< draft-ietf-httpbis-p2-semantics-20.txt   draft-ietf-httpbis-p2-semantics-21.txt >
HTTPbis Working Group R. Fielding, Ed. HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Adobe Internet-Draft Adobe
Obsoletes: 2616 (if approved) Y. Lafon, Ed. Obsoletes: 2616 (if approved) J. Reschke, Ed.
Updates: 2817 (if approved) W3C Updates: 2817 (if approved) greenbytes
Intended status: Standards Track J. Reschke, Ed. Intended status: Standards Track October 4, 2012
Expires: January 17, 2013 greenbytes Expires: April 7, 2013
July 16, 2012
HTTP/1.1, part 2: Semantics and Payloads Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content
draft-ietf-httpbis-p2-semantics-20 draft-ietf-httpbis-p2-semantics-21
Abstract Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypertext information protocol for distributed, collaborative, hypertext information
systems. This document defines the semantics of HTTP/1.1 messages, systems. This document defines the semantics of HTTP/1.1 messages,
as expressed by request methods, request header fields, response as expressed by request methods, request header fields, response
status codes, and response header fields, along with the payload of status codes, and response header fields, along with the payload of
messages (metadata and body content) and mechanisms for content messages (metadata and body content) and mechanisms for content
negotiation. negotiation.
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Discussion of this draft takes place on the HTTPBIS working group Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at mailing list (ietf-http-wg@w3.org), which is archived at
<http://lists.w3.org/Archives/Public/ietf-http-wg/>. <http://lists.w3.org/Archives/Public/ietf-http-wg/>.
The current issues list is at The current issues list is at
<http://tools.ietf.org/wg/httpbis/trac/report/3> and related <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
documents (including fancy diffs) can be found at documents (including fancy diffs) can be found at
<http://tools.ietf.org/wg/httpbis/>. <http://tools.ietf.org/wg/httpbis/>.
The changes in this draft are summarized in Appendix F.40. The changes in this draft are summarized in Appendix F.41.
Status of This Memo Status of This Memo
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7
1.1. Conformance and Error Handling . . . . . . . . . . . . . 7 1.1. Conformance and Error Handling . . . . . . . . . . . . . 7
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 8 1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 7
2. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2. Resource . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1. Safe and Idempotent Methods . . . . . . . . . . . . . . . 9 3. Representation . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.1. Safe Methods . . . . . . . . . . . . . . . . . . . . 9 3.1. Representation Metadata . . . . . . . . . . . . . . . . . 8
2.1.2. Idempotent Methods . . . . . . . . . . . . . . . . . 9 3.1.1. Data Type . . . . . . . . . . . . . . . . . . . . . . 9
2.2. Method Registry . . . . . . . . . . . . . . . . . . . . . 9 3.1.2. Data Encoding . . . . . . . . . . . . . . . . . . . . 12
2.2.1. Considerations for New Methods . . . . . . . . . . . 10 3.1.3. Audience Language . . . . . . . . . . . . . . . . . . 14
2.3. Method Definitions . . . . . . . . . . . . . . . . . . . 10 3.1.4. Identification . . . . . . . . . . . . . . . . . . . 15
2.3.1. OPTIONS . . . . . . . . . . . . . . . . . . . . . . . 11 3.2. Representation Data . . . . . . . . . . . . . . . . . . . 18
2.3.2. GET . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.3. Payload Semantics . . . . . . . . . . . . . . . . . . . . 18
2.3.3. HEAD . . . . . . . . . . . . . . . . . . . . . . . . 12 3.4. Content Negotiation . . . . . . . . . . . . . . . . . . . 19
2.3.4. POST . . . . . . . . . . . . . . . . . . . . . . . . 13 3.4.1. Proactive Negotiation . . . . . . . . . . . . . . . . 20
2.3.5. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.4.2. Reactive Negotiation . . . . . . . . . . . . . . . . 21
2.3.6. DELETE . . . . . . . . . . . . . . . . . . . . . . . 16 4. Product Tokens . . . . . . . . . . . . . . . . . . . . . . . 22
2.3.7. TRACE . . . . . . . . . . . . . . . . . . . . . . . . 16 5. Request Methods . . . . . . . . . . . . . . . . . . . . . . . 22
2.3.8. CONNECT . . . . . . . . . . . . . . . . . . . . . . . 17 5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 22
3. Header Fields . . . . . . . . . . . . . . . . . . . . . . . . 18 5.2. Common Method Properties . . . . . . . . . . . . . . . . 24
3.1. Considerations for Creating Header Fields . . . . . . . . 18 5.2.1. Safe Methods . . . . . . . . . . . . . . . . . . . . 24
3.2. Request Header Fields . . . . . . . . . . . . . . . . . . 20 5.2.2. Idempotent Methods . . . . . . . . . . . . . . . . . 25
3.3. Response Header Fields . . . . . . . . . . . . . . . . . 21 5.2.3. Cacheable Methods . . . . . . . . . . . . . . . . . . 25
4. Status Codes . . . . . . . . . . . . . . . . . . . . . . . . 22 5.3. Method Definitions . . . . . . . . . . . . . . . . . . . 25
4.1. Overview of Status Codes . . . . . . . . . . . . . . . . 22 5.3.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.2. Status Code Registry . . . . . . . . . . . . . . . . . . 24 5.3.2. HEAD . . . . . . . . . . . . . . . . . . . . . . . . 26
4.2.1. Considerations for New Status Codes . . . . . . . . . 24 5.3.3. POST . . . . . . . . . . . . . . . . . . . . . . . . 27
4.3. Informational 1xx . . . . . . . . . . . . . . . . . . . . 25 5.3.4. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.3.1. 100 Continue . . . . . . . . . . . . . . . . . . . . 25 5.3.5. DELETE . . . . . . . . . . . . . . . . . . . . . . . 30
4.3.2. 101 Switching Protocols . . . . . . . . . . . . . . . 25 5.3.6. CONNECT . . . . . . . . . . . . . . . . . . . . . . . 30
4.4. Successful 2xx . . . . . . . . . . . . . . . . . . . . . 26 5.3.7. OPTIONS . . . . . . . . . . . . . . . . . . . . . . . 32
4.4.1. 200 OK . . . . . . . . . . . . . . . . . . . . . . . 26 5.3.8. TRACE . . . . . . . . . . . . . . . . . . . . . . . . 33
4.4.2. 201 Created . . . . . . . . . . . . . . . . . . . . . 26 6. Request Header Fields . . . . . . . . . . . . . . . . . . . . 33
4.4.3. 202 Accepted . . . . . . . . . . . . . . . . . . . . 27 6.1. Controls . . . . . . . . . . . . . . . . . . . . . . . . 33
4.4.4. 203 Non-Authoritative Information . . . . . . . . . . 27 6.1.1. Max-Forwards . . . . . . . . . . . . . . . . . . . . 34
4.4.5. 204 No Content . . . . . . . . . . . . . . . . . . . 27 6.1.2. Expect . . . . . . . . . . . . . . . . . . . . . . . 34
4.4.6. 205 Reset Content . . . . . . . . . . . . . . . . . . 28 6.2. Conditionals . . . . . . . . . . . . . . . . . . . . . . 37
4.5. Redirection 3xx . . . . . . . . . . . . . . . . . . . . . 28 6.3. Content Negotiation . . . . . . . . . . . . . . . . . . . 38
4.5.1. 300 Multiple Choices . . . . . . . . . . . . . . . . 29 6.3.1. Quality Values . . . . . . . . . . . . . . . . . . . 38
4.5.2. 301 Moved Permanently . . . . . . . . . . . . . . . . 30 6.3.2. Accept . . . . . . . . . . . . . . . . . . . . . . . 38
4.5.3. 302 Found . . . . . . . . . . . . . . . . . . . . . . 30 6.3.3. Accept-Charset . . . . . . . . . . . . . . . . . . . 41
4.5.4. 303 See Other . . . . . . . . . . . . . . . . . . . . 31 6.3.4. Accept-Encoding . . . . . . . . . . . . . . . . . . . 41
4.5.5. 305 Use Proxy . . . . . . . . . . . . . . . . . . . . 31 6.3.5. Accept-Language . . . . . . . . . . . . . . . . . . . 42
4.5.6. 306 (Unused) . . . . . . . . . . . . . . . . . . . . 31 6.4. Authentication Credentials . . . . . . . . . . . . . . . 44
4.5.7. 307 Temporary Redirect . . . . . . . . . . . . . . . 32 6.5. Context . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.6. Client Error 4xx . . . . . . . . . . . . . . . . . . . . 32 6.5.1. From . . . . . . . . . . . . . . . . . . . . . . . . 44
4.6.1. 400 Bad Request . . . . . . . . . . . . . . . . . . . 32 6.5.2. Referer . . . . . . . . . . . . . . . . . . . . . . . 45
4.6.2. 402 Payment Required . . . . . . . . . . . . . . . . 32 6.5.3. User-Agent . . . . . . . . . . . . . . . . . . . . . 45
4.6.3. 403 Forbidden . . . . . . . . . . . . . . . . . . . . 32 7. Response Status Codes . . . . . . . . . . . . . . . . . . . . 46
4.6.4. 404 Not Found . . . . . . . . . . . . . . . . . . . . 33 7.1. Overview of Status Codes . . . . . . . . . . . . . . . . 47
4.6.5. 405 Method Not Allowed . . . . . . . . . . . . . . . 33 7.2. Informational 1xx . . . . . . . . . . . . . . . . . . . . 49
4.6.6. 406 Not Acceptable . . . . . . . . . . . . . . . . . 33 7.2.1. 100 Continue . . . . . . . . . . . . . . . . . . . . 49
4.6.7. 408 Request Timeout . . . . . . . . . . . . . . . . . 33 7.2.2. 101 Switching Protocols . . . . . . . . . . . . . . . 49
4.6.8. 409 Conflict . . . . . . . . . . . . . . . . . . . . 34 7.3. Successful 2xx . . . . . . . . . . . . . . . . . . . . . 50
4.6.9. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 34 7.3.1. 200 OK . . . . . . . . . . . . . . . . . . . . . . . 50
4.6.10. 411 Length Required . . . . . . . . . . . . . . . . . 34 7.3.2. 201 Created . . . . . . . . . . . . . . . . . . . . . 50
4.6.11. 413 Request Representation Too Large . . . . . . . . 35 7.3.3. 202 Accepted . . . . . . . . . . . . . . . . . . . . 51
4.6.12. 414 URI Too Long . . . . . . . . . . . . . . . . . . 35 7.3.4. 203 Non-Authoritative Information . . . . . . . . . . 51
4.6.13. 415 Unsupported Media Type . . . . . . . . . . . . . 35 7.3.5. 204 No Content . . . . . . . . . . . . . . . . . . . 51
4.6.14. 417 Expectation Failed . . . . . . . . . . . . . . . 35 7.3.6. 205 Reset Content . . . . . . . . . . . . . . . . . . 52
4.6.15. 426 Upgrade Required . . . . . . . . . . . . . . . . 35 7.4. Redirection 3xx . . . . . . . . . . . . . . . . . . . . . 52
4.7. Server Error 5xx . . . . . . . . . . . . . . . . . . . . 36 7.4.1. 300 Multiple Choices . . . . . . . . . . . . . . . . 54
4.7.1. 500 Internal Server Error . . . . . . . . . . . . . . 36 7.4.2. 301 Moved Permanently . . . . . . . . . . . . . . . . 54
4.7.2. 501 Not Implemented . . . . . . . . . . . . . . . . . 36 7.4.3. 302 Found . . . . . . . . . . . . . . . . . . . . . . 55
4.7.3. 502 Bad Gateway . . . . . . . . . . . . . . . . . . . 36 7.4.4. 303 See Other . . . . . . . . . . . . . . . . . . . . 55
4.7.4. 503 Service Unavailable . . . . . . . . . . . . . . . 36 7.4.5. 305 Use Proxy . . . . . . . . . . . . . . . . . . . . 56
4.7.5. 504 Gateway Timeout . . . . . . . . . . . . . . . . . 37 7.4.6. 306 (Unused) . . . . . . . . . . . . . . . . . . . . 56
4.7.6. 505 HTTP Version Not Supported . . . . . . . . . . . 37 7.4.7. 307 Temporary Redirect . . . . . . . . . . . . . . . 56
5. Protocol Parameters . . . . . . . . . . . . . . . . . . . . . 37 7.5. Client Error 4xx . . . . . . . . . . . . . . . . . . . . 56
5.1. Date/Time Formats . . . . . . . . . . . . . . . . . . . . 37 7.5.1. 400 Bad Request . . . . . . . . . . . . . . . . . . . 56
5.2. Product Tokens . . . . . . . . . . . . . . . . . . . . . 40 7.5.2. 402 Payment Required . . . . . . . . . . . . . . . . 56
5.3. Character Encodings (charset) . . . . . . . . . . . . . . 41 7.5.3. 403 Forbidden . . . . . . . . . . . . . . . . . . . . 57
5.4. Content Codings . . . . . . . . . . . . . . . . . . . . . 41 7.5.4. 404 Not Found . . . . . . . . . . . . . . . . . . . . 57
5.4.1. Content Coding Registry . . . . . . . . . . . . . . . 42 7.5.5. 405 Method Not Allowed . . . . . . . . . . . . . . . 57
5.5. Media Types . . . . . . . . . . . . . . . . . . . . . . . 42 7.5.6. 406 Not Acceptable . . . . . . . . . . . . . . . . . 57
5.5.1. Canonicalization and Text Defaults . . . . . . . . . 43 7.5.7. 408 Request Timeout . . . . . . . . . . . . . . . . . 58
5.5.2. Multipart Types . . . . . . . . . . . . . . . . . . . 44 7.5.8. 409 Conflict . . . . . . . . . . . . . . . . . . . . 58
5.6. Language Tags . . . . . . . . . . . . . . . . . . . . . . 44 7.5.9. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 58
6. Payload . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 7.5.10. 411 Length Required . . . . . . . . . . . . . . . . . 59
6.1. Payload Header Fields . . . . . . . . . . . . . . . . . . 45 7.5.11. 413 Request Representation Too Large . . . . . . . . 59
6.2. Payload Body . . . . . . . . . . . . . . . . . . . . . . 45 7.5.12. 414 URI Too Long . . . . . . . . . . . . . . . . . . 59
7. Representation . . . . . . . . . . . . . . . . . . . . . . . 45 7.5.13. 415 Unsupported Media Type . . . . . . . . . . . . . 59
7.1. Identifying the Resource Associated with a 7.5.14. 417 Expectation Failed . . . . . . . . . . . . . . . 60
Representation . . . . . . . . . . . . . . . . . . . . . 46 7.5.15. 426 Upgrade Required . . . . . . . . . . . . . . . . 60
7.2. Representation Header Fields . . . . . . . . . . . . . . 47 7.6. Server Error 5xx . . . . . . . . . . . . . . . . . . . . 60
7.3. Representation Data . . . . . . . . . . . . . . . . . . . 48 7.6.1. 500 Internal Server Error . . . . . . . . . . . . . . 60
8. Content Negotiation . . . . . . . . . . . . . . . . . . . . . 49 7.6.2. 501 Not Implemented . . . . . . . . . . . . . . . . . 60
8.1. Server-driven Negotiation . . . . . . . . . . . . . . . . 50 7.6.3. 502 Bad Gateway . . . . . . . . . . . . . . . . . . . 61
8.2. Agent-driven Negotiation . . . . . . . . . . . . . . . . 51 7.6.4. 503 Service Unavailable . . . . . . . . . . . . . . . 61
9. Header Field Definitions . . . . . . . . . . . . . . . . . . 52 7.6.5. 504 Gateway Timeout . . . . . . . . . . . . . . . . . 61
9.1. Accept . . . . . . . . . . . . . . . . . . . . . . . . . 52 7.6.6. 505 HTTP Version Not Supported . . . . . . . . . . . 61
9.2. Accept-Charset . . . . . . . . . . . . . . . . . . . . . 54 8. Response Header Fields . . . . . . . . . . . . . . . . . . . 61
9.3. Accept-Encoding . . . . . . . . . . . . . . . . . . . . . 55 8.1. Control Data . . . . . . . . . . . . . . . . . . . . . . 62
9.4. Accept-Language . . . . . . . . . . . . . . . . . . . . . 56 8.1.1. Origination Date . . . . . . . . . . . . . . . . . . 62
9.5. Allow . . . . . . . . . . . . . . . . . . . . . . . . . . 57 8.1.2. Location . . . . . . . . . . . . . . . . . . . . . . 65
9.6. Content-Encoding . . . . . . . . . . . . . . . . . . . . 57 8.1.3. Retry-After . . . . . . . . . . . . . . . . . . . . . 66
9.7. Content-Language . . . . . . . . . . . . . . . . . . . . 58 8.2. Selected Representation Header Fields . . . . . . . . . . 67
9.8. Content-Location . . . . . . . . . . . . . . . . . . . . 59 8.2.1. Vary . . . . . . . . . . . . . . . . . . . . . . . . 67
9.9. Content-Type . . . . . . . . . . . . . . . . . . . . . . 61 8.3. Authentication Challenges . . . . . . . . . . . . . . . . 68
9.10. Date . . . . . . . . . . . . . . . . . . . . . . . . . . 61 8.4. Informative . . . . . . . . . . . . . . . . . . . . . . . 68
9.11. Expect . . . . . . . . . . . . . . . . . . . . . . . . . 62 8.4.1. Allow . . . . . . . . . . . . . . . . . . . . . . . . 69
9.12. From . . . . . . . . . . . . . . . . . . . . . . . . . . 63 8.4.2. Server . . . . . . . . . . . . . . . . . . . . . . . 69
9.13. Location . . . . . . . . . . . . . . . . . . . . . . . . 63 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 70
9.14. Max-Forwards . . . . . . . . . . . . . . . . . . . . . . 65 9.1. Method Registry . . . . . . . . . . . . . . . . . . . . . 70
9.15. Referer . . . . . . . . . . . . . . . . . . . . . . . . . 65 9.1.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 70
9.16. Retry-After . . . . . . . . . . . . . . . . . . . . . . . 66 9.1.2. Considerations for New Methods . . . . . . . . . . . 70
9.17. Server . . . . . . . . . . . . . . . . . . . . . . . . . 66 9.1.3. Registrations . . . . . . . . . . . . . . . . . . . . 71
9.18. User-Agent . . . . . . . . . . . . . . . . . . . . . . . 67 9.2. Status Code Registry . . . . . . . . . . . . . . . . . . 71
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 67 9.2.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 71
10.1. Method Registry . . . . . . . . . . . . . . . . . . . . . 67 9.2.2. Considerations for New Status Codes . . . . . . . . . 71
10.2. Status Code Registry . . . . . . . . . . . . . . . . . . 68 9.2.3. Registrations . . . . . . . . . . . . . . . . . . . . 72
10.3. Header Field Registration . . . . . . . . . . . . . . . . 69 9.3. Header Field Registry . . . . . . . . . . . . . . . . . . 73
10.4. Content Coding Registry . . . . . . . . . . . . . . . . . 70 9.3.1. Considerations for New Header Fields . . . . . . . . 74
11. Security Considerations . . . . . . . . . . . . . . . . . . . 71 9.3.2. Registrations . . . . . . . . . . . . . . . . . . . . 75
11.1. Transfer of Sensitive Information . . . . . . . . . . . . 71
11.2. Encoding Sensitive Information in URIs . . . . . . . . . 72 9.4. Content Coding Registry . . . . . . . . . . . . . . . . . 76
11.3. Location Header Fields: Spoofing and Information 9.4.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 76
Leakage . . . . . . . . . . . . . . . . . . . . . . . . . 72 9.4.2. Registrations . . . . . . . . . . . . . . . . . . . . 77
11.4. Security Considerations for CONNECT . . . . . . . . . . . 73 10. Security Considerations . . . . . . . . . . . . . . . . . . . 77
11.5. Privacy Issues Connected to Accept Header Fields . . . . 73 10.1. Transfer of Sensitive Information . . . . . . . . . . . . 77
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 74 10.2. Encoding Sensitive Information in URIs . . . . . . . . . 78
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 74 10.3. Location Header Fields: Spoofing and Information
13.1. Normative References . . . . . . . . . . . . . . . . . . 74 Leakage . . . . . . . . . . . . . . . . . . . . . . . . . 79
13.2. Informative References . . . . . . . . . . . . . . . . . 75 10.4. Security Considerations for CONNECT . . . . . . . . . . . 79
Appendix A. Differences between HTTP and MIME . . . . . . . . . 77 10.5. Privacy Issues Connected to Accept Header Fields . . . . 79
A.1. MIME-Version . . . . . . . . . . . . . . . . . . . . . . 78 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 80
A.2. Conversion to Canonical Form . . . . . . . . . . . . . . 78 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 80
A.3. Conversion of Date Formats . . . . . . . . . . . . . . . 79 12.1. Normative References . . . . . . . . . . . . . . . . . . 80
A.4. Introduction of Content-Encoding . . . . . . . . . . . . 79 12.2. Informative References . . . . . . . . . . . . . . . . . 81
A.5. No Content-Transfer-Encoding . . . . . . . . . . . . . . 79 Appendix A. Differences between HTTP and MIME . . . . . . . . . 83
A.6. MHTML and Line Length Limitations . . . . . . . . . . . . 80 A.1. MIME-Version . . . . . . . . . . . . . . . . . . . . . . 84
Appendix B. Additional Features . . . . . . . . . . . . . . . . 80 A.2. Conversion to Canonical Form . . . . . . . . . . . . . . 84
Appendix C. Changes from RFC 2616 . . . . . . . . . . . . . . . 80 A.3. Conversion of Date Formats . . . . . . . . . . . . . . . 84
Appendix D. Imported ABNF . . . . . . . . . . . . . . . . . . . 82 A.4. Introduction of Content-Encoding . . . . . . . . . . . . 85
Appendix E. Collected ABNF . . . . . . . . . . . . . . . . . . . 83 A.5. No Content-Transfer-Encoding . . . . . . . . . . . . . . 85
A.6. MHTML and Line Length Limitations . . . . . . . . . . . . 85
Appendix B. Additional Features . . . . . . . . . . . . . . . . 85
Appendix C. Changes from RFC 2616 . . . . . . . . . . . . . . . 86
Appendix D. Imported ABNF . . . . . . . . . . . . . . . . . . . 88
Appendix E. Collected ABNF . . . . . . . . . . . . . . . . . . . 88
Appendix F. Change Log (to be removed by RFC Editor before Appendix F. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 85 publication) . . . . . . . . . . . . . . . . . . . . 91
F.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . 85 F.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . 91
F.2. Since draft-ietf-httpbis-p2-semantics-00 . . . . . . . . 86 F.2. Since draft-ietf-httpbis-p2-semantics-00 . . . . . . . . 91
F.3. Since draft-ietf-httpbis-p3-payload-00 . . . . . . . . . 86 F.3. Since draft-ietf-httpbis-p3-payload-00 . . . . . . . . . 92
F.4. Since draft-ietf-httpbis-p2-semantics-01 . . . . . . . . 87 F.4. Since draft-ietf-httpbis-p2-semantics-01 . . . . . . . . 93
F.5. Since draft-ietf-httpbis-p3-payload-01 . . . . . . . . . 88 F.5. Since draft-ietf-httpbis-p3-payload-01 . . . . . . . . . 93
F.6. Since draft-ietf-httpbis-p2-semantics-02 . . . . . . . . 88 F.6. Since draft-ietf-httpbis-p2-semantics-02 . . . . . . . . 93
F.7. Since draft-ietf-httpbis-p3-payload-02 . . . . . . . . . 89 F.7. Since draft-ietf-httpbis-p3-payload-02 . . . . . . . . . 94
F.8. Since draft-ietf-httpbis-p2-semantics-03 . . . . . . . . 89 F.8. Since draft-ietf-httpbis-p2-semantics-03 . . . . . . . . 95
F.9. Since draft-ietf-httpbis-p3-payload-03 . . . . . . . . . 89 F.9. Since draft-ietf-httpbis-p3-payload-03 . . . . . . . . . 95
F.10. Since draft-ietf-httpbis-p2-semantics-04 . . . . . . . . 90 F.10. Since draft-ietf-httpbis-p2-semantics-04 . . . . . . . . 95
F.11. Since draft-ietf-httpbis-p3-payload-04 . . . . . . . . . 90 F.11. Since draft-ietf-httpbis-p3-payload-04 . . . . . . . . . 96
F.12. Since draft-ietf-httpbis-p2-semantics-05 . . . . . . . . 91 F.12. Since draft-ietf-httpbis-p2-semantics-05 . . . . . . . . 96
F.13. Since draft-ietf-httpbis-p3-payload-05 . . . . . . . . . 91 F.13. Since draft-ietf-httpbis-p3-payload-05 . . . . . . . . . 96
F.14. Since draft-ietf-httpbis-p2-semantics-06 . . . . . . . . 91 F.14. Since draft-ietf-httpbis-p2-semantics-06 . . . . . . . . 97
F.15. Since draft-ietf-httpbis-p3-payload-06 . . . . . . . . . 92 F.15. Since draft-ietf-httpbis-p3-payload-06 . . . . . . . . . 97
F.16. Since draft-ietf-httpbis-p2-semantics-07 . . . . . . . . 92 F.16. Since draft-ietf-httpbis-p2-semantics-07 . . . . . . . . 97
F.17. Since draft-ietf-httpbis-p3-payload-07 . . . . . . . . . 92 F.17. Since draft-ietf-httpbis-p3-payload-07 . . . . . . . . . 98
F.18. Since draft-ietf-httpbis-p2-semantics-08 . . . . . . . . 93 F.18. Since draft-ietf-httpbis-p2-semantics-08 . . . . . . . . 99
F.19. Since draft-ietf-httpbis-p3-payload-08 . . . . . . . . . 93 F.19. Since draft-ietf-httpbis-p3-payload-08 . . . . . . . . . 99
F.20. Since draft-ietf-httpbis-p2-semantics-09 . . . . . . . . 93 F.20. Since draft-ietf-httpbis-p2-semantics-09 . . . . . . . . 99
F.21. Since draft-ietf-httpbis-p3-payload-09 . . . . . . . . . 94 F.21. Since draft-ietf-httpbis-p3-payload-09 . . . . . . . . . 99
F.22. Since draft-ietf-httpbis-p2-semantics-10 . . . . . . . . 94 F.22. Since draft-ietf-httpbis-p2-semantics-10 . . . . . . . . 100
F.23. Since draft-ietf-httpbis-p3-payload-10 . . . . . . . . . 95 F.23. Since draft-ietf-httpbis-p3-payload-10 . . . . . . . . . 100
F.24. Since draft-ietf-httpbis-p2-semantics-11 . . . . . . . . 95 F.24. Since draft-ietf-httpbis-p2-semantics-11 . . . . . . . . 101
F.25. Since draft-ietf-httpbis-p3-payload-11 . . . . . . . . . 96 F.25. Since draft-ietf-httpbis-p3-payload-11 . . . . . . . . . 101
F.26. Since draft-ietf-httpbis-p2-semantics-12 . . . . . . . . 96 F.26. Since draft-ietf-httpbis-p2-semantics-12 . . . . . . . . 101
F.27. Since draft-ietf-httpbis-p3-payload-12 . . . . . . . . . 97 F.27. Since draft-ietf-httpbis-p3-payload-12 . . . . . . . . . 103
F.28. Since draft-ietf-httpbis-p2-semantics-13 . . . . . . . . 97 F.28. Since draft-ietf-httpbis-p2-semantics-13 . . . . . . . . 103
F.29. Since draft-ietf-httpbis-p3-payload-13 . . . . . . . . . 98 F.29. Since draft-ietf-httpbis-p3-payload-13 . . . . . . . . . 103
F.30. Since draft-ietf-httpbis-p2-semantics-14 . . . . . . . . 98 F.30. Since draft-ietf-httpbis-p2-semantics-14 . . . . . . . . 103
F.31. Since draft-ietf-httpbis-p3-payload-14 . . . . . . . . . 98 F.31. Since draft-ietf-httpbis-p3-payload-14 . . . . . . . . . 104
F.32. Since draft-ietf-httpbis-p2-semantics-15 . . . . . . . . 98 F.32. Since draft-ietf-httpbis-p2-semantics-15 . . . . . . . . 104
F.33. Since draft-ietf-httpbis-p3-payload-15 . . . . . . . . . 99 F.33. Since draft-ietf-httpbis-p3-payload-15 . . . . . . . . . 104
F.34. Since draft-ietf-httpbis-p2-semantics-16 . . . . . . . . 99 F.34. Since draft-ietf-httpbis-p2-semantics-16 . . . . . . . . 104
F.35. Since draft-ietf-httpbis-p3-payload-16 . . . . . . . . . 99 F.35. Since draft-ietf-httpbis-p3-payload-16 . . . . . . . . . 104
F.36. Since draft-ietf-httpbis-p2-semantics-17 . . . . . . . . 99 F.36. Since draft-ietf-httpbis-p2-semantics-17 . . . . . . . . 105
F.37. Since draft-ietf-httpbis-p3-payload-17 . . . . . . . . . 100 F.37. Since draft-ietf-httpbis-p3-payload-17 . . . . . . . . . 105
F.38. Since draft-ietf-httpbis-p2-semantics-18 . . . . . . . . 100 F.38. Since draft-ietf-httpbis-p2-semantics-18 . . . . . . . . 105
F.39. Since draft-ietf-httpbis-p3-payload-18 . . . . . . . . . 101 F.39. Since draft-ietf-httpbis-p3-payload-18 . . . . . . . . . 106
F.40. Since draft-ietf-httpbis-p2-semantics-19 and F.40. Since draft-ietf-httpbis-p2-semantics-19 and
draft-ietf-httpbis-p3-payload-19 . . . . . . . . . . . . 101 draft-ietf-httpbis-p3-payload-19 . . . . . . . . . . . . 106
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 F.41. Since draft-ietf-httpbis-p2-semantics-20 . . . . . . . . 107
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
1. Introduction 1. Introduction
Each HTTP message is either a request or a response. A server Each Hypertext Transfer Protocol (HTTP) message is either a request
listens on a connection for a request, parses each message received, or a response. A server listens on a connection for a request,
interprets the message semantics in relation to the identified parses each message received, interprets the message semantics in
request target, and responds to that request with one or more relation to the identified request target, and responds to that
response messages. This document defines HTTP/1.1 request and request with one or more response messages. A client constructs
response semantics in terms of the architecture, syntax notation, and request messages to communicate specific intentions, and examines
conformance criteria defined in [Part1]. received responses to see if the intentions were carried out and
determine how to interpret the results. This document defines
HTTP/1.1 request and response semantics in terms of the architecture
defined in [Part1].
HTTP provides a uniform interface for interacting with resources HTTP provides a uniform interface for interacting with a resource
regardless of their type, nature, or implementation. HTTP semantics (Section 2), regardless of its type, nature, or implementation, and
includes the intentions defined by each request method, extensions to for transferring content in message payloads in the form of a
those semantics that might be described in request header fields, the representation (Section 3).
meaning of status codes to indicate a machine-readable response, and
additional control data and resource metadata that might be given in
response header fields.
In addition, this document defines the payload of messages (a.k.a., HTTP semantics include the intentions defined by each request method
content), the associated metadata header fields that define how the (Section 5), extensions to those semantics that might be described in
payload is intended to be interpreted by a recipient, the request request header fields (Section 6), the meaning of status codes to
indicate a machine-readable response (Section 7), and the meaning of
other control data and resource metadata that might be given in
response header fields (Section 8).
This document also defines representation metadata that describe how
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". negotiation" (Section 3.4).
Note: This document is currently disorganized in order to minimize
changes between drafts and enable reviewers to see the smaller
errata changes. A future draft will reorganize the sections to
better reflect the content. In particular, the sections will be
ordered according to the typical processing of an HTTP request
message (after message parsing): resource mapping, methods,
request modifying header fields, response status, status modifying
header fields, and resource metadata. The current mess reflects
how widely dispersed these topics and associated requirements had
become in [RFC2616].
1.1. Conformance and Error Handling 1.1. Conformance and Error Handling
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
This specification targets conformance criteria according to the role Conformance criteria and considerations regarding error handling are
of a participant in HTTP communication. Hence, HTTP requirements are defined in Section 2.5 of [Part1].
placed on senders, recipients, clients, servers, user agents,
intermediaries, origin servers, proxies, gateways, or caches,
depending on what behavior is being constrained by the requirement.
See Section 2 of [Part1] for definitions of these terms.
The verb "generate" is used instead of "send" where a requirement
differentiates between creating a protocol element and merely
forwarding a received element downstream.
An implementation is considered conformant if it complies with all of
the requirements associated with the roles it partakes in HTTP. Note
that SHOULD-level requirements are relevant here, unless one of the
documented exceptions is applicable.
This document also uses ABNF to define valid protocol elements
(Section 1.2). In addition to the prose requirements placed upon
them, senders MUST NOT generate protocol elements that do not match
the grammar defined by the ABNF rules for those protocol elements
that are applicable to the sender's role. If a received protocol
element is processed, the recipient MUST be able to parse any value
that would match the ABNF rules for that protocol element, excluding
only those rules not applicable to the recipient's role.
Unless noted otherwise, a recipient MAY attempt to recover a usable
protocol element from an invalid construct. HTTP does not define
specific error handling mechanisms except when they have a direct
impact on security, since different applications of the protocol
require different error handling strategies. For example, a Web
browser might wish to transparently recover from a response where the
Location header field doesn't parse according to the ABNF, whereas a
systems control client might consider any form of error recovery to
be dangerous.
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] with the list rule extension defined in Section notation of [RFC5234] with the list rule extension defined in Section
1.2 of [Part1]. Appendix D describes rules imported from other 1.2 of [Part1]. Appendix D describes rules imported from other
documents. Appendix E shows the collected ABNF with the list rule documents. Appendix E shows the collected ABNF with the list rule
expanded. expanded.
2. Methods 2. Resource
The method token indicates the request method to be performed on the The target of each HTTP request is called a resource. HTTP does not
target resource (Section 5.5 of [Part1]). The method is case- limit the nature of a resource; it merely defines an interface that
sensitive. might be used to interact with resources. Each resource is
identified by a Uniform Resource Identifier (URI), as described in
Section 2.7 of [Part1].
method = token When a client constructs an HTTP/1.1 request message, it sends the
"target URI" in one of various forms, as defined in (Section 5.3 of
[Part1]). When a request is received, the server reconstructs an
"effective request URI" for the target resource (Section 5.5 of
[Part1]).
The list of methods allowed by a resource can be specified in an One design goal of HTTP is to separate resource identification from
Allow header field (Section 9.5). The status code of the response request semantics, which is made possible by vesting the request
always notifies the client whether a method is currently allowed on a semantics in the request method (Section 5) and a few request-
resource, since the set of allowed methods can change dynamically. modifying header fields (Section 6). Resource owners SHOULD NOT
include request semantics within a URI, such as by specifying an
action to invoke within the path or query components of the effective
request URI, unless those semantics are disabled when they are
inconsistent with the request method.
An origin server SHOULD respond with the status code 405 (Method Not 3. Representation
Allowed) if the method is known by the origin server but not allowed
for the resource, and 501 (Not Implemented) if the method is
unrecognized or not implemented by the origin server. The methods
GET and HEAD MUST be supported by all general-purpose servers. All
other methods are OPTIONAL; however, if the above methods are
implemented, they MUST be implemented with the same semantics as
those specified in Section 2.3.
2.1. Safe and Idempotent Methods If we consider that a resource could be anything, and that the
uniform interface provided by HTTP is similar to a window through
which one can observe and act upon such a thing only through the
communication of messages to some independent actor on the other
side, then we need an abstraction to represent ("take the place of")
the current or desired state of that thing in our communications. We
call that abstraction a "representation" [REST].
2.1.1. Safe Methods For the purposes of HTTP, a representation is information that
reflects the current or desired state of a given resource, in a
format that can be readily communicated via the protocol, consisting
of a set of representation metadata and a potentially unbounded
stream of representation data.
Implementers need to be aware that the software represents the user 3.1. Representation Metadata
in their interactions over the Internet, and need to allow the user
to be aware of any actions they take which might have an unexpected
significance to themselves or others.
In particular, the convention has been established that the GET, Representation header fields provide metadata about the
HEAD, OPTIONS, and TRACE request methods SHOULD NOT have the representation. When a message includes a payload body, the
significance of taking an action other than retrieval. These request representation header fields describe how to interpret the
methods ought to be considered "safe". This allows user agents to representation data enclosed in the payload body. In a response to a
represent other methods, such as POST, PUT and DELETE, in a special HEAD request, the representation header fields describe the
way, so that the user is made aware of the fact that a possibly representation data that would have been enclosed in the payload body
unsafe action is being requested. if the same request had been a GET.
Naturally, it is not possible to ensure that the server does not The following header fields are defined to convey representation
generate side-effects as a result of performing a GET request; in metadata:
fact, some dynamic resources consider that a feature. The important
distinction here is that the user did not request the side-effects,
so therefore cannot be held accountable for them.
2.1.2. Idempotent Methods +-------------------+------------------------+
| Header Field Name | Defined in... |
+-------------------+------------------------+
| Content-Type | Section 3.1.1.5 |
| Content-Encoding | Section 3.1.2.2 |
| Content-Language | Section 3.1.3.2 |
| Content-Location | Section 3.1.4.2 |
| Expires | Section 7.3 of [Part6] |
+-------------------+------------------------+
Request methods can also have the property of "idempotence" in that, 3.1.1. Data Type
aside from error or expiration issues, the intended effect of
multiple identical requests is the same as for a single request.
PUT, DELETE, and all safe request methods are idempotent. It is
important to note that idempotence refers only to changes requested
by the client: a server is free to change its state due to multiple
requests for the purpose of tracking those requests, versioning of
results, etc.
2.2. Method Registry 3.1.1.1. Media Types
The HTTP Method Registry defines the name space for the method token HTTP uses Internet Media Types [RFC2046] in the Content-Type
in the Request line of an HTTP request. (Section 3.1.1.5) and Accept (Section 6.3.2) header fields in order
to provide open and extensible data typing and type negotiation.
Registrations MUST include the following fields: media-type = type "/" subtype *( OWS ";" OWS parameter )
type = token
subtype = token
o Method Name (see Section 2) The type/subtype MAY be followed by parameters in the form of
attribute/value pairs.
o Safe ("yes" or "no", see Section 2.1.1) parameter = attribute "=" value
attribute = token
value = word
o Idempotent ("yes" or "no", see Section 2.1.1) The type, subtype, and parameter attribute names are case-
insensitive. Parameter values might or might not be case-sensitive,
depending on the semantics of the parameter name. The presence or
absence of a parameter might be significant to the processing of a
media-type, depending on its definition within the media type
registry.
o Pointer to specification text A parameter value that matches the token production can be
transmitted as either a token or within a quoted-string. The quoted
and unquoted values are equivalent.
Values to be added to this name space require IETF Review (see Media-type values are registered with the Internet Assigned Number
[RFC5226], Section 4.1). Authority (IANA). The media type registration process is outlined in
[RFC4288]. Use of non-registered media types is discouraged.
The registry itself is maintained at 3.1.1.2. Character Encodings (charset)
<http://www.iana.org/assignments/http-methods>.
2.2.1. Considerations for New Methods HTTP uses charset names to indicate the character encoding of a
textual representation.
When it is necessary to express new semantics for a HTTP request that A character encoding is identified by a case-insensitive token. The
aren't specific to a single application or media type, and currently complete set of tokens is defined by the IANA Character Set registry
defined methods are inadequate, it might be appropriate to register a (<http://www.iana.org/assignments/character-sets>).
new method.
HTTP methods are generic; that is, they are potentially applicable to charset = token
any resource, not just one particular media type, "type" of resource,
or application. As such, it is preferred that new HTTP methods be
registered in a document that isn't specific to a single application,
so that this is clear.
Due to the parsing rules defined in Section 3.3 of [Part1], Although HTTP allows an arbitrary token to be used as a charset
definitions of HTTP methods cannot prohibit the presence of a message value, any token that has a predefined value within the IANA
body on either the request or the response message (with responses to Character Set registry MUST represent the character encoding defined
HEAD requests being the single exception). Definitions of new by that registry. Applications SHOULD limit their use of character
methods cannot change this rule, but they can specify that only zero- encodings to those defined within the IANA registry.
length bodies (as opposed to absent bodies) are allowed.
New method definitions need to indicate whether they are safe HTTP uses charset in two contexts: within an Accept-Charset request
(Section 2.1.1), what semantics (if any) the request body has, and header field (in which the charset value is an unquoted token) and as
whether they are idempotent (Section 2.1.2). They also need to state the value of a parameter in a Content-Type header field (within a
whether they can be cached ([Part6]); in particular under what request or response), in which case the parameter value of the
conditions a cache can store the response, and under what conditions charset parameter can be quoted.
such a stored response can be used to satisfy a subsequent request.
2.3. Method Definitions Implementers need to be aware of IETF character set requirements
2.3.1. OPTIONS [RFC3629] [RFC2277].
The OPTIONS method requests information about the communication 3.1.1.3. Canonicalization and Text Defaults
options available on the request/response chain identified by the
effective request URI. This method allows a client to determine the
options and/or requirements associated with a resource, or the
capabilities of a server, without implying a resource action or
initiating a resource retrieval.
Responses to the OPTIONS method are not cacheable. Internet media types are registered with a canonical form. A
representation transferred via HTTP messages MUST be in the
appropriate canonical form prior to its transmission except for
"text" types, as defined in the next paragraph.
If the OPTIONS request includes a message body (as indicated by the When in canonical form, media subtypes of the "text" type use CRLF as
presence of Content-Length or Transfer-Encoding), then the media type the text line break. HTTP relaxes this requirement and allows the
MUST be indicated by a Content-Type field. Although this transport of text media with plain CR or LF alone representing a line
specification does not define any use for such a body, future break when it is done consistently for an entire representation.
extensions to HTTP might use the OPTIONS body to make more detailed HTTP applications MUST accept CRLF, bare CR, and bare LF as
queries on the server. indicating a line break in text media received via HTTP. In
addition, if the text is in a character encoding that does not use
octets 13 and 10 for CR and LF respectively, as is the case for some
multi-byte character encodings, HTTP allows the use of whatever octet
sequences are defined by that character encoding to represent the
equivalent of CR and LF for line breaks. This flexibility regarding
line breaks applies only to text media in the payload body; a bare CR
or LF MUST NOT be substituted for CRLF within any of the HTTP control
structures (such as header fields and multipart boundaries).
If the request-target (Section 5.3 of [Part1]) is an asterisk ("*"), If a representation is encoded with a content-coding, the underlying
the OPTIONS request is intended to apply to the server in general data MUST be in a form defined above prior to being encoded.
rather than to a specific resource. Since a server's communication
options typically depend on the resource, the "*" request is only
useful as a "ping" or "no-op" type of method; it does nothing beyond
allowing the client to test the capabilities of the server. For
example, this can be used to test a proxy for HTTP/1.1 conformance
(or lack thereof).
If the request-target is not an asterisk, the OPTIONS request applies 3.1.1.4. Multipart Types
only to the options that are available when communicating with that
resource.
A 200 (OK) response SHOULD include any header fields that indicate MIME provides for a number of "multipart" types -- encapsulations of
optional features implemented by the server and applicable to that one or more representations within a single message body. All
resource (e.g., Allow), possibly including extensions not defined by multipart types share a common syntax, as defined in Section 5.1.1 of
this specification. The response body, if any, SHOULD also include [RFC2046], and include a boundary parameter as part of the media type
information about the communication options. The format for such a value. The message body is itself a protocol element; a sender MUST
body is not defined by this specification, but might be defined by generate only CRLF to represent line breaks between body-parts.
future extensions to HTTP. Content negotiation MAY be used to select
the appropriate response format. If no response body is included,
the response MUST include a Content-Length field with a field-value
of "0".
The Max-Forwards header field MAY be used to target a specific proxy In general, HTTP treats a multipart message body no differently than
in the request chain (see Section 9.14). If no Max-Forwards field is any other media type: strictly as payload. HTTP does not use the
present in the request, then the forwarded request MUST NOT include a multipart boundary as an indicator of message body length. In all
Max-Forwards field. other respects, an HTTP user agent SHOULD follow the same or similar
behavior as a MIME user agent would upon receipt of a multipart type.
The MIME header fields within each body-part of a multipart message
body do not have any significance to HTTP beyond that defined by
their MIME semantics.
2.3.2. GET A recipient MUST treat an unrecognized multipart subtype as being
equivalent to "multipart/mixed".
Note: The "multipart/form-data" type has been specifically defined
for carrying form data suitable for processing via the POST
request method, as described in [RFC2388].
3.1.1.5. Content-Type
The "Content-Type" header field indicates the media type of the
representation, which defines both the data format and how that data
SHOULD be processed by the recipient (within the scope of the request
method semantics) after any Content-Encoding is decoded. For
responses to the HEAD method, the media type is that which would have
been sent had the request been a GET.
Content-Type = media-type
Media types are defined in Section 3.1.1.1. An example of the field
is
Content-Type: text/html; charset=ISO-8859-4
A sender SHOULD include a Content-Type header field in a message
containing a payload body, defining the media type of the enclosed
representation, unless the intended media type is unknown to the
sender. If a Content-Type header field is not present, recipients
MAY either assume a media type of "application/octet-stream"
([RFC2046], Section 4.5.1) or examine the representation data to
determine its type.
In practice, resource owners do not always properly configure their
origin server to provide the correct Content-Type for a given
representation, with the result that some clients will examine a
payload's content and override the specified type. Clients that do
so risk drawing incorrect conclusions, which might expose additional
security risks (e.g., "privilege escalation"). Furthermore, it is
impossible to determine the sender's intent by examining the data
format: many data formats match multiple media types that differ only
in processing semantics. Implementers are encouraged to provide a
means of disabling such "content sniffing" when it is used.
3.1.2. Data Encoding
3.1.2.1. Content Codings
Content coding values indicate an encoding transformation that has
been or can be applied to a representation. Content codings are
primarily used to allow a representation to be compressed or
otherwise usefully transformed without losing the identity of its
underlying media type and without loss of information. Frequently,
the representation is stored in coded form, transmitted directly, and
only decoded by the recipient.
content-coding = token
All content-coding values are case-insensitive and SHOULD be
registered within the HTTP Content Coding registry, as defined in
Section 9.4. They are used in the Accept-Encoding (Section 6.3.4)
and Content-Encoding (Section 3.1.2.2) header fields.
The following content-coding values are defined by this
specification:
compress (and x-compress): See Section 4.2.1 of [Part1].
deflate: See Section 4.2.2 of [Part1].
gzip (and x-gzip): See Section 4.2.3 of [Part1].
3.1.2.2. Content-Encoding
The "Content-Encoding" header field indicates what content codings
have been applied to the representation, beyond those inherent in the
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
header field. Content-Encoding is primarily used to allow a
representation's data to be compressed without losing the identity of
its underlying media type.
Content-Encoding = 1#content-coding
An example of its use is
Content-Encoding: gzip
If multiple encodings have been applied to a representation, the
content codings MUST be listed in the order in which they were
applied. Additional information about the encoding parameters MAY be
provided by other header fields not defined by this specification.
Unlike Transfer-Encoding (Section 3.3.1 of [Part1]), the codings
listed in Content-Encoding are a characteristic of the
representation; the representation is defined in terms of the coded
form, and all other metadata about the representation is about the
coded form unless otherwise noted in the metadata definition.
Typically, the representation is only decoded just prior to rendering
or analogous usage.
A transforming proxy MAY modify the content coding if the new coding
is known to be acceptable to the recipient, unless the "no-transform"
cache-control directive is present in the message.
If the media type includes an inherent encoding, such as a data
format that is always compressed, then that encoding would not be
restated as a Content-Encoding even if it happens to be the same
algorithm as one of the content codings. Such a content coding would
only be listed if, for some bizarre reason, it is applied a second
time to form the representation. Likewise, an origin server might
choose to publish the same payload data as multiple representations
that differ only in whether the coding is defined as part of Content-
Type or Content-Encoding, since some user agents will behave
differently in their handling of each response (e.g., open a "Save as
..." dialog instead of automatic decompression and rendering of
content).
If the content-coding of a representation in a request message is not
acceptable to the origin server, the server SHOULD respond with a
status code of 415 (Unsupported Media Type).
3.1.3. Audience Language
3.1.3.1. Language Tags
A language tag, as defined in [RFC5646], identifies a natural
language spoken, written, or otherwise conveyed by human beings for
communication of information to other human beings. Computer
languages are explicitly excluded. HTTP uses language tags within
the Accept-Language and Content-Language fields.
In summary, a language tag is composed of one or more parts: A
primary language subtag followed by a possibly empty series of
subtags:
language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>
White space is not allowed within the tag and all tags are case-
insensitive. The name space of language subtags is administered by
the IANA (see
<http://www.iana.org/assignments/language-subtag-registry>).
Example tags include:
en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
See [RFC5646] for further information.
3.1.3.2. Content-Language
The "Content-Language" header field describes the natural language(s)
of the intended audience for the representation. Note that this
might not be equivalent to all the languages used within the
representation.
Content-Language = 1#language-tag
Language tags are defined in Section 3.1.3.1. The primary purpose of
Content-Language is to allow a user to identify and differentiate
representations according to the user's own preferred language.
Thus, if the content is intended only for a Danish-literate audience,
the appropriate field is
Content-Language: da
If no Content-Language is specified, the default is that the content
is intended for all language audiences. This might mean that the
sender does not consider it to be specific to any natural language,
or that the sender does not know for which language it is intended.
Multiple languages MAY be listed for content that is intended for
multiple audiences. For example, a rendition of the "Treaty of
Waitangi", presented simultaneously in the original Maori and English
versions, would call for
Content-Language: mi, en
However, just because multiple languages are present within a
representation does not mean that it is intended for multiple
linguistic audiences. An example would be a beginner's language
primer, such as "A First Lesson in Latin", which is clearly intended
to be used by an English-literate audience. In this case, the
Content-Language would properly only include "en".
Content-Language MAY be applied to any media type -- it is not
limited to textual documents.
3.1.4. Identification
3.1.4.1. Identifying a Representation
When a complete or partial representation is transferred in a message
payload, it is often desirable for the sender to supply, or the
recipient to determine, an identifier for a resource corresponding to
that representation.
The following rules are used to determine such a URI for the payload
of a request message:
o If the request has a Content-Location header field, then the
sender asserts that the payload is a representation of the
resource identified by the Content-Location field-value. However,
such an assertion cannot be trusted unless it can be verified by
other means (not defined by HTTP). The information might still be
useful for revision history links.
o Otherwise, the payload is unidentified.
The following rules, to be applied in order until a match is found,
are used to determine such a URI for the payload of a response
message:
1. If the request is GET or HEAD and the response status code is 200
(OK), 204 (No Content), 206 (Partial Content), or 304 (Not
Modified), the payload's identifier is the effective request URI
(Section 5.5 of [Part1]).
2. If the request is GET or HEAD and the response status code is 203
(Non-Authoritative Information), the payload is a potentially
modified representation of the target resource; as such, the
effective request URI might only act as an identifier for the
payload's representation when a request is made via the same
chain of intermediaries.
3. If the response has a Content-Location header field and its
field-value is a reference to the same URI as the effective
request URI, the payload's identifier is the effective request
URI.
4. If the response has a Content-Location header field and its
field-value is a reference to a URI different from the effective
request URI, then the sender asserts that the payload is a
representation of the resource identified by the Content-Location
field-value. However, such an assertion cannot be trusted unless
it can be verified by other means (not defined by HTTP).
5. Otherwise, the payload is unidentified.
3.1.4.2. Content-Location
The "Content-Location" header field references a URI that can be used
as a specific identifier for the representation in this message
payload. In other words, if one were to perform a GET on this URI at
the time of this message's generation, then a 200 (OK) response would
contain the same representation that is enclosed as payload in this
message.
Content-Location = absolute-URI / partial-URI
The Content-Location value is not a replacement for the effective
Request URI (Section 5.5 of [Part1]). It is representation metadata.
It has the same syntax and semantics as the header field of the same
name defined for MIME body parts in Section 4 of [RFC2557]. However,
its appearance in an HTTP message has some special implications for
HTTP recipients.
If Content-Location is included in a 2xx (Successful) response
message and its value refers (after conversion to absolute form) to a
URI that is the same as the effective request URI, then the response
payload SHOULD be considered a current representation of that
resource. For a GET or HEAD request, this is the same as the default
semantics when no Content-Location is provided by the server. For a
state-changing request like PUT or POST, it implies that the server's
response contains the new representation of that resource, thereby
distinguishing it from representations that might only report about
the action (e.g., "It worked!"). This allows authoring applications
to update their local copies without the need for a subsequent GET
request.
If Content-Location is included in a 2xx (Successful) response
message and its field-value refers to a URI that differs from the
effective request URI, then the origin server claims that the field-
value is an identifier for the payload's representation. Such a
claim can only be trusted if both identifiers share the same resource
owner, which cannot be programmatically determined via HTTP.
o For a response to a GET or HEAD request, this is an indication
that the effective request URI identifies a resource that is
subject to content negotiation and the Content-Location field-
value is a more specific identifier for the selected
representation.
o For a 201 (Created) response to a state-changing method, a
Content-Location field-value that is identical to the Location
field-value indicates that this payload is a current
representation of the newly created resource.
o Otherwise, such a Content-Location indicates that this payload is
a representation reporting on the requested action's status and
that the same report is available (for future access with GET) at
the given URI. For example, a purchase transaction made via a
POST request might include a receipt document as the payload of
the 200 (OK) response; the Content-Location field-value provides
an identifier for retrieving a copy of that same receipt in the
future.
If Content-Location is included in a request message, then it MAY be
interpreted by the origin server as an indication of where the user
agent originally obtained the content of the enclosed representation
(prior to any subsequent modification of the content by that user
agent). In other words, the user agent is providing the same
representation metadata that it received with the original
representation. However, such interpretation MUST NOT be used to
alter the semantics of the method requested by the client. For
example, if a client makes a PUT request on a negotiated resource and
the origin server accepts that PUT (without redirection), then the
new set of values for that resource is expected to be consistent with
the one representation supplied in that PUT; the Content-Location
cannot be used as a form of reverse content selection that identifies
only one of the negotiated representations to be updated. If the
user agent had wanted the latter semantics, it would have applied the
PUT directly to the Content-Location URI.
A Content-Location field received in a request message is transitory
information that SHOULD NOT be saved with other representation
metadata for use in later responses. The Content-Location's value
might be saved for use in other contexts, such as within source links
or other metadata.
A cache cannot assume that a representation with a Content-Location
different from the URI used to retrieve it can be used to respond to
later requests on that Content-Location URI.
3.2. Representation Data
The representation data associated with an HTTP message is either
provided as the payload body of the message or referred to by the
message semantics and the effective request URI. The representation
data is in a format and encoding defined by the representation
metadata header fields.
The data type of the representation data is determined via the header
fields Content-Type and Content-Encoding. These define a two-layer,
ordered encoding model:
representation-data := Content-Encoding( Content-Type( bits ) )
3.3. Payload Semantics
Some HTTP messages transfer a complete or partial representation as
the message "payload". In some cases, a payload might only contain
the associated representation's header fields (e.g., responses to
HEAD) or only some part(s) of the representation data (e.g., the 206
(Partial Content) status code).
The purpose of a payload in a request is defined by the method
semantics. In a response, the payload's purpose is defined by both
the request method and the response status code.
For example, a representation in the payload of a PUT request
(Section 5.3.4) represents the desired state of the target resource
if the request is successfully applied, whereas a representation in
the payload of a POST request (Section 5.3.3) represents an anonymous
resource for providing data to be processed, such as the information
that a user entered within an HTML form.
Likewise, the payload of a 200 (OK) response to GET (Section 5.3.1)
contains a representation of the target resource, as observed at the
time of the message origination date (Section 8.1.1.2), whereas the
same status code in a response to POST might contain either a
representation of the processing result or a current representation
of the target resource after applying the processing. Response
messages with an error status code usually contain a representation
that describes the error and what next steps are suggested for
resolving it.
Header fields that specifically describe the payload, rather than the
associated representation, are referred to as "payload header
fields". Payload header fields are defined in other parts of this
specification, due to their impact on message parsing.
+-------------------+--------------------------+
| Header Field Name | Defined in... |
+-------------------+--------------------------+
| Content-Length | Section 3.3.2 of [Part1] |
| Content-Range | Section 5.2 of [Part5] |
| Transfer-Encoding | Section 3.3.1 of [Part1] |
+-------------------+--------------------------+
3.4. Content Negotiation
HTTP responses include a representation which contains information
for interpretation, whether by a human user or for further
processing. Often, the server has different ways of representing the
same information; for example, in different formats, languages, or
using different character encodings.
HTTP clients and their users might have different or variable
capabilities, characteristics or preferences which would influence
which representation, among those available from the server, would be
best for the server to deliver. For this reason, HTTP provides
mechanisms for "content negotiation" -- a process of allowing
selection of a representation of a given resource, when more than one
is available.
This specification defines two patterns of content negotiation;
"proactive", where the server selects the representation based upon
the client's stated preferences, and "reactive" negotiation, where
the server provides a list of representations for the client to
choose from, based upon their metadata. In addition, there are other
patterns: some applications use an "active content" pattern, where
the server returns active content which runs on the client and, based
on client available parameters, selects additional resources to
invoke. "Transparent Content Negotiation" ([RFC2295]) has also been
proposed.
These patterns are all widely used, and have trade-offs in
applicability and practicality. In particular, when the number of
preferences or capabilities to be expressed by a client are large
(such as when many different formats are supported by a user-agent),
proactive negotiation becomes unwieldy, and might not be appropriate.
Conversely, when the number of representations to choose from is very
large, reactive negotiation might not be appropriate.
Note that, in all cases, the supplier of representations has the
responsibility for determining which representations might be
considered to be the "same information".
3.4.1. Proactive Negotiation
If the selection of the best representation for a response is made by
an algorithm located at the server, it is called proactive
negotiation. Selection is based on the available representations of
the response (the dimensions over which it can vary; e.g., language,
content-coding, etc.) and the contents of particular header fields in
the request message or on other information pertaining to the request
(such as the network address of the client).
Proactive negotiation is advantageous when the algorithm for
selecting from among the available representations is difficult to
describe to the user agent, or when the server desires to send its
"best guess" to the client along with the first response (hoping to
avoid the round-trip delay of a subsequent request if the "best
guess" is good enough for the user). In order to improve the
server's guess, the user agent MAY include request header fields
(Accept, Accept-Language, Accept-Encoding, etc.) which describe its
preferences for such a response.
Proactive negotiation has disadvantages:
1. It is impossible for the server to accurately determine what
might be "best" for any given user, since that would require
complete knowledge of both the capabilities of the user agent and
the intended use for the response (e.g., does the user want to
view it on screen or print it on paper?).
2. Having the user agent describe its capabilities in every request
can be both very inefficient (given that only a small percentage
of responses have multiple representations) and a potential
violation of the user's privacy.
3. It complicates the implementation of an origin server and the
algorithms for generating responses to a request.
4. It might limit a public cache's ability to use the same response
for multiple user's requests.
Proactive negotiation allows the user agent to specify its
preferences, but it cannot expect responses to always honor them.
For example, the origin server might not implement proactive
negotiation, or it might decide that sending a response that doesn't
conform to them is better than sending a 406 (Not Acceptable)
response.
HTTP/1.1 includes the following header fields for enabling proactive
negotiation through description of user agent capabilities and user
preferences: Accept (Section 6.3.2), Accept-Charset (Section 6.3.3),
Accept-Encoding (Section 6.3.4), Accept-Language (Section 6.3.5), and
User-Agent (Section 6.5.3). However, an origin server is not limited
to these dimensions and MAY vary the response based on any aspect of
the request, including aspects of the connection (e.g., IP address)
or information within extension header fields not defined by this
specification.
Note: In practice, User-Agent based negotiation is fragile,
because new clients might not be recognized.
The Vary header field (Section 8.2.1) can be used to express the
parameters the server uses to select a representation that is subject
to proactive negotiation.
3.4.2. Reactive Negotiation
With reactive negotiation, selection of the best representation for a
response is performed by the user agent after receiving an initial
response from the origin server. Selection is based on a list of the
available representations of the response included within the header
fields or body of the initial response, with each representation
identified by its own URI. Selection from among the representations
can be performed automatically (if the user agent is capable of doing
so) or manually by the user selecting from a generated (possibly
hypertext) menu.
Reactive negotiation is advantageous when the response would vary
over commonly-used dimensions (such as type, language, or encoding),
when the origin server is unable to determine a user agent's
capabilities from examining the request, and generally when public
caches are used to distribute server load and reduce network usage.
Reactive negotiation suffers from the disadvantage of needing a
second request to obtain the best alternate representation. This
second request is only efficient when caching is used. In addition,
this specification does not define any mechanism for supporting
automatic selection, though it also does not prevent any such
mechanism from being developed as an extension and used within
HTTP/1.1.
This specification defines the 300 (Multiple Choices) and 406 (Not
Acceptable) status codes for enabling reactive negotiation when the
server is unwilling or unable to provide a varying response using
proactive negotiation.
4. Product Tokens
Product tokens are used to allow communicating applications to
identify themselves by software name and version. Most fields using
product tokens also allow sub-products which form a significant part
of the application to be listed, separated by whitespace. By
convention, the products are listed in order of their significance
for identifying the application.
product = token ["/" product-version]
product-version = token
Examples:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
Server: Apache/0.8.4
Product tokens SHOULD be short and to the point. They MUST NOT be
used for advertising or other non-essential information. Although
any token octet MAY appear in a product-version, this token SHOULD
only be used for a version identifier (i.e., successive versions of
the same product SHOULD only differ in the product-version portion of
the product value).
5. Request Methods
5.1. Overview
The request method token is the primary source of request semantics;
it indicates the purpose for which the client has made this request
and what is expected by the client as a successful result. The
request semantics MAY be further specialized by the semantics of some
header fields when present in a request (Section 6) if those
additional semantics do not conflict with the method.
method = token
HTTP was originally designed to be usable as an interface to
distributed object systems. The request method was envisioned as
applying semantics to a target resource in much the same way as
invoking a defined method on an identified object would apply
semantics. The method token is case-sensitive because it might be
used as a gateway to object-based systems with case-sensitive method
names.
Unlike distributed objects, the standardized request methods in HTTP
are not resource-specific, since uniform interfaces provide for
better visibility and reuse in network-based systems [REST]. Once
defined, a standardized method MUST have the same semantics when
applied to any resource, though each resource determines for itself
whether those semantics are implemented or allowed.
This specification defines a number of standardized methods that are
commonly used in HTTP, as outlined by the following table. By
convention, standardized methods are defined in all-uppercase ASCII
letters.
+---------+-------------------------------------------------+-------+
| Method | Description | Sec. |
+---------+-------------------------------------------------+-------+
| GET | Transfer a current representation of the target | 5.3.1 |
| | resource. | |
| HEAD | Same as GET, but do not include a message body | 5.3.2 |
| | in the response. | |
| POST | Perform resource-specific processing on the | 5.3.3 |
| | request payload. | |
| PUT | Replace all current representations of the | 5.3.4 |
| | target resource with the request payload. | |
| DELETE | Remove all current representations of the | 5.3.5 |
| | target resource. | |
| CONNECT | Establish a tunnel to the server identified by | 5.3.6 |
| | the target resource. | |
| OPTIONS | Describe the communication options for the | 5.3.7 |
| | target resource. | |
| TRACE | Perform a message loop-back test along the path | 5.3.8 |
| | to the target resource. | |
+---------+-------------------------------------------------+-------+
The methods GET and HEAD MUST be supported by all general-purpose
servers. All other methods are OPTIONAL. When implemented, a server
MUST implement the above methods according to the semantics defined
for them in Section 5.3.
Additional methods MAY be used in HTTP; many have already been
standardized outside the scope of this specification and registered
within the HTTP Method Registry maintained by IANA, as defined in
Section 9.1.
The set of methods allowed by a target resource can be listed in an
Allow header field (Section 8.4.1). However, the set of allowed
methods can change dynamically. When a request message is received
that is unrecognized or not implemented by an origin server, the
origin server SHOULD respond with the 501 (Not Implemented) status
code. When a request message is received that is known by an origin
server but not allowed for the target resource, the origin server
SHOULD respond with the 405 (Method Not Allowed) status code.
5.2. Common Method Properties
5.2.1. Safe Methods
Request methods are considered "safe" if their defined semantics are
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
applying a safe method to a target resource. Likewise, reasonable
use of a safe method is not expected to cause any harm, loss of
property, or unusual burden on the origin server.
This definition of safe methods does not prevent an implementation
from including behavior that is potentially harmful, not entirely
read-only, or which causes side-effects while invoking a safe method.
What is important, however, is that the client did not request that
additional behavior and cannot be held accountable for it. For
example, most servers append request information to access log files
at the completion of every response, regardless of the method, and
that is considered safe even though the log storage might become full
and crash the server. Likewise, a safe request initiated by
selecting an advertisement on the Web will often have the side-effect
of charging an advertising account.
The GET, HEAD, OPTIONS, and TRACE request methods are defined to be
safe.
The purpose of distinguishing between safe and unsafe methods is to
allow automated retrieval processes (spiders) and cache performance
optimization (pre-fetching) to work without fear of causing harm. In
addition, it allows a user agent to apply appropriate constraints on
the automated use of unsafe methods when processing potentially
untrusted content.
A user agent SHOULD distinguish between safe and unsafe methods when
presenting potential actions to a user, such that the user can be
made aware of an unsafe action before it is requested.
When a resource is constructed such that parameters within the
effective request URI have the effect of selecting an action, it is
the resource owner's responsibility to ensure that the action is
consistent with the request method semantics. For example, it is
common for Web-based content editing software to use actions within
query parameters, such as "page?do=delete". If the purpose of such a
resource is to perform an unsafe action, then the resource MUST
disable or disallow that action when it is accessed using a safe
request method. Failure to do so will result in unfortunate side-
effects when automated processes perform a GET on every URI reference
for the sake of link maintenance, pre-fetching, building a search
index, etc.
5.2.2. Idempotent Methods
Request methods are considered "idempotent" if the intended effect of
multiple identical requests is the same as for a single request.
PUT, DELETE, and all safe request methods are idempotent.
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
request separately, retain a revision control history, or implement
other non-idempotent side-effects for each idempotent request.
Idempotent methods are distinguished because the request can be
repeated automatically if a communication failure occurs before the
client is able to read the server's response. For example, if a
client sends a PUT request and the underlying connection is closed
before any response is received, then it can establish a new
connection and retry the idempotent request because it knows that
repeating the request will have the same effect even if the original
request succeeded. Note, however, that repeated failures would
indicate a problem within the server.
5.2.3. Cacheable Methods
Request methods are considered "cacheable" if it is possible and
useful to answer a current client request with a stored response from
a prior request. GET and HEAD are defined to be cacheable. In
general, safe methods that do not depend on a current or
authoritative response are cacheable, though the overwhelming
majority of caches only support GET and HEAD. HTTP requirements for
cache behavior and cacheable responses are defined in [Part6].
5.3. Method Definitions
5.3.1. GET
The GET method requests transfer of a current representation of the The GET method requests transfer of a current representation of the
target resource. target resource.
If the target resource is a data-producing process, it is the If the target resource is a data-producing process, it is the
produced data which shall be returned as the representation in the produced data which shall be returned as the representation in the
response and not the source text of the process, unless that text response and not the source text of the process, unless that text
happens to be the output of the process. happens to be the output of the process.
The semantics of the GET method change to a "conditional GET" if the The semantics of the GET method change to a "conditional GET" if the
skipping to change at page 12, line 33 skipping to change at page 26, line 28
client. client.
The semantics of the GET method change to a "partial GET" if the The semantics of the GET method change to a "partial GET" if the
request message includes a Range header field ([Part5]). A partial request message includes a Range header field ([Part5]). A partial
GET requests that only part of the representation be transferred, as GET requests that only part of the representation be transferred, as
described in Section 5.4 of [Part5]. The partial GET request is described in Section 5.4 of [Part5]. The partial GET request is
intended to reduce unnecessary network usage by allowing partially- intended to reduce unnecessary network usage by allowing partially-
retrieved representations to be completed without transferring data retrieved representations to be completed without transferring data
already held by the client. already held by the client.
Bodies on GET requests have no defined semantics. Note that sending A payload within a GET request message has no defined semantics;
a body on a GET request might cause some existing implementations to sending a payload body on a GET request might cause some existing
reject the request. implementations to reject the request.
The response to a GET request is cacheable and MAY be used to satisfy The response to a GET request is cacheable and MAY be used to satisfy
subsequent GET and HEAD requests (see [Part6]). subsequent GET and HEAD requests (see [Part6]).
See Section 11.2 for security considerations when used for forms. See Section 10.2 for security considerations when used for forms.
2.3.3. HEAD 5.3.2. 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
return a message body in the response. The metadata contained in the return a message body in the response. The metadata contained in the
HTTP header fields in response to a HEAD request SHOULD be identical HTTP header fields in response to a HEAD request SHOULD be identical
to the information sent in response to a GET request. This method to the information sent in response to a GET request. This method
can be used for obtaining metadata about the representation implied can be used for obtaining metadata about the representation implied
by the request without transferring the representation body. This by the request without transferring the representation data. This
method is often used for testing hypertext links for validity, method is often used for testing hypertext links for validity,
accessibility, and recent modification. accessibility, and recent modification.
The response to a HEAD request is cacheable and MAY be used to The response to a HEAD request is cacheable and MAY be used to
satisfy a subsequent HEAD request. It also has potential side satisfy a subsequent HEAD request. It also has potential side
effects on previously stored responses to GET; see Section 5 of effects on previously stored responses to GET; see Section 5 of
[Part6]. [Part6].
Bodies on HEAD requests have no defined semantics. Note that sending A payload within a HEAD request message has no defined semantics;
a body on a HEAD request might cause some existing implementations to sending a payload body on a HEAD request might cause some existing
reject the request. implementations to reject the request.
2.3.4. POST 5.3.3. POST
The POST method requests that the origin server accept the The POST method requests that the origin server accept the
representation enclosed in the request as data to be processed by the representation enclosed in the request as data to be processed by the
target resource. POST is designed to allow a uniform method to cover target resource. POST is designed to allow a uniform method to cover
the following functions: the following functions:
o Annotation of existing resources; o Annotation of existing resources;
o Posting a message to a bulletin board, newsgroup, mailing list, or o Posting a message to a bulletin board, newsgroup, mailing list, or
similar group of articles; similar group of articles;
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The action performed by the POST method might not result in a The action performed by the POST method might not result in a
resource that can be identified by a URI. In this case, either 200 resource that can be identified by a URI. In this case, either 200
(OK) or 204 (No Content) is the appropriate response status code, (OK) or 204 (No Content) is the appropriate response status code,
depending on whether or not the response includes a representation depending on whether or not the response includes a representation
that describes the result. that describes the result.
If a resource has been created on the origin server, the response If a resource has been created on the origin server, the response
SHOULD be 201 (Created) and contain a representation which describes SHOULD be 201 (Created) and contain a representation which describes
the status of the request and refers to the new resource, and a the status of the request and refers to the new resource, and a
Location header field (see Section 9.13). Location header field (see Section 8.1.2).
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 4.1.1 of [Part6]). A explicit freshness information (see Section 4.1.1 of [Part6]). A
cached POST response with a Content-Location header field (see cached POST response with a Content-Location header field (see
Section 9.8) whose value is the effective Request URI MAY be used to Section 3.1.4.2) whose value is the effective Request URI MAY be used
satisfy subsequent GET and HEAD requests. to satisfy subsequent GET and HEAD (not POST) requests.
Note that POST caching is not widely implemented. However, the 303 Note that POST caching is not widely implemented. However, the 303
(See Other) response can be used to direct the user agent to retrieve (See Other) response can be used to direct the user agent to retrieve
a cacheable representation of the resource. a cacheable representation of the resource.
2.3.5. PUT 5.3.4. 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
returned in a 200 (OK) response. However, there is no guarantee that returned 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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content updates are possible by targeting a separately identified content updates are possible by targeting a separately identified
resource with state that overlaps a portion of the larger resource, resource with state that overlaps a portion of the larger resource,
or by using a different method that has been specifically defined for or by using a different method that has been specifically defined for
partial updates (for example, the PATCH method defined in [RFC5789]). partial updates (for example, the PATCH method defined in [RFC5789]).
Responses to the PUT method are not cacheable. If a PUT request Responses to the PUT method are not cacheable. If a PUT request
passes through a cache that has one or more stored responses for the passes through a cache that has one or more stored responses for the
effective request URI, those stored responses will be invalidated effective request URI, those stored responses will be invalidated
(see Section 6 of [Part6]). (see Section 6 of [Part6]).
2.3.6. DELETE 5.3.5. DELETE
The DELETE method requests that the origin server delete the target The DELETE method requests that the origin server delete the target
resource. This method MAY be overridden by human intervention (or resource. This method MAY be overridden by human intervention (or
other means) on the origin server. The client cannot be guaranteed other means) on the origin server. The client cannot be guaranteed
that the operation has been carried out, even if the status code that the operation has been carried out, even if the status code
returned from the origin server indicates that the action has been returned from the origin server indicates that the action has been
completed successfully. However, the server SHOULD NOT indicate completed successfully. However, the server SHOULD NOT indicate
success unless, at the time the response is given, it intends to success unless, at the time the response is given, it intends to
delete the resource or move it to an inaccessible location. delete the resource or move it to an inaccessible location.
A successful response SHOULD be 200 (OK) if the response includes a A successful response SHOULD be 200 (OK) if the response includes a
representation describing the status, 202 (Accepted) if the action representation describing the status, 202 (Accepted) if the action
has not yet been enacted, or 204 (No Content) if the action has been has not yet been enacted, or 204 (No Content) if the action has been
enacted but the response does not include a representation. enacted but the response does not include a representation.
Bodies on DELETE requests have no defined semantics. Note that A payload within a DELETE request message has no defined semantics;
sending a body on a DELETE request might cause some existing sending a payload body on a DELETE request might cause some existing
implementations to reject the request. implementations to reject the request.
Responses to the DELETE method are not cacheable. If a DELETE Responses to the DELETE method are not cacheable. If a DELETE
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 effective request URI, those stored responses will be for the effective request URI, those stored responses will be
invalidated (see Section 6 of [Part6]). invalidated (see Section 6 of [Part6]).
2.3.7. TRACE 5.3.6. CONNECT
The TRACE method requests a remote, application-layer loop-back of
the request message. The final recipient of the request SHOULD
reflect the message received back to the client as the message body
of a 200 (OK) response. The final recipient is either the origin
server or the first proxy to receive a Max-Forwards value of zero (0)
in the request (see Section 9.14). A TRACE request MUST NOT include
a message body.
TRACE allows the client to see what is being received at the other
end of the request chain and use that data for testing or diagnostic
information. The value of the Via header field (Section 6.2 of
[Part1]) is of particular interest, since it acts as a trace of the
request chain. Use of the Max-Forwards header field allows the
client to limit the length of the request chain, which is useful for
testing a chain of proxies forwarding messages in an infinite loop.
If the request is valid, the response SHOULD have a Content-Type of
"message/http" (see Section 7.3.1 of [Part1]) and contain a message
body that encloses a copy of the entire request message. Responses
to the TRACE method are not cacheable.
2.3.8. CONNECT
The CONNECT method requests that the proxy establish a tunnel to the The CONNECT method requests that the proxy establish a tunnel to the
request-target and, if successful, thereafter restrict its behavior request-target and, if successful, thereafter restrict its behavior
to blind forwarding of packets until the connection is closed. to blind forwarding of packets until the connection is closed.
When using CONNECT, the request-target MUST use the authority form When using CONNECT, the request-target MUST use the authority form
(Section 5.3 of [Part1]); i.e., the request-target consists of only (Section 5.3 of [Part1]); i.e., the request-target consists of only
the host name and port number of the tunnel destination, separated by the host name and port number of the tunnel destination, separated by
a colon. For example, a colon. For example,
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tunnel has not yet been formed and that the connection remains tunnel has not yet been formed and that the connection remains
governed by HTTP. governed by HTTP.
Proxy authentication might be used to establish the authority to Proxy authentication might be used to establish the authority to
create a tunnel: create a tunnel:
CONNECT server.example.com:80 HTTP/1.1 CONNECT server.example.com:80 HTTP/1.1
Host: server.example.com:80 Host: server.example.com:80
Proxy-Authorization: basic aGVsbG86d29ybGQ= Proxy-Authorization: basic aGVsbG86d29ybGQ=
A message body on a CONNECT request has no defined semantics. A payload within a CONNECT request message has no defined semantics;
Sending a body on a CONNECT request might cause existing sending a payload body on a CONNECT request might cause some existing
implementations to reject the request. implementations to reject the request.
Similar to a pipelined HTTP/1.1 request, data to be tunneled from Similar to a pipelined HTTP/1.1 request, data to be tunneled from
client to server MAY be sent immediately after the request (before a client to server MAY be sent immediately after the request (before a
response is received). The usual caveats also apply: data can be response is received). The usual caveats also apply: data can be
discarded if the eventual response is negative, and the connection discarded if the eventual response is negative, and the connection
can be reset with no response if more than one TCP segment is can be reset with no response if more than one TCP segment is
outstanding. outstanding.
It might be the case that the proxy itself can only reach the It might be the case that the proxy itself can only reach the
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If at any point either one of the peers gets disconnected, any If at any point either one of the peers gets disconnected, any
outstanding data that came from that peer will be passed to the other outstanding data that came from that peer will be passed to the other
one, and after that also the other connection will be terminated by one, and after that also the other connection will be terminated by
the proxy. If there is outstanding data to that peer undelivered, the proxy. If there is outstanding data to that peer undelivered,
that data will be discarded. that data will be discarded.
An origin server which receives a CONNECT request for itself MAY An origin server which receives a CONNECT request for itself MAY
respond with a 2xx status code to indicate that a connection is respond with a 2xx 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.
3. Header Fields 5.3.7. OPTIONS
Header fields are key value pairs that can be used to communicate The OPTIONS method requests information about the communication
data about the message, its payload, the target resource, or about options available on the request/response chain identified by the
the connection itself (i.e., control data). See Section 3.2 of effective request URI. This method allows a client to determine the
[Part1] for a general definition of their syntax. options and/or requirements associated with a resource, or the
capabilities of a server, without implying a resource action or
initiating a resource retrieval.
3.1. Considerations for Creating Header Fields Responses to the OPTIONS method are not cacheable.
New header fields are registered using the procedures described in If the OPTIONS request includes a payload, then the media type MUST
[RFC3864]. be indicated by a Content-Type field. Although this specification
does not define any use for such a body, future extensions to HTTP
might use the OPTIONS body to make more detailed queries on the
server.
The requirements for header field names are defined in Section 4.1 of If the request-target (Section 5.3 of [Part1]) is an asterisk ("*"),
[RFC3864]. Authors of specifications defining new fields are advised the OPTIONS request is intended to apply to the server in general
to keep the name as short as practical, and not to prefix them with rather than to a specific resource. Since a server's communication
"X-" if they are to be registered (either immediately or in the options typically depend on the resource, the "*" request is only
future). useful as a "ping" or "no-op" type of method; it does nothing beyond
allowing the client to test the capabilities of the server. For
example, this can be used to test a proxy for HTTP/1.1 conformance
(or lack thereof).
New header field values typically have their syntax defined using If the request-target is not an asterisk, the OPTIONS request applies
ABNF ([RFC5234]), using the extension defined in Appendix B of only to the options that are available when communicating with that
[Part1] as necessary, and are usually constrained to the range of resource.
ASCII characters. Header fields needing a greater range of
characters can use an encoding such as the one defined in [RFC5987].
Because commas (",") are used as a generic delimiter between field- A 200 (OK) response SHOULD include any header fields that indicate
values, they need to be treated with care if they are allowed in the optional features implemented by the server and applicable to that
field-value's payload. Typically, components that might contain a resource (e.g., Allow), possibly including extensions not defined by
comma are protected with double-quotes using the quoted-string ABNF this specification. The response payload, if any, SHOULD also
production (Section 3.2.4 of [Part1]). include information about the communication options. The format for
such a payload is not defined by this specification, but might be
defined by future extensions to HTTP. Content negotiation MAY be
used to select the appropriate representation. If no payload body is
included, the response MUST include a Content-Length field with a
field-value of "0".
For example, a textual date and a URI (either of which might contain The Max-Forwards header field MAY be used to target a specific proxy
a comma) could be safely carried in field-values like these: in the request chain (see Section 6.1.1). If no Max-Forwards field
is present in the request, then the forwarded request MUST NOT
include a Max-Forwards field.
Example-URI-Field: "http://example.com/a.html,foo", 5.3.8. TRACE
"http://without-a-comma.example.com/"
Example-Date-Field: "Sat, 04 May 1996", "Wed, 14 Sep 2005"
Note that double quote delimiters almost always are used with the The TRACE method requests a remote, application-level loop-back of
quoted-string production; using a different syntax inside double the request message. The final recipient of the request SHOULD
quotes will likely cause unnecessary confusion. reflect the message received back to the client as the message body
of a 200 (OK) response. The final recipient is either the origin
server or the first proxy to receive a Max-Forwards value of zero (0)
in the request (see Section 6.1.1). A TRACE request MUST NOT include
a message body.
Many header fields use a format including (case-insensitively) named TRACE allows the client to see what is being received at the other
parameters (for instance, Content-Type, defined in Section 9.9). end of the request chain and use that data for testing or diagnostic
Allowing both unquoted (token) and quoted (quoted-string) syntax for information. The value of the Via header field (Section 5.7 of
the parameter value enables recipients to use existing parser [Part1]) is of particular interest, since it acts as a trace of the
components. When allowing both forms, the meaning of a parameter request chain. Use of the Max-Forwards header field allows the
value ought to be independent of the syntax used for it (for an client to limit the length of the request chain, which is useful for
example, see the notes on parameter handling for media types in testing a chain of proxies forwarding messages in an infinite loop.
Section 5.5).
Authors of specifications defining new header fields are advised to If the request is valid, the response SHOULD have a Content-Type of
consider documenting: "message/http" (see Section 7.3.1 of [Part1]) and contain a message
body that encloses a copy of the entire request message. Responses
to the TRACE method are not cacheable.
o Whether the field is a single value, or whether it can be a list 6. Request Header Fields
(delimited by commas; see Section 3.2 of [Part1]).
If it does not use the list syntax, document how to treat messages A client sends request header fields to provide more information
where the header field occurs multiple times (a sensible default about the request context, make the request conditional based on the
would be to ignore the header field, but this might not always be target resource state, suggest preferred formats for the response,
the right choice). supply authentication credentials, or modify the expected request
processing. These fields act as request modifiers, similar to the
parameters on a programming language method invocation.
Note that intermediaries and software libraries might combine 6.1. Controls
multiple header field instances into a single one, despite the
header field not allowing this. A robust format enables
recipients to discover these situations (good example: "Content-
Type", as the comma can only appear inside quoted strings; bad
example: "Location", as a comma can occur inside a URI).
o Under what conditions the header field can be used; e.g., only in Controls are request header fields that direct specific handling of
responses or requests, in all messages, only on responses to a the request.
particular request method.
o Whether it is appropriate to list the field-name in the Connection +-------------------+------------------------+
header field (i.e., if the header field is to be hop-by-hop, see | Header Field Name | Defined in... |
Section 6.1 of [Part1]). +-------------------+------------------------+
| Host | Section 5.4 of [Part1] |
| Max-Forwards | Section 6.1.1 |
| Expect | Section 6.1.2 |
| Range | Section 5.4 of [Part5] |
+-------------------+------------------------+
o Under what conditions intermediaries are allowed to modify the 6.1.1. Max-Forwards
header field's value, insert or delete it.
o How the header field might interact with caching (see [Part6]). The "Max-Forwards" header field provides a mechanism with the TRACE
(Section 5.3.8) and OPTIONS (Section 5.3.7) methods to limit the
number of times that the request is forwarded by proxies. This can
be useful when the client is attempting to trace a request which
appears to be failing or looping mid-chain.
o Whether the header field is useful or allowable in trailers (see Max-Forwards = 1*DIGIT
Section 4.1 of [Part1]).
o Whether the header field ought to be preserved across redirects. The Max-Forwards value is a decimal integer indicating the remaining
number of times this request message can be forwarded.
3.2. Request Header Fields Each recipient of a TRACE or OPTIONS request containing a Max-
Forwards header field MUST check and update its value prior to
forwarding the request. If the received value is zero (0), the
recipient MUST NOT forward the request; instead, it MUST respond as
the final recipient. If the received Max-Forwards value is greater
than zero, then the forwarded message MUST contain an updated Max-
Forwards field with a value decremented by one (1).
The request header fields allow the client to pass additional The Max-Forwards header field MAY be ignored for all other request
information about the request, and about the client itself, to the methods.
server. These fields act as request modifiers, with semantics
equivalent to the parameters on a programming language method 6.1.2. Expect
invocation.
The "Expect" header field is used to indicate that particular server
behaviors are required by the client.
Expect = 1#expectation
expectation = expect-name [ BWS "=" BWS expect-value ]
*( OWS ";" [ OWS expect-param ] )
expect-param = expect-name [ BWS "=" BWS expect-value ]
expect-name = token
expect-value = token / quoted-string
If all received Expect header field(s) are syntactically valid but
contain an expectation that the recipient does not understand or
cannot comply with, the recipient MUST respond with a 417
(Expectation Failed) status code. A recipient of a syntactically
invalid Expectation header field MUST respond with a 4xx status code
other than 417.
The only expectation defined by this specification is:
100-continue
The "100-continue" expectation is defined below. It does not
support any expect-params.
Comparison is case-insensitive for names (expect-name), and case-
sensitive for values (expect-value).
The Expect mechanism is hop-by-hop: the above requirements apply to
any server, including proxies. However, the Expect header field
itself is end-to-end; it MUST be forwarded if the request is
forwarded.
Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
Expect header field.
6.1.2.1. Use of the 100 (Continue) Status
The purpose of the 100 (Continue) status code (Section 7.2.1) is to
allow a client that is sending a request message with a payload to
determine if the origin server is willing to accept the request
(based on the request header fields) before the client sends the
payload body. In some cases, it might either be inappropriate or
highly inefficient for the client to send the payload body if the
server will reject the message without looking at the body.
Requirements for HTTP/1.1 clients:
o If a client will wait for a 100 (Continue) response before sending
the payload body, it MUST send an Expect header field with the
"100-continue" expectation.
o A client MUST NOT send an Expect header field with the "100-
continue" expectation if it does not intend to send a payload
body.
Because of the presence of older implementations, the protocol allows
ambiguous situations in which a client might send "Expect: 100-
continue" without receiving either a 417 (Expectation Failed) or a
100 (Continue) status code. Therefore, when a client sends this
header field to an origin server (possibly via a proxy) from which it
has never seen a 100 (Continue) status code, the client SHOULD NOT
wait for an indefinite period before sending the payload body.
Requirements for HTTP/1.1 origin servers:
o Upon receiving a request which includes an Expect header field
with the "100-continue" expectation, an origin server MUST either
respond with 100 (Continue) status code and continue to read from
the input stream, or respond with a final status code. The origin
server MUST NOT wait for the payload body before sending the 100
(Continue) response. If it responds with a final status code, it
MAY close the transport connection or it MAY continue to read and
discard the rest of the request. It MUST NOT perform the request
method if it returns a final status code.
o An origin server SHOULD NOT send a 100 (Continue) response if the
request message does not include an Expect header field with the
"100-continue" expectation, and MUST NOT send a 100 (Continue)
response if such a request comes from an HTTP/1.0 (or earlier)
client. There is an exception to this rule: for compatibility
with [RFC2068], a server MAY send a 100 (Continue) status code in
response to an HTTP/1.1 PUT or POST request that does not include
an Expect header field with the "100-continue" expectation. This
exception, the purpose of which is to minimize any client
processing delays associated with an undeclared wait for 100
(Continue) status code, applies only to HTTP/1.1 requests, and not
to requests with any other HTTP-version value.
o An origin server MAY omit a 100 (Continue) response if it has
already received some or all of the payload body for the
corresponding request.
o An origin server that sends a 100 (Continue) response MUST
ultimately send a final status code, once the payload body is
received and processed, unless it terminates the transport
connection prematurely.
o If an origin server receives a request that does not include an
Expect header field with the "100-continue" expectation, the
request includes a payload body, and the server responds with a
final status code before reading the entire payload body from the
transport connection, then the server SHOULD NOT close the
transport connection until it has read the entire request, or
until the client closes the connection. Otherwise, the client
might not reliably receive the response message. However, this
requirement ought not be construed as preventing a server from
defending itself against denial-of-service attacks, or from badly
broken client implementations.
Requirements for HTTP/1.1 proxies:
o If a proxy receives a request that includes an Expect header field
with the "100-continue" expectation, and the proxy either knows
that the next-hop server complies with HTTP/1.1 or higher, or does
not know the HTTP version of the next-hop server, it MUST forward
the request, including the Expect header field.
o If the proxy knows that the version of the next-hop server is
HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
respond with a 417 (Expectation Failed) status code.
o Proxies SHOULD maintain a record of the HTTP version numbers
received from recently-referenced next-hop servers.
o A proxy MUST NOT forward a 100 (Continue) response if the request
message was received from an HTTP/1.0 (or earlier) client and did
not include an Expect header field with the "100-continue"
expectation. This requirement overrides the general rule for
forwarding of 1xx responses (see Section 7.2.1).
6.2. Conditionals
Conditionals are request header fields that indicate a precondition
to be tested before applying the method semantics to the target
resource. Each precondition is based on metadata that is expected to
change if the selected representation of the target resource is
changed. The HTTP/1.1 conditional request mechanisms are defined in
[Part4].
+---------------------+------------------------+ +---------------------+------------------------+
| Header Field Name | Defined in... | | Header Field Name | Defined in... |
+---------------------+------------------------+ +---------------------+------------------------+
| Accept | Section 9.1 |
| Accept-Charset | Section 9.2 |
| Accept-Encoding | Section 9.3 |
| Accept-Language | Section 9.4 |
| Authorization | Section 4.1 of [Part7] |
| Expect | Section 9.11 |
| From | Section 9.12 |
| Host | Section 5.4 of [Part1] |
| If-Match | Section 3.1 of [Part4] | | If-Match | Section 3.1 of [Part4] |
| If-Modified-Since | Section 3.3 of [Part4] |
| If-None-Match | Section 3.2 of [Part4] | | If-None-Match | Section 3.2 of [Part4] |
| If-Range | Section 5.3 of [Part5] | | If-Modified-Since | Section 3.3 of [Part4] |
| If-Unmodified-Since | Section 3.4 of [Part4] | | If-Unmodified-Since | Section 3.4 of [Part4] |
| Max-Forwards | Section 9.14 | | If-Range | Section 5.3 of [Part5] |
| Proxy-Authorization | Section 4.3 of [Part7] |
| Range | Section 5.4 of [Part5] |
| Referer | Section 9.15 |
| TE | Section 4.3 of [Part1] |
| User-Agent | Section 9.18 |
+---------------------+------------------------+ +---------------------+------------------------+
3.3. Response Header Fields 6.3. Content Negotiation
The response header fields allow the server to pass additional +-------------------+---------------+
information about the response which cannot be placed in the status- | Header Field Name | Defined in... |
line. These header fields give information about the server and +-------------------+---------------+
about further access to the target resource (Section 5.5 of [Part1]). | Accept | Section 6.3.2 |
| Accept-Charset | Section 6.3.3 |
| Accept-Encoding | Section 6.3.4 |
| Accept-Language | Section 6.3.5 |
+-------------------+---------------+
+--------------------+------------------------+ 6.3.1. Quality Values
Many of the request header fields for proactive content negotiation
use a common parameter, named "q" (case-insensitive), to assign a
relative "weight" to the preference for that associated kind of
content. This weight is referred to as a "quality value" (or
"qvalue") because the same parameter name is often used within server
configurations to assign a weight to the relative quality of the
various representations that can be selected for a resource.
The weight is normalized to a real number in the range 0 through 1,
where 0.001 is the least preferred and 1 is the most preferred; a
value of 0 means "not acceptable". If no "q" parameter is present,
the default weight is 1.
weight = OWS ";" OWS "q=" qvalue
qvalue = ( "0" [ "." 0*3DIGIT ] )
/ ( "1" [ "." 0*3("0") ] )
A sender of qvalue MUST NOT generate more than three digits after the
decimal point. User configuration of these values ought to be
limited in the same fashion.
6.3.2. Accept
The "Accept" header field can be used by user agents to specify
response media types that are acceptable. Accept header fields can
be used to indicate that the request is specifically limited to a
small set of desired types, as in the case of a request for an in-
line image.
Accept = #( media-range [ accept-params ] )
media-range = ( "*/*"
/ ( type "/" "*" )
/ ( type "/" subtype )
) *( OWS ";" OWS parameter )
accept-params = weight *( accept-ext )
accept-ext = OWS ";" OWS token [ "=" word ]
The asterisk "*" character is used to group media types into ranges,
with "*/*" indicating all media types and "type/*" indicating all
subtypes of that type. The media-range MAY include media type
parameters that are applicable to that range.
Each media-range MAY be followed by one or more accept-params,
beginning with the "q" parameter for indicating a relative weight, as
defined in Section 6.3.1. The first "q" parameter (if any) separates
the media-range parameter(s) from the accept-params.
Note: Use of the "q" parameter name to separate media type
parameters from Accept extension parameters is due to historical
practice. Although this prevents any media type parameter named
"q" from being used with a media range, such an event is believed
to be unlikely given the lack of any "q" parameters in the IANA
media type registry and the rare usage of any media type
parameters in Accept. Future media types are discouraged from
registering any parameter named "q".
The example
Accept: audio/*; q=0.2, audio/basic
SHOULD be interpreted as "I prefer audio/basic, but send me any audio
type if it is the best available after an 80% mark-down in quality".
A request without any Accept header field implies that the user agent
will accept any media type in response. If an Accept header field is
present in a request and none of the available representations for
the response have a media type that is listed as acceptable, the
origin server MAY either honor the Accept header field by sending a
406 (Not Acceptable) response or disregard the Accept header field by
treating the response as if it is not subject to content negotiation.
A more elaborate example is
Accept: text/plain; q=0.5, text/html,
text/x-dvi; q=0.8, text/x-c
Verbally, this would be interpreted as "text/html and text/x-c are
the preferred media types, but if they do not exist, then send the
text/x-dvi representation, and if that does not exist, send the text/
plain representation".
Media ranges can be overridden by more specific media ranges or
specific media types. If more than one media range applies to a
given type, the most specific reference has precedence. For example,
Accept: text/*, text/plain, text/plain;format=flowed, */*
have the following precedence:
1. text/plain;format=flowed
2. text/plain
3. text/*
4. */*
The media type quality factor associated with a given type is
determined by finding the media range with the highest precedence
which matches that type. For example,
Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
text/html;level=2;q=0.4, */*;q=0.5
would cause the following values to be associated:
+-------------------+---------------+
| Media Type | Quality Value |
+-------------------+---------------+
| text/html;level=1 | 1 |
| text/html | 0.7 |
| text/plain | 0.3 |
| image/jpeg | 0.5 |
| text/html;level=2 | 0.4 |
| text/html;level=3 | 0.7 |
+-------------------+---------------+
Note: A user agent might be provided with a default set of quality
values for certain media ranges. However, unless the user agent is a
closed system which cannot interact with other rendering agents, this
default set ought to be configurable by the user.
6.3.3. Accept-Charset
The "Accept-Charset" header field can be used by user agents to
indicate what character encodings are acceptable in a response
payload. This field allows clients capable of understanding more
comprehensive or special-purpose character encodings to signal that
capability to a server which is capable of representing documents in
those character encodings.
Accept-Charset = 1#( ( charset / "*" ) [ weight ] )
Character encoding values (a.k.a., charsets) are described in
Section 3.1.1.2. Each charset MAY be given an associated quality
value which represents the user's preference for that charset, as
defined in Section 6.3.1. An example is
Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
The special value "*", if present in the Accept-Charset field,
matches every character encoding which is not mentioned elsewhere in
the Accept-Charset field. If no "*" is present in an Accept-Charset
field, then any character encodings not explicitly mentioned in the
field are considered "not acceptable" to the client.
A request without any Accept-Charset header field implies that the
user agent will accept any character encoding in response.
If an Accept-Charset header field is present in a request and none of
the available representations for the response have a character
encoding that is listed as acceptable, the origin server MAY either
honor the Accept-Charset header field by sending a 406 (Not
Acceptable) response or disregard the Accept-Charset header field by
treating the response as if it is not subject to content negotiation.
6.3.4. Accept-Encoding
The "Accept-Encoding" header field can be used by user agents to
indicate what response content-codings (Section 3.1.2.1) are
acceptable in the response. An "identity" token is used as a synonym
for "no encoding" in order to communicate when no encoding is
preferred.
Accept-Encoding = #( codings [ weight ] )
codings = content-coding / "identity" / "*"
Each codings value MAY be given an associated quality value which
represents the preference for that encoding, as defined in
Section 6.3.1.
For example,
Accept-Encoding: compress, gzip
Accept-Encoding:
Accept-Encoding: *
Accept-Encoding: compress;q=0.5, gzip;q=1.0
Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
A server tests whether a content-coding for a given representation is
acceptable, according to an Accept-Encoding field, using these rules:
1. The special "*" symbol in an Accept-Encoding field matches any
available content-coding not explicitly listed in the header
field.
2. If the representation has no content-coding, then it is
acceptable by default unless specifically excluded by the Accept-
Encoding field stating either "identity;q=0" or "*;q=0" without a
more specific entry for "identity".
3. If the representation's content-coding is one of the content-
codings listed in the Accept-Encoding field, then it is
acceptable unless it is accompanied by a qvalue of 0. (As
defined in Section 6.3.1, a qvalue of 0 means "not acceptable".)
4. If multiple content-codings are acceptable, then the acceptable
content-coding with the highest non-zero qvalue is preferred.
An Accept-Encoding header field with a combined field-value that is
empty implies that the user agent does not want any content-coding in
response. If an Accept-Encoding header field is present in a request
and none of the available representations for the response have a
content-coding that is listed as acceptable, the origin server SHOULD
send a response without any content-coding.
A request without an Accept-Encoding header field implies that the
user agent will accept any content-coding in response.
Note: Most HTTP/1.0 applications do not recognize or obey qvalues
associated with content-codings. This means that qvalues will not
work and are not permitted with x-gzip or x-compress.
6.3.5. Accept-Language
The "Accept-Language" header field can be used by user agents to
indicate the set of natural languages that are preferred in the
response. Language tags are defined in Section 3.1.3.1.
Accept-Language = 1#( language-range [ weight ] )
language-range =
<language-range, defined in [RFC4647], Section 2.1>
Each language-range can be given an associated quality value which
represents an estimate of the user's preference for the languages
specified by that range, as defined in Section 6.3.1. For example,
Accept-Language: da, en-gb;q=0.8, en;q=0.7
would mean: "I prefer Danish, but will accept British English and
other types of English". (see also Section 2.3 of [RFC4647])
For matching, Section 3 of [RFC4647] defines several matching
schemes. Implementations can offer the most appropriate matching
scheme for their requirements.
Note: The "Basic Filtering" scheme ([RFC4647], Section 3.3.1) is
identical to the matching scheme that was previously defined in
Section 14.4 of [RFC2616].
It might be contrary to the privacy expectations of the user to send
an Accept-Language header field with the complete linguistic
preferences of the user in every request. For a discussion of this
issue, see Section 10.5.
As intelligibility is highly dependent on the individual user, it is
recommended that client applications make the choice of linguistic
preference available to the user. If the choice is not made
available, then the Accept-Language header field MUST NOT be given in
the request.
Note: When making the choice of linguistic preference available to
the user, we remind implementers of the fact that users are not
familiar with the details of language matching as described above,
and ought to be provided appropriate guidance. As an example,
users might assume that on selecting "en-gb", they will be served
any kind of English document if British English is not available.
A user agent might suggest in such a case to add "en" to get the
best matching behavior.
6.4. Authentication Credentials
+---------------------+------------------------+
| Header Field Name | Defined in... | | Header Field Name | Defined in... |
+--------------------+------------------------+ +---------------------+------------------------+
| Accept-Ranges | Section 5.1 of [Part5] | | Authorization | Section 4.1 of [Part7] |
| Age | Section 7.1 of [Part6] | | Proxy-Authorization | Section 4.3 of [Part7] |
| Allow | Section 9.5 | +---------------------+------------------------+
| Date | Section 9.10 |
| ETag | Section 2.3 of [Part4] |
| Location | Section 9.13 |
| Proxy-Authenticate | Section 4.2 of [Part7] |
| Retry-After | Section 9.16 |
| Server | Section 9.17 |
| Vary | Section 7.5 of [Part6] |
| WWW-Authenticate | Section 4.4 of [Part7] |
+--------------------+------------------------+
4. Status Codes 6.5. Context
+-------------------+------------------------+
| Header Field Name | Defined in... |
+-------------------+------------------------+
| From | Section 6.5.1 |
| Referer | Section 6.5.2 |
| TE | Section 4.3 of [Part1] |
| User-Agent | Section 6.5.3 |
+-------------------+------------------------+
6.5.1. From
The "From" header field, if given, SHOULD contain an Internet e-mail
address for the human user who controls the requesting user agent.
The address SHOULD be machine-usable, as defined by "mailbox" in
Section 3.4 of [RFC5322]:
From = mailbox
mailbox = <mailbox, defined in [RFC5322], Section 3.4>
An example is:
From: webmaster@example.org
This header field MAY be used for logging purposes and as a means for
identifying the source of invalid or unwanted requests. It SHOULD
NOT be used as an insecure form of access protection. The
interpretation of this field is that the request is being performed
on behalf of the person given, who accepts responsibility for the
method performed. In particular, robot agents SHOULD include this
header field so that the person responsible for running the robot can
be contacted if problems occur on the receiving end.
The Internet e-mail address in this field MAY be separate from the
Internet host which issued the request. For example, when a request
is passed through a proxy the original issuer's address SHOULD be
used.
The client SHOULD NOT send the From header field without the user's
approval, as it might conflict with the user's privacy interests or
their site's security policy. It is strongly recommended that the
user be able to disable, enable, and modify the value of this field
at any time prior to a request.
6.5.2. Referer
The "Referer" [sic] header field allows the client to specify the URI
of the resource from which the target URI was obtained (the
"referrer", although the header field is misspelled.).
The Referer header field allows servers to generate lists of back-
links to resources for interest, logging, optimized caching, etc. It
also allows obsolete or mistyped links to be traced for maintenance.
Some servers use Referer as a means of controlling where they allow
links from (so-called "deep linking"), but legitimate requests do not
always contain a Referer header field.
If the target URI was obtained from a source that does not have its
own URI (e.g., input from the user keyboard), the Referer field MUST
either be sent with the value "about:blank", or not be sent at all.
Note that this requirement does not apply to sources with non-HTTP
URIs (e.g., FTP).
Referer = absolute-URI / partial-URI
Example:
Referer: http://www.example.org/hypertext/Overview.html
If the field value is a relative URI, it SHOULD be interpreted
relative to the effective request URI. The URI MUST NOT include a
fragment. See Section 10.2 for security considerations.
6.5.3. User-Agent
The "User-Agent" header field contains information about the user
agent originating the request. User agents SHOULD include this field
with requests.
Typically, it is used for statistical purposes, the tracing of
protocol violations, and tailoring responses to avoid particular user
agent limitations.
The field can contain multiple product tokens (Section 4) and
comments (Section 3.2 of [Part1]) identifying the agent and its
significant subproducts. By convention, the product tokens are
listed in order of their significance for identifying the
application.
Because this field is usually sent on every request a user agent
makes, implementations are encouraged not to include needlessly fine-
grained detail, and to limit (or even prohibit) the addition of
subproducts by third parties. Overly long and detailed User-Agent
field values make requests larger and can also be used to identify
("fingerprint") the user against their wishes.
Likewise, implementations are encouraged not to use the product
tokens of other implementations in order to declare compatibility
with them, as this circumvents the purpose of the field. Finally,
they are encouraged not to use comments to identify products; doing
so makes the field value more difficult to parse.
User-Agent = product *( RWS ( product / comment ) )
Example:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
7. Response Status Codes
The status-code element is a 3-digit integer result code of the The status-code element is a 3-digit integer result code of the
attempt to understand and satisfy the request. attempt to understand and satisfy the request.
HTTP status codes are extensible. HTTP applications are not required HTTP status codes are extensible. HTTP applications are not required
to understand the meaning of all registered status codes, though such to understand the meaning of all registered status codes, though such
understanding is obviously desirable. However, applications MUST understanding is obviously desirable. However, applications MUST
understand the class of any status code, as indicated by the first understand the class of any status code, as indicated by the first
digit, and treat any unrecognized response as being equivalent to the digit, and treat any unrecognized response as being equivalent to the
x00 status code of that class, with the exception that an x00 status code of that class, with the exception that an
skipping to change at page 22, line 45 skipping to change at page 47, line 18
complete the request complete the request
o 4xx (Client Error): The request contains bad syntax or cannot be o 4xx (Client Error): The request contains bad syntax or cannot be
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
For most status codes the response can carry a payload, in which case For most status codes the response can carry a payload, in which case
a Content-Type header field indicates the payload's media type a Content-Type header field indicates the payload's media type
(Section 9.9). (Section 3.1.1.5).
4.1. Overview of Status Codes 7.1. Overview of Status Codes
The status codes listed below are defined in this specification, The status codes listed below are defined in this specification,
Section 4 of [Part4], Section 3 of [Part5], and Section 3 of [Part7]. Section 4 of [Part4], Section 3 of [Part5], and Section 3 of [Part7].
The reason phrases listed here are only recommendations -- they can The reason phrases listed here are only recommendations -- they can
be replaced by local equivalents without affecting the protocol. be replaced by local equivalents without affecting the protocol.
+-------------+------------------------------+----------------------+ +-------------+------------------------------+----------------------+
| status-code | reason-phrase | Defined in... | | status-code | reason-phrase | Defined in... |
+-------------+------------------------------+----------------------+ +-------------+------------------------------+----------------------+
| 100 | Continue | Section 4.3.1 | | 100 | Continue | Section 7.2.1 |
| 101 | Switching Protocols | Section 4.3.2 | | 101 | Switching Protocols | Section 7.2.2 |
| 200 | OK | Section 4.4.1 | | 200 | OK | Section 7.3.1 |
| 201 | Created | Section 4.4.2 | | 201 | Created | Section 7.3.2 |
| 202 | Accepted | Section 4.4.3 | | 202 | Accepted | Section 7.3.3 |
| 203 | Non-Authoritative | Section 4.4.4 | | 203 | Non-Authoritative | Section 7.3.4 |
| | Information | | | | Information | |
| 204 | No Content | Section 4.4.5 | | 204 | No Content | Section 7.3.5 |
| 205 | Reset Content | Section 4.4.6 | | 205 | Reset Content | Section 7.3.6 |
| 206 | Partial Content | Section 3.1 of | | 206 | Partial Content | Section 3.1 of |
| | | [Part5] | | | | [Part5] |
| 300 | Multiple Choices | Section 4.5.1 | | 300 | Multiple Choices | Section 7.4.1 |
| 301 | Moved Permanently | Section 4.5.2 | | 301 | Moved Permanently | Section 7.4.2 |
| 302 | Found | Section 4.5.3 | | 302 | Found | Section 7.4.3 |
| 303 | See Other | Section 4.5.4 | | 303 | See Other | Section 7.4.4 |
| 304 | Not Modified | Section 4.1 of | | 304 | Not Modified | Section 4.1 of |
| | | [Part4] | | | | [Part4] |
| 305 | Use Proxy | Section 4.5.5 | | 305 | Use Proxy | Section 7.4.5 |
| 307 | Temporary Redirect | Section 4.5.7 | | 307 | Temporary Redirect | Section 7.4.7 |
| 400 | Bad Request | Section 4.6.1 | | 400 | Bad Request | Section 7.5.1 |
| 401 | Unauthorized | Section 3.1 of | | 401 | Unauthorized | Section 3.1 of |
| | | [Part7] | | | | [Part7] |
| 402 | Payment Required | Section 4.6.2 | | 402 | Payment Required | Section 7.5.2 |
| 403 | Forbidden | Section 4.6.3 | | 403 | Forbidden | Section 7.5.3 |
| 404 | Not Found | Section 4.6.4 | | 404 | Not Found | Section 7.5.4 |
| 405 | Method Not Allowed | Section 4.6.5 | | 405 | Method Not Allowed | Section 7.5.5 |
| 406 | Not Acceptable | Section 4.6.6 | | 406 | Not Acceptable | Section 7.5.6 |
| 407 | Proxy Authentication | Section 3.2 of | | 407 | Proxy Authentication | Section 3.2 of |
| | Required | [Part7] | | | Required | [Part7] |
| 408 | Request Time-out | Section 4.6.7 | | 408 | Request Time-out | Section 7.5.7 |
| 409 | Conflict | Section 4.6.8 | | 409 | Conflict | Section 7.5.8 |
| 410 | Gone | Section 4.6.9 | | 410 | Gone | Section 7.5.9 |
| 411 | Length Required | Section 4.6.10 | | 411 | Length Required | Section 7.5.10 |
| 412 | Precondition Failed | Section 4.2 of | | 412 | Precondition Failed | Section 4.2 of |
| | | [Part4] | | | | [Part4] |
| 413 | Request Representation Too | Section 4.6.11 | | 413 | Request Representation Too | Section 7.5.11 |
| | Large | | | | Large | |
| 414 | URI Too Long | Section 4.6.12 | | 414 | URI Too Long | Section 7.5.12 |
| 415 | Unsupported Media Type | Section 4.6.13 | | 415 | Unsupported Media Type | Section 7.5.13 |
| 416 | Requested range not | Section 3.2 of | | 416 | Requested range not | Section 3.2 of |
| | satisfiable | [Part5] | | | satisfiable | [Part5] |
| 417 | Expectation Failed | Section 4.6.14 | | 417 | Expectation Failed | Section 7.5.14 |
| 426 | Upgrade Required | Section 4.6.15 | | 426 | Upgrade Required | Section 7.5.15 |
| 500 | Internal Server Error | Section 4.7.1 | | 500 | Internal Server Error | Section 7.6.1 |
| 501 | Not Implemented | Section 4.7.2 | | 501 | Not Implemented | Section 7.6.2 |
| 502 | Bad Gateway | Section 4.7.3 | | 502 | Bad Gateway | Section 7.6.3 |
| 503 | Service Unavailable | Section 4.7.4 | | 503 | Service Unavailable | Section 7.6.4 |
| 504 | Gateway Time-out | Section 4.7.5 | | 504 | Gateway Time-out | Section 7.6.5 |
| 505 | HTTP Version not supported | Section 4.7.6 | | 505 | HTTP Version not supported | Section 7.6.6 |
+-------------+------------------------------+----------------------+ +-------------+------------------------------+----------------------+
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. extension status codes defined in other specifications.
4.2. Status Code Registry 7.2. Informational 1xx
The HTTP Status Code Registry defines the name space for the status-
code token in the status-line of an HTTP response.
Values to be added to this name space require IETF Review (see
[RFC5226], Section 4.1).
The registry itself is maintained at
<http://www.iana.org/assignments/http-status-codes>.
4.2.1. Considerations for New Status Codes
When it is necessary to express new semantics for a HTTP response
that aren't specific to a single application or media type, and
currently defined status codes are inadequate, a new status code can
be registered.
HTTP status codes are generic; that is, they are potentially
applicable to any resource, not just one particular media type,
"type" of resource, or application. As such, it is preferred that
new HTTP status codes be registered in a document that isn't specific
to a single application, so that this is clear.
Definitions of new HTTP status codes typically explain the request
conditions that produce a response containing the status code (e.g.,
combinations of request header fields and/or method(s)), along with
any interactions with response header fields (e.g., those that are
required, those that modify the semantics of the response).
New HTTP status codes are required to fall under one of the
categories defined in Section 4. To allow existing parsers to
properly handle them, new status codes cannot disallow a response
body, although they can mandate a zero-length response body. They
can require the presence of one or more particular HTTP response
header field(s).
Likewise, their definitions can specify that caches are allowed to
use heuristics to determine their freshness (see [Part6]; by default,
it is not allowed), and can define how to determine the resource
which they carry a representation for (see Section 7.1; by default,
it is anonymous).
4.3. Informational 1xx
This class of status code indicates a provisional response, This class of status code indicates a provisional response,
consisting only of the status-line and optional header fields, and is consisting only of the status-line and optional header fields, and is
terminated by an empty line. There are no required header fields for terminated by an empty line. There are no required header fields for
this class of status code. Since HTTP/1.0 did not define any 1xx this class of status code. Since HTTP/1.0 did not define any 1xx
status codes, servers MUST NOT send a 1xx response to an HTTP/1.0 status codes, servers MUST NOT send a 1xx response to an HTTP/1.0
client except under experimental conditions. client except under experimental conditions.
A client MUST be prepared to accept one or more 1xx status responses A client MUST be prepared to accept one or more 1xx status responses
prior to a regular response, even if the client does not expect a 100 prior to a regular response, even if the client does not expect a 100
(Continue) status message. Unexpected 1xx status responses MAY be (Continue) status message. Unexpected 1xx status responses MAY be
ignored by a user agent. ignored by a user agent.
Proxies MUST forward 1xx responses, unless the connection between the Proxies MUST forward 1xx responses, unless the connection between the
proxy and its client has been closed, or unless the proxy itself proxy and its client has been closed, or unless the proxy itself
requested the generation of the 1xx response. (For example, if a requested the generation of the 1xx response. (For example, if a
proxy adds an "Expect: 100-continue" field when it forwards a proxy adds an "Expect: 100-continue" field when it forwards a
request, then it need not forward the corresponding 100 (Continue) request, then it need not forward the corresponding 100 (Continue)
response(s).) response(s).)
4.3.1. 100 Continue 7.2.1. 100 Continue
The client SHOULD continue with its request. This interim response The client SHOULD continue with its request. This interim response
is used to inform the client that the initial part of the request has is used to inform the client that the initial part of the request has
been received and has not yet been rejected by the server. The been received and has not yet been rejected by the server. The
client SHOULD continue by sending the remainder of the request or, if client SHOULD continue by sending the remainder of the request or, if
the request has already been completed, ignore this response. The the request has already been completed, ignore this response. The
server MUST send a final response after the request has been server MUST send a final response after the request has been
completed. See Section 6.4.3 of [Part1] for detailed discussion of completed. See Section 6.1.2.1 for detailed discussion of the use
the use and handling of this status code. and handling of this status code.
4.3.2. 101 Switching Protocols 7.2.2. 101 Switching Protocols
The server understands and is willing to comply with the client's The server understands and is willing to comply with the client's
request, via the Upgrade message header field (Section 6.5 of request, via the Upgrade message header field (Section 6.3 of
[Part1]), for a change in the application protocol being used on this [Part1]), for a change in the application protocol being used on this
connection. The server will switch protocols to those defined by the connection. The server will switch protocols to those defined by the
response's Upgrade header field immediately after the empty line response's Upgrade header field immediately after the empty line
which terminates the 101 response. which terminates the 101 response.
The protocol SHOULD be switched only when it is advantageous to do The protocol SHOULD be switched only when it is advantageous to do
so. For example, switching to a newer version of HTTP is so. For example, switching to a newer version of HTTP is
advantageous over older versions, and switching to a real-time, advantageous over older versions, and switching to a real-time,
synchronous protocol might be advantageous when delivering resources synchronous protocol might be advantageous when delivering resources
that use such features. that use such features.
4.4. Successful 2xx 7.3. Successful 2xx
This class of status code indicates that the client's request was This class of status code indicates that the client's request was
successfully received, understood, and accepted. successfully received, understood, and accepted.
4.4.1. 200 OK 7.3.1. 200 OK
The request has succeeded. The payload returned with the response is The request has succeeded. The payload returned with the response is
dependent on the method used in the request, for example: dependent on the method used in the request, for example:
GET a representation of the target resource is sent in the response; GET a representation of the target resource is sent in the response;
HEAD the same representation as GET, except without the message HEAD the same representation as GET, except without the message
body; body;
POST a representation describing or containing the result of the POST a representation describing or containing the result of the
action; action;
TRACE a representation containing the request message as received by TRACE a representation containing the request message as received by
the end server. the end server.
Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to
determine freshness for 200 responses. determine freshness for 200 responses.
4.4.2. 201 Created 7.3.2. 201 Created
The request has been fulfilled and has resulted in one or more new The request has been fulfilled and has resulted in one or more new
resources being created. resources being created.
Newly created resources are typically linked to from the response Newly created resources are typically linked to from the response
payload, with the most relevant URI also being carried in the payload, with the most relevant URI also being carried in the
Location header field. If the newly created resource's URI is the Location header field. If the newly created resource's URI is the
same as the Effective Request URI, this information can be omitted same as the Effective Request URI, this information can be omitted
(e.g., in the case of a response to a PUT request). (e.g., in the case of a response to a PUT request).
The origin server MUST create the resource(s) before returning the The origin server MUST create the resource(s) before returning the
201 status code. If the action cannot be carried out immediately, 201 status code. If the action cannot be carried out immediately,
the server SHOULD respond with 202 (Accepted) response instead. the server SHOULD respond with 202 (Accepted) response instead.
A 201 response MAY contain an ETag response header field indicating A 201 response MAY contain an ETag response header field indicating
the current value of the entity-tag for the representation of the the current value of the entity-tag for the representation of the
resource identified by the Location header field or, in case the resource identified by the Location header field or, in case the
Location header field was omitted, by the Effective Request URI (see Location header field was omitted, by the Effective Request URI (see
Section 2.3 of [Part4]). Section 2.3 of [Part4]).
4.4.3. 202 Accepted 7.3.3. 202 Accepted
The request has been accepted for processing, but the processing has The request has been accepted for processing, but the processing has
not been completed. The request might or might not eventually be not been completed. The request might or might not eventually be
acted upon, as it might be disallowed when processing actually takes acted upon, as it might be disallowed when processing actually takes
place. There is no facility for re-sending a status code from an place. There is no facility for re-sending a status code from an
asynchronous operation such as this. asynchronous operation such as this.
The 202 response is intentionally non-committal. Its purpose is to The 202 response is intentionally non-committal. Its purpose is to
allow a server to accept a request for some other process (perhaps a allow a server to accept a request for some other process (perhaps a
batch-oriented process that is only run once per day) without batch-oriented process that is only run once per day) without
requiring that the user agent's connection to the server persist requiring that the user agent's connection to the server persist
until the process is completed. The representation returned with until the process is completed. The representation returned with
this response SHOULD include an indication of the request's current this response SHOULD include an indication of the request's current
status and either a pointer to a status monitor or some estimate of status and either a pointer to a status monitor or some estimate of
when the user can expect the request to be fulfilled. when the user can expect the request to be fulfilled.
4.4.4. 203 Non-Authoritative Information 7.3.4. 203 Non-Authoritative Information
The representation in the response has been transformed or otherwise The representation in the response has been transformed or otherwise
modified by a transforming proxy (Section 2.4 of [Part1]). Note that modified by a transforming proxy (Section 2.3 of [Part1]). Note that
the behavior of transforming intermediaries is controlled by the no- the behavior of transforming intermediaries is controlled by the no-
transform Cache-Control directive (Section 7.2 of [Part6]). transform Cache-Control directive (Section 7.2 of [Part6]).
This status code is only appropriate when the response status code This status code is only appropriate when the response status code
would have been 200 (OK) otherwise. When the status code before would have been 200 (OK) otherwise. When the status code before
transformation would have been different, the 214 Transformation transformation would have been different, the 214 Transformation
Applied warn-code (Section 7.6 of [Part6]) is appropriate. Applied warn-code (Section 7.5 of [Part6]) is appropriate.
Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to
determine freshness for 203 responses. determine freshness for 203 responses.
4.4.5. 204 No Content 7.3.5. 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 return in the response payload body. Metadata in the content to return in the response payload body. Metadata in the
response header fields refer to the target resource and its current response header fields refer to the target resource and its current
representation after the requested action. representation after the requested action.
For example, if a 204 status code is received in response to a PUT For example, if a 204 status code is received in response to a PUT
request and the response contains an ETag header field, then the PUT request and the response contains an ETag header field, then the PUT
was successful and the ETag field-value contains the entity-tag for was successful and the ETag field-value contains the entity-tag for
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For example, a 204 status code is commonly used with document editing For example, a 204 status code is commonly used with document editing
interfaces corresponding to a "save" action, such that the document interfaces corresponding to a "save" action, such that the document
being saved remains available to the user for editing. It is also being saved remains available to the user for editing. It is also
frequently used with interfaces that expect automated data transfers frequently used with interfaces that expect automated data transfers
to be prevalent, such as within distributed version control systems. to be prevalent, such as within distributed version control systems.
The 204 response MUST NOT include a message body, and thus is always The 204 response MUST NOT include a message body, and thus is always
terminated by the first empty line after the header fields. terminated by the first empty line after the header fields.
4.4.6. 205 Reset Content 7.3.6. 205 Reset Content
The server has fulfilled the request and the user agent SHOULD reset The server has fulfilled the request and the user agent SHOULD reset
the document view which caused the request to be sent. This response the document view which caused the request to be sent. This response
is primarily intended to allow input for actions to take place via is primarily intended to allow input for actions to take place via
user input, followed by a clearing of the form in which the input is user input, followed by a clearing of the form in which the input is
given so that the user can easily initiate another input action. given so that the user can easily initiate another input action.
The message body included with the response MUST be empty. Note that The message body included with the response MUST be empty. Note that
receivers still need to parse the response according to the algorithm receivers still need to parse the response according to the algorithm
defined in Section 3.3 of [Part1]. defined in Section 3.3 of [Part1].
4.5. Redirection 3xx 7.4. Redirection 3xx
This class of status code indicates that further action needs to be This class of status code indicates that further action needs to be
taken by the user agent in order to fulfill the request. If the taken by the user agent in order to fulfill the request. If the
required action involves a subsequent HTTP request, it MAY be carried required action involves a subsequent HTTP request, it MAY be carried
out by the user agent without interaction with the user if and only out by the user agent without interaction with the user if and only
if the method used in the second request is known to be "safe", as if the method used in the second request is known to be "safe", as
defined in Section 2.1.1. defined in Section 5.2.1.
There are several types of redirects: There are several types of redirects:
1. Redirects of the request to another URI, either temporarily or 1. Redirects of the request to another URI, either temporarily or
permanently. The new URI is specified in the Location header permanently. The new URI is specified in the Location header
field. In this specification, the status codes 301 (Moved field. In this specification, the status codes 301 (Moved
Permanently), 302 (Found), and 307 (Temporary Redirect) fall Permanently), 302 (Found), and 307 (Temporary Redirect) fall
under this category. under this category.
2. Redirection to a new location that represents an indirect 2. Redirection to a new location that represents an indirect
response to the request, such as the result of a POST operation response to the request, such as the result of a POST operation
to be retrieved with a subsequent GET request. This is status to be retrieved with a subsequent GET request. This is status
code 303 (See Other). code 303 (See Other).
3. Redirection offering a choice of matching resources for use by 3. Redirection offering a choice of matching resources for use by
agent-driven content negotiation (Section 8.2). This is status reactive content negotiation (Section 3.4.2). This is status
code 300 (Multiple Choices). code 300 (Multiple Choices).
4. Other kinds of redirection, such as to a cached result (status 4. Other kinds of redirection, such as to a cached result (status
code 304 (Not Modified), see Section 4.1 of [Part4]). code 304 (Not Modified), see Section 4.1 of [Part4]).
Note: In HTTP/1.0, only the status codes 301 (Moved Permanently) Note: In HTTP/1.0, only the status codes 301 (Moved Permanently)
and 302 (Found) were defined for the first type of redirect, and and 302 (Found) were defined for the first type of redirect, and
the second type did not exist at all ([RFC1945], Section 9.3). the second type did not exist at all ([RFC1945], Section 9.3).
However it turned out that web forms using POST expected redirects However it turned out that web forms using POST expected redirects
to change the operation for the subsequent request to retrieval to change the operation for the subsequent request to retrieval
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Section 10.3.4). As user agents did not change their behavior to Section 10.3.4). As user agents did not change their behavior to
maintain backwards compatibility, the first revision of HTTP/1.1 maintain backwards compatibility, the first revision of HTTP/1.1
added yet another status code, 307 (Temporary Redirect), for which added yet another status code, 307 (Temporary Redirect), for which
the backwards compatibility problems did not apply ([RFC2616], the backwards compatibility problems did not apply ([RFC2616],
Section 10.3.8). Over 10 years later, most user agents still do Section 10.3.8). Over 10 years later, most user agents still do
method rewriting for status codes 301 and 302, therefore this method rewriting for status codes 301 and 302, therefore this
specification makes that behavior conformant in case the original specification makes that behavior conformant in case the original
request was POST. request was POST.
A Location header field on a 3xx response indicates that a client MAY A Location header field on a 3xx response indicates that a client MAY
automatically redirect to the URI provided; see Section 9.13. automatically redirect to the URI provided; see Section 8.1.2.
Note that for methods not known to be "safe", as defined in Note that for methods not known to be "safe", as defined in
Section 2.1.1, automatic redirection needs to done with care, since Section 5.2.1, automatic redirection needs to done with care, since
the redirect might change the conditions under which the request was the redirect might change the conditions under which the request was
issued. issued.
Clients SHOULD detect and intervene in cyclical redirections (i.e., Clients SHOULD detect and intervene in cyclical redirections (i.e.,
"infinite" redirection loops). "infinite" redirection loops).
Note: An earlier version of this specification recommended a Note: An earlier version of this specification recommended a
maximum of five redirections ([RFC2068], Section 10.3). Content maximum of five redirections ([RFC2068], Section 10.3). Content
developers need to be aware that some clients might implement such developers need to be aware that some clients might implement such
a fixed limitation. a fixed limitation.
4.5.1. 300 Multiple Choices 7.4.1. 300 Multiple Choices
The target resource has more than one representation, each with its The target resource has more than one representation, each with its
own specific location, and agent-driven negotiation information own specific location, and reactive negotiation information
(Section 8) is being provided so that the user (or user agent) can (Section 3.4) is being provided so that the user (or user agent) can
select a preferred representation by redirecting its request to that select a preferred representation by redirecting its request to that
location. location.
Unless it was a HEAD request, the response SHOULD include a Unless it was a HEAD request, the response SHOULD include a
representation containing a list of representation metadata and representation containing a list of representation metadata and
location(s) from which the user or user agent can choose the one most location(s) from which the user or user agent can choose the one most
appropriate. Depending upon the format and the capabilities of the appropriate. Depending upon the format and the capabilities of the
user agent, selection of the most appropriate choice MAY be performed user agent, selection of the most appropriate choice MAY be performed
automatically. However, this specification does not define any automatically. However, this specification does not define any
standard for such automatic selection. standard for such automatic selection.
If the server has a preferred choice of representation, it SHOULD If the server has a preferred choice of representation, it SHOULD
include the specific URI for that representation in the Location include the specific URI for that representation in the Location
field; user agents MAY use the Location field value for automatic field; user agents MAY use the Location field value for automatic
redirection. redirection.
Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to
determine freshness for 300 responses. determine freshness for 300 responses.
4.5.2. 301 Moved Permanently 7.4.2. 301 Moved Permanently
The target resource has been assigned a new permanent URI and any The target resource has been assigned a new permanent URI and any
future references to this resource SHOULD use one of the returned future references to this resource SHOULD use one of the returned
URIs. Clients with link editing capabilities ought to automatically URIs. Clients with link editing capabilities ought to automatically
re-link references to the effective request URI to one or more of the re-link references to the effective request URI to one or more of the
new references returned by the server, where possible. new references returned by the server, where possible.
Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to
determine freshness for 301 responses. determine freshness for 301 responses.
The new permanent URI SHOULD be given by the Location field in the The new permanent URI SHOULD be given by the Location field in the
response. A response payload can contain a short hypertext note with response. A response payload can contain a short hypertext note with
a hyperlink to the new URI(s). a hyperlink to the new URI(s).
Note: For historic reasons, user agents MAY change the request Note: For historic reasons, user agents MAY change the request
method from POST to GET for the subsequent request. If this method from POST to GET for the subsequent request. If this
behavior is undesired, status code 307 (Temporary Redirect) can be behavior is undesired, status code 307 (Temporary Redirect) can be
used instead. used instead.
4.5.3. 302 Found 7.4.3. 302 Found
The target resource resides temporarily under a different URI. Since The target resource resides temporarily under a different URI. Since
the redirection might be altered on occasion, the client SHOULD the redirection might be altered on occasion, the client SHOULD
continue to use the effective request URI for future requests. continue to use the effective request URI for future requests.
The temporary URI SHOULD be given by the Location field in the The temporary URI SHOULD be given by the Location field in the
response. A response payload can contain a short hypertext note with response. A response payload can contain a short hypertext note with
a hyperlink to the new URI(s). a hyperlink to the new URI(s).
Note: For historic reasons, user agents MAY change the request Note: For historic reasons, user agents MAY change the request
method from POST to GET for the subsequent request. If this method from POST to GET for the subsequent request. If this
behavior is undesired, status code 307 (Temporary Redirect) can be behavior is undesired, status code 307 (Temporary Redirect) can be
used instead. used instead.
4.5.4. 303 See Other 7.4.4. 303 See Other
The 303 status code indicates that the server is redirecting the user The 303 status code indicates that the server is redirecting the user
agent to a different resource, as indicated by a URI in the Location agent to a different resource, as indicated by a URI in the Location
header field, that is intended to provide an indirect response to the header field, that is intended to provide an indirect response to the
original request. In order to satisfy the original request, a user original request. In order to satisfy the original request, a user
agent SHOULD perform a retrieval request using the Location URI (a agent SHOULD perform a retrieval request using the Location URI (a
GET or HEAD request if using HTTP), which can itself be redirected GET or HEAD request if using HTTP), which can itself be redirected
further, and present the eventual result as an answer to the original further, and present the eventual result as an answer to the original
request. Note that the new URI in the Location header field is not request. Note that the new URI in the Location header field is not
considered equivalent to the effective request URI. considered equivalent to the effective request URI.
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representation might be useful to recipients without implying that it representation might be useful to recipients without implying that it
adequately represents the target resource. Note that answers to the adequately represents the target resource. Note that answers to the
questions of what can be represented, what representations are questions of what can be represented, what representations are
adequate, and what might be a useful description are outside the adequate, and what might be a useful description are outside the
scope of HTTP and thus entirely determined by the URI owner(s). scope of HTTP and thus entirely determined by the URI owner(s).
Except for responses to a HEAD request, the representation of a 303 Except for responses to a HEAD request, the representation of a 303
response SHOULD contain a short hypertext note with a hyperlink to response SHOULD contain a short hypertext note with a hyperlink to
the Location URI. the Location URI.
4.5.5. 305 Use Proxy 7.4.5. 305 Use Proxy
The 305 status code was defined in a previous version of this The 305 status code was defined in a previous version of this
specification (see Appendix C), and is now deprecated. specification (see Appendix C), and is now deprecated.
4.5.6. 306 (Unused) 7.4.6. 306 (Unused)
The 306 status code was used in a previous version of the The 306 status code was used in a previous version of the
specification, is no longer used, and the code is reserved. specification, is no longer used, and the code is reserved.
4.5.7. 307 Temporary Redirect 7.4.7. 307 Temporary Redirect
The target resource resides temporarily under a different URI. Since The target resource resides temporarily under a different URI. Since
the redirection can change over time, the client SHOULD continue to the redirection can change over time, the client SHOULD continue to
use the effective request URI for future requests. use the effective request URI for future requests.
The temporary URI SHOULD be given by the Location field in the The temporary URI SHOULD be given by the Location field in the
response. A response payload can contain a short hypertext note with response. A response payload can contain a short hypertext note with
a hyperlink to the new URI(s). a hyperlink to the new URI(s).
Note: This status code is similar to 302 (Found), except that it Note: This status code is similar to 302 (Found), except that it
does not allow rewriting the request method from POST to GET. does not allow rewriting the request method from POST to GET.
This specification defines no equivalent counterpart for 301 This specification defines no equivalent counterpart for 301
(Moved Permanently) ([draft-reschke-http-status-308], however, (Moved Permanently) ([status-308], however, defines the status
defines the status code 308 (Permanent Redirect) for this code 308 (Permanent Redirect) for this purpose).
purpose).
4.6. Client Error 4xx 7.5. Client Error 4xx
The 4xx class of status code is intended for cases in which the The 4xx class of status code is intended for cases in which the
client seems to have erred. Except when responding to a HEAD client seems to have erred. Except when responding to a HEAD
request, the server SHOULD include a representation containing an request, the server SHOULD include a representation containing an
explanation of the error situation, and whether it is a temporary or explanation of the error situation, and whether it is a temporary or
permanent condition. These status codes are applicable to any permanent condition. These status codes are applicable to any
request method. User agents SHOULD display any included request method. User agents SHOULD display any included
representation to the user. representation to the user.
4.6.1. 400 Bad Request 7.5.1. 400 Bad Request
The server cannot or will not process the request, due to a client The server cannot or will not process the request, due to a client
error (e.g., malformed syntax). error (e.g., malformed syntax).
4.6.2. 402 Payment Required 7.5.2. 402 Payment Required
This code is reserved for future use. This code is reserved for future use.
4.6.3. 403 Forbidden 7.5.3. 403 Forbidden
The server understood the request, but refuses to authorize it. The server understood the request, but refuses to authorize it.
Providing different user authentication credentials might be Providing different user authentication credentials might be
successful, but any credentials that were provided in the request are successful, but any credentials that were provided in the request are
insufficient. The request SHOULD NOT be repeated with the same insufficient. The request SHOULD NOT be repeated with the same
credentials. credentials.
If the request method was not HEAD and the server wishes to make If the request method was not HEAD and the server wishes to make
public why the request has not been fulfilled, it SHOULD describe the public why the request has not been fulfilled, it SHOULD describe the
reason for the refusal in the representation. If the server does not reason for the refusal in the representation. If the server does not
wish to make this information available to the client, the status wish to make this information available to the client, the status
code 404 (Not Found) MAY be used instead. code 404 (Not Found) MAY be used instead.
4.6.4. 404 Not Found 7.5.4. 404 Not Found
The server has not found anything matching the effective request URI. The server has not found anything matching the effective request URI.
No indication is given of whether the condition is temporary or No indication is given of whether the condition is temporary or
permanent. The 410 (Gone) status code SHOULD be used if the server permanent. The 410 (Gone) status code SHOULD be used if the server
knows, through some internally configurable mechanism, that an old knows, through some internally configurable mechanism, that an old
resource is permanently unavailable and has no forwarding address. resource is permanently unavailable and has no forwarding address.
This status code is commonly used when the server does not wish to This status code is commonly used when the server does not wish to
reveal exactly why the request has been refused, or when no other reveal exactly why the request has been refused, or when no other
response is applicable. response is applicable.
4.6.5. 405 Method Not Allowed 7.5.5. 405 Method Not Allowed
The method specified in the request-line is not allowed for the The method specified in the request-line is not allowed for the
target resource. The response MUST include an Allow header field target resource. The response MUST include an Allow header field
containing a list of valid methods for the requested resource. containing a list of valid methods for the requested resource.
4.6.6. 406 Not Acceptable 7.5.6. 406 Not Acceptable
The resource identified by the request is only capable of generating The resource identified by the request is only capable of generating
response representations which have content characteristics not response representations which have content characteristics not
acceptable according to the Accept and Accept-* header fields sent in acceptable according to the Accept and Accept-* header fields sent in
the request. the request.
Unless it was a HEAD request, the response SHOULD include a Unless it was a HEAD request, the response SHOULD include a
representation containing a list of available representation representation containing a list of available representation
characteristics and location(s) from which the user or user agent can characteristics and location(s) from which the user or user agent can
choose the one most appropriate. Depending upon the format and the choose the one most appropriate. Depending upon the format and the
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Note: HTTP/1.1 servers are allowed to return responses which are Note: HTTP/1.1 servers are allowed to return responses which are
not acceptable according to the accept header fields sent in the not acceptable according to the accept header fields sent in the
request. In some cases, this might even be preferable to sending request. In some cases, this might even be preferable to sending
a 406 response. User agents are encouraged to inspect the header a 406 response. User agents are encouraged to inspect the header
fields of an incoming response to determine if it is acceptable. fields of an incoming response to determine if it is acceptable.
If the response could be unacceptable, a user agent SHOULD If the response could be unacceptable, a user agent SHOULD
temporarily stop receipt of more data and query the user for a temporarily stop receipt of more data and query the user for a
decision on further actions. decision on further actions.
4.6.7. 408 Request Timeout 7.5.7. 408 Request Timeout
The client did not produce a request within the time that the server The client did not produce a request within the time that the server
was prepared to wait. The client MAY repeat the request without was prepared to wait. The client MAY repeat the request without
modifications at any later time. modifications at any later time.
4.6.8. 409 Conflict 7.5.8. 409 Conflict
The request could not be completed due to a conflict with the current The request could not be completed due to a conflict with the current
state of the resource. This code is only allowed in situations where state of the resource. This code is only allowed in situations where
it is expected that the user might be able to resolve the conflict it is expected that the user might be able to resolve the conflict
and resubmit the request. The response body SHOULD include enough and resubmit the request. The payload SHOULD include enough
information for the user to recognize the source of the conflict. information for the user to recognize the source of the conflict.
Ideally, the response representation would include enough information Ideally, the response representation would include enough information
for the user or user agent to fix the problem; however, that might for the user or user agent to fix the problem; however, that might
not be possible and is not required. not be possible and is not required.
Conflicts are most likely to occur in response to a PUT request. For Conflicts are most likely to occur in response to a PUT request. For
example, if versioning were being used and the representation being example, if versioning were being used and the representation being
PUT included changes to a resource which conflict with those made by PUT included changes to a resource which conflict with those made by
an earlier (third-party) request, the server might use the 409 an earlier (third-party) request, the server might use the 409
response to indicate that it can't complete the request. In this response to indicate that it can't complete the request. In this
case, the response representation would likely contain a list of the case, the response representation would likely contain a list of the
differences between the two versions. differences between the two versions.
4.6.9. 410 Gone 7.5.9. 410 Gone
The target resource is no longer available at the server and no The target resource is no longer available at the server and no
forwarding address is known. This condition is expected to be forwarding address is known. This condition is expected to be
considered permanent. Clients with link editing capabilities SHOULD considered permanent. Clients with link editing capabilities SHOULD
delete references to the effective request URI after user approval. delete references to the effective request URI after user approval.
If the server does not know, or has no facility to determine, whether If the server does not know, or has no facility to determine, whether
or not the condition is permanent, the status code 404 (Not Found) or not the condition is permanent, the status code 404 (Not Found)
SHOULD be used instead. SHOULD be used instead.
The 410 response is primarily intended to assist the task of web The 410 response is primarily intended to assist the task of web
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remote links to that resource be removed. Such an event is common remote links to that resource be removed. Such an event is common
for limited-time, promotional services and for resources belonging to for limited-time, promotional services and for resources belonging to
individuals no longer working at the server's site. It is not individuals no longer working at the server's site. It is not
necessary to mark all permanently unavailable resources as "gone" or necessary to mark all permanently unavailable resources as "gone" or
to keep the mark for any length of time -- that is left to the to keep the mark for any length of time -- that is left to the
discretion of the server owner. discretion of the server owner.
Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to
determine freshness for 410 responses. determine freshness for 410 responses.
4.6.10. 411 Length Required 7.5.10. 411 Length Required
The server refuses to accept the request without a defined Content- The server refuses to accept the request without a defined Content-
Length. The client MAY repeat the request if it adds a valid Length. The client MAY repeat the request if it adds a valid
Content-Length header field containing the length of the message body Content-Length header field containing the length of the message body
in the request message. in the request message.
4.6.11. 413 Request Representation Too Large 7.5.11. 413 Request Representation Too Large
The server is refusing to process a request because the request The server is refusing to process a request because the request
representation is larger than the server is willing or able to representation is larger than the server is willing or able to
process. The server MAY close the connection to prevent the client process. The server MAY close the connection to prevent the client
from continuing the request. from continuing the request.
If the condition is temporary, the server SHOULD include a Retry- If the condition is temporary, the server SHOULD include a Retry-
After header field to indicate that it is temporary and after what After header field to indicate that it is temporary and after what
time the client MAY try again. time the client MAY try again.
4.6.12. 414 URI Too Long 7.5.12. 414 URI Too Long
The server is refusing to service the request because the effective The server is refusing to service the request because the effective
request URI is longer than the server is willing to interpret. This request URI is longer than the server is willing to interpret. This
rare condition is only likely to occur when a client has improperly rare condition is only likely to occur when a client has improperly
converted a POST request to a GET request with long query converted a POST request to a GET request with long query
information, when the client has descended into a URI "black hole" of information, when the client has descended into a URI "black hole" of
redirection (e.g., a redirected URI prefix that points to a suffix of redirection (e.g., a redirected URI prefix that points to a suffix of
itself), or when the server is under attack by a client attempting to itself), or when the server is under attack by a client attempting to
exploit security holes present in some servers using fixed-length exploit security holes present in some servers using fixed-length
buffers for reading or manipulating the request-target. buffers for reading or manipulating the request-target.
4.6.13. 415 Unsupported Media Type 7.5.13. 415 Unsupported Media Type
The server is refusing to service the request because the request The server is refusing to service the request because the request
payload is in a format not supported by this request method on the payload is in a format not supported by this request method on the
target resource. target resource.
4.6.14. 417 Expectation Failed 7.5.14. 417 Expectation Failed
The expectation given in an Expect header field (see Section 9.11) The expectation given in an Expect header field (see Section 6.1.2)
could not be met by this server, or, if the server is a proxy, the could not be met by this server, or, if the server is a proxy, the
server has unambiguous evidence that the request could not be met by server has unambiguous evidence that the request could not be met by
the next-hop server. the next-hop server.
4.6.15. 426 Upgrade Required 7.5.15. 426 Upgrade Required
The request can not be completed without a prior protocol upgrade. The request can not be completed without a prior protocol upgrade.
This response MUST include an Upgrade header field (Section 6.5 of This response MUST include an Upgrade header field (Section 6.3 of
[Part1]) specifying the required protocols. [Part1]) specifying the required protocols.
Example: Example:
HTTP/1.1 426 Upgrade Required HTTP/1.1 426 Upgrade Required
Upgrade: HTTP/3.0 Upgrade: HTTP/3.0
Connection: Upgrade Connection: Upgrade
Content-Length: 53 Content-Length: 53
Content-Type: text/plain Content-Type: text/plain
This service requires use of the HTTP/3.0 protocol. This service requires use of the HTTP/3.0 protocol.
The server SHOULD include a message body in the 426 response which The server SHOULD include a message body in the 426 response which
indicates in human readable form the reason for the error and indicates in human readable form the reason for the error and
describes any alternative courses which might be available to the describes any alternative courses which might be available to the
user. user.
4.7. Server Error 5xx 7.6. Server Error 5xx
Response status codes beginning with the digit "5" indicate cases in Response status codes beginning with the digit "5" indicate cases in
which the server is aware that it has erred or is incapable of which the server is aware that it has erred or is incapable of
performing the request. Except when responding to a HEAD request, performing the request. Except when responding to a HEAD request,
the server SHOULD include a representation containing an explanation the server SHOULD include a representation containing an explanation
of the error situation, and whether it is a temporary or permanent of the error situation, and whether it is a temporary or permanent
condition. User agents SHOULD display any included representation to condition. User agents SHOULD display any included representation to
the user. These response codes are applicable to any request method. the user. These response codes are applicable to any request method.
4.7.1. 500 Internal Server Error 7.6.1. 500 Internal Server Error
The server encountered an unexpected condition which prevented it The server encountered an unexpected condition which prevented it
from fulfilling the request. from fulfilling the request.
4.7.2. 501 Not Implemented 7.6.2. 501 Not Implemented
The server does not support the functionality required to fulfill the The server does not support the functionality required to fulfill the
request. This is the appropriate response when the server does not request. This is the appropriate response when the server does not
recognize the request method and is not capable of supporting it for recognize the request method and is not capable of supporting it for
any resource. any resource.
4.7.3. 502 Bad Gateway 7.6.3. 502 Bad Gateway
The server, while acting as a gateway or proxy, received an invalid The server, while acting as a gateway or proxy, received an invalid
response from the upstream server it accessed in attempting to response from the upstream server it accessed in attempting to
fulfill the request. fulfill the request.
4.7.4. 503 Service Unavailable 7.6.4. 503 Service Unavailable
The server is currently unable to handle the request due to a The server is currently unable to handle the request due to a
temporary overloading or maintenance of the server. temporary overloading or maintenance of the server.
The implication is that this is a temporary condition which will be The implication is that this is a temporary condition which will be
alleviated after some delay. If known, the length of the delay MAY alleviated after some delay. If known, the length of the delay MAY
be indicated in a Retry-After header field (Section 9.16). If no be indicated in a Retry-After header field (Section 8.1.3). If no
Retry-After is given, the client SHOULD handle the response as it Retry-After is given, the client SHOULD handle the response as it
would for a 500 (Internal Server Error) response. would for a 500 (Internal Server Error) response.
Note: The existence of the 503 status code does not imply that a Note: The existence of the 503 status code does not imply that a
server has to use it when becoming overloaded. Some servers might server has to use it when becoming overloaded. Some servers might
wish to simply refuse the connection. wish to simply refuse the connection.
4.7.5. 504 Gateway Timeout 7.6.5. 504 Gateway Timeout
The server, while acting as a gateway or proxy, did not receive a The server, while acting as a gateway or proxy, did not receive a
timely response from the upstream server specified by the URI (e.g., timely response from the upstream server specified by the URI (e.g.,
HTTP, FTP, LDAP) or some other auxiliary server (e.g., DNS) it needed HTTP, FTP, LDAP) or some other auxiliary server (e.g., DNS) it needed
to access in attempting to complete the request. to access in attempting to complete the request.
Note to implementers: some deployed proxies are known to return Note to implementers: some deployed proxies are known to return
400 (Bad Request) or 500 (Internal Server Error) when DNS lookups 400 (Bad Request) or 500 (Internal Server Error) when DNS lookups
time out. time out.
4.7.6. 505 HTTP Version Not Supported 7.6.6. 505 HTTP Version Not Supported
The server does not support, or refuses to support, the protocol The server does not support, or refuses to support, the protocol
version that was used in the request message. The server is version that was used in the request message. The server is
indicating that it is unable or unwilling to complete the request indicating that it is unable or unwilling to complete the request
using the same major version as the client, as described in Section using the same major version as the client, as described in Section
2.7 of [Part1], other than with this error message. The response 2.6 of [Part1], other than with this error message. The response
SHOULD contain a representation describing why that version is not SHOULD contain a representation describing why that version is not
supported and what other protocols are supported by that server. supported and what other protocols are supported by that server.
5. Protocol Parameters 8. Response Header Fields
5.1. Date/Time Formats The response header fields allow the server to pass additional
information about the response which cannot be placed in the status-
line. These header fields give information about the server and
about further access to the target resource (Section 5.5 of [Part1]).
8.1. Control Data
Response header fields can supply control data that supplements the
status code or instructs the client where to go next.
+-------------------+------------------------+
| Header Field Name | Defined in... |
+-------------------+------------------------+
| Age | Section 7.1 of [Part6] |
| Date | Section 8.1.1.2 |
| Location | Section 8.1.2 |
| Retry-After | Section 8.1.3 |
+-------------------+------------------------+
8.1.1. Origination Date
8.1.1.1. Date/Time Formats
HTTP applications have historically allowed three different formats HTTP applications have historically allowed three different formats
for date/time stamps. However, the preferred format is a fixed- for date/time stamps. However, the preferred format is a fixed-
length subset of that defined by [RFC1123]: length subset of that defined by [RFC1123]:
Sun, 06 Nov 1994 08:49:37 GMT ; RFC 1123 Sun, 06 Nov 1994 08:49:37 GMT ; RFC 1123
The other formats are described here only for compatibility with The other formats are described here only for compatibility with
obsolete implementations. obsolete implementations.
skipping to change at page 40, line 30 skipping to change at page 64, line 30
Note: Recipients of date values are encouraged to be robust in Note: Recipients of date values are encouraged to be robust in
accepting date values that might have been sent by non-HTTP accepting date values that might have been sent by non-HTTP
applications, as is sometimes the case when retrieving or posting applications, as is sometimes the case when retrieving or posting
messages via proxies/gateways to SMTP or NNTP. messages via proxies/gateways to SMTP or NNTP.
Note: HTTP requirements for the date/time stamp format apply only Note: HTTP requirements for the date/time stamp format apply only
to their usage within the protocol stream. Clients and servers to their usage within the protocol stream. Clients and servers
are not required to use these formats for user presentation, are not required to use these formats for user presentation,
request logging, etc. request logging, etc.
5.2. Product Tokens 8.1.1.2. Date
Product tokens are used to allow communicating applications to
identify themselves by software name and version. Most fields using
product tokens also allow sub-products which form a significant part
of the application to be listed, separated by whitespace. By
convention, the products are listed in order of their significance
for identifying the application.
product = token ["/" product-version]
product-version = token
Examples:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
Server: Apache/0.8.4
Product tokens SHOULD be short and to the point. They MUST NOT be
used for advertising or other non-essential information. Although
any token octet MAY appear in a product-version, this token SHOULD
only be used for a version identifier (i.e., successive versions of
the same product SHOULD only differ in the product-version portion of
the product value).
5.3. Character Encodings (charset)
HTTP uses charset names to indicate the character encoding of a
textual representation.
A character encoding is identified by a case-insensitive token. The
complete set of tokens is defined by the IANA Character Set registry
(<http://www.iana.org/assignments/character-sets>).
charset = token
Although HTTP allows an arbitrary token to be used as a charset
value, any token that has a predefined value within the IANA
Character Set registry MUST represent the character encoding defined
by that registry. Applications SHOULD limit their use of character
encodings to those defined within the IANA registry.
HTTP uses charset in two contexts: within an Accept-Charset request
header field (in which the charset value is an unquoted token) and as
the value of a parameter in a Content-Type header field (within a
request or response), in which case the parameter value of the
charset parameter can be quoted.
Implementers need to be aware of IETF character set requirements
[RFC3629] [RFC2277].
5.4. Content Codings
Content coding values indicate an encoding transformation that has
been or can be applied to a representation. Content codings are
primarily used to allow a representation to be compressed or
otherwise usefully transformed without losing the identity of its
underlying media type and without loss of information. Frequently,
the representation is stored in coded form, transmitted directly, and
only decoded by the recipient.
content-coding = token
All content-coding values are case-insensitive. HTTP/1.1 uses
content-coding values in the Accept-Encoding (Section 9.3) and
Content-Encoding (Section 9.6) header fields. Although the value
describes the content-coding, what is more important is that it
indicates what decoding mechanism will be required to remove the
encoding.
compress
See Section 4.2.1 of [Part1].
deflate
See Section 4.2.2 of [Part1].
gzip
See Section 4.2.3 of [Part1].
5.4.1. Content Coding Registry
The HTTP Content Coding Registry defines the name space for the
content coding names.
Registrations MUST include the following fields:
o Name
o Description
o Pointer to specification text
Names of content codings MUST NOT overlap with names of transfer
codings (Section 4 of [Part1]), unless the encoding transformation is
identical (as is the case for the compression codings defined in
Section 4.2 of [Part1]).
Values to be added to this name space require IETF Review (see
Section 4.1 of [RFC5226]), and MUST conform to the purpose of content
coding defined in this section.
The registry itself is maintained at
<http://www.iana.org/assignments/http-parameters>.
5.5. Media Types
HTTP uses Internet Media Types [RFC2046] in the Content-Type
(Section 9.9) and Accept (Section 9.1) header fields in order to
provide open and extensible data typing and type negotiation.
media-type = type "/" subtype *( OWS ";" OWS parameter )
type = token
subtype = token
The type/subtype MAY be followed by parameters in the form of
attribute/value pairs.
parameter = attribute "=" value
attribute = token
value = word
The type, subtype, and parameter attribute names are case-
insensitive. Parameter values might or might not be case-sensitive,
depending on the semantics of the parameter name. The presence or
absence of a parameter might be significant to the processing of a
media-type, depending on its definition within the media type
registry.
A parameter value that matches the token production can be
transmitted as either a token or within a quoted-string. The quoted
and unquoted values are equivalent.
Note that some older HTTP applications do not recognize media type
parameters. When sending data to older HTTP applications,
implementations SHOULD only use media type parameters when they are
required by that type/subtype definition.
Media-type values are registered with the Internet Assigned Number
Authority (IANA). The media type registration process is outlined in
[RFC4288]. Use of non-registered media types is discouraged.
5.5.1. Canonicalization and Text Defaults
Internet media types are registered with a canonical form. A
representation transferred via HTTP messages MUST be in the
appropriate canonical form prior to its transmission except for
"text" types, as defined in the next paragraph.
When in canonical form, media subtypes of the "text" type use CRLF as
the text line break. HTTP relaxes this requirement and allows the
transport of text media with plain CR or LF alone representing a line
break when it is done consistently for an entire representation.
HTTP applications MUST accept CRLF, bare CR, and bare LF as
indicating a line break in text media received via HTTP. In
addition, if the text is in a character encoding that does not use
octets 13 and 10 for CR and LF respectively, as is the case for some
multi-byte character encodings, HTTP allows the use of whatever octet
sequences are defined by that character encoding to represent the
equivalent of CR and LF for line breaks. This flexibility regarding
line breaks applies only to text media in the payload body; a bare CR
or LF MUST NOT be substituted for CRLF within any of the HTTP control
structures (such as header fields and multipart boundaries).
If a representation is encoded with a content-coding, the underlying
data MUST be in a form defined above prior to being encoded.
5.5.2. Multipart Types
MIME provides for a number of "multipart" types -- encapsulations of
one or more representations within a single message body. All
multipart types share a common syntax, as defined in Section 5.1.1 of
[RFC2046], and MUST include a boundary parameter as part of the media
type value. The message body is itself a protocol element and MUST
therefore use only CRLF to represent line breaks between body-parts.
In general, HTTP treats a multipart message body no differently than
any other media type: strictly as payload. HTTP does not use the
multipart boundary as an indicator of message body length. In all
other respects, an HTTP user agent SHOULD follow the same or similar
behavior as a MIME user agent would upon receipt of a multipart type.
The MIME header fields within each body-part of a multipart message
body do not have any significance to HTTP beyond that defined by
their MIME semantics.
If an application receives an unrecognized multipart subtype, the
application MUST treat it as being equivalent to "multipart/mixed".
Note: The "multipart/form-data" type has been specifically defined
for carrying form data suitable for processing via the POST
request method, as described in [RFC2388].
5.6. Language Tags
A language tag, as defined in [RFC5646], identifies a natural
language spoken, written, or otherwise conveyed by human beings for
communication of information to other human beings. Computer
languages are explicitly excluded. HTTP uses language tags within
the Accept-Language and Content-Language fields.
In summary, a language tag is composed of one or more parts: A
primary language subtag followed by a possibly empty series of
subtags:
language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>
White space is not allowed within the tag and all tags are case-
insensitive. The name space of language subtags is administered by
the IANA (see
<http://www.iana.org/assignments/language-subtag-registry>).
Example tags include:
en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
See [RFC5646] for further information.
6. Payload
HTTP messages MAY transfer a payload if not otherwise restricted by
the request method or response status code. The payload consists of
metadata, in the form of header fields, and data, in the form of the
sequence of octets in the message body after any transfer-coding has
been decoded.
A "payload" in HTTP is always a partial or complete representation of
some resource. We use separate terms for payload and representation
because some messages contain only the associated representation's
header fields (e.g., responses to HEAD) or only some part(s) of the
representation (e.g., the 206 (Partial Content) status code).
6.1. Payload Header Fields
HTTP header fields that specifically define the payload, rather than
the associated representation, are referred to as "payload header
fields". The following payload header fields are defined by
HTTP/1.1:
+-------------------+--------------------------+
| Header Field Name | Defined in... |
+-------------------+--------------------------+
| Content-Length | Section 3.3.2 of [Part1] |
| Content-Range | Section 5.2 of [Part5] |
+-------------------+--------------------------+
6.2. Payload Body
A payload body is only present in a message when a message body is
present, as described in Section 3.3 of [Part1]. The payload body is
obtained from the message body by decoding any Transfer-Encoding that
might have been applied to ensure safe and proper transfer of the
message.
7. Representation
A "representation" is information in a format that can be readily
communicated from one party to another. A resource representation is
information that reflects the state of that resource, as observed at
some point in the past (e.g., in a response to GET) or to be desired
at some point in the future (e.g., in a PUT request).
Most, but not all, representations transferred via HTTP are intended
to be a representation of the target resource (the resource
identified by the effective request URI). The precise semantics of a
representation are determined by the type of message (request or
response), the request method, the response status code, and the
representation metadata. For example, the above semantic is true for
the representation in any 200 (OK) response to GET and for the
representation in any PUT request. A 200 response to PUT, in
contrast, contains either a representation that describes the
successful action or a representation of the target resource, with
the latter indicated by a Content-Location header field with the same
value as the effective request URI. Likewise, response messages with
an error status code usually contain a representation that describes
the error and what next steps are suggested for resolving it.
Request and Response messages MAY transfer a representation if not
otherwise restricted by the request method or response status code.
A representation consists of metadata (representation header fields)
and data (representation body). When a complete or partial
representation is enclosed in an HTTP message, it is referred to as
the payload of the message.
A representation body is only present in a message when a message
body is present, as described in Section 3.3 of [Part1]. The
representation body is obtained from the message body by decoding any
Transfer-Encoding that might have been applied to ensure safe and
proper transfer of the message.
7.1. Identifying the Resource Associated with a Representation
It is sometimes necessary to determine an identifier for the resource
associated with a representation.
An HTTP request representation, when present, is always associated
with an anonymous (i.e., unidentified) resource.
In the common case, an HTTP response is a representation of the
target resource (see Section 5.5 of [Part1]). However, this is not
always the case. To determine the URI of the resource a response is
associated with, the following rules are used (with the first
applicable one being selected):
1. If the response status code is 200 (OK) or 203 (Non-Authoritative
Information) and the request method was GET, the response payload
is a representation of the target resource.
2. If the response status code is 204 (No Content), 206 (Partial
Content), or 304 (Not Modified) and the request method was GET or
HEAD, the response payload is a partial representation of the
target resource.
3. If the response has a Content-Location header field, and that URI
is the same as the effective request URI, the response payload is
a representation of the target resource.
4. If the response has a Content-Location header field, and that URI
is not the same as the effective request URI, then the response
asserts that its payload is a representation of the resource
identified by the Content-Location URI. However, such an
assertion cannot be trusted unless it can be verified by other
means (not defined by HTTP).
5. Otherwise, the response is a representation of an anonymous
(i.e., unidentified) resource.
[[TODO-req-uri: The comparison function is going to have to be
defined somewhere, because we already need to compare URIs for things
like cache invalidation.]]
7.2. Representation Header Fields
Representation header fields define metadata about the representation
data enclosed in the message body or, if no message body is present,
about the representation that would have been transferred in a 200
(OK) response to a simultaneous GET request with the same effective
request URI.
The following header fields are defined as representation metadata:
+-------------------+------------------------+
| Header Field Name | Defined in... |
+-------------------+------------------------+
| Content-Encoding | Section 9.6 |
| Content-Language | Section 9.7 |
| Content-Location | Section 9.8 |
| Content-Type | Section 9.9 |
| Expires | Section 7.3 of [Part6] |
+-------------------+------------------------+
We use the term "selected representation" to refer to the the current
representation of a target resource that would have been selected in
a successful response if the same request had used the method GET and
excluded any conditional request header fields.
Additional header fields define metadata about the selected
representation, which might differ from the representation included
in the message for responses to some state-changing methods. The
following header fields are defined as selected representation
metadata:
+-------------------+------------------------+
| Header Field Name | Defined in... |
+-------------------+------------------------+
| ETag | Section 2.3 of [Part4] |
| Last-Modified | Section 2.2 of [Part4] |
+-------------------+------------------------+
7.3. Representation Data
The representation body associated with an HTTP message is either
provided as the payload body of the message or referred to by the
message semantics and the effective request URI. The representation
data is in a format and encoding defined by the representation
metadata header fields.
The data type of the representation data is determined via the header
fields Content-Type and Content-Encoding. These define a two-layer,
ordered encoding model:
representation-data := Content-Encoding( Content-Type( bits ) )
Content-Type specifies the media type of the underlying data, which
defines both the data format and how that data SHOULD be processed by
the recipient (within the scope of the request method semantics).
Any HTTP/1.1 message containing a payload body SHOULD include a
Content-Type header field defining the media type of the associated
representation unless that metadata is unknown to the sender. If the
Content-Type header field is not present, it indicates that the
sender does not know the media type of the representation; recipients
MAY either assume that the media type is "application/octet-stream"
([RFC2046], Section 4.5.1) or examine the content to determine its
type.
In practice, resource owners do not always properly configure their
origin server to provide the correct Content-Type for a given
representation, with the result that some clients will examine a
response body's content and override the specified type. Clients
that do so risk drawing incorrect conclusions, which might expose
additional security risks (e.g., "privilege escalation").
Furthermore, it is impossible to determine the sender's intent by
examining the data format: many data formats match multiple media
types that differ only in processing semantics. Implementers are
encouraged to provide a means of disabling such "content sniffing"
when it is used.
Content-Encoding is used to indicate any additional content codings
applied to the data, usually for the purpose of data compression,
that are a property of the representation. If Content-Encoding is
not present, then there is no additional encoding beyond that defined
by the Content-Type header field.
8. Content Negotiation
HTTP responses include a representation which contains information
for interpretation, whether by a human user or for further
processing. Often, the server has different ways of representing the
same information; for example, in different formats, languages, or
using different character encodings.
HTTP clients and their users might have different or variable
capabilities, characteristics or preferences which would influence
which representation, among those available from the server, would be
best for the server to deliver. For this reason, HTTP provides
mechanisms for "content negotiation" -- a process of allowing
selection of a representation of a given resource, when more than one
is available.
This specification defines two patterns of content negotiation;
"server-driven", where the server selects the representation based
upon the client's stated preferences, and "agent-driven" negotiation,
where the server provides a list of representations for the client to
choose from, based upon their metadata. In addition, there are other
patterns: some applications use an "active content" pattern, where
the server returns active content which runs on the client and, based
on client available parameters, selects additional resources to
invoke. "Transparent Content Negotiation" ([RFC2295]) has also been
proposed.
These patterns are all widely used, and have trade-offs in
applicability and practicality. In particular, when the number of
preferences or capabilities to be expressed by a client are large
(such as when many different formats are supported by a user-agent),
server-driven negotiation becomes unwieldy, and might not be
appropriate. Conversely, when the number of representations to
choose from is very large, agent-driven negotiation might not be
appropriate.
Note that in all cases, the supplier of representations has the
responsibility for determining which representations might be
considered to be the "same information".
8.1. Server-driven Negotiation
If the selection of the best representation for a response is made by
an algorithm located at the server, it is called server-driven
negotiation. Selection is based on the available representations of
the response (the dimensions over which it can vary; e.g., language,
content-coding, etc.) and the contents of particular header fields in
the request message or on other information pertaining to the request
(such as the network address of the client).
Server-driven negotiation is advantageous when the algorithm for
selecting from among the available representations is difficult to
describe to the user agent, or when the server desires to send its
"best guess" to the client along with the first response (hoping to
avoid the round-trip delay of a subsequent request if the "best
guess" is good enough for the user). In order to improve the
server's guess, the user agent MAY include request header fields
(Accept, Accept-Language, Accept-Encoding, etc.) which describe its
preferences for such a response.
Server-driven negotiation has disadvantages:
1. It is impossible for the server to accurately determine what
might be "best" for any given user, since that would require
complete knowledge of both the capabilities of the user agent and
the intended use for the response (e.g., does the user want to
view it on screen or print it on paper?).
2. Having the user agent describe its capabilities in every request
can be both very inefficient (given that only a small percentage
of responses have multiple representations) and a potential
violation of the user's privacy.
3. It complicates the implementation of an origin server and the
algorithms for generating responses to a request.
4. It might limit a public cache's ability to use the same response
for multiple user's requests.
Server-driven negotiation allows the user agent to specify its
preferences, but it cannot expect responses to always honor them.
For example, the origin server might not implement server-driven
negotiation, or it might decide that sending a response that doesn't
conform to them is better than sending a 406 (Not Acceptable)
response.
Many of the mechanisms for expressing preferences use quality values
to declare relative preference. See Section 4.3.1 of [Part1] for
more information.
HTTP/1.1 includes the following header fields for enabling server-
driven negotiation through description of user agent capabilities and
user preferences: Accept (Section 9.1), Accept-Charset (Section 9.2),
Accept-Encoding (Section 9.3), Accept-Language (Section 9.4), and
User-Agent (Section 9.18). However, an origin server is not limited
to these dimensions and MAY vary the response based on any aspect of
the request, including aspects of the connection (e.g., IP address)
or information within extension header fields not defined by this
specification.
Note: In practice, User-Agent based negotiation is fragile,
because new clients might not be recognized.
The Vary header field (Section 7.5 of [Part6]) can be used to express
the parameters the server uses to select a representation that is
subject to server-driven negotiation.
8.2. Agent-driven Negotiation
With agent-driven negotiation, selection of the best representation
for a response is performed by the user agent after receiving an
initial response from the origin server. Selection is based on a
list of the available representations of the response included within
the header fields or body of the initial response, with each
representation identified by its own URI. Selection from among the
representations can be performed automatically (if the user agent is
capable of doing so) or manually by the user selecting from a
generated (possibly hypertext) menu.
Agent-driven negotiation is advantageous when the response would vary
over commonly-used dimensions (such as type, language, or encoding),
when the origin server is unable to determine a user agent's
capabilities from examining the request, and generally when public
caches are used to distribute server load and reduce network usage.
Agent-driven negotiation suffers from the disadvantage of needing a
second request to obtain the best alternate representation. This
second request is only efficient when caching is used. In addition,
this specification does not define any mechanism for supporting
automatic selection, though it also does not prevent any such
mechanism from being developed as an extension and used within
HTTP/1.1.
This specification defines the 300 (Multiple Choices) and 406 (Not
Acceptable) status codes for enabling agent-driven negotiation when
the server is unwilling or unable to provide a varying response using
server-driven negotiation.
9. Header Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header
fields related to request and response semantics and to the payload
of messages.
9.1. Accept
The "Accept" header field can be used by user agents to specify
response media types that are acceptable. Accept header fields can
be used to indicate that the request is specifically limited to a
small set of desired types, as in the case of a request for an in-
line image.
Accept = #( media-range [ accept-params ] )
media-range = ( "*/*"
/ ( type "/" "*" )
/ ( type "/" subtype )
) *( OWS ";" OWS parameter )
accept-params = OWS ";" OWS "q=" qvalue *( accept-ext )
accept-ext = OWS ";" OWS token [ "=" word ]
The asterisk "*" character is used to group media types into ranges,
with "*/*" indicating all media types and "type/*" indicating all
subtypes of that type. The media-range MAY include media type
parameters that are applicable to that range.
Each media-range MAY be followed by one or more accept-params,
beginning with the "q" parameter for indicating a relative quality
factor. The first "q" parameter (if any) separates the media-range
parameter(s) from the accept-params. Quality factors allow the user
or user agent to indicate the relative degree of preference for that
media-range, using the qvalue scale from 0 to 1 (Section 4.3.1 of
[Part1]). The default value is q=1.
Note: Use of the "q" parameter name to separate media type
parameters from Accept extension parameters is due to historical
practice. Although this prevents any media type parameter named
"q" from being used with a media range, such an event is believed
to be unlikely given the lack of any "q" parameters in the IANA
media type registry and the rare usage of any media type
parameters in Accept. Future media types are discouraged from
registering any parameter named "q".
The example
Accept: audio/*; q=0.2, audio/basic
SHOULD be interpreted as "I prefer audio/basic, but send me any audio
type if it is the best available after an 80% mark-down in quality".
A request without any Accept header field implies that the user agent
will accept any media type in response. If an Accept header field is
present in a request and none of the available representations for
the response have a media type that is listed as acceptable, the
origin server MAY either honor the Accept header field by sending a
406 (Not Acceptable) response or disregard the Accept header field by
treating the response as if it is not subject to content negotiation.
A more elaborate example is
Accept: text/plain; q=0.5, text/html,
text/x-dvi; q=0.8, text/x-c
Verbally, this would be interpreted as "text/html and text/x-c are
the preferred media types, but if they do not exist, then send the
text/x-dvi representation, and if that does not exist, send the text/
plain representation".
Media ranges can be overridden by more specific media ranges or
specific media types. If more than one media range applies to a
given type, the most specific reference has precedence. For example,
Accept: text/*, text/plain, text/plain;format=flowed, */*
have the following precedence:
1. text/plain;format=flowed
2. text/plain
3. text/*
4. */*
The media type quality factor associated with a given type is
determined by finding the media range with the highest precedence
which matches that type. For example,
Accept: text/*;q=0.3, text/html;q=0.7, text/html;level=1,
text/html;level=2;q=0.4, */*;q=0.5
would cause the following values to be associated:
+-------------------+---------------+
| Media Type | Quality Value |
+-------------------+---------------+
| text/html;level=1 | 1 |
| text/html | 0.7 |
| text/plain | 0.3 |
| image/jpeg | 0.5 |
| text/html;level=2 | 0.4 |
| text/html;level=3 | 0.7 |
+-------------------+---------------+
Note: A user agent might be provided with a default set of quality
values for certain media ranges. However, unless the user agent is a
closed system which cannot interact with other rendering agents, this
default set ought to be configurable by the user.
9.2. Accept-Charset
The "Accept-Charset" header field can be used by user agents to
indicate what character encodings are acceptable in a response
payload. This field allows clients capable of understanding more
comprehensive or special-purpose character encodings to signal that
capability to a server which is capable of representing documents in
those character encodings.
Accept-Charset = 1#( ( charset / "*" )
[ OWS ";" OWS "q=" qvalue ] )
Character encoding values (a.k.a., charsets) are described in
Section 5.3. Each charset MAY be given an associated quality value
which represents the user's preference for that charset. The default
value is q=1. An example is
Accept-Charset: iso-8859-5, unicode-1-1;q=0.8
The special value "*", if present in the Accept-Charset field,
matches every character encoding which is not mentioned elsewhere in
the Accept-Charset field. If no "*" is present in an Accept-Charset
field, then all character encodings not explicitly mentioned get a
quality value of 0.
A request without any Accept-Charset header field implies that the
user agent will accept any character encoding in response. If an
Accept-Charset header field is present in a request and none of the
available representations for the response have a character encoding
that is listed as acceptable, the origin server MAY either honor the
Accept-Charset header field by sending a 406 (Not Acceptable)
response or disregard the Accept-Charset header field by treating the
response as if it is not subject to content negotiation.
9.3. Accept-Encoding
The "Accept-Encoding" header field can be used by user agents to
indicate what response content-codings (Section 5.4) are acceptable
in the response. An "identity" token is used as a synonym for "no
encoding" in order to communicate when no encoding is preferred.
Accept-Encoding = #( codings [ OWS ";" OWS "q=" qvalue ] )
codings = content-coding / "identity" / "*"
Each codings value MAY be given an associated quality value which
represents the preference for that encoding. The default value is
q=1.
For example,
Accept-Encoding: compress, gzip
Accept-Encoding:
Accept-Encoding: *
Accept-Encoding: compress;q=0.5, gzip;q=1.0
Accept-Encoding: gzip;q=1.0, identity; q=0.5, *;q=0
A server tests whether a content-coding for a given representation is
acceptable, according to an Accept-Encoding field, using these rules:
1. The special "*" symbol in an Accept-Encoding field matches any
available content-coding not explicitly listed in the header
field.
2. If the representation has no content-coding, then it is
acceptable by default unless specifically excluded by the Accept-
Encoding field stating either "identity;q=0" or "*;q=0" without a
more specific entry for "identity".
3. If the representation's content-coding is one of the content-
codings listed in the Accept-Encoding field, then it is
acceptable unless it is accompanied by a qvalue of 0. (As
defined in Section 4.3.1 of [Part1], a qvalue of 0 means "not
acceptable".)
4. If multiple content-codings are acceptable, then the acceptable
content-coding with the highest non-zero qvalue is preferred.
An Accept-Encoding header field with a combined field-value that is
empty implies that the user agent does not want any content-coding in
response. If an Accept-Encoding header field is present in a request
and none of the available representations for the response have a
content-coding that is listed as acceptable, the origin server SHOULD
send a response without any content-coding.
A request without an Accept-Encoding header field implies that the
user agent will accept any content-coding in response, but a
representation without content-coding is preferred for compatibility
with the widest variety of user agents.
Note: Most HTTP/1.0 applications do not recognize or obey qvalues
associated with content-codings. This means that qvalues will not
work and are not permitted with x-gzip or x-compress.
9.4. Accept-Language
The "Accept-Language" header field can be used by user agents to
indicate the set of natural languages that are preferred in the
response. Language tags are defined in Section 5.6.
Accept-Language =
1#( language-range [ OWS ";" OWS "q=" qvalue ] )
language-range =
<language-range, defined in [RFC4647], Section 2.1>
Each language-range can be given an associated quality value which
represents an estimate of the user's preference for the languages
specified by that range. The quality value defaults to "q=1". For
example,
Accept-Language: da, en-gb;q=0.8, en;q=0.7
would mean: "I prefer Danish, but will accept British English and
other types of English". (see also Section 2.3 of [RFC4647])
For matching, Section 3 of [RFC4647] defines several matching
schemes. Implementations can offer the most appropriate matching
scheme for their requirements.
Note: The "Basic Filtering" scheme ([RFC4647], Section 3.3.1) is
identical to the matching scheme that was previously defined in
Section 14.4 of [RFC2616].
It might be contrary to the privacy expectations of the user to send
an Accept-Language header field with the complete linguistic
preferences of the user in every request. For a discussion of this
issue, see Section 11.5.
As intelligibility is highly dependent on the individual user, it is
recommended that client applications make the choice of linguistic
preference available to the user. If the choice is not made
available, then the Accept-Language header field MUST NOT be given in
the request.
Note: When making the choice of linguistic preference available to
the user, we remind implementers of the fact that users are not
familiar with the details of language matching as described above,
and ought to be provided appropriate guidance. As an example,
users might assume that on selecting "en-gb", they will be served
any kind of English document if British English is not available.
A user agent might suggest in such a case to add "en" to get the
best matching behavior.
9.5. Allow
The "Allow" header field lists the set of methods advertised as
supported by the target resource. The purpose of this field is
strictly to inform the recipient of valid request methods associated
with the resource.
Allow = #method
Example of use:
Allow: GET, HEAD, PUT
The actual set of allowed methods is defined by the origin server at
the time of each request.
A proxy MUST NOT modify the Allow header field -- it does not need to
understand all the methods specified in order to handle them
according to the generic message handling rules.
9.6. Content-Encoding
The "Content-Encoding" header field indicates what content-codings
have been applied to the representation beyond those inherent in the
media type, and thus what decoding mechanisms have to be applied in
order to obtain the media-type referenced by the Content-Type header
field. Content-Encoding is primarily used to allow a representation
to be compressed without losing the identity of its underlying media
type.
Content-Encoding = 1#content-coding
Content codings are defined in Section 5.4. An example of its use is
Content-Encoding: gzip
The content-coding is a characteristic of the representation.
Typically, the representation body is stored with this encoding and
is only decoded before rendering or analogous usage. However, a
transforming proxy MAY modify the content-coding if the new coding is
known to be acceptable to the recipient, unless the "no-transform"
cache-control directive is present in the message.
If the media type includes an inherent encoding, such as a data
format that is always compressed, then that encoding would not be
restated as a Content-Encoding even if it happens to be the same
algorithm as one of the content-codings. Such a content-coding would
only be listed if, for some bizarre reason, it is applied a second
time to form the representation. Likewise, an origin server might
choose to publish the same payload data as multiple representations
that differ only in whether the coding is defined as part of Content-
Type or Content-Encoding, since some user agents will behave
differently in their handling of each response (e.g., open a "Save as
..." dialog instead of automatic decompression and rendering of
content).
A representation that has a content-coding applied to it MUST include
a Content-Encoding header field that lists the content-coding(s)
applied.
If multiple encodings have been applied to a representation, the
content codings MUST be listed in the order in which they were
applied. Additional information about the encoding parameters MAY be
provided by other header fields not defined by this specification.
If the content-coding of a representation in a request message is not
acceptable to the origin server, the server SHOULD respond with a
status code of 415 (Unsupported Media Type).
9.7. Content-Language
The "Content-Language" header field describes the natural language(s)
of the intended audience for the representation. Note that this
might not be equivalent to all the languages used within the
representation.
Content-Language = 1#language-tag
Language tags are defined in Section 5.6. The primary purpose of
Content-Language is to allow a user to identify and differentiate
representations according to the user's own preferred language.
Thus, if the body content is intended only for a Danish-literate
audience, the appropriate field is
Content-Language: da
If no Content-Language is specified, the default is that the content
is intended for all language audiences. This might mean that the
sender does not consider it to be specific to any natural language,
or that the sender does not know for which language it is intended.
Multiple languages MAY be listed for content that is intended for
multiple audiences. For example, a rendition of the "Treaty of
Waitangi", presented simultaneously in the original Maori and English
versions, would call for
Content-Language: mi, en
However, just because multiple languages are present within a
representation does not mean that it is intended for multiple
linguistic audiences. An example would be a beginner's language
primer, such as "A First Lesson in Latin", which is clearly intended
to be used by an English-literate audience. In this case, the
Content-Language would properly only include "en".
Content-Language MAY be applied to any media type -- it is not
limited to textual documents.
9.8. Content-Location
The "Content-Location" header field supplies a URI that can be used
as a specific identifier for the representation in this message. In
other words, if one were to perform a GET on this URI at the time of
this message's generation, then a 200 (OK) response would contain the
same representation that is enclosed as payload in this message.
Content-Location = absolute-URI / partial-URI
The Content-Location value is not a replacement for the effective
Request URI (Section 5.5 of [Part1]). It is representation metadata.
It has the same syntax and semantics as the header field of the same
name defined for MIME body parts in Section 4 of [RFC2557]. However,
its appearance in an HTTP message has some special implications for
HTTP recipients.
If Content-Location is included in a response message and its value
is the same as the effective request URI, then the response payload
SHOULD be considered a current representation of that resource. For
a GET or HEAD request, this is the same as the default semantics when
no Content-Location is provided by the server. For a state-changing
request like PUT or POST, it implies that the server's response
contains the new representation of that resource, thereby
distinguishing it from representations that might only report about
the action (e.g., "It worked!"). This allows authoring applications
to update their local copies without the need for a subsequent GET
request.
If Content-Location is included in a response message and its value
differs from the effective request URI, then the origin server is
informing recipients that this representation has its own, presumably
more specific, identifier. For a GET or HEAD request, this is an
indication that the effective request URI identifies a resource that
is subject to content negotiation and the selected representation for
this response can also be found at the identified URI. For other
methods, such a Content-Location indicates that this representation
contains a report on the action's status and the same report is
available (for future access with GET) at the given URI. For
example, a purchase transaction made via a POST request might include
a receipt document as the payload of the 200 (OK) response; the
Content-Location value provides an identifier for retrieving a copy
of that same receipt in the future.
If Content-Location is included in a request message, then it MAY be
interpreted by the origin server as an indication of where the user
agent originally obtained the content of the enclosed representation
(prior to any subsequent modification of the content by that user
agent). In other words, the user agent is providing the same
representation metadata that it received with the original
representation. However, such interpretation MUST NOT be used to
alter the semantics of the method requested by the client. For
example, if a client makes a PUT request on a negotiated resource and
the origin server accepts that PUT (without redirection), then the
new set of values for that resource is expected to be consistent with
the one representation supplied in that PUT; the Content-Location
cannot be used as a form of reverse content selection that identifies
only one of the negotiated representations to be updated. If the
user agent had wanted the latter semantics, it would have applied the
PUT directly to the Content-Location URI.
A Content-Location field received in a request message is transitory
information that SHOULD NOT be saved with other representation
metadata for use in later responses. The Content-Location's value
might be saved for use in other contexts, such as within source links
or other metadata.
A cache cannot assume that a representation with a Content-Location
different from the URI used to retrieve it can be used to respond to
later requests on that Content-Location URI.
If the Content-Location value is a partial URI, the partial URI is
interpreted relative to the effective request URI.
9.9. Content-Type
The "Content-Type" header field indicates the media type of the
representation. In the case of responses to the HEAD method, the
media type is that which would have been sent had the request been a
GET.
Content-Type = media-type
Media types are defined in Section 5.5. An example of the field is
Content-Type: text/html; charset=ISO-8859-4
Further discussion of Content-Type is provided in Section 7.3.
9.10. Date
The "Date" header field represents the date and time at which the The "Date" header field represents the date and time at which the
message was originated, having the same semantics as the Origination message was originated, having the same semantics as the Origination
Date Field (orig-date) defined in Section 3.6.1 of [RFC5322]. The Date Field (orig-date) defined in Section 3.6.1 of [RFC5322]. The
field value is an HTTP-date, as defined in Section 5.1; it MUST be field value is an HTTP-date, as defined in Section 8.1.1.1; it MUST
sent in rfc1123-date format. be sent in rfc1123-date format.
Date = HTTP-date Date = HTTP-date
An example is An example is
Date: Tue, 15 Nov 1994 08:12:31 GMT Date: Tue, 15 Nov 1994 08:12:31 GMT
Origin servers MUST include a Date header field in all responses, Origin servers MUST include a Date header field in all responses,
except in these cases: except in these cases:
skipping to change at page 62, line 21 skipping to change at page 65, line 31
The HTTP-date sent in a Date header field SHOULD NOT represent a date The HTTP-date sent in a Date header field SHOULD NOT represent a date
and time subsequent to the generation of the message. It SHOULD and time subsequent to the generation of the message. It SHOULD
represent the best available approximation of the date and time of represent the best available approximation of the date and time of
message generation, unless the implementation has no means of message generation, unless the implementation has no means of
generating a reasonably accurate date and time. In theory, the date generating a reasonably accurate date and time. In theory, the date
ought to represent the moment just before the payload is generated. ought to represent the moment just before the payload is generated.
In practice, the date can be generated at any time during the message In practice, the date can be generated at any time during the message
origination without affecting its semantic value. origination without affecting its semantic value.
9.11. Expect 8.1.2. Location
The "Expect" header field is used to indicate that particular server
behaviors are required by the client.
Expect = 1#expectation
expectation = expect-name [ BWS "=" BWS expect-value ]
*( OWS ";" [ OWS expect-param ] )
expect-param = expect-name [ BWS "=" BWS expect-value ]
expect-name = token
expect-value = token / quoted-string
If all received Expect header field(s) are syntactically valid but
contain an expectation that the recipient does not understand or
cannot comply with, the recipient MUST respond with a 417
(Expectation Failed) status code. A recipient of a syntactically
invalid Expectation header field MUST respond with a 4xx status code
other than 417.
The only expectation defined by this specification is:
100-continue
The "100-continue" expectation is defined Section 6.4.3 of
[Part1]. It does not support any expect-params.
Comparison is case-insensitive for names (expect-name), and case-
sensitive for values (expect-value).
The Expect mechanism is hop-by-hop: the above requirements apply to
any server, including proxies. However, the Expect header field
itself is end-to-end; it MUST be forwarded if the request is
forwarded.
Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
Expect header field.
9.12. From
The "From" header field, if given, SHOULD contain an Internet e-mail
address for the human user who controls the requesting user agent.
The address SHOULD be machine-usable, as defined by "mailbox" in
Section 3.4 of [RFC5322]:
From = mailbox
mailbox = <mailbox, defined in [RFC5322], Section 3.4>
An example is:
From: webmaster@example.org
This header field MAY be used for logging purposes and as a means for
identifying the source of invalid or unwanted requests. It SHOULD
NOT be used as an insecure form of access protection. The
interpretation of this field is that the request is being performed
on behalf of the person given, who accepts responsibility for the
method performed. In particular, robot agents SHOULD include this
header field so that the person responsible for running the robot can
be contacted if problems occur on the receiving end.
The Internet e-mail address in this field MAY be separate from the
Internet host which issued the request. For example, when a request
is passed through a proxy the original issuer's address SHOULD be
used.
The client SHOULD NOT send the From header field without the user's
approval, as it might conflict with the user's privacy interests or
their site's security policy. It is strongly recommended that the
user be able to disable, enable, and modify the value of this field
at any time prior to a request.
9.13. Location
The "Location" header field MAY be sent in responses to refer to a The "Location" header field MAY be sent in responses to refer to a
specific resource in accordance with the semantics of the status specific resource in accordance with the semantics of the status
code. code.
Location = URI-reference Location = URI-reference
For 201 (Created) responses, the Location is the URI of the new For 201 (Created) responses, the Location is the URI of the new
resource which was created by the request. For 3xx (Redirection) resource which was created by the request. For 3xx (Redirection)
responses, the location SHOULD indicate the server's preferred URI responses, the location SHOULD indicate the server's preferred URI
skipping to change at page 64, line 46 skipping to change at page 66, line 31
Note: Some recipients attempt to recover from Location fields that Note: Some recipients attempt to recover from Location fields that
are not valid URI references. This specification does not mandate are not valid URI references. This specification does not mandate
or define such processing, but does allow it. or define such processing, but does allow it.
There are circumstances in which a fragment identifier in a Location There are circumstances in which a fragment identifier in a Location
URI would not be appropriate. For instance, when it appears in a 201 URI would not be appropriate. For instance, when it appears in a 201
(Created) response, where the Location header field specifies the URI (Created) response, where the Location header field specifies the URI
for the entire created resource. for the entire created resource.
Note: The Content-Location header field (Section 9.8) differs from Note: The Content-Location header field (Section 3.1.4.2) differs
Location in that the Content-Location identifies the most specific from Location in that the Content-Location identifies the most
resource corresponding to the enclosed representation. It is specific resource corresponding to the enclosed representation.
therefore possible for a response to contain header fields for It is therefore possible for a response to contain header fields
both Location and Content-Location. for both Location and Content-Location.
9.14. Max-Forwards
The "Max-Forwards" header field provides a mechanism with the TRACE
(Section 2.3.7) and OPTIONS (Section 2.3.1) methods to limit the
number of times that the request is forwarded by proxies. This can
be useful when the client is attempting to trace a request which
appears to be failing or looping mid-chain.
Max-Forwards = 1*DIGIT
The Max-Forwards value is a decimal integer indicating the remaining
number of times this request message can be forwarded.
Each recipient of a TRACE or OPTIONS request containing a Max-
Forwards header field MUST check and update its value prior to
forwarding the request. If the received value is zero (0), the
recipient MUST NOT forward the request; instead, it MUST respond as
the final recipient. If the received Max-Forwards value is greater
than zero, then the forwarded message MUST contain an updated Max-
Forwards field with a value decremented by one (1).
The Max-Forwards header field MAY be ignored for all other request
methods.
9.15. Referer
The "Referer" [sic] header field allows the client to specify the URI
of the resource from which the target URI was obtained (the
"referrer", although the header field is misspelled.).
The Referer header field allows servers to generate lists of back-
links to resources for interest, logging, optimized caching, etc. It
also allows obsolete or mistyped links to be traced for maintenance.
Some servers use Referer as a means of controlling where they allow
links from (so-called "deep linking"), but legitimate requests do not
always contain a Referer header field.
If the target URI was obtained from a source that does not have its
own URI (e.g., input from the user keyboard), the Referer field MUST
either be sent with the value "about:blank", or not be sent at all.
Note that this requirement does not apply to sources with non-HTTP
URIs (e.g., FTP).
Referer = absolute-URI / partial-URI
Example:
Referer: http://www.example.org/hypertext/Overview.html
If the field value is a relative URI, it SHOULD be interpreted
relative to the effective request URI. The URI MUST NOT include a
fragment. See Section 11.2 for security considerations.
9.16. Retry-After 8.1.3. Retry-After
The header "Retry-After" field can be used with a 503 (Service The header "Retry-After" field can be used with a 503 (Service
Unavailable) response to indicate how long the service is expected to Unavailable) response to indicate how long the service is expected to
be unavailable to the requesting client. This field MAY also be used be unavailable to the requesting client. This field MAY also be used
with any 3xx (Redirection) response to indicate the minimum time the with any 3xx (Redirection) response to indicate the minimum time the
user-agent is asked to wait before issuing the redirected request. user-agent is asked to wait before issuing the redirected request.
The value of this field can be either an HTTP-date or an integer The value of this field can be either an HTTP-date or an integer
number of seconds (in decimal) after the time of the response. number of seconds (in decimal) after the time of the response.
skipping to change at page 66, line 34 skipping to change at page 67, line 14
delta-seconds = 1*DIGIT delta-seconds = 1*DIGIT
Two examples of its use are Two examples of its use are
Retry-After: Fri, 31 Dec 1999 23:59:59 GMT Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
Retry-After: 120 Retry-After: 120
In the latter example, the delay is 2 minutes. In the latter example, the delay is 2 minutes.
9.17. Server 8.2. Selected Representation Header Fields
We use the term "selected representation" to refer to the the current
representation of a target resource that would have been selected in
a successful response if the same request had used the method GET and
excluded any conditional request header fields.
Additional header fields define metadata about the selected
representation, which might differ from the representation included
in the message for responses to some state-changing methods. The
following header fields are defined as selected representation
metadata:
+-------------------+------------------------+
| Header Field Name | Defined in... |
+-------------------+------------------------+
| ETag | Section 2.3 of [Part4] |
| Last-Modified | Section 2.2 of [Part4] |
| Vary | Section 8.2.1 |
+-------------------+------------------------+
8.2.1. Vary
The "Vary" header field conveys the set of header fields that were
used to select the representation.
Caches use this information as part of determining whether a stored
response can be used to satisfy a given request (Section 4.3 of
[Part6]).
In uncacheable or stale responses, the Vary field value advises the
user agent about the criteria that were used to select the
representation.
Vary = "*" / 1#field-name
The set of header fields named by the Vary field value is known as
the selecting header fields.
A server SHOULD include a Vary header field with any cacheable
response that is subject to proactive negotiation. Doing so allows a
cache to properly interpret future requests on that resource and
informs the user agent about the presence of negotiation on that
resource. A server MAY include a Vary header field with a non-
cacheable response that is subject to proactive negotiation, since
this might provide the user agent with useful information about the
dimensions over which the response varies at the time of the
response.
A Vary field value of "*" signals that unspecified parameters not
limited to the header fields (e.g., the network address of the
client), play a role in the selection of the response representation;
therefore, a cache cannot determine whether this response is
appropriate. A proxy MUST NOT generate the "*" value.
The field-names given are not limited to the set of standard header
fields defined by this specification. Field names are case-
insensitive.
8.3. Authentication Challenges
Authentication challenges indicate what mechanisms are available for
the client to provide authentication credentials in future requests.
+--------------------+------------------------+
| Header Field Name | Defined in... |
+--------------------+------------------------+
| WWW-Authenticate | Section 4.4 of [Part7] |
| Proxy-Authenticate | Section 4.2 of [Part7] |
+--------------------+------------------------+
8.4. Informative
The remaining response header fields provide more information about
the target resource for potential use in later requests.
+-------------------+------------------------+
| Header Field Name | Defined in... |
+-------------------+------------------------+
| Accept-Ranges | Section 5.1 of [Part5] |
| Allow | Section 8.4.1 |
| Server | Section 8.4.2 |
+-------------------+------------------------+
8.4.1. Allow
The "Allow" header field lists the set of methods advertised as
supported by the target resource. The purpose of this field is
strictly to inform the recipient of valid request methods associated
with the resource.
Allow = #method
Example of use:
Allow: GET, HEAD, PUT
The actual set of allowed methods is defined by the origin server at
the time of each request.
A proxy MUST NOT modify the Allow header field -- it does not need to
understand all the methods specified in order to handle them
according to the generic message handling rules.
8.4.2. Server
The "Server" header field contains information about the software The "Server" header field contains information about the software
used by the origin server to handle the request. used by the origin server to handle the request.
The field can contain multiple product tokens (Section 5.2) and The field can contain multiple product tokens (Section 4) and
comments (Section 3.2 of [Part1]) identifying the server and any comments (Section 3.2 of [Part1]) identifying the server and any
significant subproducts. The product tokens are listed in order of significant subproducts. The product tokens are listed in order of
their significance for identifying the application. their significance for identifying the application.
Server = product *( RWS ( product / comment ) ) Server = product *( RWS ( product / comment ) )
Example: Example:
Server: CERN/3.0 libwww/2.17 Server: CERN/3.0 libwww/2.17
If the response is being forwarded through a proxy, the proxy If the response is being forwarded through a proxy, the proxy
application MUST NOT modify the Server header field. Instead, it application MUST NOT modify the Server header field. Instead, it
MUST include a Via field (as described in Section 6.2 of [Part1]). MUST include a Via field (as described in Section 5.7 of [Part1]).
Note: Revealing the specific software version of the server might Note: Revealing the specific software version of the server might
allow the server machine to become more vulnerable to attacks allow the server machine to become more vulnerable to attacks
against software that is known to contain security holes. Server against software that is known to contain security holes. Server
implementers are encouraged to make this field a configurable implementers are encouraged to make this field a configurable
option. option.
9.18. User-Agent 9. IANA Considerations
The "User-Agent" header field contains information about the user 9.1. Method Registry
agent originating the request. User agents SHOULD include this field
with requests.
Typically, it is used for statistical purposes, the tracing of The HTTP Method Registry defines the name space for the request
protocol violations, and tailoring responses to avoid particular user method token (Section 5). The method registry is maintained at
agent limitations. <http://www.iana.org/assignments/http-methods>.
The field can contain multiple product tokens (Section 5.2) and 9.1.1. Procedure
comments (Section 3.2 of [Part1]) identifying the agent and its
significant subproducts. By convention, the product tokens are
listed in order of their significance for identifying the
application.
Because this field is usually sent on every request a user agent HTTP method registrations MUST include the following fields:
makes, implementations are encouraged not to include needlessly fine-
grained detail, and to limit (or even prohibit) the addition of
subproducts by third parties. Overly long and detailed User-Agent
field values make requests larger and can also be used to identify
("fingerprint") the user against their wishes.
Likewise, implementations are encouraged not to use the product o Method Name (see Section 5)
tokens of other implementations in order to declare compatibility
with them, as this circumvents the purpose of the field. Finally,
they are encouraged not to use comments to identify products; doing
so makes the field value more difficult to parse.
User-Agent = product *( RWS ( product / comment ) ) o Safe ("yes" or "no", see Section 5.2.1)
Example: o Idempotent ("yes" or "no", see Section 5.2.2)
User-Agent: CERN-LineMode/2.15 libwww/2.17b3 o Pointer to specification text
10. IANA Considerations Values to be added to this name space require IETF Review (see
[RFC5226], Section 4.1).
10.1. Method Registry 9.1.2. Considerations for New Methods
The registration procedure for HTTP request methods is defined by Standardized methods are generic; that is, they are potentially
Section 2.2 of this document. applicable to any resource, not just one particular media type, kind
of resource, or application. As such, it is preferred that new
methods be registered in a document that isn't specific to a single
application or data format, since orthogonal technologies deserve
orthogonal specification.
The HTTP Method Registry shall be created at Since message parsing (Section 3.3 of [Part1]) needs to be
<http://www.iana.org/assignments/http-methods> and be populated with independent of method semantics (aside from responses to HEAD),
the registrations below: definitions of new methods cannot change the parsing algorithm or
prohibit the presence of a message body on either the request or the
response message. Definitions of new methods can specify that only a
zero-length message body is allowed by requiring a Content-Length
header field with a value of "0".
New method definitions need to indicate whether they are safe
(Section 5.2.1), idempotent (Section 5.2.2), cacheable
(Section 5.2.3), and what semantics are to be associated with the
payload body if any is present in the request. If a method is
cacheable, the method definition ought to describe how, and under
what conditions, a cache can store a response and use it to satisfy a
subsequent request.
9.1.3. Registrations
The HTTP Method Registry shall be populated with the registrations
below:
+---------+------+------------+---------------+ +---------+------+------------+---------------+
| Method | Safe | Idempotent | Reference | | Method | Safe | Idempotent | Reference |
+---------+------+------------+---------------+ +---------+------+------------+---------------+
| CONNECT | no | no | Section 2.3.8 | | CONNECT | no | no | Section 5.3.6 |
| DELETE | no | yes | Section 2.3.6 | | DELETE | no | yes | Section 5.3.5 |
| GET | yes | yes | Section 2.3.2 | | GET | yes | yes | Section 5.3.1 |
| HEAD | yes | yes | Section 2.3.3 | | HEAD | yes | yes | Section 5.3.2 |
| OPTIONS | yes | yes | Section 2.3.1 | | OPTIONS | yes | yes | Section 5.3.7 |
| POST | no | no | Section 2.3.4 | | POST | no | no | Section 5.3.3 |
| PUT | no | yes | Section 2.3.5 | | PUT | no | yes | Section 5.3.4 |
| TRACE | yes | yes | Section 2.3.7 | | TRACE | yes | yes | Section 5.3.8 |
+---------+------+------------+---------------+ +---------+------+------------+---------------+
10.2. Status Code Registry 9.2. Status Code Registry
The registration procedure for HTTP Status Codes -- previously The HTTP Status Code Registry defines the name space for the response
defined in Section 7.1 of [RFC2817] -- is now defined by Section 4.2 status-code token (Section 7). The status code registry is
of this document. maintained at <http://www.iana.org/assignments/http-status-codes>.
The HTTP Status Code Registry located at This section replaces the registration procedure for HTTP Status
<http://www.iana.org/assignments/http-status-codes> shall be updated Codes previously defined in Section 7.1 of [RFC2817].
with the registrations below:
9.2.1. Procedure
Values to be added to the HTTP status code name space require IETF
Review (see [RFC5226], Section 4.1).
9.2.2. Considerations for New Status Codes
When it is necessary to express semantics for a response that are not
defined by current status codes, a new status code can be registered.
HTTP status codes are generic; they are potentially applicable to any
resource, not just one particular media type, "type" of resource, or
application. As such, it is preferred that new status codes be
registered in a document that isn't specific to a single application.
New status codes are required to fall under one of the categories
defined in Section 7. To allow existing parsers to properly handle
them, new status codes cannot disallow a payload, although they can
mandate a zero-length payload body.
A definition for a new status code ought to explain the request
conditions that produce a response containing that status code (e.g.,
combinations of request header fields and/or method(s)) along with
any dependencies on response header fields (e.g., what fields are
required and what fields can modify the semantics). A response that
can transfer a payload ought to specify expected cache behavior
(e.g., cacheability and freshness criteria, as described in [Part6])
and whether the payload has any implied association with an
identified resource (Section 3.1.4.1).
9.2.3. Registrations
The HTTP Status Code Regis