission except for appropriate canonical form prior to its transmission except for
"text" types, as defined in the next paragraph. "text" types, as defined in the next paragraph.
When in canonical form, media subtypes of the "text" type use CRLF as When in canonical form, media subtypes of the "text" type use CRLF as
the text line break. HTTP relaxes this requirement and allows the the text line break. HTTP relaxes this requirement and allows the
transport of text media with plain CR or LF alone representing a line 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 break when it is done consistently for an entire entity-body. HTTP
applications MUST accept CRLF, bare CR, and bare LF as being applications MUST accept CRLF, bare CR, and bare LF as being
representative of a line break in text media received via HTTP. In 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 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 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 some multi-byte character sets, HTTP allows the use of whatever octet
sequences are defined by that character set to represent the sequences are defined by that character set to represent the
equivalent of CR and LF for line breaks. This flexibility regarding 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 line breaks applies only to text media in the entity-body; a bare CR
Fielding, et al Expires May, 2004 [Page 25]
or LF MUST NOT be substituted for CRLF within any of the HTTP control or LF MUST NOT be substituted for CRLF within any of the HTTP control
structures (such as header fields and multipart boundaries). structures (such as header fields and multipart boundaries).
If an entity-body is encoded with a content-coding, the underlying If an entity-body is encoded with a content-coding, the underlying
data MUST be in a form defined above prior to being encoded. data MUST be in a form defined above prior to being encoded.
The "charset" parameter is used with some media types to define the The "charset" parameter is used with some media types to define the
character set (section 3.4) of the data. When no explicit charset character set (section 3.4) of the data. When no explicit charset
parameter is provided by the sender, media subtypes of the "text" 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 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 received via HTTP. Data in character sets other than "ISO-8859-1" or
its subsets MUST be labeled with an appropriate charset value. See its subsets MUST be labeled with an appropriate charset value. See
section 3.4.1 for compatibility problems. section 0 for compatibility problems.
3.7.2 Multipart Types 3.7.2 Multipart Types
MIME provides for a number of "multipart" types -- encapsulations of MIME provides for a number of "multipart" types -- encapsulations of
one or more entities within a single message-body. All multipart 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 types share a common syntax, as defined in section 5.1.1 of RFC 2046
[40], and MUST include a boundary parameter as part of the media type [N8], and MUST include a boundary parameter as part of the media type
value. The message body is itself a protocol element and MUST value. The message body is itself a protocol element and MUST
therefore use only CRLF to represent line breaks between body-parts. therefore use only CRLF to represent line breaks between body-parts.
Unlike in RFC 2046, the epilogue of any multipart message MUST be Unlike in RFC 2046, the epilogue of any multipart message MUST be
empty; HTTP applications MUST NOT transmit the epilogue (even if the empty; HTTP applications MUST NOT transmit the epilogue (even if the
original multipart contains an epilogue). These restrictions exist in original multipart contains an epilogue). These restrictions exist in
order to preserve the self-delimiting nature of a multipart message- order to preserve the self-delimiting nature of a multipart message-
body, wherein the "end" of the message-body is indicated by the body, wherein the "end" of the message-body is indicated by the
ending multipart boundary. ending multipart boundary.
In general, HTTP treats a multipart message-body no differently than In general, HTTP treats a multipart message-body no differently than
any other media type: strictly as payload. The one exception is the any other media type: strictly as payload. The one exception is the
"multipart/byteranges" type (appendix 19.2) when it appears in a 206 "multipart/byteranges" type (appendix 17.2) when it appears in a 206
(Partial Content) response, which will be interpreted by some HTTP (Partial Content) response, which will be interpreted by some HTTP
caching mechanisms as described in sections 13.5.4 and 14.16. In all 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 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. 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- The MIME header fields within each body-part of a multipart message-
body do not have any significance to HTTP beyond that defined by body do not have any significance to HTTP beyond that defined by
their MIME semantics. their MIME semantics.
In general, an HTTP user agent SHOULD follow the same or similar 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. behavior as a MIME user agent would upon receipt of a multipart type.
If an application receives an unrecognized multipart subtype, the If an application receives an unrecognized multipart subtype, the
application MUST treat it as being equivalent to "multipart/mixed". application MUST treat it as being equivalent to "multipart/mixed".
Note: The "multipart/form-data" type has been specifically defined Note: The "multipart/form-data" type has been specifically defined
for carrying form data suitable for processing via the POST for carrying form data suitable for processing via the POST request
request method, as described in RFC 1867 [15]. method, as described in RFC 2388 [I11].
3.8 Product Tokens 3.8 Product Tokens
Product tokens are used to allow communicating applications to Product tokens are used to allow communicating applications to
identify themselves by software name and version. Most fields using identify themselves by software name and version. Most fields using
product tokens also allow sub-products which form a significant part product tokens also allow sub-products which form a significant part
Fielding, et al Expires May, 2004 [Page 26]
of the application to be listed, separated by white space. By of the application to be listed, separated by white space. By
convention, the products are listed in order of their significance convention, the products are listed in order of their significance
for identifying the application. for identifying the application.
product = token ["/" product-version] product = token ["/" product-version]
product-version = token product-version = token
Examples: Examples:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3 User-Agent: CERN-LineMode/2.15 libwww/2.17b3
Server: Apache/0.8.4 Server: Apache/0.8.4
Product tokens SHOULD be short and to the point. They MUST NOT be Product tokens SHOULD be short and to the point. They MUST NOT be
used for advertising or other non-essential information. Although any used for advertising or other non-essential information. Although any
token character MAY appear in a product-version, this token SHOULD token character MAY appear in a product-version, this token SHOULD
only be used for a version identifier (i.e., successive versions of 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 same product SHOULD only differ in the product-version portion of
the product value). the product value).
3.9 Quality Values 3.9 Quality Values
HTTP content negotiation (section 12) uses short "floating point" HTTP content negotiation (section 12) uses short "floating point"
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the same product SHOULD only differ in the product-version portion of the same product SHOULD only differ in the product-version portion of
the product value). the product value).
3.9 Quality Values 3.9 Quality Values
HTTP content negotiation (section 12) uses short "floating point" HTTP content negotiation (section 12) uses short "floating point"
numbers to indicate the relative importance ("weight") of various numbers to indicate the relative importance ("weight") of various
negotiable parameters. A weight is normalized to a real number in 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 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 value. If a parameter has a quality value of 0, then content with
this parameter is `not acceptable' for the client. HTTP/1.1 this parameter is "not acceptable" for the client. HTTP/1.1
applications MUST NOT generate more than three digits after the applications MUST NOT generate more than three digits after the
decimal point. User configuration of these values SHOULD also be decimal point. User configuration of these values SHOULD also be
limited in this fashion. limited in this fashion.
qvalue = ( "0" [ "." 0*3DIGIT ] ) qvalue = ( "0" [ "." 0*3DIGIT ] )
| ( "1" [ "." 0*3("0") ] ) | ( "1" [ "." 0*3("0") ] )
"Quality values" is a misnomer, since these values merely represent "Quality values" is a misnomer, since these values merely represent
relative degradation in desired quality. relative degradation in desired quality.
3.10 Language Tags 3.10 Language Tags
A language tag identifies a natural language spoken, written, or A language tag identifies a natural language spoken, written, or
otherwise conveyed by human beings for communication of information otherwise conveyed by human beings for communication of information
to other human beings. Computer languages are explicitly excluded. to other human beings. Computer languages are explicitly excluded.
HTTP uses language tags within the Accept-Language and Content- HTTP uses language tags within the Accept-Language and Content-
Language fields. Language fields.
The syntax and registry of HTTP language tags is the same as that 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 defined by RFC 3066 [I1]. In summary, a language tag is composed of 1
or more parts: A primary language tag and a possibly empty series of or more parts: A primary language tag and a possibly empty series of
subtags: subtags:
language-tag = primary-tag *( "-" subtag ) language-tag = primary-tag *( "-" subtag )
primary-tag = 1*8ALPHA primary-tag = 1*8ALPHA
subtag = 1*8ALPHA subtag = 1*8(ALPHA / DIGIT)
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White space is not allowed within the tag and all tags are case- White space is not allowed within the tag and all tags are case-
insensitive. The name space of language tags is administered by the insensitive. The name space of language tags is administered by the
IANA. Example tags include: IANA. Example tags include:
en, en-US, en-cockney, i-cherokee, x-pig-latin en, en-US, en-cockney, i-cherokee, x-pig-latin
where any two-letter primary-tag is an ISO-639 language abbreviation 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 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 last three tags above are not registered tags; all but the last are
examples of tags which could be registered in future.) examples of tags which could be registered in future.)
3.11 Entity Tags 3.11 Entity Tags
Entity tags are used for comparing two or more entities from the same 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 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 14.19), If-Match (section 14.24), If-None-Match (section 14.26), and
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header fields. An entity can be broken down into subranges according header fields. An entity can be broken down into subranges according
to various structural units. to various structural units.
range-unit = bytes-unit | other-range-unit range-unit = bytes-unit | other-range-unit
bytes-unit = "bytes" bytes-unit = "bytes"
other-range-unit = token other-range-unit = token
The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1 The only range unit defined by HTTP/1.1 is "bytes". HTTP/1.1
implementations MAY ignore ranges specified using other units. implementations MAY ignore ranges specified using other units.
Fielding, et al Expires May, 2004 [Page 28]
HTTP/1.1 has been designed to allow implementations of applications HTTP/1.1 has been designed to allow implementations of applications
that do not depend on knowledge of ranges. that do not depend on knowledge of ranges.
4 HTTP Message 4 HTTP Message
4.1 Message Types 4.1 Message Types
HTTP messages consist of requests from client to server and responses HTTP messages consist of requests from client to server and responses
from server to client. from server to client.
HTTP-message = Request | Response ; HTTP/1.1 messages HTTP-message = Request | Response ; HTTP/1.1 messages
Request (section 5) and Response (section 6) messages use the generic Request (section 5) and Response (section 6) messages use the generic
message format of RFC 822 [9] for transferring entities (the payload message format of RFC 822 [I9] for transferring entities (the payload
of the message). Both types of message consist of a start-line, zero 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., 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 a line with nothing preceding the CRLF) indicating the end of the
header fields, and possibly a message-body. header fields, and possibly a message-body.
generic-message = start-line generic-message = start-line
*(message-header CRLF) *(message-header CRLF)
CRLF CRLF
[ message-body ] [ message-body ]
start-line = Request-Line | Status-Line start-line = Request-Line | Status-Line
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Certain buggy HTTP/1.0 client implementations generate extra CRLF's Certain buggy HTTP/1.0 client implementations generate extra CRLF's
after a POST request. To restate what is explicitly forbidden by the 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 BNF, an HTTP/1.1 client MUST NOT preface or follow a request with an
extra CRLF. extra CRLF.
4.2 Message Headers 4.2 Message Headers
HTTP header fields, which include general-header (section 4.5), HTTP header fields, which include general-header (section 4.5),
request-header (section 5.3), response-header (section 6.2), and request-header (section 5.3), response-header (section 6.2), and
entity-header (section 7.1) fields, follow the same generic format as 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 that given in Section 3.1 of RFC 822 [I9]. Each header field consists
of a name followed by a colon (":") and the field value. Field names 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 are case-insensitive. The field value MAY be preceded by any amount
of LWS, though a single SP is preferred. Header fields can be of LWS, though a single SP is preferred. Header fields can be
extended over multiple lines by preceding each extra line with at extended over multiple lines by preceding each extra line with at
least one SP or HT. Applications ought to follow "common form", where least one SP or HT. Applications ought to follow "common form", where
one is known or indicated, when generating HTTP constructs, since one is known or indicated, when generating HTTP constructs, since
there might exist some implementations that fail to accept anything there might exist some implementations that fail to accept anything
beyond the common forms. beyond the common forms.
message-header = field-name ":" [ field-value ] message-header = field-name ":" [ field-value ]
field-name = token field-name = token
field-value = *( field-content | LWS ) field-value = *( field-content | LWS )
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field-content = <the OCTETs making up the field-value field-content = <the OCTETs making up the field-value
and consisting of either *TEXT or combinations and consisting of either *TEXT or
of token, separators, and quoted-string> combinations of token,
separators, and quoted-string>
The field-content does not include any leading or trailing LWS: The field-content does not include any leading or trailing LWS:
linear white space occurring before the first non-whitespace 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 or after the last non-whitespace
character of the field-value. Such leading or trailing LWS MAY be character of the field-value. Such leading or trailing LWS MAY be
removed without changing the semantics of the field value. Any LWS removed without changing the semantics of the field value. Any LWS
that occurs between field-content MAY be replaced with a single SP that occurs between field-content MAY be replaced with a single SP
before interpreting the field value or forwarding the message before interpreting the field value or forwarding the message
downstream. downstream.
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present in a message if and only if the entire field-value for that 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)]. header field is defined as a comma-separated list [i.e., #(values)].
It MUST be possible to combine the multiple header fields into one It MUST be possible to combine the multiple header fields into one
"field-name: field-value" pair, without changing the semantics of the "field-name: field-value" pair, without changing the semantics of the
message, by appending each subsequent field-value to the first, each message, by appending each subsequent field-value to the first, each
separated by a comma. The order in which header fields with the same separated by a comma. The order in which header fields with the same
field-name are received is therefore significant to the field-name are received is therefore significant to the
interpretation of the combined field value, and thus a proxy MUST NOT interpretation of the combined field value, and thus a proxy MUST NOT
change the order of these field values when a message is forwarded. change the order of these field values when a message is forwarded.
All HTTP header field-names are registered according to the procedure
in [I40].
4.3 Message Body 4.3 Message Body
The message-body (if any) of an HTTP message is used to carry the 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 entity-body associated with the request or response. The message-body
differs from the entity-body only when a transfer-coding has been differs from the entity-body only when a transfer-coding has been
applied, as indicated by the Transfer-Encoding header field (section applied, as indicated by the Transfer-Encoding header field (section
14.41). 14.41).
message-body = entity-body message-body = entity-body
| <entity-body encoded as per Transfer-Encoding> | <entity-body encoded as per Transfer-Encoding>
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Transfer-Encoding MUST be used to indicate any transfer-codings Transfer-Encoding MUST be used to indicate any transfer-codings
applied by an application to ensure safe and proper transfer of the applied by an application to ensure safe and proper transfer of the
message. Transfer-Encoding is a property of the message, not of the message. Transfer-Encoding is a property of the message, not of the
entity, and thus MAY be added or removed by any application along the entity, and thus MAY be added or removed by any application along the
request/response chain. (However, section 3.6 places restrictions on request/response chain. (However, section 3.6 places restrictions on
when certain transfer-codings may be used.) when certain transfer-codings may be used.)
The rules for when a message-body is allowed in a message differ for The rules for when a message-body is allowed in a message differ for
requests and responses. requests and responses.
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The presence of a message-body in a request is signaled by the The presence of a message-body in a request is signaled by the
inclusion of a Content-Length or Transfer-Encoding header field in inclusion of a Content-Length or Transfer-Encoding header field in
the request's message-headers. A message-body MUST NOT be included in the requests message-headers. A message-body MUST NOT be included in
a request if the specification of the request method (section 5.1.1) 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 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 read and forward a message-body on any request; if the request method
does not include defined semantics for an entity-body, then the does not include defined semantics for an entity-body, then the
message-body SHOULD be ignored when handling the request. message-body SHOULD be ignored when handling the request.
For response messages, whether or not a message-body is included with For response messages, whether or not a message-body is included with
a message is dependent on both the request method and the response 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 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- MUST NOT include a message-body, even though the presence of entity-
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been applied. When a message-body is included with a message, the been applied. When a message-body is included with a message, the
transfer-length of that body is determined by one of the following transfer-length of that body is determined by one of the following
(in order of precedence): (in order of precedence):
1.Any response message which "MUST NOT" include a message-body (such 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 as the 1xx, 204, and 304 responses and any response to a HEAD
request) is always terminated by the first empty line after the request) is always terminated by the first empty line after the
header fields, regardless of the entity-header fields present in header fields, regardless of the entity-header fields present in
the message. the message.
2.If a Transfer-Encoding header field (section 14.41) is present and 2. If a Transfer-Encoding header field (section 14.41) is present then
has any value other than "identity", then the transfer-length is the transfer-length is defined by use of the "chunked" transfer-
defined by use of the "chunked" transfer-coding (section 3.6), coding (section 3.6), unless the message is terminated by closing
unless the message is terminated by closing the connection. the connection.
3.If a Content-Length header field (section 14.13) is present, its 3.If a Content-Length header field (section 14.13) is present, its
decimal value in OCTETs represents both the entity-length and the decimal value in OCTETs represents both the entity-length and the
transfer-length. The Content-Length header field MUST NOT be sent transfer-length. The Content-Length header field MUST NOT be sent
if these two lengths are different (i.e., if a Transfer-Encoding if these two lengths are different (i.e., if a Transfer-Encoding
header field is present). If a message is received with both a header field is present). If a message is received with both a
Transfer-Encoding header field and a Content-Length header field, Transfer-Encoding header field and a Content-Length header field,
the latter MUST be ignored. the latter MUST be ignored.
4.If the message uses the media type "multipart/byteranges", and the 4.If the message uses the media type "multipart/byteranges", and the
ransfer-length is not otherwise specified, then this self- transfer-length is not otherwise specified, then this self-
elimiting media type defines the transfer-length. This media type delimiting media type defines the transfer-length. This media type
UST NOT be used unless the sender knows that the recipient can arse MUST NOT be used unless the sender knows that the recipient can
it; the presence in a request of a Range header with ultiple byte- parse it; the presence in a request of a Range header with multiple
range specifiers from a 1.1 client implies that the lient can parse byte-range specifiers from a 1.1 client implies that the client can
multipart/byteranges responses. parse multipart/byteranges responses.
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A range header might be forwarded by a 1.0 proxy that does not A range header might be forwarded by a 1.0 proxy that does not
understand multipart/byteranges; in this case the server MUST understand multipart/byteranges; in this case the server MUST
delimit the message using methods defined in items 1,3 or 5 of delimit the message using methods defined in items 1,3 or 5 of this
this section. section.
5.By the server closing the connection. (Closing the connection 5.By the server closing the connection. (Closing the connection
cannot be used to indicate the end of a request body, since that 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.) would leave no possibility for the server to send back a response.)
For compatibility with HTTP/1.0 applications, HTTP/1.1 requests For compatibility with HTTP/1.0 applications, HTTP/1.1 requests
containing a message-body MUST include a valid Content-Length header 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 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, request contains a message-body and a Content-Length is not given,
the server SHOULD respond with 400 (bad request) if it cannot the server SHOULD respond with 400 (bad request) if it cannot
determine the length of the message, or with 411 (length required) if determine the length of the message, or with 411 (length required) if
it wishes to insist on receiving a valid Content-Length. it wishes to insist on receiving a valid Content-Length.
All HTTP/1.1 applications that receive entities MUST accept the All HTTP/1.1 applications that receive entities MUST accept the
"chunked" transfer-coding (section 3.6), thus allowing this mechanism "chunked" transfer-coding (section 3.6), thus allowing this mechanism
to be used for messages when the message length cannot be determined to be used for messages when the message length cannot be determined
in advance. in advance.
Messages MUST NOT include both a Content-Length header field and a Messages MUST NOT include both a Content-Length header field and a
non-identity transfer-coding. If the message does include a non- transfer-coding. If the message does include a non-identity transfer-
identity transfer-coding, the Content-Length MUST be ignored. coding, the Content-Length MUST be ignored.
When a Content-Length is given in a message where a message-body is 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 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 the message-body. HTTP/1.1 user agents MUST notify the user when an
invalid length is received and detected. invalid length is received and detected.
4.5 General Header Fields 4.5 General Header Fields
There are a few header fields which have general applicability for There are a few header fields which have general applicability for
both request and response messages, but which do not apply to the both request and response messages, but which do not apply to the
skipping to change at page 35, line 23 skipping to change at line 1731
| Via ; Section 14.45 | Via ; Section 14.45
| Warning ; Section 14.46 | Warning ; Section 14.46
General-header field names can be extended reliably only in General-header field names can be extended reliably only in
combination with a change in the protocol version. However, new or combination with a change in the protocol version. However, new or
experimental header fields may be given the semantics of general experimental header fields may be given the semantics of general
header fields if all parties in the communication recognize them to header fields if all parties in the communication recognize them to
be general-header fields. Unrecognized header fields are treated as be general-header fields. Unrecognized header fields are treated as
entity-header fields. entity-header fields.
Fielding, et al Expires May, 2004 [Page 32]
5 Request 5 Request
A request message from a client to a server includes, within the 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, first line of that message, the method to be applied to the resource,
the identifier of the resource, and the protocol version in use. the identifier of the resource, and the protocol version in use.
Request = Request-Line ; Section 5.1 Request = Request-Line ; Section 5.1
*(( general-header ; Section 4.5 *(( general-header ; Section 4.5
| request-header ; Section 5.3 | request-header ; Section 5.3
| entity-header ) CRLF) ; Section 7.1 | entity-header ) CRLF) ; Section 7.1
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A request message from a client to a server includes, within the 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, first line of that message, the method to be applied to the resource,
the identifier of the resource, and the protocol version in use. the identifier of the resource, and the protocol version in use.
Request = Request-Line ; Section 5.1 Request = Request-Line ; Section 5.1
*(( general-header ; Section 4.5 *(( general-header ; Section 4.5
| request-header ; Section 5.3 | request-header ; Section 5.3
| entity-header ) CRLF) ; Section 7.1 | entity-header ) CRLF) ; Section 7.1
CRLF CRLF
[ message-body ] ; Section 4.3 [ message-body ] ; Section 4.3
5.1 Request-Line 5.1 Request-Line
The Request-Line begins with a method token, followed by the The Request-Line begins with a method token, followed by the Request-
Request-URI and the protocol version, and ending with CRLF. The URI and the protocol version, and ending with CRLF. The elements are
elements are separated by SP characters. No CR or LF is allowed separated by SP characters. No CR or LF is allowed except in the
except in the final CRLF sequence. final CRLF sequence.
Request-Line = Method SP Request-URI SP HTTP-Version CRLF Request-Line = Method SP Request-URI SP HTTP-Version CRLF
5.1.1 Method 5.1.1 Method
The Method token indicates the method to be performed on the The Method token indicates the method to be performed on the
resource identified by the Request-URI. The method is case-sensitive. resource identified by the Request-URI. The method is case-sensitive.
Method = "OPTIONS" ; Section 9.2 Method = "OPTIONS" ; Section 9.2
| "GET" ; Section 9.3 | "GET" ; Section 9.3
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and HEAD MUST be supported by all general-purpose servers. All other and HEAD MUST be supported by all general-purpose servers. All other
methods are OPTIONAL; however, if the above methods are implemented, methods are OPTIONAL; however, if the above methods are implemented,
they MUST be implemented with the same semantics as those specified they MUST be implemented with the same semantics as those specified
in section 9. in section 9.
5.1.2 Request-URI 5.1.2 Request-URI
The Request-URI is a Uniform Resource Identifier (section 3.2) and The Request-URI is a Uniform Resource Identifier (section 3.2) and
identifies the resource upon which to apply the request. identifies the resource upon which to apply the request.
Request-URI = "*" | absoluteURI | abs_path | authority Fielding, et al Expires May, 2004 [Page 33]
Request-URI = "*" | absoluteURI
| abs_path ["?" query ]| authority
The four options for Request-URI are dependent on the nature of the 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 request. The asterisk "*" means that the request does not apply to a
particular resource, but to the server itself, and is only allowed particular resource, but to the server itself, and is only allowed
when the method used does not necessarily apply to a resource. One when the method used does not necessarily apply to a resource. One
example would be example would be
OPTIONS * HTTP/1.1 OPTIONS * HTTP/1.1
The absoluteURI form is REQUIRED when the request is being made to a The absoluteURI form is REQUIRED when the request is being made to a
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GET /pub/WWW/TheProject.html HTTP/1.1 GET /pub/WWW/TheProject.html HTTP/1.1
Host: www.w3.org Host: www.w3.org
followed by the remainder of the Request. Note that the absolute path 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 cannot be empty; if none is present in the original URI, it MUST be
given as "/" (the server root). given as "/" (the server root).
The Request-URI is transmitted in the format specified in section 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 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 [N9], the origin server MUST decode the Request-URI in order to
properly interpret the request. Servers SHOULD respond to invalid properly interpret the request. Servers SHOULD respond to invalid
Request-URIs with an appropriate status code. Request-URIs with an appropriate status code.
A transparent proxy MUST NOT rewrite the "abs_path" part of the A transparent proxy MUST NOT rewrite the "abs_path" part of the
received Request-URI when forwarding it to the next inbound server, received Request-URI when forwarding it to the next inbound server,
except as noted above to replace a null abs_path with "/". except as noted above to replace a null abs_path with "/".
Fielding, et al Expires May, 2004 [Page 34]
Note: The "no rewrite" rule prevents the proxy from changing the Note: The "no rewrite" rule prevents the proxy from changing the
meaning of the request when the origin server is improperly using meaning of the request when the origin server is improperly using a
a non-reserved URI character for a reserved purpose. Implementors non-reserved URI character for a reserved purpose. Implementors
should be aware that some pre-HTTP/1.1 proxies have been known to should be aware that some pre-HTTP/1.1 proxies have been known to
rewrite the Request-URI. rewrite the Request-URI.
5.2 The Resource Identified by a Request 5.2 The Resource Identified by a Request
The exact resource identified by an Internet request is determined by The exact resource identified by an Internet request is determined by
examining both the Request-URI and the Host header field. examining both the Request-URI and the Host header field.
An origin server that does not allow resources to differ by the An origin server that does not allow resources to differ by the
requested host MAY ignore the Host header field value when requested host MAY ignore the Host header field value when
determining the resource identified by an HTTP/1.1 request. (But see 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.) section 17.6.1.1 for other requirements on Host support in HTTP/1.1.)
An origin server that does differentiate resources based on the host An origin server that does differentiate resources based on the host
requested (sometimes referred to as virtual hosts or vanity host requested (sometimes referred to as virtual hosts or vanity host
names) MUST use the following rules for determining the requested names) MUST use the following rules for determining the requested
resource on an HTTP/1.1 request: resource on an HTTP/1.1 request:
1. If Request-URI is an absoluteURI, the host is part of the 1. If Request-URI is an absoluteURI, the host is part of the Request-
Request-URI. Any Host header field value in the request MUST be URI. Any Host header field value in the request MUST be ignored.
ignored.
2. If the Request-URI is not an absoluteURI, and the request includes 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 a Host header field, the host is determined by the Host header
field value. field value.
3. If the host as determined by rule 1 or 2 is not a valid host on 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. 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 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 attempt to use heuristics (e.g., examination of the URI path for
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invocation. invocation.
request-header = Accept ; Section 14.1 request-header = Accept ; Section 14.1
| Accept-Charset ; Section 14.2 | Accept-Charset ; Section 14.2
| Accept-Encoding ; Section 14.3 | Accept-Encoding ; Section 14.3
| Accept-Language ; Section 14.4 | Accept-Language ; Section 14.4
| Authorization ; Section 14.8 | Authorization ; Section 14.8
| Expect ; Section 14.20 | Expect ; Section 14.20
| From ; Section 14.22 | From ; Section 14.22
| Host ; Section 14.23 | Host ; Section 14.23
Fielding, et al Expires May, 2004 [Page 35]
| If-Match ; Section 14.24 | If-Match ; Section 14.24
| If-Modified-Since ; Section 14.25 | If-Modified-Since ; Section 14.25
| If-None-Match ; Section 14.26 | If-None-Match ; Section 14.26
| If-Range ; Section 14.27 | If-Range ; Section 14.27
| If-Unmodified-Since ; Section 14.28 | If-Unmodified-Since ; Section 14.28
| Max-Forwards ; Section 14.31 | Max-Forwards ; Section 14.31
| Proxy-Authorization ; Section 14.34 | Proxy-Authorization ; Section 14.34
| Range ; Section 14.35 | Range ; Section 14.35
| Referer ; Section 14.36 | Referer ; Section 14.36
| TE ; Section 14.39 | TE ; Section 14.39
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defined in section 10. The Reason-Phrase is intended to give a short defined in section 10. The Reason-Phrase is intended to give a short
textual description of the Status-Code. The Status-Code is intended textual description of the Status-Code. The Status-Code is intended
for use by automata and the Reason-Phrase is intended for the human 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- user. The client is not required to examine or display the Reason-
Phrase. Phrase.
The first digit of the Status-Code defines the class of response. The 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 last two digits do not have any categorization role. There are 5
values for the first digit: values for the first digit:
- 1xx: Informational - Request received, continuing process Fielding, et al Expires May, 2004 [Page 36]
o 1xx: Informational - Request received, continuing process
- 2xx: Success - The action was successfully received, o 2xx: Success - The action was successfully received, understood,
understood, and accepted and accepted
o 3xx: Redirection - Further action must be taken in order to
- 3xx: Redirection - Further action must be taken in order to
complete the request complete the request
o 4xx: Client Error - The request contains bad syntax or cannot be
- 4xx: Client Error - The request contains bad syntax or cannot fulfilled
be fulfilled o 5xx: Server Error - The server failed to fulfill an apparently
- 5xx: Server Error - The server failed to fulfill an apparently
valid request valid request
The individual values of the numeric status codes defined for The individual values of the numeric status codes defined for
HTTP/1.1, and an example set of corresponding Reason-Phrase's, are HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
presented below. The reason phrases listed here are only presented below. The reason phrases listed here are only
recommendations -- they MAY be replaced by local equivalents without recommendations -- they MAY be replaced by local equivalents without
affecting the protocol. affecting the protocol.
Status-Code = Status-Code =
"100" ; Section 10.1.1: Continue "100" ; Section 10.1.1: Continue
| "101" ; Section 10.1.2: Switching Protocols | "101" ; Section 10.1.2: Switching Protocols
| "200" ; Section 10.2.1: OK | "200" ; Section 10.2.1: OK
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| "409" ; Section 10.4.10: Conflict | "409" ; Section 10.4.10: Conflict
| "410" ; Section 10.4.11: Gone | "410" ; Section 10.4.11: Gone
| "411" ; Section 10.4.12: Length Required | "411" ; Section 10.4.12: Length Required
| "412" ; Section 10.4.13: Precondition Failed | "412" ; Section 10.4.13: Precondition Failed
| "413" ; Section 10.4.14: Request Entity Too Large | "413" ; Section 10.4.14: Request Entity Too Large
| "414" ; Section 10.4.15: Request-URI Too Large | "414" ; Section 10.4.15: Request-URI Too Large
| "415" ; Section 10.4.16: Unsupported Media Type | "415" ; Section 10.4.16: Unsupported Media Type
| "416" ; Section 10.4.17: Requested range not satisfiable | "416" ; Section 10.4.17: Requested range not satisfiable
| "417" ; Section 10.4.18: Expectation Failed | "417" ; Section 10.4.18: Expectation Failed
| "500" ; Section 10.5.1: Internal Server Error | "500" ; Section 10.5.1: Internal Server Error
Fielding, et al Expires May, 2004 [Page 37]
| "501" ; Section 10.5.2: Not Implemented | "501" ; Section 10.5.2: Not Implemented
| "502" ; Section 10.5.3: Bad Gateway | "502" ; Section 10.5.3: Bad Gateway
| "503" ; Section 10.5.4: Service Unavailable | "503" ; Section 10.5.4: Service Unavailable
| "504" ; Section 10.5.5: Gateway Time-out | "504" ; Section 10.5.5: Gateway Time-out
| "505" ; Section 10.5.6: HTTP Version not supported | "505" ; Section 10.5.6: HTTP Version not supported
| extension-code | extension-code
extension-code = 3DIGIT extension-code = 3DIGIT
Reason-Phrase = *<TEXT, excluding CR, LF> Reason-Phrase = *<TEXT, excluding CR, LF>
HTTP status codes are extensible. HTTP applications are not required HTTP status codes are extensible. HTTP applications are not required
to understand the meaning of all registered status codes, though such to understand the meaning of all registered status codes, though such
understanding is obviously desirable. However, applications MUST understanding is obviously desirable. However, applications MUST
understand the class of any status code, as indicated by the first understand the class of any status code, as indicated by the first
digit, and treat any unrecognized response as being equivalent to the digit, and treat any unrecognized response as being equivalent to the
x00 status code of that class, with the exception that an x00 status code of that class, with the exception that an
unrecognized response MUST NOT be cached. For example, if an unrecognized response MUST NOT be cached. For example, if an
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be response-header fields. Unrecognized header fields are treated as be response-header fields. Unrecognized header fields are treated as
entity-header fields. entity-header fields.
7 Entity 7 Entity
Request and Response messages MAY transfer an entity if not otherwise Request and Response messages MAY transfer an entity if not otherwise
restricted by the request method or response status code. An entity restricted by the request method or response status code. An entity
consists of entity-header fields and an entity-body, although some consists of entity-header fields and an entity-body, although some
responses will only include the entity-headers. responses will only include the entity-headers.
Fielding, et al Expires May, 2004 [Page 38]
In this section, both sender and recipient refer to either the client In this section, both sender and recipient refer to either the client
or the server, depending on who sends and who receives the entity. or the server, depending on who sends and who receives the entity.
7.1 Entity Header Fields 7.1 Entity Header Fields
Entity-header fields define metainformation about the entity-body or, Entity-header fields define metainformation about the entity-body or,
if no body is present, about the resource identified by the request. if no body is present, about the resource identified by the request.
Some of this metainformation is OPTIONAL; some might be REQUIRED by Some of this metainformation is OPTIONAL; some might be REQUIRED by
portions of this specification. portions of this specification.
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Encoding. These define a two-layer, ordered encoding model: Encoding. These define a two-layer, ordered encoding model:
entity-body := Content-Encoding( Content-Type( data ) ) entity-body := Content-Encoding( Content-Type( data ) )
Content-Type specifies the media type of the underlying data. Content-Type specifies the media type of the underlying data.
Content-Encoding may be used to indicate any additional content Content-Encoding may be used to indicate any additional content
codings applied to the data, usually for the purpose of data codings applied to the data, usually for the purpose of data
compression, that are a property of the requested resource. There is compression, that are a property of the requested resource. There is
no default encoding. no default encoding.
Fielding, et al Expires May, 2004 [Page 39]
Any HTTP/1.1 message containing an entity-body SHOULD include a Any HTTP/1.1 message containing an entity-body SHOULD include a
Content-Type header field defining the media type of that body. If 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 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 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 content and/or the name extension(s) of the URI used to identify the
resource. If the media type remains unknown, the recipient SHOULD resource. If the media type remains unknown, the recipient SHOULD
treat it as type "application/octet-stream". treat it as type "application/octet-stream".
7.2.2 Entity Length 7.2.2 Entity Length
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8.1 Persistent Connections 8.1 Persistent Connections
8.1.1 Purpose 8.1.1 Purpose
Prior to persistent connections, a separate TCP connection was Prior to persistent connections, a separate TCP connection was
established to fetch each URL, increasing the load on HTTP servers established to fetch each URL, increasing the load on HTTP servers
and causing congestion on the Internet. The use of inline images and and causing congestion on the Internet. The use of inline images and
other associated data often require a client to make multiple other associated data often require a client to make multiple
requests of the same server in a short amount of time. Analysis of requests of the same server in a short amount of time. Analysis of
these performance problems and results from a prototype these performance problems and results from a prototype
implementation are available [26] [30]. Implementation experience and implementation are available [I19] [I23]. Implementation experience
measurements of actual HTTP/1.1 (RFC 2068) implementations show good and measurements of actual HTTP/1.1 (RFC 2068) implementations show
results [39]. Alternatives have also been explored, for example, good results [I31]. Alternatives have also been explored, for
T/TCP [27]. example, T/TCP [I20].
Persistent HTTP connections have a number of advantages: Persistent HTTP connections have a number of advantages:
- By opening and closing fewer TCP connections, CPU time is saved o By opening and closing fewer TCP connections, CPU time is saved in
in routers and hosts (clients, servers, proxies, gateways, routers and hosts (clients, servers, proxies, gateways, tunnels, or
tunnels, or caches), and memory used for TCP protocol control caches), and memory used for TCP protocol control blocks can be
blocks can be saved in hosts. saved in hosts.
o HTTP requests and responses can be pipelined on a connection.
- HTTP requests and responses can be pipelined on a connection.
Pipelining allows a client to make multiple requests without Pipelining allows a client to make multiple requests without
waiting for each response, allowing a single TCP connection to waiting for each response, allowing a single TCP connection to be
be used much more efficiently, with much lower elapsed time. used much more efficiently, with much lower elapsed time.
o Network congestion is reduced by reducing the number of packets
- Network congestion is reduced by reducing the number of packets
caused by TCP opens, and by allowing TCP sufficient time to caused by TCP opens, and by allowing TCP sufficient time to
determine the congestion state of the network. determine the congestion state of the network.
o Latency on subsequent requests is reduced since there is no time
- Latency on subsequent requests is reduced since there is no time spent in TCP∆s connection opening handshake.
spent in TCP's connection opening handshake. o HTTP can evolve more gracefully, since errors can be reported
- HTTP can evolve more gracefully, since errors can be reported
without the penalty of closing the TCP connection. Clients using without the penalty of closing the TCP connection. Clients using
future versions of HTTP might optimistically try a new feature, future versions of HTTP might optimistically try a new feature, but
but if communicating with an older server, retry with old
semantics after an error is reported. Fielding, et al Expires May, 2004 [Page 40]
if communicating with an older server, retry with old semantics
after an error is reported.
HTTP implementations SHOULD implement persistent connections. HTTP implementations SHOULD implement persistent connections.
8.1.2 Overall Operation 8.1.2 Overall Operation
A significant difference between HTTP/1.1 and earlier versions of A significant difference between HTTP/1.1 and earlier versions of
HTTP is that persistent connections are the default behavior of any HTTP is that persistent connections are the default behavior of any
HTTP connection. That is, unless otherwise indicated, the client HTTP connection. That is, unless otherwise indicated, the client
SHOULD assume that the server will maintain a persistent connection, SHOULD assume that the server will maintain a persistent connection,
even after error responses from the server. even after error responses from the server.
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maintain a persistent connection unless a Connection header including maintain a persistent connection unless a Connection header including
the connection-token "close" was sent in the request. If the server the connection-token "close" was sent in the request. If the server
chooses to close the connection immediately after sending the chooses to close the connection immediately after sending the
response, it SHOULD send a Connection header including the response, it SHOULD send a Connection header including the
connection-token close. connection-token close.
An HTTP/1.1 client MAY expect a connection to remain open, but would 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 decide to keep it open based on whether the response from a server
contains a Connection header with the connection-token close. In case contains a Connection header with the connection-token close. In case
the client does not want to maintain a connection for more than that 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. 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 If either the client or the server sends the close token in the
Connection header, that request becomes the last one for the Connection header, that request becomes the last one for the
connection. connection.
Clients and servers SHOULD NOT assume that a persistent connection is Clients and servers SHOULD NOT assume that a persistent connection is
maintained for HTTP versions less than 1.1 unless it is explicitly maintained for HTTP versions less than 1.1 unless it is explicitly
signaled. See section 19.6.2 for more information on backward signaled. See section 17.6.2 for more information on backward
compatibility with HTTP/1.0 clients. compatibility with HTTP/1.0 clients.
In order to remain persistent, all messages on the connection MUST In order to remain persistent, all messages on the connection MUST
have a self-defined message length (i.e., one not defined by closure have a self-defined message length (i.e., one not defined by closure
of the connection), as described in section 4.4. of the connection), as described in section 4.4.
8.1.2.2 Pipelining 8.1.2.2 Pipelining
Fielding, et al Expires May, 2004 [Page 41]
A client that supports persistent connections MAY "pipeline" its A client that supports persistent connections MAY "pipeline" its
requests (i.e., send multiple requests without waiting for each requests (i.e., send multiple requests without waiting for each
response). A server MUST send its responses to those requests in the response). A server MUST send its responses to those requests in the
same order that the requests were received. same order that the requests were received.
Clients which assume persistent connections and pipeline immediately Clients which assume persistent connections and pipeline immediately
after connection establishment SHOULD be prepared to retry their after connection establishment SHOULD be prepared to retry their
connection if the first pipelined attempt fails. If a client does connection if the first pipelined attempt fails. If a client does
such a retry, it MUST NOT pipeline before it knows the connection is such a retry, it MUST NOT pipeline before it knows the connection is
persistent. Clients MUST also be prepared to resend their requests if persistent. Clients MUST also be prepared to resend their requests if
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It is especially important that proxies correctly implement the It is especially important that proxies correctly implement the
properties of the Connection header field as specified in section properties of the Connection header field as specified in section
14.10. 14.10.
The proxy server MUST signal persistent connections separately with The proxy server MUST signal persistent connections separately with
its clients and the origin servers (or other proxy servers) that it its clients and the origin servers (or other proxy servers) that it
connects to. Each persistent connection applies to only one transport connects to. Each persistent connection applies to only one transport
link. link.
A proxy server MUST NOT establish a HTTP/1.1 persistent connection 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 with an HTTP/1.0 client (but see RFC 2068 [I25] for information and
discussion of the problems with the Keep-Alive header implemented by discussion of the problems with the Keep-Alive header implemented by
many HTTP/1.0 clients). many HTTP/1.0 clients).
8.1.4 Practical Considerations 8.1.4 Practical Considerations
Servers will usually have some time-out value beyond which they will Servers will usually have some time-out value beyond which they will
no longer maintain an inactive connection. Proxy servers might make no longer maintain an inactive connection. Proxy servers might make
this a higher value since it is likely that the client will be making this a higher value since it is likely that the client will be making
more connections through the same server. The use of persistent more connections through the same server. The use of persistent
connections places no requirements on the length (or existence) of connections places no requirements on the length (or existence) of
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no longer maintain an inactive connection. Proxy servers might make no longer maintain an inactive connection. Proxy servers might make
this a higher value since it is likely that the client will be making this a higher value since it is likely that the client will be making
more connections through the same server. The use of persistent more connections through the same server. The use of persistent
connections places no requirements on the length (or existence) of connections places no requirements on the length (or existence) of
this time-out for either the client or the server. this time-out for either the client or the server.
When a client or server wishes to time-out it SHOULD issue a graceful When a client or server wishes to time-out it SHOULD issue a graceful
close on the transport connection. Clients and servers SHOULD both close on the transport connection. Clients and servers SHOULD both
constantly watch for the other side of the transport close, and constantly watch for the other side of the transport close, and
respond to it as appropriate. If a client or server does not detect respond to it as appropriate. If a client or server does not detect
the other side's close promptly it could cause unnecessary resource the other side's close promptly it could cause unnecessary resource
drain on the network. drain on the network.
Fielding, et al Expires May, 2004 [Page 42]
A client, server, or proxy MAY close the transport connection at any 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 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" 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 connection. From the servers point of view, the connection is being
closed while it was idle, but from the client's point of view, a closed while it was idle, but from the client's point of view, a
request is in progress. request is in progress.
This means that clients, servers, and proxies MUST be able to recover This means that clients, servers, and proxies MUST be able to recover
from asynchronous close events. Client software SHOULD reopen the from asynchronous close events. Client software SHOULD reopen the
transport connection and retransmit the aborted sequence of requests transport connection and retransmit the aborted sequence of requests
without user interaction so long as the request sequence is without user interaction so long as the request sequence is
idempotent (see section 9.1.2). Non-idempotent methods or sequences idempotent (see section 9.1.2). Non-idempotent methods or sequences
MUST NOT be automatically retried, although user agents MAY offer a MUST NOT be automatically retried, although user agents MAY offer a
human operator the choice of retrying the request(s). Confirmation by human operator the choice of retrying the request(s). Confirmation by
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An HTTP/1.1 (or later) client sending a message-body SHOULD monitor 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 network connection for an error status while it is transmitting
the request. If the client sees an error status, it SHOULD the request. If the client sees an error status, it SHOULD
immediately cease transmitting the body. If the body is being sent immediately cease transmitting the body. If the body is being sent
using a "chunked" encoding (section 3.6), a zero length chunk and 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. 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 If the body was preceded by a Content-Length header, the client MUST
close the connection. close the connection.
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8.2.3 Use of the 100 (Continue) Status 8.2.3 Use of the 100 (Continue) Status
The purpose of the 100 (Continue) status (see section 10.1.1) is to 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 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 to determine if the origin server is willing to accept the request
(based on the request headers) before the client sends the request (based on the request headers) before the client sends the request
body. In some cases, it might either be inappropriate or highly body. In some cases, it might either be inappropriate or highly
inefficient for the client to send the body if the server will reject inefficient for the client to send the body if the server will reject
the message without looking at the body. the message without looking at the body.
Requirements for HTTP/1.1 clients: Requirements for HTTP/1.1 clients:
o If a client will wait for a 100 (Continue) response before sending
- If a client will wait for a 100 (Continue) response before the request body, it MUST send an Expect request-header field
sending the request body, it MUST send an Expect request-header (section 14.20) with the "100-continue" expectation.
field (section 14.20) with the "100-continue" expectation. o A client MUST NOT send an Expect request-header field (section
14.20) with the "100-continue" expectation if it does not intend to
- A client MUST NOT send an Expect request-header field (section send a request body.
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 Because of the presence of older implementations, the protocol allows
ambiguous situations in which a client may send "Expect: 100- ambiguous situations in which a client may send "Expect: 100-
continue" without receiving either a 417 (Expectation Failed) status continue" without receiving either a 417 (Expectation Failed) status
or a 100 (Continue) status. Therefore, when a client sends this or a 100 (Continue) status. Therefore, when a client sends this
header field to an origin server (possibly via a proxy) from which it 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 has never seen a 100 (Continue) status, the client SHOULD NOT wait
for an indefinite period before sending the request body. for an indefinite period before sending the request body.
Requirements for HTTP/1.1 origin servers: Requirements for HTTP/1.1 origin servers:
- Upon receiving a request which includes an Expect request-header o Upon receiving a request which includes an Expect request-header
field with the "100-continue" expectation, an origin server MUST field with the "100-continue" expectation, an origin server MUST
either respond with 100 (Continue) status and continue to read either respond with 100 (Continue) status and continue to read from
from the input stream, or respond with a final status code. The the input stream, or respond with a final status code. The origin
origin server MUST NOT wait for the request body before sending server MUST NOT wait for the request body before sending the 100
the 100 (Continue) response. If it responds with a final status (Continue) response. If it responds with a final status code, it
code, it MAY close the transport connection or it MAY continue MAY close the transport connection or it MAY continue to read and
to read and discard the rest of the request. It MUST NOT discard the rest of the request. It MUST NOT perform the requested
perform the requested method if it returns a final status code. method if it returns a final status code.
- An origin server SHOULD NOT send a 100 (Continue) response if o An origin server SHOULD NOT send a 100 (Continue) response if the
the request message does not include an Expect request-header request message does not include an Expect request-header field
field with the "100-continue" expectation, and MUST NOT send a with the "100-continue" expectation, and MUST NOT send a 100
100 (Continue) response if such a request comes from an HTTP/1.0 (Continue) response if such a request comes from an HTTP/1.0 (or
(or earlier) client. There is an exception to this rule: for earlier) client. There is an exception to this rule: for
compatibility with RFC 2068, a server MAY send a 100 (Continue) 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 status in response to an HTTP/1.1 PUT or POST request that does not
not include an Expect request-header field with the "100- include an Expect request-header field with the "100-continue"
continue" expectation. This exception, the purpose of which is expectation. This exception, the purpose of which is to minimize
to minimize any client processing delays associated with an any client processing delays associated with an undeclared wait for
undeclared wait for 100 (Continue) status, applies only to 100 (Continue) status, applies only to HTTP/1.1 requests, and not
HTTP/1.1 requests, and not to requests with any other HTTP- to requests with any other HTTP-version value.
version value.
- An origin server MAY omit a 100 (Continue) response if it has Fielding, et al Expires May, 2004 [Page 44]
o An origin server MAY omit a 100 (Continue) response if it has
already received some or all of the request body for the already received some or all of the request body for the
corresponding request. corresponding request.
- An origin server that sends a 100 (Continue) response MUST o An origin server that sends a 100 (Continue) response MUST
ultimately send a final status code, once the request body is ultimately send a final status code, once the request body is
received and processed, unless it terminates the transport received and processed, unless it terminates the transport
connection prematurely. connection prematurely.
- If an origin server receives a request that does not include an o If an origin server receives a request that does not include an
Expect request-header field with the "100-continue" expectation, Expect request-header field with the "100-continue" expectation,
the request includes a request body, and the server responds the request includes a request body, and the server responds with a
with a final status code before reading the entire request body final status code before reading the entire request body from the
from the transport connection, then the server SHOULD NOT close transport connection, then the server SHOULD NOT close the
the transport connection until it has read the entire request, transport connection until it has read the entire request, or until
or until the client closes the connection. Otherwise, the client the client closes the connection. Otherwise, the client might not
might not reliably receive the response message. However, this reliably receive the response message. However, this requirement is
requirement is not be construed as preventing a server from not be construed as preventing a server from defending itself
defending itself against denial-of-service attacks, or from against denial-of-service attacks, or from badly broken client
badly broken client implementations. implementations.
Requirements for HTTP/1.1 proxies: Requirements for HTTP/1.1 proxies:
- If a proxy receives a request that includes an Expect request- o If a proxy receives a request that includes an Expect request-
header field with the "100-continue" expectation, and the proxy header field with the "100-continue" expectation, and the proxy
either knows that the next-hop server complies with HTTP/1.1 or 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 higher, or does not know the HTTP version of the next-hop server,
server, it MUST forward the request, including the Expect header it MUST forward the request, including the Expect header field.
field.
- If the proxy knows that the version of the next-hop server is o 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 HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
respond with a 417 (Expectation Failed) status. respond with a 417 (Expectation Failed) status.
- Proxies SHOULD maintain a cache recording the HTTP version o Proxies SHOULD maintain a cache recording the HTTP version numbers
numbers received from recently-referenced next-hop servers. received from recently-referenced next-hop servers.
- A proxy MUST NOT forward a 100 (Continue) response if the o A proxy MUST NOT forward a 100 (Continue) response if the request
request message was received from an HTTP/1.0 (or earlier) message was received from an HTTP/1.0 (or earlier) client and did
client and did not include an Expect request-header field with not include an Expect request-header field with the "100-continue"
the "100-continue" expectation. This requirement overrides the expectation. This requirement overrides the general rule for
general rule for forwarding of 1xx responses (see section 10.1). forwarding of 1xx responses (see section 10.1).
8.2.4 Client Behavior if Server Prematurely Closes Connection 8.2.4 Client Behavior if Server Prematurely Closes Connection
If an HTTP/1.1 client sends a request which includes a request body, 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 but which does not include an Expect request-header field with the
"100-continue" expectation, and if the client is not directly "100-continue" expectation, and if the client is not directly
connected to an HTTP/1.1 origin server, and if the client sees the connected to an HTTP/1.1 origin server, and if the client sees the
connection close before receiving any status from the server, the connection close before receiving any status from the server, the
client SHOULD retry the request. If the client does retry this client SHOULD retry the request. If the client does retry this
request, it MAY use the following "binary exponential backoff" request, it MAY use the following "binary exponential backoff"
algorithm to be assured of obtaining a reliable response: algorithm to be assured of obtaining a reliable response:
Fielding, et al Expires May, 2004 [Page 45]
1. Initiate a new connection to the server 1. Initiate a new connection to the server
2. Transmit the request-headers 2. Transmit the request-headers
3. Initialize a variable R to the estimated round-trip time to the 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 server (e.g., based on the time it took to establish the
connection), or to a constant value of 5 seconds if the round- connection), or to a constant value of 5 seconds if the round-trip
trip time is not available. time is not available.
4. Compute T = R * (2**N), where N is the number of previous 4. Compute T = R * (2**N), where N is the number of previous retries
retries of this request. of this request.
5. Wait either for an error response from the server, or for T 5. Wait either for an error response from the server, or for T seconds
seconds (whichever comes first) (whichever comes first)
6. If no error response is received, after T seconds transmit the 6. If no error response is received, after T seconds transmit the body
body of the request. of the request.
7. If client sees that the connection is closed prematurely, 7. If client sees that the connection is closed prematurely, repeat
repeat from step 1 until the request is accepted, an error from step 1 until the request is accepted, an error response is
response is received, or the user becomes impatient and received, or the user becomes impatient and terminates the retry
terminates the retry process. process.
If at any point an error status is received, the client If at any point an error status is received, the client
- SHOULD NOT continue and o SHOULD NOT continue and
o SHOULD close the connection if it has not completed sending the
- SHOULD close the connection if it has not completed sending the
request message. request message.
9 Method Definitions 9 Method Definitions
The set of common methods for HTTP/1.1 is defined below. Although The set of common methods for HTTP/1.1 is defined below. Although
this set can be expanded, additional methods cannot be assumed to this set can be expanded, additional methods cannot be assumed to
share the same semantics for separately extended clients and servers. share the same semantics for separately extended clients and servers.
The Host request-header field (section 14.23) MUST accompany all The Host request-header field (section 14.23) MUST accompany all
HTTP/1.1 requests. HTTP/1.1 requests.
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9.1.1 Safe Methods 9.1.1 Safe Methods
Implementors should be aware that the software represents the user in Implementors should be aware that the software represents the user in
their interactions over the Internet, and should be careful to allow 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 the user to be aware of any actions they might take which may have an
unexpected significance to themselves or others. unexpected significance to themselves or others.
In particular, the convention has been established that the GET and In particular, the convention has been established that the GET and
HEAD methods SHOULD NOT have the significance of taking an action HEAD methods SHOULD NOT have the significance of taking an action
other than retrieval. These methods ought to be considered "safe". other than retrieval. These methods ought to be considered "safe".
Fielding, et al Expires May, 2004 [Page 46]
This allows user agents to represent other methods, such as POST, PUT 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 and DELETE, in a special way, so that the user is made aware of the
fact that a possibly unsafe action is being requested. fact that a possibly unsafe action is being requested.
Naturally, it is not possible to ensure that the server does not Naturally, it is not possible to ensure that the server does not
generate side-effects as a result of performing a GET request; in generate side-effects as a result of performing a GET request; in
fact, some dynamic resources consider that a feature. The important fact, some dynamic resources consider that a feature. The important
distinction here is that the user did not request the side-effects, distinction here is that the user did not request the side-effects,
so therefore cannot be held accountable for them. so therefore cannot be held accountable for them.
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MUST be indicated by a Content-Type field. Although this MUST be indicated by a Content-Type field. Although this
specification does not define any use for such a body, future specification does not define any use for such a body, future
extensions to HTTP might use the OPTIONS body to make more detailed extensions to HTTP might use the OPTIONS body to make more detailed
queries on the server. A server that does not support such an queries on the server. A server that does not support such an
extension MAY discard the request body. extension MAY discard the request body.
If the Request-URI is an asterisk ("*"), the OPTIONS request is If the Request-URI is an asterisk ("*"), the OPTIONS request is
intended to apply to the server in general rather than to a specific intended to apply to the server in general rather than to a specific
resource. Since a server's communication options typically depend on resource. Since a server's communication options typically depend on
the resource, the "*" request is only useful as a "ping" or "no-op" 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 Fielding, et al Expires May, 2004 [Page 47]
a proxy for HTTP/1.1 compliance (or lack thereof). type of method. For example, this can be used to test a proxy for
OPTIONS method support (or lack thereof).
If the Request-URI is not an asterisk, the OPTIONS request applies If the Request-URI is not an asterisk, the OPTIONS request applies
only to the options that are available when communicating with that only to the options that are available when communicating with that
resource. resource.
A 200 response SHOULD include any header fields that indicate A 200 response SHOULD include any header fields that indicate
optional features implemented by the server and applicable to that optional features implemented by the server and applicable to that
resource (e.g., Allow), possibly including extensions not defined by resource (e.g., Allow), possibly including extensions not defined by
this specification. The response body, if any, SHOULD also include this specification. The response body, if any, SHOULD also include
information about the communication options. The format for such a information about the communication options. The format for such a
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usage by allowing cached entities to be refreshed without requiring usage by allowing cached entities to be refreshed without requiring
multiple requests or transferring data already held by the client. multiple requests or transferring data already held by the client.
The semantics of the GET method change to a "partial GET" if the The semantics of the GET method change to a "partial GET" if the
request message includes a Range header field. A partial GET requests request message includes a Range header field. A partial GET requests
that only part of the entity be transferred, as described in section that only part of the entity be transferred, as described in section
14.35. The partial GET method is intended to reduce unnecessary 14.35. The partial GET method is intended to reduce unnecessary
network usage by allowing partially-retrieved entities to be network usage by allowing partially-retrieved entities to be
completed without transferring data already held by the client. completed without transferring data already held by the client.
Fielding, et al Expires May, 2004 [Page 48]
The response to a GET request is cacheable if and only if it meets The response to a GET request is cacheable if and only if it meets
the requirements for HTTP caching described in section 13. the requirements for HTTP caching described in section 13.
See section 15.1.3 for security considerations when used for forms. See section 15.1.3 for security considerations when used for forms.
9.4 HEAD 9.4 HEAD
The HEAD method is identical to GET except that the server MUST NOT The HEAD method is identical to GET except that the server MUST NOT
return a message-body in the response. The metainformation contained return a message-body in the response. The metainformation contained
in the HTTP headers in response to a HEAD request SHOULD be identical in the HTTP headers in response to a HEAD request SHOULD be identical
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information contained in the response MAY be used to update a information contained in the response MAY be used to update a
previously cached entity from that resource. If the new field values previously cached entity from that resource. If the new field values
indicate that the cached entity differs from the current entity (as indicate that the cached entity differs from the current entity (as
would be indicated by a change in Content-Length, Content-MD5, ETag would be indicated by a change in Content-Length, Content-MD5, ETag
or Last-Modified), then the cache MUST treat the cache entry as or Last-Modified), then the cache MUST treat the cache entry as
stale. stale.
9.5 POST 9.5 POST
The POST method is used to request that the origin server accept the 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 entity enclosed in the request as data to be processed by the
identified by the Request-URI in the Request-Line. POST is designed resource identified by the Request-URI in the Request-Line. POST is
to allow a uniform method to cover the following functions: 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. o Annotation of existing resources;
o Posting a message to a bulletin board, newsgroup, mailing list, or
similar group of articles;
o Providing a block of data, such as the result of submitting a form,
to a data-handling process;
o Extending a database through an append operation.
The actual function performed by the POST method is determined by the The actual function performed by the POST method is determined by the
server and is usually dependent on the Request-URI. The posted entity server and is usually dependent on the Request-URI.
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 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 resource that can be identified by a URI. In this case, either 200
(OK) or 204 (No Content) is the appropriate response status, (OK) or 204 (No Content) is the appropriate response status,
depending on whether or not the response includes an entity that depending on whether or not the response includes an entity that
describes the result. describes the result.
If a resource has been created on the origin server, the response If a resource has been created on the origin server, the response
SHOULD be 201 (Created) and contain an entity which describes the SHOULD be 201 (Created) and contain an entity which describes the
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status of the request and refers to the new resource, and a Location status of the request and refers to the new resource, and a Location
header (see section 14.30). header (see section 14.30).
Responses to this method are not cacheable, unless the response Responses to this method are not cacheable, unless the response
includes appropriate Cache-Control or Expires header fields. However, includes appropriate Cache-Control or Expires header fields. However,
the 303 (See Other) response can be used to direct the user agent to the 303 (See Other) response can be used to direct the user agent to
retrieve a cacheable resource. retrieve a cacheable resource.
POST requests MUST obey the message transmission requirements set out POST requests MUST obey the message transmission requirements set out
in section 8.2. in section 8.2.
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it MUST send a 301 (Moved Permanently) response; the user agent MAY it MUST send a 301 (Moved Permanently) response; the user agent MAY
then make its own decision regarding whether or not to redirect the then make its own decision regarding whether or not to redirect the
request. request.
A single resource MAY be identified by many different URIs. For A single resource MAY be identified by many different URIs. For
example, an article might have a URI for identifying "the current example, an article might have a URI for identifying "the current
version" which is separate from the URI identifying each particular version" which is separate from the URI identifying each particular
version. In this case, a PUT request on a general URI might result in version. In this case, a PUT request on a general URI might result in
several other URIs being defined by the origin server. several other URIs being defined by the origin server.
Fielding, et al Expires May, 2004 [Page 50]
HTTP/1.1 does not define how a PUT method affects the state of an HTTP/1.1 does not define how a PUT method affects the state of an
origin server. origin server.
PUT requests MUST obey the message transmission requirements set out PUT requests MUST obey the message transmission requirements set out
in section 8.2. in section 8.2.
Unless otherwise specified for a particular entity-header, the Unless otherwise specified for a particular entity-header, the
entity-headers in the PUT request SHOULD be applied to the resource entity-headers in the PUT request SHOULD be applied to the resource
created or modified by the PUT. created or modified by the PUT.
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but the response does not include an entity. but the response does not include an entity.
If the request passes through a cache and the Request-URI identifies If the request passes through a cache and the Request-URI identifies
one or more currently cached entities, those entries SHOULD be one or more currently cached entities, those entries SHOULD be
treated as stale. Responses to this method are not cacheable. treated as stale. Responses to this method are not cacheable.
9.8 TRACE 9.8 TRACE
The TRACE method is used to invoke a remote, application-layer loop- The TRACE method is used to invoke a remote, application-layer loop-
back of the request message. The final recipient of the request back of the request message. The final recipient of the request
SHOULD reflect the message received back to the client as the SHOULD reflect the message received back to the client as the entity-
entity-body of a 200 (OK) response. The final recipient is either the body of a 200 (OK) response. The final recipient is either the origin
origin server or the first proxy or gateway to receive a Max-Forwards server or the first proxy or gateway to receive a Max-Forwards value
value of zero (0) in the request (see section 14.31). A TRACE request of zero (0) in the request (see section 14.31). A TRACE request MUST
MUST NOT include an entity. NOT include an entity.
TRACE allows the client to see what is being received at the other 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 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 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. 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 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 length of the request chain, which is useful for testing a chain of
proxies forwarding messages in an infinite loop. proxies forwarding messages in an infinite loop.
If the request is valid, the response SHOULD contain the entire If the request is valid, the response SHOULD contain the entire
request message in the entity-body, with a Content-Type of request message in the entity-body, with a Content-Type of
"message/http". Responses to this method MUST NOT be cached. "message/http". Responses to this method MUST NOT be cached.
Fielding, et al Expires May, 2004 [Page 51]
9.9 CONNECT 9.9 CONNECT
This specification reserves the method name CONNECT for use with a This specification reserves the method name CONNECT for use with a
proxy that can dynamically switch to being a tunnel (e.g. SSL proxy that can dynamically switch to being a tunnel (e.g. SSL
tunneling [44]). tunneling [I33]).
10 Status Code Definitions 10 Status Code Definitions
Each Status-Code is described below, including a description of which Each Status-Code is described below, including a description of which
method(s) it can follow and any metainformation required in the method(s) it can follow and any metainformation required in the
response. response.
10.1 Informational 1xx 10.1 Informational 1xx
This class of status code indicates a provisional response, This class of status code indicates a provisional response,
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then it need not forward the corresponding 100 (Continue) then it need not forward the corresponding 100 (Continue)
response(s).) response(s).)
10.1.1 100 Continue 10.1.1 100 Continue
The client SHOULD continue with its request. This interim response is The client SHOULD continue with its request. This interim response is
used to inform the client that the initial part of the request has 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 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 SHOULD continue by sending the remainder of the request or, if the
request has already been completed, ignore this response. The server request has already been completed, ignore this response. The server
MUST send a final response after the request has been completed. See 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 section 8.2.3 for detailed discussion of the use and handling of this
status code. status code.
10.1.2 101 Switching Protocols 10.1.2 101 Switching Protocols
The server understands and is willing to comply with the client's The server understands and is willing to comply with the client's
request, via the Upgrade message header field (section 14.42), for a request, via the Upgrade message header field (section 14.42), for a
change in the application protocol being used on this connection. The change in the application protocol being used on this connection. The
server will switch protocols to those defined by the response's server will switch protocols to those defined by the responses
Upgrade header field immediately after the empty line which Upgrade header field immediately after the empty line which
terminates the 101 response. terminates the 101 response.
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The protocol SHOULD be switched only when it is advantageous to do The protocol SHOULD be switched only when it is advantageous to do
so. For example, switching to a newer version of HTTP is advantageous so. For example, switching to a newer version of HTTP is advantageous
over older versions, and switching to a real-time, synchronous over older versions, and switching to a real-time, synchronous
protocol might be advantageous when delivering resources that use protocol might be advantageous when delivering resources that use
such features. such features.
10.2 Successful 2xx 10.2 Successful 2xx
This class of status code indicates that the client's request was This class of status code indicates that the client's request was
successfully received, understood, and accepted. successfully received, understood, and accepted.
10.2.1 200 OK 10.2.1 200 OK
The request has succeeded. The information returned with the response The request has succeeded. The information returned with the response
is dependent on the method used in the request, for example: is dependent on the method used in the request, for example:
GET an entity corresponding to the requested resource is sent in GET an entity corresponding to the requested resource is sent in the
the response; response;
HEAD the entity-header fields corresponding to the requested HEAD the entity-header fields corresponding to the requested resource
resource are sent in the response without any message-body; are sent in the response without any message-body;
POST an entity describing or containing the result of the action; POST an entity describing or containing the result of the action;
TRACE an entity containing the request message as received by the
end server. TRACE an entity containing the request message as received by the end
server.
10.2.2 201 Created 10.2.2 201 Created
The request has been fulfilled and resulted in a new resource being The request has been fulfilled and resulted in a new resource being
created. The newly created resource can be referenced by the URI(s) created. The newly created resource can be referenced by the URI(s)
returned in the entity of the response, with the most specific URI returned in the entity of the response, with the most specific URI
for the resource given by a Location header field. The response for the resource given by a Location header field. The response
SHOULD include an entity containing a list of resource SHOULD include an entity containing a list of resource
characteristics and location(s) from which the user or user agent can characteristics and location(s) from which the user or user agent can
choose the one most appropriate. The entity format is specified by choose the one most appropriate. The entity format is specified by
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A 201 response MAY contain an ETag response header field indicating A 201 response MAY contain an ETag response header field indicating
the current value of the entity tag for the requested variant just the current value of the entity tag for the requested variant just
created, see section 14.19. created, see section 14.19.
10.2.3 202 Accepted 10.2.3 202 Accepted
The request has been accepted for processing, but the processing has The request has been accepted for processing, but the processing has
not been completed. The request might or might not eventually be not been completed. The request might or might not eventually be
acted upon, as it might be disallowed when processing actually takes acted upon, as it might be disallowed when processing actually takes
Fielding, et al Expires May, 2004 [Page 53]
place. There is no facility for re-sending a status code from an place. There is no facility for re-sending a status code from an
asynchronous operation such as this. asynchronous operation such as this.
The 202 response is intentionally non-committal. Its purpose is to The 202 response is intentionally non-committal. Its purpose is to
allow a server to accept a request for some other process (perhaps a allow a server to accept a request for some other process (perhaps a
batch-oriented process that is only run once per day) without batch-oriented process that is only run once per day) without
requiring that the user agent's connection to the server persist requiring that the user agent's connection to the server persist
until the process is completed. The entity returned with this until the process is completed. The entity returned with this
response SHOULD include an indication of the request's current status response SHOULD include an indication of the requests current status
and either a pointer to a status monitor or some estimate of when the and either a pointer to a status monitor or some estimate of when the
user can expect the request to be fulfilled. user can expect the request to be fulfilled.
10.2.4 203 Non-Authoritative Information 10.2.4 203 Non-Authoritative Information
The returned metainformation in the entity-header is not the The returned metainformation in the entity-header is not the
definitive set as available from the origin server, but is gathered 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 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 or superset of the original version. For example, including local
annotation information about the resource might result in a superset annotation information about the resource might result in a superset
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The server has fulfilled the request but does not need to return an The server has fulfilled the request but does not need to return an
entity-body, and might want to return updated metainformation. The entity-body, and might want to return updated metainformation. The
response MAY include new or updated metainformation in the form of response MAY include new or updated metainformation in the form of
entity-headers, which if present SHOULD be associated with the entity-headers, which if present SHOULD be associated with the
requested variant. requested variant.
If the client is a user agent, it SHOULD NOT change its document view 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 from that which caused the request to be sent. This response is
primarily intended to allow input for actions to take place without primarily intended to allow input for actions to take place without
causing a change to the user agent's active document view, although causing a change to the user agents active document view, although
any new or updated metainformation SHOULD be applied to the document any new or updated metainformation SHOULD be applied to the document
currently in the user agent's active view. currently in the user agents active view.
The 204 response MUST NOT include a message-body, and thus is always The 204 response MUST NOT include a message-body, and thus is always
terminated by the first empty line after the header fields. terminated by the first empty line after the header fields.
10.2.6 205 Reset Content 10.2.6 205 Reset Content
The server has fulfilled the request and the user agent SHOULD reset The server has fulfilled the request and the user agent SHOULD reset
the document view which caused the request to be sent. This response the document view which caused the request to be sent. This response
is primarily intended to allow input for actions to take place via 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 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 given so that the user can easily initiate another input action. The
response MUST NOT include an entity. response MUST NOT include an entity.
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10.2.7 206 Partial Content 10.2.7 206 Partial Content
The server has fulfilled the partial GET request for the resource. The server has fulfilled the partial GET request for the resource.
The request MUST have included a Range header field (section 14.35) The request MUST have included a Range header field (section 14.35)
indicating the desired range, and MAY have included an If-Range indicating the desired range, and MAY have included an If-Range
header field (section 14.27) to make the request conditional. header field (section 14.27) to make the request conditional.
The response MUST include the following header fields: The response MUST include the following header fields:
- Either a Content-Range header field (section 14.16) indicating o Either a Content-Range header field (section 14.16) indicating the
the range included with this response, or a multipart/byteranges range included with this response, or a multipart/byteranges
Content-Type including Content-Range fields for each part. If a Content-Type including Content-Range fields for each part. If a
Content-Length header field is present in the response, its Content-Length header field is present in the response, its value
value MUST match the actual number of OCTETs transmitted in the MUST match the actual number of OCTETs transmitted in the message-
message-body. body.
o Date
- Date o ETag and/or Content-Location, if the header would have been sent in
a 200 response to the same request
- ETag and/or Content-Location, if the header would have been sent o Expires, Cache-Control, and/or Vary, if the field-value might
in a 200 response to the same request differ from that sent in any previous response for the same variant
- 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 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 strong cache validator (see section 13.3.3), the response SHOULD NOT
include other entity-headers. If the response is the result of an include other entity-headers. If the response is the result of an If-
If-Range request that used a weak validator, the response MUST NOT Range request that used a weak validator, the response MUST NOT
include other entity-headers; this prevents inconsistencies between include other entity-headers; this prevents inconsistencies between
cached entity-bodies and updated headers. Otherwise, the response cached entity-bodies and updated headers. Otherwise, the response
MUST include all of the entity-headers that would have been returned MUST include all of the entity-headers that would have been returned
with a 200 (OK) response to the same request. with a 200 (OK) response to the same request.
A cache MUST NOT combine a 206 response with other previously cached A cache MUST NOT combine a 206 response with other previously cached
content if the ETag or Last-Modified headers do not match exactly, content if the ETag or Last-Modified headers do not match exactly,
see 13.5.4. see 13.5.4.
A cache that does not support the Range and Content-Range headers A cache that does not support the Range and Content-Range headers
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10.3 Redirection 3xx 10.3 Redirection 3xx
This class of status code indicates that further action needs to be 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 taken by the user agent in order to fulfill the request. The action
required MAY be carried out by the user agent without interaction 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 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 GET or HEAD. A client SHOULD detect infinite redirection loops, since
such loops generate network traffic for each redirection. such loops generate network traffic for each redirection.
Note: previous versions of this specification recommended a Note: previous versions of this specification recommended a maximum
maximum of five redirections. Content developers should be aware of five redirections. Content developers should be aware that there
that there might be clients that implement such a fixed might be clients that implement such a fixed limitation.
limitation.
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10.3.1 300 Multiple Choices 10.3.1 300 Multiple Choices
The requested resource corresponds to any one of a set of The requested resource corresponds to any one of a set of
representations, each with its own specific location, and agent- representations, each with its own specific location, and agent-
driven negotiation information (section 12) is being provided so that driven negotiation information (section 12) is being provided so that
the user (or user agent) can select a preferred representation and the user (or user agent) can select a preferred representation and
redirect its request to that location. redirect its request to that location.
Unless it was a HEAD request, the response SHOULD include an entity Unless it was a HEAD request, the response SHOULD include an entity
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URIs. Clients with link editing capabilities ought to automatically URIs. Clients with link editing capabilities ought to automatically
re-link references to the Request-URI to one or more of the new re-link references to the Request-URI to one or more of the new
references returned by the server, where possible. This response is references returned by the server, where possible. This response is
cacheable unless indicated otherwise. cacheable unless indicated otherwise.
The new permanent URI SHOULD be given by the Location field in the 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. Unless the request method was HEAD, the entity of the
response SHOULD contain a short hypertext note with a hyperlink to response SHOULD contain a short hypertext note with a hyperlink to
the new URI(s). the new URI(s).
If the 301 status code is received in response to a request other If the 301 status code is received in response to a request method
than GET or HEAD, the user agent MUST NOT automatically redirect the that is known to be "safe", as defined in section 9.1.1, then the
request unless it can be confirmed by the user, since this might request MAY be automatically redirected by the user agent without
change the conditions under which the request was issued. confirmation. Otherwise, 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 Note: When automatically redirecting a POST request after receiving
receiving a 301 status code, some existing HTTP/1.0 user agents a 301 status code, some existing HTTP/1.0 user agents will
will erroneously change it into a GET request. erroneously change it into a GET request.
10.3.3 302 Found 10.3.3 302 Found
The requested resource resides temporarily under a different URI. The requested resource resides temporarily under a different URI.
Since the redirection might be altered on occasion, the client SHOULD Since the redirection might be altered on occasion, the client SHOULD
continue to use the Request-URI for future requests. This response continue to use the Request-URI for future requests. This response
Fielding, et al Expires May, 2004 [Page 56]
is only cacheable if indicated by a Cache-Control or Expires header is only cacheable if indicated by a Cache-Control or Expires header
field. field.
The temporary URI SHOULD be given by the Location field in the The temporary URI SHOULD be given by the Location field in the
response. Unless the request method was HEAD, the entity of the response. Unless the request method was HEAD, the entity of the
response SHOULD contain a short hypertext note with a hyperlink to response SHOULD contain a short hypertext note with a hyperlink to
the new URI(s). the new URI(s).
If the 302 status code is received in response to a request other If the 302 status code is received in response to a request method
than GET or HEAD, the user agent MUST NOT automatically redirect the that is known to be "safe", as defined in section 9.1.1, then the
request unless it can be confirmed by the user, since this might request MAY be automatically redirected by the user agent without
change the conditions under which the request was issued. confirmation. Otherwise, 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 Note: RFC 1945 and RFC 2068 specify that the client is not allowed
to change the method on the redirected request. However, most to change the method on the redirected request. However, most
existing user agent implementations treat 302 as if it were a 303 existing user agent implementations treat 302 as if it were a 303
response, performing a GET on the Location field-value regardless response, performing a GET on the Location field-value regardless
of the original request method. The status codes 303 and 307 have of the original request method. The status codes 303 and 307 have
been added for servers that wish to make unambiguously clear which been added for servers that wish to make unambiguously clear which
kind of reaction is expected of the client. kind of reaction is expected of the client.
10.3.4 303 See Other 10.3.4 303 See Other
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10.3.5 304 Not Modified 10.3.5 304 Not Modified
If the client has performed a conditional GET request and access is If the client has performed a conditional GET request and access is
allowed, but the document has not been modified, the server SHOULD allowed, but the document has not been modified, the server SHOULD
respond with this status code. The 304 response MUST NOT contain a respond with this status code. The 304 response MUST NOT contain a
message-body, and thus is always terminated by the first empty line message-body, and thus is always terminated by the first empty line
after the header fields. after the header fields.
The response MUST include the following header fields: The response MUST include the following header fields:
- Date, unless its omission is required by section 14.18.1 Fielding, et al Expires May, 2004 [Page 57]
o Date, unless its omission is required by section 14.18.1
If a clockless origin server obeys these rules, and proxies and If a clockless origin server obeys these rules, and proxies and
clients add their own Date to any response received without one (as clients add their own Date to any response received without one (as
already specified by [RFC 2068], section 14.19), caches will operate already specified by [RFC 2068], section 14.19), caches will operate
correctly. correctly.
o ETag and/or Content-Location, if the header would have been sent in
- ETag and/or Content-Location, if the header would have been sent a 200 response to the same request
in a 200 response to the same request o Expires, Cache-Control, and/or Vary, if the field-value might
differ from that sent in any previous response for the same variant
- 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 If the conditional GET used a strong cache validator (see section
13.3.3), the response SHOULD NOT include other entity-headers. 13.3.3), the response SHOULD NOT include other entity-headers.
Otherwise (i.e., the conditional GET used a weak validator), the Otherwise (i.e., the conditional GET used a weak validator), the
response MUST NOT include other entity-headers; this prevents response MUST NOT include other entity-headers; this prevents
inconsistencies between cached entity-bodies and updated headers. inconsistencies between cached entity-bodies and updated headers.
If a 304 response indicates an entity not currently cached, then the If a 304 response indicates an entity not currently cached, then the
cache MUST disregard the response and repeat the request without the cache MUST disregard the response and repeat the request without the
conditional. conditional.
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The requested resource resides temporarily under a different URI. The requested resource resides temporarily under a different URI.
Since the redirection MAY be altered on occasion, the client SHOULD Since the redirection MAY be altered on occasion, the client SHOULD
continue to use the Request-URI for future requests. This response continue to use the Request-URI for future requests. This response
is only cacheable if indicated by a Cache-Control or Expires header is only cacheable if indicated by a Cache-Control or Expires header
field. field.
The temporary URI SHOULD be given by the Location field in the The temporary URI SHOULD be given by the Location field in the
response. Unless the request method was HEAD, the entity of the response. Unless the request method was HEAD, the entity of the
response SHOULD contain a short hypertext note with a hyperlink to response SHOULD contain a short hypertext note with a hyperlink to
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the new URI(s) , since many pre-HTTP/1.1 user agents do not the new URI(s) , since many pre-HTTP/1.1 user agents do not
understand the 307 status. Therefore, the note SHOULD contain the understand the 307 status. Therefore, the note SHOULD contain the
information necessary for a user to repeat the original request on information necessary for a user to repeat the original request on
the new URI. the new URI.
If the 307 status code is received in response to a request other If the 307 status code is received in response to a request method
than GET or HEAD, the user agent MUST NOT automatically redirect the that is known to be "safe", as defined in section 9.1.1, then the
request unless it can be confirmed by the user, since this might request MAY be automatically redirected by the user agent without
change the conditions under which the request was issued. confirmation. Otherwise, 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 10.4 Client Error 4xx
The 4xx class of status code is intended for cases in which the 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, client seems to have erred. Except when responding to a HEAD request,
the server SHOULD include an entity containing an explanation of the the server SHOULD include an entity containing an explanation of the
error situation, and whether it is a temporary or permanent error situation, and whether it is a temporary or permanent
condition. These status codes are applicable to any request method. condition. These status codes are applicable to any request method.
User agents SHOULD display any included entity to the user. User agents SHOULD display any included entity to the user.
If the client is sending data, a server implementation using TCP If the client is sending data, a server implementation using TCP
SHOULD be careful to ensure that the client acknowledges receipt of SHOULD be careful to ensure that the client acknowledges receipt of
the packet(s) containing the response, before the server closes the the packet(s) containing the response, before the server closes the
input connection. If the client continues sending data to the server 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 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 the client, which may erase the client's unacknowledged input buffers
before they can be read and interpreted by the HTTP application. before they can be read and interpreted by the HTTP application.
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applicable to the requested resource. The client MAY repeat the applicable to the requested resource. The client MAY repeat the
request with a suitable Authorization header field (section 14.8). If request with a suitable Authorization header field (section 14.8). If
the request already included Authorization credentials, then the 401 the request already included Authorization credentials, then the 401
response indicates that authorization has been refused for those response indicates that authorization has been refused for those
credentials. If the 401 response contains the same challenge as the credentials. If the 401 response contains the same challenge as the
prior response, and the user agent has already attempted prior response, and the user agent has already attempted
authentication at least once, then the user SHOULD be presented the authentication at least once, then the user SHOULD be presented the
entity that was given in the response, since that entity might entity that was given in the response, since that entity might
include relevant diagnostic information. HTTP access authentication include relevant diagnostic information. HTTP access authentication
is explained in "HTTP Authentication: Basic and Digest Access is explained in "HTTP Authentication: Basic and Digest Access
Authentication" [43]. Authentication" [N10].
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10.4.3 402 Payment Required 10.4.3 402 Payment Required
This code is reserved for future use. This code is reserved for future use.
10.4.4 403 Forbidden 10.4.4 403 Forbidden
The server understood the request, but is refusing to fulfill it. The server understood the request, but is refusing to fulfill it.
Authorization will not help and the request SHOULD NOT be repeated. Authorization will not help and the request SHOULD NOT be repeated.
If the request method was not HEAD and the server wishes to make 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 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 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 make this information available to the client, the status code 404
(Not Found) can be used instead. (Not Found) can be used instead.
10.4.5 404 Not Found 10.4.5 404 Not Found
The server has not found anything matching the Request-URI. No The server has not found anything matching the Request-URI. No
indication is given of whether the condition is temporary or indication is given of whether the condition is temporary or
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Unless it was a HEAD request, the response SHOULD include an entity Unless it was a HEAD request, the response SHOULD include an entity
containing a list of available entity characteristics and location(s) containing a list of available entity characteristics and location(s)
from which the user or user agent can choose the one most from which the user or user agent can choose the one most
appropriate. The entity format is specified by the media type given appropriate. The entity format is specified by the media type given
in the Content-Type header field. Depending upon the format and the in the Content-Type header field. Depending upon the format and the
capabilities of the user agent, selection of the most appropriate capabilities of the user agent, selection of the most appropriate
choice MAY be performed automatically. However, this specification choice MAY be performed automatically. However, this specification
does not define any standard for such automatic selection. does not define any standard for such automatic selection.
Note: HTTP/1.1 servers are allowed to return responses which are Note: HTTP/1.1 servers are allowed to return responses which are
not acceptable according to the accept headers sent in the not acceptable according to the accept headers sent in the request.
request. In some cases, this may even be preferable to sending a
406 response. User agents are encouraged to inspect the headers of In some cases, this may even be preferable to sending a 406
an incoming response to determine if it is acceptable.
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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 If the response could be unacceptable, a user agent SHOULD
temporarily stop receipt of more data and query the user for a temporarily stop receipt of more data and query the user for a
decision on further actions. decision on further actions.
10.4.8 407 Proxy Authentication Required 10.4.8 407 Proxy Authentication Required
This code is similar to 401 (Unauthorized), but indicates that the This code is similar to 401 (Unauthorized), but indicates that the
client must first authenticate itself with the proxy. The proxy MUST client must first authenticate itself with the proxy. The proxy MUST
return a Proxy-Authenticate header field (section 14.33) containing a return a Proxy-Authenticate header field (section 14.33) containing a
challenge applicable to the proxy for the requested resource. The challenge applicable to the proxy for the requested resource. The
client MAY repeat the request with a suitable Proxy-Authorization client MAY repeat the request with a suitable Proxy-Authorization
header field (section 14.34). HTTP access authentication is explained header field (section 14.34). HTTP access authentication is explained
in "HTTP Authentication: Basic and Digest Access Authentication" in "HTTP Authentication: Basic and Digest Access Authentication"
[43]. [N10].
10.4.9 408 Request Timeout 10.4.9 408 Request Timeout
The client did not produce a request within the time that the server The client did not produce a request within the time that the server
was prepared to wait. The client MAY repeat the request without was prepared to wait. The client MAY repeat the request without
modifications at any later time. modifications at any later time.
10.4.10 409 Conflict 10.4.10 409 Conflict
The request could not be completed due to a conflict with the current The request could not be completed due to a conflict with the current
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and resubmit the request. The response body SHOULD include enough and resubmit the request. The response body SHOULD include enough
information for the user to recognize the source of the conflict. information for the user to recognize the source of the conflict.
Ideally, the response entity would include enough information for the Ideally, the response entity would include enough information for the
user or user agent to fix the problem; however, that might not be user or user agent to fix the problem; however, that might not be
possible and is not required. possible and is not required.
Conflicts are most likely to occur in response to a PUT request. For Conflicts are most likely to occur in response to a PUT request. For
example, if versioning were being used and the entity being PUT example, if versioning were being used and the entity being PUT
included changes to a resource which conflict with those made by an included changes to a resource which conflict with those made by an
earlier (third-party) request, the server might use the 409 response earlier (third-party) request, the server might use the 409 response
to indicate that it can't complete the request. In this case, the to indicate that it cant complete the request. In this case, the
response entity would likely contain a list of the differences response entity would likely contain a list of the differences
between the two versions in a format defined by the response between the two versions in a format defined by the response Content-
Content-Type. Type.
10.4.11 410 Gone 10.4.11 410 Gone
The requested resource is no longer available at the server and no The requested resource is no longer available at the server and no
forwarding address is known. This condition is expected to be forwarding address is known. This condition is expected to be
considered permanent. Clients with link editing capabilities SHOULD considered permanent. Clients with link editing capabilities SHOULD
delete references to the Request-URI after user approval. If the delete references to the Request-URI after user approval. If the
server does not know, or has no facility to determine, whether or not server does not know, or has no facility to determine, whether or not
Fielding, et al Expires May, 2004 [Page 61]
the condition is permanent, the status code 404 (Not Found) SHOULD be the condition is permanent, the status code 404 (Not Found) SHOULD be
used instead. This response is cacheable unless indicated otherwise. used instead. This response is cacheable unless indicated otherwise.
The 410 response is primarily intended to assist the task of web The 410 response is primarily intended to assist the task of web
maintenance by notifying the recipient that the resource is maintenance by notifying the recipient that the resource is
intentionally unavailable and that the server owners desire that intentionally unavailable and that the server owners desire that
remote links to that resource be removed. Such an event is common for remote links to that resource be removed. Such an event is common for
limited-time, promotional services and for resources belonging to limited-time, promotional services and for resources belonging to
individuals no longer working at the server's site. It is not individuals no longer working at the servers site. It is not
necessary to mark all permanently unavailable resources as "gone" or necessary to mark all permanently unavailable resources as "gone" or
to keep the mark for any length of time -- that is left to the to keep the mark for any length of time -- that is left to the
discretion of the server owner. discretion of the server owner.
10.4.12 411 Length Required 10.4.12 411 Length Required
The server refuses to accept the request without a defined Content- The server refuses to accept the request without a defined Content-
Length. The client MAY repeat the request if it adds a valid Length. The client MAY repeat the request if it adds a valid Content-
Content-Length header field containing the length of the message-body Length header field containing the length of the message-body in the
in the request message. request message.
10.4.13 412 Precondition Failed 10.4.13 412 Precondition Failed
The precondition given in one or more of the request-header fields The precondition given in one or more of the request-header fields
evaluated to false when it was tested on the server. This response evaluated to false when it was tested on the server. This response
code allows the client to place preconditions on the current resource code allows the client to place preconditions on the current resource
metainformation (header field data) and thus prevent the requested metainformation (header field data) and thus prevent the requested
method from being applied to a resource other than the one intended. method from being applied to a resource other than the one intended.
10.4.14 413 Request Entity Too Large 10.4.14 413 Request Entity Too Large
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The server is refusing to service the request because the Request-URI The server is refusing to service the request because the Request-URI
is longer than the server is willing to interpret. This rare is longer than the server is willing to interpret. This rare
condition is only likely to occur when a client has improperly condition is only likely to occur when a client has improperly
converted a POST request to a GET request with long query converted a POST request to a GET request with long query
information, when the client has descended into a URI "black hole" of 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 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 itself), or when the server is under attack by a client attempting to
exploit security holes present in some servers using fixed-length exploit security holes present in some servers using fixed-length
buffers for reading or manipulating the Request-URI. buffers for reading or manipulating the Request-URI.
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10.4.16 415 Unsupported Media Type 10.4.16 415 Unsupported Media Type
The server is refusing to service the request because the entity of The server is refusing to service the request because the entity of
the request is in a format not supported by the requested resource the request is in a format not supported by the requested resource
for the requested method. for the requested method.
10.4.17 416 Requested Range Not Satisfiable 10.4.17 416 Requested Range Not Satisfiable
A server SHOULD return a response with this status code if a request 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 included a Range request-header field (section 14.35), and none of
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The server encountered an unexpected condition which prevented it The server encountered an unexpected condition which prevented it
from fulfilling the request. from fulfilling the request.
10.5.2 501 Not Implemented 10.5.2 501 Not Implemented
The server does not support the functionality required to fulfill the The server does not support the functionality required to fulfill the
request. This is the appropriate response when the server does not request. This is the appropriate response when the server does not
recognize the request method and is not capable of supporting it for recognize the request method and is not capable of supporting it for
any resource. any resource.
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10.5.3 502 Bad Gateway 10.5.3 502 Bad Gateway
The server, while acting as a gateway or proxy, received an invalid The server, while acting as a gateway or proxy, received an invalid
response from the upstream server it accessed in attempting to response from the upstream server it accessed in attempting to
fulfill the request. fulfill the request.
10.5.4 503 Service Unavailable 10.5.4 503 Service Unavailable
The server is currently unable to handle the request due to a The server is currently unable to handle the request due to a
temporary overloading or maintenance of the server. The implication temporary overloading or maintenance of the server. The implication
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an entity describing why that version is not supported and what other an entity describing why that version is not supported and what other
protocols are supported by that server. protocols are supported by that server.
11 Access Authentication 11 Access Authentication
HTTP provides several OPTIONAL challenge-response authentication HTTP provides several OPTIONAL challenge-response authentication
mechanisms which can be used by a server to challenge a client mechanisms which can be used by a server to challenge a client
request and by a client to provide authentication information. The request and by a client to provide authentication information. The
general framework for access authentication, and the specification of general framework for access authentication, and the specification of
"basic" and "digest" authentication, are specified in "HTTP "basic" and "digest" authentication, are specified in "HTTP
Authentication: Basic and Digest Access Authentication" [43]. This Authentication: Basic and Digest Access Authentication" [N10]. This
specification adopts the definitions of "challenge" and "credentials" specification adopts the definitions of "challenge" and "credentials"
from that specification. from that specification.
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12 Content Negotiation 12 Content Negotiation
Most HTTP responses include an entity which contains information for Most HTTP responses include an entity which contains information for
interpretation by a human user. Naturally, it is desirable to supply interpretation by a human user. Naturally, it is desirable to supply
the user with the "best available" entity corresponding to the the user with the "best available" entity corresponding to the
request. Unfortunately for servers and caches, not all users have the request. Unfortunately for servers and caches, not all users have the
same preferences for what is "best," and not all user agents are same preferences for what is "best," and not all user agents are
equally capable of rendering all entity types. For that reason, HTTP equally capable of rendering all entity types. For that reason, HTTP
has provisions for several mechanisms for "content negotiation" -- has provisions for several mechanisms for "content negotiation" --
the process of selecting the best representation for a given response the process of selecting the best representation for a given response
when there are multiple representations available. when there are multiple representations available.
Note: This is not called "format negotiation" because the Note: This is not called "format negotiation" because the alternate
alternate representations may be of the same media type, but use representations may be of the same media type, but use different
different capabilities of that type, be in different languages, capabilities of that type, be in different languages, etc.
etc.
Any response containing an entity-body MAY be subject to negotiation, Any response containing an entity-body MAY be subject to negotiation,
including error responses. including error responses.
There are two kinds of content negotiation which are possible in There are two kinds of content negotiation which are possible in
HTTP: server-driven and agent-driven negotiation. These two kinds of HTTP: server-driven and agent-driven negotiation. These two kinds of
negotiation are orthogonal and thus may be used separately or in negotiation are orthogonal and thus may be used separately or in
combination. One method of combination, referred to as transparent combination. One method of combination, referred to as transparent
negotiation, occurs when a cache uses the agent-driven negotiation negotiation, occurs when a cache uses the agent-driven negotiation
information provided by the origin server in order to provide information provided by the origin server in order to provide server-
server-driven negotiation for subsequent requests. driven negotiation for subsequent requests.
12.1 Server-driven Negotiation 12.1 Server-driven Negotiation
If the selection of the best representation for a response is made by If the selection of the best representation for a response is made by
an algorithm located at the server, it is called server-driven an algorithm located at the server, it is called server-driven
negotiation. Selection is based on the available representations of negotiation. Selection is based on the available representations of
the response (the dimensions over which it can vary; e.g. language, the response (the dimensions over which it can vary; e.g. language,
content-coding, etc.) and the contents of particular header fields in content-coding, etc.) and the contents of particular header fields in
the request message or on other information pertaining to the request the request message or on other information pertaining to the request
(such as the network address of the client). (such as the network address of the client).
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describe to the user agent, or when the server desires to send its 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 "best guess" to the client along with the first response (hoping to
avoid the round-trip delay of a subsequent request if the "best 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" is good enough for the user). In order to improve the server's
guess, the user agent MAY include request header fields (Accept, guess, the user agent MAY include request header fields (Accept,
Accept-Language, Accept-Encoding, etc.) which describe its Accept-Language, Accept-Encoding, etc.) which describe its
preferences for such a response. preferences for such a response.
Server-driven negotiation has disadvantages: Server-driven negotiation has disadvantages:
1. It is impossible for the server to accurately determine what 1. It is impossible for the server to accurately determine what might
might be "best" for any given user, since that would require be "best" for any given user, since that would require complete
complete knowledge of both the capabilities of the user agent knowledge of both the capabilities of the user agent and the
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 Fielding, et al Expires May, 2004 [Page 65]
request can be both very inefficient (given that only a small intended use for the response (e.g., does the user want to view it
percentage of responses have multiple representations) and a on screen or print it on paper?).
potential violation of the user's privacy.
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 3. It complicates the implementation of an origin server and the
algorithms for generating responses to a request. algorithms for generating responses to a request.
4. It may limit a public cache's ability to use the same response 4. It may limit a public cache∆s ability to use the same response for
for multiple user's requests. multiple user∆s requests.
HTTP/1.1 includes the following request-header fields for enabling HTTP/1.1 includes the following request-header fields for enabling
server-driven negotiation through description of user agent server-driven negotiation through description of user agent
capabilities and user preferences: Accept (section 14.1), Accept- capabilities and user preferences: Accept (section 14.1), Accept-
Charset (section 14.2), Accept-Encoding (section 14.3), Accept- Charset (section 14.2), Accept-Encoding (section 14.3), Accept-
Language (section 14.4), and User-Agent (section 14.43). However, an Language (section 14.4), and User-Agent (section 14.43). However, an
origin server is not limited to these dimensions and MAY vary the origin server is not limited to these dimensions and MAY vary the
response based on any aspect of the request, including information response based on any aspect of the request, including information
outside the request-header fields or within extension header fields outside the request-header fields or within extension header fields
not defined by this specification. not defined by this specification.
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over commonly-used dimensions (such as type, language, or encoding), over commonly-used dimensions (such as type, language, or encoding),
when the origin server is unable to determine a user agent's when the origin server is unable to determine a user agent's
capabilities from examining the request, and generally when public capabilities from examining the request, and generally when public
caches are used to distribute server load and reduce network usage. caches are used to distribute server load and reduce network usage.
Agent-driven negotiation suffers from the disadvantage of needing a Agent-driven negotiation suffers from the disadvantage of needing a
second request to obtain the best alternate representation. This second request to obtain the best alternate representation. This
second request is only efficient when caching is used. In addition, second request is only efficient when caching is used. In addition,
this specification does not define any mechanism for supporting this specification does not define any mechanism for supporting
automatic selection, though it also does not prevent any such automatic selection, though it also does not prevent any such
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mechanism from being developed as an extension and used within mechanism from being developed as an extension and used within
HTTP/1.1. HTTP/1.1.
HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable) HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)
status codes for enabling agent-driven negotiation when the server is status codes for enabling agent-driven negotiation when the server is
unwilling or unable to provide a varying response using server-driven unwilling or unable to provide a varying response using server-driven
negotiation. negotiation.
12.3 Transparent Negotiation 12.3 Transparent Negotiation
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network round-trips required for many operations; we use an network round-trips required for many operations; we use an
"expiration" mechanism for this purpose (see section 13.2). The "expiration" mechanism for this purpose (see section 13.2). The
latter reduces network bandwidth requirements; we use a "validation" latter reduces network bandwidth requirements; we use a "validation"
mechanism for this purpose (see section 13.3). mechanism for this purpose (see section 13.3).
Requirements for performance, availability, and disconnected Requirements for performance, availability, and disconnected
operation require us to be able to relax the goal of semantic operation require us to be able to relax the goal of semantic
transparency. The HTTP/1.1 protocol allows origin servers, caches, transparency. The HTTP/1.1 protocol allows origin servers, caches,
and clients to explicitly reduce transparency when necessary. and clients to explicitly reduce transparency when necessary.
However, because non-transparent operation may confuse non-expert However, because non-transparent operation may confuse non-expert
Fielding, et al Expires May, 2004 [Page 67]
users, and might be incompatible with certain server applications users, and might be incompatible with certain server applications
(such as those for ordering merchandise), the protocol requires that (such as those for ordering merchandise), the protocol requires that
transparency be relaxed transparency be relaxed
- only by an explicit protocol-level request when relaxed by o only by an explicit protocol-level request when relaxed by client
client or origin server or origin server
o only with an explicit warning to the end user when relaxed by cache
- only with an explicit warning to the end user when relaxed by or client
cache or client
Therefore, the HTTP/1.1 protocol provides these important elements: Therefore, the HTTP/1.1 protocol provides these important elements:
1. Protocol features that provide full semantic transparency when 1. Protocol features that provide full semantic transparency when this
this is required by all parties. is required by all parties.
2. Protocol features that allow an origin server or user agent to 2. Protocol features that allow an origin server or user agent to
explicitly request and control non-transparent operation. explicitly request and control non-transparent operation.
3. Protocol features that allow a cache to attach warnings to 3. Protocol features that allow a cache to attach warnings to
responses that do not preserve the requested approximation of responses that do not preserve the requested approximation of
semantic transparency. semantic transparency.
A basic principle is that it must be possible for the clients to A basic principle is that it must be possible for the clients to
detect any potential relaxation of semantic transparency. detect any potential relaxation of semantic transparency.
Note: The server, cache, or client implementor might be faced with Note: The server, cache, or client implementor might be faced with
design decisions not explicitly discussed in this specification. design decisions not explicitly discussed in this specification. If
If a decision might affect semantic transparency, the implementor a decision might affect semantic transparency, the implementor
ought to err on the side of maintaining transparency unless a ought to err on the side of maintaining transparency unless a
careful and complete analysis shows significant benefits in careful and complete analysis shows significant benefits in
breaking transparency. breaking transparency.
13.1.1 Cache Correctness 13.1.1 Cache Correctness
A correct cache MUST respond to a request with the most up-to-date 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 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 sections 13.2.5, 13.2.6, and 13.12) which meets one of the following
conditions: conditions:
1. It has been checked for equivalence with what the origin server 1. It has been checked for equivalence with what the origin server
would have returned by revalidating the response with the would have returned by revalidating the response with the origin
origin server (section 13.3); server (section 13.3);
2. It is "fresh enough" (see section 13.2). In the default case,
this means it meets the least restrictive freshness requirement
of the client, origin server, and cache (see section 14.9); if
the origin server so specifies, it is the freshness requirement
of the origin server alone.
If a stored response is not "fresh enough" by the most 2. It is "fresh enough" (see section 13.2). In the default case, this
restrictive freshness requirement of both the client and the means it meets the least restrictive freshness requirement of the
origin server, in carefully considered circumstances the cache client, origin server, and cache (see section 14.9); if the origin
MAY still return the response with the appropriate Warning server so specifies, it is the freshness requirement of the origin
header (see section 13.1.5 and 14.46), unless such a response server alone.
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), If a stored response is not "fresh enough" by the most restrictive
or error (4xx or 5xx) response message. freshness requirement of both the client and the origin server, in
carefully considered circumstances the cache MAY still return the
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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 If the cache can not communicate with the origin server, then a
correct cache SHOULD respond as above if the response can be correct cache SHOULD respond as above if the response can be
correctly served from the cache; if not it MUST return an error or correctly served from the cache; if not it MUST return an error or
warning indicating that there was a communication failure. warning indicating that there was a communication failure.
If a cache receives a response (either an entire response, or a 304 If a cache receives a response (either an entire response, or a 304
(Not Modified) response) that it would normally forward to the (Not Modified) response) that it would normally forward to the
requesting client, and the received response is no longer fresh, the requesting client, and the received response is no longer fresh, the
cache SHOULD forward it to the requesting client without adding a new cache SHOULD forward it to the requesting client without adding a new
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action. action.
Warnings MAY be used for other purposes, both cache-related and Warnings MAY be used for other purposes, both cache-related and
otherwise. The use of a warning, rather than an error status code, otherwise. The use of a warning, rather than an error status code,
distinguish these responses from true failures. distinguish these responses from true failures.
Warnings are assigned three digit warn-codes. The first digit Warnings are assigned three digit warn-codes. The first digit
indicates whether the Warning MUST or MUST NOT be deleted from a indicates whether the Warning MUST or MUST NOT be deleted from a
stored cache entry after a successful revalidation: stored cache entry after a successful revalidation:
1xx Warnings that describe the freshness or revalidation status of 1xx Warnings that describe the freshness or revalidation status of the
the response, and so MUST be deleted after a successful response, and so MUST be deleted after a successful revalidation.
revalidation. 1XX warn-codes MAY be generated by a cache only when
validating a cached entry. It MUST NOT be generated by clients. 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 2xx Warnings that describe some aspect of the entity body or entity
headers that is not rectified by a revalidation (for example, a headers that is not rectified by a revalidation (for example, a lossy
lossy compression of the entity bodies) and which MUST NOT be compression of the entity bodies) and which MUST NOT be deleted after
deleted after a successful revalidation. a successful revalidation.
See section 14.46 for the definitions of the codes themselves. See section 14.46 for the definitions of the codes themselves.
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HTTP/1.0 caches will cache all Warnings in responses, without HTTP/1.0 caches will cache all Warnings in responses, without
deleting the ones in the first category. Warnings in responses that deleting the ones in the first category. Warnings in responses that
are passed to HTTP/1.0 caches carry an extra warning-date field, are passed to HTTP/1.0 caches carry an extra warning-date field,
which prevents a future HTTP/1.1 recipient from believing an which prevents a future HTTP/1.1 recipient from believing an
erroneously cached Warning. erroneously cached Warning.
Warnings also carry a warning text. The text MAY be in any Warnings also carry a warning text. The text MAY be in any
appropriate natural language (perhaps based on the client's Accept appropriate natural language (perhaps based on the client's Accept
headers), and include an OPTIONAL indication of what character set is headers), and include an OPTIONAL indication of what character set is
used. used.
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Many user agents make it possible for users to override the basic Many user agents make it possible for users to override the basic
caching mechanisms. For example, the user agent might allow the user caching mechanisms. For example, the user agent might allow the user
to specify that cached entities (even explicitly stale ones) are to specify that cached entities (even explicitly stale ones) are
never validated. Or the user agent might habitually add "Cache- never validated. Or the user agent might habitually add "Cache-
Control: max-stale=3600" to every request. The user agent SHOULD NOT Control: max-stale=3600" to every request. The user agent SHOULD NOT
default to either non-transparent behavior, or behavior that results default to either non-transparent behavior, or behavior that results
in abnormally ineffective caching, but MAY be explicitly configured in abnormally ineffective caching, but MAY be explicitly configured
to do so by an explicit action of the user. to do so by an explicit action of the user.
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If the user has overridden the basic caching mechanisms, the user If the user has overridden the basic caching mechanisms, the user
agent SHOULD explicitly indicate to the user whenever this results in agent SHOULD explicitly indicate to the user whenever this results in
the display of information that might not meet the server's the display of information that might not meet the servers
transparency requirements (in particular, if the displayed entity is transparency requirements (in particular, if the displayed entity is
known to be stale). Since the protocol normally allows the user agent known to be stale). Since the protocol normally allows the user agent
to determine if responses are stale or not, this indication need only to determine if responses are stale or not, this indication need only
be displayed when this actually happens. The indication need not be a 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 dialog box; it could be an icon (for example, a picture of a rotting
fish) or some other indicator. fish) or some other indicator.
If the user has overridden the caching mechanisms in a way that would If the user has overridden the caching mechanisms in a way that would
abnormally reduce the effectiveness of caches, the user agent SHOULD abnormally reduce the effectiveness of caches, the user agent SHOULD
continually indicate this state to the user (for example, by a 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 display of a picture of currency in flames) so that the user does not
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accept of an unvalidated response; specifying a value of zero forces accept of an unvalidated response; specifying a value of zero forces
the cache(s) to revalidate all responses. A client MAY also specify the cache(s) to revalidate all responses. A client MAY also specify
the minimum time remaining before a response expires. Both of these the minimum time remaining before a response expires. Both of these
options increase constraints on the behavior of caches, and so cannot options increase constraints on the behavior of caches, and so cannot
further relax the cache's approximation of semantic transparency. further relax the cache's approximation of semantic transparency.
A client MAY also specify that it will accept stale responses, up to A client MAY also specify that it will accept stale responses, up to
some maximum amount of staleness. This loosens the constraints on the some maximum amount of staleness. This loosens the constraints on the
caches, and so might violate the origin server's specified caches, and so might violate the origin server's specified
constraints on semantic transparency, but might be necessary to constraints on semantic transparency, but might be necessary to
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support disconnected operation, or high availability in the face of support disconnected operation, or high availability in the face of
poor connectivity. poor connectivity.
13.2 Expiration Model 13.2 Expiration Model
13.2.1 Server-Specified Expiration 13.2.1 Server-Specified Expiration
HTTP caching works best when caches can entirely avoid making HTTP caching works best when caches can entirely avoid making
requests to the origin server. The primary mechanism for avoiding requests to the origin server. The primary mechanism for avoiding
requests is for an origin server to provide an explicit expiration requests is for an origin server to provide an explicit expiration
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See section 13.13 for an explanation of the difference between caches See section 13.13 for an explanation of the difference between caches
and history mechanisms. and history mechanisms.
13.2.2 Heuristic Expiration 13.2.2 Heuristic Expiration
Since origin servers do not always provide explicit expiration times, Since origin servers do not always provide explicit expiration times,
HTTP caches typically assign heuristic expiration times, employing HTTP caches typically assign heuristic expiration times, employing
algorithms that use other header values (such as the Last-Modified algorithms that use other header values (such as the Last-Modified
time) to estimate a plausible expiration time. The HTTP/1.1 time) to estimate a plausible expiration time. The HTTP/1.1
specification does not provide specific algorithms, but does impose specification does not provide specific algorithms, but does impose
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worst-case constraints on their results. Since heuristic expiration worst-case constraints on their results. Since heuristic expiration
times might compromise semantic transparency, they ought to used times might compromise semantic transparency, they ought to used
cautiously, and we encourage origin servers to provide explicit cautiously, and we encourage origin servers to provide explicit
expiration times as much as possible. expiration times as much as possible.
13.2.3 Age Calculations 13.2.3 Age Calculations
In order to know if a cached entry is fresh, a cache needs to know if 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 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 latter in section 13.2.4; this section describes how to calculate
the age of a response or cache entry. the age of a response or cache entry.
In this discussion, we use the term "now" to mean "the current value 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 of the clock at the host performing the calculation." Hosts that use
HTTP, but especially hosts running origin servers and caches, SHOULD HTTP, but especially hosts running origin servers and caches, SHOULD
use NTP [28] or some similar protocol to synchronize their clocks to use NTP [I21] or some similar protocol to synchronize their clocks to
a globally accurate time standard. a globally accurate time standard.
HTTP/1.1 requires origin servers to send a Date header, if possible, HTTP/1.1 requires origin servers to send a Date header, if possible,
with every response, giving the time at which the response was with every response, giving the time at which the response was
generated (see section 14.18). We use the term "date_value" to denote 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 the value of the Date header, in a form appropriate for arithmetic
operations. operations.
HTTP/1.1 uses the Age response-header to convey the estimated age of 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 the response message when obtained from a cache. The Age field value
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We use the term "age_value" to denote the value of the Age header, in We use the term "age_value" to denote the value of the Age header, in
a form appropriate for arithmetic operations. a form appropriate for arithmetic operations.
A response's age can be calculated in two entirely independent ways: A response's age can be calculated in two entirely independent ways:
1. now minus date_value, if the local clock is reasonably well 1. now minus date_value, if the local clock is reasonably well
synchronized to the origin server's clock. If the result is synchronized to the origin server's clock. If the result is
negative, the result is replaced by zero. negative, the result is replaced by zero.
2. age_value, if all of the caches along the response path 2. age_value, if all of the caches along the response path implement
implement HTTP/1.1. HTTP/1.1.
Given that we have two independent ways to compute the age of a Given that we have two independent ways to compute the age of a
response when it is received, we can combine these as response when it is received, we can combine these as
corrected_received_age = max(now - date_value, age_value) corrected_received_age = max(now - date_value, age_value)
and as long as we have either nearly synchronized clocks or all- and as long as we have either nearly synchronized clocks or all-
HTTP/1.1 paths, one gets a reliable (conservative) result. HTTP/1.1 paths, one gets a reliable (conservative) result.
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Because of network-imposed delays, some significant interval might Because of network-imposed delays, some significant interval might
pass between the time that a server generates a response and the time 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, it is received at the next outbound cache or client. If uncorrected,
this delay could result in improperly low ages. this delay could result in improperly low ages.
Because the request that resulted in the returned Age value must have Because the request that resulted in the returned Age value must have
been initiated prior to that Age value's generation, we can correct been initiated prior to that Age values generation, we can correct
for delays imposed by the network by recording the time at which the 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 request was initiated. Then, when an Age value is received, it MUST
be interpreted relative to the time the request was initiated, not be interpreted relative to the time the request was initiated, not
the time that the response was received. This algorithm results in the time that the response was received. This algorithm results in
conservative behavior no matter how much delay is experienced. So, we conservative behavior no matter how much delay is experienced. So, we
compute: compute:
corrected_initial_age = corrected_received_age corrected_initial_age = corrected_received_age
+ (now - request_time) + (now - request_time)
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* is the value of the origin server's Date: header * is the value of the origin server's Date: header
* request_time * request_time
* is the (local) time when the cache made the request * is the (local) time when the cache made the request
* that resulted in this cached response * that resulted in this cached response
* response_time * response_time
* is the (local) time when the cache received the * is the (local) time when the cache received the
* response * response
* now * now
* is the current (local) time * is the current (local) time
*/ */
apparent_age = max(0, response_time - date_value); apparent_age = max(0, response_time - date_value);
corrected_received_age = max(apparent_age, age_value); corrected_received_age = max(apparent_age, age_value);
response_delay = response_time - request_time; response_delay = response_time - request_time;
corrected_initial_age = corrected_received_age + response_delay; corrected_initial_age = corrected_received_age +
response_delay;
resident_time = now - response_time; resident_time = now - response_time;
current_age = corrected_initial_age + resident_time; current_age = corrected_initial_age + resident_time;
The current_age of a cache entry is calculated by adding the amount 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 of time (in seconds) since the cache entry was last validated by the
origin server to the corrected_initial_age. When a response is origin server to the corrected_initial_age. When a response is
generated from a cache entry, the cache MUST include a single Age 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 header field in the response with a value equal to the cache entry's
current_age. current_age.
The presence of an Age header field in a response implies that a 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 response is not first-hand. However, the converse is not true, since
Fielding, et al Expires May, 2004 [Page 74]
the lack of an Age header field in a response does not imply that the the lack of an Age header field in a response does not imply that the
response is first-hand unless all caches along the request path are 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 compliant with HTTP/1.1 (i.e., older HTTP caches did not implement
the Age header field). the Age header field).
13.2.4 Expiration Calculations 13.2.4 Expiration Calculations
In order to decide whether a response is fresh or stale, we need to 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 compare its freshness lifetime to its age. The age is calculated as
described in section 13.2.3; this section describes how to calculate described in section 13.2.3; this section describes how to calculate
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simple: simple:
response_is_fresh = (freshness_lifetime > current_age) response_is_fresh = (freshness_lifetime > current_age)
13.2.5 Disambiguating Expiration Values 13.2.5 Disambiguating Expiration Values
Because expiration values are assigned optimistically, it is possible Because expiration values are assigned optimistically, it is possible
for two caches to contain fresh values for the same resource that are for two caches to contain fresh values for the same resource that are
different. different.
Fielding, et al Expires May, 2004 [Page 75]
If a client performing a retrieval receives a non-first-hand response 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 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 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 response, then the client MAY ignore the response. If so, it MAY
retry the request with a "Cache-Control: max-age=0" directive (see retry the request with a "Cache-Control: max-age=0" directive (see
section 14.9), to force a check with the origin server. section 14.9), to force a check with the origin server.
If a cache has two fresh responses for the same representation with If a cache has two fresh responses for the same representation with
different validators, it MUST use the one with the more recent Date different validators, it MUST use the one with the more recent Date
header. This situation might arise because the cache is pooling header. This situation might arise because the cache is pooling
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If the Date values are equal, then the client MAY use either response 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). (or MAY, if it is being extremely prudent, request a new response).
Servers MUST NOT depend on clients being able to choose Servers MUST NOT depend on clients being able to choose
deterministically between responses generated during the same second, deterministically between responses generated during the same second,
if their expiration times overlap. if their expiration times overlap.
13.3 Validation Model 13.3 Validation Model
When a cache has a stale entry that it would like to use as a 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 response to a clients request, it first has to check with the origin
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server (or possibly an intermediate cache with a fresh response) to server (or possibly an intermediate cache with a fresh response) to
see if its cached entry is still usable. We call this "validating" 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 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 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 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 entry is invalid, the HTTP/1.1 protocol supports the use of
conditional methods. conditional methods.
The key protocol features for supporting conditional methods are The key protocol features for supporting conditional methods are
those concerned with "cache validators." When an origin server those concerned with "cache validators." When an origin server
generates a full response, it attaches some sort of validator to it, 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 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 proxy cache) makes a conditional request for a resource for which it
has a cache entry, it includes the associated validator in the has a cache entry, it includes the associated validator in the
request. request.
The server then checks that validator against the current validator The server then checks that validator against the current validator
for the entity, and, if they match (see section 13.3.3), it responds 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 with a special status code (usually, 304 (Not Modified)) and no
entity-body. Otherwise, it returns a full response (including entity-body. Otherwise, it returns a full response (including entity-
entity-body). Thus, we avoid transmitting the full response if the body). Thus, we avoid transmitting the full response if the validator
validator matches, and we avoid an extra round trip if it does not matches, and we avoid an extra round trip if it does not match.
match.
In HTTP/1.1, a conditional request looks exactly the same as a normal 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 request for the same resource, except that it carries a special
header (which includes the validator) that implicitly turns the header (which includes the validator) that implicitly turns the
method (usually, GET) into a conditional. method (usually, GET) into a conditional.
The protocol includes both positive and negative senses of cache- The protocol includes both positive and negative senses of cache-
validating conditions. That is, it is possible to request either that 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 a method be performed if and only if a validator matches or if and
only if no validators match. only if no validators match.
Note: a response that lacks a validator may still be cached, and Note: a response that lacks a validator may still be cached, and
served from cache until it expires, unless this is explicitly served from cache until it expires, unless this is explicitly
prohibited by a cache-control directive. However, a cache cannot prohibited by a cache-control directive. However, a cache cannot do
do a conditional retrieval if it does not have a validator for the a conditional retrieval if it does not have a validator for the
entity, which means it will not be refreshable after it expires. entity, which means it will not be refreshable after it expires.
13.3.1 Last-Modified Dates 13.3.1 Last-Modified Dates
The Last-Modified entity-header field value is often used as a cache 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 validator. In simple terms, a cache entry is considered to be valid
if the entity has not been modified since the Last-Modified value. if the entity has not been modified since the Last-Modified value.
13.3.2 Entity Tag Cache Validators 13.3.2 Entity Tag Cache Validators
The ETag response-header field value, an entity tag, provides for an The ETag response-header field value, an entity tag, provides for an
"opaque" cache validator. This might allow more reliable validation "opaque" cache validator. This might allow more reliable validation
in situations where it is inconvenient to store modification dates, in situations where it is inconvenient to store modification dates,
where the one-second resolution of HTTP date values is not where the one-second resolution of HTTP date values is not
sufficient, or where the origin server wishes to avoid certain sufficient, or where the origin server wishes to avoid certain
paradoxes that might arise from the use of modification dates. paradoxes that might arise from the use of modification dates.
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Entity Tags are described in section 3.11. The headers used with 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. entity tags are described in sections 14.19, 14.24, 14.26 and 14.44.
13.3.3 Weak and Strong Validators 13.3.3 Weak and Strong Validators
Since both origin servers and caches will compare two validators to Since both origin servers and caches will compare two validators to
decide if they represent the same or different entities, one normally decide if they represent the same or different entities, one normally
would expect that if the entity (the entity-body or any entity- would expect that if the entity (the entity-body or any entity-
headers) changes in any way, then the associated validator would headers) changes in any way, then the associated validator would
change as well. If this is true, then we call this validator a change as well. If this is true, then we call this validator a
"strong validator." "strong validator."
However, there might be cases when a server prefers to change the However, there might be cases when a server prefers to change the
validator only on semantically significant changes, and not when validator only on semantically significant changes, and not when
insignificant aspects of the entity change. A validator that does not insignificant aspects of the entity change. A validator that does not
always change when the resource changes is a "weak validator." always change when the resource changes is a "weak validator."
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usable in contexts that do not depend on exact equality of an entity. 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 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 entity. However, only a strong validator is usable for a sub-range
retrieval, since otherwise the client might end up with an internally retrieval, since otherwise the client might end up with an internally
inconsistent entity. inconsistent entity.
Clients MAY issue simple (non-subrange) GET requests with either weak Clients MAY issue simple (non-subrange) GET requests with either weak
validators or strong validators. Clients MUST NOT use weak validators validators or strong validators. Clients MUST NOT use weak validators
in other forms of request. in other forms of request.
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The only function that the HTTP/1.1 protocol defines on validators is The only function that the HTTP/1.1 protocol defines on validators is
comparison. There are two validator comparison functions, depending comparison. There are two validator comparison functions, depending
on whether the comparison context allows the use of weak validators on whether the comparison context allows the use of weak validators
or not: or not:
- The strong comparison function: in order to be considered equal, o The strong comparison function: in order to be considered equal,
both validators MUST be identical in every way, and both MUST both validators MUST be identical in every way, and both MUST NOT
NOT be weak. be weak.
o The weak comparison function: in order to be considered equal, both
- The weak comparison function: in order to be considered equal, validators MUST be identical in every way, but either or both of
both validators MUST be identical in every way, but either or them MAY be tagged as "weak" without affecting the result.
both of them MAY be tagged as "weak" without affecting the
result.
An entity tag is strong unless it is explicitly tagged as weak. An entity tag is strong unless it is explicitly tagged as weak.
Section 3.11 gives the syntax for entity tags. Section 3.11 gives the syntax for entity tags.
A Last-Modified time, when used as a validator in a request, is 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, implicitly weak unless it is possible to deduce that it is strong,
using the following rules: using the following rules:
- The validator is being compared by an origin server to the o The validator is being compared by an origin server to the actual
actual current validator for the entity and, current validator for the entity and,
- That origin server reliably knows that the associated entity did o That origin server reliably knows that the associated entity did
not change twice during the second covered by the presented not change twice during the second covered by the presented
validator. validator.
or or
o The validator is about to be used by a client in an If-Modified-
- The validator is about to be used by a client in an If- Since or If-Unmodified-Since header, because the client has a cache
Modified-Since or If-Unmodified-Since header, because the client entry for the associated entity, and
has a cache entry for the associated entity, and o That cache entry includes a Date value, which gives the time when
the origin server sent the original response, and
- That cache entry includes a Date value, which gives the time o The presented Last-Modified time is at least 60 seconds before the
when the origin server sent the original response, and Date value.
- The presented Last-Modified time is at least 60 seconds before
the Date value.
or or
o The validator is being compared by an intermediate cache to the
- The validator is being compared by an intermediate cache to the
validator stored in its cache entry for the entity, and validator stored in its cache entry for the entity, and
o That cache entry includes a Date value, which gives the time when
- That cache entry includes a Date value, which gives the time the origin server sent the original response, and
when the origin server sent the original response, and o The presented Last-Modified time is at least 60 seconds before the
Date value.
- 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 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 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 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- have a Date value equal to its Last-Modified time. The arbitrary 60-
second limit guards against the possibility that the Date and Last- second limit guards against the possibility that the Date and Last-
Modified values are generated from different clocks, or at somewhat Modified values are generated from different clocks, or at somewhat
different times during the preparation of the response. An different times during the preparation of the response. An
implementation MAY use a value larger than 60 seconds, if it is implementation MAY use a value larger than 60 seconds, if it is
believed that 60 seconds is too short. believed that 60 seconds is too short.
Fielding, et al Expires May, 2004 [Page 79]
If a client wishes to perform a sub-range retrieval on a value for 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 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 MAY do this only if the Last-Modified time is strong in the sense
described here. described here.
A cache or origin server receiving a conditional request, other than A cache or origin server receiving a conditional request, other than
a full-body GET request, MUST use the strong comparison function to a full-body GET request, MUST use the strong comparison function to
evaluate the condition. evaluate the condition.
These rules allow HTTP/1.1 caches and clients to safely perform sub- These rules allow HTTP/1.1 caches and clients to safely perform sub-
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servers. servers.
13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates 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, We adopt a set of rules and recommendations for origin servers,
clients, and caches regarding when various validator types ought to clients, and caches regarding when various validator types ought to
be used, and for what purposes. be used, and for what purposes.
HTTP/1.1 origin servers: HTTP/1.1 origin servers:
- SHOULD send an entity tag validator unless it is not feasible to o SHOULD send an entity tag validator unless it is not feasible to
generate one. generate one.
o MAY send a weak entity tag instead of a strong entity tag, if
- MAY send a weak entity tag instead of a strong entity tag, if performance considerations support the use of weak entity tags, or
performance considerations support the use of weak entity tags, if it is unfeasible to send a strong entity tag.
or if it is unfeasible to send a strong entity tag. o SHOULD send a Last-Modified value if it is feasible to send one,
unless the risk of a breakdown in semantic transparency that could
- SHOULD send a Last-Modified value if it is feasible to send one, result from using this date in an If-Modified-Since header would
unless the risk of a breakdown in semantic transparency that lead to serious problems.
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 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. 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 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 associated entity value changes in any way. A weak entity tag SHOULD
change whenever the associated entity changes in a semantically change whenever the associated entity changes in a semantically
significant way. significant way.
Note: in order to provide semantically transparent caching, an Note: in order to provide semantically transparent caching, an
origin server must avoid reusing a specific strong entity tag origin server must avoid reusing a specific strong entity tag value
value for two different entities, or reusing a specific weak
entity tag value for two semantically different entities. Cache for two different entities, or reusing a specific weak entity tag
entries might persist for arbitrarily long periods, regardless of value for two semantically different entities. Cache entries might
expiration times, so it might be inappropriate to expect that a persist for arbitrarily long periods, regardless of expiration
cache will never again attempt to validate an entry using a times, so it might be inappropriate to expect that a cache will
validator that it obtained at some point in the past. never again attempt to validate an entry using a validator that it
obtained at some point in the past.
HTTP/1.1 clients: HTTP/1.1 clients:
- If an entity tag has been provided by the origin server, MUST o If an entity tag has been provided by the origin server, MUST use
use that entity tag in any cache-conditional request (using If- that entity tag in any cache-conditional request (using If-Match or
Match or If-None-Match). If-None-Match).
- If only a Last-Modified value has been provided by the origin Fielding, et al Expires May, 2004 [Page 80]
o If only a Last-Modified value has been provided by the origin
server, SHOULD use that value in non-subrange cache-conditional server, SHOULD use that value in non-subrange cache-conditional
requests (using If-Modified-Since). requests (using If-Modified-Since).
o If only a Last-Modified value has been provided by an HTTP/1.0
- If only a Last-Modified value has been provided by an HTTP/1.0
origin server, MAY use that value in subrange cache-conditional origin server, MAY use that value in subrange cache-conditional
requests (using If-Unmodified-Since:). The user agent SHOULD requests (using If-Unmodified-Since:). The user agent SHOULD
provide a way to disable this, in case of difficulty. provide a way to disable this, in case of difficulty.
o If both an entity tag and a Last-Modified value have been provided
- If both an entity tag and a Last-Modified value have been by the origin server, SHOULD use both validators in cache-
provided by the origin server, SHOULD use both validators in conditional requests. This allows both HTTP/1.0 and HTTP/1.1 caches
cache-conditional requests. This allows both HTTP/1.0 and to respond appropriately.
HTTP/1.1 caches to respond appropriately.
An HTTP/1.1 origin server, upon receiving a conditional request that 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 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., 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 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) validators, MUST NOT return a response status of 304 (Not Modified)
unless doing so is consistent with all of the conditional header unless doing so is consistent with all of the conditional header
fields in the request. fields in the request.
An HTTP/1.1 caching proxy, upon receiving a conditional request that An HTTP/1.1 caching proxy, upon receiving a conditional request that
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client unless that cached response is consistent with all of the client unless that cached response is consistent with all of the
conditional header fields in the request. conditional header fields in the request.
Note: The general principle behind these rules is that HTTP/1.1 Note: The general principle behind these rules is that HTTP/1.1
servers and clients should transmit as much non-redundant servers and clients should transmit as much non-redundant
information as is available in their responses and requests. information as is available in their responses and requests.
HTTP/1.1 systems receiving this information will make the most HTTP/1.1 systems receiving this information will make the most
conservative assumptions about the validators they receive. conservative assumptions about the validators they receive.
HTTP/1.0 clients and caches will ignore entity tags. Generally, HTTP/1.0 clients and caches will ignore entity tags. Generally,
last-modified values received or used by these systems will last-modified values received or used by these systems will support
support transparent and efficient caching, and so HTTP/1.1 origin transparent and efficient caching, and so HTTP/1.1 origin servers
servers should provide Last-Modified values. In those rare cases should provide Last-Modified values. In those rare cases where the
where the use of a Last-Modified value as a validator by an use of a Last-Modified value as a validator by an HTTP/1.0 system
HTTP/1.0 system could result in a serious problem, then HTTP/1.1 could result in a serious problem, then HTTP/1.1 origin servers
origin servers should not provide one. should not provide one.
13.3.5 Non-validating Conditionals 13.3.5 Non-validating Conditionals
The principle behind entity tags is that only the service author The principle behind entity tags is that only the service author
knows the semantics of a resource well enough to select an knows the semantics of a resource well enough to select an
appropriate cache validation mechanism, and the specification of any appropriate cache validation mechanism, and the specification of any
validator comparison function more complex than byte-equality would validator comparison function more complex than byte-equality would
open up a can of worms. Thus, comparisons of any other headers open up a can of worms. Thus, comparisons of any other headers
(except Last-Modified, for compatibility with HTTP/1.0) are never (except Last-Modified, for compatibility with HTTP/1.0) are never
used for purposes of validating a cache entry. used for purposes of validating a cache entry.
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13.4 Response Cacheability 13.4 Response Cacheability
Unless specifically constrained by a cache-control (section 14.9) Unless specifically constrained by a cache-control (section 14.9)
directive, a caching system MAY always store a successful response directive, a caching system MAY always store a successful response
(see section 13.8) as a cache entry, MAY return it without validation (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 if it is fresh, and MAY return it after successful validation. If
there is neither a cache validator nor an explicit expiration time there is neither a cache validator nor an explicit expiration time
associated with a response, we do not expect it to be cached, but associated with a response, we do not expect it to be cached, but
certain caches MAY violate this expectation (for example, when little certain caches MAY violate this expectation (for example, when little
or no network connectivity is available). A client can usually detect or no network connectivity is available). A client can usually detect
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13.5 Constructing Responses From Caches 13.5 Constructing Responses From Caches
The purpose of an HTTP cache is to store information received in The purpose of an HTTP cache is to store information received in
response to requests for use in responding to future requests. In response to requests for use in responding to future requests. In
many cases, a cache simply returns the appropriate parts of a many cases, a cache simply returns the appropriate parts of a
response to the requester. However, if the cache holds a cache entry 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 based on a previous response, it might have to combine parts of a new
response with what is held in the cache entry. response with what is held in the cache entry.
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13.5.1 End-to-end and Hop-by-hop Headers 13.5.1 End-to-end and Hop-by-hop Headers
For the purpose of defining the behavior of caches and non-caching For the purpose of defining the behavior of caches and non-caching
proxies, we divide HTTP headers into two categories: proxies, we divide HTTP headers into two categories:
- End-to-end headers, which are transmitted to the ultimate o End-to-end headers, which are transmitted to the ultimate
recipient of a request or response. End-to-end headers in recipient of a request or response. End-to-end headers in responses
responses MUST be stored as part of a cache entry and MUST be MUST be stored as part of a cache entry and MUST be transmitted in
transmitted in any response formed from a cache entry. any response formed from a cache entry.
o Hop-by-hop headers, which are meaningful only for a single
- Hop-by-hop headers, which are meaningful only for a single
transport-level connection, and are not stored by caches or transport-level connection, and are not stored by caches or
forwarded by proxies. forwarded by proxies.
The following HTTP/1.1 headers are hop-by-hop headers: The following HTTP/1.1 headers are hop-by-hop headers:
- Connection o Connection
- Keep-Alive o Keep-Alive
- Proxy-Authenticate o Proxy-Authenticate
- Proxy-Authorization o Proxy-Authorization
- TE o TE
- Trailers o Trailer
- Transfer-Encoding o Transfer-Encoding
- Upgrade o Upgrade
All other headers defined by HTTP/1.1 are end-to-end headers. 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, Other hop-by-hop headers MUST be listed in a Connection header,
(section 14.10) to be introduced into HTTP/1.1 (or later). (section 14.10) to be introduced into HTTP/1.1 (or later).
13.5.2 Non-modifiable Headers 13.5.2 Non-modifiable Headers
Some features of the HTTP/1.1 protocol, such as Digest Some features of the HTTP/1.1 protocol, such as Digest
Authentication, depend on the value of certain end-to-end headers. A Authentication, depend on the value of certain end-to-end headers. A
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Other hop-by-hop headers MUST be listed in a Connection header, Other hop-by-hop headers MUST be listed in a Connection header,
(section 14.10) to be introduced into HTTP/1.1 (or later). (section 14.10) to be introduced into HTTP/1.1 (or later).
13.5.2 Non-modifiable Headers 13.5.2 Non-modifiable Headers
Some features of the HTTP/1.1 protocol, such as Digest Some features of the HTTP/1.1 protocol, such as Digest
Authentication, depend on the value of certain end-to-end headers. A Authentication, depend on the value of certain end-to-end headers. A
transparent proxy SHOULD NOT modify an end-to-end header unless the transparent proxy SHOULD NOT modify an end-to-end header unless the
definition of that header requires or specifically allows that. definition of that header requires or specifically allows that.
A transparent proxy MUST NOT modify any of the following fields in a 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 request or response, and it MUST NOT add any of these fields if not
already present: already present:
- Content-Location o Content-Location
o Content-MD5
- Content-MD5 o ETag
o Last-Modified
- ETag
- Last-Modified
A transparent proxy MUST NOT modify any of the following fields in a A transparent proxy MUST NOT modify any of the following fields in a
response: response:
- Expires Fielding, et al Expires May, 2004 [Page 83]
o Expires
but it MAY add any of these fields if not already present. If an but it MAY add any of these fields if not already present. If an Expires
Expires header is added, it MUST be given a field-value identical to header is added, it MUST be given a field-value identical to that of the
that of the Date header in that response. Date header in that response.
A proxy MUST NOT modify or add any of the following fields in a 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 message that contains the no-transform cache-control directive, or in
any request: any request:
- Content-Encoding o Content-Encoding
o Content-Range
- Content-Range o Content-Type
- Content-Type
A non-transparent proxy MAY modify or add these fields to a message 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 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 Warning 214 (Transformation applied) if one does not already appear
in the message (see section 14.46). in the message (see section 14.46).
Warning: unnecessary modification of end-to-end headers might Warning: unnecessary modification of end-to-end headers might cause
cause authentication failures if stronger authentication authentication failures if stronger authentication mechanisms are
mechanisms are introduced in later versions of HTTP. Such introduced in later versions of HTTP. Such authentication
authentication mechanisms MAY rely on the values of header fields mechanisms MAY rely on the values of header fields not listed here.
not listed here.
The Content-Length field of a request or response is added or deleted 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 according to the rules in section 4.4. A transparent proxy MUST
preserve the entity-length (section 7.2.2) of the entity-body, preserve the entity-length (section 7.2.2) of the entity-body,
although it MAY change the transfer-length (section 4.4). although it MAY change the transfer-length (section 4.4).
13.5.3 Combining Headers 13.5.3 Combining Headers
When a cache makes a validating request to a server, and the server When a cache makes a validating request to a server, and the server
provides a 304 (Not Modified) response or a 206 (Partial Content) provides a 304 (Not Modified) response or a 206 (Partial Content)
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If the status code is 304 (Not Modified), the cache uses the entity- 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 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 response. If the status code is 206 (Partial Content) and the ETag or
Last-Modified headers match exactly, the cache MAY combine the Last-Modified headers match exactly, the cache MAY combine the
contents stored in the cache entry with the new contents received in 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 the response and use the result as the entity-body of this outgoing
response, (see 13.5.4). response, (see 13.5.4).
The end-to-end headers stored in the cache entry are used for the The end-to-end headers stored in the cache entry are used for the
constructed response, except that constructed response, except that
o any stored Warning headers with warn-code 1xx (see section 14.46)
MUST be deleted from the cache entry and the forwarded response.
o any stored Warning headers with warn-code 2xx MUST be retained in
the cache entry and the forwarded response.
- any stored Warning headers with warn-code 1xx (see section Fielding, et al Expires May, 2004 [Page 84]
14.46) MUST be deleted from the cache entry and the forwarded o any end-to-end headers provided in the 304 or 206 response MUST
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. replace the corresponding headers from the cache entry.
Unless the cache decides to remove the cache entry, it MUST also Unless the cache decides to remove the cache entry, it MUST also
replace the end-to-end headers stored with the cache entry with replace the end-to-end headers stored with the cache entry with
corresponding headers received in the incoming response, except for corresponding headers received in the incoming response, except for
Warning headers as described immediately above. If a header field- Warning headers as described immediately above. If a header field-
name in the incoming response matches more than one header in the name in the incoming response matches more than one header in the
cache entry, all such old headers MUST be replaced. cache entry, all such old headers MUST be replaced.
In other words, the set of end-to-end headers received in the In other words, the set of end-to-end headers received in the
incoming response overrides all corresponding end-to-end headers incoming response overrides all corresponding end-to-end headers
stored with the cache entry (except for stored Warning headers with stored with the cache entry (except for stored Warning headers with
warn-code 1xx, which are deleted even if not overridden). warn-code 1xx, which are deleted even if not overridden).
Note: this rule allows an origin server to use a 304 (Not Note: this rule allows an origin server to use a 304 (Not Modified)
Modified) or a 206 (Partial Content) response to update any header or a 206 (Partial Content) response to update any header associated
associated with a previous response for the same entity or sub- with a previous response for the same entity or sub-ranges thereof,
ranges thereof, although it might not always be meaningful or although it might not always be meaningful or correct to do so.
correct to do so. This rule does not allow an origin server to use This rule does not allow an origin server to use a 304 (Not
a 304 (Not Modified) or a 206 (Partial Content) response to Modified) or a 206 (Partial Content) response to entirely delete a
entirely delete a header that it had provided with a previous header that it had provided with a previous response.
response.
13.5.4 Combining Byte Ranges 13.5.4 Combining Byte Ranges
A response might transfer only a subrange of the bytes of an entity- A response might transfer only a subrange of the bytes of an entity-
body, either because the request included one or more Range body, either because the request included one or more Range
specifications, or because a connection was broken prematurely. After specifications, or because a connection was broken prematurely. After
several such transfers, a cache might have received several ranges of several such transfers, a cache might have received several ranges of
the same entity-body. the same entity-body.
If a cache has a stored non-empty set of subranges for an entity, and If a cache has a stored non-empty set of subranges for an entity, and
an incoming response transfers another subrange, the cache MAY an incoming response transfers another subrange, the cache MAY
combine the new subrange with the existing set if both the following combine the new subrange with the existing set if both the following
conditions are met: conditions are met:
- Both the incoming response and the cache entry have a cache o Both the incoming response and the cache entry have a cache
validator. validator.
o The two cache validators match using the strong comparison function
- The two cache validators match using the strong comparison (see section 13.3.3).
function (see section 13.3.3).
If either requirement is not met, the cache MUST use only the most If either requirement is not met, the cache MUST use only the most
recent partial response (based on the Date values transmitted with recent partial response (based on the Date values transmitted with
every response, and using the incoming response if these values are every response, and using the incoming response if these values are
equal or missing), and MUST discard the other partial information. equal or missing), and MUST discard the other partial information.
13.6 Caching Negotiated Responses 13.6 Caching Negotiated Responses
Use of server-driven content negotiation (section 12.1), as indicated Use of server-driven content negotiation (section 12.1), as indicated
by the presence of a Vary header field in a response, alters the 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 conditions and procedure by which a cache can use the response for
Fielding, et al Expires May, 2004 [Page 85]
subsequent requests. See section 14.44 for use of the Vary header subsequent requests. See section 14.44 for use of the Vary header
field by servers. field by servers.
A server SHOULD use the Vary header field to inform a cache of what A server SHOULD use the Vary header field to inform a cache of what
request-header fields were used to select among multiple request-header fields were used to select among multiple
representations of a cacheable response subject to server-driven representations of a cacheable response subject to server-driven
negotiation. The set of header fields named by the Vary field value negotiation. The set of header fields named by the Vary field value
is known as the "selecting" request-headers. is known as the "selecting" request-headers.
When the cache receives a subsequent request whose Request-URI When the cache receives a subsequent request whose Request-URI
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entry, the new response SHOULD be used to update the header fields of 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. the existing entry, and the result MUST be returned to the client.
If any of the existing cache entries contains only partial content If any of the existing cache entries contains only partial content
for the associated entity, its entity-tag SHOULD NOT be included in 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 the If-None-Match header field unless the request is for a range that
would be fully satisfied by that entry. would be fully satisfied by that entry.
If a cache receives a successful response whose Content-Location If a cache receives a successful response whose Content-Location
field matches that of an existing cache entry for the same Request- field matches that of an existing cache entry for the same Request-
]URI, whose entity-tag differs from that of the existing entry, and URI, whose entity-tag differs from that of the existing entry, and
whose Date is more recent than that of the existing entry, the whose Date is more recent than that of the existing entry, the
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existing entry SHOULD NOT be returned in response to future requests existing entry SHOULD NOT be returned in response to future requests
and SHOULD be deleted from the cache. and SHOULD be deleted from the cache.
13.7 Shared and Non-Shared Caches 13.7 Shared and Non-Shared Caches
For reasons of security and privacy, it is necessary to make a For reasons of security and privacy, it is necessary to make a
distinction between "shared" and "non-shared" caches. A non-shared distinction between "shared" and "non-shared" caches. A non-shared
cache is one that is accessible only to a single user. Accessibility cache is one that is accessible only to a single user. Accessibility
in this case SHOULD be enforced by appropriate security mechanisms. in this case SHOULD be enforced by appropriate security mechanisms.
All other caches are considered to be "shared." Other sections of All other caches are considered to be "shared." Other sections of
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14.9). 14.9).
13.9 Side Effects of GET and HEAD 13.9 Side Effects of GET and HEAD
Unless the origin server explicitly prohibits the caching of their Unless the origin server explicitly prohibits the caching of their
responses, the application of GET and HEAD methods to any resources responses, the application of GET and HEAD methods to any resources
SHOULD NOT have side effects that would lead to erroneous behavior if SHOULD NOT have side effects that would lead to erroneous behavior if
these responses are taken from a cache. They MAY still have side 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 effects, but a cache is not required to consider such side effects in
its caching decisions. Caches are always expected to observe an its caching decisions. Caches are always expected to observe an
origin server's explicit restrictions on caching. origin servers explicit restrictions on caching.
We note one exception to this rule: since some applications have We note one exception to this rule: since some applications have
traditionally used GETs and HEADs with query URLs (those containing a traditionally used GETs and HEADs with query URLs (those containing a
"?" in the rel_path part) to perform operations with significant side "?" in the rel_path part) to perform operations with significant side
effects, caches MUST NOT treat responses to such URIs as fresh unless effects, caches MUST NOT treat responses to such URIs as fresh unless
the server provides an explicit expiration time. This specifically the server provides an explicit expiration time. This specifically
means that responses from HTTP/1.0 servers for such URIs SHOULD NOT 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. be taken from a cache. See section 9.1.1 for related information.
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13.10 Invalidation After Updates or Deletions 13.10 Invalidation After Updates or Deletions
The effect of certain methods performed on a resource at the origin The effect of certain methods performed on a resource at the origin
server might cause one or more existing cache entries to become non- server might cause one or more existing cache entries to become non-
transparently invalid. That is, although they might continue to be transparently invalid. That is, although they might continue to be
"fresh," they do not accurately reflect what the origin server would "fresh," they do not accurately reflect what the origin server would
return for a new request on that resource. return for a new request on that resource.
There is no way for the HTTP protocol to guarantee that all such There is no way for the HTTP protocol to guarantee that all such
cache entries are marked invalid. For example, the request that cache entries are marked invalid. For example, the request that
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In this section, the phrase "invalidate an entity" means that the In this section, the phrase "invalidate an entity" means that the
cache will either remove all instances of that entity from its cache will either remove all instances of that entity from its
storage, or will mark these as "invalid" and in need of a mandatory storage, or will mark these as "invalid" and in need of a mandatory
revalidation before they can be returned in response to a subsequent revalidation before they can be returned in response to a subsequent
request. request.
Some HTTP methods MUST cause a cache to invalidate an entity. This is 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 either the entity referred to by the Request-URI, or by the Location
or Content-Location headers (if present). These methods are: or Content-Location headers (if present). These methods are:
- PUT o PUT
o DELETE
- DELETE o POST
- POST
In order to prevent denial of service attacks, an invalidation based An invalidation based on the URI in a Location or Content-Location
on the URI in a Location or Content-Location header MUST only be header MUST NOT be performed if the host part of that URI differs
performed if the host part is the same as in the Request-URI. from the host part in the Request-URI. This helps prevent denial of
service attacks.
A cache that passes through requests for methods it does not A cache that passes through requests for methods it does not
understand SHOULD invalidate any entities referred to by the understand SHOULD invalidate any entities referred to by the Request-
Request-URI. URI.
13.11 Write-Through Mandatory 13.11 Write-Through Mandatory
All methods that might be expected to cause modifications to the All methods that might be expected to cause modifications to the
origin server's resources MUST be written through to the origin origin server's resources MUST be written through to the origin
server. This currently includes all methods except for GET and HEAD. 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 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 transmitted the request to the inbound server, and having received a
corresponding response from the inbound server. This does not prevent corresponding response from the inbound server. This does not prevent
a proxy cache from sending a 100 (Continue) response before the a proxy cache from sending a 100 (Continue) response before the
inbound server has sent its final reply. inbound server has sent its final reply.
The alternative (known as "write-back" or "copy-back" caching) is not The alternative (known as "write-back" or "copy-back" caching) is not
allowed in HTTP/1.1, due to the difficulty of providing consistent allowed in HTTP/1.1, due to the difficulty of providing consistent
updates and the problems arising from server, cache, or network updates and the problems arising from server, cache, or network
failure prior to write-back. failure prior to write-back.
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transmitted the request to the inbound server, and having received a transmitted the request to the inbound server, and having received a
corresponding response from the inbound server. This does not prevent corresponding response from the inbound server. This does not prevent
a proxy cache from sending a 100 (Continue) response before the a proxy cache from sending a 100 (Continue) response before the
inbound server has sent its final reply. inbound server has sent its final reply.
The alternative (known as "write-back" or "copy-back" caching) is not The alternative (known as "write-back" or "copy-back" caching) is not
allowed in HTTP/1.1, due to the difficulty of providing consistent allowed in HTTP/1.1, due to the difficulty of providing consistent
updates and the problems arising from server, cache, or network updates and the problems arising from server, cache, or network
failure prior to write-back. failure prior to write-back.
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13.12 Cache Replacement 13.12 Cache Replacement
If a new cacheable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8) 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 response is received from a resource while any existing responses for
the same resource are cached, the cache SHOULD use the new response 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 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 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 future requests that would previously have caused the old response to
be returned. If it inserts the new response into cache storage the be returned. If it inserts the new response into cache storage the
rules in section 13.5.3 apply. rules in section 13.5.3 apply.
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history lists, which can be used to redisplay an entity retrieved history lists, which can be used to redisplay an entity retrieved
earlier in a session. earlier in a session.
History mechanisms and caches are different. In particular history History mechanisms and caches are different. In particular history
mechanisms SHOULD NOT try to show a semantically transparent view of mechanisms SHOULD NOT try to show a semantically transparent view of
the current state of a resource. Rather, a history mechanism is meant 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 to show exactly what the user saw at the time when the resource was
retrieved. retrieved.
By default, an expiration time does not apply to history mechanisms. By default, an expiration time does not apply to history mechanisms.
If the entity is still in storage, a history mechanism SHOULD display If the entity is still in storage, a history mechanism SHOULD display
it even if the entity has expired, unless the user has specifically it even if the entity has expired, unless the user has specifically
configured the agent to refresh expired history documents. configured the agent to refresh expired history documents.
This is not to be construed to prohibit the history mechanism from This is not to be construed to prohibit the history mechanism from
telling the user that a view might be stale. telling the user that a view might be stale.
Note: if history list mechanisms unnecessarily prevent users from Note: if history list mechanisms unnecessarily prevent users from
viewing stale resources, this will tend to force service authors viewing stale resources, this will tend to force service authors to
to avoid using HTTP expiration controls and cache controls when avoid using HTTP expiration controls and cache controls when they
they would otherwise like to. Service authors may consider it would otherwise like to. Service authors may consider it important
important that users not be presented with error messages or that users not be presented with error messages or warning messages
warning messages when they use navigation controls (such as BACK) when they use navigation controls (such as BACK) to view previously
to view previously fetched resources. Even though sometimes such fetched resources. Even though sometimes such resources ought not
resources ought not to cached, or ought to expire quickly, user to cached, or ought to expire quickly, user interface
interface considerations may force service authors to resort to considerations may force service authors to resort to other means
other means of preventing caching (e.g. "once-only" URLs) in order of preventing caching (e.g. "once-only" URLs) in order not to
not to suffer the effects of improperly functioning history suffer the effects of improperly functioning history mechanisms.
mechanisms.
14 Header Field Definitions 14 Header Field Definitions
This section defines the syntax and semantics of all standard This section defines the syntax and semantics of all standard
HTTP/1.1 header fields. For entity-header fields, both sender and HTTP/1.1 header fields. For entity-header fields, both sender and
recipient refer to either the client or the server, depending on who recipient refer to either the client or the server, depending on who
sends and who receives the entity. sends and who receives the entity.
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14.1 Accept 14.1 Accept
The Accept request-header field can be used to specify certain media The Accept request-header field can be used to specify certain media
types which are acceptable for the response. Accept headers can be types which are acceptable for the response. Accept headers can be
used to indicate that the request is specifically limited to a small 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 set of desired types, as in the case of a request for an in-line
image. image.
Accept = "Accept" ":" Accept = "Accept" ":"
#( media-range [ accept-params ] ) #( media-range [ accept-params ] )
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Each media-range MAY be followed by one or more accept-params, Each media-range MAY be followed by one or more accept-params,
beginning with the "q" parameter for indicating a relative quality beginning with the "q" parameter for indicating a relative quality
factor. The first "q" parameter (if any) separates the media-range factor. The first "q" parameter (if any) separates the media-range
parameter(s) from the accept-params. Quality factors allow the user parameter(s) from the accept-params. Quality factors allow the user
or user agent to indicate the relative degree of preference for that 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 media-range, using the qvalue scale from 0 to 1 (section 3.9). The
default value is q=1. default value is q=1.
Note: Use of the "q" parameter name to separate media type Note: Use of the "q" parameter name to separate media type
parameters from Accept extension parameters is due to historical parameters from Accept extension parameters is due to historical
practice. Although this prevents any media type parameter named practice. Although this prevents any media type parameter named "q"
"q" from being used with a media range, such an event is believed from being used with a media range, such an event is believed to be
to be unlikely given the lack of any "q" parameters in the IANA unlikely given the lack of any "q" parameters in the IANA media
media type registry and the rare usage of any media type type registry and the rare usage of any media type parameters in
parameters in Accept. Future media types are discouraged from Accept. Future media types are discouraged from registering any
registering any parameter named "q". parameter named "q".
The example The example
Accept: audio/*; q=0.2, audio/basic Accept: audio/*; q=0.2, audio/basic
SHOULD be interpreted as "I prefer audio/basic, but send me any audio 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." 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 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, client accepts all media types. If an Accept header field is present,
and if the server cannot send a response which is acceptable and if the server cannot send a response which is acceptable
according to the combined Accept field value, then the server SHOULD according to the combined Accept field value, then the server SHOULD
send a 406 (not acceptable) response. send a 406 (not acceptable) response.
A more elaborate example is A more elaborate example is
Accept: text/plain; q=0.5, text/html, Accept: text/plain; q=0.5, text/html,
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text/x-dvi; q=0.8, text/x-c text/x-dvi; q=0.8, text/x-c
Verbally, this would be interpreted as "text/html and text/x-c are 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 the preferred media types, but if they do not exist, then send the