rfc2616-orig.txt   draft-rfc2616bis-00.txt 
Network Working Group R. Fielding Network Working Group Y. Lafon
Request for Comments: 2616 UC Irvine Internet-Draft W3C
Obsoletes: 2068 J. Gettys Obsoletes: 2616 (if approved) J. Reschke
Category: Standards Track Compaq/W3C Intended status: Standards Track greenbytes
J. Mogul Expires: April 16, 2007 October 13, 2006
Compaq
H. Frystyk
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
draft-lafon-rfc2616bis-00
Status of this Memo Status of this Memo
This document specifies an Internet standards track protocol for the By submitting this Internet-Draft, each author represents that any
Internet community, and requests discussion and suggestions for applicable patent or other IPR claims of which he or she is aware
improvements. Please refer to the current edition of the "Internet have been or will be disclosed, and any of which he or she becomes
Official Protocol Standards" (STD 1) for the standardization state aware will be disclosed, in accordance with Section 6 of BCP 79.
and status of this protocol. Distribution of this memo is unlimited.
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved. Copyright (C) The Internet Society (2006).
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 RFC2616.
Editorial Note (To be removed by RFC Editor before publication)
Distribution of this document is unlimited. Please send comments to
the Hypertext Transfer Protocol (HTTP) mailing list at
ietf-http-wg@w3.org [51], which may be joined by sending a message
with subject "subscribe" to ietf-http-wg-request@w3.org [52].
Discussions of the HTTP working group are archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>. XML versions,
latest edits and the issues list for this document are available from
<http://www.w3.org/Protocols/HTTP/1.1/>.
The purpose of this document is to revise RFC2616 ([50]), doing only
minimal corrections. For now, it is not planned to advance the
standards level of HTTP, thus - if published - the specification will
still be a "Proposed Standard" (see [46]).
The current plan is to incorporate known errata, and to update the
specification text according to the current IETF publication
guidelines. In particular:
o Incorporate the corrections collected in the RFC2616 errata
document (<http://skrb.org/ietf/http_errata.html>) and potentially
newly discovered and agreed-upon errata.
o Update references, and re-classify them into "Normative" and
"Informative", based on the prior work done by Jim Gettys in
<http://tools.ietf.org/html/draft-gettys-http-v11-spec-rev-00>.
This document is based on a variant of the original RFC2616
specification formatted using Marshall T. Rose's "xml2rfc" tool (see
<http://xml.resource.org>) and therefore deviates from the original
text in word wrapping, page breaks, list formatting, reference
formatting, whitespace usage and appendix numbering. Otherwise, it
is supposed to contain an accurate copy of the original specification
text. See <http://www.w3.org/Protocols/HTTP/1.1/
rfc2616bis-00-from-rfc2616.diff.html> for a comparison between both
documents, as generated by "rfcdiff"
(<http://tools.ietf.org/tools/rfcdiff/>).
Table of Contents Table of Contents
1 Introduction ...................................................7 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 10
1.1 Purpose......................................................7 1.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.2 Requirements .................................................8 1.2 Requirements . . . . . . . . . . . . . . . . . . . . . . 10
1.3 Terminology ..................................................8 1.3 Terminology . . . . . . . . . . . . . . . . . . . . . . . 11
1.4 Overall Operation ...........................................12 1.4 Overall Operation . . . . . . . . . . . . . . . . . . . . 15
2 Notational Conventions and Generic Grammar ....................14 2 Notational Conventions and Generic Grammar . . . . . . . . . 18
2.1 Augmented BNF ...............................................14 2.1 Augmented BNF . . . . . . . . . . . . . . . . . . . . . . 18
2.2 Basic Rules .................................................15 2.2 Basic Rules . . . . . . . . . . . . . . . . . . . . . . . 20
3 Protocol Parameters ...........................................17 3 Protocol Parameters . . . . . . . . . . . . . . . . . . . . . 22
3.1 HTTP Version ................................................17 3.1 HTTP Version . . . . . . . . . . . . . . . . . . . . . . 22
3.2 Uniform Resource Identifiers ................................18 3.2 Uniform Resource Identifiers . . . . . . . . . . . . . . 23
3.2.1 General Syntax ...........................................19 3.2.1 General Syntax . . . . . . . . . . . . . . . . . . . 23
3.2.2 http URL .................................................19 3.2.2 http URL . . . . . . . . . . . . . . . . . . . . . . 23
3.2.3 URI Comparison ...........................................20 3.2.3 URI Comparison . . . . . . . . . . . . . . . . . . . 24
3.3 Date/Time Formats ...........................................20 3.3 Date/Time Formats . . . . . . . . . . . . . . . . . . . . 24
3.3.1 Full Date ................................................20 3.3.1 Full Date . . . . . . . . . . . . . . . . . . . . . . 24
3.3.2 Delta Seconds ............................................21 3.3.2 Delta Seconds . . . . . . . . . . . . . . . . . . . . 26
3.4 Character Sets ..............................................21 3.4 Character Sets . . . . . . . . . . . . . . . . . . . . . 26
3.4.1 Missing Charset ..........................................22 3.4.1 Missing Charset . . . . . . . . . . . . . . . . . . . 27
3.5 Content Codings .............................................23 3.5 Content Codings . . . . . . . . . . . . . . . . . . . . . 27
3.6 Transfer Codings ............................................24 3.6 Transfer Codings . . . . . . . . . . . . . . . . . . . . 28
3.6.1 Chunked Transfer Coding ..................................25 3.6.1 Chunked Transfer Coding . . . . . . . . . . . . . . . 29
3.7 Media Types .................................................26 3.7 Media Types . . . . . . . . . . . . . . . . . . . . . . . 31
3.7.1 Canonicalization and Text Defaults .......................27 3.7.1 Canonicalization and Text Defaults . . . . . . . . . 31
3.7.2 Multipart Types ..........................................27 3.7.2 Multipart Types . . . . . . . . . . . . . . . . . . . 32
3.8 Product Tokens ..............................................28 3.8 Product Tokens . . . . . . . . . . . . . . . . . . . . . 33
3.9 Quality Values ..............................................29 3.9 Quality Values . . . . . . . . . . . . . . . . . . . . . 33
3.10 Language Tags ...............................................29 3.10 Language Tags . . . . . . . . . . . . . . . . . . . . . . 34
3.11 Entity Tags .................................................30 3.11 Entity Tags . . . . . . . . . . . . . . . . . . . . . . . 34
3.12 Range Units .................................................30 3.12 Range Units . . . . . . . . . . . . . . . . . . . . . . . 35
4 HTTP Message ..................................................31 4 HTTP Message . . . . . . . . . . . . . . . . . . . . . . . . 36
4.1 Message Types ...............................................31 4.1 Message Types . . . . . . . . . . . . . . . . . . . . . . 36
4.2 Message Headers .............................................31 4.2 Message Headers . . . . . . . . . . . . . . . . . . . . . 36
4.3 Message Body ................................................32 4.3 Message Body . . . . . . . . . . . . . . . . . . . . . . 37
4.4 Message Length ..............................................33 4.4 Message Length . . . . . . . . . . . . . . . . . . . . . 38
4.5 General Header Fields .......................................34 4.5 General Header Fields . . . . . . . . . . . . . . . . . . 39
5 Request .......................................................35 5 Request . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
5.1 Request-Line ................................................35 5.1 Request-Line . . . . . . . . . . . . . . . . . . . . . . 41
5.1.1 Method ...................................................36 5.1.1 Method . . . . . . . . . . . . . . . . . . . . . . . 41
5.1.2 Request-URI ..............................................36 5.1.2 Request-URI . . . . . . . . . . . . . . . . . . . . . 42
5.2 The Resource Identified by a Request ........................38 5.2 The Resource Identified by a Request . . . . . . . . . . 43
5.3 Request Header Fields .......................................38 5.3 Request Header Fields . . . . . . . . . . . . . . . . . . 44
6 Response ......................................................39 6 Response . . . . . . . . . . . . . . . . . . . . . . . . . . 45
6.1 Status-Line .................................................39 6.1 Status-Line . . . . . . . . . . . . . . . . . . . . . . . 45
6.1.1 Status Code and Reason Phrase ............................39 6.1.1 Status Code and Reason Phrase . . . . . . . . . . . . 45
6.2 Response Header Fields ......................................41 6.2 Response Header Fields . . . . . . . . . . . . . . . . . 48
7 Entity ........................................................42 7 Entity . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
7.1 Entity Header Fields ........................................42 7.1 Entity Header Fields . . . . . . . . . . . . . . . . . . 49
7.2 Entity Body .................................................43 7.2 Entity Body . . . . . . . . . . . . . . . . . . . . . . . 49
7.2.1 Type .....................................................43 7.2.1 Type . . . . . . . . . . . . . . . . . . . . . . . . 50
7.2.2 Entity Length ............................................43 7.2.2 Entity Length . . . . . . . . . . . . . . . . . . . . 50
8 Connections ...................................................44 8 Connections . . . . . . . . . . . . . . . . . . . . . . . . . 51
8.1 Persistent Connections ......................................44 8.1 Persistent Connections . . . . . . . . . . . . . . . . . 51
8.1.1 Purpose ..................................................44 8.1.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . 51
8.1.2 Overall Operation ........................................45 8.1.2 Overall Operation . . . . . . . . . . . . . . . . . . 51
8.1.3 Proxy Servers ............................................46 8.1.3 Proxy Servers . . . . . . . . . . . . . . . . . . . . 53
8.1.4 Practical Considerations .................................46 8.1.4 Practical Considerations . . . . . . . . . . . . . . 53
8.2 Message Transmission Requirements ...........................47 8.2 Message Transmission Requirements . . . . . . . . . . . . 54
8.2.1 Persistent Connections and Flow Control ..................47 8.2.1 Persistent Connections and Flow Control . . . . . . . 54
8.2.2 Monitoring Connections for Error Status Messages .........48 8.2.2 Monitoring Connections for Error Status Messages . . 54
8.2.3 Use of the 100 (Continue) Status .........................48 8.2.3 Use of the 100 (Continue) Status . . . . . . . . . . 55
8.2.4 Client Behavior if Server Prematurely Closes Connection ..50 8.2.4 Client Behavior if Server Prematurely Closes
9 Method Definitions ............................................51 Connection . . . . . . . . . . . . . . . . . . . . . 57
9.1 Safe and Idempotent Methods .................................51 9 Method Definitions . . . . . . . . . . . . . . . . . . . . . 58
9.1.1 Safe Methods .............................................51 9.1 Safe and Idempotent Methods . . . . . . . . . . . . . . . 58
9.1.2 Idempotent Methods .......................................51 9.1.1 Safe Methods . . . . . . . . . . . . . . . . . . . . 58
9.2 OPTIONS .....................................................52 9.1.2 Idempotent Methods . . . . . . . . . . . . . . . . . 58
9.3 GET .........................................................53 9.2 OPTIONS . . . . . . . . . . . . . . . . . . . . . . . . . 59
9.4 HEAD ........................................................54 9.3 GET . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
9.5 POST ........................................................54 9.4 HEAD . . . . . . . . . . . . . . . . . . . . . . . . . . 60
9.6 PUT .........................................................55 9.5 POST . . . . . . . . . . . . . . . . . . . . . . . . . . 61
9.7 DELETE ......................................................56 9.6 PUT . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
9.8 TRACE .......................................................56 9.7 DELETE . . . . . . . . . . . . . . . . . . . . . . . . . 63
9.9 CONNECT .....................................................57 9.8 TRACE . . . . . . . . . . . . . . . . . . . . . . . . . . 63
10 Status Code Definitions ......................................57 9.9 CONNECT . . . . . . . . . . . . . . . . . . . . . . . . . 64
10.1 Informational 1xx ...........................................57 10 Status Code Definitions . . . . . . . . . . . . . . . . . . . 65
10.1.1 100 Continue .............................................58 10.1 Informational 1xx . . . . . . . . . . . . . . . . . . . . 65
10.1.2 101 Switching Protocols ..................................58 10.1.1 100 Continue . . . . . . . . . . . . . . . . . . . . 65
10.2 Successful 2xx ..............................................58 10.1.2 101 Switching Protocols . . . . . . . . . . . . . . . 65
10.2.1 200 OK ...................................................58 10.2 Successful 2xx . . . . . . . . . . . . . . . . . . . . . 66
10.2.2 201 Created ..............................................59 10.2.1 200 OK . . . . . . . . . . . . . . . . . . . . . . . 66
10.2.3 202 Accepted .............................................59 10.2.2 201 Created . . . . . . . . . . . . . . . . . . . . . 66
10.2.4 203 Non-Authoritative Information ........................59 10.2.3 202 Accepted . . . . . . . . . . . . . . . . . . . . 66
10.2.5 204 No Content ...........................................60 10.2.4 203 Non-Authoritative Information . . . . . . . . . . 67
10.2.6 205 Reset Content ........................................60 10.2.5 204 No Content . . . . . . . . . . . . . . . . . . . 67
10.2.7 206 Partial Content ......................................60 10.2.6 205 Reset Content . . . . . . . . . . . . . . . . . . 67
10.3 Redirection 3xx .............................................61 10.2.7 206 Partial Content . . . . . . . . . . . . . . . . . 68
10.3.1 300 Multiple Choices .....................................61 10.3 Redirection 3xx . . . . . . . . . . . . . . . . . . . . . 68
10.3.2 301 Moved Permanently ....................................62 10.3.1 300 Multiple Choices . . . . . . . . . . . . . . . . 69
10.3.3 302 Found ................................................62 10.3.2 301 Moved Permanently . . . . . . . . . . . . . . . . 69
10.3.4 303 See Other ............................................63 10.3.3 302 Found . . . . . . . . . . . . . . . . . . . . . . 70
10.3.5 304 Not Modified .........................................63 10.3.4 303 See Other . . . . . . . . . . . . . . . . . . . . 70
10.3.6 305 Use Proxy ............................................64 10.3.5 304 Not Modified . . . . . . . . . . . . . . . . . . 71
10.3.7 306 (Unused) .............................................64 10.3.6 305 Use Proxy . . . . . . . . . . . . . . . . . . . . 71
10.3.8 307 Temporary Redirect ...................................65 10.3.7 306 (Unused) . . . . . . . . . . . . . . . . . . . . 72
10.4 Client Error 4xx ............................................65 10.3.8 307 Temporary Redirect . . . . . . . . . . . . . . . 72
10.4.1 400 Bad Request .........................................65 10.4 Client Error 4xx . . . . . . . . . . . . . . . . . . . . 72
10.4.2 401 Unauthorized ........................................66 10.4.1 400 Bad Request . . . . . . . . . . . . . . . . . . . 73
10.4.3 402 Payment Required ....................................66 10.4.2 401 Unauthorized . . . . . . . . . . . . . . . . . . 73
10.4.4 403 Forbidden ...........................................66 10.4.3 402 Payment Required . . . . . . . . . . . . . . . . 73
10.4.5 404 Not Found ...........................................66 10.4.4 403 Forbidden . . . . . . . . . . . . . . . . . . . . 73
10.4.6 405 Method Not Allowed ..................................66 10.4.5 404 Not Found . . . . . . . . . . . . . . . . . . . . 73
10.4.7 406 Not Acceptable ......................................67 10.4.6 405 Method Not Allowed . . . . . . . . . . . . . . . 74
10.4.8 407 Proxy Authentication Required .......................67 10.4.7 406 Not Acceptable . . . . . . . . . . . . . . . . . 74
10.4.9 408 Request Timeout .....................................67 10.4.8 407 Proxy Authentication Required . . . . . . . . . . 74
10.4.10 409 Conflict ............................................67 10.4.9 408 Request Timeout . . . . . . . . . . . . . . . . . 75
10.4.11 410 Gone ................................................68 10.4.10 409 Conflict . . . . . . . . . . . . . . . . . . . . 75
10.4.12 411 Length Required .....................................68 10.4.11 410 Gone . . . . . . . . . . . . . . . . . . . . . . 75
10.4.13 412 Precondition Failed .................................68 10.4.12 411 Length Required . . . . . . . . . . . . . . . . . 76
10.4.14 413 Request Entity Too Large ............................69 10.4.13 412 Precondition Failed . . . . . . . . . . . . . . . 76
10.4.15 414 Request-URI Too Long ................................69 10.4.14 413 Request Entity Too Large . . . . . . . . . . . . 76
10.4.16 415 Unsupported Media Type ..............................69 10.4.15 414 Request-URI Too Long . . . . . . . . . . . . . . 76
10.4.17 416 Requested Range Not Satisfiable .....................69 10.4.16 415 Unsupported Media Type . . . . . . . . . . . . . 76
10.4.18 417 Expectation Failed ..................................70 10.4.17 416 Requested Range Not Satisfiable . . . . . . . . . 76
10.5 Server Error 5xx ............................................70 10.4.18 417 Expectation Failed . . . . . . . . . . . . . . . 77
10.5.1 500 Internal Server Error ................................70 10.5 Server Error 5xx . . . . . . . . . . . . . . . . . . . . 77
10.5.2 501 Not Implemented ......................................70 10.5.1 500 Internal Server Error . . . . . . . . . . . . . . 77
10.5.3 502 Bad Gateway ..........................................70 10.5.2 501 Not Implemented . . . . . . . . . . . . . . . . . 77
10.5.4 503 Service Unavailable ..................................70 10.5.3 502 Bad Gateway . . . . . . . . . . . . . . . . . . . 77
10.5.5 504 Gateway Timeout ......................................71 10.5.4 503 Service Unavailable . . . . . . . . . . . . . . . 78
10.5.6 505 HTTP Version Not Supported ...........................71 10.5.5 504 Gateway Timeout . . . . . . . . . . . . . . . . . 78
11 Access Authentication ........................................71 10.5.6 505 HTTP Version Not Supported . . . . . . . . . . . 78
12 Content Negotiation ..........................................71 11 Access Authentication . . . . . . . . . . . . . . . . . . . . 79
12.1 Server-driven Negotiation ...................................72 12 Content Negotiation . . . . . . . . . . . . . . . . . . . . . 80
12.2 Agent-driven Negotiation ....................................73 12.1 Server-driven Negotiation . . . . . . . . . . . . . . . . 80
12.3 Transparent Negotiation .....................................74 12.2 Agent-driven Negotiation . . . . . . . . . . . . . . . . 81
13 Caching in HTTP ..............................................74 12.3 Transparent Negotiation . . . . . . . . . . . . . . . . . 82
13.1.1 Cache Correctness ........................................75 13 Caching in HTTP . . . . . . . . . . . . . . . . . . . . . . . 83
13.1.2 Warnings .................................................76 13.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
13.1.3 Cache-control Mechanisms .................................77 13.1.1 Cache Correctness . . . . . . . . . . . . . . . . . . 84
13.1.4 Explicit User Agent Warnings .............................78 13.1.2 Warnings . . . . . . . . . . . . . . . . . . . . . . 85
13.1.5 Exceptions to the Rules and Warnings .....................78 13.1.3 Cache-control Mechanisms . . . . . . . . . . . . . . 86
13.1.6 Client-controlled Behavior ...............................79 13.1.4 Explicit User Agent Warnings . . . . . . . . . . . . 86
13.2 Expiration Model ............................................79 13.1.5 Exceptions to the Rules and Warnings . . . . . . . . 87
13.2.1 Server-Specified Expiration ..............................79 13.1.6 Client-controlled Behavior . . . . . . . . . . . . . 87
13.2.2 Heuristic Expiration .....................................80 13.2 Expiration Model . . . . . . . . . . . . . . . . . . . . 88
13.2.3 Age Calculations .........................................80 13.2.1 Server-Specified Expiration . . . . . . . . . . . . . 88
13.2.4 Expiration Calculations ..................................83 13.2.2 Heuristic Expiration . . . . . . . . . . . . . . . . 88
13.2.5 Disambiguating Expiration Values .........................84 13.2.3 Age Calculations . . . . . . . . . . . . . . . . . . 89
13.2.6 Disambiguating Multiple Responses ........................84 13.2.4 Expiration Calculations . . . . . . . . . . . . . . . 91
13.3 Validation Model ............................................85 13.2.5 Disambiguating Expiration Values . . . . . . . . . . 92
13.3.1 Last-Modified Dates ......................................86 13.2.6 Disambiguating Multiple Responses . . . . . . . . . . 93
13.3.2 Entity Tag Cache Validators ..............................86 13.3 Validation Model . . . . . . . . . . . . . . . . . . . . 93
13.3.3 Weak and Strong Validators ...............................86 13.3.1 Last-Modified Dates . . . . . . . . . . . . . . . . . 94
13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates.89 13.3.2 Entity Tag Cache Validators . . . . . . . . . . . . . 94
13.3.5 Non-validating Conditionals ..............................90 13.3.3 Weak and Strong Validators . . . . . . . . . . . . . 95
13.4 Response Cacheability .......................................91 13.3.4 Rules for When to Use Entity Tags and
13.5 Constructing Responses From Caches ..........................92 Last-Modified Dates . . . . . . . . . . . . . . . . . 97
13.5.1 End-to-end and Hop-by-hop Headers ........................92 13.3.5 Non-validating Conditionals . . . . . . . . . . . . . 99
13.5.2 Non-modifiable Headers ...................................92 13.4 Response Cacheability . . . . . . . . . . . . . . . . . . 99
13.5.3 Combining Headers ........................................94 13.5 Constructing Responses From Caches . . . . . . . . . . . 100
13.5.4 Combining Byte Ranges ....................................95 13.5.1 End-to-end and Hop-by-hop Headers . . . . . . . . . . 100
13.6 Caching Negotiated Responses ................................95 13.5.2 Non-modifiable Headers . . . . . . . . . . . . . . . 101
13.7 Shared and Non-Shared Caches ................................96 13.5.3 Combining Headers . . . . . . . . . . . . . . . . . . 102
13.8 Errors or Incomplete Response Cache Behavior ................97 13.5.4 Combining Byte Ranges . . . . . . . . . . . . . . . . 103
13.9 Side Effects of GET and HEAD ................................97 13.6 Caching Negotiated Responses . . . . . . . . . . . . . . 104
13.10 Invalidation After Updates or Deletions ...................97 13.7 Shared and Non-Shared Caches . . . . . . . . . . . . . . 105
13.11 Write-Through Mandatory ...................................98 13.8 Errors or Incomplete Response Cache Behavior . . . . . . 105
13.12 Cache Replacement .........................................99 13.9 Side Effects of GET and HEAD . . . . . . . . . . . . . . 106
13.13 History Lists .............................................99 13.10 Invalidation After Updates or Deletions . . . . . . . . . 106
14 Header Field Definitions ....................................100 13.11 Write-Through Mandatory . . . . . . . . . . . . . . . . . 107
14.1 Accept .....................................................100 13.12 Cache Replacement . . . . . . . . . . . . . . . . . . . . 107
14.2 Accept-Charset .............................................102 13.13 History Lists . . . . . . . . . . . . . . . . . . . . . . 108
14.3 Accept-Encoding ............................................102 14 Header Field Definitions . . . . . . . . . . . . . . . . . . 109
14.4 Accept-Language ............................................104 14.1 Accept . . . . . . . . . . . . . . . . . . . . . . . . . 109
14.5 Accept-Ranges ..............................................105 14.2 Accept-Charset . . . . . . . . . . . . . . . . . . . . . 111
14.6 Age ........................................................106 14.3 Accept-Encoding . . . . . . . . . . . . . . . . . . . . . 111
14.7 Allow ......................................................106 14.4 Accept-Language . . . . . . . . . . . . . . . . . . . . . 113
14.8 Authorization ..............................................107 14.5 Accept-Ranges . . . . . . . . . . . . . . . . . . . . . . 114
14.9 Cache-Control ..............................................108 14.6 Age . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
14.9.1 What is Cacheable .......................................109 14.7 Allow . . . . . . . . . . . . . . . . . . . . . . . . . . 115
14.9.2 What May be Stored by Caches ............................110 14.8 Authorization . . . . . . . . . . . . . . . . . . . . . . 116
14.9.3 Modifications of the Basic Expiration Mechanism .........111 14.9 Cache-Control . . . . . . . . . . . . . . . . . . . . . . 116
14.9.4 Cache Revalidation and Reload Controls ..................113 14.9.1 What is Cacheable . . . . . . . . . . . . . . . . . . 118
14.9.5 No-Transform Directive ..................................115 14.9.2 What May be Stored by Caches . . . . . . . . . . . . 119
14.9.6 Cache Control Extensions ................................116 14.9.3 Modifications of the Basic Expiration Mechanism . . . 120
14.10 Connection ...............................................117 14.9.4 Cache Revalidation and Reload Controls . . . . . . . 122
14.11 Content-Encoding .........................................118 14.9.5 No-Transform Directive . . . . . . . . . . . . . . . 125
14.12 Content-Language .........................................118 14.9.6 Cache Control Extensions . . . . . . . . . . . . . . 125
14.13 Content-Length ...........................................119 14.10 Connection . . . . . . . . . . . . . . . . . . . . . . . 126
14.14 Content-Location .........................................120 14.11 Content-Encoding . . . . . . . . . . . . . . . . . . . . 127
14.15 Content-MD5 ..............................................121 14.12 Content-Language . . . . . . . . . . . . . . . . . . . . 128
14.16 Content-Range ............................................122 14.13 Content-Length . . . . . . . . . . . . . . . . . . . . . 128
14.17 Content-Type .............................................124 14.14 Content-Location . . . . . . . . . . . . . . . . . . . . 129
14.18 Date .....................................................124 14.15 Content-MD5 . . . . . . . . . . . . . . . . . . . . . . . 130
14.18.1 Clockless Origin Server Operation ......................125 14.16 Content-Range . . . . . . . . . . . . . . . . . . . . . . 131
14.19 ETag .....................................................126 14.17 Content-Type . . . . . . . . . . . . . . . . . . . . . . 133
14.20 Expect ...................................................126 14.18 Date . . . . . . . . . . . . . . . . . . . . . . . . . . 133
14.21 Expires ..................................................127 14.18.1 Clockless Origin Server Operation . . . . . . . . . . 134
14.22 From .....................................................128 14.19 ETag . . . . . . . . . . . . . . . . . . . . . . . . . . 135
14.23 Host .....................................................128 14.20 Expect . . . . . . . . . . . . . . . . . . . . . . . . . 135
14.24 If-Match .................................................129 14.21 Expires . . . . . . . . . . . . . . . . . . . . . . . . . 136
14.25 If-Modified-Since ........................................130 14.22 From . . . . . . . . . . . . . . . . . . . . . . . . . . 137
14.26 If-None-Match ............................................132 14.23 Host . . . . . . . . . . . . . . . . . . . . . . . . . . 137
14.27 If-Range .................................................133 14.24 If-Match . . . . . . . . . . . . . . . . . . . . . . . . 138
14.28 If-Unmodified-Since ......................................134 14.25 If-Modified-Since . . . . . . . . . . . . . . . . . . . . 139
14.29 Last-Modified ............................................134 14.26 If-None-Match . . . . . . . . . . . . . . . . . . . . . . 141
14.30 Location .................................................135 14.27 If-Range . . . . . . . . . . . . . . . . . . . . . . . . 142
14.31 Max-Forwards .............................................136 14.28 If-Unmodified-Since . . . . . . . . . . . . . . . . . . . 143
14.32 Pragma ...................................................136 14.29 Last-Modified . . . . . . . . . . . . . . . . . . . . . . 143
14.33 Proxy-Authenticate .......................................137 14.30 Location . . . . . . . . . . . . . . . . . . . . . . . . 144
14.34 Proxy-Authorization ......................................137 14.31 Max-Forwards . . . . . . . . . . . . . . . . . . . . . . 144
14.35 Range ....................................................138 14.32 Pragma . . . . . . . . . . . . . . . . . . . . . . . . . 145
14.35.1 Byte Ranges ...........................................138 14.33 Proxy-Authenticate . . . . . . . . . . . . . . . . . . . 146
14.35.2 Range Retrieval Requests ..............................139 14.34 Proxy-Authorization . . . . . . . . . . . . . . . . . . . 146
14.36 Referer ..................................................140 14.35 Range . . . . . . . . . . . . . . . . . . . . . . . . . . 147
14.37 Retry-After ..............................................141 14.35.1 Byte Ranges . . . . . . . . . . . . . . . . . . . . . 147
14.38 Server ...................................................141 14.35.2 Range Retrieval Requests . . . . . . . . . . . . . . 148
14.39 TE .......................................................142 14.36 Referer . . . . . . . . . . . . . . . . . . . . . . . . . 149
14.40 Trailer ..................................................143 14.37 Retry-After . . . . . . . . . . . . . . . . . . . . . . . 150
14.41 Transfer-Encoding..........................................143 14.38 Server . . . . . . . . . . . . . . . . . . . . . . . . . 150
14.42 Upgrade ..................................................144 14.39 TE . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
14.43 User-Agent ...............................................145 14.40 Trailer . . . . . . . . . . . . . . . . . . . . . . . . . 152
14.44 Vary .....................................................145 14.41 Transfer-Encoding . . . . . . . . . . . . . . . . . . . . 152
14.45 Via ......................................................146 14.42 Upgrade . . . . . . . . . . . . . . . . . . . . . . . . . 153
14.46 Warning ..................................................148 14.43 User-Agent . . . . . . . . . . . . . . . . . . . . . . . 154
14.47 WWW-Authenticate .........................................150 14.44 Vary . . . . . . . . . . . . . . . . . . . . . . . . . . 154
15 Security Considerations .......................................150 14.45 Via . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
15.1 Personal Information....................................151 14.46 Warning . . . . . . . . . . . . . . . . . . . . . . . . . 157
15.1.1 Abuse of Server Log Information .........................151 14.47 WWW-Authenticate . . . . . . . . . . . . . . . . . . . . 159
15.1.2 Transfer of Sensitive Information .......................151 15 Security Considerations . . . . . . . . . . . . . . . . . . . 160
15.1.3 Encoding Sensitive Information in URI's .................152 15.1 Personal Information . . . . . . . . . . . . . . . . . . 160
15.1.4 Privacy Issues Connected to Accept Headers ..............152 15.1.1 Abuse of Server Log Information . . . . . . . . . . . 160
15.2 Attacks Based On File and Path Names .......................153 15.1.2 Transfer of Sensitive Information . . . . . . . . . . 160
15.3 DNS Spoofing ...............................................154 15.1.3 Encoding Sensitive Information in URI's . . . . . . . 161
15.4 Location Headers and Spoofing ..............................154 15.1.4 Privacy Issues Connected to Accept Headers . . . . . 162
15.5 Content-Disposition Issues .................................154 15.2 Attacks Based On File and Path Names . . . . . . . . . . 162
15.6 Authentication Credentials and Idle Clients ................155 15.3 DNS Spoofing . . . . . . . . . . . . . . . . . . . . . . 163
15.7 Proxies and Caching ........................................155 15.4 Location Headers and Spoofing . . . . . . . . . . . . . . 163
15.7.1 Denial of Service Attacks on Proxies....................156 15.5 Content-Disposition Issues . . . . . . . . . . . . . . . 164
16 Acknowledgments .............................................156 15.6 Authentication Credentials and Idle Clients . . . . . . . 164
17 References ..................................................158 15.7 Proxies and Caching . . . . . . . . . . . . . . . . . . . 164
18 Authors' Addresses ..........................................162 15.7.1 Denial of Service Attacks on Proxies . . . . . . . . 165
19 Appendices ..................................................164 16 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 166
19.1 Internet Media Type message/http and application/http ......164 16.1 (RFC2616) . . . . . . . . . . . . . . . . . . . . . . . . 166
19.2 Internet Media Type multipart/byteranges ...................165 16.2 (This Document) . . . . . . . . . . . . . . . . . . . . . 168
19.3 Tolerant Applications ......................................166 17. References . . . . . . . . . . . . . . . . . . . . . . . . . 169
19.4 Differences Between HTTP Entities and RFC 2045 Entities ....167 17.1 References . . . . . . . . . . . . . . . . . . . . . . . 169
19.4.1 MIME-Version ............................................167 17.2 Normative References . . . . . . . . . . . . . . . . . . 172
19.4.2 Conversion to Canonical Form ............................167 Appendix A Internet Media Type message/http and
19.4.3 Conversion of Date Formats ..............................168 application/http . . . . . . . . . . . . . . . . . . 174
19.4.4 Introduction of Content-Encoding ........................168 Appendix B Internet Media Type multipart/byteranges . . . . . . 176
19.4.5 No Content-Transfer-Encoding ............................168 Appendix C Tolerant Applications . . . . . . . . . . . . . . . . 178
19.4.6 Introduction of Transfer-Encoding .......................169 Appendix D Differences Between HTTP Entities and RFC 2045
19.4.7 MHTML and Line Length Limitations .......................169 Entities . . . . . . . . . . . . . . . . . . . . . . 179
19.5 Additional Features ........................................169 D.1 MIME-Version . . . . . . . . . . . . . . . . . . . . . . 179
19.5.1 Content-Disposition .....................................170 D.2 Conversion to Canonical Form . . . . . . . . . . . . . . 179
19.6 Compatibility with Previous Versions .......................170 D.3 Conversion of Date Formats . . . . . . . . . . . . . . . 180
19.6.1 Changes from HTTP/1.0 ...................................171 D.4 Introduction of Content-Encoding . . . . . . . . . . . . 180
19.6.2 Compatibility with HTTP/1.0 Persistent Connections ......172 D.5 No Content-Transfer-Encoding . . . . . . . . . . . . . . 180
19.6.3 Changes from RFC 2068 ...................................172 D.6 Introduction of Transfer-Encoding . . . . . . . . . . . . 181
20 Index .......................................................175 D.7 MHTML and Line Length Limitations . . . . . . . . . . . . 181
21 Full Copyright Statement ....................................176 Appendix E Additional Features . . . . . . . . . . . . . . . . . 182
E.1 Content-Disposition . . . . . . . . . . . . . . . . . . . 182
Appendix F Compatibility with Previous Versions . . . . . . . . 183
F.1 Changes from HTTP/1.0 . . . . . . . . . . . . . . . . . . 183
F.1.1 Changes to Simplify Multi-homed Web Servers and
Conserve IP Addresses . . . . . . . . . . . . . . . . 183
F.2 Compatibility with HTTP/1.0 Persistent Connections . . . 184
F.3 Changes from RFC 2068 . . . . . . . . . . . . . . . . . . 185
Appendix G Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 188
G.1 Since RFC2616 . . . . . . . . . . . . . . . . . . . . . . 188
Appendix H Open issues (to be removed by RFC Editor prior to
publication) . . . . . . . . . . . . . . . . . . . . 189
H.1 rfc2616bis . . . . . . . . . . . . . . . . . . . . . . . 189
H.2 edit . . . . . . . . . . . . . . . . . . . . . . . . . . 189
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 201
Intellectual Property and Copyright Statements . . . . . . . . . 202
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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access to resources available from diverse applications. access to resources available from diverse applications.
1.2 Requirements 1.2 Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [34]. document are to be interpreted as described in RFC 2119 [34].
An implementation is not compliant if it fails to satisfy one or more An implementation is not compliant if it fails to satisfy one or more
of the MUST or REQUIRED level requirements for the protocols it of the MUST or REQUIRED level requirements for the protocols it
implements. An implementation that satisfies all the MUST or REQUIRED implements. An implementation that satisfies all the MUST or
level and all the SHOULD level requirements for its protocols is said REQUIRED level and all the SHOULD level requirements for its
to be "unconditionally compliant"; one that satisfies all the MUST protocols is said to be "unconditionally compliant"; one that
level requirements but not all the SHOULD level requirements for its satisfies all the MUST level requirements but not all the SHOULD
protocols is said to be "conditionally compliant." level requirements for its 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.
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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
the request or response in order to provide some added service to modifies the request or response in order to provide some added
the user agent, such as group annotation services, media type service to the user agent, such as group annotation services,
transformation, protocol reduction, or anonymity filtering. Except media type transformation, protocol reduction, or anonymity
where either transparent or non-transparent behavior is explicitly filtering. Except where either transparent or non-transparent
stated, the HTTP proxy requirements apply to both types of behavior is explicitly stated, the HTTP proxy requirements apply
proxies. to both types of 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
ends of the relayed connections are closed. both 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
cannot be used by a server that is acting as a tunnel. cache cannot be used by a server that is acting as a tunnel.
cacheable cacheable
A response is cacheable if a cache is allowed to store a copy of A response is cacheable if a cache is allowed to store a copy of
the response message for use in answering subsequent requests. The the response message for use in answering subsequent requests.
rules for determining the cacheability of HTTP responses are The 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.
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1.4 Overall Operation 1.4 Overall Operation
The HTTP protocol is a request/response protocol. A client sends a The HTTP protocol is a request/response protocol. A client sends a
request to the server in the form of a request method, URI, and request to the server in the form of a request method, URI, and
protocol version, followed by a MIME-like message containing request protocol version, followed by a MIME-like message containing request
modifiers, client information, and possible body content over a modifiers, client information, and possible body content over a
connection with a server. The server responds with a status line, connection with a server. The server responds with a status line,
including the message's protocol version and a success or error code, including the message's protocol version and a success or error code,
followed by a MIME-like message containing server information, entity followed by a MIME-like message containing server information, entity
metainformation, and possible entity-body content. The relationship metainformation, and possible entity-body content. The relationship
between HTTP and MIME is described in appendix 19.4. between HTTP and MIME is described in appendix D.
Most HTTP communication is initiated by a user agent and consists of Most HTTP communication is initiated by a user agent and consists of
a request to be applied to a resource on some origin server. In the a request to be applied to a resource on some origin server. In the
simplest case, this may be accomplished via a single connection (v) simplest case, this may be accomplished via a single connection (v)
between the user agent (UA) and the origin server (O). between the user agent (UA) and the origin server (O).
request chain ------------------------> request chain ------------------------>
UA -------------------v------------------- O UA -------------------v------------------- O
<----------------------- response chain <----------------------- response chain
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request chain --------------------------------------> request chain -------------------------------------->
UA -----v----- A -----v----- B -----v----- C -----v----- O UA -----v----- A -----v----- B -----v----- C -----v----- O
<------------------------------------- response chain <------------------------------------- response chain
The figure above shows three intermediaries (A, B, and C) between the The figure above shows three intermediaries (A, B, and C) between the
user agent and origin server. A request or response message that user agent and origin server. A request or response message that
travels the whole chain will pass through four separate connections. travels the whole chain will pass through four separate connections.
This distinction is important because some HTTP communication options This distinction is important because some HTTP communication options
may apply only to the connection with the nearest, non-tunnel may apply only to the connection with the nearest, non-tunnel
neighbor, only to the end-points of the chain, or to all connections neighbor, only to the end-points of the chain, or to all connections
along the chain. Although the diagram is linear, each participant may along the chain. Although the diagram is linear, each participant
be engaged in multiple, simultaneous communications. For example, B may be engaged in multiple, simultaneous communications. For
may be receiving requests from many clients other than A, and/or example, B may be receiving requests from many clients other than A,
forwarding requests to servers other than C, at the same time that it and/or forwarding requests to servers other than C, at the same time
is handling A's request. that it is handling A's request.
Any party to the communication which is not acting as a tunnel may Any party to the communication which is not acting as a tunnel may
employ an internal cache for handling requests. The effect of a cache employ an internal cache for handling requests. The effect of a
is that the request/response chain is shortened if one of the cache is that the request/response chain is shortened if one of the
participants along the chain has a cached response applicable to that participants along the chain has a cached response applicable to that
request. The following illustrates the resulting chain if B has a request. The following illustrates the resulting chain if B has a
cached copy of an earlier response from O (via C) for a request which cached copy of an earlier response from O (via C) for a request which
has not been cached by UA or A. has not been cached by UA or A.
request chain ----------> request chain ---------->
UA -----v----- A -----v----- B - - - - - - C - - - - - - O UA -----v----- A -----v----- B - - - - - - C - - - - - - O
<--------- response chain <--------- response chain
Not all responses are usefully cacheable, and some requests may Not all responses are usefully cacheable, and some requests may
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one or more request/response exchanges, although connections may be one or more request/response exchanges, although connections may be
closed for a variety of reasons (see section 8.1). closed for a variety of reasons (see section 8.1).
2 Notational Conventions and Generic Grammar 2 Notational Conventions and Generic Grammar
2.1 Augmented BNF 2.1 Augmented BNF
All of the mechanisms specified in this document are described in All of the mechanisms specified in this document are described in
both prose and an augmented Backus-Naur Form (BNF) similar to that both prose and an augmented Backus-Naur Form (BNF) similar to that
used by RFC 822 [9]. Implementors will need to be familiar with the used by RFC 822 [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
includes the following constructs: BNF 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"
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DIGIT = <any US-ASCII digit "0".."9"> DIGIT = <any US-ASCII digit "0".."9">
CTL = <any US-ASCII control character CTL = <any US-ASCII control character
(octets 0 - 31) and DEL (127)> (octets 0 - 31) and DEL (127)>
CR = <US-ASCII CR, carriage return (13)> CR = <US-ASCII CR, carriage return (13)>
LF = <US-ASCII LF, linefeed (10)> LF = <US-ASCII LF, linefeed (10)>
SP = <US-ASCII SP, space (32)> SP = <US-ASCII SP, space (32)>
HT = <US-ASCII HT, horizontal-tab (9)> HT = <US-ASCII HT, horizontal-tab (9)>
<"> = <US-ASCII double-quote mark (34)> <"> = <US-ASCII double-quote mark (34)>
HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
protocol elements except the entity-body (see appendix 19.3 for protocol elements except the entity-body (see appendix C for tolerant
tolerant applications). The end-of-line marker within an entity-body applications). The end-of-line marker within an entity-body is
is defined by its associated media type, as described in section 3.7. defined by its associated media type, as described in section 3.7.
CRLF = CR LF CRLF = CR LF
HTTP/1.1 header field values can be folded onto multiple lines if the HTTP/1.1 header field values can be folded onto multiple lines if the
continuation line begins with a space or horizontal tab. All linear continuation line begins with a space or horizontal tab. All linear
white space, including folding, has the same semantics as SP. A white space, including folding, has the same semantics as SP. A
recipient MAY replace any linear white space with a single SP before recipient MAY replace any linear white space with a single SP before
interpreting the field value or forwarding the message downstream. interpreting the field value or forwarding the message downstream.
LWS = [CRLF] 1*( SP | HT ) LWS = [CRLF] 1*( SP | HT )
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but including LWS> but including LWS>
A CRLF is allowed in the definition of TEXT only as part of a header A CRLF is allowed in the definition of TEXT only as part of a header
field continuation. It is expected that the folding LWS will be field continuation. It is expected that the folding LWS will be
replaced with a single SP before interpretation of the TEXT value. replaced with a single SP before interpretation of the TEXT value.
Hexadecimal numeric characters are used in several protocol elements. Hexadecimal numeric characters are used in several protocol elements.
HEX = "A" | "B" | "C" | "D" | "E" | "F" HEX = "A" | "B" | "C" | "D" | "E" | "F"
| "a" | "b" | "c" | "d" | "e" | "f" | DIGIT | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
Many HTTP/1.1 header field values consist of words separated by LWS Many HTTP/1.1 header field values consist of words separated by LWS
or special characters. These special characters MUST be in a quoted or special characters. These special characters MUST be in a quoted
string to be used within a parameter value (as defined in section string to be used within a parameter value (as defined in
3.6). section 3.6).
token = 1*<any CHAR except CTLs or separators> token = 1*<any CHAR except CTLs or separators>
separators = "(" | ")" | "<" | ">" | "@" separators = "(" | ")" | "<" | ">" | "@"
| "," | ";" | ":" | "\" | <"> | "," | ";" | ":" | "\" | <">
| "/" | "[" | "]" | "?" | "=" | "/" | "[" | "]" | "?" | "="
| "{" | "}" | SP | HT | "{" | "}" | SP | HT
Comments can be included in some HTTP header fields by surrounding Comments can be included in some HTTP header fields by surrounding
the comment text with parentheses. Comments are only allowed in the comment text with parentheses. Comments are only allowed in
fields containing "comment" as part of their field value definition. fields containing "comment" as part of their field value definition.
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changed. See RFC 2145 [36] for a fuller explanation. changed. See RFC 2145 [36] for a fuller explanation.
The version of an HTTP message is indicated by an HTTP-Version field The version of an HTTP message is indicated by an HTTP-Version field
in the first line of the message. in the first line of the message.
HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT
Note that the major and minor numbers MUST be treated as separate Note that the major and minor numbers MUST be treated as separate
integers and that each MAY be incremented higher than a single digit. integers and that each MAY be incremented higher than a single digit.
Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is
lower than HTTP/12.3. Leading zeros MUST be ignored by recipients and lower than HTTP/12.3. Leading zeros MUST be ignored by recipients
MUST NOT be sent. and MUST NOT be sent.
An application that sends a request or response message that includes An application that sends a request or response message that includes
HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant HTTP-Version of "HTTP/1.1" MUST be at least conditionally compliant
with this specification. Applications that are at least conditionally with this specification. Applications that are at least
compliant with this specification SHOULD use an HTTP-Version of conditionally compliant with this specification SHOULD use an HTTP-
"HTTP/1.1" in their messages, and MUST do so for any message that is Version of "HTTP/1.1" in their messages, and MUST do so for any
not compatible with HTTP/1.0. For more details on when to send message that is not compatible with HTTP/1.0. For more details on
specific HTTP-Version values, see RFC 2145 [36]. when to send specific HTTP-Version values, see RFC 2145 [36].
The HTTP version of an application is the highest HTTP version for The HTTP version of an application is the highest HTTP version for
which the application is at least conditionally compliant. which the application is at least conditionally compliant.
Proxy and gateway applications need to be careful when forwarding Proxy and gateway applications need to be careful when forwarding
messages in protocol versions different from that of the application. messages in protocol versions different from that of the application.
Since the protocol version indicates the protocol capability of the Since the protocol version indicates the protocol capability of the
sender, a proxy/gateway MUST NOT send a message with a version sender, a proxy/gateway MUST NOT send a message with a version
indicator which is greater than its actual version. If a higher indicator which is greater than its actual version. If a higher
version request is received, the proxy/gateway MUST either downgrade version request is received, the proxy/gateway MUST either downgrade
the request version, or respond with an error, or switch to tunnel the request version, or respond with an error, or switch to tunnel
behavior. behavior.
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,
MAY, and tunnels MUST NOT upgrade the request to the highest version gateways MAY, and tunnels MUST NOT upgrade the request to the highest
they support. The proxy/gateway's response to that request MUST be in version they support. The proxy/gateway's response to that request
the same major version as the request. MUST be in the same major version as the request.
Note: Converting between versions of HTTP may involve modification Note: Converting between versions of HTTP may involve modification
of header fields required or forbidden by the versions involved. of header fields required or forbidden by the versions involved.
3.2 Uniform Resource Identifiers 3.2 Uniform Resource Identifiers
URIs have been known by many names: WWW addresses, Universal Document URIs have been known by many names: WWW addresses, Universal Document
Identifiers, Universal Resource Identifiers [3], and finally the Identifiers, Universal Resource Identifiers [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.
3.2.1 General Syntax 3.2.1 General Syntax
URIs in HTTP can be represented in absolute form or relative to some URIs in HTTP can be represented in absolute form or relative to some
known base URI [11], depending upon the context of their use. The two known base URI [11], depending upon the context of their use. The
forms are differentiated by the fact that absolute URIs always begin two forms are differentiated by the fact that absolute URIs always
with a scheme name followed by a colon. For definitive information on begin with a scheme name followed by a colon. For definitive
URL syntax and semantics, see "Uniform Resource Identifiers (URI): information on URL syntax and semantics, see "Uniform Resource
Generic Syntax and Semantics," RFC 2396 [42] (which replaces RFCs Identifiers (URI): Generic Syntax and Semantics," RFC 2396 [42]
1738 [4] and RFC 1808 [11]). This specification adopts the (which replaces RFCs 1738 [4] and RFC 1808 [11]). This specification
definitions of "URI-reference", "absoluteURI", "relativeURI", "port", adopts the definitions of "URI-reference", "absoluteURI",
"host","abs_path", "rel_path", and "authority" from that "relativeURI", "port", "host","abs_path", "rel_path", and "authority"
specification. from that specification.
The HTTP protocol does not place any a priori limit on the length of The HTTP protocol does not place any a priori limit on the length of
a URI. Servers MUST be able to handle the URI of any resource they a URI. Servers MUST be able to handle the URI of any resource they
serve, and SHOULD be able to handle URIs of unbounded length if they serve, and SHOULD be able to handle URIs of unbounded length if they
provide GET-based forms that could generate such URIs. A server provide GET-based forms that could generate such URIs. A server
SHOULD return 414 (Request-URI Too Long) status if a URI is longer SHOULD return 414 (Request-URI Too Long) status if a URI is longer
than the server can handle (see section 10.4.15). than the server can handle (see section 10.4.15).
Note: Servers ought to be cautious about depending on URI lengths Note: Servers ought to be cautious about depending on URI lengths
above 255 bytes, because some older client or proxy above 255 bytes, because some older client or proxy
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The "http" scheme is used to locate network resources via the HTTP The "http" scheme is used to locate network resources via the HTTP
protocol. This section defines the scheme-specific syntax and protocol. This section defines the scheme-specific syntax and
semantics for http URLs. semantics for http URLs.
http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]] http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
If the port is empty or not given, port 80 is assumed. The semantics If the port is empty or not given, port 80 is assumed. The semantics
are that the identified resource is located at the server listening are that the identified resource is located at the server listening
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
in URLs SHOULD be avoided whenever possible (see RFC 1900 [24]). If addresses in URLs SHOULD be avoided whenever possible (see RFC 1900
the abs_path is not present in the URL, it MUST be given as "/" when [24]). If the abs_path is not present in the URL, it MUST be given
used as a Request-URI for a resource (section 5.1.2). If a proxy as "/" when used as a Request-URI for a resource (section 5.1.2). If
receives a host name which is not a fully qualified domain name, it a proxy receives a host name which is not a fully qualified domain
MAY add its domain to the host name it received. If a proxy receives name, it MAY add its domain to the host name it received. If a proxy
a fully qualified domain name, the proxy MUST NOT change the host receives a fully qualified domain name, the proxy MUST NOT change the
name. host name.
3.2.3 URI Comparison 3.2.3 URI Comparison
When comparing two URIs to decide if they match or not, a client When comparing two URIs to decide if they match or not, a client
SHOULD use a case-sensitive octet-by-octet comparison of the entire SHOULD use a case-sensitive octet-by-octet comparison of the entire
URIs, with these exceptions: URIs, with these exceptions:
- A port that is empty or not given is equivalent to the default o A port that is empty or not given is equivalent to the default
port for that URI-reference; port for that URI-reference;
- Comparisons of host names MUST be case-insensitive; o Comparisons of host names MUST be case-insensitive;
- Comparisons of scheme names MUST be case-insensitive; o Comparisons of scheme names MUST be case-insensitive;
- An empty abs_path is equivalent to an abs_path of "/". o 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
RFC 2396 [42]) 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
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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
obsolete RFC 850 [12] date format and lacks a four-digit year. the obsolete RFC 850 [12] date format and lacks a four-digit year.
HTTP/1.1 clients and servers that parse the date value MUST accept 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 appendix C 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 posting applications, as is sometimes the case when retrieving or posting
messages via proxies/gateways to SMTP or NNTP. messages via proxies/gateways to SMTP or NNTP.
All HTTP date/time stamps MUST be represented in Greenwich Mean Time All HTTP date/time stamps MUST be represented in Greenwich Mean Time
(GMT), without exception. For the purposes of HTTP, GMT is exactly (GMT), without exception. For the purposes of HTTP, GMT is exactly
equal to UTC (Coordinated Universal Time). This is indicated in the equal to UTC (Coordinated Universal Time). This is indicated in the
first two formats by the inclusion of "GMT" as the three-letter first two formats by the inclusion of "GMT" as the three-letter
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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 only Note: HTTP requirements for the date/time stamp format apply only to
to their usage within the protocol stream. Clients and servers are their usage within the protocol stream. Clients and servers are not
not required to use these formats for user presentation, request required to use these formats for user presentation, request logging,
logging, etc. 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
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content-coding values in the Accept-Encoding (section 14.3) and content-coding values in the Accept-Encoding (section 14.3) and
Content-Encoding (section 14.11) header fields. Although the value Content-Encoding (section 14.11) header fields. Although the value
describes the content-coding, what is more important is that it describes the content-coding, what is more important is that it
indicates what decoding mechanism will be required to remove the indicates what decoding mechanism will be required to remove the
encoding. encoding.
The Internet Assigned Numbers Authority (IANA) acts as a registry for The Internet Assigned Numbers Authority (IANA) acts as a registry for
content-coding value tokens. Initially, the registry contains the content-coding value tokens. Initially, the registry contains the
following tokens: following tokens:
gzip An encoding format produced by the file compression program gzip
"gzip" (GNU zip) as described in RFC 1952 [25]. This format is a
Lempel-Ziv coding (LZ77) with a 32 bit CRC. An encoding format produced by the file compression program "gzip"
(GNU zip) as described in RFC 1952 [25]. This format is a Lempel-
Ziv coding (LZ77) with a 32 bit CRC.
compress compress
The encoding format produced by the common UNIX file compression The encoding format produced by the common UNIX file compression
program "compress". This format is an adaptive Lempel-Ziv-Welch program "compress". This format is an adaptive Lempel-Ziv-Welch
coding (LZW). coding (LZW).
Use of program names for the identification of encoding formats Use of program names for the identification of encoding formats is
is not desirable and is discouraged for future encodings. Their not desirable and is discouraged for future encodings. Their use
use here is representative of historical practice, not good here is representative of historical practice, not good design.
design. For compatibility with previous implementations of HTTP, For compatibility with previous implementations of HTTP,
applications SHOULD consider "x-gzip" and "x-compress" to be applications SHOULD consider "x-gzip" and "x-compress" to be
equivalent to "gzip" and "compress" respectively. equivalent to "gzip" and "compress" respectively.
deflate deflate
The "zlib" format defined in RFC 1950 [31] in combination with
the "deflate" compression mechanism described in RFC 1951 [29]. The "zlib" format defined in RFC 1950 [31] in combination with the
"deflate" compression mechanism described in RFC 1951 [29].
identity identity
The default (identity) encoding; the use of no transformation The default (identity) encoding; the use of no transformation
whatsoever. This content-coding is used only in the Accept- whatsoever. This content-coding is used only in the Accept-
Encoding header, and SHOULD NOT be used in the Content-Encoding Encoding header, and SHOULD NOT be used in the Content-Encoding
header. header.
New content-coding value tokens SHOULD be registered; to allow New content-coding value tokens SHOULD be registered; to allow
interoperability between clients and servers, specifications of the interoperability between clients and servers, specifications of the
content coding algorithms needed to implement a new value SHOULD be content coding algorithms needed to implement a new value SHOULD be
publicly available and adequate for independent implementation, and publicly available and adequate for independent implementation, and
conform to the purpose of content coding defined in this section. conform to the purpose of content coding defined in this section.
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A server which receives an entity-body with a transfer-coding it does A server which receives an entity-body with a transfer-coding it does
not understand SHOULD return 501 (Unimplemented), and close the not understand SHOULD return 501 (Unimplemented), and close the
connection. A server MUST NOT send transfer-codings to an HTTP/1.0 connection. A server MUST NOT send transfer-codings to an HTTP/1.0
client. client.
3.6.1 Chunked Transfer Coding 3.6.1 Chunked Transfer Coding
The chunked encoding modifies the body of a message in order to The chunked encoding modifies the body of a message in order to
transfer it as a series of chunks, each with its own size indicator, transfer it as a series of chunks, each with its own size indicator,
followed by an OPTIONAL trailer containing entity-header fields. This followed by an OPTIONAL trailer containing entity-header fields.
allows dynamically produced content to be transferred along with the This allows dynamically produced content to be transferred along with
information necessary for the recipient to verify that it has the information necessary for the recipient to verify that it has
received the full message. received the full message.
Chunked-Body = *chunk Chunked-Body = *chunk
last-chunk last-chunk
trailer trailer
CRLF CRLF
chunk = chunk-size [ chunk-extension ] CRLF chunk = chunk-size [ chunk-extension ] CRLF
chunk-data CRLF chunk-data CRLF
chunk-size = 1*HEX chunk-size = 1*HEX
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The trailer allows the sender to include additional HTTP header The trailer allows the sender to include additional HTTP header
fields at the end of the message. The Trailer header field can be fields at the end of the message. The Trailer header field can be
used to indicate which header fields are included in a trailer (see used to indicate which header fields are included in a trailer (see
section 14.40). section 14.40).
A server using chunked transfer-coding in a response MUST NOT use the A server using chunked transfer-coding in a response MUST NOT use the
trailer for any header fields unless at least one of the following is trailer for any header fields unless at least one of the following is
true: true:
a)the request included a TE header field that indicates "trailers" is 1. the request included a TE header field that indicates "trailers"
acceptable in the transfer-coding of the response, as described in is acceptable in the transfer-coding of the response, as
section 14.39; or, described in section 14.39; or,
b)the server is the origin server for the response, the trailer 2. the server is the origin server for the response, the trailer
fields consist entirely of optional metadata, and the recipient fields consist entirely of optional metadata, and the recipient
could use the message (in a manner acceptable to the origin server) could use the message (in a manner acceptable to the origin
without receiving this metadata. In other words, the origin server server) without receiving this metadata. In other words, the
is willing to accept the possibility that the trailer fields might origin server is willing to accept the possibility that the
be silently discarded along the path to the client. trailer fields might be silently discarded along the path to the
client.
This requirement prevents an interoperability failure when the This requirement prevents an interoperability failure when the
message is being received by an HTTP/1.1 (or later) proxy and message is being received by an HTTP/1.1 (or later) proxy and
forwarded to an HTTP/1.0 recipient. It avoids a situation where forwarded to an HTTP/1.0 recipient. It avoids a situation where
compliance with the protocol would have necessitated a possibly compliance with the protocol would have necessitated a possibly
infinite buffer on the proxy. infinite buffer on the proxy.
An example process for decoding a Chunked-Body is presented in An example process for decoding a Chunked-Body is presented in
appendix 19.4.6. appendix D.6.
All HTTP/1.1 applications MUST be able to receive and decode the All HTTP/1.1 applications MUST be able to receive and decode the
"chunked" transfer-coding, and MUST ignore chunk-extension extensions "chunked" transfer-coding, and MUST ignore chunk-extension extensions
they do not understand. they do not understand.
3.7 Media Types 3.7 Media Types
HTTP uses Internet Media Types [17] in the Content-Type (section HTTP uses Internet Media Types [17] in the Content-Type
14.17) and Accept (section 14.1) header fields in order to provide (section 14.17) and Accept (section 14.1) header fields in order to
open and extensible data typing and type negotiation. provide open and extensible data typing and type negotiation.
media-type = type "/" subtype *( ";" parameter ) media-type = type "/" subtype *( ";" parameter )
type = token type = token
subtype = token subtype = token
Parameters MAY follow the type/subtype in the form of attribute/value Parameters MAY follow the type/subtype in the form of attribute/value
pairs (as defined in section 3.6). pairs (as defined in section 3.6).
The type, subtype, and parameter attribute names are case- The type, subtype, and parameter attribute names are case-
insensitive. Parameter values might or might not be case-sensitive, insensitive. Parameter values might or might not be case-sensitive,
depending on the semantics of the parameter name. Linear white space depending on the semantics of the parameter name. Linear white space
(LWS) MUST NOT be used between the type and subtype, nor between an (LWS) MUST NOT be used between the type and subtype, nor between an
attribute and its value. The presence or absence of a parameter might attribute and its value. The presence or absence of a parameter
be significant to the processing of a media-type, depending on its might be significant to the processing of a media-type, depending on
definition within the media type registry. its definition within the media type registry.
Note that some older HTTP applications do not recognize media type Note that some older HTTP applications do not recognize media type
parameters. When sending data to older HTTP applications, parameters. When sending data to older HTTP applications,
implementations SHOULD only use media type parameters when they are implementations SHOULD only use media type parameters when they are
required by that type/subtype definition. required by that type/subtype definition.
Media-type values are registered with the Internet Assigned Number Media-type values are registered with the Internet Assigned Number
Authority (IANA [19]). The media type registration process is Authority (IANA [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.
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3.7.2 Multipart Types 3.7.2 Multipart Types
MIME provides for a number of "multipart" types -- encapsulations of MIME provides for a number of "multipart" types -- encapsulations of
one or more entities within a single message-body. All multipart one or more entities within a single message-body. All multipart
types share a common syntax, as defined in section 5.1.1 of RFC 2046 types share a common syntax, as defined in section 5.1.1 of RFC 2046
[40], and MUST include a boundary parameter as part of the media type [40], and MUST include a boundary parameter as part of the media type
value. The message body is itself a protocol element and MUST value. The message body is itself a protocol element and MUST
therefore use only CRLF to represent line breaks between body-parts. therefore use only CRLF to represent line breaks between body-parts.
Unlike in RFC 2046, the epilogue of any multipart message MUST be Unlike in RFC 2046, the epilogue of any multipart message MUST be
empty; HTTP applications MUST NOT transmit the epilogue (even if the empty; HTTP applications MUST NOT transmit the epilogue (even if the
original multipart contains an epilogue). These restrictions exist in original multipart contains an epilogue). These restrictions exist
order to preserve the self-delimiting nature of a multipart message- in order to preserve the self-delimiting nature of a multipart
body, wherein the "end" of the message-body is indicated by the message-body, wherein the "end" of the message-body is indicated by
ending multipart boundary. the ending multipart boundary.
In general, HTTP treats a multipart message-body no differently than In general, HTTP treats a multipart message-body no differently than
any other media type: strictly as payload. The one exception is the any other media type: strictly as payload. The one exception is the
"multipart/byteranges" type (appendix 19.2) when it appears in a 206 "multipart/byteranges" type (appendix B) when it appears in a 206
(Partial Content) response, which will be interpreted by some HTTP (Partial Content) response, which will be interpreted by some HTTP
caching mechanisms as described in sections 13.5.4 and 14.16. In all caching mechanisms as described in sections 13.5.4 and 14.16. In all
other cases, an HTTP user agent SHOULD follow the same or similar other cases, an HTTP user agent SHOULD follow the same or similar
behavior as a MIME user agent would upon receipt of a multipart type. behavior as a MIME user agent would upon receipt of a multipart type.
The MIME header fields within each body-part of a multipart message- The MIME header fields within each body-part of a multipart message-
body do not have any significance to HTTP beyond that defined by body do not have any significance to HTTP beyond that defined by
their MIME semantics. their MIME semantics.
In general, an HTTP user agent SHOULD follow the same or similar In general, an HTTP user agent SHOULD follow the same or similar
behavior as a MIME user agent would upon receipt of a multipart type. behavior as a MIME user agent would upon receipt of a multipart type.
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convention, the products are listed in order of their significance convention, the products are listed in order of their significance
for identifying the application. for identifying the application.
product = token ["/" product-version] product = token ["/" product-version]
product-version = token product-version = token
Examples: Examples:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3 User-Agent: CERN-LineMode/2.15 libwww/2.17b3
Server: Apache/0.8.4 Server: Apache/0.8.4
Product tokens SHOULD be short and to the point. They MUST NOT be Product tokens SHOULD be short and to the point. They MUST NOT be
used for advertising or other non-essential information. Although any used for advertising or other non-essential information. Although
token character MAY appear in a product-version, this token SHOULD any token character MAY appear in a product-version, this token
only be used for a version identifier (i.e., successive versions of SHOULD only be used for a version identifier (i.e., successive
the same product SHOULD only differ in the product-version portion of versions of the same product SHOULD only differ in the product-
the product value). version portion of the product value).
3.9 Quality Values 3.9 Quality Values
HTTP content negotiation (section 12) uses short "floating point" HTTP content negotiation (section 12) uses short "floating point"
numbers to indicate the relative importance ("weight") of various numbers to indicate the relative importance ("weight") of various
negotiable parameters. A weight is normalized to a real number in negotiable parameters. A weight is normalized to a real number in
the range 0 through 1, where 0 is the minimum and 1 the maximum the range 0 through 1, where 0 is the minimum and 1 the maximum
value. If a parameter has a quality value of 0, then content with value. If a parameter has a quality value of 0, then content with
this parameter is `not acceptable' for the client. HTTP/1.1 this parameter is `not acceptable' for the client. HTTP/1.1
applications MUST NOT generate more than three digits after the applications MUST NOT generate more than three digits after the
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language-tag = primary-tag *( "-" subtag ) language-tag = primary-tag *( "-" subtag )
primary-tag = 1*8ALPHA primary-tag = 1*8ALPHA
subtag = 1*8ALPHA subtag = 1*8ALPHA
White space is not allowed within the tag and all tags are case- White space is not allowed within the tag and all tags are case-
insensitive. The name space of language tags is administered by the insensitive. The name space of language tags is administered by the
IANA. Example tags include: IANA. Example tags include:
en, en-US, en-cockney, i-cherokee, x-pig-latin en, en-US, en-cockney, i-cherokee, x-pig-latin
where any two-letter primary-tag is an ISO-639 language abbreviation where any two-letter primary-tag is an ISO-639 language abbreviation
and any two-letter initial subtag is an ISO-3166 country code. (The and any two-letter initial subtag is an ISO-3166 country code. (The
last three tags above are not registered tags; all but the last are last three tags above are not registered tags; all but the last are
examples of tags which could be registered in future.) examples of tags which could be registered in future.)
3.11 Entity Tags 3.11 Entity Tags
Entity tags are used for comparing two or more entities from the same Entity tags are used for comparing two or more entities from the same
requested resource. HTTP/1.1 uses entity tags in the ETag (section requested resource. HTTP/1.1 uses entity tags in the ETag
14.19), If-Match (section 14.24), If-None-Match (section 14.26), and (section 14.19), If-Match (section 14.24), If-None-Match
If-Range (section 14.27) header fields. The definition of how they (section 14.26), and If-Range (section 14.27) header fields. The
are used and compared as cache validators is in section 13.3.3. An definition of how they are used and compared as cache validators is
entity tag consists of an opaque quoted string, possibly prefixed by in section 13.3.3. An entity tag consists of an opaque quoted
a weakness indicator. string, possibly prefixed by a weakness indicator.
entity-tag = [ weak ] opaque-tag entity-tag = [ weak ] opaque-tag
weak = "W/" weak = "W/"
opaque-tag = quoted-string opaque-tag = quoted-string
A "strong entity tag" MAY be shared by two entities of a resource A "strong entity tag" MAY be shared by two entities of a resource
only if they are equivalent by octet equality. only if they are equivalent by octet equality.
A "weak entity tag," indicated by the "W/" prefix, MAY be shared by A "weak entity tag," indicated by the "W/" prefix, MAY be shared by
two entities of a resource only if the entities are equivalent and two entities of a resource only if the entities are equivalent and
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a line with nothing preceding the CRLF) indicating the end of the a line with nothing preceding the CRLF) indicating the end of the
header fields, and possibly a message-body. header fields, and possibly a message-body.
generic-message = start-line generic-message = start-line
*(message-header CRLF) *(message-header CRLF)
CRLF CRLF
[ message-body ] [ message-body ]
start-line = Request-Line | Status-Line start-line = Request-Line | Status-Line
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,
the server is reading the protocol stream at the beginning of a if 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
extra CRLF. extra CRLF.
4.2 Message Headers 4.2 Message Headers
HTTP header fields, which include general-header (section 4.5), HTTP header fields, which include general-header (section 4.5),
request-header (section 5.3), response-header (section 6.2), and request-header (section 5.3), response-header (section 6.2), and
entity-header (section 7.1) fields, follow the same generic format as entity-header (section 7.1) fields, follow the same generic format as
that given in Section 3.1 of RFC 822 [9]. Each header field consists that given in Section 3.1 of RFC 822 [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",
one is known or indicated, when generating HTTP constructs, since where one is known or indicated, when generating HTTP constructs,
there might exist some implementations that fail to accept anything since there might exist some implementations that fail to accept
beyond the common forms. anything beyond the common forms.
message-header = field-name ":" [ field-value ] message-header = field-name ":" [ field-value ]
field-name = token field-name = token
field-value = *( field-content | LWS ) field-value = *( field-content | LWS )
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
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"field-name: field-value" pair, without changing the semantics of the "field-name: field-value" pair, without changing the semantics of the
message, by appending each subsequent field-value to the first, each message, by appending each subsequent field-value to the first, each
separated by a comma. The order in which header fields with the same separated by a comma. The order in which header fields with the same
field-name are received is therefore significant to the field-name are received is therefore significant to the
interpretation of the combined field value, and thus a proxy MUST NOT interpretation of the combined field value, and thus a proxy MUST NOT
change the order of these field values when a message is forwarded. change the order of these field values when a message is forwarded.
4.3 Message Body 4.3 Message Body
The message-body (if any) of an HTTP message is used to carry the The message-body (if any) of an HTTP message is used to carry the
entity-body associated with the request or response. The message-body entity-body associated with the request or response. The message-
differs from the entity-body only when a transfer-coding has been body differs from the entity-body only when a transfer-coding has
applied, as indicated by the Transfer-Encoding header field (section been applied, as indicated by the Transfer-Encoding header field
14.41). (section 14.41).
message-body = entity-body message-body = entity-body
| <entity-body encoded as per Transfer-Encoding> | <entity-body encoded as per Transfer-Encoding>
Transfer-Encoding MUST be used to indicate any transfer-codings Transfer-Encoding MUST be used to indicate any transfer-codings
applied by an application to ensure safe and proper transfer of the applied by an application to ensure safe and proper transfer of the
message. Transfer-Encoding is a property of the message, not of the message. Transfer-Encoding is a property of the message, not of the
entity, and thus MAY be added or removed by any application along the entity, and thus MAY be added or removed by any application along the
request/response chain. (However, section 3.6 places restrictions on request/response chain. (However, section 3.6 places restrictions on
when certain transfer-codings may be used.) when certain transfer-codings may be used.)
The rules for when a message-body is allowed in a message differ for The rules for when a message-body is allowed in a message differ for
requests and responses. requests and responses.
The presence of a message-body in a request is signaled by the The presence of a message-body in a request is signaled by the
inclusion of a Content-Length or Transfer-Encoding header field in inclusion of a Content-Length or Transfer-Encoding header field in
the request's message-headers. A message-body MUST NOT be included in the request's message-headers. A message-body MUST NOT be included
a request if the specification of the request method (section 5.1.1) in a request if the specification of the request method
does not allow sending an entity-body in requests. A server SHOULD (section 5.1.1) does not allow sending an entity-body in requests. A
read and forward a message-body on any request; if the request method server SHOULD read and forward a message-body on any request; if the
does not include defined semantics for an entity-body, then the request method does not include defined semantics for an entity-body,
message-body SHOULD be ignored when handling the request. then the message-body SHOULD be ignored when handling the request.
For response messages, whether or not a message-body is included with For response messages, whether or not a message-body is included with
a message is dependent on both the request method and the response a message is dependent on both the request method and the response
status code (section 6.1.1). All responses to the HEAD request method status code (section 6.1.1). All responses to the HEAD request
MUST NOT include a message-body, even though the presence of entity- method MUST NOT include a message-body, even though the presence of
header fields might lead one to believe they do. All 1xx entity-header fields might lead one to believe they do. All 1xx
(informational), 204 (no content), and 304 (not modified) responses (informational), 204 (no content), and 304 (not modified) responses
MUST NOT include a message-body. All other responses do include a MUST NOT include a message-body. All other responses do include a
message-body, although it MAY be of zero length. message-body, although it MAY be of zero length.
4.4 Message Length 4.4 Message Length
The transfer-length of a message is the length of the message-body as The transfer-length of a message is the length of the message-body as
it appears in the message; that is, after any transfer-codings have it appears in the message; that is, after any transfer-codings have
been applied. When a message-body is included with a message, the been applied. When a message-body is included with a message, the
transfer-length of that body is determined by one of the following transfer-length of that body is determined by one of the following
(in order of precedence): (in order of precedence):
1.Any response message which "MUST NOT" include a message-body (such 1. Any response message which "MUST NOT" include a message-body
as the 1xx, 204, and 304 responses and any response to a HEAD (such as the 1xx, 204, and 304 responses and any response to a
request) is always terminated by the first empty line after the HEAD request) is always terminated by the first empty line after
header fields, regardless of the entity-header fields present in the header fields, regardless of the entity-header fields present
the message. in the message.
2.If a Transfer-Encoding header field (section 14.41) is present and 2. If a Transfer-Encoding header field (section 14.41) is present
has any value other than "identity", then the transfer-length is and has any value other than "identity", then the transfer-length
defined by use of the "chunked" transfer-coding (section 3.6), is defined by use of the "chunked" transfer-coding (section 3.6),
unless the message is terminated by closing the connection. unless the message is terminated by closing the connection.
3.If a Content-Length header field (section 14.13) is present, its 3.If a Content-Length header field (section 14.13) is present, its
decimal value in OCTETs represents both the entity-length and the decimal value in OCTETs represents both the entity-length and the
transfer-length. The Content-Length header field MUST NOT be sent transfer-length. The Content-Length header field MUST NOT be
if these two lengths are different (i.e., if a Transfer-Encoding sent if these two lengths are different (i.e., if a Transfer-
header field is present). If a message is received with both a Encoding header field is present). If a message is received with
Transfer-Encoding header field and a Content-Length header field, both a Transfer-Encoding header field and a Content-Length header
the latter MUST be ignored. field, 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
ransfer-length is not otherwise specified, then this self- the ransfer-length is not otherwise specified, then this self-
elimiting media type defines the transfer-length. This media type elimiting media type defines the transfer-length. This media
UST NOT be used unless the sender knows that the recipient can arse type UST NOT be used unless the sender knows that the recipient
it; the presence in a request of a Range header with ultiple byte- can arse it; the presence in a request of a Range header with
range specifiers from a 1.1 client implies that the lient can parse ultiple byte-range specifiers from a 1.1 client implies that the
multipart/byteranges responses. lient can parse 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
this section. of 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.
All HTTP/1.1 applications that receive entities MUST accept the All HTTP/1.1 applications that receive entities MUST accept the
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Request = Request-Line ; Section 5.1 Request = Request-Line ; Section 5.1
*(( general-header ; Section 4.5 *(( general-header ; Section 4.5
| request-header ; Section 5.3 | request-header ; Section 5.3
| entity-header ) CRLF) ; Section 7.1 | entity-header ) CRLF) ; Section 7.1
CRLF CRLF
[ message-body ] ; Section 4.3 [ message-body ] ; Section 4.3
5.1 Request-Line 5.1 Request-Line
The Request-Line begins with a method token, followed by the The Request-Line begins with a method token, followed by the Request-
Request-URI and the protocol version, and ending with CRLF. The URI and the protocol version, and ending with CRLF. The elements are
elements are separated by SP characters. No CR or LF is allowed separated by SP characters. No CR or LF is allowed except in the
except in the final CRLF sequence. final CRLF sequence.
Request-Line = Method SP Request-URI SP HTTP-Version CRLF Request-Line = Method SP Request-URI SP HTTP-Version CRLF
5.1.1 Method 5.1.1 Method
The Method token indicates the method to be performed on the The Method token indicates the method to be performed on the resource
resource identified by the Request-URI. The method is case-sensitive. 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
| "PUT" ; Section 9.6 | "PUT" ; Section 9.6
| "DELETE" ; Section 9.7 | "DELETE" ; Section 9.7
| "TRACE" ; Section 9.8 | "TRACE" ; Section 9.8
| "CONNECT" ; Section 9.9 | "CONNECT" ; Section 9.9
| extension-method | extension-method
extension-method = token extension-method = token
The list of methods allowed by a resource can be specified in an The list of methods allowed by a resource can be specified in an
Allow header field (section 14.7). The return code of the response Allow header field (section 14.7). The return code of the response
always notifies the client whether a method is currently allowed on a always notifies the client whether a method is currently allowed on a
resource, since the set of allowed methods can change dynamically. An resource, since the set of allowed methods can change dynamically.
origin server SHOULD return the status code 405 (Method Not Allowed) An origin server SHOULD return the status code 405 (Method Not
if the method is known by the origin server but not allowed for the Allowed) if the method is known by the origin server but not allowed
requested resource, and 501 (Not Implemented) if the method is for the requested resource, and 501 (Not Implemented) if the method
unrecognized or not implemented by the origin server. The methods GET is unrecognized or not implemented by the origin server. The methods
and HEAD MUST be supported by all general-purpose servers. All other GET and HEAD MUST be supported by all general-purpose servers. All
methods are OPTIONAL; however, if the above methods are implemented, other methods are OPTIONAL; however, if the above methods are
they MUST be implemented with the same semantics as those specified implemented, they MUST be implemented with the same semantics as
in section 9. those specified in section 9.
5.1.2 Request-URI 5.1.2 Request-URI
The Request-URI is a Uniform Resource Identifier (section 3.2) and The Request-URI is a Uniform Resource Identifier (section 3.2) and
identifies the resource upon which to apply the request. identifies the resource upon which to apply the request.
Request-URI = "*" | absoluteURI | abs_path | authority Request-URI = "*" | absoluteURI | abs_path | authority
The four options for Request-URI are dependent on the nature of the The four options for Request-URI are dependent on the nature of the
request. The asterisk "*" means that the request does not apply to a request. The asterisk "*" means that the request does not apply to a
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example would be example would be
OPTIONS * HTTP/1.1 OPTIONS * HTTP/1.1
The absoluteURI form is REQUIRED when the request is being made to a The absoluteURI form is REQUIRED when the request is being made to a
proxy. The proxy is requested to forward the request or service it proxy. The proxy is requested to forward the request or service it
from a valid cache, and return the response. Note that the proxy MAY from a valid cache, and return the response. Note that the proxy MAY
forward the request on to another proxy or directly to the server forward the request on to another proxy or directly to the server
specified by the absoluteURI. In order to avoid request loops, a specified by the absoluteURI. In order to avoid request loops, a
proxy MUST be able to recognize all of its server names, including proxy MUST be able to recognize all of its server names, including
any aliases, local variations, and the numeric IP address. An example any aliases, local variations, and the numeric IP address. An
Request-Line would be: example Request-Line would be:
GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1 GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1
To allow for transition to absoluteURIs in all requests in future To allow for transition to absoluteURIs in all requests in future
versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI
form in requests, even though HTTP/1.1 clients will only generate form in requests, even though HTTP/1.1 clients will only generate
them in requests to proxies. them in requests to proxies.
The authority form is only used by the CONNECT method (section 9.9). The authority form is only used by the CONNECT method (section 9.9).
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resource on an origin server or gateway. In this case the absolute resource on an origin server or gateway. In this case the absolute
path of the URI MUST be transmitted (see section 3.2.1, abs_path) as path of the URI MUST be transmitted (see section 3.2.1, abs_path) as
the Request-URI, and the network location of the URI (authority) MUST the Request-URI, and the network location of the URI (authority) MUST
be transmitted in a Host header field. For example, a client wishing be transmitted in a Host header field. For example, a client wishing
to retrieve the resource above directly from the origin server would to retrieve the resource above directly from the origin server would
create a TCP connection to port 80 of the host "www.w3.org" and send create a TCP connection to port 80 of the host "www.w3.org" and send
the lines: the lines:
GET /pub/WWW/TheProject.html HTTP/1.1 GET /pub/WWW/TheProject.html HTTP/1.1
Host: www.w3.org Host: www.w3.org
followed by the remainder of the Request. Note that the absolute
followed by the remainder of the Request. Note that the absolute path path cannot be empty; if none is present in the original URI, it MUST
cannot be empty; if none is present in the original URI, it MUST be be given as "/" (the server root).
given as "/" (the server root).
The Request-URI is transmitted in the format specified in section The Request-URI is transmitted in the format specified in section
3.2.1. If the Request-URI is encoded using the "% HEX HEX" encoding 3.2.1. If the Request-URI is encoded using the "% HEX HEX" encoding
[42], the origin server MUST decode the Request-URI in order to [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 "/".
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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.) appendix F.1.1 for other requirements on Host support in HTTP/1.1.)
An origin server that does differentiate resources based on the host An origin server that does differentiate resources based on the host
requested (sometimes referred to as virtual hosts or vanity host requested (sometimes referred to as virtual hosts or vanity host
names) MUST use the following rules for determining the requested names) MUST use the following rules for determining the requested
resource on an HTTP/1.1 request: resource on an HTTP/1.1 request:
1. If Request-URI is an absoluteURI, the host is part of the 1. If Request-URI is an absoluteURI, the host is part of the
Request-URI. Any Host header field value in the request MUST be Request-URI. Any Host header field value in the request MUST be
ignored. ignored.
2. If the Request-URI is not an absoluteURI, and the request includes 2. If the Request-URI is not an absoluteURI, and the request
a Host header field, the host is determined by the Host header includes a Host header field, the host is determined by the Host
field value. header 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
information about the request, and about the client itself, to the information about the request, and about the client itself, to the
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| response-header ; Section 6.2 | response-header ; Section 6.2
| entity-header ) CRLF) ; 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.
Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF
6.1.1 Status Code and Reason Phrase 6.1.1 Status Code and Reason Phrase
The Status-Code element is a 3-digit integer result code of the The Status-Code element is a 3-digit integer result code of the
attempt to understand and satisfy the request. These codes are fully attempt to understand and satisfy the request. These codes are fully
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.
last two digits do not have any categorization role. There are 5 The 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 o 1xx: Informational - Request received, continuing process
- 2xx: Success - The action was successfully received, o 2xx: Success - The action was successfully received, understood,
understood, and accepted and accepted
- 3xx: Redirection - Further action must be taken in order to o 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 o 4xx: Client Error - The request contains bad syntax or cannot be
be fulfilled fulfilled
o 5xx: Server Error - The server failed to fulfill an apparently
- 5xx: Server Error - The server failed to fulfill an apparently
valid request valid request
The individual values of the numeric status codes defined for The individual values of the numeric status codes defined for
HTTP/1.1, and an example set of corresponding Reason-Phrase's, are HTTP/1.1, and an example set of corresponding Reason-Phrase's, are
presented below. The reason phrases listed here are only presented below. The reason phrases listed here are only
recommendations -- they MAY be replaced by local equivalents without recommendations -- they MAY be replaced by local equivalents without
affecting the protocol. affecting the protocol.
Status-Code = Status-Code =
"100" ; Section 10.1.1: Continue "100" ; Section 10.1.1: Continue
| "101" ; Section 10.1.2: Switching Protocols | "101" ; Section 10.1.2: Switching Protocols
| "200" ; Section 10.2.1: OK | "200" ; Section 10.2.1: OK
| "201" ; Section 10.2.2: Created | "201" ; Section 10.2.2: Created
| "202" ; Section 10.2.3: Accepted | "202" ; Section 10.2.3: Accepted
| "203" ; Section 10.2.4: Non-Authoritative Information | "203" ; Section 10.2.4: Non-Authoritative Information
| "204" ; Section 10.2.5: No Content | "204" ; Section 10.2.5: No Content
| "205" ; Section 10.2.6: Reset Content | "205" ; Section 10.2.6: Reset Content
| "206" ; Section 10.2.7: Partial Content | "206" ; Section 10.2.7 Partial Content
| "300" ; Section 10.3.1: Multiple Choices | "300" ; Section 10.3.1: Multiple Choices
| "301" ; Section 10.3.2: Moved Permanently | "301" ; Section 10.3.2: Moved Permanently
| "302" ; Section 10.3.3: Found | "302" ; Section 10.3.3: Found
| "303" ; Section 10.3.4: See Other | "303" ; Section 10.3.4: See Other
| "304" ; Section 10.3.5: Not Modified | "304" ; Section 10.3.5: Not Modified
| "305" ; Section 10.3.6: Use Proxy | "305" ; Section 10.3.6: Use Proxy
| "307" ; Section 10.3.8: Temporary Redirect | "307" ; Section 10.3.8: Temporary Redirect
| "400" ; Section 10.4.1: Bad Request | "400" ; Section 10.4.1: Bad Request
| "401" ; Section 10.4.2: Unauthorized | "401" ; Section 10.4.2: Unauthorized
| "402" ; Section 10.4.3: Payment Required | "402" ; Section 10.4.3: Payment Required
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safely assume that there was something wrong with its request and safely assume that there was something wrong with its request and
treat the response as if it had received a 400 status code. In such treat the response as if it had received a 400 status code. In such
cases, user agents SHOULD present to the user the entity returned cases, user agents SHOULD present to the user the entity returned
with the response, since that entity is likely to include human- with the response, since that entity is likely to include human-
readable information which will explain the unusual status. readable information which will explain the unusual status.
6.2 Response Header Fields 6.2 Response Header Fields
The response-header fields allow the server to pass additional The response-header fields allow the server to pass additional
information about the response which cannot be placed in the Status- information about the response which cannot be placed in the Status-
Line. These header fields give information about the server and about Line. These header fields give information about the server and
further access to the resource identified by the Request-URI. about further access to the resource identified by the Request-URI.
response-header = Accept-Ranges ; Section 14.5 response-header = Accept-Ranges ; Section 14.5
| Age ; Section 14.6 | Age ; Section 14.6
| ETag ; Section 14.19 | ETag ; Section 14.19
| Location ; Section 14.30 | Location ; Section 14.30
| Proxy-Authenticate ; Section 14.33 | Proxy-Authenticate ; Section 14.33
| Retry-After ; Section 14.37 | Retry-After ; Section 14.37
| Server ; Section 14.38 | Server ; Section 14.38
| Vary ; Section 14.44 | Vary ; Section 14.44
| WWW-Authenticate ; Section 14.47 | WWW-Authenticate ; Section 14.47
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Content-Type header field defining the media type of that body. If Content-Type header field defining the media type of that body. If
and only if the media type is not given by a Content-Type field, the and only if the media type is not given by a Content-Type field, the
recipient MAY attempt to guess the media type via inspection of its recipient MAY attempt to guess the media type via inspection of its
content and/or the name extension(s) of the URI used to identify the content and/or the name extension(s) of the URI used to identify the
resource. If the media type remains unknown, the recipient SHOULD resource. If the media type remains unknown, the recipient SHOULD
treat it as type "application/octet-stream". treat it as type "application/octet-stream".
7.2.2 Entity Length 7.2.2 Entity Length
The entity-length of a message is the length of the message-body The entity-length of a message is the length of the message-body
before any transfer-codings have been applied. Section 4.4 defines before any transfer-codings have been applied. section 4.4 defines
how the transfer-length of a message-body is determined. how the transfer-length of a message-body is determined.
8 Connections 8 Connections
8.1 Persistent Connections 8.1 Persistent Connections
8.1.1 Purpose 8.1.1 Purpose
Prior to persistent connections, a separate TCP connection was Prior to persistent connections, a separate TCP connection was
established to fetch each URL, increasing the load on HTTP servers established to fetch each URL, increasing the load on HTTP servers
and causing congestion on the Internet. The use of inline images and and causing congestion on the Internet. The use of inline images and
other associated data often require a client to make multiple other associated data often require a client to make multiple
requests of the same server in a short amount of time. Analysis of requests of the same server in a short amount of time. Analysis of
these performance problems and results from a prototype these performance problems and results from a prototype
implementation are available [26] [30]. Implementation experience and implementation are available [26] [30]. Implementation experience
measurements of actual HTTP/1.1 (RFC 2068) implementations show good and measurements of actual HTTP/1.1 (RFC 2068) implementations show
results [39]. Alternatives have also been explored, for example, good results [39]. Alternatives have also been explored, for
T/TCP [27]. example, 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 o By opening and closing fewer TCP connections, CPU time is saved in
in routers and hosts (clients, servers, proxies, gateways, routers and hosts (clients, servers, proxies, gateways, tunnels,
tunnels, or caches), and memory used for TCP protocol control or caches), and memory used for TCP protocol control blocks can be
blocks can be saved in hosts. saved in hosts.
- HTTP requests and responses can be pipelined on a connection. o HTTP requests and responses can be pipelined on a connection.
Pipelining allows a client to make multiple requests without Pipelining allows a client to make multiple requests without
waiting for each response, allowing a single TCP connection to waiting for each response, allowing a single TCP connection to be
be used much more efficiently, with much lower elapsed time. used much more efficiently, with much lower elapsed time.
- Network congestion is reduced by reducing the number of packets o 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 time o Latency on subsequent requests is reduced since there is no time
spent in TCP's connection opening handshake. spent in TCP's connection opening handshake.
- HTTP can evolve more gracefully, since errors can be reported o HTTP can evolve more gracefully, since errors can be reported
without the penalty of closing the TCP connection. Clients using without the penalty of closing the TCP connection. Clients using
future versions of HTTP might optimistically try a new feature, future versions of HTTP might optimistically try a new feature,
but if communicating with an older server, retry with old but if communicating with an older server, retry with 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.
Persistent connections provide a mechanism by which a client and a Persistent connections provide a mechanism by which a client and a
server can signal the close of a TCP connection. This signaling takes server can signal the close of a TCP connection. This signaling
place using the Connection header field (section 14.10). Once a close takes place using the Connection header field (section 14.10). Once
has been signaled, the client MUST NOT send any more requests on that a close has been signaled, the client MUST NOT send any more requests
connection. on that connection.
8.1.2.1 Negotiation 8.1.2.1 Negotiation
An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to
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
the client does not want to maintain a connection for more than that case the client does not want to maintain a connection for more than
request, it SHOULD send a Connection header including the that request, it SHOULD send a Connection header including the
connection-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 appendix F.2 for more information on backward
compatibility with HTTP/1.0 clients. compatibility with HTTP/1.0 clients.
In order to remain persistent, all messages on the connection MUST In order to remain persistent, all messages on the connection MUST
have a self-defined message length (i.e., one not defined by closure have a self-defined message length (i.e., one not defined by closure
of the connection), as described in section 4.4. of the connection), as described in section 4.4.
8.1.2.2 Pipelining 8.1.2.2 Pipelining
A client that supports persistent connections MAY "pipeline" its A client that supports persistent connections MAY "pipeline" its
requests (i.e., send multiple requests without waiting for each requests (i.e., send multiple requests without waiting for each
response). A server MUST send its responses to those requests in the response). A server MUST send its responses to those requests in the
same order that the requests were received. same order that the requests were received.
Clients which assume persistent connections and pipeline immediately Clients which assume persistent connections and pipeline immediately
after connection establishment SHOULD be prepared to retry their after connection establishment SHOULD be prepared to retry their
connection if the first pipelined attempt fails. If a client does connection if the first pipelined attempt fails. If a client does
such a retry, it MUST NOT pipeline before it knows the connection is such a retry, it MUST NOT pipeline before it knows the connection is
persistent. Clients MUST also be prepared to resend their requests if persistent. Clients MUST also be prepared to resend their requests
the server closes the connection before sending all of the if the server closes the connection before sending all of the
corresponding responses. corresponding responses.
Clients SHOULD NOT pipeline requests using non-idempotent methods or Clients SHOULD NOT pipeline requests using non-idempotent methods or
non-idempotent sequences of methods (see section 9.1.2). Otherwise, a non-idempotent sequences of methods (see section 9.1.2). Otherwise,
premature termination of the transport connection could lead to a premature termination of the transport connection could lead to
indeterminate results. A client wishing to send a non-idempotent indeterminate results. A client wishing to send a non-idempotent
request SHOULD wait to send that request until it has received the request SHOULD wait to send that request until it has received the
response status for the previous request. response status for the previous request.
8.1.3 Proxy Servers 8.1.3 Proxy Servers
It is especially important that proxies correctly implement the It is especially important that proxies correctly implement the
properties of the Connection header field as specified in section properties of the Connection header field as specified in
14.10. section 14.10.
The proxy server MUST signal persistent connections separately with The proxy server MUST signal persistent connections separately with
its clients and the origin servers (or other proxy servers) that it its clients and the origin servers (or other proxy servers) that it
connects to. Each persistent connection applies to only one transport connects to. Each persistent connection applies to only one
link. transport link.
A proxy server MUST NOT establish a HTTP/1.1 persistent connection A proxy server MUST NOT establish a HTTP/1.1 persistent connection
with an HTTP/1.0 client (but see RFC 2068 [33] for information and with an HTTP/1.0 client (but see RFC 2068 [33] for information and
discussion of the problems with the Keep-Alive header implemented by discussion of the problems with the Keep-Alive header implemented by
many HTTP/1.0 clients). many HTTP/1.0 clients).
8.1.4 Practical Considerations 8.1.4 Practical Considerations
Servers will usually have some time-out value beyond which they will Servers will usually have some time-out value beyond which they will
no longer maintain an inactive connection. Proxy servers might make no longer maintain an inactive connection. Proxy servers might make
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connection. From the server's point of view, the connection is being connection. From the server's point of view, the connection is being
closed while it was idle, but from the client's point of view, a closed while it was idle, but from the client's point of view, a
request is in progress. request is in progress.
This means that clients, servers, and proxies MUST be able to recover This means that clients, servers, and proxies MUST be able to recover
from asynchronous close events. Client software SHOULD reopen the from asynchronous close events. Client software SHOULD reopen the
transport connection and retransmit the aborted sequence of requests transport connection and retransmit the aborted sequence of requests
without user interaction so long as the request sequence is without user interaction so long as the request sequence is
idempotent (see section 9.1.2). Non-idempotent methods or sequences idempotent (see section 9.1.2). Non-idempotent methods or sequences
MUST NOT be automatically retried, although user agents MAY offer a MUST NOT be automatically retried, although user agents MAY offer a
human operator the choice of retrying the request(s). Confirmation by human operator the choice of retrying the request(s). Confirmation
user-agent software with semantic understanding of the application by user-agent software with semantic understanding of the application
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
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The purpose of the 100 (Continue) status (see section 10.1.1) is to The purpose of the 100 (Continue) status (see section 10.1.1) is to
allow a client that is sending a request message with a request body allow a client that is sending a request message with a request body
to determine if the origin server is willing to accept the request to determine if the origin server is willing to accept the request
(based on the request headers) before the client sends the request (based on the request headers) before the client sends the request
body. In some cases, it might either be inappropriate or highly body. In some cases, it might either be inappropriate or highly
inefficient for the client to send the body if the server will reject inefficient for the client to send the body if the server will reject
the message without looking at the body. the message without looking at the body.
Requirements for HTTP/1.1 clients: Requirements for HTTP/1.1 clients:
- If a client will wait for a 100 (Continue) response before o If a client will wait for a 100 (Continue) response before sending
sending the request body, it MUST send an Expect request-header the request body, it MUST send an Expect request-header field
field (section 14.20) with the "100-continue" expectation. (section 14.20) with the "100-continue" expectation.
- A client MUST NOT send an Expect request-header field (section o A client MUST NOT send an Expect request-header field
14.20) with the "100-continue" expectation if it does not intend (section 14.20) with the "100-continue" expectation if it does not
to send a request body. intend to send a request body.
Because of the presence of older implementations, the protocol allows Because of the presence of older implementations, the protocol allows
ambiguous situations in which a client may send "Expect: 100- ambiguous situations in which a client may send "Expect: 100-
continue" without receiving either a 417 (Expectation Failed) status continue" without receiving either a 417 (Expectation Failed) status
or a 100 (Continue) status. Therefore, when a client sends this or a 100 (Continue) status. Therefore, when a client sends this
header field to an origin server (possibly via a proxy) from which it header field to an origin server (possibly via a proxy) from which it
has never seen a 100 (Continue) status, the client SHOULD NOT wait has never seen a 100 (Continue) status, the client SHOULD NOT wait
for an indefinite period before sending the request body. for an indefinite period before sending the request body.
Requirements for HTTP/1.1 origin servers: Requirements for HTTP/1.1 origin servers:
- Upon receiving a request which includes an Expect request-header o Upon receiving a request which includes an Expect request-header
field with the "100-continue" expectation, an origin server MUST field with the "100-continue" expectation, an origin server MUST
either respond with 100 (Continue) status and continue to read either respond with 100 (Continue) status and continue to read
from the input stream, or respond with a final status code. The from the input stream, or respond with a final status code. The
origin server MUST NOT wait for the request body before sending origin server MUST NOT wait for the request body before sending
the 100 (Continue) response. If it responds with a final status the 100 (Continue) response. If it responds with a final status
code, it MAY close the transport connection or it MAY continue code, it MAY close the transport connection or it MAY continue to
to read and discard the rest of the request. It MUST NOT read and discard the rest of the request. It MUST NOT perform the
perform the requested method if it returns a final status code. requested method if it returns a final status code.
- An origin server SHOULD NOT send a 100 (Continue) response if o An origin server SHOULD NOT send a 100 (Continue) response if the
the request message does not include an Expect request-header request message does not include an Expect request-header field
field with the "100-continue" expectation, and MUST NOT send a with the "100-continue" expectation, and MUST NOT send a 100
100 (Continue) response if such a request comes from an HTTP/1.0 (Continue) response if such a request comes from an HTTP/1.0 (or
(or earlier) client. There is an exception to this rule: for earlier) client. There is an exception to this rule: for
compatibility with RFC 2068, a server MAY send a 100 (Continue) compatibility with RFC 2068, a server MAY send a 100 (Continue)
status in response to an HTTP/1.1 PUT or POST request that does status in response to an HTTP/1.1 PUT or POST request that does
not include an Expect request-header field with the "100- not include an Expect request-header field with the "100-continue"
continue" expectation. This exception, the purpose of which is expectation. This exception, the purpose of which is to minimize
to minimize any client processing delays associated with an any client processing delays associated with an undeclared wait
undeclared wait for 100 (Continue) status, applies only to for 100 (Continue) status, applies only to HTTP/1.1 requests, and
HTTP/1.1 requests, and not to requests with any other HTTP- not to requests with any other HTTP-version value.
version value.
- An origin server MAY omit a 100 (Continue) response if it has o An origin server MAY omit a 100 (Continue) response if it has
already received some or all of the request body for the already received some or all of the request body for the
corresponding request. corresponding request.
- An origin server that sends a 100 (Continue) response MUST o An origin server that sends a 100 (Continue) response MUST
ultimately send a final status code, once the request body is ultimately send a final status code, once the request body is
received and processed, unless it terminates the transport received and processed, unless it terminates the transport
connection prematurely. connection prematurely.
- If an origin server receives a request that does not include an o If an origin server receives a request that does not include an
Expect request-header field with the "100-continue" expectation, Expect request-header field with the "100-continue" expectation,
the request includes a request body, and the server responds the request includes a request body, and the server responds with
with a final status code before reading the entire request body a final status code before reading the entire request body from
from the transport connection, then the server SHOULD NOT close the transport connection, then the server SHOULD NOT close the
the transport connection until it has read the entire request, transport connection until it has read the entire request, or
or until the client closes the connection. Otherwise, the client 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
badly broken client implementations. 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- o If a proxy receives a request that includes an Expect request-
header field with the "100-continue" expectation, and the proxy header field with the "100-continue" expectation, and the proxy
either knows that the next-hop server complies with HTTP/1.1 or either knows that the next-hop server complies with HTTP/1.1 or
higher, or does not know the HTTP version of the next-hop higher, or does not know the HTTP version of the next-hop server,
server, it MUST forward the request, including the Expect header it MUST forward the request, including the Expect header field.
field.
- If the proxy knows that the version of the next-hop server is o If the proxy knows that the version of the next-hop server is
HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
respond with a 417 (Expectation Failed) status. respond with a 417 (Expectation Failed) status.
- Proxies SHOULD maintain a cache recording the HTTP version o Proxies SHOULD maintain a cache recording the HTTP version numbers
numbers received from recently-referenced next-hop servers. received from recently-referenced next-hop servers.
- A proxy MUST NOT forward a 100 (Continue) response if the o A proxy MUST NOT forward a 100 (Continue) response if the request
request message was received from an HTTP/1.0 (or earlier) message was received from an HTTP/1.0 (or earlier) client and did
client and did not include an Expect request-header field with not include an Expect request-header field with the "100-continue"
the "100-continue" expectation. This requirement overrides the expectation. This requirement overrides the general rule for
general rule for forwarding of 1xx responses (see section 10.1). forwarding of 1xx responses (see section 10.1).
8.2.4 Client Behavior if Server Prematurely Closes Connection 8.2.4 Client Behavior if Server Prematurely Closes Connection
If an HTTP/1.1 client sends a request which includes a request body, If an HTTP/1.1 client sends a request which includes a request body,
but which does not include an Expect request-header field with the but which does not include an Expect request-header field with the
"100-continue" expectation, and if the client is not directly "100-continue" expectation, and if the client is not directly
connected to an HTTP/1.1 origin server, and if the client sees the connected to an HTTP/1.1 origin server, and if the client sees the
connection close before receiving any status from the server, the connection close before receiving any status from the server, the
client SHOULD retry the request. If the client does retry this client SHOULD retry the request. If the client does retry this
request, it MAY use the following "binary exponential backoff" request, it MAY use the following "binary exponential backoff"
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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
retries of this request. of this request.
5. Wait either for an error response from the server, or for T 5. Wait either for an error response from the server, or for T
seconds (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, 7. If client sees that the connection is closed prematurely, repeat
repeat from step 1 until the request is accepted, an error from step 1 until the request is accepted, an error response is
response is received, or the user becomes impatient and received, or the user becomes impatient and terminates the retry
terminates the retry process. process.
If at any point an error status is received, the client If at any point an error status is received, the client
- SHOULD NOT continue and o SHOULD NOT continue and
- SHOULD close the connection if it has not completed sending the o 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.
9.1 Safe and Idempotent Methods 9.1 Safe and Idempotent Methods
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
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specification does not define any use for such a body, future specification does not define any use for such a body, future
extensions to HTTP might use the OPTIONS body to make more detailed extensions to HTTP might use the OPTIONS body to make more detailed
queries on the server. A server that does not support such an queries on the server. A server that does not support such an
extension MAY discard the request body. extension MAY discard the request body.
If the Request-URI is an asterisk ("*"), the OPTIONS request is If the Request-URI is an asterisk ("*"), the OPTIONS request is
intended to apply to the server in general rather than to a specific intended to apply to the server in general rather than to a specific
resource. Since a server's communication options typically depend on resource. Since a server's communication options typically depend on
the resource, the "*" request is only useful as a "ping" or "no-op" the resource, the "*" request is only useful as a "ping" or "no-op"
type of method; it does nothing beyond allowing the client to test type of method; it does nothing beyond allowing the client to test
the capabilities of the server. For example, this can be used to test the capabilities of the server. For example, this can be used to
a proxy for HTTP/1.1 compliance (or lack thereof). test a proxy for HTTP/1.1 compliance (or lack thereof).
If the Request-URI is not an asterisk, the OPTIONS request applies If the Request-URI is not an asterisk, the OPTIONS request applies
only to the options that are available when communicating with that only to the options that are available when communicating with that
resource. resource.
A 200 response SHOULD include any header fields that indicate A 200 response SHOULD include any header fields that indicate
optional features implemented by the server and applicable to that optional features implemented by the server and applicable to that
resource (e.g., Allow), possibly including extensions not defined by resource (e.g., Allow), possibly including extensions not defined by
this specification. The response body, if any, SHOULD also include this specification. The response body, if any, SHOULD also include
information about the communication options. The format for such a information about the communication options. The format for such a
body is not defined by this specification, but might be defined by body is not defined by this specification, but might be defined by
future extensions to HTTP. Content negotiation MAY be used to select future extensions to HTTP. Content negotiation MAY be used to select
the appropriate response format. If no response body is included, the the appropriate response format. If no response body is included,
response MUST include a Content-Length field with a field-value of the response MUST include a Content-Length field with a field-value
"0". of "0".
The Max-Forwards request-header field MAY be used to target a The Max-Forwards request-header field MAY be used to target a
specific proxy in the request chain. When a proxy receives an OPTIONS specific proxy in the request chain. When a proxy receives an
request on an absoluteURI for which request forwarding is permitted, OPTIONS request on an absoluteURI for which request forwarding is
the proxy MUST check for a Max-Forwards field. If the Max-Forwards permitted, the proxy MUST check for a Max-Forwards field. If the
field-value is zero ("0"), the proxy MUST NOT forward the message; Max-Forwards field-value is zero ("0"), the proxy MUST NOT forward
instead, the proxy SHOULD respond with its own communication options. the message; instead, the proxy SHOULD respond with its own
If the Max-Forwards field-value is an integer greater than zero, the communication options. If the Max-Forwards field-value is an integer
proxy MUST decrement the field-value when it forwards the request. If greater than zero, the proxy MUST decrement the field-value when it
no Max-Forwards field is present in the request, then the forwarded forwards the request. If no Max-Forwards field is present in the
request MUST NOT include a Max-Forwards field. request, then the forwarded request MUST NOT include a Max-Forwards
field.
9.3 GET 9.3 GET
The GET method means retrieve whatever information (in the form of an The GET method means retrieve whatever information (in the form of an
entity) is identified by the Request-URI. If the Request-URI refers entity) is identified by the Request-URI. If the Request-URI refers
to a data-producing process, it is the produced data which shall be to a data-producing process, it is the produced data which shall be
returned as the entity in the response and not the source text of the returned as the entity in the response and not the source text of the
process, unless that text happens to be the output of the process. process, unless that text happens to be the output of the process.
The semantics of the GET method change to a "conditional GET" if the The semantics of the GET method change to a "conditional GET" if the
request message includes an If-Modified-Since, If-Unmodified-Since, request message includes an If-Modified-Since, If-Unmodified-Since,
If-Match, If-None-Match, or If-Range header field. A conditional GET If-Match, If-None-Match, or If-Range header field. A conditional GET
method requests that the entity be transferred only under the method requests that the entity be transferred only under the
circumstances described by the conditional header field(s). The circumstances described by the conditional header field(s). The
conditional GET method is intended to reduce unnecessary network conditional GET method is intended to reduce unnecessary network
usage by allowing cached entities to be refreshed without requiring usage by allowing cached entities to be refreshed without requiring
multiple requests or transferring data already held by the client. multiple requests or transferring data already held by the client.
The semantics of the GET method change to a "partial GET" if the The semantics of the GET method change to a "partial GET" if the
request message includes a Range header field. A partial GET requests request message includes a Range header field. A partial GET
that only part of the entity be transferred, as described in section requests that only part of the entity be transferred, as described in
14.35. The partial GET method is intended to reduce unnecessary section 14.35. The partial GET method is intended to reduce
network usage by allowing partially-retrieved entities to be unnecessary network usage by allowing partially-retrieved entities to
completed without transferring data already held by the client. be completed without transferring data already held by the client.
The response to a GET request is cacheable if and only if it meets The response to a GET request is cacheable if and only if it meets
the requirements for HTTP caching described in section 13. the requirements for HTTP caching described in section 13.
See section 15.1.3 for security considerations when used for forms. See section 15.1.3 for security considerations when used for forms.
9.4 HEAD 9.4 HEAD
The HEAD method is identical to GET except that the server MUST NOT The HEAD method is identical to GET except that the server MUST NOT
return a message-body in the response. The metainformation contained return a message-body in the response. The metainformation contained
in the HTTP headers in response to a HEAD request SHOULD be identical in the HTTP headers in response to a HEAD request SHOULD be identical
to the information sent in response to a GET request. This method can to the information sent in response to a GET request. This method
be used for obtaining metainformation about the entity implied by the can be used for obtaining metainformation about the entity implied by
request without transferring the entity-body itself. This method is the request without transferring the entity-body itself. This method
often used for testing hypertext links for validity, accessibility, is often used for testing hypertext links for validity,
and recent modification. accessibility, and recent modification.
The response to a HEAD request MAY be cacheable in the sense that the The response to a HEAD request MAY be cacheable in the sense that the
information contained in the response MAY be used to update a information contained in the response MAY be used to update a
previously cached entity from that resource. If the new field values previously cached entity from that resource. If the new field values
indicate that the cached entity differs from the current entity (as indicate that the cached entity differs from the current entity (as
would be indicated by a change in Content-Length, Content-MD5, ETag would be indicated by a change in Content-Length, Content-MD5, ETag
or Last-Modified), then the cache MUST treat the cache entry as or Last-Modified), then the cache MUST treat the cache entry as
stale. stale.
9.5 POST 9.5 POST
The POST method is used to request that the origin server accept the The POST method is used to request that the origin server accept the
entity enclosed in the request as a new subordinate of the resource entity enclosed in the request as 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; o Annotation of existing resources;
- Posting a message to a bulletin board, newsgroup, mailing list, o Posting a message to a bulletin board, newsgroup, mailing list, or
or similar group of articles; similar group of articles;
- Providing a block of data, such as the result of submitting a o 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. o Extending a database through an append operation.
The actual function performed by the POST method is determined by the The actual function performed by the POST method is determined by the
server and is usually dependent on the Request-URI. The posted entity server and is usually dependent on the Request-URI. The posted
is subordinate to that URI in the same way that a file is subordinate entity is subordinate to that URI in the same way that a file is
to a directory containing it, a news article is subordinate to a subordinate to a directory containing it, a news article is
newsgroup to which it is posted, or a record is subordinate to a subordinate to a newsgroup to which it is posted, or a record is
database. subordinate to a database.
The action performed by the POST method might not result in a The action performed by the POST method might not result in a
resource that can be identified by a URI. In this case, either 200 resource that can be identified by a URI. In this case, either 200
(OK) or 204 (No Content) is the appropriate response status, (OK) or 204 (No Content) is the appropriate response status,
depending on whether or not the response includes an entity that depending on whether or not the response includes an entity that
describes the result. describes the result.
If a resource has been created on the origin server, the response If a resource has been created on the origin server, the response
SHOULD be 201 (Created) and contain an entity which describes the SHOULD be 201 (Created) and contain an entity which describes the
status of the request and refers to the new resource, and a Location status of the request and refers to the new resource, and a Location
header (see section 14.30). header (see section 14.30).
Responses to this method are not cacheable, unless the response Responses to this method are not cacheable, unless the response
includes appropriate Cache-Control or Expires header fields. However, includes appropriate Cache-Control or Expires header fields.
the 303 (See Other) response can be used to direct the user agent to However, the 303 (See Other) response can be used to direct the user
retrieve a cacheable resource. agent to retrieve a cacheable resource.
POST requests MUST obey the message transmission requirements set out POST requests MUST obey the message transmission requirements set out
in section 8.2. in section 8.2.
See section 15.1.3 for security considerations. See section 15.1.3 for security considerations.
9.6 PUT 9.6 PUT
The PUT method requests that the enclosed entity be stored under the The PUT method requests that the enclosed entity be stored under the
supplied Request-URI. If the Request-URI refers to an already supplied Request-URI. If the Request-URI refers to an already
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Request-URI does not point to an existing resource, and that URI is Request-URI does not point to an existing resource, and that URI is
capable of being defined as a new resource by the requesting user capable of being defined as a new resource by the requesting user
agent, the origin server can create the resource with that URI. If a agent, the origin server can create the resource with that URI. If a
new resource is created, the origin server MUST inform the user agent new resource is created, the origin server MUST inform the user agent
via the 201 (Created) response. If an existing resource is modified, via the 201 (Created) response. If an existing resource is modified,
either the 200 (OK) or 204 (No Content) response codes SHOULD be sent either the 200 (OK) or 204 (No Content) response codes SHOULD be sent
to indicate successful completion of the request. If the resource to indicate successful completion of the request. If the resource
could not be created or modified with the Request-URI, an appropriate could not be created or modified with the Request-URI, an appropriate
error response SHOULD be given that reflects the nature of the error response SHOULD be given that reflects the nature of the
problem. The recipient of the entity MUST NOT ignore any Content-* problem. The recipient of the entity MUST NOT ignore any Content-*
(e.g. Content-Range) headers that it does not understand or implement (e.g. Content-Range) headers that it does not understand or
and MUST return a 501 (Not Implemented) response in such cases. implement and MUST return a 501 (Not Implemented) response in such
cases.
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.
The fundamental difference between the POST and PUT requests is The fundamental difference between the POST and PUT requests is
reflected in the different meaning of the Request-URI. The URI in a reflected in the different meaning of the Request-URI. The URI in a
POST request identifies the resource that will handle the enclosed POST request identifies the resource that will handle the enclosed
entity. That resource might be a data-accepting process, a gateway to entity. That resource might be a data-accepting process, a gateway
some other protocol, or a separate entity that accepts annotations. to some other protocol, or a separate entity that accepts
In contrast, the URI in a PUT request identifies the entity enclosed annotations. In contrast, the URI in a PUT request identifies the
with the request -- the user agent knows what URI is intended and the entity enclosed with the request -- the user agent knows what URI is
server MUST NOT attempt to apply the request to some other resource. intended and the server MUST NOT attempt to apply the request to some
If the server desires that the request be applied to a different URI, other resource. If the server desires that the request be applied to
it MUST send a 301 (Moved Permanently) response; the user agent MAY a different URI, it MUST send a 301 (Moved Permanently) response; the
then make its own decision regarding whether or not to redirect the user agent MAY then make its own decision regarding whether or not to
request. redirect the request.
A single resource MAY be identified by many different URIs. For A single resource MAY be identified by many different URIs. For
example, an article might have a URI for identifying "the current example, an article might have a URI for identifying "the current
version" which is separate from the URI identifying each particular version" which is separate from the URI identifying each particular
version. In this case, a PUT request on a general URI might result in version. In this case, a PUT request on a general URI might result
several other URIs being defined by the origin server. in several other URIs being defined by the origin server.
HTTP/1.1 does not define how a PUT method affects the state of an HTTP/1.1 does not define how a PUT method affects the state of an
origin server. origin server.
PUT requests MUST obey the message transmission requirements set out PUT requests MUST obey the message transmission requirements set out
in section 8.2. in section 8.2.
Unless otherwise specified for a particular entity-header, the Unless otherwise specified for a particular entity-header, the
entity-headers in the PUT request SHOULD be applied to the resource entity-headers in the PUT request SHOULD be applied to the resource
created or modified by the PUT. created or modified by the PUT.
9.7 DELETE 9.7 DELETE
The DELETE method requests that the origin server delete the resource The DELETE method requests that the origin server delete the resource
identified by the Request-URI. This method MAY be overridden by human identified by the Request-URI. This method MAY be overridden by
intervention (or other means) on the origin server. The client cannot human intervention (or other means) on the origin server. The client
be guaranteed that the operation has been carried out, even if the cannot be guaranteed that the operation has been carried out, even if
status code returned from the origin server indicates that the action the status code returned from the origin server indicates that the
has been completed successfully. However, the server SHOULD NOT action has been completed successfully. However, the server SHOULD
indicate success unless, at the time the response is given, it NOT indicate success unless, at the time the response is given, it
intends to delete the resource or move it to an inaccessible intends to delete the resource or move it to an inaccessible
location. location.
A successful response SHOULD be 200 (OK) if the response includes an A successful response SHOULD be 200 (OK) if the response includes an
entity describing the status, 202 (Accepted) if the action has not entity describing the status, 202 (Accepted) if the action has not
yet been enacted, or 204 (No Content) if the action has been enacted yet been enacted, or 204 (No Content) if the action has been enacted
but the response does not include an entity. but the response does not include an entity.
If the request passes through a cache and the Request-URI identifies If the request passes through a cache and the Request-URI identifies
one or more currently cached entities, those entries SHOULD be one or more currently cached entities, those entries SHOULD be
treated as stale. Responses to this method are not cacheable. treated as stale. Responses to this method are not cacheable.
9.8 TRACE 9.8 TRACE
The TRACE method is used to invoke a remote, application-layer loop- The TRACE method is used to invoke a remote, application-layer loop-
back of the request message. The final recipient of the request back of the request message. The final recipient of the request
SHOULD reflect the message received back to the client as the SHOULD reflect the message received back to the client as the entity-
entity-body of a 200 (OK) response. The final recipient is either the body of a 200 (OK) response. The final recipient is either the
origin server or the first proxy or gateway to receive a Max-Forwards origin server or the first proxy or gateway to receive a Max-Forwards
value of zero (0) in the request (see section 14.31). A TRACE request value of zero (0) in the request (see section 14.31). A TRACE
MUST NOT include an entity. request 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
request message in the entity-body, with a Content-Type of request message in the entity-body, with a Content-Type of "message/
"message/http". Responses to this method MUST NOT be cached. http". Responses to this method MUST NOT be cached.
9.9 CONNECT 9.9 CONNECT
This specification reserves the method name CONNECT for use with a This specification reserves the method name CONNECT for use with a
proxy that can dynamically switch to being a tunnel (e.g. SSL proxy that can dynamically switch to being a tunnel (e.g. SSL
tunneling [44]). tunneling [44]).
10 Status Code Definitions 10 Status Code Definitions
Each Status-Code is described below, including a description of which Each Status-Code is described below, including a description of which
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Proxies MUST forward 1xx responses, unless the connection between the Proxies MUST forward 1xx responses, unless the connection between the
proxy and its client has been closed, or unless the proxy itself proxy and its client has been closed, or unless the proxy itself
requested the generation of the 1xx response. (For example, if a requested the generation of the 1xx response. (For example, if a
proxy adds a "Expect: 100-continue" field when it forwards a request, proxy adds a "Expect: 100-continue" field when it forwards a request,
then it need not forward the corresponding 100 (Continue) then it need not forward the corresponding 100 (Continue)
response(s).) response(s).)
10.1.1 100 Continue 10.1.1 100 Continue
The client SHOULD continue with its request. This interim response is The client SHOULD continue with its request. This interim response
used to inform the client that the initial part of the request has is used to inform the client that the initial part of the request has
been received and has not yet been rejected by the server. The client been received and has not yet been rejected by the server. The
SHOULD continue by sending the remainder of the request or, if the client SHOULD continue by sending the remainder of the request or, if
request has already been completed, ignore this response. The server the request has already been completed, ignore this response. The
MUST send a final response after the request has been completed. See server MUST send a final response after the request has been
section 8.2.3 for detailed discussion of the use and handling of this completed. See section 8.2.3 for detailed discussion of the use and
status code. handling of this status code.
10.1.2 101 Switching Protocols 10.1.2 101 Switching Protocols
The server understands and is willing to comply with the client's The server understands and is willing to comply with the client's
request, via the Upgrade message header field (section 14.42), for a request, via the Upgrade message header field (section 14.42), for a
change in the application protocol being used on this connection. The change in the application protocol being used on this connection.
server will switch protocols to those defined by the response's The server will switch protocols to those defined by the response's
Upgrade header field immediately after the empty line which Upgrade header field immediately after the empty line which
terminates the 101 response. terminates the 101 response.
The protocol SHOULD be switched only when it is advantageous to do The protocol SHOULD be switched only when it is advantageous to do
so. For example, switching to a newer version of HTTP is advantageous so. For example, switching to a newer version of HTTP is
over older versions, and switching to a real-time, synchronous advantageous over older versions, and switching to a real-time,
protocol might be advantageous when delivering resources that use synchronous protocol might be advantageous when delivering resources
such features. that use such features.
10.2 Successful 2xx 10.2 Successful 2xx
This class of status code indicates that the client's request was This class of status code indicates that the client's request was
successfully received, understood, and accepted. successfully received, understood, and accepted.
10.2.1 200 OK 10.2.1 200 OK
The request has succeeded. The information returned with the response The request has succeeded. The information returned with the
is dependent on the method used in the request, for example: response is dependent on the method used in the request, for example:
GET an entity corresponding to the requested resource is sent in GET an entity corresponding to the requested resource is sent in the
the response; response;
HEAD the entity-header fields corresponding to the requested HEAD the entity-header fields corresponding to the requested
resource are sent in the response without any message-body; resource are sent in the response without any message-body;
POST an entity describing or containing the result of the action; POST an entity describing or containing the result of the action;
TRACE an entity containing the request message as received by the TRACE an entity containing the request message as received by the
end server. 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
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requiring that the user agent's connection to the server persist requiring that the user agent's connection to the server persist
until the process is completed. The entity returned with this until the process is completed. The entity returned with this
response SHOULD include an indication of the request's current status response SHOULD include an indication of the request's current status
and either a pointer to a status monitor or some estimate of when the and either a pointer to a status monitor or some estimate of when the
user can expect the request to be fulfilled. user can expect the request to be fulfilled.
10.2.4 203 Non-Authoritative Information 10.2.4 203 Non-Authoritative Information
The returned metainformation in the entity-header is not the The returned metainformation in the entity-header is not the
definitive set as available from the origin server, but is gathered definitive set as available from the origin server, but is gathered
from a local or a third-party copy. The set presented MAY be a subset from a local or a third-party copy. The set presented MAY be a
or superset of the original version. For example, including local subset or superset of the original version. For example, including
annotation information about the resource might result in a superset local annotation information about the resource might result in a
of the metainformation known by the origin server. Use of this superset of the metainformation known by the origin server. Use of
response code is not required and is only appropriate when the this response code is not required and is only appropriate when the
response would otherwise be 200 (OK). response would otherwise be 200 (OK).
10.2.5 204 No Content 10.2.5 204 No Content
The server has fulfilled the request but does not need to return an The server has fulfilled the request but does not need to return an
entity-body, and might want to return updated metainformation. The entity-body, and might want to return updated metainformation. The
response MAY include new or updated metainformation in the form of response MAY include new or updated metainformation in the form of
entity-headers, which if present SHOULD be associated with the entity-headers, which if present SHOULD be associated with the
requested variant. requested variant.
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10.2.7 206 Partial Content 10.2.7 206 Partial Content
The server has fulfilled the partial GET request for the resource. The server has fulfilled the partial GET request for the resource.
The request MUST have included a Range header field (section 14.35) The request MUST have included a Range header field (section 14.35)
indicating the desired range, and MAY have included an If-Range indicating the desired range, and MAY have included an If-Range
header field (section 14.27) to make the request conditional. header field (section 14.27) to make the request conditional.
The response MUST include the following header fields: The response MUST include the following header fields:
- Either a Content-Range header field (section 14.16) indicating o Either a Content-Range header field (section 14.16) indicating the
the range included with this response, or a multipart/byteranges range included with this response, or a multipart/byteranges
Content-Type including Content-Range fields for each part. If a Content-Type including Content-Range fields for each part. If a
Content-Length header field is present in the response, its Content-Length header field is present in the response, its value
value MUST match the actual number of OCTETs transmitted in the MUST match the actual number of OCTETs transmitted in the message-
message-body. body.
- Date o Date
- ETag and/or Content-Location, if the header would have been sent o 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
o 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 include other entity-headers. If the response is the result of an
If-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
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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,
such loops generate network traffic for each redirection. since 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 aware maximum of five redirections. Content developers should be 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.
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 resource characteristics and location(s) from containing a list of resource characteristics and location(s) from
which the user or user agent can choose the one most appropriate. The which the user or user agent can choose the one most appropriate.
entity format is specified by the media type given in the Content- The entity format is specified by the media type given in the
Type header field. Depending upon the format and the capabilities of Content-Type header field. Depending upon the format and the
the user agent, selection of the most appropriate choice MAY be capabilities of the user agent, selection of the most appropriate
performed automatically. However, this specification does not define choice MAY be performed automatically. However, this specification
any standard for such automatic selection. does not define any standard for such automatic selection.
If the server has a preferred choice of representation, it SHOULD If the server has a preferred choice of representation, it SHOULD
include the specific URI for that representation in the Location include the specific URI for that representation in the Location
field; user agents MAY use the Location field value for automatic field; user agents MAY use the Location field value for automatic
redirection. This response is cacheable unless indicated otherwise. redirection. This response is cacheable unless indicated otherwise.
10.3.2 301 Moved Permanently 10.3.2 301 Moved Permanently
The requested resource has been assigned a new permanent URI and any The requested resource has been assigned a new permanent URI and any
future references to this resource SHOULD use one of the returned future references to this resource SHOULD use one of the returned
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10.3.5 304 Not Modified 10.3.5 304 Not Modified
If the client has performed a conditional GET request and access is If the client has performed a conditional GET request and access is
allowed, but the document has not been modified, the server SHOULD allowed, but the document has not been modified, the server SHOULD
respond with this status code. The 304 response MUST NOT contain a respond with this status code. The 304 response MUST NOT contain a
message-body, and thus is always terminated by the first empty line message-body, and thus is always terminated by the first empty line
after the header fields. after the header fields.
The response MUST include the following header fields: The response MUST include the following header fields:
- Date, unless its omission is required by section 14.18.1 o Date, unless its omission is required by section 14.18.1
If a clockless origin server obeys these rules, and proxies and If a clockless origin server obeys these rules, and proxies and
clients add their own Date to any response received without one (as clients add their own Date to any response received without one (as
already specified by [RFC 2068], section 14.19), caches will operate already specified by [RFC 2068], section 14.19), caches will operate
correctly. correctly.
- ETag and/or Content-Location, if the header would have been sent o 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 o 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
13.3.3), the response SHOULD NOT include other entity-headers. section 13.3.3), the response SHOULD NOT include other entity-
Otherwise (i.e., the conditional GET used a weak validator), the headers. Otherwise (i.e., the conditional GET used a weak
response MUST NOT include other entity-headers; this prevents validator), the response MUST NOT include other entity-headers; this
inconsistencies between cached entity-bodies and updated headers. prevents 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
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the new URI. the new URI.
If the 307 status code is received in response to a request other If the 307 status code is received in response to a request 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.
10.4 Client Error 4xx 10.4 Client Error 4xx
The 4xx class of status code is intended for cases in which the The 4xx class of status code is intended for cases in which the
client seems to have erred. Except when responding to a HEAD request, client seems to have erred. Except when responding to a HEAD
the server SHOULD include an entity containing an explanation of the request, the server SHOULD include an entity containing an
error situation, and whether it is a temporary or permanent explanation of the error situation, and whether it is a temporary or
condition. These status codes are applicable to any request method. permanent condition. These status codes are applicable to any
User agents SHOULD display any included entity to the user. request method. User agents SHOULD display any included entity to
the user.
If the client is sending data, a server implementation using TCP If the client is sending data, a server implementation using TCP
SHOULD be careful to ensure that the client acknowledges receipt of SHOULD be careful to ensure that the client acknowledges receipt of
the packet(s) containing the response, before the server closes the the packet(s) containing the response, before the server closes the
input connection. If the client continues sending data to the server input connection. If the client continues sending data to the server
after the close, the server's TCP stack will send a reset packet to after the close, the server's TCP stack will send a reset packet to
the client, which may erase the client's unacknowledged input buffers the client, which may erase the client's unacknowledged input buffers
before they can be read and interpreted by the HTTP application. before they can be read and interpreted by the HTTP application.
10.4.1 400 Bad Request 10.4.1 400 Bad Request
The request could not be understood by the server due to malformed The request could not be understood by the server due to malformed
syntax. The client SHOULD NOT repeat the request without syntax. The client SHOULD NOT repeat the request without
modifications. modifications.
10.4.2 401 Unauthorized 10.4.2 401 Unauthorized
The request requires user authentication. The response MUST include a The request requires user authentication. The response MUST include
WWW-Authenticate header field (section 14.47) containing a challenge a WWW-Authenticate header field (section 14.47) containing a
applicable to the requested resource. The client MAY repeat the challenge applicable to the requested resource. The client MAY
request with a suitable Authorization header field (section 14.8). If repeat the request with a suitable Authorization header field
the request already included Authorization credentials, then the 401 (section 14.8). If the request already included Authorization
response indicates that authorization has been refused for those credentials, then the 401 response indicates that authorization has
credentials. If the 401 response contains the same challenge as the been refused for those credentials. If the 401 response contains the
prior response, and the user agent has already attempted same challenge as the prior response, and the user agent has already
authentication at least once, then the user SHOULD be presented the attempted authentication at least once, then the user SHOULD be
entity that was given in the response, since that entity might presented the entity that was given in the response, since that
include relevant diagnostic information. HTTP access authentication entity might include relevant diagnostic information. HTTP access
is explained in "HTTP Authentication: Basic and Digest Access authentication is explained in "HTTP Authentication: Basic and Digest
Authentication" [43]. Access Authentication" [43].
10.4.3 402 Payment Required 10.4.3 402 Payment Required
This code is reserved for future use. This code is reserved for future use.
10.4.4 403 Forbidden 10.4.4 403 Forbidden
The server understood the request, but is refusing to fulfill it. The server understood the request, but is refusing to fulfill it.
Authorization will not help and the request SHOULD NOT be repeated. Authorization will not help and the request SHOULD NOT be repeated.
If the request method was not HEAD and the server wishes to make If the request method was not HEAD and the server wishes to make
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from which the user or user agent can choose the one most from which the user or user agent can choose the one most
appropriate. The entity format is specified by the media type given appropriate. The entity format is specified by the media type given
in the Content-Type header field. Depending upon the format and the in the Content-Type header field. Depending upon the format and the
capabilities of the user agent, selection of the most appropriate capabilities of the user agent, selection of the most appropriate
choice MAY be performed automatically. However, this specification choice MAY be performed automatically. However, this specification
does not define any standard for such automatic selection. does not define any standard for such automatic selection.
Note: HTTP/1.1 servers are allowed to return responses which are Note: HTTP/1.1 servers are allowed to return responses which are
not acceptable according to the accept headers sent in the not acceptable according to the accept headers sent in the
request. In some cases, this may even be preferable to sending a request. In some cases, this may even be preferable to sending a
406 response. User agents are encouraged to inspect the headers of 406 response. User agents are encouraged to inspect the headers
an incoming response to determine if it is acceptable. of an incoming response to determine if it is acceptable.
If the response could be unacceptable, a user agent SHOULD If the response could be unacceptable, a user agent SHOULD
temporarily stop receipt of more data and query the user for a temporarily stop receipt of more data and query the user for a
decision on further actions. decision on further actions.
10.4.8 407 Proxy Authentication Required 10.4.8 407 Proxy Authentication Required
This code is similar to 401 (Unauthorized), but indicates that the This code is similar to 401 (Unauthorized), but indicates that the
client must first authenticate itself with the proxy. The proxy MUST client must first authenticate itself with the proxy. The proxy MUST
return a Proxy-Authenticate header field (section 14.33) containing a return a Proxy-Authenticate header field (section 14.33) containing a
challenge applicable to the proxy for the requested resource. The challenge applicable to the proxy for the requested resource. The
client MAY repeat the request with a suitable Proxy-Authorization client MAY repeat the request with a suitable Proxy-Authorization
header field (section 14.34). HTTP access authentication is explained header field (section 14.34). HTTP access authentication is
in "HTTP Authentication: Basic and Digest Access Authentication" explained in "HTTP Authentication: Basic and Digest Access
[43]. Authentication" [43].
10.4.9 408 Request Timeout 10.4.9 408 Request Timeout
The client did not produce a request within the time that the server The client did not produce a request within the time that the server
was prepared to wait. The client MAY repeat the request without was prepared to wait. The client MAY repeat the request without
modifications at any later time. modifications at any later time.
10.4.10 409 Conflict 10.4.10 409 Conflict
The request could not be completed due to a conflict with the current The request could not be completed due to a conflict with the current
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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 between the two versions in a format defined by the response Content-
Content-Type. Type.
10.4.11 410 Gone 10.4.11 410 Gone
The requested resource is no longer available at the server and no The requested resource is no longer available at the server and no
forwarding address is known. This condition is expected to be forwarding address is known. This condition is expected to be
considered permanent. Clients with link editing capabilities SHOULD considered permanent. Clients with link editing capabilities SHOULD
delete references to the Request-URI after user approval. If the delete references to the Request-URI after user approval. If the
server does not know, or has no facility to determine, whether or not server does not know, or has no facility to determine, whether or not
the condition is permanent, the status code 404 (Not Found) SHOULD be the condition is permanent, the status code 404 (Not Found) SHOULD be
used instead. This response is cacheable unless indicated otherwise. used instead. This response is cacheable unless indicated otherwise.
The 410 response is primarily intended to assist the task of web The 410 response is primarily intended to assist the task of web
maintenance by notifying the recipient that the resource is maintenance by notifying the recipient that the resource is
intentionally unavailable and that the server owners desire that intentionally unavailable and that the server owners desire that
remote links to that resource be removed. Such an event is common for remote links to that resource be removed. Such an event is common
limited-time, promotional services and for resources belonging to for 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 Length. The client MAY repeat the request if it adds a valid
Content-Length header field containing the length of the message-body Content-Length header field containing the length of the message-body
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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
14.16). This response MUST NOT use the multipart/byteranges content- section 14.16). This response MUST NOT use the multipart/byteranges
type. content-type.
10.4.18 417 Expectation Failed 10.4.18 417 Expectation Failed
The expectation given in an Expect request-header field (see section The expectation given in an Expect request-header field (see
14.20) could not be met by this server, or, if the server is a proxy, section 14.20) could not be met by this server, or, if the server is
the server has unambiguous evidence that the request could not be met a proxy, the server has unambiguous evidence that the request could
by the next-hop server. not be met by the next-hop server.
10.5 Server Error 5xx 10.5 Server Error 5xx
Response status codes beginning with the digit "5" indicate cases in Response status codes beginning with the digit "5" indicate cases in
which the server is aware that it has erred or is incapable of which the server is aware that it has erred or is incapable of
performing the request. Except when responding to a HEAD request, the performing the request. Except when responding to a HEAD request,
server SHOULD include an entity containing an explanation of the the server SHOULD include an entity containing an explanation of the
error situation, and whether it is a temporary or permanent error situation, and whether it is a temporary or permanent
condition. User agents SHOULD display any included entity to the condition. User agents SHOULD display any included entity to the
user. These response codes are applicable to any request method. user. These response codes are applicable to any request method.
10.5.1 500 Internal Server Error 10.5.1 500 Internal Server Error
The server encountered an unexpected condition which prevented it The server encountered an unexpected condition which prevented it
from fulfilling the request. from fulfilling the request.
10.5.2 501 Not Implemented 10.5.2 501 Not Implemented
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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 a Note: The existence of the 503 status code does not imply that a
server must use it when becoming overloaded. Some servers may wish server must use it when becoming overloaded. Some servers may
to simply refuse the connection. wish 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.
10.5.6 505 HTTP Version Not Supported 10.5.6 505 HTTP Version Not Supported
The server does not support, or refuses to support, the HTTP protocol The server does not support, or refuses to support, the HTTP protocol
version that was used in the request message. The server is version that was used in the request message. The server is
indicating that it is unable or unwilling to complete the request indicating that it is unable or unwilling to complete the request
using the same major version as the client, as described in section using the same major version as the client, as described in
3.1, other than with this error message. The response SHOULD contain section 3.1, other than with this error message. The response SHOULD
an entity describing why that version is not supported and what other contain an entity describing why that version is not supported and
protocols are supported by that server. what other protocols are supported by that server.
11 Access Authentication 11 Access Authentication
HTTP provides several OPTIONAL challenge-response authentication HTTP provides several OPTIONAL challenge-response authentication
mechanisms which can be used by a server to challenge a client mechanisms which can be used by a server to challenge a client
request and by a client to provide authentication information. The request and by a client to provide authentication information. The
general framework for access authentication, and the specification of general framework for access authentication, and the specification of
"basic" and "digest" authentication, are specified in "HTTP "basic" and "digest" authentication, are specified in "HTTP
Authentication: Basic and Digest Access Authentication" [43]. This Authentication: Basic and Digest Access Authentication" [43]. This
specification adopts the definitions of "challenge" and "credentials" specification adopts the definitions of "challenge" and "credentials"
from that specification. from that specification.
12 Content Negotiation 12 Content Negotiation
Most HTTP responses include an entity which contains information for Most HTTP responses include an entity which contains information for
interpretation by a human user. Naturally, it is desirable to supply interpretation by a human user. Naturally, it is desirable to supply
the user with the "best available" entity corresponding to the the user with the "best available" entity corresponding to the
request. Unfortunately for servers and caches, not all users have the request. Unfortunately for servers and caches, not all users have
same preferences for what is "best," and not all user agents are the 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 use alternate representations may be of the same media type, but use
different capabilities of that type, be in different languages, different capabilities of that type, be in different languages,
etc. etc.
Any response containing an entity-body MAY be subject to negotiation, Any response containing an entity-body MAY be subject to negotiation,
including error responses. including error responses.
There are two kinds of content negotiation which are possible in There are two kinds of content negotiation which are possible in
HTTP: server-driven and agent-driven negotiation. These two kinds of HTTP: server-driven and agent-driven negotiation. These two kinds of
negotiation are orthogonal and thus may be used separately or in negotiation are orthogonal and thus may be used separately or in
combination. One method of combination, referred to as transparent combination. One method of combination, referred to as transparent
negotiation, occurs when a cache uses the agent-driven negotiation negotiation, occurs when a cache uses the agent-driven negotiation
information provided by the origin server in order to provide information provided by the origin server in order to provide server-
server-driven negotiation for subsequent requests. driven negotiation for subsequent requests.
12.1 Server-driven Negotiation 12.1 Server-driven Negotiation
If the selection of the best representation for a response is made by If the selection of the best representation for a response is made by
an algorithm located at the server, it is called server-driven an algorithm located at the server, it is called server-driven
negotiation. Selection is based on the available representations of negotiation. Selection is based on the available representations of
the response (the dimensions over which it can vary; e.g. language, the response (the dimensions over which it can vary; e.g. language,
content-coding, etc.) and the contents of particular header fields in content-coding, etc.) and the contents of particular header fields in
the request message or on other information pertaining to the request the request message or on other information pertaining to the request
(such as the network address of the client). (such as the network address of the client).
Server-driven negotiation is advantageous when the algorithm for Server-driven negotiation is advantageous when the algorithm for
selecting from among the available representations is difficult to selecting from among the available representations is difficult to
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
guess, the user agent MAY include request header fields (Accept, server's guess, the user agent MAY include request header fields
Accept-Language, Accept-Encoding, etc.) which describe its (Accept, 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
and the intended use for the response (e.g., does the user want the intended use for the response (e.g., does the user want to
to view it on screen or print it on paper?). view it on screen or print it on paper?).
2. Having the user agent describe its capabilities in every 2. Having the user agent describe its capabilities in every request
request can be both very inefficient (given that only a small can be both very inefficient (given that only a small percentage
percentage of responses have multiple representations) and a of responses have multiple representations) and a potential
potential violation of the user's privacy. 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.
The Vary header field can be used to express the parameters the The Vary header field can be used to express the parameters the
server uses to select a representation that is subject to server- server uses to select a representation that is subject to server-
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
by caches and section 14.44 for use of the Vary header field by field 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
of the available representations of the response included within the list of the available representations of the response included within
header fields or entity-body of the initial response, with each the header fields 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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HTTP/1.1. HTTP/1.1.
HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable) HTTP/1.1 defines the 300 (Multiple Choices) and 406 (Not Acceptable)
status codes for enabling agent-driven negotiation when the server is status codes for enabling agent-driven negotiation when the server is
unwilling or unable to provide a varying response using server-driven unwilling or unable to provide a varying response using server-driven
negotiation. negotiation.
12.3 Transparent Negotiation 12.3 Transparent Negotiation
Transparent negotiation is a combination of both server-driven and Transparent negotiation is a combination of both server-driven and
agent-driven negotiation. When a cache is supplied with a form of the agent-driven negotiation. When a cache is supplied with a form of
list of available representations of the response (as in agent-driven the list of available representations of the response (as in agent-
negotiation) and the dimensions of variance are completely understood driven negotiation) and the dimensions of variance are completely
by the cache, then the cache becomes capable of performing server- understood by the cache, then the cache becomes capable of performing
driven negotiation on behalf of the origin server for subsequent server-driven negotiation on behalf of the origin server for
requests on that resource. subsequent requests on that resource.
Transparent negotiation has the advantage of distributing the Transparent negotiation has the advantage of distributing the
negotiation work that would otherwise be required of the origin negotiation work that would otherwise be required of the origin
server and also removing the second request delay of agent-driven server and also removing the second request delay of agent-driven
negotiation when the cache is able to correctly guess the right negotiation when the cache is able to correctly guess the right
response. response.
This specification does not define any mechanism for transparent This specification does not define any mechanism for transparent
negotiation, though it also does not prevent any such mechanism from negotiation, though it also does not prevent any such mechanism from
being developed as an extension that could be used within HTTP/1.1. being developed as an extension that could be used within HTTP/1.1.
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HTTP is typically used for distributed information systems, where HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. The performance can be improved by the use of response caches. The
HTTP/1.1 protocol includes a number of elements intended to make HTTP/1.1 protocol includes a number of elements intended to make
caching work as well as possible. Because these elements are caching work as well as possible. Because these elements are
inextricable from other aspects of the protocol, and because they inextricable from other aspects of the protocol, and because they
interact with each other, it is useful to describe the basic caching interact with each other, it is useful to describe the basic caching
design of HTTP separately from the detailed descriptions of methods, design of HTTP separately from the detailed descriptions of methods,
headers, response codes, etc. headers, response codes, etc.
Caching would be useless if it did not significantly improve Caching would be useless if it did not significantly improve
performance. The goal of caching in HTTP/1.1 is to eliminate the need performance. The goal of caching in HTTP/1.1 is to eliminate the
to send requests in many cases, and to eliminate the need to send need to send requests in many cases, and to eliminate the need to
full responses in many other cases. The former reduces the number of send full responses in many other cases. The former reduces the
network round-trips required for many operations; we use an number of network round-trips required for many operations; we use an
"expiration" mechanism for this purpose (see section 13.2). The "expiration" mechanism for this purpose (see section 13.2). The
latter reduces network bandwidth requirements; we use a "validation" latter reduces network bandwidth requirements; we use a "validation"
mechanism for this purpose (see section 13.3). mechanism for this purpose (see section 13.3).
Requirements for performance, availability, and disconnected Requirements for performance, availability, and disconnected
operation require us to be able to relax the goal of semantic operation require us to be able to relax the goal of semantic
transparency. The HTTP/1.1 protocol allows origin servers, caches, transparency. The HTTP/1.1 protocol allows origin servers, caches,
and clients to explicitly reduce transparency when necessary. and clients to explicitly reduce transparency when necessary.
However, because non-transparent operation may confuse non-expert However, because non-transparent operation may confuse non-expert
users, and might be incompatible with certain server applications users, and might be incompatible with certain server applications
(such as those for ordering merchandise), the protocol requires that (such as those for ordering merchandise), the protocol requires that
transparency be relaxed transparency be relaxed
- only by an explicit protocol-level request when relaxed by o only by an explicit protocol-level request when relaxed by client
client or origin server or origin server
- only with an explicit warning to the end user when relaxed by o 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.
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A basic principle is that it must be possible for the clients to A basic principle is that it must be possible for the clients to
detect any potential relaxation of semantic transparency. detect any potential relaxation of semantic transparency.
Note: The server, cache, or client implementor might be faced with Note: The server, cache, or client implementor might be faced with
design decisions not explicitly discussed in this specification. design decisions not explicitly discussed in this specification.
If a decision might affect semantic transparency, the implementor If a decision might affect semantic transparency, the implementor
ought to err on the side of maintaining transparency unless a ought to err on the side of maintaining transparency unless a
careful and complete analysis shows significant benefits in careful and complete analysis shows significant benefits in
breaking transparency. breaking transparency.
13.1
13.1.1 Cache Correctness 13.1.1 Cache Correctness
A correct cache MUST respond to a request with the most up-to-date A correct cache MUST respond to a request with the most up-to-date
response held by the cache that is appropriate to the request (see response held by the cache that is appropriate to the request (see
sections 13.2.5, 13.2.6, and 13.12) which meets one of the following sections 13.2.5, 13.2.6, and 13.12) which meets one of the following
conditions: conditions:
1. It has been checked for equivalence with what the origin server 1. It has been checked for equivalence with what the origin server
would have returned by revalidating the response with the would have returned by revalidating the response with the origin
origin server (section 13.3); server (section 13.3);
2. It is "fresh enough" (see section 13.2). In the default case, 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 restrictive freshness requirement of both the
If a stored response is not "fresh enough" by the most client and the origin server, in carefully considered
restrictive freshness requirement of both the client and the circumstances the cache MAY still return the response with the
origin server, in carefully considered circumstances the cache appropriate Warning header (see section 13.1.5 and 14.46), unless
MAY still return the response with the appropriate Warning such a response is prohibited (e.g., by a "no-store" cache-
header (see section 13.1.5 and 14.46), unless such a response directive, or by a "no-cache" cache-request-directive; see
is prohibited (e.g., by a "no-store" cache-directive, or by a 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
or error (4xx or 5xx) response message. 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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practical or reasonable to display all of them to the user. This practical or reasonable to display all of them to the user. This
version of HTTP does not specify strict priority rules for deciding version of HTTP does not specify strict priority rules for deciding
which warnings to display and in what order, but does suggest some which warnings to display and in what order, but does suggest some
heuristics. heuristics.
13.1.3 Cache-control Mechanisms 13.1.3 Cache-control Mechanisms
The basic cache mechanisms in HTTP/1.1 (server-specified expiration The basic cache mechanisms in HTTP/1.1 (server-specified expiration
times and validators) are implicit directives to caches. In some times and validators) are implicit directives to caches. In some
cases, a server or client might need to provide explicit directives cases, a server or client might need to provide explicit directives
to the HTTP caches. We use the Cache-Control header for this purpose. to the HTTP caches. We use the Cache-Control header for this
purpose.
The Cache-Control header allows a client or server to transmit a The Cache-Control header allows a client or server to transmit a
variety of directives in either requests or responses. These variety of directives in either requests or responses. These
directives typically override the default caching algorithms. As a directives typically override the default caching algorithms. As a
general rule, if there is any apparent conflict between header general rule, if there is any apparent conflict between header
values, the most restrictive interpretation is applied (that is, the values, the most restrictive interpretation is applied (that is, the
one that is most likely to preserve semantic transparency). However, one that is most likely to preserve semantic transparency). However,
in some cases, cache-control directives are explicitly specified as in some cases, cache-control directives are explicitly specified as
weakening the approximation of semantic transparency (for example, weakening the approximation of semantic transparency (for example,
"max-stale" or "public"). "max-stale" or "public").
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never validated. Or the user agent might habitually add "Cache- never validated. Or the user agent might habitually add "Cache-
Control: max-stale=3600" to every request. The user agent SHOULD NOT Control: max-stale=3600" to every request. The user agent SHOULD NOT
default to either non-transparent behavior, or behavior that results default to either non-transparent behavior, or behavior that results
in abnormally ineffective caching, but MAY be explicitly configured in abnormally ineffective caching, but MAY be explicitly configured
to do so by an explicit action of the user. to do so by an explicit action of the user.
If the user has overridden the basic caching mechanisms, the user If the user has overridden the basic caching mechanisms, the user
agent SHOULD explicitly indicate to the user whenever this results in agent SHOULD explicitly indicate to the user whenever this results in
the display of information that might not meet the server's the display of information that might not meet the server's
transparency requirements (in particular, if the displayed entity is transparency requirements (in particular, if the displayed entity is
known to be stale). Since the protocol normally allows the user agent known to be stale). Since the protocol normally allows the user
to determine if responses are stale or not, this indication need only agent to determine if responses are stale or not, this indication
be displayed when this actually happens. The indication need not be a need only be displayed when this actually happens. The indication
dialog box; it could be an icon (for example, a picture of a rotting need not be a dialog box; it could be an icon (for example, a picture
fish) or some other indicator. of a rotting fish) or some other indicator.
If the user has overridden the caching mechanisms in a way that would If the user has overridden the caching mechanisms in a way that would
abnormally reduce the effectiveness of caches, the user agent SHOULD abnormally reduce the effectiveness of caches, the user agent SHOULD
continually indicate this state to the user (for example, by a continually indicate this state to the user (for example, by a
display of a picture of currency in flames) so that the user does not display of a picture of currency in flames) so that the user does not
inadvertently consume excess resources or suffer from excessive inadvertently consume excess resources or suffer from excessive
latency. latency.
13.1.5 Exceptions to the Rules and Warnings 13.1.5 Exceptions to the Rules and Warnings
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directives of the Cache-Control header. directives of the Cache-Control header.
A client's request MAY specify the maximum age it is willing to A client's request MAY specify the maximum age it is willing to
accept of an unvalidated response; specifying a value of zero forces accept of an unvalidated response; specifying a value of zero forces
the cache(s) to revalidate all responses. A client MAY also specify the cache(s) to revalidate all responses. A client MAY also specify
the minimum time remaining before a response expires. Both of these the minimum time remaining before a response expires. Both of these
options increase constraints on the behavior of caches, and so cannot options increase constraints on the behavior of caches, and so cannot
further relax the cache's approximation of semantic transparency. further relax the cache's approximation of semantic transparency.
A client MAY also specify that it will accept stale responses, up to A client MAY also specify that it will accept stale responses, up to
some maximum amount of staleness. This loosens the constraints on the some maximum amount of staleness. This loosens the constraints on
caches, and so might violate the origin server's specified the caches, and so might violate the origin server's specified
constraints on semantic transparency, but might be necessary to constraints on semantic transparency, but might be necessary to
support disconnected operation, or high availability in the face of support disconnected operation, or high availability in the face of
poor connectivity. poor connectivity.
13.2 Expiration Model 13.2 Expiration Model
13.2.1 Server-Specified Expiration 13.2.1 Server-Specified Expiration
HTTP caching works best when caches can entirely avoid making HTTP caching works best when caches can entirely avoid making
requests to the origin server. The primary mechanism for avoiding requests to the origin server. The primary mechanism for avoiding
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expiration time is reached. This normally preserves semantic expiration time is reached. This normally preserves semantic
transparency, as long as the server's expiration times are carefully transparency, as long as the server's expiration times are carefully
chosen. chosen.
The expiration mechanism applies only to responses taken from a cache The expiration mechanism applies only to responses taken from a cache
and not to first-hand responses forwarded immediately to the and not to first-hand responses forwarded immediately to the
requesting client. requesting client.
If an origin server wishes to force a semantically transparent cache If an origin server wishes to force a semantically transparent cache
to validate every request, it MAY assign an explicit expiration time to validate every request, it MAY assign an explicit expiration time
in the past. This means that the response is always stale, and so the in the past. This means that the response is always stale, and so
cache SHOULD validate it before using it for subsequent requests. See the cache SHOULD validate it before using it for subsequent requests.
section 14.9.4 for a more restrictive way to force revalidation. See section 14.9.4 for a more restrictive way to force revalidation.
If an origin server wishes to force any HTTP/1.1 cache, no matter how If an origin server wishes to force any HTTP/1.1 cache, no matter how
it is configured, to validate every request, it SHOULD use the "must- it is configured, to validate every request, it SHOULD use the "must-
revalidate" cache-control directive (see section 14.9). revalidate" cache-control directive (see section 14.9).
Servers specify explicit expiration times using either the Expires Servers specify explicit expiration times using either the Expires
header, or the max-age directive of the Cache-Control header. header, or the max-age directive of the Cache-Control header.
An expiration time cannot be used to force a user agent to refresh An expiration time cannot be used to force a user agent to refresh
its display or reload a resource; its semantics apply only to caching its display or reload a resource; its semantics apply only to caching
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the age of a response or cache entry. the age of a response or cache entry.
In this discussion, we use the term "now" to mean "the current value In this discussion, we use the term "now" to mean "the current value
of the clock at the host performing the calculation." Hosts that use of the clock at the host performing the calculation." Hosts that use
HTTP, but especially hosts running origin servers and caches, SHOULD HTTP, but especially hosts running origin servers and caches, SHOULD
use NTP [28] or some similar protocol to synchronize their clocks to use NTP [28] or some similar protocol to synchronize their clocks to
a globally accurate time standard. a globally accurate time standard.
HTTP/1.1 requires origin servers to send a Date header, if possible, HTTP/1.1 requires origin servers to send a Date header, if possible,
with every response, giving the time at which the response was with every response, giving the time at which the response was
generated (see section 14.18). We use the term "date_value" to denote generated (see section 14.18). We use the term "date_value" to
the value of the Date header, in a form appropriate for arithmetic denote the value of the Date header, in a form appropriate for
operations. arithmetic operations.
HTTP/1.1 uses the Age response-header to convey the estimated age of HTTP/1.1 uses the Age response-header to convey the estimated age of
the response message when obtained from a cache. The Age field value the response message when obtained from a cache. The Age field value
is the cache's estimate of the amount of time since the response was is the cache's estimate of the amount of time since the response was
generated or revalidated by the origin server. generated or revalidated by the origin server.
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
implement HTTP/1.1. HTTP/1.1.
Given that we have two independent ways to compute the age of a Given that we have two independent ways to compute the age of a
response when it is received, we can combine these as response when it is received, we can combine these as
corrected_received_age = max(now - date_value, age_value) corrected_received_age = max(now - date_value, age_value)
and as long as we have either nearly synchronized clocks or all- and as long as we have either nearly synchronized clocks or all-
HTTP/1.1 paths, one gets a reliable (conservative) result. HTTP/1.1 paths, one gets a reliable (conservative) result.
Because of network-imposed delays, some significant interval might Because of network-imposed delays, some significant interval might
pass between the time that a server generates a response and the time pass between the time that a server generates a response and the time
it is received at the next outbound cache or client. If uncorrected, it is received at the next outbound cache or client. If uncorrected,
this delay could result in improperly low ages. this delay could result in improperly low ages.
Because the request that resulted in the returned Age value must have Because the request that resulted in the returned Age value must have
been initiated prior to that Age value's generation, we can correct been initiated prior to that Age value's generation, we can correct
for delays imposed by the network by recording the time at which the for delays imposed by the network by recording the time at which the
request was initiated. Then, when an Age value is received, it MUST request was initiated. Then, when an Age value is received, it MUST
be interpreted relative to the time the request was initiated, not be interpreted relative to the time the request was initiated, not
the time that the response was received. This algorithm results in the time that the response was received. This algorithm results in
conservative behavior no matter how much delay is experienced. So, we conservative behavior no matter how much delay is experienced. So,
compute: we compute:
corrected_initial_age = corrected_received_age corrected_initial_age = corrected_received_age
+ (now - request_time) + (now - request_time)
where "request_time" is the time (according to the local clock) when where "request_time" is the time (according to the local clock) when
the request that elicited this response was sent. the request that elicited this response was sent.
Summary of age calculation algorithm, when a cache receives a Summary of age calculation algorithm, when a cache receives a
response: response:
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freshness_lifetime = max_age_value freshness_lifetime = max_age_value
Otherwise, if Expires is present in the response, the calculation is: Otherwise, if Expires is present in the response, the calculation is:
freshness_lifetime = expires_value - date_value freshness_lifetime = expires_value - date_value
Note that neither of these calculations is vulnerable to clock skew, Note that neither of these calculations is vulnerable to clock skew,
since all of the information comes from the origin server. since all of the information comes from the origin server.
If none of Expires, Cache-Control: max-age, or Cache-Control: s- If none of Expires, Cache-Control: max-age, or Cache-Control:
maxage (see section 14.9.3) appears in the response, and the response s-maxage (see section 14.9.3) appears in the response, and the
does not include other restrictions on caching, the cache MAY compute response does not include other restrictions on caching, the cache
a freshness lifetime using a heuristic. The cache MUST attach Warning MAY compute a freshness lifetime using a heuristic. The cache MUST
113 to any response whose age is more than 24 hours if such warning attach Warning 113 to any response whose age is more than 24 hours if
has not already been added. such warning has not already been added.
Also, if the response does have a Last-Modified time, the heuristic Also, if the response does have a Last-Modified time, the heuristic
expiration value SHOULD be no more than some fraction of the interval expiration value SHOULD be no more than some fraction of the interval
since that time. A typical setting of this fraction might be 10%. since that time. A typical setting of this fraction might be 10%.
The calculation to determine if a response has expired is quite The calculation to determine if a response has expired is quite
simple: simple:
response_is_fresh = (freshness_lifetime > current_age) response_is_fresh = (freshness_lifetime > current_age)
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Because a client might be receiving responses via multiple paths, so Because a client might be receiving responses via multiple paths, so
that some responses flow through one set of caches and other that some responses flow through one set of caches and other
responses flow through a different set of caches, a client might responses flow through a different set of caches, a client might
receive responses in an order different from that in which the origin receive responses in an order different from that in which the origin
server sent them. We would like the client to use the most recently server sent them. We would like the client to use the most recently
generated response, even if older responses are still apparently generated response, even if older responses are still apparently
fresh. fresh.
Neither the entity tag nor the expiration value can impose an Neither the entity tag nor the expiration value can impose an
ordering on responses, since it is possible that a later response ordering on responses, since it is possible that a later response
intentionally carries an earlier expiration time. The Date values are intentionally carries an earlier expiration time. The Date values
ordered to a granularity of one second. are ordered to a granularity of one second.
When a client tries to revalidate a cache entry, and the response it When a client tries to revalidate a cache entry, and the response it
receives contains a Date header that appears to be older than the one receives contains a Date header that appears to be older than the one
for the existing entry, then the client SHOULD repeat the request for the existing entry, then the client SHOULD repeat the request
unconditionally, and include unconditionally, and include
Cache-Control: max-age=0 Cache-Control: max-age=0
to force any intermediate caches to validate their copies directly to force any intermediate caches to validate their copies directly
with the origin server, or with the origin server, or
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entity-body). Thus, we avoid transmitting the full response if the entity-body). Thus, we avoid transmitting the full response if the
validator matches, and we avoid an extra round trip if it does not validator matches, and we avoid an extra round trip if it does not
match. match.
In HTTP/1.1, a conditional request looks exactly the same as a normal In HTTP/1.1, a conditional request looks exactly the same as a normal
request for the same resource, except that it carries a special request for the same resource, except that it carries a special
header (which includes the validator) that implicitly turns the header (which includes the validator) that implicitly turns the
method (usually, GET) into a conditional. method (usually, GET) into a conditional.
The protocol includes both positive and negative senses of cache- The protocol includes both positive and negative senses of cache-
validating conditions. That is, it is possible to request either that validating conditions. That is, it is possible to request either
a method be performed if and only if a validator matches or if and that a method be performed if and only if a validator matches or if
only if no validators match. and only if no validators match.
Note: a response that lacks a validator may still be cached, and Note: a response that lacks a validator may still be cached, and
served from cache until it expires, unless this is explicitly served from cache until it expires, unless this is explicitly
prohibited by a cache-control directive. However, a cache cannot prohibited by a cache-control directive. However, a cache cannot
do a conditional retrieval if it does not have a validator for the do a conditional retrieval if it does not have a validator for the
entity, which means it will not be refreshable after it expires. entity, which means it will not be refreshable after it expires.
13.3.1 Last-Modified Dates 13.3.1 Last-Modified Dates
The Last-Modified entity-header field value is often used as a cache The Last-Modified entity-header field value is often used as a cache
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Since both origin servers and caches will compare two validators to Since both origin servers and caches will compare two validators to
decide if they represent the same or different entities, one normally decide if they represent the same or different entities, one normally
would expect that if the entity (the entity-body or any entity- would expect that if the entity (the entity-body or any entity-
headers) changes in any way, then the associated validator would headers) changes in any way, then the associated validator would
change as well. If this is true, then we call this validator a change as well. If this is true, then we call this validator a
"strong validator." "strong validator."
However, there might be cases when a server prefers to change the However, there might be cases when a server prefers to change the
validator only on semantically significant changes, and not when validator only on semantically significant changes, and not when
insignificant aspects of the entity change. A validator that does not insignificant aspects of the entity change. A validator that does
always change when the resource changes is a "weak validator." not always change when the resource changes is a "weak validator."
Entity tags are normally "strong validators," but the protocol Entity tags are normally "strong validators," but the protocol
provides a mechanism to tag an entity tag as "weak." One can think of provides a mechanism to tag an entity tag as "weak." One can think
a strong validator as one that changes whenever the bits of an entity of a strong validator as one that changes whenever the bits of an
changes, while a weak value changes whenever the meaning of an entity entity changes, while a weak value changes whenever the meaning of an
changes. Alternatively, one can think of a strong validator as part entity changes. Alternatively, one can think of a strong validator
of an identifier for a specific entity, while a weak validator is as part of an identifier for a specific entity, while a weak
part of an identifier for a set of semantically equivalent entities. validator is part of an identifier for a set of semantically
equivalent entities.
Note: One example of a strong validator is an integer that is 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 that resolution, could be a weak validator, since it is possible that
the resource might be modified twice during a single second. the resource might be modified twice during a single second.
Support for weak validators is optional. However, weak validators Support for weak validators is optional. However, weak validators
allow for more efficient caching of equivalent objects; for allow for more efficient caching of equivalent objects; for
example, a hit counter on a site is probably good enough if it is example, a hit counter on a site is probably good enough if it is
updated every few days or weeks, and any value during that period updated every few days or weeks, and any value 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
usable in contexts that do not depend on exact equality of an entity. only usable in contexts that do not depend on exact equality of an
For example, either kind is usable for a conditional GET of a full entity. For example, either kind is usable for a conditional GET of
entity. However, only a strong validator is usable for a sub-range a full entity. However, only a strong validator is usable for a sub-
retrieval, since otherwise the client might end up with an internally range retrieval, since otherwise the client might end up with an
inconsistent entity. internally inconsistent entity.
Clients MAY issue simple (non-subrange) GET requests with either weak Clients MAY issue simple (non-subrange) GET requests with either weak
validators or strong validators. Clients MUST NOT use weak validators validators or strong validators. Clients MUST NOT use weak
in other forms of request. validators 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 equal, o The strong comparison function: in order to be considered equal,
both validators MUST be identical in every way, and both MUST both validators MUST be identical in every way, and both MUST NOT
NOT be weak. be weak.
- The weak comparison function: in order to be considered equal, o 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
both of them MAY be tagged as "weak" without affecting the 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 o The validator is being compared by an origin server to the actual
actual current validator for the entity and, current validator for the entity and,
- That origin server reliably knows that the associated entity did
o That origin server reliably knows that the associated entity did
not change twice during the second covered by the presented not change twice during the second covered by the presented
validator. validator.
or or
- The validator is about to be used by a client in an If- o The validator is about to be used by a client in an If-Modified-
Modified-Since or If-Unmodified-Since header, because the client Since or If-Unmodified-Since header, because the client has a
has a cache entry for the associated entity, and cache entry for the associated entity, and
- That cache entry includes a Date value, which gives the time o That cache entry includes a Date value, which gives the time when
when the origin server sent the original response, and the origin server sent the original response, and
- The presented Last-Modified time is at least 60 seconds before o The presented Last-Modified time is at least 60 seconds before the
the Date value. Date value.
or or
o The validator is being compared by an intermediate cache to the
- The validator is being compared by an intermediate cache to the
validator stored in its cache entry for the entity, and validator stored in its cache entry for the entity, and
- That cache entry includes a Date value, which gives the time o That cache entry includes a Date value, which gives the time when
when the origin server sent the original response, and the origin server sent the original response, and
- The presented Last-Modified time is at least 60 seconds before o The presented Last-Modified time is at least 60 seconds before the
the Date value. Date value.
This method relies on the fact that if two different responses were This method relies on the fact that if two different responses were
sent by the origin server during the same second, but both had the sent by the origin server during the same second, but both had the
same Last-Modified time, then at least one of those responses would same Last-Modified time, then at least one of those responses would
have a Date value equal to its Last-Modified time. The arbitrary 60- have a Date value equal to its Last-Modified time. The arbitrary 60-
second limit guards against the possibility that the Date and Last- second limit guards against the possibility that the Date and Last-
Modified values are generated from different clocks, or at somewhat Modified values are generated from different clocks, or at somewhat
different times during the preparation of the response. An different times during the preparation of the response. An
implementation MAY use a value larger than 60 seconds, if it is implementation MAY use a value larger than 60 seconds, if it is
believed that 60 seconds is too short. believed that 60 seconds is too short.
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servers. servers.
13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates 13.3.4 Rules for When to Use Entity Tags and Last-Modified Dates
We adopt a set of rules and recommendations for origin servers, We adopt a set of rules and recommendations for origin servers,
clients, and caches regarding when various validator types ought to clients, and caches regarding when various validator types ought to
be used, and for what purposes. be used, and for what purposes.
HTTP/1.1 origin servers: HTTP/1.1 origin servers:
- SHOULD send an entity tag validator unless it is not feasible to o SHOULD send an entity tag validator unless it is not feasible to
generate one. generate one.
- MAY send a weak entity tag instead of a strong entity tag, if o 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
or if it is unfeasible to send a strong entity tag. if it is unfeasible to send a strong entity tag.
- SHOULD send a Last-Modified value if it is feasible to send one, o SHOULD send a Last-Modified value if it is feasible to send one,
unless the risk of a breakdown in semantic transparency that unless the risk of a breakdown in semantic transparency that could
could result from using this date in an If-Modified-Since header result from using this date in an If-Modified-Since header would
would lead to serious problems. 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 of entries might persist for arbitrarily long periods, regardless of
expiration times, so it might be inappropriate to expect that a expiration times, so it might be inappropriate to expect that a
cache will never again attempt to validate an entry using a cache will never again attempt to validate an entry 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 o If an entity tag has been provided by the origin server, MUST use
use that entity tag in any cache-conditional request (using If- that entity tag in any cache-conditional request (using If-Match
Match or If-None-Match). or If-None-Match).
- If only a Last-Modified value has been provided by the origin o If only a Last-Modified value has been provided by the origin
server, SHOULD use that value in non-subrange cache-conditional server, SHOULD use that value in non-subrange cache-conditional
requests (using If-Modified-Since). requests (using If-Modified-Since).
- If only a Last-Modified value has been provided by an HTTP/1.0 o If only a Last-Modified value has been provided by an HTTP/1.0
origin server, MAY use that value in subrange cache-conditional 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 o If both an entity tag and a Last-Modified value have been provided
provided by the origin server, SHOULD use both validators in by the origin server, SHOULD use both validators in cache-
cache-conditional requests. This allows both HTTP/1.0 and conditional requests. This allows both HTTP/1.0 and HTTP/1.1
HTTP/1.1 caches to respond appropriately. caches to respond appropriately.
An HTTP/1.1 origin server, upon receiving a conditional request that An HTTP/1.1 origin server, upon receiving a conditional request that
includes both a Last-Modified date (e.g., in an If-Modified-Since or includes both a Last-Modified date (e.g., in an If-Modified-Since or
If-Unmodified-Since header field) and one or more entity tags (e.g., If-Unmodified-Since header field) and one or more entity tags (e.g.,
in an If-Match, If-None-Match, or If-Range header field) as cache in an If-Match, If-None-Match, or If-Range header field) as cache
validators, MUST NOT return a response status of 304 (Not Modified) validators, MUST NOT return a response status of 304 (Not Modified)
unless doing so is consistent with all of the conditional header unless doing so is consistent with all of the conditional header
fields in the request. fields in the request.
An HTTP/1.1 caching proxy, upon receiving a conditional request that An HTTP/1.1 caching proxy, upon receiving a conditional request that
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13.4 Response Cacheability 13.4 Response Cacheability
Unless specifically constrained by a cache-control (section 14.9) Unless specifically constrained by a cache-control (section 14.9)
directive, a caching system MAY always store a successful response directive, a caching system MAY always store a successful response
(see section 13.8) as a cache entry, MAY return it without validation (see section 13.8) as a cache entry, MAY return it without validation
if it is fresh, and MAY return it after successful validation. If if it is fresh, and MAY return it after successful validation. If
there is neither a cache validator nor an explicit expiration time there is neither a cache validator nor an explicit expiration time
associated with a response, we do not expect it to be cached, but associated with a response, we do not expect it to be cached, but
certain caches MAY violate this expectation (for example, when little certain caches MAY violate this expectation (for example, when little
or no network connectivity is available). A client can usually detect or no network connectivity is available). A client can usually
that such a response was taken from a cache by comparing the Date detect that such a response was taken from a cache by comparing the
header to the current time. Date header to the current time.
Note: some HTTP/1.0 caches are known to violate this expectation Note: some HTTP/1.0 caches are known to violate this 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
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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 o End-to-end headers, which are transmitted to the ultimate
recipient of a request or response. End-to-end headers in recipient of a request or response. End-to-end headers in
responses 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 o Hop-by-hop headers, which are meaningful only for a single
transport-level connection, and are not stored by caches or transport-level connection, and are not stored by caches or
forwarded by proxies. forwarded by proxies.
The following HTTP/1.1 headers are hop-by-hop headers: The following HTTP/1.1 headers are hop-by-hop headers:
- Connection o Connection
- Keep-Alive
- Proxy-Authenticate o Keep-Alive
- Proxy-Authorization
- TE o Proxy-Authenticate
- Trailers
- Transfer-Encoding o Proxy-Authorization
- Upgrade
o TE
o Trailers
o Transfer-Encoding
o Upgrade
All other headers defined by HTTP/1.1 are end-to-end headers. All other headers defined by HTTP/1.1 are end-to-end headers.
Other hop-by-hop headers MUST be listed in a Connection header, Other hop-by-hop headers MUST be listed in a Connection header,
(section 14.10) to be introduced into HTTP/1.1 (or later). (section 14.10) to be introduced into HTTP/1.1 (or later).
13.5.2 Non-modifiable Headers 13.5.2 Non-modifiable Headers
Some features of the HTTP/1.1 protocol, such as Digest Some features of the HTTP/1.1 protocol, such as Digest
Authentication, depend on the value of certain end-to-end headers. A Authentication, depend on the value of certain end-to-end headers. A
transparent proxy SHOULD NOT modify an end-to-end header unless the transparent proxy SHOULD NOT modify an end-to-end header unless the
definition of that header requires or specifically allows that. definition of that header requires or specifically allows that.
A transparent proxy MUST NOT modify any of the following fields in a A transparent proxy MUST NOT modify any of the following fields in a
request or response, and it MUST NOT add any of these fields if not request or response, and it MUST NOT add any of these fields if not
already present: already present:
- Content-Location o Content-Location
- Content-MD5 o Content-MD5
- ETag o ETag
- Last-Modified o 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 o Expires
but it MAY add any of these fields if not already present. If an but it MAY add any of these fields if not already present. If an
Expires 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 o Content-Encoding
- Content-Range o Content-Range
- Content-Type o 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 fields authentication mechanisms MAY rely on the values of header fields
not listed here. not listed here.
The Content-Length field of a request or response is added or deleted The Content-Length field of a request or response is added or deleted
according to the rules in section 4.4. A transparent proxy MUST according to the rules in section 4.4. A transparent proxy MUST
preserve the entity-length (section 7.2.2) of the entity-body, preserve the entity-length (section 7.2.2) of the entity-body,
although it MAY change the transfer-length (section 4.4). although it MAY change the transfer-length (section 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)
response, the cache then constructs a response to send to the response, the cache then constructs a response to send to the
requesting client. requesting client.
If the status code is 304 (Not Modified), the cache uses the entity- If the status code is 304 (Not Modified), the cache uses the entity-
body stored in the cache entry as the entity-body of this outgoing body stored in the cache entry as the entity-body of this outgoing
response. If the status code is 206 (Partial Content) and the ETag or response. If the status code is 206 (Partial Content) and the ETag
Last-Modified headers match exactly, the cache MAY combine the or 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 o any stored Warning headers with warn-code 1xx (see section 14.46)
14.46) MUST be deleted from the cache entry and the forwarded MUST be deleted from the cache entry and the forwarded response.
response.
- any stored Warning headers with warn-code 2xx MUST be retained o any stored Warning headers with warn-code 2xx MUST be retained in
in the cache entry and the forwarded response. the cache entry and the forwarded response.
- any end-to-end headers provided in the 304 or 206 response MUST o 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 header Modified) or a 206 (Partial Content) response to update any header
associated with a previous response for the same entity or sub- associated with a previous response for the same entity or sub-
ranges thereof, although it might not always be meaningful or ranges thereof, although it might not always be meaningful or
correct to do so. This rule does not allow an origin server to use correct to do so. This rule does not allow an origin server to
a 304 (Not Modified) or a 206 (Partial Content) response to use a 304 (Not Modified) or a 206 (Partial Content) response to
entirely delete a header that it had provided with a previous entirely delete a header that it had provided with a previous
response. response.
13.5.4 Combining Byte Ranges 13.5.4 Combining Byte Ranges
A response might transfer only a subrange of the bytes of an entity- A response might transfer only a subrange of the bytes of an entity-
body, either because the request included one or more Range body, either because the request included one or more Range
specifications, or because a connection was broken prematurely. After specifications, or because a connection was broken prematurely.
several such transfers, a cache might have received several ranges of After several such transfers, a cache might have received several
the same entity-body. ranges of the same entity-body.
If a cache has a stored non-empty set of subranges for an entity, and If a cache has a stored non-empty set of subranges for an entity, and
an incoming response transfers another subrange, the cache MAY an incoming response transfers another subrange, the cache MAY
combine the new subrange with the existing set if both the following combine the new subrange with the existing set if both the following
conditions are met: conditions are met:
- Both the incoming response and the cache entry have a cache o Both the incoming response and the cache entry have a cache
validator. validator.
- The two cache validators match using the strong comparison o 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.
All other caches are considered to be "shared." Other sections of All other caches are considered to be "shared." Other sections of
this specification place certain constraints on the operation of this specification place certain constraints on the operation of
shared caches in order to prevent loss of privacy or failure of shared caches in order to prevent loss of privacy or failure of
access controls. access controls.
13.8 Errors or Incomplete Response Cache Behavior 13.8 Errors or Incomplete Response Cache Behavior
A cache that receives an incomplete response (for example, with fewer A cache that receives an incomplete response (for example, with fewer
bytes of data than specified in a Content-Length header) MAY store bytes of data than specified in a Content-Length header) MAY store
the response. However, the cache MUST treat this as a partial the response. However, the cache MUST treat this as a partial
response. Partial responses MAY be combined as described in section response. Partial responses MAY be combined as described in
13.5.4; the result might be a full response or might still be section 13.5.4; the result might be a full response or might still be
partial. A cache MUST NOT return a partial response to a client partial. A cache MUST NOT return a partial response to a client
without explicitly marking it as such, using the 206 (Partial without explicitly marking it as such, using the 206 (Partial
Content) status code. A cache MUST NOT return a partial response Content) status code. A cache MUST NOT return a partial response
using a status code of 200 (OK). using a status code of 200 (OK).
If a cache receives a 5xx response while attempting to revalidate an If a cache receives a 5xx response while attempting to revalidate an
entry, it MAY either forward this response to the requesting client, entry, it MAY either forward this response to the requesting client,
or act as if the server failed to respond. In the latter case, it MAY or act as if the server failed to respond. In the latter case, it
return a previously received response unless the cached entry MAY return a previously received response unless the cached entry
includes the "must-revalidate" cache-control directive (see section includes the "must-revalidate" cache-control directive (see
14.9). section 14.9).
13.9 Side Effects of GET and HEAD 13.9 Side Effects of GET and HEAD
Unless the origin server explicitly prohibits the caching of their Unless the origin server explicitly prohibits the caching of their
responses, the application of GET and HEAD methods to any resources responses, the application of GET and HEAD methods to any resources
SHOULD NOT have side effects that would lead to erroneous behavior if SHOULD NOT have side effects that would lead to erroneous behavior if
these responses are taken from a cache. They MAY still have side these responses are taken from a cache. They MAY still have side
effects, but a cache is not required to consider such side effects in effects, but a cache is not required to consider such side effects in
its caching decisions. Caches are always expected to observe an its caching decisions. Caches are always expected to observe an
origin server's explicit restrictions on caching. origin server's explicit restrictions on caching.
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caused the change at the origin server might not have gone through caused the change at the origin server might not have gone through
the proxy where a cache entry is stored. However, several rules help the proxy where a cache entry is stored. However, several rules help
reduce the likelihood of erroneous behavior. reduce the likelihood of erroneous behavior.
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
either the entity referred to by the Request-URI, or by the Location is either the entity referred to by the Request-URI, or by the
or Content-Location headers (if present). These methods are: Location or Content-Location headers (if present). These methods
are:
- PUT o PUT
- DELETE o DELETE
- POST o 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 understand SHOULD invalidate any entities referred to by the Request-
Request-URI. URI.
13.11 Write-Through Mandatory 13.11 Write-Through Mandatory
All methods that might be expected to cause modifications to the All methods that might be expected to cause modifications to the
origin server's resources MUST be written through to the origin origin server's resources MUST be written through to the origin
server. This currently includes all methods except for GET and HEAD. server. This currently includes all methods except for GET and HEAD.
A cache MUST NOT reply to such a request from a client before having A cache MUST NOT reply to such a request from a client before having
transmitted the request to the inbound server, and having received a transmitted the request to the inbound server, and having received a
corresponding response from the inbound server. This does not prevent corresponding response from the inbound server. This does not
a proxy cache from sending a 100 (Continue) response before the prevent a proxy cache from sending a 100 (Continue) response before
inbound server has sent its final reply. the inbound server has sent its final reply.
The alternative (known as "write-back" or "copy-back" caching) is not The alternative (known as "write-back" or "copy-back" caching) is not
allowed in HTTP/1.1, due to the difficulty of providing consistent allowed in HTTP/1.1, due to the difficulty of providing consistent
updates and the problems arising from server, cache, or network updates and the problems arising from server, cache, or network
failure prior to write-back. failure prior to write-back.
13.12 Cache Replacement 13.12 Cache Replacement
If a new cacheable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8) If a new cacheable (see sections 14.9.2, 13.2.5, 13.2.6 and 13.8)
response is received from a resource while any existing responses for response is received from a resource while any existing responses for
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existing cached responses is not cacheable. existing cached responses is not cacheable.
13.13 History Lists 13.13 History Lists
User agents often have history mechanisms, such as "Back" buttons and User agents often have history mechanisms, such as "Back" buttons and
history lists, which can be used to redisplay an entity retrieved history lists, which can be used to redisplay an entity retrieved
earlier in a session. earlier in a session.
History mechanisms and caches are different. In particular history History mechanisms and caches are different. In particular history
mechanisms SHOULD NOT try to show a semantically transparent view of mechanisms SHOULD NOT try to show a semantically transparent view of
the current state of a resource. Rather, a history mechanism is meant the current state of a resource. Rather, a history mechanism is
to show exactly what the user saw at the time when the resource was meant to show exactly what the user saw at the time when the resource
retrieved. was retrieved.
By default, an expiration time does not apply to history mechanisms. By default, an expiration time does not apply to history mechanisms.
If the entity is still in storage, a history mechanism SHOULD display If the entity is still in storage, a history mechanism SHOULD display
it even if the entity has expired, unless the user has specifically it even if the entity has expired, unless the user has specifically
configured the agent to refresh expired history documents. configured the agent to refresh expired history documents.
This is not to be construed to prohibit the history mechanism from This is not to be construed to prohibit the history mechanism from
telling the user that a view might be stale. telling the user that a view might be stale.
Note: if history list mechanisms unnecessarily prevent users from Note: if history list mechanisms unnecessarily prevent users from
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Note: Use of the "q" parameter name to separate media type Note: Use of the "q" parameter name to separate media type
parameters from Accept extension parameters is due to historical parameters from Accept extension parameters is due to historical
practice. Although this prevents any media type parameter named practice. Although this prevents any media type parameter named
"q" from being used with a media range, such an event is believed "q" from being used with a media range, such an event is believed
to be unlikely given the lack of any "q" parameters in the IANA to be unlikely given the lack of any "q" parameters in the IANA
media type registry and the rare usage of any media type media type registry and the rare usage of any media type
parameters in Accept. Future media types are discouraged from parameters in Accept. Future media types are 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
and if the server cannot send a response which is acceptable present, and if the server cannot send a response which is acceptable
according to the combined Accept field value, then the server SHOULD according to the combined Accept field value, then the server SHOULD
send a 406 (not acceptable) response. send a 406 (not acceptable) response.
A more elaborate example is A more elaborate example is
Accept: text/plain; q=0.5, text/html, Accept: text/plain; q=0.5, text/html,
text/x-dvi; q=0.8, text/x-c text/x-dvi; q=0.8, text/x-c
Verbally, this would be interpreted as "text/html and text/x-c are Verbally, this would be interpreted as "text/html and text/x-c are
the preferred media types, but if they do not exist, then send the the preferred media types, but if they do not exist, then send the
text/x-dvi entity, and if that does not exist, send the text/plain text/x-dvi entity, and if that does not exist, send the text/plain
entity." entity."
Media ranges can be overridden by more specific media ranges or Media ranges can be overridden by more specific media ranges or
specific media types. If more than one media range applies to a given specific media types. If more than one media range applies to a
type, the most specific reference has precedence. For example, given type, the most specific reference has precedence. For example,
Accept: text/*, text/html, text/html;level=1, */* Accept: text/*, text/html, text/html;level=1, */*
have the following precedence: have the following precedence:
1) text/html;level=1 1) text/html;level=1
2) text/html 2) text/html
3) text/* 3) text/*
4) */* 4) */*
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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 quality Note: A user agent might be provided with a default set of quality
values for certain media ranges. However, unless the user agent is values for certain media ranges. However, unless the user agent is a
a closed system which cannot interact with other rendering agents, closed system which cannot interact with other rendering agents, this
this default set ought to be configurable by the user. default set ought to be configurable by 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" ":"
1#( ( charset | "*" )[ ";" "q" "=" qvalue ] ) 1#( ( charset | "*" )[ ";" "q" "=" qvalue ] )
Character set values are described in section 3.4. Each charset MAY Character set values are described in section 3.4. Each charset MAY
be given an associated quality value which represents the user's be given an associated quality value which represents the user's
preference for that charset. The default value is q=1. An example is preference for that charset. The default value is q=1. An example
is
Accept-Charset: iso-8859-5, unicode-1-1;q=0.8 Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
The special value "*", if present in the Accept-Charset field, The special value "*", if present in the Accept-Charset field,
matches every character set (including ISO-8859-1) which is not matches every character set (including ISO-8859-1) which is not
mentioned elsewhere in the Accept-Charset field. If no "*" is present mentioned elsewhere in the Accept-Charset field. If no "*" is
in an Accept-Charset field, then all character sets not explicitly present in an Accept-Charset field, then all character sets not
mentioned get a quality value of 0, except for ISO-8859-1, which gets explicitly mentioned get a quality value of 0, except for ISO-8859-1,
a quality value of 1 if not explicitly mentioned. which gets a quality value of 1 if not explicitly mentioned.
If no Accept-Charset header is present, the default is that any If no Accept-Charset header is present, the default is that any
character set is acceptable. If an Accept-Charset header is present, character set is acceptable. If an Accept-Charset header is present,
and if the server cannot send a response which is acceptable and if the server cannot send a response which is acceptable
according to the Accept-Charset header, then the server SHOULD send according to the Accept-Charset header, then the server SHOULD send
an error response with the 406 (not acceptable) status code, though an error response with the 406 (not acceptable) status code, though
the sending of an unacceptable response is also allowed. the sending of an unacceptable response is also allowed.
14.3 Accept-Encoding 14.3 Accept-Encoding
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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
the Accept-Encoding field, then it is acceptable, unless it is Accept-Encoding field, then it is acceptable, unless it is
accompanied by a qvalue of 0. (As defined in section 3.9, a 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.")
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 available content-coding not explicitly listed in the header
field. 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.
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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-
content-codings commonly understood by HTTP/1.0 clients (i.e., codings commonly understood by HTTP/1.0 clients (i.e., "gzip" and
"gzip" and "compress") are preferred; some older clients "compress") are preferred; some older clients improperly display
improperly display messages sent with other content-codings. The messages sent with other content-codings. The server might also
server might also make this decision based on information about make this decision based on information about the particular user-
the particular user-agent or client. agent or client.
Note: Most HTTP/1.0 applications do not recognize or obey qvalues Note: Most HTTP/1.0 applications do not recognize or obey qvalues
associated with content-codings. This means that qvalues will not associated with content-codings. This means that qvalues will not
work and are not permitted with x-gzip or x-compress. work and are not permitted with x-gzip or x-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
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 ) ) | "*" )
Each language-range MAY be given an associated quality value which Each language-range MAY be given an associated quality value which
represents an estimate of the user's preference for the languages represents an estimate of the user's preference for the languages
specified by that range. The quality value defaults to "q=1". For specified by that range. The quality value defaults to "q=1". For
example, example,
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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 is language tags are assigned to languages in such a way that it is
always true that if a user understands a language with a certain always true that if a user understands a language with a certain
tag, then this user will also understand all languages with tags tag, then this user will also understand all languages with tags
for which this tag is a prefix. The prefix rule simply allows the for which this tag is a prefix. The prefix rule 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 The language quality factor assigned to a language-tag by the Accept-
Accept-Language field is the quality value of the longest language- Language field is the quality value of the longest language-range in
range in the field that matches the language-tag. If no language- the field that matches the language-tag. If no language-range in the
range in the field matches the tag, the language quality factor field matches the tag, the language quality factor assigned is 0. If
assigned is 0. If no Accept-Language header is present in the no Accept-Language header is present in the request, the server
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
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the user, we remind implementors of the fact that users are not the user, we remind implementors of the fact that users are not
familiar with the details of language matching as described above, familiar with the details of language matching as described above,
and should provide appropriate guidance. As an example, users and should provide appropriate guidance. As an example, users
might assume that on selecting "en-gb", they will be served any might assume that on selecting "en-gb", they will be served any
kind of English document if British English is not available. A kind of English document if British English is not available. A
user agent might suggest in such a case to add "en" to get the user agent might suggest in such a case to add "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 The Accept-Ranges response-header field allows the server to indicate
indicate its acceptance of range requests for a resource: 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 Servers that do not accept any kind of range request for a resource
resource MAY send 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 amount of time since the response (or its revalidation) was generated
generated at the origin server. A cached response is "fresh" if at the origin server. A cached response is "fresh" if its age does
its age does not exceed its freshness lifetime. Age values are not exceed its freshness lifetime. Age values are calculated as
calculated as specified in section 13.2.3. 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 If a cache receives a value larger than the largest positive integer
integer it can represent, or if any of its age calculations it can represent, or if any of its age calculations overflows, it
overflows, it MUST transmit an Age header with a value of MUST transmit an Age header with a value of 2147483648 (2^31). An
2147483648 (2^31). An HTTP/1.1 server that includes a cache MUST HTTP/1.1 server that includes a cache MUST include an Age header
include an Age header field in every response generated from its field in every response generated from its own cache. Caches SHOULD
own cache. Caches SHOULD use an arithmetic type of at least 31 use an arithmetic type of at least 31 bits of range.
bits of range.
14.7 Allow 14.7 Allow
The Allow entity-header field lists the set of methods supported The Allow entity-header field lists the set of methods supported by
by the resource identified by the Request-URI. The purpose of this the resource identified by the Request-URI. The purpose of this
field is strictly to inform the recipient of valid methods field is strictly to inform the recipient of valid methods associated
associated with the resource. An Allow header field MUST be with the resource. An Allow header field MUST be present in a 405
present in a 405 (Method Not Allowed) response. (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 However, the indications given by the Allow header field value SHOULD
SHOULD be followed. The actual set of allowed methods is defined be followed. The actual set of allowed methods is defined by the
by the origin server at the time of each request. 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 understand all the methods specified, since the user agent might have
have other means of communicating with the origin server. 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 usually, but not necessarily, after receiving a 401 response--does so
so by including an Authorization request-header field with the by including an Authorization request-header field with the request.
request. The Authorization field value consists of credentials The Authorization field value consists of credentials containing the
containing the authentication information of the user agent for authentication information of the user agent for the realm of the
the realm of the resource being requested. 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 authenticated and a realm specified, the same credentials SHOULD be
be valid for all other requests within this realm (assuming that valid for all other requests within this realm (assuming that the
the authentication scheme itself does not require otherwise, such authentication scheme itself does not require otherwise, such as
as credentials that vary according to a challenge value or using 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 When a shared cache (see section 13.7) receives a request containing
containing an Authorization field, it MUST NOT return the an Authorization field, it MUST NOT return the corresponding response
corresponding response as a reply to any other request, unless one as a reply to any other request, unless one of the following specific
of the following specific exceptions holds: exceptions holds:
1. If the response includes the "s-maxage" cache-control 1. If the response includes the "s-maxage" cache-control directive,
directive, the cache MAY use that response in replying to a the cache MAY use that response in replying to a subsequent
subsequent request. But (if the specified maximum age has request. But (if the specified maximum age has passed) a proxy
passed) a proxy cache MUST first revalidate it with the origin cache MUST first revalidate it with the origin server, using the
server, using the request-headers from the new request to allow request-headers from the new request to allow the origin server
the origin server to authenticate the new request. (This is the to authenticate the new request. (This is the defined behavior
defined behavior for s-maxage.) If the response includes "s- for s-maxage.) If the response includes "s-maxage=0", the proxy
maxage=0", the proxy MUST always revalidate it before re-using MUST always revalidate it before re-using it.
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
it MAY be returned in reply to any subsequent request. 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/
request/response chain. The directives specify behavior intended to response chain. The directives specify behavior intended to prevent
prevent caches from adversely interfering with the request or caches from adversely interfering with the request or response.
response. These directives typically override the default caching These directives typically override the default caching algorithms.
algorithms. Cache directives are unidirectional in that the presence Cache directives are unidirectional in that the presence of a
of a directive in a request does not imply that the same directive is directive in a request does not imply that the same directive is to
to be given in the response. be given in the response.
Note that HTTP/1.0 caches might not implement Cache-Control and Note that HTTP/1.0 caches might not implement Cache-Control and
might only implement Pragma: no-cache (see section 14.32). might only implement Pragma: no-cache (see section 14.32).
Cache directives MUST be passed through by a proxy or gateway Cache directives MUST be passed through by a proxy or gateway
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 cache- request/response chain. It is not possible to specify a cache-
directive for a specific cache. directive for a specific cache.
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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 by o Restrictions on what are cacheable; these may only be imposed by
the origin server. the origin server.
- Restrictions on what may be stored by a cache; these may be o 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 o 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 o 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. o Control over transformation of entities.
- Extensions to the caching system. o 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)
MAY cache the response. cache 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
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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
response is cacheable (i.e., "public") unless some other, more response is cacheable (i.e., "public") unless some other, more
restrictive cache directive is also present. restrictive cache directive is also present.
If a response includes both an Expires header and a max-age If a response includes both an Expires header and a max-age
directive, the max-age directive overrides the Expires header, even directive, the max-age directive overrides the Expires header, even
if the Expires header is more restrictive. This rule allows an origin if the Expires header is more restrictive. This rule allows an
server to provide, for a given response, a longer expiration time to origin server to provide, for a given response, a longer expiration
an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This
useful if certain HTTP/1.0 caches improperly calculate ages or might be useful if certain HTTP/1.0 caches improperly calculate ages
expiration times, perhaps due to desynchronized clocks. or 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
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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- without first revalidating it with the origin server. The
maxage directive is always ignored by a private cache. s-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
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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
a new entity and cache validator, however, the intermediate cache with a new entity and cache validator, however, the intermediate
can compare the returned validator with the one provided in the cache can compare the returned validator with the one provided in
client's request, using the strong comparison function. If the the client's request, using the strong comparison function. If
client's validator is equal to the origin server's, then the 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
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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 Message headers listed in the Connection header MUST NOT include end-
end-to-end headers, such as Cache-Control. 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.
HTTP/1.1 applications that do not support persistent connections MUST HTTP/1.1 applications that do not support persistent connections MUST
include the "close" connection option in every message. include the "close" connection option in every message.
A system receiving an HTTP/1.0 (or lower-version) message that A system receiving an HTTP/1.0 (or lower-version) message that
includes a Connection header MUST, for each connection-token in this includes a Connection header MUST, for each connection-token in this
field, remove and ignore any header field(s) from the message with field, remove and ignore any header field(s) from the message with
the same name as the connection-token. This protects against mistaken the same name as the connection-token. This protects against
forwarding of such header fields by pre-HTTP/1.1 proxies. See section mistaken forwarding of such header fields by pre-HTTP/1.1 proxies.
19.6.2. See appendix F.2.
14.11 Content-Encoding 14.11 Content-Encoding
The Content-Encoding entity-header field is used as a modifier to the The Content-Encoding entity-header field is used as a modifier to the
media-type. When present, its value indicates what additional content media-type. When present, its value indicates what additional
codings have been applied to the entity-body, and thus what decoding content codings have been applied to the entity-body, and thus what
mechanisms must be applied in order to obtain the media-type decoding mechanisms must be applied in order to obtain the media-type
referenced by the Content-Type header field. Content-Encoding is referenced by the Content-Type header field. Content-Encoding is
primarily used to allow a document to be compressed without losing primarily used to allow a document to be compressed without losing
the identity of its underlying media type. the identity of its underlying media type.
Content-Encoding = "Content-Encoding" ":" 1#content-coding Content-Encoding = "Content-Encoding" ":" 1#content-coding
Content codings are defined in section 3.5. An example of its use is Content codings are defined in section 3.5. An example of its use is
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 include a Content-Encoding entity-header (section response MUST include a Content-Encoding entity-header
14.11) that lists the non-identity content-coding(s) used. (section 14.11) that lists the non-identity content-coding(s) used.
If the content-coding of an entity in a request message is not 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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by other entity-header fields not defined by this specification. by other entity-header fields not defined by this specification.
14.12 Content-Language 14.12 Content-Language
The Content-Language entity-header field describes the natural The Content-Language entity-header field describes the natural
language(s) of the intended audience for the enclosed entity. Note language(s) of the intended audience for the enclosed entity. Note
that this might not be equivalent to all the languages used within that this might not be equivalent to all the languages used within
the entity-body. the entity-body.
Content-Language = "Content-Language" ":" 1#language-tag Content-Language = "Content-Language" ":" 1#language-tag
Language tags are defined in section 3.10. The primary purpose of Language tags are defined in section 3.10. The primary purpose of
Content-Language is to allow a user to identify and differentiate Content-Language is to allow a user to identify and differentiate
entities according to the user's own preferred language. Thus, if the entities according to the user's own preferred language. Thus, if
body content is intended only for a Danish-literate audience, the the body content is intended only for a Danish-literate audience, the
appropriate field is appropriate field is
Content-Language: da Content-Language: da
If no Content-Language is specified, the default is that the content If no Content-Language is specified, the default is that the content
is intended for all language audiences. This might mean that the is intended for all language audiences. This might mean that the
sender does not consider it to be specific to any natural language, sender does not consider it to be specific to any natural language,
or that the sender does not know for which language it is intended. or that the sender does not know for which language it is intended.
Multiple languages MAY be listed for content that is intended for Multiple 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 Lesson in Latin," which is clearly intended to be used by an English-
English-literate audience. In this case, the Content-Language would literate audience. In this case, the Content-Language would properly
properly only include "en". 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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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.
Content-Length = "Content-Length" ":" 1*DIGIT Content-Length = "Content-Length" ":" 1*DIGIT
An example is An example is
Content-Length: 3495 Content-Length: 3495
Applications SHOULD use this field to indicate the transfer-length of Applications SHOULD use this field to indicate the transfer-length of
the message-body, unless this is prohibited by the rules in section the message-body, unless this is prohibited by the rules in
4.4. section 4.4.
Any Content-Length greater than or equal to zero is a valid value. Any Content-Length greater than or equal to zero is a valid value.
Section 4.4 describes how to determine the length of a message-body section 4.4 describes how to determine the length of a message-body
if a Content-Length is not given. if a Content-Length is not given.
Note that the meaning of this field is significantly different from Note that the meaning of this field is significantly different from
the corresponding definition in MIME, where it is an optional field the corresponding definition in MIME, where it is an optional field
used within the "message/external-body" content-type. In HTTP, it used within the "message/external-body" content-type. In HTTP, it
SHOULD be sent whenever the message's length can be determined prior SHOULD be sent whenever the message's length can be determined prior
to being transferred, unless this is prohibited by the rules in to being transferred, unless this is prohibited by the rules in