< draft-ietf-http-v11-spec-rev   rfc2616.txt 
HTTP Working Group R. Fielding, UC Irvine Network Working Group R. Fielding
INTERNET-DRAFT J. Gettys, Compaq/W3C Request for Comments: 2616 UC Irvine
<draft-ietf-http-v11-spec-rev-06> J. C. Mogul, Compaq Obsoletes: 2068 J. Gettys
H. Frystyk, W3C/MIT Category: Standards Track Compaq/W3C
L. Masinter, Xerox J. Mogul
P. Leach, Microsoft Compaq
T. Berners-Lee, W3C/MIT H. Frystyk
Expires May 18, 1999 November 18, 1998 W3C/MIT
L. Masinter
Xerox
P. Leach
Microsoft
T. Berners-Lee
W3C/MIT
June 1999
Hypertext Transfer Protocol -- HTTP/1.1 Hypertext Transfer Protocol -- HTTP/1.1
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document specifies an Internet standards track protocol for the
documents of the Internet Engineering Task Force (IETF), its areas, Internet community, and requests discussion and suggestions for
and its working groups. Note that other groups may also distribute improvements. Please refer to the current edition of the "Internet
working documents as Internet-Drafts. Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or made obsolete by other documents at
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Distribution of this document is unlimited. Please send comments to Copyright Notice
the HTTP working group at <http-wg@hplb.hpl.hp.com>. Discussions of
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http://www.ics.uci.edu/pub/ietf/http/. General discussions about HTTP
and the applications which use HTTP should take place on the <www-
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The IETF takes no position regarding the validity or scope of any Copyright (C) The Internet Society (1999). All Rights Reserved.
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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 [47]. 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.
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 is an update to RFC 2068 [33].
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved. See
section 20 for the full copyright notice.
Table of Contents Table of Contents
HYPERTEXT TRANSFER PROTOCOL -- HTTP/1.1..............................1 1 Introduction ...................................................7
1.1 Purpose......................................................7
Status of this Memo..................................................1 1.2 Requirements .................................................8
1.3 Terminology ..................................................8
Abstract.............................................................2 1.4 Overall Operation ...........................................12
2 Notational Conventions and Generic Grammar ....................14
Copyright Notice.....................................................2 2.1 Augmented BNF ...............................................14
2.2 Basic Rules .................................................15
Table of Contents....................................................3 3 Protocol Parameters ...........................................17
3.1 HTTP Version ................................................17
1 Introduction ....................................................8 3.2 Uniform Resource Identifiers ................................18
1.1 Purpose.......................................................8 3.2.1 General Syntax ...........................................19
1.2 Requirements ..................................................9 3.2.2 http URL .................................................19
1.3 Terminology ...................................................9 3.2.3 URI Comparison ...........................................20
1.4 Overall Operation ............................................12 3.3 Date/Time Formats ...........................................20
3.3.1 Full Date ................................................20
2 Notational Conventions and Generic Grammar .....................14 3.3.2 Delta Seconds ............................................21
2.1 Augmented BNF ................................................14 3.4 Character Sets ..............................................21
2.2 Basic Rules ..................................................16 3.4.1 Missing Charset ..........................................22
3.5 Content Codings .............................................23
3 Protocol Parameters ............................................17 3.6 Transfer Codings ............................................24
3.1 HTTP Version .................................................17 3.6.1 Chunked Transfer Coding ..................................25
3.2 Uniform Resource Identifiers .................................18 3.7 Media Types .................................................26
3.2.1 General Syntax ............................................18 3.7.1 Canonicalization and Text Defaults .......................27
3.2.2 http URL ..................................................19 3.7.2 Multipart Types ..........................................27
3.2.3 URI Comparison ............................................19 3.8 Product Tokens ..............................................28
3.3 Date/Time Formats ............................................20 3.9 Quality Values ..............................................29
3.3.1 Full Date .................................................20 3.10 Language Tags ...............................................29
3.3.2 Delta Seconds .............................................21 3.11 Entity Tags .................................................30
3.4 Character Sets ...............................................21 3.12 Range Units .................................................30
3.4.1 Missing Charset ...........................................22 4 HTTP Message ..................................................31
3.5 Content Codings ..............................................22 4.1 Message Types ...............................................31
3.6 Transfer Codings .............................................23 4.2 Message Headers .............................................31
3.6.1 Chunked Transfer Coding ...................................24 4.3 Message Body ................................................32
3.7 Media Types ..................................................25 4.4 Message Length ..............................................33
3.7.1 Canonicalization and Text Defaults ........................26 4.5 General Header Fields .......................................34
3.7.2 Multipart Types ...........................................26 5 Request .......................................................35
3.8 Product Tokens ...............................................27 5.1 Request-Line ................................................35
3.9 Quality Values ...............................................27 5.1.1 Method ...................................................36
3.10 Language Tags ................................................28 5.1.2 Request-URI ..............................................36
3.11 Entity Tags ..................................................28 5.2 The Resource Identified by a Request ........................38
3.12 Range Units ..................................................29 5.3 Request Header Fields .......................................38
6 Response ......................................................39
4 HTTP Message ...................................................29 6.1 Status-Line .................................................39
4.1 Message Types ................................................29 6.1.1 Status Code and Reason Phrase ............................39
4.2 Message Headers ..............................................30 6.2 Response Header Fields ......................................41
4.3 Message Body .................................................31 7 Entity ........................................................42
4.4 Message Length ...............................................31 7.1 Entity Header Fields ........................................42
4.5 General Header Fields ........................................33 7.2 Entity Body .................................................43
5 Request ........................................................33 7.2.1 Type .....................................................43
5.1 Request-Line .................................................33 7.2.2 Entity Length ............................................43
5.1.1 Method ....................................................34 8 Connections ...................................................44
5.1.2 Request-URI ...............................................34 8.1 Persistent Connections ......................................44
5.2 The Resource Identified by a Request .........................35 8.1.1 Purpose ..................................................44
5.3 Request Header Fields ........................................36 8.1.2 Overall Operation ........................................45
8.1.3 Proxy Servers ............................................46
6 Response .......................................................36 8.1.4 Practical Considerations .................................46
6.1 Status-Line ..................................................37 8.2 Message Transmission Requirements ...........................47
6.1.1 Status Code and Reason Phrase .............................37 8.2.1 Persistent Connections and Flow Control ..................47
6.2 Response Header Fields .......................................39 8.2.2 Monitoring Connections for Error Status Messages .........48
8.2.3 Use of the 100 (Continue) Status .........................48
7 Entity .........................................................39 8.2.4 Client Behavior if Server Prematurely Closes Connection ..50
7.1 Entity Header Fields .........................................39 9 Method Definitions ............................................51
7.2 Entity Body ..................................................40 9.1 Safe and Idempotent Methods .................................51
7.2.1 Type ......................................................40 9.1.1 Safe Methods .............................................51
7.2.2 Entity Length .............................................40 9.1.2 Idempotent Methods .......................................51
9.2 OPTIONS .....................................................52
8 Connections ....................................................41 9.3 GET .........................................................53
8.1 Persistent Connections .......................................41 9.4 HEAD ........................................................54
8.1.1 Purpose ...................................................41 9.5 POST ........................................................54
8.1.2 Overall Operation .........................................41 9.6 PUT .........................................................55
8.1.3 Proxy Servers .............................................43 9.7 DELETE ......................................................56
8.1.4 Practical Considerations ..................................43 9.8 TRACE .......................................................56
8.2 Message Transmission Requirements ............................44 9.9 CONNECT .....................................................57
8.2.1 Persistent Connections and Flow Control ...................44 10 Status Code Definitions ......................................57
8.2.2 Monitoring Connections for Error Status Messages ..........44 10.1 Informational 1xx ...........................................57
8.2.3 Use of the 100 (Continue) Status ..........................44 10.1.1 100 Continue .............................................58
8.2.4 Client Behavior if Server Prematurely Closes Connection ...46 10.1.2 101 Switching Protocols ..................................58
10.2 Successful 2xx ..............................................58
9 Method Definitions .............................................47 10.2.1 200 OK ...................................................58
9.1 Safe and Idempotent Methods ..................................47 10.2.2 201 Created ..............................................59
9.1.1 Safe Methods ..............................................47 10.2.3 202 Accepted .............................................59
9.1.2 Idempotent Methods ........................................48 10.2.4 203 Non-Authoritative Information ........................59
9.2 OPTIONS ......................................................48 10.2.5 204 No Content ...........................................60
9.3 GET ..........................................................49 10.2.6 205 Reset Content ........................................60
9.4 HEAD .........................................................49 10.2.7 206 Partial Content ......................................60
9.5 POST .........................................................50 10.3 Redirection 3xx .............................................61
9.6 PUT ..........................................................51 10.3.1 300 Multiple Choices .....................................61
9.7 DELETE .......................................................52 10.3.2 301 Moved Permanently ....................................62
9.8 TRACE ........................................................52 10.3.3 302 Found ................................................62
9.9 CONNECT ......................................................52 10.3.4 303 See Other ............................................63
10.3.5 304 Not Modified .........................................63
10 Status Code Definitions .......................................53 10.3.6 305 Use Proxy ............................................64
10.1 Informational 1xx ............................................53 10.3.7 306 (Unused) .............................................64
10.1.1 100 Continue ..............................................53 10.3.8 307 Temporary Redirect ...................................65
10.1.2 101 Switching Protocols ...................................53 10.4 Client Error 4xx ............................................65
10.2 Successful 2xx ...............................................54 10.4.1 400 Bad Request .........................................65
10.2.1 200 OK ....................................................54 10.4.2 401 Unauthorized ........................................66
10.2.2 201 Created ...............................................54 10.4.3 402 Payment Required ....................................66
10.2.3 202 Accepted ..............................................54 10.4.4 403 Forbidden ...........................................66
10.2.4 203 Non-Authoritative Information .........................55 10.4.5 404 Not Found ...........................................66
10.2.5 204 No Content ............................................55 10.4.6 405 Method Not Allowed ..................................66
10.2.6 205 Reset Content .........................................55 10.4.7 406 Not Acceptable ......................................67
10.2.7 206 Partial Content .......................................56 10.4.8 407 Proxy Authentication Required .......................67
10.3 Redirection 3xx ..............................................56 10.4.9 408 Request Timeout .....................................67
10.3.1 300 Multiple Choices ......................................57 10.4.10 409 Conflict ............................................67
10.3.2 301 Moved Permanently .....................................57 10.4.11 410 Gone ................................................68
10.3.3 302 Found .................................................57 10.4.12 411 Length Required .....................................68
10.3.4 303 See Other .............................................58 10.4.13 412 Precondition Failed .................................68
10.3.5 304 Not Modified ..........................................58 10.4.14 413 Request Entity Too Large ............................69
10.3.6 305 Use Proxy .............................................59 10.4.15 414 Request-URI Too Long ................................69
10.3.7 306 (Unused) ..............................................59 10.4.16 415 Unsupported Media Type ..............................69
10.3.8 307 Temporary Redirect ....................................59 10.4.17 416 Requested Range Not Satisfiable .....................69
10.4 Client Error 4xx .............................................60 10.4.18 417 Expectation Failed ..................................70
10.4.1 400 Bad Request ..........................................60 10.5 Server Error 5xx ............................................70
10.4.2 401 Unauthorized .........................................60 10.5.1 500 Internal Server Error ................................70
10.4.3 402 Payment Required .....................................61 10.5.2 501 Not Implemented ......................................70
10.4.4 403 Forbidden ............................................61 10.5.3 502 Bad Gateway ..........................................70
10.4.5 404 Not Found ............................................61 10.5.4 503 Service Unavailable ..................................70
10.4.6 405 Method Not Allowed ...................................61 10.5.5 504 Gateway Timeout ......................................71
10.4.7 406 Not Acceptable .......................................61 10.5.6 505 HTTP Version Not Supported ...........................71
10.4.8 407 Proxy Authentication Required ........................62 11 Access Authentication ........................................71
10.4.9 408 Request Timeout ......................................62 12 Content Negotiation ..........................................71
10.4.10 409 Conflict .............................................62 12.1 Server-driven Negotiation ...................................72
10.4.11 410 Gone .................................................62 12.2 Agent-driven Negotiation ....................................73
10.4.12 411 Length Required ......................................63 12.3 Transparent Negotiation .....................................74
10.4.13 412 Precondition Failed ..................................63 13 Caching in HTTP ..............................................74
10.4.14 413 Request Entity Too Large .............................63 13.1.1 Cache Correctness ........................................75
10.4.15 414 Request-URI Too Long .................................63 13.1.2 Warnings .................................................76
10.4.16 415 Unsupported Media Type ...............................64 13.1.3 Cache-control Mechanisms .................................77
10.4.17 416 Requested Range Not Satisfiable ......................64 13.1.4 Explicit User Agent Warnings .............................78
10.4.18 417 Expectation Failed ...................................64 13.1.5 Exceptions to the Rules and Warnings .....................78
10.5 Server Error 5xx .............................................64 13.1.6 Client-controlled Behavior ...............................79
10.5.1 500 Internal Server Error .................................64 13.2 Expiration Model ............................................79
10.5.2 501 Not Implemented .......................................65 13.2.1 Server-Specified Expiration ..............................79
10.5.3 502 Bad Gateway ...........................................65 13.2.2 Heuristic Expiration .....................................80
10.5.4 503 Service Unavailable ...................................65 13.2.3 Age Calculations .........................................80
10.5.5 504 Gateway Timeout .......................................65 13.2.4 Expiration Calculations ..................................83
10.5.6 505 HTTP Version Not Supported ............................65 13.2.5 Disambiguating Expiration Values .........................84
13.2.6 Disambiguating Multiple Responses ........................84
11 Access Authentication .........................................65 13.3 Validation Model ............................................85
13.3.1 Last-Modified Dates ......................................86
12 Content Negotiation ...........................................66 13.3.2 Entity Tag Cache Validators ..............................86
12.1 Server-driven Negotiation ....................................66 13.3.3 Weak and Strong Validators ...............................86
12.2 Agent-driven Negotiation .....................................67 13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates.89
12.3 Transparent Negotiation ......................................68 13.3.5 Non-validating Conditionals ..............................90
13.4 Response Cacheability .......................................91
13 Caching in HTTP ...............................................68 13.5 Constructing Responses From Caches ..........................92
13.1.1 Cache Correctness .........................................69 13.5.1 End-to-end and Hop-by-hop Headers ........................92
13.1.2 Warnings ..................................................70 13.5.2 Non-modifiable Headers ...................................92
13.1.3 Cache-control Mechanisms ..................................71 13.5.3 Combining Headers ........................................94
13.1.4 Explicit User Agent Warnings ..............................72 13.5.4 Combining Byte Ranges ....................................95
13.1.5 Exceptions to the Rules and Warnings ......................72 13.6 Caching Negotiated Responses ................................95
13.1.6 Client-controlled Behavior ................................72 13.7 Shared and Non-Shared Caches ................................96
13.2 Expiration Model .............................................73 13.8 Errors or Incomplete Response Cache Behavior ................97
13.2.1 Server-Specified Expiration ...............................73 13.9 Side Effects of GET and HEAD ................................97
13.2.2 Heuristic Expiration ......................................74 13.10 Invalidation After Updates or Deletions ...................97
13.2.3 Age Calculations ..........................................74 13.11 Write-Through Mandatory ...................................98
13.2.4 Expiration Calculations ...................................76 13.12 Cache Replacement .........................................99
13.2.5 Disambiguating Expiration Values ..........................77 13.13 History Lists .............................................99
13.2.6 Disambiguating Multiple Responses .........................77 14 Header Field Definitions ....................................100
13.3 Validation Model .............................................78 14.1 Accept .....................................................100
13.3.1 Last-Modified Dates .......................................79 14.2 Accept-Charset .............................................102
13.3.2 Entity Tag Cache Validators ...............................79 14.3 Accept-Encoding ............................................102
13.3.3 Weak and Strong Validators ................................79 14.4 Accept-Language ............................................104
13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates .81 14.5 Accept-Ranges ..............................................105
13.3.5 Non-validating Conditionals ...............................83 14.6 Age ........................................................106
13.4 Response Cacheability ........................................83 14.7 Allow ......................................................106
13.5 Constructing Responses From Caches ...........................84 14.8 Authorization ..............................................107
13.5.1 End-to-end and Hop-by-hop Headers .........................84 14.9 Cache-Control ..............................................108
13.5.2 Non-modifiable Headers ....................................85 14.9.1 What is Cacheable .......................................109
13.5.3 Combining Headers .........................................86 14.9.2 What May be Stored by Caches ............................110
13.5.4 Combining Byte Ranges .....................................87 14.9.3 Modifications of the Basic Expiration Mechanism .........111
13.6 Caching Negotiated Responses .................................87 14.9.4 Cache Revalidation and Reload Controls ..................113
13.7 Shared and Non-Shared Caches .................................88 14.9.5 No-Transform Directive ..................................115
13.8 Errors or Incomplete Response Cache Behavior .................89 14.9.6 Cache Control Extensions ................................116
13.9 Side Effects of GET and HEAD .................................89 14.10 Connection ...............................................117
13.10 Invalidation After Updates or Deletions ....................89 14.11 Content-Encoding .........................................118
13.11 Write-Through Mandatory ....................................90 14.12 Content-Language .........................................118
13.12 Cache Replacement ..........................................90 14.13 Content-Length ...........................................119
13.13 History Lists ..............................................91 14.14 Content-Location .........................................120
14.15 Content-MD5 ..............................................121
14 Header Field Definitions ......................................91 14.16 Content-Range ............................................122
14.1 Accept .......................................................91 14.17 Content-Type .............................................124
14.2 Accept-Charset ...............................................93 14.18 Date .....................................................124
14.3 Accept-Encoding ..............................................94 14.18.1 Clockless Origin Server Operation ......................125
14.4 Accept-Language ..............................................95 14.19 ETag .....................................................126
14.5 Accept-Ranges ................................................96 14.20 Expect ...................................................126
14.6 Age ..........................................................97 14.21 Expires ..................................................127
14.7 Allow ........................................................97 14.22 From .....................................................128
14.8 Authorization ................................................98 14.23 Host .....................................................128
14.9 Cache-Control ................................................98 14.24 If-Match .................................................129
14.9.1 What is Cacheable ........................................100 14.25 If-Modified-Since ........................................130
14.9.2 What May be Stored by Caches .............................101 14.26 If-None-Match ............................................132
14.9.3 Modifications of the Basic Expiration Mechanism ..........101 14.27 If-Range .................................................133
14.9.4 Cache Revalidation and Reload Controls ...................103 14.28 If-Unmodified-Since ......................................134
14.9.5 No-Transform Directive ...................................105 14.29 Last-Modified ............................................134
14.9.6 Cache Control Extensions .................................106 14.30 Location .................................................135
14.10 Connection ................................................106 14.31 Max-Forwards .............................................136
14.11 Content-Encoding ..........................................107 14.32 Pragma ...................................................136
14.12 Content-Language ..........................................108 14.33 Proxy-Authenticate .......................................137
14.13 Content-Length ............................................109 14.34 Proxy-Authorization ......................................137
14.14 Content-Location ..........................................109 14.35 Range ....................................................138
14.15 Content-MD5 ...............................................110 14.35.1 Byte Ranges ...........................................138
14.16 Content-Range .............................................111 14.35.2 Range Retrieval Requests ..............................139
14.17 Content-Type ..............................................113 14.36 Referer ..................................................140
14.18 Date ......................................................113 14.37 Retry-After ..............................................141
14.18.1 Clockless Origin Server Operation .......................114 14.38 Server ...................................................141
14.19 ETag ......................................................114 14.39 TE .......................................................142
14.20 Expect ....................................................115 14.40 Trailer ..................................................143
14.21 Expires ...................................................115 14.41 Transfer-Encoding..........................................143
14.22 From ......................................................116 14.42 Upgrade ..................................................144
14.23 Host ......................................................117 14.43 User-Agent ...............................................145
14.24 If-Match ..................................................118 14.44 Vary .....................................................145
14.25 If-Modified-Since .........................................119 14.45 Via ......................................................146
14.26 If-None-Match .............................................120 14.46 Warning ..................................................148
14.27 If-Range ..................................................121 14.47 WWW-Authenticate .........................................150
14.28 If-Unmodified-Since .......................................122 15 Security Considerations .......................................150
14.29 Last-Modified .............................................122 15.1 Personal Information....................................151
14.30 Location ..................................................123 15.1.1 Abuse of Server Log Information .........................151
14.31 Max-Forwards ..............................................123 15.1.2 Transfer of Sensitive Information .......................151
14.32 Pragma ....................................................124 15.1.3 Encoding Sensitive Information in URI's .................152
14.33 Proxy-Authenticate ........................................124 15.1.4 Privacy Issues Connected to Accept Headers ..............152
14.34 Proxy-Authorization .......................................125 15.2 Attacks Based On File and Path Names .......................153
14.35 Range .....................................................125 15.3 DNS Spoofing ...............................................154
14.35.1 Byte Ranges ............................................125 15.4 Location Headers and Spoofing ..............................154
14.35.2 Range Retrieval Requests ...............................127 15.5 Content-Disposition Issues .................................154
14.36 Referer ...................................................127 15.6 Authentication Credentials and Idle Clients ................155
14.37 Retry-After ...............................................128 15.7 Proxies and Caching ........................................155
14.38 Server ....................................................128 15.7.1 Denial of Service Attacks on Proxies....................156
14.39 TE ........................................................129 16 Acknowledgments .............................................156
14.40 Trailer ...................................................130 17 References ..................................................158
14.41 Transfer-Encoding...........................................130 18 Authors' Addresses ..........................................162
14.42 Upgrade ...................................................131 19 Appendices ..................................................164
14.43 User-Agent ................................................132 19.1 Internet Media Type message/http and application/http ......164
14.44 Vary ......................................................132 19.2 Internet Media Type multipart/byteranges ...................165
14.45 Via .......................................................133 19.3 Tolerant Applications ......................................166
14.46 Warning ...................................................134 19.4 Differences Between HTTP Entities and RFC 2045 Entities ....167
14.47 WWW-Authenticate ..........................................136 19.4.1 MIME-Version ............................................167
19.4.2 Conversion to Canonical Form ............................167
15 Security Considerations ........................................136 19.4.3 Conversion of Date Formats ..............................168
15.1 Personal Information.....................................137 19.4.4 Introduction of Content-Encoding ........................168
15.1.1 Abuse of Server Log Information ..........................137 19.4.5 No Content-Transfer-Encoding ............................168
15.1.2 Transfer of Sensitive Information ........................137 19.4.6 Introduction of Transfer-Encoding .......................169
15.1.3 Encoding Sensitive Information in URI's ..................138 19.4.7 MHTML and Line Length Limitations .......................169
15.1.4 Privacy Issues Connected to Accept Headers ...............138 19.5 Additional Features ........................................169
15.2 Attacks Based On File and Path Names ........................139 19.5.1 Content-Disposition .....................................170
15.3 DNS Spoofing ................................................139 19.6 Compatibility with Previous Versions .......................170
15.4 Location Headers and Spoofing ...............................140 19.6.1 Changes from HTTP/1.0 ...................................171
15.5 Content-Disposition Issues ..................................140 19.6.2 Compatibility with HTTP/1.0 Persistent Connections ......172
15.6 Authentication Credentials and Idle Clients .................140 19.6.3 Changes from RFC 2068 ...................................172
15.7 Proxies and Caching .........................................141 20 Index .......................................................175
15.7.1 Denial of Service Attacks on Proxies.....................142 21 Full Copyright Statement ....................................176
16 Acknowledgments ..............................................142
17 References ...................................................143
18 Authors' Addresses ...........................................147
19 Appendices ...................................................148
19.1 Internet Media Type message/http and application/http .......148
19.2 Internet Media Type multipart/byteranges ....................149
19.3 Tolerant Applications .......................................150
19.4 Differences Between HTTP Entities and RFC 2045 Entities .....151
19.4.1 MIME-Version .............................................151
19.4.2 Conversion to Canonical Form .............................151
19.4.3 Conversion of Date Formats ...............................152
19.4.4 Introduction of Content-Encoding .........................152
19.4.5 No Content-Transfer-Encoding .............................152
19.4.6 Introduction of Transfer-Encoding ........................152
19.4.7 MHTML and Line Length Limitations ........................153
19.5 Additional Features .........................................153
19.5.1 Content-Disposition ......................................153
19.6 Compatibility with Previous Versions ........................154
19.6.1 Changes from HTTP/1.0 ....................................155
19.6.2 Compatibility with HTTP/1.0 Persistent Connections .......155
19.6.3 Changes from RFC 2068 ....................................156
19.7 Notes to the RFC Editor and IANA ............................158
19.7.1 Transfer-coding Values ...................................158
19.7.2 Definition of application/http ...........................159
19.7.3 Addition of "identity" content-coding to content-coding
Registry 159
20 Full Copyright Statement .....................................159
21 Index ........................................................160
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
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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."
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
The basic unit of HTTP communication, consisting of a structured The basic unit of HTTP communication, consisting of a structured
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 defined in section 3.2. Resources may be available in multiple as 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. An entity consists of metainformation in the form of response. An entity consists of metainformation in the form of
entity-header fields and content in the form of an entity-body, as entity-header fields and content in the form of an entity-body, as
described in section 7. described in 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 of entities in any response can be negotiated representation of entities in any response can be negotiated
(including error responses). (including error 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 `variant.' Use of the term `variant' representations is termed a `varriant'. Use of the term `variant'
does not necessarily imply that the resource is subject to content does not necessarily imply that the resource is subject to content
negotiation. negotiation.
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 requests by sending back responses. Any given program may service requests by sending back responses. Any given program may
be capable of being both a client and a server; our use of these be capable of being both a client and a server; our use of these
terms refers only to the role being performed by the program for a terms refers only to the role being performed by the program for a
particular connection, rather than to the program's capabilities particular connection, rather than to the program's capabilities
in general. Likewise, any server may act as an origin server, in general. Likewise, any server may act as an origin server,
proxy, gateway, or tunnel, switching behavior based on the nature proxy, gateway, or 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 the purpose of making requests on behalf of other clients. for the purpose of making requests on behalf of other clients.
Requests are serviced internally or by passing them on, with Requests are serviced internally or by passing them on, with
possible translation, to other servers. A proxy MUST implement possible translation, to other servers. A proxy MUST implement
both the client and server requirements of this specification. A both the client and server requirements of this specification. A
"transparent proxy" is a proxy that does not modify the request or "transparent proxy" is a proxy that does not modify the request or
response beyond what is required for proxy authentication and response beyond what is required for proxy authentication and
identification. A "non-transparent proxy" is a proxy that modifies identification. A "non-transparent proxy" is a proxy that modifies
the request or response in order to provide some added service to the request or response in order to provide some added service to
the user agent, such as group annotation services, media type the user agent, such as group annotation services, media type
transformation, protocol reduction, or anonymity filtering. Except transformation, protocol reduction, or anonymity filtering. Except
where either transparent or non-transparent behavior is explicitly where either transparent or non-transparent behavior is explicitly
stated, the HTTP proxy requirements apply to both types of stated, the HTTP proxy 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 a proxy, a gateway receives requests as if it were the Unlike a proxy, a gateway receives requests as if it were the
origin server for the requested resource; the requesting client origin server for the requested resource; the requesting client
may not be aware that it is communicating with a gateway. may not be aware 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 connections. Once active, a tunnel is not considered a party two connections. Once active, a tunnel is not considered a party
to the HTTP communication, though the tunnel may have been to the HTTP communication, though the tunnel may have been
initiated by an HTTP request. The tunnel ceases to exist when both initiated by an HTTP request. The tunnel ceases to exist when both
ends of the relayed connections are closed. ends of the 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 controls its message storage, retrieval, and deletion. A that controls its message storage, retrieval, and deletion. A
cache stores cacheable responses in order to reduce the response cache stores cacheable responses in order to reduce the response
time and network bandwidth consumption on future, equivalent time and network bandwidth consumption on future, equivalent
requests. Any client or server may include a cache, though a cache requests. Any client or server may include a cache, though a cache
cannot be used by a server that is acting as a tunnel. 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 response message for use in answering subsequent requests. The the response message for use in answering subsequent requests. The
rules for determining the cacheability of HTTP responses are rules for determining the cacheability of HTTP responses are
defined in section 13. Even if a resource is cacheable, there may defined in section 13. Even if a resource is cacheable, there may
be additional constraints on whether a cache can use the cached be additional constraints on whether a cache can use the cached
copy for a particular request. copy for a 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 delay from the origin server, perhaps via one or more unnecessary delay from the origin server, perhaps via one or more
proxies. A response is also first-hand if its validity has just proxies. A response is also first-hand if its validity has just
been checked directly with the origin server. been checked 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 longer be returned by a cache without further validation. no 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
The length of time between the generation of a response and its The length of time between the generation of a response and its
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 to a particular response, when its use affects neither the respect to a particular response, when its use affects neither the
requesting client nor the origin server, except to improve requesting client nor the origin server, except to improve
performance. When a cache is semantically transparent, the client performance. When a cache is semantically transparent, the client
receives exactly the same response (except for hop-by-hop headers) receives exactly the same response (except for hop-by-hop headers)
that it would have received had its request been handled directly that it would have 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 is used to find out whether a cache entry is an equivalent that is used to find out whether a cache entry is an equivalent
copy of an entity. copy of an entity.
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 flow from upstream to downstream. messages 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 "outbound" means "traveling toward the user agent" and "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
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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 [9]. 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 "<" and ">") and is separated from its definition by the enclosing "<" and ">") and is separated from its definition by the
equal "=" character. White space is only significant in that equal "=" character. White space is only significant in that
indentation of continuation lines is used to indicate a rule indentation of continuation lines is used to indicate a rule
definition that spans more than one line. Certain basic rules are definition that spans more than one line. Certain basic rules are
in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle
brackets are used within definitions whenever their presence will brackets are used within definitions whenever their presence will
facilitate discerning the use of rule names. facilitate discerning the use of rule names.
"literal" "literal"
Quotation marks surround literal text. Unless stated otherwise, Quotation marks surround literal text. Unless stated otherwise,
the text is case-insensitive. the 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" will accept yes or no. 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 form is "<n>*<m>element" indicating at least <n> and at most full form is "<n>*<m>element" indicating at least <n> and at most
<m> occurrences of element. Default values are 0 and infinity so <m> occurrences of element. Default values are 0 and infinity so
that "*(element)" allows any number, including zero; "1*element" that "*(element)" allows any number, including zero; "1*element"
requires at least one; and "1*2element" allows one or two. requires 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 to "*1(foo bar)". equivalent 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 elements. The full form is "<n>#<m>element" indicating at least
<n> and at most <m> elements, each separated by one or more commas <n> and at most <m> elements, each separated by one or more commas
(",") and OPTIONAL linear white space (LWS). This makes the usual (",") and OPTIONAL linear white space (LWS). This makes the usual
form of lists very easy; a rule such as 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) " is permitted, but counts as only two "(element), , (element) " is permitted, but counts as only two
elements. Therefore, where at least one element is required, at elements. Therefore, where at least one element is required, at
least one non-null element MUST be present. Default values are 0 least one non-null element MUST be present. Default values are 0
and infinity so that "#element" allows any number, including zero; and infinity so that "#element" allows any number, including zero;
"1#element" requires at least one; and "1#2element" allows one or "1#element" 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 a comment that continues to the end of line. This is a starts a comment that continues to the end of line. This is a
simple way of including useful notes in parallel with the simple way of 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 adjacent words and separators, without changing the between adjacent words and separators, without changing the
interpretation of a field. At least one delimiter (LWS and/or interpretation of a field. At least one delimiter (LWS and/or
separators) MUST exist between any two tokens (for the definition
of "token" below), since they would otherwise be interpreted as a separators) MUST exist between any two tokens (for the definition
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 [21].
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">
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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.
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[33], caching proxies MUST, gateways
MAY, and tunnels MUST NOT upgrade the request to the highest version MAY, and tunnels MUST NOT upgrade the request to the highest version
they support. The proxy/gateway's response to that request MUST be in they support. The proxy/gateway's response to that request MUST be in
the same major version as the request. the same major version as the request.
Note: Converting between versions of HTTP may involve Note: Converting between versions of HTTP may involve modification
modification of header fields required or forbidden by the of header fields required or forbidden by the versions involved.
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 [3], and finally the
combination of Uniform Resource Locators (URL) [4] and Names (URN) combination of Uniform Resource Locators (URL) [4] and Names (URN)
[20]. As far as HTTP is concerned, Uniform Resource Identifiers are [20]. As far as HTTP is concerned, Uniform Resource Identifiers are
simply formatted strings which identify--via name, location, or any simply formatted strings which identify--via name, location, or any
other characteristic--a resource. other characteristic--a resource.
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"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 Note: Servers ought to be cautious about depending on URI lengths
lengths above 255 bytes, because some older client or proxy above 255 bytes, because some older client or proxy
implementations might not properly support these lengths. implementations 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 ] 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
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 [24]). 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 - A port that is empty or not given is equivalent to the default
port for that URI-reference; port for that URI-reference;
. Comparisons of host names MUST be case-insensitive; - Comparisons of host names MUST be case-insensitive;
. Comparisons of scheme names MUST be case-insensitive; - Comparisons of scheme names MUST be case-insensitive;
. An empty abs_path is equivalent to an abs_path of "/". - 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" and "unsafe" sets (see
section 3.2) are equivalent to their ""%" HEX HEX" encoding. RFC 2396 [42]) 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 [8] (an update to
RFC 822 [9]). The second format is in common use, but is based on the RFC 822 [9]). The second format is in common use, but is based on the
obsolete RFC 850 [12] date format and lacks a four-digit year. obsolete RFC 850 [12] date format and lacks a four-digit year.
HTTP/1.1 clients and servers that parse the date value MUST accept 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 19.3 for further information.
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 applications, as is sometimes the case when retrieving or posting
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
asctime format. HTTP-date is case sensitive and MUST NOT include asctime format. HTTP-date is case sensitive and MUST NOT include
additional LWS beyond that specifically included as SP in the additional LWS beyond that specifically included as SP in the
grammar. grammar.
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time = 2DIGIT ":" 2DIGIT ":" 2DIGIT time = 2DIGIT ":" 2DIGIT ":" 2DIGIT
; 00:00:00 - 23:59:59 ; 00:00:00 - 23:59:59
wkday = "Mon" | "Tue" | "Wed" wkday = "Mon" | "Tue" | "Wed"
| "Thu" | "Fri" | "Sat" | "Sun" | "Thu" | "Fri" | "Sat" | "Sun"
weekday = "Monday" | "Tuesday" | "Wednesday" weekday = "Monday" | "Tuesday" | "Wednesday"
| "Thursday" | "Friday" | "Saturday" | "Sunday" | "Thursday" | "Friday" | "Saturday" | "Sunday"
month = "Jan" | "Feb" | "Mar" | "Apr" month = "Jan" | "Feb" | "Mar" | "Apr"
| "May" | "Jun" | "Jul" | "Aug" | "May" | "Jun" | "Jul" | "Aug"
| "Sep" | "Oct" | "Nov" | "Dec" | "Sep" | "Oct" | "Nov" | "Dec"
Note: HTTP requirements for the date/time stamp format apply Note: HTTP requirements for the date/time stamp format apply only
only to their usage within the protocol stream. Clients and to their usage within the protocol stream. Clients and servers are
servers are not required to use these formats for user not required to use these formats for user presentation, request
presentation, request logging, etc. logging, etc.
3.3.2 Delta Seconds 3.3.2 Delta Seconds
Some HTTP header fields allow a time value to be specified as an Some HTTP header fields allow a time value to be specified as an
integer number of seconds, represented in decimal, after the time integer number of seconds, represented in decimal, after the time
that the message was received. that the message was received.
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
3.4 Character Sets 3.4 Character Sets
skipping to change at page 21, line 47 skipping to change at page 22, line 21
This definition is intended to allow various kinds of character This definition is intended to allow various kinds of character
encoding, from simple single-table mappings such as US-ASCII to encoding, from simple single-table mappings such as US-ASCII to
complex table switching methods such as those that use ISO-2022's complex table switching methods such as those that use ISO-2022's
techniques. However, the definition associated with a MIME character techniques. However, the definition associated with a MIME character
set name MUST fully specify the mapping to be performed from octets set name MUST fully specify the mapping to be performed from octets
to characters. In particular, use of external profiling information to characters. In particular, use of external profiling information
to determine the exact mapping is not permitted. 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 share the same registry, it is important that the MIME share the same registry, it is important that the terminology
terminology also be shared. also 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]. [19].
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 [19] MUST represent the character set defined
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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 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. The chunked encoding is ended by any chunk whose size is
zero, followed by the trailer, which is terminated by an empty line. 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" a)the request included a TE header field that indicates "trailers" is
is acceptable in the transfer-coding of the response, as acceptable in the transfer-coding of the response, as described in
described in section 14.39; or, 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 could use the message (in a manner acceptable to the origin server)
server) without receiving this metadata. In other words, the without receiving this metadata. In other words, the origin server
origin server is willing to accept the possibility that the is willing to accept the possibility that the trailer fields might
trailer fields might be silently discarded along the path be silently discarded along the path to the client.
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 19.4.6.
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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 [19]). The media type registration process is
outlined in RFC 1590 [17]. Use of non-registered media types is outlined in RFC 1590 [17]. 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 entity- Internet media types are registered with a canonical form. An
body transferred via HTTP messages MUST be represented in the entity-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
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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 Note: The "multipart/form-data" type has been specifically defined
defined for carrying form data suitable for processing via the for carrying form data suitable for processing via the POST
POST request method, as described in RFC 1867 [15]. request method, as described in RFC 1867 [15].
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
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.
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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 [9] 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 *(message-header CRLF)
CRLF CRLF
[ message-body ] [ message-body ]
start-line = Request-Line | Status-Line start-line = Request-Line | Status-Line
In the interest of robustness, servers SHOULD ignore any empty In the interest of robustness, servers SHOULD ignore any empty
line(s) received where a Request-Line is expected. In other words, if line(s) received where a Request-Line is expected. In other words, if
the server is reading the protocol stream at the beginning of a the server is reading the protocol stream at the beginning of a
message and receives a CRLF first, it should ignore the CRLF. message and receives a CRLF first, it should ignore the CRLF.
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
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that given in Section 3.1 of RFC 822 [9]. Each header field consists that given in Section 3.1 of RFC 822 [9]. Each header field consists
of a name followed by a colon (":") and the field value. Field names 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 ] CRLF message-header = field-name ":" [ field-value ]
field-name = token field-name = token
field-value = *( field-content | LWS ) field-value = *( field-content | LWS )
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 combinations
of token, separators, and quoted-string> 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
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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
transfer-length is not otherwise specified, then this self- ransfer-length is not otherwise specified, then this self-
delimiting media type defines the transfer-length. This media type elimiting media type defines the transfer-length. This media type
MUST NOT be used unless the sender knows that the recipient can UST NOT be used unless the sender knows that the recipient can arse
parse it; the presence in a request of a Range header with it; the presence in a request of a Range header with ultiple byte-
multiple byte-range specifiers from a 1.1 client implies that the range specifiers from a 1.1 client implies that the lient can parse
client can parse multipart/byteranges responses. multipart/byteranges responses.
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 section. this 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.
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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.
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 ) ; 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 Request- The Request-Line begins with a method token, followed by the
URI and the protocol version, and ending with CRLF. The elements are Request-URI and the protocol version, and ending with CRLF. The
separated by SP characters. No CR or LF is allowed except in the elements are separated by SP characters. No CR or LF is allowed
final CRLF sequence. except in the 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
| "HEAD" ; Section 9.4 | "HEAD" ; Section 9.4
| "POST" ; Section 9.5 | "POST" ; Section 9.5
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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 [42], 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 "/".
Note: The "no rewrite" rule prevents the proxy from changing Note: The "no rewrite" rule prevents the proxy from changing the
the meaning of the request when the origin server is improperly meaning of the request when the origin server is improperly using
using a non-reserved URI character for a reserved purpose. a non-reserved URI character for a reserved purpose. Implementors
Implementors should be aware that some pre-HTTP/1.1 proxies should be aware that some pre-HTTP/1.1 proxies have been known to
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 19.6.1.1 for other requirements on Host support in HTTP/1.1.)
An origin server that does differentiate resources based on the host 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 Request- 1. If Request-URI is an absoluteURI, the host is part of the
URI. Any Host header field value in the request MUST be ignored. Request-URI. Any Host header field value in the request MUST be
ignored.
2. If the Request-URI is not an absoluteURI, and the request includes 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
something unique to a particular host) in order to determine what something unique to a particular host) in order to determine what
exact resource is being requested. exact resource is being requested.
5.3 Request Header Fields 5.3 Request Header Fields
The request-header fields allow the client to pass additional The request-header fields allow the client to pass additional
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| User-Agent ; Section 14.43 | User-Agent ; Section 14.43
Request-header field names can be extended reliably only in Request-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 request- experimental header fields MAY be given the semantics of request-
header fields if all parties in the communication recognize them to header fields if all parties in the communication recognize them to
be request-header fields. Unrecognized header fields are treated as be request-header fields. Unrecognized header fields are treated as
entity-header fields. entity-header fields.
6 Response 6 Response
After receiving and interpreting a request message, a server responds After receiving and interpreting a request message, a server responds
with an HTTP response message. with an HTTP response message.
Response = Status-Line ; Section 6.1 Response = Status-Line ; Section 6.1
*( general-header ; Section 4.5 *(( general-header ; Section 4.5
| response-header ; Section 6.2 | response-header ; Section 6.2
| entity-header ) ; Section 7.1 | entity-header ) CRLF) ; Section 7.1
CRLF CRLF
[ message-body ] ; Section 7.2 [ message-body ] ; Section 7.2
6.1 Status-Line 6.1 Status-Line
The first line of a Response message is the Status-Line, consisting The first line of a Response message is the Status-Line, consisting
of the protocol version followed by a numeric status code and its of the protocol version followed by a numeric status code and its
associated textual phrase, with each element separated by SP associated textual phrase, with each element separated by SP
characters. No CR or LF is allowed except in the final CRLF sequence. characters. No CR or LF is allowed except in the final CRLF sequence.
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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 - 1xx: Informational - Request received, continuing process
. 2xx: Success - The action was successfully received, - 2xx: Success - The action was successfully received,
understood, and accepted understood, and accepted
. 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
. 4xx: Client Error - The request contains bad syntax or cannot - 4xx: Client Error - The request contains bad syntax or cannot
be fulfilled be fulfilled
. 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
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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 [26] [30]. Implementation experience and
measurements of actual HTTP/1.1 (RFC 2068) implementations show good measurements of actual HTTP/1.1 (RFC 2068) implementations show good
results [39]. Alternatives have also been explored, for example, results [39]. Alternatives have also been explored, for example,
T/TCP [27]. T/TCP [27].
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
- By opening and closing fewer TCP connections, CPU time is saved
in routers and hosts (clients, servers, proxies, gateways, in routers and hosts (clients, servers, proxies, gateways,
tunnels, or caches), and memory used for TCP protocol control tunnels, or caches), and memory used for TCP protocol control
blocks can be saved in hosts. blocks can be saved in hosts.
. 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 used much more efficiently, with much lower elapsed time. be used much more efficiently, with much lower elapsed time.
. 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.
. Latency on subsequent requests is reduced since there is no - Latency on subsequent requests is reduced since there is no time
time spent in TCP's connection opening handshake. spent in TCP's connection opening handshake.
. 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 without the penalty of closing the TCP connection. Clients using
using future versions of HTTP might optimistically try a new future versions of HTTP might optimistically try a new feature,
feature, but if communicating with an older server, retry with but if communicating with an older server, retry with old
old semantics after an error is reported. 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- request, it SHOULD send a Connection header including the
token close. 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 19.6.2 for more information on backward
compatibility with HTTP/1.0 clients. compatibility with HTTP/1.0 clients.
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MAY substitute for user confirmation. The automatic retry SHOULD NOT MAY substitute for user confirmation. The automatic retry SHOULD NOT
be repeated if the second sequence of requests fails. be repeated if the second sequence of requests fails.
Servers SHOULD always respond to at least one request per connection, Servers SHOULD always respond to at least one request per connection,
if at all possible. Servers SHOULD NOT close a connection in the if at all possible. Servers SHOULD NOT close a connection in the
middle of transmitting a response, unless a network or client failure middle of transmitting a response, unless a network or client failure
is suspected. is suspected.
Clients that use persistent connections SHOULD limit the number of Clients that use persistent connections SHOULD limit the number of
simultaneous connections that they maintain to a given server. A simultaneous connections that they maintain to a given server. A
single-user client SHOULD NOT maintain more than connections with any single-user client SHOULD NOT maintain more than 2 connections with
server or proxy. A proxy SHOULD use up to 2*N connections to another any server or proxy. A proxy SHOULD use up to 2*N connections to
server or proxy, where N is the number of simultaneously active another server or proxy, where N is the number of simultaneously
users. These guidelines are intended to improve HTTP response times active users. These guidelines are intended to improve HTTP response
and avoid congestion. times and avoid congestion.
8.2 Message Transmission Requirements 8.2 Message Transmission Requirements
8.2.1 Persistent Connections and Flow Control 8.2.1 Persistent Connections and Flow Control
HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's
flow control mechanisms to resolve temporary overloads, rather than flow control mechanisms to resolve temporary overloads, rather than
terminating connections with the expectation that clients will retry. terminating connections with the expectation that clients will retry.
The latter technique can exacerbate network congestion. The latter technique can exacerbate network congestion.
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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.
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 an 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:
. If a client will wait for a 100 (Continue) response before - If a client will wait for a 100 (Continue) response before
sending the request body, it MUST send an Expect request-header sending the request body, it MUST send an Expect request-header
field (section 14.20) with the "100-continue" expectation. field (section 14.20) with the "100-continue" expectation.
. A client MUST NOT send an Expect request-header field (section - A client MUST NOT send an Expect request-header field (section
14.20) with the "100-continue" expectation if it does not intend 14.20) with the "100-continue" expectation if it does not intend
to send a request body. 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- - Upon receiving a request which includes an Expect request-header
header field with the "100-continue" expectation, an origin field with the "100-continue" expectation, an origin server MUST
server MUST either respond with 100 (Continue) status and either respond with 100 (Continue) status and continue to read
continue to read from the input stream, or respond with a final from the input stream, or respond with a final status code. The
status code. The origin server MUST NOT wait for the request origin server MUST NOT wait for the request body before sending
body before sending the 100 (Continue) response. If it responds the 100 (Continue) response. If it responds with a final status
with a final status code, it MAY close the transport connection code, it MAY close the transport connection or it MAY continue
or it MAY continue to read and discard the rest of the request. to read and discard the rest of the request. It MUST NOT
It MUST NOT perform the requested method if it returns a final perform the requested method if it returns a final status code.
status code.
. An origin server SHOULD NOT send a 100 (Continue) response if - An origin server SHOULD NOT send a 100 (Continue) response if
the request message does not include an Expect request-header the request message does not include an Expect request-header
field with the "100-continue" expectation, and MUST NOT send a field with the "100-continue" expectation, and MUST NOT send a
100 (Continue) response if such a request comes from an HTTP/1.0 100 (Continue) response if such a request comes from an HTTP/1.0
(or earlier) client. There is an exception to this rule: for (or 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 include an Expect request-header field with the "100- not include an Expect request-header field with the "100-
continue" expectation. This exception, the purpose of which is continue" expectation. This exception, the purpose of which is
to minimize any client processing delays associated with an to minimize any client processing delays associated with an
undeclared wait for 100 (Continue) status, applies only to undeclared wait for 100 (Continue) status, applies only to
HTTP/1.1 requests, and not to requests with any other HTTP- HTTP/1.1 requests, and not to requests with any other HTTP-
version value. version value.
. An origin server MAY omit a 100 (Continue) response if it has - 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 - 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 - 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 final status code before reading the entire request body with a final status code before reading the entire request body
from the transport connection, then the server SHOULD NOT close from the transport connection, then the server SHOULD NOT close
the transport connection until it has read the entire request, the transport connection until it has read the entire request,
or until the client closes the connection. Otherwise, the client or until the client closes the connection. Otherwise, the client
might not reliably receive the response message. However, this might not reliably receive the response message. However, this
requirement is not be construed as preventing a server from requirement is not be construed as preventing a server from
defending itself against denial-of-service attacks, or from defending itself against denial-of-service attacks, or from
badly broken client implementations. badly broken client implementations.
Requirements for HTTP/1.1 proxies: Requirements for HTTP/1.1 proxies:
. If a proxy receives a request that includes an Expect request- - 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, it MUST forward the request, including the Expect header server, it MUST forward the request, including the Expect header
field. field.
. If the proxy knows that the version of the next-hop server is - 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 - Proxies SHOULD maintain a cache recording the HTTP version
numbers received from recently-referenced next-hop servers. numbers received from recently-referenced next-hop servers.
. A proxy MUST NOT forward a 100 (Continue) response if the - A proxy MUST NOT forward a 100 (Continue) response if the
request message was received from an HTTP/1.0 (or earlier) request message was received from an HTTP/1.0 (or earlier)
client and did not include an Expect request-header field with client and did not include an Expect request-header field with
the "100-continue" expectation. This requirement overrides the the "100-continue" expectation. This requirement overrides the
general rule for forwarding of 1xx responses (see section 10.1). general rule for 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, subject to the restrictions in client SHOULD retry the request. If the client does retry this
section 8.2.3. If the client does retry this request, it MAY use the request, it MAY use the following "binary exponential backoff"
following "binary exponential backoff" algorithm to be assured of algorithm to be assured of obtaining a reliable response:
obtaining a reliable response:
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 time is not available. trip 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 of this request. retries 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 (whichever comes first) seconds (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 of the request. body of the request.
7. If client sees that the connection is closed prematurely, repeat 7. If client sees that the connection is closed prematurely,
from step 1 until the request is accepted, an error response is repeat from step 1 until the request is accepted, an error
received, or the user becomes impatient and terminates the retry response is received, or the user becomes impatient and
process. terminates the retry 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 - SHOULD NOT continue and
. 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".
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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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 a new subordinate of the resource
identified by the Request-URI in the Request-Line. POST is designed identified by the Request-URI in the Request-Line. POST is designed
to allow a uniform method to cover the following functions: to allow a uniform method to cover the following functions:
. Annotation of existing resources; - Annotation of existing resources;
. Posting a message to a bulletin board, newsgroup, mailing list, - Posting a message to a bulletin board, newsgroup, mailing list,
or similar group of articles; or similar group of articles;
. Providing a block of data, such as the result of submitting a - Providing a block of data, such as the result of submitting a
form, to a data-handling process; form, to a data-handling process;
. Extending a database through an append operation. - 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. The posted entity
is subordinate to that URI in the same way that a file is subordinate 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 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 newsgroup to which it is posted, or a record is subordinate to a
database. 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
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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 entity- SHOULD reflect the message received back to the client as the
body of a 200 (OK) response. The final recipient is either the origin entity-body of a 200 (OK) response. The final recipient is either the
server or the first proxy or gateway to receive a Max-Forwards value origin server or the first proxy or gateway to receive a Max-Forwards
of zero (0) in the request (see section 14.31). A TRACE request MUST value of zero (0) in the request (see section 14.31). A TRACE request
NOT include an entity. MUST 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
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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 the GET an entity corresponding to the requested resource is sent in
response; the response;
HEAD the entity-header fields corresponding to the requested resource
are sent in the response without any message-body;
POST an entity describing or containing the result of the action; HEAD the entity-header fields corresponding to the requested
resource are sent in the response without any message-body;
TRACE an entity containing the request message as received by POST an entity describing or containing the result of the action;
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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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 - Either a Content-Range header field (section 14.16) indicating
the range included with this response, or a multipart/byteranges the 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 MUST match the actual number of OCTETs transmitted in the value MUST match the actual number of OCTETs transmitted in the
message-body. message-body.
. Date - Date
. ETag and/or Content-Location, if the header would have been sent - ETag and/or Content-Location, if the header would have been sent
in a 200 response to the same request in a 200 response to the same request
. Expires, Cache-Control, and/or Vary, if the field-value might - Expires, Cache-Control, and/or Vary, if the field-value might
differ from that sent in any previous response for the same differ from that sent in any previous response for the same
variant 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 If- include other entity-headers. If the response is the result of an
Range request that used a weak validator, the response MUST NOT If-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
MUST NOT cache 206 (Partial) responses. MUST NOT cache 206 (Partial) responses.
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 of five redirections. Content developers should be maximum of five redirections. Content developers should be aware
aware that there might be clients that implement such a fixed that there might be clients that implement such a fixed
limitation. limitation.
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.
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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 other
than GET or HEAD, the user agent MUST NOT automatically redirect the than GET or HEAD, the user agent MUST NOT automatically redirect the
request unless it can be confirmed by the user, since this might request unless it can be confirmed by the user, since this might
change the conditions under which the request was issued. change the conditions under which the request was issued.
Note: RFC 1945 and RFC 2068 specify that the client is not Note: RFC 1945 and RFC 2068 specify that the client is not allowed
allowed to change the method on the redirected request. to change the method on the redirected request. However, most
However, most existing user agent implementations treat 302 as existing user agent implementations treat 302 as if it were a 303
if it were a 303 response, performing a GET on the Location response, performing a GET on the Location field-value regardless
field-value regardless of the original request method. The of the original request method. The status codes 303 and 307 have
status codes 303 and 307 have been added for servers that wish been added for servers that wish to make unambiguously clear which
to make unambiguously clear which kind of reaction is expected kind of reaction is expected of the client.
of the client.
10.3.4 303 See Other 10.3.4 303 See Other
The response to the request can be found under a different URI and The response to the request can be found under a different URI and
SHOULD be retrieved using a GET method on that resource. This method SHOULD be retrieved using a GET method on that resource. This method
exists primarily to allow the output of a POST-activated script to exists primarily to allow the output of a POST-activated script to
redirect the user agent to a selected resource. The new URI is not a redirect the user agent to a selected resource. The new URI is not a
substitute reference for the originally requested resource. The 303 substitute reference for the originally requested resource. The 303
response MUST NOT be cached, but the response to the second response MUST NOT be cached, but the response to the second
(redirected) request might be cacheable. (redirected) request might be cacheable.
The different URI SHOULD be given by the Location field in the The different 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).
Note: Many pre-HTTP/1.1 user agents do not understand the 303 Note: Many pre-HTTP/1.1 user agents do not understand the 303
status. When interoperability with such clients is a concern, status. When interoperability with such clients is a concern, the
the 302 status code may be used instead, since most user agents 302 status code may be used instead, since most user agents react
react to a 302 response as described here for 303. to a 302 response as described here for 303.
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 - 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.
. ETag and/or Content-Location, if the header would have been sent - ETag and/or Content-Location, if the header would have been sent
in a 200 response to the same request in a 200 response to the same request
. Expires, Cache-Control, and/or Vary, if the field-value might - Expires, Cache-Control, and/or Vary, if the field-value might
differ from that sent in any previous response for the same differ from that sent in any previous response for the same
variant 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.
If a cache uses a received 304 response to update a cache entry, the If a cache uses a received 304 response to update a cache entry, the
cache MUST update the entry to reflect any new field values given in cache MUST update the entry to reflect any new field values given in
the response. the response.
10.3.6 305 Use Proxy 10.3.6 305 Use Proxy
The requested resource MUST be accessed through the proxy given by The requested resource MUST be accessed through the proxy given by
the Location field. The Location field gives the URI of the proxy. the Location field. The Location field gives the URI of the proxy.
The recipient is expected to repeat this single request via the The recipient is expected to repeat this single request via the
proxy. 305 responses MUST only be generated by origin servers. proxy. 305 responses MUST only be generated by origin servers.
Note: RFC 2068 was not clear that 305 was intended to redirect Note: RFC 2068 was not clear that 305 was intended to redirect a
a single request, and to be generated by origin servers only. single request, and to be generated by origin servers only. Not
Not observing these limitations has significant security observing these limitations has significant security consequences.
consequences.
10.3.7 306 (Unused) 10.3.7 306 (Unused)
The 306 status code was used in a previous version of the The 306 status code was used in a previous version of the
specification, is no longer used, and the code is reserved. specification, is no longer used, and the code is reserved.
10.3.8 307 Temporary Redirect 10.3.8 307 Temporary Redirect
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
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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 Note: HTTP/1.1 servers are allowed to return responses which are
are not acceptable according to the accept headers sent in the not acceptable according to the accept headers sent in the
request. In some cases, this may even be preferable to sending request. In some cases, this may even be preferable to sending a
a 406 response. User agents are encouraged to inspect the 406 response. User agents are encouraged to inspect the headers of
headers of an incoming response to determine if it is an incoming response to determine if it is acceptable.
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
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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 can't 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 Content- between the two versions in a format defined by the response
Type. Content-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
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.
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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 server's 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 Content- Length. The client MAY repeat the request if it adds a valid
Length header field containing the length of the message-body in the Content-Length header field containing the length of the message-body
request message. in the 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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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
the range-specifier values in this field overlap the current extent the range-specifier values in this field overlap the current extent
of the selected resource, and the request did not include an If-Range of the selected resource, and the request did not include an If-Range
request-header field. (For byte-ranges, this means that the first- request-header field. (For byte-ranges, this means that the first-
byte-pos of all of the byte-range-spec values were greater than the byte-pos of all of the byte-range-spec values were greater than the
current length of the selected resource.) current length of the selected resource.)
When this status code is returned for a byte-range request, the When this status code is returned for a byte-range request, the
response SHOULD include a Content-Range entity-header field response SHOULD include a Content-Range entity-header field
specifying the current length of the selected resource (see section specifying the current length of the selected resource (see section
14.16). This response MUST NOT use the multipart/byteranges content- 14.16). This response MUST NOT use the multipart/byteranges content-
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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
is that this is a temporary condition which will be alleviated after is that this is a temporary condition which will be alleviated after
some delay. If known, the length of the delay MAY be indicated in a some delay. If known, the length of the delay MAY be indicated in a
Retry-After header. If no Retry-After is given, the client SHOULD Retry-After header. If no Retry-After is given, the client SHOULD
handle the response as it would for a 500 response. handle the response as it would for a 500 response.
Note: The existence of the 503 status code does not imply that Note: The existence of the 503 status code does not imply that a
a server must use it when becoming overloaded. Some servers may server must use it when becoming overloaded. Some servers may wish
wish to simply refuse the connection. to simply refuse the connection.
10.5.5 504 Gateway Timeout 10.5.5 504 Gateway Timeout
The server, while acting as a gateway or proxy, did not receive a The server, while acting as a gateway or proxy, did not receive a
timely response from the upstream server specified by the URI (e.g. timely response from the upstream server specified by the URI (e.g.
HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed HTTP, FTP, LDAP) or some other auxiliary server (e.g. DNS) it needed
to access in attempting to complete the request. to access in attempting to complete the request.
Note: Note to implementors: some deployed proxies are known to Note: Note to implementors: some deployed proxies are known to
return 400 or 500 when DNS lookups time out. return 400 or 500 when DNS lookups time out.
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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 representations may be of the same media type, but alternate representations may be of the same media type, but use
use different capabilities of that type, be in different different capabilities of that type, be in different languages,
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 server- information provided by the origin server in order to provide
driven negotiation for subsequent requests. server-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 be "best" for any given user, since that would require might be "best" for any given user, since that would require
complete knowledge of both the capabilities of the user agent complete knowledge of both the capabilities of the user agent
and the intended use for the response (e.g., does the user want and the intended use for the response (e.g., does the user want
to view it on screen or print it on paper?). to view it on screen or print it on paper?).
2. Having the user agent describe its capabilities in every request 2. Having the user agent describe its capabilities in every
can be both very inefficient (given that only a small percentage request can be both very inefficient (given that only a small
of responses have multiple representations) and a potential percentage of responses have multiple representations) and a
violation of the user's privacy. 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 multiple user's requests. for 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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driven negotiation. See section 13.6 for use of the Vary header field driven negotiation. See section 13.6 for use of the Vary header field
by caches and section 14.44 for use of the Vary header field by by caches and section 14.44 for use of the Vary header field by
servers. servers.
12.2 Agent-driven Negotiation 12.2 Agent-driven Negotiation
With agent-driven negotiation, selection of the best representation With agent-driven negotiation, selection of the best representation
for a response is performed by the user agent after receiving an for a response is performed by the user agent after receiving an
initial response from the origin server. Selection is based on a list initial response from the origin server. Selection is based on a list
of the available representations of the response included within the of the available representations of the response included within the
header fields (this specification reserves the header name header fields or entity-body of the initial response, with each
Alternates) or entity-body of the initial response, with each
representation identified by its own URI. Selection from among the representation identified by its own URI. Selection from among the
representations may be performed automatically (if the user agent is representations may be performed automatically (if the user agent is
capable of doing so) or manually by the user selecting from a capable of doing so) or manually by the user selecting from a
generated (possibly hypertext) menu. generated (possibly hypertext) menu.
Agent-driven negotiation is advantageous when the response would vary Agent-driven negotiation is advantageous when the response would vary
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.
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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
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 - only by an explicit protocol-level request when relaxed by
client or origin server client or origin server
. only with an explicit warning to the end user when relaxed by - only with an explicit warning to the end user when relaxed by
cache 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 is required by all parties. this 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 Note: The server, cache, or client implementor might be faced with
with design decisions not explicitly discussed in this design decisions not explicitly discussed in this specification.
specification. If a decision might affect semantic If a decision might affect semantic transparency, the implementor
transparency, the implementor ought to err on the side of ought to err on the side of maintaining transparency unless a
maintaining transparency unless a careful and complete analysis careful and complete analysis shows significant benefits in
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 origin would have returned by revalidating the response with the
server (section 13.3); origin server (section 13.3);
2. It is "fresh enough" (see section 13.2). In the default case, 2. It is "fresh enough" (see section 13.2). In the default case,
this means it meets the least restrictive freshness requirement this means it meets the least restrictive freshness requirement
of the client, origin server, and cache (see section 14.9); if of the client, origin server, and cache (see section 14.9); if
the origin server so specifies, it is the freshness requirement the origin server so specifies, it is the freshness requirement
of the origin server alone. of the origin server alone.
If a stored response is not "fresh enough" by the most If a stored response is not "fresh enough" by the most
restrictive freshness requirement of both the client and the restrictive freshness requirement of both the client and the
origin server, in carefully considered circumstances the cache origin server, in carefully considered circumstances the cache
MAY still return the response with the appropriate Warning MAY still return the response with the appropriate Warning
header (see section 13.1.5 and 14.46), unless such a response is header (see section 13.1.5 and 14.46), unless such a response
prohibited (e.g., by a "no-store" cache-directive, or by a "no- is prohibited (e.g., by a "no-store" cache-directive, or by a
cache" cache-request-directive; see section 14.9). "no-cache" cache-request-directive; see section 14.9).
3. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect), 3. It is an appropriate 304 (Not Modified), 305 (Proxy Redirect),
or error (4xx or 5xx) response message. 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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In essence, the Age value is the sum of the time that the response In essence, the Age value is the sum of the time that the response
has been resident in each of the caches along the path from the has been resident in each of the caches along the path from the
origin server, plus the amount of time it has been in transit along origin server, plus the amount of time it has been in transit along
network paths. network paths.
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 HTTP/1.1. implement 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.
Because of network-imposed delays, some significant interval might Because of network-imposed delays, some significant interval might
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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- entity-body. Otherwise, it returns a full response (including
body). Thus, we avoid transmitting the full response if the validator entity-body). Thus, we avoid transmitting the full response if the
matches, and we avoid an extra round trip if it does not match. validator matches, and we avoid an extra round trip if it does not
match.
In HTTP/1.1, a conditional request looks exactly the same as a normal 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, Note: a response that lacks a validator may still be cached, and
and served from cache until it expires, unless this is served from cache until it expires, unless this is explicitly
explicitly prohibited by a cache-control directive. However, a prohibited by a cache-control directive. However, a cache cannot
cache cannot do a conditional retrieval if it does not have a do a conditional retrieval if it does not have a validator for the
validator for the entity, which means it will not be entity, which means it will not be refreshable after it expires.
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
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a strong validator as one that changes whenever the bits of an entity a strong validator as one that changes whenever the bits of an entity
changes, while a weak value changes whenever the meaning of an entity changes, while a weak value changes whenever the meaning of an entity
changes. Alternatively, one can think of a strong validator as part changes. Alternatively, one can think of a strong validator as part
of an identifier for a specific entity, while a weak validator is of an identifier for a specific entity, while a weak validator is
part of an identifier for a set of semantically equivalent entities. part of an identifier for a set of semantically equivalent entities.
Note: One example of a strong validator is an integer that is Note: One example of a strong validator is an integer that is
incremented in stable storage every time an entity is changed. incremented in stable storage every time an entity is changed.
An entity's modification time, if represented with one-second An entity's modification time, if represented with one-second
resolution, could be a weak validator, since it is possible resolution, could be a weak validator, since it is possible that
that the resource might be modified twice during a single the resource might be modified twice during a single second.
second.
Support for weak validators is optional. However, weak Support for weak validators is optional. However, weak validators
validators allow for more efficient caching of equivalent allow for more efficient caching of equivalent objects; for
objects; for example, a hit counter on a site is probably good example, a hit counter on a site is probably good enough if it is
enough if it is updated every few days or weeks, and any value updated every few days or weeks, and any value during that period
during that period is likely "good enough" to be equivalent. is likely "good enough" to be equivalent.
A "use" of a validator is either when a client generates a request A "use" of a validator is either when a client generates a request
and includes the validator in a validating header field, or when a and includes the validator in a validating header field, or when a
server compares two validators. server compares two validators.
Strong validators are usable in any context. Weak validators are only Strong validators are usable in any context. Weak validators are only
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
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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.
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 - The strong comparison function: in order to be considered equal,
equal, both validators MUST be identical in every way, and both both validators MUST be identical in every way, and both MUST
MUST NOT be weak. NOT be weak.
. The weak comparison function: in order to be considered equal, - The weak comparison function: in order to be considered equal,
both validators MUST be identical in every way, but either or both validators MUST be identical in every way, but either or
both of them MAY be tagged as "weak" without affecting the both of them MAY be tagged as "weak" without affecting the
result. 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 - The validator is being compared by an origin server to the
actual current validator for the entity and, actual current validator for the entity and,
. That origin server reliably knows that the associated entity - That origin server reliably knows that the associated entity did
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
. The validator is about to be used by a client in an If- - The validator is about to be used by a client in an If-
Modified-Since or If-Unmodified-Since header, because the Modified-Since or If-Unmodified-Since header, because the client
client has a cache entry for the associated entity, and has a cache entry for the associated entity, and
. That cache entry includes a Date value, which gives the time - That cache entry includes a Date value, which gives the time
when the origin server sent the original response, and when the origin server sent the original response, and
. The presented Last-Modified time is at least 60 seconds before - The presented Last-Modified time is at least 60 seconds before
the Date value. the Date value.
or or
. 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
. That cache entry includes a Date value, which gives the time - That cache entry includes a Date value, which gives the time
when the origin server sent the original response, and when the origin server sent the original response, and
. The presented Last-Modified time is at least 60 seconds before - The presented Last-Modified time is at least 60 seconds before
the Date value. 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
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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 - SHOULD send an entity tag validator unless it is not feasible to
to generate one. generate one.
. 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, performance considerations support the use of weak entity tags,
or if it is unfeasible to send a strong entity tag. or if it is unfeasible to send a strong entity tag.
. SHOULD send a Last-Modified value if it is feasible to send - SHOULD send a Last-Modified value if it is feasible to send one,
one, unless the risk of a breakdown in semantic transparency unless the risk of a breakdown in semantic transparency that
that could result from using this date in an If-Modified-Since could result from using this date in an If-Modified-Since header
header would lead to serious problems. 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 for two different entities, or reusing a specific weak value for two different entities, or reusing a specific weak
entity tag value for two semantically different entities. Cache entity tag value for two semantically different entities. Cache
entries might persist for arbitrarily long periods, regardless entries might persist for arbitrarily long periods, regardless of
of expiration times, so it might be inappropriate to expect expiration times, so it might be inappropriate to expect that a
that a cache will never again attempt to validate an entry cache will never again attempt to validate an entry using a
using a validator that it obtained at some point in the past. 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 - If an entity tag has been provided by the origin server, MUST
use that entity tag in any cache-conditional request (using If- use that entity tag in any cache-conditional request (using If-
Match or If-None-Match). Match or If-None-Match).
. If only a Last-Modified value has been provided by the origin - 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).
. 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.
. If both an entity tag and a Last-Modified value have been - If both an entity tag and a Last-Modified value have been
provided by the origin server, SHOULD use both validators in provided by the origin server, SHOULD use both validators in
cache-conditional requests. This allows both HTTP/1.0 and cache-conditional requests. This allows both HTTP/1.0 and
HTTP/1.1 caches 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
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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 transparent and efficient caching, and so HTTP/1.1 support transparent and efficient caching, and so HTTP/1.1 origin
origin servers should provide Last-Modified values. In those servers should provide Last-Modified values. In those rare cases
rare cases where the use of a Last-Modified value as a where the use of a Last-Modified value as a validator by an
validator by an HTTP/1.0 system could result in a serious HTTP/1.0 system could result in a serious problem, then HTTP/1.1
problem, then HTTP/1.1 origin servers should not provide one. origin servers 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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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
that such a response was taken from a cache by comparing the Date that such a response was taken from a cache by comparing the Date
header to the current time. header to the current time.
Note: some HTTP/1.0 caches are known to violate this Note: some HTTP/1.0 caches are known to violate this expectation
expectation without providing any Warning. without providing any Warning.
However, in some cases it might be inappropriate for a cache to However, in some cases it might be inappropriate for a cache to
retain an entity, or to return it in response to a subsequent retain an entity, or to return it in response to a subsequent
request. This might be because absolute semantic transparency is request. This might be because absolute semantic transparency is
deemed necessary by the service author, or because of security or deemed necessary by the service author, or because of security or
privacy considerations. Certain cache-control directives are privacy considerations. Certain cache-control directives are
therefore provided so that the server can indicate that certain therefore provided so that the server can indicate that certain
resource entities, or portions thereof, are not to be cached resource entities, or portions thereof, are not to be cached
regardless of other considerations. regardless of other considerations.
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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.
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 - 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 MUST be stored as part of a cache entry and MUST be responses MUST be stored as part of a cache entry and MUST be
transmitted in any response formed from a cache entry. transmitted in any response formed from a cache entry.
. 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 - Connection
- Keep-Alive
. Keep-Alive - Proxy-Authenticate
- Proxy-Authorization
. Proxy-Authenticate - TE
. Proxy-Authorization - Trailers
- Transfer-Encoding
. Transfer-Encoding - Upgrade
. 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
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 - Content-Location
. Content-MD5 - Content-MD5
. ETag - ETag
. Last-Modified - 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 - 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 header is added, it MUST be given a field-value identical to Expires header is added, it MUST be given a field-value identical to
that of the Date header in that response. that of the 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 - Content-Encoding
. Content-Range - Content-Range
. 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 authentication failures if stronger authentication cause authentication failures if stronger authentication
mechanisms are introduced in later versions of HTTP. Such mechanisms are introduced in later versions of HTTP. Such
authentication mechanisms MAY rely on the values of header authentication mechanisms MAY rely on the values of header fields
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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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
. any stored Warning headers with warn-code 1xx (see section - any stored Warning headers with warn-code 1xx (see section
14.46) MUST be deleted from the cache entry and the forwarded 14.46) MUST be deleted from the cache entry and the forwarded
response. response.
. any stored Warning headers with warn-code 2xx MUST be retained - any stored Warning headers with warn-code 2xx MUST be retained
in the cache entry and the forwarded response. in the cache entry and the forwarded response.
. any end-to-end headers provided in the 304 or 206 response MUST - 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) or a 206 (Partial Content) response to update any Modified) or a 206 (Partial Content) response to update any header
header associated with a previous response for the same entity associated with a previous response for the same entity or sub-
or sub-ranges thereof, although it might not always be ranges thereof, although it might not always be meaningful or
meaningful or correct to do so. This rule does not allow an correct to do so. This rule does not allow an origin server to use
origin server to use a 304 (Not Modified) or a 206 (Partial a 304 (Not Modified) or a 206 (Partial Content) response to
Content) response to entirely delete a header that it had entirely delete a header that it had provided with a previous
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 - Both the incoming response and the cache entry have a cache
validator. validator.
. The two cache validators match using the strong comparison - The two cache validators match using the strong comparison
function (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
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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
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.
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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 - PUT
. DELETE - DELETE
. POST - POST
In order to prevent denial of service attacks, an invalidation based In order to prevent denial of service attacks, an invalidation based
on the URI in a Location or Content-Location header MUST only be on the URI in a Location or Content-Location header MUST only be
performed if the host part is the same as in the Request-URI. performed if the host part is the same as in the Request-URI.
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 Request- understand SHOULD invalidate any entities referred to by the
URI. Request-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
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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 Note: if history list mechanisms unnecessarily prevent users from
from viewing stale resources, this will tend to force service viewing stale resources, this will tend to force service authors
authors to avoid using HTTP expiration controls and cache to avoid using HTTP expiration controls and cache controls when
controls when they would otherwise like to. Service authors may they would otherwise like to. Service authors may consider it
consider it important that users not be presented with error important that users not be presented with error messages or
messages or warning messages when they use navigation controls warning messages when they use navigation controls (such as BACK)
(such as BACK) to view previously fetched resources. Even to view previously fetched resources. Even though sometimes such
though sometimes such resources ought not to cached, or ought resources ought not to cached, or ought to expire quickly, user
to expire quickly, user interface considerations may force interface considerations may force service authors to resort to
service authors to resort to other means of preventing caching other means of preventing caching (e.g. "once-only" URLs) in order
(e.g. "once-only" URLs) in order not to suffer the effects of not to suffer the effects of improperly functioning history
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.
14.1 Accept 14.1 Accept
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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 parameters from Accept extension parameters is due to historical
historical practice. Although this prevents any media type practice. Although this prevents any media type parameter named
parameter named "q" from being used with a media range, such an "q" from being used with a media range, such an event is believed
event is believed to be unlikely given the lack of any "q" to be unlikely given the lack of any "q" parameters in the IANA
parameters in the IANA media type registry and the rare usage media type registry and the rare usage of any media type
of any media type parameters in Accept. Future media types are parameters in Accept. Future media types are discouraged from
discouraged from registering any parameter named "q". registering any 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,
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would cause the following values to be associated: would cause the following values to be associated:
text/html;level=1 = 1 text/html;level=1 = 1
text/html = 0.7 text/html = 0.7
text/plain = 0.3 text/plain = 0.3
image/jpeg = 0.5 image/jpeg = 0.5
text/html;level=2 = 0.4 text/html;level=2 = 0.4
text/html;level=3 = 0.7 text/html;level=3 = 0.7
Note: A user agent might be provided with a default set of Note: A user agent might be provided with a default set of quality
quality values for certain media ranges. However, unless the values for certain media ranges. However, unless the user agent is
user agent is a closed system which cannot interact with other a closed system which cannot interact with other rendering agents,
rendering agents, this default set ought to be configurable by this default set ought to be configurable by the user.
the user.
14.2 Accept-Charset 14.2 Accept-Charset
The Accept-Charset request-header field can be used to indicate what The Accept-Charset request-header field can be used to indicate what
character sets are acceptable for the response. This field allows character sets are acceptable for the response. This field allows
clients capable of understanding more comprehensive or special- clients capable of understanding more comprehensive or special-
purpose character sets to signal that capability to a server which is purpose character sets to signal that capability to a server which is
capable of representing documents in those character sets. capable of representing documents in those character sets.
Accept-Charset = "Accept-Charset" ":" Accept-Charset = "Accept-Charset" ":"
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Accept-Encoding: compress, gzip Accept-Encoding: compress, gzip
Accept-Encoding: Accept-Encoding:
Accept-Encoding: * Accept-Encoding: *
Accept-Encoding: compress;q=0.5, gzip;q=1.0 Accept-Encoding: compress;q=0.5, gzip;q=1.0
Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0 Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
A server tests whether a content-coding is acceptable, according to A server tests whether a content-coding is acceptable, according to
an Accept-Encoding field, using these rules: an Accept-Encoding field, using these rules:
1. If the content-coding is one of the content-codings listed in 1. If the content-coding is one of the content-codings listed in
the Accept-Encoding field, then it is acceptable, unless it is the Accept-Encoding field, then it is acceptable, unless it is
accompanied by a qvalue of 0. (As defined in section 3.9, a qvalue accompanied by a qvalue of 0. (As defined in section 3.9, a
of 0 means "not acceptable.") qvalue of 0 means "not acceptable.")
2. The special "*" symbol in an Accept-Encoding field matches any 2. The special "*" symbol in an Accept-Encoding field matches any
available content-coding not explicitly listed in the header field. available content-coding not explicitly listed in the header
field.
3. If multiple content-codings are acceptable, then the acceptable 3. If multiple content-codings are acceptable, then the acceptable
content-coding with the highest non-zero qvalue is preferred. content-coding with the highest non-zero qvalue is preferred.
4. The "identity" content-coding is always acceptable, unless 4. The "identity" content-coding is always acceptable, unless
specifically refused because the Accept-Encoding field includes specifically refused because the Accept-Encoding field includes
"identity;q=0", or because the field includes "*;q=0" and does not "identity;q=0", or because the field includes "*;q=0" and does
explicitly include the "identity" content-coding. If the Accept- not explicitly include the "identity" content-coding. If the
Encoding field-value is empty, then only the "identity" encoding is Accept-Encoding field-value is empty, then only the "identity"
acceptable. encoding is acceptable.
If an Accept-Encoding field is present in a request, and if the If an Accept-Encoding field is present in a request, and if the
server cannot send a response which is acceptable according to the server cannot send a response which is acceptable according to the
Accept-Encoding header, then the server SHOULD send an error response Accept-Encoding header, then the server SHOULD send an error response
with the 406 (Not Acceptable) status code. with the 406 (Not Acceptable) status code.
If no Accept-Encoding field is present in a request, the server MAY If no Accept-Encoding field is present in a request, the server MAY
assume that the client will accept any content coding. In this case, assume that the client will accept any content coding. In this case,
if "identity" is one of the available content-codings, then the if "identity" is one of the available content-codings, then the
server SHOULD use the "identity" content-coding, unless it has server SHOULD use the "identity" content-coding, unless it has
additional information that a different content-coding is meaningful additional information that a different content-coding is meaningful
to the client. to the client.
Note: If the request does not include an Accept-Encoding field, Note: If the request does not include an Accept-Encoding field,
and if the "identity" content-coding is unavailable, then and if the "identity" content-coding is unavailable, then
content-codings commonly understood by HTTP/1.0 clients (i.e., content-codings commonly understood by HTTP/1.0 clients (i.e.,
"gzip" and "compress") are preferred; some older clients "gzip" and "compress") are preferred; some older clients
improperly display messages sent with other content-codings. improperly display messages sent with other content-codings. The
The server might also make this decision based on information server might also make this decision based on information about
about the particular user-agent or client. the particular user-agent or client.
Note: Most HTTP/1.0 applications do not recognize or obey Note: Most HTTP/1.0 applications do not recognize or obey qvalues
qvalues associated with content-codings. This means that associated with content-codings. This means that qvalues will not
qvalues will not work and are not permitted with x-gzip or x- work and are not permitted with x-gzip or x-compress.
compress.
14.4 Accept-Language 14.4 Accept-Language
The Accept-Language request-header field is similar to Accept, but The Accept-Language request-header field is similar to Accept, but
restricts the set of natural languages that are preferred as a restricts the set of natural languages that are preferred as a
response to the request. Language tags are defined in section 3.10. response to the request. Language tags are defined in section 3.10.
Accept-Language = "Accept-Language" ":" Accept-Language = "Accept-Language" ":"
1#( language-range [ ";" "q" "=" qvalue ] ) 1#( language-range [ ";" "q" "=" qvalue ] )
language-range = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" ) language-range = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )
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would mean: "I prefer Danish, but will accept British English and would mean: "I prefer Danish, but will accept British English and
other types of English." A language-range matches a language-tag if other types of English." A language-range matches a language-tag if
it exactly equals the tag, or if it exactly equals a prefix of the it exactly equals the tag, or if it exactly equals a prefix of the
tag such that the first tag character following the prefix is "-". tag such that the first tag character following the prefix is "-".
The special range "*", if present in the Accept-Language field, The special range "*", if present in the Accept-Language field,
matches every tag not matched by any other range present in the matches every tag not matched by any other range present in the
Accept-Language field. Accept-Language field.
Note: This use of a prefix matching rule does not imply that Note: This use of a prefix matching rule does not imply that
language tags are assigned to languages in such a way that it language tags are assigned to languages in such a way that it is
is always true that if a user understands a language with a always true that if a user understands a language with a certain
certain tag, then this user will also understand all languages tag, then this user will also understand all languages with tags
with tags for which this tag is a prefix. The prefix rule for which this tag is a prefix. The prefix rule simply allows the
simply allows the use of prefix tags if this is the case. use of prefix tags if this is the case.
The language quality factor assigned to a language-tag by the Accept- The language quality factor assigned to a language-tag by the
Language field is the quality value of the longest language-range in Accept-Language field is the quality value of the longest language-
the field that matches the language-tag. If no language-range in the range in the field that matches the language-tag. If no language-
field matches the tag, the language quality factor assigned is 0. If range in the field matches the tag, the language quality factor
no Accept-Language header is present in the request, the server assigned is 0. If no Accept-Language header is present in the
request, the server
SHOULD assume that all languages are equally acceptable. If an SHOULD assume that all languages are equally acceptable. If an
Accept-Language header is present, then all languages which are Accept-Language header is present, then all languages which are
assigned a quality factor greater than 0 are acceptable. assigned a quality factor greater than 0 are acceptable.
It might be contrary to the privacy expectations of the user to send It might be contrary to the privacy expectations of the user to send
an Accept-Language header with the complete linguistic preferences of an Accept-Language header with the complete linguistic preferences of
the user in every request. For a discussion of this issue, see the user in every request. For a discussion of this issue, see
section 15.1.4. section 15.1.4.
As intelligibility is highly dependent on the individual user, it is As intelligibility is highly dependent on the individual user, it is
recommended that client applications make the choice of linguistic recommended that client applications make the choice of linguistic
preference available to the user. If the choice is not made preference available to the user. If the choice is not made
available, then the Accept-Language header field MUST NOT be given in available, then the Accept-Language header field MUST NOT be given in
the request. the request.
Note: When making the choice of linguistic preference available Note: When making the choice of linguistic preference available to
to the user, we remind implementors of the fact that users are the user, we remind implementors of the fact that users are not
not familiar with the details of language matching as described familiar with the details of language matching as described above,
above, and should provide appropriate guidance. As an example, and should provide appropriate guidance. As an example, users
users might assume that on selecting "en-gb", they will be might assume that on selecting "en-gb", they will be served any
served any kind of English document if British English is not kind of English document if British English is not available. A
available. A user agent might suggest in such a case to add user agent might suggest in such a case to add "en" to get the
"en" to get the best matching behavior. best matching behavior.
14.5 Accept-Ranges 14.5 Accept-Ranges
The Accept-Ranges response-header field allows the server to indicate The Accept-Ranges response-header field allows the server to
its acceptance of range requests for a resource: indicate its acceptance of range requests for a resource:
Accept-Ranges = "Accept-Ranges" ":" acceptable-ranges Accept-Ranges = "Accept-Ranges" ":" acceptable-ranges
acceptable-ranges = 1#range-unit | "none" acceptable-ranges = 1#range-unit | "none"
Origin servers that accept byte-range requests MAY send Origin servers that accept byte-range requests MAY send
Accept-Ranges: bytes Accept-Ranges: bytes
but are not required to do so. Clients MAY generate byte-range but are not required to do so. Clients MAY generate byte-range
requests without having received this header for the resource requests without having received this header for the resource
involved. Range units are defined in section 3.12. involved. Range units are defined in section 3.12.
Servers that do not accept any kind of range request for a resource Servers that do not accept any kind of range request for a
MAY send resource MAY send
Accept-Ranges: none Accept-Ranges: none
to advise the client not to attempt a range request. to advise the client not to attempt a range request.
14.6 Age 14.6 Age
The Age response-header field conveys the sender's estimate of the The Age response-header field conveys the sender's estimate of the
amount of time since the response (or its revalidation) was generated amount of time since the response (or its revalidation) was
at the origin server. A cached response is "fresh" if its age does generated at the origin server. A cached response is "fresh" if
not exceed its freshness lifetime. Age values are calculated as its age does not exceed its freshness lifetime. Age values are
specified in section 13.2.3. calculated as specified in section 13.2.3.
Age = "Age" ":" age-value Age = "Age" ":" age-value
age-value = delta-seconds age-value = delta-seconds
Age values are non-negative decimal integers, representing time in Age values are non-negative decimal integers, representing time in
seconds. seconds.
If a cache receives a value larger than the largest positive integer If a cache receives a value larger than the largest positive
it can represent, or if any of its age calculations overflows, it integer it can represent, or if any of its age calculations
MUST transmit an Age header with a value of 2147483648 (2^31). An overflows, it MUST transmit an Age header with a value of
HTTP/1.1 server that includes a cache MUST include an Age header 2147483648 (2^31). An HTTP/1.1 server that includes a cache MUST
field in every response generated from its own cache. Caches SHOULD include an Age header field in every response generated from its
use an arithmetic type of at least 31 bits of range. own cache. Caches SHOULD use an arithmetic type of at least 31
bits of range.
14.7 Allow 14.7 Allow
The Allow entity-header field lists the set of methods supported by The Allow entity-header field lists the set of methods supported
the resource identified by the Request-URI. The purpose of this field by the resource identified by the Request-URI. The purpose of this
is strictly to inform the recipient of valid methods associated with field is strictly to inform the recipient of valid methods
the resource. An Allow header field MUST be present in a 405 (Method associated with the resource. An Allow header field MUST be
Not Allowed) response. present in a 405 (Method Not Allowed) response.
Allow = "Allow" ":" #Method Allow = "Allow" ":" #Method
Example of use: Example of use:
Allow: GET, HEAD, PUT Allow: GET, HEAD, PUT
This field cannot prevent a client from trying other methods. This field cannot prevent a client from trying other methods.
However, the indications given by the Allow header field value SHOULD However, the indications given by the Allow header field value
be followed. The actual set of allowed methods is defined by the SHOULD be followed. The actual set of allowed methods is defined
origin server at the time of each request. by the origin server at the time of each request.
The Allow header field MAY be provided with a PUT request to The Allow header field MAY be provided with a PUT request to
recommend the methods to be supported by the new or modified recommend the methods to be supported by the new or modified
resource. The server is not required to support these methods and resource. The server is not required to support these methods and
SHOULD include an Allow header in the response giving the actual SHOULD include an Allow header in the response giving the actual
supported methods. supported methods.
A proxy MUST NOT modify the Allow header field even if it does not A proxy MUST NOT modify the Allow header field even if it does not
understand all the methods specified, since the user agent might have understand all the methods specified, since the user agent might
other means of communicating with the origin server. have other means of communicating with the origin server.
14.8 Authorization 14.8 Authorization
A user agent that wishes to authenticate itself with a server-- A user agent that wishes to authenticate itself with a server--
usually, but not necessarily, after receiving a 401 response--does so usually, but not necessarily, after receiving a 401 response--does
by including an Authorization request-header field with the request. so by including an Authorization request-header field with the
The Authorization field value consists of credentials containing the request. The Authorization field value consists of credentials
authentication information of the user agent for the realm of the containing the authentication information of the user agent for
resource being requested. the realm of the resource being requested.
Authorization = "Authorization" ":" credentials Authorization = "Authorization" ":" credentials
HTTP access authentication is described in "HTTP Authentication: HTTP access authentication is described in "HTTP Authentication:
Basic and Digest Access Authentication" [43]. If a request is Basic and Digest Access Authentication" [43]. If a request is
authenticated and a realm specified, the same credentials SHOULD be authenticated and a realm specified, the same credentials SHOULD
valid for all other requests within this realm (assuming that the be valid for all other requests within this realm (assuming that
authentication scheme itself does not require otherwise, such as the authentication scheme itself does not require otherwise, such
credentials that vary according to a challenge value or using as credentials that vary according to a challenge value or using
synchronized clocks). synchronized clocks).
When a shared cache (see section 13.7) receives a request containing When a shared cache (see section 13.7) receives a request
an Authorization field, it MUST NOT return the corresponding response containing an Authorization field, it MUST NOT return the
as a reply to any other request, unless one of the following specific corresponding response as a reply to any other request, unless one
exceptions holds: of the following specific exceptions holds:
1. If the response includes the "s-maxage" cache-control directive, 1. If the response includes the "s-maxage" cache-control
the cache MAY use that response in replying to a subsequent directive, the cache MAY use that response in replying to a
request. But (if the specified maximum age has passed) a proxy subsequent request. But (if the specified maximum age has
cache MUST first revalidate it with the origin server, using the passed) a proxy cache MUST first revalidate it with the origin
request-headers from the new request to allow the origin server server, using the request-headers from the new request to allow
to authenticate the new request. (This is the defined behavior the origin server to authenticate the new request. (This is the
for s-maxage.) If the response includes "s-maxage=0", the proxy defined behavior for s-maxage.) If the response includes "s-
MUST always revalidate it before re-using it. maxage=0", the proxy MUST always revalidate it before re-using
it.
2. If the response includes the "must-revalidate" cache-control 2. If the response includes the "must-revalidate" cache-control
directive, the cache MAY use that response in replying to a directive, the cache MAY use that response in replying to a
subsequent request. But if the response is stale, all caches subsequent request. But if the response is stale, all caches
MUST first revalidate it with the origin server, using the MUST first revalidate it with the origin server, using the
request-headers from the new request to allow the origin server request-headers from the new request to allow the origin server
to authenticate the new request. to authenticate the new request.
3. If the response includes the "public" cache-control directive, 3. If the response includes the "public" cache-control directive,
it MAY be returned in reply to any subsequent request. it MAY be returned in reply to any subsequent request.
14.9 Cache-Control 14.9 Cache-Control
The Cache-Control general-header field is used to specify directives The Cache-Control general-header field is used to specify directives
that MUST be obeyed by all caching mechanisms along the that MUST be obeyed by all caching mechanisms along the
request/response chain. The directives specify behavior intended to request/response chain. The directives specify behavior intended to
prevent caches from adversely interfering with the request or prevent caches from adversely interfering with the request or
response. These directives typically override the default caching response. These directives typically override the default caching
algorithms. Cache directives are unidirectional in that the presence algorithms. Cache directives are unidirectional in that the presence
of a directive in a request does not imply that the same directive is of a directive in a request does not imply that the same directive is
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| cache-extension ; Section 14.9.6 | cache-extension ; Section 14.9.6
cache-response-directive = cache-response-directive =
"public" ; Section 14.9.1 "public" ; Section 14.9.1
| "private" [ "=" <"> 1#field-name <"> ] ; Section 14.9.1 | "private" [ "=" <"> 1#field-name <"> ] ; Section 14.9.1
| "no-cache" [ "=" <"> 1#field-name <"> ]; Section 14.9.1 | "no-cache" [ "=" <"> 1#field-name <"> ]; Section 14.9.1
| "no-store" ; Section 14.9.2 | "no-store" ; Section 14.9.2
| "no-transform" ; Section 14.9.5 | "no-transform" ; Section 14.9.5
| "must-revalidate" ; Section 14.9.4 | "must-revalidate" ; Section 14.9.4
| "proxy-revalidate" ; Section 14.9.4 | "proxy-revalidate" ; Section 14.9.4
| "max-age" "=" delta-seconds ; Section 14.9.4 | "max-age" "=" delta-seconds ; Section 14.9.3
| "s-maxage" "=" delta-seconds ; Section 14.9.3 | "s-maxage" "=" delta-seconds ; Section 14.9.3
| cache-extension ; Section 14.9.6 | cache-extension ; Section 14.9.6
cache-extension = token [ "=" ( token | quoted-string ) ] cache-extension = token [ "=" ( token | quoted-string ) ]
When a directive appears without any 1#field-name parameter, the When a directive appears without any 1#field-name parameter, the
directive applies to the entire request or response. When such a directive applies to the entire request or response. When such a
directive appears with a 1#field-name parameter, it applies only to directive appears with a 1#field-name parameter, it applies only to
the named field or fields, and not to the rest of the request or the named field or fields, and not to the rest of the request or
response. This mechanism supports extensibility; implementations of response. This mechanism supports extensibility; implementations of
future versions of the HTTP protocol might apply these directives to future versions of the HTTP protocol might apply these directives to
header fields not defined in HTTP/1.1. header fields not defined in HTTP/1.1.
The cache-control directives can be broken down into these general The cache-control directives can be broken down into these general
categories: categories:
. Restrictions on what are cacheable; these may only be imposed - Restrictions on what are cacheable; these may only be imposed by
by the origin server. the origin server.
. Restrictions on what may be stored by a cache; these may be - Restrictions on what may be stored by a cache; these may be
imposed by either the origin server or the user agent. imposed by either the origin server or the user agent.
. Modifications of the basic expiration mechanism; these may be - Modifications of the basic expiration mechanism; these may be
imposed by either the origin server or the user agent. imposed by either the origin server or the user agent.
. Controls over cache revalidation and reload; these may only be - Controls over cache revalidation and reload; these may only be
imposed by a user agent. imposed by a user agent.
. Control over transformation of entities. - Control over transformation of entities.
. Extensions to the caching system. - Extensions to the caching system.
14.9.1 What is Cacheable 14.9.1 What is Cacheable
By default, a response is cacheable if the requirements of the By default, a response is cacheable if the requirements of the
request method, request header fields, and the response status request method, request header fields, and the response status
indicate that it is cacheable. Section 13.4 summarizes these defaults indicate that it is cacheable. Section 13.4 summarizes these defaults
for cacheability. The following Cache-Control response directives for cacheability. The following Cache-Control response directives
allow an origin server to override the default cacheability of a allow an origin server to override the default cacheability of a
response: response:
public public
Indicates that the response MAY be cached by any cache, even if it Indicates that the response MAY be cached by any cache, even if it
would normally be non-cacheable or cacheable only within a non- would normally be non-cacheable or cacheable only within a non-
shared cache. (See also Authorization, section 14.8, for shared cache. (See also Authorization, section 14.8, for
additional details.) additional details.)
private private
Indicates that all or part of the response message is intended for Indicates that all or part of the response message is intended for
a single user and MUST NOT be cached by a shared cache. This a single user and MUST NOT be cached by a shared cache. This
allows an origin server to state that the specified parts of the allows an origin server to state that the specified parts of the
response are intended for only one user and are not a valid response are intended for only one user and are not a valid
response for requests by other users. A private (non-shared) cache response for requests by other users. A private (non-shared) cache
MAY cache the response. MAY cache the response.
Note: This usage of the word private only controls where the Note: This usage of the word private only controls where the
response may be cached, and cannot ensure the privacy of the response may be cached, and cannot ensure the privacy of the
message content. message content.
no-cache no-cache
If the no-cache directive does not specify a field-name, then a If the no-cache directive does not specify a field-name, then a
cache MUST NOT use the response to satisfy a subsequent request cache MUST NOT use the response to satisfy a subsequent request
without successful revalidation with the origin server. This without successful revalidation with the origin server. This
allows an origin server to prevent caching even by caches that allows an origin server to prevent caching even by caches that
have been configured to return stale responses to client requests. have been configured to return stale responses to client requests.
If the no-cache directive does specify one or more field-names, If the no-cache directive does specify one or more field-names,
then a cache MAY use the response to satisfy a subsequent request, then a cache MAY use the response to satisfy a subsequent request,
subject to any other restrictions on caching. However, the subject to any other restrictions on caching. However, the
specified field-name(s) MUST NOT be sent in the response to a specified field-name(s) MUST NOT be sent in the response to a
subsequent request without successful revalidation with the origin subsequent request without successful revalidation with the origin
server. This allows an origin server to prevent the re-use of server. This allows an origin server to prevent the re-use of
certain header fields in a response, while still allowing caching certain header fields in a response, while still allowing caching
of the rest of the response. of the rest of the response.
Note: Most HTTP/1.0 caches will not recognize or obey this Note: Most HTTP/1.0 caches will not recognize or obey this
directive. directive.
14.9.2 What May be Stored by Caches 14.9.2 What May be Stored by Caches
no-store no-store
The purpose of the no-store directive is to prevent the The purpose of the no-store directive is to prevent the
inadvertent release or retention of sensitive information (for inadvertent release or retention of sensitive information (for
example, on backup tapes). The no-store directive applies to the example, on backup tapes). The no-store directive applies to the
entire message, and MAY be sent either in a response or in a entire message, and MAY be sent either in a response or in a
request. If sent in a request, a cache MUST NOT store any part of request. If sent in a request, a cache MUST NOT store any part of
either this request or any response to it. If sent in a response, either this request or any response to it. If sent in a response,
a cache MUST NOT store any part of either this response or the a cache MUST NOT store any part of either this response or the
request that elicited it. This directive applies to both non- request that elicited it. This directive applies to both non-
shared and shared caches. "MUST NOT store" in this context means shared and shared caches. "MUST NOT store" in this context means
that the cache MUST NOT intentionally store the information in that the cache MUST NOT intentionally store the information in
non-volatile storage, and MUST make a best-effort attempt to non-volatile storage, and MUST make a best-effort attempt to
remove the information from volatile storage as promptly as remove the information from volatile storage as promptly as
possible after forwarding it. possible after forwarding it.
Even when this directive is associated with a response, users Even when this directive is associated with a response, users
might explicitly store such a response outside of the caching might explicitly store such a response outside of the caching
system (e.g., with a "Save As" dialog). History buffers MAY store system (e.g., with a "Save As" dialog). History buffers MAY store
such responses as part of their normal operation. such responses as part of their normal operation.
The purpose of this directive is to meet the stated requirements The purpose of this directive is to meet the stated requirements
of certain users and service authors who are concerned about of certain users and service authors who are concerned about
accidental releases of information via unanticipated accesses to accidental releases of information via unanticipated accesses to
cache data structures. While the use of this directive might cache data structures. While the use of this directive might
improve privacy in some cases, we caution that it is NOT in any improve privacy in some cases, we caution that it is NOT in any
way a reliable or sufficient mechanism for ensuring privacy. In way a reliable or sufficient mechanism for ensuring privacy. In
particular, malicious or compromised caches might not recognize or particular, malicious or compromised caches might not recognize or
obey this directive, and communications networks might be obey this directive, and communications networks might be
vulnerable to eavesdropping. vulnerable to eavesdropping.
14.9.3 Modifications of the Basic Expiration Mechanism 14.9.3 Modifications of the Basic Expiration Mechanism
The expiration time of an entity MAY be specified by the origin The expiration time of an entity MAY be specified by the origin
server using the Expires header (see section 14.21). Alternatively, server using the Expires header (see section 14.21). Alternatively,
it MAY be specified using the max-age directive in a response. When it MAY be specified using the max-age directive in a response. When
the max-age cache-control directive is present in a cached response, the max-age cache-control directive is present in a cached response,
the response is stale if its current age is greater than the age the response is stale if its current age is greater than the age
value given (in seconds) at the time of a new request for that value given (in seconds) at the time of a new request for that
resource. The max-age directive on a response implies that the resource. The max-age directive on a response implies that the
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useful if certain HTTP/1.0 caches improperly calculate ages or useful if certain HTTP/1.0 caches improperly calculate ages or
expiration times, perhaps due to desynchronized clocks. expiration times, perhaps due to desynchronized clocks.
Many HTTP/1.0 cache implementations will treat an Expires value that Many HTTP/1.0 cache implementations will treat an Expires value that
is less than or equal to the response Date value as being equivalent is less than or equal to the response Date value as being equivalent
to the Cache-Control response directive "no-cache". If an HTTP/1.1 to the Cache-Control response directive "no-cache". If an HTTP/1.1
cache receives such a response, and the response does not include a cache receives such a response, and the response does not include a
Cache-Control header field, it SHOULD consider the response to be Cache-Control header field, it SHOULD consider the response to be
non-cacheable in order to retain compatibility with HTTP/1.0 servers. non-cacheable in order to retain compatibility with HTTP/1.0 servers.
Note: An origin server might wish to use a relatively new HTTP Note: An origin server might wish to use a relatively new HTTP
cache control feature, such as the "private" directive, on a cache control feature, such as the "private" directive, on a
network including older caches that do not understand that network including older caches that do not understand that
feature. The origin server will need to combine the new feature feature. The origin server will need to combine the new feature
with an Expires field whose value is less than or equal to the with an Expires field whose value is less than or equal to the
Date value. This will prevent older caches from improperly Date value. This will prevent older caches from improperly
caching the response. caching the response.
s-maxage s-maxage
If a response includes an s-maxage directive, then for a shared If a response includes an s-maxage directive, then for a shared
cache (but not for a private cache), the maximum age specified by cache (but not for a private cache), the maximum age specified by
this directive overrides the maximum age specified by either the this directive overrides the maximum age specified by either the
max-age directive or the Expires header. The s-maxage directive max-age directive or the Expires header. The s-maxage directive
also implies the semantics of the proxy-revalidate directive (see also implies the semantics of the proxy-revalidate directive (see
section 14.9.4), i.e., that the shared cache must not use the section 14.9.4), i.e., that the shared cache must not use the
entry after it becomes stale to respond to a subsequent request entry after it becomes stale to respond to a subsequent request
without first revalidating it with the origin server. The s-maxage without first revalidating it with the origin server. The s-
directive is always ignored by a private cache. maxage directive is always ignored by a private cache.
Note that most older caches, not compliant with this specification, Note that most older caches, not compliant with this specification,
do not implement any cache-control directives. An origin server do not implement any cache-control directives. An origin server
wishing to use a cache-control directive that restricts, but does not wishing to use a cache-control directive that restricts, but does not
prevent, caching by an HTTP/1.1-compliant cache MAY exploit the prevent, caching by an HTTP/1.1-compliant cache MAY exploit the
requirement that the max-age directive overrides the Expires header, requirement that the max-age directive overrides the Expires header,
and the fact that pre-HTTP/1.1-compliant caches do not observe the and the fact that pre-HTTP/1.1-compliant caches do not observe the
max-age directive. max-age directive.
Other directives allow a user agent to modify the basic expiration Other directives allow a user agent to modify the basic expiration
mechanism. These directives MAY be specified on a request: mechanism. These directives MAY be specified on a request:
max-age max-age
Indicates that the client is willing to accept a response whose Indicates that the client is willing to accept a response whose
age is no greater than the specified time in seconds. Unless max- age is no greater than the specified time in seconds. Unless max-
stale directive is also included, the client is not willing to stale directive is also included, the client is not willing to
accept a stale response. accept a stale response.
min-fresh min-fresh
Indicates that the client is willing to accept a response whose Indicates that the client is willing to accept a response whose
freshness lifetime is no less than its current age plus the freshness lifetime is no less than its current age plus the
specified time in seconds. That is, the client wants a response specified time in seconds. That is, the client wants a response
that will still be fresh for at least the specified number of that will still be fresh for at least the specified number of
seconds. seconds.
max-stale max-stale
Indicates that the client is willing to accept a response that has Indicates that the client is willing to accept a response that has
exceeded its expiration time. If max-stale is assigned a value, exceeded its expiration time. If max-stale is assigned a value,
then the client is willing to accept a response that has exceeded then the client is willing to accept a response that has exceeded
its expiration time by no more than the specified number of its expiration time by no more than the specified number of
seconds. If no value is assigned to max-stale, then the client is seconds. If no value is assigned to max-stale, then the client is
willing to accept a stale response of any age. willing to accept a stale response of any age.
If a cache returns a stale response, either because of a max-stale If a cache returns a stale response, either because of a max-stale
directive on a request, or because the cache is configured to directive on a request, or because the cache is configured to
override the expiration time of a response, the cache MUST attach a override the expiration time of a response, the cache MUST attach a
Warning header to the stale response, using Warning 110 (Response is Warning header to the stale response, using Warning 110 (Response is
stale). stale).
A cache MAY be configured to return stale responses without A cache MAY be configured to return stale responses without
validation, but only if this does not conflict with any "MUST"-level validation, but only if this does not conflict with any "MUST"-level
requirements concerning cache validation (e.g., a "must-revalidate" requirements concerning cache validation (e.g., a "must-revalidate"
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End-to-end revalidation may be requested either when the client does End-to-end revalidation may be requested either when the client does
not have its own local cached copy, in which case we call it not have its own local cached copy, in which case we call it
"unspecified end-to-end revalidation", or when the client does have a "unspecified end-to-end revalidation", or when the client does have a
local cached copy, in which case we call it "specific end-to-end local cached copy, in which case we call it "specific end-to-end
revalidation." revalidation."
The client can specify these three kinds of action using Cache- The client can specify these three kinds of action using Cache-
Control request directives: Control request directives:
End-to-end reload End-to-end reload
The request includes a "no-cache" cache-control directive or, for The request includes a "no-cache" cache-control directive or, for
compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
names MUST NOT be included with the no-cache directive in a names MUST NOT be included with the no-cache directive in a
request. The server MUST NOT use a cached copy when responding to request. The server MUST NOT use a cached copy when responding to
such a request. such a request.
Specific end-to-end revalidation Specific end-to-end revalidation
The request includes a "max-age=0" cache-control directive, which The request includes a "max-age=0" cache-control directive, which
forces each cache along the path to the origin server to forces each cache along the path to the origin server to
revalidate its own entry, if any, with the next cache or server. revalidate its own entry, if any, with the next cache or server.
The initial request includes a cache-validating conditional with
The initial request includes a cache-validating conditional with the client's current validator.
the client's current validator.
Unspecified end-to-end revalidation Unspecified end-to-end revalidation
The request includes "max-age=0" cache-control directive, which The request includes "max-age=0" cache-control directive, which
forces each cache along the path to the origin server to forces each cache along the path to the origin server to
revalidate its own entry, if any, with the next cache or server. revalidate its own entry, if any, with the next cache or server.
The initial request does not include a cache-validating The initial request does not include a cache-validating
conditional; the first cache along the path (if any) that holds a conditional; the first cache along the path (if any) that holds a
cache entry for this resource includes a cache-validating cache entry for this resource includes a cache-validating
conditional with its current validator. conditional with its current validator.
max-age max-age
When an intermediate cache is forced, by means of a max-age=0 When an intermediate cache is forced, by means of a max-age=0
directive, to revalidate its own cache entry, and the client has directive, to revalidate its own cache entry, and the client has
supplied its own validator in the request, the supplied validator supplied its own validator in the request, the supplied validator
might differ from the validator currently stored with the cache might differ from the validator currently stored with the cache
entry. In this case, the cache MAY use either validator in making entry. In this case, the cache MAY use either validator in making
its own request without affecting semantic transparency. its own request without affecting semantic transparency.
However, the choice of validator might affect performance. The However, the choice of validator might affect performance. The
best approach is for the intermediate cache to use its own best approach is for the intermediate cache to use its own
validator when making its request. If the server replies with 304 validator when making its request. If the server replies with 304
(Not Modified), then the cache can return its now validated copy (Not Modified), then the cache can return its now validated copy
to the client with a 200 (OK) response. If the server replies with to the client with a 200 (OK) response. If the server replies with
a new entity and cache validator, however, the intermediate cache a new entity and cache validator, however, the intermediate cache
can compare the returned validator with the one provided in the can compare the returned validator with the one provided in the
client's request, using the strong comparison function. If the client's request, using the strong comparison function. If the
client's validator is equal to the origin server's, then the client's validator is equal to the origin server's, then the
intermediate cache simply returns 304 (Not Modified). Otherwise, intermediate cache simply returns 304 (Not Modified). Otherwise,
it returns the new entity with a 200 (OK) response. it returns the new entity with a 200 (OK) response.
If a request includes the no-cache directive, it SHOULD NOT If a request includes the no-cache directive, it SHOULD NOT
include min-fresh, max-stale, or max-age. include min-fresh, max-stale, or max-age.
only-if-cached only-if-cached
In some cases, such as times of extremely poor network In some cases, such as times of extremely poor network
connectivity, a client may want a cache to return only those connectivity, a client may want a cache to return only those
responses that it currently has stored, and not to reload or responses that it currently has stored, and not to reload or
revalidate with the origin server. To do this, the client may revalidate with the origin server. To do this, the client may
include the only-if-cached directive in a request. If it receives include the only-if-cached directive in a request. If it receives
this directive, a cache SHOULD either respond using a cached entry this directive, a cache SHOULD either respond using a cached entry
that is consistent with the other constraints of the request, or that is consistent with the other constraints of the request, or
respond with a 504 (Gateway Timeout) status. However, if a group respond with a 504 (Gateway Timeout) status. However, if a group
of caches is being operated as a unified system with good internal of caches is being operated as a unified system with good internal
connectivity, such a request MAY be forwarded within that group of connectivity, such a request MAY be forwarded within that group of
caches. caches.
must-revalidate must-revalidate
Because a cache MAY be configured to ignore a server's specified Because a cache MAY be configured to ignore a server's specified
expiration time, and because a client request MAY include a max- expiration time, and because a client request MAY include a max-
stale directive (which has a similar effect), the protocol also stale directive (which has a similar effect), the protocol also
includes a mechanism for the origin server to require revalidation includes a mechanism for the origin server to require revalidation
of a cache entry on any subsequent use. When the must-revalidate of a cache entry on any subsequent use. When the must-revalidate
directive is present in a response received by a cache, that cache directive is present in a response received by a cache, that cache
MUST NOT use the entry after it becomes stale to respond to a MUST NOT use the entry after it becomes stale to respond to a
subsequent request without first revalidating it with the origin subsequent request without first revalidating it with the origin
server. (I.e., the cache MUST do an end-to-end revalidation every server. (I.e., the cache MUST do an end-to-end revalidation every
time, if, based solely on the origin server's Expires or max-age time, if, based solely on the origin server's Expires or max-age
value, the cached response is stale.) value, the cached response is stale.)
The must-revalidate directive is necessary to support reliable The must-revalidate directive is necessary to support reliable
operation for certain protocol features. In all circumstances an operation for certain protocol features. In all circumstances an
HTTP/1.1 cache MUST obey the must-revalidate directive; in HTTP/1.1 cache MUST obey the must-revalidate directive; in
particular, if the cache cannot reach the origin server for any particular, if the cache cannot reach the origin server for any
reason, it MUST generate a 504 (Gateway Timeout) response. reason, it MUST generate a 504 (Gateway Timeout) response.
Servers SHOULD send the must-revalidate directive if and only if Servers SHOULD send the must-revalidate directive if and only if
failure to revalidate a request on the entity could result in failure to revalidate a request on the entity could result in
incorrect operation, such as a silently unexecuted financial incorrect operation, such as a silently unexecuted financial
transaction. Recipients MUST NOT take any automated action that transaction. Recipients MUST NOT take any automated action that
violates this directive, and MUST NOT automatically provide an violates this directive, and MUST NOT automatically provide an
unvalidated copy of the entity if revalidation fails. unvalidated copy of the entity if revalidation fails.
Although this is not recommended, user agents operating under Although this is not recommended, user agents operating under
severe connectivity constraints MAY violate this directive but, if severe connectivity constraints MAY violate this directive but, if
so, MUST explicitly warn the user that an unvalidated response has so, MUST explicitly warn the user that an unvalidated response has
been provided. The warning MUST be provided on each unvalidated been provided. The warning MUST be provided on each unvalidated
access, and SHOULD require explicit user confirmation. access, and SHOULD require explicit user confirmation.
proxy-revalidate proxy-revalidate
The proxy-revalidate directive has the same meaning as the must- The proxy-revalidate directive has the same meaning as the must-
revalidate directive, except that it does not apply to non-shared revalidate directive, except that it does not apply to non-shared
user agent caches. It can be used on a response to an user agent caches. It can be used on a response to an
authenticated request to permit the user's cache to store and authenticated request to permit the user's cache to store and
later return the response without needing to revalidate it (since later return the response without needing to revalidate it (since
it has already been authenticated once by that user), while still it has already been authenticated once by that user), while still
requiring proxies that service many users to revalidate each time requiring proxies that service many users to revalidate each time
(in order to make sure that each user has been authenticated). (in order to make sure that each user has been authenticated).
Note that such authenticated responses also need the public cache Note that such authenticated responses also need the public cache
control directive in order to allow them to be cached at all. control directive in order to allow them to be cached at all.
14.9.5 No-Transform Directive 14.9.5 No-Transform Directive
no-transform no-transform
Implementors of intermediate caches (proxies) have found it useful Implementors of intermediate caches (proxies) have found it useful
to convert the media type of certain entity bodies. A non- to convert the media type of certain entity bodies. A non-
transparent proxy might, for example, convert between image transparent proxy might, for example, convert between image
formats in order to save cache space or to reduce the amount of formats in order to save cache space or to reduce the amount of
traffic on a slow link. traffic on a slow link.
Serious operational problems occur, however, when these Serious operational problems occur, however, when these
transformations are applied to entity bodies intended for certain transformations are applied to entity bodies intended for certain
kinds of applications. For example, applications for medical kinds of applications. For example, applications for medical
imaging, scientific data analysis and those using end-to-end imaging, scientific data analysis and those using end-to-end
authentication, all depend on receiving an entity body that is bit authentication, all depend on receiving an entity body that is bit
for bit identical to the original entity-body. for bit identical to the original entity-body.
Therefore, if a message includes the no-transform directive, an Therefore, if a message includes the no-transform directive, an
intermediate cache or proxy MUST NOT change those headers that are intermediate cache or proxy MUST NOT change those headers that are
listed in section 13.5.2 as being subject to the no-transform listed in section 13.5.2 as being subject to the no-transform
directive. This implies that the cache or proxy MUST NOT change directive. This implies that the cache or proxy MUST NOT change
any aspect of the entity-body that is specified by these headers, any aspect of the entity-body that is specified by these headers,
including the value of the entity-body itself. including the value of the entity-body itself.
14.9.6 Cache Control Extensions 14.9.6 Cache Control Extensions
The Cache-Control header field can be extended through the use of one The Cache-Control header field can be extended through the use of one
or more cache-extension tokens, each with an optional assigned value. or more cache-extension tokens, each with an optional assigned value.
Informational extensions (those which do not require a change in Informational extensions (those which do not require a change in
cache behavior) MAY be added without changing the semantics of other cache behavior) MAY be added without changing the semantics of other
directives. Behavioral extensions are designed to work by acting as directives. Behavioral extensions are designed to work by acting as
modifiers to the existing base of cache directives. Both the new modifiers to the existing base of cache directives. Both the new
directive and the standard directive are supplied, such that directive and the standard directive are supplied, such that
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HTTP/1.1 proxies MUST parse the Connection header field before a HTTP/1.1 proxies MUST parse the Connection header field before a
message is forwarded and, for each connection-token in this field, message is forwarded and, for each connection-token in this field,
remove any header field(s) from the message with the same name as the remove any header field(s) from the message with the same name as the
connection-token. Connection options are signaled by the presence of connection-token. Connection options are signaled by the presence of
a connection-token in the Connection header field, not by any a connection-token in the Connection header field, not by any
corresponding additional header field(s), since the additional header corresponding additional header field(s), since the additional header
field may not be sent if there are no parameters associated with that field may not be sent if there are no parameters associated with that
connection option. connection option.
Message headers listed in the Connection header MUST NOT include end- Message headers listed in the Connection header MUST NOT include
to-end headers, such as Cache-Control. end-to-end headers, such as Cache-Control.
HTTP/1.1 defines the "close" connection option for the sender to HTTP/1.1 defines the "close" connection option for the sender to
signal that the connection will be closed after completion of the signal that the connection will be closed after completion of the
response. For example, response. For example,
Connection: close Connection: close
in either the request or the response header fields indicates that in either the request or the response header fields indicates that
the connection SHOULD NOT be considered `persistent' (section 8.1) the connection SHOULD NOT be considered `persistent' (section 8.1)
after the current request/response is complete. after the current request/response is complete.
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Content-Encoding: gzip Content-Encoding: gzip
The content-coding is a characteristic of the entity identified by The content-coding is a characteristic of the entity identified by
the Request-URI. Typically, the entity-body is stored with this the Request-URI. Typically, the entity-body is stored with this
encoding and is only decoded before rendering or analogous usage. encoding and is only decoded before rendering or analogous usage.
However, a non-transparent proxy MAY modify the content-coding if the However, a non-transparent proxy MAY modify the content-coding if the
new coding is known to be acceptable to the recipient, unless the new coding is known to be acceptable to the recipient, unless the
"no-transform" cache-control directive is present in the message. "no-transform" cache-control directive is present in the message.
If the content-coding of an entity is not "identity", then the If the content-coding of an entity is not "identity", then the
response MUST including a Content-Encoding entity-header (section response MUST include a Content-Encoding entity-header (section
14.11) that lists the non-identity content-coding(s) used. 14.11) that lists the non-identity content-coding(s) used.
If the content-coding of an entity in a request message is not If the content-coding of an entity in a request message is not
acceptable to the origin server, the server SHOULD respond with a acceptable to the origin server, the server SHOULD respond with a
status code of 415 (Unsupported Media Type). status code of 415 (Unsupported Media Type).
If multiple encodings have been applied to an entity, the content If multiple encodings have been applied to an entity, the content
codings MUST be listed in the order in which they were applied. codings MUST be listed in the order in which they were applied.
Additional information about the encoding parameters MAY be provided Additional information about the encoding parameters MAY be provided
by other entity-header fields not defined by this specification. by other entity-header fields not defined by this specification.
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Multiple languages MAY be listed for content that is intended for Multiple languages MAY be listed for content that is intended for
multiple audiences. For example, a rendition of the "Treaty of multiple audiences. For example, a rendition of the "Treaty of
Waitangi," presented simultaneously in the original Maori and English Waitangi," presented simultaneously in the original Maori and English
versions, would call for versions, would call for
Content-Language: mi, en Content-Language: mi, en
However, just because multiple languages are present within an entity However, just because multiple languages are present within an entity
does not mean that it is intended for multiple linguistic audiences. does not mean that it is intended for multiple linguistic audiences.
An example would be a beginner's language primer, such as "A First An example would be a beginner's language primer, such as "A First
Lesson in Latin," which is clearly intended to be used by an English- Lesson in Latin," which is clearly intended to be used by an
literate audience. In this case, the Content-Language would properly English-literate audience. In this case, the Content-Language would
only include "en". properly only include "en".
Content-Language MAY be applied to any media type -- it is not Content-Language MAY be applied to any media type -- it is not
limited to textual documents. limited to textual documents.
14.13 Content-Length 14.13 Content-Length
The Content-Length entity-header field indicates the size of the The Content-Length entity-header field indicates the size of the
entity-body, in decimal number of OCTETs, sent to the recipient or, entity-body, in decimal number of OCTETs, sent to the recipient or,
in the case of the HEAD method, the size of the entity-body that in the case of the HEAD method, the size of the entity-body that
would have been sent had the request been a GET. would have been sent had the request been a GET.
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If the Content-Location is a relative URI, the relative URI is If the Content-Location is a relative URI, the relative URI is
interpreted relative to the Request-URI. interpreted relative to the Request-URI.
The meaning of the Content-Location header in PUT or POST requests is The meaning of the Content-Location header in PUT or POST requests is
undefined; servers are free to ignore it in those cases. undefined; servers are free to ignore it in those cases.
14.15 Content-MD5 14.15 Content-MD5
The Content-MD5 entity-header field, as defined in RFC 1864 [23], is The Content-MD5 entity-header field, as defined in RFC 1864 [23], is
an MD5 digest of the entity-body for the purpose of providing an end- an MD5 digest of the entity-body for the purpose of providing an
to-end message integrity check (MIC) of the entity-body. (Note: a MIC end-to-end message integrity check (MIC) of the entity-body. (Note: a
is good for detecting accidental modification of the entity-body in MIC is good for detecting accidental modification of the entity-body
transit, but is not proof against malicious attacks.) in transit, but is not proof against malicious attacks.)
Content-MD5 = "Content-MD5" ":" md5-digest Content-MD5 = "Content-MD5" ":" md5-digest
md5-digest = <base64 of 128 bit MD5 digest as per RFC 1864> md5-digest = <base64 of 128 bit MD5 digest as per RFC 1864>
The Content-MD5 header field MAY be generated by an origin server or The Content-MD5 header field MAY be generated by an origin server or
client to function as an integrity check of the entity-body. Only client to function as an integrity check of the entity-body. Only
origin servers or clients MAY generate the Content-MD5 header field; origin servers or clients MAY generate the Content-MD5 header field;
proxies and gateways MUST NOT generate it, as this would defeat its proxies and gateways MUST NOT generate it, as this would defeat its
value as an end-to-end integrity check. Any recipient of the entity- value as an end-to-end integrity check. Any recipient of the entity-
body, including gateways and proxies, MAY check that the digest value body, including gateways and proxies, MAY check that the digest value
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of the body-part has had the encoding applied, and the body-part is of the body-part has had the encoding applied, and the body-part is
included in the Content-MD5 digest as is -- i.e., after the included in the Content-MD5 digest as is -- i.e., after the
application. The Transfer-Encoding header field is not allowed within application. The Transfer-Encoding header field is not allowed within
body-parts. body-parts.
Conversion of all line breaks to CRLF MUST NOT be done before Conversion of all line breaks to CRLF MUST NOT be done before
computing or checking the digest: the line break convention used in computing or checking the digest: the line break convention used in
the text actually transmitted MUST be left unaltered when computing the text actually transmitted MUST be left unaltered when computing
the digest. the digest.
Note: while the definition of Content-MD5 is exactly the same Note: while the definition of Content-MD5 is exactly the same for
for HTTP as in RFC 1864 for MIME entity-bodies, there are HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
several ways in which the application of Content-MD5 to HTTP in which the application of Content-MD5 to HTTP entity-bodies
entity-bodies differs from its application to MIME entity- differs from its application to MIME entity-bodies. One is that
bodies. One is that HTTP, unlike MIME, does not use Content- HTTP, unlike MIME, does not use Content-Transfer-Encoding, and
Transfer-Encoding, and does use Transfer-Encoding and Content- does use Transfer-Encoding and Content-Encoding. Another is that
Encoding. Another is that HTTP more frequently uses binary HTTP more frequently uses binary content types than MIME, so it is
content types than MIME, so it is worth noting that, in such worth noting that, in such cases, the byte order used to compute
cases, the byte order used to compute the digest is the the digest is the transmission byte order defined for the type.
transmission byte order defined for the type. Lastly, HTTP Lastly, HTTP allows transmission of text types with any of several
allows transmission of text types with any of several line line break conventions and not just the canonical form using CRLF.
break conventions and not just the canonical form using CRLF.
14.16 Content-Range 14.16 Content-Range
The Content-Range entity-header is sent with a partial entity-body to The Content-Range entity-header is sent with a partial entity-body to
specify where in the full entity-body the partial body should be specify where in the full entity-body the partial body should be
applied. Range units are defined in section 3.12. applied. Range units are defined in section 3.12.
Content-Range = "Content-Range" ":" content-range-spec Content-Range = "Content-Range" ":" content-range-spec
content-range-spec = byte-content-range-spec content-range-spec = byte-content-range-spec
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When an HTTP message includes the content of multiple ranges (for When an HTTP message includes the content of multiple ranges (for
example, a response to a request for multiple non-overlapping example, a response to a request for multiple non-overlapping
ranges), these are transmitted as a multipart message. The multipart ranges), these are transmitted as a multipart message. The multipart
media type used for this purpose is "multipart/byteranges" as defined media type used for this purpose is "multipart/byteranges" as defined
in appendix 19.2. See appendix 19.6.3 for a compatibility issue. in appendix 19.2. See appendix 19.6.3 for a compatibility issue.
A response to a request for a single range MUST NOT be sent using the A response to a request for a single range MUST NOT be sent using the
multipart/byteranges media type. A response to a request for multipart/byteranges media type. A response to a request for
multiple ranges, whose result is a single range, MAY be sent as a multiple ranges, whose result is a single range, MAY be sent as a
multipart/byteranges media type with one part. A client that cannot multipart/byteranges media type with one part. A client that cannot
decode a multipart/byteranges message MUST NOT ask for multiple byte- decode a multipart/byteranges message MUST NOT ask for multiple
ranges in a single request. byte-ranges in a single request.
When a client requests multiple byte-ranges in one request, the When a client requests multiple byte-ranges in one request, the
server SHOULD return them in the order that they appeared in the server SHOULD return them in the order that they appeared in the
request. request.
If the server ignores a byte-range-spec because it is syntactically If the server ignores a byte-range-spec because it is syntactically
invalid, the server SHOULD treat the request as if the invalid Range invalid, the server SHOULD treat the request as if the invalid Range
header field did not exist. (Normally, this means return a 200 header field did not exist. (Normally, this means return a 200
response containing the full entity). response containing the full entity).
If the server receives a request (other than one including an If- If the server receives a request (other than one including an If-
Range request-header field) with an unsatisfiable Range request- Range request-header field) with an unsatisfiable Range request-
header field (that is, all of whose byte-range-spec values have a header field (that is, all of whose byte-range-spec values have a
first-byte-pos value greater than the current length of the selected first-byte-pos value greater than the current length of the selected
resource), it SHOULD return a response code of 416 (Requested range resource), it SHOULD return a response code of 416 (Requested range
not satisfiable) (section 10.4.17). not satisfiable) (section 10.4.17).
Note: clients cannot depend on servers to send a 416 (Requested Note: clients cannot depend on servers to send a 416 (Requested
range not satisfiable) response instead of a 200 (OK) response range not satisfiable) response instead of a 200 (OK) response for
for an unsatisfiable Range request-header, since not all an unsatisfiable Range request-header, since not all servers
servers implement this request-header. implement this request-header.
14.17 Content-Type 14.17 Content-Type
The Content-Type entity-header field indicates the media type of the The Content-Type entity-header field indicates the media type of the
entity-body sent to the recipient or, in the case of the HEAD method, entity-body sent to the recipient or, in the case of the HEAD method,
the media type that would have been sent had the request been a GET. the media type that would have been sent had the request been a GET.
Content-Type = "Content-Type" ":" media-type Content-Type = "Content-Type" ":" media-type
Media types are defined in section 3.7. An example of the field is Media types are defined in section 3.7. An example of the field is
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Date = "Date" ":" HTTP-date Date = "Date" ":" HTTP-date
An example is An example is
Date: Tue, 15 Nov 1994 08:12:31 GMT Date: Tue, 15 Nov 1994 08:12:31 GMT
Origin servers MUST include a Date header field in all responses, Origin servers MUST include a Date header field in all responses,
except in these cases: except in these cases:
1. If the response status code is 100 (Continue) or 101 (Switching 1. If the response status code is 100 (Continue) or 101 (Switching
Protocols), the response MAY include a Date header field, at the Protocols), the response MAY include a Date header field, at
server's option. the server's option.
2. If the response status code conveys a server error, e.g. 500 2. If the response status code conveys a server error, e.g. 500
(Internal Server Error) or 503 (Service Unavailable), and it is (Internal Server Error) or 503 (Service Unavailable), and it is
inconvenient or impossible to generate a valid Date. inconvenient or impossible to generate a valid Date.
3. If the server does not have a clock that can provide a 3. If the server does not have a clock that can provide a
reasonable approximation of the current time, its responses MUST reasonable approximation of the current time, its responses
NOT include a Date header field. In this case, the rules in MUST NOT include a Date header field. In this case, the rules
section 14.18.1 MUST be followed. in section 14.18.1 MUST be followed.
A received message that does not have a Date header field MUST be A received message that does not have a Date header field MUST be
assigned one by the recipient if the message will be cached by that assigned one by the recipient if the message will be cached by that
recipient or gatewayed via a protocol which requires a Date. An HTTP recipient or gatewayed via a protocol which requires a Date. An HTTP
implementation without a clock MUST NOT cache responses without implementation without a clock MUST NOT cache responses without
revalidating them on every use. An HTTP cache, especially a shared revalidating them on every use. An HTTP cache, especially a shared
cache, SHOULD use a mechanism, such as NTP [28], to synchronize its cache, SHOULD use a mechanism, such as NTP [28], to synchronize its
clock with a reliable external standard. clock with a reliable external standard.
Clients SHOULD only send a Date header field in messages that include Clients SHOULD only send a Date header field in messages that include
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future extensions. If a server receives a request containing an future extensions. If a server receives a request containing an
Expect field that includes an expectation-extension that it does not Expect field that includes an expectation-extension that it does not
support, it MUST respond with a 417 (Expectation Failed) status. support, it MUST respond with a 417 (Expectation Failed) status.
Comparison of expectation values is case-insensitive for unquoted Comparison of expectation values is case-insensitive for unquoted
tokens (including the 100-continue token), and is case-sensitive for tokens (including the 100-continue token), and is case-sensitive for
quoted-string expectation-extensions. quoted-string expectation-extensions.
The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST
return a 417 (Expectation Failed) status if it receives a request return a 417 (Expectation Failed) status if it receives a request
with an expectation that it cannot meet. However, the Expect request- with an expectation that it cannot meet. However, the Expect
header itself is end-to-end; it MUST be forwarded if the request is request-header itself is end-to-end; it MUST be forwarded if the
forwarded. request is forwarded.
Many older HTTP/1.0 and HTTP/1.1 applications do not understand the Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
Expect header. Expect header.
See section 8.2.3 for the use of the 100 (continue) status. See section 8.2.3 for the use of the 100 (continue) status.
14.21 Expires 14.21 Expires
The Expires entity-header field gives the date/time after which the The Expires entity-header field gives the date/time after which the
response is considered stale. A stale cache entry may not normally be response is considered stale. A stale cache entry may not normally be
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The format is an absolute date and time as defined by HTTP-date in The format is an absolute date and time as defined by HTTP-date in
section 3.3.1; it MUST be in RFC 1123 date format: section 3.3.1; it MUST be in RFC 1123 date format:
Expires = "Expires" ":" HTTP-date Expires = "Expires" ":" HTTP-date
An example of its use is An example of its use is
Expires: Thu, 01 Dec 1994 16:00:00 GMT Expires: Thu, 01 Dec 1994 16:00:00 GMT
Note: if a response includes a Cache-Control field with the Note: if a response includes a Cache-Control field with the max-
max-age directive (see section 14.9.3), that directive age directive (see section 14.9.3), that directive overrides the
overrides the Expires field. Expires field.
HTTP/1.1 clients and caches MUST treat other invalid date formats, HTTP/1.1 clients and caches MUST treat other invalid date formats,
especially including the value "0", as in the past (i.e., "already especially including the value "0", as in the past (i.e., "already
expired"). expired").
To mark a response as "already expired," an origin server sends an To mark a response as "already expired," an origin server sends an
Expires date that is equal to the Date header value. (See the rules Expires date that is equal to the Date header value. (See the rules
for expiration calculations in section 13.2.4.) for expiration calculations in section 13.2.4.)
To mark a response as "never expires," an origin server sends an To mark a response as "never expires," an origin server sends an
Expires date approximately one year from the time the response is Expires date approximately one year from the time the response is
sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one
year in the future. year in the future.
The presence of an Expires header field with a date value of some The presence of an Expires header field with a date value of some
time in the future on a response that otherwise would by default be time in the future on a response that otherwise would by default be
non-cacheable indicates that the response is cacheable, unless non-cacheable indicates that the response is cacheable, unless
indicated otherwise by a Cache-Control header field (section 14.9). indicated otherwise by a Cache-Control header field (section 14.9).
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An example of the field is: An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A GET method with an If-Modified-Since header and no Range header A GET method with an If-Modified-Since header and no Range header
requests that the identified entity be transferred only if it has requests that the identified entity be transferred only if it has
been modified since the date given by the If-Modified-Since header. been modified since the date given by the If-Modified-Since header.
The algorithm for determining this includes the following cases: The algorithm for determining this includes the following cases:
a) If the request would normally result in anything other than a a) If the request would normally result in anything other than a
200 (OK) status, or if the passed If-Modified-Since date is 200 (OK) status, or if the passed If-Modified-Since date is
invalid, the response is exactly the same as for a normal GET. A invalid, the response is exactly the same as for a normal GET.
date which is later than the server's current time is invalid. A date which is later than the server's current time is
invalid.
b) If the variant has been modified since the If-Modified-Since b) If the variant has been modified since the If-Modified-Since
date, the response is exactly the same as for a normal GET. date, the response is exactly the same as for a normal GET.
c) If the variant has not been modified since a valid If-Modified- c) If the variant has not been modified since a valid If-
Since date, the server SHOULD return a 304 (Not Modified) Modified-Since date, the server SHOULD return a 304 (Not
response. Modified) response.
The purpose of this feature is to allow efficient updates of cached The purpose of this feature is to allow efficient updates of cached
information with a minimum amount of transaction overhead. information with a minimum amount of transaction overhead.
Note: The Range request-header field modifies the meaning of Note: The Range request-header field modifies the meaning of If-
If-Modified-Since; see section 14.35 for full details. Modified-Since; see section 14.35 for full details.
Note: If-Modified-Since times are interpreted by the server, Note: If-Modified-Since times are interpreted by the server, whose
whose clock might not be synchronized with the client. clock might not be synchronized with the client.
Note: When handling an If-Modified-Since header field, some Note: When handling an If-Modified-Since header field, some
servers will use an exact date comparison function, rather than servers will use an exact date comparison function, rather than a
a less-than function, for deciding whether to send a 304 (Not less-than function, for deciding whether to send a 304 (Not
Modified) response. To get best results when sending an If- Modified) response. To get best results when sending an If-
Modified-Since header field for cache validation, clients are Modified-Since header field for cache validation, clients are
advised to use the exact date string received in a previous advised to use the exact date string received in a previous Last-
Last-Modified header field whenever possible. Modified header field whenever possible.
Note: If a client uses an arbitrary date in the If-Modified- Note: If a client uses an arbitrary date in the If-Modified-Since
Since header instead of a date taken from the Last-Modified header instead of a date taken from the Last-Modified header for
header for the same request, the client should be aware of the the same request, the client should be aware of the fact that this
fact that this date is interpreted in the server's date is interpreted in the server's understanding of time. The
understanding of time. The client should consider client should consider unsynchronized clocks and rounding problems
unsynchronized clocks and rounding problems due to the due to the different encodings of time between the client and
different encodings of time between the client and server. This server. This includes the possibility of race conditions if the
includes the possibility of race conditions if the document has document has changed between the time it was first requested and
changed between the time it was first requested and the If- the If-Modified-Since date of a subsequent request, and the
Modified-Since date of a subsequent request, and the
possibility of clock-skew-related problems if the If-Modified- possibility of clock-skew-related problems if the If-Modified-
Since date is derived from the client's clock without Since date is derived from the client's clock without correction
correction to the server's clock. Corrections for different to the server's clock. Corrections for different time bases
time bases between client and server are at best approximate between client and server are at best approximate due to network
due to network latency. latency.
The result of a request having both an If-Modified-Since header field The result of a request having both an If-Modified-Since header field
and either an If-Match or an If-Unmodified-Since header fields is and either an If-Match or an If-Unmodified-Since header fields is
undefined by this specification. undefined by this specification.
14.26 If-None-Match 14.26 If-None-Match
The If-None-Match request-header field is used with a method to make The If-None-Match request-header field is used with a method to make
it conditional. A client that has one or more entities previously it conditional. A client that has one or more entities previously
obtained from the resource can verify that none of those entities is obtained from the resource can verify that none of those entities is
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to have an up-to-date copy of the entire entity in its cache, it to have an up-to-date copy of the entire entity in its cache, it
could use the Range request-header with a conditional GET (using could use the Range request-header with a conditional GET (using
either or both of If-Unmodified-Since and If-Match.) However, if the either or both of If-Unmodified-Since and If-Match.) However, if the
condition fails because the entity has been modified, the client condition fails because the entity has been modified, the client
would then have to make a second request to obtain the entire current would then have to make a second request to obtain the entire current
entity-body. entity-body.
The If-Range header allows a client to "short-circuit" the second The If-Range header allows a client to "short-circuit" the second
request. Informally, its meaning is `if the entity is unchanged, send request. Informally, its meaning is `if the entity is unchanged, send
me the part(s) that I am missing; otherwise, send me the entire new me the part(s) that I am missing; otherwise, send me the entire new
entity.' entity'.
If-Range = "If-Range" ":" ( entity-tag | HTTP-date ) If-Range = "If-Range" ":" ( entity-tag | HTTP-date )
If the client has no entity tag for an entity, but does have a Last- If the client has no entity tag for an entity, but does have a Last-
Modified date, it MAY use that date in an If-Range header. (The Modified date, it MAY use that date in an If-Range header. (The
server can distinguish between a valid HTTP-date and any form of server can distinguish between a valid HTTP-date and any form of
entity-tag by examining no more than two characters.) The If-Range entity-tag by examining no more than two characters.) The If-Range
header SHOULD only be used together with a Range header, and MUST be header SHOULD only be used together with a Range header, and MUST be
ignored if the request does not include a Range header, or if the ignored if the request does not include a Range header, or if the
server does not support the sub-range operation. server does not support the sub-range operation.
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server's preferred URI for automatic redirection to the resource. The server's preferred URI for automatic redirection to the resource. The
field value consists of a single absolute URI. field value consists of a single absolute URI.
Location = "Location" ":" absoluteURI Location = "Location" ":" absoluteURI
An example is: An example is:
Location: http://www.w3.org/pub/WWW/People.html Location: http://www.w3.org/pub/WWW/People.html
Note: The Content-Location header field (section 14.14) differs Note: The Content-Location header field (section 14.14) differs
from Location in that the Content-Location identifies the from Location in that the Content-Location identifies the original
original location of the entity enclosed in the request. It is location of the entity enclosed in the request. It is therefore
therefore possible for a response to contain header fields for possible for a response to contain header fields for both Location
both Location and Content-Location. Also see section 13.10 for and Content-Location. Also see section 13.10 for cache
cache requirements of some methods. requirements of some methods.
14.31 Max-Forwards 14.31 Max-Forwards
The Max-Forwards request-header field provides a mechanism with the The Max-Forwards request-header field provides a mechanism with the
TRACE (section 9.8) and OPTIONS (section 9.2) methods to limit the TRACE (section 9.8) and OPTIONS (section 9.2) methods to limit the
number of proxies or gateways that can forward the request to the number of proxies or gateways that can forward the request to the
next inbound server. This can be useful when the client is attempting next inbound server. This can be useful when the client is attempting
to trace a request chain which appears to be failing or looping in to trace a request chain which appears to be failing or looping in
mid-chain. mid-chain.
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application, regardless of their significance to that application, application, regardless of their significance to that application,
since the directives might be applicable to all recipients along the since the directives might be applicable to all recipients along the
request/response chain. It is not possible to specify a pragma for a request/response chain. It is not possible to specify a pragma for a
specific recipient; however, any pragma directive not relevant to a specific recipient; however, any pragma directive not relevant to a
recipient SHOULD be ignored by that recipient. recipient SHOULD be ignored by that recipient.
HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
sent "Cache-Control: no-cache". No new Pragma directives will be sent "Cache-Control: no-cache". No new Pragma directives will be
defined in HTTP. defined in HTTP.
Note: because the meaning of "Pragma: no-cache as a response
header field is not actually specified, it does not provide a
reliable replacement for "Cache-Control: no-cache" in a response
14.33 Proxy-Authenticate 14.33 Proxy-Authenticate
The Proxy-Authenticate response-header field MUST be included as part The Proxy-Authenticate response-header field MUST be included as part
of a 407 (Proxy Authentication Required) response. The field value of a 407 (Proxy Authentication Required) response. The field value
consists of a challenge that indicates the authentication scheme and consists of a challenge that indicates the authentication scheme and
parameters applicable to the proxy for this Request-URI. parameters applicable to the proxy for this Request-URI.
Proxy-Authenticate = "Proxy-Authenticate" ":" 1#challenge Proxy-Authenticate = "Proxy-Authenticate" ":" 1#challenge
The HTTP access authentication process is described in "HTTP The HTTP access authentication process is described in "HTTP
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of ranges within a single entity. of ranges within a single entity.
ranges-specifier = byte-ranges-specifier ranges-specifier = byte-ranges-specifier
byte-ranges-specifier = bytes-unit "=" byte-range-set byte-ranges-specifier = bytes-unit "=" byte-range-set
byte-range-set = 1#( byte-range-spec | suffix-byte-range-spec ) byte-range-set = 1#( byte-range-spec | suffix-byte-range-spec )
byte-range-spec = first-byte-pos "-" [last-byte-pos] byte-range-spec = first-byte-pos "-" [last-byte-pos]
first-byte-pos = 1*DIGIT first-byte-pos = 1*DIGIT
last-byte-pos = 1*DIGIT last-byte-pos = 1*DIGIT
The first-byte-pos value in a byte-range-spec gives the byte-offset The first-byte-pos value in a byte-range-spec gives the byte-offset
of the first byte in a range. The last-byte-pos value gives the byte- of the first byte in a range. The last-byte-pos value gives the
offset of the last byte in the range; that is, the byte positions byte-offset of the last byte in the range; that is, the byte
specified are inclusive. Byte offsets start at zero. positions specified are inclusive. Byte offsets start at zero.
If the last-byte-pos value is present, it MUST be greater than or If the last-byte-pos value is present, it MUST be greater than or
equal to the first-byte-pos in that byte-range-spec, or the byte- equal to the first-byte-pos in that byte-range-spec, or the byte-
range-spec is syntactically invalid. The recipient of a byte-range- range-spec is syntactically invalid. The recipient of a byte-range-
set that includes one or more syntactically invalid byte-range-spec set that includes one or more syntactically invalid byte-range-spec
values MUST ignore the header field that includes that byte-range- values MUST ignore the header field that includes that byte-range-
set. set.
If the last-byte-pos value is absent, or if the value is greater than If the last-byte-pos value is absent, or if the value is greater than
or equal to the current length of the entity-body, last-byte-pos is or equal to the current length of the entity-body, last-byte-pos is
taken to be equal to one less than the current length of the entity- taken to be equal to one less than the current length of the entity-
body in bytes. body in bytes.
By its choice of last-byte-pos, a client can limit the number of By its choice of last-byte-pos, a client can limit the number of
bytes retrieved without knowing the size of the entity. bytes retrieved without knowing the size of the entity.
suffix-byte-range-spec = "-" suffix-length suffix-byte-range-spec = "-" suffix-length
suffix-length = 1*DIGIT suffix-length = 1*DIGIT
A suffix-byte-range-spec is used to specify the suffix of the entity- A suffix-byte-range-spec is used to specify the suffix of the
body, of a length given by the suffix-length value. (That is, this entity-body, of a length given by the suffix-length value. (That is,
form specifies the last N bytes of an entity-body.) If the entity is this form specifies the last N bytes of an entity-body.) If the
shorter than the specified suffix-length, the entire entity-body is entity is shorter than the specified suffix-length, the entire
used. entity-body is used.
If a syntactically valid byte-range-set includes at least one byte- If a syntactically valid byte-range-set includes at least one byte-
range-spec whose first-byte-pos is less than the current length of range-spec whose first-byte-pos is less than the current length of
the entity-body, or at least one suffix-byte-range-spec with a non- the entity-body, or at least one suffix-byte-range-spec with a non-
zero suffix-length, then the byte-range-set is satisfiable. zero suffix-length, then the byte-range-set is satisfiable.
Otherwise, the byte-range-set is unsatisfiable. If the byte-range-set Otherwise, the byte-range-set is unsatisfiable. If the byte-range-set
is unsatisfiable, the server SHOULD return a response with a status is unsatisfiable, the server SHOULD return a response with a status
of 416 (Requested range not satisfiable). Otherwise, the server of 416 (Requested range not satisfiable). Otherwise, the server
SHOULD return a response with a status of 206 (Partial Content) SHOULD return a response with a status of 206 (Partial Content)
containing the satisfiable ranges of the entity-body. containing the satisfiable ranges of the entity-body.
Examples of byte-ranges-specifier values (assuming an entity-body of Examples of byte-ranges-specifier values (assuming an entity-body of
length 10000): length 10000):
. The first 500 bytes (byte offsets 0-499, inclusive): - The first 500 bytes (byte offsets 0-499, inclusive): bytes=0-
bytes=0-499 499
. The second 500 bytes (byte offsets 500-999, inclusive): - The second 500 bytes (byte offsets 500-999, inclusive):
bytes=500-999 bytes=500-999
. The final 500 bytes (byte offsets 9500-9999, inclusive): - The final 500 bytes (byte offsets 9500-9999, inclusive):
bytes=-500 bytes=-500
. Or - Or bytes=9500-
bytes=9500-
. The first and last bytes only (bytes 0 and 9999): - The first and last bytes only (bytes 0 and 9999): bytes=0-0,-1
bytes=0-0,-1
. Several legal but not canonical specifications of the second - Several legal but not canonical specifications of the second 500
500 bytes (byte offsets 500-999, inclusive): bytes (byte offsets 500-999, inclusive):
bytes=500-600,601-999 bytes=500-600,601-999
bytes=500-700,601-999 bytes=500-700,601-999
14.35.2 Range Retrieval Requests 14.35.2 Range Retrieval Requests
HTTP retrieval requests using conditional or unconditional GET HTTP retrieval requests using conditional or unconditional GET
methods MAY request one or more sub-ranges of the entity, instead of methods MAY request one or more sub-ranges of the entity, instead of
the entire entity, using the Range request header, which applies to the entire entity, using the Range request header, which applies to
the entity returned as the result of the request: the entity returned as the result of the request:
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A server MAY ignore the Range header. However, HTTP/1.1 origin A server MAY ignore the Range header. However, HTTP/1.1 origin
servers and intermediate caches ought to support byte ranges when servers and intermediate caches ought to support byte ranges when
possible, since Range supports efficient recovery from partially possible, since Range supports efficient recovery from partially
failed transfers, and supports efficient partial retrieval of large failed transfers, and supports efficient partial retrieval of large
entities. entities.
If the server supports the Range header and the specified range or If the server supports the Range header and the specified range or
ranges are appropriate for the entity: ranges are appropriate for the entity:
. The presence of a Range header in an unconditional GET modifies - The presence of a Range header in an unconditional GET modifies
what is returned if the GET is otherwise successful. In other what is returned if the GET is otherwise successful. In other
words, the response carries a status code of 206 (Partial words, the response carries a status code of 206 (Partial
Content) instead of 200 (OK). Content) instead of 200 (OK).
. The presence of a Range header in a conditional GET (a request - The presence of a Range header in a conditional GET (a request
using one or both of If-Modified-Since and If-None-Match, or using one or both of If-Modified-Since and If-None-Match, or
one or both of If-Unmodified-Since and If-Match) modifies what one or both of If-Unmodified-Since and If-Match) modifies what
is returned if the GET is otherwise successful and the is returned if the GET is otherwise successful and the
condition is true. It does not affect the 304 (Not Modified) condition is true. It does not affect the 304 (Not Modified)
response returned if the conditional is false. response returned if the conditional is false.
In some cases, it might be more appropriate to use the If-Range In some cases, it might be more appropriate to use the If-Range
header (see section 14.27) in addition to the Range header. header (see section 14.27) in addition to the Range header.
If a proxy that supports ranges receives a Range request, forwards If a proxy that supports ranges receives a Range request, forwards
skipping to change at page 128, line 55 skipping to change at page 141, line 47
Server = "Server" ":" 1*( product | comment ) Server = "Server" ":" 1*( product | comment )
Example: Example:
Server: CERN/3.0 libwww/2.17 Server: CERN/3.0 libwww/2.17
If the response is being forwarded through a proxy, the proxy If the response is being forwarded through a proxy, the proxy
application MUST NOT modify the Server response-header. Instead, it application MUST NOT modify the Server response-header. Instead, it
SHOULD include a Via field (as described in section 14.45). SHOULD include a Via field (as described in section 14.45).
Note: Revealing the specific software version of the server Note: Revealing the specific software version of the server might
might allow the server machine to become more vulnerable to allow the server machine to become more vulnerable to attacks
attacks against software that is known to contain security against software that is known to contain security holes. Server
holes. Server implementors are encouraged to make this field a implementors are encouraged to make this field a configurable
configurable option. option.
14.39 TE 14.39 TE
The TE request-header field indicates what extension transfer-codings The TE request-header field indicates what extension transfer-codings
it is willing to accept in the response and whether or not it is it is willing to accept in the response and whether or not it is
willing to accept trailer fields in a chunked transfer-coding. Its willing to accept trailer fields in a chunked transfer-coding. Its
value may consist of the keyword "trailers" and/or a comma-separated value may consist of the keyword "trailers" and/or a comma-separated
list of extension transfer-coding names with optional accept list of extension transfer-coding names with optional accept
parameters (as described in section 3.6). parameters (as described in section 3.6).
skipping to change at page 129, line 38 skipping to change at page 142, line 36
TE: TE:
TE: trailers, deflate;q=0.5 TE: trailers, deflate;q=0.5
The TE header field only applies to the immediate connection. The TE header field only applies to the immediate connection.
Therefore, the keyword MUST be supplied within a Connection header Therefore, the keyword MUST be supplied within a Connection header
field (section 14.10) whenever TE is present in an HTTP/1.1 message. field (section 14.10) whenever TE is present in an HTTP/1.1 message.
A server tests whether a transfer-coding is acceptable, according to A server tests whether a transfer-coding is acceptable, according to
a TE field, using these rules: a TE field, using these rules:
1. 1. The "chunked" transfer-coding is always acceptable. If the
The "chunked" transfer-coding is always acceptable. If the keyword "trailers" is listed, the client indicates that it is
keyword "trailers" is listed, the client indicates that it is willing to accept trailer fields in the chunked response on
willing to accept trailer fields in the chunked response on behalf of itself and any downstream clients. The implication is
behalf of itself and any downstream clients. The implication is that, if given, the client is stating that either all
that, if given, the client is stating that either all downstream downstream clients are willing to accept trailer fields in the
clients are willing to accept trailer fields in the forwarded forwarded response, or that it will attempt to buffer the
response, or that it will attempt to buffer the response on response on behalf of downstream recipients.
behalf of downstream recipients.
Note: HTTP/1.1 does not define any means to limit the size of a Note: HTTP/1.1 does not define any means to limit the size of a
chunked response such that a client can be assured of buffering chunked response such that a client can be assured of buffering
the entire response. the entire response.
2. If the transfer-coding being tested is one of the transfer- 2. If the transfer-coding being tested is one of the transfer-
codings listed in the TE field, then it is acceptable unless it codings listed in the TE field, then it is acceptable unless it
is accompanied by a qvalue of 0. (As defined in section 3.9, a is accompanied by a qvalue of 0. (As defined in section 3.9, a
qvalue of 0 means "not acceptable.") qvalue of 0 means "not acceptable.")
3. If multiple transfer-codings are acceptable, then the acceptable 3. If multiple transfer-codings are acceptable, then the
transfer-coding with the highest non-zero qvalue is preferred. acceptable transfer-coding with the highest non-zero qvalue is
The "chunked" transfer-coding always has a qvalue of 1. preferred. The "chunked" transfer-coding always has a qvalue
of 1.
If the TE field-value is empty or if no TE field is present, the only If the TE field-value is empty or if no TE field is present, the only
transfer-coding is "chunked". A message with no transfer-coding is transfer-coding is "chunked". A message with no transfer-coding is
always acceptable. always acceptable.
14.40 Trailer 14.40 Trailer
The Trailer general field value indicates that the given set of The Trailer general field value indicates that the given set of
header fields is present in the trailer of a message encoded with header fields is present in the trailer of a message encoded with
chunked transfer-coding. chunked transfer-coding.
skipping to change at page 132, line 56 skipping to change at page 146, line 18
response. response.
A Vary field value consisting of a list of field-names signals that A Vary field value consisting of a list of field-names signals that
the representation selected for the response is based on a selection the representation selected for the response is based on a selection
algorithm which considers ONLY the listed request-header field values algorithm which considers ONLY the listed request-header field values
in selecting the most appropriate representation. A cache MAY assume in selecting the most appropriate representation. A cache MAY assume
that the same selection will be made for future requests with the that the same selection will be made for future requests with the
same values for the listed field names, for the duration of time for same values for the listed field names, for the duration of time for
which the response is fresh. which the response is fresh.
The field-names given are not limited to the set of standard request- The field-names given are not limited to the set of standard
header fields defined by this specification. Field names are case- request-header fields defined by this specification. Field names are
insensitive. case-insensitive.
A Vary field value of "*" signals that unspecified parameters not A Vary field value of "*" signals that unspecified parameters not
limited to the request-headers (e.g., the network address of the limited to the request-headers (e.g., the network address of the
client), play a role in the selection of the response representation. client), play a role in the selection of the response representation.
The "*" value MUST NOT be generated by a proxy server; it may only be The "*" value MUST NOT be generated by a proxy server; it may only be
generated by an origin server. generated by an origin server.
14.45 Via 14.45 Via
The Via general-header field MUST be used by gateways and proxies to The Via general-header field MUST be used by gateways and proxies to
skipping to change at page 135, line 28 skipping to change at page 149, line 14
specific Warning codes. It MUST then add any Warning headers received specific Warning codes. It MUST then add any Warning headers received
in the validating response. In other words, Warning headers are those in the validating response. In other words, Warning headers are those
that would be attached to the most recent relevant response. that would be attached to the most recent relevant response.
When multiple Warning headers are attached to a response, the user When multiple Warning headers are attached to a response, the user
agent ought to inform the user of as many of them as possible, in the agent ought to inform the user of as many of them as possible, in the
order that they appear in the response. If it is not possible to order that they appear in the response. If it is not possible to
inform the user of all of the warnings, the user agent SHOULD follow inform the user of all of the warnings, the user agent SHOULD follow
these heuristics: these heuristics:
. Warnings that appear early in the response take priority over - Warnings that appear early in the response take priority over
those appearing later in the response. those appearing later in the response.
. Warnings in the user's preferred character set take priority - Warnings in the user's preferred character set take priority
over warnings in other character sets but with identical warn- over warnings in other character sets but with identical warn-
codes and warn-agents. codes and warn-agents.
Systems that generate multiple Warning headers SHOULD order them with Systems that generate multiple Warning headers SHOULD order them with
this user agent behavior in mind. this user agent behavior in mind.
Requirements for the behavior of caches with respect to Warnings are Requirements for the behavior of caches with respect to Warnings are
stated in section 13.1.2. stated in section 13.1.2.
This is a list of the currently-defined warn-codes, each with a This is a list of the currently-defined warn-codes, each with a
skipping to change at page 135, line 53 skipping to change at page 149, line 39
110 Response is stale 110 Response is stale
MUST be included whenever the returned response is stale. MUST be included whenever the returned response is stale.
111 Revalidation failed 111 Revalidation failed
MUST be included if a cache returns a stale response because an MUST be included if a cache returns a stale response because an
attempt to revalidate the response failed, due to an inability to attempt to revalidate the response failed, due to an inability to
reach the server. reach the server.
112 Disconnected operation 112 Disconnected operation
SHOULD be included if the cache is intentionally disconnected SHOULD be included if the cache is intentionally disconnected from
from the rest of the network for a period of time. the rest of the network for a period of time.
113 Heuristic expiration 113 Heuristic expiration
MUST be included if the cache heuristically chose a freshness MUST be included if the cache heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater lifetime greater than 24 hours and the response's age is greater
than 24 hours. than 24 hours.
199 Miscellaneous warning 199 Miscellaneous warning
The warning text MAY include arbitrary information to be presented The warning text MAY include arbitrary information to be presented
to a human user, or logged. A system receiving this warning MUST to a human user, or logged. A system receiving this warning MUST
NOT take any automated action, besides presenting the warning to NOT take any automated action, besides presenting the warning to
the user. the user.
214 Transformation applied 214 Transformation applied
MUST be added by an intermediate cache or proxy if it applies any MUST be added by an intermediate cache or proxy if it applies any
transformation changing the content-coding (as specified in the transformation changing the content-coding (as specified in the
Content-Encoding header) or media-type (as specified in the Content-Encoding header) or media-type (as specified in the
Content-Type header) of the response, or the entity-body of the Content-Type header) of the response, or the entity-body of the
response, unless this Warning code already appears in the response, unless this Warning code already appears in the response.
response.
299 Miscellaneous persistent warning 299 Miscellaneous persistent warning
The warning text MAY include arbitrary information to be presented The warning text MAY include arbitrary information to be presented
to a human user, or logged. A system receiving this warning MUST to a human user, or logged. A system receiving this warning MUST
NOT take any automated action. NOT take any automated action.
If an implementation sends a message with one or more Warning headers If an implementation sends a message with one or more Warning headers
whose version is HTTP/1.0 or lower, then the sender MUST include in whose version is HTTP/1.0 or lower, then the sender MUST include in
each warning-value a warn-date that matches the date in the response. each warning-value a warn-date that matches the date in the response.
skipping to change at page 139, line 52 skipping to change at page 154, line 8
be accessible via the HTTP server. Similarly, files intended for be accessible via the HTTP server. Similarly, files intended for
reference only internally to the server (such as access control reference only internally to the server (such as access control
files, configuration files, and script code) MUST be protected from files, configuration files, and script code) MUST be protected from
inappropriate retrieval, since they might contain sensitive inappropriate retrieval, since they might contain sensitive
information. Experience has shown that minor bugs in such HTTP server information. Experience has shown that minor bugs in such HTTP server
implementations have turned into security risks. implementations have turned into security risks.
15.3 DNS Spoofing 15.3 DNS Spoofing
Clients using HTTP rely heavily on the Domain Name Service, and are Clients using HTTP rely heavily on the Domain Name Service, and are
thus generally prone to security attacks based on the deliberate mis- thus generally prone to security attacks based on the deliberate
association of IP addresses and DNS names. Clients need to be mis-association of IP addresses and DNS names. Clients need to be
cautious in assuming the continuing validity of an IP number/DNS name cautious in assuming the continuing validity of an IP number/DNS name
association. association.
In particular, HTTP clients SHOULD rely on their name resolver for In particular, HTTP clients SHOULD rely on their name resolver for
confirmation of an IP number/DNS name association, rather than confirmation of an IP number/DNS name association, rather than
caching the result of previous host name lookups. Many platforms caching the result of previous host name lookups. Many platforms
already can cache host name lookups locally when appropriate, and already can cache host name lookups locally when appropriate, and
they SHOULD be configured to do so. It is proper for these lookups to they SHOULD be configured to do so. It is proper for these lookups to
be cached, however, only when the TTL (Time To Live) information be cached, however, only when the TTL (Time To Live) information
reported by the name server makes it likely that the cached reported by the name server makes it likely that the cached
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15.6 Authentication Credentials and Idle Clients 15.6 Authentication Credentials and Idle Clients
Existing HTTP clients and user agents typically retain authentication Existing HTTP clients and user agents typically retain authentication
information indefinitely. HTTP/1.1. does not provide a method for a information indefinitely. HTTP/1.1. does not provide a method for a
server to direct clients to discard these cached credentials. This is server to direct clients to discard these cached credentials. This is
a significant defect that requires further extensions to HTTP. a significant defect that requires further extensions to HTTP.
Circumstances under which credential caching can interfere with the Circumstances under which credential caching can interfere with the
application's security model include but are not limited to: application's security model include but are not limited to:
. Clients which have been idle for an extended period following - Clients which have been idle for an extended period following
which the server might wish to cause the client to reprompt the which the server might wish to cause the client to reprompt the
user for credentials. user for credentials.
. Applications which include a session termination indication - Applications which include a session termination indication
(such as a `logout' or `commit' button on a page) after which (such as a `logout' or `commit' button on a page) after which
the server side of the application `knows' that there is no the server side of the application `knows' that there is no
further reason for the client to retain the credentials. further reason for the client to retain the credentials.
This is currently under separate study. There are a number of work- This is currently under separate study. There are a number of work-
arounds to parts of this problem, and we encourage the use of arounds to parts of this problem, and we encourage the use of
password protection in screen savers, idle time-outs, and other password protection in screen savers, idle time-outs, and other
methods which mitigate the security problems inherent in this methods which mitigate the security problems inherent in this
problem. In particular, user agents which cache credentials are problem. In particular, user agents which cache credentials are
encouraged to provide a readily accessible mechanism for discarding encouraged to provide a readily accessible mechanism for discarding
skipping to change at page 142, line 38 skipping to change at page 157, line 5
Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois
Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob Groff, Phillip M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob
McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc
VanHeyningen deserve special recognition for their efforts in VanHeyningen deserve special recognition for their efforts in
defining early aspects of the protocol. defining early aspects of the protocol.
This document has benefited greatly from the comments of all those This document has benefited greatly from the comments of all those
participating in the HTTP-WG. In addition to those already mentioned, participating in the HTTP-WG. In addition to those already mentioned,
the following individuals have contributed to this specification: the following individuals have contributed to this specification:
Gary Adams Albert Lunde Gary Adams Ross Patterson
Harald Tveit Alvestrand John C. Mallery Harald Tveit Alvestrand Albert Lunde
Keith Ball Jean-Philippe Martin-Flatin Keith Ball John C. Mallery
Brian Behlendorf Larry Masinter Brian Behlendorf Jean-Philippe Martin-Flatin
Paul Burchard Mitra Paul Burchard Mitra
Maurizio Codogno David Morris Maurizio Codogno David Morris
Mike Cowlishaw Gavin Nicol Mike Cowlishaw Gavin Nicol
Roman Czyborra Bill Perry Roman Czyborra Bill Perry
Michael A. Dolan Jeffrey Perry Michael A. Dolan Jeffrey Perry
David J. Fiander Scott Powers David J. Fiander Scott Powers
Alan Freier Owen Rees Alan Freier Owen Rees
Marc Hedlund Luigi Rizzo Marc Hedlund Luigi Rizzo
Greg Herlihy David Robinson Greg Herlihy David Robinson
Koen Holtman Marc Salomon Koen Holtman Marc Salomon
skipping to change at page 143, line 11 skipping to change at page 157, line 31
Shel Kaphan Jim Seidman Shel Kaphan Jim Seidman
Rohit Khare Chuck Shotton Rohit Khare Chuck Shotton
John Klensin Eric W. Sink John Klensin Eric W. Sink
Martijn Koster Simon E. Spero Martijn Koster Simon E. Spero
Alexei Kosut Richard N. Taylor Alexei Kosut Richard N. Taylor
David M. Kristol Robert S. Thau David M. Kristol Robert S. Thau
Daniel LaLiberte Bill (BearHeart) Weinman Daniel LaLiberte Bill (BearHeart) Weinman
Ben Laurie Francois Yergeau Ben Laurie Francois Yergeau
Paul J. Leach Mary Ellen Zurko Paul J. Leach Mary Ellen Zurko
Daniel DuBois Josh Cohen Daniel DuBois Josh Cohen
Ross Patterson
Much of the content and presentation of the caching design is due to Much of the content and presentation of the caching design is due to
suggestions and comments from individuals including: Shel Kaphan, suggestions and comments from individuals including: Shel Kaphan,
Paul Leach, Koen Holtman, David Morris, and Larry Masinter. Paul Leach, Koen Holtman, David Morris, and Larry Masinter.
Most of the specification of ranges is based on work originally done Most of the specification of ranges is based on work originally done
by Ari Luotonen and John Franks, with additional input from Steve by Ari Luotonen and John Franks, with additional input from Steve
Zilles. Zilles.
Thanks to the "cave men" of Palo Alto. You know who you are. Thanks to the "cave men" of Palo Alto. You know who you are.
skipping to change at page 143, line 37 skipping to change at page 158, line 12
Larry Masinter for their help. And thanks go particularly to Jeff Larry Masinter for their help. And thanks go particularly to Jeff
Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit. Mogul and Scott Lawrence for performing the "MUST/MAY/SHOULD" audit.
The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
Frystyk implemented RFC 2068 early, and we wish to thank them for the Frystyk implemented RFC 2068 early, and we wish to thank them for the
discovery of many of the problems that this document attempts to discovery of many of the problems that this document attempts to
rectify. rectify.
17 References 17 References
[1]Alvestrand, H., "Tags for the Identification of Languages" RFC [1] Alvestrand, H., "Tags for the Identification of Languages", RFC
1766, UNINETT, March 1995. 1766, March 1995.
[2] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey, [2] Anklesaria, F., McCahill, M., Lindner, P., Johnson, D., Torrey,
D., and B. Alberti. "The Internet Gopher Protocol (a distributed D. and B. Alberti, "The Internet Gopher Protocol (a distributed
document search and retrieval protocol)", RFC 1436, University document search and retrieval protocol)", RFC 1436, March 1993.
of Minnesota, March 1993.
[3] Berners-Lee, T., "Universal Resource Identifiers in WWW," RFC [3] Berners-Lee, T., "Universal Resource Identifiers in WWW", RFC
1630, CERN, June 1994. 1630, June 1994.
[4] Berners-Lee, T., Masinter, L., and M. McCahill. "Uniform [4] Berners-Lee, T., Masinter, L. and M. McCahill, "Uniform Resource
Resource Locators (URL)," RFC 1738, CERN, Xerox PARC, University Locators (URL)", RFC 1738, December 1994.
of Minnesota, December 1994.
[5] Berners-Lee, T. and D. Connolly. "Hypertext Markup Language - [5] Berners-Lee, T. and D. Connolly, "Hypertext Markup Language -
2.0," RFC 1866, MIT/LCS, November 1995. 2.0", RFC 1866, November 1995.
[6] Berners-Lee, T., Fielding, R. and H. Frystyk. "Hypertext [6] Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext Transfer
Transfer Protocol -- HTTP/1.0," RFC 1945, MIT/LCS, UC Irvine, Protocol -- HTTP/1.0", RFC 1945, May 1996.
May 1996.
[7] Freed, N., and N. Borenstein. "Multipurpose Internet Mail [7] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies." Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, Innosoft, First Virtual, November 1996. RFC 2045, November 1996.
[8] Braden, R., "Requirements for Internet Hosts -- Communication [8] Braden, R., "Requirements for Internet Hosts -- Communication
Layers," STD 3, RFC 1123, IETF, October 1989. Layers", STD 3, RFC 1123, October 1989.
[9] D. H. Crocker, "Standard for The Format of ARPA Internet Text [9] Crocker, D., "Standard for The Format of ARPA Internet Text
Messages," STD 11, RFC 822, UDEL, August 1982. Messages", STD 11, RFC 822, August 1982.
[10]Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R., [10] Davis, F., Kahle, B., Morris, H., Salem, J., Shen, T., Wang, R.,
Sui, J., and M. Grinbaum, "WAIS Interface Protocol Prototype Sui, J., and M. Grinbaum, "WAIS Interface Protocol Prototype
Functional Specification." (v1.5), Thinking Machines Functional Specification," (v1.5), Thinking Machines
Corporation, April 1990. Corporation, April 1990.
[11]Fielding, R., "Relative Uniform Resource Locators," RFC 1808, UC [11] Fielding, R., "Relative Uniform Resource Locators", RFC 1808,
Irvine, June 1995. June 1995.
[12]Horton, M., and R. Adams. "Standard for Interchange of USENET [12] Horton, M. and R. Adams, "Standard for Interchange of USENET
Messages," RFC 1036 (Obsoletes RFC 850), AT&T Bell Laboratories, Messages", RFC 1036, December 1987.
Center for Seismic Studies, December 1987.
[13]Kantor, B. and P. Lapsley. "Network News Transfer Protocol," RFC [13] Kantor, B. and P. Lapsley, "Network News Transfer Protocol", RFC
977, UC San Diego, UC Berkeley, February 1986. 977, February 1986.
[14]Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part [14] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC 2047, Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
University of Tennessee, November 1996. November 1996.
[15]Nebel, E., and L. Masinter. "Form-based File Upload in HTML," [15] Nebel, E. and L. Masinter, "Form-based File Upload in HTML", RFC
RFC 1867, Xerox Corporation, November 1995. 1867, November 1995.
[16]Postel, J., "Simple Mail Transfer Protocol," STD 10, RFC 821, [16] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
USC/ISI, August 1982. August 1982.
[17]Postel, J., "Media Type Registration Procedure," RFC 1590, [17] Postel, J., "Media Type Registration Procedure", RFC 1590,
USC/ISI, November 1996. November 1996.
[18]Postel, J. and J. Reynolds. "File Transfer Protocol," STD 9, RFC [18] Postel, J. and J. Reynolds, "File Transfer Protocol", STD 9, RFC
959, USC/ISI, October 1985. 959, October 1985.
[19]Reynolds, J. and J. Postel. "Assigned Numbers," STD 2, RFC 1700, [19] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
USC/ISI, October 1994. October 1994.
[20]Sollins, K. and L. Masinter. "Functional Requirements for [20] Sollins, K. and L. Masinter, "Functional Requirements for
Uniform Resource Names," RFC 1737, MIT/LCS, Xerox Corporation, Uniform Resource Names", RFC 1737, December 1994.
December 1994.
[21]US-ASCII. Coded Character Set - 7-Bit American Standard Code for [21] US-ASCII. Coded Character Set - 7-Bit American Standard Code for
Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986. Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.
[22]ISO-8859. International Standard -- Information Processing -- [22] ISO-8859. International Standard -- Information Processing --
8-bit Single-Byte Coded Graphic Character Sets -- 8-bit Single-Byte Coded Graphic Character Sets --
Part 1: Latin alphabet No. 1, ISO-8859-1:1987. Part 1: Latin alphabet No. 1, ISO-8859-1:1987.
Part 2: Latin alphabet No. 2, ISO-8859-2, 1987. Part 2: Latin alphabet No. 2, ISO-8859-2, 1987.
Part 3: Latin alphabet No. 3, ISO-8859-3, 1988. Part 3: Latin alphabet No. 3, ISO-8859-3, 1988.
Part 4: Latin alphabet No. 4, ISO-8859-4, 1988. Part 4: Latin alphabet No. 4, ISO-8859-4, 1988.
Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988. Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988.
Part 6: Latin/Arabic alphabet, ISO-8859-6, 1987. Part 6: Latin/Arabic alphabet, ISO-8859-6, 1987.
Part 7: Latin/Greek alphabet, ISO-8859-7, 1987. Part 7: Latin/Greek alphabet, ISO-8859-7, 1987.
Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988. Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988.
Part 9: Latin alphabet No. 5, ISO-8859-9, 1990. Part 9: Latin alphabet No. 5, ISO-8859-9, 1990.
[23]Meyers, J., and M. Rose. "The Content-MD5 Header Field," RFC [23] Meyers, J. and M. Rose, "The Content-MD5 Header Field", RFC
1864, Carnegie Mellon, Dover Beach Consulting, October, 1995. 1864, October 1995.
[24]Carpenter, B. and Y. Rekhter. "Renumbering Needs Work," RFC [24] Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", RFC
1900, IAB, February 1996. 1900, February 1996.
[25]Deutsch, P., "GZIP file format specification version 4.3,." RFC [25] Deutsch, P., "GZIP file format specification version 4.3", RFC
1952, Aladdin Enterprises, May, 1996. 1952, May 1996.
[26]Venkata N. Padmanabhan, and Jeffrey C. Mogul. "Improving HTTP [26] Venkata N. Padmanabhan, and Jeffrey C. Mogul. "Improving HTTP
Latency", Computer Networks and ISDN Systems, v. 28, pp. 25-35, Latency", Computer Networks and ISDN Systems, v. 28, pp. 25-35,
Dec. 1995. Slightly revised version of paper in Proc. 2nd Dec. 1995. Slightly revised version of paper in Proc. 2nd
International WWW Conference '94: Mosaic and the Web, Oct. 1994, International WWW Conference '94: Mosaic and the Web, Oct. 1994,
which is available at which is available at
http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLat http://www.ncsa.uiuc.edu/SDG/IT94/Proceedings/DDay/mogul/HTTPLat
ency.html. ency.html.
[27]Joe Touch, John Heidemann, and Katia Obraczka. "Analysis of HTTP [27] Joe Touch, John Heidemann, and Katia Obraczka. "Analysis of HTTP
Performance", <URL: http://www.isi.edu/touch/pubs/http-perf96/>, Performance", <URL: http://www.isi.edu/touch/pubs/http-perf96/>,
ISI Research Report ISI/RR-98-463, (original report dated Aug. ISI Research Report ISI/RR-98-463, (original report dated Aug.
1996), USC/Information Sciences Institute, August 1998. 1996), USC/Information Sciences Institute, August 1998.
[28]Mills, D., "Network Time Protocol (Version 3) Specification, [28] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation and Analysis." RFC 1305, University of Delaware, Implementation and Analysis", RFC 1305, March 1992.
March, 1992.
[29]Deutsch, P., "DEFLATE Compressed Data Format Specification [29] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3." RFC 1951, Aladdin Enterprises, May 1996. version 1.3", RFC 1951, May 1996.
[30]S. Spero, "Analysis of HTTP Performance Problems," [30] S. Spero, "Analysis of HTTP Performance Problems,"
http://sunsite.unc.edu/mdma-release/http-prob.html. http://sunsite.unc.edu/mdma-release/http-prob.html.
[31]Deutsch, P. and J-L. Gailly. "ZLIB Compressed Data Format [31] Deutsch, P. and J. Gailly, "ZLIB Compressed Data Format
Specification version 3.3," RFC 1950, Aladdin Enterprises, Info- Specification version 3.3", RFC 1950, May 1996.
ZIP, May 1996.
[32]Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P., [32] Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P.,
Luotonen, A., Sink, E., and L. Stewart. "An Extension to HTTP: Luotonen, A., Sink, E. and L. Stewart, "An Extension to HTTP:
Digest Access Authentication," RFC 2069, January 1997. Digest Access Authentication", RFC 2069, January 1997.
[33]Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Berners-Lee, [33] Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.
T., "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2068, UC Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
Irvine, Digital Equipment Corporation, M.I.T., January, 1997. 2068, January 1997.
[34]Bradner, S., "Key words for use in RFCs to Indicate Requirement [34] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels," RFC 2119, Harvard University, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[35]Troost, R., and Dorner, S., "Communicating Presentation [35] Troost, R. and Dorner, S., "Communicating Presentation
Information in Internet Messages: The Content-Disposition Information in Internet Messages: The Content-Disposition
Header," RFC 1806, New Century Systems, QUALCOMM, Inc., June Header", RFC 1806, June 1995.
1995.
[36]Mogul, J.C., Fielding, R., Gettys, J, Frystyk, H., "Use and [36] Mogul, J., Fielding, R., Gettys, J. and H. Frystyk, "Use and
Interpretation of HTTP Version Numbers", RFC 2145, Digital Interpretation of HTTP Version Numbers", RFC 2145, May 1997.
Equipment Corporation, U.C. Irvine, M.I.T., May 1997.[jg639] [jg639]
[37]Palme, J, "Common Internet Message Headers," RFC 2076, Stockholm [37] Palme, J., "Common Internet Message Headers", RFC 2076, February
University, KTH, February, 1997[jg640]. 1997. [jg640]
[38]Yergeau, F., "UTF-8, a transformation format of Unicode and ISO- [38] Yergeau, F., "UTF-8, a transformation format of Unicode and
10646," RFC 2279 (obsoleted RFC 2044), Alis Technologies, ISO-10646", RFC 2279, January 1998. [jg641]
January 1998. [jg641]
[39]Nielsen, H.F., Gettys, J., Baird-Smith, A., Prud'hommeaux, E., [39] Nielsen, H.F., Gettys, J., Baird-Smith, A., Prud'hommeaux, E.,
Lie, H., and C. Lilley. "Network Performance Effects of Lie, H., and C. Lilley. "Network Performance Effects of
HTTP/1.1, CSS1, and PNG," Proceedings of ACM SIGCOMM '97, Cannes HTTP/1.1, CSS1, and PNG," Proceedings of ACM SIGCOMM '97, Cannes
France, September 1997.[jg642] France, September 1997.[jg642]
[40]Freed, N., and N. Borenstein. "Multipurpose Internet Mail [40] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types." RFC 2046, Innosoft, Extensions (MIME) Part Two: Media Types", RFC 2046, November
First Virtual, November 1996. [jg643] 1996. [jg643]
[41]Alvestrand, H. T., "IETF Policy on Character Sets and [41] Alvestrand, H., "IETF Policy on Character Sets and Languages",
Languages," RFC 2277, BCP 18, UNINETT, January, 1998. [jg644] BCP 18, RFC 2277, January 1998. [jg644]
[42]Berners-Lee, T., Fielding, R., Masinter, L., "Uniform Resource [42] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax and Semantics," RFC 2396, Identifiers (URI): Generic Syntax and Semantics", RFC 2396,
August, 1998.[jg645] August 1998. [jg645]
[43]Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., [43] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., Sink, E., Stewart, L., "HTTP Leach, P., Luotonen, A., Sink, E. and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication," Work in Authentication: Basic and Digest Access Authentication", RFC
Progress, September, 1998.[jg646] 2617, June 1999. [jg646]
[44]Luotonen, A., "Tunneling TCP based protocols through Web proxy [44] Luotonen, A., "Tunneling TCP based protocols through Web proxy
servers," Work in Progress, February, 1998.[jg647] servers," Work in Progress. [jg647]
[45]Palme, J., Hopmann, A., "MIME E-mail Encapsulation of Aggregate [45] Palme, J. and A. Hopmann, "MIME E-mail Encapsulation of
Documents, such as HTML (MHTML)," RFC 2110, March 1997 Aggregate Documents, such as HTML (MHTML)", RFC 2110, March
1997.
[46]Bradner, S., "The Internet Standards Process -- Revision 3," BCP [46] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, Harvard University, October, 1996. 9, RFC 2026, October 1996.
[47]Masinter, L., "Hyper Text Coffee Pot Control Protocol [47] Masinter, L., "Hyper Text Coffee Pot Control Protocol
(HTCPCP/1.0)," RFC 2324, April, 1998. (HTCPCP/1.0)", RFC 2324, 1 April 1998.
[48]Freed, N., Borenstein, N., "Multipurpose Internet Mail [48] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and Examples," Extensions (MIME) Part Five: Conformance Criteria and Examples",
RFC 2049, November, 1996. RFC 2049, November 1996.
[49]Troost, R., Dorner, S., Moore, K., "Communicating Presentation [49] Troost, R., Dorner, S. and K. Moore, "Communicating Presentation
Information in Internet Messages: The Content-Disposition Header Information in Internet Messages: The Content-Disposition Header
Field," RFC 2183, August, 1997. Field", RFC 2183, August 1997.
18 Authors' Addresses 18 Authors' Addresses
Roy T. Fielding Roy T. Fielding
Department of Information and Computer Science Information and Computer Science
University of California University of California, Irvine
Irvine, CA 92697-3425, USA Irvine, CA 92697-3425, USA
Fax: +1 (714) 824-1715
Email: fielding@ics.uci.edu Fax: +1 (949) 824-1715
EMail: fielding@ics.uci.edu
James Gettys James Gettys
World Wide Web Consortium World Wide Web Consortium
MIT Laboratory for Computer Science MIT Laboratory for Computer Science
545 Technology Square 545 Technology Square
Cambridge, MA 02139, USA Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682 Fax: +1 (617) 258 8682
Email: jg@w3.org EMail: jg@w3.org
Jeffrey C. Mogul Jeffrey C. Mogul
Western Research Laboratory Western Research Laboratory
Compaq Computer Corporation Compaq Computer Corporation
250 University Avenue 250 University Avenue
Palo Alto, California, 94305, USA Palo Alto, California, 94305, USA
Email: mogul@wrl.dec.com
EMail: mogul@wrl.dec.com
Henrik Frystyk Nielsen Henrik Frystyk Nielsen
World Wide Web Consortium World Wide Web Consortium
MIT Laboratory for Computer Science MIT Laboratory for Computer Science
545 Technology Square 545 Technology Square
Cambridge, MA 02139, USA Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682 Fax: +1 (617) 258 8682
Email: frystyk@w3.org EMail: frystyk@w3.org
Larry Masinter Larry Masinter
Xerox PARC Xerox Corporation
3333 Coyote Hill Road 3333 Coyote Hill Road
Palo Alto, CA 94034, USA Palo Alto, CA 94034, USA
Fax:+1 (415) 812-4333
Email: masinter@parc.xerox.com
EMail: masinter@parc.xerox.com
Paul J. Leach Paul J. Leach
Microsoft Corporation Microsoft Corporation
1 Microsoft Way 1 Microsoft Way
Redmond, WA 98052, USA Redmond, WA 98052, USA
Email: paulle@microsoft.com
EMail: paulle@microsoft.com
Tim Berners-Lee Tim Berners-Lee
Director, World Wide Web Consortium Director, World Wide Web Consortium
MIT Laboratory for Computer Science MIT Laboratory for Computer Science
545 Technology Square 545 Technology Square
Cambridge, MA 02139, USA Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682 Fax: +1 (617) 258 8682
Email: timbl@w3.org EMail: timbl@w3.org
19 Appendices 19 Appendices
19.1 Internet Media Type message/http and application/http 19.1 Internet Media Type message/http and application/http
In addition to defining the HTTP/1.1 protocol, this document serves In addition to defining the HTTP/1.1 protocol, this document serves
as the specification for the Internet media type "message/http" and as the specification for the Internet media type "message/http" and
"application/http". The message/http type can be used to enclose a "application/http". The message/http type can be used to enclose a
single HTTP request or response message, provided that it obeys the single HTTP request or response message, provided that it obeys the
MIME restrictions for all "message" types regarding line length and MIME restrictions for all "message" types regarding line length and
skipping to change at page 149, line 4 skipping to change at page 164, line 46
version: The HTTP-Version number of the enclosed messages version: The HTTP-Version number of the enclosed messages
(e.g., "1.1"). If not present, the version can be (e.g., "1.1"). If not present, the version can be
determined from the first line of the body. determined from the first line of the body.
msgtype: The message type -- "request" or "response". If not msgtype: The message type -- "request" or "response". If not
present, the type can be determined from the first present, the type can be determined from the first
line of the body. line of the body.
Encoding considerations: HTTP messages enclosed by this type Encoding considerations: HTTP messages enclosed by this type
are in "binary" format; use of an appropriate are in "binary" format; use of an appropriate
Content-Transfer-Encoding is required when Content-Transfer-Encoding is required when
transmitted via E-mail. transmitted via E-mail.
Security considerations: none Security considerations: none
19.2 Internet Media Type multipart/byteranges 19.2 Internet Media Type multipart/byteranges
When an HTTP 206 (Partial Content) response message includes the When an HTTP 206 (Partial Content) response message includes the
content of multiple ranges (a response to a request for multiple non- content of multiple ranges (a response to a request for multiple
overlapping ranges), these are transmitted as a multipart message- non-overlapping ranges), these are transmitted as a multipart
body. The media type for this purpose is called message-body. The media type for this purpose is called
"multipart/byteranges". "multipart/byteranges".
The multipart/byteranges media type includes two or more parts, each The multipart/byteranges media type includes two or more parts, each
with its own Content-Type and Content-Range fields. The required with its own Content-Type and Content-Range fields. The required
boundary parameter specifies the boundary string used to separate boundary parameter specifies the boundary string used to separate
each body-part. each body-part.
Media Type name: multipart Media Type name: multipart
Media subtype name: byteranges Media subtype name: byteranges
Required parameters: boundary Required parameters: boundary
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...the first range... ...the first range...
--THIS_STRING_SEPARATES --THIS_STRING_SEPARATES
Content-type: application/pdf Content-type: application/pdf
Content-range: bytes 7000-7999/8000 Content-range: bytes 7000-7999/8000
...the second range ...the second range
--THIS_STRING_SEPARATES-- --THIS_STRING_SEPARATES--
Notes: Notes:
1) Additional CRLFs may precede the first boundary string in 1) Additional CRLFs may precede the first boundary string in the
the entity. entity.
2) Although RFC 2046 [40] permits the boundary string to be 2) Although RFC 2046 [40] permits the boundary string to be
quoted, some existing implementations handle a quoted boundary quoted, some existing implementations handle a quoted boundary
string incorrectly. string incorrectly.
3) A number of browsers and servers were coded to an early 3) A number of browsers and servers were coded to an early draft
draft of the byteranges specification to use a media type of of the byteranges specification to use a media type of
multipart/x-byteranges, which is almost, but not quite multipart/x-byteranges, which is almost, but not quite
compatible with the version documented in HTTP/1.1. compatible with the version documented in HTTP/1.1.
19.3 Tolerant Applications 19.3 Tolerant Applications
Although this document specifies the requirements for the generation Although this document specifies the requirements for the generation
of HTTP/1.1 messages, not all applications will be correct in their of HTTP/1.1 messages, not all applications will be correct in their
implementation. We therefore recommend that operational applications implementation. We therefore recommend that operational applications
be tolerant of deviations whenever those deviations can be be tolerant of deviations whenever those deviations can be
interpreted unambiguously. interpreted unambiguously.
Clients SHOULD be tolerant in parsing the Status-Line and servers Clients SHOULD be tolerant in parsing the Status-Line and servers
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The character set of an entity-body SHOULD be labeled as the lowest The character set of an entity-body SHOULD be labeled as the lowest
common denominator of the character codes used within that body, with common denominator of the character codes used within that body, with
the exception that not labeling the entity is preferred over labeling the exception that not labeling the entity is preferred over labeling
the entity with the labels US-ASCII or ISO-8859-1. See section 3.7.1 the entity with the labels US-ASCII or ISO-8859-1. See section 3.7.1
and 3.4.1. and 3.4.1.
Additional rules for requirements on parsing and encoding of dates Additional rules for requirements on parsing and encoding of dates
and other potential problems with date encodings include: and other potential problems with date encodings include:
. HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date - HTTP/1.1 clients and caches SHOULD assume that an RFC-850 date
which appears to be more than 50 years in the future is in fact which appears to be more than 50 years in the future is in fact
in the past (this helps solve the "year 2000" problem). in the past (this helps solve the "year 2000" problem).
. An HTTP/1.1 implementation MAY internally represent a parsed - An HTTP/1.1 implementation MAY internally represent a parsed
Expires date as earlier than the proper value, but MUST NOT Expires date as earlier than the proper value, but MUST NOT
internally represent a parsed Expires date as later than the internally represent a parsed Expires date as later than the
proper value. proper value.
. All expiration-related calculations MUST be done in GMT. The - All expiration-related calculations MUST be done in GMT. The
local time zone MUST NOT influence the calculation or local time zone MUST NOT influence the calculation or comparison
comparison of an age or expiration time. of an age or expiration time.
. If an HTTP header incorrectly carries a date value with a time - If an HTTP header incorrectly carries a date value with a time
zone other than GMT, it MUST be converted into GMT using the zone other than GMT, it MUST be converted into GMT using the
most conservative possible conversion. most conservative possible conversion.
19.4 Differences Between HTTP Entities and RFC 2045 Entities 19.4 Differences Between HTTP Entities and RFC 2045 Entities
HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC
822 [9]) and the Multipurpose Internet Mail Extensions (MIME [7]) to 822 [9]) and the Multipurpose Internet Mail Extensions (MIME [7]) to
allow entities to be transmitted in an open variety of allow entities to be transmitted in an open variety of
representations and with extensible mechanisms. However, RFC 2045 representations and with extensible mechanisms. However, RFC 2045
discusses mail, and HTTP has a few features that are different from discusses mail, and HTTP has a few features that are different from
skipping to change at page 154, line 37 skipping to change at page 170, line 50
See section 15.5 for Content-Disposition security issues. See section 15.5 for Content-Disposition security issues.
19.6 Compatibility with Previous Versions 19.6 Compatibility with Previous Versions
It is beyond the scope of a protocol specification to mandate It is beyond the scope of a protocol specification to mandate
compliance with previous versions. HTTP/1.1 was deliberately compliance with previous versions. HTTP/1.1 was deliberately
designed, however, to make supporting previous versions easy. It is designed, however, to make supporting previous versions easy. It is
worth noting that, at the time of composing this specification worth noting that, at the time of composing this specification
(1996), we would expect commercial HTTP/1.1 servers to: (1996), we would expect commercial HTTP/1.1 servers to:
. recognize the format of the Request-Line for HTTP/0.9, 1.0, and - recognize the format of the Request-Line for HTTP/0.9, 1.0, and
1.1 requests; 1.1 requests;
. understand any valid request in the format of HTTP/0.9, 1.0, or - understand any valid request in the format of HTTP/0.9, 1.0, or
1.1; 1.1;
. respond appropriately with a message in the same major version - respond appropriately with a message in the same major version
used by the client. used by the client.
And we would expect HTTP/1.1 clients to: And we would expect HTTP/1.1 clients to:
. recognize the format of the Status-Line for HTTP/1.0 and 1.1 - recognize the format of the Status-Line for HTTP/1.0 and 1.1
responses; responses;
. understand any valid response in the format of HTTP/0.9, 1.0, - understand any valid response in the format of HTTP/0.9, 1.0, or
or 1.1. 1.1.
For most implementations of HTTP/1.0, each connection is established For most implementations of HTTP/1.0, each connection is established
by the client prior to the request and closed by the server after by the client prior to the request and closed by the server after
sending the response. Some implementations implement the Keep-Alive sending the response. Some implementations implement the Keep-Alive
version of persistent connections described in section 19.7.1 of RFC version of persistent connections described in section 19.7.1 of RFC
2068 [33]. 2068 [33].
19.6.1 Changes from HTTP/1.0 19.6.1 Changes from HTTP/1.0
This section summarizes major differences between versions HTTP/1.0 This section summarizes major differences between versions HTTP/1.0
and HTTP/1.1. and HTTP/1.1.
19.6.1.1 Changes to Simplify Multi-homed Web Servers and Conserve IP 19.6.1.1 Changes to Simplify Multi-homed Web Servers and Conserve IP
Addresses Addresses
The requirements that clients and servers support the Host request- The requirements that clients and servers support the Host request-
header, report an error if the Host request-header (section 14.23) is header, report an error if the Host request-header (section 14.23) is
missing from an HTTP/1.1 request, and accept absolute URIs (section missing from an HTTP/1.1 request, and accept absolute URIs (section
5.1.2) are among the most important changes defined by this 5.1.2) are among the most important changes defined by this
specification. specification.
Older HTTP/1.0 clients assumed a one-to-one relationship of IP Older HTTP/1.0 clients assumed a one-to-one relationship of IP
addresses and servers; there was no other established mechanism for addresses and servers; there was no other established mechanism for
distinguishing the intended server of a request than the IP address distinguishing the intended server of a request than the IP address
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simplifying large operational Web servers, where allocation of many simplifying large operational Web servers, where allocation of many
IP addresses to a single host has created serious problems. The IP addresses to a single host has created serious problems. The
Internet will also be able to recover the IP addresses that have been Internet will also be able to recover the IP addresses that have been
allocated for the sole purpose of allowing special-purpose domain allocated for the sole purpose of allowing special-purpose domain
names to be used in root-level HTTP URLs. Given the rate of growth of names to be used in root-level HTTP URLs. Given the rate of growth of
the Web, and the number of servers already deployed, it is extremely the Web, and the number of servers already deployed, it is extremely
important that all implementations of HTTP (including updates to important that all implementations of HTTP (including updates to
existing HTTP/1.0 applications) correctly implement these existing HTTP/1.0 applications) correctly implement these
requirements: requirements:
. Both clients and servers MUST support the Host request-header. - Both clients and servers MUST support the Host request-header.
. A client that sends an HTTP/1.1 request MUST send a Host - A client that sends an HTTP/1.1 request MUST send a Host header.
header.
. Servers MUST report a 400 (Bad Request) error if an HTTP/1.1 - Servers MUST report a 400 (Bad Request) error if an HTTP/1.1
request does not include a Host request-header. request does not include a Host request-header.
. Servers MUST accept absolute URIs. - Servers MUST accept absolute URIs.
19.6.2 Compatibility with HTTP/1.0 Persistent Connections 19.6.2 Compatibility with HTTP/1.0 Persistent Connections
Some clients and servers might wish to be compatible with some Some clients and servers might wish to be compatible with some
previous implementations of persistent connections in HTTP/1.0 previous implementations of persistent connections in HTTP/1.0
clients and servers. Persistent connections in HTTP/1.0 are clients and servers. Persistent connections in HTTP/1.0 are
explicitly negotiated as they are not the default behavior. HTTP/1.0 explicitly negotiated as they are not the default behavior. HTTP/1.0
experimental implementations of persistent connections are faulty, experimental implementations of persistent connections are faulty,
and the new facilities in HTTP/1.1 are designed to rectify these and the new facilities in HTTP/1.1 are designed to rectify these
problems. The problem was that some existing 1.0 clients may be problems. The problem was that some existing 1.0 clients may be
skipping to change at page 156, line 29 skipping to change at page 172, line 51
The original HTTP/1.0 form of persistent connections (the Connection: The original HTTP/1.0 form of persistent connections (the Connection:
Keep-Alive and Keep-Alive header) is documented in RFC 2068. [33] Keep-Alive and Keep-Alive header) is documented in RFC 2068. [33]
19.6.3 Changes from RFC 2068 19.6.3 Changes from RFC 2068
This specification has been carefully audited to correct and This specification has been carefully audited to correct and
disambiguate key word usage; RFC 2068 had many problems in respect to disambiguate key word usage; RFC 2068 had many problems in respect to
the conventions laid out in RFC 2119 [34]. the conventions laid out in RFC 2119 [34].
Clarified which error code should be used for inbound server failures Clarified which error code should be used for inbound server failures
(e.g. DNS failures). (Section 10.5.5) (e.g. DNS failures). (Section 10.5.5).
CREATE had a race that required an Etag be sent when a resource is CREATE had a race that required an Etag be sent when a resource is
first created. (Section 10.2.2). first created. (Section 10.2.2).
Content-Base was deleted from the specification: it was not Content-Base was deleted from the specification: it was not
implemented widely, and there is no simple, safe way to introduce it implemented widely, and there is no simple, safe way to introduce it
without a robust extension mechanism. In addition, it is used in a without a robust extension mechanism. In addition, it is used in a
similar, but not identical fashion in MHTML [45]. similar, but not identical fashion in MHTML [45].
Transfer-coding and message lengths all interact in ways that Transfer-coding and message lengths all interact in ways that
skipping to change at page 157, line 36 skipping to change at page 174, line 15
syntactic problems, and range doesn't exist in the document. The 416 syntactic problems, and range doesn't exist in the document. The 416
status code was needed to resolve this ambiguity needed to indicate status code was needed to resolve this ambiguity needed to indicate
an error for a byte range request that falls outside of the actual an error for a byte range request that falls outside of the actual
contents of a document. (Section 10.4.17, 14.16) contents of a document. (Section 10.4.17, 14.16)
Rewrite of message transmission requirements to make it much harder Rewrite of message transmission requirements to make it much harder
for implementors to get it wrong, as the consequences of errors here for implementors to get it wrong, as the consequences of errors here
can have significant impact on the Internet, and to deal with the can have significant impact on the Internet, and to deal with the
following problems: following problems:
1. Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where 1. Changing "HTTP/1.1 or later" to "HTTP/1.1", in contexts where
this was incorrectly placing a requirement on the behavior of an this was incorrectly placing a requirement on the behavior of
implementation of a future version of HTTP/1.x an implementation of a future version of HTTP/1.x
2. Made it clear that user-agents should retry requests, not 2. Made it clear that user-agents should retry requests, not
"clients" in general. "clients" in general.
3. Converted requirements for clients to ignore unexpected 100 3. Converted requirements for clients to ignore unexpected 100
(Continue) responses, and for proxies to forward 100 responses, (Continue) responses, and for proxies to forward 100 responses,
into a general requirement for 1xx responses. into a general requirement for 1xx responses.
4. Modified some TCP-specific language, to make it clearer that 4. Modified some TCP-specific language, to make it clearer that
non-TCP transports are possible for HTTP. non-TCP transports are possible for HTTP.
5. Require that the origin server MUST NOT wait for the request 5. Require that the origin server MUST NOT wait for the request
body before it sends a required 100 (Continue) response. body before it sends a required 100 (Continue) response.
6. 6. Allow, rather than require, a server to omit 100 (Continue) if
Allow, rather than require, a server to omit 100 (Continue) if it has already seen some of the request body.
it has already seen some of the request body.
7. 7. Allow servers to defend against denial-of-service attacks and
Allow servers to defend against denial-of-service attacks and broken clients.
broken clients.
This change adds the Expect header and 417 status code. The message This change adds the Expect header and 417 status code. The message
transmission requirements fixes are in sections 8.2, 10.4.18, transmission requirements fixes are in sections 8.2, 10.4.18,
8.1.2.2, 13.11, and 14.20. 8.1.2.2, 13.11, and 14.20.
Proxies should be able to add Content-Length when appropriate. Proxies should be able to add Content-Length when appropriate.
(Section 13.5.2) (Section 13.5.2)
Clean up confusion between 403 and 404 responses. (Section 10.4.4, Clean up confusion between 403 and 404 responses. (Section 10.4.4,
10.4.5, and 10.4.11) 10.4.5, and 10.4.11)
skipping to change at page 158, line 39 skipping to change at page 175, line 24
clients.(Section 3.6, 3.6.1, and 14.39) clients.(Section 3.6, 3.6.1, and 14.39)
The PATCH, LINK, UNLINK methods were defined but not commonly The PATCH, LINK, UNLINK methods were defined but not commonly
implemented in previous versions of this specification. See RFC 2068 implemented in previous versions of this specification. See RFC 2068
[33]. [33].
The Alternates, Content-Version, Derived-From, Link, URI, Public and The Alternates, Content-Version, Derived-From, Link, URI, Public and
Content-Base header fields were defined in previous versions of this Content-Base header fields were defined in previous versions of this
specification, but not commonly implemented. See RFC 2068 [33]. specification, but not commonly implemented. See RFC 2068 [33].
19.7 Notes to the RFC Editor and IANA 20 Index
This section of the document should be DELETED! It calls for the RFC
editor and IANA to take some actions before the draft becomes a Draft
Standard. After those actions are taken, please delete this section
of the specification.
19.7.1 Transfer-coding Values
This document defines a new class of registry for its transfer-coding
values as part of the solution to solve problems discovered in RFC
2068 with the caching of transfer encoded documents. Initially, the
registry should contain the following tokens: "chunked" (section
3.6.1), "gzip" (section 3.5), "compress" (section 3.5), and
"deflate" (section 3.5) and the special keyword "trailers" (section
14.39). The registry should note that "specifications of the
transfer-coding algorithms needed to implement a new value should be
publicly available and adequate for independent implementation, and
conform to the purpose of content coding defined RFC XXXX." where RFC
XXXX is the number assigned to this document.
19.7.2 Definition of application/http
Appendix 19.1 defines Internet Media Type application/http in
addition to the Internet Media Type message/http defined by RFC 2068.
19.7.3 Addition of "identity" content-coding to content-coding Registry
The "identity" content coding is added to the content-coding registry Please see the PostScript version of this RFC for the INDEX.
by this document (section 3.5) to solve a problem discovered in RFC
2068.
20 Full Copyright Statement 21. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved. Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than followed, or as required to translate it into languages other than
English. English.
The limited permissions granted above are perpetual and will not be The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns. revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
21 Index Acknowledgement
While some care was taken producing this index, there is no guarantee
that all occurrences of an index term have been entered into the
index. Bold face italic is used for the definition of a term.
"literal", 14 411, 32, 38, 63
#rule, 15 412, 38, 63, 118, 120, 122
(rule1 rule2), 14 413, 38, 63
*rule, 15 414, 19, 38, 63
; comment, 15 415, 38, 64, 108
[rule], 15 416, 38, 64, 112, 113, 126, 157
<">, 16 417, 38, 64, 115, 158
100, 38, 44, 45, 46, 53, 90, 114, 4xx Client Error Status Codes, 60
115 500, 38, 64, 65, 114
101, 38, 53, 114, 131 501, 24, 34, 38, 51, 65
1xx Informational Status Codes, 502, 38, 65
53 503, 38, 65, 114, 128
200, 38, 48, 50, 51, 52, 54, 55, 504, 38, 65, 104, 105
56, 59, 84, 89, 104, 113, 119, 505, 38, 65
122, 127 5xx Server Error Status Codes, 64
201, 38, 50, 51, 54, 123 abs_path, 19, 34, 35
202, 38, 52, 54, 55 absoluteURI, 19, 34, 35, 36, 109,
203, 38, 55, 84 123, 128
204, 31, 32, 38, 50, 51, 52, 55 Accept, 25, 36, 67, 71, 91, 92,
205, 38, 55 94, 95, 138
206, 38, 56, 84, 86, 89, 112, acceptable-ranges, 96
122, 126, 127, 149, 157 Accept-Charset, 36, 67, 93, 94
2xx, 121 Accept-Encoding, 22, 23, 36, 67,
2xx Successful Status Codes, 54 94, 95
300, 38, 57, 68, 84 accept-extension, 92
301, 38, 51, 57, 84, 131 Accept-Language, 28, 36, 67, 95,
302, 38, 57, 58, 59, 84, 131 96, 135, 138, 139
303, 38, 50, 58, 131 accept-params, 92, 129
304, 31, 32, 38, 58, 59, 70, 78, Accept-Ranges, 39, 96
82, 86, 88, 104, 119, 120, 121, Access Authentication, 66
127 Basic and Digest. See [43]
305, 38, 59, 70, 131 Acknowledgements, 142
306, 59 age, 11
307, 38, 58, 60, 84 Age, 39, 74, 75, 76, 97
3xx Redirection Status Codes, 56 age-value, 97
400, 32, 36, 38, 39, 60, 117, 155 Allow, 34, 39, 48, 61, 97
401, 38, 60, 62, 98, 136 ALPHA, 14, 16
402, 38, 61 Alternates. See RFC 2068
403, 38, 61, 158 ANSI X3.4-1986, 16, 144
404, 38, 61, 63, 158 asctime-date, 20
405, 34, 38, 61, 97 attribute, 23
406, 38, 61, 62, 68, 92, 94 authority, 19, 34, 35
407, 38, 62, 124 Authorization, 36, 60, 61, 84,
408, 38, 62 98, 100, 125
409, 38, 62 Backus-Naur Form, 14
410, 38, 62, 63, 84 Basic Authentication. See [43]
BCP 18, 146 cache-extension, 99
BCP 9, 1, 146 extensions, 106
byte-content-range-spec, 111, 112 max-age, 73, 76, 77, 84, 99,
byte-range, 125 101, 102, 103, 104, 105, 116,
byte-range-resp-spec, 111, 112 157
byte-range-set, 125, 126 max-stale, 71, 72, 99, 102, 103,
byte-range-spec, 64, 113, 125, 104
126 min-fresh, 99, 102
byte-ranges-specifier, 125 must-revalidate, 98, 99, 103,
bytes, 96 104, 105
bytes-unit, 29 no-cache, 70, 78, 99, 100, 102,
cachable, 11 103, 104, 124
cache, 11 no-store, 70, 99, 101
Cache no-transform, 99, 105, 108
cachability of responses, 83 only-if-cached, 99, 104
calculating the age of a private, 84, 99, 100, 102, 106
response, 74 proxy-revalidate, 84, 99, 105
combining byte ranges, 87 public, 71, 84, 98, 99, 100,
byte-range-spec, 64, 113, 125,
combining headers, 86 101, 105
combining negotiated responses, s-maxage, 76, 84, 98, 99, 102,
87 157
constructing responses, 84 cache-directive, 99, 106, 124
correctness, 69 cache-request-directive, 70, 99
disambiguating expiration Changes from HTTP/1.0. See RFC
values, 77 1945 and RFC 2068
disambiguating multiple Host requirement, 155
responses, 77 CHAR, 16
entity tags used as cache charset, 21, 93
validators, 79 chunk, 24
entry validation, 78 chunk-data, 24
errors or incomplete responses, chunked, 129, 130
89 Chunked-Body, 24
expiration calculation, 76 chunk-extension, 24
explicit expiration time, 73 chunk-ext-name, 24
GET and HEAD cannot affect chunk-ext-val, 24
caching, 89 chunk-size, 24
heuristic expiration, 74 client, 10
history list behavior, 91 codings, 94
invalidation cannot be complete, comment, 17, 132, 133
89 Compatibility
Last-Modified values used as missing charset, 22
validators, 79 multipart/x-byteranges, 150
mechanisms, 71 Compatibility with previous HTTP
replacement of cached responses, versions, 154
90 CONNECT, 34, 35. See [44].
shared and non-shared, 88 connection, 9
Warnings, 70 Connection, 33, 42, 43, 84, 85,
weak and strong cache 106, 107, 129, 131, 156
validators, 79 close, 42, 107, 156
write-through mandatory, 90 Keep-Alive, 156. See RFC 2068
Cache-Control, 33, 50, 56, 57, connection-token, 107
59, 71, 72, 73, 76, 77, 78, 83, Content Codings
84, 85, 89, 98, 99, 100, 102, compress, 22
103, 104, 107, 116, 124 deflate, 23
gzip, 22 Content-Transfer-Encoding, 152
identity, 23 date formats, 152
content negotiation, 10 MIME-Version, 151
Content Negotiation, 66 Transfer-Encoding, 152
Content-Base, 156. See RFC 2068 Digest Authentication, 85.
content-cncoding, 108 [43]
content-coding, 22, 23, 24, 26, DIGIT, 14, 15, 16, 18, 20, 28,
66, 94, 95, 108, 130, 136, 158 123, 151
identity, 156 disp-extension-token, 154
new tokens SHOULD be registered disposition-parm, 154
with IANA, 23 disposition-type, 154
qvalues used with, 95 DNS, 139, 140, 156
content-disposition, 154 HTTP applications MUST obey TTL
Content-Disposition, 140, 146, information, 140
153, 154 downstream, 12
Content-Encoding, 22, 23, 39, 40, End-to-end headers, 84
85, 107, 108, 110, 111, 136, entity, 9
151, 152 Entity, 39
Content-Language, 28, 39, 108, Entity body, 40
109, 135 Entity Tags, 28, 79
Content-Length, 31, 32, 33, 39, entity-body, 40
44, 48, 50, 56, 63, 86, 89, 109, entity-header, 33, 37, 39
112, 130, 153, 158 Entity-header fields, 39
Content-Location, 39, 56, 59, 85, entity-length, 40, 86
88, 90, 109, 110, 123, 140 entity-tag, 28, 120, 121
Content-MD5, 39, 50, 85, 110, Etag, 156
111, 145 ETag, 28, 39, 50, 54, 56, 59, 79,
Content-Range, 56, 84, 111 85, 86, 88, 114, 115, 120
content-range-spec, 111 Expect, 36, 44, 45, 46, 53, 64,
Content-Transfer-Encoding, 24, 115, 158
111, 152 expectation, 115
Content-Type, 22, 25, 40, 48, 52, expectation-extension, 115
54, 56, 57, 61, 62, 85, 107, expect-params, 115
112, 113, 136, 149, 152 Expires, 40, 50, 56, 57, 59, 73,
Content-Version. See RFC 2068 76, 84, 85, 101, 102, 105, 114,
CR, 16, 26, 33, 37, 38, 150, 151, 115, 116, 150
152 explicit expiration time, 11
CRLF, 14, 16, 24, 26, 29, 30, 33, extension-code, 38
37, 111, 150, 151 extension-header, 40
ctext, 17 extension-pragma, 124
CTL, 16 field-content, 30
Date, 33, 56, 58, 59, 74, 77, 78, field-name, 30
81, 83, 87, 88, 90, 102, 113, field-value, 30
114, 116, 123, 136, 152 filename-parm, 154
date1, 20 first-byte-pos, 64, 112, 113,
date2, 20 125, 126
date3, 20 first-hand, 11
DELETE, 34, 47, 48, 52, 90 fresh, 12
delta-seconds, 21, 128 freshness lifetime, 12
Derived-From. See RFC 2068 freshness_lifetime, 76
Differences between MIME and From, 36, 43, 116, 117, 137, 138
HTTP, 151 gateway, 11
canonical form, 151 General Header Fields, 33
Content-Encoding, 152 general-header, 33, 37
generic-message, 29 Larry Masinter, 147
GET, 19, 34, 35, 47, 48, 49, 54, last-byte-pos, 112, 125, 126
56, 57, 58, 59, 60, 63, 78, 80, last-chunk, 24
81, 89, 90, 97, 109, 113, 118, Last-Modified, 12, 40, 50, 56,
119, 120, 121, 127, 138 74, 77, 79, 80, 81, 82, 83, 85,
HEAD, 31, 32, 34, 47, 48, 49, 50, 86, 114, 119, 121, 122, 123
54, 56, 57, 58, 60, 61, 64, 89, LF, 16, 26, 33, 37, 38, 150, 151,
90, 97, 109, 113, 120 152
Headers lifetime, 12, 74, 76, 77, 97,
end-to-end, 84, 85, 86, 107, 115 102, 136
hop-by-hop, 12, Link. See RFC 2068
non-modifiable headers, 85 LINK. See RFC 2068
Henrik Frystyk Nielsen, 147 LOALPHA, 16
heuristic expiration time, 11 Location, 39, 50, 54, 57, 58, 59,
HEX, 16, 20, 24 60, 90, 123, 140
Hop-by-hop headers, 84, 85 LWS, 14, 16, 30
host, 19, 133, 134 Max-Forwards, 36, 49, 52, 123,
Host, 35, 36, 47, 117, 155 124
HT, 14, 16, 17, 30, 150 MAY, 9
http_URL, 19 media type, 16, 22, 26, 32, 40,
HTTP-date, 20, 113, 114, 116, 54, 57, 61, 66, 92, 93, 105,
119, 121, 122, 128, 134 107, 109, 113, 148, 149, 150,
HTTP-message, 29 151, 152
HTTP-Version, 18, 33, 37 Media Types, 25
IANA, 21, 22, 24, 26, 28, 92, 148 media-range, 91
identity, 23, 94, 95, 108, 156 media-type, 25, 26, 107, 110, 136
If-Match, 28, 36, 49, 82, 118, message, 9
119, 120, 121, 127 Message Body, 31
If-Modified-Since, 36, 49, 80, Message Headers, 30
82, 119, 120, 121, 122, 127 Message Length, 31
If-None-Match, 28, 36, 49, 82, Message Transmission
88, 119, 120, 121, 122, 127 Requirements, 44
If-Range, 28, 36, 49, 56, 64, 82, Message Types, 29
113, 121, 122, 127 message-body, 29, 31, 33, 37, 40
If-Unmodified-Since, 36, 49, 80, message-header, 29, 30, 40
82, 120, 121, 122, 127 Method, 33, 34, 97
If-Unmodified-Since, 122 Method Definitions, 47
implied *LWS, 15 Methods
inbound, 12 Idempotent, 48
instance-length, 111, 112 Safe and Idempotent, 47
ISO-10646, 146 MIME, 8, 9, 12, 21, 24, 26, 27,
ISO-2022, 21 109, 110, 111, 142, 144, 146,
ISO-3166, 28 151, 152, 153
ISO-639, 28 multipart, 26
ISO-8859, 145 MIME-Version, 151
ISO-8859-1, 16, 22, 26, 93, 94, month, 21
135, 150 multipart/byteranges, 27, 32, 56,
James Gettys, 147 64, 112, 149
Jeffrey C. Mogul, 147 multipart/x-byteranges, 150
Keep-Alive, 43, 84, 154, 156. See MUST, 9
RFC 2068 MUST NOT, 9
Language Tags, 28 N rule, 15
language-range, 95, 96 name, 14
language-tag, 28, 95 non-shared cache, 88, 100, 106
non-transparent proxy. See proxy: range-unit, 29, 96
non-transparent Reason-Phrase, 37, 38
OCTET, received-by, 133, 134
opaque-tag, 28 received-protocol, 133, 134
OPTIONAL, 9 RECOMMENDED, 9
OPTIONS, 34, 48, 49, 123, 124 References, 143
origin server, 10 Referer, 36, 127, 128, 137, 138
other-range-unit, 29 rel_path, 19, 89
outbound, 12 relativeURI, 19, 109, 128
parameter, 23 representation,10
PATCH. See RFC 2068 request, 9
Paul J. Leach, 147 Request, 33
Persistent Connections, 41 Request header fields, 36
Overall Operation, 41 request-header, 33, 36
Purpose, 41 Request-Line, 30, 33, 34, 50, 61,
Use of Connection Header, 42 150, 154
Pipelining, 42 Request-URI, 19, 33, 34, 35, 36,
port, 19, 133, 134 38, 39, 48, 49, 50, 51, 52, 57,
POST, 27, 30, 34, 45, 47, 50, 51, 59, 61, 63, 87, 88, 90, 97, 108,
54, 57, 58, 63, 90, 114, 138 110, 123, 124, 127, 128, 136,
Pragma, 33, 99, 103, 124 138, 139
no-cache, 70, 78, 99, 124 REQUIRED, 9
pragma-directive, 124 Requirements
primary-tag, 28 compliance, 9
product, 27, 132 key words, 9
Product tokens, 27 resource, 9
product-version, 27 response, 9
protocol-name, 133 Response, 37
protocol-version, 133 Response Header Fields, 39
proxy, 10 response-header, 37, 39
non-transparent, 10, 85, 105, Retry-After, 39, 63, 65, 128
108 Revalidation
transparent, 10, 40, 85 end-to-end, 103
Proxy-Authenticate, 39, 62, 84, end-to-end reload, 103
124, 125 end-to-end specific
Proxy-Authorization, 36, 62, 85, revalidation, 103
125 end-to-end unspecific
pseudonym, 133, 134 revalidation, 104
Public. See RFC 2068 RFC 1036, 20, 144
public cache, 67, 68 RFC 1123, 20, 113, 116, 144
PUT, 34, 45, 47, 48, 51, 52, 62, RFC 1305, 145
90, 97, 114, 118, 121 RFC 1436, 143
qdtext, 17 RFC 1590, 26, 144
Quality Values, 27 RFC 1630, 143
quoted-pair, 17 RFC 1700, 144
quoted-string, 15, 17, 24, 28, RFC 1737, 144
30, 92, 99, 115, 124, 134, 154 RFC 1738, 19, 143
qvalue, 28, 92, 93, 94 RFC 1766, 28, 143
Range, 29, 36, 40, 49, 51, 56, RFC 1806, 140, 146, 153
64, 84, 85, 87, 112, 113, 119, RFC 1808, 19, 144
121, 125, 127, 149 RFC 1864, 110, 111, 145
Range Units, 29 RFC 1866, 143
ranges-specifier, 111, 125, 126, RFC 1867, 27, 144
127 RFC 1900, 19, 145
RFC 1945, 8, 58, 144, 153 Referer header, 137, 138
RFC 1950, 23, 145 sensitive headers, 137
RFC 1951, 23, 145 Server header, 137
RFC 1952, 145 Transfer of Sensitive
RFC 2026, 146 Information, 137
RFC 2044, 146 Via header, 137
RFC 2045, 144, 151, 152 selecting request-headers, 87
RFC 2046, 26, 146, 150, 151 semantically transparent, 12
RFC 2047, 16, 135, 144 separators, 17
RFC 2049, 147, 151 server, 10
RFC 2068, 2, 18, 41, 43, 45, 58, Server, 27, 39, 128, 133, 137,
59, 143, 146, 153, 155, 156 138
changes from, 156 SHALL, 9
RFC 2069, 145 SHALL NOT, 9
RFC 2076, 146, 153 shared caches, 88, 101
RFC 2110, 146 SHOULD, 9
RFC 2119, 9, 146, 156 SHOULD NOT, 9
RFC 2145, 17, 146, 156 SP, 14, 16, 17, 20, 30, 33, 37,
RFC 2277, 146 111, 134, 150
RFC 2279, 146 stale, 12
RFC 2324, 147 start-line, 30
RFC 2396, 19, 146 Status Code Definitions, 53
RFC 821, 144 Status-Code, 37, 38, 53
RFC 822, 14, 20, 29, 30, 113, Status-Line, 30, 37, 39, 53, 150,
116, 133, 142, 144, 151 154
RFC 850, 20, 144 strong entity tag, 29
RFC 959, 144 strong validators, 80
RFC 977, 144 subtag, 28
rfc1123-date, 20 subtype, 25
RFC-850, 150 suffix-byte-range-spec, 125, 126
rfc850-date, 20 suffix-length, 126
Roy T. Fielding, 147 T/TCP, 41
rule1 | rule2, 14 t-codings, 129
Safe and Idempotent Methods, 47 TE, 25, 36, 129, 130, 158
Security Considerations, 137 TEXT, 16
abuse of server logs, 137 Tim Berners-Lee, 148
Accept header, 138 time, 20
Accept headers can reveal ethnic token, 15, 17, 21, 22, 23, 24,
information, 138 25, 27, 29, 30, 33, 34, 92, 99,
attacks based on path names, 139 107, 115, 124, 131, 133, 154
Authentication Credentials and Tolerant Applications, 150
Idle Clients, 140 bad dates, 150
be careful about personal should tolerate whitespace in
information, 137 request and status lines, 150
Content-Disposition Header, 140 tolerate LF and ignore CR in
Content-Location header, 140 line terminators, 150
encoding information in URI's, use lowest common denominator of
138 character set, 150
From header, 138 TRACE, 34, 48, 52, 54, 123, 124
GET method, 138 trailer, 24
Location header, 140 Trailer, 24, 33, 130
Location headers and spoofing, trailers, 129
140 Transfer Encoding
Proxies and Caching, 141 chunked, 23
transfer-coding validators, 12, 28, 71, 77, 78, Funding for the RFC Editor function is currently provided by the
chunked, 24 79, 80, 82, 83, 87 Internet Society.
deflate, 24 rules on use of, 81
gzip, 24 value, 23
identity, 24 variant, 10
transfer-coding, 23, 24, 25, 31, Vary, 39, 56, 59, 67, 87, 118,
32, 40, 110, 129, 130, 152, 153, 121, 132, 139
156, 158 Via, 33, 52, 128, 133, 134, 137
chunked, 23, 24, 32, 44, 129, warn-agent, 134
130, 153, 158 warn-code, 86, 134
chunked REQUIRED, 32 warn-codes, 70
compress, 24, 158 warn-date, 134, 136
identity, 32 Warning, 33, 70, 71, 72, 76, 83,
trailers, 129 85, 86, 103, 134, 135, 136, 158
Transfer-Encoding, 23, 31, 32, Warnings
33, 40, 48, 85, 130, 131, 152, 110 Response is stale, 135
153 111 Revalidation failed, 135
transfer-extension, 23, 129 112 Disconnected operation, 135
transfer-length, 40, 86 113 Heuristic expiration, 135
transparent 199 Miscellaneous warning, 136
proxy, 85 214 Transformation applied, 136
transparent proxy. See proxy: 299 Miscellaneous persistent
transparent warning, 136
tunnel, 11 warning-value, 134, 136
type, 25 warn-text, 134, 135
UNLINK. See RFC 2068 weak, 28
UPALPHA, 16 weak entity tag, 29
Upgrade, 33, 53, 85, 131 weak validators, 80
upstream, 12 weekday, 21
URI. See RFC 2068 wkday, 21
URI-reference, 19 WWW-Authenticate, 39, 60, 125,
US-ASCII, 16, 21, 150 136
user agent, 10 x-compress, 95
User-Agent, 27, 36, 67, 132, 133, x-gzip, 95
138
 End of changes. 428 change blocks. 
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