HTTP Semantics
draft-ietf-httpbis-semantics-02
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9110.
|
|
|---|---|---|---|
| Authors | Roy T. Fielding , Mark Nottingham , Julian Reschke | ||
| Last updated | 2018-07-02 (Latest revision 2018-05-31) | ||
| Replaces | draft-fielding-httpbis-http-semantics, draft-ietf-httpbis-auth, draft-ietf-httpbis-conditional, draft-ietf-httpbis-range | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
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by Dale Worley
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
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| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 9110 (Internet Standard) | |
| Consensus boilerplate | Unknown | ||
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draft-ietf-httpbis-semantics-02
HTTP Working Group R. Fielding, Ed.
Internet-Draft Adobe
Obsoletes: 7230,7231,7232,7233,7235 (if M. Nottingham, Ed.
approved) Fastly
Intended status: Standards Track J. Reschke, Ed.
Expires: January 3, 2019 greenbytes
July 2, 2018
HTTP Semantics
draft-ietf-httpbis-semantics-02
Abstract
The Hypertext Transfer Protocol (HTTP) is a stateless application-
level protocol for distributed, collaborative, hypertext information
systems. This document defines the semantics of HTTP: its
architecture, terminology, the "http" and "https" Uniform Resource
Identifier (URI) schemes, core request methods, request header
fields, response status codes, response header fields, and content
negotiation.
This document obsoletes RFC 7231, RFC 7232, RFC 7233, RFC 7235, and
portions of RFC 7230.
Editorial Note
This note is to be removed before publishing as an RFC.
Discussion of this draft takes place on the HTTP working group
mailing list (ietf-http-wg@w3.org), which is archived at
<https://lists.w3.org/Archives/Public/ietf-http-wg/>.
Working Group information can be found at <https://httpwg.org/>;
source code and issues list for this draft can be found at
<https://github.com/httpwg/http-core>.
The changes in this draft are summarized in Appendix G.3.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Fielding, et al. Expires January 3, 2019 [Page 1]
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on January 3, 2019.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 9
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 9
2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1. Client/Server Messaging . . . . . . . . . . . . . . . . . 10
2.2. Intermediaries . . . . . . . . . . . . . . . . . . . . . 11
2.3. Caches . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4. Uniform Resource Identifiers . . . . . . . . . . . . . . 14
2.5. Resources . . . . . . . . . . . . . . . . . . . . . . . . 15
2.5.1. http URI Scheme . . . . . . . . . . . . . . . . . . . 16
2.5.2. https URI Scheme . . . . . . . . . . . . . . . . . . 17
2.5.3. http and https URI Normalization and Comparison . . . 18
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3. Conformance . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1. Implementation Diversity . . . . . . . . . . . . . . . . 18
3.2. Role-based Requirements . . . . . . . . . . . . . . . . . 19
3.3. Parsing Elements . . . . . . . . . . . . . . . . . . . . 20
3.4. Error Handling . . . . . . . . . . . . . . . . . . . . . 20
3.5. Protocol Versioning . . . . . . . . . . . . . . . . . . . 21
4. Message Abstraction . . . . . . . . . . . . . . . . . . . . . 22
4.1. Field Names . . . . . . . . . . . . . . . . . . . . . . . 22
4.1.1. Field Name Registry . . . . . . . . . . . . . . . . . 24
4.1.2. Field Extensibility . . . . . . . . . . . . . . . . . 24
4.1.3. Considerations for New Fields . . . . . . . . . . . . 24
4.2. Field Values . . . . . . . . . . . . . . . . . . . . . . 26
4.2.1. Field Order . . . . . . . . . . . . . . . . . . . . . 26
4.2.2. Field Limits . . . . . . . . . . . . . . . . . . . . 27
4.2.3. Field Value Components . . . . . . . . . . . . . . . 27
4.2.4. Designing New Field Values . . . . . . . . . . . . . 28
4.3. Whitespace . . . . . . . . . . . . . . . . . . . . . . . 29
4.4. Trailer . . . . . . . . . . . . . . . . . . . . . . . . . 30
5. Message Routing . . . . . . . . . . . . . . . . . . . . . . . 30
5.1. Identifying a Target Resource . . . . . . . . . . . . . . 30
5.2. Routing Inbound . . . . . . . . . . . . . . . . . . . . . 30
5.3. Effective Request URI . . . . . . . . . . . . . . . . . . 31
5.4. Host . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5.5. Associating a Response to a Request . . . . . . . . . . . 33
5.6. Message Forwarding . . . . . . . . . . . . . . . . . . . 33
5.6.1. Via . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.6.2. Transformations . . . . . . . . . . . . . . . . . . . 35
6. Representations . . . . . . . . . . . . . . . . . . . . . . . 37
6.1. Representation Data . . . . . . . . . . . . . . . . . . . 37
6.1.1. Media Type . . . . . . . . . . . . . . . . . . . . . 37
6.1.2. Content Codings . . . . . . . . . . . . . . . . . . . 40
6.1.3. Language Tags . . . . . . . . . . . . . . . . . . . . 41
6.1.4. Range Units . . . . . . . . . . . . . . . . . . . . . 42
6.2. Representation Metadata . . . . . . . . . . . . . . . . . 45
6.2.1. Content-Type . . . . . . . . . . . . . . . . . . . . 45
6.2.2. Content-Encoding . . . . . . . . . . . . . . . . . . 46
6.2.3. Content-Language . . . . . . . . . . . . . . . . . . 47
6.2.4. Content-Length . . . . . . . . . . . . . . . . . . . 48
6.2.5. Content-Location . . . . . . . . . . . . . . . . . . 49
6.3. Payload . . . . . . . . . . . . . . . . . . . . . . . . . 51
6.3.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . 51
6.3.2. Identification . . . . . . . . . . . . . . . . . . . 52
6.3.3. Content-Range . . . . . . . . . . . . . . . . . . . . 53
6.3.4. Media Type multipart/byteranges . . . . . . . . . . . 55
6.4. Content Negotiation . . . . . . . . . . . . . . . . . . . 57
6.4.1. Proactive Negotiation . . . . . . . . . . . . . . . . 57
6.4.2. Reactive Negotiation . . . . . . . . . . . . . . . . 58
7. Request Methods . . . . . . . . . . . . . . . . . . . . . . . 59
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7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 59
7.2. Common Method Properties . . . . . . . . . . . . . . . . 61
7.2.1. Safe Methods . . . . . . . . . . . . . . . . . . . . 61
7.2.2. Idempotent Methods . . . . . . . . . . . . . . . . . 62
7.2.3. Cacheable Methods . . . . . . . . . . . . . . . . . . 62
7.3. Method Definitions . . . . . . . . . . . . . . . . . . . 63
7.3.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 63
7.3.2. HEAD . . . . . . . . . . . . . . . . . . . . . . . . 63
7.3.3. POST . . . . . . . . . . . . . . . . . . . . . . . . 64
7.3.4. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 65
7.3.5. DELETE . . . . . . . . . . . . . . . . . . . . . . . 68
7.3.6. CONNECT . . . . . . . . . . . . . . . . . . . . . . . 69
7.3.7. OPTIONS . . . . . . . . . . . . . . . . . . . . . . . 70
7.3.8. TRACE . . . . . . . . . . . . . . . . . . . . . . . . 71
7.4. Method Extensibility . . . . . . . . . . . . . . . . . . 72
7.4.1. Method Registry . . . . . . . . . . . . . . . . . . . 72
7.4.2. Considerations for New Methods . . . . . . . . . . . 72
8. Request Header Fields . . . . . . . . . . . . . . . . . . . . 73
8.1. Controls . . . . . . . . . . . . . . . . . . . . . . . . 73
8.1.1. Expect . . . . . . . . . . . . . . . . . . . . . . . 74
8.1.2. Max-Forwards . . . . . . . . . . . . . . . . . . . . 76
8.2. Preconditions . . . . . . . . . . . . . . . . . . . . . . 76
8.2.1. Evaluation . . . . . . . . . . . . . . . . . . . . . 77
8.2.2. Precedence . . . . . . . . . . . . . . . . . . . . . 78
8.2.3. If-Match . . . . . . . . . . . . . . . . . . . . . . 80
8.2.4. If-None-Match . . . . . . . . . . . . . . . . . . . . 81
8.2.5. If-Modified-Since . . . . . . . . . . . . . . . . . . 82
8.2.6. If-Unmodified-Since . . . . . . . . . . . . . . . . . 83
8.2.7. If-Range . . . . . . . . . . . . . . . . . . . . . . 85
8.3. Range . . . . . . . . . . . . . . . . . . . . . . . . . . 86
8.4. Content Negotiation . . . . . . . . . . . . . . . . . . . 87
8.4.1. Quality Values . . . . . . . . . . . . . . . . . . . 88
8.4.2. Accept . . . . . . . . . . . . . . . . . . . . . . . 88
8.4.3. Accept-Charset . . . . . . . . . . . . . . . . . . . 90
8.4.4. Accept-Encoding . . . . . . . . . . . . . . . . . . . 91
8.4.5. Accept-Language . . . . . . . . . . . . . . . . . . . 93
8.5. Authentication Credentials . . . . . . . . . . . . . . . 94
8.5.1. Challenge and Response . . . . . . . . . . . . . . . 94
8.5.2. Protection Space (Realm) . . . . . . . . . . . . . . 96
8.5.3. Authorization . . . . . . . . . . . . . . . . . . . . 97
8.5.4. Proxy-Authorization . . . . . . . . . . . . . . . . . 97
8.5.5. Authentication Scheme Extensibility . . . . . . . . . 98
8.6. Request Context . . . . . . . . . . . . . . . . . . . . . 100
8.6.1. From . . . . . . . . . . . . . . . . . . . . . . . . 100
8.6.2. Referer . . . . . . . . . . . . . . . . . . . . . . . 101
8.6.3. User-Agent . . . . . . . . . . . . . . . . . . . . . 102
9. Response Status Codes . . . . . . . . . . . . . . . . . . . . 103
9.1. Overview of Status Codes . . . . . . . . . . . . . . . . 104
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9.2. Informational 1xx . . . . . . . . . . . . . . . . . . . . 106
9.2.1. 100 Continue . . . . . . . . . . . . . . . . . . . . 106
9.2.2. 101 Switching Protocols . . . . . . . . . . . . . . . 106
9.3. Successful 2xx . . . . . . . . . . . . . . . . . . . . . 107
9.3.1. 200 OK . . . . . . . . . . . . . . . . . . . . . . . 107
9.3.2. 201 Created . . . . . . . . . . . . . . . . . . . . . 108
9.3.3. 202 Accepted . . . . . . . . . . . . . . . . . . . . 108
9.3.4. 203 Non-Authoritative Information . . . . . . . . . . 108
9.3.5. 204 No Content . . . . . . . . . . . . . . . . . . . 109
9.3.6. 205 Reset Content . . . . . . . . . . . . . . . . . . 109
9.3.7. 206 Partial Content . . . . . . . . . . . . . . . . . 110
9.4. Redirection 3xx . . . . . . . . . . . . . . . . . . . . . 113
9.4.1. 300 Multiple Choices . . . . . . . . . . . . . . . . 114
9.4.2. 301 Moved Permanently . . . . . . . . . . . . . . . . 115
9.4.3. 302 Found . . . . . . . . . . . . . . . . . . . . . . 116
9.4.4. 303 See Other . . . . . . . . . . . . . . . . . . . . 116
9.4.5. 304 Not Modified . . . . . . . . . . . . . . . . . . 117
9.4.6. 305 Use Proxy . . . . . . . . . . . . . . . . . . . . 117
9.4.7. 306 (Unused) . . . . . . . . . . . . . . . . . . . . 118
9.4.8. 307 Temporary Redirect . . . . . . . . . . . . . . . 118
9.5. Client Error 4xx . . . . . . . . . . . . . . . . . . . . 118
9.5.1. 400 Bad Request . . . . . . . . . . . . . . . . . . . 118
9.5.2. 401 Unauthorized . . . . . . . . . . . . . . . . . . 118
9.5.3. 402 Payment Required . . . . . . . . . . . . . . . . 119
9.5.4. 403 Forbidden . . . . . . . . . . . . . . . . . . . . 119
9.5.5. 404 Not Found . . . . . . . . . . . . . . . . . . . . 119
9.5.6. 405 Method Not Allowed . . . . . . . . . . . . . . . 120
9.5.7. 406 Not Acceptable . . . . . . . . . . . . . . . . . 120
9.5.8. 407 Proxy Authentication Required . . . . . . . . . . 120
9.5.9. 408 Request Timeout . . . . . . . . . . . . . . . . . 120
9.5.10. 409 Conflict . . . . . . . . . . . . . . . . . . . . 121
9.5.11. 410 Gone . . . . . . . . . . . . . . . . . . . . . . 121
9.5.12. 411 Length Required . . . . . . . . . . . . . . . . . 121
9.5.13. 412 Precondition Failed . . . . . . . . . . . . . . . 122
9.5.14. 413 Payload Too Large . . . . . . . . . . . . . . . . 122
9.5.15. 414 URI Too Long . . . . . . . . . . . . . . . . . . 122
9.5.16. 415 Unsupported Media Type . . . . . . . . . . . . . 122
9.5.17. 416 Range Not Satisfiable . . . . . . . . . . . . . . 123
9.5.18. 417 Expectation Failed . . . . . . . . . . . . . . . 123
9.5.19. 426 Upgrade Required . . . . . . . . . . . . . . . . 123
9.6. Server Error 5xx . . . . . . . . . . . . . . . . . . . . 124
9.6.1. 500 Internal Server Error . . . . . . . . . . . . . . 124
9.6.2. 501 Not Implemented . . . . . . . . . . . . . . . . . 124
9.6.3. 502 Bad Gateway . . . . . . . . . . . . . . . . . . . 124
9.6.4. 503 Service Unavailable . . . . . . . . . . . . . . . 125
9.6.5. 504 Gateway Timeout . . . . . . . . . . . . . . . . . 125
9.6.6. 505 HTTP Version Not Supported . . . . . . . . . . . 125
9.7. Status Code Extensibility . . . . . . . . . . . . . . . . 125
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9.7.1. Status Code Registry . . . . . . . . . . . . . . . . 125
9.7.2. Considerations for New Status Codes . . . . . . . . . 126
10. Response Header Fields . . . . . . . . . . . . . . . . . . . 127
10.1. Control Data . . . . . . . . . . . . . . . . . . . . . . 127
10.1.1. Origination Date . . . . . . . . . . . . . . . . . . 127
10.1.2. Location . . . . . . . . . . . . . . . . . . . . . . 131
10.1.3. Retry-After . . . . . . . . . . . . . . . . . . . . 132
10.1.4. Vary . . . . . . . . . . . . . . . . . . . . . . . . 133
10.2. Validators . . . . . . . . . . . . . . . . . . . . . . . 134
10.2.1. Weak versus Strong . . . . . . . . . . . . . . . . . 135
10.2.2. Last-Modified . . . . . . . . . . . . . . . . . . . 137
10.2.3. ETag . . . . . . . . . . . . . . . . . . . . . . . . 139
10.2.4. When to Use Entity-Tags and Last-Modified Dates . . 142
10.3. Authentication Challenges . . . . . . . . . . . . . . . 143
10.3.1. WWW-Authenticate . . . . . . . . . . . . . . . . . . 143
10.3.2. Proxy-Authenticate . . . . . . . . . . . . . . . . . 144
10.4. Response Context . . . . . . . . . . . . . . . . . . . . 144
10.4.1. Accept-Ranges . . . . . . . . . . . . . . . . . . . 145
10.4.2. Allow . . . . . . . . . . . . . . . . . . . . . . . 145
10.4.3. Server . . . . . . . . . . . . . . . . . . . . . . . 146
11. ABNF List Extension: #rule . . . . . . . . . . . . . . . . . 146
12. Security Considerations . . . . . . . . . . . . . . . . . . . 148
12.1. Establishing Authority . . . . . . . . . . . . . . . . . 148
12.2. Risks of Intermediaries . . . . . . . . . . . . . . . . 149
12.3. Attacks Based on File and Path Names . . . . . . . . . . 149
12.4. Attacks Based on Command, Code, or Query Injection . . . 150
12.5. Attacks via Protocol Element Length . . . . . . . . . . 151
12.6. Disclosure of Personal Information . . . . . . . . . . . 151
12.7. Privacy of Server Log Information . . . . . . . . . . . 151
12.8. Disclosure of Sensitive Information in URIs . . . . . . 152
12.9. Disclosure of Fragment after Redirects . . . . . . . . . 152
12.10. Disclosure of Product Information . . . . . . . . . . . 153
12.11. Browser Fingerprinting . . . . . . . . . . . . . . . . . 153
12.12. Validator Retention . . . . . . . . . . . . . . . . . . 154
12.13. Denial-of-Service Attacks Using Range . . . . . . . . . 154
12.14. Authentication Considerations . . . . . . . . . . . . . 155
12.14.1. Confidentiality of Credentials . . . . . . . . . . 155
12.14.2. Credentials and Idle Clients . . . . . . . . . . . 155
12.14.3. Protection Spaces . . . . . . . . . . . . . . . . . 156
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 156
13.1. URI Scheme Registration . . . . . . . . . . . . . . . . 156
13.2. Method Registration . . . . . . . . . . . . . . . . . . 157
13.3. Status Code Registration . . . . . . . . . . . . . . . . 157
13.4. Header Field Registration . . . . . . . . . . . . . . . 157
13.5. Authentication Scheme Registration . . . . . . . . . . . 157
13.6. Content Coding Registration . . . . . . . . . . . . . . 157
13.7. Range Unit Registration . . . . . . . . . . . . . . . . 157
13.8. Media Type Registration . . . . . . . . . . . . . . . . 157
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14. References . . . . . . . . . . . . . . . . . . . . . . . . . 158
14.1. Normative References . . . . . . . . . . . . . . . . . . 158
14.2. Informative References . . . . . . . . . . . . . . . . . 159
Appendix A. Collected ABNF . . . . . . . . . . . . . . . . . . . 165
Appendix B. Changes from RFC 7230 . . . . . . . . . . . . . . . 169
Appendix C. Changes from RFC 7231 . . . . . . . . . . . . . . . 170
Appendix D. Changes from RFC 7232 . . . . . . . . . . . . . . . 170
Appendix E. Changes from RFC 7233 . . . . . . . . . . . . . . . 170
Appendix F. Changes from RFC 7235 . . . . . . . . . . . . . . . 170
Appendix G. Change Log . . . . . . . . . . . . . . . . . . . . . 170
G.1. Between RFC723x and draft 00 . . . . . . . . . . . . . . 170
G.2. Since draft-ietf-httpbis-semantics-00 . . . . . . . . . . 170
G.3. Since draft-ietf-httpbis-semantics-01 . . . . . . . . . . 171
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 180
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 180
1. Introduction
The Hypertext Transfer Protocol (HTTP) is a stateless application-
level request/response protocol that uses extensible semantics and
self-descriptive messages for flexible interaction with network-based
hypertext information systems. HTTP is defined by a series of
documents that collectively form the HTTP/1.1 specification:
o "HTTP Semantics" (this document)
o "HTTP Caching" [Caching]
o "HTTP/1.1 Messaging" [Messaging]
HTTP is a generic interface protocol for information systems. It is
designed to hide the details of how a service is implemented by
presenting a uniform interface to clients that is independent of the
types of resources provided. Likewise, servers do not need to be
aware of each client's purpose: an HTTP request can be considered in
isolation rather than being associated with a specific type of client
or a predetermined sequence of application steps. The result is a
protocol that can be used effectively in many different contexts and
for which implementations can evolve independently over time.
HTTP is also designed for use as an intermediation protocol for
translating communication to and from non-HTTP information systems.
HTTP proxies and gateways can provide access to alternative
information services by translating their diverse protocols into a
hypertext format that can be viewed and manipulated by clients in the
same way as HTTP services.
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One consequence of this flexibility is that the protocol cannot be
defined in terms of what occurs behind the interface. Instead, we
are limited to defining the syntax of communication, the intent of
received communication, and the expected behavior of recipients. If
the communication is considered in isolation, then successful actions
ought to be reflected in corresponding changes to the observable
interface provided by servers. However, since multiple clients might
act in parallel and perhaps at cross-purposes, we cannot require that
such changes be observable beyond the scope of a single response.
Each HTTP message is either a request or a response. A server
listens on a connection for a request, parses each message received,
interprets the message semantics in relation to the identified
request target, and responds to that request with one or more
response messages. A client constructs request messages to
communicate specific intentions, examines received responses to see
if the intentions were carried out, and determines how to interpret
the results.
HTTP provides a uniform interface for interacting with a resource
(Section 2.5), regardless of its type, nature, or implementation, via
the manipulation and transfer of representations (Section 6).
This document defines semantics that are common to all versions of
HTTP. HTTP semantics include the intentions defined by each request
method (Section 7), extensions to those semantics that might be
described in request header fields (Section 8), the meaning of status
codes to indicate a machine-readable response (Section 9), and the
meaning of other control data and resource metadata that might be
given in response header fields (Section 10).
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
selection algorithms that are collectively referred to as "content
negotiation" (Section 6.4).
This document defines HTTP range requests, partial responses, and the
multipart/byteranges media type.
This document obsoletes the portions of RFC 7230 that are independent
of the HTTP/1.1 messaging syntax and connection management, with the
changes being summarized in Appendix B. The other parts of RFC 7230
are obsoleted by "HTTP/1.1 Messaging" [Messaging]. This document
also obsoletes RFC 7231 (see Appendix C), RFC 7232 (see Appendix D),
RFC 7233 (see Appendix E), and RFC 7235 (see Appendix F).
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1.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Conformance criteria and considerations regarding error handling are
defined in Section 3.
1.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with a list extension, defined in Section 11,
that allows for compact definition of comma-separated lists using a
'#' operator (similar to how the '*' operator indicates repetition).
Appendix A shows the collected grammar with all list operators
expanded to standard ABNF notation.
As a convention, ABNF rule names prefixed with "obs-" denote
"obsolete" grammar rules that appear for historical reasons.
The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF
(line feed), OCTET (any 8-bit sequence of data), SP (space), and
VCHAR (any visible US-ASCII character).
The rules below are defined in [Messaging]:
obs-fold = <obs-fold, see [Messaging], Section 5.2>
protocol-name = <protocol-name, see [Messaging], Section 9.7>
protocol-version = <protocol-version, see [Messaging], Section 9.7>
request-target = <request-target, see [Messaging], Section 3.2>
This specification uses the terms "character", "character encoding
scheme", "charset", and "protocol element" as they are defined in
[RFC6365].
2. Architecture
HTTP was created for the World Wide Web (WWW) architecture and has
evolved over time to support the scalability needs of a worldwide
hypertext system. Much of that architecture is reflected in the
terminology and syntax productions used to define HTTP.
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2.1. Client/Server Messaging
HTTP is a stateless request/response protocol that operates by
exchanging messages (Section 2 of [Messaging]) across a reliable
transport- or session-layer "connection" (Section 9 of [Messaging]).
An HTTP "client" is a program that establishes a connection to a
server for the purpose of sending one or more HTTP requests. An HTTP
"server" is a program that accepts connections in order to service
HTTP requests by sending HTTP responses.
The terms "client" and "server" refer only to the roles that these
programs perform for a particular connection. The same program might
act as a client on some connections and a server on others. The term
"user agent" refers to any of the various client programs that
initiate a request, including (but not limited to) browsers, spiders
(web-based robots), command-line tools, custom applications, and
mobile apps. The term "origin server" refers to the program that can
originate authoritative responses for a given target resource. The
terms "sender" and "recipient" refer to any implementation that sends
or receives a given message, respectively.
HTTP relies upon the Uniform Resource Identifier (URI) standard
[RFC3986] to indicate the target resource (Section 5.1) and
relationships between resources.
Most HTTP communication consists of a retrieval request (GET) for a
representation of some resource identified by a URI. In the simplest
case, this might be accomplished via a single bidirectional
connection (===) between the user agent (UA) and the origin server
(O).
request >
UA ======================================= O
< response
A client sends an HTTP request to a server in the form of a request
message, beginning with a request-line that includes a method, URI,
and protocol version (Section 3 of [Messaging]), followed by header
fields containing request modifiers, client information, and
representation metadata (Section 5 of [Messaging]), an empty line to
indicate the end of the header section, and finally a message body
containing the payload body (if any, Section 6 of [Messaging]).
A server responds to a client's request by sending one or more HTTP
response messages, each beginning with a status line that includes
the protocol version, a success or error code, and textual reason
phrase (Section 4 of [Messaging]), possibly followed by header fields
containing server information, resource metadata, and representation
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metadata (Section 5 of [Messaging]), an empty line to indicate the
end of the header section, and finally a message body containing the
payload body (if any, Section 6 of [Messaging]).
A connection might be used for multiple request/response exchanges,
as defined in Section 9.3 of [Messaging].
The following example illustrates a typical message exchange for a
GET request (Section 7.3.1) on the URI "http://www.example.com/
hello.txt":
Client request:
GET /hello.txt HTTP/1.1
User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
Host: www.example.com
Accept-Language: en, mi
Server response:
HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
Server: Apache
Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
ETag: "34aa387-d-1568eb00"
Accept-Ranges: bytes
Content-Length: 51
Vary: Accept-Encoding
Content-Type: text/plain
Hello World! My payload includes a trailing CRLF.
2.2. Intermediaries
HTTP enables the use of intermediaries to satisfy requests through a
chain of connections. There are three common forms of HTTP
intermediary: proxy, gateway, and tunnel. In some cases, a single
intermediary might act as an origin server, proxy, gateway, or
tunnel, switching behavior based on the nature of each request.
> > > >
UA =========== A =========== B =========== C =========== O
< < < <
The figure above shows three intermediaries (A, B, and C) between the
user agent and origin server. A request or response message that
travels the whole chain will pass through four separate connections.
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Some HTTP communication options might apply only to the connection
with the nearest, non-tunnel neighbor, only to the endpoints of the
chain, or to all connections along the chain. Although the diagram
is linear, each participant might be engaged in multiple,
simultaneous communications. For example, B might be receiving
requests from many clients other than A, and/or forwarding requests
to servers other than C, at the same time that it is handling A's
request. Likewise, later requests might be sent through a different
path of connections, often based on dynamic configuration for load
balancing.
The terms "upstream" and "downstream" are used to describe
directional requirements in relation to the message flow: all
messages flow from upstream to downstream. The terms "inbound" and
"outbound" are used to describe directional requirements in relation
to the request route: "inbound" means toward the origin server and
"outbound" means toward the user agent.
A "proxy" is a message-forwarding agent that is selected by the
client, usually via local configuration rules, to receive requests
for some type(s) of absolute URI and attempt to satisfy those
requests via translation through the HTTP interface. Some
translations are minimal, such as for proxy requests for "http" URIs,
whereas other requests might require translation to and from entirely
different application-level protocols. Proxies are often used to
group an organization's HTTP requests through a common intermediary
for the sake of security, annotation services, or shared caching.
Some proxies are designed to apply transformations to selected
messages or payloads while they are being forwarded, as described in
Section 5.6.2.
A "gateway" (a.k.a. "reverse proxy") is an intermediary that acts as
an origin server for the outbound connection but translates received
requests and forwards them inbound to another server or servers.
Gateways are often used to encapsulate legacy or untrusted
information services, to improve server performance through
"accelerator" caching, and to enable partitioning or load balancing
of HTTP services across multiple machines.
All HTTP requirements applicable to an origin server also apply to
the outbound communication of a gateway. A gateway communicates with
inbound servers using any protocol that it desires, including private
extensions to HTTP that are outside the scope of this specification.
However, an HTTP-to-HTTP gateway that wishes to interoperate with
third-party HTTP servers ought to conform to user agent requirements
on the gateway's inbound connection.
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A "tunnel" acts as a blind relay between two connections without
changing the messages. Once active, a tunnel is not considered a
party to the HTTP communication, though the tunnel might have been
initiated by an HTTP request. A tunnel ceases to exist when both
ends of the relayed connection are closed. Tunnels are used to
extend a virtual connection through an intermediary, such as when
Transport Layer Security (TLS, [RFC5246]) is used to establish
confidential communication through a shared firewall proxy.
The above categories for intermediary only consider those acting as
participants in the HTTP communication. There are also
intermediaries that can act on lower layers of the network protocol
stack, filtering or redirecting HTTP traffic without the knowledge or
permission of message senders. Network intermediaries are
indistinguishable (at a protocol level) from a man-in-the-middle
attack, often introducing security flaws or interoperability problems
due to mistakenly violating HTTP semantics.
For example, an "interception proxy" [RFC3040] (also commonly known
as a "transparent proxy" [RFC1919] or "captive portal") differs from
an HTTP proxy because it is not selected by the client. Instead, an
interception proxy filters or redirects outgoing TCP port 80 packets
(and occasionally other common port traffic). Interception proxies
are commonly found on public network access points, as a means of
enforcing account subscription prior to allowing use of non-local
Internet services, and within corporate firewalls to enforce network
usage policies.
HTTP is defined as a stateless protocol, meaning that each request
message can be understood in isolation. Many implementations depend
on HTTP's stateless design in order to reuse proxied connections or
dynamically load balance requests across multiple servers. Hence, a
server MUST NOT assume that two requests on the same connection are
from the same user agent unless the connection is secured and
specific to that agent. Some non-standard HTTP extensions (e.g.,
[RFC4559]) have been known to violate this requirement, resulting in
security and interoperability problems.
2.3. Caches
A "cache" is a local store of previous response messages and the
subsystem that controls its message storage, retrieval, and deletion.
A cache stores cacheable responses in order to reduce the response
time and network bandwidth consumption on future, equivalent
requests. Any client or server MAY employ a cache, though a cache
cannot be used by a server while it is acting as a tunnel.
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The effect of a cache is that the request/response chain is shortened
if one of the participants along the chain has a cached response
applicable to that request. The following illustrates the resulting
chain if B has a cached copy of an earlier response from O (via C)
for a request that has not been cached by UA or A.
> >
UA =========== A =========== B - - - - - - C - - - - - - O
< <
A response is "cacheable" if a cache is allowed to store a copy of
the response message for use in answering subsequent requests. Even
when a response is cacheable, there might be additional constraints
placed by the client or by the origin server on when that cached
response can be used for a particular request. HTTP requirements for
cache behavior and cacheable responses are defined in Section 2 of
[Caching].
There is a wide variety of architectures and configurations of caches
deployed across the World Wide Web and inside large organizations.
These include national hierarchies of proxy caches to save
transoceanic bandwidth, collaborative systems that broadcast or
multicast cache entries, archives of pre-fetched cache entries for
use in off-line or high-latency environments, and so on.
2.4. Uniform Resource Identifiers
Uniform Resource Identifiers (URIs) [RFC3986] are used throughout
HTTP as the means for identifying resources (Section 2.5). URI
references are used to target requests, indicate redirects, and
define relationships.
The definitions of "URI-reference", "absolute-URI", "relative-part",
"authority", "port", "host", "path-abempty", "segment", "query", and
"fragment" are adopted from the URI generic syntax. An "absolute-
path" rule is defined for protocol elements that can contain a non-
empty path component. (This rule differs slightly from the path-
abempty rule of RFC 3986, which allows for an empty path to be used
in references, and path-absolute rule, which does not allow paths
that begin with "//".) A "partial-URI" rule is defined for protocol
elements that can contain a relative URI but not a fragment
component.
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URI-reference = <URI-reference, see [RFC3986], Section 4.1>
absolute-URI = <absolute-URI, see [RFC3986], Section 4.3>
relative-part = <relative-part, see [RFC3986], Section 4.2>
authority = <authority, see [RFC3986], Section 3.2>
uri-host = <host, see [RFC3986], Section 3.2.2>
port = <port, see [RFC3986], Section 3.2.3>
path-abempty = <path-abempty, see [RFC3986], Section 3.3>
segment = <segment, see [RFC3986], Section 3.3>
query = <query, see [RFC3986], Section 3.4>
fragment = <fragment, see [RFC3986], Section 3.5>
absolute-path = 1*( "/" segment )
partial-URI = relative-part [ "?" query ]
Each protocol element in HTTP that allows a URI reference will
indicate in its ABNF production whether the element allows any form
of reference (URI-reference), only a URI in absolute form (absolute-
URI), only the path and optional query components, or some
combination of the above. Unless otherwise indicated, URI references
are parsed relative to the effective request URI (Section 5.3).
2.5. Resources
The target of an HTTP request is called a "resource". HTTP does not
limit the nature of a resource; it merely defines an interface that
might be used to interact with resources. Each resource is
identified by a Uniform Resource Identifier (URI), as described in
Section 2.4.
One design goal of HTTP is to separate resource identification from
request semantics, which is made possible by vesting the request
semantics in the request method (Section 7) and a few request-
modifying header fields (Section 8). If there is a conflict between
the method semantics and any semantic implied by the URI itself, as
described in Section 7.2.1, the method semantics take precedence.
IANA maintains the registry of URI Schemes [BCP35] at
<https://www.iana.org/assignments/uri-schemes/>. Although requests
might target any URI scheme, the following schemes are inherent to
HTTP servers:
+------------+------------------------------------+---------------+
| URI Scheme | Description | Reference |
+------------+------------------------------------+---------------+
| http | Hypertext Transfer Protocol | Section 2.5.1 |
| https | Hypertext Transfer Protocol Secure | Section 2.5.2 |
+------------+------------------------------------+---------------+
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2.5.1. http URI Scheme
The "http" URI scheme is hereby defined for the purpose of minting
identifiers according to their association with the hierarchical
namespace governed by a potential HTTP origin server listening for
TCP ([RFC0793]) connections on a given port.
http-URI = "http:" "//" authority path-abempty [ "?" query ]
[ "#" fragment ]
The origin server for an "http" URI is identified by the authority
component, which includes a host identifier and optional TCP port
([RFC3986], Section 3.2.2). The hierarchical path component and
optional query component serve as an identifier for a potential
target resource within that origin server's name space. The optional
fragment component allows for indirect identification of a secondary
resource, independent of the URI scheme, as defined in Section 3.5 of
[RFC3986].
A sender MUST NOT generate an "http" URI with an empty host
identifier. A recipient that processes such a URI reference MUST
reject it as invalid.
If the host identifier is provided as an IP address, the origin
server is the listener (if any) on the indicated TCP port at that IP
address. If host is a registered name, the registered name is an
indirect identifier for use with a name resolution service, such as
DNS, to find an address for that origin server. If the port
subcomponent is empty or not given, TCP port 80 (the reserved port
for WWW services) is the default.
Note that the presence of a URI with a given authority component does
not imply that there is always an HTTP server listening for
connections on that host and port. Anyone can mint a URI. What the
authority component determines is who has the right to respond
authoritatively to requests that target the identified resource. The
delegated nature of registered names and IP addresses creates a
federated namespace, based on control over the indicated host and
port, whether or not an HTTP server is present. See Section 12.1 for
security considerations related to establishing authority.
When an "http" URI is used within a context that calls for access to
the indicated resource, a client MAY attempt access by resolving the
host to an IP address, establishing a TCP connection to that address
on the indicated port, and sending an HTTP request message (Section 2
of [Messaging]) containing the URI's identifying data to the server.
If the server responds to that request with a non-interim HTTP
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response message, as described in Section 9, then that response is
considered an authoritative answer to the client's request.
Although HTTP is independent of the transport protocol, the "http"
scheme is specific to TCP-based services because the name delegation
process depends on TCP for establishing authority. An HTTP service
based on some other underlying connection protocol would presumably
be identified using a different URI scheme, just as the "https"
scheme (below) is used for resources that require an end-to-end
secured connection. Other protocols might also be used to provide
access to "http" identified resources -- it is only the authoritative
interface that is specific to TCP.
The URI generic syntax for authority also includes a deprecated
userinfo subcomponent ([RFC3986], Section 3.2.1) for including user
authentication information in the URI. Some implementations make use
of the userinfo component for internal configuration of
authentication information, such as within command invocation
options, configuration files, or bookmark lists, even though such
usage might expose a user identifier or password. A sender MUST NOT
generate the userinfo subcomponent (and its "@" delimiter) when an
"http" URI reference is generated within a message as a request
target or header field value. Before making use of an "http" URI
reference received from an untrusted source, a recipient SHOULD parse
for userinfo and treat its presence as an error; it is likely being
used to obscure the authority for the sake of phishing attacks.
2.5.2. https URI Scheme
The "https" URI scheme is hereby defined for the purpose of minting
identifiers according to their association with the hierarchical
namespace governed by a potential HTTP origin server listening to a
given TCP port for TLS-secured connections ([RFC5246]).
All of the requirements listed above for the "http" scheme are also
requirements for the "https" scheme, except that TCP port 443 is the
default if the port subcomponent is empty or not given, and the user
agent MUST ensure that its connection to the origin server is secured
through the use of strong encryption, end-to-end, prior to sending
the first HTTP request.
https-URI = "https:" "//" authority path-abempty [ "?" query ]
[ "#" fragment ]
Note that the "https" URI scheme depends on both TLS and TCP for
establishing authority. Resources made available via the "https"
scheme have no shared identity with the "http" scheme even if their
resource identifiers indicate the same authority (the same host
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listening to the same TCP port). They are distinct namespaces and
are considered to be distinct origin servers. However, an extension
to HTTP that is defined to apply to entire host domains, such as the
Cookie protocol [RFC6265], can allow information set by one service
to impact communication with other services within a matching group
of host domains.
The process for authoritative access to an "https" identified
resource is defined in [RFC2818].
2.5.3. http and https URI Normalization and Comparison
Since the "http" and "https" schemes conform to the URI generic
syntax, such URIs are normalized and compared according to the
algorithm defined in Section 6 of [RFC3986], using the defaults
described above for each scheme.
If the port is equal to the default port for a scheme, the normal
form is to omit the port subcomponent. When not being used in
absolute form as the request target of an OPTIONS request, an empty
path component is equivalent to an absolute path of "/", so the
normal form is to provide a path of "/" instead. The scheme and host
are case-insensitive and normally provided in lowercase; all other
components are compared in a case-sensitive manner. Characters other
than those in the "reserved" set are equivalent to their percent-
encoded octets: the normal form is to not encode them (see Sections
2.1 and 2.2 of [RFC3986]).
For example, the following three URIs are equivalent:
http://example.com:80/~smith/home.html
http://EXAMPLE.com/%7Esmith/home.html
http://EXAMPLE.com:/%7esmith/home.html
3. Conformance
3.1. Implementation Diversity
When considering the design of HTTP, it is easy to fall into a trap
of thinking that all user agents are general-purpose browsers and all
origin servers are large public websites. That is not the case in
practice. Common HTTP user agents include household appliances,
stereos, scales, firmware update scripts, command-line programs,
mobile apps, and communication devices in a multitude of shapes and
sizes. Likewise, common HTTP origin servers include home automation
units, configurable networking components, office machines,
autonomous robots, news feeds, traffic cameras, ad selectors, and
video-delivery platforms.
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The term "user agent" does not imply that there is a human user
directly interacting with the software agent at the time of a
request. In many cases, a user agent is installed or configured to
run in the background and save its results for later inspection (or
save only a subset of those results that might be interesting or
erroneous). Spiders, for example, are typically given a start URI
and configured to follow certain behavior while crawling the Web as a
hypertext graph.
The implementation diversity of HTTP means that not all user agents
can make interactive suggestions to their user or provide adequate
warning for security or privacy concerns. In the few cases where
this specification requires reporting of errors to the user, it is
acceptable for such reporting to only be observable in an error
console or log file. Likewise, requirements that an automated action
be confirmed by the user before proceeding might be met via advance
configuration choices, run-time options, or simple avoidance of the
unsafe action; confirmation does not imply any specific user
interface or interruption of normal processing if the user has
already made that choice.
3.2. Role-based Requirements
This specification targets conformance criteria according to the role
of a participant in HTTP communication. Hence, HTTP requirements are
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.
Additional (social) requirements are placed on implementations,
resource owners, and protocol element registrations when they apply
beyond the scope of a single communication.
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.
Conformance includes both the syntax and semantics of protocol
elements. A sender MUST NOT generate protocol elements that convey a
meaning that is known by that sender to be false. A sender MUST NOT
generate protocol elements that do not match the grammar defined by
the corresponding ABNF rules. Within a given message, a sender MUST
NOT generate protocol elements or syntax alternatives that are only
allowed to be generated by participants in other roles (i.e., a role
that the sender does not have for that message).
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3.3. Parsing Elements
When a received protocol element is parsed, the recipient MUST be
able to parse any value of reasonable length that is applicable to
the recipient's role and that matches the grammar defined by the
corresponding ABNF rules. Note, however, that some received protocol
elements might not be parsed. For example, an intermediary
forwarding a message might parse a header-field into generic field-
name and field-value components, but then forward the header field
without further parsing inside the field-value.
HTTP does not have specific length limitations for many of its
protocol elements because the lengths that might be appropriate will
vary widely, depending on the deployment context and purpose of the
implementation. Hence, interoperability between senders and
recipients depends on shared expectations regarding what is a
reasonable length for each protocol element. Furthermore, what is
commonly understood to be a reasonable length for some protocol
elements has changed over the course of the past two decades of HTTP
use and is expected to continue changing in the future.
At a minimum, a recipient MUST be able to parse and process protocol
element lengths that are at least as long as the values that it
generates for those same protocol elements in other messages. For
example, an origin server that publishes very long URI references to
its own resources needs to be able to parse and process those same
references when received as a request target.
3.4. Error Handling
A recipient MUST interpret a received protocol element according to
the semantics defined for it by this specification, including
extensions to this specification, unless the recipient has determined
(through experience or configuration) that the sender incorrectly
implements what is implied by those semantics. For example, an
origin server might disregard the contents of a received Accept-
Encoding header field if inspection of the User-Agent header field
indicates a specific implementation version that is known to fail on
receipt of certain content codings.
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
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systems control client might consider any form of error recovery to
be dangerous.
3.5. Protocol Versioning
The HTTP version number consists of two decimal digits separated by a
"." (period or decimal point). The first digit ("major version")
indicates the HTTP messaging syntax, whereas the second digit ("minor
version") indicates the highest minor version within that major
version to which the sender is conformant and able to understand for
future communication.
The protocol version as a whole indicates the sender's conformance
with the set of requirements laid out in that version's corresponding
specification of HTTP. For example, the version "HTTP/1.1" is
defined by the combined specifications of this document, "HTTP
Caching" [Caching], and "HTTP/1.1 Messaging" [Messaging].
The minor version advertises the sender's communication capabilities
even when the sender is only using a backwards-compatible subset of
the protocol, thereby letting the recipient know that more advanced
features can be used in response (by servers) or in future requests
(by clients).
A client SHOULD send a request version equal to the highest version
to which the client is conformant and whose major version is no
higher than the highest version supported by the server, if this is
known. A client MUST NOT send a version to which it is not
conformant.
A client MAY send a lower request version if it is known that the
server incorrectly implements the HTTP specification, but only after
the client has attempted at least one normal request and determined
from the response status code or header fields (e.g., Server) that
the server improperly handles higher request versions.
A server SHOULD send a response version equal to the highest version
to which the server is conformant that has a major version less than
or equal to the one received in the request. A server MUST NOT send
a version to which it is not conformant. A server can send a 505
(HTTP Version Not Supported) response if it wishes, for any reason,
to refuse service of the client's major protocol version.
HTTP's major version number is incremented when an incompatible
message syntax is introduced. The minor number is incremented when
changes made to the protocol have the effect of adding to the message
semantics or implying additional capabilities of the sender.
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When an HTTP message is received with a major version number that the
recipient implements, but a higher minor version number than what the
recipient implements, the recipient SHOULD process the message as if
it were in the highest minor version within that major version to
which the recipient is conformant. A recipient can assume that a
message with a higher minor version, when sent to a recipient that
has not yet indicated support for that higher version, is
sufficiently backwards-compatible to be safely processed by any
implementation of the same major version.
[[CREF1: When a major version of HTTP does not define any minor
versions, the minor version "0" is implied and ought to be used when
referring to that protocol within a protocol element that requires
sending a minor version. ]]
4. Message Abstraction
Each major version of HTTP defines its own syntax for the inclusion
of information in messages. Nevertheless, a common abstraction is
that a message includes some form of envelope/framing, a potential
set of named data fields, and a potential body. This section defines
the abstraction for message fields as field-name and field-value
pairs.
4.1. Field Names
Header fields are key:value pairs that can be used to communicate
data about the message, its payload, the target resource, or the
connection (i.e., control data).
The requirements for header field names are defined in [BCP90].
The field-name token labels the corresponding field-value as having
the semantics defined by that header field. For example, the Date
header field is defined in Section 10.1.1.2 as containing the
origination timestamp for the message in which it appears.
field-name = token
The interpretation of a header field does not change between minor
versions of the same major HTTP version, though the default behavior
of a recipient in the absence of such a field can change. Unless
specified otherwise, header fields are defined for all versions of
HTTP. In particular, the Host and Connection header fields ought to
be implemented by all HTTP/1.x implementations whether or not they
advertise conformance with HTTP/1.1.
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New header fields can be introduced without changing the protocol
version if their defined semantics allow them to be safely ignored by
recipients that do not recognize them. Header field extensibility is
discussed in Section 4.1.2.
The following field names are defined by this document:
+---------------------+----------+----------+-------------------+
| Header Field Name | Protocol | Status | Reference |
+---------------------+----------+----------+-------------------+
| Accept | http | standard | Section 8.4.2 |
| Accept-Charset | http | standard | Section 8.4.3 |
| Accept-Encoding | http | standard | Section 8.4.4 |
| Accept-Language | http | standard | Section 8.4.5 |
| Accept-Ranges | http | standard | Section 10.4.1 |
| Allow | http | standard | Section 10.4.2 |
| Authorization | http | standard | Section 8.5.3 |
| Content-Encoding | http | standard | Section 6.2.2 |
| Content-Language | http | standard | Section 6.2.3 |
| Content-Length | http | standard | Section 6.2.4 |
| Content-Location | http | standard | Section 6.2.5 |
| Content-Range | http | standard | Section 6.3.3 |
| Content-Type | http | standard | Section 6.2.1 |
| Date | http | standard | Section 10.1.1.2 |
| ETag | http | standard | Section 10.2.3 |
| Expect | http | standard | Section 8.1.1 |
| From | http | standard | Section 8.6.1 |
| Host | http | standard | Section 5.4 |
| If-Match | http | standard | Section 8.2.3 |
| If-Modified-Since | http | standard | Section 8.2.5 |
| If-None-Match | http | standard | Section 8.2.4 |
| If-Range | http | standard | Section 8.2.7 |
| If-Unmodified-Since | http | standard | Section 8.2.6 |
| Last-Modified | http | standard | Section 10.2.2 |
| Location | http | standard | Section 10.1.2 |
| Max-Forwards | http | standard | Section 8.1.2 |
| Proxy-Authenticate | http | standard | Section 10.3.2 |
| Proxy-Authorization | http | standard | Section 8.5.4 |
| Range | http | standard | Section 8.3 |
| Referer | http | standard | Section 8.6.2 |
| Retry-After | http | standard | Section 10.1.3 |
| Server | http | standard | Section 10.4.3 |
| Trailer | http | standard | Section 4.4 |
| User-Agent | http | standard | Section 8.6.3 |
| Vary | http | standard | Section 10.1.4 |
| Via | http | standard | Section 5.6.1 |
| WWW-Authenticate | http | standard | Section 10.3.1 |
+---------------------+----------+----------+-------------------+
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4.1.1. Field Name Registry
HTTP header fields are registered within the "Message Headers"
registry located at <https://www.iana.org/assignments/message-
headers>, as defined by [BCP90], with the protocol "http".
4.1.2. Field Extensibility
Header fields are fully extensible: there is no limit on the
introduction of new field names, each presumably defining new
semantics, nor on the number of header fields used in a given
message. Existing fields are defined in each part of this
specification and in many other specifications outside this document
set.
New header fields can be defined such that, when they are understood
by a recipient, they might override or enhance the interpretation of
previously defined header fields, define preconditions on request
evaluation, or refine the meaning of responses.
A proxy MUST forward unrecognized header fields unless the field-name
is listed in the Connection header field (Section 9.1 of [Messaging])
or the proxy is specifically configured to block, or otherwise
transform, such fields. Other recipients SHOULD ignore unrecognized
header fields. These requirements allow HTTP's functionality to be
enhanced without requiring prior update of deployed intermediaries.
All defined header fields ought to be registered with IANA in the
"Message Headers" registry.
4.1.3. Considerations for New Fields
Authors of specifications defining new fields are advised to keep the
name as short as practical and not to prefix the name with "X-"
unless the header field will never be used on the Internet. (The
"X-" prefix idiom has been extensively misused in practice; it was
intended to only be used as a mechanism for avoiding name collisions
inside proprietary software or intranet processing, since the prefix
would ensure that private names never collide with a newly registered
Internet name; see [BCP178] for further information).
Authors of specifications defining new header fields are advised to
consider documenting:
o Whether the field is a single value or whether it can be a list
(delimited by commas; see Section 5 of [Messaging]).
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If it does not use the list syntax, document how to treat messages
where the field occurs multiple times (a sensible default would be
to ignore the field, but this might not always be the right
choice).
Note that intermediaries and software libraries might combine
multiple header field instances into a single one, despite the
field's definition not allowing the list syntax. 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
responses or requests, in all messages, only on responses to a
particular request method, etc.
o Whether the field should be stored by origin servers that
understand it upon a PUT request.
o Whether the field semantics are further refined by the context,
such as by existing request methods or status codes.
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
Section 9.1 of [Messaging]).
o Under what conditions intermediaries are allowed to insert,
delete, or modify the field's value.
o Whether it is appropriate to list the field-name in a Vary
response header field (e.g., when the request header field is used
by an origin server's content selection algorithm; see
Section 10.1.4).
o Whether the header field is useful or allowable in trailers (see
Section 7.1 of [Messaging]).
o Whether the header field ought to be preserved across redirects.
o Whether it introduces any additional security considerations, such
as disclosure of privacy-related data.
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4.2. Field Values
This specification does not use ABNF rules to define each "Field-
Name: Field Value" pair, as was done in earlier editions. Instead,
this specification uses ABNF rules that are named according to each
registered field name, wherein the rule defines the valid grammar for
that field's corresponding field values (i.e., after the field-value
has been extracted by a generic field parser).
field-value = *( field-content / obs-fold )
field-content = field-vchar [ 1*( SP / HTAB ) field-vchar ]
field-vchar = VCHAR / obs-text
Historically, HTTP header field values could be extended over
multiple lines by preceding each extra line with at least one space
or horizontal tab (obs-fold). [[CREF2: This document assumes that
any such obs-fold has been replaced with one or more SP octets prior
to interpreting the field value, as described in Section 5.2 of
[Messaging].]]
Historically, HTTP has allowed field content with text in the
ISO-8859-1 charset [ISO-8859-1], supporting other charsets only
through use of [RFC2047] encoding. In practice, most HTTP header
field values use only a subset of the US-ASCII charset [USASCII].
Newly defined header fields SHOULD limit their field values to
US-ASCII octets. A recipient SHOULD treat other octets in field
content (obs-text) as opaque data.
4.2.1. Field Order
The order in which header fields with differing field names are
received is not significant. However, it is good practice to send
header fields that contain control data first, such as Host on
requests and Date on responses, so that implementations can decide
when not to handle a message as early as possible. A server MUST NOT
apply a request to the target resource until the entire request
header section is received, since later header fields might include
conditionals, authentication credentials, or deliberately misleading
duplicate header fields that would impact request processing.
A sender MUST NOT generate multiple header fields with the same field
name in a message unless either the entire field value for that
header field is defined as a comma-separated list [i.e., #(values)]
or the header field is a well-known exception (as noted below).
A recipient MAY combine multiple header fields with the same field
name into one "field-name: field-value" pair, without changing the
semantics of the message, by appending each subsequent field value to
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the combined field value in order, separated by a comma. The order
in which header fields with the same field name are received is
therefore significant to the interpretation of the combined field
value; a proxy MUST NOT change the order of these field values when
forwarding a message.
Note: In practice, the "Set-Cookie" header field ([RFC6265]) often
appears multiple times in a response message and does not use the
list syntax, violating the above requirements on multiple header
fields with the same name. Since it cannot be combined into a
single field-value, recipients ought to handle "Set-Cookie" as a
special case while processing header fields. (See Appendix A.2.3
of [Kri2001] for details.)
4.2.2. Field Limits
HTTP does not place a predefined limit on the length of each header
field or on the length of the header section as a whole, as described
in Section 3. Various ad hoc limitations on individual header field
length are found in practice, often depending on the specific field
semantics.
A server that receives a request header field, or set of fields,
larger than it wishes to process MUST respond with an appropriate 4xx
(Client Error) status code. Ignoring such header fields would
increase the server's vulnerability to request smuggling attacks
(Section 11.2 of [Messaging]).
A client MAY discard or truncate received header fields that are
larger than the client wishes to process if the field semantics are
such that the dropped value(s) can be safely ignored without changing
the message framing or response semantics.
4.2.3. Field Value Components
Most HTTP header field values are defined using common syntax
components (token, quoted-string, and comment) separated by
whitespace or specific delimiting characters. Delimiters are chosen
from the set of US-ASCII visual characters not allowed in a token
(DQUOTE and "(),/:;<=>?@[\]{}").
token = 1*tchar
tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*"
/ "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
/ DIGIT / ALPHA
; any VCHAR, except delimiters
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A string of text is parsed as a single value if it is quoted using
double-quote marks.
quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
qdtext = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text
obs-text = %x80-FF
Comments can be included in some HTTP header fields by surrounding
the comment text with parentheses. Comments are only allowed in
fields containing "comment" as part of their field value definition.
comment = "(" *( ctext / quoted-pair / comment ) ")"
ctext = HTAB / SP / %x21-27 / %x2A-5B / %x5D-7E / obs-text
The backslash octet ("\") can be used as a single-octet quoting
mechanism within quoted-string and comment constructs. Recipients
that process the value of a quoted-string MUST handle a quoted-pair
as if it were replaced by the octet following the backslash.
quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
A sender SHOULD NOT generate a quoted-pair in a quoted-string except
where necessary to quote DQUOTE and backslash octets occurring within
that string. A sender SHOULD NOT generate a quoted-pair in a comment
except where necessary to quote parentheses ["(" and ")"] and
backslash octets occurring within that comment.
4.2.4. Designing New Field Values
New header field values typically have their syntax defined using
ABNF ([RFC5234]), using the extension defined in Section 11 as
necessary, and are usually constrained to the range of US-ASCII
characters. Header fields needing a greater range of characters can
use an encoding such as the one defined in [RFC8187].
Leading and trailing whitespace in raw field values is removed upon
field parsing (Section 5.1 of [Messaging]). Field definitions where
leading or trailing whitespace in values is significant will have to
use a container syntax such as quoted-string (Section 4.2.3).
Because commas (",") are used as a generic delimiter between field-
values, they need to be treated with care if they are allowed in the
field-value. Typically, components that might contain a comma are
protected with double-quotes using the quoted-string ABNF production.
For example, a textual date and a URI (either of which might contain
a comma) could be safely carried in field-values like these:
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Example-URI-Field: "http://example.com/a.html,foo",
"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
quoted-string production; using a different syntax inside double-
quotes will likely cause unnecessary confusion.
Many header fields use a format including (case-insensitively) named
parameters (for instance, Content-Type, defined in Section 6.2.1).
Allowing both unquoted (token) and quoted (quoted-string) syntax for
the parameter value enables recipients to use existing parser
components. When allowing both forms, the meaning of a parameter
value ought to be independent of the syntax used for it (for an
example, see the notes on parameter handling for media types in
Section 6.1.1).
4.3. Whitespace
This specification uses three rules to denote the use of linear
whitespace: OWS (optional whitespace), RWS (required whitespace), and
BWS ("bad" whitespace).
The OWS rule is used where zero or more linear whitespace octets
might appear. For protocol elements where optional whitespace is
preferred to improve readability, a sender SHOULD generate the
optional whitespace as a single SP; otherwise, a sender SHOULD NOT
generate optional whitespace except as needed to white out invalid or
unwanted protocol elements during in-place message filtering.
The RWS rule is used when at least one linear whitespace octet is
required to separate field tokens. A sender SHOULD generate RWS as a
single SP.
The BWS rule is used where the grammar allows optional whitespace
only for historical reasons. A sender MUST NOT generate BWS in
messages. A recipient MUST parse for such bad whitespace and remove
it before interpreting the protocol element.
OWS = *( SP / HTAB )
; optional whitespace
RWS = 1*( SP / HTAB )
; required whitespace
BWS = OWS
; "bad" whitespace
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4.4. Trailer
[[CREF3: The "Trailer" header field in a message indicates fields
that the sender anticipates sending after the message header block
(i.e., during or after the payload is sent). This is typically used
to supply metadata that might be dynamically generated while the data
is sent, such as a message integrity check, digital signature, or
post-processing status. ]]
Trailer = 1#field-name
[[CREF4: How, where, and when trailer fields might be sent depends on
both the protocol in use (HTTP version and/or transfer coding) and
the semantics of each named header field. Many header fields cannot
be processed outside the header section because their evaluation is
necessary for message routing, authentication, or configuration prior
to receiving the representation data. ]]
5. Message Routing
HTTP request message routing is determined by each client based on
the target resource, the client's proxy configuration, and
establishment or reuse of an inbound connection. The corresponding
response routing follows the same connection chain back to the
client.
5.1. Identifying a Target Resource
HTTP is used in a wide variety of applications, ranging from general-
purpose computers to home appliances. In some cases, communication
options are hard-coded in a client's configuration. However, most
HTTP clients rely on the same resource identification mechanism and
configuration techniques as general-purpose Web browsers.
HTTP communication is initiated by a user agent for some purpose.
The purpose is a combination of request semantics and a target
resource upon which to apply those semantics. A URI reference
(Section 2.4) is typically used as an identifier for the "target
resource", which a user agent would resolve to its absolute form in
order to obtain the "target URI". The target URI excludes the
reference's fragment component, if any, since fragment identifiers
are reserved for client-side processing ([RFC3986], Section 3.5).
5.2. Routing Inbound
Once the target URI is determined, a client needs to decide whether a
network request is necessary to accomplish the desired semantics and,
if so, where that request is to be directed.
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If the client has a cache [Caching] and the request can be satisfied
by it, then the request is usually directed there first.
If the request is not satisfied by a cache, then a typical client
will check its configuration to determine whether a proxy is to be
used to satisfy the request. Proxy configuration is implementation-
dependent, but is often based on URI prefix matching, selective
authority matching, or both, and the proxy itself is usually
identified by an "http" or "https" URI. If a proxy is applicable,
the client connects inbound by establishing (or reusing) a connection
to that proxy.
If no proxy is applicable, a typical client will invoke a handler
routine, usually specific to the target URI's scheme, to connect
directly to an authority for the target resource. How that is
accomplished is dependent on the target URI scheme and defined by its
associated specification, similar to how this specification defines
origin server access for resolution of the "http" (Section 2.5.1) and
"https" (Section 2.5.2) schemes.
HTTP requirements regarding connection management are defined in
Section 9 of [Messaging].
5.3. Effective Request URI
Once an inbound connection is obtained, the client sends an HTTP
request message (Section 2 of [Messaging]).
Depending on the nature of the request, the client's target URI might
be split into components and transmitted (or implied) within various
parts of a request message. These parts are recombined by each
recipient, in accordance with their local configuration and incoming
connection context, to form an "effective request URI" for
identifying the intended target resource with respect to that server.
Section 3.3 of [Messaging] defines how a server determines the
effective request URI for an HTTP/1.1 request.
For a user agent, the effective request URI is the target URI.
Once the effective request URI has been constructed, an origin server
needs to decide whether or not to provide service for that URI via
the connection in which the request was received. For example, the
request might have been misdirected, deliberately or accidentally,
such that the information within a received request-target or Host
header field differs from the host or port upon which the connection
has been made. If the connection is from a trusted gateway, that
inconsistency might be expected; otherwise, it might indicate an
attempt to bypass security filters, trick the server into delivering
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non-public content, or poison a cache. See Section 12 for security
considerations regarding message routing.
5.4. Host
The "Host" header field in a request provides the host and port
information from the target URI, enabling the origin server to
distinguish among resources while servicing requests for multiple
host names on a single IP address.
Host = uri-host [ ":" port ] ; Section 2.4
A client MUST send a Host header field in all HTTP/1.1 request
messages. If the target URI includes an authority component, then a
client MUST send a field-value for Host that is identical to that
authority component, excluding any userinfo subcomponent and its "@"
delimiter (Section 2.5.1). If the authority component is missing or
undefined for the target URI, then a client MUST send a Host header
field with an empty field-value.
Since the Host field-value is critical information for handling a
request, a user agent SHOULD generate Host as the first header field
following the request-line.
For example, a GET request to the origin server for
<http://www.example.org/pub/WWW/> would begin with:
GET /pub/WWW/ HTTP/1.1
Host: www.example.org
A client MUST send a Host header field in an HTTP/1.1 request even if
the request-target is in the absolute-form, since this allows the
Host information to be forwarded through ancient HTTP/1.0 proxies
that might not have implemented Host.
When a proxy receives a request with an absolute-form of request-
target, the proxy MUST ignore the received Host header field (if any)
and instead replace it with the host information of the request-
target. A proxy that forwards such a request MUST generate a new
Host field-value based on the received request-target rather than
forward the received Host field-value.
Since the Host header field acts as an application-level routing
mechanism, it is a frequent target for malware seeking to poison a
shared cache or redirect a request to an unintended server. An
interception proxy is particularly vulnerable if it relies on the
Host field-value for redirecting requests to internal servers, or for
use as a cache key in a shared cache, without first verifying that
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the intercepted connection is targeting a valid IP address for that
host.
A server MUST respond with a 400 (Bad Request) status code to any
HTTP/1.1 request message that lacks a Host header field and to any
request message that contains more than one Host header field or a
Host header field with an invalid field-value.
5.5. Associating a Response to a Request
HTTP does not include a request identifier for associating a given
request message with its corresponding one or more response messages.
Hence, it relies on the order of response arrival to correspond
exactly to the order in which requests are made on the same
connection. More than one response message per request only occurs
when one or more informational responses (1xx, see Section 9.2)
precede a final response to the same request.
A client that has more than one outstanding request on a connection
MUST maintain a list of outstanding requests in the order sent and
MUST associate each received response message on that connection to
the highest ordered request that has not yet received a final (non-
1xx) response.
5.6. Message Forwarding
As described in Section 2.2, intermediaries can serve a variety of
roles in the processing of HTTP requests and responses. Some
intermediaries are used to improve performance or availability.
Others are used for access control or to filter content. Since an
HTTP stream has characteristics similar to a pipe-and-filter
architecture, there are no inherent limits to the extent an
intermediary can enhance (or interfere) with either direction of the
stream.
An intermediary not acting as a tunnel MUST implement the Connection
header field, as specified in Section 9.1 of [Messaging], and exclude
fields from being forwarded that are only intended for the incoming
connection.
An intermediary MUST NOT forward a message to itself unless it is
protected from an infinite request loop. In general, an intermediary
ought to recognize its own server names, including any aliases, local
variations, or literal IP addresses, and respond to such requests
directly.
An HTTP message can be parsed as a stream for incremental processing
or forwarding downstream. However, recipients cannot rely on
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incremental delivery of partial messages, since some implementations
will buffer or delay message forwarding for the sake of network
efficiency, security checks, or payload transformations.
5.6.1. Via
The "Via" header field indicates the presence of intermediate
protocols and recipients between the user agent and the server (on
requests) or between the origin server and the client (on responses),
similar to the "Received" header field in email (Section 3.6.7 of
[RFC5322]). Via can be used for tracking message forwards, avoiding
request loops, and identifying the protocol capabilities of senders
along the request/response chain.
Via = 1#( received-protocol RWS received-by [ RWS comment ] )
received-protocol = [ protocol-name "/" ] protocol-version
; see [Messaging], Section 9.7
received-by = ( uri-host [ ":" port ] ) / pseudonym
pseudonym = token
Multiple Via field values represent each proxy or gateway that has
forwarded the message. Each intermediary appends its own information
about how the message was received, such that the end result is
ordered according to the sequence of forwarding recipients.
A proxy MUST send an appropriate Via header field, as described
below, in each message that it forwards. An HTTP-to-HTTP gateway
MUST send an appropriate Via header field in each inbound request
message and MAY send a Via header field in forwarded response
messages.
For each intermediary, the received-protocol indicates the protocol
and protocol version used by the upstream sender of the message.
Hence, the Via field value records the advertised protocol
capabilities of the request/response chain such that they remain
visible to downstream recipients; this can be useful for determining
what backwards-incompatible features might be safe to use in
response, or within a later request, as described in Section 3.5.
For brevity, the protocol-name is omitted when the received protocol
is HTTP.
The received-by portion of the field value is normally the host and
optional port number of a recipient server or client that
subsequently forwarded the message. However, if the real host is
considered to be sensitive information, a sender MAY replace it with
a pseudonym. If a port is not provided, a recipient MAY interpret
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that as meaning it was received on the default TCP port, if any, for
the received-protocol.
A sender MAY generate comments in the Via header field to identify
the software of each recipient, analogous to the User-Agent and
Server header fields. However, all comments in the Via field are
optional, and a recipient MAY remove them prior to forwarding the
message.
For example, a request message could be sent from an HTTP/1.0 user
agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
forward the request to a public proxy at p.example.net, which
completes the request by forwarding it to the origin server at
www.example.com. The request received by www.example.com would then
have the following Via header field:
Via: 1.0 fred, 1.1 p.example.net
An intermediary used as a portal through a network firewall SHOULD
NOT forward the names and ports of hosts within the firewall region
unless it is explicitly enabled to do so. If not enabled, such an
intermediary SHOULD replace each received-by host of any host behind
the firewall by an appropriate pseudonym for that host.
An intermediary MAY combine an ordered subsequence of Via header
field entries into a single such entry if the entries have identical
received-protocol values. For example,
Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
could be collapsed to
Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
A sender SHOULD NOT combine multiple entries unless they are all
under the same organizational control and the hosts have already been
replaced by pseudonyms. A sender MUST NOT combine entries that have
different received-protocol values.
5.6.2. Transformations
Some intermediaries include features for transforming messages and
their payloads. A proxy might, for example, convert between image
formats in order to save cache space or to reduce the amount of
traffic on a slow link. However, operational problems might occur
when these transformations are applied to payloads intended for
critical applications, such as medical imaging or scientific data
analysis, particularly when integrity checks or digital signatures
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are used to ensure that the payload received is identical to the
original.
An HTTP-to-HTTP proxy is called a "transforming proxy" if it is
designed or configured to modify messages in a semantically
meaningful way (i.e., modifications, beyond those required by normal
HTTP processing, that change the message in a way that would be
significant to the original sender or potentially significant to
downstream recipients). For example, a transforming proxy might be
acting as a shared annotation server (modifying responses to include
references to a local annotation database), a malware filter, a
format transcoder, or a privacy filter. Such transformations are
presumed to be desired by whichever client (or client organization)
selected the proxy.
If a proxy receives a request-target with a host name that is not a
fully qualified domain name, it MAY add its own domain to the host
name it received when forwarding the request. A proxy MUST NOT
change the host name if the request-target contains a fully qualified
domain name.
A proxy MUST NOT modify the "absolute-path" and "query" parts of the
received request-target when forwarding it to the next inbound
server, except as noted above to replace an empty path with "/" or
"*".
A proxy MAY modify the message body through application or removal of
a transfer coding (Section 7 of [Messaging]).
A proxy MUST NOT transform the payload (Section 6.3) of a message
that contains a no-transform cache-control directive (Section 5.2 of
[Caching]).
A proxy MAY transform the payload of a message that does not contain
a no-transform cache-control directive. A proxy that transforms a
payload MUST add a Warning header field with the warn-code of 214
("Transformation Applied") if one is not already in the message (see
Section 5.5 of [Caching]). A proxy that transforms the payload of a
200 (OK) response can further inform downstream recipients that a
transformation has been applied by changing the response status code
to 203 (Non-Authoritative Information) (Section 9.3.4).
A proxy SHOULD NOT modify header fields that provide information
about the endpoints of the communication chain, the resource state,
or the selected representation (other than the payload) unless the
field's definition specifically allows such modification or the
modification is deemed necessary for privacy or security.
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6. Representations
Considering 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, an
abstraction is needed to represent ("take the place of") the current
or desired state of that thing in our communications. That
abstraction is called a representation [REST].
For the purposes of HTTP, a "representation" is information that is
intended to reflect a past, current, or desired state of a given
resource, in a format that can be readily communicated via the
protocol, and that consists of a set of representation metadata and a
potentially unbounded stream of representation data.
An origin server might be provided with, or be capable of generating,
multiple representations that are each intended to reflect the
current state of a target resource. In such cases, some algorithm is
used by the origin server to select one of those representations as
most applicable to a given request, usually based on content
negotiation. This "selected representation" is used to provide the
data and metadata for evaluating conditional requests Section 8.2 and
constructing the payload for 200 (OK) and 304 (Not Modified)
responses to GET (Section 7.3.1).
6.1. 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 ) )
6.1.1. Media Type
HTTP uses media types [RFC2046] in the Content-Type (Section 6.2.1)
and Accept (Section 8.4.2) header fields in order to provide open and
extensible data typing and type negotiation. Media types define both
a data format and various processing models: how to process that data
in accordance with each context in which it is received.
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media-type = type "/" subtype *( OWS ";" OWS parameter )
type = token
subtype = token
The type/subtype MAY be followed by parameters in the form of
name=value pairs.
parameter = token "=" ( token / quoted-string )
The type, subtype, and parameter name tokens 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 either as a token or within a quoted-string. The quoted
and unquoted values are equivalent. For example, the following
examples are all equivalent, but the first is preferred for
consistency:
text/html;charset=utf-8
text/html;charset=UTF-8
Text/HTML;Charset="utf-8"
text/html; charset="utf-8"
Media types ought to be registered with IANA according to the
procedures defined in [BCP13].
Note: Unlike some similar constructs in other header fields, media
type parameters do not allow whitespace (even "bad" whitespace)
around the "=" character.
6.1.1.1. Charset
HTTP uses charset names to indicate or negotiate the character
encoding scheme of a textual representation [RFC6365]. A charset is
identified by a case-insensitive token.
charset = token
Charset names ought to be registered in the IANA "Character Sets"
registry (<https://www.iana.org/assignments/character-sets>)
according to the procedures defined in [RFC2978].
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6.1.1.2. Canonicalization and Text Defaults
Media types are registered with a canonical form in order to be
interoperable among systems with varying native encoding formats.
Representations selected or transferred via HTTP ought to be in
canonical form, for many of the same reasons described by the
Multipurpose Internet Mail Extensions (MIME) [RFC2045]. However, the
performance characteristics of email deployments (i.e., store and
forward messages to peers) are significantly different from those
common to HTTP and the Web (server-based information services).
Furthermore, MIME's constraints for the sake of compatibility with
older mail transfer protocols do not apply to HTTP (see Appendix B of
[Messaging]).
MIME's canonical form requires that media subtypes of the "text" type
use CRLF as the text line break. HTTP allows the transfer of text
media with plain CR or LF alone representing a line break, when such
line breaks are consistent for an entire representation. An HTTP
sender MAY generate, and a recipient MUST be able to parse, line
breaks in text media that consist of CRLF, bare CR, or bare LF. In
addition, text media in HTTP is not limited to charsets that use
octets 13 and 10 for CR and LF, respectively. This flexibility
regarding line breaks applies only to text within a representation
that has been assigned a "text" media type; it does not apply to
"multipart" types or HTTP elements outside the payload body (e.g.,
header fields).
If a representation is encoded with a content-coding, the underlying
data ought to be in a form defined above prior to being encoded.
6.1.1.3. 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 include a boundary parameter as part of the media type
value. The message body is itself a protocol element; a sender MUST
generate only CRLF to represent line breaks between body parts.
HTTP message framing does not use the multipart boundary as an
indicator of message body length, though it might be used by
implementations that generate or process the payload. For example,
the "multipart/form-data" type is often used for carrying form data
in a request, as described in [RFC7578], and the "multipart/
byteranges" type is defined by this specification for use in some 206
(Partial Content) responses (see Section 9.3.7).
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6.1.2. 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 final recipient.
content-coding = token
Content-coding values are used in the Accept-Encoding (Section 8.4.4)
and Content-Encoding (Section 6.2.2) header fields.
The following content-coding values are defined by this
specification:
+------------+------------------------------------------+-----------+
| Name | Description | Reference |
+------------+------------------------------------------+-----------+
| compress | UNIX "compress" data format [Welch] | Section 6 |
| | | .1.2.1 |
| deflate | "deflate" compressed data ([RFC1951]) | Section 6 |
| | inside the "zlib" data format | .1.2.2 |
| | ([RFC1950]) | |
| gzip | GZIP file format [RFC1952] | Section 6 |
| | | .1.2.3 |
| identity | Reserved (synonym for "no encoding" in | Section 8 |
| | Accept-Encoding) | .4.4 |
| x-compress | Deprecated (alias for compress) | Section 6 |
| | | .1.2.1 |
| x-gzip | Deprecated (alias for gzip) | Section 6 |
| | | .1.2.3 |
+------------+------------------------------------------+-----------+
6.1.2.1. Compress Coding
The "compress" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
[Welch] that is commonly produced by the UNIX file compression
program "compress". A recipient SHOULD consider "x-compress" to be
equivalent to "compress".
6.1.2.2. Deflate Coding
The "deflate" coding is a "zlib" data format [RFC1950] containing a
"deflate" compressed data stream [RFC1951] that uses a combination of
the Lempel-Ziv (LZ77) compression algorithm and Huffman coding.
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Note: Some non-conformant implementations send the "deflate"
compressed data without the zlib wrapper.
6.1.2.3. Gzip Coding
The "gzip" coding is an LZ77 coding with a 32-bit Cyclic Redundancy
Check (CRC) that is commonly produced by the gzip file compression
program [RFC1952]. A recipient SHOULD consider "x-gzip" to be
equivalent to "gzip".
6.1.2.4. Content Coding Extensibility
Additional content codings, outside the scope of this specification,
have been specified for use in HTTP. All such content codings ought
to be registered within the "HTTP Content Coding Registry".
6.1.2.4.1. Content Coding Registry
The "HTTP Content Coding Registry", maintained by IANA at
<https://www.iana.org/assignments/http-parameters/>, registers
content-coding names.
Content coding 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 7 of [Messaging]), unless the encoding
transformation is identical (as is the case for the compression
codings defined in Section 6.1.2).
Values to be added to this namespace require IETF Review (see
Section 4.8 of [RFC8126]) and MUST conform to the purpose of content
coding defined in Section 6.1.2.
6.1.3. 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 header fields. Accept-Language uses the broader language-
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range production defined in Section 8.4.5, whereas Content-Language
uses the language-tag production defined below.
language-tag = <Language-Tag, see [RFC5646], Section 2.1>
A language tag is a sequence of one or more case-insensitive subtags,
each separated by a hyphen character ("-", %x2D). In most cases, a
language tag consists of a primary language subtag that identifies a
broad family of related languages (e.g., "en" = English), which is
optionally followed by a series of subtags that refine or narrow that
language's range (e.g., "en-CA" = the variety of English as
communicated in Canada). Whitespace is not allowed within a language
tag. Example tags include:
fr, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN
See [RFC5646] for further information.
6.1.4. Range Units
A representation can be partitioned into subranges according to
various structural units, depending on the structure inherent in the
representation's media type. This "range unit" is used in the
Accept-Ranges (Section 10.4.1) response header field to advertise
support for range requests, the Range (Section 8.3) request header
field to delineate the parts of a representation that are requested,
and the Content-Range (Section 6.3.3) payload header field to
describe which part of a representation is being transferred.
range-unit = bytes-unit / other-range-unit
The following range unit names are defined by this document:
+-------------+---------------------------------------+-------------+
| Range Unit | Description | Reference |
| Name | | |
+-------------+---------------------------------------+-------------+
| bytes | a range of octets | Section 6.1 |
| | | .4.1 |
| none | reserved as keyword, indicating no | Section 10. |
| | ranges are supported | 4.1 |
+-------------+---------------------------------------+-------------+
6.1.4.1. Byte Ranges
Since representation data is transferred in payloads as a sequence of
octets, a byte range is a meaningful substructure for any
representation transferable over HTTP (Section 6). The "bytes" range
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unit is defined for expressing subranges of the data's octet
sequence.
bytes-unit = "bytes"
A byte-range request can specify a single range of bytes or a set of
ranges within a single representation.
byte-ranges-specifier = bytes-unit "=" byte-range-set
byte-range-set = 1#( byte-range-spec / suffix-byte-range-spec )
byte-range-spec = first-byte-pos "-" [ last-byte-pos ]
first-byte-pos = 1*DIGIT
last-byte-pos = 1*DIGIT
The first-byte-pos value in a byte-range-spec gives the byte-offset
of the first byte in a range. The last-byte-pos value gives the
byte-offset of the last byte in the range; that is, the byte
positions specified are inclusive. Byte offsets start at zero.
Examples of byte-ranges-specifier values:
o The first 500 bytes (byte offsets 0-499, inclusive):
bytes=0-499
o The second 500 bytes (byte offsets 500-999, inclusive):
bytes=500-999
A byte-range-spec is invalid if the last-byte-pos value is present
and less than the first-byte-pos.
A client can limit the number of bytes requested without knowing the
size of the selected representation. If the last-byte-pos value is
absent, or if the value is greater than or equal to the current
length of the representation data, the byte range is interpreted as
the remainder of the representation (i.e., the server replaces the
value of last-byte-pos with a value that is one less than the current
length of the selected representation).
A client can request the last N bytes of the selected representation
using a suffix-byte-range-spec.
suffix-byte-range-spec = "-" suffix-length
suffix-length = 1*DIGIT
If the selected representation is shorter than the specified suffix-
length, the entire representation is used.
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Additional examples, assuming a representation of length 10000:
o The final 500 bytes (byte offsets 9500-9999, inclusive):
bytes=-500
Or:
bytes=9500-
o The first and last bytes only (bytes 0 and 9999):
bytes=0-0,-1
o Other valid (but not canonical) specifications of the second 500
bytes (byte offsets 500-999, inclusive):
bytes=500-600,601-999
bytes=500-700,601-999
If a valid byte-range-set includes at least one byte-range-spec with
a first-byte-pos that is less than the current length of the
representation, or at least one suffix-byte-range-spec with a non-
zero suffix-length, then the byte-range-set is satisfiable.
Otherwise, the byte-range-set is unsatisfiable.
In the byte-range syntax, first-byte-pos, last-byte-pos, and suffix-
length are expressed as decimal number of octets. Since there is no
predefined limit to the length of a payload, recipients MUST
anticipate potentially large decimal numerals and prevent parsing
errors due to integer conversion overflows.
6.1.4.2. Other Range Units
Range units are intended to be extensible. New range units ought to
be registered with IANA, as defined in Section 6.1.4.3.
other-range-unit = token
6.1.4.3. Range Unit Registry
The "HTTP Range Unit Registry" defines the namespace for the range
unit names and refers to their corresponding specifications. It is
maintained at <https://www.iana.org/assignments/http-parameters>.
Registration of an HTTP Range Unit MUST include the following fields:
o Name
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o Description
o Pointer to specification text
Values to be added to this namespace require IETF Review (see
[RFC8126], Section 4.8).
6.2. Representation Metadata
Representation header fields provide metadata about the
representation. When a message includes a payload body, the
representation header fields describe how to interpret the
representation data enclosed in the payload body. In a response to a
HEAD request, the representation header fields describe the
representation data that would have been enclosed in the payload body
if the same request had been a GET.
The following header fields convey representation metadata:
+-------------------+---------------+
| Header Field Name | Defined in... |
+-------------------+---------------+
| Content-Type | Section 6.2.1 |
| Content-Encoding | Section 6.2.2 |
| Content-Language | Section 6.2.3 |
| Content-Length | Section 6.2.4 |
| Content-Location | Section 6.2.5 |
+-------------------+---------------+
6.2.1. Content-Type
The "Content-Type" header field indicates the media type of the
associated representation: either the representation enclosed in the
message payload or the selected representation, as determined by the
message semantics. The indicated media type defines both the data
format and how that data is intended to be processed by a recipient,
within the scope of the received message semantics, after any content
codings indicated by Content-Encoding are decoded.
Content-Type = media-type
Media types are defined in Section 6.1.1. An example of the field is
Content-Type: text/html; charset=ISO-8859-4
A sender that generates a message containing a payload body SHOULD
generate a Content-Type header field in that message unless the
intended media type of the enclosed representation is unknown to the
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sender. If a Content-Type header field is not present, the recipient
MAY either assume a media type of "application/octet-stream"
([RFC2046], Section 4.5.1) or examine the 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.
6.2.2. 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 one or more encodings have been applied to a representation, the
sender that applied the encodings MUST generate a Content-Encoding
header field that lists the content codings in the order in which
they were applied. Additional information about the encoding
parameters can be provided by other header fields not defined by this
specification.
Unlike Transfer-Encoding (Section 6.1 of [Messaging]), the codings
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.
If the media type includes an inherent encoding, such as a data
format that is always compressed, then that encoding would not be
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restated in 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 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).
An origin server MAY respond with a status code of 415 (Unsupported
Media Type) if a representation in the request message has a content
coding that is not acceptable.
6.2.3. 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 6.1.3. The primary purpose of
Content-Language is to allow a user to identify and differentiate
representations according to the users' 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
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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.
6.2.4. Content-Length
[[CREF5: The "Content-Length" header field indicates the number of
data octets (body length) for the representation. In some cases,
Content-Length is used to define or estimate message framing. ]]
Content-Length = 1*DIGIT
An example is
Content-Length: 3495
A sender MUST NOT send a Content-Length header field in any message
that contains a Transfer-Encoding header field.
A user agent SHOULD send a Content-Length in a request message when
no Transfer-Encoding is sent and the request method defines a meaning
for an enclosed payload body. For example, a Content-Length header
field is normally sent in a POST request even when the value is 0
(indicating an empty payload body). A user agent SHOULD NOT send a
Content-Length header field when the request message does not contain
a payload body and the method semantics do not anticipate such a
body.
A server MAY send a Content-Length header field in a response to a
HEAD request (Section 7.3.2); a server MUST NOT send Content-Length
in such a response unless its field-value equals the decimal number
of octets that would have been sent in the payload body of a response
if the same request had used the GET method.
A server MAY send a Content-Length header field in a 304 (Not
Modified) response to a conditional GET request (Section 9.4.5); a
server MUST NOT send Content-Length in such a response unless its
field-value equals the decimal number of octets that would have been
sent in the payload body of a 200 (OK) response to the same request.
A server MUST NOT send a Content-Length header field in any response
with a status code of 1xx (Informational) or 204 (No Content). A
server MUST NOT send a Content-Length header field in any 2xx
(Successful) response to a CONNECT request (Section 7.3.6).
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Aside from the cases defined above, in the absence of Transfer-
Encoding, an origin server SHOULD send a Content-Length header field
when the payload body size is known prior to sending the complete
header section. This will allow downstream recipients to measure
transfer progress, know when a received message is complete, and
potentially reuse the connection for additional requests.
Any Content-Length field value greater than or equal to zero is
valid. Since there is no predefined limit to the length of a
payload, a recipient MUST anticipate potentially large decimal
numerals and prevent parsing errors due to integer conversion
overflows (Section 12.5).
If a message is received that has multiple Content-Length header
fields with field-values consisting of the same decimal value, or a
single Content-Length header field with a field value containing a
list of identical decimal values (e.g., "Content-Length: 42, 42"),
indicating that duplicate Content-Length header fields have been
generated or combined by an upstream message processor, then the
recipient MUST either reject the message as invalid or replace the
duplicated field-values with a single valid Content-Length field
containing that decimal value prior to determining the message body
length or forwarding the message.
6.2.5. Content-Location
The "Content-Location" header field references a URI that can be used
as an identifier for a specific resource corresponding to the
representation in this message's payload. In other words, if one
were to perform a GET request on this URI at the time of this
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 5.3). 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 recipient
MAY consider the payload to be a current representation of that
resource at the time indicated by the message origination date. For
a GET (Section 7.3.1) or HEAD (Section 7.3.2) request, this is the
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same as the default semantics when no Content-Location is provided by
the server. For a state-changing request like PUT (Section 7.3.4) or
POST (Section 7.3.3), 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 URI is
an identifier for a different resource corresponding to the enclosed
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 refers to 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.
A user agent that sends Content-Location in a request message is
stating that its value refers to where the user agent originally
obtained the content of the enclosed representation (prior to any
modifications made by that user agent). In other words, the user
agent is providing a back link to the source of the original
representation.
An origin server that receives a Content-Location field in a request
message MUST treat the information as transitory request context
rather than as metadata to be saved verbatim as part of the
representation. An origin server MAY use that context to guide in
processing the request or to save it for other uses, such as within
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source links or versioning metadata. However, an origin server MUST
NOT use such context information to alter the request semantics.
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 state of 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 identifier to update
only one of the negotiated representations. If the user agent had
wanted the latter semantics, it would have applied the PUT directly
to the Content-Location URI.
6.3. Payload
Some HTTP messages transfer a complete or partial representation as
the message "payload". In some cases, a payload might contain only
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).
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-Range | Section 6.3.3 |
| Trailer | Section 4.4 |
| Transfer-Encoding | Section 6.1 of [Messaging] |
+-------------------+----------------------------+
6.3.1. Purpose
The purpose of a payload in a request is defined by the method
semantics. For example, a representation in the payload of a PUT
request (Section 7.3.4) 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 7.3.3)
represents information to be processed by the target resource.
In a response, the payload's purpose is defined by both the request
method and the response status code. For example, the payload of a
200 (OK) response to GET (Section 7.3.1) represents the current state
of the target resource, as observed at the time of the message
origination date (Section 10.1.1.2), whereas the payload of the same
status code in a response to POST might represent either the
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processing result or the new state of the target resource after
applying the processing. Response messages with an error status code
usually contain a payload that represents the error condition, such
that it describes the error state and what next steps are suggested
for resolving it.
6.3.2. Identification
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.
For 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 this specification). The information
might still be useful for revision history links.
o Otherwise, the payload is unidentified.
For a response message, the following rules are applied in order
until a match is found:
1. If the request method is GET or HEAD and the response status code
is 200 (OK), 204 (No Content), 206 (Partial Content), or 304 (Not
Modified), the payload is a representation of the resource
identified by the effective request URI (Section 5.3).
2. If the request method is GET or HEAD and the response status code
is 203 (Non-Authoritative Information), the payload is a
potentially modified or enhanced representation of the target
resource as provided by an intermediary.
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 is a representation of the resource
identified by 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
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it can be verified by other means (not defined by this
specification).
5. Otherwise, the payload is unidentified.
6.3.3. Content-Range
The "Content-Range" header field is sent in a single part 206
(Partial Content) response to indicate the partial range of the
selected representation enclosed as the message payload, sent in each
part of a multipart 206 response to indicate the range enclosed
within each body part, and sent in 416 (Range Not Satisfiable)
responses to provide information about the selected representation.
Content-Range = byte-content-range
/ other-content-range
byte-content-range = bytes-unit SP
( byte-range-resp / unsatisfied-range )
byte-range-resp = byte-range "/" ( complete-length / "*" )
byte-range = first-byte-pos "-" last-byte-pos
unsatisfied-range = "*/" complete-length
complete-length = 1*DIGIT
other-content-range = other-range-unit SP other-range-resp
other-range-resp = *VCHAR
If a 206 (Partial Content) response contains a Content-Range header
field with a range unit (Section 6.1.4) that the recipient does not
understand, the recipient MUST NOT attempt to recombine it with a
stored representation. A proxy that receives such a message SHOULD
forward it downstream.
For byte ranges, a sender SHOULD indicate the complete length of the
representation from which the range has been extracted, unless the
complete length is unknown or difficult to determine. An asterisk
character ("*") in place of the complete-length indicates that the
representation length was unknown when the header field was
generated.
The following example illustrates when the complete length of the
selected representation is known by the sender to be 1234 bytes:
Content-Range: bytes 42-1233/1234
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and this second example illustrates when the complete length is
unknown:
Content-Range: bytes 42-1233/*
A Content-Range field value is invalid if it contains a byte-range-
resp that has a last-byte-pos value less than its first-byte-pos
value, or a complete-length value less than or equal to its last-
byte-pos value. The recipient of an invalid Content-Range MUST NOT
attempt to recombine the received content with a stored
representation.
A server generating a 416 (Range Not Satisfiable) response to a byte-
range request SHOULD send a Content-Range header field with an
unsatisfied-range value, as in the following example:
Content-Range: bytes */1234
The complete-length in a 416 response indicates the current length of
the selected representation.
The Content-Range header field has no meaning for status codes that
do not explicitly describe its semantic. For this specification,
only the 206 (Partial Content) and 416 (Range Not Satisfiable) status
codes describe a meaning for Content-Range.
The following are examples of Content-Range values in which the
selected representation contains a total of 1234 bytes:
o The first 500 bytes:
Content-Range: bytes 0-499/1234
o The second 500 bytes:
Content-Range: bytes 500-999/1234
o All except for the first 500 bytes:
Content-Range: bytes 500-1233/1234
o The last 500 bytes:
Content-Range: bytes 734-1233/1234
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6.3.4. Media Type multipart/byteranges
When a 206 (Partial Content) response message includes the content of
multiple ranges, they are transmitted as body parts in a multipart
message body ([RFC2046], Section 5.1) with the media type of
"multipart/byteranges".
The multipart/byteranges media type includes one or more body parts,
each with its own Content-Type and Content-Range fields. The
required boundary parameter specifies the boundary string used to
separate each body part.
Implementation Notes:
1. Additional CRLFs might precede the first boundary string in the
body.
2. Although [RFC2046] permits the boundary string to be quoted, some
existing implementations handle a quoted boundary string
incorrectly.
3. A number of clients and servers were coded to an early draft of
the byteranges specification that used a media type of multipart/
x-byteranges, which is almost (but not quite) compatible with
this type.
Despite the name, the "multipart/byteranges" media type is not
limited to byte ranges. The following example uses an "exampleunit"
range unit:
HTTP/1.1 206 Partial Content
Date: Tue, 14 Nov 1995 06:25:24 GMT
Last-Modified: Tue, 14 July 04:58:08 GMT
Content-Length: 2331785
Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES
--THIS_STRING_SEPARATES
Content-Type: video/example
Content-Range: exampleunit 1.2-4.3/25
...the first range...
--THIS_STRING_SEPARATES
Content-Type: video/example
Content-Range: exampleunit 11.2-14.3/25
...the second range
--THIS_STRING_SEPARATES--
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The following information serves as the registration form for the
multipart/byteranges media type.
Type name: multipart
Subtype name: byteranges
Required parameters: boundary
Optional parameters: N/A
Encoding considerations: only "7bit", "8bit", or "binary" are
permitted
Security considerations: see Section 12
Interoperability considerations: N/A
Published specification: This specification (see Section 6.3.4).
Applications that use this media type: HTTP components supporting
multiple ranges in a single request.
Fragment identifier considerations: N/A
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): N/A
Macintosh file type code(s): N/A
Person and email address to contact for further information: See Aut
hors' Addresses section.
Intended usage: COMMON
Restrictions on usage: N/A
Author: See Authors' Addresses section.
Change controller: IESG
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6.4. Content Negotiation
When responses convey payload information, whether indicating a
success or an error, the origin server often has different ways of
representing that information; for example, in different formats,
languages, or encodings. Likewise, different users or user agents
might have differing capabilities, characteristics, or preferences
that could influence which representation, among those available,
would be best to deliver. For this reason, HTTP provides mechanisms
for content negotiation.
This specification defines two patterns of content negotiation that
can be made visible within the protocol: "proactive", where the
server selects the representation based upon the user agent's stated
preferences, and "reactive" negotiation, where the server provides a
list of representations for the user agent to choose from. Other
patterns of content negotiation include "conditional content", where
the representation consists of multiple parts that are selectively
rendered based on user agent parameters, "active content", where the
representation contains a script that makes additional (more
specific) requests based on the user agent characteristics, and
"Transparent Content Negotiation" ([RFC2295]), where content
selection is performed by an intermediary. These patterns are not
mutually exclusive, and each has trade-offs in applicability and
practicality.
Note that, in all cases, HTTP is not aware of the resource semantics.
The consistency with which an origin server responds to requests,
over time and over the varying dimensions of content negotiation, and
thus the "sameness" of a resource's observed representations over
time, is determined entirely by whatever entity or algorithm selects
or generates those responses. HTTP pays no attention to the man
behind the curtain.
6.4.1. Proactive Negotiation
When content negotiation preferences are sent by the user agent in a
request to encourage an algorithm located at the server to select the
preferred representation, it is called proactive negotiation (a.k.a.,
server-driven negotiation). Selection is based on the available
representations for a response (the dimensions over which it might
vary, such as language, content-coding, etc.) compared to various
information supplied in the request, including both the explicit
negotiation fields of Section 8.4 and implicit characteristics, such
as the client's network address or parts of the User-Agent field.
Proactive negotiation is advantageous when the algorithm for
selecting from among the available representations is difficult to
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describe to a user agent, or when the server desires to send its
"best guess" to the user agent along with the first response (hoping
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, a user agent MAY send request header fields that
describe its preferences.
Proactive negotiation has serious disadvantages:
o 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?);
o 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 risk
to the user's privacy;
o It complicates the implementation of an origin server and the
algorithms for generating responses to a request; and,
o It limits the reusability of responses for shared caching.
A user agent cannot rely on proactive negotiation preferences being
consistently honored, since the origin server might not implement
proactive negotiation for the requested resource or might decide that
sending a response that doesn't conform to the user agent's
preferences is better than sending a 406 (Not Acceptable) response.
A Vary header field (Section 10.1.4) is often sent in a response
subject to proactive negotiation to indicate what parts of the
request information were used in the selection algorithm.
6.4.2. Reactive Negotiation
With reactive negotiation (a.k.a., agent-driven negotiation),
selection of the best response representation (regardless of the
status code) is performed by the user agent after receiving an
initial response from the origin server that contains a list of
resources for alternative representations. If the user agent is not
satisfied by the initial response representation, it can perform a
GET request on one or more of the alternative resources, selected
based on metadata included in the list, to obtain a different form of
representation for that response. Selection of alternatives might be
performed automatically by the user agent or manually by the user
selecting from a generated (possibly hypertext) menu.
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Note that the above refers to representations of the response, in
general, not representations of the resource. The alternative
representations are only considered representations of the target
resource if the response in which those alternatives are provided has
the semantics of being a representation of the target resource (e.g.,
a 200 (OK) response to a GET request) or has the semantics of
providing links to alternative representations for the target
resource (e.g., a 300 (Multiple Choices) response to a GET request).
A server might choose not to send an initial representation, other
than the list of alternatives, and thereby indicate that reactive
negotiation by the user agent is preferred. For example, the
alternatives listed in responses with the 300 (Multiple Choices) and
406 (Not Acceptable) status codes include information about the
available representations so that the user or user agent can react by
making a selection.
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 disadvantages of transmitting a
list of alternatives to the user agent, which degrades user-perceived
latency if transmitted in the header section, and needing a second
request to obtain an alternate representation. Furthermore, this
specification does not define a mechanism for supporting automatic
selection, though it does not prevent such a mechanism from being
developed as an extension.
7. Request Methods
7.1. 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 method's semantics might be further specialized by the
semantics of some header fields when present in a request (Section 8)
if those additional semantics do not conflict with the method. For
example, a client can send conditional request header fields
(Section 8.2) to make the requested action conditional on the current
state of the target resource.
method = token
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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 ought to 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 US-
ASCII letters.
+---------+-------------------------------------------------+-------+
| Method | Description | Sec. |
+---------+-------------------------------------------------+-------+
| GET | Transfer a current representation of the target | 7.3.1 |
| | resource. | |
| HEAD | Same as GET, but only transfer the status line | 7.3.2 |
| | and header section. | |
| POST | Perform resource-specific processing on the | 7.3.3 |
| | request payload. | |
| PUT | Replace all current representations of the | 7.3.4 |
| | target resource with the request payload. | |
| DELETE | Remove all current representations of the | 7.3.5 |
| | target resource. | |
| CONNECT | Establish a tunnel to the server identified by | 7.3.6 |
| | the target resource. | |
| OPTIONS | Describe the communication options for the | 7.3.7 |
| | target resource. | |
| TRACE | Perform a message loop-back test along the path | 7.3.8 |
| | to the target resource. | |
+---------+-------------------------------------------------+-------+
All general-purpose servers MUST support the methods GET and HEAD.
All other methods are OPTIONAL.
The set of methods allowed by a target resource can be listed in an
Allow header field (Section 10.4.2). However, the set of allowed
methods can change dynamically. When a request method is received
that is unrecognized or not implemented by an origin server, the
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origin server SHOULD respond with the 501 (Not Implemented) status
code. When a request method 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.
7.2. Common Method Properties
+---------+------+------------+----------------+
| Method | Safe | Idempotent | Reference |
+---------+------+------------+----------------+
| CONNECT | no | no | Section 7.3.6 |
| DELETE | no | yes | Section 7.3.5 |
| GET | yes | yes | Section 7.3.1 |
| HEAD | yes | yes | Section 7.3.2 |
| OPTIONS | yes | yes | Section 7.3.7 |
| POST | no | no | Section 7.3.3 |
| PUT | no | yes | Section 7.3.4 |
| TRACE | yes | yes | Section 7.3.8 |
+---------+------+------------+----------------+
7.2.1. 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, that is not
entirely read-only, or that 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.
Of the request methods defined by this specification, the GET, HEAD,
OPTIONS, and TRACE 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
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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 owner 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.
7.2.2. Idempotent Methods
A request method is considered "idempotent" if the intended effect on
the server of multiple identical requests with that method is the
same as the effect for a single such request. Of the request methods
defined by this specification, PUT, DELETE, and 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 the client can establish a new
connection and retry the idempotent request. It knows that repeating
the request will have the same intended effect, even if the original
request succeeded, though the response might differ.
7.2.3. Cacheable Methods
Request methods can be defined as "cacheable" to indicate that
responses to them are allowed to be stored for future reuse; for
specific requirements see [Caching]. In general, safe methods that
do not depend on a current or authoritative response are defined as
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cacheable; this specification defines GET, HEAD, and POST as
cacheable, although the overwhelming majority of cache
implementations only support GET and HEAD.
7.3. Method Definitions
7.3.1. GET
The GET method requests transfer of a current selected representation
for the target resource. GET is the primary mechanism of information
retrieval and the focus of almost all performance optimizations.
Hence, when people speak of retrieving some identifiable information
via HTTP, they are generally referring to making a GET request.
It is tempting to think of resource identifiers as remote file system
pathnames and of representations as being a copy of the contents of
such files. In fact, that is how many resources are implemented (see
Section 12.3 for related security considerations). However, there
are no such limitations in practice. The HTTP interface for a
resource is just as likely to be implemented as a tree of content
objects, a programmatic view on various database records, or a
gateway to other information systems. Even when the URI mapping
mechanism is tied to a file system, an origin server might be
configured to execute the files with the request as input and send
the output as the representation rather than transfer the files
directly. Regardless, only the origin server needs to know how each
of its resource identifiers corresponds to an implementation and how
each implementation manages to select and send a current
representation of the target resource in a response to GET.
A client can alter the semantics of GET to be a "range request",
requesting transfer of only some part(s) of the selected
representation, by sending a Range header field in the request
(Section 8.3).
A payload within a GET request message has no defined semantics;
sending a payload body on a GET request might cause some existing
implementations to reject the request.
The response to a GET request is cacheable; a cache MAY use it to
satisfy subsequent GET and HEAD requests unless otherwise indicated
by the Cache-Control header field (Section 5.2 of [Caching]).
7.3.2. HEAD
The HEAD method is identical to GET except that the server MUST NOT
send a message body in the response (i.e., the response terminates at
the end of the header section). The server SHOULD send the same
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header fields in response to a HEAD request as it would have sent if
the request had been a GET, except that the payload header fields
(Section 6.3) MAY be omitted. This method can be used for obtaining
metadata about the selected representation without transferring the
representation data and is often used for testing hypertext links for
validity, accessibility, and recent modification.
A payload within a HEAD request message has no defined semantics;
sending a payload body on a HEAD request might cause some existing
implementations to reject the request.
The response to a HEAD request is cacheable; a cache MAY use it to
satisfy subsequent HEAD requests unless otherwise indicated by the
Cache-Control header field (Section 5.2 of [Caching]). A HEAD
response might also have an effect on previously cached responses to
GET; see Section 4.3.5 of [Caching].
7.3.3. POST
The POST method requests that the target resource process the
representation enclosed in the request according to the resource's
own specific semantics. For example, POST is used for the following
functions (among others):
o Providing a block of data, such as the fields entered into an HTML
form, to a data-handling process;
o Posting a message to a bulletin board, newsgroup, mailing list,
blog, or similar group of articles;
o Creating a new resource that has yet to be identified by the
origin server; and
o Appending data to a resource's existing representation(s).
An origin server indicates response semantics by choosing an
appropriate status code depending on the result of processing the
POST request; almost all of the status codes defined by this
specification might be received in a response to POST (the exceptions
being 206 (Partial Content), 304 (Not Modified), and 416 (Range Not
Satisfiable)).
If one or more resources has been created on the origin server as a
result of successfully processing a POST request, the origin server
SHOULD send a 201 (Created) response containing a Location header
field that provides an identifier for the primary resource created
(Section 10.1.2) and a representation that describes the status of
the request while referring to the new resource(s).
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Responses to POST requests are only cacheable when they include
explicit freshness information (see Section 4.2.1 of [Caching]).
However, POST caching is not widely implemented. For cases where an
origin server wishes the client to be able to cache the result of a
POST in a way that can be reused by a later GET, the origin server
MAY send a 200 (OK) response containing the result and a Content-
Location header field that has the same value as the POST's effective
request URI (Section 6.2.5).
If the result of processing a POST would be equivalent to a
representation of an existing resource, an origin server MAY redirect
the user agent to that resource by sending a 303 (See Other) response
with the existing resource's identifier in the Location field. This
has the benefits of providing the user agent a resource identifier
and transferring the representation via a method more amenable to
shared caching, though at the cost of an extra request if the user
agent does not already have the representation cached.
7.3.4. PUT
The PUT method requests that the state of the target resource be
created or replaced with the state defined by the representation
enclosed in the request message payload. A successful PUT of a given
representation would suggest that a subsequent GET on that same
target resource will result in an equivalent representation being
sent in a 200 (OK) response. However, there is no guarantee that
such a state change will be observable, since the target resource
might be acted upon by other user agents in parallel, or might be
subject to dynamic processing by the origin server, before any
subsequent GET is received. A successful response only implies that
the user agent's intent was achieved at the time of its processing by
the origin server.
If the target resource does not have a current representation and the
PUT successfully creates one, then the origin server MUST inform the
user agent by sending a 201 (Created) response. If the target
resource does have a current representation and that representation
is successfully modified in accordance with the state of the enclosed
representation, then the origin server MUST send either a 200 (OK) or
a 204 (No Content) response to indicate successful completion of the
request.
An origin server SHOULD ignore unrecognized header fields received in
a PUT request (i.e., do not save them as part of the resource state).
An origin server SHOULD verify that the PUT representation is
consistent with any constraints the server has for the target
resource that cannot or will not be changed by the PUT. This is
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particularly important when the origin server uses internal
configuration information related to the URI in order to set the
values for representation metadata on GET responses. When a PUT
representation is inconsistent with the target resource, the origin
server SHOULD either make them consistent, by transforming the
representation or changing the resource configuration, or respond
with an appropriate error message containing sufficient information
to explain why the representation is unsuitable. The 409 (Conflict)
or 415 (Unsupported Media Type) status codes are suggested, with the
latter being specific to constraints on Content-Type values.
For example, if the target resource is configured to always have a
Content-Type of "text/html" and the representation being PUT has a
Content-Type of "image/jpeg", the origin server ought to do one of:
a. reconfigure the target resource to reflect the new media type;
b. transform the PUT representation to a format consistent with that
of the resource before saving it as the new resource state; or,
c. reject the request with a 415 (Unsupported Media Type) response
indicating that the target resource is limited to "text/html",
perhaps including a link to a different resource that would be a
suitable target for the new representation.
HTTP does not define exactly how a PUT method affects the state of an
origin server beyond what can be expressed by the intent of the user
agent request and the semantics of the origin server response. It
does not define what a resource might be, in any sense of that word,
beyond the interface provided via HTTP. It does not define how
resource state is "stored", nor how such storage might change as a
result of a change in resource state, nor how the origin server
translates resource state into representations. Generally speaking,
all implementation details behind the resource interface are
intentionally hidden by the server.
An origin server MUST NOT send a validator header field
(Section 10.2), such as an ETag or Last-Modified field, in a
successful response to PUT unless the request's representation data
was saved without any transformation applied to the body (i.e., the
resource's new representation data is identical to the representation
data received in the PUT request) and the validator field value
reflects the new representation. This requirement allows a user
agent to know when the representation body it has in memory remains
current as a result of the PUT, thus not in need of being retrieved
again from the origin server, and that the new validator(s) received
in the response can be used for future conditional requests in order
to prevent accidental overwrites (Section 8.2).
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The fundamental difference between the POST and PUT methods is
highlighted by the different intent for the enclosed representation.
The target resource in a POST request is intended to handle the
enclosed representation according to the resource's own semantics,
whereas the enclosed representation in a PUT request is defined as
replacing the state of the target resource. Hence, the intent of PUT
is idempotent and visible to intermediaries, even though the exact
effect is only known by the origin server.
Proper interpretation of a PUT request presumes that the user agent
knows which target resource is desired. A service that selects a
proper URI on behalf of the client, after receiving a state-changing
request, SHOULD be implemented using the POST method rather than PUT.
If the origin server will not make the requested PUT state change to
the target resource and instead wishes to have it applied to a
different resource, such as when the resource has been moved to a
different URI, then the origin server MUST send an appropriate 3xx
(Redirection) response; the user agent MAY then make its own decision
regarding whether or not to redirect the request.
A PUT request applied to the target resource can have side effects on
other resources. For example, an article might have a URI for
identifying "the current version" (a resource) that is separate from
the URIs identifying each particular version (different resources
that at one point shared the same state as the current version
resource). A successful PUT request on "the current version" URI
might therefore create a new version resource in addition to changing
the state of the target resource, and might also cause links to be
added between the related resources.
An origin server that allows PUT on a given target resource MUST send
a 400 (Bad Request) response to a PUT request that contains a
Content-Range header field (Section 6.3.3), since the payload is
likely to be partial content that has been mistakenly PUT as a full
representation. Partial content updates are possible by targeting a
separately identified resource with state that overlaps a portion of
the larger resource, or by using a different method that has been
specifically defined for partial updates (for example, the PATCH
method defined in [RFC5789]).
Responses to the PUT method are not cacheable. If a successful PUT
request passes through a cache that has one or more stored responses
for the effective request URI, those stored responses will be
invalidated (see Section 4.4 of [Caching]).
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7.3.5. DELETE
The DELETE method requests that the origin server remove the
association between the target resource and its current
functionality. In effect, this method is similar to the rm command
in UNIX: it expresses a deletion operation on the URI mapping of the
origin server rather than an expectation that the previously
associated information be deleted.
If the target resource has one or more current representations, they
might or might not be destroyed by the origin server, and the
associated storage might or might not be reclaimed, depending
entirely on the nature of the resource and its implementation by the
origin server (which are beyond the scope of this specification).
Likewise, other implementation aspects of a resource might need to be
deactivated or archived as a result of a DELETE, such as database or
gateway connections. In general, it is assumed that the origin
server will only allow DELETE on resources for which it has a
prescribed mechanism for accomplishing the deletion.
Relatively few resources allow the DELETE method -- its primary use
is for remote authoring environments, where the user has some
direction regarding its effect. For example, a resource that was
previously created using a PUT request, or identified via the
Location header field after a 201 (Created) response to a POST
request, might allow a corresponding DELETE request to undo those
actions. Similarly, custom user agent implementations that implement
an authoring function, such as revision control clients using HTTP
for remote operations, might use DELETE based on an assumption that
the server's URI space has been crafted to correspond to a version
repository.
If a DELETE method is successfully applied, the origin server SHOULD
send
o a 202 (Accepted) status code if the action will likely succeed but
has not yet been enacted,
o a 204 (No Content) status code if the action has been enacted and
no further information is to be supplied, or
o a 200 (OK) status code if the action has been enacted and the
response message includes a representation describing the status.
A payload within a DELETE request message has no defined semantics;
sending a payload body on a DELETE request might cause some existing
implementations to reject the request.
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Responses to the DELETE method are not cacheable. If a DELETE
request passes through a cache that has one or more stored responses
for the effective request URI, those stored responses will be
invalidated (see Section 4.4 of [Caching]).
7.3.6. CONNECT
The CONNECT method requests that the recipient establish a tunnel to
the destination origin server identified by the request-target and,
if successful, thereafter restrict its behavior to blind forwarding
of packets, in both directions, until the tunnel is closed. Tunnels
are commonly used to create an end-to-end virtual connection, through
one or more proxies, which can then be secured using TLS (Transport
Layer Security, [RFC5246]).
CONNECT is intended only for use in requests to a proxy. An origin
server that receives a CONNECT request for itself MAY respond with a
2xx (Successful) status code to indicate that a connection is
established. However, most origin servers do not implement CONNECT.
A client sending a CONNECT request MUST send the authority form of
request-target (Section 3.2 of [Messaging]); i.e., the request-target
consists of only the host name and port number of the tunnel
destination, separated by a colon. For example,
CONNECT server.example.com:80 HTTP/1.1
Host: server.example.com:80
The recipient proxy can establish a tunnel either by directly
connecting to the request-target or, if configured to use another
proxy, by forwarding the CONNECT request to the next inbound proxy.
Any 2xx (Successful) response indicates that the sender (and all
inbound proxies) will switch to tunnel mode immediately after the
blank line that concludes the successful response's header section;
data received after that blank line is from the server identified by
the request-target. Any response other than a successful response
indicates that the tunnel has not yet been formed and that the
connection remains governed by HTTP.
A tunnel is closed when a tunnel intermediary detects that either
side has closed its connection: the intermediary MUST attempt to send
any outstanding data that came from the closed side to the other
side, close both connections, and then discard any remaining data
left undelivered.
Proxy authentication might be used to establish the authority to
create a tunnel. For example,
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CONNECT server.example.com:80 HTTP/1.1
Host: server.example.com:80
Proxy-Authorization: basic aGVsbG86d29ybGQ=
There are significant risks in establishing a tunnel to arbitrary
servers, particularly when the destination is a well-known or
reserved TCP port that is not intended for Web traffic. For example,
a CONNECT to a request-target of "example.com:25" would suggest that
the proxy connect to the reserved port for SMTP traffic; if allowed,
that could trick the proxy into relaying spam email. Proxies that
support CONNECT SHOULD restrict its use to a limited set of known
ports or a configurable whitelist of safe request targets.
A server MUST NOT send any Transfer-Encoding or Content-Length header
fields in a 2xx (Successful) response to CONNECT. A client MUST
ignore any Content-Length or Transfer-Encoding header fields received
in a successful response to CONNECT.
A payload within a CONNECT request message has no defined semantics;
sending a payload body on a CONNECT request might cause some existing
implementations to reject the request.
Responses to the CONNECT method are not cacheable.
7.3.7. OPTIONS
The OPTIONS method requests information about the communication
options available for the target resource, at either the origin
server or an intervening intermediary. 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.
An OPTIONS request with an asterisk ("*") as the request-target
(Section 3.2 of [Messaging]) applies to the server in general 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
to the options that are available when communicating with the target
resource.
A server generating a successful response to OPTIONS SHOULD send any
header fields that might indicate optional features implemented by
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the server and applicable to the target resource (e.g., Allow),
including potential extensions not defined by this specification.
The response payload, if any, might also describe the communication
options in a machine or human-readable representation. A standard
format for such a representation is not defined by this
specification, but might be defined by future extensions to HTTP. A
server MUST generate a Content-Length field with a value of "0" if no
payload body is to be sent in the response.
A client MAY send a Max-Forwards header field in an OPTIONS request
to target a specific recipient in the request chain (see
Section 8.1.2). A proxy MUST NOT generate a Max-Forwards header
field while forwarding a request unless that request was received
with a Max-Forwards field.
A client that generates an OPTIONS request containing a payload body
MUST send a valid Content-Type header field describing the
representation media type. Although this specification does not
define any use for such a payload, future extensions to HTTP might
use the OPTIONS body to make more detailed queries about the target
resource.
Responses to the OPTIONS method are not cacheable.
7.3.8. TRACE
The TRACE method requests a remote, application-level loop-back of
the request message. The final recipient of the request SHOULD
reflect the message received, excluding some fields described below,
back to the client as the message body of a 200 (OK) response with a
Content-Type of "message/http" (Section 10.1 of [Messaging]). The
final recipient is either the origin server or the first server to
receive a Max-Forwards value of zero (0) in the request
(Section 8.1.2).
A client MUST NOT generate header fields in a TRACE request
containing sensitive data that might be disclosed by the response.
For example, it would be foolish for a user agent to send stored user
credentials Section 8.5 or cookies [RFC6265] in a TRACE request. The
final recipient of the request SHOULD exclude any request header
fields that are likely to contain sensitive data when that recipient
generates the response 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 5.6.1) 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
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length of the request chain, which is useful for testing a chain of
proxies forwarding messages in an infinite loop.
A client MUST NOT send a message body in a TRACE request.
Responses to the TRACE method are not cacheable.
7.4. Method Extensibility
Additional methods, outside the scope of this specification, have
been specified for use in HTTP. All such methods ought to be
registered within the "Hypertext Transfer Protocol (HTTP) Method
Registry".
7.4.1. Method Registry
The "Hypertext Transfer Protocol (HTTP) Method Registry", maintained
by IANA at <https://www.iana.org/assignments/http-methods>, registers
method names.
HTTP method registrations MUST include the following fields:
o Method Name (see Section 7)
o Safe ("yes" or "no", see Section 7.2.1)
o Idempotent ("yes" or "no", see Section 7.2.2)
o Pointer to specification text
Values to be added to this namespace require IETF Review (see
[RFC8126], Section 4.8).
7.4.2. Considerations for New Methods
Standardized methods are generic; that is, they are potentially
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.
Since message parsing (Section 6 of [Messaging]) needs to be
independent of method semantics (aside from responses to HEAD),
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
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zero-length message body is allowed by requiring a Content-Length
header field with a value of "0".
A new method definition needs to indicate whether it is safe
(Section 7.2.1), idempotent (Section 7.2.2), cacheable
(Section 7.2.3), what semantics are to be associated with the payload
body if any is present in the request and what refinements the method
makes to header field or status code semantics. If the new method is
cacheable, its definition ought to describe how, and under what
conditions, a cache can store a response and use it to satisfy a
subsequent request. The new method ought to describe whether it can
be made conditional (Section 8.2) and, if so, how a server responds
when the condition is false. Likewise, if the new method might have
some use for partial response semantics (Section 8.3), it ought to
document this, too.
Note: Avoid defining a method name that starts with "M-", since
that prefix might be misinterpreted as having the semantics
assigned to it by [RFC2774].
8. Request Header Fields
A client sends request header fields to provide more information
about the request context, make the request conditional based on the
target resource state, suggest preferred formats for the response,
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.
8.1. Controls
Controls are request header fields that direct specific handling of
the request.
+-------------------+----------------------------+
| Header Field Name | Defined in... |
+-------------------+----------------------------+
| Cache-Control | Section 5.2 of [Caching] |
| Expect | Section 8.1.1 |
| Host | Section 5.4 |
| Max-Forwards | Section 8.1.2 |
| Pragma | Section 5.4 of [Caching] |
| TE | Section 7.4 of [Messaging] |
+-------------------+----------------------------+
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8.1.1. Expect
The "Expect" header field in a request indicates a certain set of
behaviors (expectations) that need to be supported by the server in
order to properly handle this request. The only such expectation
defined by this specification is 100-continue.
Expect = "100-continue"
The Expect field-value is case-insensitive.
A server that receives an Expect field-value other than 100-continue
MAY respond with a 417 (Expectation Failed) status code to indicate
that the unexpected expectation cannot be met.
A 100-continue expectation informs recipients that the client is
about to send a (presumably large) message body in this request and
wishes to receive a 100 (Continue) interim response if the request-
line and header fields are not sufficient to cause an immediate
success, redirect, or error response. This allows the client to wait
for an indication that it is worthwhile to send the message body
before actually doing so, which can improve efficiency when the
message body is huge or when the client anticipates that an error is
likely (e.g., when sending a state-changing method, for the first
time, without previously verified authentication credentials).
For example, a request that begins with
PUT /somewhere/fun HTTP/1.1
Host: origin.example.com
Content-Type: video/h264
Content-Length: 1234567890987
Expect: 100-continue
allows the origin server to immediately respond with an error
message, such as 401 (Unauthorized) or 405 (Method Not Allowed),
before the client starts filling the pipes with an unnecessary data
transfer.
Requirements for clients:
o A client MUST NOT generate a 100-continue expectation in a request
that does not include a message body.
o A client that will wait for a 100 (Continue) response before
sending the request message body MUST send an Expect header field
containing a 100-continue expectation.
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o A client that sends a 100-continue expectation is not required to
wait for any specific length of time; such a client MAY proceed to
send the message body even if it has not yet received a response.
Furthermore, since 100 (Continue) responses cannot be sent through
an HTTP/1.0 intermediary, such a client SHOULD NOT wait for an
indefinite period before sending the message body.
o A client that receives a 417 (Expectation Failed) status code in
response to a request containing a 100-continue expectation SHOULD
repeat that request without a 100-continue expectation, since the
417 response merely indicates that the response chain does not
support expectations (e.g., it passes through an HTTP/1.0 server).
Requirements for servers:
o A server that receives a 100-continue expectation in an HTTP/1.0
request MUST ignore that expectation.
o A server MAY omit sending a 100 (Continue) response if it has
already received some or all of the message body for the
corresponding request, or if the framing indicates that there is
no message body.
o A server that sends a 100 (Continue) response MUST ultimately send
a final status code, once the message body is received and
processed, unless the connection is closed prematurely.
o A server that responds with a final status code before reading the
entire message body SHOULD indicate in that response whether it
intends to close the connection or continue reading and discarding
the request message (see Section 9.6 of [Messaging]).
An origin server MUST, upon receiving an HTTP/1.1 (or later) request-
line and a complete header section that contains a 100-continue
expectation and indicates a request message body will follow, either
send an immediate response with a final status code, if that status
can be determined by examining just the request-line and header
fields, or send an immediate 100 (Continue) response to encourage the
client to send the request's message body. The origin server MUST
NOT wait for the message body before sending the 100 (Continue)
response.
A proxy MUST, upon receiving an HTTP/1.1 (or later) request-line and
a complete header section that contains a 100-continue expectation
and indicates a request message body will follow, either send an
immediate response with a final status code, if that status can be
determined by examining just the request-line and header fields, or
begin forwarding the request toward the origin server by sending a
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corresponding request-line and header section to the next inbound
server. If the proxy believes (from configuration or past
interaction) that the next inbound server only supports HTTP/1.0, the
proxy MAY generate an immediate 100 (Continue) response to encourage
the client to begin sending the message body.
Note: The Expect header field was added after the original
publication of HTTP/1.1 [RFC2068] as both the means to request an
interim 100 (Continue) response and the general mechanism for
indicating must-understand extensions. However, the extension
mechanism has not been used by clients and the must-understand
requirements have not been implemented by many servers, rendering
the extension mechanism useless. This specification has removed
the extension mechanism in order to simplify the definition and
processing of 100-continue.
8.1.2. Max-Forwards
The "Max-Forwards" header field provides a mechanism with the TRACE
(Section 7.3.8) and OPTIONS (Section 7.3.7) request methods to limit
the number of times that the request is forwarded by proxies. This
can be useful when the client is attempting to trace a request that
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 intermediary that receives 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
intermediary MUST NOT forward the request; instead, the intermediary
MUST respond as the final recipient. If the received Max-Forwards
value is greater than zero, the intermediary MUST generate an updated
Max-Forwards field in the forwarded message with a field-value that
is the lesser of a) the received value decremented by one (1) or b)
the recipient's maximum supported value for Max-Forwards.
A recipient MAY ignore a Max-Forwards header field received with any
other request methods.
8.2. Preconditions
A conditional request is an HTTP request with one or more request
header fields that indicate a precondition to be tested before
applying the request method to the target resource. Section 8.2.1
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defines when preconditions are applied. Section 8.2.2 defines the
order of evaluation when more than one precondition is present.
Conditional GET requests are the most efficient mechanism for HTTP
cache updates [Caching]. Conditionals can also be applied to state-
changing methods, such as PUT and DELETE, to prevent the "lost
update" problem: one client accidentally overwriting the work of
another client that has been acting in parallel.
Conditional request preconditions are based on the state of the
target resource as a whole (its current value set) or the state as
observed in a previously obtained representation (one value in that
set). A resource might have multiple current representations, each
with its own observable state. The conditional request mechanisms
assume that the mapping of requests to a "selected representation"
(Section 6) will be consistent over time if the server intends to
take advantage of conditionals. Regardless, if the mapping is
inconsistent and the server is unable to select the appropriate
representation, then no harm will result when the precondition
evaluates to false.
The following request header fields allow a client to place a
precondition on the state of the target resource, so that the action
corresponding to the method semantics will not be applied if the
precondition evaluates to false. Each precondition defined by this
specification consists of a comparison between a set of validators
obtained from prior representations of the target resource to the
current state of validators for the selected representation
(Section 10.2). Hence, these preconditions evaluate whether the
state of the target resource has changed since a given state known by
the client. The effect of such an evaluation depends on the method
semantics and choice of conditional, as defined in Section 8.2.1.
+---------------------+---------------+
| Header Field Name | Defined in... |
+---------------------+---------------+
| If-Match | Section 8.2.3 |
| If-None-Match | Section 8.2.4 |
| If-Modified-Since | Section 8.2.5 |
| If-Unmodified-Since | Section 8.2.6 |
| If-Range | Section 8.2.7 |
+---------------------+---------------+
8.2.1. Evaluation
Except when excluded below, a recipient cache or origin server MUST
evaluate received request preconditions after it has successfully
performed its normal request checks and just before it would perform
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the action associated with the request method. A server MUST ignore
all received preconditions if its response to the same request
without those conditions would have been a status code other than a
2xx (Successful) or 412 (Precondition Failed). In other words,
redirects and failures take precedence over the evaluation of
preconditions in conditional requests.
A server that is not the origin server for the target resource and
cannot act as a cache for requests on the target resource MUST NOT
evaluate the conditional request header fields defined by this
specification, and it MUST forward them if the request is forwarded,
since the generating client intends that they be evaluated by a
server that can provide a current representation. Likewise, a server
MUST ignore the conditional request header fields defined by this
specification when received with a request method that does not
involve the selection or modification of a selected representation,
such as CONNECT, OPTIONS, or TRACE.
Conditional request header fields that are defined by extensions to
HTTP might place conditions on all recipients, on the state of the
target resource in general, or on a group of resources. For
instance, the "If" header field in WebDAV can make a request
conditional on various aspects of multiple resources, such as locks,
if the recipient understands and implements that field ([RFC4918],
Section 10.4).
Although conditional request header fields are defined as being
usable with the HEAD method (to keep HEAD's semantics consistent with
those of GET), there is no point in sending a conditional HEAD
because a successful response is around the same size as a 304 (Not
Modified) response and more useful than a 412 (Precondition Failed)
response.
8.2.2. Precedence
When more than one conditional request header field is present in a
request, the order in which the fields are evaluated becomes
important. In practice, the fields defined in this document are
consistently implemented in a single, logical order, since "lost
update" preconditions have more strict requirements than cache
validation, a validated cache is more efficient than a partial
response, and entity tags are presumed to be more accurate than date
validators.
A recipient cache or origin server MUST evaluate the request
preconditions defined by this specification in the following order:
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1. When recipient is the origin server and If-Match is present,
evaluate the If-Match precondition:
* if true, continue to step 3
* if false, respond 412 (Precondition Failed) unless it can be
determined that the state-changing request has already
succeeded (see Section 8.2.3)
2. When recipient is the origin server, If-Match is not present, and
If-Unmodified-Since is present, evaluate the If-Unmodified-Since
precondition:
* if true, continue to step 3
* if false, respond 412 (Precondition Failed) unless it can be
determined that the state-changing request has already
succeeded (see Section 8.2.6)
3. When If-None-Match is present, evaluate the If-None-Match
precondition:
* if true, continue to step 5
* if false for GET/HEAD, respond 304 (Not Modified)
* if false for other methods, respond 412 (Precondition Failed)
4. When the method is GET or HEAD, If-None-Match is not present, and
If-Modified-Since is present, evaluate the If-Modified-Since
precondition:
* if true, continue to step 5
* if false, respond 304 (Not Modified)
5. When the method is GET and both Range and If-Range are present,
evaluate the If-Range precondition:
* if the validator matches and the Range specification is
applicable to the selected representation, respond 206
(Partial Content)
6. Otherwise,
* all conditions are met, so perform the requested action and
respond according to its success or failure.
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Any extension to HTTP/1.1 that defines additional conditional request
header fields ought to define its own expectations regarding the
order for evaluating such fields in relation to those defined in this
document and other conditionals that might be found in practice.
8.2.3. If-Match
The "If-Match" header field makes the request method conditional on
the recipient origin server either having at least one current
representation of the target resource, when the field-value is "*",
or having a current representation of the target resource that has an
entity-tag matching a member of the list of entity-tags provided in
the field-value.
An origin server MUST use the strong comparison function when
comparing entity-tags for If-Match (Section 10.2.3.2), since the
client intends this precondition to prevent the method from being
applied if there have been any changes to the representation data.
If-Match = "*" / 1#entity-tag
Examples:
If-Match: "xyzzy"
If-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-Match: *
If-Match is most often used with state-changing methods (e.g., POST,
PUT, DELETE) to prevent accidental overwrites when multiple user
agents might be acting in parallel on the same resource (i.e., to
prevent the "lost update" problem). It can also be used with safe
methods to abort a request if the selected representation does not
match one already stored (or partially stored) from a prior request.
An origin server that receives an If-Match header field MUST evaluate
the condition prior to performing the method (Section 8.2.1). If the
field-value is "*", the condition is false if the origin server does
not have a current representation for the target resource. If the
field-value is a list of entity-tags, the condition is false if none
of the listed tags match the entity-tag of the selected
representation.
An origin server MUST NOT perform the requested method if a received
If-Match condition evaluates to false; instead, the origin server
MUST respond with either a) the 412 (Precondition Failed) status code
or b) one of the 2xx (Successful) status codes if the origin server
has verified that a state change is being requested and the final
state is already reflected in the current state of the target
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resource (i.e., the change requested by the user agent has already
succeeded, but the user agent might not be aware of it, perhaps
because the prior response was lost or a compatible change was made
by some other user agent). In the latter case, the origin server
MUST NOT send a validator header field in the response unless it can
verify that the request is a duplicate of an immediately prior change
made by the same user agent.
The If-Match header field can be ignored by caches and intermediaries
because it is not applicable to a stored response.
8.2.4. If-None-Match
The "If-None-Match" header field makes the request method conditional
on a recipient cache or origin server either not having any current
representation of the target resource, when the field-value is "*",
or having a selected representation with an entity-tag that does not
match any of those listed in the field-value.
A recipient MUST use the weak comparison function when comparing
entity-tags for If-None-Match (Section 10.2.3.2), since weak entity-
tags can be used for cache validation even if there have been changes
to the representation data.
If-None-Match = "*" / 1#entity-tag
Examples:
If-None-Match: "xyzzy"
If-None-Match: W/"xyzzy"
If-None-Match: "xyzzy", "r2d2xxxx", "c3piozzzz"
If-None-Match: W/"xyzzy", W/"r2d2xxxx", W/"c3piozzzz"
If-None-Match: *
If-None-Match is primarily used in conditional GET requests to enable
efficient updates of cached information with a minimum amount of
transaction overhead. When a client desires to update one or more
stored responses that have entity-tags, the client SHOULD generate an
If-None-Match header field containing a list of those entity-tags
when making a GET request; this allows recipient servers to send a
304 (Not Modified) response to indicate when one of those stored
responses matches the selected representation.
If-None-Match can also be used with a value of "*" to prevent an
unsafe request method (e.g., PUT) from inadvertently modifying an
existing representation of the target resource when the client
believes that the resource does not have a current representation
(Section 7.2.1). This is a variation on the "lost update" problem
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that might arise if more than one client attempts to create an
initial representation for the target resource.
An origin server that receives an If-None-Match header field MUST
evaluate the condition prior to performing the method
(Section 8.2.1). If the field-value is "*", the condition is false
if the origin server has a current representation for the target
resource. If the field-value is a list of entity-tags, the condition
is false if one of the listed tags match the entity-tag of the
selected representation.
An origin server MUST NOT perform the requested method if the
condition evaluates to false; instead, the origin server MUST respond
with either a) the 304 (Not Modified) status code if the request
method is GET or HEAD or b) the 412 (Precondition Failed) status code
for all other request methods.
Requirements on cache handling of a received If-None-Match header
field are defined in Section 4.3.2 of [Caching].
8.2.5. If-Modified-Since
The "If-Modified-Since" header field makes a GET or HEAD request
method conditional on the selected representation's modification date
being more recent than the date provided in the field-value.
Transfer of the selected representation's data is avoided if that
data has not changed.
If-Modified-Since = HTTP-date
An example of the field is:
If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A recipient MUST ignore If-Modified-Since if the request contains an
If-None-Match header field; the condition in If-None-Match is
considered to be a more accurate replacement for the condition in If-
Modified-Since, and the two are only combined for the sake of
interoperating with older intermediaries that might not implement If-
None-Match.
A recipient MUST ignore the If-Modified-Since header field if the
received field-value is not a valid HTTP-date, or if the request
method is neither GET nor HEAD.
A recipient MUST interpret an If-Modified-Since field-value's
timestamp in terms of the origin server's clock.
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If-Modified-Since is typically used for two distinct purposes: 1) to
allow efficient updates of a cached representation that does not have
an entity-tag and 2) to limit the scope of a web traversal to
resources that have recently changed.
When used for cache updates, a cache will typically use the value of
the cached message's Last-Modified field to generate the field value
of If-Modified-Since. This behavior is most interoperable for cases
where clocks are poorly synchronized or when the server has chosen to
only honor exact timestamp matches (due to a problem with Last-
Modified dates that appear to go "back in time" when the origin
server's clock is corrected or a representation is restored from an
archived backup). However, caches occasionally generate the field
value based on other data, such as the Date header field of the
cached message or the local clock time that the message was received,
particularly when the cached message does not contain a Last-Modified
field.
When used for limiting the scope of retrieval to a recent time
window, a user agent will generate an If-Modified-Since field value
based on either its own local clock or a Date header field received
from the server in a prior response. Origin servers that choose an
exact timestamp match based on the selected representation's Last-
Modified field will not be able to help the user agent limit its data
transfers to only those changed during the specified window.
An origin server that receives an If-Modified-Since header field
SHOULD evaluate the condition prior to performing the method
(Section 8.2.1). The origin server SHOULD NOT perform the requested
method if the selected representation's last modification date is
earlier than or equal to the date provided in the field-value;
instead, the origin server SHOULD generate a 304 (Not Modified)
response, including only those metadata that are useful for
identifying or updating a previously cached response.
Requirements on cache handling of a received If-Modified-Since header
field are defined in Section 4.3.2 of [Caching].
8.2.6. If-Unmodified-Since
The "If-Unmodified-Since" header field makes the request method
conditional on the selected representation's last modification date
being earlier than or equal to the date provided in the field-value.
This field accomplishes the same purpose as If-Match for cases where
the user agent does not have an entity-tag for the representation.
If-Unmodified-Since = HTTP-date
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An example of the field is:
If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT
A recipient MUST ignore If-Unmodified-Since if the request contains
an If-Match header field; the condition in If-Match is considered to
be a more accurate replacement for the condition in If-Unmodified-
Since, and the two are only combined for the sake of interoperating
with older intermediaries that might not implement If-Match.
A recipient MUST ignore the If-Unmodified-Since header field if the
received field-value is not a valid HTTP-date.
A recipient MUST interpret an If-Unmodified-Since field-value's
timestamp in terms of the origin server's clock.
If-Unmodified-Since is most often used with state-changing methods
(e.g., POST, PUT, DELETE) to prevent accidental overwrites when
multiple user agents might be acting in parallel on a resource that
does not supply entity-tags with its representations (i.e., to
prevent the "lost update" problem). It can also be used with safe
methods to abort a request if the selected representation does not
match one already stored (or partially stored) from a prior request.
An origin server that receives an If-Unmodified-Since header field
MUST evaluate the condition prior to performing the method
(Section 8.2.1). The origin server MUST NOT perform the requested
method if the selected representation's last modification date is
more recent than the date provided in the field-value; instead the
origin server MUST respond with either a) the 412 (Precondition
Failed) status code or b) one of the 2xx (Successful) status codes if
the origin server has verified that a state change is being requested
and the final state is already reflected in the current state of the
target resource (i.e., the change requested by the user agent has
already succeeded, but the user agent might not be aware of that
because the prior response message was lost or a compatible change
was made by some other user agent). In the latter case, the origin
server MUST NOT send a validator header field in the response unless
it can verify that the request is a duplicate of an immediately prior
change made by the same user agent.
The If-Unmodified-Since header field can be ignored by caches and
intermediaries because it is not applicable to a stored response.
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8.2.7. If-Range
The "If-Range" header field provides a special conditional request
mechanism that is similar to the If-Match and If-Unmodified-Since
header fields but that instructs the recipient to ignore the Range
header field if the validator doesn't match, resulting in transfer of
the new selected representation instead of a 412 (Precondition
Failed) response.
If a client has a partial copy of a representation and wishes to have
an up-to-date copy of the entire representation, it could use the
Range header field with a conditional GET (using either or both of
If-Unmodified-Since and If-Match.) However, if the precondition
fails because the representation has been modified, the client would
then have to make a second request to obtain the entire current
representation.
The "If-Range" header field allows a client to "short-circuit" the
second request. Informally, its meaning is as follows: if the
representation is unchanged, send me the part(s) that I am requesting
in Range; otherwise, send me the entire representation.
If-Range = entity-tag / HTTP-date
A client MUST NOT generate an If-Range header field in a request that
does not contain a Range header field. A server MUST ignore an If-
Range header field received in a request that does not contain a
Range header field. An origin server MUST ignore an If-Range header
field received in a request for a target resource that does not
support Range requests.
A client MUST NOT generate an If-Range header field containing an
entity-tag that is marked as weak. A client MUST NOT generate an If-
Range header field containing an HTTP-date unless the client has no
entity-tag for the corresponding representation and the date is a
strong validator in the sense defined by Section 10.2.2.2.
A server that evaluates an If-Range precondition MUST use the strong
comparison function when comparing entity-tags (Section 10.2.3.2) and
MUST evaluate the condition as false if an HTTP-date validator is
provided that is not a strong validator in the sense defined by
Section 10.2.2.2. A valid entity-tag can be distinguished from a
valid HTTP-date by examining the first two characters for a DQUOTE.
If the validator given in the If-Range header field matches the
current validator for the selected representation of the target
resource, then the server SHOULD process the Range header field as
requested. If the validator does not match, the server MUST ignore
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the Range header field. Note that this comparison by exact match,
including when the validator is an HTTP-date, differs from the
"earlier than or equal to" comparison used when evaluating an If-
Unmodified-Since conditional.
8.3. Range
The "Range" header field on a GET request modifies the method
semantics to request transfer of only one or more subranges of the
selected representation data, rather than the entire selected
representation data.
Range = byte-ranges-specifier / other-ranges-specifier
other-ranges-specifier = other-range-unit "=" other-range-set
other-range-set = 1*VCHAR
Clients often encounter interrupted data transfers as a result of
canceled requests or dropped connections. When a client has stored a
partial representation, it is desirable to request the remainder of
that representation in a subsequent request rather than transfer the
entire representation. Likewise, devices with limited local storage
might benefit from being able to request only a subset of a larger
representation, such as a single page of a very large document, or
the dimensions of an embedded image.
Range requests are an OPTIONAL feature of HTTP, designed so that
recipients not implementing this feature (or not supporting it for
the target resource) can respond as if it is a normal GET request
without impacting interoperability. Partial responses are indicated
by a distinct status code to not be mistaken for full responses by
caches that might not implement the feature.
A server MAY ignore the Range header field. However, origin servers
and intermediate caches ought to support byte ranges when possible,
since Range supports efficient recovery from partially failed
transfers and partial retrieval of large representations. A server
MUST ignore a Range header field received with a request method other
than GET.
Although the range request mechanism is designed to allow for
extensible range types, this specification only defines requests for
byte ranges.
An origin server MUST ignore a Range header field that contains a
range unit it does not understand. A proxy MAY discard a Range
header field that contains a range unit it does not understand.
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A server that supports range requests MAY ignore or reject a Range
header field that consists of more than two overlapping ranges, or a
set of many small ranges that are not listed in ascending order,
since both are indications of either a broken client or a deliberate
denial-of-service attack (Section 12.13). A client SHOULD NOT
request multiple ranges that are inherently less efficient to process
and transfer than a single range that encompasses the same data.
A client that is requesting multiple ranges SHOULD list those ranges
in ascending order (the order in which they would typically be
received in a complete representation) unless there is a specific
need to request a later part earlier. For example, a user agent
processing a large representation with an internal catalog of parts
might need to request later parts first, particularly if the
representation consists of pages stored in reverse order and the user
agent wishes to transfer one page at a time.
The Range header field is evaluated after evaluating the precondition
header fields defined in Section 8.2, and only if the result in
absence of the Range header field would be a 200 (OK) response. In
other words, Range is ignored when a conditional GET would result in
a 304 (Not Modified) response.
The If-Range header field (Section 8.2.7) can be used as a
precondition to applying the Range header field.
If all of the preconditions are true, the server supports the Range
header field for the target resource, and the specified range(s) are
valid and satisfiable (as defined in Section 6.1.4.1), the server
SHOULD send a 206 (Partial Content) response with a payload
containing one or more partial representations that correspond to the
satisfiable ranges requested.
If all of the preconditions are true, the server supports the Range
header field for the target resource, and the specified range(s) are
invalid or unsatisfiable, the server SHOULD send a 416 (Range Not
Satisfiable) response.
8.4. Content Negotiation
The following request header fields are sent by a user agent to
engage in proactive negotiation of the response content, as defined
in Section 6.4.1. The preferences sent in these fields apply to any
content in the response, including representations of the target
resource, representations of error or processing status, and
potentially even the miscellaneous text strings that might appear
within the protocol.
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+-------------------+---------------+
| Header Field Name | Defined in... |
+-------------------+---------------+
| Accept | Section 8.4.2 |
| Accept-Charset | Section 8.4.3 |
| Accept-Encoding | Section 8.4.4 |
| Accept-Language | Section 8.4.5 |
+-------------------+---------------+
8.4.1. Quality Values
Many of the request header fields for proactive 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.
8.4.2. 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 [ "=" ( token / quoted-string ) ]
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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 can include media type
parameters that are applicable to that range.
Each media-range might be followed by zero or more applicable media
type parameters (e.g., charset), an optional "q" parameter for
indicating a relative weight (Section 8.4.1), and then zero or more
extension parameters. The "q" parameter is necessary if any
extensions (accept-ext) are present, since it acts as a separator
between the two parameter sets.
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
is interpreted as "I prefer audio/basic, but send me any audio type
if it is the best available after an 80% markdown in quality".
A request without any Accept header field implies that the user agent
will accept any media type in response. If the 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 can either honor the header field by sending a 406 (Not
Acceptable) response or disregard the header field by treating the
response as if it is not subject to 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 equally 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".
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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
that matches the 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 that cannot interact with other rendering agents, this
default set ought to be configurable by the user.
8.4.3. Accept-Charset
The "Accept-Charset" header field can be sent by a user agent to
indicate what charsets are acceptable in textual response content.
This field allows user agents capable of understanding more
comprehensive or special-purpose charsets to signal that capability
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to an origin server that is capable of representing information in
those charsets.
Accept-Charset = 1#( ( charset / "*" ) [ weight ] )
Charset names are defined in Section 6.1.1.1. A user agent MAY
associate a quality value with each charset to indicate the user's
relative preference for that charset, as defined in Section 8.4.1.
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 charset that is not mentioned elsewhere in the Accept-
Charset field. If no "*" is present in an Accept-Charset field, then
any charsets 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 charset in response. Most general-purpose
user agents do not send Accept-Charset, unless specifically
configured to do so, because a detailed list of supported charsets
makes it easier for a server to identify an individual by virtue of
the user agent's request characteristics (Section 12.11).
If an Accept-Charset header field is present in a request and none of
the available representations for the response has a charset that is
listed as acceptable, the origin server can either honor the header
field, by sending a 406 (Not Acceptable) response, or disregard the
header field by treating the resource as if it is not subject to
content negotiation.
8.4.4. Accept-Encoding
The "Accept-Encoding" header field can be used by user agents to
indicate what response content-codings (Section 6.1.2) 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
representing the preference for that encoding, as defined in
Section 8.4.1. The asterisk "*" symbol in an Accept-Encoding field
matches any available content-coding not explicitly listed in the
header field.
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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 request without an Accept-Encoding header field implies that the
user agent has no preferences regarding content-codings. Although
this allows the server to use any content-coding in a response, it
does not imply that the user agent will be able to correctly process
all encodings.
A server tests whether a content-coding for a given representation is
acceptable using these rules:
1. If no Accept-Encoding field is in the request, any content-coding
is considered acceptable by the user agent.
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 8.4.1, 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.
Note: Most HTTP/1.0 applications do not recognize or obey qvalues
associated with content-codings. This means that qvalues might
not work and are not permitted with x-gzip or x-compress.
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8.4.5. 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 6.1.3.
Accept-Language = 1#( language-range [ weight ] )
language-range =
<language-range, see [RFC4647], Section 2.1>
Each language-range can be given an associated quality value
representing an estimate of the user's preference for the languages
specified by that range, as defined in Section 8.4.1. 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".
A request without any Accept-Language header field implies that the
user agent will accept any language in response. If the header field
is present in a request and none of the available representations for
the response have a matching language tag, the origin server can
either disregard the header field by treating the response as if it
is not subject to content negotiation or honor the header field by
sending a 406 (Not Acceptable) response. However, the latter is not
encouraged, as doing so can prevent users from accessing content that
they might be able to use (with translation software, for example).
Note that some recipients treat the order in which language tags are
listed as an indication of descending priority, particularly for tags
that are assigned equal quality values (no value is the same as q=1).
However, this behavior cannot be relied upon. For consistency and to
maximize interoperability, many user agents assign each language tag
a unique quality value while also listing them in order of decreasing
quality. Additional discussion of language priority lists can be
found in 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. The "Basic Filtering" scheme
([RFC4647], Section 3.3.1) is identical to the matching scheme that
was previously defined for HTTP 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 (Section 12.11).
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Since intelligibility is highly dependent on the individual user,
user agents need to allow user control over the linguistic preference
(either through configuration of the user agent itself or by
defaulting to a user controllable system setting). A user agent that
does not provide such control to the user MUST NOT send an Accept-
Language header field.
Note: User agents ought to provide guidance to users when setting
a preference, since users are rarely familiar with the details of
language matching as described above. For 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 the list for
better matching behavior.
8.5. Authentication Credentials
HTTP provides a general framework for access control and
authentication, via an extensible set of challenge-response
authentication schemes, which can be used by a server to challenge a
client request and by a client to provide authentication information.
Two header fields are used for carrying authentication credentials.
Note that various custom mechanisms for user authentication use the
Cookie header field for this purpose, as defined in [RFC6265].
+---------------------+---------------+
| Header Field Name | Defined in... |
+---------------------+---------------+
| Authorization | Section 8.5.3 |
| Proxy-Authorization | Section 8.5.4 |
+---------------------+---------------+
8.5.1. Challenge and Response
HTTP provides a simple challenge-response authentication framework
that can be used by a server to challenge a client request and by a
client to provide authentication information. It uses a case-
insensitive token as a means to identify the authentication scheme,
followed by additional information necessary for achieving
authentication via that scheme. The latter can be either a comma-
separated list of parameters or a single sequence of characters
capable of holding base64-encoded information.
Authentication parameters are name=value pairs, where the name token
is matched case-insensitively, and each parameter name MUST only
occur once per challenge.
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auth-scheme = token
auth-param = token BWS "=" BWS ( token / quoted-string )
token68 = 1*( ALPHA / DIGIT /
"-" / "." / "_" / "~" / "+" / "/" ) *"="
The token68 syntax allows the 66 unreserved URI characters
([RFC3986]), plus a few others, so that it can hold a base64,
base64url (URL and filename safe alphabet), base32, or base16 (hex)
encoding, with or without padding, but excluding whitespace
([RFC4648]).
A 401 (Unauthorized) response message is used by an origin server to
challenge the authorization of a user agent, including a WWW-
Authenticate header field containing at least one challenge
applicable to the requested resource.
A 407 (Proxy Authentication Required) response message is used by a
proxy to challenge the authorization of a client, including a Proxy-
Authenticate header field containing at least one challenge
applicable to the proxy for the requested resource.
challenge = auth-scheme [ 1*SP ( token68 / #auth-param ) ]
Note: Many clients fail to parse a challenge that contains an
unknown scheme. A workaround for this problem is to list well-
supported schemes (such as "basic") first.
A user agent that wishes to authenticate itself with an origin server
-- usually, but not necessarily, after receiving a 401 (Unauthorized)
-- can do so by including an Authorization header field with the
request.
A client that wishes to authenticate itself with a proxy -- usually,
but not necessarily, after receiving a 407 (Proxy Authentication
Required) -- can do so by including a Proxy-Authorization header
field with the request.
Both the Authorization field value and the Proxy-Authorization field
value contain the client's credentials for the realm of the resource
being requested, based upon a challenge received in a response
(possibly at some point in the past). When creating their values,
the user agent ought to do so by selecting the challenge with what it
considers to be the most secure auth-scheme that it understands,
obtaining credentials from the user as appropriate. Transmission of
credentials within header field values implies significant security
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considerations regarding the confidentiality of the underlying
connection, as described in Section 12.14.1.
credentials = auth-scheme [ 1*SP ( token68 / #auth-param ) ]
Upon receipt of a request for a protected resource that omits
credentials, contains invalid credentials (e.g., a bad password) or
partial credentials (e.g., when the authentication scheme requires
more than one round trip), an origin server SHOULD send a 401
(Unauthorized) response that contains a WWW-Authenticate header field
with at least one (possibly new) challenge applicable to the
requested resource.
Likewise, upon receipt of a request that omits proxy credentials or
contains invalid or partial proxy credentials, a proxy that requires
authentication SHOULD generate a 407 (Proxy Authentication Required)
response that contains a Proxy-Authenticate header field with at
least one (possibly new) challenge applicable to the proxy.
A server that receives valid credentials that are not adequate to
gain access ought to respond with the 403 (Forbidden) status code
(Section 9.5.4).
HTTP does not restrict applications to this simple challenge-response
framework for access authentication. Additional mechanisms can be
used, such as authentication at the transport level or via message
encapsulation, and with additional header fields specifying
authentication information. However, such additional mechanisms are
not defined by this specification.
8.5.2. Protection Space (Realm)
The "realm" authentication parameter is reserved for use by
authentication schemes that wish to indicate a scope of protection.
A protection space is defined by the canonical root URI (the scheme
and authority components of the effective request URI; see
Section 5.3) of the server being accessed, in combination with the
realm value if present. These realms allow the protected resources
on a server to be partitioned into a set of protection spaces, each
with its own authentication scheme and/or authorization database.
The realm value is a string, generally assigned by the origin server,
that can have additional semantics specific to the authentication
scheme. Note that a response can have multiple challenges with the
same auth-scheme but with different realms.
The protection space determines the domain over which credentials can
be automatically applied. If a prior request has been authorized,
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the user agent MAY reuse the same credentials for all other requests
within that protection space for a period of time determined by the
authentication scheme, parameters, and/or user preferences (such as a
configurable inactivity timeout). Unless specifically allowed by the
authentication scheme, a single protection space cannot extend
outside the scope of its server.
For historical reasons, a sender MUST only generate the quoted-string
syntax. Recipients might have to support both token and quoted-
string syntax for maximum interoperability with existing clients that
have been accepting both notations for a long time.
8.5.3. Authorization
The "Authorization" header field allows a user agent to authenticate
itself with an origin server -- usually, but not necessarily, after
receiving a 401 (Unauthorized) response. Its value consists of
credentials containing the authentication information of the user
agent for the realm of the resource being requested.
Authorization = credentials
If a request is authenticated and a realm specified, the same
credentials are presumed to be valid for all other requests within
this realm (assuming that the authentication scheme itself does not
require otherwise, such as credentials that vary according to a
challenge value or using synchronized clocks).
A proxy forwarding a request MUST NOT modify any Authorization fields
in that request. See Section 3.2 of [Caching] for details of and
requirements pertaining to handling of the Authorization field by
HTTP caches.
8.5.4. Proxy-Authorization
The "Proxy-Authorization" header field allows the client to identify
itself (or its user) to a proxy that requires authentication. Its
value consists of credentials containing the authentication
information of the client for the proxy and/or realm of the resource
being requested.
Proxy-Authorization = credentials
Unlike Authorization, the Proxy-Authorization header field applies
only to the next inbound proxy that demanded authentication using the
Proxy-Authenticate field. When multiple proxies are used in a chain,
the Proxy-Authorization header field is consumed by the first inbound
proxy that was expecting to receive credentials. A proxy MAY relay
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the credentials from the client request to the next proxy if that is
the mechanism by which the proxies cooperatively authenticate a given
request.
8.5.5. Authentication Scheme Extensibility
Aside from the general framework, this document does not specify any
authentication schemes. New and existing authentication schemes are
specified independently and ought to be registered within the
"Hypertext Transfer Protocol (HTTP) Authentication Scheme Registry".
For example, the "basic" and "digest" authentication schemes are
defined by RFC 7617 and RFC 7616, respectively.
8.5.5.1. Authentication Scheme Registry
The "Hypertext Transfer Protocol (HTTP) Authentication Scheme
Registry" defines the namespace for the authentication schemes in
challenges and credentials. It is maintained at
<https://www.iana.org/assignments/http-authschemes>.
Registrations MUST include the following fields:
o Authentication Scheme Name
o Pointer to specification text
o Notes (optional)
Values to be added to this namespace require IETF Review (see
[RFC8126], Section 4.8).
8.5.5.2. Considerations for New Authentication Schemes
There are certain aspects of the HTTP Authentication framework that
put constraints on how new authentication schemes can work:
o HTTP authentication is presumed to be stateless: all of the
information necessary to authenticate a request MUST be provided
in the request, rather than be dependent on the server remembering
prior requests. Authentication based on, or bound to, the
underlying connection is outside the scope of this specification
and inherently flawed unless steps are taken to ensure that the
connection cannot be used by any party other than the
authenticated user (see Section 2.2).
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o The authentication parameter "realm" is reserved for defining
protection spaces as described in Section 8.5.2. New schemes MUST
NOT use it in a way incompatible with that definition.
o The "token68" notation was introduced for compatibility with
existing authentication schemes and can only be used once per
challenge or credential. Thus, new schemes ought to use the auth-
param syntax instead, because otherwise future extensions will be
impossible.
o The parsing of challenges and credentials is defined by this
specification and cannot be modified by new authentication
schemes. When the auth-param syntax is used, all parameters ought
to support both token and quoted-string syntax, and syntactical
constraints ought to be defined on the field value after parsing
(i.e., quoted-string processing). This is necessary so that
recipients can use a generic parser that applies to all
authentication schemes.
Note: The fact that the value syntax for the "realm" parameter is
restricted to quoted-string was a bad design choice not to be
repeated for new parameters.
o Definitions of new schemes ought to define the treatment of
unknown extension parameters. In general, a "must-ignore" rule is
preferable to a "must-understand" rule, because otherwise it will
be hard to introduce new parameters in the presence of legacy
recipients. Furthermore, it's good to describe the policy for
defining new parameters (such as "update the specification" or
"use this registry").
o Authentication schemes need to document whether they are usable in
origin-server authentication (i.e., using WWW-Authenticate), and/
or proxy authentication (i.e., using Proxy-Authenticate).
o The credentials carried in an Authorization header field are
specific to the user agent and, therefore, have the same effect on
HTTP caches as the "private" Cache-Control response directive
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(Section 5.2.2.6 of [Caching]), within the scope of the request in
which they appear.
Therefore, new authentication schemes that choose not to carry
credentials in the Authorization header field (e.g., using a newly
defined header field) will need to explicitly disallow caching, by
mandating the use of either Cache-Control request directives
(e.g., "no-store", Section 5.2.1.5 of [Caching]) or response
directives (e.g., "private").
8.6. Request Context
The following request header fields provide additional information
about the request context, including information about the user, user
agent, and resource behind the request.
+-------------------+---------------+
| Header Field Name | Defined in... |
+-------------------+---------------+
| From | Section 8.6.1 |
| Referer | Section 8.6.2 |
| User-Agent | Section 8.6.3 |
+-------------------+---------------+
8.6.1. From
The "From" header field contains an Internet email address for a
human user who controls the requesting user agent. The address ought
to be machine-usable, as defined by "mailbox" in Section 3.4 of
[RFC5322]:
From = mailbox
mailbox = <mailbox, see [RFC5322], Section 3.4>
An example is:
From: webmaster@example.org
The From header field is rarely sent by non-robotic user agents. A
user agent SHOULD NOT send a From header field without explicit
configuration by the user, since that might conflict with the user's
privacy interests or their site's security policy.
A robotic user agent SHOULD send a valid From header field so that
the person responsible for running the robot can be contacted if
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problems occur on servers, such as if the robot is sending excessive,
unwanted, or invalid requests.
A server SHOULD NOT use the From header field for access control or
authentication, since most recipients will assume that the field
value is public information.
8.6.2. Referer
The "Referer" [sic] header field allows the user agent to specify a
URI reference for the resource from which the target URI was obtained
(i.e., the "referrer", though the field name is misspelled). A user
agent MUST NOT include the fragment and userinfo components of the
URI reference [RFC3986], if any, when generating the Referer field
value.
Referer = absolute-URI / partial-URI
The Referer header field allows servers to generate back-links to
other resources for simple analytics, logging, optimized caching,
etc. It also allows obsolete or mistyped links to be found for
maintenance. Some servers use the Referer header field as a means of
denying links from other sites (so-called "deep linking") or
restricting cross-site request forgery (CSRF), but not all requests
contain it.
Example:
Referer: http://www.example.org/hypertext/Overview.html
If the target URI was obtained from a source that does not have its
own URI (e.g., input from the user keyboard, or an entry within the
user's bookmarks/favorites), the user agent MUST either exclude the
Referer field or send it with a value of "about:blank".
The Referer field has the potential to reveal information about the
request context or browsing history of the user, which is a privacy
concern if the referring resource's identifier reveals personal
information (such as an account name) or a resource that is supposed
to be confidential (such as behind a firewall or internal to a
secured service). Most general-purpose user agents do not send the
Referer header field when the referring resource is a local "file" or
"data" URI. A user agent MUST NOT send a Referer header field in an
unsecured HTTP request if the referring page was received with a
secure protocol. See Section 12.8 for additional security
considerations.
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Some intermediaries have been known to indiscriminately remove
Referer header fields from outgoing requests. This has the
unfortunate side effect of interfering with protection against CSRF
attacks, which can be far more harmful to their users.
Intermediaries and user agent extensions that wish to limit
information disclosure in Referer ought to restrict their changes to
specific edits, such as replacing internal domain names with
pseudonyms or truncating the query and/or path components. An
intermediary SHOULD NOT modify or delete the Referer header field
when the field value shares the same scheme and host as the request
target.
8.6.3. User-Agent
The "User-Agent" header field contains information about the user
agent originating the request, which is often used by servers to help
identify the scope of reported interoperability problems, to work
around or tailor responses to avoid particular user agent
limitations, and for analytics regarding browser or operating system
use. A user agent SHOULD send a User-Agent field in each request
unless specifically configured not to do so.
User-Agent = product *( RWS ( product / comment ) )
The User-Agent field-value consists of one or more product
identifiers, each followed by zero or more comments (Section 5 of
[Messaging]), which together identify the user agent software and its
significant subproducts. By convention, the product identifiers are
listed in decreasing order of their significance for identifying the
user agent software. Each product identifier consists of a name and
optional version.
product = token ["/" product-version]
product-version = token
A sender SHOULD limit generated product identifiers to what is
necessary to identify the product; a sender MUST NOT generate
advertising or other nonessential information within the product
identifier. A sender SHOULD NOT generate information in product-
version that is not a version identifier (i.e., successive versions
of the same product name ought to differ only in the product-version
portion of the product identifier).
Example:
User-Agent: CERN-LineMode/2.15 libwww/2.17b3
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A user agent SHOULD NOT generate a User-Agent field containing
needlessly fine-grained detail and SHOULD limit the addition of
subproducts by third parties. Overly long and detailed User-Agent
field values increase request latency and the risk of a user being
identified against their wishes ("fingerprinting").
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. If a user
agent masquerades as a different user agent, recipients can assume
that the user intentionally desires to see responses tailored for
that identified user agent, even if they might not work as well for
the actual user agent being used.
9. Response Status Codes
The (response) status code is a three-digit integer code giving the
result of the attempt to understand and satisfy the request.
HTTP status codes are extensible. HTTP clients are not required to
understand the meaning of all registered status codes, though such
understanding is obviously desirable. However, a client MUST
understand the class of any status code, as indicated by the first
digit, and treat an unrecognized status code as being equivalent to
the x00 status code of that class, with the exception that a
recipient MUST NOT cache a response with an unrecognized status code.
For example, if an unrecognized status code of 471 is received by a
client, the client can assume that there was something wrong with its
request and treat the response as if it had received a 400 (Bad
Request) status code. The response message will usually contain a
representation that explains the status.
The first digit of the status code defines the class of response.
The last two digits do not have any categorization role. There are
five values for the first digit:
o 1xx (Informational): The request was received, continuing process
o 2xx (Successful): The request was successfully received,
understood, and accepted
o 3xx (Redirection): Further action needs to be taken in order to
complete the request
o 4xx (Client Error): The request contains bad syntax or cannot be
fulfilled
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o 5xx (Server Error): The server failed to fulfill an apparently
valid request
9.1. Overview of Status Codes
The status codes listed below are defined in this specification. The
reason phrases listed here are only recommendations -- they can be
replaced by local equivalents without affecting the protocol.
Responses with status codes that are defined as cacheable by default
(e.g., 200, 203, 204, 206, 300, 301, 404, 405, 410, 414, and 501 in
this specification) can be reused by a cache with heuristic
expiration unless otherwise indicated by the method definition or
explicit cache controls [Caching]; all other status codes are not
cacheable by default.
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+-------+-------------------------------+-----------------+
| Value | Description | Reference |
+-------+-------------------------------+-----------------+
| 100 | Continue | Section 9.2.1 |
| 101 | Switching Protocols | Section 9.2.2 |
| 200 | OK | Section 9.3.1 |
| 201 | Created | Section 9.3.2 |
| 202 | Accepted | Section 9.3.3 |
| 203 | Non-Authoritative Information | Section 9.3.4 |
| 204 | No Content | Section 9.3.5 |
| 205 | Reset Content | Section 9.3.6 |
| 206 | Partial Content | Section 9.3.7 |
| 300 | Multiple Choices | Section 9.4.1 |
| 301 | Moved Permanently | Section 9.4.2 |
| 302 | Found | Section 9.4.3 |
| 303 | See Other | Section 9.4.4 |
| 304 | Not Modified | Section 9.4.5 |
| 305 | Use Proxy | Section 9.4.6 |
| 306 | (Unused) | Section 9.4.7 |
| 307 | Temporary Redirect | Section 9.4.8 |
| 400 | Bad Request | Section 9.5.1 |
| 401 | Unauthorized | Section 9.5.2 |
| 402 | Payment Required | Section 9.5.3 |
| 403 | Forbidden | Section 9.5.4 |
| 404 | Not Found | Section 9.5.5 |
| 405 | Method Not Allowed | Section 9.5.6 |
| 406 | Not Acceptable | Section 9.5.7 |
| 407 | Proxy Authentication Required | Section 9.5.8 |
| 408 | Request Timeout | Section 9.5.9 |
| 409 | Conflict | Section 9.5.10 |
| 410 | Gone | Section 9.5.11 |
| 411 | Length Required | Section 9.5.12 |
| 412 | Precondition Failed | Section 9.5.13 |
| 413 | Payload Too Large | Section 9.5.14 |
| 414 | URI Too Long | Section 9.5.15 |
| 415 | Unsupported Media Type | Section 9.5.16 |
| 416 | Range Not Satisfiable | Section 9.5.17 |
| 417 | Expectation Failed | Section 9.5.18 |
| 426 | Upgrade Required | Section 9.5.19 |
| 500 | Internal Server Error | Section 9.6.1 |
| 501 | Not Implemented | Section 9.6.2 |
| 502 | Bad Gateway | Section 9.6.3 |
| 503 | Service Unavailable | Section 9.6.4 |
| 504 | Gateway Timeout | Section 9.6.5 |
| 505 | HTTP Version Not Supported | Section 9.6.6 |
+-------+-------------------------------+-----------------+
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Note that this list is not exhaustive -- it does not include
extension status codes defined in other specifications (Section 9.7).
9.2. Informational 1xx
The 1xx (Informational) class of status code indicates an interim
response for communicating connection status or request progress
prior to completing the requested action and sending a final
response. 1xx responses are terminated by the first empty line after
the status-line (the empty line signaling the end of the header
section). Since HTTP/1.0 did not define any 1xx status codes, a
server MUST NOT send a 1xx response to an HTTP/1.0 client.
A client MUST be able to parse one or more 1xx responses received
prior to a final response, even if the client does not expect one. A
user agent MAY ignore unexpected 1xx responses.
A proxy MUST forward 1xx responses unless the proxy itself requested
the generation of the 1xx response. For example, if a proxy adds an
"Expect: 100-continue" field when it forwards a request, then it need
not forward the corresponding 100 (Continue) response(s).
9.2.1. 100 Continue
The 100 (Continue) status code indicates that the initial part of a
request has been received and has not yet been rejected by the
server. The server intends to send a final response after the
request has been fully received and acted upon.
When the request contains an Expect header field that includes a
100-continue expectation, the 100 response indicates that the server
wishes to receive the request payload body, as described in
Section 8.1.1. The client ought to continue sending the request and
discard the 100 response.
If the request did not contain an Expect header field containing the
100-continue expectation, the client can simply discard this interim
response.
9.2.2. 101 Switching Protocols
The 101 (Switching Protocols) status code indicates that the server
understands and is willing to comply with the client's request, via
the Upgrade header field (Section 9.7 of [Messaging]), for a change
in the application protocol being used on this connection. The
server MUST generate an Upgrade header field in the response that
indicates which protocol(s) will be switched to immediately after the
empty line that terminates the 101 response.
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It is assumed that the server will only agree to switch protocols
when it is advantageous to do so. For example, switching to a newer
version of HTTP might be advantageous over older versions, and
switching to a real-time, synchronous protocol might be advantageous
when delivering resources that use such features.
9.3. Successful 2xx
The 2xx (Successful) class of status code indicates that the client's
request was successfully received, understood, and accepted.
9.3.1. 200 OK
The 200 (OK) status code indicates that the request has succeeded.
The payload sent in a 200 response depends on the request method.
For the methods defined by this specification, the intended meaning
of the payload can be summarized as:
GET a representation of the target resource;
HEAD the same representation as GET, but without the representation
data;
POST a representation of the status of, or results obtained from,
the action;
PUT, DELETE a representation of the status of the action;
OPTIONS a representation of the communications options;
TRACE a representation of the request message as received by the end
server.
Aside from responses to CONNECT, a 200 response always has a payload,
though an origin server MAY generate a payload body of zero length.
If no payload is desired, an origin server ought to send 204 (No
Content) instead. For CONNECT, no payload is allowed because the
successful result is a tunnel, which begins immediately after the 200
response header section.
A 200 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [Caching]).
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9.3.2. 201 Created
The 201 (Created) status code indicates that the request has been
fulfilled and has resulted in one or more new resources being
created. The primary resource created by the request is identified
by either a Location header field in the response or, if no Location
field is received, by the effective request URI.
The 201 response payload typically describes and links to the
resource(s) created. See Section 10.2 for a discussion of the
meaning and purpose of validator header fields, such as ETag and
Last-Modified, in a 201 response.
9.3.3. 202 Accepted
The 202 (Accepted) status code indicates that the request has been
accepted for processing, but the processing has not been completed.
The request might or might not eventually be acted upon, as it might
be disallowed when processing actually takes place. There is no
facility in HTTP for re-sending a status code from an asynchronous
operation.
The 202 response is intentionally noncommittal. Its purpose is to
allow a server to accept a request for some other process (perhaps a
batch-oriented process that is only run once per day) without
requiring that the user agent's connection to the server persist
until the process is completed. The representation sent with this
response ought to describe the request's current status and point to
(or embed) a status monitor that can provide the user with an
estimate of when the request will be fulfilled.
9.3.4. 203 Non-Authoritative Information
The 203 (Non-Authoritative Information) status code indicates that
the request was successful but the enclosed payload has been modified
from that of the origin server's 200 (OK) response by a transforming
proxy (Section 5.6.2). This status code allows the proxy to notify
recipients when a transformation has been applied, since that
knowledge might impact later decisions regarding the content. For
example, future cache validation requests for the content might only
be applicable along the same request path (through the same proxies).
The 203 response is similar to the Warning code of 214 Transformation
Applied (Section 5.5 of [Caching]), which has the advantage of being
applicable to responses with any status code.
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A 203 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [Caching]).
9.3.5. 204 No Content
The 204 (No Content) status code indicates that the server has
successfully fulfilled the request and that there is no additional
content to send in the response payload body. Metadata in the
response header fields refer to the target resource and its selected
representation after the requested action was applied.
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
was successful and the ETag field-value contains the entity-tag for
the new representation of that target resource.
The 204 response allows a server to indicate that the action has been
successfully applied to the target resource, while implying that the
user agent does not need to traverse away from its current "document
view" (if any). The server assumes that the user agent will provide
some indication of the success to its user, in accord with its own
interface, and apply any new or updated metadata in the response to
its active representation.
For example, a 204 status code is commonly used with document editing
interfaces corresponding to a "save" action, such that the document
being saved remains available to the user for editing. It is also
frequently used with interfaces that expect automated data transfers
to be prevalent, such as within distributed version control systems.
A 204 response is terminated by the first empty line after the header
fields because it cannot contain a message body.
A 204 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [Caching]).
9.3.6. 205 Reset Content
The 205 (Reset Content) status code indicates that the server has
fulfilled the request and desires that the user agent reset the
"document view", which caused the request to be sent, to its original
state as received from the origin server.
This response is intended to support a common data entry use case
where the user receives content that supports data entry (a form,
notepad, canvas, etc.), enters or manipulates data in that space,
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causes the entered data to be submitted in a request, and then the
data entry mechanism is reset for the next entry so that the user can
easily initiate another input action.
Since the 205 status code implies that no additional content will be
provided, a server MUST NOT generate a payload in a 205 response. In
other words, a server MUST do one of the following for a 205
response: a) indicate a zero-length body for the response by
including a Content-Length header field with a value of 0; b)
indicate a zero-length payload for the response by including a
Transfer-Encoding header field with a value of chunked and a message
body consisting of a single chunk of zero-length; or, c) close the
connection immediately after sending the blank line terminating the
header section.
9.3.7. 206 Partial Content
The 206 (Partial Content) status code indicates that the server is
successfully fulfilling a range request for the target resource by
transferring one or more parts of the selected representation.
When a 206 response is generated, the server MUST generate the
following header fields, in addition to those required in the
subsections below, if the field would have been sent in a 200 (OK)
response to the same request: Date, Cache-Control, ETag, Expires,
Content-Location, and Vary.
If a 206 is generated in response to a request with an If-Range
header field, the sender SHOULD NOT generate other representation
header fields beyond those required, because the client is understood
to already have a prior response containing those header fields.
Otherwise, the sender MUST generate all of the representation header
fields that would have been sent in a 200 (OK) response to the same
request.
A 206 response is cacheable by default; i.e., unless otherwise
indicated by explicit cache controls (see Section 4.2.2 of
[Caching]).
9.3.7.1. Single Part
If a single part is being transferred, the server generating the 206
response MUST generate a Content-Range header field, describing what
range of the selected representation is enclosed, and a payload
consisting of the range. For example:
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HTTP/1.1 206 Partial Content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-Range: bytes 21010-47021/47022
Content-Length: 26012
Content-Type: image/gif
... 26012 bytes of partial image data ...
9.3.7.2. Multiple Parts
If multiple parts are being transferred, the server generating the
206 response MUST generate a "multipart/byteranges" payload, as
defined in Section 6.3.4, and a Content-Type header field containing
the multipart/byteranges media type and its required boundary
parameter. To avoid confusion with single-part responses, a server
MUST NOT generate a Content-Range header field in the HTTP header
section of a multiple part response (this field will be sent in each
part instead).
Within the header area of each body part in the multipart payload,
the server MUST generate a Content-Range header field corresponding
to the range being enclosed in that body part. If the selected
representation would have had a Content-Type header field in a 200
(OK) response, the server SHOULD generate that same Content-Type
field in the header area of each body part. For example:
HTTP/1.1 206 Partial Content
Date: Wed, 15 Nov 1995 06:25:24 GMT
Last-Modified: Wed, 15 Nov 1995 04:58:08 GMT
Content-Length: 1741
Content-Type: multipart/byteranges; boundary=THIS_STRING_SEPARATES
--THIS_STRING_SEPARATES
Content-Type: application/pdf
Content-Range: bytes 500-999/8000
...the first range...
--THIS_STRING_SEPARATES
Content-Type: application/pdf
Content-Range: bytes 7000-7999/8000
...the second range
--THIS_STRING_SEPARATES--
When multiple ranges are requested, a server MAY coalesce any of the
ranges that overlap, or that are separated by a gap that is smaller
than the overhead of sending multiple parts, regardless of the order
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in which the corresponding byte-range-spec appeared in the received
Range header field. Since the typical overhead between parts of a
multipart/byteranges payload is around 80 bytes, depending on the
selected representation's media type and the chosen boundary
parameter length, it can be less efficient to transfer many small
disjoint parts than it is to transfer the entire selected
representation.
A server MUST NOT generate a multipart response to a request for a
single range, since a client that does not request multiple parts
might not support multipart responses. However, a server MAY
generate a multipart/byteranges payload with only a single body part
if multiple ranges were requested and only one range was found to be
satisfiable or only one range remained after coalescing. A client
that cannot process a multipart/byteranges response MUST NOT generate
a request that asks for multiple ranges.
When a multipart response payload is generated, the server SHOULD
send the parts in the same order that the corresponding byte-range-
spec appeared in the received Range header field, excluding those
ranges that were deemed unsatisfiable or that were coalesced into
other ranges. A client that receives a multipart response MUST
inspect the Content-Range header field present in each body part in
order to determine which range is contained in that body part; a
client cannot rely on receiving the same ranges that it requested,
nor the same order that it requested.
9.3.7.3. Combining Parts
A response might transfer only a subrange of a representation if the
connection closed prematurely or if the request used one or more
Range specifications. After several such transfers, a client might
have received several ranges of the same representation. These
ranges can only be safely combined if they all have in common the
same strong validator (Section 10.2.1).
A client that has received multiple partial responses to GET requests
on a target resource MAY combine those responses into a larger
continuous range if they share the same strong validator.
If the most recent response is an incomplete 200 (OK) response, then
the header fields of that response are used for any combined response
and replace those of the matching stored responses.
If the most recent response is a 206 (Partial Content) response and
at least one of the matching stored responses is a 200 (OK), then the
combined response header fields consist of the most recent 200
response's header fields. If all of the matching stored responses
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are 206 responses, then the stored response with the most recent
header fields is used as the source of header fields for the combined
response, except that the client MUST use other header fields
provided in the new response, aside from Content-Range, to replace
all instances of the corresponding header fields in the stored
response.
The combined response message body consists of the union of partial
content ranges in the new response and each of the selected
responses. If the union consists of the entire range of the
representation, then the client MUST process the combined response as
if it were a complete 200 (OK) response, including a Content-Length
header field that reflects the complete length. Otherwise, the
client MUST process the set of continuous ranges as one of the
following: an incomplete 200 (OK) response if the combined response
is a prefix of the representation, a single 206 (Partial Content)
response containing a multipart/byteranges body, or multiple 206
(Partial Content) responses, each with one continuous range that is
indicated by a Content-Range header field.
9.4. Redirection 3xx
The 3xx (Redirection) class of status code indicates that further
action needs to be taken by the user agent in order to fulfill the
request. If a Location header field (Section 10.1.2) is provided,
the user agent MAY automatically redirect its request to the URI
referenced by the Location field value, even if the specific status
code is not understood. Automatic redirection needs to be done with
care for methods not known to be safe, as defined in Section 7.2.1,
since the user might not wish to redirect an unsafe request.
There are several types of redirects:
1. Redirects that indicate the resource might be available at a
different URI, as provided by the Location field, as in the
status codes 301 (Moved Permanently), 302 (Found), and 307
(Temporary Redirect).
2. Redirection that offers a choice of matching resources, each
capable of representing the original request target, as in the
300 (Multiple Choices) status code.
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3. Redirection to a different resource, identified by the Location
field, that can represent an indirect response to the request, as
in the 303 (See Other) status code.
4. Redirection to a previously cached result, as in the 304 (Not
Modified) status code.
Note: In HTTP/1.0, the status codes 301 (Moved Permanently) and
302 (Found) were defined for the first type of redirect
([RFC1945], Section 9.3). Early user agents split on whether the
method applied to the redirect target would be the same as the
original request or would be rewritten as GET. Although HTTP
originally defined the former semantics for 301 and 302 (to match
its original implementation at CERN), and defined 303 (See Other)
to match the latter semantics, prevailing practice gradually
converged on the latter semantics for 301 and 302 as well. The
first revision of HTTP/1.1 added 307 (Temporary Redirect) to
indicate the former semantics without being impacted by divergent
practice. Over 10 years later, most user agents still do method
rewriting for 301 and 302; therefore, this specification makes
that behavior conformant when the original request is POST.
A client SHOULD detect and intervene in cyclical redirections (i.e.,
"infinite" redirection loops).
Note: An earlier version of this specification recommended a
maximum of five redirections ([RFC2068], Section 10.3). Content
developers need to be aware that some clients might implement such
a fixed limitation.
9.4.1. 300 Multiple Choices
The 300 (Multiple Choices) status code indicates that the target
resource has more than one representation, each with its own more
specific identifier, and information about the alternatives is being
provided so that the user (or user agent) can select a preferred
representation by redirecting its request to one or more of those
identifiers. In other words, the server desires that the user agent
engage in reactive negotiation to select the most appropriate
representation(s) for its needs (Section 6.4).
If the server has a preferred choice, the server SHOULD generate a
Location header field containing a preferred choice's URI reference.
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The user agent MAY use the Location field value for automatic
redirection.
For request methods other than HEAD, the server SHOULD generate a
payload in the 300 response containing a list of representation
metadata and URI reference(s) from which the user or user agent can
choose the one most preferred. The user agent MAY make a selection
from that list automatically if it understands the provided media
type. A specific format for automatic selection is not defined by
this specification because HTTP tries to remain orthogonal to the
definition of its payloads. In practice, the representation is
provided in some easily parsed format believed to be acceptable to
the user agent, as determined by shared design or content
negotiation, or in some commonly accepted hypertext format.
A 300 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [Caching]).
Note: The original proposal for the 300 status code defined the
URI header field as providing a list of alternative
representations, such that it would be usable for 200, 300, and
406 responses and be transferred in responses to the HEAD method.
However, lack of deployment and disagreement over syntax led to
both URI and Alternates (a subsequent proposal) being dropped from
this specification. It is possible to communicate the list using
a set of Link header fields [RFC8288], each with a relationship of
"alternate", though deployment is a chicken-and-egg problem.
9.4.2. 301 Moved Permanently
The 301 (Moved Permanently) status code indicates that the target
resource has been assigned a new permanent URI and any future
references to this resource ought to use one of the enclosed URIs.
Clients with link-editing capabilities ought to automatically re-link
references to the effective request URI to one or more of the new
references sent by the server, where possible.
The server SHOULD generate a Location header field in the response
containing a preferred URI reference for the new permanent URI. The
user agent MAY use the Location field value for automatic
redirection. The server's response payload usually contains a short
hypertext note with a hyperlink to the new URI(s).
Note: For historical reasons, a user agent MAY change the request
method from POST to GET for the subsequent request. If this
behavior is undesired, the 307 (Temporary Redirect) status code
can be used instead.
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A 301 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [Caching]).
9.4.3. 302 Found
The 302 (Found) status code indicates that the target resource
resides temporarily under a different URI. Since the redirection
might be altered on occasion, the client ought to continue to use the
effective request URI for future requests.
The server SHOULD generate a Location header field in the response
containing a URI reference for the different URI. The user agent MAY
use the Location field value for automatic redirection. The server's
response payload usually contains a short hypertext note with a
hyperlink to the different URI(s).
Note: For historical reasons, a user agent MAY change the request
method from POST to GET for the subsequent request. If this
behavior is undesired, the 307 (Temporary Redirect) status code
can be used instead.
9.4.4. 303 See Other
The 303 (See Other) status code indicates that the server is
redirecting the user agent to a different resource, as indicated by a
URI in the Location header field, which is intended to provide an
indirect response to the original request. A user agent can perform
a retrieval request targeting that URI (a GET or HEAD request if
using HTTP), which might also be redirected, and present the eventual
result as an answer to the original request. Note that the new URI
in the Location header field is not considered equivalent to the
effective request URI.
This status code is applicable to any HTTP method. It is primarily
used to allow the output of a POST action to redirect the user agent
to a selected resource, since doing so provides the information
corresponding to the POST response in a form that can be separately
identified, bookmarked, and cached, independent of the original
request.
A 303 response to a GET request indicates that the origin server does
not have a representation of the target resource that can be
transferred by the server over HTTP. However, the Location field
value refers to a resource that is descriptive of the target
resource, such that making a retrieval request on that other resource
might result in a representation that is useful to recipients without
implying that it represents the original target resource. Note that
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answers to the questions of what can be represented, what
representations are adequate, and what might be a useful description
are outside the scope of HTTP.
Except for responses to a HEAD request, the representation of a 303
response ought to contain a short hypertext note with a hyperlink to
the same URI reference provided in the Location header field.
9.4.5. 304 Not Modified
The 304 (Not Modified) status code indicates that a conditional GET
or HEAD request has been received and would have resulted in a 200
(OK) response if it were not for the fact that the condition
evaluated to false. In other words, there is no need for the server
to transfer a representation of the target resource because the
request indicates that the client, which made the request
conditional, already has a valid representation; the server is
therefore redirecting the client to make use of that stored
representation as if it were the payload of a 200 (OK) response.
The server generating a 304 response MUST generate any of the
following header fields that would have been sent in a 200 (OK)
response to the same request: Cache-Control, Content-Location, Date,
ETag, Expires, and Vary.
Since the goal of a 304 response is to minimize information transfer
when the recipient already has one or more cached representations, a
sender SHOULD NOT generate representation metadata other than the
above listed fields unless said metadata exists for the purpose of
guiding cache updates (e.g., Last-Modified might be useful if the
response does not have an ETag field).
Requirements on a cache that receives a 304 response are defined in
Section 4.3.4 of [Caching]. If the conditional request originated
with an outbound client, such as a user agent with its own cache
sending a conditional GET to a shared proxy, then the proxy SHOULD
forward the 304 response to that client.
A 304 response cannot contain a message-body; it is always terminated
by the first empty line after the header fields.
9.4.6. 305 Use Proxy
The 305 (Use Proxy) status code was defined in a previous version of
this specification and is now deprecated (Appendix B of [RFC7231]).
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9.4.7. 306 (Unused)
The 306 status code was defined in a previous version of this
specification, is no longer used, and the code is reserved.
9.4.8. 307 Temporary Redirect
The 307 (Temporary Redirect) status code indicates that the target
resource resides temporarily under a different URI and the user agent
MUST NOT change the request method if it performs an automatic
redirection to that URI. Since the redirection can change over time,
the client ought to continue using the original effective request URI
for future requests.
The server SHOULD generate a Location header field in the response
containing a URI reference for the different URI. The user agent MAY
use the Location field value for automatic redirection. The server's
response payload usually contains a short hypertext note with a
hyperlink to the different URI(s).
Note: This status code is similar to 302 (Found), except that it
does not allow changing the request method from POST to GET. This
specification defines no equivalent counterpart for 301 (Moved
Permanently) ([RFC7538], however, defines the status code 308
(Permanent Redirect) for this purpose).
9.5. Client Error 4xx
The 4xx (Client Error) class of status code indicates that the client
seems to have erred. Except when responding to a HEAD request, the
server SHOULD send a representation containing an explanation of the
error situation, and whether it is a temporary or permanent
condition. These status codes are applicable to any request method.
User agents SHOULD display any included representation to the user.
9.5.1. 400 Bad Request
The 400 (Bad Request) status code indicates that the server cannot or
will not process the request due to something that is perceived to be
a client error (e.g., malformed request syntax, invalid request
message framing, or deceptive request routing).
9.5.2. 401 Unauthorized
The 401 (Unauthorized) status code indicates that the request has not
been applied because it lacks valid authentication credentials for
the target resource. The server generating a 401 response MUST send
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a WWW-Authenticate header field (Section 10.3.1) containing at least
one challenge applicable to the target resource.
If the request included authentication credentials, then the 401
response indicates that authorization has been refused for those
credentials. The user agent MAY repeat the request with a new or
replaced Authorization header field (Section 8.5.3). If the 401
response contains the same challenge as the prior response, and the
user agent has already attempted authentication at least once, then
the user agent SHOULD present the enclosed representation to the
user, since it usually contains relevant diagnostic information.
9.5.3. 402 Payment Required
The 402 (Payment Required) status code is reserved for future use.
9.5.4. 403 Forbidden
The 403 (Forbidden) status code indicates that the server understood
the request but refuses to authorize it. A server that wishes to
make public why the request has been forbidden can describe that
reason in the response payload (if any).
If authentication credentials were provided in the request, the
server considers them insufficient to grant access. The client
SHOULD NOT automatically repeat the request with the same
credentials. The client MAY repeat the request with new or different
credentials. However, a request might be forbidden for reasons
unrelated to the credentials.
An origin server that wishes to "hide" the current existence of a
forbidden target resource MAY instead respond with a status code of
404 (Not Found).
9.5.5. 404 Not Found
The 404 (Not Found) status code indicates that the origin server did
not find a current representation for the target resource or is not
willing to disclose that one exists. A 404 status code does not
indicate whether this lack of representation is temporary or
permanent; the 410 (Gone) status code is preferred over 404 if the
origin server knows, presumably through some configurable means, that
the condition is likely to be permanent.
A 404 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [Caching]).
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9.5.6. 405 Method Not Allowed
The 405 (Method Not Allowed) status code indicates that the method
received in the request-line is known by the origin server but not
supported by the target resource. The origin server MUST generate an
Allow header field in a 405 response containing a list of the target
resource's currently supported methods.
A 405 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [Caching]).
9.5.7. 406 Not Acceptable
The 406 (Not Acceptable) status code indicates that the target
resource does not have a current representation that would be
acceptable to the user agent, according to the proactive negotiation
header fields received in the request (Section 8.4), and the server
is unwilling to supply a default representation.
The server SHOULD generate a payload containing a list of available
representation characteristics and corresponding resource identifiers
from which the user or user agent can choose the one most
appropriate. A user agent MAY automatically select the most
appropriate choice from that list. However, this specification does
not define any standard for such automatic selection, as described in
Section 9.4.1.
9.5.8. 407 Proxy Authentication Required
The 407 (Proxy Authentication Required) status code is similar to 401
(Unauthorized), but it indicates that the client needs to
authenticate itself in order to use a proxy. The proxy MUST send a
Proxy-Authenticate header field (Section 10.3.2) containing a
challenge applicable to that proxy for the target resource. The
client MAY repeat the request with a new or replaced Proxy-
Authorization header field (Section 8.5.4).
9.5.9. 408 Request Timeout
The 408 (Request Timeout) status code indicates that the server did
not receive a complete request message within the time that it was
prepared to wait. A server SHOULD send the "close" connection option
(Section 9.1 of [Messaging]) in the response, since 408 implies that
the server has decided to close the connection rather than continue
waiting. If the client has an outstanding request in transit, the
client MAY repeat that request on a new connection.
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9.5.10. 409 Conflict
The 409 (Conflict) status code indicates that the request could not
be completed due to a conflict with the current state of the target
resource. This code is used in situations where the user might be
able to resolve the conflict and resubmit the request. The server
SHOULD generate a payload that includes enough information for a user
to recognize the source of the conflict.
Conflicts are most likely to occur in response to a PUT request. For
example, if versioning were being used and the representation being
PUT included changes to a resource that conflict with those made by
an earlier (third-party) request, the origin server might use a 409
response to indicate that it can't complete the request. In this
case, the response representation would likely contain information
useful for merging the differences based on the revision history.
9.5.11. 410 Gone
The 410 (Gone) status code indicates that access to the target
resource is no longer available at the origin server and that this
condition is likely to be permanent. If the origin server does not
know, or has no facility to determine, whether or not the condition
is permanent, the status code 404 (Not Found) ought to be used
instead.
The 410 response is primarily intended to assist the task of web
maintenance by notifying the recipient that the resource is
intentionally unavailable and that the server owners desire that
remote links to that resource be removed. Such an event is common
for limited-time, promotional services and for resources belonging to
individuals no longer associated with the origin server's site. It
is not necessary to mark all permanently unavailable resources as
"gone" or to keep the mark for any length of time -- that is left to
the discretion of the server owner.
A 410 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [Caching]).
9.5.12. 411 Length Required
The 411 (Length Required) status code indicates that the server
refuses to accept the request without a defined Content-Length
(Section 6.2.4). The client MAY repeat the request if it adds a
valid Content-Length header field containing the length of the
message body in the request message.
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9.5.13. 412 Precondition Failed
The 412 (Precondition Failed) status code indicates that one or more
conditions given in the request header fields evaluated to false when
tested on the server. This response status code allows the client to
place preconditions on the current resource state (its current
representations and metadata) and, thus, prevent the request method
from being applied if the target resource is in an unexpected state.
9.5.14. 413 Payload Too Large
The 413 (Payload Too Large) status code indicates that the server is
refusing to process a request because the request payload is larger
than the server is willing or able to process. The server MAY close
the connection to prevent the client from continuing the request.
If the condition is temporary, the server SHOULD generate a Retry-
After header field to indicate that it is temporary and after what
time the client MAY try again.
9.5.15. 414 URI Too Long
The 414 (URI Too Long) status code indicates that the server is
refusing to service the request because the request-target
(Section 3.2 of [Messaging]) is longer than the server is willing to
interpret. This rare condition is only likely to occur when a client
has improperly converted a POST request to a GET request with long
query information, when the client has descended into a "black hole"
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 exploit potential security holes.
A 414 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [Caching]).
9.5.16. 415 Unsupported Media Type
The 415 (Unsupported Media Type) status code indicates that the
origin server is refusing to service the request because the payload
is in a format not supported by this method on the target resource.
The format problem might be due to the request's indicated Content-
Type or Content-Encoding, or as a result of inspecting the data
directly.
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9.5.17. 416 Range Not Satisfiable
The 416 (Range Not Satisfiable) status code indicates that none of
the ranges in the request's Range header field (Section 8.3) overlap
the current extent of the selected representation or that the set of
ranges requested has been rejected due to invalid ranges or an
excessive request of small or overlapping ranges.
For byte ranges, failing to overlap the current extent means that the
first-byte-pos of all of the byte-range-spec values were greater than
the current length of the selected representation. When this status
code is generated in response to a byte-range request, the sender
SHOULD generate a Content-Range header field specifying the current
length of the selected representation (Section 6.3.3).
For example:
HTTP/1.1 416 Range Not Satisfiable
Date: Fri, 20 Jan 2012 15:41:54 GMT
Content-Range: bytes */47022
Note: Because servers are free to ignore Range, many
implementations will simply respond with the entire selected
representation in a 200 (OK) response. That is partly because
most clients are prepared to receive a 200 (OK) to complete the
task (albeit less efficiently) and partly because clients might
not stop making an invalid partial request until they have
received a complete representation. Thus, clients cannot depend
on receiving a 416 (Range Not Satisfiable) response even when it
is most appropriate.
9.5.18. 417 Expectation Failed
The 417 (Expectation Failed) status code indicates that the
expectation given in the request's Expect header field
(Section 8.1.1) could not be met by at least one of the inbound
servers.
9.5.19. 426 Upgrade Required
The 426 (Upgrade Required) status code indicates that the server
refuses to perform the request using the current protocol but might
be willing to do so after the client upgrades to a different
protocol. The server MUST send an Upgrade header field in a 426
response to indicate the required protocol(s) (Section 9.7 of
[Messaging]).
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Example:
HTTP/1.1 426 Upgrade Required
Upgrade: HTTP/3.0
Connection: Upgrade
Content-Length: 53
Content-Type: text/plain
This service requires use of the HTTP/3.0 protocol.
9.6. Server Error 5xx
The 5xx (Server Error) class of status code indicates that the server
is aware that it has erred or is incapable of performing the
requested method. Except when responding to a HEAD request, the
server SHOULD send a representation containing an explanation of the
error situation, and whether it is a temporary or permanent
condition. A user agent SHOULD display any included representation
to the user. These response codes are applicable to any request
method.
9.6.1. 500 Internal Server Error
The 500 (Internal Server Error) status code indicates that the server
encountered an unexpected condition that prevented it from fulfilling
the request.
9.6.2. 501 Not Implemented
The 501 (Not Implemented) status code indicates that the server does
not support the functionality required to fulfill the request. This
is the appropriate response when the server does not recognize the
request method and is not capable of supporting it for any resource.
A 501 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [Caching]).
9.6.3. 502 Bad Gateway
The 502 (Bad Gateway) status code indicates that the server, while
acting as a gateway or proxy, received an invalid response from an
inbound server it accessed while attempting to fulfill the request.
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9.6.4. 503 Service Unavailable
The 503 (Service Unavailable) status code indicates that the server
is currently unable to handle the request due to a temporary overload
or scheduled maintenance, which will likely be alleviated after some
delay. The server MAY send a Retry-After header field
(Section 10.1.3) to suggest an appropriate amount of time for the
client to wait before retrying the request.
Note: The existence of the 503 status code does not imply that a
server has to use it when becoming overloaded. Some servers might
simply refuse the connection.
9.6.5. 504 Gateway Timeout
The 504 (Gateway Timeout) status code indicates that the server,
while acting as a gateway or proxy, did not receive a timely response
from an upstream server it needed to access in order to complete the
request.
9.6.6. 505 HTTP Version Not Supported
The 505 (HTTP Version Not Supported) status code indicates that the
server does not support, or refuses to support, the major version of
HTTP that was used in the request message. The server is indicating
that it is unable or unwilling to complete the request using the same
major version as the client, as described in Section 3.5, other than
with this error message. The server SHOULD generate a representation
for the 505 response that describes why that version is not supported
and what other protocols are supported by that server.
9.7. Status Code Extensibility
Additional status codes, outside the scope of this specification,
have been specified for use in HTTP. All such status codes ought to
be registered within the "Hypertext Transfer Protocol (HTTP) Status
Code Registry".
9.7.1. Status Code Registry
The "Hypertext Transfer Protocol (HTTP) Status Code Registry",
maintained by IANA at <https://www.iana.org/assignments/http-status-
codes>, registers status code numbers.
A registration MUST include the following fields:
o Status Code (3 digits)
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o Short Description
o Pointer to specification text
Values to be added to the HTTP status code namespace require IETF
Review (see [RFC8126], Section 4.8).
9.7.2. 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.
Status codes are generic; they are potentially applicable to any
resource, not just one particular media type, kind of resource, or
application of HTTP. 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 9. To allow existing parsers to process the
response message, new status codes cannot disallow a payload,
although they can mandate a zero-length payload body.
Proposals for new status codes that are not yet widely deployed ought
to avoid allocating a specific number for the code until there is
clear consensus that it will be registered; instead, early drafts can
use a notation such as "4NN", or "3N0" .. "3N9", to indicate the
class of the proposed status code(s) without consuming a number
prematurely.
The definition of a new status code ought to explain the request
conditions that would cause 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, what fields can modify the semantics, and what header
field semantics are further refined when used with the new status
code).
The definition of a new status code ought to specify whether or not
it is cacheable. Note that all status codes can be cached if the
response they occur in has explicit freshness information; however,
status codes that are defined as being cacheable are allowed to be
cached without explicit freshness information. Likewise, the
definition of a status code can place constraints upon cache
behavior. See [Caching] for more information.
Finally, the definition of a new status code ought to indicate
whether the payload has any implied association with an identified
resource (Section 6.3.2).
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10. Response Header Fields
The response header fields allow the server to pass additional
information about the response beyond what is placed in the status-
line. These header fields give information about the server, about
further access to the target resource, or about related resources.
Although each response header field has a defined meaning, in
general, the precise semantics might be further refined by the
semantics of the request method and/or response status code.
10.1. Control Data
Response header fields can supply control data that supplements the
status code, directs caching, or instructs the client where to go
next.
+-------------------+--------------------------+
| Header Field Name | Defined in... |
+-------------------+--------------------------+
| Age | Section 5.1 of [Caching] |
| Cache-Control | Section 5.2 of [Caching] |
| Expires | Section 5.3 of [Caching] |
| Date | Section 10.1.1.2 |
| Location | Section 10.1.2 |
| Retry-After | Section 10.1.3 |
| Vary | Section 10.1.4 |
| Warning | Section 5.5 of [Caching] |
+-------------------+--------------------------+
10.1.1. Origination Date
10.1.1.1. Date/Time Formats
Prior to 1995, there were three different formats commonly used by
servers to communicate timestamps. For compatibility with old
implementations, all three are defined here. The preferred format is
a fixed-length and single-zone subset of the date and time
specification used by the Internet Message Format [RFC5322].
HTTP-date = IMF-fixdate / obs-date
An example of the preferred format is
Sun, 06 Nov 1994 08:49:37 GMT ; IMF-fixdate
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Examples of the two obsolete formats are
Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format
A recipient that parses a timestamp value in an HTTP header field
MUST accept all three HTTP-date formats. When a sender generates a
header field that contains one or more timestamps defined as HTTP-
date, the sender MUST generate those timestamps in the IMF-fixdate
format.
An HTTP-date value represents time as an instance of Coordinated
Universal Time (UTC). The first two formats indicate UTC by the
three-letter abbreviation for Greenwich Mean Time, "GMT", a
predecessor of the UTC name; values in the asctime format are assumed
to be in UTC. A sender that generates HTTP-date values from a local
clock ought to use NTP ([RFC5905]) or some similar protocol to
synchronize its clock to UTC.
Preferred format:
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IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT
; fixed length/zone/capitalization subset of the format
; see Section 3.3 of [RFC5322]
day-name = %x4D.6F.6E ; "Mon", case-sensitive
/ %x54.75.65 ; "Tue", case-sensitive
/ %x57.65.64 ; "Wed", case-sensitive
/ %x54.68.75 ; "Thu", case-sensitive
/ %x46.72.69 ; "Fri", case-sensitive
/ %x53.61.74 ; "Sat", case-sensitive
/ %x53.75.6E ; "Sun", case-sensitive
date1 = day SP month SP year
; e.g., 02 Jun 1982
day = 2DIGIT
month = %x4A.61.6E ; "Jan", case-sensitive
/ %x46.65.62 ; "Feb", case-sensitive
/ %x4D.61.72 ; "Mar", case-sensitive
/ %x41.70.72 ; "Apr", case-sensitive
/ %x4D.61.79 ; "May", case-sensitive
/ %x4A.75.6E ; "Jun", case-sensitive
/ %x4A.75.6C ; "Jul", case-sensitive
/ %x41.75.67 ; "Aug", case-sensitive
/ %x53.65.70 ; "Sep", case-sensitive
/ %x4F.63.74 ; "Oct", case-sensitive
/ %x4E.6F.76 ; "Nov", case-sensitive
/ %x44.65.63 ; "Dec", case-sensitive
year = 4DIGIT
GMT = %x47.4D.54 ; "GMT", case-sensitive
time-of-day = hour ":" minute ":" second
; 00:00:00 - 23:59:60 (leap second)
hour = 2DIGIT
minute = 2DIGIT
second = 2DIGIT
Obsolete formats:
obs-date = rfc850-date / asctime-date
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rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT
date2 = day "-" month "-" 2DIGIT
; e.g., 02-Jun-82
day-name-l = %x4D.6F.6E.64.61.79 ; "Monday", case-sensitive
/ %x54.75.65.73.64.61.79 ; "Tuesday", case-sensitive
/ %x57.65.64.6E.65.73.64.61.79 ; "Wednesday", case-sensitive
/ %x54.68.75.72.73.64.61.79 ; "Thursday", case-sensitive
/ %x46.72.69.64.61.79 ; "Friday", case-sensitive
/ %x53.61.74.75.72.64.61.79 ; "Saturday", case-sensitive
/ %x53.75.6E.64.61.79 ; "Sunday", case-sensitive
asctime-date = day-name SP date3 SP time-of-day SP year
date3 = month SP ( 2DIGIT / ( SP 1DIGIT ))
; e.g., Jun 2
HTTP-date is case sensitive. A sender MUST NOT generate additional
whitespace in an HTTP-date beyond that specifically included as SP in
the grammar. The semantics of day-name, day, month, year, and time-
of-day are the same as those defined for the Internet Message Format
constructs with the corresponding name ([RFC5322], Section 3.3).
Recipients of a timestamp value in rfc850-date format, which uses a
two-digit year, MUST interpret a timestamp that appears to be more
than 50 years in the future as representing the most recent year in
the past that had the same last two digits.
Recipients of timestamp values are encouraged to be robust in parsing
timestamps unless otherwise restricted by the field definition. For
example, messages are occasionally forwarded over HTTP from a non-
HTTP source that might generate any of the date and time
specifications defined by the Internet Message Format.
Note: HTTP requirements for the date/time stamp format apply only
to their usage within the protocol stream. Implementations are
not required to use these formats for user presentation, request
logging, etc.
10.1.1.2. Date
The "Date" header field represents the date and time at which the
message was originated, having the same semantics as the Origination
Date Field (orig-date) defined in Section 3.6.1 of [RFC5322]. The
field value is an HTTP-date, as defined in Section 10.1.1.1.
Date = HTTP-date
An example is
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Date: Tue, 15 Nov 1994 08:12:31 GMT
When a Date header field is generated, the sender SHOULD generate its
field value as the best available approximation of the date and time
of message generation. In theory, the date ought to represent the
moment just before the payload is generated. In practice, the date
can be generated at any time during message origination.
An origin server MUST NOT send a Date header field if it does not
have a clock capable of providing a reasonable approximation of the
current instance in Coordinated Universal Time. An origin server MAY
send a Date header field if the response is in the 1xx
(Informational) or 5xx (Server Error) class of status codes. An
origin server MUST send a Date header field in all other cases.
A recipient with a clock that receives a response message without a
Date header field MUST record the time it was received and append a
corresponding Date header field to the message's header section if it
is cached or forwarded downstream.
A user agent MAY send a Date header field in a request, though
generally will not do so unless it is believed to convey useful
information to the server. For example, custom applications of HTTP
might convey a Date if the server is expected to adjust its
interpretation of the user's request based on differences between the
user agent and server clocks.
10.1.2. Location
The "Location" header field is used in some responses to refer to a
specific resource in relation to the response. The type of
relationship is defined by the combination of request method and
status code semantics.
Location = URI-reference
The field value consists of a single URI-reference. When it has the
form of a relative reference ([RFC3986], Section 4.2), the final
value is computed by resolving it against the effective request URI
([RFC3986], Section 5).
For 201 (Created) responses, the Location value refers to the primary
resource created by the request. For 3xx (Redirection) responses,
the Location value refers to the preferred target resource for
automatically redirecting the request.
If the Location value provided in a 3xx (Redirection) response does
not have a fragment component, a user agent MUST process the
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redirection as if the value inherits the fragment component of the
URI reference used to generate the request target (i.e., the
redirection inherits the original reference's fragment, if any).
For example, a GET request generated for the URI reference
"http://www.example.org/~tim" might result in a 303 (See Other)
response containing the header field:
Location: /People.html#tim
which suggests that the user agent redirect to
"http://www.example.org/People.html#tim"
Likewise, a GET request generated for the URI reference
"http://www.example.org/index.html#larry" might result in a 301
(Moved Permanently) response containing the header field:
Location: http://www.example.net/index.html
which suggests that the user agent redirect to
"http://www.example.net/index.html#larry", preserving the original
fragment identifier.
There are circumstances in which a fragment identifier in a Location
value would not be appropriate. For example, the Location header
field in a 201 (Created) response is supposed to provide a URI that
is specific to the created resource.
Note: Some recipients attempt to recover from Location fields that
are not valid URI references. This specification does not mandate
or define such processing, but does allow it for the sake of
robustness.
Note: The Content-Location header field (Section 6.2.5) differs
from Location in that the Content-Location refers to the most
specific resource corresponding to the enclosed representation.
It is therefore possible for a response to contain both the
Location and Content-Location header fields.
10.1.3. Retry-After
Servers send the "Retry-After" header field to indicate how long the
user agent ought to wait before making a follow-up request. When
sent with a 503 (Service Unavailable) response, Retry-After indicates
how long the service is expected to be unavailable to the client.
When sent with any 3xx (Redirection) response, Retry-After indicates
the minimum time that the user agent is asked to wait before issuing
the redirected request.
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The value of this field can be either an HTTP-date or a number of
seconds to delay after the response is received.
Retry-After = HTTP-date / delay-seconds
A delay-seconds value is a non-negative decimal integer, representing
time in seconds.
delay-seconds = 1*DIGIT
Two examples of its use are
Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
Retry-After: 120
In the latter example, the delay is 2 minutes.
10.1.4. Vary
The "Vary" header field in a response describes what parts of a
request message, aside from the method, Host header field, and
request target, might influence the origin server's process for
selecting and representing this response. The value consists of
either a single asterisk ("*") or a list of header field names (case-
insensitive).
Vary = "*" / 1#field-name
A Vary field value of "*" signals that anything about the request
might play a role in selecting the response representation, possibly
including elements outside the message syntax (e.g., the client's
network address). A recipient will not be able to determine whether
this response is appropriate for a later request without forwarding
the request to the origin server. A proxy MUST NOT generate a Vary
field with a "*" value.
A Vary field value consisting of a comma-separated list of names
indicates that the named request header fields, known as the
selecting header fields, might have a role in selecting the
representation. The potential selecting header fields are not
limited to those defined by this specification.
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For example, a response that contains
Vary: accept-encoding, accept-language
indicates that the origin server might have used the request's
Accept-Encoding and Accept-Language fields (or lack thereof) as
determining factors while choosing the content for this response.
An origin server might send Vary with a list of fields for two
purposes:
1. To inform cache recipients that they MUST NOT use this response
to satisfy a later request unless the later request has the same
values for the listed fields as the original request (Section 4.1
of [Caching]). In other words, Vary expands the cache key
required to match a new request to the stored cache entry.
2. To inform user agent recipients that this response is subject to
content negotiation (Section 8.4) and that a different
representation might be sent in a subsequent request if
additional parameters are provided in the listed header fields
(proactive negotiation).
An origin server SHOULD send a Vary header field when its algorithm
for selecting a representation varies based on aspects of the request
message other than the method and request target, unless the variance
cannot be crossed or the origin server has been deliberately
configured to prevent cache transparency. For example, there is no
need to send the Authorization field name in Vary because reuse
across users is constrained by the field definition (Section 8.5.3).
Likewise, an origin server might use Cache-Control directives
(Section 5.2 of [Caching]) to supplant Vary if it considers the
variance less significant than the performance cost of Vary's impact
on caching.
10.2. Validators
Validator header fields convey metadata about the selected
representation (Section 6). In responses to safe requests, validator
fields describe the selected representation chosen by the origin
server while handling the response. Note that, depending on the
status code semantics, the selected representation for a given
response is not necessarily the same as the representation enclosed
as response payload.
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In a successful response to a state-changing request, validator
fields describe the new representation that has replaced the prior
selected representation as a result of processing the request.
For example, an ETag header field in a 201 (Created) response
communicates the entity-tag of the newly created resource's
representation, so that it can be used in later conditional requests
to prevent the "lost update" problem Section 8.2.
+-------------------+----------------+
| Header Field Name | Defined in... |
+-------------------+----------------+
| ETag | Section 10.2.3 |
| Last-Modified | Section 10.2.2 |
+-------------------+----------------+
This specification defines two forms of metadata that are commonly
used to observe resource state and test for preconditions:
modification dates (Section 10.2.2) and opaque entity tags
(Section 10.2.3). Additional metadata that reflects resource state
has been defined by various extensions of HTTP, such as Web
Distributed Authoring and Versioning (WebDAV, [RFC4918]), that are
beyond the scope of this specification. A resource metadata value is
referred to as a "validator" when it is used within a precondition.
10.2.1. Weak versus Strong
Validators come in two flavors: strong or weak. Weak validators are
easy to generate but are far less useful for comparisons. Strong
validators are ideal for comparisons but can be very difficult (and
occasionally impossible) to generate efficiently. Rather than impose
that all forms of resource adhere to the same strength of validator,
HTTP exposes the type of validator in use and imposes restrictions on
when weak validators can be used as preconditions.
A "strong validator" is representation metadata that changes value
whenever a change occurs to the representation data that would be
observable in the payload body of a 200 (OK) response to GET.
A strong validator might change for reasons other than a change to
the representation data, such as when a semantically significant part
of the representation metadata is changed (e.g., Content-Type), but
it is in the best interests of the origin server to only change the
value when it is necessary to invalidate the stored responses held by
remote caches and authoring tools.
Cache entries might persist for arbitrarily long periods, regardless
of expiration times. Thus, a cache might attempt to validate an
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entry using a validator that it obtained in the distant past. A
strong validator is unique across all versions of all representations
associated with a particular resource over time. However, there is
no implication of uniqueness across representations of different
resources (i.e., the same strong validator might be in use for
representations of multiple resources at the same time and does not
imply that those representations are equivalent).
There are a variety of strong validators used in practice. The best
are based on strict revision control, wherein each change to a
representation always results in a unique node name and revision
identifier being assigned before the representation is made
accessible to GET. A collision-resistant hash function applied to
the representation data is also sufficient if the data is available
prior to the response header fields being sent and the digest does
not need to be recalculated every time a validation request is
received. However, if a resource has distinct representations that
differ only in their metadata, such as might occur with content
negotiation over media types that happen to share the same data
format, then the origin server needs to incorporate additional
information in the validator to distinguish those representations.
In contrast, a "weak validator" is representation metadata that might
not change for every change to the representation data. This
weakness might be due to limitations in how the value is calculated,
such as clock resolution, an inability to ensure uniqueness for all
possible representations of the resource, or a desire of the resource
owner to group representations by some self-determined set of
equivalency rather than unique sequences of data. An origin server
SHOULD change a weak entity-tag whenever it considers prior
representations to be unacceptable as a substitute for the current
representation. In other words, a weak entity-tag ought to change
whenever the origin server wants caches to invalidate old responses.
For example, the representation of a weather report that changes in
content every second, based on dynamic measurements, might be grouped
into sets of equivalent representations (from the origin server's
perspective) with the same weak validator in order to allow cached
representations to be valid for a reasonable period of time (perhaps
adjusted dynamically based on server load or weather quality).
Likewise, a representation's modification time, if defined with only
one-second resolution, might be a weak validator if it is possible
for the representation to be modified twice during a single second
and retrieved between those modifications.
Likewise, a validator is weak if it is shared by two or more
representations of a given resource at the same time, unless those
representations have identical representation data. For example, if
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the origin server sends the same validator for a representation with
a gzip content coding applied as it does for a representation with no
content coding, then that validator is weak. However, two
simultaneous representations might share the same strong validator if
they differ only in the representation metadata, such as when two
different media types are available for the same representation data.
Strong validators are usable for all conditional requests, including
cache validation, partial content ranges, and "lost update"
avoidance. Weak validators are only usable when the client does not
require exact equality with previously obtained representation data,
such as when validating a cache entry or limiting a web traversal to
recent changes.
10.2.2. Last-Modified
The "Last-Modified" header field in a response provides a timestamp
indicating the date and time at which the origin server believes the
selected representation was last modified, as determined at the
conclusion of handling the request.
Last-Modified = HTTP-date
An example of its use is
Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT
10.2.2.1. Generation
An origin server SHOULD send Last-Modified for any selected
representation for which a last modification date can be reasonably
and consistently determined, since its use in conditional requests
and evaluating cache freshness ([Caching]) results in a substantial
reduction of HTTP traffic on the Internet and can be a significant
factor in improving service scalability and reliability.
A representation is typically the sum of many parts behind the
resource interface. The last-modified time would usually be the most
recent time that any of those parts were changed. How that value is
determined for any given resource is an implementation detail beyond
the scope of this specification. What matters to HTTP is how
recipients of the Last-Modified header field can use its value to
make conditional requests and test the validity of locally cached
responses.
An origin server SHOULD obtain the Last-Modified value of the
representation as close as possible to the time that it generates the
Date field value for its response. This allows a recipient to make
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an accurate assessment of the representation's modification time,
especially if the representation changes near the time that the
response is generated.
An origin server with a clock MUST NOT send a Last-Modified date that
is later than the server's time of message origination (Date). If
the last modification time is derived from implementation-specific
metadata that evaluates to some time in the future, according to the
origin server's clock, then the origin server MUST replace that value
with the message origination date. This prevents a future
modification date from having an adverse impact on cache validation.
An origin server without a clock MUST NOT assign Last-Modified values
to a response unless these values were associated with the resource
by some other system or user with a reliable clock.
10.2.2.2. Comparison
A Last-Modified time, when used as a validator in a request, is
implicitly weak unless it is possible to deduce that it is strong,
using the following rules:
o The validator is being compared by an origin server to the actual
current validator for the representation and,
o That origin server reliably knows that the associated
representation did not change twice during the second covered by
the presented validator.
or
o The validator is about to be used by a client in an If-Modified-
Since, If-Unmodified-Since, or If-Range header field, because the
client has a cache entry for the associated representation, and
o That cache entry includes a Date value, which gives the time when
the origin server sent the original response, and
o The presented Last-Modified time is at least 60 seconds before the
Date value.
or
o The validator is being compared by an intermediate cache to the
validator stored in its cache entry for the representation, and
o That cache entry includes a Date value, which gives the time when
the origin server sent the original response, and
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o The presented Last-Modified time is at least 60 seconds before the
Date value.
This method relies on the fact that if two different responses were
sent by the origin server during the same second, but both had the
same Last-Modified time, then at least one of those responses would
have a Date value equal to its Last-Modified time. The arbitrary
60-second limit guards against the possibility that the Date and
Last-Modified values are generated from different clocks or at
somewhat different times during the preparation of the response. An
implementation MAY use a value larger than 60 seconds, if it is
believed that 60 seconds is too short.
10.2.3. ETag
The "ETag" header field in a response provides the current entity-tag
for the selected representation, as determined at the conclusion of
handling the request. An entity-tag is an opaque validator for
differentiating between multiple representations of the same
resource, regardless of whether those multiple representations are
due to resource state changes over time, content negotiation
resulting in multiple representations being valid at the same time,
or both. An entity-tag consists of an opaque quoted string, possibly
prefixed by a weakness indicator.
ETag = entity-tag
entity-tag = [ weak ] opaque-tag
weak = %x57.2F ; "W/", case-sensitive
opaque-tag = DQUOTE *etagc DQUOTE
etagc = %x21 / %x23-7E / obs-text
; VCHAR except double quotes, plus obs-text
Note: Previously, opaque-tag was defined to be a quoted-string
([RFC2616], Section 3.11); thus, some recipients might perform
backslash unescaping. Servers therefore ought to avoid backslash
characters in entity tags.
An entity-tag can be more reliable for validation than a modification
date in situations where it is inconvenient to store modification
dates, where the one-second resolution of HTTP date values is not
sufficient, or where modification dates are not consistently
maintained.
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Examples:
ETag: "xyzzy"
ETag: W/"xyzzy"
ETag: ""
An entity-tag can be either a weak or strong validator, with strong
being the default. If an origin server provides an entity-tag for a
representation and the generation of that entity-tag does not satisfy
all of the characteristics of a strong validator (Section 10.2.1),
then the origin server MUST mark the entity-tag as weak by prefixing
its opaque value with "W/" (case-sensitive).
10.2.3.1. Generation
The principle behind entity-tags is that only the service author
knows the implementation of a resource well enough to select the most
accurate and efficient validation mechanism for that resource, and
that any such mechanism can be mapped to a simple sequence of octets
for easy comparison. Since the value is opaque, there is no need for
the client to be aware of how each entity-tag is constructed.
For example, a resource that has implementation-specific versioning
applied to all changes might use an internal revision number, perhaps
combined with a variance identifier for content negotiation, to
accurately differentiate between representations. Other
implementations might use a collision-resistant hash of
representation content, a combination of various file attributes, or
a modification timestamp that has sub-second resolution.
An origin server SHOULD send an ETag for any selected representation
for which detection of changes can be reasonably and consistently
determined, since the entity-tag's use in conditional requests and
evaluating cache freshness ([Caching]) can result in a substantial
reduction of HTTP network traffic and can be a significant factor in
improving service scalability and reliability.
10.2.3.2. Comparison
There are two entity-tag comparison functions, depending on whether
or not the comparison context allows the use of weak validators:
o Strong comparison: two entity-tags are equivalent if both are not
weak and their opaque-tags match character-by-character.
o Weak comparison: two entity-tags are equivalent if their opaque-
tags match character-by-character, regardless of either or both
being tagged as "weak".
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The example below shows the results for a set of entity-tag pairs and
both the weak and strong comparison function results:
+--------+--------+-------------------+-----------------+
| ETag 1 | ETag 2 | Strong Comparison | Weak Comparison |
+--------+--------+-------------------+-----------------+
| W/"1" | W/"1" | no match | match |
| W/"1" | W/"2" | no match | no match |
| W/"1" | "1" | no match | match |
| "1" | "1" | match | match |
+--------+--------+-------------------+-----------------+
10.2.3.3. Example: Entity-Tags Varying on Content-Negotiated Resources
Consider a resource that is subject to content negotiation
(Section 6.4), and where the representations sent in response to a
GET request vary based on the Accept-Encoding request header field
(Section 8.4.4):
>> Request:
GET /index HTTP/1.1
Host: www.example.com
Accept-Encoding: gzip
In this case, the response might or might not use the gzip content
coding. If it does not, the response might look like:
>> Response:
HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-a"
Content-Length: 70
Vary: Accept-Encoding
Content-Type: text/plain
Hello World!
Hello World!
Hello World!
Hello World!
Hello World!
An alternative representation that does use gzip content coding would
be:
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>> Response:
HTTP/1.1 200 OK
Date: Fri, 26 Mar 2010 00:05:00 GMT
ETag: "123-b"
Content-Length: 43
Vary: Accept-Encoding
Content-Type: text/plain
Content-Encoding: gzip
...binary data...
Note: Content codings are a property of the representation data,
so a strong entity-tag for a content-encoded representation has to
be distinct from the entity tag of an unencoded representation to
prevent potential conflicts during cache updates and range
requests. In contrast, transfer codings (Section 7 of
[Messaging]) apply only during message transfer and do not result
in distinct entity-tags.
10.2.4. When to Use Entity-Tags and Last-Modified Dates
In 200 (OK) responses to GET or HEAD, an origin server:
o SHOULD send an entity-tag validator unless it is not feasible to
generate one.
o MAY send a weak entity-tag instead of a strong entity-tag, if
performance considerations support the use of weak entity-tags, or
if it is unfeasible to send a strong entity-tag.
o SHOULD send a Last-Modified value if it is feasible to send one.
In other words, the preferred behavior for an origin server is to
send both a strong entity-tag and a Last-Modified value in successful
responses to a retrieval request.
A client:
o MUST send that entity-tag in any cache validation request (using
If-Match or If-None-Match) if an entity-tag has been provided by
the origin server.
o SHOULD send the Last-Modified value in non-subrange cache
validation requests (using If-Modified-Since) if only a Last-
Modified value has been provided by the origin server.
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o MAY send the Last-Modified value in subrange cache validation
requests (using If-Unmodified-Since) if only a Last-Modified value
has been provided by an HTTP/1.0 origin server. The user agent
SHOULD provide a way to disable this, in case of difficulty.
o SHOULD send both validators in cache validation requests if both
an entity-tag and a Last-Modified value have been provided by the
origin server. This allows both HTTP/1.0 and HTTP/1.1 caches to
respond appropriately.
10.3. 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 10.3.1 |
| Proxy-Authenticate | Section 10.3.2 |
+--------------------+----------------+
10.3.1. WWW-Authenticate
The "WWW-Authenticate" header field indicates the authentication
scheme(s) and parameters applicable to the target resource.
WWW-Authenticate = 1#challenge
A server generating a 401 (Unauthorized) response MUST send a WWW-
Authenticate header field containing at least one challenge. A
server MAY generate a WWW-Authenticate header field in other response
messages to indicate that supplying credentials (or different
credentials) might affect the response.
A proxy forwarding a response MUST NOT modify any WWW-Authenticate
fields in that response.
User agents are advised to take special care in parsing the field
value, as it might contain more than one challenge, and each
challenge can contain a comma-separated list of authentication
parameters. Furthermore, the header field itself can occur multiple
times.
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For instance:
WWW-Authenticate: Newauth realm="apps", type=1,
title="Login to \"apps\"", Basic realm="simple"
This header field contains two challenges; one for the "Newauth"
scheme with a realm value of "apps", and two additional parameters
"type" and "title", and another one for the "Basic" scheme with a
realm value of "simple".
Note: The challenge grammar production uses the list syntax as
well. Therefore, a sequence of comma, whitespace, and comma can
be considered either as applying to the preceding challenge, or to
be an empty entry in the list of challenges. In practice, this
ambiguity does not affect the semantics of the header field value
and thus is harmless.
10.3.2. Proxy-Authenticate
The "Proxy-Authenticate" header field consists of at least one
challenge that indicates the authentication scheme(s) and parameters
applicable to the proxy for this effective request URI (Section 5.3).
A proxy MUST send at least one Proxy-Authenticate header field in
each 407 (Proxy Authentication Required) response that it generates.
Proxy-Authenticate = 1#challenge
Unlike WWW-Authenticate, the Proxy-Authenticate header field applies
only to the next outbound client on the response chain. This is
because only the client that chose a given proxy is likely to have
the credentials necessary for authentication. However, when multiple
proxies are used within the same administrative domain, such as
office and regional caching proxies within a large corporate network,
it is common for credentials to be generated by the user agent and
passed through the hierarchy until consumed. Hence, in such a
configuration, it will appear as if Proxy-Authenticate is being
forwarded because each proxy will send the same challenge set.
Note that the parsing considerations for WWW-Authenticate apply to
this header field as well; see Section 10.3.1 for details.
10.4. Response Context
The remaining response header fields provide more information about
the target resource for potential use in later requests.
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+-------------------+----------------+
| Header Field Name | Defined in... |
+-------------------+----------------+
| Accept-Ranges | Section 10.4.1 |
| Allow | Section 10.4.2 |
| Server | Section 10.4.3 |
+-------------------+----------------+
10.4.1. Accept-Ranges
The "Accept-Ranges" header field allows a server to indicate that it
supports range requests for the target resource.
Accept-Ranges = acceptable-ranges
acceptable-ranges = 1#range-unit / "none"
An origin server that supports byte-range requests for a given target
resource MAY send
Accept-Ranges: bytes
to indicate what range units are supported. A client MAY generate
range requests without having received this header field for the
resource involved. Range units are defined in Section 6.1.4.
A server that does not support any kind of range request for the
target resource MAY send
Accept-Ranges: none
to advise the client not to attempt a range request.
10.4.2. 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. An origin server MUST generate an Allow
field in a 405 (Method Not Allowed) response and MAY do so in any
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other response. An empty Allow field value indicates that the
resource allows no methods, which might occur in a 405 response if
the resource has been temporarily disabled by configuration.
A proxy MUST NOT modify the Allow header field -- it does not need to
understand all of the indicated methods in order to handle them
according to the generic message handling rules.
10.4.3. Server
The "Server" header field contains information about the software
used by the origin server to handle the request, which is often used
by clients to help identify the scope of reported interoperability
problems, to work around or tailor requests to avoid particular
server limitations, and for analytics regarding server or operating
system use. An origin server MAY generate a Server field in its
responses.
Server = product *( RWS ( product / comment ) )
The Server field-value consists of one or more product identifiers,
each followed by zero or more comments (Section 5 of [Messaging]),
which together identify the origin server software and its
significant subproducts. By convention, the product identifiers are
listed in decreasing order of their significance for identifying the
origin server software. Each product identifier consists of a name
and optional version, as defined in Section 8.6.3.
Example:
Server: CERN/3.0 libwww/2.17
An origin server SHOULD NOT generate a Server field containing
needlessly fine-grained detail and SHOULD limit the addition of
subproducts by third parties. Overly long and detailed Server field
values increase response latency and potentially reveal internal
implementation details that might make it (slightly) easier for
attackers to find and exploit known security holes.
11. ABNF List Extension: #rule
A #rule extension to the ABNF rules of [RFC5234] is used to improve
readability in the definitions of some header field values.
A construct "#" is defined, similar to "*", for defining comma-
delimited lists of elements. The full form is "<n>#<m>element"
indicating at least <n> and at most <m> elements, each separated by a
single comma (",") and optional whitespace (OWS).
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In any production that uses the list construct, a sender MUST NOT
generate empty list elements. In other words, a sender MUST generate
lists that satisfy the following syntax:
1#element => element *( OWS "," OWS element )
and:
#element => [ 1#element ]
and for n >= 1 and m > 1:
<n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )
For compatibility with legacy list rules, a recipient MUST parse and
ignore a reasonable number of empty list elements: enough to handle
common mistakes by senders that merge values, but not so much that
they could be used as a denial-of-service mechanism. In other words,
a recipient MUST accept lists that satisfy the following syntax:
#element => [ ( "," / element ) *( OWS "," [ OWS element ] ) ]
1#element => *( "," OWS ) element *( OWS "," [ OWS element ] )
Empty elements do not contribute to the count of elements present.
For example, given these ABNF productions:
example-list = 1#example-list-elmt
example-list-elmt = token ; see Section 4.2.3
Then the following are valid values for example-list (not including
the double quotes, which are present for delimitation only):
"foo,bar"
"foo ,bar,"
"foo , ,bar,charlie"
In contrast, the following values would be invalid, since at least
one non-empty element is required by the example-list production:
""
","
", ,"
Appendix A shows the collected ABNF for recipients after the list
constructs have been expanded.
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12. Security Considerations
This section is meant to inform developers, information providers,
and users of known security concerns relevant to HTTP semantics and
its use for transferring information over the Internet.
Considerations related to message syntax, parsing, and routing are
discussed in Section 11 of [Messaging].
The list of considerations below is not exhaustive. Most security
concerns related to HTTP semantics are about securing server-side
applications (code behind the HTTP interface), securing user agent
processing of payloads received via HTTP, or secure use of the
Internet in general, rather than security of the protocol. Various
organizations maintain topical information and links to current
research on Web application security (e.g., [OWASP]).
12.1. Establishing Authority
HTTP relies on the notion of an authoritative response: a response
that has been determined by (or at the direction of) the authority
identified within the target URI to be the most appropriate response
for that request given the state of the target resource at the time
of response message origination. Providing a response from a non-
authoritative source, such as a shared cache, is often useful to
improve performance and availability, but only to the extent that the
source can be trusted or the distrusted response can be safely used.
Unfortunately, establishing authority can be difficult. For example,
phishing is an attack on the user's perception of authority, where
that perception can be misled by presenting similar branding in
hypertext, possibly aided by userinfo obfuscating the authority
component (see Section 2.5.1). User agents can reduce the impact of
phishing attacks by enabling users to easily inspect a target URI
prior to making an action, by prominently distinguishing (or
rejecting) userinfo when present, and by not sending stored
credentials and cookies when the referring document is from an
unknown or untrusted source.
When a registered name is used in the authority component, the "http"
URI scheme (Section 2.5.1) relies on the user's local name resolution
service to determine where it can find authoritative responses. This
means that any attack on a user's network host table, cached names,
or name resolution libraries becomes an avenue for attack on
establishing authority. Likewise, the user's choice of server for
Domain Name Service (DNS), and the hierarchy of servers from which it
obtains resolution results, could impact the authenticity of address
mappings; DNS Security Extensions (DNSSEC, [RFC4033]) are one way to
improve authenticity.
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Furthermore, after an IP address is obtained, establishing authority
for an "http" URI is vulnerable to attacks on Internet Protocol
routing.
The "https" scheme (Section 2.5.2) is intended to prevent (or at
least reveal) many of these potential attacks on establishing
authority, provided that the negotiated TLS connection is secured and
the client properly verifies that the communicating server's identity
matches the target URI's authority component (see [RFC2818]).
Correctly implementing such verification can be difficult (see
[Georgiev]).
12.2. Risks of Intermediaries
By their very nature, HTTP intermediaries are men-in-the-middle and,
thus, represent an opportunity for man-in-the-middle attacks.
Compromise of the systems on which the intermediaries run can result
in serious security and privacy problems. Intermediaries might have
access to security-related information, personal information about
individual users and organizations, and proprietary information
belonging to users and content providers. A compromised
intermediary, or an intermediary implemented or configured without
regard to security and privacy considerations, might be used in the
commission of a wide range of potential attacks.
Intermediaries that contain a shared cache are especially vulnerable
to cache poisoning attacks, as described in Section 7 of [Caching].
Implementers need to consider the privacy and security implications
of their design and coding decisions, and of the configuration
options they provide to operators (especially the default
configuration).
Users need to be aware that intermediaries are no more trustworthy
than the people who run them; HTTP itself cannot solve this problem.
12.3. Attacks Based on File and Path Names
Origin servers frequently make use of their local file system to
manage the mapping from effective request URI to resource
representations. Most file systems are not designed to protect
against malicious file or path names. Therefore, an origin server
needs to avoid accessing names that have a special significance to
the system when mapping the request target to files, folders, or
directories.
For example, UNIX, Microsoft Windows, and other operating systems use
".." as a path component to indicate a directory level above the
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current one, and they use specially named paths or file names to send
data to system devices. Similar naming conventions might exist
within other types of storage systems. Likewise, local storage
systems have an annoying tendency to prefer user-friendliness over
security when handling invalid or unexpected characters,
recomposition of decomposed characters, and case-normalization of
case-insensitive names.
Attacks based on such special names tend to focus on either denial-
of-service (e.g., telling the server to read from a COM port) or
disclosure of configuration and source files that are not meant to be
served.
12.4. Attacks Based on Command, Code, or Query Injection
Origin servers often use parameters within the URI as a means of
identifying system services, selecting database entries, or choosing
a data source. However, data received in a request cannot be
trusted. An attacker could construct any of the request data
elements (method, request-target, header fields, or body) to contain
data that might be misinterpreted as a command, code, or query when
passed through a command invocation, language interpreter, or
database interface.
For example, SQL injection is a common attack wherein additional
query language is inserted within some part of the request-target or
header fields (e.g., Host, Referer, etc.). If the received data is
used directly within a SELECT statement, the query language might be
interpreted as a database command instead of a simple string value.
This type of implementation vulnerability is extremely common, in
spite of being easy to prevent.
In general, resource implementations ought to avoid use of request
data in contexts that are processed or interpreted as instructions.
Parameters ought to be compared to fixed strings and acted upon as a
result of that comparison, rather than passed through an interface
that is not prepared for untrusted data. Received data that isn't
based on fixed parameters ought to be carefully filtered or encoded
to avoid being misinterpreted.
Similar considerations apply to request data when it is stored and
later processed, such as within log files, monitoring tools, or when
included within a data format that allows embedded scripts.
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12.5. Attacks via Protocol Element Length
Because HTTP uses mostly textual, character-delimited fields, parsers
are often vulnerable to attacks based on sending very long (or very
slow) streams of data, particularly where an implementation is
expecting a protocol element with no predefined length (Section 3.3).
To promote interoperability, specific recommendations are made for
minimum size limits on request-line (Section 3 of [Messaging]) and
header fields (Section 5 of [Messaging]). These are minimum
recommendations, chosen to be supportable even by implementations
with limited resources; it is expected that most implementations will
choose substantially higher limits.
A server can reject a message that has a request-target that is too
long (Section 9.5.15) or a request payload that is too large
(Section 9.5.14). Additional status codes related to capacity limits
have been defined by extensions to HTTP [RFC6585].
Recipients ought to carefully limit the extent to which they process
other protocol elements, including (but not limited to) request
methods, response status phrases, header field-names, numeric values,
and body chunks. Failure to limit such processing can result in
buffer overflows, arithmetic overflows, or increased vulnerability to
denial-of-service attacks.
12.6. Disclosure of Personal Information
Clients are often privy to large amounts of personal information,
including both information provided by the user to interact with
resources (e.g., the user's name, location, mail address, passwords,
encryption keys, etc.) and information about the user's browsing
activity over time (e.g., history, bookmarks, etc.). Implementations
need to prevent unintentional disclosure of personal information.
12.7. Privacy of Server Log Information
A server is in the position to save personal data about a user's
requests over time, which might identify their reading patterns or
subjects of interest. In particular, log information gathered at an
intermediary often contains a history of user agent interaction,
across a multitude of sites, that can be traced to individual users.
HTTP log information is confidential in nature; its handling is often
constrained by laws and regulations. Log information needs to be
securely stored and appropriate guidelines followed for its analysis.
Anonymization of personal information within individual entries
helps, but it is generally not sufficient to prevent real log traces
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from being re-identified based on correlation with other access
characteristics. As such, access traces that are keyed to a specific
client are unsafe to publish even if the key is pseudonymous.
To minimize the risk of theft or accidental publication, log
information ought to be purged of personally identifiable
information, including user identifiers, IP addresses, and user-
provided query parameters, as soon as that information is no longer
necessary to support operational needs for security, auditing, or
fraud control.
12.8. Disclosure of Sensitive Information in URIs
URIs are intended to be shared, not secured, even when they identify
secure resources. URIs are often shown on displays, added to
templates when a page is printed, and stored in a variety of
unprotected bookmark lists. It is therefore unwise to include
information within a URI that is sensitive, personally identifiable,
or a risk to disclose.
Authors of services ought to avoid GET-based forms for the submission
of sensitive data because that data will be placed in the request-
target. Many existing servers, proxies, and user agents log or
display the request-target in places where it might be visible to
third parties. Such services ought to use POST-based form submission
instead.
Since the Referer header field tells a target site about the context
that resulted in a request, it has the potential to reveal
information about the user's immediate browsing history and any
personal information that might be found in the referring resource's
URI. Limitations on the Referer header field are described in
Section 8.6.2 to address some of its security considerations.
12.9. Disclosure of Fragment after Redirects
Although fragment identifiers used within URI references are not sent
in requests, implementers ought to be aware that they will be visible
to the user agent and any extensions or scripts running as a result
of the response. In particular, when a redirect occurs and the
original request's fragment identifier is inherited by the new
reference in Location (Section 10.1.2), this might have the effect of
disclosing one site's fragment to another site. If the first site
uses personal information in fragments, it ought to ensure that
redirects to other sites include a (possibly empty) fragment
component in order to block that inheritance.
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12.10. Disclosure of Product Information
The User-Agent (Section 8.6.3), Via (Section 5.6.1), and Server
(Section 10.4.3) header fields often reveal information about the
respective sender's software systems. In theory, this can make it
easier for an attacker to exploit known security holes; in practice,
attackers tend to try all potential holes regardless of the apparent
software versions being used.
Proxies that serve as a portal through a network firewall ought to
take special precautions regarding the transfer of header information
that might identify hosts behind the firewall. The Via header field
allows intermediaries to replace sensitive machine names with
pseudonyms.
12.11. Browser Fingerprinting
Browser fingerprinting is a set of techniques for identifying a
specific user agent over time through its unique set of
characteristics. These characteristics might include information
related to its TCP behavior, feature capabilities, and scripting
environment, though of particular interest here is the set of unique
characteristics that might be communicated via HTTP. Fingerprinting
is considered a privacy concern because it enables tracking of a user
agent's behavior over time without the corresponding controls that
the user might have over other forms of data collection (e.g.,
cookies). Many general-purpose user agents (i.e., Web browsers) have
taken steps to reduce their fingerprints.
There are a number of request header fields that might reveal
information to servers that is sufficiently unique to enable
fingerprinting. The From header field is the most obvious, though it
is expected that From will only be sent when self-identification is
desired by the user. Likewise, Cookie header fields are deliberately
designed to enable re-identification, so fingerprinting concerns only
apply to situations where cookies are disabled or restricted by the
user agent's configuration.
The User-Agent header field might contain enough information to
uniquely identify a specific device, usually when combined with other
characteristics, particularly if the user agent sends excessive
details about the user's system or extensions. However, the source
of unique information that is least expected by users is proactive
negotiation (Section 8.4), including the Accept, Accept-Charset,
Accept-Encoding, and Accept-Language header fields.
In addition to the fingerprinting concern, detailed use of the
Accept-Language header field can reveal information the user might
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consider to be of a private nature. For example, understanding a
given language set might be strongly correlated to membership in a
particular ethnic group. An approach that limits such loss of
privacy would be for a user agent to omit the sending of Accept-
Language except for sites that have been whitelisted, perhaps via
interaction after detecting a Vary header field that indicates
language negotiation might be useful.
In environments where proxies are used to enhance privacy, user
agents ought to be conservative in sending proactive negotiation
header fields. General-purpose user agents that provide a high
degree of header field configurability ought to inform users about
the loss of privacy that might result if too much detail is provided.
As an extreme privacy measure, proxies could filter the proactive
negotiation header fields in relayed requests.
12.12. Validator Retention
The validators defined by this specification are not intended to
ensure the validity of a representation, guard against malicious
changes, or detect man-in-the-middle attacks. At best, they enable
more efficient cache updates and optimistic concurrent writes when
all participants are behaving nicely. At worst, the conditions will
fail and the client will receive a response that is no more harmful
than an HTTP exchange without conditional requests.
An entity-tag can be abused in ways that create privacy risks. For
example, a site might deliberately construct a semantically invalid
entity-tag that is unique to the user or user agent, send it in a
cacheable response with a long freshness time, and then read that
entity-tag in later conditional requests as a means of re-identifying
that user or user agent. Such an identifying tag would become a
persistent identifier for as long as the user agent retained the
original cache entry. User agents that cache representations ought
to ensure that the cache is cleared or replaced whenever the user
performs privacy-maintaining actions, such as clearing stored cookies
or changing to a private browsing mode.
12.13. Denial-of-Service Attacks Using Range
Unconstrained multiple range requests are susceptible to denial-of-
service attacks because the effort required to request many
overlapping ranges of the same data is tiny compared to the time,
memory, and bandwidth consumed by attempting to serve the requested
data in many parts. Servers ought to ignore, coalesce, or reject
egregious range requests, such as requests for more than two
overlapping ranges or for many small ranges in a single set,
particularly when the ranges are requested out of order for no
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apparent reason. Multipart range requests are not designed to
support random access.
12.14. Authentication Considerations
Everything about the topic of HTTP authentication is a security
consideration, so the list of considerations below is not exhaustive.
Furthermore, it is limited to security considerations regarding the
authentication framework, in general, rather than discussing all of
the potential considerations for specific authentication schemes
(which ought to be documented in the specifications that define those
schemes). Various organizations maintain topical information and
links to current research on Web application security (e.g.,
[OWASP]), including common pitfalls for implementing and using the
authentication schemes found in practice.
12.14.1. Confidentiality of Credentials
The HTTP authentication framework does not define a single mechanism
for maintaining the confidentiality of credentials; instead, each
authentication scheme defines how the credentials are encoded prior
to transmission. While this provides flexibility for the development
of future authentication schemes, it is inadequate for the protection
of existing schemes that provide no confidentiality on their own, or
that do not sufficiently protect against replay attacks.
Furthermore, if the server expects credentials that are specific to
each individual user, the exchange of those credentials will have the
effect of identifying that user even if the content within
credentials remains confidential.
HTTP depends on the security properties of the underlying transport-
or session-level connection to provide confidential transmission of
header fields. In other words, if a server limits access to
authenticated users using this framework, the server needs to ensure
that the connection is properly secured in accordance with the nature
of the authentication scheme used. For example, services that depend
on individual user authentication often require a connection to be
secured with TLS ("Transport Layer Security", [RFC5246]) prior to
exchanging any credentials.
12.14.2. Credentials and Idle Clients
Existing HTTP clients and user agents typically retain authentication
information indefinitely. HTTP does not provide a mechanism for the
origin server to direct clients to discard these cached credentials,
since the protocol has no awareness of how credentials are obtained
or managed by the user agent. The mechanisms for expiring or
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revoking credentials can be specified as part of an authentication
scheme definition.
Circumstances under which credential caching can interfere with the
application's security model include but are not limited to:
o Clients that have been idle for an extended period, following
which the server might wish to cause the client to re-prompt the
user for credentials.
o Applications that include a session termination indication (such
as a "logout" or "commit" button on a page) after which the server
side of the application "knows" that there is no further reason
for the client to retain the credentials.
User agents that cache credentials are encouraged to provide a
readily accessible mechanism for discarding cached credentials under
user control.
12.14.3. Protection Spaces
Authentication schemes that solely rely on the "realm" mechanism for
establishing a protection space will expose credentials to all
resources on an origin server. Clients that have successfully made
authenticated requests with a resource can use the same
authentication credentials for other resources on the same origin
server. This makes it possible for a different resource to harvest
authentication credentials for other resources.
This is of particular concern when an origin server hosts resources
for multiple parties under the same canonical root URI
(Section 8.5.2). Possible mitigation strategies include restricting
direct access to authentication credentials (i.e., not making the
content of the Authorization request header field available), and
separating protection spaces by using a different host name (or port
number) for each party.
13. IANA Considerations
The change controller for the following registrations is: "IETF
(iesg@ietf.org) - Internet Engineering Task Force".
13.1. URI Scheme Registration
Please update the registry of URI Schemes [BCP35] at
<https://www.iana.org/assignments/uri-schemes/> with the permanent
schemes listed in the first table of Section 2.5.
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13.2. Method Registration
Please update the "Hypertext Transfer Protocol (HTTP) Method
Registry" at <https://www.iana.org/assignments/http-methods> with the
registration procedure of Section 7.4.1 and the method names
summarized in the table of Section 7.2.
13.3. Status Code Registration
Please update the "Hypertext Transfer Protocol (HTTP) Status Code
Registry" at <https://www.iana.org/assignments/http-status-codes>
with the registration procedure of Section 9.7.1 and the status code
values summarized in the table of Section 9.1.
13.4. Header Field Registration
Please update the "Message Headers" registry of "Permanent Message
Header Field Names" at <https://www.iana.org/assignments/message-
headers> with the header field names listed in the table of
Section 4.1.
13.5. Authentication Scheme Registration
Please update the "Hypertext Transfer Protocol (HTTP) Authentication
Scheme Registry" at <https://www.iana.org/assignments/http-
authschemes> with the registration procedure of Section 8.5.5.1. No
authentication schemes are defined in this document.
13.6. Content Coding Registration
Please update the "HTTP Content Coding Registry" at
<https://www.iana.org/assignments/http-parameters/> with the
registration procedure of Section 6.1.2.4.1 and the content coding
names summarized in the table of Section 6.1.2.
13.7. Range Unit Registration
Please update the "HTTP Range Unit Registry" at
<https://www.iana.org/assignments/http-parameters/> with the
registration procedure of Section 6.1.4.3 and the range unit names
summarized in the table of Section 6.1.4.
13.8. Media Type Registration
Please update the "Media Types" registry at
<https://www.iana.org/assignments/media-types> with the registration
information in Section 6.3.4 for the media type "multipart/
byteranges".
Fielding, et al. Expires January 3, 2019 [Page 157]
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14. References
14.1. Normative References
[Caching] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Caching", draft-ietf-httpbis-cache-02 (work in
progress), July 2018.
[Messaging]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1 Messaging", draft-ietf-httpbis-messaging-02
(work in progress), July 2018.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC1950] Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data Format
Specification version 3.3", RFC 1950,
DOI 10.17487/RFC1950, May 1996,
<https://www.rfc-editor.org/info/rfc1950>.
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, DOI 10.17487/RFC1951, May 1996,
<https://www.rfc-editor.org/info/rfc1951>.
[RFC1952] Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G.
Randers-Pehrson, "GZIP file format specification version
4.3", RFC 1952, DOI 10.17487/RFC1952, May 1996,
<https://www.rfc-editor.org/info/rfc1952>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC4647] Phillips, A., Ed. and M. Davis, Ed., "Matching of Language
Tags", BCP 47, RFC 4647, DOI 10.17487/RFC4647, September
2006, <https://www.rfc-editor.org/info/rfc4647>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5646] Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
September 2009, <https://www.rfc-editor.org/info/rfc5646>.
[RFC6365] Hoffman, P. and J. Klensin, "Terminology Used in
Internationalization in the IETF", BCP 166, RFC 6365,
DOI 10.17487/RFC6365, September 2011,
<https://www.rfc-editor.org/info/rfc6365>.
[USASCII] American National Standards Institute, "Coded Character
Set -- 7-bit American Standard Code for Information
Interchange", ANSI X3.4, 1986.
[Welch] Welch, T., "A Technique for High-Performance Data
Compression", IEEE Computer 17(6),
DOI 10.1109/MC.1984.1659158, June 1984,
<https://ieeexplore.ieee.org/document/1659158/>.
14.2. Informative References
[BCP13] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, January 2013,
<https://www.rfc-editor.org/info/bcp13>.
[BCP178] Saint-Andre, P., Crocker, D., and M. Nottingham,
"Deprecating the "X-" Prefix and Similar Constructs in
Application Protocols", BCP 178, RFC 6648, June 2012,
<https://www.rfc-editor.org/info/bcp178>.
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[BCP35] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
and Registration Procedures for URI Schemes", BCP 35,
RFC 7595, June 2015,
<https://www.rfc-editor.org/info/bcp35>.
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004, <https://www.rfc-editor.org/info/bcp90>.
[Georgiev]
Georgiev, M., Iyengar, S., Jana, S., Anubhai, R., Boneh,
D., and V. Shmatikov, "The Most Dangerous Code in the
World: Validating SSL Certificates in Non-browser
Software", In Proceedings of the 2012 ACM Conference on
Computer and Communications Security (CCS '12), pp. 38-49,
October 2012,
<http://doi.acm.org/10.1145/2382196.2382204>.
[ISO-8859-1]
International Organization for Standardization,
"Information technology -- 8-bit single-byte coded graphic
character sets -- Part 1: Latin alphabet No. 1", ISO/
IEC 8859-1:1998, 1998.
[Kri2001] Kristol, D., "HTTP Cookies: Standards, Privacy, and
Politics", ACM Transactions on Internet Technology 1(2),
November 2001, <http://arxiv.org/abs/cs.SE/0105018>.
[OWASP] van der Stock, A., Ed., "A Guide to Building Secure Web
Applications and Web Services", The Open Web Application
Security Project (OWASP) 2.0.1, July 2005,
<https://www.owasp.org/>.
[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures",
Doctoral Dissertation, University of California, Irvine,
September 2000,
<https://roy.gbiv.com/pubs/dissertation/top.htm>.
[RFC1919] Chatel, M., "Classical versus Transparent IP Proxies",
RFC 1919, DOI 10.17487/RFC1919, March 1996,
<https://www.rfc-editor.org/info/rfc1919>.
[RFC1945] Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945,
DOI 10.17487/RFC1945, May 1996,
<https://www.rfc-editor.org/info/rfc1945>.
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[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions)
Part Three: Message Header Extensions for Non-ASCII Text",
RFC 2047, DOI 10.17487/RFC2047, November 1996,
<https://www.rfc-editor.org/info/rfc2047>.
[RFC2068] Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2068, DOI 10.17487/RFC2068, January 1997,
<https://www.rfc-editor.org/info/rfc2068>.
[RFC2145] Mogul, J., Fielding, R., Gettys, J., and H. Nielsen, "Use
and Interpretation of HTTP Version Numbers", RFC 2145,
DOI 10.17487/RFC2145, May 1997,
<https://www.rfc-editor.org/info/rfc2145>.
[RFC2295] Holtman, K. and A. Mutz, "Transparent Content Negotiation
in HTTP", RFC 2295, DOI 10.17487/RFC2295, March 1998,
<https://www.rfc-editor.org/info/rfc2295>.
[RFC2557] Palme, F., Hopmann, A., Shelness, N., and E. Stefferud,
"MIME Encapsulation of Aggregate Documents, such as HTML
(MHTML)", RFC 2557, DOI 10.17487/RFC2557, March 1999,
<https://www.rfc-editor.org/info/rfc2557>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999,
<https://www.rfc-editor.org/info/rfc2616>.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, DOI 10.17487/RFC2617, June 1999,
<https://www.rfc-editor.org/info/rfc2617>.
[RFC2774] Frystyk, H., Leach, P., and S. Lawrence, "An HTTP
Extension Framework", RFC 2774, DOI 10.17487/RFC2774,
February 2000, <https://www.rfc-editor.org/info/rfc2774>.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818,
DOI 10.17487/RFC2818, May 2000,
<https://www.rfc-editor.org/info/rfc2818>.
[RFC2978] Freed, N. and J. Postel, "IANA Charset Registration
Procedures", BCP 19, RFC 2978, DOI 10.17487/RFC2978,
October 2000, <https://www.rfc-editor.org/info/rfc2978>.
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[RFC3040] Cooper, I., Melve, I., and G. Tomlinson, "Internet Web
Replication and Caching Taxonomy", RFC 3040,
DOI 10.17487/RFC3040, January 2001,
<https://www.rfc-editor.org/info/rfc3040>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.
[RFC4559] Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
Kerberos and NTLM HTTP Authentication in Microsoft
Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006,
<https://www.rfc-editor.org/info/rfc4559>.
[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918,
DOI 10.17487/RFC4918, June 2007,
<https://www.rfc-editor.org/info/rfc4918>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC5322] Resnick, P., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.
[RFC5789] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
RFC 5789, DOI 10.17487/RFC5789, March 2010,
<https://www.rfc-editor.org/info/rfc5789>.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
DOI 10.17487/RFC6265, April 2011,
<https://www.rfc-editor.org/info/rfc6265>.
[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status
Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
<https://www.rfc-editor.org/info/rfc6585>.
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[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
DOI 10.17487/RFC7232, June 2014,
<https://www.rfc-editor.org/info/rfc7232>.
[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP): Range Requests",
RFC 7233, DOI 10.17487/RFC7233, June 2014,
<https://www.rfc-editor.org/info/rfc7233>.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235,
DOI 10.17487/RFC7235, June 2014,
<https://www.rfc-editor.org/info/rfc7235>.
[RFC7538] Reschke, J., "The Hypertext Transfer Protocol Status Code
308 (Permanent Redirect)", RFC 7538, DOI 10.17487/RFC7538,
April 2015, <https://www.rfc-editor.org/info/rfc7538>.
[RFC7578] Masinter, L., "Returning Values from Forms: multipart/
form-data", RFC 7578, DOI 10.17487/RFC7578, July 2015,
<https://www.rfc-editor.org/info/rfc7578>.
[RFC7616] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
Digest Access Authentication", RFC 7616,
DOI 10.17487/RFC7616, September 2015,
<https://www.rfc-editor.org/info/rfc7616>.
[RFC7617] Reschke, J., "The 'Basic' HTTP Authentication Scheme",
RFC 7617, DOI 10.17487/RFC7617, September 2015,
<https://www.rfc-editor.org/info/rfc7617>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
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[RFC8187] Reschke, J., "Indicating Character Encoding and Language
for HTTP Header Field Parameters", RFC 8187,
DOI 10.17487/RFC8187, September 2017,
<https://www.rfc-editor.org/info/rfc8187>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
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Appendix A. Collected ABNF
In the collected ABNF below, list rules are expanded as per
Section 11.
Accept = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [
OWS ( media-range [ accept-params ] ) ] ) ]
Accept-Charset = *( "," OWS ) ( ( charset / "*" ) [ weight ] ) *( OWS
"," [ OWS ( ( charset / "*" ) [ weight ] ) ] )
Accept-Encoding = [ ( "," / ( codings [ weight ] ) ) *( OWS "," [ OWS
( codings [ weight ] ) ] ) ]
Accept-Language = *( "," OWS ) ( language-range [ weight ] ) *( OWS
"," [ OWS ( language-range [ weight ] ) ] )
Accept-Ranges = acceptable-ranges
Allow = [ ( "," / method ) *( OWS "," [ OWS method ] ) ]
Authorization = credentials
BWS = OWS
Content-Encoding = *( "," OWS ) content-coding *( OWS "," [ OWS
content-coding ] )
Content-Language = *( "," OWS ) language-tag *( OWS "," [ OWS
language-tag ] )
Content-Length = 1*DIGIT
Content-Location = absolute-URI / partial-URI
Content-Range = byte-content-range / other-content-range
Content-Type = media-type
Date = HTTP-date
ETag = entity-tag
Expect = "100-continue"
From = mailbox
GMT = %x47.4D.54 ; GMT
HTTP-date = IMF-fixdate / obs-date
Host = uri-host [ ":" port ]
IMF-fixdate = day-name "," SP date1 SP time-of-day SP GMT
If-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
entity-tag ] ) )
If-Modified-Since = HTTP-date
If-None-Match = "*" / ( *( "," OWS ) entity-tag *( OWS "," [ OWS
entity-tag ] ) )
If-Range = entity-tag / HTTP-date
If-Unmodified-Since = HTTP-date
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Last-Modified = HTTP-date
Location = URI-reference
Max-Forwards = 1*DIGIT
OWS = *( SP / HTAB )
Proxy-Authenticate = *( "," OWS ) challenge *( OWS "," [ OWS
challenge ] )
Proxy-Authorization = credentials
RWS = 1*( SP / HTAB )
Range = byte-ranges-specifier / other-ranges-specifier
Referer = absolute-URI / partial-URI
Retry-After = HTTP-date / delay-seconds
Server = product *( RWS ( product / comment ) )
Trailer = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
URI-reference = <URI-reference, see [RFC3986], Section 4.1>
User-Agent = product *( RWS ( product / comment ) )
Vary = "*" / ( *( "," OWS ) field-name *( OWS "," [ OWS field-name ]
) )
Via = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
comment ] ) ] )
WWW-Authenticate = *( "," OWS ) challenge *( OWS "," [ OWS challenge
] )
absolute-URI = <absolute-URI, see [RFC3986], Section 4.3>
absolute-path = 1*( "/" segment )
accept-ext = OWS ";" OWS token [ "=" ( token / quoted-string ) ]
accept-params = weight *accept-ext
acceptable-ranges = ( *( "," OWS ) range-unit *( OWS "," [ OWS
range-unit ] ) ) / "none"
asctime-date = day-name SP date3 SP time-of-day SP year
auth-param = token BWS "=" BWS ( token / quoted-string )
auth-scheme = token
authority = <authority, see [RFC3986], Section 3.2>
byte-content-range = bytes-unit SP ( byte-range-resp /
unsatisfied-range )
byte-range = first-byte-pos "-" last-byte-pos
byte-range-resp = byte-range "/" ( complete-length / "*" )
byte-range-set = *( "," OWS ) ( byte-range-spec /
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suffix-byte-range-spec ) *( OWS "," [ OWS ( byte-range-spec /
suffix-byte-range-spec ) ] )
byte-range-spec = first-byte-pos "-" [ last-byte-pos ]
byte-ranges-specifier = bytes-unit "=" byte-range-set
bytes-unit = "bytes"
challenge = auth-scheme [ 1*SP ( token68 / [ ( "," / auth-param ) *(
OWS "," [ OWS auth-param ] ) ] ) ]
charset = token
codings = content-coding / "identity" / "*"
comment = "(" *( ctext / quoted-pair / comment ) ")"
complete-length = 1*DIGIT
content-coding = token
credentials = auth-scheme [ 1*SP ( token68 / [ ( "," / auth-param )
*( OWS "," [ OWS auth-param ] ) ] ) ]
ctext = HTAB / SP / %x21-27 ; '!'-'''
/ %x2A-5B ; '*'-'['
/ %x5D-7E ; ']'-'~'
/ obs-text
date1 = day SP month SP year
date2 = day "-" month "-" 2DIGIT
date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
day = 2DIGIT
day-name = %x4D.6F.6E ; Mon
/ %x54.75.65 ; Tue
/ %x57.65.64 ; Wed
/ %x54.68.75 ; Thu
/ %x46.72.69 ; Fri
/ %x53.61.74 ; Sat
/ %x53.75.6E ; Sun
day-name-l = %x4D.6F.6E.64.61.79 ; Monday
/ %x54.75.65.73.64.61.79 ; Tuesday
/ %x57.65.64.6E.65.73.64.61.79 ; Wednesday
/ %x54.68.75.72.73.64.61.79 ; Thursday
/ %x46.72.69.64.61.79 ; Friday
/ %x53.61.74.75.72.64.61.79 ; Saturday
/ %x53.75.6E.64.61.79 ; Sunday
delay-seconds = 1*DIGIT
entity-tag = [ weak ] opaque-tag
etagc = "!" / %x23-7E ; '#'-'~'
/ obs-text
field-content = field-vchar [ 1*( SP / HTAB ) field-vchar ]
field-name = token
field-value = *( field-content / obs-fold )
field-vchar = VCHAR / obs-text
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first-byte-pos = 1*DIGIT
fragment = <fragment, see [RFC3986], Section 3.5>
hour = 2DIGIT
http-URI = "http://" authority path-abempty [ "?" query ] [ "#"
fragment ]
https-URI = "https://" authority path-abempty [ "?" query ] [ "#"
fragment ]
language-range = <language-range, see [RFC4647], Section 2.1>
language-tag = <Language-Tag, see [RFC5646], Section 2.1>
last-byte-pos = 1*DIGIT
mailbox = <mailbox, see [RFC5322], Section 3.4>
media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
";" OWS parameter )
media-type = type "/" subtype *( OWS ";" OWS parameter )
method = token
minute = 2DIGIT
month = %x4A.61.6E ; Jan
/ %x46.65.62 ; Feb
/ %x4D.61.72 ; Mar
/ %x41.70.72 ; Apr
/ %x4D.61.79 ; May
/ %x4A.75.6E ; Jun
/ %x4A.75.6C ; Jul
/ %x41.75.67 ; Aug
/ %x53.65.70 ; Sep
/ %x4F.63.74 ; Oct
/ %x4E.6F.76 ; Nov
/ %x44.65.63 ; Dec
obs-date = rfc850-date / asctime-date
obs-fold = <obs-fold, see [Messaging], Section 5.2>
obs-text = %x80-FF
opaque-tag = DQUOTE *etagc DQUOTE
other-content-range = other-range-unit SP other-range-resp
other-range-resp = *VCHAR
other-range-set = 1*VCHAR
other-range-unit = token
other-ranges-specifier = other-range-unit "=" other-range-set
parameter = token "=" ( token / quoted-string )
partial-URI = relative-part [ "?" query ]
path-abempty = <path-abempty, see [RFC3986], Section 3.3>
port = <port, see [RFC3986], Section 3.2.3>
product = token [ "/" product-version ]
product-version = token
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protocol-name = <protocol-name, see [Messaging], Section 9.7>
protocol-version = <protocol-version, see [Messaging], Section 9.7>
pseudonym = token
qdtext = HTAB / SP / "!" / %x23-5B ; '#'-'['
/ %x5D-7E ; ']'-'~'
/ obs-text
query = <query, see [RFC3986], Section 3.4>
quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
qvalue = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
range-unit = bytes-unit / other-range-unit
received-by = ( uri-host [ ":" port ] ) / pseudonym
received-protocol = [ protocol-name "/" ] protocol-version
relative-part = <relative-part, see [RFC3986], Section 4.2>
request-target = <request-target, see [Messaging], Section 3.2>
rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT
second = 2DIGIT
segment = <segment, see [RFC3986], Section 3.3>
subtype = token
suffix-byte-range-spec = "-" suffix-length
suffix-length = 1*DIGIT
tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
"^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
time-of-day = hour ":" minute ":" second
token = 1*tchar
token68 = 1*( ALPHA / DIGIT / "-" / "." / "_" / "~" / "+" / "/" )
*"="
type = token
unsatisfied-range = "*/" complete-length
uri-host = <host, see [RFC3986], Section 3.2.2>
weak = %x57.2F ; W/
weight = OWS ";" OWS "q=" qvalue
year = 4DIGIT
Appendix B. Changes from RFC 7230
Most of the sections introducing HTTP's design goals, history,
architecture, conformance criteria, protocol versioning, URIs,
message routing, and header field values have been moved here
(without substantive change).
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Appendix C. Changes from RFC 7231
None yet.
Appendix D. Changes from RFC 7232
None yet.
Appendix E. Changes from RFC 7233
None yet.
Appendix F. Changes from RFC 7235
None yet.
Appendix G. Change Log
This section is to be removed before publishing as an RFC.
G.1. Between RFC723x and draft 00
The changes were purely editorial:
o Change boilerplate and abstract to indicate the "draft" status,
and update references to ancestor specifications.
o Remove version "1.1" from document title, indicating that this
specification applies to all HTTP versions.
o Adjust historical notes.
o Update links to sibling specifications.
o Replace sections listing changes from RFC 2616 by new empty
sections referring to RFC 723x.
o Remove acknowledgements specific to RFC 723x.
o Move "Acknowledgements" to the very end and make them unnumbered.
G.2. Since draft-ietf-httpbis-semantics-00
The changes in this draft are editorial, with respect to HTTP as a
whole, to merge core HTTP semantics into this document:
o Merged introduction, architecture, conformance, and ABNF
extensions from RFC 7230 (Messaging).
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o Rearranged architecture to extract conformance, http(s) schemes,
and protocol versioning into a separate major section.
o Moved discussion of MIME differences to [Messaging] since that is
primarily concerned with transforming 1.1 messages.
o Merged entire content of RFC 7232 (Conditional Requests).
o Merged entire content of RFC 7233 (Range Requests).
o Merged entire content of RFC 7235 (Auth Framework).
o Moved all extensibility tips, registration procedures, and
registry tables from the IANA considerations to normative
sections, reducing the IANA considerations to just instructions
that will be removed prior to publication as an RFC.
G.3. Since draft-ietf-httpbis-semantics-01
o Improve [Welch] citation (<https://github.com/httpwg/http-core/
issues/63>)
o Remove HTTP/1.1-ism about Range Requests
(<https://github.com/httpwg/http-core/issues/71>)
o Cite RFC 8126 instead of RFC 5226 (<https://github.com/httpwg/
http-core/issues/75>)
o Cite RFC 7538 instead of RFC 7238 (<https://github.com/httpwg/
http-core/issues/76>)
o Cite RFC 8288 instead of RFC 5988 (<https://github.com/httpwg/
http-core/issues/77>)
o Cite RFC 8187 instead of RFC 5987 (<https://github.com/httpwg/
http-core/issues/78>)
o Cite RFC 7578 instead of RFC 2388 (<https://github.com/httpwg/
http-core/issues/79>)
o Cite RFC 7595 instead of RFC 4395 (<https://github.com/httpwg/
http-core/issues/80>)
o improve ABNF readability for qdtext (<https://github.com/httpwg/
http-core/issues/81>, <https://www.rfc-editor.org/errata/eid4891>)
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o Clarify "resource" vs "representation" in definition of status
code 416 (<https://github.com/httpwg/http-core/issues/83>,
<https://www.rfc-editor.org/errata/eid4664>)
o Resolved erratum 4072, no change needed here
(<https://github.com/httpwg/http-core/issues/84>,
<https://www.rfc-editor.org/errata/eid4072>)
o Clarify DELETE status code suggestions
(<https://github.com/httpwg/http-core/issues/85>,
<https://www.rfc-editor.org/errata/eid4436>)
o In Section 6.3.3, fix ABNF for "other-range-resp" to use VCHAR
instead of CHAR (<https://github.com/httpwg/http-core/issues/86>,
<https://www.rfc-editor.org/errata/eid4707>)
o Resolved erratum 5162, no change needed here
(<https://github.com/httpwg/http-core/issues/89>,
<https://www.rfc-editor.org/errata/eid5162>)
o Replace "response code" with "response status code" and "status-
code" (the ABNF production name from the HTTP/1.1 message format)
by "status code" (<https://github.com/httpwg/http-core/issues/94>,
<https://www.rfc-editor.org/errata/eid4050>)
o Added a missing word in Section 9.4 (<https://github.com/httpwg/
http-core/issues/98>, <https://www.rfc-editor.org/errata/eid4452>)
o In Section 11, fixed an example that had trailing whitespace where
it shouldn't (<https://github.com/httpwg/http-core/issues/104>,
<https://www.rfc-editor.org/errata/eid4169>)
o In Section 9.3.7, remove words that were potentially misleading
with respect to the relation to the requested ranges
(<https://github.com/httpwg/http-core/issues/102>,
<https://www.rfc-editor.org/errata/eid4358>)
Index
1
100 Continue (status code) 106
100-continue (expect value) 74
101 Switching Protocols (status code) 106
1xx Informational (status code class) 106
2
200 OK (status code) 107
201 Created (status code) 108
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202 Accepted (status code) 108
203 Non-Authoritative Information (status code) 108
204 No Content (status code) 109
205 Reset Content (status code) 109
206 Partial Content (status code) 110
2xx Successful (status code class) 107
3
300 Multiple Choices (status code) 114
301 Moved Permanently (status code) 115
302 Found (status code) 116
303 See Other (status code) 116
304 Not Modified (status code) 117
305 Use Proxy (status code) 117
306 (Unused) (status code) 118
307 Temporary Redirect (status code) 118
3xx Redirection (status code class) 113
4
400 Bad Request (status code) 118
401 Unauthorized (status code) 118
402 Payment Required (status code) 119
403 Forbidden (status code) 119
404 Not Found (status code) 119
405 Method Not Allowed (status code) 120
406 Not Acceptable (status code) 120
407 Proxy Authentication Required (status code) 120
408 Request Timeout (status code) 120
409 Conflict (status code) 121
410 Gone (status code) 121
411 Length Required (status code) 121
412 Precondition Failed (status code) 122
413 Payload Too Large (status code) 122
414 URI Too Long (status code) 122
415 Unsupported Media Type (status code) 122
416 Range Not Satisfiable (status code) 123
417 Expectation Failed (status code) 123
426 Upgrade Required (status code) 123
4xx Client Error (status code class) 118
5
500 Internal Server Error (status code) 124
501 Not Implemented (status code) 124
502 Bad Gateway (status code) 124
503 Service Unavailable (status code) 125
504 Gateway Timeout (status code) 125
505 HTTP Version Not Supported (status code) 125
5xx Server Error (status code class) 124
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A
Accept header field 88
Accept-Charset header field 90
Accept-Encoding header field 91
Accept-Language header field 93
Accept-Ranges header field 145
Allow header field 145
Authorization header field 97
accelerator 12
authoritative response 148
B
browser 10
C
CONNECT method 69
Canonical Root URI 96
Content-Encoding header field 46
Content-Language header field 47
Content-Length header field 48
Content-Location header field 49
Content-Range header field 53
Content-Type header field 45
cache 13
cacheable 14, 62
captive portal 13
client 10
compress (Coding Format) 40
compress (content coding) 40
conditional request 76
connection 10
content coding 40
content negotiation 8
D
DELETE method 68
Date header field 130
Delimiters 27
deflate (Coding Format) 40
deflate (content coding) 40
downstream 12
E
ETag header field 139
Expect header field 74
effective request URI 31
F
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From header field 100
G
GET method 63
Grammar
absolute-path 15
absolute-URI 15
Accept 88
Accept-Charset 91
Accept-Encoding 91
accept-ext 88
Accept-Language 93
accept-params 88
Accept-Ranges 145
acceptable-ranges 145
Allow 145
ALPHA 9
asctime-date 130
auth-param 95
auth-scheme 95
authority 15
Authorization 97
BWS 29
byte-content-range 53
byte-range 53
byte-range-resp 53
byte-range-set 43
byte-range-spec 43
byte-ranges-specifier 43
bytes-unit 42-43
challenge 95
charset 38
codings 91
comment 28
complete-length 53
content-coding 40
Content-Encoding 46
Content-Language 47
Content-Length 48
Content-Location 49
Content-Range 53
Content-Type 45
CR 9
credentials 96
CRLF 9
ctext 28
CTL 9
Date 130
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date1 129
day 129
day-name 129
day-name-l 129
delay-seconds 133
DIGIT 9
DQUOTE 9
entity-tag 139
ETag 139
etagc 139
Expect 74
field-content 26
field-name 22, 30
field-value 26
field-vchar 26
first-byte-pos 43
fragment 15
From 100
GMT 129
HEXDIG 9
Host 32
hour 129
HTAB 9
HTTP-date 127
http-URI 16
https-URI 17
If-Match 80
If-Modified-Since 82
If-None-Match 81
If-Range 85
If-Unmodified-Since 83
IMF-fixdate 129
language-range 93
language-tag 42
last-byte-pos 43
Last-Modified 137
LF 9
Location 131
Max-Forwards 76
media-range 88
media-type 38
method 59
minute 129
month 129
obs-date 129
obs-text 28
OCTET 9
opaque-tag 139
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other-content-range 53
other-range-resp 53
other-range-unit 42, 44
OWS 29
parameter 38
partial-URI 15
port 15
product 102
product-version 102
protocol-name 34
protocol-version 34
Proxy-Authenticate 144
Proxy-Authorization 97
pseudonym 34
qdtext 28
query 15
quoted-pair 28
quoted-string 28
qvalue 88
Range 86
range-unit 42
ranges-specifier 43
received-by 34
received-protocol 34
Referer 101
Retry-After 133
rfc850-date 130
RWS 29
second 129
segment 15
Server 146
SP 9
subtype 38
suffix-byte-range-spec 43
suffix-length 43
tchar 27
time-of-day 129
token 27
token68 95
Trailer 30
type 38
unsatisfied-range 53
uri-host 15
URI-reference 15
User-Agent 102
Vary 133
VCHAR 9
Via 34
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weak 139
weight 88
WWW-Authenticate 143
year 129
gateway 12
gzip (Coding Format) 41
gzip (content coding) 40
H
HEAD method 63
Host header field 32
http URI scheme 16
https URI scheme 17
I
If-Match header field 80
If-Modified-Since header field 82
If-None-Match header field 81
If-Range header field 85
If-Unmodified-Since header field 83
idempotent 62
inbound 12
interception proxy 13
intermediary 11
L
Last-Modified header field 137
Location header field 131
M
Max-Forwards header field 76
Media Type
multipart/byteranges 55
multipart/x-byteranges 55
message 10
metadata 134
multipart/byteranges Media Type 55
multipart/x-byteranges Media Type 55
N
non-transforming proxy 35
O
OPTIONS method 70
origin server 10
outbound 12
P
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POST method 64
PUT method 65
Protection Space 96
Proxy-Authenticate header field 144
Proxy-Authorization header field 97
payload 51
phishing 148
proxy 12
R
Range header field 86
Realm 96
Referer header field 101
Retry-After header field 132
recipient 10
representation 37
request 10
resource 14
response 10
reverse proxy 12
S
Server header field 146
Status Codes Classes
1xx Informational 106
2xx Successful 107
3xx Redirection 113
4xx Client Error 118
5xx Server Error 124
safe 61
selected representation 37, 77, 134
sender 10
server 10
spider 10
T
TRACE method 71
Trailer header field 30
target URI 30
target resource 30
transforming proxy 35
transparent proxy 13
tunnel 13
U
URI scheme
http 16
https 17
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User-Agent header field 102
upstream 12
user agent 10
V
Vary header field 133
Via header field 34
validator 134
strong 135
weak 135
W
WWW-Authenticate header field 143
X
x-compress (content coding) 40
x-gzip (content coding) 40
Acknowledgments
This edition of the HTTP specification builds on the many
contributions that went into RFC 1945, RFC 2068, RFC 2145, and RFC
2616, including substantial contributions made by the previous
authors, editors, and Working Group Chairs: Tim Berners-Lee, Ari
Luotonen, Roy T. Fielding, Henrik Frystyk Nielsen, Jim Gettys,
Jeffrey C. Mogul, Larry Masinter, Paul J. Leach, and Yves Lafon.
See Section 10 of [RFC7230] for further acknowledgements from prior
revisions.
In addition, this document has reincorporated the HTTP Authentication
Framework, previously defined in RFC 7235 and RFC 2617. We thank
John Franks, Phillip M. Hallam-Baker, Jeffery L. Hostetler, Scott
D. Lawrence, Paul J. Leach, Ari Luotonen, and Lawrence C. Stewart
for their work on that specification. See Section 6 of [RFC2617] for
further acknowledgements.
[[newacks: New acks to be added here.]]
Authors' Addresses
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Roy T. Fielding (editor)
Adobe
345 Park Ave
San Jose, CA 95110
USA
EMail: fielding@gbiv.com
URI: https://roy.gbiv.com/
Mark Nottingham (editor)
Fastly
EMail: mnot@mnot.net
URI: https://www.mnot.net/
Julian F. Reschke (editor)
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
EMail: julian.reschke@greenbytes.de
URI: https://greenbytes.de/tech/webdav/
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