draft-ietf-httpbis-semantics-08.txt   draft-ietf-httpbis-semantics-09.txt 
HTTP Working Group R. Fielding, Ed. HTTP Working Group R. Fielding, Ed.
Internet-Draft Adobe Internet-Draft Adobe
Obsoletes: 2818,7230,7231,7232,7233,7235 M. Nottingham, Ed. Obsoletes: 2818,7230,7231,7232,7233,7235 M. Nottingham, Ed.
,7538,7615,7694 (if approved) Fastly ,7538,7615,7694 (if approved) Fastly
Intended status: Standards Track J. Reschke, Ed. Intended status: Standards Track J. Reschke, Ed.
Expires: November 27, 2020 greenbytes Expires: January 12, 2021 greenbytes
May 26, 2020 July 11, 2020
HTTP Semantics HTTP Semantics
draft-ietf-httpbis-semantics-08 draft-ietf-httpbis-semantics-09
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is a stateless application- The Hypertext Transfer Protocol (HTTP) is a stateless application-
level protocol for distributed, collaborative, hypertext information level protocol for distributed, collaborative, hypertext information
systems. This document defines the semantics of HTTP: its systems. This document defines the semantics of HTTP: its
architecture, terminology, the "http" and "https" Uniform Resource architecture, terminology, the "http" and "https" Uniform Resource
Identifier (URI) schemes, core request methods, request header Identifier (URI) schemes, core request methods, request header
fields, response status codes, response header fields, and content fields, response status codes, response header fields, and content
negotiation. negotiation.
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This note is to be removed before publishing as an RFC. This note is to be removed before publishing as an RFC.
Discussion of this draft takes place on the HTTP working group Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at mailing list (ietf-http-wg@w3.org), which is archived at
<https://lists.w3.org/Archives/Public/ietf-http-wg/>. <https://lists.w3.org/Archives/Public/ietf-http-wg/>.
Working Group information can be found at <https://httpwg.org/>; Working Group information can be found at <https://httpwg.org/>;
source code and issues list for this draft can be found at source code and issues list for this draft can be found at
<https://github.com/httpwg/http-core>. <https://github.com/httpwg/http-core>.
The changes in this draft are summarized in Appendix D.9. The changes in this draft are summarized in Appendix D.10.
Status of This Memo Status of This Memo
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Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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2.5.1. http URI Scheme . . . . . . . . . . . . . . . . . . . 18 2.5.1. http URI Scheme . . . . . . . . . . . . . . . . . . . 18
2.5.2. https URI Scheme . . . . . . . . . . . . . . . . . . 18 2.5.2. https URI Scheme . . . . . . . . . . . . . . . . . . 18
2.5.3. http and https URI Normalization and Comparison . . . 19 2.5.3. http and https URI Normalization and Comparison . . . 19
2.5.4. Deprecated userinfo . . . . . . . . . . . . . . . . . 20 2.5.4. Deprecated userinfo . . . . . . . . . . . . . . . . . 20
2.5.5. Fragment Identifiers on http(s) URI References . . . 20 2.5.5. Fragment Identifiers on http(s) URI References . . . 20
3. Conformance . . . . . . . . . . . . . . . . . . . . . . . . . 21 3. Conformance . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.1. Implementation Diversity . . . . . . . . . . . . . . . . 21 3.1. Implementation Diversity . . . . . . . . . . . . . . . . 21
3.2. Role-based Requirements . . . . . . . . . . . . . . . . . 21 3.2. Role-based Requirements . . . . . . . . . . . . . . . . . 21
3.3. Parsing Elements . . . . . . . . . . . . . . . . . . . . 22 3.3. Parsing Elements . . . . . . . . . . . . . . . . . . . . 22
3.4. Error Handling . . . . . . . . . . . . . . . . . . . . . 23 3.4. Error Handling . . . . . . . . . . . . . . . . . . . . . 23
3.5. Protocol Versioning . . . . . . . . . . . . . . . . . . . 23 4. Extending and Versioning HTTP . . . . . . . . . . . . . . . . 23
4. Header and Trailer Fields . . . . . . . . . . . . . . . . . . 24 4.1. Extending HTTP . . . . . . . . . . . . . . . . . . . . . 23
4.1. Field Ordering and Combination . . . . . . . . . . . . . 25 4.2. Protocol Versioning . . . . . . . . . . . . . . . . . . . 24
4.2. Field Limits . . . . . . . . . . . . . . . . . . . . . . 26 5. Header and Trailer Fields . . . . . . . . . . . . . . . . . . 25
4.3. Field Names . . . . . . . . . . . . . . . . . . . . . . . 27 5.1. Field Ordering and Combination . . . . . . . . . . . . . 26
4.3.1. Field Extensibility . . . . . . . . . . . . . . . . . 27 5.2. Field Limits . . . . . . . . . . . . . . . . . . . . . . 27
4.3.2. Field Name Registry . . . . . . . . . . . . . . . . . 28 5.3. Field Names . . . . . . . . . . . . . . . . . . . . . . . 28
4.4. Field Values . . . . . . . . . . . . . . . . . . . . . . 29 5.3.1. Field Extensibility . . . . . . . . . . . . . . . . . 28
4.4.1. Common Field Value Components . . . . . . . . . . . . 30 5.3.2. Field Name Registry . . . . . . . . . . . . . . . . . 29
4.5. ABNF List Extension: #rule . . . . . . . . . . . . . . . 32 5.4. Field Values . . . . . . . . . . . . . . . . . . . . . . 30
4.5.1. Sender Requirements . . . . . . . . . . . . . . . . . 32 5.4.1. Common Field Value Components . . . . . . . . . . . . 31
4.5.2. Recipient Requirements . . . . . . . . . . . . . . . 32 5.5. ABNF List Extension: #rule . . . . . . . . . . . . . . . 35
4.6. Trailer Fields . . . . . . . . . . . . . . . . . . . . . 33 5.5.1. Sender Requirements . . . . . . . . . . . . . . . . . 35
4.6.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . 33 5.5.2. Recipient Requirements . . . . . . . . . . . . . . . 36
4.6.2. Limitations . . . . . . . . . . . . . . . . . . . . . 34 5.6. Trailer Fields . . . . . . . . . . . . . . . . . . . . . 36
4.6.3. Trailer . . . . . . . . . . . . . . . . . . . . . . . 34 5.6.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . 36
4.7. Considerations for New HTTP Fields . . . . . . . . . . . 35 5.6.2. Limitations . . . . . . . . . . . . . . . . . . . . . 37
4.8. Fields Defined In This Document . . . . . . . . . . . . . 36 5.6.3. Trailer . . . . . . . . . . . . . . . . . . . . . . . 37
5. Message Routing . . . . . . . . . . . . . . . . . . . . . . . 38 5.7. Considerations for New HTTP Fields . . . . . . . . . . . 38
5.1. Identifying a Target Resource . . . . . . . . . . . . . . 38 5.8. Fields Defined In This Document . . . . . . . . . . . . . 39
5.2. Determining Origin . . . . . . . . . . . . . . . . . . . 38 6. Message Routing . . . . . . . . . . . . . . . . . . . . . . . 41
5.3. Routing Inbound . . . . . . . . . . . . . . . . . . . . . 39 6.1. Identifying a Target Resource . . . . . . . . . . . . . . 41
5.4. Direct Authoritative Access . . . . . . . . . . . . . . . 40 6.2. Determining Origin . . . . . . . . . . . . . . . . . . . 42
5.4.1. http origins . . . . . . . . . . . . . . . . . . . . 40 6.3. Routing Inbound . . . . . . . . . . . . . . . . . . . . . 42
5.4.2. https origins . . . . . . . . . . . . . . . . . . . . 41 6.4. Direct Authoritative Access . . . . . . . . . . . . . . . 43
5.4.3. Initiating HTTP Over TLS . . . . . . . . . . . . . . 42 6.4.1. http origins . . . . . . . . . . . . . . . . . . . . 43
5.5. Reconstructing the Target URI . . . . . . . . . . . . . . 44 6.4.2. https origins . . . . . . . . . . . . . . . . . . . . 44
5.6. Host . . . . . . . . . . . . . . . . . . . . . . . . . . 44 6.4.3. Initiating HTTP Over TLS . . . . . . . . . . . . . . 45
5.7. Message Forwarding . . . . . . . . . . . . . . . . . . . 45 6.5. Reconstructing the Target URI . . . . . . . . . . . . . . 47
5.7.1. Via . . . . . . . . . . . . . . . . . . . . . . . . . 46 6.6. Host . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.7.2. Transformations . . . . . . . . . . . . . . . . . . . 47 6.7. Message Forwarding . . . . . . . . . . . . . . . . . . . 48
6. Representations . . . . . . . . . . . . . . . . . . . . . . . 49 6.7.1. Via . . . . . . . . . . . . . . . . . . . . . . . . . 49
6.1. Representation Data . . . . . . . . . . . . . . . . . . . 49 6.7.2. Transformations . . . . . . . . . . . . . . . . . . . 51
6.1.1. Media Type . . . . . . . . . . . . . . . . . . . . . 49 7. Representations . . . . . . . . . . . . . . . . . . . . . . . 52
6.1.2. Content Codings . . . . . . . . . . . . . . . . . . . 52 7.1. Representation Data . . . . . . . . . . . . . . . . . . . 52
6.1.3. Language Tags . . . . . . . . . . . . . . . . . . . . 53 7.1.1. Media Type . . . . . . . . . . . . . . . . . . . . . 53
6.1.4. Range Units . . . . . . . . . . . . . . . . . . . . . 54 7.1.2. Content Codings . . . . . . . . . . . . . . . . . . . 55
6.2. Representation Metadata . . . . . . . . . . . . . . . . . 58 7.1.3. Language Tags . . . . . . . . . . . . . . . . . . . . 57
6.2.1. Content-Type . . . . . . . . . . . . . . . . . . . . 59 7.1.4. Range Units . . . . . . . . . . . . . . . . . . . . . 57
6.2.2. Content-Encoding . . . . . . . . . . . . . . . . . . 60 7.2. Representation Metadata . . . . . . . . . . . . . . . . . 61
6.2.3. Content-Language . . . . . . . . . . . . . . . . . . 61 7.2.1. Content-Type . . . . . . . . . . . . . . . . . . . . 62
6.2.4. Content-Length . . . . . . . . . . . . . . . . . . . 61 7.2.2. Content-Encoding . . . . . . . . . . . . . . . . . . 63
6.2.5. Content-Location . . . . . . . . . . . . . . . . . . 63 7.2.3. Content-Language . . . . . . . . . . . . . . . . . . 64
6.3. Payload . . . . . . . . . . . . . . . . . . . . . . . . . 64 7.2.4. Content-Length . . . . . . . . . . . . . . . . . . . 64
6.3.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . 65 7.2.5. Content-Location . . . . . . . . . . . . . . . . . . 66
6.3.2. Identification . . . . . . . . . . . . . . . . . . . 65 7.3. Payload . . . . . . . . . . . . . . . . . . . . . . . . . 68
6.3.3. Payload Body . . . . . . . . . . . . . . . . . . . . 66 7.3.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . 68
6.3.4. Content-Range . . . . . . . . . . . . . . . . . . . . 67 7.3.2. Identification . . . . . . . . . . . . . . . . . . . 68
6.3.5. Media Type multipart/byteranges . . . . . . . . . . . 69 7.3.3. Payload Body . . . . . . . . . . . . . . . . . . . . 69
6.4. Content Negotiation . . . . . . . . . . . . . . . . . . . 71 7.3.4. Content-Range . . . . . . . . . . . . . . . . . . . . 70
6.4.1. Proactive Negotiation . . . . . . . . . . . . . . . . 72 7.3.5. Media Type multipart/byteranges . . . . . . . . . . . 72
6.4.2. Reactive Negotiation . . . . . . . . . . . . . . . . 73 7.4. Content Negotiation . . . . . . . . . . . . . . . . . . . 74
6.4.3. Request Payload Negotiation . . . . . . . . . . . . . 74 7.4.1. Proactive Negotiation . . . . . . . . . . . . . . . . 75
6.4.4. Quality Values . . . . . . . . . . . . . . . . . . . 74 7.4.2. Reactive Negotiation . . . . . . . . . . . . . . . . 76
7. Request Methods . . . . . . . . . . . . . . . . . . . . . . . 74 7.4.3. Request Payload Negotiation . . . . . . . . . . . . . 77
7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 74 7.4.4. Quality Values . . . . . . . . . . . . . . . . . . . 77
7.2. Common Method Properties . . . . . . . . . . . . . . . . 76 8. Request Methods . . . . . . . . . . . . . . . . . . . . . . . 77
7.2.1. Safe Methods . . . . . . . . . . . . . . . . . . . . 77 8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 77
7.2.2. Idempotent Methods . . . . . . . . . . . . . . . . . 78 8.2. Common Method Properties . . . . . . . . . . . . . . . . 79
7.2.3. Methods and Caching . . . . . . . . . . . . . . . . . 79 8.2.1. Safe Methods . . . . . . . . . . . . . . . . . . . . 80
7.3. Method Definitions . . . . . . . . . . . . . . . . . . . 79 8.2.2. Idempotent Methods . . . . . . . . . . . . . . . . . 81
7.3.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 79 8.2.3. Methods and Caching . . . . . . . . . . . . . . . . . 82
7.3.2. HEAD . . . . . . . . . . . . . . . . . . . . . . . . 80 8.3. Method Definitions . . . . . . . . . . . . . . . . . . . 82
7.3.3. POST . . . . . . . . . . . . . . . . . . . . . . . . 81 8.3.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 82
7.3.4. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 82 8.3.2. HEAD . . . . . . . . . . . . . . . . . . . . . . . . 83
7.3.5. DELETE . . . . . . . . . . . . . . . . . . . . . . . 84 8.3.3. POST . . . . . . . . . . . . . . . . . . . . . . . . 84
7.3.6. CONNECT . . . . . . . . . . . . . . . . . . . . . . . 85 8.3.4. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 85
7.3.7. OPTIONS . . . . . . . . . . . . . . . . . . . . . . . 87 8.3.5. DELETE . . . . . . . . . . . . . . . . . . . . . . . 87
7.3.8. TRACE . . . . . . . . . . . . . . . . . . . . . . . . 88 8.3.6. CONNECT . . . . . . . . . . . . . . . . . . . . . . . 88
7.4. Method Extensibility . . . . . . . . . . . . . . . . . . 88 8.3.7. OPTIONS . . . . . . . . . . . . . . . . . . . . . . . 90
7.4.1. Method Registry . . . . . . . . . . . . . . . . . . . 88 8.3.8. TRACE . . . . . . . . . . . . . . . . . . . . . . . . 91
7.4.2. Considerations for New Methods . . . . . . . . . . . 89 8.4. Method Extensibility . . . . . . . . . . . . . . . . . . 91
8. Request Header Fields . . . . . . . . . . . . . . . . . . . . 89 8.4.1. Method Registry . . . . . . . . . . . . . . . . . . . 92
8.1. Controls . . . . . . . . . . . . . . . . . . . . . . . . 90 8.4.2. Considerations for New Methods . . . . . . . . . . . 92
8.1.1. Expect . . . . . . . . . . . . . . . . . . . . . . . 90 9. Request Header Fields . . . . . . . . . . . . . . . . . . . . 93
8.1.2. Max-Forwards . . . . . . . . . . . . . . . . . . . . 92 9.1. Controls . . . . . . . . . . . . . . . . . . . . . . . . 93
8.2. Preconditions . . . . . . . . . . . . . . . . . . . . . . 93 9.1.1. Expect . . . . . . . . . . . . . . . . . . . . . . . 93
8.2.1. Evaluation . . . . . . . . . . . . . . . . . . . . . 94 9.1.2. Max-Forwards . . . . . . . . . . . . . . . . . . . . 96
8.2.2. Precedence . . . . . . . . . . . . . . . . . . . . . 95 9.2. Preconditions . . . . . . . . . . . . . . . . . . . . . . 96
8.2.3. If-Match . . . . . . . . . . . . . . . . . . . . . . 97 9.2.1. Evaluation . . . . . . . . . . . . . . . . . . . . . 97
8.2.4. If-None-Match . . . . . . . . . . . . . . . . . . . . 98 9.2.2. Precedence . . . . . . . . . . . . . . . . . . . . . 98
8.2.5. If-Modified-Since . . . . . . . . . . . . . . . . . . 99 9.2.3. If-Match . . . . . . . . . . . . . . . . . . . . . . 100
8.2.6. If-Unmodified-Since . . . . . . . . . . . . . . . . . 101 9.2.4. If-None-Match . . . . . . . . . . . . . . . . . . . . 101
8.2.7. If-Range . . . . . . . . . . . . . . . . . . . . . . 102 9.2.5. If-Modified-Since . . . . . . . . . . . . . . . . . . 103
8.3. Range . . . . . . . . . . . . . . . . . . . . . . . . . . 103 9.2.6. If-Unmodified-Since . . . . . . . . . . . . . . . . . 104
8.4. Negotiation . . . . . . . . . . . . . . . . . . . . . . . 105 9.2.7. If-Range . . . . . . . . . . . . . . . . . . . . . . 105
8.4.1. Accept . . . . . . . . . . . . . . . . . . . . . . . 106 9.3. Range . . . . . . . . . . . . . . . . . . . . . . . . . . 106
8.4.2. Accept-Charset . . . . . . . . . . . . . . . . . . . 108 9.4. Negotiation . . . . . . . . . . . . . . . . . . . . . . . 108
8.4.3. Accept-Encoding . . . . . . . . . . . . . . . . . . . 108 9.4.1. Accept . . . . . . . . . . . . . . . . . . . . . . . 109
8.4.4. Accept-Language . . . . . . . . . . . . . . . . . . . 110 9.4.2. Accept-Charset . . . . . . . . . . . . . . . . . . . 111
8.5. Authentication Credentials . . . . . . . . . . . . . . . 111 9.4.3. Accept-Encoding . . . . . . . . . . . . . . . . . . . 112
8.5.1. Challenge and Response . . . . . . . . . . . . . . . 112 9.4.4. Accept-Language . . . . . . . . . . . . . . . . . . . 114
8.5.2. Protection Space (Realm) . . . . . . . . . . . . . . 113 9.5. Authentication Credentials . . . . . . . . . . . . . . . 115
8.5.3. Authorization . . . . . . . . . . . . . . . . . . . . 114 9.5.1. Challenge and Response . . . . . . . . . . . . . . . 115
8.5.4. Proxy-Authorization . . . . . . . . . . . . . . . . . 115 9.5.2. Protection Space (Realm) . . . . . . . . . . . . . . 117
8.5.5. Authentication Scheme Extensibility . . . . . . . . . 115 9.5.3. Authorization . . . . . . . . . . . . . . . . . . . . 118
8.6. Request Context . . . . . . . . . . . . . . . . . . . . . 117 9.5.4. Proxy-Authorization . . . . . . . . . . . . . . . . . 118
8.6.1. From . . . . . . . . . . . . . . . . . . . . . . . . 118 9.5.5. Authentication Scheme Extensibility . . . . . . . . . 118
8.6.2. Referer . . . . . . . . . . . . . . . . . . . . . . . 118 9.6. Request Context . . . . . . . . . . . . . . . . . . . . . 121
8.6.3. User-Agent . . . . . . . . . . . . . . . . . . . . . 119 9.6.1. From . . . . . . . . . . . . . . . . . . . . . . . . 121
9. Response Status Codes . . . . . . . . . . . . . . . . . . . . 120 9.6.2. Referer . . . . . . . . . . . . . . . . . . . . . . . 122
9.1. Overview of Status Codes . . . . . . . . . . . . . . . . 121 9.6.3. User-Agent . . . . . . . . . . . . . . . . . . . . . 123
9.2. Informational 1xx . . . . . . . . . . . . . . . . . . . . 123 10. Response Status Codes . . . . . . . . . . . . . . . . . . . . 124
9.2.1. 100 Continue . . . . . . . . . . . . . . . . . . . . 123 10.1. Overview of Status Codes . . . . . . . . . . . . . . . . 125
9.2.2. 101 Switching Protocols . . . . . . . . . . . . . . . 123 10.2. Informational 1xx . . . . . . . . . . . . . . . . . . . 126
9.3. Successful 2xx . . . . . . . . . . . . . . . . . . . . . 124 10.2.1. 100 Continue . . . . . . . . . . . . . . . . . . . . 127
9.3.1. 200 OK . . . . . . . . . . . . . . . . . . . . . . . 124 10.2.2. 101 Switching Protocols . . . . . . . . . . . . . . 127
9.3.2. 201 Created . . . . . . . . . . . . . . . . . . . . . 124 10.3. Successful 2xx . . . . . . . . . . . . . . . . . . . . . 127
9.3.3. 202 Accepted . . . . . . . . . . . . . . . . . . . . 125 10.3.1. 200 OK . . . . . . . . . . . . . . . . . . . . . . . 127
9.3.4. 203 Non-Authoritative Information . . . . . . . . . . 125 10.3.2. 201 Created . . . . . . . . . . . . . . . . . . . . 128
9.3.5. 204 No Content . . . . . . . . . . . . . . . . . . . 125 10.3.3. 202 Accepted . . . . . . . . . . . . . . . . . . . . 128
9.3.6. 205 Reset Content . . . . . . . . . . . . . . . . . . 126 10.3.4. 203 Non-Authoritative Information . . . . . . . . . 129
9.3.7. 206 Partial Content . . . . . . . . . . . . . . . . . 127 10.3.5. 204 No Content . . . . . . . . . . . . . . . . . . . 129
9.4. Redirection 3xx . . . . . . . . . . . . . . . . . . . . . 130 10.3.6. 205 Reset Content . . . . . . . . . . . . . . . . . 130
9.4.1. 300 Multiple Choices . . . . . . . . . . . . . . . . 131 10.3.7. 206 Partial Content . . . . . . . . . . . . . . . . 130
9.4.2. 301 Moved Permanently . . . . . . . . . . . . . . . . 132 10.4. Redirection 3xx . . . . . . . . . . . . . . . . . . . . 133
9.4.3. 302 Found . . . . . . . . . . . . . . . . . . . . . . 132 10.4.1. 300 Multiple Choices . . . . . . . . . . . . . . . . 135
9.4.4. 303 See Other . . . . . . . . . . . . . . . . . . . . 133 10.4.2. 301 Moved Permanently . . . . . . . . . . . . . . . 136
9.4.5. 304 Not Modified . . . . . . . . . . . . . . . . . . 133 10.4.3. 302 Found . . . . . . . . . . . . . . . . . . . . . 136
9.4.6. 305 Use Proxy . . . . . . . . . . . . . . . . . . . . 134 10.4.4. 303 See Other . . . . . . . . . . . . . . . . . . . 137
9.4.7. 306 (Unused) . . . . . . . . . . . . . . . . . . . . 134 10.4.5. 304 Not Modified . . . . . . . . . . . . . . . . . . 137
9.4.8. 307 Temporary Redirect . . . . . . . . . . . . . . . 134 10.4.6. 305 Use Proxy . . . . . . . . . . . . . . . . . . . 138
9.4.9. 308 Permanent Redirect . . . . . . . . . . . . . . . 135 10.4.7. 306 (Unused) . . . . . . . . . . . . . . . . . . . . 138
9.5. Client Error 4xx . . . . . . . . . . . . . . . . . . . . 135 10.4.8. 307 Temporary Redirect . . . . . . . . . . . . . . . 138
9.5.1. 400 Bad Request . . . . . . . . . . . . . . . . . . . 135 10.4.9. 308 Permanent Redirect . . . . . . . . . . . . . . . 139
9.5.2. 401 Unauthorized . . . . . . . . . . . . . . . . . . 135 10.5. Client Error 4xx . . . . . . . . . . . . . . . . . . . . 139
9.5.3. 402 Payment Required . . . . . . . . . . . . . . . . 136 10.5.1. 400 Bad Request . . . . . . . . . . . . . . . . . . 139
9.5.4. 403 Forbidden . . . . . . . . . . . . . . . . . . . . 136 10.5.2. 401 Unauthorized . . . . . . . . . . . . . . . . . . 139
9.5.5. 404 Not Found . . . . . . . . . . . . . . . . . . . . 136 10.5.3. 402 Payment Required . . . . . . . . . . . . . . . . 140
9.5.6. 405 Method Not Allowed . . . . . . . . . . . . . . . 137 10.5.4. 403 Forbidden . . . . . . . . . . . . . . . . . . . 140
9.5.7. 406 Not Acceptable . . . . . . . . . . . . . . . . . 137 10.5.5. 404 Not Found . . . . . . . . . . . . . . . . . . . 140
9.5.8. 407 Proxy Authentication Required . . . . . . . . . . 137 10.5.6. 405 Method Not Allowed . . . . . . . . . . . . . . . 141
9.5.9. 408 Request Timeout . . . . . . . . . . . . . . . . . 137 10.5.7. 406 Not Acceptable . . . . . . . . . . . . . . . . . 141
9.5.10. 409 Conflict . . . . . . . . . . . . . . . . . . . . 138 10.5.8. 407 Proxy Authentication Required . . . . . . . . . 141
9.5.11. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 138 10.5.9. 408 Request Timeout . . . . . . . . . . . . . . . . 141
9.5.12. 411 Length Required . . . . . . . . . . . . . . . . . 138 10.5.10. 409 Conflict . . . . . . . . . . . . . . . . . . . . 142
9.5.13. 412 Precondition Failed . . . . . . . . . . . . . . . 139 10.5.11. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 142
9.5.14. 413 Payload Too Large . . . . . . . . . . . . . . . . 139 10.5.12. 411 Length Required . . . . . . . . . . . . . . . . 142
9.5.15. 414 URI Too Long . . . . . . . . . . . . . . . . . . 139 10.5.13. 412 Precondition Failed . . . . . . . . . . . . . . 143
9.5.16. 415 Unsupported Media Type . . . . . . . . . . . . . 139 10.5.14. 413 Payload Too Large . . . . . . . . . . . . . . . 143
9.5.17. 416 Range Not Satisfiable . . . . . . . . . . . . . . 140 10.5.15. 414 URI Too Long . . . . . . . . . . . . . . . . . . 143
9.5.18. 417 Expectation Failed . . . . . . . . . . . . . . . 140 10.5.16. 415 Unsupported Media Type . . . . . . . . . . . . . 143
9.5.19. 418 (Unused) . . . . . . . . . . . . . . . . . . . . 140 10.5.17. 416 Range Not Satisfiable . . . . . . . . . . . . . 144
9.5.20. 422 Unprocessable Payload . . . . . . . . . . . . . . 141 10.5.18. 417 Expectation Failed . . . . . . . . . . . . . . . 144
9.5.21. 426 Upgrade Required . . . . . . . . . . . . . . . . 141 10.5.19. 418 (Unused) . . . . . . . . . . . . . . . . . . . . 145
9.6. Server Error 5xx . . . . . . . . . . . . . . . . . . . . 141 10.5.20. 422 Unprocessable Payload . . . . . . . . . . . . . 145
9.6.1. 500 Internal Server Error . . . . . . . . . . . . . . 142 10.5.21. 426 Upgrade Required . . . . . . . . . . . . . . . . 145
9.6.2. 501 Not Implemented . . . . . . . . . . . . . . . . . 142 10.6. Server Error 5xx . . . . . . . . . . . . . . . . . . . . 145
9.6.3. 502 Bad Gateway . . . . . . . . . . . . . . . . . . . 142 10.6.1. 500 Internal Server Error . . . . . . . . . . . . . 146
9.6.4. 503 Service Unavailable . . . . . . . . . . . . . . . 142 10.6.2. 501 Not Implemented . . . . . . . . . . . . . . . . 146
9.6.5. 504 Gateway Timeout . . . . . . . . . . . . . . . . . 142 10.6.3. 502 Bad Gateway . . . . . . . . . . . . . . . . . . 146
9.6.6. 505 HTTP Version Not Supported . . . . . . . . . . . 142 10.6.4. 503 Service Unavailable . . . . . . . . . . . . . . 146
9.7. Status Code Extensibility . . . . . . . . . . . . . . . . 143 10.6.5. 504 Gateway Timeout . . . . . . . . . . . . . . . . 146
9.7.1. Status Code Registry . . . . . . . . . . . . . . . . 143 10.6.6. 505 HTTP Version Not Supported . . . . . . . . . . . 147
9.7.2. Considerations for New Status Codes . . . . . . . . . 143 10.7. Status Code Extensibility . . . . . . . . . . . . . . . 147
10. Response Header Fields . . . . . . . . . . . . . . . . . . . 144 10.7.1. Status Code Registry . . . . . . . . . . . . . . . . 147
10.1. Control Data . . . . . . . . . . . . . . . . . . . . . . 144 10.7.2. Considerations for New Status Codes . . . . . . . . 147
10.1.1. Origination Date . . . . . . . . . . . . . . . . . . 145 11. Response Header Fields . . . . . . . . . . . . . . . . . . . 148
10.1.2. Location . . . . . . . . . . . . . . . . . . . . . . 148 11.1. Control Data . . . . . . . . . . . . . . . . . . . . . . 149
10.1.3. Retry-After . . . . . . . . . . . . . . . . . . . . 149 11.1.1. Date . . . . . . . . . . . . . . . . . . . . . . . . 149
10.1.4. Vary . . . . . . . . . . . . . . . . . . . . . . . . 149 11.1.2. Location . . . . . . . . . . . . . . . . . . . . . . 150
10.2. Validators . . . . . . . . . . . . . . . . . . . . . . . 151 11.1.3. Retry-After . . . . . . . . . . . . . . . . . . . . 151
10.2.1. Weak versus Strong . . . . . . . . . . . . . . . . . 152 11.1.4. Vary . . . . . . . . . . . . . . . . . . . . . . . . 152
10.2.2. Last-Modified . . . . . . . . . . . . . . . . . . . 153 11.2. Validators . . . . . . . . . . . . . . . . . . . . . . . 153
10.2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . 155 11.2.1. Weak versus Strong . . . . . . . . . . . . . . . . . 154
10.2.4. When to Use Entity-Tags and Last-Modified Dates . . 159 11.2.2. Last-Modified . . . . . . . . . . . . . . . . . . . 155
10.3. Authentication Challenges . . . . . . . . . . . . . . . 159 11.2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . 157
10.3.1. WWW-Authenticate . . . . . . . . . . . . . . . . . . 160 11.2.4. When to Use Entity-Tags and Last-Modified Dates . . 161
10.3.2. Proxy-Authenticate . . . . . . . . . . . . . . . . . 161 11.3. Authentication Challenges . . . . . . . . . . . . . . . 161
10.3.3. Authentication-Info . . . . . . . . . . . . . . . . 161 11.3.1. WWW-Authenticate . . . . . . . . . . . . . . . . . . 162
10.3.4. Proxy-Authentication-Info . . . . . . . . . . . . . 162 11.3.2. Proxy-Authenticate . . . . . . . . . . . . . . . . . 163
10.4. Response Context . . . . . . . . . . . . . . . . . . . . 163 11.3.3. Authentication-Info . . . . . . . . . . . . . . . . 163
10.4.1. Accept-Ranges . . . . . . . . . . . . . . . . . . . 163 11.3.4. Proxy-Authentication-Info . . . . . . . . . . . . . 164
10.4.2. Allow . . . . . . . . . . . . . . . . . . . . . . . 163 11.4. Response Context . . . . . . . . . . . . . . . . . . . . 165
10.4.3. Server . . . . . . . . . . . . . . . . . . . . . . . 164 11.4.1. Accept-Ranges . . . . . . . . . . . . . . . . . . . 165
11. Security Considerations . . . . . . . . . . . . . . . . . . . 165 11.4.2. Allow . . . . . . . . . . . . . . . . . . . . . . . 165
11.1. Establishing Authority . . . . . . . . . . . . . . . . . 165 11.4.3. Server . . . . . . . . . . . . . . . . . . . . . . . 166
11.2. Risks of Intermediaries . . . . . . . . . . . . . . . . 166 12. Security Considerations . . . . . . . . . . . . . . . . . . . 167
11.3. Attacks Based on File and Path Names . . . . . . . . . . 167 12.1. Establishing Authority . . . . . . . . . . . . . . . . . 167
11.4. Attacks Based on Command, Code, or Query Injection . . . 167 12.2. Risks of Intermediaries . . . . . . . . . . . . . . . . 168
11.5. Attacks via Protocol Element Length . . . . . . . . . . 168 12.3. Attacks Based on File and Path Names . . . . . . . . . . 169
11.6. Disclosure of Personal Information . . . . . . . . . . . 168 12.4. Attacks Based on Command, Code, or Query Injection . . . 169
11.7. Privacy of Server Log Information . . . . . . . . . . . 168 12.5. Attacks via Protocol Element Length . . . . . . . . . . 170
11.8. Disclosure of Sensitive Information in URIs . . . . . . 169 12.6. Disclosure of Personal Information . . . . . . . . . . . 170
11.9. Disclosure of Fragment after Redirects . . . . . . . . . 169 12.7. Privacy of Server Log Information . . . . . . . . . . . 170
11.10. Disclosure of Product Information . . . . . . . . . . . 170 12.8. Disclosure of Sensitive Information in URIs . . . . . . 171
11.11. Browser Fingerprinting . . . . . . . . . . . . . . . . . 170 12.9. Disclosure of Fragment after Redirects . . . . . . . . . 171
11.12. Validator Retention . . . . . . . . . . . . . . . . . . 171 12.10. Disclosure of Product Information . . . . . . . . . . . 172
11.13. Denial-of-Service Attacks Using Range . . . . . . . . . 172 12.11. Browser Fingerprinting . . . . . . . . . . . . . . . . . 172
11.14. Authentication Considerations . . . . . . . . . . . . . 172 12.12. Validator Retention . . . . . . . . . . . . . . . . . . 173
11.14.1. Confidentiality of Credentials . . . . . . . . . . 172 12.13. Denial-of-Service Attacks Using Range . . . . . . . . . 174
11.14.2. Credentials and Idle Clients . . . . . . . . . . . 173 12.14. Authentication Considerations . . . . . . . . . . . . . 174
11.14.3. Protection Spaces . . . . . . . . . . . . . . . . . 173 12.14.1. Confidentiality of Credentials . . . . . . . . . . 174
11.14.4. Additional Response Fields . . . . . . . . . . . . 174 12.14.2. Credentials and Idle Clients . . . . . . . . . . . 175
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 174 12.14.3. Protection Spaces . . . . . . . . . . . . . . . . . 175
12.1. URI Scheme Registration . . . . . . . . . . . . . . . . 174 12.14.4. Additional Response Fields . . . . . . . . . . . . 176
12.2. Method Registration . . . . . . . . . . . . . . . . . . 174 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 176
12.3. Status Code Registration . . . . . . . . . . . . . . . . 174 13.1. URI Scheme Registration . . . . . . . . . . . . . . . . 176
12.4. HTTP Field Name Registration . . . . . . . . . . . . . . 175 13.2. Method Registration . . . . . . . . . . . . . . . . . . 176
12.5. Authentication Scheme Registration . . . . . . . . . . . 175 13.3. Status Code Registration . . . . . . . . . . . . . . . . 176
12.6. Content Coding Registration . . . . . . . . . . . . . . 175 13.4. HTTP Field Name Registration . . . . . . . . . . . . . . 177
12.7. Range Unit Registration . . . . . . . . . . . . . . . . 176 13.5. Authentication Scheme Registration . . . . . . . . . . . 177
12.8. Media Type Registration . . . . . . . . . . . . . . . . 176 13.6. Content Coding Registration . . . . . . . . . . . . . . 178
12.9. Port Registration . . . . . . . . . . . . . . . . . . . 176 13.7. Range Unit Registration . . . . . . . . . . . . . . . . 178
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 176 13.8. Media Type Registration . . . . . . . . . . . . . . . . 178
13.1. Normative References . . . . . . . . . . . . . . . . . . 176 13.9. Port Registration . . . . . . . . . . . . . . . . . . . 178
13.2. Informative References . . . . . . . . . . . . . . . . . 178 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 178
Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 184 14.1. Normative References . . . . . . . . . . . . . . . . . . 178
Appendix B. Changes from previous RFCs . . . . . . . . . . . . . 188 14.2. Informative References . . . . . . . . . . . . . . . . . 180
B.1. Changes from RFC 2818 . . . . . . . . . . . . . . . . . . 188 Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 187
B.2. Changes from RFC 7230 . . . . . . . . . . . . . . . . . . 188 Appendix B. Changes from previous RFCs . . . . . . . . . . . . . 191
B.3. Changes from RFC 7231 . . . . . . . . . . . . . . . . . . 189 B.1. Changes from RFC 2818 . . . . . . . . . . . . . . . . . . 191
B.4. Changes from RFC 7232 . . . . . . . . . . . . . . . . . . 190 B.2. Changes from RFC 7230 . . . . . . . . . . . . . . . . . . 191
B.5. Changes from RFC 7233 . . . . . . . . . . . . . . . . . . 190 B.3. Changes from RFC 7231 . . . . . . . . . . . . . . . . . . 192
B.6. Changes from RFC 7235 . . . . . . . . . . . . . . . . . . 190 B.4. Changes from RFC 7232 . . . . . . . . . . . . . . . . . . 193
B.7. Changes from RFC 7538 . . . . . . . . . . . . . . . . . . 190 B.5. Changes from RFC 7233 . . . . . . . . . . . . . . . . . . 193
B.8. Changes from RFC 7615 . . . . . . . . . . . . . . . . . . 190 B.6. Changes from RFC 7235 . . . . . . . . . . . . . . . . . . 193
Appendix C. Changes from RFC 7694 . . . . . . . . . . . . . . . 190 B.7. Changes from RFC 7538 . . . . . . . . . . . . . . . . . . 193
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 191 B.8. Changes from RFC 7615 . . . . . . . . . . . . . . . . . . 193
D.1. Between RFC723x and draft 00 . . . . . . . . . . . . . . 191 Appendix C. Changes from RFC 7694 . . . . . . . . . . . . . . . 193
D.2. Since draft-ietf-httpbis-semantics-00 . . . . . . . . . . 191 Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 193
D.3. Since draft-ietf-httpbis-semantics-01 . . . . . . . . . . 192 D.1. Between RFC723x and draft 00 . . . . . . . . . . . . . . 194
D.4. Since draft-ietf-httpbis-semantics-02 . . . . . . . . . . 193 D.2. Since draft-ietf-httpbis-semantics-00 . . . . . . . . . . 194
D.5. Since draft-ietf-httpbis-semantics-03 . . . . . . . . . . 194 D.3. Since draft-ietf-httpbis-semantics-01 . . . . . . . . . . 195
D.6. Since draft-ietf-httpbis-semantics-04 . . . . . . . . . . 195 D.4. Since draft-ietf-httpbis-semantics-02 . . . . . . . . . . 196
D.7. Since draft-ietf-httpbis-semantics-05 . . . . . . . . . . 195 D.5. Since draft-ietf-httpbis-semantics-03 . . . . . . . . . . 197
D.8. Since draft-ietf-httpbis-semantics-06 . . . . . . . . . . 196 D.6. Since draft-ietf-httpbis-semantics-04 . . . . . . . . . . 198
D.9. Since draft-ietf-httpbis-semantics-07 . . . . . . . . . . 198 D.7. Since draft-ietf-httpbis-semantics-05 . . . . . . . . . . 198
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 D.8. Since draft-ietf-httpbis-semantics-06 . . . . . . . . . . 199
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 209 D.9. Since draft-ietf-httpbis-semantics-07 . . . . . . . . . . 201
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 210 D.10. Since draft-ietf-httpbis-semantics-08 . . . . . . . . . . 202
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 213
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 214
1. Introduction 1. Introduction
The Hypertext Transfer Protocol (HTTP) is a stateless application- The Hypertext Transfer Protocol (HTTP) is a stateless application-
level request/response protocol that uses extensible semantics and level request/response protocol that uses extensible semantics and
self-descriptive messages for flexible interaction with network-based self-descriptive messages for flexible interaction with network-based
hypertext information systems. HTTP is defined by a series of hypertext information systems. HTTP is defined by a series of
documents that collectively form the HTTP/1.1 specification: documents that collectively form the HTTP/1.1 specification:
o "HTTP Semantics" (this document) o "HTTP Semantics" (this document)
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listens on a connection for a request, parses each message received, listens on a connection for a request, parses each message received,
interprets the message semantics in relation to the identified target interprets the message semantics in relation to the identified target
resource, and responds to that request with one or more response resource, and responds to that request with one or more response
messages. A client constructs request messages to communicate messages. A client constructs request messages to communicate
specific intentions, examines received responses to see if the specific intentions, examines received responses to see if the
intentions were carried out, and determines how to interpret the intentions were carried out, and determines how to interpret the
results. results.
HTTP provides a uniform interface for interacting with a resource HTTP provides a uniform interface for interacting with a resource
(Section 2.5), regardless of its type, nature, or implementation, via (Section 2.5), regardless of its type, nature, or implementation, via
the manipulation and transfer of representations (Section 6). the manipulation and transfer of representations (Section 7).
This document defines semantics that are common to all versions of This document defines semantics that are common to all versions of
HTTP. HTTP semantics include the intentions defined by each request HTTP. HTTP semantics include the intentions defined by each request
method (Section 7), extensions to those semantics that might be method (Section 8), extensions to those semantics that might be
described in request header fields (Section 8), the meaning of status described in request header fields (Section 9), the meaning of status
codes to indicate a machine-readable response (Section 9), and the codes to indicate a machine-readable response (Section 10), and the
meaning of other control data and resource metadata that might be meaning of other control data and resource metadata that might be
given in response header fields (Section 10). given in response header fields (Section 11).
This document also defines representation metadata that describe how This document also defines representation metadata that describe how
a payload is intended to be interpreted by a recipient, the request a payload is intended to be interpreted by a recipient, the request
header fields that might influence content selection, and the various header fields that might influence content selection, and the various
selection algorithms that are collectively referred to as "content selection algorithms that are collectively referred to as "content
negotiation" (Section 6.4). negotiation" (Section 7.4).
This document defines HTTP range requests, partial responses, and the This document defines HTTP range requests, partial responses, and the
multipart/byteranges media type. multipart/byteranges media type.
This document obsoletes the portions of RFC 7230 that are independent This document obsoletes the portions of RFC 7230 that are independent
of the HTTP/1.1 messaging syntax and connection management, with the of the HTTP/1.1 messaging syntax and connection management, with the
changes being summarized in Appendix B.2. The other parts of RFC changes being summarized in Appendix B.2. The other parts of RFC
7230 are obsoleted by "HTTP/1.1 Messaging" [Messaging]. This 7230 are obsoleted by "HTTP/1.1 Messaging" [Messaging]. This
document also obsoletes RFC 2818 (see Appendix B.1), RFC 7231 (see document also obsoletes RFC 2818 (see Appendix B.1), RFC 7231 (see
Appendix B.3), RFC 7232 (see Appendix B.4), RFC 7233 (see Appendix B.3), RFC 7232 (see Appendix B.4), RFC 7233 (see
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Conformance criteria and considerations regarding error handling are Conformance criteria and considerations regarding error handling are
defined in Section 3. defined in Section 3.
1.2. Syntax Notation 1.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF) This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234], extended with the notation for case- notation of [RFC5234], extended with the notation for case-
sensitivity in strings defined in [RFC7405]. sensitivity in strings defined in [RFC7405].
It also uses a list extension, defined in Section 4.5, that allows It also uses a list extension, defined in Section 5.5, that allows
for compact definition of comma-separated lists using a '#' operator for compact definition of comma-separated lists using a '#' operator
(similar to how the '*' operator indicates repetition). Appendix A (similar to how the '*' operator indicates repetition). Appendix A
shows the collected grammar with all list operators expanded to shows the collected grammar with all list operators expanded to
standard ABNF notation. standard ABNF notation.
As a convention, ABNF rule names prefixed with "obs-" denote As a convention, ABNF rule names prefixed with "obs-" denote
"obsolete" grammar rules that appear for historical reasons. "obsolete" grammar rules that appear for historical reasons.
The following core rules are included by reference, as defined in The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return), Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF
(line feed), OCTET (any 8-bit sequence of data), SP (space), and (line feed), OCTET (any 8-bit sequence of data), SP (space), and
VCHAR (any visible US-ASCII character). VCHAR (any visible US-ASCII character).
Section 4.4.1 defines some generic syntactic components for field Section 5.4.1 defines some generic syntactic components for field
values. values.
The rules below are defined in [Messaging]: The rules below are defined in [Messaging]:
protocol-name = <protocol-name, see [Messaging], Section 9.9> protocol-name = <protocol-name, see [Messaging], Section 9.9>
protocol-version = <protocol-version, see [Messaging], Section 9.9> protocol-version = <protocol-version, see [Messaging], Section 9.9>
This specification uses the terms "character", "character encoding This specification uses the terms "character", "character encoding
scheme", "charset", and "protocol element" as they are defined in scheme", "charset", and "protocol element" as they are defined in
[RFC6365]. [RFC6365].
skipping to change at page 12, line 8 skipping to change at page 12, line 8
act as a client on some connections and a server on others. The term act as a client on some connections and a server on others. The term
"user agent" refers to any of the various client programs that "user agent" refers to any of the various client programs that
initiate a request, including (but not limited to) browsers, spiders initiate a request, including (but not limited to) browsers, spiders
(web-based robots), command-line tools, custom applications, and (web-based robots), command-line tools, custom applications, and
mobile apps. The term "origin server" refers to the program that can mobile apps. The term "origin server" refers to the program that can
originate authoritative responses for a given target resource. The originate authoritative responses for a given target resource. The
terms "sender" and "recipient" refer to any implementation that sends terms "sender" and "recipient" refer to any implementation that sends
or receives a given message, respectively. or receives a given message, respectively.
HTTP relies upon the Uniform Resource Identifier (URI) standard HTTP relies upon the Uniform Resource Identifier (URI) standard
[RFC3986] to indicate the target resource (Section 5.1) and [RFC3986] to indicate the target resource (Section 6.1) and
relationships between resources. relationships between resources.
Most HTTP communication consists of a retrieval request (GET) for a Most HTTP communication consists of a retrieval request (GET) for a
representation of some resource identified by a URI. In the simplest representation of some resource identified by a URI. In the simplest
case, this might be accomplished via a single bidirectional case, this might be accomplished via a single bidirectional
connection (===) between the user agent (UA) and the origin server connection (===) between the user agent (UA) and the origin server
(O). (O).
request > request >
UA ======================================= O UA ======================================= O
< response < response
Each major version of HTTP defines its own syntax for the inclusion Each major version of HTTP defines its own syntax for the inclusion
of information in messages. Nevertheless, a common abstraction is of information in messages. Nevertheless, a common abstraction is
that a message includes some form of envelope/framing, a potential that a message includes some form of envelope/framing, a potential
set of named fields up front (a header section), a potential body, set of named fields up front (a header section), a potential body,
and a potential following set of named fields (a trailer section). and a potential following set of named fields (a trailer section).
A client sends an HTTP request to a server in the form of a request A client sends an HTTP request to a server in the form of a request
message, beginning with a method (Section 7) and request target, message with a method (Section 8) and request target. The request
followed by header fields containing request modifiers, client might also contain header fields for request modifiers, client
information, and representation metadata (Section 4), and finally a information, and representation metadata (Section 5), a payload body
payload body (if any, Section 6.3.3). (Section 7.3.3) to be processed in accordance with the method, and
trailer fields for metadata collected while sending the payload.
A server responds to a client's request by sending one or more HTTP A server responds to a client's request by sending one or more HTTP
response messages, each beginning with a success or error code response messages, each including a success or error code
(Section 9), possibly followed by header fields containing server (Section 10). The response might also contain header fields for
information, resource metadata, and representation metadata server information, resource metadata, and representation metadata
(Section 4), and finally a payload body (if any, Section 6.3.3). (Section 5), a payload body (Section 7.3.3) to be interpreted in
accordance with the status code, and trailer fields for metadata
collected while sending the payload.
One of the functions of the message framing mechanism is to assure One of the functions of the message framing mechanism is to assure
that messages are complete. A message is considered complete when that messages are complete. A message is considered complete when
all of the octets indicated by its framing are available. Note that, all of the octets indicated by its framing are available. Note that,
when no explicit framing is used, a response message that is ended by when no explicit framing is used, a response message that is ended by
the transport connection's close is considered complete even though the transport connection's close is considered complete even though
it might be indistinguishable from an incomplete response, unless a it might be indistinguishable from an incomplete response, unless a
transport-level error indicates that it is not complete. transport-level error indicates that it is not complete.
A connection might be used for multiple request/response exchanges. A connection might be used for multiple request/response exchanges.
The mechanism used to correlate between request and response messages The mechanism used to correlate between request and response messages
is version dependent; some versions of HTTP use implicit ordering of is version dependent; some versions of HTTP use implicit ordering of
messages, while others use an explicit identifier. messages, while others use an explicit identifier.
Responses (both final and interim) can be sent at any time after a Responses (both final and interim) can be sent at any time after a
request is received, even if it is not yet complete. However, request is received, even if it is not yet complete. However,
clients (including intermediaries) might abandon a request if the clients (including intermediaries) might abandon a request if the
response is not forthcoming within a reasonable period of time. response is not forthcoming within a reasonable period of time.
The following example illustrates a typical message exchange for a The following example illustrates a typical message exchange for a
GET request (Section 7.3.1) on the URI "http://www.example.com/ GET request (Section 8.3.1) on the URI "http://www.example.com/
hello.txt": hello.txt":
Client request: Client request:
GET /hello.txt HTTP/1.1 GET /hello.txt HTTP/1.1
User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3 User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
Host: www.example.com Host: www.example.com
Accept-Language: en, mi Accept-Language: en, mi
Server response: Server response:
skipping to change at page 14, line 31 skipping to change at page 14, line 34
client, usually via local configuration rules, to receive requests client, usually via local configuration rules, to receive requests
for some type(s) of absolute URI and attempt to satisfy those for some type(s) of absolute URI and attempt to satisfy those
requests via translation through the HTTP interface. Some requests via translation through the HTTP interface. Some
translations are minimal, such as for proxy requests for "http" URIs, translations are minimal, such as for proxy requests for "http" URIs,
whereas other requests might require translation to and from entirely whereas other requests might require translation to and from entirely
different application-level protocols. Proxies are often used to different application-level protocols. Proxies are often used to
group an organization's HTTP requests through a common intermediary group an organization's HTTP requests through a common intermediary
for the sake of security, annotation services, or shared caching. for the sake of security, annotation services, or shared caching.
Some proxies are designed to apply transformations to selected Some proxies are designed to apply transformations to selected
messages or payloads while they are being forwarded, as described in messages or payloads while they are being forwarded, as described in
Section 5.7.2. Section 6.7.2.
A "gateway" (a.k.a. "reverse proxy") is an intermediary that acts as A "gateway" (a.k.a. "reverse proxy") is an intermediary that acts as
an origin server for the outbound connection but translates received an origin server for the outbound connection but translates received
requests and forwards them inbound to another server or servers. requests and forwards them inbound to another server or servers.
Gateways are often used to encapsulate legacy or untrusted Gateways are often used to encapsulate legacy or untrusted
information services, to improve server performance through information services, to improve server performance through
"accelerator" caching, and to enable partitioning or load balancing "accelerator" caching, and to enable partitioning or load balancing
of HTTP services across multiple machines. of HTTP services across multiple machines.
All HTTP requirements applicable to an origin server also apply to All HTTP requirements applicable to an origin server also apply to
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query = <query, see [RFC3986], Section 3.4> query = <query, see [RFC3986], Section 3.4>
absolute-path = 1*( "/" segment ) absolute-path = 1*( "/" segment )
partial-URI = relative-part [ "?" query ] partial-URI = relative-part [ "?" query ]
Each protocol element in HTTP that allows a URI reference will Each protocol element in HTTP that allows a URI reference will
indicate in its ABNF production whether the element allows any form indicate in its ABNF production whether the element allows any form
of reference (URI-reference), only a URI in absolute form (absolute- of reference (URI-reference), only a URI in absolute form (absolute-
URI), only the path and optional query components, or some URI), only the path and optional query components, or some
combination of the above. Unless otherwise indicated, URI references combination of the above. Unless otherwise indicated, URI references
are parsed relative to the target URI (Section 5.1). are parsed relative to the target URI (Section 6.1).
It is RECOMMENDED that all senders and recipients support, at a It is RECOMMENDED that all senders and recipients support, at a
minimum, URIs with lengths of 8000 octets in protocol elements. Note minimum, URIs with lengths of 8000 octets in protocol elements. Note
that this implies some structures and on-wire representations (for that this implies some structures and on-wire representations (for
example, the request line in HTTP/1.1) will necessarily be larger in example, the request line in HTTP/1.1) will necessarily be larger in
some cases. some cases.
2.5. Resources 2.5. Resources
The target of an HTTP request is called a "resource". HTTP does not The target of an HTTP request is called a "resource". HTTP does not
limit the nature of a resource; it merely defines an interface that limit the nature of a resource; it merely defines an interface that
might be used to interact with resources. Most resources are might be used to interact with resources. Most resources are
identified by a Uniform Resource Identifier (URI), as described in identified by a Uniform Resource Identifier (URI), as described in
Section 2.4. Section 2.4.
One design goal of HTTP is to separate resource identification from One design goal of HTTP is to separate resource identification from
request semantics, which is made possible by vesting the request request semantics, which is made possible by vesting the request
semantics in the request method (Section 7) and a few request- semantics in the request method (Section 8) and a few request-
modifying header fields (Section 8). If there is a conflict between modifying header fields (Section 9). If there is a conflict between
the method semantics and any semantic implied by the URI itself, as the method semantics and any semantic implied by the URI itself, as
described in Section 7.2.1, the method semantics take precedence. described in Section 8.2.1, the method semantics take precedence.
IANA maintains the registry of URI Schemes [BCP35] at IANA maintains the registry of URI Schemes [BCP35] at
<https://www.iana.org/assignments/uri-schemes/>. Although requests <https://www.iana.org/assignments/uri-schemes/>. Although requests
might target any URI scheme, the following schemes are inherent to might target any URI scheme, the following schemes are inherent to
HTTP servers: HTTP servers:
+------------+------------------------------------+---------------+ +------------+------------------------------------+---------------+
| URI Scheme | Description | Reference | | URI Scheme | Description | Reference |
+------------+------------------------------------+---------------+ +------------+------------------------------------+---------------+
| http | Hypertext Transfer Protocol | Section 2.5.1 | | http | Hypertext Transfer Protocol | Section 2.5.1 |
skipping to change at page 18, line 34 skipping to change at page 18, line 34
server listening for TCP ([RFC0793]) connections on a given port. server listening for TCP ([RFC0793]) connections on a given port.
http-URI = "http" "://" authority path-abempty [ "?" query ] http-URI = "http" "://" authority path-abempty [ "?" query ]
The origin server for an "http" URI is identified by the authority The origin server for an "http" URI is identified by the authority
component, which includes a host identifier and optional port number component, which includes a host identifier and optional port number
([RFC3986], Section 3.2.2). If the port subcomponent is empty or not ([RFC3986], Section 3.2.2). If the port subcomponent is empty or not
given, TCP port 80 (the reserved port for WWW services) is the given, TCP port 80 (the reserved port for WWW services) is the
default. The origin determines who has the right to respond default. The origin determines who has the right to respond
authoritatively to requests that target the identified resource, as authoritatively to requests that target the identified resource, as
defined in Section 5.4.1. defined in Section 6.4.1.
A sender MUST NOT generate an "http" URI with an empty host A sender MUST NOT generate an "http" URI with an empty host
identifier. A recipient that processes such a URI reference MUST identifier. A recipient that processes such a URI reference MUST
reject it as invalid. reject it as invalid.
The hierarchical path component and optional query component identify The hierarchical path component and optional query component identify
the target resource within that origin server's name space. the target resource within that origin server's name space.
2.5.2. https URI Scheme 2.5.2. https URI Scheme
skipping to change at page 19, line 15 skipping to change at page 19, line 15
strong encryption. strong encryption.
https-URI = "https" "://" authority path-abempty [ "?" query ] https-URI = "https" "://" authority path-abempty [ "?" query ]
The origin server for an "https" URI is identified by the authority The origin server for an "https" URI is identified by the authority
component, which includes a host identifier and optional port number component, which includes a host identifier and optional port number
([RFC3986], Section 3.2.2). If the port subcomponent is empty or not ([RFC3986], Section 3.2.2). If the port subcomponent is empty or not
given, TCP port 443 (the reserved port for HTTP over TLS) is the given, TCP port 443 (the reserved port for HTTP over TLS) is the
default. The origin determines who has the right to respond default. The origin determines who has the right to respond
authoritatively to requests that target the identified resource, as authoritatively to requests that target the identified resource, as
defined in Section 5.4.2. defined in Section 6.4.2.
A sender MUST NOT generate an "https" URI with an empty host A sender MUST NOT generate an "https" URI with an empty host
identifier. A recipient that processes such a URI reference MUST identifier. A recipient that processes such a URI reference MUST
reject it as invalid. reject it as invalid.
The hierarchical path component and optional query component identify The hierarchical path component and optional query component identify
the target resource within that origin server's name space. the target resource within that origin server's name space.
A client MUST ensure that its HTTP requests for an "https" resource A client MUST ensure that its HTTP requests for an "https" resource
are secured, prior to being communicated, and that it only accepts are secured, prior to being communicated, and that it only accepts
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authority for the sake of phishing attacks. authority for the sake of phishing attacks.
2.5.5. Fragment Identifiers on http(s) URI References 2.5.5. Fragment Identifiers on http(s) URI References
Fragment identifiers allow for indirect identification of a secondary Fragment identifiers allow for indirect identification of a secondary
resource, independent of the URI scheme, as defined in Section 3.5 of resource, independent of the URI scheme, as defined in Section 3.5 of
[RFC3986]. Some protocol elements that refer to a URI allow [RFC3986]. Some protocol elements that refer to a URI allow
inclusion of a fragment, while others do not. They are distinguished inclusion of a fragment, while others do not. They are distinguished
by use of the ABNF rule for elements where fragment is allowed; by use of the ABNF rule for elements where fragment is allowed;
otherwise, a specific rule that excludes fragments is used (see otherwise, a specific rule that excludes fragments is used (see
Section 5.1). Section 6.1).
Note: the fragment identifier component is not part of the actual Note: the fragment identifier component is not part of the actual
scheme definition for a URI scheme (see Section 4.3 of [RFC3986]), scheme definition for a URI scheme (see Section 4.3 of [RFC3986]),
thus does not appear in the ABNF definitions for the "http" and thus does not appear in the ABNF definitions for the "http" and
"https" URI schemes above. "https" URI schemes above.
3. Conformance 3. Conformance
3.1. Implementation Diversity 3.1. Implementation Diversity
skipping to change at page 23, line 28 skipping to change at page 23, line 28
protocol element from an invalid construct. HTTP does not define protocol element from an invalid construct. HTTP does not define
specific error handling mechanisms except when they have a direct specific error handling mechanisms except when they have a direct
impact on security, since different applications of the protocol impact on security, since different applications of the protocol
require different error handling strategies. For example, a Web require different error handling strategies. For example, a Web
browser might wish to transparently recover from a response where the browser might wish to transparently recover from a response where the
Location header field doesn't parse according to the ABNF, whereas a Location header field doesn't parse according to the ABNF, whereas a
systems control client might consider any form of error recovery to systems control client might consider any form of error recovery to
be dangerous. be dangerous.
Some requests can be automatically retried by a client in the event Some requests can be automatically retried by a client in the event
of an underlying connection failure, as described in Section 7.2.2. of an underlying connection failure, as described in Section 8.2.2.
3.5. Protocol Versioning 4. Extending and Versioning HTTP
While HTTP's core semantics don't change between protocol versions,
the expression of them "on the wire" can change, and so the HTTP
version number changes when incompatible changes are made to the wire
format. Additionally, HTTP allows incremental, backwards-compatible
changes to be made to the protocol without changing its version
through the use of defined extension points.
4.1. Extending HTTP
HTTP defines a number of generic extension points that can be used to
introduce capabilities to the protocol without introducing a new
version, including methods (Section 8.4), status codes
(Section 10.7), header and trailer fields (Section 5.7), and further
extensibility points within defined fields (such as Cache-Control in
Section 5.2.3 of [Caching]). Because the semantics of HTTP are not
versioned, these extension points are persistent; the version of the
protocol in use does not affect their semantics.
Version-independent extensions are discouraged from depending on or
interacting with the specific version of the protocol in use. When
this is unavoidable, careful consideration needs to be given to how
the extension can interoperate across versions.
Additionally, specific versions of HTTP might have their own
extensibility points, such as transfer-codings in HTTP/1.1
(Section 6.1 of [Messaging]) and HTTP/2 ([RFC7540]) SETTINGS or frame
types. These extension points are specific to the version of the
protocol they occur within.
Version-specific extensions cannot override or modify the semantics
of a version-independent mechanism or extension point (like a method
or header field) without explicitly being allowed by that protocol
element. For example, the CONNECT method (Section 8.3.6) allows
this.
These guidelines assure that the protocol operates correctly and
predictably, even when parts of the path implement different versions
of HTTP.
4.2. Protocol Versioning
The HTTP version number consists of two decimal digits separated by a The HTTP version number consists of two decimal digits separated by a
"." (period or decimal point). The first digit ("major version") "." (period or decimal point). The first digit ("major version")
indicates the HTTP messaging syntax, whereas the second digit ("minor indicates the HTTP messaging syntax, whereas the second digit ("minor
version") indicates the highest minor version within that major version") indicates the highest minor version within that major
version to which the sender is conformant and able to understand for version to which the sender is conformant and able to understand for
future communication. future communication.
The protocol version as a whole indicates the sender's conformance The protocol version as a whole indicates the sender's conformance
with the set of requirements laid out in that version's corresponding with the set of requirements laid out in that version's corresponding
skipping to change at page 24, line 41 skipping to change at page 25, line 35
message with a higher minor version, when sent to a recipient that message with a higher minor version, when sent to a recipient that
has not yet indicated support for that higher version, is has not yet indicated support for that higher version, is
sufficiently backwards-compatible to be safely processed by any sufficiently backwards-compatible to be safely processed by any
implementation of the same major version. implementation of the same major version.
When a major version of HTTP does not define any minor versions, the When a major version of HTTP does not define any minor versions, the
minor version "0" is implied and is used when referring to that minor version "0" is implied and is used when referring to that
protocol within a protocol element that requires sending a minor protocol within a protocol element that requires sending a minor
version. version.
4. Header and Trailer Fields 5. Header and Trailer Fields
HTTP messages use key/value pairs to convey data about the message, HTTP messages use key/value pairs to convey data about the message,
its payload, the target resource, or the connection (i.e., control its payload, the target resource, or the connection (i.e., control
data). They are called "HTTP fields" or just "fields". data). They are called "HTTP fields" or just "fields".
Every message can have two separate areas that such fields can occur Every message can have two separate areas that such fields can occur
within; the "header field section" (or just "header section") within; the "header field section" (or just "header section")
preceding the message body and containing "header fields" (or just preceding the message body and containing "header fields" (or just
"headers", colloquially) and the "trailer field section" (or just "headers", colloquially) and the "trailer field section" (or just
"trailer section") after the message body containing "trailer fields" "trailer section") after the message body containing "trailer fields"
(or just "trailers" colloquially). Header fields are more common; (or just "trailers" colloquially). Header fields are more common;
see Section 4.6 for discussion of the applicability and limitations see Section 5.6 for discussion of the applicability and limitations
of trailer fields. of trailer fields.
Both sections are composed of any number of "field lines", each with Both sections are composed of any number of "field lines", each with
a "field name" (see Section 4.3) identifying the field, and a "field a "field name" (see Section 5.3) identifying the field, and a "field
line value" that conveys data for the field. line value" that conveys data for the field.
Each field name present in a section has a corresponding "field Each field name present in a section has a corresponding "field
value" for that section, composed from all field line values with value" for that section, composed from all field line values with
that given field name in that section, concatenated together and that given field name in that section, concatenated together and
separated with commas. See Section 4.1 for further discussion of the separated with commas. See Section 5.1 for further discussion of the
semantics of field ordering and combination in messages, and semantics of field ordering and combination in messages, and
Section 4.4 for more discussion of field values. Section 5.4 for more discussion of field values.
For example, this section: For example, this section:
Example-Field: Foo, Bar Example-Field: Foo, Bar
Example-Field: Baz Example-Field: Baz
contains two field lines, both with the field name "Example-Field". contains two field lines, both with the field name "Example-Field".
The first field line has a field line value of "Foo, Bar", while the The first field line has a field line value of "Foo, Bar", while the
second field line value is "Baz". The field value for "Example- second field line value is "Baz". The field value for "Example-
Field" is a list with three members: "Foo", "Bar", and "Baz". Field" is a list with three members: "Foo", "Bar", and "Baz".
skipping to change at page 25, line 39 skipping to change at page 26, line 36
The interpretation of a field does not change between minor versions The interpretation of a field does not change between minor versions
of the same major HTTP version, though the default behavior of a of the same major HTTP version, though the default behavior of a
recipient in the absence of such a field can change. Unless recipient in the absence of such a field can change. Unless
specified otherwise, fields are defined for all versions of HTTP. In specified otherwise, fields are defined for all versions of HTTP. In
particular, the Host and Connection fields ought to be implemented by particular, the Host and Connection fields ought to be implemented by
all HTTP/1.x implementations whether or not they advertise all HTTP/1.x implementations whether or not they advertise
conformance with HTTP/1.1. conformance with HTTP/1.1.
New fields can be introduced without changing the protocol version if New fields can be introduced without changing the protocol version if
their defined semantics allow them to be safely ignored by recipients their defined semantics allow them to be safely ignored by recipients
that do not recognize them; see Section 4.3.1. that do not recognize them; see Section 5.3.1.
4.1. Field Ordering and Combination 5.1. Field Ordering and Combination
The order in which field lines with differing names are received in a The order in which field lines with differing names are received in a
message is not significant. However, it is good practice to send message is not significant. However, it is good practice to send
header fields that contain control data first, such as Host on header fields that contain control data first, such as Host on
requests and Date on responses, so that implementations can decide requests and Date on responses, so that implementations can decide
when not to handle a message as early as possible. A server MUST NOT when not to handle a message as early as possible. A server MUST NOT
apply a request to the target resource until the entire request apply a request to the target resource until the entire request
header section is received, since later header field lines might header section is received, since later header field lines might
include conditionals, authentication credentials, or deliberately include conditionals, authentication credentials, or deliberately
misleading duplicate header fields that would impact request misleading duplicate header fields that would impact request
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proxy MUST NOT change the order of these field line values when proxy MUST NOT change the order of these field line values when
forwarding a message. forwarding a message.
This means that, aside from the well-known exception noted below, a This means that, aside from the well-known exception noted below, a
sender MUST NOT generate multiple field lines with the same name in a sender MUST NOT generate multiple field lines with the same name in a
message (whether in the headers or trailers), or append a field line message (whether in the headers or trailers), or append a field line
when a field line of the same name already exists in the message, when a field line of the same name already exists in the message,
unless that field's definition allows multiple field line values to unless that field's definition allows multiple field line values to
be recombined as a comma-separated list [i.e., at least one be recombined as a comma-separated list [i.e., at least one
alternative of the field's definition allows a comma-separated list, alternative of the field's definition allows a comma-separated list,
such as an ABNF rule of #(values) defined in Section 4.5]. such as an ABNF rule of #(values) defined in Section 5.5].
Note: In practice, the "Set-Cookie" header field ([RFC6265]) often Note: In practice, the "Set-Cookie" header field ([RFC6265]) often
appears in a response message across multiple field lines and does appears in a response message across multiple field lines and does
not use the list syntax, violating the above requirements on not use the list syntax, violating the above requirements on
multiple field lines with the same field name. Since it cannot be multiple field lines with the same field name. Since it cannot be
combined into a single field value, recipients ought to handle combined into a single field value, recipients ought to handle
"Set-Cookie" as a special case while processing fields. (See "Set-Cookie" as a special case while processing fields. (See
Appendix A.2.3 of [Kri2001] for details.) Appendix A.2.3 of [Kri2001] for details.)
4.2. Field Limits 5.2. Field Limits
HTTP does not place a predefined limit on the length of each field HTTP does not place a predefined limit on the length of each field
line, field value, or on the length of the header or trailer section line, field value, or on the length of the header or trailer section
as a whole, as described in Section 3. Various ad hoc limitations on as a whole, as described in Section 3. Various ad hoc limitations on
individual lengths are found in practice, often depending on the individual lengths are found in practice, often depending on the
specific field's semantics. specific field's semantics.
A server that receives a request header field line, field value, or A server that receives a request header field line, field value, or
set of fields larger than it wishes to process MUST respond with an set of fields larger than it wishes to process MUST respond with an
appropriate 4xx (Client Error) status code. Ignoring such header appropriate 4xx (Client Error) status code. Ignoring such header
fields would increase the server's vulnerability to request smuggling fields would increase the server's vulnerability to request smuggling
attacks (Section 11.2 of [Messaging]). attacks (Section 11.2 of [Messaging]).
A client MAY discard or truncate received field lines that are larger A client MAY discard or truncate received field lines that are larger
than the client wishes to process if the field semantics are such than the client wishes to process if the field semantics are such
that the dropped value(s) can be safely ignored without changing the that the dropped value(s) can be safely ignored without changing the
message framing or response semantics. message framing or response semantics.
4.3. Field Names 5.3. Field Names
The field-name token labels the corresponding field value as having The field-name token labels the corresponding field value as having
the semantics defined by that field. For example, the Date header the semantics defined by that field. For example, the Date header
field is defined in Section 10.1.1.2 as containing the origination field is defined in Section 11.1.1 as containing the origination
timestamp for the message in which it appears. timestamp for the message in which it appears.
field-name = token field-name = token
Field names are case-insensitive and ought to be registered within Field names are case-insensitive and ought to be registered within
the "Hypertext Transfer Protocol (HTTP) Field Name Registry"; see the "Hypertext Transfer Protocol (HTTP) Field Name Registry"; see
Section 4.3.2. Section 5.3.2.
Authors of specifications defining new fields are advised to choose a Authors of specifications defining new fields are advised to choose a
short but descriptive field name. Short names avoid needless data short but descriptive field name. Short names avoid needless data
transmission; descriptive names avoid confusion and "squatting" on transmission; descriptive names avoid confusion and "squatting" on
names that might have broader uses. names that might have broader uses.
To that end, limited-use fields (such as a header confined to a To that end, limited-use fields (such as a header confined to a
single application or use case) are encouraged to use a name that single application or use case) are encouraged to use a name that
includes its name (or an abbreviation) as a prefix; for example, if includes its name (or an abbreviation) as a prefix; for example, if
the Foo Application needs a Description field, it might use "Foo- the Foo Application needs a Description field, it might use "Foo-
skipping to change at page 27, line 46 skipping to change at page 28, line 44
SHOULD begin with an alphanumeric character. SHOULD begin with an alphanumeric character.
Field names ought not be prefixed with "X-"; see [BCP178] for further Field names ought not be prefixed with "X-"; see [BCP178] for further
information. information.
Other prefixes are sometimes used in HTTP field names; for example, Other prefixes are sometimes used in HTTP field names; for example,
"Accept-" is used in many content negotiation headers. These "Accept-" is used in many content negotiation headers. These
prefixes are only an aid to recognizing the purpose of a field, and prefixes are only an aid to recognizing the purpose of a field, and
do not trigger automatic processing. do not trigger automatic processing.
4.3.1. Field Extensibility 5.3.1. Field Extensibility
There is no limit on the introduction of new field names, each There is no limit on the introduction of new field names, each
presumably defining new semantics. presumably defining new semantics.
New fields can be defined such that, when they are understood by a New fields can be defined such that, when they are understood by a
recipient, they might override or enhance the interpretation of recipient, they might override or enhance the interpretation of
previously defined fields, define preconditions on request previously defined fields, define preconditions on request
evaluation, or refine the meaning of responses. evaluation, or refine the meaning of responses.
A proxy MUST forward unrecognized header fields unless the field name A proxy MUST forward unrecognized header fields unless the field name
is listed in the Connection header field (Section 9.1 of [Messaging]) is listed in the Connection header field (Section 9.1 of [Messaging])
or the proxy is specifically configured to block, or otherwise or the proxy is specifically configured to block, or otherwise
transform, such fields. Other recipients SHOULD ignore unrecognized transform, such fields. Other recipients SHOULD ignore unrecognized
header and trailer fields. These requirements allow HTTP's header and trailer fields. These requirements allow HTTP's
functionality to be enhanced without requiring prior update of functionality to be enhanced without requiring prior update of
deployed intermediaries. deployed intermediaries.
4.3.2. Field Name Registry 5.3.2. Field Name Registry
The "Hypertext Transfer Protocol (HTTP) Field Name Registry" defines The "Hypertext Transfer Protocol (HTTP) Field Name Registry" defines
the namespace for HTTP field names. the namespace for HTTP field names.
Any party can request registration of a HTTP field. See Section 4.7 Any party can request registration of a HTTP field. See Section 5.7
for considerations to take into account when creating a new HTTP for considerations to take into account when creating a new HTTP
field. field.
The "Hypertext Transfer Protocol (HTTP) Field Name Registry" is The "Hypertext Transfer Protocol (HTTP) Field Name Registry" is
located at "https://www.iana.org/assignments/http-fields/". located at <https://www.iana.org/assignments/http-fields/>.
Registration requests can be made by following the instructions Registration requests can be made by following the instructions
located there or by sending an email to the "ietf-http-wg@ietf.org" located there or by sending an email to the "ietf-http-wg@ietf.org"
mailing list. mailing list.
Field names are registered on the advice of a Designated Expert Field names are registered on the advice of a Designated Expert
(appointed by the IESG or their delegate). Fields with the status (appointed by the IESG or their delegate). Fields with the status
'permanent' are Specification Required (using terminology from 'permanent' are Specification Required ([RFC8126], Section 4.6).
[RFC8126]).
Registration requests consist of at least the following information: Registration requests consist of at least the following information:
o Field name: The requested field name. It MUST conform to the Field name:
field-name syntax defined in Section 4.3, and SHOULD be restricted The requested field name. It MUST conform to the field-name
to just letters, digits, hyphen ('-') and underscore ('_') syntax defined in Section 5.3, and SHOULD be restricted to just
characters, with the first character being a letter. letters, digits, hyphen ('-') and underscore ('_') characters,
with the first character being a letter.
o Status: "permanent" or "provisional" Status:
"permanent" or "provisional".
o Specification document(s): Reference to the document that Specification document(s):
specifies the field, preferably including a URI that can be used Reference to the document that specifies the field, preferably
to retrieve a copy of the document. An indication of the relevant including a URI that can be used to retrieve a copy of the
section(s) can also be included, but is not required. document. An indication of the relevant section(s) can also be
included, but is not required.
And, optionally:
Comments: Additional information, such as about reserved entries.
The Expert(s) can define additional fields to be collected in the The Expert(s) can define additional fields to be collected in the
registry, in consultation with the community. registry, in consultation with the community.
Standards-defined names have a status of "permanent". Other names Standards-defined names have a status of "permanent". Other names
can also be registered as permanent, if the Expert(s) find that they can also be registered as permanent, if the Expert(s) find that they
are in use, in consultation with the community. Other names should are in use, in consultation with the community. Other names should
be registered as "provisional". be registered as "provisional".
Provisional entries can be removed by the Expert(s) if -- in Provisional entries can be removed by the Expert(s) if -- in
consultation with the community -- the Expert(s) find that they are consultation with the community -- the Expert(s) find that they are
not in use. The Experts can change a provisional entry's status to not in use. The Experts can change a provisional entry's status to
permanent at any time. permanent at any time.
Note that names can be registered by third parties (including the Note that names can be registered by third parties (including the
Expert(s)), if the Expert(s) determines that an unregistered name is Expert(s)), if the Expert(s) determines that an unregistered name is
widely deployed and not likely to be registered in a timely manner widely deployed and not likely to be registered in a timely manner
otherwise. otherwise.
4.4. Field Values 5.4. Field Values
HTTP field values typically have their syntax defined using ABNF HTTP field values typically have their syntax defined using ABNF
([RFC5234]), using the extension defined in Section 4.5 as necessary, ([RFC5234]), using the extension defined in Section 5.5 as necessary,
and are usually constrained to the range of US-ASCII characters. and are usually constrained to the range of US-ASCII characters.
Fields needing a greater range of characters can use an encoding such Fields needing a greater range of characters can use an encoding such
as the one defined in [RFC8187]. as the one defined in [RFC8187].
field-value = *field-content field-value = *field-content
field-content = field-vchar field-content = field-vchar
[ 1*( SP / HTAB / field-vchar ) field-vchar ] [ 1*( SP / HTAB / field-vchar ) field-vchar ]
field-vchar = VCHAR / obs-text field-vchar = VCHAR / obs-text
Historically, HTTP allowed field content with text in the ISO-8859-1 Historically, HTTP allowed field content with text in the ISO-8859-1
charset [ISO-8859-1], supporting other charsets only through use of charset [ISO-8859-1], supporting other charsets only through use of
[RFC2047] encoding. In practice, most HTTP field values use only a [RFC2047] encoding. In practice, most HTTP field values use only a
subset of the US-ASCII charset [USASCII]. Newly defined fields subset of the US-ASCII charset [USASCII]. Newly defined fields
SHOULD limit their values to US-ASCII octets. A recipient SHOULD SHOULD limit their values to US-ASCII octets. A recipient SHOULD
treat other octets in field content (obs-text) as opaque data. treat other octets in field content (obs-text) as opaque data.
Field values containing control (CTL) characters such as CR or LF are
invalid; recipients MUST either reject a field value containing
control characters, or convert them to SP before processing or
forwarding the message.
Leading and trailing whitespace in raw field values is removed upon Leading and trailing whitespace in raw field values is removed upon
field parsing (Section 5.1 of [Messaging]). Field definitions where field parsing (Section 5.1 of [Messaging]). Field definitions where
leading or trailing whitespace in values is significant will have to leading or trailing whitespace in values is significant will have to
use a container syntax such as quoted-string (Section 4.4.1.2). use a container syntax such as quoted-string (Section 5.4.1.2).
Because commas (",") are used as a generic delimiter between members Because commas (",") are used as a generic delimiter between members
of a list-based field value, they need to be treated with care if of a list-based field value, they need to be treated with care if
they are allowed as data within those members. Typically, list they are allowed as data within those members. Typically, list
members that might contain a comma are enclosed in a quoted-string. members that might contain a comma are enclosed in a quoted-string.
For example, a textual date and a URI (either of which might contain For example, a textual date and a URI (either of which might contain
a comma) could be safely carried in list-based field values like a comma) could be safely carried in list-based field values like
these: these:
Example-URI-Field: "http://example.com/a.html,foo", Example-URI-Field: "http://example.com/a.html,foo",
"http://without-a-comma.example.com/" "http://without-a-comma.example.com/"
Example-Date-Field: "Sat, 04 May 1996", "Wed, 14 Sep 2005" Example-Date-Field: "Sat, 04 May 1996", "Wed, 14 Sep 2005"
Note that double-quote delimiters almost always are used with the Note that double-quote delimiters almost always are used with the
quoted-string production; using a different syntax inside double- quoted-string production; using a different syntax inside double-
quotes will likely cause unnecessary confusion. quotes will likely cause unnecessary confusion.
Many fields (such as Content-Type, defined in Section 6.2.1) use a Many fields (such as Content-Type, defined in Section 7.2.1) use a
common syntax for parameters that allows both unquoted (token) and common syntax for parameters that allows both unquoted (token) and
quoted (quoted-string) syntax for a parameter value quoted (quoted-string) syntax for a parameter value
(Section 4.4.1.4). Use of common syntax allows recipients to reuse (Section 5.4.1.4). Use of common syntax allows recipients to reuse
existing parser components. When allowing both forms, the meaning of existing parser components. When allowing both forms, the meaning of
a parameter value ought to be the same whether it was received as a a parameter value ought to be the same whether it was received as a
token or a quoted string. token or a quoted string.
Historically, HTTP field values could be extended over multiple lines Historically, HTTP field values could be extended over multiple lines
by preceding each extra line with at least one space or horizontal by preceding each extra line with at least one space or horizontal
tab (obs-fold). This document assumes that any such obsolete line tab (obs-fold). This document assumes that any such obsolete line
folding has been replaced with one or more SP octets prior to folding has been replaced with one or more SP octets prior to
interpreting the field value, as described in Section 5.2 of interpreting the field value, as described in Section 5.2 of
[Messaging]. [Messaging].
This specification does not use ABNF rules to define each "Field Note: This specification does not use ABNF rules to define each
Name: Field Value" pair, as was done in earlier editions. "Field Name: Field Value" pair, as was done in earlier editions
Instead, this specification uses ABNF rules that are named (published before [RFC7230]). Instead, ABNF rules are named
according to each registered field name, wherein the rule defines according to each registered field name, wherein the rule defines
the valid grammar for that field's corresponding field values the valid grammar for that field's corresponding field values
(i.e., after the field value has been extracted by a generic field (i.e., after the field value has been extracted by a generic field
parser). parser).
4.4.1. Common Field Value Components 5.4.1. Common Field Value Components
Many HTTP field values are defined using common syntax components, Many HTTP field values are defined using common syntax components,
separated by whitespace or specific delimiting characters. separated by whitespace or specific delimiting characters.
Delimiters are chosen from the set of US-ASCII visual characters not Delimiters are chosen from the set of US-ASCII visual characters not
allowed in a token (DQUOTE and "(),/:;<=>?@[\]{}"). allowed in a token (DQUOTE and "(),/:;<=>?@[\]{}").
4.4.1.1. Tokens 5.4.1.1. Tokens
Tokens are short textual identifiers that do not include whitespace Tokens are short textual identifiers that do not include whitespace
or delimiters. or delimiters.
token = 1*tchar token = 1*tchar
tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*"
/ "+" / "-" / "." / "^" / "_" / "`" / "|" / "~" / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
/ DIGIT / ALPHA / DIGIT / ALPHA
; any VCHAR, except delimiters ; any VCHAR, except delimiters
4.4.1.2. Quoted Strings 5.4.1.2. Quoted Strings
A string of text is parsed as a single value if it is quoted using A string of text is parsed as a single value if it is quoted using
double-quote marks. double-quote marks.
quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
qdtext = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text qdtext = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text
obs-text = %x80-FF obs-text = %x80-FF
The backslash octet ("\") can be used as a single-octet quoting The backslash octet ("\") can be used as a single-octet quoting
mechanism within quoted-string and comment constructs. Recipients mechanism within quoted-string and comment constructs. Recipients
skipping to change at page 31, line 34 skipping to change at page 32, line 39
as if it were replaced by the octet following the backslash. as if it were replaced by the octet following the backslash.
quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text ) quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
A sender SHOULD NOT generate a quoted-pair in a quoted-string except A sender SHOULD NOT generate a quoted-pair in a quoted-string except
where necessary to quote DQUOTE and backslash octets occurring within where necessary to quote DQUOTE and backslash octets occurring within
that string. A sender SHOULD NOT generate a quoted-pair in a comment that string. A sender SHOULD NOT generate a quoted-pair in a comment
except where necessary to quote parentheses ["(" and ")"] and except where necessary to quote parentheses ["(" and ")"] and
backslash octets occurring within that comment. backslash octets occurring within that comment.
4.4.1.3. Comments 5.4.1.3. Comments
Comments can be included in some HTTP fields by surrounding the Comments can be included in some HTTP fields by surrounding the
comment text with parentheses. Comments are only allowed in fields comment text with parentheses. Comments are only allowed in fields
containing "comment" as part of their field value definition. containing "comment" as part of their field value definition.
comment = "(" *( ctext / quoted-pair / comment ) ")" comment = "(" *( ctext / quoted-pair / comment ) ")"
ctext = HTAB / SP / %x21-27 / %x2A-5B / %x5D-7E / obs-text ctext = HTAB / SP / %x21-27 / %x2A-5B / %x5D-7E / obs-text
4.4.1.4. Parameters 5.4.1.4. Parameters
A parameter is a name=value pair that is often defined within field A parameter is a name=value pair that is often defined within field
values as a common syntax for appending auxiliary information to an values as a common syntax for appending auxiliary information to an
item. Each parameter is usually delimited by an immediately item. Each parameter is usually delimited by an immediately
preceding semicolon. preceding semicolon.
parameter = parameter-name "=" parameter-value parameter = parameter-name "=" parameter-value
parameter-name = token parameter-name = token
parameter-value = ( token / quoted-string ) parameter-value = ( token / quoted-string )
Parameter names are case-insensitive. Parameter values might or Parameter names are case-insensitive. Parameter values might or
might not be case-sensitive, depending on the semantics of the might not be case-sensitive, depending on the semantics of the
parameter name. Examples of parameters and some equivalent forms can parameter name. Examples of parameters and some equivalent forms can
be seen in media types (Section 6.1.1) and the Accept header field be seen in media types (Section 7.1.1) and the Accept header field
(Section 8.4.1). (Section 9.4.1).
A parameter value that matches the token production can be A parameter value that matches the token production can be
transmitted either as a token or within a quoted-string. The quoted transmitted either as a token or within a quoted-string. The quoted
and unquoted values are equivalent. and unquoted values are equivalent.
Note: Parameters do not allow whitespace (not even "bad" Note: Parameters do not allow whitespace (not even "bad"
whitespace) around the "=" character. whitespace) around the "=" character.
4.5. ABNF List Extension: #rule 5.4.1.5. Date/Time Formats
Prior to 1995, there were three different formats commonly used by
servers to communicate timestamps. For compatibility with old
implementations, all three are defined here. The preferred format is
a fixed-length and single-zone subset of the date and time
specification used by the Internet Message Format [RFC5322].
HTTP-date = IMF-fixdate / obs-date
An example of the preferred format is
Sun, 06 Nov 1994 08:49:37 GMT ; IMF-fixdate
Examples of the two obsolete formats are
Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
A recipient that parses a timestamp value in an HTTP field MUST
accept all three HTTP-date formats. When a sender generates a field
that contains one or more timestamps defined as HTTP-date, the sender
MUST generate those timestamps in the IMF-fixdate format.
An HTTP-date value represents time as an instance of Coordinated
Universal Time (UTC). The first two formats indicate UTC by the
three-letter abbreviation for Greenwich Mean Time, "GMT", a
predecessor of the UTC name; values in the asctime format are assumed
to be in UTC. A sender that generates HTTP-date values from a local
clock ought to use NTP ([RFC5905]) or some similar protocol to
synchronize its clock to UTC.
Preferred format:
IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT
; fixed length/zone/capitalization subset of the format
; see Section 3.3 of [RFC5322]
day-name = %s"Mon" / %s"Tue" / %s"Wed"
/ %s"Thu" / %s"Fri" / %s"Sat" / %s"Sun"
date1 = day SP month SP year
; e.g., 02 Jun 1982
day = 2DIGIT
month = %s"Jan" / %s"Feb" / %s"Mar" / %s"Apr"
/ %s"May" / %s"Jun" / %s"Jul" / %s"Aug"
/ %s"Sep" / %s"Oct" / %s"Nov" / %s"Dec"
year = 4DIGIT
GMT = %s"GMT"
time-of-day = hour ":" minute ":" second
; 00:00:00 - 23:59:60 (leap second)
hour = 2DIGIT
minute = 2DIGIT
second = 2DIGIT
Obsolete formats:
obs-date = rfc850-date / asctime-date
rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT
date2 = day "-" month "-" 2DIGIT
; e.g., 02-Jun-82
day-name-l = %s"Monday" / %s"Tuesday" / %s"Wednesday"
/ %s"Thursday" / %s"Friday" / %s"Saturday" / %s"Sunday"
asctime-date = day-name SP date3 SP time-of-day SP year
date3 = month SP ( 2DIGIT / ( SP 1DIGIT ))
; e.g., Jun 2
HTTP-date is case sensitive. A sender MUST NOT generate additional
whitespace in an HTTP-date beyond that specifically included as SP in
the grammar. The semantics of day-name, day, month, year, and time-
of-day are the same as those defined for the Internet Message Format
constructs with the corresponding name ([RFC5322], Section 3.3).
Recipients of a timestamp value in rfc850-date format, which uses a
two-digit year, MUST interpret a timestamp that appears to be more
than 50 years in the future as representing the most recent year in
the past that had the same last two digits.
Recipients of timestamp values are encouraged to be robust in parsing
timestamps unless otherwise restricted by the field definition. For
example, messages are occasionally forwarded over HTTP from a non-
HTTP source that might generate any of the date and time
specifications defined by the Internet Message Format.
Note: HTTP requirements for the date/time stamp format apply only
to their usage within the protocol stream. Implementations are
not required to use these formats for user presentation, request
logging, etc.
5.5. ABNF List Extension: #rule
A #rule extension to the ABNF rules of [RFC5234] is used to improve A #rule extension to the ABNF rules of [RFC5234] is used to improve
readability in the definitions of some list-based field values. readability in the definitions of some list-based field values.
A construct "#" is defined, similar to "*", for defining comma- A construct "#" is defined, similar to "*", for defining comma-
delimited lists of elements. The full form is "<n>#<m>element" delimited lists of elements. The full form is "<n>#<m>element"
indicating at least <n> and at most <m> elements, each separated by a indicating at least <n> and at most <m> elements, each separated by a
single comma (",") and optional whitespace (OWS). single comma (",") and optional whitespace (OWS).
4.5.1. Sender Requirements 5.5.1. Sender Requirements
In any production that uses the list construct, a sender MUST NOT In any production that uses the list construct, a sender MUST NOT
generate empty list elements. In other words, a sender MUST generate generate empty list elements. In other words, a sender MUST generate
lists that satisfy the following syntax: lists that satisfy the following syntax:
1#element => element *( OWS "," OWS element ) 1#element => element *( OWS "," OWS element )
and: and:
#element => [ 1#element ] #element => [ 1#element ]
and for n >= 1 and m > 1: and for n >= 1 and m > 1:
<n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element ) <n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )
4.5.2. Recipient Requirements Appendix A shows the collected ABNF for senders after the list
constructs have been expanded.
5.5.2. Recipient Requirements
Empty elements do not contribute to the count of elements present. A Empty elements do not contribute to the count of elements present. A
recipient MUST parse and ignore a reasonable number of empty list recipient MUST parse and ignore a reasonable number of empty list
elements: enough to handle common mistakes by senders that merge elements: enough to handle common mistakes by senders that merge
values, but not so much that they could be used as a denial-of- values, but not so much that they could be used as a denial-of-
service mechanism. In other words, a recipient MUST accept lists service mechanism. In other words, a recipient MUST accept lists
that satisfy the following syntax: that satisfy the following syntax:
#element => [ element ] *( OWS "," OWS [ element ] ) #element => [ element ] *( OWS "," OWS [ element ] )
Note that because of the potential presence of empty list elements, Note that because of the potential presence of empty list elements,
the RFC 5234 ABNF cannot enforce the cardinality of list elements, the RFC 5234 ABNF cannot enforce the cardinality of list elements,
and consequently all cases are mapped is if there was no cardinality and consequently all cases are mapped is if there was no cardinality
specified. specified.
For example, given these ABNF productions: For example, given these ABNF productions:
example-list = 1#example-list-elmt example-list = 1#example-list-elmt
example-list-elmt = token ; see Section 4.4.1.1 example-list-elmt = token ; see Section 5.4.1.1
Then the following are valid values for example-list (not including Then the following are valid values for example-list (not including
the double quotes, which are present for delimitation only): the double quotes, which are present for delimitation only):
"foo,bar" "foo,bar"
"foo ,bar," "foo ,bar,"
"foo , ,bar,charlie" "foo , ,bar,charlie"
In contrast, the following values would be invalid, since at least In contrast, the following values would be invalid, since at least
one non-empty element is required by the example-list production: one non-empty element is required by the example-list production:
"" ""
"," ","
", ," ", ,"
Appendix A shows the collected ABNF for recipients after the list 5.6. Trailer Fields
constructs have been expanded.
4.6. Trailer Fields
4.6.1. Purpose 5.6.1. Purpose
In some HTTP versions, additional metadata can be sent after the In some HTTP versions, additional metadata can be sent after the
initial header section has been completed (during or after initial header section has been completed (during or after
transmission of the payload body), such as a message integrity check, transmission of the payload body), such as a message integrity check,
digital signature, or post-processing status. For example, the digital signature, or post-processing status. For example, the
chunked coding in HTTP/1.1 allows a trailer section after the payload chunked coding in HTTP/1.1 allows a trailer section after the payload
body (Section 7.1.2 of [Messaging]) which can contain trailer fields: body (Section 7.1.2 of [Messaging]) which can contain trailer fields:
field names and values that share the same syntax and namespace as field names and values that share the same syntax and namespace as
header fields but that are received after the header section. header fields but that are received after the header section.
Trailer fields ought to be processed and stored separately from the Trailer fields ought to be processed and stored separately from the
fields in the header section to avoid contradicting message semantics fields in the header section to avoid contradicting message semantics
known at the time the header section was complete. The presence or known at the time the header section was complete. The presence or
absence of certain header fields might impact choices made for the absence of certain header fields might impact choices made for the
routing or processing of the message as a whole before the trailers routing or processing of the message as a whole before the trailers
are received; those choices cannot be unmade by the later discovery are received; those choices cannot be unmade by the later discovery
of trailer fields. of trailer fields.
4.6.2. Limitations 5.6.2. Limitations
Many fields cannot be processed outside the header section because Many fields cannot be processed outside the header section because
their evaluation is necessary prior to receiving the message body, their evaluation is necessary prior to receiving the message body,
such as those that describe message framing, routing, authentication, such as those that describe message framing, routing, authentication,
request modifiers, response controls, or payload format. A sender request modifiers, response controls, or payload format. A sender
MUST NOT generate a trailer field unless the sender knows the MUST NOT generate a trailer field unless the sender knows the
corresponding header field name's definition permits the field to be corresponding header field name's definition permits the field to be
sent in trailers. sent in trailers.
Trailer fields can be difficult to process by intermediaries that Trailer fields can be difficult to process by intermediaries that
skipping to change at page 34, line 41 skipping to change at page 37, line 47
downstream clients are willing to accept trailer fields in the downstream clients are willing to accept trailer fields in the
forwarded response. Note that the presence of "trailers" does not forwarded response. Note that the presence of "trailers" does not
mean that the client(s) will process any particular trailer field in mean that the client(s) will process any particular trailer field in
the response; only that the trailer section as a whole will not be the response; only that the trailer section as a whole will not be
dropped by any of the clients. dropped by any of the clients.
Because of the potential for trailer fields to be discarded in Because of the potential for trailer fields to be discarded in
transit, a server SHOULD NOT generate trailer fields that it believes transit, a server SHOULD NOT generate trailer fields that it believes
are necessary for the user agent to receive. are necessary for the user agent to receive.
4.6.3. Trailer 5.6.3. Trailer
The "Trailer" header field provides a list of field names that the The "Trailer" header field provides a list of field names that the
sender anticipates sending as trailer fields within that message. sender anticipates sending as trailer fields within that message.
This allows a recipient to prepare for receipt of the indicated This allows a recipient to prepare for receipt of the indicated
metadata before it starts processing the body. metadata before it starts processing the body.
Trailer = 1#field-name Trailer = 1#field-name
For example, a sender might indicate that a message integrity check For example, a sender might indicate that a message integrity check
will be computed as the payload is being streamed and provide the will be computed as the payload is being streamed and provide the
final signature as a trailer field. This allows a recipient to final signature as a trailer field. This allows a recipient to
perform the same check on the fly as the payload data is received. perform the same check on the fly as the payload data is received.
A sender that intends to generate one or more trailer fields in a A sender that intends to generate one or more trailer fields in a
message SHOULD generate a Trailer header field in the header section message SHOULD generate a Trailer header field in the header section
of that message to indicate which fields might be present in the of that message to indicate which fields might be present in the
trailers. trailers.
4.7. Considerations for New HTTP Fields 5.7. Considerations for New HTTP Fields
See Section 4.3 for a general requirements for field names, and See Section 5.3 for a general requirements for field names, and
Section 4.4 for a discussion of field values. Section 5.4 for a discussion of field values.
Authors of specifications defining new fields are advised to consider Authors of specifications defining new fields are advised to consider
documenting: documenting:
o Whether the field is a single value or whether it can be a list o Whether the field is a single value or whether it can be a list
(delimited by commas; see Section 4.4). (delimited by commas; see Section 5.4).
If it is not a list, document how to treat messages where the If it is not a list, document how to treat messages where the
field occurs multiple times (a sensible default would be to ignore field occurs multiple times (a sensible default would be to ignore
the field, but this might not always be the right choice). the field, but this might not always be the right choice).
Note that intermediaries and software libraries might combine Note that intermediaries and software libraries might combine
multiple field instances into a single one, despite the field's multiple field instances into a single one, despite the field's
definition not allowing the list syntax. A robust format enables definition not allowing the list syntax. A robust format enables
recipients to discover these situations (good example: "Content- recipients to discover these situations (good example: "Content-
Type", as the comma can only appear inside quoted strings; bad Type", as the comma can only appear inside quoted strings; bad
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o Whether it is appropriate to list the field name in the Connection o Whether it is appropriate to list the field name in the Connection
header field (i.e., if the field is to be hop-by-hop; see header field (i.e., if the field is to be hop-by-hop; see
Section 9.1 of [Messaging]). Section 9.1 of [Messaging]).
o Under what conditions intermediaries are allowed to insert, o Under what conditions intermediaries are allowed to insert,
delete, or modify the field's value. delete, or modify the field's value.
o Whether it is appropriate to list the field name in a Vary o Whether it is appropriate to list the field name in a Vary
response header field (e.g., when the request header field is used response header field (e.g., when the request header field is used
by an origin server's content selection algorithm; see by an origin server's content selection algorithm; see
Section 10.1.4). Section 11.1.4).
o Whether the field is allowable in trailers (see Section 4.6). o Whether the field is allowable in trailers (see Section 5.6).
o Whether the field ought to be preserved across redirects. o Whether the field ought to be preserved across redirects.
o Whether it introduces any additional security considerations, such o Whether it introduces any additional security considerations, such
as disclosure of privacy-related data. as disclosure of privacy-related data.
4.8. Fields Defined In This Document 5.8. Fields Defined In This Document
The following fields are defined by this document: The following fields are defined by this document:
+---------------------------+------------+-------------------+ +---------------------------+------------+-----------------+
| Field Name | Status | Reference | | Field Name | Status | Reference |
+---------------------------+------------+-------------------+ +---------------------------+------------+-----------------+
| Accept | standard | Section 8.4.1 | | Accept | standard | Section 9.4.1 |
| Accept-Charset | deprecated | Section 8.4.2 | | Accept-Charset | deprecated | Section 9.4.2 |
| Accept-Encoding | standard | Section 8.4.3 | | Accept-Encoding | standard | Section 9.4.3 |
| Accept-Language | standard | Section 8.4.4 | | Accept-Language | standard | Section 9.4.4 |
| Accept-Ranges | standard | Section 10.4.1 | | Accept-Ranges | standard | Section 11.4.1 |
| Allow | standard | Section 10.4.2 | | Allow | standard | Section 11.4.2 |
| Authentication-Info | standard | Section 10.3.3 | | Authentication-Info | standard | Section 11.3.3 |
| Authorization | standard | Section 8.5.3 | | Authorization | standard | Section 9.5.3 |
| Content-Encoding | standard | Section 6.2.2 | | Content-Encoding | standard | Section 7.2.2 |
| Content-Language | standard | Section 6.2.3 | | Content-Language | standard | Section 7.2.3 |
| Content-Length | standard | Section 6.2.4 | | Content-Length | standard | Section 7.2.4 |
| Content-Location | standard | Section 6.2.5 | | Content-Location | standard | Section 7.2.5 |
| Content-Range | standard | Section 6.3.4 | | Content-Range | standard | Section 7.3.4 |
| Content-Type | standard | Section 6.2.1 | | Content-Type | standard | Section 7.2.1 |
| Date | standard | Section 10.1.1.2 | | Date | standard | Section 11.1.1 |
| ETag | standard | Section 10.2.3 | | ETag | standard | Section 11.2.3 |
| Expect | standard | Section 8.1.1 | | Expect | standard | Section 9.1.1 |
| From | standard | Section 8.6.1 | | From | standard | Section 9.6.1 |
| Host | standard | Section 5.6 | | Host | standard | Section 6.6 |
| If-Match | standard | Section 8.2.3 | | If-Match | standard | Section 9.2.3 |
| If-Modified-Since | standard | Section 8.2.5 | | If-Modified-Since | standard | Section 9.2.5 |
| If-None-Match | standard | Section 8.2.4 | | If-None-Match | standard | Section 9.2.4 |
| If-Range | standard | Section 8.2.7 | | If-Range | standard | Section 9.2.7 |
| If-Unmodified-Since | standard | Section 8.2.6 | | If-Unmodified-Since | standard | Section 9.2.6 |
| Last-Modified | standard | Section 10.2.2 | | Last-Modified | standard | Section 11.2.2 |
| Location | standard | Section 10.1.2 | | Location | standard | Section 11.1.2 |
| Max-Forwards | standard | Section 8.1.2 | | Max-Forwards | standard | Section 9.1.2 |
| Proxy-Authenticate | standard | Section 10.3.2 | | Proxy-Authenticate | standard | Section 11.3.2 |
| Proxy-Authentication-Info | standard | Section 10.3.4 | | Proxy-Authentication-Info | standard | Section 11.3.4 |
| Proxy-Authorization | standard | Section 8.5.4 | | Proxy-Authorization | standard | Section 9.5.4 |
| Range | standard | Section 8.3 | | Range | standard | Section 9.3 |
| Referer | standard | Section 8.6.2 | | Referer | standard | Section 9.6.2 |
| Retry-After | standard | Section 10.1.3 | | Retry-After | standard | Section 11.1.3 |
| Server | standard | Section 10.4.3 | | Server | standard | Section 11.4.3 |
| Trailer | standard | Section 4.6.3 | | Trailer | standard | Section 5.6.3 |
| User-Agent | standard | Section 8.6.3 | | User-Agent | standard | Section 9.6.3 |
| Vary | standard | Section 10.1.4 | | Vary | standard | Section 11.1.4 |
| Via | standard | Section 5.7.1 | | Via | standard | Section 6.7.1 |
| WWW-Authenticate | standard | Section 10.3.1 | | WWW-Authenticate | standard | Section 11.3.1 |
+---------------------------+------------+-------------------+ +---------------------------+------------+-----------------+
Table 1 Table 1
5. Message Routing Furthermore, the field name "*" is reserved, since using that name as
an HTTP header field might conflict with its special semantics in the
Vary header field (Section 11.1.4).
+------------+----------+--------------+-------------+
| Field Name | Status | Reference | Comments |
+------------+----------+--------------+-------------+
| * | standard | Section 5.8 | (reserved) |
+------------+----------+--------------+-------------+
6. Message Routing
HTTP request message routing is determined by each client based on HTTP request message routing is determined by each client based on
the target resource, the client's proxy configuration, and the target resource, the client's proxy configuration, and
establishment or reuse of an inbound connection. The corresponding establishment or reuse of an inbound connection. The corresponding
response routing follows the same connection chain back to the response routing follows the same connection chain back to the
client. client.
5.1. Identifying a Target Resource 6.1. Identifying a Target Resource
HTTP is used in a wide variety of applications, ranging from general- HTTP is used in a wide variety of applications, ranging from general-
purpose computers to home appliances. In some cases, communication purpose computers to home appliances. In some cases, communication
options are hard-coded in a client's configuration. However, most options are hard-coded in a client's configuration. However, most
HTTP clients rely on the same resource identification mechanism and HTTP clients rely on the same resource identification mechanism and
configuration techniques as general-purpose Web browsers. configuration techniques as general-purpose Web browsers.
HTTP communication is initiated by a user agent for some purpose. HTTP communication is initiated by a user agent for some purpose.
The purpose is a combination of request semantics and a target The purpose is a combination of request semantics and a target
resource upon which to apply those semantics. The "request target" resource upon which to apply those semantics. The "request target"
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Typically, the request target is a URI reference (Section 2.4) which Typically, the request target is a URI reference (Section 2.4) which
a user agent would resolve to its absolute form in order to obtain a user agent would resolve to its absolute form in order to obtain
the "target URI". The target URI excludes the reference's fragment the "target URI". The target URI excludes the reference's fragment
component, if any, since fragment identifiers are reserved for component, if any, since fragment identifiers are reserved for
client-side processing ([RFC3986], Section 3.5). client-side processing ([RFC3986], Section 3.5).
However, there are two special, method-specific forms allowed for the However, there are two special, method-specific forms allowed for the
request target in specific circumstances: request target in specific circumstances:
o For CONNECT (Section 7.3.6), the request target is the host name o For CONNECT (Section 8.3.6), the request target is the host name
and port number of the tunnel destination, separated by a colon. and port number of the tunnel destination, separated by a colon.
o For OPTIONS (Section 7.3.7), the request target can be a single o For OPTIONS (Section 8.3.7), the request target can be a single
asterisk ("*"). asterisk ("*").
See the respective method definitions for details. These forms MUST See the respective method definitions for details. These forms MUST
NOT be used with other methods. NOT be used with other methods.
5.2. Determining Origin 6.2. Determining Origin
The "origin" for a given URI is the triple of scheme, host, and port The "origin" for a given URI is the triple of scheme, host, and port
after normalizing the scheme and host to lowercase and normalizing after normalizing the scheme and host to lowercase and normalizing
the port to remove any leading zeros. If port is elided from the the port to remove any leading zeros. If port is elided from the
URI, the default port for that scheme is used. For example, the URI URI, the default port for that scheme is used. For example, the URI
https://Example.Com/happy.js https://Example.Com/happy.js
would have the origin would have the origin
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future. future.
Two origins are distinct if they differ in scheme, host, or port. Two origins are distinct if they differ in scheme, host, or port.
Even when it can be verified that the same entity controls two Even when it can be verified that the same entity controls two
distinct origins, the two namespaces under those origins are distinct distinct origins, the two namespaces under those origins are distinct
unless explicitly aliased by a server authoritative for that origin. unless explicitly aliased by a server authoritative for that origin.
Origin is also used within HTML and related Web protocols, beyond the Origin is also used within HTML and related Web protocols, beyond the
scope of this document, as described in [RFC6454]. scope of this document, as described in [RFC6454].
5.3. Routing Inbound 6.3. Routing Inbound
Once the target URI and its origin are determined, a client decides Once the target URI and its origin are determined, a client decides
whether a network request is necessary to accomplish the desired whether a network request is necessary to accomplish the desired
semantics and, if so, where that request is to be directed. semantics and, if so, where that request is to be directed.
If the client has a cache [Caching] and the request can be satisfied If the client has a cache [Caching] and the request can be satisfied
by it, then the request is usually directed there first. by it, then the request is usually directed there first.
If the request is not satisfied by a cache, then a typical client If the request is not satisfied by a cache, then a typical client
will check its configuration to determine whether a proxy is to be will check its configuration to determine whether a proxy is to be
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to that proxy. to that proxy.
If no proxy is applicable, a typical client will invoke a handler If no proxy is applicable, a typical client will invoke a handler
routine, usually specific to the target URI's scheme, to connect routine, usually specific to the target URI's scheme, to connect
directly to an origin for the target resource. How that is directly to an origin for the target resource. How that is
accomplished is dependent on the target URI scheme and defined by its accomplished is dependent on the target URI scheme and defined by its
associated specification, similar to how this specification defines associated specification, similar to how this specification defines
origin server access for resolution of the "http" (Section 2.5.1) and origin server access for resolution of the "http" (Section 2.5.1) and
"https" (Section 2.5.2) schemes. "https" (Section 2.5.2) schemes.
5.4. Direct Authoritative Access 6.4. Direct Authoritative Access
5.4.1. http origins 6.4.1. http origins
Although HTTP is independent of the transport protocol, the "http" Although HTTP is independent of the transport protocol, the "http"
scheme is specific to associating authority with whomever controls scheme is specific to associating authority with whomever controls
the origin server listening for TCP connections on the indicated port the origin server listening for TCP connections on the indicated port
of whatever host is identified within the authority component. This of whatever host is identified within the authority component. This
is a very weak sense of authority because it depends on both client- is a very weak sense of authority because it depends on both client-
specific name resolution mechanisms and communication that might not specific name resolution mechanisms and communication that might not
be secured from man-in-the-middle attacks. Nevertheless, it is a be secured from man-in-the-middle attacks. Nevertheless, it is a
sufficient minimum for binding "http" identifiers to an origin server sufficient minimum for binding "http" identifiers to an origin server
for consistent resolution within a trusted environment. for consistent resolution within a trusted environment.
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DNS, to find an address for an appropriate origin server. DNS, to find an address for an appropriate origin server.
When an "http" URI is used within a context that calls for access to When an "http" URI is used within a context that calls for access to
the indicated resource, a client MAY attempt access by resolving the the indicated resource, a client MAY attempt access by resolving the
host identifier to an IP address, establishing a TCP connection to host identifier to an IP address, establishing a TCP connection to
that address on the indicated port, and sending an HTTP request that address on the indicated port, and sending an HTTP request
message to the server containing the URI's identifying data message to the server containing the URI's identifying data
(Section 2 of [Messaging]). (Section 2 of [Messaging]).
If the server responds to such a request with a non-interim HTTP If the server responds to such a request with a non-interim HTTP
response message, as described in Section 9, then that response is response message, as described in Section 10, then that response is
considered an authoritative answer to the client's request. considered an authoritative answer to the client's request.
Note, however, that the above is not the only means for obtaining an Note, however, that the above is not the only means for obtaining an
authoritative response, nor does it imply that an authoritative authoritative response, nor does it imply that an authoritative
response is always necessary (see [Caching]). For example, the Alt- response is always necessary (see [Caching]). For example, the Alt-
Svc header field [RFC7838] allows an origin server to identify other Svc header field [RFC7838] allows an origin server to identify other
services that are also authoritative for that origin. Access to services that are also authoritative for that origin. Access to
"http" identified resources might also be provided by protocols "http" identified resources might also be provided by protocols
outside the scope of this document. outside the scope of this document.
See Section 11.1 for security considerations related to establishing See Section 12.1 for security considerations related to establishing
authority. authority.
5.4.2. https origins 6.4.2. https origins
The "https" scheme associates authority based on the ability of a The "https" scheme associates authority based on the ability of a
server to use a private key associated with a certificate that the server to use a private key associated with a certificate that the
client considers to be trustworthy for the identified host. If a client considers to be trustworthy for the identified host. If a
server presents a certificate that verifiably applies to the host, server presents a certificate that verifiably applies to the host,
along with proof that it controls the corresponding private key, then along with proof that it controls the corresponding private key, then
a client will accept a secured connection to that server as being a client will accept a secured connection to that server as being
authoritative for all origins with the same scheme and host. authoritative for all origins with the same scheme and host.
A client is therefore relying upon a chain of trust, conveyed from A client is therefore relying upon a chain of trust, conveyed from
some trust anchor (which is usually prearranged or configured), some trust anchor (which is usually prearranged or configured),
through a chain of certificates (e.g., [RFC5280]) to a final through a chain of certificates (e.g., [RFC5280]) to a final
certificate that binds a private key to the host name of the origin. certificate that binds a private key to the host name of the origin.
The handshake and certificate validation in Section 5.4.3 describe The handshake and certificate validation in Section 6.4.3 describe
how that final certificate can be used to initiate a secured how that final certificate can be used to initiate a secured
connection. connection.
Note that the "https" scheme does not rely on TCP and the connected Note that the "https" scheme does not rely on TCP and the connected
port number for associating authority, since both are outside the port number for associating authority, since both are outside the
secured communication and thus cannot be trusted as definitive. secured communication and thus cannot be trusted as definitive.
Hence, the HTTP communication might take place over any channel that Hence, the HTTP communication might take place over any channel that
has been secured, as defined in Section 2.5.2, including protocols has been secured, as defined in Section 2.5.2, including protocols
that don't use TCP. It is the origin's responsibility to ensure that that don't use TCP. It is the origin's responsibility to ensure that
any services provided with control over its certificate's private key any services provided with control over its certificate's private key
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When an "https" URI is used within a context that calls for access to When an "https" URI is used within a context that calls for access to
the indicated resource, a client MAY attempt access by resolving the the indicated resource, a client MAY attempt access by resolving the
host identifier to an IP address, establishing a TCP connection to host identifier to an IP address, establishing a TCP connection to
that address on the indicated port, securing the connection end-to- that address on the indicated port, securing the connection end-to-
end by successfully initiating TLS over TCP with confidentiality and end by successfully initiating TLS over TCP with confidentiality and
integrity protection, and sending an HTTP request message to the integrity protection, and sending an HTTP request message to the
server over that secured connection containing the URI's identifying server over that secured connection containing the URI's identifying
data (Section 2 of [Messaging]). data (Section 2 of [Messaging]).
If the server responds to such a request with a non-interim HTTP If the server responds to such a request with a non-interim HTTP
response message, as described in Section 9, then that response is response message, as described in Section 10, then that response is
considered an authoritative answer to the client's request. considered an authoritative answer to the client's request.
Note, however, that the above is not the only means for obtaining an Note, however, that the above is not the only means for obtaining an
authoritative response, nor does it imply that an authoritative authoritative response, nor does it imply that an authoritative
response is always necessary (see [Caching]). response is always necessary (see [Caching]).
5.4.3. Initiating HTTP Over TLS 6.4.3. Initiating HTTP Over TLS
Conceptually, HTTP/TLS is very simple. Simply use HTTP over TLS Conceptually, HTTP/TLS is very simple. Simply use HTTP over TLS
precisely as you would use HTTP over TCP. precisely as you would use HTTP over TCP.
The agent acting as the HTTP client should also act as the TLS The agent acting as the HTTP client should also act as the TLS
client. It should initiate a connection to the server on the client. It should initiate a connection to the server on the
appropriate port and then send the TLS ClientHello to begin the TLS appropriate port and then send the TLS ClientHello to begin the TLS
handshake. When the TLS handshake has finished. The client may then handshake. When the TLS handshake has finished. The client may then
initiate the first HTTP request. All HTTP data MUST be sent as TLS initiate the first HTTP request. All HTTP data MUST be sent as TLS
"application data". Normal HTTP behavior, including retained "application data". Normal HTTP behavior, including retained
connections should be followed. connections should be followed.
5.4.3.1. Identifying HTTPS Servers 6.4.3.1. Identifying HTTPS Servers
In general, HTTP/TLS requests are generated by dereferencing a URI. In general, HTTP/TLS requests are generated by dereferencing a URI.
As a consequence, the hostname for the server is known to the client. As a consequence, the hostname for the server is known to the client.
If the hostname is available, the client MUST check it against the If the hostname is available, the client MUST check it against the
server's identity as presented in the server's Certificate message, server's identity as presented in the server's Certificate message,
in order to prevent man-in-the-middle attacks. in order to prevent man-in-the-middle attacks.
If the client has external information as to the expected identity of If the client has external information as to the expected identity of
the server, the hostname check MAY be omitted. (For instance, a the server, the hostname check MAY be omitted. (For instance, a
client may be connecting to a machine whose address and hostname are client may be connecting to a machine whose address and hostname are
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Note that in many cases the URI itself comes from an untrusted Note that in many cases the URI itself comes from an untrusted
source. The above-described check provides no protection against source. The above-described check provides no protection against
attacks where this source is compromised. For example, if the URI attacks where this source is compromised. For example, if the URI
was obtained by clicking on an HTML page which was itself obtained was obtained by clicking on an HTML page which was itself obtained
without using HTTP/TLS, a man in the middle could have replaced the without using HTTP/TLS, a man in the middle could have replaced the
URI. In order to prevent this form of attack, users should carefully URI. In order to prevent this form of attack, users should carefully
examine the certificate presented by the server to determine if it examine the certificate presented by the server to determine if it
meets their expectations. meets their expectations.
5.4.3.2. Identifying HTTPS Clients 6.4.3.2. Identifying HTTPS Clients
Typically, the server has no external knowledge of what the client's Typically, the server has no external knowledge of what the client's
identity ought to be and so checks (other than that the client has a identity ought to be and so checks (other than that the client has a
certificate chain rooted in an appropriate CA) are not possible. If certificate chain rooted in an appropriate CA) are not possible. If
a server has such knowledge (typically from some source external to a server has such knowledge (typically from some source external to
HTTP or TLS) it SHOULD check the identity as described above. HTTP or TLS) it SHOULD check the identity as described above.
5.5. Reconstructing the Target URI 6.5. Reconstructing the Target URI
Once an inbound connection is obtained, the client sends an HTTP Once an inbound connection is obtained, the client sends an HTTP
request message (Section 2 of [Messaging]). request message (Section 2 of [Messaging]).
Depending on the nature of the request, the client's target URI might Depending on the nature of the request, the client's target URI might
be split into components and transmitted (or implied) within various be split into components and transmitted (or implied) within various
parts of a request message. These parts are recombined by each parts of a request message. These parts are recombined by each
recipient, in accordance with their local configuration and incoming recipient, in accordance with their local configuration and incoming
connection context, to determine the target URI. Appendix of connection context, to determine the target URI. Appendix of
[Messaging] defines how a server determines the target URI for an [Messaging] defines how a server determines the target URI for an
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Once the target URI has been reconstructed, an origin server needs to Once the target URI has been reconstructed, an origin server needs to
decide whether or not to provide service for that URI via the decide whether or not to provide service for that URI via the
connection in which the request was received. For example, the connection in which the request was received. For example, the
request might have been misdirected, deliberately or accidentally, request might have been misdirected, deliberately or accidentally,
such that the information within a received Host header field differs such that the information within a received Host header field differs
from the host or port upon which the connection has been made. If from the host or port upon which the connection has been made. If
the connection is from a trusted gateway, that inconsistency might be the connection is from a trusted gateway, that inconsistency might be
expected; otherwise, it might indicate an attempt to bypass security expected; otherwise, it might indicate an attempt to bypass security
filters, trick the server into delivering non-public content, or filters, trick the server into delivering non-public content, or
poison a cache. See Section 11 for security considerations regarding poison a cache. See Section 12 for security considerations regarding
message routing. message routing.
Note: previous specifications defined the recomposed target URI as Note: previous specifications defined the recomposed target URI as
a distinct concept, the effective request URI. a distinct concept, the effective request URI.
5.6. Host 6.6. Host
The "Host" header field in a request provides the host and port The "Host" header field in a request provides the host and port
information from the target URI, enabling the origin server to information from the target URI, enabling the origin server to
distinguish among resources while servicing requests for multiple distinguish among resources while servicing requests for multiple
host names on a single IP address. host names on a single IP address.
Host = uri-host [ ":" port ] ; Section 2.4 Host = uri-host [ ":" port ] ; Section 2.4
A client MUST send a Host header field in all HTTP/1.1 request A client MUST send a Host header field in all HTTP/1.1 request
messages. If the target URI includes an authority component, then a messages. If the target URI includes an authority component, then a
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Host field value for redirecting requests to internal servers, or for Host field value for redirecting requests to internal servers, or for
use as a cache key in a shared cache, without first verifying that use as a cache key in a shared cache, without first verifying that
the intercepted connection is targeting a valid IP address for that the intercepted connection is targeting a valid IP address for that
host. host.
A server MUST respond with a 400 (Bad Request) status code to any A server MUST respond with a 400 (Bad Request) status code to any
HTTP/1.1 request message that lacks a Host header field and to any HTTP/1.1 request message that lacks a Host header field and to any
request message that contains more than one Host header field or a request message that contains more than one Host header field or a
Host header field with an invalid field value. Host header field with an invalid field value.
5.7. Message Forwarding 6.7. Message Forwarding
As described in Section 2.2, intermediaries can serve a variety of As described in Section 2.2, intermediaries can serve a variety of
roles in the processing of HTTP requests and responses. Some roles in the processing of HTTP requests and responses. Some
intermediaries are used to improve performance or availability. intermediaries are used to improve performance or availability.
Others are used for access control or to filter content. Since an Others are used for access control or to filter content. Since an
HTTP stream has characteristics similar to a pipe-and-filter HTTP stream has characteristics similar to a pipe-and-filter
architecture, there are no inherent limits to the extent an architecture, there are no inherent limits to the extent an
intermediary can enhance (or interfere) with either direction of the intermediary can enhance (or interfere) with either direction of the
stream. stream.
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ought to recognize its own server names, including any aliases, local ought to recognize its own server names, including any aliases, local
variations, or literal IP addresses, and respond to such requests variations, or literal IP addresses, and respond to such requests
directly. directly.
An HTTP message can be parsed as a stream for incremental processing An HTTP message can be parsed as a stream for incremental processing
or forwarding downstream. However, recipients cannot rely on or forwarding downstream. However, recipients cannot rely on
incremental delivery of partial messages, since some implementations incremental delivery of partial messages, since some implementations
will buffer or delay message forwarding for the sake of network will buffer or delay message forwarding for the sake of network
efficiency, security checks, or payload transformations. efficiency, security checks, or payload transformations.
5.7.1. Via 6.7.1. Via
The "Via" header field indicates the presence of intermediate The "Via" header field indicates the presence of intermediate
protocols and recipients between the user agent and the server (on protocols and recipients between the user agent and the server (on
requests) or between the origin server and the client (on responses), requests) or between the origin server and the client (on responses),
similar to the "Received" header field in email (Section 3.6.7 of similar to the "Received" header field in email (Section 3.6.7 of
[RFC5322]). Via can be used for tracking message forwards, avoiding [RFC5322]). Via can be used for tracking message forwards, avoiding
request loops, and identifying the protocol capabilities of senders request loops, and identifying the protocol capabilities of senders
along the request/response chain. along the request/response chain.
Via = 1#( received-protocol RWS received-by [ RWS comment ] ) Via = 1#( received-protocol RWS received-by [ RWS comment ] )
skipping to change at page 46, line 47 skipping to change at page 50, line 11
MUST send an appropriate Via header field in each inbound request MUST send an appropriate Via header field in each inbound request
message and MAY send a Via header field in forwarded response message and MAY send a Via header field in forwarded response
messages. messages.
For each intermediary, the received-protocol indicates the protocol For each intermediary, the received-protocol indicates the protocol
and protocol version used by the upstream sender of the message. and protocol version used by the upstream sender of the message.
Hence, the Via field value records the advertised protocol Hence, the Via field value records the advertised protocol
capabilities of the request/response chain such that they remain capabilities of the request/response chain such that they remain
visible to downstream recipients; this can be useful for determining visible to downstream recipients; this can be useful for determining
what backwards-incompatible features might be safe to use in what backwards-incompatible features might be safe to use in
response, or within a later request, as described in Section 3.5. response, or within a later request, as described in Section 4.2.
For brevity, the protocol-name is omitted when the received protocol For brevity, the protocol-name is omitted when the received protocol
is HTTP. is HTTP.
The received-by portion is normally the host and optional port number The received-by portion is normally the host and optional port number
of a recipient server or client that subsequently forwarded the of a recipient server or client that subsequently forwarded the
message. However, if the real host is considered to be sensitive message. However, if the real host is considered to be sensitive
information, a sender MAY replace it with a pseudonym. If a port is information, a sender MAY replace it with a pseudonym. If a port is
not provided, a recipient MAY interpret that as meaning it was not provided, a recipient MAY interpret that as meaning it was
received on the default TCP port, if any, for the received-protocol. received on the default TCP port, if any, for the received-protocol.
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could be collapsed to could be collapsed to
Via: 1.0 ricky, 1.1 mertz, 1.0 lucy Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
A sender SHOULD NOT combine multiple list members unless they are all A sender SHOULD NOT combine multiple list members unless they are all
under the same organizational control and the hosts have already been under the same organizational control and the hosts have already been
replaced by pseudonyms. A sender MUST NOT combine members that have replaced by pseudonyms. A sender MUST NOT combine members that have
different received-protocol values. different received-protocol values.
5.7.2. Transformations 6.7.2. Transformations
Some intermediaries include features for transforming messages and Some intermediaries include features for transforming messages and
their payloads. A proxy might, for example, convert between image their payloads. A proxy might, for example, convert between image
formats in order to save cache space or to reduce the amount of formats in order to save cache space or to reduce the amount of
traffic on a slow link. However, operational problems might occur traffic on a slow link. However, operational problems might occur
when these transformations are applied to payloads intended for when these transformations are applied to payloads intended for
critical applications, such as medical imaging or scientific data critical applications, such as medical imaging or scientific data
analysis, particularly when integrity checks or digital signatures analysis, particularly when integrity checks or digital signatures
are used to ensure that the payload received is identical to the are used to ensure that the payload received is identical to the
original. original.
skipping to change at page 48, line 34 skipping to change at page 51, line 46
received when forwarding the request. A proxy MUST NOT change the received when forwarding the request. A proxy MUST NOT change the
host name if the target URI contains a fully qualified domain name. host name if the target URI contains a fully qualified domain name.
A proxy MUST NOT modify the "absolute-path" and "query" parts of the A proxy MUST NOT modify the "absolute-path" and "query" parts of the
received target URI when forwarding it to the next inbound server, received target URI when forwarding it to the next inbound server,
except as noted above to replace an empty path with "/" or "*". except as noted above to replace an empty path with "/" or "*".
A proxy MAY modify the message body through application or removal of A proxy MAY modify the message body through application or removal of
a transfer coding (Section 7 of [Messaging]). a transfer coding (Section 7 of [Messaging]).
A proxy MUST NOT transform the payload (Section 6.3) of a message A proxy MUST NOT transform the payload (Section 7.3) of a message
that contains a no-transform cache-control response directive that contains a no-transform cache-control response directive
(Section 5.2 of [Caching]). (Section 5.2 of [Caching]).
A proxy MAY transform the payload of a message that does not contain A proxy MAY transform the payload of a message that does not contain
a no-transform cache-control directive. A proxy that transforms the a no-transform cache-control directive. A proxy that transforms the
payload of a 200 (OK) response can inform downstream recipients that payload of a 200 (OK) response can inform downstream recipients that
a transformation has been applied by changing the response status a transformation has been applied by changing the response status
code to 203 (Non-Authoritative Information) (Section 9.3.4). code to 203 (Non-Authoritative Information) (Section 10.3.4).
A proxy SHOULD NOT modify header fields that provide information A proxy SHOULD NOT modify header fields that provide information
about the endpoints of the communication chain, the resource state, about the endpoints of the communication chain, the resource state,
or the selected representation (other than the payload) unless the or the selected representation (other than the payload) unless the
field's definition specifically allows such modification or the field's definition specifically allows such modification or the
modification is deemed necessary for privacy or security. modification is deemed necessary for privacy or security.
6. Representations 7. Representations
Considering that a resource could be anything, and that the uniform Considering that a resource could be anything, and that the uniform
interface provided by HTTP is similar to a window through which one interface provided by HTTP is similar to a window through which one
can observe and act upon such a thing only through the communication can observe and act upon such a thing only through the communication
of messages to some independent actor on the other side, an of messages to some independent actor on the other side, an
abstraction is needed to represent ("take the place of") the current abstraction is needed to represent ("take the place of") the current
or desired state of that thing in our communications. That or desired state of that thing in our communications. That
abstraction is called a representation [REST]. abstraction is called a representation [REST].
For the purposes of HTTP, a "representation" is information that is For the purposes of HTTP, a "representation" is information that is
skipping to change at page 49, line 27 skipping to change at page 52, line 35
resource, in a format that can be readily communicated via the resource, in a format that can be readily communicated via the
protocol, and that consists of a set of representation metadata and a protocol, and that consists of a set of representation metadata and a
potentially unbounded stream of representation data. potentially unbounded stream of representation data.
An origin server might be provided with, or be capable of generating, An origin server might be provided with, or be capable of generating,
multiple representations that are each intended to reflect the multiple representations that are each intended to reflect the
current state of a target resource. In such cases, some algorithm is current state of a target resource. In such cases, some algorithm is
used by the origin server to select one of those representations as used by the origin server to select one of those representations as
most applicable to a given request, usually based on content most applicable to a given request, usually based on content
negotiation. This "selected representation" is used to provide the negotiation. This "selected representation" is used to provide the
data and metadata for evaluating conditional requests (Section 8.2) data and metadata for evaluating conditional requests (Section 9.2)
and constructing the payload for 200 (OK), 206 (Partial Content), and and constructing the payload for 200 (OK), 206 (Partial Content), and
304 (Not Modified) responses to GET (Section 7.3.1). 304 (Not Modified) responses to GET (Section 8.3.1).
6.1. Representation Data 7.1. Representation Data
The representation data associated with an HTTP message is either The representation data associated with an HTTP message is either
provided as the payload body of the message or referred to by the provided as the payload body of the message or referred to by the
message semantics and the target URI. The representation data is in message semantics and the target URI. The representation data is in
a format and encoding defined by the representation metadata header a format and encoding defined by the representation metadata header
fields. fields.
The data type of the representation data is determined via the header The data type of the representation data is determined via the header
fields Content-Type and Content-Encoding. These define a two-layer, fields Content-Type and Content-Encoding. These define a two-layer,
ordered encoding model: ordered encoding model:
representation-data := Content-Encoding( Content-Type( bits ) ) representation-data := Content-Encoding( Content-Type( bits ) )
6.1.1. Media Type 7.1.1. Media Type
HTTP uses media types [RFC2046] in the Content-Type (Section 6.2.1) HTTP uses media types [RFC2046] in the Content-Type (Section 7.2.1)
and Accept (Section 8.4.1) header fields in order to provide open and and Accept (Section 9.4.1) header fields in order to provide open and
extensible data typing and type negotiation. Media types define both extensible data typing and type negotiation. Media types define both
a data format and various processing models: how to process that data a data format and various processing models: how to process that data
in accordance with each context in which it is received. in accordance with each context in which it is received.
media-type = type "/" subtype *( OWS ";" OWS parameter ) media-type = type "/" subtype *( OWS ";" OWS parameter )
type = token type = token
subtype = token subtype = token
The type and subtype tokens are case-insensitive. The type and subtype tokens are case-insensitive.
The type/subtype MAY be followed by semicolon-delimited parameters The type/subtype MAY be followed by semicolon-delimited parameters
(Section 4.4.1.4) in the form of name=value pairs. The presence or (Section 5.4.1.4) in the form of name=value pairs. The presence or
absence of a parameter might be significant to the processing of a absence of a parameter might be significant to the processing of a
media type, depending on its definition within the media type media type, depending on its definition within the media type
registry. Parameter values might or might not be case-sensitive, registry. Parameter values might or might not be case-sensitive,
depending on the semantics of the parameter name. depending on the semantics of the parameter name.
For example, the following media types are equivalent in describing For example, the following media types are equivalent in describing
HTML text data encoded in the UTF-8 character encoding scheme, but HTML text data encoded in the UTF-8 character encoding scheme, but
the first is preferred for consistency (the "charset" parameter value the first is preferred for consistency (the "charset" parameter value
is defined as being case-insensitive in [RFC2046], Section 4.1.2): is defined as being case-insensitive in [RFC2046], Section 4.1.2):
text/html;charset=utf-8 text/html;charset=utf-8
Text/HTML;Charset="utf-8" Text/HTML;Charset="utf-8"
text/html; charset="utf-8" text/html; charset="utf-8"
text/html;charset=UTF-8 text/html;charset=UTF-8
Media types ought to be registered with IANA according to the Media types ought to be registered with IANA according to the
procedures defined in [BCP13]. procedures defined in [BCP13].
6.1.1.1. Charset 7.1.1.1. Charset
HTTP uses charset names to indicate or negotiate the character HTTP uses charset names to indicate or negotiate the character
encoding scheme of a textual representation [RFC6365]. A charset is encoding scheme of a textual representation [RFC6365]. A charset is
identified by a case-insensitive token. identified by a case-insensitive token.
charset = token charset = token
Charset names ought to be registered in the IANA "Character Sets" Charset names ought to be registered in the IANA "Character Sets"
registry (<https://www.iana.org/assignments/character-sets>) registry (<https://www.iana.org/assignments/character-sets>)
according to the procedures defined in Section 2 of [RFC2978]. according to the procedures defined in Section 2 of [RFC2978].
skipping to change at page 50, line 45 skipping to change at page 54, line 4
identified by a case-insensitive token. identified by a case-insensitive token.
charset = token charset = token
Charset names ought to be registered in the IANA "Character Sets" Charset names ought to be registered in the IANA "Character Sets"
registry (<https://www.iana.org/assignments/character-sets>) registry (<https://www.iana.org/assignments/character-sets>)
according to the procedures defined in Section 2 of [RFC2978]. according to the procedures defined in Section 2 of [RFC2978].
Note: In theory, charset names are defined by the "mime-charset" Note: In theory, charset names are defined by the "mime-charset"
ABNF rule defined in Section 2.3 of [RFC2978] (as corrected in ABNF rule defined in Section 2.3 of [RFC2978] (as corrected in
[Err1912]). That rule allows two characters that are not included [Err1912]). That rule allows two characters that are not included
in "token" ("{" and "}"), but no charset name registered at the in "token" ("{" and "}"), but no charset name registered at the
time of this writing includes braces (see [Err5433]). time of this writing includes braces (see [Err5433]).
6.1.1.2. Canonicalization and Text Defaults 7.1.1.2. Canonicalization and Text Defaults
Media types are registered with a canonical form in order to be Media types are registered with a canonical form in order to be
interoperable among systems with varying native encoding formats. interoperable among systems with varying native encoding formats.
Representations selected or transferred via HTTP ought to be in Representations selected or transferred via HTTP ought to be in
canonical form, for many of the same reasons described by the canonical form, for many of the same reasons described by the
Multipurpose Internet Mail Extensions (MIME) [RFC2045]. However, the Multipurpose Internet Mail Extensions (MIME) [RFC2045]. However, the
performance characteristics of email deployments (i.e., store and performance characteristics of email deployments (i.e., store and
forward messages to peers) are significantly different from those forward messages to peers) are significantly different from those
common to HTTP and the Web (server-based information services). common to HTTP and the Web (server-based information services).
Furthermore, MIME's constraints for the sake of compatibility with Furthermore, MIME's constraints for the sake of compatibility with
skipping to change at page 51, line 35 skipping to change at page 54, line 39
addition, text media in HTTP is not limited to charsets that use addition, text media in HTTP is not limited to charsets that use
octets 13 and 10 for CR and LF, respectively. This flexibility octets 13 and 10 for CR and LF, respectively. This flexibility
regarding line breaks applies only to text within a representation regarding line breaks applies only to text within a representation
that has been assigned a "text" media type; it does not apply to that has been assigned a "text" media type; it does not apply to
"multipart" types or HTTP elements outside the payload body (e.g., "multipart" types or HTTP elements outside the payload body (e.g.,
header fields). header fields).
If a representation is encoded with a content-coding, the underlying If a representation is encoded with a content-coding, the underlying
data ought to be in a form defined above prior to being encoded. data ought to be in a form defined above prior to being encoded.
6.1.1.3. Multipart Types 7.1.1.3. 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 representations within a single message body. All one or more representations within a single message body. All
multipart types share a common syntax, as defined in Section 5.1.1 of multipart types share a common syntax, as defined in Section 5.1.1 of
[RFC2046], and include a boundary parameter as part of the media type [RFC2046], and include a boundary parameter as part of the media type
value. The message body is itself a protocol element; a sender MUST value. The message body is itself a protocol element; a sender MUST
generate only CRLF to represent line breaks between body parts. generate only CRLF to represent line breaks between body parts.
HTTP message framing does not use the multipart boundary as an HTTP message framing does not use the multipart boundary as an
indicator of message body length, though it might be used by indicator of message body length, though it might be used by
implementations that generate or process the payload. For example, implementations that generate or process the payload. For example,
the "multipart/form-data" type is often used for carrying form data the "multipart/form-data" type is often used for carrying form data
in a request, as described in [RFC7578], and the "multipart/ in a request, as described in [RFC7578], and the "multipart/
byteranges" type is defined by this specification for use in some 206 byteranges" type is defined by this specification for use in some 206
(Partial Content) responses (see Section 9.3.7). (Partial Content) responses (see Section 10.3.7).
6.1.2. Content Codings 7.1.2. Content Codings
Content coding values indicate an encoding transformation that has Content coding values indicate an encoding transformation that has
been or can be applied to a representation. Content codings are been or can be applied to a representation. Content codings are
primarily used to allow a representation to be compressed or primarily used to allow a representation to be compressed or
otherwise usefully transformed without losing the identity of its otherwise usefully transformed without losing the identity of its
underlying media type and without loss of information. Frequently, underlying media type and without loss of information. Frequently,
the representation is stored in coded form, transmitted directly, and the representation is stored in coded form, transmitted directly, and
only decoded by the final recipient. only decoded by the final recipient.
content-coding = token content-coding = token
All content codings are case-insensitive and ought to be registered All content codings are case-insensitive and ought to be registered
within the "HTTP Content Coding Registry", as defined in within the "HTTP Content Coding Registry", as defined in
Section 6.1.2.4 Section 7.1.2.4
Content-coding values are used in the Accept-Encoding (Section 8.4.3) Content-coding values are used in the Accept-Encoding (Section 9.4.3)
and Content-Encoding (Section 6.2.2) header fields. and Content-Encoding (Section 7.2.2) header fields.
The following content-coding values are defined by this The following content-coding values are defined by this
specification: specification:
+------------+------------------------------------------+-----------+ +------------+------------------------------------------+-----------+
| Name | Description | Reference | | Name | Description | Reference |
+------------+------------------------------------------+-----------+ +------------+------------------------------------------+-----------+
| compress | UNIX "compress" data format [Welch] | Section 6 | | compress | UNIX "compress" data format [Welch] | Section 7 |
| | | .1.2.1 | | | | .1.2.1 |
| deflate | "deflate" compressed data ([RFC1951]) | Section 6 | | deflate | "deflate" compressed data ([RFC1951]) | Section 7 |
| | inside the "zlib" data format | .1.2.2 | | | inside the "zlib" data format | .1.2.2 |
| | ([RFC1950]) | | | | ([RFC1950]) | |
| gzip | GZIP file format [RFC1952] | Section 6 | | gzip | GZIP file format [RFC1952] | Section 7 |
| | | .1.2.3 | | | | .1.2.3 |
| identity | Reserved (synonym for "no encoding" in | Section 8 | | identity | Reserved | Section 9 |
| | Accept-Encoding) | .4.3 | | | | .4.3 |
| x-compress | Deprecated (alias for compress) | Section 6 | | x-compress | Deprecated (alias for compress) | Section 7 |
| | | .1.2.1 | | | | .1.2.1 |
| x-gzip | Deprecated (alias for gzip) | Section 6 | | x-gzip | Deprecated (alias for gzip) | Section 7 |
| | | .1.2.3 | | | | .1.2.3 |
+------------+------------------------------------------+-----------+ +------------+------------------------------------------+-----------+
Table 2 Table 2
6.1.2.1. Compress Coding 7.1.2.1. Compress Coding
The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
[Welch] that is commonly produced by the UNIX file compression [Welch] that is commonly produced by the UNIX file compression
program "compress". A recipient SHOULD consider "x-compress" to be program "compress". A recipient SHOULD consider "x-compress" to be
equivalent to "compress". equivalent to "compress".
6.1.2.2. Deflate Coding 7.1.2.2. Deflate Coding
The "deflate" coding is a "zlib" data format [RFC1950] containing a The "deflate" coding is a "zlib" data format [RFC1950] containing a
"deflate" compressed data stream [RFC1951] that uses a combination of "deflate" compressed data stream [RFC1951] that uses a combination of
the Lempel-Ziv (LZ77) compression algorithm and Huffman coding. the Lempel-Ziv (LZ77) compression algorithm and Huffman coding.
Note: Some non-conformant implementations send the "deflate" Note: Some non-conformant implementations send the "deflate"
compressed data without the zlib wrapper. compressed data without the zlib wrapper.
6.1.2.3. Gzip Coding 7.1.2.3. Gzip Coding
The "gzip" coding is an LZ77 coding with a 32-bit Cyclic Redundancy The "gzip" coding is an LZ77 coding with a 32-bit Cyclic Redundancy
Check (CRC) that is commonly produced by the gzip file compression Check (CRC) that is commonly produced by the gzip file compression
program [RFC1952]. A recipient SHOULD consider "x-gzip" to be program [RFC1952]. A recipient SHOULD consider "x-gzip" to be
equivalent to "gzip". equivalent to "gzip".
6.1.2.4. Content Coding Registry 7.1.2.4. Content Coding Registry
The "HTTP Content Coding Registry", maintained by IANA at The "HTTP Content Coding Registry", maintained by IANA at
<https://www.iana.org/assignments/http-parameters/>, registers <https://www.iana.org/assignments/http-parameters/>, registers
content-coding names. content-coding names.
Content coding registrations MUST include the following fields: Content coding registrations MUST include the following fields:
o Name o Name
o Description o Description
o Pointer to specification text o Pointer to specification text
Names of content codings MUST NOT overlap with names of transfer Names of content codings MUST NOT overlap with names of transfer
codings (Section 7 of [Messaging]), unless the encoding codings (Section 7 of [Messaging]), unless the encoding
transformation is identical (as is the case for the compression transformation is identical (as is the case for the compression
codings defined in Section 6.1.2). codings defined in Section 7.1.2).
Values to be added to this namespace require IETF Review (see Values to be added to this namespace require IETF Review (see
Section 4.8 of [RFC8126]) and MUST conform to the purpose of content Section 4.8 of [RFC8126]) and MUST conform to the purpose of content
coding defined in Section 6.1.2. coding defined in Section 7.1.2.
6.1.3. Language Tags 7.1.3. Language Tags
A language tag, as defined in [RFC5646], identifies a natural A language tag, as defined in [RFC5646], identifies a natural
language spoken, written, or otherwise conveyed by human beings for language spoken, written, or otherwise conveyed by human beings for
communication of information to other human beings. Computer communication of information to other human beings. Computer
languages are explicitly excluded. languages are explicitly excluded.
HTTP uses language tags within the Accept-Language and Content- HTTP uses language tags within the Accept-Language and Content-
Language header fields. Accept-Language uses the broader language- Language header fields. Accept-Language uses the broader language-
range production defined in Section 8.4.4, whereas Content-Language range production defined in Section 9.4.4, whereas Content-Language
uses the language-tag production defined below. uses the language-tag production defined below.
language-tag = <Language-Tag, see [RFC5646], Section 2.1> language-tag = <Language-Tag, see [RFC5646], Section 2.1>
A language tag is a sequence of one or more case-insensitive subtags, A language tag is a sequence of one or more case-insensitive subtags,
each separated by a hyphen character ("-", %x2D). In most cases, a each separated by a hyphen character ("-", %x2D). In most cases, a
language tag consists of a primary language subtag that identifies a language tag consists of a primary language subtag that identifies a
broad family of related languages (e.g., "en" = English), which is broad family of related languages (e.g., "en" = English), which is
optionally followed by a series of subtags that refine or narrow that optionally followed by a series of subtags that refine or narrow that
language's range (e.g., "en-CA" = the variety of English as language's range (e.g., "en-CA" = the variety of English as
communicated in Canada). Whitespace is not allowed within a language communicated in Canada). Whitespace is not allowed within a language
tag. Example tags include: tag. Example tags include:
fr, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN fr, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
See [RFC5646] for further information. See [RFC5646] for further information.
6.1.4. Range Units 7.1.4. Range Units
Representation data can be partitioned into subranges when there are Representation data can be partitioned into subranges when there are
addressable structural units inherent to that data's content coding addressable structural units inherent to that data's content coding
or media type. For example, octet (a.k.a., byte) boundaries are a or media type. For example, octet (a.k.a., byte) boundaries are a
structural unit common to all representation data, allowing structural unit common to all representation data, allowing
partitions of the data to be identified as a range of bytes at some partitions of the data to be identified as a range of bytes at some
offset from the start or end of that data. offset from the start or end of that data.
This general notion of a "range unit" is used in the Accept-Ranges This general notion of a "range unit" is used in the Accept-Ranges
(Section 10.4.1) response header field to advertise support for range (Section 11.4.1) response header field to advertise support for range
requests, the Range (Section 8.3) request header field to delineate requests, the Range (Section 9.3) request header field to delineate
the parts of a representation that are requested, and the Content- the parts of a representation that are requested, and the Content-
Range (Section 6.3.4) payload header field to describe which part of Range (Section 7.3.4) payload header field to describe which part of
a representation is being transferred. a representation is being transferred.
range-unit = token range-unit = token
All range unit names are case-insensitive and ought to be registered All range unit names are case-insensitive and ought to be registered
within the "HTTP Range Unit Registry", as defined in Section 6.1.4.4 within the "HTTP Range Unit Registry", as defined in Section 7.1.4.4
The following range unit names are defined by this document: The following range unit names are defined by this document:
+------------+-----------------------------------------+------------+ +------------+-----------------------------------------+------------+
| Range Unit | Description | Reference | | Range Unit | Description | Reference |
| Name | | | | Name | | |
+------------+-----------------------------------------+------------+ +------------+-----------------------------------------+------------+
| bytes | a range of octets | Section 6. | | bytes | a range of octets | Section 7. |
| | | 1.4.2 | | | | 1.4.2 |
| none | reserved as keyword to indicate range | Section 10 | | none | reserved as keyword to indicate range | Section 11 |
| | requests are not supported | .4.1 | | | requests are not supported | .4.1 |
+------------+-----------------------------------------+------------+ +------------+-----------------------------------------+------------+
Table 3 Table 3
6.1.4.1. Range Specifiers 7.1.4.1. Range Specifiers
Ranges are expressed in terms of a range unit paired with a set of Ranges are expressed in terms of a range unit paired with a set of
range specifiers. The range unit name determines what kinds of range specifiers. The range unit name determines what kinds of
range-spec are applicable to its own specifiers. Hence, the range-spec are applicable to its own specifiers. Hence, the
following gramar is generic: each range unit is expected to specify following gramar is generic: each range unit is expected to specify
requirements on when int-range, suffix-range, and other-range are requirements on when int-range, suffix-range, and other-range are
allowed. allowed.
A range request can specify a single range or a set of ranges within A range request can specify a single range or a set of ranges within
a single representation. a single representation.
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suffix-range = "-" suffix-length suffix-range = "-" suffix-length
suffix-length = 1*DIGIT suffix-length = 1*DIGIT
To provide for extensibility, the other-range rule is a mostly To provide for extensibility, the other-range rule is a mostly
unconstrained grammar that allows application-specific or future unconstrained grammar that allows application-specific or future
range units to define additional range specifiers. range units to define additional range specifiers.
other-range = 1*( %x21-2B / %x2D-7E ) other-range = 1*( %x21-2B / %x2D-7E )
; 1*(VCHAR excluding comma) ; 1*(VCHAR excluding comma)
6.1.4.2. Byte Ranges 7.1.4.2. Byte Ranges
The "bytes" range unit is used to express subranges of a The "bytes" range unit is used to express subranges of a
representation data's octet sequence. Each byte range is expressed representation data's octet sequence. Each byte range is expressed
as an integer range at some offset, relative to either the beginning as an integer range at some offset, relative to either the beginning
(int-range) or end (suffix-range) of the representation data. Byte (int-range) or end (suffix-range) of the representation data. Byte
ranges do not use the other-range specifier. ranges do not use the other-range specifier.
The first-pos value in a bytes int-range gives the offset of the The first-pos value in a bytes int-range gives the offset of the
first byte in a range. The last-pos value gives the offset of the first byte in a range. The last-pos value gives the offset of the
last byte in the range; that is, the byte positions specified are last byte in the range; that is, the byte positions specified are
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bytes=500-999 bytes=500-999
A client can limit the number of bytes requested without knowing the A client can limit the number of bytes requested without knowing the
size of the selected representation. If the last-pos value is size of the selected representation. If the last-pos value is
absent, or if the value is greater than or equal to the current absent, or if the value is greater than or equal to the current
length of the representation data, the byte range is interpreted as length of the representation data, the byte range is interpreted as
the remainder of the representation (i.e., the server replaces the the remainder of the representation (i.e., the server replaces the
value of last-pos with a value that is one less than the current value of last-pos with a value that is one less than the current
length of the selected representation). length of the selected representation).
A client can request the last N bytes of the selected representation A client can request the last N bytes (N > 0) of the selected
using a suffix-range. If the selected representation is shorter than representation using a suffix-range. If the selected representation
the specified suffix-length, the entire representation is used. is shorter than the specified suffix-length, the entire
representation is used.
Additional examples, assuming a representation of length 10000: Additional examples, assuming a representation of length 10000:
o The final 500 bytes (byte offsets 9500-9999, inclusive): o The final 500 bytes (byte offsets 9500-9999, inclusive):
bytes=-500 bytes=-500
Or: Or:
bytes=9500- bytes=9500-
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bytes=500-600,601-999 bytes=500-600,601-999
bytes=500-700,601-999 bytes=500-700,601-999
If a valid bytes range-set includes at least one range-spec with a If a valid bytes range-set includes at least one range-spec with a
first-pos that is less than the current length of the representation, first-pos that is less than the current length of the representation,
or at least one suffix-range with a non-zero suffix-length, then the or at least one suffix-range with a non-zero suffix-length, then the
bytes range-set is satisfiable. Otherwise, the bytes range-set is bytes range-set is satisfiable. Otherwise, the bytes range-set is
unsatisfiable. unsatisfiable.
If the selected representation has zero length, the only satisfiable
form of range-spec is a suffix-range with a non-zero suffix-length.
In the byte-range syntax, first-pos, last-pos, and suffix-length are In the byte-range syntax, first-pos, last-pos, and suffix-length are
expressed as decimal number of octets. Since there is no predefined expressed as decimal number of octets. Since there is no predefined
limit to the length of a payload, recipients MUST anticipate limit to the length of a payload, recipients MUST anticipate
potentially large decimal numerals and prevent parsing errors due to potentially large decimal numerals and prevent parsing errors due to
integer conversion overflows. integer conversion overflows.
6.1.4.3. Other Range Units 7.1.4.3. Other Range Units
Other range units, such as format-specific boundaries like pages, Other range units, such as format-specific boundaries like pages,
sections, records, rows, or time, are potentially usable in HTTP for sections, records, rows, or time, are potentially usable in HTTP for
application-specific purposes, but are not commonly used in practice. application-specific purposes, but are not commonly used in practice.
Implementors of alternative range units ought to consider how they Implementors of alternative range units ought to consider how they
would work with content codings and general-purpose intermediaries. would work with content codings and general-purpose intermediaries.
Range units are intended to be extensible. New range units ought to Range units are intended to be extensible. New range units ought to
be registered with IANA, as defined in Section 6.1.4.4. be registered with IANA, as defined in Section 7.1.4.4.
6.1.4.4. Range Unit Registry 7.1.4.4. Range Unit Registry
The "HTTP Range Unit Registry" defines the namespace for the range The "HTTP Range Unit Registry" defines the namespace for the range
unit names and refers to their corresponding specifications. It is unit names and refers to their corresponding specifications. It is
maintained at <https://www.iana.org/assignments/http-parameters>. maintained at <https://www.iana.org/assignments/http-parameters>.
Registration of an HTTP Range Unit MUST include the following fields: Registration of an HTTP Range Unit MUST include the following fields:
o Name o Name
o Description o Description
o Pointer to specification text o Pointer to specification text
Values to be added to this namespace require IETF Review (see Values to be added to this namespace require IETF Review (see
[RFC8126], Section 4.8). [RFC8126], Section 4.8).
6.2. Representation Metadata 7.2. Representation Metadata
Representation header fields provide metadata about the Representation header fields provide metadata about the
representation. When a message includes a payload body, the representation. When a message includes a payload body, the
representation header fields describe how to interpret the representation header fields describe how to interpret the
representation data enclosed in the payload body. In a response to a representation data enclosed in the payload body. In a response to a
HEAD request, the representation header fields describe the HEAD request, the representation header fields describe the
representation data that would have been enclosed in the payload body representation data that would have been enclosed in the payload body
if the same request had been a GET. if the same request had been a GET.
The following header fields convey representation metadata: The following header fields convey representation metadata:
+------------------+---------------+ +------------------+---------------+
| Field Name | Defined in... | | Field Name | Defined in... |
+------------------+---------------+ +------------------+---------------+
| Content-Type | Section 6.2.1 | | Content-Type | Section 7.2.1 |
| Content-Encoding | Section 6.2.2 | | Content-Encoding | Section 7.2.2 |
| Content-Language | Section 6.2.3 | | Content-Language | Section 7.2.3 |
| Content-Length | Section 6.2.4 | | Content-Length | Section 7.2.4 |
| Content-Location | Section 6.2.5 | | Content-Location | Section 7.2.5 |
+------------------+---------------+ +------------------+---------------+
6.2.1. Content-Type 7.2.1. Content-Type
The "Content-Type" header field indicates the media type of the The "Content-Type" header field indicates the media type of the
associated representation: either the representation enclosed in the associated representation: either the representation enclosed in the
message payload or the selected representation, as determined by the message payload or the selected representation, as determined by the
message semantics. The indicated media type defines both the data message semantics. The indicated media type defines both the data
format and how that data is intended to be processed by a recipient, format and how that data is intended to be processed by a recipient,
within the scope of the received message semantics, after any content within the scope of the received message semantics, after any content
codings indicated by Content-Encoding are decoded. codings indicated by Content-Encoding are decoded.
Content-Type = media-type Content-Type = media-type
Media types are defined in Section 6.1.1. An example of the field is Media types are defined in Section 7.1.1. An example of the field is
Content-Type: text/html; charset=ISO-8859-4 Content-Type: text/html; charset=ISO-8859-4
A sender that generates a message containing a payload body SHOULD A sender that generates a message containing a payload body SHOULD
generate a Content-Type header field in that message unless the generate a Content-Type header field in that message unless the
intended media type of the enclosed representation is unknown to the intended media type of the enclosed representation is unknown to the
sender. If a Content-Type header field is not present, the recipient sender. If a Content-Type header field is not present, the recipient
MAY either assume a media type of "application/octet-stream" MAY either assume a media type of "application/octet-stream"
([RFC2046], Section 4.5.1) or examine the data to determine its type. ([RFC2046], Section 4.5.1) or examine the data to determine its type.
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representation. Some user agents examine a payload's content and, in representation. Some user agents examine a payload's content and, in
certain cases, override the received type (for example, see certain cases, override the received type (for example, see
[Sniffing]). This "MIME sniffing" risks drawing incorrect [Sniffing]). This "MIME sniffing" risks drawing incorrect
conclusions about the data, which might expose the user to additional conclusions about the data, which might expose the user to additional
security risks (e.g., "privilege escalation"). Furthermore, it is security risks (e.g., "privilege escalation"). Furthermore, it is
impossible to determine the sender's intended processing model by impossible to determine the sender's intended processing model by
examining the data format: many data formats match multiple media examining the data format: many data formats match multiple media
types that differ only in processing semantics. Implementers are types that differ only in processing semantics. Implementers are
encouraged to provide a means to disable such sniffing. encouraged to provide a means to disable such sniffing.
6.2.2. Content-Encoding 7.2.2. Content-Encoding
The "Content-Encoding" header field indicates what content codings The "Content-Encoding" header field indicates what content codings
have been applied to the representation, beyond those inherent in the have been applied to the representation, beyond those inherent in the
media type, and thus what decoding mechanisms have to be applied in media type, and thus what decoding mechanisms have to be applied in
order to obtain data in the media type referenced by the Content-Type order to obtain data in the media type referenced by the Content-Type
header field. Content-Encoding is primarily used to allow a header field. Content-Encoding is primarily used to allow a
representation's data to be compressed without losing the identity of representation's data to be compressed without losing the identity of
its underlying media type. its underlying media type.
Content-Encoding = 1#content-coding Content-Encoding = 1#content-coding
An example of its use is An example of its use is
Content-Encoding: gzip Content-Encoding: gzip
If one or more encodings have been applied to a representation, the If one or more encodings have been applied to a representation, the
sender that applied the encodings MUST generate a Content-Encoding sender that applied the encodings MUST generate a Content-Encoding
header field that lists the content codings in the order in which header field that lists the content codings in the order in which
they were applied. Additional information about the encoding they were applied. Note that the coding named "identity" is reserved
parameters can be provided by other header fields not defined by this for its special role in Accept-Encoding, and thus SHOULD NOT be
specification. included.
Additional information about the encoding parameters can be provided
by other header fields not defined by this specification.
Unlike Transfer-Encoding (Section 6.1 of [Messaging]), the codings Unlike Transfer-Encoding (Section 6.1 of [Messaging]), the codings
listed in Content-Encoding are a characteristic of the listed in Content-Encoding are a characteristic of the
representation; the representation is defined in terms of the coded representation; the representation is defined in terms of the coded
form, and all other metadata about the representation is about the form, and all other metadata about the representation is about the
coded form unless otherwise noted in the metadata definition. coded form unless otherwise noted in the metadata definition.
Typically, the representation is only decoded just prior to rendering Typically, the representation is only decoded just prior to rendering
or analogous usage. or analogous usage.
If the media type includes an inherent encoding, such as a data If the media type includes an inherent encoding, such as a data
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choose to publish the same data as multiple representations that choose to publish the same data as multiple representations that
differ only in whether the coding is defined as part of Content-Type differ only in whether the coding is defined as part of Content-Type
or Content-Encoding, since some user agents will behave differently or Content-Encoding, since some user agents will behave differently
in their handling of each response (e.g., open a "Save as ..." dialog in their handling of each response (e.g., open a "Save as ..." dialog
instead of automatic decompression and rendering of content). instead of automatic decompression and rendering of content).
An origin server MAY respond with a status code of 415 (Unsupported An origin server MAY respond with a status code of 415 (Unsupported
Media Type) if a representation in the request message has a content Media Type) if a representation in the request message has a content
coding that is not acceptable. coding that is not acceptable.
6.2.3. Content-Language 7.2.3. Content-Language
The "Content-Language" header field describes the natural language(s) The "Content-Language" header field describes the natural language(s)
of the intended audience for the representation. Note that this of the intended audience for the representation. Note that this
might not be equivalent to all the languages used within the might not be equivalent to all the languages used within the
representation. representation.
Content-Language = 1#language-tag Content-Language = 1#language-tag
Language tags are defined in Section 6.1.3. The primary purpose of Language tags are defined in Section 7.1.3. 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
representations according to the users' own preferred language. representations according to the users' own preferred language.
Thus, if the content is intended only for a Danish-literate audience, Thus, if the content is intended only for a Danish-literate audience,
the appropriate field is the 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,
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However, just because multiple languages are present within a However, just because multiple languages are present within a
representation does not mean that it is intended for multiple representation does not mean that it is intended for multiple
linguistic audiences. An example would be a beginner's language linguistic audiences. An example would be a beginner's language
primer, such as "A First Lesson in Latin", which is clearly intended primer, such as "A First Lesson in Latin", which is clearly intended
to be used by an English-literate audience. In this case, the to be used by an English-literate audience. In this case, the
Content-Language would properly only include "en". Content-Language would properly only include "en".
Content-Language MAY be applied to any media type -- it is not Content-Language MAY be applied to any media type -- it is not
limited to textual documents. limited to textual documents.
6.2.4. Content-Length 7.2.4. Content-Length
[[CREF1: The "Content-Length" header field indicates the number of [[CREF1: The "Content-Length" header field indicates the number of
data octets (body length) for the representation. In some cases, data octets (body length) for the representation. In some cases,
Content-Length is used to define or estimate message framing. ]] Content-Length is used to define or estimate message framing. ]]
Content-Length = 1*DIGIT Content-Length = 1*DIGIT
An example is An example is
Content-Length: 3495 Content-Length: 3495
A sender MUST NOT send a Content-Length header field in any message A sender MUST NOT send a Content-Length header field in any message
that contains a Transfer-Encoding header field. that contains a Transfer-Encoding header field.
A user agent SHOULD send a Content-Length in a request message when A user agent SHOULD send a Content-Length in a request message when
no Transfer-Encoding is sent and the request method defines a meaning no Transfer-Encoding is sent and the request method defines a meaning
for an enclosed payload body. For example, a Content-Length header for an enclosed payload body. For example, a Content-Length header
field is normally sent in a POST request even when the value is 0 field is normally sent in a POST request even when the value is 0
(indicating an empty payload body). A user agent SHOULD NOT send a (indicating an empty payload body). A user agent SHOULD NOT send a
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A user agent SHOULD send a Content-Length in a request message when A user agent SHOULD send a Content-Length in a request message when
no Transfer-Encoding is sent and the request method defines a meaning no Transfer-Encoding is sent and the request method defines a meaning
for an enclosed payload body. For example, a Content-Length header for an enclosed payload body. For example, a Content-Length header
field is normally sent in a POST request even when the value is 0 field is normally sent in a POST request even when the value is 0
(indicating an empty payload body). A user agent SHOULD NOT send a (indicating an empty payload body). A user agent SHOULD NOT send a
Content-Length header field when the request message does not contain Content-Length header field when the request message does not contain
a payload body and the method semantics do not anticipate such a a payload body and the method semantics do not anticipate such a
body. body.
A server MAY send a Content-Length header field in a response to a A server MAY send a Content-Length header field in a response to a
HEAD request (Section 7.3.2); a server MUST NOT send Content-Length HEAD request (Section 8.3.2); a server MUST NOT send Content-Length
in such a response unless its field value equals the decimal number in such a response unless its field value equals the decimal number
of octets that would have been sent in the payload body of a response of octets that would have been sent in the payload body of a response
if the same request had used the GET method. if the same request had used the GET method.
A server MAY send a Content-Length header field in a 304 (Not A server MAY send a Content-Length header field in a 304 (Not
Modified) response to a conditional GET request (Section 9.4.5); a Modified) response to a conditional GET request (Section 10.4.5); a
server MUST NOT send Content-Length in such a response unless its server MUST NOT send Content-Length in such a response unless its
field value equals the decimal number of octets that would have been field value equals the decimal number of octets that would have been
sent in the payload body of a 200 (OK) response to the same request. sent in the payload body of a 200 (OK) response to the same request.
A server MUST NOT send a Content-Length header field in any response A server MUST NOT send a Content-Length header field in any response
with a status code of 1xx (Informational) or 204 (No Content). A with a status code of 1xx (Informational) or 204 (No Content). A
server MUST NOT send a Content-Length header field in any 2xx server MUST NOT send a Content-Length header field in any 2xx
(Successful) response to a CONNECT request (Section 7.3.6). (Successful) response to a CONNECT request (Section 8.3.6).
Aside from the cases defined above, in the absence of Transfer- Aside from the cases defined above, in the absence of Transfer-
Encoding, an origin server SHOULD send a Content-Length header field Encoding, an origin server SHOULD send a Content-Length header field
when the payload body size is known prior to sending the complete when the payload body size is known prior to sending the complete
header section. This will allow downstream recipients to measure header section. This will allow downstream recipients to measure
transfer progress, know when a received message is complete, and transfer progress, know when a received message is complete, and
potentially reuse the connection for additional requests. potentially reuse the connection for additional requests.
Any Content-Length field value greater than or equal to zero is Any Content-Length field value greater than or equal to zero is
valid. Since there is no predefined limit to the length of a valid. Since there is no predefined limit to the length of a
payload, a recipient MUST anticipate potentially large decimal payload, a recipient MUST anticipate potentially large decimal
numerals and prevent parsing errors due to integer conversion numerals and prevent parsing errors due to integer conversion
overflows (Section 11.5). overflows (Section 12.5).
If a message is received that has a Content-Length header field value If a message is received that has a Content-Length header field value
consisting of the same decimal value as a comma-separated list consisting of the same decimal value as a comma-separated list
(Section 4.5) -- for example, "Content-Length: 42, 42" -- indicating (Section 5.5) -- for example, "Content-Length: 42, 42" -- indicating
that duplicate Content-Length header fields have been generated or that duplicate Content-Length header fields have been generated or
combined by an upstream message processor, then the recipient MUST combined by an upstream message processor, then the recipient MUST
either reject the message as invalid or replace the duplicated field either reject the message as invalid or replace the duplicated field
values with a single valid Content-Length field containing that values with a single valid Content-Length field containing that
decimal value prior to determining the message body length or decimal value prior to determining the message body length or
forwarding the message. forwarding the message.
6.2.5. Content-Location 7.2.5. Content-Location
The "Content-Location" header field references a URI that can be used The "Content-Location" header field references a URI that can be used
as an identifier for a specific resource corresponding to the as an identifier for a specific resource corresponding to the
representation in this message's payload. In other words, if one representation in this message's payload. In other words, if one
were to perform a GET request on this URI at the time of this were to perform a GET request on this URI at the time of this
message's generation, then a 200 (OK) response would contain the same message's generation, then a 200 (OK) response would contain the same
representation that is enclosed as payload in this message. representation that is enclosed as payload in this message.
Content-Location = absolute-URI / partial-URI Content-Location = absolute-URI / partial-URI
The field value is either an absolute-URI or a partial-URI. In the The field value is either an absolute-URI or a partial-URI. In the
latter case (Section 2.4), the referenced URI is relative to the latter case (Section 2.4), the referenced URI is relative to the
target URI ([RFC3986], Section 5). target URI ([RFC3986], Section 5).
The Content-Location value is not a replacement for the target URI The Content-Location value is not a replacement for the target URI
(Section 5.1). It is representation metadata. It has the same (Section 6.1). It is representation metadata. It has the same
syntax and semantics as the header field of the same name defined for syntax and semantics as the header field of the same name defined for
MIME body parts in Section 4 of [RFC2557]. However, its appearance MIME body parts in Section 4 of [RFC2557]. However, its appearance
in an HTTP message has some special implications for HTTP recipients. in an HTTP message has some special implications for HTTP recipients.
If Content-Location is included in a 2xx (Successful) response If Content-Location is included in a 2xx (Successful) response
message and its value refers (after conversion to absolute form) to a message and its value refers (after conversion to absolute form) to a
URI that is the same as the target URI, then the recipient MAY URI that is the same as the target URI, then the recipient MAY
consider the payload to be a current representation of that resource consider the payload to be a current representation of that resource
at the time indicated by the message origination date. For a GET at the time indicated by the message origination date. For a GET
(Section 7.3.1) or HEAD (Section 7.3.2) request, this is the same as (Section 8.3.1) or HEAD (Section 8.3.2) request, this is the same as
the default semantics when no Content-Location is provided by the the default semantics when no Content-Location is provided by the
server. For a state-changing request like PUT (Section 7.3.4) or server. For a state-changing request like PUT (Section 8.3.4) or
POST (Section 7.3.3), it implies that the server's response contains POST (Section 8.3.3), it implies that the server's response contains
the new representation of that resource, thereby distinguishing it the new representation of that resource, thereby distinguishing it
from representations that might only report about the action (e.g., from representations that might only report about the action (e.g.,
"It worked!"). This allows authoring applications to update their "It worked!"). This allows authoring applications to update their
local copies without the need for a subsequent GET request. local copies without the need for a subsequent GET request.
If Content-Location is included in a 2xx (Successful) response If Content-Location is included in a 2xx (Successful) response
message and its field value refers to a URI that differs from the message and its field value refers to a URI that differs from the
target URI, then the origin server claims that the URI is an target URI, then the origin server claims that the URI is an
identifier for a different resource corresponding to the enclosed identifier for a different resource corresponding to the enclosed
representation. Such a claim can only be trusted if both identifiers representation. Such a claim can only be trusted if both identifiers
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For example, if a client makes a PUT request on a negotiated resource For example, if a client makes a PUT request on a negotiated resource
and the origin server accepts that PUT (without redirection), then and the origin server accepts that PUT (without redirection), then
the new state of that resource is expected to be consistent with the the new state of that resource is expected to be consistent with the
one representation supplied in that PUT; the Content-Location cannot one representation supplied in that PUT; the Content-Location cannot
be used as a form of reverse content selection identifier to update be used as a form of reverse content selection identifier to update
only one of the negotiated representations. If the user agent had only one of the negotiated representations. If the user agent had
wanted the latter semantics, it would have applied the PUT directly wanted the latter semantics, it would have applied the PUT directly
to the Content-Location URI. to the Content-Location URI.
6.3. Payload 7.3. Payload
Some HTTP messages transfer a complete or partial representation as Some HTTP messages transfer a complete or partial representation as
the message "payload". In some cases, a payload might contain only the message "payload". In some cases, a payload might contain only
the associated representation's header fields (e.g., responses to the associated representation's header fields (e.g., responses to
HEAD) or only some part(s) of the representation data (e.g., the 206 HEAD) or only some part(s) of the representation data (e.g., the 206
(Partial Content) status code). (Partial Content) status code).
Header fields that specifically describe the payload, rather than the Header fields that specifically describe the payload, rather than the
associated representation, are referred to as "payload header associated representation, are referred to as "payload header
fields". Payload header fields are defined in other parts of this fields". Payload header fields are defined in other parts of this
specification, due to their impact on message parsing. specification, due to their impact on message parsing.
+-------------------+----------------------------+ +-------------------+----------------------------+
| Field Name | Defined in... | | Field Name | Defined in... |
+-------------------+----------------------------+ +-------------------+----------------------------+
| Content-Range | Section 6.3.4 | | Content-Range | Section 7.3.4 |
| Trailer | Section 4.6.3 | | Trailer | Section 5.6.3 |
| Transfer-Encoding | Section 6.1 of [Messaging] | | Transfer-Encoding | Section 6.1 of [Messaging] |
+-------------------+----------------------------+ +-------------------+----------------------------+
6.3.1. Purpose 7.3.1. Purpose
The purpose of a payload in a request is defined by the method The purpose of a payload in a request is defined by the method
semantics. For example, a representation in the payload of a PUT semantics. For example, a representation in the payload of a PUT
request (Section 7.3.4) represents the desired state of the target request (Section 8.3.4) represents the desired state of the target
resource if the request is successfully applied, whereas a resource if the request is successfully applied, whereas a
representation in the payload of a POST request (Section 7.3.3) representation in the payload of a POST request (Section 8.3.3)
represents information to be processed by the target resource. represents information to be processed by the target resource.
In a response, the payload's purpose is defined by both the request In a response, the payload's purpose is defined by both the request
method and the response status code. For example, the payload of a method and the response status code. For example, the payload of a
200 (OK) response to GET (Section 7.3.1) represents the current state 200 (OK) response to GET (Section 8.3.1) represents the current state
of the target resource, as observed at the time of the message of the target resource, as observed at the time of the message
origination date (Section 10.1.1.2), whereas the payload of the same origination date (Section 11.1.1), whereas the payload of the same
status code in a response to POST might represent either the status code in a response to POST might represent either the
processing result or the new state of the target resource after processing result or the new state of the target resource after
applying the processing. Response messages with an error status code applying the processing. Response messages with an error status code
usually contain a payload that represents the error condition, such usually contain a payload that represents the error condition, such
that it describes the error state and what next steps are suggested that it describes the error state and what next steps are suggested
for resolving it. for resolving it.
6.3.2. Identification 7.3.2. Identification
When a complete or partial representation is transferred in a message When a complete or partial representation is transferred in a message
payload, it is often desirable for the sender to supply, or the payload, it is often desirable for the sender to supply, or the
recipient to determine, an identifier for a resource corresponding to recipient to determine, an identifier for a resource corresponding to
that representation. that representation.
For a request message: For a request message:
o If the request has a Content-Location header field, then the o If the request has a Content-Location header field, then the
sender asserts that the payload is a representation of the sender asserts that the payload is a representation of the
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might still be useful for revision history links. might still be useful for revision history links.
o Otherwise, the payload is unidentified. o Otherwise, the payload is unidentified.
For a response message, the following rules are applied in order For a response message, the following rules are applied in order
until a match is found: until a match is found:
1. If the request method is GET or HEAD and the response status code 1. If the request method is GET or HEAD and the response status code
is 200 (OK), 204 (No Content), 206 (Partial Content), or 304 (Not is 200 (OK), 204 (No Content), 206 (Partial Content), or 304 (Not
Modified), the payload is a representation of the resource Modified), the payload is a representation of the resource
identified by the target URI (Section 5.1). identified by the target URI (Section 6.1).
2. If the request method is GET or HEAD and the response status code 2. If the request method is GET or HEAD and the response status code
is 203 (Non-Authoritative Information), the payload is a is 203 (Non-Authoritative Information), the payload is a
potentially modified or enhanced representation of the target potentially modified or enhanced representation of the target
resource as provided by an intermediary. resource as provided by an intermediary.
3. If the response has a Content-Location header field and its field 3. If the response has a Content-Location header field and its field
value is a reference to the same URI as the target URI, the value is a reference to the same URI as the target URI, the
payload is a representation of the target resource. payload is a representation of the target resource.
4. If the response has a Content-Location header field and its field 4. If the response has a Content-Location header field and its field
value is a reference to a URI different from the target URI, then value is a reference to a URI different from the target URI, then
the sender asserts that the payload is a representation of the the sender asserts that the payload is a representation of the
resource identified by the Content-Location field value. resource identified by the Content-Location field value.
However, such an assertion cannot be trusted unless it can be However, such an assertion cannot be trusted unless it can be
verified by other means (not defined by this specification). verified by other means (not defined by this specification).
5. Otherwise, the payload is unidentified. 5. Otherwise, the payload is unidentified.
6.3.3. Payload Body 7.3.3. Payload Body
The payload body contains the data of a request or response. This is The payload body contains the data of a request or response. This is
distinct from the message body (e.g., Section 6 of [Messaging]), distinct from the message body (e.g., Section 6 of [Messaging]),
which is how the payload body is transferred "on the wire", and might which is how the payload body is transferred "on the wire", and might
be encoded, depending on the HTTP version in use. be encoded, depending on the HTTP version in use.
It is also distinct from a request or response's representation data It is also distinct from a request or response's representation data
(Section 6.1), which can be inferred from protocol operation, rather (Section 7.1), which can be inferred from protocol operation, rather
than necessarily appearing "on the wire." than necessarily appearing "on the wire."
The presence of a payload body in a request depends on whether the The presence of a payload body in a request depends on whether the
request method used defines semantics for it. request method used defines semantics for it.
The presence of a payload body in a response depends on both the The presence of a payload body in a response depends on both the
request method to which it is responding and the response status code request method to which it is responding and the response status code
(Section 9). (Section 10).
Responses to the HEAD request method (Section 7.3.2) never include a Responses to the HEAD request method (Section 8.3.2) never include a
payload body because the associated response header fields indicate payload body because the associated response header fields indicate
only what their values would have been if the request method had been only what their values would have been if the request method had been
GET (Section 7.3.1). GET (Section 8.3.1).
2xx (Successful) responses to a CONNECT request method 2xx (Successful) responses to a CONNECT request method
(Section 7.3.6) switch the connection to tunnel mode instead of (Section 8.3.6) switch the connection to tunnel mode instead of
having a payload body. having a payload body.
All 1xx (Informational), 204 (No Content), and 304 (Not Modified) All 1xx (Informational), 204 (No Content), and 304 (Not Modified)
responses do not include a payload body. responses do not include a payload body.
All other responses do include a payload body, although that body All other responses do include a payload body, although that body
might be of zero length. might be of zero length.
6.3.4. Content-Range 7.3.4. Content-Range
The "Content-Range" header field is sent in a single part 206 The "Content-Range" header field is sent in a single part 206
(Partial Content) response to indicate the partial range of the (Partial Content) response to indicate the partial range of the
selected representation enclosed as the message payload, sent in each selected representation enclosed as the message payload, sent in each
part of a multipart 206 response to indicate the range enclosed part of a multipart 206 response to indicate the range enclosed
within each body part, and sent in 416 (Range Not Satisfiable) within each body part, and sent in 416 (Range Not Satisfiable)
responses to provide information about the selected representation. responses to provide information about the selected representation.
Content-Range = range-unit SP Content-Range = range-unit SP
( range-resp / unsatisfied-range ) ( range-resp / unsatisfied-range )
range-resp = incl-range "/" ( complete-length / "*" ) range-resp = incl-range "/" ( complete-length / "*" )
incl-range = first-pos "-" last-pos incl-range = first-pos "-" last-pos
unsatisfied-range = "*/" complete-length unsatisfied-range = "*/" complete-length
complete-length = 1*DIGIT complete-length = 1*DIGIT
If a 206 (Partial Content) response contains a Content-Range header If a 206 (Partial Content) response contains a Content-Range header
field with a range unit (Section 6.1.4) that the recipient does not field with a range unit (Section 7.1.4) that the recipient does not
understand, the recipient MUST NOT attempt to recombine it with a understand, the recipient MUST NOT attempt to recombine it with a
stored representation. A proxy that receives such a message SHOULD stored representation. A proxy that receives such a message SHOULD
forward it downstream. forward it downstream.
For byte ranges, a sender SHOULD indicate the complete length of the For byte ranges, a sender SHOULD indicate the complete length of the
representation from which the range has been extracted, unless the representation from which the range has been extracted, unless the
complete length is unknown or difficult to determine. An asterisk complete length is unknown or difficult to determine. An asterisk
character ("*") in place of the complete-length indicates that the character ("*") in place of the complete-length indicates that the
representation length was unknown when the header field was representation length was unknown when the header field was
generated. generated.
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Content-Range: bytes 500-999/1234 Content-Range: bytes 500-999/1234
o All except for the first 500 bytes: o All except for the first 500 bytes:
Content-Range: bytes 500-1233/1234 Content-Range: bytes 500-1233/1234
o The last 500 bytes: o The last 500 bytes:
Content-Range: bytes 734-1233/1234 Content-Range: bytes 734-1233/1234
6.3.5. Media Type multipart/byteranges 7.3.5. Media Type multipart/byteranges
When a 206 (Partial Content) response message includes the content of When a 206 (Partial Content) response message includes the content of
multiple ranges, they are transmitted as body parts in a multipart multiple ranges, they are transmitted as body parts in a multipart
message body ([RFC2046], Section 5.1) with the media type of message body ([RFC2046], Section 5.1) with the media type of
"multipart/byteranges". "multipart/byteranges".
The multipart/byteranges media type includes one or more body parts, The multipart/byteranges media type includes one or more body parts,
each with its own Content-Type and Content-Range fields. The each with its own Content-Type and Content-Range fields. The
required boundary parameter specifies the boundary string used to required boundary parameter specifies the boundary string used to
separate each body part. separate each body part.
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Subtype name: byteranges Subtype name: byteranges
Required parameters: boundary Required parameters: boundary
Optional parameters: N/A Optional parameters: N/A
Encoding considerations: only "7bit", "8bit", or "binary" are Encoding considerations: only "7bit", "8bit", or "binary" are
permitted permitted
Security considerations: see Section 11 Security considerations: see Section 12
Interoperability considerations: N/A Interoperability considerations: N/A
Published specification: This specification (see Section 6.3.5). Published specification: This specification (see Section 7.3.5).
Applications that use this media type: HTTP components supporting Applications that use this media type: HTTP components supporting
multiple ranges in a single request. multiple ranges in a single request.
Fragment identifier considerations: N/A Fragment identifier considerations: N/A
Additional information: Additional information:
Deprecated alias names for this type: N/A Deprecated alias names for this type: N/A
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hors' Addresses section. hors' Addresses section.
Intended usage: COMMON Intended usage: COMMON
Restrictions on usage: N/A Restrictions on usage: N/A
Author: See Authors' Addresses section. Author: See Authors' Addresses section.
Change controller: IESG Change controller: IESG
6.4. Content Negotiation 7.4. Content Negotiation
When responses convey payload information, whether indicating a When responses convey payload information, whether indicating a
success or an error, the origin server often has different ways of success or an error, the origin server often has different ways of
representing that information; for example, in different formats, representing that information; for example, in different formats,
languages, or encodings. Likewise, different users or user agents languages, or encodings. Likewise, different users or user agents
might have differing capabilities, characteristics, or preferences might have differing capabilities, characteristics, or preferences
that could influence which representation, among those available, that could influence which representation, among those available,
would be best to deliver. For this reason, HTTP provides mechanisms would be best to deliver. For this reason, HTTP provides mechanisms
for content negotiation. for content negotiation.
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These patterns are not mutually exclusive, and each has trade-offs in These patterns are not mutually exclusive, and each has trade-offs in
applicability and practicality. applicability and practicality.
Note that, in all cases, HTTP is not aware of the resource semantics. Note that, in all cases, HTTP is not aware of the resource semantics.
The consistency with which an origin server responds to requests, The consistency with which an origin server responds to requests,
over time and over the varying dimensions of content negotiation, and over time and over the varying dimensions of content negotiation, and
thus the "sameness" of a resource's observed representations over thus the "sameness" of a resource's observed representations over
time, is determined entirely by whatever entity or algorithm selects time, is determined entirely by whatever entity or algorithm selects
or generates those responses. or generates those responses.
6.4.1. Proactive Negotiation 7.4.1. Proactive Negotiation
When content negotiation preferences are sent by the user agent in a When content negotiation preferences are sent by the user agent in a
request to encourage an algorithm located at the server to select the request to encourage an algorithm located at the server to select the
preferred representation, it is called proactive negotiation (a.k.a., preferred representation, it is called proactive negotiation (a.k.a.,
server-driven negotiation). Selection is based on the available server-driven negotiation). Selection is based on the available
representations for a response (the dimensions over which it might representations for a response (the dimensions over which it might
vary, such as language, content-coding, etc.) compared to various vary, such as language, content-coding, etc.) compared to various
information supplied in the request, including both the explicit information supplied in the request, including both the explicit
negotiation fields of Section 8.4 and implicit characteristics, such negotiation fields of Section 9.4 and implicit characteristics, such
as the client's network address or parts of the User-Agent field. as the client's network address or parts of the User-Agent field.
Proactive negotiation is advantageous when the algorithm for Proactive 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 a user agent, or when the server desires to send its describe to a user agent, or when the server desires to send its
"best guess" to the user agent along with the first response (hoping "best guess" to the user agent along with the first response (hoping
to avoid the round trip delay of a subsequent request if the "best to avoid the round trip delay of a subsequent request if the "best
guess" is good enough for the user). In order to improve the guess" is good enough for the user). In order to improve the
server's guess, a user agent MAY send request header fields that server's guess, a user agent MAY send request header fields that
describe its preferences. describe its preferences.
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algorithms for generating responses to a request; and, algorithms for generating responses to a request; and,
o It limits the reusability of responses for shared caching. o It limits the reusability of responses for shared caching.
A user agent cannot rely on proactive negotiation preferences being A user agent cannot rely on proactive negotiation preferences being
consistently honored, since the origin server might not implement consistently honored, since the origin server might not implement
proactive negotiation for the requested resource or might decide that proactive negotiation for the requested resource or might decide that
sending a response that doesn't conform to the user agent's sending a response that doesn't conform to the user agent's
preferences is better than sending a 406 (Not Acceptable) response. preferences is better than sending a 406 (Not Acceptable) response.
A Vary header field (Section 10.1.4) is often sent in a response A Vary header field (Section 11.1.4) is often sent in a response
subject to proactive negotiation to indicate what parts of the subject to proactive negotiation to indicate what parts of the
request information were used in the selection algorithm. request information were used in the selection algorithm.
6.4.2. Reactive Negotiation 7.4.2. Reactive Negotiation
With reactive negotiation (a.k.a., agent-driven negotiation), With reactive negotiation (a.k.a., agent-driven negotiation),
selection of the best response representation (regardless of the selection of the best response representation (regardless of the
status code) is performed by the user agent after receiving an status code) is performed by the user agent after receiving an
initial response from the origin server that contains a list of initial response from the origin server that contains a list of
resources for alternative representations. If the user agent is not resources for alternative representations. If the user agent is not
satisfied by the initial response representation, it can perform a satisfied by the initial response representation, it can perform a
GET request on one or more of the alternative resources, selected GET request on one or more of the alternative resources, selected
based on metadata included in the list, to obtain a different form of based on metadata included in the list, to obtain a different form of
representation for that response. Selection of alternatives might be representation for that response. Selection of alternatives might be
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caches are used to distribute server load and reduce network usage. caches are used to distribute server load and reduce network usage.
Reactive negotiation suffers from the disadvantages of transmitting a Reactive negotiation suffers from the disadvantages of transmitting a
list of alternatives to the user agent, which degrades user-perceived list of alternatives to the user agent, which degrades user-perceived
latency if transmitted in the header section, and needing a second latency if transmitted in the header section, and needing a second
request to obtain an alternate representation. Furthermore, this request to obtain an alternate representation. Furthermore, this
specification does not define a mechanism for supporting automatic specification does not define a mechanism for supporting automatic
selection, though it does not prevent such a mechanism from being selection, though it does not prevent such a mechanism from being
developed as an extension. developed as an extension.
6.4.3. Request Payload Negotiation 7.4.3. Request Payload Negotiation
When content negotiation preferences are sent in a server's response, When content negotiation preferences are sent in a server's response,
the listed preferences are called request payload negotiation because the listed preferences are called request payload negotiation because
they intend to influence selection of an appropriate payload for they intend to influence selection of an appropriate payload for
subsequent requests to that resource. For example, the Accept- subsequent requests to that resource. For example, the Accept-
Encoding field (Section 8.4.3) can be sent in a response to indicate Encoding field (Section 9.4.3) can be sent in a response to indicate
preferred content codings for subsequent requests to that resource preferred content codings for subsequent requests to that resource
[RFC7694]. [RFC7694].
Similarly, Section 3.1 of [RFC5789] defines the "Accept-Patch" Similarly, Section 3.1 of [RFC5789] defines the "Accept-Patch"
response header field which allows discovery of which content response header field which allows discovery of which content
types are accepted in PATCH requests. types are accepted in PATCH requests.
6.4.4. Quality Values 7.4.4. Quality Values
The content negotiation fields defined by this specification use a The content negotiation fields defined by this specification use a
common parameter, named "q" (case-insensitive), to assign a relative common parameter, named "q" (case-insensitive), to assign a relative
"weight" to the preference for that associated kind of content. This "weight" to the preference for that associated kind of content. This
weight is referred to as a "quality value" (or "qvalue") because the weight is referred to as a "quality value" (or "qvalue") because the
same parameter name is often used within server configurations to same parameter name is often used within server configurations to
assign a weight to the relative quality of the various assign a weight to the relative quality of the various
representations that can be selected for a resource. representations that can be selected for a resource.
The weight is normalized to a real number in the range 0 through 1, The weight is normalized to a real number in the range 0 through 1,
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the default weight is 1. the default weight is 1.
weight = OWS ";" OWS "q=" qvalue weight = OWS ";" OWS "q=" qvalue
qvalue = ( "0" [ "." 0*3DIGIT ] ) qvalue = ( "0" [ "." 0*3DIGIT ] )
/ ( "1" [ "." 0*3("0") ] ) / ( "1" [ "." 0*3("0") ] )
A sender of qvalue MUST NOT generate more than three digits after the A sender of qvalue MUST NOT generate more than three digits after the
decimal point. User configuration of these values ought to be decimal point. User configuration of these values ought to be
limited in the same fashion. limited in the same fashion.
7. Request Methods 8. Request Methods
7.1. Overview 8.1. Overview
The request method token is the primary source of request semantics; The request method token is the primary source of request semantics;
it indicates the purpose for which the client has made this request it indicates the purpose for which the client has made this request
and what is expected by the client as a successful result. and what is expected by the client as a successful result.
The request method's semantics might be further specialized by the The request method's semantics might be further specialized by the
semantics of some header fields when present in a request (Section 8) semantics of some header fields when present in a request (Section 9)
if those additional semantics do not conflict with the method. For if those additional semantics do not conflict with the method. For
example, a client can send conditional request header fields example, a client can send conditional request header fields
(Section 8.2) to make the requested action conditional on the current (Section 9.2) to make the requested action conditional on the current
state of the target resource. state of the target resource.
method = token method = token
HTTP was originally designed to be usable as an interface to HTTP was originally designed to be usable as an interface to
distributed object systems. The request method was envisioned as distributed object systems. The request method was envisioned as
applying semantics to a target resource in much the same way as applying semantics to a target resource in much the same way as
invoking a defined method on an identified object would apply invoking a defined method on an identified object would apply
semantics. semantics.
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defined, a standardized method ought to have the same semantics when defined, a standardized method ought to have the same semantics when
applied to any resource, though each resource determines for itself applied to any resource, though each resource determines for itself
whether those semantics are implemented or allowed. whether those semantics are implemented or allowed.
This specification defines a number of standardized methods that are This specification defines a number of standardized methods that are
commonly used in HTTP, as outlined by the following table. commonly used in HTTP, as outlined by the following table.
+---------+-------------------------------------------------+-------+ +---------+-------------------------------------------------+-------+
| Method | Description | Sec. | | Method | Description | Sec. |
+---------+-------------------------------------------------+-------+ +---------+-------------------------------------------------+-------+
| GET | Transfer a current representation of the target | 7.3.1 | | GET | Transfer a current representation of the target | 8.3.1 |
| | resource. | | | | resource. | |
| HEAD | Same as GET, but do not transfer the response | 7.3.2 | | HEAD | Same as GET, but do not transfer the response | 8.3.2 |
| | body. | | | | body. | |
| POST | Perform resource-specific processing on the | 7.3.3 | | POST | Perform resource-specific processing on the | 8.3.3 |
| | request payload. | | | | request payload. | |
| PUT | Replace all current representations of the | 7.3.4 | | PUT | Replace all current representations of the | 8.3.4 |
| | target resource with the request payload. | | | | target resource with the request payload. | |
| DELETE | Remove all current representations of the | 7.3.5 | | DELETE | Remove all current representations of the | 8.3.5 |
| | target resource. | | | | target resource. | |
| CONNECT | Establish a tunnel to the server identified by | 7.3.6 | | CONNECT | Establish a tunnel to the server identified by | 8.3.6 |
| | the target resource. | | | | the target resource. | |
| OPTIONS | Describe the communication options for the | 7.3.7 | | OPTIONS | Describe the communication options for the | 8.3.7 |
| | target resource. | | | | target resource. | |
| TRACE | Perform a message loop-back test along the path | 7.3.8 | | TRACE | Perform a message loop-back test along the path | 8.3.8 |
| | to the target resource. | | | | to the target resource. | |
+---------+-------------------------------------------------+-------+ +---------+-------------------------------------------------+-------+
Table 4 Table 4
All general-purpose servers MUST support the methods GET and HEAD. All general-purpose servers MUST support the methods GET and HEAD.
All other methods are OPTIONAL. All other methods are OPTIONAL.
The set of methods allowed by a target resource can be listed in an The set of methods allowed by a target resource can be listed in an
Allow header field (Section 10.4.2). However, the set of allowed Allow header field (Section 11.4.2). However, the set of allowed
methods can change dynamically. When a request method is received methods can change dynamically. When a request method is received
that is unrecognized or not implemented by an origin server, the that is unrecognized or not implemented by an origin server, the
origin server SHOULD respond with the 501 (Not Implemented) status origin server SHOULD respond with the 501 (Not Implemented) status
code. When a request method is received that is known by an origin code. When a request method is received that is known by an origin
server but not allowed for the target resource, the origin server server but not allowed for the target resource, the origin server
SHOULD respond with the 405 (Method Not Allowed) status code. SHOULD respond with the 405 (Method Not Allowed) status code.
7.2. Common Method Properties 8.2. Common Method Properties
+---------+------+------------+----------------+ +---------+------+------------+----------------+
| Method | Safe | Idempotent | Reference | | Method | Safe | Idempotent | Reference |
+---------+------+------------+----------------+ +---------+------+------------+----------------+
| CONNECT | no | no | Section 7.3.6 | | CONNECT | no | no | Section 8.3.6 |
| DELETE | no | yes | Section 7.3.5 | | DELETE | no | yes | Section 8.3.5 |
| GET | yes | yes | Section 7.3.1 | | GET | yes | yes | Section 8.3.1 |
| HEAD | yes | yes | Section 7.3.2 | | HEAD | yes | yes | Section 8.3.2 |
| OPTIONS | yes | yes | Section 7.3.7 | | OPTIONS | yes | yes | Section 8.3.7 |
| POST | no | no | Section 7.3.3 | | POST | no | no | Section 8.3.3 |
| PUT | no | yes | Section 7.3.4 | | PUT | no | yes | Section 8.3.4 |
| TRACE | yes | yes | Section 7.3.8 | | TRACE | yes | yes | Section 8.3.8 |
+---------+------+------------+----------------+ +---------+------+------------+----------------+
Table 5 Table 5
7.2.1. Safe Methods 8.2.1. Safe Methods
Request methods are considered "safe" if their defined semantics are Request methods are considered "safe" if their defined semantics are
essentially read-only; i.e., the client does not request, and does essentially read-only; i.e., the client does not request, and does
not expect, any state change on the origin server as a result of not expect, any state change on the origin server as a result of
applying a safe method to a target resource. Likewise, reasonable applying a safe method to a target resource. Likewise, reasonable
use of a safe method is not expected to cause any harm, loss of use of a safe method is not expected to cause any harm, loss of
property, or unusual burden on the origin server. property, or unusual burden on the origin server.
This definition of safe methods does not prevent an implementation This definition of safe methods does not prevent an implementation
from including behavior that is potentially harmful, that is not from including behavior that is potentially harmful, that is not
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the request method semantics. For example, it is common for Web- the request method semantics. For example, it is common for Web-
based content editing software to use actions within query based content editing software to use actions within query
parameters, such as "page?do=delete". If the purpose of such a parameters, such as "page?do=delete". If the purpose of such a
resource is to perform an unsafe action, then the resource owner MUST resource is to perform an unsafe action, then the resource owner MUST
disable or disallow that action when it is accessed using a safe disable or disallow that action when it is accessed using a safe
request method. Failure to do so will result in unfortunate side request method. Failure to do so will result in unfortunate side
effects when automated processes perform a GET on every URI reference effects when automated processes perform a GET on every URI reference
for the sake of link maintenance, pre-fetching, building a search for the sake of link maintenance, pre-fetching, building a search
index, etc. index, etc.
7.2.2. Idempotent Methods 8.2.2. Idempotent Methods
A request method is considered "idempotent" if the intended effect on A request method is considered "idempotent" if the intended effect on
the server of multiple identical requests with that method is the the server of multiple identical requests with that method is the
same as the effect for a single such request. Of the request methods same as the effect for a single such request. Of the request methods
defined by this specification, PUT, DELETE, and safe request methods defined by this specification, PUT, DELETE, and safe request methods
are idempotent. are idempotent.
Like the definition of safe, the idempotent property only applies to Like the definition of safe, the idempotent property only applies to
what has been requested by the user; a server is free to log each what has been requested by the user; a server is free to log each
request separately, retain a revision control history, or implement request separately, retain a revision control history, or implement
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retrying the requests that failed. retrying the requests that failed.
Some clients use weaker signals to initiate automatic retries. For Some clients use weaker signals to initiate automatic retries. For
example, when a POST request is sent, but the underlying transport example, when a POST request is sent, but the underlying transport
connection is closed before any part of the response is received. connection is closed before any part of the response is received.
Although this is commonly implemented, it is not recommended. Although this is commonly implemented, it is not recommended.
A proxy MUST NOT automatically retry non-idempotent requests. A A proxy MUST NOT automatically retry non-idempotent requests. A
client SHOULD NOT automatically retry a failed automatic retry. client SHOULD NOT automatically retry a failed automatic retry.
7.2.3. Methods and Caching 8.2.3. Methods and Caching
For a cache to store and use a response, the associated method needs For a cache to store and use a response, the associated method needs
to explicitly allow caching, and detail under what conditions a to explicitly allow caching, and detail under what conditions a
response can be used to satisfy subsequent requests; a method response can be used to satisfy subsequent requests; a method
definition which does not do so cannot be cached. For additional definition which does not do so cannot be cached. For additional
requirements see [Caching]. requirements see [Caching].
This specification defines caching semantics for GET, HEAD, and POST, This specification defines caching semantics for GET, HEAD, and POST,
although the overwhelming majority of cache implementations only although the overwhelming majority of cache implementations only
support GET and HEAD. support GET and HEAD.
7.3. Method Definitions 8.3. Method Definitions
7.3.1. GET 8.3.1. GET
The GET method requests transfer of a current selected representation The GET method requests transfer of a current selected representation
for the target resource. GET is the primary mechanism of information for the target resource. GET is the primary mechanism of information
retrieval and the focus of almost all performance optimizations. retrieval and the focus of almost all performance optimizations.
Hence, when people speak of retrieving some identifiable information Hence, when people speak of retrieving some identifiable information
via HTTP, they are generally referring to making a GET request. via HTTP, they are generally referring to making a GET request.
The GET method is specifically intended to reflect the quality of The GET method is specifically intended to reflect the quality of
"sameness" identified by the request URI as if it were referenced as "sameness" identified by the request URI as if it were referenced as
an ordinary hypertext link. an ordinary hypertext link.
It is tempting to think of resource identifiers as remote file system It is tempting to think of resource identifiers as remote file system
pathnames and of representations as being a copy of the contents of pathnames and of representations as being a copy of the contents of
such files. In fact, that is how many resources are implemented (see such files. In fact, that is how many resources are implemented (see
Section 11.3 for related security considerations). However, there Section 12.3 for related security considerations). However, there
are no such limitations in practice. The HTTP interface for a are no such limitations in practice. The HTTP interface for a
resource is just as likely to be implemented as a tree of content resource is just as likely to be implemented as a tree of content
objects, a programmatic view on various database records, or a objects, a programmatic view on various database records, or a
gateway to other information systems. Even when the URI mapping gateway to other information systems. Even when the URI mapping
mechanism is tied to a file system, an origin server might be mechanism is tied to a file system, an origin server might be
configured to execute the files with the request as input and send configured to execute the files with the request as input and send
the output as the representation rather than transfer the files the output as the representation rather than transfer the files
directly. Regardless, only the origin server needs to know how each directly. Regardless, only the origin server needs to know how each
of its resource identifiers corresponds to an implementation and how of its resource identifiers corresponds to an implementation and how
each implementation manages to select and send a current each implementation manages to select and send a current
representation of the target resource in a response to GET. representation of the target resource in a response to GET.
A client can alter the semantics of GET to be a "range request", A client can alter the semantics of GET to be a "range request",
requesting transfer of only some part(s) of the selected requesting transfer of only some part(s) of the selected
representation, by sending a Range header field in the request representation, by sending a Range header field in the request
(Section 8.3). (Section 9.3).
A client SHOULD NOT generate a body in a GET request. A payload A client SHOULD NOT generate a body in a GET request. A payload
received in a GET request has no defined semantics, cannot alter the received in a GET request has no defined semantics, cannot alter the
meaning or target of the request, and might lead some implementations meaning or target of the request, and might lead some implementations
to reject the request and close the connection because of its to reject the request and close the connection because of its
potential as a request smuggling attack (Section 11.2 of potential as a request smuggling attack (Section 11.2 of
[Messaging]). [Messaging]).
The response to a GET request is cacheable; a cache MAY use it to The response to a GET request is cacheable; a cache MAY use it to
satisfy subsequent GET and HEAD requests unless otherwise indicated satisfy subsequent GET and HEAD requests unless otherwise indicated
by the Cache-Control header field (Section 5.2 of [Caching]). A by the Cache-Control header field (Section 5.2 of [Caching]). A
cache that receives a payload in a GET request is likely to ignore cache that receives a payload in a GET request is likely to ignore
that payload and cache regardless of the payload contents. that payload and cache regardless of the payload contents.
7.3.2. HEAD 8.3.2. 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
send a message body in the response (i.e., the response terminates at send a message body in the response (i.e., the response terminates at
the end of the header section). The server SHOULD send the same the end of the header section). The server SHOULD send the same
header fields in response to a HEAD request as it would have sent if header fields in response to a HEAD request as it would have sent if
the request had been a GET, except that the payload header fields the request had been a GET, except that the payload header fields
(Section 6.3) MAY be omitted. This method can be used for obtaining (Section 7.3) MAY be omitted. This method can be used for obtaining
metadata about the selected representation without transferring the metadata about the selected representation without transferring the
representation data and is often used for testing hypertext links for representation data and is often used for testing hypertext links for
validity, accessibility, and recent modification. validity, accessibility, and recent modification.
A payload within a HEAD request message has no defined semantics; A payload within a HEAD request message has no defined semantics;
sending a payload body on a HEAD request might cause some existing sending a payload body on a HEAD request might cause some existing
implementations to reject the request. implementations to reject the request.
The response to a HEAD request is cacheable; a cache MAY use it to The response to a HEAD request is cacheable; a cache MAY use it to
satisfy subsequent HEAD requests unless otherwise indicated by the satisfy subsequent HEAD requests unless otherwise indicated by the
Cache-Control header field (Section 5.2 of [Caching]). A HEAD Cache-Control header field (Section 5.2 of [Caching]). A HEAD
response might also have an effect on previously cached responses to response might also have an effect on previously cached responses to
GET; see Section 4.3.5 of [Caching]. GET; see Section 4.3.5 of [Caching].
7.3.3. POST 8.3.3. POST
The POST method requests that the target resource process the The POST method requests that the target resource process the
representation enclosed in the request according to the resource's representation enclosed in the request according to the resource's
own specific semantics. For example, POST is used for the following own specific semantics. For example, POST is used for the following
functions (among others): functions (among others):
o Providing a block of data, such as the fields entered into an HTML o Providing a block of data, such as the fields entered into an HTML
form, to a data-handling process; form, to a data-handling process;
o Posting a message to a bulletin board, newsgroup, mailing list, o Posting a message to a bulletin board, newsgroup, mailing list,
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appropriate status code depending on the result of processing the appropriate status code depending on the result of processing the
POST request; almost all of the status codes defined by this POST request; almost all of the status codes defined by this
specification might be received in a response to POST (the exceptions specification might be received in a response to POST (the exceptions
being 206 (Partial Content), 304 (Not Modified), and 416 (Range Not being 206 (Partial Content), 304 (Not Modified), and 416 (Range Not
Satisfiable)). Satisfiable)).
If one or more resources has been created on the origin server as a If one or more resources has been created on the origin server as a
result of successfully processing a POST request, the origin server result of successfully processing a POST request, the origin server
SHOULD send a 201 (Created) response containing a Location header SHOULD send a 201 (Created) response containing a Location header
field that provides an identifier for the primary resource created field that provides an identifier for the primary resource created
(Section 10.1.2) and a representation that describes the status of (Section 11.1.2) and a representation that describes the status of
the request while referring to the new resource(s). the request while referring to the new resource(s).
Responses to POST requests are only cacheable when they include Responses to POST requests are only cacheable when they include
explicit freshness information (see Section 4.2.1 of [Caching]) and a explicit freshness information (see Section 4.2.1 of [Caching]) and a
Content-Location header field that has the same value as the POST's Content-Location header field that has the same value as the POST's
target URI (Section 6.2.5). A cached POST response can be reused to target URI (Section 7.2.5). A cached POST response can be reused to
satisfy a later GET or HEAD request, but not a POST request, since satisfy a later GET or HEAD request, but not a POST request, since
POST is required to be written through to the origin server, because POST is required to be written through to the origin server, because
it is unsafe; see Section 4 of [Caching]. it is unsafe; see Section 4 of [Caching].
If the result of processing a POST would be equivalent to a If the result of processing a POST would be equivalent to a
representation of an existing resource, an origin server MAY redirect representation of an existing resource, an origin server MAY redirect
the user agent to that resource by sending a 303 (See Other) response the user agent to that resource by sending a 303 (See Other) response
with the existing resource's identifier in the Location field. This with the existing resource's identifier in the Location field. This
has the benefits of providing the user agent a resource identifier has the benefits of providing the user agent a resource identifier
and transferring the representation via a method more amenable to and transferring the representation via a method more amenable to
shared caching, though at the cost of an extra request if the user shared caching, though at the cost of an extra request if the user
agent does not already have the representation cached. agent does not already have the representation cached.
7.3.4. PUT 8.3.4. PUT
The PUT method requests that the state of the target resource be The PUT method requests that the state of the target resource be
created or replaced with the state defined by the representation created or replaced with the state defined by the representation
enclosed in the request message payload. A successful PUT of a given enclosed in the request message payload. A successful PUT of a given
representation would suggest that a subsequent GET on that same representation would suggest that a subsequent GET on that same
target resource will result in an equivalent representation being target resource will result in an equivalent representation being
sent in a 200 (OK) response. However, there is no guarantee that sent in a 200 (OK) response. However, there is no guarantee that
such a state change will be observable, since the target resource such a state change will be observable, since the target resource
might be acted upon by other user agents in parallel, or might be might be acted upon by other user agents in parallel, or might be
subject to dynamic processing by the origin server, before any subject to dynamic processing by the origin server, before any
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agent request and the semantics of the origin server response. It agent request and the semantics of the origin server response. It
does not define what a resource might be, in any sense of that word, does not define what a resource might be, in any sense of that word,
beyond the interface provided via HTTP. It does not define how beyond the interface provided via HTTP. It does not define how
resource state is "stored", nor how such storage might change as a resource state is "stored", nor how such storage might change as a
result of a change in resource state, nor how the origin server result of a change in resource state, nor how the origin server
translates resource state into representations. Generally speaking, translates resource state into representations. Generally speaking,
all implementation details behind the resource interface are all implementation details behind the resource interface are
intentionally hidden by the server. intentionally hidden by the server.
An origin server MUST NOT send a validator header field An origin server MUST NOT send a validator header field
(Section 10.2), such as an ETag or Last-Modified field, in a (Section 11.2), such as an ETag or Last-Modified field, in a
successful response to PUT unless the request's representation data successful response to PUT unless the request's representation data
was saved without any transformation applied to the body (i.e., the was saved without any transformation applied to the body (i.e., the
resource's new representation data is identical to the representation resource's new representation data is identical to the representation
data received in the PUT request) and the validator field value data received in the PUT request) and the validator field value
reflects the new representation. This requirement allows a user reflects the new representation. This requirement allows a user
agent to know when the representation body it has in memory remains agent to know when the representation body it has in memory remains
current as a result of the PUT, thus not in need of being retrieved current as a result of the PUT, thus not in need of being retrieved
again from the origin server, and that the new validator(s) received again from the origin server, and that the new validator(s) received
in the response can be used for future conditional requests in order in the response can be used for future conditional requests in order
to prevent accidental overwrites (Section 8.2). to prevent accidental overwrites (Section 9.2).
The fundamental difference between the POST and PUT methods is The fundamental difference between the POST and PUT methods is
highlighted by the different intent for the enclosed representation. highlighted by the different intent for the enclosed representation.
The target resource in a POST request is intended to handle the The target resource in a POST request is intended to handle the
enclosed representation according to the resource's own semantics, enclosed representation according to the resource's own semantics,
whereas the enclosed representation in a PUT request is defined as whereas the enclosed representation in a PUT request is defined as
replacing the state of the target resource. Hence, the intent of PUT replacing the state of the target resource. Hence, the intent of PUT
is idempotent and visible to intermediaries, even though the exact is idempotent and visible to intermediaries, even though the exact
effect is only known by the origin server. effect is only known by the origin server.
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identifying "the current version" (a resource) that is separate from identifying "the current version" (a resource) that is separate from
the URIs identifying each particular version (different resources the URIs identifying each particular version (different resources
that at one point shared the same state as the current version that at one point shared the same state as the current version
resource). A successful PUT request on "the current version" URI resource). A successful PUT request on "the current version" URI
might therefore create a new version resource in addition to changing might therefore create a new version resource in addition to changing
the state of the target resource, and might also cause links to be the state of the target resource, and might also cause links to be
added between the related resources. added between the related resources.
An origin server that allows PUT on a given target resource MUST send An origin server that allows PUT on a given target resource MUST send
a 400 (Bad Request) response to a PUT request that contains a a 400 (Bad Request) response to a PUT request that contains a
Content-Range header field (Section 6.3.4), since the payload is Content-Range header field (Section 7.3.4), since the payload is
likely to be partial content that has been mistakenly PUT as a full likely to be partial content that has been mistakenly PUT as a full
representation. Partial content updates are possible by targeting a representation. Partial content updates are possible by targeting a
separately identified resource with state that overlaps a portion of separately identified resource with state that overlaps a portion of
the larger resource, or by using a different method that has been the larger resource, or by using a different method that has been
specifically defined for partial updates (for example, the PATCH specifically defined for partial updates (for example, the PATCH
method defined in [RFC5789]). method defined in [RFC5789]).
Responses to the PUT method are not cacheable. If a successful PUT Responses to the PUT method are not cacheable. If a successful PUT
request passes through a cache that has one or more stored responses request passes through a cache that has one or more stored responses
for the target URI, those stored responses will be invalidated (see for the target URI, those stored responses will be invalidated (see
Section 4.4 of [Caching]). Section 4.4 of [Caching]).
7.3.5. DELETE 8.3.5. DELETE
The DELETE method requests that the origin server remove the The DELETE method requests that the origin server remove the
association between the target resource and its current association between the target resource and its current
functionality. In effect, this method is similar to the rm command functionality. In effect, this method is similar to the rm command
in UNIX: it expresses a deletion operation on the URI mapping of the in UNIX: it expresses a deletion operation on the URI mapping of the
origin server rather than an expectation that the previously origin server rather than an expectation that the previously
associated information be deleted. associated information be deleted.
If the target resource has one or more current representations, they If the target resource has one or more current representations, they
might or might not be destroyed by the origin server, and the might or might not be destroyed by the origin server, and the
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A client SHOULD NOT generate a body in a DELETE request. A payload A client SHOULD NOT generate a body in a DELETE request. A payload
received in a DELETE request has no defined semantics, cannot alter received in a DELETE request has no defined semantics, cannot alter
the meaning or target of the request, and might lead some the meaning or target of the request, and might lead some
implementations to reject the request. implementations to reject the request.
Responses to the DELETE method are not cacheable. If a successful Responses to the DELETE method are not cacheable. If a successful
DELETE request passes through a cache that has one or more stored DELETE request passes through a cache that has one or more stored
responses for the target URI, those stored responses will be responses for the target URI, those stored responses will be
invalidated (see Section 4.4 of [Caching]). invalidated (see Section 4.4 of [Caching]).
7.3.6. CONNECT 8.3.6. CONNECT
The CONNECT method requests that the recipient establish a tunnel to The CONNECT method requests that the recipient establish a tunnel to
the destination origin server identified by the request target and, the destination origin server identified by the request target and,
if successful, thereafter restrict its behavior to blind forwarding if successful, thereafter restrict its behavior to blind forwarding
of data, in both directions, until the tunnel is closed. Tunnels are of data, in both directions, until the tunnel is closed. Tunnels are
commonly used to create an end-to-end virtual connection, through one commonly used to create an end-to-end virtual connection, through one
or more proxies, which can then be secured using TLS (Transport Layer or more proxies, which can then be secured using TLS (Transport Layer
Security, [RFC8446]). Security, [RFC8446]).
Because CONNECT changes the request/response nature of an HTTP
connection, specific HTTP versions might have different ways of
mapping its semantics into the protocol's wire format.
CONNECT is intended only for use in requests to a proxy. An origin CONNECT is intended only for use in requests to a proxy. An origin
server that receives a CONNECT request for itself MAY respond with a server that receives a CONNECT request for itself MAY respond with a
2xx (Successful) status code to indicate that a connection is 2xx (Successful) status code to indicate that a connection is
established. However, most origin servers do not implement CONNECT. established. However, most origin servers do not implement CONNECT.
A client sending a CONNECT request MUST send the authority component A client sending a CONNECT request MUST send the authority component
(described in Section 3.2 of [RFC3986]) as the request target; i.e., (described in Section 3.2 of [RFC3986]) as the request target; i.e.,
the request target consists of only the host name and port number of the request target consists of only the host name and port number of
the tunnel destination, separated by a colon. For example, the tunnel destination, separated by a colon. For example,
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fields in a 2xx (Successful) response to CONNECT. A client MUST fields in a 2xx (Successful) response to CONNECT. A client MUST
ignore any Content-Length or Transfer-Encoding header fields received ignore any Content-Length or Transfer-Encoding header fields received
in a successful response to CONNECT. in a successful response to CONNECT.
A payload within a CONNECT request message has no defined semantics; A payload within a CONNECT request message has no defined semantics;
sending a payload body on a CONNECT request might cause some existing sending a payload body on a CONNECT request might cause some existing
implementations to reject the request. implementations to reject the request.
Responses to the CONNECT method are not cacheable. Responses to the CONNECT method are not cacheable.
7.3.7. OPTIONS 8.3.7. OPTIONS
The OPTIONS method requests information about the communication The OPTIONS method requests information about the communication
options available for the target resource, at either the origin options available for the target resource, at either the origin
server or an intervening intermediary. This method allows a client server or an intervening intermediary. This method allows a client
to determine the options and/or requirements associated with a to determine the options and/or requirements associated with a
resource, or the capabilities of a server, without implying a resource, or the capabilities of a server, without implying a
resource action. resource action.
An OPTIONS request with an asterisk ("*") as the request target An OPTIONS request with an asterisk ("*") as the request target
(Section 5.1) applies to the server in general rather than to a (Section 6.1) applies to the server in general rather than to a
specific resource. Since a server's communication options typically specific resource. Since a server's communication options typically
depend on the resource, the "*" request is only useful as a "ping" or depend on the resource, the "*" request is only useful as a "ping" or
"no-op" type of method; it does nothing beyond allowing the client to "no-op" type of method; it does nothing beyond allowing the client to
test the capabilities of the server. For example, this can be used test the capabilities of the server. For example, this can be used
to test a proxy for HTTP/1.1 conformance (or lack thereof). to test a proxy for HTTP/1.1 conformance (or lack thereof).
If the request target is not an asterisk, the OPTIONS request applies If the request target is not an asterisk, the OPTIONS request applies
to the options that are available when communicating with the target to the options that are available when communicating with the target
resource. resource.
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header that might indicate optional features implemented by the header that might indicate optional features implemented by the
server and applicable to the target resource (e.g., Allow), including server and applicable to the target resource (e.g., Allow), including
potential extensions not defined by this specification. The response potential extensions not defined by this specification. The response
payload, if any, might also describe the communication options in a payload, if any, might also describe the communication options in a
machine or human-readable representation. A standard format for such machine or human-readable representation. A standard format for such
a representation is not defined by this specification, but might be a representation is not defined by this specification, but might be
defined by future extensions to HTTP. defined by future extensions to HTTP.
A client MAY send a Max-Forwards header field in an OPTIONS request A client MAY send a Max-Forwards header field in an OPTIONS request
to target a specific recipient in the request chain (see to target a specific recipient in the request chain (see
Section 8.1.2). A proxy MUST NOT generate a Max-Forwards header Section 9.1.2). A proxy MUST NOT generate a Max-Forwards header
field while forwarding a request unless that request was received field while forwarding a request unless that request was received
with a Max-Forwards field. with a Max-Forwards field.
A client that generates an OPTIONS request containing a payload body A client that generates an OPTIONS request containing a payload body
MUST send a valid Content-Type header field describing the MUST send a valid Content-Type header field describing the
representation media type. Note that this specification does not representation media type. Note that this specification does not
define any use for such a payload. define any use for such a payload.
Responses to the OPTIONS method are not cacheable. Responses to the OPTIONS method are not cacheable.
7.3.8. TRACE 8.3.8. TRACE
The TRACE method requests a remote, application-level loop-back of The TRACE method requests a remote, application-level loop-back of
the request message. The final recipient of the request SHOULD the request message. The final recipient of the request SHOULD
reflect the message received, excluding some fields described below, reflect the message received, excluding some fields described below,
back to the client as the message body of a 200 (OK) response with a back to the client as the message body of a 200 (OK) response with a
Content-Type of "message/http" (Section 10.1 of [Messaging]). The Content-Type of "message/http" (Section 10.1 of [Messaging]). The
final recipient is either the origin server or the first server to final recipient is either the origin server or the first server to
receive a Max-Forwards value of zero (0) in the request receive a Max-Forwards value of zero (0) in the request
(Section 8.1.2). (Section 9.1.2).
A client MUST NOT generate fields in a TRACE request containing A client MUST NOT generate fields in a TRACE request containing
sensitive data that might be disclosed by the response. For example, sensitive data that might be disclosed by the response. For example,
it would be foolish for a user agent to send stored user credentials it would be foolish for a user agent to send stored user credentials
Section 8.5 or cookies [RFC6265] in a TRACE request. The final Section 9.5 or cookies [RFC6265] in a TRACE request. The final
recipient of the request SHOULD exclude any request fields that are recipient of the request SHOULD exclude any request fields that are
likely to contain sensitive data when that recipient generates the likely to contain sensitive data when that recipient generates the
response body. response body.
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 5.7.1) is of information. The value of the Via header field (Section 6.7.1) 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.
A client MUST NOT send a message body in a TRACE request. A client MUST NOT send a message body in a TRACE request.
Responses to the TRACE method are not cacheable. Responses to the TRACE method are not cacheable.
7.4. Method Extensibility 8.4. Method Extensibility
Additional methods, outside the scope of this specification, have Additional methods, outside the scope of this specification, have
been specified for use in HTTP. All such methods ought to be been specified for use in HTTP. All such methods ought to be
registered within the "Hypertext Transfer Protocol (HTTP) Method registered within the "Hypertext Transfer Protocol (HTTP) Method
Registry". Registry".
7.4.1. Method Registry 8.4.1. Method Registry
The "Hypertext Transfer Protocol (HTTP) Method Registry", maintained The "Hypertext Transfer Protocol (HTTP) Method Registry", maintained
by IANA at <https://www.iana.org/assignments/http-methods>, registers by IANA at <https://www.iana.org/assignments/http-methods>, registers
method names. method names.
HTTP method registrations MUST include the following fields: HTTP method registrations MUST include the following fields:
o Method Name (see Section 7) o Method Name (see Section 8)
o Safe ("yes" or "no", see Section 7.2.1)
o Idempotent ("yes" or "no", see Section 7.2.2) o Safe ("yes" or "no", see Section 8.2.1)
o Idempotent ("yes" or "no", see Section 8.2.2)
o Pointer to specification text o Pointer to specification text
Values to be added to this namespace require IETF Review (see Values to be added to this namespace require IETF Review (see
[RFC8126], Section 4.8). [RFC8126], Section 4.8).
7.4.2. Considerations for New Methods 8.4.2. Considerations for New Methods
Standardized methods are generic; that is, they are potentially Standardized methods are generic; that is, they are potentially
applicable to any resource, not just one particular media type, kind applicable to any resource, not just one particular media type, kind
of resource, or application. As such, it is preferred that new of resource, or application. As such, it is preferred that new
methods be registered in a document that isn't specific to a single methods be registered in a document that isn't specific to a single
application or data format, since orthogonal technologies deserve application or data format, since orthogonal technologies deserve
orthogonal specification. orthogonal specification.
Since message parsing (Section 6 of [Messaging]) needs to be Since message parsing (Section 6 of [Messaging]) needs to be
independent of method semantics (aside from responses to HEAD), independent of method semantics (aside from responses to HEAD),
definitions of new methods cannot change the parsing algorithm or definitions of new methods cannot change the parsing algorithm or
prohibit the presence of a message body on either the request or the prohibit the presence of a message body on either the request or the
response message. Definitions of new methods can specify that only a response message. Definitions of new methods can specify that only a
zero-length message body is allowed by requiring a Content-Length zero-length message body is allowed by requiring a Content-Length
header field with a value of "0". header field with a value of "0".
A new method definition needs to indicate whether it is safe A new method definition needs to indicate whether it is safe
(Section 7.2.1), idempotent (Section 7.2.2), cacheable (Section 8.2.1), idempotent (Section 8.2.2), cacheable
(Section 7.2.3), what semantics are to be associated with the payload (Section 8.2.3), what semantics are to be associated with the payload
body if any is present in the request and what refinements the method body if any is present in the request and what refinements the method
makes to header field or status code semantics. If the new method is makes to header field or status code semantics. If the new method is
cacheable, its definition ought to describe how, and under what cacheable, its definition ought to describe how, and under what
conditions, a cache can store a response and use it to satisfy a conditions, a cache can store a response and use it to satisfy a
subsequent request. The new method ought to describe whether it can subsequent request. The new method ought to describe whether it can
be made conditional (Section 8.2) and, if so, how a server responds be made conditional (Section 9.2) and, if so, how a server responds
when the condition is false. Likewise, if the new method might have when the condition is false. Likewise, if the new method might have
some use for partial response semantics (Section 8.3), it ought to some use for partial response semantics (Section 9.3), it ought to
document this, too. document this, too.
Note: Avoid defining a method name that starts with "M-", since Note: Avoid defining a method name that starts with "M-", since
that prefix might be misinterpreted as having the semantics that prefix might be misinterpreted as having the semantics
assigned to it by [RFC2774]. assigned to it by [RFC2774].
8. Request Header Fields 9. Request Header Fields
A client sends request header fields to provide more information A client sends request header fields to provide more information
about the request context, make the request conditional based on the about the request context, make the request conditional based on the
target resource state, suggest preferred formats for the response, target resource state, suggest preferred formats for the response,
supply authentication credentials, or modify the expected request supply authentication credentials, or modify the expected request
processing. These fields act as request modifiers, similar to the processing. These fields act as request modifiers, similar to the
parameters on a programming language method invocation. parameters on a programming language method invocation.
8.1. Controls 9.1. Controls
Controls are request header fields that direct specific handling of Controls are request header fields that direct specific handling of
the request. the request.
+---------------+----------------------------+ +---------------+----------------------------+
| Field Name | Defined in... | | Field Name | Defined in... |
+---------------+----------------------------+ +---------------+----------------------------+
| Cache-Control | Section 5.2 of [Caching] | | Cache-Control | Section 5.2 of [Caching] |
| Expect | Section 8.1.1 | | Expect | Section 9.1.1 |
| Host | Section 5.6 | | Host | Section 6.6 |
| Max-Forwards | Section 8.1.2 | | Max-Forwards | Section 9.1.2 |
| Pragma | Section 5.4 of [Caching] | | Pragma | Section 5.4 of [Caching] |
| TE | Section 7.4 of [Messaging] | | TE | Section 7.4 of [Messaging] |
+---------------+----------------------------+ +---------------+----------------------------+
8.1.1. Expect 9.1.1. Expect
The "Expect" header field in a request indicates a certain set of The "Expect" header field in a request indicates a certain set of
behaviors (expectations) that need to be supported by the server in behaviors (expectations) that need to be supported by the server in
order to properly handle this request. The only such expectation order to properly handle this request. The only such expectation
defined by this specification is 100-continue. defined by this specification is 100-continue.
Expect = "100-continue" Expect = "100-continue"
The Expect field value is case-insensitive. The Expect field value is case-insensitive.
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Note: The Expect header field was added after the original Note: The Expect header field was added after the original
publication of HTTP/1.1 [RFC2068] as both the means to request an publication of HTTP/1.1 [RFC2068] as both the means to request an
interim 100 (Continue) response and the general mechanism for interim 100 (Continue) response and the general mechanism for
indicating must-understand extensions. However, the extension indicating must-understand extensions. However, the extension
mechanism has not been used by clients and the must-understand mechanism has not been used by clients and the must-understand
requirements have not been implemented by many servers, rendering requirements have not been implemented by many servers, rendering
the extension mechanism useless. This specification has removed the extension mechanism useless. This specification has removed
the extension mechanism in order to simplify the definition and the extension mechanism in order to simplify the definition and
processing of 100-continue. processing of 100-continue.
8.1.2. Max-Forwards 9.1.2. Max-Forwards
The "Max-Forwards" header field provides a mechanism with the TRACE The "Max-Forwards" header field provides a mechanism with the TRACE
(Section 7.3.8) and OPTIONS (Section 7.3.7) request methods to limit (Section 8.3.8) and OPTIONS (Section 8.3.7) request methods to limit
the number of times that the request is forwarded by proxies. This the number of times that the request is forwarded by proxies. This
can be useful when the client is attempting to trace a request that can be useful when the client is attempting to trace a request that
appears to be failing or looping mid-chain. appears to be failing or looping mid-chain.
Max-Forwards = 1*DIGIT Max-Forwards = 1*DIGIT
The Max-Forwards value is a decimal integer indicating the remaining The Max-Forwards value is a decimal integer indicating the remaining
number of times this request message can be forwarded. number of times this request message can be forwarded.
Each intermediary that receives a TRACE or OPTIONS request containing Each intermediary that receives a TRACE or OPTIONS request containing
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intermediary MUST NOT forward the request; instead, the intermediary intermediary MUST NOT forward the request; instead, the intermediary
MUST respond as the final recipient. If the received Max-Forwards MUST respond as the final recipient. If the received Max-Forwards
value is greater than zero, the intermediary MUST generate an updated value is greater than zero, the intermediary MUST generate an updated
Max-Forwards field in the forwarded message with a field value that Max-Forwards field in the forwarded message with a field value that
is the lesser of a) the received value decremented by one (1) or b) is the lesser of a) the received value decremented by one (1) or b)
the recipient's maximum supported value for Max-Forwards. the recipient's maximum supported value for Max-Forwards.
A recipient MAY ignore a Max-Forwards header field received with any A recipient MAY ignore a Max-Forwards header field received with any
other request methods. other request methods.
8.2. Preconditions 9.2. Preconditions
A conditional request is an HTTP request with one or more request A conditional request is an HTTP request with one or more request
header fields that indicate a precondition to be tested before header fields that indicate a precondition to be tested before
applying the request method to the target resource. Section 8.2.1 applying the request method to the target resource. Section 9.2.1
defines when preconditions are applied. Section 8.2.2 defines the defines when preconditions are applied. Section 9.2.2 defines the
order of evaluation when more than one precondition is present. order of evaluation when more than one precondition is present.
Conditional GET requests are the most efficient mechanism for HTTP Conditional GET requests are the most efficient mechanism for HTTP
cache updates [Caching]. Conditionals can also be applied to state- cache updates [Caching]. Conditionals can also be applied to state-
changing methods, such as PUT and DELETE, to prevent the "lost changing methods, such as PUT and DELETE, to prevent the "lost
update" problem: one client accidentally overwriting the work of update" problem: one client accidentally overwriting the work of
another client that has been acting in parallel. another client that has been acting in parallel.
Conditional request preconditions are based on the state of the Conditional request preconditions are based on the state of the
target resource as a whole (its current value set) or the state as target resource as a whole (its current value set) or the state as
observed in a previously obtained representation (one value in that observed in a previously obtained representation (one value in that
set). A resource might have multiple current representations, each set). A resource might have multiple current representations, each
with its own observable state. The conditional request mechanisms with its own observable state. The conditional request mechanisms
assume that the mapping of requests to a selected representation assume that the mapping of requests to a selected representation
(Section 6) will be consistent over time if the server intends to (Section 7) will be consistent over time if the server intends to
take advantage of conditionals. Regardless, if the mapping is take advantage of conditionals. Regardless, if the mapping is
inconsistent and the server is unable to select the appropriate inconsistent and the server is unable to select the appropriate
representation, then no harm will result when the precondition representation, then no harm will result when the precondition
evaluates to false. evaluates to false.
The following request header fields allow a client to place a The following request header fields allow a client to place a
precondition on the state of the target resource, so that the action precondition on the state of the target resource, so that the action
corresponding to the method semantics will not be applied if the corresponding to the method semantics will not be applied if the
precondition evaluates to false. Each precondition defined by this precondition evaluates to false. Each precondition defined by this
specification consists of a comparison between a set of validators specification consists of a comparison between a set of validators
obtained from prior representations of the target resource to the obtained from prior representations of the target resource to the
current state of validators for the selected representation current state of validators for the selected representation
(Section 10.2). Hence, these preconditions evaluate whether the (Section 11.2). Hence, these preconditions evaluate whether the
state of the target resource has changed since a given state known by state of the target resource has changed since a given state known by
the client. The effect of such an evaluation depends on the method the client. The effect of such an evaluation depends on the method
semantics and choice of conditional, as defined in Section 8.2.1. semantics and choice of conditional, as defined in Section 9.2.1.
+---------------------+---------------+ +---------------------+---------------+
| Field Name | Defined in... | | Field Name | Defined in... |
+---------------------+---------------+ +---------------------+---------------+
| If-Match | Section 8.2.3 | | If-Match | Section 9.2.3 |
| If-None-Match | Section 8.2.4 | | If-None-Match | Section 9.2.4 |
| If-Modified-Since | Section 8.2.5 | | If-Modified-Since | Section 9.2.5 |
| If-Unmodified-Since | Section 8.2.6 | | If-Unmodified-Since | Section 9.2.6 |
| If-Range | Section 8.2.7 | | If-Range | Section 9.2.7 |
+---------------------+---------------+ +---------------------+---------------+
8.2.1. Evaluation 9.2.1. Evaluation
Except when excluded below, a recipient cache or origin server MUST Except when excluded below, a recipient cache or origin server MUST
evaluate received request preconditions after it has successfully evaluate received request preconditions after it has successfully
performed its normal request checks and just before it would perform performed its normal request checks and just before it would perform
the action associated with the request method. A server MUST ignore the action associated with the request method. A server MUST ignore
all received preconditions if its response to the same request all received preconditions if its response to the same request
without those conditions would have been a status code other than a without those conditions would have been a status code other than a
2xx (Successful) or 412 (Precondition Failed). In other words, 2xx (Successful) or 412 (Precondition Failed). In other words,
redirects and failures take precedence over the evaluation of redirects and failures take precedence over the evaluation of
preconditions in conditional requests. preconditions in conditional requests.
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if the recipient understands and implements that field ([RFC4918], if the recipient understands and implements that field ([RFC4918],
Section 10.4). Section 10.4).
Although conditional request header fields are defined as being Although conditional request header fields are defined as being
usable with the HEAD method (to keep HEAD's semantics consistent with usable with the HEAD method (to keep HEAD's semantics consistent with
those of GET), there is no point in sending a conditional HEAD those of GET), there is no point in sending a conditional HEAD
because a successful response is around the same size as a 304 (Not because a successful response is around the same size as a 304 (Not
Modified) response and more useful than a 412 (Precondition Failed) Modified) response and more useful than a 412 (Precondition Failed)
response. response.
8.2.2. Precedence 9.2.2. Precedence
When more than one conditional request header field is present in a When more than one conditional request header field is present in a
request, the order in which the fields are evaluated becomes request, the order in which the fields are evaluated becomes
important. In practice, the fields defined in this document are important. In practice, the fields defined in this document are
consistently implemented in a single, logical order, since "lost consistently implemented in a single, logical order, since "lost
update" preconditions have more strict requirements than cache update" preconditions have more strict requirements than cache
validation, a validated cache is more efficient than a partial validation, a validated cache is more efficient than a partial
response, and entity tags are presumed to be more accurate than date response, and entity tags are presumed to be more accurate than date
validators. validators.
A recipient cache or origin server MUST evaluate the request A recipient cache or origin server MUST evaluate the request
preconditions defined by this specification in the following order: preconditions defined by this specification in the following order:
1. When recipient is the origin server and If-Match is present, 1. When recipient is the origin server and If-Match is present,
evaluate the If-Match precondition: evaluate the If-Match precondition:
* if true, continue to step 3 * if true, continue to step 3
* if false, respond 412 (Precondition Failed) unless it can be * if false, respond 412 (Precondition Failed) unless it can be
determined that the state-changing request has already determined that the state-changing request has already
succeeded (see Section 8.2.3) succeeded (see Section 9.2.3)
2. When recipient is the origin server, If-Match is not present, and 2. When recipient is the origin server, If-Match is not present, and
If-Unmodified-Since is present, evaluate the If-Unmodified-Since If-Unmodified-Since is present, evaluate the If-Unmodified-Since
precondition: precondition:
* if true, continue to step 3 * if true, continue to step 3
* if false, respond 412 (Precondition Failed) unless it can be * if false, respond 412 (Precondition Failed) unless it can be
determined that the state-changing request has already determined that the state-changing request has already
succeeded (see Section 8.2.6) succeeded (see Section 9.2.6)
3. When If-None-Match is present, evaluate the If-None-Match 3. When If-None-Match is present, evaluate the If-None-Match
precondition: precondition:
* if true, continue to step 5 * if true, continue to step 5
* if false for GET/HEAD, respond 304 (Not Modified) * if false for GET/HEAD, respond 304 (Not Modified)
* if false for other methods, respond 412 (Precondition Failed) * if false for other methods, respond 412 (Precondition Failed)
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6. Otherwise, 6. Otherwise,
* all conditions are met, so perform the requested action and * all conditions are met, so perform the requested action and
respond according to its success or failure. respond according to its success or failure.
Any extension to HTTP that defines additional conditional request Any extension to HTTP that defines additional conditional request
header fields ought to define its own expectations regarding the header fields ought to define its own expectations regarding the
order for evaluating such fields in relation to those defined in this order for evaluating such fields in relation to those defined in this
document and other conditionals that might be found in practice. document and other conditionals that might be found in practice.
8.2.3. If-Match 9.2.3. If-Match
The "If-Match" header field makes the request method conditional on The "If-Match" header field makes the request method conditional on
the recipient origin server either having at least one current the recipient origin server either having at least one current
representation of the target resource, when the field value is "*", representation of the target resource, when the field value is "*",
or having a current representation of the target resource that has an or having a current representation of the target resource that has an
entity-tag matching a member of the list of entity-tags provided in entity-tag matching a member of the list of entity-tags provided in
the field value. the field value.
An origin server MUST use the strong comparison function when An origin server MUST use the strong comparison function when
comparing entity-tags for If-Match (Section 10.2.3.2), since the comparing entity-tags for If-Match (Section 11.2.3.2), since the
client intends this precondition to prevent the method from being client intends this precondition to prevent the method from being
applied if there have been any changes to the representation data. applied if there have been any changes to the representation data.
If-Match = "*" / 1#entity-tag If-Match = "*" / 1#entity-tag
Examples: Examples:
If-Match: "xyzzy" If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: * If-Match: *
If-Match is most often used with state-changing methods (e.g., POST, If-Match is most often used with state-changing methods (e.g., POST,
PUT, DELETE) to prevent accidental overwrites when multiple user PUT, DELETE) to prevent accidental overwrites when multiple user
agents might be acting in parallel on the same resource (i.e., to agents might be acting in parallel on the same resource (i.e., to
prevent the "lost update" problem). It can also be used with safe prevent the "lost update" problem). It can also be used with safe
methods to abort a request if the selected representation does not methods to abort a request if the selected representation does not
match one already stored (or partially stored) from a prior request. match one already stored (or partially stored) from a prior request.
An origin server that receives an If-Match header field MUST evaluate An origin server that receives an If-Match header field MUST evaluate
the condition prior to performing the method (Section 8.2.1). the condition as per Section 9.2.1 prior to performing the method.
To evaluate a received If-Match header field: To evaluate a received If-Match header field:
1. If the field value is "*", the condition is true if the origin 1. If the field value is "*", the condition is true if the origin
server has a current representation for the target resource. server has a current representation for the target resource.
2. If the field value is a list of entity-tags, the condition is 2. If the field value is a list of entity-tags, the condition is
true if any of the listed tags match the entity-tag of the true if any of the listed tags match the entity-tag of the
selected representation. selected representation.
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succeeded, but the user agent might not be aware of it, perhaps succeeded, but the user agent might not be aware of it, perhaps
because the prior response was lost or a compatible change was made because the prior response was lost or a compatible change was made
by some other user agent). In the latter case, the origin server by some other user agent). In the latter case, the origin server
MUST NOT send a validator header field in the response unless it can MUST NOT send a validator header field in the response unless it can
verify that the request is a duplicate of an immediately prior change verify that the request is a duplicate of an immediately prior change
made by the same user agent. made by the same user agent.
The If-Match header field can be ignored by caches and intermediaries The If-Match header field can be ignored by caches and intermediaries
because it is not applicable to a stored response. because it is not applicable to a stored response.
8.2.4. If-None-Match Note that an If-Match header field with a list value containing "*"
and other values (including other instances of "*") is unlikely to be
interoperable.
9.2.4. If-None-Match
The "If-None-Match" header field makes the request method conditional The "If-None-Match" header field makes the request method conditional
on a recipient cache or origin server either not having any current on a recipient cache or origin server either not having any current
representation of the target resource, when the field value is "*", representation of the target resource, when the field value is "*",
or having a selected representation with an entity-tag that does not or having a selected representation with an entity-tag that does not
match any of those listed in the field value. match any of those listed in the field value.
A recipient MUST use the weak comparison function when comparing A recipient MUST use the weak comparison function when comparing
entity-tags for If-None-Match (Section 10.2.3.2), since weak entity- entity-tags for If-None-Match (Section 11.2.3.2), since weak entity-
tags can be used for cache validation even if there have been changes tags can be used for cache validation even if there have been changes
to the representation data. to the representation data.
If-None-Match = "*" / 1#entity-tag If-None-Match = "*" / 1#entity-tag
Examples: Examples:
If-None-Match: "xyzzy" If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy" If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz" If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
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stored responses that have entity-tags, the client SHOULD generate an stored responses that have entity-tags, the client SHOULD generate an
If-None-Match header field containing a list of those entity-tags If-None-Match header field containing a list of those entity-tags
when making a GET request; this allows recipient servers to send a when making a GET request; this allows recipient servers to send a
304 (Not Modified) response to indicate when one of those stored 304 (Not Modified) response to indicate when one of those stored
responses matches the selected representation. responses matches the selected representation.
If-None-Match can also be used with a value of "*" to prevent an If-None-Match can also be used with a value of "*" to prevent an
unsafe request method (e.g., PUT) from inadvertently modifying an unsafe request method (e.g., PUT) from inadvertently modifying an
existing representation of the target resource when the client existing representation of the target resource when the client
believes that the resource does not have a current representation believes that the resource does not have a current representation
(Section 7.2.1). This is a variation on the "lost update" problem (Section 8.2.1). This is a variation on the "lost update" problem
that might arise if more than one client attempts to create an that might arise if more than one client attempts to create an
initial representation for the target resource. initial representation for the target resource.
An origin server that receives an If-None-Match header field MUST An origin server that receives an If-None-Match header field MUST
evaluate the condition prior to performing the method evaluate the condition as per Section 9.2.1 prior to performing the
(Section 8.2.1). method.
To evaluate a received If-None-Match header field: To evaluate a received If-None-Match header field:
1. If the field value is "*", the condition is false if the origin 1. If the field value is "*", the condition is false if the origin
server has a current representation for the target resource. server has a current representation for the target resource.
2. If the field value is a list of entity-tags, the condition is 2. If the field value is a list of entity-tags, the condition is
false if one of the listed tags matches the entity-tag of the false if one of the listed tags matches the entity-tag of the
selected representation. selected representation.
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An origin server MUST NOT perform the requested method if the An origin server MUST NOT perform the requested method if the
condition evaluates to false; instead, the origin server MUST respond condition evaluates to false; instead, the origin server MUST respond
with either a) the 304 (Not Modified) status code if the request with either a) the 304 (Not Modified) status code if the request
method is GET or HEAD or b) the 412 (Precondition Failed) status code method is GET or HEAD or b) the 412 (Precondition Failed) status code
for all other request methods. for all other request methods.
Requirements on cache handling of a received If-None-Match header Requirements on cache handling of a received If-None-Match header
field are defined in Section 4.3.2 of [Caching]. field are defined in Section 4.3.2 of [Caching].
8.2.5. If-Modified-Since Note that an If-None-Match header field with a list value containing
"*" and other values (including other instances of "*") is unlikely
to be interoperable.
9.2.5. If-Modified-Since
The "If-Modified-Since" header field makes a GET or HEAD request The "If-Modified-Since" header field makes a GET or HEAD request
method conditional on the selected representation's modification date method conditional on the selected representation's modification date
being more recent than the date provided in the field value. being more recent than the date provided in the field value.
Transfer of the selected representation's data is avoided if that Transfer of the selected representation's data is avoided if that
data has not changed. data has not changed.
If-Modified-Since = HTTP-date If-Modified-Since = HTTP-date
An example of the field is: An example of the field is:
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When used for limiting the scope of retrieval to a recent time When used for limiting the scope of retrieval to a recent time
window, a user agent will generate an If-Modified-Since field value window, a user agent will generate an If-Modified-Since field value
based on either its own local clock or a Date header field received based on either its own local clock or a Date header field received
from the server in a prior response. Origin servers that choose an from the server in a prior response. Origin servers that choose an
exact timestamp match based on the selected representation's Last- exact timestamp match based on the selected representation's Last-
Modified field will not be able to help the user agent limit its data Modified field will not be able to help the user agent limit its data
transfers to only those changed during the specified window. transfers to only those changed during the specified window.
An origin server that receives an If-Modified-Since header field An origin server that receives an If-Modified-Since header field
SHOULD evaluate the condition prior to performing the method SHOULD evaluate the condition as per Section 9.2.1 prior to
(Section 8.2.1). The origin server SHOULD NOT perform the requested performing the method. The origin server SHOULD NOT perform the
method if the selected representation's last modification date is requested method if the selected representation's last modification
earlier than or equal to the date provided in the field value; date is earlier than or equal to the date provided in the field
instead, the origin server SHOULD generate a 304 (Not Modified) value; instead, the origin server SHOULD generate a 304 (Not
response, including only those metadata that are useful for Modified) response, including only those metadata that are useful for
identifying or updating a previously cached response. identifying or updating a previously cached response.
Requirements on cache handling of a received If-Modified-Since header Requirements on cache handling of a received If-Modified-Since header
field are defined in Section 4.3.2 of [Caching]. field are defined in Section 4.3.2 of [Caching].
8.2.6. If-Unmodified-Since 9.2.6. If-Unmodified-Since
The "If-Unmodified-Since" header field makes the request method The "If-Unmodified-Since" header field makes the request method
conditional on the selected representation's last modification date conditional on the selected representation's last modification date
being earlier than or equal to the date provided in the field value. being earlier than or equal to the date provided in the field value.
This field accomplishes the same purpose as If-Match for cases where This field accomplishes the same purpose as If-Match for cases where
the user agent does not have an entity-tag for the representation. the user agent does not have an entity-tag for the representation.
If-Unmodified-Since = HTTP-date If-Unmodified-Since = HTTP-date
An example of the field is: An example of the field is:
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If-Unmodified-Since is most often used with state-changing methods If-Unmodified-Since is most often used with state-changing methods
(e.g., POST, PUT, DELETE) to prevent accidental overwrites when (e.g., POST, PUT, DELETE) to prevent accidental overwrites when
multiple user agents might be acting in parallel on a resource that multiple user agents might be acting in parallel on a resource that
does not supply entity-tags with its representations (i.e., to does not supply entity-tags with its representations (i.e., to
prevent the "lost update" problem). It can also be used with safe prevent the "lost update" problem). It can also be used with safe
methods to abort a request if the selected representation does not methods to abort a request if the selected representation does not
match one already stored (or partially stored) from a prior request. match one already stored (or partially stored) from a prior request.
An origin server that receives an If-Unmodified-Since header field An origin server that receives an If-Unmodified-Since header field
MUST evaluate the condition (Section 8.2.1) prior to performing the MUST evaluate the condition as per Section 9.2.1 prior to performing
method. the method.
If the selected representation has a last modification date, the If the selected representation has a last modification date, the
origin server MUST NOT perform the requested method if that date is origin server MUST NOT perform the requested method if that date is
more recent than the date provided in the field value. Instead, the more recent than the date provided in the field value. Instead, the
origin server MUST respond with either a) the 412 (Precondition origin server MUST respond with either a) the 412 (Precondition
Failed) status code or b) one of the 2xx (Successful) status codes if Failed) status code or b) one of the 2xx (Successful) status codes if
the origin server has verified that a state change is being requested the origin server has verified that a state change is being requested
and the final state is already reflected in the current state of the and the final state is already reflected in the current state of the
target resource (i.e., the change requested by the user agent has target resource (i.e., the change requested by the user agent has
already succeeded, but the user agent might not be aware of that already succeeded, but the user agent might not be aware of that
because the prior response message was lost or a compatible change because the prior response message was lost or a compatible change
was made by some other user agent). In the latter case, the origin was made by some other user agent). In the latter case, the origin
server MUST NOT send a validator header field in the response unless server MUST NOT send a validator header field in the response unless
it can verify that the request is a duplicate of an immediately prior it can verify that the request is a duplicate of an immediately prior
change made by the same user agent. change made by the same user agent.
The If-Unmodified-Since header field can be ignored by caches and The If-Unmodified-Since header field can be ignored by caches and
intermediaries because it is not applicable to a stored response. intermediaries because it is not applicable to a stored response.
8.2.7. If-Range 9.2.7. If-Range
The "If-Range" header field provides a special conditional request The "If-Range" header field provides a special conditional request
mechanism that is similar to the If-Match and If-Unmodified-Since mechanism that is similar to the If-Match and If-Unmodified-Since
header fields but that instructs the recipient to ignore the Range header fields but that instructs the recipient to ignore the Range
header field if the validator doesn't match, resulting in transfer of header field if the validator doesn't match, resulting in transfer of
the new selected representation instead of a 412 (Precondition the new selected representation instead of a 412 (Precondition
Failed) response. Failed) response.
If a client has a partial copy of a representation and wishes to have If a client has a partial copy of a representation and wishes to have
an up-to-date copy of the entire representation, it could use the an up-to-date copy of the entire representation, it could use the
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does not contain a Range header field. A server MUST ignore an If- does not contain a Range header field. A server MUST ignore an If-
Range header field received in a request that does not contain a Range header field received in a request that does not contain a
Range header field. An origin server MUST ignore an If-Range header Range header field. An origin server MUST ignore an If-Range header
field received in a request for a target resource that does not field received in a request for a target resource that does not
support Range requests. support Range requests.
A client MUST NOT generate an If-Range header field containing an A client MUST NOT generate an If-Range header field containing an
entity-tag that is marked as weak. A client MUST NOT generate an If- entity-tag that is marked as weak. A client MUST NOT generate an If-
Range header field containing an HTTP-date unless the client has no Range header field containing an HTTP-date unless the client has no
entity-tag for the corresponding representation and the date is a entity-tag for the corresponding representation and the date is a
strong validator in the sense defined by Section 10.2.2.2. strong validator in the sense defined by Section 11.2.2.2.
A server that evaluates an If-Range precondition MUST use the strong A server that evaluates an If-Range precondition MUST use the strong
comparison function when comparing entity-tags (Section 10.2.3.2) and comparison function when comparing entity-tags (Section 11.2.3.2) and
MUST evaluate the condition as false if an HTTP-date validator is MUST evaluate the condition as false if an HTTP-date validator is
provided that is not a strong validator in the sense defined by provided that is not a strong validator in the sense defined by
Section 10.2.2.2. A valid entity-tag can be distinguished from a Section 11.2.2.2. A valid entity-tag can be distinguished from a
valid HTTP-date by examining the first two characters for a DQUOTE. valid HTTP-date by examining the first two characters for a DQUOTE.
If the validator given in the If-Range header field matches the If the validator given in the If-Range header field matches the
current validator for the selected representation of the target current validator for the selected representation of the target
resource, then the server SHOULD process the Range header field as resource, then the server SHOULD process the Range header field as
requested. If the validator does not match, the server MUST ignore requested. If the validator does not match, the server MUST ignore
the Range header field. Note that this comparison by exact match, the Range header field. Note that this comparison by exact match,
including when the validator is an HTTP-date, differs from the including when the validator is an HTTP-date, differs from the
"earlier than or equal to" comparison used when evaluating an If- "earlier than or equal to" comparison used when evaluating an If-
Unmodified-Since conditional. Unmodified-Since conditional.
8.3. Range 9.3. Range
The "Range" header field on a GET request modifies the method The "Range" header field on a GET request modifies the method
semantics to request transfer of only one or more subranges of the semantics to request transfer of only one or more subranges of the
selected representation data (Section 6.1), rather than the entire selected representation data (Section 7.1), rather than the entire
selected representation. selected representation.
Range = ranges-specifier Range = ranges-specifier
Clients often encounter interrupted data transfers as a result of Clients often encounter interrupted data transfers as a result of
canceled requests or dropped connections. When a client has stored a canceled requests or dropped connections. When a client has stored a
partial representation, it is desirable to request the remainder of partial representation, it is desirable to request the remainder of
that representation in a subsequent request rather than transfer the that representation in a subsequent request rather than transfer the
entire representation. Likewise, devices with limited local storage entire representation. Likewise, devices with limited local storage
might benefit from being able to request only a subset of a larger might benefit from being able to request only a subset of a larger
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byte ranges. byte ranges.
An origin server MUST ignore a Range header field that contains a An origin server MUST ignore a Range header field that contains a
range unit it does not understand. A proxy MAY discard a Range range unit it does not understand. A proxy MAY discard a Range
header field that contains a range unit it does not understand. header field that contains a range unit it does not understand.
A server that supports range requests MAY ignore or reject a Range A server that supports range requests MAY ignore or reject a Range
header field that consists of more than two overlapping ranges, or a header field that consists of more than two overlapping ranges, or a
set of many small ranges that are not listed in ascending order, set of many small ranges that are not listed in ascending order,
since both are indications of either a broken client or a deliberate since both are indications of either a broken client or a deliberate
denial-of-service attack (Section 11.13). A client SHOULD NOT denial-of-service attack (Section 12.13). A client SHOULD NOT
request multiple ranges that are inherently less efficient to process request multiple ranges that are inherently less efficient to process
and transfer than a single range that encompasses the same data. and transfer than a single range that encompasses the same data.
A server that supports range requests MAY ignore a Range header field
when the selected representation has no body (i.e., the selected
representation data is of zero length).
A client that is requesting multiple ranges SHOULD list those ranges A client that is requesting multiple ranges SHOULD list those ranges
in ascending order (the order in which they would typically be in ascending order (the order in which they would typically be
received in a complete representation) unless there is a specific received in a complete representation) unless there is a specific
need to request a later part earlier. For example, a user agent need to request a later part earlier. For example, a user agent
processing a large representation with an internal catalog of parts processing a large representation with an internal catalog of parts
might need to request later parts first, particularly if the might need to request later parts first, particularly if the
representation consists of pages stored in reverse order and the user representation consists of pages stored in reverse order and the user
agent wishes to transfer one page at a time. agent wishes to transfer one page at a time.
The Range header field is evaluated after evaluating the precondition The Range header field is evaluated after evaluating the precondition
header fields defined in Section 8.2, and only if the result in header fields defined in Section 9.2, and only if the result in
absence of the Range header field would be a 200 (OK) response. In absence of the Range header field would be a 200 (OK) response. In
other words, Range is ignored when a conditional GET would result in other words, Range is ignored when a conditional GET would result in
a 304 (Not Modified) response. a 304 (Not Modified) response.
The If-Range header field (Section 8.2.7) can be used as a The If-Range header field (Section 9.2.7) can be used as a
precondition to applying the Range header field. precondition to applying the Range header field.
If all of the preconditions are true, the server supports the Range If all of the preconditions are true, the server supports the Range
header field for the target resource, and the specified range(s) are header field for the target resource, and the specified range(s) are
valid and satisfiable (as defined in Section 6.1.4.2), the server valid and satisfiable (as defined in Section 7.1.4.2), the server
SHOULD send a 206 (Partial Content) response with a payload SHOULD send a 206 (Partial Content) response with a payload
containing one or more partial representations that correspond to the containing one or more partial representations that correspond to the
satisfiable ranges requested. satisfiable ranges requested.
If all of the preconditions are true, the server supports the Range If all of the preconditions are true, the server supports the Range
header field for the target resource, and the specified range(s) are header field for the target resource, and the specified range(s) are
invalid or unsatisfiable, the server SHOULD send a 416 (Range Not invalid or unsatisfiable, the server SHOULD send a 416 (Range Not
Satisfiable) response. Satisfiable) response.
8.4. Negotiation 9.4. Negotiation
The following request header fields can be sent by a user agent to The following request header fields can be sent by a user agent to
engage in proactive negotiation of the response content, as defined engage in proactive negotiation of the response content, as defined
in Section 6.4.1. The preferences sent in these fields apply to any in Section 7.4.1. The preferences sent in these fields apply to any
content in the response, including representations of the target content in the response, including representations of the target
resource, representations of error or processing status, and resource, representations of error or processing status, and
potentially even the miscellaneous text strings that might appear potentially even the miscellaneous text strings that might appear
within the protocol. within the protocol.
+-----------------+---------------+ +-----------------+---------------+
| Field Name | Defined in... | | Field Name | Defined in... |
+-----------------+---------------+ +-----------------+---------------+
| Accept | Section 8.4.1 | | Accept | Section 9.4.1 |
| Accept-Charset | Section 8.4.2 | | Accept-Charset | Section 9.4.2 |
| Accept-Encoding | Section 8.4.3 | | Accept-Encoding | Section 9.4.3 |
| Accept-Language | Section 8.4.4 | | Accept-Language | Section 9.4.4 |
+-----------------+---------------+ +-----------------+---------------+
For each of these header fields, a request that does not contain it For each of these header fields, a request that does not contain it
implies that the user agent has no preference on that axis of implies that the user agent has no preference on that axis of
negotiation. If the header field is present in a request and none of negotiation. If the header field is present in a request and none of
the available representations for the response can be considered the available representations for the response can be considered
acceptable according to it, the origin server can either honor the acceptable according to it, the origin server can either honor the
header field by sending a 406 (Not Acceptable) response or disregard header field by sending a 406 (Not Acceptable) response or disregard
the header field by treating the response as if it is not subject to the header field by treating the response as if it is not subject to
content negotiation for that request header field. This does not content negotiation for that request header field. This does not
imply, however, that the client will be able to use the imply, however, that the client will be able to use the
representation. representation.
Note: Sending these header fields makes it easier for a server to Note: Sending these header fields makes it easier for a server to
identify an individual by virtue of the user agent's request identify an individual by virtue of the user agent's request
characteristics (Section 11.11). characteristics (Section 12.11).
Each of these header fields defines a wildcard value (often, "*") to Each of these header fields defines a wildcard value (often, "*") to
select unspecified values. If no wildcard is present, all values not select unspecified values. If no wildcard is present, all values not
explicitly mentioned in the field are considered "not acceptable" to explicitly mentioned in the field are considered "not acceptable" to
the client. the client.
Note: In practice, using wildcards in content negotiation has limited Note: In practice, using wildcards in content negotiation has limited
practical value, because it is seldom useful to say, for example, "I practical value, because it is seldom useful to say, for example, "I
prefer image/* more or less than (some other specific value)". prefer image/* more or less than (some other specific value)".
Clients can explicitly request a 406 (Not Acceptable) response if a Clients can explicitly request a 406 (Not Acceptable) response if a
more preferred format is not available by sending Accept: */*;q=0, more preferred format is not available by sending Accept: */*;q=0,
but they still need to be able to handle a different response, since but they still need to be able to handle a different response, since
the server is allowed to ignore their preference. the server is allowed to ignore their preference.
8.4.1. Accept 9.4.1. Accept
The "Accept" header field can be used by user agents to specify their The "Accept" header field can be used by user agents to specify their
preferences regarding response media types. For example, Accept preferences regarding response media types. For example, Accept
header fields can be used to indicate that the request is header fields can be used to indicate that the request is
specifically limited to a small set of desired types, as in the case specifically limited to a small set of desired types, as in the case
of a request for an in-line image. of a request for an in-line image.
When sent by a server in a response, Accept provides information
about what content types are preferred in the payload of a subsequent
request to the same resource.
Accept = #( media-range [ accept-params ] ) Accept = #( media-range [ accept-params ] )
media-range = ( "*/*" media-range = ( "*/*"
/ ( type "/" "*" ) / ( type "/" "*" )
/ ( type "/" subtype ) / ( type "/" subtype )
) *( OWS ";" OWS parameter ) ) *( OWS ";" OWS parameter )
accept-params = weight *( accept-ext ) accept-params = weight *( accept-ext )
accept-ext = OWS ";" OWS token [ "=" ( token / quoted-string ) ] accept-ext = OWS ";" OWS token [ "=" ( token / quoted-string ) ]
The asterisk "*" character is used to group media types into ranges, The asterisk "*" character is used to group media types into ranges,
with "*/*" indicating all media types and "type/*" indicating all with "*/*" indicating all media types and "type/*" indicating all
subtypes of that type. The media-range can include media type subtypes of that type. The media-range can include media type
parameters that are applicable to that range. parameters that are applicable to that range.
Each media-range might be followed by zero or more applicable media Each media-range might be followed by zero or more applicable media
type parameters (e.g., charset), an optional "q" parameter for type parameters (e.g., charset), an optional "q" parameter for
indicating a relative weight (Section 6.4.4), and then zero or more indicating a relative weight (Section 7.4.4), and then zero or more
extension parameters. The "q" parameter is necessary if any extension parameters. The "q" parameter is necessary if any
extensions (accept-ext) are present, since it acts as a separator extensions (accept-ext) are present, since it acts as a separator
between the two parameter sets. between the two parameter sets.
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
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have the following precedence: have the following precedence:
1. text/plain;format=flowed 1. text/plain;format=flowed
2. text/plain 2. text/plain
3. text/* 3. text/*
4. */* 4. */*
The media type quality factor associated with a given type is The media type quality factor associated with a given type is
determined by finding the media range with the highest precedence determined by finding the media range with the highest precedence
that matches the type. For example, that matches the type. For example,
Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1, Accept: text/*;q=0.3, text/plain;q=0.7, text/plain;format=flowed,
text/html;level=2;q=0.4, */*;q=0.5 text/plain;format=fixed;q=0.4, */*;q=0.5
would cause the following values to be associated: would cause the following values to be associated:
+-------------------+---------------+ +--------------------------+---------------+
| Media Type | Quality Value | | Media Type | Quality Value |
+-------------------+---------------+ +--------------------------+---------------+
| text/html;level=1 | 1 | | text/plain;format=flowed | 1 |
| text/html | 0.7 | | text/plain | 0.7 |
| text/plain | 0.3 | | text/html | 0.3 |
| image/jpeg | 0.5 | | image/jpeg | 0.5 |
| text/html;level=2 | 0.4 | | text/plain;format=fixed | 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 a values for certain media ranges. However, unless the user agent is a
closed system that cannot interact with other rendering agents, this closed system that cannot interact with other rendering agents, this
default set ought to be configurable by the user. default set ought to be configurable by the user.
8.4.2. Accept-Charset 9.4.2. Accept-Charset
The "Accept-Charset" header field can be sent by a user agent to The "Accept-Charset" header field can be sent by a user agent to
indicate its preferences for charsets in textual response content. indicate its preferences for charsets in textual response content.
For example, this field allows user agents capable of understanding For example, this field allows user agents capable of understanding
more comprehensive or special-purpose charsets to signal that more comprehensive or special-purpose charsets to signal that
capability to an origin server that is capable of representing capability to an origin server that is capable of representing
information in those charsets. information in those charsets.
Accept-Charset = 1#( ( charset / "*" ) [ weight ] ) Accept-Charset = 1#( ( charset / "*" ) [ weight ] )
Charset names are defined in Section 6.1.1.1. A user agent MAY Charset names are defined in Section 7.1.1.1. A user agent MAY
associate a quality value with each charset to indicate the user's associate a quality value with each charset to indicate the user's
relative preference for that charset, as defined in Section 6.4.4. relative preference for that charset, as defined in Section 7.4.4.
An example is 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 charset that is not mentioned elsewhere in the Accept- matches every charset that is not mentioned elsewhere in the Accept-
Charset field. Charset field.
Note: Accept-Charset is deprecated because UTF-8 has become nearly Note: Accept-Charset is deprecated because UTF-8 has become nearly
ubiquitous and sending a detailed list of user-preferred charsets ubiquitous and sending a detailed list of user-preferred charsets
wastes bandwidth, increases latency, and makes passive fingerprinting wastes bandwidth, increases latency, and makes passive fingerprinting
far too easy (Section 11.11). Most general-purpose user agents do far too easy (Section 12.11). Most general-purpose user agents do
not send Accept-Charset, unless specifically configured to do so. not send Accept-Charset, unless specifically configured to do so.
8.4.3. Accept-Encoding 9.4.3. Accept-Encoding
The "Accept-Encoding" header field can be used to indicate The "Accept-Encoding" header field can be used to indicate
preferences regarding the use of content codings (Section 6.1.2). preferences regarding the use of content codings (Section 7.1.2).
When sent by a user agent in a request, Accept-Encoding indicates the When sent by a user agent in a request, Accept-Encoding indicates the
content codings acceptable in a response. content codings acceptable in a response.
When sent by a server in a response, Accept-Encoding provides When sent by a server in a response, Accept-Encoding provides
information about what content codings are preferred in the payload information about what content codings are preferred in the payload
of a subsequent request to the same resource. of a subsequent request to the same resource.
An "identity" token is used as a synonym for "no encoding" in order An "identity" token is used as a synonym for "no encoding" in order
to communicate when no encoding is preferred. to communicate when no encoding is preferred.
Accept-Encoding = #( codings [ weight ] ) Accept-Encoding = #( codings [ weight ] )
codings = content-coding / "identity" / "*" codings = content-coding / "identity" / "*"
Each codings value MAY be given an associated quality value Each codings value MAY be given an associated quality value
representing the preference for that encoding, as defined in representing the preference for that encoding, as defined in
Section 6.4.4. The asterisk "*" symbol in an Accept-Encoding field Section 7.4.4. The asterisk "*" symbol in an Accept-Encoding field
matches any available content-coding not explicitly listed in the matches any available content-coding not explicitly listed in the
header field. header field.
For example, For example,
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
skipping to change at page 109, line 30 skipping to change at page 113, line 8
is considered acceptable by the user agent. is considered acceptable by the user agent.
2. If the representation has no content-coding, then it is 2. If the representation has no content-coding, then it is
acceptable by default unless specifically excluded by the Accept- acceptable by default unless specifically excluded by the Accept-
Encoding field stating either "identity;q=0" or "*;q=0" without a Encoding field stating either "identity;q=0" or "*;q=0" without a
more specific entry for "identity". more specific entry for "identity".
3. If the representation's content-coding is one of the content- 3. If the representation's content-coding is one of the content-
codings listed in the Accept-Encoding field value, then it is codings listed in the Accept-Encoding field value, then it is
acceptable unless it is accompanied by a qvalue of 0. (As acceptable unless it is accompanied by a qvalue of 0. (As
defined in Section 6.4.4, a qvalue of 0 means "not acceptable".) defined in Section 7.4.4, a qvalue of 0 means "not acceptable".)
4. If multiple content-codings are acceptable, then the acceptable 4. 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.
An Accept-Encoding header field with a field value that is empty An Accept-Encoding header field with a field value that is empty
implies that the user agent does not want any content-coding in implies that the user agent does not want any content-coding in
response. If an Accept-Encoding header field is present in a request response. If an Accept-Encoding header field is present in a request
and none of the available representations for the response have a and none of the available representations for the response have a
content-coding that is listed as acceptable, the origin server SHOULD content-coding that is listed as acceptable, the origin server SHOULD
send a response without any content-coding. send a response without any content-coding.
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be used to indicate to clients that content codings are supported, to be used to indicate to clients that content codings are supported, to
optimize future interactions. For example, a resource might include optimize future interactions. For example, a resource might include
it in a 2xx (Successful) response when the request payload was big it in a 2xx (Successful) response when the request payload was big
enough to justify use of a compression coding but the client failed enough to justify use of a compression coding but the client failed
do so. do so.
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 might associated with content-codings. This means that qvalues might
not work and are not permitted with x-gzip or x-compress. not work and are not permitted with x-gzip or x-compress.
8.4.4. Accept-Language 9.4.4. Accept-Language
The "Accept-Language" header field can be used by user agents to The "Accept-Language" header field can be used by user agents to
indicate the set of natural languages that are preferred in the indicate the set of natural languages that are preferred in the
response. Language tags are defined in Section 6.1.3. response. Language tags are defined in Section 7.1.3.
Accept-Language = 1#( language-range [ weight ] ) Accept-Language = 1#( language-range [ weight ] )
language-range = language-range =
<language-range, see [RFC4647], Section 2.1> <language-range, see [RFC4647], Section 2.1>
Each language-range can be given an associated quality value Each language-range can be given an associated quality value
representing an estimate of the user's preference for the languages representing an estimate of the user's preference for the languages
specified by that range, as defined in Section 6.4.4. For example, specified by that range, as defined in Section 7.4.4. For example,
Accept-Language: da, en-gb;q=0.8, en;q=0.7 Accept-Language: da, en-gb;q=0.8, en;q=0.7
would mean: "I prefer Danish, but will accept British English and would mean: "I prefer Danish, but will accept British English and
other types of English". other types of English".
Note that some recipients treat the order in which language tags are Note that some recipients treat the order in which language tags are
listed as an indication of descending priority, particularly for tags listed as an indication of descending priority, particularly for tags
that are assigned equal quality values (no value is the same as q=1). that are assigned equal quality values (no value is the same as q=1).
However, this behavior cannot be relied upon. For consistency and to However, this behavior cannot be relied upon. For consistency and to
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found in Section 2.3 of [RFC4647]. found in Section 2.3 of [RFC4647].
For matching, Section 3 of [RFC4647] defines several matching For matching, Section 3 of [RFC4647] defines several matching
schemes. Implementations can offer the most appropriate matching schemes. Implementations can offer the most appropriate matching
scheme for their requirements. The "Basic Filtering" scheme scheme for their requirements. The "Basic Filtering" scheme
([RFC4647], Section 3.3.1) is identical to the matching scheme that ([RFC4647], Section 3.3.1) is identical to the matching scheme that
was previously defined for HTTP in Section 14.4 of [RFC2616]. was previously defined for HTTP in Section 14.4 of [RFC2616].
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 field with the complete linguistic an Accept-Language header field with the complete linguistic
preferences of the user in every request (Section 11.11). preferences of the user in every request (Section 12.11).
Since intelligibility is highly dependent on the individual user, Since intelligibility is highly dependent on the individual user,
user agents need to allow user control over the linguistic preference user agents need to allow user control over the linguistic preference
(either through configuration of the user agent itself or by (either through configuration of the user agent itself or by
defaulting to a user controllable system setting). A user agent that defaulting to a user controllable system setting). A user agent that
does not provide such control to the user MUST NOT send an Accept- does not provide such control to the user MUST NOT send an Accept-
Language header field. Language header field.
Note: User agents ought to provide guidance to users when setting Note: User agents ought to provide guidance to users when setting
a preference, since users are rarely familiar with the details of a preference, since users are rarely familiar with the details of
language matching as described above. For example, users might language matching as described above. For example, users might
assume that on selecting "en-gb", they will be served any kind of assume that on selecting "en-gb", they will be served any kind of
English document if British English is not available. A user English document if British English is not available. A user
agent might suggest, in such a case, to add "en" to the list for agent might suggest, in such a case, to add "en" to the list for
better matching behavior. better matching behavior.
8.5. Authentication Credentials 9.5. Authentication Credentials
HTTP provides a general framework for access control and HTTP provides a general framework for access control and
authentication, via an extensible set of challenge-response authentication, via an extensible set of challenge-response
authentication schemes, which can be used by a server to challenge a authentication schemes, which can be used by a server to challenge a
client request and by a client to provide authentication information. client request and by a client to provide authentication information.
Two header fields are used for carrying authentication credentials. Two header fields are used for carrying authentication credentials.
Note that various custom mechanisms for user authentication use the Note that various custom mechanisms for user authentication use the
Cookie header field for this purpose, as defined in [RFC6265]. Cookie header field for this purpose, as defined in [RFC6265].
+---------------------+---------------+ +---------------------+---------------+
| Field Name | Defined in... | | Field Name | Defined in... |
+---------------------+---------------+ +---------------------+---------------+
| Authorization | Section 8.5.3 | | Authorization | Section 9.5.3 |
| Proxy-Authorization | Section 8.5.4 | | Proxy-Authorization | Section 9.5.4 |
+---------------------+---------------+ +---------------------+---------------+
8.5.1. Challenge and Response 9.5.1. Challenge and Response
HTTP provides a simple challenge-response authentication framework HTTP provides a simple challenge-response authentication framework
that can be used by a server to challenge a client request and by a that can be used by a server to challenge a client request and by a
client to provide authentication information. It uses a case- client to provide authentication information. It uses a case-
insensitive token as a means to identify the authentication scheme, insensitive token as a means to identify the authentication scheme,
followed by additional information necessary for achieving followed by additional information necessary for achieving
authentication via that scheme. The latter can be either a comma- authentication via that scheme. The latter can be either a comma-
separated list of parameters or a single sequence of characters separated list of parameters or a single sequence of characters
capable of holding base64-encoded information. capable of holding base64-encoded information.
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Both the Authorization field value and the Proxy-Authorization field Both the Authorization field value and the Proxy-Authorization field
value contain the client's credentials for the realm of the resource value contain the client's credentials for the realm of the resource
being requested, based upon a challenge received in a response being requested, based upon a challenge received in a response
(possibly at some point in the past). When creating their values, (possibly at some point in the past). When creating their values,
the user agent ought to do so by selecting the challenge with what it the user agent ought to do so by selecting the challenge with what it
considers to be the most secure auth-scheme that it understands, considers to be the most secure auth-scheme that it understands,
obtaining credentials from the user as appropriate. Transmission of obtaining credentials from the user as appropriate. Transmission of
credentials within header field values implies significant security credentials within header field values implies significant security
considerations regarding the confidentiality of the underlying considerations regarding the confidentiality of the underlying
connection, as described in Section 11.14.1. connection, as described in Section 12.14.1.
credentials = auth-scheme [ 1*SP ( token68 / #auth-param ) ] credentials = auth-scheme [ 1*SP ( token68 / #auth-param ) ]
Upon receipt of a request for a protected resource that omits Upon receipt of a request for a protected resource that omits
credentials, contains invalid credentials (e.g., a bad password) or credentials, contains invalid credentials (e.g., a bad password) or
partial credentials (e.g., when the authentication scheme requires partial credentials (e.g., when the authentication scheme requires
more than one round trip), an origin server SHOULD send a 401 more than one round trip), an origin server SHOULD send a 401
(Unauthorized) response that contains a WWW-Authenticate header field (Unauthorized) response that contains a WWW-Authenticate header field
with at least one (possibly new) challenge applicable to the with at least one (possibly new) challenge applicable to the
requested resource. requested resource.
Likewise, upon receipt of a request that omits proxy credentials or Likewise, upon receipt of a request that omits proxy credentials or
contains invalid or partial proxy credentials, a proxy that requires contains invalid or partial proxy credentials, a proxy that requires
authentication SHOULD generate a 407 (Proxy Authentication Required) authentication SHOULD generate a 407 (Proxy Authentication Required)
response that contains a Proxy-Authenticate header field with at response that contains a Proxy-Authenticate header field with at
least one (possibly new) challenge applicable to the proxy. least one (possibly new) challenge applicable to the proxy.
A server that receives valid credentials that are not adequate to A server that receives valid credentials that are not adequate to
gain access ought to respond with the 403 (Forbidden) status code gain access ought to respond with the 403 (Forbidden) status code
(Section 9.5.4). (Section 10.5.4).
HTTP does not restrict applications to this simple challenge-response HTTP does not restrict applications to this simple challenge-response
framework for access authentication. Additional mechanisms can be framework for access authentication. Additional mechanisms can be
used, such as authentication at the transport level or via message used, such as authentication at the transport level or via message
encapsulation, and with additional header fields specifying encapsulation, and with additional header fields specifying
authentication information. However, such additional mechanisms are authentication information. However, such additional mechanisms are
not defined by this specification. not defined by this specification.
8.5.2. Protection Space (Realm) 9.5.2. Protection Space (Realm)
The "realm" authentication parameter is reserved for use by The "realm" authentication parameter is reserved for use by
authentication schemes that wish to indicate a scope of protection. authentication schemes that wish to indicate a scope of protection.
A protection space is defined by the canonical root URI (the scheme A protection space is defined by the canonical root URI (the scheme
and authority components of the target URI; see Section 5.1) of the and authority components of the target URI; see Section 6.1) of the
server being accessed, in combination with the realm value if server being accessed, in combination with the realm value if
present. These realms allow the protected resources on a server to present. These realms allow the protected resources on a server to
be partitioned into a set of protection spaces, each with its own be partitioned into a set of protection spaces, each with its own
authentication scheme and/or authorization database. The realm value authentication scheme and/or authorization database. The realm value
is a string, generally assigned by the origin server, that can have is a string, generally assigned by the origin server, that can have
additional semantics specific to the authentication scheme. Note additional semantics specific to the authentication scheme. Note
that a response can have multiple challenges with the same auth- that a response can have multiple challenges with the same auth-
scheme but with different realms. scheme but with different realms.
The protection space determines the domain over which credentials can The protection space determines the domain over which credentials can
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authentication scheme, parameters, and/or user preferences (such as a authentication scheme, parameters, and/or user preferences (such as a
configurable inactivity timeout). Unless specifically allowed by the configurable inactivity timeout). Unless specifically allowed by the
authentication scheme, a single protection space cannot extend authentication scheme, a single protection space cannot extend
outside the scope of its server. outside the scope of its server.
For historical reasons, a sender MUST only generate the quoted-string For historical reasons, a sender MUST only generate the quoted-string
syntax. Recipients might have to support both token and quoted- syntax. Recipients might have to support both token and quoted-
string syntax for maximum interoperability with existing clients that string syntax for maximum interoperability with existing clients that
have been accepting both notations for a long time. have been accepting both notations for a long time.
8.5.3. Authorization 9.5.3. Authorization
The "Authorization" header field allows a user agent to authenticate The "Authorization" header field allows a user agent to authenticate
itself with an origin server -- usually, but not necessarily, after itself with an origin server -- usually, but not necessarily, after
receiving a 401 (Unauthorized) response. Its value consists of receiving a 401 (Unauthorized) response. Its value consists of
credentials containing the authentication information of the user credentials containing the authentication information of the user
agent for the realm of the resource being requested. agent for the realm of the resource being requested.
Authorization = credentials Authorization = credentials
If a request is authenticated and a realm specified, the same If a request is authenticated and a realm specified, the same
credentials are presumed to be valid for all other requests within credentials are presumed to be valid for all other requests within
this realm (assuming that the authentication scheme itself does not this realm (assuming that the authentication scheme itself does not
require otherwise, such as credentials that vary according to a require otherwise, such as credentials that vary according to a
challenge value or using synchronized clocks). challenge value or using synchronized clocks).
A proxy forwarding a request MUST NOT modify any Authorization fields A proxy forwarding a request MUST NOT modify any Authorization fields
in that request. See Section 3.3 of [Caching] for details of and in that request. See Section 3.3 of [Caching] for details of and
requirements pertaining to handling of the Authorization field by requirements pertaining to handling of the Authorization field by
HTTP caches. HTTP caches.
8.5.4. Proxy-Authorization 9.5.4. Proxy-Authorization
The "Proxy-Authorization" header field allows the client to identify The "Proxy-Authorization" header field allows the client to identify
itself (or its user) to a proxy that requires authentication. Its itself (or its user) to a proxy that requires authentication. Its
value consists of credentials containing the authentication value consists of credentials containing the authentication
information of the client for the proxy and/or realm of the resource information of the client for the proxy and/or realm of the resource
being requested. being requested.
Proxy-Authorization = credentials Proxy-Authorization = credentials
Unlike Authorization, the Proxy-Authorization header field applies Unlike Authorization, the Proxy-Authorization header field applies
only to the next inbound proxy that demanded authentication using the only to the next inbound proxy that demanded authentication using the
Proxy-Authenticate field. When multiple proxies are used in a chain, Proxy-Authenticate field. When multiple proxies are used in a chain,
the Proxy-Authorization header field is consumed by the first inbound the Proxy-Authorization header field is consumed by the first inbound
proxy that was expecting to receive credentials. A proxy MAY relay proxy that was expecting to receive credentials. A proxy MAY relay
the credentials from the client request to the next proxy if that is the credentials from the client request to the next proxy if that is
the mechanism by which the proxies cooperatively authenticate a given the mechanism by which the proxies cooperatively authenticate a given
request. request.
8.5.5. Authentication Scheme Extensibility 9.5.5. Authentication Scheme Extensibility
Aside from the general framework, this document does not specify any Aside from the general framework, this document does not specify any
authentication schemes. New and existing authentication schemes are authentication schemes. New and existing authentication schemes are
specified independently and ought to be registered within the specified independently and ought to be registered within the
"Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry". "Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry".
For example, the "basic" and "digest" authentication schemes are For example, the "basic" and "digest" authentication schemes are
defined by RFC 7617 and RFC 7616, respectively. defined by RFC 7617 and RFC 7616, respectively.
8.5.5.1. Authentication Scheme Registry 9.5.5.1. Authentication Scheme Registry
The "Hypertext Transfer Protocol (HTTP) Authentication Scheme The "Hypertext Transfer Protocol (HTTP) Authentication Scheme
Registry" defines the namespace for the authentication schemes in Registry" defines the namespace for the authentication schemes in
challenges and credentials. It is maintained at challenges and credentials. It is maintained at
<https://www.iana.org/assignments/http-authschemes>. <https://www.iana.org/assignments/http-authschemes>.
Registrations MUST include the following fields: Registrations MUST include the following fields:
o Authentication Scheme Name o Authentication Scheme Name
o Pointer to specification text o Pointer to specification text
o Notes (optional) o Notes (optional)
Values to be added to this namespace require IETF Review (see Values to be added to this namespace require IETF Review (see
[RFC8126], Section 4.8). [RFC8126], Section 4.8).
8.5.5.2. Considerations for New Authentication Schemes 9.5.5.2. Considerations for New Authentication Schemes
There are certain aspects of the HTTP Authentication framework that There are certain aspects of the HTTP Authentication framework that
put constraints on how new authentication schemes can work: put constraints on how new authentication schemes can work:
o HTTP authentication is presumed to be stateless: all of the o HTTP authentication is presumed to be stateless: all of the
information necessary to authenticate a request MUST be provided information necessary to authenticate a request MUST be provided
in the request, rather than be dependent on the server remembering in the request, rather than be dependent on the server remembering
prior requests. Authentication based on, or bound to, the prior requests. Authentication based on, or bound to, the
underlying connection is outside the scope of this specification underlying connection is outside the scope of this specification
and inherently flawed unless steps are taken to ensure that the and inherently flawed unless steps are taken to ensure that the
connection cannot be used by any party other than the connection cannot be used by any party other than the
authenticated user (see Section 2.2). authenticated user (see Section 2.2).
o The authentication parameter "realm" is reserved for defining o The authentication parameter "realm" is reserved for defining
protection spaces as described in Section 8.5.2. New schemes MUST protection spaces as described in Section 9.5.2. New schemes MUST
NOT use it in a way incompatible with that definition. NOT use it in a way incompatible with that definition.
o The "token68" notation was introduced for compatibility with o The "token68" notation was introduced for compatibility with
existing authentication schemes and can only be used once per existing authentication schemes and can only be used once per
challenge or credential. Thus, new schemes ought to use the auth- challenge or credential. Thus, new schemes ought to use the auth-
param syntax instead, because otherwise future extensions will be param syntax instead, because otherwise future extensions will be
impossible. impossible.
o The parsing of challenges and credentials is defined by this o The parsing of challenges and credentials is defined by this
specification and cannot be modified by new authentication specification and cannot be modified by new authentication
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Therefore, new authentication schemes that choose not to carry Therefore, new authentication schemes that choose not to carry
credentials in the Authorization header field (e.g., using a newly credentials in the Authorization header field (e.g., using a newly
defined header field) will need to explicitly disallow caching, by defined header field) will need to explicitly disallow caching, by
mandating the use of Cache-Control response directives (e.g., mandating the use of Cache-Control response directives (e.g.,
"private"). "private").
o Schemes using Authentication-Info, Proxy-Authentication-Info, or o Schemes using Authentication-Info, Proxy-Authentication-Info, or
any other authentication related response header field need to any other authentication related response header field need to
consider and document the related security considerations (see consider and document the related security considerations (see
Section 11.14.4). Section 12.14.4).
8.6. Request Context 9.6. Request Context
The following request header fields provide additional information The following request header fields provide additional information
about the request context, including information about the user, user about the request context, including information about the user, user
agent, and resource behind the request. agent, and resource behind the request.
+------------+---------------+ +------------+---------------+
| Field Name | Defined in... | | Field Name | Defined in... |
+------------+---------------+ +------------+---------------+
| From | Section 8.6.1 | | From | Section 9.6.1 |
| Referer | Section 8.6.2 | | Referer | Section 9.6.2 |
| User-Agent | Section 8.6.3 | | User-Agent | Section 9.6.3 |
+------------+---------------+ +------------+---------------+
8.6.1. From 9.6.1. From
The "From" header field contains an Internet email address for a The "From" header field contains an Internet email address for a
human user who controls the requesting user agent. The address ought human user who controls the requesting user agent. The address ought
to be machine-usable, as defined by "mailbox" in Section 3.4 of to be machine-usable, as defined by "mailbox" in Section 3.4 of
[RFC5322]: [RFC5322]:
From = mailbox From = mailbox
mailbox = <mailbox, see [RFC5322], Section 3.4> mailbox = <mailbox, see [RFC5322], Section 3.4>
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A robotic user agent SHOULD send a valid From header field so that A robotic user agent SHOULD send a valid From header field so that
the person responsible for running the robot can be contacted if the person responsible for running the robot can be contacted if
problems occur on servers, such as if the robot is sending excessive, problems occur on servers, such as if the robot is sending excessive,
unwanted, or invalid requests. unwanted, or invalid requests.
A server SHOULD NOT use the From header field for access control or A server SHOULD NOT use the From header field for access control or
authentication, since most recipients will assume that the field authentication, since most recipients will assume that the field
value is public information. value is public information.
8.6.2. Referer 9.6.2. Referer
The "Referer" [sic] header field allows the user agent to specify a The "Referer" [sic] header field allows the user agent to specify a
URI reference for the resource from which the target URI was obtained URI reference for the resource from which the target URI was obtained
(i.e., the "referrer", though the field name is misspelled). A user (i.e., the "referrer", though the field name is misspelled). A user
agent MUST NOT include the fragment and userinfo components of the agent MUST NOT include the fragment and userinfo components of the
URI reference [RFC3986], if any, when generating the Referer field URI reference [RFC3986], if any, when generating the Referer field
value. value.
Referer = absolute-URI / partial-URI Referer = absolute-URI / partial-URI
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The Referer field has the potential to reveal information about the The Referer field has the potential to reveal information about the
request context or browsing history of the user, which is a privacy request context or browsing history of the user, which is a privacy
concern if the referring resource's identifier reveals personal concern if the referring resource's identifier reveals personal
information (such as an account name) or a resource that is supposed information (such as an account name) or a resource that is supposed
to be confidential (such as behind a firewall or internal to a to be confidential (such as behind a firewall or internal to a
secured service). Most general-purpose user agents do not send the secured service). Most general-purpose user agents do not send the
Referer header field when the referring resource is a local "file" or Referer header field when the referring resource is a local "file" or
"data" URI. A user agent MUST NOT send a Referer header field in an "data" URI. A user agent MUST NOT send a Referer header field in an
unsecured HTTP request if the referring page was received with a unsecured HTTP request if the referring page was received with a
secure protocol. See Section 11.8 for additional security secure protocol. See Section 12.8 for additional security
considerations. considerations.
Some intermediaries have been known to indiscriminately remove Some intermediaries have been known to indiscriminately remove
Referer header fields from outgoing requests. This has the Referer header fields from outgoing requests. This has the
unfortunate side effect of interfering with protection against CSRF unfortunate side effect of interfering with protection against CSRF
attacks, which can be far more harmful to their users. attacks, which can be far more harmful to their users.
Intermediaries and user agent extensions that wish to limit Intermediaries and user agent extensions that wish to limit
information disclosure in Referer ought to restrict their changes to information disclosure in Referer ought to restrict their changes to
specific edits, such as replacing internal domain names with specific edits, such as replacing internal domain names with
pseudonyms or truncating the query and/or path components. An pseudonyms or truncating the query and/or path components. An
intermediary SHOULD NOT modify or delete the Referer header field intermediary SHOULD NOT modify or delete the Referer header field
when the field value shares the same scheme and host as the target when the field value shares the same scheme and host as the target
URI. URI.
8.6.3. User-Agent 9.6.3. User-Agent
The "User-Agent" header field contains information about the user The "User-Agent" header field contains information about the user
agent originating the request, which is often used by servers to help agent originating the request, which is often used by servers to help
identify the scope of reported interoperability problems, to work identify the scope of reported interoperability problems, to work
around or tailor responses to avoid particular user agent around or tailor responses to avoid particular user agent
limitations, and for analytics regarding browser or operating system limitations, and for analytics regarding browser or operating system
use. A user agent SHOULD send a User-Agent field in each request use. A user agent SHOULD send a User-Agent field in each request
unless specifically configured not to do so. unless specifically configured not to do so.
User-Agent = product *( RWS ( product / comment ) ) User-Agent = product *( RWS ( product / comment ) )
The User-Agent field value consists of one or more product The User-Agent field value consists of one or more product
identifiers, each followed by zero or more comments identifiers, each followed by zero or more comments
(Section 4.4.1.3), which together identify the user agent software (Section 5.4.1.3), which together identify the user agent software
and its significant subproducts. By convention, the product and its significant subproducts. By convention, the product
identifiers are listed in decreasing order of their significance for identifiers are listed in decreasing order of their significance for
identifying the user agent software. Each product identifier identifying the user agent software. Each product identifier
consists of a name and optional version. consists of a name and optional version.
product = token ["/" product-version] product = token ["/" product-version]
product-version = token product-version = token
A sender SHOULD limit generated product identifiers to what is A sender SHOULD limit generated product identifiers to what is
necessary to identify the product; a sender MUST NOT generate necessary to identify the product; a sender MUST NOT generate
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identified against their wishes ("fingerprinting"). identified against their wishes ("fingerprinting").
Likewise, implementations are encouraged not to use the product Likewise, implementations are encouraged not to use the product
tokens of other implementations in order to declare compatibility tokens of other implementations in order to declare compatibility
with them, as this circumvents the purpose of the field. If a user with them, as this circumvents the purpose of the field. If a user
agent masquerades as a different user agent, recipients can assume agent masquerades as a different user agent, recipients can assume
that the user intentionally desires to see responses tailored for that the user intentionally desires to see responses tailored for
that identified user agent, even if they might not work as well for that identified user agent, even if they might not work as well for
the actual user agent being used. the actual user agent being used.
9. Response Status Codes 10. Response Status Codes
The (response) status code is a three-digit integer code giving the The (response) status code is a three-digit integer code giving the
result of the attempt to understand and satisfy the request. result of the attempt to understand and satisfy the request.
HTTP status codes are extensible. HTTP clients are not required to HTTP status codes are extensible. HTTP clients are not required to
understand the meaning of all registered status codes, though such understand the meaning of all registered status codes, though such
understanding is obviously desirable. However, a client MUST understanding is obviously desirable. However, a client MUST
understand the class of any status code, as indicated by the first understand the class of any status code, as indicated by the first
digit, and treat an unrecognized status code as being equivalent to digit, and treat an unrecognized status code as being equivalent to
the x00 status code of that class. the x00 status code of that class.
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fulfilled fulfilled
o 5xx (Server Error): The server failed to fulfill an apparently o 5xx (Server Error): The server failed to fulfill an apparently
valid request valid request
A single request can have multiple associated responses: zero or more A single request can have multiple associated responses: zero or more
interim (non-final) responses with status codes in the interim (non-final) responses with status codes in the
"informational" (1xx) range, followed by exactly one final response "informational" (1xx) range, followed by exactly one final response
with a status code in one of the other ranges. with a status code in one of the other ranges.
9.1. Overview of Status Codes 10.1. Overview of Status Codes
The status codes listed below are defined in this specification. The The status codes listed below are defined in this specification. The
reason phrases listed here are only recommendations -- they can be reason phrases listed here are only recommendations -- they can be
replaced by local equivalents without affecting the protocol. replaced by local equivalents without affecting the protocol.
Responses with status codes that are defined as heuristically Responses with status codes that are defined as heuristically
cacheable (e.g., 200, 203, 204, 206, 300, 301, 404, 405, 410, 414, cacheable (e.g., 200, 203, 204, 206, 300, 301, 308, 404, 405, 410,
and 501 in this specification) can be reused by a cache with 414, and 501 in this specification) can be reused by a cache with
heuristic expiration unless otherwise indicated by the method heuristic expiration unless otherwise indicated by the method
definition or explicit cache controls [Caching]; all other status definition or explicit cache controls [Caching]; all other status
codes are not heuristically cacheable. codes are not heuristically cacheable.
+-------+-------------------------------+-----------------+ +-------+-------------------------------+------------------+
| Value | Description | Reference | | Value | Description | Reference |
+-------+-------------------------------+-----------------+ +-------+-------------------------------+------------------+
| 100 | Continue | Section 9.2.1 | | 100 | Continue | Section 10.2.1 |
| 101 | Switching Protocols | Section 9.2.2 | | 101 | Switching Protocols | Section 10.2.2 |
| 200 | OK | Section 9.3.1 | | 200 | OK | Section 10.3.1 |
| 201 | Created | Section 9.3.2 | | 201 | Created | Section 10.3.2 |
| 202 | Accepted | Section 9.3.3 | | 202 | Accepted | Section 10.3.3 |
| 203 | Non-Authoritative Information | Section 9.3.4 | | 203 | Non-Authoritative Information | Section 10.3.4 |
| 204 | No Content | Section 9.3.5 | | 204 | No Content | Section 10.3.5 |
| 205 | Reset Content | Section 9.3.6 | | 205 | Reset Content | Section 10.3.6 |
| 206 | Partial Content | Section 9.3.7 | | 206 | Partial Content | Section 10.3.7 |
| 300 | Multiple Choices | Section 9.4.1 | | 300 | Multiple Choices | Section 10.4.1 |
| 301 | Moved Permanently | Section 9.4.2 | | 301 | Moved Permanently | Section 10.4.2 |
| 302 | Found | Section 9.4.3 | | 302 | Found | Section 10.4.3 |
| 303 | See Other | Section 9.4.4 | | 303 | See Other | Section 10.4.4 |
| 304 | Not Modified | Section 9.4.5 | | 304 | Not Modified | Section 10.4.5 |
| 305 | Use Proxy | Section 9.4.6 | | 305 | Use Proxy | Section 10.4.6 |
| 306 | (Unused) | Section 9.4.7 | | 306 | (Unused) | Section 10.4.7 |
| 307 | Temporary Redirect | Section 9.4.8 | | 307 | Temporary Redirect | Section 10.4.8 |
| 308 | Permanent Redirect | Section 9.4.9 | | 308 | Permanent Redirect | Section 10.4.9 |
| 400 | Bad Request | Section 9.5.1 | | 400 | Bad Request | Section 10.5.1 |
| 401 | Unauthorized | Section 9.5.2 | | 401 | Unauthorized | Section 10.5.2 |
| 402 | Payment Required | Section 9.5.3 | | 402 | Payment Required | Section 10.5.3 |
| 403 | Forbidden | Section 9.5.4 | | 403 | Forbidden | Section 10.5.4 |
| 404 | Not Found | Section 9.5.5 | | 404 | Not Found | Section 10.5.5 |
| 405 | Method Not Allowed | Section 9.5.6 | | 405 | Method Not Allowed | Section 10.5.6 |
| 406 | Not Acceptable | Section 9.5.7 | | 406 | Not Acceptable | Section 10.5.7 |
| 407 | Proxy Authentication Required | Section 9.5.8 | | 407 | Proxy Authentication Required | Section 10.5.8 |
| 408 | Request Timeout | Section 9.5.9 | | 408 | Request Timeout | Section 10.5.9 |
| 409 | Conflict | Section 9.5.10 | | 409 | Conflict | Section 10.5.10 |
| 410 | Gone | Section 9.5.11 | | 410 | Gone | Section 10.5.11 |
| 411 | Length Required | Section 9.5.12 | | 411 | Length Required | Section 10.5.12 |
| 412 | Precondition Failed | Section 9.5.13 | | 412 | Precondition Failed | Section 10.5.13 |
| 413 | Payload Too Large | Section 9.5.14 | | 413 | Payload Too Large | Section 10.5.14 |
| 414 | URI Too Long | Section 9.5.15 | | 414 | URI Too Long | Section 10.5.15 |
| 415 | Unsupported Media Type | Section 9.5.16 | | 415 | Unsupported Media Type | Section 10.5.16 |
| 416 | Range Not Satisfiable | Section 9.5.17 | | 416 | Range Not Satisfiable | Section 10.5.17 |
| 417 | Expectation Failed | Section 9.5.18 | | 417 | Expectation Failed | Section 10.5.18 |
| 418 | (Unused) | Section 9.5.19 | | 418 | (Unused) | Section 10.5.19 |
| 422 | Unprocessable Payload | Section 9.5.20 | | 422 | Unprocessable Payload | Section 10.5.20 |
| 426 | Upgrade Required | Section 9.5.21 | | 426 | Upgrade Required | Section 10.5.21 |
| 500 | Internal Server Error | Section 9.6.1 | | 500 | Internal Server Error | Section 10.6.1 |
| 501 | Not Implemented | Section 9.6.2 | | 501 | Not Implemented | Section 10.6.2 |
| 502 | Bad Gateway | Section 9.6.3 | | 502 | Bad Gateway | Section 10.6.3 |
| 503 | Service Unavailable | Section 9.6.4 | | 503 | Service Unavailable | Section 10.6.4 |
| 504 | Gateway Timeout | Section 9.6.5 | | 504 | Gateway Timeout | Section 10.6.5 |
| 505 | HTTP Version Not Supported | Section 9.6.6 | | 505 | HTTP Version Not Supported | Section 10.6.6 |
+-------+-------------------------------+-----------------+ +-------+-------------------------------+------------------+
Table 6 Table 6
Note that this list is not exhaustive -- it does not include Note that this list is not exhaustive -- it does not include
extension status codes defined in other specifications (Section 9.7). extension status codes defined in other specifications
(Section 10.7).
9.2. Informational 1xx 10.2. Informational 1xx
The 1xx (Informational) class of status code indicates an interim The 1xx (Informational) class of status code indicates an interim
response for communicating connection status or request progress response for communicating connection status or request progress
prior to completing the requested action and sending a final prior to completing the requested action and sending a final
response. 1xx responses are terminated by the end of the header response. 1xx responses are terminated by the end of the header
section. Since HTTP/1.0 did not define any 1xx status codes, a section. Since HTTP/1.0 did not define any 1xx status codes, a
server MUST NOT send a 1xx response to an HTTP/1.0 client. server MUST NOT send a 1xx response to an HTTP/1.0 client.
A client MUST be able to parse one or more 1xx responses received A client MUST be able to parse one or more 1xx responses received
prior to a final response, even if the client does not expect one. A prior to a final response, even if the client does not expect one. A
user agent MAY ignore unexpected 1xx responses. user agent MAY ignore unexpected 1xx responses.
A proxy MUST forward 1xx responses unless the proxy itself requested A proxy MUST forward 1xx responses unless the proxy itself requested
the generation of the 1xx response. For example, if a proxy adds an the generation of the 1xx response. For example, if a proxy adds an
"Expect: 100-continue" field when it forwards a request, then it need "Expect: 100-continue" field when it forwards a request, then it need
not forward the corresponding 100 (Continue) response(s). not forward the corresponding 100 (Continue) response(s).
9.2.1. 100 Continue 10.2.1. 100 Continue
The 100 (Continue) status code indicates that the initial part of a The 100 (Continue) status code indicates that the initial part of a
request has been received and has not yet been rejected by the request has been received and has not yet been rejected by the
server. The server intends to send a final response after the server. The server intends to send a final response after the
request has been fully received and acted upon. request has been fully received and acted upon.
When the request contains an Expect header field that includes a When the request contains an Expect header field that includes a
100-continue expectation, the 100 response indicates that the server 100-continue expectation, the 100 response indicates that the server
wishes to receive the request payload body, as described in wishes to receive the request payload body, as described in
Section 8.1.1. The client ought to continue sending the request and Section 9.1.1. The client ought to continue sending the request and
discard the 100 response. discard the 100 response.
If the request did not contain an Expect header field containing the If the request did not contain an Expect header field containing the
100-continue expectation, the client can simply discard this interim 100-continue expectation, the client can simply discard this interim
response. response.
9.2.2. 101 Switching Protocols 10.2.2. 101 Switching Protocols
The 101 (Switching Protocols) status code indicates that the server The 101 (Switching Protocols) status code indicates that the server
understands and is willing to comply with the client's request, via understands and is willing to comply with the client's request, via
the Upgrade header field (Section 9.9 of [Messaging]), for a change the Upgrade header field (Section 9.9 of [Messaging]), for a change
in the application protocol being used on this connection. The in the application protocol being used on this connection. The
server MUST generate an Upgrade header field in the response that server MUST generate an Upgrade header field in the response that
indicates which protocol(s) will be switched to immediately after the indicates which protocol(s) will be switched to immediately after the
empty line that terminates the 101 response. empty line that terminates the 101 response.
It is assumed that the server will only agree to switch protocols It is assumed that the server will only agree to switch protocols
when it is advantageous to do so. For example, switching to a newer when it is advantageous to do so. For example, switching to a newer
version of HTTP might be advantageous over older versions, and version of HTTP might be advantageous over older versions, and
switching to a real-time, synchronous protocol might be advantageous switching to a real-time, synchronous protocol might be advantageous
when delivering resources that use such features. when delivering resources that use such features.
9.3. Successful 2xx 10.3. Successful 2xx
The 2xx (Successful) class of status code indicates that the client's The 2xx (Successful) class of status code indicates that the client's
request was successfully received, understood, and accepted. request was successfully received, understood, and accepted.
9.3.1. 200 OK 10.3.1. 200 OK
The 200 (OK) status code indicates that the request has succeeded. The 200 (OK) status code indicates that the request has succeeded.
The payload sent in a 200 response depends on the request method. The payload sent in a 200 response depends on the request method.
For the methods defined by this specification, the intended meaning For the methods defined by this specification, the intended meaning
of the payload can be summarized as: of the payload can be summarized as:
GET a representation of the target resource; GET a representation of the target resource;
HEAD the same representation as GET, but without the representation HEAD the same representation as GET, but without the representation
data; data;
POST a representation of the status of, or results obtained from, POST a representation of the status of, or results obtained from,
the action; the action;
PUT, DELETE a representation of the status of the action; PUT, DELETE a representation of the status of the action;
OPTIONS a representation of the communications options; OPTIONS a representation of the communications options;
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though an origin server MAY generate a payload body of zero length. though an origin server MAY generate a payload body of zero length.
If no payload is desired, an origin server ought to send 204 (No If no payload is desired, an origin server ought to send 204 (No
Content) instead. For CONNECT, no payload is allowed because the Content) instead. For CONNECT, no payload is allowed because the
successful result is a tunnel, which begins immediately after the 200 successful result is a tunnel, which begins immediately after the 200
response header section. response header section.
A 200 response is heuristically cacheable; i.e., unless otherwise A 200 response is heuristically cacheable; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [Caching]). Section 4.2.2 of [Caching]).
9.3.2. 201 Created 10.3.2. 201 Created
The 201 (Created) status code indicates that the request has been The 201 (Created) status code indicates that the request has been
fulfilled and has resulted in one or more new resources being fulfilled and has resulted in one or more new resources being
created. The primary resource created by the request is identified created. The primary resource created by the request is identified
by either a Location header field in the response or, if no Location by either a Location header field in the response or, if no Location
field is received, by the target URI. field is received, by the target URI.
The 201 response payload typically describes and links to the The 201 response payload typically describes and links to the
resource(s) created. See Section 10.2 for a discussion of the resource(s) created. See Section 11.2 for a discussion of the
meaning and purpose of validator header fields, such as ETag and meaning and purpose of validator header fields, such as ETag and
Last-Modified, in a 201 response. Last-Modified, in a 201 response.
9.3.3. 202 Accepted 10.3.3. 202 Accepted
The 202 (Accepted) status code indicates that the request has been The 202 (Accepted) status code indicates that the request has been
accepted for processing, but the processing has not been completed. accepted for processing, but the processing has not been completed.
The request might or might not eventually be acted upon, as it might The request might or might not eventually be acted upon, as it might
be disallowed when processing actually takes place. There is no be disallowed when processing actually takes place. There is no
facility in HTTP for re-sending a status code from an asynchronous facility in HTTP for re-sending a status code from an asynchronous
operation. operation.
The 202 response is intentionally noncommittal. Its purpose is to The 202 response is intentionally noncommittal. Its purpose is to
allow a server to accept a request for some other process (perhaps a allow a server to accept a request for some other process (perhaps a
batch-oriented process that is only run once per day) without batch-oriented process that is only run once per day) without
requiring that the user agent's connection to the server persist requiring that the user agent's connection to the server persist
until the process is completed. The representation sent with this until the process is completed. The representation sent with this
response ought to describe the request's current status and point to response ought to describe the request's current status and point to
(or embed) a status monitor that can provide the user with an (or embed) a status monitor that can provide the user with an
estimate of when the request will be fulfilled. estimate of when the request will be fulfilled.
9.3.4. 203 Non-Authoritative Information 10.3.4. 203 Non-Authoritative Information
The 203 (Non-Authoritative Information) status code indicates that The 203 (Non-Authoritative Information) status code indicates that
the request was successful but the enclosed payload has been modified the request was successful but the enclosed payload has been modified
from that of the origin server's 200 (OK) response by a transforming from that of the origin server's 200 (OK) response by a transforming
proxy (Section 5.7.2). This status code allows the proxy to notify proxy (Section 6.7.2). This status code allows the proxy to notify
recipients when a transformation has been applied, since that recipients when a transformation has been applied, since that
knowledge might impact later decisions regarding the content. For knowledge might impact later decisions regarding the content. For
example, future cache validation requests for the content might only example, future cache validation requests for the content might only
be applicable along the same request path (through the same proxies). be applicable along the same request path (through the same proxies).
The 203 response is similar to the Warning code of 214 Transformation The 203 response is similar to the Warning code of 214 Transformation
Applied (Section 5.5 of [Caching]), which has the advantage of being Applied (Section 5.5 of [Caching]), which has the advantage of being
applicable to responses with any status code. applicable to responses with any status code.
A 203 response is heuristically cacheable; i.e., unless otherwise A 203 response is heuristically cacheable; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [Caching]). Section 4.2.2 of [Caching]).
9.3.5. 204 No Content 10.3.5. 204 No Content
The 204 (No Content) status code indicates that the server has The 204 (No Content) status code indicates that the server has
successfully fulfilled the request and that there is no additional successfully fulfilled the request and that there is no additional
content to send in the response payload body. Metadata in the content to send in the response p